Coactivator peptides have a differential stabilizing effect on the binding of estrogens and antiestrogens with the estrogen receptor

Interaction of the aryl hydrocarbon receptor ligand 6-methyl-1,3,8-trichlorodibenzofuran with estrogen receptor alpha

The polycyclic aromatic hydrocarbon 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) is related to the industrial byproduct dioxin and is a weak agonist and partial antagonist at the aryl hydrocarbon receptor (AhR). Tamoxifen is used for the treatment and prevention of breast cancer and interferes with the interaction of estrogen with estrogen receptor alpha (ER). The combination of MCDF and tamoxifen lowered the effective dose of both drugs required to inhibit 7,12-dimethylbenz(a)anthracene-induced mammary tumor growth in rats and protected against the estrogenic effects of tamoxifen on the uterus in rats (A. McDougal et al., Cancer Res 2001;61:3902-7), pointing to the potential use of MCDF in breast cancer treatment. Potential AhR-ER cross-talk is evidenced by the antiestrogenic activity of MCDF and the degradative effect of MCDF on ER protein levels. Our studies confirmed that MCDF degraded the ER. MCDF displayed antiestrogenic activity at higher concentrations in MCF-7 human breast cancer cells, but MCDF alone (10(-6) M) stimulated the growth of MCF-7 cells. MCDF also activated an estrogen response element (ERE)-luciferase reporter and increased mRNA levels of the estrogen-responsive gene transforming growth factor (TGF)-alpha. The estrogenic effects of MCDF are ER dependent because they were blocked by the pure antiestrogen ICI 182,780. MCDF induced ER-coactivator interaction in glutathione S-transferase pull-down assays and the formation of an ER.ERE complex in gel mobility shift assays, further indicating that the estrogenic actions of MCDF are mediated by the ER. In addition, knockdown of the AhR with small interfering RNA did not affect MCDF-induced ERE-luciferase activity. Overall, these data support the conclusion that MCDF is a partial agonist at the ER. This study provides the first evidence for the direct interaction of the ER with MCDF and challenges the view that MCDF is simply an AhR-specific ligand.

Estrogen-induced smooth muscle cell growth is regulated by tuberin and associated with altered activation of platelet-derived growth factor receptor-beta and ERK-1/2

The mechanisms that regulate the diverse responses to estrogen (E2) are unknown. Loss of function of the tuberous sclerosis 2 gene (TSC2), a tumor suppressor gene, has been associated with a growth-promoting effect of E2. We hypothesized that tuberin, the protein product of TSC2, binds to estrogen receptors (ER) and regulates the growth effect of E2. An in vivo association between full-length tuberin and ERalpha was observed in HEK 293 cells and ELT-3 smooth muscle cells. In contrast, poor association was observed between tuberin and ERbeta. Complex formation with ERalpha and the C-terminal end of tuberin was also observed in vivo and in vitro, indicating that binding between ERalpha and tuberin occurs at the C-terminal end of the tuberin molecule. We examined the effect of tuberin expression in ELT-3 smooth muscle cells on the growth response to E2. The growth-promoting effect of E2 in tuberin-null ELT-3 smooth muscle cells was ERalpha-specific, associated with up-regulation and activation of platelet-derived growth factor receptor-beta (PDGFRbeta) and activation of the signaling intermediate, extracellular signal-regulated kinase-1/-2 (ERK-1/2). In contrast, the expression of tuberin in ELT-3 smooth muscle cells resulted in significant abrogation of E2-stimulated growth. In parallel with this observation, the expression of tuberin in ELT-3 cells also resulted in significant inhibition of PDGFRbeta and ERK-1/2 activation in response to E2. These results demonstrate that tuberin binds specifically to ERalpha and inhibits E2-induced proliferation of ELT-3 cells. Furthermore, the opposing effects of tuberin on estrogen-induced activation of PDGFRbeta and ERK-1/-2 suggest a pivotal role for tuberin in directing the signaling events that dictate the growth response to E2.

