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Changes in Proinflammatory Cytokine Activity after Menopause

Department of Internal Medicine, Berufsgenossenschaftliche Kliniken Bergmannsheil, University of Bochum, D-44789 Bochum, Germany

Correspondence: Address all correspondence and requests for reprints to: Johannes Pfeilschifter, M.D., Berufsgenossenschaftliche Kliniken Bergmannsheil, University of Bochum, Department of Internal Medicine, Bürkle-de-la-Camp-Platz 1, D-44789 Bochum, Germany. E-mail: Johannes.Pfeilschifter{at}ruhr-uni-bochum.de

	Abstract

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
There is now a large body of evidence suggesting that the decline in ovarian function with menopause is associated with spontaneous increases in proinflammatory cytokines. The cytokines that have obtained the most attention are IL-1, IL-6, and TNF-. The exact mechanisms by which estrogen interferes with cytokine activity are still incompletely known but may potentially include interactions of the ER with other transcription factors, modulation of nitric oxide activity, antioxidative effects, plasma membrane actions, and changes in immune cell function. Experimental and clinical studies strongly support a link between the increased state of proinflammatory cytokine activity and postmenopausal bone loss. Preliminary evidence suggests that these changes also might be relevant to vascular homeostasis and the development of atherosclerosis. Better knowledge of the mechanisms and the time course of these interactions may open new avenues for the prevention and treatment of some of the most prevalent and important disorders in postmenopausal women.

I. Introduction

II. Evidence for Increases in IL-1, TNF-, and IL-6 after Natural or Surgical Menopause

III. Effect of Estrogen Treatment on IL-1, IL-6, and TNF

IV. Potential Mechanisms of the Interaction between Estrogen and Proinflammatory Cytokines at the Molecular Level (Fig. 3)

A. Interactions between estrogen and the IL-6 gene

B. Interactions between estrogen and the TNF- gene

C. Estrogen, nitric oxide (NO), and cytokines

D. Direct antioxidant effects of estrogen

E. Rapid nongenomic actions

F. Effects of estrogen on lymphocytes

G. Central interactions between the gonadal axis and inflammatory processes

H. Changes in visceral fat with menopause

V. Effects of Estrogen on RANKL and Osteoprotegerin (OPG)

VI. Effects of Estrogen on Other Cellular Mediators Relevant to Inflammatory Processes

A. Effects of estrogen on TGF-ß

B. Effects of estrogen on PG synthesis

C. Effects of estrogen on macrophage colony-stimulating factor (M-CSF) and granulocyte-macrophage-colony stimulating factor (GM-CSF)

VII. Implications for Postmenopausal Bone Loss (Fig. 4)

A. Impact of cytokine changes with estrogen deficiency on osteoclastogenesis

B. Evidence for an essential role of proinflammatory cytokines in bone loss with estrogen deficiency

C. Evidence for a role of cytokines in human bone loss after menopause

VIII. Potential Implications for Vascular Function (Fig. 4)

IX. Individual Susceptibility to Estrogen-Dependent Cytokine Changes

X. Time Course of Cytokine Increases after Menopause

XI. Role of Progesterone

XII. Role of Androgens

XIII. Potential Implications for the Prophylaxis of Menopause-Associated Diseases

XIV. Summary and Conclusions

	I. Introduction

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
NATURAL MENOPAUSE IS associated with a rapid decline in circulating estrogen (1). There is now a large body of evidence suggesting that, apart from the loss of reproductive function, the decline in sex hormones has many implications for nonreproductive tissues (2, 3). For example, it is well known that estrogen contributes to the maintenance of bone mass. Despite recent doubts arising from short-term placebo-controlled studies in women with established coronary artery disease, there is still substantial reason to believe that estrogen may prevent coronary heart disease in the long run. Because the average life expectancy for women in Western countries exceeds 80 yr and women spend more than a third of their lifetime in postmenopause, the possible implications of estrogen deficiency on the rates of cardiovascular disease and osteoporosis are of enormous public health importance.

There is no unifying mechanism that would be able to explain all consequences of menopause on the metabolism of organs as diverse as bone, blood vessels, or adipose tissue (4, 5, 6). However, menopause-triggered changes in the activity of proinflammatory cytokines are beginning to emerge as a common theme that may have a significant impact on the function of all of these tissues. The following review focuses on what is currently known about the mechanisms of the interactions between estrogen and proinflammatory cytokines and attempts to outline possible implications for bone and the cardiovascular system.

	II. Evidence for Increases in IL-1, TNF-, and IL-6 after Natural or Surgical Menopause

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
Spontaneous increases in the expression and secretion of the proinflammatory cytokines IL-1, IL-6, and TNF- with estrogen deficiency were first noted several years ago in ex vivo cultures of circulating monocytes (Refs. 7 and 8 and Fig. 1), bone marrow macrophages (9, 10, 11, 12), and osteoblasts (13). Increases in these cytokines with estrogen deficiency are, however, subtle compared with the increases observed as a host reaction to infection or major tissue injury (14). Efforts to directly demonstrate cytokine increases with estrogen deficiency in tissue samples in vivo (15, 16) or in circulation (17, 18, 19, 20) have been less successful. Nevertheless, using extremely sensitive techniques, such as IL-6 promoter luciferase constructs, increases in IL-6 promoter activity have been observed in the spleens of ovariectomized transgenic mice harboring these constructs (21). Moreover, several authors have noted increases (22) or at least a trend for increases (23, 24) in circulating IL-6 and TNF- (25) after natural or surgical menopause.

View larger version (14K): [in this window] [in a new window]   Figure 1. Effect of oophorectomy and subsequent estrogen therapy on human mononuclear cell secretion of IL-1 (A), TNF- (B), and phytohemagglutinin-induced secretion of GM-CSF (C). *, P < 0.05; **, P < 0.005; ***, P < 0.0005 compared with baseline. [Reproduced with permission from R. Pacifici et al.: Proc Natl Acad Sci USA 88:5134–5138, 1991 (8 ).]

