Partial purification and characterization of oestrogen receptors in subfractions of hepatocyte plasma membranes

Membrane-Initiated Estrogen, Androgen, and Progesterone Receptor Signaling in Health and Disease

Rapid effects of steroid hormones were discovered in the early 1950s, but the subject was dominated in the 1970s by discoveries of estradiol and progesterone stimulating protein synthesis. This led to the paradigm that steroid hormones regulate growth, differentiation, and metabolism via binding a receptor in the nucleus. It took 30 years to appreciate not only that some cellular functions arise solely from membrane-localized steroid receptor (SR) actions, but that rapid sex steroid signaling from membrane-localized SRs is a prerequisite for the phosphorylation, nuclear import, and potentiation of the transcriptional activity of nuclear SR counterparts. Here, we provide a review and update on the current state of knowledge of membrane-initiated estrogen (ER), androgen (AR) and progesterone (PR) receptor signaling, the mechanisms of membrane-associated SR potentiation of their nuclear SR homologues, and the importance of this membrane-nuclear SR collaboration in physiology and disease. We also highlight potential clinical implications of pathway-selective modulation of membrane-associated SR.

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

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

The role of Gα protein signaling in the membrane estrogen receptor-mediated signaling

Estrogens exert rapid, extranuclear effects by their action on the plasma membrane estrogen receptors (mERs). Gα protein associated with the cell membrane is involved in many important processes regulated by estrogens. However, the Gα's role in the mER-mediated signaling and the signaling pathways involved are poorly understood. This review aims to outline the Gα's role in the mER-mediated signaling. Immunoblotting, immunofluorescence, co-immunoprecipitation, and RNA interference were carried out using vascular endothelial cells (ECs) and human breast carcinoma cell lines as experimental models. Electrophysiology and immunocytochemistry were carried out using guinea pigs as animal models. Recent advances suggest that the signaling of mERα through Gα is required for vascular EC migration or endothelial H₂S release, while Gα₁₃ is involved in estrogen-induced breast cancer cell invasion. Besides, the Gα_q-coupled PLC-PKC-PKA pathway is critical for the neural regulation of energy homeostasis. This review summarizes the contributions of Gα to mER-mediated signaling, including cardiovascular protection, breast cancer metastasis, neural regulation of homeostatic functions, and osteogenesis.

Estrogen and thrombosis: A bench to bedside review

Estrogen, in the clinical setting is used primarily for contraception and hormone replacement therapy. It has been well established that estrogen increases the risk of both arterial and venous thrombosis. While estrogen is known to induce a prothrombotic milieu through various effects on the hemostatic pathways, the exact molecular mechanism leading to those effects is not known. The most common clinical presentation of estrogen-related thrombosis is venous thromboembolism (VTE) of the deep veins of the legs or pulmonary vessels, usually within the first few months of use. Estrogen has also been associated with increased risk of "unusual site" thromboses, as well as arterial thrombosis. Women at high-risk of thrombosis need careful evaluation and counseling for contraception, pregnancy, menopausal hormonal therapy and other estrogen-related conditions or treatments in order to lower the risk of thromboses. We review the most recent evidence on management of high-estrogen states in women at high-risk of thrombosis, as well as emerging data on unique populations such as transgender women. More studies are needed to better understand the pathophysiology of hormone-related thrombosis, as well as more comprehensive techniques to stratify risks for thrombosis so as to enable tailoring of recommendations for each individual.

Twenty years of the G protein-coupled estrogen receptor GPER: Historical and personal perspectives

Estrogens play a critical role in many aspects of physiology, particularly female reproductive function, but also in pathophysiology, and are associated with protection from numerous diseases in premenopausal women. Steroids and the effects of estrogen have been known for ∼90 years, with the first evidence for a receptor for estrogen presented ∼50 years ago. The original ancestral steroid receptor, extending back into evolution more than 500 million years, was likely an estrogen receptor, whereas G protein-coupled receptors (GPCRs) trace their origins back into history more than one billion years. The classical estrogen receptors (ERα and ERβ) are ligand-activated transcription factors that confer estrogen sensitivity upon many genes. It was soon apparent that these, or novel receptors may also be responsible for the "rapid"/"non-genomic" membrane-associated effects of estrogen. The identification of an orphan GPCR (GPR30, published in 1996) opened a new field of research with the description in 2000 that GPR30 expression is required for rapid estrogen signaling. In 2005-2006, the field was greatly stimulated by two studies that described the binding of estrogen to GPR30-expressing cell membranes, followed by the identification of a GPR30-selective agonist (that lacked binding and activity towards ERα and ERβ). Renamed GPER (G protein-coupled estrogen receptor) by IUPHAR in 2007, the total number of articles in PubMed related to this receptor recently surpassed 1000. In this article, the authors present personal perspectives on how they became involved in the discovery and/or advancement of GPER research. These areas include non-genomic effects on vascular tone, receptor cloning, molecular and cellular biology, signal transduction mechanisms and pharmacology of GPER, highlighting the roles of GPER and GPER-selective compounds in diseases such as obesity, diabetes, and cancer and the obligatory role of GPER in propagating cardiovascular aging, arterial hypertension and heart failure through the stimulation of Nox expression.

The Estrogen Receptors: An Overview from Different Perspectives

The estrogen receptors, ERα, ERβ, and GPER, mediate the effects of estrogenic compounds on their target tissues. Estrogen receptors are located in the tissues of the female reproductive tract and breast as one would expect, but also in tissues as diverse as bone, brain, liver, colon, skin, and salivary gland. The purpose of this discussion of the estrogen receptors is to provide a brief overview of the estrogen receptors and estrogen action from perspectives such as the historical, physiological, pharmacological, pathological, structural, and ligand perspectives.

Regulation of endothelial nitric oxide synthase and high-density lipoprotein quality by estradiol in cardiovascular pathology

Estrogens have been recognized, in the last 3 decades, as important hormones in direct and indirect modulation of vascular health. In addition to their direct benefit on cardiovascular health, the presence of esterified estrogen in the lipid core of high-density lipoprotein (HDL) particles indirectly contributes to atheroprotection by significantly improving HDL quality and functionality. Estrogens modulate their physiological activity via genomic and nongenomic mechanisms. Genomic mechanisms are thought to be mediated directly by interaction of the hormone receptor complex with the hormone response elements that regulate gene expression. Nongenomic mechanisms are thought to occur via interaction of the estrogen with membrane-bound receptors, which rapidly activate intracellular signaling without binding of the hormone receptor complex to its hormone response elements. Estradiol in particular mediates early and late endothelial nitric oxide synthase (eNOS) activation via interaction with estrogen receptors through both nongenomic and genomic mechanisms. In the vascular system, the primary endogenous source of nitric oxide (NO) generation is eNOS. Nitric oxide primarily influences blood vessel relaxation, the heart rate, and myocyte contractility. The abnormalities in expression and/or functions of eNOS lead to the development of cardiovascular diseases, both in animals and in humans. Although considerable research efforts have been dedicated to understanding the mechanisms of action of estradiol in regulating cardiac eNOS, more research is needed to fully understand the details of such mechanisms. This review focuses on recent findings from animal and human studies on the regulation of eNOS and HDL quality by estradiol in cardiovascular pathology.

