Corticotrope responsiveness to glucocorticoids is modulated via rapid CRF-mediated induction of the proto-oncogene c-fos

Incensole acetate reduces depressive-like behavior and modulates hippocampal BDNF and CRF expression of submissive animals

Incensole acetate (IA), a constituent of Boswellia resin ('frankincense'), was previously demonstrated to exhibit an antidepressive-like effect in the Forced Swim Test (FST) in mice following single dose administration (50 mg/kg). Here, we show that acute administration of considerably lower dose (10 mg/kg) IA to selectively bred mice, showing prominent submissive behavior, exerted significant antidepressant-like effects in the FST. Furthermore, chronic administration of 1 or 5 mg/kg per day of IA for three consecutive weeks dose- and time-dependently reduced the submissiveness of the mice in the Dominant-Submissive Relationship test, developed to screen the chronic effect of antidepressants. This behavioral effect was concomitant to reduced serum corticosterone levels, dose-dependent down-regulation of corticotropin releasing factor and up-regulation of brain derived neurotrophic factor transcripts IV and VI expression in the hippocampus. These data suggest that IA modulates the hypothalamic-pituitary-adrenal (HPA) axis and influences hippocampal gene expression, leading to beneficial behavioral effects supporting its potential as a novel treatment of depressive-like disorders.

Moderate hypercapnia-induced anesthetic effects and endogenous opioids

The purpose of this report is to explore the mechanisms of hypercapnia-induced antinociception. We carried out three experiments, the first to confirm whether moderate hypercapnia induces anesthetic effects, the second to determine whether naloxone reverses the anesthetic effects, and the third to evaluate whether beta-endorphin is related to the anesthetic effects. In a pre-test, we determined the optimal CO(2) concentration in a chamber which would cause moderate hypercapnia in rats. Eighteen rats were divided into control, hypercapnia, and hypercapnia plus naloxone groups in experiment 1. The naloxone group rats were injected with naloxone (10 mg/kg) intraperitoneally before gas inhalation. After 60 min gas inhalation, 10% formalin was injected into the left rear paw of all rats, and nociceptive behaviors were observed for 1 h. In experiment 2, 11 rats were divided into control and hypercapnia groups. The brain was removed and fixed under pentobarbital anesthesia. Sections were immunostained for c-Fos and beta-endorphin (ACTH) with the ABC method. All neurons double-labeled for c-Fos and beta-endorphin (ACTH) in the arcuate nucleus were counted by blinded investigators. Moderate hypercapnia (PaCO(2) 83+/-7 mmHg) reduced nociceptive behavior in the formalin test and naloxone pre-treatment attenuated this phenomenon. However, beta-endorphin-producing neurons were not activated by CO(2) inhalation. Endogenous opioids are related to moderate, hypercapnia-induced anesthetic effects, but, beta-endorphin-producing neurons in the hypothalamus were not activated by the CO(2) inhalation stress.

Estrogen and tamoxifen differentially regulate beta-endorphin and cFos expression and neuronal colocalization in the arcuate nucleus of the rat

Estrogen regulates hypothalamic gene expression, synthesis and release of the endogenous opioid peptide beta-endorphin (betaEND), although a consensus estrogen response element sequence has not been identified in the rat proopiomelanocortin (POMC) gene. POMC gene expression is also regulated by the activation of AP-1 promoter elements, which are known to be estrogen sensitive. The present studies examine whether estrogen modulates the hypothalamic POMC system through a non-classical mechanism involving AP-1 binding proteins such as cFos. Immunohistochemical double-labeling for betaEND and cFos was used and immunoreactive (-ir) populations were quantified in the arcuate nucleus and periarcuate area across time using unbiased stereological methods. Ovariectomized rats were injected with 50 microg estradiol (E2), 500 microg tamoxifen citrate (TAM) or both (E2+TAM) and were perfused 1, 2, 4 or 48 h later. E2 rapidly increased numbers of cFos-ir, betaEND-ir and doubly-labeled cells after 4 h, and the number of betaEND-ir cells remained high 48 h later, suggesting that the stimulatory effects of cFos on POMC in the hypothalamus persist after the cFos signal decays. Treatment with TAM alone did not affect the numbers of immunoreactive cells, although E2+TAM blocked the E2-mediated induction in all immunoreactive populations. Similar effects were seen at the transcriptional level. E2 increased hypothalamic POMC mRNA after 4 h, while TAM treatment or coadministration of E2+TAM did not significantly change the levels of POMC mRNA. Cellular colocalization of betaEND-ir and cFos-ir supports a possible intracellular co-regulation of these peptides by an estrogen-dependent mechanism within a subset of hypothalamic neurons. It does not, however, appear that E2 acts directly through an AP-1 site within the POMC gene.