	III. Effect of Estrogen Treatment on IL-1, IL-6, and TNF

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
Because withdrawal of estrogen is associated with increases in cytokine activity, administration of estrogen might be expected to cause corresponding decreases in cytokine activity. Indeed, E2 treatment inhibited IL-1ß and TNF- expression and/or release in osteoblast-like cells (31, 32), monocytes/macrophages (8, 33, 34, 35, 36, 37), and whole-blood cultures (38). Likewise, E2 inhibited the release of IL-6 from a variety of cell species, including macrophages (37, 39), bone marrow cells (40), whole-blood cultures (38), bone marrow stromal cells, synoviocytes (41), osteoblasts (Refs. 42, 43, 44, 45 and Fig. 2), and endothelial cells (46, 47, 48). E2 attenuated increases in circulating IL-6 levels in mice in vivo triggered by hemorrhage (49, 50). Estrogen-treated ovariectomized rhesus monkeys showed an attenuation of the peak ACTH level and the IL-6 response to an iv infusion of recombinant human IL-1 ß (51). Similar inhibitory effects of estrogen on cytokine responses and the hypothalamic-pituitary-adrenal axis toward inflammatory stimuli were observed in postmenopausal women (52). Moreover, serum IL-6 levels appear to be lower in postmenopausal women on hormonal replacement therapy (22, 53, 54) and in estrogen-treated ovariectomized mice (19) compared with untreated controls. Other endogenous estrogens such as estrone, which is the predominant estrogen in the menopausal years (55), and environmental phytoestrogens (56) appear to share these repressive effects of E2 on IL-6 gene expression.

View larger version (33K): [in this window] [in a new window]   Figure 2. Effect of withdrawal of 17ß-E2 on basal and stimulated IL-6 production by primary cultures of calvaria cells. Primary cultures of calvaria cells were treated with 17ß-E2, as indicated by the black bars at the bottom of the figure. At -24 h, medium was changed in all the cultures. At zero time, cells were stimulated with 20 pM recombinant human IL-1ß (A), 10 nM bovine PTH(1–34) (B), left untreated (C), or treated with 30 pM recombinant murine TNF- (D). After 48 h, supernatants were collected. Bars represent the mean amount of IL-6 (± SD) in supernatants from duplicate cultures. *, Significantly different from other treatment groups (P < 0.01). **, Significantly different from cells never treated with 17ß-E2 (P < 0.01). [Reproduced with permission from G. Passeri et al.: Endocrinology 133:822–828, 1993 (13 ). © The Endocrine Society.]

Nevertheless, the relation between estrogen and these cytokines is far from being that simple. In fact, the existing literature is replete with seemingly contradictory data, because an equally large number of studies failed to demonstrate any effects of estrogen on the expression or protein concentrations of these cytokines. A lack of estrogen action has been reported for macrophages (59, 60, 61), bone marrow cells (45, 62, 63), bone marrow stromal cells/osteoblasts (31, 64, 65, 66), vascular smooth muscle cells (67), or on circulating levels of these cytokines (27, 63, 68, 69). Even more puzzling, some researchers have even reported biphasic (70) or frankly stimulatory effects of estrogen on the release of IL-1, TNF-, or IL-6 from monocytes/macrophages (19, 71, 72, 73, 74, 75) and fibroblast-like synoviocytes (76). Similar conflicting data have been reported for the effects of estrogen on the concentration of circulating TNF- (39), IL-1, and IL-6 (77).

These discrepancies are not readily explained by differences in species, cell type, time after menopause, estrogen concentrations, or culture conditions. They raise the hypothesis that there may be several distinct pathways by which estrogen may affect cytokine gene expression, eliciting either net increases or decreases in cytokine production, depending on the activation of these pathways in the individual cellular context. Alternatively, there may be some intrinsic or extrinsic coregulators of estrogen action that are able to switch the response from suppression to stimulation, again largely depending on the individual cellular context. Part of the discrepancies, particularly those involving in vivo studies, may also be due to pharmacological differences between hormonal replacement and endogenous estrogen secretion. There is no study as yet that would have reported spontaneous decreases in cytokine activity with natural menopause, suggesting that stimulatory effects of estrogen on cytokine production in nonreproductive tissues may perhaps only occur with nonphysiological conditions. For example, in a recent study, monocyte IL-6 secretion was inhibited by estrogen in healthy rats, but inhibition tended to be reversed under conditions of activated immune function (19). Better comprehension of the complex effects of estrogen on cytokine production will largely depend on the success in unraveling the underlying mechanisms of these effects at the molecular level.

	IV. Potential Mechanisms of the Interaction between Estrogen and Proinflammatory Cytokines at the Molecular Level (Fig. 3)

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
At present, it appears as though most, if not all, effects of estrogen on cytokine activity in nonreproductive tissues occur by activating the ER. There are two well-characterized intracellular ERs, - and -ß (78). The tripartite structure of the ERs is defined by a central sequence-specific DNA binding domain, an N-terminal constitutive activation function (AF)-1, and a C-terminal E/F domain (AF-2). In the best understood mode of action, or classical pathway, ERs function as ligand-dependent transcription factors. Ligand binding is thought to cause a conformational shift in helix 12 in the C-terminal AF-2 domain. This is followed by the dissociation of tissue-specific corepressors and association of coactivators that favor the binding to specific consensus regulatory sequences, referred to as estrogen response elements (EREs), located within the regulatory regions of target genes (79). The effects of estrogen on reproductive tissues are mediated this way. In contrast, the promoters of IL-1, IL-6, and TNF- lack a classical ERE. Activation of the ER, therefore, is unlikely to result in a direct interaction with these promoters (43). Studies with the new class of selective ER modulators (SERMs) have provided further evidence for a different mode of interaction of the ER with target genes in nonreproductive tissues that is not mediated by a direct transcriptional effect of the ER (80, 81, 82). Consistent with such ERE-independent actions of the ER, several interactions of the ER with other transcription factors have been discovered in the past few years, some of which may also be involved in the inhibitory effects of estrogen on proinflammatory cytokine activity.

