The nuclear receptor corepressor (N-CoR) contains three isoleucine motifs (I/LXXII) that serve as receptor interaction domains (IDs)

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.

Signaling by estrogens

By regulating activities and expression levels of key signaling molecules, estrogens control mechanisms that are responsible for crucial cellular functions. Ligand binding to estrogen receptor (ER) leads to conformational changes that regulate the receptor activity, its interaction with other proteins and DNA. In the cytoplasm, receptor interactions with kinases and scaffolding molecules regulate cell signaling cascades (extranuclear/nongenomic action). In the nucleus, estrogens control a repertoire of coregulators and other auxiliary proteins that are associated with ER, which in turn determines the nature of regulated genes and level of their expression (genomic action). The combination of genomic and nongenomic actions of estrogens ultimately confers the cell-type and tissue-type selectivity. Recent studies have revealed some important new insights into the molecular mechanisms underlying ER action, which may help to explain the functional basis of existing selective ER modulators (SERMs) and provide evidence into how ER might be selectively targeted to achieve specific therapeutic goals. In this review, we will summarize some new molecular details that relate to estrogen signaling. We will also discuss some new strategies that may potentially lead to the development of functionally selective ER modulators that can separate between the beneficial, prodifferentiative effects in bone, the cardiovascular system and the CNS as well as the "detrimental," proliferative effects in reproductive tissues and organs.

Nuclear compartmentalization of N-CoR and its interactions with steroid receptors

The repression mechanisms by the nuclear receptor corepressor (N-CoR) of steroid hormone receptor (SHR)-mediated transactivation were examined. Yellow fluorescent protein (YFP)-N-CoR was distributed as intranuclear discrete dots, while coexpression of androgen receptor (AR), glucocorticoid receptor alpha, and estrogen receptor alpha ligand-dependently triggered redistribution of YFP-N-CoR. In fluorescence recovery after photobleaching analysis, mobility of the N-CoR was reduced by 5alpha-dihydrotestosterone (DHT)-bound AR. The middle region of N-CoR mostly contributed to the interaction with agonist-bound SHRs and the suppression of their transactivation function. N-CoR impaired the DHT-induced N-C interaction of AR, and the impaired interaction was dose-dependently recovered by coexpression of SRC-1 and CBP. N-CoR also impaired the intranuclear complete (distinct) focus formation of SHRs. Coexpression of SRC-1 or CBP released YFP-N-CoR or endogenous N-CoR from incomplete foci and simultaneously recovered complete foci of AR-green fluorescent protein. These results indicate that the relative ratio of coactivators and corepressors determines the conformational equilibrium between transcriptionally active and inactive SHRs in the presence of agonists. The intranuclear foci formed by agonist-bound SHRs were completely destroyed by actinomycin D and alpha-amanitin, indicating that the focus formation does not precede the transcriptional activation. The focus formation may reflect the accumulation of SHR/coactivator complexes released from the transcriptionally active sites and thus be a mirror of transcriptionally active complex formation.

The N-CoR complex enables chromatin remodeler SNF2H to enhance repression by thyroid hormone receptor

Unliganded thyroid hormone receptor (TR) actively represses transcription via the nuclear receptor corepressor (N-CoR)/histone deacetylase 3 (HDAC3) complex. Although transcriptional activation by liganded receptors involves chromatin remodeling, the role of ATP-dependent remodeling in receptor-mediated repression is unknown. Here we report that SNF2H, the mammalian ISWI chromatin remodeling ATPase, is critical for repression of a genomically integrated, TR-regulated reporter gene. N-CoR and HDAC3 are both required for recruitment of SNF2H to the repressed gene. SNF2H does not interact directly with the N-CoR/HDAC3 complex, but binds to unacetylated histone H4 tails, suggesting that deacetylase activity of the corepressor complex is critical to SNF2H function. Indeed, HDAC3 as well as SNF2H are required for nucleosomal organization on the TR target gene. Consistent with these findings, reduction of SNF2H induces expression of an endogenous TR-regulated gene, dio1, in liver cells. Thus, although not apparent from studies of transiently transfected reporter genes, gene repression by TR involves the targeting of chromatin remodeling factors to repressed genes by the HDAC activity of nuclear receptor corepressors.

Combinatorial roles of nuclear receptors in inflammation and immunity

Members of the nuclear-receptor superfamily have well-documented regulatory effects on inflammatory processes. Recent work has highlighted the roles of peroxisome-proliferator-activated receptors (PPARs) and liver X receptors (LXRs) in controlling metabolic and inflammatory programmes of gene expression in macrophages and lymphocytes. Here, we describe recent studies that extend our understanding of how these nuclear receptors, through their interactions with transcription factors and other cell-signalling systems, have important regulatory roles in innate and adaptive immunity. We suggest that by using receptor-specific mechanisms, PPARs and LXRs function in a combinatorial manner with the glucocorticoid receptor to integrate local and systemic responses to inflammation.

Xenopus embryos lacking specific isoforms of the corepressor SMRT develop abnormal heads

The corepressor SMRT acts on a range of transcription factors, including the retinoid and thyroid hormone nuclear receptors. The carboxy-terminal region of SMRT contains CoRNR box motifs that mediate these interactions. We have shown, in Xenopus, that SMRT can exist as isoforms containing either two or three CoRNR boxes depending on the alternative splicing of exon 37b. The number of SMRT transcript isoforms expressed increases during development until all sixteen possible isoforms are identified in the swimming tadpole. To eliminate specific SMRT isoforms, we have developed a process that uses an antisense morpholino oligonucleotide in Xenopus to dictate the outcome of alternative splicing at a defined exon and used this to inhibit the formation of transcripts containing exon 37b. These embryos are therefore limited to the expression of SMRT isoforms that contain two rather than three CoRNR boxes. Analysis of responsive genes in these embryos shows that targets of thyroid hormone, but not retinoid signaling are affected by the elimination of exon 37b. Morpholino-injected embryos have swimming abnormalities and develop altered head morphology, an expanded olfactory epithelium and disorganized peripheral axons. These experiments indicate a critical role for the alternative splicing of SMRT in development.

