Estrogen receptor domains E and F: role in dimerization and interaction with coactivator RIP-140

Molecular basis and dual ligand regulation of tetrameric Estrogen Receptor α/14-3-3ζ protein complex

Therapeutic strategies targeting Nuclear Receptors (NRs) beyond their endogenous ligand binding pocket have gained significant scientific interest, driven by a need to circumvent problems associated with drug resistance and pharmacological profile. The hub protein 14-3-3 is an endogenous regulator of various NRs, providing a novel entry point for small molecule modulation of NR activity. Exemplified, 14-3-3 binding to the C-terminal F-domain of the Estrogen Receptor alpha (ERα), and small molecule stabilization of the ERα/14-3-3ζ protein complex by the natural product Fusicoccin A (FC-A), was demonstrated to downregulate ERα-mediated breast cancer proliferation. This presents a novel drug discovery approach to target ERα, however, structural and mechanistic insights into ERα/14-3-3 complex formation are lacking. Here, we provide an in-depth molecular understanding of the ERα/14-3-3ζ complex by isolating 14-3-3ζ in complex with an ERα protein construct comprising its Ligand Binding Domain (LBD) and phosphorylated F-domain. Bacterial co-expression and co-purification of the ERα/14-3-3ζ complex, followed by extensive biophysical and structural characterization, revealed a tetrameric complex between the ERα homodimer and the 14-3-3ζ homodimer. 14-3-3ζ binding to ERα, and ERα/14-3-3ζ complex stabilization by FC-A, appeared to be orthogonal to ERα endogenous agonist (E2) binding, E2-induced conformational changes, and cofactor recruitment. Similarly, the ERα antagonist 4-hydroxytamoxifen inhibited cofactor recruitment to the ERα LBD while ERα was bound to 14-3-3ζ. Furthermore, stabilization of the ERα/14-3-3ζ protein complex by FC-A was not influenced by the disease-associated and 4-hydroxytamoxifen resistant ERα-Y537S mutant. Together, these molecular and mechanistic insights provide direction for targeting ERα via the ERα/14-3-3 complex as an alternative drug discovery approach.

Segregation of nuclear and membrane-initiated actions of estrogen receptor using genetically modified animals and pharmacological tools

Estrogen receptor alpha (ERα) and beta (ERβ) are members of the nuclear receptor superfamily, playing widespread functions in reproductive and non-reproductive tissues. Beside the canonical function of ERs as nuclear receptors, in this review, we summarize our current understanding of extra-nuclear, membrane-initiated functions of ERs with a specific focus on ERα. Over the last decade, in vivo evidence has accumulated to demonstrate the physiological relevance of this ERα membrane-initiated-signaling from mouse models to selective pharmacological tools. Finally, we discuss the perspectives and future challenges opened by the integration of extra-nuclear ERα signaling in physiology and pathology of estrogens.

Estrogen signaling: An emanating therapeutic target for breast cancer treatment

Breast cancer, a most common malignancy in women, was known to be associated with steroid hormone estrogen. The discovery of estrogen receptor (ER) gave us not only a powerful predictive and prognostic marker, but also an efficient target for the treatment of hormone-dependent breast cancer with various estrogen ligands. ER consists of two subtypes i.e. ERα and ERβ, that are mostly G-protein-coupled receptors and activated by estrogen, specially 17β-estradiol. The activation is followed by translocation into the nucleus and binding with DNA to modulate activities of different genes. ERs can manage synthesis of RNA through genomic actions without directly binding to DNA. Receptors are tethered by protein-protein interactions to a transcription factor complex to communicate with DNA. Estrogens also exhibit nongenomic actions, a characteristic feature of steroid hormones, which are so rapid to be considered by the activation of RNA and translation. These are habitually related to stimulation of different protein kinase cascades. Majority of post-menopausal breast cancer is estrogen dependent, mostly potent biological estrogen (E2) for continuous growth and proliferation. Estrogen helps in regulating the differentiation and proliferation of normal breast epithelial cells. In this review we have investigated the important role of ER in development and progression of breast cancer, which is complicated by receptor's interaction with co-regulatory proteins, cross-talk with other signal transduction pathways and development of treatment strategies viz. selective estrogen receptor modulators (SERMs), selective estrogen receptor down regulators (SERDs), aromatase and sulphatase inhibitors.

