Steroid/nuclear receptor coactivators

Sexual partner preference in animals and humans

Sex differences in brain and behavior of animals including humans result from an interaction between biological and environmental influences. This is also true for the differences between men and women concerning sexual orientation. Sexual differentiation is mediated by three groups of biological mechanisms: early actions of sex steroids, more direct actions of sex-specific genes not mediated by gonadal sex steroids and epigenetic mechanisms. Differential interactions with parents and conspecifics have additionally long-term influences on behavior. This presentation reviews available evidence indicating that these different mechanisms play a significant role in the control of sexual partner preference in animals and humans, in other words the homosexual versus heterosexual orientation. Clinical and epidemiological studies of phenotypically selected populations indicate that early actions of hormones and genetic factors clearly contribute to the determination of sexual orientation. The maternal embryonic environment also modifies the incidence of male homosexuality via immunological mechanisms. The relative contribution of each of these mechanisms remains however to be determined.

Integrative genomic approaches to dissect clinically-significant relationships between the VDR cistrome and gene expression in primary colon cancer

Recently, we undertook a pan-cancer analyses of the nuclear hormone receptor (NR) superfamily in The Cancer Genome Atlas (TCGA), and revealed that the vitamin D receptor (NR1I1/VDR) was commonly and significantly down-regulated specifically in colon adenocarcinoma cohort (COAD). To examine the consequence of down-regulated VDR expression we re-analyzed VDR chromatin immunoprecipitation sequencing (ChIP-Seq) data from LS180 colon cancer cells (GSE31939). This analysis identified 1809 loci that displayed significant (p.adj<0.01) differential binding of the VDR in response 1,25(OH)₂D₃ treatment; 947 peaks annotated to 672 genes. We examined expression patterns in the COAD cohort of 286 tumors compared to 41 normal samples and revealed that VDR bound genes were significantly positively correlated to VDR expression compared to the background transcriptome, suggesting direct regulation by VDR. Gene set enrichment analyses revealed significant enrichment for genes known to be regulated by a number of other transcription factors including SMADs and JUN. Filtering VDR associated genes for those that were commonly and significantly altered in COAD revealed a cohort of 27 differentially expressed genes. The expression patterns of these genes clustered tumors and significantly associated with disease free survival. For instance, males with low expression of Lectin, Galactoside Binding Soluble 4 (LGALS4, encodes the colon tumor suppressor, Galactin 4) had significantly shorted disease free survival. These analyses suggest that reduced expression of VDR in colon cancer (but neither loss nor mutation) changes the actions of the VDR by both dampening the expression of tumor suppressors (e.g. LGALS4) whilst either stabilizing or not down-regulating expression of oncogenes (e.g. Carbonic Anhydrase 9 (CA9)). These integrative genomic approaches are relatively generic and applicable to the study of any transcription factor.

Bioinformatic approaches to interrogating vitamin D receptor signaling

Bioinformatics applies unbiased approaches to develop statistically-robust insight into health and disease. At the global, or "20,000 foot" view bioinformatic analyses of vitamin D receptor (NR1I1/VDR) signaling can measure where the VDR gene or protein exerts a genome-wide significant impact on biology; VDR is significantly implicated in bone biology and immune systems, but not in cancer. With a more VDR-centric, or "2000 foot" view, bioinformatic approaches can interrogate events downstream of VDR activity. Integrative approaches can combine VDR ChIP-Seq in cell systems where significant volumes of publically available data are available. For example, VDR ChIP-Seq studies can be combined with genome-wide association studies to reveal significant associations to immune phenotypes. Similarly, VDR ChIP-Seq can be combined with data from Cancer Genome Atlas (TCGA) to infer the impact of VDR target genes in cancer progression. Therefore, bioinformatic approaches can reveal what aspects of VDR downstream networks are significantly related to disease or phenotype.

Pan-cancer analyses of the nuclear receptor superfamily

Nuclear receptors (NR) act as an integrated conduit for environmental and hormonal signals to govern genomic responses, which relate to cell fate decisions. We review how their integrated actions with each other, shared co-factors and other transcription factors are disrupted in cancer. Steroid hormone nuclear receptors are oncogenic drivers in breast and prostate cancer and blockade of signaling is a major therapeutic goal. By contrast to blockade of receptors, in other cancers enhanced receptor function is attractive, as illustrated initially with targeting of retinoic acid receptors in leukemia. In the post-genomic era large consortia, such as The Cancer Genome Atlas, have developed a remarkable volume of genomic data with which to examine multiple aspects of nuclear receptor status in a pan-cancer manner. Therefore to extend the review of NR function we have also undertaken bioinformatics analyses of NR expression in over 3000 tumors, spread across six different tumor types (bladder, breast, colon, head and neck, liver and prostate). Specifically, to ask how the NR expression was distorted (altered expression, mutation and CNV) we have applied bootstrapping approaches to simulate data for comparison, and also compared these NR findings to 12 other transcription factor families. Nuclear receptors were uniquely and uniformly downregulated across all six tumor types, more than predicted by chance. These approaches also revealed that each tumor type had a specific NR expression profile but these were most similar between breast and prostate cancer. Some NRs were down-regulated in at least five tumor types (e.g. NR3C2/MR and NR5A2/LRH-1)) whereas others were uniquely down-regulated in one tumor (e.g. NR1B3/RARG). The downregulation was not driven by copy number variation or mutation and epigenetic mechanisms maybe responsible for the altered nuclear receptor expression.

