Alternative initiation of translation accounts for a 67/45 kDa dimorphism of the human estrogen receptor ERalpha

Progress in the Understanding of Estrogen Receptor Alpha Signaling in Triple-Negative Breast Cancer: Reactivation of Silenced ER-α and Signaling through ER-α36

Triple-negative breast cancer (TNBC) is an aggressive tumor that accounts for approximately 15% of total breast cancer cases. It is characterized by poor prognosis and high rate of recurrence compared to other types of breast cancer. TNBC has a limited range of treatment options that include chemotherapy, surgery, and radiation due to the absence of estrogen receptor alpha (ER-α) rendering hormonal therapy ineffective. However, possible targets for improving the clinical outcomes in TNBC exist, such as targeting estrogen signaling through membranous ER-α36 and reactivating silenced ER-α. It has been shown that epigenetic drugs such as DNA methyltransferase and histone deacetylase inhibitors can restore the expression of ER-α. This reactivation of ER-α, presents a potential strategy to re-sensitize TNBC to hormonal therapy. Also, this review provides up-to-date information related to the direct involvement of miRNA in regulating the translation of ER-α mRNA. Specific epi-miRNAs can regulate ER-α expression indirectly by post-transcriptional targeting of mRNAs of enzymes that are involved in DNA methylation and histone deacetylation. Furthermore, ER-α36, an alternative splice variant of ER-α66, is highly expressed in ER-negative breast tumors and activates MAPK/ERK pathway, promoting cell proliferation, escaping apoptosis, and enhancing metastasis. In the future, these recent advances may be helpful for researchers working in the field to obtain novel treatment options for TNBC, utilizing epigenetic drugs and epi-miRNAs that regulate ER-α expression. Also, there is some evidence to suggest that drugs that decrease the expression of ER-α36 may be effective in treating TNBC.

Alternative promoters and splicing create multiple functionally distinct isoforms of oestrogen receptor alpha in breast cancer and healthy tissues

BACKGROUND

Oestrogen receptor alpha (ER) is involved in cell growth and proliferation and functions as a transcription factor, a transcriptional coregulator, and in cytoplasmic signalling. It affects, for example, bone, endometrium, ovaries and mammary epithelium. It is a key biomarker in clinical management of breast cancer, where it is used as a prognostic and treatment-predictive factor, and a therapeutical target. Several ER isoforms have been described, but transcript annotation in public databases is incomplete and inconsistent, and functional differences are not well understood.

METHODS

We have analysed short- and long-read RNA sequencing data from breast tumours, breast cancer cell lines, and normal tissues to create a comprehensive annotation of ER transcripts and combined it with experimental studies of full-length protein and six alternative isoforms.

RESULTS

The isoforms have varying transcription factor activity, subcellular localisation, and response to the ER-targeting drugs tamoxifen and fulvestrant. Antibodies differ in ability to detect alternative isoforms, which raises concerns for the interpretation of ER-status in routine pathology.

CONCLUSIONS

Future work should investigate the effects of alternative isoforms on patient survival and therapy response. An accurate annotation of ER isoforms will aid in interpretation of clinical data and inform functional studies to improve our understanding of the ER in health and disease.

Estradiol Signaling at the Heart of Folliculogenesis: Its Potential Deregulation in Human Ovarian Pathologies

Estradiol (E2) is a major hormone controlling women fertility, in particular folliculogenesis. This steroid, which is locally produced by granulosa cells (GC) within ovarian follicles, controls the development and selection of dominant preovulatory follicles. E2 effects rely on a complex set of nuclear and extra-nuclear signal transduction pathways principally triggered by its nuclear receptors, ERα and ERβ. These transcription factors are differentially expressed within follicles, with ERβ being the predominant ER in GC. Several ERβ splice isoforms have been identified and display specific structural features, which greatly complicates the nature of ERβ-mediated E2 signaling. This review aims at providing a concise overview of the main actions of E2 during follicular growth, maturation, and selection in human. It also describes the current understanding of the various roles of ERβ splice isoforms, especially their influence on cell fate. We finally discuss how E2 signaling deregulation could participate in two ovarian pathogeneses characterized by either a follicular arrest, as in polycystic ovary syndrome, or an excess of GC survival and proliferation, leading to granulosa cell tumors. This review emphasizes the need for further research to better understand the molecular basis of E2 signaling throughout folliculogenesis and to improve the efficiency of ovarian-related disease therapies.

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.

Targeting Estrogens and Various Estrogen-Related Receptors against Non-Small Cell Lung Cancers: A Perspective

Non-small cell lung cancers (NSCLCs) account for ~85% of lung cancer cases worldwide. Mammalian lungs are exposed to both endogenous and exogenous estrogens. The expression of estrogen receptors (ERs) in lung cancer cells has evoked the necessity to evaluate the role of estrogens in the disease progression. Estrogens, specifically 17β-estradiol, promote maturation of several tissue types including lungs. Recent epidemiologic data indicate that women have a higher risk of lung adenocarcinoma, a type of NSCLC, when compared to men, independent of smoking status. Besides ERs, pulmonary tissues both in healthy physiology and in NSCLCs also express G-protein-coupled ERs (GPERs), epidermal growth factor receptor (EGFRs), estrogen-related receptors (ERRs) and orphan nuclear receptors. Premenopausal females between the ages of 15 and 50 years synthesize a large contingent of estrogens and are at a greater risk of developing NSCLCs. Estrogen-ER/GPER/EGFR/ERR-mediated activation of various cell signaling molecules regulates NSCLC cell proliferation, survival and apoptosis. This article sheds light on the most recent achievements in the elucidation of sequential biochemical events in estrogen-activated cell signaling pathways involved in NSCLC severity with insight into the mechanism of regulation by ERs/GPERs/EGFRs/ERRs. It further discusses the success of anti-estrogen therapies against NSCLCs.

Estrogen receptor-α signaling in post-natal mammary development and breast cancers

17β-estradiol controls post-natal mammary gland development and exerts its effects through Estrogen Receptor ERα, a member of the nuclear receptor family. ERα is also critical for breast cancer progression and remains a central therapeutic target for hormone-dependent breast cancers. In this review, we summarize the current understanding of the complex ERα signaling pathways that involve either classical nuclear “genomic” or membrane “non-genomic” actions and regulate in concert with other hormones the different stages of mammary development. We describe the cellular and molecular features of the luminal cell lineage expressing ERα and provide an overview of the transgenic mouse models impacting ERα signaling, highlighting the pivotal role of ERα in mammary gland morphogenesis and function and its implication in the tumorigenic processes. Finally, we describe the main features of the ERα-positive luminal breast cancers and their modeling in mice.

