Estrogen action via the cAMP signaling pathway: stimulation of adenylate cyclase and cAMP-regulated gene transcription

Dopamine D(1) receptor activation of adenylyl cyclase, not phospholipase C, in the nucleus accumbens promotes maternal behavior onset in rats

A body of evidence supports the idea that the mesolimbic dopamine (DA) system modulates the natural increase in responsiveness female rats show toward offspring (biological or foster) at birth. In the absence of the full hormonal changes associated with pregnancy and birth, female rats do not show immediate responsiveness toward foster offspring. Activation of the mesolimbic DA system can produce an immediate onset of maternal behavior in these females. For example, female rats that are hysterectomized and ovariectomized on day 15 of pregnancy (15HO) and presented with pups 48 hours later normally show maternal behavior after 2-3 days of pup exposure, but will show maternal behavior on day 0 of testing after microinjection of the DA D(1) receptor agonist, SKF 38393, into the nucleus accumbens (NA) at the time of pup presentation. DA D(1) receptor stimulation is known to activate cAMP intracellular signaling cascades via its stimulation of adenylyl cyclase (AC). However, some DA D(1) receptors are also linked to phospholipase C (PLC) and are capable of activating phosphatidylinositol signaling cascades. SKF 38393 stimulates both types of D(1) receptors. Here we provide evidence that the facilitatory effects of DA D(1) receptor stimulation in the NA on maternal behavior are mediated by AC-linked DA D(1) receptors. By examining the effects of intra-NA application of SKF 83822, a drug which selectively binds DA D(1)-AC receptors, or SKF 83959, a drug which selectively activates D(1)-PLC-linked receptors, we find that only SKF 83822 facilitates maternal behavior onset.

Mating changes the subcellular distribution and the functionality of estrogen receptors in the rat oviduct

BACKGROUND

Mating changes the mode of action of 17beta-estradiol (E2) to accelerate oviductal egg transport from a nongenomic to a genomic mode, although in both pathways estrogen receptors (ER) are required. This change was designated as intracellular path shifting (IPS).

METHODS

Herein, we examined the subcellular distribution of ESR1 and ESR2 (formerly known as ER-alpha and ER-beta) in oviductal epithelial cells of rats on day 1 of cycle (C1) or pregnancy (P1) using immunoelectron microscopy for ESR1 and ESR2. The effect of mating on intraoviductal ESR1 or ESR2 signaling was then explored comparing the expression of E2-target genes c-fos, brain creatine kinase (Ckb) and calbindin 9 kDa (s100g) in rats on C1 or P1 treated with selective agonists for ESR1 (PPT) or ESR2 (DPN). The effect of ER agonists on egg transport was also evaluated on C1 or P1 rats.

RESULTS

Receptor immunoreactivity was associated with the nucleus, cytoplasm and plasma membrane of the epithelial cells. Mating affected the subcellular distribution of both receptors as well as the response to E2. In C1 and P1 rats, PPT increased Ckb while both agonists increased c-fos. DPN increased Ckb and s100g only in C1 and P1 rats, respectively. PPT accelerated egg transport in both groups and DPN accelerated egg transport only in C1 rats.

CONCLUSION

Estrogen receptors present a subcellular distribution compatible with E2 genomic and nongenomic signaling in the oviductal epithelial cells of C1 and P1 although IPS occurs independently of changes in the distribution of ESR1 and ESR2 in the oviductal epithelial cells. Mating affected intraoviductal ER-signaling and induced loss of functional involvement of ESR2 on E2-induced accelerated egg transport. These findings reveal a profound influence on the ER signaling pathways exerted by mating in the oviduct.

Mitochondrial mechanisms of estrogen neuroprotection

Mitochondria have become a primary focus in our search not only for the mechanism(s) of neuronal death but also for neuroprotective drugs and therapies that can delay or prevent Alzheimer's disease and other chronic neurodegenerative conditions. This is because mitochrondria play a central role in regulating viability and death of neurons, and mitochondrial dysfunction has been shown to contribute to neuronal death seen in neurodegenerative diseases. In this article, we review the evidence for the role of mitochondria in cell death and neurodegeneration and provide evidence that estrogens have multiple effects on mitochondria that enhance or preserve mitochondrial function during pathologic circumstances such as excitotoxicity, oxidative stress, and others. As such, estrogens and novel non-hormonal analogs have come to figure prominently in our efforts to protect neurons against both acute brain injury and chronic neurodegeneration.

Effects of estrogen on AF64A-induced apoptosis in NG108-15 cells

In this study, we show that pretreatment with physiological concentrations (1-100 nM) of 17beta-estradiol decreased apoptosis induced by ethylcholine aziridinium (AF64A), a choline toxin, in the cholinergic neuronal cell line NG108-15. These protective effects were observed after short-term (30 min) pretreatment, and were blocked by treatment with an estrogen receptor antagonist and inhibitors of phosphatidylinositol 3-kinase (PI3K) and mitogen-activated protein kinase kinase (MEK). The protective effects were, however, not reversed by a protein synthesis inhibitor. Furthermore, we examined the effects of 17beta-estradiol on choline uptake in NG108-15 cells. Although choline uptake was inhibited by a selective inhibitor of choline uptake, hemicholinium-3, it was not altered by treatment with 17beta-estradiol. These results indicated that the protective effect of 17beta-estradiol on AF64A-induced apoptosis could be nongenomic, and that this effect may be due to the activation of PI3K/Akt and/or MEK/extracellular signal-regulated kinase (ERK) pathways.

Membrane-initiated estradiol signaling increases tyrosine hydroxylase promoter activity with ER alpha in PC12 cells

Tyrosine hydroxylase (TH) promoter activity is induced by 17beta-estradiol (E(2)) in PC12 cells expressing estradiol receptor-alpha (ERalpha) requiring a cAMP/calcium response element (CRE/CaRE) at -45. To examine whether membrane-initiated estradiol signaling is underlying this induction, cells co-transfected with TH reporter construct and ERalpha expression vector were exposed to membrane-impermeant estradiol conjugate (beta-estradiol-6-(O-carboxy-methyl) oxime-bovine serum albumin, E(2)BSA). TH promoter activity was elevated by E(2)BSA in dose- and time-dependent manner. E(2)BSA also elicited rapid phosphorylation of CRE binding protein (CREB) and increased CRE-driven promoter activity. Over-expression of dominant negative forms of CREB, with mutations in DNA binding or phosphorylation site, prevented TH promoter response to E(2)BSA. Pre-treatment with protein kinase A (PKA) and MEK inhibitors reduced E(2) dependent phosphorylation of CREB and ERK, and also decreased induction of TH promoter activity by E(2) or E(2)BSA. Blocking S-palmitoylation of ERalpha with C451A mutation and/or pre-treatment with 2-Bromopalmitate did not prevent but instead enhanced E(2) or E(2)BSA-elicited induction of TH promoter activity. These findings reveal, for the first time, that estradiol induction of TH gene transcription with ERalpha in PC12 cells involves membrane-initiated estradiol signaling, rapid activation of dual PKA/MEK signaling pathways, leading to CREB phosphorylation, acting at CRE/CaRE. The data demonstrate possible mechanism whereby estradiol affects catecholaminergic systems in vivo.

