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Department of Molecular and Cellular Biology, Baylor College of Medicine, Houston, Texas 77030
Correspondence: Address all correspondence and requests for reprints to: Carolyn L. Smith, Ph.D., Department of Molecular and Cellular Biology, One Baylor Plaza, Houston, Texas 77030. E-mail: carolyns{at}bcm.tmc.edu
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 Top Abstract I. IntroductionII. Biology of Selective...III. Mechanisms of SRM...IV. Molecular Basis of...V. Lessons Learned from...VI. Concluding RemarksReferences |
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I. Introduction |
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TopAbstractI. IntroductionII. Biology of Selective...III. Mechanisms of SRM...IV. Molecular Basis of...V. Lessons Learned from...VI. Concluding RemarksReferences |
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Careful examination of the selective biological effects of tamoxifen (e.g., estrogen-like activity in the uterus but antiestrogen-like effects in the breast) led to the emergence of the concept of selective ER modulators or SERMs (see below). The molecular mechanisms through which selective effects are obtained has been the topic of intense investigation with the result that not only do we have at least a basic understanding of how the selective nature of SERM activities is achieved, we have now progressed to a point at which the stage is set to pursue the identification and development of selective receptor modulators (SRMs) for a host of other nuclear receptors. The driving force in this research is the desire to obtain agents that can better separate desired nuclear receptor effects from those that are undesirable. Indeed, the lessons learned from SERMs provide a framework in which to pursue the development of other SRM ligands with high binding affinity to the receptor of interest. The experience with SERMs suggests that compounds with selective activities are likely to be of intense clinical and economic interest, thereby stimulating significant research in this area in both the basic science and pharmacological arenas.
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II. Biology of Selective Receptor Modulators (SRMs) |
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TopAbstractI. IntroductionII. Biology of Selective...III. Mechanisms of SRM...IV. Molecular Basis of...V. Lessons Learned from...VI. Concluding RemarksReferences |
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and ERß, plays a significant role in normal biology and pathophysiology. Estrogens are widely used clinically to control reproduction (i.e., oral contraceptives) and for hormone therapy and the management of menopausal symptoms in women. Although beneficial in these contexts, estrogen use also has been implicated as a risk factor in breast and uterine cancer, particularly since the first published report from the Women’s Health Initiative (3), suggesting that a greater measure of flexibility to control unwanted side effects would be desirable. Consequently, the recognition of SERMs as agents able to elicit estrogenic effects in a tissue-specific manner has expanded the potential population that could benefit from ER ligand therapies. SERMs have been important for their clinical potential as SRMs as well as serving as the focus of a vast body of research defining the molecular mechanisms through which cell and tissue selectivity is achieved. The trans isomer of tamoxifen is the prototypic SERM (4, 5, 6, 7, 8). Although one of its first proposed uses was to regulate fertility, it has been employed primarily as an agent used to treat and, more recently, prevent breast cancer (9, 10, 11, 12). The ability of tamoxifen to inhibit ER action has long been considered integral to its utility in the breast cancer arena, and this is consistent with numerous studies and clinical trials demonstrating an effect of tamoxifen in ER-positive cells or breast tumors and an absence of any significant activity in those lacking ER expression (8, 13). However, the demonstration of tamoxifen’s estrogen-like effects in the rodent uterus and skeleton (7, 14) suggested that this drug may have distinct biological properties depending on the tissue environment. The subsequent observation of the estrogen-like effects of tamoxifen in the human skeleton (15) was important to the conceptualization of SERMs as potential drugs for indications other than breast cancer.
Tamoxifen’s activity profile has long been thought to be a reflection of its partial agonist/antagonist activity. For instance, in the absence of endogenous estrogens, tamoxifen frequently exhibits weak estrogenic activity, such as modest stimulation of uterine wet weight and bone density in ovariectomized rats, whereas in the presence of estradiol, it can serve as an antiestrogen, inhibiting responses to a level corresponding to the comparatively modest agonist activity of tamoxifen itself (7, 16). Much effort has been devoted toward understanding the molecular mechanisms through which selective ER actions are achieved, and this is the focus of much of this review. It is appropriate, however, to recognize that the success of tamoxifen as a SERM has been a driving force in the search for new SERMs as well as selective modulators for other nuclear/steroid receptors. Raloxifene-like tamoxifen exhibits antiestrogen activity in the breast and estrogen activity in the skeleton. However, raloxifene lacks the significant uterotropic activity associated with tamoxifen and therefore represents an improved agonist/antagonist profile (6, 17, 18, 19). However, because neither tamoxifen nor raloxifene possesses significant estrogen-like activity in the CNS, there is clearly a market for other SERMs to fill this niche. Indeed, a number of other compounds, including lasofoxifene, arzoxifene, and bazedoxifene, are under development, which may one day be of clinical use for chemoprevention of breast cancer or treatment and prevention of osteoporosis (20, 21, 22). It is also noteworthy that SERMs likely exist in nature. For instance, the estrone metabolite, 
8, 9-dehydroestrone sulfate suppresses hot flushes in postmenopausal women, an estrogenic action. However, it is unable to significantly affect certain other parameters associated with estrogenic responses such as total cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol (23).
B. Selective tissue estrogenic activity regulators (STEARs)
Although distinct from SRMs, a new classification of compounds with tissue-selective biological activity, called selective tissue estrogenic activity regulators (STEARs), has arisen. This class of compounds does not interact directly with receptors such as the ERs, progesterone receptor (PR), or androgen receptor (AR), thus distinguishing them from SRMs. Instead, a precursor (prohormone) compound relies on tissue-selective metabolism to generate hormonal metabolites that have a tissue-specific functional profile. An example of such a drug is tibolone, which currently is the leader in worldwide sales for hormone therapy outside of the United States (24). The oral form of this steroid prohormone is inactive; once in the body, however, it is metabolized to 3- and 3ß-hydroxy-derivative forms, which are estrogenic, and a 4-isomer, which has weaker androgenic and progestational activities (25). Moreover, tibolone is a sulfatase inhibitor (e.g., blocks conversion of estrone sulfate into estrone) and can also stimulate local sulfotransferase activity (26). The resultant activity profile in humans shows estrogenic activity in bone, as well as CNS vasomotor suppression of hot flushes, but no significant uterotropic or mammotropic activities (27). With the exception of the notable CNS effects, tibolone has a SERM profile, but achieves this without directly modulating the estrogen, androgen, or PRs as it is the various metabolites of tibolone that exert their effects directly on these receptors (24, 26).
C. Selective PR modulators (SPRMs)
PRs control a number of processes critical for reproduction (28). In addition to the natural ligand progesterone, there presently are good synthetic PR agonists and antagonists available for use in humans, and these are used to control reproduction and the function of various reproductive organs such as the uterus. Although the need for a SPRM may be less obvious than for SERMs and selective AR modulators (SARMs) (see below), it has been suggested that a SPRM may be useful for treatment of endometriosis. Current therapies for endometriosis include the use of GnRH antagonists, PR agonists, or androgens. However, these options are not without side effects. GnRH antagonists, by virtue of their ability to induce a hypoestrogenic state, induce hot flushes and urogenital symptoms and may increase the risk of osteoporosis. Progestins may produce breakthrough bleeding, bloating, breast tenderness, mood changes, and breast epithelial proliferation, and androgen use is associated with seborrhea, acne, hirsutism, negative changes in lipid profiles, and, potentially, virilization. As an alternative, an agent that can more specifically target the endometrium may be an effective treatment for endometriosis, while minimizing side effects. A SPRM in combination with an ER ligand also may be useful in menopausal hormone therapy, particularly with respect to hormonal effects on the breast.
Several potential SPRMs have been identified [e.g., dexamethasone-oxetanone and J867 (29, 30)]. The biological properties of some of these agents have been shown to exert partial PR agonist activity in that they can stimulate the proliferation and differentiation of epithelial endometrial cells in juvenile, estrogen-primed rabbit uterus, although not as effectively as progesterone (30). In contrast, in the presence of progesterone these agents partially inhibit the activity of the PR and thus also possess antagonist properties. Various other measures of progestin/antiprogestin activity, such as induction of cervical ripening and parturition in pregnant guinea pigs, and the ability to inhibit mammary gland proliferation, indicate that the activity of these agents falls between that of antiprogestins such as onapristone (ZK 98,229) and mifepristone (RU486) and progestins such as progesterone and R5020. Differences in the action of SPRMs in comparison with antiprogestins also have been observed at the gene level; whereas medroxyprogesterone acetate (a progestin) and ZK137,316 (an antiprogestin) inhibit vascular endothelial growth factor expression in endometrial fibroblasts, the SPRM J867 is unable to do so (31). Moreover, at least one agent, J1042, is able to induce a marked reduction in endometrial thickness in cynomolgus macaques, suggesting it or a related compound may have potential as an endometriosis therapy (32). Recently, it has been shown that the SPRM asopril can inhibit estrogen-dependent uterine growth but is devoid of the breast stimulatory effects of progesterone. Such a compound is being used to treat endometriosis and uterine fibroids, while sparing the breast of significant stimulation (33).
D. Selective AR modulators (SARMs)
AR expression is widespread throughout the body, and androgens play a desirable role in promoting and maintaining bone strength, increasing muscle mass, decreasing fat tissue, and enhancing libido (34). Although androgen therapies are currently available, they are primarily based on delivery of testosterone or its derivatives by injections or skin patches (35). Neither approach is optimal because injections result in undesirable fluctuations in serum testosterone levels, and skin patches are associated with irritation and rashes. Oral preparations of currently available androgens are not recommended because of their relatively low efficacy and potential hepatic toxicity. There is, therefore, a desire to develop a form of androgen therapy that is easily administered orally and that will avoid the considerable fluctuations of serum androgens observed for injectables. More importantly, the goal exists to obtain androgenic therapies that do not exert undesirable side effects such as alterations in lipid profiles (e.g., high- and low-density lipoproteins), fluid retention, liver toxicity, prostatic hypertrophy, and gynecomastia. The more severe side effects associated with supraphysiological doses of androgens taken by body builders and athletes, such as increased aggression, decreased testicular size, and azoospermia, are unacceptable under all conditions. Simply stated, the goal of preservation of positive androgen effects in some tissues, while minimizing negative side effects in other tissues, has stimulated a search for SARMs.
Although the use of estrogens and SERMs is widespread, there has not been an equivalent trend observed for androgen therapies. This is not due to a lack of indication for androgen treatment, as this type of therapy would be of benefit for treatment of men with primary or secondary hypogonadism, osteopenia and osteoporosis, HIV wasting and cancer-related cachexia, anemias, various muscle dystrophies, and, potentially, male contraception. Although there is a paucity of well-designed studies supporting an indication for androgen therapy in women, androgen therapy has been advocated for improvement of bone strength, libido and other sexual parameters, as well as a sense of well-being in postmenopausal women (36).
Indeed, the current lack of SARM-based therapies results from a lack of a suitable agent. However, the greater understanding of the molecular events through which SERM actions are achieved established a rational basis for identifying and characterizing SARMs. Ideally, such an agent would be orally active, and, as a treatment for hypogonadism, should be capable of stimulating muscle mass and strength, bone strength, libido, and virilization but with minimal hypertrophic effects on the prostate. For osteopenia or osteoporosis indications, a SARM with anabolic activity in bone and possibly muscle, but with relatively little activity on sex-accessory tissues, would be desirable. SARMs for females might target libido and other sexuality parameters while avoiding virilization. Progress has been made in this area, and several compounds that possess a mixture of agonist- and antagonist-like activities in transient transfect assays measuring AR trans-activation of a target gene in cells have been identified (37, 38). Assessments of the in vivo SARM activity of these compounds is underway in animals as well as in humans, and they show a promising tissue-selective activity profile. Animal experiments with one such SARM, LGD2226, revealed that it prevented loss of bone mineral density associated with orchidectomy in rats and exerted anabolic activity in the levator ani muscle; in contrast, LGD2226 did not stimulate prostate weights above those observed for intact rats (39).
E. Selective peroxisome proliferator-activated receptor modulators (SPARMs)
Peroxisome proliferator-activated receptors (PPARs) are nuclear receptors that bind to DNA as heterodimers with the retinoid X receptors. The PPARs have been shown to play an important role in regulating genes involved in lipid metabolism and may well be exploited in the management of various other aspects of health and disease including inflammation and cancer (see Refs. 40, 41, 42, 43 for review). The synthetic ligands for the PPAR
receptor, thiazolidinediones, have been exploited for their antidiabetic activity, but due to their side effects (e.g., weight gain and increase in LDL-cholesterol), the search for additional PPAR agonists has continued (40). Several potential compounds have been identified. One of these, N-9-fluorenylmethyloxycarbonyl (FMOC)-L-leucine improves insulin sensitivity without greatly activating PPAR adipogenic pathways (44). A second compound, the SR-202 phosphonophosphate, inhibits thiazolidinedione-induced PPAR activity as measured by trans-activation assays and 3T3-L1 cell differentiation to adipocytes, yet is still able to improve insulin sensitivity in vivo (45). Most recently, a novel non-thiazolidinedione-selective PPAR modulator, nTZDpa, which activates a unique profile of PPAR target genes in comparison with full agonists in white adipose tissue, was shown to ameliorate hyperglycemia and hyperinsulinemia in mice fed a high-fat diet while reducing body weight gain and adipose depot size (46). Moreover, nTZDpa did not cause cardiac hypertrophy, an effect associated with several other PPAR agonists (47). Although it is not yet clear the extent to which the effects of either of these two compounds are mediated by PPAR in vivo, these agents represent an advancement in the separation of insulin sensitization and adipocyte differentiation properties associated with thiazolidinediones, and they or agents modeled on their structures have potential as a future source of antidiabetic drugs.