Involvement of the Androgen and Glucocorticoid Receptors in Bladder Cancer

Bladder cancer is encountered worldwide having been associated with a host of environmental and lifestyle risk factors. The disease has a male to female prevalence of 3 : 1. This disparity has raised the possibility of the androgen receptor (AR) pathway being involved in the genesis of the disease; indeed, research has shown that AR is involved in and is likely a driver of bladder cancer. Similarly, an inflammatory response has been implicated as a major player in bladder carcinogenesis. Consistent with this concept, recent work on anti-inflammatory glucocorticoid signaling points to a pathway that may impact bladder cancer. The glucocorticoid receptor- (GR-) α isoform has an important role in suppressing inflammatory processes, which may be attenuated by AR in the development of bladder cancer. In addition, a GR isoform that is inhibitory to GRα, GRβ, is proinflammatory and has been shown to induce cancer growth. In this paper, we review the evidence of inflammatory mediators and the relationship of AR and GR isoforms as they relate to the propensity for bladder cancer.

Retinoic acid receptors: from molecular mechanisms to cancer therapy

Retinoic acid (RA), the major bioactive metabolite of retinol or vitamin A, induces a spectrum of pleiotropic effects in cell growth and differentiation that are relevant for embryonic development and adult physiology. The RA activity is mediated primarily by members of the retinoic acid receptor (RAR) subfamily, namely RARα, RARβ and RARγ, which belong to the nuclear receptor (NR) superfamily of transcription factors. RARs form heterodimers with members of the retinoid X receptor (RXR) subfamily and act as ligand-regulated transcription factors through binding specific RA response elements (RAREs) located in target genes promoters. RARs also have non-genomic effects and activate kinase signaling pathways, which fine-tune the transcription of the RA target genes. The disruption of RA signaling pathways is thought to underlie the etiology of a number of hematological and non-hematological malignancies, including leukemias, skin cancer, head/neck cancer, lung cancer, breast cancer, ovarian cancer, prostate cancer, renal cell carcinoma, pancreatic cancer, liver cancer, glioblastoma and neuroblastoma. Of note, RA and its derivatives (retinoids) are employed as potential chemotherapeutic or chemopreventive agents because of their differentiation, anti-proliferative, pro-apoptotic, and anti-oxidant effects. In humans, retinoids reverse premalignant epithelial lesions, induce the differentiation of myeloid normal and leukemic cells, and prevent lung, liver, and breast cancer. Here, we provide an overview of the biochemical and molecular mechanisms that regulate the RA and retinoid signaling pathways. Moreover, mechanisms through which deregulation of RA signaling pathways ultimately impact on cancer are examined. Finally, the therapeutic effects of retinoids are reported.

Embryo implantation failure and other reproductive defects in Ube2q1-deficient female mice

The ubiquitination process is indispensable for proteome regulation. Three classes of ubiquitin (Ub)-related proteins can be distinguished: E1, E2 and E3. Proteins from the E2 class are responsible for the transfer of Ubls from E1 to the target protein. For this activity, interaction with class E3 ligases is usually required. Ub-conjugating enzyme E2Q 1 (UBE2Q1) belongs to the E2 class of Ub-related enzymes and is demonstrated to be involved in the regulation of membrane B4GALT1 protein. Here, we demonstrate that human UBE2Q1 and mouse Ube2q1 are widely expressed and highly conserved genes. To elucidate the function of UBE2Q1 protein, we generated knockout mouse model. No overt phenotype was detected in UBE2Q1-deficient males, but in mutant females, pleiotropic reproductive defects were observed including altered oestrus cycle, abnormal sexual behaviour and reduced offspring care. Moreover, in the uterus of mutant females, significantly increased embryonic lethality and decreased implantation capacity of homozygous mutant embryos were noticed. We found that Ube2q1 is not expressed in the uterus of non-pregnant females but its expression is up-regulated during pregnancy. Taken together, Ube2q1 is involved in different aspects of female fertility.

Nuclear hormone receptors in podocytes

Nuclear receptors are a family of ligand-activated, DNA sequence-specific transcription factors that regulate various aspects of animal development, cell proliferation, differentiation, and homeostasis. The physiological roles of nuclear receptors and their ligands have been intensively studied in cancer and metabolic syndrome. However, their role in kidney diseases is still evolving, despite their ligands being used clinically to treat renal diseases for decades. This review will discuss the progress of our understanding of the role of nuclear receptors and their ligands in kidney physiology with emphasis on their roles in treating glomerular disorders and podocyte injury repair responses.

The role of androgen receptor mutations in prostate cancer progression

Prostate tumour growth is almost always dependent upon the androgen receptor pathway and hence therapies aimed at blocking this signalling axis are useful tools in the management of this disease. Unfortunately such therapies invariably fail; and the tumour progresses to an “androgen-independent” stage. In such cases androgen receptor expression is almost always maintained and much evidence exists to suggest that it may still be driving growth. One mechanism by which the receptor is thought to remain active is mutation. This review summarises the present data on androgen receptor mutations in prostate cancer, and how such substitutions offer a growth advantage by affecting cofactor interactions or by reducing ligand specificity. Such alterations appear to have a subsequent effect upon gene expression suggesting that tumours may “behave” differently dependent upon the ligand promoting growth and if a mutation is present.