Effects of diverse dietary phytoestrogens on cell growth, cell cycle and apoptosis in estrogen-receptor-positive breast cancer cells

Phytoestrogens have attracted attention as being safer alternatives to hormone replacement therapy (HRT) and as chemopreventive reagents for breast cancer because dietary soy isoflavone intake has been correlated with reduction in risk. To identify safe and effective phytoestrogen candidates for HRT and breast cancer prevention, we investigated the effects of daidzein, genistein, coumestrol, resveratrol and glycitein on cell growth, cell cycle, cyclin D1 expression, apoptosis, Bcl-2/Bax expression ratio and p53-dependent or NF-kappaB-dependent transcriptional activity in MCF-7 breast cancer cells. Phytoestrogens, except for glycitein, significantly enhanced estrogen-response-element-dependent transcriptional activity up to a level similar to that of 17beta-estradiol (E(2)). E(2) increased cell growth significantly, coumestrol increased cell growth moderately, and resveratrol and glycitein reduced cell growth. Phytoestrogens, except for glycitein, stimulated the promotion of cells to G(1)/S transition in cell cycle analysis, similar to E(2). This stimulation was accompanied by transient up-regulation of cyclin D1. While genistein, resveratrol and glycitein all increased apoptosis and reduced the Bcl-2/Bax ratio, resveratrol reduced this ratio more than either genistein or glycitein. Moreover, resveratrol significantly enhanced p53-dependent transcriptional activity, but slightly reduced NF-kappaB-dependent transcriptional activity. On knockdown analysis, genistein, resveratrol and glycitein all reduced the Bcl-2/Bax ratio in the presence of apoptosis-inducing stimuli, and estrogen receptor (ER) alpha silencing had no effect on these reductions. In contrast, in the absence of apoptosis-inducing stimuli, only resveratrol reduced the ratio, and ERalpha silencing abolished this reduction. Thus, resveratrol might be the most promising candidate for HRT and chemoprevention of breast cancer due to its estrogenic activity and high antitumor activity.

Erk signaling and chromatin remodeling in MMTV promoter activation by progestins

Transcription from the mouse mammary tumor virus (MMTV) promoter can be induced by progestins. The progesterone receptor (PR) binds to a cluster of five hormone responsive elements (HREs) and activates the promoter by synergistic interactions with the ubiquitous transcription factor, nuclear factor 1 (NF1). Progesterone treatment of cells in culture leads to activation of the Src/Ras/Erk/Msk1 cascade. Selective inhibition of Erk, or its target kinase Msk1, interferes with chromatin remodeling and blocks MMTV activation. A complex of activated PR, Erk and Msk1 is recruited to promoter after 5 min of hormone treatment and phosphorylates histone H3 at serine 10. This modification promotes the displacement of HP1γ and subsequent chromatin remodeling. Progestin treatment leads to the recruitment of the BAF complex, which selectively displaces histones H2A and H2B from the nucleosome containing the HREs. The acetyltransferase PCAF is also required for induction of progesterone target genes and acetylates histone H3 at K14, an epigenetic mark, which interacts with Brg1 and Brm, anchoring the BAF complex to chromatin. In nucleosomes assembled on either MMTV or mouse rDNA promoter sequences, SWI/SNF displaces histones H2A and H2B from MMTV, but not from the rDNA nucleosome. Thus, the outcome of nucleosome remodeling by purified SWI/SNF depends on DNA sequence. The resultant H3/H4 tetramer particle is then the substrate for subsequent events in induction. Thus, initial activation of the MMTV promoter requires activation of several kinases and PCAF leading to phosphoacetylation of H3, and recruitment of BAF with subsequent removal of H2A/H2B.

Non-genomic regulation of vascular cell function and growth by estrogen

Estrogens exert rapid, non-genomic effects, which are mediated by plasma membrane-associated estrogen receptors (mER) mERalpha and mERbeta, and the intracellular transmembrane G protein-coupled estrogen receptor (GPER). Membrane-initiated responses contribute to transcriptional activation, resulting in a complex interplay of nuclear and extra-nuclear mechanisms that mediate the acute physiological responses to estrogens. Non-genomic estrogen signaling also activates a variety of intracellular estrogen signaling pathways that regulate vascular function and cell growth involving rapid but also long-term effects. This review discusses recent advances in understanding of the mechanisms of non-genomic estrogen receptor signaling in the vascular wall.

Presence of a putative steroidal allosteric site on glycoprotein hormone receptors

In a previous work we found that the insecticide 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT), inhibits the accumulation of cAMP as induced by the bovine thyroid stimulating hormone (bTSH) in cells transfected with the TSH receptor. In this work, we demonstrate that the DDT molecular analogues, diethylstilbestrol and quercetine, are more potent inhibitors of the TSH receptor activity than DDT itself. The notion that all these compounds interfere with nuclear estrogen receptors, as either agonists (DDT and diethylstilbestrol) or antagonists (quercetin), prompted us to test the ability of the steroid hormone 17-beta-estradiol to inhibit the TSH receptor activity. We found that estrogen exposure causes a modest but significant inhibition of the bTSH induced cAMP accumulation both in transfected CHO-TSH receptor and Fischer Rat Thyroid Low Serum 5% (FRTL-5) cells. When applied to CHO cells transfected with the luteinizing hormone receptor, 17-beta-estradiol proved capable of inhibiting the hCG induced cAMP accumulation at a concentration as low as 10nM, though the effect was not greater than 35%. The effect of 17-beta-estradiol was not estrogen receptors mediated, as co-transfection of the estrogen receptor alpha and beta subunits with LH receptor caused cAMP to increase above the level attained by the sole hCG stimulation, and not to decrease it as expected. These data suggest the presence of a steroidal-like allosteric binding site on glycoprotein hormone receptors.

Role of cyclic AMP response element-binding protein in insulin-like growth factor-i receptor up-regulation by sex steroids in prostate cancer cells

Insulin-like growth factor-I receptor (IGF-IR) overexpression may play a role in prostate cancer progression. We found previously that, in prostate cancer cells, IGF-IR is up-regulated by both androgens and estrogens via a nongenotropic pathway. We now show that, in prostate cancer cells, stimulation with either androgens or estrogens up-regulates IGF-IR by inducing cyclic AMP response element-binding protein (CREB) activation. Both sex steroids phosphorylated CREB at Ser(133) in a dose-dependent manner in androgen receptor (AR)-positive LNCaP cells, whereas only estrogens phosphorylated CREB in AR-negative PC3 cells. CREB phosphorylation involved c-Src-dependent extracellular signal-regulated kinase 1/2 activation, but not protein kinase A, protein kinase C, or calmodulin-dependent kinase II, and occurred also in cells transfected with AR or estrogen receptor mutants that do not localize into the nucleus. CREB silencing abrogated IGF-IR up-regulation and promoter activation. We also showed that CREB binds to IGF-IR promoter region and identified the relevant CREB-binding site at the 5'-untranslated region fragment of IGF-IR promoter. In conclusion, we describe a novel mechanism of IGF-IR up-regulation and promoter activity by CREB activation, induced by sex steroids, through a nongenotropic signaling.

Membrane estradiol signaling in the brain

While the physiology of membrane-initiated estradiol signaling in the nervous system has remained elusive, a great deal of progress has been made toward understanding the activation of cell signaling. Membrane-initiated estradiol signaling activates G proteins and their downstream cascades, but the identity of membrane receptors and the proximal signaling mechanism(s) have been more difficult to elucidate. Mounting evidence suggests that classical intracellular estrogen receptor-alpha (ERalpha) and ERbeta are trafficked to the membrane to mediate estradiol cell signaling. Moreover, an interaction of membrane ERalpha and ERbeta with metabotropic glutamate receptors has been identified that explains the pleomorphic actions of membrane-initiated estradiol signaling. This review focuses on the mechanism of actions initiated by membrane estradiol receptors and discusses the role of scaffold proteins and signaling cascades involved in the regulation of nociception, sexual receptivity and the synthesis of neuroprogesterone, an important component in the central nervous system signaling.

