Nuclear factor RIP140 modulates transcriptional activation by the estrogen receptor

The nuclear receptor transcriptional coregulator RIP140

The nuclear receptor superfamily comprises ligand-regulated transcription factors that control various developmental and physiological pathways. These receptors share a common modular structure and regulate gene expression through the recruitment of a large set of coregulatory proteins. These transcription cofactors regulate, either positively or negatively, chromatin structure and transcription initiation. One of the first proteins to be identified as a hormone-recruited cofactor was RIP140. Despite its recruitment by agonist-liganded receptors, RIP140 exhibits a strong transcriptional repressive activity which involves several inhibitory domains and different effectors. Interestingly, the RIP140 gene, located on chromosome 21 in humans, is finely regulated at the transcriptional level by various nuclear receptors. In addition, the protein undergoes several post-translational modifications which control its repressive activity. Finally, experiments performed in mice devoid of the RIP140 gene indicate that this transcriptional cofactor is essential for female fertility and energy homeostasis. RIP140 therefore appears to be an important modulator of nuclear receptor activity which could play major roles in physiological processes and hormone-dependent diseases.

Development of a coactivator displacement assay for the orphan receptor estrogen-related receptor-γ using time-resolved fluorescence resonance energy transfer

The estrogen-related receptor-gamma (ERRgamma) is a constitutively active orphan receptor that belongs to the nuclear receptor superfamily and is most closely related to the estrogen receptors. Although its physiological ligand is unknown, ERRgamma has been shown to interact with synthetic estrogenic compounds such as 4-hydroxytamoxifen (4-OHT), tamoxifen, and diethylstilbestrol (DES). To assess how coregulator proteins interact with ERRgamma in response to ligand, an in vitro interaction methodology using time-resolved fluorescence resonance energy transfer (TR-FRET) was developed using glutathione S-transferase (GST)-tagged ERRgamma ligand-binding domain (LBD), a terbium-labeled anti-GST antibody, a fluorescein-labeled peptide containing sequences derived from coregulator proteins, and various ligands. An initial screen of these coregulator peptides bearing the coactivator LXXLL motif, the corepressor LXXI/HIXXXI/L motif, or other interaction motifs from natural coactivator sequences or random phage display peptides indicated that the peptides PGC1alpha, D22, and SRC1-4, known as class III coregulators, interacted most strongly with ERRgamma in the absence of ligand. Given its assay window and biological relevance in energy metabolism and obesity, further studies were conducted with PGC1alpha. Fluorescein-labeled PGC1alpha peptide was displaced from the ERRgamma LBD in the presence of increasing concentrations of 4-OHT and tamoxifen, but DES was less effective in PGC1alpha displacement. The statistical parameter Z' factor that measures the robustness of the assay was greater than 0.8 for displacement of PGC1alpha from ERRgamma LBD in the presence of saturating 4-OHT over an assay incubation time of 1-6 h, indicating an excellent assay. These findings also suggest that binding of 4-OHT, tamoxifen, or DES to ERRgamma results in differential affinity of coregulators for ERRgamma due to unique ligand-induced conformations.

RIP140 Expression Is Stimulated by Estrogen-related Receptor α during Adipogenesis

RIP140 is a corepressor for nuclear receptors that regulates energy expenditure in adipose tissue by suppressing the expression of clusters of metabolic genes involved in glucose and lipid metabolism. The gene encoding RIP140/Nrip1 contains only one coding exon but has multiple promoters and 5' non-coding exons that are subject to alternative splicing. In adipocytes we have defined a promoter, referred to as P2, that is preferentially utilized and activated during adipogenesis. Expression studies and chromatin immunoprecipitation experiments indicate that estrogen-related receptor alpha (ERRalpha), the level of which increases during adipogenesis in parallel with RIP140, stimulates transcription from the P2 promoter. Further analysis indicates that ERRalpha is capable of activating RIP140 gene transcription by two mechanisms, directly by binding to an estrogen receptor element/ERR element at -650/-633 and indirectly through Sp1 binding sites in the proximal promoter. Thus, the up-regulation of RIP140 by ERRalpha during adipogenesis may provide an inhibitory feedback mechanism to control the expression of many nuclear receptor target genes.

Regulatory circuits controlling white versus brown adipocyte differentiation

Adipose tissue is a major endocrine organ that exerts a profound influence on whole-body homoeostasis. Two types of adipose tissue exist in mammals: WAT (white adipose tissue) and BAT (brown adipose tissue). WAT stores energy and is the largest energy reserve in mammals, whereas BAT, expressing UCP1 (uncoupling protein 1), can dissipate energy through adaptive thermogenesis. In rodents, ample evidence supports BAT as an organ counteracting obesity, whereas less is known about the presence and significance of BAT in humans. Despite the different functions of white and brown adipocytes, knowledge of factors differentially influencing the formation of white and brown fat cells is sparse. Here we summarize recent progress in the molecular understanding of white versus brown adipocyte differentiation, including novel insights into transcriptional and signal transduction pathways. Since expression of UCP1 is the hallmark of BAT and a key factor determining energy expenditure, we also review conditions associated with enhanced energy expenditure and UCP1 expression in WAT that may provide information on processes involved in brown adipocyte differentiation.

Nuclear receptors as drug targets in metabolic diseases: new approaches to therapy

Nuclear receptors represent novel targets for the development of therapeutic agents for the treatment of numerous diseases, including type 2 diabetes, obesity dyslipidemia, atherosclerosis and the metabolic syndrome. There have been many recent advances in the development of new therapeutic agents for a subset of these receptors, including the peroxisome proliferator-activated receptors, the liver X receptors and the farnesoid X receptor. To date, the synthesis of selective modulators that regulate the activity of these receptors has been empirical. However, a detailed understanding of the molecular basis for selective modulation, as well as new insights into the biology of these receptors, might open the door to the rational design of a new generation of therapeutic agents with improved safety and efficacy.

Novel role of the RET finger protein in estrogen receptor-mediated transcription in MCF-7 cells

The Scaffold attachment factor B1 (SAFB1) is an estrogen receptor (ESR1) repressor that has been proposed to inhibit breast tumorigenesis. To obtain insight into the functions of SAFB1 we utilized a yeast two-hybrid screen and identified the Ret finger protein (RFP) as interacting with the SAFB1 C-terminus. RFP is a member of the trimotif (TRIM) family of proteins, which we found widely expressed in a series of breast cancer cell lines. We confirmed the interaction between SAFB1 and RFP through in vitro (GST-pull-down) and in vivo (coimmunoprecipitations) assays. We hypothesized that SAFB1 functions as a scaffolding protein to recruit proteins such as RFP into proximity with ESR1. Consequently, we asked whether RFP would modulate ESR1 activity and we discovered that RFP was important for the ESR1-dependent expression of cyclin D1 (CCND1) and the progesterone receptor (PR), but not IRS1 or MYC. Although RFP did not interact with ESR1 directly, it does coimmunoprecipitate with ESR1, demonstrating that RFP is found within the same protein complex. Chromatin immunoprecipitation assays (ChIP) located RFP to the TFF1 promoter, a known ESR1-regulated gene. Taken together, our study provides further evidence that coactivation and corepression are integrally linked processes and that RFP is a component of an ESR1 regulatory complex.