Expression and clinical implications of estrogen receptors in thoracic malignancies: a narrative review

Thoracic malignancies represent a significant global health burden with incidence and mortality increasing year by year. Thoracic cancer prognosis and treatment options depend on several factors, including the type and size of the tumor, its location, and the overall health status of patients. Gender represents an important prognostic variable in thoracic malignancies. One of the greatest biological differences between women and men is the presence of female sex hormones, and an increasing number of studies suggest that estrogens may play either a causative or a protective role in thoracic malignancies. Over the past 60 years since the discovery of the first nuclear estrogen receptor (ER) isoform α and the almost 20 years since the discovery of the second estrogen receptor, ERβ, different mechanisms governing estrogen action have been identified and characterized. This literature review reports the published data regarding the expression and function of ERs in different thoracic malignancies and discuss sex disparity in clinical outcomes. From this analysis emerges that further efforts are warranted to better elucidate the role of sex hormones in thoracic malignancies, and to reduce disparities in care between genders. Understanding the mechanisms by which gender-related differences can affect and interfere with the onset and evolution of thoracic malignancies and impact on response to therapies could help to improve the knowledge needed to develop increasingly personalized and targeted treatments.

HMGA1a Induces Alternative Splicing of the Estrogen Receptor-αlpha Gene by Trapping U1 snRNP to an Upstream Pseudo-5' Splice Site

Objectives: The high-mobility group A protein 1a (HMGA1a) protein is known as a transcription factor that binds to DNA, but recent studies have shown it exerts novel functions through RNA-binding. We were prompted to decipher the mechanism of HMGA1a-induced alternative splicing of the estrogen receptor alpha (ERα) that we recently reported would alter tamoxifen sensitivity in MCF-7 TAMR1 cells.

Methods: Endogenous expression of full length ERα66 and its isoform ERα46 were evaluated in MCF-7 breast cancer cells by transient expression of HMGA1a and an RNA decoy (2′-O-methylated RNA of the HMGA1a RNA-binding site) that binds to HMGA1a. RNA-binding of HMGA1a was checked by RNA-EMSA. In vitro splicing assay was performed to check the direct involvement of HMGA1a in splicing regulation. RNA-EMSA assay in the presence of purified U1 snRNP was performed with psoralen UV crosslinking to check complex formation of HMGA1a-U1 snRNP at the upstream pseudo-5′ splice site of exon 1.

Results: HMGA1a induced exon skipping of a shortened exon 1 of ERα in in vitro splicing assays that was blocked by the HMGA1a RNA decoy and sequence-specific RNA-binding was confirmed by RNA-EMSA. RNA-EMSA combined with psoralen UV crosslinking showed that HMGA1a trapped purified U1 snRNP at the upstream pseudo-5′ splice site.

Conclusions: Regulation of ERα alternative splicing by an HMGA1a-trapped U1 snRNP complex at the upstream 5′ splice site of exon 1 offers novel insight on 5′ splice site regulation by U1 snRNP as well as a promising target in breast cancer therapy where alternative splicing of ERα is involved.

Translational control of the undifferentiated phenotype in ER‑positive breast tumor cells: Cytoplasmic localization of ERα and impact of IRES inhibition

Using a series of potential biomarkers relevant to mechanisms of protein synthesis, we observed that estrogen receptor (ER)-positive breast tumor cells exist in two distinct yet interconvertible phenotypic states (of roughly equal proportion) which differ in the degree of differentiation and use of IRES-mediated translation. Nascently translated IGF1R in the cytoplasm positively correlated with IRES activity and the undifferentiated phenotype, while epitope accessibility of RACK1, an integral component of the 40S ribosomal subunit, aligned with the more differentiated IRES-off state. When deprived of soluble growth factors, the entire tumor cell population shifted to the undifferentiated phenotype in which IRES-mediated translation was active, facilitating survival under these adverse microenvironmental conditions. However, if IRES-mediated translation was inhibited, the cells instead were forced to transition uniformly to the more differentiated state. Notably, cytoplasmic localization of estrogen receptor α (ERα/ESR1) precisely mirrored the pattern observed with nascent IGF1R, correlating with the undifferentiated IRES-active phenotype. Inhibition of IRES-mediated translation resulted in both a shift in ERα to the nucleus (consistent with differentiation) and a marked decrease in ERα abundance (consistent with the inhibition of ERα synthesis via its IRES). Although breast tumor cells tolerated forced differentiation without extensive loss of their viability, their reproductive capacity was severely compromised. In addition, CDK1 was decreased, connexin 43 eliminated and Myc translation altered as a consequence of IRES inhibition. Isolated or low-density ER-positive breast tumor cells were particularly vulnerable to IRES inhibition, losing the ability to generate viable cohesive colonies, or undergoing massive cell death. Collectively, these results provide further evidence for the integral relationship between IRES-mediated translation and the undifferentiated phenotype and demonstrate how therapeutic manipulation of this specialized mode of protein synthesis may be used to limit the phenotypic plasticity and incapacitate or eliminate these otherwise highly resilient breast tumor cells.

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.

PRMT5 regulates IRES-dependent translation via methylation of hnRNP A1

The type II arginine methyltransferase PRMT5 is responsible for the symmetric dimethylation of histone to generate the H3R8me2s and H4R3me2s marks, which correlate with the repression of transcription. However, the protein level of a number of genes (MEP50, CCND1, MYC, HIF1a, MTIF and CDKN1B) are reported to be downregulated by the loss of PRMT5, while their mRNA levels remain unchanged, which is counterintuitive for PRMT5's proposed role as a transcription repressor. We noticed that the majority of the genes regulated by PRMT5, at the posttranscriptional level, express mRNA containing an internal ribosome entry site (IRES). Using an IRES-dependent reporter system, we established that PRMT5 facilitates the translation of a subset of IRES-containing genes. The heterogeneous nuclear ribonucleoprotein, hnRNP A1, is an IRES transacting factor (ITAF) that regulates the IRES-dependent translation of Cyclin D1 and c-Myc. We showed that hnRNP A1 is methylated by PRMT5 on two residues, R218 and R225, and that this methylation facilitates the interaction of hnRNP A1 with IRES RNA to promote IRES-dependent translation. This study defines a new role for PRMT5 regulation of cellular protein levels, which goes beyond the known functions of PRMT5 as a transcription and splicing regulator.

