Role Of Corepressors Research Articles

Steroid-receptor coactivator complexes in thyroid hormone-regulation of Xenopus metamorphosis.

Anuran metamorphosis is perhaps the most drastic developmental change regulated by thyroid hormone (T3) in vertebrate. It mimics the postembryonic development in mammals when many organs/tissues mature into adult forms and plasma T3 level peaks. T3 functions by regulating target gene transcription through T3 receptors (TRs), which can recruit corepressor or coactivator complexes to target genes in the absence or presence of T3, respectively. By using molecular and genetic approaches, we and others have investigated the role of corepressor or coactivator complexes in TR function during the development of two highly related anuran species, the pseudo-tetraploid Xenopus laevis and diploid Xenopus tropicalis. Here we will review some of these studies that demonstrate a critical role of coactivator complexes, particularly those containing steroid receptor coactivator (SRC) 3, in regulating metamorphic rate and ensuring the completion of metamorphosis.

Epigeneitc silencing of ribosomal RNA genes by Mybbp1a

BackgroundTranscription of the ribosomal RNA gene repeats by Pol I occurs in the nucleolus and is a fundamental step in ribosome biogenesis and protein translation. Due to tight coordination between ribosome biogenesis and cell proliferation, transcription of rRNA and stable maintenance of rDNA clusters are thought to be under intricate control by intercalated mechanisms, particularly at the epigenetic level.Methods and ResultsHere we identify the nucleolar protein Myb-binding protein 1a (Mybbp1a) as a novel negative regulator of rRNA expression. Suppression of rDNA transcription by Mybbp1a was linked to promoter regulation as illustrated by its binding to the chromatin around the hypermethylated, inactive rDNA gene promoters. Our data further showed that downregulation of Mybbp1a abrogated the local DNA methylation levels and histone marks associated with gene silencing, and altered the promoter occupancy of various factors such UBF and HDACs, consequently leading to elevated rRNA expression. Mechanistically, we propose that Mybbp1a maintains rDNA repeats in a silenced state while in association with the negative epigenetic modifiers HDAC1/2.ConclusionsResults from our present work reveal a previously unrecognized co-repressor role of Mybbp1a in rRNA expression. They are further consistent with the scenario that Mybbp1a is an integral constituent of the rDNA epigenetic regulation that underlies the balanced state of rDNA clusters.

Regulation of mouse-renin gene by apurinic/apyrimidinic-endonuclease 1 (APE1/Ref-1) via recruitment of histone deacetylase 1 corepressor complex

Apurinic/apyrimidinic-endonuclease 1 (APE1) heterozygous mice have chronically elevated blood pressure. Renin of the renin-angiotensin (ANG) system for blood pressure maintenance regulates production of ANG II, a vasoactive hormone. Renin expression and secretion from kidney juxtaglomerular cells are regulated by intracellular calcium. Our objective in this study is to investigate APE1's regulatory role in renin expression. Effect of APE1 on calcium-mediated modulation of renin expression was examined by real-time reverse transcriptase-PCR, Western analysis and renin promoter-dependent luciferase activity in APE1-knockdown, APE1-overexpressing or control mouse kidney As4.1 cells. Furthermore, coimmunoprecipitation and chromatin immunoprecipitation assays were utilized to examine the association of APE1 with histone deacetylase (HDAC)1 corepressor complex and their recruitment to renin enhancer. Finally, kidney renin mRNA level and plasma-renin activity were measured in wild-type and APE1-heterozygous mice. Here we show that APE1 is involved in calcium-mediated repression of renin gene. Our results further indicate that APE1 is a component of HDAC1 corepressor complex bound to renin-enhancer region. Increase in intracellular calcium ion concentration enhances the association of APE1 with HDAC1 corepressor complex and their recruitment to the enhancer region. Furthermore, APE1's N-terminal region is critical for formation and recruitment of the enhancer-bound corepressor complex. Increased renin expression in kidneys and higher plasma-renin activity in APE1 heterozygous mice further supports APE1's corepressor role in vivo. This study uncovers APE1's function as a novel negative regulator of renin expression, and thereby in blood pressure maintenance.

