Nongenomic mechanisms of glucocorticoid inhibition of nicotine-induced calcium influx in PC12 cells: involvement of protein kinase C

The acute effects of hydrocortisone on cardiac electrocardiography, action potentials, intracellular calcium, and contraction: The role of protein kinase C

Hydrocortisone exerts adverse effects on various organs, including the heart. This study investigated the still unclear effects of hydrocortisone on electrophysiological and biochemical aspects of cardiac excitation-contraction coupling. In guinea pigs' hearts, hydrocortisone administration reduced the QT interval of ECG and the action potential duration (APD). In guinea pig ventricular myocytes, hydrocortisone reduced contraction and Ca²⁺ transient amplitudes. These reductions and the effects on APD were prevented by pretreatment with the protein kinase C (PKC) inhibitor staurosporine. In an overexpression system of Xenopus oocytes, hydrocortisone increased hERG K⁺ currents and reduced Kv1.5 K⁺ currents; these effects were negated by pretreatment with staurosporine. Western blot analysis revealed dose- and time-dependent changes in PKCα/βII, PKCε, and PKCγ phosphorylation by hydrocortisone in guinea pig ventricular myocytes. Therefore, hydrocortisone can acutely affect cardiac excitation-contraction coupling, including ion channel activity, APD, ECG, Ca²⁺ transients, and contraction, possibly via biochemical changes in PKC.

Nongenomic glucocorticoid effects and their mechanisms of action in vertebrates

Glucocorticoids (GC) act on multiple organ systems to regulate a variety of physiological processes in vertebrates. Due to their immunosuppressive and anti-inflammatory actions, glucocorticoids are an attractive target for pharmaceutical development. Accordingly, they are one of the most widely prescribed classes of therapeutics. Through the classical mechanism of steroid action, glucocorticoids are thought to mainly affect gene transcription, both in a stimulatory and suppressive fashion, regulating de novo protein synthesis that subsequently leads to the physiological response. However, over the past three decades multiple lines of evidence demonstrate that glucocorticoids may work through rapid, nonclassical mechanisms that do not require alterations in gene transcription or translation. This review assimilates evidence across the vertebrate taxa on the diversity of nongenomic actions of glucocorticoids and the membrane-associated cellular mechanisms that may underlie rapid glucocorticoid responses to include potential binding sites characterized to date.

Nongenomic cortisol signaling in fish

Glucocorticoids are critical regulators of the cellular processes that allow animals to cope with stressors. In teleosts, cortisol is the primary circulating glucocorticoid and this hormone mediates a suite of physiological responses, most importantly energy substrate mobilization that is essential for acute stress adaptation. Cortisol signaling has been extensively studied and the majority of work has been on the activation of the glucocorticoid receptor (GR), a ligand-bound transcription factor, and the associated downstream transcriptional and protein responses. Despite the role of this hormone in acute stress adaptation, very few studies have examined the rapid effects of this hormone on cellular function. The nongenomic corticosteroid effects, which are rapid (seconds to minutes) and independent of transcription and translation, involve changes to second-messenger pathways and effector proteins, but the primary receptors involved in this pathway activation remain elusive. In teleosts, a few studies suggested the possibility that GR located on the membrane may be initiating a rapid response based on the abrogation of this effect with RU486, a GR antagonist. However, studies have also proposed other signaling mechanisms, including a putative novel membrane receptor and changes to membrane biophysical properties as initiators of rapid signaling in response to cortisol stimulation. Emerging evidence suggests that cortisol activates multiple signaling pathways in cells to bring about rapid effects, but the underlying physiological implications on acute stress adaptation are far from clear.

MicroRNAs 29b and 181a down-regulate the expression of the norepinephrine transporter and glucocorticoid receptors in PC12 cells

MicroRNAs are short non-coding RNAs that provide global regulation of gene expression at the post-transcriptional level. Such regulation has been found to play a role in stress-induced epigenetic responses in the brain. The norepinephrine transporter (NET) and glucocorticoid receptors are closely related to the homeostatic integration and regulation after stress. Our previous studies demonstrated that NET mRNA and protein levels in rats are regulated by chronic stress and by administration of corticosterone, which is mediated through glucocorticoid receptors. Whether miRNAs are intermediaries in the regulation of these proteins remains to be elucidated. This study was undertaken to determine possible regulatory effects of miRNAs on the expression of NET and glucocorticoid receptors in the noradrenergic neuronal cell line. Using computational target prediction, we identified several candidate miRNAs potentially targeting NET and glucocorticoid receptors. Western blot results showed that over-expression of miR-181a and miR-29b significantly repressed protein levels of NET, which is accompanied by a reduced [³ H] norepinephrine uptake, and glucocorticoid receptors in PC12 cells. Luciferase reporter assays verified that both miR-181a and miR-29b bind the 3'UTR of mRNA of NET and glucocorticoid receptors. Furthermore, exposure of PC12 cells to corticosterone markedly reduced the endogenous levels of miR-29b, which was not reversed by the application of glucocorticoid receptor antagonist mifepristone. These observations indicate that miR-181a and miR-29b can function as the negative regulators of NET and glucocorticoid receptor translation in vitro. This regulatory effect may be related to stress-induced up-regulation of the noradrenergic phenotype, a phenomenon observed in stress models and depressive patients. This study demonstrated that miR-29b and miR-181a, two short non-coding RNAs that provide global regulation of gene expression, markedly repressed protein levels of norepinephrine (NE) transporter and glucocorticoid receptor (GR), as well as NE uptake by binding the 3'UTR of their mRNAs in PC12 cells. Also, exposure of cells to corticosterone significantly reduced miR-29b levels through a GR-independent way.

