Rapid effects of corticosteroids on cytosolic protein kinase C and intracellular calcium concentration in human distal colon

Non-genomic actions of steroid hormones on the contractility of non-vascular smooth muscles

Steroid hormones play an important role in physiological processes. The classical pathway of steroid actions is mediated by nuclear receptors, which regulate genes to modify biological processes. Non-genomic pathways of steroid actions are also known, mediated by cell membrane-located seven transmembrane domain receptors. Sex steroids and glucocorticoids have several membrane receptors already identified to mediate their rapid actions. However, mineralocorticoids have no identified membrane receptors, although their rapid actions are also measurable. In non-vascular smooth muscles (bronchial, uterine, gastrointestinal, and urinary), the rapid actions of steroids are mediated through the modification of the intracellular Ca2+ level by various Ca-channels and the cAMP and IP3 system. The non-genomic action can be converted into a genomic one, suggesting that these distinct pathways may interconnect, resulting in convergence between them. Sex steroids mostly relax all the non-vascular smooth muscles, except androgens and progesterone, which contract colonic and urinary bladder smooth muscles, respectively. Corticosteroids also induce relaxation in bronchial and uterine tissues, but their actions on gastrointestinal and urinary bladder smooth muscles have not been investigated yet. Bile acids also contribute to the smooth muscle contractility. Although the therapeutic application of the rapid effects of steroid hormones and their analogues for smooth muscle contractility disorders seems remote, the actions and mechanism discovered so far are promising. Further research is needed to expand our knowledge in this field by using existing experience. One of the greatest challenges is to separate genomic and non-genomic effects, but model molecules are available to start this line of research.

Mineralocorticoid receptor signaling: crosstalk with membrane receptors and other modulators

The mineralocorticoid receptor (MR) belongs to the steroid receptor superfamily. Classically, it acts as a ligand-bound transcription factor in epithelial tissues, where it regulates water and electrolyte homeostasis and controls blood pressure. Additionally, the MR has been shown to elicit pathophysiological effects including inflammation, fibrosis and remodeling processes in the cardiovascular system and the kidneys and MR antagonists have proven beneficial for patients with certain cardiovascular and renal disease. The underlying molecular mechanisms that mediate MR effects have not been fully elucidated but very likely rely on interactions with other signaling pathways in addition to genomic actions at hormone response elements. In this review we will focus on interactions of MR signaling with different membrane receptors, namely receptor tyrosine kinases and the angiotensin II receptor because of their potential relevance for disease. In addition, GPR30 is discussed as a new aldosterone receptor. To gain insights into the problem why the MR only seems to mediate pathophysiological effects in the presence of additional permissive factors we will also briefly discuss factors that lead to modulation of MR activity as well. Overall, MR signaling is part of an intricate network that still needs to be investigated further.

Aldosterone increases cardiac vagal tone via G protein-coupled oestrogen receptor activation

In addition to acting on mineralocorticoid receptors, aldosterone has been recently shown to activate the G protein-coupled oestrogen receptor (GPER) in vascular cells. In light of the newly identified role for GPER in vagal cardiac control, we examined whether or not aldosterone activates GPER in rat nucleus ambiguus. Aldosterone produced a dose-dependent increase in cytosolic Ca(2+) concentration in retrogradely labelled cardiac vagal neurons of nucleus ambiguus; the response was abolished by pretreatment with the GPER antagonist G-36, but was not affected by the mineralocorticoid receptor antagonists, spironolactone and eplerenone. In Ca(2+)-free saline, the response to aldosterone was insensitive to blockade of the Ca(2+) release from lysosomes, while it was reduced by blocking the Ca(2+) release via ryanodine receptors and abolished by blocking the IP3 receptors. Aldosterone induced Ca(2+) influx via P/Q-type Ca(2+) channels, but not via L-type and N-type Ca(2+) channels. Aldosterone induced depolarization of cardiac vagal neurons of nucleus ambiguus that was sensitive to antagonism of GPER but not of mineralocorticoid receptor. in vivo studies, using telemetric measurement of heart rate, indicate that microinjection of aldosterone into the nucleus ambiguus produced a dose-dependent bradycardia in conscious, freely moving rats. Aldosterone-induced bradycardia was blocked by the GPER antagonist, but not by the mineralocorticoid receptor antagonists. In summary, we report for the first time that aldosterone decreases heart rate by activating GPER in cardiac vagal neurons of nucleus ambiguus.

Non-genomic actions of aldosterone: from receptors and signals to membrane targets

In tissues which express the mineralocorticoid receptor (MR), aldosterone modulates the expression of membrane targets such as the subunits of the epithelial Na(+) channel, in combination with important signalling intermediates such as serum and glucocorticoid-regulated kinase-1. In addition, the rapid 'non-genomic' activation of protein kinases and secondary messenger signalling cascades has also been detected in aldosterone-sensitive tissues of the nephron, distal colon and cardiovascular system. These rapid actions are variously described as being coupled to MR or to an as yet unidentified, membrane-associated aldosterone receptor. The rapidly activated signalling cascades add a level of fine-tuning to the activity of aldosterone-responsive membrane transporters and also modulate the aldosterone-induced changes in gene expression through receptor and transcription factor phosphorylation.

