This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Riggs, B. L. Articles by Melton, L. J. Search for Related Content PubMed PubMed Citation Articles by Riggs, B. L. Articles by Melton, L. J., III

Sex Steroids and the Construction and Conservation of the Adult Skeleton

Division of Endocrinology and Metabolism (B.L.R., S.K., L.J.M.), Department of Internal Medicine, and Section of Clinical Epidemiology (L.J.M.), Department of Health Sciences Research, Mayo Clinic and Mayo Foundation, 200 First Street SW, Rochester, Minnesota 55905

Correspondence: Address all correspondence and requests for reprints to: B. Lawrence Riggs, M.D., Mayo Clinic and Mayo Foundation, 200 First Street SW, North 6 Plummer, Rochester, Minnesota 55905.

	Abstract

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
Here we review and extend a new unitary model for the pathophysiology of involutional osteoporosis that identifies estrogen (E) as the key hormone for maintaining bone mass and E deficiency as the major cause of age-related bone loss in both sexes. Also, both E and testosterone (T) are key regulators of skeletal growth and maturation, and E, together with GH and IGF-I, initiate a 3- to 4-yr pubertal growth spurt that doubles skeletal mass. Although E is required for the attainment of maximal peak bone mass in both sexes, the additional action of T on stimulating periosteal apposition accounts for the larger size and thicker cortices of the adult male skeleton. Aging women undergo two phases of bone loss, whereas aging men undergo only one. In women, the menopause initiates an accelerated phase of predominantly cancellous bone loss that declines rapidly over 4–8 yr to become asymptotic with a subsequent slow phase that continues indefinitely. The accelerated phase results from the loss of the direct restraining effects of E on bone turnover, an action mediated by E receptors in both osteoblasts and osteoclasts. In the ensuing slow phase, the rate of cancellous bone loss is reduced, but the rate of cortical bone loss is unchanged or increased. This phase is mediated largely by secondary hyperparathyroidism that results from the loss of E actions on extraskeletal calcium metabolism. The resultant external calcium losses increase the level of dietary calcium intake that is required to maintain bone balance. Impaired osteoblast function due to E deficiency, aging, or both also contributes to the slow phase of bone loss. Although both serum bioavailable (Bio) E and Bio T decline in aging men, Bio E is the major predictor of their bone loss. Thus, both sex steroids are important for developing peak bone mass, but E deficiency is the major determinant of age-related bone loss in both sexes.

I. Introduction

A. Overview of methodology

B. Applications of microarray technology

II. Use of Microarrays in Immune Cell Characterization

A. Characterization of T and B cell lineages

B. Characterization of immune cell activation

III. Microarray-Aided Cancer Diagnosis and Identification of Prognostic Markers

A. Cancer classification

B. Diagnostic markers for cancer

C. Development of a comprehensive leukemia database

D. Experimental limitations

IV. Microarry Analysis of Gene Expression in Response to Various Stimuli

A. Glucocorticoid-mediated changes in gene expression

B. Signaling in calcium-mediated lymphocyte activation

C. Stress responses

D. Cell growth and differentiation

E. Apoptosis

V. Conclusions

	I. Introduction

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
INVOLUTIONAL OSTEOPOROSIS IS one of the most serious diseases facing the aging population. Age-related bone loss is universal, affecting older women and men in every population. By one analysis, 35% of postmenopausal white women and 19% of white men have osteoporosis as assessed by bone mineral density (BMD) measurements at the hip, spine, or distal forearm (1). Nonwhite men and women are affected to a lesser, but still substantial, degree (2). In the United States alone, osteoporosis leads to an estimated 1.3 million fractures each year, at a cost to the health care system at least 14 billion dollars annually for only direct expenditures (3). Moreover, the lifetime risk for these fractures is 40% for women and nearly 15% for men, and these figures will rise substantially based on projected increases in life expectancy (4). Few serious diseases have such a high penetrance in their target populations.

Sixty years ago, Albright et al. (5) related the causation of postmenopausal osteoporosis to estrogen (E) deficiency and found that E treatment improved calcium balance in postmenopausal women. These pioneering studies were validated some 30 yr later by densitometric studies demonstrating that the accelerated bone loss induced by ovariectomy could be prevented by E therapy (6, 7). Although the menopause came to be well accepted as a cause of postmenopausal bone loss, a number of other age-related factors were also implicated in both women and men. These included secondary hyperparathyroidism (8), impaired vitamin D metabolism (9), and impaired osteoblast function (10). In addition, nutritional vitamin D deficiency (11) and inadequate calcium intake (12) were found to cause bone loss in subsets of the aging population. Thus, E deficiency was believed to be but one of the multiple causes of involutional osteoporosis and its effect largely limited to bone loss in women during the first decade after menopause.

In 1998, however, we (13) proposed a new unitary model for the pathophysiology of involutional osteoporosis that identified E deficiency as the major cause of both the early, accelerated and the late, slow phases of bone loss in postmenopausal women and as a contributing cause of bone loss in elderly men. We now update this model based on new data that have been published subsequently and extend it by examining the effect of sex steroids on skeletal growth and maturation and on the sensing of biomechanical strain by bone cells.

	II. Skeletal Effects of Sex Steroids

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
A. Synthesis and metabolism of sex steroids
In premenopausal women, more than 95% of serum estradiol (E₂) and most of serum estrone (E₁) is derived from ovarian secretion. Peripheral conversion of steroid precursors accounts for the remainder in premenopausal women and for almost all of the circulating estrogens in postmenopausal women. Also, in men, more than 95% of the major potent circulating androgen, testosterone (T), is derived from testicular secretion. For serum T in premenopausal women, 25% is derived from ovarian secretion, 25% from adrenal secretion, and 50% from peripheral conversion, and the sources are similar in postmenopausal women except that ovarian secretion of T decreases. In many target tissues, 5-dihydrotestosterone (DHT), formed from T through the action of the enzyme 5-reductase, is the main source of androgenic activity. 5-Reductase is present as two isoforms. Almost all of the circulating DHT arises from back diffusion into the circulation from this extragonadal conversion, rather than from direct gonadal secretion. In addition, the adrenal cortex and, to a lesser extent, the gonads secrete large amounts of C19 androgens, chiefly dehydroepiandrosterone (DHEA), DHEA sulfate (DHEA-S), and ⁴-androstenedione. Although only weakly androgenic themselves, they are an important source of substrate for the extragonadal synthesis of potent sex steroids [see reviews (14, 15) for details of sex steroid biosynthesis]. Table 1 gives mean values for circulating sex steroids in young adult and elderly women and men based on the data of Labrie et al. (16) and of Khosla et al. (17).

Extragonadal biosynthesis plays a minor role in sex steroid biosynthesis in lower mammals (except for the brain in most mammals and the placenta in some ruminants), but in humans and higher primates, extragonadal biosynthesis is remarkably well developed (19, 20). Thus, multiple peripheral tissues including bone can synthesize E₁ from circulating C19 steroids, and E₂ and DHT can be synthesized directly from T. The concentrations of circulating C19 precursors are high. For example, serum DHEA-S levels in adult men and women are 100- to 500-fold higher than T and 1,000- to 10,000-fold more than E₂ (19). Thus, although the conversion rate is only 1–2%, the quantity of active new steroids generated extragonadally is appreciable. The principal site of this conversion is adipose tissue. The rate of extragonadal biosynthesis is increased in obese persons and is also increased in aging postmenopausal women (20).

Labrie et al. (16) have given the name "intracrinology" to the process by which active steroids are synthesized by a peripheral target cell, in which the action of the steroid is exerted without its release into the extracellular fluid. The extragonadal intracrine tissues that synthesize E₁ and E₂ utilize the same enzymatic pathways that are employed for gonadal synthesis except that they are unable to synthesize C₁₉ steroids and must depend on circulating precursors for substrate. Figure 1 shows the major pathways for the extragonadal synthesis of the potent sex steroids. The key enzymes involved in this process—the seven isoforms of 17ß-hydroxysteroid dehydrogenase (17ß-HSD) (21), aromatase (CYP19) (22, 23, 24), steroid sulfatase (25), 3ß-hydroxysteroid dehydrogenase (3ß-HSD) (26), and 5-reductase (27, 28)—are present in osteoblast-lineage cells. Aromatase is also expressed in chondrocytes (29). The sex steroids synthesized extragonadally undoubtedly also have paracrine actions.

View larger version (24K): [in this window] [in a new window]   Figure 1. Main pathways for extragonadal synthesis of active androgens (boxes) and potent estrogens (circles) in humans. The various 17ß-HSD isoforms are given as numbers within the broken outline. The arrows indicate whether the reaction is reversible or irreversible. Asterisks indicate the enzymes that have been shown to be present in osteoblasts. S, Sulfate; AND, androstenediol. See text for derivation of other abbreviations. [Adapted with permission from F. Labrie et al.: J Mol Endocrinol 25:1–16, 2000 (19 ). © the Society for Endocrinology.]

B. Physiological effects of estrogen
E has specific functions at the organ, tissue, and cellular levels of the skeleton. At the organ level, E acts to conserve bone mass. Indeed, the actions of E and those of biomechanical strain are the major physiological mechanisms for bone mass conservation. In fact, with a few exceptions, such as states of corticosteroid excess, major decreases in bone mass do not occur unless one of these two homeostatic mechanisms is affected. At the tissue level, E tonically suppresses bone turnover and maintains balanced rates of bone formation and bone resorption (as reviewed in Ref. 32). At the cellular level, E affects the generation, lifespan, and functional activity of both osteoclasts and osteoblasts. E decreases osteoclast formation and activity and, by increasing apoptosis, it decreases osteoclast lifespan (33). As will be discussed later, controversy exists about the action of E on osteoblasts. Some evidence suggests that E increases osteoblast formation, differentiation, proliferation, and function, although results have varied among different model systems (34, 35, 36). Recently, two groups (32, 37) have demonstrated that E antagonizes glucocorticoid-induced osteoblast apoptosis and, thus, extends osteoblast lifespan.

As originally pointed out by Frost (38), the activities of osteoclasts and osteoblasts are combined into functional assemblies called basic multicellular units (BMUs). A remodeling cycle begins with formation of a new BMU on a previously inactive surface of bone. The lining cells disappear and are replaced by multinucleated osteoclasts that construct a resorption lacunae on the endosteal surface of bone over a 2-wk interval. The resorption phase then is terminated, probably by osteoclast apoptosis, and after a brief reversal phase, a team of osteoblasts is recruited that fill in the resorption cavity with new bone. In cortical bone, osteoclasts form the leading edge of a cutting cone that creates a resorption tunnel, and osteoblasts follow in their wake to convert it into a structural osteon [Haversian system (for reviews, see Refs. 32 and 39)].

E deficiency affects remodeling in several ways. First, it increases the activation frequency ("birth rate") of BMUs, which leads to higher bone turnover. Second, it induces a remodeling imbalance by prolonging the resorption phase [osteoclast apoptosis is reduced (33)] and shortening the formation phase [osteoblast apoptosis is increased (32)]. Also, increased osteoclast recruitment extends the progression of the BMU. As a consequence of these changes, the volume of the resorption cavity is increased beyond the capacity of the osteoblasts to refill it. In cancellous bone, the extended osteoclast lifespan increases resorption depth, leading to trabecular plate perforation and loss of trabecular connectivity (39, 40, 41). In cortical bone, the rapid phase is associated with subendocortical cavitation, and eventually, the inner third of the cortex may assume cancellous-like characteristics (39). The consequences of the effects of perforative resorption on cancellous bone are shown in Fig. 2, which compares the three-dimensional microstructure of lumbar spine bone samples from an E-replete premenopausal woman with those from an E-deficient woman with postmenopausal osteoporosis. In contrast to the osteoclast-mediated disruption of the cancellous bone microarchitecture during the rapid phase, the subsequent slow phase of bone loss is characterized by trabecular thinning in which impaired osteoblast activity plays a prominent causal role (39).

View larger version (66K): [in this window] [in a new window]   Figure 2. Three-dimensional reconstruction by microcomputed tomography of a lumbar spine sample from a young-adult normal woman and from a woman with postmenopausal osteoporosis. In the osteoporotic woman, not only is bone mass reduced, but there is microarchitectural deterioration of bone structure. Whereas the rod-like structure in the normal case is very isotropic, the structure in the osteoporotic case shows preferential loss of horizontal struts and a concomitant loss of trabecular connectivity. These changes lead to a reduction in bone strength that is more than would be predicted by the decrease in BMD. Images courtesy of Ralph Müller, Ph.D., Beth Israel Deaconess Medical Center and Harvard Medical School (Boston, MA).

D. Transduction by sex steroid receptors
Before 1988, sex steroids were believed to affect the skeleton only indirectly by regulating secretion of systemic calcitrophic hormones. However, it now is firmly established that osteoblasts (48, 49), osteoclasts (50, 51), and osteocytes (52, 53) contain functional E receptors (ERs), although their concentration is lower than in reproductive tissues. In addition to the classical ER, a genetically distinct second receptor, ERß, has recently been discovered that has extensive homology with the ligand and DNA binding domains of ER (54). ER/ERß heterodimers also have been described (55). ER mediates most of the actions of E on bone cells, whereas ERß, in some circumstances, can act as a dominant negative antagonist to ER (56, 57). Bone cells contain both receptors, although their distributions within bone differ. Immunohistological studies of developing human bone have demonstrated that ER is the predominant species in cortical bone but that ERß is the predominant species in cancellous bone (58). Moreover, variation in the sequence of expression of ER and ERß during osteoblast differentiation could contribute to developmental differences in expression of ER-responsive genes. Thus, in human fetal osteoblastic cells, ER mRNA increases only slightly (3-fold), whereas ERß increases markedly (20-fold) and exponentially during osteoblast differentiation (59). Chondrocytes in human growth plate cartilage also contain both ER and ERß (60, 61). Finally, both osteoblasts (62) and osteoclasts (63) also contain high affinity androgen receptors (ARs).

Much has been learned from studies of the skeletal phenotypes of ER knockout (ERKO) (64), ERß knockout (BERKO) (65), and double ER knockout (DERKO) (65) mice. However, the findings in these mutant mice do not completely reproduce those of E-deficient women. For example, the ERKO and DERKO mice have shortened femoral length (64, 65), whereas E-deficient girls and the single reported case of a male with homozygous null mutations of the ER gene (64) have elongated limb bones due to failure of the epiphyseal growth plate to fuse. In ERKO or DERKO mice, there is a decrease in appendicular bone growth (associated with and possibly due to a decrease in serum IGF-I levels) that is greater in females than in males (64, 66). However, ERKO mice have a cortical osteopenia and increased bone turnover that is greater in the male than in the female (64). In contrast, the skeletal phenotype in BERKO males is similar to that of the wild-type males (65). However, the BERKO females have an increase in cortical bone associated with increased periosteal apposition that develops during growth (3–6 months old) and is maintained in adults (12–13 months old; Refs. 67 and 68). The adult BERKO females are also protected against the age-related cancellous bone loss that occurs in the wild-type mice, and although the growth plate width is unaffected, histological indices of formation and resorption are decreased (67, 68). Interestingly, after ovariectomy, adult DERKO females undergo the same degree of cancellous bone loss as wild-type females, and the bone loss can be prevented by E treatment, but at a 5-fold higher dosage than is required to prevent bone loss in wild-type mice (69). The reason for these observations is unclear at present, but they could be caused by the persistence of ER splice variants in the DERKO mice, or possibly, by a sex-nonspecific, nongenomic mechanism involving the AR (70).

Although the exact meaning of the data from these mutant mice is unclear and more studies are needed, several tentative interpretations can be made. First, most of E-dependent bone growth is mediated through ER because growth is disrupted in the ERKO and DERKO mice, but not in the BERKO mice. Second, ERß may account for at least part of the sexual dimorphic changes in the skeleton because BERKO females, but not BERKO males, have larger cortical bone width than the wild-type females. These changes may be the result of ERß-antagonism of ER-stimulated periosteal bone formation. Third, because 1-yr-old BERKO females have more cancellous bone than do wild-type females, ERß may be permissive for age-related bone loss in females, possibly by stimulating bone resorption on cancellous and endocortical surfaces or by inhibiting a stimulatory effect of ER on bone formation. Alternatively, the deletion of ERß may lead to enhanced sensitivity of bone to ER and, hence, to an increase in E action despite age-related decreases in serum E levels.

The testicular feminized male (TFM) rat has a spontaneous homozygous null mutation of the AR gene, resulting in androgen resistance (71). The animals have a female skeletal phenotype, but are not osteoporotic. The aromatase knockout (ArKO) mouse is E deficient because of targeted deletion of the CYP19 aromatase gene. Both ArKO males and females are osteoporotic. However, as assessed by histomorphometry and serum osteocalcin levels, the ArKO females have high bone turnover, whereas ArKO males have decreased bone turnover (72). The explanation for this sexually dimorphic response in the ArKO mice is not clear at present.

E. Molecular mediators of sex steroid action on bone cells
During the last decade, but especially during the past 3 yr, major progress has been made on elucidating the molecular mechanisms of E action on bone cells. Early studies focused on the role of E deficiency in increasing the production in bone of the proinflammatory cytokines IL-1, IL-6, TNF, granulocyte-macrophage colony-stimulating factor, macrophage colony-stimulating factor (M-CSF), and prostaglandin-E₂ (PGE₂). These cytokines increase bone resorption, mainly by increasing the pool size of preosteoclasts in bone marrow (31, 32, 73). Moreover, ovariectomy-induced increases in osteoclastogenesis are attenuated or prevented by measures that impair the synthesis or response to IL-1, IL-6, TNF, or PGE₂ (31, 32, 74). E also up-regulates TGF-ß (75), an inhibitor of bone resorption that acts directly on osteoclasts to decrease their activity (33) and rate of apoptosis (32).

However, E regulation of bone resorption must now be re-evaluated because of the discovery of three new members of the TNF ligand and receptor signaling family that are the final effectors of osteoclast differentiation and function (76, 77). The long-sought osteoblast-derived paracrine effector of osteoclast differentiation was identified as the receptor activator of nuclear factor-B ligand (RANKL), which is expressed by stromal-osteoblastic lineage cells. Cell-to-cell contact between these cells and osteoclast lineage cells allows RANKL to bind its membrane receptor, RANK, potently stimulating all aspects of osteoclast function: in response to RANKL signaling, osteoclast differentiation and activity increase and osteoclast apoptosis decreases. Indeed, RANKL is both necessary and sufficient for osteoclast formation, provided that permissive concentrations of M-CSF are present. The stromal-osteoblast lineage cells also secrete osteoprotegerin (OPG), a soluble decoy receptor that neutralizes RANKL. E increases OPG (78) and decreases M-CSF (79) and RANK (80). However, part of its effect on this signaling system may be indirect through E-stimulated intermediaries. Thus, IL-1 and TNF increase RANKL, OPG, and M-CSF, whereas PGE₂ increases RANKL and decreases OPG (31, 76, 77). E has not yet been shown to regulate RANKL or RANK directly. E also blocks the activity of Jun NH₂-terminal kinase and the resulting production of c-Jun and JunD in osteoclast lineage cells (80, 81). Thus, it seems likely that E inhibits bone resorption by inducing small but cumulative changes in multiple E-dependent regulatory factors, as is shown by the model in Fig. 3.

View larger version (47K): [in this window] [in a new window]   Figure 3. Model for mediation of effects of E on osteoclast formation and function by cytokines in bone marrow microenvironment. Stimulatory factors are shown in orange and inhibitory factors are shown in blue. Positive (+) or negative (-) effects of E on these regulatory factors are shown in red. The model assumes that regulation is accomplished by multiple cytokines working together in concert. [Modified with permission from B. L. Riggs: J Clin Invest 106:1203–1204, 2000 (73 ).]

The effects of androgens on inhibiting bone resorption may be mediated, at least in part, by decreased IL-6 production, because 5-DHT suppresses constitutive and cytokine-stimulated IL-6 production in murine marrow stromal cells (42) and in human osteoblastic cells (87), and this inhibition is quantitatively similar to that achieved with E. Both in osteoblastic cells in vitro and in elderly men in vivo, androgen decreases (88, 89) and E increases (78, 89) OPG production, which may partly explain why the antiresorptive action of T is weaker than the antiresorptive action of E.

Finally, Kousteni et al. (70) have reported that the antiapoptotic effects of E and T on osteoblasts and osteocytes may be mediated by rapid, nongenomic, and sex-nonspecific signaling through the ligand binding domain of ER, ERß, or AR. This action is distinct from the classical actions of these receptors, which are sex-specific, genomic, and transcriptional.

Thus, the autocrine and paracrine basis for sex steroid action on bone cells has recently come into sharper focus. However, how to weigh the importance of the various cytokines and how to quantify their complex interactions in mediating the sex steroid effects are subjects for further research.

