A splice variant of estrogen receptor beta missing exon 3 displays altered subnuclear localization and capacity for transcriptional activation

Roles of estrogen receptors during sexual reversal in Pelodiscus sinensis

BACKGROUND

The Chinese soft-shelled turtle, Pelodiscus sinensis, exhibits distinct sexual dimorphism, with the males growing faster and larger than the females. During breeding, all-male offspring can be obtained using 17β-estradiol (E2). However, the molecular mechanisms underlying E2-induced sexual reversal have not yet been elucidated. Previous studies have investigated the molecular sequence and expression characteristics of estrogen receptors (ERs).

METHODS AND RESULTS

In this study, primary liver cells and embryos of P. sinensis were treated with ER agonists or inhibitors. Cell incubation experiments revealed that nuclear ERs (nERs) were the main pathway for the transmission of estrogen signals. Our results showed that ERα agonist (ERα-ag) upregulated the expression of Rspo1, whereas ERα inhibitor (ERα-Inh) downregulated its expression. The expression of Dmrt1 was enhanced after ERα-Inh + G-ag treatment, indicating that the regulation of male genes may not act through a single estrogen receptor, but a combination of ERs. In embryos, only the ERα-ag remarkably promoted the expression levels of Rspo1, Wnt4, and β-catenin, whereas the ERα-Inh had a suppressive effect. Additionally, Dmrt1, Amh, and Sox9 expression levels were downregulated after ERβ inhibitor (ERβ-Inh) treatment. GPER agonist (G-ag) has a significant promotion effect on Rspo1, Wnt4, and β-catenin, while the inhibitor G-Inh does not affect male-related genes.

CONCLUSIONS

Overall, these results suggest that ERs play different roles during sexual reversal in P. sinensis and ERα may be the main carrier of estrogen-induced sexual reversal in P. sinensis. Further studies need to be performed to analyze the mechanism of ER action.

The Isoforms of Estrogen Receptor Alpha and Beta in Thyroid Cancer

The incidence of thyroid cancer was predominant in women, indicating that the sex hormone may have a role in thyroid cancer development. Generally, the sex hormone exerts its function by binding to the correspondent nuclear receptors. Therefore, aberrant of these receptors may be involved in the development of thyroid cancer. Estrogen receptor alpha (ERα) and beta (ERβ), two main estrogen receptors, have been reported to have an important role in the pathogenesis of thyroid cancer. When the ERα and ERβ genes undergo the alternative RNA splicing, some ERα and ERβ isoforms with incomplete functional domains may be formed. To date, several isoforms of ERα and ERβ have been identified. However, their expression and roles in thyroid cancer are far from clear. In this review, we summarized the expressions and roles of ERα and ERβ isoforms in thyroid cancer, aiming to provide the perspective of modulating the alternative RNA splicing of ERα and ERβ against thyroid cancer.

Structural and functional characteristics of oestrogen receptor β splice variants: Implications for the ageing brain

Oestrogen receptor (ER)β is a multifunctional nuclear receptor that mediates the actions of oestrogenic compounds. Despite its well defined role in mediating the actions of oestrogens, a substantial body of evidence demonstrates that ERβ has a broad range of physiological functions independent of those normally attributed to oestrogen signalling. These functions can partly be achieved by the activity of several alternatively spliced isoforms that have been identified for ERβ. This short review describes structural differences between the ERβ splice variants that are known to be translated into proteins. Moreover, we discuss how these alternative structures contribute to functional differences in the context of both healthy and pathological conditions. Our review also describes the principal factors that regulate alternative RNA splicing. The alternatively spliced isoforms of ERβ are differentially expressed according to brain region, age and hormonal milieu, emphasising the likelihood that there are precise cell-specific mechanisms regulating ERβ alternative splicing. However, despite these correlative data, the molecular factors regulating alternative ERβ splicing in the brain remain unknown. We also review the basic mechanisms that regulate alternative RNA splicing and use that framework to make logical predictions about ERβ alternative splicing in the brain.

17β-estradiol regulates the RNA-binding protein Nova1, which then regulates the alternative splicing of estrogen receptor β in the aging female rat brain

Alternative RNA splicing results in the translation of diverse protein products arising from a common nucleotide sequence. These alternative protein products are often functional and can have widely divergent actions from the canonical protein. Studies in humans and other vertebrate animals have demonstrated that alternative splicing events increase with advanced age, sometimes resulting in pathological consequences. Menopause represents a critical transition for women, where the beneficial effects of estrogens are no longer evident; therefore, factors underlying increased pathological conditions in women are confounded by the dual factors of aging and declining estrogens. Estrogen receptors (ERs) are subject to alternative splicing, the spliced variants increase following menopause, and they fail to efficiently activate estrogen-dependent signaling pathways. However, the factors that regulate the alternative splicing of ERs remain unknown. We demonstrate novel evidence supporting a potential biological feedback loop where 17β-estradiol regulates the RNA-binding protein Nova1, which, in turn, regulates the alternative splicing of ERβ. These data increase our understanding of ER alternative splicing and could have potential implications for women taking hormone replacement therapy after menopause.

Hypothalamic-pituitary-adrenal and hypothalamic-pituitary-gonadal axes: sex differences in regulation of stress responsivity

Gonadal hormones play a key role in the establishment, activation, and regulation of the hypothalamic-pituitary-adrenal (HPA) axis. By influencing the response and sensitivity to releasing factors, neurotransmitters, and hormones, gonadal steroids help orchestrate the gain of the HPA axis to fine-tune the levels of stress hormones in the general circulation. From early life to adulthood, gonadal steroids can differentially affect the HPA axis, resulting in sex differences in the responsivity of this axis. The HPA axis influences many physiological functions making an organism's response to changes in the environment appropriate for its reproductive status. Although the acute HPA response to stressors is a beneficial response, constant activation of this circuitry by chronic or traumatic stressful episodes may lead to a dysregulation of the HPA axis and cause pathology. Compared to males, female mice and rats show a more robust HPA axis response, as a result of circulating estradiol levels which elevate stress hormone levels during non-threatening situations, and during and after stressors. Fluctuating levels of gonadal steroids in females across the estrous cycle are a major factor contributing to sex differences in the robustness of HPA activity in females compared to males. Moreover, gonadal steroids may also contribute to epigenetic and organizational influences on the HPA axis even before puberty. Correspondingly, crosstalk between the hypothalamic-pituitary-gonadal (HPG) and HPA axes could lead to abnormalities of stress responses. In humans, a dysregulated stress response is one of the most common symptoms seen across many neuropsychiatric disorders, and as a result, such interactions may exacerbate peripheral pathologies. In this review, we discuss the HPA and HPG axes and review how gonadal steroids interact with the HPA axis to regulate the stress circuitry during all stages in life.