New aspects of rapid aldosterone signaling

Aldosterone, the endogenous ligand of the mineralocorticoid receptor (MR) in humans, is a steroid hormone that regulates salt and water homeostasis. Recently, additional pathophysiological effects in the renocardiovascular system have been identified. Besides genomic effects mediated by activated MR, rapid aldosterone actions that are independent of translation and transcription have been documented. While these nongenomic actions influence electrolyte homeostasis, pH and cell volume in classical MR target organs, they also participate in pathophysiological effects in the renocardiovascular system causing endothelial dysfunction, inflammation and remodeling. The mechanisms conveying these rapid effects consist of a multitude of signaling molecules and include a cross-talk with genomic aldosterone effects as well as with angiotensin II and epidermal growth factor receptor signaling. Rapid corticosteroid signaling via the MR has also been demonstrated in the brain. Altogether, the function of nongenomic aldosterone effects seems to be to modulate other signaling cascades, depending on the surrounding milieu.

Rapid responses to aldosterone in the kidney and colon

Aldosterone is a crucial modulator of ion transport across high resistance epithelia and regulates whole body electrolyte balance through its effects on the kidney and colon. The net consequence of aldosterone release is to promote salt conservation. The genomic mechanism of aldosterone action is relatively well characterized and the role of the classical mineralocorticoid receptor as a ligand-dependent transcription factor is well established. The rapid effects of aldosterone on target tissues are less well understood and there is still controversy over the identity of the aldosterone non-genomic receptor. Greater understanding of the physiological consequences of aldosterone's rapid responses in the kidney and colon has been achieved through the identification of definite and putative membrane targets and their signaling regulators.

17beta-estradiol rapidly mobilizes intracellular calcium from ryanodine-receptor-gated stores via a PKC-PKA-Erk-dependent pathway in the human eccrine sweat gland cell line NCL-SG3

We describe a novel rapid non-genomic effect of 17beta-estradiol (E2) on intracellular Ca2+ ([Ca2+]i) signalling in the eccrine sweat gland epithelial cell line NCL-SG3. E2 had no observable effect on basal [Ca2+]i, however exposure of cells to E2 in the presence of the microsomal Ca2+ ATPase pump inhibitor, thapsigargin, produced a secondary, sustained increase in [Ca2+]i compared to thapsigargin treatment alone, where cells responded with a transient single spike-like increase in [Ca2+]i. The E2-induced increase in [Ca2+]i was not dependent on the presence of extracellular calcium and was completely abolished by ryanodine (100 microM). The estrogen receptor antagonist ICI 182,780 (1 microM) prevented the E2-induced effects suggesting a role for the estrogen receptor in the release of [Ca2+]i from ryanodine-receptor-gated stores. The E2-induced effect on [Ca2+]i could also be prevented by the protein kinase C delta (PKCdelta)-specific inhibitor rottlerin (10 microM), the protein kinase A (PKA) inhibitor Rp-adenosine 3',5'-cyclic monophosphorothioate (200 microM) and the MEK inhibitor PD98059 (10 microM). We established E2 rapidly activates the novel PKC isoform PKCepsilon, PKA and Erk 1/2 MAPK in a PKCdelta and estrogen-receptor-dependent manner. The E2-induced effect was specific to 17beta-estradiol, as other steroids had no effect on [Ca2+]i. We have demonstrated a novel mechanism by which E2 rapidly modulates [Ca2+]i release from ryanodine-receptor-gated intracellular Ca2+ stores. The signal transduction pathway involves the estrogen receptor coupled to a PKC-PKA-Erk 1/2 signalling pathway.

Role of protein kinase C in aldosterone-induced non-genomic inhibition of basolateral potassium channels in human colonic crypts

Aldosterone produces rapid, non-genomic, inhibition of basolateral intermediate conductance K(+) (IK(Ca)) channels in human colonic crypt cells but the intracellular second messengers involved are unclear. We therefore evaluated the role of protein kinase C (PKC) in aldosterone's non-genomic inhibitory effect on basolateral IK(Ca) channels in crypt cells from normal human sigmoid colon. Patch clamp studies revealed that in cell-attached patches, IK(Ca) channel activity decreased progressively to 38+/-8% (P<0.001) of the basal value 10 min after the addition of 1 nmol/L aldosterone, and decreased further to 23+/-6% (P<0.02) of the basal value 5 min after increasing the aldosterone concentration to 10 nmol/L. Pre-incubation of crypts with 1 micromol/L chelerythrine chloride or 1 micromol/L Gö 6976 (PKC inhibitors) prevented the inhibitory effect of aldosterone. Conversely, channel activity decreased to 60+/-9% (P<0.02) of the basal value 10 min after the addition of 500 nmol/L PMA (a PKC activator), whereas 4alpha-PMA (an inactive ester) had no effect. When aldosterone (10 nmol/L) and PMA were added together, IK(Ca) channel activity was inhibited to the same extent as with aldosterone alone. These results indicate that aldosterone's non-genomic inhibitory effect on the macroscopic basolateral K(+) conductance in human colonic crypts reflects PKC-mediated inhibition of IK(Ca) channels.

