Membrane estrogen receptors identified by multiple antibody labeling and impeded-ligand binding

Steroid hormone receptor signaling in tumorigenesis

Excessive activation of the hormone signaling pathways is implicated in several disorders of the target tissues, with cancer being one of the most serious fallouts. Steroid hormone receptors are key proteins through which steroid hormones convey their signals to the cells. Deregulated activity of the hormone receptors due to their altered activation; stability or sub-cellular localization is heavily implicated in the onset and progress of cancers. The role played by estrogen and its receptors in breast cancer remains the most thoroughly investigated steroid-dependent cancer system till date. Choosing it as an example, we have summarized the molecular mechanisms underlying the action of the estrogen receptors (ERs) in manifesting the effects of the estrogens in the cells. A special emphasis is placed on the molecular mechanism of their functionality, role of the coactivator proteins, and the reasons for the deregulated signaling. The therapeutic approaches resulting from the mechanistic study of the ER action and their efficacies are also discussed.

Biology of progesterone receptor loss in breast cancer and its implications for endocrine therapy

The response to endocrine therapy in breast cancer correlates with estrogen receptor (ER) and progesterone receptor (PR) status. ER-positive/PR-negative breast cancers respond less well to selective ER modulator (SERM) therapy than ER-positive/PR-positive tumors. The predictive value of PR has long been attributed to the dependence of PR expression on ER activity, with the absence of PR reflecting a nonfunctional ER and resistance to hormonal therapy. However, recent clinical and laboratory evidence suggests that ER-positive/PR-negative breast cancers may be specifically resistant to SERMs, whereas they may be less resistant to estrogen withdrawal therapy with aromatase inhibitors, which is a result inconsistent with the nonfunctional ER theory. Novel alternative molecular mechanisms potentially explaining SERM resistance in ER-positive/PR-negative tumors have been suggested by recent experimental indications that growth factors may downregulate PR levels. Thus, the absence of PR may not simply indicate a lack of ER activity, but rather may reflect hyperactive cross talk between ER and growth factor signaling pathways that downregulate PR even as they activate other ER functions. Therefore, ER-positive/PR-negative breast tumors might best be treated by completely blocking ER action via estrogen withdrawal with aromatase inhibitors, by targeted ER degradation, or by combined therapy targeting both ER and growth factor signaling pathways. In this review, we will discuss the biology and etiology of ER-positive/PR-negative breast cancer, highlighting recent data on molecular cross talk between ER and growth factor signaling pathways and demonstrating how PR might be a useful marker of these activities. Finally, we will consider the clinical implications of these observations.

Estrogen prevents glutamate-induced apoptosis in C6 glioma cells by a receptor-mediated mechanism

Estrogen-mediated neuroprotection is well established; however, no single mechanism of action for this effect has yet been established. As glial cells are integral for both the intact and injured nervous system, we hypothesized that estrogen-mediated neuroprotection may partly be attributed to attenuation of glial cell apoptosis, allowing them to protect neurons following injury. To assess the protective effects of estrogen on glia, C6 rat glioma cells were treated for 24 h with 500 microM glutamate. Cell viability was determined by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and apoptosis was confirmed by cell morphology and DNA fragmentation. Pretreatment with 10 nM 17beta-estradiol (estrogen) increased cell viability and attenuated apoptosis. Treatment with the stereoisomer 17alpha-estradiol, or estrogen plus estrogen receptor antagonist ICI 182,780, was significantly less effective, indicating that cytoprotection was receptor-mediated. Estrogen treatment upregulated expression of estrogen receptor alpha. Cell impermeable bovine serum albumin-conjugated estrogen was also protective, indicating activation of estrogen receptors on the cell membrane. Intracellular free [Ca2+] was increased after glutamate treatment. This increase was attenuated in cells pretreated with estrogen. Glutamate increased the activity of pro-apoptotic proteases, such as calpain and caspase-3, and these protease activities were significantly attenuated by estrogen. The mechanism by which estrogen decreased intracellular Ca2+ was examined by assaying cell viability after using inhibitors that either blocked extracellular Ca2+ influx or prevented the release of intracellular Ca2+ stores. While several inhibitors increased cell viability in glutamate-treated cells, none were as protective as estrogen, and estrogen co-treatment significantly increased cell viability. These findings indicate that estrogen-mediated cytoprotection may be related to effects on Ca2+ entry but that these effects are not limited to any one of these Ca2+ entry points alone.

Spotlight on the role of hormonal factors in the emergence of autoreactive B-lymphocytes

Pathogenic autoimmunity requires a combination of inherited and acquired factors. In as much as hormones influence the sexual dimorphism of the immune system, it is possible that they can initiate or accelerate an autoimmune process, and contribute to gender-biased autoimmune disorders. Not only natural hormones, but also endocrine disruptors, such as environmental estrogens, may act in conjunction with other factors to override immune tolerance to self-antigens. In lupus, murine and human studies demonstrate that female sex hormones are implicated in disease pathogenesis. In the B cell compartment, both prolactin and estrogen are immunomodulators that affect maturation, selection and antibody secretion. Their impact may be based on their capacity to allow autoreactive B cells to escape the normal mechanisms of tolerance and to accumulate in sufficient numbers to cause clinically apparent disease. Both hormones lead to the survival and activation of autoreactive B cells, but they skew B cell maturation towards different directions, with prolactin inducing T cell-dependent autoreactive follicular B cells and estrogen eliciting T cell-independent autoreactive marginal zone B cells. Differential modulation of the cytokine milieu by hormones may also affect the development and activation of specific mature B cell subsets. This novel insight suggests that targeted manipulation of these pathways may represent a promising avenue in the treatment of lupus and other gender-biased autoimmune diseases.

The role of aromatase inhibitors in ameliorating deleterious effects of ovarian stimulation on outcome of infertility treatment

Clinical utilization of ovulation stimulation to facilitate the ability of a couple to conceive has not only provided a valuable therapeutic approach, but has also yielded extensive information on the physiology of ovarian follicular recruitment, endometrial receptivity and early embryo competency. One of the consequences of the use of fertility enhancing agents for ovarian stimulation has been the creation of a hyperestrogenic state, which may influence each of these parameters. Use of aromatase inhibitors reduces hyperestrogenism inevitably attained during ovarian stimulation. In addition, the adjunct use of aromatase inhibitors during ovarian stimulation reduces amount of gonadotropins required for optimum stimulation. The unique approach of reducing hyperestrogenism, as well as lowering amount of gonadotropins without affecting the number of mature ovarian follicles is an exciting strategy that could result in improvement in the treatment outcome by ameliorating the deleterious effects of the ovarian stimulation on follicular development, endometrial receptivity, as well as oocyte and embryo quality.

The red wine polyphenol, resveratrol, exerts acute direct actions on guinea-pig ventricular myocytes

Epidemiological evidence suggests that moderate consumption of red wine may be cardioprotective, although the precise mechanism(s) responsible remains poorly understood. We hypothesized that the red wine polyphenol, resveratrol, may exert direct actions on the heart and thus potentially contribute to cardioprotection. We show that resveratrol acutely decreases Ca2+ transient amplitude in isolated cardiac myocytes. Intriguingly, resveratrol simultaneously increases cell shortening in half the cells tested, while decreasing shortening in the other half. The former could be attributed to heightened myofilament Ca2+ sensitivity. This was no longer observed in myocytes that had been incubated with the oestrogen receptor antagonist, ICI 182,780, suggesting an oestrogen-receptor dependent mechanism of action. In addition, resveratrol significantly decreased action potential duration and the peak L-type Ca2+ current. Our findings provide evidence that resveratrol exerts multiple direct actions on cardiac myocytes, the net result of which is no overall change in cell contraction. The clinical significance of these results remains to be determined.