F. Other SRMs
With our increased understanding of the mechanisms through which SRM activities are achieved (see following sections), it is now possible to extend the pharmacological concept of executable modulation to other members of the nuclear receptor superfamily. For example, the identification of a selective glucocorticoid receptor (GR) modulator (SGRM) with good antiinflammatory activity and reduced side effects (e.g., diabetogenic, osteoporotic) would be highly beneficial. The recent identification of the nonsteroidal GR ligand, AL-438, indicates that selective modulation of GR activity is obtainable (48). This compound retains potent antiinflammatory activity, as measured by a carrageenan-induced paw edema assay, yet has reduced the severity of side effects associated with glucocorticoid use. In rats, AL-438 did not induce hyperglycemia, nor did it exert the same negative effects on bone associated with use of the potent glucocorticoid, prednisolone. These differences are accompanied by differential regulation of GR target genes (i.e., osteocalcin and aromatase) by prednisolone in comparison with AL-438.
The appearance of tissue selectivity may also be achieved through the use of ligands that distinguish between receptor isoforms and, consequently, regulate biological processes in an apparently tissue-specific manner. For example, therapeutic use of thyroid hormone (T3) will reduce serum cholesterol, but this is accompanied by various cardiac side effects, such as tachycardia and increased cardiac output. Studies from thyroid hormone receptor- (TR) and TRß knockout mice reveal that the effects of T3 on the heart are primarily TR regulated, whereas the effects on cholesterol are mediated via TRß (49, 50). The development of thyromimetics that selectively target TRß offers the opportunity to achieve cholesterol-lowering effects with a reduced risk of negative effects on the heart (51). One such agent, KB-141, caused a significant reduction in cholesterol, lipoprotein(a), and body weight with no effect on heart rate in a short-term treatment of primates and suggests that selective TRß activation may have utility in the treatment of obesity and hypercholesterolemia (52).
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III. Mechanisms of SRM Action on Steroid Receptors |
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TopAbstractI. IntroductionII. Biology of Selective...III. Mechanisms of SRM...IV. Molecular Basis of...V. Lessons Learned from...VI. Concluding RemarksReferences |
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, ERß, AR, PR, PPAR, and others have been revealed through research conducted over the last 15+ years. In general, the effects of these ligands are mediated via their cognate nuclear receptors, which are members of a superfamily of transcription factors. For clarity and because SERMs are the founding members of this class of compound, the basic model of steroid receptor action will be presented relative to the ERs. There are two ERs, ER and ERß (53, 54). It has long been established that 17ß-estradiol (E2) binding to ER induces a conformational change in the receptor’s hormone-binding domain and enhances receptor dimerization and the ability of receptors to bind to estrogen response elements (EREs) generally located in the promoter region of target genes (1, 55). Studies of the effect of ligands on the structure of the ERß hormone-binding domain reveal ligand-induced conformational alterations (56, 57, 58), and it is clear that ERß also binds with high affinity to EREs (59, 60). Two distinct regions within ER, apart from the centrally located DNA-binding domain, specifically contribute to transcriptional activity: the constitutively active, activation function-1 (AF-1), which is located in the amino terminus, and the ligand-regulatable AF-2 found within the hormone-binding domain. The ligand-binding domain (LBD) of ERß exhibits a moderately high degree of homology to the corresponding region of ER, and like all other steroid receptors, it possesses an AF-2 domain the activity of which is sensitive to ligand. However, its AF-1 domain is poorly conserved with that of ER and is less active (61); certain data suggest that the ERß A/B domain possesses a repressive function (61, 62, 63). Once at the promoter, the "activated," ligand-bound receptor, either ER or ERß, interacts with coactivator proteins to form a multiprotein complex that activates the general transcriptional machinery and increases the expression of target genes through processes involving chromatin remodeling, formation of stable preinitiation complexes, and enhanced rates of RNA polymerase II reinitiation (2, 64, 65, 66, 67, 68, 69).
The activities of the ERs, when assessed on EREs, can be inhibited by binding to antagonistic ligands, of which there are two types. Class I antiestrogens, such as 4-hydroxytamoxifen (4HT) and raloxifene, are referred to as partial or mixed agonists/antagonists, whereas type II antiestrogens, such as ICI 182,780, are called "pure" antiestrogens. These compounds bind to the ERs with high affinity (70, 71, 72); raloxifene and 4HT block the ligand-activated AF-2 domain and particularly in the case of 4HT, leave AF-1 able to initiate gene expression (73, 74). Depending on the cell type and promoter examined, AF-1 and AF-2 can mediate E2-induced transcription independently or synergistically (75, 76); their relative abilities to stimulate gene expression vary in a promoter- and cell type-specific manner (73, 77). As a result, 4HT and raloxifene stimulate gene expression in some, but not all, contexts, and these antiestrogens are therefore classified as SERMs. Importantly, the ability of 4HT or raloxifene to either activate or inhibit gene expression in a context-specific manner indicates that intrinsic cellular differences in processes or factors, such as cell-signaling pathways, accessory transcription factors, and/or transcription factor modulatory proteins, may account for the distinct interpretations of SERM biocharacter (i.e., agonist vs. antagonist activity). For ERß, 4HT and raloxifene also inhibit its AF-2 domain, and due to the relatively poor AF-1 activity of the receptor, these ligands generally block ERß transcriptional activity measured on EREs (61, 78, 79). In contrast to 4HT and raloxifene, the pure antiestrogens, ICI 164,384 and ICI 182,780, inhibit ER and ERß transcriptional activity in a context-indiscriminate manner. It should be noted that 4HT and raloxifene also exert effects on the transcriptional activity of both ER and ERß tethered to DNA indirectly through interaction with other transcription factors such as activator protein 1 (AP-1) and Sp1 (80, 81). In this context, the agonist activities of these ligands also is apparent in a cell-specific manner.
B. Effect of ligand on receptor structure
The first results indicating that ligands affect the structure of steroid receptors were obtained from antibody epitope mapping and partial proteolysis experiments (64, 77, 82, 83). For the latter, receptors were incubated with various ligands, subjected to limited digestion with enzymes such as chymotrypsin or trypsin, and, depending on whether ERs were bound to estradiol or antiestrogens, protected polypeptide fragments of different sizes were obtained. This was taken as an indication that these different classes of ligands induced distinct conformational changes in the C-terminal LBD of the receptor, later termed helix 12 (58, 64, 77); similar approaches have revealed ligand-induced conformational changes for AR (84), PR (83), GR (85), and PPAR (86).
Subsequently, the crystal structures of the LBDs of a number of nuclear receptors, including ER and ERß complexed with the agonistic ligands 17ß-estradiol or diethylstilbestrol (87, 88), were solved, and, like other members of the steroid receptor superfamily, these regions were found to be composed of 12 -helices. Surrounding a tightly packed central core composed of helices 5, 6, 9, and 10 are helices 2, 3, 4, 7, 8, and 11; helix 1 is not part of the conical nuclear receptor LBD "sandwich" motif (helices 2–11), and the position of helix 12 is variable. The region on the surface of the LBD to which coactivators bind via their NR boxes (see Section III.C for more details) is referred to as the coactivator-binding groove and is composed of residues from helices 3, 4, 5, and 12. The bottom and sides of this groove are nonpolar, but the ends are charged (87). When agonists occupy the LBD, helix 12 packs against helices 3, 5/6, and 11, thus forming part of the coactivator-binding groove. However, relative to the agonist-bound structures of ER and ERß, the position of the 12th helix in relation to the remainder of the LBD differs when mixed ER agonists/antagonists, such as 4HT or raloxifene, occupy the ligand-binding pocket (56, 87, 88). In these structures helix 12 is reoriented to partially occlude the coactivator-binding groove, therefore enabling it to block certain AF-2-dependent interactions with coactivators (56, 87). Thus, crystallography substantiates that ER ligands are determinants of the conformation of the LBD of the receptor. It is important to note that structures for the A/B domain or full-length ERs, including information on how ligands affect amino- and carboxy-terminal interactions, are not available. These will undoubtedly be important for fully understanding SERM action.
The conformation of ERß LBD bound to the pure antiestrogen ICI 164,384 is distinct as revealed by crystallography; helix 12 is disordered and therefore does not appear in the structure (89). However, the bulky side chain of ICI extends out of the ligand-binding pocket and makes contact with a portion of the coactivator-binding groove, therefore likely precluding productive LBD interaction with coactivators. As noted above, ICI antiestrogens, unlike SERMS such as tamoxifen and raloxifene, do not possess partial agonist activity. This has been suggested to result from the ability of ICI 164,384 to inhibit ER dimerization (90). However, recent data suggest that this is not the case; the ERß-ICI 164,384 crystal structure reveals a dimer (89), and fluorescence resonance energy transfer experiments demonstrate that both ICI 164,384 and ICI 182,780 induce ER LBD dimerization (91). The ICI compounds also have been shown to promote a modest nuclear to cytoplasmic shuttling of ER and induce the degradation of this receptor (92, 93, 94). The ICI 182,780 antagonist also has been shown to significantly retard the intranuclear mobility of ER and to render the receptor resistant to extraction, both suggesting a tight association with a subnuclear compartment (95, 96). The potent antiestrogen activity of the ICI compounds therefore resides in their ability to efficiently block coactivator interactions as well as other aspects of receptor function and expression required for transcriptional activity.
A second approach used recently to characterize the effect of ligands on the conformation of steroid receptors utilizes affinity selection of phage-displayed peptides (reviewed in Ref. 97). Full-length recombinant ER or ERß immobilized on tissue culture plates incubated with phage in the absence or presence of various ER ligands revealed that ligand-induced ER-phage interactions varied depending on the ligand and type of receptor (57, 98, 99). These patterns of interactions are consistent with the ability of each ligand to induce a receptor conformation that exposes a unique peptide-binding surface. These results are notable not only because they are consistent with the above mentioned crystallography studies, but also because they reveal structural differences in the context of full-length ERs instead of only the LBDs, as is the case for the crystallographic analyses. In addition, they clearly reveal differences in the structures of ERs bound to either 4HT or raloxifene, which are difficult to discern in the crystallographic analyses. This is important for understanding the molecular basis for differences in the biological activities of these two SERMs. Peptide-based approaches also have been used to evaluate the conformation of AR (100).
C. Coactivators
1. Coactivator interactions with ERs.
The hypothesized ability of coactivators to bind to steroid receptors in an agonist-dependent manner was exploited in the initial predictions of coactivators and corepressors (101, 102, 103). Upon cloning of the first authentic steroid receptor coactivator (SRC)-1, its interaction with PR or ER was demonstrated to be promoted by agonist and inhibited by antagonist (104). SRC-1 (NCoA-1) was the progenitor molecule for the SRC-1/p160 family of coactivators, which includes SRC-2 [transcriptional intermediary factor 2 (TIF2)/GR-interacting protein 1 (GRIP-1)] and SRC-3 [(ACTR/pCIP/receptor associated coactivator (RAC3)/TRAM-1/amplified in breast cancer 1 (AIB1) (104, 105, 106, 107, 108, 109, 110, 111, 112)]. This slightly confusing nomenclature will vary in this review, depending on the laboratory source for the data. Subsequent analyses have defined the NR box motifs (LxxLL, where L = leucine and x is any amino acid) found within many coactivators including the p160s as critical for their ability to bind to steroid receptors via their coactivator-binding groove within the LBD (113, 114). The structure of the co-crystal complex of the ER agonist, diethylstilbestrol, with the ER LBD and a NR box-containing portion of the GRIP1 coactivator reveals that residues within helix 12 as well as within helices 3 and 5 are important for mediating interactions between ERs and coactivators (87, 88, 115). As discussed above, helix 12, by virtue of the ability of ligands to alter its position relative to the remainder of the LBD, plays a critical role in regulating coactivator interactions with this region of the receptor (87, 116, 117). Thus, agonists promote coactivator binding to ERs by inducing a LBD structure favorable for this interaction. However, it is important to note that steroid receptors and coactivators can utilize other regions within their structures to bind to one another. For example, SRC family coactivators as well as cAMP-response element binding protein (CREB)-binding protein (CBP) and p300 also bind to the A/B domain of ER and ERß in a hormone-independent fashion (118, 119, 120); in the case of at least GRIP1 this interaction with the A/B domain is not dependent on the coactivator’s LxxLL motifs.
2. Coactivators functional roles.
Experiments performed in yeast provided some of the first indications that there may be competition among transcription factors (squelching) for binding to a limiting pool of accessory factors necessary for gene expression (121). Work in transient transfection systems utilizing cotransfection of PRs and ERs extended this concept to the nuclear receptor superfamily and strongly suggested that, in order to activate gene expression, receptors had to interact with some unknown factors in the cell (122). After initial biochemical identification of several ER-interacting proteins (103, 123), molecular biological approaches, chief among them yeast two-hybrid assays, resulted in the cloning of more than 50 coactivators in a relatively short period of time (66, 68, 69). In general, coactivator proteins (note: there is one RNA coactivator; see Ref. 124) do not bind to DNA, but interact indirectly through association with other DNA-binding proteins (e.g., nuclear receptors). Once recruited to the promoter, coactivators enhance transcriptional activity through a combination of mechanisms, including efficient recruitment of basal transcription factors such as template-activating factors and TATA-binding protein. In addition, nuclear receptor-interacting coactivators possess themselves, or recruit other nuclear proteins that possess, enzymatic activities crucial for efficient gene expression including the ATP-coupled chromatin-remodeling SWI-SNF complex, a number of acetyltransferase proteins (e.g., CBP/p300, pCAF, and p160s), methyltransferases [e.g., coactivator-associated arginine (R) methyltransferase-1 (CARM1) and PRMT-1/2] and ubiquitin ligases (e.g., E6-AP and Rsp5) (125, 126, 127, 128). A general model of coregulator interactions with ER is presented in Fig. 1
(see accompanying legend for more details).
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Several recent reports primarily employing chromatin immunoprecipitation (ChIP) assays suggest that receptor and coregulator association in gene promoters is temporally regulated. For instance, ER appears to cycle off and on the pS2 promoter in certain cells (e.g., MCF-7) in response to continuous E2 stimulation (132). In vitro transcriptional assays indicate that SRC-1 must be recruited to the promoter before p300 for efficient gene expression (133). In addition, a recent report has shown that SRC-1 and the DRIP205/TRAP220 coactivator cycle off and on the pS2 promoter after E2 treatment; notably when SRC-1 is bound, TRAP220 is absent and vice versa (134). Although the rapid kinetics of receptor and coactivators are evident in cellular imaging experiments (95, 135), it is unknown what controls the dynamic association of nuclear receptors and coregulators with target genes, or whether these processes can be regulated in a cell-specific fashion. Nonetheless, the ability of steroid receptors to activate transcription is a product of the ability of the receptor to interact with coactivators and other proteins required for gene expression, and the effect of various enzymatic activities on the formation, function, and disassembly of the receptor-coactivator complex.
