Membrane Steroid Receptors Research Articles

Convergence of Multiple Mechanisms of Steroid Hormone Action

Steroid hormones modulate a wide array of physiological processes including development, metabolism, and reproduction in various species. It is generally believed that these biological effects are predominantly mediated by their binding to specific intracellular receptors resulting in conformational change, dimerization, and recruitment of coregulators for transcription-dependent genomic actions (classical mechanism). In addition, to their cognate ligands, intracellular steroid receptors can also be activated in a "ligand-independent" manner by other factors including neurotransmitters. Recent studies indicate that rapid, nonclassical steroid effects involve extranuclear steroid receptors located at the membrane, which interact with cytoplasmic kinase signaling molecules and G-proteins. The current review deals with various mechanisms that function together in an integrated manner to promote hormone-dependent actions on the central and sympathetic nervous systems.

Minireview: Recent Advances in Extranuclear Steroid Receptor Actions

The participation of extranuclear steroid receptor signaling in organ physiology and the impact for pathobiology has increasingly been demonstrated. Important functions of membrane estrogen receptors in the cardiovascular system demonstrate new mechanisms of rapid steroid signaling to gene regulation, preventing cardiovascular disease and maintaining healthy arterial function. In cancer cells, kinase signaling initiated by extranuclear estrogen, progesterone, and androgen receptors modulates transcriptional events in the nucleus, which in turn regulate proliferation, migration, and invasion. Important mediators of cross talk between cytoplasmic and nuclear steroid receptor signaling are the proline-, glutamic acid-, and leucine-rich protein-1 and paxillin proteins, both of which modulate membrane and nuclear receptor pool signaling to promote a variety of cell biological functions.

Neurotoxic properties of the anabolic androgenic steroids nandrolone and methandrostenolone in primary neuronal cultures

Anabolic-androgenic steroid (AAS) abuse is associated with multiple neurobehavioral disturbances. The sites of action and the neurobiological sequels of AAS abuse are unclear at present. We investigated whether two different AASs, nandrolone and methandrostenolone, could affect neuronal survival in culture. The endogenous androgenic steroid testosterone was used for comparison. Both testosterone and nandrolone were neurotoxic at micromolar concentrations, and their effects were prevented by blockade of androgen receptors (ARs) with flutamide. Neuronal toxicity developed only over a 48-hr exposure to the steroids. The cell-impermeable analogues testosterone-BSA and nandrolone-BSA, which preferentially target membrane-associated ARs, were also neurotoxic in a time-dependent and flutamide-sensitive manner. Testosterone-BSA and nandrolone-BSA were more potent than their parent compounds, suggesting that membrane-associated ARs were the relevant sites for the neurotoxic actions of the steroids. Unlike testosterone and nandrolone, toxicity by methandrostenolone and methandrostenolone-BSA was insensitive to flutamide, but it was prevented by the glucocorticoid receptor (GR) antagonist RU-486. Methandrostenolone-BSA was more potent than the parent compound, suggesting that its toxicity relied on the preferential activation of putative membrane-associated GRs. Consistently with the evidence that membrane-associated GRs can mediate rapid effects, a brief challenge with methandrostenolone-BSA was able to promote neuronal toxicity. Activation of putative membrane steroid receptors by nontoxic (nanomolar) concentrations of either nandrolone-BSA or methandrostenolone-BSA became sufficient to increase neuronal susceptibility to the apoptotic stimulus provided by β-amyloid (the main culprit of AD). We speculate that AAS abuse might facilitate the onset or progression of neurodegenerative diseases not usually linked to drug abuse.

Minireview: Extranuclear Steroid Receptors: Roles in Modulation of Cell Functions

Steroid receptors existing outside the nucleus are increasingly being recognized in many organs and cell types, impacting the biology of bone, the heart and blood vessels, and the central nervous system. Some controversy exists as to the nature of the receptors at the plasma membrane. However, compelling evidence has been advanced that at least some classical steroid receptors mediate steroid ligand actions originating as signaling from the cell surface. Here I review the recent findings in this evolving field emphasizing the in vivo impact of these receptor pools with a focus on estrogen receptors.

