The steroid receptor coactivator SRC-3 (p/CIP/RAC3/AIB1/ACTR/TRAM-1) is required for normal growth, puberty, female reproductive function, and mammary gland development

Expression and regulation of nuclear receptor coactivators in glucocorticoid action

Nuclear receptor coactivators are involved in receptor-mediated transcriptional activation of target genes in a hormone-sensitive manner, and the mechanism of their transactivation has been studied in recent years. The glucocorticoid receptor (GR) interacts with several coactivators, including steroid receptor coactivator-1 (SRC-1) family and CREB-binding protein (CBP). Since coactivators function as transcription amplifiers, subtle changes in expression levels of coactivators in certain cells would markedly intensify receptor-mediated transcriptional activity. The regulation of coactivators by glucocorticoid action, however, has not yet been clarified. In this study, we have shown that one of the coactivators interacting with GR, SRC-1, is downregulated by dexamethasone (DEX) both in vivo and in vitro. In experiments on Sprague-Dawley rats in vivo, the downregulation of SRC-1 was observed in heart, stomach, kidney, liver, and cerebrum, and in experiments on two types of kidney-derived cells in vitro, similar downregulation of SRC-1 was demonstrated in both types of cells. DEX-mediated downregulation of SRC-1 mRNA recovered in 4-8 h, while the downregulation of SRC-1 protein lasted for 12 h and its levels returned to the basal level, 24 h after DEX treatment. Other coactivators examined in this study showed no remarkable changes in expression by DEX treatment, implying that ligand-mediated downregulation of SRC-1 has a pivotal role in the physiology of glucocorticoid action.

Oestrogens and puberty

Oestrogens induce the development of female reproductive tissues. Endogenous human oestrogens include oestradiol, oestrone and oestriol. Oestrogen signalling in target tissues is dependent on the tissue concentration of oestrogen and the interaction of oestrogen receptors with an array of cell-specific co-regulator proteins. The diverse mechanisms of oestrogen signalling are complex and incompletely understood. In puberty, oestrogen is derived from both gonadal and peripheral sources. Originally, oestrogen was only thought to drive feminization in females; now, oestrogen is known to be important for pubertal development of males as well. Oestrogen is required for normal maturation of the neuroendocrine-gonadal axis and bone in both sexes, and a variety of other tissues are also responsive to oestrogen. Abnormal puberty can be associated with either excessive or inadequate oestrogen production. Girls deficient in oestrogen should receive replacement in physiological doses. Aromatase inhibitors and anti-oestrogens may prove to be useful therapeutic tools in some types of abnormal puberty.

Combinatorial control of gene expression by nuclear receptors and coregulators

The nuclear receptor (NR) superfamily of transcription factors regulates gene expression in response to endocrine signaling, and recruitment of coregulators affords these receptors considerable functional flexibility. We will place historical aspects of NR research in context with current opinions on their mechanism of signal transduction, and we will speculate upon future trends in the field.

Reduction of coactivator expression by antisense oligodeoxynucleotides inhibits ERalpha transcriptional activity and MCF-7 proliferation

Steroid receptor RNA activator (SRA) is a novel coactivator for steroid receptors that acts as an RNA molecule, whereas steroid receptor coactivator (SRC) family members, such as steroid receptor coactivator-1 (SRC-1) and transcriptional intermediary factor 2 (TIF2) exert their biological effects as proteins. Individual overexpression of each of these coactivators, which can form multimeric complexes in vivo, results in stimulated ERalpha transcriptional activity in transient transfection assays. However there is no information on the consequences of reducing SRC-1, TIF2, or SRA expression, singly or in combination, on ERalpha transcriptional activity. We therefore developed antisense oligodeoxynucleotides (asODNs) to SRA, SRC-1, and TIF2 mRNAs, which rapidly and specifically reduced the expression of each of these coactivators. ERalpha-dependent gene expression was reduced in a dose-dependent fashion by up to 80% in cells transfected with these oligonucleotides. Furthermore, treatment of cells with combinations of SRA, SRC-1, and TIF2 asODNs reduced ERalpha transcriptional activity to an extent greater than individual asODN treatment alone, suggesting that these coactivators cooperate, in at least an additive fashion, to activate ERalpha-dependent target gene expression. Finally, treatment of MCF-7 cells with asODN against SRC-1 and TIF2 revealed a requirement of these coactivators, but not SRA, for hormone-dependent DNA synthesis and induction of estrogen-dependent pS2 gene expression, indicating that SRA and SRC family coactivators can fulfill specific functional roles. Taken together, we have developed a rapid method to reduce endogenous coactivator expression that enables an assessment of the in vivo role of specific coactivators on ERalpha biological action and avoids potential artifacts arising from overexpression of coactivators in transient transfection assays.