D. Corepressors
Although there are far fewer nuclear receptor corepressors, these molecules serve important roles in negatively regulating receptor-dependent gene expression. Unliganded retinoic acid receptor (RAR) and thyroid-hormone receptor (TR) repress basal transcription in the absence of their cognate ligands, and this function is mediated, at least in part, by two, large (
270 kDa) nuclear proteins, silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) and nuclear receptor corepressor (NCoR). These corepressors bind to the unliganded receptors via CoRNR boxes, which consist of LxxxI/HIxxxI/L motifs (136, 137, 138, 139). Upon hormone binding, these corepressors dissociate from receptor and enable TR and RAR to associate with coactivator(s) and stimulate gene expression (138). Accordingly, the occupancy of the LBD, and therefore its conformation (140), dictates whether these receptors interact with coactivators or corepressors and activate or repress transcription (141). Although these corepressors do not appear to possess intrinsic repressive activity, they, like coactivators, also function as part of larger protein complexes that include histone deacetylases, which enhance tight nucleosome-DNA interactions and inhibit transcription factor recruitment and gene expression.
Although corepressors bind very well to some nuclear receptors in the absence of their cognate ligands, this is less the case for steroid receptors such as ER, PR, GR, and AR. Rather, corepressors bind to these receptors in the presence of their respective antagonists, 4HT, RU486 (for PR and GR), and cyproterone acetate (119, 142, 143, 144, 145). Both 4HT and raloxifene have been shown to recruit NCoR and SMRT to certain ER target gene promoters (132, 146). The molecular basis of the interactions between steroid receptors and corepressors is not well defined, but a CoRNR box-containing peptide can bind to ERs in the presence of tamoxifen, and mutations within helices 3 and 5 inhibit this interaction (147). Transient transfection assays first demonstrated that both NCoR and SMRT can selectively repress the agonist activity of 4HT and RU486 on ER and PR, respectively (142, 143). Subsequently, it was demonstrated that injecting inhibitory antibodies to NCoR or SMRT promoted the agonist activity of 4HT (119) and that 4HT was a relatively potent ER agonist in fibroblasts derived from NCoR null mice (148). Taken together, the data indicate that SMRT and NCoR are weak ER corepressors, but that these proteins inhibit the agonist potential of antiestrogen-liganded ER activity. Evidence for coregulator inhibition of agonist-bound steroid receptor by the DEAD box RNA helicase DP97 and a novel corepressor, LCoR, also has been obtained (149, 150). Whether corepressor binding represses the ligand-independent activity of these receptors is less clear, but is possible (151). The existence of a cellular equilibrium of coactivators and corepressors that can be shifted toward corepressor preference by antagonist is most likely. Interaction of NCoR and/or SMRT with AR, GR, and PR also has been demonstrated (142, 144, 145, 152, 153).
In addition to NCoR and SMRT, several other molecules have been associated with negative regulation of steroid receptor activity. SAP30 has been shown to be important for NCoR-mediated repression of antagonist bound ER whereas Sharp may play a role in repressing estrogen-induced ER activity indirectly via its effects on the SRA coactivator (154, 155, 156, 157). Corepressor activity also has been shown for MTA (158) receptor-interacting protein (RIP140)/Nrip140 (159, 160), REA (161, 162), RTA (163), and DAX (164).
E. SRM hypothesis
With the identification of coactivators and corepressors, and the biochemical demonstrations that ligands regulate the interactions of receptors with coregulator proteins, it has become possible to more fully consider the role of coregulators in regulating receptor function. Early work demonstrated that overexpression of SRC-1 significantly enhanced 4HT-stimulated ER activity in a cell environment (e.g., HepG2 cells) where 4HT exhibits agonist activity (143). However, exogenous SRC-1 does not robustly increase 4HT agonist activity in all cells, suggesting some component of cell specificity (143). Nonetheless, the ability of SRC-1 to modulate ER-4HT activity in HepG2 cells suggests that productive ER-SRC-1 interactions do occur in some cells in the presence of 4HT. It is possible that in vitro interactions between 4HT-occupied receptor and SRC-1 are generally not observed because studies often fail to consider the contribution of the AF-1 or DNA-binding domains (103). When the corepressors NCoR or SMRT were ectopically expressed in cells, the agonist activity of 4HT was reduced (142, 143, 148), and this is consistent with antiestrogens promoting interactions between ER and corepressors. Collectively, these data suggest that perturbing the expression of coactivators and corepressors within a cell affects the relative agonist and antagonist activity of the SERM, 4HT (see model in Fig. 2). Many studies have followed this line of reasoning, and the activity of a potential corepressor is typically assessed by determining the ability of the candidate molecule to reduce the agonist activity of the SERM, 4HT.
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IV. Molecular Basis of Cellular Selectivity |
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TopAbstractI. IntroductionII. Biology of Selective...III. Mechanisms of SRM...IV. Molecular Basis of...V. Lessons Learned from...VI. Concluding RemarksReferences |
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or ERß. Work from many investigators has characterized functional similarities and differences between these two receptors (170). Although both receptors bind estradiol and SERMS with similar affinity and interact with the same DNA response element, the transcriptional activity of these receptors is distinct. For instance, estradiol generally stimulates greater transcriptional activity via ER than through ERß (61, 171, 172). More pronounced differences are observed in the case of SERM-bound receptors. For ERE-dependent gene expression, tamoxifen is a partial agonist of ER but is generally unable to stimulate ERß transcriptional activity (61, 171, 173). Conversely, when assessing ER activity on AP-1 containing reporter genes, tamoxifen will stimulate ER and ERß transcriptional activity (174).
It has been postulated that differences in the activities between the respective forms of each of these receptors is due to differences in the abilities of the receptors to interact with coregulatory proteins. In support of this, a number of differences in the ability of ER and ERß to interact with coactivators have been noted. For estrogen-bound receptors, ERß, but not ER, binds well to the receptor-interacting component of the mammalian mediator complex, TRAP220 (175). In contrast, the PPAR coactivator-1 (PGC-1)-related coactivator, PERC, coactivates ER, but not ERß, transcriptional activity (176). There are also several AF-1-specific coactivators, such as p68/p72 and MMS19; in the case of the former, they enhance ER, but not ERß, transcriptional activity (177, 178, 179).
There are differences between the relative affinities of ER and ERß for members of the p160 coactivator family. The ER-specific agonists, propyl pyrazole triol and R,R-tetrahydrochrysenes bind to both ER and ERß, but induce interaction of p160 coactivators only with ER, not ERß (180). Other differences in binding are more subtle. Using an approach in which the NR box regions of the SRC family are assessed for their ability to bind to ERs in the presence of estradiol, ER, compared with ERß, was shown to bind with greater affinity to each of these coactivators (181). Real-time interactions between ERs and NR box domains of each of the p160s assessed by BIAcore technology substantiate that result (182). However, this approach also revealed that differences between ERs and full-length SRC family members were more similar with a relative strength of RAC3/SRC-3 greater than SRC-1 greater than TIF2/SRC-2. Differences in binding also can be observed in analyses of the ability of ER and ERß to interact with peptides encompassing each of the individual NR boxes of the p160 coactivator family (183, 184). For instance, NR box IV of SRC-1 binds much better to ERß than to ER, regardless of ligand (e.g., estradiol, diethylstilbestrol, or genistein). In addition, the nature of the ligand influences the relative affinity of the receptor for particular NR boxes; interaction of the estradiol-bound ERß is approximately two times greater than genistein-bound ERß to SRC-2 NR box I, whereas genistein-bound ERß binds approximately five times better to SRC-2 NR box III than the same receptor bound to estradiol. This is also consistent with genistein-bound ERß binding better to p160 coactivators than genistein-bound ER (185).
In the case of progestins and glucocorticoids, there are two hormone-binding receptors for each class of ligand. However, these do not arise from separate genes, but rather from variations in transcriptional start sites and translation initiation (186, 187, 188, 189). The A form of PR and the B form of GR (PRA and GRB) represent N-terminal deletions of the larger proteins, PRB and GRA, respectively. Regardless of the route in which the different receptors are derived, they have distinct biological activity. In both cases, the activity of the B forms of each receptor is greater than the A receptor isoform (186, 190). Moreover, PRA can also repress the transcription of other transcription factors, and this extends to other steroid receptors such as ER (190, 191, 192). In the case of PRs, an inhibitor domain has been mapped within the amino terminus of PRA (193) that is required for the ability of PRA to bind with greater affinity than PRB to the corepressor SMRT (194). In contrast, PRB binds better to NR box peptides of SRC-1 and GRIP1, and this undoubtedly contributes to this receptor’s greater transcriptional activity.
Another example of the specificity of nuclear receptor interactions with coactivators comes from a study examining the requirement of specific p160 coactivators for activation of an integrated chromosomal reporter gene, MMTV-CAT, by GR and PRs and their respective ligands (195). ChIP assays revealed that both receptors recruited SRC-3 to the target gene. In addition, PR recruited SRC-1 and CBP, and this was associated with acetylation of histone H4 on Lys5. In contrast, ligand activation of GR led to recruitment of SRC-2 and pCAF followed by Ser10 phosphorylation and Lys14 acetylation of histone H3. Thus, even for identical cell and promoter contexts, closely related receptors can utilize different complements of coactivators in the process of activating gene expression. Differences in the interactions of PPAR with p160s also have been observed (44). In the presence of rosiglitazone, TIF2 interaction with this receptor is greater than for SRC-1, whereas in the presence of the selective PPAR modulator, FMOC-L-leucine, which activates insulin-sensitizing but not adipogenesis pathways, PPAR preferentially interacts with SRC-1. Taken together, these results suggest that p160s may substitute for one another to the extent that they promote overall target gene expression, but that differences in downstream events (e.g., histone modification or biological responses) are associated with different coactivator usage.
B. Influence of DNA on coregulator interaction
Although consensus DNA response element sequences have been defined for members of the steroid receptor superfamily, it is clear that not all target genes contain the ideal sequence required to mediate receptor-DNA interactions. For instance, for the 38 estrogen-responsive genes reviewed by Klinge (196), most of the functional EREs located within the promoters or 3'-untranslated regions are not the traditional consensus sequence. Thus, many target genes contain response elements that bear little similarity to consensus EREs. It has been demonstrated that the sequence of the response element affects the affinity that a given receptor has for binding DNA. As might be expected, ER binds with the greatest affinity to the consensus ERE sequence found within the vitellogenin A2 gene and less well to the imperfect EREs found within the vitellogenin B1, pS2, and oxytocin genes (197). This explains, at least in part, how the sequence of the response element can be one important determinant of the extent to which ERs can activate gene expression (197, 198, 199, 200).
However, the conformation of transcription factors can be altered through binding to DNA (reviewed in Ref. 201). The specific sequence of the receptor response elements for ERs and TR can exert distinct allosteric effects on the conformation of ER, ERß, and TR. This has been shown by experiments in which DNA-bound ERs or TR are subjected to limited proteolysis (197, 202) as well as experiments in which LXXLL-containing peptides sensitive to the conformation of the ER or ERß vary in their relative ability to bind to receptor depending on the nature of the ERE (203). Just as ligand-induced changes in receptor conformation influence receptor interactions with coactivators, consensus and imperfect EREs also influence the relative ability of ERs to bind to coactivators. Using an approach based on the relative ability of ERs bound to various response elements to bind to coactivators in HeLa cell nuclear extracts, it has been shown that TIF2 interacts better with ER bound to EREs from the vitellogenin A2, pS2, or oxytocin genes than from the vitellogenin B1 gene (198). In contrast, the response element sequence did not affect ER interaction with AIB1 (198). A similar theme of ERE sequence affecting ERß interaction with coactivators in U2OS nuclear extracts also has been documented (199). However, in this case, interaction between ERß bound to EREs from either the pS2 or vitellogenin B1 genes bound to TIF2 and AIB1 less well than ERß bound to the vitellogenin A2 gene (199). This also correlated with the inability of overexpressed AIB1 to enhance ERß-mediated transcription of synthetic target genes containing either a pS2 or vitellogenin B1 ERE.
In addition to the nature of the steroid response element itself, the context in which the response element resides also determines the ability of steroid receptors to interact with coactivators. The estrogen responsiveness of the pS2 gene is dependent upon both an ERE and an AP-1 site located adjacent to one another; mutation of either one of these sites significantly compromises, but does not block, induction of target gene expression by estrogens (204, 205). It is known that SRC-1 and TIF2 both can coactivate pS2 gene expression (204, 206). However, SRC-1 coactivation of estrogen-stimulated pS2 expression is more dependent on a functional AP-1 site than is coactivation by TIF2 (204). The difference in the relative ability of SRC-1 and TIF2 to coactivate pS2 expression is dependent on the sequence of the ERE; substitution of the pS2 ERE with a consensus ERE enables SRC-1 to coactivate gene expression regardless of whether or not the AP-1 site is mutated.
It should be noted that many estrogen-responsive genes do not appear to contain functional ERE sequences, and the ability of ERs to regulate gene expression is achieved via indirect tethering of the receptor to DNA via other transcription factors, such as AP-1 and Sp1 (reviewed in Refs. 80 and 81). ChIP experiments have demonstrated distinct patterns of tamoxifen-induced association of the promotors of the c-Myc and cathepsin D genes with coactivators and corepressors (146). In Ishikawa cells, tamoxifen stimulates the expression of c-Myc but not cathepsin D mRNA; this result correlates with the recruitment of coactivators to the c-Myc promoter and corepressors to the cathepsin D promoter. Intriguingly, ER regulation of c-Myc expression is dependent upon a discrete non-ERE-containing site (207), whereas ER regulation of cathepsin D is directly mediated via an ERE (208). Although not formally proven, these data raise the interesting hypothesis that tamoxifen cell specificity may be influenced by the mechanism by which ER is tethered to the promoters of target genes, and therefore the ability of the receptor to recruit coactivators or corepressors. It is interesting to note in this study that raloxifene recruited corepressors to both target gene promoters, thus clearly distinguishing itself from tamoxifen.
