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

Hormonal regulation of metabolism-recent lessons learned from insulin and estrogen

Hormonal signaling plays key roles in tissue and metabolic homeostasis. Accumulated evidence has revealed a great deal of insulin and estrogen signaling pathways and their interplays in the regulation of mitochondrial, cellular remodeling, and macronutrient metabolism. Insulin signaling regulates nutrient and mitochondrial metabolism by targeting the IRS-PI3K-Akt-FoxOs signaling cascade and PGC1α. Estrogen signaling fine-tunes protein turnover and mitochondrial metabolism through its receptors (ERα, ERβ, and GPER). Insulin and estrogen signaling converge on Sirt1, mTOR, and PI3K in the joint regulation of autophagy and mitochondrial metabolism. Dysregulated insulin and estrogen signaling lead to metabolic diseases. This article reviews the up-to-date evidence that depicts the pathways of insulin signaling and estrogen-ER signaling in the regulation of metabolism. In addition, we discuss the cross-talk between estrogen signaling and insulin signaling via Sirt1, mTOR, and PI3K, as well as new therapeutic options such as agonists of GLP1 receptor, GIP receptor, and β3-AR. Mapping the molecular pathways of insulin signaling, estrogen signaling, and their interplays advances our understanding of metabolism and discovery of new therapeutic options for metabolic disorders.

Codon adaptation by synonymous mutations impacts the functional properties of the estrogen receptor-alpha protein in breast cancer cells

Oestrogen receptor-alpha (ERα) positivity is intimately associated with the development of hormone-dependent breast cancers. A major challenge in the treatment of these cancers is to understand and overcome the mechanisms of endocrine resistance. Recently, two distinct translation programmes using specific transfer RNA (tRNA) repertoires and codon usage frequencies were evidenced during cell proliferation and differentiation. Considering the phenotype switch of cancer cells to more proliferating and less-differentiated states, we can speculate that the changes in the tRNA pool and codon usage that likely occur make the ERα coding sequence no longer adapted, impacting translational rate, co-translational folding and the resulting functional properties of the protein. To verify this hypothesis, we generated an ERα synonymous coding sequence whose codon usage was optimized to the frequencies observed in genes expressed specifically in proliferating cells and then investigated the functional properties of the encoded receptor. We demonstrate that such a codon adaptation restores ERα activities to levels observed in differentiated cells, including: (a) an enhanced contribution exerted by transactivation function 1 (AF1) in ERα transcriptional activity; (b) enhanced interactions with nuclear receptor corepressor 1 and 2 [NCoR1 and NCoR2 (also known as SMRT) respectively], promoting repressive capability; and (c) reduced interactions with SRC proto-oncogene, non-receptor tyrosine kinase (Src) and phosphoinositide 3-kinase (PI3K) p85 kinases, inhibiting MAPK and AKT signalling pathway.

Hyperactivation of MAPK Induces Tamoxifen Resistance in SPRED2-Deficient ERα-Positive Breast Cancer

Simple Summary

Tamoxifen has been used for more than 40 years to treat breast tumors that are dependent on the hormone estrogen for their growth. However, resistance and recurrence of the tumors during the course of the treatment are common. Understanding the mechanisms that drive tamoxifen resistance and discovering new biomarkers for early detection are keys for designing appropriate personalized therapies. Here, we show that low levels of SPRED2 may be useful as a novel biomarker of tamoxifen resistance. We found that SPRED2 deficiency causes a hyperactivation of the mitogen-activated protein kinases (MAPKs) ERK1/ERK2, which in turn enhances estrogen signaling and diminishes the toxic effects of tamoxifen on breast cancer cells. Treatment with the ERK1/2 inhibitor, ulixertinib, could restore their sensitivity to tamoxifen. Therefore, we propose that patients with estrogen-dependent breast cancer characterized by low expression levels of SPRED2 may be candidates for a combination therapy with tamoxifen and ulixertinib.

Abstract

Breast cancer is the number one cause of cancer-related mortality in women worldwide. Most breast tumors depend on the expression of the estrogen receptor α (ERα) for their growth. For this reason, targeting ERα with antagonists such as tamoxifen is the therapy of choice for most patients. Although initially responsive to tamoxifen, about 40% of the patients will develop resistance and ultimately a recurrence of the disease. Thus, finding new biomarkers and therapeutic approaches to treatment-resistant tumors is of high significance. SPRED2, an inhibitor of the MAPK signal transduction pathway, has been found to be downregulated in various cancers. In the present study, we found that SPRED2 is downregulated in a large proportion of breast-cancer patients. Moreover, the knockdown of SPRED2 significantly increases cell proliferation and leads to tamoxifen resistance of breast-cancer cells that are initially tamoxifen-sensitive. We found that resistance occurs through increased activation of the MAPKs ERK1/ERK2, which enhances the transcriptional activity of ERα. Treatment of SPRED2-deficient breast cancer cells with a combination of the ERK 1/2 inhibitor ulixertinib and 4-hydroxytamoxifen (4-OHT) can inhibit cell growth and proliferation and overcome the induced tamoxifen resistance. Taken together, these results indicate that SPRED2 may also be a tumor suppressor for breast cancer and that it is a key regulator of cellular sensitivity to 4-OHT.

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

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

The genomic regulatory elements for estrogen receptor alpha transactivation-function-1 regulated genes

Estrogen receptor alpha (ERα) is a ligand-dependent transcription regulator, containing two transactivation functional domains, AF-1 and AF-2. The selective estrogen receptor modulators (SERMs), including 4-hydroxytamoxifen (4OHT), activate AF-1 preferentially rather than AF-2. However, it is unclear whether this specific function is related to the tissue-selective functionality of SERMs. Moreover, there is no information determining AF-1-dependent estrogenic-genes existing in tissues. We sought to identify AF-1-dependent estrogenic-genes using the AF-2 mutated knock-in (KI) mouse model, AF2ERKI. AF2ER is an AF-2 disrupted estradiol (E2)-insensitive mutant ERα, but AF-1-dependent transcription can be activated by the estrogen-antagonists, fulvestrant (ICI) and 4OHT. Gene profiling and ChIP-Seq analysis identified Klk1b21 as an ICI-inducible gene in AF2ERKI uterus. The regulatory activity was analyzed further using a cell-based reporter assay. The 5'-flanking 0.4k bp region of Klk1b21 gene responded as an ERα AF-1-dependent estrogen-responsive promoter. The 150 bp minimum ERα binding element (EBE) consists of three direct repeats. These three half-site sequences were essential for the ERα-dependent transactivation and were differentially recognized by E2 and 4OHT for the gene activation. This response was impaired when the minimum EBE was fused with a thymidine-kinase promoter but could be restored by fusion with the 100 bp minimum transcription initiation element (TIE) of Klk1b21, suggesting that the cooperative function of EBE and TIE is essential for mediating AF-1-dependent transactivation. These findings provide the first in vivo evidence that endogenous ERα AF-1 dominant estrogenic-genes exist in estrogen-responsive organs. Such findings will aid in understanding the mechanism of ERα-dependent tissue-selective activity of SERMs.