Nuclear receptor coactivators modulate hormone-dependent gene expression in brain and female reproductive behavior in rats

Gonadal steroid hormones act in the brain to elicit changes in gene expression that result in profound effects on behavior and physiology. A variety of in vitro studies indicate that nuclear receptor coactivators are required for efficient transcriptional activity of steroid receptors. Two nuclear receptor coactivators, steroid receptor coactivator-1 (SRC-1) and cAMP response element binding protein-binding protein (CBP), have been shown to act in concert to enhance ER activity in vitro. In the present study, we investigated the function of these important nuclear receptor coactivators in estrogen action in rodent brain. Reduction of SRC-1 and CBP protein in brain disrupted ER-mediated activation of the behaviorally relevant progestin receptor gene. Furthermore, we found that SRC-1 and CBP function in brain to modulate the expression of hormone-dependent female sexual behavior. These findings indicate that these nuclear receptor coactivators function in brain to modulate ER transcriptional activity and the expression of hormone-dependent behavior.

Modifiers of risk of hereditary breast and ovarian cancer

Hereditary breast and ovarian cancer is among the most commonly encountered adult genetic disease, and it is increasingly important that geneticists, oncologists, surgeons and gynaecologists are aware of the issues regarding risk assessment, prevention and management of women with inherited susceptibility to cancer. Genetic risk can be modified by external factors, but what are these factors, and how might our knowledge of them help us to better define the risks for individual women and to develop strategies for cancer prevention?

Genetic ablation of the steroid receptor coactivator-ubiquitin ligase, E6-AP, results in tissue-selective steroid hormone resistance and defects in reproduction

The E6-associated protein (E6-AP), although originally identified as a ubiquitin ligase, has recently been shown to function as a coactivator of steroid receptor-dependent gene expression in in vitro assays. In order to determine whether E6-AP acts as a coactivator in vivo, physiological parameters associated with male and female sex steroid action were assessed in the E6-AP null mouse. Gonadal size was reduced in E6-AP null male and female mice in comparison to wild-type controls in conjunction with reduced fertility in both genders. Consistent with this observation, defects in sperm production and function, as well as ovulation were observed. In comparison to wild-type controls, induction of prostate gland growth induced by testosterone and uterine growth by estradiol were significantly reduced. In contrast, estrogen and progesterone-stimulated growth of virgin mammary gland was not compromised by E6-AP ablation despite E6-AP expression in this tissue. This latter finding contrasts with the impaired estrogen and progesterone-induced mammary gland development observed previously for steroid receptor coactivator type 1 (SRC-1) and SRC-3 female knockout mice. Taken together, these results are consistent with a role for E6-AP in mediating a subset of steroid hormone actions in vivo. Nevertheless, differences observed between SRC and E6-AP knockout phenotypes indicate that these two families of steroid receptor coactivators are not functionally equivalent and supports the hypothesis that coactivators contribute to tissue-specific steroid hormone action.

Molecular perspectives on selective estrogen receptor modulators (SERMs): progress in understanding their tissue-specific agonist and antagonist actions

Synthetic estrogen receptor ligands such as tamoxifen and raloxifene produce biologic responses which can be either estrogenic or anti-estrogenic, depending upon the tissue in which their action is examined. To reflect the fact that they are not 'pure' antagonists, such ligands have been more accurately termed selective estrogen receptor modulators (SERMs). Recent progress in our understanding of the molecular biology of estrogen receptor (ER) action has provided a great deal of evidence which promises to increase our understanding of the mechanism through which SERMs elicit their tissue-specific effects. The identification of numerous coactivators and corepressors which modulate receptor function and the realization of two subtypes of ER attest to the potential complexity through which SERMs produce diverse tissue-specific responses. Evidence from co-crystal structures of ER ligand-binding domains complexed with SERMs provides additional information as to how this class of ligands can elicit diverse biologic responses. SERMs also influence the stability of the ER protein, and recent information on the determinants of receptor stability and the role of proteasome-mediated protein degradation in ER-driven transcription also promises to give a fuller understanding of SERM biology. These aspects of the molecular biology of estrogen receptor action may help clarify the mechanism(s) of SERM biologic action and will be addressed in further detail in this review.