Structural and functional characteristics of oestrogen receptor β splice variants: Implications for the ageing brain

Oestrogen receptor (ER)β is a multifunctional nuclear receptor that mediates the actions of oestrogenic compounds. Despite its well defined role in mediating the actions of oestrogens, a substantial body of evidence demonstrates that ERβ has a broad range of physiological functions independent of those normally attributed to oestrogen signalling. These functions can partly be achieved by the activity of several alternatively spliced isoforms that have been identified for ERβ. This short review describes structural differences between the ERβ splice variants that are known to be translated into proteins. Moreover, we discuss how these alternative structures contribute to functional differences in the context of both healthy and pathological conditions. Our review also describes the principal factors that regulate alternative RNA splicing. The alternatively spliced isoforms of ERβ are differentially expressed according to brain region, age and hormonal milieu, emphasising the likelihood that there are precise cell-specific mechanisms regulating ERβ alternative splicing. However, despite these correlative data, the molecular factors regulating alternative ERβ splicing in the brain remain unknown. We also review the basic mechanisms that regulate alternative RNA splicing and use that framework to make logical predictions about ERβ alternative splicing in the brain.

Flexible small molecular anti-estrogens with N,N-dialkylated-2,5-diethoxy-4-morpholinoaniline scaffold targets multiple estrogen receptor conformations

Estrogen mediates various cellular processes including cell proliferation, differentiation, growth and mammary gland function. Estrogen Receptors (ERs) are expressed in 70% of breast cancers. Consequently, estrogen mediated ER signaling plays a critical role in breast cancer diagnosis, prognosis, and treatment. ERs are ligand-triggered transcription factors. However, in the absence of a cognate estrogenic ligand, ERs can be activated by a variety of other extracellular signals. Tamoxifen, an anti-estrogen that selectively targets ER, induces substantial regression of breast tumors and an increase in disease-free survival. Tamoxifen mimics estrogen effects in other tissues thereby providing some beneficial effects including reduced risk of osteoporosis. However, breast cancers that initially respond well to tamoxifen tend to develop resistance and resume growth despite the continued presence of the antagonist. Library of compounds with substituted morpholinoaniline scaffold, a set of structurally divergent potential ER antagonists that fit the tamoxifen pharmacophore, were designed to target ER Ligand Binding Domain (LBD) and to recruit co-regulator proteins including BRCA1 over a range of conformational changes. Two of the lead compounds in the library, BR46 and BR47, were found to inhibit estrogen induced cell proliferation and cell viability. Discovery of novel lead molecules targeting ligand binding pockets of hER has provided structural clues toward the development of new breed of small molecule therapeutics for tamoxifen-resistant breast cancers and would complement already existent anti-estrogen therapy.

Membrane and Nuclear Estrogen Receptor Alpha Actions: From Tissue Specificity to Medical Implications

Estrogen receptor alpha (ERα) has been recognized now for several decades as playing a key role in reproduction and exerting functions in numerous nonreproductive tissues. In this review, we attempt to summarize the in vitro studies that are the basis of our current understanding of the mechanisms of action of ERα as a nuclear receptor and the key roles played by its two activation functions (AFs) in its transcriptional activities. We then depict the consequences of the selective inactivation of these AFs in mouse models, focusing on the prominent roles played by ERα in the reproductive tract and in the vascular system. Evidence has accumulated over the two last decades that ERα is also associated with the plasma membrane and activates non-nuclear signaling from this site. These rapid/nongenomic/membrane-initiated steroid signals (MISS) have been characterized in a variety of cell lines, and in particular in endothelial cells. The development of selective pharmacological tools that specifically activate MISS and the generation of mice expressing an ERα protein impeded for membrane localization have begun to unravel the physiological role of MISS in vivo. Finally, we discuss novel perspectives for the design of tissue-selective ER modulators based on the integration of the physiological and pathophysiological roles of MISS actions of estrogens.

Modulation of nuclear receptor activity by the F domain

The F domain located at the C-terminus of proteins is one of the least conserved regions of the estrogen receptors alpha and beta, members of the nuclear hormone receptor superfamily. Indeed, many members of the superfamily lack the F domain. However, when present, removing the F domain entirely or mutating it alters transactivation, dimerization, and the responses to agonist and antagonist ligands. This review focuses on the functions of the F domain of the estrogen receptors, particularly in relation to other members of the superfamily.