Membrane estrogen receptors activate metabotropic glutamate receptors to influence nervous system physiology

Until recently, the idea that estradiol could affect cellular processes independent of nuclear estrogen receptors was often dismissed as artifact. This in spite of a large number of carefully controlled studies performed both within and outside the nervous system demonstrating estrogens regulate various intracellular signaling pathways by acting at the membrane surface of cells and/or at biological rates incompatible with the time course of genomic-initiated events. The concept that estradiol can act on surface membrane receptors to regulate nervous system function is now far less controversial. However, there is evidence that there may be multiple types of estrogen receptors on the membrane surface of cells. Determining the physiological relevance of each of these receptors is currently underway. Two important membrane estrogen receptors are in fact the classical estrogen receptor-alpha (ERalpha) and estrogen receptor-beta (ERbeta) proteins, which is somewhat surprising based upon their well-established role in nuclear gene transcription. This review will focus on the mechanism by which surface-localized ERalpha and ERbeta stimulate intracellular signaling events in cells of the nervous system through activation of metabotropic glutamate receptors (mGluRs). This mechanism of estrogen receptor function also requires caveolin proteins, which provide the subcellular compartmentalization of the particular signaling components required for appropriate cell stimulation. The review will conclude with several examples of physiological processes under the apparent regulation of ER/mGluR signaling.

A single injection of 17beta-estradiol at the time of pup presentation promotes the onset of maternal behavior in pregnancy-terminated rats

Female rats that are hysterectomized and ovariectomized on day 15 of pregnancy (15HO) and presented with pups 48 h later show maternal behavior after 2 or 3 days of pup exposure. In contrast, if 15HO females are administered (sc) 20 microg/kg of estradiol benzoate (EB) on day 15 of pregnancy after HO, they show near immediate maternal behavior when pups are presented 48 h later. EB has typically been administered on day 15 because of the underlying assumption that EB exerts genomic effects which require a long duration before being expressed in changes in neuronal phenotype. In light of the more recent evidence that estradiol can generate rapid changes in cellular function, we examined whether injection of a water-soluble form of 17beta-estradiol (E(2)) can facilitate maternal behavior in pregnancy-terminated females when it is administered at the time of pup presentation rather than at the time of HO. Female rats treated with 100 microg/kg of E(2) showed a robust facilitation of maternal behavior, requiring a median of 1 day of pup exposure before showing maternal behavior, compared with 3 days in vehicle-treated rats.

Estradiol shapes auditory processing in the adult brain by regulating inhibitory transmission and plasticity-associated gene expression

Estradiol impacts a wide variety of brain processes, including sex differentiation, mood, and learning. Here we show that estradiol regulates auditory processing of acoustic signals in the vertebrate brain, more specifically in the caudomedial nidopallium (NCM), the songbird analog of the mammalian auditory association cortex. Multielectrode recordings coupled with local pharmacological manipulations in awake animals reveal that both exogenous and locally generated estradiol increase auditory-evoked activity in NCM. This enhancement in neuronal responses is mediated by suppression of local inhibitory transmission. Surprisingly, we also found that estradiol is both necessary and sufficient for the induction of multiple mitogen-activated protein kinase (MAPK)-dependent genes thought to be required for synaptic plasticity and memorization of birdsong. Specifically, we show that local blockade of estrogen receptors or aromatase activity in awake birds decrease song-induced MAPK-dependent gene expression. Infusions of estradiol in acoustically isolated birds induce transcriptional activation of these genes to levels comparable with song-stimulated animals. Our results reveal acute and rapid nongenomic functions for estradiol in central auditory physiology and suggest that such roles may be ubiquitously expressed across sensory systems.

Signaling, physiological functions and clinical relevance of the G protein-coupled estrogen receptor GPER

GPR30, now named GPER1 (G protein-coupled estrogen receptor1) or GPER here, was first identified as an orphan 7-transmembrane G protein-coupled receptor by multiple laboratories using either homology cloning or differential expression and subsequently shown to be required for estrogen-mediated signaling in certain cancer cells. The actions of estrogen are extensive in the body and are thought to be mediated predominantly by classical nuclear estrogen receptors that act as transcription factors/regulators. Nevertheless, certain aspects of estrogen function remain incompatible with the generally accepted mechanisms of classical estrogen receptor action. Many recent studies have revealed that GPER contributes to some of the actions of estrogen, including rapid signaling events and rapid transcriptional activation. With the introduction of GPER-selective ligands and GPER knockout mice, the functions of GPER are becoming more clearly defined. In many cases, there appears to be a complex interplay between the two receptor systems, suggesting that estrogen-mediated physiological responses may be mediated by either receptor or a combination of both receptor types, with important medical implications.

Targeting aromatase and estrogen signaling in human non-small cell lung cancer

Lung cancer has become increasingly common in women, and gender differences in the physiology and pathogenesis of the disease have suggested a role for estrogens. In the lung recent data have shown local production of estrogens from androgens via the action of aromatase enzyme and higher levels of estrogen in tumor tissue as compared with surrounding normal lung tissue. High levels of aromatase expression are also maintained in metastases as compared with primary tumors. Consistent with these findings, clinical studies suggest that aromatase expression may be a useful predictive biomarker for prognosis in the management of non-small cell lung cancer (NSCLC), the most common form of lung malignancy. Low levels of aromatase associate with a higher probability of long-term survival in older women with early stage NSCLC. Treatment of lung NSCLC xenografts in vivo with an aromatase inhibitor (exemestane) alone or combined with standard cisplatin chemotherapy elicits a significant reduction in tumor progression as compared to paired controls. Further, lung cancer progression is also governed by complex interactions between estrogen and growth factor signaling pathways to stimulate the growth of NSCLC as well as tumor-associated angiogenesis. We find that combination therapy with the multitargeted growth factor receptor inhibitor vandetanib and the estrogen receptor antagonist fulvestrant inhibit tumor growth more effectively than either treatment administered alone. Thus, incorporation of antiestrogen treatment strategies in standard antitumor therapies for NSCLC may contribute to improved patient outcome, an approach that deserves to be tested in clinical trials.

Sex differences in stress responses: focus on ovarian hormones

Women in the reproductive age are more vulnerable to develop affective disorders than men. This difference may attribute to anatomical differences, hormonal influences and environmental factors such as stress. However, the higher prevalence in women normalizes once menopause is established, suggesting that ovarian hormones may play an important role in the development of depression in women. Ovarian hormones such as estrogen can pass the brain-blood barrier and bind to cytoplasmatic estrogen receptor (ER)-alpha and ER-beta in different areas of the limbic system. During stress, estrogen can modulate the behavioral and neurobiological response depending on the concentrations of estrogen. In this review we present evidence for disparate effects of chronic stress on neuroplasticity and brain activity in male and female rats. Furthermore, we will demonstrate that effects of social support on coping with stress can be mimicked by social housing of rats and that this model can be used for identification of underlying neurobiological mechanisms, including behavior, phosphorylation of CREB and ERK1/2, and brain activity changes as measured with fos expression. Using cyclic administration of estrogen in ovariectomized female rats we could specifically address effects of different plasma estrogen levels and antidepressants on stress-induced neuroplasticity and activity changes. In this model we also studied effects of estrogen on recovery after chronic stress. We conclude that the female brain has a different innate strategy to handle stress than the male brain and that female animal models are necessary for studying the underlying mechanisms and options for treatment.