View larger version (43K): [in this window] [in a new window]   Figure 3. Schematic presentation of the potential actions of estrogen on proinflammatory cytokine activity. The activated ER directly inhibits IL-6 and TNF- gene expression via NF-B- and AP-1-dependent mechanisms. Estrogen also modulates the activity of NO via eNOS and iNOS, exerts direct and indirect antioxidative actions, and interferes with the activity of the hypothalamic-pituitary-adrenal (HPA)-axis and lymphocyte function. Membrane ERs may also be involved in cytokine regulation. Cytokines that have been shown to be directly and/or indirectly modulated by estrogen include IL-1, IL-6, TNF-, M-CSF, GM-CSF, OPG, and TGF-ß. , Stimulatory effects; , inhibitory effects.

The inhibitory action of the activated ER on binding of NF-B to the IL-6-gene is one of the best-examined examples for a cross-talk between ligand-activated ERs and proinflammatory transcription factors. Expression of IL-6 at inflammatory sites is largely controlled by NF-B (84). Antagonism of NF-B activity has been observed with many nuclear receptors, including the receptors for glucocorticoids (85, 86), androgens (87), progesterone (88), the RXR (89), and the peroxisome proliferator-activated receptor (90). There is also ample evidence that the p65 subunit of NF-B represses ER--mediated transactivation (86). However, in some cells, this repression appears to be reciprocal, and E2 can in return block the ability of NF-B to bind to target sequences located within the regulatory regions of the IL-6 gene (44). These repressive effects of 17ß-E2 on NF-B activation have now been confirmed for a large number of different cell species, including astroglia cells (91), HeLa cells (92, 93), murine macrophages (94), human osteoblasts (44), human hepatoma HepG2 cells (95), and rat cardiac myocytes (96).

The cell type-specific manner in which this occurs is consistent with a cofactor-modulated process. Deletion analysis and the use of ER-/ß chimeras suggest that in osteoblasts, the A/B domain of the ER containing AF-1 is essential for preventing NF-B-induced gene transcription (97). The exact mechanism by which the ER blocks NF-B-induced IL-6 expression is, however, not agreed upon (98, 99). The association between ER and NF-B may introduce conformational changes in both proteins that lead to the inability to bind DNA (98). Alternatively, the association may result in the formation of inactive complexes on the DNA by preventing the interaction with essential cofactors or the basal transcriptional machinery. The NF-B protein RelA (p65) can recruit histone acetyltransferase activity to the promoter through interactions with the p300 and cAMP response element binding protein-binding protein (CBP) coactivator proteins (100). A competitive mechanism for limiting amounts of CBP and other coactivators has been proposed for the interaction between NF-B and the GR to explain glucocorticoid-mediated inhibition of NF-B (101). A similar competition may be involved in the estrogen-mediated inhibition of NF-B-induced IL-6 transcription, because the ER also interacts with the p300 coactivator. In support of this hypothesis, Harnish et al. (95) showed that in HepG2 cells, ligand-bound ER- decreased histone acetyltransferase activity required for NF-B transcriptional activity. CBP reversed NF-B inhibition, supporting the hypothesis that reduced NF-B function may occur by limiting the availability of this coactivator. Moreover, overexpression of p300 in human coronary smooth muscle cells significantly reduced the inhibitory effect of ER on RelA-dependent transcription in these cells (102). Nevertheless, some questions remain, because the latter effects were ligand dependent and may be explained by a direct inhibitory effect of p300 on ligand sensitivity (103).

In some cell species, estrogen may also act by modifying the expression of factors binding to the DNA or by altering nuclear translocation of NF-B. Thus, in a study by Sun et al. (104), estrogen was observed to block the degradation of IB, suggesting that the inhibitory effect of estrogen on NF-B function may also be, in part, due to the sustained presence of the inhibitory subunit of NF-B.

It has been suggested that the effects of the ER on IL-6 transcription may extend beyond NF-B, because the transcription factor CCAAT/enhancer-binding protein (C/EBP)ß (nuclear factor-IL-6) has also been shown to interact with the human ER in vitro (44). Many immune response genes and acute-phase response genes contain both C/EBPß and NF-B sites, and cooperative interactions between nuclear factor-IL-6 and NF-B may play an important role in the expression of these genes (105). Indeed, studies in HeLa cells transiently transfected with a pIL-6/chloramphenicol acetyltransferase plasmid and an ER expression vector, suggested that, in addition to an intact NF-B binding site, estrogen repression of IL-6 responses requires the binding of C/EBP-family factors to the C/EBP site of the IL-6 promoter (93). However, the need for C/EBP for the inhibitory effects of estrogen on IL-6 in HeLa cells was not confirmed by Galien and Garcia (98) in a second study.

Estrogen-induced down-regulation of IL-6 expression has been independently observed in cells that only express ER- or ER-ß (96, 106), suggesting that both receptor types are capable of repressing NF-B signaling to the IL-6 gene upon activation. Whether this may apply to all forms of the ER-ß is not clear yet, because Bhat et al. (107) have identified a form of a human ER-ß containing an N-terminal extension that attenuated cytokine-mediated NF-B activation in contrast to a receptor form lacking this N-terminal extension. In human osteoblastic U2-OS cells that were transiently or stably transfected with ER- or ER-ß, ER- was the major ER through which transcription of NF-B-regulated genes was inhibited, suggesting that the extent of estrogen-induced IL-6 suppression may depend on the cell-specific pattern of ER expression (97).

The described interaction between the ER and NF-B may also be relevant to the expression of other NF-B-regulated proinflammatory cytokine genes (108). However, because many of these interactions appear to be cell- and gene-specific, extrapolations from the above findings to other cytokine genes are difficult to draw without detailed examination. This is illustrated by the observation that NF-B complexes cooperate with ER- to recruit cofactors into the complex and thereby synergistically activate the serotonin-1A receptor promoter (109).

B. Interactions between estrogen and the TNF- gene
The activator protein (AP)-1 site in the TNF-responsive element of the -125 to -82 region of the TNF--promoter is thought to be critical for the ability of IL-1 and TNF- to induce TNF- gene expression. AP-1 consists of homodimers of Jun family proteins or heterodimers of the Jun family with Fos family proteins (110). Depending on the cellular context and the transcribed gene, estrogen has been shown to either activate (111, 112, 113, 114, 115, 116, 117) or suppress (118, 119) AP-1-mediated gene transcription. In monocytic cells, estrogen blocks AP-1-mediated transcription of the TNF- gene. Interestingly, estrogen also repressed the collagenase promoter when cotransfected into the cells, despite the fact that estrogen is known to activate collagenase transcription in other cell types via an AP-1 element in the collagenase promoter. This clearly indicates that estrogen-mediated stimulation or repression of gene transcription at AP-1 sites occurs in a cell-specific fashion (120). Of note, soybean isoflavones are also capable of suppressing TNF- transcription in monocytes by binding to the ER (121).