The tissue-specific effects of different 17β-estradiol doses reveal the key sensitizing role of AF1 domain in ERα activity

17β-Estradiol (E2) action can be mediated by the full-length estrogen receptor alpha (ERα66), and also by the AF1 domain-deficient ERα (ERα46) isoform, but their respective sensitivity to E2 is essentially unknown. We first performed a dose response study using subcutaneous home-made pellets mimicking either metestrus, proestrus or a pharmacological doses of E2, which resulted in plasma concentrations around 3, 30 and 600 pM, respectively. Analysis of the uterus, vagina and bone after chronic exposure to E2 demonstrated dose-dependent effects, with a maximal response reached at the proestrus-dose in wild type mice expressing mainly ERα66. In contrast, in transgenic mice harbouring only an ERα deleted in AF1, these effects of E2 were either strongly shifted rightward (10-100-fold) and/or attenuated, depending on the tissue studied. Finally, experiments in different cell lines transfected with ERα66 or ERα46 also delineated varying profiles of ERα AF1 sensitivity to E2. Altogether, this work emphasizes the importance of dose in the tissue-specific actions of E2 and demonstrates the key sensitizing role of AF1 in ERα activity.

Novel mechanism for estrogen receptor alpha modulation of murine lupus

Female sex is a risk factor for lupus. Sex hormones, sex chromosomes and hormone receptors are implicated in the pathogenic pathways in lupus. Estrogen receptor alpha (ERα) knockout (KO) mice are used for defining hormone receptor effects in lupus. Prior studies of ERα KO in lupus have conflicting results, likely due to sex hormone levels, different lupus strains and different ERα KO constructs. Our objective was to compare a complete KO of ERα vs. the original functional KO of ERα (expressing a short ERα) on disease expression and immune phenotype, while controlling sex hormone levels. We studied female lupus prone NZM2410 WT and ERα mutant mice. All mice (n = 44) were ovariectomized (OVX) for hormonal control. Groups of each genotype were estrogen (E2)-repleted after OVX. We found that OVXed NZM mice expressing the truncated ERα (ERα short) had significantly reduced nephritis and prolonged survival compared to both wildtype and the complete ERαKO (ERα null) mice, but surprisingly only if E2-repleted. ERα null mice were not protected regardless of E2 status. We observed significant differences in splenic B cells and dendritic cells and a decrease in cDC2 (CD11b+CD8^-) dendritic cells, without a concomitant decrease in cDC1 (CD11b-CD8a+) cells comparing ERα short to ERα null or WT mice. Our data support a protective role for the ERα short protein. ERα short is similar to an endogenously expressed ERα variant (ERα46). Modulating its expression/activity represents a potential approach for treating female-predominant autoimmune diseases.

Tyrosine phosphorylation regulates ERβ ubiquitination, protein turnover, and inhibition of breast cancer

Unlike estrogen receptor α (ERα) that predominantly promotes hormone-dependent breast tumor growth, ERβ exhibits antitumor effects in a variety of cancer types. We recently identified a phosphotyrosine residue in ERβ, but not ERα, that dictates ERβ transcriptional activity and antitumor function. We show here that this ER isotype-specific phosphotyrosine switch is important for regulating ERβ activity in cell proliferation, migration, and invasion. At the mechanistic level, phosphorylated ERβ, which recruits transcriptional coactivator p300, is in turn targeted by p300 for ubiquitination and proteasome-dependent protein turnover. Furthermore, ERβ-specific agonists such as S-equol enhance ERβ phosphorylation, suggesting a crosstalk between ligand- and posttranslational modification-dependent ERβ activation. Inhibition of xenograft tumor growth by S-equol is associated with reduced tumor Ki-67 expression and elevated ERβ tyrosine phosphorylation. Taken together, our data support the notion that phosphotyrosine-dependent ERβ signaling is an attractive target for anticancer treatment.

Protein folding and tRNA biology

Polypeptides can fold into tertiary structures while they are synthesized by the ribosome. In addition to the amino acid sequence, protein folding is determined by several factors within the cell. Among others, the folding pathway of a nascent polypeptide can be affected by transient interactions with other proteins, ligands, or the ribosome, as well as by the translocation through membrane pores. Particularly, the translation machinery and the population of tRNA under different physiological or adaptive responses can dramatically affect protein folding. This review summarizes the scientific evidence describing the role of translation kinetics and tRNA populations on protein folding and addresses current efforts to better understand tRNA biology. It is organized into three main parts, which are focused on: (i) protein folding in the cellular context; (ii) tRNA biology and the complexity of the tRNA population; and (iii) available methods and technical challenges in the characterization of tRNA pools. In this manner, this work illustrates the ways by which functional properties of proteins may be modulated by cellular tRNA populations.

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

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

Sex Hormones and Cardiometabolic Health: Role of Estrogen and Estrogen Receptors

With increased life expectancy, women will spend over three decades of life postmenopause. The menopausal transition increases susceptibility to metabolic diseases such as obesity, diabetes, cardiovascular disease, and cancer. Thus, it is more important than ever to develop effective hormonal treatment strategies to protect aging women. Understanding the role of estrogens, and their biological actions mediated by estrogen receptors (ERs), in the regulation of cardiometabolic health is of paramount importance to discover novel targeted therapeutics. In this brief review, we provide a detailed overview of the literature, from basic science findings to human clinical trial evidence, supporting a protective role of estrogens and their receptors, specifically ERα, in maintenance of cardiometabolic health. In so doing, we provide a concise mechanistic discussion of some of the major tissue-specific roles of estrogens signaling through ERα. Taken together, evidence suggests that targeted, perhaps receptor-specific, hormonal therapies can and should be used to optimize the health of women as they transition through menopause, while reducing the undesired complications that have limited the efficacy and use of traditional hormone replacement interventions.

Estrogen Receptor-Dependent Regulation of Dendritic Cell Development and Function

Autoimmunity, infectious diseases and cancer affect women and men differently. Because they tend to develop more vigorous adaptive immune responses than men, women are less susceptible to some infectious diseases but also at higher risk of autoimmunity. The regulation of immune responses by sex-dependent factors probably involves several non-redundant mechanisms. A privileged area of study, however, concerns the role of sex steroid hormones in the biology of innate immune cells, especially dendritic cells (DCs). In recent years, our understanding of the lineage origin of DC populations has expanded, and the lineage-committing transcription factors shaping peripheral DC subsets have been identified. Both progenitor cells and mature DC subsets express estrogen receptors (ERs), which are ligand-dependent transcription factors. This suggests that estrogens may contribute to the reported sex differences in immunity by regulating DC biology. Here, we review the recent literature and highlight evidence that estrogen-dependent activation of ERα regulates the development or the functional responses of particular DC subsets. The in vitro model of GM-CSF-induced DC differentiation shows that CD11c⁺ CD11b^int Ly6c^neg cells depend on ERα activation by estrogen for their development, and for the acquisition of competence to activate naive CD4⁺ T lymphocytes and mount a robust pro-inflammatory cytokine response to CD40 stimulation. In this model, estrogen signaling in conjunction with GM-CSF is necessary to promote early interferon regulatory factor (Irf)-4 expression in macrophage-DC progenitors and their subsequent differentiation into IRF-4^hi CD11c⁺ CD11b^int Ly6c^neg cells, closely related to the cDC2 subset. The Flt3L-induced model of DC differentiation in turn shows that ERα signaling promotes the development of conventional DC (cDC) and plasmacytoid DC (pDC) with higher capability of pro-inflammatory cytokine production in response to TLR stimulation. Likewise, cell-intrinsic ER signaling positively regulates the TLR-driven production of type I interferons (IFNs) in mouse pDCs in vivo. This effect of estrogens likely contributes to the greater proficiency of women's pDCs than men's as regards the production of type I IFNs elicited by TLR7 ligands. In summary, evidence is emerging in support of the notion that estrogen signaling regulates important aspects of cDC and pDC development and/or effector functions, in both mice and humans.

