Estrogen regulates the expression of several different estrogen receptor mRNA isoforms in rat pituitary

Estrogen Receptor α Isoforms Generated by Alternative Use of Cryptic Exons

Estrogen receptor α (ERα) regulates several physiological functions. In pathophysiological conditions, ERα is involved in the development and progression of estrogen-sensitive tumors. The ERα gene contains multiple 5'-untranslated exons and eight conventional coding exons and presents multiple isoforms generated by alternative promoter usage and alternative splicing. This gene also possesses non-conventional exons in the 3'- and intronic regions, and alternative use of cryptic exons contributes to further diversity of ERα mRNAs and proteins. Recently, the genomic organization of ERα genes and the splicing profiles of their transcripts were comparatively analyzed in humans, mice, and rats, and multiple ERα isoforms with distinct structures and functions were identified. These transcripts contain cryptic sequences that encode insertion-containing or truncated ERα proteins. In particular, alternative cryptic exons with in-frame stop codons yield transcripts encoding C-terminally-truncated ERα proteins. The C-terminally-truncated ERα isoforms lack part or all of the ligand-binding domain but possess an isoform-specific sequence. Some of these isoforms exhibit constitutive transactivation and resistance to estrogen receptor antagonists. Although numerous studies have reported conflicting results regarding their functions, the critical determinant for their gain-of-function has been identified structurally. Here we review recent progress in ERα variant research concerning the genomic organization of ERα genes, splicing profiles of ERα transcripts, and transactivation properties of ERα isoforms.

Estrogen Receptor Alpha Splice Variants, Post-Translational Modifications, and Their Physiological Functions

The importance of estrogenic signaling for a broad spectrum of biological processes, including reproduction, cancer development, energy metabolism, memory and learning, and so on, has been well documented. Among reported estrogen receptors, estrogen receptor alpha (ERα) has been known to be a major mediator of cellular estrogenic signaling. Accumulating evidence has shown that the regulations of ERα gene transcription, splicing, and expression across the tissues are highly complex. The ERα promoter region is composed of multiple leader exons and 5′-untranslated region (5′-UTR) exons. Differential splicing results in multiple ERα proteins with different molecular weights and functional domains. Furthermore, various post-translational modifications (PTMs) further impact ERα cellular localization, ligand affinity, and therefore functionality. These splicing isoforms and PTMs are differentially expressed in a tissue-specific manner, mediate certain aspects of ERα signaling, and may work even antagonistically against the full-length ERα. The fundamental understanding of the ERα splicing isoforms in normal physiology is limited and association studies of the splicing isoforms and the PTMs are scarce. This review aims to summarize the functional diversity of these ERα variants and the PTMs in normal physiological processes, particularly as studied in transgenic mouse models.

Stress and steroid interaction modulates expression of estrogen receptor alpha in the brain, pituitary, and testes of immature Gallus gallus domesticus

In nature, food availability stimulates hypothalamo-pituitary-gonadal (HPG) axis while its scarcity induces stress, which further stimulates hypothalamo-pituitary-adrenal (HPA) axis producing a detrimental effect on the avian reproductive physiology. The present experiment was designed to examine the interaction of stress like food restriction and estradiol on male reproductive physiology with special emphasis on estrogen receptor alpha (ERα) as these play crucial role in reproduction. To achieve this, 60 day old White Leghorn immature cockrels were taken and divided into four groups (n  =  8 per group). One group was provided with food and water ad libitum. Second group was food restricted (FR) for 9 h/day after 5 days, third and fourth were administered with estradiol benzoate (EB 0.5 mg/100g/day) for 12 days. Fourth group was FR for 9 h/day after 5 days of EB treatment till last day of experiment (EB + FR). Immunofluorescent localization of ERα was principally in the pre-optic area and paraventricular nuclei of hypothalamus and in anterior pituitary gland. ERα expression was highly reduced (from 40 AU to 20 AU) after FR in testis but it increased (50 AU) after EB administration, EB + FR reflects a diminishing pattern in the increment after EB. FR decreased plasma estradiol while EB increased it. Increased plasma corticosterone, hydrogen peroxide, malondialdehyde, and decreased anti-oxidant enzymes in brain and testis of all groups indicate oxidative stress in the HPG axis. The increased ERα after EB and a decrease with FR and EB + FR support their reproductive function. Estrogen and its receptor alpha are responsible for maintaining epithelial morphology but FR along with EB administration modulates the testicular development by significantly decreasing its size (p<.0001) and seminiferous tubules (p<.0001) and no sperm formation via highly reduced expression of ir-ERα in HPG axis. Our findings led us to conclude that stress like FR and estradiol induces testicular regression immature male chickens by modulating ir-ERα expression in the HPG axis thereby resulting in reduction in reproductive physiology.

N-terminal truncations in sex steroid receptors and rapid steroid actions

Sex steroid receptors act as ligand activated nuclear transcription factors throughout the body, including the brain. However, post-translational modification of these receptors can direct them to extranuclear sites, including the plasma membrane, where they are able to initiate rapid signaling. Because of the conserved domain structure of these receptors, alternative exon splicing can result in proteins with altered nuclear and extranuclear actions. Although much attention has focused on internal and C-terminal splice variants, both estrogen and androgen receptors undergo N-terminal truncations, as well. These truncated proteins not only influence the transcriptional activity of the full-length receptors, but also associate with caveolin and initiate signaling at the plasma membrane. Such actions may have important physiological consequences in neuronal, endothelial, and cancer signaling and cell survival.

Characterization of rodent constitutively active estrogen receptor α variants and their constitutive transactivation mechanisms

Estrogen receptor α (ERα) mRNAs exhibit remarkable heterogeneity owing to complicated alternative splicing. Some encode C-terminally-truncated ERα proteins, which display ligand-independent transactivation or dominant-negative activity. We previously characterized C-terminally-truncated ERα mRNA variants with cryptic sequences in humans and mice, and demonstrated that helices in the ligand-binding domains (LBDs) of ERα variants contribute to ligand-independent transcriptional activity. However, existence of non-conventional coding exons and generation of constitutively active ERα variants have remained to be examined in rats. To comparatively analyze modular organization and splicing profiles of the ERα genes, the range of C-terminally-truncated ERα variants was explored in rats and mice using rapid amplification of cDNA ends and RT-PCR cloning. Furthermore, their functions were determined in transiently transfected cells using expression constructs and luciferase reporter assays. Multiple cryptic exons and C-terminally-truncated ERα variant mRNAs were identified in rats and mice. Naturally occurring and artificially truncated variants/constructs lacking helix 5 potentially exhibited gain-of-function in transfected cells. Although cryptic exons and splicing profiles were poorly conserved among humans, mice, and rats, constitutively active variants were generated from the ERα genes. Moreover, the primary mechanism of ligand-independent activation by C-terminally-truncated ERα variants is C-terminus to helix 5 deletion in the LBD. This comparative study documented the complexity of ERα genes, mRNAs, and proteins, and further determined the underlying structural basis of ligand-independent activation by C-terminally-truncated ERα variants.

Spatiotemporal expression analysis of nuclear estrogen receptors in the zebrafish ovary and their regulation in vitro by endocrine hormones and paracrine factors

Estradiol (E2) stimulates luteinizing hormone receptor (lhcgr) expression via nuclear estrogen receptors (nERs) in the zebrafish ovary. We have demonstrated that endocrine hormones such as gonadotropin (hCG) and paracrine factors such as epidermal growth factor (EGF) and pituitary adenylate cyclase-activating peptide (PACAP) can modulate E2-induced lhcgr expression in vitro. These observations raised a question on whether these hormones and factors exert their effects via regulating the expression of nERs. In this study, we first characterized the spatiotemporal expression profiles of three nER subtypes in the zebrafish ovary, including esr1 (ERα), esr2a (ERβ2) and esr2b (ERβ1). All three nERs increased their expression at the pre-vitellogenic stage and peaked at mid- (esr1 and esr2a) or late vitellogenic (esr2b) stage, followed by a significant decline at the full-grown stage. RT-PCR analysis showed that esr1 and esr2b were exclusively expressed in the follicle layer while esr2a was expressed in both compartments. We then examined how E2, hCG, PACAP and EGF regulated the expression of nERs in cultured zebrafish follicle cells. E2 quickly increased esr1 but reduced esr2a and esr2b expression from 1.5 to 12h of treatment. Similarly, EGF down-regulated esr2a significantly at 1.5h and this effect was further intensified at 24h. hCG decreased the expression of all three nER subtypes with similar potency throughout the 24-h time-course. Interestingly, PACAP exerted a biphasic regulation on esr2a. Our present study suggests that nERs, especially esr2a, provide potential target points for other hormones and factors to modulate E2 activity during folliculogenesis in the zebrafish.