Integrative Genomics Identifies the Corepressor SMRT as a Gatekeeper of Adipogenesis through the Transcription Factors C/EBPβ and KAISO

The molecular role of corepressors is poorly understood. Here, we studied the transcriptional function of the corepressor SMRT during terminal adipogenesis. Genome-wide DNA-binding profiling revealed that this corepressor is predominantly located in active chromatin regions and that most distal SMRT binding events are lost after differentiation induction. Promoter-proximal tethering of SMRT in preadipocytes is primarily mediated by KAISO through the conserved TCTCGCGAGA motif. Further characterization revealed that KAISO, similar to SMRT, accelerates the cell cycle and increases fat accumulation upon knockdown, identifying KAISO as an adipogenic repressor that likely modulates the mitotic clonal expansion phase of this process. SMRT-bound promoter-distal sites tend to overlap with C/EBPβ-bound regions, which become occupied by proadipogenic transcription factors after SMRT clearance. This reveals a role for SMRT in masking enhancers from proadipogenic factors in preadipocytes. Finally, we identified SMRT as an adipogenic gatekeeper as it directly fine-tunes transcription of pro- and antiadipogenic genes.

Abstract 2223: Epigenetic silencing of ribosomal RNA genes by Mybbp1a

Abstract Transcription of the ribosomal RNA gene repeats by Pol I occurs in the nucleolus and is a fundamental step in ribosome biogenesis and protein translation. Due to tight coordination between ribosome biogenesis and cell proliferation, transcription of rRNA and stable maintenance of rDNA clusters are thought to be under intricate control by intercalated mechanisms, particularly at the epigenetic level. Here we identify the nucleolar protein Myb-binding protein 1a (Mybbp1a) as a novel negative regulator of rRNA expression. Suppression of rDNA transcription by Mybbp1a was linked to promoter regulation as illustrated by its binding to the chromatin around the hypermethylated, inactive rDNA gene promoters. Our data further showed that downregulation of Mybbp1a abrogated the local DNA methylation levels and histone marks associated with gene silencing, and altered the promoter occupancy of various factors such UBF and HDACs, consequently leading to elevated rRNA expression. Mechanistically, we propose that Mybbp1a maintains rDNA repeats in a silenced state while in association with the negative epigenetic modifiers HDAC1/2. Results from our present work reveal a previously unrecognized co-repressor role of Mybbp1a in rRNA expression. They are further consistent with the scenario that Mybbp1a is an integral constituent of the rDNA epigenetic regulation that underlies the balanced state of rDNA clusters. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2223. doi:1538-7445.AM2012-2223

Epigenetic silencing of myogenic gene program by Myb-binding protein 1a suppresses myogenesis

Skeletal myogenesis involves highly coordinated steps that integrate developmental cues at the chromatin of muscle progenitors. Here, we identify Myb-binding protein 1a (Mybbp1a) as a novel negative regulator of muscle-specific gene expression and myoblast differentiation. The mode of action of Mybbp1a was linked to promoter regulation as illustrated by its interaction with MyoD at the genomic regions of silent muscle-specific genes as well as its negative effect on MyoD-mediated transcriptional activity. We propose that Mybbp1a exerts its repressive role by inducing a less permissible chromatin structure following recruitment of negative epigenetic modifiers such as HDAC1/2 and Suv39h1. At the onset of differentiation, Mybbp1a undergoes a promoter disengagement that may be due to the differentiation-responsive, miR-546-mediated downregulation of Mybbp1a expression. Moreover, such alteration gave rise to promoter enrichment of activators and histone acetylation, an epigenetic status amenable to gene activation. Together, these findings unveil a hitherto unrecognized transcriptional co-repressor role of Mybbp1a in proliferating muscle progenitor cells, and highlight an epigenetic mechanism by which Mybbp1a and miR-546 interplay to control myoblast differentiation transition.

Thyroid hormone receptor β1 domains responsible for the antagonism with the ras oncogene: role of corepressors

The thyroid hormone receptor (TR) is a suppressor of ras-mediated responses. To characterize the receptor domains involved in this function, we analyzed a panel of TRβ1 mutants for their ability to interfere with ras-driven cyclin D1 activation, formation of transformation foci and tumor growth in nude mice. Our results show that the domains and mechanisms responsible for the anti-transforming and anti-tumorigenic actions of the receptor are divergent from those operating in classical T3-dependent transcriptional activation. TRβ1 mutants that do not bind coactivators and do not transactivate retained the capacity of suppressing cellular transformation and tumor growth, whereas selective mutations in the hinge region affecting corepressors recruitment abolished these actions, while preserving ligand-dependent transcription. There was a strict parallelism between anti-transforming activity of the various mutants and their ability to antagonize cyclin D1 stimulation by ras, indicating that transrepression mechanisms may have an important function in suppression of the transforming effects of the oncogene by TRβ1. The inhibitory action of T3 on transformation was further enhanced after over-expression of corepressors, while corepressor depletion by means of small-interference RNA reversed significantly hormonal action. This shows an important functional role of endogenous corepressors in suppression of ras-mediated transformation and tumorigenesis by TRβ1.

