Integration of the non-genomic and genomic actions of estrogen. Membrane-initiated signaling by steroid to transcription and cell biology

Elimination of Neoangiogenesis for Plaque Stabilization

Emerging data suggest that intraplaque hemorrhage is critical in promoting atherosclerotic lesion instability. Because red blood cell membranes are a rich source of free cholesterol and accumulated red blood cells within plaques promote inflammation, intraplaque hemorrhage is associated with expansion of the necrotic core. Plaque hemorrhage results from the development of immature neointimal vasa vasorum. Therefore, it is proposed that molecular therapies designed to eliminate pathologic neovascularization within developing lesions will interrupt the process of hemorrhage and decrease the rate of necrotic core expansion. The elimination of intraplaque neovascularization would involve targeting of pre-existing and new vessel development. The concept of vascular regression has met some success in other neovascular-dependent diseases, including macular degeneration and malignancies. The efficacy of this novel approach is dependent on gaining critical knowledge of the environment required to support development and maturation of the vasa vasorum within varying plaque types. A multitargeted approach involving selective local antiangiogenic agents should contribute to prevention of plaque progression and its clinical consequences.

The antioxidant effect of estrogen and Selective Estrogen Receptor Modulators in the inhibition of osteocyte apoptosis in vitro

Withdrawal of estrogen represents the primary factor determining post menopausal bone loss and has been associated with negative indicators of bone quality including the apoptotic death of osteocytes in vivo. While hormone replacement therapy in the form of Estrogen or Selective Estrogen Receptor Modulators (SERMs) demonstrates clear estrogen receptor (ER)-mediated benefits to bone mass, less is known regarding the mechanism of action of these compounds in the maintenance of bone cell populations. We have investigated the potential antioxidant effects of estrogen, estrogen derivatives and the SERMs Raloxifene and LY117018 in the prevention of oxidative stress induced apoptosis in the osteocyte like cell line MLO-Y4. Treatment of MLO-Y4 with 0.3 mM H(2)O(2) induced apoptosis that was significantly inhibited (p< or =0.002) when the cells were pre-treated for 1 h with either 17beta-estradiol, Raloxifene or LY117018 (10 nM). The stereoisomer 17alpha-estradiol also prevented H(2)O(2) induced apoptosis in MLO-Y4. Importantly, pre-treatment of ER-negative HEK293 cells with either 1 microM, 100 nM or 10 nM 17beta-estradiol, Raloxifene or LY117018 significantly inhibited H(2)O(2) induced apoptosis in these cells (p< or =4.2x10(-5)) indicating an estrogen receptor-independent effect of these compounds. Comparisons of 17beta-estradiol and similar molecules containing the putative free radical scavenger C3-OH moiety on the steroid A-ring (17alpha-estradiol, 17alpha-ethinylestradiol; 10 nM) with structurally related molecules lacking the C3-OH grouping (Mestranol and Quinestrol; 10 nM) demonstrated that only compounds containing the C3-OH moiety showed anti-apoptotic behavior in these studies (p< or =0.0033). Similarly the identification of the presence of reactive oxygen species (ROS) in cells as evidenced by the free radical indicator 2'7'-dichlorodihydrofluorescein diacetate demonstrated that 17beta-estradiol, SERMs and related molecules with C3-OH moiety were capable of blocking ROS generated in cells by H(2)O(2) (p< or =0.002) while Mestranol and Quinestrol showed no such blockade. It is possible that the loss of osteocytes during estrogen insufficiency may occur through a failure to suppress the activity of naturally occurring or disease associated oxidant molecules. These data suggest that the osteocyte protective effects of estrogen and SERMs may operate through a common receptor-independent mechanism which may be related to the antioxidant activity of these molecules.

Membrane-initiated actions of estrogens in neuroendocrinology: emerging principles

Hormonal ligands for the nuclear receptor superfamily have at least two interacting mechanisms of action: 1) classical transcriptional regulation of target genes (genomic mechanisms); and 2) nongenomic actions that are initiated at the cell membrane, which could impact transcription. Although transcriptional mechanisms are increasingly well understood, membrane-initiated actions of these ligands are incompletely understood. Historically, this has led to a considerable divergence of thought in the molecular endocrine field. We have attempted to uncover principles of hormone action that are relevant to membrane-initiated actions of estrogens. There is evidence that the membrane-limited actions of hormones, particularly estrogens, involve the rapid activation of kinases and the release of calcium. Membrane actions of estrogens, which activate these rapid signaling cascades, can also potentiate nuclear transcription. These signaling cascades may occur in parallel or in series but subsequently converge at the level of modification of transcriptionally relevant molecules such as nuclear receptors and/or coactivators. In addition, other hormones or neurotransmitters may also activate cascades to crosstalk with estrogen receptor-mediated transcription. The idea of synergistic coupling between membrane-initiated and genomic actions of hormones fundamentally revises the paradigms of cell signaling in neuroendocrinology.

Modulation of the BK channel by estrogens: examination at single channel level

BK channels regulate vascular tone by hyperpolarizing smooth muscle in response to fluctuating calcium concentrations. Oestrogen has been reported to lower blood pressure by increasing BK channel open probability through direct binding to the regulatory beta1-subunit(s) associated with the channel. The present investigation demonstrates that 17beta-oestradiol activates the BK channel complex by increasing the burst duration of channel openings. A subconductance state was observed in 25% of recordings following the addition of 17beta-oestradiol and could reflect uncoupling between the pore forming alpha1-subunit and the regulatory beta1-subunit. We also present evidence that more than one beta1-subunit is required to facilitate binding of 17beta-oestradiol to the channel complex.

Estrogen-dependent growth and estrogen receptor (ER)-α concentration in T47D breast cancer cells are inhibited by VACM-1, a cul 5 gene

Vasopressin-activated calcium mobilizing receptor (VACM-1)/cullin 5 (cul 5) inhibits growth when expressed in T47D breast cancer cells by a mechanism that involves a decrease in MAPK phosphorylation and a decrease in the early growth response element (egr-1) concentration in the nucleus. Since both MAPK and egr-1 pathways can be regulated by 17beta-estradiol, we next examined the effects of VACM-1 cDNA expression on estrogen-dependent growth in T47D cells and on estrogen receptor (ER) concentrations. Our results demonstrate that in T47D cells, both basal and 17beta-estradiol-dependent increase in cell growth and MAPK phosphorylation were inhibited in cells transfected with VACM-1 cDNA. Further, Western blot and immunocytochemistry data analyses indicate that ER concentrations and its nuclear localization are significantly lower in cells transfected with VACM-1 cDNA when compared to controls. These data indicate that in the T47D cancer cell line VACM-1 inhibits growth by attenuating estrogen-dependent signaling responses. These findings may have implications in the development of cancer treatments.

Rapid Estrogen Actions in the Cardiovascular System

In the last two decades, several studies have unveiled a series of original signaling mechanisms through which so-called "nuclear" receptors can mediate rapid actions of steroid hormones. These rapid signaling actions are independent of the synthesis of mRNA or protein, and are therefore known as "nontranscriptional" or "nongenomic" as opposed to the classical genomic mechanisms. Nongenomic signaling of estrogens plays a prominent role in nonreproductive tissues, and between these is the vascular wall. At this level, estrogen triggers rapid vasodilatation, exerts anti-inflammatory effects, stimulates endothelial growth and migration, and protects the vessels from atherosclerotic degeneration. Nongenomic signaling mechanisms have been involved in many of these actions and are increasingly considered to be of importance for vascular function in physiological and pathophysiological conditions. Rapid actions of steroid hormones have been implicated with vascular as well as with myocardial protection in animal experimental models. Moreover, the nongenomic signaling of estrogens is tightly interconnected with the nuclear pathways, and there are several indications that, through nongenomic modulation of signaling cascades, estrogens are also able to modulate the expression of several relevant genes in endothelial cells. In conclusion, while we are still in an early phase of the investigations of the nontranscriptional actions of steroid hormone receptors, it is clear that this newly recognized category of signaling mechanisms is responsible for critical steroid actions in nonreproductive tissues.