C. Effect of cell signaling on receptor-coregulator interactions
A number of cellular signaling pathways influence the ability of coregulators to exert their effects on nuclear receptor-dependent gene expression. A summary of enzymatic, protein-protein, and regulatory effects for a selected group of coactivators is located in Table 1. More details are presented in the following sections.
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is phosphorylated in response to estrogen treatment in cells as well as stimulation with various growth factors, and that this phosphorylation is associated with an increase in the receptor’s transcriptional activity (209, 212). Several studies relate this to alterations in coregulator interactions. For instance, in breast cancer cells, phosphorylation of Ser118 in human ER leads to reduced interaction with the SMRT corepressor (119), whereas another laboratory has demonstrated that this posttranslational modification is required for the interaction of this receptor with the amino-terminal-specific coactivator, p68 (177). Although an early report indicated that phosphorylation of the A/B domain of ER did not affect interactions between GAL-A/B and the p160 and CBP/p300 coactivators (118), recent work has shown that phosphorylation of serines 104, 106, and 118 does positively modulate this interaction, particularly in the context of the full-length receptor (213). It is not clear to what extent, if any, that the phosphorylation-induced loss of interaction between ER and corepressors contributes to the phosphorylation-induced interaction between this receptor and coactivators. Phosphorylation of serines in the amino terminus of ERß also has been shown to enhance interactions between this receptor and SRC-1 and CBP (214, 215). Therefore, to the extent that nuclear receptors can be differentially phosphorylated in various cellular environments, this can directly affect the ability of coregulators to interact with these receptors and affect their transcriptional activity.
2. Coregulator phosphorylation.
Although there is a long history of steroid receptor phosphorylation and the effect of this posttranslation modification on receptor function (209, 210), it has been recognized only recently that both coactivators and corepressors are also substrates for kinases (Fig. 3). For instance, SRC-1 is a phosphoprotein in which seven phosphorylation sites have been identified (216), two of which (Thr1179 and Ser1185) can be phosphorylated in vitro by the MAPK, Erk2 (216). Although treatment of cells with a steroid receptor ligand, progesterone, does not increase SRC-1 phosphorylation, cAMP treatment of cells does; however, this probably occurs as a result of cAMP stimulating the activity of the Erk1/2 kinases (217). SRC-1 phosphorylation contributes to its ability to coactivate cAMP- or progesterone-induced PR transcriptional activity, as evidenced by reduced coactivation by a SRC-1 phosphorylation mutant and the ability of the MAPK kinase (MEK)1/2 inhibitor U0126 to reduce SRC-1 coactivation (217). This is not due to any effect on the intrinsic transcriptional activity of SRC-1, or the ability of this coactivator to bind to PR or CBP, but rather a reduction in SRC-1 interaction with the pCAF coactivator, and a loss of SRC-1 functional cooperation with CBP has been observed (217). Phosphorylation of SRC-1 is also important for its coactivation of AR ligand independently activated via IL-6 signaling (218). In contrast, mutation of all seven known SRC-1 phosphorylation sites does not specifically inhibit coactivation of ER function stimulated by either E2- or cAMP-signaling pathway; rather this results in a general reduction in coactivation function (219).
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increases the intrinsic transcriptional activity of GAL-GRIP1 (220). Moreover, mutation of Ser736 to an alanine residue reduces the ability of EGF to stimulate the intrinsic transcriptional activity of this coactivator, as well as the ability of GRIP1 to coactivate PR stimulated by either R5020 or EGF. Similar results are obtained with ER in which mutation of the ERK2 phosphorylation site on GRIP1 inhibits the ability of this coactivator to stimulate ER transcriptional activity stimulated by EGF and E2. The mechanism by which this occurs is not well defined, but there is evidence to suggest that functional interactions between GRIP1 and CBP are compromised by mutation of the Ser736 site. Note that the serine736 to alanine736 mutant form of GRIP1 is nuclear in the absence or presence of EGF (220). In addition, GRIP1 also can be phosphorylated by c-Jun N-terminal kinase 1 in vitro (220), and this appears to occur through sites other than Ser736 or Ser554 (220). AIB1 (SRC-3) can also be phosphorylated in vitro with Erk2, and transfection of MCF-7 cells with a constitutively active form of MEK1 results in increased intrinsic transcriptional activity of AIB1 as well as enhanced interaction with p300 (221). Moreover, SRC-3 is found as a complex with I
B kinase (IKK), the kinase responsible for phosphorylation and subsequent degradation of IB and consequently the activation of nuclear factor-B (NF-B); IKK complexes can phosphorylate SRC-3 in vitro (222). Intriguingly, elevated SRC-3 phosphorylation is observed in cells treated with TNF, an agent that activates the IKK/NF-B pathway, and this correlates with a shift of compartmental equilibrium for SRC-3 from the cytoplasm to the nucleus, suggesting that phosphorylation influences the subcellular localization of SRC-3 (222). A second report confirms that p/CIP (SRC-3) subcellular localization is regulatable, as evidenced by the treatment of serum-starved cells with growth factors or phorbol esters promoting the translocation of p/CIP from the cytoplasm to nuclear compartments (223).
Phosphorylation of coactivators also may affect their ability to interact with steroid receptors. For instance, TRBP/ASC-2/RAP250/AIB3/PRIP is a coactivator that in addition to binding to nuclear receptors via a single LXXLL motif (224) associates with CBP/p300, the DRIP130 component of DRIP/TRAP complexes, the RNA recognition motif-containing coactivator CoAA, DNA-dependent protein kinase, and poly(ADP-ribose) polymerase complexes (224, 225, 226, 227). Although wild-type TRBP binds well to both ER and ERß, mutational analyses of the region surrounding the LXXLL motif within TRBP revealed that the amino acid located at the -3 position relative to the NR box could influence the relative binding of TRBP mutants to ER vs. ERß (228). Moreover, in vitro experiments demonstrate that MAPK phosphorylation of the wild-type -3 position amino acid, Ser884, inhibits the interaction between a TRBP peptide and both ER and ERß, thereby suggesting that phosphorylation of TRBP may be a negative regulator of its coactivator function (228). Likewise, phosphorylation of the AR coactivator, ARA55, by the proline-rich tyrosine kinase 2 (Pyk2) reduces ARA55 coactivation of AR-dependent gene expression (229). This appears to be related, at least in part, to a reduction in interaction between AR and ARA55.
Finally, phosphorylation of CBP/p300 may also affect the activity of these cointegrators of nuclear receptor function. For example, phosphorylation of p300 on Ser89 by PKC
reduces its coactivation ability as well as its histone acetyltransferase activity measured on histones in vitro (230). Moreover, phosphorylation of this same serine residue by the AMP-activated protein kinase reduces interaction between p300 and the nuclear receptors PPAR, TR, RAR, and retinoid X receptor (231). Thus, phosphorylation of Ser89 by either of these two kinases is a negative regulatory event that may become important during fluctuations in cellular metabolism reflected by elevated AMP (for AMP kinase) or during differentiation or apoptosis (for PKC). Conversely, phosphorylation of CBP or p300 by members of the MAPK signaling pathway, p42/p44 and MAPK kinase kinase 1, positively regulates the activity of these coactivators (232, 233, 234). In addition, cAMP can also increase the intrinsic transcriptional activity of CBP (233).
Just as phosphorylation can regulate the ability of coactivators to affect nuclear receptor activity, by either affecting coactivator function or the ability to interact with receptor, so too can phosphorylation influence the interaction of corepressors with nuclear receptors. The SMRT corepressor may be phosphorylated by the MEK1 kinase, MAPK kinase kinase-1 (as well as MEK-1) resulting in loss of repression due to reduction in the interaction between SMRT and TR, which is accompanied by nuclear export of SMRT to the perinuclear or cytoplasmic compartments (235). Phosphorylation by the p38 MAPK also increases the activity of the PGC-1 coactivator, and this appears to be the result of a phosphorylation event that impairs the functional interaction between PGC-1 and a putative repressor of this coactivator’s activity; estrogen-related receptor- has been shown to possess this activity (236, 237, 238). A nuclear to cytoplasmic shift in the localization of the corepressor RIP140 by the 14–3-3 protein and by cytoplasmic sequestration of SMRT and NCoR by p65 and IB also may affect gene expression positively by reducing levels of corepressors in their functional compartments of interest (239, 240).
3. Regulation of coregulator function by ubiquitinylation or sumolyation.
Steroid receptors, such as ER, PR, PPAR, and GR can be down-regulated by their cognate ligands in cells. The ability of ligands to induce polyubiquitination of these steroid receptors and the ability of inhibitors of the 26S proteasome such as MG132 and lactacystin to block ligand-dependent degradation of these steroid receptors argues that ligand-dependent degradation of these receptors occurs via the 26S proteasome (131, 241, 242, 243, 244). The 26S proteasome is a multiprotein entity, which possesses protease activity that ultimately leads to the cleavage and degradation of polyubiquitinated target proteins (see Refs. 245 and 246 for review). In seeming contradiction to the ability of proteasome inhibitors to increase receptor levels, they also block the transcriptional activity of ER and a number of other members of the nuclear receptor superfamily including progesterone, T3, ARs, and RARs (131, 247, 248). There is not a simple correlation between the ability of ligands to down-regulate nuclear receptor expression and the requirement of proteasome activity for receptor-dependent gene expression. Notably, the transcriptional activity of the human GR, which is a ligand-dependent target of the 26S proteasome, is not compromised by proteasome inhibitors such as MG132 or lactacystin (243, 249, 250, 251). In contrast, the transcriptional activity of the AR is blocked by proteasome inhibitors, even though ligand stabilizes this molecule instead of inducing its down-regulation (248). Thus, the mechanisms by which proteasome inhibitors block transcription are not defined. This inhibition may signify that receptor turnover is required for its efficient transcriptional activity and/or that proteasome activity is required for other aspects of the transcription process. In this regard, MG132 blocks association of the phosphorylated RNA polymerase II with the pS2 gene promoter in MCF-7 cells, and also reduces the frequency with which ER and other factors cycle off and on this promoter in response to estradiol treatment (252).
In this regard, the p160 family of coactivators, as well as CBP, have been shown to be targets of the ubiquitin-dependent 26S proteasome pathway by virtue of their increased expression in cells treated with MG132 and ubiquitination in vivo (131, 253). However, proteasome activity is not required for the intrinsic transcriptional activity of these molecules. This would suggest that reductions in steroid receptor activity induced by treatment with inhibitors of the proteasome are not linked to loss of coactivator activity per se, but instead result from a block in some other reaction required for receptor-dependent gene expression. In support of this, it has been demonstrated that proteasome inhibitor treatment of prostate cancer cells blocks cytoplasmic-to-nuclear translocation of AR (but not GR) after ligand treatment. Moreover, mammalian two-hybrid assays suggest that the MG132 may reduce ligand-dependent interactions between the AR LBD and the ARA70 and TIF2 coactivators (248). Proteasome function also appears to be required for the dynamic occupancy of the prostate-specific antigen promoter by AR in prostate cells; ChIP assays reveal the presence of the S1 subunit of the 19S proteasome subcomplex at the prostate-specific antigen promoter and that MG132 blocks release of AR from the promoter (254). Similar results, as described above, have been obtained for ER (252).
It should be noted that the nuclear interaction domain of GRIP1 and SRC-1 can be modified by sumoylation (255, 256). SUMO (small ubiquitin-like modifier) modification results from the covalent linkage of SUMO to specific lysines of target proteins (257, 258, 259). It does not, however, promote degradation of target proteins, but instead appears to regulate protein-protein interactions and protein targeting. Mutation of GRIP1 sumoylation sites reduces the colocalization of AR with GRIP1 in the nucleus after testosterone treatment (260). Moreover, GRIP1 sumoylation mutants are relatively poor coactivators of AR-dependent gene expression. There are two major sumoylation sites in SRC-1. Sumoylation was reported to increase PR-SRC-1 interaction and prolong SRC-1 retention in the nucleus (256).
A number of molecules associated with the proteasome-dependent degradation pathway have been identified as either coactivators [e.g., E6-AP (127), the SUG1 ATPase which is a member of the base portion of the 19S proteasome regulatory complex (261), and yRsp5/hRPF1 (128)] or corepressors [e.g., the ubiquitin ligase BRCA1 (262, 263)] of nuclear receptor function. The extent to which the activities of these proteins relate to proteasome function and contribute to their effect on nuclear receptor transcriptional activities in most cases has not been determined. One notable exception to this is for BRCA1, which strongly inhibits ER transcriptional activity in T-47D (breast) and DU145 (prostate) but not in three cervical cell lines, C33A, CaSki, or SiHa (262). This appears to be related to the ability of wild-type BRCA1 to down-regulate expression levels of p300, but only in the cells in which BRCA1 inhibits ER transcriptional activity [e.g., breast and prostate but not cervical cells (264)]. Interestingly, this effect of BRCA1 does not appear to influence expression levels of CBP, suggesting a high degree of specificity considering the similarity of CBP and p300 structure and function (264).
Proteasomal regulation of the corepressor NCoR has been documented (265). Treatment of cells with MG132 increases NCoR expression levels, indicating that this protein is degraded by the 26S proteasome. This degradation results from the interaction of mSiah2, the mammalian homolog of Drosophila Seven in absentia (sina) with the amino terminus of NCoR. Although experiments examining the ability of mSiah2 to affect SMRT degradation revealed no such regulation, they were performed with the originally identified version of SMRT, lacking the amino terminus later shown to be part of the full-length protein (265, 266). Intriguingly, expression of mSiah2 is variable, being most abundant in the nervous system (267). Therefore, in cells in which mSiah2 is relatively high (e.g., N18 neuroblastoma cells) NCoR expression is relatively low, whereas in 293T kidney cells in which mSiah2 expression is relatively low, NCoR expression is relatively abundant (265). Transfection of 293T cells with a mSiah2 expression vector significantly decreases the half-life of NCoR protein.