The Interaction between Nuclear Receptor Corepressor and Histone Deacetylase 3 Regulates Both Positive and Negative Thyroid Hormone Action in Vivo

Thyroid hormone (TH) plays a critical role in development, growth, and metabolism by binding to nuclear TH receptors to modulate gene expression. In the absence of TH, TH receptors repress genes that are TH-activated by recruiting the nuclear receptor corepressor (NCoR), which exists in a tight complex with histone deacetylase 3 (HDAC3). Here we explored the actions of TH in the deacetylase activating domain mutant (DADm) mouse, whose NCoR-HDAC3 interaction is genetically disrupted. Several TH-activated genes were derepressed in the liver of euthyroid and hypothyroid DADm mice, consistent with the corepressor paradigm and a critical role of the NCoR-HDAC3 interaction in basal repression. The role of corepressors in genes that are down-regulated by TH is less well understood. Remarkably, circulating TSH levels were increased in euthyroid DADm mice, and the pituitary expression of TSHalpha, a classic TH-down-regulated gene, was modestly but significantly elevated regardless of TH status. Thus, the NCoR interaction with HDAC3 modulates expression of both positively- and negatively-regulated genes by TH in vivo.

The Interaction between Nuclear Receptor Corepressor and Histone Deacetylase 3 Regulates Both Positive and Negative Thyroid Hormone Actionin Vivo

Thyroid hormone (TH) plays a critical role in development, growth, and metabolism by binding to nuclear TH receptors to modulate gene expression. In the absence of TH, TH receptors repress genes that are TH-activated by recruiting the nuclear receptor corepressor (NCoR), which exists in a tight complex with histone deacetylase 3 (HDAC3). Here we explored the actions of TH in the deacetylase activating domain mutant (DADm) mouse, whose NCoR-HDAC3 interaction is genetically disrupted. Several TH-activated genes were derepressed in the liver of euthyroid and hypothyroid DADm mice, consistent with the corepressor paradigm and a critical role of the NCoR-HDAC3 interaction in basal repression. The role of corepressors in genes that are down-regulated by TH is less well understood. Remarkably, circulating TSH levels were increased in euthyroid DADm mice, and the pituitary expression of TSHalpha, a classic TH-down-regulated gene, was modestly but significantly elevated regardless of TH status. Thus, the NCoR interaction with HDAC3 modulates expression of both positively- and negatively-regulated genes by TH in vivo.

Cooperative Activation of Cyclin D1 and Progesterone Receptor Gene Expression by the SRC-3 Coactivator and SMRT Corepressor

Although the ability of coactivators to enhance the expression of estrogen receptor-alpha (ERalpha) target genes is well established, the role of corepressors in regulating 17beta-estradiol (E2)-induced gene expression is poorly understood. Previous studies revealed that the silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) corepressor is required for full ERalpha transcriptional activity in MCF-7 breast cancer cells, and we report herein the E2-dependent recruitment of SMRT to the regulatory regions of the progesterone receptor (PR) and cyclin D1 genes. Individual depletion of SMRT or steroid receptor coactivator (SRC)-3 modestly decreased E2-induced PR and cyclin D1 expression; however, simultaneous depletion revealed a cooperative effect of this coactivator and corepressor on the expression of these genes. SMRT and SRC-3 bind directly in an ERalpha-independent manner, and this interaction promotes E2-dependent SRC-3 binding to ERalpha measured by co-IP and SRC-3 recruitment to the cyclin D1 gene as measured by chromatin IP assays. Moreover, SMRT stimulates the intrinsic transcriptional activity of all of the SRC family (p160) coactivators. Our data link the SMRT corepressor directly with SRC family coactivators in positive regulation of ERalpha-dependent gene expression and, taken with the positive correlation found for SMRT and SRC-3 in human breast tumors, suggest that SMRT can promote ERalpha- and SRC-3-dependent gene expression in breast cancer.