The direct interaction between the ER and the AP-1 site in the TNF- promoter requires binding of the ER to other promoter-bound proteins, such as the GR-interacting protein 1 (121, 122), and the AF-2 surface in the ligand-binding domain of the ER (120, 121). This may explain why raloxifene, which prevents the formation of an active AF-2 surface, antagonized the AP-1-mediated repression of the TNF- promoter (120, 122).

Estrogen may also suppress AP-1-dependent TNF--transcription by inhibiting Jun expression and suppressing the Jun N-terminal kinase pathway. Binding of estrogen to ER-ß in the murine RAW 264.7 monocytic cells reduced the activity of the Jun NH₂-terminal kinase, diminished phosphorylation of c-Jun and JunD at their NH₂ termini, and resulted in a consecutive decrease in the ability of these nuclear proteins to autostimulate the expression of c-Jun and JunD genes, thus leading to lower production of c-Jun and JunD (123, 124).

Decreases in TNF- transcription in monocytes were observed with transfection of both the estrogen - and ß-receptor, albeit the latter appears to be more efficient in suppressing TNF- transcription (120, 123). There is increasing awareness of such a selective regulation of ER- and -ß mode and amplitude of gene transcription by different receptor ligands, as both receptors appear to have strong affinity preferences for particular coactivators (125). The manner by which the two different ERs are regulated is just about to be explored, but differential effects of ER- and ER-ß on both NF-B-induced IL-6 production (97) and AP-1-dependent TNF- gene transcription may offer a glimpse at one of several possible mechanisms explaining the cell-specific differences in the effects of estrogen on cytokine production.

Until now, no direct molecular mechanism has been proposed for a direct suppressive effect of estrogen on IL-1 transcription. NF-B is a potent stimulator of IL-1 gene transcription (126), raising the possibility that repression of IL-1 transcription by estrogen may work through a similar mechanism as that implicated in the repression of IL-6 gene transcription. However, it is also possible that the increases in IL-1 activity with estrogen deficiency may be merely secondary to an increase in TNF-, as the two cytokines are known to stimulate the expression of each other (127, 128) and the secretion of the two cytokines is usually closely correlated. Consistent with this hypothesis, TNF- neutralization blocked IL-1 production, whereas IL-1 neutralization did not block TNF- (129). Likewise, in cell cultures prepared from neonatal calvariae, TNF--stimulated IL-6 production was inhibited by 17ß-E2, whereas IL-1-induced IL-6 production was unaffected by 17ß-E2 (42).

C. Estrogen, nitric oxide (NO), and cytokines
NO may be another potential target through which estrogen may regulate cytokine activity. NO has been the focus of much attention in the last few years. As a diffusible universal messenger, it plays a central role in the pathogenesis of inflammatory diseases (130). NO is an extremely pleiotropic molecule, and there are many contradictory reports in the literature concerning its role as an anti- or proinflammatory agent. These inconsistencies may be due to the multiple cellular actions of this molecule, the level and site of NO production, and the redox milieu into which it is released (131). Estrogen causes rapid stimulation of NO production in endothelial cells (132, 133), osteoblasts (134), and monocytes (135). This is achieved through up-regulation of the activity of the constitutively expressed endothelial NO synthase (eNOS/NOS III) and does not appear to be associated with changes in eNOS mRNA or protein (136). Correspondingly, ER--knockout mice have a significantly reduced basal release of NO in aorta (137). Moreover, plasma nitrate and nitrite levels, which are related to NO production, are decreased in a state of estrogen deficiency (16, 138, 139), and serum NO levels increase with estrogen replacement (140, 141).

Determining the precise mechanism through which estrogen regulates eNOS activity is an active area of investigation (142). There is increasing evidence that at least part of the acute stimulation of eNOS enzymatic activity may be mediated by coupling of specific plasma membrane ERs to G proteins (134, 143, 144). In human endothelial cells, the ER- also appears to bind to the p85 regulatory subunit of PI3K, leading to the activation of protein kinase B/Akt (145, 146).

Interestingly, and emphasizing the complexity of estrogen actions on NO production, estrogen inhibits another form of NOS that is induced by IL-1 and TNF- and is therefore called cytokine-inducible NO synthase [iNOS/NOS II (147, 148, 149, 150)]. We still do not exactly understand the physiological implications of the differential effects of estrogen on the two NO synthase isoforms. However, the inhibitory effects of estrogen on iNOS activity might well contribute to its suppressive effects on proinflammatory cytokine activity (149).

The effects of NO on proinflammatory cytokines and cytokine-dependent tissue activities are mixed and may critically depend on cofactors in the cellular environment. Both stimulatory (151, 152, 153, 154, 155, 156, 157) and inhibitory effects (158, 159, 160, 161, 162, 163, 164, 165, 166) on the expression and/or secretion or proinflammatory cytokines have been described. These dual effects are nicely illustrated by the differential effects of NO in bone tissue. iNOS-deficient mice exhibit profound defects of IL-1-induced osteoclastic bone resorption through abnormalities in the translocation of the p65 component of NF-B and in NF-B-DNA binding, suggesting that the iNOS pathway is essential for IL-1-induced bone resorption, which is again thought to play a major role in menopause-associated bone loss (167). In contrast, NO donors have been rather shown to reverse ovariectomy-induced bone loss in rats (168, 169), and nitrate use appears to protect postmenopausal women against bone loss as effectively as estrogen use (140, 170).

At present, there are no data that would prove any relevance of these changes in NO activity to cytokine production with menopause. Nevertheless, given the central role of NO as a cellular mediator, the differential effects of estrogen on eNOS and iNOS activity on the one hand, and the variable interactions between NO and cytokine activity on the other hand, may be part of the "Janus-faced" effects of estrogen on cytokine production that have been encountered in some of the described experimental models.