Selective Agonists of Estrogen Receptor Isoforms

The cloning of estrogen receptors (ERs) and generation of ER-deficient mice have increased our understanding of the molecular mechanisms underlying the cardiovascular effects of estrogen. It is conceivable that clinical trials of estrogens so far failed to improve cardiovascular health because of the poor ER isoform selectivity and tissue specificity of endogenous hormones as well as incorrect treatment timing and regimens. Tissue-selective ER modulators (SERMs) may be safer agents than endogenous estrogens for cardiovascular disease. Yet, designing isoform-selective ER ligands (I-SERMs) with agonist or antagonist activity is required to pursue improved pharmacological control of ERs, especially taking into account emerging evidence for the beneficial role of vascular ER alpha activation. Ideally, the quest for unique ER ligands targeted to the vascular wall should lead to compounds that merge the pharmacological profiles of SERM and I-SERM agents. This review highlights the current bases for and approaches to selective ER modulation in the cardiovascular system.

The hair follicle as an estrogen target and source

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

Dietary isoflavones differentially induce gene expression changes in lymphocytes from postmenopausal women who form equol as compared with those who do not

Human and animal studies suggest that dietary soy isoflavones reduce cancer risk, ameliorate postmenopausal syndrome and decrease bone resorption in postmenopausal women. The capacity to form the metabolite equol from daidzein is suggested as an important modulator of response to isoflavones; this capacity depends on gut colonization with appropriate bacteria. We administered a dietary supplement containing high-dose purified soy isoflavones (genistein, 558 mg/day; daidzein, 296 mg/day; and glycitein, 44 mg/day) to 30 postmenopausal women for 84 days and collected peripheral lymphocytes at timed intervals. Using microarray analysis, we determined whether changes in gene expression associated with this treatment support existing hypotheses as to isoflavones' mechanisms of action. Expression of a large number of genes was altered by isoflavone treatment, including induction of genes associated with cyclic adenosine 3',5'-monophosphate (cAMP) signaling and cell differentiation and decreased expression of genes associated with cyclin-dependent kinase activity and cell division. We report that isoflavone treatment in subjects who have the capacity to produce equol differentially affects gene expression as compared with nonproducers, supporting the plausibility of the importance of equol production. In general, isoflavones had a stronger effect on some putative estrogen-responsive genes in equol producers than in nonproducers. Our study suggests that, in humans, isoflavone changes are related to increased cell differentiation, increased cAMP signaling and G-protein-coupled protein metabolism and increased steroid hormone receptor activity and have some estrogen agonist effects; equol-production status is likely to be an important modulator of responses to isoflavones.

17 beta-estradiol modifies nitric oxide-sensitive guanylyl cyclase expression and down-regulates its activity in rat anterior pituitary gland

Previous studies showed that 17 beta-estradiol (17 beta-E2) regulates the nitric oxide (NO)/soluble guanylyl cyclase (sGC)/cGMP pathway in many tissues. Evidence from our laboratory indicates that 17 beta-E2 disrupts the inhibitory effect of NO on prolactin release, decreasing sGC activity and affecting the cGMP pathway in anterior pituitary gland of adult ovariectomized and estrogenized rats. To ascertain the mechanisms by which 17 beta-E2 affects sGC activity, we investigated the in vivo and in vitro effects of 17 beta-E2 on sGC protein and mRNA expression in anterior pituitary gland from immature female rats. In the present work, we showed that 17 beta-E2 acute treatment exerted opposite effects on the two sGC subunits, increasing alpha1 and decreasing beta1 subunit protein and mRNA expression. This action on sGC protein expression was maximal 6-9 h after 17 beta-E2 administration. 17beta-E2 also caused the same effect on mRNA expression at earlier times. Concomitantly, 17 beta-E2 dramatically decreased sGC activity 6 and 9 h after injection. These effects were specific of 17 beta-E2, because they were not observed with the administration of other steroids such as progesterone and 17 alpha-estradiol. This inhibitory action of 17beta-E2 on sGC also required the activation of estrogen receptor (ER), because treatment with the pure ER antagonist ICI 182,780 completely blocked 17 beta-E2 action. 17 beta-E2 acute treatment caused the same effects on pituitary cells in culture. These results suggest that 17 beta-E2 exerts an acute inhibitory effect on sGC in anterior pituitary gland by down-regulating sGC beta 1 subunit and sGC activity in a specific, ER-dependent manner.

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

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

Cellular mechanisms underlying the cardiovascular actions of oestrogens

Although pre-menopausal women enjoy relative cardiovascular protection, hormone (oestrogen±progestin)-replacement therapy has not shown cardiovascular benefits in post-menopausal women, suggesting that the effects of oestrogens on the cardiovascular system are much more complex than previously expected. Endothelial cells, smooth muscle cells, cardiac myocytes and fibroblasts, the cellular components of blood vessels and the heart, play important roles in cardiovascular health and disease. During the development and progression of cardiovascular disease, changes occur both in the structure and function of these cells, resulting in a wide range of abnormalities, which affect growth, death and physiological function. These cells contain functional oestrogen receptors and are targets for oestrogen action. This review focuses on recent studies on the effects of oestrogen on cardiovascular cell function. Oestrogens, particularly 17β-oestradiol, exert multiple effects on cardiovascular cells, and these effects may contribute to the gender-associated protection against cardiovascular diseases.

Multiple biological activities for two peptides derived from the nerve growth factor precursor

ProNGF can be cleaved proteolytically at dibasic residues and liberates two other peptides beside NGF, LIP1 a 29 amino acid (aa) peptide and LIP2 a 38 aa peptide. These peptides were found present in the rat intestine and shown to induce rapid phosphorylation of the Trk receptor in cell lines. The present study describes several novel biological properties for these peptides. They exert an anti-proliferative effect on the mitogenic activity of estrogen and IGF in MCF-7 cells. They protect against in vivo induction of excitotoxic lesions by the glutamatergic analogue ibotenate injected into the developing mouse brain and against in vitro NMDA-induced cell death in primary neuronal cultures. They bind to murine microglial cells and induce phosphorylation of Akt. These results suggest a role for LIP1 and LIP2 in cell survival.

Estrogen regulation of nitric oxide and inducible nitric oxide synthase (iNOS) in immune cells: implications for immunity, autoimmune diseases, and apoptosis

Nitric oxide plays a central role in the physiology and pathology of diverse tissues including the immune system. It is clear that the levels of nitric oxide must be carefully regulated to maintain homeostasis. Appropriate levels of nitric oxide derived from iNOS assist in mounting an effective defense against invading microbes. Conversely, inability to generate nitric oxide results in serious, even fatal, susceptibility to infections. Further, dysregulation or overproduction of nitric oxide has been implicated in the pathogenesis of many disorders, including atherosclerosis, neurodegenerative diseases, inflammatory autoimmune diseases, and cancer. Therefore, depending upon the levels of nitric oxide generated, the potential exists for nitric oxide to behave like a "double-edged" biological sword. Taking these issues into consideration, it is thus pivotal to understand the regulation of nitric oxide. Nitric oxide is regulated by many endogenous factors including hormones such as estrogens. While the effects of estrogen on the generation of nitric oxide in non-immune tissues are relatively well documented, the effect of estrogen on iNOS/nitric oxide in immune cells is only now becoming apparent. Our laboratory has recently shown that estrogen treatment of mice markedly upregulates the levels of iNOS mRNA, iNOS protein, and nitric oxide in activated splenocytes. This upregulation of nitric oxide is in part mediated through interferon-gamma (IFN-gamma), a pro-inflammatory cytokine that is enhanced by estrogen. These findings are important considering that estrogens are not only involved in regulation of normal immune responses, but also are implicated in many autoimmune and inflammatory diseases. To date, there are no reviews on the effects of estrogen on immune tissue-derived nitric oxide and therefore this review will address this critical gap in the literature. Given the increasing importance of immune-tissue-derived iNOS in health and disease, studies on estrogen-induced regulation of iNOS may offer a better understanding of diseases and aid in devising new therapeutic interventions.