Multiple mutations contribute to repression by the v-Erb A oncoprotein

The v-Erb A oncoprotein of avian erythroblastosis virus is derived from c-Erb A, a hormone-activated transcription factor. Notably, v-Erb A has sustained multiple mutations relative to c-Erb A and functions as a constitutive transcriptional repressor. We report here an analysis of the contributions of these different mutations to v-Erb A function. Our experiments demonstrate that two amino-acid differences between v-Erb A and c-Erb A, located in the 'I-box,' alter the dimerization properties of the viral protein, resulting in more stable homodimer formation, increased corepressor binding, and increased target gene repression. An additional amino-acid difference between v- and c-Erb A, located in helix 3 of the hormone binding domain, renders corepressor binding by the viral protein more resistant to release by thyroid hormone. Finally, we report that a C-terminal truncation in v-Erb A not only inhibits exchange of corepressor and coactivator, as previously noted, but also permits v-Erb A to recruit both SMRT and N-CoR corepressors, whereas c-Erb A is selective for N-CoR. The latter two mutations in v-Erb A also impair its ability to suppress c-Jun function in response to T3 hormone. We propose that the acquisition of oncogenic potential by the v-Erb A protein was a multistep process involving a series of mutations that alter the transcriptional repressive properties of the viral protein through multiple mechanisms.

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.

SMRT has tissue-specific isoform profiles that include a form containing one CoRNR box

SMRT acts as a corepressor for a range of transcription factors. The amino-terminal part of the protein includes domains that mainly mediate transcriptional repression whilst the carboxy-terminal part includes domains that interact with nuclear receptors using up to three motifs called CoRNR boxes. The region of the SMRT primary transcript encoding the interaction domains is subject to alternative splicing that varies the inclusion of the third CoRNR box. The profile in mice includes an abundant, novel SMRT isoform that possesses just one CoRNR box. Mouse tissues therefore express SMRT isoforms containing one, two or three CoRNR boxes. In frogs, the SMRT isoform profile is tissue-specific. The mouse also shows distinct profiles generated by differential expression levels of the SMRT transcript isoforms. The formation of multiple SMRT isoforms and their tissue-specific regulation indicates a mechanism, whereby cells can define the repertoire of transcription factors regulated by SMRT.

Noncalcemic actions of vitamin D receptor ligands

1alpha,25-Dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)], the active metabolite of vitamin D(3), is known for the maintenance of mineral homeostasis and normal skeletal architecture. However, apart from these traditional calcium-related actions, 1,25-(OH)(2)D(3) and its synthetic analogs are being increasingly recognized for their potent antiproliferative, prodifferentiative, and immunomodulatory activities. These actions of 1,25-(OH)(2)D(3) are mediated through vitamin D receptor (VDR), which belongs to the superfamily of steroid/thyroid hormone nuclear receptors. Physiological and pharmacological actions of 1,25-(OH)(2)D(3) in various systems, along with the detection of VDR in target cells, have indicated potential therapeutic applications of VDR ligands in inflammation (rheumatoid arthritis, psoriatic arthritis), dermatological indications (psoriasis, actinic keratosis, seborrheic dermatitis, photoaging), osteoporosis (postmenopausal and steroid-induced osteoporosis), cancers (prostate, colon, breast, myelodysplasia, leukemia, head and neck squamous cell carcinoma, and basal cell carcinoma), secondary hyperparathyroidism, and autoimmune diseases (systemic lupus erythematosus, type I diabetes, multiple sclerosis, and organ transplantation). As a result, VDR ligands have been developed for the treatment of psoriasis, osteoporosis, and secondary hyperparathyroidism. Furthermore, encouraging results have been obtained with VDR ligands in clinical trials of prostate cancer and hepatocellular carcinoma. This review deals with the molecular aspects of noncalcemic actions of vitamin D analogs that account for the efficacy of VDR ligands in the above-mentioned indications.

E1A and a nuclear receptor corepressor splice variant (N-CoRI) are thyroid hormone receptor coactivators that bind in the corepressor mode

Unliganded thyroid hormone (TH) receptors (TRs) and other nuclear receptors (NRs) repress transcription of hormone-activated genes by recruiting corepressors (CoRs), such as NR CoR (N-CoR) and SMRT. Unliganded TRs also activate transcription of TH-repressed genes. Some evidence suggests that these effects also involve TR/CoR contacts; however, the precise reasons that CoRs activate transcription in these contexts are obscure. Unraveling these mechanisms is complicated by the fact that it is difficult to decipher direct vs. indirect effects of TR-coregulator contacts in mammalian cells. In this study, we used yeast, Saccharomyces cerevisiae, which lack endogenous NRs and NR coregulators, to determine how unliganded TRs can activate transcription. We previously showed that adenovirus 5 early-region 1A coactivates unliganded TRs in yeast, and that these effects are blocked by TH. We show here that human adenovirus type 5 early region 1A (E1A) contains a short peptide (LDQLIEEVL amino acids 20-28) that resembles CoR-NR interaction motifs (CoRNR boxes), and that this motif is required for TR binding and coactivation. Although full-length N-CoR does not coactivate TR in yeast, a naturally occurring N-CoR variant (N-CoR(I)) and an artificial N-CoR truncation (N-CoR(C)) that retain CoRNR boxes but lack N-terminal repressor domains behave as potent and direct TH-repressed coactivators for unliganded TRs. We conclude that E1A and N-CoR(I) are naturally occurring TR coactivators that bind in the typical CoR mode and suggest that similar factors could mediate transcriptional activation by unliganded TRs in mammals.

