Membrane oestrogen receptors on rat pituitary tumour cells: immuno-identification and responses to oestradiol and xenoestrogens

MAP kinase mediates growth factor-induced nuclear translocation of estrogen receptor alpha

In addition to mediating the classical transcriptional effects of estrogen, estrogen receptors (ERs) are now known to regulate gene expression in the absence of estrogen by ligand-independent activation pathways, and to mediate the rapid, non-genomic effects of estrogen as well. ERs have been shown to associate with the cell membrane, and recent studies demonstrate that this subpopulation of membrane-associated ER mediates the rapid effects of estrogen. To date, however, little is known regarding the pathways that regulate the distribution of the ER between the nuclear and membrane fractions. In the current study, we demonstrate membrane localization of transiently transfected ERalpha in human vascular smooth muscle cells, and translocation of ERalpha from the membrane to the nucleus in response to both estrogen-dependent and estrogen-independent stimulation. Mutational analyses identified serine 118 as the critical residue regulating nuclear localization following estrogen-independent stimulation, but not following estrogen stimulation. Induction of nuclear localization of ERalpha by estrogen-independent, but not estrogen-dependent stimulation was blocked by both pharmacologic and genetic inhibition of mitogen-activated protein (MAP) kinase activation. Furthermore, constitutive activation of MAP kinase resulted in nuclear translocation of ERalpha. These overexpression studies support that MAP kinase-mediated phosphorylation of ERalpha induces nuclear localization of the ER in response to estrogen-independent, but not estrogen-dependent stimulation, demonstrating stimulus-specific molecular pathways regulate the nuclear localization of the ER. These findings identify a previously unrecognized pathway that regulates the intracellular localization of the ER, and represent the first demonstration that the distribution of the ER between membrane and nuclear compartments is regulated by physiologic stimuli.

A nonclassical estrogen membrane receptor triggers rapid differential actions in the endocrine pancreas

Glucose homeostasis in blood is mainly maintained by insulin released from beta-cells and glucagon released from alpha-cells, both integrated within the pancreatic islet of Langerhans. The secretory processes in both types of cells are triggered by a rise in intracellular calcium concentration ([Ca2+](i)). In this study, rapid effects of the natural hormone E2 on [Ca2+](i) were studied in both types of cells within intact islets using laser scanning confocal microscopy. alpha- And beta-cells showed opposite [Ca2+](i) responses when stimulated with physiological concentrations of 17beta-E2. Although the estrogen produced an increase in the frequency of glucose-induced [Ca2+](i) oscillations in insulin-releasing beta-cells, it prevented the low glucose-induced [Ca2+](i) oscillations in glucagon-releasing alpha-cells. The effects of 17beta-E2 on alpha-cells were mimicked by the cGMP permeable analog 8bromo-cGMP and blocked by the cGMP-dependent protein kinase (PKG) inhibitor KT5823. Evidence indicated that these were membrane actions mediated by a nonclassical ER. Both effects were rapid in onset and were reproduced by 17beta-E2 linked to horseradish peroxidase, a cell-impermeable molecule. Furthermore, these actions were not blocked by the specific ER blocker ICI 182,780. Competition studies performed with 17beta-E2 linked to horseradish peroxidase binding in alpha-cells supported the idea that the membrane receptor involved is neither ERalpha nor ERbeta. Additionally, the binding site was shared by the neurotransmitters epinephrine, norepinephrine, and dopamine and had the same pharmacological profile as the receptor previously described for beta-cells. Therefore, rapid estrogen actions in islet cells are initiated by a nonclassical estrogen membrane receptor.