Isoform-specific degradation of PR-B by E6-AP is critical for normal mammary gland development

E6-associated protein (E6-AP), which was originally identified as an ubiquitin-protein ligase, also functions as a coactivator of estrogen (ER-α) and progesterone (PR) receptors. To investigate the in vivo role of E6-AP in mammary gland development, we generated transgenic mouse lines that either overexpress wild-type (WT) human E6-AP (E6-AP(WT)) or ubiquitin-protein ligase-defective E6-AP (E6-AP(C833S)) in the mammary gland. Here we show that overexpression of E6-AP(WT) results in impaired mammary gland development. In contrast, overexpression of E6-AP(C833S) or loss of E6-AP (E6-AP(KO)) increases lateral branching and alveolus-like protuberances in the mammary gland. We also show that the mammary phenotypes observed in the E6-AP transgenic and knockout mice are due, in large part, to the alteration of PR-B protein levels. We also observed alteration in ER-α protein level, which might contribute to the observed mammary phenotype by regulating PR expression. Furthermore, E6-AP regulates PR-B protein levels via the ubiquitin-proteasome pathway. Additionally, we also show that E6-AP impairs progesterone-induced Wnt-4 expression by decreasing the steady state level of PR-B in both mice and in human breast cancer cells. In conclusion, we present the novel observation that E6-AP controls mammary gland development by regulating PR-B protein turnover via the ubiquitin proteasome pathway. For the first time, we show that the E3-ligase activity rather than the coactivation function of E6-AP plays an important role in the mammary gland development, and the ubiquitin-dependent PR-B degradation is not required for its transactivation functions. This mechanism appears to regulate normal mammogenesis, and dysregulation of this process may be an important contributor to mammary cancer development and progression.

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.

Vitamin A regulation of gene expression: molecular mechanism of a prototype gene

Vitamin A regulation of gene expression is a well-characterized example of direct nutrient regulation of gene expression. The downstream metabolites of retinol, all-trans and 9-cis retinoic acids are the bioactive components that bind and activate their cognate nuclear receptors to regulate target genes. There are multiple retinoid receptor subtypes that are encoded by separate genes and each subtype has different isoforms. These receptors are Class II members of the thyroid/retinoid/vitamin D superfamily of nuclear receptors. The characterization of the retinoid receptors and the DNA response elements of target genes that bind these receptors have vastly expanded our knowledge of the mechanism of retinoid regulation of target genes. The basic regulatory mechanism of retinoids interacting with their cognate receptors is further complicated by the interaction of coactivators and corepressors, nuclear proteins that are involved in activation or repression of transcription, respectively. Most of these coregulators are involved in modifying chromatin and nucleosome structure such that chromatin is relaxed or condensed, and in bridging between the upstream enhancer domains and the transcription preinitiation complex. Retinoid regulation of the rate of transcription of target genes and the duration of the retinoid response is further complicated by covalent modification of the retinoid receptors by phosphorylation involved in coactivator association and ubiquitinylation involved in the degradation of retinoid receptors. This review presents a prototype retinoid responsive gene that encodes the phosphoenolpyruate carboxykinase (PEPCK) gene as an example of a specific mechanism of retinoid regulation of a metabolic gene. The retinoid response elements and overall mechanism of retinoid regulation of the PEPCK gene have been well documented by both in vitro and in vivo methods. We provide detailed information on the specific nuclear receptors, coactivators and chromatin modification events that occur when vitamin A is deficient and, therefore, retinoids are not available to activate the nuclear retinoid-signaling cascade.

The Intersection Between the Aryl Hydrocarbon Receptor (AhR)‐ and Retinoic Acid‐Signaling Pathways

Data from a variety of animal and cell culture model systems have demonstrated an interaction between the aryl hydrocarbon receptor (AhR)- and retinoic acid (RA)-signaling pathways. The AhR(1) was originally identified as the receptor for the polycyclic aromatic hydrocarbon family of environmental contaminants; however, recent data indicate that the AhR binds to a variety of endogenous and exogenous compounds, including some synthetic retinoids. In addition, activation of the AhR pathway alters the function of nuclear hormone-signaling pathways, including the estrogen, thyroid, and RA pathways. Activation of the AhR pathway through exposure to environmental compounds results in significant changes in RA synthesis, catabolism, transport, and excretion. Some effects on retinoid homeostasis mediated by the AhR pathway may result from the interactions of these two pathways at the level of activating or repressing the expression of specific genes. This chapter will review these two pathways, the evidence demonstrating a link between them, and the data indicating the molecular basis of the interactions between these two pathways.

The competence factor beta Ftz-F1 potentiates ecdysone receptor activity via recruiting a p160/SRC coactivator

Hormones provide generalized signals that are interpreted in a specific spatial and temporal manner by a developing or reproducing multicellular organism. The ability to respond to hormones is determined by the competence of a cell or a tissue. The betaFtz-F1 orphan nuclear receptor acts as a competence factor for the steroid hormone 20-hydroxyecdysone (20E) in Drosophila melanogaster metamorphosis and mosquito reproduction. The molecular nature of the betaFtz-F1 action remains unclear. We report that the protein-protein interaction between betaFtz-F1 and a p160/SRC coactivator of the ecdysone receptor, FISC, is crucial for the stage-specific expression of the 20E effector genes during mosquito reproduction. This interaction dramatically increases recruitment of FISC to the functional ecdysone receptor in a 20E-dependent manner. The presence of betaFtz-F1 facilitates loading of FISC and the ecdysone receptor on the target promoters, leading to enhanced local histone H4 acetylation and robust activation of the target genes. Thus, our results reveal the molecular basis of competence for the stage-specific 20E response.

