The Relative Contribution Exerted by AF-1 and AF-2 Transactivation Functions in Estrogen Receptor α Transcriptional Activity Depends upon the Differentiation Stage of the Cell

COUP-TFI modulates estrogen signaling and influences proliferation, survival and migration of breast cancer cells

We previously showed that COUP-TFI interacts with the Estrogen Receptor alpha (ER alpha) to recruit Extracellular signal Regulated Kinases (ERKs) in an Estradiol (E2)-independent manner, resulting in an enhancement of ER alpha transcriptional activity. However, the involvement of COUP-TFI in physiologically relevant functions of ER alpha, such as the mitogenic activity that E2 has on breast cancer cells, remains poorly understood. Here, we first showed that the amounts of COUP-TFI protein are higher in dedifferentiated mammary cell lines (MDA-MB-231) and tumor breast cells as compared to the differentiated MCF-7 cell line and normal breast cells. To evaluate the functional relevance of the COUP-TFI/ER alpha interplay in mammary cells, we generated MCF-7 cells that stably over-express COUP-TFI. We found that the over-expression of COUP-TFI enhances motility and invasiveness of MCF-7 cells. COUP-TFI also promotes the proliferation of MCF-7 cells through ER alpha-dependent mechanisms that target cell cycle progression and cell survival. To further investigate the mechanisms underlying these effects of COUP-TFI, we evaluated the expression of known E2-target genes in breast cancer, and found that COUP-TFI differentially regulated genes involved in cell proliferation, apoptosis, and migration/invasion. Notably, Cathepsin D (CTSD) transcript and protein levels were significantly higher in presence and absence of E2 in MCF-7 over-expressing COUP-TFI. Chromatin Immunoprecipitation assays showed that ER alpha, phospho-RNA Polymerase II, as well as p68 RNA Helicase, a phospho-Serine 118 dependent co-activator of ER alpha, were preferentially recruited onto the CTSD gene proximal promoter in COUP-TFI over-expressing cells. These results suggest that COUP-TFI selectively regulates the expression of endogenous E2-target genes and consequently modifies ER alpha positive mammary cells response to E2.

Oestrogen receptors pathways to oestrogen responsive elements: the transactivation function-1 acts as the keystone of oestrogen receptor (ER)beta-mediated transcriptional repression of ERalpha

Oestrogen receptors (ER)alpha and beta modify the expression of genes involved in cell growth, proliferation and differentiation through binding to oestrogen response elements (EREs) located in a number of gene promoters. Transient transfection of different luciferase reporter vectors 3xEREs-Vit, 2xEREs-tk and ERE-C3 showed that the transactivation capacity of both ER subtypes was influenced by 1) the nature of the inducer (oestradiol (E2), phyto- and anti-oestrogen (AE)), 2) the structure of the promoter (nucleotidic sequence, number of ERE, length of the promoter sequence) and 3) the cell line (containing endogenous ER (MCF-7) or in which ER was stably expressed (MDA-MB-231-HE-5 (ERalpha+) or MDA-MB-231-HERB (ERbeta+)). ER subtype did not display the same efficacy on the different constructions in the presence of E2 and of AE according to the cell (e.g. in MCF-7 cells: tk>>Vit>>C3 approximately 0 while in MDA-MB-231 cells: Vit>>tk approximately C3). E2 response was higher in MCF-7 cells, probably due to higher ER expression level (maximal at 10(-10)M instead of 10(-8)M for E2 in HE-5 cells). Finally, the same ligand could exert opposite activities on the same promoter according to the ER isoform expressed: in the MDA-MB-231 cells, AE acted as inducers of the C3 promoter via ERbeta whereas ERalpha/AE complexes down-regulated this promoter. Approximately 70% of breast tumours express ER and most tumour cells coexpress both ER isotypes. Thus, different types of ER dimers can be formed in such tumours (ERbeta or ERalpha homodimers or ERalpha/ERbeta heterodimers). We therefore studied the influence of the coexistence of the two ERs on the ligand-induced transcriptional process following transient transfection of ERalpha in ERbeta+ cells, and inversely ERbeta in ERalpha+ cells. ERbeta-transfection inhibited the E2- and genistein-induced ERalpha-dependent transcription on all promoters in all cell lines except C3 in MCF-7; this inhibitory effect was lost following transfection of ERbeta deleted of its AF-1 (ERbeta-AF-2). These results suggest that the dominant negative properties of ERbeta are mainly due to its AF-1 function. Interestingly, transfection of an ERbeta-AF-2 construct into MCF-7 cells potentiated the transcription inhibitory capacity of 4-OH-tamoxifen (OHT) on the Vit and tk promoters. Thus, (1) OHT exerts an agonistic activity through the AF-1 function of ER and (2) expression of ERbeta in breast cancer cells seems to favour the AE treatment. Contrary to ERbeta, ERalpha-transfection had little effect on ERbeta transactivation capacity in HERB cells. Finally, the ratio ERalpha/ERbeta constitutes one decisive parameters to orientate the transcriptional mechanism of a target gene in the presence of agonist as well as of antagonist ligands.

