Unique forms of human and mouse nuclear receptor corepressor SMRT

Nuclear receptor corepressors non-canonically drive glucocorticoid receptor-dependent activation of hepatic gluconeogenesis

Nuclear receptor corepressors (NCoRs) function in multiprotein complexes containing histone deacetylase 3 (HDAC3) to alter transcriptional output primarily through repressive chromatin remodelling at target loci1-5. In the liver, loss of HDAC3 causes a marked hepatosteatosis largely because of de-repression of genes involved in lipid metabolism6,7; however, the individual roles and contribution of other complex members to hepatic and systemic metabolic regulation are unclear. Here we show that adult loss of both NCoR1 and NCoR2 (double knockout (KO)) in hepatocytes phenocopied the hepatomegalic fatty liver phenotype of HDAC3 KO. In addition, double KO livers exhibited a dramatic reduction in glycogen storage and gluconeogenic gene expression that was not observed with hepatic KO of individual NCoRs or HDAC3, resulting in profound fasting hypoglycaemia. This surprising HDAC3-independent activation function of NCoR1 and NCoR2 is due to an unexpected loss of chromatin accessibility on deletion of NCoRs that prevented glucocorticoid receptor binding and stimulatory effect on gluconeogenic genes. These studies reveal an unanticipated, non-canonical activation function of NCoRs that is required for metabolic health.

Acetylation of nuclear receptors in health and disease: an update

Lysine acetylation is a common reversible post-translational modification of proteins that plays a key role in regulating gene expression. Nuclear receptors (NRs) include ligand-inducible transcription factors and orphan receptors for which the ligand is undetermined, which together regulate the expression of genes involved in development, metabolism, homeostasis, reproduction and human diseases including cancer. Since the original finding that the ERα, AR and HNF4 are acetylated, we now understand that the vast majority of NRs are acetylated and that this modification has profound effects on NR function. Acetylation sites are often conserved and involve both ordered and disordered regions of NRs. The acetylated residues function as part of an intramolecular signalling platform intersecting phosphorylation, methylation and other modifications. Acetylation of NR has been shown to impact recruitment into chromatin, co-repressor and coactivator complex formation, sensitivity and specificity of regulation by ligand and ligand antagonists, DNA binding, subcellular distribution and transcriptional activity. A growing body of evidence in mice indicates a vital role for NR acetylation in metabolism. Additionally, mutations of the NR acetylation site occur in human disease. This review focuses on the role of NR acetylation in coordinating signalling in normal physiology and disease.

FUS-ERG induces late-onset azacitidine resistance in acute myeloid leukaemia cells

FUS-ERG is a chimeric gene with a poor prognosis, found in myelodysplastic syndromes (MDS) and acute myeloid leukaemia (AML). It remains unclear whether DNA hypomethylating agents, including azacitidine (Aza), are effective in FUS-ERG-harbouring AML and how FUS-ERG induces chemoresistance. Stable Ba/F3 transfectants with FUS-ERG were repeatedly exposed to Aza for 7 days of treatment and at 21-day intervals to investigate Aza sensitivity. Stable FUS-ERG transfectants acquired resistance acquired resistance after three courses of Aza exposure. RNA sequencing (RNA-seq) was performed when Aza susceptibility began to change; genes with altered expression or transcript variants were identified. Molecular signatures of these genes were analysed using gene ontology. RNA-seq analyses identified 74 upregulated and 320 downregulated genes involved in cell motility, cytokine production, and kinase activity. Additionally, 1321 genes with altered transcript variants were identified, revealing their involvement in chromatin organisation. In a clinical case of AML with FUS-ERG, we compared whole-genome alterations between the initial MDS diagnosis and AML recurrence after Aza treatment. Genes with non-synonymous or near mutations in transcription regulatory areas (TRAs), additionally detected in AML recurrence, were collated with the gene list from RNA-seq to identify genes involved in acquiring Aza resistance in the presence of FUS-ERG. Whole-genome sequencing of clinical specimens identified 29 genes with non-synonymous mutations, including BCOR, and 48 genes located within 20 kb of 54 TRA mutations in AML recurrence. These genes were involved in chromatin organisation and included NCOR2 as an overlapping gene with RNA-seq data. Transcription regulators involved in mutated TRAs were skewed and included RCOR1 in AML recurrence. We tested the efficacy of BH3 mimetics, including venetoclax and S63845, in primary Aza-resistant AML cells treated with FUS-ERG. Primary FUS-ERG-harbouring AML cells acquiring Aza resistance affected the myeloid cell leukaemia-1 (MCL1) inhibitor S63845 but not while using venetoclax, despite no mutations in BCL2. FUS-ERG promoted Aza resistance after several treatments. The disturbance of chromatin organisation might induce this by co-repressors, including BCOR, NCOR2, and RCOR1. MCL1 inhibition could partially overcome Aza resistance in FUS-ERG-harbouring AML cells.

Genetic Alterations in Papillary Thyroid Carcinoma With Hashimoto's Thyroiditis： ANK3, an Indolent Maintainer of Papillary Thyroid Carcinoma

Hashimoto’s thyroiditis (TH) is a risk factor for the occurrence of papillary thyroid carcinoma (PTC), which is considered to be the most common type of thyroid cancer. In recent years, the prevalence of PTC with TH has been increasing, but little is known about the genetic alteration in PTC with TH. This study analyzed the mutation spectrum and mutation signature of somatic single nucleotide variants (SNV) for 10 non-tumor and tumor pair tissues of PTC with TH using whole-exome sequencing. The ANK3 protein expression was evaluated by immunohistochemistry in PTC with TH and PTC samples. Moreover, the functional role of ANK3 in PTC cells was determined by CCK-8 proliferation assay, colony formation assays, cell cycle analysis, cell invasion and migration and in vivo study through overexpression assay. Our results showed three distinct mutational signatures and the C>T/G>A substitution was the most common type of SNV. Gene-set enrichment analysis showed that most of the significantly mutated genes were enriched in the regulation of actin cytoskeleton signaling. Moreover, NCOR2, BPTF, ANK3, and PCSK5 were identified as the significantly mutated genes in PTC with TH, most of which have not been previously characterized. Unexpectedly, it was found that ANK3 was overexpressed in cytoplasm close to the membrane of PTC cells with TH and in almost all PTC cases, suggesting its role as a diagnostic marker of PTC. Ectopic expression of ANK3 suppressed invasion and migration, increased apoptosis of B-CPAP and TPC-1 cells. Moreover, our findings revealed that enhanced ANK3 expression inhibits growth of PTC cells both in vitro and in vivo. Ectopic expression of ANK3 significantly enhanced E-cadherin protein expression and inhibited PTC progression, at least in part, by suppression of epithelial-mesenchymal transition (EMT). Our study shows that ANK3 exerts an anti-oncogenic role in the development of PTC and might be an indolent maintainer of PTC.