Rapid regulation of adrenomedullin in metabolically compromised vascular smooth muscle cells

OBJECTIVE

The prepro-adrenomedullin gene encodes the biologically active peptide adrenomedullin, which acts as a potent vasodilator as well as a modulator of vascular smooth muscle cell growth. We investigated the question of whether adrenomedullin is regulated in response to metabolic perturbations in vascular smooth muscle.

MATERIALS AND METHODS

Acute inhibition of glycolysis, leading to partial depletion of cellular ATP, was produced in cultured rat aortic vascular smooth muscle cells by replacing glucose with 2-deoxyglucose. Solution hybridization/RNase protection analysis was used to quantitate changes in expression of the prepro-adreno-medullin messenger RNA and a specific radioimmunoassay was used to assess levels of secreted adrenomedullin.

RESULTS

Acute incubation of rat aortic vascular smooth muscle cells with 2-deoxyglucose caused a rapid and sustained induction of low basal levels of adrenomedullin messenger RNA, which reached twice the control levels by 1 h and four times control levels by 6 h. The induction of adrenomedullin messenger RNA expression was dependent upon de-novo gene transcription and was reversed by the re-introduction of glucose. Despite the sustained increase in adrenomedullin messenger RNA, secretion of immunoreactive-adrenomedullin from vascular smooth muscle cells was reduced by as much as 75% and paralleled the inhibition of radiolabeled amino acid incorporation into protein during glycolytic inhibition; both parameters recovered towards control levels following re-introduction of glucose.

CONCLUSIONS

The rapid and reversible activation of the adrenomedullin gene and inhibition of adrenomedullin peptide release in response to metabolic inhibition suggest that adrenomedullin represents a novel localized mechanism that may modulate regional blood flow and vascular smooth muscle cell proliferation in response to perturbations of normal metabolism.

Corticotropin releasing hormone, receptor regulation and the stress response

Corticotropin releasing hormone (CRH) coordinates behavioral, autonomic and hormonal responses to stress, including activation of the hypothalamic-pituitary-adrenal (HPA) axis with stimulation of adrenocorticotropin (ACTH) and glucocorticoids. Differential changes of expression of CRH and vasopressin(VP) in the parvicellular hypothalamic paraventricular nucleus (PVN), as well as regulation of CRH and VP receptors, are critical for the responsiveness of the HPA axis during stress. Pituitary CRH receptor (CRH-R)expression and content is controlled by the coordinated action of CRH, VP and glucocorticoids. Marked changes in hypothalamic and pituitary CRH-R expression support a key regulatory role for CRH in the HPA axis and the integrated stress response.

Regulation of c-fos expression by EGF and GnRH in specific anterior pituitary cells from proestrous female rats