Estrogen Receptor Isoform-specific Regulation of the Retinoblastoma-binding Protein 1 (RBBP1) Gene

Estrogen (E2) is involved in mediating many important functions relevant to osteoblast biology through the actions of the estrogen receptors (ER) alpha and beta. To further understand the mechanisms of ER-specific regulation, we used microarray and reverse transcription-PCR analyses of E2-treated U2OS-ERalpha or -ERbeta cells and identified retinoblastoma-binding protein 1 (RBBP1) as a major E2-regulated gene. RBBP1 is a retinoblastoma cofactor involved in the control of osteoblastic proliferation. Although RBBP1 mRNA levels rapidly increased after 2 h of E2 treatment in both U2OS-ER-expressing lines, a sustained induction was only observed in U2OS-ERalpha cells. Examination of the RBBP1 genomic sequence revealed an ER response element and a Sp1 site located within the first intron. Chromatin immunoprecipitation analyses demonstrated that E2-dependent ERalpha binding to the intron 1 enhancer region was constitutive, whereas ERbeta binding was transient, consistent with the mRNA time course. Interestingly, transient transfection and receptor mutational studies revealed that RBBP1 induction by ERalpha only requires the Sp1 site, whereas ERbeta utilizes both the Sp1 and estrogen response elements binding sites for maximal E2-dependent activation. Stable U2OS transfectants containing a deletion of the ERalpha activation function 1 (AF1) resulted in a temporal mRNA induction profile similar to that of wild type ERbeta. Further, overexpression and chromatin immunoprecipitation analyses also demonstrated that E2-dependent RBBP1 induction is SRC2-dependent for both ER isoforms. These results describe an E2-dependent, ER isoform-specific transcriptional activation of the RBBP1 gene, which in part, is explained by the differential activity of ER AF1 and enhancer element binding.

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

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

Signaling regulation of genomic and nongenomic functions of estrogen receptors

Estrogen receptors (ERs) mediate the effects of 17beta-estradiol under physiologic and pathologic conditions. ERs trigger 17beta-estradiol-sensitive gene transcription by binding to specific estrogen response elements (i.e. genomic mechanism). The cellular effects of estrogen are also influenced by membrane- or cytoplasm-initiated responses (i.e. nongenomic mechanism). Both ER-evoked genomic and nongenomic effects originate from a unique signaling network. Furthermore, estrogen-initiated rapid pathways and ERalpha interactions with specific partners (e.g. AIB1, PELP1/MNAR; MTA1, MTA1s and p130Cas) influence both ER functions. Here, we summarize the recent findings related to multiple regulatory levels of the signaling networks responsible for ERs-mediated responses in breast cancer cells.

Unliganded estrogen receptor-alpha activates transcription of the mammary gland Na+/I- symporter gene

The function of sodium iodide symporter (Na(+)/I(-) symporter, or NIS) in mammary epithelial cells is essential for the accumulation of I(-) in milk; the newborn's first source of I(-) for thyroid hormone synthesis. Furthermore, increased mammary gland NIS expression has previously been shown in human breast cancer. Several hormones and factors including all-trans-retinoic acid (tRA) regulate the expression of NIS. In this study, using breast cancer cell lines, we established that tRA-responsive NIS expression is confined to estrogen receptor-alpha (ERalpha) positive cells and we investigated the role of ERalpha in the regulation of NIS expression. We showed that the suppression of endogenous ERalpha by RNA interference downregulates NIS expression in ERalpha positive mammary cells. Besides, in an ERalpha negative cell line, reintroduction of ERalpha resulted in the expression of NIS in a ligand-independent manner. We also identified a novel estrogen-responsive element in the promoter region of NIS that specifically binds ERalpha and mediates ERalpha-dependent activation of transcription. Our results indicate that unliganded ERalpha (apo-ERalpha) contributes to the regulation of NIS gene expression.