Rapid actions of 17beta-oestradiol on a subset of lactotrophs in the rat pituitary

Increasingly the role of rapid mechanisms of steroid action in physiological regulation are being recognised. We have investigated rapid effects of 17beta-oestradiol (E) on prolactin (PRL) release in vitro. Pituitary segments from male rats were incubated for 5, 10 or 20 min in Earle's balanced salt solution containing 1.2 mM tannic acid (to enable visualisation of exocytosed secretory granules by electron microscopy) either alone (control) or containing 10(-10)-10(-8) M E conjugated to bovine serum albumin (E-BSA). PRL and leuteinising hormone (LH) release from pituitary segments were also determined in response to E and E-BSA by radioimmunoassay. Within 10 min E-BSA and E (10(-12)-10(-6) M) stimulated a significant (P < 0.05) concentration-dependent release of PRL but not LH. After exposure to experimental media for 5 min, only occasional exocytosis from type I lactotrophs (characterised by large polymorphic secretory granules) was observed in either control or E-BSA treated tissue. In contrast, E-BSA (10(-10)-10(-8) M) induced a significant (P < 0.05) increase in the number of exocytotic profiles from type II lactotrophs (characterized by smaller, spherical granules). This effect was not inhibited by removal of extracellular calcium, or by pre-treatment of cells with the RNA synthesis inhibitor actinomycin-D (0.5 microg ml(-1)), the protein synthesis inhibitor cycloheximide (1 microg ml(-1)) or the anti-oestrogen ICI 182,780 (1 microM). FACS analysis demonstrated binding of E-BSA-fluorescein isothiocyanate (FITC) (10(-10)-10(-7) M) to a subpopulation of anterior pituitary cells. The E-BSA-FITC binding sites assumed a patchy distribution across the cell surface. In conclusion, we report for the first time a rapid, non-genomic effect of E on PRL secretion in normal pituitary tissue.

Rapid non-genomic and genomic responses to progestogens, estrogens, and glucocorticoids in the endocrine pancreatic B cell, the adipocyte and other cell types

Rapid biologic responses to injected steroids were described as early as 60 years ago. More recently, evidence has been presented that 17beta-estradiol given i.v. will double the uterine cAMP activity within 15 s (Proc Natl Acad Sci USA 1967;58:1711-8), and also that estrogens will bind to the outer surfaces of endometrial cells (Nature 1977;265:69-72), suggesting that these steroids can both engage and direct intracellular events. Unfortunately, studies of such rapid membrane effects of steroids have languished due to the accumulation of compelling data for the more slowly manifest actions of these compounds at the level of nuclear DNA. We report a number of observations in women, in experimental animals, and in isolated organ or cell systems using 17beta-estradiol, progesterone or glucocorticoids which provide ample evidence for rapid intracellular metabolic responses to these steroids, mediated by their actions at the cellular plasma membrane. Such rapid responses have been shown in various classic targets or not, such as the B cell of the endocrine pancreas and the fat cell. They involve plasma membrane binding, changes in membrane electrical activity, Ca2+ handling, G and Ras proteins, cAMP, cGMP, IP(3), DAG, phosphodiesterases, protein kinases, tyrosine kinases, ER kinases, and mitogen activated protein kinases (MAPks) and nitric oxide synthase. These recent findings are discussed in detail and should lead to a fuller understanding of the cellular effects of the steroid hormones.

Linkage of rapid estrogen action to MAPK activation by ERalpha-Shc association and Shc pathway activation

E2 rapidly activates MAPK in breast cancer cells, and the mechanism for this effect has not been fully identified. Since growth factor-induced MAPK activation involves signaling via the adapter protein Shc (Src-homology and collagen homology) and its association with membrane receptors, we hypothesized that breast cancer cells utilize similar signaling mechanisms in response to E2. In the present study, we demonstrated that E2 rapidly induced Shc phosphorylation and Shc-Grb2 (growth factor receptor binding protein 2)-Sos (son of sevenless) complex formation in MCF-7 cells. Overexpression of dominant negative Shc blocked the effect of E2 on MAPK, indicating a critical role of Shc in E2 action. Using selective inhibitors, we also demonstrated that ERalpha and Src are upstream regulators of Shc. A rapid physical association between ERalpha and Shc upon E2 stimulation further evidenced the role of ERalpha on Shc activation. Mutagenesis studies showed that the phosphotyrosine binding and SH2 domains of Shc are required to interact with the activation function 1, but not activation function 2, domain of ERalpha. Using a glutathione-S-transferase-Shc pull-down assay, we demonstrated that this ERalpha-Shc association was direct. Biological consequences of this pathway were further investigated at the genomic and nongenomic levels. E2 stimulated MAPK-mediated Elk-1 transcriptional activity. Confocal microscopy studies showed that E2 rapidly induced formation of membrane ruffles, pseudopodia, and ERalpha membrane translocation. The E2-induced morphological changes were prevented by antiestrogen. Together our results demonstrate that ERalpha can mediate the rapid effects of E2 on Shc, MAPK, Elk-1, and morphological changes in breast cancer cells