Allosteric control of ligand selectivity between estrogen receptors alpha and beta: implications for other nuclear receptors

Allosteric communication between interacting molecules is fundamental to signal transduction and many other cellular processes. To better understand the relationship between nuclear receptor (NR) ligand positioning and the formation of the coactivator binding pocket, we investigated the determinants of ligand selectivity between the two estrogen receptor subtypes ERalpha and ERbeta. Chimeric receptors and structurally guided amino acid substitutions were used to demonstrate that distinct "hot spot" amino acids are required for ligand selectivity. Residues within the ligand binding pocket as well as distal secondary structural interactions contribute to subtype-specific positioning of the ligand and transcriptional output. Examination of other NRs suggests a mechanism of communication between the ligand and coactivator binding pockets, accounting for partial agonist and dimer-specific activity. These results demonstrate the importance of long-range interactions in the transmission of information through the ligand binding domain as well as in determining the ligand selectivity of closely related NR receptor subtypes.

Urban Renewal in the Nucleus: Is Protein Turnover by Proteasomes Absolutely Required for Nuclear Receptor-Regulated Transcription?

The importance of the ubiquitin proteasome pathway in higher eukaryotes has been well established in cell cycle regulation, signal transduction, and cell differentiation, but has only recently been linked to nuclear hormone receptor-regulated gene transcription. Characterization of a number of ubiquitin proteasome pathway enzymes as coactivators and observations that several nuclear receptors are ubiquitinated and degraded in the course of their nuclear activities provide evidence that ubiquitin proteasome-mediated protein degradation plays an integral role in eukaryotic transcription. In addition to receptors, studies have revealed that coactivators are ubiquitinated and degraded via the proteasome. The notion that the ubiquitin proteasome pathway is involved in gene transcription is further strengthened by the fact that ubiquitin proteasome pathway enzymes are recruited to the promoters of target genes and that proteasome-dependent degradation of nuclear receptors is required for efficient transcriptional activity. These findings suggest that protein degradation is coupled with nuclear receptor coactivation activity. It is possible that the ubiquitin proteasome pathway modulates transcription by promoting remodeling and turnover of the nuclear receptor-transcription complex. In this review, we discus the possible role of the ubiquitin proteasome pathway in nuclear hormone receptor-regulated gene transcription.

Novel activation step required for transcriptional competence of progesterone receptor on chromatin templates

To elucidate the earliest molecular steps in the activation of transcription by the progesterone receptor (PR), we investigated its activity in a cell-free transcription system utilizing chromatin templates. PR prepared as a ligand-free, recombinant protein failed to induce transcription on chromatin templates. However, transcriptional competence could be restored by coincubation with rabbit reticulocyte lysate (RRL). The interaction of PR with chaperones results in a receptor conformation competent to bind ligand and RRL contains abundant chaperone-mediated protein folding activity. Blocking this activity with the specific inhibitor geldanamycin inhibited receptor-dependent transcriptional activity. However, recombinant chaperones could not replace RRL in the restoration of transcriptional activity on chromatin templates, suggesting the presence of an additional activity in the lysate. Under chromatin assembly conditions, PR could bind naked DNA and RRL did not increase that binding. In contrast, PR bound to a chromatin template only poorly. Interestingly, RRL stimulated sequence-specific binding by PR to target sites in chromatin and the concomitant recruitment of the steroid receptor coactivator 1 to the promoter. Thus, our results indicate that a novel protein-mediated activity in RRL is involved in an additional, heretofore unrecognized, activation step required for PR to become transcriptionally competent on chromatin templates.

Development of a homogeneous, fluorescence resonance energy transfer-based in vitro recruitment assay for peroxisome proliferator-activated receptor delta via selection of active LXXLL coactivator peptides

The peroxisome proliferator-activated receptors (PPARs) are nuclear receptors activated by fatty acids and their metabolites. The PPARdelta subtype is believed to be involved in lipoprotein regulation and may have a role in reverse cholesterol transport. While the range of biological roles of PPARdelta still remains unclear, it is of therapeutic interest in cardiovascular diseases. Here we report a homogeneous in vitro assay for studying ligand activation of PPARdelta. We surveyed a panel of peptides containing the LXXLL motifs derived from coactivator protein sequences. Peptides with the best response were used to develop a sensitive and homogeneous recruitment assay for PPARdelta. The optimized assay has a signal-to-background ratio of about 8:1 and an assay quality parameter Z'-factor value of 0.8. The assay signal generated is stable for hours to even overnight. This simple recruitment assay can provide agonist and/or antagonist information that cannot be assessed by receptor-binding assay, and can be used for characterization and screening of ligands that modulate the activation of PPARdelta.

