Regulation of SMRT and N-CoR corepressor function

CK2-NCoR signaling cascade promotes prostate tumorigenesis

The aberrant expressions of casein kinase 2 (CK2) was found in prostate cancer patient and cell lines, but little is known of the detailed mechanisms implicated in prostate tumorigenesis. In this study, we report that both CK2 activity and CK2-mediated NCoR phosphorylation are significantly elevated in the androgen-independent prostate cancer cell line DU145 and PC-3 compared with RWPE1 and LNCaP cells. Increased phosphorylation inversely correlates with the mRNA level of the NCoR-regulated gene, interferon-γ-inducible protein 10 (IP-10). CK2 inhibition abrogated NCoR phosphorylation, IP-10 transcriptional repression, and the invasion activity of PC-3 cells. Inhibition of the CK2-NCoR network significantly reduced in vivo PC-3 cell tumorigenicity, likely due to transcriptional derepression of IP-10. Clinicopathological analyses revealed that increased CK2-mediated NCoR phosphorylation significantly correlates with poor survival among prostate cancer patients. These findings elucidate a CK2-modulated oncogenic cascade in prostate tumorigenesis.

Identification of cellular proteins that interact with human cytomegalovirus immediate-early protein 1 by protein array assay

Human cytomegalovirus (HCMV) gene expression during infection is characterized as a sequential process including immediate-early (IE), early (E), and late (L)-stage gene expression. The most abundantly expressed gene at the IE stage of infection is the major IE (MIE) gene that produces IE1 and IE2. IE1 has been the focus of study because it is an important protein, not only for viral gene expression but also for viral replication. It is believed that IE1 plays important roles in viral gene regulation by interacting with cellular proteins. In the current study, we performed protein array assays and identified 83 cellular proteins that interact with IE1. Among them, seven are RNA-binding proteins that are important in RNA processing; more than half are nuclear proteins that are involved in gene regulations. Tumorigenesis-related proteins are also found to interact with IE1, implying that the role of IE1 in tumorigenesis might need to be reevaluated. Unexpectedly, cytoplasmic proteins, such as Golgi autoantigen and GGA1 (both related to the Golgi trafficking protein), are also found to be associated with IE1. We also employed a coimmunoprecipitation assay to test the interactions of IE1 and some of the proteins identified in the protein array assays and confirmed that the results from the protein array assays are reliable. Many of the proteins identified by the protein array assay have not been previously reported. Therefore, the functions of the IE1-protein interactions need to be further explored in the future.

Protein kinase A phosphorylates NCoR to enhance its nuclear translocation and repressive function in human prostate cancer cells

Protein kinase A (PKA) phosphorylates diverse protein substrates to modulate their function. In this study, we found that PKA specifically phosphorylates the RD1 (repression domain 1) domain of nuclear receptor corepressor (NCoR). We demonstrated that the Serine-70 of NCoR is identified the critical amino acid for PKA-dependent NCoR phosphorylation. Importantly, we found that PKA-dependent phosphorylation enhances the nuclear translocation of NCoR. More importantly, the activation of PKA enhanced the repressive activity of NCoR in a reporter assay and potentiated the antagonist activity in the androgen receptor (AR)-mediated transcription. Taken together, these results uncover a regulatory mechanism by which PKA positively modulates NCoR function in transcriptional regulation in prostate cancer.

GEI-8, a homologue of vertebrate nuclear receptor corepressor NCoR/SMRT, regulates gonad development and neuronal functions in Caenorhabditis elegans

NCoR and SMRT are two paralogous vertebrate proteins that function as corepressors with unliganded nuclear receptors. Although C. elegans has a large number of nuclear receptors, orthologues of the corepressors NCoR and SMRT have not unambiguously been identified in Drosophila or C. elegans. Here, we identify GEI-8 as the closest homologue of NCoR and SMRT in C. elegans and demonstrate that GEI-8 is expressed as at least two isoforms throughout development in multiple tissues, including neurons, muscle and intestinal cells. We demonstrate that a homozygous deletion within the gei-8 coding region, which is predicted to encode a truncated protein lacking the predicted NR domain, results in severe mutant phenotypes with developmental defects, slow movement and growth, arrested gonadogenesis and defects in cholinergic neurotransmission. Whole genome expression analysis by microarrays identified sets of de-regulated genes consistent with both the observed mutant phenotypes and a role of GEI-8 in regulating transcription. Interestingly, the upregulated transcripts included a predicted mitochondrial sulfide:quinine reductase encoded by Y9C9A.16. This locus also contains non-coding, 21-U RNAs of the piRNA class. Inhibition of the expression of the region coding for 21-U RNAs leads to irregular gonadogenesis in the homozygous gei-8 mutants, but not in an otherwise wild-type background, suggesting that GEI-8 may function in concert with the 21-U RNAs to regulate gonadogenesis. Our results confirm that GEI-8 is the orthologue of the vertebrate NCoR/SMRT corepressors and demonstrate important roles for this putative transcriptional corepressor in development and neuronal function.