Dopamine as a prolactin (PRL) inhibitor

Dopamine is a small and relatively simple molecule that fulfills diverse functions. Within the brain, it acts as a classical neurotransmitter whose attenuation or overactivity can result in disorders such as Parkinson's disease and schizophrenia. Major advances in the cloning and characterization of biosynthetic enzymes, transporters, and receptors have increased our knowledge regarding the metabolism, release, reuptake, and mechanism of action of dopamine. Dopamine reaches the pituitary via hypophysial portal blood from several hypothalamic nerve tracts that are regulated by PRL itself, estrogens, and several neuropeptides and neurotransmitters. Dopamine binds to type-2 dopamine receptors that are functionally linked to membrane channels and G proteins and suppresses the high intrinsic secretory activity of the pituitary lactotrophs. In addition to inhibiting PRL release by controlling calcium fluxes, dopamine activates several interacting intracellular signaling pathways and suppresses PRL gene expression and lactotroph proliferation. Thus, PRL homeostasis should be viewed in the context of a fine balance between the action of dopamine as an inhibitor and the many hypothalamic, systemic, and local factors acting as stimulators, none of which has yet emerged as a primary PRL releasing factor. The generation of transgenic animals with overexpressed or mutated genes expanded our understanding of dopamine-PRL interactions and the physiological consequences of their perturbations. PRL release in humans, which differs in many respects from that in laboratory animals, is affected by several drugs used in clinical practice. Hyperprolactinemia is a major neuroendocrine-related cause of reproductive disturbances in both men and women. The treatment of hyperprolactinemia has greatly benefited from the generation of progressively more effective and selective dopaminergic drugs.

Estradiol, in the CNS, targets several physiologically relevant membrane-associated proteins

We will describe the identity and function of two unexpected estrogen binding proteins from rat brain cell membranes in search for the putative membrane estrogen receptor (mER). An E-6-BSA column retained a distinctive 37-kDa protein that showed 100% homology with glyceraldehyde-3-phosphate dehydrogenase (GAPDH). A P-3-BSA column also retained the same protein but with less affinity. E-6-BSA bound to GAPDH with an IC50 of 50 nM, whereas the IC50 for P-3-BSA was about 500 nM. A dose of 10 nM 17beta-estradiol stimulated the catalysis of GAPDH, whereas progesterone at 100 nM inhibited it. Other steroids were ineffective. We examined if GAPDH activity would change during the rat estrous cycle, and what would be the effect of ovariectomy and estrogen treatment. The hippocampus and cerebellum were collected and GAPDH catalysis in both cytosolic and plasmalemmal-microsomal fractions was tested. The highest activity was found in Proestrus morning and the lowest in Estrus in both fractions. After ovariectomy (3 weeks) the hippocampus membrane fraction showed significantly reduced activity compared to that of Diestrus. An injection of estradiol in ovariectomized rats (10 microg/rat, s.c.) increased GAPDH activity in the hippocampus membrane fractions close to 60% from that of ovariectomized oil-treated controls 24 h after treatment maintaining similar levels by 48 h. No changes were detected in the preparations from the cerebellum of the same rats. The other protein retained by E-BSA columns was a 55-kDa protein identified as beta-tubulin. Two other proteins were also co-purified from the rat hippocampus: a 37-kDa (GAPDH) and a 45-kDa (actin). A purified brain tubulin (Cytoskeleton) was also retained with high affinity by the E-6-BSA, but with less affinity by an E-17-BSA column and not retained by either BSA, P-3-BSA or C-21-BSA columns. E-6-[125I]BSA bound with high affinity to tubulin (1 microg) and 17beta-estradiol completely displaced the binding at 10(-7) M. 17alpha-estradiol was ineffective and neither progesterone, corticosterone, DES nor 2-methoxyestradiol (2-ME) was able to displace the ligand. The T-3-[125I]BSA also bound to tubulin. But it seems to interact with another binding site, because colchicine at 10(-5) M completely eliminated the binding of T-3-[125 I]BSA to tubulin but did not displace the E-6-BSA site. Taxol competed off both ligands but only by 50%. None of the two ligands bound actin. These novel findings add new information to be considered in the intracellular actions of estradiol, particularly in the remodeling and functions of the cytoskeleton.