Daidzein-estrogen interaction in the rat uterus and its effect on human breast cancer cell growth

Sex hormone replacement therapy provides several advantages in the quality of life for climacteric women. However, estrogen-induced cell proliferation in the uterus and mammary gland increases the risk of cancer development in these organs. The lower incidence of mammary cancer in Asian women as compared with Western women has been attributed to high intake of soy isoflavones, including genistein. We have previously shown that genistein induces an estradiol-like hypertrophy of uterine cells, but does not induce cell proliferation, uterine eosinophilia, or endometrial edema. It also inhibits estradiol-induced mitosis in uterine cells and hormone-induced uterine eosinophilia and endometrial edema. Nevertheless, genistein stimulates growth of human breast cancer cells in culture; therefore, it is not an ideal estrogen for use in hormone replacement therapy (HRD). The present study investigated the effect of another soy isoflavone, daidzein (subcutaneous, 0.066 mg/kg body weight), in the same animal model, and its effect on responses induced by subsequent treatment (1 h later) with estradiol-17β (E(2); subcutaneous, 0.33 mg/kg body weight). In addition, we investigated the effects of daidzein (1 μg/mL) or E(2) on the growth of human breast cancer cells in culture. Results indicate that daidzein stimulates growth of breast cancer cells and potentiates estrogen-induced cell proliferation in the uterus. We suggest caution for the use of daidzein or formulas containing this compound in HRD. Future research strategies should be addressed in the search for new phytoestrogens that selectively inhibit cell proliferation in the uterus and breast.

Estradiol signaling in the regulation of reproduction and energy balance

Our knowledge of membrane estrogenic signaling mechanisms and their interactions that regulate physiology and behavior has grown rapidly over the past three decades. The discovery of novel membrane estrogen receptors and their signaling mechanisms has started to reveal the complex timing and interactions of these various signaling mechanisms with classical genomic steroid actions within the nervous system to regulate physiology and behavior. The activation of the various estrogenic signaling mechanisms is site specific and differs across the estrous cycle acting through both classical genomic mechanisms and rapid membrane-initiated signaling to coordinate reproductive behavior and physiology. This review focuses on our current understanding of estrogenic signaling mechanisms to promote: (1) sexual receptivity within the arcuate nucleus of the hypothalamus, (2) estrogen positive feedback that stimulates de novo neuroprogesterone synthesis to trigger the luteinizing hormone surge important for ovulation and estrous cyclicity, and (3) alterations in energy balance.

Enhanced estradiol-induced vasorelaxation in aortas from type 2 diabetic mice may reflect a compensatory role of p38 MAPK-mediated eNOS activation

Cardiovascular problems are a major cause of morbidity and mortality, mainly due to coronary artery disease and atherosclerosis, in type 2 diabetes mellitus. However, female gender is a protective factor in the development of, for example, atherosclerosis and hypertension. One of the female hormones, 17β-estradiol (E2), is known to protect against the cardiovascular injury resulting from endothelial dysfunction, but the mechanism by which it does so remains unknown. Our hypothesis was that E2-mediated activation of Akt and mitogen-activated protein kinase (MAPK), and the subsequent endothelial NO synthase (eNOS) phosphorylation, might protect the aorta in diabetic mellitus. The experimental type 2 diabetic model we employed to test that hypothesis (female mice given streptozotocin and nicotinamide) is here termed fDM. In fDM aortas, we examined the E2-induced relaxation response and the associated protein activities. In control (age-matched, nondiabetic) aortas, E2 induced a vascular relaxation response that was mediated via Akt/eNOS and mitogen-activated/ERK-activating kinase (MEK)/eNOS pathways. In fDM aortas (vs. control aortas), (a) the E2-induced relaxation was enhanced, (b) the mediation of the response was different (via Akt/eNOS and p38 MAPK/eNOS pathways), and (c) E2 stimulation increased p38 MAPK and eNOS phosphorylations, decreased MEK phosphorylation, but did not alter estrogen receptor activity. We infer that at least in fDM aortas, E2 has beneficial effects (enhanced vascular relaxation and protection) that are mediated through Akt activation and (compensating for reduced MEK activation) p38 MAPK activation, leading to enhanced eNOS phosphorylation.

Genistein selectively inhibits estrogen-induced cell proliferation and other responses to hormone stimulation in the prepubertal rat uterus

Sex hormone replacement therapy helps improve quality of life in climacteric women. However, estrogen-induced cell proliferation in the uterus and mammary gland increases the risk for cancer in these organs. The lower incidence of mammary cancer in Asian women than in western women has been attributed to high intake of soy isoflavones, including genistein. Our previous work in the prepubertal rat uterus model showed that genistein (0.5 mg/kg body weight subcutaneously) caused an estradiol-like hypertrophy in myometrial and uterine luminal epithelial cells and an increase in RNA content in luminal epithelium; however, it did not induce cell proliferation, uterine eosinophilia, or endometrial edema. The present study investigated, in the same animal model, the effect of genistein administration (0.5 mg/kg body weight subcutaneously) before treatment with estradiol-17β (0.33 mg/kg body weight subcutaneously) on uterine responses that were not induced by genistein. Pretreatment with this phytoestrogen completely inhibited estradiol-induced mitoses in uterine luminal epithelium, endometrial stroma, and myometrium and partially inhibited estradiol-induced uterine eosinophilia and endometrial edema. These findings indicate that genistein protects against estrogen-induced cell proliferation in the uterus and suggest that future studies should investigate the possibility of using this agent to decrease the risk for uterine cancer after hormone replacement therapy in climacteric women.

Suppression of estrogen receptor transcriptional activity by connective tissue growth factor

Secreted growth factors have been shown to stimulate the transcriptional activity of estrogen receptors (ER) that are responsible for many biological processes. However, whether these growth factors physically interact with ER remains unclear. Here, we show for the first time that connective tissue growth factor (CTGF) physically and functionally associates with ER. CTGF interacted with ER both in vitro and in vivo. CTGF interacted with ER DNA-binding domain. ER interaction region in CTGF was mapped to the thrombospondin type I repeat, a cell attachment motif. Overexpression of CTGF inhibited ER transcriptional activity as well as the expression of estrogen-responsive genes, including pS2 and cathepsin D. Reduction of endogenous CTGF with CTGF small interfering RNA enhanced ER transcriptional activity. The interaction between CTGF and ER is required for the repression of estrogen-responsive transcription by CTGF. Moreover, CTGF reduced ER protein expression, whereas the CTGF mutant that did not repress ER transcriptional activity also did not alter ER protein levels. The results suggested the transcriptional regulation of estrogen signaling through interaction between CTGF and ER, and thus may provide a novel mechanism by which cross-talk between secreted growth factor and ER signaling pathways occurs.

Akt/protein B and estradiol receptor alpha expressions in postmenopausal endometrium

OBJECTIVE

Estrogen receptor interacts in several types of cells with phosphatidyl-inositol 3-kinase/Akt pathway regulating cell survival and apoptosis. No data are available of the Akt/PKB signaling and its role in the endometrial homeostasis of the postmenopausal uterus. The aim of the present investigation was to study the Akt/protein kinase B signaling in tissue samples retrieved from postmenopausal endometrium of the human uterus with parallel observation of the changes of the expression and phosphorylation of ERalpha.

STUDY DESIGN

Sixty disease-free postmenopausal women were enrolled in the study. Endometrial tissue samples were obtained from diagnostic curettage or direct from the uterus after hysterectomy done for benign uterine lesions other than endometrial disease. For comparison, the studied parameters were also analyzed in endometrial samples of women with regular menstrual cycles (n=16). In each individual tissue sample the expression and phosphorylation of ERalpha, Akt, and cyclin D1 was analyzed by Western blotting.

RESULTS

The level of Akt protein did not show significant change, however, the activation of Akt proteins and the expression of ERalpha increased parallel with serum estrogen (E2) levels, suggesting the role of E2 in Akt activation and ERalpha expression. The level of pERalpha(Ser167) changed parallel with pAkt(Ser473) levels. Significant correlation was found between the changes of pERalpha and ERalpha (r=0.650399, p<0.005), and in that of pERalpha and pAkt (r=0.639643, p<0.007), respectively. The expression of cyclin D1 was increased in samples with elevated pAkt levels.