Androgen receptor (AR) coregulators: an overview

The biological action of androgens is mediated through the androgen receptor (AR). Androgen-bound AR functions as a transcription factor to regulate genes involved in an array of physiological processes, most notably male sexual differentiation and maturation, and the maintenance of spermatogenesis. The transcriptional activity of AR is affected by coregulators that influence a number of functional properties of AR, including ligand selectivity and DNA binding capacity. As the promoter of target genes, coregulators participate in DNA modification, either directly through modification of histones or indirectly by the recruitment of chromatin-modifying complexes, as well as functioning in the recruitment of the basal transcriptional machinery. Aberrant coregulator activity due to mutation or altered expression levels may be a contributing factor in the progression of diseases related to AR activity, such as prostate cancer. AR demonstrates distinct differences in its interaction with coregulators from other steroid receptors due to differences in the functional interaction between AR domains, possibly resulting in alterations in the dynamic interactions between coregulator complexes.

Ligand-selective interactions of ER detected in living cells by fluorescence resonance energy transfer

Some aspects of ligand-regulated transcription activation by the estrogen receptor (ER) are associated with the estrogen-dependent formation of a hydrophobic cleft on the receptor surface. At least in vitro, this cleft is required for direct interaction of ER with an alpha helix, containing variants of the sequence LXXLL, found in many coactivators. In cells, it is unknown whether ER interactions with the different LXXLL-containing helices are uniformly similar or whether they vary with LXXLL sequence or activating ligand. Using fluorescence resonance energy transfer (FRET), we confirm in the physiological environment a direct interaction between the estradiol (E2)-bound ER and LXXLL peptides expressed in living cells as fusions with spectral variants of the green fluorescent protein. This interaction was blocked by a single amino acid mutation in the hydrophobic cleft. No FRET was detected when cells were incubated with the antiestrogenic ligands tamoxifen and ICI 182,780. E2, diethylstilbestrol, ethyl indenestrol A, and 6,4'-dihydroxyflavone all promoted FRET and activated ER-dependent transcription. Measurement of the level of FRET of ER with different LXXLL-containing peptides suggested that the orientations or affinities of the LXXLL interactions with the hydrophobic cleft were globally similar but slightly different for some activating ligands.

Mechanisms of estrogen action

Our appreciation of the physiological functions of estrogens and the mechanisms through which estrogens bring about these functions has changed during the past decade. Just as transgenic mice were produced in which estrogen receptors had been inactivated and we thought that we were about to understand the role of estrogen receptors in physiology and pathology, it was found that there was not one but two distinct and functional estrogen receptors, now called ER alpha and ER beta. Transgenic mice in which each of the receptors or both the receptors are inactive have revealed a much broader role for estrogens in the body than was previously thought. This decade also saw the description of a male patient who had no functional ER alpha and whose continued bone growth clearly revealed an important function of estrogen in men. The importance of estrogen in both males and females was also demonstrated in the laboratory in transgenic mice in which the aromatase gene was inactivated. Finally, crystal structures of the estrogen receptors with agonists and antagonists have revealed much about how ligand binding influences receptor conformation and how this conformation influences interaction of the receptor with coactivators or corepressors and hence determines cellular response to ligands.

Selective estrogen receptor modulators and coronary heart disease

The vasculature has been recognized as an important target of estrogen action through rapid non-genomic effects and/or via the classic pathway (genomic effects) involving estrogen receptors (ER-alpha and ER-beta). Multiple mechanisms participate in the regulation of different estrogen-controlled genes, providing a wide spectrum of possibilities for development of drugs, including pure agonists or antagonists or mixed agonists/antagonists, the so-called selective estrogen receptor modulators (SERM). In theory, an ideal SERM should reduce the risks of coronary heart disease (CHD) and preserve bone density, without or with very low incidences of breast and endometrial neoplasms or venous thromboembolism (VTE). The precise mechanism for the protective effects of estrogens and their receptors on cardiovascular diseases is not yet fully established. In this review, we summarize the recent advances in understanding the action of ERs/ligands, the therapeutic implications for CHD, and highlight the recent progress of both clinical and basic studies on the protection issue. Finally, a number of newly developed SERMs and their clinical applications as well as the laboratory investigations are discussed.

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.