The nuclear receptor corepressors NCoR and SMRT decrease peroxisome proliferator-activated receptor gamma transcriptional activity and repress 3T3-L1 adipogenesis

The peroxisome proliferator-activated receptor gamma (PPARgamma) is a central regulator of adipogenesis and recruits coactivator proteins in response to ligand. However, the role of another class of nuclear cofactors, the nuclear receptor corepressors, in modulating PPARgamma transcriptional activity is less clear. Such corepressors include the nuclear receptor corepressor (NCoR) and the silencing mediator of retinoid and thyroid hormone receptors (SMRT). Our data suggest that PPARgamma recruits SMRT and NCoR in the absence of ligand and that these corepressors are capable of down-regulating PPARgamma-mediated transcriptional activity. The addition of the PPARgamma ligand pioglitazone results in dissociation of the PPARgamma-corepressor complex. To define the role of SMRT and NCoR in PPARgamma action, 3T3-L1 cells deficient in SMRT or NCoR were generated by RNA interference. When these cells are exposed to differentiation media, they exhibit increased expression of adipocyte-specific genes and increased production of lipid droplets, as compared with control cells. These data suggest that the nuclear receptor corepressors decrease PPARgamma transcriptional activity and repress the adipogenic program in 3T3-L1 cells.

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.

Alternative mRNA splicing of SMRT creates functional diversity by generating corepressor isoforms with different affinities for different nuclear receptors

Many eukaryotic transcription factors are bimodal in their regulatory properties and can both repress and activate expression of their target genes. These divergent transcriptional properties are conferred through recruitment of auxiliary proteins, denoted coactivators and corepressors. Repression plays a particularly critical role in the functions of the nuclear receptors, a large family of ligand-regulated transcription factors involved in metazoan development, differentiation, reproduction, and homeostasis. The SMRT corepressor interacts directly with nuclear receptors and serves, in turn, as a platform for the assembly of a larger corepressor complex. We report here that SMRT is expressed in cells by alternative mRNA splicing to yield two distinct variants or isoforms. We designate these isoforms SMRTalpha and SMRTtau and demonstrate that these isoforms have significantly different affinities for different nuclear receptors. These isoforms are evolutionarily conserved and are expressed in a tissue-specific manner. Our results suggest that differential mRNA splicing serves to customize corepressor function in different cells, allowing the transcriptional properties of nuclear receptors to be adapted to different contexts.

Transgenic analysis reveals that thyroid hormone receptor is sufficient to mediate the thyroid hormone signal in frog metamorphosis

Thyroid hormone (T3) has long been known to be important for vertebrate development and adult organ function. Whereas thyroid hormone receptor (TR) knockout and transgenic studies of mice have implicated TR involvement in mammalian development, the underlying molecular bases for the resulting phenotypes remain to be determined in vivo, especially considering that T3 is known to have both genomic, i.e., through TRs, and nongenomic effects on cells. Amphibian metamorphosis is an excellent model for studying the role of TR in vertebrate development because of its total dependence on T3. Here we investigated the role of TR in metamorphosis by developing a dominant positive mutant thyroid hormone receptor (dpTR). In the frog oocyte transcription system, dpTR bound a T3-responsive promoter and activated the promoter independently of T3. Transgenic expression of dpTR under the control of a heat shock-inducible promoter in premetamorphic tadpoles led to precocious metamorphic transformations. Molecular analyses showed that dpTR induced metamorphosis by specifically binding to known T3 target genes, leading to increased local histone acetylation and gene activation, similar to T3-bound TR during natural metamorphosis. Our experiments indicated that the metamorphic role of T3 is through genomic action of the hormone, at least on the developmental parameters tested. They further provide the first example where TR is shown to mediate directly and sufficiently these developmental effects of T3 in individual organs by regulating target gene expression in these organs.

The androgen receptor recruits nuclear receptor CoRepressor (N-CoR) in the presence of mifepristone via its N and C termini revealing a novel molecular mechanism for androgen receptor antagonists

The androgen receptor (AR) activates target gene expression in the presence of agonist ligands via the recruitment of transcriptional coactivators, but recent work shows that overexpression of the nuclear corepressors NCoR and SMRT attenuates this agonist-mediated AR activation. Here we demonstrate using NCoR siRNA and chromatin immunoprecipitation that endogenous NCoR is recruited to and represses the dihydrotestosterone (DHT)-liganded AR. Furthermore this study shows that NCoR and coactivators compete for AR in the presence of DHT. AR antagonists such as bicalutamide that are currently in use for prostate cancer treatment can also mediate NCoR recruitment, but mifepristone (RU486) at nanomolar concentrations is unique in its ability to markedly enhance the AR-NCoR interaction. The RU486-liganded AR interacted with a C-terminal fragment of NCoR, and this interaction was mediated by the two most C-terminal nuclear receptor interacting domains (RIDs) present in NCoR. Significantly, in addition to the AR ligand binding domain, the AR N terminus was also required for this interaction. Mutagenesis studies demonstrate that the N-terminal surface of the AR-mediating NCoR recruitment was distinct from tau5 and from the FXXLF motif that mediates agonist-induced N-C-terminal interaction. Taken together these data demonstrate that NCoR is a physiological regulator of the AR and reveal a new mechanism for AR antagonism that may be exploited for the development of more potent AR antagonists.

