The estrogen receptor enhances AP-1 activity by two distinct mechanisms with different requirements for receptor transactivation functions

Differential and Opposing Regulation of PAI-1 Promoter Activity by Estrogen Receptor α and Estrogen Receptor β in Endothelial Cells

To investigate the molecular mechanisms involved in the estrogen-dependent control of plasminogen activator inhibitor-1 (PAI-1) gene expression in vascular cells, we compared the transactivation properties of estrogen receptors (ERalpha and ERbeta) in regulating the activity of a human PAI-1 promoter reporter construct in transfected bovine aortic endothelial cells (BAECs). ERalpha increased PAI-1 promoter activity in BAECs by an estrogen-dependent mechanism, whereas ERbeta suppressed PAI-1 promoter activity by an estrogen-independent mechanism. The suppressive activity of ERbeta was dominant over the inductive activity of ERalpha. Mutation of a putative estrogen response element (ERE) located at position -427 in the proximal promoter abolished the ERalpha action without influencing the suppressive effects of ERbeta. Mutation of either AP1-like site did not eliminate the ERalpha or ERbeta actions at the PAI-1 promoter, suggesting that other promoter elements are involved in these responses. These mutations significantly reduced the -3.4kbp PAI-1 promoter response to serum. We concluded that ERalpha and ERbeta exert differential effects on the PAI-1 promoter activity in transfected BAECs. ERalpha activated the PAI-1 promoter through a proximal ERE (-427) and possibly additional EREs located within the PAI-1 promoter, whereas ERbeta suppressed the promoter construct via an unidentified mechanism. This is the first demonstration of the differential regulation of a vascular gene promoter by ERalpha and ERbeta.

Estrogen receptor-dependent activation of AP-1 via non-genomic signalling

Background

Ligand-bound estrogen receptor α (ERα) and estrogen receptor β (ERβ) modulate AP-1-dependent transcription via protein-protein interactions on DNA, in a manner that depends on the type of cells and the subtype of ER. We present here evidence for an additional mechanism by which ERs modulate the transcriptional activity of AP-1.

Results

We show that ERs located in the cytoplasm efficiently activate transcription at AP-1 sites in response to 17β-estradiol, while ERs present in the nucleus repress transcription under the same conditions. 17β-estradiol-induced activation of the coll-73-luc reporter correlated with cytoplasmic localization of various ERα and ERβ mutant receptors, and was inhibited in the presence of the full estrogen antagonist ICI 182,780 and the MAP-kinase inhibitor UO126. We also show that the selective estrogen receptor modulator (SERM) tamoxifen is as potent as 17β-estradiol in inducing activation of AP-1 when ERα is present in the cytoplasm.

Conclusions

These results suggest that non-genomic signalling is involved in the mechanism by which ERα and ERβ influence AP-1-dependent transcription. We have previously shown that Stat3 and Stat5 are targeted by non-genomic actions of ERs, and the results presented here allow us to conclude that ERs bound to 17β-estradiol mediate the transcriptional activation of promoters regulated by AP-1 and by Stat proteins via different combinations of signal transduction pathways. Our observations thereby provide new insights into the mechanisms by which ERs act at alternate response elements, and suggest a mechanism by which tamoxifen exerts its action as a tissue-selective agonist.

Identification of tamoxifen-induced coregulator interaction surfaces within the ligand-binding domain of estrogen receptors

Tamoxifen is a selective estrogen receptor (ER) modulator that is clinically used as an antagonist to treat estrogen-dependent breast cancers but displays unwanted agonistic effects in other tissues. Previous studies on ERalpha have delineated a role of the N-terminal activation function AF-1 in mediating the agonistic effects of tamoxifen, while the mechanisms for how ERbeta mediates tamoxifen action remain to be elucidated. As peptides can be used to detect distinct receptor conformations and binding surfaces for coactivators and corepressors, we attempted in this study to identify previously unrecognized peptides that interact specifically with ERs in the presence of tamoxifen. We identified two distinct peptides among others that are highly selective for tamoxifen-bound ERalpha or ERbeta. Domain mapping and mutation analysis suggest that these peptides recognize a novel tamoxifen-induced binding surface within the C-terminal ligand-binding domain that is distinct from the agonist-induced AF-2 surface. Peptide expression specifically inhibited transcriptional ER activity in response to tamoxifen, presumably by preventing the binding of endogenous coactivators. Moreover, tamoxifen-responsive and ER subtype-selective coactivators were engineered by replacing the LXXLL motifs in the coactivator TIF2 with either of the two peptides. Finally, our results indicate that related coactivators may act via the novel tamoxifen-induced binding surface, referred to as AF-T, allowing us to propose a revised model of tamoxifen agonism.

A conserved lysine in the estrogen receptor DNA binding domain regulates ligand activation profiles at AP-1 sites, possibly by controlling interactions with a modulating repressor

Background

Estrogen receptors alpha and beta (ERα and ERβ) differentially activate genes with AP-1 elements. ERα activates AP-1 targets via activation functions with estrogens (the AF-dependent pathway), whereas ERβ, and a short version of ERα (ERα DBD-LBD) activate only with anti-estrogens (AF-independent pathway). The DNA binding domain (DBD) plays an important role in both pathways, even though neither pathway requires ERE recognition.

Results

Mutations of a highly conserved DBD lysine (ERα.K206A/G), lead to super-activation of AP-1 through activation function dependent pathways, up to 200 fold. This super-activity can be elicited either through ER AFs 1 or 2, or that of a heterologous activation function (VP16). The homologous substitution in ERβ, K170A, or in ERα DBD-LBD leads to estrogen-dependent AP-1 activation and loss of the usually potent anti-estrogen effects. Each of numerous K206 substitutions in ERα, except K206R, eliminates anti-estrogen activation and this loss correlates perfectly with a loss of ability to titrate a repressive function from the RU486 bound progesterone receptor.

Conclusion

We conclude that ER DBDs contain a complex regulatory function that influences ligand activation profiles at AP-1. This function, which requires the integrity of the conserved lysine, both allows for activation at AP-1 with anti-estrogens (with ERβ and ERα DBD-LBD), and prevents ERα from becoming superactive at AP-1 with estrogens. We discuss the possibility that a repressor interaction with the DBD both mediates the AF-independent pathway and dampens the AF dependent pathway. Mutations in the conserved lysine might, by this model, disrupt the binding or function of the repressor.

Estrogen and neuroprotection: higher constitutive expression of glutaredoxin in female mice offers protection against MPTP-mediated neurodegeneration

Incidence of Parkinson's disease is lower in women as compared with men. Although neuroprotective effect of estrogen is recognized, the underlying molecular mechanisms are unclear. MPTP (1-methyl-4-phenyl-1, 2, 3, 6, tetrahydro-pyridine), a neurotoxin that causes Parkinson's disease-like symptoms acts through inhibition of mitochondrial complex I. Administration of MPTP to male mice results in loss of dopaminergic neurons in substantia nigra, whereas female mice are unaffected. Oxidation of critical thiol groups by MPTP disrupts mitochondrial complex I, and up-regulation of glutaredoxin (a thiol disulfide oxidoreductase) is essential for recovery of complex I. Early events following MPTP exposure, such as increased AP1 transcription, loss of glutathione, and up-regulation of glutaredoxin mRNA is seen only in male mice, indicating that early response to neurotoxic insult does not occur in females. Pretreatment of female mice with ICI 182,780, estrogen receptor (ER) antagonist sensitizes them to MPTP-mediated complex I dysfunction. Constitutive expression of glutaredoxin is significantly higher in female mice as compared with males. ICI 182,780 down-regulates glutaredoxin activity in female mouse brain regions (midbrain and striatum), indicating that glutaredoxin expression is regulated through estrogen receptor signaling. Higher constitutive expression of glutaredoxin could potentially contribute to the neuroprotection seen in female mouse following exposure to neurotoxins, such as MPTP.