F. Interaction with biomechanical forces
Frost (90) has emphasized the role of biomechanical strain, especially that induced by muscle contraction, in determining the level of bone mass. He suggests that the strain is sensed by an internal skeletal mechanostat that initiates changes in bone remodeling to adjust bone mass and distribution to a level that is appropriate for the ambient biomechanical forces. At normal adult activity levels, bone remodeling is maintained by what he terms a conservation mode that suppresses BMU activity on all bone surfaces. The higher strain levels associated with growth or extreme physical activity will induce a modeling mode that increases bone mass by accretion on bone surfaces. However, chronically low strain levels will induce a disuse mode of bone remodeling. In this mode, there is increased bone turnover on all bone surfaces, but on endosteal surfaces, which are in contact with bone marrow, more bone is resorbed than is formed. From these observations, Frost (91) theorized that the remodeling imbalance on endosteal surfaces induced by inactivity was mediated by a factor released by bone marrow termed rho. He also noted that the histomorphometric changes induced by either E deficiency or the disuse mode were very similar. In both, the bone loss is confined to the endosteal surface and does not involve the intracortical and periosteal surfaces (92). Thus, he hypothesized that E deficiency alters the set point of the mechanostat by decreasing the sensing of strain signals. This then switches bone remodeling from the conservation to the disuse mode. The bone loss will continue until a new steady state is reached where once again strain is sensed as high enough to return to the conservation mode of bone remodeling. This model is illustrated by the schematic in Fig. 4.

View larger version (35K): [in this window] [in a new window]   Figure 4. Schematic of mechanism of E interaction with biomechanical strain to regulate bone mass based on publications of Frost (90 91 92 ). Internal mechanostat in bone senses strain generated by muscle contraction or skeletal loading. Mechanostat provides cybernetic feedback by activating bone cells that induce either bone gain or bone loss until a new steady state is achieved. High bone mass is associated with low strain signals, and low bone mass is associated with high strain signals. However, the sensing of strain by the mechanostat is modulated by ambient E concentration in the bone microenvironment. At menopause, low E levels lead to impaired sensing. Because of the resultant low strain signals, the mechanostat erroneously senses that bone mass is increased. This leads to rapid bone loss that continues until strains are sensed as being the same as those present before menopause, at which point bone loss ceases.

	III. Patterns of Skeletal Growth and Maturation

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
Sex steroids are responsible for the maturation and the sexual dimorphism of the skeleton. Skeletal size and volumetric BMD are similar in prepubertal girls and boys. Between the onset of puberty and young adulthood, however, skeletal mass doubles (100). The rates of increase in statural height and bone remodeling are greatest in early puberty and then decline progressively (100, 101, 102, 103, 104). In contrast, maximal increases in volumetric BMD occur 2 yr later—at menarche in girls and late puberty in boys. The pattern of growth of boys differs from that of girls in two ways: boys have two more years of prepubertal growth because of their later puberty (age 14, rather than age 12 as in girls), and their pubertal growth spurt lasts for 4 yr rather than the 3 yr that it lasts in girls (100, 101, 102, 103, 104). These differences largely account for the 10% greater statural height and the 25% greater peak bone mass achieved by males. For the most part, the greater bone mass in males is due to their greater bone size.

Skeletal growth occurs mainly by modeling that increases the size and shape of bones. Linear bone growth occurs by ossification of the endochondral growth plates. Radial bone growth occurs by periosteal apposition, and the marrow cavity size increases by endosteal resorption. Prepubertal growth is proportionately greater in the legs, whereas pubertal growth is proportionately greater in the trunk (105). The excess in periosteal bone apposition over endosteal bone resorption that occurs during the pubertal growth spurt increases both the size and the volumetric BMD (the total bone mass contained within a volume of bone) of the extremities (106). Puberty is terminated by epiphyseal plate closure, by which time volumetric BMD has reached about 90–95% of peak mass. A process termed "consolidation" then brings the skeleton to its maximal values by continued periosteal apposition and, possibly, also by trabecular thickening. Undoubtedly, part of the increase in BMD during consolidation relates to the decline in the high intracortical porosity associated with the rapid pubertal phase of bone growth. How long consolidation continues is disputed: some find that it lasts only until the end of the second decade (101), whereas others find that for vertebral BMD it may last until the end of the third decade (107).

	IV. Role of Sex Steroids in Skeletal Maturation

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
Before puberty, basal levels of the GH/IGF-I axis maintain slow, but continuous, bone growth. Puberty is triggered by increased pulsatile secretion of GnRH by the hypothalamus, leading to increases in serum gonadotropins and, thus, to increases in gonadal secretion of sex steroids (108). The increases in serum E enhance pulsatile GH secretion in both sexes by 1.5- to 3.0-fold; these increases, in turn, increase circulating, and possibly osteoblast, IGF-I concentrations by 1.5- to 3.0-fold (109, 110). The increases in GH, IGF-I, and E act coordinately to support the pubertal growth spurt. The high pubertal levels of GH and IGF-I are maintained during the 3–4 yr of rapid growth but then gradually decrease to prepubertal levels over several years, although the serum sex steroids remain at adult levels (109). However, it is the increase in serum E that is responsible for the pubertal growth spurt. Males with homozygous mutations in ER or aromatase genes do not undergo rapid adolescent growth, despite normal or increased levels of serum T (110, 111, 112, 113, 114). Moreover, it is the continued rise in serum E levels during puberty that is the probable cause of epiphyseal closure in both sexes, because young adult males who are unable to respond to E because of homozygous mutations of the ER gene (111) or the aromatase gene (112, 113, 114) have open epiphyses, whereas men with testicular feminization due to null mutations of AR achieve epiphyseal closure (115, 116). Experimental studies in juvenile ovariectomized rabbits have demonstrated that E accelerates the programmed senescence in the proliferation rate and number and size of chondrocytes, leading to epiphyseal plate fusion (60). Thus, E both initiates the pubertal growth spurt and then ends it by inducing epiphyseal closure. Sex steroids also appear to increase bone mass during skeletal maturation independently of the effects of circulating levels of GH and IGF-I. The 25% greater bone mass in postpubertal boys over postpubertal girls is likely due mainly to the pubertal increase in serum T, because increases in GH secretion and IGF-I production are similar or even greater in girls than in boys. However, as will be reviewed later, E contributes substantially to volumetric BMD in both sexes. Based on measurements made in a young adult male who was unable to synthesize E because of a genetic defect in aromatase activity, BMD was reduced by 25–40% of predicted values at various skeletal scanning sites (114). Thus, both T and E have substantial effects on bone size and volumetric BMD, although E appears to play the dominant role.

	V. Patterns of Age-Related Bone Loss

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
As pointed out by Seeman (106), the compartments (endosteal, intracortical, and periosteal) of bone may change differentially during bone growth and maturation and during bone loss. However, the changes in the individual compartments cannot be assessed by dual energy x-ray absorptiometry (DXA), the most commonly employed method for assessing bone density, which provides only an integral measure of BMD. Moreover, DXA provides data as areal BMD (g/cm²), which overestimates volumetric BMD (g/cm³) in larger bones by failing to account for the variable dimension of depth. Finally, bone size is ignored in the results of conventional assessments of BMD despite biomechanical data showing that, for a given bone mass, larger bones are stronger (117, 118). Thus, sex steroid sufficiency and deficiency exert differential effects on BMD and bone size that are not captured by most clinical measurements. Age and sex differences in bone size and shape are shown schematically in Fig. 5.

View larger version (30K): [in this window] [in a new window]   Figure 5. Schematic representation in cortical bone of the differential effects of gender on growth and maturation and on bone loss with aging. Before puberty there are no differences in cortical bone between sexes. The bone of the prepubertal female and that of the adult female are similar in proportions, but the bone of the latter is larger. However, owing to a greater rate of periosteal apposition during puberty, the cortical bone of the young-adult male is larger and the cortex is thicker. By old age, there has been extensive loss of cortical bone due to endosteal resorption in both sexes, but because women undergo menopause, they lose more cortical bone than men do. The endosteal resorption with aging is partially offset by a continued periosteal apposition that is 3-fold greater in men than in women. Not shown in the cartoon are age-related increases in intracortical porosity that present in both sexes, but more so in women because of menopause. Cartoons are based on published data (105 141 ).

A. Patterns in women
Women undergo two major phases of involutional bone loss: an early, but transient, accelerated phase that begins at menopause and a slow, continuous phase. The early phase declines exponentially over 4–8 yr to merge asymptotically with the subsequent slow phase. This phase accounts for losses of 20–30% in cancellous bone, but for only 5–10% in cortical bone. Because natural menopause occurs at different ages, the skeletal consequences of menopause are most clearly apparent after ovariectomy. In a 2-yr longitudinal study of middle-aged women who underwent ovariectomy, Genant et al. (7) found losses of 18% in cancellous bone by quantitative computed tomography but of only 4% in cortical bone by single photon absorptiometry. By contrast, Recker et al. (122) and Guthrie et al. (123) followed cohorts of 75 and 224 perimenopausal women, respectively, across natural menopause. In the 3 yr preceding the cessation of menses, Recker et al. (122) found losses of 4% in BMD of the lumbar spine and proximal femur associated with declining serum E levels, whereas Guthrie et al. (123) found losses of only 1–2%. After cessation of menses, Recker et al. reported losses of about 7% over 3 yr when an asymptote was reached. In their more extended follow-up, Guthrie et al. reported that the total excess bone loss was 14% and that the asymptote was not reached until after 8–10 yr. There are two reasons for the higher rate of bone loss in the study of Genant et al. (7) as compared with the latter two investigations (122, 123). First, the more precipitous fall in serum E and, to a lesser extent, T, after ovariectomy led to a more rapid rate of bone loss. Second, the high rate of bone loss found by Genant et al. was determined using quantitative computed tomography that specifically measured the more responsive cancellous bone in the vertebral centrum as compared with DXA that measured overall (both cortical and cancellous) vertebral bone loss in the latter two studies.

B. Patterns in men
Men do not undergo the equivalent of menopause and, thus, lack the early, accelerated phase of bone loss experienced by women. Castrated men (male sex offenders in Czechoslovakia) have a pattern of rapid bone loss similar to that of women after menopause (124). However, aging men exhibit a slow phase of bone loss that is virtually identical with the late slow phase that is experienced by postmenopausal women, leading to overall losses of about 20–25% in both cortical and cancellous bone. Periosteal apposition in the appendicular skeleton continues through life in both men and women, but men add 3-fold more bone by this process than do women (106). This increases the width of the long bones, including the proximal femur, and the same amount of bone distributed over a wider area is stronger. Thus, the greater biomechanical strength afforded by the wider bones partially compensates for age-related decreases in BMD. Indeed, Beck et al. (117) reanalyzed data from DXA measurements of the proximal femur on a non-Hispanic, white subgroup (2719 men and 2904 women) of subjects from the third National Health and Nutrition Examination Survey (NHANES). As a result of the increased bone width, they calculated that the femoral neck section modulus, an index of mechanical strength, was reduced over life by 14% in elderly women and by 6% in elderly men. However, these analyses did not include measurements of cancellous bone in the proximal femur and, thus, did not take into account the decrease in mechanical strength due to cancellous bone loss that occurred concomitantly with the changes in bone size. Thus, bone strength was undoubtedly reduced more than they estimated.

	VI. Mechanism of the Early Accelerated Phase of Postmenopausal Bone Loss

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
This phase begins at the menopause, can be prevented by E replacement (6, 7), and clearly results from loss of ovarian function. During the 2- or 4-yr menopausal transition, serum E2 levels fall to 10–15% of the premenopausal level, although levels of serum E₁, a 4-fold weaker E, fall to about 25–35% of the premenopausal level (125). Also, serum T decreases after menopause because of decreases of ovarian T production (126), but this decrease is only moderate, because T continues to be produced by adrenal cortex and by the ovarian interstitium. As assessed by biochemical markers, bone resorption increases by 90% at the menopause, whereas bone formation markers increase by only 45% (127). The increase in bone turnover and remodeling imbalance lead to accelerated bone loss, particularly on the endosteal surface of bone. Although the menopause induces rapid bone loss, part of the decrease in BMD that is measured by bone densitometry relates to an increase in the remodeling space induced by the large increase in BMU numbers (128). The rapid bone loss in this phase produces an increased outflow of calcium from bone into the extracellular pool, but hypercalcemia is prevented by compensatory increases in urinary calcium excretion (129) and decreases in intestinal calcium absorption (130), and by a partial suppression of PTH secretion (13). Although bone responsiveness to infused PTH is enhanced during this phase (131), this may reflect only the overall increase in BMU numbers.

As reviewed earlier, it is possible that the early rapid phase of bone loss results from a reduced sensing of biomechanical strain by bone cells induced by acute E deficiency. If this concept is correct, it would rationalize the otherwise difficult to explain observation that the rapid phase of postmenopausal bone loss subsides after 4–8 yr. Thus, when bone mass is reduced to such a level that the mechanostat again senses bone strains as similar to those present before menopause, when E was sufficient, rapid bone loss will cease. Indeed, Heshmati et al. (132) found that reducing serum E among postmenopausal women to virtually undetectable levels by administration of an aromatase inhibitor resulted in increased bone resorption and decreased serum PTH, which is evidence that the rapid phase of bone loss had been reactivated. Had the effect of increased E deficiency been on external calcium homeostasis, aromatase treatment would have increased serum PTH further. Also, had the mechanism terminating the rapid phase of bone loss been a high degree of cancellous bone depletion, induction of a more severe degree of E deficiency should not have reactivated the rapid phase of bone loss. Nonetheless, an effect of reduced bone mass per se on tapering the rate of bone loss cannot be excluded.

	VII. Mechanisms of the Late, Slow Phase of Age-Related Bone Loss in Women

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
A. Secondary hyperparathyroidism
The late, slow phase of bone loss is associated with progressive increases in levels of serum PTH and in biochemical markers of bone turnover (Fig. 6), and these increases correlate with each other (13). Moreover, when serum PTH levels were suppressed by a 24-h calcium infusion in groups of young premenopausal and elderly postmenopausal women, the increases in biochemical markers in the postmenopausal women that were present on the control day were no longer present on the calcium infusion day, strongly suggesting that the increased serum PTH was the cause of the increase in bone turnover (133). Because the increases in serum PTH are not associated with increases in serum ionic calcium levels or major abnormalities in renal function, they are indicative of secondary hyperparathyroidism caused by age-related abnormalities in extraskeletal calcium homeostasis. Indeed, many studies have shown that age impairs calcium absorption (134) and especially impairs the ability to adapt to a lower calcium intake by increasing intestinal calcium absorption (135). Aging also impairs renal calcium conservation (133, 136). Both abnormalities lead to external calcium wasting. Thus, unless dietary calcium is substantially increased to offset these losses, PTH secretion increases to maintain normal levels of serum ionic calcium by resorption of bone that contains 99% of body calcium stores. If the hypothesis that calcium wasting is the cause of the secondary hyperparathyroidism and increased bone resorption associated with aging is correct, these abnormalities should be corrected by calcium supplementation. In fact, many studies have now shown that calcium supplementation in elderly women retards bone loss and, possibly, also reduces fracture occurrence in late, postmenopausal women (12). Moreover, McKane et al. (137) demonstrated that a chronically high calcium intake reduced the elevated levels of serum PTH and bone turnover markers in elderly women to within the normal range for premenopausal women (Fig. 7). However, the level of calcium intake (2400 mg/d) in the treatment group of that study was far higher than the average calcium intake among American postmenopausal women of 700 mg/d found in the National Health and Nutrition Examination Survey (NHANES) survey (138).

View larger version (27K): [in this window] [in a new window]   Figure 6. Changes in serum PTH (upper panels) and in bone turnover markers (lower panels) as a function of age in men and women over the age of 50. Data are from a population sample from Rochester, Minnesota (17 ). Results are expressed as the percentage change from young-adult values. Serum osteocalcin is a marker for bone formation, and urine N-telopeptide of type I collagen (NTx) is a marker for bone resorption. For changes in markers of bone turnover, note that increases in women begin at menopause and continue progressively with aging. In men, the increases begin later in life. Note also that the increase in bone resorption exceeds that of bone formation at all ages, indicating a persistent remodeling imbalance. Serum PTH levels increase in both sexes. Although the proportional increase in men over midlife values is greater than in women, absolute values late in life are similar in both sexes. This discrepancy occurs because the change over life in men is parabolic. There are higher values in young adulthood, which decrease in midlife and then increase in old age. In contrast, the increase in serum PTH levels in women begins earlier and increase continuously.

View larger version (27K): [in this window] [in a new window]   Figure 7. Levels of serum PTH and bone resorption [assessed by urinary excretion of deoxypyridinoline (Dpd)] are increased (P < 0.001 for both variables) in elderly postmenopausal women as compared with premenopausal women. Either a high calcium (Ca) intake (2400 mg/d over 3 yr) or chronic E therapy reduced values to levels that were not significantly different from premenopausal women. Values were reduced to a greater degree after E replacement therapy (ERT) than after Ca supplementation. [Data are from W. R. McKane et al.: J Clin Endocrinol Metab 81:1699–1703, 1996 (137 ), and Proc Assoc Am Physicians 109:174–180, 1997 (142 ).]

We have attempted to resolve the apparent paradox of how E deficiency produces opposite types of parathyroid function in the two phases of bone loss (reviewed above) by hypothesizing that there are two types of E action on bone—a direct action on bone cells and an indirect action that is mediated by changes in PTH secretion resulting from E effects on extraskeletal calcium metabolism. E increases intestinal calcium absorption both in experimental animals (143, 144) and in humans (130, 145), acting through intestinal ER (143). E also increases renal calcium conservation (136, 146) by enhancing tubular calcium resorption (146). Thus, the loss of the direct actions of E on the gut and kidney will result in continued calcium wasting. Unless these losses are compensated for by very large increases in dietary calcium intake, they will lead to secondary hyperparathyroidism and will contribute to the slow phase of bone loss.

C. Relationship between the direct and indirect mechanisms of estrogen deficiency on bone and the resultant two phases of bone loss
Although both phases of postmenopausal bone loss are caused by E deficiency, the mechanisms by which the E deficiency produces the bone loss differ. We suggest that this accounts for the different patterns observed in the two phases of postmenopausal bone loss. The major characteristics of the early, rapid phase are that it is self-limiting and induces disproportionate cancellous bone loss. As reviewed earlier, both of these characteristics can be explained by E deficiency resetting the mechanostat. When bone strain is sensed as "normal" by the reset mechanostat, the accelerated bone loss ceases. The remodeling characteristics of this phase of bone loss follow what Frost (90, 91, 92) has termed the "disuse mode," which affects mainly bone on endosteal surfaces. Because of its greater proportion of surfaces interfacing with the bone marrow, cancellous bone, rather than cortical bone, is preferentially lost in this mode.

The major characteristics of the late, slow phase are that it continues indefinitely and that there are similar or even greater losses of cortical than of cancellous bone. Because the bone loss is driven by the PTH excess, rather than by the sensing of biomechanical strain by bone cells, it will continue as long as the secondary hyperparathyroidism persists. The action of PTH also determines the remodeling characteristics, and the bone loss is not restricted to the endosteal-marrow interface but affects all bone surfaces. These remodeling characteristics are consistent with those observed in patients with mild primary hyperparathyroidism who maintain cancellous volume and structure but lose cortical bone (147, 148). They are also consistent with the findings that transgenic mice expressing constitutively active PTH receptors in osteoblasts have increased density of cancellous bone but decreased density of cortical bone (149). The relative sparing of cancellous bone may be due to the anabolic action of PTH that is manifested in certain circumstances (150).

D. Effects of decreased bone formation
Although increased bone resorption is the predominant cause of bone loss in postmenopausal women, decreased bone formation also contributes. Because the components of bone turnover are tightly coupled, an increase in bone resorption will not cause substantial bone loss unless the compensatory increase in bone formation is impaired. In both phases of postmenopausal bone loss in women, however, bone resorption at the tissue level is higher than formation, indicating impaired compensation (Refs. 125 and 127 and Fig. 6). Moreover, Lips et al. (10) have demonstrated by histomorphometry that late postmenopausal women have decreased wall thickness of trabecular packets, which is strong evidence of decreased bone formation at the cellular level.

These abnormalities generally have been attributed to age-related factors, particularly to decreases in paracrine production of growth factors (151) or to decreases in circulating levels of GH (109, 152) and IGF-I (153, 154, 155). However, if E stimulates bone formation, postmenopausal E deficiency could also be a contributing cause. Indeed, impaired bone formation becomes apparent soon after menopause (156). E increases production of IGF-I (157), TGF-ß (75), and procollagen synthesis by osteoblastic cells in vitro (157) and increases osteoblast lifespan by decreasing osteoblast apoptosis (32, 37). Direct evidence that E can stimulate bone formation after cessation of skeletal growth was provided by Khastgir et al. (158), who obtained iliac biopsies for histomorphometry in 22 elderly women (mean age, 65 yr) before and 6 yr after percutaneous administration of high dosages of E. They found a 61% increase in cancellous bone volume and a 12% increase in the wall thickness of trabecular packets. Tobias and Compston (159) have reported similar results. It is unclear whether these results represent only pharmacological effects or are an augmentation of physiological effects of E that are ordinarily not large enough to detect.