A nuclear receptor corepressor transcriptional checkpoint controlling activator protein 1-dependent gene networks required for macrophage activation

The nuclear receptor corepressor (NCoR) and the related factor known as silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) are essential components of multiprotein complexes that mediate active repression by unliganded nuclear receptors. Recent studies suggest that NCoR and SMRT can interact with and exert repressive effects on several other classes of DNA-binding transcription factors, but the physiological importance of these interactions has not been established. Here, investigation of endogenous transcriptional programs regulated by NCoR in macrophages reveals that NCoR acts as a transcriptional checkpoint for activator protein (AP)-1-dependent gene networks that regulate diverse biological processes including inflammation, cell migration, and collagen catabolism, with loss of NCoR, resulting in derepression of AP-1 target genes. The NCoR corepressor complex imposes an active block of exchange of c-Jun for c-Jun/c-Fos heterodimers, with targeted deletion of the c-Jun locus, resulting in loss of NCoR complexes from AP-1 target genes under basal conditions. The checkpoint function of NCoR is relieved by signal-dependent phosphorylation of c-Jun, which directs removal of NCoR/HDAC3/TBL1/TBLR1 complexes through recruitment of a specific ubiquitylation complex, as a prerequisite to the default binding of c-Jun/c-Fos heterodimers and transcriptional activation. The requirement for a checkpoint function to achieve the appropriate dynamic range of transcriptional responses to inflammatory signals is likely to be used by other signal-dependent transcription factors that regulate diverse homeostatic and developmental processes.

Alternative splicing generates multiple SMRT transcripts encoding conserved repressor domains linked to variable transcription factor interaction domains

Silencing mediator for retinoid and thyroid hormone receptor (SMRT) and nuclear receptor corepressor protein (NCoR) are corepressors that interact with a range of transcription factors. They both consist of N-terminal repressor domains that associate with histone deacetylases and C-terminal interaction domains (IDs) that contain CoRNR box motifs. These motifs mediate the interaction between corepressors and nuclear receptors (NRs), such as the retinoid and thyroid hormone receptors. However, whilst NCoR produces a single transcript during Xenopus development, xSMRT is subject to alternative splicing at four sites in the 3' part of the transcript, the region encoding the C-terminal IDs. Although this provides the potential to produce 16 different transcripts, only five isoforms are found in early embryos. The sites of alternative splicing predict that the resultant isoforms will differ in their ability to interact with NRs, as one site varies the number of CoRNR boxes, the second site changes the sequence flanking CoRNR box-1 and the other sites delete amino acid residues between CoRNR boxes 1 and 2 and so alter the critical spacing between these motifs. SMRT and NCoR therefore represent paralogues in which one form, SMRT, has evolved the ability to generate multiple isoforms whereas the other, NCoR, is invariant in Xenopus development.

[Molecular mechanism of thyroid hormone action]

Thyroid hormones (TH) are involved in normal differentiation, growth, and metabolism in several tissues of all vertebrates. Their actions are mediated by the TH receptors (TRs), members of the nuclear hormone receptor superfamily. These receptors are transcription factors that bind to DNA on specific sequences, the TR response element (TREs), in promoters of target genes. Two genes encode TRs, alpha e beta, located in chromosomes 17 and 3, respectively. These isoforms show different functions and exhibit a tissue specific expression. TRs function as monomers, homodimers or heterodimers with retinoid X receptor (RXR) and modulate transcription activity (repression or activation) by interacting with co-repressor and co-activators, which associate with TR in the absence or presence of T3, respectively. Understanding the molecular mechanism of TR action and the definition of its crystallographic structure will provide new insights into transcription mechanisms and will facilitate the design of new drugs with greater therapeutic value.

A novel small peptide as a targeting ligand for receptor tyrosine kinase Tie2

Tie2 is an endothelium-specific receptor tyrosine kinase known to play an important role in tumor angiogenesis. We sought to identify a small peptide ligand against Tie2 for developing a delivery targeting agent. We used hydrophobic analysis and comparative sequence/structure analysis to select a minimal peptide based on angiopoietin-2 amino acid sequence. The resulting peptide named GA3(WTIIQRREDGSVDFQRTWKEYK) was synthesized and labeled with iodine-125 at the C-terminal tyrosine residue to characterize its binding capability. In in vitro binding assays, GA3 can not only specifically bind to SMMC7721-Tie2 but also compete with angiopoietin-2 in binding. Via mouse tail vein injection, 125I-labeled GA3 was found to favorably accumulate in SPC-A1 xenograft tumor tissues which positively express Tie2. These results demonstrated that GA3 may be useful as a drug or gene delivery ligand for targeted chemotherapy, radiotherapy, and gene therapy.

Ligand-Selective Inhibition of the Interaction of Steroid Receptor Coactivators and Estrogen Receptor Isoforms

Ligand-dependent nuclear hormone receptor (NR) signaling requires direct interaction between NR and the steroid receptor coactivators (SRC). Herein we utilize a library of SRC2 peptidomimetics to select for specific inhibitors of the interaction of SRC2 with the two estrogen receptor (ER) isoforms, ERalpha and ERbeta, in the presence of three different ligands: 17beta-estradiol, diethylstilbesterol, and genistein. The pattern of inhibitor selectivity for each ER isoform varied depending upon which ligand was present, thus demonstrating that the ligands exert unique allosteric effects upon the surface of the SRC binding pocket. Several of the lead compounds are highly (>100-fold) selective for blocking the binding of SRC2 to ERalpha, in preference to ERbeta, in the presence of one ligand and therefore may prove useful for decoupling ERbeta signaling from ERalpha signaling.

Protein inhibitor of activated signal transducer and activator of transcription 1 interacts with the N-terminal domain of mineralocorticoid receptor and represses its transcriptional activity: implication of small ubiquitin-related modifier 1 modification

Molecular mechanisms underlying mineralocorticoid receptor (MR)-mediated gene expression are not fully understood but seem to largely depend upon interactions with specific coregulators. To identify novel human MR (hMR) molecular partners, yeast two-hybrid screenings performed using the N-terminal domain as bait, allowed us to isolate protein inhibitor of activated signal transducer and activator of transcription (PIAS)1 and PIASxbeta, described as SUMO (small ubiquitin-related modifier) E3-ligases. Specific interaction between PIAS1 and hMR was confirmed by glutathione-S-transferase pull-down experiments and N-terminal subdomains responsible for physical contacts were delineated. Transient transfections demonstrated that PIAS1 is a corepressor of aldosterone-activated MR transactivation but has no significant effect on human glucocorticoid receptor transactivation. The agonist or antagonist nature of the bound ligand also determines PIAS1 corepressive action. We provided evidence that PIAS1 conjugated SUMO-1 to hMR both in vitro and in vivo. Deciphering the unique sumoylation pattern of hMR, which possesses five consensus SUMO-1 binding sites, by combinatorial lysine substitutions, revealed a major impact of sumoylation on hMR properties. Using a murine mammary tumor virus promoter, PIAS1 action was independent of sumoylation whereas with glucocorticoid response element promoter, PIAS1 corepressive action depended on hMR sumoylation status. Taken together, our results identify a novel function for PIAS1 which interacts with the N-terminal domain of hMR and represses its ligand-dependent transcriptional activity, at least in part, through SUMO modifications.