Regulation of SMRT corepressor dimerization and composition by MAP kinase phosphorylation

The SMRT (Silencing Mediator of Retinoid and Thyroid hormone receptors) corepressor mediates gene repression by nuclear receptors and other transcriptional factors. The SMRT protein serves as a key nucleating core that organizes the assembly of a larger corepressor complex. We report here that SMRT interacts with itself to form a protein dimer, and that Erk2, a mitogen-activated protein (MAP) kinase, disrupts this SMRT self-dimerization in vitro and in vivo. Notably Erk2 phosphorylation also results in a re-organization of the overall corepressor complex, characterized by a reduced sedimentation coefficient, partial release of HDAC3, TBL-1, and TBLR-1, and inhibition of transcriptional repression. We propose that SMRT dimers form the central platform on which additional corepressor components assemble, and that kinase signaling modifies the architecture, composition, and function of this complex. These observations contribute to our understanding of how the SMRT corepressor complex assembles and is regulated during cell proliferation and differentiation.

Estrogen receptors recruit SMRT and N-CoR corepressors through newly recognized contacts between the corepressor N terminus and the receptor DNA binding domain

Estrogen receptors (ERs) are hormone-regulated transcription factors that regulate key aspects of reproduction and development. ERs are unusual in that they do not typically repress transcription in the absence of hormone but instead possess otherwise cryptic repressive functions that are revealed upon binding to certain hormone antagonists. The roles of corepressors in the control of these aspects of ER function are complex and incompletely understood. We report here that ERs recruit SMRT through an unusual mode of interaction involving multiple contact surfaces. Two surfaces of SMRT, located at the N- and C-terminal domains, contribute to the recruitment of the corepressor to ERs in vitro and are crucial for the corepressor modulation of ER transcriptional activity in cells. These corepressor surfaces contact the DNA binding domain of the receptor, rather than the hormone binding domain previously elucidated for other corepressor/nuclear receptor interactions, and are modulated by the ER's recognition of cognate DNA binding sites. Several additional nuclear receptors, and at least one other corepressor, N-CoR, share aspects of this novel mode of corepressor recruitment. Our results highlight a molecular mechanism that helps explain several previously paradoxical aspects of ER-mediated transcriptional antagonism, which may have a broader significance for an understanding of target gene repression by other nuclear receptors.

Silencing mediator of retinoic acid and thyroid hormone receptor regulates enhanced activation of signal transducer and activator of transcription 3 by epstein-barr virus-derived epstein-barr nuclear antigen 2

The Epstein-Barr virus (EBV)-encoded latency protein Epstein-Barr nuclear antigen 2 (EBNA2) is a nuclear transcriptional activator that is essential for EBV-induced cellular transformation. In a previous study, we demonstrated that EBNA2 interacts with signal transducer and activator of transcription 3 (STAT3), a signal transducer for an interleukin (IL)-6 family cytokine, and enhances its transcriptional activity. Here, we show that overexpression of a corepressor, silencing mediator of retinoic acid and thyroid hormone receptor (SMRT), decreases the EBNA2-mediated enhanced STAT3 activation. Furthermore, small-interfering RNA-mediated reduction of endogenous SMRT expression augments the EBNA2-mediated enhanced STAT3 activation. Importantly, EBNA2 reduces interactions between STAT3 and SMRT. These data demonstrate that EBNA2 acts as a transcriptional coactivator of STAT3 by influencing the SMRT corepressor complex.