The Nuclear Receptor Corepressor Has Organizational Effects within the Developing Amygdala on Juvenile Social Play and Anxiety-Like Behavior

Nuclear receptor function on DNA is regulated by the balanced recruitment of coregulatory complexes. Recruited proteins that increase gene expression are called coactivators, and those that decrease gene expression are called corepressors. Little is known about the role of corepressors, such as nuclear receptor corepressor (NCoR), on the organization of behavior. We used real-time PCR to show that NCoR mRNA levels are sexually dimorphic, that females express higher levels of NCoR mRNA within the developing amygdala and hypothalamus, and that NCoR mRNA levels are reduced by estradiol treatment. To investigate the functional role of NCoR on juvenile social behavior, we infused small interfering RNA targeted against NCoR within the developing rat amygdala and assessed the enduring impact on juvenile social play behavior, sociability, and anxiety-like behavior. As expected, control males exhibited higher levels of juvenile social play than control females. Reducing NCoR expression during development further increased juvenile play in males only. Interestingly, decreased NCoR expression within the developing amygdala had lasting effects on increasing juvenile anxiety-like behavior in males and females. These data suggest that the corepressor NCoR functions to blunt sex differences in juvenile play behavior, a sexually dimorphic and hormone-dependent behavior, and appears critical for appropriate anxiety-like behavior in juvenile males and females.

Impact of SNPs in ERα corepressor SMRT on breast cancer risk and response to endocrine therapy.

Abstract Abstract #1043 Approximately 1/3 of hormone dependent breast cancers do not respond to tamoxifen treatment. The fact that these tumors are intrinsically resistant suggests that underlying genetic factors are responsible for this phenotype. The two major types of genetic variation are single nucleotide polymorphisms (SNPs) and copy number variations (CNVs). SNPs are the most common form of genetic alteration with an estimated 10 million SNPs across the genome and likely influence both disease susceptibility and response to therapy.
 Traditionally, researchers have attempted to identify SNPs associated with a clinical outcome in a “top-down” approach by conducting linkage and SNP association studies. However, here we report on a “bottom-up” approach whereby SNPs are first tested in vitro for a functional effect. This awards two benefits over a “top-down” approach: 1) identification of functional SNPs and 2) detailed characterization of the SNP so that the clinical association studies can be designed accordingly.
 Tamoxifen acts by blocking estradiol (E2) from binding to the estrogen receptor (ERα) and by recruiting corepressors, which then actively repress gene transcription. The critical role of corepressors, especially SMRT, in the proper action of tamoxifen makes them likely to be involved in tamoxifen resistance. Thus, we used a “bottom-up” approach to identify SNPs in ERα corepressors which affect the ability of tamoxifen to suppress ERα function.
 Here we report the detailed in vitro characterization of a number of SNPs in SMRT (NCoR2) with respect to altered ERα function in the presence of tamoxifen. Further, these SNPs are currently being genotyped so that associations can be made in a population of women receiving tamoxifen as either preventative or adjuvant therapy in which extensive surrogate markers for ERα function were collected during the 1-year follow (hot flash rate, hot flash intensity, lipid profiles, and bone mineral density).
 Through the discovery of common polymorphisms affecting Tamoxifen response, we hope to identify new diagnostic criteria to predict which patients will respond to anti-estrogen therapy and which will be better candidates for another line of therapy, such as aromatase inhibitors. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 1043.

Inhibition of MAPK-signaling pathway promotes the interaction of the corepressor SMRT with the human androgen receptor and mediates repression of prostate cancer cell growth in the presence of antiandrogens