The N-CoR/histone deacetylase 3 complex is required for repression by thyroid hormone receptor

Nuclear receptor corepressors (N-CoR) and silencing mediator for retinoid and thyroid receptors (SMRT) have both been implicated in thyroid hormone receptor (TR)-mediated repression. Here we show that endogenous N-CoR, TBL1, and histone deacetylase 3 (HDAC3), but not HDAC1, -2, or -4, are recruited to a stably integrated reporter gene repressed by unliganded TR as well as the orphan receptor RevErb. Unliganded TR also recruits this complex to a transiently transfected reporter, and transcriptional repression is associated with local histone deacetylation that is reversed by the presence of thyroid hormone. Knockdown of N-CoR using small interfering RNAs markedly reduces repression by the TR ligand binding domain in human 293T cells, whereas knockdown of SMRT has little effect. RevErb repression appears to involve both corepressors in this system. Knockdown of HDAC3 markedly reduces repression by both TR and RevErb, while knockdown of HDAC1 or 2 has more modest, partly nonspecific effects. Thus, HDAC3 is critical for repression by multiple nuclear receptors and the N-CoR HDAC3 complex plays a unique and necessary role in TR-mediated gene repression in human 293T cells.

ERbeta Binds N-CoR in the Presence of Estrogens via an LXXLL-like Motif in the N-CoR C-terminus

Nuclear receptors (NRs) usually bind the corepressors N-CoR and SMRT in the absence of ligand or in the presence of antagonists. Agonist binding leads to corepressor release and recruitment of coactivators. Here, we report that estrogen receptor beta (ERbeta) binds N-CoR and SMRT in the presence of agonists, but not antagonists, in vitro and in vivo. This ligand preference differs from that of ERalpha interactions with corepressors, which are inhibited by estradiol, and resembles that of ERbeta interactions with coactivators. ERbeta /N-CoR interactions involve ERbeta AF-2, which also mediates coactivator recognition. Moreover, ERbeta recognizes a sequence (PLTIRML) in the N-CoR C-terminus that resembles coactivator LXXLL motifs. Inhibition of histone deacetylase activity specifically potentiates ERbeta LBD activity, suggesting that corepressors restrict the activity of AF-2. We conclude that the ER isoforms show completely distinct modes of interaction with a physiologically important corepressor and discuss our results in terms of ER isoform specificity in vivo.

Nuclear receptor corepressors

The ability of NR LBDs to transfer repression function to a heterologous DNA binding domain, and the cross-squelching of repression by untethered LBDs, has suggested that repression is mediated by interactions with putative cellular corepressor proteins. The yeast-two hybrid screen for protein interactors has proven to be the key to the isolation and characterization of corepressors. This short review will focus on N-CoR and SMRT.

Liver X receptors interact with corepressors to regulate gene expression

Liver X receptors (LXRs) are members of the nuclear receptor superfamily that regulate gene expression in response to oxysterols and play a critical role in cholesterol homeostasis by regulating genes that are involved in cholesterol transport, catabolism, and triglyceride synthesis. Oxysterols and synthetic agonists bind LXRs and activate transcription by recruiting coactivator proteins. The role of LXRs in regulating target gene expression in the absence of ligand is unknown. Here we show that LXRs interact with corepressors, N-CoR (nuclear receptor corepressor) and SMRT (silent mediator of retinoic acid receptor and thyroid receptor), which are released upon binding agonists. The LXR-corepressor interaction is isoform selective, wherein LXRalpha has a very strong interaction with corepressors and LXRbeta only shows weak interaction. LXRs also exhibit a preference for interacting with N-CoR vs. SMRT. Similar to other nuclear receptors, mutations in the LXR helix 3 and 4 region abolish corepressor interaction. Using a transient transfection assay, we demonstrate that LXR represses transcription that can be further increased by cotransfecting N-CoR into cells. Chromatin immunoprecipitation experiments further indicated that N-CoR is recruited onto endogenous LXR target genes, and addition of LXR agonists releases N-CoR from their promoters. Collectively, these results suggest that corepressors play an important role in regulating LXR target gene expression.

Triiodothyronine increases brain natriuretic peptide (BNP) gene transcription and amplifies endothelin-dependent BNP gene transcription and hypertrophy in neonatal rat ventricular myocytes

Brain natriuretic peptide (BNP) gene expression is a well documented marker of hypertrophy in the cardiac myocyte. Triiodothyronine (T(3)), the bioactive form of thyroid hormone, triggers a unique form of hypertrophy in cardiac myocytes that accompanies the selective activation or suppression of specific gene targets. In this study, we show that the BNP gene is a target of T(3) action. BNP secretion was increased 6-fold, BNP mRNA levels 3-fold, and BNP promoter activity 3-5-fold following T(3) treatment. This was accompanied by an increase in myocyte size, sarcomeric organization, and protein synthesis. Of note, several of the responses to T(3) synergized with those to the conventional hypertrophic agonist endothelin. The response to the liganded thyroid hormone receptor (TR) was mediated by an unusual thyroid hormone response element located between -1000 and -987 relative to the transcription start site. Both TR homodimers and TR.retinoid X receptor heterodimers associated with this element in an electrophoretic mobility shift assay. Protein fragments harboring the LXXLL motifs of the coactivators GRIP1 and SRC1 or TRAP220 interacted predominantly with the TR.retinoid X receptor heterodimeric pair in a ligand-dependent fashion. Both TR homodimers and heterodimers in the unliganded state selectively associated with glutathione S-transferase-nuclear receptor corepressor fragments harboring one of three receptor interaction domains containing the sequence (I/L)XX(I/V)I. These interactions were dissociated following the addition of T(3). Collectively, these findings identify the BNP gene as a potential model for the investigation of TR-dependent gene regulation in the heart.