Estrogen regulation of adiposity and fuel partitioning. Evidence of genomic and non-genomic regulation of lipogenic and oxidative pathways

Menopause is associated with increased adiposity and greater risk of metabolic disease. In the ovariectomized (OVX) rodent model of menopause, increased adiposity is prevented by estrogen (E2) replacement, reflecting both anorexigenic and potentially metabolic actions of E2. To elucidate metabolic and molecular mechanisms by which E2 regulates fat storage and fat mobilization independently of reduced energy intake, C57 BL/6 mice were ovariectomized, randomized to estrogen (OVX-E2) or control pellet implants (OVX-C), and pairfed for 40 days. E2 treatment was associated with reduced adipose mass and adipocyte size and down-regulation of lipogenic genes in adipocytes under the control of sterol-regulatory element-binding protein 1c. Adipocytes of OVX-E2 mice contained >3-fold more perilipin protein than adipocytes of pairfed control (OVX) mice, and this difference was associated with enhanced ex vivo lipolytic response to catecholamines and with greater levels of serum-free fatty acids following fasting. As in adipose tissue, E2 decreased the expression of lipogenic genes in liver and skeletal muscle. In the latter, E2 appears to promote the partitioning of free fatty acids toward oxidation and away from triglyceride storage by up-regulating the expression of peroxisome proliferation activator receptor-delta and its downstream targets and also by directly and rapidly activating AMP-activated protein kinase. Thus, novel genomic and non-genomic actions of E2 promote leanness in OVX mice independently of reduced energy intake.

Evidence that 17alpha-estradiol is biologically active in the uterine tissue: antiuterotonic and antiuterotrophic action

BACKGROUND

17alpha-Estradiol has been considered as the hormonally inactive isomer of 17beta-estradiol. Recently, nongenomic (smooth muscle relaxation) and genomic (light estrogenic activity) effects of 17alpha-estradiol have been reported, but no reports have yet determined its possible antiestrogenic activity. Therefore, this study investigated: the nongenomic action of 17alpha-estradiol on uterine contractile activity and its potential agonist-antagonist activity on uterine growth.

METHODS

Uterine rings from rats were isometrically recorded. Different concentrations (0.2-200 microM) of 17alpha-estradiol were tested on spontaneous contraction and equimolarly compared with 17beta-estradiol. To examine the mechanism of 17alpha-estradiol action, its effect was studied in presence of beta2-antagonist (propranolol), antiestrogens (tamoxifen and ICI 182,780) or inhibitors of protein synthesis (cycloheximide) and transcription (actinomycin D). Moreover, contractions induced by high potassium (KCl) solution or calcium in depolarized tissues by KCl-calcium free solution were exposed to 17alpha-estradiol. Collaterally, we performed an uterotrophic assay in adult ovariectomized rats measuring the uterine wet weight. The administration for three days of 0.3 microM/day/Kg 17beta-estradiol was equimolarly compared with the response produced by 17alpha-estradiol. Antiuterotrophic activity was assayed by administration of 0.3 microM/day/Kg 17beta-estradiol and various doses ratios (1:1, 1:3, 1:5, and 1:100) of 17alpha-estradiol.

RESULTS

The estradiol isomers elicited an immediate relaxation, concentration-dependent and reversible on spontaneous contraction. 17alpha-Estradiol presented lower potency than 17beta-estradiol although it did not antagonize 17beta-estradiol-induced relaxation. Relaxation to 17alpha-estradiol was not inhibited by propranolol, tamoxifen, ICI 182,780, cycloheximide or actinomycin D. The KCl contractions were also sensitive to 17alpha-estradiol-induced relaxation and calcium contractions in depolarized tissues were markedly prevented by 17alpha-estradiol, implying a reduction of extracellular calcium influx through voltage-operated calcium channels (VOCCs). Uterotrophic assay detected significant increase in uterine weight using 17alpha-estradiol, which was significantly minor as compared with 17beta-estradiol. 17alpha-Estradiol, at all doses ratios, significantly antagonized the hypertrophic response of 17beta-estradiol.

CONCLUSION

17alpha-Estradiol induces a relaxing effect, which may be independent of the classical estrogen receptor, nongenomic action, apparently mediated by inactivation of VOCCs. 17alpha-Estradiol is also a weak estrogen agonist (uterotrophic response); likewise, 17alpha-estradiol may act as an antiestrogen (antiuterotrophic response). The overall data document a nongenomic relaxing action and a novel antiestrogenic action of 17alpha-estradiol, which are relevant in estrogen-mediated uterine physiology.

Endocrinology and hormone therapy in breast cancer: new insight into estrogen receptor-alpha function and its implication for endocrine therapy resistance in breast cancer

Estrogen and its receptor (ER) are critical for development and progression of breast cancer. This pathway is targeted by endocrine therapies that either block ER functions or deplete ER's estrogen ligand. While endocrine therapies are very effective, de novo and acquired resistance are still common. Laboratory and clinical data now indicate that bidirectional molecular crosstalk between nuclear or membrane ER and growth factor receptor pathways such as HER2/neu is involved in endocrine resistance. Preclinical data suggest that blockade of selected growth factor receptor signaling can overcome this type of resistance, and this strategy is already being tested in clinical trials

Rapid, estrogen receptor-mediated signaling: why is the endothelium so special?

Classical, ligand-activated genomic effects of estrogen receptors (ERs) were once thought to mediate all estrogen responses. It is now accepted that rapid, nongenomic responses, mediated by ER-containing membrane complexes, occur in many tissues. The endothelium is a major target of such responses and is the critical regulatory tissue that, when normally functional, determines a state of "vascular health." When dysfunctional, the phenotypic and functional alterations result in vascular pathology, the most common form of which is atherosclerosis. Nitric oxide (NO) is a vascular protective substance generated by endothelial NO synthase (eNOS) in endothelial cells. The engagement of membrane ERs by 17beta-estradiol (E2) is a potent stimulus to eNOS activation and NO release. Here, we describe the multimolecular components of ER-containing membrane complex assembly and the mechanisms directing ER targeting to caveolae microdomains in the plasma membrane. We discuss the possibility that various ERalpha splice forms, expressed in endothelial cells, may be particularly efficient signal transducers and may use classical receptor domains for membrane targeting and insertion. Finally, we discuss the biomedical ramifications of ER-mediated endothelial activation, including the controversies surrounding hormone replacement therapy and cardiovascular disease.

Oestrogen functions in skin and skin appendages

Oestrogens have significant effects on different cell types important in skin physiology, including the epidermal keratinocytes, dermal fibroblasts and melanocytes. In addition, they can also modulate skin appendages such as the hair follicle, the sebaceous gland and the apocrine glands. Oestrogens may also have important modulatory roles in events such as skin ageing, pigmentation, hair growth, sebum production and skin cancer. It is now recognised that oestrogens can modulate their actions via two distinct intracellular receptors (ERalpha and ERbeta) or via cell surface receptors, which activate specific second messenger signalling pathways. This paper highlights the effects of oestrogens on different components of the skin and reviews some of the more recent developments in terms of receptor expression and cell signalling pathways.

Oestrogen receptors and selective oestrogen receptor modulators: molecular and cellular pharmacology

The early termination of the two arms of the Women's Health Initiative Trials has led to an increased interest and demand for selective oestrogen receptor modulators because of their potential to retain the benefits of hormone replacement therapy (oestrogen plus a gestagen) and at the same time avoid most of its severe adverse events. Selective oestrogen receptor modulators are a class of oestrogen receptor binding, small organic molecules that take advantage of the plasticity of the oestrogen receptors (alpha and beta, respectively), modulating the surface conformation of the oestrogen receptors upon binding in the respective ligand binding cavity. By doing so they affect the binding of various co-factors to the surface of the oestrogen receptors that, at least in part, explains why selective oestrogen receptor modulators may mimic the activity of oestrogen in some tissues where so desired, while opposing its activity in tissues where oestrogen-like activity is undesirable. Although selective oestrogen receptor modulators have many properties in common they also display unique activities including oestrogen receptor surface modulation and regulation of target gene expression. Selective oestrogen receptor modulators therefore offer the opportunity to develop pharmaceuticals with very distinct pharmacology and mechanism of action. Furthermore, these modulators offer the advantage of decreased risk for the development of breast and endometrial cancer and circumvent the need for combination with a gestagen. Most selective oestrogen receptor modulators in development bind with roughly equal affinity to both oestrogen receptor alpha and beta (balanced) and our view is that it is unlikely that a balanced selective oestrogen receptor modulator will inherit all desired effects of oestrogen (e.g. 17beta-oestradiol) and at the same time be devoid of all undesired effects. We therefore propose that the development of oestrogen receptor-subtype (alpha and beta, respectively) selective pharmaceuticals for specific applications (designer drugs) would better provide the benefits of hormone replacement therapy without its associated risks.