Identification and characterization of a tissue-specific coactivator, GT198, that interacts with the DNA-binding domains of nuclear receptors

Gene activation mediated by nuclear receptors is regulated in a tissue-specific manner and requires interactions between nuclear receptors and their cofactors. Here, we identified and characterized a tissue-specific coactivator, GT198, that interacts with the DNA-binding domains of nuclear receptors. GT198 was originally described as a genomic transcript that mapped to the human breast cancer susceptibility locus 17q12-q21 with unknown function. We show that GT198 exhibits a tissue-specific expression pattern in which its mRNA is elevated in testis, spleen, thymus, pituitary cells, and several cancer cell lines. GT198 is a 217-amino-acid nuclear protein that contains a leucine zipper required for its dimerization. In vitro binding and yeast two-hybrid assays indicated that GT198 interacted with nuclear receptors through their DNA-binding domains. GT198 potently stimulated transcription mediated by estrogen receptor alpha and beta, thyroid hormone receptor beta1, androgen receptor, glucocorticoid receptor, and progesterone receptor. However, the action of GT198 was distinguishable from that of the ligand-binding domain-interacting nuclear receptor coactivators, such as TRBP, CBP, and SRC-1, with respect to basal activation and hormone sensitivity. Furthermore, protein kinase A, protein kinase C, and mitogen-activated protein kinase can phosphorylate GT198 in vitro, and cotransfection of these kinases regulated the transcriptional activity of GT198. These data suggest that GT198 is a tissue-specific, kinase-regulated nuclear receptor coactivator that interacts with the DNA-binding domains of nuclear receptors.

Expression of RAC 3, a steroid hormone receptor co-activator in prostate cancer

RAC 3, one of the p160 family of co-activators is known to enhance the transcriptional activity of a number of steroid receptors. As co-activators are also known to enhance androgen receptor (AR) activity, we investigated the role of RAC 3 in the context of prostate cancer. In prostate cancer cell lines, we found variable levels of the RAC 3 protein with highest expression seen in AR-positive LNCaP cells, moderate expression in AR-negative PC 3 cells and low-level expression in AR-negative DU 145 cells. Immuno-precipitation studies showed that endogenous RAC 3 interacted with the AR in vivo and transfection assays confirmed that RAC 3 enhanced AR transcriptional activity. In clinical prostate tissue, we found strong RAC 3 mRNA expression and immuno-histochemistry demonstrated that in benign tissue, the protein was expressed predominantly in luminal cells, while in primary malignant epithelium it was more homogeneously expressed. In a series of 37 patients, the levels of RAC 3 expression correlated significantly with tumour grade (P = 0.01) and stage of disease (P = 0.03) but not with serum PSA levels. In addition moderate or high RAC 3 expression was associated with poorer disease-specific survival (P = 0.03). We conclude that RAC 3 is an important co-activator of the AR in the prostate and may have an important role in the progression of prostate cancer.

Development of an ER action indicator mouse for the study of estrogens, selective ER modulators (SERMs), and Xenobiotics

We have developed a transgenic mouse that functions as a reporter of ER activity, termed ER action indicator (ERIN), by incorporating a transgene with an estrogen-responsive promoter (three copies of the vitellogenin estrogen response element with a minimal thymidine kinase promoter) linked to the reporter gene beta-galactosidase. Evaluation of ER activity in female ERIN mice demonstrated estrogen-inducible expression of the reporter gene in the uterus, pituitary, and hypothalamus; established targets of estrogen action. Importantly, we also identified ER activity in a number of nonclassical estrogen target tissues, including kidney, liver, adrenal, and thyroid gland. ERIN provides a system to measure the same end point (transgene regulation) in different target tissues, permitting separation of the contributions of cell- and promoter-specific factors in determining ER pharmacology. In this regard we observed that on this specific promoter the pituitary gland was 25-fold more sensitive than the uterus to the estrogen diethylstilbestrol, implying the existence of cell-specific factors that influence ligand sensitivity. Our studies also identified considerable difference in the efficacy and potency of ER ligands in the uterus when ER transcriptional activity was assayed vs. uterine weight gain. Specifically, we observed that the environmental estrogen bisphenol A was a potent agonist in stimulating ER transcriptional activity, whereas it exhibited little uterotropic activity. In contrast to bisphenol A, tamoxifen significantly increased uterine weight, but minimally induced ER reporter activity in this tissue. Given the results of these studies, we believe that ERIN will be a useful model to evaluate ER ligand pharmacology and will assist in defining the cellular and molecular mechanisms that determine agonist and antagonist activity.

Association of steroid receptor coactivator AIB1 with estrogen receptor-alpha in breast cancer cells

The steroid receptor coactivator AIB1 (amplified in breast cancer-1) is a transcriptional coactivator which has been found to be amplified in breast cancer. We have now investigated the role of the AIB1 protein in breast cancer cell lines. Although detectable levels of AIB1 were present in most cell lines, high levels of AIB1 expression were observed only in the ER-positive cell lines MCF-7 and BT-474 by western blot analysis. Newly developed monoclonal antibodies (mAbs) were used in several assays to show an association between AIBI and estrogen receptor-alpha (ER). AIB1 and ER co-localized to the nucleus of ER positive cell lines as shown by immunofluorescence microscopy, and a functional association of native AIB1 and ER in MCF-7 nuclear extracts was shown by EMSA. Recombinant ER also recruited AIB1 protein from nuclear extracts, shown by EMSA and by precipitation of ER-complex proteins bound to a biotinylated-ERE DNA target. Additionally, anti-AIB1 mAbs were able to immunoprecipitate ER from nuclear extracts of chemically cross-linked cells but not from uncross-linked cells. Both immunoprecipitation and oligonucleotide precipitation studies demonstrated the presence of p300 and CBP as part of the ER transcriptional complex. These results suggest that AIB1 and ER do associate physically in ER-positive breast cancer cell lines. We propose that in AIB1 amplified breast cancers, a heightened AIB1/ER association may play a crucial role in the progression of these tumors.