Selective estrogen receptor modulators: discrimination of agonistic versus antagonistic activities by gene expression profiling in breast cancer cells

Selective estrogen receptor modulators (SERMs) such as tamoxifen are effective in the treatment of many estrogen receptor-positive breast cancers and have also proven to be effective in the prevention of breast cancer in women at high risk for the disease. The comparative abilities of tamoxifen versus raloxifene in breast cancer prevention are currently being compared in the Study of Tamoxifen and Raloxifene trial. To better understand the actions of these compounds in breast cancer, we have examined their effects on the expression of approximately 12,000 genes, using Affymetrix GeneChip microarrays, with quantitative PCR verification in many cases, categorizing their actions as agonist, antagonist, or partial agonist/antagonist. Analysis of gene stimulation and inhibition by the SERMs trans-hydroxytamoxifen (TOT) and raloxifene (Ral) or ICI 182,780 (ICI) and by estradiol (E2) in estrogen receptor-containing MCF-7 human breast cancer cells revealed that (a) TOT was the most E2-like of the three compounds, (b) all three compounds either partially or fully antagonized the action of E2 on most genes, with the order of antagonist activity being ICI > Ral > TOT, (c) TOT and Ral, but not ICI, displayed partial agonist/partial antagonist activity on a number of E2-regulated genes, (d) several stimulatory cell cycle-related genes were down-regulated exclusively by ICI, (e) the estrogen-like activity of Ral nearly always overlapped with that of TOT, indicating that Ral has little unique agonist activity different from that of TOT, and (f) some genes were specifically up-regulated by TOT but not Ral, ICI, or E2. Hence, gene expression profiling can discern fundamental differences among SERMs and provides insight into the distinct biologies of TOT, Ral, and ICI in breast cancer.

Evidence for functional estrogen receptors α and β in human adipose cells: regional specificities and regulation by estrogens

Adipocytes are estrogen-responsive cells, but the quantitative expression and transcriptional regulation of the estrogen receptors (ER-α and ER-β) in human adipocytes and their precursor cells are unclear. Using real-time quantitative PCR, we have demonstrated that both ER-α and ER-β mRNA are expressed in human mature adipocytes with a large predominance of ER-α mRNA. Moreover, ER-α mRNA is identically expressed whatever the anatomic origin (intraabdominal and subcutaneous) of the adipocytes and the gender. ER-β mRNA levels are higher in women compared with men, without regional differences. 17β-Estradiol in vitro upregulates expression of both ER-α and ER-β mRNA in subcutaneous adipocytes from women but only the ER-α mRNA in subcutaneous and intra-abdominal adipocytes from men. In preadipocytes, only the ER-α subtype was present. In the latter cells, estrogens in vitro had no influence on ER-α expression (mRNA and protein). The present study also shows that estrogens in vitro increase the AP-1, SP-1, and estrogen response element DNA binding activities in differentiated but not in confluent preadipocytes, suggesting that ER become functional during the course of adipogenesis. On the whole, these data are consistent with a predominant role of the ER-α subtype in mediating the effects of estrogens on human adipose tissue development and metabolism.

Estrogen receptors ER alpha and ER beta in proliferation in the rodent mammary gland

Most evidence supports the view that ER alpha is responsible for estrogen (ovarian estradiol, E(2))-induced proliferation in the epithelial cells of the mammary gland, but despite this, proliferating epithelial cells do not express ER alpha. We have examined this apparent paradox by studying the role of ER alpha and ER beta in E(2)-induced proliferation in mammary glands (measured by BrdUrd incorporation into DNA) in mice with intact ER beta (WT mice) and those in which the ER beta gene has been inactivated (ER beta(-/-) mice). On treatment of ER beta(-/-) mice with E(2) or ovariectomized WT mice with E(2), tamoxifen, or a specific ER beta agonist (BAG), the number of BrdUrd-labeled cells in mammary glands increased from 3.4% in controls to 28-38% in the treated mice. This indicates that both ER alpha and ER beta can mediate E(2)-induced proliferation independently of each other. With specific antibodies, ER beta was found in both epithelial and stromal cells, whereas ER alpha was strictly epithelial. Within 4 h of a single dose of E(2), ER alpha was lost from the nuclei of epithelial cells. In WT mice, ER alpha reappeared by 24 h, but in ER beta(-/-) mice, return to the nucleus was delayed by 24 h. At 4 h after E(2), neither ER alpha nor progesterone receptor was detectable in BrdUrd-labeled nuclei but by 48 h after E(2), 29% of the BrdUrd-labeled cells expressed ER alpha, and 21-38% expressed progesterone receptor. During 3 weeks of continuous E(2) treatment, ER beta remained in the nucleus, but there was no detectable ER alpha. With tamoxifen treatment, ER alpha remained in the nucleus, but ER beta was lost. From these results, we conclude that ER alpha receives the proliferation signal from E(2), initiates DNA synthesis, and is then lost from cells. The subsequent steps in proliferation can proceed in the absence of either ER alpha or ER beta. ER beta facilitates the return of ER alpha to the nucleus and restores responsiveness to E(2). By down-regulating ER beta, tamoxifen may prolong refractoriness to E(2) in mammary epithelium.

Effects of environmental estrogenic chemicals on AP1 mediated transcription with estrogen receptors alpha and beta

There has been much discussion concerning endocrine disrupting chemicals suspected of exerting adverse effects in both wildlife and humans. Since the majority of these compounds are estrogenic, a large number of in vitro tests for estrogenic characteristics have been developed for screening purpose. One reliable and widely used method is the reporter gene assay employing estrogen receptors (ERs) and a reporter gene with a cis-acting estrogen responsive element (ERE). Other elements such as AP1 also mediate estrogenic signals and the manner of response could be quite different from that of ERE. Since this has yet to be explored, the ER mediated AP1 activity in response to a series of environmental estrogens was investigated in comparison with ERE findings. All the compounds exhibited estrogenic properties with ERE-luc and their AP1 responses were quite similar. These was one exception, however, p,p'-DDT (1,1,1,-trichloro-2,2-bis(p-chlorophenyl)ethane) did not exert any AP1-luc activity, while it appeared to be estrogenic at 10(-7) to 10(-5)M with the ERE action. None of the compounds demonstrated ER beta:AP1 activity. These data suggest that significant differences can occur in responses through the two estrogen pathways depending on environmental chemicals.

Molecular Mechanisms, Physiological Consequences and Pharmacological Implications of Estrogen Receptor Action

The estrogen receptors (ERs), ERalpha and ERbeta, play a central role in mediating the biological effects of estrogen. The transcription rate of estrogen target genes is determined by several parameters including the type of ligand, estrogen receptor subtype and isoform, as well as interactions with receptor-binding cofactor proteins. The ERs regulate gene expression by binding to specific response element sequences in the promoters of estrogen target genes. Alternative pathways have also been described in which the ERs modulate transcription indirectly, via protein : protein interactions. In this regulatory mode, which has been traced to activator protein (AP)-1-, cyclic adenosine monophosphate (cAMP)-, and Sp1-response elements, the ERs appear to be tethered to target gene promoters via heterologous transcription factors. It has been found that ERalpha and ERbeta have opposite effects on transcription mediated via the indirect mode of action. Moreover, recent studies suggest that ERbeta may inhibit the stimulatory effects of ERalpha on cellular proliferation. Estrogen is a key regulatory hormone that affects numerous physiological processes. Estrogen is required for female pubertal development and affects growth, differentiation and function of the female reproductive system. It has recently been suggested that estrogen also has an important role in the male urogenital tract. In addition, estrogens have profound effects in other tissues. For instance, in the skeleton estrogen prevents bone-resorption by inhibition of osteoclast function. Numerous reports have suggested that estrogen has a beneficial effect in the cardiovascular system and in the CNS; however, this has not been confirmed in randomized clinical trials. In fact, a large randomized trial on healthy postmenopausal women receiving oral estrogen plus progestin showed an increased incidence of cardiovascular disease. In addition, this study revealed an increased risk for dementia and impaired cognitive function in the group receiving oral estrogen/progestin. Additional clinical trials are required to determine which hormonal component causes these health risks or whether the effects were due to the combination of estrogen and progestin.