Evaluation of in vivo estrogenic potency of natural estrogen-active chemical, puerarin, on pituitary function in gonadectomized female rats

AIMS

Previous research has revealed that puerarin, the major phytoestrogen in tuberous roots of Pueraria lobata and Pueraria mirifica, acts as a selective estrogen receptor modulator that displays predominantly estrogenic potential for health benefit. However, little is known about the estrogenic potency of puerarin in pituitary, especially in the rat model of postmenopausal females.

MAIN METHODS

Plasma prolactin and growth hormone levels as well as mRNA expression levels of pituitary estrogen-regulated genes, such as estrogen receptor (ER) subtypes alpha (ERα) and beta (ERβ), truncated ER product-1 (TERP-1) and -2 (TERP-2) and gonadotropin alpha subunit, were examined using radioimmunoassay and TaqMan® real-time PCR, respectively. The effects were compared with the potent ER agonist, 17β-estradiol-3-benzoate (E2B), and both substances were supplemented at low and high doses, i.e., 0.6 or 3g puerarin and 0.0043 or 0.0173g E2B per kilogram of phytoestrogens-free rat chow, and applied to ovariectomized rats (five groups; 11-12 rats per group) for 12weeks.

KEY FINDINGS

Puerarin possessed weak E2B-like activities on pituitary function by acting as ERβ and TERP-1/-2 agonists, which resulted in the downregulation and upregulation of ERβ and TERP-1/-2 mRNA expressions, respectively, and elevation of growth hormone levels. There were trends of decreased levels of alpha subunit mRNA transcripts and increased levels of prolactin in puerarin-treated rats as observed in E2B-treated animals.

SIGNIFICANCE

This is the first report in ovariectomized rats the effects of puerarin on somatotropes and pituitary estrogen-responsive mRNA expressions, which are very weakly estrogenic by acting through ERβ- and TERP-1/-2 mediated pathways.

Effects of resveratrol on cell growth and prolactin synthesis in GH3 cells

Resveratrol (RE), a phytoestrogen, has antiestrogenic properties. Estrogen plays a key role in the development and progression of pituitary prolactinoma. Moreover, RE is a potent cancer chemopreventive agent that inhibits the initiation, promotion and progression of carcinogenesis. The present study investigated the antitumor effects of RE on GH3 pituitary tumor cells. A concentration- and treatment duration-dependent biphasic effect of RE on the proliferation of the GH3 cells was demonstrated. After three days of treatment, RE stimulated proliferation at low concentrations and inhibited proliferation at high concentrations. However, when the treatment duration was reduced to 6 h, RE inhibited proliferation in a concentration-dependent manner. In addition, RE induced apoptosis with the activation of caspase-3 and -8, and decreased the percentage of prolactin (PRL)-immunopositive GH3 cells. Furthermore, RE suppressed expression of the PRL gene and inhibited the cell proliferation and PRL synthesis induced by 17β-estradiol (E2). In GH3 cells, the proliferation response exhibited higher sensitivity to E2 compared with the PRL response; by contrast, the PRL response was more sensitive to RE than the proliferation response was. These results indicate that RE, an antiestrogenic compound, exerts its antitumor effect on GH3 cells through the suppression of GH3 cell growth and through the inhibition of PRL synthesis. The RE-induced cell apoptosis was shown to be caspase-dependent. Therefore, the present study provides support for the use of RE in the chemoprevention and chemotherapy of pituitary prolactinoma.

Identification of androgen receptor variants in testis from humans and other vertebrates

The androgen receptor (AR) is a ligand-activated transcription factor member of the nuclear receptor superfamily. The existence of alternatively spliced variants is well recognised for several members of this superfamily, most of them having functional importance. For example, several testicular oestrogen receptor variants have been suggested to play a role in the regulation of spermatogenesis. However, information on AR variants is mostly related to cancer and androgen insensitivity syndrome (AIS) cases. The objective of this study was to investigate the expression of AR variants in the testis from humans and other vertebrates. Four AR variants [ARΔ2(Stop) , ARΔ2(23Stop) , ARΔ3 and ARΔ4(120)] were identified in human testis. ARΔ2(Stop) and ARΔ3, with exon 2 or 3 deleted, respectively, were also expressed in human liver, lung, kidney and heart. In addition, ARΔ2(Stop) was expressed in rat and gilthead seabream testis, while an ARΔ3 was detected in African clawed frog testis. This is the first report revealing the existence of AR variants in the testis of evolutionarily distant vertebrate species and in nonpathological tissues. These data suggest the functional importance of these novel AR forms and demonstrate a complexity in AR signalling that is not exclusive of pathological conditions.

17β-Estradiol stimulates the translocation of endogenous estrogen receptor α at the plasma membrane of normal anterior pituitary cells

In the present work we aimed at identifying ERα in the plasma membrane of normal anterior pituitary cells and investigated if 17β-estradiol was able to induce their subcellular redistribution. Our results show that about 8% of anterior pituitary cells expressed ERα in the plasma membrane, with the geometrical mean fluorescence intensity being increased after steroid hormone treatment. 17β-Estradiol and the selective ERα agonist PPT induced an increase of ERα expression in the plasma membrane and activated the PKCα/ERK 1/2 pathway in a time-course not compatible with genomic actions, thus supporting the notion of membrane-initiated effects. These findings suggest that 17β-estradiol stimulates the translocation of endogenous ERα to the plasma membrane, consequently modulating this ER pool and leading to cellular biological effects in normal anterior pituitary gland.

Identification of C-terminally and N-terminally truncated estrogen receptor α variants in the mouse

We re-examined mouse ERα mRNA variants using rapid amplification of cDNA ends (RACE) and RT-PCR. Our analysis showed the presence of several mRNA variants containing unique 5'- or 3'-nucleotide sequences. We mapped the cDNA sequences on the mouse genome, and identified four novel 3'-terminal and 5'-leader exons in the intronic region between exons 4 and 5. RT-PCR analysis revealed that the expression patterns of the C-terminally truncated ERα products (CTERPs) were similar to that of Wild-type ERα and that the N-terminally truncated ERα products (NTERPs) appeared to have different expression profiles. Moreover, we constructed expression vectors and analyzed the subcellular localization and the transcriptional activation abilities of the variant proteins in transfected HEK293 cells using immunocytochemistry and luciferase reporter assay. The CTERP variants localized in the nuclei and constitutively activated estrogen response element (ERE)-driven promoters, while the NTERP variant was located in the extra-nuclear regions and had no ability to activate the ERE promoters in the presence or absence of 10 nM estradiol. Our results indicate that the mouse ERα gene is more complex than previously thought in terms of genomic organization and that alternative splicing and alternative usage of intronic promoters contribute to the remarkable diversity of ERα mRNAs and proteins.

Mechanisms and significance of nuclear receptor auto- and cross-regulation

The number of functional hormone receptors expressed by a cell in large part determines its responsiveness to the hormonal signal. The regulation of hormone receptor gene expression is therefore a central component of hormone action. Vertebrate steroid and thyroid hormones act by binding to nuclear receptors (NR) that function as ligand-activated transcription factors. Nuclear receptor genes are regulated by diverse and interacting intracellular signaling pathways. Nuclear receptor ligands can regulate the expression of the gene for the NR that mediates the hormone's action (autoregulation), thus influencing how a cell responds to the hormone. Autoregulation can be either positive or negative, the hormone increasing or decreasing, respectively, the expression of its own NR. Positive autoregulation (autoinduction) is often observed during postembryonic development, and during the ovarian cycle, where it enhances cellular sensitivity to the hormonal signal to drive the developmental process. By contrast, negative autoregulation (autorepression) may become important in the juvenile and adult for homeostatic negative feedback responses. In addition to autoregulation, a NR can influence the expression other types of NRs (cross-regulation), thus modifying how a cell responds to a different hormone. Cross-regulation by NRs is an important means to temporally coordinate cell responses to a subsequent (different) hormonal signal, or to allow for crosstalk between hormone signaling pathways.

17β-oestradiol acts as a negative modulator of insulin-induced lactotroph cell proliferation through oestrogen receptor α, via nitric oxide/guanylyl cyclase/cGMP

OBJECTIVES

17β-oestradiol interacts with growth factors to modulate lactotroph cell population. However, contribution of isoforms of the oestrogen receptor in these activities is not fully understood. In the present study, we have established participation of α and β oestrogen receptors in effects of 17β-oestradiol on lactotroph proliferation induced by insulin and shown involvement of the NO/sGC/cGMP pathway.

