Steroid hormone specifically binds to rat kidney plasma membrane

The effects of non-genomic glucocorticoid mechanisms on bodily functions and the central neural system. A critical evaluation of findings

Mounting evidence suggests that--beyond the well-known genomic effects--glucocorticoids affect cell function via non-genomic mechanisms. Such mechanisms operate in many major systems and organs including the cardiovascular, immune, endocrine and nervous systems, smooth and skeletal muscles, liver, and fat cells. Non-genomic effects are exerted by direct actions on membrane lipids (affecting membrane fluidity), membrane proteins (e.g. ion channels and neurotransmitter receptors), and cytoplasmic proteins (e.g. MAPKs, phospholipases, protein kinases, etc.). These actions are mediated by the glucocorticoids per se or by the proteins dissociated from the liganded glucocorticoid receptor complex. The MR and GR also activate non-genomic mechanisms in certain cases. Some effects of glucocorticoids are shared by a variety of steroids, whereas others are more selective. Moreover, "ultra-selective" effects-mediated by certain glucocorticoids only-were also shown. Disparate findings suggest that non-genomic mechanisms also show "demand-specificity", i.e. require the coincidence of two or more processes. Some of the non-genomic mechanisms activated by glucocorticoids are therapeutically relevant; moreover, the "non-genomic specificity" of certain glucocorticoids raises the possibility of therapeutic applications. Despite the large body of evidence, however, the non-genomic mechanisms of glucocorticoids are still poorly understood. Criteria for differentiating genomic and non-genomic mechanisms are often loosely applied; interactions between various mechanisms are unknown, and non-genomic mechanism-specific pharmacological (potentially therapeutic) agents are lacking. Nevertheless, the discovery of non-genomic mechanisms is a major breakthrough in stress research, and further insights into these mechanisms may open novel approaches for the therapy of various diseases.

Rapid nongenomic inhibitory effects of glucocorticoids on phagocytosis and superoxide anion production by macrophages

Traditionally, steroid hormone effects have been described as a result of the modulation of nuclear transcription, thus triggering genomic events that are responsible for physiological effects. Despite early observations of rapid steroid effects that were incompatible with this theory, nongenomic steroid effects have been widely recognized only recently. However, the nongenomic effect of glucocorticoid (GC) on anti-inflammation and immunosuppression has not been reported. Macrophages play important roles in inflammation and the immune response. The present experiment selected macrophages as experimental cells to explore the nongenomic effects and possible mechanisms of GCs on phagocytosis and superoxide anion production. Phagocytosis by macrophages was detected by the neutral red uptake assay. The superoxide anions were measured by cytochrome C reduction assay. It was found that both 10(-4) and 10(-5) mol/L corticosterone (CORT) rapidly inhibited uptake of neutral red by macrophages in less than 30 min, and the inhibition by the former was stronger than that of the latter. CORT (10(-4) to 10(-10) mol/L) rapidly inhibited superoxide anion production by macrophages in less than 30 min. The above-mentioned effects were insensitive to the GC-receptor antagonist mifepristone (RU486) and the translation inhibitor actidione. CORT coupled to bovine serum albumin (BSA-CORT) was able to mimic the rapid inhibitory effects of CORT. The results indicated that CORT could rapidly inhibit phagocytosis and superoxide anion production by mouse peritoneal macrophages in vitro in less than 30 min by a rapid, nongenomic mechanism, which contributes to the anti-inflammatory and immunosuppressive actions of GCs. These data shed a new light on the clinical application of GCs.

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.

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.