17beta-estradiol attenuates alpha, beta-meATP-induced currents in rat dorsal root ganglion neurons

The effects of 17beta-estradiol on the alpha,beta-me ATP-induced currents were studied on dorsal root ganglion (DRG) neurons using whole-cell recording technique. Three types of currents (transient, sustained or biphasic) were evoked by alpha,beta-me ATP in acutely dissociated DRG neurons. When neurons were pre-incubated with 17beta-estradiol (10-1000 nM) for 4 min, an inhibition of the transient current and the transient component of the biphasic current was observed. In contrast, 17beta-estradiol did not have any significant effect on the sustained current evoked by alpha,beta-meATP. The inhibitory effects were concentration-dependent, reversible and could be blocked by the estradiol receptor inhibitor, ICI 182,780 (1 microM). However, bovine serum albumin-conjugated 17beta-estradiol (17beta-estradiol-BSA, 10 nM) failed to mimic the effects of 17beta-estradiol. 17alpha-estradiol, the inactive isoform, did not have significant effects on alphabeta-meATP-induced currents, either. Sustained currents induced by ATP (100 microM) in nodose ganglion (NG), superior cervical ganglion (SCG) and otic ganglion (OTG) neurons were not affected by 17beta-estradiol. These results suggest that the female gonadal hormone, 17beta-estradiol, might participate in control of pain by modulating P2X3 receptor-mediated events in sensory neurons.

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.

Promoting insulin secretion in pancreatic islets by means of bisphenol A and nonylphenol via intracellular estrogen receptors

In this study, we investigated the effects of endocrine disrupters bisphenol A (BPA) and nonylphenol (NP) on insulin secretion from rat pancreatic islets. Following acute exposure to BPA and NP, neither BPA nor NP (0.1, 1, 10, 100 and 1000 microg/l) affected insulin secretion in concentrations of 16.7 mM glucose. However, insulin secretion following long-term exposure to BPA or NP for 24 h in 16.7 mM glucose was significantly higher than without exposure. To determine whether increased insulin secretion resulting from long-term exposure to BPA and NP is induced via intracellular estrogen receptors, we blocked the cytosolic/nuclear estrogen receptors, using actinomycin-D (Act-D), an inhibitor of RNA synthesis, and ICI 182,780 (ICI), an estrogen receptor inhibitor. Following long-term exposure to BPA (10 microg/l) or NP (10 microg/l), Act-D or ICI treatment eliminated the facilitation of insulin secretion. In conclusion, we have demonstrated for the first time that long-term exposure to endocrine disrupters, such as BPA and NP, promotes in vitro insulin secretion from the pancreatic islets, via cytosolic/nuclear estrogen receptors.

17beta-estradiol enhances heparin-binding epidermal growth factor-like growth factor production in human keratinocytes

Heparin-binding epidermal growth factor-like growth factor (HB-EGF) enhances reepithelialization in wounds. Estrogen is known to promote cutaneous wound repair. We examined the in vitro effects of 17beta-estradiol (E2) on HB-EGF production by human keratinocytes. E2 or membrane-impermeable BSA-conjugated E2 (E2-BSA) increased HB-EGF secretion, mRNA level, and promoter activity in keratinocytes. E2 or E2-BSA enhanced in vitro wound closure in keratinocytes, and the closure was suppressed by anti-HB-EGF antibody. Activator protein-1 (AP-1) and specificity protein 1 (Sp1) sites on HB-EGF promoter were responsible for the E2- or E2-BSA-induced transactivation. Antisense oligonucleotides against c-Fos, c-Jun, and Sp1 blocked E2- or E2-BSA-induced HB-EGF transactivation. E2 or E2-BSA enhanced DNA binding and transcriptional activity of AP-1 and generated c-Fos/c-Jun heterodimers by inducing c-Fos expression. E2 or E2-BSA enhanced DNA binding and transcriptional activity of Sp1 in parallel with the enhancement of Sp1 phosphorylation. These effects of E2 or E2-BSA were not blocked by the nuclear estrogen receptor antagonist ICI-182,780 or anti-estrogen receptor-alpha or -beta antibodies but were blocked by inhibitors of G protein, phosphatidylinositol-specific PLC, PKC-alpha, and MEK1. These results suggest that E2 or E2-BSA may enhance HB-EGF production via activation of AP-1 and Sp1. These effects of E2 or E2-BSA may be dependent on membrane G protein-coupled receptors different from nuclear estrogen receptors and on the receptor-mediated activities of phosphatidylinositol-specific PLC, PKC-alpha, and MEK1. E2 may enhance wound reepithelialization by promoting HB-EGF production in keratinocytes.

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.

Regulatory roles of sex hormones in cutaneous biology and immunology

Recent studies have revealed that sex hormones manifest a variety of biological and immunological effects in the skin. Pregnancy, menstruation and the menopause modulate the natural course of psoriasis, indicating a female hormone-induced regulation of skin inflammation. Estrogen in vitro down-regulates the production of the neutrophil, type 1 T cell and macrophage-attracting chemokines, CXCL8, CXCL10, CCL5, by keratinocytes, and suppresses IL-12 production and antigen-presenting capacity while enhancing anti-inflammatory IL-10 production by dendritic cells. These data indicate that estrogen may attenuate inflammation in psoriatic lesions. Estrogen, alone or together with progesterone, prevents or reverses skin atrophy, dryness and wrinkles associated with chronological or photo-aging. Estrogen and progesterone stimulate proliferation of keratinocytes while estrogen suppresses apoptosis and thus prevents epidermal atrophy. Estrogen also enhances collagen synthesis, and estrogen and progesterone suppress collagenolysis by reducing matrix metalloproteinase activity in fibroblasts, thereby maintaining skin thickness. Estrogen maintains skin moisture by increasing acid mucopolysaccharide or hyaluronic acid levels in the dermis. Progesterone increases sebum secretion. Estrogen accelerates cutaneous wound healing stimulating NGF production in macrophages, GM-CSF production in keratinocytes and bFGF and TGF-beta1 production in fibroblasts, leading to the enhancement of wound re-innervation, re-epithelialization and granulation tissue formation. In contrast, androgens prolong inflammation, reduce deposition of extracellular matrix in wounds, and reduce the rate of wound healing. Estrogen enhances VEGF production in macrophages, an effect that is antagonized by androgens and which may be related to the development of granuloma pyogenicum during pregnancy. These regulatory effects of sex steroids may be manipulated as therapeutic or prophylactic measures in psoriasis, aging, chronic wounds or granuloma pyogenicum.

Metabolites of progesterone and the pregnane X receptor: a novel pathway regulating uterine contractility in pregnancy?

OBJECTIVE

The purpose of this study was to determine the role of 5beta-dihydroprogesterone (5beta-DHP), acting through the nuclear receptor pregnane X receptor (PXR), in regulating uterine contractility.

STUDY DESIGN

Uterine contractility was studied in tissues from women, rats, and mice. Messenger RNA was assessed using reverse transcriptase-polymerase chain reaction (RT-PCR), and protein was measured using enzyme assays, immunofluorescence microscopy, and Western analyses.

RESULTS

Human and rat uterine tissues contain mRNA and protein for 5beta-reductase and for PXR. Acute in vitro treatment with 5beta-DHP causes rapid uterine relaxation that is not mediated by PXR. Chronic in vivo administration of 5beta-DHP to mice with intact PXR, but not in mice with disrupted PXR, causes an increased effect of 1400W, a specific inhibitor of inducible nitric oxide synthase (iNOS). This suggests that 5beta-DHP increased iNOS-modulated uterine tone, as occurs during pregnancy.

CONCLUSION

These data support the hypothesis that metabolites of progesterone may act chronically through a PXR-mediated mechanism to regulate uterine contractility.

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.