4. Coactivator methylation.
The AD2 region of p160 coactivators binds to methyltransferases (126, 268), and this interaction was suggested to be important for methylation of histones, a process involved in activation of gene expression (269, 270). An arginine residue within the CBP KIX domain also has been shown to be methylated by CARM1, and this inhibits the interaction between CBP and the kinase inducible domain of CREB (271). This could promote CBP coactivation of nuclear receptor activity by increasing the pool of CBP available for functional interaction. Under some conditions, the methyltransferase activity of CARM1 is important for the ability of this factor to coactivate ER transcriptional activity (272). Another report indicates that CARM1 can methylate CBP on three arginine residues, Arg714, Arg742, and Arg768, conserved between CBP and p300 (273). These residues are not located in the nuclear receptor or p160-interacting domains of CBP, but this methylation is important for strong CBP coactivation of ER and GRIP1 transcriptional activity (273). The mechanism for this is not clear, but enhanced CBP/p300 coactivation potential within a cell could potentially contribute to the agonist activity of SRMs.
5. Acetylation.
Although no published evidence to date implicates acetylation in the regulation of SRM activity, this posttranslational modification through its ability to regulate protein-protein interactions could affect the interaction of coregulators with nuclear receptor receptors and, in so doing, perturb the agonist/antagonist potential of this class of compounds. The SRC-1, RAC3, CBP, and p300 coactivators possess acetyltransferase activity that was originally thought to be directed solely toward histones. However, one report implicates histone acetyltransferase activity in the disassembly of receptor-coactivator complexes leading to the attenuation of gene transcription (274). This is accomplished through acetylation of ACTR (SRC-3) by CBP/p300 on lysine residues adjacent to an NR box, with the result of a loss of interaction between this p160 and ER. In this context, acetylation could be a mechanism that inhibits the agonist potential of SRMs through reducing ER interaction with a coactivator. p300 Has been shown in vitro to acetylate ER on lysines 302 and 303, residues located within the hinge region (275). Mutation of these sites to arginines, threonines, glutamines, or alanines increases E2-induced ER transcriptional activity, again suggesting that acetylation results in a loss of transcriptional activity (275). Paradoxically, template-activating factor 1ß inhibits ER acetylation and transactivation, suggesting that regulation of receptor-dependent gene expression by acetylation is complex and not yet well understood (276). Whether or even how these acetylation events may be regulated in a cell-specific manner is presently unknown.
6. Coactivator effects on splicing.
An ever-expanding role of coregulators in transcription has become evident. The original SRCs (such as SRC-1 and SRC-3) have been thought to work primarily via remodeling of promoter chromatin and initiation of transcription, a role also ascribed to CBP/p300/pCAF. The TRAP/DRIP complex of coactivators then plays a role in reinitiation of transcription and links the coactivational complex to general transcription factors and RNA polymerase II for efficient transcription of pre-mRNA. One might question, however, of what avail would be a high level of transcription if splicing were to become limited or inaccurate? Because the structural genes are designed to produce protein products and not unspliced pre-mRNAs, a growing suspicion has developed that transcription and alternative splicing might be somehow linked via information in the promoter regions of genes (277). Given the estimates that more than 60% of transcripts from eukaryotic genes are alternatively spliced (some of which can be regulated by steroid hormones), and given that nuclear receptors are the "great family of gene regulators," it would not be unreasonable that nuclear receptors coordinate their transcriptional regulatory effects with simultaneous effects on alternate splicing of the target gene mRNAs. In fact, a recent report in cultured cells substantiates this hypothesis, demonstrating that steroid hormones can affect the processing of pre-mRNA synthesized from steroid-sensitive promoters, but not from steroid-unresponsive promoters (129). This effect on alternative splicing is ligand and receptor dependent and receptor selective. The mechanism of the regulatory effect of receptors on RNA processing appears to be due to recruitment of subsets of certain coactivators, because addition of the coactivator CoAA (225) stimulated ER-mediated exon exclusion whereas the coactivator p72 (178) stimulated exon inclusion in the same target gene (129). SRC-1 had no significant effect on splicing. Similar results were demonstrated first for PGC-1 (130) and subsequently for the p68 and p72 coactivators/RNA helicases, which have been shown to act as splicing factors (278, 279). With the availability of these data, we now can conclude that steroid hormone receptors can simultaneously control gene transcription activity and exon content of the product RNA by recruiting coactivators involved in both processes (129, 278). It would not be unexpected if future experiments were to show that steroid receptors can have effects on other steps in mRNA processing and its export from the nucleus.
7. Triggering coactivators.
In addition to receptors acting as proteins that serve to specifically recruit coactivators to target gene promoters, they also can influence the transcriptional activity of the coactivators. The intrinsic transcriptional activity of the PPAR coactivator PGC-1 is relatively low when measured as a GAL4-PGC-1 fusion protein. However, expression of PPAR can significantly increase the apparent transcriptional activity of PCG-1 (280). This increase in activity, which does not require the PPAR AF2 activity, is reflected in a PPAR-induced change in the conformation of PGC-1 and an increase in the association between PGC-1 and the SRC-1 and p300 coactivators (280). These data therefore suggest that receptor binding to coactivator can induce allosteric changes in coactivators that trigger their activity via increased recruitment of other coactivators, thereby increasing receptor-dependent transcription.
The AR also has been shown to stimulate the transcriptional activity of a DNA-bound coactivator (281). The intrinsic transcriptional activity of GAL4-GRIP1, GAL4-CBP, and GAL4-p300 all can be stimulated by AR expression. Although this AR activity is not dependent on the AF-2 domain of the receptor, it does require both the amino- and carboxy-terminal regions of AR. Moreover, AR is unable to trigger the activity of an AD1-deletion mutant of GRIP1, although this form of GRIP1 still retains the capacity to coactivate AR-dependent gene expression, suggesting that unique domains of GRIP1 are required for coactivation of AR-dependent gene expression and GRIP1 activity triggered by AR (281). A similar model has been proposed for ER stimulation of AP-1-dependent gene expression in which ER interacts with AP-1 indirectly through binding to CBP/p300 and p160 coactivators (80). However, in neither of these cases has the mechanism by which AR or ER increases coactivator activity and reporter gene expression been determined.
D. Relative coregulator expression
1. Coactivator expression in normal tissues.
Based on the original hypothesis in which the relative levels of coactivators and corepressors were envisioned to control the relative agonist and antagonist activity of SRMs, it was predicted that significant differences in coactivator and corepressor expression found in various cell and tissue types would be important determinants of SRM activity (142, 143). With the continually expanding number of nuclear receptor coregulatory molecules, this has become an increasingly difficult hypothesis to test. Nonetheless, several examples of coactivators with distinct expression cell patterns have been described. For example, the DNA-binding domain-interacting coactivator, GT198, is expressed in a tissue-selective fashion; mRNA levels are very high in testis, modest in spleen and thymus, but absent in brain, heart, kidney, liver, lung, and thyroid (282). The human PGC-1 coactivator is also expressed in a tissue-restricted manner. Although detected in heart, skeletal muscle, kidney, and liver by Northern blot, it is absent in brain, colon, thymus, spleen, small intestine, placenta, lung, and peripheral blood lymphocytes (283). The expression pattern of the AR coactivator, FHL2, is restricted to the myocardium of the heart and prostate epithelial cells (284).
However, the expression pattern of most coactivators and corepressors examined appears to be quite broad. For example, the NCoR and SMRT corepressors are widely expressed, and the p160 family coactivators have been detected in most cell and tissue types (104, 139, 285, 286). There are, however, exceptions and variations to this theme. As an example, SRC-3 expression is undetectable in the ventromedial hypothalamus of mice and rats, and in 4-wk-old mouse uterus, although it is expressed in many other tissues examined (287, 288, 289). It should be noted, however, that low levels of SRC-3 have been demonstrated for human proliferating endometrium with increased expression in the late secretory phase (290), and other investigators have demonstrated the mRNA in immature and mature rat uteri (291). In addition to absolute changes in coactivator expression, relative differences that may be important for tissue/cell-specific responses to ligands, particularly SRMs, have been noted. For instance, similar expression levels of CBP, p300, AIB1, GRIP1, p300, NCoR, and SMRT have been measured for Ishikawa uterine and MCF-7 breast cells (146). However, SRC-1 expression was much greater in the Ishikawa cells, and this correlated with the agonist activity of tamoxifen in this cell line (146). Moreover, increasing SRC-1, but not GRIP1 or AIB1, expression in MCF-7 cells conferred tamoxifen agonist activity, whereas decreasing SRC-1 expression via short interfering RNA inhibited the agonist activity of tamoxifen in Ishikawa cells. Thus, relative as well as absolute changes in coactivator expression can affect SRM activity.
The demonstration that changes in coactivator expression could lead to alterations in SERM responses, as mentioned above, raises the issue of whether changes in coregulator expression within a given cell or tissue type could lead to altered responsiveness of that tissue to ligand. This has been addressed with biopsies obtained from human endometrium, which demonstrated that although SRC-1 and TIF2 levels did not change over the menstrual cycle, AIB1 expression increased as the cycle progressed (290). A subsequent report outlining differences in the relative expression of some coactivators and corepressors suggests that more studies are required to get a clear picture of the patterns of coregulator expression in this tissue (292). Specific influences may alter coactivator expression and, intriguingly, some of these responses appear to be cell and/or tissue specific. For example, estradiol decreases and T3 increases SRC-1 mRNA expression in GH3 rat pituitary cells, an effect matched in the pituitaries of T3-treated male rats (285). However, neither estradiol nor tamoxifen affects the expression of p160, RIP140, or p300 mRNAs in rat uterus (291), whereas another report indicates that estradiol decreases and antiestrogens increase AIB1 expression in MCF-7 cells (293). Estrogen also increases the expression of RIP140 mRNA in MCF-7, but not Ishikawa cells (294). The basis for the differences in response to estradiol in the different tissues and cells is unknown. Retinoic acid also has been shown to increase AIB1 expression in breast (MCF-7) and leukemia (HL-60 and NB4) cells; however, several lines of evidence support the conclusion that the retinoid effects in MCF-7 cells are not direct but, rather, are mediated through alterations in TGFß production (288, 293). Indeed, TGFß increases AIB1 expression in A549 and MCF-7 cells (293, 295). Decreases in coactivator expression also have been observed. Stimulation of protein kinase A is associated with a reduction in TIF2 protein, but not mRNA expression, whereas the deacetylase inhibitor sodium butyrate reduced p300 expression (296, 297). These are not transcriptional effects but, rather, reflect increased protein degradation which, at least for p300, is proteasome dependent (297). Thus, other signaling events within the cell may affect nuclear receptor transcriptional responses via alteration in the expression of coregulators. When the above data are viewed together, the relative quantitative changes in the cellular fingerprint of coactivator proteins in normal differentiated cells are rather minimal, usually varying by a factor of 1.
2. Alterations in coregulator expression associated with pathology.
The identification of AIB1 as a coactivator with increased expression in breast and ovarian cancer was the first indication that alterations in coactivator and corepressor expression may be associated with disease (111). As its name implies, the gene for AIB1 (amplified in breast cancer-1) was first reported to be amplified in approximately 10% of human breast cancers, and in a survey of 105 human tumors, 64% were found to express elevated levels of AIB1 mRNA in comparison with normal mammary epithelium. Another study has found the AIB1 gene to be amplified in 4.8% of breast (n = 1157) and 7.4% of ovarian (n = 122) cancers (298). Genetic amplification in cancer tissues suggests that AIB1/SRC-3 may be an oncogene. Since that time, a number of studies have examined the expression of coregulators in breast and prostate cancer and, in general, suggest that a change in the expression of selected coactivators is associated with tumorigenesis and/or progression. For example, increases in the SRA and AIB1 coactivators in breast tumors in comparison with adjacent normal tissue have been reported (299), as have changes in SRA and AIB1 expression relative to ER and PR status (300, 301). This suggests that elevated coactivator levels may increase the sensitivity of tumors to estrogens and growth factors. Another recent study indicates that, in patients receiving tamoxifen adjuvant therapy, high AIB1 expression correlates with poor disease-free survival, which is indicative of tamoxifen resistance (302). Importantly, patients with high levels of AIB1 and the growth factor receptor HER2 had worse outcomes than all other patients combined. In contrast, high levels of AIB1 in patients not receiving tamoxifen therapy were associated with better prognosis and longer disease-free survival. Taken together, a poor response to tamoxifen therapy appears to relate to high levels of both HER2 and AIB1 expression, and this suggests that AIB1 may be an important therapeutic target. Another study has linked low NCoR mRNA expression to a relative reduction in relapse-free survival (303).
Elevated TIF2 and AIB1 expression also have been found in the endometrium of women with polycystic ovarian syndrome (PCOS) in comparison with control fertile women, potentially contributing to the sensitivity of the PCOS endometrium to estrogen and the association of PCOS and endometrial adenocarcinoma (290, 304). The expression of the MTA1 corepressor and a MTA1 variant has been found at greater levels in breast tumors, particularly in those that are ER negative (158, 305). Although this might seem to confer an advantage, increased MTA1 variant expression mislocalizes ER to the cytoplasm, increases nongenomic estrogen signaling, and correlates with a more malignant phenotype (305).
Results indicating that expression of TIF2 and CBP are higher in intraductal carcinomas than normal mammary gland whereas NCoR levels are lower in invasive vs. intraductal carcinoma also have been obtained (306) and suggest that changes in coregulator expression may occur during cancer progression. Based on current models of the molecular basis of SRM tissue-selective action, it has been predicted that alterations in coregulator expression are associated with the acquisition of hormone independence and/or antihormone resistance. Although decreases in NCoR expression have been found for tamoxifen-resistant MCF-7 breast cancer cells (119), relative changes in the expression of AIB1 and SRA were not observed for de novo tamoxifen-resistant breast cancer (307). Although it may be tempting to speculate, based on these studies, that alterations in corepressors are responsible for acquisition of abnormal hormone responses, prostate cancer recurrence after androgen deprivation therapy has been associated with increased expression of SRC-1 and TIF2 coactivators (308). Collectively, the results of studies characterizing coregulator expression in hormone-responsive tissues are intriguing, but the data are still incomplete. Studies of a larger number of tumors and corresponding normal samples would be helpful. Moreover, more complete surveys of coactivator and corepressor protein content for each specimen also may help elucidate the identity of the coactivators and/or corepressors critical for tumor development and progression.