Structure-function relationship of estrogen receptor alpha and beta: impact on human health

17Beta-estradiol (E2) controls many aspects of human physiology, including development, reproduction and homeostasis, through regulation of the transcriptional activity of its cognate receptors (ERs). The crystal structures of ERs with agonists and antagonists and the use of transgenic animals have revealed much about how hormone binding influences ER conformation(s) and how this conformation(s), in turn, influences the interaction of ERs with co-activators or co-repressors and hence determines ER binding to DNA and cellular outcomes. This information has helped to shed light on the connection between E2 and the development or progression of numerous diseases. Current therapeutic strategy in the treatment of E2-related pathologies relies on the modulation of ER trancriptional activity by anti-estrogens; however, data accumulated during the last five years reveal that ER activities are not only restricted to the nucleus. ERs are very mobile proteins continuously shuttling between protein targets located within various cellular compartments (e.g., membrane, nucleus). This allows E2 to generate different and synergic signal transduction pathways (i.e., non-genomic and genomic) which provide plasticity for cell response to E2. Understanding the structural basis and the molecular mechanisms by which ER transduce E2 signals in target cells will allow to create new pharmacologic therapies aimed at the treatment of a variety of human diseases affecting the cardiovascular system, the reproductive system, the skeletal system, the nervous system, the mammary gland, and many others.

The proteins of human chromosome 21

Recent genomic sequence annotation suggests that the long arm of human chromosome 21 encodes more than 400 genes. Because there is no evidence to exclude any significant segment of 21 q from containing genes relevant to the Down syndrome (DS) cognitive phenotype, all genes in this entire set must be considered as candidates. Only a subset, however, is likely to make critical contributions. Determining which these are is both a major focus in biology and a critical step in efficient development of therapeutics. The subtle molecular abnormality in DS, the 50% increase in chromosome 21 gene expression, presents significant challenges for researchers in detection and quantitation. Another challenge is the current limitation in understanding gene functions and in interpreting biological characteristics. Here, we review information on chromosome 21-encoded proteins compiled from the literature and from genomics and proteomics databases. For each protein, we summarize their evolutionary conservation, the complexity of their known protein interactions and their level of expression in brain, and discuss the implications and limitations of these data. For a subset, we discuss neurologically relevant phenotypes of mouse models that include knockouts, mutations, or overexpression. Lastly, we highlight a small number of genes for which recent evidence suggests a function in biochemical/cellular pathways that are relevant to cognition. Until knowledge deficits are overcome, we suggest that effective development of gene-phenotype correlations in DS requires a serious and continuous effort to assimilate broad categories of information on chromosome 21 genes, plus the creation of more versatile mouse models.

Metabolic regulation by the nuclear receptor corepressor RIP140

Whereas the importance of activating gene expression in metabolic pathways to control energy homeostasis is well established, the contribution of transcriptional inhibition is less well defined. In this review we highlight a crucial role of RIP140, a transcriptional corepressor for nuclear receptors, in the regulation of energy expenditure. Mice devoid of the RIP140 gene are lean, exhibit resistance to high-fat-diet-induced obesity, and have increased glucose tolerance and insulin sensitivity. Consistent with these observations, RIP140 suppresses the expression of gene clusters that are involved in lipid and carbohydrate metabolism, including fatty acid oxidation, oxidative phosphorylation and mitochondrial uncoupling. Therefore, the functional interplay between transcriptional activators and the corepressor RIP140 is an essential process in metabolic regulation.

Human Ccr4-Not complex is a ligand-dependent repressor of nuclear receptor-mediated transcription

The Ccr4-Not complex is a highly conserved regulator of mRNA metabolism. The transcription regulatory function of this complex in higher eukaryotes, however, is largely unexplored. Here we report that CNOT1, the large human subunit, represses the ligand-dependent transcriptional activation function of oestrogen receptor (ER) alpha. Promoter recruitment assays indicate that CNOT1 contains an intrinsic ability to mediate transcriptional repression. Furthermore, CNOT1 can interact with the ligand-binding domain of ERalpha in a hormone-dependent fashion and is recruited with other Ccr4-Not subunits to endogenous oestrogen-regulated promoters dependent on the presence of ligand. In addition, siRNA-mediated depletion of endogenous CNOT1 or other Ccr4-Not subunits in breast cancer cells results in deregulation of ERalpha target genes. Finally, CNOT1 interacts in a ligand-dependent manner with RXR and represses transcription mediated by several RXR heterodimers. These findings define a function for the human Ccr4-Not complex as a transcriptional repressor of nuclear receptor signalling that is relevant for the understanding of molecular pathways involved in cancer.

Role of estrogen receptor alpha transcriptional coregulators in tamoxifen resistance in breast cancer

Tamoxifen is the endocrine agent most commonly used at all stages of breast cancer. Estrogen receptor (ER) alpha, which belongs to the superfamily of nuclear receptors, has been used to identify breast cancer patients who are likely to respond to tamoxifen, but resistance nonetheless occurs in 30-50% of treated ER alpha-positive breast cancer patients. The antiproliferative activity of tamoxifen, relying primarily on its ability to compete with estrogen for the ER alpha ligand binding site in breast tumor tissue, hypotheses forwarded to explain treatment failure include: (1) the existence of a second estrogen receptor (ER beta), (2) an imbalance in estrogen biosynthesis and catabolism, (3) altered bioavailability of tamoxifen, (4) altered cellular trafficking of ER alpha, (5) non genomic effects of ER alpha, directly interacting with several signal transduction pathways, and (6) transcriptional dysregulation of ER alpha target genes, which may involve both genomic (ERE alteration) and non genomic alterations. A first non genomic alteration involves the regulation of ER alpha activity by its phosphorylation mediated by growth factors-kinases signaling pathways. A second non genomic alteration, which is the purpose of this review, involves regulatory factors (coregulators) known as coactivators and corepressors, which activate (or repress) the transcription of ER alpha-responsive genes. The regulation process involves both chromatin remodeling and ER alpha interaction with the transcriptional machinery. Thus, dysregulated expression (coactivator overexpression or corepressor underexpression) and/or mutation of these coregulators is thought to impair the action of tamoxifen. Many altered pathways may account for tamoxifen resistance which may be best studied by multigene approaches.

Multilocus analysis of estrogen-related genes in Spanish postmenopausal women suggests an interactive role of ESR1, ESR2 and NRIP1 genes in the pathogenesis of osteoporosis

Osteoporosis is a common disease with multiple environmental and genetic risk factors involved. Using a marker-by-marker approach, the role of different estrogen-related genes has been analyzed in different populations, but most of these studies ignore the complex multigenic nature of human osteoporosis. Looking for markers related to osteoporosis, we have analyzed five single nucleotide polymorphisms located in genes related to the estrogen pathway, Follicle Stimulating Hormone Receptor (FSHR) gene, the CYP19 aromatase (CYP19A1) gene, the Estrogen Receptor alpha (ESR1) gene, the Estrogen Receptor beta (ESR2) gene and the Nuclear Receptor Interacting Protein 1 (NRIP1) gene in 265 unrelated postmenopausal women. We have obtained nominal P values for the NRIP1 Gly75Gly and ESR2 *39A>G markers (P=0.013 and P=0.02 respectively), but no gene seems to be associated after multiple test corrections. Reanalysis of this study using 437 postmenopausal women confirmed our results and only detect marginal effects for ESR2 marker (P=0.045). By contrast, multilocus analysis predicted epistatic interactions between ESR1, ESR2 and NRIP1 loci and its involvement in postmenopausal osteoporosis (P=0.003). We detected two digenic genotypes involving ESR2-NRIP1 and ESR2-ESR1 genes strongly associated with osteoporosis (P=0.007). Replication of multilocus studies using 437 patients confirmed the detected interactions (P<0.01). We proposed a non-additive non-multiplicative oligogenic model including ESR2 AG genotype modulated by NRIP1 A+ or ESR1 TT genotypes involved in osteoporosis. Our results reaffirm the polygenic nature and the genetic complexity of osteoporosis trait adding a new candidate gene (NRIP1) for association studies of bone-related traits.