Mechanistic Pathways of Sex Differences in Cardiovascular Disease

Major differences between men and women exist in epidemiology, manifestation, pathophysiology, treatment, and outcome of cardiovascular diseases (CVD), such as coronary artery disease, pressure overload, hypertension, cardiomyopathy, and heart failure. Corresponding sex differences have been studied in a number of animal models, and mechanistic investigations have been undertaken to analyze the observed sex differences. We summarize the biological mechanisms of sex differences in CVD focusing on three main areas, i.e., genetic mechanisms, epigenetic mechanisms, as well as sex hormones and their receptors. We discuss relevant subtypes of sex hormone receptors, as well as genomic and nongenomic, activational and organizational effects of sex hormones. We describe the interaction of sex hormones with intracellular signaling relevant for cardiovascular cells and the cardiovascular system. Sex, sex hormones, and their receptors may affect a number of cellular processes by their synergistic action on multiple targets. We discuss in detail sex differences in organelle function and in biological processes. We conclude that there is a need for a more detailed understanding of sex differences and their underlying mechanisms, which holds the potential to design new drugs that target sex-specific cardiovascular mechanisms and affect phenotypes. The comparison of both sexes may lead to the identification of protective or maladaptive mechanisms in one sex that could serve as a novel therapeutic target in one sex or in both.

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.

ERα36--Another piece of the estrogen puzzle

Although the nuclear action of estrogen receptors (ER) is a well-known fact, evidence supporting membrane estrogen receptors is steadily accumulating. New ER variants of unrecognized function have been discovered. ERα is a product of the ESR1 gene. It serves not only as a template for the full-length 66kDa protein, but also for smaller isoforms which exist as independent receptors. The recently discovered ERα36 (36kDa), consisting of 310 amino acids of total 595 ERα66 protein residues, is an example of that group. The transcription initiation site is identified in the first intron of the ESR1 gene. C-Terminal 27 amino acids are encoded by previously unknown exon 9. The presence of this unique C-terminal sequence creates an opportunity for the production of selective antibodies. ERα36 has been shown to have a high affinity to the cell membrane and as much as 90% of the protein can be bound with it. Post-translational palmitoylation is suspected to play a crucial role in ERα36 anchoring to the cell membrane. In silico analysis suggests the existence of a potential transmembrane domain in ERα36. ERα36 was found in most cells of animals at various ages, but its exact physiological function remains to be fully elucidated. It seems that cells traditionally considered as being deprived of ER are able to respond to hormonal stimulation via the ERα36 receptor. Moreover, ERα36 displays unique pharmacological properties and its action may be behind antiestrogen resistance. The use of ERα36 in cancer diagnosis gives rise to great expectations.

Cellular processing of the glucocorticoid receptor gene and protein: new mechanisms for generating tissue-specific actions of glucocorticoids

Glucocorticoids regulate numerous physiological processes and are mainstays in the treatment of inflammation, autoimmune disease, and cancer. The traditional view that glucocorticoids act through a single glucocorticoid receptor (GR) protein has changed in recent years with the discovery of a large cohort of receptor subtypes arising from alternative processing of the GR gene. These isoforms differ in their expression, gene regulatory, and functional profiles. Post-translational modification of these proteins further expands GR diversity. Here, we discuss the origin and molecular properties of the GR isoforms and their contribution to the sensitivity and specificity of the glucocorticoid response.

The transactivating function 1 of estrogen receptor alpha is dispensable for the vasculoprotective actions of 17beta-estradiol

Full-length 66-kDa estrogen receptor alpha (ERalpha) stimulates target gene transcription through two activation functions (AFs), AF-1 in the N-terminal domain and AF-2 in the ligand binding domain. Another physiologically expressed 46-kDa ERalpha isoform lacks the N-terminal A/B domains and is consequently devoid of AF-1. Previous studies in cultured endothelial cells showed that the N-terminal A/B domain might not be required for estradiol (E2)-elicited NO production. To evaluate the involvement of ERalpha AF-1 in the vasculoprotective actions of E2, we generated a targeted deletion of the ERalpha A/B domain in the mouse. In these ERalphaAF-1(0) mice, both basal endothelial NO production and reendothelialization process were increased by E2 administration to a similar extent than in control mice. Furthermore, exogenous E2 similarly decreased fatty streak deposits at the aortic root from both ovariectomized 18-week-old ERalphaAF-1(+/+) LDLr(-/-) (low-density lipoprotein receptor) and ERalphaAF-1(0) LDLr (-/-) mice fed with a hypercholesterolemic diet. In addition, quantification of lesion size on en face preparations of the aortic tree of 8-month-old ovariectomized or intact female mice revealed that ERalpha AF-1 is dispensable for the atheroprotective action of endogenous estrogens. We conclude that ERalpha AF-1 is not required for three major vasculoprotective actions of E2, whereas it is necessary for the effects of E2 on its reproductive targets. Thus, selective ER modulators stimulating ERalpha with minimal activation of ERalpha AF-1 could retain beneficial vascular actions, while minimizing the sexual effects.

Generation and characterization of a complete null estrogen receptor alpha mouse using Cre/LoxP technology

Conventional estrogen receptor alpha knockout (neo-ERalphaKO, neo-ERalpha(-/-)) mice contain a truncated and chimeric ERalpha fusion protein that retains 35% estrogen-dependent transactivation activity, and therefore the in vivo ERalpha function is difficult to study thoroughly. Furthermore, these neo-ERalpha(-/-) mice cannot be used for tissue and temporal specific ERalpha deletion. Therefore, there is a clear need to establish a floxed ERalpha mouse line that can knockout ERalpha specifically and completely in each selected cell type. Here we generated floxed ERalpha mice using a self-excising ACN (tACE-Cre/Neo) cassette. Mating the floxed ERalpha mice with ACTB-Cre mice produces a deletion of the floxed allele disrupting the reading frame of the ERalpha transcript so that no ERalpha protein is detected in the ACTB-Cre/ERalpha(-/-) mice. Expression of ERalpha target genes, such as G-6-PD and lactoferrin, is diminished by over 90% in the ACTB-Cre/ERalpha(-/-) uterus, but not in the neo-ERalpha(-/-) uterus. Furthermore, we also validated that ACTB-Cre/ERalpha(-/-) females have a hypoplastic internal genital tract, polycystic ovaries with hemorrhagic follicles, infertility, and higher body weight. Together, our data clearly demonstrate that the newly established floxed ERalpha mouse is a reliable mouse model for future studies of ERalpha roles in vivo in the selective estrogen target tissues. The complete knockout of ERalpha in the ACTB-Cre/ERalpha(-/-) mice will also provide an improved mouse model to study the role of ERalpha in vivo.