Crosstalk and considerations in endocrine disruptor research

The presence of endocrine disrupting chemicals in the environment has prompted action on several fronts to assess the potential health risks of these compounds. To fully understand the mechanisms behind the observed endocrine disruption, crosstalk and other factors should be considered. In this article we will discuss how crosstalk modulates estrogen action in several common assays and how this and other considerations appear to have been overlooked. In addition, a paradigm shift from theoretical linear response pathways to interaction maps should aid in the understanding and analysis of endocrine disruption.

Ligands Specify Coactivator Nuclear Receptor (NR) Box Affinity for Estrogen Receptor Subtypes

Nuclear receptors (NRs) require coactivators to efficiently activate transcription of their target genes. Many coactivators including the p160 proteins utilize a short NR box motif to recognize the ligand-binding domain of the NR when it is activated by ligand. To investigate the ability of various ligands to specify the affinity of NR boxes for a ligand-bound NR, we compared the capacity of p160 NR boxes to be recruited to estrogen receptor (ERα) and ERβ in the presence of 17β-estradiol, diethylstilbestrol, and genestein. A time-resolved fluorescence-based binding assay was used to determine the dissociation constants for the 10 NR boxes derived from the three p160 coactivators for both ER subtypes in the presence of the each of the agonists. While the affinity of some NR boxes for ER was independent of the agonist, we identified several NR boxes that had significantly different affinities for ER depending on which agonist was bound to the receptor. Therefore, an agonist may specify the affinity of an NR for various NR boxes and thus regulate the coactivator selectivity of the receptor.

Estrogen receptor beta-selective transcriptional activity and recruitment of coregulators by phytoestrogens

Estrogens used in hormone replacement therapy regimens may increase the risk of developing breast cancer. Paradoxically, high consumption of plant-derived phytoestrogens, particularly soybean isoflavones, is associated with a low incidence of breast cancer. To explore the molecular basis for these potential different clinical outcomes, we investigated whether soybean isoflavones elicit distinct transcriptional actions from estrogens. Our results demonstrate that the estrogen 17beta-estradiol effectively triggers the transcriptional activation and repression pathways with both estrogen receptors (ERs) ERalpha and ERbeta. In contrast, soybean isoflavones (genistein, daidzein, and biochanin A) are ERbeta-selective agonists of transcriptional repression and activation at physiological levels. The molecular mechanism for ERbeta selectivity by isoflavones involves their capacity to create an activation function-2 surface of ERbeta that has a greater affinity for coregulators than ERalpha. Phytoestrogens may act as natural selective estrogen receptor modulators that elicit distinct clinical effects from estrogens used for hormone replacement by selectively recruiting coregulatory proteins to ERbeta that trigger transcriptional pathways.

The human estrogen receptor alpha dimer binds a single SRC-1 coactivator molecule with an affinity dictated by agonist structure

Nuclear receptors act as ligand-inducible transcription factors. Agonist binding leads to interaction with coactivator proteins, and to the assembly of the general transcription machinery. In addition to structural information, a thorough understanding of transcriptional activation by the nuclear receptors requires the characterization of the thermodynamic parameters governing these protein/protein interactions. In this study we have quantitatively characterized the interactions of full-length baculovirus expressed human estrogen receptor alpha (ERalpha), as well as ERalpha hormone binding domain (ERHBD) with a fragment of the coactivator protein SRC-1 (amino acid residues 570 to 780). Fluorescence anisotropy and fluorescence correlation spectroscopy of fluorescently labeled SRC-1(570-780) demonstrate unambiguously that the stoichiometry of the SRC-1/ERalpha/estradiol complex is one coactivator molecule per ERalpha dimer. The affinity of the estradiol or estriol bound ERalpha/SRC-1 complexes was found to be significantly higher than that observed in the presence of estrone. No binding was observed in the absence of ligand or in the presence of antagonists. Distinct anisotropy values for the ERalpha-SRC-1 complexes with different agonists suggest distinct conformations of the complexes depending upon agonist structure.