Antiestrogens: structure-activity relationships and use in breast cancer treatment

About 70% of breast tumors express estrogen receptor alpha (ERα), which mediates the proliferative effects of estrogens on breast epithelial cells, and are candidates for treatment with antiestrogens, steroidal or non-steroidal molecules designed to compete with estrogens and antagonize ERs. The variable patterns of activity of antiestrogens (AEs) in estrogen target tissues and the lack of systematic cross-resistance between different types of molecules have provided evidence for different mechanisms of action. AEs are typically classified as selective estrogen receptor modulators (SERMs), which display tissue-specific partial agonist activity (e.g. tamoxifen and raloxifene), or as pure AEs (e.g. fulvestrant), which enhance ERα post-translational modification by ubiquitin-like molecules and accelerate its proteasomal degradation. Characterization of second- and third-generation AEs, however, suggests the induction of diverse ERα structural conformations, resulting in variable degrees of receptor downregulation and different patterns of systemic properties in animal models and in the clinic.

The AF-1-deficient estrogen receptor ERα46 isoform is frequently expressed in human breast tumors

BACKGROUND

To date, all studies conducted on breast cancer diagnosis have focused on the expression of the full-length 66-kDa estrogen receptor alpha (ERα66). However, much less attention has been paid to a shorter 46-kDa isoform (ERα46), devoid of the N-terminal region containing the transactivation function AF-1. Here, we investigated the expression levels of ERα46 in breast tumors in relation to tumor grade and size, and examined the mechanism of its generation and its specificities of coregulatory binding and its functional activities.

METHODS

Using approaches combining immunohistochemistry, Western blotting, and proteomics, antibodies allowing ERα46 detection were identified and the expression levels of ERα46 were quantified in 116 ERα-positive human breast tumors. ERα46 expression upon cellular stress was studied, and coregulator bindings, transcriptional, and proliferative response were determined to both ERα isoforms.

RESULTS

ERα46 was expressed in over 70% of breast tumors at variable levels which sometimes were more abundant than ERα66, especially in differentiated, lower-grade, and smaller-sized tumors. We also found that ERα46 can be generated via internal ribosome entry site-mediated translation in the context of endoplasmic reticulum stress. The binding affinities of both unliganded and fully-activated receptors towards co-regulator peptides revealed that the respective potencies of ERα46 and ERα66 differ significantly, contributing to the differential transcriptional activity of target genes to 17β estradiol (E2). Finally, increasing amounts of ERα46 decrease the proliferation rate of MCF7 tumor cells in response to E2.

CONCLUSIONS

We found that, besides the full-length ERα66, the overlooked ERα46 isoform is also expressed in a majority of breast tumors. This finding highlights the importance of the choice of antibodies used for the diagnosis of breast cancer, which are able or not to detect the ERα46 isoform. In addition, since the function of both ERα isoforms differs, this work underlines the need to develop new technologies in order to discriminate ERα66 and ERα46 expression in breast cancer diagnosis which could have potential clinical relevance.

Mechanisms of uterine estrogen signaling during early pregnancy in mice: an update

Adherence of an embryo to the uterus represents the most critical step of the reproductive process. Implantation is a synchronized event between the blastocyst and the uterine luminal epithelium, leading to structural and functional changes for further embryonic growth and development. The milieu comprising the complex process of implantation is mediated by estrogen through diverse but interdependent signaling pathways. Mouse models have demonstrated the relevance of the expression of estrogen-modulated paracrine factors to uterine receptivity and implantation window. More importantly, some factors seem to serve as molecular links between different estrogen pathways, promoting cell growth, acting as molecular chaperones, or amplifying estrogenic effects. Abnormal expression of these factors can lead to implantation failure and infertility. This review provides an overview of several well-characterized signaling pathways that elucidates the molecular cross talk involved in the uterus during early pregnancy.

Silent Polymorphisms: Can the tRNA Population Explain Changes in Protein Properties?

Silent mutations are being intensively studied. We previously showed that the estrogen receptor alpha Ala87’s synonymous polymorphism affects its functional properties. Whereas a link has been clearly established between the effect of silent mutations, tRNA abundance and protein folding in prokaryotes, this connection remains controversial in eukaryotic systems. Although a synonymous polymorphism can affect mRNA structure or the interaction with specific ligands, it seems that the relative frequencies of isoacceptor tRNAs could play a key role in the protein-folding process, possibly through modulation of translation kinetics. Conformational changes could be subtle but enough to cause alterations in solubility, proteolysis profiles, functional parameters or intracellular targeting. Interestingly, recent advances describe dramatic changes in the tRNA population associated with proliferation, differentiation or response to chemical, physical or biological stress. In addition, several reports reveal changes in tRNAs’ posttranscriptional modifications in different physiological or pathological conditions. In consequence, since changes in the cell state imply quantitative and/or qualitative changes in the tRNA pool, they could increase the likelihood of protein conformational variants, related to a particular codon usage during translation, with consequences of diverse significance. These observations emphasize the importance of genetic code flexibility in the co-translational protein-folding process.

The effect of glucuronidation on isoflavone induced estrogen receptor (ER)α and ERβ mediated coregulator interactions

Non-prenylated isoflavone aglycones are known to have phyto-estrogenic properties and act as agonistic ligands on ERα and ERβ due to their structural resemblance to 17β-estradiol (E2). Genistein and daidzein are the two main dietary isoflavones; upon uptake they are extensively metabolized and exist nearly exclusively as their conjugated forms in biological fluids. Little is known about the effect of conjugation on the intrinsic estrogenic activities of these isoflavones. To characterize and compare the intrinsic estrogenic activities of genistein and daidzein, and their respective 7-O-glucuronide metabolites a cell-free assay system was employed that determines the ligand-induced changes in ERα- and ERβ-ligand binding domain (LBD) interactions with 154 different binding motifs derived from 66 different nuclear receptor coregulators. The glucuronides were 8 to 4400 times less potent than their respective aglycones to modulate ERα-LBD and ERβ-LBD-coregulator interactions. Glucuronidation changed the preferential activation of genistein from ERβ-LBD to ERα-LBD and further increased the slightly preferential activation of daidzein for ERα-LBD. The tested isoflavone compounds were less potent than E2 (around 5 to 1580 times for the aglycones) but modulated the LBD-coregulator interactions in a manner similar to E2. Our results show that genistein and daidzein remain agonistic ligands of ERα-LBD and ERβ-LBD in their conjugated form with a higher relative preference for ERα-LBD than the corresponding aglycones. This shift in receptor preference is of special interest as the preferential activation of ERβ is considered one of the possible modes of action underlying the supposed beneficial instead of adverse health effects of isoflavones.

The uterine and vascular actions of estetrol delineate a distinctive profile of estrogen receptor α modulation, uncoupling nuclear and membrane activation

Estetrol (E₄) is a natural estrogen with a long half-life produced only by the human fetal liver during pregnancy. The crystal structures of the estrogen receptor α (ERα) ligand-binding domain bound to 17β-estradiol (E₂) and E₄ are very similar, as well as their capacity to activate the two activation functions AF-1 and AF-2 and to recruit the coactivator SRC3. In vivo administration of high doses of E₄ stimulated uterine gene expression, epithelial proliferation, and prevented atheroma, three recognized nuclear ERα actions. However, E₄ failed to promote endothelial NO synthase activation and acceleration of endothelial healing, two processes clearly dependent on membrane-initiated steroid signaling (MISS). Furthermore, E₄ antagonized E₂ MISS-dependent effects in endothelium but also in MCF-7 breast cancer cell line. This profile of ERα activation by E₄, uncoupling nuclear and membrane activation, characterizes E₄ as a selective ER modulator which could have medical applications that should now be considered further.