Prostate cancer is one of the most prominent malignancies of elderly males. The growth of normal prostate and prostate cancer (PCa) cells depend on functional androgen receptor (AR), a ligand controlled transcription factor and member of the nuclear receptor superfamily. Binding of agonistic ligand enhances the transactivation function of AR and hence promotes the growth of prostate epithelial cells. We have earlier shown that AR antagonistic ligands such as cyproterone acetate (CPA) promote the recruitment of transcriptional corepressors such as silencing mediator of retinoid and thyroid receptor (SMRT) leading to repression of AR transactivation in non-PCa cells. Unfortunately, however, in LNCaP PCa cells, CPA functions as an agonist and thereby increases AR transactivation function. Here, we show that activated MEK signaling cascade inhibits functional recruitment of corepressor SMRT to CPA-bound AR in PCa cells. Chemical blockade of MEK kinase using a specific inhibitor U0126 increases the interaction and hence repression of AR by the corepressor SMRT in LNCaP PCa cells. This inhibition also results in enhanced antagonistic behavior of CPA as assessed by reporter and cell-growth assays. Moreover, the growth of LNCaP cells stably overexpressing SMRT was more robustly inhibited in the presence of CPA and U1026. In line with this, the growth rate of LNCaP cells was decelerated in the presence of both CPA and U0126. This suggests that activated MEK signaling pathway attenuates the functional recruitment of corepressor SMRT to AR induced by antagonists and thus indicates the important role of corepressors in mediating repression of both AR transactivation and PCa cell growth by antagonists. Furthermore, these findings suggest that combining receptor antagonists with signaling inhibitors could be a beneficial approach for PCa treatment.

Suppressive effect of aryl hydrocarbon receptor repressor on transcriptional activity of estrogen receptor alpha by protein–protein interaction in stably and transiently expressing cell lines

Aryl hydrocarbon receptor repressor (AhRR) suppressed, in a ligand independent manner, the ability of estrogen receptor alpha (ERalpha) to enhance the transcription of heterologous estrogen-responsive reporter plasmids in transient transfection assays, as well as of endogenous estrogen-responsive genes in human breast cancer MCF-7 cells. AhRR repressed ERalpha-mediated trans-activation by interfering allosterically with the ligand-independent function of AF-1. The direct interaction between AhRR and ERalpha at the multipartite binding site of ERalpha, which ranges from a DNA binding domain to a ligand binding domain, but did not include the AF-1 moiety was confirmed by a coimmunoprecipitation assay. The AhRR/ERalpha complex was formed in the nuclear compartment and was entrapped by a cis-element in the promoter of E2-responsive genes, as determined in a chromatin immunoprecipitation assay. AhRR might play a role of co-repressor on the transcriptional activity of the ERalpha homodimer.

CtIP Activates Its Own and Cyclin D1 Promoters via the E2F/RB Pathway during G1/S Progression

Cell cycle progression from G(1) to S phase is mainly controlled by E2F transcription factors and RB family proteins. Previously we showed that the presence of CtIP is essential for G(1)/S transition in primary mouse blastocysts, as well as in NIH 3T3 cells. However, how CtIP executes this function remains to be elucidated. Here we show that in NIH 3T3 cells the expression of CtIP is regulated by the E2F/RB pathway during late G(1) and S phases. The presence of wild-type CtIP, but not the E157K mutant form, which failed to interact with RB, enhanced its own promoter activity. Chromatin immunoprecipitation analysis indicated that the recruitment of CtIP to its promoter occurs concomitantly with TFIIB, a component of the RNA polymerase II complex, and with dissociation of RB from the promoter during late G(1) and G(1)/S transition. Similar positive regulation of cyclin D1 expression by CtIP was also observed. Consistently, cells expressing the CtIP(E157K) protein alone exhibited growth retardation, an increase in the G(1) population, and a decrease in the S-phase population. Taken together, these results suggest that, contrary to the postulated universal corepressor role, CtIP activates a subset of E2F-responsive promoters by releasing RB-imposed repression and therefore promotes G(1)/S progression.

Estrogen Receptor Corepressors –Mechanism of Action and Role in Breast Tumorigenesis

Estrogen receptor α (ERα) is a critical player in normal breast development and breast cancer progression and disease. Its activity is not only regulated by ligands, but also through interaction with cofactor proteins termed coactivators and corepressors. There is increasing evidence that coactivators can function as oncogenes in breast cancer, however little is known about the role of corepressors.