17beta-Estradiol inhibits calcium-dependent, but not calcium-independent, contraction in isolated rat aorta

It is now well known that 17beta-estradiol has an endothelium-independent, non-genomic vasorelaxant effect. We hypothesized that 17beta-estradiol has its non-genomic effect on calcium-independent contraction in de-endothelialized rat aortic rings. Rat aortic ring preparations were mounted in organ baths and exposed to contractile agents. 17beta-Estradiol (8, 20 or 50 microM), but not 17alpha-estradiol, concentration-dependently decreased the tension induced by 1.0 microM phenylephrine (PE) in the presence, but not in the absence, of calcium in the solution. Pretreatment with 17beta-estradiol concentration-dependently inhibited vascular contractions induced by cumulative addition of PE or calcium and almost completely abolished those induced by cumulative addition of Bay K8644, a calcium channel opener. Furthermore, 17beta-estradiol also concentration-dependently decreased the tension induced by 0.3 microM phorbol 12,13-dibutyrate (PDBu), a protein kinase C activator, in the presence of calcium in the solution, but not in the absence of calcium in the solution. Pretreatment with 17beta-estradiol had little effect on vascular contractions induced by PDBu or PE or on PE-induced mitogen-activated protein kinase (MAPK) activation in calcium-free Krebs solution. These results suggest that 17beta-estradiol inhibits calcium-dependent, but not calcium-independent, vascular contraction.

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

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

Mechanisms of membrane estrogen receptor-alpha-mediated rapid stimulation of Ca2+ levels and prolactin release in a pituitary cell line

The role of membrane estrogen receptor-alpha (mERalpha) in rapid nongenomic responses to 17beta-estradiol (E(2)) was tested in sublines of GH3/B6 rat prolactinoma cells selected for high (GH3/B6/F10) and low (GH3/B6/D9) mERalpha expression. E(2) elicited rapid, concentration-dependent intracellular Ca(2+) concentration ([Ca(2+)](i)) increases in the F10 subline. Lack of inhibition by thapsigargin depletion of intracellular Ca(2+) pools, together with abrogation of the response in Ca(2+)-free medium, suggested an extracellular source of Ca(2+) for this response. The participation of voltage-dependent channels in the E(2)-induced [Ca(2+)](i) increase was confirmed by the specific L-type Ca(2+) channel inhibitor nifedipine. For comparison, the D9 mERalpha-depleted subline was insensitive to steroid action via this signaling mechanism. [Ca(2+)](i) elevation was correlated with prolactin (PRL) release in the F10 cell line in as little as 3 min. E(2) caused a much higher PRL release than KCl treatment (which caused maximal Ca(2+) elevation), suggesting that secretion was also controlled by additional mechanisms. Participation of mERalpha in these effects was confirmed by the ability of E(2)-peroxidase (a cell-impermeable analog of E(2)) to cause these responses, blockage of the responses with the ER antagonist ICI 182 780, and the inability of the E(2) stereoisomer 17alpha-E(2) to elicit a response. Thus rapid exocytosis of PRL is regulated in these cells by mERalpha signaling to specific Ca(2+) channels utilizing extracellular Ca(2+) sources and additional signaling mechanisms.

Estradiol acutely attenuates glutamate-induced calcium overload in primarily cultured rat hippocampal neurons through a membrane receptor-dependent mechanism

Increasing lines of evidence indicate that estrogen acts as a neuroprotective agent through a nongenomic mechanism. We tested the hypothesis that 17beta-estradiol could rapidly attenuate glutamate-induced calcium (Ca2+) overload in rat primary hippocampal neurons via a membrane receptor-dependent mechanism. The bulk cytosolic intracellular Ca2+ level was measured in neurons with fluorescent Ca2+ probe fluo3. Preexposure of primary cultured hippocampal neurons to 17beta-estradiol for 3 min attenuated intracellular Ca2+ increase induced by glutamate in a concentration-dependent manner. The action of 17beta-estradiol was reversible after washout. Administration of membrane-impermeable 17beta-estradiol conjugated to bovine serum albumin (E2-BSA) produced the same effect, suggesting possible involvement of cell membrane receptors. ICI 182,780, a specific estrogen receptor (ER) antagonist, blocked the neuronal response to 17beta-estradiol and estradiol BSA, indicating a role of specific ERs. The present study demonstrates that 17beta-estradiol acutely reduces glutamate-stimulated intracellular Ca2+ increase via ERs probably on the cell surface of the hippocampal neurons.

Estrogen signaling in the hypothalamus

Estrogen has multifaceted effects on the hypothalamus that regulate a number of homeostatic functions including reproduction, temperature, energy balance, stress, and motivated behaviors. Estrogen targets all of the major hypothalamic neuroendocrine and autonomic cellular groups to activate multiple signaling pathways. Originally it was thought that all of these actions of estrogen could be ascribed to its binding to its "classical" intracellular receptor and to alterations in gene transcription. However, we now know that this steroid hormone activates multiple signaling pathways to affect neuronal excitability and gene transcription. Although the "classical" genomic signaling pathway has been recognized for almost half a century, until recently little attention has been paid to the rapid membrane-initiated signaling by estrogen in neurons. It has been known since the 1970s that estrogen can rapidly alter neuronal firing within seconds, indicating that some cellular effects of estrogen could occur via rapid, non-transcriptional mechanisms. Therefore, this chapter reviews the current status of estrogen signaling in the hypothalamus via membrane-initiated and nuclear-mediated events that affect the excitability of hypothalamic neurons and, ultimately, neuroendocrine and autonomic functions.

Hormone multifunctionalities: a theory of endocrine signaling, command and control

A theory is presented outlining how organisms can function and benefit from multifunctionality of hormones in order to enhance greatly the information-carrying potential of endocrine signaling. Hormones are produced continuously as micropulses, and intermittently as larger pulses. It is generally believed that micropulses generate fluctuating basal hormone concentrations, which may consistently elicit particular responses among diverse variables. Evidence is discussed suggesting that in contrast to the hormone micropulses, the larger endogenous hormone pulses may elicit responses which may differ from one pulse to another and may therefore serve different physiological functions. In this paper we postulate that an endogenous hormone pulse is a specific form of a multisignal message that serves a certain physiological function. Different pulses of a hormone may be signals of diverse multisignal messages that serve different functions. A multisignal message may elicit congruous responses by selectively enhancing some actions and suppressing other actions of the component signals. Various roles of signals of multisignal messages are discussed, as well as processes that may be involved in the diversity and selectivity of actions of different pulses of a hormone. Hormones also are converted into other hormones; we analyze how precursor and derived hormones may function independently of each other, and how precursor hormones may give rise to permissive effects. Mechanisms involved in therapeutic and adverse effects of hormone administrations are analyzed, and a strategy is suggested for developing more selective hormonal therapies.

Membrane estrogen receptor-alpha levels predict estrogen-induced ERK1/2 activation in MCF-7 cells

INTRODUCTION

We examined the participation of a membrane form of estrogen receptor (mER)-alpha in the activation of mitogen-activated protein kinases (extracellular signal-regulated kinase [ERK]1 and ERK2) related to cell growth responses in MCF-7 cells.

METHODS

We immunopanned and subsequently separated MCF-7 cells (using fluorescence-activated cell sorting) into mER-alpha-enriched (mERhigh) and mER-alpha-depleted (mERlow) populations. We then measured the expression levels of mER-alpha on the surface of these separated cell populations by immunocytochemical analysis and by a quantitative 96-well plate immunoassay that distinguished between mER-alpha and intracellular ER-alpha. Western analysis was used to determine colocalized estrogen receptor (ER)-alpha and caveolins in membrane subfractions. The levels of activated ERK1 and ERK2 were determined using a fixed cell-based enzyme-linked immunosorbent assay developed in our laboratory.

RESULTS

Immunocytochemical studies revealed punctate ER-alpha antibody staining of the surface of nonpermeabilized mERhigh cells, whereas the majority of mERlow cells exhibited little or no staining. Western analysis demonstrated that mERhigh cells expressed caveolin-1 and caveolin-2, and that ER-alpha was contained in the same gradient-separated membrane fractions. The quantitative immunoassay for ER-alpha detected a significant difference in mER-alpha levels between mERhigh and mERlow cells when cells were grown at a sufficiently low cell density, but equivalent levels of total ER-alpha (membrane plus intracellular receptors). These two separated cell subpopulations also exhibited different kinetics of ERK1/2 activation with 1 pmol/l 17beta-estradiol (E2), as well as different patterns of E2 dose-dependent responsiveness. The maximal kinase activation was achieved after 10 min versus 6 min in mERhigh versus mERlow cells, respectively. After a decline in the level of phosphorylated ERKs, a reactivation was seen at 60 min in mERhigh cells but not in mERlow cells. Both 1A and 2B protein phosphatases participated in dephosphorylation of ERKs, as demonstrated by efficient reversal of ERK1/2 inactivation with okadaic acid and cyclosporin A.