Membrane-localised oestrogen receptor alpha and beta influence neuronal activity through activation of metabotropic glutamate receptors

Until recently, the idea that oestradiol could affect cellular processes independent of nuclear oestrogen receptors (ERs) was controversial. This was despite the large number of carefully controlled studies performed both within and outside the nervous system demonstrating that oestrogens regulate various intracellular signalling pathways by acting at the membrane surface of cells and/or at biological rates incompatible with the time course of genomic-initiated events. At present, it is far less controversial that oestradiol acts at surface membrane receptors to regulate nervous system function. Recent studies have demonstrated that the classical intracellular ERs, ERalpha and ERbeta, are major players in mediating the actions of oestradiol on the membrane surface. This review focuses on one potential mechanism by which surface-localised ERalpha and ERbeta stimulate intracellular signalling events in cells of the nervous system. After oestradiol treatment, both ERalpha and ERbeta are capable of activating different classes of metabotropic glutamate receptors (mGluRs). Oestradiol activation of mGluRs is independent of glutamate, but requires expression of several different caveolin proteins to compartmentalise the different ERs with mGluRs into functional signalling microdomains. ER/mGluR signalling is a potential means by which oestrogens can both rapidly and for extended periods, influence a variety of intracellular signalling processes and behaviours.

Rapid pain modulation with nuclear receptor ligands

We discuss and present new data regarding the physiological and molecular mechanisms of nuclear receptor activation in pain control, with a particular emphasis on non-genomic effects of ligands at peroxisome proliferator-activated receptor (PPAR), GPR30, and classical estrogen receptors. PPARalpha agonists rapidly reduce both acute and chronic pain in a number of pain assays. These effects precede transcriptional anti-inflammatory actions, and are mediated in part by IK(ca) and BK(ca) channels on DRG neurons. In contrast to the peripheral site of action of PPARalpha ligands, the dorsal horn supports the expression of PPARgamma. Intrathecal administration of PPARgamma ligands rapidly (< or =5 min) attenuated mechanical and thermal hypersensitivity associated with nerve injury in a dose-dependent manner that could be blocked with PPARgamma antagonists. By contrast, a PPARgamma antagonist itself rapidly increased the mechanical allodynia associated with nerve injury. These data suggest that ligand-dependent, non-genomic activation of spinal PPARgamma decreases behavioral signs of inflammatory and neuropathic pain. We also report that the GPR30 is expressed on cultured sensory neurons, that activation of the receptor elicits signaling to increase calcium accumulation. This signaling may contribute to increased neuronal sensitivity as treatment with the GPR30 agonist induces hyperalgesia. Finally, application of the membrane-impermeable 17beta-E(2)-BSA rapidly (within 15 min) enhanced BK-stimulated inositol phosphate (IP) accumulation and PGE(2)-mediated cAMP accumulation in trigeminal ganglion cultures. We conclude that nuclear receptor ligands may operate through rapid, non-genomic mechanisms to modulate inflammatory and neuropathic pain.

Protein kinase C signaling in the hypothalamic arcuate nucleus regulates sexual receptivity in female rats

Rapid membrane-mediated estradiol signaling regulating sexual receptivity requires the interaction of the estrogen receptor (ER)-alpha and the metabotropic glutamate receptor 1a (mGluR1a). A cell signaling antibody microarray revealed that estradiol activated 42 proteins in the arcuate nucleus of the hypothalamus (ARH). To begin an analysis of various signaling pathways, protein kinase A and protein kinase C (PKC)-theta, whose signaling pathways have been implicated in the estradiol regulation of sexual receptivity, were examined. In the ARH sample, the increase in phospho-protein kinase A could not be confirmed by Western blotting, in either cytosolic or membrane fractions. However, the increase in phosphorylated PKCtheta seen with the pathway array was verified by Western blotting. To study whether rapid estradiol activation of PKC regulates the ARH-medial preoptic nucleus pathway regulating lordosis, mu-opioid receptor (MOR) internalization and lordosis reflex were tested. Blocking PKC in ARH with 2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]3-(1H-indol-3-yl) maleimide significantly attenuated estradiol-induced MOR internalization. Furthermore, disruption of PKC signaling within the ARH at the time of estradiol treatment significantly diminished the lordosis reflex. Moreover, blocking PKC prevented MOR internalization when the circuit was activated by the mGluR1a agonist, (RS)-3,5-dihydroxyphenylglycine. Activation of PKC with phorbol 12, 13-dibutyrate induced MOR internalization, indicating that PKC was a critical step for membrane ERalpha-initiated mGluR1a-mediated cell signaling and phorbol 12, 13-dibutyrate significantly facilitated the lordosis reflex. Together these findings indicate that rapid membrane ERalpha-mGluR1a interactions activate PKCtheta cell signaling, which regulates female sexual receptivity.

Eicosapentaenoic acid reduces ABCA1 serine phosphorylation and impairs ABCA1-dependent cholesterol efflux through cyclic AMP/protein kinase A signaling pathway in THP-1 macrophage-derived foam cells

ABCA1 is a key mediator of cholesterol efflux to apoA-I in cholesterol loaded macrophages, a first step of RCT in vivo. Unsaturated fatty acids can inhibit cholesterol efflux from macrophages by increasing degradation of ABCA1. However, the detailed mechanisms of ABCA1 regulation by unsaturated fatty acids are not fully understood. In the present study, we investigated the effects of EPA on ABCA1 expression and ABCA1-dependent cholesterol efflux and examined the role of cAMP/PKA pathway on the regulation of ABCA1 by EPA in THP-1 macrophage-derived foam cells. Results showed that EPA significantly destabilized ABCA1 protein and reduced ABCA1-dependent cholesterol efflux but had no effect on ABCA1 mRNA expression. We also revealed that EPA markedly reduced cAMP level and PKA activity and ABCA1 serine phosphorylation. PKA-specific activation by PKA agonist markedly compensated the down-regulation of ABCA1 serine phosphorylation and ABCA1-mediated cholesterol efflux by EPA, while, siRNA of PKA leaded to reduce of ABCA1 serine phosphorylation and ABCA1-mediated cholesterol efflux more significantly than EPA. However, EPA-Induced enhancement of degradation rate of ABCA1 protein did not change by treatment with PKA agonist or PKA-siRNA. These results provide evidence that EPA may have dual negative effects on ABCA1 activity by decreasing ABCA1 protein level and by reducing PKA-mediated ABCA1 serine phosphorylation in THP-1 macrophage-derived foam cells.