Factor recruitment and TIF2/GRIP1 corepressor activity at a collagenase-3 response element that mediates regulation by phorbol esters and hormones

To investigate determinants of specific transcriptional regulation, we measured factor occupancy and function at a response element, col3A, associated with the collagenase-3 gene in human U2OS osteosarcoma cells; col3A confers activation by phorbol esters, and repression by glucocorticoid and thyroid hormones. The subunit composition and activity of AP-1, which binds col3A, paralleled the intracellular level of cFos, which is modulated by phorbol esters and glucocorticoids. In contrast, a similar AP-1 site at the collagenase-1 gene, not inducible in U2OS cells, was not bound by AP-1. The glucocorticoid receptor (GR) associated with col3A through protein-protein interactions with AP-1, regardless of AP-1 subunit composition, and repressed transcription. TIF2/GRIP1, reportedly a coactivator for GR and the thyroid hormone receptor (TR), was recruited to col3A and potentiated GR-mediated repression in the presence of a GR agonist but not antagonist. GRIP1 mutants deficient in GR binding and coactivator functions were also defective for corepression, and a GRIP1 fragment containing the GR-interacting region functioned as a dominant-negative for repression. In contrast, repression by TR was unaffected by GRIP1. Thus, the composition of regulatory complexes, and the biological activities of the bound factors, are dynamic and dependent on cell and response element contexts. Cofactors such as GRIP1 probably contain distinct surfaces for activation and repression that function in a context-dependent manner.

Signaling pathways in mammary gland development

Unlike most other organs, development of the mammary gland occurs predominantly after birth, under the control of steroid and peptide hormones. Once the gland is established, cycles of proliferation, functional differentiation, and death of alveolar epithelium occur repeatedly with each pregnancy. Although it is unique in this respect, the signaling pathways utilized by the gland are shared with other cell types, and have been tailored to meet the needs of this secretory tissue. Here we discuss the signaling pathways that have been adopted by the mammary gland for its own purposes, and the functions they perform.

The specificity of interactions between nuclear hormone receptors and corepressors is mediated by distinct amino acid sequences within the interacting domains

The thyroid hormone receptor (TR) and retinoic acid receptor (RAR) isoforms interact with the nuclear corepressors [NCoR (nuclear corepressor protein) and SMRT (silencing mediator for retinoid and thyroid hormone receptors)] in the absence of ligand to silence transcription. NCoR and SMRT contain C-terminal nuclear hormone receptor (NHR) interacting domains that each contain variations of the consensus sequence I/L-x-x-I/V-I (CoRNR box). We have previously demonstrated that TRbeta1 preferentially interacts with NCoR, whereas RARalpha prefers SMRT. Here, we demonstrate that this is due, in part, to the presence of a novel NCoR interacting domain, termed N3, upstream of the previously described domains. An analogous domain is not present in SMRT. This domain is specific for TR and interacts poorly with RAR. Our data suggest that the presence of two corepressor interacting domains are necessary for full interactions with nuclear receptors in cells. Interestingly, mutation of N3 alone specifically decreases binding of NCoR to TR in cells but does not decrease NCoR-RAR interactions. In addition, while the exact CoRNR box sequence of a SMRT interacting domain is critical for recruitment of SMRT by RAR, the CoRNR box sequences themselves do not explain the strong interaction of the N2 domain with TRbeta1. Additional regions distal to the CoRNR box sequence are needed for optimal binding. Thus, through sequence differences in known interacting domains and the presence of a newly identified interacting domain, NCoR is able to preferentially bind TRbeta1. These preferences are likely to be important in corepressor action in vivo.

Use of suppressor mutants to probe the function of estrogen receptor-p160 coactivator interactions

Estrogen-dependent recruitment of coactivators by estrogen receptor alpha (ERalpha) represents a crucial step in the transcriptional activation of target genes. However, studies of the function of individual coactivators has been hindered by the presence of endogenous coactivators, many of which are potentially recruited in the presence of agonist via a common mechanism. To circumvent this problem, we have generated second-site suppressor mutations in the nuclear receptor interaction domain of p160 coactivators which rescue their binding to a transcriptionally defective ERalpha that is refractory to wild-type coactivators. Analysis of these altered-specificity receptor-coactivator combinations, in the absence of interference from endogenous coregulators, indicated that estrogen-dependent transcription from reporter genes is critically dependent on direct recruitment of a p160 coactivator in mammalian cells and that the three p160 family members serve functionally redundant roles. Furthermore, our results suggest that such a change-of-specificity mutation may act as a transposable protein-protein interaction module which provides a novel tool with which to dissect the functional roles of other nuclear receptor coregulators at the cellular level.