Sensors and signals: a coactivator/corepressor/epigenetic code for integrating signal-dependent programs of transcriptional response

A decade of intensive investigation of coactivators and corepressors required for regulated actions of DNA-binding transcription factors has revealed a network of sequentially exchanged cofactor complexes that execute a series of enzymatic modifications required for regulated gene expression. These coregulator complexes possess "sensing" activities required for interpretation of multiple signaling pathways. In this review, we examine recent progress in understanding the functional consequences of "molecular sensor" and "molecular adaptor" actions of corepressor/coactivator complexes in integrating signal-dependent programs of transcriptional responses at the molecular level. This strategy imposes a temporal order for modifying programs of transcriptional regulation in response to the cellular milieu, which is used to mediate developmental/homeostatic and pathological events.

Expression and function of human steroid receptor RNA activator in prostate cancer cells: role of endogenous hSRA protein in androgen receptor-mediated transcription

Steroid receptor RNA activator (SRA) was first isolated as a steroid receptor co-activator that functioned as an RNA transcript. Later, we demonstrated that SRA needs to be translated in order to co-activate androgen receptor (AR). Here, we showed that three isoforms of human SRA enhanced AR activities. Small interfering RNA against SRA suppressed AR activities in PC-3 cells transfected with pSG5AR and in LNCaP cells that have an endogenous mutated-AR. Western blot showed that SRA protein was expressed at a higher level in PC-3 than in LNCaP cells, suggesting that SRA may be related to hormone-independent growth of prostate cancer.

Receptor-interacting protein 140 is a repressor of the androgen receptor activity

The androgen receptor (AR) is a ligand-activated transcription factor that controls growth and survival of prostate cancer cells. In the present study, we investigated the regulation of AR activity by the receptor-interacting protein 140 (RIP140). We first showed that RIP140 could be coimmunoprecipitated with the receptor when coexpressed in 293T cells. This interaction appeared physiologically relevant because chromatin immunoprecipitation assays revealed that, under R1881 treatment, RIP140 could be recruited to the prostate-specific antigen encoding gene in LNCaP cells. In vitro glutathione S-transferase pull-down assays provided evidence that the carboxy-terminal domain of AR could interact with different regions of RIP140. By means of fluorescent proteins, we demonstrated that ligand-activated AR was not only able to translocate to the nucleus but also to relocate RIP140 from very structured nuclear foci to a diffuse pattern. Overexpression of RIP140 strongly repressed AR-dependent transactivation by preferentially targeting the ligand binding domain-dependent activity. Moreover, disruption of RIP140 expression induced AR overactivation, thus revealing RIP140 as a strong AR repressor. We analyzed its mechanism of transrepression and first demonstrated that different regions of RIP140 could mediate AR-dependent repression. We then showed that the carboxy-terminal end of RIP140 could reverse transcriptional intermediary factor 2-dependent overactivation of AR. The use of mutants of RIP140 allowed us to suggest that C-terminal binding protein played no role in RIP140-dependent inhibition of AR activity, whereas histone deacetylases partly regulated that transrepression. Finally, we provided evidence for a stimulation of RIP140 mRNA expression in LNCaP cells under androgen treatment, further emphasizing the role of RIP140 in androgen signaling.

Transcriptional dysregulation in Down syndrome: predictions for altered protein complex stoichiometries and post-translational modifications, and consequences for learning/behavior genes ELK, CREB, and the estrogen and glucocorticoid receptors

The phenotype of Down syndrome, trisomy of chromosome 21, is hypothesized to be produced by the increased expression due to gene dosage of normal chromosome 21 genes. Chromosome 21 encodes a number of proteins that, based on experimental evidence or domain composition, are classed as transcription factors or their co-regulators. Other chromosome 21 proteins contribute to post-translational modification of transcription factors, including their phosphorylation, dephosphorylation and sumoylation. Several of these chromosome 21 proteins and the pathways in which they function have overlapping transcription factor specificities. Thus, altered stoichiometry in complexes and altered levels of activation of individual transcription factors may contribute to the Down syndrome phenotype by perturbation of downstream gene expression. Here we review recent data on four chromosome 21 proteins: NRIP1, GABPA, DYRK1A and SUMO3. We discuss the implications for activation of ELK, CREB, C/EBP alpha, beta estrogen and glucocorticoid receptors, and for expression of BDNF. Each of these proteins is relevant to learning, behavior and/or development and therefore perturbation of their activation may contribute to the Down syndrome phenotype.

Transcriptional Regulation of the Human NRIP1/RIP140 Gene by Estrogen Is Modulated by Dioxin Signalling

Receptor interacting protein 140 (RIP140) is a negative transcriptional regulator of nuclear hormone receptors that is required for the maintenance of energy homeostasis and ovulation. In this study, we investigated the mechanisms by which RIP140 expression is controlled by estrogens in breast cancer cells. We first analyzed by real time reverse transcription-polymerase chain reaction the regulation of RIP140 mRNA accumulation by estrogen receptor (ER) ligands in MCF-7 cells. We showed that the induction by estradiol (E2) was rapid and did not affect the apparent stability of the mRNA, suggesting a direct transcriptional regulation. To further study the underlying regulatory mechanisms, we then characterized the human RIP140 gene. We identified several noncoding exons with alternative splicing and localized the promoter region more than 100 kilobases upstream from the coding exon. Although we mapped a perfect consensus estrogen response element able to bind ERalpha in gel shift and in chromatin immunoprecipitation experiments, the effect of E2 on RIP140 gene transcription was very modest. This might result at least in part from the presence of an overlapping aryl hydrocarbon receptor (AhR) binding site, which interfered with the E2 response on both the transiently transfected reporter construct and the accumulation of the endogenous RIP140 mRNA. Altogether, our data indicate that the RIP140 gene exhibits a complex structure with several noncoding exons and supports transcriptional cross-talk and feedback involving the ERalpha and AhR nuclear receptors.

Multilocus analyses of estrogen-related genes reveal involvement of the ESR1 gene in male infertility and the polygenic nature of the pathology

OBJECTIVE

To examine whether polymorphisms within the ESR1, FSHR, ESR2, CYP19A1, and NRIP1 genes are susceptibility factors for human male idiopathic infertility and to test the joint effects of these genes on male reproductive function.

DESIGN

Genetic association study of male infertility with polymorphisms, using both single-gene and multilocus approaches.

SETTING

Private and public fertility units and a private center for biomedical research.

PATIENT(S)

One hundred four Spanish men with azoospermia or severe oligozoospermia and 95 unselected race-matched healthy controls from the same geographic region.

INTERVENTION(S)

Peripheral blood extraction, DNA purification, and ESR1 g.938T>C, FSHR Ser680Asn, ESR2 *39A>G, CYP19A1 *19C>T, and NRIP1 Gly75Gly polymorphism analyses.