Corticosterone suppresses vasotocin-enhanced clasping behavior in male rough-skinned newts by novel mechanisms interfering with V1a receptor availability and receptor-mediated endocytosis

In rough-skinned newts, Taricha granulosa, exposure to an acute stressor results in the rapid release of corticosterone (CORT), which suppresses the ability of vasotocin (VT) to enhance clasping behavior. CORT also suppresses VT-induced spontaneous activity and sensory responsiveness of clasp-controlling neurons in the rostromedial reticular formation (Rf). The cellular mechanisms underlying this interaction remain unclear. We hypothesized that CORT blocks VT-enhanced clasping by interfering with V1a receptor availability and/or VT-induced endocytosis. We administered a physiologically active fluorescent VT conjugated to Oregon Green (VT-OG) to the fourth ventricle 9 min after an intraperitoneal injection of CORT (0, 10, 40 μg/0.1mL amphibian Ringers). The brains were collected 30 min post-VT-OG, fixed, and imaged with confocal microscopy. CORT diminished the number of endocytosed vesicles, percent area containing VT-OG, sum intensity of VT-OG, and the amount of VT-V1a within each vesicle; indicating that CORT was interfering with V1a receptor availability and VT-V1a receptor-mediated endocytosis. CORT actions were brain location-specific and season-dependent in a manner that is consistent with the natural and context-dependent expression of clasping behavior. Furthermore, the sensitivity of the Rf to CORT was much higher in animals during the breeding season, arguing for ethologically appropriate seasonal variation in CORT's ability to prevent VT-induced endocytosis. Our data are consistent with the time course and interaction effects of CORT and VT on clasping behavior and neurophysiology. CORT interference with VT-induced endocytosis may be a common mechanism employed by hormones across taxa for mediating rapid context- and season-specific behavioral responses.

Changes in the glucocorticoid receptor and Ca²⁺/calreticulin-dependent signalling pathway in the medial prefrontal cortex of rats with post-traumatic stress disorder

The glucocorticoid receptor (GR), calreticulin (CRT) and protein kinase C (PKC) have all been implicated in the Ca(2+)-dependent signalling pathway, which plays an important role in the plasticity of the central nervous system, learning and memory. The medial prefrontal cortex (mPFC) is known to be involved in mechanisms of learning and memory. In the present study, single prolonged stress (SPS) was used as an animal model of post-traumatic stress disorder (PTSD). The Morris water maze test was used to detect rats' ability for spatial memory and learning. A fluorescence spectrophotometer was used to measure the concentration of intracellular Ca(2+) in mPFC. Immunohistochemistry, immunofluorescence, western blot and reverse transcription polymerase chain reaction were used to explore changes in GR, CRT and PKC in mPFC of SPS rats. The concentration of Ca(2+) in mPFC was increased in the SPS rats. We found increased intensity of GR and CRT immunoreactivity and increased messenger RNA (mRNA) levels of GR, CRT and PKC in mPFC of the SPS groups, although the degree and time of increase was different among them. The protein levels of cytoplasmic GR, cytoplasmic CRT and cytoplasmic pPKC increased in mPFC of the SPS groups, whereas the protein level of nuclear GR decreased in comparison with the control group. As a conclusion, changed CRT and GR/PKC were involved in the mechanism of SPS-induced dysfunctional mPFC.

G protein-coupled receptors: extranuclear mediators for the non-genomic actions of steroids

Steroids hormones possess two distinct actions, a delayed genomic effect and a rapid non-genomic effect. Rapid steroid-triggered signaling is mediated by specific receptors localized most often to the plasma membrane. The nature of these receptors is of great interest and accumulated data suggest that G protein-coupled receptors (GPCRs) are appealing candidates. Increasing evidence regarding the interaction between steroids and specific membrane proteins, as well as the involvement of G protein and corresponding downstream signaling, have led to identification of physiologically relevant GPCRs as steroid extranuclear receptors. Examples include G protein-coupled receptor 30 (GPR30) for estrogen, membrane progestin receptor for progesterone, G protein-coupled receptor family C group 6 member A (GPRC6A) and zinc transporter member 9 (ZIP9) for androgen, and trace amine associated receptor 1 (TAAR1) for thyroid hormone. These receptor-mediated biological effects have been extended to reproductive development, cardiovascular function, neuroendocrinology and cancer pathophysiology. However, although great progress have been achieved, there are still important questions that need to be answered, including the identities of GPCRs responsible for the remaining steroids (e.g., glucocorticoid), the structural basis of steroids and GPCRs' interaction and the integration of extranuclear and nuclear signaling to the final physiological function. Here, we reviewed the several significant developments in this field and highlighted a hypothesis that attempts to explain the general interaction between steroids and GPCRs.

Specificity out of clutter: a hypothetical role of G protein-coupled receptors in the non-genomic effect of steroids

The non-genomic effect has been considered to underlie the rapid action of steroids. This signaling is initiated at the plasma membrane-level and does not directly influence gene expression. Recent studies have provided detailed information on their downstream pathways, but less is known about the nature of correlated membrane-bound receptors. Here, we propose that binding of steroids to a consensus motif, namely CRAC, of G protein-coupled receptors (GPCRs) shifts the agonist-binding state of receptors and accounts for this effect to a certain extent. The interaction between steroids and GPCRs is specific, while the identities of the GPCRs involved are not restrained, which can coordinate the high heterogeneity of this signaling and reconcile multiple discrepancies in the literature.

Glucocorticoid decreases airway tone via a nongenomic pathway

Nocturnal asthma is associated with circadian rhythms. Although glucocorticoids have contributed to therapeutic success, the underlying mechanism has not been studied thoroughly in asthma. Here, we report that cortisol, a member of glucocorticoids, ameliorate guinea pig tracheal spasm via a nongenomic effect. We set a concentration gradient of cortisol to mimic the functional circadian fluctuation. When administrated over a threshold (150 ng/ml), cortisol could synergize with the spasmolytic action of β-agonist (isoprenaline) in histamine-sensitized tracheal spirals in vitro. This permissive action was abolished by the glucocorticoid receptor antagonist, RU486, indicating that cortisol acts via its receptor. Using the RNA polymerase inhibitor, actinomycin D, we showed that this permissive action was not affected by transcription. PMA, activator of protein kinase C (PKC), could partially imitate this rapid effect, while PKC inhibition also blocked this action to some extent. It is likely that this nongenomic effect of glucocorticoid underlies the onset and susceptibility of asthma, implying novel medication target in clinical practice.