Aging and Loss of Circulating 17β-Estradiol Alters the Alternative Splicing of ERβ in the Female Rat Brain

Loss of circulating 17β-estradiol (E2) that occurs during menopause can have detrimental effects on cognitive function. The efficacy of hormone replacement therapy declines as women become farther removed from the menopausal transition, yet the molecular mechanisms underlying this age-related switch in E2 efficacy are unknown. We hypothesized that aging and varying lengths of E2 deprivation alters the ratio of alternatively spliced estrogen receptor (ER)β isoforms in the brain of female rats. Further, we tested whether changes in global transcriptional activity and splicing kinetics regulate the alternative splicing of ERβ. Our results revealed brain region-specific changes in ERβ alternative splicing in both aging and E2-deprivation paradigms and showed that ERβ could mediate E2-induced alternative splicing. Global transcriptional activity, as measured by phosphorylated RNA polymerase II, was also regulated by age and E2 in specific brain regions. Finally, we show that inhibition of topoisomerase I resulted in increased ERβ2 splice variant expression.

Estrogen receptor alpha and beta in health and disease

Estrogen receptors alpha (ERα) and beta (ERβ) are transcription factors that are involved in the regulation of many complex physiological processes in humans. Abnormal ER signaling leads to development of a variety of diseases, such as cancer, metabolic and cardiovascular disease, neurodegeneration, inflammation, and osteoporosis. This review provides an overview and update on ERα and ERβ in health and disease with focus on their role in cancer and metabolic disease and in the context of recent years' success in providing genome wide data on ER function. Furthermore, potential clinical applications and challenges are also discussed.

Estrogen receptor β exon 3-deleted mouse: The importance of non-ERE pathways in ERβ signaling

In 1998, an estrogen receptor β (ERβ) knockout (KO) mouse was created by interrupting the gene at the DNA binding domain (DBD) with a neocassette. The mutant females were subfertile and there were abnormalities in the brain, prostate, lung, colon, and immune system. In 2008, another ERβ mutant mouse was generated by deleting ERβ exon 3 which encodes the first zinc finger in the DBD. The female mice of this strain were unable to ovulate but were otherwise normal. The differences in the phenotypes of the two KO strains, have led to questions about the physiological function of ERβ. In the present study, we created an ERβ exon 3-deleted mouse (ERβ-Δex3) and confirmed that the only observable defect was anovulation. Despite the two in-frame stop codons introduced by splicing between exons 2 and 4, an ERβ protein was expressed in nuclei of prostate epithelial cells. Using two different anti-ERβ antibodies, we showed that an in-frame ligand binding domain and C terminus were present in the ERβ-Δex3 protein. Moreover, with nuclear extracts from ERβ-Δex3 prostates, there was an ERβ-dependent retardation of migration of activator protein-1 response elements in EMSA. Unlike the original knockout mouse, expression of Ki67, androgen receptor, and Dachshund-1 in prostate epithelium was not altered in the ERβ-Δex3 mouse. We conclude that very little of ERβ transcriptional activity depends on binding to classical estrogen response elements (EREs).

Molecular signature of rapid estrogen regulation of synaptic connectivity and cognition

There is now a growing appreciation that estrogens are capable of rapidly activating a number of signaling cascades within the central nervous system. In addition, there are an increasing number of studies reporting that 17β-estradiol, the major biologically active estrogen, can modulate cognition within a rapid time frame. Here we review recent studies that have begun to uncover the molecular and cellular framework which contributes to estrogens ability to rapidly modulate cognition. We first describe the mechanisms by which estrogen receptors (ERs) can couple to intracellular signaling cascades, either directly, or via the transactivation of other receptors. Subsequently, we review the evidence that estrogen can rapidly modulate both neuronal function and structure in the hippocampus and the cortex. Finally, we will discuss how estrogens may influence cognitive function through the modulation of neuronal structure, and the implications this may have on the treatment of a range of brain disorders.

Gonadal steroid hormones and the hypothalamo-pituitary-adrenal axis

The hypothalamo-pituitary-adrenal (HPA) axis represents a complex neuroendocrine feedback loop controlling the secretion of adrenal glucocorticoid hormones. Central to its function is the paraventricular nucleus of the hypothalamus (PVN) where neurons expressing corticotropin releasing factor reside. These HPA motor neurons are a primary site of integration leading to graded endocrine responses to physical and psychological stressors. An important regulatory factor that must be considered, prior to generating an appropriate response is the animal's reproductive status. Thus, PVN neurons express androgen and estrogen receptors and receive input from sites that also express these receptors. Consequently, changes in reproduction and gonadal steroid levels modulate the stress response and this underlies sex differences in HPA axis function. This review examines the make up of the HPA axis and hypothalamo-pituitary-gonadal (HPG) axis and the interactions between the two that should be considered when exploring normal and pathological responses to environmental stressors.

Insights into rapid modulation of neuroplasticity by brain estrogens

Converging evidence from cellular, electrophysiological, anatomic, and behavioral studies suggests that the remodeling of synapse structure and function is a critical component of cognition. This modulation of neuroplasticity can be achieved through the actions of numerous extracellular signals. Moreover, it is thought that it is the integration of different extracellular signals regulation of neuroplasticity that greatly influences cognitive function. One group of signals that exerts powerful effects on multiple neurologic processes is estrogens. Classically, estrogens have been described to exert their effects over a period of hours to days. However, there is now increasing evidence that estrogens can rapidly influence multiple behaviors, including those that require forebrain neural circuitry. Moreover, these effects are found in both sexes. Critically, it is now emerging that the modulation of cognition by rapid estrogenic signaling is achieved by activation of specific signaling cascades and regulation of synapse structure and function, cumulating in the rewiring of neural circuits. The importance of understanding the rapid effects of estrogens on forebrain function and circuitry is further emphasized as investigations continue to consider the potential of estrogenic-based therapies for neuropathologies. This review focuses on how estrogens can rapidly influence cognition and the emerging mechanisms that underlie these effects. We discuss the potential sources and the biosynthesis of estrogens within the brain and the consequences of rapid estrogenic-signaling on the remodeling of neural circuits. Furthermore, we argue that estrogens act via distinct signaling pathways to modulate synapse structure and function in a manner that may vary with cell type, developmental stage, and sex. Finally, we present a model in which the coordination of rapid estrogenic-signaling and activity-dependent stimuli can result in long-lasting changes in neural circuits, contributing to cognition, with potential relevance for the development of novel estrogenic-based therapies for neurodevelopmental or neurodegenerative disorders.

Estrogen signaling and the aging brain: context-dependent considerations for postmenopausal hormone therapy

Recent clinical studies have spurred rigorous debate about the benefits of hormone therapy (HT) for postmenopausal women. Controversy first emerged based on a sharp increase in the risk of cardiovascular disease in participants of the Women's Health Initiative (WHI) studies, suggesting that decades of empirical research in animal models was not necessarily applicable to humans. However, a reexamination of the data from the WHI studies suggests that the timing of HT might be a critical factor and that advanced age and/or length of estrogen deprivation might alter the body's ability to respond to estrogens. Dichotomous estrogenic effects are mediated primarily by the actions of two high-affinity estrogen receptors alpha and beta (ER α & ER β ). The expression of the ERs can be overlapping or distinct, dependent upon brain region, sex, age, and exposure to hormone, and, during the time of menopause, there may be changes in receptor expression profiles, post-translational modifications, and protein:protein interactions that could lead to a completely different environment for E2 to exert its effects. In this review, factors affecting estrogen-signaling processes will be discussed with particular attention paid to the expression and transcriptional actions of ER β in brain regions that regulate cognition and affect.