Therapeutic Actions of Melatonin in Cancer: Possible Mechanisms

Melatonin is a phylogenetically well-preserved molecule with diverse physiological functions. In addition to its well-known regulatory control of the sleep/wake cycle, as well as circadian rhythms generally, melatonin is involved in immunomodulation, hematopoiesis, and antioxidative processes. Recent human and animal studies have now shown that melatonin also has important oncostatic properties. Both at physiological and pharmacological doses melatonin exerts growth inhibitory effects on breast cancer cell lines. In hepatomas, through its activation of MT 1 and MT2 receptors, melatonin inhibits linoleic acid uptake, thereby preventing the formation of the mitogenic metabolite 1,3-hydroxyoctadecadienoic acid. In animal model studies, melatonin has been shown to have preventative action against nitrosodiethylamine (NDEA)-induced liver cancer. Melatonin also inhibits the growth of prostate tumors via activation of MT1 receptors thereby inducing translocation of the androgen receptor to the cytoplasm and inhibition of the effect of endogenous androgens. There is abundant evidence indicating that melatonin is involved in preventing tumor initiation, promotion, and progression. The anticarcinogenic effect of melatonin on neoplastic cells relies on its antioxidant, immunostimulating, and apoptotic properties. Melatonin's oncostatic actions include the direct augmentation of natural killer (NK) cell activity, which increases immunosurveillance, as well as the stimulation of cytokine production, for example, of interleukin (IL)-2, IL-6, IL-12, and interferon (IFN)-γ. In addition to its direct oncostatic action, melatonin protects hematopoietic precursors from the toxic effect of anticancer chemotherapeutic drugs. Melatonin secretion is impaired in patients suffering from breast cancer, endometrial cancer, or colorectal cancer. The increased incidence of breast cancer and colorectal cancer seen in nurses and other night shift workers suggests a possible link between diminished secretion of melatonin and increased exposure to light during nighttime. The physiological surge of melatonin at night is thus considered a “natural restraint” on tumor initiation, promotion, and progression.

Converse regulatory functions of estrogen receptor-alpha and -beta subtypes expressed in hypothalamic gonadotropin-releasing hormone neurons

Estradiol (E(2)) acts as a potent feedback molecule between the ovary and hypothalamic GnRH neurons, and exerts both positive and negative regulatory actions on GnRH synthesis and secretion. However, the extent to which these actions are mediated by estrogen receptors (ERs) expressed in GnRH neurons has been controversial. In this study, Single-cell RT-PCR revealed the expression of both ERalpha and ERbeta isoforms in cultured fetal and adult rat hypothalamic GnRH neurons. Both ERalpha and ERbeta or individual ERs were expressed in 94% of cultured fetal GnRH neurons. In adult female rats at diestrus, 68% of GnRH neurons expressed ERs, followed by 54% in estrus and 19% in proestrus. Expression of individual ERs was found in 24% of adult male GnRH neurons. ERalpha exerted marked G(i)-mediated inhibitory effects on spontaneous action potential (AP) firing, cAMP production, and pulsatile GnRH secretion, indicating its capacity for negative regulation of GnRH neuronal function. In contrast, increased E(2) concentration and ERbeta agonists increase the rate of AP firing, GnRH secretion, and cAMP production, consistent with ERbeta-dependent positive regulation of GnRH secretion. Consonant with the coupling of ERalpha to pertussis toxin-sensitive G(i/o) proteins, E(2) also activates G protein-activated inwardly rectifying potassium channels, decreasing membrane excitability and slowing the firing of spontaneous APs in hypothalamic GnRH neurons. These findings demonstrate that the dual actions of E(2) on GnRH neuronal membrane excitability, cAMP production, and GnRH secretion are mediated by the dose-dependent activation of ERalpha and ERbeta expressed in hypothalamic GnRH neurons.

Estradiol induces hypothalamic dendritic spines by enhancing glutamate release: a mechanism for organizational sex differences

The naturally occurring sex difference in dendritic spine number on hypothalamic neurons offers a unique opportunity to investigate mechanisms establishing synaptic patterning during perinatal sensitive periods. A major advantage of the rat as a model of sexual differentiation is that treatment of neonatal females with estradiol will permanently induce the male phenotype. During the development of other systems, exuberant innervation is followed by activity-dependent pruning necessary for elimination of spurious synapses. In contrast, we demonstrate that estradiol-induced organization in the hypothalamus involves the induction of new synapses on dendritic spines. Activation of estrogen receptors by estradiol triggers a nongenomic activation of PI3 kinase that results in enhanced glutamate release from presynaptic neurons. Subsequent activation of ionotropic glutamate receptors activates MAP kinases, thereby inducing dendritic spine formation. These results reveal a transneuronal mechanism by which estradiol acts during a sensitive period to establish a profound and lasting sex difference in hypothalamic synaptic patterning.

Mechanisms mediating oestradiol modulation of the developing brain

The brain has been known to be a sensitive target organ for the permanent organisational effects of gonadal steroids for close to 50 years. Recent advances have revealed a variety of unexpected cellular mechanisms by which steroids impact on the synaptic profile of hypothalamic nuclei critical to the control of reproduction. This review focuses on three in particular: 1) prostaglandins in the masculinisation of the preoptic area and control of male sexual behaviour; 2) GABA in the arcuate nucleus and potential control of the anterior pituitary; and 3) non-genomic activation of phosphotydolinositol 3 (PI3) kinase and glutamate in the ventromedial nucleus, which is relevant to the control of female reproductive behaviour. The importance of cell-to-cell communication, be it between neurones or between neurones and astrocytes, is highlighted as an essential principle for expanding the impact of steroids beyond those cells that express nuclear receptors.

The Cell Surface Estrogen Receptor, G Protein- Coupled Receptor 30 (GPR30), is Markedly Down Regulated During Breast Tumorigenesis

Background:

GPR30 is a cell surface estrogen receptor that has been shown to mediate a number of non-genomic rapid effects of estrogen and appear to balance the signaling of estrogen and growth factors. In addition, progestins appear to use GPR30 for their actions. Therefore, GPR30 could play a critical role in hormonal regulation of breast epithelial cell integrity. Deregulation of the events mediated by GPR30 could contribute to tumorigenesis.

Methods:

To understand the role of GPR30 in the deregulation of estrogen signaling processes during breast carcinogenesis, we have undertaken this study to investigate its expression at mRNA levels in tumor tissues and their matched normal tissues. We compared its expression at mRNA levels by RT quantitative real-time PCR relative to GAPDH in ERα”—positive (n = 54) and ERα”—negative (n = 45) breast cancer tissues to their matched normal tissues.

Results:

We report here, for the first time, that GPR30 mRNA levels were significantly down-regulated in cancer tissues in comparison with their matched normal tissues (p < 0.0001 by two sided paired t-test). The GPR30 expression levels were significantly lower in tumor tissues from patients (n = 29) who had lymph node metastasis in comparison with tumors from patients (n = 53) who were negative for lymph node metastasis (two sample t-test, p < 0.02), but no association was found with ERα, PR and other tumor characteristics.

Conclusions:

Down-regulation of GPR30 could contribute to breast tumorigenesis and lymph node metastasis.

Cellular mechanisms of estradiol-mediated masculinization of the brain

The sexual differentiation of reproductive physiology and behavior in the rodent brain is largely determined by estradiol aromatized from testicular androgens. The cellular mechanisms by which estradiol masculinizes the brain are beginning to emerge and revealing novel features of brain development that are highly region-specific. In the preoptic area, the major site controlling male sexual behavior, estradiol increases the level of the COX-2 enzyme and its product, prostaglandin E2 which promotes dendritic spine synaptogenesis. In the ventromedial nucleus of the hypothalamus, the major site controlling female reproductive behavior, estradiol promotes glutamate release from synaptic terminals, activating NMDA receptors and the MAP kinase pathway. In the arcuate nucleus, a major regulator of anterior pituitary function, estradiol increases GABA synthesis, altering the morphology of neighboring astrocytes and reducing formation of dendritic spines synapses. Glutamate, GABA and the importance of neuronal-astrocytic cross-talk are emerging as common aspects of masculinization. Advances are also being made in the mechanistic basis of female brain development, although the challenges are far greater.