MAIN OUTCOME MEASURE(S)

Single-gene statistical analyses and multilocus statistical analyses with Sumstat, Permutation and Model-free analysis, and Estimating Haplotypes software.

RESULT(S)

We observed an excess of homozygous infertile men for the ESR1 g.938T>C marker. Multilocus analyses detected genetic interaction between the five candidate gene markers that are influential over male infertility. In addition, we detected a five-loci protector genetic pattern with a frequency of 9.4% in controls but absent in infertile men.

CONCLUSION(S)

Our results support a relevant role for the estrogenic pathway, notably the ESR1 gene, in human male reproductive function and advocate a complex trait model for male infertility.

Differential recruitment of p160 coactivators by glucocorticoid receptor between Schwann cells and astrocytes

In the nervous system, glucocorticoids can exert beneficial or noxious effects, depending on their concentration and the duration of hormonal stimulation. They exert their effects on neuronal and glial cells by means of their cognate receptor, the glucocorticoid receptor (GR), which recruits the p160 coactivator family members SRC-1 (steroid receptor coactivator 1), SRC-2, and SRC-3 after hormone binding. In this study, we investigated the molecular pathways used by the GR in cultured glial cells of the central and the peripheral nervous systems, astrocytes and Schwann cells (MSC80 cells), respectively. We performed functional studies based on transient transfection of a minimal glucocorticoid-sensitive reporter gene into the glial cells to test the influence of overexpression or selective inhibition by short interfering RNA of the three p160 coactivator family members on GR transactivation. We demonstrate that, depending on the glial cell type, GR differentially recruits p160 family members: in Schwann cells, GR recruited SRC-1a, SRC-1e, or SRC-3, whereas in astrocytes, SRC-1e and SRC-2, and to a lesser extent SRC-3, were active toward GR signaling. The C-terminal nuclear receptor-interacting domain of SRC-1a participates in its exclusion from the GR transcriptional complex in astrocytes. Immunolocalization experiments revealed a cell-specific intracellular distribution of the p160s, which was dependent on the duration of the hormonal induction. For example, within astrocytes, SRC-1 and SRC-2 were mainly nuclear, whereas SRC-3 unexpectedly localized to the lumen of the Golgi apparatus. In contrast, in Schwann cells, SRC-1 showed a nucleocytoplasmic shuttling depending on hormonal stimulation, whereas SRC-2 remained strictly nuclear and SRC-3 remained predominantly cytoplasmic. Altogether, these results highlight the cell specificity and the time dependence of p160s recruitment by the activated GR in glial cells, revealing the complexity of GR-p160 assembly in the nervous system.

Pharmacogenetics of controlled ovarian hyperstimulation

Controlled ovarian hyperstimulation (COH) is a routine treatment employed in most assisted reproductive techniques (ARTs). The existence of genetic factors involved in COH has been suspected. The main challenge for clinicians involved in ART is COH cycle cancellation, which usually occurs due to two opposing situations. On the one hand, there is the presence of a poor response during COH treatment, and on the other there is the presentation of a side effect related to gonadotropin hypersensitivity (ovarian hyperstimulation syndrome [OHSS]). Evidence for an association between single nucleotide polymorphisms and COH outcome has been obtained during the last decade. The genetic dissection of both extreme phenotypes of COH will be the main objective of this review. The development of predictive panels useful for the clinical management of COH is currently underway, and will improve the clinical management of patients undergoing ART.

Multiple signaling defects in the absence of RIP140 impair both cumulus expansion and follicle rupture

The nuclear receptor corepressor RIP140 is essential in the ovary for ovulation, but is not required for follicle growth and luteinization. To identify genes that may be subject to regulation by RIP140 or play a role in ovulation, we compared ovarian gene expression profiles in untreated immature wild-type and RIP140 null mice and after treatment with pregnant mare serum gonadotropin and human chorionic gonadotropin. Many genes involved in signaling, extracellular matrix formation, cell-cell attachment, and adhesion were aberrantly regulated in the absence of RIP140, varying according to the hormone status of the mice. Notable among these was the reduced expression of a number of genes that encode components of signaling pathways and matrix proteins required for cumulus expansion, a key remodeling process necessary for ovulation. Histological analysis confirmed that cumulus expansion in RIP140 null mice is reduced, oocyte detachment from the mural cell wall is impaired, and follicles fail to rupture in response to LH. Although the expression of many genes involved in cumulus cell expansion was reduced, there was a subset of genes involved in extracellular matrix formation and cell-cell interactions that was up-regulated and may interfere with ovarian tissue remodeling. We propose that widespread gene dysregulation in ovarian tissues in the absence of RIP140 leads to the anovulatory phenotype. This helps to define an important role for RIP140 in the regulation of multiple processes leading to ovulation.

The Human Tumor Antigen PRAME Is a Dominant Repressor of Retinoic Acid Receptor Signaling

Retinoic acid (RA) induces proliferation arrest, differentiation, and apoptosis, and defects in retinoic acid receptor (RAR) signaling have been implicated in cancer. The human tumor antigen PRAME is overexpressed in a variety of cancers, but its function has remained unclear. We identify here PRAME as a dominant repressor of RAR signaling. PRAME binds to RAR in the presence of RA, preventing ligand-induced receptor activation and target gene transcription through recruitment of Polycomb proteins. PRAME is present at RAR target promoters and inhibits RA-induced differentiation, growth arrest, and apoptosis. Conversely, knockdown of PRAME expression by RNA interference in RA-resistant human melanoma restores RAR signaling and reinstates sensitivity to the antiproliferative effects of RA in vitro and in vivo. Our data suggest that overexpression of PRAME frequently observed in human cancers confers growth or survival advantages by antagonizing RAR signaling.

Preliminary molecular genetic analysis of the Receptor Interacting Protein 140 (RIP140) in women affected by endometriosis

Background

Endometriosis is a complex disease affecting 10–15% of women at reproductive age. Very few genes are known to be altered in this pathology. RIP140 protein is an important cofactor of oestrogen receptor and many other nuclear receptors. Targeting disruption experiments of nrip1 gene in mice have demonstrated that nuclear receptor interacting protein 1 gene (nrip1), the gene encoding for rip140 protein, is essential for female fertility. Specifically, mice null for nrip1 gene are viable, but females are infertile because of complete failure of mature follicles to release oocytes at ovulation stage. The ovarian phenotype observed in mice devoid of rip140 closely resembles the luteinized unruptured follicle (LUF) syndrome that is observed in a high proportion of women affected of endometriosis or idiopathic infertility. Here we present a preliminary work that analyses the role of NRIP1 gene in humans.

Methods

We have sequenced the complete coding region of NRIP1 gene in 20 unrelated patients affected by endometriosis. We have performed genetic association studies by using the DNA variants identified during the sequencing process.

Results

We identified six DNA variants within the coding sequence of NRIP1 gene, and five of them generated amino acid changes in the protein. We observed that three of twenty sequenced patients have specific combinations of amino-acid variants within the RIP140 protein that are poorly represented in the control population (p = 0.006). Moreover, we found that Arg448Gly, a common polymorphism located within NRIP1 gene, is associated with endometriosis in a case-control study (59 cases and 141 controls, p_{allele positivity test} = 0.027).

Conclusion

Our results suggest that NRIP1 gene variants, separately or in combinations, might act as predisposing factors for human endometriosis.