Age-dependent ERα MB1 splice variant expression in discrete areas of the human brain

A role of estrogens in brain aging and Alzheimer's disease (AD) is a hot topic of research. We show in material of 71 patients that the estrogen receptor alpha (ERalpha) splice variant MB1 is expressed at the protein and mRNA level in the human brain. MB1 is mainly confined to astrocytes, membranes and cytoplasm of projecting neurons and endothelial cells. It was consistently observed in the thalamus, colliculus inferior, pontine nuclei, dorsal motor nucleus of vagus, some motor neurons in the anterior and lateral horns of the spinal cord and rarely in pyramidal neurons of the cerebral cortex. The highest level of MB1 immunoreactivity (MB1-ir) was noted in the caudal hypothalamus, in particular in the tuberomamillary nucleus (TMN). MB1-ir in the TMN increased during aging in women. MB1-ir was higher in young (<50 years of age) men than in premenopausal women. No significant changes of this variant were observed in the TMN of AD cases. In conclusion, MB1 may function as a dominant negative isoform in the human brain.

Estrogen receptor-alpha splice variants in the human brain

In the present review we discuss recent findings showing that, in addition to the canonical estrogen receptor-alpha (ERalpha), the level of various ERalpha splice variants is changed in the human brain in aging and Alzheimer's disease (AD) at both the mRNA and protein level and that they should be considered for the understanding of estrogen effects on the brain and estrogen therapy pitfalls. Indeed, the expression pattern of certain splice forms is brain area-specific. Thus, the major isoform found in the mamillary body (MB) appeared to be del.7 (deletion of exon 7), while in the hippocampus del.4 (exon 4 omitted) was expressed at the highest level. Furthermore, while transcripts missing exons 7 and 2 declined with aging in the MB of patients >60 years old, no age-related alterations were determined for a number of splice variants in the hippocampus. A novel MB1 isoform with a 168-bp deletion within the transactivation function 1 of ERalpha turned out to accumulate in the histaminergic tuberomamillary nucleus of postmenopausal women. Finally, the level of alternatively spliced ERalpha may also change in AD in a brain area-specific manner and so affect the sensitivity to estrogen therapy.

The embryonic pregnancy signal oestradiol influences gene expression at the level of translational initiation in porcine endometrial cells

In the pig, conceptus-derived oestrogens (days 11 and 12 of pregnancy) seem to be a critical component of the signalling mechanism for maternal recognition of pregnancy. Embryonic oestrogens can mediate effects on endometrial function by interactions with epithelial and stromal oestrogen receptors (ER). Recent data demonstrate that cell membrane ER interacts with the phosphatidylinositol 3-kinase/Akt pathway in several types of cells. The protein kinase Akt is involved in the control of cell growth, survival and proliferation. One distinct function of the Akt signalling cascade is its ability to phosphorylate the eukaryotic initiation factor-4E (eIF-4E)-binding protein 1 (4E-BP1). This phosphorylation suppresses the inhibitory effect of 4E-BP1 on the translation initiation factor eIF4E and in such a way potentially stimulates gene expression at the level of translational initiation. The aim of the present study was to examine if embryonic oestradiol (E(2)) transmits its effect by such a mechanism. Endometrial cells of cyclic gilts (day 13 of the oestrous cycle, n = 4) were cultured and supplemented with vehicle (control), E(2) (50 and 100 pm/l) or with the selective ER modulator raloxifen (10 and 1000 nm/l), and incubated for 24 h. The cell viability was detected by MTT assay, the abundance and phosphorylation of Akt, 4E-BP1 and ERalpha was analysed by Western blotting. Incubation with E(2) or raloxifen did not alter endometrial cell viability. The phosphorylation of Akt at Ser(473) seems to be increased by E(2) (p < 0.05) and decreased by raloxifen (p > 0.05). Raloxifen (1000 nm/l) induced a band shift in 4E-BP1 to the highest electrophoretic mobility which reflects a decrease in phosphorylation (p < 0.05), whereas an influence of E(2) on 4E-BP1 phosphorylation could not be detected. The decrease (p < 0.05) of the abundance of the 80 kDa ERalpha form both by E(2) and raloxifen indicates that the E(2)-stimulated Akt phosphorylation and the inhibition of 4E-BP1 phosphorylation by raloxifen is an E(2) ER-transmitted process. Therefore, embryonic oestrogens can potentially transmit their effect by influencing signalling cascades which modulate gene expression at the level of translational initiation.

Searching for IRES

The cell has many ways to regulate the production of proteins. One mechanism is through the changes to the machinery of translation initiation. These alterations favor the translation of one subset of mRNAs over another. It was first shown that internal ribosome entry sites (IRESes) within viral RNA genomes allowed the production of viral proteins more efficiently than most of the host proteins. The RNA secondary structure of viral IRESes has sometimes been conserved between viral species even though the primary sequences differ. These structures are important for IRES function, but no similar structure conservation has yet to be shown in cellular IRES. With the advances in mathematical modeling and computational approaches to complex biological problems, is there a way to predict an IRES in a data set of unknown sequences? This review examines what is known about cellular IRES structures, as well as the data sets and tools available to examine this question. We find that the lengths, number of upstream AUGs, and %GC content of 5'-UTRs of the human transcriptome have a similar distribution to those of published IRES-containing UTRs. Although the UTRs containing IRESes are on the average longer, almost half of all 5'-UTRs are long enough to contain an IRES. Examination of the available RNA structure prediction software and RNA motif searching programs indicates that while these programs are useful tools to fine tune the empirically determined RNA secondary structure, the accuracy of de novo secondary structure prediction of large RNA molecules and subsequent identification of new IRES elements by computational approaches, is still not possible.