MATERIALS AND METHODS

Cell cultures were prepared from anterior pituitaries of female rats to evaluate lactotroph cell proliferation using bromodeoxyuridine (BrdUrd) detection, protein expression by western blotting and cGMP by enzyme immunoassay.

RESULTS

In serum-free conditions, 17β-oestradiol and α and β oestrogen receptor agonists (PPT and DPN) failed to increase numbers of lactotroph cells undergoing mitosis. Co-incubation of 17β-oestradiol/insulin and PPT/insulin significantly decreased lactotroph mitogenic activity promoted by insulin alone. Both ICI 182780 and NOS inhibitors (L-NMMA and L-NAME) induced reversal of the anti-proliferative effect promoted by 17β-oestradiol/insulin and PPT/insulin. Moreover, 17β-oestradiol, PPT and insulin increased sGC α1 protein expression and inhibited β1, whereas co-incubation of 17β-oestradiol/insulin or PPT/insulin induced increases of the two isoforms α1 and β1. 17β-oestradiol and insulin reduced cGMP production, while 17β-oestradiol/insulin co-incubation increased this cyclic nucleotide.

CONCLUSIONS

Our results suggest that 17β-oestradiol is capable of arresting lactotroph proliferation induced by insulin through ER α with participation of the signalling NO/sGC/cGMP pathway.

Estradiol rapidly regulates membrane estrogen receptor alpha levels in hypothalamic neurons

Estrogen receptors (ERs) and estrogen-binding proteins have been localized intracellularly and on the cell surface. The membrane-associated proteins initiate signaling that activates a myriad of cellular responses including the modulation of ion channels and ultimately transcription. Although many of the downstream actions of membrane ERs, including ERα and ERβ, have been characterized, the mechanisms regulating membrane ER levels have remained elusive in the nervous system. In the present study, we used surface biotinylation to identify and study the estradiol regulation of membrane ERα in mixed-sex, cultured hypothalamic neurons from rat. Following surface biotinylation, Western blot analysis revealed full-length 66 kDa ERα and several ERα splice variants, most notably a biotinylated 52 kDa ERα-immunoreactive protein. Treatment of the neurons with estradiol caused a rapid and transient increase of the biotinylated 52 kDa and 66 kDa ERα proteins in the plasma membrane. Exposure of the neurons to estradiol also significantly increased internalization of 52 kDa and 66 kDa ERα membrane proteins, a measure of receptor activation. In the hypothalamus, membrane ERα signaling depends on transactivation of metabotropic glutamate receptor-1a (mGluR1a). Estradiol treatment increased the internalization of mGluR1a in parallel with ERα, a finding consistent with the hypothesis of an ERα-mGluR1a signaling unit. These results demonstrate that estradiol regulates the amount of ERα in the membrane, suggesting estradiol can regulate its own membrane signaling.

Estrogen actions on neuroendocrine glia

Astrocytes are the most abundant cells in the central nervous system (CNS). It appears that astrocytes are as diverse as neurons, having different phenotypes in various regions throughout the brain and participating in intercellular communication that involves signaling to neurons. It is not surprising then that astrocytes in the hypothalamus have an active role in the CNS regulation of reproduction. In addition to the traditional mechanism involving ensheathment of neurons and processes, astrocytes may have a critical role in regulating estrogen-positive feedback. Work in our laboratory has focused on the relationship between circulating estradiol and progesterone synthesized de novo in the brain. We have demonstrated that circulating estradiol stimulates the synthesis of progesterone in adult hypothalamic astrocytes, and this neuroprogesterone is critical for initiating the LH surge. Estradiol cell signaling is initiated at the cell membrane and involves the transactivation of metabotropic glutamate receptor type 1a (mGluR1a) leading to the release of intracellular stores of calcium. We used surface biotinylation to demonstrate that estrogen receptor-alpha (ERalpha) is present in the cell membrane and has an extracellular portion. Like other membrane receptors, ERalpha is inserted into the membrane and removed via internalization after agonist stimulation. This trafficking is directly regulated by estradiol, which rapidly and transiently increases the levels of membrane ERalpha, and upon activation, increases internalization that finally leads to ERalpha degradation. This autoregulation temporally limits membrane-initiated estradiol cell signaling. Thus, neuroprogesterone, the necessary signal for the LH surge, is released when circulating levels of estradiol peak on proestrus and activate progesterone receptors whose expression has been induced by the gradual rise of estradiol during follicular development.

Genetic, epigenetic and environmental impact on sex differences in social behavior

The field of behavioral neuroendocrinology has generated thousands of studies that indicate differences in brain structure and reactivity to gonadal steroids that produce sex-specific patterns of social behavior. However, rapidly emerging evidence shows that genetic polymorphisms and resulting differences in the expression of neuroactive peptides and receptors as well as early-life experience and epigenetic changes are important modifiers of social behavior. Furthermore, due to its inherent complexity, the neurochemical mechanisms underlying sex differences in social behavior are usually studied in a tightly regulated laboratory setting rather than in complex environments. Importantly, specific hormones may elicit a range of different behaviors depending on the cues present in these environments. For example, individuals exposed to a psychosocial stressor may respond differently to the effects of a gonadal steroid than those not exposed to chronic stress. The objective of this review is not to re-examine the activational effects of hormones on sex differences in social behavior but rather to consider how genetic and environmental factors modify the effects of hormones on behavior. We will focus on estrogen and its receptors but consideration is also given to the role of androgens. Furthermore, we have limited our discussions to the importance of oxytocin and vasopressin as targets of gonadal steroids and how these effects are modified by genetic and experiential situations. Taken together, the data clearly underscore the need to expand research initiatives to consider gene-environment interactions for better understanding of the neurobiology of sex differences in social behavior.

Estrogen receptor-alpha overexpression suppresses 17beta-estradiol-mediated vascular endothelial growth factor expression and activation of survival kinases

Estrogen and its receptors influence growth and differentiation by stimulating the production and secretion of growth factors. Our previous studies indicate an increased expression of estrogen receptor (ER)-alpha and decreased growth factor synthesis in the olfactory bulb of reproductive senescent female rats as compared with young animals. The present study tests the hypothesis that abnormal overexpression of ERalpha contributes to decreased growth factor synthesis. We developed the HeLa-Tet-On cell line stably transfected with ERalpha (HTERalpha) that expresses increasing amounts of ERalpha with increasing doses of doxycycline (Dox). Increasing doses of Dox had no effect on vascular endothelial growth factor (VEGF) secretion in HTERalpha cells. However, in the presence of 40 nm 17beta-estradiol, VEGF secretion increased in low-dose Dox-exposed HTERalpha cultures, which was attenuated by the ERalpha antagonist, 1,3-Bis(4-hydroxyphenyl)-4-methyl-5-[4-(2-piperidinylethoxy)phenol]1H-pyrazole dihydrochloride. However, at high-dose Dox and, consequently, high ERalpha levels, estradiol failed to increase VEGF. In the HeLa X6 cell line in which the Tet-On construct is upstream of an unrelated gene (Pitx2A), estradiol failed to induce VEGF at any Dox dose. Furthermore, in the HTERalpha cell line, estradiol selectively down-regulates phospho-ERK2 and phospho-Akt at high ERalpha expression. This study clearly demonstrates that the dose of receptor critically mediates estradiol's ability to regulate growth factors and survival kinases. The present data also support the hypothesis that 17beta-estradiol treatment to an ERalpha overexpressing system, such as the senescent brain, could reverse the normally observed beneficial effect of estrogen.

Estradiol regulates alternative splicing of estrogen receptor-alpha mRNA in differentiated NG108-15 neuronal cells

The biological actions of estrogen are mostly conveyed through interaction with two different types of estrogen receptor (ER), ER-alpha and ER-beta. With regard to ER-alpha, an alternatively spliced form and its translated product, truncated estrogen receptor product-1 (TERP-1), have been identified in the rat pituitary. TERP-1 has the ability to inhibit the ER binding to DNA response element by forming hetero-dimers with the wild-type ER. Furthermore, TERP-1 expression increased concurrently with serum estrogen levels. Although estrogen also plays important roles in the central nervous system, the existence and regulatory mechanism of alternatively spliced ER-alpha mRNA expression has remained unclear. The present study evaluated the expression of the alternatively spliced form of the ER-alpha gene, and examined the influence of a representative ER ligand, 17beta-estradiol (E2), on the expression in differentiated NG108-15 neuronal cells. A real-time RT-PCR analysis using primer sets designed to amplify from exons 3 to 4, exons 4 to 5, exons 5 to 6, exons 6 to 7, and exons 7 to 8 of the mouse ER-alpha gene revealed the existence of alternatively spliced ER-alpha mRNA and its putative transcription initiation site, located between exon 4 and exon 5. Although E2 had no apparent effect on the overall expression of ER-alpha mRNA, it reduced the incidence of the alternatively spliced form of ER-alpha. The down-regulation by E2 predominantly arose via binding to nuclear ERs. The present study demonstrated that alternatively spliced ER-alpha mRNA is expressed in differentiated NG108-15 neuronal cells, and provides evidence for the functional up-regulation of ER-alpha via the ligand-binding activation of ERs.