Nature of functional estrogen receptors at the plasma membrane

Although rapid signaling by estrogen at the plasma membrane is established, it is controversial as to the nature of the receptor protein. Estrogen may bind membrane proteins comparable to classical nuclear estrogen receptors (ERs), but some studies identify nonclassical receptors, such as G protein-coupled receptor (GPR)30. We took several approaches to define membrane-localized estrogen-binding proteins. In endothelial cells (ECs) from ERalpha/ERbeta combined-deleted mice, estradiol (E2) failed to specifically bind, and did not activate cAMP, ERK, or phosphatidyinositol 3-kinase or stimulate DNA synthesis. This is in contrast to wild-type ECs, indicating the lack of any functional estrogen-binding proteins in ERalpha/ERbeta combined-deleted ECs. To directly determine the identity of membrane and nuclear-localized ER, we isolated subcellular receptor pools from MCF7 cells. Putative ER proteins were trypsin digested and subjected to tandem array mass spectrometry. The output analysis identified membrane and nuclear E2-binding proteins as classical human ERalpha. We also determined whether GPR30 plays any role in E2 rapid actions. MCF7 (ER and GPR30 positive) and SKBR-3 (ER negative, GPR30 positive) cells were incubated with E2. Only MCF7 responded with significantly increased signaling. In MCF7, the response to E2 was not different in cells transfected with small interfering RNA to green fluorescent protein or GPR30. In contrast, interfering RNA to ERalpha or ER inhibition prevented rapid signaling and resulting biology in MCF7. In breast cancer and ECs, nuclear and membrane ERs are the same proteins. Furthermore, classical ERs mediate rapid signals induced by E2 in these cells.

Estradiol-17β stimulates proliferation of mouse embryonic stem cells: involvement of MAPKs and CDKs as well as protooncogenes

Although the importance of estradiol-17β (E2) in many physiological processes has been reported, to date no researchers have investigated the effects of E2 on embryonic stem (ES) cell proliferation. Therefore, in the present study, we have examined the effect of E2 on the DNA synthesis of murine ES (ES-E14TG2a) cells and its related signaling pathways. The results of this study show that E2 (10−9 M) significantly increased [3H]thymidine incorporation at >4 h and that E2 (>10−12 M) induced an increase of [3H]thymidine incorporation after 8-h incubation. Moreover, E2 (>10−12 M) also increased 5′-bromo-2′-deoxyuridine (BrdU) incorporation and cell number. Indeed, E2 stimulated estrogen receptor (ER)-α and -β protein levels and increased mRNA expression levels of protooncogenes (c- fos, c- jun, and c- myc). Tamoxifen (antiestrogen) completely inhibited E2-induced increases in [3H]thymidine incorporation. In addition, estradiol-6- O-carboxymethyl oxime-BSA (E2-BSA; 10−9 M) increased [3H]thymidine incorporation at >1 h, and E2-BSA (>10−12 M) increased [3H]thymidine incorporation after 1-h incubation. E2-BSA-induced increase in BrdU incorporation also occurred in a dose-dependent manner. Tamoxifen had no effect on E2-BSA-induced increase of [3H]thymidine incorporation. Also, E2 and E2-BSA displayed maximal phosphorylation of p44/42 MAPKs at 10 and 5 min, respectively. E2 increased cyclins D1 and E as well as cyclin-dependent kinase (CDK)2 and CDK4. In contrast, E2 decreased the levels of p21cip1 and p27kip1 (CDK-inhibitory proteins). Increases of these cell cycle regulators were blocked by 10−5 M PD-98059 (MEK inhibitor). Moreover, E2-induced increase of [3H]thymidine incorporation was inhibited by PD-98059 or butyrolactone I (CDK2 inhibitor). In conclusion, estradiol-17β stimulates the proliferation of murine ES cells, and this action is mediated by MAPKs, CDKs, or protooncogenes.

S-palmitoylation modulates estrogen receptor alpha localization and functions

17beta-Estradiol (E2) acts as a chemical messenger in target tissues inducing both slow nuclear and rapid extra-nuclear responses. E2 binds to its cognate nuclear receptors (ER) resulting in the activation of target gene transcription in the nucleus. In addition to these genomic effects, E2 modulates cell functions through rapid non-genomic actions. Stimulation of G-proteins, Ca(2+) influx, inositol phosphate generation as well as phospholipase C, ERK/MAPK, and PI3K/AKT activation all occur within seconds to minutes after E2 binding to a small population of ERalpha located at the plasma membrane. The great impact of these rapid signals on cell physiology renders central the knowledge of the structural bases and mechanisms that mediate extra-nuclear signaling by E2. Several laboratories, including our own, have recently elucidated the structural requirements for localization and function of plasma membrane ERalpha. This review summarizes the molecular mechanisms of E2-induced rapid non-genomic actions relevant for cell functions, highlighting the role of lipid modification (i.e., palmitoylation) in the ERalpha localization to and residence at the plasma membrane.

Activating transcription factor 3 is estrogen-responsive in utero and upregulated during sexual differentiation

BACKGROUND/AIMS

Synthetic estrogens induce hypospadias, an anomaly of genital tubercle/urethral development. Activating transcription factor 3 (ATF3), which is estrogen-responsive in vitro, is upregulated in hypospadiac human tissue. We used a mouse model of steroid-dependent genital tubercle development to elucidate the ontogeny of ATF3 expression and the developmental response of ATF3 in vivo to estrogen exposure.

METHODS

We used quantitative RT-PCR to assess ontogenic expression of ATF3 and its response to estrogen treatment in utero. Immunohistochemistry was used to localize the protein.

RESULTS

Quantitative RT-PCR showed that ATF3 mRNA is upregulated in all estrogen-exposed fetal genital tubercles compared to controls (p = 0.024), including specifically in males exposed in utero (p = 0.049). Additionally, its expression increases significantly during the period of sexual differentiation in both sexes and significantly correlates with female development (p = 0.004), a phenomenon that appears to be attributable to higher levels at birth in females. The protein localizes in the nucleus, as expected.

CONCLUSIONS

ATF3 is estrogen-responsive in vivo. The response of ATF3 to estrogenic stimulation in utero at an earlier stage may contribute to urethral abnormalities observed in estrogen-exposed male fetuses, although it is likely not the only gene involved, which supports the general understanding that hypospadias is subject to multifactorial influences. ATF3 may therefore be an appropriate gene for further investigations in an endocrine context.

Integration of membrane and nuclear estrogen receptor signaling

The classical mechanism of estradiol (E2) action is mediated by the nuclear estrogen receptors ERalpha and ERbeta, which function as ligand-dependent transcription factors that regulate transcription of target genes containing the consensus estrogen response element (ERE) in their promoter regions. However, accumulating evidence indicates that E2 can also exert its actions through a unique membrane estrogen receptor (mER). Upon activation of the mER, various signaling pathways (i.e. Ca(2+), cAMP, protein kinase cascades) are rapidly activated and ultimately influence downstream transcription factors. Some target genes of the mER pathway may be activated independently of the nuclear estrogen receptor (nER). Additionally, it has been shown that classical nER action can be modulated by mER-initiated signaling through phosphorylation of nER and its coactivators, and by induction of third messengers (i.e. cyclin D1 and c-fos). Based on current evidence, we propose a model for E2 action integrating distinct membrane receptor and nuclear receptor signaling. This membrane receptor-nuclear receptor interaction is likely to exist for other hormones. Steroid hormones and other hormones acting through hormone receptors in the steroid receptor superfamily (i.e. thyroid hormones) also activate many of the same intracellular signaling cascades, which provides the basis for extensive crosstalk networks between hormones. The model proposed serves as a framework to investigate the diverse actions of hormones and endocrine disrupting chemicals (EDCs).

Estrogen Dendrimer Conjugates that Preferentially Activate Extranuclear, Nongenomic Versus Genomic Pathways of Estrogen Action

Estrogenic hormones are classically thought to exert their effects by binding to nuclear estrogen receptors and altering target gene transcription, but estrogens can also have nongenomic effects through rapid activation of membrane-initiated kinase cascades. The development of ligands that selectively activate only the nongenomic pathways would provide useful tools to investigate the significance of these pathways. We have prepared large, abiotic, nondegradable poly(amido)amine dendrimer macromolecules that are conjugated to multiple estrogen molecules through chemically robust linkages. Because of their charge and size, these estrogen-dendrimer conjugates (EDCs) remain outside the nucleus. They stimulate ERK, Shc, and Src phosphorylation in MCF-7 breast cancer cells at low concentrations, yet they are very ineffective in stimulating transcription of endogenous estrogen target genes, being approximately 10,000-fold less potent than estradiol in genomic actions. In contrast to estradiol, EDC was not effective in stimulating breast cancer cell proliferation. Because these EDC ligands activate nongenomic activity at concentrations at which they do not alter the transcription of estrogen target genes, they should be useful in studying extranuclear initiated pathways of estrogen action in a variety of target cells.