CONCLUSION

Our results suggest that the levels of mER-alpha play a role in the temporal coordination of phosphorylation/dephosphorylation events for the ERKs in breast cancer cells, and that these signaling differences can be correlated to previously demonstrated differences in E2-induced cell proliferation outcomes in these cell types.

Membrane estrogen receptor-alpha levels in MCF-7 breast cancer cells predict cAMP and proliferation responses

Introduction

17β-estradiol (E₂) can rapidly induce cAMP production, but the conditions under which these cAMP levels are best measured and the signaling pathways responsible for the consequent proliferative effects on breast cancer cells are not fully understood. To help resolve these issues, we compared cAMP mechanistic responses in MCF-7 cell lines selected for low (mER^low) and high (mER^high) expression of the membrane form of estrogen receptor (mER)-α, and thus addressed the receptor subform involved in cAMP signaling.

Methods

MCF-7 cells were immunopanned and subsequently separated by fluorescence activated cell sorting into mER^high (mER-α-enriched) and mER^low (mER-α-depleted) populations. Unique (compared with previously reported) incubation conditions at 4°C were found to be optimal for demonstrating E₂-induced cAMP production. Time-dependent and dose-dependent effects of E₂ on cAMP production were determined for both cell subpopulations. The effects of forskolin, 8-CPT cAMP, protein kinase A inhibitor (H-89), and adenylyl cyclase inhibitor (SQ 22,536) on E₂-induced cell proliferation were assessed using the crystal violet assay.

Results

We demonstrated a rapid and transient cAMP increase after 1 pmol/l E₂ stimulation in mER^high cells; at 4°C these responses were much more reliable and robust than at 37°C (the condition most often used). The loss of cAMP at 37°C was not due to export. 3-Isobutyl-1-methylxanthine (IBMX; 1 mmol/l) only partially preserved cAMP, suggesting that multiple phosphodiesterases modulate its level. The accumulated cAMP was consistently much higher in mER^high cells than in mER^low cells, implicating mER-α levels in the process. ICI172,780 blocked the E₂-induced response and 17α-estradiol did not elicit the response, also suggesting activity through an estrogen receptor. E₂ dose-dependent cAMP production, although biphasic in both cell types, was responsive to 50-fold higher E₂ concentrations in mER^high cells. Proliferation of mER^low cells was stimulated over the whole range of E₂concentrations, whereas the number of mER^high cells was greatly decreased at concentrations above 1 nmol/l, suggesting that estrogen over-stimulation can lead to cell death, as has previously been reported, and that mER-α participates. E₂-mediated activation of adenylyl cyclase and downstream participation of protein kinase A were shown to be involved in these responses.

Conclusion

Rapid mER-α-mediated nongenomic signaling cascades generate cAMP and downstream signaling events, which contribute to the regulation of breast cancer cell number.

A subcellular distribution of estrogen receptor-alpha is changed during artificially induced senescence of PC12 pheochromocytoma cells

Although estrogen has been considered as a sex hormone for decades, recent reports suggest that estrogen might modulate the development and physiological function of the brain. In addition, the subcellular localization of estrogen receptors (ERs) has shown their presence within both the perinuclear cytoplasm and nuclei, suggesting that these ERs may differ functionally. We, therefore, assayed changes in the subcellular localization of ER-alpha immunoreactivity (IR) in rat pheochromocytoma PC12 cells during the artificial senescence induced by the telomerase inhibitor, 3'-azido-3'-deoxythymidine (AZT). After 2 months of culture with AZT, PC12 cells showed morphological and biochemical characteristics of cellular senescence. In the cells showing artificial senescence, the ER-alpha IR was mainly localized within the cytoplasm, whereas in control cells, ER-alpha IR was found only in the nuclei. Since senescence was induced by AZT, which inhibits the action of telomerase whenever the cells divide, the change in subcellular distribution of ER-alpha IR may be correlated with the length of the telomere.

An alternative mode of action of endocrine-disrupting chemicals and chemoprevention

Endocrine-disrupting chemicals (EDCs) produce a wide variety of endocrine-disruption effects, including developmental disorders, carcinogenicity, and mutagenicity. Our recent study demonstrated that about 94% of the 48 EDCs classified by Centers for Disease Control and Prevention (CDC) generated free radicals and that this free radical generation induced by EDCs might represent a common toxic mechanism of action of EDCs. The chemoprevention of endocrine-disrupting effects, such as employed in the control of caner by interfering with common toxic mechanisms of action of EDCs, represents a promising approach to this problem. In this context, it is proposed that EDCs may produce endocrine-disrupting effects including carcinogenicity via the generation of free radicals, and thus the effects may be modulated or prevented by scavenging free radicals with antioxidants, such as vitamins, curcumin, isoflavonoids, resveratrol, and plant polysaccharides. Here, an alternative mode of action of EDCs and their possible chemoprevention are proposed.

Estrogen and the brain: beyond ER-α and ER-β

17beta-Estradiol is a greatly under-appreciated neural growth and trophic factor for the mammalian brain of all ages. Like other growth factors, such as the neurotrophins, 17beta-estradiol influences neurogenesis, neuronal differentiation, and neuronal survival of its targets throughout life. Estrogen elicits developmentally regulated differentiative effects, which are not normally seen in the adult brain. However, re-expression of this developmental response occurs in the adult, following loss of trophic support, whether induced by estrogen deprivation or brain injury. In addition to the classical intranuclear estrogen receptors (ER) ER-alpha and ER-beta, we have recently identified a novel, plasma membrane-associated, putative ER that is neither ER-alpha nor ER-beta, which we have designated 'ER-X'. ER-X is a developmentally regulated estrogen-binding protein, present in wild-type, ER-alpha gene-disrupted (alphaERKO) and ER-alpha null mice, which is re-expressed following ischemic brain injury. The preferred ligand of ER-X is 17alpha-estradiol. Although ER-X shares some homology with the C-terminus of ER-alpha, it is not an alternative splicing variant and may be a new gene. While ER-X appears to mediate 17alpha- and 17beta-estradiol activation of the MAPK cascade, ER-alpha, in contrast, is inhibitory to its activation. Estradiol activation of MAPK/ERK may be particularly relevant for neuroprotection during aging and Alzheimer's disease.

Palmitoylation-dependent estrogen receptor alpha membrane localization: regulation by 17beta-estradiol

A fraction of the nuclear estrogen receptor alpha (ERalpha) is localized to the plasma membrane region of 17beta-estradiol (E2) target cells. We previously reported that ERalpha is a palmitoylated protein. To gain insight into the molecular mechanism of ERalpha residence at the plasma membrane, we tested both the role of palmitoylation and the impact of E2 stimulation on ERalpha membrane localization. The cancer cell lines expressing transfected or endogenous human ERalpha (HeLa and HepG2, respectively) or the ERalpha nonpalmitoylable Cys447Ala mutant transfected in HeLa cells were used as experimental models. We found that palmitoylation of ERalpha enacts ERalpha association with the plasma membrane, interaction with the membrane protein caveolin-1, and nongenomic activities, including activation of signaling pathways and cell proliferation (i.e., ERK and AKT activation, cyclin D1 promoter activity, DNA synthesis). Moreover, E2 reduces both ERalpha palmitoylation and its interaction with caveolin-1, in a time- and dose-dependent manner. These data point to the physiological role of ERalpha palmitoylation in the receptor localization to the cell membrane and in the regulation of the E2-induced cell proliferation.