Conserved estrogen binding and signaling functions of the G protein-coupled estrogen receptor 1 (GPER) in mammals and fish

Recent studies by several research groups have shown that G protein estrogen receptor-1 (GPER) formerly known as GPR30, mediates 17β-estradiol (E2) activation of signal transduction pathways in a variety of human cancer cells and displays E2 binding typical of a membrane estrogen receptor. However, the importance of GPER as an estrogen receptor has been questioned by Otto and co-workers. Some of the pitfalls in investigating the functions of recombinant steroid membrane receptors that may explain the negative results of these investigators are discussed. The characteristics of GPER have also been investigated in a teleost fish, Atlantic croaker, where it has been shown to mediate E2 inhibition of oocyte maturation. Investigations on newly discovered homologous proteins from distantly related vertebrate groups are valuable for determining their fundamental, evolutionarily conserved functions. Therefore, the functions of croaker and human GPERs were compared. The comparisons show that croaker and human GPER have very similar estrogen binding characteristics, typical of estrogen membrane receptors, and activate the same estrogen signaling pathways via stimulatory G proteins (Gs) resulting in increased cAMP production. These results suggest that the estrogen binding and estrogen signaling functions of GPER arose early in vertebrate evolution, prior to the divergence of the teleosts from the tetrapods, more than 200 million years ago. The finding that estrogen membrane signaling through GPER has been conserved for such a long period in two distantly related vertebrate groups, mammals and fish, suggests that this is a fundamental function of GPER in vertebrates, and likely its major physiological role.

Doing Protein Kinase C: Membrane Estrogen Receptor Signaling in a Neural Circuit

Female sexual behavior has been an exquisitely valuable model for developing an understanding of mechanisms of steroid action on nerve cells (1,2). The report by Dewing et al. (3) in this issue continues this grand tradition by identifying a set of biochemical pathways that mediate intracellular signaling stemming from interactions of estradiol with membrane binding sites. The work is of note from two points of view. The first is the inclusion of these novel signaling pathways coupled to membrane estrogen receptors with the more conventional intracellular mechanisms of estrogen action on reproduction. The second is the incorporation of these findings into a functional neuronal circuit mediating reproduction. For those new to this field, the acceptance of multiple forms of the estrogen receptor and even the existence of membrane estrogen receptors seems commonplace. Lost in today’s calm sea are the stormy history of dogmatic fervor and the battles defending each fundamentalist view of how estrogen exerts its biological actions. Jensen’s reviews of this history (4,5) are very revealing, tracing the transitions from the early beliefs that estrogen had an enzymatic-like action to the discovery and defense of the existence of intracellular receptors. The acceptance of intracellular receptors for estrogen (and other steroids of course) led to other debates about activated and inactivated receptor states, including the relocation of receptors in these states to different subcellular compartments (e.g. Refs. 6 and 7). This acceptance of intracellular receptors grew largely from the development of methodological approaches to visualize the receptors themselves using labeling of specific receptor antibodies (4) or by autoradiographic means to see the incorporation of estrogen into cells (8,9). In contrast, the existence of membrane steroid receptors was not so fondly embraced. These receptors are much like the dark side of the moon in that despite the weight of decades of physiological evidence (10), the inability to see the receptors has contributed to continued skepticism (and mystery). The appeal of the scientific approach taken in the research leading to this report is that the authors have taken a set of molecular processes identified in individual hippocampal neurons (11) and applied them to solving a fundamental question within a hypothalamic circuit. The model from hippocampal neurons suggests that estradiol-17β binds to conventional estrogen receptors that have been inserted into the cell membrane. The estrogen receptors in turn couple (directly or indirectly) with metabotropic glutamate receptors (mGluR; in this case mGluR1a), triggering signaling cascades associated with the glutamate receptor. Dewing et al. (3) took a systematic approach to investigating the role of protein kinase C (PKC) mediating the effects of estradiol and glutamate. The signaling pathway studied came from a combination of previous research and from the authors’ microarray analysis of estradiol-induced proteins in the arcuate nucleus. The arcuate nucleus sends projections to the medial preoptic area, and this circuit is part of a clearly larger neural pathway known to regulate the expression of female sexual behavior (termed lordosis in rodents). The functional endpoints included lordosis itself as well as internalization of the μ-opioid receptor in medial preoptic neurons. The rationale for this latter measure comes from the authors’ previous work showing that opioid release in the medial preoptic area is a key synaptic event regulating the expression of lordosis (12). Thus, μ-opioid receptor internalization serves as a proxy for increased synaptic opioid release, which in turn is a proxy for neural activation necessary for lordosis (when lordosis itself was not measured). Indeed, the results of the study supported the proposed model. Administration of either estradiol or an mGluR1a agonist produced opioid receptor internalization. Blocking PKC antagonized the effect mediated by the stimulation of either receptor. Finally, PKC activation mimicked the effects of estradiol or mGluR1a on opioid receptor internalization or female sexual behavior. The linchpin in all of this is the demonstration that estrogen receptors co-immunoprecipitated with mGluR1 in membrane fractions from the arcuate nucleus. Two key developments in this field loom on the horizon. The first is of course the visualization of membrane estrogen receptors. Here, Jensen’s (5) lesson of alternative approaches derived from the identification of intracellular receptors will be applicable, because little progress has been made in this arena. The other development will be the molecular details of the nature of the membrane interactions between estrogen receptors and other protein partners. Like the basic research and clinical impact of such a simple discovery as a method to immunostain the estrogen receptor, a similar discovery for membrane estrogen receptors is eagerly awaited.