D. Direct antioxidant effects of estrogen
Free radicals are potent stimuli of NF-B-mediated proinflammatory cytokine expression. As estrogen has direct antioxidant effects in vitro and in vivo, increases in free radical production with the decline in estrogen concentrations may be another source of increase in cytokine production. In vascular tissue, increases in the oxidative modifications of molecules, such as low-density lipoproteins, after menopause may stimulate cytokine production by propagating the formation of cytokine-producing foam cells that are characteristic of early atherosclerotic lesions (171).

The mechanisms of the antioxidant effect of estrogen are incompletely understood and may vary in different tissues. Estrogen might play an inhibitory role either in production and/or scavenging of reactive oxygen species, or in protecting the level of endogenous antioxidants. An ER-dependent inhibition of superoxide anion production in microglial cells has been linked to the activation of MAPK (172). Direct antioxidative effects appear to be dependent on the A-ring hydroxyl group of the estrogen molecule, which acts as a highly effective electron donor and free radical scavenger. 3-Methoxyestrone, which lacks the phenolic hydroxyl group, does not interfere with free radical generation (102, 173). However, E2 may also affect the cellular antioxidant enzyme system that is necessary to maintain an optimal redox balance. Detoxification of superoxide anion and hydrogen peroxide, catalyzed by intracellular superoxided dismutase, catalase, and glutathione peroxidase enzyme activities, represents a major line of defense. Significantly lower glutathione peroxidase activity has been seen in blood samples from late menopausal women compared with that seen in premenopausal women (174, 175, 176). Some of the antioxidative properties of estrogen on the vessel wall also appear to be mediated through down-regulation of angiotensin-1 (AT1) receptor gene expression. Up-regulation of AT1 receptor expression with estrogen deficiency is thought to be a predominant source of free radical production in the vasculature. Indeed, increases in the generation of superoxide radicals in the vessel wall with ovariectomy in the rat can be normalized by AT1 receptor antagonists (177, 178, 179).

The impact of these antioxidative effects of estrogen on cytokine production has received little attention so far. Many of the antioxidative effects of estrogen have been observed at supraphysiological levels, but there are some data showing that these effects may also be relevant at physiological estrogen levels (180). For example, 17ß-E2 inhibited in vitro vascular smooth muscle cell proliferation via a nongenomic antioxidant mechanism (181). 17ß-E2, but also its stereoisomer 17-E2, dose dependently and receptor independently inhibited reactive oxygen species generation in cytomegalovirus-infected smooth muscle cells and the consecutive increases in NFB-activity. These effects occurred at physiological estrogen concentrations (102).

E. Rapid nongenomic actions
In a variety of cell types, E2 exerts rapid nongenomic actions that appear to be mediated by plasma membrane ERs (182). For example, E2 rapidly activates MAPK in endothelial cells (183) and in fibroblasts that have been transfected with cDNA clones encoding either ER- or -ß (184). Estrogen also rapidly activates early growth response gene-1 expression in cardiomyocytes via ERK 1 and 2 (185). These rapid effects of estrogen trigger several interactive signal transduction pathways that involve the PKC and PKA pathways as well as MAPK and resemble the effects of the more traditional membrane growth factor receptors. These pathways have the potential to enhance (186, 187, 188, 189, 190, 191, 192, 193) or inhibit (194) the expression of IL-1, IL-6, and/or TNF-, depending on the cellular context. The relevance of these estrogen membrane receptors for estrogen actions is just about to be explored, but it is possible that the stimulatory effects of estrogen on the production of proinflammatory cytokines that have been observed in some in vitro settings may be in part attributable to rapid estrogen actions and to membrane-receptor-mediated effects.

F. Effects of estrogen on lymphocytes
Of considerable interest both with respect to postmenopausal changes in bone metabolism and vascular function is the action of estrogen on lymphocyte function (195, 196). Alterations in T cell subsets have already been described several years ago in women with postmenopausal osteoporosis (197, 198). A recent study by Cenci et al. (199) suggested that the increase in TNF- production in the bone marrow of ovariectomized mice may, at least in part, originate from activated T cells. Remarkably, this appeared to be due to the increase in the number of TNF--producing T cells, not to an increase in TNF- gene expression. The expansion of the T lymphocyte pool in bone marrow after loss of ovarian function may thus be an important indirect mechanism for increases in proinflammatory cytokine concentration. In line with this, the inhibitory effect of estrogen on TNF- production in an animal model of experimental autoimmune encephalomyelitis appears to be predominantly due to a reduction in the number of proinflammatory cytokine producing T cells in the central nervous system (200).

Estrogen deficiency is also associated with an increase in pre-B lymphopoiesis in bone marrow (201, 202, 203, 204, 205). Vice versa, B-lineage differentiation in highly purified early pro-B cells was abrogated when the cells were treated with E2 (195). In a mouse model of systemic lupus erythematosus, estrogen treatment altered the maturation of splenic B cell subsets with a diminished transitional population and an increase in marginal zone B cells (206). As cell-to-cell interactions between lymphocytes and mesenchymal cells greatly affect the production of IL-6 by mesenchymal cells (207, 208), the postmenopausal increases in lymphopoiesis may not only affect TNF- secretion, but IL-6 synthesis as well.

At present, it is difficult to judge which may be first: the rise in proinflammatory cytokines or the enlargement of the activated lymphocyte pool. Arguing for the latter, both T and B cells express ERs, and estrogen deficiency may remove some direct restrictions imposed on lymphocyte proliferation and function. Alternatively, the increases in lymphocyte number and function may predominantly be the result of the increased availability of lymphocyte stimulators after menopause. RANKL, a member of the TNF ligand family whose interactions with estrogen will be described in Section V in more detail, is crucial for lymphocyte development. As the activity of RANKL is up-regulated by estrogen deficiency, this may in turn promote T and B cell development and activation (209). Kanematsu et al. (210) reported that in ovariectomized mice, increases in pre-B cells were abolished by the administration of the cyclooxygenase (COX) inhibitor indomethacin. This suggests that RANKL-induced increases in pre-B cells may be in part mediated by increased PGE₂ levels. Administration of a neutralizing antibody against IL-6 also has been shown to prevent ovariectomy-induced increases in the number of several hematopoietic progenitors in the bone marrow of mice (211). Mechanisms that involve the interaction between matrix proteins, such as osteopontin and lymphocyte function, may potentially play a role as well (212).