Histone Deacetylases (HDACs): Evolution, Specificity, Role in Transcriptional Complexes, and Pharmacological Actionability

Histone deacetylases (HDACs) are evolutionary conserved enzymes which operate by removing acetyl groups from histones and other protein regulatory factors, with functional consequences on chromatin remodeling and gene expression profiles. We provide here a review on the recent knowledge accrued on the zinc-dependent HDAC protein family across different species, tissues, and human pathologies, specifically focusing on the role of HDAC inhibitors as anti-cancer agents. We will investigate the chemical specificity of different HDACs and discuss their role in the human interactome as members of chromatin-binding and regulatory complexes.

The Nuclear Receptor-Co-repressor Complex in Control of Liver Metabolism and Disease

Hepatocytes are the major cell-type in the liver responsible for the coordination of metabolism in response to multiple signaling inputs. Coordination occurs primarily at the level of gene expression via transcriptional networks composed of transcription factors, in particular nuclear receptors (NRs), and associated co-regulators, including chromatin-modifying complexes. Disturbance of these networks by genetic, environmental or nutritional factors can lead to metabolic dysregulation and has been linked to the progression of non-alcoholic fatty liver disease (NAFLD) toward steatohepatitis and even liver cancer. Since there are currently no approved therapies, major efforts are dedicated to identify the critical factors that can be employed for drug development. Amongst the identified factors with clinical significance are currently lipid-sensing NRs including PPARs, LXRs, and FXR. However, major obstacles of NR-targeting are the undesired side effects associated with the genome-wide NR activities in multiple cell-types. Thus, of particular interest are co-regulators that determine NR activities, context-selectivity, and associated chromatin states. Current research on the role of co-regulators in hepatocytes is still premature due to the large number of candidates, the limited number of available mouse models, and the technical challenges in studying their chromatin occupancy. As a result, how NR-co-regulator networks in hepatocytes are coordinated by extracellular signals, and how NR-pathway selectivity is achieved, remains currently poorly understood. We will here review a notable exception, namely a fundamental transcriptional co-repressor complex that during the past decade has become the probably most-studied and best-understood physiological relevant co-regulator in hepatocytes. This multiprotein complex contains the core subunits HDAC3, NCOR, SMRT, TBL1, TBLR1, and GPS2 and is referred to as the "NR-co-repressor complex." We will particularly discuss recent advances in characterizing hepatocyte-specific loss-of-function mouse models and in applying genome-wide sequencing approaches including ChIP-seq. Both have been instrumental to uncover the role of each of the subunits under physiological conditions and in disease models, but they also revealed insights into the NR target range and genomic mechanisms of action of the co-repressor complex. We will integrate a discussion of translational aspects about the role of the complex in NAFLD pathways and in particular about the hypothesis that patient-specific alterations of specific subunits may determine NAFLD susceptibility and the therapeutic outcomes of NR-directed treatments.

Corepressor SMRT is required to maintain Hox transcriptional memory during somitogenesis

Retinoic acid (RA) is an important transcriptional regulator during both vertebrate and invertebrate body pattern formation. The Homeobox (Hox) gene family is activated by a gradient of RA formed along the length of the embryo at specific time points during fetal development. Generation of a genetically modified mouse harboring mutations in the SMRT repressor demonstrated that SMRT-dependent repression of retinoic acid receptor (RAR) is critical to establish and maintain the somitic Hox code and segmental identity during fetal development via epigenetic marking of target loci.

Nuclear hormone receptors (NRs), such as retinoic acid receptors (RARs), play critical roles in vertebrate development and homeostasis by regulating target gene transcription. Their activity is controlled by ligand-dependent release of corepressors and subsequent recruitment of coactivators, but how these individual receptor modes contribute to development are unknown. Here, we show that mice carrying targeted knockin mutations in the corepressor Silencing Mediator of Retinoid and Thyroid hormone receptor (SMRT) that specifically disable SMRT function in NR signaling (SMRT^mRID), display defects in cranial neural crest cell-derived structures and posterior homeotic transformations of axial vertebrae. SMRT^mRID embryos show enhanced transcription of RAR targets including Hox loci, resulting in respecification of vertebral identities. Up-regulated histone acetylation and decreased H3K27 methylation are evident in the Hox loci whose somitic expression boundaries are rostrally shifted. Furthermore, enhanced recruitment of super elongation complex is evident in rapidly induced non-Pol II-paused targets in SMRT^mRID embryonic stem cells. These results demonstrate that SMRT-dependent repression of RAR is critical to establish and maintain the somitic Hox code and segmental identity during fetal development via epigenetic marking of target loci.

NCOR1-a new player on the field of T cell development

Nuclear receptor corepressor 1 (NCOR1) is a transcriptional corepressor that links chromatin-modifying enzymes with gene-specific transcription factors. Although identified more than 20 years ago as a corepressor of nuclear receptors, the role of NCOR1 in T cells remained only poorly understood. However, recent studies indicate that the survival of developing thymocytes is regulated by NCOR1, revealing an essential role for NCOR1 in the T cell lineage. In this review, we will briefly summarize basic facts about NCOR1 structure and functions. We will further summarize studies demonstrating an essential role for NCOR1 in controlling positive and negative selection of thymocytes during T cell development. Finally, we will discuss similarities and differences between the phenotypes of mice with a T cell-specific deletion of NCOR1 or histone deacetylase 3 (HDAC3), because HDAC3 is the predominant member of the HDAC family that interacts with NCOR1 corepressor complexes. With this review we aim to introduce NCOR1 as a new player in the team of transcriptional coregulators that control T cell development and thus the generation of the peripheral T cell pool.

Role of Coactivators and Corepressors in Steroid and Nuclear Receptor Signaling: Potential Markers of Tumor Growth and Drug Sensitivity

Nuclear receptors regulate target gene expression in response to steroid and thyroid hormones, retinoids, vitamin D and other ligands. These ligand-dependent transcription factors function by contacting various nuclear cooperating proteins, called coactivators and corepressors, which mediate local chromatin remodeling as well as communication with the basal transcriptional apparatus. Nuclear receptors and their coregulatory proteins play a role in cancer and other diseases, one leading example being the estrogen receptor pathway in breast cancer. Coregulators are often present in limiting amounts in cell nuclei and modifications of their level of expression and/or structure lead to alterations in nuclear receptor functioning, which may be as pronounced as a complete inversion of signaling, i.e. from stimulating to repressing certain genes in response to an identical stimulus. In addition, hemizygous knock-out of certain coactivator genes has been demonstrated to produce cancer-prone phenotypes in mice. Thus, assessment of coactivator and corepressor expression and structure in tumors may turn out to be essential to determine the role of nuclear receptors in cancer and to predict prognosis and response to therapy.

Corepressor diversification by alternative mRNA splicing is species specific

BACKGROUND

SMRT and NCoR are corepressor paralogs that help mediate transcriptional repression by a variety of transcription factors, including the nuclear hormone receptors. The functions of both corepressors are extensively diversified in mice by alternative mRNA splicing, generating a series of protein variants that differ in different tissues and that exert different, even diametrically opposite, biochemical and biological effects from one another.