Estrogen induces rapid translocation of estrogen receptor beta, but not estrogen receptor alpha, to the neuronal plasma membrane

Estrogen receptors can activate transcription in the nucleus, and activate rapid signal transduction cascades in the cytosol. Multiple reports identify estrogen receptors at the plasma membrane, while others document the dynamic responses of estrogen receptor to ligand binding. However, the function and identity of membrane estrogen receptors remain controversial. We have used confocal microscopy and cell fractionation on the murine hippocampus-derived HT22 cell line and rat primary cortical neurons transfected with estrogen receptor-green fluorescent protein constructs to address the membrane localization of these receptors. We observe translocation of estrogen receptor beta (beta) to the plasma membrane 5 min after exposure to 17beta-estradiol, whereas estrogen receptor alpha (alpha) localization remains unchanged. Membrane localization of estrogen receptor beta is transient, selective for 17beta-estradiol, and is not blocked by ICI182,780. Inhibition of the mitogen-activated protein kinase pathway does not block estrogen-mediated estrogen receptor beta membrane translocation, and in fact prolongs membrane localization. These data suggest that while both estrogen receptor alpha and estrogen receptor beta can be present at the neuronal membrane, their presence is differentially regulated.

Estrogen signaling through the transmembrane G protein-coupled receptor GPR30

Steroids play an important role in the regulation of normal physiology and the treatment of disease. Steroid receptors have classically been described as ligand-activated transcription factors mediating long-term genomic effects in hormonally regulated tissues. It is now clear that steroids also mediate rapid signaling events traditionally associated with growth factor receptors and G protein-coupled receptors. Although evidence suggests that the classical steroid receptors are capable of mediating many of these events, more recent discoveries reveal the existence of transmembrane receptors capable of responding to steroids with cellular activation. One such receptor, GPR30, is a member of the G protein-coupled receptor superfamily and mediates estrogen-dependent kinase activation as well as transcriptional responses. In this review, we provide an overview of the evidence for the cellular and physiological actions of GPR30 in estrogen-dependent processes and discuss the relationship of GPR30 with classical estrogen receptors.

Oestradiol rapidly enhances spontaneous glycinergic synaptic inhibition of hypoglossal motoneurones

Whereas oestradiol is well-known to facilitate excitatory glutamatergic synaptic transmission, its effects on fast inhibitory neurotransmission are not as well established. Possible acute modulation of the spontaneous glycinergic synaptic activity by oestradiol was investigated in voltage-clamped hypoglossal motoneurones by whole-cell patch-clamp recording in rat brainstem slices. The spontaneous glycinergic synaptic activity was continuously recorded in each neurone under control conditions, during 12-20 min of perfusion with 17beta-oestradiol and during washing. When oestradiol was diluted in ethanol, the control solution contained the same amount of ethanol. At 100 nM, oestradiol markedly increased the frequency of the total spontaneous glycinergic activity. Similar experiments were performed after blockade of action potentials by tetrodotoxin, aiming to isolate miniature glycinergic synaptic currents. Oestradiol increased the frequency of glycinergic miniatures in most slices, in some cases within less than 1 min. In some slices, oestradiol also favoured the occurrence of glycinergic miniatures of large amplitude. These effects were slowly reversible during washing. At 1 nm, oestradiol still increased the frequency of glycinergic miniatures. The results were confirmed in the absence of ethanol by using water-soluble cyclodextrin-encapsulated oestradiol. In these experiments, the control solution contained the same amount of (2-hydroxypropyl)-beta-cyclodextrin as the oestradiol-containing solution. In addition, prolonged control recordings were performed without applying oestradiol to check the stability of the glycinergic synaptic activity during prolonged whole-cell recordings. The results show, for the first time, that, within a few minutes, oestradiol can enhance the spontaneous synaptic release of a major inhibitory transmitter, glycine.

Estradiol regulates corticotropin-releasing hormone gene (crh) expression in a rapid and phasic manner that parallels estrogen receptor-alpha and -beta recruitment to a 3',5'-cyclic adenosine 5'-monophosphate regulatory region of the proximal crh promoter

In the central nervous system, CRH regulates several affective states. Dysregulation of neuronal crh expression in the paraventricular nucleus of the hypothalamus correlates with some forms of depression, and amygdalar crh expression may modulate levels of anxiety. Because estrogens modulate these states, we sought to determine 17beta-estradiol (E2) effects on crh expression. CRH mRNA levels were measured in the AR-5 amygdaloid cell line by RT-PCR analysis. They increased by 1 min of E2 treatment, suggesting that crh behaves as an immediate-early gene. After peaking at 3 min, CRH mRNA returned to basal levels and then increased by 60 min. To dissect some of the molecular mechanisms underlying these events, we measured occupancy of the crh promoter by estrogen receptors (ERs) and coactivators, using chromatin immunoprecipitation. Because this promoter does not contain palindromic estrogen response elements, we targeted the region of a cAMP regulatory element (CRE), implicated in crh regulation. The temporal pattern of the mRNA response was mimicked by recruitment of ERalpha and -beta, phospho-CRE-binding protein, coactivators steroid receptor coactivator-1 and CRE-binding protein-binding protein (CBP), and an increase in histone 3 and 4 acetylation. Lastly, ERalpha and -beta loading were temporally dissociated, peaking at 1 and 3 min, respectively. The ER peaks were associated with coactivators and acetylation patterns. ERalpha associated with phospho-CRE-binding protein, CBP, steroid receptor coactivator-1, and increased acetylated histone 3. ERbeta associated with CBP and increased acetylated histone 4. The tight temporal correlation between E2-induced CRH mRNA levels and promoter occupancy by ERs strongly suggest that E2 regulates crh expression through an ERalpha- and/or ERbeta-CRE alternate pathway.

Coregulation of estrogen receptor by ERBB4/HER4 establishes a growth-promoting autocrine signal in breast tumor cells

Although crosstalk between cell-surface and nuclear receptor signaling pathways has been implicated in the development and progression of endocrine-regulated cancers, evidence of direct coupling of these signaling pathways has remained elusive. Here we show that estrogen promotes an association between extranuclear estrogen receptor alpha (ER) and the epidermal growth factor receptor (EGFR) family member ERBB4. Ectopically expressed as well as endogenous ERBB4 interacts with and potentiates ER transactivation, indicating that the ERBB4/ER interaction is functional. Estrogen induces nuclear translocation of the proteolytic processed ERBB4 intracellular domain (4ICD) and nuclear translocation of 4ICD requires functional ligand-bound ER. The nuclear ER/4ICD complex is selectively recruited to estrogen-inducible gene promoters such as progesterone receptor (PgR) and stromal cell-derived factor 1 (SDF-1) but not to trefoil factor 1 precursor (pS2). Consistent with 4ICD-selective promoter binding, suppression of ERBB4 expression by interfering RNA shows that 4ICD coactivates ER transcription at the PgR and SDF-1 but not the pS2 promoter. Significantly, ERBB4 itself is an estrogen-inducible gene and the ERBB4 promoter harbors a consensus estrogen response element (ERE) half-site with overlapping activator protein-1 elements that bind ER and 4ICD in response to estrogen. Using a cell proliferation assay and a small interfering RNA approach, we show that ERBB4 expression is required for the growth-promoting action of estrogen in the T47D breast cancer cell line. Our results indicate that ERBB4 is a unique coregulator of ER, directly coupling extranuclear and nuclear estrogen actions in breast cancer. We propose that the contribution of an autocrine ERBB4/ER signaling pathway to tumor growth and therapeutic response should be considered when managing patients with ER-positive breast cancer.