Estrogen receptors and the regulation of neural stress responses

It is now well established that estrogens can influence a panoply of physiological and behavioral functions. In many instances, the effects of estrogens are mediated by the 'classical' actions of two different estrogen receptors (ERs), ERα or ERβ. ERα and ERβ appear to have opposing actions in the control of stress responses and modulate different neurotransmitter or neuropeptide systems. Studies elucidating the molecular mechanisms for such regulatory processes are currently in progress. Furthermore, the use of ERα and ERβ knockout mouse lines has allowed the exploration of the importance of these receptors in behavioral responses such as anxiety-like and depressive-like behaviors. This review examines some of the recent advances in our knowledge of hormonal control of neuroendocrine and behavioral responses to stress and underscore the importance of these receptors as future therapeutic targets for control of stress-related signaling pathways.

Roles for oestrogen receptor β in adult brain function

Oestradiol exerts a profound influence upon multiple brain circuits. For the most part, these effects are mediated by oestrogen receptor (ER)α. We review here the roles of ERβ, the other ER isoform, in mediating rodent oestradiol-regulated anxiety, aggressive and sexual behaviours, the control of gonadotrophin secretion, and adult neurogenesis. Evidence exists for: (i) ERβ located in the paraventricular nucleus underpinning the suppressive influence of oestradiol on the stress axis and anxiety-like behaviour; (ii) ERβ expressed in gonadotrophin-releasing hormone neurones contributing to oestrogen negative-feedback control of gonadotrophin secretion; (iii) ERβ controlling the offset of lordosis behaviour; (iv) ERβ suppressing aggressive behaviour in males; (v) ERβ modulating responses to social stimuli; and (vi) ERβ in controlling adult neurogenesis. This review highlights two major themes; first, ERβ and ERα are usually tightly inter-related in the oestradiol-dependent control of a particular brain function. For example, even though oestradiol feedback to control reproduction occurs principally through ERα-dependent mechanisms, modulatory roles for ERβ also exist. Second, the roles of ERα and ERβ within a particular neural network may be synergistic or antagonistic. Examples of the latter include the role of ERα to enhance, and ERβ to suppress, anxiety-like and aggressive behaviours. Splice variants such as ERβ2, acting as dominant negative receptors, are of further particular interest because their expression levels may reflect preceeding oestradiol exposure of relevance to oestradiol replacement therapy. Together, this review highlights the predominant modulatory, but nonetheless important, roles of ERβ in mediating the many effects of oestradiol upon adult brain function.

Membrane estrogen receptors mediate calcium signaling and MAP kinase activation in individual hippocampal neurons

Previously we demonstrated that 17β-Estradiol (E2) induced rapid Ca(2+) influx via L-type calcium channel activation, which was required for activation of Src/ERK/CREB/Bcl2 signaling cascade and subsequent induction of neuroprotective and neurotrophic responses in rat hippocampal and cortical neurons (Wu et al., 2005; Zhao et al., 2005). The current study determined the presence and specificity of membrane E2 binding sites and the functional consequence of E2 binding to membrane receptors in individual neurons. Using E2-BSA-FITC (fluorescein isothiocyanate) macromolecular complex, membrane E2 binding sites were observed in hippocampal neurons. Punctate FITC signal was observed on plasma membrane of soma and neuronal processes in E2-BSA-FITC binding neurons. No membrane binding was observed with BSA-FITC. Specificity of binding was demonstrated by competition with excess un-conjugated E2. An ERa specific agonist, PPT, and an ERb agonist, DPN, partially competed for E2-BSA-FITC binding. Imaging of intracellular Ca(2+) ([Ca(2+)]i) in live neurons, revealed rapid Ca(2+) responses in E2-BSA-FITC binding neurons within minutes that culminated in a greater [Ca(2+)]i rise and [Ca(2+)]i spikes at >20 min. The same neurons in which E2-BSA-FITC induced a [Ca(2+)]i rise also exhibited activated pERK (extracellular signal-regulated kinase) that was translocated to the nucleus. Immunofluorescent analyses demonstrated that both excitatory and inhibitory neuronal markers labeled subpopulations of E2-BSA-FITC binding neurons. All E2-BSA-FITC binding neurons expressed L-type calcium channels. These results demonstrate, at a single cell level, that E2 membrane receptors mediate the rapid signaling cascades required for E2 neuroprotective and neurotrophic effects in hippocampal neurons. These results are discussed with respect to therapeutic targets of estrogen therapy in brain.

Identification of novel transcript variants of estrogen receptor α, β and progesterone receptor gene in human endometrium

The human progesterone receptor (PR) and estrogen receptor genes (ESR1 and ESR2) are known to code for a multitude of transcript variants resulting from alternative splicing. Many of them are translated into nuclear receptor proteins with altered structure and function. Expression of these alternative estrogen and progesterone receptors modulates the cellular response to sexual steroid hormones. Recent studies also suggested their significance in development of hormone-dependent diseases like gynecological cancers. We report identification of 12 new transcript variations of the PR, ESR1, and ESR2 gene in human endometrium which result from differential exon-skipping. We succeeded in cloning of four new double or triple exon-deletion transcript variants of ERα, four single, double or triple exon-skipped mRNA isoforms of ERβ, and four new transcript variations of PR gene. Sequence analysis suggested that at least four of them, ERαΔ5/6, ERαΔ5/6/7, PRΔ7, and PRΔ6/7 are translated into receptor proteins which might exert ligand-independent effects on steroid hormone signalling. Comparison of pre- and post-menopausal endometrium revealed differential expression of PRΔ6/7, ERαΔ5/6/7, ERαΔ3/4/5, and ERβΔ1-0N. We also report differential expression of the exon-skipped isoforms in a panel of human cancer cell lines derived from the breast, ovary, and endometrium. Our identification of additional transcript variations further increases the complexity of steroid hormone receptor gene expression and signalling.

17beta-estradiol induces transthyretin expression in murine choroid plexus via an oestrogen receptor dependent pathway

Oestrogen protects against AD by multiple mechanisms, including the enhancement of Abeta clearance. Transthyretin (TTR) is a homotetrameric protein mainly synthesized by the liver and choroid plexus (CP) of the brain that sequesters the amyloid beta (Abeta) peptide. In this study we examined the effects of 17beta-estradiol (E2) on TTR protein and mRNA levels, in primary cultures of rat CP epithelial cells (CPEC) by Western blot and Real Time PCR, respectively. Moreover, the localization of oestrogen receptors alpha (ERalpha) and beta (ERbeta) in response to E2 treatment was analysed by confocal microscopy in these cells. The expression of TTR, ERalpha and ERbeta was also compared in the CP of castrated female mice treated with E2 to vehicle-treated animals by Real Time PCR. TTR concentration in the CSF of all these animals was measured by radioimmunoassay. E2 treatment induced TTR transcription and increased TTR protein content in CPEC. Pre-treatment with ICI 182,780 (ICI) abrogated E2-induced TTR expression suggesting that, TTR is up-regulated via an ER-dependent pathway. Confocal microscopy demonstrated extranuclear ERalpha and ERbeta localization in untreated CPEC. Upon E2 treatment, translocation of ERalpha to the nucleus occurred, while ERbeta remained in the cytosol. These data was concurrent with the up-regulation of TTR expression detected in the CP of castrated female mice subjected to E2 treatment. Our results highlight the importance of E2 on the regulation of TTR, which may participate in the oestrogen-induced decrease in Abeta levels and deposition described in the literature.