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V. Lessons Learned from Coregulator Knockout Mice |
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Although some differences in the biology of coactivators are related to their tissue expression profiles (e.g., SRC-1 null, but not SRC-3 null, mice have a uterine phenotype; this undoubtedly relates to the relatively low SRC-3 expression in wild-type mice), differences in the biological roles of coactivators expressed within a given tissue or organ can be observed. For example, the mammary glands of virgin SRC-1 null mice exhibit decreased ductal growth and branching (309). However, TIF2, SRC-3, and E6-AP expression within the mammary gland is not required for virgin mammary gland development, even though these coactivators are expressed in this tissue; this suggests that coactivators play spatial and temporal specific roles in vivo (287, 310, 311, 315). It is interesting to note that E6-AP is expressed in prostate as well as mammary gland, and lack of E6-AP expression does compromise prostate growth responses to androgen stimulation. Thus, this coactivator contributes to steroid-induced growth responses in a tissue-specific manner (315). Interesting differences between the SRC-1 and TIF2 knockouts have been recently documented for adipose tissues (316). TIF2-/- mice are protected against obesity and exhibit enhanced adaptive thermogenesis, whereas SRC-1-/- mice, as a consequence of their reduced energy expenditure, are prone to obesity. These responses can be influenced in response to a high-fat, Western-style diet which increases the ratio of TIF2:SRC-1 expression in adipose tissue, with the resulting increase in TIF2 expression leading to higher fat accumulation and decreased energy expenditure (316). These metabolic patterns appear due to the relative preferences of TIF2 and SRC-1 for PPAR and PGC-1.
Although the SRC-1 and TIF2 null mice have revealed aspects of their unique and overlapping functions, analyses of SRC-1 knockout mice relative to T3 action has revealed a paradoxical aspect of the function of this coactivator; specifically, it can play both positive and negative regulatory roles in gene expression. SRC-1 null mice are resistant to T3 as evidenced by elevated levels of serum TSH despite increased T3 levels (317). Moreover, the reduction in TSH levels associated with administration of T3 to wild-type animals is blunted in SRC-1 knockouts. This is reflected in a reduced ability of T3 to suppress TSHß mRNA in these animals (318). Although loss of SRC-1 expression also affects the expression of another gene positively regulated by T3, spot 14 (S14), several other T3 target genes (e.g., GH, malic enzyme, type I iodothyronine 5'-deiodinase and others) are unaffected by loss of this coactivator, providing another example of gene-selective regulation by coactivators (318).
Crosses between SRC-1 null mice and mice deficient in either TR or TRß reveal specific requirements of this coactivator relative to the ability of these receptors to activate or repress TR target genes in the liver (319). For instance, 5'-deiodinase expression is unaffected by loss of SRC-1 alone. However, loss of SRC-1 expression in SRC-1/TRß double null mice further reduces expression of this gene to levels below those observed for TRß null mice. In contrast, TR negatively regulates this gene, and loss of TR expression increases 5'-deiodinase expression by 2400%. Nonetheless, loss of TR and SRC-1 results in only a 3-fold increase in gene expression over wild-type controls. This suggests that the loss of TR-dependent repression is balanced by the loss of SRC-1 on TRß-positive regulation of this gene. Thus, animals deficient for either TR or TRß along with SRC-1 reveal a role for this coactivator that would be unsuspected from the phenotype of the SRC-1 null animal alone (319). Crosses between the SRC-1 and TIF2 null mice also reveal information on the biological function of these two coactivators (320). Unlike SRC-1 null mice, TIF2 null mice are not resistant to thyroid hormone as measured by T3, T4, and TSH levels, and this initially suggested that TIF2 did not play a role in thyroid hormone action. However, double-heterozygous mice (SRC-1+/-/TIF2+/-) are as resistant to T3 as SRC-1 null mice, although SRC-1 heterozygotes (SRC-1+/-) are not. The appearance of a TIF2 phenotype on a reduced SRC-1 expression background indicates that TIF2 does contribute to T3 action and suggests aspects of limited functional redundancy between these two coactivators (320).
It should be noted that knockouts for the coactivators CBP (321, 322), p300 (323), and TRAP220/DRIP205 (324) have been generated. In contrast to the knockout models described above, lack of expression of these coactivators results in embryonic lethality, resulting from early defects in development. Consistent with the ability of these factors, particularly CBP/p300, to stimulate the activity of a broad range of transcription factors (325, 326), the knockout data indicate that these cofactors are critical to mammalian development, and this is supported by the phenotypes of CBP, p300, and TRAP220 heterozygous mutant mice, which exhibit gross defects in morphogenetic and cell differentiation processes (323, 327).
Evidence obtained from NCoR null mice reveals biological functions that require corepressor expression for normal activity and lays the groundwork to begin to examine the role of these molecules in the regulation of tissue-specific nuclear receptor ligand function. The NCoR null mice are embryonic lethal, with the majority dying at postnatal d 15.5 due to developmental defects in the erythrocytic, thymic, and neural systems (148). However, mouse embryo fibroblasts (MEFs) derived from NCoR null embryos provide a model system to examine the importance of this corepressor for the relative agonist/antagonist activity of tamoxifen. In transient transfection assays, 4HT stimulated ER-dependent expression of a reporter gene in NCoR null, but not wild-type, MEFs (148); estrogen stimulation of the target gene was comparable between genotypes. Moreover, expression of exogenous NCoR in the null MEFs significantly attenuated the agonist activity of 4HT, consistent with the hypothesis that corepressors play an important role in defining the biocharacter of this antiestrogen. Although these results support the hypothesis that corepressors are important for the antagonistic activity of SRMs, the inability of the SMRT corepressor expressed within these cells to maintain the antagonist activity of 4HT suggests some selectivity between NCoR and SMRT function. More detailed studies of the relative expression of these and other corepressors, and analysis of endogenous target gene expression should help to clarify this issue.
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VI. Concluding Remarks |
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It is hoped that the future will hold an entirely new complement of SRMs for every occasion. SERMs, which inhibit breast and prostate response to estrogens while providing estrogen-like stimulation of bone, brain, and potentially the cardiovascular system, are already available. To date, however, we have not been able to discover SERMs that suppress hot flushes in postmenopausal women without stimulating uterine growth; nevertheless, an interesting combination of a new SERM plus conjugated equine estrogens (Premarin) may provide this desired profile (328). In the near future, we may be using oral SARMs for treatment of male osteoporosis or muscle-wasting diseases without the concomitant stimulation of prostatic growth. SPRMs that do not stimulate mammary alveolar proliferation will be employed for the treatment of uterine endometriosis. New SGRMs may give us greater abilities to suppress inflammation without concomitant fat deposition, collagen destruction, osteoporosis, and diabetogenic effects. These same concepts of ligand-driven conformational diversity and selective tissue actions will also be exploited in the future for drugs that selectively regulate the many additional orphan receptors in the nuclear receptor family. By this process of translation of fundamental research to development of pharmaceutical therapies, the field of nuclear receptors will provide a significant return for the investment by the National Institutes of Health in basic research over recent decades.
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Acknowledgments |
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Footnotes |
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Abbreviations: AF-1 and -2, Activation function 1 and 2; AIB1, amplified in breast cancer 1; AP-1, activator protein 1; AR, androgen receptor; CARM1, coactivator-associated arginine (R) methyltransferase-1; CBP, cAMP response element binding protein (CREB)-binding protein; ChIP, chromatin immunoprecipitation; CNS, central nervous system; DRIP, vitamin D receptor-interacting protein; E2, 17ß-estradiol; EGF, epidermal growth factor; ER, estrogen receptor; ERE, estrogen response element; GR, glucocorticoid receptor; GRIP, GR-interacting protein; 4HT, 4-hydroxytamoxifen; IKK, IB kinase; LBD, ligand-binding domain; MEF, mouse embryo fibroblast; MEK, MAPK kinase; NCoR, nuclear receptor corepressor; PCOS, polycystic ovarian syndrome; PGC-1, PPAR coactivator-1; PPAR, peroxisome proliferator-activated receptor; PR, progesterone receptor; RAC3, receptor associated coactivator; RAR, retinoic acid receptor; RIP, receptor-interacting protein; SARM, selective AR modulator; SERM, selective ER modulator; SGRM, selective GR modulator; SMRT, silencing mediator of retinoic acid and thyroid hormone receptor; SPRM, selective PR modulator; SRC, steroid receptor coactivator; SRM, selective receptor modulator; TIF, transcriptional intermediary factor; TR, thyroid hormone receptor; TRAP, TR-associated protein.
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References |
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|---|
8, 9-dehydroestrone sulfate in postmenopausal women. J Clin Endocrinol Metab 84:2020–2027 modulators (SPARMs). Trends Mol Med 7:395–400[CrossRef][Medline]
ligand with potent insulin-sensitizing yet weak adipogenic activity. Mol Cell 8:737–747[CrossRef][Medline]
antagonist with antiobesity and antidiabetic activity. Mol Endocrinol 16:2628–2644 selective modulator. Mol Endocrinol 17:662–676 and ERß. Proc Natl Acad Sci USA 96:3999–4004 and ß by both the antiestrogens ICI 164, 384 and ICI 182, 780 leading to opposite effects on receptor stability. Biochem Biophys Res Commun 261:1–5[CrossRef][Medline]
. J Biol Chem 272:25832–25838 and ß form heterodimers on DNA. J Biol Chem 272:19858–19862 transcriptional activity and is a key regulator of the cellular response to estrogens and antiestrogens. Endocrinology 140:5566–5578 and ß. Mol Pharmacol 58:584–590 activation by ligands, emphasizing the role of interactions between distant A and E domains. Mol Cell 10:1019–1032[CrossRef][Medline]
and ß. Endocrinology 138:863–870 and ß. Biochem Biophys Res Commun 236:140–145[CrossRef][Medline]
and ß is cell type- and promoter-dependent. J Biol Chem 274:32008–32014: binding and activation correlate with antidiabetic actions in db/db mice. Endocrinology 137:4189–4195[Abstract]
reveals a role for ligand in the nuclear distribution of the receptor. Mol Biol Cell 10:471–486 and steroid receptor coactivator-1. Mol Endocrinol 14:518–534 and ß. Mol Cell Biol 19:8226–8239
4 activation domain of the thyroid hormone receptor is required to release a corepressor(s) necessary for transcriptional silencing. Mol Cell Biol 15:76–86 and ß by interacting directly with the N-terminal A/B domains. J Biol Chem 275:15645–15651 and coactivator turnover and for efficient estrogen receptor- transactivation. Mol Cell 5:939–948[CrossRef][Medline]
provides evidence in support of a role for corepressors in regulating cellular responses to agonists and antagonists. Mol Endocrinol 16:1778–1792 receptor chimeras. Endocrinology 139:4513–4522 and ERß. J Steroid Biochem Mol Biol 69:165–175[CrossRef][Medline]
and estrogen receptor ß to partial estrogen agonists/antagonists. Mol Pharmacol 54:105–112 and ERß at AP-1 sites. Science 277:1508–1510 and ERß. J Biol Chem 276:23397–23404. J Biol Chem 277:13918–13925. Mol Cell Biol 19:5363–5372 coactivator through the N-terminal activation domain (AF-1) with an RNA coactivator, SRA. EMBO J 20:1341–1352[CrossRef][Medline]
and estrogen receptor-ß: correlations with biological character and distinct differences among SRC coactivator family members. Endocrinology 141:3534–3545 and ß interplay with SRC family coactivators. ER selective ligands. Biochemistry 40:6756–6765[CrossRef][Medline]
and ERß. J Biol Chem 275:35986–35993 and ß with estrogen response elements. Mol Cell Endocrinol 181:151–163[CrossRef][Medline]
and ERß. Mol Endocrinol 16:674–693 depends on the coactivator subtype, the type of estrogen response element, and the promoter context. Mol Endocrinol 16:2571–2581 transcriptional activity and MCF-7 proliferation. Mol Endocrinol 16:253–270: regulation by phosphorylation sites in the A/B region depends on other receptor domains. Mol Endocrinol 17:1296–1314 (ER)- and ERß-dependent gene expression by cAMP signaling pathway(s). J Biol Chem 278:12834–12845B kinase. Mol Cell Biol 22:3549–3561. Biochim Biophys Acta 1592:205–211[Medline]
coactivator-1. Mol Cell 8:971–982[CrossRef][Medline]
coactivator-1 (PGC-1). J Biol Chem 277:50991–50995B and p65 regulate the cytoplasmic shuttling of nuclear corepressors: cross-talk between notch and NFB pathways. Mol Biol Cell 14:491–502 is a ubiquitinated protein whose stability is affected differentially by agonists, antagonists, and selective estrogen receptor modulators. J Biol Chem 276:35684–35692 is linked to ligand-dependent activation. J Biol Chem 275:18527–18533 on responsive promoters is an integral feature of estrogen signaling. Mol Cell 11:695–707[CrossRef][Medline]