Glucocorticoid acts on a putative G protein-coupled receptor to rapidly regulate the activity of NMDA receptors in hippocampal neurons

Glucocorticoids (GCs) have been demonstrated to act through both genomic and nongenomic mechanisms. The present study demonstrated that corticosterone rapidly suppressed the activity of N-methyl-D-aspartate (NMDA) receptors in cultured hippocampal neurons. The effect was maintained with corticosterone conjugated to bovine serum albumin and blocked by inhibition of G protein activity with intracellular GDP-β-S application. Corticosterone increased GTP-bound G(s) protein and cyclic AMP (cAMP) production, activated phospholipase Cβ(3) (PLC-β(3)), and induced inositol-1,4,5-triphosphate (IP(3)) production. Blocking PLC and the downstream cascades with PLC inhibitor, IP(3) receptor antagonist, Ca(2+) chelator, and protein kinase C (PKC) inhibitors prevented the actions of corticosterone. Blocking adenylate cyclase (AC) and protein kinase A (PKA) caused a decrease in NMDA-evoked currents. Application of corticosterone partly reversed the inhibition of NMDA currents caused by blockage of AC and PKA. Intracerebroventricular administration of corticosterone significantly suppressed long-term potentiation (LTP) in the CA1 region of the hippocampus within 30 min in vivo, implicating the possibly physiological significance of rapid effects of GC on NMDA receptors. Taken together, our results indicate that GCs act on a putative G protein-coupled receptor to activate multiple signaling pathways in hippocampal neurons, and the rapid suppression of NMDA activity by GCs is dependent on PLC and downstream signaling.

Rapid stress-induced corticosterone rise in the hippocampus reverses serial memory retrieval pattern

We previously showed that an acute stress (electric footshocks) induced both a rapid plasma corticosterone rise and a reversal of serial memory retrieval pattern in a contextual serial discrimination (CSD) task. This study is aimed at determining (i) if the rapid stress effects on CSD performance are mediated by the hippocampus; (ii) if hippocampal corticosterone membrane receptor activation is involved in the rapid stress effects on CSD performance. In experiment 1, microdialysis in the dorsal hippocampus (dHPC) was used to measure the stress-induced corticosterone rise; in parallel, the effect of acute stress on CSD performance was evaluated. In addition, the functional involvement of corticosterone in the behavioral effects of stress was assessed by administering metyrapone, a corticosterone synthesis inhibitor, before stress. In experiment 2, the involvement of hippocampal corticosterone membrane receptors in the stress-induced reversal of CSD performance was studied by injecting corticosterone-bovine serum albumin (BSA) (a membrane-impermeable complex) in the dHPC in non stressed mice. Results showed that (i) the acute stress induced a rapid (15 min) and transitory (90 min) corticosterone rise into the hippocampus dHPC, and a reversal of serial memory retrieval pattern; (ii) both the endocrinal and memory stress-induced effects were blocked by metyrapone; (iii) corticosterone-BSA injection into the dHPC in non stressed mice mimicked the effects of stress on serial retrieval pattern. Overall, our study is first to show that (i) a rapid stress-induced corticosterone rise into the dHPC transitorily reverses serial memory retrieval pattern and (ii) hippocampal corticosterone membrane receptors activation is involved in the rapid effects of acute stress on serial memory retrieval.

Glucocorticoid rapidly enhances NMDA-evoked neurotoxicity by attenuating the NR2A-containing NMDA receptor-mediated ERK1/2 activation

Glucocorticoid (GC) has been shown to affect the neuronal survival/death through a genomic mechanism, but whether or not it does through a nongenomic mechanism is unknown. Using a previously identified GR-deficient primary hippocampal neuron culture, we show here that a 15-min coexposure of N-methyl-D-aspartate (NMDA) with corticosterone at a stress-induced level significantly enhances neuronal death compared to NMDA alone. This enhancing effect of GC can be mimicked by the BSA-conjugated corticosterone, which is plasma membrane impermeable and cannot be blocked by RU38486 spironolactone. Furthermore, using a calcium-imaging technique, we found that B could increase both the percentage of neurons showing a significant increment of intracellular free calcium ([Ca2+](i)) due to NMDA stimulation and the amplitude of [Ca2+](i) increment in the individual responsive cells. Interestingly, this boosting effect of GC on [Ca2+](i) increment could be blocked by the NMDA receptor subunit 2A (NR2A)-specific antagonist [(R)-[(S)-1-(4-bromo-phenyl)-ethylamino]-(2,3-dioxo-1,2,3,4-tetrahydro-quinoxalin-5-yl)-methyl]-phosphonic acid (NVP-AAM077) but not by the NMDA receptor subunit 2B (NR2B)-specific antagonist Ro25-6981. Moreover, we also found that GC can dramatically attenuate the NMDA-induced activation of ERK1/2 without affecting that of p38; and that the NMDA-induced ERK1/2 activation and its attenuation by GC both can be occluded by the NVP-AAM077 but not by Ro25-6981. Consistently, the enhancing effect of GC on NMDA neurotoxicity can also be blocked by NVP-AAM077 and the ERK1/2 inhibitor PD98059 but not by Ro25-6981 and p38 inhibitor SB203580. Indeed, the NMDA neurotoxicity itself can be blocked by Ro25-6981 or SB203580, whereas it is increased by NVP-AAM077 and PD98059. Therefore, it is probable that NMDA triggers a prodeath signaling through the NR2B-p38 MAPK pathway, and a prosurvival signaling through the NR2A-ERK1/2 MAPK pathway, whereas the latter was negatively regulated by rapid GC action. Taken together, the present data suggest a nongenomic action by GC that enhances NMDA neurotoxicity through facilitating [Ca2+](i) increment and attenuating the NR2A-ERK1/2-mediated neuroprotective signaling, implicating a novel pathway underlying the regulatory effect of GC on neuronal survival/death.