Fos and Jun inhibit estrogen-induced transcription of the human progesterone receptor gene through an activator protein-1 site

The progesterone receptor (PR) gene is activated by estrogen in normal reproductive tissues and in MCF-7 human breast cancer cells. Although it is typically thought that estrogen responsiveness is mediated through estrogen response elements (EREs), the human PR gene lacks a palindromic ERE sequence. We have identified an activating protein-1 (AP-1) site at +745 in the human PR gene that bound purified Fos and Jun and formed a complex with Fos/Jun heterodimers present in MCF-7 nuclear extracts. Surprisingly, mutating the +745 AP-1 site in the context of a 1.5-kb region of the PR gene significantly enhanced estrogen receptor (ER) alpha-mediated transactivation, suggesting that the wild-type +745 AP-1 site plays a role in inhibiting PR gene expression in the presence of hormone. In support of this idea, transient transfection assays demonstrated that increasing levels of Fos and Jun repressed transcription of a reporter plasmid containing the +745 AP-1 site. Fos levels were transiently increased, ERalpha levels were decreased, and Jun was dephosphorylated after MCF-7 cells were treated with estrogen. Chromatin immunoprecipitation assays demonstrated that Jun was associated with the +745 AP-1 site in the endogenous PR gene in the presence and in the absence of estrogen, but that ERalpha and Fos were only associated with the +745 AP-1 site after estrogen treatment of MCF-7 cells. Our studies suggest that the human PR gene is regulated by multiple transcription factors and that the differential binding of these dynamically regulated trans-acting factors influences gene expression.

Estrogen induces insulin-like growth factor-1 mRNA expression in the immortalized hippocampal cell: determination by quantitative real-time polymerase chain reaction

The basic mechanisms of nerve protection by estrogen remain to be clarified. This study was undertaken to confirm estrogen-induced insulin-like growth factor 1 (IGF-1) mRNA expression in the immortalized rat hippocampal cell H19-7 using a real-time quantitative polymerase chain reaction (PCR) assay, which has considerably increased accuracy and rapidity over other current methods. Upon stimulation by estradiol, the copy number of ERalpha mRNA showed a 1.4-fold increase, and that of IGF-1 mRNA showed a 38.5-fold increase when compared with the control value. ICI182,780 inhibited the estradiol-induced upregulation of ERalpha mRNA completely, while it inhibited estradiol-stimulated IGF-1 mRNA expression partially. The increase of the copy number of IGF-1 mRNA was accomanied by enhancement of IGF-1 protein as observed by Western blot analysis.

Mutation of Leu-536 in human estrogen receptor-alpha alters the coupling between ligand binding, transcription activation, and receptor conformation

The estrogen receptor (ER), of which there are two forms, ERalpha and ERbeta, is a ligand-modulated transcription factor important in both normal biology and as a target for agents to prevent and treat breast cancer. Crystallographic studies of the ERalpha ligand-binding domain suggest that Leu-536 may be involved in hydrophobic interactions at the start of a helix, "helix 12," that is crucial in the agonist-stimulated activity of ERalpha, as well as in the ability of antagonists to block the activity of ERalpha. We found that certain mutations of Leu-536 increased the ligand-independent activity of ERalpha although greatly reducing or eliminating the agonist activity of 17beta-estradiol (E2) and 4-hydroxytamoxifen (4OHT), on an estrogen response element-driven and an AP-1-driven reporter. The mutations impaired the interaction of the ER ligand-binding domain with the SRC1 receptor-interacting domain in a mammalian two-hybrid system. When tested in the yeast two-hybrid system, mutation of Leu-536 increased the basal reactivity of ERalpha to probes that recognize the agonist-bound conformation but did not significantly alter its reactivity to these probes in the presence of E2. Most interestingly, mutation of Leu-536 reduced the interaction of the 4OHT-bound ERalpha and increased the reactivity of the raloxifene- or ICI 182,780-bound ERalpha, with probes that recognize the 4OHT-bound ERalpha conformation in a yeast two-hybrid system. These results show that Leu-536 is critical in coupling the binding of ligand to the modulation of the conformation and activity of ERalpha.

ERbeta Binds N-CoR in the Presence of Estrogens via an LXXLL-like Motif in the N-CoR C-terminus

Nuclear receptors (NRs) usually bind the corepressors N-CoR and SMRT in the absence of ligand or in the presence of antagonists. Agonist binding leads to corepressor release and recruitment of coactivators. Here, we report that estrogen receptor beta (ERbeta) binds N-CoR and SMRT in the presence of agonists, but not antagonists, in vitro and in vivo. This ligand preference differs from that of ERalpha interactions with corepressors, which are inhibited by estradiol, and resembles that of ERbeta interactions with coactivators. ERbeta /N-CoR interactions involve ERbeta AF-2, which also mediates coactivator recognition. Moreover, ERbeta recognizes a sequence (PLTIRML) in the N-CoR C-terminus that resembles coactivator LXXLL motifs. Inhibition of histone deacetylase activity specifically potentiates ERbeta LBD activity, suggesting that corepressors restrict the activity of AF-2. We conclude that the ER isoforms show completely distinct modes of interaction with a physiologically important corepressor and discuss our results in terms of ER isoform specificity in vivo.

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.

Differential SERM effects on corepressor binding dictate ERalpha activity in vivo

Selective estrogen receptor modulators (SERMs) show differential effects upon ERalpha activation function 1 (AF-1). Tamoxifen allows strong ERalpha AF-1 activity, whereas raloxifene allows less and ICI 182,780 (ICI) allows none. Here, we show that blockade of corepressor histone de-acetylase (HDAC) activity reverses the differential inhibitory effect of SERMs upon AF-1 activity in MCF-7 cells. This suggests that differential SERM repression of AF-1 involves HDAC-dependent corepressors. Consistent with this, ICI and raloxifene are more potent than tamoxifen in promoting ERalpha-dependent sequestration of progesterone receptor-associated corepressors. Moreover, ICI and raloxifene are more efficient than tamoxifen in promoting ERalpha binding to the corepressor N-CoR in vivo and in vitro. An ERalpha mutation (537X) that increases N-CoR binding in the presence of all SERMs blocks AF-1 activity. An ERalpha mutation (L379R) that decreases N-CoR binding increases AF-1 activity in the presence of ICI and raloxifene and reverses the effect of the 537X mutation. The 537X and L379R mutations also alter the ligand preference of ERalpha action at AP-1 sites and C3 complement, an action that also involves AF-1. Together, our results suggest that differential SERM effects on corepressor binding can explain differences in SERM effects on ERalpha activity. We propose a model for differential effects of SERMs on N-CoR binding.

Chromatin exposes intrinsic differences in the transcriptional activities of estrogen receptors alpha and beta

The biological actions of estrogens are mediated via two distinct intranuclear estrogen receptor (ER) proteins, ERalpha and ERbeta. We have used an in vitro chromatin assembly and transcription system to compare the transcriptional activities of the two ERs in the context of chromatin, the physiological template for transcription by RNA polymerase II. We find that under conditions where many biochemical activities of the receptors are similar (e.g. ligand binding, chromatin binding, chromatin remodeling and co-activator recruitment), liganded ERalpha is a much more potent transcriptional activator than ERbeta with chromatin templates, but not with naked DNA. This difference is attributable to the N-terminal A/B region of ERalpha, which contains a transferable activation function that facilitates transcription specifically with chromatin templates. Interestingly, chromatin selectively restricts ligand-dependent transcriptional activation by ERbeta under some conditions (e.g. with a closed chromatin architecture), while allowing it under other conditions (e.g. with an open chromatin architecture). Collectively, our results define an important role for chromatin in determining signaling outcomes mediated by distinct subtypes of signal-transducing transcriptional activator proteins.