Characterisation of three variants of estrogen receptor beta mRNA in the common sole, Solea solea L. (Teleostei)

In all vertebrates, estrogen action is mediated by cognate nuclear receptors. In this study, we cloned the different transcripts of the estrogen receptor beta (ERbeta) gene of the common sole, Solea solea. 5'-RLM-RACE (RNA Ligase-Mediated 5'-Rapid Amplification of cDNA Ends) and 3'-RACE analyses revealed three isoforms of different length, called Long, Intermediate and Short isoforms, consisting of 2212, 1531 and 1207 b, respectively. The Long isoform is characterised by an open reading frame (ORF) encoding 589aa, with an estimated molecular weight of 65kDa. Phylogenetic analysis established that it belongs to the teleost ERbeta1 or ERbetaa cluster. The Intermediate isoform encodes a 490-aa protein, which lacks the first 99aa of the Long isoform, but still retains a complete DNA-binding domain (DBD). In the Short variant (363aa-long), all the N-terminal region, down to the two zinc fingers included, is missing, thus crippling DBD. ERbeta transcription was analysed by semiquantitative RT-PCR with specific primers, common to the Long and Intermediate isoforms, in various sole tissues, such as brain, gills, muscle, stomach, intestine, spleen, head kidney, kidney, liver and gonads. This analysis revealed that ERbeta displays a widespread or ubiquitous pattern of transcription, with the highest levels being found in the gonads and liver.

Reproductive stage-related effects of salmon GnRH and sex steroid hormones on expression of genes encoding fushi tarazu factor 1 homolog and estrogen receptor alpha in masu salmon pituitary cells in vitro

Expression of genes encoding gonadotropin (GTH) subunits in the salmon pituitary was regulated by salmon gonadotropin-releasing hormone (sGnRH) and sex steroid hormones in a reproductive stage-dependent manner, probably through DNA-binding transcription factors. Direct effects of these hormones on expression of genes encoding salmon fushi tarazu factor 1 homolog (sFF1-I) and estrogen receptor alpha (ERalpha) were therefore examined by use of primary pituitary cell cultures of masu salmon at different reproductive stages. Pituitaries were collected in March (before initiation of gonadal maturation), in May (early maturing), in July (late maturing), and in September (spawning period). Amounts of sFF1-I and ERalpha mRNAs in the pituitary cells were determined by real-time polymerase chain reactions after a treatment with sGnRH, estradiol-17beta (E2), testosterone (T) or 11-ketotestosterone (11KT). The amounts of sFF1-I mRNA were elevated by E2 in the males, and by sGnRH and T in the females before initiation of gonadal maturation and at the early maturing stage. The amounts of ERalpha mRNA in the early maturing females were elevated by sGnRH. Effects of sGnRH were not significant at the late maturing and spawning stages. The amounts of ERalpha mRNA in the spawning males were halved by 11KT and E2, and those of sFF1-I and ERalpha mRNAs in the late maturing females were decreased by T and 11KT. These results indicated that responsiveness of sFF1-I and ERalpha genes to sGnRH and sex steroid hormones is seasonally variable in relation to reproductive stages. Expression of sFF1 and ERalpha genes should be stimulated at the early stages of gonadal maturation prior to increases in the amounts of GTH subunit mRNAs, while attenuated after the late maturing period when stored amounts of GTH subunit mRNAs reached near the maximum.

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

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

A dose-response study on the estrogenic activity of benzophenone-2 on various endpoints in the serum, pituitary and uterus of female rats

The tetrahydroxylated biphenyl-ketone 2,2',4,4'-tetrahydroxybenzophenone (BP2), one of twelve benzophenone-derived UV-filters, is used in cosmetic products and in packaging materials to protect these products from light induced damage. Recently published studies showed that BP2 exerts estrogenic activity; thus, it is an endocrine active chemical. We present data from a pharmacodynamic dose-response experiment with five dosages of BP2 applied per gavage to adult ovariectomized (ovx) rats for 5 days. Estradiol-valerate (E2) served as a control compound. The uterotrophic assay, proposed by the OECD, was modified to have a broader view on endocrine activity outside the urogenital tract to prevent that undesirable actions in other organs regulated by estrogens are missed. The gene expression levels of marker genes of estrogenic action were measured by semi-quantitative RT-PCR. Metabolic parameters were assessed by determination of the serum concentrations of leptin, cholesterol, high- and low-density lipoproteins, and triglycerides in the serum. Administration of BP2 at dosages of 10-1,000 mg/kg bodyweight led to changes of these parameters comparable to the changes in the E2 group with 0.6 mg/kg bodyweight. For the observed estrogenic activities of BP2, the "no observed adverse effect levels" were determined. Additionally, the data were further analyzed using the benchmark approach. If BP2 is transcutaneously absorbed in the human, the obtained threshold values would suggest refraining from the further use of BP2 as UV-filter in cosmetic products although additional toxicological studies should be conducted to clarify possible adverse effects.

Stimulation of the novel estrogen receptor-alpha intronic TERP-1 promoter by estrogens, androgen, pituitary adenylate cyclase-activating peptide, and forskolin, and autoregulation by TERP-1 protein

The estrogen receptor-alpha (ERalpha) pituitary-specific variant, TERP-1, is regulated dramatically by physiological status. We examined hormonal regulation of the TERP-1 promoter in transient transfection assays in GH3 somatolactotrope cells. We found that 17beta-estradiol (E2), genistein, androgen, pituitary adenylate cyclase-activating peptide, and forskolin (FSK) all stimulated TERP-1 promoter activity, whereas progesterone had no effect. ERalpha bound to a palindromic estrogen response element (ERE) and two half-site EREs; mutation of any of these sites decreased basal expression and completely obliterated E2 stimulation. In contrast, mutation of an activator protein-1 site decreased basal and FSK-stimulated promoter activity, but not E2 or androgen stimulation. The pure antiestrogen ICI 182,780 suppressed E2 and genistein, but not FSK or androgen, stimulation. Similarly, mutation of the ERE palindrome or half-site EREs suppressed promoter stimulation by E2 and genistein, but not by androgen or FSK. Because TERP-1 levels regulate ERalpha function on model promoters, we tested TERP-1 modulation of its own and other physiological promoters. TERP-1 suppressed basal and E2-stimulated expression of its own promoter. TERP-1 suppression required the ERE regions of the promoter, and the dimerization domain of TERP-1. TERP-1 overexpression also suppressed E2 stimulation of the progesterone receptor and prolactin promoters. Thus, estrogens, androgen, and FSK can stimulate TERP-1 promoter activity, and increased TERP-1 expression modulates E2 stimulation of physiological promoters. These data suggest that TERP-1 regulation may play a significant role in modifying pituitary ERalpha responses.

Cells of proopiomelanocortin lineage from the rodent anterior pituitary lack sexually dimorphic expression of neurofilaments

Lactotrophs, gonadotrophs, thyrotrophs and somatotrophs of the rat anterior pituitary (AP) express 68-kDa neurofilaments (NF68) and other neuronal markers. NF68 expression in the AP appears to be estrogen-dependent, but its significance is unknown. The aims of this work were: (1) to establish the expression pattern of NF68 immunoreactivity in the mouse AP, and (2) discover if corticotrophs and melanotrophs from both rodent species also express NF68. Primary cultures and frozen sections of AP from sexually mature mice were immunolabeled with anti-NF68 antibodies. In separate experiments, samples were immunostained for NF68 and AP hormones. Here we report that mouse lactotrophs, gonadotrophs, thyrotrophs and somatotrophs also express NF68 in a sexually dimorphic manner. The percentages of non-expressing, weakly expressing and strongly expressing cells were similar between both rodent species, although NF68+ cells were about 50% less abundant in the mouse compared to the rat pituitary. Remarkably, our study shows for the first time that rodent pituitary cells from the proopiomelanocortin lineage nearly completely lack NF68 immunoreactivity. In this regard, they differ from the rest of the AP population. Our findings establish a foundation for experiments aimed at investigating the functional significance of estrogen-dependent regulation of NF68 expression in rodent AP cells.

