Concomitant activation of Cl- and K+ currents by secretory stimulation in human epithelial cells

Zinc inhibits cAMP-stimulated Cl secretion via basolateral K-channel blockade in rat ileum

Zn, an essential micronutrient and second most abundant trace element in cell and tissues, reduces stool output when administered to children with acute diarrhea. The mechanism by which Zn improves diarrhea is not known but could result from stimulating Na absorption and/or inhibiting anion secretion. The aim of this study was to investigate the direct effect of Zn on intestinal epithelial ion absorption and secretion. Rat ileum was partially stripped of serosal and muscle layers, and the mucosa was mounted in lucite chambers. Potential difference and short-circuit current were measured by conventional current-voltage clamp method. 86Rb efflux and uptake were assessed for serosal K channel and Na-K-2Cl cotransport activity, respectively. Efflux experiments were performed in isolated cells preloaded with 86Rb in the presence of ouabain and bumetanide, whereas uptake experiments were performed in low-Cl isotonic buffer containing Ba and ouabain. Neither mucosal nor serosal Zn affected glucose-stimulated Na absorption. In contrast, forskolin-induced Cl secretion was markedly reduced by serosal but not mucosal addition of Zn. Zn also substantially reversed the increase in Cl secretion induced by 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) with half-maximal inhibitory concentration of 0.43 mM. In contrast, serosal Zn did not alter Cl secretion stimulated by carbachol, a Ca-dependent agonist. Zn inhibited 8-BrcAMP-stimulated 86Rb efflux but not carbachol-stimulated 86Rb efflux. Zn had no effect on bumetanide-sensitive 86Rb uptake, Na-K-ATPase, or CFTR. We conclude from these studies that Zn inhibits cAMP-induced Cl secretion by blocking basolateral membrane K channels.

Chloride and potassium channel function in alveolar epithelial cells

Electrolyte transport across the adult alveolar epithelium plays an important role in maintaining a thin fluid layer along the apical surface of the alveolus that facilitates gas exchange across the epithelium. Most of the work published on the transport properties of alveolar epithelial cells has focused on the mechanisms and regulation of Na(+) transport and, in particular, the role of amiloride-sensitive Na(+) channels in the apical membrane and the Na(+)-K(+)-ATPase located in the basolateral membrane. Less is known about the identity and role of Cl(-) and K(+) channels in alveolar epithelial cells, but studies are revealing important functions for these channels in regulation of alveolar fluid volume and ionic composition. The purpose of this review is to examine previous work published on Cl(-) and K(+) channels in alveolar epithelial cells and to discuss the conclusions and speculations regarding their role in alveolar cell transport function.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

K+ channel cAMP activated in guinea pig gallbladder epithelial cells

In guinea pig gallbladder epithelial cells, an increase in intracellular cAMP levels elicits the rise of anion channel activity. We investigated by patch-clamp techniques whether K(+) channels were also activated. In a cell-attached configuration and in the presence of theophylline and forskolin or 8-Br-cAMP in the cellular incubation bath, an increase of the open probability (P(o)) values for Ca(2+)-activated K(+) channels with a single-channel conductance of about 160 pS, for inward current, was observed. The increase in P(o) of these channels was also seen in an inside-out configuration and in the presence of PKA, ATP, and cAMP, but not with cAMP alone; phosphorylation did not influence single-channel conductance. In the inside-out configuration, the opioid loperamide (10(-5) M) was able to reduce P(o) when it was present either in the microelectrode filling solution or on the cytoplasmic side. Detection in the epithelial cells by RT-PCR of the mRNA corresponding to the alpha subunit of large-conductance Ca(2+)-activated K(+) channels (BK(Ca)) indicates that this gallbladder channel could belong to the BK family. Immunohistochemistry experiments confirm that these cells express the BK alpha subunit, which is located on the apical membrane. Other K(+) channels with lower conductance (40 pS) were not activated either by 8-Br-cAMP (cell-attached) or by PKA + ATP + cAMP (inside-out). These channels were insensitive to TEA(+) and loperamide. The data demonstrate that under conditions that induce secretion, phosphorylation activates anion channels as well as Ca(2+)-dependent, loperamide-sensitive K(+) channels present on the apical membrane.