hinge region by p300 regulates transactivation and hormone sensitivity. J Biol Chem 6:18375–18383
-associated protein, template-activating factor 1ß, inhibits acetylation and transactivation. Mol Endocrinol 17:67–78 coactivator-1 through transcription factor docking. Science 286:1368–1371 coregulators in breast carcinoma: evidence that NCOR1 expression is predictive of the response to tamoxifen. Clin Cancer Res 9:1259–1266 in the cytoplasm. Nature 418:654–657[CrossRef][Medline]
, estrogen receptor-ß, coactivators, and corepressors in breast cancer. Clin Cancer Res 6:512–518 by S118 phosphorylation involves a ligand-dependent interaction with TFIIH and participation of CDK7. Mol Cell 6:127–137[CrossRef][Medline]
and phosphorylates its AF-1-activating domain through cdk7. J Biol Chem 275:21896–21904 and essential for the antiproliferative activity of ICI 182,780 in ER-positive breast cancer cells. Mol Endocrinol 17:356–365This article has been cited by other articles:
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|
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|
|
|
|
|
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|
|
|
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|
|
|
|
 |
|
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|
|
|
|
 |
|
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|
|
|
|
 |
|
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|
|
|
|
|
|
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|
|
|
|
|
|
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|
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H. Chen, T. L. Clemens, M. Hewison, and J. S. Adams Estradiol and Tamoxifen Mediate Rescue of the Dominant-Negative Effects of Estrogen Response Element-Binding Protein in Vivo and in Vitro Endocrinology, May 1, 2009; 150(5): 2429 - 2435. [Abstract] [Full Text] [PDF]
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 J. O. Jones, E. C. Bolton, Y. Huang, C. Feau, R. K. Guy, K. R. Yamamoto, B. Hann, and M. I. Diamond Non-competitive androgen receptor inhibition in vitro and in vivo PNAS, April 28, 2009; 106(17): 7233 - 7238. [Abstract] [Full Text] [PDF]
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S. Karmakar, E. A. Foster, and C. L. Smith Unique Roles of p160 Coactivators for Regulation of Breast Cancer Cell Proliferation and Estrogen Receptor-{alpha} Transcriptional Activity Endocrinology, April 1, 2009; 150(4): 1588 - 1596. [Abstract] [Full Text] [PDF]
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 V. C. Jordan A Century of Deciphering the Control Mechanisms of Sex Steroid Action in Breast and Prostate Cancer: The Origins of Targeted Therapy and Chemoprevention Cancer Res., February 15, 2009; 69(4): 1243 - 1254. [Abstract] [Full Text] [PDF]
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D. J Kojetin, T. P Burris, E. V Jensen, and S. A Khan Implications of the binding of tamoxifen to the coactivator recognition site of the estrogen receptor Endocr. Relat. Cancer, December 1, 2008; 15(4): 851 - 870. [Abstract] [Full Text] [PDF]
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 K. W. Kang and Y. G. Kim Bioequivalence Studies of Tibolone in Premenopausal Women and Effects on Expression of the Tibolone-Metabolizing Enzyme AKR1C (Aldo-Keto Reductase) Family Caused by Estradiol J. Clin. Pharmacol., December 1, 2008; 48(12): 1430 - 1437. [Abstract] [Full Text] [PDF]
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A. Coste, J.-F. Louet, M. Lagouge, C. Lerin, M. C. Antal, H. Meziane, K. Schoonjans, P. Puigserver, B. W. O'Malley, and J. Auwerx The genetic ablation of SRC-3 protects against obesity and improves insulin sensitivity by reducing the acetylation of PGC-1{alpha} PNAS, November 4, 2008; 105(44): 17187 - 17192. [Abstract] [Full Text] [PDF]
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R. Narayanan, C. C. Coss, M. Yepuru, J. D. Kearbey, D. D. Miller, and J. T. Dalton Steroidal Androgens and Nonsteroidal, Tissue-Selective Androgen Receptor Modulator, S-22, Regulate Androgen Receptor Function through Distinct Genomic and Nongenomic Signaling Pathways Mol. Endocrinol., November 1, 2008; 22(11): 2448 - 2465. [Abstract] [Full Text] [PDF]
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S. Xu, Y. Cheng, J. R. Keast, and P. B. Osborne 17{beta}-Estradiol Activates Estrogen Receptor {beta}-Signalling and Inhibits Transient Receptor Potential Vanilloid Receptor 1 Activation by Capsaicin in Adult Rat Nociceptor Neurons Endocrinology, November 1, 2008; 149(11): 5540 - 5548. [Abstract] [Full Text] [PDF]
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 J. J. Eloranta and G. A. Kullak-Ublick The Role of FXR in Disorders of Bile Acid Homeostasis Physiology, October 1, 2008; 23(5): 286 - 295. [Abstract] [Full Text] [PDF]
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L.-N. Song and E. P. Gelmann Silencing Mediator for Retinoid and Thyroid Hormone Receptor and Nuclear Receptor Corepressor Attenuate Transcriptional Activation by the {beta}-Catenin-TCF4 Complex J. Biol. Chem., September 19, 2008; 283(38): 25988 - 25999. [Abstract] [Full Text] [PDF]
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 |
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 R. Sanchez-Martinez, A. Zambrano, A. I. Castillo, and A. Aranda Vitamin D-Dependent Recruitment of Corepressors to Vitamin D/Retinoid X Receptor Heterodimers Mol. Cell. Biol., June 1, 2008; 28(11): 3817 - 3829. [Abstract] [Full Text] [PDF]
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S. Lee, J. Lee, S.-K. Lee, and J. W. Lee Activating Signal Cointegrator-2 Is an Essential Adaptor to Recruit Histone H3 Lysine 4 Methyltransferases MLL3 and MLL4 to the Liver X Receptors Mol. Endocrinol., June 1, 2008; 22(6): 1312 - 1319. [Abstract] [Full Text] [PDF]
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B. Tan, X. Long, H. Nakshatri, K. P Nephew, and R. M Bigsby Striatin-3{gamma} inhibits estrogen receptor activity by recruiting a protein phosphatase J. Mol. Endocrinol., May 1, 2008; 40(5): 199 - 210. [Abstract] [Full Text] [PDF]
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 |
|
 J. K. Nagpal, S. Nair, D. Chakravarty, R. Rajhans, S. Pothana, D. W. Brann, R. R. Tekmal, and R. K. Vadlamudi Growth Factor Regulation of Estrogen Receptor Coregulator PELP1 Functions via Protein Kinase A Pathway Mol. Cancer Res., May 1, 2008; 6(5): 851 - 861. [Abstract] [Full Text] [PDF]
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 |
|
 M. K. Glassberg, S. J. Elliot, J. Fritz, P. Catanuto, M. Potier, R. Donahue, W. Stetler-Stevenson, and M. Karl Activation of the Estrogen Receptor Contributes to the Progression of Pulmonary Lymphangioleiomyomatosis via Matrix Metalloproteinase-Induced Cell Invasiveness J. Clin. Endocrinol. Metab., May 1, 2008; 93(5): 1625 - 1633. [Abstract] [Full Text] [PDF]
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|
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 |
|
 Y. Zhang, B. Chen, Y. Li, J. Chen, G. Lou, M. Chen, and D. Zhou Transcriptional Regulation of the Human PNRC Promoter by NFY in HepG2 Cells J. Biochem., May 1, 2008; 143(5): 675 - 683. [Abstract] [Full Text] [PDF]
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M. Honorat, A. Mesnier, J. Vendrell, J. Guitton, I. Bieche, R. Lidereau, G. D Kruh, C. Dumontet, P. Cohen, and L. Payen ABCC11 expression is regulated by estrogen in MCF7 cells, correlated with estrogen receptor {alpha} expression in postmenopausal breast tumors and overexpressed in tamoxifen-resistant breast cancer cells Endocr. Relat. Cancer, March 1, 2008; 15(1): 125 - 138. [Abstract] [Full Text] [PDF]
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R. Rajhans, H. B. Nair, S. S. Nair, V. Cortez, K. Ikuko, N. B. Kirma, D. Zhou, A. E. Holden, D. W Brann, S. Chen, et al. Modulation of in Situ Estrogen Synthesis by Proline-, Glutamic Acid-, and Leucine-Rich Protein-1: Potential Estrogen Receptor Autocrine Signaling Loop in Breast Cancer Cells Mol. Endocrinol., March 1, 2008; 22(3): 649 - 664. [Abstract] [Full Text] [PDF]
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N. Levy, D. Tatomer, C. B. Herber, X. Zhao, H. Tang, T. Sargeant, L. J. Ball, J. Summers, T. P. Speed, and D. C. Leitman Differential Regulation of Native Estrogen Receptor-Regulatory Elements by Estradiol, Tamoxifen, and Raloxifene Mol. Endocrinol., February 1, 2008; 22(2): 287 - 303. [Abstract] [Full Text] [PDF]
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K. J. Higgins, S. Liu, M. Abdelrahim, K. Vanderlaag, X. Liu, W. Porter, R. Metz, and S. Safe Vascular Endothelial Growth Factor Receptor-2 Expression Is Down-Regulated by 17{beta}-Estradiol in MCF-7 Breast Cancer Cells by Estrogen Receptor {alpha}/Sp Proteins Mol. Endocrinol., February 1, 2008; 22(2): 388 - 402. [Abstract] [Full Text] [PDF]
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 |
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 F. Stormshak and C. V. Bishop BOARD-INVITED REVIEW: Estrogen and progesterone signaling: Genomic and nongenomic actions in domestic ruminants J Anim Sci, February 1, 2008; 86(2): 299 - 315. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. M. McCARTHY Estradiol and the Developing Brain Physiol Rev, January 1, 2008; 88(1): 91 - 134. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 V. C. Jordan and B. W. O'Malley Selective Estrogen-Receptor Modulators and Antihormonal Resistance in Breast Cancer J. Clin. Oncol., December 20, 2007; 25(36): 5815 - 5824. [Abstract] [Full Text] [PDF]
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|
|
A. Germaniuk-Kurowska, A. Nag, X. Zhao, M. Dimri, H. Band, and V. Band Ada3 Requirement for HAT Recruitment to Estrogen Receptors and Estrogen-Dependent Breast Cancer Cell Proliferation Cancer Res., December 15, 2007; 67(24): 11789 - 11797. [Abstract] [Full Text] [PDF]
|
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|
|
|
K. Britt, A. Ashworth, and M. Smalley Pregnancy and the risk of breast cancer Endocr. Relat. Cancer, December 1, 2007; 14(4): 907 - 933. [Abstract] [Full Text] [PDF]
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 |
|
 H. V. Heemers and D. J. Tindall Androgen Receptor (AR) Coregulators: A Diversity of Functions Converging on and Regulating the AR Transcriptional Complex Endocr. Rev., December 1, 2007; 28(7): 778 - 808. [Abstract] [Full Text] [PDF]
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|
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P. J. O'Shea, C. J. Guigon, G. R. Williams, and S.-y. Cheng Regulation of Fibroblast Growth Factor Receptor-1 (Fgfr1) by Thyroid Hormone: Identification of a Thyroid Hormone Response Element in the Murine Fgfr1 Promoter Endocrinology, December 1, 2007; 148(12): 5966 - 5976. [Abstract] [Full Text] [PDF]
|
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J. Kawagoe, M. Ohmichi, S. Tsutsumi, T. Ohta, K. Takahashi, and H. Kurachi Mechanism of the Divergent Effects of Estrogen on the Cell Proliferation of Human Umbilical Endothelial Versus Aortic Smooth Muscle Cells Endocrinology, December 1, 2007; 148(12): 6092 - 6099. [Abstract] [Full Text] [PDF]
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|
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|
J. X. Zou, A. S. Revenko, L. B. Li, A. T. Gemo, and H.-W. Chen ANCCA, an estrogen-regulated AAA+ ATPase coactivator for ER{alpha}, is required for coregulator occupancy and chromatin modification PNAS, November 13, 2007; 104(46): 18067 - 18072. [Abstract] [Full Text] [PDF]
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|
|
 |
|
 F. M. Horne and D. L. Blithe Progesterone receptor modulators and the endometrium: changes and consequences Hum. Reprod. Update, November 1, 2007; 13(6): 567 - 580. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Das, M. Schapira, M. Tomic-Canic, R. Goyanka, T. Cardozo, and H. H. Samuels Farnesyl Pyrophosphate Is a Novel Transcriptional Activator for a Subset of Nuclear Hormone Receptors Mol. Endocrinol., November 1, 2007; 21(11): 2672 - 2686. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. A. McDevitt, C. Glidewell-Kenney, J. Weiss, P. Chambon, J. L. Jameson, and J. E. Levine Estrogen Response Element-Independent Estrogen Receptor (ER)-{alpha} Signaling Does Not Rescue Sexual Behavior but Restores Normal Testosterone Secretion in Male ER{alpha} Knockout Mice Endocrinology, November 1, 2007; 148(11): 5288 - 5294. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
V. Nahoum, E. Perez, P. Germain, F. Rodriguez-Barrios, F. Manzo, S. Kammerer, G. Lemaire, O. Hirsch, C. A. Royer, H. Gronemeyer, et al. Modulators of the structural dynamics of the retinoid X receptor to reveal receptor function PNAS, October 30, 2007; 104(44): 17323 - 17328. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Khan, F. Wu, S. Liu, Q. Wu, and S. Safe Role of specificity protein transcription factors in estrogen-induced gene expression in MCF-7 breast cancer cells J. Mol. Endocrinol., October 1, 2007; 39(4): 289 - 304. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
D. Li, Q. Kang, and D.-M. Wang Constitutive Coactivator of Peroxisome Proliferator-Activated Receptor (PPAR{gamma}), a Novel Coactivator of PPAR{gamma} that Promotes Adipogenesis Mol. Endocrinol., October 1, 2007; 21(10): 2320 - 2333. [Abstract] [Full Text] [PDF]
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|
D. Kressler, M. B. Hock, and A. Kralli Coactivators PGC-1beta and SRC-1 Interact Functionally to Promote the Agonist Activity of the Selective Estrogen Receptor Modulator Tamoxifen J. Biol. Chem., September 14, 2007; 282(37): 26897 - 26907. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. E Burdette and T. K Woodruff Activin and estrogen crosstalk regulates transcription in human breast cancer cells Endocr. Relat. Cancer, September 1, 2007; 14(3): 679 - 689. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