Chromosome-wide mapping of estrogen receptor binding reveals long-range regulation requiring the forkhead protein FoxA1

Estrogen plays an essential physiologic role in reproduction and a pathologic one in breast cancer. The completion of the human genome has allowed the identification of the expressed regions of protein-coding genes; however, little is known concerning the organization of their cis-regulatory elements. We have mapped the association of the estrogen receptor (ER) with the complete nonrepetitive sequence of human chromosomes 21 and 22 by combining chromatin immunoprecipitation (ChIP) with tiled microarrays. ER binds selectively to a limited number of sites, the majority of which are distant from the transcription start sites of regulated genes. The unbiased sequence interrogation of the genuine chromatin binding sites suggests that direct ER binding requires the presence of Forkhead factor binding in close proximity. Furthermore, knockdown of FoxA1 expression blocks the association of ER with chromatin and estrogen-induced gene expression demonstrating the necessity of FoxA1 in mediating an estrogen response in breast cancer cells.

Regulation of co-repressive activity of and HDAC recruitment to RIP140 by site-specific phosphorylation

Receptor interacting protein 140 (RIP140) is a versatile transcriptional co-repressor that contains several autonomous repressive domains (RDs). The N-terminal RD acts by recruiting histone deacetylases (HDACs). In a comprehensive proteomic analysis of RIP140 by MS, 11 phosphorylation sites of RIP140 are identified; among them five sites are located in the N-terminal RD including Ser104, Thr202, Thr207, Ser358, and Ser380. The role of phosphorylation of RIP140 in regulating its biological activity and the underlying mechanism are examined using a site-directed mutagenesis approach. Mutations mimicking constitutive phosphorylation or dephosphorylation are introduced. The N-terminal RD phosphorylation, mediated by the mitogen-activated protein kinase (MAPK), enhances its repressive activity through increased recruitment of HDAC. Mutations mimicking constitutive dephosphorylation at Thr202 or Thr207 significantly impair its repressive activity and HDAC recruitment, whereas mutation at Ser358 only slightly affects its HDAC recruitment and the repressive activity. Consistently, mutations mimicking constitutive phosphorylation at either Thr202 or Thr207 convert RIP140 into a more potent repressor, which is less responsive to a disturbance in the MAPK system. Furthermore, constitutive phosphorylation at both Thr202 and Thr207 residues renders RIP140 fully repressive and strongly interacting with HDAC. The activity of this mutant is resistant to the MAPK inhibitor, indicating an essential role for Thr202 and Thr207 in MAPK-mediated modulation of RIP140 function. The study provides insights into the modulation of RIP140 biological activity through a specific cellular signaling pathway that augments phosphorylation at specific residues of RIP140 molecule and alters its cofactor recruitment.

A pp32–retinoblastoma protein complex modulates androgen receptor-mediated transcription and associates with components of the splicing machinery

We have previously shown pp32 and the retinoblastoma protein interact. pp32 and the retinoblastoma protein are nuclear receptor transcriptional coregulators: the retinoblastoma protein is a coactivator for androgen receptor, the major regulator of prostate cancer growth, while pp32, which is highly expressed in prostate cancer, is a corepressor of the estrogen receptor. We now show pp32 increases androgen receptor-mediated transcription and the retinoblastoma protein modulates this activity. Using affinity purification and mass spectrometry, we identify members of the pp32-retinoblastoma protein complex as PSF and nonO/p54nrb, proteins implicated in coordinate regulation of nuclear receptor-mediated transcription and splicing. We show that the pp32-retinoblastoma protein complex is modulated during TPA-induced K562 differentiation. Present evidence suggests that nuclear receptors assemble multiprotein complexes to coordinately regulate transcription and mRNA processing. Our results suggest that pp32 and the retinoblastoma protein may be part of a multiprotein complex that coordinately regulates nuclear receptor-mediated transcription and mRNA processing.

Transcriptional activities of retinoic acid receptors

Vitamin A derivatives plays a crucial role in embryonic development, as demonstrated by the teratogenic effect of either an excess or a deficiency in vitamin A. Retinoid effects extend however beyond embryonic development, and tissue homeostasis, lipid metabolism, cellular differentiation and proliferation are in part controlled through the retinoid signaling pathway. Retinoids are also therapeutically effective in the treatment of skin diseases (acne, psoriasis and photoaging) and of some cancers. Most of these effects are the consequences of retinoic acid receptors activation, which triggers transcriptional events leading either to transcriptional activation or repression of retinoid-controlled genes. Synthetic molecules are able to mimic part of the biological effects of the natural retinoic acid receptors, all-trans retinoic acid. Therefore, retinoic acid receptors are considered as highly valuable therapeutic targets and limiting unwanted secondary effects due to retinoid treatment requires a molecular knowledge of retinoic acid receptors biology. In this review, we will examine experimental evidence which provide a molecular basis for the pleiotropic effects of retinoids, and emphasize the crucial roles of coregulators of retinoic acid receptors, providing a conceptual framework to identify novel therapeutic targets.

Nuclear hormone receptor coregulator: role in hormone action, metabolism, growth, and development

Nuclear hormone receptor coregulator (NRC) (also referred to as activating signal cointegrator-2, thyroid hormone receptor-binding protein, peroxisome proliferator activating receptor-interacting protein, and 250-kDa receptor associated protein) belongs to a growing class of nuclear cofactors widely known as coregulators or coactivators that are necessary for transcriptional activation of target genes. The NRC gene is also amplified and overexpressed in breast, colon, and lung cancers. NRC is a 2063-amino acid protein that harbors a potent N-terminal activation domain (AD1) and a second more centrally located activation domain (AD2) that is rich in Glu and Pro. Near AD2 is a receptor-interacting domain containing an LxxLL motif (LxxLL-1), which interacts with a wide variety of ligand-bound nuclear hormone receptors with high affinity. A second LxxLL motif (LxxLL-2) located in the C-terminal region of NRC is more restricted in its nuclear hormone receptor specificity. The intrinsic activation potential of NRC is regulated by a C-terminal serine, threonine, leucine-regulatory domain. The potential role of NRC as a cointegrator is suggested by its ability to enhance transcriptional activation of a wide variety of transcription factors and from its in vivo association with a number of known transcriptional regulators including CBP/p300. Recent studies in mice indicate that deletion of both NRC alleles leads to embryonic lethality resulting from general growth retardation coupled with developmental defects in the heart, liver, brain, and placenta. NRC(-/-) mouse embryo fibroblasts spontaneously undergo apoptosis, indicating the importance of NRC as a prosurvival and antiapoptotic gene. Studies with 129S6 NRC(+/-) mice indicate that NRC is a pleiotropic regulator that is involved in growth, development, reproduction, metabolism, and wound healing.

