Calcium and protein kinase C regulation of the glucocorticoid receptor in mouse corticotrope tumor cells

Corticosteroid receptors, 11 beta-hydroxysteroid dehydrogenase, and the heart

Mineralocorticoid and glucocorticoid hormones are known as corticosteroid hormones and are synthesized mainly in the adrenal cortex; however, more recently the enzymes involved in their synthesis have been found in a variety of cells and tissues, including the heart. The effects of these hormones are mediated via both cytoplasmic mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs), which act as ligand-inducible transcription factors. In addition, rapid, nongenomically mediated effects of these steroids can occur that may be via novel corticosteroid receptors. The lipophilic nature of these hormones allows them to pass freely through the cell membrane, although the intracellular concentration of mineralocorticoids and glucocorticoids is dependent on several cellular factors. The main regulators of intracellular glucocorticoid levels are 11 beta-hydroxysteroid dehydrogenase (11 beta HSD) isoforms. 11 beta HSD1 acts predominantly as a reductase in vivo, facilitating glucocorticoid action by converting circulating receptor-inactive 11-ketoglucocorticoids to active glucocorticoids. In contrast, 11 beta HSD 2 acts exclusively as an 11 beta-dehydrogenase and decreases intracellular glucocorticoids by converting them to their receptor-inactive 11-ketometabolites. Furthermore, P-glycoproteins, by actively pumping steroids out of cells, can selectively decrease steroids and local steroid synthesis can increase steroid concentrations. Receptor concentration, receptor modification, and receptor-protein interactions can also significantly impact on the corticosteroid response. This review details the receptors and possible mechanisms involved in both mediating and modulating corticosteroid responses. In addition, direct effects of corticosteroids on the heart are described including a discussion of the corticosteroid receptors and the mechanisms involved in mediating their effects.

Chronic corticotropin-releasing hormone and vasopressin regulate corticosteroid receptors in rat hippocampus and anterior pituitary

Corticotropin-releasing hormone (CRH) and vasopressin (AVP) participate in the endocrine, autonomic, immunological and behavioral response to stress. CRH and AVP receptors are found in hippocampus and anterior pituitary, where mineralocorticoid (MR) and glucocorticoid (GR) receptors are abundant. We investigated the possible influence of CRH and AVP on the regulation of MR and GR in both tissues. CRH, AVP, or their antagonists were administered to adrenalectomized rats substituted with corticosterone, to avoid interference with adrenal secretion. Repeated i.c.v. oCRH injections (10 microgram) for 5 days significantly decreased MR and GR mRNA in hippocampus and MR mRNA in anterior pituitary. AVP significantly increased both corticosteroid receptor mRNAs, as repeated i.c.v. injections (5 microgram) for 5 days in hippocampus, and as continuous i.c.v. infusion (10 ng/h/5 days) in anterior pituitary. The i.c.v. infusion of 5 or 10 microgram/day of the alpha-helical CRH antagonist during intermittent restraint stress (5 days), induced a significant decrease in hippocampal MR binding. In anterior pituitary, 5 microgram/day significantly decreased MR binding, while 10 microgram/day significantly increased GR binding. Under the same conditions of stress, the infusion of 15 microgram/day of the vasopressin V1a/1b receptor antagonist [dP Tyr (Me)(2)AVP] significantly increased MR and GR binding in hippocampus and anterior pituitary; 5 microgram/day significantly decreased pituitary MR binding. Our results show that CRH and AVP regulate MR and GR in hippocampus and anterior pituitary. This reveals another important function of CRH and AVP, which could be relevant to understand stress adaptation and the pathophysiology of stress-related disorders like major depression.