Preferential physical and functional interaction of pregnane X receptor with the SMRTalpha isoform

The silencing mediator for retinoid and thyroid hormone receptors (SMRT) serves as a platform for transcriptional repression elicited by several steroid/nuclear receptors and transcription factors. SMRT exists in two major splicing isoforms, alpha and tau, with SMRTalpha containing only an extra 46-amino acid sequence inserted immediately downstream from the C-terminal corepressor motif. Little is known about potential functional differences between these two isoforms. Here we show that the pregnane X receptor (PXR) interacts more strongly with SMRTalpha than with SMRTtau both in vitro and in vivo. It is interesting that the PXR-SMRTalpha interaction is also resistant to PXR ligand-induced dissociation, in contrast to the PXR-SMRTtau interaction. SMRTalpha consistently inhibits PXR activity more efficiently than does SMRTtau in transfection assays, although they possess comparable intrinsic repression activity and association with histone deacetylase. We further show that the mechanism for the enhanced PXR-SMRTalpha interaction involves both the 46-amino acid insert and the C-terminal corepressor motif. In particular, the first five amino acids of the SMRTalpha insert are essential and sufficient for the enhanced binding of SMRTalpha to PXR. Furthermore, we demonstrate that Tyr2354 and Asp2355 residues of the SMRTalpha insert are most critical for the enhanced interaction. In addition, expression data show that SMRTalpha is more abundantly expressed in most human tissues and cancer cell lines, and together these data suggest that SMRTalpha may play a more important role than SMRTtau in the negative regulation of PXR.

Atrophin proteins: an overview of a new class of nuclear receptor corepressors

The normal development and physiological functions of multicellular organisms are regulated by complex gene transcriptional networks that include myriad transcription factors, their associating coregulators, and multiple chromatin-modifying factors. Aberrant gene transcriptional regulation resulting from mutations among these elements often leads to developmental defects and diseases. This review article concentrates on the Atrophin family proteins, including vertebrate Atrophin-1 (ATN1), vertebrate arginine-glutamic acid dipeptide repeats protein (RERE), and Drosophila Atrophin (Atro), which we recently identified as nuclear receptor corepressors. Disruption of Atrophin-mediated pathways causes multiple developmental defects in mouse, zebrafish, and Drosophila, while an aberrant form of ATN1 and altered expression levels of RERE are associated with neurodegenerative disease and cancer in humans, respectively. We here provide an overview of current knowledge about these Atrophin proteins. We hope that this information on Atrophin proteins may help stimulate fresh ideas about how this newly identified class of nuclear receptor corepressors aids specific nuclear receptors and other transcriptional factors in regulating gene transcription, manifesting physiological effects, and causing diseases.

Cooperative regulation in development by SMRT and FOXP1

A critical aspect of mammalian development involves the actions of dedicated repressors/corepressors to prevent unregulated gene activation programs that would initiate specific cell determination events. While the role of NCoR/SMRT corepressors in nuclear receptor actions is well documented, we here report that a previously unrecognized functional interaction between SMRT and a forkhead protein, FOXP1, is required for cardiac growth and regulation of macrophage differentiation. Our studies demonstrate that SMRT and FOXP1 define a functional biological unit required to orchestrate specific programs critical for mammalian organogenesis, linking developmental roles of FOX to a specific corepressor.

Response of SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) and N-CoR (nuclear receptor corepressor) corepressors to mitogen-activated protein kinase kinase kinase cascades is determined by alternative mRNA splicing

The SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) and N-CoR (nuclear receptor corepressor) corepressors are important mediators of transcriptional repression by nuclear hormone receptors. SMRT is regulated by MAPK kinase kinase (MAPKKK) cascades that induce its release from its receptor partners, its export from nucleus to cytoplasm, and derepression of target gene expression. Intriguingly, the otherwise closely related N-CoR is refractory to MAPKKK signaling under the same conditions. However, both SMRT and N-CoR are expressed as a series of alternatively spliced protein variants differing in structure and function. We have now characterized the impact of this alternative mRNA splicing on the corepressor response to MAPKKK signaling. Whereas the SMRTalpha, SMRTtau, and SMRTsp2 splice variants are released from their nuclear receptor partners in response to MAPKKK activation, the SMRTsp18 variant, which resembles N-CoR in its overall molecular architecture, is relatively refractory to this kinase-induced release. Alternative splicing of N-CoR, in contrast, had only minimal effects on the resistance of this corepressor to MAPKKK inhibition. Notably, all of the SMRT splice variants examined redistributed from nucleus to cytoplasm in response to MAPKKK cascade signaling, but none of the N-CoR splice variants did so. Different tiers of the MAPKKK cascade hierarchy contributed to these different aspects of corepressor regulation, with MAP/ERK kinase kinase 1 and MAP/ERK kinase 1 regulating subcellular redistribution and ERK2 regulating nuclear receptor-corepressor interaction. We conclude that cells can customize their transcriptional response to MAPKKK cascade signaling by selective expression of the SMRT or N-CoR locus, by selective utilization of a specific corepressor splice variant, and by selective exploitation of specific tiers of the MAPK cascade.