Heart estrogen receptor alpha: distinct membrane and nuclear distribution patterns and regulation by estrogen

Estrogen receptor alpha (ERalpha) is present in the heart consistent with estrogen-induced modulation of cardiac function by genomic and non-genomic mechanisms, and with estrogen-mediated cardioprotective effects. We show that, in heart from adult male rats, ERalpha is detected mainly as two distinct isoforms: (i) a approximately 66 kDa isoform with the expected mass of the classical full-length ERalpha and (ii) an additional isoform of approximately 45 kDa. Differential centrifugation separated the 66 kDa isoform into the cytosolic fraction; while the 45 kDa isoform was enriched in the membrane fraction. High-resolution confocal studies show that ERalpha is distributed in the nucleus, cytosol, and various membranes including the plasmalemma. Notoriously, ERalpha labeling was very prominent in T-tubular membranes defined by alpha-actinin staining and the intercalated disks. In the T-tubules, ERalpha degree of association to alpha-actinin depends on the distribution pattern of the receptor along the T-tubules; association is high when ERalpha pattern is "continuous," while it is low when the receptor has a discontinuous "granular" distribution. Nuclear ERalpha has a distinct trabecular distribution and it is excluded from the heterochromatin, consistent with an active transcription factor. Treatment with estrogen ( approximately 4 h) produced an overall decrease in both nuclear and non-nuclear ERalpha levels and made more evident discrete ERalpha nuclear puncta uncovering cellular mechanism(s) of short term action of estrogen in the heart. The results indicate that the levels of the cardiac ERalpha isoforms are downregulated by estrogen and are differentially distributed: the full-length ERalpha is mainly compartmentalized in the cytosol and nucleus, while the 45 kDa isoform is mainly present in membrane structures. The membrane localization of ERalpha may support the rapid effects of estrogens on heart function.

Estrogen receptors in membrane lipid rafts and signal transduction in breast cancer

Regulation of breast cancer growth by estrogen is mediated by estrogen receptors (ER) in nuclear and extranuclear compartments. We assessed the structure and functions of extranuclear ER that initiate downstream signaling to the nucleus. ER, including full-length 66-kDa ER and a 46-kDa ER splice variant, are enriched in lipid rafts from MCF-7 cells with (MCF-7/HER-2) or without (MCF-7/PAR) HER-2 gene overexpression and co-localize with HER-1 and HER-2 growth factor receptors, as well as with lipid raft marker flotillin-2. In contrast, ER-negative MCF-7 cells do not express nuclear or lipid raft ER. ER knockdown with siRNA also elicits a marked loss of ER in MCF-7 cell rafts. In MCF-7/PAR cells, estrogen enhances ER association with membrane rafts and induces rapid phosphorylation of nuclear receptor coactivator AIB1, actions not detected in ER-negative cells. Thus, nuclear and lipid raft ER derive from the same transcript, and extranuclear ER co-localizes with HER receptors in membrane signaling domains that modulate downstream nuclear events leading to cell growth.

Two internal ribosome entry sites mediate the translation of p53 isoforms

The p53 tumour suppressor protein has a crucial role in cell-cycle arrest and apoptosis. Previous reports show that the p53 messenger RNA is translated to produce an amino-terminal-deleted isoform (DeltaN-p53) from an internal initiation codon, which acts as a dominant-negative inhibitor of full-length p53. Here, we show that two internal ribosome entry sites (IRESs) mediate the translation of both full-length and DeltaN-p53 isoforms. The IRES directing the translation of full-length p53 is in the 5'-untranslated region of the mRNA, whereas the IRES mediating the translation of DeltaN-p53 extends into the protein-coding region. The two IRESs show distinct cell-cycle phase-dependent activity, with the IRES for full-length p53 being active at the G2-M transition and the IRES for DeltaN-p53 showing highest activity at the G1-S transition. These results indicate a novel translational control of p53 gene expression and activity.

p53 protein variants: structural and functional similarities with p63 and p73 isoforms

Since its discovery in 1979, many studies have reported that the p53 tumour suppressor protein could be expressed in the form of products smaller than those predicted by the full-length amino-acid sequence. These products differ from full-length p53 in their N- or C-terminal regions, but generally conserve the central, DNA-binding domain. They appear to be expressed at rather low levels and to be restricted to particular cell types and/or physiological circumstances, suggesting that they play very narrow and specific roles. Several mechanisms have been proposed to explain their timely occurrence, including alternative splicing, internal initiation of translation or proteolytic cleavage. A precise assessment of the various 'p53 isoforms' reveals striking similarities with several isoforms of the p53 homologous proteins p63 or p73, suggesting that regulated production of specific, N- or C-terminal variants may be a 'trademark' of all family members. In this review, we summarize the published evidence on the structure, mode of production, expression and function of the p53 isoforms, and discuss their properties in the light of recent data on the structure and function of p63/p73 isoforms.

Plasma membrane localization and function of the estrogen receptor alpha variant (ER46) in human endothelial cells

Estrogen receptor (ER) alpha variants have been identified in an array of nonendothelial cells. We previously demonstrated that estrogen rapidly induces nitric oxide release via a phosphatidylinositol 3-kinase/Akt/endothelial nitric-oxide synthase (eNOS) pathway in EA.hy926 cells (immortalized human endothelial cells), which express a 46-kDa ER. We now confirm that, due to alternative splicing, the 46-kDa endothelial cell protein (ER46) is an amino-terminal truncated product of full-length ER alpha (ER66). ER46 is expressed in the plasma membrane, cytosol, and nucleus of resting, estrogen-deprived cells. Flow cytometric and immunofluorescence microscopic analyses demonstrated that the ER46 C but not N terminus is Ab-accessible in the plasma membrane. Inhibition of palmitoylation with tunicamycin and [(3)H]palmitic acid labeling demonstrated an estrogen-induced, palmitoylation-dependent plasma membrane ER46 recruitment, with reorganization into caveolae. In reconstituted, estrogen-stimulated COS-7 (ER-null) cells, membrane ER46 more efficiently triggered membrane eNOS phosphorylation than ER66. Conversely, ER66 more efficiently mediated estrogen response element reporter-gene transactivation than ER46. These results demonstrate that ER46 is localized and further dynamically targeted to the plasma membrane in a palmitoylation-dependent manner. ER46 more efficiently modulates membrane-initiated estrogen actions, including eNOS activation, than full-length ER66. These findings may have important implications in vascular-specific targeting of estrogen receptor agonists.