WW domain binding protein-2, an E6-associated protein interacting protein, acts as a coactivator of estrogen and progesterone receptors

WW domain binding protein-2 (WBP-2) was cloned as an E6-associated protein interacting protein, and its role in steroid hormone receptors functions was investigated. We show that WBP-2 specifically enhanced the transactivation functions of progesterone receptor (PR) and estrogen receptor (ER), whereas it did not have any significant effect on the androgen receptor, glucocorticoid receptor, or the activation functions of p53 and VP-16. Depletion of endogenous WBP-2 with small interfering RNAs indicated that WBP-2 was required for the proper functioning of PR and ER. We also demonstrated that WBP-2 contains an intrinsic activation domain. Moreover, chromatin immunoprecipitation assays demonstrate the hormone-dependent recruitment of WBP-2 onto an estrogen-responsive promoter. Mutational analysis suggests that one of three polyproline (PY) motifs of WBP-2 is essential for its coactivation and intrinsic activation functions. We show that WBP-2 and E6-associated protein each enhance PR function, and their effect on PR action are additive when coexpressed, suggesting a common signaling pathway. In this study, we also demonstrate that the WBP-2 binding protein, Yes kinase-associated protein (YAP) enhances PR transactivation, but YAP's coactivation function is absolutely dependent on WBP-2. Taken together, our data establish the role of WBP-2 and YAP as coactivators for ER and PR transactivation pathways.

P38MAPK-dependent phosphorylation and degradation of SRC-3/AIB1 and RARalpha-mediated transcription

Nuclear retinoic acid (RA) receptors (RARs) activate gene expression through dynamic interactions with coregulators in coordination with the ligand and phosphorylation processes. Here we show that during RA-dependent activation of the RARalpha isotype, the p160 coactivator pCIP/ACTR/AIB-1/RAC-3/TRAM-1/SRC-3 is phosphorylated by p38MAPK. SRC-3 phosphorylation has been correlated to an initial facilitation of RARalpha-target genes activation, via the control of the dynamics of the interactions of the coactivator with RARalpha. Then, phosphorylation inhibits transcription via promoting the degradation of SRC-3. In line with this, inhibition of p38MAPK markedly enhances RARalpha-mediated transcription and RA-dependent induction of cell differentiation. SRC-3 phosphorylation and degradation occur only within the context of RARalpha complexes, suggesting that the RAR isotype defines a phosphorylation code through dictating the accessibility of the coactivator to p38MAPK. We propose a model in which RARalpha transcriptional activity is regulated by SRC-3 through coordinated events that are fine-tuned by RA and p38MAPK.

Nuclear hormone receptor degradation and gene transcription: An update

The ubiquitin-proteasome pathway (UPP) is known to degrade short-lived and misfolded proteins. Its role in cell cycle regulation and signal transduction is well established. However, the importance of the UPP in nuclear hormone receptor-regulated gene transcription is relatively new. Nuclear hormone receptors (NHRs) are degraded by the UPP both in the presence or absence of their cognate ligands. In recent years, it has become evident that NHR degradation and NHR-dependent transcription are interdependent processes. The link between these two processes has become stronger with the discovery of a number of ubiquitin-pathway enzymes and components of the proteasome acting as modulators of NHR function. Also, UPP enzymes and components of the proteasome are recruited to the promoters of NHR-responsive genes. Interestingly both coactivators and corepressors (coregulators) of NHRs are also targeted to the UPP for degradation. Furthermore, additional evidence also indicates that the UPP may be involved in the turnover of transcription complexes, thereby facilitating proper gene transcription. In this review we discuss and provide an update on the role of UPP in NHR-dependent gene regulation.

Kinase-independent transcriptional co-activation of peroxisome proliferator-activated receptor alpha by AMP-activated protein kinase

AMPK (AMP-activated protein kinase) responds to intracellular ATP depletion, while PPARalpha (peroxisome proliferator-activated receptor alpha) induces the expression of genes coding for enzymes and proteins involved in increasing cellular ATP yields. PPARalpha-mediated transcription is shown here to be co-activated by the alpha subunit of AMPK, as well as by kinase-deficient (Thr172Ala) and kinase-less (Asp157Ala, Asp139Ala) mutants of AMPKalpha. The Ser452Ala mutant of mPPARalpha mutated in its putative consensus AMPKalpha phosphorylation site is similarly co-activated by AMPKalpha. AMPKalpha or its kinase-less mutants bind to PPARalpha; binding is increased by MgATP, to a lesser extent by MgADP, but not at all by AMP or ZMP [AICAR (5-aminoimidazole-4-carboxamide ribonucleoside) monophosphate]. ATP-activated binding of AMPKalpha to PPARalpha is mediated primarily by the C-terminal regulatory domain of AMPKalpha. PPARalpha co-activation by AMPKalpha may, however, require its secondary interaction with the N-terminal catalytic domain of AMPKalpha, independently of its kinase activity. While AMPK catalytic activity is activated by AICAR, PPARalpha co-activation and PPARalpha-controlled transcription are robustly inhibited by AICAR, with concomitant translocation of nuclear AMPKalpha or its kinase-less mutants to the cytosol. In conclusion, AMPKalpha, independently of its kinase activity, co-activates PPARalpha both in primary rat hepatocytes and in PPARalpha-transfected cells. The kinase and transcriptional co-activation modes of AMPKalpha are both regulated by the cellular ATP/AMP ratio. Co-activation of PPARalpha by AMPKalpha may transcriptionally complement AMPK in maintaining cellular ATP status.