Chronic lithium chloride injection increases glucocorticoid receptor but not mineralocorticoid receptor mRNA expression in rat brain

Lithium has been used clinically for the treatment of bipolar disorders. However, the brain mechanisms, by which lithium acts, are still unclear. An impaired hypothalamic-pituitary-adrenal (HPA) axis has been implicated in the pathogenesis of mood disorders. In this study, we investigated the effects of chronic lithium on the corticosteroid receptors in the brain. Male Wistar rats were injected with LiCl (1.5 mEq/kg) or saline intraperitoneally (i.p.) once a day for 14 days. Twenty-four hours after the last injection, the expressions of mRNA for glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) in the brain were determined by non-radioactive in situ hybridization. Chronic administration of LiCl increased the expression of GR mRNA in the hippocampus and paraventricular nucleus of the hypothalamus (PVN). However, no significant changes were observed in the expression of either MR mRNA in the hippocampus or GR mRNA in the locus ceruleus. Since the hippocampus and PVN mediate negative feedback regulation of the HPA axis, an increased expression of GR mRNA in these regions may normalize HPA axis activity in mood disorders. Thus, the effect of chronic lithium on GR function may be involved in its antimanic and/or prophylactic activity in bipolar disorders.

Glucocorticoid block of protein kinase C signalling in mouse pituitary corticotroph AtT20 D16:16 cells

1. The regulation of large conductance calcium- and voltage-activated potassium (BK) currents by activation of the protein kinase C (PKC) and glucocorticoid signalling pathways was investigated in AtT20 D16:16 clonal mouse anterior pituitary corticotroph cells. 2. Maximal activation of PKC using the phorbol esters, 4beta-phorbol 12-myristate, 13-acetate (PMA), phorbol 12, 13 dibutyrate (PDBu) and 12-deoxyphorbol 13-phenylacetate (dPPA) elicited a rapid, and sustained, inhibition of the outward steady-state voltage- and calcium- dependent potassium current predominantly carried through BK channels. 3. The effect of PMA was blocked by the PKC inhibitors bisindolylmaleimide I (BIS; 100 nM) and chelerythrine chloride (CHE; 25 microM) and was not mimicked by the inactive phorbol ester analogue 4alpha-PMA. 4. PMA had no significant effect on the 1 mM tetraethylammonium (TEA)-insensitive outward current or pharmacologically isolated, high voltage-activated calcium current. 5. PMA had no significant effect on steady-state outward current in cells pre-treated for 2 h with 1 microM of the glucocorticoid agonist dexamethasone. Dexamethasone had no significant effect on steady-state outward current amplitude or sensitivity to 1 mM TEA and did not block PMA-induced translocation of the phorbol ester-sensitive PKC isoforms, PKCalpha and PKCepsilon, to membrane fractions. 6. Taken together these data suggest that in AtT20 D16:16 corticotroph cells BK channels are important targets for PKC action and that glucocorticoids inhibit PKC signalling downstream of PKC activation.

An FK506-sensitive transporter selectively decreases intracellular levels and potency of steroid hormones

Steroid hormones bind and activate intracellular receptors that are ligand-regulated transcription factors. Mammalian steroid receptors can confer hormone-dependent transcriptional enhancement when expressed in yeast, thereby enabling the genetic identification of nonreceptor proteins that function in the hormone signal transduction pathway. Pdr5p (Lem1/Sts1/Ydr1p), a yeast ATP-binding cassette transporter, selectively decreases the intracellular levels of particular steroid hormones, indicating that active processes can affect the passage of steroids across biological membranes. In yeast, the immunosuppressive drug FK506 inhibited Pdr5p, thereby potentiating activation of the glucocorticoid receptor by dexamethasone, a ligand that is exported by Pdr5p. In mammalian L929 cells but not in HeLa cells, FK506 potentiated dexamethasone responsiveness and increased dexamethasone accumulation, without altering the hormone-binding properties of the glucocorticoid receptor. We suggest that an FK506-sensitive transporter in L929 cells selectively decreases intracellular hormone levels and, consequently, the potency of particular steroids. Thus, steroid transporters may modulate, in a cell-specific manner, an initial step in signaling, the availability of hormone to the receptor.

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.