C-fos is an early expression oncogene that can be stimulated by a variety of regulators. It is expressed by subsets of all pituitary cells, with increased expression seen in proestrous rats. However, in freshly dispersed pituitary cells studied during different stages of the cycle, there is limited expression of fos by luteinizing hormone (LH) cells and little basal expression by cells with follicle-stimulating hormone (FSH) antigens. Proestrus is a time during which pituitary gonadotropes express peak levels of receptors for gonadotropin-releasing hormone (GnRH) and epidermal growth factor (EGF). We hypothesized that if GnRH or EGF stimulated fos activity in gonadotropes they would be most effective during the peak expression of their receptors. Anterior pituitaries were removed, cut into small pieces, and stimulated for 30 min. Total RNA was then collected and analyzed by Northern analysis. Both EGF and GnRH caused an increase in c-fos mRNA levels in the anterior pituitary gland compared with unstimulated pituitary glands assayed immediately after removal from the pituitary. However, the stimulatory effects were no greater than those seen with medium alone. This suggested that fos expression could be stimulated by local factors either in the pituitary or the medium itself. The second phase of the study focused on pituitary cells plated for 1 hr and then stimulated with EGF and GnRH for 15 min. Dual immunocytochemistry was done to learn which cell types expressed the fos proteins. After 15 min, EGF and GnRH both increased the percentages of fos-bearing cells above levels seen in medium alone. EGF stimulated fos proteins in subsets of FSH, adrenocorticotropin (ACTH), and growth hormone (GH) cells. GnRH increased fos proteins in subsets of ACTH and GH cells. These results suggest that EGF and GnRH may regulate fos expression, but not necessarily in gonadotropes. They also highlight the need for carefully timed experiments because endogenous factors in the pituitary itself may stimulate immediate early gene expression. (J Histochem Cytochem 46:935-943, 1998)

Vasopressin and the regulation of hypothalamic-pituitary-adrenal axis function: implications for the pathophysiology of depression

The role of arginine vasopressin (AVPNP) in the control of adrenocorticotropic hormone (ACTH) secretion is explored, and in particular, its involvement in various stress response paradigms which may be of relevance in our understanding of the pathophysiology of depression. VP is released from two sites in the hypothalamus; the parvicellular division of the paraventricular nucleus (PVN), where corticotropin releasing hormone (CRH) is also formed, and from the magnocellular neurons of the supraoptic nucleus (SON) and the PVN. The intricate interaction with CRH, the other main ACTH secretagogue, and with glucocorticoids, the inhibitory feedback component of hypothalamic-pituitary-adrenal-axis (HPA) activity, is outlined. That VP plays an important role in the stress response is now beyond doubt. Examination of the impact of psychological stressors on the differential expression of VP and CRH at a hypothalamic and pituitary level has been facilitated by advances in molecular biological techniques. Of importance has been the cloning of the V1b receptor gene, the receptor at which AVP is active in the anterior pituitary. Chronic stress paradigms, associated with HPA hyperresponsiveness, and ACTH release following a novel superimposed stress, have been found with relative consistency to show a shift in the CRH:AVP ratio. This may relate to differing feedback sensitivity of AVP to glucocorticoid feedback restraint and the greater responsivity of AVP over CRH to chronic stimulatory stress input. Evidence for functionally distinct pools of ACTH releasing corticotropes, and the finding that AVP levels more closely correlate with ACTH levels than do CRH levels, suggest a more dynamic role for AVP in activity of the stress axis, and a primarily permissive function for CRH. The renewed interest in the role of VP in HPA axis activity may have important implications for furthering our understanding of psychiatric conditions such as depression, where significant dysregulation of this axis is seen. Elevated baseline cortisol, dexamethasone non-suppression and blunted CRH/ACTH release have been consistently documented. The possible contribution of VP to this hyperactivity, despite its known synergy with CRH, has been largely neglected. In animal models there is clear evidence that chronic psychological stressors increase the ratio of AVP to CRH production. Psychosocial stressors are intrinsically linked with depressive illness. The finding of elevated levels of AVP in postmortem studies of depressives and the lowering of CSF AVP levels by antidepressants, raises the question of the precise role of AVP in the overactivity of the HPA in depression, a finding that is currently attributed to overdrive of its HPA regulatory companion, CRH.

Stress-induced stimulation of pituitary POMC gene expression is associated with activation of transcription factor AP-1 in hypothalamus and pituitary