The Book of Opposites: The Role of the Nuclear Receptor Co-regulators in the Suppression of Epidermal Genes by Retinoic Acid and Thyroid Hormone Receptors

Transcriptional regulation by nuclear receptors occurs through complex interactions that involve DNA response elements, co-activators/co-repressors, and histone modifying enzymes. Very little is known about how molecular interplay of these components may determine tissue specificity of hormone action. We have shown previously that retinoic acid (RA) and thyroid hormone (T3) repress transcription of a specific group of epidermal keratin genes through a novel mechanism that utilizes receptors homodimers. In this paper, we have analyzed the epidermal specificity of RA/T3 action by testing the role of co-repressors and co-activators in regulation of epidermal genes. Using transient co-transfections, northern blots, antisense oligonucleotides, and a histone deacetylase (HDAC) inhibitor, trichostatin A, we found that in the context of specific keratin RE (KRE), co-activators and histone acetylase become co-repressors of the RA/T3 receptors in the presence of their respective ligands. Conversely, co-repressors and HDAC become co-activators of unliganded T3Ralpha. The receptor-co-activator interaction is intact and occurs through the NR-box. Therefore, the role of co-activator is to associate with liganded receptors whereas the KRE-receptor interaction determines specific transcriptional signal, in this case repression. This novel molecular mechanism of transcriptional repression conveys how RA and T3 target specific groups of epidermal genes, thus exerting intrinsic tissue specificity.

Gene repression by nuclear hormone receptors.

Repression by nuclear hormone receptors (NHRs) plays an important role in development, immune response and cellular function. We review mechanisms of how NHRs act as repressors of gene transcription either by direct contact with basal transcription factors or through recruitment of cofactors and enzymic activities that modulate chromatin accessibility. We describe also the role and biochemical mechanism of the cognate hormone that switches a NHR from a transcriptional silencer into an activator. This includes data from crystal structure, functional receptor domain analyses and the role of co-repressors in chromatin modification and remodelling. Furthermore, the comparison of negative response elements with classical response elements unravels the role of co-repressors in this context. We also describe the inhibition of the nuclear factor kappaB and Jun/Fos pathway by NHRs, as well as the molecular mechanism of anti-hormone therapies. Anti-hormones are commonly used in breast and prostate cancer therapy to inhibit cancer proliferation through repression of the oestrogen or androgen receptor, respectively. Here we provide a comprehensive overview of the various mechanism of NHR repression.

The role of corepressors in transcriptional regulation by nuclear hormone receptors.

Nuclear receptors (also known as nuclear hormone receptors) are hormone-regulated transcription factors that control many important physiological and developmental processes in animals and humans. Defects in receptor function result in disease. The diverse biological roles of these receptors reflect their surprisingly versatile transcriptional properties, with many receptors possessing the ability to both repress and activate target gene expression. These bipolar transcriptional properties are mediated through the interactions of the receptors with two distinct classes of auxiliary proteins: corepressors and coactivators. This review focuses on how corepressors work together with nuclear receptors to repress gene transcription in the normal organism and on the aberrations in this process that lead to neoplasia and endocrine disorders. The actions of coactivators and the contributions of the same corepressors to the functions of nonreceptor transcription factors are also touched on.

Estrogen receptor corepressors -- a role in human breast cancer?

Estrogen receptor alpha (ERalpha) has an established role in promoting breast cancer. Transcriptional activation by ERalpha is a complex and multistep process, and it is influenced by coactivator and corepressor proteins that can either positively or negatively modulate ERalpha-mediated transcriptional activity. Corepressors are proposed to provide a counterbalance to the estrogen-induced transactivation, and represent a potential mechanism employed by the cell to regulate hormonal responses. In this review, we present evidence from tissue culture, animal and clinical studies, supporting the hypothesis that corepressors are crucial regulators of ERalpha-mediated action, and that their loss could promote breast cancer development and resistance to endocrine therapy. We propose that ERalpha corepressors play an important biological role by controlling the magnitude of the estrogen response, mediating antiestrogen inhibition of ERalpha, repressing DNA-bound ERalpha in the absence of the ligand, and conferring active repression of ERalpha-downregulated genes. Different ERalpha corepressors regulate steroid receptor activity through a variety of mechanisms, including formation of multiprotein complexes that are able to affect chromatin remodeling, histone deacetylation, or basal transcription. Other mechanisms include competition with coactivators, interference with DNA binding and ERalpha homodimerization, alteration of ERalpha stability, sequestration of ERalpha in the cytoplasm, and effects on RNA processing. Most ERalpha corepressors can control the receptor's activity through more than one mechanism, and it is possible that the synergy between different pathways cooperates to fully inhibit ERalpha transcriptional activity, and create an integrated response to a variety of different cellular signaling pathways. We will discuss the role of corepressors in tumor suppression and the link they might present between ERalpha regulation and DNA repair. Finally, we will discuss major challenges in the field and speculate on the exciting findings that await us in the next few years.