Thus, accumulating data implicate E deficiency as a contributing cause of decreased bone formation with aging. Nonetheless, there is not a clear consensus on whether E stimulates osteoblast function, and, if it does, what is the relative contribution of increased proliferation and decreased apoptosis.

	VIII. Mechanism of Age-Related Bone Loss in Men

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
A. Age-related bone loss and osteoporosis in men
Although osteoporosis is often considered to be mainly a disease of women, men lose half as much bone with aging and have one third as many fragility fractures that women do (13). Except in the infrequent older man who develops overt hypogonadism, levels of total serum E and T decrease in men only slightly with aging. Thus, the prevailing opinion has been that sex steroid deficiency is not a major cause of age-related bone loss in men. However, in the last few years, thinking on this issue has undergone a paradigm shift.

B. Changes in serum sex steroids with age
It is now clear that the failure of earlier studies to find major decreases in serum levels of total sex steroid in aging men was due to their failure to account for the confounding effect of a 2-fold age-related rise in levels of serum SHBG (Ref. 160 and Table 3). Circulating sex steroids that are bound to SHBG have restricted access to target tissues, whereas the 1–3% fraction that is free and the 35–55% fraction that is loosely bound to albumin are readily accessible. Although there is controversy about the reliability of bioavailable (Bio; non-SHBG-bound) sex steroid measurements, they correlate well with the more well accepted measurement of free levels. Several groups have reported substantial decreases in serum levels of free or Bio sex steroid levels with aging (17, 160, 161). Figure 8 and Table 3 show changes with age of serum SHBG, Bio E, and Bio T in 350 women and 350 men of a population-based, age-stratified sample from Rochester, Minnesota (161). In aging men, Bio T and E are decreased substantially due to progressive increases in serum SHBG. The physiological importance of these decreases is reinforced by the reciprocal increases in serum FSH and LH.

View larger version (19K): [in this window] [in a new window]   Figure 8. Patterns of age-related changes in serum values for SHBG (panel A), Bio E (panel B), and Bio T (panel C) among an age-stratified sample of Rochester, Minnesota, men (solid lines) and women (dashed lines). Note that changes in serum Bio T are plotted logarithmically to accommodate large differences in levels between sexes. [Data are from Khosla et al.: J Clin Endocrinol Metab 83:2226–2274, 1998 (17 ).]

View larger version (13K): [in this window] [in a new window]   Figure 9. Model for causation of increases in serum SHBG in aging men. This is a complex interaction driven by a reduced secretory capacity of GH by the pituitary and T by the testes. Because T decreases SHBG synthesis, the progressive decreases in Bio T lead to higher values, which further reduce Bio T. Decreased GH secretion reduces IGF-I production that also reduces SHBG synthesis. This leads to a vicious cycle: as SHBG increases progressively, it will further reduce circulating levels of Bio T.

D. Relative effects of estrogen and testosterone on the male skeleton
The traditional beliefs that bone mass is regulated by androgens in men and by E in women have recently been called into question by three "experiments of nature." Smith et al. (111) reported that a 28-yr-old man with homozygous mutations of the ER gene was eunuchoid, had unfused epiphyses, and was severely osteopenic despite normal levels of serum T and elevated levels of serum E. Carani et al. (113) and Bilezikian et al. (114) each studied a young adult male with homozygous null mutations of the gene for the P-450 enzyme, aromatase, which is required for E synthesis from androgen precursors. Both men had undetectable levels of serum E, elevated levels of serum T, unfused epiphyses, and osteopenia. In both, E treatment fused the epiphyses and increased BMD. Thus, either impaired responsiveness of bone to E or impaired E synthesis leads to osteopenia in young adult men despite T sufficiency. These important case reports fulfill Koch’s postulates for a major effect of E on the male skeleton in humans.

In a relevant experimental study in aged male rats, Vanderschueren et al. (166) found that both orchiectomy and treatment with an aromatase inhibitor produced comparable decreases in bone density, suggesting that the aromatization of androgens to E was playing a major role in skeletal maintenance. Moreover, targeted deletion of the gene for either ER (65, 167) or aromatase (72) results in decreased BMD in male mice. In rats, the nonaromatizable androgen DHT decreased biochemical markers of bone turnover and urinary calcium excretion in immature rats, although it is unclear whether these effects were due to its skeletal or extraskeletal actions (169). In an in vitro system, E, T, and DHT stimulated osteoblast proliferation. However, the ER antagonist ICI 182,780 blocked the effects of E and T, but not that of DHT (96). Finally, T has been shown to prevent orchiectomy- induced bone loss in ER-knockout mice (170). One possible interpretation of these data is that aromatization of T to E followed by binding of E to the ER is the preferred pathway for androgen action, but when this is blocked or when a high dosage of an androgen is administered, the AR-mediated pathway is used as a default pathway to modulate bone cell function.

Nonetheless, eight recent, community-based, observational studies (17, 160, 161, 162, 171, 172, 173, 174, 175) have uniformly demonstrated by multivariate analysis that E, rather than T, was the main predictor of BMD at all sites, except for certain cortical bone sites in the appendicular skeleton. However, because the prevailing BMD of elderly men is the algebraic summation of the amount of bone that is gained during growth and maturation and the amount lost with aging, these correlations could reflect either or both processes. In a population-based cohort, Khosla et al. (176) found that the 4-yr rates of loss from the radius and ulna in aging men correlated with Bio E rather than with Bio T, confirming a major role for E deficiency in the bone loss of aging men. Moreover, they found that this inverse correlation occurred only when the baseline level of serum Bio E₂ was below a level of 40 pmol/liter [11 pg/ml; serum total E₂, 114 pmol/liter (31 pg/ml)]. Also, when 50 elderly men were treated for 6 months with raloxifene or placebo, Doran et al. (177) found that a baseline serum E₂ level of 96 pmol/liter (26 pg/ml) delineated a nonbeneficial from a beneficial effect of raloxifene therapy: when values were above this level, raloxifene therapy tended to increase biochemical markers for bone resorption, whereas when they were below this level, they tended to decrease them and did so with an inverse relationship to serum Bio E₂ levels. One interpretation of these data is that a serum E₂ level of 96–114 pmol/liter (26–31 pg/ml) represents the threshold level below which the ERs in bone cells are unoccupied by E, leading to functional skeletal E deficiency. Interestingly, population-based studies (160) show that only about half of men aged 70 are below this level, whereas almost all postmenopausal women are. This may explain, in part, why all aging women lose bone but only some aging men do.

Finally, Falahati-Nini et al. (43) assessed the relative effects of E and T on bone turnover (and, by inference, on inducing bone loss) by direct intervention. Fifty-nine elderly men (mean age, 68 yr) were made pharmacologically hypogonadal by administration of the GnRH agonist leuprolide and had the conversion of androgens to E blocked by administration of the aromatase inhibitor letrozole. During a 3-wk lead-in, all subjects received replacement dosages of T and E by patch. The sex steroids were then withdrawn and the subjects were randomly assigned to treatment groups of E alone, T alone, both, or neither. Bone turnover markers were assessed before randomization and after 3 wk of treatment. By two-factor ANOVA, E prevented the increase in the bone resorption markers, whereas T had only a small, nonsignificant effect. Based on these data, we inferred that E accounted for at least 70% of the effect of sex steroids on bone resorption and that T accounted for no more than 30% of the effect. An effect of androgens on bone resorption is consistent with the presence of AR in human osteoclasts (63). For bone formation markers, however, serum osteocalcin was maintained by both E and T, whereas serum COOH-terminal type I procollagen peptide was maintained only by E. Because osteocalcin is a late marker of osteoblast differentiation, these data are consistent with the observation in vitro by Kousteni et al. (70) that both E and T regulate apoptosis in mature osteoblasts via a rapid nongenomic action. Collectively, these results (Fig. 10) strongly suggest that E is the dominant sex steroid regulating bone resorption, but that both E and T may be important in maintaining bone formation. Thus, age-related decreases in serum Bio E may be the major cause of bone loss and osteoporosis in aging men. Finally, Lanyon and Skerry (178) have suggested that the effect of bone strain on maintaining bone mass in men also is mediated by the ER.

View larger version (27K): [in this window] [in a new window]   Figure 10. Experimental testing of the relative importance of E and T in suppressing bone turnover in elderly men. After suppression of sex steroid production by GnRH agonist treatment and blocking conversion of androgens to E with aromatase inhibition, subjects were randomly assigned to groups treated with T, E, both, or neither. Panel A shows the effects of treatment on the resorption markers, urinary deoxypyridinoline (Dpd) and N-telopeptide of type I collagen (NTx). By ANOVA, E, but not T, prevented increases in bone resorption markers. The possibility of a small effect on T on opposing this increase cannot be excluded. Panel B shows the effects on bone formation markers, serum osteocalcin, and N-terminal extension peptide of type I procollagen (PINP). Levels of serum bone alkaline phosphatase did not change (data not shown). Withdrawal of E and T leads to a decrease in markers (indicating that bone formation was being stimulated). For serum osteocalcin, a marker of late osteoblast function, ANOVA showed that T and E were equally effective in reversing decreases, whereas for serum PINP, a marker of all stages of osteoblast function, E, but not T, was effective. For significance of change from baseline: *, P < 0.05; **, P < 0.01; and ***, P < 0.001. [Reprinted with permission from A. Falahati-Nini et al.: J Clin Invest 106:1553–1560, 2000 (43 ).]

	IX. Causes of Individual Differences in Skeletal Responsiveness

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
The patterns of bone gain and bone loss described earlier and the mechanisms that cause them occur in everyone. Yet only about one in two women and one in six men develop fractures due to osteoporosis. Even accounting for the occurrence of falls and trauma that predispose certain individuals to fracture, there remains a wide variability of peak bone mass and of rates of bone loss with aging. Because of the overriding importance of sex steroids in determining and in maintaining bone mass in both sexes, variability in the levels of serum sex steroids, in the responsiveness of bone to a given serum level, or both could contribute to the variability of bone gain during puberty and bone loss with aging.

A. Differences in serum sex steroid levels
The relationship between individual differences in sex steroid levels and rates of bone acquisition during puberty needs further examination. Cadogan et al. (104) found that 64% of the variance in total body bone mineral in pubertal girls could be explained by serum E₂ levels and lean body mass. However, Lorentzon et al. (181) were unable to relate changes in linear growth to serum E levels in pubertal boys. See the review by Grumbach (110) for more details.

There also is insufficient information about the relationship between individual differences in sex steroid levels and the rate and duration of the accelerated phase of bone loss in the early postmenopause. We have hypothesized that women who develop vertebral or distal forearm fractures during the first 15–20 yr after menopause are those who have experienced disproportionate cancellous bone loss. We have termed this clinical syndrome "type I osteoporosis" and have suggested that it may be caused by E deficiency plus some additional factor(s) that increases the rate or extends the duration of the accelerated, early phase of postmenopausal bone loss (182). This contrasts with type II osteoporosis, which occurs in the entire population of aging women and men, is associated with hip and other fractures later in life, and can be attributed to the effects of the slow phase of bone loss. Women with type I osteoporosis have higher bone turnover and a larger remodeling imbalance (183) but do not have consistently lower levels of serum sex steroids (184) as compared with nonosteoporotic control women. However, these earlier studies could be criticized because the assays for assessing sex steroid levels then available were relatively insensitive. Thus, we have recently (185) reexamined this issue using new ultrasensitive assays in 40 typical type I osteoporotic women with vertebral fractures and in 40 age-matched control women. We found that serum levels of E₂, E₁, and T were indistinguishable between the groups, whereas bone turnover markers were increased by up to 50% in the osteoporotic group. Previous studies (186) have shown that E replacement will normalize bone turnover in these patients. Thus, the data are consistent with the hypothesis that the type I osteoporosis fracture syndrome is mainly the result of increased responsiveness of bone to E deficiency that is evident in the presence of low serum E levels but that is overcome by restoring premenopausal high serum E levels. This is likely to be caused by a genetically determined change such as polymorphism(s) of a gene or genes involved in receptor or postreceptor sex steroid signaling (see next section). Moreover, it is possible that these same polymorphisms also may lead to impaired E enhancement of skeletal growth and maturation, resulting in reduced peak bone mass.

More information is available on the relationship between late postmenopausal bone loss and serum E levels assessed by ultrasensitive assays. In three nested case-control studies from the Study of Osteoporotic Fractures, elderly women with lower levels of serum E and higher levels of serum SHBG had lower cross-sectional BMD values at the calcaneus, proximal radius, proximal femur, and lumbar spine (187); higher rates of bone loss from the calcaneus and proximal femur (188); and increased risk for vertebral and hip fractures (189) after adjusting for age. We have reanalyzed our population-based data from Rochester, Minnesota. After adjusting for age, we also find a positive correlation between proximal femur BMD and serum Bio E₂ (r = 0.31, P < 0.001) in untreated elderly women.

Although the correlations between serum E levels and bone loss in late postmenopausal women were significant, they may underestimate the restraining effect on bone loss of extragonadal E synthesis, which is virtually the exclusive source of circulating E levels in women after menopause. Depending on the gradient between circulating concentrations and intracellular concentrations in intracrine cells, local synthesis could play a major role in sex steroid action. Based on the effect on bone turnover markers induced by administration of the potent aromatase inhibitor letrozole to postmenopausal women, Heshmati et al. (132, 190) estimated that the remodeling imbalance present in postmenopausal women would be 50% higher except for the presence of aromatase-dependent extragonadal E synthesis.

Moreover, because the process is substrate limited, it is likely that the large age-related decreases in levels of circulating C19 adrenal precursors reduce extragonadal E synthesis and, thus, further enhance bone loss. As shown in Table 1, these decrease by as much as 75% between young adulthood and old age in both sexes. Interestingly, the decline begins in young adulthood and, for the most part, continues throughout life (191). This raises the possibility that part of the bone loss that has been documented to occur in premenopausal women (120, 121) may relate to these decreases. Finally, it is possible that changes in the pattern of serum E metabolism could affect bone loss. After the reversible 17ß-HSD-mediated conversion of E₂ to E₁, there is a mutually exclusive C-2 or C-16 hydroxylation. Using the ovariectomized mouse model, Westerlind et al. (18) demonstrated that the 2-hydroxyestrone (2-OHE₁) metabolite was inactive, whereas the 16-OHE₁ metabolite was equipotent with E₂. Indeed, Lim et al. (193) reported that women with postmenopausal osteoporosis had a low 16-OHE₁/2-OHE₁ ratio, although the validity of these measurements has been challenged (194).

B. Differences in bone responsiveness to sex steroids
After T or E binds to its respective receptor, the hormone-receptor complex disassociates from heat shock proteins, dimerizes, forms complexes with various coactivator proteins (195), and binds to E or T response elements in DNA directly or by protein-protein bridging to other DNA binding sites. Binding of the various E/ER complexes to DNA activates genes involved in several signaling systems. Genetic polymorphisms could modify any step of this complex pathway, thus affecting the responsiveness of bone cells to E. Also, individual differences in the concentration or ratio of ER and ERß in bone cells could alter bone responsiveness to E, but this has not been systematically studied.

Recently, a number of investigators have related various allelic variants in the ER gene to BMD or to fracture risk. In a multivariate analysis in healthy adolescent boys, Lorentzon et al. (181) found that the XbaI and PvuII genotypes independently predicted volumetric BMD of the lumbar spine, and that the PP allelic variant predicted statural height. In postmenopausal women, a TA repeat polymorphism was associated with lower BMD (197, 198) and increased risk for osteoporotic fractures (198). Others (199, 200) found that the Px haplotype was associated with significantly lower BMD values in postmenopausal women and, in one study (196), accounted for 16% and 23% of the observed variance in BMD at the lumbar spine and proximal femur, respectively. In a group of 322 early postmenopausal Finnish women followed for 5 yr, Salmen et al. (201) found that those expressing the pp variant of the PvuII genotype lost less bone than did those expressing the Pp or PP variants. In a group of Italian postmenopausal women with normal BMD, osteopenia, or osteoporosis, the combination of the PPXX ER and vitamin D receptor AABBtt genotypes (202) predicted lumbar spine BMD best. Others, however, have been unable to demonstrate a relationship between ER polymorphisms and BMD (203, 204). Allelic variants of the ERß gene have thus far not been reported to increase the risk for osteoporosis. Although these associations are intriguing, they are far from conclusive. Moreover, there is, as yet, no direct experimental evidence that polymorphisms of the ER gene alter phenotype such as has been demonstrated between the polymorphic binding site of the collagen type I A1 gene and altered collagen synthesis in vitro (205).

Finally, individual differences in the effect of E deficiency on extraskeletal calcium metabolism could affect osteoporosis risk. This possibility was suggested by Heshmati et al. (206), who found that a group of 20 women with postmenopausal osteoporosis had impaired renal tubular reabsorption of calcium as compared with 20 postmenopausal women without osteoporosis. Whether this defect is part of an alteration in the E-signaling system is not known. Variability in the concentration of serum T, the rate of aromatization of T to E, or bone responsiveness to T also could affect the rate of bone loss in aging men. Polymorphisms of the aromatase gene have been related to both female (207) and male (208) osteoporosis.

	X. Causes of Bone Loss Other Than Sex Steroid Deficiency

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
A. Age-related decreases in muscle mass
As a corollary to his mechanostat hypothesis, Frost has suggested in various publications (summarized in Refs. 209 and 210) that the loss of muscle mass with aging is the principal cause of involutional osteoporosis. He cites studies of 1066 persons by three groups that show high correlations (r = 0.89–0.94) between lean body mass and total body bone mineral (210). Indeed, in a population sample, Proctor et al. (211) found that physical activity declined by 34% and 38% and lean body mass declined by 18% and 17% with aging in women and men, respectively. Moreover, Center et al. (171) have shown that a 1 SD decrease in quadricep muscle strength is associated with a 3-fold increase in the risk of fracture.

Nonetheless, there are a number of reasons for believing that the action of sex steroids on bone is of equal, or of even greater, importance to the conservation of bone mass. First, the high correlation between total skeletal muscle mass and total body bone mass is an overestimate because both variables are highly correlated with body size. In our age-stratified, population-based sample of 348 adult men and 351 adult women from Rochester, Minnesota, when both height and body mass index were entered into the multivariate model, total skeletal muscle mass accounted for 18% of the variance in proximal femur BMD in both women and men (211). In the same multivariate model, serum sex steroid levels accounted for 21% and 20% of the variance of proximal femur BMD in women and men, respectively (160). Second, part of the high correlation between muscle and bone mass may be related to changes in age-related factors that affect both correlates, such as decreases in serum T, GH, and IGF-I. Third, E therapy initiated soon after menopause essentially prevents significant bone loss for at least 8 yr after menopause (212), but a regular exercise program has not been demonstrated to do so. Indeed, in a 2-yr intervention study in early postmenopausal women, the rate of change in distal forearm BMD in the group receiving only a formal exercise program was -2.6%, whereas in the group receiving both exercise and E therapy, it was +2.7% (213). Even older postmenopausal osteoporotic women receiving E therapy will gain 12% in lumbar spine BMD over 3 yr (214), whereas exercise programs in older postmenopausal women generally increase BMD by only 1–3% (215). Finally, elite woman distance runners who become amenorrheic develop severe bone loss despite subjecting their skeleton to large biomechanical loads (216). This is consistent with the concept that the major function of the mechanostat is to add bone during growth but that it is less able to add bone to the adult skeleton (217). It should be also noted that T has a direct effect on increasing muscle mass and strength (218). Studies should be made to quantify the independent effects of biomechanical strains and sex steroid action on the maintenance of bone mass and, especially, to determine in vivo the interaction between these two positive determinants.

B. Other endocrine abnormalities
Other than changes in serum levels of sex steroids and PTH, the two most important causes of age-related bone loss are abnormalities in the vitamin D-endocrine and in the GH-IGF-I regulatory systems. Reduced serum concentrations of both of the active vitamin D metabolites—25- hydroxyvitamin D [25(OH)D] and 1,25-(OH)₂D—occur with aging in both sexes. In several population-based studies, 25(OH)D, an indicator of vitamin D nutrition, decreased by 30–60% (219). Nutritional vitamin D deficiency may contribute to the secondary hyperparathyroidism and bone loss with aging because decreases in serum 25(OH)D correlate inversely with serum PTH levels and directly with BMD (125). However, nutritional vitamin D deficiency is unlikely to be the major cause of secondary hyperparathyroidism in most elderly women because, as mentioned earlier, E replacement normalizes the increase in serum PTH levels (125, 142). Nonetheless, house-bound persons with inadequate exposure to UV radiation and poor nutrition, especially populations who reside in countries with higher latitudes, such as Great Britain and France, and where dairy products are not fortified with vitamin D, may be at risk for vitamin D deficiency bone loss. Chapuy et al. (11) found that supplementing the diet of elderly, house-bound French women with 800 U/d of vitamin D and 1000 mg/d of calcium decreased the incidence of hip fracture by 43% over the next 18 months. However, Lips et al. (220) could not demonstrate this in a Dutch cohort receiving somewhat smaller supplements. Serum levels of the physiologically active vitamin D metabolite, 1,25-(OH)₂D, also decrease with aging, at least relative to the concomitant increases in serum PTH (134). Elderly women infused with PTH had a blunting of the stimulated increases in serum 1,25-(OH)₂D levels relative to changes in young adults (221). Thus, reduction in the activity of 25(OH)D 1-hydroxylase, the renal enzyme that is responsible for the conversion of 25(OH)D to 1,25-(OH)₂D, may also contribute to the secondary hyperparathyroidism and increased bone resorption associated with aging.