The response to environmental stimuli such as stress involves changes in gene transcription in both brain and pituitary, which in turn, facilitate adaptive phenotypic alterations favoring survival. In the present study we have examined the expression of the inducible immediate-early genes of the fos and jun families, and the activity of transcription factor AP-1 in the hypothalamus and anterior pituitary gland of rats, after a single restraint challenge. Restraint led to a rapid transient increase in c-fos but not c-jun expression in hypothalamus and pituitary. Changes in jun-B expression in hypothalamus were qualitatively similar to c-fos, though not statistically significant at 30 min. Furthermore, a single episode of restraint stress led to significant increases (50-100%) in nuclear AP-1 DNA binding activity in both hypothalamus and pituitary, while DNA binding of an unrelated transcription factor (Sp1) was unchanged. Associated with the stress-induced activation of pituitary AP-1 was a parallel three- to fourfold transcriptional stimulation of pituitary POMC gene expression. These data demonstrate that the rapidly inducible members of the fos and jun gene families contribute to increased activity of transcription factor AP-1 in both hypothalamus and pituitary following stress, and suggest that AP-1 may be a crucial factor involved in rapid transcriptional responses during stress.

Glucocorticoid regulation of hypothalamic proopiomelanocortin

Although glucocorticoids clearly inhibit proopiomelanocortin (POMC) gene transcription and peptide synthesis in the anterior pituitary, the effects of glucocorticoids on POMC in the hypothalamus are still unclear, even though most POMC neurons in the arcuate nucleus are known to have glucocorticoid receptors. In this study, we have therefore examined the effect of adrenalectomy (ADX) and glucocorticoid replacement on POMC mRNA and peptide (beta-EP and alpha-MSH) levels in the medial basal hypothalamus (MBH) of the rat. POMC mRNA was measured by a sensitive solution hybridization S1 nuclease protection assay, and beta-EP and alpha-MSH were measured by radioimmunoassay. In a first experiment, animals were studied 7 days after ADX or sham surgery. The mean POMC mRNA concentration was 1.01+/-0.14 pg/microg RNA (means+/-SE) in the intact animals and decreased to 0.55+/-0.07 pg/microg RNA in the MBH of the ADX animals (p < 0.005). Beta-EP levels decreased in parallel from 4.30+/-0.18 to 3.36+/-0.11 ng/mg protein (p < 0.001); alpha-MSH levels decreased from 3.25+/-0.21 to 2.41+/-0.16 ng/mg protein (p < 0.005). In a second experiment, animals were studied 2 weeks after ADX. POMC mRNA levels again fell significantly from 1.15+/-0.19 pg/microg RNA in the intact animals to 0.51+/-0.06 pg/microg in the ADX animals (p < 0.01). Beta-EP levels fell also, but this was not significant. In a third experiment, all animals underwent ADX, and half of them received daily subcutaneous injections of dexamethasone (20 microg). Nine days after ADX, the mean POMC mRNA level was 0.66+/-0.04 pg/microg RNA in the saline-treated animals and increased to 0.98+/-0.08 pg/microg RNA in the dexamethasone-treated animals (p < 0.005). A parallel increase in beta-EP levels from 5.03+/-0.41 to 6.01+/-0.53 ng/mg protein was also noted, but this was not statistically significant. We conclude that POMC gene expression is significantly inhibited in the MBH at 1 and 2 weeks after ADX. This effect was reversed by glucocorticoid replacement with doses close to the physiological range. The parallel changes in POMC mRNA and peptide levels strongly suggest that, in contrast to the anterior pituitary, low doses of glucocorticoids stimulate the biosynthesis of POMC in the MBH of ADX rats.

Patterns of Fos-Immunoreactivity in the CNS Induced by Repeated Hemorrhage in Conscious Rats: Correlations with Pituitary-Adrenal Axis Activity

While the present understanding of pituitary-adrenal function predicts attenuation of responses to a repeated stressor, experimental observations often show occurrence of potentiation rather than inhibition. The role of the CNS in this phenomenon was investigated in rats sustaining either a single (S-HEM) or a double episode (R-HEM) of hemorrhage. For S-HEM, blood was withdrawn over 3min and retransfused at 10min; for R-HEM, the stimulus was repeated at 90 min. S-HEM elicited 26- and 9-fold increases in circulating adrenocorticotropin (ACTH) and corticosterone, respectively. After R-HEM the plasma ACTH response was potentiated by 82%. Sixty min after S-HEM, Fos-like immunoreactivity (Fos-IR) was increased in medullary (solitary nucleus, NTS and ventrolateral medulla, VLM), pontine (locus coeruleus, LC and parabrachial nucleus, PBN), limbic (central amygdala, CNA and bed nucleus, BNST), and hypothalamic (supraoptic nucleus, SON and paraventricular nucleus, PVN) regions activated by hemodynamic stimuli. However after R-HEM, the Fos-IR response was significantly potentiated only in the VLM and PVN, while only a moderate increase was evident in the NTS. In other brain regions (LC, PBN, CNA, BNST, HPC and SON), Fos-IR either did not change or the increases were less than those observed after S-HEM. It is suggested that this plasticity in the pattern of neuronal activation following repetition of a stimulus may account for the maintenance of pituitary-adrenal secretory responses and its potentiation after R-HEM.