Estrogen receptor (ER) was regulated by RNPC1 stabilizing mRNA in ER positive breast cancer

Estrogen receptors (ERs), including ERα and ERβ, mainly mediate the genotype effect of estrogen. ERα is highly expressed in most breast cancers. Endocrine therapy is the most effective and safety adjunctive therapy for ER positive breast cancers. RNPC1, an RNA binding protein (RBP), post-transcriptionally regulating gene expression, is emerging as a critical mechanism for gene regulation in mammalian cells. In this study, we revealed RNPC1's capability of regulating ERα expression. There was a significant correlation between RNPC1 and ERα expression in breast cancer tissues. Ectopic expression of RNPC1 could increase ERα transcript and expression in breast cancer cells, and vice versa. Consistent with this, RNPC1 was able to bind to ERα transcript to increase its stability. Furthermore, overexpression of ERα could decrease the level of RNPC1 transcript and protein. It suggested a novel mechanism by which ERα expression was regulated via stabilizing mRNA. A regulatory feedback loop between RNPC1 and ERα was proved. It indicated that RNPC1 played a crucial role in ERα regulation in ER-positive breast cancers via binding to ERα mRNA. These findings might provide new insights into breast cancer endocrine therapy and ERα research.

The transcriptional activities and cellular localization of the human estrogen receptor alpha are affected by the synonymous Ala87 mutation

Until recently, synonymous mutations (which do not change amino acids) have been much neglected. Some evidence suggests that this kind of mutations could affect mRNA secondary structure or stability, translation kinetics and protein structure. To explore deeper the role of synonymous mutations, we studied their consequence on the functional activity of the estrogen receptor alpha (ERα). The ERα is a ligand-inducible transcription factor that orchestrates pleiotropic cellular effects, at both genomic and non-genomic levels in response to estrogens. In this work we analyzed in transient transfection experiments, the activity of ERα carrying the synonymous mutation Ala87, a polymorphism involving about 5-10% of the population. In comparison to the wild type receptor, our results show that ERαA87 mutation reduces the transactivation efficiency of ERα on an ERE reporter gene while its expression level remains similar. This mutation enhances 4-OHT-induced transactivation of ERα on an AP1 reporter gene. Finally, the mutation affects the subcellular localization of ERα in a cell type specific manner. It enhances the cytoplasmic location of ERα without significant changes in non-genomic effects of E2. The functional alteration of the ERαA87 determined in this work highlights the relevance of synonymous mutations for biomedical and pharmacological points of view.

The actin/MKL1 signalling pathway influences cell growth and gene expression through large-scale chromatin reorganization and histone post-translational modifications

In addition to soluble factors, mechanical constraints and extracellular matrix stiffness are important regulators of cell fate that are mediated by cytoskeletal modifications. The EMT (epithelial-mesenchymal transition) that occurs during normal development and malignant progression is a typical example of the phenotypic switch associated with profound actin remodelling and changes in gene expression. For instance, actin dynamics control motile cell functions in EMT, in part, through regulating the subcellular localization of the myocardin-related transcription factor MKL1 (megakaryoblastic leukaemia translocation 1), a co-activator of SRF (serum-responsive factor). In the present paper, we show that MKL1 participates also to the control of the cellular switch between growth and quiescence. Experimental disconnection between MKL1 and G-actin (globular actin), by using an MKL1 mutant or enhancing the F (filamentous)-/G-actin ratio, generates a widely open chromatin state and a global increase in biosynthetic activity, classically associated with cell growth. Conversely, G-actin accumulation favours nuclear condensation and cell quiescence. These large-scale chromatin changes rely upon extensive histone modifications, exemplified by that of H3K9 (H3 Lys9) shifting from trimethylation, a heterochromatin mark, to acetylation, a mark of euchromatin. The present study provides the first evidence for a global reversible hetero/euchromatinization phenomenon triggered by the actin/MKL1 signalling pathway.

Activation of the MKL1/actin signaling pathway induces hormonal escape in estrogen-responsive breast cancer cell lines

Estrogen receptor alpha (ERα) is generally considered to be a good prognostic marker because almost 70% of ERα-positive tumors respond to anti-hormone therapies. Unfortunately, during cancer progression, mammary tumors can escape from estrogen control, resulting in resistance to treatment. In this study, we demonstrate that activation of the actin/megakaryoblastic leukemia 1 (MKL1) signaling pathway promotes the hormonal escape of estrogen-sensitive breast cancer cell lines. The actin/MKL1 signaling pathway is silenced in differentiated ERα-positive breast cancer MCF-7 and T47D cell lines and active in ERα-negative HMT-3522 T4-2 and MDA-MB-231 breast cancer cells, which have undergone epithelial-mesenchymal transition. We showed that MKL1 activation in MCF-7 cells, either by modulating actin dynamics or using MKL1 mutants, down-regulates ERα expression and abolishes E2-dependent cell growth. Interestingly, the constitutively active form of MKL1 represses PR and HER2 expression in these cells and increases the expression of HB-EGF, TGFβ, and amphiregulin growth factors in an E2-independent manner. The resulting expression profile (ER-, PR-, HER2-) typically corresponds to the triple-negative breast cancer expression profile.

SIP1/NHERF2 enhances estrogen receptor alpha transactivation in breast cancer cells

The estrogen receptor alpha (ERα) is a ligand-activated transcription factor that possesses two activating domains designated AF-1 and AF-2 that mediate its transcriptional activity. The role of AF-2 is to recruit coregulator protein complexes capable of modifying chromatin condensation status. In contrast, the mechanism responsible for the ligand-independent AF-1 activity and for its synergistic functional interaction with AF-2 is unclear. In this study, we have identified the protein Na+/H+ Exchanger RegulatoryFactor 2 (NHERF2) as an ERα-associated coactivator that interacts predominantly with the AF-1 domain of the nuclear receptor. Overexpression of NHERF2 in breast cancer MCF7 cells produced an increase in ERα transactivation. Interestingly, the presence of SRC-1 in NHERF2 stably overexpressing MCF7 cells produced a synergistic increase in ERα activity. We show further that NHERF2 interacts with ERα and SRC-1 in the promoter region of ERα target genes. The binding of NHERF2 to ERα in MCF7 cells increased cell proliferation and the ability of MCF7 cells to form tumors in a mouse model. We analyzed the expression of NHERF2 in breast cancer tumors finding a 2- to 17-fold increase in its mRNA levels in 50% of the tumor samples compared to normal breast tissue. These results indicate that NHERF2 is a coactivator of ERα that may participate in the development of estrogen-dependent breast cancer tumors.