Assessment of xenoestrogens using three distinct estrogen receptors and the zebrafish brain aromatase gene in a highly responsive glial cell system

The brain cytochrome P450 aromatase (Aro-B) in zebrafish is expressed in radial glial cells and is strongly stimulated by estrogens (E2); thus, it can be used in vivo as a biomarker of xenoestrogen effects on the central nervous system. By quantitative real-time polymerase chain reaction, we first confirmed that the expression of Aro-B gene is robustly stimulated in juvenile zebrafish exposed to several xenoestrogens. To investigate the impact of environmental estrogenic chemicals on distinct estrogen receptor (ER) activity, we developed a glial cell-based assay using Aro-B as the target gene. To this end, the ER-negative glial cell line U251-MG was transfected with the three zebrafish ER subtypes and the Aro-B promoter linked to a luciferase reporter gene. E2 treatment of U251-MG glial cells cotransfected with zebrafish ER-alpha and the Aro-B promoter-luciferase reporter resulted in a 60- to 80-fold stimulation of luciferase activity. The detection limit was <0.05 nM, and the EC50 (median effective concentration) was 1.4 nM. Interestingly, in this glial cell context, maximal induction achieved with the Aro-B reporter was three times greater than that observed with a classical estrogen-response-element reporter gene (ERE-tk-Luc). Dose-response analyses with ethynylestradiol (EE2), estrone (E1), alpha-zeralenol, and genistein showed that estrogenic potency of these agents markedly differed depending on the ER subtype in the assay. Moreover, the combination of these agents showed an additive effect according to the concept of concentration addition. This confirmed that the combined additive effect of the xenoestrogens leads to an enhancement of the estrogenic potency, even when each single agent might be present at low effect concentrations. In conclusion, we demonstrate that our bioassay provides a fast, reliable, sensitive, and efficient test for evaluating estrogenic potency of endocrine disruptors on ER subtypes in a glial context.

Gender difference in the activity but not expression of estrogen receptors alpha and beta in human lung adenocarcinoma cells

The higher frequency of lung adenocarcinoma in women smokers than in men smokers suggests a role for gender-dependent factors in the etiology of lung cancer. We evaluated estrogen receptor (ER) alpha and beta expression and activity in human lung adenocarcinoma cell lines and normal lung fibroblasts. Full-length ERalpha and ERbeta proteins were expressed in all cell lines with higher ERbeta than ERalpha. Although estradiol (E(2)) binding was similar, E(2) stimulated proliferation only in cells from females, and this response was inhibited by anti-estrogens 4-hydroxytamoxifen (4-OHT) and ICI 182,780. In contrast, E(2) did not stimulate replication of lung adenocarcinoma cells from males and 4-OHT or ICI did not block cell proliferation. Similarly, transcription of an estrogen response element-driven reporter gene was stimulated by E(2) in lung adenocarcinoma cells from females, but not males. Progesterone receptor (PR) expression was increased by E(2) in two out of five adenocarcinoma cell lines from females, but none from males. E(2) decreased E-cadherin protein expression in some of the cell lines from females, as it did in MCF-7 breast cancer cells, but not in the cell lines from males. Thus, ERalpha and ERbeta expression does not correlate with the effect of ER ligands on cellular activities in lung adenocarcinoma cells. On the other hand, coactivator DRIP205 expression was higher in lung adenocarcinoma cells from females versus males and higher in adenocarcinoma cells than in normal human bronchial epithelial cells. DRIP205 and other ER coregulators may contribute to differences in estrogen responsiveness between lung adenocarcinoma cells in females and males.