Rapid non-genomic inhibition of ATP-induced Cl- secretion by dexamethasone in human bronchial epithelium

A non-genomic antisecretory role for dexamethasone at low concentrations (0.1 nM to1 microM) is described in monolayers of human bronchial epithelial cells in primary culture and in a continuous cell line (16HBE14o- cells). Dexamethasone produced a rapid decrease of [Ca(2+)](i) (measured with fura-2 spectrofluorescence) to a new steady-state concentration. After 15 min exposure to dexamethasone (1 nM), [Ca(2+)](i) was reduced by 32 +/- 11 nM (n = 7, P < 0.0001) from a basal value of 213 +/- 36 nM (n = 7). We have shown previously that aldosterone (1 nM) also produces a rapid fall in [Ca(2+)](i); however, after the decrease in [Ca(2+)](i) induced by dexamethasone, subsequent addition of aldosterone did not produced any further lowering of [Ca(2+)](i). The rapid response to dexamethasone was insensitive to pretreatment with cycloheximide and unaffected by the glucocorticoid type II and mineralocorticoid receptor antagonists RU486 and spironolactone, respectively. The rapid [Ca(2+)](i) decrease induced by dexamethasone was inhibited by the Ca(2+)-ATPase pump inhibitor thapsigargin (1 microM), the adenylate cyclase inhibitor MDL hydrochloride (500 microM) and the protein kinase A inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate (200 microM), but was not affected by the protein kinase C inhibitor, chelerythrine chloride (0.1 microM). Treatment of 16HBE14o- cell monolayers with dexamethasone (1 nM) inhibited the large and transient [Ca(2+)](i) increase induced by apical exposure to ATP (10(-4) M). Dexamethasone (1 nM) also reduced by 30 % the Ca(2+)-dependant Cl(-) secretion induced by apical exposure to ATP (measured as the Cl(-)-sensitive short-circuit current across monolayers mounted in Ussing chambers). Our results demonstrate, for the first time, that dexamethasone at low concentrations inhibits Cl(-) secretion in human bronchial epithelial cells. The rapid inhibition of Cl(-) secretion induced by the synthetic glucocorticoid is associated with a rapid decrease in [Ca(2+)](i) via a non-genomic mechanism that does not involve the classical glucocorticoid or mineralocorticoid receptor. Rather, it is a result of rapid non-genomic stimulation of thapsigargin-sensitive Ca(2+)-ATPase, via adenylate cyclase and protein kinase A signalling.

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.

Rapid androgen actions on calcium signaling in rat sertoli cells and two human prostatic cell lines: similar biphasic responses between 1 picomolar and 100 nanomolar concentrations

Androgen-induced calcium fluxes and gap junctional intercellular communication (GJIC) were studied in three different cell types. A transient (2-3 min duration) increase in intracellular calcium levels was observed within 20-30 sec of androgen addition, which was followed by a plateau phase with steroid concentrations higher than 1 nM. The kinetics of the calcium responses were similar in immature rat Sertoli cells, which contain normal nuclear receptors; the human prostatic tumor cell line, LNCaP, which contains a mutated nuclear receptor; and the human prostatic cell line, PC3, which does not contain a nuclear receptor. The human A431 tumor cell line did not respond to androgens. Concentrations of testosterone and the synthetic androgen, R1881, between 1-1000 pM induced transient calcium increases with ED(50) values near 1 pM and 1 nM, whereas dihydrotestosterone (DHT) was not active at these concentrations. At concentrations higher than 1 nM, testosterone, R1881, and DHT were equipotent in stimulating an increase in calcium that lasted for more than 10 min, with ED(50) values between 5 and 20 nM. Testosterone covalently bound to albumin was also active, whereas 11 related androstane compounds as well as progesterone and estradiol-17beta were inactive at 1000 nM. The calcium response induced by the three androgens (10 nM) was abolished in all cell types by hydroxyflutamide (1000 nM) and finasteride (1000 nM), but not by cyproterone acetate (1000 nM). The calcium response was also abolished in the absence of extracellular calcium and strongly inhibited by the presence of verapamil. Exposure of the responsive cells to brief (150-sec) pulses of androgens generated calcium responses that were similar to those after continuous exposure. After exposure of Sertoli cells for only 30 sec to 100 nM testosterone, the calcium response lasted for at least 50 min. Although nuclear binding of androgens could be demonstrated, there was no evidence for tight binding to the plasma membrane under similar conditions. When protein synthesis was inhibited, an enhancement of GJIC between rat Sertoli cells, but not between LNCaP cells or PC3 cells, was observed within 15 min of the addition of 10 nM testosterone. Because nuclear androgens are not present in PC3 cells and many functional properties of the responsive system are different from the nuclear receptor in all three cell types, we postulate the existence of an alternative cell surface receptor system with biphasic response characteristics (high and low affinity). The calcium signals are probably coupled to the regulation of gap junctional efficiency between Sertoli cells. The low-affinity receptors may convey complementary androgen signals at elevated local levels such as in the testis, when nuclear receptors are (over)saturated.