G. Central interactions between the gonadal axis and inflammatory processes
Apart from direct cellular interactions between estrogen and cytokines at the molecular level, several menopause-related changes in cell and tissue function have the potential to indirectly affect proinflammatory cytokine activity, although the relevance of these interactions is largely unknown at the present time. For example, estrogen is known to alter the hypothalamic-pituitary-adrenal axis through modulating the release of both hypothalamic and pituitary peptides, such as PRL (213) and GH, which in turn may modify immune responses and cytokine secretion (214). There are many other reciprocal interactions between the hypothalamic-pituitary-adrenal axis and immune-mediated inflammatory processes on the one hand and the gonadal axis on the other hand, which may potentially contribute to changes in inflammatory parameters with menopause. Estrogen directly enhances CRH gene expression and stimulates the central noradrenergic system, thereby positively affecting the two principal components of the stress system. (215). This may be associated with a suppression of the immune-inflammatory reaction and a shift from the T helper cell 1 to the T helper cell 2 profile in lymphocytes (216). It may also, in part, explain the preponderance of some autoimmune disorders in women as compared with men.

H. Changes in visceral fat with menopause
Women tend to accumulate visceral fat with menopause. This appears to involve an estrogen-dependent mechanism of reduced energy expenditure (217, 218, 219) and can be prevented by hormonal replacement therapy (220, 221). Although still speculative at the present time, adipose tissue might turn out to be a significant source of increased proinflammatory cytokines in postmenopausal women. Indeed, there is increasing evidence that adipose tissue is a major determinant of circulating IL-6 (222). As adipocytes from visceral adipose tissue release 2–3 times more IL-6 than those from the sc depot (223), even small increases in visceral fat with menopause might have a large impact on circulating IL-6 levels. In accordance with this hypothesis, Straub et al. (54) observed a positive association between body mass index and serum IL-6 only in postmenopausal women, and this relationship was lost among women with hormone replacement. It appears that to some extent, IL-6 may in return increase estrogen synthesis in adipose tissue by stimulating the aromatase activity and, thus, the conversion of C19 androgenic steroids to estrogen (224). As adipose tissue is the major site of estrogen biosynthesis in postmenopausal women, this may serve as a negative feedback mechanism to limit IL-6 increases.

	V. Effects of Estrogen on RANKL and Osteoprotegerin (OPG)

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
RANKL is a recently discovered member of the TNF superfamily. It is a membrane-associated factor that is expressed in a variety of cell species, including osteoblast/stromal cells, endothelial cells, and lymphocytes. As a result of its independent discovery by several investigators, it is also known as TNF-related activation-induced cytokine, OPG ligand (OPG-L), and osteoclast differentiation factor (225, 226). RANKL exerts its effects through binding to RANK, a TNF receptor family member. RANK is also known as osteoclast differentiation and activation receptor. The biological activity of RANKL is neutralized by binding to OPG, another member of the TNF receptor superfamily. OPG is also known as osteoclastogenesis inhibitory factor, follicular dendritic cell-derived receptor-1, or TNF receptor-like molecule 1. OPG does not contain a transmembrane domain and acts as a soluble receptor.

RANKL and OPG have emerged as essential mediators of proinflammatory cytokines on osteoclastogenesis and lymphocyte maturation. RANKL- and RANK-deficient mice have a complete block in osteoclast development and show significant alterations in both immune cells and immune cell organs with a lack of lymph node development (227, 228). Moreover, several studies have implicated OPG in the regulation of vascular homeostasis (229, 230).

Estrogen dose dependently increased levels of OPG mRNA and protein in human osteoblastic cells (231) and stromal cells (232). Cotreatment with the antiestrogen ICI 182,780 abrogated these effects completely, demonstrating that they are ER mediated. In the mouse stromal cell line ST-2, enhanced induction of OPG expression was only observed in ER--overexpressing cells, but not in ER-ß-transfected cells, indicating that, at least in some cell species, the stimulatory effects of estrogen on OPG are dependent on ER- (233). Estrogen deficiency results in decreased OPG expression in bone, whereas estrogen replacement therapy prevents it (234). In accordance with these findings, postmenopausal women who use hormone replacement therapy were reported to have higher circulating OPG levels than those without hormone replacement (235). Apart from its direct effect on OPG synthesis, estrogen may indirectly block RANKL activity by limiting the availability of the major RANKL stimulators IL-1, IL-6, and TNF- (236). However, IL-1- and TNF- are known to stimulate OPG secretion as well (237, 238, 239, 240). RANK expression itself appears to be unaffected by estrogen deficiency, but estrogen deficiency may enhance RANK signaling in monocytes by down-regulating RANK-induced Jun N-terminal kinase 1 activity (241).

	VI. Effects of Estrogen on Other Cellular Mediators Relevant to Inflammatory Processes

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
A. Effects of estrogen on TGF-ß
TGF-ß plays an important role in tissue recycling and tissue repair in response to injury. It exerts a large number of suppressive effects on inflammatory processes, ranging from the suppression of the growth and differentiation of hematopoietic precursor cells to inhibition of proliferation and apoptosis of immunocompetent cells. TGF-ß thereby often antagonizes the effects of proinflammatory cytokines. The essential role of TGF-ß in suppressing inflammatory processes is impressively demonstrated by the appearance of massive infiltrations of inflammatory cells in several organs of mice deficient in TGF-ß1 activity (242, 243, 244).

Findings that estrogen positively affects TGF-ß function have been very consistent for various tissues. Estrogen stimulates TGF-ß expression and/or production by osteoclasts (245, 246), osteoblasts (247, 248), vascular smooth muscle cells (249), and fibroblasts (250). Transdermal 17ß-E2 increased serum levels of TGF-ß1 in postmenopausal women (251). With the exception of a single study (252), ovariectomized rats were observed to have lower protein and mRNA levels of TGF-ß1 in bone tissue compared with sham-operated rats (253, 254, 255). Interestingly, in contrast to rats, whose capacity for bone remodeling is limited and in whom the primary effects of estrogen on TGF-ß production are likely to predominate, estrogen deficiency in humans is associated with an increase in skeletal TGF-ß, and this increase can be prevented by hormone replacement therapy (256). However, this is most likely a consequence of the reactive increases in bone formation and not a direct effect on TGF-ß expression.