Ca(2+) signaling in porcine duodenal glands by muscarinic receptor activation

The duodenal glands have been thought to play an important role in defense against proximal duodenal ulcer; however, the secretory mechanisms of these glands remain to be determined. In isolated duodenal acinar cells of the pig, we investigated the effects of ACh on intracellular Ca(2+) concentration ([Ca(2+)](i)) and on membrane currents with fura 2 fluorometry and the patch clamp technique. ACh caused a transient increase in [Ca(2+)](i), and the increase was markedly inhibited by atropine or 4-diphenylacetoxy-N-methylpiperidine methiodide but not by hexamethonium, pirenzepine, or methoctramine. The expression of mRNA for the M(3) subtype far exceeded that for either M(1) or M(2) as revealed by real-time quantitative PCR and in situ hybridization. The rise in [Ca(2+)](i) evoked by ACh was largely inhibited by thapsigargin but slightly affected by extracellular Ca(2+) deprivation. Caffeine had no effect on [Ca(2+)](i). ACh elicited Ca(2+)-dependent K(+) currents, a finding similar to the response to inositol 1,4,5,-trisphosphate applied intracellularly. These results suggest the presence of M(3) receptors linked to Ca(2+) release in porcine duodenal glands.

Differential expression of KvLQT1 and its regulator IsK in mouse epithelia

KCNQ1 is the human gene responsible in most cases for the long QT syndrome, a genetic disorder characterized by anomalies in cardiac repolarization leading to arrhythmias and sudden death. KCNQ1 encodes a pore-forming K+ channel subunit termed KvLQT1 which, in association with its regulatory beta-subunit IsK (also called minK), produces the slow component of the delayed-rectifier cardiac K+ current. We used in situ hybridization to localize KvLQT1 and IsK mRNAs in various tissues from adult mice. We showed that KvLQT1 mRNA expression is widely distributed in epithelial tissues, in the absence (small intestine, lung, liver, thymus) or presence (kidney, stomach, exocrine pancreas) of its regulator IsK. In the kidney and the stomach, however, the expression patterns of KvLQT1 and IsK do not coincide. In many tissues, in situ data obtained with the IsK probe coincide with beta-galactosidase expression in IsK-deficient mice in which the bacterial lacZ gene has been substituted for the IsK coding region. Because expression of KvLQT1 in the presence or absence of its regulator generates a K+ current with different biophysical characteristics, the role of KvLQT1 in epithelial cells may vary depending on the expression of its regulator IsK. The high level of KvLQT1 expression in epithelial tissues is consistent with its potential role in K+ secretion and recycling, in maintaining the resting potential, and in regulating Cl- secretion and/or Na+ absorption.

Ca2+-activated Cl- channels can substitute for CFTR in stimulation of pancreatic duct bicarbonate secretion

This study addresses the mechanisms by which a defect in CFTR impairs pancreatic duct bicarbonate secretion in cystic fibrosis. We used control (PANC-1) and CFTR-deficient (CFPAC-1; DeltaF508 mutation) cell lines and measured HCO3- extrusion by the rate of recovery of intracellular pH after an alkaline load and recorded whole cell membrane currents using patch clamp techniques. 1) In PANC-1 cells, cAMP causes parallel activation of Cl- channels and of HCO3- extrusion by DIDS-sensitive and Na+-independent Cl-/HCO3- exchange, both effects being inhibited by Cl- channel blockers NPPB and glibenclamide. 2) In CFPAC-1 cells, cAMP fails to stimulate Cl-/HCO3- exchange and Cl- channels, except after promoting surface expression of DeltaF508-CFTR by glycerol treatment. Instead, raising intracellular Ca2+ concentration to 1 micromol/l or stimulating purinergic receptors with ATP (10 and 100 micromol/l) leads to parallel activation of Cl- channels and HCO3- extrusion. 3) K+ channel function is required for coupling cAMP- and Ca2+-dependent Cl- channel activation to effective stimulation of Cl-/HCO3- exchange in control and CF cells, respectively. It is concluded that stimulation of pancreatic duct bicarbonate secretion via Cl-/HCO3- exchange is directly correlated to activation of apical membrane Cl- channels. Reduced bicarbonate secretion in cystic fibrosis results from defective cAMP-activated Cl- channels. This defect is partially compensated for by an increased sensitivity of CF cells to purinergic stimulation and by alternative activation of Ca2+-dependent Cl- channels, mechanisms of interest with respect to possible treatment of cystic fibrosis and of related chronic pancreatic diseases.