Lysine methylation of progesterone receptor at activation function 1 regulates both ligand-independent activity and ligand sensitivity of the receptor

Progesterone receptor (PR) exists in two isoforms, PRA and PRB, and both contain activation functions AF-1 and AF-2. It is believed that AF-1 is primarily responsible for the ligand-independent activity, whereas AF-2 mediates ligand-dependent PR activation. Although more than a dozen post-translational modifications of PR have been reported, no post-translational modification on AF-1 or AF-2 has been reported. Using LC-MS/MS-based proteomic analysis, this study revealed AF-1 monomethylation at Lys-464. Mutational analysis revealed the remarkable importance of Lys-464 in regulating PR activity. Single point mutation K464Q or K464A led to ligand-independent PR gel upshift similar to the ligand-induced gel upshift. This upshift was associated with increases in both ligand-dependent and ligand-independent PR phosphorylation and PR activity due to the hyperactivation of AF-1. In contrast, mutation of Lys-464 to the bulkier phenylalanine to mimic the effect of methylation caused a drastic decrease in PR activity. Importantly, PR-K464Q also showed heightened ligand sensitivity, and this was associated with increases in its functional interaction with transcription co-regulators NCoR1 and SRC-1. These results suggest that monomethylation of PR at Lys-464 probably has a repressive effect on AF-1 activity and ligand sensitivity.

Prevention of obesity and insulin resistance by estrogens requires ERα activation function-2 (ERαAF-2), whereas ERαAF-1 is dispensable

The beneficial metabolic actions of estrogen-based therapies are mainly mediated by estrogen receptor α (ERα), a nuclear receptor that regulates gene transcription through two activation functions (AFs): AF-1 and AF-2. Using mouse models deleted electively for ERαAF-1 (ERαAF-1°) or ERαAF-2 (ERαAF-2°), we determined their respective roles in the actions of estrogens on body composition and glucose homeostasis in response to either a normal diet or a high-fat diet (HFD). ERαAF-2° males and females developed accelerated weight gain, massive adiposity, severe insulin resistance, and glucose intolerance--quite reminiscent of the phenotype observed in mice deleted for the entire ERα protein (ERα(-/-)). In striking contrast, ERαAF-1° and wild-type (wt) mice shared a similar metabolic phenotype. Accordingly, 17β-estradiol administration regulated key metabolic genes in insulin-sensitive tissues and conferred a strong protection against HFD-induced metabolic disturbances in wt and ERαAF-1° ovariectomized mice, whereas these actions were totally abrogated in ERαAF-2° and ERα(-/-) mice. Thus, whereas both AFs have been previously shown to contribute to endometrial and breast cancer cell proliferation, the protective effect of estrogens against obesity and insulin resistance depends on ERαAF-2 but not ERαAF-1, thereby delineating new options for selective modulation of ERα.

The AF-1 activation function of estrogen receptor α is necessary and sufficient for uterine epithelial cell proliferation in vivo

Estrogen receptor-α (ERα) regulates gene transcription through the 2 activation functions (AFs) AF-1 and AF-2. The crucial role of ERαAF-2 was previously demonstrated for endometrial proliferative action of 17β-estradiol (E2). Here, we investigated the role of ERαAF-1 in the regulation of gene transcription and cell proliferation in the uterus. We show that acute treatment with E2 or tamoxifen, which selectively activates ERαAF-1, similarly regulate the expression of a uterine set of estrogen-dependent genes as well as epithelial cell proliferation in the uterus of wild-type mice. These effects were abrogated in mice lacking ERαAF-1 (ERαAF-1(0)). Four weeks of E2 treatment led to uterine hypertrophy and sustained luminal epithelial and stromal cell proliferation in wild-type mice, but not in ERαAF-1(0) mice. However, ERαAF-1(0) mice still presented a moderate uterine hypertrophy essentially due to a stromal edema, potentially due to the persistence of Vegf-a induction. Epithelial apoptosis is largely decreased in these ERαAF-1(0) uteri, and response to progesterone is also altered. Finally, E2-induced proliferation of an ERα-positive epithelial cancer cell line was also inhibited by overexpression of an inducible ERα isoform lacking AF-1. Altogether, these data highlight the crucial role of ERαAF-1 in the E2-induced proliferative response in vitro and in vivo. Because ERαAF-1 was previously reported to be dispensable for several E2 extrareproductive protective effects, an optimal ERα modulation could be obtained using molecules activating ERα with a minimal ERαAF-1 action.

Estradiol promotes functional responses in inflammatory and steady-state dendritic cells through differential requirement for activation function-1 of estrogen receptor α

17β-Estradiol (E2) has been shown to regulate GM-CSF- or Flt3 ligand-driven dendritic cell (DC) development through estrogen receptor (ER) α signaling in myeloid progenitors. ERα regulates transcription of target genes through two distinct activation functions (AFs), AF-1 and AF-2, whose respective involvement varies in a cell type- or tissue-specific manner. In this study, we investigated the role of ERα AFs in the development and effector functions of inflammatory DCs, steady-state conventional DCs, and plasmacytoid DCs (pDC), using mouse lacking either AF-1 or AF-2. In agreement with previous works, we showed that E2 fostered the differentiation and effector functions of inflammatory DCs through ERα-dependent upregulation of IFN regulatory factor (IRF)-4 in GM-CSF-stimulated myeloid progenitors. Interestingly, whereas AF-1 was required for early IRF-4 upregulation in DC precursors, it was dispensable to enhance IRF-4 expression in differentiated DCs to a level compatible with the development of the more functional Ly6C(-) CD11b(+) DC subset. Presence of E2 had no effect on progenitors from either knock-in mice with 7-aa deletion in helix 12 of ERα, lacking AF-2, or ERα(-/-) mice. By contrast, in Flt3 ligand-driven DC differentiation, activation of AF-1 domain was required to promote the development of more functionally competent conventional DCs and pDCs. Moreover, lack of ERα AF-1 blunted the TLR7-mediated IFN-α response of female pDCs in vivo. Thus, our study demonstrates that ERα uses AF-1 differently in steady-state and inflammatory DC lineages to regulate their innate functions, suggesting that selective ER modulators could be used to target specific DC subsets.

Estrogenic potency of benzophenone UV filters in breast cancer cells: proliferative and transcriptional activity substantiated by docking analysis

The results from recent studies show that some benzophenones (BPs) and their hydroxylated metabolites can function as weak estrogens (E2) in the environment. However, little is known about the structure-activity relationship of these molecules. We have examined the effects of exposure to ten different BPs on the proliferation of estrogen receptor (ER)-positive breast cancer cells and on the transcriptional activity of E2-target genes. We analyzed two genes that are tightly linked with estrogen-mediated proliferation, the CXCL12 and amphiregulin genes and two classical estrogen-responsive genes, the pS2 and progesterone receptor. Significant differences in the BPs efficiency to induce cell proliferation and endogenous E2-target gene expressions were observed. Using ERE-, Sp1-, AP1- and C3-reporter genes that contain different ER-binding sites in their promoter, we also showed significant differences in the BPs efficiency in activation of the ER transactivation. Together, our analyzes showed that the most active molecule is 4-hydroxy-BP. Docking analysis of the interaction of BPs in the ligand-binding pocket of ERα suggests that the minimum structural requirement for the estrogenic activity of BPs is a hydroxyl (OH) group in the phenyl A-ring that allows interaction with Glu-353, Arg-394 or Phe-404, which enhances the stability between BPs and ERα. Our modeling also indicates a loss of interaction between the OH groups of the phenyl B-ring and His-524. In addition, the presence of some OH groups in the phenyl B-ring can create repulsion forces, which may constrain helix 12 in an unfavorable position, explaining the differential estrogenic effects of BPs. These results, together with our analysis of BPs for their potency in activation of cell proliferation and ER-mediated transcription, report an improved understanding of the mechanism and structure–activity relationship of BPs.