HOXA10 expression is repressed by progesterone in the myometrium: differential tissue-specific regulation of HOX gene expression in the reproductive tract

HOX genes are essential regulators of development in all multicellular organisms, including humans. We have previously shown that HOXA10 is expressed in the developing uterus and later in the adult human endometrium. HOX genes regulate endometrial development in response to sex steroids. Here, we demonstrate that HOXA10 is expressed in the myometrium as well. In situ hybridization reveals abundant HOXA10 expression, and Northern analysis demonstrates differential HOX gene expression in the myometrium throughout the menstrual cycle. HOXA10 expression decreases in the midsecretory phase, coinciding with high serum progesterone levels. Treatment of primary myometrial cell cultures with progesterone decreases HOXA10 expression in vitro-paralleling the expression seen in vivo. Despite the presence of progesterone receptors in the endometrium and myometrium, HOXA10 is differentially regulated in each tissue by progesterone. HOXA10 expression is induced in the stroma and decreased in the myometrium by progesterone. The differential tissue-specific response of this gene in response to progesterone is likely mediated by sex steroid receptor coactivators or corepressors. Decreased myometrial expression of developmental regulatory genes such as HOXA10 in the nonpregnant uterus may dedifferentiate the myometrium and allow growth in preparation for pregnancy.

Nuclear hormone receptors and gene expression

The nuclear hormone receptor superfamily includes receptors for thyroid and steroid hormones, retinoids and vitamin D, as well as different "orphan" receptors of unknown ligand. Ligands for some of these receptors have been recently identified, showing that products of lipid metabolism such as fatty acids, prostaglandins, or cholesterol derivatives can regulate gene expression by binding to nuclear receptors. Nuclear receptors act as ligand-inducible transcription factors by directly interacting as monomers, homodimers, or heterodimers with the retinoid X receptor with DNA response elements of target genes, as well as by "cross-talking" to other signaling pathways. The effects of nuclear receptors on transcription are mediated through recruitment of coregulators. A subset of receptors binds corepressor factors and actively represses target gene expression in the absence of ligand. Corepressors are found within multicomponent complexes that contain histone deacetylase activity. Deacetylation leads to chromatin compactation and transcriptional repression. Upon ligand binding, the receptors undergo a conformational change that allows the recruitment of multiple coactivator complexes. Some of these proteins are chromatin remodeling factors or possess histone acetylase activity, whereas others may interact directly with the basic transcriptional machinery. Recruitment of coactivator complexes to the target promoter causes chromatin decompactation and transcriptional activation. The characterization of corepressor and coactivator complexes, in concert with the identification of the specific interaction motifs in the receptors, has demonstrated the existence of a general molecular mechanism by which different receptors elicit their transcriptional responses in target genes.

Ribozyme targeting demonstrates that the nuclear receptor coactivator AIB1 is a rate-limiting factor for estrogen-dependent growth of human MCF-7 breast cancer cells

Human breast tumorigenesis is promoted by the estrogen receptor pathway, and nuclear receptor coactivators are thought to participate in this process. Here we studied whether one of these coactivators, AIB1 (amplified in breast cancer 1), was rate-limiting for hormone-dependent growth of human MCF-7 breast cancer cells. We developed MCF-7 breast cancer cell lines in which the expression of AIB1 can be modulated by regulatable ribozymes directed against AIB1 mRNA. We found that depletion of endogenous AIB1 levels reduced steroid hormone signaling via the estrogen receptor alpha or progesterone receptor beta on transiently transfected reporter templates. Down-regulation of AIB1 levels in MCF-7 cells did not affect estrogen-stimulated cell cycle progression but reduced estrogen-mediated inhibition of apoptosis and cell growth. Finally, upon reduction of endogenous AIB1 expression, estrogen-dependent colony formation in soft agar and tumor growth of MCF-7 cells in nude mice was decreased. From these findings we conclude that, despite the presence of different estrogen receptor coactivators in breast cancer cells, AIB1 exerts a rate-limiting role for hormone-dependent human breast tumor growth.