CONCLUSION

The results are indicating that the postmenopausal endometrium responds to E2 by both genomic and nongenomic mechanism. The interaction between ERalpha and Akt plays crucial role in the regulation of proliferative activity in postmenopausal endometrium.

The role of nuclear endothelial nitric oxide synthase in the endothelial and prostate microenvironments

This review is based on novel observations from our laboratory on the nuclear translocation and functional role of endothelial nitric oxide synthase (eNOS) in endothelial and prostate cancer (PCa) epithelial cells. Nitric oxide (NO), the product of eNOS, is a free radical involved in the physiology and pathophysiology of living organisms and in a variety of biological processes including the maintenance of vascular homeostasis. Of relevance in this context is the role that estrogens play in the apoptotic process and the migration of endothelial cells through the regulation of target genes such as eNOS itself. It has been shown that both estrogen and NO signaling, mediated respectively by the estrogen receptors (ERs) and eNOS, can strongly counteract endothelial senescence through a common effector, the catalytic subunit of human telomerase. Therefore, this protein has been identified as a key molecule in the aging process which, intriguingly, is considered the only risk factor in the development of PCa and one of the major determinants of cardiovascular diseases. Indeed, in both these contexts we have defined a molecular mechanism involving activation of eNOS and hypoxia-inducible factors in association with ERβ that characterizes the most aggressive form of PCa or influences endothelial cell differentiation. Altogether these data led us to postulate that activation of eNOS is a crucial requirement for the delaying of endothelial senescence as well as for the acquisition of androgen-independence and for tumor progression in the prostate microenvironment.

17Beta-estradiol elevates cGMP and, via plasma membrane recruitment of protein kinase GIalpha, stimulates Ca2+ efflux from rat hepatocytes

Rapid non-genomic effects of 17beta-estradiol, the principal circulating estrogen, have been observed in a wide variety of cell types. Here we investigate rapid signaling effects of 17beta-estradiol in rat hepatocytes. We show that, above a threshold concentration of 1 nm, 17beta-estradiol, but not 17alpha-estradiol, stimulates particulate guanylyl cyclase to elevate cGMP, which through activation and plasma membrane recruitment of protein kinase G isoform Ialpha, stimulates plasma membrane Ca(2+)-ATPase-mediated Ca(2+) efflux from rat hepatocytes. These effects are extremely rapid in onset and are mimicked by a membrane-impermeant 17beta-estradiol-BSA conjugate, suggesting that 17beta-estradiol acts at the extracellular face of the plasma membrane. We also show that 17beta-estradiol binds specifically to the intact hepatocyte plasma membrane through an interaction that is competed by an excess of atrial natriuretic peptide but also shows many similarities to the pharmacological characteristics of the putative gamma-adrenergic receptor. We, therefore, propose that the observed rapid signaling effects of 17beta-estradiol are mediated either through the guanylyl cyclase A receptor for atrial natriuretic peptide or through the gamma-adrenergic receptor, which is either itself a transmembrane guanylyl cyclase or activates a transmembrane guanylyl cyclase through cross-talk signaling.

Membrane-initiated actions of estrogen on the endothelium

Estrogen-induced rapid, membrane-initiated activation of numerous signal transduction cascades has been shown in animal, cellular and molecular vascular studies, which support the favorable effects of estrogen on vascular structure and function. These effects are mediated by distinct forms of estrogen receptor (ER) alpha. This includes estrogen-stimulated, rapid activation of endothelial nitric oxide synthase (eNOS), resulting in elaboration of the athero-protective, angiogenesis-promoting product nitric oxide (NO). An N-terminus truncated short isoform of ERalpha, ER46, plays a critical role in membrane-initiated, rapid responses to 17beta-estradiol (E2) in human endothelial cells (ECs). We have proposed a ER46-centered, eNOS-activating molecular complex in human EC caveolar membranes, containing c-Src, phosphatidylinositol 3-kinase (PI3K), Akt and eNOS. In this review, we describe estrogen-induced, rapid, non-genomic actions in the endothelium.

Properties of estrogen binding components in the plasma membrane of neurohypophysis in hens and changes in its binding before and after oviposition

The present study was performed to elucidate whether the estrogen binding component regarded as a receptor exists in the plasma membrane fraction of neurohypophysis in hens and whether the binding of receptor changes with relation to oviposition. The specific binding for estradiol-17beta (E2) in the neurohypophysis of hens was demonstrated by the use of radioligand binding assays on the plasma membrane fraction of the tissue. The binding to [3H]E2 had a binding specificity to E2 and diethylstilbestrol, reversibility, and saturation. Scatchard analysis revealed that the binding sites are of a single class. The equilibrium dissociation constant obtained by Scatchard analysis and kinetic analysis suggested a high affinity, and the maximum binding capacity obtained by Scatchard analysis suggested a limited capacity. These properties are characteristics of a receptor, which suggests that an estrogen receptor exists in the plasma membrane of hen neurohypophysis. The equilibrium dissociation constant value of estrogen receptor of the neurohypophysis was not significantly different between laying hens and nonlaying hens, but the maximum binding capacity value was statistically smaller (the binding affinity is higher) in laying hens than in nonlaying hens. The specific binding of estrogen receptor showed a decrease at 1 h after an injection of diethylstilbestrol in nonlaying hens. The specific binding also decreased 3 h before oviposition in laying hens and maintained low value until just after oviposition. The present study suggests that estrogen may act directly on the neurohypophysis during 3 h before oviposition in hens.

Role of sex steroid receptors in pathobiology of hepatocellular carcinoma

The striking gender disparity observed in the incidence of hepatocellular carcinoma (HCC) suggests an important role of sex hormones in HCC pathogenesis. Though the studies began as early as in 1980s, the precise role of sex hormones and the significance of their receptors in HCC still remain poorly understood and perhaps contribute to current controversies about the potential use of hormonal therapy in HCC. A comprehensive review of the existing literature revealed several shortcomings associated with the studies on estrogen receptor (ER) and androgen receptor (AR) in normal liver and HCC. These shortcomings include the use of less sensitive receptor ligand binding assays and immunohistochemistry studies for ERα alone until 1996 when ERβ isoform was identified. The animal models of HCC utilized for studies were primarily based on chemical-induced hepatocarcinogenesis with less similarity to virus-induced HCC pathogenesis. However, recent in vitro studies in hepatoma cells provide newer insights for hormonal regulation of key cellular processes including interaction of ER and AR with viral proteins. In light of the above facts, there is an urgent need for a detailed investigation of sex hormones and their receptors in normal liver and HCC. In this review, we systematically present the information currently available on androgens, estrogens and their receptors in normal liver and HCC obtained from in vitro, in vivo experimental models and clinical studies. This information will direct future basic and clinical research to bridge the gap in knowledge to explore the therapeutic potential of hormonal therapy in HCC.