A rapid, nongenomic pathway facilitates the synaptic transmission induced by retinoic acid at the developing synapse

We have previously shown that retinoic acid (RA), a factor highly expressed in spinal cord, rapidly and specifically enhances the spontaneous acetylcholine release at developing neuromuscular synapses in Xenopus cell culture, using whole-cell patch-clamp recording. We have now further investigated the underlying mechanisms that are involved in RA-induced facilitation on the frequency of spontaneous synaptic currents (SSCs). Buffering the rise of intracellular Ca2+ with BAPTA-AM hampered the facilitation of SSC frequency induced by RA. The prompt RA-enhanced SSC frequency was not abolished when Ca2+ was eliminated from the culture medium or there was bath application of the pharmacological Ca2+ channel inhibitor Cd2+, indicating that Ca2+ influx through voltage-activated Ca2+ channels are not required. Application of membrane-permeable inhibitors of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] or ryanodine receptors effectively blocked the increase of SSC frequency elicited by RA. Treating cells with either wortmannin or LY294002, two structurally different inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase) and with the phospholipase Cgamma (PLCgamma) inhibitor U73122, abolished RA-induced facilitation of synaptic transmission. Preincubation of the cultures with pharmacological inhibitors, either genistein, a broad-spectrum tyrosine kinase inhibitor, or PP2, which predominantly inhibits the Src family of nonreceptor tyrosine kinase, completely abolished RA-induced synaptic facilitation. Taken collectively, these results suggest that RA elicits Ca2+ release from Ins1,4,5P3 and/or ryanodine-sensitive intracellular Ca2+ stores of the presynaptic nerve terminal. This is done via PLCgamma/PI 3-kinase signaling cascades and Src tyrosine kinase activation, leading to an enhancement of spontaneous transmitter release.

The nongenomic actions of aldosterone

Aldosterone has physiological effects to regulate fluid and electrolyte homeostasis across epithelia and proinflammatory effects on a variety of nonepithelial cells in the context of inappropriate salt status. These effects are mediated by mineralocorticoid receptors, members of a large family of nuclear transcription factors, by DNA-directed, RNA-mediated protein synthesis. Rapid effects of aldosterone, insensitive to actinomycin D or cycloheximide and thus clearly nongenomic, have been convincingly documented in a variety of epithelial and nonepithelial tissues. Despite strenuous attempts, isolation of a nonclassical membrane receptor for aldosterone has proven unsuccessful, and rapid nongenomic effects mediated by classical mineralocorticoid receptors are increasingly recognized in the kidney, heart, and vascular wall. The mechanism of rapid nongenomic actions of aldosterone may vary between tissues in terms of pathways; in addition, what remains to be established is the physiological role of aldosterone action via such rapid nongenomic mechanisms and how they might synergize with the longer time course genomic actions of mineralocorticoids.

Mechanisms for aldosterone and spironolactone-induced positive inotropic actions in the rat heart

Previously, we reported that aldosterone and spironolactone have inotropic effects in the isolated perfused heart. To address the mechanisms underlying these inotropic effects, we examined the effects of aldosterone and spironolactone on isolated cardiac myocyte shortening, intracellular calcium ([Ca+2]i), pHi, and calcium-dependent actinomyosin ATPase activity. Aldosterone significantly increased shortening in cardiac myocytes (8.0+/-1.0 versus 16.0+/-1.3%, P<0.01) but neither diastolic [Ca+2]i (61.0+/-1.1 versus 66.0+/-4.4 nmol/L) nor peak systolic [Ca+2]i (302+/-11 versus 304+/-17 nmol/L) was affected. Spironolactone-increased shortening was also not coupled with changes in peak systolic calcium; however, diastolic calcium was significantly increased by spironolactone. Aldosterone, but not spironolactone, increased pHi from 7.23+/-0.03 to 7.59+/-0.02 (P<0.01); this was completely blocked by coadministration of 100 micromol/L of ethyl-isopropyl amiloride (EIPA), an inhibitor of the Na+/H+ exchanger (P<0.01). Consistent with this finding, aldosterone increased cytosolic sodium concentration ([Na+]i) from 9.2+/-0.15 to 11.4+/-0.2 mmol/L and produced a leftward shift in the pCa ATPase curve (pCa=5.82+/-0.02 versus 6.35+/-0.02, P<0.01) without affecting maximal myosin ATPase activity. Conversely, spironolactone, but not aldosterone, significantly increases maximal actomyosin ATPase activity (837+/-59 versus 355+/-52 nmol inorganic phosphate (P(i)) x min(-1) x g tissue(-1)). Collectively, these data strongly suggest that the inotropic actions of aldosterone and spironolactone are caused by different mechanisms of action. Aldosterone appeared to increase inotropy primarily through increased cytosolic pH, whereas spironolactone increased myosin ATPase calcium sensitivity and diastolic calcium concentration.