A role for the androgen metabolite, 5alpha-androstane-3beta,17beta-diol, in modulating oestrogen receptor beta-mediated regulation of hormonal stress reactivity

Activation of the hypothalamic-pituitary-adrenal (HPA) axis is a basic response of animals to environmental perturbations that threaten homeostasis. These responses are regulated by neurones in the paraventricular nucleus of the hypothalamus (PVN) that synthesise and secrete corticotrophin-releasing hormone (CRH). Other PVN neuropeptides, such as arginine vasopressin and oxytocin, can also modulate activity of CRH neurones in the PVN and enhance CRH secretagogue activity of the anterior pituitary gland. In rodents, sex differences in HPA reactivity are well established; females exhibit a more robust activation of the HPA axis after stress than do males. These sex differences primarily result from opposing actions of sex steroids, testosterone and oestrogen, on HPA function. Ostreogen enhances stress activated adrenocorticotrophic hormone (ACTH) and corticosterone (CORT) secretion, whereas testosterone decreases the gain of the HPA axis and inhibits ACTH and CORT responses to stress. Data show that androgens can act directly on PVN neurones in the male rat through a novel pathway involving oestrogen receptor (ER)beta, whereas oestrogen acts predominantly through ERalpha. Thus, we examined the hypothesis that, in males, testosterone suppresses HPA function via an androgen metabolite that binds ERbeta. Clues to the neurobiological mechanisms underlying such a novel action can be gleaned from studies showing extensive colocalisation of ERbeta in oxytocin-containing cells of the PVN. Hence, in this review, we address the possibility that testosterone inhibits HPA reactivity by metabolising to 5alpha-androstane-3beta,17beta-diol, a compound that binds ERbeta and regulates oxytocin containing neurones of the PVN. These findings suggest a re-evaluation of studies examining pathways for androgen receptor signalling.

Estrogen and its receptors in cancer

The involvement of estrogen and its receptors in the development of cancer has been known for years. However, the exact mechanism responsible is far from clear. The estrogen-mediated carcinogenic process is complicated by recent findings, which reveal that estrogens have multiple functions in cells, which can be either adverse or beneficial, and that the effects of estrogen may be cell-type or organ dependent. The estrogenic effect may be also greatly influenced by the state of two estrogen receptors, ERalpha and ERbeta. This review will discuss the role and function of estrogens and its receptors in cancers of three categories: (1) Breast cancer and gynecologic cancers, (2) Cancers of endocrine organs, (3) Lung cancer and cancers of digestive system. We will also review some novel treatments aiming to interfere with relevant pathways mediated by estrogens and its receptors.

Do Estrogen Receptor beta Polymorphisms Play A Role in the Pharmacogenetics of Estrogen Signaling?

Estrogen hormones play critical roles in the regulation of many tissue functions. The effects of estrogens are primarily mediated by the estrogen receptors (ER) alpha and beta. ERs are ligand-activated transcription factors that regulate a complex array of genomic events that orchestrate cellular growth, differentiation and death. Although many factors contribute to their etiology, estrogens are thought to be the primary agents for the development and/or progression of target tissue malignancies. Many of the current modalities for the treatment of estrogen target tissue malignancies are based on agents with diverse pharmacology that alter or prevent ER functions by acting as estrogen competitors. Although these compounds have been successfully used in clinical settings, the efficacy of treatment shows variability. An increasing body of evidence implicates ERalpha polymorphisms as one of the contributory factors for differential responses to estrogen competitors. This review aims to highlight the recent findings on polymorphisms of the lately identified ERbeta in order to provide a functional perspective with potential pharmacogenomic implications.

An alternate pathway for androgen regulation of brain function: activation of estrogen receptor beta by the metabolite of dihydrotestosterone, 5alpha-androstane-3beta,17beta-diol

The complexity of gonadal steroid hormone actions is reflected in their broad and diverse effects on a host of integrated systems including reproductive physiology, sexual behavior, stress responses, immune function, cognition, and neural protection. Understanding the specific contributions of androgens and estrogens in neurons that mediate these important biological processes is central to the study of neuroendocrinology. Of particular interest in recent years has been the biological role of androgen metabolites. The goal of this review is to highlight recent data delineating the specific brain targets for the dihydrotestosterone metabolite, 5alpha-androstane, 3beta,17beta-diol (3beta-Diol). Studies using both in vitro and in vivo approaches provide compelling evidence that 3beta-Diol is an important modulator of the stress response mediated by the hypothalmo-pituitary-adrenal axis. Furthermore, the actions of 3beta-Diol are mediated by estrogen receptors, and not androgen receptors, often through a canonical estrogen response element in the promoter of a given target gene. These novel findings compel us to re-evaluate the interpretation of past studies and the design of future experiments aimed at elucidating the specific effects of androgen receptor signaling pathways.

Estrogen induces rapid translocation of estrogen receptor beta, but not estrogen receptor alpha, to the neuronal plasma membrane

Estrogen receptors can activate transcription in the nucleus, and activate rapid signal transduction cascades in the cytosol. Multiple reports identify estrogen receptors at the plasma membrane, while others document the dynamic responses of estrogen receptor to ligand binding. However, the function and identity of membrane estrogen receptors remain controversial. We have used confocal microscopy and cell fractionation on the murine hippocampus-derived HT22 cell line and rat primary cortical neurons transfected with estrogen receptor-green fluorescent protein constructs to address the membrane localization of these receptors. We observe translocation of estrogen receptor beta (beta) to the plasma membrane 5 min after exposure to 17beta-estradiol, whereas estrogen receptor alpha (alpha) localization remains unchanged. Membrane localization of estrogen receptor beta is transient, selective for 17beta-estradiol, and is not blocked by ICI182,780. Inhibition of the mitogen-activated protein kinase pathway does not block estrogen-mediated estrogen receptor beta membrane translocation, and in fact prolongs membrane localization. These data suggest that while both estrogen receptor alpha and estrogen receptor beta can be present at the neuronal membrane, their presence is differentially regulated.