Vascular cell signaling by membrane estrogen receptors

The definition of estrogen's actions has expanded from transcriptional regulation to the rapid, membrane-initiated activation of numerous signal transduction cascades. Multiple biological effects of estrogen have been shown in numerous animals, cellular and molecular studies, which support the favorable effects of estrogen on vascular structure, function, and cell signaling. Work from several laboratories has shown that these effects are mediated by distinct forms of estrogen receptor (ER) alpha. This includes estrogen-stimulated rapid activation of endothelial nitric oxide synthase (eNOS), resulting in the elaboration of the athero-protective, angiogenesis-promoting product nitric oxide (NO). We have described the expression of ER46, an N-terminus truncated isoform of the ERalpha, in human endothelial cells (EC), and its critical role in membrane-initiated, rapid responses to 17beta-estradiol (E2). We have proposed an ER46-centered, eNOS activating molecular complex in human EC caveolar membranes, containing c-Src, phosphatidylinositol 3-kinase (PI3K), Akt and eNOS. Our previous studies support estrogen-induced rapid eNOS activation via a sequential c-Src/PI3K/Akt cascade in EC. In this review, we describe estrogen-induced, rapid, non-genomic actions in endothelium, driven by c-Src-ER46-caveolin-1 interactions, with consequent activation of eNOS. Amidst ongoing controversies in hormone replacement therapy, these molecular and cellular data, defining favorable estrogenic effects on the endothelium, provide a strong impetus to resolve these clinical questions.

Extra-nuclear signaling of estrogen receptors

Estrogen controls multiple biological functions through binding to estrogen receptors (ERs). Traditionally, ERs have been regarded as transcription factors regulating the expression of target genes. However, growing evidence of rapid estrogen's actions in a number of tissues has been accumulating and alternative mechanisms of signal transduction have been proposed. These so called "extra-nuclear actions" do not require gene expression or protein synthesis and are independent of the nuclear localization of ERs. Indeed, some of these actions are elicited by ERs residing at or near the plasma membrane. Membrane-associated molecules such as ion channels, G proteins, the tyrosine kinase c-Src as well as growth factor receptors are modulated by liganded ERs within the membrane, leading to the activation of downstream cascades such as mitogen-activated protein kinase, phosphatidylinositol 3-OH kinase, protein kinase A, and protein kinase C. These cascades mediate some important rapid actions of estrogen, such as the activation of nitric oxide synthesis or the remodeling of actin cytoskeleton. In addition, these pathways are critical for the regulation of the expression of a number of target proteins implicated in cell proliferation, apoptosis, differentiation, movement, and homeostasis. In this manner, the extra-nuclear pathways are tightly integrated with the genomic pathways to orchestrate the full spectrum of estrogen's biological functions. The recent advancements in the characterization of the molecular basis of the extra-nuclear signaling of estrogen helps to understand the role of estrogen on human cells, and may in future turn out to be of relevance for clinical purposes.

17beta-estradiol rapidly mobilizes intracellular calcium from ryanodine-receptor-gated stores via a PKC-PKA-Erk-dependent pathway in the human eccrine sweat gland cell line NCL-SG3

We describe a novel rapid non-genomic effect of 17beta-estradiol (E2) on intracellular Ca2+ ([Ca2+]i) signalling in the eccrine sweat gland epithelial cell line NCL-SG3. E2 had no observable effect on basal [Ca2+]i, however exposure of cells to E2 in the presence of the microsomal Ca2+ ATPase pump inhibitor, thapsigargin, produced a secondary, sustained increase in [Ca2+]i compared to thapsigargin treatment alone, where cells responded with a transient single spike-like increase in [Ca2+]i. The E2-induced increase in [Ca2+]i was not dependent on the presence of extracellular calcium and was completely abolished by ryanodine (100 microM). The estrogen receptor antagonist ICI 182,780 (1 microM) prevented the E2-induced effects suggesting a role for the estrogen receptor in the release of [Ca2+]i from ryanodine-receptor-gated stores. The E2-induced effect on [Ca2+]i could also be prevented by the protein kinase C delta (PKCdelta)-specific inhibitor rottlerin (10 microM), the protein kinase A (PKA) inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate (200 microM) and the MEK inhibitor PD98059 (10 microM). We established E2 rapidly activates the novel PKC isoform PKCepsilon, PKA and Erk 1/2 MAPK in a PKCdelta and estrogen-receptor-dependent manner. The E2-induced effect was specific to 17beta-estradiol, as other steroids had no effect on [Ca2+]i. We have demonstrated a novel mechanism by which E2 rapidly modulates [Ca2+]i release from ryanodine-receptor-gated intracellular Ca2+ stores. The signal transduction pathway involves the estrogen receptor coupled to a PKC-PKA-Erk 1/2 signalling pathway.

Effect of oestradiol on dopamine receptors and protein kinase C activity in the rat pituitary: binding of oestradiol to pituitary membranes

Oestradiol exerts an important modulatory influence on the release of prolactin which is accomplished partly through disruption of the inhibitory influence of dopamine. We have focused on the status of the anterior pituitary D2 dopamine receptor in female rats treated chronically with oestradiol or progesterone. A direct membrane effect of these steroids on the dopamine system was also investigated in vitro. Both steroids affected the status of the D2 receptor, oestradiol decreasing the number of sites in vitro and progesterone increasing it both in vitro and in vivo. The in vitro studies demonstrated that these steroids exert a direct membrane effect on the D2 receptor. These results correlated with an in vitro short-term physiological effect of oestradiol and progesterone on the dopaminergic inhibition of prolactin release, oestradiol decreasing it while progesterone had the opposite effect. Binding studies with [3H] oestradiol on pituitary membranes revealed a site for oestradiol of high affinity and low capacity, indicating that oestradiol's membrane effects could be mediated by a specific receptor. In vivo treatment with oestradiol also induces proliferation of prolactin-secreting cells (lactotrophs). We focused on the effect of oestradiol on protein kinase C activity, which is involved in both secretion and proliferation. In female rats treated with oestradiol total protein kinase C activity was increased by 74% (particulate 90%, soluble 71%) in comparison with controls. This effect was reversed by concomitant treatment with a dopamine agonist. Thus in the pituitary oestradiol and progesterone affect the characteristics of membrane components that are implicated in the physiological control of the cell. Whether these effects are post-transcriptional only or are also mediated through direct membrane mechanisms needs further investigation.

Non-genomic mechanisms of action of steroid hormones

Sex steroid hormones are known to act through intracellular receptors and their cognate hormone response elements, located in the promoters of hormone-regulated genes. However, this classical mechanism of action cannot account for a variety of rapid effects of steroids (within seconds or minutes). In this review, non-genomic modes of target cell responses to sex steroids are described. The prototypical example is the resumption of meiosis in amphibian oocytes, triggered by progesterone at the plasma membrane level. Membrane effects of progesterone may also account for sperm maturation. Other membrane-mediated effects of steroids are reviewed. Whether a steroid hormone might elicit responses from a single cell through both genomic and membrane mechanisms remains an open question.

Estrogen receptors outside the nucleus in breast cancer

The estrogen receptor (ER) is the single most powerful predictor of breast cancer prognosis as well as an important contributor to the biology of carcinogenesis. In addition, endocrine therapy targeting ER directly (SERMS) or indirectly (aromatase inhibitors) forms the mainstay of adjuant therapy. Traditionally, human tumors are scored for the amount and presence of ER. However, this has centered on the population of ER found in the transformed epithelial cell nucleus. Over the last 40 years, it has been appreciated that additional cellular ER pools exist, in cytoplasm and at the plasma membrane. In this review, we discuss the important functions of extra-nuclear ER in breast cancer, including integration of function with nuclear ER.