Some organotin compounds enhance histone acetyltransferase activity

Eukaryotic DNA is packaged into chromatin, whose basic subunit is the nucleosome, which consists of DNA and a core histone octamer. Histone acetylation is important for the regulation of gene expression and is catalyzed by histone acetyltransferase (HAT). We observed the effects of suspected endocrine-disrupting chemicals (EDCs) on HAT activity. We showed that some organotin compounds--tributyltin (TBT) and triphenyltin (TPT)--enhanced HAT activity of core histones in a dose-dependent way and other EDCs did not affect HAT activity. Organotin compounds have various influences on physical function including the hormone and immune systems, embryogenesis, and development. Dibutyltin and diphenyltin, metabolites of TBT and TPT, respectively, also promoted HAT activity, but monobutyltin, monophenyltin, and inorganic tin had no effect. Further, TBT and TPT enhanced HAT activity when nucleosomal histones were used as substrates. These data indicate that the organotin compounds have unique effects on HATs independent of their EDC activities and suggest that the varied toxicities of the organotin compounds may be caused by aberrant gene expression following altered histone acetylation.

Characterization of the interaction between retinoic acid receptor/retinoid X receptor (RAR/RXR) heterodimers and transcriptional coactivators through structural and fluorescence anisotropy studies

Retinoid receptors (RARs and RXRs) are ligand-activated transcription factors that regulate the transcription of target genes by recruiting coregulator complexes at cognate promoters. To understand the effects of heterodimerization and ligand binding on coactivator recruitment, we solved the crystal structure of the complex between the RARbeta/RXRalpha ligand-binding domain heterodimer, its 9-cis retinoic acid ligand, and an LXXLL-containing peptide (termed NR box 2) derived from the nuclear receptor interaction domain (NID) of the TRAP220 coactivator. In parallel, we measured the binding affinities of the isolated NR box 2 peptide or the full-length NID of the coactivator SRC-1 for retinoid receptors in the presence of various types of ligands. Our correlative analysis of three-dimensional structures and fluorescence data reveals that heterodimerization does not significantly alter the structure of individual subunits or their intrinsic capacity to interact with NR box 2. Similarly, we show that the ability of a protomer to recruit NR box 2 does not vary as a function of the ligand binding status of the partner receptor. In contrast, the strength of the overall association between the heterodimer and the full-length SRC-1 NID is dictated by the combinatorial action of RAR and RXR ligands, the simultaneous presence of the two receptor agonists being required for highest binding affinity. We identified an LXXLL peptide-driven mechanism by which the concerted reorientation of three phenylalanine side chains generates an "aromatic clamp" that locks the RXR activation helix H12 in the transcriptionally active conformation. Finally, we show how variations of helix H11-ligand interactions can alter the communication pathway linking helices H11, H12, and the connecting loop L11-12 to the coactivator-binding site. Together, our results reveal molecular and structural features that impact on the ligand-dependent interaction of the RAR/RXR heterodimer with nuclear receptor coactivators.

The ubiquitin-conjugating enzyme UBCH7 acts as a coactivator for steroid hormone receptors

We investigated the role of the ubiquitin-conjugating enzyme UBCH7 in nuclear receptor transactivation. Using transient transfection assays, we demonstrated that UBCH7 modulates the transcriptional activity of progesterone receptor (PR) and glucocorticoid, androgen, and retinoic acid receptors in a hormone-dependent manner and that the ubiquitin conjugation activity of UBCH7 is required for its ability to potentiate transactivation by steroid hormone receptors (SHR). However, UBCH7 showed no significant effect on the transactivation functions of p53 and VP-16 activation domain. Depletion of endogenous UBCH7 protein by small interfering RNAs suggests that UBCH7 is required for the proper function of SHR. Furthermore, a chromatin immunoprecipitation assay demonstrated the hormone-dependent recruitment of UBCH7 onto estrogen receptor- and PR-responsive promoters. Additionally, we show that UBCH7 and E6-associated protein (E6-AP) synergistically enhance PR transactivation. We also demonstrate that UBCH7 interacts with steroid receptor coactivator 1 (SRC-1) and that UBCH7 coactivation function is dependent on SRC-1. Taken together, our results reveal the possible role of UBCH7 in steroid receptor transactivation and provide insights into the mechanism of action of UBCH7 in receptor function.

Acetylation of nuclear receptors in cellular growth and apoptosis

Post-translational modification of chromatin histones governs a key mechanism of transcriptional regulation. Histone acetylation, together with methylation, phosphorylation, ubiquitylation, sumoylation, glycosylation, and ADP ribosylation, modulate the activity of many genes by modifying both core histones and non-histone transcription factors. Epigenetic protein modification plays an important role in multiple cellular processes including DNA repair, protein stability, nuclear translocation, protein-protein interactions, and in regulation of cellular proliferation, differentiation and apoptosis. Histone acetyltransferases modify histones, coactivators, nuclear transport proteins, structural proteins, cell cycle components and transcription factors including p53 and nuclear receptors. The estrogen, PPARgamma and androgen receptor are members of the nuclear receptor (NR) superfamily. The androgen receptor (AR) and estrogen receptor alpha (ERalpha) are directly acetylated by histone acetyltransferases at a motif that is conserved between species and other NR. Point mutations at the lysine residue within the acetylation motif of the AR and ERalpha have been identified in prostate cancer as well as in breast cancer tissue. Acetylation of the NR governs ligand sensitivity and hormone antagonist responses. The AR is acetylated by p300, P/CAF and TIP60 and acetylation of the AR regulates co-regulator recruitment and growth properties of the receptors in cultured cells and in vivo. AR acetylation mimic mutants convey reduced apoptosis and enhanced growth properties correlating with altered promoter specificity for cell-cycle target genes. Cell-cycle control proteins, including cyclins, in turn alter the access of transcription factors and nuclear receptors to the promoters of target genes.