Multi-organic risk assessment of estrogenic properties of octyl-methoxycinnamate in vivo

Sun protection products contain a variety of UV-filters, among others, octyl-methoxycinnamate (OMC). Recently, an uterotrophic effect in immature rats has been reported, indicating that OMC might have estrogenic properties and thus is an endocrine active chemical (EAC). However, determination of an estrogenic activity in the uterus only is a restricted approach with the potential risk of missing undesirable actions in other organs regulated by estrogens. A pharmacodynamic experiment with 5 dosages of OMC in adult ovariectomized (ovx) rats was carried out to quantify the multi-organic estrogenic properties of OMC. As control compound, estradiol-valerate (E2) was included. Animals were treated per gavage for 5 days. The expression levels of markers of estrogenic action in several organs were measured by RT-PCR. Effects on metabolic parameters were assessed by determination of the serum concentrations of leptin, cholesterol, high and low density lipoproteins (HDL and LDL), glucose and triglycerides. Observed changes upon OMC treatment were analyzed using the NO(A)EL and the benchmark dose approach. From the obtained pharmacodynamic data of the most sensitive parameter (truncated estrogen receptor protein 1 gene expression in the pituitary) we obtained threshold values that are exceeded by the recommended use of OMC containing formulations for skin protection in humans, therefore we propose to reduce the use of OMC in cosmetic products. In addition to estrogenic actions of OMC, non-estrogenic effects have been found for this chemical supporting the need of a multi-organic risk assessment of putative EACs.

A rat endometrial cell line (R1-49E1) expressing estrogen receptor-alpha regulated by the tet-off system

BACKGROUND

Estrogens exert profound effects on target tissues. These effects are mediated by two estrogen receptors (ER(alpha) and ER(beta)) that bind to specific DNA sequences in estrogen-dependent genes. Other molecules such as growth factors, transcription factors and some oncoproteins might interact with the estrogen receptors and thus regulate the transcription of these genes. Currently there is no adequate cellular model to study these interactions.

METHODS

We transfected the human wild-type ER(alpha) to an ER-negative rat epithelial endometrial cell line (Rentr01) using a tetracycline-regulated gene expression system. The exogenous receptor was correctly translated, had an appropriate hormone-binding affinity, and bound well to estrogen response elements containing DNA.

RESULTS

We obtained a new stable cell line that is ER(beta) negative but ER(alpha) positive (R1-49E1). The expression of receptor alpha can be regulated in a dose-response manner by addition of tetracycline in the culture medium. Estradiol treatment of ER(alpha)-containing cells apparently diminished cellular proliferation, and the exogenous receptor can induce the transcription of the endogenous progesterone receptor isoform B (PgR-B) gene.

CONCLUSIONS

This epithelial cellular model may be useful to study the interaction between estrogens and other cell signaling pathways in epithelial endometrial cell physiology.

Ontogeny of estrogen receptor (ER) alpha and its co-localization with pituitary hormones in the pituitary gland of chick embryos

Estrogen is involved in regulating the development and hormone secretion of the anterior pituitary gland following its binding to estrogen receptors (ERs) expressed on pituitary cells. However, the pituitary is comprised of several cell types, and to date, there is no data about the specific cell types expressing ERs in embyonic chick pituitary. We therefore followed, by immunohistochemistry, the ontogeny of the pituitary ER alpha (ERalpha), and the cell types expressing ERalpha throughout chick embryo development. ERalpha immunoreacitivity was restricted to the nuclei of pituitary cells. ERalpha-immunopositive (ERalpha(+)) cells were first detected at embryonic day 6.5 (E6.5), after which ERalpha(+) cells were consistently detected throughout the anterior pituitary gland, although the density of ERalpha(+) cells in the caudal lobe of the pars distalis was higher than that in the cephalic lobe. The proportion of ERalpha(+) cells in the pituitary was about 6% at E8.5; expression increased to 22% by E18.5 of gestation, with no additional change until hatching. Double-labeling of ERalpha and pituitary hormones showed that the dominant cell types expressing ERalpha were gonadotrophs immunopositive for luteinizing hormone (LH); the proportion of ERalpha(+) cells expressing LH increased throughout gestation and reached approximately 57% at hatching. About 2%-6% of thyroid-stimulating-hormone-immunopositive and 1%-2% prolactin-immunopositive cells expressed ERalpha at later stages of embryonic development, but no growth-hormone-positive or adrenocorticotropic-hormone-positive cells expressed ERalpha during the embryonic period. Thus, gonadotrophs are the main cell population expressing ERalpha in the anterior pituitary gland of chick embryo, and ERalpha is involved in regulating the development of the pituitary gland and the maturation of the hormone-secreting function.

The hamster androgen receptor promoter: a molecular analysis

The steroid/thyroid hormone receptors are members of a very large family of nuclear-activated transcription factors. These receptors play a crucial role in most biological function, including regulation of development, metabolism, behaviour and reproduction. Among androgen receptor (AR), we have recently demonstrated that its expression in the Harderian gland (HG) of the male hamster is under a well-co-ordinated cross-talk between various steroid hormone receptors. Here, are presented data on the sequence of hamster AR promoter region (5'UTR) and the molecular tools of its regulation. The 5'UTR is 1585 bp. The promoter region shows various responsive elements. Two putative CREM elements are present at -71 and -1576 bp. A putative retinoic acid responsive element is present at -1476 bp. An androgen/glucocorticoid responsive element is present at -473 bp. A putative thyroid hormone-responsive element at -381 bp and an estrogen responsive element at -230 bp. Also, a homopurinic stretch is evident between -1199 and -1118. Furthermore, Sp1 sites are also spread along the sequence. As well as for human, mouse, rat and pig, the hamster lacks the canonical promoter TATA and CCAAT boxes. Gel retardation experiments confirm the presence of active responsive elements for AR, estrogen receptor, glucocorticoid receptor and thyroid hormone receptor. Previous data on the regulation of expression of AR by other members of steroid/thyroid hormone receptors well correlate with sequence analysis and gel retardation experiments. Thus, androgens, thyroid hormone, stimulate the AR transcription, while synthetic glucocorticoid (Dex) and estrogen are potent inhibitors of AR expression. The comparison of hamster AR promoter sequence with other AR promoter shows an 89, 82, 84 and 84% identity with human, rat, mouse and pig AR promoter, respectively. These results, in the light of the extreme plasticity of hamster HG, suggest that the comparative study of expression and regulation of AR gene in the HG of the hamster offers a useful tool to approach the normal and pathological phenotype in human.

Developmental effects of prenatal exposure to bisphenol a on the uterus of rat offspring

Exposure to estrogenic compounds during critical periods of fetal development could result in adverse effects on the development of reproductive organs that are not apparent until later in life. Bisphenol A (BPA), which is employed in the manufacture of a wide range of consumer products, is a prime candidate for endocrine disruption. We examined BPA to address the question of whether in utero exposure affects the uterus of the offspring and studied the expression and distribution of the estrogen receptors alpha (ERalpha) and beta (ERbeta), because estrogens influence the development, growth, and function of the uterus through both receptors. Gravid Sprague-Dawley dams were administered by gavage either 0.1 or 50 mg/kg per day BPA or 0.2 mg/kg per day 17alpha-ethinyl estradiol (EE2) as reference dose on gestation days 6 through 21. Female offspring were killed in estrus. Uterine morphologic changes as well as ERalpha and ERbeta distribution and expression were measured by immunohistochemistry and Western blot analysis. Striking morphologic changes were observed in the uterine epithelium of postpubertal offspring during estrus of the in utero BPA-treated animals (the thickness of the total epithelium was significantly reduced). ERalpha expression was increased in the 50-mg BPA and EE2-treated group. In contrast, we observed significantly decreased ERbeta expression in all BPA- and EE2-treated animals when compared with the control. In summary, these results clearly indicate that in utero exposure of rats to BPA promotes uterine disruption in offspring. We hypothesize that the uterine disruption could possibly be provoked by a dysregulation of ERalpha and ERbeta.