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.

Estradiol regulates different genes in human breast tumor xenografts compared with the identical cells in culture

In breast cancers, estrogen receptor (ER) levels are highly correlated with response to endocrine therapies. We sought to define mechanisms of estrogen (E) signaling in a solid breast tumor model using gene expression profiling. ER(+) T47D-Y human breast cancer cells were grown as xenografts in ovariectomized nude mice under four conditions: 1) 17beta-estradiol for 8 wk (E); 2) without E for 8 wk (control); 3) E for 7 wk followed by 1 wk of E withdrawal (Ewd); or 4) E for 8 wk plus tamoxifen for the last week. E-regulated genes were defined as those that differed significantly between control and E and/or between E and Ewd or control and Ewd. These protocols generated 188 in vivo E-regulated genes that showed two major patterns of regulation. Approximately 46% returned to basal states after Ewd (class I genes); 53% did not (class II genes). In addition, more than 70% of class II-regulated genes also failed to reverse in response to tamoxifen. These genes may be interesting for the study of hormone-resistance issues. A subset of in vivo E-regulated genes appears on lists of clinical ER discriminator genes. These may be useful therapeutic targets or markers of E activity. Comparison of in vivo E-regulated genes with those regulated in identical cells in vitro after 6 and 24 h of E treatment demonstrate only 11% overlap. This indicates the extent to which gene expression profiles are uniquely dependent on hormone-treatment times and the cellular microenvironment.

Ovariectomy prevents the recovery of atrophied gastrocnemius skeletal muscle mass

The recovery of atrophied muscle mass in animals is thought to be dependent on a number of factors including hormones, cytokines, and/or growth factor expression. The Akt/mammalian target of rapamycin (mTOR) signaling pathway is believed to be activated by these various factors, resulting in skeletal muscle growth through the initiation of protein synthesis. It was hypothesized that surgical removal of the ovaries (Ovx) may alter activation of the Akt/mTOR signaling pathway, a mechanism necessary for muscle regrowth. To test this, 36 Sprague-Dawley rats underwent Ovx or sham surgeries. A portion of the animals were then subjected to hindlimb unloading (HLU) for 28 days. After HLU, one group of Sham and Ovx rats underwent a 14-day recovery period in which the animals were allowed free cage ambulation. The HLU animals demonstrated ∼21–27% reduction in medial gastrocnemius muscle mass irrespective of whether the ovaries were intact or not. The Sham animals that were reloaded recovered their atrophied muscle mass; however, the Ovx group failed to recover any of the atrophied muscle mass with reloading. The failure to recover muscle mass in the Ovx group was associated with reduced phosphorylation levels of both Akt and p70s6k, whereas in the Sham recovery animals no reductions were found in Akt phosphorylation and significant increases in p70s6k activation were detected. Finally, no differences were detected in mTOR phosphorylation in any of Sham or Ovx groups. These results suggest that ovariectomy surgeries could be detrimental to the recovery of atrophied muscle mass.

Steering estrogen signals from the plasma membrane to the nucleus: Two sides of the coin

Estrogen mediate its biological effects through its association with estrogen receptors (ERs). They also regulate the expression of a variety of genes involved in distinct physiological processes, including development, metabolism, and reproduction. In addition, emerging data suggest that the estrogen-estrogen receptor complex can also function as a cytoplasmic signaling molecule and may influence processes such as cardiovascular protection, bone preservation, neuroprotection, and proliferation of various cell types. Such extranuclear or nongenomic signaling pathways are rapid and supposedly independent of transcription. A recent exciting finding was that G-coupled membrane protein receptor, GPR30, an alternative to the classical ERs, is also involved in the rapid signaling of estrogen through its direct association with estrogen. These new findings combined with the recent advances in the cytoplasmic functions of proline, glutamic acid, luecine rich protein 1 (PELP1), and metastatic tumor antigen 1 short form (MTA1s) have opened a new spectrum and raised several new concerns in the field of estrogen biology and put the attention to unveil many unknown mechanistic actions of estrogen in cellular physiology. In this review, we briefly summarize what is currently known of the cellular mechanisms and physiology of estrogen's nongenomic actions in various cellular systems used by ERs.

Estrogen signaling multiple pathways to impact gene transcription

Steroid hormones exert profound effects on cell growth, development, differentiation, and homeostasis. Their effects are mediated through specific intracellular steroid receptors that act via multiple mechanisms. Among others, the action mechanism starting upon 17beta-estradiol (E2) binds to its receptors (ER) is considered a paradigmatic example of how steroid hormones function. Ligand-activated ER dimerizes and translocates in the nucleus where it recognizes specific hormone response elements located in or near promoter DNA regions of target genes. Behind the classical genomic mechanism shared with other steroid hormones, E2 also modulates gene expression by a second indirect mechanism that involves the interaction of ER with other transcription factors which, in turn, bind their cognate DNA elements. In this case, ER modulates the activities of transcription factors such as the activator protein (AP)-1, nuclear factor-kappaB (NF-kappaB) and stimulating protein-1 (Sp-1), by stabilizing DNA-protein complexes and/or recruiting co-activators. In addition, E2 binding to ER may also exert rapid actions that start with the activation of a variety of signal transduction pathways (e.g. ERK/MAPK, p38/MAPK, PI3K/AKT, PLC/PKC). The debate about the contribution of different ER-mediated signaling pathways to coordinate the expression of specific sets of genes is still open. This review will focus on the recent knowledge about the mechanism by which ERs regulate the expression of target genes and the emerging field of integration of membrane and nuclear receptor signaling, giving examples of the ways by which the genomic and non-genomic actions of ERs on target genes converge.

Activating transcription factor 3 is up-regulated in patients with hypospadias

Hypospadias is a congenital anomaly of the genitalia characterized by abnormalities of the urethra and foreskin, with the urethral meatus located in an abnormal position anywhere from the distal ventral penile shaft to the perineum. Because the incidence of hypospadias is approximately 1/200-1/300 live male births, it is one of the most common congenital malformations, but its etiology is largely uncharacterized. Genomic analysis of hypospadic tissue indicated a potential role for activating transcription factor 3 (ATF3) in the development of this anomaly. ATF3 may be involved in homeostasis, wound healing, cell adhesion, or apoptosis, and normally it is expressed at a steady-state in quiescent cells. Additionally, it has been shown to be an estrogen-responsive gene, and the etiology of hypospadias may be related to in utero exposure to estrogenic or anti-androgenic compounds. We examined the expression of ATF3 in tissues from 28 children with hypospadias compared with 20 normal penile skin tissue samples from elective circumcision. Eighty-six percent of the hypospadias samples were immunohistochemically positive, compared with 13% of normal tissue samples. Seventy-five percent of hypospadias samples were positive from in situ hybridization, compared with 1% of circumcision samples. Our results indicate that ATF3 is up-regulated in the penile skin tissues of boys with hypospadias, suggesting a role for this transcription factor in the development of this abnormality. Because the etiology of hypospadias may include exposure to estrogenic compounds, the responsiveness of ATF3 to estrogen is also discussed.

Sex and estrogenic effects on coexpression of mRNAs in single ventromedial hypothalamic neurons

Regulated gene expression in single neurons can be linked to biophysical events and behavior in the case of estrogen-regulated gene expression in neurons in the ventrolateral portion of the ventromedial nucleus (VMN) of the hypothalamus. These cells are essential for lordosis behavior. What genes are coexpressed in neurons that have high levels of mRNAs for estrogen receptors (ERs)? We have been able to isolate and measure certain mRNAs from individual VMN neurons collected from rat hypothalamus. Large numbers of neurons express mRNA for ERalpha, but these neurons are not identical with the population of VMN neurons expressing the likely gene duplication product, ERbeta. An extremely high proportion of neurons expressing either ER also coexpress mRNA for the oxytocin receptor (OTR). This fact matches the known participation of oxytocin binding and signaling in sexual and affiliative behaviors. In view of data that ER and OTR can signal through PKCs, we looked at coexpression of selected PKCs in the same individual neurons. The most discriminating analysis was for triple coexpression of ERs, OTR, and each selected PKC isoform. These patterns of triple coexpression were significantly different for male vs. female VMN neurons. Further, individual neurons expressing ERalpha could distribute their signaling across the various PKC isoforms differently in different cells, whereas the reverse was not true. These findings and this methodology establish the basis for systematic linkage of the brain's hormone-sensitive signaling pathways to biophysical and behavioral mechanisms in a well studied mammalian system.