Aging decreases the amplitude and frequency of GH secretion (152), which leads to decreased hepatic production of IGF-I. Indeed, serum IGF-I levels decrease by 60% with aging in women, and there are also smaller decreases in serum IGF-II levels (153, 154). Thus, decreased systemic and skeletal production of IGF-I may contribute to decreases in bone formation with aging.

Other changes in endocrine function with aging appear to make smaller contributions to bone loss. Among the weak adrenal androgens, levels of serum DHEA and DHEA-SO4 decrease by about 80% (16). Because cortisol secretion remains constant or increases throughout life, the decrease in adrenal androgenic steroids leads to an increase in the catabolic/anabolic ratio of circulating adrenal steroid hormones with aging that could also contribute to bone loss (222, 223).

C. Peak bone mass
Those persons who achieve a higher peak bone mass in young adulthood are less likely to develop osteoporosis as age-related bone loss ensues, whereas those with low levels are clearly at greater risk (105, 111). The relative contribution of peak bone mass and bone loss to the BMD in an elderly woman or man is unclear. Some have estimated, however, that about half of the variance in cancellous BMD and one third of the variance of cortical BMD in women by age 70 is due to bone loss (224, 225, 226). In a study of women with vertebral fractures and their daughters, Tabensky et al. (227) found that the daughters had half of the deficit in Z-scores that the mothers did, indicating an important contribution of peak bone mass. As reviewed in Section IX, differences in serum levels of or responsiveness to sex steroids could contribute to the large variability in peak bone mass. Nonetheless, the combined effects of nonhormonal factors such as heredity, activity, calcium intake, and protein and caloric nutrition are substantial (228).

D. Genetic polymorphisms not affecting sex steroids
The heritable component of peak bone mass has been estimated to be about 50–70% and that for age-related bone loss is thought to be much less. In addition to genetic polymorphisms involving sex steroids that affect BMD, other allelic variations have been described. These include polymorphic variants for the vitamin D receptor gene, the TGF-ß gene, and the Sp1 binding site in the collagen type I A1 gene. In addition, studies in inbred mice have made quantitative trait localizations of additional genes that affect bone density, size, and structure [see the review by Nguyen et al. (229) for details].

E. Behavioral and environmental causes
Sporadic factors that affect some, but not other, members of the aging population may contribute to fracture risk in about 40% of men and 20% of women (230). These include use of certain drugs such as corticosteroids; diseases such as malabsorption, anorexia nervosa, and renal hypercalciuria; and behavioral factors such as smoking, alcohol abuse, and inactivity. Some of these sporadic factors, however, may exert their effect on bone by impairing production of sex steroids. For example, smoking increases the catabolism of E (231) and anorexia nervosa (232) or excessive exercise (216) may result in hypothalamic-induced amenorrhea. It is important to recognize that bone loss from these secondary factors is superimposable on that induced by decreases in sex steroid production.

	XI. Summation of Mechanisms

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
We propose that E deficiency is the major cause of both the early, accelerated and the late, slow phases of bone loss in postmenopausal women and contributes substantially to the continuous phase of bone loss in aging men. To this can be added the important roles of E and T in the development of peak bone mass during and after the pubertal growth spurt. Although E deficiency is the primary cause of the two phases of bone loss in women and the single phase in men, the downstream mediators differ, as is shown schematically in Fig. 11. The acute loss of the restraining effect of E on bone turnover and remodeling imbalance at menopause initiates the early, accelerated phase of bone loss. The predominantly cancellous bone loss may be mediated by a resetting of the mechanostat induced by E deficiency, leading to a reduction in bone mass to such a level that bone strain is again sensed as normal. When this level is reached after 4–8 yr, further bone loss from this mechanism ceases. During the accelerated bone loss, the outpouring of calcium from bone into the extracellular fluids is compensated for by decreases in PTH secretion and 1,25-(OH)₂D production, leading to excretion of the excess calcium (Fig. 11A).

View larger version (55K): [in this window] [in a new window]   Figure 11. Flow diagram of the suggested mechanisms for the two phases of bone loss in elderly women and the single phase in elderly men. They are similar in that all three are driven by E deficiency, but the downstream effects differ. In panel A (early postmenopausal women), the loss of the direct inhibitory effects of E on bone leads to increased outflow of bone and inhibition of PTH secretion and 1,25-(OH)2D production. In panel B (late postmenopausal women), the loss of effects of E on external calcium metabolism leads to calcium wasting and to increased PTH secretion that secondarily induces bone loss. In panel C (aging men), the mechanism is the same as in phase B in women except that there are additional effects due to T deficiency (open arrows). OB, Osteoblast; TRCa, renal tubular reabsorption of calcium.

In aging men, the continuous phase of bone loss with increases in serum PTH and bone resorption markers has a pattern that is similar to that of the late, slow phase in aging women. Because men do not have the equivalent of menopause, they lack the early, accelerated phase that is induced in women by the precipitous fall in serum estrogens soon after menopause. However, serum levels of both Bio E and Bio T decrease substantially in aging men, although the decrease in Bio E appears to be the dominant cause of their bone loss. A major difference between the mechanisms of bone loss in aging men and that of the slow phase of bone loss in aging women, however, is the added effects of T deficiency, as shown in Fig. 11C. First, and not shown in the figure, young adult men have larger, denser bones because of the added effect of T during the pubertal growth spurt, and thus, men will have more bone late in life than women for a similar rate of loss. Moreover, because larger bones are stronger, men have additional protection against fractures than women who have smaller skeletons. Second, because aromatase can convert T to E₂, T can be considered to be a prohormone for E (20). Thus, a deficiency in T will exacerbate E deficiency by reducing substrate. Third, T clearly enhances bone formation through its antiapoptotic effect on osteoblasts and, possibly, through increased osteoblast proliferation. Moreover, T deficiency decreases the rate of periosteal bone apposition, whereas E deficiency opposes it. Fourth, T has an anti-resorptive action, although it is not as great as that of E. Some of the antiresorptive effect of T is probably mediated by enhancing calcium absorption (180). However, because osteoclasts contain functional AR (63), undoubtedly there is also a direct effect.

Thus, sex steroids play key roles in the construction and in the conservation of the adult skeleton, and E deficiency is an important cause of involutional osteoporosis in both sexes.

	XII. Epilogue

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 
The view that E plays a central role in the regulation of bone mass in both sexes leads to a number of unanswered questions. Why are sex steroids rather than the primary calcitrophic hormones the major regulators of bone mass? Why does E have major regulatory effects on external calcium homeostasis? Why is E, rather than T, the major regulator of bone mass in men? Why are the onset of puberty and the accelerated bone growth leading to adult bone mass so closely linked? And why is the overall system so complex?

It is often said that biology can be understood only in the context of evolution. We speculate that the major role of E in regulating bone mass evolved from its primary role in supporting reproduction. When a new function is needed, the evolutionary process characteristically adapts an existing mechanism rather than developing a completely new one. The therapsips (mammal-like reptiles) of the Triassic era were the immediate ancestors of mammals and were oviparous. In birds, up to 38% of bone mineral is mobilized to supply calcium for eggshell mineralization (234). Once egg laying is completed, bone mineral of the skeleton is rapidly restored by enhanced utilization of dietary calcium and reduced renal calcium excretion. When mammals became viviparous, it is likely that this earlier system was co-opted to provide calcium for mineralizing the fetal skeleton and for subsequent lactation. Once in place in females, it could also be used to conserve bone mass in males. Indeed, ER appears to have been the first steroid nuclear receptor to evolve in vertebrates and was present long before the AR evolved (235). Moreover, even the role of the sex steroids in inducing and supporting the pubertal growth spurt can be understood in evolutionary terms. The tight coupling between the onset of puberty and the skeletal growth spurt ensures that reproduction cannot occur until there is sufficient skeletal mass to support pregnancy. Finally, because reproductive success is the keystone of natural selection, the surprising complexity of sex steroid regulation of bone mass in mammals can be explained by its evolutionary linkage to ancient reproductive mechanisms.

	Acknowledgments

 
We are grateful to Drs. Shreyasee Amin, Richard Eastell, A. Michael Parfitt, Gideon A. Rodan, and Ego Seeman for their helpful suggestions during the preparation of this manuscript.

	Footnotes

 
This work was supported in part by NIH Grants P01-AG04875 and R01-AR27065. E-mail: riggs.lawrence{at}mayo.edu

This work was presented, in part (by B.L.R.), as a plenary lecture at the 82nd Annual Meeting of the Endocrine Society, Toronto, Ontario, Canada, June 21–24, 2000.

Abbreviations: AR, Androgen receptor; ArKO, aromatase knockout; BERKO, ERß knockout; Bio, bioavailable; BMD, bone mineral density; BMU, basic multicellular units; DERKO, double ER knockout; DHEA, dehydroepiandrosterone; DHT, dihydrotestosterone; DXA, dual energy x-ray absorptiometry; E, estrogen; E₁, estrone; E₂, estradiol; ER, E receptor; ERKO, ER knockout; HSD, hydroxysteroid dehydrogenase; M-CSF, macrophage colony-stimulating factor; OHE₁, hydroxyestrone; 25(OH)D, 25-hydroxyvitamin D; 1,25(OH)₂D, 1,25-dihydroxyvitamin D; OPG, osteoprotegerin; PGE₂, prostaglandin E₂; RANK, receptor activator of nuclear factor-B; RANKL, RANK ligand; T, testosterone.

	References

Top Abstract I. Introduction II. Skeletal Effects of... III. Patterns of Skeletal... IV. Role of Sex... V. Patterns of Age-Related... VI. Mechanism of the... VII. Mechanisms of the... VIII. Mechanism of Age-Related... IX. Causes of Individual... X. Causes of Bone... XI. Summation of Mechanisms XII. Epilogue References

 