Estradiol-induced IP(3) mediates the estrogen receptor activity expressed in human cells

The recent findings that estradiol-induced IP(3)/PKC-alpha signalling pathway triggers DNA synthesis in HepG2 cells, containing estrogen receptor unable to stimulate gene transactivation, raises the hypothesis that this pathway represents an alternative signalling present when the amount of estrogen receptor (ER) is insufficient to mediate genomic effects. beta-estradiol-stimulated DNA synthesis and target gene expression have been studied in HepG2 and, ER-alpha or ER-beta negative, HeLa cells. We also examined whether either receptor is required for rapid effects of estrogen on DNA synthesis. Finally, the consequences of increased ER expression on estrogen-induced DNA synthesis and synthetic target gene expression have been evaluated. Our data indicate that the E2-induced IP(3) production is dependent on expression of either ER-alpha or ER-beta in both HepG2 and HeLa cells. Moreover, inhibition of the IP(3) second messenger pathway blocks E2-induced cellular actions suggesting that this second messenger is responsible for estrogen's rapid, non-genomic effects on both DNA synthesis and gene expression.

Membrane estrogen and glucocorticoid receptors--implications for hormonal control of immune function and autoimmunity

Membrane steroid receptors (mSRs) have recently re-emerged as candidates for mediating steroid effects which do not fit the paradigm of nuclear transcription factor mechanisms. We have studied two steroid-binding classes of mSRs, and have noted striking similarities in their characteristics (immunocytochemical appearance, biochemical properties, proteolytic sensitivity, signaling pathways, regulation, and molecular origins). These observations strengthen the conclusion that mSRs can be modified versions of intracellular steroid receptors. The membrane estrogen receptors (mERs) we studied are involved in estrogen-induced release of prolactin. Membrane glucocorticoid receptors (mGRs) in both mouse and human lymphoma cells are necessary for the initiation of glucocorticoid-induced therapeutic apoptosis which is related to the developmental phenomenon of thymic involution. Diseases of autoimmunity such as systemic lupus erythematosus and arthritis are related to estrogen status. Since both of these mSRs have recently been found in both normal and cancerous lymphoid cells, actions of these mSRs may have important consequences for functions and diseases of the immune system. Therefore, the study of these forms of steroid receptors may present novel therapeutic opportunities for the use of steroids and steroid analogs.

Environmental signaling: what embryos and evolution teach us about endocrine disrupting chemicals