Estrogen response element-independent estrogen receptor (ER)-alpha signaling does not rescue sexual behavior but restores normal testosterone secretion in male ERalpha knockout mice

Estrogen receptor (ER)-alpha mediates estradiol (E(2)) actions in the male gonads and brain and is critical for normal male reproductive function. In the classical pathway, ERalpha binds to estrogen response elements (EREs) to regulate gene transcription. ERalpha can also regulate gene transcription independently of EREs via protein-protein interactions with transcription factors and additionally signal via rapid, nongenomic pathways originating at the cell membrane. This study assessed the degree to which ERE-independent ERalpha signaling can rescue the disrupted masculine sexual behaviors and elevated serum testosterone (T) levels that have been shown to result from ERalpha gene deletion. We utilized male ERalpha null mice that possess a ER knock-in mutation (E207A/G208A; AA), in which the mutant ERalpha is incapable of binding to DNA and can signal only through ERE-independent pathways (ERalpha(-/AA) mice). We found that sexual behavior, including mounting, is virtually absent in ERalpha(-/-) and ERalpha(-/AA) males, suggesting that ERE-independent signaling is insufficient to maintain any degree of normal sexual behavior in the absence of ERE binding. By contrast, ERE-independent signaling in the ERalpha(-/AA) mouse is sufficient to restore serum T levels to values observed in wild-type males. These data indicate that binding of ERs to EREs mediates most if not all of E(2)'s effects on male sexual behavior, whereas ERE-independent ERalpha signaling may mediate E(2)'s inhibitory effects on T production.

Dual effects of daidzein, a soy isoflavone, on catecholamine synthesis and secretion in cultured bovine adrenal medullary cells

We recently demonstrated the occurrence and functional roles of plasma membrane estrogen receptors in cultured bovine adrenal medullary cells. Here we report the effects of daidzein, a phytoestrogen of soybeans, on catecholamine synthesis and secretion in the cells. Incubation of cells with daidzein for 20 min increased the synthesis of (14)C-catecholamines from [(14)C]tyrosine but not [(14)C]dihydroxyphenylalanine, in a concentration-dependent manner (10-1000 nm). The stimulatory effect of daidzein on (14)C-catecholamine synthesis was not inhibited by ICI182,780, a classical estrogen receptor inhibitor. Acetylcholine, a physiological secretagogue, stimulated the synthesis of (14)C-catecholamines, which was suppressed by daidzein at 1 mum. Daidzein at high concentrations (1-100 microm) suppressed catecholamine secretion induced by acetylcholine. Furthermore, daidzein (10-1000 nm) inhibited the specific binding of [(3)H]17beta-estradiol to plasma membranes isolated from bovine adrenal medulla. The present findings suggest that daidzein at low concentrations stimulates catecholamine synthesis through plasma membrane estrogen receptors but at high concentrations inhibits catecholamine synthesis and secretion induced by acetylcholine in bovine adrenal medulla. The latter effect of daidzein may be a beneficial action on the cardiovascular system.

Estrogen receptor DNA binding is not required for estrogen-induced breast cell growth

In this study, we determined whether ER DNA binding is necessary for estrogen to stimulate the growth of breast cancer cells. To investigate the requirement of ER DNA binding we expressed either wild-type or a DNA-binding mutant ERalpha in a clone of the MCF-7 breast cancer cell line that no longer expressed endogenous ERalpha. Estrogen did not activate non-genomic kinase cascades in the parental MCF-7 cells or in cells expressing ERalpha mutant. In cells expressing the ERalpha mutant, estrogen did not induce ERE-dependent gene expression but did induce AP-1- and Sp1-dependent gene expression and the cell cycle regulatory genes cyclin D1 and c-myc. However, we demonstrated that estrogen still induced cell proliferation in MCF-7 cells expressing the ERalpha mutant. These results demonstrate that ER DNA binding is not absolutely required for estrogen to induce breast cancer cell growth.

The discovery and mechanism of action of letrozole

Because estrogen contributes to the promotion and progression of breast cancer, a greater understanding of the role of estrogen in breast cancer has led to therapeutic strategies targeting estrogen synthesis, the estrogen receptor, and intracellular signaling pathways. The enzyme aromatase catalyses the final step in estrogen biosynthesis and was identified as an attractive target for selective inhibition. Modern third-generation aromatase inhibitors (AIs) effectively block the production of estrogen without exerting effects on other steroidogenic pathways. The discovery of letrozole (Femara^®) achieved the goal of discovering a highly potent and totally selective AI. Letrozole has greater potency than other AIs, including anastrozole, exemestane, formestane, and aminoglutethimide. Moreover, letrozole produces near complete inhibition of aromatase in peripheral tissues and is associated with greater suppression of estrogen than is achieved with other AIs. The potent anti-tumor effects of letrozole were demonstrated in several animal models. Studies with MCF-7Ca xenografts successfully predicted that letrozole would be clinically superior to the previous gold standard tamoxifen and also indicated that it may be more effective than other AIs. An extensive program of randomized clinical trials has demonstrated the clinical benefits of letrozole across the spectrum of hormone-responsive breast cancer in postmenopausal women.

Membrane estrogen receptor-alpha interactions with metabotropic glutamate receptor 1a modulate female sexual receptivity in rats

In rats, female sexual behavior is regulated by a well defined limbic-hypothalamic circuit that integrates sensory and hormonal information. Estradiol activation of this circuit results in mu-opioid receptor (MOR) internalization in the medial preoptic nucleus, an important step for full expression of sexual receptivity. Estradiol acts through both membrane and intracellular receptors to influence neuronal activity and behavior, yet the mechanism(s) and physiological significance of estradiol-mediated membrane responses in vivo have remained elusive. Recent in vitro evidence found that stimulation of membrane-associated estrogen receptor-alpha (ER alpha) led to activation of metabotropic glutamate receptor 1a (mGluR1a). Furthermore, mGluR1a signaling was responsible for the observed downstream effects of estradiol. Here we present data that show that ER alpha and mGluR1a directly interact to mediate a rapid estradiol-induced activation of MOR in the medial preoptic nucleus, leading to female sexual receptivity. In addition, blockade of mGluR1a in the arcuate nucleus of the hypothalamus resulted in a significant attenuation of estradiol-induced MOR internalization, leading to diminished female sexual behavior. These results link membrane-initiated estradiol actions to neural events modulating behavior, demonstrating the physiological importance of ER alpha-to-mGluR1a signaling.

Differential regulation of cyclic AMP synthesis by estrogen in MCF7 cells

The classical view of the molecular actions of estrogen is described by its interaction with the intracellular estrogen receptor (ER), the binding of hormone receptor complex to the estrogen response element (ERE) on the DNA and followed by the alterations of gene expressions. Recently it has been reported that membrane estrogen receptor (mER) exist and it is suggested to be G protein linked receptor. In this report we show that under steroid-free culture conditions supplemented with low percentage of charcoal-stripped serum, differential estrogen treatments of human breast cancer MCF7 cells induce different responses of cyclic AMP (cAMP) productions. Treating [2-(3)H]adenine-labeled MCF7 cells with 1 nM estrogen for 30 min stimulates cAMP production by measuring the ratio of [3H]cAMP:Total [3H]adenine nucleotides (ATP+ADP+cAMP), as determined by column chromatography, when compared with the control. This short-term estrogen treatment also significantly enhanced forskolin stimulated cAMP production when compared with the ratio of cAMP/Total measured in cells stimulated with forskolin alone. Pre-treating MCF7 cells with the same concentration of estrogen for 24h before the assay, on the contrary, significantly decreased the basal cAMP level and it also suppressed cAMP production stimulated with forskolin when compared with its respective value under short-term estrogen treatment. Estrogen receptor antagonist ICI 182780 abolished both the stimulatory and suppressive effect of estrogen on cAMP synthesis indicating both effects were mediated through ER. Pre-treating cells with pertussis toxin relieved the suppression of cAMP synthesis by chronic estrogen treatment. Our data suggest that estrogen exerts differential effects on the cAMP production in MCF7 cells, involving the activations Galpha(i) and Galpha(s) family of G proteins, depending on the length of time of hormone treatment.