17beta-estradiol stimulates the growth of human keratinocytes by inducing cyclin D2 expression

Estrogen is reported to prevent age-associated epidermal thinning in the skin. We examined if 17beta-estradiol (E2) may enhance the growth of human keratinocytes, focusing on its effects on the expression of cell cycle-regulatory proteins. E2 enhanced proliferation, bromodeoxyuridine incorporation of keratinocytes, and increased the proportion of cells in the S phase. The E2-induced stimulation of proliferation and bromodeoxyuridine incorporation was suppressed by antisense oligonucleotide against cyclin D2, which induces G1 to S phase progression. E2 increased protein and mRNA levels of cyclin D2, and resultantly enhanced assembly and kinase activities of cyclin D2-cyclin-dependent kinases 4 or 6 complexes. E2 enhanced cyclin D2 promoter activity, and the element homologous to cAMP response element (CRE) on the promoter was responsible for the effect. Cyclin D2 expression was enhanced by antiestrogens, ICI 182,780 and 4-hydroxytamoxifen, and membrane-impermeable bovine serum albumin-conjugated E2, indicating the effects via membrane E2-binding sites. E2 increased the enhancer activity of CRE-like element and the amount of phosphorylated cAMP response element binding protein (CREB) binding this element, and the increases were suppressed by H-89, an inhibitor of cAMP-dependent protein kinase A. H-89 also suppressed E2-induced cyclin D2 expression, proliferation, and bromodeoxyuridine incorporation in keratinocytes. Antisense oligonucleotide against G-protein-coupled receptor GPR30 suppressed the E2-induced increases of phosphorylated CREB, cyclin D2 level, proliferation, and bromodeoxyuridine incorporation in keratinocytes. These results suggest that E2 may stimulate the growth of keratinocytes by inducing cyclin D2 expression via CREB phosphorylation by protein kinase A, dependent on cAMP. These effects of E2 may be mediated via cell surface GPR30.

17beta-estradiol enhances the production of granulocyte-macrophage colony-stimulating factor in human keratinocytes

Granulocyte-macrophage colony-stimulating factor (GM-CSF) is effective for impaired wound repair. Estrogen is known to enhance wound repair. We examined if 17beta-estradiol (E2) may in vitro enhance GM-CSF production in human keratinocytes. E2 and membrane-impermeable bovine serum albumin-conjugated E2 increased GM-CSF secretion, mRNA stability, and promoter activity. The element homologous to activator protein-1 (AP-1) on the promoter was responsible for the activation. E2 enhanced transcriptional activity and DNA binding of AP-1. E2 transiently generated c-Fos protein, and shifted AP-1 composition from c-Jun homodimers to c-Fos/c-Jun heterodimers in keratinocytes. E2-induced enhancement of GM-CSF secretion, mRNA stability, and promoter activity were not suppressed by estrogen receptor antagonist ICI 182,780, however, suppressed by conventional protein kinase C inhibitor Gö6976 and PD98059, an inhibitor of mitogen-activated protein kinase kinase (MEK). Gö6976 and PD98059 suppressed E2-induced c-Fos expression and enhancement of DNA-binding and transcriptional activity at AP-1. E2 induced membrane translocation of protein kinase Calpha, which was suppressed by phosphatidylinositol (PI)-specific phospholipase C (PLC) inhibitor U73122. E2 stimulated the phosphorylation of extracellular signal-regulated kinase (ERK), which was suppressed by PD98059, Gö6976, and U73122. E2 transiently generated inositol 1,4,5-triphosphate in keratinocytes, which was suppressed by U73122 and guanine nucleotide-binding protein inhibitor. These results suggest that E2 may enhance GM-CSF production via guanine nucleotide-binding protein-coupled membrane receptors and signaling cascade of PI-specific PLC/protein kinase Calpha/MEK/ERK.

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

Background

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

Results

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

Conclusions

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

Mineralocorticoid receptor antagonists do not block rapid ERK activation by aldosterone

Aldosterone can elicit rapid nongenomic effects both in vivo and in vitro, often mediated by signal transduction cascades. However, it is not understood how these rapid effects are initiated. In this study we show that aldosterone leads to rapid activation of mitogen activated protein kinases ERK1/2 in the cortical collecting duct cell line M-1. Inhibitors of transcription and translation could not block this activation, which suggests an extranuclear (nongenomic) mechanism. Although it is known that M-1 cells do not contain a transcriptionally functional MR, it is not known whether a closely related protein still could mediate the effects, or an unrelated nonclassic receptor. To test this hypothesis, the effects of four classical mineralocorticoid receptor antagonists were studied. None of the compounds could block the response to aldosterone. Altogether, the data suggest that rapid aldosterone effects in M-1 cells are initiated by a receptor different from the classical mineralocorticoid receptor.

Stimulatory effect of naturally occurring flavonols quercetin and kaempferol on alkaline phosphatase activity in MG-63 human osteoblasts through ERK and estrogen receptor pathway

Many plant-derived substances have estrogenic activities. Due to their ability to bind the estrogen receptor (ER), these compounds have the potential to counteract the deleterious effects of estrogen deficiency on bone. In this study, we investigated the in vitro effect of two widespread flavonols, quercetin and kaempferol, on alkaline phosphatase (ALP) activity in MG-63 cultured human osteoblasts. We found that both flavonols significantly increased ALP activity. This effect was markedly reduced by PD 98059, an inhibitor of the extracellular regulated kinase (ERK) pathway, and by ICI 182780, an antagonist of ERs. Western blot studies confirmed that ERK is rapidly activated in cells treated by both flavonols. Finally, ICI 182780 markedly inhibits the flavonol-induced ERK activation. The data presented in this study support the conclusion that, in MG-63 osteoblasts (i) the increase in ALP activity by flavonols involves a rapid stimulation of ERK activation but also involves the ER, and that (ii) the activation of ERK by flavonols occurs most likely downstream of the ERs activation. Taken together, these results suggest that flavonols derivatives as quercetin and kaempferol can stimulate osteoblastic activity. Such compounds may represent new pharmacological tools for the treatment of osteoporosis.

Cortisol rapidly suppresses intracellular calcium and voltage-gated calcium channel activity in prolactin cells of the tilapia (Oreochromis mossambicus)

Cortisol was previously shown to rapidly (10-20 min) reduce the release of prolactin (PRL) from pituitary glands of tilapia (Oreochromis mossambicus). This inhibition of PRL release by cortisol is accompanied by rapid reductions in (45)Ca(2+) and cAMP accumulation. Cortisol's early actions occur through a protein synthesis-independent pathway and are mimicked by a membrane-impermeable analog. The signaling pathway that mediates rapid, nongenomic membrane effects of glucocorticoids is poorly understood. Using the advantageous characteristics of the teleost pituitary gland from which a nearly pure population of PRL cells can be isolated and incubated in defined medium, we examined whether cortisol rapidly reduces intracellular free calcium (Ca(i)(2+)) and suppresses L-type voltage-gated ion channel activity in events that lead to reduced PRL release. Microspectrofluorometry, used in combination with the Ca(2+)-sensitive dye fura 2 revealed that cortisol reversibly reduces basal and hyposmotically induced Ca(i)(2+) within seconds (P < 0.001) in dispersed pituitary cells. Somatostatin, a peptide known to inhibit PRL release through a membrane receptor-coupled mechanism, similarly reduces Ca(i)(2+). Under depolarizing [K(+)], the L-type calcium channel agonist BAY K 8644, a factor known to delay the closing of L-type Ca(2+) channels, stimulates PRL release in a concentration-dependent fashion (P < 0.01). Cortisol (and somatostatin) blocks BAY K 8644-induced PRL release (P < 0.01; 30 min), well within the time course over which its actions occur, independent of protein synthesis and at the level of the plasma membrane. Results indicate that cortisol inhibits tilapia PRL release through rapid reductions in Ca(i)(2+) that likely involve an attenuation of Ca(2+) entry through L-type voltage-gated Ca(2+) channels. These results provide further evidence that glucocorticoids rapidly modulate hormone secretion via a membrane-associated mechanism similar to that observed with the fast effects of peptides and neurotransmitters.