Interaction of 14-3-3 proteins with the estrogen receptor alpha F domain provides a drug target interface

Estrogen receptor alpha (ERα) is involved in numerous physiological and pathological processes, including breast cancer. Breast cancer therapy is therefore currently directed at inhibiting the transcriptional potency of ERα, either by blocking estrogen production through aromatase inhibitors or antiestrogens that compete for hormone binding. Due to resistance, new treatment modalities are needed and as ERα dimerization is essential for its activity, interference with receptor dimerization offers a new opportunity to exploit in drug design. Here we describe a unique mechanism of how ERα dimerization is negatively controlled by interaction with 14-3-3 proteins at the extreme C terminus of the receptor. Moreover, the small-molecule fusicoccin (FC) stabilizes this ERα/14-3-3 interaction. Cocrystallization of the trimeric ERα/14-3-3/FC complex provides the structural basis for this stabilization and shows the importance of phosphorylation of the penultimate Threonine (ERα-T(594)) for high-affinity interaction. We confirm that T(594) is a distinct ERα phosphorylation site in the breast cancer cell line MCF-7 using a phospho-T(594)-specific antibody and by mass spectrometry. In line with its ERα/14-3-3 interaction stabilizing effect, fusicoccin reduces the estradiol-stimulated ERα dimerization, inhibits ERα/chromatin interactions and downstream gene expression, resulting in decreased cell proliferation. Herewith, a unique functional phosphosite and an alternative regulation mechanism of ERα are provided, together with a small molecule that selectively targets this ERα/14-3-3 interface.

Nutritional flavonoids impact on nuclear and extranuclear estrogen receptor activities

Flavonoids are a large group of nonnutrient compounds naturally produced from plants as part of their defence mechanisms against stresses of different origins. They emerged from being considered an agricultural oddity only after it was observed that these compounds possess a potential protective function against several human degenerative diseases. This has led to recommending the consumption of food containing high concentrations of flavonoids, which at present, especially as soy isoflavones, are even available as overthecounter nutraceuticals. The increased use of flavonoids has occurred even though their mechanisms are not completely understood, in particular those involving the flavonoid impact on estrogen signals. In fact, most of the human health protective effects of flavonoids are described either as estrogenmimetic, or as antiestrogenic, while others do not involve estrogen signaling at all. Thus, the same molecule is reported as an endocrine disruptor, an estrogen mimetic or as an antioxidant without estrogenic effects. This is due in part to the complexity of the estrogen mechanism, which is conducted by different pathways and involves two different receptor isoforms. These pathways can be modulated by flavonoids and should be considered for a reliable evaluation of flavonoid, both estrogenicity and antiestrogenicity, and for a correct prediction of their effects on human health.

Characterisation of human oestrogen receptor beta (ERβ) splice variants in neuronal cells

Oestrogen receptor (ER)α and ERβ are members of the ligand-activated superfamily of nuclear receptors and mediate most facets of oestrogen signalling. Several naturally occurring splice variants of each ER have been identified in the human brain, yet the biological significance of these splice variants in the brain remains unknown. In the present study, we exploit the unique structural differences of the human ERβ splice variants to determine the functional significance of individual ER domains in the brain. We previously established that full-length rodent ERβ (i.e. rERβ1) has constitutive transcriptional activity in neuronal cells in the absence of ligand. By contrast to the rodent splice variants, the human ERβ splice variants used in the present study contain varying length truncations of exon 8, which encodes for the E/F domains. Our results reveal that, in neuronal cells, each human-specific ERβ splice variant constitutively activated promoters mediated by a canonical oestrogen response element and repressed promoters mediated by activator protein-1 sites via p38 activity. From these data, we conclude that the C-terminus, encoding the AF-2 region and F domain, is not essential for the constitutive properties of human ERβ. Taken together, these studies show that human-specific ERβ variants are constitutively active and also provide novel insight into the contributions of the functional domains of ERβ towards mediating constitutive transcription at various promoters in neuronal cells.

Serum estrogen and tumor-positive estrogen receptor-alpha are strong prognostic classifiers of non-small-cell lung cancer survival in both men and women

The role of tumor estrogen receptors (ERs) and serum estrogen in lung cancer is inconclusive. We investigated the hypothesis that ERs and functional single-nucleotide polymorphisms in the estrogen biosynthesis pathway are associated with poorer lung cancer survival. Lung cancer patients (n = 305) from a National Cancer Institute-Maryland (NCI-MD) case-case cohort in the Baltimore metropolitan area were used as a test cohort. To validate, 227 cases from the NCI-MD case-control cohort and 293 cases from a Norwegian lung cancer cohort were studied. Information on demographics, tobacco and reproductive histories was collected in an interviewer-administered questionnaire. Serum estrogen, progesterone, tumor messenger RNA expression of hormone receptors and germ line DNA polymorphisms were analyzed for associations with lung cancer survival. Patients in the highest tertile of serum estrogen had worse survival in all three cohorts (P combined < 0.001). Furthermore, the variant allele of estrogen receptor alpha (ER-α) polymorphism (rs2228480) was significantly associated with increased tumor ER-α levels and worse survival in all three cohorts [hazard ratio (HR) = 2.59, 95% confidence interval (CI): 1.20- 4.01; HR = 1.76, 95% CI: 1.08-2.87 and HR = 2.85, 95% CI: 1.31-4.36). Other polymorphisms associated with lower serum estrogen correlated with improved survival. Results were independent of gender and hormone replacement therapy. We report a significant association of increased serum estrogen with poorer survival among lung cancer male and female patients. Understanding the genetic control of estrogen biosynthesis and response in lung cancer could lead to improved prognosis and therapy.