Effects of exon-deleted estrogen receptor β transcript variants on growth, apoptosis and gene expression of human breast cancer cell lines

Estrogen receptor beta gene codes for a variety of transcript isoforms resulting from alternative splicing, which are expressed both in mammary gland and in breast cancer cells. We studied the function of two exon-deleted ERbeta isoforms recently identified by our group in comparison to ERbeta1 in regulation of growth, apoptosis and gene expression of two breast cancer cell lines with different ERalpha status. Overexpression of ERbeta1, but not of the exon-deleted variants exerted strong antitumoral effects both on ERalpha-positive MCF-7 and ERalpha-negative SK-BR-3 cells. ERbeta1 overexpression slowed growth of MCF-7 and SK-BR-3 cells in the absence of E2 and also inhibited E2-triggered growth stimulation of MCF-7 cells, but overexpression of the exon-skipped variants did not affect cell growth. Whereas overexpression of ERbeta1 triggered an increased basal and tamoxifen-induced apoptosis of MCF-7 and SK-BR-3 cells, the isoforms ERbetadelta125 or ERbetadelta1256 did not affect cellular tamoxifen response. The observed lack of function of the exon-deleted variants in terms of regulation of proliferation was accompanied both by their inability to affect expression of cyclins D1 and A2, p21 (WAF1) and PR and their disability to modulate estrogen response element (ERE) activation. In contrast, our results demonstrating antitumoral effects of ERbeta1 on breast cancer cells with different ERalpha-status support the hypothesis that ERbeta is able to exert antitumoral actions both on ERalpha-positive and -negative breast cancer cells.

Genetics and pharmacogenetics of estrogen response

Estrogens are a steroid hormone group distributed widely in animals and human beings. Estrogens diffuse across cell phospholipidic membranes and interact with estrogen receptors. Their highest concentration is found in target tissues with reproductive function (breast, ovary, vagina and uterus). High estrogen levels are usually associated with tumor onset and progression, while loss of estrogen or its receptor(s) contributes to development and/or progression of various diseases (osteoporosis, neurodegenerative disease and cardiovascular disease). Despite the numerous efforts to highlight estrogen's mechanism of action, recent discoveries showed an unexpected degree of complexity of estrogenic response.

Estrogen receptor-beta mediates dihydrotestosterone-induced stimulation of the arginine vasopressin promoter in neuronal cells

Arginine vasopressin (AVP) is a neuropeptide involved in the regulation of fluid balance, stress, circadian rhythms, and social behaviors. In the brain, AVP is tightly regulated by gonadal steroid hormones in discrete regions with gonadectomy abolishing and testosterone replacement restoring normal AVP expression in adult males. Previous studies demonstrated that 17beta-estradiol, a primary metabolite of testosterone, is responsible for restoring most of the AVP expression in the brain after castration. However, 5alpha-dihydrotestosterone (DHT) has also been shown to play a role in the regulation of AVP expression, thus implicating the involvement of both androgen and estrogen receptors (ER). Furthermore, DHT, through its conversion to 5alpha-androstane-3beta,17beta-diol, has been shown to modulate estrogen response element-mediated promoter activity through an ER pathway. The present study addressed two central hypotheses: 1) that androgens directly modulate AVP promoter activity and 2) the effect is mediated by an estrogen or androgen receptor pathway. To that end, we overexpressed androgen receptor, ERbeta, and ERbeta splice variants in a neuronal cell line and measured AVP promoter activity using a firefly luciferase reporter assay. Our results demonstrate that DHT and its metabolite 5alpha-androstane-3beta,17beta-diol stimulate AVP promoter activity through ERbeta in a neuronal cell line.

Estrogen receptor beta in the brain: from form to function

Estrogens have numerous effects on the brain, both in adulthood and during development. These actions of estrogen are mediated by two distinct estrogen receptor (ER) systems, ER alpha (ERalpha) and ER beta (ERbeta). In brain, ERalpha plays a critical role in regulating reproductive neuroendocrine function and behavior, however, a definitive role for ERbeta in any neurobiological function has been slow in forthcoming. Clues to the function of ERbeta in the central nervous system can be gleaned from the neuroanatomical distribution of ERbeta and the phenotypes of neurons that express ERbeta. ERbeta immunoreactivity has been found in populations of GnRH, CRH, vasopressin, oxytocin and prolactin containing neurons in the hypothalamus. Utilizing subtype-selective estrogen receptor agonists can help determine the roles for ERbeta in non-reproductive behaviors in rat models. ERbeta-selective agonists exert potent anxiolytic activity when animals were tested in a number of behavioral paradigms. Consistent with this, ERbeta-selective agonists also inhibited the ACTH and corticosterone response to stress. In contrast, ERalpha selective agonists were found to be anxiogenic and correspondingly increased the hormonal stress response. Taken together, our studies implicate ERbeta as an important modulator of some non-reproductive neurobiological systems. The molecular and neuroanatomical targets of estrogen that are mediated by ERbeta remain to be determined. A number of splice variants of ERbeta mRNA have been reported in brain tissue. Imaging of eGFP labeled chimeric receptor proteins transfected into cell lines shows that ERbeta splice variation can alter trafficking patterns and function. The originally described ERbeta (herein termed ERbeta1) is characterized by possessing a high affinity for estradiol. Similar to ERalpha, it is localized in the nucleus and is trafficked to nuclear sites termed "hyperspeckles" following ligand binding. In contrast, ERbeta2 contains an 18 amino acid insert within the ligand-binding domain and as a result can be best described as a low affinity form of ERbeta. A delta3 (delta3) variant of ERbeta has a deletion of the 3rd exon (coding for the second half of the DNA-binding domain) and as a result does not bind an estrogen response element in DNA. delta3 variants are trafficked to a unique low abundance and larger nuclear site following ligand binding. A delta4 (delta4) variant lacks exon 4 and as a result is localized to the cytoplasm. The amount of individual splice variant mRNAs varies depending upon brain region. Examination of neuropeptide promoter regulation by ERbeta splice variants demonstrates that ERbeta functions as a constitutively active transcription factor. Moreover, it appears that splice variation of ERbeta alters its ability to regulate transcription in a promoter-dependent and ligand-dependent fashion.

Detection and localization of an estrogen receptor beta splice variant protein (ERβ2) in the adult female rat forebrain and midbrain regions

Estrogens regulate neural processes such as neuronal development, reproductive behavior, and hormone secretion, and signal through estrogen receptor (ER) alpha and ERbeta (here called ERbeta1). Recent studies have found variations in ERalpha and ERbeta1 mRNA splicing in rodents and humans. Functional reporter gene assays suggest that these splicing variations alter ER-mediated transcriptional regulation. Estrogen receptor beta 2 (ERbeta2), an ERbeta1 splice variant containing an 18 amino acid (AA) insert in the ligand binding domain, binds estradiol with approximately 10-fold lower affinity than ERbeta1, suggesting that it may serve as a low-affinity ER. Moreover, ERbeta2 reportedly acts in a dominant-negative fashion when heterodimerized with ERbeta1 or ERalpha. To explore the function of ERbeta2 in brain, an antiserum (TwobetaER.1) targeting the 18 AA insert was developed and characterized. Western blot analysis and transient expression of ERbeta2 in cell lines demonstrated that TwobetaER.1 recognizes ERbeta2. In the adult female rat brain, ERbeta2 immunoreactivity is localized in the cell nucleus and is expressed with a distribution similar to that of ERbeta1 mRNA. ERbeta2 immunoreactive cell numbers were high in, for example, piriform cortex, paraventricular nucleus, supraoptic nucleus, arcuate nucleus, and hippocampal CA regions, whereas it was low in the dentate gyrus. Moreover, ERbeta2 is coexpressed in gonadotropin-releasing hormone and oxytocin neurons. These studies demonstrate ERbeta splice variant proteins in brain and support the hypothesis that ER signaling diversity depends not only on ligand or coregulatory proteins, but also on regional and phenotypic selectivity of ER splice variant proteins.