The term "endocrine disrupting chemicals" is commonly used to describe environmental agents that alter the endocrine system. Laboratories working in this emerging field-environmental endocrine research-have looked at chemicals that mimic or block endogenous vertebrate steroid hormones by interacting with the hormone's receptor. Environmental chemicals known to do this do so most often with receptors derived from the steroid/thyroid/retinoid gene family. They include ubiquitous and persistent organochlorines, as well as plasticizers, pharmaceuticals, and natural hormones. These chemicals function as estrogens, antiestrogens, and antiandrogens but have few, if any, structural similarities. Therefore, receptor-based or functional assays have the best chance of detecting putative biological activity of environmental chemicals. Three nuclear estrogen receptor forms-alpha, beta, and gamma-as well as multiple membrane forms and a possible mitochondrial form have been reported, suggesting a previously unknown diversity of signaling pathways available to estrogenic chemicals. Examples of environmental or ambient estrogenization occur in laboratory experiments, zoo animals, domestic animals, wildlife, and humans. Environmentally estrogenized phenotypes may differ depending upon the time of exposure-i.e., whether the exposure occurred at a developmental (organizational and irreversible) or postdevelopmental (activational and reversible) stage. The term "estrogen" must be defined in each case, since steroidal estrogens differ among themselves and from synthetic or plant-derived chemicals. An "estrogen-like function" seems to be an evolutionarily ancient signal that has been retained in a number of chemicals, some of which are vertebrate hormones. Signaling, required for symbiosis between plants and bacteria, may be viewed, therefore, as an early example of hormone cross-talk. Developmental feminization at the structural or functional level is an emerging theme in species exposed, during embryonic or fetal life, to estrogenic compounds. Human experience as well as studies in experimental animals with the potent estrogen diethylstilbestrol provide informative models. Advances in the molecular genetics of sex differentiation in vertebrates facilitate mechanistic understanding. Experiments addressing the concept of gene imprinting or induction of epigenetic memory by estrogen or other hormones suggest a link to persistent, heritable phenotypic changes seen after developmental estrogenization, independent of mutagenesis. Environmental endocrine science provides a new context in which to examine the informational content of ecosystem-wide communication networks. As common features come to light, this research may allow us to predict environmentally induced alterations in internal signaling systems of vertebrates and some invertebrates and eventually to explicate environmental contributions to human reproductive and developmental health.

Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions

The evolution of novelty in tightly integrated biological systems, such as hormones and their receptors, seems to challenge the theory of natural selection: it has not been clear how a new function for any one part (such as a ligand) can be selected for unless the other members of the system (e.g., a receptor) are already present. Here I show-based on identification and phylogenetic analysis of steroid receptors in basal vertebrates and reconstruction of the sequences and functional attributes of ancestral proteins-that the first steroid receptor was an estrogen receptor, followed by a progesterone receptor. Genome mapping and phylogenetic analyses indicate that the full complement of mammalian steroid receptors evolved from these ancient receptors by two large-scale genome expansions, one before the advent of jawed vertebrates and one after. Specific regulation of physiological processes by androgens and corticoids are relatively recent innovations that emerged after these duplications. These findings support a model of ligand exploitation in which the terminal ligand in a biosynthetic pathway is the first for which a receptor evolves; selection for this hormone also selects for the synthesis of intermediates despite the absence of receptors, and duplicated receptors then evolve affinity for these substances. In this way, novel hormone-receptor pairs are created, and an integrated system of increasing complexity elaborated. This model suggests that ligands for some "orphan" receptors may be found among intermediates in the synthesis of ligands for phylogenetically related receptors.

Nongenomic actions of estrogens and xenoestrogens by binding at a plasma membrane receptor unrelated to estrogen receptor alpha and estrogen receptor beta

The molecular mechanism used by environmental chemicals to exert their hormone-like actions is still only partially resolved. Although it generally is accepted that xenoestrogens act at the genomic level by binding to intracellular estrogen receptors, we have shown here that they trigger nongenomic effects in pancreatic beta cells. Both xenoestrogens and the circulating hormone, 17beta-estradiol, bind with high affinity to a common membrane binding site unrelated to the intracellular estrogen receptors ERalpha and ERbeta. This binding site is shared by dopamine, epinephrine, and norepinephrine and has the pharmacological profile of the gammaadrenergic receptor. This study provides an outline of the membrane receptor involved in rapid xenoestrogen actions.

Non-competitive inhibition of kainate-induced currents by diethylstilbestrol in acutely isolated mouse CA1 hippocampal neurons

The effect of a synthetic estrogen, diethylstilbestrol (DES), on kainate-induced currents was investigated in the hippocampal CA1 pyramidal neurons acutely dissociated from the mice using the nystatin-perforated patch-clamp recording configuration under voltage-clamp conditions. DES inhibited the current evoked by 100 microM kainate in a concentration-dependent manner with a half-maximum inhibitory concentration of 8.8 microM. The action of DES was voltage-independent. Since DES produced a suppression of the maximum response of the kainate concentration-response curve, the inhibition by DES of the kainate-induced current appears to be non-competitive.