Cooperative activation of gene expression by agonists and antagonists mediated by estrogen receptor heteroligand dimer complexes

Estrogen receptor (ER) antagonists are generally thought to inhibit estrogen action through competitive inhibition, resulting in receptor binding to antagonist rather than agonist. However, microarray analyses reveal a group of genes for which ER agonist and antagonist cooperatively regulate expression, suggesting additional models of combined agonist/antagonist action must exist. In conjunction with a chimeric reporter gene and two modified ERs, one [ERα(GSCKV)] with a mutation in the DNA-binding domain and the other (ERα-G521R) with a ligand-binding specificity mutation, we herein demonstrate that ER agonist and antagonist cooperatively activate gene expression through an ER heteroligand dimer complex (ER-HLD) consisting of one subunit of the receptor dimer bound to agonist and another occupied by antagonist. Coimmunoprecipitation experiments confirmed interaction between the agonist-bound and antagonist-bound receptors. This cooperative activation of gene expression was enhanced by steroid receptor coactivator 3 coactivator, and required each ligand-bound subunit of the dimer to bind to DNA, as well as both activation function 1 domains for maximal transcriptional activity. Ligand combinations able to induce ER-HLD transcriptional activity include the agonists 17β-estradiol or conjugated estrogens with the antagonists tamoxifen, raloxifene, bazedoxifene, or fulvestrant. Moreover, ER-HLD can activate transcription in the context of a natural promoter. Taken together, these findings broaden our understanding of the complex relationship between ER agonist and antagonist, and suggest a novel model by which cell and tissue selective effects of antiestrogens may be achieved.

From in vivo gene targeting of oestrogen receptors to optimization of their modulation in menopause

The ancestral status of oestrogen receptor (ER) in the family of the steroid receptors has probably contributed to the pleiotropic actions of oestrogens, and in particular, that of 17β-oestradiol (E2). Indeed, in addition to their well-described role in sexual development and reproduction, they influence most of the physiological processes. The pathophysiological counterpart of these actions includes prevention of osteoporosis, atheroma and type 2 diabetes, and also the promotion of uterus and breast cancer growth. Thus, the major challenge consists in uncoupling some beneficial actions from other deleterious ones, that is, selective ER modulation. Tamoxifen and raloxifene are already used, as they prevent the recurrence of breast cancer and mimic oestrogen action mainly on bone. Both E2 and tamoxifen exhibit a proliferative and, thus, a protumoural action on the endometrium. Activation of ERα and ERβ regulates target gene transcription (genomic action) through two independent activation functions, AF-1 and AF-2, but can also elicit rapid membrane-initiated steroid signals. In the present review, we attempted to summarize recent advances provided by the in vivo molecular 'dissection' of ERα, allowing the uncoupling of some of its actions and potentially paving the way to optimized selective ER modulators.

Allosteric modulators of steroid hormone receptors: structural dynamics and gene regulation

Steroid hormones are synthesized from cholesterol primarily in the adrenal gland and the gonads and play vital roles in normal physiology, the control of development, differentiation, metabolic homeostasis, and reproduction. The actions of these small lipophilic molecules are mediated by intracellular receptor proteins. It is just over 25 yr since the first cDNA for steroid receptors were cloned, a development that led to the birth of a superfamily of ligand-activated transcription factors: the nuclear receptors. The receptor proteins share structurally and functionally related ligand binding and DNA-binding domains but possess distinct N-terminal domains and hinge regions that are intrinsically disordered. Since the original cloning experiments, considerable progress has been made in our understanding of the structure, mechanisms of action, and biology of this important class of ligand-activated transcription factors. In recent years, there has been interest in the structural plasticity and function of the N-terminal domain of steroid hormone receptors and in the allosteric regulation of protein folding and function in response to hormone, DNA response element architecture, and coregulatory protein binding partners. The N-terminal domain can exist as an ensemble of conformers, having more or less structure, which prime this region of the receptor to rapidly respond to changes in the intracellular environment through hormone binding and posttranslation modifications. In this review, we address the question of receptor structure and function dynamics with particular emphasis on the structurally flexible N-terminal domain, intra- and interdomain communications, and the allosteric regulation of receptor action.

Activation function 2 (AF2) of estrogen receptor-alpha is required for the atheroprotective action of estradiol but not to accelerate endothelial healing

17β-Estradiol (E2) regulates estrogen receptor-α (ERα) target gene transcription through the two independent activation functions (AFs), AF1 and AF2, located in the N-terminal and ligand binding domain of ERα, respectively. We previously reported that ERα is required for the E2 atheroprotective action as well as for its accelerative action on endothelial healing, but its AF1 function is dispensable. Here, we investigated the role of ERαAF2 in these two major beneficial actions of E2 by electively targeting ERαAF2 (named ERαAF2(0)). Our results prove four points. (i) Compared with WT ERα, the ability of ERαAF2(0) to stimulate the C3 complement or the estrogen response element-thymidine kinase promoter in two cell lines was dramatically decreased, confirming the importance of AF2 in the E2-induced transcriptional activity of ERα. (ii) The uterotrophic action of E2 was totally absent in ERαAF2(0) mice, showing the crucial role of ERαAF2 in E2-induced uterus hyperplasia. (iii) ERαAF2 was dispensable for the accelerative action of E2 on endothelial healing, underlining the functionality of ERαAF2(0) in vivo. (iv) Finally, the atheroprotective effect of E2 was abrogated in ERαAF2(0) LDL-r(-/-) mice. Thus, whereas ERαAF1 and ERαAF2 are both required for the uterotrophic action of E2, we show that only ERαAF2 is necessary for its atheroprotective effect.

Multiple Routes to Oestrogen Antagonism

Several lines of evidence attest to the existence of alternative ligand binding sites on the oestrogen receptor (ER), including non-competitive inhibition by trilostane or tamoxifen. It is possible that the inhibitory action of conventional oestrogen agonists at high concentrations may indicate that they too interact at alternative ER sites, albeit at low affinity. To test this possibility an oestrogen reporter assay was used to compare the activity of different oestrogens and antagonists in breast cancer and prostate cell lines. All four cell lines tested contained different amounts of oestrogen receptor α (ERα), ERβ, progesterone receptor and coregulator mRNA. Though differences were observed in response to stimulation and inhibition, these correlated only with the presence or absence of ERα, and not with the other components. Thus stimulation of the reporter by oestradiol and oestrone was biphasic in the breast cancer cells, while prostate cells were unable to respond. Only T47D cells were stimulated by oestriol or diethylstilboestrol, however reporter activity of all the cell lines was repressed by 10μM diethylstilboestrol. Reporter activity of MCF-7 cells was inhibited by tamoxifen, raloxifene and ICI 182,780, but stimulated by trilostane, yet all these antioestrogens inhibited agonist-stimulated activity. Trilostane also inhibited the agonism seen in cells co-treated with E2 and tamoxifen. It is clear that several of the compounds tested may have either agonist or antagonist effects under different conditions and at different concentrations, acting through ERα alone. Though biphasic dose response curves, or hormesis, have been attributed to various mechanisms, we here provide evidence that alternative ligand binding sites may contribute to this phenomenon.