RESULTS

We report here that the alternative splicing previously reported for SMRT appears to be a relatively recent evolutionary phenomenon, with only one of these previously identified sites utilized in a teleost fish and a limited additional number of the additional known sites utilized in a bird, reptile, and marsupial. In contrast, extensive SMRT alternative splicing at these sites was detected among the placental mammals. The alternative splicing of NCoR previously identified in mice (and shown to regulate lipid and carbohydrate metabolism) is likely to have arisen separately and after that of SMRT, and includes an example of convergent evolution.

CONCLUSIONS

We propose that the functions of both SMRT and NCoR have been diversified by alternative splicing during evolution to allow customization for different purposes in different tissues and different species.

ROS1 fusions in cancer: a review

The ROS1 gene belongs to the sevenless subfamily of tyrosine kinase insulin receptor genes. A literature review identified a ROS1 fusion in 2.54% of the patients with lung adenocarcinoma and even higher frequencies in spitzoid neoplasms and inflammatory myofibroblastic tumors. At present, 26 genes were found to fuse with ROS1, some of them already known to fuse with RET and ALK. All the fusion proteins retain the ROS1 kinase domain, but rarely its transmembrane domain. Most of the partners have dimerization domains that are retained in the fusion, presumably leading to constitutive ROS1 tyrosine kinase activation. Some partners have transmembrane domains that are retained or not in the chimeric proteins. Therefore, different ROS1 fusions have distinct subcellular localization, suggesting that they may activate different substrates in vivo.

Evaluation of NF-κB subunit expression and signaling pathway activation demonstrates that p52 expression confers better outcome in germinal center B-cell-like diffuse large B-cell lymphoma in association with CD30 and BCL2 functions

Nuclear factor-κB (NF-κB) is a transcription factor with a well-described oncogenic role. Study for each of five NF-κB pathway subunits was only reported on small cohorts in diffuse large B-cell lymphoma (DLBCL). In this large cohort (n=533) of patients with de novo DLBCL, we evaluated the protein expression frequency, gene expression signature, and clinical implication for each of these five NF-κB subunits. Expression of p50, p52, p65, RELB, and c-Rel was 34%, 12%, 20%, 14%, and 23%, whereas p50/p65, p50/c-Rel, and p52/RELB expression was 11%, 11%, and 3%, respectively. NF-κB subunits were expressed in both germinal center B-cell-like (GCB) and activated B-cell-like (ABC) DLBCL, but p50 and p50/c-Rel were associated with ABC-DLBCL. p52, RELB, and p52/RELB expressions were associated with CD30 expression. p52 expression was negatively associated with BCL2 (B-cell lymphoma 2) expression and BCL2 rearrangement. Although p52 expression was associated with better progression-free survival (PFS) (P=0.0170), singular expression of the remaining NF-κB subunits alone did not show significant prognostic impact in the overall DLBCL cohort. Expression of p52/RELB was associated with better overall survival (OS) and PFS (P=0.0307 and P=0.0247). When cases were stratified into GCB- and ABC-DLBCL, p52 or p52/RELB dimer expression status was associated with better OS and PFS (P=0.0134 and P=0.0124) only within the GCB subtype. However, multivariate analysis did not show p52 expression to be an independent prognostic factor. Beneficial effect of p52 in GCB-DLBC appears to be its positive correlation with CD30 and negative correlation with BCL2 expression. Gene expression profiling (GEP) showed that p52(+) GCB-DLBCL was distinct from p52(-) GCB-DLBCL. Collectively, our data suggest that DLBCL patients with p52 expression might not benefit from therapy targeting the NF-κB pathway.

Nuclear receptor corepressor complexes in cancer: mechanism, function and regulation

Nuclear receptor corepressor (NCoR) and silencing mediator for retinoid and thyroid hormone receptors (SMRT) function as corepressors for diverse transcription factors including nuclear receptors such as estrogen receptors and androgen receptors. Deregulated functions of NCoR and SMRT have been observed in many types of cancers and leukemias. NCoR and SMRT directly bind to transcription factors and nucleate the formation of stable complexes that include histone deacetylase 3, transducin b-like protein 1/TBL1-related protein 1, and G-protein pathway suppressor 2. These NCoR/SMRT-interacting proteins also show deregulated functions in cancers. In this review, we summarize the literature on the mechanism, regulation, and function of the core components of NCoR/SMRT complexes in the context of their involvement in cancers and leukemias. While the current studies support the view that the corepressors are promising targets for cancer treatment, elucidation of the mechanisms of corepressors involved in individual types of cancers is likely required for effective therapy.

Activation of p53 transcriptional activity by SMRT: a histone deacetylase 3-independent function of a transcriptional corepressor

The silencing mediator of retinoic acid and thyroid hormone receptors (SMRT) is an established histone deacetylase 3 (HDAC3)-dependent transcriptional corepressor. Microarray analyses of MCF-7 cells transfected with control or SMRT small interfering RNA revealed SMRT regulation of genes involved in DNA damage responses, and the levels of the DNA damage marker γH2AX as well as poly(ADP-ribose) polymerase cleavage were elevated in SMRT-depleted cells treated with doxorubicin. A number of these genes are established p53 targets. SMRT knockdown decreased the activity of two p53-dependent reporter genes as well as the expression of p53 target genes, such as CDKN1A (which encodes p21). SMRT bound directly to p53 and was recruited to p53 binding sites within the p21 promoter. Depletion of GPS2 and TBL1, components of the SMRT corepressor complex, but not histone deacetylase 3 (HDAC3) decreased p21-luciferase activity. p53 bound to the SMRT deacetylase activation domain (DAD), which mediates HDAC3 binding and activation, and HDAC3 could attenuate p53 binding to the DAD region of SMRT. Moreover, an HDAC3 binding-deficient SMRT DAD mutant coactivated p53 transcriptional activity. Collectively, these data highlight a biological role for SMRT in mediating DNA damage responses and suggest a model where p53 binding to the DAD limits HDAC3 interaction with this coregulator, thereby facilitating SMRT coactivation of p53-dependent gene expression.

Rescue of a primary myelofibrosis model by retinoid-antagonist therapy

Molecular targeting of the two receptor interaction domains of the epigenetic repressor silencing mediator of retinoid and thyroid hormone receptors (SMRT(mRID)) produced a transplantable skeletal syndrome that reduced radial bone growth, increased numbers of bone-resorbing periosteal osteoclasts, and increased bone fracture risk. Furthermore, SMRT(mRID) mice develop spontaneous primary myelofibrosis, a chronic, usually idiopathic disorder characterized by progressive bone marrow fibrosis. Frequently linked to polycythemia vera and chronic myeloid leukemia, myelofibrosis displays high patient morbidity and mortality, and current treatment is mostly palliative. To decipher the etiology of this disease, we identified the thrombopoietin (Tpo) gene as a target of the SMRT-retinoic acid receptor signaling pathway in bone marrow stromal cells. Chronic induction of Tpo in SMRT(mRID) mice results in up-regulation of TGF-β and PDGF in megakaryocytes, uncontrolled proliferation of bone marrow reticular cells, and fibrosis of the marrow compartment. Of therapeutic relevance, we show that this syndrome can be rescued by retinoid antagonists, demonstrating that the physical interface between SMRT and retinoic acid receptor can be a potential therapeutic target to block primary myelofibrosis disease progression.