Non-genomic regulation of transmitter release by retinoic acid at developing motoneurons in Xenopus cell culture

Although the long-term effects of all-trans retinoic acid (RA) on neuronal growth and differentiation have been intensively studied, nothing is known about its effect on synaptic transmission. Here we show that RA rapidly and specifically enhances the spontaneous acetylcholine release at developing neuromuscular synapses in Xenopus cell culture using whole-cell patch-clamp recording. Acute addition of RA dose-dependently and reversibly enhances the frequency of spontaneous synaptic currents (SSCs). Application of the lipophilic RA analogue all-trans retinol or RA metabolites produced by light-induced decomposition failed to provoke similar changes in SSC frequency, indicating the specificity of RA-induced facilitation of spontaneous transmitter release. Protein synthesis inhibitors anisomycin or cycloheximide had no effect on RA-induced SSC frequency facilitation. Treating cells with pan RA receptor (RAR) selective agonist or RARbeta-selective agonist, but not RARalpha-, RARgamma- or retinoid X receptor (RXR)-selective agonists, mimicked the action of RA. These results suggest that RA acts through the activation of RARbeta, to induce a rapid, non-genomic increase in the frequency of spontaneous transmitter release at developing neuromuscular synapses.

Identification of a truncated ratp28-related protein expressed in kidney

An RT-PCR based strategy to clone the membrane-associated steroid binding protein ratp28 additionally amplified a novel sequence-related PCR product termed HC5. The HC5 PCR product was cloned and sequenced and showed 94% nucleotide sequence similarity to ratp28. The HC5 cDNA sequence open reading frame encodes a predicted 75 amino acid (8.0kDa) protein, and is therefore truncated compared to ratp28 (195 amino acids, 21.6kDa). In vitro transcription and translation of the HC5 cDNA resulted in the production of 2 proteins of approximately 8 and 6kDa. Restriction digests from various tissues demonstrated that liver and heart expressed primarily ratp28 mRNA whereas kidney and blood contained both ratp28 and HC5 transcripts. Phage display was employed to generate an antibody fragment to a peptide sequence conserved in ratp28 and HC5. Western blotting identified a 10kDa protein in cytosolic fractions of rat kidney. The function of HC5 remains to be determined.

Proteins of multiple classes may participate in nongenomic steroid actions

Responses to steroids initiated from non-nuclear receptors impinge on a wide variety of cellular responses and utilize nearly all known signal transduction webs. While the mechanisms by which steroid receptors localize in the membrane are still unclear, it is apparent that this alternative localization allows steroid receptors to participate in a wide range of complex functions influencing cell proliferation, death, and differentiation. The central debate still remains the identity of the protein class or classes that mediate membrane-initiated (nongenomic) responses. The data thus far have supported several possibilities, including: nuclear steroid receptor-like forms in non-nuclear locations; other known (nonsteroid) membrane receptors or channels with additional steroid-binding sites; enzymes; transporters; receptors for serum steroid-binding proteins; unique and previously undescribed proteins; or chimeras of typical steroid receptor domains with other unique or known protein domains. Categorizing membrane steroid receptor proteins based exclusively on the actions of antagonists and agonists, without considering cell context and protein partnering issues, may mislead us into predicting more receptor subtypes than really exist. However, the plethora of signaling and functional outcomes may indicate the participation of more than one kind of steroid-binding protein. Resolving such unanswered questions will require future investigative focus on this alternative arm of steroid action, which is likely to yield as many therapeutic opportunities as have nuclear steroid mechanisms.

Rapid, nongenomic effects of aldosterone in the heart mediated by epsilon protein kinase C

Aldosterone elevates Na+/K+/2Cl- cotransporter activity in rabbit cardiomyocytes within 15 min, an effect blocked by K-canrenoate and thus putatively mineralocorticoid receptor mediated. Increased cotransporter activity raises intracellular [Na+] sufficient to produce a secondary increase in Na+-K+ pump activity; when this increase in intracellular [Na+] is prevented, a rapid effect of aldosterone to lower pump activity is seen. Addition of transcription inhibitor actinomycin D did not change basal or aldosterone-induced lowered pump activity, indicating a direct, nongenomic action of aldosterone. We examined a possible role for protein kinase C (PKC) in the rapid nongenomic effects of aldosterone. Single ventricular myocytes and pipette solutions containing 10 mm intracellular [Na+] were used in patch clamp studies to measure Na+-K+ pump activity. Aldosterone lowered pump current, an effect abolished by epsilon PKC (epsilonPKC) inhibition but neither alphaPKC nor scrambled epsilonPKC; addition of epsilonPKC activator peptide mimicked the rapid aldosterone effect. In rabbits chronically infused with aldosterone, the lowered pump current in cardiomyocytes was acutely (< or =15 min) restored by epsilonPKC inhibition. These studies show that rapid effects of aldosterone on Na+-K+ pump activity are nongenomic and specifically epsilonPKC mediated; in addition, such effects may be prolonged (7 d) and long-lived ( approximately 4 h isolated cardiomyocyte preparation time). The rapid, prolonged, long-lived effects can be rapidly (< or =15 min) reversed by epsilonPKC blockade, suggesting a hitherto unrecognized complexity of aldosterone action in the heart and perhaps by extension other tissues.