Identification of a novel testicular orphan receptor-4 (TR4)-associated protein as repressor for the selective suppression of TR4-mediated transactivation

Although many co-activators have been identified for various nuclear receptors, relatively fewer co-repressors have been isolated and characterized. Here we report the identification of a novel testicular orphan nuclear receptor-4 (TR4)-associated protein (TRA16) that is mainly localized in the nucleus of cells as a repressor to suppress TR4-mediated transactivation. The suppression of TR4-mediated transactivation is selective because TRA16 shows only a slight influence on the transactivation of androgen receptor, glucocorticoid receptor, and progesterone receptor. Sequence analysis shows that TRA16 is a novel gene with 139 amino acids in an open reading frame with a molecular mass of 16 kDa, which did not match any published gene sequences. Mammalian two-hybrid system and co-immunoprecipitation assays both demonstrate that TRA16 can interact strongly with TR4. The electrophoretic mobility shift assay suggests that TRA16 may suppress TR4-mediated transactivation via decreased binding between the TR4 protein and the TR4 response element on the target gene(s). Furthermore, TRA16 can also block the interaction between TR4 and TR4 ligand-binding domain through interacting with TR4-DNA-binding and ligand-binding domains. These unique suppression mechanisms suggest that TRA16 may function as a novel repressor to selectively suppress the TR4-mediated transactivation.

Differential SERM effects on corepressor binding dictate ERalpha activity in vivo

Selective estrogen receptor modulators (SERMs) show differential effects upon ERalpha activation function 1 (AF-1). Tamoxifen allows strong ERalpha AF-1 activity, whereas raloxifene allows less and ICI 182,780 (ICI) allows none. Here, we show that blockade of corepressor histone de-acetylase (HDAC) activity reverses the differential inhibitory effect of SERMs upon AF-1 activity in MCF-7 cells. This suggests that differential SERM repression of AF-1 involves HDAC-dependent corepressors. Consistent with this, ICI and raloxifene are more potent than tamoxifen in promoting ERalpha-dependent sequestration of progesterone receptor-associated corepressors. Moreover, ICI and raloxifene are more efficient than tamoxifen in promoting ERalpha binding to the corepressor N-CoR in vivo and in vitro. An ERalpha mutation (537X) that increases N-CoR binding in the presence of all SERMs blocks AF-1 activity. An ERalpha mutation (L379R) that decreases N-CoR binding increases AF-1 activity in the presence of ICI and raloxifene and reverses the effect of the 537X mutation. The 537X and L379R mutations also alter the ligand preference of ERalpha action at AP-1 sites and C3 complement, an action that also involves AF-1. Together, our results suggest that differential SERM effects on corepressor binding can explain differences in SERM effects on ERalpha activity. We propose a model for differential effects of SERMs on N-CoR binding.

Co-repressor release but not ligand binding is a prerequisite for transcription activation by human retinoid acid receptor alpha ligand-binding domain

Nuclear hormone receptors coordinately regulate the activity of genetic networks through the recruitment of transcriptional co-regulators, including co-repressors and co-activators. Allosteric modulation of the ligand-binding domain by hormonal activators shifts the co-factor binding preference by defined structural changes in overlapping docking sites. We report here that mutations at conserved residues within the docking motif of the retinoic acid receptor alpha cause defects in dimerization, co-regulator association, and transcriptional regulation. Furthermore, although a minimal co-repressor receptor interaction domain is sufficient for receptor binding, flanking sequences appear to stabilize this interaction without interfering with ligand sensitivity. However, ligand sensitivity is changed by the K262A mutation, which requires much higher concentrations of all-trans-retinoic acid to promote co-repressor dissociation. Consequently, K262A functions as a dominant-negative mutant at low concentrations of all-trans-retinoic acid. As a result, transcriptional activation is mechanistically linked to co-repressor release.

Determination of nuclear receptor corepressor interactions with the thyroid hormone receptor

The thyroid hormone receptor (TR) recruits the nuclear corepressors, nuclear receptor corepressor (NCoR) and silencing mediator of retinoid and thyroid hormone receptors (SMRT), to target DNA elements in the absence of ligand. While the TR preferentially recruits NCoR, the mechanism remains unclear. The corepressors interact with the TR via interacting domains (IDs) present in their C terminus which contain a conserved motif termed a CoRNR box. Despite their similarity, the corepressor IDs allow for nuclear receptor specificity. Here we demonstrate that NCoR stabilizes the TR homodimer when bound to DNA by preventing its dissociation from thyroid hormone response elements. This suggests that NCoR acts to hold the repression complex in place on target elements. The TR homodimer recruits NCoR through two of its three IDs, one of which is not present in SMRT. This unique ID, N3, contains a CoRNR box but lacks the extended helical motif present in each of the other IDs. Instead, N3 contains an isoleucine just proximal to this motif. This isoleucine is also conserved in N2 but not in the corresponding S2 domain in SMRT. On thyroid hormone response elements and in mammalian cells this residue is critical in both N3 and N2 for high-affinity TR binding. In addition, this residue also controls specificity for the interactions of TR with NCoR. Together these data suggest that the specific recruitment of NCoR by the TR through a unique motif allows for stabilization of the repression complex on target elements.