Glucocorticoid-mediated inhibition of Lck modulates the pattern of T cell receptor-induced calcium signals by down-regulating inositol 1,4,5-trisphosphate receptors

Glucocorticoids are potent immunosuppressive agents that block upstream signaling events required for T cell receptor (TCR) activation. However, the mechanism by which glucocorticoids inhibit downstream responses, such as inositol 1,4,5-trisphosphate (IP(3))-induced calcium signals, is not completely understood. Here we demonstrate that low concentrations of dexamethasone rapidly convert transient calcium elevations to oscillations after strong TCR stimulation. Dexamethasone converted the pattern of calcium signaling by inhibiting the Src family kinase Lck, which was shown to interact with and positively regulate Type I IP(3) receptor. In addition, low concentrations of dexamethasone were sufficient to inhibit calcium oscillations and interleukin-2 mRNA after weak TCR stimulation. Together, these findings indicate that by inhibiting Lck and subsequently down-regulating IP(3) receptors, glucocorticoids suppress immune responses by weakening the strength of the TCR signal.

Nongenomic glucocorticoid effects on activity-dependent potentiation of catecholamine release in chromaffin cells

Adrenal medulla chromaffin cells are neuroendocrine and modified sympathetic ganglion cells. Catecholamines released from chromaffin cells mediate the fight-or-flight response or alert reaction against dangerous conditions. Here we report that short-term treatment with glucocorticoids, released from adrenal cortex cells in response to chronic stress, inhibits activity-dependent potentiation (ADP) of catecholamine release. First, short-term treatment with dexamethasone (DEX), a synthetic glucocorticoid, reduces ADP in a concentration-dependent manner (IC50 324.2+/-54.5 nM). The inhibitory effect of DEX is not reversed by RU-486 treatment, suggesting that the rapid inhibitory effect of DEX on ADP of catecholamine release is independent of glucocorticoid receptors. Second, DEX treatment reduces the frequency of fusion between vesicles and plasma membrane without affecting calcium influx. DEX disrupts activity-induced vesicle translocation and F-actin disassembly, thereby leading to inhibition of the vesicle fusion frequency. Third, we provide evidence that DEX reduces F-actin disassembly via inhibiting phosphorylation and translocation of myristoylated alanine-rich C kinase substrate and its upstream kinase protein kinase Cepsilon. Altogether, we suggest that glucocorticoids inhibit ADP of catecholamine release by decreasing myristoylated alanine-rich C kinase substrate phosphorylation, which inhibits F-actin disassembly and vesicle translocation.

Dexamethasone-induced expression of endothelial mitogen-activated protein kinase phosphatase-1 involves activation of the transcription factors activator protein-1 and 3',5'-cyclic adenosine 5'-monophosphate response element-binding protein and the generation of reactive oxygen species

We have recently identified the MAPK phosphatase (MKP)-1 as a novel mediator of the antiinflammatory properties of glucocorticoids (dexamethasone) in the human endothelium. However, nothing is as yet known about the signaling pathways responsible for the up-regulation of MKP-1 by dexamethasone in endothelial cells. Knowledge of the molecular basis of this new alternative way of glucocorticoid action could facilitate the identification of new antiinflammatory drug targets. Thus, the aim of our study was to elucidate the underlying molecular mechanisms. Using Western blot analysis, we found that dexamethasone rapidly activates ERK, c-jun N-terminal kinase (JNK), and p38 MAPK in human umbilical vein endothelial cells. By applying the kinase inhibitors PD98059 (MAPK kinase-1) and SP600125 (JNK), ERK and JNK were shown to be crucial for the induction of MKP-1. Using EMSA and a decoy oligonucleotide approach, the transcription factors activator protein-1 (activated by ERK and JNK) and cAMP response element-binding protein (activated by ERK) were found to be involved in the up-regulation of MKP-1 by dexamethasone. Interestingly, dexamethasone induces the generation of reactive oxygen species (measured by dihydrofluorescein assay), which participate in the signaling process by triggering JNK activation. Our work elucidates a novel alternative mechanism for transducing antiinflammatory effects of glucocorticoids in the human endothelium. Thus, our study adds valuable information to the efforts made to find new antiinflammatory principles utilized by glucocorticoids. This might help to gain new therapeutic options to limit glucocorticoid side effects and to overcome resistance.

Dehydroepiandrosterone stimulates endothelial proliferation and angiogenesis through extracellular signal-regulated kinase 1/2-mediated mechanisms

Dehydroepiandrosterone (DHEA) activates a plasma membrane receptor on vascular endothelial cells and phosphorylates ERK 1/2. We hypothesize that ERK1/2-dependent vascular endothelial proliferation underlies part of the beneficial vascular effect of DHEA. DHEA (0.1-10 nm) activated ERK1/2 in bovine aortic endothelial cells (BAECs) by 15 min, causing nuclear translocation of phosphorylated ERK1/2 and phosphorylation of nuclear p90 ribosomal S6 kinase. ERK1/2 phosphorylation was dependent on plasma membrane-initiated activation of Gi/o proteins and the upstream MAPK kinase because the effect was seen with albumin-conjugated DHEA and was blocked by pertussis toxin or PD098059. A 15-min incubation of BAECs with 1 nm DHEA (or albumin-conjugated DHEA) increased endothelial proliferation by 30% at 24 h. This effect was not altered by inhibition of estrogen or androgen receptors or nitric oxide production. There was a similar effect of DHEA to increase endothelial migration. DHEA also increased the formation of primitive capillary tubes of BAECs in vitro in solubilized basement membrane. These rapid DHEA-induced effects were reversed by the inhibition of either Gi/o-proteins or ERK1/2. Additionally, DHEA enhanced angiogenesis in vivo in a chick embryo chorioallantoic membrane assay. These findings indicate that exposure to DHEA, at concentrations found in human blood, causes vascular endothelial proliferation by a plasma membrane-initiated activity that is Gi/o and ERK1/2 dependent. These data, along with previous findings, define an important vascular endothelial cell signaling pathway that is activated by DHEA and suggest that this steroid may play a role in vascular function.