Corepressors: custom tailoring and alterations while you wait

A diverse cadre of metazoan transcription factors mediate repression by recruiting protein complexes containing the SMRT (silencing mediator of retinoid and thyroid hormone receptor) or N-CoR (nuclear receptor corepressor) corepressors. SMRT and N-CoR nucleate the assembly of still larger corepressor complexes that perform the specific molecular incantations necessary to confer transcriptional repression. Although SMRT and N-CoR are paralogs and possess similar molecular architectures and mechanistic strategies, they nonetheless exhibit distinct molecular and biological properties. It is now clear that the functions of both SMRT and N-CoR are further diversified through alternative mRNA splicing, yielding a series of corepressor protein variants that participate in distinctive transcription factor partnerships and display distinguishable repression properties. This review will discuss what is known about the structure and actions of SMRT, N-CoR, and their splicing variants, and how alternative splicing may allow the functions of these corepressors to be adapted and tailored to different cells and to different developmental stages.

Androgen receptor corepressors: an overview

Androgens play pivotal roles in sex differentiation and development, in reproductive functions, and sexual behavior. The actions of androgens are mediated through the intracellular androgen receptor (AR), a member of the nuclear receptor (NR) superfamily, which regulates a wide range of target gene expression. Recent studies indicate that the proper transcriptional activity of AR is modulated by AR coregulators, including coactivators that can enhance AR transactivation and corepressors that can suppress AR transactivation. Here, we summarize the recent discoveries relating to AR corepressor function with the following different mechanisms: (1) corepressors that inhibit the DNA binding or nuclear translocation of AR; (2) corepressors that recruit histone deacetylases; (3) corepressors that interrupt the interaction between AR and its coactivators; (4) corepressors that interrupt the interaction between the N-terminus and C-terminus of AR; (5) corepressors that function as scaffolds for other AR coregulators; (6) corepressors that target the basal transcriptional machinery; (7) other mechanisms. The potential impact and future directions of AR corepressors are also discussed.

Heterodimers of retinoic acid receptors and thyroid hormone receptors display unique combinatorial regulatory properties

Nuclear receptors are ligand-regulated transcription factors that regulate key aspects of metazoan development, differentiation, and homeostasis. Nuclear receptors recognize target genes by binding to specific DNA recognition sequences, denoted hormone response elements (HREs). Many nuclear receptors can recognize HREs as either homodimers or heterodimers. Retinoid X receptors (RXRs), in particular, serve as important heterodimer partners for many other nuclear receptors, including thyroid hormone receptors (TRs), and RXR/TR heterodimers have been proposed to be the primary mediators of target gene regulation by T3 hormone. Here, we report that the retinoic acid receptors (RARs), a distinct class of nuclear receptors, are also efficient heterodimer partners for TRs. These RAR/TR heterodimers form with similar affinities as RXR/TR heterodimers on an assortment of consensus and natural HREs, and preferentially assemble with the RAR partner 5' of the TR moiety. The corepressor and coactivator recruitment properties of these RAR/TR heterodimers and their transcriptional activities in vivo are distinct from those observed with the corresponding RXR heterodimers. Our studies indicate that RXRs are not unique in their ability to partner with TRs, and that RARs can also serve as robust heterodimer partners and combinatorial regulators of T3-modulated gene expression.