1. Melton III LJ, Chrischilles EA, Cooper C, Lane AW, Riggs BL 1992 Perspective. How many women have osteoporosis? J Bone Miner Res 7:1005–1010[Medline]
2. Looker AC, Orwoll ES, Johnston Jr CC, Lindsay RL, Wahner HW, Dunn WL, Calvo MS, Harris TB, Heyse SP 1997 Prevalence of low femoral bone density in older U.S. adults from NHANES III. J Bone Miner Res 12:1761–1768[CrossRef][Medline]
3. Ray NF, Chan JK, Thamer M, Melton III LJ 1997 Medical expenditures for the treatment of osteoporotic fractures in the United States in 1995: report from the National Osteoporosis Foundation. J Bone Miner Res 12:24–35[CrossRef][Medline]
4. Cooper C, Campion G, Melton III LJ 1992 Hip fractures in the elderly: a world-wide projection. Osteoporos Int 2:285–289[CrossRef][Medline]
5. Albright F, Smith PH, Richardson AM 1941 Postmenopausal osteoporosis. JAMA 116:2465–2474[Abstract/Free Full Text]
6. Lindsay R, Aitken JM, Anderson JB, Hart DM, MacDonald EB, Clarke AC 1976 Long-term prevention of postmenopausal osteoporosis by oestrogen: evidence for an increased bone mass after delayed onset of oestrogen treatment. Lancet 1:1038–1041[CrossRef][Medline]
7. Genant HK, Cann CE, Ettinger B, Gordan GS 1982 Quantitative computed tomography of vertebral spongiosa: a sensitive method for detecting early bone loss after oophorectomy. Ann Intern Med 97:699–705
8. Wiske PS, Epstein S, Bell NH, Queener SF, Edmondson J, Johnston CC 1979 Increases in immunoreactive parathyroid hormone with age. N Engl J Med 300:1419–1421[Medline]
9. Gallagher JC, Riggs BL, Eisman J, Hamstra A, Arnaud SB, DeLuca HF 1979 Intestinal calcium absorption and serum vitamin D metabolites in normal subjects and osteoporotic patients: effect of age and dietary calcium. J Clin Invest 64:729–736
10. Lips P, Coupron P, Meunier PJ 1978 Mean wall thickness of trabecular bone packets in the human iliac crest: changes with age. Calcif Tissue Res 26:13–17[CrossRef][Medline]
11. Chapuy MC, Arlot ME, Duboeuf F, Brun J, Crouzet B, Arnaud S, Delmas PD, Meunier PJ 1992 Vitamin D₃ and calcium to prevent hip fractures in elderly women. N Engl J Med 327:1637–1642[Abstract]
12. Heaney RP 2000 Calcium, dairy products and osteoporosis. J Am Coll Nutr 19:83S–99S
13. Riggs BL, Khosla S, Melton III LJ 1998 A unitary model for involutional osteoporosis: estrogen deficiency causes both type I and type II osteoporosis in postmenopausal women and contributes to bone loss in aging men. J Bone Miner Res 13:763–773[CrossRef][Medline]
14. Longcope C 1998 Metabolism of estrogens and progestins. In: Fraser IS, Jansen RP, Lobo RA, Whitehead MI, eds. Estrogens and progestens in clinical practice. New York: Churchill Livingstone; 89–94
15. Griffin JE, Wilson JD 1998 Disorders of the testis and male reproductive tract. In: Wilson JD, Foster DW, Kronenberg HM, Larsen PR, eds. Williams textbook of endocrinology. 9th ed. Philadelphia: W.B. Saunders Co; chap 16:819–875
16. Labrie F, Belanger A, Cusan L, Gomez JL, Candas B 1997 Marked decline in serum concentrations of adrenal C19 sex steroid precursors and conjugated androgen metabolites during aging. J Clin Endocrinol Metab 82:2396–2402[Abstract/Free Full Text]
17. Khosla S, Melton III LJ, Atkinson EJ, O’Fallon WM, Klee GG, Riggs BL 1998 Relationship of serum sex steroid levels and bone turnover markers with bone mineral density in men and women: a key role for bioavailable estrogen. J Clin Endocrinol Metab 83:2266–2274[Abstract/Free Full Text]
18. Westerlind KC, Gibson KJ, Malone P, Evans GL, Turner RT 1998 Differential effects of estrogen metabolites on bone and reproductive tissues of ovariectomized rats. J Bone Miner Res 13:1023–1031[CrossRef][Medline]
19. Labrie F, Luu-The V, Lin SX, Simard J, Labrie C, El-Alfy M, Pelletier G, Belanger A 2000 Intracrinology: role of the family of 17ß-hydroxysteroid dehydrogenases in human physiology and disease. J Mol Endocrinol 25:1–16[Abstract]
20. Simpson E, Rubin G, Clyne C, Robertson K, O’Donnell L, Jones M, Davis S 2000 The role of local estrogen biosynthesis in males and females. Trends Endocrinol Metab 11:184–188[CrossRef][Medline]
21. Eyre LJ, Bland R, Bujalska IJ, Sheppard MC, Stewart PM, Hewison M 1998 Characterization of aromatase and 17ß-hydroxysteroid dehydrogenase expression in rat osteoblastic cells. J Bone Miner Res 13:996–1004[CrossRef][Medline]
22. Bruch H-R, Wolf L, Budde R, Romalo G, Schweikert HU 1992 Androstenedione metabolism in cultured human osteoblast-like cells. J Clin Endocrinol Metab 75:101–105[Abstract]
23. Roa-Pena L, Zreik T, Harada N, Spelsberg TC, Riggs BL, Naftolin F 1994 Human osteoblast-like cells express aromatase immunoreactivity. Menopause 1:73–77
24. Shozu M, Simpson ER 1998 Aromatase expression of human osteoblast-like cells. Mol Cell Endocrinol 139:117–129[CrossRef][Medline]
25. Fujikawa H, Okura F, Kuwano Y, Sekizawa A, Chiba H, Shimodaira K, Saito H, Yanaihara T 1997 Steroid sulfatase activity in osteoblast cells. Biochem Biophys Res Commun 231:42–47[CrossRef][Medline]
26. Kuwano Y, Fujikawa H, Watanabe A, Shimodaira K, Sekizawa A, Saito H, Yanaihara T 1997 3ß-Hydroxysteroid dehydrogenase activity in human osteoblast-like cells. Endocr J 44:847–853[Medline]
27. Nakano Y, Morimoto I, Ishida O, Fujihira T, Mizokami A, Tanimoto A, Yanagihara N, Izumi F, Eto S 1994 The receptor, metabolism and effects of androgen in osteoblastic MC3T3–E1 cells. Bone Miner 26:245–259[Medline]
28. Shimodaira K, Fujikawa H, Okura F, Shimizu Y, Saito H, Yanaihara T 1996 Osteoblast cells (MG-63 and HOS) have aromatase and 5-reductase activities. Biochem Mol Biol Int 39:109–116[Medline]
29. Sasano H, Uzuki M, Sawai T, Nagura H, Matsunaga G, Kashimoto O, Harada N 1997 Aromatase in human bone tissue. J Bone Miner Res 12:1416–1423[CrossRef][Medline]
30. Turgeon C, Gingras S, Carriere MC, Blais Y, Labrie F, Simard J 1998 Regulation of sex steroid formation by interleukin-4 and interleukin-6 in breast cancer cells. J Steroid Biochem Molec Biol 65:151–162[CrossRef][Medline]
31. Pacifici R 1996 Estrogen, cytokines, and pathogenesis of postmenopausal osteoporosis. J Bone Miner Res 11:1043–1051[Medline]
32. Manolagas SC 2000 Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis. Endocr Rev 21:115–137[Abstract/Free Full Text]
33. Hughes DE, Dai A, Tiffee JC, Li HH, Mundy GR, Boyce BF 1996 Estrogen promotes apoptosis of murine osteoclasts mediated by TGF-ß. Nat Med 2:1132–1136[CrossRef][Medline]
34. Chow J, Tobias JH, Colston KW, Chambers TJ 1992 Estrogen maintains trabecular bone volume in rats not only by suppression of bone resorption but also by stimulation of bone formation. J Clin Invest 89:74–78
35. Majeska RJ, Ryaby JT, Einhorn TA 1994 Direct modulation of osteoblastic activity with estrogen. J Bone Joint Surg Am 76A: 713–721
36. Qu Q, Perala-Heape M, Kapanen A, Dahllund J, Salo J, Vaananen HK, Harkonen P 1998 Estrogen enhances differentiation of osteoblasts in mouse bone marrow culture. Bone 22:201–209[Medline]
37. Gohel A, McCarthy MB, Gronowicz G 1999 Estrogen prevents glucocorticoid-induced apoptosis in osteoblasts in vivo and in vitro. Endocrinology 140:5339–5347[Abstract/Free Full Text]
38. Frost HM 1966 Bone dynamics in metabolic bone disease. J Bone Joint Surg 48:1192–1203[Free Full Text]
39. Parfitt AM 2000 Skeletal heterogeneity and the purposes of bone remodeling: implications for the understanding of osteoporosis. In: Marcus R, Feldman D, Kelsey J, eds. Osteoporosis. 2nd ed. San Diego: Academic Press; 433–447
40. Parfitt AM, Mathews CHE, Villaneuva AR, Kleerekoper M, Frame B, Rao DS 1983 Relationships between surface, volume, and thickness of iliac trabecular bone in aging and in osteoporosis. J Clin Invest 72:1396–1409
41. Eriksen EF, Landgahl B, Vesterby A, Rungby J, Kassem M 1999 Hormone replacement therapy prevents osteoclastic hyperactivity: a histomorphometric study in early postmenopausal women. J Bone Miner Res 14:1217–1221[CrossRef][Medline]
42. Bellido T, Jilka R, Boyce B, Girasole G, Broxmeyer H, Dalrymple S, Murray R, Manolagas S 1995 Regulation of interleukin-6, osteoclastogenesis, and bone mass by androgens. J Clin Invest 95:2886–2895
43. Falahati-Nini A, Riggs BL, Atkinson EJ, O’Fallon WM, Eastell R, Khosla S 2000 Relative contributions of testosterone and estrogen in regulating bone resorption and formation in normal elderly men. J Clin Invest 106:1553–1560[Medline]
44. Chen JR, Kousteni S, Bellido T, Han L, DiGregorio GB, Jilka RL, Manolagas SC 2001 Gender-independent induction of murine osteoclast apoptosis in vitro by either estrogens or non-aromatizable androgens. J Bone Miner Res 16(Suppl 1):S159 (Abstract)
45. Kasperk CH, Wergedal JE, Farley JR, Linkhart TA, Turner RT, Baylink DJ 1989 Androgens directly stimulate proliferation of bone cells in vitro. Endocrinology 124:1576–1578[Abstract/Free Full Text]
46. Wakley GK, Schutte Jr HD, Hannon KS, Turner RT 1991 Androgen treatment prevents loss of cancellous bone in the orchidectomized rat. J Bone Miner Res 6:325–330[Medline]
47. Turner RT, Colvard DS, Spelsberg TC 1990 Estrogen inhibition of periosteal bone formation in rat long bones: down-regulation of gene expression for bone matrix proteins. Endocrinology 127:1346–1351[Abstract/Free Full Text]
48. Eriksen EF, Colvard DS, Berg NJ, Graham ML, Mann KG, Spelsberg TC, Riggs BL 1988 Evidence of estrogen receptors in normal human osteoblast-like cells. Science 241:84–86[Abstract/Free Full Text]
49. Komm BS, Terpening CM, Benz DJ, Graeme KA, O’Malley BW, Haussler MR 1988 Estrogen binding receptor mRNA, and biologic response in osteoblast-like osteosarcoma cells. Science 241:81–84[Abstract/Free Full Text]
50. Oursler MJ, Osdoby P, Pyfferoen J, Riggs BL, Spelsberg TC 1991 Avian osteoclasts as estrogen target cells. Proc Natl Acad Sci USA 88:6613–6617[Abstract/Free Full Text]
51. Oursler MJ, Pederson L, Fitzpatrick L, Riggs BL 1994 Human giant cell tumors of the bone (osteoclastomas) are estrogen target cells. Proc Natl Acad Sci USA 91:5227–5231[Abstract/Free Full Text]
52. Braidman I, Baris C, Wood L, Selby P, Adams J, Freemont A, Hoyland J 2000 Preliminary evidence for impaired estrogen receptor- protein expression in osteoblasts and osteocytes from men with idiopathic osteoporosis. Bone 26:423–427[Medline]
53. Tomkinson A, Gevers EF, Wit JM, Reeve J, Noble BS 1998 The role of estrogen in the control of rat osteocyte apoptosis. J Bone Miner Res 13:1243–1250[CrossRef][Medline]
54. Kuiper GG, Enmark E, Pelto-Huikko M, Nilsson S, Gustafsson JA 1996 Cloning of a novel estrogen receptor expressed in rat prostate and ovary. Proc Natl Acad Sci USA 93:5925–5930[Abstract/Free Full Text]
55. Pattersson K, Grandien K, Kuiper GGJM, Gustafsson JA 1997 Mouse estrogen receptor beta forms estrogen response element binding heterodimers with estrogen receptor-. Mol Endocrinol 11:1486–1496[Abstract/Free Full Text]
56. Barkhem T, Carlsson B, Nilsson Y, Enmark E, Gustafsson JA, Nilsson S 1998 Differential response of estrogen receptor and estrogen receptor ß to partial estrogen agonists/antagonists. Mol Pharmacol 54:105–112[Abstract/Free Full Text]
57. Hall JM, McDonnell DP 1999 The estrogen receptor ß-isoform (ERß) of the human estrogen receptor modulates ER transcriptional activity and is a key regulator of the cellular response to estrogens and antiestrogens. Endocrinology 140:5566–5578[Abstract/Free Full Text]
58. Bord S, Horner A, Beavan S, Compston J 2001 Estrogen receptors and ß are differentially expressed in developing human bone. Endocrinology 86:2309–2314
59. Arts J, Kuiper GG, Janssen JM, Gustafsson JA, Lowik CW, Pols HA, Van Leeuwen JP 1997 Differential expression of estrogen receptors and ß mRNA during differentiation of human osteoblast SV-HFO cells. Endocrinology 138:5067–5070[Abstract/Free Full Text]
60. Weise M, DeLevi S, Barnes KM, Gafni RI, Abad V, Baron J 2001 Effects of estrogen on growth plate senescence and epiphyseal fusion. Proc Natl Acad Sci USA 98:6871–6876[Abstract/Free Full Text]
61. Nilsson LO, Boman A, Savendahl L, Grigelioniene G, Ohlsson C, Ritzen EM, Wroblewski J 1999 Demonstration of estrogen receptor-ß immunoreactivity in human growth plate cartilage. J Clin Endocrinol Metab 84:370–373[Abstract/Free Full Text]
62. Colvard DS, Eriksen EF, Keeting PE, Wilson EM, Lubahn DB, French FS, Riggs BL, Spelsberg TC 1989 Identification of androgen receptors in normal human osteoblast-like cells. Proc Natl Acad Sci USA 86:854–857[Abstract/Free Full Text]
63. Pederson L, Kremer M, Judd J, Pascoe D, Spelsberg TC, Riggs BL, Oursler MJ 1999 Androgens regulate bone resorption activity of isolated osteoclasts in vitro. Proc Natl Acad Sci USA 96:505–510[Abstract/Free Full Text]
64. Couse JF, Korach KS 1999 Estrogen receptor null mice: what have we learned and where will they lead us? Endocr Rev 20:358–417[Abstract/Free Full Text]
65. Vidal O, Lindberg MK, Hollberg K, Baylink DJ, Andersson G, Lubahn DB, Mohan S, Gustafsson J, Ohlsson C 2000 Estrogen receptor specificity in the regulation of skeletal growth and maturation in male mice. Proc Natl Acad Sci USA 97:5474–5479[Abstract/Free Full Text]
66. Vidal O, Lindberg M, Savendahl L, Lubahn DB, Ritzen EM, Gusfafsson JA, Ohlsson C 1999 Disproportional body growth in female estrogen receptor--inactivated mice. Biochem Biophys Res Commun 265:569–571[CrossRef][Medline]
67. Windahl SH, Hollberg K, Vidal O, Gustafsson J-A, Ohlsson C, Andersson G 2001 Female estrogen receptor ß-/- mice are partially protected against age-related trabecular bone loss. J Bone Miner Res 16:1388–1398[CrossRef][Medline]
68. Ke HZ, Chidsey-Frink KL, Qi H, Crawford DT, Simmons HA, Brown TA, Petersen DN, Allen MR, McNeish JD, Thompson DD 2001 The role of estrogen receptor-ß (ER-ß) in the early age-related bone gain and later age-related bone loss. J Bone Miner Res 16(Suppl 1):S160 (Abstract)
69. Sims NA, Dupont S, Krust A, Clement-Lacroix P, Minet D, Resche-Rigon M, Gaillard-Kelly M, Baron R 2002 Deletion of estrogen receptors reveals a regulatory role for estrogen receptors-ß in bone remodeling in females but not in males. Bone 30:18–25[Medline]
70. Kousteni S, Bellido T, Plotkin LI, O’Brien CA, Bodenner DL, Han L, Han K, DiGregorio GB, Katzenellenbogen JA, Katzenellenbogen BS, Roberson PK, Weinstein RS, Jilka RL, Manolagas SC 2001 Nongenotropic, sex-nonspecific signaling through the estrogen or androgen receptors: dissociation from transcriptional activity. Cell 104:1–20[CrossRef][Medline]
71. Vanderschueren D, Van Herck E, Suiker AMH, Visser WJ, Schot LPC, Chung K, Lucas RS, Einhorn TA, Bouillon R 1993 Bone and mineral metabolism in the androgen-resistant (testicular feminized) male rat. J Bone Miner Res 8:801–809[Medline]
72. Oz OK, Zerwekh JE, Fisher C, Graves K, Nanu L, Millsaps R, Simpson ER 2000 Bone has a sexually dimorphic response to aromatase deficiency. J Bone Miner Res 15:507–514[CrossRef][Medline]
73. Riggs BL 2000 The mechanisms of estrogen regulation of bone resorption. J Clin Invest 106:1203–1204[Medline]
74. Kawaguchi H, Pilbeam CC, Vargas SJ, Morse EE, Lorenzo JA, Raisz LG 1995 Ovariectomy enhances and estrogen replacement inhibits the activity of bone marrow factors that stimulate prostaglandin production in cultured mouse calvariae. J Clin Invest 96:539–548
75. Oursler MJ, Cortese C, Keeting PE, Anderson MA, Bonde SK, Riggs BL, Spelsberg TC 1991 Modulation of transforming growth factor-ß production in normal human osteoblast-like cells by 17ß-estradiol and parathyroid hormone. Endocrinology 129:3313–3320[Abstract/Free Full Text]
76. Suda T, Takahashi N, Udagawa N, Jimi E, Gillespie MT, Martin TJ 1999 Modulation of osteoclast differentiation and function by the new members of the tumor necrosis factor receptor and ligand families. Endocr Rev 20:345–357[Abstract/Free Full Text]
77. Hofbauer LC, Khosla S, Dunstan CR, Lacey DL, Boyle WJ, Riggs BL 2000 The roles of osteoprotegerin and osteoprotegerin ligand in the paracrine regulation of bone resorption. J Bone Miner Res 15:2–12[CrossRef][Medline]
78. Hofbauer LC, Khosla S, Dunstan CR, Lacey DL, Spelsberg TC, Riggs BL 1999 Estrogen stimulates gene expression and protein production of osteoprotegerin in human osteoblastic cells. Endocrinology 140:4367–4370[Abstract/Free Full Text]
79. Kimble RB, Srivastava S, Ross FP, Matayoshi A, Pacifici R 1996 Estrogen deficiency increases the ability of stromal cells to support murine osteoclastogenesis via an interleukin-1- and tumor necrosis factor-mediated stimulation of macrophage colony-stimulating factor production. J Biol Chem 271:28890–28897[Abstract/Free Full Text]
80. Shevde NK, Bendixen AC, Dienger KM, Pike JW 2000 Estrogens suppress RANK ligand-induced osteoclast differentiation via a stromal cell independent mechanism involving c-Jun repression. Proc Natl Acad Sci USA 97:7829–7834[Abstract/Free Full Text]
81. Srivastava S, Weitzmann MN, Cenci S, Ross FP, Adler S, Pacifici R 1999 Estrogen decreases TNF gene expression by blocking JNK activity and the resulting production of c-Jun and JunD. J Clin Invest 104:503–513[Medline]
82. Kasperk C, Fitzsimmons R, Strong D, Mohan S, Jennings J, Wergedal J, Baylink D 1990 Studies of the mechanism by which androgens enhance mitogenesis and differentiation in bone cells. J Clin Endocrinol Metab 71:1322–1329[Abstract/Free Full Text]
83. Benz DJ, Haussler MR, Thomas MA, Speelman B, Komm BS 1991 High-affinity androgen binding and androgenic regulation of 1(I)-procollagen and transforming growth factor-ß steady state messenger ribonucleic acid levels in human osteoblast-like osteosarcoma cells. Endocrinology 128:2723–2730[Abstract/Free Full Text]
84. Gori F, Hofbauer LC, Conover CA, Khosla S 1999 Effects of androgens on the insulin-like growth factor system in an androgen-responsive human osteoblastic cell line. Endocrinology 140:5579–5586[Abstract/Free Full Text]
85. Bodine PVN, Riggs BL, Spelsberg TC 1995 Regulation of c-fos expression and TGF-ß production by gonadal and adrenal androgens in normal human osteoblastic cells. J Steroid Biochem Mol Biol 52:149–158[CrossRef][Medline]
86. Gill RK, Turner RT, Wronski TJ, Nell NH 1998 Orchiectomy markedly reduces the concentration of the three isoforms of transforming growth factor ß in rat bone, and reduction is prevented by testosterone. Endocrinology 139:546–550[Abstract/Free Full Text]
87. Hofbauer L, Ten RM, Khosla S 1999 The anti-androgen hydroxyflutamide and androgens inhibit interleukin-6 production by an androgen-responsive human osteoblastic cell line. J Bone Miner Res 14:1330–1337[CrossRef][Medline]
88. Hofbauer LC, Hicok K, Chen D, Khosla S Regulation of osteoprotegerin gene expression and protein production by androgens and anti-androgens in human osteoblastic cells. Program of the 83rd Annual Meeting of the Endocrine Society, Denver, CO, 2001, pp 254–255 (Abstract P1-496)
89. Khosla SK, Atkinson EJ, Dunstan CR, O’Fallon WM 2002 Effect of estrogen versus testosterone on circulating osteoprotegerin and other cytokine levels in normal elderly men. J Clin Endocrinol Metab 87:1550–1554[Abstract/Free Full Text]
90. Frost HM 1992 The role of changes in mechanical usage set points in the pathogenesis of osteoporosis. J Bone Miner Res 7:253–261[Medline]
91. Frost HM 1998 On rho, a marrow mediator, and estrogen: their roles in bone strength and "mass" in human females, osteopenias, and osteoporoses–insights from a new paradigm. J Bone Miner Metab 16:113–123[CrossRef]
92. Frost HM 1999 Perspective: on the estrogen-bone relationship and postmenopausal bone loss. J Bone Miner Res 14:1473–1477[CrossRef][Medline]
93. Lanyon LE 1993 Osteocytes, strain detection, bone modeling and remodeling. Calcif Tissue Int 53:S102–S107
94. Jilka RL, Takahashi K, Munshi M, Williams DC, Roberson PK, Manolagas SC 1998 Loss of estrogen upregulates osteoblastogenesis in the murine bone marrow: evidence for autonomy from factors released during bone resorption. J Clin Invest 101:1942–1950[Medline]
95. Westerlind KC, Wronski TJ, Ritman EL, Luo ZP, An KN, Bell NH, Turner RT 1997 Estrogen regulates the rate of bone turnover but bone balance in ovariectomized rats is modulated by prevailing mechanical strain. Proc Natl Acad Sci USA 94:4199–4204[Abstract/Free Full Text]
96. Damien E, Price JS, Lanyon LE 2000 Mechanical strain stimulates osteoblast proliferation through the estrogen receptor in males as well as females. J Bone Miner Res 15:2169–2177[CrossRef][Medline]
97. Zaman G, Cheng MZ, Jessop HL, White R, Lanyon LE 2000 Mechanical strain activates estrogen response elements in bone cells. Bone 27:233–239[Medline]
98. Jessop HL, Sjoberg M, Cheng MZ, Zaman G, Wheeler-Jones CPD, Lanyon LE 2001 Mechanical strain and estrogen activate estrogen receptor in bone cells. J Bone Miner Res 16:1045–1055[CrossRef][Medline]
99. Deleted in proof
100. Katzman DK, Bachrach LK, Carter DR, Marcus R 1991 Clinical and anthropometric correlates of bone mineral acquisition in healthy adolescent girls. J Clin Endocrinol Metab 73:1332–1339[Abstract/Free Full Text]
101. Theintz G, Buchs B, Rizzoli R, Slosman D, Clavien H, Sizonenko PC, Bonjour JP 1992 Longitudinal monitoring of bone mass accumulation in healthy adolescents: evidence for a marked reduction after 16 years of age at the levels of lumbar spine and femoral neck in female subjects. J Clin Endocrinol Metab 75:1060–1065[Abstract]
102. Lu P, Cowell CT, Lloyd-Jones SA, Briody JN, Howman-Giles R 1996 Volumetric bone mineral density in normal subjects, aged 5–27 years. J Clin Endocrinol Metab 81:1586–1590[Abstract]
103. Gilsanz V, Roe TF, Mora S, Costin G, Goodman WG 1991 Changes in vertebral bone density in black girls and white girls during childhood and puberty. N Engl J Med 352:1597–1600
104. Cadogan J, Blumsohn A, Barker ME, Eastell R 1998 A longitudinal study of bone gain in pubertal girls: anthropometric and biochemical correlates. J Bone Miner Res 13:1602–1612[CrossRef][Medline]
105. Bass S, Delmas PD, Pearce G, Hendrich E, Tabensky A, Seeman E 1999 The differing tempo of growth in bone size, mass, and density in girls is region-specific. J Clin Invest 104:795–804[Medline]
106. Seeman E 1997 From density to structure: growing up and growing old on the surfaces of bone. J Bone Miner Res 12:509–521[CrossRef][Medline]
107. Matkovic V, Jelic T, Wardlaw GM, Ilch JZ, Goel PK, Wright JK, Andon MB, Smith KT, Heaney RP 1994 Timing of peak bone mass in Caucasian females and its implication for the prevention of osteoporosis. J Clin Invest 93:799–808
108. Terasawa E, Fernandez DL 2001 Neurobiological mechanisms of the onset of puberty in primates. Endocr Rev 22:111–151[Abstract/Free Full Text]
109. Giustina A, Veldhuis JD 1998 Pathophysiology of the neuroregulation of growth hormone secretion in experimental animals and the human. Endocr Rev 19:717–797[Abstract/Free Full Text]
110. Grumbach MM 2000 Estrogen, bone, growth and sex: a sea change in conventional wisdom. J Pediatr Endocrinol Metab 13:1439–1455
111. Smith EP, Boyd J, Frank GR, Takahashi H, Cohen RM, Specker B, Williams TC, Lubahn DB, Korach KS 1994 Estrogen resistance caused by a mutation in the estrogen-receptor gene in a man. N Engl J Med 331:1056–1061[Abstract/Free Full Text]
112. Morishima A, Grumbach MM, Simpson ER, Fisher C, Qin K 1995 Aromatase deficiency in male and female siblings caused by a novel mutation and the physiological role of estrogens. J Clin Endocrinol Metab 80:3689–3698[Abstract]
113. Carani C, Qin K, Simoni M, Faustini-Fustini M, Serpente S, Boyd J, Korach KS, Simpson ER 1997 Effect of testosterone and estradiol in a man with aromatase deficiency. N Engl J Med 337:91–95[Free Full Text]
114. Bilezikian JP, Morishima A, Bell J, Grumbach MM 1998 Increased bone mass as a result of estrogen therapy in a man with aromatase deficiency. N Engl J Med 339:599–603[Free Full Text]
115. Zachman M, Prader A, Sobel EH, Crigler Jr JF, Ritzen EM, Atares M, Fernandez A 1986 Pubertal growth in patients with androgen insensitivity: indirect evidence for the importance of estrogen in pubertal growth in girls. J Pediatr 108:694–697[CrossRef][Medline]
116. Marcus R, Leary D, Schneider DL, Shane E, Favus M, Quigley CA 2000 The contribution of testosterone to skeletal development and maintenance: lessons from the androgen insensitivity syndrome. J Clin Endocrinol Metab 85:1032–1037[Abstract/Free Full Text]
117. Beck TJ, Looker AC, Ruff CB, Sievanen H, Wahner HW 2000 Structural trends in the aging femoral neck and proximal shaft: analysis of the third national health and nutrition examination survey dual-energy x-ray absorptiometry data. J Bone Miner Res 15:2297–2304[CrossRef][Medline]