Detection of a raft-located estrogen receptor-like protein distinct from ER alpha

17Beta-estradiol (17beta-E2) elicits at the cell membrane rapid actions that remain insensitive to the inhibitory effect of ICI 182,780, a pure estrogen antagonist, and therefore cannot be attributed to the classic nuclear receptors. We addressed the question of the identity of the protein involved in these rapid actions. We first examined the responses of several cell lines for intracellular calcium mobilization, an effect not inhibited by ICI 182,780, tamoxifen and raloxifen. We then demonstrated the presence of binding sites in the membranes, by incubating them with antibodies directed against different domains of ER alpha, and by flow cytometry analysis. The membrane proteins were eluted by affinity chromatography using E2 conjugated to bovine serum albumin as a ligand. Western blots of the elution fractions using an antibody directed against the ligand binding site of ER alpha showed the existence of a protein of approximately 50 kDa. The protein was concentrated in the lipid rafts, together with another heavier form of approximately 66 kDa. The 50 kDa protein was immunoprecipitable, and co-immunoprecipitation experiments showed that it was associated with the Gbeta(1-4) protein, but not with caveolin-1. The protein was expressed in ER alpha-null cells, like HO-23 and Cos-7 cells. Therefore, in the lipid rafts, there exists a protein, similar to, but molecularly distinct from ER alpha.

Chromosome-wide mapping of estrogen receptor binding reveals long-range regulation requiring the forkhead protein FoxA1

Estrogen plays an essential physiologic role in reproduction and a pathologic one in breast cancer. The completion of the human genome has allowed the identification of the expressed regions of protein-coding genes; however, little is known concerning the organization of their cis-regulatory elements. We have mapped the association of the estrogen receptor (ER) with the complete nonrepetitive sequence of human chromosomes 21 and 22 by combining chromatin immunoprecipitation (ChIP) with tiled microarrays. ER binds selectively to a limited number of sites, the majority of which are distant from the transcription start sites of regulated genes. The unbiased sequence interrogation of the genuine chromatin binding sites suggests that direct ER binding requires the presence of Forkhead factor binding in close proximity. Furthermore, knockdown of FoxA1 expression blocks the association of ER with chromatin and estrogen-induced gene expression demonstrating the necessity of FoxA1 in mediating an estrogen response in breast cancer cells.

Effects of estradiol on lipopolysaccharide and Pam3Cys stimulation of CCL20/macrophage inflammatory protein 3 alpha and tumor necrosis factor alpha production by uterine epithelial cells in culture

We have previously demonstrated that rat uterine epithelial cells (UEC) produce CCL20/macrophage inflammatory protein 3 alpha (MIP3alpha) and tumor necrosis factor alpha (TNF-alpha) in response to live and heat-killed Escherichia coli and to the pathogen-associated molecular patterns (PAMP) lipopolysaccharide (LPS) and Pam3Cys. To determine whether estradiol (E2) modulates PAMP-induced CCL20/MIP3alpha and TNF-alpha secretion, primary cultures of rat UEC were incubated with E2 for 24 h and then treated with LPS or Pam3Cys or not treated for an additional 12 h. E2 inhibited the constitutive secretion of TNF-alpha and CCL20/MIP3alpha into culture media. Interestingly, E2 pretreatment enhanced CCL20/MIP3alpha secretion due to LPS and Pam3Cys administration. In contrast, and at the same time, E2 lowered the TNF-alpha response to both PAMP. To determine whether estrogen receptors (ER) mediated the effects of E2, epithelial cells were incubated with E2 and/or ICI 182,780, a known ER antagonist. ICI 182,780 had no effect on E2 inhibition of constitutive TNF-alpha and CCL20/MIP3alpha secretion. In contrast, ICI 182,780 reversed the stimulatory effect of E2 on LPS- and/or Pam3Cys-induced CCL20/MIP3alpha secretion as well as partially reversed the inhibitory effect of E2 on TNF-alpha production by epithelial cells. Overall, these results indicate that E2 regulates the production of TNF-alpha and CCL20/MIP3alpha by UEC in the absence as well as presence of PAMP. Since CCL20/MIP3alpha has antimicrobial activity and is chemotactic for immune cells, these studies suggest that regulation of CCL20/MIP3alpha and TNF-alpha by E2 and PAMP may have profound effects on innate and adaptive immune responses to microbial challenge in the female reproductive tract.

Human Mineralocorticoid Receptor Expression Renders Cells Responsive for Nongenotropic Aldosterone Actions

The steroid hormone aldosterone is important for salt and water homeostasis as well as for pathological tissue modifications in the cardiovascular system and the kidney. The mechanisms of action include a classical genomic pathway, but physiological relevant nongenotropic effects have also been described. Unlike for estrogens or progesterone, the mechanisms for these nongenotropic effects are not well understood, although pharmacological studies suggest a role for the mineralocorticoid receptor (MR). Here we investigated whether the MR contributes to nongenotropic effects. After transfection with human MR, aldosterone induced a rapid and dose-dependent phosphorylation of ERK1/2 and c-Jun NH2-terminal kinase (JNK) 1/2 kinases in Chinese hamster ovary or human embryonic kidney cells, which was reduced by the MR-antagonist spironolactone and involved cSrc kinase as well as the epidermal growth factor receptor. In primary human aortic endothelial cells, similar results were obtained for ERK1/2 and JNK1/2. Inhibition of MAPK kinase (MEK) kinase but not of protein kinase C prevented the rapid action of aldosterone and also reduced aldosterone-induced transactivation, most probably due to impaired nuclear-cytoplasmic shuttling of MR. Cytosolic Ca2+ was increased by aldosterone in mock- and in human MR-transfected cells to the same extend due to Ca2+ influx, whereas dexamethasone had virtually no effect. Spironolactone did not prevent the Ca2+ response. We conclude that some nongenotropic effects of aldosterone are MR dependent and others are MR independent (e.g. Ca2+), indicating a higher degree of complexity of rapid aldosterone signaling. According to this model, we have to distinguish three aldosterone signaling pathways: 1) genomic via MR, 2) nongenotropic via MR, and 3) nongenotropic MR independent.

A Nongenomic Action of 17β-Estradiol as the Mechanism Underlying the Acute Suppression of Secretion of Luteinizing Hormone1

The objective of the present study was to determine the ability of 17beta-estradiol (E(2)) and conjugated forms of E(2) (E(2) conjugated to BSA [E(2)-BSA] and a novel conjugate, E(2) conjugated to a small peptide [E(2)-PEP]) to prevent the GnRH-induced secretion of LH and to determine the role of estradiol receptors (ERs) and ER subtypes (ERalpha, also known as ESR1, and ERbeta, also known as ESR2) in the mediation of the acute action of E(2) in primary cultures of ovine pituitary cells. Preincubation of cells for 15 min with E(2), E(2)-BSA, or E(2)-PEP prevented the GnRH-induced secretion of LH (P < 0.01). Treatment of cells with nonestrogenic steroid hormones did not affect secretion of LH when given alone, nor did these steroids impair the E(2)-induced inhibition of LH secretion (P > 0.1). Likewise, treatment of cells with the ER-antagonists tamoxifen, hydroxytamoxifen, or ICI 182 780 did not affect (P > 0.1) secretion of LH when given alone but did prevent (P < 0.01) the inhibition by E(2) and the E(2)-conjugates on GnRH-induced secretion of LH. When cells were treated with subtype-selective ER agonists, the ERalpha agonist (propylpyrazole-triol), but not the ERbeta agonist (diarylpropionitrile), decreased (P < 0.01) the GnRH-induced secretion of LH. In conclusion, the rapidity by which E(2) prevented GnRH-induced release of LH in ovine pituitary cells suggests that this inhibition is mediated via a nongenomic action of E(2). The inhibition of GnRH-induced secretion of LH proved to be steroid specific and mediated by ERs. It may occur specifically through ERalpha. The fact that E(2)-BSA or E(2)-PEP mimicked the action of E(2) suggests that this effect was mediated by an ER associated with the plasma membrane.