Estradiol prevents amyloid-beta peptide-induced cell death in a cholinergic cell line via modulation of a classical estrogen receptor

The pathology of Alzheimer's disease includes amyloid-beta peptide aggregation that contributes to degeneration of cholinergic neurons. Even though the underlying molecular mechanisms remain unclear, recent in vitro evidence supports a protective role for estrogens against several neurotoxic agents. Here we report that, in a murine cholinergic cell line (SN56), the massive cell death induced by 1-40 fragment of amyloid-beta peptide was prevented by 17beta-estradiol through a mechanism that may involve estrogen receptor activation. The protective effect of estradiol was observed in a dose-dependent manner, and was completely blocked by the pure antiestrogen ICI 182,780. In contrast, the inactive isomer 17alpha-estradiol consistently showed weaker neuroprotection than the native hormone that was unaffected by ICI 182,780 treatment. In addition, equivalent concentrations of 17beta-estradiol enhanced luciferase activity in cells transfected with a luciferase reporter gene driven by tandem estrogen response elements. Estrogen-induced luciferase activity was blocked by ICI 182,780, indicating estrogen receptor-dependent transcriptional activity. We also observed by reverse transcription-polymerase chain reaction, Western blot and immunocytochemistry that increasing concentrations of 17beta-estradiol enhanced the expression of estrogen receptor alpha mRNA and protein during amyloid-beta-induced toxicity. Under these conditions, it was found by confocal microscopy that the localization of estrogen receptor alpha in the absence of hormone was mainly extranuclear. However, the receptor was consistently observed also at the nuclear region after estrogen exposure. Overall, these data suggest that estrogen may exert neuroprotective effects against amyloid-beta-induced toxicity by activation of estrogen receptor-mediated pathways. In addition, intracellular estrogen receptors are up-regulated by their cognate hormone even during exposure to neurotoxic agents.

Rapid effects of 17beta-estradiol on cell signaling and function of Mytilus hemocytes

Estrogens affect the functioning of several non-reproductive tissues, the immune system in particular. In mammalian immunocytes, 17beta-estradiol (E2) has both dose- and cell-type specific effects and the responses to E2 seem to be mediated by rapid, non-genomic mechanisms; these may be initiated at either membrane or cytosolic locations, and can result in both direct local effects, such as modification of ion fluxes, and regulation of gene transcription secondary to activation of different kinase cascades, including mitogen activated protein kinases (MAPKs). In this work, the short-term effects of E(2) and the possible mechanisms of estrogen-mediated cell signaling were investigated in the hemocytes, the immune cells of the bivalve mollusc, the mussel Mytilus galloprovincialis Lam. The results show that E2 (25nM) caused a rapid and significant increase in hemocyte cytosolic [Ca2+]; lower concentrations (5 nM) showed a smaller, not significant effect. Both E2 concentrations affected the phosphorylation state of the components of tyrosine kinase-mediated signal transduction MAPK- and STAT- (signal transducers and activators of transcription) like proteins within 5-15 min from E2 addition. A greater effect and clearer time course were observed with 25 nM E2: in particular, E2 induced a transient increase in p-ERK2 MAPK and a persistent increase in p-p38 MAPK. Moreover, both STAT3 and STAT5 were tyrosine phosphorylated in response to E2. E2 (5 nM) induced both morphological (as evaluated by SEM) and functional changes (such as extracellular release of hydrolytic enzymes, lysosomal membrane destabilisation, and stimulation of the bactericidal activity) within 10-30 min from addition. Lysosomal membrane destabilisation induced by both E2 concentrations was abolished by hemocyte preincubation with the p38 MAPK inhibitor SB203580, and significantly reduced by PD98059 and Wortmannin (inhibitors of ERK MAPK and PI3-K, respectively), this suggesting that rapid activation of kinase cascades is involved in mediating the effects of E2 in mussel hemocytes. The antiestrogen Tamoxifen prevented or strongly reduced most, but not all, the effects of E2. Western blotting with heterologous anti-ERalpha-anti-ERbeta-antibodies revealed the presence of immunoreactive ERalpha- and ERbeta-like proteins in hemocyte protein extracts. Overall, our data support the hypothesis that the rapid effects and mechanisms of action of 17beta-estradiol are extremely conserved and that they may play a crucial role in endocrine-immune interactions in invertebrates.

Quantitative measurement of estrogen-induced ERK 1 and 2 activation via multiple membrane-initiated signaling pathways

Estradiol (E2) and other steroids have recently been shown to initiate various intracellular signaling cascades from the plasma membrane, including those stimulating mitogen-activated protein kinases (MAPKs), and particularly extracellular-regulated kinases (ERKs). In this study we demonstrated the ability of E2 to activate ERKs in the GH3/B6/F10 pituitary tumor cell line, originally selected for its enhanced expression of membrane estrogen receptor-alpha (mERalpha). We compared E2 to its cell-impermeable analog (E2 conjugated to peroxidase, E2-P), and to the synthetic estrogen diethylstilbestrol (DES). Time-dependent ERK activation was quantified with a novel fixed cell-based immunoassay developed to efficiently determine activation by multiple compounds over multiple parameters. Both E2 and DES produced bimodal responses, but with distinctly different time courses of enzyme phosphorylation (activation) and inactivation; E2-P induced a monophasic ERK activation. E2 also phosphorylated ERKs in concentration-dependent manner with two concentration optima (10(-14) and 10(-8)M). Inhibitors were employed to determine pathway (ER, EGFR, membrane organization, PI3 kinase, Src kinase, Ca2+) involvement and timing of pathway activations; all affected ERK activation as early as 3-6 min, suggesting simultaneous, not sequential, activation. Therefore, E2 and other estrogenic compounds can produce rapid ERK phosphorylations via nongenomic pathways, using more than one pathway for signal generation.

Estrogens in the Nervous System: Mechanisms and Nonreproductive Functions

The past decade has witnessed a growing interest in estrogens and their activity in the central nervous system, which was originally believed to be restricted to the control of reproduction. It is now well accepted that estrogens modulate the activity of all types of neural cells through a multiplicity of mechanisms. Estrogens, by binding to two cognate receptors ERalpha and ERbeta, may interact with selected promoters to initiate the synthesis of target proteins. Alternatively, the hormone receptor complex may interfere with intracellular signaling at both cytoplasmic and nuclear levels. The generation of cellular and animal models, combined with clinical and epidemiological studies, has allowed us to appreciate the neurotrophic and neuroprotective effects of estrogens. These findings are of major interest because estradiol might become an important therapeutic agent to maintain neural functions during aging and in selected neural diseases.

Proteins of multiple classes may participate in nongenomic steroid actions

Responses to steroids initiated from non-nuclear receptors impinge on a wide variety of cellular responses and utilize nearly all known signal transduction webs. While the mechanisms by which steroid receptors localize in the membrane are still unclear, it is apparent that this alternative localization allows steroid receptors to participate in a wide range of complex functions influencing cell proliferation, death, and differentiation. The central debate still remains the identity of the protein class or classes that mediate membrane-initiated (nongenomic) responses. The data thus far have supported several possibilities, including: nuclear steroid receptor-like forms in non-nuclear locations; other known (nonsteroid) membrane receptors or channels with additional steroid-binding sites; enzymes; transporters; receptors for serum steroid-binding proteins; unique and previously undescribed proteins; or chimeras of typical steroid receptor domains with other unique or known protein domains. Categorizing membrane steroid receptor proteins based exclusively on the actions of antagonists and agonists, without considering cell context and protein partnering issues, may mislead us into predicting more receptor subtypes than really exist. However, the plethora of signaling and functional outcomes may indicate the participation of more than one kind of steroid-binding protein. Resolving such unanswered questions will require future investigative focus on this alternative arm of steroid action, which is likely to yield as many therapeutic opportunities as have nuclear steroid mechanisms.

Therapeutic targeting in the estrogen receptor hormonal pathway

Estrogens work along with genetic changes to promote the development and growth of breast cancers. Because estrogenic hormones act via the estrogen receptors (ERs), ER-alpha and ER-beta, and the ER is present in more than half of breast tumors, this receptor has been the most widely targeted protein in breast cancer therapy. The presence of the ER in breast tumors predicts improved disease-free survival and response to selective ER modulators (SERMs), such as tamoxifen, or other forms of endocrine therapy. Suppression of ER activity by SERMs has proven to be a great benefit in the treatment of breast cancers and also in the prevention of breast cancer in women at high risk for the disease. The Study of Tamoxifen and Raloxifene trial comparing tamoxifen versus raloxifene effectiveness in breast cancer prevention is currently under way. To understand the balance of beneficial and undesirable effects of SERMs and to optimize their effectiveness, current investigations seek to characterize the genes activated or suppressed by these agents. Elucidation of the gene networks and cell signaling pathways under estrogen and SERM regulation and a clearer definition of the respective roles of ER-alpha and ER-beta and their coregulators in the actions of selective ER ligands, should enable the identification of new gene targets for therapeutic intervention and the development of novel drugs for the optimal treatment and prevention of breast cancer.