Novel dimeric Nur77 signaling mechanism in endocrine and lymphoid cells

Within the nuclear receptor family, Nur77 (also known as NGFI-B) distinguishes itself by its ability to bind a target sequence (the NBRE) as a monomer and by its role in T-cell receptor (TCR)-induced apoptosis in T cells. We now report on a novel mechanism of Nur77 action that is mediated by homodimers. These dimers bind a Nur77 response element (NurRE), which has been identified as a target of CRH-induced Nur77 in the pro-opiomelanocortin (POMC) gene promoter. Both halves of the palindromic NurRE are required for responsiveness to physiological signals, like CRH in pituitary-derived AtT-20 cells. Similarly, in T-cell hybridomas, TCR activation induced NurRE but not NBRE reporters. The in vivo signaling function of Nur77 thus appears to be mediated by dimers acting on a palindromic response element of unusual spacing between its half-sites. This mechanism may represent the biologically relevant paradigm of action for this subfamily of orphan nuclear receptors.

Differential expression of c-fos in vitro by all anterior pituitary cell types during the estrous cycle: enhanced expression by luteinizing hormone but not by follicle-stimulating hormone cells

C-fos expression appears in some activated cell types. Because of dynamic changes in gonadotropes during the estrous cycle, this study was initiated to determine if fos might be expressed in gonadotropes before any period of activation. We detected c-fos and pituitary antigens in dissociated anterior pituitary cells by dual-labeling immunocytochemistry. The highest percentage of cells with fos protein were found in proestrous rat populations. In diestrous and proestrous populations, dual labeling showed that 6-9% of pituitary cells contained fos with adrenocorticotropin, thyroid-stimulating hormone, prolactin, or growth hormone antigens. In contrast, only 0.8-3% contained fos with luteinizing hormone (LH) or follicle-stimulating hormone (FSH) antigens. We then tested the hypothesis that gonadotropes might increase fos expression earlier in the cycle. In populations from metestrous rats, c-fos labeling was found in 45% of LH cells compared to only 23% of LH cells in the proestrous group. This suggests that proportionately more LH cells are being activated to produce fos early in the cycle. Perhaps fos is used in translation of LH beta antigens or gonadotropin-releasing hormone (GnRH) receptor mRNAs. In contrast, less than 1% of all pituitary cells expressed fos with FSH at all stages of the cycle (only 6-12% of FSH cells). This differential expression suggests one mechanism behind the regulation of non-parallel storage and release of gonadotropin antigens.