Expression and ecdysteroid responsiveness of the nuclear receptors HR3 and E75 in the crustacean Daphnia magna

Ecdysteroids initiate signaling along multiple pathways that regulate various aspects of development, maturation, and reproduction in arthropods. Signaling often involves the induction of downstream transcription factors that either positively or negatively regulate aspects of the pathway. We tested the hypothesis that crustaceans express the nuclear receptors HR3 (ortholog to vertebrate ROR) and E75 (ortholog to vertebrate rev-erb) in response to ecdysteroid signaling. HR3 and E75 cDNAs were cloned from the crustacean Daphnia magna. The DNA-binding domain and ligand-binding domain of the daphnid HR3 were 95% and 61% identical to those of Drosophila melanogaster. The DNA-binding domain and ligand-binding domain of the daphnid E75 were 100% and 71% identical to those of D. melanogaster. Both receptors exhibited structural characteristics of binding to DNA as a monomer. The expression of these receptor mRNAs was evaluated through the adult molt cycle and during embryo development. E75 levels were relatively constant throughout the adult molt cycle and through embryo development. HR3 levels were comparable to those of E75 during the initial phases of the adult molt cycle but were elevated approximately 30-fold at a time in the cycle co-incident with the pre-molt surge in ecdysteroid levels. HR3 mRNA levels in embryos also varied co-incident with ecdysteroid levels. To substantiate a role of ecdysteroids in the expression of HR3, daphnids were continuously exposed to 20-hydroxyecdysone and changes in gene expression were measured. HR3 levels were significantly induced by 20-hydroxyecdysone; while E75 levels were minimally affected. These results are consistent with the premise that transcription of HR3 is regulated by ecdysteroids in the crustacean D. magna and that HR3 likely serves as a mediator of ecdysteroid regulatory action in crustaceans. The marginal induction of E75 by 20-hydroxyecdysone may represent limited, tissue or cell-type-specific induction of this transcription factor.

A hypersensitive estrogen receptor alpha mutation that alters dynamic protein interactions

Estrogen receptor alpha (ERalpha) is highly regulated through multiple mechanisms including cell signaling, posttranslational modifications, and protein-protein interactions. We have previously identified a K303R ERalpha mutation within the hinge region of ERalpha. This mutation results in an altered posttranslational regulation and increased in vitro growth in the presence of low estrogen concentrations. We sought to determine if cells expressing this mutant ERalpha would display hypersensitive tumor growth in in vivo athymic ovariectomized nude mice. MCF-7 cells, stably expressing the K303R ERalpha, formed tumors in nude mice faster than cells expressing wild-type ERalpha in the presence of low levels of estrogen. When estrogen was withdrawn, all tumors regressed but half of the K303R ERalpha-expressing tumors became estrogen-independent and regrew. We evaluated potential mechanisms for the observed hypersensitivity. The mutant ERalpha did not demonstrate increased estrogen binding affinity, but did exhibit increased interactions with members of the SRC family of coactivators. The mutant ERalpha demonstrated increased levels and occupancy time on the pS2 promoter. In the presence of the K303R ERalpha, the SRC-3 and p300 coactivators also displayed increased levels and time on the pS2 promoter. The K303R ERalpha has, in part, lost critical negative regulation by the F domain. Collectively, these data demonstrate an important role for the K303R ERalpha mutation in hormonal regulation of tumor growth and estrogen-regulated promoter dynamics in human breast cancer.

Ligand dissociation from estrogen receptor is mediated by receptor dimerization: evidence from molecular dynamics simulations