17beta-estradiol inhibits oxidative stress-induced apoptosis in keratinocytes by promoting Bcl-2 expression

We examined in vitro effects of 17beta-estradiol on H2O2-induced apoptosis in human keratinocytes. 17beta-estradiol prevented the H2O2-induced apoptosis. H2O2 decreased, whereas 17beta-estradiol increased Bcl-2 protein and mRNA levels in keratinocytes, and H2O2 plus 17beta-estradiol led to basal levels. Overexpression of Bcl-2 protected keratinocytes against H2O2-induced apoptosis, indicating the anti-apoptotic effect of Bcl-2. H2O2 suppressed, whereas 17beta-estradiol enhanced bcl-2 promoter activity, and H2O2 plus 17beta-estradiol led to basal activity. Cyclic adenosine monophosphate (cAMP) response element on bcl-2 promoter was responsible for the effects of 17beta-estradiol and H2O2. Bcl-2 expression was enhanced by membrane-impermeable bovine serum albumin-conjugated 17beta-estradiol, indicating the effects via membrane 17beta-estradiol-binding sites. H2O2 decreased, whereas 17beta-estradiol increased the amount of phosphorylated cAMP response element-binding protein and cAMP response element-dependent transcriptional activity, and H2O2 plus 17beta-estradiol led to basal levels. H-89, an inhibitor of cAMP-dependent protein kinase A, suppressed basal and 17beta-estradiol-induced cAMP response element-binding protein phosphorylation, cAMP response element-dependent transcriptional activity, Bcl-2 expression, and apoptosis resistance. The cAMP analog, dibutyryl cAMP, enhanced cAMP response element-binding protein phosphorylation, cAMP response element-dependent transcriptional activity, Bcl-2 expression, and apoptosis resistance. 17Beta-estradiol increased intracellular cAMP level and protein kinase A activity, whereas these were not altered by H2O2. Keratinocytes expressed mRNA for estrogen receptor beta and guanine nucleotide-binding protein-coupled receptor, GPR30. GPR30 anti-sense oligonucleotide did, but anti-sense estrogen receptor beta did not suppress 17beta-estradiol-induced cAMP signal, cAMP response element-binding protein phosphorylation, Bcl-2 expression, and apoptosis resistance. These results suggest that 17beta-estradiol may enhance Bcl-2 expression and prevent H2O2-induced apoptosis by phosphorylating cAMP response element-binding protein via cAMP/protein kinase A pathway in keratinocytes. These effects of 17beta-estradiol may be mediated via membrane GPR30.

Regulation of cell signalling cascades by steroid hormones

Steroid hormones control a wide variety of cellular functions important for cell homeostasis, proliferation, differentiation, and apoptosis. Evidence collected in the last few years indicates that this regulation is mediated by a complex interface of direct control of gene expression (so-called "transcriptional" action, driven by receptors that are localized in cell nucleus) and by regulation of cell signaling/phosphorylation cascades ("extranuclear" action, mediated by receptors that are localized in close proximity to cellular membrane). Regulation of gene expression takes place via ligand-dependent receptor binding to target gene promoters as part of the preinitiation complex, which leads to chromatin remodeling and ultimately regulates the rate of gene expression. Steroid-mediated regulation of cell signaling does not depend on gene transcription or protein biosynthesis. The molecular mechanism of this phenomenon is not fully understood. This review will focus on recent developments related to our understanding of the molecular mechanism of the extranuclear action of steroid hormones.

Neuroimmunoprotective effects of estrogen and derivatives in experimental autoimmune encephalomyelitis: Therapeutic implications for multiple sclerosis

The extensive literature and the work from our laboratory illustrate the large number of complex processes affected by estrogen that might contribute to the striking ability of 17beta-estradiol (E2) and its derivatives to inhibit clinical and histological signs of experimental autoimmune encephalomyelitis (EAE) in mice. These effects require sustained exposure to relatively low doses of exogenous hormone and offer better protection when initiated prior to induction of EAE. However, oral ethinyl estradiol (EE) and fluasterone, which lacks estrogenic side effects, could partially reverse clinical EAE when given after the onset of disease. The three main areas discussed in this review include E2-mediated inhibition of encephalitogenic T cells, inhibition of cell migration into central nervous system tissue, and neuroprotective effects that promote axon and myelin survival. E2 effects on EAE were mediated through Esr1 (alpha receptor for E2) but not Esr2 (beta receptor for E2), as were its antiinflammatory and neuroprotective effects. A novel finding is that E2 up-regulated the expression of Foxp3 and CTLA-4 that contribute to the activity of CD4+CD25+ Treg cells. The protective effects of E2 in EAE suggest its use as therapy for MS, although the risk of cardiovascular disease may complicate treatment in postmenopausal women. This risk could be minimized by using subpregnancy levels of exogenous E2 that produced synergistic effects when used in combination another immunoregulatory therapy. Alternatively, one might envision using EE or fluasterone metabolites alone or in combination therapies in both male and female MS patients.

17Beta-estradiol enhances the production of nerve growth factor in THP-1-derived macrophages or peripheral blood monocyte-derived macrophages

We examined in vitro effects of 17beta-estradiol (E2) on nerve growth factor production by macrophages derived from monocytic cell line THP-1-or periphereal blood monocytes. E2 and membrane-impermeable bovine serum albumin-conjugated E2 (E2-BSA) enhanced nerve growth factor secretion and mRNA expression in both types of macrophages E2 enhanced nerve growth factor promotor activity in THP-1-derived macrophages and two activator protein-1 binding sites on the promoter were responsible for the enhancement. E2 and E2-BSA enhanced transcriptional activity and DNA binding of activator protein-1. E2 and E2-BSA shifted the activator protein-1 composition from c-Jun homodimers to c-Fos/c-Jun heterodimers. E2 and E2-BSA transiently induced c-Fos mRNA, which was constitutively undetectable in both types of macrophages. Adenylate cyclase inhibitor SQ22536 suppressed E2-induced nerve growth factor production and c-Fos expression. E2 and E2-BSA increased intracellular cyclic adenosine monophosphate level in both types of macrophages. Antisense oligonucleotide against guanine nucleotide-binding protein-coupled receptor, GPR30 suppressed the E2-induced cyclic adenosine monophosphate signal, c-Fos expression, and nerve growth factor secretion in both types of macrophages. These results suggest that E2 may enhance nerve growth factor production by inducing c-Fos expression via cyclic adenosine monophosphate signal in macrophages. These effects may be mediated via GPR30.

Membrane restraint of estrogen receptor alpha enhances estrogen-dependent nuclear localization and genomic function

Estrogen receptor (ER) alpha localizes to both the nucleus and the plasma membrane, mediating estrogen-dependent genomic and nongenomic signaling, respectively. In some cells, ERalpha appears to be excluded from the nucleus, and it is unclear whether genomic signaling takes place. The purpose of this study was to determine whether membrane-associated ERalpha is capable of genomic signaling, or whether this pool of receptors strictly serves membrane-mediated signaling. ERalpha fused to the C-terminal cytoplasmic tail of bovine rhodopsin (Rh-ERalpha) activates ER response element-dependent transcription only in the presence of estrogen; the activity is antagonized by the estrogen antagonist ICI 182,780 and by the dominant-negative mutant of ERalpha and is unaffected by inhibitors of MAPKs and Akt signaling, indicating that this was due to direct genomic action. The activity of Rh-ERalpha containing the activating Y537S mutation was also estrogen dependent, suggesting that estrogen gated the entry of Rh-ERalpha into the nucleus. Indeed, cell fractionation studies demonstrated that Rh-ERalpha protein, in contrast to ERalpha that was nuclear at baseline, was excluded from the nucleus in the absence of hormone, and localized to the inner nuclear membrane on incubation with estrogen. These data demonstrate that membrane tethered ERalpha is capable of nuclear function and that its transcriptional activity is regulated by hormone-dependent entry into the inner nuclear membrane. Furthermore, these experiments provide evidence that under certain circumstances, membrane proteins are capable of nuclear function without detectable nucleoplasmic localization.