Cell-specific induction of c-fos expression in the pituitary gland by estrogen

Estrogens regulate many functions of pituitary lactotrophs, including PRL gene expression, release, storage, and cellular proliferation. The mechanism by which estrogens exert such a variety of functions is poorly understood. In the uterus, estrogens rapidly and transiently induce the expression of the immediate early genes c-fos and c-jun in specific cell types. The Fos/Jun proteins form the activating protein-1 (AP1) transcription factor that mediates ligand-activated cell proliferation, differentiation, and secretion. Here we used Fischer 344 (F344) rats that develop hyperprolactinemia and prolactinomas in response to estrogens. The objectives were to: 1) determine whether estrogen induces c-fos expression in the pituitary gland and identify the responsive cells; 2) compare the dynamics of c-fos induction in the pituitary and uterus; and 3) examine the temporal relationship between c-fos expression and PRL release. Ovariectomized F344 rats were injected with 1 microg estradiol and killed at different times thereafter. Pituitaries were subjected to in situ hybridization for c-fos and immunostaining for selected pituitary cells. Estradiol stimulated c-fos expression in lactotrophs and folliculo-stellate cells within the anterior lobe without affecting either the intermediate or neural lobes. In a second experiment, c-fos messenger RNA levels were measured by solution hybridization in anterior pituitaries and uteri from estradiol-treated rats. Trunk blood was analyzed for PRL by RIA. The estrogen-induced c-fos rise in the uterus was rapid, robust, and transient, whereas that in the anterior pituitary was delayed, lower, and sustained. The profile of serum PRL levels resembles that of c-fos induction in the anterior pituitary. We conclude that: 1) both lactotrophs and folliculo-stellate cells increase c-fos expression in response to estrogens; 2) induction of c-fos expression may mediate some estrogenic effects on PRL synthesis and release and lactotroph proliferation in F344 rats; and 3) the atypical dynamics of c-fos induction in the pituitary could be due to indirect effects of estrogens on PRL-regulating factors within the hypothalamo-pituitary complex as well as to pituitary-specific estrogen receptor isoforms, coactivators, or repressors.

CRF stimulates expression of multiple fos and jun related genes in the AtT-20 corticotroph cell

Recent studies have shown that corticotropin releasing factor (CRF) stimulates c-fos gene expression in the AtT-20 corticotroph cell line, and that overexpression of c-Fos results in activation of POMC gene transcription. Since transactivation by c-Fos requires dimerization with a Jun family member to form the active transcription factor AP-1, we have examined the expression of multiple fos and jun related genes and have correlated their expression with AP-1 DNA binding activity in AtT-20 nuclear extracts after stimulation with CRF. Although basal expression of c-fos mRNA was extremely low, it was rapidly and transiently stimulated in AtT-20 cells following administration of either constant or a single pulse of CRF. In contrast, basal expression of c-jun mRNA was slightly higher and underwent little or no change in response to CRF. Specific ribonuclease protection analysis showed that in addition to c-fos, mRNA transcripts encoding fos B and jun B were rapidly stimulated in response to CRF, though levels of induced fos B mRNA were 20-40 times lower than c-fos or jun B, respectively. Gel shift analysis demonstrated that CRF caused a sustained increase in AP-1 DNA binding to both a canonical AP-1 element as well as to the POMC exon-1 AP-1 site. Studies with specific antisera directed against c-Fos revealed that although no c-Fos could be detected in AP-1 complexes in basal cell extracts, c-Fos became a prominent component of AP-1 following CRF stimulation, reaching maximal levels by 4 h. Despite the fact that AP-1 DNA binding activity remained elevated for at least 24 h after CRF, c-Fos was most prominent during the early phase of the response. Similarly, JunB was shown to be a major component of AP-1 DNA binding activity in CRF-stimulated AtT-20 nuclear extracts that persisted for at least 24h after stimulation. Despite the obvious induction of fos B mRNA in response to CRF, FosB protein was not detected in DNA bound AP-1 complexes. These data demonstrate that CRF is a potent stimulus for corticotroph expression of c-fos, jun B and fos B, and suggest that the subsequent increase in AP-1 may play a role in activation of gene expression and/or as a modulator of glucocorticoid receptor function.

Cyclosporin A and FK506 are potent activators of proopiomelanocortin-derived peptide secretion without affecting corticotrope glucocorticoid receptor function