The cell-specific activity of the estrogen receptor alpha may be fine-tuned by phosphorylation-induced structural gymnastics

The estrogen receptor α (ERα) regulates the transcription of target genes by recruiting coregulator proteins through several domains including the two activation functions AF1 and AF2. The contribution of the N-terminally located AF1 activity is particularly important in differentiated cells, and for ERα to integrate inputs from other signaling pathways. However, how the phosphorylation of key residues influences AF1 activity has long remained mysterious, in part because the naturally disordered AF1 domain has resisted a structural characterization. The recent discovery of two coregulators that are specific for a phosphorylated form of AF1 suggests that phosphorylation, possibly in conjunction with the subsequent binding of these coregulators, may enforce a stable structure. The binding of the "pioneer" coregulators might facilitate the subsequent recruitment of yet other coregulators. Different AF1 folds may be enabled by the combinatorial action of posttranslational modifications and coregulator binding thereby fine-tuning ERα activities in a cell- and promoter-specific fashion.

The human estrogen receptor-alpha isoform hERalpha46 antagonizes the proliferative influence of hERalpha66 in MCF7 breast cancer cells

The expression of two human estrogen receptor-alpha (hERalpha) isoforms has been characterized within estrogen receptor-alpha-positive breast cancer cell lines such as MCF7: the full-length hERalpha66 and the N terminally deleted hERalpha46, which is devoid of activation function (AF)-1. Although hERalpha66 is known to mediate the mitogenic effects that estrogens have on MCF7 cells, the exact function of hERalpha46 in these cells remains undefined. Here we show that, during MCF7 cell growth, hERalpha46 is mainly expressed in the nucleus at relatively low levels, whereas hERalpha66 accumulates in the nucleus. When cells reach confluence, the situation reverses, with hERalpha46 accumulating within the nucleus. Although hERalpha46 expression remains rather stable during an estrogen-induced cell cycle, its overexpression in proliferating MCF7 cells provokes a cell-cycle arrest in G(0)/G(1) phases. To gain further details on the influence of hERalpha46 on cell growth, we used PC12 estrogen receptor-alpha-negative cell line, in which stable transfection of hERalpha66 but not hERalpha46 allows estrogens to behave as mitogens. We next demonstrate that, in MCF7 cells, overexpression of hERalpha46 inhibits the hERalpha66-mediated estrogenic induction of all AF-1-sensitive reporters: c-fos and cyclin D1 as well as estrogen-responsive element-driven reporters. Our data indicate that this inhibition occurs likely through functional competitions between both isoforms. In summary, hERalpha46 antagonizes the proliferative action of hERalpha66 in MCF7 cells in part by inhibiting hERalpha66 AF-1 activity.

Aryl hydrocarbon receptor-mediated transcription: ligand-dependent recruitment of estrogen receptor alpha to 2,3,7,8-tetrachlorodibenzo-p-dioxin-responsive promoters

Using chromatin immunoprecipitation assays, we studied the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-mediated recruitment of the aryl hydrocarbon receptor (AhR) and several co-regulators to the CYP1A1 promoter. AhR displayed a time-dependent recruitment, reaching a peak at 75 min and maintaining promoter occupancy for the remainder of the time course. Recruitment of AhR was followed by TIF2/SRC2, which preceded CBP, histone H3 acetylation, and RNA polymerase II (RNAPII). Simultaneous recruitment to the enhancer and the TATA box region suggests the formation of a large multiprotein complex bridging the two promoter regions. Interestingly, estrogen receptor alpha (ERalpha) displayed a TCDD- and time-dependent recruitment to the CYP1A1 promoter, which was increased by co-treatment with estradiol. Transfection in HuH7 human liver cells confirmed previously reported ERalpha enhancement of AhR activity. In contrast, TCDD did not induce the recruitment of ERalpha to the estrogen-responsive pS2 promoter, and after 120 min of co-treatment with estradiol, ERalpha is still present on the CYP1A1 promoter but no longer at pS2. RNA interference studies with T47D cells support a role for ERalpha in TCDD-dependent CYP1A1 expression. Our data suggest that ERalpha acts as a coregulator of AhR-mediated transcriptional activation and that the recruitment of ERalpha by AhR represents a novel mechanism AhR-ERalpha cross talk.