Effect of low dose irradiation on estrogen receptor level in MCF-7 breast cancer cells

Exposure of MCF-7 cells to single and/or repeated low gamma-ray doses (0.5 to 8 Gy) resulted in a decrease in the capacity of these cells to concentrate tritiated estradiol ([3H]E2) (reduction of the number of binding sites). The decrease in the [3H]E2-binding capacity was higher than the survival rate, indicating that it could not be ascribed to cell death. Moreover, such low irradiation doses failed to similarly affect the specific incorporation of [3H]ORG 2058, even when the progesterone receptor was induced by E2, a finding that rejects the hypothesis of a nonspecific effect on all steroid hormone receptors. This loss of [3H]E2 binding was reflected by the elimination of the estrogen receptor alpha (ER) when the latter was assessed by immunocytochemistry. However, additional immunochemical studies (Western blot data) performed on cell extracts under denaturing conditions failed to show any similar elimination of the ER peptide, suggesting that the loss of E2-binding capacity would be relevant to subtle changes in the ER structure and/or ER-associated proteins. The loss of binding capacity, produced by a 3-Gy irradiation, failed to decrease the sensitivity of the cells to E2, since progesterone receptor induction and growth stimulation were maintained. Insufficient ER diminution may explain this observation.

Temporally distinct and ligand-specific recruitment of nuclear receptor-interacting peptides and cofactors to subnuclear domains containing the estrogen receptor

Ligand binding to estrogen receptor (ER) is presumed to regulate the type and timing of ER interactions with different cofactors. Using fluorescence microscopy in living cells, we characterized the recruitment of five different green fluorescent protein (GFP)-labeled ER-interacting peptides to the distinct subnuclear compartment occupied by blue fluorescent protein (BFP)-labeled ER alpha. Different ligands promoted the recruitment of different peptides. One peptide was recruited in response to estradiol (E2), tamoxifen, raloxifene, or ICI 182,780 incubation whereas other peptides were recruited specifically by E2 or tamoxifen. Peptides containing different sequences surrounding the ER-interacting motif LXXLL were recruited with different time courses after E2 addition. Complex temporal kinetics also were observed for recruitment of the full-length, ER cofactor glucocorticoid receptor-interacting protein 1 (GRIP1); rapid, E2-dependent recruitment of GRIP1 was blocked by mutation of the GRIP1 LXXLL motifs to LXXAA whereas slower E2 recruitment persisted for the GRIP1 LXXAA mutant. This suggested the presence of multiple, temporally distinct GRIP 1 recruitment mechanisms. E2 recruitment of GRIP1 and LXXLL peptides was blocked by coincubation with excess ICI 182,780. In contrast, preformed E2/ER/GRIP1 and E2/ER/LXXLL complexes were resistant to subsequent ICI 182,780 addition whereas ICI 182,780 dispersed preformed complexes containing the GRIP1 LXXAA mutant. This suggested that E2-induced LXXLL binding altered subsequent ligand/ER interactions. Thus, alternative, ligand-selective recruitment and dissociation mechanisms with distinct temporal sequences are available for ER alpha action in vivo.

Evidence for a common step in three different processes for modulating the kinetic properties of glucocorticoid receptor-induced gene transcription

The dose-response curve of steroid hormones and the associated EC(50) value are critical parameters both in the development of new pharmacologically active compounds and in the endocrine therapy of various disease states. We have recently described three different variables that can reposition the dose-response curve of agonist-bound glucocorticoid receptors (GRs): a 21-base pair sequence of the rat tyrosine aminotransferase gene called a glucocorticoid modulatory element (GME), GR concentration, and coactivator concentration. At the same time, each of these three components was found to influence the partial agonist activity of antiglucocorticoids. In an effort to determine whether these three processes proceed via independent pathways or a common intermediate, we have examined several mechanistic details. The effects of increasing concentrations of both GR and the coactivator TIF2 are found to be saturable. Furthermore, saturating levels of either GR or TIF2 inhibit the ability of each protein, and the GME, to affect further changes in the dose-response curve or partial agonist activity of antisteroids. This competitive inhibition suggests that all three modulators proceed through a common step involving a titratable factor. Support for this hypothesis comes from the observation that a fragment of the coactivator TIF2 retaining intrinsic transactivation activity is a dominant negative inhibitor of each component (GME, GR, and coactivator). This inhibition was not due to nonspecific effects on the general transcription machinery as the VP16 transactivation domain was inactive. The viral protein E1A also prevented the action of each of the three components in a manner that was independent of E1A's ability to block the histone acetyltransferase activity of CBP. Collectively, these results suggest that three different inputs (GME, GR, and coactivator) for perturbing the dose-response curve, and partial agonist activity, of GR-steroid complexes act by converging at a single step that involves a limiting factor prior to transcription initiation.