Estrogen Receptor (ER) is an important target for pharmaceutical design. Like other ligand-dependent transcription factors, hormone binding regulates ER transcriptional activity. Nevertheless, the mechanisms by which ligands enter and leave ERs and other nuclear receptors remain poorly understood. Here, we report results of locally enhanced sampling molecular dynamics simulations to identify dissociation pathways of two ER ligands [the natural hormone 17beta-estradiol (E(2)) and the selective ER modulator raloxifene (RAL)] from the human ERalpha ligand-binding domain in monomeric and dimeric forms. E(2) dissociation occurs via three different pathways in ER monomers. One resembles the mousetrap mechanism (Path I), involving repositioning of helix 12 (H12), others involve the separation of H8 and H11 (Path II), and a variant of this pathway at the bottom of the ligand-binding domain (Path II'). RAL leaves the receptor through Path I and a Path I variant in which the ligand leaves the receptor through the loop region between H11 and H12 (Path I'). Remarkably, ER dimerization strongly suppresses Paths II and II' for E(2) dissociation and modifies RAL escape routes. We propose that differences in ligand release pathways detected in the simulations for ER monomers and dimers provide an explanation for previously observed effects of ER quaternary state on ligand dissociation rates and suggest that dimerization may play an important, and hitherto unexpected, role in regulation of ligand dissociation rates throughout the nuclear receptor family.

The multifunctional estrogen receptor-alpha F domain

The members of the nuclear receptor superfamily act as transcriptional regulatory factors and exhibit a multidomain structure characterized as domains A-E/F. This review focuses on a small, relatively understudied region at the extreme carboxy-terminus of the estrogen receptor (ER) alpha, the F domain. The F domain contributes to differences in the activity of ER alpha and beta subtypes; it is required for tamoxifen's agonist activity on an estrogen response element, and it modifies the receptor's interactions with coregulators including steroid receptor coactivator-1. The differences between the F domains of the ER alpha and beta subtypes and among the other members of the nuclear hormone receptor superfamily may offer opportunities for selective control of the activity of these proteins.

SUMOylation modulates the transcription repressor function of RIP140

RIP140/NRIP1 (receptor-interacting protein 140) functions as a corepressor of nuclear receptors. It plays an important role in the transcriptional control of energy metabolism and female fertility. RIP140 contains four distinct repression domains (RD1-RD4), and the repressive activity of RIP140 involves complex mechanisms. The function of both RD1 and RD2 is linked to recruitment of histone deacetylases and C-terminal binding protein, respectively, but the mechanism of repression for RD3 and RD4 has remained elusive. Because covalent modification by small ubiquitin-like modifiers (SUMO-1, -2, and -3; SUMOylation) is often associated with transcriptional repression, we studied whether SUMOylation is involved in the repressive activity of RIP140. We show that two conserved lysines, Lys(756) and Lys(1154), located in RD3 and RD4, respectively, are subject to reversible SUMOylation, with SUMO-1 being more efficiently conjugated than SUMO-2. Interestingly, mutations of the RIP140 SUMOylation sites compromised the transcription repressor function of RIP140 and blunted its capacity to repress estrogen receptor alpha-dependent transcription. Conjugation of SUMO-1 also influenced the subnuclear distribution pattern of RIP140. In sum, our demonstration that the function of RIP140 repression domains 3 and 4 can be modulated by reversible SUMO modification thus adds a novel level to the regulation of RIP140 activity, which may have ramifications in the control of gene networks exerted by RIP140.

An F-domain introduced by alternative splicing regulates activity of the zebrafish thyroid hormone receptor alpha

Thyroid hormones (THs) play an important role in vertebrate development; however, the underlying mechanisms of their actions are still poorly understood. Zebrafish (Danio rerio) is an emerging vertebrate model system to study the roles of THs during development. In general, the response to THs relies on closely related proteins and mechanisms across vertebrate species, however some species-specific differences exist. In contrast to mammals, zebrafish has two TRalpha genes (thraa, thrab). Moreover, the zebrafish thraa gene expresses a TRalpha isoform (TRalphaA1) that differs from other TRs by containing additional C-terminal amino acids. C-terminal extensions, called "F domains", are common in other members of the nuclear receptor superfamily and modulate the response of these receptors to hormones. Here we demonstrate that the F-domain constrains the transcriptional activity of zebrafish TRalpha by altering the selectivity of this receptor for certain coactivator binding motifs. We found that the F-domain of zebrafish TRalphaA1 is encoded on a separate exon whose inclusion is regulated by alternative splicing, indicating a regulatory role of the F-domain in vivo. Quantitative expression analyses revealed that TRalphaA1 is primarily expressed in reproductive organs whereas TRalphaB and the TRalphaA isoform that lacks the F-domain (TRalphaA1-2) appear to be ubiquitous. The relative expression levels of these TRalpha transcripts differ in a tissue-specific manner suggesting that zebrafish uses both alternative splicing and differential expression of TRalpha genes to diversify the cellular response to THs.