Meeting review: Extra-nuclear steroid receptors—Integration with multiple signaling pathways

The FASEB Summer Research Meeting entitled “Extra-Nuclear Steroid Receptors: Integration with Multiple Signaling Pathways,” was held from July 27 through August 1, 2008 in Carefree, Arizona. The meeting served as an excellent opportunity for scientists interested in both nuclear and extra-nuclear steroid signaling to exchange ideas in a relatively small, informal setting. The meeting began with a short talk by Richard Pietras (University of California at Los Angeles) on the history of extra-nuclear, or nongenomic, steroid signaling. His presentation covered work from the 1970s up to the present, and served as a wonderful introduction to the field. After Dr. Pietras, Margaret Shupnik (University of Virginia) presented the keynote lecture on the integration of membrane and nuclear receptor functionality. The talk focused on the role of membrane-initiated kinase cascades in regulating nuclear steroid receptor signaling, and introduced many important concepts that were addressed throughout the meeting.

Nongenomic steroid-triggered oocyte maturation: Of mice and frogs

Luteinizing hormone (LH) mediates many important processes in ovarian follicles, including cumulus cell expansion, changes in gap junction expression and activity, sterol and steroid production, and the release of paracrine signaling molecules. All of these functions work together to trigger oocyte maturation (meiotic progression) and subsequent ovulation. Many laboratories are interested in better understanding both the extra-oocyte follicular processes that trigger oocyte maturation, as well as the intra-oocyte molecules and signals that regulate meiosis. Multiple model systems have been used to study LH-effects in the ovary, including fish, frogs, mice, rats, pigs, and primates. Here we provide a brief summary of oocyte maturation, focusing primarily on steroid-triggered meiotic progression in frogs and mice. Furthermore, we present new studies that implicate classical steroid receptors rather than alternative non-classical membrane steroid receptors as the primary regulators of steroid-mediated oocyte maturation in both of these model systems.

Pancreatic Expression and Mitochondrial Localization of the Progestin-AdipoQ Receptor PAQR10

Steroid hormones induce changes in gene expression by binding to intracellular receptors that then translocate to the nucleus. Steroids have also been shown to rapidly modify cell function by binding to surface membrane receptors. We identified a candidate steroid membrane receptor, the progestin and adipoQ receptor (PAQR) 10, a member of the PAQR family, in a screen for genes differentially expressed in mouse pancreatic beta-cells. PAQR10 gene expression was tissue restricted compared with other PAQRs. In the mouse embryonic pancreas, PAQR10 expression mirrored development of the endocrine lineage, with PAQR10 protein expression confined to endocrine islet-duct structures in the late embryo and neonate. In the adult mouse pancreas, PAQR10 was expressed exclusively in islet cells except for its reappearance in ducts of maternal islets during pregnancy. PAQR10 has a predicted molecular mass of 29 kDa, comprises seven transmembrane domains, and, like other PAQRs, is predicted to have an intracellular N-terminus and an extracellular C-terminus. In silico analysis indicated that three members of the PAQR family, PAQRs 9, 10, and 11, have a candidate mitochondrial localization signal (MLS) at the N-terminus. We showed that PAQR10 has a functional N-terminal MLS and that the native protein localizes to mitochondria. PAQR10 is structurally related to some bacterial hemolysins, pore-forming virulence factors that target mitochondria and regulate apoptosis. We propose that PAQR10 may act at the level of the mitochondrion to regulate pancreatic endocrine cell development/survival.