118. Bouxsein ML 1999 Application of biomechanics to the aging human skeleton. In: Glowacki J, Rosen CJ, Bilezikian JP, eds. The aging skeleton. San Diego: Academic Press; chap 27:315–330
119. Slemenda C, Hui SL, Longcope C, Johnston CC 1987 Sex steroids and bone mass: a study of changes about the time of menopause. J Clin Invest 80:1261–1269
120. Riggs BL, Wahner HW, Melton III LJ, Richelson LS, Judd HL, Offord KP 1986 Rates of bone loss in the appendicular and axial skeletons of women: evidence of substantial vertebral bone loss before menopause. J Clin Invest 77:1487–1491
121. Melton III LJ, Atkinson EJ, O’Connor MK, O’Fallon WM, Riggs BL 2000 Determinants of bone loss from the femoral neck in women of different ages. J Bone Miner Res 15:24–31[CrossRef][Medline]
122. Recker R, Lappe J, Davies K, Heaney R 2000 Characterization of perimenopausal bone loss: a prospective study. J Bone Miner Res 15:1965–1973[CrossRef][Medline]
123. Guthrie JR, Ebeling PR, Hopper JL, Barrett-Connor E, Dennerstein L, Dudley EC, Burger HG, Wark JD 1998 A prospective study of bone loss in menopausal Australian-born women. Osteoporos Int 8:282–290[CrossRef][Medline]
124. Stepan JJ, Lachman M, Zverina J, Pacovsky V 1989 Castrated men exhibit bone loss: effect of calcitonin treatment on biochemical indices of bone remodeling. J Clin Endocrinol Metab 69:523–527[Abstract/Free Full Text]
125. Khosla SK, Atkinson EJ, Melton III LJ, Riggs BL 1997 Effects of age and estrogen status on serum parathyroid hormone levels and biochemical markers of bone turnover in women: a population-based study. J Clin Endocrinol Metab 82:1522–1527[Abstract/Free Full Text]
126. Horton R, Romanoff E, Walker J 1966 Androstenedione and testosterone in ovarian venous and peripheral plasma during ovariectomy for breast cancer. J Clin Endocrinol Metab 26:1267–1269[Abstract/Free Full Text]
127. Garnero P, Sornay-Rendu E, Chapuy M, Delmas PD 1996 Increased bone turnover in late postmenopausal women is a major determinant of osteoporosis. J Bone Miner Res 11:337–349[Medline]
128. Heaney RP 1994 The bone-remodeling transient: implications for the interpretation of clinical studies of bone mass change. J Bone Miner Res 9:1515–1523[Medline]
129. Young MM Nordin BEC 1967 Effects of natural and artificial menopause on plasma and urinary calcium and phosphorus. Lancet 2:118–120
130. Gennari C, Agnusdei D, Nardi P, Civitelli R 1990 Estrogen preserves a normal intestinal responsiveness to 1,25-dihydroxyvitamin D₃ in oophorectomized women. J Clin Endocrinol Metab 71:1288–1293[Abstract/Free Full Text]
131. Cosman F, Shen V, Xie F, Seibel M, Ratcliffe A, Lindsay R 1993 Estrogen protection against bone resorbing effects of parathyroid hormone infusion. Ann Intern Med 118:337–343[Abstract/Free Full Text]
132. Heshmati HM, Khosla S, Robins SP, Geller N, McAlister CA, Riggs BL 2002 Endogenous residual estrogen levels determine bone resorption even in late postmenopausal women. J Bone Miner Res 17:172–178[CrossRef][Medline]
133. Ledger GA, Burritt MF, Kao PC, O’Fallon WM, Riggs BL, Khosla S 1995 Role of parathyroid hormone in mediating nocturnal and age-related increases in bone resorption. J Clin Endocrinol Metab 80:3304–3310[Abstract]
134. Eastell R, Yergey AL, Vieira N, Cedel SL, Kumar R, Riggs BL 1991 Interrelationship among vitamin D metabolism, true calcium absorption, parathyroid function and age in women: evidence of an age-related intestinal resistance to 1,25(OH)₂D action. J Bone Miner Res 6:125–132[Medline]
135. Ireland P, Fordtran JS 1973 Effect of dietary calcium and age on jejunal calcium absorption in humans studied by intestinal perfusion. J Clin Invest 52:2672–2681
136. Nordin BEC, Need AG, Morris HA, Horowitz M, Robertson WG 1991 Evidence for a renal calcium leak in postmenopausal women. J Clin Endocrinol Metab 72:401–407[Abstract/Free Full Text]
137. McKane WR, Khosla S, Egan KS, Robins SP, Burritt MF, Riggs BL 1996 Role of calcium intake in modulating age-related increases in parathyroid function and bone resorption. J Clin Endocrinol Metab 81:1699–1703[Abstract]
138. Looker AC, Harris TB, Madans JH, Sempos CT 1993 Dietary calcium and hip fracture risk: the NHANES I epidemiologic follow-up study. Osteoporos Int 3:177–184[CrossRef][Medline]
139. Ledger GA, Burritt MF, Kao PC, O’Fallon WM, O’Fallon WM, Riggs BL, Khosla S 1994 Abnormalities of parathyroid hormone secretion in elderly women that are reversible by short term therapy with 1,25-dihydroxyvitamin D₃. J Clin Endocrinol Metab 79:211–216[Abstract]
140. Akerstrom G, Rudberg C, Grimelius L, Bergstrom R, Johansson H, Ljunghall S, Rastad J 1986 Histologic parathyroid abnormalities in an autopsy series. Hum Pathol 17:520–527[Medline]
141. Garn SM 1972 The course of bone gain and the phases of bone loss. Orthop Clin North Am 3:503–520[Medline]
142. McKane RW, Khosla S, Risteli J, Robins SP, Muhs JM, Riggs BL 1997 Role of estrogen deficiency in pathogenesis of secondary hyperparathyroidism and increased bone resorption in elderly women. Proc Assoc Am Physicians 109:174–180[Medline]
143. Arjmandi BH, Salih MA, Herbert DC, Sims SH, Kalu DN 1993 Evidence for estrogen receptor-linked calcium transport in the intestine. Bone Miner 21:63–74[Medline]
144. Bolscher MT, Netelenbos JC, Barto R, Van Buuren LM, Van Der Vijgh WJF 1999 Estrogen regulation of intestinal calcium absorption in the intact ovariectomized adult rat. J Bone Miner Res 14:1197–1202[CrossRef][Medline]
145. Gallagher JC, Riggs BL, DeLuca HF 1980 Effect of estrogen on calcium absorption and serum vitamin D metabolites in postmenopausal osteoporosis. J Clin Endocrinol Metab 51:1359–1364[Abstract/Free Full Text]
146. McKane WR, Khosla S, Burritt MF, Kao PC, Wilson DM, Ory SJ, Riggs BL 1995 Mechanism of renal calcium conservation with estrogen replacement therapy in women in early postmenopause—a clinical research center study. J Clin Endocrinol Metab 80:3458–3464[Abstract]
147. Parisien M, Mellish RWE, Silverberg SJ, Shane E, Lindsay R, Bilezikian JP, Dempster DW 1992 Maintenance of cancellous bone connectivity in primary hyperparathyroidism: trabecular strut analysis. J Bone Miner Res 7:913–919[Medline]
148. Silverberg SJ, Gartenberg F, Jacobs TP, Shane E, Siris E, Staron RB, Bilezikian JP 1995 Longitudinal measurements of bone density and biochemical indices in untreated primary hyperparathyroidism. J Clin Endocrinol Metab 80:723–728[Abstract]
149. Calvi LM, Sims NA, Hunzelman JL, Knight MC, Giovannetti A, Saxton JM, Kronenberg HM, Baron R, Schipani E 2001 Activated parathyroid hormone/parathyroid hormone-related protein receptor in osteoblastic cells differentially affects cortical and trabecular bone. J Clin Invest 107:277–286[Medline]
150. Dempster DW, Cosman F, Parisien M, Shen V, Lindsay R 1993 Anabolic actions of parathyroid hormone on bone. Endocr Rev 14:690–709[Abstract/Free Full Text]
151. Marie PJ, Hott M, Launay JM, Graulet AM, Gueris J 1993 In vitro production of cytokines by bone surface-derived osteoblastic cells in normal and osteoporotic postmenopausal women: relationship with cell proliferation. J Clin Endocrinol Metab 77:824–830[Abstract]
152. Ho KY, Evans WS, Blizzard RM, Veldhuis JD, Merriam GR, Samojlik E, Furlanetto R, Rogol AD, Kaiser DL, Thorner MO 1987 Effects of sex and age on the 24-hour profile of growth hormone secretion in man: importance of endogenous estradiol concentrations. J Clin Endocrinol Metab 64:51–58[Abstract/Free Full Text]
153. Bennet A, Wahner HW, Riggs BL, Hintz RL 1984 Insulin-like growth factors I and II, aging and bone density in women. J Clin Endocrinol Metab 59:701–704[Abstract/Free Full Text]
154. Boonen S, Mohan S, Dequeker J, Aerssens J, Vanderschueren D, Verbeke G, Broos P, Bouillon R, Baylink DJ 1999 Down-regulation of the serum stimulatory components of the insulin-like growth factor (IGF) system (IGF-I, IGF-II, IGF binding protein [BP]-3, and IGFBP-5) in age-related (type II) femoral neck osteoporosis. J Bone Miner Res 14:2150–2158[CrossRef][Medline]
155. Pfeilschifter J, Diel I, Kloppinger T, Bismar H, Schuster EM, Balbach S, Ziegler R, Baylink DJ, Mohan S 2000 Concentrations of insulin-like growth factor (IGF)-I, IGF-II, and IGF binding protein-4 and -5 in human bone cell conditioned media do not change with age. Mech Ageing Dev 117:109–114[CrossRef][Medline]
156. Heaney RP, Recker RR, Saville PD 1978 Menopausal changes in bone remodeling. J Lab Clin Med 92:964–970[Medline]
157. Ernst M, Heath JK, Rodan GA 1989 Estradiol effects on proliferation, messenger ribonucleic acid for collagen and insulin-like growth factor-I, and parathyroid hormone-stimulated adenylate cyclase activity in osteoblastic cells from calvariae and long bones. Endocrinology 125:825–833[Abstract/Free Full Text]
158. Khastgir G, Studd J, Holland N, Alaghband-Zadeh J, Fox S, Chow J 2001 Anabolic effect of estrogen replacement on bone in postmenopausal women with osteoporosis: histomorphometric evidence in a longitudinal study. J Clin Endocrinol Metab 86:289–295[Abstract/Free Full Text]
159. Tobias JH, Compston JE 1999 Does estrogen stimulate osteoblast function in postmenopausal women? Bone 24:121–124[Medline]
160. Slemenda CW, Longcope C, Zhou L, Hui SL, Peacock M, Johnston C 1997 Sex steroids and bone mass in older men: positive associations with serum estrogens and negative associations with androgens. J Clin Invest 100:1755–1759[Medline]
161. Greendale GA, Edelstein S, Barrett-Connor E 1997 Endogenous sex steroids and bone mineral density in older women and men: the Rancho Bernardo study. J Bone Miner Res 12:1833–1843[CrossRef][Medline]
162. Tenover JS, Matsumoto AM, Plymate SR, Bremner WJ 1987 The effects of aging in normal men on bioavailable testosterone and luteinizing hormone secretion: response to clomiphene citrate. J Endocrinol Metab 65:1118–1126[Abstract/Free Full Text]
163. Veldhuis JD, Metzger DL, Martha Jr PM, Mauras N, Kerrigan JR, Keenan B, Rogol AD, Pincus SM 1997 Estrogen and testosterone, but not a nonaromatizable androgen, direct network integration of hypothalamo-somatotrope (growth hormone)-insulin-like growth factor I axis in the human: evidence from pubertal pathophysiology and sex-steroid hormone replacement. J Clin Endocrinol Metab 82:3414–3420[Abstract/Free Full Text]
164. Rajaram S, Baylink DJ, Mohan S 1997 Insulin-like growth factor-binding proteins in serum and other biological fluids: regulation and functions. Endocr Rev 18:801–831[Abstract/Free Full Text]
165. von Schoultz B, Carlstrom K 1989 On the regulation of sex- hormone-binding globulin–a challenge of an old dogma and outlines of an alternative mechanism. J Steroid Biochem 32:327–334[CrossRef][Medline]
166. Vanderschueren D, Van Herck E, Nijs J, Ederveen AGH, De Coster R, Bouillon R 1997 Aromatase inhibition impairs skeletal modeling and decreases bone mineral density in growing male rats. Endocrinology 138:2301–2307[Abstract/Free Full Text]
167. Schmidt A, Seedor JG, Gentile MA, Pennypacker BL, Rodan GA, Kimmel DB 1999 Femoral bone density and length in male and female estrogen receptor- (ER) knockout mice. J Bone Miner Res 14(Suppl 1):S456
168. Deleted in proof
169. Mason RA, Morris HA 1997 Effects of dihydrotestosterone on bone biochemical markers in sham and oophorectomized rats. J Bone Miner Res 12:1431–1437[CrossRef][Medline]
170. Vandenput L, Ederveen AGH, Erben RG, Stahr K, Swinnen JV, Van Herck E, Verstuyf A, Boonen S, Bouillon R, Vanderschueren D 2001 Testosterone prevents orchidectomy-induced bone loss in estrogen receptor- knockout mice. Biochem Biophys Res Commun 285:70–76[CrossRef][Medline]
171. Center JR, Nguyen TV, White CP, Eisman JA 1997 Male osteoporosis predictors: sex hormones and calcitropic hormones. J Bone Miner Res 11:S368
172. Ongphiphadhanakul B, Rajatanavin R, Chanprasertyothin S, Piaseau N, Chailurkit L 1998 Serum oestradiol and oestrogen- receptor gene polymorphism are associated with bone mineral density independently of serum testosterone in normal males. Clin Endocrinol (Oxf) 49:803–809[CrossRef][Medline]
173. van den Beld AW, de Jong FH, Grobbee DE, Pols HAP, Lamberts SWJ 200 Measures of bioavailable serum testosterone and estradiol and their relationships with muscle strength, bone density, and body composition in elderly men. J Clin Endocrinol Metab 85:3276–3282
174. Amin S, Zhang Y, Sawin CT, Evans SR, Hannan MT, Kiel DP, Wilson PWF, Felson DT 2000 Association of hypogonadism and estradiol levels with bone mineral density in elderly men from the Framingham study. Ann Intern Med 133:951–963[Abstract/Free Full Text]
175. Szulc P, Munoz F, Claustrat B, Garnero P, Marchand F 2001 Bioavailable estradiol may be an important determinant of osteoporosis in men: the MINOS study. J Clin Endocrinol Metab 86:192–199[Abstract/Free Full Text]
176. Khosla S, Melton III LJ, Atkinson EJ, O’Fallon WM 2001 Relationship of sex steroids to longitudinal changes in bone mineral density and bone resorption in young versus elderly men: effects of estrogen on peak bone mass and on age-related bone loss. J Clin Endocrinol Metab 86:3555–3561[Abstract/Free Full Text]
177. Doran PM, Riggs BL, Atkinson EJ, Khosla S 2001 Effects of raloxifene, a selective estrogen receptor modulator, on bone turnover markers and serum sex steroid and lipid levels in elderly men. J Bone Miner Res 16:2118–2125[CrossRef][Medline]
178. Lanyon L, Skerry T 2001 Postmenopausal osteoporosis as a failure of bone’s adaptation to functional loading: a hypothesis. J Bone Miner Res 16:1937–1947[CrossRef][Medline]
179. Windahl SH, Vidal O, Andersson G, Gustafsson JA, Ohlsson C 1999 Increased cortical bone mineral content but unchanged trabecular bone mineral density in female ERß-/- mice. J Clin Invest 104:895–901[Medline]
180. Hope, WG, Ibarra MJ, Thomas ML 1992 Testosterone alters duodenal calcium transport and longitudinal bone growth rate in parallel in the male rat. Proc Soc Exp Biol Med 200:536–541[CrossRef][Medline]
181. Lorentzon M, Lorentzon R, Backstrom T, Nordstrom P 1999 Estrogen receptor gene polymorphism, but not estradiol levels, is related to bone density in healthy adolescent boys: a cross-sectional and longitudinal study. J Clin Endocrinol Metab 84:4597–4601[Abstract/Free Full Text]
182. Riggs BL, Melton III LJ 1983 Evidence for two distinct syndromes of involutional osteoporosis. Am J Med 75:899–901[CrossRef][Medline]
183. Eriksen EF, Hodgson SF, Eastell R, Cedel SL, O’Fallon WM, Riggs BL 1990 Cancellous bone remodeling in type I (postmenopausal) osteoporosis: quantitative assessment of rates of formation, resorption, and bone loss at tissue and cellular levels. J Bone Miner Res 5:311–319[Medline]
184. Davidson BJ, Riggs BL, Wahner HW, Judd HL 1983 Endogenous cortisol and sex steroids in patients with osteoporotic spinal fractures. Obstet Gynecol 61:275–278[Medline]
185. Riggs BL, Melton III LJ, Atkinson EJ, O’Fallon WM, Dunstan C, Khosla S 2001 Evidence that postmenopausal women with vertebral fractures due to type I osteoporosis have enhanced bone responsiveness to estrogen deficiency. J Bone Miner Res 17(Suppl 1):S281
186. Lufkin EG, Wahner HW, O’Fallon WM, Hodgson SF, Kotowicz MA, Lane AW, Judd HL, Caplan RH, Riggs BL 1992 Treatment of postmenopausal osteoporosis with transdermal estrogen. Ann Intern Med 117:1–9
187. Ettinger B, Pressman A, Sklarin P, Bauer DC, Cautley JA, Cummings SR 1998 Associations between low levels of serum estradiol, bone density, and fractures among elderly women: the study of osteoporotic fractures. J Clin Endocrinol Metab 83:2239–2243[Abstract/Free Full Text]
188. Stone K, Bauer DC, Black DM, Sklarin P, Ensrud KE, Cummings SR 1998 Hormonal predictors of bone loss in elderly women: a prospective study. J Bone Miner Res 13:1167–1174[CrossRef][Medline]
189. Cummings SR, Browner WS, Bauer D, Stone K, Ensrud K, Jamal S, Ettinger B 1998 Endogenous hormones and the risk of hip and vertebral fractures among older women. N Engl J Med 339:733–738[Abstract/Free Full Text]
190. Heshmati HM, Khosla S, Robins SP, O’Fallon WM, Melton III LJ, Riggs BL 2002 Role of low levels of endogenous estrogen in regulation of bone resorption in late postmenopausal women. J Bone Miner Res 17:172–178
191. Arlt W, Callies F, Koehler I, van Vlijmen JC, Fassnacht M, Strasburger CJ, Seibel MJ, Huebler D, Ernst M, Oettel M, Reincke M, Schulte HM, Allolio B 2001 Dehydroepiandrosterone supplementation in healthy men with an age-related decline of dehydroepiandrosterone secretion. J Clin Endocrinol Metab 86:4686–4692[Abstract/Free Full Text]
192. Deleted in proof
193. Lim SK, Won YJ, Lee JH, Kwon SH, Lee EJ, Kim KR, Lee HC, Huh KB, Chung BC 1997 Altered hydroxylation of estrogen in patients with postmenopausal osteopenia. J Clin Endocrinol Metab 82:1001–1006[Abstract/Free Full Text]
194. Adlercreutz H, Fotsis T 1998 Comment on altered hydroxylation of estrogen in patients with postmenopausal osteopenia. J Clin Endocrinol Metab 83:4170–4171[Free Full Text]
195. McKenna NJ, Lanz RB, O’Malley BW 1999 Nuclear receptor coregulators: cellular and molecular biology. Endocr Rev 20:321–344[Abstract/Free Full Text]
196. Deleted in proof
197. Sano M, Inoue S, Hosoi T, Ouchi Y, Emi M, Shiraki M, Orimo H 1995 Association of estrogen receptor dinucleotide repeat polymorphism with osteoporosis. Biochem Biophys Res Commun 217:378–383[CrossRef][Medline]
198. Langdahl BL, Lokke E, Carstens M, Stenkjaer LL, Eriksen EF 2000 A TA repeat polymorphism in the estrogen receptor gene is associated with osteoporotic fractures but polymorphisms in the first exon and intron are not. J Bone Miner Res 15:2222–2230[CrossRef][Medline]
199. Kobayashi S, Inoue S, Hosoi T, Ouchi Y, Shiraki M, Orimo H 1996 Association of bone mineral density with polymorphism of the estrogen receptor gene. J Bone Miner Res 11:306–311[Medline]
200. Albagha OME, McGuigan FEA, Reid DM, Ralston SH 2001 Estrogen receptor gene polymorphisms and bone mineral density: haplotype analysis in women from the United Kingdom. J Bone Miner Res 16:128–134[CrossRef][Medline]
201. Salmen T, Heikkinen A-M, Mahonen A, Kroger H, Komulainen M, Saarikoski S, Honkanen R, Maenpaa PH 2000 Early postmenopausal bone loss is associated with PvuII estrogen receptor gene polymorphism in Finnish women: effect of hormone replacement therapy. J Bone Miner Res 15:315–321[CrossRef][Medline]
202. Gennari L, Becherini L, Masi L, Mansani R, Gonnelli S, Cepollaro C, Martini S, Montagnani A, Lentini G, Becorpi AM, Brandi ML 1998 Vitamin D and estrogen receptor allelic variants in Italian postmenopausal women: evidence of multiple gene contribution to bone mineral density. J Clin Endocrinol Metab 83:939–944[Abstract/Free Full Text]
203. Bagger YZ, Jorgensen HL, Heegaard AM, Bayer L, Hansen L, Hassager C 2000 No major effect of estrogen receptor gene polymorphisms on bone mineral density or bone loss in postmenopausal Danish women. Bone 26:111–116[Medline]
204. Brown MA, Haughton MA, Grant SFA, Gunnell AS, Henderson NK, Eisman JA 2001 Genetic control of bone density and turnover: role of the collagen 1 -1, estrogen receptor, and vitamin D receptor genes. J Bone Miner Res 16:758–764[CrossRef][Medline]
205. Mann V, Hobson EE, Li B, Stewart TL, Grant SFA, Robins SP, Aspden RM, Ralston SH 2001 A COL1A1 Sp1 binding site polymorphism predisposes to osteoporotic fracture by affecting bone density and quality. J Clin Invest 107:899–907[Medline]
206. Heshmati HM, Khosla S, Burritt MF, O’Fallon WM, Riggs BL 1998 A defect in renal calcium conservation may contribute to the pathogenesis of postmenopausal osteoporosis. J Clin Endocrinol Metab 83:1916–1920[Abstract/Free Full Text]
207. Masi L, Becherini L, Gennari L, Amedei A, Colli E, Falchetti A, Farci M, Silvestri S, Gonnelli S, Brandi ML 2001 Polymorphism of the aromatase gene in postmenopausal Italian women: distribution and correlation with bone mass and fracture risk. J Clin Endocrinol Metab 86:2263–2269[Abstract/Free Full Text]
208. Gennari L, Brandi ML 2001 Genetics of male osteoporosis. Calcif Tissue Int 69:200–204[CrossRef][Medline]
209. Frost HM 1997 On our age-related bone loss: insights from a new paradigm. J Bone Miner Res 12:1539–1546[CrossRef][Medline]
210. Frost HM 2000 Why the ISMNI and the Utah paradigm? Their role in skeletal and extraskeletal disorders. J Musculoskel Interact 1:4–8
211. Proctor DN, Melton LJ, Khosla S, Crowson CS, O’Connor MK, Riggs BL 2000 Relative influence of physical activity, muscle mass and strength on bone density. Osteoporos Int 11:944–952[CrossRef][Medline]
212. Lindsay R, Hart DM, Forrest C, Baird C 1980 Prevention of spinal osteoporosis in oophorectomised women. Lancet 2:1151–1154[Medline]
213. Prince RL, Smith M, Dick IM, Price RI, Webb PG, Henderson NK, Harris MM 1991 A comparative study of exercise, calcium supplementation, and hormone-replacement therapy. N Engl J Med 325:1189–1195[Abstract]
214. Lufkin EG, Riggs BL 1996 Three-year follow-up on effects of transdermal estrogen. Ann Intern Med 125:77 (letter)[Free Full Text]
215. Marcus R 1998 Exercise: moving in the right direction. J Bone Miner Res 13:1793–1796[CrossRef][Medline]
216. Drinkwater BL, Nilson K, Chesnut III CH, Bremner WJ, Shainholtz S, Southworth MB 1984 Bone mineral content of amenorrheic and eumenorrheic athletes. N Engl J Med 311:277–281[Abstract]
217. Parfitt AM 1994 The two faces of growth: benefits and risks to bone integrity. Osteoporos Int 4:382–398[CrossRef][Medline]
218. Bhasin S, Storer TW, Berman N, Calliegari C, Clevenger B, Phillips J, Bunnell TJ, Tricker R, Shirazi A, Casaburi R 1996 The effects of supraphysiologic doses of testosterone on muscle size and strength in normal men. N Engl J Med 335:1–7[Abstract/Free Full Text]
219. Eastell R, Riggs BL 1997 Vitamin D and osteoporosis. In: Feldman D, Glorieux FH, Pike JW, eds. Vitamin D. San Diego: Academic press; 695–711
220. Lips P, Graafmans WC, Ooms ME, Bezemer PD, Bouter LM 1996 Vitamin D supplementation and fracture incidence in elderly persons. A randomized, placebo-controlled clinical trial. Ann Intern Med 124:400–406[Abstract/Free Full Text]
221. Tsai K-S, Heath III H, Kumar R, Riggs BL 1984 Impaired vitamin D metabolism with aging in women: possible role in pathogenesis of senile osteoporosis. J Clin Invest 73:1668–1672
222. Kalmijn S, Launer LJ, Stolk RP, de Jong FH, Pols AP, Hofman A, Breteler MMB, Lamberts SWJ 1998 A prospective study on cortisol, dehydroepiandrosterone sulfate, and cognitive function in the elderly. J Clin Endocrinol Metab 83:3487–3492[Abstract/Free Full Text]
223. Murakami K, Nakagawa T, Shozu M, Uchide K, Koike K, Inoue M 1999 Changes with aging of steroidal levels in the cerebrospinal fluid of women. Maturitas 33:71–80[CrossRef][Medline]
224. Hui SL, Slemenda CW, Johnston CC 1990 The contribution of bone loss to postmenopausal osteoporosis. Osteoporos Int 1:30–34[CrossRef][Medline]
225. Hansen MA, Overgaard K, Riis BJ, Christiansen C 1991 Role of peak bone mass and bone loss in postmenopausal osteoporosis: 12 year study. BMJ 303:961–964
226. Block JE, Smith R, Gluer C, Steiger P, Ettinger B, Genant HK 1989 Models of spinal trabecular bone loss as determined by quantitative computed tomography. J Bone Miner Res 4:249–257[Medline]
227. Tabensky A, Duan Y, Edmonds J, Seeman E 2001 The contribution of reduced peak accrual of bone and age-related bone loss to osteoporosis at the spine and hip: insights from the daughters of women with vertebral or hip fractures. J Bone Miner Res 161101–1107
228. Rizzoli R, Bonjour J-P 1999 Determinants of peak bone mass and mechanisms of bone loss. Osteoporos Int 9 S2:S17–S23
229. Nguyen TV, Blangero J, Eisman JA 2000 Genetic epidemiological approaches to the search for osteoporosis genes. J Bone Miner Res 15:392–401[CrossRef][Medline]
230. Riggs BL, Melton LJ 1986 Medical progress series: involutional osteoporosis. N Engl J Med 314:1676–1686[Medline]
231. Jensen J, Christiansen C, Rodbro P 1985 Cigarette smoking, serum estrogens, and bone loss during hormone-replacement therapy early after menopause. N Engl J Med 313:973–975[Abstract]
232. Rigotti NA, Nussbaum SR, Herzog DB, Neer RM 1984 Osteoporosis in women with anorexia nervosa. N Engl J Med 311:1601–1606[Abstract]
233. Riis B, Thomsen K, Christiansen C 1987 Does calcium supplementation prevent postmenopausal bone loss: a double-blind, controlled clinical study. N Engl J Med 316:173–177[Abstract]
234. Sturkie PD 1976 Reproduction in the female and egg production. In: Sturkie PD, ed. Avian physiology. Ithaca, NY: Springer; 302–330
235. Thornton JW 2001 Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions. Proc Natl Acad Sci USA 98:5671–5676[Abstract/Free Full Text]