4-Hydroxytamoxifen inhibits proliferation of multiple myeloma cells in vitro through down-regulation of c-Myc, up-regulation of p27Kip1, and modulation of Bcl-2 family members

PURPOSE

Multiple myeloma is an incurable B-cell malignancy requiring new therapeutic strategies. Our approach was to analyze the in vitro effects of a selective estrogen receptor modulator, 4-hydroxytamoxifen (4-OHT), on six multiple myeloma cell lines.

EXPERIMENTAL DESIGN

Cultured multiple myeloma cells were treated with various 4-OHT concentrations and the cellular response was studied: cell proliferation, cell viability, induction of apoptosis, caspase activities, and expression of signaling proteins.

RESULTS

We found that pharmacologic concentrations of 4-OHT inhibit cell proliferation (4 of 6 cell lines). This inhibition is achieved by two independent events: a block at the G(1) phase of the cell cycle and the induction of apoptotic death. The cellular response to 4-OHT depends on the presence of functional estrogen receptors. 4-OHT treatment activates an intrinsic mitochondrial caspase-9-dependent pathway but not the Fas/FasL death pathway. Signaling pathways known to be involved in the survival and/or proliferation of multiple myeloma cells are not affected by 4-OHT treatment. 4-OHT-induced G(1) arrest is accompanied by the up-regulation of the cell cycle inhibitor p27(Kip1) and the down-regulation of c-Myc. Among the Bcl-2 family members tested, the proapoptotic BimS protein is induced whereas the antiapoptotic protein Mcl-1 is decreased.

CONCLUSIONS

Although the effects of 4-OHT are observed at micromolar concentrations, cellular mechanisms responsible for G(1) arrest, as well as apoptosis induction, are similar to those observed in breast cancer cells. Our data support the concept that 4-OHT may represent an alternative approach to inhibit proliferation and induce apoptosis of multiple myeloma cells.

Estrogen inhibits cardiomyocyte hypertrophy in vitro. Antagonism of calcineurin-related hypertrophy through induction of MCIP1

Evidence from in vivo studies suggests that some inputs to cardiac hypertrophy are opposed by the actions of estrogen. However, the mechanisms of E2 action in this respect are mainly unknown. An important pathway that is utilized by multiple hypertrophic stimuli involves the activation of the tyrosine phosphatase, calcineurin (PP2B). Here we show that 17beta-estradiol (E2) significantly prevents angiotensin II (AngII)- or endothelin-1 (ET-1)-induced new protein synthesis, skeletal muscle actin expression, and increased surface area in cultured rat cardiomyocytes. ET-1 stimulated calcineurin phosphatase activity, resulting in new protein synthesis, and both were prevented by E2. E2 induced the MCIP1 gene, an inhibitor of calcineurin activity, via phosphatidylinositol 3-kinase, transcriptional, and mRNA stability mechanisms. Small interfering RNA for MCIP1 significantly reversed both the E2 restraint of protein synthesis and the inhibition of AngII-induced calcineurin activity. AngII-induced the translocation of the hypertrophic transcription factor, NF-AT, to the nucleus of the cardiomyocyte and stimulated NF-AT transcriptional activity. Both were prevented by E2. AngII also stimulated the activation of ERK and protein kinase C, contributing to cardiac hypertrophy. E2 inhibited these pathways, related to the stimulation of atrial natriuretic peptide production and secretion. Thus, restraint of calcineurin and kinase signaling to the hypertrophic program underlie these important effects of E2.

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.

Estrogen Induces Vascular Wall Dilation

Estrogen has been shown to affect vascular cell and arterial function in vitro and in vivo. Here we examined the ability of estradiol (E(2)) to cause rapid arterial dilation of elastic and muscular arteries in vivo and the mechanisms involved. E(2) administration caused a rapid increase in the outer wall diameter of both types of arteries in ovariectomized female mice. This resulted from estrogen receptor (ER)-mediated stimulation of nitric oxide production, demonstrated by preinjecting the mice arteries with a soluble inhibitor of nitric oxide (monomethyl l-arginine) and by showing the absence of E(2) action in eNOS-/- mice. Rapid activation of both ERK/MAP kinase and phosphatidylinositol 3-kinase activity was found in the E(2)-exposed arteries, and inhibiting either kinase prevented the vasodilatory action of E(2). Kinase activation and vasodilator responses to E(2) were absent in either ERalpha or ERbeta knock-out mice, implicating both receptor subtypes as mediating this E(2) action. These results indicate that E(2) modulation of arterial tonus through plasma membrane ER and rapid signaling could underlie many previously observed actions of estrogen reported to occur in women.

Differential estrogen signaling in endothelial cells upon pulsed or continuous administration

While experimental evidence demonstrates that estrogen protects vascular cells, clinical trials on hormone replacement therapies (HRT) fail to report cardiovascular benefits. This discrepancy may indicate that estrogen signaling during HRT may not be fully effective in vascular cells, possibly due to the way of delivering estrogens to vascular tissues. We therefore, tested whether a different kinetics of exposure of endothelial cells to estrogens may alter the balance between transcriptional and non-transcriptional signaling.

METHODS AND RESULTS

Cultured human umbilical vein endothelial cells (HUVEC) were exposed to equal amounts of 17beta-estradiol administered transiently (7 nM for 1 h) or continuously (0.29 nM for 24 h), using endothelial nitric oxide synthase (eNOS) expression and activity as functional endpoints. Transient and continuous administrations equally increased eNOS expression, implying that a short contact between hormone and cells is sufficient to trigger genomic pathways. In agreement, in ovariectomized rats pulse-like changes in plasma estradiol or stable levels equally increased aortic eNOS. More importantly, we extensively show that transient estradiol administration results in preferential recruitment of non-genomic pathways, leading to more vivid activation of MAPK and phosphatidylinositol-3 kinase (PI3K) cascades.

CONCLUSIONS

Our data suggest that the kinetics of contact of estrogen with the cells could represent a sensor for estrogen receptor (ER) signaling towards non-genomic or genomic pathways. These findings deepen our understanding of estrogen signaling, and have important implications for the design of safer and more effective HRT.

Regulation of the estrogen-inducible gene expression profile by the breast cancer susceptibility gene BRCA1

The tumor suppressor gene BRCA1 functions in part as a caretaker in preserving the integrity of the genome, but also exhibits tissue-specific function by inhibiting estrogen receptor activity. Because estrogen (E2) induces a wide range of gene expression changes (by nongenomic and several transcriptional pathways), we sought to determine how comprehensive is the BRCA1-mediated inhibition of E2-induced gene expression alterations. Using cDNA-spotted microarrays, we identified a relatively large number of gene expression alterations (both increased and decreased expression) in MCF-7 cells caused by E2, some of which have been reported in previous studies. However, in the presence of exogenous wild-type BRCA1 (wtBRCA1), the response to E2 was severely blunted, with only about 10% the number of gene expression changes as that found in the absence of wtBRCA1. Examples of these findings were confirmed by semiquantitative and quantitative RT-PCR assays. In contrast to wtBRCA1, the induction by E2 of several E2-responsive genes was not inhibited by a full-length tumor-associated mutant BRCA1 protein [T300G (or (61)Cys-->Gly)]. For three E2-responsive genes whose induction by E2 was inhibited by wtBRCA1, wtBRCA1 had little or no effect on the mRNA half-life in the presence of E2. Consistent with these findings, wtBRCA1 inhibited E2-stimulated proliferation of MCF-7 cells, but wtBRCA1 failed to inhibit the proliferation of MCF-7 cells stimulated by IGF-I. Our findings suggest that BRCA1 globally inhibits the response to estrogen in a dose- and time-dependent fashion. The implications of these findings for understanding how BRCA1 may act to restrain E2 action in vivo are considered.