The stimulatory effects of estrogen on TGF-ß activity are also ERE independent and can be mimicked by the SERMs tamoxifen and raloxifene (246, 248, 257). Yang et al. (248) demonstrated a direct interaction of the ER with a sequence in the TGF-ß3 gene promoter termed "raloxifene response element," which did not require the DNA binding domain of the ER. Astonishingly, activation of TGF-ß3 transcription was also observed with the pure ER antagonist ICI 164,384 (248), but this was not confirmed in another study (245).

B. Effects of estrogen on PG synthesis
COX is the key step in the conversion of arachidonic acid to a series of bioactive prostanoids. Two COX genes have been identified: a constitutive form (COX-1), which is expressed ubiquitously, and a second form (COX 2), which is highly inducible in response to proinflammatory cytokines (258). COX 2 expression is greatly elevated in both acute and chronic inflammation. PGE₂ is one of the most abundant COX metabolites. Estrogen deficiency increased PGE₂ concentrations in ex vivo cultures of human and rat bone marrow cells (12, 259) and in organ cultures of calvariae from ovariectomized rats (260). Postmenopausal women also excrete larger amounts of urinary PGE₂ compared with premenopausal women (261). Conversely, E2 inhibited PGE₂ production in stromal cells and monocytes (259, 261, 262, 263).

At present, there is no evidence for a direct inhibitory effect of estrogen on COX activity. Rather, estrogen increases the release of another COX product, PGI₂, by vascular endothelium (264, 265, 266). It is likely that the increased production of PGE₂ with menopause is predominantly due to the up-regulation of its major cytokine inducers. This is supported by the finding that IL-1 receptor antagonist (IL-1ra) and anti-IL-1-neutralizing antibody were able to decrease exaggerated PG production and COX-2 expression in cultured neonatal mouse calvariae when stimulated with bone marrow supernatants from ovariectomized mice (259).

C. Effects of estrogen on macrophage colony-stimulating factor (M-CSF) and granulocyte-macrophage-colony stimulating factor (GM-CSF)
GM-CSF and M-CSF control the production, maturation, and function of granulocytes and monocyte-macrophages (267). Increases in GM-CSF activity were observed after surgical menopause in ex vivo cultures of peripheral blood mononuclear cells (7). Bone marrow cells from women who had discontinued estrogen replacement shortly before aspiration or who were within 5 yr of natural menopause also secreted more GM-CSF than bone marrow cells from premenopausal women (12). As with PG production, the increases in GM-CSF with estrogen deficiency may be predominantly mediated by IL-1 and TNF-. However, other mechanisms must be involved as well, as the increase in GM-CSF production in mononuclear cells from ovariectomized women was very rapid and preceded the increase in IL-1 and TNF (7).

Estrogen also down-regulates membrane-bound and soluble M-CSF. Whereas the expression of membrane-bound M-CSF on bone marrow cells is directly inhibited by estrogen (268, 269), down-regulation of soluble M-CSF secretion by estrogen may involve the differentiation of stroma cell precursors toward a mature phenotype characterized by a lower production of soluble M-CSF. Vice versa, estrogen deficiency increases the production of soluble M-CSF in bone marrow by an IL-1- and TNF-mediated expansion of a stromal cell population that produces larger amounts of soluble M-CSF (34, 270).

	VII. Implications for Postmenopausal Bone Loss (Fig. 4)

Top Abstract I. Introduction II. Evidence for Increases... III. Effect of Estrogen... IV. Potential Mechanisms of... V. Effects of Estrogen... VI. Effects of Estrogen... VII. Implications for... VIII. Potential Implications for... IX. Individual Susceptibility to... X. Time Course of... XI. Role of Progesterone XII. Role of Androgens XIII. Potential Implications for... XIV. Summary and Conclusions References

 
A. Impact of cytokine changes with estrogen deficiency on osteoclastogenesis
There is progressive loss of bone tissue after natural or surgical menopause, leading to increased fractures within 15–20 yr from the cessation of ovarian function (271). ERs have been detected in many cells that reside in bone tissue (272, 273, 274, 275, 276, 277, 278), suggesting that menopause may have direct consequences on cytokine secretion by cells located within the bone microenvironment. Bone marrow cells of the monocyte/macrophage lineage are believed to be the major source of the postmenopausal increases in TNF- and IL-1 secretion in bone tissue (279). However, in the past few years it has been increasingly recognized that activated T cells are also an important source of increased TNF- production in the bone marrow after menopause (195, 196, 209, 280, 281, 282, 283). In contrast, stromal cells/osteoblasts are considered to be the major producers of IL-6 in bone tissue (284).

View larger version (33K): [in this window] [in a new window]   Figure 4. Overview of the multiple interactions by which cytokines and estrogen regulate bone resorption. The arrows indicate stimulatory () or inhibitory () effects of a cytokine on the synthesis of another cytokine or on a particular step in osteoclastogenesis. Macrophages (IL-1 and TNF-), lymphocytes (TNF- and RANKL), and stromal cells/osteoblasts (IL-6, TGF-ß, RANKL, M-CSF, GM-CSF, and PGE2) are thought to contribute to the increased cytokine production in the bone microenvironment with estrogen deficiency. M-CSF, GM-CSF, and IL-6 facilitate osteoclast precursor proliferation. RANKL, TNF-, PGE2, and TGF-ß participate in the differentiation of osteoclast precursors into mature osteoclasts. RANKL, IL-1, and IL-6 directly stimulate the bone resorbing activity of the mature osteoclast. IL-1, M-CSF, RANKL, and TGF-ß modulate osteoclast apoptosis. IL-1, TNF-, PGE2, and TGF-ß also have potent effects on osteoblast function. Most cytokines are stimulated or inhibited by other cytokines, thereby creating a network of synergistic cytokine actions, which ultimately lead to increased bone resorption. OB, Osteoblast/stromal cell; L, lymphocyte; M, monocyte/macrophage.