Hydrocortisone has a protective effect on CyclosporinA-induced cardiotoxicity

CyclosporinA (CsA) is an immunosuppressive drug which induces severe adverse effects such as cardiotoxicity and nephrotoxicity. In several therapeutic protocols CsA is used in association with corticosteroids to obtain better therapeutic results. Recently, our studies showed that CsA increases blood pressure while inhibit Nitric Oxide (NO) production in vivo. In this study we evaluated in rat cardiomyocytes the effects of CsA, used alone or in association with Hydrocortisone (HY), on intracellular calcium concentration, NO production and lipid peroxidation (MDA level). Our results demonstrated that CsA increased intracellular calcium and such effect was dose-dependent. HY used alone, slightly decreased intracellular calcium, while dramatically reduced CsA-induced calcium fluxes. CsA (3.2 microM) increased lipid peroxidation and this effect was blunted by HY. Both CsA and HY inhibited NO production in rat cardiomyocytes acting on this pathway synergically. Our results demonstrated that in rat cardiomyocytes, CsA toxicity is due to a calcium overload, which in turn induce lipid peroxidation and determines oxidative stress-induced cell injury. Treatment with HY effectively inhibits CsA-induced toxicity, decreasing lipid peroxidation as well as calcium intracellular concentration. Our findings seem to suggest that glucocorticoids may be effective in reducing CsA-induced cardiotoxicity at concentrations which are consistent with current therapeutic doses.

Aldosterone

Aldosterone plays a pivotal role in electrolyte and fluid homeostasis and thus control of blood pressure. The "classical" view of aldosterone action is that it targets epithelia of the distal colon and renal nephron to stimulate Na(+) (re)absorption and K(+) secretion. In these cells, aldosterone binds steroid receptors, promoting translocation to the nucleus, where they modulate gene expression with the induced proteins stimulating transport. This "genomic" action is dependent on transcription and translation and has a latency of 0.5-1.0 h. Recently, more rapid actions of aldosterone that are independent of transcription and translation have been described. These "nongenomic" actions are mediated by a distinct receptor that is insensitive to inhibitors of the classical mineralocorticoid receptor, such as spironolactone. The present review describes advances in our understanding of the classical model of aldosterone action as well as those that broaden this model to encompass nongenomic actions, nonepithelial targets, production of aldosterone outside of the adrenal gland, novel mechanisms of specificity, and novel mechanisms for mediating genomic actions.

Rapid responses to steroid hormones: from frog skin to human colon. A homage to Hans Ussing

Fifty years ago, Hans Ussing described the mechanism by which ions are actively transported across frog skin. Since then, an enormous amount of effort has been invested in determining the cellular and molecular specifics of the transport mechanisms and their regulatory pathways. Ion transport in high-resistance epithelia is regulated by a variety of hormonal and non-hormonal factors. In vertebrates, steroid hormones such as mineralocorticoids, glucocorticoids and estrogens are major regulators of ion and water transport and hence are central to the control of extracellular fluid volume and blood pressure. Steroid hormones act through nuclear receptors to control the transcriptional activity of specific target genes, such as ion channels, ion transporters and ion pumps. These effects are observed after a latency of several hours and can last for days leading to cellular differentiation that allows a higher transport activity. This pathway is the so-called genomic phase. However, in the past 10 years, it has become apparent that steroid hormones can regulate electrolyte and water transport in tight epithelia independently of the transcription of these ion channels and transporters by regulating ion transporter activity in a non-genomic fashion via modulation of various signal transduction pathways. The molecular mechanisms underlying the steroid hormone-induced activation of signal transduction pathways such as protein kinase C (PKC), protein kinase A (PKA), intracellular calcium, intracellular pH and mitogen-activated protein kinases (MAPKs) and how non-genomic activation of these pathways influences epithelial ion transport will be discussed in this review.

Dexamethasone induces the secretion of annexin I in immature lymphoblastic cells by a calcium-dependent mechanism

The mechanisms by which glucocorticoids (GC) regulate annexin I (ANXA1) secretion in different cells are still a matter of debate. The aims of this study were to evaluate the ability of dexamethasone (Dex) to induce ANXA1 secretion and to investigate the roles of the intracellular free Ca2+ concentration ([Ca2+]i), and of the GC receptor, on that process. For this purpose, the human immature lymphoblastic CCRF-CEM cell line was used. Treatment of the cells with Dex, for up to 4 h, significantly reduced the intracellular content of ANXA1 and increased the amount of this protein bound to the outer surface of the plasma membrane, whereas exposure of cells to Dex, for 12 h, induced the synthesis of ANXA1. At the same short time periods, Dex also induced a significant increase in the [Ca2+]i. Incubation of the cells with BAPTA-AM (10 microM), a cell-permeant high affinity Ca2+ chelator, completely inhibited Dex-induced ANXA1 secretion. Furthermore, the Ca2+ ionophore, ionomycin, alone induced ANXA1 cleavage, but not its secretion. Additionally, we used brefeldin A to investigate the involvement of the classical endoplasmic reticulum (ER)-Golgi pathway of protein secretion in the release of ANXA1. The GC receptor antagonist, RU486, neither reverted the Dex-dependent ANXA1 secretion nor inhibited the increase of the [Ca2+]i induced by Dex. Together, our results indicate that Dex induces ANXA1 synthesis and secretion in CCRF-CEM cells. ANXA1 secretion in this cell type show the following characteristics: (i) is unlikely to involve the classical ER-Golgi pathway; (ii) requires a Ca(2+)-dependent cleavage of ANXA1; (iii) involves both Ca(2+)-dependent and independent mechanisms; and (iv) is apparently independent of the GC receptor alpha isoform.