Identification of regions within the F domain of the human estrogen receptor alpha that are important for modulating transactivation and protein-protein interactions

The estrogen receptor (ER)alpha is a biologically and clinically important ligand-modulated transcription factor. The F domain of the ERalpha modulates its functions in a ligand-, promoter-, and cell-specific manner. To identify the region(s) responsible for these functions, we characterized the effects of serial truncations within the F domain. We found that truncating the last 16 residues of the F domain altered the activity of the human ERalpha (hERalpha) on an estrogen response element-driven promoter in response to estradiol or 4-hydroxytamoxifen (4-OHT), its sensitivity to overexpression of the coactivator steroid receptor coactivator-1 in mammalian cells, and its interaction with a receptor-interacting domain of the coactivator steroid receptor coactivator-1 or engineered proteins ("monobodies") that specifically bind to ERalpha/ligand complexes in a yeast two-hybrid system. Most importantly, the ability of the ER to induce pS2 was reduced in MDA-MB-231 cells stably expressing this truncated ER vs. the wild-type ER. The region includes a distinctive segment (residues 579-584; LQKYYIT) having a high content of bulky and/or hydrophobic amino acids that was previously predicted to adopt a beta-strand-like structure. As previously reported, removal of the entire F domain was necessary to eliminate the agonist activity of 4-OHT. In addition, mutation of the vicinal glycine residues between the ligand-binding domain and F domains specifically reduced the 4-OHT-dependent interactions of the hERalpha ligand-binding domain and F domains with monobodies. These results show that regions within the F domain of the hERalpha selectively modulate its activity and its interactions with other proteins.

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.

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.

ERalpha and STAT5a cross-talk: interaction through C-terminal portions of the proteins decreases STAT5a phosphorylation, nuclear translocation and DNA-binding

Cross-talk between ERalpha and STAT5a was demonstrated to mediate through a direct physical association between the two proteins. By GST pull-down assays and functional assays with various constructs of ERalpha and STAT5a, it was shown that the C-termini of these two proteins were mainly responsible for this interaction. Furthermore, the interaction between ERalpha and STAT5a was demonstrated to give rise to functional changes in their signaling events. In cell transfection studies, it was shown that ERalpha activation could attenuate PRLR signaling through STAT5a. This ERalpha-mediated attenuation of PRLR signaling was substantiated by observed decreases in the phosphorylation of JAK2 and STAT5a, reduced translocation of STAT5a into the nucleus, and reduced binding of STAT5a onto a GAS-containing nucleotide. Apart from transfected cells, the interaction between ERalpha and STAT5a could also be observed in established breast cancer cell lines of MCF-7 and T-47D in co-immunoprecipitation studies. However, the functional consequence of the interaction in these cancer cells was very different from the transfected HEK293 cells. ER activation could lead to potentiation of PRLR signaling in MCF-7 cells but not in T-47D cells. Conversely, in both MCF-7 and T-47D cells, PRLR activation could lead to attenuation of ER signaling. These data serve to elucidate the mechanisms underlying the ERalpha-STAT5a cross-talk and in demonstrating that the functional consequence of this cross-talk depends on the precise milieus of the intracellular environment.

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.

Mechanism of a transcriptional cross talk between transforming growth factor-beta-regulated Smad3 and Smad4 proteins and orphan nuclear receptor hepatocyte nuclear factor-4

We have shown previously that the transforming growth factor-beta (TGFbeta)-regulated Sma-Mad (Smad) protein 3 and Smad4 proteins transactivate the apolipoprotein C-III promoter in hepatic cells via a hormone response element that binds the nuclear receptor hepatocyte nuclear factor 4 (HNF-4). In the present study, we show that Smad3 and Smad4 but not Smad2 physically interact with HNF-4 via their Mad homology 1 domains both in vitro and in vivo. The synergistic transactivation of target promoters by Smads and HNF-4 was shown to depend on the specific promoter context and did not require an intact beta-hairpin/DNA binding domain of the Smads. Using glutathione S-transferase interaction assays, we established that two regions of HNF-4, the N-terminal activation function 1 (AF-1) domain (aa 1-24) and the C-terminal F domain (aa 388-455) can mediate physical Smad3/HNF-4 interactions in vitro. In vivo, Smad3 and Smad4 proteins enhanced the transactivation function of various GAL4-HNF-4 fusion proteins via the AF-1 and the adjacent DNA binding domain, whereas a single tyrosine to alanine substitution in AF-1 abolished coactivation by Smads. The findings suggest that the transcriptional cross talk between the TGFbeta-regulated Smads and HNF-4 is mediated by specific functional domains in the two types of transcription factors. Furthermore, the specificity of this interaction for certain target promoters may play an important role in various hepatocyte functions, which are regulated by TGFbeta and the Smads.

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.