Dehydroepiandrosterone protects vascular endothelial cells against apoptosis through a Galphai protein-dependent activation of phosphatidylinositol 3-kinase/Akt and regulation of antiapoptotic Bcl-2 expression

The adrenal steroid dehydroepiandrosterone (DHEA) may improve vascular function, but the mechanism is unclear. In the present study, we show that DHEA significantly increased cell viability, reduced caspase-3 activity, and protected both bovine and human vascular endothelial cells against serum deprivation-induced apoptosis. This effect was dose dependent and maximal at physiological concentrations (0.1-10 nM). DHEA stimulation of bovine aortic endothelial cells resulted in rapid and dose-dependent phosphorylation of Akt, which was blocked by LY294002, a specific inhibitor of phosphatidylinositol 3-kinase (PI3K), the upstream kinase of Akt. Accordingly, inhibition of PI3K or transfection of the cells with dominant-negative Akt ablated the antiapoptotic effect of DHEA. The induced Akt phosphorylation and subsequent cytoprotective effect of DHEA were dependent on activation of Galphai proteins, but were estrogen receptor independent, because these effects were blocked by pertussis toxin but not by the estrogen receptor inhibitor ICI182,780 or the aromatase inhibitor aminoglutethimide. Finally, DHEA enhanced antiapoptotic Bcl-2 protein expression, its promoter activity, and gene transcription attributable to the activation of the PI3K/Akt pathway. Neutralization of Bcl-2 by antibody transfection significantly decreased the antiapoptotic effect of DHEA. These findings provide the first evidence that DHEA acts as a survival factor for endothelial cells by triggering the Galphai-PI3K/Akt-Bcl-2 pathway to protect cells against apoptosis. This may represent an important mechanism underlying the vascular protective effect of DHEA.

ERK 1/2 signaling pathway is involved in nicotine-mediated neuroprotection in spinal cord neurons

Evidence indicates that agonists of neuronal nicotinic receptors (nAChRs), including nicotine, can induce neuroprotective and anti-apoptotic effects in the CNS. To study these mechanisms, the present study focused on nicotine-mediated modulation of the extracellular regulated kinase 1 and 2 (ERK1/2) pathway in cultured spinal cord neurons. Exposure to nicotine (0.1-10 microM) for as short as 1 min markedly upregulated levels of phosphorylated ERK1/2 (pERK1/2) and increased total ERK1/2 activity. Inhibition studies with mecamylamine and alpha-bungarotoxin revealed that these effects were mediated by the alpha7 nicotinic receptor. In addition, pre-exposure to U0126, a specific inhibitor of the ERK1/2 signaling, prevented nicotine-mediated anti-apoptotic effects. To indicate if treatment with nicotine also can activate ERK1/2 in vivo, a moderate spinal cord injury (SCI) was induced in rats using a weight-drop device and nicotine was injected 2 h post-trauma. Consistent with in vitro data, nicotine increased levels of pERK1/2 in this animal model of spinal cord trauma. Results of the present study indicate that the ERK1/2 pathway is involved in anti-apoptotic effects of nicotine in spinal cord neurons and may be involved in therapeutic effects of nicotine in spinal cord trauma.

Minireview: rapid glucocorticoid signaling via membrane-associated receptors

Glucocorticoids are secreted into the systemic circulation from the adrenal cortex and initiate a broad range of actions throughout the organism that regulate the function of multiple organ systems, including the liver, muscle, the immune system, the pancreas, fat tissue, and the brain. Delayed glucocorticoid effects are mediated by classical steroid mechanisms involving transcriptional regulation. Relatively rapid effects of glucocorticoids also occur that are incompatible with genomic regulation and invoke a noncanonical mode of steroid action. Studies conducted in several labs and on different species suggest that the rapid effects of glucocorticoids are mediated by the activation of one or more membrane-associated receptors. Here, we provide a brief review focused on multiple lines of evidence suggesting that rapid glucocorticoid actions are triggered by, or at least dependent on, membrane-associated G protein-coupled receptors and activation of downstream signaling cascades. We also discuss the possibility that membrane-initiated actions of glucocorticoids may provide an additional mechanism for the regulation of gene transcription.

Cultured embryonic hippocampal neurons deficient in glucocorticoid (GC) receptor: a novel model for studying nongenomic effects of GC in the neural system

Glucocorticoid (GC) acts through both genomic and nongenomic mechanisms. It affects the structure and function of the central nervous system, especially the hippocampus. Here we report an in vitro culture system that can yield embryonic hippocampal neurons deficient in the expression of GC receptor as demonstrated by immunoblotting, immunocytochemistry, and RT-PCR. Owing to this unique feature, those neuron preparations can serve as an ideal model for studying the nongenomic actions of GC on neural cells. In this study, we found that the Erk1/2, c-Jun N-terminal kinase (JNK), and p38 MAPKs were activated in these neurons by BSA-conjugated corticosterone within 15 min of treatment. This activation was not blocked by RU38486, spironolactone, or cycloheximide. Therefore, it is concluded that the activation of MAPKs observed here was due to the nongenomic action of GC. Furthermore, a 24-h incubation with corticosterone at concentrations ranged from 10(-11)-10(-5) M did not have an effect on the viability of GC receptor-deficient neurons.

Expression and activity of C/EBPbeta and delta are upregulated by dexamethasone in skeletal muscle

The influence of glucocorticoids on the expression and activity of the transcription factors CCAAT/enhancer binding protein (C/EBP)beta and delta in skeletal muscle was examined by treating rats or cultured L6 myotubes with dexamethasone. Treatment of rats with 10 mg/kg of dexamethasone resulted in increased C/EBPbeta and delta DNA binding activity in the extensor digitorum longus muscle as determined by electrophoretic mobility shift assay (EMSA) and supershift analysis. A similar response was noticed in dexamethasone-treated myotubes. In other experiments, myocytes were transfected with a plasmid containing a promoter construct consisting of multiple C/EBP binding elements upstream of a luciferase reporter gene. Treatment of these cells with dexamethasone resulted in a fourfold increase in luciferase activity, suggesting that glucocorticoids increase C/EBP-dependent gene activation in muscle cells. In addition, dexamethasone upregulated the protein and gene expression of C/EBPbeta and delta in the myotubes in a time- and dose-dependent fashion as determined by Western blotting and real-time PCR, respectively. The results suggest that glucocorticoids increase C/EBPbeta and delta activity and expression through a direct effect in skeletal muscle.