Differential regulation of gonadotropin-releasing hormone (GnRH)I and GnRHII messenger ribonucleic acid by gonadal steroids in human granulosa luteal cells

In humans, reproduction was generally believed to be controlled by only one form of GnRH (called mammalian GnRH or GnRHI). However, recently, a second form of GnRH, analogous to chicken GnRHII, was discovered in several tissues, including the human ovary. The regulation and function of GnRHI in the hypothalamus has been well studied. However, the function and regulation of GnRHI, and particularly GnRHII in the ovary, is less well understood. Because gonadal sex steroids are one of the main regulators of reproduction, we investigated, in the present study, the regulation of GnRHI and GnRHII mRNA expression by 17beta-estradiol (E2) and RU486 (a progesterone antagonist) in human granulosa luteal cells (hGLCs). The levels of the mRNA transcripts encoding the two GnRH forms were examined using semiquantitative RT-PCR followed by Southern blot analysis. With time in culture, GnRHI and GnRHII mRNA levels significantly increased, by 120% and 210%, at d 8 and d 1, respectively. The levels remained elevated until the termination of these experiments at d 10. A 24-h treatment of hGLCs with E2 (10(-9) to 10(-7) M) resulted in a dose-dependent decrease and increase in mRNA expression of GnRHI and GnRHII, respectively. E2 (10(-9) M) significantly decreased GnRHI mRNA levels (by 55%) and increased GnRHII mRNA levels (by 294%). Time-course studies demonstrated that E2 (10(-9) M) significantly decreased GnRHI mRNA levels in a time-dependent manner, with maximal inhibition of 77% at 48 h. In contrast, GnRHII mRNA levels significantly increased in a time-dependent fashion, reaching a maximum level of 280% at 24 h. Cotreatment of hGLCs with E2 and tamoxifen (an E2 antagonist) reversed the inhibitory and stimulatory effects of E2 on the mRNA expression of GnRHI and GnRHII, respectively. Time- and dose-dependent treatment with RU486 did not affect GnRHI mRNA levels in hGLCs. In contrast, RU486 treatment significantly increased GnRHII mRNA levels in hGLCs in a time- and dose-dependent fashion, with a maximum increase being observed at 24 h (with 10(-5)M RU486). In summary, the present study demonstrated that the expression of GnRHI and GnRHII at the transcriptional level is differently regulated by E2 and P4 in hGLCs.

Estrogens activate bone morphogenetic protein-2 gene transcription in mouse mesenchymal stem cells

Estrogens exert their physiological effects on target tissues by interacting with the estrogen receptors, ERalpha and ERbeta. Estrogen replacement is one the most common and effective strategies used to prevent osteoporosis in postmenopausal women. Whereas it was thought that estrogens work exclusively by inhibiting bone resorption, our previous results show that 17beta-estradiol (E2) increases mouse bone morphogenetic protein (BMP)-2 mRNA, suggesting that estrogens may also enhance bone formation. In this study, we used quantitative real-time RT-PCR analysis to demonstrate that estrogens increase BMP-2 mRNA in mouse mesenchymal stem cells. The selective ER modulators, tamoxifen, raloxifene, and ICI-182,780 (ICI), failed to enhance BMP-2 mRNA, whereas ICI inhibited E2 stimulation of expression. To investigate if estrogens increase BMP-2 expression by transcriptional mechanisms and if the response is mediated by ERalpha and/or ERbeta, we studied the effects of estrogens on BMP-2 promoter activity in transient transfected C3H10T1/2 cells. E2 produced a dose-dependent induction of the mouse -2712 BMP-2 promoter activity in cells cotransfected with ERalpha and ERbeta. At a dose of 10 nM E2, ERalpha induced mouse BMP-2 promoter activity 9-fold, whereas a 3-fold increase was observed in cells cotransfected with ERbeta. Tamoxifen and raloxifene were weak activators of the mouse BMP-2 promoter via ERalpha, but not via ERbeta. ICI blocked the activation of BMP-2 promoter activity by E2 acting via both ERalpha and ERbeta, indicating that mouse BMP-2 promoter activation is ER dependent. In contrast to E2 and selective ER modulators, the phytoestrogen, genistein was more effective at activating the mouse BMP-2 promoter with ERbeta, compared with ERalpha. Using a deletion series of the BMP-2 promoter, we determined that AP-1 or Sp1 sites are not required for E2 activation. A mutation in a sequence at -415 to -402 (5'-GGGCCActcTGACCC-3') that resembles the classical estrogen-responsive element abolished the activation of the BMP-2 promoter in response to E2. Our studies demonstrate that E2 activation of mouse BMP-2 gene transcription requires ERalpha or ERbeta acting via a variant estrogen-responsive element binding site in the promoter, with ERalpha being the more efficacious regulator. Estrogenic compounds may enhance bone formation by increasing the transcription of the BMP-2 gene.

Mutations in the estrogen receptor DNA-binding domain discriminate between the classical mechanism of action and cross-talk with Stat5b and activating protein 1 (AP-1)

Estrogen receptors (ERs) efficiently potentiate the transcriptional activity of prolactin-activated Stat5b through a mechanism that involves the ER DNA-binding domain (DBD) and the hinge domain. We have identified residues within the DBD of ER that are critical for the functional interaction of ER with Stat5b. We show that disruption of the second zinc finger structure abrogated cross-talk between ER and Stat5b, while the structure of the first zinc finger was not important. Furthermore, we confirm that intact DNA binding activity was not required for potentiation of Stat5b activity and that the dimerization of ER did not seem to be involved. Ligand-bound ERs also modulated activating protein 1-dependent transcription, and our data demonstrate that both zinc finger structures of the ER DBD are important for an intact response. We show that introduction of various point mutations within the DBD altered the response of the receptor to 17beta-estradiol and to the estrogen antagonists 4-hydroxytamoxifen and ICI 182,870 on the collagenase promoter. These findings provide new insights into the mechanisms by which ERs act in cross-talk with non-related transcription factors.

Design of thyroid hormone receptor antagonists from first principles

It is desirable to obtain TR antagonists for treatment of hyperthyroidism and other conditions. We have designed TR antagonists from first principles based on TR crystal structures. Since agonist ligands are buried in the fold of the TR ligand binding domain (LBD), we reasoned that ligands that resemble agonists with large extensions should bind the LBD, but would prevent its folding into an active conformation. In particular, we predicted that extensions at the 5' aryl position of ligand should reposition helix (H) 12, which forms part of the co-activator binding surface, and thereby inhibit TR activity. We have found that some synthetic ligands with 5' aryl ring extensions behave as antagonists (DIBRT, NH-3), or partial antagonists (GC-14, NH-4). Moreover, one compound (NH-3) represents the first potent TR antagonist with nanomolar affinity that also inhibits TR action in an animal model. However, the properties of the ligands also reveal unexpected aspects of TR behavior. While nuclear receptor antagonists generally promote binding of co-repressors, NH-3 blocks co-activator binding and also prevents co-repressor binding. More surprisingly, many compounds with extensions behave as full or partial agonists. We present hypotheses to explain both behaviors in terms of dynamic equilibrium of H12 position.