Mutations targeted to a predicted helix in the extreme carboxyl-terminal region of the human estrogen receptor-alpha alter its response to estradiol and 4-hydroxytamoxifen

The human estrogen receptor-alpha, a member of the nuclear receptor superfamily, is a ligand-regulated transcriptional modulator. Because comparatively little is known about the extreme carboxyl-terminal region of the estrogen receptor (F domain), we used secondary structure prediction to design mutations that delete the F domain (S554stop), disrupt a possible turn (G556L/G557L), and alter a predicted helix (S559A/E562A, Q565P), and we evaluated the effects of these mutations on hormone binding and transcription activation in response to estradiol and the mixed agonist/antagonist 4-hydroxytamoxifen. Mutations that deleted the F domain (S554stop) or targeted the predicted helix (S559A/E562A, Q565P) greatly reduced or eliminated the agonist activity of 4-hydroxytamoxifen. Deleting the F domain increased the affinity of the receptor for estradiol and decreased the antagonist activity of 4-hydroxytamoxifen. The Q565P mutant exhibited a non-cooperative hormone-binding mechanism, as well as an impaired response to estradiol and increased antagonist activity of 4-hydroxytamoxifen. Our results show that mutations in the F domain alter not only the response to estradiol, the affinity for hormone, and the interaction between receptor subunits but can uncouple the agonist and antagonist activities of 4-hydroxytamoxifen. These results suggest that the F domain modulates the activity of the estrogen receptor-alpha by multiple mechanisms.

Estrogen response elements alter coactivator recruitment through allosteric modulation of estrogen receptor beta conformation

Estrogen receptor beta (ERbeta) activates transcription by binding to estrogen response elements (EREs) and coactivator proteins that act as bridging proteins between the receptor and the basal transcription machinery. Although the imperfect vitellogenin B1, pS2, and oxytocin (OT) EREs each differ from the consensus vitellogenin A2 ERE sequence by a single base pair, ERbeta activates transcription of reporter plasmids containing A2, pS2, B1, and OT EREs to different extents. To explain how these differences in transactivation might occur, we have examined the interaction of ERbeta with these EREs and monitored recruitment of the coactivators amplified in breast cancer (AIB1) and transcription intermediary factor 2 (TIF2). Protease sensitivity, antibody interaction, and DNA pull-down assays demonstrated that ERbeta undergoes ERE-dependent changes in conformation resulting in differential recruitment of AIB1 and TIF2 to the DNA-bound receptor. Overexpression of TIF2 or AIB1 in transient transfection assays differentially enhanced ERbeta-mediated transcription of reporter plasmids containing the A2, pS2, B1, and OT EREs. Our studies demonstrate that individual ERE sequences induce changes in conformation of the DNA-bound receptor and influence coactivator recruitment. DNA-induced modulation of receptor conformation may contribute to the ability of ERbeta to differentially activate transcription of genes containing divergent ERE sequences.

Estrogen receptor and breast cancer

Breast cancer, the most common malignancy in women, was already known to be associated with the steroid hormone estrogen more than a century ago. The discovery of the estrogen receptor (ER) provided us not only with a powerful predictive and prognostic marker, but also an efficient target for the treatment of hormone-dependent breast cancer with antiestrogens. In this paper we will sketch the important role of ER in the development, progression, and treatment of the disease, which is complicated by the receptor's interaction with co-regulatory proteins, its cross-talk with other signal transduction pathways, and its involvement in the development of antiestrogen resistance.

Differential recruitment of the mammalian mediator subunit TRAP220 by estrogen receptors ERalpha and ERbeta

Estrogen receptors (ERs) associate with distinct transcriptional coactivators to mediate activation of target genes in response to estrogens. Previous work has provided multiple evidence for a critical role of p160 coactivators and associated histone acetyltransferases in estrogen signaling. In contrast, the involvement of the mammalian mediator complex remains to be established. Further, although the two subtypes ERalpha and ERbeta appear to be similar in regard to principles of LXXLL-mediated coactivator binding to the AF-2 activation domain, there are indications that the context-dependent transcriptional activation profiles of the two ERs can be quite distinct. Potentially, this could be attributed to differences with regard to coregulator recruitment. We have here studied the interactions of the nuclear receptor-binding subunit of the mammalian mediator complex, referred to as TRAP220, with ERalpha and ERbeta. In comparison to the p160 coactivator TIF2, we find that TRAP220 displays ERbeta preference. Here, we show that this is a feature of the binding specificity of the TRAP220 LXXLL motifs and demonstrate that the ER subtype-specific F-domain influences TRAP220 interaction. Such differences with regard to coactivator recruitment indicate that the relative importance of individual coregulators in estrogen signaling could depend on the dominant ER subtype.