Post-translational modification of nuclear co-repressor receptor-interacting protein 140 by acetylation

Receptor-interacting protein 140 (RIP140) is a versatile co-regulator for nuclear receptors and many transcription factors and contains several autonomous repressive domains. RIP140 can be acetylated, and acetylation affects its biological activity. In this study, a comprehensive proteomic analysis using liquid chromatography-tandem mass spectroscopy was conducted to identify the in vivo acetylation sites on RIP140 purified from Sf21 insect cells. Eight acetylation sites were found within the amino-terminal and the central regions, including Lys111, Lys158, Lys287, Lys311, Lys482, Lys529, Lys607, and Lys932. Reporter assays were conducted to examine the effects of acetylation on various domains of RIP140. Green fluorescent protein-tagged fusion proteins were used to demonstrate the effect on nuclear translocation of these domains. A general inhibitor of reversible protein deacetylation was used to enrich the acetylated population of RIP140. The amino-terminal region (amino acids (aa) 1-495) was more repressive and accumulated more in the nuclei under hyperacetylated conditions, whereas hyperacetylation reduced the repressive activity and nuclear translocation of the central region (aa 336-1006). The deacetylase inhibitor had no effect on the carboxyl-terminal region (aa 977-1161) where no acetylation sites were found. Hyperacetylation also enhanced the repressive activity of the full-length protein but triggered its export into the cytosol in a small population of cells. This study revealed differential effects of post-translational modification on various domains of RIP140 through acetylation, including its effects on repressive activity and nuclear translocation of the full-length protein and its subdomains.

Mapping of phosphorylation sites of nuclear corepressor receptor interacting protein 140 by liquid chromatography-tandem mass spectroscopy

Receptor interacting protein (RIP140) is a versatile coregulator for many nuclear receptors and transcription factors. Analysis by liqid chromatography tandem mass spectroscopy led to the identification of 11 phosphopeptides from tryptic digests of His6-RIP140 purified from Sf21 insect cells. No phosphopeptides were detected on RIP140 expressed in E. coli in a parallel experiment, suggesting that RIP140 phosphorylation occurred specifically only in eukaryotic cells. The tandem mass spectra of the precursor ions of the phosphopeptides were analyzed to map the exact phosphorylation sites on RIP140. All the phosphopeptides displayed intact phosphate containing y- or b-ion signals along with their beta-eliminated product ions, due to neutral loss of phosphoric acid. Phosphorylation occurred specifically on nine serine and a single threonine residues, including Ser-104, Thr-207, Ser-358, Ser-380, Ser-488, Ser-519, Ser-531, Ser-543, Ser-672, and Ser-1003. No tyrosine phosphorylation was found. These data suggested that the central region of RIP140, one major repressive domain, was extensively modified by phosphorylation. These phosphorylation sites can be the targets in future studies addressing post-translational modification of RIP140 with regards to its biological activities.

9-cis-Retinoic Acid Inhibits Androgen Receptor Activity through Activation of Retinoid X Receptor

Although the retinoic X receptor (RXR) forms heterodimers with many members of the estrogen receptor subfamily, the interaction between RXR and the members of the glucocorticoid receptor subfamily remains unclear. Here we show that the RXR can form a heterodimer with the androgen receptor (AR) under in vitro and in vivo conditions. Functional analyses further demonstrated that the AR, in the presence or absence of androgen, can function as a repressor to suppress RXR target genes, thereby preventing the RXR binding to the RXR DNA response element. In contrast, RXR can function as a repressor to suppress AR target genes in the presence of 9-cis-retinoic acid, but unliganded RXR can function as a weak coactivator to moderately enhance AR transactivation. Together, these results not only reveal a unique interaction between members of the two nuclear receptor subfamilies, but also represent the first evidence showing a nuclear receptor (RXR) may function as either a repressor or a coactivator based on the ligand binding status.

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.

p21WAF1/CIP1 selectively controls the transcriptional activity of estrogen receptor alpha

Estrogen receptors (ER) are ligand-dependent transcription factors that regulate growth, differentiation, and maintenance of cellular functions in a wide variety of tissues. We report here that p21WAF1/CIP1, a cyclin-dependent kinase (Cdk) inhibitor, cooperates with CBP to regulate the ERalpha-mediated transcription of endogenous target genes in a promoter-specific manner. The estrogen-induced expression of the progesterone receptor and WISP-2 mRNA transcripts in MCF-7 cells was enhanced by p21WAF1/CIP1, whereas that of the cyclin D1 mRNA was reduced and the pS2 mRNA was not affected. Chromatin immunoprecipitation assays revealed that p21WAF1/CIP1 was recruited simultaneously with ERalpha and CBP to the endogenous progesterone receptor gene promoter in an estrogen-dependent manner. Experiments in which the p21WAF1/CIP1 protein was knocked down by RNA interference showed that the induction of the expression of the gene encoding the progesterone receptor required p21WAF1/CIP1, in contrast with that of the cyclin D1 and pS2 genes. p21WAF1/CIP1 induced not only cell cycle arrest in breast cancer cells but also milk fat globule protein and lipid droplets, indicators of the differentiated phenotype, as well as cell flattening and increase of the volume of the cytoplasm. These results indicate that p21WAF1/CIP1, in addition to its Cdk-regulatory role, behaves as a transcriptional coactivator in a gene-specific manner implicated in cell differentiation.

Coregulator Interactions with the Thyroid Hormone Receptor

The thyroid hormone receptor (TR) directly regulates the transcription of thyroid hormone-responsive genes in response to changing levels of thyroid hormone. Mechanistically TR utilizes a complex set of binding interactions, with hormone, response elements, and coregulatory proteins, to provide specific local control of patterns of transcriptional response that are partially responsible for inducing the tissue-selective responses to the circulating hormone. One of the apparently dominant phenomena in the regulation of thyroid hormone responses is the protein interactions between TR and its coregulators. This review summarizes the current state of knowledge with respect to the identity of these coregulators, their interaction with TR, and the consequences of those interactions.

[RIP140 and hormone signaling]

Nuclear hormone receptors belong to a superfamily of ligand-activated transcription factors which regulate fundamental physiological processes. Their activity is controlled by a large number of coregulatory proteins which are, in most cases, recruited by nuclear receptors in the presence of ligand. RIP140 (receptor interacting protein of 140 kDa) was one of the first transcription cofactors to be identified almost ten years ago. This molecule is an atypical cofactor which interacts with agonist-liganded nuclear receptors but negatively regulates their transactivation potential. RIP140 exhibits nine leucine-rich motifs (LxxLL) which mediate the specific docking on the nuclear receptor ligand-binding domain. Transcription repression exerted by this cofactor implicates different mechanisms. Not only it involves a competition with coactivators such as those belonging to the p160 family, but also relies on active intrinsic repression through at least four different domains which allow recruitement of downstream repressors such as histone deacetylases (HDACs) or C-terminal binding proteins (CtBPs). The biological role of RIP140 has been investigated by disrupting the gene in mice. The lack of RIP140 expression in ovaries prevents follicle rupture and ovulation, rising to female infertility. In addition, this cofactor is also required for the control of fat storage and utilization through the regulation of genes involved in thermogenesis. Finally, RIP140 could play a role in the hormonal control of cancer cell proliferation by negatively regulating the activity of estrogen and retinoic acid receptors which are key actors in cancer growth. Interestingly, both estrogens and retinoic acid regulate RIP140 gene expression, revealing an increased level of complexity. In conclusion, RIP140 is an atypical transcription inhibitor which, by repressing nuclear hormone receptor activity, plays fundamental physiopathological roles.