Mitogenic activity of estrogens in human breast cancer cells does not rely on direct induction of mitogen-activated protein kinase/extracellularly regulated kinase or phosphatidylinositol 3-kinase

We have addressed the question of rapid, nongenomic mechanisms that may be involved in the mitogenic action of estrogens in hormone-dependent breast cancer cells. In quiescent, estrogen-deprived MCF-7 cells, estradiol did not induce a rapid activation of either the MAPK/ERK or phosphatidylinositol-3 kinase (PI-3K)/Akt pathway, whereas the entry into the cell cycle was documented by the successive inductions of cyclin D1 expression, hyperphosphorylation of the retinoblastoma protein (Rb), activity of the promoter of the cyclin A gene, and DNA synthesis. However, pharmacological inhibitors of the src family kinases, 4-amino-5-(4-methylphenyl)-7-(t-butyl) pyrazolo[3,4-d] pyrimidine (PP1) or of the PI-3K (LY294002) did prevent the entry of the cells into the cell cycle and inhibited the late G1 phase progression, whereas the inhibitor of MAPK/ERK activation (U0126) had only a partial inhibitory effect in the early G1 phase. In agreement with these results, small interfering RNA targeting Akt strongly inhibited the estradiolinduced cell cycle progression monitored by the activation of the promoter of the cyclin A gene. The expression of small interfering RNA targeting MAPK 1 and 2 also had a clear inhibitory effect on the estradiol-induced activation of the cyclin A promoter and also antagonized the estradiol-induced transcription directed by the estrogen response element. Finally, transfection of the estrogen receptor into NIH3T3 fibroblasts did not confer to the cells sensitivity to a mitogenic action of estradiol. We conclude that the induction of the cell cycle by estradiol does not require a direct activation of MAPK/ERK or PI-3K signaling protein kinase cascades, but that these kinases appear to have a permissive role in the cell cycle progression.

Nuclear retinoid receptors and the transcription of retinoid-target genes

The pleiotropic effects of retinoids are mediated by nuclear retinoid receptors (RARs and RXRs) which are ligand-activated transcription factors. In response to retinoid binding, RAR/RXR heterodimers undergo major conformational changes and orchestrate the transcription of specific gene networks, through binding to specific DNA response elements and recruiting cofactor complexes that act to modify local chromatin structure and/or engage the basal transcription machinery. Then the degradation of RARs and RXRs by the ubiquitin-proteasome controls the magnitude and the duration of the retinoid response. RARs and RXRs also integrate a variety of signaling pathways through phosphorylation events which cooperate with the ligand for the control of retinoid-target genes transcription. These different modes of regulation reveal unexpected levels of complexity in the dynamics of retinoid-dependent transcription.

Steroid receptor action

The ovarian hormones oestradiol and progesterone exert their actions on target cells predominantly through the binding and activation of the oestrogen receptor (ER) and progesterone receptor (PR), respectively. These receptors are members of the steroid/thyroid hormone superfamily of ligand-dependent transcription factors and bind to the control regions (promoters) of specific genes, where they recruit co-activators or co-repressors and the transcriptional machinery necessary to elicit gene expression. The ability of a nuclear receptor to modulate gene transcription is further dependent on its interaction with other transcription factors, which in turn can be regulated by either distinct or multiple cytoplasmic signalling pathways. This chapter summarises the extraordinary diversity of factors involved in determining the cellular response to a hormonal signal and emphasises the role of ER and PR in regulating ovarian and uterine functions.

Nuclear Receptor Recruitment of Histone-Modifying Enzymes to Target Gene Promoters

Nuclear receptors (NRs) compose one of the largest known families of eukaryotic transcription factors and, as such, serve as a paradigm for understanding the fundamental molecular mechanisms of eukaryotic transcriptional regulation. The packaging of eukaryotic genomic DNA into a higher ordered chromatin structure, which generally acts as a barrier to transcription by inhibiting transcription factor accessibility, has a major influence on the mechanisms by which NRs activate or repress gene expression. A major breakthrough in the field's understanding of these mechanisms comes from the recent identification of NR-associated coregulatory factors (i.e., coactivators and corepressors). Although several of these NR cofactors are involved in chromatin remodeling and facilitating the recruitment of the basal transcription machinery, the focus of this chapter is on NR coactivators and corepressors that act to covalently modify the amino-terminal tails of core histones. These modifications (acetylation, methylation, and phosphorylation) are thought to directly affect chromatin structure and?or serve as binding surfaces for other coregulatory proteins. This chapter presents the most current models for NR recruitment of histone-modifying enzymes and then summarizes their functional importance in NR-associated gene expression.

Domains of estrogen receptor alpha (ERalpha) required for ERalpha/Sp1-mediated activation of GC-rich promoters by estrogens and antiestrogens in breast cancer cells

Estrogen receptor alpha (ERalpha)/Sp1 activation of GC-rich gene promoters in breast cancer cells is dependent, in part, on activation function 1 (AF1) of ERalpha, and this study investigates contributions of the DNA binding domain (C) and AF2 (DEF) regions of ERalpha on activation of ERalpha/Sp1. 17Beta-estradiol (E2) and the antiestrogens 4-hydroxytamoxifen and ICI 182,780 induced reporter gene activity in MCF-7 and MDA-MB-231 cells cotransfected with human or mouse ERalpha (hERalpha or MOR), but not ERbeta and GC-rich constructs containing three tandem Sp1 binding sites (pSp13) or other E2-responsive GC-rich promoters. Estrogen and antiestrogen activation of hERalpha/Sp1 was dependent on overlapping and different regions of the C, D, E, and F domains of ERalpha. Antiestrogen-induced activation of hERalpha/Sp1 was lost using hERalpha mutants deleted in zinc finger 1 [amino acids (aa) 185-205], zinc finger 2 (aa 218-245), and the hinge/helix 1 (aa 265-330) domains. In contrast with antiestrogens, E2-dependent activation of hERalpha/Sp1 required the C-terminal F domain (aa 579-595), which contains a beta-strand structural motif. Moreover, in peptide competition experiments overexpression of a C-terminal (aa 575-595) F domain peptide specifically blocked E2-dependent activation of hERalpha/Sp1, suggesting that F domain interactions with nuclear cofactors are required for ERalpha/Sp1 action.