Estrogen receptor signaling pathways in human non-small cell lung cancer

Lung cancer is the most common cause of cancer mortality in male and female patients in the US. The etiology of non-small cell lung cancer (NSCLC) is not fully defined, but new data suggest that estrogens and growth factors promote tumor progression. In this work, we confirm that estrogen receptors (ER), both ERalpha and ERbeta, occur in significant proportions of archival NSCLC specimens from the clinic, with receptor expression in tumor cell nuclei and in extranuclear sites. Further, ERalpha in tumor nuclei was present in activated forms as assessed by detection of ER phosphorylation at serines-118 and -167, residues commonly modulated by growth factor receptor as well as steroid signaling. In experiments using small interfering RNA (siRNA) constructs, we find that suppressing expression of either ERalpha or ERbeta elicits a significant reduction in NSCLC cell proliferation in vitro. Estrogen signaling in NSCLC cells may also include steroid receptor coactivators (SRC), as SRC-3 and MNAR/PELP1 are both expressed in several lung cell lines, and both EGF and estradiol elicit serine phosphorylation of SRC-3 in vitro. EGFR and ER also cooperate in promoting early activation of p42/p44 MAP kinase in NSCLC cells. To assess new strategies to block NSCLC growth, we used Faslodex alone and with erlotinib, an EGFR kinase inhibitor. The drug tandem elicited enhanced blockade of the growth of NSCLC xenografts in vivo, and antitumor activity exceeded that of either agent given alone. The potential for use of antiestrogens alone and with growth factor receptor antagonists is now being pursued further in clinical trials.

Biology of estrogens in skin: implications for skin aging

Estrogens have a profound influence on skin. The relative hypoestrogenism that accompanies menopause exacerbates the deleterious effects of both intrinsic and environmental aging. Estrogens clearly have a key role in skin aging homeostasis as evidenced by the accelerated decline in skin appearance seen in the perimenopausal years. Estrogens improve skin in many ways. Among these, they increase collagen content and skin thickness and improve skin moisture. However, despite the knowledge that estrogens have such important effects on skin, the cellular and subcellular sites and mechanisms of estrogen action are still poorly understood. Estrogen receptors (ERs) have been detected in skin, and recent studies suggest that estrogens exert their effect in skin through the same molecular pathways used in other non-reproductive tissues. Although systemic hormone replacement therapy (HRT) has been used for many years, recent trials have reported a significant increased risk of breast cancer and other pathologies with this treatment. This has led to reconsider the risks and benefits of HRT. For this reason, systemic HRT cannot be recommended today to treat skin aging. Currently, intensive research is conducted to develop new drugs called selective ER modulators (SERMs). These drugs exert mixed estrogenic and antiestrogenic effects depending on the tissue and cell type. One might expect in the future such a drug targeting specifically the skin without systemic side effects.

Membrane estrogen receptors: genomic actions and post transcriptional regulation

The primary cellular location of the nuclear estrogen receptor II (nER II) is the plasma membrane. A number of reports that have appeared in the recent past indicate that plasma membrane localized estrogen receptor alpha (ERalpha) also exists. Whether the membrane localized ERalpha represents the receptor that binds to the estrogen responsive element (ERE) remains to be known. The mechanisms that underlie the internalization of nER II (non-activated estrogen receptor, deglycosylated) have been identified to a certain extent. The question remains: is the primary location of the ERalpha also the plasma membrane? If that is the case, it will be a challenging task to identify the molecular events that underlie the plasma membrane-to-nucleus movement of ERalpha. The internalization mechanisms for the two 66kDa plasma membrane ERs, following hormone binding, appear to be distinct and without any overlaps. Interestingly, while the major gene regulatory role for ERalpha appears to be at the level of transcription, the nER II has its major functional role in post transcriptional mechanisms. The endoplasmic reticulum associated anchor protein-55 (ap55) that was recently reported from the author's laboratory needs a closer look. It is a high affinity estrogen binding protein that anchors the estrogen receptor activation factor (E-RAF) in an estrogen-mediated event. It will be interesting to examine whether ap55 bears any structural similarity with either ERalpha or ERbeta.

Membrane steroid receptor signaling in normal and neoplastic cells

Rapid, non-genomic, steroid actions have been identified for more than 20 years. In the last decade however, a great expansion of research was observed. In the present review we report the identification and the subsequent signaling cascades involved in these rapid steroid effects. In the current state of knowledge, with the exception of progesterone for which a seven-loop G protein-coupled receptor has been identified, two major lines of evidence exist for membrane-related steroid actions: (1) a binding to the intracellular receptor, coupled to the plasma membrane, or interacting with other growth factor receptors, and (2) the existence of specific membrane steroid receptors. In addition, major intracellular signaling cascades involved in cell survival and/or apoptosis are activated by non-genomic steroid actions. Finally, it appears that cancer cells and tumors express membrane steroid sites, related to cancer aggressiveness. These lines of evidence may implicate, in the forthcoming years, membrane steroid receptors in cancer control as major or adjuvant chemotherapeutic agents, providing new possible targets for cancer chemotherapy.

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

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

Activation of membrane estrogen receptors induce pro-survival kinases

Experimental and epidemiological data suggest a neuroprotective role for estrogen (E(2)). We have recently shown that, in PC12 cells, non-permeable estradiol conjugated to bovine serum albumin (BSA) prevent serum-deprivation induced apoptosis through activation of specific membrane estrogen receptors (mER). In the present study, we explored in detail the early signaling events involved in this anti-apoptotic action, downstream to activation of mER. Our findings suggest that mER is associated to G-proteins, and its activation with non-permeable E(2)-BSA results in the activation of the following downstream pro-survival kinases pathways: (1) the PKB/Akt pathway, (2) the Src-->MEK-->ERK kinases and finally (3) the MAPK-->ERK kinases. Activation of these pro-survival signals leads to CREB phosphorylation and NFkappaB nuclear translocation, two transcription factors controlling the expression of anti-apoptotic Bcl-2 proteins. These data suggest that major pro-survival kinases are involved in the mER-mediated anti-apoptotic effects of estrogen. This is further supported by experiments with specific kinases inhibitors, which partially but significantly reversed the mER-mediated anti-apoptotic effect of E(2)-BSA. Our findings suggest that estrogen act via mER as potent cytoprotective factors, downstream activating pro-survival kinases, assuring thus an efficient and multipotent activation of the anti-apoptotic machinery.

Effects of estrogen in the male rat medial amygdala: infusion of an aromatase inhibitor lowers mating and bovine serum albumin-conjugated estradiol implants do not promote mating

In male rats, copulatory behavior depends on estrogen-responsive neurons located in brain areas known to be crucial for mating. Blocking the aromatization of testosterone (T) to estradiol (E(2)) either throughout the brain or within the medial preoptic area (MPO) reduces mating, whereas E(2) treatment of either the MPO or the medial amygdala (MEA) maintains sexual behavior. The effects of T aromatization in the MEA have received less attention; therefore, 2 studies were done to further elucidate the effects of E(2) in the MEA. In experiment 1, gonadally intact male rats that showed robust mating behavior were administered chronic fadrozole, a nonsteroidal aromatase inhibitor, to the MEA to stop the conversion of T to E(2) and then paired with receptive females. Infusion of fadrozole to the MEA significantly lowered mating behavior in experimental males compared to vehicle-infused control males. To further investigate the mechanism by which E(2) acts in the MEA, in experiment 2, E(2) conjugated to bovine serum albumin (BSA-E(2): a complex of E(2 )and a large protein that does not cross the plasma membrane, thereby restricting the action of E(2) to cell-surface signaling) was chronically administered bilaterally to the MEA of castrated, dihydrotestosterone-treated males. This treatment did not maintain mating behavior. These studies show that E(2) acts in the MEA to promote male sexual behavior and suggest an intercellular mechanism of E(2) action.