Inhibition of AP-1 transcription factor causes blockade of multiple signal transduction pathways and inhibits breast cancer growth

AP-1 transcription factors play a critical role in signal transduction pathways in many cells. We have investigated the role of AP-1 in controlling proliferative signals in breast cells, and have previously shown that AP-1 complexes are activated by peptide and steroid growth factors in both normal and malignant breast cells. In this study, we investigated the role of AP-1 in transducing proliferative signals induced by peptide and steroid growth factors. We used MCF-7 clones that express a specific inhibitor of AP-1, a dominant-negative cJun mutant (TAM67), under the control of an inducible promoter to investigate the role of AP-1 in regulating breast cancer growth. In the presence of doxycycline (Dox), the AP-1 inhibitor was not expressed, and the MCF-7 clones proliferated normally in response to serum stimulation. However, when Dox was withdrawn, TAM67 was expressed, AP-1 activity was inhibited, and serum-induced proliferation was blocked. We next investigated whether the mitogenic response to specific growth factors also requires AP-1. MCF-7 Tet-Off-TAM67 cells were grown in the presence of increasing concentrations of IGF-1, EGF, heregulin-beta, bFGF, or estrogen under un-induced and induced conditions. These studies showed that the AP-1 inhibitor completely blocked proliferation in response to the peptide growth factors (IGF-1, EGF, heregulin-beta, and bFGF), and partially blocked the response to estrogen. To investigate the effect of AP-1 blockade on in vivo tumor growth, we injected the MCF-7 Tet-Off TAM67 cells into nude mice receiving doxycycline to suppress the expression of the AP-1 inhibitor. After the mice developed tumors, they were randomized to either continue to receive Dox or not. In mice not receiving Dox, the expression of TAM67 was induced, and tumor growth was inhibited, while the tumors in mice receiving Dox continued to grow. Analysis of the tumors from these mice showed that the expression of TAM67 caused reduced proliferation of the breast cancer cells without inducing apoptosis. These results demonstrate that AP-1 blockade supresses mitogenic signals from multiple different peptide growth factors as well as estrogen, and inhibits the growth of MCF-7 breast cancer cells both in vitro and in vivo. These results suggest that novel agents specifically targeting AP-1 or its activating kinases could be promising agents for the treatment of breast cancer.

An estrogen receptor (ER)alpha deoxyribonucleic acid-binding domain knock-in mutation provides evidence for nonclassical ER pathway signaling in vivo

We created a nonclassical estrogen receptor (ER) knock-in mouse model by introducing a mutation that selectively eliminates classical ER signaling through estrogen response elements, while preserving the nonclassical ER pathway. Heterozygous nonclassical ER knock-in (NERKI) females are infertile. Their ovaries contain no corpora lutea, reflecting a defect in ovulation, and the stromal cells contain lipid droplets, suggesting altered steroidogenesis. The uteri are enlarged with evidence of cystic endometrial hyperplasia, and the mammary glands are hypoplastic. These phenotypic features indicate differential ER effects on growth and development in various estrogen-responsive tissues. These findings suggest that nonclassical ER signaling pathways play an important physiological role in the development and function of the reproductive system.

Distinct nongenomic signal transduction pathways controlled by 17beta-estradiol regulate DNA synthesis and cyclin D(1) gene transcription in HepG2 cells

Estrogens induce cell proliferation in target tissues by stimulating progression through the G1 phase of the cell cycle. Activation of cyclin D(1) gene expression is a critical feature of this hormonal action. The existence of rapid/nongenomic estradiol-regulated protein kinase C (PKC-alpha) and extracellular signal-regulated kinase (ERK) signal transduction pathways, their cross talk, and role played in DNA synthesis and cyclin D(1) gene transcription have been studied herein in human hepatoma HepG2 cells. 17Beta-estradiol was found to rapidly activate PKC-alpha translocation and ERK-2/mitogen-activated protein kinase phosphorylation in this cell line. These actions were independent of each other, preceding the increase of thymidine incorporation into DNA and cyclin D(1) expression, and did not involve DNA binding by estrogen receptor. The results obtained with specific inhibitors indicated that PKC-alpha pathway is necessary to mediate the estradiol-induced G1-S progression of HepG2 cells, but it does not exert any effect(s) on cyclin D(1) gene expression. On the contrary, ERK-2 cascade was strongly involved in both G1-S progression and cyclin D(1) gene transcription. Deletion of its activating protein-1 responsive element motif resulted in attenuation of cyclin D(1) promoter responsiveness to estrogen. These results indicate that estrogen-induced cyclin D(1) transcription can occur in HepG2 cells independently of the transcriptional activity of estrogen receptor, sustaining the pivotal role played by nongenomic pathways of estrogen action in hormone-induced proliferation.

Estrogen action and cytoplasmic signaling cascades. Part I: membrane-associated signaling complexes

Remarkable progress in recent years has suggested that estrogen action in vivo is complex and often involves activation of cytoplasmic signaling cascades in addition to genomic actions mediated directly through estrogen receptors alpha and beta. Rather than a linear response mediated solely through estrogen-responsive DNA elements, in vivo estrogen might simultaneously activate distinct signaling cascades that function as networks to coordinate tissue responses to estrogen. This complex signaling system provides for exquisite control and plasticity of response to estrogen at the tissue level, and undoubtedly contributes to the remarkable tissue-specific responses to estrogens. In part I of this series, we summarize cytoplasmic signaling modules involving estrogen or estrogen receptors, with particular focus on recently described membrane-associated signaling complexes.

Estrogen up-regulation of p53 gene expression in MCF-7 breast cancer cells is mediated by calmodulin kinase IV-dependent activation of a nuclear factor kappaB/CCAAT-binding transcription factor-1 complex

This study investigates the mechanism of hormonal regulation of p53 gene expression in MCF-7 human breast cancer cells. 17beta-Estradiol (E2) induced a 2-fold increase in p53 mRNA levels and a 2- to 3-fold increase in p53 protein. Analysis of the p53 gene promoter has identified a minimal E2-responsive region at -106 to -40, and mutation/deletion analysis of the promoter showed that motifs that bind CCAAT-binding transcription factor-1 (CTF-1) and nuclear factor kappaB (NFkappaB) proteins are required for hormone responsiveness. The p65 subunit of NFkappaB was identified in both nuclear and cytosolic fractions of untreated MCF-7 cells; however, formation of the nuclear NFkappaB complex was E2 independent. Hormonal activation of constructs containing p53 promoter inserts (-106 to -40) and the GAL4-p65 fusion proteins was inhibited by the intracellular Ca2+ ion chelator EGTA-AM and Ca2+/calmodulin-dependent protein kinase (CaMK) inhibitor KN-93. Constitutively active CaMKIV but not CaMKI activated p65, and treatment of MCF-7 cells with E2 induced phosphorylation of CaMKIV but not CaMKI. The results indicate that hormonal activation of p53 though nongenomic pathways was CaMKIV-dependent and involved cooperative p65-CTF-1 interactions.

Estrogen receptor beta. Potential functional significance of a variety of mRNA isoforms

Recent cloning of estrogen receptor beta (ERbeta) was followed by the discovery of a variety of its isoforms. This review describes the complexity of ERbeta mRNAs in various species for which most data have been gathered so far. The most surprising finding is the great variation in isoform structure among various mammalian species. This may reflect either the fact that only a very limited number of isoforms have been described so far or between-species specificity, especially as common elements in closely related species could still be noted. Isoform variations, as detected mainly at the mRNA sequence level, should result in profound functional differences at the level of proteins as already shown in selected cases. Thus, it is proposed that the diversity of ERbeta isoforms implies a functional role of this phenomenon in cellular physiology and pathology of estrogen response.

Opposing action of estrogen receptors alpha and beta on cyclin D1 gene expression

Induction of cyclin D1 gene transcription by estrogen receptor alpha (ERalpha) plays an important role in estrogen-mediated proliferation. There is no classical estrogen response element in the cyclin D1 promoter, and induction by ERalpha has been mapped to an alternative response element, a cyclic AMP-response element at -57, with possible participation of an activating protein-1 site at -954. The action of ERbeta at the cyclin D1 promoter is unknown, although evidence suggests that ERbeta may inhibit the proliferative action of ERalpha. We examined the response of cyclin D1 promoter constructs by luciferase assay and the response of the endogenous protein by Western blot in HeLa cells transiently expressing ERalpha, ERalphaK206A (a derivative that is superactive at alternative response elements), or ERbeta. In each case, ER activation at the cyclin D1 promoter is mediated by both the cyclic AMP-response element and the activating protein-1 site, which play partly redundant roles. The activation by ERbeta occurs only with antiestrogens. Estrogens, which activate cyclin D1 gene expression with ERalpha, inhibit expression with ERbeta. Strikingly, the presence of ERbeta completely inhibits cyclin D1 gene activation by estrogen and ERalpha or even by estrogen and the superactive ERalphaK206A. The observation of the opposing action and dominance of ERbeta over ERalpha in activation of cyclin D1 gene expression has implications for the postulated role of ERbeta as a modulator of the proliferative effects of estrogen.