Differential effects of all-trans-retinoic acid (RA) on Erk1/2 phosphorylation and cAMP accumulation in normal and malignant human prostate epithelial cells: Erk1/2 inhibition restores RA-induced decrease of cell growth in malignant prostate cells

OBJECTIVE

All-trans-retinoic acid (RA) regulates cellular growth, differentiation and apoptosis in human prostate by binding to RA receptors. Non-genomic retinoid effects on signal transduction kinases in the cytoplasm are also described in several cells but they are still unknown in prostate cells.

METHODS

Using an epithelial cell line derived from normal human prostate (EPN), and normal (NPEC) and malignant (CPEC) epithelial primary cultures of human prostate, we have examined effects of RA on both extracellular signal-regulated kinase 1/2 (Erk1/2) and cAMP accumulation. Then we have verified the effect of the inhibition of Erk1/2 on RA-induced growth arrest and apoptosis in malignant cells.

RESULTS

In NPEC and in EPN treated with RA for up to 24 h, Western blot analyses of Erk1/2 phosphorylation show that RA causes a rapid activation of Erk1/2 within 5 min, which is maintained for 30 min, followed by a return to basal levels. In CPEC, the activated phosphorylation levels persist up to 24 h. While basal cAMP levels are not affected by 30 min treatment with RA in both EPN and NPEC, levels are increased in CPEC. Forskolin-induced cAMP levels are decreased by RA in all cell types. CPEC were incubated for up to 96 h with RA with and without the inhibitor of Erk1/2, UO126. CPEC incubated with RA and UO126 for 72 h showed a significant arrest of cell growth and after 96 h apoptosis in 11% of cells.

CONCLUSIONS

We show rapid effects of RA on cytoplasmic messenger pathways in human prostate, and that responses can differ between normal and malignant cells. The inhibition of these pathways could improve the efficiency of RA in prostate cancer growth control.

Gene expression signature of estrogen receptor alpha status in breast cancer

Background

Estrogens are known to regulate the proliferation of breast cancer cells and to modify their phenotypic properties. Identification of estrogen-regulated genes in human breast tumors is an essential step toward understanding the molecular mechanisms of estrogen action in cancer. To this end we generated and compared the Serial Analysis of Gene Expression (SAGE) profiles of 26 human breast carcinomas based on their estrogen receptor α (ER) status. Thus, producing a breast cancer SAGE database of almost 2.5 million tags, representing over 50,000 transcripts.

Results

We identified 520 transcripts differentially expressed between ERα-positive (+) and ERα-negative (-) primary breast tumors (Fold change ≥ 2; p < 0.05). Furthermore, we identified 220 high-affinity Estrogen Responsive Elements (EREs) distributed on the promoter regions of 163 out of the 473 up-modulated genes in ERα (+) breast tumors. In brief, we observed predominantly up-regulation of cell growth related genes, DNA binding and transcription factor activity related genes based on Gene Ontology (GO) biological functional annotation. GO terms over-representation analysis showed a statistically significant enrichment of various transcript families including: metal ion binding related transcripts (p = 0.011), calcium ion binding related transcripts (p = 0.033) and steroid hormone receptor activity related transcripts (p = 0.031). SAGE data associated with ERα status was compared with reported information from breast cancer DNA microarrays studies. A significant proportion of ERα associated gene expression changes was validated by this cross-platform comparison. However, our SAGE study also identified novel sets of genes as highly expressed in ERα (+) invasive breast tumors not previously reported. These observations were further validated in an independent set of human breast tumors by means of real time RT-PCR.

Conclusion

The integration of the breast cancer comparative transcriptome analysis based on ERα status coupled to the genome-wide identification of high-affinity EREs and GO over-representation analysis, provide useful information for validation and discovery of signaling networks related to estrogen response in this malignancy.

Transcription factor cross-talk: the estrogen receptor and NF-kappaB

The NF-kappaB family of transcription factors regulates many genes that are essential primarily for the development, maintenance and function of the innate and adaptive immune systems. Thus, aberrant activity of the nuclear factor NF-kappaB has a role in many pathological conditions with inflammatory and autoimmune components. Estrogen receptors (ERs) are transcription factors that mediate the biological responses to the sex hormone estrogen and are essential for reproduction and for functions of the cardiovascular, skeletal and nervous systems. Recent studies have demonstrated molecular cross-talk between these families of transcription factors in which the ER mediates inhibition of NF-kappaB activity at several levels. Such cross-talk between these important regulators of the endocrine and immune systems might be exploited for the treatment of cancer and inflammatory and autoimmune diseases.

Integration of the extranuclear and nuclear actions of estrogen

Estrogen receptors (ERs) are localized to many sites within the cell, potentially contributing to overall estrogen action. In the nucleus, estrogen mainly modulates gene transcription, and the resulting protein products determine the cell biological actions of the sex steroid. In addition, a small pool of ERs localize to the plasma membrane and signal mainly though coupling, directly or indirectly, to G proteins. In response to steroid, signal transduction modulates both nontranscriptional and transcriptional events and impacts both the rapid and more prolonged actions of estrogen. Cross-talk from membrane-localized ERs to nuclear ERs can be mediated through growth factor receptor tyrosine kinases, such as epidermal growth factor receptor and IGF-I receptor. Growth factor receptors enact signal transduction to kinases such as ERK and phosphatidylinositol 3-kinase that phosphorylate and activate nuclear ERs, and this can also occur in the absence of sex steroid. A complex relationship between the membrane and nuclear effects of estrogen also involves membrane-initiated phosphorylation of coactivators, recruiting these proteins to the nuclear transcriptosome. Finally, large pools of cytoplasmic ERs exist, and some are localized to mitochondria. The integration of sex steroid effects at distinct cellular locations of its receptor leads to important cellular physiological outcomes and are manifest in both reproductive and nonreproductive organs.

'In vivo' effects of Bisphenol A in Mytilus hemocytes: modulation of kinase-mediated signalling pathways

Endocrine disrupting chemicals (EDCs) include a variety of natural and synthetic estrogens, as well as estrogen-mimicking chemicals. We have previously shown that in the hemocytes of the mussel Mytilus galloprovincialis Lam. both natural and environmental estrogens in vitro can rapidly affect the phosphorylation state of components of tyrosine kinase-mediated cell signalling, in particular of mitogen activated protein kinases (MAPKs) and signal transducers and activators of transcription (STAT), that are involved in mediating the hemocyte immune response. These effects were consistent with the hypothesis that 'alternative' modes of estrogen action involving kinase-mediated pathways similar to those described in mammalian systems are also present in invertebrate cells. This possibility was investigated in vivo with Bisphenol A (BPA): mussels were injected with BPA and hemocytes sampled at 6, 12, and 24 h post-injection. The results show that BPA (25 nM nominal concentration in the hemolymph) lead to a significant lysosomal membrane destabilisation at all times post-injection, indicating BPA-induced stress conditions in the hemocytes, whereas lower concentrations were ineffective. BPA induced significant changes in the phosphorylation state of MAPK and STAT members, as evaluated by SDS-PAGE and WB of hemocyte protein extracts with specific antibodies, although to a different degree at different exposure times. In particular, BPA induced a dramatic decrease in phosphorylation of the stress-activated p38 MAPK, whose activation is crucial in mediating the bactericidal activity. Moreover, BPA decreased the phosphorylation of a CREB-like transcription factor (cAMP-responsive element binding protein). The results demonstrate that BPA can affect kinase-mediated cell signalling in mussel hemocytes also in vivo, and suggest that EDCs may affect gene expression in mussel cells through modulation of the activity of transcription factors secondary to cytosolic kinase cascades. Overall, these data address the importance of investigating full range responses to EDCs in ecologically relevant marine invertebrate species.