Influence of polyhydroxysteroids on [Ca(2+)](i)

Recently, we have shown that two biologically active, disulfated polyhydroxysteroids from the Pacific brittle star Ophiopholis aculeata stimulate Ca(2+) influx into different cell types. In the present study, 45Ca(2+) and two fluorescent calcium probes, quin-2/AM and fura-2/AM, were employed to investigate the course and amplitude of calcium signals induced in different mouse cells using an radio-isotope, spectrofluorimetry, and microcytofluorimetry techniques. The cytotoxic and hemolytic effects were not observed for both steroids at the wide range of concentrations. Steroids did not influence [3H]-uridine incorporation in a variety of cells. The investigated steroids stimulated a rapid increase in cytosolic Ca(2+) content in Ehrlich mouse carcinoma cells, mouse spleen lymphocytes, and mouse peritoneal macrophages in the concentration range 1-100 microg/ml on a dose-dependent basis. Blockers of L-type calcium channels, such as verapamil, diltiazem, nifedipine (1 x 10(-7)M), and 1mM EGTA, inhibited this process and reduced the response of cells to steroid application. The stimulatory effect of steroids on human fibroblast proliferation and mouse macrophage lysosome activity was observed also. It is suggested that the investigated compounds may act as Ca(2+)-agonists and increase Ca(2+)-transport across cell membranes.

Non-genomic convergent and divergent signalling of rapid responses to aldosterone and estradiol in mammalian colon

Studies from our laboratory have demonstrated rapid ( < 1 min) non-genomic activation of Na(+)-H(+) exchange, K(+) recycling, PKC activity and a PKC-dependent Ca(2+) entry through L-type Ca(2+) channels specifically by mineralocorticoids in distal colon. Aldosterone directly stimulates the activity of the PKC alpha isoform (but not PKC delta, PKC epsilon and PKC zeta) in a cell-free assay system containing only purified commercially available enzyme, appropriate substrate peptide, co-factors and lipid vesicles. The primary ion transport target of the non-genomic signal transduction cascade elicited by aldosterone in epithelia is the Na(+)-H(+) exchanger. In isolated colonic crypts, aldosterone produced a PKC alpha sensitive intracellular alkalinisation within 1 min of hormone addition. Intracellular alkalinisation upregulates an ATP-dependent K(+) channel, which is involved in K(+) recycling to maintain the electrical driving force for Na(+) absorption, while inhibiting a Ca(2+) -dependent K(+) channel, which generates the charge balance for Cl(-) secretion. The non-genomic response to aldosterone in distal colon appears to enhance the capacity for absorption while down-regulating the potential for secretion. We have also demonstrated rapid (< 1 min) non-genomic activation of Na(+)-H(+) exchange, K(+) recycling, PKC alpha activity, and a PKC delta- and PKA-dependent Ca(2+) entry through di-hydropyridine-blockable Ca(2+) channels specifically by 17beta-estradiol in distal colon. These rapid effects are female gender specific and are insensitive to inhibitors of the classical estrogen receptor (ER). 17 beta-Estradiol directly stimulated the activity of both PKC delta and PKC alpha (but not PKC epsilon or PKC zeta) in a cell-free assay system. E2 rapidly inhibited basolateral K(Ca) channel activity which would be expected to result in an acute inhibition of Cl(-) secretion. Physiological concentrations of E2 (0.1-10 nM) reduced both basal and secretagogue-induced Cl(-) secretion. This anti-secretory effect of E2 is sensitive to PKC inhibition, intracellular Ca(2+) chelation, and is female gender specific and insensitive to inhibitors of the classical ER. These observations link rapid non-genomic activation of second messengers with a rapid gender-specific physiological effect in the whole tissue. Aldosterone and E2 differ in their protein kinase signal transduction and both hormones stimulate specific PKC isoforms indicating both common and divergent signalling systems for salt-retaining steroid hormones. The physiological function of non-genomic effects of aldosterone and estradiol is to shift the balance from net secretion to net absorption in a pluripotential epithelium.

Sex and salt hormones: rapid effects in epithelia

Recent evidence points to protein kinase C isoforms as highly specific receptors for aldosterone and estradiol in epithelia. The end targets of the kinase activation are Na(+)/H(+) exchange and K(+) and Ca(2+) channels. The physiological role of the nongenomic response is to increase electrolyte absorption and inhibit secretion in pluripotential epithelia.