A synthetic channel-forming peptide induces Cl(-) secretion: modulation by Ca(2+)-dependent K(+) channels

A synthetic Cl(-) channel-forming peptide, C-K4-M2GlyR, applied to the apical membrane of human epithelial cell monolayers induces transepithelial Cl(-) and fluid secretion. The sequence of the core peptide, M2GlyR, corresponds to the second membrane-spanning region of the glycine receptor, a domain thought to line the pore of the ligand-gated Cl(-) channel. Using a pharmacological approach, we show that the flux of Cl(-) through the artificial Cl(-) channel can be regulated by modulating basolateral K(+) efflux through Ca(2+)-dependent K(+) channels. Application of C-K4-M2GlyR to the apical surface of monolayers composed of human colonic cells of the T84 cell line generated a sustained increase in short-circuit current (I(SC)) and caused net fluid secretion. The current was inhibited by the application of clotrimazole, a non-specific inhibitor of K(+) channels, and charybdotoxin, a potent inhibitor of Ca(2+)-dependent K(+) channels. Direct activation of these channels with 1-ethyl-2-benzimidazolinone (1-EBIO) greatly amplified the Cl(-) secretory current induced by C-K4-M2GlyR. The effect of the combination of C-K4-M2GlyR and 1-EBIO on I(SC) was significantly greater than the sum of the individual effects of the two compounds and was independent of cAMP. Treatment with 1-EBIO also increased the magnitude of fluid secretion induced by the peptide. The cooperative action of C-K4-M2GlyR and 1-EBIO on I(SC) was attenuated by Cl(-) transport inhibitors, by removing Cl(-) from the bathing solution and by basolateral treatment with K(+) channel blockers. These results indicate that apical membrane insertion of Cl(-) channel-forming peptides such as C-K4-M2GlyR and direct activation of basolateral K(+) channels with benzimidazolones may coordinate the apical Cl(-) conductance and the basolateral K(+) conductance, thereby providing a pharmacological approach to modulating Cl(-) and fluid secretion by human epithelia deficient in cystic fibrosis transmembrane conductance regulator Cl(-) channels.

Basolateral K+ channel involvement in forskolin-activated chloride secretion in human colon

1. In this study we investigated the role of basolateral potassium transport in maintaining cAMP-activated chloride secretion in human colonic epithelium. 2. Ion transport was quantified in isolated human colonic epithelium using the short-circuit current technique. Basolateral potassium transport was studied using nystatin permeabilization. Intracellular calcium measurements were obtained from isolated human colonic crypts using fura-2 spectrofluorescence imaging. 3. In intact isolated colonic strips, forskolin and prostaglandin E2 (PGE2) activated an inward transmembrane current (ISC) consistent with anion secretion (for forskolin DeltaISC = 63.8+/-6.2 microA cm(-2), n = 6; for PGE2 DeltaISC = 34.3+/-5.2 microA cm(-2), n = 6). This current was inhibited in chloride-free Krebs solution or by inhibiting basolateral chloride uptake with bumetanide and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid DIDS). 4. The forskolin- and PGE2-induced chloride secretion was inhibited by basolateral exposure to barium (5 mM), tetrapentylammonium (10 microM) and tetraethylammonium (10 mM). 5. The transepithelial current produced under an apical to serosal K+ gradient in nystatin-perforated colon is generated at the basolateral membrane by K+ transport. Forskolin failed to activate this current under conditions of high or low calcium and failed to increase the levels of intracellular calcium in isolated crypts 6. In conclusion, we propose that potassium recycling through basolateral K+ channels is essential for cAMP-activated chloride secretion.

beta3-adrenoceptor control the cystic fibrosis transmembrane conductance regulator through a cAMP/protein kinase A-independent pathway