Because estrogen is a potent stimulator of OPG production (237) and suppresses M-CSF production, loss of estrogen would also be expected to promote the signaling and gene expression cascade that leads to the differentiation of osteoclast progenitors to mature osteoclasts. A decrease in OPG production with estrogen deficiency is bound to increase the ratio of RANKL/OPG activity. It thus facilitates the binding of RANKL-expressing osteoblast/stromal cells and lymphocytes to osteoclast progenitors as the key signal for initiating osteoclast differentiation (227, 315, 316, 317, 318, 319). Estrogen deficiency is associated with the loss of inhibitory effects on a closely interrelated network of cytokine stimuli that are suited to tip the balance of RANKL-to-OPG production further toward increased RANKL activity. In particular, PGE₂, which increases RANKL expression and decreases OPG expression in osteoblastic/stromal cells (240, 320, 321), may be involved in the increase in the expression of RANKL in bone marrow pre-B cells from ovariectomized mice, as the increase could be mimicked by PGE₂ and was abolished by indomethacin (210, 322). The PGE₂-induced increases in the RANKL/OPG ratio with estrogen deficiency may again only be a consequence of increases in monocyte and T cell-derived IL-1 and TNF- secretion (238, 318, 323, 324, 325, 326). Although the data are still somewhat conflicting (226, 237, 239, 327), IL-6 also appears to contribute to this up-regulation of RANKL activity in estrogen-deficient osteoblastic cells (318, 328).

Both TNF- (316, 318, 329, 330) and PGE₂ (331) may have a direct, RANKL-independent effect on osteoclast differentiation. This effect may be small compared with that of RANKL, but it may be important for potentiating RANKL-induced osteoclast formation (332). Indeed, it has been proposed that because minuscule amounts of RANKL are sufficient to maximally stimulate osteoclastogenesis, RANKL-independent effects may be the true rate-limiting steps of increased osteoclast differentiation with menopause (332).

Whereas all of the consequences of estrogen deficiency on proinflammatory cytokine activity described so far are clearly synergistic with respect to osteoclast formation and strongly support a major role of these cytokines in postmenopausal bone loss, the role of TGF-ß is more ambivalent. Proliferation of osteoclast progenitors may be facilitated by the relative decline in TGF-ß activity with estrogen deficiency, as TGF-ß has been shown to be a major inhibitor of osteoclast generation (333, 334). Relative decreases in TGF-ß with menopause may also contribute to the increases in the RANKL/OPG ratio with estrogen deficiency (236, 326, 335). On the other hand, TGF-ß has recently been observed to be essential for the differentiation of osteoclast precursors cells into osteoclasts (336, 337, 338). Moreover, TGF-ß has been described as both stimulating (339, 340, 341) and inhibiting (73, 342, 343) proinflammatory cytokine production.

IL-1, TNF- (344, 345), and PGE₂ (346) may not only promote osteoclast generation, but they also appear to stimulate mature osteoclasts to perform more resorption cycles via modulation of RANKL activity (347). IL-1 (318, 329, 348, 349) and IL-6 (350) also directly enhance osteoclast activity by RANKL-independent mechanisms. Finally, increases in IL-1 (351, 352), M-CSF (353), RANKL (225), and a relative decrease in TGF-ß (246) may directly extend the lifespan of the osteoclast by inhibiting osteoclast apoptosis.

In premenopause, variations in bone resorption are usually compensated by appropriate changes in bone formation. This is thought to be the result of locally released anabolic growth factors, such as TGF-ß (354) and IGFs (355). Efforts to compensate the increased bone resorption are also evident with estrogen deficiency (256) but obviously do not suffice to achieve a neutral bone balance. Thus, depending on the perspective, postmenopausal bone loss may also be viewed as resulting from inadequate bone formation. It is well known that proinflammatory cytokines have potent effects on osteoblast function. Both TNF- and IL-1 inhibit collagen synthesis in osteoblasts (299, 356, 357, 358, 359, 360, 361) and stimulate or inhibit bone cell proliferation, depending on the experimental systems employed (325, 357, 362, 363, 364, 365). Both TGF-ß and PGE₂ are potent stimulators of bone formation (366, 367). However, the relevance of postmenopausal changes in these cytokines with respect to bone formation has been much less explored compared with bone resorption.

B. Evidence for an essential role of proinflammatory cytokines in bone loss with estrogen deficiency
There is now compelling evidence from studies in mice and rats that changes in cytokine activity with estrogen deficiency are indeed essential for the bone loss that is observed in these animals. Mice insensitive to IL-1 due to the lack of IL-1 receptor type I are protected against ovariectomy-induced bone loss (368). Treatment with IL-1ra decreased bone resorption (11) and prevented bone loss in ovariectomized rats (10). The essential role of TNF- in ovariectomy-associated bone loss is suggested by the finding that treatment of ovariectomized mice with TNF- binding protein, a potent inhibitor of TNF-, completely prevented bone loss (11, 369). Likewise, transgenic mice insensitive to TNF- due to the overexpression of soluble TNF receptor (370), as well as rats receiving an orally active inhibitor of IL-1 and TNF production (371), have been shown to be protected against bone loss with ovariectomy. Increases in osteoclast numbers in vitro and in vivo in ovariectomized mice were also prevented by an antibody to IL-6 (Refs. 9 and 372 and Fig. 5). In accordance, IL-6-knockout mice are protected against the loss of trabecular bone induced by ovariectomy (373). The critical involvement of PGs is supported by the findings that the nonsteroidal COX inhibitor naproxen inhibited bone loss induced by up to 70% ovariectomy in rats (374). T cell deficiency also effectively prevented bone loss in ovariectomized mice (199).

View larger version (32K): [in this window] [in a new window]   Figure 5. Inhibitory effect of a neutralizing antibody to IL-6 (IL-6 MAb) or estrogen administration (E2) on osteoclast numbers per millimeter of secondary spongiosa of the tibia of ovariectomized (OVX) female mice. Data are mean ± SEM. In contrast, osteoclast numbers in OVX mice and OVX mice receiving a monoclonal IgG antibody to ß-galactosidase (OVX + IgG) were significantly increased compared with those of sham-operated animals (Sham). [Derived from R. L.