The hair follicle as an estrogen target and source

For many decades, androgens have dominated endocrine research in hair growth control. Androgen metabolism and the androgen receptor currently are the key targets for systemic, pharmacological hair growth control in clinical medicine. However, it has long been known that estrogens also profoundly alter hair follicle growth and cycling by binding to locally expressed high-affinity estrogen receptors (ERs). Besides altering the transcription of genes with estrogen-responsive elements, 17beta-estradiol (E2) also modifies androgen metabolism within distinct subunits of the pilosebaceous unit (i.e., hair follicle and sebaceous gland). The latter displays prominent aromatase activity, the key enzyme for androgen conversion to E2, and is both an estrogen source and target. Here, we chart the recent renaissance of estrogen research in hair research; explain why the hair follicle offers an ideal, clinically relevant test system for studying the role of sex steroids, their receptors, and interactions in neuroectodermal-mesodermal interaction systems in general; and illustrate how it can be exploited to identify novel functions and signaling cross talks of ER-mediated signaling. Emphasizing the long-underestimated complexity and species-, gender-, and site-dependence of E2-induced biological effects on the hair follicle, we explore targets for pharmacological intervention in clinically relevant hair cycle manipulation, ranging from androgenetic alopecia and hirsutism via telogen effluvium to chemotherapy-induced alopecia. While defining major open questions, unsolved clinical challenges, and particularly promising research avenues in this area, we argue that the time has come to pay estrogen-mediated signaling the full attention it deserves in future endocrinological therapy of common hair growth disorders.

Evidence for estrogen receptor α and β expression in skeletal muscle of pigs

Recent research suggests that estrogen receptors (ERs) are of significance in skeletal muscle function. The aim of the present study was to investigate, whether ERalpha and ERbeta are expressed in different porcine skeletal muscles and in satellite cells derived from semimembranosus muscle (SM) at the protein and mRNA level. Immunohistochemistry demonstrated positive staining for ERalpha in the nuclei of skeletal muscle cells, while the ERbeta stain showed positive signals in nuclei and cytoplasm of skeletal myofibers and myoblasts derived from satellite cells. Additionally, a weak expression of both ER subtypes was seen in skeletal muscle tissue and SM satellite cells with Western blot analysis. A clear expression of the ERalpha mRNA and a weak expression of the ERbeta mRNA was seen in skeletal muscle tissue and SM satellite cell cultures, as determined by reverse transcription (RT)-PCR. The present study shows for the first time that both ERalpha and ERbeta are expressed in porcine skeletal muscle, which, consequently, could be considered as a target tissue for estrogens or estrogen-like compounds. However, more detailed studies on the functional impact of both receptor subtypes in skeletal muscle are necessary. The porcine SM satellite cell culture provides a suitable in vitro model to investigate estrogenic effects on pig skeletal muscle.

Subnuclear compartmentalization of sequence-specific transcription factors and regulation of eukaryotic gene expression

To date, the majority of the research regarding eukaryotic transcription factors has focused on characterizing their function primarily through in vitro methods. These studies have revealed that transcription factors are essentially modular structures, containing separate regions that participate in such activities as DNA binding, protein-protein interaction, and transcriptional activation or repression. To fully comprehend the behavior of a given transcription factor, however, these domains must be analyzed in the context of the entire protein, and in certain cases the context of a multiprotein complex. Furthermore, it must be appreciated that transcription factors function in the nucleus, where they must contend with a variety of factors, including the nuclear architecture, chromatin domains, chromosome territories, and cell-cycle-associated processes. Recent examinations of transcription factors in the nucleus have clarified the behavior of these proteins in vivo and have increased our understanding of how gene expression is regulated in eukaryotes. Here, we review the current knowledge regarding sequence-specific transcription factor compartmentalization within the nucleus and discuss its impact on the regulation of such processes as activation or repression of gene expression and interaction with coregulatory factors.

ERbeta protein expression in female cynomolgus monkey and CF-1 mouse brain: Western analysis

In humans and rodents, multiple ERbeta variants with sizes ranging from 477-549 amino acids (aa) have been described. The identification of these variants in target tissues has important implications for estrogen signaling and cellular responsiveness. Western blot analysis using two anti-ERbeta antibodies specific for mammalian ERbeta sequences (PA1-310B and PA1-311) was employed to examine ERbeta protein expression in neural tissues from ovariectomized (OVX) cynomolgus macaques and CF-1 mice as well as to assess potential regulatory effects of acute and extended estradiol (E(2)) treatment. In hypothalamic extracts from both species, a single ERbeta immunoreactive (ERbeta-ir) band was detected at approximately 54 kDa, corresponding to the expected molecular weight for ERbeta477 and/or 485. In cynomolgus females, oral E(2) administration for 16 weeks had no apparent effect on hypothalamic ERbeta protein expression. In mouse, a single injection of E(2) did not change hypothalamic ERbeta protein levels 1.5, 4, 8, 16, or 24 h after injection. Extending the hormonal treatment to 4 or 21 days in OVX female mice also had no effect on the level of hypothalamic ERbeta protein. Additional regional analyses in female mouse brain with PA1-310B antibody showed that a second, 59 kDa ERbeta-ir band was present in cortex, striatum, hippocampus, and amygdala that could represent one or both of the larger ERbeta variants (530 and 549aa). The expression level of the second ERbeta isoform exhibited regional variation, with the strongest immunoreactivity detected in cortex and amygdala. Elucidating the functions of these ERbeta isoforms in the CNS will facilitate our understanding of the tissue- and promoter-specific actions of estrogen.

Estrogen receptor (ER)beta isoforms rather than ERalpha regulate corticotropin-releasing hormone promoter activity through an alternate pathway

The hypothalamic-pituitary-adrenal axis regulates mammalian stress responses by secreting glucocorticoids. The magnitude of the response is in part determined by gender, for in response to a given stressor, circulating glucocorticoids reach higher levels in female rats than in males. This gender difference could result from estrogen regulation of the corticotropin-releasing hormone (CRH) promoter via either of its receptors: estrogen receptor (ER)alpha or ERbeta. Immunocytochemistry revealed that a subset (12%) of medial parvocellular CRH neurons in the rat hypothalamus contain ERbeta but not ERalpha. To determine whether ERs could regulate CRH promoter activity, we cotransfected cells with a CRH promoter construct and either ERalpha or individual ERbeta isoforms. ERalpha weakly stimulated CRH promoter transcriptional activity in a ligand-independent manner. Conversely, all ERbeta isoforms tested stimulated CRH promoter activity with different ligand profiles. ERbeta1 and ERbeta2delta3 displayed constitutive activity (ERbeta1 more than ERbeta2delta3). Ligand-dependent activity of beta isoforms 1 and 2 was altered by an Exon3 splice variant (delta3) or by the additional 18 amino acids in the ligand-binding domain of ERbeta2 isoforms. Lastly, we suggest that ER regulation of CRH takes place through an alternate pathway, one that requires protein-protein interactions with other transcription factors or their associated complexes. However, a pure ER-activator protein-1 alternate pathway does not appear to be involved.