Unliganded glucocorticoid receptors (GR) are localized in the cytoplasm and are associated with heat shock protein (hsp)90, hsp70, and a member of the immunophilin family, FK506 binding protein 59 (FKBP59). Several members of the cyclophilin and FKBP families have now been shown to associate with unactivated steroid receptors, however the physiological role these immunophilins play in steroid receptor function is questionable. In the present study we have measured GR binding and nuclear translocation of activated receptor in corticotrope cells following treatment with the immunophilin ligands FK506 and cyclospcrin A (CsA). Extensive GR binding studies in AtT20 cells, a mouse corticotrope tumor cell line failed to demonstrate an effect of FK506 or CsA on either the ability of GR to bind ligand, or on nuclear translocation of the liganded receptor at either a saturating or subsaturating dose of dexamethasone (DEX). Consistent with the binding data, functionally, neither CsA nor FK506 altered the glucocorticoid induced decrease in either proopiomelanocortin (POMC) derived peptide secretion or POMC heteronuclear (hn) RNA expression. Despite the fact these drugs did not modulate the actions of glucocorticoids on corticotrope cells, both FK506 and CsA were potent stimulators of basal beta-endorphin secretion (4-6 fold) from rat anterior pituitary cultures and AtT20 cells. In addition, FK506 and CsA potentiated beta-endorphin secretion induced by corticotropin releasing factor (CRF) and phorbol ester, but had no apparent acute (60 min) effect on POMC hnRNA levels. Unlike the acute actions of these immunosuppressant drugs, chronic (24 h) treatment lead to a decrease in cytoplasmic POMC mRNA with no apparent change in the amount of secreted beta-endorphin. Taken together these data suggest that FK506 and CsA do not alter GR activation or function in corticotrope cells, however, they are potent but short lived stimulators of POMC-derived peptide secretion. The observation that CsA and FK506 stimulate POMC-derived peptide secretion, and potentiate both phorbol ester and CRF induced secretion, suggests that these immunosuppressant drugs are acting upon a common point within these intracellular pathways.

Mechanism(s) of early glucocorticoid inhibition of adrenocorticotropin secretion from anterior pituitary corticotropes

Adrenal glucocorticoid hormones, released in response to stress activation of the hypothalamic-pituitary-adrenal axis (HPA), are powerful regulators of cellular function. Analysis of early (10 min to <3 h) glucocorticoid inhibition of ACTH secretion from anterior pituitary corticotropes is providing insight into potentially generic genomic mechanisms by which glucocorticoids regulate cellular excitability. Early glucocorticoid inhibition is dependent upon activation of intracellular type II glucocorticoid receptors and induction of new proteins, including the calcium-binding protein calmodulin. Glucocorticoids inhibit ACTH secretion stimulated by neuropeptide activation of the cAMP/protein kinase A (PKA) or protein kinase C (PKC) signaling pathways through mechanisms acting at, or beyond, the level of intracellular free calcium mobilization. Increasing evidence also suggests that the efficacy of early glucocorticoid inhibition is selectively modulated by the PKA pathways. The integration of molecular, electrophysiological, imaging and classic neuroendocrine techniques will further expose the molecular basis of early glucocorticoid inhibition.

Proliferation of the murine corticotropic tumour cell line AtT20 is affected by hypophysiotrophic hormones, growth factors and glucocorticoids

In pituitary-dependent hyperadrenocorticism (Cushing's disease), the disturbed regulation of ACTH secretion is associated with neoplastic transformation of corticotropic cells. As these two phenomena are almost indissolubly connected, it is of prime importance to elucidate the factor(s) that induce corticotropic cell proliferation. Here we report on the effects of hypophysiotrophic hormones and intrapituitary growth factors on the proliferation and hormone secretion of the murine corticotropic tumour cell line AtT20/D16v, as measured by DNA content, and ACTH concentration in culture media. In addition, sensitivity to the inhibitory effect of cortisol was assessed under various conditions. Corticotropin releasing hormone (CRH) and vasopressin (AVP) induced proliferation of AtT20-cells. In contrast to that caused by AVP, the CRH-induced proliferation was associated with increased ACTH secretion, which could be inhibited by cortisol. Insulin-like growth factor-I (IGF-I), epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) also stimulated the proliferation of AtT20-cells. The proliferation of AtT20-cells was significantly inhibited by cortisol in all tests. The IGF-I-induced proliferation was the least sensitive to inhibition by cortisol. The growth factors did not stimulate ACTH secretion but IGF-I differed in that it prevented the inhibition of basal ACTH secretion by cortisol. Additional experiments (Western ligand blot analysis) concerning the relative insensitivity of IGF-I induced proliferation to inhibition by cortisol revealed that IGF-I increased the concentration of a 29 kDa IGF binding protein (IGFBP) in the culture medium. The concentration of the 29 kDa IGFBP was slightly decreased by cortisol.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoid regulation of c-fos, c-jun and transcription factor AP-1 in the AtT-20 corticotrope cell