Design of thyroid hormone receptor antagonists from first principles

It is desirable to obtain TR antagonists for treatment of hyperthyroidism and other conditions. We have designed TR antagonists from first principles based on TR crystal structures. Since agonist ligands are buried in the fold of the TR ligand binding domain (LBD), we reasoned that ligands that resemble agonists with large extensions should bind the LBD, but would prevent its folding into an active conformation. In particular, we predicted that extensions at the 5' aryl position of ligand should reposition helix (H) 12, which forms part of the co-activator binding surface, and thereby inhibit TR activity. We have found that some synthetic ligands with 5' aryl ring extensions behave as antagonists (DIBRT, NH-3), or partial antagonists (GC-14, NH-4). Moreover, one compound (NH-3) represents the first potent TR antagonist with nanomolar affinity that also inhibits TR action in an animal model. However, the properties of the ligands also reveal unexpected aspects of TR behavior. While nuclear receptor antagonists generally promote binding of co-repressors, NH-3 blocks co-activator binding and also prevents co-repressor binding. More surprisingly, many compounds with extensions behave as full or partial agonists. We present hypotheses to explain both behaviors in terms of dynamic equilibrium of H12 position.

Nuclear receptor corepressor recruitment by unliganded thyroid hormone receptor in gene repression during Xenopus laevis development

Thyroid hormone receptors (TR) act as activators of transcription in the presence of the thyroid hormone (T(3)) and as repressors in its absence. While many in vitro approaches have been used to study the molecular mechanisms of TR action, their physiological relevance has not been addressed. Here we investigate how TR regulates gene expression during vertebrate postembryonic development by using T(3)-dependent amphibian metamorphosis as a model. Earlier studies suggest that TR acts as a repressor during premetamorphosis when T(3) is absent. We hypothesize that corepressor complexes containing the nuclear receptor corepressor (N-CoR) are key factors in this TR-dependent gene repression, which is important for premetamorphic tadpole growth. To test this hypothesis, we isolated Xenopus laevis N-CoR (xN-CoR) and showed that it was present in pre- and metamorphic tadpoles. Using a chromatin immunoprecipitation assay, we demonstrated that xN-CoR was recruited to the promoters of T(3) response genes during premetamorphosis and released upon T(3) treatment, accompanied by a local increase in histone acetylation. Furthermore, overexpression of a dominant-negative N-CoR in tadpole tail muscle led to increased transcription from a T(3)-dependent promoter. Our data indicate that N-CoR is recruited by unliganded TR to repress target gene expression during premetamorphic animal growth, an important process that prepares the tadpole for metamorphosis.

Phosphorylation and intramolecular stabilization of the ligand binding domain in the nuclear receptor steroidogenic factor 1

Steroidogenic factor 1 (SF-1) is an orphan nuclear receptor with no known ligand. We showed previously that phosphorylation at serine 203 located N'-terminal to the ligand binding domain (LBD) enhanced cofactor recruitment, analogous to the ligand-mediated recruitment in ligand-dependent receptors. In this study, results of biochemical analyses and an LBD helix assembly assay suggest that the SF-1 LBD adopts an active conformation, with helices 1 and 12 packed against the predicted alpha-helical bundle, in the apparent absence of ligand. Fine mapping of the previously defined proximal activation function in SF-1 showed that the activation function mapped fully to helix 1 of the LBD. Limited proteolyses demonstrate that phosphorylation of S203 in the hinge region mimics the stabilizing effects of ligand on the LBD. Moreover, similar effects were observed in an SF-1/thyroid hormone LBD chimera receptor, illustrating that the S203 phosphorylation effects are transferable to a heterologous ligand-dependent receptor. Our collective data suggest that the hinge together with helix 1 is an individualized specific motif, which is tightly associated with its cognate LBD. For SF-1, we find that this intramolecular association and hence receptor activity are further enhanced by mitogen-activated protein kinase phosphorylation, thus mimicking many of the ligand-induced changes observed for ligand-dependent receptors.

Putative role of the orphan nuclear receptor SXR (steroid and xenobiotic receptor) in the mechanism of CYP3A4 inhibition by xenobiotics

Cytochrome P450 monooxygenase 3A4 (CYP3A4) is responsible for the metabolism of endogenous steroids and drugs in liver. Many inducers of human CYP3A4, such as rifampicin, bind to the orphan nuclear receptor SXR (steroid and xenobiotic receptor) as ligands and stimulate transcription on xenobiotic response elements located in the CYP3A4 promoter. Conversely, it is not known whether SXR mediates the transcriptional repression. We thus examined transcriptional repression of SXR and its interaction with corepressors, NCoR (nuclear receptor corepressor) and SMRT (silencing mediator for retinoid and thyroid receptors) using reporter assays in the absence and presence of ligand. Cotransfection of SMRT, but not NCoR, inhibited not only basal but also rifampicin-induced transcriptional activity of SXR on the CYP3A4 promoter through specific SMRT-SXR interaction in HepG2 cells. Interestingly, rifampicin also increased the interaction of SXR with SMRT as well as with coactivator SRC-1. On the other hand, the anti-fungal agent ketoconazole decreased SXR interaction with both SRC-1 and SMRT. Ketoconazole partially inhibited corticosterone-induced SXR-mediated transcription on the CYP3A4 promoter. Taken together, our results suggest that the differential interaction of coactivators and corepressors induced by various xenobiotics may alter SXR-mediated transcription. Further, the effects of ketoconazole on the CYP3A4 gene suppression may explain, in part, drug-induced inhibition of the CYP3A4 action at the transcriptional level.