Emerging roles of the corepressors NCoR1 and SMRT in homeostasis

Epigenetic regulation of gene expression is strongly influenced by the accessibility of nucleosomal DNA or the state of chromatin compaction. In this context, coregulators, including both coactivators and corepressors, are pivotal intermediates that bridge chromatin-modifying enzymes and transcription factors. NCoR1 (nuclear receptor corepressor) and SMRT (silencing mediator of retinoic acid and thyroid hormone receptor) are among the best-characterized corepressors from a molecular point of view. These coregulators have conserved orthologs in lower organisms, which underscores their functional importance. Here we summarize the results from recent in vivo studies that reveal the wide-ranging roles of NCoR1 and SMRT in developmental as well as homeostatic processes, including metabolism, inflammation, and circadian rhythms. We also discuss the potential implications of NCoR1 and SMRT regulation of pathways ranging from genomic stability and carcinogenesis to metabolic diseases such as type 2 diabetes.

The NuRD architecture

The nucleosome remodeling and deacetylase (NuRD) complex regulates chromatin organization, gene transcription, genomic stability and developmental signaling. NuRD has a unique dual enzymatic activity, containing an ATPase and a histone deacetylase among its six core subunits. Recent studies indicate that NuRD composition and the interplay between subunits may dictate the diverse functions of the complex. In this review, we examine the structures and biological roles of the NuRD subunits and discuss new avenues of research to advance our understanding of the NuRD-mediated signaling network.

PA1 protein, a new competitive decelerator acting at more than one step to impede glucocorticoid receptor-mediated transactivation

Numerous cofactors modulate the gene regulatory activity of glucocorticoid receptors (GRs) by affecting one or more of the following three major transcriptional properties: the maximal activity of agonists (A(max)), the potency of agonists (EC(50)), and the partial agonist activity of antisteroids (PAA). Here, we report that the recently described nuclear protein, Pax2 transactivation domain interaction protein (PTIP)-associated protein 1 (PA1), is a new inhibitor of GR transactivation. PA1 suppresses A(max), increases the EC(50), and reduces the PAA of an exogenous reporter gene in a manner that is independent of associated PTIP. PA1 is fully active with, and strongly binds to, the C-terminal half of GR. PA1 reverses the effects of the coactivator TIF2 on GR-mediated gene induction but is unable to augment the actions of the corepressor SMRT. Analysis of competition assays between PA1 and TIF2 with an exogenous reporter indicates that the kinetic definition of PA1 action is a competitive decelerator at two sites upstream from where TIF2 acts. With the endogenous genes IGFBP1 and IP6K3, PA1 also represses GR induction, increases the EC(50), and decreases the PAA. ChIP and re-ChIP experiments indicate that PA1 accomplishes this inhibition of the two genes via different mechanisms as follows: PA1 appears to increase GR dissociation from and reduce GR transactivation at the IGFBP1 promoter regions but blocks GR binding to the IP6K3 promoter. We conclude that PA1 is a new competitive decelerator of GR transactivation and can act at more than one molecularly defined step in a manner that depends upon the specific gene.

Identification of location and kinetically defined mechanism of cofactors and reporter genes in the cascade of steroid-regulated transactivation

A currently obscure area of steroid hormone action is where the component factors, including receptor and reporter gene, act. The DNA binding of factors can be precisely defined, but the location and timing of factor binding and action are usually not equivalent. These questions are addressed for several factors (e.g. glucocorticoid receptor (GR), reporter, TIF2, NCoR, NELF-A, sSMRT, and STAMP) using our recently developed competition assay. This assay reveals both the kinetically defined mechanism of factor action and where the above factors act relative to both each other and the equilibrium equivalent to the rate-limiting step, which we call the concentration limiting step (CLS). The utility of this competition assay would be greatly increased if the position of the CLS is invariant and if the factor acting at the CLS is known. Here we report that the exogenous GREtkLUC reporter acts at the CLS as an accelerator for gene induction by GRs in U2OS cells. This mechanism of reporter function at the CLS persists with different reporters, factors, receptors, and cell types. We, therefore, propose that the reporter gene always acts at the CLS during gene induction and constitutes a landmark around which one can order the actions of all other factors. Current data suggest that how and where GR and the short form of SMRT act is also constant. These results validate a novel and rational methodology for identifying distally acting factors that would be attractive targets for pharmaceutical intervention in the treatment of diseases involving GR-regulated genes.

SMRTε, a corepressor variant, interacts with a restricted subset of nuclear receptors, including the retinoic acid receptors α and β

The SMRT and NCoR corepressors bind to, and mediate transcriptional repression by, many nuclear receptors. Both SMRT and NCoR are expressed by alternative mRNA splicing, generating a series of structurally and functionally distinct corepressor "variants". We report that a splice variant of SMRT, SMRTε, recognizes a restricted subset of nuclear receptors. Unlike the other corepressor variants characterized, SMRTε possesses only a single receptor interaction domain (RID) and exhibits an unusual specificity for a subset of nuclear receptors that includes the retinoic acid receptors (RARs). The ability of the single RID in SMRTε to efficiently interact with RARs appears to be enhanced by a recently recognized β-strand/β-strand interaction between corepressor and receptor. We suggest that alternative mRNA splicing of corepressors can restrict their function to specific nuclear receptor partnerships, and we propose that this may serve to customize the transcriptional repression properties of different cell types for different biological purposes.

Deducing the temporal order of cofactor function in ligand-regulated gene transcription: theory and experimental verification

Cofactors are intimately involved in steroid-regulated gene expression. Two critical questions are (1) the steps at which cofactors exert their biological activities and (2) the nature of that activity. Here we show that a new mathematical theory of steroid hormone action can be used to deduce the kinetic properties and reaction sequence position for the functioning of any two cofactors relative to a concentration limiting step (CLS) and to each other. The predictions of the theory, which can be applied using graphical methods similar to those of enzyme kinetics, are validated by obtaining internally consistent data for pair-wise analyses of three cofactors (TIF2, sSMRT, and NCoR) in U2OS cells. The analysis of TIF2 and sSMRT actions on GR-induction of an endogenous gene gave results identical to those with an exogenous reporter. Thus new tools to determine previously unobtainable information about the nature and position of cofactor action in any process displaying first-order Hill plot kinetics are now available.

Alternative mRNA splicing of corepressors generates variants that play opposing roles in adipocyte differentiation

The SMRT and NCoR corepressors partner with, and help mediate repression by, a wide variety of nuclear receptors and non-receptor transcription factors. Both SMRT and NCoR are expressed by alternative mRNA splicing, resulting in the production of a series of interrelated corepressor variants that differ in their tissue distribution and in their biochemical properties. We report here that different corepressor splice variants can exert opposing transcriptional and biological effects during adipocyte differentiation. Most notably, the NCoRω splice variant inhibits, whereas the NCoRδ splice variant promotes, adipogenesis. Furthermore, the ratio of NCoRω to NCoRδ decreases during adipogenic differentiation. We propose that this alteration in corepressor splicing helps convert the cellular transcriptional program from one that maintains the pre-adipocyte in an undifferentiated state to a new transcriptional context that promotes differentiation and helps establish the proper physiology of the mature adipocyte.