The human homologue of the yeast DNA repair and TFIIH regulator MMS19 is an AF-1-specific coactivator of estrogen receptor

Steroid/nuclear hormone receptors are ligand-dependent transcriptional regulators that control gene expression in a wide array of biological processes. The transcriptional activity of the receptors is mediated by an N-terminal ligand-independent transcriptional activation function AF-1 and a C-terminal ligand-dependent transcriptional activation function AF-2. The nuclear receptor coactivator RAC3 (also known as AIB1/ACTR/pCIP/TRAM-1/SRC-3) is amplified in breast cancer cells, where it forms a complex with estrogen receptor (ER) and enhances AF-2 activity of the receptor. Here, we identify a putative human homologue of the yeast DNA repair and transcriptional regulator MMS19 as a RAC3-interacting protein. The human MMS19 interacts with the N-terminal PAS-A/B domain of RAC3 in vivo and in vitro through a conserved C-terminal domain. Interestingly, the human MMS19 also interacts with estrogen receptors in a ligand-independent manner but not with retinoic acid receptor or thyroid hormone receptor. Overexpression of the interacting domain of hMMS19 strongly inhibits ER-mediated transcriptional activation, indicating a dominant negative activity. In contrast, over expression of the full-length hMMS19 enhances ER-mediated transcriptional activation. We find that hMMS19 stimulates the AF-1 activity of ERalpha, but not the AF-2 activity, suggesting that hMMS19 may be an AF-1-specific transcriptional coactivator of estrogen receptor.

Differential recruitment of the mammalian mediator subunit TRAP220 by estrogen receptors ERalpha and ERbeta

Estrogen receptors (ERs) associate with distinct transcriptional coactivators to mediate activation of target genes in response to estrogens. Previous work has provided multiple evidence for a critical role of p160 coactivators and associated histone acetyltransferases in estrogen signaling. In contrast, the involvement of the mammalian mediator complex remains to be established. Further, although the two subtypes ERalpha and ERbeta appear to be similar in regard to principles of LXXLL-mediated coactivator binding to the AF-2 activation domain, there are indications that the context-dependent transcriptional activation profiles of the two ERs can be quite distinct. Potentially, this could be attributed to differences with regard to coregulator recruitment. We have here studied the interactions of the nuclear receptor-binding subunit of the mammalian mediator complex, referred to as TRAP220, with ERalpha and ERbeta. In comparison to the p160 coactivator TIF2, we find that TRAP220 displays ERbeta preference. Here, we show that this is a feature of the binding specificity of the TRAP220 LXXLL motifs and demonstrate that the ER subtype-specific F-domain influences TRAP220 interaction. Such differences with regard to coactivator recruitment indicate that the relative importance of individual coregulators in estrogen signaling could depend on the dominant ER subtype.

Ligands Specify Coactivator Nuclear Receptor (NR) Box Affinity for Estrogen Receptor Subtypes

Nuclear receptors (NRs) require coactivators to efficiently activate transcription of their target genes. Many coactivators including the p160 proteins utilize a short NR box motif to recognize the ligand-binding domain of the NR when it is activated by ligand. To investigate the ability of various ligands to specify the affinity of NR boxes for a ligand-bound NR, we compared the capacity of p160 NR boxes to be recruited to estrogen receptor (ERα) and ERβ in the presence of 17β-estradiol, diethylstilbestrol, and genestein. A time-resolved fluorescence-based binding assay was used to determine the dissociation constants for the 10 NR boxes derived from the three p160 coactivators for both ER subtypes in the presence of the each of the agonists. While the affinity of some NR boxes for ER was independent of the agonist, we identified several NR boxes that had significantly different affinities for ER depending on which agonist was bound to the receptor. Therefore, an agonist may specify the affinity of an NR for various NR boxes and thus regulate the coactivator selectivity of the receptor.

Role of protein methylation in chromatin remodeling and transcriptional regulation

Recent findings suggest that lysine and arginine-specific methylation of histones may cooperate with other types of post-translational histone modification to regulate chromatin structure and gene transcription. Proteins that methylate histones on arginine residues can collaborate with other coactivators to enhance the activity of specific transcriptional activators such as nuclear receptors. Lysine methylation of histones is associated with transcriptionally active nuclei, regulates other types of histone modifications, and is necessary for proper mitotic cell divisions. The fact that some transcription factors and proteins involved in RNA processing can also be methylated suggests that protein methylation may also contribute in other ways to regulation of transcription and post-transcriptional steps in gene regulation. In future work, it will be important to develop methods for evaluating the precise roles of protein methylation in the regulation of native genes in physiological settings, e.g. by using chromatin immunoprecipitation assays, differentiating cell culture systems, and genetically altered cells and animals. It will also be important to isolate additional protein methyltransferases by molecular cloning and to characterize new methyltransferase substrates, the regulation of methyltransferase activities, and the roles of new methyltransferases and substrates.