Functional analysis of the rat bile salt export pump gene promoter

The 5' flanking region of the bile salt export pump (Bsep) gene was systematically analysed to provide the basis for understanding the mechanisms which regulate Bsep transcription. In addition substrates and drugs were investigated for their ability to alter Bsep promoter activity. Bsep promoter function was restricted to hepatocyte derived HepG2 cells. The 5' deletional analysis revealed a biphasic shape of reporter gene activities, indicating a suppressive element between nucleotides -800 and -512. Two consensus sites for the farnesoid X receptor (FXR) were located at nucleotides -473 and -64. The latter was characterized as functionally active in bile acid-mediated feed-back regulation of Bsep transcription. Bsep promoter activity was reduced by rifampin and beta-estradiol. The anti-estrogen tamoxifen stimulated promoter activity. Dexamethasone, hydrocortisone and phenobarbital had no effect on Bsep promoter activity. In conclusion, the data suggest that transcriptional regulation of the Bsep gene can be modulated by a number of endogenous compounds and xenobiotics. FXR was a major regulatory factor, mediating bile acid feed-back stimulation of Bsep transcription.

A novel mechanism of gene regulation and tumor suppression by the transcription factor FKHR

The mammalian DAF-16-like transcription factors, FKHR, FKHRL1, and AFX, function as key regulators of insulin signaling, cell cycle progression, and apoptosis downstream of phosphoinositide 3-kinase. Gene activation through binding to insulin response sequences (IRS) has been thought to be essential for mediating these functions. However, using transcriptional profiling, chromatin immunoprecipitation, and functional experiments, we demonstrate that rather than activation of IRS regulated genes (Class I transcripts), transcriptional repression of D-type cyclins (in Class III) is required for FKHR mediated inhibition of cell cycle progression and transformation. These data suggest that a novel mechanism of FKHR-mediated gene regulation is linked to its activity as a suppressor of tumor growth.

Estrogen regulates the development of brain-derived neurotrophic factor mRNA and protein in the rat hippocampus

During development, estrogen has a variety of effects on morphological and electrophysiological properties of hippocampal neurons. Brain-derived neurotrophic factor (BDNF) also plays an important role in the survival and differentiation of neurons during development. We examined the effects of gonadectomy with and without estrogen replacement on the mRNA and protein of BDNF and its receptor, trkB, during early postnatal development of the rat hippocampus. We used immunocytochemistry to demonstrate that estrogen receptor alpha (ERalpha) and BDNF were localized to the same cells within the developing hippocampus. BDNF and ERalpha were colocalized in pyramidal cells of the CA3 subregion and to a lesser extent in CA1. To determine whether BDNF mRNA was regulated by estrogen during development, we gonadectomized male rat pups at postnatal day 0 (P0) and examined mRNA and protein levels from P0 to P25 using real-time reverse transcription-PCR and Western blot analysis. After gonadectomy, BDNF mRNA levels are significantly reduced on P7, but after treatment of gonadectomized animals with estradiol benzoate on P0, levels at all ages were similar to those in intact animals. BDNF mRNA changes after gonadectomy are accompanied by an increase in the levels of BDNF protein, which were reduced by estrogen treatment at P0. We also examined the effect of postnatal estrogen treatment on trkB. There were no significant changes in trkB mRNA or protein in gonadectomized or estrogen-replaced animals. These results suggest that a direct interaction may exist between ERalpha and BDNF to alter hippocampal physiology during development in the rat.

Contribution of estrogen receptor alpha to oncogenic K-Ras-mediated NIH3T3 cell transformation and its implication for escape from senescence by modulating the p53 pathway

We previously reported that enhanced transcriptional activation of estrogen receptor alpha (ERalpha) contributed to [(12)Val]K-Ras-mediated NIH3T3 cell transformation. Functional inactivation of ERalpha by a dominant negative mutant of ERalpha (DNER) in the presence of activated K-Ras 4B mutant arrested the cell cycle at G(0)/G(1), subsequently provoking replicative cell senescence, finally abrogating tumorigenic potential. p53-dependent up-regulation of p21 was implicated in this cell senescence induction. Alterations in the MDM2 protein in response to DNER accounted for this p21-mediated cell senescence induction. An oncogenic K-Ras 4B mutant significantly increased MDM2 proteins coprecipitated with p53, and suppressed p53 transcriptional activity. In turn, DNER exerted its function to decrease MDM2 proteins coprecipitated with p53, followed by the stimulation of p53 activity in the presence of the oncogenic K-Ras 4B mutant. In addition, overexpression of wild type ERalpha in NIH3T3 cells resulted in the significant increase in the MDM2 protein level and the resultant suppression of p53 transcriptional activity. Finally, we demonstrated that c-Jun expression overcame the suppression and resultant enhancement of p21 protein level in response to DNER. The data imply that the ERalpha-AP1 pathway activated by oncogenic K-Ras 4B mutant contributes to the NIH3T3 cells' transformation by modulating p53 transcriptional activity through MDM2.

SKF-82958 is a subtype-selective estrogen receptor-alpha (ERalpha ) agonist that induces functional interactions between ERalpha and AP-1

The transcriptional activity of estrogen receptors (ERs) can be regulated by ligands as well as agents such as dopamine, which stimulate intracellular signaling pathways able to communicate with these receptors. We examined the ability of SKF-82958 (SKF), a previously characterized full dopamine D1 receptor agonist, to stimulate the transcriptional activity of ERalpha and ERbeta. Treatment of HeLa cells with SKF-82958 stimulated robust ERalpha-dependent transcription from an estrogen-response element-E1b-CAT reporter in the absence of estrogen, and this was accompanied by increased receptor phosphorylation. However, induction of ERbeta-directed gene expression under the same conditions was negligible. In our cell model, SKF treatment did not elevate cAMP levels nor enhance transcription from a cAMP-response element-linked reporter. Control studies revealed that SKF-82958, but not dopamine, competes with 17beta-estradiol for binding to ERalpha or ERbeta with comparable relative binding affinities. Therefore, SKF-82958 is an ERalpha-selective agonist. Transcriptional activation of ERalpha by SKF was more potent than expected from its relative binding activity, and further examination revealed that this synthetic compound induced expression of an AP-1 target gene in a tetradecanoylphorbol-13-acetate-response element (TRE)-dependent manner. A putative TRE site upstream of the estrogen-response element and the amino-terminal domain of the receptor contributed to, but were not required for, SKF-induced expression of an ERalpha-dependent reporter gene. Overexpression of the AP-1 protein c-Jun, but not c-Fos, strongly enhanced SKF-induced ERalpha target gene expression but only when the TRE was present. These studies provide information on the ability of a ligand that weakly stimulates ERalpha to yield strong stimulation of ERalpha-dependent gene expression through cross-talk with other intracellular signaling pathways producing a robust combinatorial response within the cell.