In human cardiac myocytes, we have previously identified a functional beta3-adrenoceptor in which stimulation reduces action potential duration. Surprisingly, in cardiac biopsies obtained from cystic fibrosis patients, beta3-adrenoceptor agonists produced no effects on action potential duration. This result suggests the involvement of cystic fibrosis transmembrane conductance regulator (CFTR) chloride current in the electrophysiological effects of beta3-adrenoceptor stimulation in non-cystic fibrosis tissues. We therefore investigated the control of CFTR activity by human beta3-adrenoceptors in a recombinant system: A549 human cells were intranuclearly injected with plasmids encoding CFTR and beta3-adrenoceptors. CFTR activity was functionally assayed using the 6-methoxy-N-(3-sulfopropyl)quinolinium fluorescent probe and the patch-clamp technique. Injection of CFTR-cDNA alone led to the expression of a functional CFTR protein activated by cAMP or cGMP. Co-expression of CFTR (but not of mutated DeltaF508-CFTR) with high levels of beta3-adrenoceptor produced an increased halide permeability under base-line conditions that was not further sensitive to cAMP or beta3-adrenoceptor stimulation. Patch-clamp experiments confirmed that CFTR channels were permanently activated in cells co-expressing CFTR and a high level of beta3-adrenoceptor. Permanent CFTR activation was not associated with elevated intracellular cAMP or cGMP levels. When the expression level of beta3-adrenoceptor was lowered, CFTR was not activated under base-line conditions but became sensitive to beta3-adrenoceptor stimulation (isoproterenol plus nadolol, SR 58611, or CGP 12177). This later effect was not prevented by protein kinase A inhibitors. Our results provide molecular evidence that CFTR but not mutated DeltaF508-CFTR is regulated by beta3-adrenoceptors expression through a protein kinase A-independent pathway.

Berberine inhibits ion transport in human colonic epithelia

The effects of berberine on ion transport in both human colonic mucosal epithelia and an intestinal epithelial cell line (T84) were examined. Berberine (concentration range 0-500 microM) reduced both basal and stimulated ion transport responses in human colonic mucosae in a manner which was non-specific for Ca2+ -or cAMP-mediated signals. Similarly, in cultured intestinal epithelial monolayers, berberine inhibited Ca2+ -and cAMP-mediated responses indicating an inhibitory activity directly at the level of the epithelium rather than an indirect effect through other mucosal element(s). Berberine did not alter the rate of generation of cAMP by adenylyl cyclase or the activity of protein kinase A, the effector enzyme of the cAMP pathway. Berberine inhibited carbachol-stimulated 86Rb+ efflux from T84 monolayers. Berberine also inhibited K+ conductance in apically-permeabilised re-sected mucosae. These results indicate i) that berberine exerts an anti-secretory action directly upon epithelial cells and ii) the mechanism of action may be at the level of blockade of K+ channels.

Levamisole inhibits intestinal Cl- secretion via basolateral K+ channel blockade

BACKGROUND & AIMS

Phenylimidazothiazoles have recently been shown to activate wild-type and mutant cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels in transfected cells and were proposed as therapy for cystic fibrosis. The aim of this study was to investigate the effects of phenylimidazothiazoles on regulated transepithelial Cl- transport in intact epithelia.

METHODS

T84 intestinal epithelial cells grown on permeable supports and stripped human colonic mucosal sheets were studied by conventional current-voltage clamping. Selective permeabilization of apical or basolateral membranes with the monovalent ionophore nystatin was used to isolate basolateral K+ and apical Cl- channel activity, respectively. 86Rb+ uptake was assessed for Na/K/2Cl cotransporter and Na+,K(+)-adenosine triphosphatase activity.

RESULTS

In T84 monolayers and human colon, levamisole and its brominated derivative bromotetramisole failed to activate transepithelial secretion. In fact, these compounds dose-dependently inhibited secretory responses to the cyclic adenosine monophosphate agonist forskolin and the Ca2+ agonist carbachol. In permeabilized T84 monolayers, phenylimidazothiazoles weakly activated apical Cl- currents (consistent with their reported action on CFTR) and did not affect bumetanide-sensitive or bumetanide-insensitive 86+Rb+ uptake. Instead, they profoundly inhibited the basolateral Ba(2+)-sensitive and Ba(2+)-insensitive K+ currents.

CONCLUSIONS

Phenylimidazothiazoles block K+ channels required for Cl(-)-secretory responses elicited by diverse pathways in model epithelia and native colon, an effect that outweighs their ability to activate apical Cl- channels.