MicroRNA-184 downregulates nuclear receptor corepressor 2 in mouse spermatogenesis

Background

There have been increasing attentions on the role of small RNAs, especially microRNAs in post-transcriptional gene regulation during spermatogenesis. MicroRNA-184 (miR-184) has been shown to be mainly expressed in the testis and brain, and that its expression levels are by far the highest in the testis. However, the role of miR-184 in mammalian spermatogenesis remains unclear.

Results

In this study, we demonstrated that miR-184 levels were increased during mouse postnatal testis development. Specifically, miR-184 expression was restricted to the germ cells from spermatogonia to round spermatids. Overexpression of miR-184 promoted the proliferation of a germ cell line, GC-1spg. Moreover, miR-184 downregulated nuclear receptor corepressor 2 (Ncor2) by targeting its 3' untranslated region through inhibiting NCOR2 protein translation.

Conclusions

MiR-184 may be involved in the post-transcription regulation of mRNAs such as Ncor2 in mammalian spermatogenesis.

The negative side of retinoic acid receptors

This is a review of research that supports a hypothesis regarding early restriction of gene expression in the vertebrate embryo. We hypothesize that vertebrate retinoic acid receptors (RARs for several vertebrates but rars for zebrafish) are part of an embryonic, epigenetic switch whose default position, at the time of fertilization is "OFF". This is due to the assemblage of a rar-corepressor-histone deacetylase complex on retinoic acid response elements (RAREs) in regulatory regions of a subset of genes. In addition, selective and precise allocation of retinoic acid during early development through the interaction of Phase I enzymes throws the switch "ON" in a predictable, developmental manner. We are proposing that this is a basic, early embryonic switch that can cause the initiation of cascades of gene expression that are responsible for at least some early, diversification of cell phenotypes. Dehydrogenases and a subset of cytochrome p450 genes (cyp26a1, cyp26b1, and cyp26c1) play the major role in providing the retinoic acid and limiting its access. We also suggest that this mechanism may be playing a significant role in the repression of genes in undifferentiated stem cells.

Role of xenobiotic metabolism in cancer: involvement of transcriptional and miRNA regulation of P450s

Cytochrome P450 enzymes (P450s) are important targets in cancer, due to their role in xenobiotic metabolism. Since P450s are the "bridges" between the environment and our body, their function can be linked in many ways to carcinogenesis: they activate dietary and environmental components to ultimate carcinogens (i), the cancer tissue maintains its drug resistance with altered expression of P450s (ii), P450s metabolize (sometimes activate) drugs used for cancer treatment (iii) and they are potential targets for anticancer therapy (iiii). These highly polymorphic enzymes are regulated at multiple molecular levels. Regulation is as important as genetic difference in the existing individual variability in P450 activity. In this review, examples of the transcriptional (DNA methylation, histone modification, modulation by xenosensors) and post-transcriptional (miRNA) regulation will be presented and thereby introduce potential molecular targets at which the metabolism of anticancer drugs, the elimination of cancerogenes or the progress of carcinogenesis could be affected.

Physiological roles of class I HDAC complex and histone demethylase

Epigenetic gene silencing is one of the fundamental mechanisms for ensuring proper gene expression patterns during cellular differentiation and development. Histone deacetylases (HDACs) are evolutionally conserved enzymes that remove acetyl modifications from histones and play a central role in epigenetic gene silencing. In cells, HDAC forms a multiprotein complex (HDAC complex) in which the associated proteins are believed to help HDAC carry out its cellular functions. Though each HDAC complex contains distinct components, the presence of isoforms for some of the components expands the variety of complexes and the diversity of their cellular roles. Recent studies have also revealed a functional link between HDAC complexes and specific histone demethylases. In this paper, we summarize the distinct and cooperative roles of four class I HDAC complexes, Sin3, NuRD, CoREST, and NCoR/SMRT, with respect to their component diversity and their relationship with specific histone demethylases.

Aberrant expression and modification of silencing mediator of retinoic acid and thyroid hormone receptors involved in the pathogenesis of tumoral cortisol resistance

Ectopic ACTH syndrome (EAS) accounts for 10-15% of cases of Cushing's syndrome and is mostly caused by small cell lung cancers or thymic carcinoids. EAS is characterized by tumoral cortisol resistance, whose underlying mechanism remains unknown. In this study, we reported that silencing mediator of retinoic acid and thyroid hormone receptors (SMRT), a major nuclear corepressor, was aberrantly expressed in ACTH-secreting thymic carcinoids. Overexpression and knockdown of SMRT in the ACTH-secreting AtT-20 cell line demonstrated that SMRT participated in the negative feedback of dexamethasone-mediated suppression of proopiomelanocortin. Posttranslational modification by the small ubiquitin-like modifiers (SUMO), i.e. SUMOylation plays an important role in fine-tuning transcriptional activities. SUMOylation of SMRT was observed in dexamethasone-resistant cell lines. Moreover, overexpression of the deSUMOylation enzyme enhanced the suppression of proopiomelanocortin by dexamethasone in AtT-20 cells. An evolutionarily conserved consensus SUMOylation site was identified close to the histone deacetylase 3 recruiting domain of SMRT, which might interfere with the recruiting process. These results suggested that aberrant expression and modification of SMRT might be involved in the pathogenesis of tumoral cortisol resistance. A therapeutic approach targeting SMRT SUMOylation might be developed for EAS patients.

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.

Hairless is a nuclear receptor corepressor essential for skin function

The activity of nuclear receptors is modulated by numerous coregulatory factors. Corepressors can either mediate the ability of nuclear receptors to repress transcription, or can inhibit transactivation by nuclear receptors. As we learn more about the mechanisms of transcriptional repression, the importance of repression by nuclear receptors in development and disease has become clear. The protein encoded by the mammalian Hairless (Hr) gene was shown to be a corepressor by virtue of its functional similarity to the well-established corepressors N-CoR and SMRT. Mutation of the Hr gene results in congenital hair loss in both mice and men. Investigation of Hairless function both in vitro and in mouse models in vivo has revealed a critical role in maintaining skin and hair by regulating the differentiation of epithelial stem cells, as well as a putative role in regulating gene expression via chromatin remodeling.