17beta-oestradiol stimulates capacitative Ca2+ entry in human endometrial cells

Oestrogen plays an essential role in regulating growth and differentiation in the human endometrium which undergoes dynamic morphological and functional changes during the menstrual cycle in preparation for implantation. In this tissue, it has been suggested that intracellular calcium could be a key signal in transducing early responses to steroid hormones. Here, we have investigated the rapid effects of 17beta-oestradiol on [Ca2+]i in a human endometrial cell line (RL95-2). Using confocal imaging microscopy, we show that physiological concentrations of 17beta-oestradiol trigger rapid and transient increases in [Ca2+]i. Our results demonstrate that 17beta-oestradiol-induced [Ca2+]i variations are critically dependent on calcium influx via lanthanum-sensitive calcium channels. Moreover, the 17beta-oestradiol-induced Ca2+ influx is significantly increased by the depletion of intracellular stores by thapsigargin and decreased by chelerythrine chloride, an inhibitor of protein kinase C. These data indicate a non-genomic action of 17beta-oestradiol to stimulate capacitative Ca2+ entry through store-operated calcium channels via a PKC-sensitive pathway.

Basic fibroblast growth factor inhibits apoptosis of spontaneously immortalized granulosa cells by regulating intracellular free calcium levels through a protein kinase Cdelta-dependent pathway

Previous studies have shown that basic fibroblast growth factor (bFGF) inhibits primary granulosa cells from undergoing apoptosis. The present studies were designed to determine whether spontaneously immortalized granulosa cells (SIGCs) undergo apoptosis when deprived of growth factors and whether bFGF prevents apoptosis. In the absence of serum, the SIGCs lost cell contact and underwent apoptosis as indicated by the presence of annexin V binding, DNA ladders, and nuclear fragmentation. Basic FGF maintained cell contact and reduced the percentage of apoptotic cells. This antiapoptotic action was not observed ifbFGF was added 30 min after serum withdrawal. Further, intracellular free calcium ([Ca2+]i) levels gradually increased 3- to 4-fold within 10 min of serum withdrawal. This increase was inhibited by bFGF. The intracellular calcium chelator, BAPTA, completely prevented the SIGCs from undergoing apoptosis in the absence of serum. These observations suggest that bFGF's ability to regulate [Ca2+]i is an essential component of its antiapoptotic action. The phorbol ester TPA, an activator of protein kinase C (PKC), blocked apoptosis due to serum deprivation. Conversely, bisindolylmaleimide II, an inhibitor of PKC, completely attenuated, whereas bisindolylmaleimide V, an inactive bisindolylmaleimide analog, did not influence bFGF's antiapoptotic action. Also, treatment with the PKC inhibitor, chelerythrine chloride, interfered with bFGF's ability to maintain calcium homeostasis. Western blot analysis revealed that SIGCs expressed PKCdelta, tau, lambda, and zeta. Of these PKC isoforms, only PKCdelta has been shown to be activated by TPA. In apoptotic SIGCs, PKCdelta levels were depleted. When PKCdelta levels were reduced by pretreatment with 500 nM TPA, neither bFGF nor 10 nM TPA suppressed apoptosis. Collectively, these observations suggest that bFGF maintains [Ca2+]i and thereby SIGC viability through a PKCdelta-dependent pathway.

Nongenomic effects of aldosterone on Ca2+ in M-1 cortical collecting duct cells

BACKGROUND

Aldosterone at physiological levels induces rapid (<5 min) increases in intracellular protein kinase C (PKC) activity and a rise in calcium and pH in mineralocorticoid hormone target epithelia, such as distal colon and sweat gland. The end targets of these rapid responses in epithelia are Na+/H+ exchange and K+ channels.

METHODS

The mouse cortical collecting duct (CCD) M-1 cell line was grown to confluency and loaded with Fura-2 for spectrofluorescence measurements of intracellular free calcium at 37 degrees C bathed in Krebs solution.

RESULTS

Aldosterone (1 nmol/L) produced a rapid, transient peak increase in [Ca2+]i in M-1 cells. This effect was abolished upon removal of extracellular Ca2+, but was unaffected by pretreatment with spironolactone (10 micromol/L) or actinomycin D (10 micromol/L). However, pretreatment with the specific PKC inhibitor chelerythrine chloride (1 micromol/L) prevented the aldosterone-induced rise in [Ca2+]i. Dexamethasone, at a concentration 10,000-fold higher than aldosterone (10 micromol/L), also produced a transient increase in [Ca2+]i, but this response was significantly smaller than that of aldosterone. In contrast, hydrocortisone had no effect on [Ca2+]i at either nmol/L or micromol/L concentrations. Both of the sex steroids, 17beta-estradiol (10 nmol/L) and progesterone (10 nmol/L), induced protein kinase C-dependent increases in [Ca2+]i.

CONCLUSIONS

Aldosterone and sex steroid hormones activate intracellular calcium signaling in CCD cells via a nongenomic PKC-dependent pathway, which may have important implications for renal transport.

Rapid, nongenomic steroid actions: A new age?