Multiple forms of estrogen receptor-alpha in cerebral blood vessels: regulation by estrogen

The cerebral vasculature is an important target tissue for estrogen, as evidenced by significant effects of estrogen on vascular reactivity and protein levels of endothelial nitric oxide synthase and prostacyclin synthase. However, the presence, localization, and regulation of estrogen receptors in the cerebral vasculature have not been investigated. In this study, we identified the presence of estrogen receptor-alpha (ER-alpha) in female rat cerebral blood vessels and localized this receptor to both smooth muscle and endothelial cells by use of immunohistochemistry and confocal microscopy. With immunoblot analysis, multiple forms of ER-alpha were detected at 110, 93, 82, 50, and 45 kDa in addition to a relatively weak band corresponding to the 66-kDa putative unmodified receptor. The 82-kDa band was identified as Ser(118)-phosphorylated ER-alpha, whereas the 50-kDa band lacks the normal NH(2) terminus, suggestive of an ER-alpha splice variant. Lower molecular mass bands persisted after in vivo inhibition of 26S proteasome activity with lactacystin, whereas the 110- and 93-kDa bands increased. All forms of ER-alpha in cerebral vessels were decreased after ovariectomy but significantly increased after chronic estrogen exposure in vivo.

Translational control of gene expression: role of IRESs and consequences for cell transformation and angiogenesis

Translational control of gene expression has, over the last 10 years, become appreciated as an important process in its regulation in eukaryotes. Among a series of control mechanisms exerted at the translational level, the use of alternative codons provides a very subtle means of increasing gene diversity by expressing several proteins from a single mRNA. The internal ribosome entry sites (IRESs) act as specific translational enhancers that allow translation initiation to occur independently of the classic cap-dependent mechanism, in response to specific stimuli and under the control of different trans-acting factors. It is striking to observe that the two processes mostly concern genes coding for control proteins such as growth factors, protooncogenes, angiogenesis factors, and apoptosis regulators. Here, we focus on the translational regulation of four mRNAs, with both IRESs and alternative initiation codons, which are the messengers of retroviral murine leukemia virus, fibroblast growth factor 2, vascular endothelial growth factor, and protooncogene c-myc. Four of them are involved in cell transformation and/or angiogenesis, with important consequences for such translation regulations in these pathophysiological processes.

DeltaN-p53, a natural isoform of p53 lacking the first transactivation domain, counteracts growth suppression by wild-type p53

The tumor suppressor protein p53 is ubiquitously expressed as a major isoform of 53 kD, but several forms of lower molecular weight have been observed. Here, we describe a new isoform, DeltaN-p53, produced by internal initiation of translation at codon 40 and lacking the N-terminal first transactivation domain. This isoform has impaired transcriptional activation capacity, and does not complex with the p53 regulatory protein Mdm2. Furthermore, DeltaN-p53 oligomerizes with full-length p53 (FL-p53) and negatively regulates its transcriptional and growth-suppressive activities. Consistent with the lack of Mdm2 binding, DeltaN-p53 does not accumulate in response to DNA-damage, suggesting that this isoform is not involved in the response to genotoxic stress. However, in serum-starved cells expressing wild-type p53, DeltaN-p53 becomes the predominant p53 form during the synchronous progression into S phase after serum stimulation. These results suggest that DeltaN-p53 may play a role as a transient, negative regulator of p53 during cell cycle progression.

The AF-1 activation-function of ERalpha may be dispensable to mediate the effect of estradiol on endothelial NO production in mice

Two isoforms of estrogen receptor (ER) have been described: ERalpha and ERbeta. The initial gene targeting of ERalpha, consisting in the introduction of a Neo cassette in exon 1 [alphaERKO, hereafter called ERalpha-Neo KO (knockout)], was reported in 1993. More recently, another mouse deficient in ERalpha because of the deletion of exon 2 (ERalphaKO, hereafter called ERalpha-delta2 KO) was generated. In ovariectomized ERalpha-wild-type mice, estradiol (E(2)) increases uterine weight and basal production of endothelial nitric oxide (NO). Both of these effects are abolished in ERalpha-delta2 KO mice. In contrast, we show here that both of these effects of E(2) are partially (uterine weight) or totally (endothelial NO production) preserved in ERalpha-Neo KO. We also confirm the presence of two ERalpha mRNA splice variants in uterus and aorta from ERalpha-Neo KO mice. One of them encodes a chimeric ERalpha protein (ERalpha55), partially deleted in the A/B domain, that was detected in both uterus and aorta by Western blot analysis. The other ERalpha mRNA splice variant codes for an isoform deleted for the A/B domain (ERalpha46), which was detected in uterus of ERalpha-Neo KO, and wild-type mice. This protein isoform was not detected in aorta. The identification of these two N-terminal modified isoforms in uterus, and at least one of them in aorta, probably explains the persistence of the E(2) effects in ERalpha-Neo KO mice. Furthermore, ERalpha-Neo KO mice may help in the elucidation of the specific functions of full-length ERalpha (ERalpha66) and ERalpha46, both shown to be physiologically generated in vivo.

Rapid vascular cell responses to estrogen and membrane receptors

There is a growing interest in the effects of estrogen on the vascular wall, due to the marked gender difference in the incidence of clinically apparent coronary heart disease, when comparing premenopausal women with age-matched males. Estrogen has numerous effects on vascular endothelial and smooth muscle cells, both of which express estrogen receptors (ERs). Although ERs are classically defined as ligand-activated transcription factors, it has become increasingly clear that estrogen-stimulated, ER-dependent cellular responses can be rapid consequences of signal transduction cascades. The cellular localization and molecular form of the ER(s) which mediates rapid signaling are poorly defined. In this review, we describe the mounting evidence for membrane-localized ERs that vary in structure from classical forms. We also discuss ER-catalyzed molecular complex formations and a variety of estrogen-triggered signal transduction cascades, including those involving phosphatidylinositol 3-kinase/Akt, MAP kinase and G-protein-coupled receptors, all of which may induce "protective" profiles in vascular cells.