Rapid activation of JNK and p38 by glucocorticoids in primary cultured hippocampal cells

Rapid activation of JNK and p38 and their translocation to the cell nucleus by glucocorticoids, corticosterone (Cort), and bovine serum-conjugated corticosterone (Cort-BSA) were studied in primary cultured hippocampal cells by using immunoblotting and immunofluorescence confocal microscopy. The rapid activation occurred 5 min after stimulation and was maintained at plateau for as long as 2-4 hr; i.e., the response persisted for 2 hr after washing out the 15-min application of Cort-BSA. The activation occurred at a minimal concentration of 10(-9) M for Cort and 10(-8) M for Cort-BSA. GDPbetaS blocked the activation, but RU38486, a nuclear glucocorticoid receptor antagonist, could not block the activation, indicating the involvement of the membrane-delineated receptor in this reaction. The protein kinase C (PKC) inhibitor Go6976 blocked the response, whereas the protein kinase A inhibitor H89 could not, implying the involvement of PKC in the intracellular signal transduction pathway. The nongenomic nature of the responses and the transduction pathway and the significance of persistent action and biological significance are discussed.

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

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

Corticosterone enhances adrenocorticotropin-induced calcium signals in bovine adrenocortical cells

The rapid effects of steroid hormones on Ca(2+) signals have been examined in bovine adrenocortical cells. Among the steroid molecules tested, only corticosterone rapidly stimulated Ca(2+) signals upon addition of ACTH, although corticosterone alone did not induce Ca(2+) signals. Corticosterone also enhanced steroidogenesis induced by ACTH. The enhancement of ACTH-induced Ca(2+) signals was also observed with membrane-impermeable corticosterone conjugated to BSA and was not inhibited by cycloheximide. In addition, corticosterone did not enhance Ca(2+) signals induced by ATP or angiotensin II. These results suggest that corticosterone selectively stimulates ACTH-induced Ca(2+) signals in a nongenomic way by acting on a target in the plasma membrane. Furthermore, the supernatants of cells incubated with ACTH or ATP enhanced Ca(2+) signals, suggesting that steroids produced by such treatment act in an autocrine fashion. Consistent with this idea, these effects were inhibited by inhibitors of steroidogenesis (aminoglutethimide or metyrapone). These results show that steroid molecules synthesized in adrenocortical cells facilitate ACTH-induced Ca(2+) signals. Taken together, corticosterone secreted from adrenocortical cells activates ACTH-induced Ca(2+) signals and steroidogenesis by nongenomic means.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Nongenomic mechanism of glucocorticoid inhibition of bradykinin-induced calcium influx in PC12 cells: possible involvement of protein kinase C

Many stimulants, including bradykinin (BK), can induce increase in [Ca(2+)](i) in PC12 cells. Bradykinin induces an increase in [Ca(2+)](i) via intracellular Ca(2+) release and extracellular Ca(2+) influx through the transduction of G protein, but not through voltage-sensitive calcium channels. In this experiment, We analyzed how corticosterone (Cort) influences BK-induced intracellular Ca(2+) release and extracellular Ca(2+) influx, and further studied the mechanism of glucocorticoid's action. To dissociate the intracellular Ca(2+) release and extracellular Ca(2+) influx induced by BK, the Ca(2+)-free/Ca(2+)- reintroduction protocol was used. The results were as follows: (1) The Ca(2+) influx induced by BK could be rapidly inhibited by Cort, but intracellular Ca(2+) release could not be affected significantly. (2) The inhibitory effect of Cort-BSA (BSA -conjugated Cort) on Ca(2+) influx induced by BK was the same as the effect of free Cort. (3) Protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate) could mimic and PKC inhibitor Gö6976 could reverse the inhibitory effect of Cort. (4) There was no inhibitory effect of Cort on Ca(2+) influx induced by BK when pretreated with pertussis toxin. The results suggested, for the first time, that Cort might act via a putative membrane receptor and inhibit the Ca(2+) influx induced by BK through the pertussis toxin -sensitive G protein-PKC pathway.

Cloning, expression, and characterization of a membrane progestin receptor and evidence it is an intermediary in meiotic maturation of fish oocytes

The structures of membrane receptors mediating rapid, nongenomic actions of steroids have not been identified. We describe the cloning of a cDNA from spotted seatrout ovaries encoding a protein that satisfies the following seven criteria for its designation as a steroid membrane receptor: plausible structure, tissue specificity, cellular distribution, steroid binding, signal transduction, hormonal regulation, and biological relevance. For plausible structure, computer modeling predicts that the protein has seven transmembrane domains, typical of G protein-coupled receptors. The mRNA (4.0 kb) is only detected in the brain and reproductive tissues on Northern blots. Antisera only detect the protein (40 kDa) in plasma membranes of reproductive tissues. The recombinant protein produced in an Escherichia coli expression system has a high affinity (K(d) = 30 nM), saturable, displaceable, single binding site specific for progestins. Progestins alter signal transduction pathways, activating mitogen-activated protein kinase and inhibiting adenylyl cyclase, in a transfected mammalian cell line. Inhibition of adenylyl cyclase is pertussis toxin sensitive, suggesting the receptor may be coupled to an inhibitory G protein. Progestins and gonadotropin up-regulate both mRNA and protein levels in seatrout ovaries. Changes in receptor abundance in response to hormones and at various stages of oocyte development, its probable coupling to an inhibitory G protein and inhibition of progestin induction of oocyte maturation upon microinjection of antisense oligonucleotides are consistent with the identity of the receptor as an intermediary in oocyte maturation. These characteristics suggest the fish protein is a membrane progestin receptor mediating a "nonclassical" action of progestins to induce oocyte maturation in fish.