Development and validation of a test for environmental estrogens: Checking xeno-estrogen activity by CXCL12 secretion in BREAST CANCER CELL LINES (CXCL-test)

Several methods have been developed to evaluate and quantify the effects of Endocrine disruptor chemicals (EDC). Nevertheless, most of these methods are time-consuming or not enough sensitive to detect EDC at the environmental range. To link the biological effect of tested EDC to natural protein secretion, we have developed a new screening method based on the secretion of the cytokine CXCL12 (or SDF-1, Stroma-cell Derived Factor 1), which plays a capital role in cell survival and migration. We have demonstrated that CXCL12 secretion is regulated by estrogenic compounds in a dose-dependent way in ER-positive breast cancer cell lines (MCF-7 and T47D). By combining cell culture and ELISA test, we used this up-regulation of CXCL12 secretion to test several major environmental contaminants. Our results showed that 17β-estradiol (from 10(-11) M), 17α-ethynylestradiol (from 10(-12) M), genistein (from 10(-8) M) and bisphenol A (from 10(-6) M) dose-regulate CXCL12 secretion in T47D. In contrast, antiestrogens, raloxifen and 4-hydroxytamoxifen, had no effect on the CXCL12 secretion, but were able to inhibit E2 effect. Moreover, we used cell proliferation assays to evaluate the effect of these different compounds on the growth of T47D cells. We found strong correlation (P = 0.7) between proliferation and CXCL12 secretion. However CXCL12 secretion was as sensitive as cell proliferation assays but appeared more rapid. Thus, this bioassay named CXCL-test (for Checking Xeno-estrogen activity by CXCL12 secretion in breast cancer cell Lines) constitutes a fast and sensitive method for the detection of estrogenic compounds allowing in 14 h to achieve a detection limit of 10(-11) M of E2 (2.7 ng/L).

The hinge region of the human estrogen receptor determines functional synergy between AF-1 and AF-2 in the quantitative response to estradiol and tamoxifen

Human estrogen receptors α and β (ERα and ERβ) greatly differ in their target genes, transcriptional potency and cofactor-binding capacity, and are differentially expressed in various tissues. In classical estrogen response element (ERE)-mediated transactivation, ERβ has a markedly reduced activation potential compared with ERα; the mechanism underlying this difference is unclear. Here, we report that the binding of steroid receptor coactivator-1 (SRC-1) to the AF-1 domain of ERα is essential but not sufficient to facilitate synergy between the AF-1 and AF-2 domains, which is required for a full agonistic response to estradiol (E2). Complete synergy is achieved through the distinct hinge domain of ERα, which enables combined action of the AF-1 and AF-2 domains. AF-1 of ERβ lacks the capacity to interact with SRC-1, which prevents hinge-mediated synergy between AF-1 and AF-2, thereby explaining the reduced E2-mediated transactivation of ERβ. Transactivation of ERβ by E2 requires only the AF-2 domain. A weak agonistic response to tamoxifen occurs for ERα, but not for ERβ, and depends on AF-1 and the hinge-region domain of ERα.

Comparative effects of R- and S-equol and implication of transactivation functions (AF-1 and AF-2) in estrogen receptor-induced transcriptional activity

Equol, one of the main metabolites of daidzein, is a chiral compound with pleiotropic effects on cellular signaling. This property may induce activation/inhibition of the estrogen receptors (ER) a or b, and therefore, explain the beneficial/deleterious effects of equol on estrogen-dependent diseases. With its asymmetric centre at position C-3, equol can exist in two enantiomeric forms (R- and S-equol). To elucidate the yet unclear mechanisms of ER activation/inhibition by equol, we performed a comprehensive analysis of ERa and ERb transactivation by racemic equol, as well as by enantiomerically pure forms. Racemic equol was prepared by catalytic hydrogenation from daidzein and separated into enantiomers by chiral HPLC. The configuration assignment was performed by optical rotatory power measurements. The ER-induced transactivation by R- and S-equol (0.1–10 µM) and 17b-estradiol (E2, 10 nM) was studied using transient transfections of ERα and ERβ in CHO, HepG2 and HeLa cell lines. R- and S-equol induce ER transactivation in an opposite fashion according to the cellular context. R-equol and S-equol are more potent in inducing ERα in an AF-2 and AF-1 permissive cell line, respectively. Involvement of ERα transactivation functions (AF-1 and AF-2) in these effects has been examined. Both AF-1 and AF-2 are involved in racemic equol, R-equol and S-equol induced ERα transcriptional activity. These results could be of interest to find a specific ligand modulating ER transactivation and could contribute to explaining the diversity of equol actions in vivo.

Repression of the estrogen receptor-alpha transcriptional activity by the Rho/megakaryoblastic leukemia 1 signaling pathway

Although involved in processes leading to the emergence and development of hormone-dependent breast cancers, the estrogen receptor alpha (ERalpha) also prevents transformed cells from progressing toward a more aggressive phenotype. The transcriptional activity of ERalpha is mediated through two transactivation functions, called activation function 1 and 2, whose respective involvement varies in a cell-specific manner. Here, we identify the Rho/megakaryoblastic leukemia 1 (MKL1) signaling pathway as a main actor in controlling the cell-specific activity of both transactivation functions of ERalpha. Notably, we show that, when the coregulator MKL1 is sequestered in an inactive form by unpolymerized actin, the transcriptional activity of ERalpha mainly relies on the activation function 1. The activation of MKL1, which results from its dissociation from unpolymerized actin, promoted by the ability of Rho to support polymeric actin accumulation, silences the activation function 1 of ERalpha and allows the receptor to mainly act through its activation function 2. Importantly, this switch in the respective contribution exerted by both transactivation functions is correlated with an impaired ability of ERalpha to efficiently transactivate estrogen-regulated reporter genes. MKL1 is further shown to be present on estrogen-responsive genes in vivo. Interestingly, the Rho/MKL1 signaling pathway is activated during the epithelial-mesenchymal transition. A reduced transactivation efficiency of ERalpha, resulting from the activation of this pathway, may therefore suppress the protective role exerted by ERalpha toward tumor progression and invasiveness.

Respective contribution exerted by AF-1 and AF-2 transactivation functions in estrogen receptor alpha induced transcriptional activity by isoflavones and equol: consequence on breast cancer cell proliferation

Estrogens used in hormone replacement therapy regimens may increase the risk of developing breast cancer. Paradoxically, high consumption of plant-derived phytoestrogens, particularly soybean isoflavones, is associated with a low incidence of breast cancer. To explore the molecular basis for these potentially different experimental/clinical outcomes, we investigated whether soybean isoflavones elicit distinct transcriptional actions from estrogens by performing transient transfections in different cell lines. Our results demonstrate that 17beta estradiol (E2), isoflavones, and equol (EQ) effectively trigger the transcriptional activation with both estrogen receptors (ER), ER alpha and ER beta. ER alpha transcriptional activity is mediated through two transactivation domains AF-1 and AF-2, whose activity is tightly regulated in a cell-type and promoter-specific manner. Isoflavones, genistein, and daidzein (DAI), and EQ, the main estrogenic metabolite of DAI, are ER alpha agonists for transcriptional activation. The molecular mechanisms for ER alpha-induced transcriptional activity by isoflavones and EQ involve their capacity to act mainly through AF-1 regardless of the cell type. Therefore, our data indicate that estrogenic ligands, such as isoflavones and EQ, exert their effects on ER alpha transactivation similarly to the endogenous ligand E2, and suggest that the risk of estrogen-related diseases might not be reduced by soy-rich food and/or isoflavone- or EQ-based supplements.