Activation of the novel estrogen receptor G protein-coupled receptor 30 (GPR30) at the plasma membrane

G protein-coupled receptor 30 (GPR30), a seven-transmembrane receptor (7TMR), is associated with rapid estrogen-dependent, G protein signaling and specific estrogen binding. At present, the subcellular site of GPR30 action is unclear. Previous studies using antibodies and fluorochrome-labeled estradiol (E2) have failed to detect GPR30 on the cell surface, suggesting that GPR30 may function uniquely among 7TMRs as an intracellular receptor. Here, we show that detectable expression of GPR30 on the surface of transfected HEK-293 cells can be selected by fluorescence-activated cell sorting. Expression of GPR30 on the cell surface was confirmed by confocal microscopy using the lectin concanavalin A as a plasma membrane marker. Stimulation of GPR30-expressing HEK-293 cells with 17beta-E2 caused sequestration of GPR30 from the cell surface and resulted in its codistribution with clathrin and mobilization of intracellular calcium stores. Evidence that GPR30 signals from the cell surface was obtained from experiments demonstrating that the cell-impermeable E2-protein conjugates E2-BSA and E2-horseradish peroxidase promote GPR30-dependent elevation of intracellular cAMP concentrations. Subcellular fractionation studies further support the plasma membrane as a site of GPR30 action with specific [3H]17beta-E2 binding and G protein activation associated with plasma membrane but not microsomal, or other fractions, prepared from HEK-293 or SKBR3 breast cancer cells. These results suggest that GPR30, like other 7TMRs, functions as a plasma membrane receptor.

Nongenomic estrogen effects on nitric oxide synthase activity in rat adipocytes

Estrogens exert multiple genomic effects on adipose tissue through binding to nuclear estrogen receptors. However, there is evidence for additional nongenomic mechanisms whereby estrogens may exert their control on adipose tissue metabolism through rapid activation of various membrane-initiated kinase cascades. Here, we tested rapid effects of estrogens on nitric oxide production in white adipose tissue using 17-beta estradiol (E2) and its membrane impermeant albumin conjugated form (17-beta estradiol hemisuccinate BSA, E2-BSA). We found that both E2 and E2-BSA stimulate nitric oxide synthase (NOS) activity in adipocytes. These effects were abolished by 1) ICI 182-780, a selective estrogen receptor antagonist; 2) wortmannin, an inhibitor of phosphatidylinositol 3-kinase; and 3) N-[2-(p-bromocinnamylamino) ethyl]-5-isoquinolinesulfonamide (H-89) an inhibitor of protein kinase A. In contrast to NOS activation by E2, E2-BSA-induced NOS activity was abolished by UO126, an inhibitor of MAPK kinase/ERK (p42/p44 MAPKs). Immunoblotting studies have shown that both estrogens phosphorylate endothelial NOS (NOS III) on Ser(1179), an effect that is prevented by wortmannin and H89, suggesting that NOS III is the target for estrogen-induced NOS activity. Furthermore, only the E2-BSA-induced NOS III phosphorylation on Ser(1179) was totally abolished by UO126. These results indicate that the signaling cascades involved in adipocyte NOS stimulation by estrogens are different depending on whether estrogens are free or conjugated to albumin and therefore underline the importance of estrogen receptor locations in the nongenomic actions of estrogens in these cells.

Overview of nomenclature of nuclear receptors

Nuclear receptor pharmacology has, to a certain extent, led the way, compared with other receptor systems, in the appreciation that ligands may exert very diverse pharmacology, based on their individual chemical structure and the allosteric changes induced in the receptor/accessory protein complex. This can lead to very selective pharmacological effects, which may not necessarily be predicted from the experience with other agonists/partial agonists/antagonists. If this is the case, then drug discovery may be back to drug-specific pharmacology (where each drug may have an original profile), rather than specific-drug pharmacology (where agents specific for a receptor have a distinct profile). As functional selectivity is indeed a crucial mechanism to be considered when going through the drug discovery development process, then initial screens using reconstituted systems may not show the appropriate pharmacology, simply because the required stoichiometry of corepressors and coactivators may not be present to select the best compounds; therefore, multiple effector systems are necessary to screen for differential activation, and, even then, screening with in vivo pathophysiological models may ultimately be required for the selection process-a massive but necessary task for pharmacologists. Thus, the characterization of nuclear receptors and their associated proteins and the ligands that interact with them will remain a challenge to pharmacologists.

Effect of estrogens on skin aging and the potential role of SERMs

In humans, structural and functional changes attributable to aging are more visibly evident in the skin than in any other organ. Estrogens have significant effects on skin physiology and modulate epidermal keratinocytes, dermal fibroblasts and melanocytes, in addition to skin appendages including the hair follicle and the sebaceous gland. Importantly, skin aging can be significantly delayed by the administration of estrogen. This paper reviews the effects of estrogens on skin and the mechanisms by which estrogens can alleviate the changes due to aging that occur in human skin. The relevance of estrogen replacement therapy (HRT) in postmenopausal women and the potential value of selective estrogen receptor modulators (SERMs) as a therapy for diminishing skin aging are also highlighted.

Retinoic acid inhibits HIV-1-induced podocyte proliferation through the cAMP pathway

HIV-associated nephropathy is characterized by renal podocyte proliferation and dedifferentiation. This study found that all-trans retinoic acid (atRA) reverses the effects of HIV-1 infection in podocytes. Treatment with atRA reduced cell proliferation rate by causing G1 arrest and restored the expression of the differentiation markers (synaptopodin, nephrin, podocin, and WT-1) in HIV-1-infected podocytes. It is interesting that both atRA and 9-cis RA increased intracellular cAMP levels in podocytes. Podocytes expressed most isoforms of retinoic acid receptors (RAR) and retinoid X receptors (RXR) with the exception of RXRgamma. RARalpha antagonists blocked atRA-induced cAMP production and its antiproliferative and prodifferentiation effects on podocytes, suggesting that RARalpha is required. For determination of the effect of increased intracellular cAMP on HIV-infected podocytes, cells were stimulated with either forskolin or 8-bromo-cAMP. Both compounds inhibited cell proliferation significantly and restored synaptopodin expression in HIV-infected podocytes. The effects of atRA were abolished by Rp-cAMP, an inhibitor of the cAMP/protein kinase A pathway and were enhanced by rolipram, an inhibitor of phosphodiesterase 4, suggesting that the antiproliferative and prodifferentiation effects of atRA on HIV-infected podocytes are cAMP dependent. Furthermore, both atRA and forskolin suppressed HIV-induced mitogen-activated protein kinase 1 and 2 and Stat3 phosphorylation. In vivo, atRA reduced proteinuria, cell proliferation, and glomerulosclerosis in HIV-1-transgenic mice. These findings suggest that atRA reverses the abnormal phenotype in HIV-1-infected podocytes by stimulating RARalpha-mediated intracellular cAMP production. These results demonstrate the mechanism by which atRA reverses the proliferation of podocytes that is induced by HIV-1.