A rapid, nongenomic action of glucocorticoids in rat B103 neuroblastoma cells

We report here a new example in which glucocorticoids (GCs) acted in a rapid, nongenomic way. In rat B103 neuroblastoma cells, 5-hydroxytryptamine (5-HT) was found to evoke an immediate rise in intracellular free calcium concentration ([Ca(2+)](i)). Pre-incubation of B103 cells for 5 min with corticosterone (B) or bovine serum albumin-conjugated corticosterone (B-BSA) concentration-dependently (10(-4)-10(-8) M) inhibited the peak increments in [Ca(2+)](i). Cortisol and dexamethasone had a similar effect, while deoxycorticosterone and cholesterol were ineffective. This rapid inhibitory effect of corticosterone could be mimicked by protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and abolished completely by PKC inhibitors Ro31-8220 or GF-109203X. Neither pertussis toxin (PTX) nor nuclear GC receptor (GR) antagonist RU38486 influenced the rapid action of B. Our results suggest that GCs can modulate the 5-HT-induced Ca(2+) response in B103 cells in a membrane-initiated, nongenomic, and PKC-dependent manner.

Inhibitory effects of cortical steroids and adrenocorticotropic hormone on catecholamine secretion in guinea-pig perfused adrenal glands

1 We investigated the effects of exogenously applied steroids and endogenously released cortisol on catecholamine (CA) secretion induced by cholinergic agonists in perfused guinea-pig adrenal glands. 2 Acetylcholine (ACh) and electrical stimulation induced CA secretion, which was reversibly inhibited by cortisol. Adrenocorticotropic hormone (ACTH) increased the concentration of cortisol in the perfusion effluent and partly inhibited the secretory response to ACh. 3 Cortisol or aldosterone dose-dependently inhibited secretory responses to nicotine and muscarine. These inhibitory effects were not antagonized by mifepristone and spironolactone, respective cortisol and aldosterone receptor blockers. 4 Dexamethasone, cortisone, corticosterone, 11-deoxycortisol, 11-deoxycorticosterone, prednisolone and cholesterol inhibited nicotine-evoked CA secretion. The secretory response to muscarine was inhibited by these compounds except for dexamethasone and prednisolone. 5 Dexamethasone, cortisol and aldosterone had no effect on CA secretion induced by high KCl. 6 These results suggest that steroids affect nicotinic and muscarinic ACh receptor-mediated responses through distinct mechanisms, and that cortisol released from the adrenal cortex inhibits CA secretion from the adrenal medulla.

Comparison of the mechanisms of nongenomic actions of thyroid hormone and steroid hormones

Steroids and thyroid hormone are thought primarily to act via binding to hormone-specific nuclear receptor superfamily members. The nuclear ligand-receptor complexes then initiate transcriptional activity. Actions of steroids and iodothyronines that are nongenomic or extranuclear in mechanism have been recognized recently and new insights into such mechanisms are available. Despite their distinct structures and biologic effects, the two families of hormones have similarities in the mechanisms of their nongenomic actions. That is, both steroids and thyroid hormone appear to interact with specific cell surface G protein-coupled receptors and to activate signal transducing kinases such as those involved in the mitogen-activated protein kinase (MAPK) pathway. Much is known about the ability of certain steroids such as estrogen and mineralocorticoids to increase [Ca2+]i acutely and stimulation of the MAPK cascade by L-T4 appears to depend upon a hormone-induced increase in [Ca2+]i via phosphoinositide pathway activation. At least in the case of iodothyronines, hormone activation of the MAPK pathway modulates the cellular activities of certain cytokines and growth factors. One of the two cell surface estrogen receptors (ERs) may be an expression of the same transcript as that for nuclear ER, whereas the mineralocorticoid and progesterone-binding proteins in the plasma membrane appear to be products of genes different from those of nuclear receptors. Iodothyronine structure-activity relationships at the plasma membrane binding site for thyroid hormone suggest that the cell surface receptor for T4 that also binds 3,5,3'-triiodo-L-T3 is different from the nuclear T3 receptor (TR). There are interfaces of nongenomic and genomic mechanisms for both steroids and thyroid hormone. For example, by nongenomic mechanisms, estrogen and thyroid hormone can promote serine phosphorylation, respectively, of nuclear ER and TR. Transcriptional activity of the nuclear receptor proteins can be altered by such phosphorylation.

Possible genomic consequence of nongenomic action of glucocorticoids in neural cells

The nongenomic, rapid effects of glucocorticoid activate multiple intracellular transduction pathways. This review proposes a possible genomic consequence of the nongenomic action of steroids. The genomic actions of hormonal steroids may be twofold: classic genomic and nongenomically induced genomic.

Rapid activation of ERK1/2 mitogen-activated protein kinase by corticosterone in PC12 cells

Although the nongenomic effects of glucocorticoids have been well acknowledged, its precise intracellular signal transduction pathway remains to be elucidated. The present study using Western immunoblot and protein kinase activity assay, for the first time, showed that corticosterone (B) can induce a rapid activation of Erk1/2 mitogen-activated protein kinase (MAPK) in PC12 cells. The dose-response curve was bell shaped, with the maximal activation at 10(-9) M in 15 min. The results from immunofluorescence staining also revealed that the activated Erk1/2 MAPK was translocated from cytoplasm to nucleus of PC12 cells in 15 min. Activation of Erk1/2 MAPK by B was apparently not mediated by the classical cytosolic steroid receptors, for B-BSA can induce the phosphorylation of Erk1/2 MAPK, but the antagonist (RU38486) cannot block the phosphorylation of Erk1/2 MAPK induced by B. Phosphorylation of Erk1/2 MAPK induced by B was not affected by a tyrosine kinase inhibitor (genistein), suggesting that the pathway did not involve the tyrosine kinase activity. On the other hand, protein kinase C activator (PMA) can activate and protein kinase C inhibitor (Gö6976) can block the activation of Erk1/2 MAPK induced by B. Taken together, these data clearly demonstrated that B might act via putative membrane receptor and rapidly activate Erk1/2 MAPK through protein kinase C alpha in PC12 cells.