Mechanisms of estrogen receptor signaling: convergence of genomic and nongenomic actions on target genes

Estrogen receptors (ERs) act by regulating transcriptional processes. The classical mechanism of ER action involves estrogen binding to receptors in the nucleus, after which the receptors dimerize and bind to specific response elements known as estrogen response elements (EREs) located in the promoters of target genes. However, ERs can also regulate gene expression without directly binding to DNA. This occurs through protein-protein interactions with other DNA-binding transcription factors in the nucleus. In addition, membrane-associated ERs mediate nongenomic actions of estrogens, which can lead both to altered functions of proteins in the cytoplasm and to regulation of gene expression. The latter two mechanisms of ER action enable a broader range of genes to be regulated than the range that can be regulated by the classical mechanism of ER action alone. This review surveys our knowledge about the molecular mechanism by which ERs regulate the expression of genes that do not contain EREs, and it gives examples of the ways in which the genomic and nongenomic actions of ERs on target genes converge. Genomic and nongenomic actions of ERs that do not depend on EREs influence the physiology of many target tissues, and thus, increasing our understanding of the molecular mechanisms behind these actions is highly relevant for the development of novel drugs that target specific receptor actions.

The androgen metabolite, 5alpha-androstane-3beta, 17beta-diol, is a potent modulator of estrogen receptor-beta1-mediated gene transcription in neuronal cells

5alpha-Androstane-3beta, 17beta-diol (3betaAdiol) is a metabolite of the potent androgen, 5alpha-dihydrotestosterone. Recent studies showed that 3betaAdiol binds to estrogen receptor (ER)-beta and regulates growth of the prostate gland through an estrogen, and not androgen, receptor-mediated pathway. These data raise the possibility that 3betaAdiol could regulate important physiological processes in other tissues that produce 3betaAdiol, such as the brain. Although it is widely accepted that the brain is a target for 5alpha-dihydrotestosterone action, there is no evidence that 3betaAdiol has a direct action in neurons. To explore the molecular mechanisms by which 3betaAdiol might act to modulate gene transcription in neuronal cells, we examined whether 3betaAdiol activates ER-mediated promoter activity and whether ER transactivation is facilitated by a classical estrogen response element (ERE) or an AP-1 complex. The HT-22 neuronal cell line was cotransfected with an expression vector containing ERalpha, ER-beta1, or the ERbeta splice variant, ER-beta2 and one of two luciferase-reporter constructs containing either a consensus ERE or an AP-1 enhancer site. Cells were treated with 100 nM 17beta-estradiol, 100 nM 3betaAdiol, or vehicle for 15 h. We show that 3betaAdiol activated ER-beta1-induced transcription mediated by an ERE equivalent to that of 17beta-estradiol. By contrast, 3betaAdiol had no effect on ERalpha- or ER-beta2-mediated promoter activity. Moreover, ER-beta1 stimulated transcription mediated by an ERE and inhibited transcription by an AP-1 site in the absence of ligand binding. These data provide evidence for activation of ER signaling pathways by an androgen metabolite in neuronal cells.

Loss of ERbeta expression as a common step in estrogen-dependent tumor progression

The characterization of estrogen receptor beta (ERbeta) brought new insight into the mechanisms underlying estrogen signaling. Estrogen induction of cell proliferation is a crucial step in carcinogenesis of gynecologic target tissues, and the mitogenic effects of estrogen in these tissues (such as breast, endometrium and ovary) are well documented both in vitro and in vivo. There is also an emerging body of evidence that colon and prostate cancer growth is influenced by estrogens. In all of these tissues, most studies have shown decreased ERbeta expression in cancer as compared with benign tumors or normal tissues, whereas ERalpha expression persists. The loss of ERbeta expression in cancer cells could reflect tumor cell dedifferentiation but may also represent a critical stage in estrogen-dependent tumor progression. Modulation of the expression of ERalpha target genes by ERbeta or ERbeta-specific gene induction could explain that ERbeta has a differential effect on proliferation as compared with ERalpha. ERbeta may exert a protective effect and thus constitute a new target for hormone therapy, such as ligand specific activation. The potential distinct roles of ERalpha and ERbeta expression in carcinogenesis, as suggested by experimental and clinical data, are discussed in this review.

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

Background

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

Results

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

Conclusions

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

Molecular assays to profile 10 estrogen receptor beta isoform mRNA copy numbers in ovary, breast, uterus, and bone tissues

Estrogens regulate various biological processes in a diverse range of reproductive and nonreproductive tissues through two genetically distinct but structurally related high affinity nuclear receptors, the estrogen receptor alpha and beta (ERalpha and ERbeta). The physiological significance of the presence of two ERs that have redundant functions is not known. Several unique properties of ERbeta together with its distinct expression patterns are considered to be, in part, the basis for diverse functional actions of estrogens and opposing actions of selective estrogen receptor modulators (SERMs) in different tissues. To understand how relative expression levels of two ERs correlate to seemingly dissimilar actions of estrogens and SERMs, quantitative methods are required that can precisely measure the levels of every isoform. Previously, methods to quantify eight ERalpha isoforms have been described [Poola I. (2003) Anal. Biochem. 314, 217-226]. In this article, real-time PCRbased molecular assays are described that can distinguish and quantify as low as 100 copies of 10 ERbeta isoform mRNAs, the ERbeta1, ERbeta2, ERbeta4, ERbeta5, and ERbeta exon 2Delta, exon 3Delta, exon 4Delta, exon 5Delta, exon 6Delta, and exons 5-6Delta. Each isoform mRNA is quantified using a specific primer pair and a 5'FAM (carboxy-fluorescein)- and 3'TAMARA (6-carboxy tetraethyl-rhodamine)-labeled probe and in comparison with a standard curve constructed with known copy numbers of its respective reverse transcribed cRNA. The devised assays were applied to profile 10 ERbeta isoforms in four estrogen-sensitive tissues-ovary, breast, uterus, and bone. The sensitivity of detection of each isoform in these tissues varied from picograms to nanograms of reverse-transcribed total RNA depending on the isoform and the tissue. The results presented also show that each tissue has a distinct profile of 10 isoform mRNAs. Interestingly, ERalpha- negative breast cancer cell lines and tumors expressed significant amounts of ERbeta isoforms suggesting that mitogenic stimulation by estrogen exists in these tissues. Bone tissues expressed several isoforms, although wild type was not present. In addition to the assay development, evidence is presented to demonstrate for the first time that ERbeta4 and ERbeta5 are full length receptors, contrary to previous reports that they are short receptors of exon 7-8.