Hyperexpression of recombinant CFTR in heterologous cells alters its physiological properties

We investigated whether high levels of expression of the cystic fibrosis transmembrane conductance regulator (CFTR) would alter the functional properties of newly synthesized recombinant proteins. COS-7, CFPAC-1, and A549 cells were intranuclearly injected with a Simian virus 40-driven pECE-CFTR plasmid and assayed for halide permeability using the 6-methoxy-N-(3-sulfopropyl)quinolinium fluorescent probe. With increasing numbers of microinjected pECE-CFTR copies, the baseline permeability to halide dose dependently increased, and the response to adenosine 3',5'-cyclic monophosphate (cAMP) stimulation decreased. In cells hyperexpressing CFTR, the high level of halide permeability was reduced when a cell metabolism poisoning cocktail was applied to decrease intracellular ATP and, inversely, was increased by orthovanadate. In CFPAC-1 cells investigated with the patch-clamp technique, CFTR hyperexpression led to a time-independent nonrectifying chloride current that was not sensitive to cAMP stimulation. CFPAC-1 cells hyperexpressing CFTR exhibited no outward rectifying chloride current nor inward rectifying potassium current either spontaneously or under cAMP stimulation. We conclude that hyperexpression of recombinant CFTR proteins modifies their properties inasmuch as 1) CFTR channels are permanently activated and not susceptible to cAMP regulation and 2) they lose their capacity to regulate heterologous ionic channels.

Modulation of K+ channels by arachidonic acid in T84 cells. I. Inhibition of the Ca(2+)-dependent K+ channel

The Cl- secretory response of colonic cells to Ca(2+)-mediated agonists is transient despite a sustained elevation of intracellular Ca2+. We evaluated the effects of second messengers proposed to limit Ca(2+)-mediated Cl- secretion on the basolateral membrane, Ca(2+)-dependent K+ channel (Kca) in colonic secretory cells, T84. Neither protein kinase C (PKC) nor inositol tetrakisphosphate (1,3,4,5 or 3,4,5,6 form) affected Kca in excised inside-out patches. In contrast, arachidonic acid (AA; 3 microM) potently inhibited Kca, reducing NP0, the product of number of channels and channel open probability, by 95%. The apparent inhibition constant for this AA effect was 425 nM. AA inhibited Kca in the presence of both indomethacin and nordihydroguaiaretic acid, blockers of the cyclooxygenase and lipoxygenase pathways. In the presence of albumin, the effect of AA on Kca was reversed. A similar effect of AA was observed on Kca during outside-out recording. We determined also the effect of the cis-unsaturated fatty acid linoleate, the trans-unsaturated fatty acid elaidate, and the saturated fatty acid myristate. At 3 microM, all of these fatty acids inhibited Kca, reducing NP0 by 72-86%. Finally, the effect of the cytosolic phospholipase A2 inhibitor arachidonyltrifluoromethyl ketone (AACOCF3) on the carbachol-induced short-circuit current (Isc) response was determined. In the presence of AACOCF3, the peak carbachol-induced Isc response was increased approximately 2.5-fold. Our results suggest that AA generation induced by Ca(2+)-mediated agonists may contribute to the dissociation observed between the rise in intracellular Ca2+ evoked by these agonists and the associated Cl- secretory response.

The antifungal antibiotic, clotrimazole, inhibits Cl- secretion by polarized monolayers of human colonic epithelial cells

Clotrimazole (CLT) prevents dehydration of the human HbSS red cell through inhibition of Ca++-dependent (Gardos) K+ channels in vitro (1993. J. Clin Invest. 92:520-526.) and in patients (1996. J. Clin Invest. 97:1227-1234.). Basolateral membrane K+ channels of intestinal crypt epithelial cells also participate in secretagogue-stimulated Cl- secretion. We examined the ability of CLT to block intestinal Cl- secretion by inhibition of K+ transport. Cl- secretion was measured as short-circuit current (Isc) across monolayers of T84 cells. CLT reversibly inhibited Cl- secretory responses to both cAMP- and Ca2+-dependent agonists with IC50 values of approximately 5 microM. Onset of inhibition was more rapid when CLT was applied to the basolateral cell surface. Apical Cl- channel and basolateral NaK2Cl cotransporter activities were unaffected by CLT treatment as assessed by isotopic flux measurement. In contrast, CLT strongly inhibited basolateral 86Rb efflux. These data provide evidence that CLT reversibly inhibits Cl- secretion elicited by cAMP-, cGMP-, or Ca2+-dependent agonists in T84 cells. CLT acts distal to the generation of cAMP and Ca2+ signals, and appears to inhibit basolateral K+ channels directly. CLT and related drugs may serve as novel antidiarrheal agents in humans and animals.