Membrane-initiated estrogen signaling in hypothalamic neurons.

It is well known that many of the actions of 17beta-estradiol (E2) in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there is compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. But it is not well understood how estrogen signals via membrane receptors, and how these signals impact not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for sometime that E2 can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane delimited events. In addition, E2 can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review will consider our current knowledge of rapid membrane-initiated and intracellular signaling by E2 in the hypothalamus, the nature of receptors involved and how they contribute to homeostatic functions.

Expression of membrane progesterone receptors on human T lymphocytes and Jurkat cells and activation of G-proteins by progesterone

Although there is significant evidence for progesterone's role as an immunomodulator, nuclear progesterone receptors have not been consistently identified in immune cells. Recently, three new putative membrane progesterone receptors (mPRs), mPRalpha, mPRbeta, and mPRgamma have been described. The objective of this study was to examine whether mPRs are expressed in peripheral blood leukocytes (PBLs) in women of reproductive age, and to further characterize them in T lymphocytes and immortalized T cells (Jurkat cells). Transcripts for mPRalpha and mPRbeta but not mPRgamma, were detected by RT-PCR in PBLs, T lymphocytes, and Jurkat cells. Western blot analysis showed the presence of the mPRalpha and mPRbeta proteins on cell membranes of T lymphocytes and Jurkat cells. Expression of the mPRalpha mRNA was upregulated in the luteal phase of the menstrual cycle in cluster of differentiation (CD)8+, but not in CD4+, T lymphocytes. Radioreceptor assays revealed specific [(3)H]progesterone binding to T- and Jurkat cell membranes (K(d) 4.25 nM) characteristic of steroid membrane receptors. Progesterone activated an inhibitory G-protein (G(i)), suggesting that mPRs are coupled to G(i) in Jurkat cells. These results suggest a potential novel mechanism for progesterone's immunoregulatory function through activation of mPRs.

Extranuclear Steroid Receptors: Nature and Actions

Rapid effects of steroid hormones result from the actions of specific receptors localized most often to the plasma membrane. Fast-acting membrane-initiated steroid signaling (MISS) leads to the modification of existing proteins and cell behaviors. Rapid steroid-triggered signaling through calcium, amine release, and kinase activation also impacts the regulation of gene expression by steroids, sometimes requiring integration with nuclear steroid receptor function. In this and other ways, the integration of all steroid actions in the cell coordinates outcomes such as cell fate, proliferation, differentiation, and migration. The nature of the receptors is of intense interest, and significant data suggest that extranuclear and nuclear steroid receptor pools are the same proteins. Insights regarding the structural determinants for membrane localization and function, as well as the nature of interactions with G proteins and other signaling molecules in confined areas of the membrane, have led to a fuller understanding of how steroid receptors effect rapid actions. Increasingly, the relevance of rapid signaling for the in vivo functions of steroid hormones has been established. Examples include steroid effects on reproductive organ development and function, cardiovascular responsiveness, and cancer biology. However, although great strides have been made, much remains to be understood concerning the integration of extranuclear and nuclear receptor functions to organ biology. In this review, we highlight the significant progress that has been made in these areas.

Erythropoietin and Its Receptor in Breast Cancer: Correlation with Steroid Receptors and Outcome

Autocrine/paracrine erythropoietin (EPO) action, promoting cell survival and mediated by its receptor (EPOR) in various solid tumors, including breast carcinoma, questions about the prognostic and therapeutic interest of this system. The expression of EPO/EPOR is steroid dependent in some tissues; however, a clear relationship of EPO/EPOR and steroid receptors in breast cancer has not been established thus far. Recently, the field of steroid receptors has expanded, including rapid effects mediated by membrane-associated receptors, regulating cell survival or apoptosis. The aim of this study was to evaluate EPO/EPOR and membrane-associated steroid receptor expression in breast carcinoma, in view of their prognostic significance, compared with other established markers [estrogen receptor (ER)-progesterone receptor (PR) status and Her2 expression] and hypoxia-induced factor 1 nuclear localization in 61 breast cancer specimens followed for <or=90 months. We report that EPO-EPOR were expressed in 80% and 84% of samples, although 8% and 2% of nontumoral fields expressed EPO/EPOR too. Membrane-associated receptors for estrogen (mER), progesterone (mPR), and androgen (mAR) were expressed in 96%, 94%, and 93% of cases. Significant correlations between EPO-hypoxia-induced factor 1alpha, mER-ER, mER-EPO, mAR-EPOR, and mER-mPR-Her2 were found. Finally, EPO, EPOR, and mAR are inversely related to disease-free and overall survival. However, in view of the above correlations, we conclude that EPO/EPOR and membrane steroid receptors are not independent prognostic markers as they are closely related to other established markers. In contrast, they may represent possible new therapeutic targets.