Interaction of tetrahydrocrysene ketone with estrogen receptors alpha and beta indicates conformational differences in the receptor subtypes

Estrogen receptors (ER) alpha and beta bind estradiol (E2) and other estrogenic ligands with different affinities. To measure the rate of E2 association with ERa and ERbeta, we employed tetrahydrocrysene ketone (THCK), a fluorescent ligand that is an agonist with ERalpha and an antagonist with ERbeta. We report that THCK binds E2-liganded and unliganded ERalpha and ERbeta, indicating a THCK binding site(s) other than the E2 binding pocket. THCK fluorescence was greater for ligand-occupied ERbeta than ERalpha, suggesting differences in the microenvironment of the THCK binding site(s). THCK fluorescence was also significantly greater for E2-, 4-hydroxytamoxifen-, and tamoxifen aziridine-liganded versus unliganded ER, allowing calculations of E2 association rate constants (ka) of 7.60 +/- 0.75 and 5.12 +/- 0.30 x 10(5) M(-1) s(-1) for E2-ERalpha and E2-ERbeta, respectively. THCK did not affect ERalpha binding to estrogen response element (ERE) DNA, but decreased ERbeta-ERE binding. We conclude that THCK binding site(s) on ERalpha versus ERbeta are different and important for ER function.

Prothymosin alpha selectively enhances estrogen receptor transcriptional activity by interacting with a repressor of estrogen receptor activity

We find that prothymosin alpha (PTalpha) selectively enhances transcriptional activation by the estrogen receptor (ER) but not transcriptional activity of other nuclear hormone receptors. This selectivity for ER is explained by PTalpha interaction not with ER, but with a 37-kDa protein denoted REA, for repressor of estrogen receptor activity, a protein that we have previously shown binds to ER, blocking coactivator binding to ER. We isolated PTalpha, known to be a chromatin-remodeling protein associated with cell proliferation, using REA as bait in a yeast two-hybrid screen with a cDNA library from MCF-7 human breast cancer cells. PTalpha increases the magnitude of ERalpha transcriptional activity three- to fourfold. It shows lesser enhancement of ERbeta transcriptional activity and has no influence on the transcriptional activity of other nuclear hormone receptors (progesterone receptor, glucocorticoid receptor, thyroid hormone receptor, or retinoic acid receptor) or on the basal activity of ERs. In contrast, the steroid receptor coactivator SRC-1 increases transcriptional activity of all of these receptors. Cotransfection of PTalpha or SRC-1 with increasing amounts of REA, as well as competitive glutathione S-transferase pulldown and mammalian two-hybrid studies, show that REA competes with PTalpha (or SRC-1) for regulation of ER transcriptional activity and suppresses the ER stimulation by PTalpha or SRC-1, indicating that REA can function as an anticoactivator in cells. Our data support a model in which PTalpha, which does not interact with ER, selectively enhances the transcriptional activity of the ER but not that of other nuclear receptors by recruiting the repressive REA protein away from ER, thereby allowing effective coactivation of ER with SRC-1 or other coregulators. The ability of PTalpha to directly interact in vitro and in vivo with REA, a selective coregulator of the ER, thereby enabling the interaction of ER with coactivators, appears to explain its ability to selectively enhance ER transcriptional activity. These findings highlight a new role for PTalpha as a coregulator activity-modulating protein that confers receptor specificity. Proteins such as PTalpha represent an additional regulatory component that defines a novel paradigm enabling receptor-selective enhancement of transcriptional activity by coactivators.