Expression of CFTR controls cAMP-dependent activation of epithelial K+ currents

The perforated-patch configuration of the patch-clamp technique was used to record whole cell currents from human epithelial CFPAC-1 cells defective for functional cystic fibrosis transmembrane conductance regulator (CFTR). In CFPAC-1 cells, adenosine 3',5'-cyclic monophosphate (cAMP) stimulation with forskolin (10 microM) plus 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (400 microM) activated neither Cl- nor K+ currents. In the same cells transfected with wild-type CFTR gene, cAMP stimulation produced activation of both Cl- and K+ currents. In Cl(-)-depleted medium (gluconate as a substitute), cAMP stimulation evoked a K+ current in CFTR-transfected but not in untransfected CFPAC-1 cells. This cAMP-evoked K+ current was the sum of two components: 1) a time-independent inwardly rectifying component, and 2) a slowly relaxing component activated at positive voltages. Increasing intracellular Ca2+ with ionomycin (1 microM) activated K+ currents in either transfected or untransfected cells. In transfected cells, blocking the CFTR conductance with high-concentration glibenclamide (100 microM) reduced the K+ current when activated by cAMP but not when activated by Ca2+. Pretreating CFTR-transfected cells for 48 h with interferon-gamma downregulated CFTR gene expression and reduced cAMP but not Ca2+ activation of the whole cell K+ current. From these results, we conclude that functional membrane CFTR protein influences activation by cAMP of epithelial K+ currents.

ATP-sensitive K+ channels regulated by intracellular Ca2+ and phosphorylation in normal (T84) and cystic fibrosis (CFPAC-1) epithelial cells

The elementary K+ conductance activated by the cAMP or the Ca2+ second messenger pathways was investigated in the model salt-secreting epithelium, the human T84 cell line. Under Cl(-)-free conditions, an inwardly rectifying whole-cell K+ current was evoked by either forskolin 10 (mumol/l) or acetylcholine 1 (mumol/l) and blocked by extracellular charybdotoxin 10 (nmol/l). In the cell-attached mode, both secretory agonists induced the opening of a channel showing inward rectification with a unitary chord conductance of 36.8 +/- 2.5 pS (n = 26) for inward currents. In inside-out patches, a 35-pS inwardly rectifying K+ channel that corresponded to the channel recorded in the cell-attached configuration was recorded in the presence of 0.3 mumol/l free Ca2+ at the inner side of the membrane. This channel was blocked by Ba2+ (5 mumol/l) and by charybdotoxin (50 nmol/l). Its open probability was enhanced by intracellular Ca2+ with and EC50 of 0.25 mumol/l and strongly reduced by intracellular MgATP with an IC50 of 600 mumol/l. In the continuous presence of ATP, the channel activity was consistently increased by 125 kU/l catalytic subunit of cAMP-dependent protein kinase. In the cystic fibrosis pancreatic duct cell line CFPAC-1, a K+ channel was also recorded, with similar characteristics and regulation as the 35-pS channel in T84 cells. We conclude that an ATP-sensitive K+ channel regulated by intracellular Ca2+ and phosphorylation supports the main K+ current activated by secretory agonists in normal cystic fibrosis cell lines.(ABSTRACT TRUNCATED AT 250 WORDS)

Interferon-gamma downregulates CFTR gene expression in epithelial cells

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene, resulting in defective transepithelial Cl- transport. The regulation of CF gene expression is not fully understood. We report that interferon-gamma (IFN-gamma), but not IFN-alpha or -beta, downregulates CFTR mRNA levels in two colon-derived epithelial cell lines, HT-29 and T84, in a time- and concentration (from 0.1 IU/ml)-dependent manner. IFN-gamma has no effect on the transcription rate of the CFTR gene but reduces CFTR mRNA half-life, indicating that it exerts a posttranscriptional regulation of CFTR expression, at least partly, through destabilization of the transcripts. Cells treated with IFN-gamma contain subnormal amounts of 165-kDa CFTR protein. Assays of adenosine 3',5'-cyclic monophosphate-stimulated 36Cl- efflux and whole cell currents show that CFTR function is diminished in IFN-gamma-treated cells. IFN-gamma and tumor necrosis factor-alpha synergistically reduce CFTR gene expression. Our results suggest that production of these cytokines in response to bacterial infections and inflammatory disorders may alter transmembrane Cl- transport.