Characterization of Progenitor-Cell-Specific Genes Identified by Subtractive Suppression Hybridization

We have utilized subtractive suppression hybridization (SSH) to identify differentially expressed genes present in either neuroepithelial (NEP) cells or glial restricted precursor (GRP) cells. Eighteen clones enriched in GRP cells and 28 in NEP cells were identified. Five of the GRP-specific clones (tenascin C, cystatin C, GABA transporter 3, extracellular matrix molecule 2 and H2-4) were characterized further, and their glial specificity was confirmed by RT-PCR, in situ hybridization and immunocytochemistry. H2-4 (an expressed sequence tag) was shown to be part of chondroitin sulfate proteoglycan 3. Overall, our results show that SSH can be used to identify lineage- and stage-specific markers and that extracellular matrix molecules likely play important roles in the migration and differentiation of GRPs.

Limiting effects of RIP140 in estrogen signaling: potential mediation of anti-estrogenic effects of retinoic acid

The receptor interacting protein 140 (RIP140) belongs to a unique subclass of nuclear receptor coregulators with the ability to bind and repress the action of a number of agonist-bound hormone receptors. We have previously demonstrated that all-trans-retinoic acid (RA) induction of RIP140 constitutes a rate-limiting step in the regulation of retinoid receptor signaling. Here we demonstrate that RIP140 is also a limiting regulator of estrogen receptor signaling. Overexpression of RIP140 dose dependently inhibits estrogen-dependent reporter activity in human breast cancer cells. Furthermore, small interfering RNA to RIP140 enhances estrogen-dependent signaling. Our previous studies indicate that RIP140 is a direct target of RA. We report here that RA can abrogate estrogen-mediated cell cycle re-entry. In addition, RA treatment of estrogen-dependent breast cancer cells opposes estrogen receptor-dependent reporter activity, implying that a proportion of RA effects are anti-estrogenic. We provide evidence for a role for RIP140 in mediating anti-estrogenic effects of RA. RIP140 small interfering RNA blocks RA-mediated repression of estrogen receptor activity and provides a growth advantage to estrogen-dependent cells. Together these data implicate a regulatory role for RIP140 in mediating anti-estrogenic effects of RA in estrogen-dependent breast cancer cells and suggest that acute regulation of coregulator expression may be a general mechanism to integrate diverse hormone signals.

Expression of nuclear receptor coactivators in androgen-responsive and -unresponsive motoneurons

Adult rat lumbar motoneurons in the spinal nucleus of the bulbocavernosus (SNB) respond to androgens with an increase in soma size. This response is mediated by the androgen receptor (AR) in these motoneurons. Interestingly, other lumbar motoneurons in the rat possess the AR, yet do not respond to androgens in this fashion. This paradox suggests the existence and participation of nuclear receptor coregulators in conferring direct androgen-responsiveness to select motoneurons in the adult rat spinal cord. Nuclear receptor coregulators have received much attention recently for their proposed role in enhancing or repressing the transcriptional activity of steroid hormone receptors. The present study used immunocytochemistry to identify a number of nuclear receptor coactivators that are expressed by adult lumbar motoneurons: SRC-1, SRC-2, CBP, p300, and cJUN. Results of this study indicate that all five of these coactivators are abundantly expressed in the androgen-responsive SNB, and in two adjacent motor pools, the androgen-responsive dorsolateral nucleus (DLN), and the androgen-unresponsive retrodorsolateral nucleus (RDLN). While we detected significant regional differences for only SRC-1 and cJUN, the SNB consistently contained the highest percentage of immunoreactive motoneurons for all five cofactors examined. Our results indicate five different putative cofactors have the potential to participate in motoneuronal responses to androgens, since their distribution overlaps well with the distribution of ARs in these motoneurons.

Distinct gene expression patterns in a tamoxifen-sensitive human mammary carcinoma xenograft and its tamoxifen-resistant subline MaCa 3366/TAM

The reasons why human mammary tumors become resistant to tamoxifen therapy are mainly unknown. Changes in gene expression may occur as cells acquire resistance to antiestrogens. We therefore undertook a comparative gene expression analysis of tamoxifen-sensitive and tamoxifen-resistant human breast cancer in vivo models using Affymetrix oligonucleotide arrays to analyze differential gene expression. Total RNAs from the tamoxifen-sensitive patient-derived mammary carcinoma xenograft MaCa 3366 and the tamoxifen-resistant model MaCa 3366/TAM were hybridized to Affymetrix HuGeneFL and to Hu95Av2 arrays. Pairwise comparisons and clustering algorithms were applied to identify differentially expressed genes and patterns of gene expression. As revealed by cluster analysis, the tamoxifen-sensitive and the tamoxifen-resistant breast carcinomas differed regarding their gene expression pattern. More than 100 transcripts are changed in abundance in MaCa 3366/TAM as compared with MaCa 3366. Among the genes that are differentially expressed in the tamoxifen-resistant tumors, there are several IFN-inducible and estrogen-responsive genes, and genes known to be involved in breast carcinogenesis. The genes neuronatin (NNAT) and bone marrow stem cell antigen 2 (BST2) were sharply up-regulated in MaCa 3366/TAM. The differential expression of four genes (NNAT, BST2, IGFBP5, and BCAS1) was confirmed by Taqman PCR. Our results provide the starting point for deriving markers for tamoxifen resistance by differential gene expression profiling in a human breast cancer model of acquired tamoxifen resistance. Finally, genes whose expression profiles are distinctly changed between the two xenograft lines will be further evaluated as potential targets for diagnostic or therapeutic approaches of tamoxifen-resistant breast cancer.

The Transcriptional Corepressor, PELP1, Recruits HDAC2 and Masks Histones Using Two Separate Domains

PELP1 (proline-, glutamic acid-, and leucine-rich protein 1) has been recognized as a coactivator of estrogen receptor (ER)-recruiting p300/CREB-binding protein histone acetyltransferase to the target chromosome. The present study shows that PELP1 does indeed coactivate ER-mediated transcription but also serves as a corepressor of other nuclear hormone receptors (NR)- and non-NR sequence-specific transcription factors tested, including GR, Nur77, AP1, NF-kappaB, and TCF/SRF. PELP1 expression also retarded the proliferation of mouse fibroblast cell lines in which there was no detectable ER. This was due, at least in part, to the suppressed activation of serum-response genes such as c-fos that in turn resulted from the blocked histone hyperacetylation of nucleosomes containing the c-fos promoter region. The N-terminal leucine-abundant region of PELP1 was observed to interact with HDAC2 and exhibited repressive activity when tethered to the chromatin. In addition, the C-terminal glutamic acid-abundant region bound to the hypoacetylated histones H3 and H4 and prevented them from becoming substrates of histone acetyltransferase. Thus PELP1 promotes and maintains the hypoacetylated state of histones at the target genomic site, and ER binding reverses its role to hyperacetylate histones through an as yet unidentified mechanism.

Molecular interaction of retinoic acid receptors with coregulators PCAF and RIP140

p300/CBP-associating factor (PCAF) is a ligand-dependent coactivator, whereas receptor-interacting protein 140 (RIP140) is a ligand-dependent negative coregulator for retinoic acid (RA) receptor (RAR) and retinoid X receptor (RXR). To compare these molecular interactions and to determine the effect of RXR ligands, we focus on PCAF/RAR/RXR complex formation in this study for a comparison to RIP140/RAR/RXR complex formation. The LBD of RXR is identified as its primary PCAF-interacting motif. BIAcore studies determine the Kd of RAR/RXR association with PCAF as 9.35 nM in the presence of RXR ligand AGN194204, and 47.2 nM in the absence of ligand. Cross-linking study demonstrates tri-molecular complex consisting of one RAR/RXR pair and one PCAF. In competition experiments, RIP140 strongly competes with PCAF for interaction with RAR/RXR both in vitro and in vivo. Chromatin immunoprecipitation demonstrates recruitment of RIP140 and PCAF to the endogenous RA-regulated gene, the RARbeta2 promoter. This study presents kinetic evidence for competition of RIP140 with PCAF for ligand-dependent interactions with RAR/RXR, and provides kinetic explanation for the suppressive activity of RIP140 in RA-activated gene expression.