In the traditional theory of steroid action, steroids bind to intracellular receptors and modulate nuclear transcription after translocation of steroid-receptor complexes into the nucleus. Due to similarities of molecular structure, specific receptors for steroids, vitamin D(3) derivatives, and thyroid hormone are considered to represent a superfamily of steroid receptors. While genomic steroid effects characterized by their delayed onset of action and their sensitivity to blockers of transcription and protein synthesis have been known for several decades, rapid actions of steroids have been more widely recognized and characterized in detail only recently. Rapid effects of steroids, thyroid hormones, and the steroid hormone metabolite of vitamin D(3), 1alpha, 25-dihydroxyvitamin D(3), on cellular signaling and function may be transmitted by specific membrane receptors. Binding sites in membranes have been characterized, exposing binding features compatible with an involvement in rapid steroid signaling. Characteristics of putative membrane receptors are completely distinct from intracellular steroid receptors, a fact which is further supported by the inability of classic steroid receptor antagonists to block nongenomic steroid actions. A putative progesterone membrane receptor has been cloned and functionally expressed with regard to progesterone binding. Development of drugs that specifically affect nongenomic action alone or even both modes of action may find applications in various, areas such as in the cardiovascular and central nervous systems and treatment of preterm labor, infertility, and electrolyte abnormalities.

Rapid nongenomic effects of aldosterone in mineralocorticoid-receptor-knockout mice

In addition to genomic effects of aldosterone, rapid nongenomic effects of steroids have been reported in various tissues that were clearly incompatible with a genomic action of aldosterone. Rapid effects of aldosterone involve second messengers such as calcium and cAMP. Specific high affinity binding sites for aldosterone have been characterized in membranes for different cells, which probably transmit those rapid steroid effects. To date, it is unclear if these binding sites are modified classical mineralocorticoid receptors (MR) or if they represent an unrelated receptor protein. The aim of the present study was to investigate whether rapid aldosterone action still occurs in the absence of the classical MR. For this purpose we used the model of MR knockout mice. Rapid effects were analyzed in skin cells, measuring intracellular calcium and cAMP levels after stimulation with aldosterone. We found that rapid effects are not only present in MR knockout mice, but that the effects are even larger than in wild-type mice cells. The results of the present study demonstrate that the classic MR is dispensable for rapid aldosterone action. The study, thus, proves that a receptor different from the classic intracellular receptor is involved in rapid aldosterone signaling.

Regulatory mechanism of polarized membrane transport by glucocorticoid in renal proximal tubule cells: involvement of [Ca2+]i

We examined the effect of glucocorticoids on brush border membrane transporters and, furthermore, the involvement of Ca2+ in its action in the primary cultured rabbit renal proximal tubule cells (PTCs). Dexamethasone (DEX, 10(-9) M) decreased Pi uptake by 17%; whereas DEX affected neither alpha-methyl-glucopyranoside (alpha-MG) uptake nor Na+ uptake. The DEX-induced inhibition of Pi uptake was due to a decrease of V(max). In contrast, other steroid hormones such as progesterone, testosterone, and 17beta-estradiol (10(-9) M) did not induce inhibition of Pi uptake. In order to examine the involvement of Ca2+ in DEX-induced inhibition of Pi uptake, PTCs were treated with A 23187 (10(-6) M, Ca2+ ionophore). A 23187 also inhibited Pi uptake, mimicking DEX action in Pi uptake. Treatments with W-7 (10(-4) M, calmodulin dependent kinase inhibitor), KN-62 (10(-6) M, Ca2+/calmodulin-dependent protein kinase II inhibitor), and BAPTA/AM (10(-6) M) or TMB-8 (10(-4) M) (intracellular Ca2+ mobilization blockers) blocked the DEX-induced inhibition of Pi uptake. However, nifedifine, methoxyverapamil (10(-6) M, L-type Ca2+ channel blockers), and EGTA (1 mM, extracellular Ca2+ chelator) did not block it. In conclusion, DEX inhibited Pi uptake via, in part, Ca2+/calmodulin pathway mediated by intracellular Ca2+ mobilization in the PTCs.

Rapid responses to aldosterone in human distal colon

Aldosterone at normal physiological levels induces rapid increases in intracellular calcium and pH in human distal colon. The end target of these rapid signaling responses are basolateral K+ channels. Using spectrofluorescence microscopy and Ussing chamber techniques, we have shown that aldosterone activates basolateral Na/H exchange via a protein kinase C and calcium-dependent signaling pathway. The resultant intracellular alkalinization up-regulates an adenosine triphosphate (ATP)-dependent K+ channel (K(ATP)) and inhibits a Ca2+ -dependent K+ channel (K(Ca)). In Ussing chamber experiments, we have shown that the K(ATP) channel is required to drive sodium absorption, whereas the K(Ca) channel is necessary for both cyclic adenosine monophosphate and calcium-dependent chloride secretion. The rapid effects of aldosterone on intracellular calcium, pH, protein kinase C and K(ATP), K(Ca) channels are insensitive to cycloheximide, actinomycin D, and spironalactone, indicating a nongenomic mechanism of action. We propose that the physiological role for the rapid nongenomic effect of aldosterone is to prime pluripotential epithelia for absorption by simultaneously up-regulating K(ATP) channels to drive absorption through surface cells and down-regulating the secretory capacity by inhibiting K(Ca) channels involved in secretion through crypt cells.