Corticosterone-induced rapid phosphorylation of p38 and JNK mitogen-activated protein kinases in PC12 cells

The present study showed that corticosterone (B) could induce a rapid activation of p38 and c-Jun NH(2)-terminal protein kinase (JNK) in PC12 cells. The dose-response and time-response curves were bell-shaped with maximal activation at 10(-9) M and at 15 min. RU38486 had no effect, and bovine serum albumin-coupled B could induce the activation. Genistein failed to block the phosphorylation, suggesting the pathway was not involved in tyrosine kinase activity. Phorbol 12-myristate 13-acetate could mimic, while Gö6976 could abolish the actions. These results demonstrated that B might act via a putative membrane receptor to activate p38 and JNK rapidly through a protein kinase C-dependent pathway.

Regulation of phosphate uptake in primary cultured rabbit renal proximal tubule cells by glucocorticoids: evidence for nongenomic as well as genomic mechanisms

We have investigated the nongenomic as well as the genomic effects of glucocorticoids on phosphate (Pi) uptake in primary rabbit renal proximal tubule cells (PTCs) and have defined the involved signaling pathways. In the present study, cortisol-BSA (cortisol-BSA) (>10(-9) M, 30 min) was found to inhibit Pi uptake in a time- and concentration-dependent manner. However, progesterone-BSA (P(4)-BSA), 17ss-estradiol-BSA (E(2)-BSA), testosterone-BSA (T(4)-BSA), aldosterone, P(4), E(2), and T(4) (10(-9) M, 1 h) had no effect on Pi uptake. In addition, cortisol-BSA (10(-9) M) did not affect either Na(+) uptake or alpha-methylglucopyranoside (alpha-MG) uptake. The cortisol-BSA-induced inhibition of Pi uptake was associated with a decrease in the V(max) for Pi uptake, rather than the K(m). The inhibitory effect of cortisol-BSA was not blocked either by actinomycin D (an inhibitor of transcription), cycloheximide (an inhibitor of translation), or classical glucocorticoid receptor antagonists (RU 486 or P(4)). The cortisol-BSA-induced inhibition of Pi uptake was blocked by two phospholipase C (PLC) inhibitors (neomycin or U73122), and two protein kinase C (PKC) inhibitors (staurosporine or bisindolylmaleimide I) but not by two adenylate cyclase/protein kinase A inhibitors [SQ 22536 (an adenylate cyclase inhibitor) or myristoylated protein kinase A inhibitor amide 14-22]. Furthermore, cortisol-BSA promoted the translocation of PKC from the cytosolic fraction to the membrane fraction, while having no effect on the activity of adenylate cyclase. Our observations may thus be interpreted as indicating that cortisol does indeed inhibit renal Pi uptake via a nongenomic mechanism, which involves the PLC/PKC pathway.

Upregulation of the maturation-inducing steroid membrane receptor in spotted seatrout ovaries by gonadotropin during oocyte maturation and its physiological significance

Changes in ovarian maturation-inducing steroid (MIS; 17,20 beta, 21-trihydroxy-4-pregnen-3-one [20 beta-S]) membrane receptor concentrations during the reproductive cycle were investigated in spotted seatrout (Cynoscion nebulosus) captured at their spawning grounds. Ovarian receptor concentrations increased gradually during ovarian recrudescence and subsequently increased rapidly during oocyte maturation, reaching 3.5-fold the prematuration values by the beginning of ovulation. The significant elevation of receptor concentrations by the germinal vesicle migration stage of oocyte maturation was accompanied by increases in circulating levels of gonadotropin (LH, GTH II) and MIS (20 beta-S). The regulation and physiological significance of the increase in ovarian MIS membrane receptor concentrations were investigated in a double in vitro incubation system. Incubation of fully grown, follicle-enclosed oocytes with hCG (10 IU/ml) for 6 h caused a two- to fourfold increase in oocyte and ovarian MIS receptor concentrations and the development of oocyte maturational competence (OMC; ability to complete oocyte maturation in vitro in response to exogenous 20 beta-S in a second incubation). Both upregulation of the MIS receptor and development of OMC in response to gonadotropin were blocked by coincubation with actinomycin D or cycloheximide, which are inhibitors of mRNA and protein synthesis, respectively, but not by cyanoketone, which is an inhibitor of 3 beta-hydroxysteroid dehydrogenase-dependent steroid synthesis. Incubation with a variety of steroids, including 20 beta-S, failed to increase receptor concentrations or to induce OMC, further supporting a steroid-independent mechanism of gonadotropin action. In contrast, insulin-like growth factor I (IGF-I) mimicked the actions of gonadotropin, which suggests IGF-I may be a component of the hormone signaling pathway. A close correlation was found between the relative increase in MIS receptor concentrations and the percentage of oocytes that became maturationally competent after treatment with different concentrations of gonadotropins and drugs that elevate cAMP levels. The finding that upregulation of the MIS receptor in response to gonadotropin and other treatments is invariably associated with the development of OMC indicates that these two processes are intimately related, and it suggests that the increase in MIS receptor concentrations is a critical regulatory step in the hormonal control of oocyte maturation.

Pleiotropic signaling pathways in rapid, nongenomic action of glucocorticoid

The traditional genomic theory of steroid action does not fully explain the rapid effects of hormonal steroids, and it is thought that the nongenomic actions mediated by a putative membrane receptor may provide a plausible explanation. Although there is a rich body of evidence to substantiate the rapid, nongenomic effects of steroid hormones, the signal transduction pathways involved have proved to be complex and pleiotropic. Based on previous studies on the rapid, nongenomic actions of glucocorticoid (GC) and the G-protein-protein kinase pathways involved, including our own studies on PC12, SK-N-SH, BT-325 cells, and synaptosomes, in this review we will discuss the issue of multiple signal transduction pathways involved in the rapid, nongenomic effects of GC.