The activation function-1 domain of estrogen receptor α in uterine stromal cells is required for mouse but not human uterine epithelial response to estrogen

The activation function-1 (AF-1) domain of the estrogen receptor alpha (ERalpha) in stromal cells has been shown to be required for epithelial responses to estrogen in the mouse uterus. To investigate the role of the stroma in estrogenic responses of human uterine epithelium (hUtE), human/mouse chimeric uteri composed of human epithelium and mouse stroma were prepared as tissue recombinants (TR) that were grown in vivo under the renal capsule of female nude mouse hosts. In association with mouse uterine stroma (mUtS), hUtE formed normal glands surrounded by mouse endometrial stroma and the human epithelium influenced the differentiation of stroma into myometrium, such that a histologically normal appearing uterine tissue was formed. The hUtE showed a similar proliferative response and increase in progesterone receptors (PR) in response to 17beta-estradiol (E2) in association with either human or mUtS, as TRs. However, under identical endocrine and micro-environmental conditions, hUtE required 5-7 days exposure to E2 rather than 1 day, as shown for mouse uterine epithelium, to obtain a maximal proliferative response. Moreover, this extended length of E2 exposure inhibited mouse epithelial proliferation in the presence of mouse stroma. In addition, unlike the mouse epithelium, which does not proliferate or show regulation of PR expression in response to E2 in association with uterine stroma derived from mice that are null for the AF-1 domain of ERalpha, hUtE proliferates and PR are up-regulated in response to E2 in association genetically identical ERalpha knock-out mouse stromal cells. These results clearly demonstrate fundamental differences between mouse and human uterine epithelia with respect to the mechanisms that regulate estrogen-induced proliferation and expression of PR. Moreover, we show that genetically engineered mouse models could potentially aid in dissecting molecular pathways of stromal epithelial interactions in the human uterus.

SPBP is a phosphoserine-specific repressor of estrogen receptor alpha

Multiple signaling pathways stimulate the activity of estrogen receptor alpha (ERalpha) by direct phosphorylation within its N-terminal activation function 1 (AF1). How phosphorylation affects AF1 activity remains poorly understood. We performed a phage display screen for human proteins that are exclusively recruited to the phosphorylated form of AF1 and found the stromelysin-1 platelet-derived growth factor-responsive element-binding protein (SPBP). In a purified system, SPBP bound only the in vitro-phosphorylated form of the ERalpha AF1 or the phosphoserine mimic S118E, and the interaction domain could be mapped to a 42-amino-acid fragment of SPBP. In cells, SPBP preferentially interacted with liganded and phosphorylated ERalpha. Functionally, SPBP behaved as a repressor of activated ERalpha, which extends its previously demonstrated roles as a DNA binding transactivation factor and coactivator of other transcription factors. By targeting the phosphorylated form of AF1, SPBP may contribute to attenuating and fine-tuning ERalpha activity. A functional consequence is that SPBP inhibits the proliferation of ERalpha-dependent but not ERalpha-independent breast cancer cell lines, mirroring a reported negative correlation with the ERalpha status of breast tumors.

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

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

Transcriptional complexes engaged by apo-estrogen receptor-alpha isoforms have divergent outcomes

Unliganded (apo-) estrogen receptor alpha (ERalpha, NR3A1) is classically considered as transcriptionally unproductive. Reassessing this paradigm demonstrated that apo-human ERalpha (ERalpha66) and its N-terminally truncated isoform (ERalpha46) are both predominantly nuclear transcription factors that cycle on the endogenous estrogen-responsive pS2 gene promoter in vivo. Importantly, isoform-specific consequences occur in terms of poising the promoter for transcription, as evaluated by determining (i) the engagement of several cofactors and the resulting nucleosomal organization; and (ii) the CpG methylation state of the pS2 promoter. Although transcriptionally unproductive, cycling of apo-ERalpha66 prepares the promoter to respond to ligand, through sequentially targeting chromatin remodeling complexes and general transcription factors. Additionally, apo-ERalpha46 recruits corepressors, following engagement of cofactors identical to those recruited by apo-ERalpha66. Together, these data describe differential activities of ERalpha isoforms. Furthermore, they depict the maintenance of a promoter in a repressed state as a cyclical process that is intrinsically dependent on initial poising of the promoter.