ISOLATION AND CHARACTERIZATION OF A cDNA FROM ATLANTIC CROAKER (Micropogonias undulatus) OVARIES ENCODING A NOVEL PROTEIN WITH THE CHARACTERISTICS OF A MEMBRANE ANDROGEN RECEPTOR

There is now extensive evidence that in addition to the wellcharacterized genomic actions mediated through nuclear receptors, steroids can also exert “nonclassical” rapid actions initiated at the cellsurface. However, the identities of the receptors that mediate many of these nonclassical steroid actions remain unresolved. Both nuclear receptors and novel receptors for estrogens and progestins have been identified in the plasma membranes of a variety of target cells. In contrast, there is a relative lack of information on the identities of membrane receptors mediating nonclassical androgen actions. In the present study, a protein displaying androgen binding characteristics typical of androgen membrane receptors was partially purified from the ovaries of Atlantic croaker. The purified protein was used to produce polyclonal antibodies that were subsequently used to screen an ovarian cDNA library. Positive clones were sequenced and a novel cDNA encoding a protein of approximately 40 kDa with several putative transmembrane domains was identified. Structural analyses revealed no sequence similarities on nucleotide or amino acid levels between this novel gene and nuclear steroid receptors or steroid membrane receptors. The novel cDNA was stably transfected into the breast cancer cell line SKBR-3 to examine androgen binding characteristics, expression patterns and cellsignaling. Western blot and immunocytochemical analyses of transfected cells showed that the recombinant protein had a molecular weight of approximately 40 kDa and was located in the plasma membranes. Testosterone (T) binding to plasma membranes prepared from transfected cells was increased 303 ± 87% compared to that in untransfected cells. Saturation and Scatchard analyses indicated the presence of a single, high affinity (Kd – 12.75 ± 3.22 nM), low-capacity (Bmax – 2.8 ± 0.5 pmol/mg protein) testosterone binding site on the membranes of transfected cells. The specific testosterone binding to the membrane fraction was readily displaceable with rapid association and dissociation kinetics (t½ – 1.5 ± 0.4 and 2.2 ± 0.7 minutes respectively). Steroid binding to the recombinant protein was specific for androgens. Progesterone displayed low affinity, and 17-beta estradiol (E2) and cortisol showed no affinity for the putative receptor in competitive binding assays. Highest binding affinity was observed with T, but other androgens such as 5alpha-dihydrotestosterone (DHT), androstenedione and 11-ketoandrostenedione also displayed significant binding affinities. Interestingly, the teleost androgen, 11- ketotestosterone, showed no binding affinity for the recombinant protein. Western blot and RT-PCR analyses of croaker tissues showed expression of the putative androgen membrane receptor protein and mRNA in reproductive tissues as well as in the brain and liver, whereas there was little or no expression in heart, gills, or intestine. Moreover, the expression of the novel protein in oocyte membranes varied during the ovarian cycle and was upregulated by gonadotropin and steroid treatments in vitro, suggesting it has a physiological role in croaker reproduction. Taken together, the results are consistent with the suggestion that the cDNA and protein discovered in Atlantic croaker ovaries is a novel androgen membrane receptor, unrelated to any known steroid receptors. (poster)

A comprehensive review on the stinging nettle effect and efficacy profiles. Part II: Urticae radix