G protein pathway suppressor 2 (GPS2) is a transcriptional corepressor important for estrogen receptor alpha-mediated transcriptional regulation

We have identified G protein suppressor 2 (GPS2) as a stable component of the SMRT corepressor complexes. GPS2 potently represses basal transcription, with the repression domain mapped to the N-terminal silencing mediator of retinoic acid and thyroid hormone receptor (SMRT)-interacting domain. Knockdown of GPS2 abrogates, whereas overexpression potentiates, SMRT-mediated repression activity. The SMRT complexes are involved in 4-hydroxyl-tamoxifen (4OHT)-mediated gene repression by estrogen receptor alpha (ERalpha). We show that 4OHT recruits SMRT and GPS2 to the promoter of pS2, an ERalpha target gene, in a dynamic manner. Unexpectedly, we also found that estradiol (E2) promotes promoter recruitment of the SMRT complexes. While knockdown of GPS2 compromised 4OHT-mediated repression, it enhanced E2-induced expression of a reporter gene and several endogenous ERalpha target genes, including pS2, cyclin D1 (CCND1), progesterone receptor (PR), and c-MYC. Finally, we show that depletion of GPS2 or SMRT by siRNA promotes cell proliferation in MCF-7 breast cancer cells. Thus, we concluded that GPS2 is an integral component of the SMRT complexes, important for ligand-dependent gene regulations by ERalpha and a suppressor for MCF-7 cell proliferation.

Chromatin remodeling and nuclear receptor signaling

Nuclear receptors (NRs) constitute a large family of ligand-dependent transcription factors that play key roles in development, differentiation, metabolism, and homeostasis. They participate in these processes by coordinating and regulating the expression of their target genes. The eukaryotic genome is packaged as chromatin and is generally inhibitory to the process of transcription. NRs overcome this barrier by recruiting two classes of chromatin remodelers, histone modifying enzymes and ATP-dependent chromatin remodelers. These remodelers alter chromatin structure at target gene promoters by posttranslational modification of histone tails and by disrupting DNA-histone interactions, respectively. In the presence of ligand, NRs promote transcription by recruiting remodeling enzymes that increase promoter accessibility to the basal transcription machinery. In the absence of ligand a subset of NRs recruit remodelers that establish and maintain a closed chromatin environment, to ensure efficient gene silencing. This chapter reviews the chromatin remodeling enzymes associated with NR gene control, with an emphasis on the mechanisms of NR-mediated repression.

N-CoR is required for patterning the anterior-posterior axis of zebrafish hindbrain by actively repressing retinoid signaling

Active repression of gene expression mediated by unliganded nuclear receptors plays crucial roles in early development of vertebrates. N-CoR (nuclear receptor co-repressor) is the first identified co-repressor that can repress retinoic acid (RA) inducible gene transcription in the absence of RA. Previously, N-CoR was reported to be required for late-stage organogenesis in mouse but whether N-CoR can affect RA-responsive early embryonic patterning is unknown. In this study, we report molecular cloning of zebrafish orthologue of N-CoR and its wide distribution pattern during zebrafish early development. Knocking down n-cor elevates endogenous RA signaling in zebrafish embryos and posteriorizes the neural ectoderm. Overexpressing or knocking down n-cor in zebrafish embryos alters the length of hindbrain in a manner similar to decreasing or increasing RA signaling in embryos, respectively. Taken together, our results demonstrate that N-CoR is essential for early hindbrain patterning by actively repressing retinoid signaling.

Alternative splicing determines the interaction of SMRT isoforms with nuclear receptor-DNA complexes

Signalling by small molecules, such as retinoic acid, is mediated by heterodimers comprising a class II nuclear receptor and an RXR (retinoid X receptor) subunit. The receptors bind to DNA response elements and act as ligand-dependent transcription factors, but, in the absence of signal, the receptors bind the co-repressors SMRT [silencing mediator for RAR (retinoic acid receptor) and TR (thyroid hormone receptor)] and NCoR (nuclear receptor co-repressor) and repress gene expression. Alternative splicing of the SMRT transcript in mammals generates six isoforms containing 1, 2 or 3 CoRNR (co-repressor for nuclear receptor) box motifs which are responsible for the interactions with nuclear receptors. We show that human cell lines express all six SMRT isoforms and then determine the binding affinity of mouse SMRT isoforms for RAR/RXR and three additional class II nuclear receptor-DNA complexes. This approach demonstrates the importance of the full complement of CoRNR boxes within each SMRT protein, rather than the identity of individual CoRNR boxes, in directing the interaction of SMRT with nuclear receptors. Each class of SMRT isoform displays a distinct feature, as the 1-box isoform discriminates between DNA response elements, the 2-box isoforms promote high-affinity binding to TR complexes and the 3-box isoforms show differential binding to nuclear receptors. Consequently, the differential deployment of SMRT isoforms observed in vivo could significantly expand the regulatory capacity of nuclear receptor signalling.

Activity and intracellular location of estrogen receptors alpha and beta in human bronchial epithelial cells

Gender differences in lung disease and cancer are well-established. We reported estrogenic transcriptional responses in lung adenocarcinoma cells from females but not males despite similar estrogen receptor (ER) expression. Here we tested the hypothesis that normal human bronchial epithelial cells (HBECs) show gender-independent estrogenic responses. We report that a small sample of HBECs express approximately twice as much ERbeta as ERalpha. ERalpha and ERbeta were located in the cytoplasm, nucleus, and mitochondria. In contrast to lung adenocarcinoma cells, estradiol (E2) induced estrogen response element (ERE)-mediated luciferase reporter activity in transiently transfected HBECs regardless of donor gender. Overexpression of ERalpha-VP16 increased ERE-mediated transcriptional activity in all HBECs. E2 increased and 4-hydroxytamoxifen and ICI 182,780 inhibited HBEC proliferation and cyclin D1 expression in a cell line-specific manner. In conclusion, the response of HBECs to ER ligands is gender-independent suggesting that estrogenic sensitivity may be acquired during lung carcinogenesis.

New insights into the functions and regulation of the transcriptional corepressors SMRT and N-CoR

Corepressors are large proteins that facilitate transcriptional repression through recruitment of histone-modifying enzymes. Two major corepressors, SMRT (silencing mediator for retinoid and thyroid hormone receptors) and N-CoR (nuclear receptor corepressor), have been shown to mediate repression associated with nuclear receptors and a myriad of other transcription factors. This review will focus on recent studies on these proteins, including newly discovered physiological roles of the corepressors, their modes of regulation, their roles in antiestrogen-resistant breast cancer and their functions during the cell cycle.

Cdk2 and Pin1 negatively regulate the transcriptional corepressor SMRT

Silencing mediator for retinoic acid and thyroid hormone receptor (SMRT) is a transcriptional corepressor that participates in diverse signaling pathways and human diseases. However, regulation of SMRT stability remains largely unexplored. We show that the peptidyl-prolyl isomerase Pin1 interacts with SMRT both in vitro and in mammalian cells. This interaction requires the WW domain of Pin1 and SMRT phosphorylation. Pin1 regulates SMRT protein stability, thereby affecting SMRT-dependent transcriptional repression. SMRT phosphorylation at multiple sites is required for Pin1 interaction, and these sites can be phosphorylated by Cdk2, which interacts with SMRT. Cdk2-mediated phosphorylation of SMRT is required for Pin1 binding and decreases SMRT stability, whereas mutation of these phosphorylation sites abrogates Pin1 binding and stabilizes SMRT. Finally, decreases in SMRT stability occur in response to the activation of Her2/Neu/ErbB2, and this receptor functions upstream of both Pin1 and Cdk2 in the signaling cascade that regulates SMRT stability and cellular response to tamoxifen.