Glucocorticoid modulates Na+/H+ exchange activity in vascular smooth muscle cells by nongenomic and genomic mechanisms

BACKGROUND

In vascular smooth muscle cells (VSMCs), Na+/H+ exchange (NHE) plays an important role in intracellular pH (pHi) regulation. The genomic effect of glucocorticoid (GC) on NHE activity has been suggested in VSMCs. However, the nongenomic and genomic effects of GC on NHE activity and the underlying intracellular signaling mechanisms have not yet been demonstrated in VSMCs. Also, it is not known whether there are specific surface-binding sites of GC to the plasma membrane of VSMCs.

METHODS

The effects of short (3 h)- and long (24 h)-term exposure to corticosterone (CORTI) on NHE activity were studied in cultured rat aortic VSMCs by using pHi measurement with the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. The NHE activity was calculated from the initial rate of Na+-dependent pHi recovery after the acid load.

RESULTS

Short-term exposure of VSMCs to CORTI (10-6 mol/L) increased NHE activity, whereas long-term exposure to CORTI decreased it. The inhibitors of gene transcription (actinomycin D) and of protein synthesis (cycloheximide) did not affect the short-term effect of CORTI on NHE activity, but inhibited the long-term effect of CORTI on NHE activity. The cytosolic GC receptor (GR) antagonist (RU38486) inhibited both the short- and long-term effects of CORTI on NHE activity, but the cytosolic mineralocorticoid receptor antagonist (spironolactone) did not influence either the short- or long-term CORTI effects. Two protein kinase C (PKC) inhibitors (staurosporine A and calphostin C) and PKC down-regulation [24-h pre-exposure to phorbol 12-myristate 13-acetate (PMA)] inhibited both short- and long-term CORTI effects. Exposure to PMA for three hours mimicked the short-term CORTI effect. The short-term CORTI effect was inhibited by the disruptor of microtubule (colchicine), but not by the disruptor of filamentous-actin (cytochalasin B). The long-term exposure to CORTI decreased NHE (NHE-1) mRNA levels to 0.65 times the control level, whereas the short-term exposure to CORTI caused no effect. Scatchard analysis of [3H]CORTI surface binding to VSMCs showed a single class of CORTI binding sites with a Bmax of 876.2 fmol per mg of cell protein and a Kd of 12.2 nmol/L. RU38486 also inhibited [3H]CORTI surface binding to VSMCs.

CONCLUSIONS

In VSMCs, NHE activity is stimulated by short-term exposure to CORTI, but is inhibited by long-term exposure to CORTI. The short-term stimulatory effect of CORTI on NHE activity is independent of gene transcription and protein synthesis, is mediated through the CORTI surface receptor, and occurs through a microtubule-dependent process. The long-term inhibitory effect of CORTI on NHE activity requires gene transcription and protein synthesis and occurs only through the cytosolic GR. The short- and long-term effects of CORTI on NHE activity occur via PKC activation. Therefore, CORTI differentially modulates NHE activity in VSMCs by nongenomic and genomic mechanisms.

Purification of a cortisol binding protein from hepatic plasma membrane

A cortisol binding protein from rat liver plasma membranes has been solubilized in active form by using the zwitterionic detergent CHAPS. Two types of binding sites have been characterised in both native and solubilized membranes. The first is of high affinity and low binding capacity (12 nM; 946 fmol/mg) and the other one is of low affinity and high capacity of binding (344 nM; 12677 fmol/mg) for solubilized membranes. The purified material retained a binding activity comparable to that displayed by the original membrane. The specific binding activity was enriched about 12700-fold, with an 8% yield. Analysis of the purified preparation on sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed two protein subunits with molecular mass of 52000 and 57000 Da. The new cortisol-specific binding membrane protein could be related to the nongenomic effects previously described for this hormone.