Linkage of rapid estrogen action to MAPK activation by ERalpha-Shc association and Shc pathway activation

E2 rapidly activates MAPK in breast cancer cells, and the mechanism for this effect has not been fully identified. Since growth factor-induced MAPK activation involves signaling via the adapter protein Shc (Src-homology and collagen homology) and its association with membrane receptors, we hypothesized that breast cancer cells utilize similar signaling mechanisms in response to E2. In the present study, we demonstrated that E2 rapidly induced Shc phosphorylation and Shc-Grb2 (growth factor receptor binding protein 2)-Sos (son of sevenless) complex formation in MCF-7 cells. Overexpression of dominant negative Shc blocked the effect of E2 on MAPK, indicating a critical role of Shc in E2 action. Using selective inhibitors, we also demonstrated that ERalpha and Src are upstream regulators of Shc. A rapid physical association between ERalpha and Shc upon E2 stimulation further evidenced the role of ERalpha on Shc activation. Mutagenesis studies showed that the phosphotyrosine binding and SH2 domains of Shc are required to interact with the activation function 1, but not activation function 2, domain of ERalpha. Using a glutathione-S-transferase-Shc pull-down assay, we demonstrated that this ERalpha-Shc association was direct. Biological consequences of this pathway were further investigated at the genomic and nongenomic levels. E2 stimulated MAPK-mediated Elk-1 transcriptional activity. Confocal microscopy studies showed that E2 rapidly induced formation of membrane ruffles, pseudopodia, and ERalpha membrane translocation. The E2-induced morphological changes were prevented by antiestrogen. Together our results demonstrate that ERalpha can mediate the rapid effects of E2 on Shc, MAPK, Elk-1, and morphological changes in breast cancer cells

Structure-function relationships in estrogen receptors and the characterization of novel selective estrogen receptor modulators with unique pharmacological profiles

This article summarizes recent research on the development of estrogen receptor alpha (ER alpha) and estrogen receptor beta (ER beta) subtype-selective ligands based on our understanding of structure-activity relationships in these two estrogen receptors and differences in their ligand binding domains and activation function domains. The use of these ligands should enable greater understanding of the unique biologies mediated by ER alpha versus ER beta and may, as well, provide selective estrogen receptor modulators having unique biological and pharmacological profiles optimal for prevention and treatment of breast cancer, for menopausal hormone replacement, for prevention of osteoporosis, and for potential cardiovascular benefit.

Differential modulation of transcriptional activity of estrogen receptors by direct protein-protein interactions with the T cell factor family of transcription factors

Two major signaling pathways, those triggered by estrogen (E(2)) and by the Wnt family, interact in the breast to cause growth and differentiation. The estrogen receptors ER(alpha) and ER(beta) are activated by binding E(2) and act as ligand-dependent transcription factors. The effector for the Wnt family is the Tcf family of transcription factors. Both sets of transcription factors recognize discrete but different nucleotide sequences in the promoters of their target genes. By using transient transfections of reporter constructs for the osteopontin and thymidine kinase promoters in rat mammary cells, we show that Tcf-4 antagonizes and Tcf-1 stimulates the effects of activated ER/E(2). For mutants of the former promoter, the stimulatory effects of ER(alpha)/E(2) can be made to be dependent on Tcf-1, and for the latter promoter the effects of the T cell factors (TCFs) are dependent on ER/E(2). Direct interaction between ERs and Tcfs either at the Tcf/ER(alpha)-binding site on the DNA or in the absence of DNA is established by gel retardation assays or by coimmunoprecipitation/biosensor methods, respectively. These results show that the two sets of transcription factors can interact directly, the interaction between ERs and Tcf-4 being antagonistic and that between ERs and Tcf-1 being synergistic on the activity of the promoters employed. Since Tcf-4 is the major Tcf family member in the breast, it is suggested that the antagonistic interaction is normally dominant in vivo in this tissue.

Estradiol increases IRS-1 gene expression and insulin signaling in breast cancer cells

This study demonstrates how the potentiating effects of E2 on insulin signaling in ER-positive breast cancer cells are consequent to an enhanced IRS-1 expression [corrected]. It induces an increase of both PI-3K/AKT and ERK1/2 activities. A direct action of E2 in the regulating mouse IRS-1 gene is also investigated in both Chinese hamster ovary and MCF-7 cells that are transfected with mouse IRS-1 regulatory sequences. The authors have reported, for the first time, how E2 induction of IRS-1 mRNA was correlated with a direct positive regulatory role of E2 on the IRS-1 promoter. This effect seems to be not strictly related to the cell type.

Development of an ER action indicator mouse for the study of estrogens, selective ER modulators (SERMs), and Xenobiotics

We have developed a transgenic mouse that functions as a reporter of ER activity, termed ER action indicator (ERIN), by incorporating a transgene with an estrogen-responsive promoter (three copies of the vitellogenin estrogen response element with a minimal thymidine kinase promoter) linked to the reporter gene beta-galactosidase. Evaluation of ER activity in female ERIN mice demonstrated estrogen-inducible expression of the reporter gene in the uterus, pituitary, and hypothalamus; established targets of estrogen action. Importantly, we also identified ER activity in a number of nonclassical estrogen target tissues, including kidney, liver, adrenal, and thyroid gland. ERIN provides a system to measure the same end point (transgene regulation) in different target tissues, permitting separation of the contributions of cell- and promoter-specific factors in determining ER pharmacology. In this regard we observed that on this specific promoter the pituitary gland was 25-fold more sensitive than the uterus to the estrogen diethylstilbestrol, implying the existence of cell-specific factors that influence ligand sensitivity. Our studies also identified considerable difference in the efficacy and potency of ER ligands in the uterus when ER transcriptional activity was assayed vs. uterine weight gain. Specifically, we observed that the environmental estrogen bisphenol A was a potent agonist in stimulating ER transcriptional activity, whereas it exhibited little uterotropic activity. In contrast to bisphenol A, tamoxifen significantly increased uterine weight, but minimally induced ER reporter activity in this tissue. Given the results of these studies, we believe that ERIN will be a useful model to evaluate ER ligand pharmacology and will assist in defining the cellular and molecular mechanisms that determine agonist and antagonist activity.

The role of estrogen receptors in breast cancer metastasis

It has long been appreciated that the estrogen receptor (ER) plays an important role in the biology of breast cancer. It is an accepted factor predicting favorable disease outcome and treatment response, and as such is generally considered to represent a "good" prognostic marker in breast cancer. In this review we present data suggesting that the ER may also play a pivotal role in the metastatic behavior of breast cancer, and present an argument that the up-regulation of ER and/or the selection of specific ER mutations are early events important for facilitating tumor progression. Thus, ER could serve dual roles in breast cancer, acting as a "bad" prognostic marker later in the disease process.

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.

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.

Ligand-independent activation of oestrogen receptor alpha by caveolin-1

Expression of caveolin-1 in the human mammary adenocarcinoma cell line MCF-7 causes ligand-independent concentration of oestrogen receptor alpha (ERalpha) in the nucleus, and potentiates ligand-independent and ligand-dependent transcription from an oestrogen response element-driven reporter gene. Furthermore, caveolin-1 co-immunoprecipitates with ERalpha [Schlegel, Wang, Katzenellenbogen, Pestell and Lisanti (1999) J. Biol. Chem. 274, 33551-33556]. In the present study we show that caveolin-1 binds directly to ERalpha. This interaction is mediated by residues 82-101 of caveolin-1 (i.e. the caveolin scaffolding domain) and residues 1-282 of ERalpha. The caveolin-binding domain of ERalpha includes the ligand-independent transactivation domain, activation function (AF)-1, but lacks the hormone-binding domain and the ligand-gated transactivation domain, AF-2. In co-transfection studies, caveolin-1 potentiates the transcriptional activation of ERalpha(1-282), a truncation mutant that has intact AF-1 and DNA-binding domains. Since AF-1 activity is regulated largely by phosphorylation we determined that co-expression with caveolin-1 increased the basal phosphorylation of ERalpha(1-282), but blocked the epidermal growth factor-dependent increase in phosphorylation. Indeed, caveolin-1 interacted with and potentiated the transactivation of an ERalpha mutant that cannot be phosphorylated by extracellular signal-regulated kinase (ERK)1/2 [ERalpha(Ser(118)-->Ala)]. Thus caveolin-1 is a novel ERalpha regulator that drives ERK1/2-independent phosphorylation and activation of AF-1.