Nuclear receptors: integration of multiple signalling pathways through phosphorylation

Nuclear receptors (NRs) orchestrate the transcription of specific gene networks in response to binding of their cognate ligand. They also act as mediators in a variety of signalling pathways through integrating diverse phosphorylation events. NR phosphorylation concerns all three major domains, the N-terminal activation function (AF-1), the ligand-binding and the DNA binding domains. Often, phosphorylation of NRs by kinases that are associated with general transcription factors (e.g. cdk7 within TFIIH), or activated in response to a variety of signals (MAPKs, Akt, PKA, PKC), facilitates the recruitment of coactivators or of components of the transcription machinery and, therefore, cooperates with the ligand to enhance transcription activation. But phosphorylation can also contribute to the termination of the ligand response through inducing DNA dissociation or NR degradation or through decreasing ligand affinity. These different modes of regulation reveal an unexpected complexity of the dynamics of NR-mediated transcription. In addition, deregulation of NR phosphorylation may impact their action in certain diseases or cancers.

Border-cell migration: the race is on

The conversion of stationary epithelial cells into migratory, invasive cells is important for normal embryonic development and tumour metastasis. Border-cell migration in the ovary of Drosophila melanogaster has emerged as a simple, genetically tractable model for studying this process. Three distinct signals, which are also upregulated in cancer, control border-cell migration, so identifying further genes that are involved in border-cell migration could provide new insights into tumour invasion.

Nuclear receptor superfamily: Principles of signaling

Nuclear receptors (NRs) comprise a family of 49 members that share a common structural organization and act as ligand-inducible transcription factors with major (patho)physiological impact. For some NRs (“orphan receptors”), cognate ligands have not yet been identified or may not exist. The principles of DNA recognition and ligand binding are well understood from both biochemical and crystal structure analyses. The 3D structures of several DNA-binding domains (DBDs),in complexes with a variety of cognate response elements, and multiple ligand-binding domains (LBDs), in the absence (apoLBD)and presence (holoLBD) of agonist, have been established and reveal canonical structural organization. Agonist binding induces a structural transition in the LBD whose most striking feature is the relocation of helix H12, which is required for establishing a coactivator complex, through interaction with members of the p160 family (SRC1, TIF2, AIB1) and/or the TRAP/DRIP complex. The p160-dependent coactivator complex is a multiprotein complex that comprises histone acetyltransferases (HATs), such as CBP,methyltransferases, such as CARM1, and other enzymes (SUMO ligase,etc.). The agonist-dependent recruitment of the HAT complex results in chromatin modification in the environment of the target gene promoters, which is requisite to, or may in some cases be sufficient for, transcription activation. In the absence of ligands, or in the presence of some antagonists, certain NRs are bound to distinct multiprotein complexes through the interaction with corepressors, such as NCoR and SMRT. Corepressor complexes comprise histone deacetylases (HDACs) that have the capacity to condense chromatin over target gene promoters. Ligands have been designed that selectively modulate the interaction between NRs and their coregulators. Both HATs and HDACs can also modify the acetylation status of nonhistone proteins, but the significance in the context of NR signaling is unclear. NRs communicate with other intracellular signaling pathways on a mutual basis, and their functionality may be altered, positively or negatively, by post-translational modification. The majority of NRs act as retinoid X receptor (RXR) heterodimers in which RXR cannot a priori respond autonomously to its cognate ligand to activate target gene transcription. This RXR subordination allows signaling pathway identity for the RXR partner. The corresponding mechanism is understood and reveals cell and NR selectivity, indicating that RXR can, under certain conditions, act autonomously. NRs are regulators of cell life and death,and NR malfunction can be at the basis of both disease and therapy, as is impressively documented in the case of acute promyelocytic leukemia. Recently, several pathways have been uncovered that link NR action with cell proliferation and apoptosis.

Phosphorylation by p38MAPK and recruitment of SUG-1 are required for RA-induced RAR gamma degradation and transactivation

The nuclear retinoic acid receptor RAR gamma 2 undergoes proteasome-dependent degradation upon ligand binding. Here we provide evidence that the domains that signal proteasome-mediated degradation overlap with those that activate transcription, i.e. the activation domains AF-1 and AF-2. The AF-1 domain signals RAR gamma 2 degradation through its phosphorylation by p38MAPK in response to RA. The AF-2 domain acts via the recruitment of SUG-1, which belongs to the 19S regulatory subunit of the 26S proteasome. Blocking RAR gamma 2 degradation through inhibition of either the p38MAPK pathway or the 26S proteasome function impairs its RA-induced transactivation activity. Thus, the turnover of RAR gamma 2 is linked to transactivation.