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.

Silencing mediator for retinoid and thyroid hormone receptor and nuclear receptor corepressor attenuate transcriptional activation by the beta-catenin-TCF4 complex

beta-Catenin is a multifunctional mediator of cellular signaling and an oncogene. Nuclear beta-catenin, when complexed with members of the T-cell factor (TCF)/leukocyte enhancer factor family of DNA-binding proteins, mediates transcriptional activation important for embryonic development and adult cell homeostasis. Deregulation of intracellular levels of beta-catenin is an early event in the development of a variety of cancers. We observed that the proteins silencing mediator for retinoid and thyroid hormone receptor (SMRT) and the nuclear receptor corepressor (NCoR) are negative regulators of transcription induced by the beta-catenin-TCF4 complex. Overexpression of SMRT and NCoR attenuated the transcription of beta-catenin-TCF4-specific reporter gene and of CCND1, an endogenous beta-catenin target gene. Knockdown of endogenous SMRT or NCoR by short interfering RNA augmented the beta-catenin-TCF4-mediated reporter gene expression. Glutathione S-transferase pulldown experiments showed there was a direct physical association of SMRT and NCoR with both beta-catenin and TCF4. DNA-protein interaction studies revealed that the interactions between either SMRT or NCoR and beta-catenin or TCF4 occurred at the promoter regions of CCND1 and other target genes. These findings demonstrate an important role for corepressors SMRT and NCoR in the regulation of beta-catenin-TCF4-mediated gene transcription.

Interplay of pregnane X receptor with other nuclear receptors on gene regulation

Human body needs to protect itself from a diverse array of harmful chemicals. These chemicals are also involved in drug metabolism, enzyme induction, and can cause adverse drug-drug interactions. Being a member of nuclear receptors (NRs), pregnane X receptor (PXR) has recently emerged as transcriptional regulators of cytochrome P450 (CYP) and transporters expression so as to against xenobiotics exposure. This review describes some common nuclear receptors, i.e. farnesoid X receptor (FXR), small heterodimer partner (SHP), hepatocyte nuclear factor-4alpha (HNF-4alpha), liver X receptor (LXR), glucocorticoid receptor (GR), constitutive androstane receptor (CAR) that crosstalk with PXR and involvement of coregulators thus control target genes expression.

A critical role for the co-repressor N-CoR in erythroid differentiation and heme synthesis

Co-repressor N-CoR (nuclear receptor co-repressor) has important roles in different biological processes, including proliferation, differentiation and development. Mutant mice lacking N-CoR are embryonically lethal and appear to die from anemia owing to defects in definitive erythropoiesis. However, the underlying molecular mechanisms of N-CoR-mediated erythroid differentiation are largely unknown. Using the human erythroleukemic K562 cell line, which can be chemically induced to differentiate into either erythroid or megakaryocytic lineages depending on the inducers used, we have investigated the role of N-CoR in erythroid differentiation. We show that knockdown of N-CoR either transiently (siRNA) or permanently (shRNA) impairs the cytosine arabinoside (Ara-C)- but not hemin-induced erythroid differentiation of K562 cells. RT-PCR analysis reveals that N-CoR is required for induction by Ara-C of 5-aminolevulinate synthase (ALA-S2), a key enzyme involved in heme biosynthesis. Furthermore, the amount of N-CoR proteins increases significantly during Ara-C-induced K562 differentiation, apparently through a post-transcriptional mechanism. Consistent with the data from N-CoR-null mice, N-CoR is not required for the differentiation of K562 cells into megakaryocytic lineages, induced by phorbol 12-myristate 13-acetate. Thus, our in vitro study confirms a role for N-CoR in erythroid differentiation and reveals for the first time that N-CoR is required for the induction of a key enzyme involved in heme synthesis.

HDAC3: taking the SMRT-N-CoRrect road to repression

Known histone deacetylases (HDACs) are divided into different classes, and HDAC3 belongs to Class I. Through forming multiprotein complexes with the corepressors SMRT and N-CoR, HDAC3 regulates the transcription of a plethora of genes. A growing list of nonhistone substrates extends the role of HDAC3 beyond transcriptional repression. Here, we review data on the composition, regulation and mechanism of action of the SMRT/N-CoR-HDAC3 complexes and provide several examples of nontranscriptional functions, to illustrate the wide variety of physiological processes affected by this deacetylase. Furthermore, we discuss the implication of HDAC3 in cancer, focusing on leukemia. We conclude with some thoughts about the potential therapeutic efficacies of HDAC3 activity modulation.

Characterization of transcriptional regulatory domains of ankyrin repeat cofactor-1

The ankyrin repeats cofactor-1 (ANCO-1) was recently identified as a p160 coactivator-interacting protein that may inhibit transcriptional activity of nuclear receptors. Here, we have characterized the transcriptional regulatory domains of ANCO-1. Two intrinsic repression domains (RD) were identified: an N-terminal RD1 at residues 318-611 and a C-terminal RD2 at 2369-2663. ANCO-1 also contains an activation domain (AD) capable of stimulating transcription in both mammalian and yeast cells. The minimal AD was delimited to a 70-amino acid region at residues 2076-2145. Overall, full-length ANCO-1 exhibited transcriptional repressor activity, suggesting that RD domains may suppress the AD activity. We further demonstrated that ANCO-1 silencing by siRNA enhanced progesterone receptor-mediated transcription. Together, these results indicate that the transcriptional potential of ANCO-1 may be modulated by a combination of repression and activation signals.

Transcriptional activity of androgen receptor is modulated by two RNA splicing factors, PSF and p54nrb

Nuclear receptors regulate gene activation or repression through dynamic interactions with coregulators. The interactions between nuclear receptors and RNA splicing factors link gene transcription initiation with pre-mRNA splicing, providing a coordinated control of the products of gene transcription. Here we report that two RNA splicing factors, PTB-associated splicing factor (PSF) and p54nrb, synergistically form protein complexes with the androgen receptor (AR) in a ligand-independent manner and inhibit its transcriptional activity. PSF does not affect AR protein stability, as in the case of the progesterone receptor, but impedes the interaction of AR with the androgen response element. Both splicing factors interact directly with mSin3A and attract mSin3A to the AR complex in a synergistic manner. The suppression of AR transcriptional activity by PSF and p54nrb is reversed by the inhibition of histone deacetylase activity. These data demonstrated that PSF and p54nrb complex with AR and play a key role in modulating AR-mediated gene transcription.

The silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) corepressor is required for full estrogen receptor alpha transcriptional activity

Multiple factors influence estrogen receptor alpha (ERalpha) transcriptional activity. Current models suggest that the silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) corepressor functions within a histone deactylase-containing protein complex that binds to antiestrogen-bound ERalpha and contributes to negative regulation of gene expression. In this report, we demonstrate that SMRT is required for full agonist-dependent ERalpha activation. Chromatin immunoprecipitation assays demonstrate that SMRT, like ERalpha and the SRC-3 coactivator, is recruited to an estrogen-responsive promoter in estrogen-treated MCF-7 cells. Depletion of SMRT, but not histone deacetylases 1 or 3, negatively impacts estradiol-stimulated ERalpha transcriptional activity, while exogenous expression of SMRT's receptor interaction domains blocks ERalpha activity, indicating a functional interaction between this corepressor and agonist-bound ERalpha. Stimulation of estradiol-induced ERalpha activity by SMRT overexpression occurred in HeLa and MCF-7 cells, but not HepG2 cells, indicating that these positive effects are cell type specific. Similarly, the ability of SMRT depletion to promote the agonist activity of tamoxifen was observed for HeLa but not MCF-7 cells. Furthermore, impairment of agonist-stimulated activity by SMRT depletion is specific to ERalpha and not observed for receptors for vitamin D, androgen, or thyroid hormone. Nuclear receptor corepressor (N-CoR) depletion increased the transcriptional activity of all four tested receptors. SMRT is required for full expression of the ERalpha target genes cyclin D1, BCL-2, and progesterone receptor but not pS2, and its depletion significantly attenuated estrogen-dependent proliferation of MCF-7 cells. Taken together, these data indicate that SMRT, in conjunction with gene-specific and cell-dependent factors, is required for positively regulating agonist-dependent ERalpha transcriptional activity.

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.

Overview of nomenclature of nuclear receptors

Nuclear receptor pharmacology has, to a certain extent, led the way, compared with other receptor systems, in the appreciation that ligands may exert very diverse pharmacology, based on their individual chemical structure and the allosteric changes induced in the receptor/accessory protein complex. This can lead to very selective pharmacological effects, which may not necessarily be predicted from the experience with other agonists/partial agonists/antagonists. If this is the case, then drug discovery may be back to drug-specific pharmacology (where each drug may have an original profile), rather than specific-drug pharmacology (where agents specific for a receptor have a distinct profile). As functional selectivity is indeed a crucial mechanism to be considered when going through the drug discovery development process, then initial screens using reconstituted systems may not show the appropriate pharmacology, simply because the required stoichiometry of corepressors and coactivators may not be present to select the best compounds; therefore, multiple effector systems are necessary to screen for differential activation, and, even then, screening with in vivo pathophysiological models may ultimately be required for the selection process-a massive but necessary task for pharmacologists. Thus, the characterization of nuclear receptors and their associated proteins and the ligands that interact with them will remain a challenge to pharmacologists.

Subcellular localization of ankyrin repeats cofactor-1 regulates its corepressor activity

The ankyrin repeats cofactor-1 (ANCO-1) was recently identified as a novel nuclear receptor corepressor that regulates receptor-mediated transcription through interactions with p160 coactivators and histone deacetylases. Interestingly, exogenously expressed ANCO-1 is localized at distinct subnuclear domains. The relevance of these subnuclear domains and the mechanisms of nucleocytoplasmic translocation of ANCO-1 have not been determined. We report here the identification of an N-terminal signaling motif that is essential for both nuclear/subnuclear localization and transcription corepressor function of ANCO-1. This N-terminal motif at residues 80-86 of ANCO-1 constitutes a classical nuclear localization signal (NLS1). Disruption of NLS1 causes complete cytoplasmic accumulation of the full-length ANCO-1, and abolishes its corepressor function on receptor-mediated transcription. A second NLS (NLS2) is found at the C-terminal residues 2384-2390; however, its disruption abolishes only nuclear localization of isolated C-terminal fragments. We also identify a leucine-rich nuclear export signal (NES) at residues 2415-2424 of ANCO-1, and show that both the NLSs and NES sequences are capable of mediating nuclear import and export of heterologous protein, respectively. In addition, attachment of the NES sequence to a transcription factor impairs its activation function. These results suggest that ANCO-1 subnuclear localization is regulated by both nuclear import and export signals, and that proper subcellular localization of ANCO-1 is essential for its corepressor function.

Corepressor/coactivator paradox: potential constitutive coactivation by corepressor splice variants

The functional consequences of the interaction of transcriptional coregulators with the human thyroid hormone receptor (TR) in mammalian cells are complex. We have used the yeast, Saccharomyces cerevisiae, which lack endogenous nuclear receptors (NRs) and NR coregulators, as a model to decipher mechanisms regulating transcriptional activation by TR. In effect, this system allows the reconstitution of TR mediated transcription complexes by the expression of specific combinations of mammalian proteins in yeast. In this yeast system, human adenovirus 5 early region 1A (E1A), a natural N-CoR splice variant (N-CoR(I)) or an artificial N-CoR truncation (N-CoR(C)) coactivate unliganded TRs and these effects are inhibited by thyroid hormone (TH). E1A contains a short peptide sequence that resembles known corepressor-NR interaction motifs (CoRNR box motif, CBM), and this motif is required for TR binding and coactivation. N-CoR(I) and N-CoR(C) contain three CBMs, but only the C-terminal CBM1 is critical for coactivation. These observations in a yeast model system suggest that E1A and N-CoR(I) are naturally occurring TR coactivators that bind in the typical corepressor mode. These findings also raise the possibility that alternative splicing events which form corepressor proteins containing only C-terminal CBM motifs could represent a novel mechanism in mammalian cells for regulating constitutive transcriptional activation by TRs.

Dimerization-induced corepressor binding and relaxed DNA-binding specificity are critical for PML/RARA-induced immortalization

The pathogenesis of acute promyelocytic leukemia involves the transcriptional repression of master genes of myeloid differentiation by the promyelocytic leukemia-retinoic acid receptor alpha (PML/RARA) oncogene. PML-enforced RARA homodimerization allows the tighter binding of corepressors, silencing RARA target genes. In addition, homodimerization dramatically extends the spectrum of DNA-binding sites of the fusion protein compared with those of normal RARA. Yet, any contribution of these two properties of PML/RARA to differentiation arrest and immortalization of primary mouse hematopoietic progenitors was unknown. We demonstrate that dimerization-induced silencing mediator of retinoid and thyroid receptors (SMRT)-enhanced binding and relaxed DNA-binding site specificity are both required for efficient immortalization. Thus, enforced RARA dimerization is critical not only for triggering transcriptional repression but also for extending the repertoire of target genes. Our studies exemplify how dimerization-induced gain of functions converts an unessential transcription factor into a dominant oncogenic protein.

NCOR1 mRNA is an independent prognostic factor for breast cancer

The transcriptional function of estrogen receptor alpha (ERalpha) can be modulated by co-regulatory proteins. In the present study, therefore, the level of expression of one of the co-regulator Nuclear Receptor Co-repressor 1 (NCOR1) mRNA has been assessed by quantitative real-time RT-PCR in 160 cases of invasive breast carcinoma. It was found that NCOR1 mRNA was expressed at significantly higher levels in patients over 50 years of age, without axillary lymph node involvement, with tumor size less than 2 cm, with low or intermediate histological grade, with ERalpha/PgR-positive and with HER2 negative tumors. Patients with high levels of expression of NCOR1 mRNA have a better prognosis than those with low expression. Univariate and multivariate prognostic analysis demonstrated that NCOR1 mRNA is an independent prognostic factor for breast cancer.