This article has been cited by other articles:

				L. M. McNamara Perspective on post-menopausal osteoporosis: establishing an interdisciplinary understanding of the sequence of events from the molecular level to whole bone fractures J R Soc Interface, October 21, 2009; (2009) rsif.2009.0282v1. [Abstract] [Full Text] [PDF]

				N. Carrillo-Lopez, P. Roman-Garcia, A. Rodriguez-Rebollar, J. L. Fernandez-Martin, M. Naves-Diaz, and J. B. Cannata-Andia Indirect Regulation of PTH by Estrogens May Require FGF23 J. Am. Soc. Nephrol., September 1, 2009; 20(9): 2009 - 2017. [Abstract] [Full Text] [PDF]

				I. Mohamed and J. K Yeh Alfacalcidol prevents aromatase inhibitor (Letrozole)-induced bone mineral loss in young growing female rats J. Endocrinol., August 1, 2009; 202(2): 317 - 325. [Abstract] [Full Text] [PDF]

				S. K. Baniwal, O. Khalid, D. Sir, G. Buchanan, G. A. Coetzee, and B. Frenkel Repression of Runx2 by Androgen Receptor (AR) in Osteoblasts and Prostate Cancer Cells: AR Binds Runx2 and Abrogates Its Recruitment to DNA Mol. Endocrinol., August 1, 2009; 23(8): 1203 - 1214. [Abstract] [Full Text] [PDF]

				U. I. Modder, D. G. Monroe, D. G. Fraser, T. C. Spelsberg, C. J. Rosen, M. Gehin, P. Chambon, B. W. O'Malley, and S. Khosla Skeletal Consequences of Deletion of Steroid Receptor Coactivator-2/Transcription Intermediary Factor-2 J. Biol. Chem., July 10, 2009; 284(28): 18767 - 18777. [Abstract] [Full Text] [PDF]

				L. E. Lanyon, T. Sugiyama, and J. S. Price Regulation of Bone Mass: Local Control or Systemic Influence or Both? IBMS BoneKEy, June 1, 2009; 6(6): 218 - 226. [Abstract] [Full Text] [PDF]

				C. Ohlsson and L. Vandenput The role of estrogens for male bone health Eur. J. Endocrinol., June 1, 2009; 160(6): 883 - 889. [Abstract] [Full Text] [PDF]

				N. M. Teplyuk, Y. Zhang, Y. Lou, J. R. Hawse, M. Q. Hassan, V. I. Teplyuk, J. Pratap, M. Galindo, J. L. Stein, G. S. Stein, et al. The Osteogenic Transcription Factor Runx2 Controls Genes Involved in Sterol/Steroid Metabolism, Including Cyp11a1 in Osteoblasts Mol. Endocrinol., June 1, 2009; 23(6): 849 - 861. [Abstract] [Full Text] [PDF]

				T. G. Travison, A. B. Araujo, T. J. Beck, R. E. Williams, R. V. Clark, B. Z. Leder, and J. B. McKinlay Relation between Serum Testosterone, Serum Estradiol, Sex Hormone-Binding Globulin, and Geometrical Measures of Adult Male Proximal Femur Strength J. Clin. Endocrinol. Metab., March 1, 2009; 94(3): 853 - 860. [Abstract] [Full Text] [PDF]

				A. Paterson and T. Baker Bisphosphonates and Bone Turnover in Premenopausal Women Receiving Adjuvant Chemotherapy J. Clin. Oncol., March 1, 2009; 27(7): 1005 - 1006. [Full Text] [PDF]

				A. L. Eriksson, M. Lorentzon, L. Vandenput, F. Labrie, M. Lindersson, A.-C. Syvanen, E. S. Orwoll, S. R. Cummings, J. M. Zmuda, O. Ljunggren, et al. Genetic Variations in Sex Steroid-Related Genes as Predictors of Serum Estrogen Levels in Men J. Clin. Endocrinol. Metab., March 1, 2009; 94(3): 1033 - 1041. [Abstract] [Full Text] [PDF]

				S. H. Windahl, N. Andersson, A. S. Chagin, U. E. A. Martensson, H. Carlsten, B. Olde, C. Swanson, S. Moverare-Skrtic, L. Savendahl, M. K. Lagerquist, et al. The role of the G protein-coupled receptor GPR30 in the effects of estrogen in ovariectomized mice Am J Physiol Endocrinol Metab, March 1, 2009; 296(3): E490 - E496. [Abstract] [Full Text] [PDF]

				J. A. Hyder, M. A. Allison, N. Wong, A. Papa, T. F. Lang, C. Sirlin, S. M. Gapstur, P. Ouyang, J. J. Carr, and M. H. Criqui Association of Coronary Artery and Aortic Calcium With Lumbar Bone Density: The MESA Abdominal Aortic Calcium Study Am. J. Epidemiol., January 15, 2009; 169(2): 186 - 194. [Abstract] [Full Text] [PDF]

				I. Bab and R. Yirmiya Depression, Selective Serotonin Re-Uptake Inhibitors and the Regulation of Bone Mass IBMS BoneKEy, January 1, 2009; 6(1): 8 - 15. [Abstract] [Full Text] [PDF]

				G. N. Farhat, E. Taioli, J. A. Cauley, J. M. Zmuda, E. Orwoll, D. C. Bauer, T. J. Wilt, A. R. Hoffman, T. M. Beer, J. M. Shikany, et al. The Association of Bone Mineral Density with Prostate Cancer Risk in the Osteoporotic Fractures in Men (MrOS) Study Cancer Epidemiol. Biomarkers Prev., January 1, 2009; 18(1): 148 - 154. [Abstract] [Full Text] [PDF]

				F. Callewaert, K. Venken, J. Ophoff, K. De Gendt, A. Torcasio, G. H. van Lenthe, H. Van Oosterwyck, S. Boonen, R. Bouillon, G. Verhoeven, et al. Differential regulation of bone and body composition in male mice with combined inactivation of androgen and estrogen receptor-{alpha} FASEB J, January 1, 2009; 23(1): 232 - 240. [Abstract] [Full Text] [PDF]

				C. M. Jankowski, W. S. Gozansky, J. M. Kittelson, R. E. Van Pelt, R. S. Schwartz, and W. M. Kohrt Increases in Bone Mineral Density in Response to Oral Dehydroepiandrosterone Replacement in Older Adults Appear to Be Mediated by Serum Estrogens J. Clin. Endocrinol. Metab., December 1, 2008; 93(12): 4767 - 4773. [Abstract] [Full Text] [PDF]

				O. Khalid, S. K. Baniwal, D. J. Purcell, N. Leclerc, Y. Gabet, M. R. Stallcup, G. A. Coetzee, and B. Frenkel Modulation of Runx2 Activity by Estrogen Receptor-{alpha}: Implications for Osteoporosis and Breast Cancer Endocrinology, December 1, 2008; 149(12): 5984 - 5995. [Abstract] [Full Text] [PDF]

				M. E. McGee-Lawrence, H. V. Carey, and S. W. Donahue Mammalian hibernation as a model of disuse osteoporosis: the effects of physical inactivity on bone metabolism, structure, and strength Am J Physiol Regulatory Integrative Comp Physiol, December 1, 2008; 295(6): R1999 - R2014. [Abstract] [Full Text] [PDF]

				P. M. Camacho, A. S. Dayal, J. L. Diaz, F. A. Nabhan, M. Agarwal, J. G. Norton, P. A. Robinson, and K. S. Albain Prevalence of Secondary Causes of Bone Loss Among Breast Cancer Patients With Osteopenia and Osteoporosis J. Clin. Oncol., November 20, 2008; 26(33): 5380 - 5385. [Abstract] [Full Text] [PDF]

				A. Fausto-Sterling The bare bones of race. Social Studies of Science, October 1, 2008; 38(5): 657 - 694. [Abstract] [PDF]

				C. Prakash, K. A. Johnson, C. M. Schroeder, and M. J. Potchoiba Metabolism, Distribution, and Excretion of a Next Generation Selective Estrogen Receptor Modulator, Lasofoxifene, in Rats and Monkeys Drug Metab. Dispos., September 1, 2008; 36(9): 1753 - 1769. [Abstract] [Full Text] [PDF]

				A. Giustina, G. Mazziotti, and E. Canalis Growth Hormone, Insulin-Like Growth Factors, and the Skeleton Endocr. Rev., August 1, 2008; 29(5): 535 - 559. [Abstract] [Full Text] [PDF]

				M. Misra, D. K. Katzman, J. Cord, S. J. Manning, N. Mendes, D. B. Herzog, K. K. Miller, and A. Klibanski Bone Metabolism in Adolescent Boys with Anorexia Nervosa J. Clin. Endocrinol. Metab., August 1, 2008; 93(8): 3029 - 3036. [Abstract] [Full Text] [PDF]

				C. Prakash, K. A. Johnson, and M. J. Gardner Disposition of Lasofoxifene, a Next-Generation Selective Estrogen Receptor Modulator, in Healthy Male Subjects Drug Metab. Dispos., July 1, 2008; 36(7): 1218 - 1226. [Abstract] [Full Text] [PDF]

				T. Chevalley, J.-P. Bonjour, S. Ferrari, and R. Rizzoli Influence of Age at Menarche on Forearm Bone Microstructure in Healthy Young Women J. Clin. Endocrinol. Metab., July 1, 2008; 93(7): 2594 - 2601. [Abstract] [Full Text] [PDF]

				J. R. Hawse, M. Subramaniam, D. G. Monroe, A. H. Hemmingsen, J. N. Ingle, S. Khosla, M. J. Oursler, and T. C. Spelsberg Estrogen Receptor {beta} Isoform-Specific Induction of Transforming Growth Factor {beta}-Inducible Early Gene-1 in Human Osteoblast Cells: An Essential Role for the Activation Function 1 Domain Mol. Endocrinol., July 1, 2008; 22(7): 1579 - 1595. [Abstract] [Full Text] [PDF]

				H. G. Bone, M. A. Bolognese, C. K. Yuen, D. L. Kendler, H. Wang, Y. Liu, and J. San Martin Effects of Denosumab on Bone Mineral Density and Bone Turnover in Postmenopausal Women J. Clin. Endocrinol. Metab., June 1, 2008; 93(6): 2149 - 2157. [Abstract] [Full Text] [PDF]

				K. Christo, R. Prabhakaran, B. Lamparello, J. Cord, K. K. Miller, M. A. Goldstein, N. Gupta, D. B. Herzog, A. Klibanski, and M. Misra Bone Metabolism in Adolescent Athletes With Amenorrhea, Athletes With Eumenorrhea, and Control Subjects Pediatrics, June 1, 2008; 121(6): 1127 - 1136. [Abstract] [Full Text] [PDF]

				B. J. Edwards and C. A. Migliorati Osteoporosis and Its Implications for Dental Patients J Am Dent Assoc, May 1, 2008; 139(5): 545 - 552. [Abstract] [Full Text] [PDF]

				J. S. Lee, A. Z. LaCroix, L. Wu, J. A. Cauley, R. D. Jackson, C. Kooperberg, M. S. Leboff, J. Robbins, C. E. Lewis, D. C. Bauer, et al. Associations of Serum Sex Hormone-Binding Globulin and Sex Hormone Concentrations with Hip Fracture Risk in Postmenopausal Women J. Clin. Endocrinol. Metab., May 1, 2008; 93(5): 1796 - 1803. [Abstract] [Full Text] [PDF]

				S. M. Ott Reproductive Hormones and Skeletal Health in Young Women J. Clin. Endocrinol. Metab., April 1, 2008; 93(4): 1175 - 1177. [Full Text] [PDF]

				M. Misra, R. Prabhakaran, K. K. Miller, M. A. Goldstein, D. Mickley, L. Clauss, P. Lockhart, J. Cord, D. B. Herzog, D. K. Katzman, et al. Prognostic Indicators of Changes in Bone Density Measures in Adolescent Girls with Anorexia Nervosa-II J. Clin. Endocrinol. Metab., April 1, 2008; 93(4): 1292 - 1297. [Abstract] [Full Text] [PDF]

				S. H. Windahl, M. K. Lagerquist, N. Andersson, C. Jochems, A. Kallkopf, C. Hakansson, J. Inzunza, J.-A. Gustafsson, P. T. van der Saag, H. Carlsten, et al. Identification of Target Cells for the Genomic Effects of Estrogens in Bone Endocrinology, December 1, 2007; 148(12): 5688 - 5695. [Abstract] [Full Text] [PDF]

				T. Bao and N. E. Davidson How We Maintain Bone Health in Early-Stage Breast Cancer Patients on Aromatase Inhibitors J. Oncol. Pract, November 1, 2007; 3(6): 323 - 325. [Full Text] [PDF]

				S. Khosla Estrogen and the Death of Osteoclasts: A Fascinating Story IBMS BoneKEy, October 1, 2007; 4(10): 267 - 272. [Full Text] [PDF]

				C. Swanson, M. Lorentzon, L. Vandenput, F. Labrie, A. Rane, J. Jakobsson, S. Chouinard, A. Belanger, and C. Ohlsson Sex Steroid Levels and Cortical Bone Size in Young Men Are Associated with a Uridine Diphosphate Glucuronosyltransferase 2B7 Polymorphism (H268Y) J. Clin. Endocrinol. Metab., September 1, 2007; 92(9): 3697 - 3704. [Abstract] [Full Text] [PDF]

				S. B. Goodman, W. Jiranek, E. Petrow, and A. W. Yasko The Effects of Medications on Bone J. Am. Acad. Ortho. Surg., August 1, 2007; 15(8): 450 - 460. [Abstract] [Full Text] [PDF]

				V. J. Armstrong, M. Muzylak, A. Sunters, G. Zaman, L. K. Saxon, J. S. Price, and L. E. Lanyon Wnt/beta-Catenin Signaling Is a Component of Osteoblastic Bone Cell Early Responses to Load-bearing and Requires Estrogen Receptor {alpha} J. Biol. Chem., July 13, 2007; 282(28): 20715 - 20727. [Abstract] [Full Text] [PDF]

				C. J Lees, J. R Kaplan, H. Chen, C. P Jerome, T. C Register, and A. A Franke Bone mass and soy isoflavones in socially housed, premenopausal macaques Am. J. Clinical Nutrition, July 1, 2007; 86(1): 245 - 250. [Abstract] [Full Text] [PDF]

				J. Enriquez, A. E. Lemus, J. Chimal-Monroy, H. Arzate, G. A Garcia, B. Herrero, F. Larrea, and G. Perez-Palacios The effects of synthetic 19-norprogestins on osteoblastic cell function are mediated by their non-phenolic reduced metabolites J. Endocrinol., June 1, 2007; 193(3): 493 - 504. [Abstract] [Full Text] [PDF]

				G. R. Williams Hypogonadal Bone Loss: Sex Steroids or Gonadotropins? Endocrinology, June 1, 2007; 148(6): 2610 - 2612. [Full Text] [PDF]

				M. Schumacher, R. Guennoun, A. Ghoumari, C. Massaad, F. Robert, M. El-Etr, Y. Akwa, K. Rajkowski, and E.-E. Baulieu Novel Perspectives for Progesterone in Hormone Replacement Therapy, with Special Reference to the Nervous System Endocr. Rev., June 1, 2007; 28(4): 387 - 439. [Abstract] [Full Text] [PDF]