RU486-induced glucocorticoid receptor agonism is controlled by the receptor N terminus and by corepressor binding

Glucocorticoid-induced gene transcription has been shown to be mediated by coactivators bound to the glucocorticoid receptor (GR). The glucocorticoid antagonist RU486 interferes with the steroid-mediated activation and can also exhibit partial agonist activity, a response in which corepressors have been implicated. Here we have shown that deletion of the N terminus of GR totally abolishes the agonist activity of RU486. Furthermore, we have demonstrated that corepressors bind directly to the RU486-bound GR as determined by glutathione S-transferase pull-down, mammalian two-hybrid assay, and coimmunoprecipitation. Fine mapping of the interaction regions within GR and the corepressor NCoR reveals a complex interaction profile that involves a number of domains in each protein. Notably, the N and the C termini of GR are both involved in corepressor binding. Thus, the N terminus of GR is a major determinant for RU486-dependent NCoR interaction as well as for RU486-mediated agonist activity.

Rational design and synthesis of a novel thyroid hormone antagonist that blocks coactivator recruitment

Recent efforts have focused on the design and synthesis of thyroid hormone (T(3)) antagonists as potential therapeutic agents and chemical probes to understand hormone-signaling pathways. We previously reported the development of novel first-generation T(3) antagonists DIBRT, HY-4, and GC-14 using the "extension hypothesis" as a general guideline in hormone antagonist design.(1-3) These compounds contain extensions at the 5'-position (DIBRT, GC-14) of the outer thyronine ring or from the bridging carbon (HY-4). All of these compounds have only a modest affinity and potency for the thyroid hormone receptor (TR) that limits studies of their antagonistic actions. Here, we report the design and synthesis of a novel series of 5'-phenylethynyl derivatives sharing the GC-1 halogen-free thyronine scaffold.(4) One compound (NH-3) is a T(3) antagonist with negligible TR agonist activity and improved TR binding affinity and potency that allow for further characterization of its observed activity. One mechanism for antagonism appears to be the ability of NH-3 to block TR-coactivator interactions. NH-3 will be a useful pharmacological tool for further study of T(3) signaling and TR function.

Inhibition of the dihydrotestosterone-activated androgen receptor by nuclear receptor corepressor

Nuclear receptor corepressor (NCoR) mediates transcriptional repression by unliganded nuclear receptors and certain steroid hormone receptors (SHRs) bound to nonphysiological antagonists, but has not been found to regulate SHRs bound to their natural ligands. This report demonstrates that NCoR interacts directly with the androgen receptor (AR) and represses dihydrotestosterone-stimulated AR transcriptional activity. The NCoR C terminus, containing the receptor interacting domains, was necessary for repression, which was ablated by mutations in the corepressor nuclear receptor (CoRNR) boxes. In contrast, the NCoR N terminus, containing domains that can recruit histone deacetylases, was not necessary for repression. Binding studies in vitro with a series of glutathione-S-transferase-NCoR and -AR fusion proteins demonstrated a direct interaction that was similarly dependent upon the NCoR corepressor nuclear receptor boxes and AR ligand binding domain and was independent of ligand and helix 12 in the AR ligand binding domain. This NCoR-AR interaction was further demonstrated in mammalian two-hybrid assays and by coimmunoprecipitation of the endogenous proteins from a prostate cancer cell line. Finally, AR transcriptional activity could be enhanced in vivo by sequestration of endogenous NCoR with unliganded thyroid hormone receptor. These results demonstrate that AR, in contrast to other SHRs, is regulated by NCoR and suggest the possibility of developing selective AR modulators that enhance this interaction.

Nuclear receptor corepressor-dependent repression of peroxisome-proliferator-activated receptor delta-mediated transactivation

The nuclear receptor corepressor (NCoR) was isolated as a peroxisome-proliferator-activated receptor (PPAR) delta interacting protein using the yeast two-hybrid system. NCoR interacted strongly with the ligand-binding domain of PPAR delta, whereas interactions with the ligand-binding domains of PPAR gamma and PPAR alpha were significantly weaker. PPAR-NCoR interactions were antagonized by ligands in the two-hybrid system, but were ligand-insensitive in in vitro pull-down assays. Interaction between PPAR delta and NCoR was unaffected by coexpression of retinoid X receptor (RXR) alpha. The PPAR delta-RXR alpha heterodimer bound to an acyl-CoA oxidase (ACO)-type peroxisome-proliferator response element recruited a glutathione S-transferase-NCoR fusion protein in a ligand-independent manner. Contrasting with most other nuclear receptors, PPAR delta was found to interact equally well with interaction domains I and II of NCoR. In transient transfection experiments, NCoR and the related silencing mediator for retinoid and thyroid hormone receptor (SMRT) were shown to exert a marked dose-dependent repression of ligand-induced PPAR delta-mediated transactivation; in addition, transactivation induced by the cAMP-elevating agent forskolin was efficiently reduced to basal levels by NCoR as well as SMRT coexpression. Our results suggest that the transactivation potential of liganded PPAR delta can be fine-tuned by interaction with NCoR and SMRT in a manner determined by the expression levels of corepressors and coactivators.

The hairless gene mutated in congenital hair loss disorders encodes a novel nuclear receptor corepressor

The mammalian hairless (hr) gene plays a critical role in the maintenance of hair growth. Although the hr gene has been identified, the biochemical function of its encoded protein (Hr) has remained obscure. Here, we show that Hr functions as a transcriptional corepressor for thyroid hormone receptors (TRs). We find that two independent regions of Hr mediate TR binding and that interaction requires a cluster of hydrophobic residues similar to the binding motifs proposed for nuclear receptor corepressors (N-CoR and SMRT). Similarly, we show that Hr binds to the same region of TR as known corepressors. We show that Hr interacts with histone deacetylases (HDACs) and is localized to matrix-associated deacetylase (MAD) bodies, indicating that the mechanism of Hr-mediated repression is likely through associated HDAC activity. Thus, Hr is a component of the corepressor machinery, and despite its lack of sequence identity with previously described corepressors, its mode of action is remarkably conserved. On the basis of its thyroid hormone-inducible and tissue- and developmental-specific expression, Hr likely defines a new class of nuclear receptor corepressors that serve a more specialized role than ubiquitous corepressors. The discovery that Hr is a corepressor provides a molecular basis for specific hair loss syndromes in both humans and mice.

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.