Glucocorticoids (GC) are potent repressors of both basal and corticotropin releasing factor (CRF) stimulated transcription of the proopiomelanocortin (POMC) gene in corticotrope cells of the anterior pituitary. Despite the finding of a novel, high affinity glucocorticoid receptor (GR) binding site within the proximal region of the POMC promoter, the mechanism by which GC inhibit POMC transcription is still uncertain. Recent studies have described mechanisms whereby GC inhibit transcription of other genes via a direct interaction with components of the transcription factor AP-1. Since it has been shown that CRF stimulates c-fos in AtT-20 corticotrope cells, and that c-fos over-expression elevates POMC transcription, the current study has investigated whether GC can repress c-fos and c-jun gene expression and AP-1 DNA binding activity in AtT-20 corticotrope cells. Acute treatment with doses of dexamethasone (DEX) that markedly inhibited nuclear POMC hnRNA had no effect on basal c-fos mRNA expression, but resulted in a transient down regulation of c-jun. In addition, acute DEX pretreatment significantly lowered CRF stimulation of POMC gene expression and attenuated the CRF stimulation of c-fos mRNA by 25%. Although DEX treatment of AtT-20 cells did not affect AP-1 DNA binding capacity of nuclear extracts, DEX pretreatment blunted the stimulation of AP-1 binding in response to CRF. In further studies, nuclear extracts from CRF-treated cells were coincubated with nuclear extracts from control or DEX treated cells. High levels of DEX treated extracts led to a relative repression of CRF-induced AP-1 binding, suggesting that ligand-activated GR may lower available AP-1 levels by direct protein: protein interaction. Finally, the composition of AP-1 in AtT-20 nuclear extracts was found to be heterogeneous, with the variation dependent upon hormonal treatment. These data suggest that in the corticotrope cell relatively high levels of activated GR may influence CRF-induced AP-1 DNA binding via transient genomic actions on basal c-jun and stimulated c-fos and/or via direct protein:protein interactions.

Corticosteroid receptors and the central nervous system

In mammalian systems, the physiological mineralocorticoid is aldosterone (aldo), and the physiological glucocorticoid cortisol (F), or corticosterone (B) in rats and mice. Receptors (MR) with high affinity for aldo, B and F are found in both epithelia and the central nervous system (CNS); receptors (GR) with lower affinity for F and B, and still lower for aldo, are found in essentially all cells. Both MR and GR bind to and activate canonical pentadecamer response elements in transfected cells and in epithelia, wherein MR aldo, B and F all act as agonists. In vivo, in epithelial cells a low Km, NAD-dependent, 11 beta hydroxysteroid dehydrogenase (11 beta OHSD) converts B and F, but not aldo, to receptor-inactive 11-keto congeners, thus allowing aldo to occupy epithelial MR and produce sodium retention. The CNS differs markedly in terms of MR/GR in a number of ways: (i) most but not all MR in the CNS are functionally unprotected, despite the presence of a low Km, NADP-preferring 11 beta OHSD, so that they operate as high-affinity GR; (ii) in such CNS 'MR', aldo antagonizes the effects of B, and vice versa, in contrast with epithelia; (iii) also in contrast with epithelia, activated GR in the CNS do not mimic activated MR, suggesting considerable if not total specificity at the response element level. These differences suggest that glucocorticoids have two distinct domains of action in the CNS, mediated by 'MR' at low B/F concentrations, and GR at higher concentrations; secondly, they suggest that the nuclear recognition and response elements mediating these effects are other than canonical pentadecamer sequences.

Enzymes and receptors: challenges and future directions

In many ways, the present paper will cover much of the same ground as those that precede it. As its title suggests, it is an overview--in part to highlight various aspects of the areas which have been discussed, and in part to put a personal view of where the most fruitful avenues for exploration over the next two years lie. It is very clearly not a comprehensive summary of the topic; given the delightful unpredictability of biological advance, there is every likelihood such predictions may prove substantially wrong. On the other hand, nothing ventured, nothing won; and so with this caveat, there follows one man's reckoning of the directions for the immediate future.