				J. Gao, R. Tiwari-Pandey, R. Samadfam, Y. Yang, D. Miao, A. C. Karaplis, M. R. Sairam, and D. Goltzman Altered Ovarian Function Affects Skeletal Homeostasis Independent of the Action of Follicle-Stimulating Hormone Endocrinology, June 1, 2007; 148(6): 2613 - 2621. [Abstract] [Full Text] [PDF]

				M. Q. Hassan, R. Tare, S. H. Lee, M. Mandeville, B. Weiner, M. Montecino, A. J. van Wijnen, J. L. Stein, G. S. Stein, and J. B. Lian HOXA10 Controls Osteoblastogenesis by Directly Activating Bone Regulatory and Phenotypic Genes Mol. Cell. Biol., May 1, 2007; 27(9): 3337 - 3352. [Abstract] [Full Text] [PDF]

				J. H. D. Bassett, P. J. O'Shea, S. Sriskantharajah, B. Rabier, A. Boyde, P. G. T. Howell, R. E. Weiss, J.-P. Roux, L. Malaval, P. Clement-Lacroix, et al. Thyroid Hormone Excess Rather Than Thyrotropin Deficiency Induces Osteoporosis in Hyperthyroidism Mol. Endocrinol., May 1, 2007; 21(5): 1095 - 1107. [Abstract] [Full Text] [PDF]

				D. S. Perrien, N. S. Akel, P. K. Edwards, A. A. Carver, M. S. Bendre, F. L. Swain, R. A. Skinner, W. R. Hogue, K. M. Nicks, T. M. Pierson, et al. Inhibin A Is an Endocrine Stimulator of Bone Mass and Strength Endocrinology, April 1, 2007; 148(4): 1654 - 1665. [Abstract] [Full Text] [PDF]

				G. I. Perez, A. Jurisicova, L. Wise, T. Lipina, M. Kanisek, A. Bechard, Y. Takai, P. Hunt, J. Roder, M. Grynpas, et al. Absence of the proapoptotic Bax protein extends fertility and alleviates age-related health complications in female mice PNAS, March 20, 2007; 104(12): 5229 - 5234. [Abstract] [Full Text] [PDF]

				A. Brufsky, W. G. Harker, J. T. Beck, R. Carroll, E. Tan-Chiu, C. Seidler, J. Hohneker, L. Lacerna, S. Petrone, and E. A. Perez Zoledronic Acid Inhibits Adjuvant Letrozole-Induced Bone Loss in Postmenopausal Women With Early Breast Cancer J. Clin. Oncol., March 1, 2007; 25(7): 829 - 836. [Abstract] [Full Text] [PDF]

				J.-P. Bonjour and T. Chevalley Pubertal Timing, Peak Bone Mass and Fragility Fracture Risk IBMS BoneKEy, February 1, 2007; 4(2): 30 - 48. [Abstract] [Full Text] [PDF]

				J. A. Riancho, C. Valero, A. Naranjo, D. J. Morales, C. Sanudo, and M. T. Zarrabeitia Identification of an Aromatase Haplotype That Is Associated with Gene Expression and Postmenopausal Osteoporosis J. Clin. Endocrinol. Metab., February 1, 2007; 92(2): 660 - 665. [Abstract] [Full Text] [PDF]

				M. T Zarrabeitia, J. L Hernandez, C. Valero, A. Zarrabeitia, J. A Amado, J. Gonzalez-Macias, and J. A Riancho Adiposity, estradiol, and genetic variants of steroid-metabolizing enzymes as determinants of bone mineral density Eur. J. Endocrinol., January 1, 2007; 156(1): 117 - 122. [Abstract] [Full Text] [PDF]

				A. M. Pino, J. M. Rodriguez, S. Rios, P. Astudillo, L. Leiva, G. Seitz, M. Fernandez, and J P. Rodriguez Aromatase activity of human mesenchymal stem cells is stimulated by early differentiation, vitamin D and leptin J. Endocrinol., December 1, 2006; 191(3): 715 - 725. [Abstract] [Full Text] [PDF]

				J.-R. Chen, R. L. Haley, M. Hidestrand, K. Shankar, X. Liu, C. K. Lumpkin, P. M. Simpson, T. M. Badger, and M. J. J. Ronis Estradiol Protects against Ethanol-Induced Bone Loss by Inhibiting Up-Regulation of Receptor Activator of Nuclear Factor-{kappa}B Ligand in Osteoblasts J. Pharmacol. Exp. Ther., December 1, 2006; 319(3): 1182 - 1190. [Abstract] [Full Text] [PDF]

				G. Jasienska, M. Kapiszewska, P. T. Ellison, M. Kalemba-Drozdz, I. Nenko, I. Thune, and A. Ziomkiewicz CYP17 Genotypes Differ in Salivary 17-{beta} Estradiol Levels: A Study Based on Hormonal Profiles from Entire Menstrual Cycles. Cancer Epidemiol. Biomarkers Prev., November 1, 2006; 15(11): 2131 - 2135. [Abstract] [Full Text] [PDF]

				A. Hirbe, E. A. Morgan, O. Uluckan, and K. Weilbaecher Skeletal complications of breast cancer therapies. Clin. Cancer Res., October 15, 2006; 12(20): 6309s - 6314s. [Abstract] [Full Text] [PDF]

				A. Bjornerem, B. Straume, P. Oian, and G. K. R. Berntsen Seasonal Variation of Estradiol, Follicle Stimulating Hormone, and Dehydroepiandrosterone Sulfate in Women and Men J. Clin. Endocrinol. Metab., October 1, 2006; 91(10): 3798 - 3802. [Abstract] [Full Text] [PDF]

				U.H. Lerner Bone Remodeling in Post-menopausal Osteoporosis Journal of Dental Research, July 1, 2006; 85(7): 584 - 595. [Abstract] [Full Text] [PDF]

				N. Rasheed, X. Wang, Q.-T. Niu, J. Yeh, and B. Li Atm-deficient mice: an osteoporosis model with defective osteoblast differentiation and increased osteoclastogenesis Hum. Mol. Genet., June 15, 2006; 15(12): 1938 - 1948. [Abstract] [Full Text] [PDF]

				S. Bhasin, G. R. Cunningham, F. J. Hayes, A. M. Matsumoto, P. J. Snyder, R. S. Swerdloff, and V. M. Montori Testosterone Therapy in Adult Men with Androgen Deficiency Syndromes: An Endocrine Society Clinical Practice Guideline J. Clin. Endocrinol. Metab., June 1, 2006; 91(6): 1995 - 2010. [Abstract] [Full Text] [PDF]

				P. R. Ebeling Inhibin in bone--new tricks for an old dog. J. Clin. Endocrinol. Metab., May 1, 2006; 91(5): 1669 - 1670. [Full Text] [PDF]

				D. S. Perrien, S. J. Achenbach, S. E. Bledsoe, B. Walser, L. J. Suva, S. Khosla, and D. Gaddy Bone Turnover across the Menopause Transition: Correlations with Inhibins and Follicle-Stimulating Hormone J. Clin. Endocrinol. Metab., May 1, 2006; 91(5): 1848 - 1854. [Abstract] [Full Text] [PDF]

				M. C M Bunck, A. W F T Toorians, P. Lips, and L. J G Gooren The effects of the aromatase inhibitor anastrozole on bone metabolism and cardiovascular risk indices in ovariectomized, androgen-treated female-to-male transsexuals. Eur. J. Endocrinol., April 1, 2006; 154(4): 569 - 575. [Abstract] [Full Text] [PDF]

				M. R. Sowers, M. Jannausch, D. McConnell, R. Little, G. A. Greendale, J. S. Finkelstein, R. M. Neer, J. Johnston, and B. Ettinger Hormone Predictors of Bone Mineral Density Changes during the Menopausal Transition J. Clin. Endocrinol. Metab., April 1, 2006; 91(4): 1261 - 1267. [Abstract] [Full Text] [PDF]

				M. Misra, K. K. Miller, P. Tsai, K. Gallagher, A. Lin, N. Lee, D. B. Herzog, and A. Klibanski Elevated Peptide YY Levels in Adolescent Girls with Anorexia Nervosa J. Clin. Endocrinol. Metab., March 1, 2006; 91(3): 1027 - 1033. [Abstract] [Full Text] [PDF]

				N. A. Sims, K. Brennan, J. Spaliviero, D. J. Handelsman, and M. J. Seibel Perinatal testosterone surge is required for normal adult bone size but not for normal bone remodeling Am J Physiol Endocrinol Metab, March 1, 2006; 290(3): E456 - E462. [Abstract] [Full Text] [PDF]

				S L Liu and C M Lebrun Effect of oral contraceptives and hormone replacement therapy on bone mineral density in premenopausal and perimenopausal women: a systematic review Br. J. Sports Med., January 1, 2006; 40(1): 11 - 24. [Abstract] [Full Text] [PDF]

				I. J. Kerber, R. J. Turner, V. M. Miller, T. B. Clarkson, S. M. Harman, E. A. Brinton, M. Cedars, R. Lobo, J. E. Manson, G. R. Merriam, et al. Eu-estrogenemia J Appl Physiol, December 1, 2005; 99(6): 2471 - 2472. [Full Text] [PDF]

				M. R. Ryan, R. Shepherd, J. K. Leavey, Y. Gao, F. Grassi, F. J. Schnell, W.-P. Qian, G. J. Kersh, M. N. Weitzmann, and R. Pacifici An IL-7-dependent rebound in thymic T cell output contributes to the bone loss induced by estrogen deficiency PNAS, November 15, 2005; 102(46): 16735 - 16740. [Abstract] [Full Text] [PDF]

				N. Andersson, U. Islander, E. Egecioglu, E. Lof, C. Swanson, S. Moverare-Skrtic, K. Sjogren, M. K Lindberg, H. Carlsten, and C. Ohlsson Investigation of central versus peripheral effects of estradiol in ovariectomized mice J. Endocrinol., November 1, 2005; 187(2): 303 - 309. [Abstract] [Full Text] [PDF]

				K. Templeton Secondary Osteoporosis J. Am. Acad. Ortho. Surg., November 1, 2005; 13(7): 475 - 486. [Abstract] [Full Text] [PDF]

				S. Ichikawa, D. L. Koller, M. Peacock, M. L. Johnson, D. Lai, S. L. Hui, C. C. Johnston, T. M. Foroud, and M. J. Econs Polymorphisms in the Estrogen Receptor {beta} (ESR2) Gene Are Associated with Bone Mineral Density in Caucasian Men and Women J. Clin. Endocrinol. Metab., November 1, 2005; 90(11): 5921 - 5927. [Abstract] [Full Text] [PDF]

				G. Spohn, K. Schwarz, P. Maurer, H. Illges, N. Rajasekaran, Y. Choi, G. T. Jennings, and M. F. Bachmann Protection against Osteoporosis by Active Immunization with TRANCE/RANKL Displayed on Virus-Like Particles J. Immunol., November 1, 2005; 175(9): 6211 - 6218. [Abstract] [Full Text] [PDF]

				J. M. Kaufman and A. Vermeulen The Decline of Androgen Levels in Elderly Men and Its Clinical and Therapeutic Implications Endocr. Rev., October 1, 2005; 26(6): 833 - 876. [Abstract] [Full Text] [PDF]

				S. Khosla, B. L. Riggs, R. A. Robb, J. J. Camp, S. J. Achenbach, A. L. Oberg, P. A. Rouleau, and L. J. Melton III Relationship of Volumetric Bone Density and Structural Parameters at Different Skeletal Sites to Sex Steroid Levels in Women J. Clin. Endocrinol. Metab., September 1, 2005; 90(9): 5096 - 5103. [Abstract] [Full Text] [PDF]

				E. Rzewuska-Lech, M. Jayachandran, L. A. Fitzpatrick, and V. M. Miller Differential effects of 17{beta}-estradiol and raloxifene on VSMC phenotype and expression of osteoblast-associated proteins Am J Physiol Endocrinol Metab, July 1, 2005; 289(1): E105 - E112. [Abstract] [Full Text] [PDF]

				H. Z. Ring, C. N. Lessov, T. Reed, R. Marcus, L. Holloway, G. E. Swan, and D. Carmelli Heritability of Plasma Sex Hormones and Hormone Binding Globulin in Adult Male Twins J. Clin. Endocrinol. Metab., June 1, 2005; 90(6): 3653 - 3658. [Abstract] [Full Text] [PDF]

				L. S. Acheson Bone Density and the Risk of Fractures: Should Treatment Thresholds Vary by Race? JAMA, May 4, 2005; 293(17): 2151 - 2154. [Full Text] [PDF]

				R. Eastell Role of oestrogen in the regulation of bone turnover at the menarche J. Endocrinol., May 1, 2005; 185(2): 223 - 234. [Abstract] [Full Text] [PDF]

				E. Bonnelye and J. E. Aubin Estrogen Receptor-Related Receptor {alpha}: A Mediator of Estrogen Response in Bone J. Clin. Endocrinol. Metab., May 1, 2005; 90(5): 3115 - 3121. [Abstract] [Full Text] [PDF]

				K. E. Ensrud, R. L. Fullman, E. Barrett-Connor, J. A. Cauley, M. L. Stefanick, H. A. Fink, C. E. Lewis, E. Orwoll, and for the Osteoporotic Fractures in Men Study Resear Voluntary Weight Reduction in Older Men Increases Hip Bone Loss: The Osteoporotic Fractures in Men Study J. Clin. Endocrinol. Metab., April 1, 2005; 90(4): 1998 - 2004. [Abstract] [Full Text] [PDF]

				S. Bonadonna, A. Burattin, M. Nuzzo, G. Bugari, E. A. Rosei, D. Valle, N. Iori, J. P Bilezikian, J. D Veldhuis, and A. Giustina Chronic glucocorticoid treatment alters spontaneous pulsatile parathyroid hormone secretory dynamics in human subjects Eur. J. Endocrinol., February 1, 2005; 152(2): 199 - 205. [Abstract] [Full Text] [PDF]

				P. V. Nantermet, P. Masarachia, M. A. Gentile, B. Pennypacker, J. Xu, D. Holder, D. Gerhold, D. Towler, A. Schmidt, D. B. Kimmel, et al. Androgenic Induction of Growth and Differentiation in the Rodent Uterus Involves the Modulation of Estrogen-Regulated Genetic Pathways Endocrinology, February 1, 2005; 146(2): 564 - 578. [Abstract] [Full Text] [PDF]

				J. M. Lean, C. J. Jagger, B. Kirstein, K. Fuller, and T. J. Chambers Hydrogen Peroxide Is Essential for Estrogen-Deficiency Bone Loss and Osteoclast Formation Endocrinology, February 1, 2005; 146(2): 728 - 735. [Abstract] [Full Text] [PDF]

				D. A. M. Salih, S. Mohan, Y. Kasukawa, G. Tripathi, F. A. Lovett, N. F. Anderson, E. J. Carter, J. E. Wergedal, D. J. Baylink, and J. M. Pell Insulin-Like Growth Factor-Binding Protein-5 Induces a Gender-Related Decrease in Bone Mineral Density in Transgenic Mice Endocrinology, February 1, 2005; 146(2): 931 - 940. [Abstract] [Full Text] [PDF]

				C. J. Jagger, J. M. Lean, J. T. Davies, and T. J. Chambers Tumor Necrosis Factor-{alpha} Mediates Osteopenia Caused by Depletion of Antioxidants Endocrinology, January 1, 2005; 146(1): 113 - 118. [Abstract] [Full Text] [PDF]

				A Martinetti, E Bajetta, L Ferrari, N Zilembo, E Seregni, M Del Vecchio, R Longarini, I La Torre, L Toffolatti, D Paleari, et al. Osteoprotegerin and osteopontin serum values in postmenopausal advanced breast cancer patients treated with anastrozole Endocr. Relat. Cancer, December 1, 2004; 11(4): 771 - 779. [Abstract] [Full Text] [PDF]

				L. Gennari, R. Nuti, and J. P. Bilezikian Aromatase Activity and Bone Homeostasis in Men J. Clin. Endocrinol. Metab., December 1, 2004; 89(12): 5898 - 5907. [Abstract] [Full Text] [PDF]

				Y. Gao, W.-P. Qian, K. Dark, G. Toraldo, A. S. P. Lin, R. E. Guldberg, R. A. Flavell, M. N. Weitzmann, and R. Pacifici Estrogen prevents bone loss through transforming growth factor {beta} signaling in T cells PNAS, November 23, 2004; 101(47): 16618 - 16623. [Abstract] [Full Text] [PDF]

				K. B. Markou, P. Mylonas, A. Theodoropoulou, A. Kontogiannis, M. Leglise, A. G. Vagenakis, and N. A. Georgopoulos The Influence of Intensive Physical Exercise on Bone Acquisition in Adolescent Elite Female and Male Artistic Gymnasts J. Clin. Endocrinol. Metab., September 1, 2004; 89(9): 4383 - 4387. [Abstract] [Full Text] [PDF]

				M. Muir, G. Romalo, L. Wolf, W. Elger, and H.-U. Schweikert Estrone Sulfate Is a Major Source of Local Estrogen Formation in Human Bone J. Clin. Endocrinol. Metab., September 1, 2004; 89(9): 4685 - 4692. [Abstract] [Full Text] [PDF]

				H. W. Goderie-Plomp, M. van der Klift, W. de Ronde, A. Hofman, F. H. de Jong, and H. A. P. Pols Endogenous Sex Hormones, Sex Hormone-Binding Globulin, and the Risk of Incident Vertebral Fractures in Elderly Men and Women: The Rotterdam Study J. Clin. Endocrinol. Metab., July 1, 2004; 89(7): 3261 - 3269. [Abstract] [Full Text] [PDF]

				F. Stossi, D. H. Barnett, J. Frasor, B. Komm, C. R. Lyttle, and B. S. Katzenellenbogen Transcriptional Profiling of Estrogen-Regulated Gene Expression via Estrogen Receptor (ER) {alpha} or ER{beta} in Human Osteosarcoma Cells: Distinct and Common Target Genes for These Receptors Endocrinology, July 1, 2004; 145(7): 3473 - 3486. [Abstract] [Full Text] [PDF]

				D. Vanderschueren, L. Vandenput, S. Boonen, M. K. Lindberg, R. Bouillon, and C. Ohlsson Androgens and Bone Endocr. Rev., June 1, 2004; 25(3): 389 - 425. [Abstract] [Full Text] [PDF]

				U. I. L. Modder, A. Sanyal, A. E. Kearns, J. D. Sibonga, E. Nishihara, J. Xu, B. W. O'Malley, E. L. Ritman, B. L. Riggs, T. C. Spelsberg, et al. Effects of Loss of Steroid Receptor Coactivator-1 on the Skeletal Response to Estrogen in Mice Endocrinology, February 1, 2004; 145(2): 913 - 921. [Abstract] [Full Text] [PDF]

				J. Somner, S. McLellan, J. Cheung, Y. T. Mak, M. L. Frost, K. M. Knapp, A. S. Wierzbicki, M. Wheeler, I. Fogelman, S. H. Ralston, et al. Polymorphisms in the P450 c17 (17-Hydroxylase/17,20-Lyase) and P450 c19 (Aromatase) Genes: Association with Serum Sex Steroid Concentrations and Bone Mineral Density in Postmenopausal Women J. Clin. Endocrinol. Metab., January 1, 2004; 89(1): 344 - 351. [Abstract] [Full Text] [PDF]

				S. Bhasin, W. E. Taylor, R. Singh, J. Artaza, I. Sinha-Hikim, R. Jasuja, H. Choi, and N. F. Gonzalez-Cadavid The Mechanisms of Androgen Effects on Body Composition: Mesenchymal Pluripotent Cell as the Target of Androgen Action J. Gerontol. A Biol. Sci. Med. Sci., December 1, 2003; 58(12): M1103 - 1110. [Abstract] [Full Text] [PDF]

				Y. Kasukawa, D. J. Baylink, J. E. Wergedal, Y. Amaar, A. K. Srivastava, R. Guo, and S. Mohan Lack of Insulin-Like Growth Factor I Exaggerates the Effect of Calcium Deficiency on Bone Accretion in Mice Endocrinology, November 1, 2003; 144(11): 4682 - 4689. [Abstract] [Full Text] [PDF]

				S. Cenci, G. Toraldo, M. N. Weitzmann, C. Roggia, Y. Gao, W. P. Qian, O. Sierra, and R. Pacifici Estrogen deficiency induces bone loss by increasing T cell proliferation and lifespan through IFN-{gamma}-induced class II transactivator PNAS, September 2, 2003; 100(18): 10405 - 10410. [Abstract] [Full Text] [PDF]

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Copyright Permission Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Riggs, B. L. Articles by Melton, L. J. Search for Related Content PubMed PubMed Citation Articles by Riggs, B. L. Articles by Melton, L. J., III