Immunolocalization of estrogen receptor beta in the mouse brain: comparison with estrogen receptor alpha

Estrogen receptor alpha (ER alpha) and ER beta are members of the steroid nuclear receptor family that modulate gene transcription in an estrogen-dependent manner. ER mRNA and protein have been detected both peripherally and in the central nervous system, with most data having come from the rat. Here we report the development of an ER beta-selective antibody that cross-reacts with mouse, rat, and human ER beta protein and its use to determine the distribution of ER beta in the murine brain. Further, a previously characterized polyclonal antibody to ER alpha was used to compare the distribution of the two receptors in the first comprehensive description of ER distribution specifically in the mouse brain. ER beta immunoreactivity (ir) was primarily localized to cell nuclei within select regions of the brain, including the olfactory bulb, cerebral cortex, septum, preoptic area, bed nucleus of the stria terminalis, amygdala, paraventricular hypothalamic nucleus, thalamus, ventral tegmental area, substantia nigra, dorsal raphe, locus coeruleus, and cerebellum. Extranuclear-ir was detected in several areas, including fibers of the olfactory bulb, CA3 stratum lucidum, and CA1 stratum radiatum of the hippocampus and cerebellum. Although both receptors were generally expressed in a similar distribution through the brain, nuclear ER alpha-ir was the predominant subtype in the hippocampus, preoptic area, and most of the hypothalamus, whereas it was sparse or absent from the cerebral cortex and cerebellum. Collectively, these findings demonstrate the region-selective expression of ER beta and ER alpha in the adult ovariectomized mouse brain. These data provide an anatomical framework for understanding the mechanisms by which estrogen regulates specific neural systems in the mouse.

Isoform/variant mRNAs for sex steroid hormone receptors in humans

The open reading frames of human sex steroid hormone receptors (hSSHRs) are composed of eight exons. In addition, the presence of various exons - including 5'-untranslated exons, alternative coding exons and novel 'intronic' exons - has been demonstrated in the genes encoding hSSHRs. The isoform/variant hSSHR mRNAs generated from thes e exons can be tentatively classified into seven types. In type 1, different mRNAs are generated with the use of alternative transcription start sites. In type 2, one or more exons are skipped. In type 3, one or more exons are duplicated. In type 4, distinct mRNAs containing different 5'-untranslated exon(s) are synthesized. In type 5, distinct mRNAs possessing different coding exon(s) are generated. In type 6, mRNA is synthesized by intronic exons and coding exons 4/5-8. In type 7, mRNA with insertion of intronic exon(s) is generated. Here, we review the isoform/variant hSSHR mRNAs and the structure of the genes encoding them.

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

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

Cloning and characterization of porcine ovarian estrogen receptor beta isoforms

The cDNA for the full-length porcine estrogen receptor beta (ER beta) and an alternatively spliced transcript with a deletion of exon 5 (ER beta delta 5) was cloned from pig ovary. RNase protection assays revealed that ER beta mRNA was expressed in the preovulatory follicles and early, midluteal, and regressing corpora lutea (CL) of eCG +/- hCG-primed gilts. ER beta and ER beta delta 5 transcripts were shown by semiquantitative reverse transcription polymerase chain reaction to be expressed at a ratio of approximately 2:1 in granulosa cells, small, medium, and large antral follicles, and midluteal phase corpora lutea of unprimed animals. Immunoreactive ER beta proteins corresponding to the size of in vitro translated ER beta and ER beta delta 5 were detected by immunoblot. Full-length ER beta was detected in granulosa, small, medium, and large antral follicles, and midluteal phase CL of unprimed animals. Putative ER beta delta 5 immunoreactive bands were abundant only in granulosa cell extracts. In COS-1 cells, transfected ER beta delta 5 had no effect on basal transcription of an estrogen-responsive reporter construct but did repress wild-type ER beta transactivation when cotransfected at 10-fold excess plasmid. No repression of ER alpha transactivation was observed. In primary granulosa cell cultures, transfected ER beta delta 5 plasmid did not inhibit basal reporter activation. ER beta delta 5 was shown by immunofluorescence to localize to the nucleus in transfected COS-1 cells. In vitro translated ER beta delta 5 proteins bound estrogen response elements in DNA in electrophoretic mobility shift assays, as indicated by supershift analysis. ER beta is abundant in porcine ovary, and a naturally occurring splice variant missing exon 5 may have biological function.

Oestrogen receptors, receptor variants and oestrogen actions in the hypothalamic-pituitary axis

Information on oestrogen action has grown exponentially in the past decade, and recent studies have begun to define the mechanism of ligand-dependent activation and cell-specific effects. Oestrogen-mediated gene transcription in a specific tissue depends on several factors, the most important of which is the presence of at least one of the two nuclear oestrogen receptor (ER) isoforms, ER(alpha) and ERbeta. The presence and levels of specific ER isoform variants, along with receptor coactivator, corepressor and integrator proteins, directly modulate overall nuclear ER activity. The structure of the ligand, including both physiological oestrogens and synthetic oestrogen receptor modulators, influences ER interactions with these other proteins and thus determines the biological response. Furthermore, peptide and neurotransmitter-stimulated intracellular signalling pathways activate specific enzyme cascades and may modify the receptors and their interacting proteins, resulting in either independent or ligand-enhanced ER-mediated responses. Finally, several rapid effects of oestrogen probably occur at the membrane through nongenomic pathways that may or may not require the same ER proteins that are found in the nucleus. This review concentrates on the pituitary-hypothalamic axis and the genomic effects of oestrogen, and discusses the current knowledge of each of these factors in determining oestrogen actions in the neuroendocrine system.

Oestrogen receptor beta-immunoreactivity in gonadotropin releasing hormone-expressing neurones: regulation by oestrogen

Double-label immunohistochemistry was employed to establish whether immunoreactivity for the beta subtype of the oestrogen receptor (ER beta-IR) is present in gonadotropin releasing hormone (GnRH)-containing cells. In the immortalized GnRH cell line, GT1-7, almost all nuclei were immunoreactive for ER beta. In the preoptic area of ovariectomized rats, more than one-half of the GnRH neurones (52.0-63.5%) contained ER beta-IR within the nucleus; a smaller proportion of these neurones (5-10%) displayed a particularly intense nuclear signal for ER beta. The presence of ER beta-IR in the nuclei of GT1-7 cells and GnRH neurones is consistent with recent reports of ER beta mRNA in these cells. Oestrogen treatment reduced the percentage of GnRH neurones with detectable ER beta-IR. The range of signal intensity for ER beta and the incidence of the ER beta signal in GnRH neurones were comparable following double-label immunohistochemistry involving either bright field or fluorescent techniques. These findings raise the possibility that ER beta receptors mediate direct effects of oestrogen on GnRH neurones.