Regulation of signal transduction pathways by estrogen and progesterone

The female sex steroid hormones 17beta-estradiol and progesterone mediate their biological effects on development, differentiation, and maintenance of reproductive tract and other target tissues through gene regulation by nuclear steroid receptors that function as ligand-dependent transcription factors. However, not all effects of 17beta-estradiol and progesterone are mediated by direct control of gene expression. These hormones also have rapid stimulatory effects on the activities of a variety of signal transduction molecules and pathways and, in many cases, these effects appear to be initiated from the plasma cell membrane. There is growing evidence that a subpopulation of the conventional nuclear steroid receptor localized at the cell membrane mediates many of the rapid signaling actions of steroid hormones; however, novel membrane receptors unrelated to conventional steroid receptors have also been implicated. This chapter reviews the nature of the receptors that mediate rapid signaling actions of estrogen and progesterone and describes the signaling molecules and pathways involved, the mechanisms by which receptors couple with components of signaling complexes and trigger responses, and the target tissues and cell functions regulated by this mode of steroid hormone action.

Distinctive actions of membrane-targeted versus nuclear localized estrogen receptors in breast cancer cells

Estrogens regulate multiple activities in breast cancer cells, including proliferation. Whereas these hormones are most commonly known to regulate gene transcription through direct interaction with estrogen receptors (ERs) and with specific DNA sequences of target genes, recent studies show that ER also activates a number of rapid signaling events that are initiated at the cell membrane. To study the membrane-initiated effects of estrogen and separate them from the activities initiated by the nuclear localized ER in human breast cancer cells, we generated MDA-MB-231 breast cancer cell lines that have stably integrated either the wild-type nuclear form of ER (WT-ER) or a modified, membrane-targeted ER (MT-ER) that lacks a nuclear localization sequence and is dually acylated with a myristoylation sequence at the N terminus and a palmitoylation sequence at the C terminus. We demonstrate that MT-ER is membrane localized in the absence of estradiol (E2), showing punctate membrane and cytoplasmic speckles after E2 exposure. In contrast to WT-ER, MT-ER was not down-regulated by E2 or by antiestrogen ICI 182,780 exposure, and MT-ER failed to regulate endogenous E2-responsive genes highly up-regulated by WT-ER. Cells expressing MT-ER showed a greater serum response element-mediated transcriptional response that was partially inhibited by antiestrogen ICI 182,780. The MT-ER and WT-ER differentially altered ERK1/2 and Akt activities and the proliferation of breast cancer cells in response to E2. Hence, this study reveals distinct actions of the MT-ER vs. the WT-ER in effecting estrogen actions in breast cancer cells.

Rapid nitric oxide-mediated S-nitrosylation of estrogen receptor: regulation of estrogen-dependent gene transcription

Nitric oxide (NO) and estrogen receptor (ER) are both important mediators of signal transduction in cardiovascular and reproductive tissues. In this study, we evaluated NO-mediated S-nitrosylation of ER and assessed the effect of this structural modification on transcription-related functions of ER. We have found selective inhibitory effects of NO on specific binding of ER to specific estrogen-responsive elements (ERE) that can be reversed in the presence of the reducing agent, DTT, thus suggesting that S-nitrosylation of thiolate-zinc centers may occur within the ER molecule. Furthermore, we examined inhibitory effects of NO on ER-dependent transcriptional activity by using an ERE-driven reporter gene system. By monitoring biophysical changes in the structure of NO-treated or untreated human recombinant ERalpha,we obtained evidence for the formation of S-nitrosothiols in the ER molecule. In addition, we have detected specific S-nitrosylation of cysteine residues within the ER molecule by immunodetection of S-nitrosocysteine moieties in ER. Collectively, these findings suggest an important physiological role for NO in modification of human ER structure by S-nitrosylation, an effect that leads, in turn, to impaired DNA-binding activity of ER and subsequent blockade of estrogen-dependent gene transcription. Thus, NO-induced S-nitrosylation of ER can occur at cysteine residues that coordinate Zn2+ within the two major DNA-binding Zn-finger domains of ER, resulting in selective inhibition of DNA-binding at specific ERE. This cross-communication between NO and ER may favor activation of rapid (nongenomic) signaling pathways and subsequent modulation of downstream genomic activity.

Estradiol Protects Against Oxygen and Glucose Deprivation in Rat Hippocampal Organotypic Cultures and Activates Akt and Inactivates GSK-3?

Here we investigated the neuroprotective effect of 17beta-estradiol in an in vitro model of ischemia. We used organotypic hippocampal slice cultures, acute or chronically treated with 17beta-estradiol (10 nM), and exposed to oxygen and glucose deprivation (OGD). Cellular death was quantified by measuring uptake of propidium iodide (PI), a marker of dead cells. In OGD exposed cultures, treated only with vehicle, about 70% of the CA1 area of hippocampus was labeled with PI, indicating a great percentage of cellular death. When cultures were treated with 17beta-estradiol (acute or chronically), this cellular death was reduced to 15%. This effect was prevented by LY294002 but was not by PD98059. Immunoblotting revealed that both, chronic and acute, treatments with 17beta-estradiol induced the phosphorylation/activation of Akt and the phosphorylation/inactivation of GSK-3beta. Our results show a clear neuroprotective effect of 17beta-estradiol and suggest that this effect could involve PI3-K pathway.

Estradiol-mediated internalisation of the non-activated estrogen receptor from the goat uterine plasma membrane: identification of the proteins involved

An indirect approach has been made to study the molecular details associated with the estradiol-induced internalisation of the non-activated estrogen receptor (naER) from the goat uterine plasma membrane. The internalisation of naER appears to be an energy dependent process. Exposure of the plasma membrane to estradiol results in the activation of a Mg2+ dependent ATPase associated with the membrane fraction. Presence of quercetin in the medium prevented the activation of the Mg2+ ATPase as well as the dissociation of naER from the plasma membrane. Using isolated plasma membrane preparations it has been possible to identify the proteins which interact with naER during various stages of its internalisation. The main proteins identified are: (1) a 58 kDa protein, p58, which apparently recognizes the nuclear localization signals on the naER and transports it to the nucleus: (2) hsp70: (3) hsp90, the functional roles of which remain unknown at this stage; (4) a 50 kDa protein associated with the clathrin coated vesicles, presumed to be involved in recognizing the tyrosine based internalisation signals on the naER; (5) actin which mediates the plasma membrane-to-nucleus movement of the naER-p58 complex.

Phosphorylation of Y845 on the epidermal growth factor receptor mediates binding to the mitochondrial protein cytochrome c oxidase subunit II

When co-overexpressed, the epidermal growth factor receptor (EGFR) and c-Src cooperate to cause synergistic increases in EGF-induced DNA synthesis, soft agar colony growth, and tumor formation in nude mice. This synergy is dependent upon c-Src-mediated phosphorylation of a unique tyrosine on the EGFR, namely, tyrosine 845 (Y845). Phenylalanine substitution of Y845 (Y845F) was found to inhibit EGF-induced DNA synthesis without affecting the catalytic activity of the receptor or its ability to phosphorylate Shc or activate mitogen-activated protein kinase. These results suggest that synergism may occur through alternate signaling pathways mediated by phosphorylated Y845 (pY845). One such pathway involves the transcription factor Stat5b. Here we describe another pathway that involves cytochrome c oxidase subunit II (CoxII). CoxII was identified as a specific binding partner of a pY845-containing peptide in a phage display screen. EGF-dependent binding of CoxII to the wild type but not to the mutant Y845F-EGFR was confirmed by coimmunoprecipitation experiments. This association also required the kinase activity of c-Src. Confocal microscopy, as well as biochemical fractionation, indicated that the EGFR translocates to the mitochondria after EGF stimulation, where it colocalizes with CoxII. Such translocation required the catalytic activity of the receptor but not phosphorylation of Y845. However, ectopic expression of the Y845F-EGFR prevented the EGF from protecting MDA-MB-231 breast cancer cells from adriamycin-induced apoptosis, whereas two mutants of Stat5b, a dominant-interfering mutant (DNstat5b) and a tyrosine mutation at 699 (Y699F-Stat5b) did not. Taken together, these data suggest that, through the ability of EGFR to translocate to the mitochondria, the binding of proteins such as CoxII to pY845 on the EGFR may positively regulate survival pathways that contribute to oncogenesis.