ERalpha gene expression in human primary osteoblasts: evidence for the expression of two receptor proteins

The beneficial influence of E2 in the maintenance of healthy bone is well recognized. However, the way in which the actions of this hormone are mediated is less clearly understood. Western blot analysis of ERalpha in osteoblasts clearly demonstrated that the well characterized 66-kDa ERalpha was only one of the ERalpha isoforms present. Here we describe a 46-kDa isoform of ERalpha, expressed at a level similar to the 66-kDa isoform, that is also present in human primary osteoblasts. This shorter isoform is generated by alternative splicing of an ERalpha gene product, which results in exon 1 being skipped with a start codon in exon 2 used to initiate translation of the protein. Consequently, the transactivation domain AF-1 of this ERalpha isoform is absent. Functional analysis revealed that human (h)ERalpha46 is able to heterodimerize with the full-length ERalpha and also with ERbeta. Further, a DNA-binding complex that corresponds to hERalpha46 is detectable in human osteoblasts. We have shown that hERalpha46 is a strong inhibitor of hERalpha66 when they are coexpressed in the human osteosarcoma cell line SaOs. As a functional consequence, proliferation of the transfected cells is inhibited when increasing amounts of hERalpha46 are cotransfected with hERalpha66. In addition to human bone, the expression of the alternatively spliced ERalpha mRNA variant is also detectable in bone of ERalpha knockout mice. These data suggest that, in osteoblasts, E2 can act in part through an ERalpha isoform that is markedly different from the 66-kDa receptor. The expression of two ERalpha protein isoforms may account, in part, for the differential action that estrogens and estrogen analogs have in different tissues. In particular, the current models of the action of estrogens should be reevaluated to take account of the presence of at least two ERalpha protein isoforms in bone and perhaps in other tissues.

Novel intronic promoter in the rat ER alpha gene responsible for the transient transcription of a variant receptor

To analyze the molecular origin of an ER variant, the truncated ER product-1, transiently expressed at the proestrus in lactotrope cells, we generated a 2.5-kb sequence of a genomic region upstream and downstream the specific sequence truncated ER product-1. Genomic Southern blot analysis showed that truncated ER product-1 is spliced from a noncoding leader exon localized within the intron 4 of the ER alpha gene. Analysis of the promoter sequence revealed the presence of a major transcriptional start site, a canonical TATA box and putative cis regulatory elements for pituitary specific expression as well as an E-responsive element. In transient transfection, the truncated ER product-1 promoter was transcriptionally the most active in the lactotrope cell lines (MMQ). Analysis of truncated ER product-1 functionality showed that: 1) the protein inhibited ER alpha binding to the E-responsive element in electromobility shift assays, 2) inhibited the E2 binding to ER alpha in binding assays, 3) the truncated ER product-1/ER alpha complex antagonized the transcriptional activity elicited by E2, 4) nuclear localization of green fluorescent protein-ER alpha was altered in Chinese hamster ovary cell lines stably expressing truncated ER product-1. Collectively, these data demonstrated that the protein exerts full dominant negative activity against ER alpha. Moreover, truncated ER product-1/ER alpha complex also repressed the activity of all promoters tested to date, suggesting a general inhibitory effect toward transcription. In conclusion, the data suggest that truncated ER product-1 could regulate estrogen signaling via a specific promoter in lactotrope cells.

Demonstration of estrogen receptor subtypes alpha and beta in human adipose tissue: influences of adipose cell differentiation and fat depot localization

A novel ER-subtype, the ER-beta has recently been characterized in various tissues, furthermore five isoforms of the ER-beta are known (ER-beta1--ER-beta5). Using immunoblotting and real- time RT-PCR, ER-alpha and beta were studied in human adipose tissue. The expression of ER-alpha mRNA was equal in subcutaneous gluteal adipose tissue, subcutaneous abdominal and intra-abdominal adipose tissue, similar findings were obtained at the protein level. In contrast the amount of ER-beta1 (protein and mRNA) was significantly lower in intra-abdominal adipose tissue as compared with the subcutaneous adipose tissue (five-fold lower in women, P<0.005 and three-fold lower in men, P<0.005) whereas the expression of ER-beta4 and -beta5 mRNA isoforms were significantly higher in gluteal adipose tissue compared to subcutaneous abdominal adipose tissue. No significant gender differences in ER expression was detected in any of the fat depots investigated. During adipocyte differentiation the expression of ER-alpha, -beta4 and -beta5 mRNA declined, whereas, the expression of ER-beta1 mRNA was constant. In conclusion, the existence of ER-beta isoforms in human adipose tissue was demonstrated and the amount of these receptors was dependent upon fat depot localization, with much reduced expression of ER-beta1 in intra-abdominal adipose tissue compared to subcutaneous adipose tissue. These findings may indicate that estrogens could have differentiation and depot specific effects in human adipose tissue.

Tissue-specific expression of human ERalpha and ERbeta in the male

The important role of estrogens in women in physiological and pathological processes is well accepted, but recently it has become evident that estrogens are also important in male physiology, in particular, within bone metabolism and reproduction. Consequently, it is necessary to identify and to characterize the molecular mechanisms of estrogen action in order to evaluate how the pleiotropic effects of estrogens are mediated in a variety of tissues. We have recently shown that human estrogen receptor alpha (ERalpha) mRNA is transcribed from at least six different promoters (1A-1F). Transcription of ERalpha in bone is exclusively dependent on the F-promoter. To study the regulation of ER expression in this tissue, we examined 1 kbp of the F-promoter region of human ERalpha, which is located more than 70 kbp upstream of the transcription start site of the ERalpha gene. Transient transfection experiments demonstrated a basal activity from the F-promoter, which was further increased when ERalpha was cotransfected. We have shown recently that the F-promoter can give rise to at least two ERalpha isoforms in bone. On the contrary, ERbeta expression in primary osteoblasts is extremely low, indicating that this ER isoform plays only a minor role in these cells. In contrast to bone, we have demonstrated that both ERalpha and ERbeta transcripts are readily detected in testis. Here, we report that besides ERalpha, ERbeta transcripts can give rise to two protein isoforms and that this complex situation could have important functional consequences for the signalling of estrogens and their analogs.