SMRT and N-CoR corepressors are regulated by distinct kinase signaling pathways

N-CoR and SMRT are corepressor paralogs that partner with and mediate transcriptional repression by a wide variety of metazoan transcription factors, including nuclear hormone receptors. Although encoded by distinct genetic loci, N-CoR and SMRT share substantial sequence interrelatedness, form analogous assemblies with histone deacetylases and auxiliary factors, can interact with overlapping sets of transcription factor partners, and exert overlapping functions in cells. SMRT is subject to negative regulation by MAPK signaling pathways operating downstream of growth factor and stress signaling pathways. We report here that whereas activation of MEKK1 leads to phosphorylation of SMRT, its dissociation from its transcription factor partners in vivo and in vitro, and its redistribution from the cell nucleus to a cytoplasmic compartment, N-CoR is refractory to all these forms of regulation. In contrast to this MAPK cascade, other signal transduction pathways operating downstream of growth factor/cytokine receptors appear able to affect both corepressor paralogs. Our results indicate that SMRT and N-CoR are embedded in distinct regulatory networks and that the two corepressors interpret growth factor, cytokine, differentiation, and prosurvival signals differently.

Fundamentals of transcription factors and their impact on pancreatic development and cancer

Transcription factors are proteins that regulate gene expression by modulating the synthesis of messenger RNA. Since this process, known as gene transcription, is often the dominant control point in the production of many proteins, transcription factors are key regulators of numerous cellular functions, including secretion, proliferation, differentiation, and apoptosis. Most transcription factors are also the final effectors of signaling pathways that transduce signals from the cell membrane to the nucleus. Therefore alterations in the activity or expression of some transcription factors have a significant impact on the biology of human cells and may lead to the development of diseases. In this article we review this field of research with a particular emphasis on the role of transcription factors in pancreatic development and cancer.

Retinoids in cancer therapy and chemoprevention: promise meets resistance

Retinoids (natural and synthetic derivatives of vitamin A) signal potent differentiation and growth-suppressive effects in diverse normal, premalignant, and malignant cells. A strong rationale exists for the use of retinoids in cancer treatment and chemoprevention based on preclinical, epidemiological, and early clinical findings. Despite the success of all-trans-retinoic acid (RA)-based differentiation therapy in acute promyelocytic leukemia (APL), the broad promise of retinoids in the clinic has not yet been realized. In addition to the expected limited activity of any single therapeutic agent, translation of retinoid activities from the laboratory to the clinic has met with intrinsic or acquired retinoid resistance. Evidence suggests that solid tumors develop intrinsic resistance to retinoids during carcinogenesis. In contrast, relapse of APL is often associated with acquired resistance to retinoid maturation induction. This review discusses what is known about retinoid resistance mechanisms in cancer therapy and chemoprevention. Strategies to overcome this resistance will be discussed, including combination therapy with other differentiation-inducing, cytotoxic or chromatin-remodeling agents, as well as the use of receptor-selective and nonclassical retinoids. Opportunities exist in the post-genomic era to bypass resistance to classical retinoids by identifying target genes and associated pathways that directly mediate the antineoplastic effects of retinoids. In this regard, the retinoids are useful pharmacological tools to reveal important pathways targeted in cancer therapy and chemoprevention.

Repression of the luteinizing hormone receptor gene promoter by cross talk among EAR3/COUP-TFI, Sp1/Sp3, and TFIIB

Transcription of luteinizing hormone receptor (LHR) gene is activated by Sp1/Sp3 at two Sp1 sites and is repressed by nuclear orphan receptors EAR2 and EAR3 through a direct-repeat (DR) motif. To elucidate the mechanism of the orphan receptor-mediated gene repression, we explored the functional connection between the orphan receptors and Sp1/Sp3 complex, and its impact on the basal transcription machinery. The Sp1(I) site was identified as critical for the repression since its mutation reduced the inhibition by EAR2 and abolished the inhibition by EAR3. Cotransfection analyses in SL2 cells showed that both Sp1 and Sp3 were required for this process since EAR3 displayed a complete Sp1/Sp3-dependent inhibitory effect. Functional cooperation between Sp1 and DR domains was further supported by mutual recruitment of EAR3 and Sp1/Sp3 bound to their cognate sites. Deletion of EAR3 N-terminal and DNA-binding domains that reduced its interaction with Sp1 impaired its inhibitory effect on human LHR (hLHR) gene transcription. Furthermore, we demonstrate interaction of TFIIB with Sp1/Sp3 at the Sp1(I) site besides its association with EAR3 and the TATA-less core promoter region. Such interaction relied on Sp1 site-bound Sp1/Sp3 complex and adaptor protein(s) present in the JAR nuclear extracts. We further demonstrated that EAR3 specifically decreased association of TFIIB to the Sp1(I) site without interfering on its interaction with the hLHR core promoter. The C-terminal region of EAR3, which did not participate in its interaction with Sp1, was required for its inhibitory function and may affect the association of TFIIB with Sp1. Moreover, perturbation of the association of TFIIB with Sp1 by EAR3 was reflected in the reduced recruitment of RNA polymerase II to the promoter. Overexpression of TFIIB counteracted the inhibitory effect of EAR3 and activated hLHR gene transcription in an Sp1 site-dependent manner. These findings therefore indicate that TFIIB is a key component in the regulatory control of EAR3 and Sp1/Sp3 on the initiation complex. Such cross talk among EAR3, TFIIB, and Sp1/Sp3 reveals repression of hLHR gene transcription by nuclear orphan receptors is achieved via perturbation of communication between Sp1/Sp3 at the Sp1-1 site and the basal transcription initiator complex.