FXXLF and WXXLF sequences mediate the NH2-terminal interaction with the ligand binding domain of the androgen receptor

The nuclear receptor superfamily members of eukaryotic transcriptional regulators contain a highly conserved activation function 2 (AF2) in the hormone binding carboxyl-terminal domain and, for some, an additional activation function 1 in the NH(2)-terminal region which is not conserved. Recent biochemical and crystallographic studies revealed the molecular basis of AF2 is hormone-dependent recruitment of LXXLL motif-containing coactivators, including the p160 family, to a hydrophobic cleft in the ligand binding domain. Our previous studies demonstrated that AF2 in the androgen receptor (AR) binds only weakly to LXXLL motif-containing coactivators and instead mediates an androgen-dependent interaction with the AR NH(2)-terminal domain required for its physiological function. Here we demonstrate in a mammalian two-hybrid assay, glutathione S-transferase fusion protein binding studies, and functional assays that two predicted alpha-helical regions that are similar, but functionally distinct from the p160 coactivator interaction sequence, mediate the androgen-dependent, NH(2)- and carboxyl-terminal interaction. FXXLF in the AR NH(2)-terminal domain with the sequence (23)FQNLF(27) mediates interaction with AF2 and is the predominant androgen-dependent interaction site. This FXXLF sequence and a second NH(2)-terminal WXXLF sequence (433)WHTLF(437) interact with different regions of the ligand binding domain to stabilize the hormone-receptor complex and may compete with AF2 recruitment of LXXLL motif-containing coactivators. The results suggest a unique mechanism for AR-mediated transcriptional activation.

The 26S proteasome is required for estrogen receptor-alpha and coactivator turnover and for efficient estrogen receptor-alpha transactivation

Estrogen receptor-alpha (ER alpha) is downregulated in the presence of its cognate ligand, estradiol (E2), through the ubiquitin proteasome pathway. Here, we show that ubiquitin proteasome function is required for ER alpha to serve as a transcriptional activator. Deletion of the last 61 amino acids of ER alpha, including residues that form helix 12, abolishes ligand-mediated downregulation of the receptor as do point mutations in the ligand binding domain that impair coactivator binding. In addition, coactivators also are subject to degradation by the 26S proteasome, but their intrinsic transcriptional activity is not affected. These data provide evidence that protein interactions with ER alpha coactivator binding surfaces are important for ligand-mediated receptor down-regulation and suggest that receptor and coactivator turnover contributes to ER alpha transcriptional activity.

Estrogen receptor transcription and transactivation: Estrogen receptor alpha and estrogen receptor beta: regulation by selective estrogen receptor modulators and importance in breast cancer

Estrogens display intriguing tissue-selective action that is of great biomedical importance in the development of optimal therapeutics for the prevention and treatment of breast cancer, for menopausal hormone replacement, and for fertility regulation. Certain compounds that act through the estrogen receptor (ER), now referred to as selective estrogen receptor modulators (SERMs), can demonstrate remarkable differences in activity in the various estrogen target tissues, functioning as agonists in some tissues but as antagonists in others. Recent advances elucidating the tripartite nature of the biochemical and molecular actions of estrogens provide a good basis for understanding these tissue-selective actions. As discussed in this thematic review, the development of optimal SERMs should now be viewed in the context of two estrogen receptor subtypes, ERalpha and ERbeta, that have differing affinities and responsiveness to various SERMs, and differing tissue distribution and effectiveness at various gene regulatory sites. Cellular, biochemical, and structural approaches have also shown that the nature of the ligand affects the conformation assumed by the ER-ligand complex, thereby regulating its state of phosphorylation and the recruitment of different coregulator proteins. Growth factors and protein kinases that control the phosphorylation state of the complex also regulate the bioactivity of the ER. These interactions and changes determine the magnitude of the transcriptional response and the potency of different SERMs. As these critical components are becoming increasingly well defined, they provide a sound basis for the development of novel SERMs with optimal profiles of tissue selectivity as medical therapeutic agents.

Activation function 2 in the human androgen receptor ligand binding domain mediates interdomain communication with the NH(2)-terminal domain

Activation function 2 in the ligand binding domain of nuclear receptors forms a hydrophobic cleft that binds the LXXLL motif of p160 transcriptional coactivators. Here we provide evidence that activation function 2 in the androgen receptor serves as the contact site for the androgen dependent NH(2)- and carboxyl-terminal interaction of the androgen receptor and only weakly interacts with p160 coactivators in an LXXLL-dependent manner. Mutagenesis studies indicate that it is the NH(2)-/carboxyl-terminal interaction that is required by activation function 2 to stabilize helix 12 and slow androgen dissociation critical for androgen receptor activity in vivo. The androgen receptor recruits p160 coactivators through its NH(2)-terminal and DNA binding domains in an LXXLL motif-independent manner. The results suggest a novel function for activation function 2 and a unique mechanism of nuclear receptor transactivation.