Decryption of the retinoid death code in leukemia

The recent elucidation of several molecular paradigms by which retinoids regulate growth, differentiation, and apoptosis highlights their promise as drugs for cancer therapy and prevention. Several novel signaling pathways by which retinoids induce cell death have been identified recently. They comprise (a) the induction by RARalpha-selective retinoids of the tumor-selective death ligand TRAIL that kills acute promyelocytic leukemia (APL) cells in a paracrine mode of action, which is the cause of retinoic acid-induced apoptosis after maturation: (b) a novel RARalpha-independent rexinoid-PKA cross-talk that induces maturation of both ATRA-sensitive and ATRA-resistant APL cells and does not invoke ligand-induced alteration of PML-RARalpha signaling, stability, or compartmentalization; and (c) a novel rexinoid signaling pathway that triggers apoptosis of immature APL cells and may correspond to a default death pathway that is operative in the absence of "survival" factors. This rexinoid apoptosis is inhibited by RXR but not RAR antagonists and is distinct from that triggered by RAR agonists, which control cell maturation and postmaturation apoptosis. Here we discuss the promise of retinoids for cancer treatment and prevention with an emphasis on the recently identified mechanisms by which they control (cancer) cell proliferation.

Androgen-selective gene regulation in the prostate

Androgens are essential for normal prostate growth and function but are also intimately associated with prostate cancer, an important cause of mortality in the ageing male population. The effects of androgens are mediated via a specific androgen receptor (AR) belonging to the nuclear receptor family and acting as a ligand-dependent transcription factor. The AR is built in a modular fashion and composed of a long N-terminal region with transactivation functions, a central DNA-binding domain, an intermediate hinge region and a C-terminal ligand-binding domain with additional transactivation functions. In its inactive form, the AR is complexed to heat-shock proteins, and mainly cytoplasmic. Following activation, the AR enters the nucleus, binds to its cognate DNA response elements as a homodimer and stimulates gene transcription. Various cofactors directly interact with the AR to modulate gene transcription. In addition, cross-talk between the AR and other signalling pathways has been proven for several prostate-expressed genes. Understanding the intricate networks underlying androgen-selective gene regulation represents a formidable challenge but might also offer the chance to identify new drug targets for the treatment of prostate carcinoma.

Effects of currently used pesticides in assays for estrogenicity, androgenicity, and aromatase activity in vitro

Twenty-four pesticides were tested for interactions with the estrogen receptor (ER) and the androgen receptor (AR) in transactivation assays. Estrogen-like effects on MCF-7 cell proliferation and effects on CYP19 aromatase activity in human placental microsomes were also investigated. Pesticides (endosulfan, methiocarb, methomyl, pirimicarb, propamocarb, deltamethrin, fenpropathrin, dimethoate, chlorpyriphos, dichlorvos, tolchlofos-methyl, vinclozolin, iprodion, fenarimol, prochloraz, fosetyl-aluminum, chlorothalonil, daminozid, paclobutrazol, chlormequat chlorid, and ethephon) were selected according to their frequent use in Danish greenhouses. In addition, the metabolite mercaptodimethur sulfoxide, the herbicide tribenuron-methyl, and the organochlorine dieldrin, were included. Several of the pesticides, dieldrin, endosulfan, methiocarb, and fenarimol, acted both as estrogen agonists and androgen antagonists. Prochloraz reacted as both an estrogen and an androgen antagonist. Furthermore, fenarimol and prochloraz were potent aromatase inhibitors while endosulfan was a weak inhibitor. Hence, these three pesticides possess at least three different ways to potentially disturb sex hormone actions. In addition, chlorpyrifos, deltamethrin, tolclofos-methyl, and tribenuron-methyl induced weak responses in one or both estrogenicity assays. Upon cotreatment with 17beta-estradiol, the response was potentiated by endosulfan in the proliferation assay and by pirimicarb, propamocarb, and daminozid in the ER transactivation assay. Vinclozolin reacted as a potent AR antagonist and dichlorvos as a very weak one. Methomyl, pirimicarb, propamocarb, and iprodion weakly stimulated aromatase activity. Although the potencies of the pesticides to react as hormone agonists or antagonists are low compared to the natural ligands, the integrated response in the organism might be amplified by the ability of the pesticides to act via several mechanism and the frequent simultaneous exposure to several pesticides.

The SMRT corepressor is a target of phosphorylation by protein kinase CK2 (casein kinase II)

The Silencing-Mediator for Retinoid/Thyroid hormone receptors (SMRT) interacts with, and mediates transcriptional repression by, a variety of eukaryotic transcription factors, including the nuclear hormone receptors. The ability of SMRT to function as a transcriptional 'corepressor' is regulated by a variety of signal transduction pathways. We report here that SMRT is a phosphoprotein in vivo, and is also phosphorylated in vitro by unfractionated cell extracts. A major site of phosphorylation of SMRT is a protein kinase CK2 motif centered on serine 1492, and located within a C-terminal SMRT domain that mediates interaction of the corepressor with the nuclear hormone receptors. Phosphorylation of SMRT by CK2 stabilizes the ability of the SMRT protein to interact with nuclear hormone receptors. Our results indicate that SMRT is a member of an expanding family of transcriptional regulators that are modified, and potentially regulated, in response to protein kinase CK2.

Peroxisome proliferator-activated receptors in vascular biology and atherosclerosis: emerging insights for evolving paradigms

Peroxisome Proliferator-Activated Receptors (PPARs), members of the steroid hormone nuclear receptor superfamily, act as ligand-activated transcription factors controlling the expression of specific target genes. Known PPAR isoforms include PPAR gamma, important in adipogenesis and lipid metabolism, PPAR alpha, implicated in fatty acid metabolism, and PPAR delta, about which the least is known. Recent work implicates PPAR alpha and gamma in vascular biology and atherosclerosis, and will be reviewed here. Such effects may have clinical implications given PPAR agonists in use as pharmacologic agents (eg, thiazolidinediones as insulin sensitizers [gamma] and fibric acids as lipid lowering agents [alpha]).