17beta-estradiol protects oligodendrocytes from cytotoxicity induced cell death

During pregnancy, changes in circulating levels of hormones, including estrogens, correlates with a significant decrease in the relapse incidence in women with Multiple Sclerosis (MS). In the present study, we demonstrate that both primary and cell line cultures of rat oligodendrocytes express the estrogen receptor (ER)-alpha and ERbeta estrogen receptors in the cytosol and nucleus, and that nuclear compartmentalization becomes more pronounced as the cells mature. Moreover, 17beta-estradiol significantly decreases the cytotoxic effects of the peroxynitrite generator 3-(4-morpholinyl)-sydnonimine (SIN-1) in both immature and mature oligodendrocytes in a dose dependent manner. This protective mechanism requires pretreatment with 17beta-estradiol and is blocked by ICI 182,780, a selective ERalpha/ERbeta antagonist. These results strongly suggest that 17beta-estradiol protects oligodendrocytes against SIN-1 mediated cytotoxicity through the activation of the estrogen receptors and provides new insights into the roles of the estrogen signaling pathways in myelin forming cells that are lost in demyelinating disorders.

Regulation of Estrogen Receptor (ER) Isoform Messenger RNA Expression by Different ER Ligands in Female Rat Pituitary1

Net estrogen sensitivity in target tissues critically depends on the regulated expression of full-length and alternately processed estrogen receptor (ER) isoforms. However, the molecular mechanisms for the control of pituitary responsiveness to estrogen remain partially unknown. In the present communication, we report the ability of different ligands, with distinct agonistic or antagonistic properties at the ER, to modulate the expression of the transcripts encoding ERalpha and ERbeta isoforms, as well as those for the truncated ERalpha product (TERP), and the variant ERbeta2, in pituitaries from ovariectomized rats, i.e., a background devoid of endogenous estrogen. Compared with expression levels at the morning of proestrus, ovariectomy (OVX) resulted in increased pituitary expression of ERbeta and ERbeta2 mRNAs, whereas it decreased TERP-1 and -2 levels without affecting those of ERalpha. Administration of estradiol benzoate (as potent agonist for alpha and beta forms of ER) or the selective ERalpha agonist, propyl pyrazole triol, fully reversed the responses to OVX, while the ERbeta ligand, diarylpropionitrile, failed to induce any significant effect except for a partial stimulation of TERP-1 and -2 mRNA expression levels. To note, the ERbeta agonist was also ineffective in altering pituitary expression of progesterone receptor-B mRNA, i.e., a major estrogen-responsive target. In all parameters tested, tamoxifen, a selective ER modulator with mixed agonist/antagonist activity, behaved as ERalpha agonist, although the magnitude of tamoxifen effects was significantly lower than those of the ERalpha ligand, except for TERP induction. In contrast, the pure antiestrogen RU-58668 did not modify the expression of any of the targets under analysis. Overall, our results indicate that endogenous estrogen differentially regulates pituitary expression of the mRNAs encoding several ER isoforms with distinct functional properties, by a mechanism that is mostly conducted through ERalpha. Differential regulation of ER isoforms may represent a relevant system for the self-tuning of estrogen responsiveness in female pituitary.

Pituitary and gonadal effects of GnRH (gonadotropin releasing hormone) analogues in two peripubertal female rat models

Central precocious puberty is commonly treated by gonadotropin releasing hormone (GnRH) agonists. To compare modes of action and effectiveness of GnRH analogues and assess treatment combinations of agonistic (triptorelin) and antagonistic (cetrorelix acetate) GnRH analogues with established treatment, we used prepubertal 31-d-old ovariectomized female rats. Strongest inhibition of LH and FSH occurred after 2-d treatment with antagonist alone (LH 0.08 +/- 0.02 versus 3.2 +/- 0.56 ng/mL in controls; FSH 10.8 +/- 2.8 versus 44.2 +/- 5.0 ng/mL in controls, p < 0.001). Combined agonist/antagonist was second most effective of the treatments (after 5 d treatment, LH 0.52 +/- 0.15 versus 4.9 +/- 1.1 ng/mL in controls; p < 0.01). Pituitary gonadotropin subunit LHbeta mRNA levels were inhibited in all groups except controls, but pituitary GnRH receptor mRNA was stimulated by agonist yet unaffected by combined analogues. Explanted ovaries were incubated with either analogue, both 10-6 M. After 4 h, GnRH receptor mRNA levels were significantly reduced by antagonist but not agonist. To verify puberty-inhibiting effects of GnRH analogues, we used 26-d-old female rats with androgen-induced precocious puberty after injecting subcutaneously single 300 microg danazol on postnatal d 5. Single application of cetrorelix depot (cetrorelix embonate) reduced serum estradiol levels and pituitary LHbeta expression; GnRH receptor mRNA levels were down-regulated in the pituitary and ovary (p < 0.05). In androgen-induced precocious puberty model, single injection of antagonist effectively arrests premature hormonal activation and down-regulates pituitary and ovarian GnRH receptors. We conclude that GnRH analogue combination and especially antagonist alone treatment most directly suppress gonadotropin levels. This implies that early treatment gonadotropin flare-up associated with agonist treatment is avoidable.

Characterization of a membrane-associated estrogen receptor in a rat hypothalamic cell line (D12)

The ability of estrogens to produce rapid changes in cellular function has been firmly established. The question remains whether these changes are mediated by a modified form of the nuclear estrogen receptor (ER) that is associated with the plasma membrane (mER) or by a completely novel membrane receptor. Therefore, we characterized the biochemical properties of the nuclear and membrane-associated ERs expressed endogenously in a rat hypothalamic endothelial cell line (D12). Radioligand binding experiments using D12 membrane fractions showed that these cells exhibit properties consistent with a binding site specific for estrogens (mER). Equilibrium binding assays using [125I]16-alpha-iodo-3,17- beta-estradiol revealed saturable binding to mER, an affinity value similar to nuclear ER, with differing receptor expression levels. Competition assays revealed that 9 of 12 ER ligands tested had comparable affinities for mER and ER. For example, 17-alpha-estradiol and estrone had similar binding characteristics for both receptors while differences were noted for raloxifene, 17beta-estradiol (E2), and genistein. Western blot and immunocytochemical analyses using antibodies specific for ERalpha confirmed that D12 cells expressed a membrane-associated protein with a molecular mass (67 kDa) similar to that of ERalpha that colocalized with caveolae-enriched membranes. A rapid increase in intracellar Ca2+ levels in the presence of E2 suggests that mER can mediate physiologic changes through calcium mobilization. These data support the expression of mER in these brain-derived endothelial cells that is similar to, but biochemically distinguishable from, nuclear ERalpha.

Truncated estrogen receptor product-1 stimulates estrogen receptor alpha transcriptional activity by titration of repressor proteins

The truncated estrogen receptor product-1 (TERP-1, or TERP) is a pituitary-specific isoform of estrogen receptor alpha (ERalpha), and its expression is regulated by estrogen. TERP modulates the transcriptional activity of ERalpha but has no independent effect on transcription of estrogen-response element-containing promoters. At low concentrations, TERP stimulates ERalpha transcriptional activity in transient transfection assays. At TERP concentrations equal to or greater than full-length ERalpha, TERP forms dimers with ERalpha and reduces both ligand-dependent and -independent transcription. A dimerization mutant of TERP, TERP L509R, stimulated ERalpha transcription at all concentrations. We hypothesized that TERP stimulates ERalpha transcriptional activity by titrating suppressors of ERalpha activity. We found that repressor of estrogen receptor activity (REA), originally isolated from human breast cancer cells, is present in mouse pituitary gonadotrope cell lines. Levels of REA vary slightly throughout the rat reproductive cycle, but TERP mRNA and protein vary much more dramatically. In transfection experiments, REA suppressed ERalpha transcriptional activity, and TERP L509R was able to alleviate transcriptional suppression by REA. In glutathione S-transferase pull-down assays, TERP bound to REA more efficiently than did ERalpha at equivalent concentrations, suggesting that REA will preferentially bind to TERP. Our findings suggest that the stimulation of pituitary ERalpha activity by low concentrations of TERP can occur by titration of corepressors such as REA.