Activity and intracellular location of estrogen receptors alpha and beta in human bronchial epithelial cells

Gender differences in lung disease and cancer are well-established. We reported estrogenic transcriptional responses in lung adenocarcinoma cells from females but not males despite similar estrogen receptor (ER) expression. Here we tested the hypothesis that normal human bronchial epithelial cells (HBECs) show gender-independent estrogenic responses. We report that a small sample of HBECs express approximately twice as much ERbeta as ERalpha. ERalpha and ERbeta were located in the cytoplasm, nucleus, and mitochondria. In contrast to lung adenocarcinoma cells, estradiol (E2) induced estrogen response element (ERE)-mediated luciferase reporter activity in transiently transfected HBECs regardless of donor gender. Overexpression of ERalpha-VP16 increased ERE-mediated transcriptional activity in all HBECs. E2 increased and 4-hydroxytamoxifen and ICI 182,780 inhibited HBEC proliferation and cyclin D1 expression in a cell line-specific manner. In conclusion, the response of HBECs to ER ligands is gender-independent suggesting that estrogenic sensitivity may be acquired during lung carcinogenesis.

Characterization of a cis-acting element involved in cell-specific expression of the zebrafish brain aromatase gene

The cytochrome P450 Aromatase is the key enzyme catalyzing the conversion of androgens into estrogens. In zebrafish, the brain aromatase is encoded by cyp19b. Expression of cyp19b is restricted to radial glial cells bordering forebrain ventricles and is strongly stimulated by estrogens during development. At the promoter level, we have previously shown that an estrogen responsive element (ERE) is required for induction by estrogens. Here, we investigated the role of ERE flanking regions in the control of cell-specific expression. First, we show that a 20 bp length motif, named G x RE (glial x responsive element), acts in synergy with the ERE to mediate the estrogenic induction specifically in glial cells. Second, we demonstrate that, in vitro, this sequence binds factors exclusively present in glial or neuro-glial cells and is able to confer a glial specificity to an artificial estrogen-dependent gene. Taken together, these results contribute to the understanding of the molecular mechanisms allowing cyp19b regulation by estrogens and allowed to identify a promoter sequence involved in the strong estrogen inducibility of cyp19b which is specific for glial cells. The exceptional aromatase activity measured in the brain of teleost fish could rely on such mechanisms.

High-throughput cell-based screening reveals a role for ZNF131 as a repressor of ERalpha signaling

BACKGROUND

Estrogen receptor alpha (ERalpha) is a transcription factor whose activity is affected by multiple regulatory cofactors. In an effort to identify the human genes involved in the regulation of ERalpha, we constructed a high-throughput, cell-based, functional screening platform by linking a response element (ERE) with a reporter gene. This allowed the cellular activity of ERalpha, in cells cotransfected with the candidate gene, to be quantified in the presence or absence of its cognate ligand E2.

RESULTS

From a library of 570 human cDNA clones, we identified zinc finger protein 131 (ZNF131) as a repressor of ERalpha mediated transactivation. ZNF131 is a typical member of the BTB/POZ family of transcription factors, and shows both ubiquitous expression and a high degree of sequence conservation. The luciferase reporter gene assay revealed that ZNF131 inhibits ligand-dependent transactivation by ERalpha in a dose-dependent manner. Electrophoretic mobility shift assay clearly demonstrated that the interaction between ZNF131 and ERalpha interrupts or prevents ERalpha binding to the estrogen response element (ERE). In addition, ZNF131 was able to suppress the expression of pS2, an ERalpha target gene.

CONCLUSION

We suggest that the functional screening platform we constructed can be applied for high-throughput genomic screening candidate ERalpha-related genes. This in turn may provide new insights into the underlying molecular mechanisms of ERalpha regulation in mammalian cells.

Estrogen receptor beta isoform-specific induction of transforming growth factor beta-inducible early gene-1 in human osteoblast cells: an essential role for the activation function 1 domain

The estrogen receptors (ER) alpha and beta are important ligand-mediated transcription factors known to play significant biological roles in numerous tissues including bone. Despite the high homology shared by these receptors, recent studies have suggested that their function is largely unique. Although these receptors have been studied in detail for more than a decade, little data exist concerning the mechanisms by which these two proteins regulate distinct sets of genes. Using the TGFbeta-inducible early gene-1 (TIEG) as a model, we demonstrate that TIEG is rapidly induced in response to estrogen in osteoblasts by ERbeta, but not ERalpha. We have identified the regulatory elements utilized by ERbeta and have demonstrated that ERbeta recruits steroid receptor coactivator (SRC)1 and SRC2 to this regulatory region. Additionally, deletion of the ERbeta-activation function 1 (AF1) domain drastically decreases the estrogen induction of TIEG. Through the use of chimeric receptors, we have demonstrated that the AF1 domain of ERbeta is responsible for recruiting SRC1 and SRC2 and inducing the expression of TIEG in osteoblasts. Finally, SRC1, but not SRC2, is essential for TIEG induction by ERbeta. Overall, these data demonstrate that the estrogen induction of TIEG is ERbeta specific and that the AF1 domain of ERbeta confers this specificity. Finally, a novel and important role for ERbeta's AF1 is implicated in the recruitment of specific coactivators, suggesting that the AF1 may play a significant role in conferring the differences in regulation of gene expression by these two receptors.

Enterodiol and enterolactone, two major diet-derived polyphenol metabolites have different impact on ERalpha transcriptional activation in human breast cancer cells

Lignans are plant compounds metabolized in the mammalian gut to produce the estrogenic enterolignans, enterodiol (ED) and enterolactone (EL). Because estrogens have been linked to breast cancer etiology, enterolignans could affect breast cancer risk, but to our knowledge, the mechanisms by which they exert their estrogenic and/or anti-estrogenic effects in humans are still unclear. To better understand how estrogenic compounds from the food, such as the enterolignans, might influence breast cancer progression and their mechanisms to interfere with human estrogen receptor (ER) signalling in hormone-dependant diseases, we examined and compared the ability of ED, EL and 17beta-estradiol (E2) to induce the transactivation of ERalpha and ERbeta, to modulate ERalpha target genes, to exert either growth stimulatory or anti-proliferative effects and finally to modulate MCF-7 cell migration by acting on matrix metalloproteases (MMP)-2 and -9, at concentrations that are achievable through a lignan-rich diet. This study indicates that enterolignans show distinct properties for transactivation of ERalpha and ERbeta. ED, as E2, induces ERalpha transcriptional activation through transactivation functions AF-1 and AF-2, while EL is less efficient in inducing AF-1, acting predominantly through AF-2. Furthermore, ED and EL modulate ERalpha mRNA and protein contents as well as MCF-7 cell proliferation and secreted MMP activities in a different way. Enterolignans are compounds of wide interest nowadays and our results help to unveil their mechanisms of action on ER, emphasizing the fact that the dietary load in lignans could be of importance in the balance between being risk or chemopreventive factors for breast cancer and women's health.

Translational machinery and protein folding: evidence of conformational variants of the estrogen receptor alpha

As an approach to understand how translation may affect protein folding, we analyzed structural and functional properties of the human estrogen receptor alpha synthesized by different eukaryotic translation systems. A minimum of three conformations of the receptor were detected using limited proteolysis and a sterol ligand-binding assay. The receptor in vitro translated in rabbit reticulocyte lysate was rapidly degraded by protease, produced major bands of about 34kDa and showed a high affinity for estradiol. In a wheat germ translation system, the receptor was more slowly digested. Two soluble co-existing conformations were evident by different degradation patterns and estradiol binding. Our data show that differences in the translation machinery may result in alternative conformations of the receptor with distinct sterol binding properties. These studies suggest that components of the cellular translation machinery itself might influence the protein folding pathways and the relative abundance of different receptor conformers.