Estrogen Receptor-Mediated Vascular Responsiveness to Nebivolol: A Novel Endothelium-Related Mechanism of Therapeutic Vasorelaxation

Nebivolol is a highly selective and lipophilic beta1-adrenergic receptor antagonist with vasodilating characteristics attributed mainly to endothelial generation of nitric oxide (NO). Coincidently, rapid vascular vasodilating effects of estradiol are also widely reported and membrane-initiated signaling by estrogen receptor (ER), leading to generation of NO, parallels the vasodilating effects observed for nebivolol. Thus, we hypothesized that the NO-dependent vasodilating effect attributed to nebivolol may be partially mediated through its interaction with the membrane-associated form of ER. The objective of this study was to examine the ER-mediated/endothelium-dependent vascular responses to nebivolol and the specific binding properties of nebivolol to ER. In isolated rat aortic rings, the endothelium-dependent vasodilating effect of nebivolol was significantly blocked by the use of the potent ER antagonist, ICI 182,780. In contrast, in the absence of intact endothelium, nebivolol-induced vasorelaxation was not blocked by ICI 182,780, strongly suggesting that nebivolol-elicited vasorelaxation is partially dependent on ER-mediated pathways. Further, we examined the binding of nebivolol to ER (MCF-7 cells) by radioligand binding assay and revealed specific binding kinetics with an estimated DC50 of 5 x 10 M, coinciding with the approximate EC50 of nebivolol as a vasorelaxant. In conclusion, the endothelium-dependent vascular response to nebivolol is attributed partially to its interaction with ER.

Evidence on the operation of ATP-induced capacitative calcium entry in breast cancer cells and its blockade by 17beta-estradiol

Little is known about the regulation of cytosolic calcium Ca(2+) levels ([Ca(2+)](i)) in breast cancer cells. We investigated the existence of capacitative calcium entry (CCE) in the tumorigenic cell line MCF-7 and its responsiveness to ATP. MCF-7 cells express purinergic receptors as well as estrogen receptors (ER). Depletion of calcium stores with thapsigargin (TG, 500 nM) or ATP (10 microM) in the absence of extracellular Ca(2+), resulted in a rapid and transient elevation in [Ca(2+)](i). After recovery of basal levels, Ca(2+) readmission (1.5 mM) to the medium increased Ca(2+) influx (twofold over basal), reflecting pre-activation of a CCE pathway. Cells pretreated with TG were unable to respond to ATP, thus indicating that the same Ca(2+) store is involved in their response. Moreover, IP(3)-dependent ATP-induced calcium mobilization and CCE were completely blocked using compound U-73122, an inhibitor of phospholipase C. Compound 2-APB (75 microM) and Gd(3+) (10 microM), antagonists of the CCE pathway, completely prevented ATP-stimulated capacitative Ca(2+) entry. CCE in MCF-7 cells was highly permeable to Mn(2+) and to the Ca(2+) surrogate Sr(2+). Mn(2+) entry sensitivity to Gd(3+) matched that of the Ca(2+) entry pathway. 17Beta-estradiol blocked ATP-induced CCE, but was without effect on TG-induced CCE. Besides, the estrogen blockade of the ATP-induced CCE was completely abolished by preincubation of the cells with an ER monoclonal antibody. ER alpha immunoreactivity could also be detected in a purified plasma membrane fraction of MCF-7 cells. These results represent the first evidence on the operation of a ATP-responsive CCE pathway in MCF-7 cells and also indicate that 17beta-estradiol interferes with this mechanism by acting at the cell surface level.

Nongenomic effects of 17beta-estradiol--diversity of membrane binding sites

The classical model of the action of 17beta-estradiol comprises binding of this gonadal steroid hormone to nuclear estrogen receptors leading to the modulation of gene transcription and protein synthesis. However, in the last few years several evidences about the rapid nongenomic action of 17beta-estradiol via the stimulation of putative receptors located in the cell membrane have also been reported. These nongenomic responses occur within a few minutes and are insensitive to the inhibitors of transcription and translation; however, no such membrane receptors have been cloned so far. In this review article, we present a survey showing that such membrane binding sites of 17beta-estradiol have rather different ligand specificity properties than that of classical genomic estrogen receptors concerning the potential activity of different estrogens and other steroid hormones, supporting the conception of structurally distinct receptors for genomic and nongenomic pathways. The fact that rapid responses to 17beta-estradiol could be induced by a wide concentration range from some picomolar to high micromolar doses points to the diversity of these nongenomic membrane binding sites as well as the complexity of their functioning.

Differential expression of Akt/protein kinase B, Bcl-2 and Bax proteins in human leiomyoma and myometrium

The expression and activation of serine/threonine protein kinase, Akt, in leiomyoma and in adjacent myometrium of human uteri was studied parallel with the changes of Bcl-2, Bax proteins, estrogen and progesterone receptors during menstrual cycle and early stage of the menopause. Abundant expression of Akt protein was detected in the studied tissues during menstrual cycle, the rate of increase was higher in leiomyoma than in corresponding myometrium. The expression of estrogen receptor alpha, progesterone receptor and of Bcl-2 protein changed parallel with that of Akt protein. The level of phosphorylated Akt (pAkt(473)) was seen only in leiomyoma samples from the growing period of tumors. At early stage of menopause levels of all studied proteins were lower than that in the menstrual cycle with the exception of Bax protein expression, which was high in leiomyoma. Our data suggest the involvement of phosphatidylinositol 3-kinase/Akt signaling in the pathomechanism of leiomyoma.

Uncoupling of 5-HT1A receptors in the brain by estrogens: regional variations in antagonism by ICI 182,780

Previously we have shown that 17beta-estradiol (in vivo and in vitro) rapidly decreases the function of serotonin(1A) (5-HT(1A)) receptors, allowing us to hypothesize that 17beta-estradiol accomplished this via activation of a membrane estrogen receptor. Hippocampus and frontal cortex obtained from ovariectomized rats were incubated with 17beta-estradiol or bovine serum albumin (BSA)-estradiol in the presence or absence of the estrogen receptor (ER) antagonist ICI 182,780. Membranes were prepared to measure R(+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding (a measure of 5-HT(1A) receptor coupling and function). In both hippocampus and frontal cortex, 17beta-estradiol and BSA-estradiol (50 nM) decreased R(+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding. ICI 182,780 blocked the effect of both the estrogens in hippocampus, but only the effect of 17beta-estradiol in frontal cortex. Due to the inability of ICI 182,780 to block the effects of BSA-estradiol in frontal cortex, similar experiments were performed using the selective estrogen receptor modulator tamoxifen as the agonist. Tamoxifen (100 nM and 1 microM) decreased R(+)8-OH-DPAT-stimulated [(35)S]GTPgammaS binding. ICI 182,780 (1 microM) blocked the ability of tamoxifen to decrease 5-HT(1A) receptor coupling in the hippocampus, but not in the frontal cortex. Taken together, these data support the existence of a pharmacologically distinct ER in hippocampus vs. frontal cortex that might be responsible for rapid uncoupling of 5-HT(1A) receptors.