Characterization of the physical interaction between estrogen receptor alpha and JUN proteins

Activated estrogen receptor alpha (ERalpha) modulates transcription triggered by the transcription factor activator protein-1 (AP-1), which consists of Jun-Jun homodimers and Jun-Fos heterodimers. Previous studies have demonstrated that the interference occurs without binding of ERalpha to DNA but probably results from protein.protein interactions. However, involvement of a direct interaction between ERalpha and AP-1 is still debated. Using glutathione S-transferase pull-down assays, we demonstrated that ERalpha bound directly to c-Jun and JunB but not to FOS family members, in a ligand-independent manner. The interaction could occur when c-Jun was bound onto DNA, as shown in a protein-protein-DNA assay. It implicated the C-terminal part of c-Jun and amino acids 259-302 present in the ERalpha hinge domain. ERalpha but not an ERalpha mutant deleted of amino acids 250-303 (ER241G), also associated with c-Jun in intact cells, in the presence of estradiol, as shown by two-hybrid and coimmunoprecipitation assays. We also show that ERalpha, c-Jun, and the p160 coactivator GRIP1 can form a multiprotein complex in vitro and in intact cells and that the ERalpha.c-Jun interaction could be crucial for the stability of this complex. VP16-ERalpha and c-Jun, which both interact with GRIP1, had synergistic effect on GAL4-GRIP1-induced transcription in the presence of estradiol, and this synergistic effect was not observed with the ERalpha mutant VP16-ER241G or when c-Fos, which bound GRIP1 but not ERalpha, was used instead of c-Jun. Finally, ER241G was inefficient for regulation of AP-1 activity, and an ERalpha truncation mutant encompassing the hinge domain had a dominant negative effect on ERalpha action. These results altogether demonstrate that ERalpha can bind to c-Jun in vitro and in intact cells and that this interaction, by stabilizing a multiprotein complex containing p160 coactivator, is likely to be involved in estradiol regulation of AP-1 responses.

ER beta inhibits proliferation and invasion of breast cancer cells

Recent studies indicate that the expression of ER beta in breast cancer is lower than in the normal breast, suggesting that ER beta could play an important role in carcinogenesis. To investigate this hypothesis, we engineered ER-negative MDA-MB-231 (human breast cancer cells) to reintroduce either ER alpha or ER beta protein with an adenoviral vector. In these cells, ER beta (as ER alpha) expression was monitored using RT-PCR and Western blot. ER beta protein was localized in the nucleus (immunocytochemistry) and able to transactivate estrogen-responsive reporter constructs in the presence of E2. ER beta and ER alpha induced the expression of several endogenous genes such as pS2, TGF alpha, or the cyclin kinase inhibitor p21 but, in contrast to ER alpha, ER beta was unable to regulate c-myc proto-oncogene expression. The pure antiestrogen ICI 164, 384 completely blocked ER alpha and ER beta estrogen-induced activities. ER beta inhibited MDA-MB-231 cell proliferation in a ligand-independent manner, whereas ER alpha inhibition of proliferation is hormone dependent. Moreover, ER beta and ER alpha decreased cell motility and invasion. Our data bring the first evidence that ER beta is an important modulator of proliferation and invasion of breast cancer cells and support the hypothesis that the loss of ER beta expression could be one of the events leading to the development of breast cancer.

An association between the estrogen-dependent hypotensive effect of ethanol and an elevated brainstem c-jun mRNA in female rats

We have recently demonstrated that chronic ethanol administration lowers blood pressure (BP) in female rats and this effect is significantly attenuated by ovariectomy. The present study investigated whether ethanol hypotension is estrogen dependent. Further, since estrogen regulates AP-1 activity, the study was extended to determine whether estrogen/c-jun interaction is involved in the estrogen-dependent hypotensive effect of ethanol. Changes in BP and heart rate (HR) were evaluated in radiotelemetered pair-fed sham-operated (SO), ovariectomized (OVX), and OVX estradiol (E2)-treated rats receiving liquid diet with or without ethanol (5%, w/v) for 12 weeks. The in situ hybridization technique was used to measure the c-jun mRNA expression in two brainstem areas, the nucleus tractus solitarius (NTS) and the rostral ventrolateral medulla (RVLM). Ethanol feeding caused significant (P<0.05) decreases in BP in SO rats that started at week 1 and reached its maximum (approximately 10 mmHg) at week 6 and remained at that level till the end of week 12. In OVX rats, ethanol had no effect on BP during the first 5 weeks after which a decrease of 5 mmHg was demonstrated and remained thereafter. Estrogen replacement (17beta-estradiol subcutaneous pellet, 14.2 microg/day) restored the hypotensive effect of ethanol to a level similar to that of SO rats both in terms of magnitude and duration. Densitometric analysis of the in situ hybridization autoradiograms revealed that OVX and E2 replacement had no effect on c-jun mRNA expression in the NTS or RVLM. Ethanol feeding produced a significant (twofold) increase in c-jun mRNA expression in the RVLM of SO rats versus no effect in the NTS. The increased expression of c-jun mRNA observed following ethanol treatment in the RVLM of SO rats was abolished in OVX rats and restored to SO levels after E2 replacement. These findings suggest a link between the estrogen-dependent hypotensive effect of chronically administered ethanol and the increased expression of c-jun mRNA in the brainstem of female rats.

Activation of estrogen receptor beta is a prerequisite for estrogen-dependent upregulation of nitric oxide synthases in neonatal rat cardiac myocytes

Physiological effects of estrogen on myocardium are mediated by two intracellular estrogen receptors, ERalpha and ERbeta, that regulate transcription of target genes through binding to specific DNA target sequences. To define the role of ERbeta in the transcriptional activation of both endothelial (eNOS) and inducible nitric oxide synthase (iNOS) in cardiac myocytes, we used the complete ER-specific antagonist R,R-tetrahydrochrysene (R,R-THC). R,R-THC inhibited activation of iNOS/eNOS promoter-luciferase reporter constructs (iNOS/eNOS-Luc) in a dose-dependent fashion in COS7 cells selectively transfected with ERbeta, but failed to influence ERalpha-mediated increase of iNOS/ eNOS-Luc. In neonatal rat cardiomyocytes transfected with eNOS-Luc or iNOS-Luc, incubation with 17betaestradiol (E2, 10(-8) M) for 24 h stimulated expression of eNOS and iNOS. R,R-THC (10(-5) M) completely inhibited this effect. Furthermore, eNOS and iNOS protein expression in cardiac myocytes induced by E2 was completely blocked by R,R-THC as shown by immunoblot analysis. Taken together, these results show that ERbeta mediates transcriptional activation of eNOS and iNOS by E2.

Estrogen receptor interaction with estrogen response elements

The estrogen receptor (ER) is a ligand-activated enhancer protein that is a member of the steroid/nuclear receptor superfamily. Two genes encode mammalian ER: ERalpha and ERbeta. ER binds to specific DNA sequences called estrogen response elements (EREs) with high affinity and transactivates gene expression in response to estradiol (E(2)). The purpose of this review is to summarize how natural and synthetic variations in the ERE sequence impact the affinity of ER-ERE binding and E(2)-induced transcriptional activity. Surprisingly, although the consensus ERE sequence was delineated in 1989, there are only seven natural EREs for which both ERalpha binding affinity and transcriptional activation have been examined. Even less information is available regarding how variations in ERE sequence impact ERbeta binding and transcriptional activity. Review of data from our own laboratory and those in the literature indicate that ERalpha binding affinity does not relate linearly with E(2)-induced transcriptional activation. We suggest that the reasons for this discord include cellular amounts of coactivators and adaptor proteins that play roles both in ER binding and transcriptional activation; phosphorylation of ER and other proteins involved in transcriptional activation; and sequence-specific and protein-induced alterations in chromatin architecture.