Regulation of the intronic promoter of rat estrogen receptor alpha gene, responsible for truncated estrogen receptor product-1 expression

We have characterized the intronic promoter of the rat estrogen receptor (ER) alpha gene, responsible for the lactotrope-specific truncated ER product (TERP)-1 isoform expression. Transcriptional regulation was investigated by transient transfections using 5'-deletion constructs. TERP promoter constructs were highly active in MMQ cells, a pure lactotrope cell line, whereas a low basal activity was detected in alphaT3-1 gonadotrope cells or in COS-7 monkey kidney cells. Serial deletion analysis revealed that 1) a minimal -693-bp region encompassing the TATA box is sufficient to allow lactotrope-specific expression; 2) the promoter contains strong positive cis-acting elements both in the distal and proximal regions, and 3) the region spanning the -1698/-1194 region includes repressor elements. Transient transfection studies, EMSAs, and gel shifts demonstrated that estrogen activates the TERP promoter via an estrogen-responsive element (ERE1) located within the proximal region. Mutation of ERE1 site completely abolishes the estradiol-dependent transcription, indicating that ERE1 site is sufficient to confer estrogen responsiveness to TERP promoter. In addition, ERalpha action was synergized by transfection of the pituitary-specific factor Pit-1. EMSAs showed that a single Pit-1 DNA binding element in the vicinity of the TATA box is sufficient to confer response by the TERP promoter. In conclusion, we demonstrated, for the first time, that TERP promoter regulation involves ERE and Pit-1 cis-elements and corresponding trans-acting factors, which could play a role in the physiological changes that occur in TERP-1 transcription in lactotrope cells.

Sex differences in androgen and estrogen receptor expression in rat substantia nigra during development: an immunohistochemical study

Gonadal hormones are important regulators of sexual differentiation of the CNS. Exposure to testosterone and estrogen during development causes permanent organizational differences between males and females. We previously described functional sex-related differences of the GABA(A)ergic circuits of the rat substantia nigra pars reticulata (SNR) involved in the control of flurothyl seizures. This sexual differentiation of the SNR is regulated by postnatal testosterone. To assess whether the organizing effects of testosterone in the SNR are mediated via the androgen receptor (AR) and/or estrogen receptors (ER), we used immunohistochemistry to study the ontogeny of AR, ERalpha and ERbeta expression in SNR and substantia nigra pars compacta (SNC) of male and female rats. Rats on the day of birth [postnatal day (PN) 0] and at PN1, PN5, PN15 and PN30 were used. AR- and ERbeta-immunopositive cells were present in SNR and SNC in both sexes and at all ages. ERalpha was not detected in male and female SNC at PN0-PN1. In both substantia nigra (SN) regions, there were developmentally regulated sex differences in AR, ERalpha and ERbeta immunoreactivity. In the SN, each receptor showed specific intracellular localization: AR was present in the nucleus, ERalpha and ERbeta were present both in nuclear and extranuclear compartments. ERalpha was detected also in processes. At PN0-PN1, quantitative analysis revealed sex and regional differences in the distribution of SN cells expressing AR and ERalpha, while ERbeta were equally present in both sexes. The presence of gonadal steroid receptors in the SN suggests that the biological effects of gonadal hormones in the CNS extend beyond reproduction-related functions and may affect and modify motor behaviors (including seizures) in a sex-specific manner. Based on the ontogeny of SNR ERbeta, we hypothesize that postnatal injections of testosterone may regulate the nigral GABA(A) system through the aromatization pathway and activation of ERbeta.

Regulation of the membrane estrogen receptor-alpha: role of cell density, serum, cell passage number, and estradiol

We used modified immunocytochemical conditions to quantify a membrane form of estrogen receptor-alpha (mERalpha) in a rat pituitary tumor cell line, GH3/B6/F10. We studied the regulation of mERalpha vs. levels of intracellular ERalpha (iERalpha) using our 96-well plate immunoassay. The anti-ERalpha antibody C542 was used to label the ERalpha (via conjugated alkaline phosphatase) in fixed permeabilized (for iERalpha) vs. nonpermeabilized cells (for mERalpha). Expression of mERalpha was highest at low cell densities (<1000 cells/well) and decreased significantly at densities where cellular processes touched, whereas the more abundant iERalpha increased with increasing cell density over the same range. Serum starvation for 48 h caused increases in mERalpha, whereas iERalpha levels showed no significant changes. A large decline in mERalpha and iERalpha levels with cell passage number was observed. Minutes after nM 17beta-estradiol (E2) treatment, a portion of the cells rounded up and detached from the culture plate, whereas nM cholesterol had no such effect. Although E2 treatment did not change mERalpha levels, the antigen was reorganized from a fine particulate to aggregation into asymmetric large granules of staining. That common culturing conditions favor down-regulation of mERalpha may explain the relatively few reports of this protein in other experimental systems.

The C-terminal extension (CTE) of the nuclear hormone receptor DNA binding domain determines interactions and functional response to the HMGB-1/-2 co-regulatory proteins

Previously, we and others reported that the high mobility group proteins, HMGB-1/-2, enhance DNA binding in vitro and transactivation in situ by the steroid hormone subgroup of nuclear receptors but did not influence these functions of class II receptors. We show here that the DNA binding domain (DBD) is sufficient to account for the selective influence of HMGB-1/-2 on the steroid class of receptors. Furthermore, the use of chimeric DBDs reveals that this selectivity is dependent on the C-terminal extension (CTE), amino acid sequences adjacent to the zinc finger core DBD. HMGB-1/-2 interact directly with the DBDs of steroid but not class II receptors, and this interaction requires the CTE. This in vitro interaction correlates with a requirement of the CTE for maximal HMGB-1/-2 enhancement of DNA binding in vitro and transcriptional activation in cells. Finally, class II receptor DBDs have a much higher intrinsic affinity for DNA than steroid receptor DBDs, and this affinity difference is also dependent on the CTE. These results reveal the importance of the steroid receptor CTE for DNA binding affinity and functional response to HMGB-1/-2.

Multifarious effects of estrogen on the pituitary gonadotrope with special emphasis on studies in the ovine species

The gonadotrope is a complex cell that expresses receptors for gonadotropin releasing hormone (GnRH) and estrogen. It has synthetic machinery for the production of 3 gonadotropin subunits which are assembled into two gonadotropins, luteinising hormone (LH) and follicle stimulating hormone (FSH). The production and secretion of LH and FSH are differentially regulated by GnRH and estrogen. Patterns of secretion of LH are dictated by the pulsatile release of GnRH from the median eminence as well as the feedback effects of estrogen. The means by which estrogen plays such an important role in the regulation of LH and FSH is reviewed in this chapter, with emphasis on work that has been done in the sheep. Estrogen regulates the second messenger systems in the gonadotrope as well as the number of GnRH receptors and the function of ion channels in the plasma membrane. Estrogen also regulates gene expression in these cells. Additionally, GnRH appears to regulate the level of estrogen receptor in the ovine gonadotrope, so there is substantial cross-talk between the signalling pathways for GnRH and estrogen. No clear picture has emerged as to how estrogen exerts a positive feedback effect on the gonadotrope and it is suggested that this might be forthcoming from more definitive studies on the way that estrogen regulates the second messenger systems and the trafficking of secretory vesicles.

In utero exposure to low doses of bisphenol A lead to long-term deleterious effects in the vagina

The origins of the "endocrine disrupter hypothesis" may be traced to reports on adolescent daughters born to women who had taken the highly potent synthetic estrogen, diethylstilbestrol, while pregnant, and who developed a rare form of vaginal cancer and adenocarcinoma. Bisphenol A (BPA) is an estrogenic chemical that is highly employed in the manufacture of a wide range of consumer products. Some observational studies have suggested that the amounts of BPA to which we are exposed could alter the reproductive organs of developing rodents. We examined the influence of BPA at low doses to address the questions of (a) whether in utero exposure affects the vagina of the offspring and (b) which mechanisms cause the toxic effects. Gravid Sprague-Dawley dams were administered either 0.1 (low dose) or 50 mg/kg per day BPA, the no observed effect level, or 0.2 mg/kg per day 17 alpha-ethinyl estradiol by gavage. Striking morphological changes were observed in the vagina of postpubertal offspring leading us to examine vaginal estrogen receptor (ER) expression because BPA binds to the ER alpha, which is important for growth of the vaginal epithelium. We show that the full-length ER alpha is not expressed during estrus in the vagina of female offspring exposed to either dose of BPA when compared to the control group, whereas ER alpha expression does not differ from the control group during the diestrus stage. ER alpha downregulation seems to be responsible for the observed altered vaginal morphology.

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.