Phytoestrogens as modulators of steroid action in target cells

Although numerous reports exist on the potential beneficial role of nutritional phytoestrogens in human health, their molecular mechanism in target cells is still not completely understood. Phytoestrogens promote estrogen and antiestrogen effects by interacting with numerous molecules, carrier proteins, enzymes and membrane and nuclear receptors, directly or indirectly involved in the transfer of estrogen signals. The hypothesis that the ER beta subtype plays a key role in antiproliferative effect of phytoestrogens, especially in breast cancer, is examined here. This review focus on the effects of phytoestrogens in developmental processes such as those linked to reproductive function, tumorigenesis and angiogenesis.

Conventional estrogen receptors are found in the plasma membrane of vaginal epithelial cells of the rat

Estrogens induce rapid (non-genomic) and delayed (genomic) effects on the target cells. The early effects include induction of signal transduction pathway within seconds, whereas the delayed responses require hours and involve transcription and translation. The rapid effects of estradiol (E) on the vaginal epithelial cells (VEC) involved calcium uptake within seconds via the induction of phosphoinositol lipid metabolism as reported in our earlier studies. In this study, we demonstrate the presence of classical estrogen receptors (ER) on the plasma membrane of VEC of the rats. Immunoreactive bands of 67, 56 and 35 kDa are detectable in the membrane fractions (mf) using antibodies recognizing different epitopes of ER alpha. We have also been able to purify a protein having a mass of 67 kDa from the detergent-soluble fraction of the plasma membrane of VEC, which shows properties identical to the classical receptor purified from the cytosolic fraction of the cells. The membrane receptors get dissociated upon binding to the ligand. Besides a role in signal transduction events induced by estradiol, the membrane estrogen receptors may have an important role to play in translocation of the steroid to the cytosolic compartment.

Cellular functions of plasma membrane estrogen receptors

Strong evidence now exists for the presence and importance of plasma membrane estrogen receptors (ER) in a variety of cells that are targets for steroid action. When estradiol (E2) binds cell surface proteins, the initiation of signal transduction triggers downstream signaling cascades that contribute to important functions. These functions include cell growth and survival, migration, and new blood vessel formation. In some instances these effects result from the initiation of gene transcription, upregulated through signaling from the membrane. The membrane ER probably originates from the same gene and transcript that produces the nuclear receptor. In the membrane, ER appear to localize mainly to discrete domains of the plasma membrane, known as caveolae, but the mechanisms by which this small pool of ER translocates to this site are currently unknown. At the caveolae, a cross talk with signaling molecules facilitates E2/ER cell biologic actions. This both includes direct stimulation of signaling via G protein activation, and a cross-activation of the epidermal growth factor receptor (EGFR). This review article highlights some of the important advances in understanding the cell biology of estrogen action that emanates from the membrane.

Evaluation of potential implication of membrane estrogen binding sites on ERE-dependent transcriptional activity and intracellular estrogen receptor-alpha regulation in MCF-7 breast cancer cells

The potential involvement of membrane estrogen binding sites in the induction of ERE-dependent transcriptional activity as well as in the regulation of intracellular estrogen receptor alpha (ER-alpha) level under estradiol (E2) stimulation was investigated. Our approach relied upon the use of two DCC-treated E2-BSA (bovine serum albumin) solutions (E2-6-BSA and E2-17-BSA). The absence of detectable free E2 in these solutions was established. Both E2-BSA conjugates led to a transient dose-dependent stimulation of the expression of ERE-luciferase (LUC) reporter gene in MVLN cells (MCF-7 cells stably transfected with a pVit-tk-LUC reporter plasmid), a property not recorded with free E2, which maintained enhanced transcriptional activity during the whole experiment. A very low concentration of E2 (10 pM) synergistically acted with E2-BSA conjugates. Hence, ERE-dependent transcriptional activity induced by these conjugates appeared to result from their known interactions with membrane estrogen binding sites. Anti-estrogens (AEs: 4-OH-TAM and RU 58,668), which antagonize genomic ER responses, abrogated the luciferase activity induced by E2-BSA conjugates, confirming a potential relationship between membrane-related signals and intracellular ER. Moreover, induction of luciferase was recorded when the cells were exposed to IBMX (3-isobutyl-1-methylxanthine) and cyclic nucleotides (cAMP/cGMP), suggesting the implication of the latter in the signal transduction pathway leading to the expression of the reporter gene. Growth factors (IGF-I, EGF and TGF-alpha) also slightly stimulated luciferase and synergistically acted with 10 pM E2, or 1 microM E2-BSA conjugates, in agreement with the concept of a cross-talk between steroids and peptides acting on the cell membrane. Remarkably, E2-BSA conjugates, IBMX and all investigated growth factors failed to down-regulate intracellular ER in MCF-7 cells, indicating the need for a direct intracellular interaction of the ligand with the receptor to regulate its level. ER elimination was, however, found in the presence of conditioned media (CMs) prepared from cells pre-exposed to E2-BSA conjugates, suggesting that they may produce (a) modulator(s) that may enhance receptor down-regulation when released within the medium.

Subcellular distribution of native estrogen receptor alpha and beta isoforms in rabbit uterus and ovary

The association of estrogen receptors with non-nuclear/cytoplasmic compartments in target tissues has been documented. However, limited information is available on the distribution of estrogen receptor isoforms, specially with regard to the newly described beta isotype. The subcellular localization of estrogen receptor alpha and beta isoforms was investigated in rabbit uterus and ovary. Native alpha and beta subtypes were immunodetected using specific antibodies after subjecting the tissue to fractionation by differential centrifugation. The ovary expressed alpha and beta estrogen receptors in predominant association to cytosolic components. However, in the uterus, a substantial proportion of the total estrogen binding capacity and coexpression of the two isoforms was detected in mitochondria and microsomes. The mitochondrial-enriched subfraction represented an important source of 17beta-estradiol binding, where the steroid was recognized in a stereospecific and high affinity manner. The existence of mitochondrial and membrane estrogen binding sites correlated with the presence of estrogen receptor alpha but mainly with estrogen receptor beta proteins. Using macromolecular 17beta-estradiol derivatives in Ligand Blot studies, we could confirm that both alpha and beta isoforms were expressed as the major estrogen binding proteins in the uterus, while estrogen receptor alpha was clearly the dominant isoform in the ovary. Other low molecular weight estrogen receptor alpha-like proteins were found to represent an independent subpopulation of uterine binding sites, expressed to a lesser extent. This differential cellular partitioning of estrogen receptor alpha and beta forms may contribute to the known diversity of 17beta-estradiol effects in target organs. Both estrogen receptor alpha and beta expression levels and cellular localization patterns among tissues, add complexity to the whole estrogen signaling system, in which membrane and mitochondrial events could also be implicated.

Cell localization, physiology, and nongenomic actions of estrogen receptors

The existence of binding proteins for the female sex steroid, 17beta-estradiol, has been known for almost 50 years. Presently, two estrogen receptors (ERs), ER-alpha and ER-beta, have been cloned in mammals, and they are expressed in many cell types of metazoans. ERs act primarily as nuclear transcription factors, and this effect is enhanced by ligand binding. Emerging data have identified a separate pool of receptors for this steroid in the plasma membrane, but the mechanisms of action and cellular functions of these proteins are just beginning to be defined. In this review, the known details of the nuclear and plasma membrane ER functions will be discussed. A particular focus will be to define the signaling pathways from the membrane that lead to important cell physiology effects of estrogen. The potential interactions of membrane ER with other local proteins will also be discussed, and the unique but often complementary roles of the receptor pools will be highlighted. These details may be of additional relevance to other steroid receptors, since there is evidence of their existence in the cell membrane.