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

Dexamethasone reduces airway epithelial Cl- secretion by rapid non-genomic inhibition of KCNQ1, KCNN4 and KATP K+ channels

Basolateral membrane K⁺ channels play a key role in basal and agonist stimulated Cl^- transport across airway epithelial cells by generating a favourable electrical driving force for Cl^- efflux. The K⁺ channel sub-types and molecular mechanisms of regulation by hormones and secretagoues are still poorly understood. Here we have identified the type of K⁺ channels involved in cAMP and Ca²⁺ stimulated Cl^- secretion and uncovered a novel anti-secretory effect of dexamethasone mediated by inhibition of basolateral membrane K⁺ channels in a human airway cell model of 16HBE14o^- cells commonly used for ion transport studies. Dexamethasone produced a rapid inhibition of transepithelial chloride ion secretion under steady state conditions and after stimulation with cAMP agonist (forskolin) or a Ca²⁺ mobilizing agonist (ATP). Our results show three different types of K⁺ channels are targeted by dexamethasone to reduce airway secretion, namely Ca²⁺-activated secretion via KCNN4 (KCa3.1) channels and cAMP-activated secretion via KCNQ1 (Kv7.1) and KATP (Kir6.1,6.2) channels. The down-regulation of KCNN4 and KCNQ1 channel activities by dexamethasone involves rapid non-genomic activation of PKCα and PKA signalling pathways, respectively. Dexamethasone signal transduction for PKC and PKA activation was demonstrated to occur through a rapid non-genomic pathway that did not implicate the classical nuclear receptors for glucocorticoids or mineralocorticoids but occurred via a novel signalling cascade involving sequentially a G_i-protein coupled receptor, PKC, adenylyl cyclase Type IV, cAMP, PKA and ERK1/2 activation. The rapid, non-genomic, effects of dexamethasone on airway epithelial ion transport and cell signalling introduces a new paradigm for glucocorticoid actions in lung epithelia which may serve to augment the anti-inflammatory activity of the steroid and enhance its therapeutic potential in treating airway hypersecretion in asthma and COPD.

Rapid anti-secretory effects of glucocorticoids in human airway epithelium

Glucocorticoids are anti-inflammatory molecules classically described as acting through a genomic pathway. Similar to many steroid hormones, glucocorticoids also induce rapid non-genomic responses. The present paper provides a general overview of the rapid non-genomic effects of glucocorticoids in airway and will be mainly focused on a retrospective of the authors work on rapid effects of glucocorticoids in airway epithelial cell transport. Using fluorescence microscopy, short circuit current measurements in human bronchial epithelial (16HBE14o(-)) cells, we reported rapid non-genomic effects of dexamethasone on cell signalling and ion transport. Dexamethasone (1 nM) rapidly stimulated Na(+)/H(+) exchanger activity and pH(i) regulation in 16HBE14o(-) cells. Dexamethasone also produced a rapid decrease of intracellular [Ca(2+)](i) to a new steady state concentration and inhibited the large and transient [Ca(2+)](i) increase induced by apical adenosine tri-phosphate (ATP). Dexamethasone also reduced by 1/3 the Ca(2+)-dependent Cl(-) secretion induced by apical ATP. The rapid effects of dexamethasone on intracellular pH and Ca(2+) were not affected by inhibitors of transcription, cycloheximide or by the classical glucocorticoid and mineralocorticoid receptors antagonists, RU486 and spironolactone, respectively. The rapid responses to glucocorticoid were reduced by the inhibitors of adenylated cyclase, cAMP-dependent protein kinase (PKA) and mitogen-activated protein kinase (ERK1/2). Our results demonstrate, that dexamethasone at low concentrations, rapidly regulates intracellular pH, Ca(2+) and PKA activity and inhibits Cl(-) secretion in human bronchial epithelial cells via a non-genomic mechanism which neither involve the classical glucocorticoid nor mineralocorticoid receptor.

Role of ATP in DNA synthesis of renal proximal tubule cells: involvement of calcium, MAPKs, and CDKs

Although ATP has been shown to act as a modulator in various kidney functions, its effect on renal proximal tubule cell (PTC) proliferation has not been elucidated. This study investigated the effect of ATP on cell proliferation and the effect of its related signal pathways on primary cultured PTCs. Treatment with >10(-5) M ATP for 1 h stimulated incorporation of thymidine and bromodeoxyuridine. ATP (10(-4) M)-induced stimulation of thymidine incorporation was blocked by suramin (a P2X and P2Y receptor antagonist), reactive blue 2 (a P2Y receptor antagonist), MRS-2159 (a P2X1 receptor antagonist), and MRS-2179 (a P2Y1 receptor antagonist). ATP increased intracellular Ca2+ concentration, which was blocked by suramin, methoxyverapamil, and EGTA. ATP-induced stimulation of cell proliferation was also blocked by EGTA (an extracellular Ca2+ chelator), methoxyverapamil (a Ca2+ antagonist), and nifedipine (an L-type Ca2+ channel blocker), suggesting a role for Ca2+ influx. ATP-induced phosphorylation of p38 and p44/42 MAPKs was blocked by nifedipine. ATP increased expression levels of cyclin-dependent kinase (CDK)-2, CDK-4, and cyclin E, which were blocked by suramin, reactive blue 2, MRS-2179, MRS-2159, and nifedipine. However, ATP decreased expression levels of p21WAF1/Cip1 and p27kip1. ATP-induced stimulation of thymidine incorporation and increase of CDK-2 and CDK-4 expression were blocked by SB-203580 (a p38 MAPK inhibitor) and PD-98059 (an MEK inhibitor), but not by SP-600125 (a JNK inhibitor). In conclusion, ATP stimulates proliferation by increasing intracellular Ca2+ concentration and activating p38, p44/42 MAPKs, and CDKs in PTCs.

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.