Tissue-specific expression of two structurally different estrogen receptor alpha isoforms along the female reproductive axis of an oviparous species, the rainbow trout

In oviparous species, in addition to a full-length estrogen receptor alpha (ER alpha), another ER alpha isoform lacking the A domain and exhibiting a ligand-independent transactivation function has been consistently reported. Although both isoforms are expressed in the liver, their respective sites of expression in other potential target tissues are unknown. In contrast to the situation in Xenopus and chicken, the two isoforms of rainbow trout (Oncorhynchus mykiss) are generated from two classes of transcripts with different 5' untranslated sequences issued from the same gene by alternative splicing and promoter usage. The aim of this study was to take advantage of the unique organization of the rainbow trout ER alpha gene to investigate the tissue distribution of these two messenger species along the reproductive axis of female trout. The S1 nuclease assay and in situ hybridization were used, with probes specific for each of the transcripts. Reverse transcription polymerase chain reaction (RT-PCR) with primers specific for each of the isoforms also was performed. The data indicated that the full-length ER alpha is expressed in liver, brain, pituitary, and ovary, whereas expression of the isoform with the truncated A domain is restricted to the liver, demonstrating a tissue-specific expression of these two ER alpha isoforms. The presence of a short liver-specific isoform in oviparous species suggests its role in the development and/or maintenance of the unique function of the liver in the vitellogenesis process.

The percentage of pituitary gonadotropes with immunoreactive oestradiol receptors increases in the follicular phase of the ovine oestrous cycle

During the oestrous cycle, there is an alteration in gonadotrope responsiveness to gonadotropin releasing hormone (GnRH). One cellular mechanism that may be involved in these changes at the pituitary level is the hormonal regulation of oestrogen receptor (ER) expression. Using double-label immunohistochemistry, we examined the proportion of gonadotropes, lactotropes and somatotropes with immunoreactive (ir) oestrogen receptor alpha (ERalpha) in pituitary sections from ewes at three stages of the ovine oestrous cycle (n = 8 per group). The percentage of ERalpha positive cells that also stained positive for luteinizing hormone (LH) increased in the transition from the luteal phase to the follicular phase (n = 8), with no further increase at the time of oestrus (n = 8). In the pituitaries from the luteal phase sheep, only a small number (15%) of lactotropes and 4% of somatotropes were found to contain ir-ERalpha and there were no alterations across the oestrous cycle. When we examined pituitaries from ovariectomized (OVX) ewes treated (i.m.) with either oestradiol benzoate (50 microg) or oil vehicle for 2, 4, 6 or 16 h (n = 4 per group), there was no effect of treatment. In fact, the percentage of gonadotropes that were ERalpha-positive in OVX ewes was similar to that observed in the pituitaries from the follicular phase ewes, both of which display a high frequency of pulsatile GnRH secretion. We conclude that the number of gonadotropes that contain ir-ERalpha increases in the follicular phase of the oestrous cycle and this may enhance the responsiveness of these cells to oestrogen and GnRH. We suggest that this may be due to increased pulsatile GnRH input rather than rising oestrogen levels.

Mechanisms of estrogen action

Our appreciation of the physiological functions of estrogens and the mechanisms through which estrogens bring about these functions has changed during the past decade. Just as transgenic mice were produced in which estrogen receptors had been inactivated and we thought that we were about to understand the role of estrogen receptors in physiology and pathology, it was found that there was not one but two distinct and functional estrogen receptors, now called ER alpha and ER beta. Transgenic mice in which each of the receptors or both the receptors are inactive have revealed a much broader role for estrogens in the body than was previously thought. This decade also saw the description of a male patient who had no functional ER alpha and whose continued bone growth clearly revealed an important function of estrogen in men. The importance of estrogen in both males and females was also demonstrated in the laboratory in transgenic mice in which the aromatase gene was inactivated. Finally, crystal structures of the estrogen receptors with agonists and antagonists have revealed much about how ligand binding influences receptor conformation and how this conformation influences interaction of the receptor with coactivators or corepressors and hence determines cellular response to ligands.

Distribution analysis of the two chicken estrogen receptor-alpha isoforms and their transcripts in the hypothalamus and anterior pituitary gland

Estrogen plays a key role in the control of reproductive behavior and in the regulation of the neuroendocrine system. To elucidate the mechanisms by which it controls these functions it is important to understand how estrogenic effects are mediated. We have investigated the distribution of the two isoforms of the chicken estrogen receptor alpha (cER-alpha) protein; the previously characterized cER-alpha 66 and a new N-terminal truncated isoform, cER-alpha 61. Immunolocalization demonstrated the presence of cER-alpha 66 protein in hypothalamic areas, principally the nucleus septalis lateralis, bed nucleus striae terminalis medialis, nucleus preopticus medialis, and nucleus infundibuli hypothalami, and in the anterior pituitary gland. When the distribution of ER-alpha immunoreactive cells was compared using the antibodies H 222 (directed against the hormone-binding domain) and ER 221 (directed against the 21-amino acid N-terminus), no apparent differences could be detected. Because this immunocytochemical approach was not able to distinguish whether full-length cER-alpha 66 is the only isoform observed in the ER-positive regions or whether both cER-alpha receptor isoforms are present, SI nuclease assays were performed to compare the relative abundance in these regions of the two distinct classes of cER-alpha mRNA variants (A1-D and A2), which encode the cER-alpha 66 and cER-alpha 61 protein isoforms, respectively. In cockerels and hens, both variants of cER-alpha mRNA are expressed in the anterior pituitary gland and basal hypothalamus with a dominance of the mRNA that encodes cER-alpha 66, whereas the mRNA that encodes cER-alpha 61 was not detectable in the anterior hypothalamus. Therefore, because both receptor isoforms differ in their ability to modulate estrogen target gene expression in a promoter and cell type-specific manner, these differences may mediate the pleiotropic actions of estrogen in reproductive behavior and neuroendocrine functions.

Oestrogenic and antiandrogenic chemicals in the environment: effects on male reproductive health

Exposures of human populations to pesticides and industrial pollutants, and to synthetic chemicals present in foods, beverages, and plastics, have raised concern that these substances can interfere with endogenous sex hormone function. Interference with sex hormone action can, in turn, result in a variety of developmental and reproductive anomalies. Compounds in this class are thus referred to as endocrine disruptors (EDs). EDs that affect reproductive processes in vertebrates act primarily by altering oestrogenic and antiandrogenic activities. The recent cloning of a second oestrogen receptor (ER) subtype (ERbeta) and its widespread tissue distribution pattern indicates that the first ER to be cloned, ERalpha, may not be the only, or even the primary, mediator of oestrogen action. It is anticipated that this discovery will lead to development of antagonist compounds specific to either ER subtype, and help to determine the function of each receptor subtype in reproductive and other tissues. Growing evidence suggests that EDs interfere with reproductive function at low exposure levels and cause distinct effects at different concentrations within the same organ. Developing organisms have increased susceptibility to the actions of EDs because differentiating tissues are more vulnerable to changes in hormonal milieu. Thus, children are at greater risk of toxicant-related illnesses than adults. However, most data are collected from laboratory studies, and it remains to be determined that the levels of chemicals in the environment can impair human reproductive health. There is also significant genetic variability between human and animal species in their reactions to chemicals. The effects of low-dose, chronic, and multiple chemical exposures warrant further investigation in order to characterize the risk of environmental agents to humans. The aims of this review, which will focus on male reproduction, are to: 1) identify synthetic chemicals in the environment that fall into the ED class; 2) describe their mechanisms of toxicity in reproductive tissues; and, 3) outline the direction of future research efforts with respect to EDs.

Dominant negative ER induces apoptosis in GH(4) pituitary lactotrope cells and inhibits tumor growth in nude mice

The ER plays an important role in the proliferation and differentiation of lactotrope tumor cells. GH(4) cells were infected with adenoviral vectors (AdL540Q and Ad1-536) to investigate the ability of dominant negative ER mutants to affect the regulation of gene expression and cell growth by endogenous ER. The dominant negative mutants suppressed estradiol stimulation of an estrogen-responsive reporter gene and the PRL promoter in these cells. AdL540Q or Ad1--536 infection also inhibited GH(4) cell growth and induced apoptosis, increasing the expression of the proapoptotic Bax protein and decreasing the expression of antiapoptotic Bcl-2. AdwtER-infected cells also showed decreased Bcl-2 protein. E2-induced activation of p38 MAPK, an enzyme that may participate in apoptosis, was observed in cells infected with AdwtER, AdL540Q, and Ad1--536. Consistent with the apoptotic effects in vitro, infection of GH(4) cells with AdL540Q or Ad1--536 inhibited the ability of the cells to form tumors in nude mice. These results indicate that dominant negative ER mutants induce apoptosis of GH(4) cells and suppress tumor formation and development. The delivery of dominant negative ERs by adenoviral vectors may provide an alternative modality for the targeted therapy of pituitary lactotrope adenomas and other estrogen-responsive tumors.