T. J. Peterson, S. Karmakar, M. C. Pace, T. Gao, and C. L. Smith The Silencing Mediator of Retinoic Acid and Thyroid Hormone Receptor (SMRT) Corepressor Is Required for Full Estrogen Receptor {alpha} Transcriptional Activity Mol. Cell. Biol., September 1, 2007; 27(17): 5933 - 5948. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. Heldring, A. Pike, S. Andersson, J. Matthews, G. Cheng, J. Hartman, M. Tujague, A. Strom, E. Treuter, M. Warner, et al. Estrogen Receptors: How Do They Signal and What Are Their Targets Physiol Rev, July 1, 2007; 87(3): 905 - 931. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. Levy, X. Zhao, H. Tang, R. B. Jaffe, T. P. Speed, and D. C. Leitman Multiple Transcription Factor Elements Collaborate with Estrogen Receptor {alpha} to Activate an Inducible Estrogen Response Element in the NKG2E Gene Endocrinology, July 1, 2007; 148(7): 3449 - 3458. [Abstract] [Full Text] [PDF]
|
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|
|
|
|
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]
|
|
|
|
|
|
R. Rajhans, S. Nair, A. H. Holden, R. Kumar, R. R. Tekmal, and R. K. Vadlamudi Oncogenic Potential of the Nuclear Receptor Coregulator Proline-, Glutamic Acid-, Leucine-Rich Protein 1/Modulator of the Nongenomic Actions of the Estrogen Receptor Cancer Res., June 1, 2007; 67(11): 5505 - 5512. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. Li, M. E Andersen, S. Heber, and Q. Zhang Non-monotonic dose-response relationship in steroid hormone receptor-mediated gene expression J. Mol. Endocrinol., May 1, 2007; 38(5): 569 - 585. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. Martin, M. B. Taylor, G. Krikun, C. Lockwood, G. E. Akbas, and H. S. Taylor Differential Cell-Specific Modulation of HOXA10 by Estrogen and Specificity Protein 1 Response Elements J. Clin. Endocrinol. Metab., May 1, 2007; 92(5): 1920 - 1926. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. P. Madauss, E. T. Grygielko, S.-J. Deng, A. C. Sulpizio, T. B. Stanley, C. Wu, S. A. Short, S. K. Thompson, E. L. Stewart, N. J. Laping, et al. A Structural and in Vitro Characterization of Asoprisnil: A Selective Progesterone Receptor Modulator Mol. Endocrinol., May 1, 2007; 21(5): 1066 - 1081. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. J. Han, S. Y. Tsai, M.-J. Tsai, and B. W. O'Malley Distinct Temporal and Spatial Activities of RU486 on Progesterone Receptor Function in Reproductive Organs of Ovariectomized Mice Endocrinology, May 1, 2007; 148(5): 2471 - 2486. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. Chintharlapalli, S. Papineni, I. Jutooru, A. McAlees, and S. Safe Structure-dependent activity of glycyrrhetinic acid derivatives as peroxisome proliferator-activated receptor {gamma} agonists in colon cancer cells Mol. Cancer Ther., May 1, 2007; 6(5): 1588 - 1598. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. C Garrido-Gracia, A. Gordon, C. Bellido, R. Aguilar, I. Barranco, Y. Millan, J. M. de las Mulas, and J. E Sanchez-Criado The integrated action of oestrogen receptor isoforms and sites with progesterone receptor in the gonadotrope modulates LH secretion: evidence from tamoxifen-treated ovariectomized rats J. Endocrinol., April 1, 2007; 193(1): 107 - 119. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 S. Hirano, D. Furutama, and T. Hanafusa Physiologically high concentrations of 17beta-estradiol enhance NF-{kappa}B activity in human T cells Am J Physiol Regulatory Integrative Comp Physiol, April 1, 2007; 292(4): R1465 - R1471. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Jeyakumar, X.-f. Liu, H. Erdjument-Bromage, P. Tempst, and M. K. Bagchi Phosphorylation of Thyroid Hormone Receptor-associated Nuclear Receptor Corepressor Holocomplex by the DNA-dependent Protein Kinase Enhances Its Histone Deacetylase Activity J. Biol. Chem., March 30, 2007; 282(13): 9312 - 9322. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. J. Li, Z. Haque, Q. Lu, L. Li, R. Karas, and M. Mendelsohn Steroid receptor coactivator 3 is a coactivator for myocardin, the regulator of smooth muscle transcription and differentiation PNAS, March 6, 2007; 104(10): 4065 - 4070. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Galluzzo, F. Caiazza, S. Moreno, and M. Marino Role of ER{beta} palmitoylation in the inhibition of human colon cancer cell proliferation Endocr. Relat. Cancer, March 1, 2007; 14(1): 153 - 167. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. E. Wierman Sex steroid effects at target tissues: mechanisms of action Advan Physiol Educ, March 1, 2007; 31(1): 26 - 33. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
W. Yong, Z. Yang, S. Periyasamy, H. Chen, S. Yucel, W. Li, L. Y. Lin, I. M. Wolf, M. J. Cohn, L. S. Baskin, et al. Essential Role for Co-chaperone Fkbp52 but Not Fkbp51 in Androgen Receptor-mediated Signaling and Physiology J. Biol. Chem., February 16, 2007; 282(7): 5026 - 5036. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Nichols The Fight Against Tamoxifen Resistance in Breast Cancer Therapy: A New Target in the Battle? Mol. Interv., February 1, 2007; 7(1): 13 - 16. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. Flamant, K. Gauthier, and J. Samarut Thyroid Hormones Signaling Is Getting More Complex: STORMs Are Coming Mol. Endocrinol., February 1, 2007; 21(2): 321 - 333. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Sasaki, N. Ohara, Q. Xu, J. Wang, D. A. DeManno, K. Chwalisz, S. Yoshida, and T. Maruo A Novel Selective Progesterone Receptor Modulator Asoprisnil Activates Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL)-Mediated Signaling Pathway in Cultured Human Uterine Leiomyoma Cells in the Absence of Comparable Effects on Myometrial Cells J. Clin. Endocrinol. Metab., February 1, 2007; 92(2): 616 - 623. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. B. Dammer, A. Leon, and M. B. Sewer Coregulator Exchange and Sphingosine-Sensitive Cooperativity of Steroidogenic Factor-1, General Control Nonderepressed 5, p54, and p160 Coactivators Regulate Cyclic Adenosine 3',5'-Monophosphate-Dependent Cytochrome P450c17 Transcription Rate Mol. Endocrinol., February 1, 2007; 21(2): 415 - 438. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
A. Cvoro, S. Paruthiyil, J. O. Jones, C. Tzagarakis-Foster, N. J. Clegg, D. Tatomer, R. T. Medina, M. Tagliaferri, F. Schaufele, T. S. Scanlan, et al. Selective Activation of Estrogen Receptor-{beta} Transcriptional Pathways by an Herbal Extract Endocrinology, February 1, 2007; 148(2): 538 - 547. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Rayner, Y.-X. Chen, B. Hibbert, D. White, H. Miller, E. H. Postel, and E. R. O'Brien NM23-H2, an estrogen receptor beta-associated protein, shows diminished expression with progression of atherosclerosis Am J Physiol Regulatory Integrative Comp Physiol, February 1, 2007; 292(2): R743 - R750. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Yokota, H. Shibata, I. Kurihara, S. Kobayashi, N. Suda, A. Murai-Takeda, I. Saito, H. Kitagawa, S. Kato, T. Saruta, et al. Coactivation of the N-terminal Transactivation of Mineralocorticoid Receptor by Ubc9 J. Biol. Chem., January 19, 2007; 282(3): 1998 - 2010. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Ostrowski, J. E. Kuhns, J. A. Lupisella, M. C. Manfredi, B. C. Beehler, S. R. Krystek Jr., Y. Bi, C. Sun, R. Seethala, R. Golla, et al. Pharmacological and X-Ray Structural Characterization of a Novel Selective Androgen Receptor Modulator: Potent Hyperanabolic Stimulation of Skeletal Muscle with Hypostimulation of Prostate in Rats Endocrinology, January 1, 2007; 148(1): 4 - 12. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Folkertsma, P. I. van Noort, A. de Heer, P. Carati, R. Brandt, A. Visser, G. Vriend, and J. de Vlieg The Use of in Vitro Peptide Binding Profiles and in Silico Ligand-Receptor Interaction Profiles to Describe Ligand-Induced Conformations of the Retinoid X Receptor {alpha} Ligand-Binding Domain Mol. Endocrinol., January 1, 2007; 21(1): 30 - 48. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 P. Germain, B. Staels, C. Dacquet, M. Spedding, and V. Laudet Overview of Nomenclature of Nuclear Receptors Pharmacol. Rev., December 1, 2006; 58(4): 685 - 704. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
N. Z. Lu, S. E. Wardell, K. L. Burnstein, D. Defranco, P. J. Fuller, V. Giguere, R. B. Hochberg, L. McKay, J.-M. Renoir, N. L. Weigel, et al. International Union of Pharmacology. LXV. The Pharmacology and Classification of the Nuclear Receptor Superfamily: Glucocorticoid, Mineralocorticoid, Progesterone, and Androgen Receptors Pharmacol. Rev., December 1, 2006; 58(4): 782 - 797. [Full Text] [PDF]
|
|
|
|
|
|
A. I. Castillo, R. Sanchez-Martinez, A. M. Jimenez-Lara, A. Steinmeyer, U. Zugel, and A. Aranda Characterization of Vitamin D Receptor Ligands with Cell-Specific and Dissociated Activity Mol. Endocrinol., December 1, 2006; 20(12): 3093 - 3104. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. M. Jaber, T. Gao, L. Huang, S. Karmakar, and C. L. Smith The Pure Estrogen Receptor Antagonist ICI 182,780 Promotes a Novel Interaction of Estrogen Receptor-{alpha} with the 3',5'-Cyclic Adenosine Monophosphate Response Element-Binding Protein-Binding Protein/p300 Coactivators Mol. Endocrinol., November 1, 2006; 20(11): 2695 - 2710. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. Gizard, R. Robillard, B. Gross, O. Barbier, F. Revillion, J.-P. Peyrat, G. Torpier, D. W. Hum, and B. Staels TReP-132 Is a Novel Progesterone Receptor Coactivator Required for the Inhibition of Breast Cancer Cell Growth and Enhancement of Differentiation by Progesterone Mol. Cell. Biol., October 15, 2006; 26(20): 7632 - 7644. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. L. Turgeon, M. C. Carr, P. M. Maki, M. E. Mendelsohn, and P. M. Wise Complex Actions of Sex Steroids in Adipose Tissue, the Cardiovascular System, and Brain: Insights from Basic Science and Clinical Studies Endocr. Rev., October 1, 2006; 27(6): 575 - 605. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
U. Ohnemus, M. Uenalan, J. Inzunza, J.-A. Gustafsson, and R. Paus The Hair Follicle as an Estrogen Target and Source Endocr. Rev., October 1, 2006; 27(6): 677 - 706. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. J. Brayman, J. Julian, B. Mulac-Jericevic, O. M. Conneely, D. P. Edwards, and D. D. Carson Progesterone Receptor Isoforms A and B Differentially Regulate MUC1 Expression in Uterine Epithelial Cells Mol. Endocrinol., October 1, 2006; 20(10): 2278 - 2291. [Abstract] [Full Text] [PDF]
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D. G. Monroe, F. J. Secreto, J. R. Hawse, M. Subramaniam, S. Khosla, and T. C. Spelsberg Estrogen Receptor Isoform-specific Regulation of the Retinoblastoma-binding Protein 1 (RBBP1) Gene: ROLES OF AF1 AND ENHANCER ELEMENTS J. Biol. Chem., September 29, 2006; 281(39): 28596 - 28604. [Abstract] [Full Text] [PDF]
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J. E. O'Brien, T. J. Peterson, M. H. Tong, E.-J. Lee, L. E. Pfaff, S. C. Hewitt, K. S. Korach, J. Weiss, and J. L. Jameson Estrogen-induced Proliferation of Uterine Epithelial Cells Is Independent of Estrogen Receptor {alpha} Binding to Classical Estrogen Response Elements J. Biol. Chem., September 8, 2006; 281(36): 26683 - 26692. [Abstract] [Full Text] [PDF]
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J. L Turgeon and D. W Waring Differential expression and regulation of progesterone receptor isoforms in rat and mouse pituitary cells and L{beta}T2 gonadotropes. J. Endocrinol., September 1, 2006; 190(3): 837 - 846. [Abstract] [Full Text] [PDF]
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S. Kim, M. Yamazaki, L. A. Zella, N. K. Shevde, and J. W. Pike Activation of Receptor Activator of NF-{kappa}B Ligand Gene Expression by 1,25-Dihydroxyvitamin D3 Is Mediated through Multiple Long-Range Enhancers Mol. Cell. Biol., September 1, 2006; 26(17): 6469 - 6486. [Abstract] [Full Text] [PDF]
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H. Tian, M. A. Mahajan, C. T. Wong, I. Habeos, and H. H. Samuels The N-Terminal A/B Domain of the Thyroid Hormone Receptor-{beta}2 Isoform Influences Ligand-Dependent Recruitment of Coactivators to the Ligand-Binding Domain Mol. Endocrinol., September 1, 2006; 20(9): 2036 - 2051. [Abstract] [Full Text] [PDF]
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 V. C. Jordan The Science of Selective Estrogen Receptor Modulators: Concept to Clinical Practice Clin. Cancer Res., September 1, 2006; 12(17): 5010 - 5013. [Full Text] [PDF]
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M. Doshida, M. Ohmichi, S. Tsutsumi, J. Kawagoe, T. Takahashi, B. Du, A. Mori-Abe, T. Ohta, M. Saitoh-Sekiguchi, K. Takahashi, et al. Raloxifene Increases Proliferation and Up-regulates Telomerase Activity in Human Umbilical Vein Endothelial Cells J. Biol. Chem., August 25, 2006; 281(34): 24270 - 24278. [Abstract] [Full Text] [PDF]
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S. Ray, X. Hou, H.-E. Zhou, H. Wang, and S. K. Das Bip Is a Molecular Link between the Phase I and Phase II Estrogenic Responses in Uterus Mol. Endocrinol., August 1, 2006; 20(8): 1825 - 1837. [Abstract] [Full Text] [PDF]
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M. Adler, Y. Hou, P. Sandrock, C. Y. Meyers, T. A. Winters, W. J. Banz, and S. Adler Derivatives of Z-Bisdehydrodoisynolic Acid Provide a New Description of the Binding-Activity Paradox and Selective Estrogen Receptor Modulator Activity Endocrinology, August 1, 2006; 147(8): 3952 - 3960. [Abstract] [Full Text] [PDF]
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B.-S. An, D. M. Selva, G. L. Hammond, A. Rivero-Muller, N. Rahman, and P. C. K. Leung Steroid Receptor Coactivator-3 Is Required for Progesterone Receptor Trans-activation of Target Genes in Response to Gonadotropin-releasing Hormone Treatment of Pituitary Cells J. Biol. Chem., July 28, 2006; 281(30): 20817 - 20824. [Abstract] [Full Text] [PDF]
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