Selective recruitment of p160 coactivators on glucocorticoid-regulated promoters in Schwann cells

In the nervous system, glucocorticoid hormones play a major role during development and throughout life. We studied the mechanisms of action of the glucocorticoid receptor (GR) and its interactions with p160 coactivator family members [steroid receptor coactivator (SRC)-1 (a and e), SRC-2 and SRC-3] in mouse Schwann cells (MSC80). We found that the three p160s were expressed in MSC80 cells. We have shown by functional overexpression and RNA interference experiments that the recruitment of these coactivators by the GR is promoter dependent. A minimal promoter containing two glucocorticoid response elements, (GRE)2-TATA, recruits SRC-1 (a and e) and SRC-3, whereas SRC-2 is excluded. Within the context of the more complex mouse mammary tumor virus promoter, GR recruits SRC-1e and SRC-2, whereas SRC-1a and SRC-3 are not implicated. Furthermore, we have identified cytosolic aspartate aminotransferase as a GR target gene in MSC80 cells by microarray experiments. The GR recruits exclusively SRC-1e in the context of the cytosolic aspartate aminotransferase promoter. Because SRC-1 is the omnipresent coactivator of GR, we further investigated the interactions between GR and this coactivator in Schwann cells by reporter assays and immunocytochemistry experiments with deleted forms of SRC-1. We have shown that SRC-1 unexpectedly interacts with GR via its two nuclear receptor binding domains, thus providing a novel mechanism of GR signaling within the nervous system.

Quantitative Proteomics of the Thyroid Hormone Receptor-Coregulator Interactions

The thyroid hormone receptor regulates a diverse set of genes that control processes from embryonic development to adult homeostasis. Upon binding of thyroid hormone, the thyroid receptor releases corepressor proteins and undergoes a conformational change that allows for the interaction of coactivating proteins necessary for gene transcription. This interaction is mediated by a conserved motif, termed the NR box, found in many coregulators. Recent work has demonstrated that differentially assembled coregulator complexes can elicit specific biological responses. However, the mechanism for the selective assembly of these coregulator complexes has yet to be elucidated. To further understand the principles underlying thyroid receptor-coregulator selectivity, we designed a high-throughput in vitro binding assay to measure the equilibrium affinity of thyroid receptor to a library of potential coregulators in the presence of different ligands including the endogenous thyroid hormone T3, synthetic thyroid receptor beta-selective agonist GC-1, and antagonist NH-3. Using this homogenous method several coregulator NR boxes capable of associating with thyroid receptor at physiologically relevant concentrations were identified including ones found in traditional coactivating proteins such as SRC1, SRC2, TRAP220, TRBP, p300, and ARA70; and those in coregulators known to repress gene activation including RIP140 and DAX-1. In addition, it was discovered that the thyroid receptor-coregulator binding patterns vary with ligand and that this differential binding can be used to predict biological responses. Finally, it is demonstrated that this is a general method that can be applied to other nuclear receptors and can be used to establish rules for nuclear receptor-coregulator selectivity.

The nuclear hormone receptor coactivator NRC is a pleiotropic modulator affecting growth, development, apoptosis, reproduction, and wound repair

Nuclear hormone receptor coregulator (NRC) is a 2,063-amino-acid coregulator of nuclear hormone receptors and other transcription factors (e.g., c-Fos, c-Jun, and NF-kappaB). We and others have generated C57BL/6-129S6 hybrid (C57/129) NRC(+/-) mice that appear outwardly normal and grow and reproduce. In contrast, homozygous deletion of the NRC gene is embryonic lethal. NRC(-/-) embryos are always smaller than NRC(+/+) embryos, and NRC(-/-) embryos die between 8.5 and 12.5 days postcoitus (dpc), suggesting that NRC has a pleotrophic effect on growth. To study this, we derived mouse embryonic fibroblasts (MEFs) from 12.5-dpc embryos, which revealed that NRC(-/-) MEFs exhibit a high rate of apoptosis. Furthermore, a small interfering RNA that targets mouse NRC leads to enhanced apoptosis of wild-type MEFs. The finding that C57/129 NRC(+/-) mice exhibit no apparent phenotype prompted us to develop 129S6 NRC(+/-) mice, since the phenotype(s) of certain gene deletions may be strain dependent. In contrast with C57/129 NRC(+/-) females, 20% of 129S6 NRC(+/-) females are infertile while 80% are hypofertile. The 129S6 NRC(+/-) males produce offspring when crossed with wild-type 129S6 females, although fertility is reduced. The 129S6 NRC(+/-) mice tend to be stunted in their growth compared with their wild-type littermates and exhibit increased postnatal mortality. Lastly, both C57/129 NRC(+/-) and 129S6 NRC(+/-) mice exhibit a spontaneous wound healing defect, indicating that NRC plays an important role in that process. Our findings reveal that NRC is a coregulator that controls many cellular and physiologic processes ranging from growth and development to reproduction and wound repair.

Predicting pathway perturbations in Down syndrome

Comparative annotation of human chromosome 21 genomic sequence with homologous regions of mouse chromosomes 16, 17 and 10 has identified 170 orthologous gene pairs. Functional annotation of these genes, based on literature reports and computationally-derived predictions, shows that a broad range of cellular processes are represented. A goal of Down syndrome research is to determine which of these processes are perturbed by overexpression of chromosome 21 genes, and which may, therefore, contribute to the cognitive deficits that characterize Down syndrome. Eleven chromosome 21 genes are annotated to interact with or be affected by components of the MAP Kinase pathway and eight are involved in Ca2+/calcineurin signaling. Both pathways are critical for normal neurological function, and consequently their perturbations are proposed as candidates for phenotypic relevance. We present evidence suggesting that the MAP Kinase pathway is perturbed in the Ts65Dn mouse model of Down syndrome at 4-6 months of age. Analysis is complicated by the observation that overexpression of chromosome 21 genes in trisomy may be affected by method of detection, organism, tissue or brain region, and/or developmental age.

Nuclear receptor corepressor RIP140 regulates fat accumulation

Nuclear receptors and their coactivators have been shown to function as key regulators of adipose tissue biology. Here we show that a ligand-dependent transcriptional repressor for nuclear receptors plays a crucial role in regulating the balance between energy storage and energy expenditure. Mice devoid of the corepressor protein RIP140 are lean, show resistance to high-fat diet-induced obesity and hepatic steatosis, and have increased oxygen consumption. Although the process of adipogenesis is unaffected, expression of certain lipogenic enzymes is reduced. In contrast, genes involved in energy dissipation and mitochondrial uncoupling, including uncoupling protein 1, are markedly increased. Therefore, the maintenance of energy homeostasis requires the action of a transcriptional repressor in white adipose tissue, and ligand-dependent recruitment of RIP140 to nuclear receptors may provide a therapeutic target in the treatment of obesity and related disorders.

The role of corepressors in transcriptional regulation by nuclear hormone receptors

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