Novel transcripts of the estrogen receptor alpha gene in channel catfish

Complementary DNA libraries from liver and ovary of an immature female channel catfish were screened with a homologous ERalpha cDNA probe. The hepatic library yielded two new channel catfish ER cDNAs that encode N-terminal ERalpha variants of different sizes. Relative to the catfish ERalpha (medium size; 581 residues) previously reported, these new cDNAs encode Long-ERalpha (36 residues longer) and Short-ERalpha (389 residues shorter). The 5'-end of Long-ERalpha cDNA is identical to that of Medium-ERalpha but has an additional 503-bp segment with an upstream, in-frame translation-start codon. Recombinant Long-ERalpha binds estrogen with high affinity (K(d) = 3. 4 nM), similar to that previously reported for Medium-ERalpha but lower than reported for catfish ERbeta. Short-ERalpha cDNA encodes a protein that lacks most of the receptor protein and does not bind estrogen. Northern hybridization confirmed the existence of multiple hepatic ERalpha RNAs that include the size range of the ERalpha cDNAs obtained from the libraries as well as additional sizes. Using primers for RT-PCR that target locations internal to the protein-coding sequence, we also established the presence of several ERalpha cDNA variants with in-frame insertions in the ligand-binding and DNA-binding domains and in-frame or out-of-frame deletions in the ligand-binding domain. These internal variants showed patterns of expression that differed between the ovary and liver. Further, the ovarian library yielded a full-length, ERalpha antisense cDNA containing a poly(A) signal and tail. A limited survey of histological preparations from juvenile catfish by in situ hybridization using directionally synthesized cRNA probes also suggested the expression of ERalpha antisense RNA in a tissue-specific manner. In conclusion, channel catfish seemingly have three broad classes of ERalpha mRNA variants: those encoding N-terminal truncated variants, those encoding internal variants (including C-terminal truncated variants), and antisense mRNA. The sense variants may encode functional ERalpha or related proteins that modulate ERalpha or ERbeta activity. The existence of ER antisense mRNA is reported in this study for the first time. Its role may be to participate in the regulation of ER gene expression.

Identification of a new isoform of the human estrogen receptor-alpha (hER-alpha) that is encoded by distinct transcripts and that is able to repress hER-alpha activation function 1

A new isoform of the human estrogen receptor-alpha (hER-alpha) has been identified and characterized. This 46 kDa isoform (hERalpha46) lacks the N-terminal 173 amino acids present in the previously characterized 66 kDa isoform (hERalpha66). hERalpha46 is encoded by a new class of hER-alpha transcript that lacks the first coding exon (exon 1A) of the ER-alpha gene. We demonstrated that these Delta1A hER-alpha transcripts originate from the E and F hER-alpha promoters and are produced by the splicing of exon 1E directly to exon 2. Functional analysis of hERalpha46 showed that, in a cell context sensitive to the transactivation function AF-2, this receptor is an effective ligand-inducible transcription factor. In contrast, hERalpha46 is a powerful inhibitor of hERalpha66 in a cell context where the transactivating function of AF-1 predominates over AF-2. The mechanisms by which the AF-1 dominant-negative action is exerted may involve heterodimeri zation of the two receptor isoforms and/or direct competition for the ER-alpha DNA-binding site. hERalpha66/hERalpha46 ratios change with the cell growth status of the breast carcinoma cell line MCF7, suggesting a role of hERalpha46 in cellular proliferation.

Identification of a new isoform of the human estrogen receptor-alpha (hER-alpha) that is encoded by distinct transcripts and that is able to repress hER-alpha activation function 1

A new isoform of the human estrogen receptor-alpha (hER-alpha) has been identified and characterized. This 46 kDa isoform (hERalpha46) lacks the N-terminal 173 amino acids present in the previously characterized 66 kDa isoform (hERalpha66). hERalpha46 is encoded by a new class of hER-alpha transcript that lacks the first coding exon (exon 1A) of the ER-alpha gene. We demonstrated that these Delta1A hER-alpha transcripts originate from the E and F hER-alpha promoters and are produced by the splicing of exon 1E directly to exon 2. Functional analysis of hERalpha46 showed that, in a cell context sensitive to the transactivation function AF-2, this receptor is an effective ligand-inducible transcription factor. In contrast, hERalpha46 is a powerful inhibitor of hERalpha66 in a cell context where the transactivating function of AF-1 predominates over AF-2. The mechanisms by which the AF-1 dominant-negative action is exerted may involve heterodimeri zation of the two receptor isoforms and/or direct competition for the ER-alpha DNA-binding site. hERalpha66/hERalpha46 ratios change with the cell growth status of the breast carcinoma cell line MCF7, suggesting a role of hERalpha46 in cellular proliferation.

The analysis of chimeric human/rainbow trout estrogen receptors reveals amino acid residues outside of P- and D-boxes important for the transactivation function

The amino acid sequence of rainbow trout estrogen receptor (rtER) is highly conserved in the C domain but presents few similarities in the A/B and E domains with human estrogen receptor alpha (hER) [NR3A1]. A previous study has shown that rtER and hER have differential functional activities in yeast Saccharomyces cerevisiae. To determine the domain(s) responsible for these differences, chimeric human/rainbow trout estrogen receptors were constructed. The A/B, C/D or E/F regions of rtER were replaced by corresponding regions of hER and expressed in yeast cells. Ligand-binding and transcription activation abilities of these hybrid receptors were compared with those of wild-type rtER or hER. Surprisingly, our data revealed that the human C/D domains play an important role in the magnitude of transactivation of ER. Two other chimeric ERs carrying either a C or D domain of hER showed that the C domain was responsible for this effect whereas the D domain did not affect hybrid receptor activities. Moreover, a chimeric hER carrying the C domain of rtER showed maximal transcriptional activity similar to that observed with rtER. Gel shift assays showed that, whereas rtER and hER present a similar binding affinity to an estrogen response element (ERE) element, the rtER C domain is responsible for a weaker DNA binding stability compared to those of hER. In addition, the human C domain allows approximately 2 times faster association of ER to an ERE. Utilization of reporter genes containing one or three EREs confirms that rtER requires protein-protein interactions for its stabilization on DNA and that the C domain is involved in this stabilization. Moreover, AF-1 may be implicated in this synergistic effect of EREs. Interestingly, although E domains of these two receptors are much less conserved, replacement of this domain in rtER by its human counterpart resulted in higher estradiol sensitivity but no increase in the magnitude of transactivation. Data from the chimeric receptors, rtER(hC) and hER(rtC), demonstrated that rtER AF-1 and AF-2 activation domains activated transcription in the presence of estradiol similar to both AF-1 and AF-2 hER. This implies that these domains, which show poor sequence homology, may interact with similar basal transcription factors.