Sharp, an inducible cofactor that integrates nuclear receptor repression and activation

A yeast two-hybrid screen using the conserved carboxyl terminus of the nuclear receptor corepressor SMRT as a bait led to the isolation of a novel human gene termed SHARP (SMRT/HDAC1 Associated Repressor Protein). SHARP is a potent transcriptional repressor whose repression domain (RD) interacts directly with SMRT and at least five members of the NuRD complex including HDAC1 and HDAC2. In addition, SHARP binds to the steroid receptor RNA coactivator SRA via an intrinsic RNA binding domain and suppresses SRA-potentiated steroid receptor transcription activity. Accordingly, SHARP has the capacity to modulate both liganded and nonliganded nuclear receptors. Surprisingly, the expression of SHARP is itself steroid inducible, suggesting a simple feedback mechanism for attenuation of the hormonal response.

Novel glucocorticoid receptor coactivator effector mechanisms

Glucocorticoids regulate numerous distinct physiological processes, most of which rely on the ability of the hormone-bound glucocorticoid receptor (GR) to change the expression of target genes in a cell- and promoter-dependent manner. The transcriptional activity of GR depends on coactivators that regulate transcription by remodeling chromatin or by facilitating the recruitment of the basal transcriptional machinery. Coactivators are often part of multiprotein complexes that are not specific for GR but also mediate the activity of other nuclear receptors (NRs) and unrelated transcription factors. Surprisingly, recent results reveal that the activity of coactivators might contribute to the receptor, promoter and cell specificity of NR action. The emerging picture shows coactivators as flexible, but precise, coordinators of complex and dynamic networks, in which transcriptional regulation by GR and other NRs is linked to other signaling pathways.

Control of cardiac-specific transcription by p300 through myocyte enhancer factor-2D

The transcriptional integrator p300 regulates gene expression by interaction with sequence-specific DNA-binding proteins and local remodeling of chromatin. p300 is required for cardiac-specific gene transcription, but the molecular basis of this requirement is unknown. Here we report that the MADS (MCM-1, agamous, deficiens, serum response factor) box transcription factor myocyte enhancer factor-2D (MEF-2D) acts as the principal conduit for cardiac transcriptional activation by p300. p300 activation of the native 2130-base pair human skeletal alpha-actin promoter required a single hybrid MEF-2/GATA-4 DNA motif centered at -1256 base pairs. Maximal expression of the promoter in cultured myocytes and in vivo correlated with binding of both MEF-2 and p300, but not GATA-4, to this AT-rich motif. p300 and MEF-2 were coprecipitated from cardiac nuclear extracts by an oligomer containing this element. p300 was found exclusively in a complex with MEF-2D at this and related sites in other cardiac-restricted promoters. MEF-2D, but not other MEFs, significantly potentiated cardiac-specific transcription by p300. No physical or functional interaction was observed between p300 and other factors implicated in skeletal actin transcription, including GATA-4, TEF-1, or SRF. These results show that, in the intact cell, p300 interactions with its protein targets are highly selective and that MEF-2D is the preferred channel for p300-mediated transcriptional control in the heart.

Roles of growth hormone and insulin-like growth factor 1 in mouse postnatal growth

To examine the relationship between growth hormone (GH) and insulin-like growth factor 1 (IGF1) in controlling postnatal growth, we performed a comparative analysis of dwarfing phenotypes manifested in mouse mutants lacking GH receptor, IGF1, or both. This genetic study has provided conclusive evidence demonstrating that GH and IGF1 promote postnatal growth by both independent and common functions, as the growth retardation of double Ghr/Igf1 nullizygotes is more severe than that observed with either class of single mutant. In fact, the body weight of these double-mutant mice is only approximately 17% of normal and, in absolute magnitude ( approximately 5 g), only twice that of the smallest known mammal. Thus, the growth control pathway in which the components of the GH/IGF1 signaling systems participate constitutes the major determinant of body size. To complement this conclusion mainly based on extensive growth curve analyses, we also present details concerning the involvement of the GH/IGF1 axis in linear growth derived by a developmental study of long bone ossification in the mutants.

Regulation of somatic growth by the p160 coactivator p/CIP

A family of p160 coactivators was initially identified based on ligand-dependent interactions with nuclear receptors and thought to function, in part, by recruiting CREB-binding protein/p300 to several classes of transcription factors. One of the p160 factors, p/CIP/AIB1, often amplified and overexpressed in breast cancer, also exhibits particularly strong interaction with CREB-binding protein/p300. In this manuscript, we report that p/CIP, which exhibits regulated transfer from cytoplasm to nucleus, is required for normal somatic growth from embryonic day 13.5 through maturity. Our data suggest that a short stature phenotype of p/CIP gene-deleted mice reflect both altered regulation of insulin-like growth factor-1 (IGF-1) gene expression in specific tissues and a cell-autonomous defect of response to IGF-1, including ineffective transcriptional activities by several classes of regulated transcription factors under specific conditions. The actions of p/CIP are therefore required for full expression of a subset of genes critical for regulating physiological patterns of somatic growth in mammals.