AIB1 is a conduit for kinase-mediated growth factor signaling to the estrogen receptor

Growth factor modulation of estrogen receptor (ER) activity plays an important role in both normal estrogen physiology and the pathogenesis of breast cancer. Growth factors are known to stimulate the ligand-independent activity of ER through the activation of mitogen-activated protein kinase (MAPK) and the direct phosphorylation of ER. We found that the transcriptional activity of AIB1, a ligand-dependent ER coactivator and a gene amplified preferentially in ER-positive breast cancers, is enhanced by MAPK phosphorylation. We demonstrate that AIB1 is a phosphoprotein in vivo and can be phosphorylated in vitro by MAPK. Finally, we observed that MAPK activation of AIB1 stimulates the recruitment of p300 and associated histone acetyltransferase activity. These results suggest that the ability of growth factors to modulate estrogen action may be mediated through MAPK activation of the nuclear receptor coactivator AIB1.

p53 down-regulates ER-responsive genes by interfering with the binding of ER to ERE

Overexpression of the tumor suppressor p53 in HeLa cells leads to loss of the estradiol- and genistein-induced human estrogen receptor (ERalpha) transactivity. The coactivator p300, which binds to both ERalpha and p53, does not prevent this loss of hERalpha function. In this report we demonstrate that p53 physically binds to multiple domains of the hERalpha. This binding did not interfere with either the ERalpha dimerization or the interaction between hERalpha and its coactivator SRC-1. However, p53 did interfere with the hERalpha-ERE binding. These results may explain how p53 down-regulates the expression of some estrogen-responsive genes such as c-fos, c-jun, TPA, and bcl-2. This study supports the cross-talk between the p53 and the ERalpha signaling pathways.

Estrogen receptor null mice: what have we learned and where will they lead us?

All scientific investigations begin with distinct objectives: first is the hypothesis upon which studies are undertaken to disprove, and second is the overall aim of obtaining further information, from which future and more precise hypotheses may be drawn. Studies focusing on the generation and use of gene-targeted animal models also apply these goals and may be loosely categorized into sequential phases that become apparent as the use of the model progresses. Initial studies of knockout models often focus on the plausibility of the model based on prior knowledge and whether the generation of an animal lacking the particular gene will prove lethal or not. Upon the successful generation of a knockout, confirmatory studies are undertaken to corroborate previously established hypotheses of the function of the disrupted gene product. As these studies continue, observations of unpredicted phenotypes or, more likely, the lack of a phenotype that was expected based on models put forth from past investigations are noted. Often the surprising phenotype is due to the loss of a gene product that is downstream from the functions of the disrupted gene, whereas the lack of an expected phenotype may be due to compensatory roles filled by alternate mechanisms. As the descriptive studies of the knockout continue, use of the model is often shifted to the role as a unique research reagent, to be used in studies that 1) were not previously possible in a wild-type model; 2) aimed at finding related proteins or pathways whose existence or functions were previously masked; or 3) the subsequent effects of the gene disruption on related physiological and biochemical systems. The alpha ERKO mice continue to satisfy the confirmatory role of a knockout quite well. As summarized in Table 4, the phenotypes observed in the alpha ERKO due to estrogen insensitivity have definitively illustrated several roles that were previously believed to be dependent on functional ER alpha, including 1) the proliferative and differentiative actions critical to the function of the adult female reproductive tract and mammary gland; 2) as an obligatory component in growth factor signaling in the uterus and mammary gland; 3) as the principal steroid involved in negative regulation of gonadotropin gene transcription and LH levels in the hypothalamic-pituitary axis; 4) as a positive regulator of PR expression in several tissues; 5) in the positive regulation of PRL synthesis and secretion from the pituitary; 6) as a promotional factor in oncogene-induced mammary neoplasia; and 7) as a crucial component in the differentiation and activation of several behaviors in both the female and male. The list of unpredictable phenotypes in the alpha ERKO must begin with the observation that generation of an animal lacking a functional ER alpha gene was successful and produced animals of both sexes that exhibit a life span comparable to wild-type. The successful generation of beta ERKO mice suggests that this receptor is also not essential to survival and was most likely not a compensatory factor in the survival of the alpha ERKO. In support of this is our recent successful generation of double knockout, or alpha beta ERKO mice of both sexes. The precise defects in certain components of male reproduction, including the production of abnormal sperm and the loss of intromission and ejaculatory responses that were observed in the alpha ERKO, were quite surprising. In turn, certain estrogen pathways in the alpha ERKO female appear intact or unaffected, such as the ability of the uterus to successfully exhibit a progesterone-induced decidualization response, and the possible maintenance of an LH surge system in the hypothalamus. [ABSTRACT TRUNCATED]