Nettle root is recommended for complaints associated with benign prostatic hyperplasia (BPH). We therefore conducted a comprehensive review of the literature to summarise the pharmacological and clinical effects of this plant material. Only a few components of the active principle have been identified and the mechanism of action is still unclear. It seems likely that sex hormone binding globulin (SHBG), aromatase, epidermal growth factor and prostate steroid membrane receptors are involved in the anti-prostatic effect, but less likely that 5alpha-reductase or androgen receptors are involved. Extract and a polysaccharide fraction were shown to exert anti-inflammatory activity. A proprietary methanolic nettle root extract and particular fractions inhibited cell proliferation. Isolated lectins (UDA) were shown to be promising immunomodulatory agents, having also anti-viral and fungistatic effects. However, despite these in vitro studies it is unclear whether the in-vitro or animal data are a surrogate for clinical effects. The clinical evidence of effectiveness for nettle root in the treatment of BPH is based on many open studies. A small number of randomised controlled studies indicate that a proprietary methanolic extract is effective in improving BPH complaints. However, the significance and magnitude of the effect remains to be established in further confirmatory studies before nettle root treatment may be accepted in the guidelines for BPH treatment. The risk for adverse events during nettle root treatment is very low, as is its toxicity. Pre-clinical safety data remain to be completed.

Membrane-Initiated Signaling of Estrogen in the Brain

It is well known that many of the actions of estrogen in the central nervous system are mediated via intracellular receptor/transcription factors that interact with steroid response elements on target genes. However, there now exists compelling evidence for membrane steroid receptors for estrogen in hypothalamic and other brain neurons. It is not well understood how estrogen signals via membrane receptors, and how these signals influence not only membrane excitability but also gene transcription in neurons. Indeed, it has been known for some time that estrogen can rapidly alter neuronal activity within seconds, indicating that some cellular effects can occur via membrane-delimited events. In addition, estrogen can affect second messenger systems including calcium mobilization and a plethora of kinases to alter cell signaling. Therefore, this review considers our current knowledge of rapid membrane-initiated and intracellular signaling by estrogen in the brain, and the nature of receptors involved and how they contribute to homeostatic functions.

P8 Potentiation of erythropoietin (EPO) action by membrane steroid receptor agonists

The present invention relates to the use of membrane steroid receptor agonists as potentiators of the action of erythropoietin (EPO). The present invention also relates to the combined use of membrane steroid receptor agonists and erythropoietin to control apoptosis, proliferation, differentiation, migration and regeneration of cells, in different organs and tissues. Compositions comprising (i) a membrane steroid receptor agonist and (ii) erythropoietin are also provided, as are kits comprising (i) a membrane steroid receptor agonist and (ii) erythropoietin.

Crosstalk between EGFR and Extranuclear Steroid Receptors

Epidermal growth factor (EGF) stimulates DNA synthesis and cytoskeletal rearrangement in human breast cancer (MCF-7) and human prostate cancer (LNCaP) cells. Both effects are inhibited by estrogen (ICI 182,780) and androgen (Casodex) antagonists. This supports the view that crosstalk exists between EGF and estradiol (ER) and androgen (AR) receptors and suggests that these receptors are directly involved in the EGF action. Our recent work shows that EGF stimulates ER phosphorylation on tyrosine and promotes the association of a complex between EGFR, AR/ER, and the kinase Src. The complex assembly triggers Src activity, epidermal growth factor receptor (EGFR) phosphorylation on tyrosine, and the EGF-dependent signaling pathway activation. In these cells, the AR/ER/Src complex is required for the EGF action, as the growth factor effects are abolished upon receptor silencing by specific SiRNAs and steroid antagonists or Src inhibition by the kinase inhibitor PP2.

Membrane steroid receptor signaling in normal and neoplastic cells

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

Progestin, estrogen and androgen G-protein coupled receptors in fish gonads

The identities of the membrane receptors mediating the majority of rapid, cell surface-initiated, nongenomic (i.e. nonclassical) steroid actions described to date are unclear. Two novel 7-transmembrane spanning proteins, representing two distinct classes of steroid membrane receptors, membrane progestin receptor alpha (mPRα) and a membrane estrogen receptor (mER), GPR30, have recently been identified in several vertebrate species. Evidence that both receptors activate G-proteins and function as G-protein coupled receptors (GPCRs) is briefly reviewed. New data on progestin actions on fish gametes suggest a widespread involvement of mPRα in oocyte maturation and sperm hyperactivity in this vertebrate group. Information on the second messenger pathways activated upon estrogen binding to a membrane estrogen receptor in croaker gonads and preliminary evidence for the presence of a GPR30-like protein in fish gonads are discussed. Finally, initial characterization of the ligand binding, G-protein activation and molecular size of a membrane androgen receptor (mAR) in croaker ovaries suggests the presence of a third unique steroid receptor in fish gonads that also may function as a GPCR.