Ablation of phosphoinositide 3-kinase-gamma reduces the severity of acute pancreatitis

In pancreatic acini, the G-protein-activated phosphoinositide 3-kinase-gamma (PI3K gamma) regulates several key pathological responses to cholecystokinin hyperstimulation in vitro. Thus, using mice lacking PI3K gamma, we studied the function of this enzyme in vivo in two different models of acute pancreatitis. The disease was induced by supramaximal concentrations of cerulein and by feeding mice a choline-deficient/ethionine-supplemented diet. Although the secretive function of isolated pancreatic acini was identical in mutant and control samples, in both models, genetic ablation of PI3K gamma significantly reduced the extent of acinar cell injury/necrosis. In agreement with a protective role of apoptosis in pancreatitis, PI3K gamma-deficient pancreata showed an increased number of apoptotic acinar cells, as determined by terminal dUTP nick-end labeling and caspase-3 activity. In addition, neutrophil infiltration within the pancreatic tissue was also reduced, suggesting a dual action of PI3K gamma, both in the triggering events within acinar cells and in the subsequent neutrophil recruitment and activation. Finally, the lethality of the choline-deficient/ethionine-supplemented diet-induced pancreatitis was significantly reduced in mice lacking PI3K gamma. Our results thus suggest that inhibition of PI3K gamma may be of therapeutic value in acute pancreatitis.

17beta-estradiol-dependent activation of signal transducer and activator of transcription-1 in human fetal osteoblasts is dependent on Src kinase activity

Estrogen is essential for normal growth and remodeling of bone. Although the mechanism of estrogen action on bone cells has been widely investigated, the full spectrum of signal transduction pathways activated by estrogen is unknown. In this report, we investigate the effects of the gonadal hormone 17beta-estradiol on the regulation of signal transducer and activator of transcription-1 (Stat1) protein in cultured human fetal osteoblast cells, devoid of the classical estrogen receptors (ERs). 17beta-estradiol (10 nM) led to rapid (within 15 min) activation of Stat1 protein as indicated by increases in tyrosine phosphorylation and DNA binding activity. Also, 17beta-estradiol increased gamma-activated sequence-dependent transcription in transient transfection assays, suggesting an increase in Stat protein-dependent transcription. Estrogen-dependent Stat1 activation was blocked in cells that transiently express dominant-negative Stat1 mutant protein. Activation of Stat1 by 17beta-estradiol was not inhibited by ER antagonist ICI 182,780, providing further evidence that it is not dependent on classical ERs. 17beta-Estradiol induced rapid (within 15 min) Stat1 phosphorylation and stimulated gamma-activated sequence-dependent transcription in ER-negative breast cancer cells, indicating that these results are not unique to bone cells. The rapid estrogenic effect involving the phosphorylation and activation of Stat1 was blocked in the presence of Src family kinase inhibitor PP2; activated Stat1 was associated with Src protein in estrogen-treated cells. These findings indicate the requirement for Src kinase pathways in estrogen-mediated Stat1 activation. Thus, the ER-independent activation of Stat1 in 17beta-estradiol-treated osteoblast and breast cancer cells may partially mediate the actions of estrogen on target cells.

Direct effects of sex steroid hormones on adipose tissues and obesity

Sex steroid hormones are involved in the metabolism, accumulation and distribution of adipose tissues. It is now known that oestrogen receptor, progesterone receptor and androgen receptor exist in adipose tissues, so their actions could be direct. Sex steroid hormones carry out their function in adipose tissues by both genomic and nongenomic mechanisms. In the genomic mechanism, the sex steroid hormone binds to its receptor and the steroid-receptor complex regulates the transcription of given genes. Leptin and lipoprotein lipase are two key proteins in adipose tissues that are regulated by transcriptional control with sex steroid hormones. In the nongenomic mechanism, the sex steroid hormone binds to its receptor in the plasma membrane, and second messengers are formed. This involves both the cAMP cascade and the phosphoinositide cascade. Activation of the cAMP cascade by sex steroid hormones would activate hormone-sensitive lipase leading to lipolysis in adipose tissues. In the phosphoinositide cascade, diacylglycerol and inositol 1,4,5-trisphosphate are formed as second messengers ultimately causing the activation of protein kinase C. Their activation appears to be involved in the control of preadipocyte proliferation and differentiation. In the presence of sex steroid hormones, a normal distribution of body fat exists, but with a decrease in sex steroid hormones, as occurs with ageing or gonadectomy, there is a tendency to increase central obesity, a major risk for cardiovascular disease, type 2 diabetes and certain cancers. Because sex steroid hormones regulate the amount and distribution of adipose tissues, they or adipose tissue-specific selective receptor modulators might be used to ameliorate obesity. In fact, hormone replacement therapy in postmenopausal women and testosterone replacement therapy in older men appear to reduce the degree of central obesity. However, these therapies have numerous side effects limiting their use, and selective receptor modulators of sex steroid hormones are needed that are more specific for adipose tissues with fewer side effects.

Ovariectomy enhances renal cortical expression and function of cyclooxygenase-2

BACKGROUND

Cyclooxygenase-2 (COX-2) inhibitors are used as analgesics in postmenopausal women, who develop edema and require a salt-restricted diet. This study was performed to determine the renal expression of COX-2 and on COX-2-dependent regulation of renal blood flow (RBF) in ovariectomized rats.

METHODS

Sprague-Dawley rats were divided into 4 groups: sham-operated rats fed a normal-salt diet (Sh+NS) or a low-salt diet (Sh+LS), and bilaterally ovariectomized rats fed a normal-salt diet (Ox+NS) or a low-salt diet (Ox+LS) (N= 6 in each group). Estrogen replacement therapy was performed on other ovariectomized rats. A renal clearance study was performed in anesthetized animals.

RESULTS

Ovariectomy increased renal cortical COX-2 expression independently of dietary salt intake (Sh+NS <Ox+N; Sh+LS <Ox+LS). Inhibition of COX-2 by NS398 reduced the urinary excretion of 6-keto-prostaglandin F1alpha in all 4 groups, although the reduction was greater in the Ox+LS group than in the Ox+NS and Sh+LS groups, which in turn had a greater reduction than the Sh+NS group. RBF significantly decreased in every group except the Sh+NS group, but no effect on blood pressure, inulin clearance, or urinary sodium excretion was seen. The decrease in RBF was significantly greater in the Ox+LS group than in the Sh+LS and Ox+NS group. The decrease in RBF was dependent on cortical RBF in the Sh+LS and Ox+NS groups, and on both cortical and medullary RBF in the Ox+LS group. Estrogen replacement therapy reversed the ovariectomy-induced changes.

CONCLUSION

Estrogen-dependent COX-2 expression plays an important role in the RBF regulation in female rats.

Estrogen Receptor Activation of Phosphoinositide-3 Kinase, Akt, and Nitric Oxide Signaling in Cerebral Blood Vessels: Rapid and Long-Term Effects

Estrogen receptor regulation of nitric oxide production by vascular endothelium may involve rapid, membrane-initiated signaling pathways in addition to classic genomic mechanisms. In this study, we demonstrate using intact cerebral blood vessels that 17beta-estradiol rapidly activates endothelial nitric-oxide synthase (eNOS) via a phosphoinositide-3 (PI-3) kinase-dependent pathway. The effect is mediated by estrogen receptors (ERs), consistent with colocalization of ERalpha and caveolin-1 immunoreactivity at the plasma membrane of endothelial cells lining cerebral arteries. Treatment with 10 nM 17beta-estradiol for 30 min increased NO production, as measured by total nitrite assay, in cerebral vessels isolated from ovariectomized rats. This effect was significantly decreased by membrane cholesterol depletion with beta-methyl-cyclodextrin, the ER antagonist ICI 182,780 [fulvestrant (Faslodex)], and two inhibitors of PI-3 kinase: wortmannin and LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride]. In parallel with NO production, 17beta-estradiol treatment rapidly increased phosphorylation of both eNOS (p-eNOS) and Akt (p-Akt). PI-3 kinase inhibitors also blocked the latter effects; together, these data are consistent with ER activation of the PI-3 kinase-p-Akt-p-eNOS pathway. ERalpha protein (66 and 50 kDa) coimmunoprecipitated with eNOS as well as with the p85alpha regulatory subunit of PI-3 kinase, further implicating ERalpha in kinase activation of eNOS. Little is known regarding the effects of estrogen on cellular kinase pathways in vivo; therefore, we compared cerebral blood vessels isolated from ovariectomized rats that were either untreated or given estrogen replacement for 4 weeks. Long-term estrogen exposure increased levels of cerebrovascular p-Akt and p-eNOS as well as basal NO production. Thus, in addition to the rapid activation of PI-3 kinase, p-Akt, and p-eNOS, estrogen signaling via nontranscriptional, kinase mechanisms has long-term consequences for vascular function.