Isotype-restricted corepressor recruitment: a constitutively closed helix 12 conformation in retinoic acid receptors beta and gamma interferes with corepressor recruitment and prevents transcriptional repression

Retinoic acid receptors (RARs) are ligand-regulated transcription factors that play multiple roles in vertebrate development and differentiation. RARs as a class are capable of both repressing and activating target gene expression. Transcriptional repression is mediated through the recruitment of corepressor proteins such as SMRT. Notably, vertebrates encode three major forms of RARs, alpha, beta, and gamma, and these distinct RAR isotypes differ in the ability to recruit a corepressor. RAR alpha strongly interacts with SMRT and can repress target gene transcription, whereas RAR beta and -gamma interact with SMRT only weakly and fail to repress. We report here the use of a genetic suppressor approach, based on a yeast two-hybrid interaction assay using Saccharomyces cerevisiae, for the isolation of RAR beta mutants that have gained the RAR alpha-like corepressor phenotype, i.e., a strong interaction with SMRT and the ability to repress gene expression in vertebrate cells. Analysis of these gain-of-function mutants indicates that the different corepressor interaction properties of RAR alpha, -beta and -gamma are determined by a gating mechanism through which amino acid differences in the helix 3 region of these receptors influence the position of the receptor C-terminal helix 12 domain. As a consequence, the RAR beta and RAR gamma receptors appear to adopt a constitutively closed helix 12 conformation in the absence of hormone that may approximate the conformation of RAR alpha when bound to hormone agonist. This closed helix 12 conformation in RAR beta and RAR gamma blocks corepressor binding, prevents repression, and permits significant levels of target gene activation even in the absence of hormone. We refer to this phenomenon as a "gate-latch" model of corepressor regulation.

Active repression of RAR signaling is required for head formation

The retinoic acid receptors (RARs) recruit coactivator and corepressor proteins to activate or repress the transcription of target genes depending on the presence of retinoic acid (RA). Despite a detailed molecular understanding of how corepressor complexes function, there is no in vivo evidence to support a necessary function for RAR-mediated repression. Signaling through RARs is required for patterning along the anteroposterior (A-P) axis, particularly in the hindbrain and posterior, although the absence of RA is required for correct anterior patterning. Because RARs and corepressors are present in regions in which RA is absent, we hypothesized that repression mediated through unliganded RARs might be important for anterior patterning. To test this hypothesis, specific reagents were used that either reduce or augment RAR-mediated repression. Derepression of RAR signaling by expressing a dominant-negative corepressor resulted in embryos that exhibited phenotypes similar to those treated by RA. Anterior structures such as forebrain and cement gland were greatly reduced, as was the expression of molecular markers. Enhancement of target gene repression using an RAR inverse agonist resulted in up-regulation of anterior neural markers and expansion of anterior structures. Morpholino antisense oligonucleotide-mediated RARalpha loss-of-function phenocopied the effects of RA treatment and dominant-negative corepressor expression. Microinjection of wild-type or dominant-negative RARalpha rescued the morpholino phenotype, confirming that RAR is functioning anteriorly as a transcriptional repressor. Lastly, increasing RAR-mediated repression potentiated head-inducing activity of the growth factor inhibitor cerberus, whereas releasing RAR-mediated repression blocked cerberus from inducing ectopic heads. We conclude that RAR-mediated repression of target genes is critical for head formation. This requirement establishes an important biological role for active repression of target genes by nuclear hormone receptors and illustrates a novel function for RARs during vertebrate development.