The nuclear receptor co-repressor nrip1 (RIP140) is essential for female fertility

Ovulatory dysfunction is the commonest cause of female infertility. Here we show that the co-repressor nuclear-receptor-interacting protein 1 (Nrip1; encoded by the gene Nrip1) is essential for ovulation. Mice null for this protein are viable, but female mice are infertile because of complete failure of mature follicles to release the oocyte at ovulation. In contrast, luteinization proceeds normally, resulting in a phenotype closely resembling that of luteinized unruptured follicle syndrome, often associated with infertility in women. Therefore, whereas the pre-ovulatory surge of luteinizing hormone induces both ovulation and luteinization, the ability to suppress the action of nuclear receptors is essential for the coordinated control of ovarian function with the essential process of oocyte release dependent on the activity of the transcriptional co-repressor Nrip1 (RIP40).

Endogenously expressed estrogen receptor and coactivator AIB1 interact in MCF-7 human breast cancer cells

Coactivators are believed to mediate estrogen-induced gene responses via interaction with estrogen receptors (ER). Currently, a major challenge is to determine the importance of each coactivator in a specific cell type and promoter context in response to a particular ligand. The potential of ER to interact with a growing list of coactivators has been shown in a variety of in vitro and gene transfer assays, yet very few data have demonstrated the interaction of endogenous coactivators with ER in intact cells. We report here a ligand-specific interaction of endogenous human ER (hER) and the AIB1 coactivator in MCF-7 human breast cancer cells by using immunoprecipitation analyses. Complexes between endogenously expressed hER and AIB1 were detected in estradiol-treated cells and to a much lesser extent in cells treated with the partial agonist, monohydroxytamoxifen. We were unable to detect an hER-SRC-1 complex in our immunoprecipitations from MCF-7 cells. The in vitro-binding affinity for mouse ER interaction with AIB1 was estimated to be 40-120 nM. We conclude that AIB1 is a major coactivator for hER in MCF-7 human breast cancer cells.

The role of coactivators and corepressors in the biology and mechanism of action of steroid hormone receptors

Steroid hormone receptors are members of a superfamily of ligand-dependent transcription factors. As such they have a DNA binding domain that recognizes specific target gene sequences along with separate transcriptional activation domains. What sets steroid hormone receptors (and other nuclear hormone receptors) apart from other families of sequence specific transcriptional activators is the presence of a ligand binding domain (LBD) that acts as a molecular switch to turn on transcriptional activity when a hormonal ligand induces a conformational change in the receptor. Upon binding hormone, steroid receptors recruit a novel coactivator protein complex with an essential role in receptor-mediated transcriptional activation. Coactivators function as adaptors in a signaling pathway that transmits transcriptional responses from the DNA bound receptor to the basal transcriptional machinery. Hormone agonists induce a conformational change in the carboxyl-terminal transcriptional activation domain, AF-2, that creates a new protein interaction site on the surface of the LBD that is recognized by LXXLL motifs in the p160 family of coactivators. In contrast, steroid antagonists such as the antiestrogen tamoxifen for the estrogen receptor induce an alternate conformation in AF-2 that occludes the coactivator binding site and recruits corepressors that can actively silence steroid responsive genes. Thus, the cellular availability of coactivators and corepressors is an important determinant in the biological response to both steroid hormone agonists and antagonists. This paper provides an update on the properties and mechanism of action of nuclear receptor coactivators, the nature of the coactivator-binding site, and the structural and mechanistic basis for ligand-dependent binding of coactivators to receptors.

Estrogen receptor alpha in human breast cancer: occurrence and significance

Estrogens have long been recognized as being important for stimulating the growth of a large proportion of breast cancers. Now it is recognized that estrogen action is mediated by two receptors, and the presence of estrogen receptor alpha (ER alpha) correlates with better prognosis and the likelihood of response to hormonal therapy. Over half of all breast cancers overexpress ER alpha and around 70% of these respond to anti-estrogen (for example tamoxifen) therapy. In addition, the presence of elevated levels of ER alpha in benign breast epithelium appears to indicate an increased risk of breast cancer, suggesting a role for ER alpha in breast cancer initiation, as well as progression. However, a proportion of ER alpha-positive tumors does not respond to endocrine therapy and the majority of those that do respond eventually become resistant. Most resistant tumors remain ER alpha-positive and frequently respond to alternative endocrine treatment, indicative of a continued role for ER alpha in breast cancer cell proliferation. The problem of resistance has resulted in the search for and the development of diverse hormonal therapies designed to inhibit ER alpha action, while research on the mechanisms which underlie resistance has shed light on the cellular mechanisms, other than ligand binding, which control ER alpha function.