Mechanisms of anion secretion in Calu-3 human airway epithelial cells by 7,8-benzoquinoline

A Novel Role of Arrhythmia-Related Gene KCNQ1 Revealed by Multi-Omic Analysis: Theragnostic Value and Potential Mechanisms in Lung Adenocarcinoma

The early diagnosis, prognostic prediction, and personalized therapy of lung adenocarcinoma (LUAD) remains a challenging issue. KCNQ1 (potassium voltage-gated channel subfamily Q Member 1) is implicated in long QT syndrome (LQTS) and cardiac arrhythmia, while its significance in LUAD remains unclear. In this study, we aimed to explore the significance of KCNQ1 in terms of clinical value, tumor immunity, underlying mechanisms, and a precision medicine approach by means of multi-omics analysis. The association of KCNQ1 with LUAD was first explored. Both altered variants and high expression of KCNQ1 in a TCGA-LUAD cohort indicated a favorable outcome. KCNQ1 levels had a negative correlation with tumor proliferation index Ki67 levels. siRNA-knockdown of KCNQ1 promoted the migration ability of lung cancer cells. KCNQ1 levels were decreased in LUAD tissue compared to normal tissue. A receiver operating characteristic (ROC) curve indicated good diagnostic efficiency of KCNQ1. High KCNQ1 is associated with an immunoactive profile of immune infiltration and immunomodulators and is involved in the inhibition of the cell cycle and DNA replication. Lapatinib was identified as a potent drug for LUAD in the context of low KCNQ1. This study unveiled the significance of KCNQ1 in diagnosis and prognosis and provided a corresponding precision medicine strategy for LUAD.

The ubiquitous flavonoid quercetin is an atypical KCNQ potassium channel activator

Many commonly consumed plants are used as folk medicines, often with unclear molecular mechanisms. Recent studies uncovered the ubiquitous and influential KCNQ family of voltage-gated potassium (Kv) channels as a therapeutic target for several medicinal plant compounds. Capers - immature flower buds of Capparis spinosa - have been consumed for food and medicinal purposes for millennia. Here, we show that caper extract hyperpolarizes cells expressing KCNQ1 or KCNQ2/3 Kv channels. Capers are the richest known natural source of quercetin, the most consumed dietary flavonoid. Quercetin potentiated KCNQ1/KCNE1, KCNQ2/3 and KCNQ4 currents but, unusually, not KCNQ5. Strikingly, quercetin augmented both activation and inactivation of KCNQ1, via a unique KCNQ activation mechanism involving sites atop the voltage sensor and in the pore. The findings uncover a novel potential molecular basis for therapeutic effects of quercetin-rich foods and a new chemical space for atypical modes of KCNQ channel modulation.

Kaitlyn E. Redford and Geoffrey W. Abbott show that quercetin, a flavonoid highly expressed in capers, potentiates KCNQ currents to varying degrees depending on the subunit composition of the channel complex. By combining in silico docking, mutagenesis, and electrophysiology they show that this flavonoid can bind KCNQ channels atop the voltage sensor and within the pore module, highlighting an atypical mode of channel modulation.

The KCNE2 potassium channel β subunit is required for normal lung function and resilience to ischemia and reperfusion injury

The KCNE2 single transmembrane-spanning voltage-gated potassium (K_v) channel β subunit is ubiquitously expressed and essential for normal function of a variety of cell types, often via regulation of the KCNQ1 K_v channel. A polymorphism upstream of KCNE2 is associated with reduced lung function in human populations, but the pulmonary consequences of KCNE2 gene disruption are unknown. Here, germline deletion of mouse Kcne2 reduced pulmonary expression of potassium channel α subunits Kcnq1 and Kcnb1 but did not alter expression of other Kcne genes. Kcne2 colocalized and coimmunoprecipitated with Kcnq1 in mouse lungs, suggesting the formation of pulmonary Kcnq1-Kcne2 potassium channel complexes. Kcne2 deletion reduced blood O₂, increased CO₂, increased pulmonary apoptosis, and increased inflammatory mediators TNF-α, IL-6, and leukocytes in bronchoalveolar lavage (BAL) fluids. Consistent with increased pulmonary vascular leakage, Kcne2 deletion increased plasma, BAL albumin, and the BAL:plasma albumin concentration ratio. Kcne2^-/- mouse lungs exhibited baseline induction of the reperfusion injury salvage kinase pathway but were less able to respond via this pathway to imposed pulmonary ischemia/reperfusion injury (IRI). We conclude that KCNE2 regulates KCNQ1 in the lungs and is required for normal lung function and resistance to pulmonary IRI. Our data support a causal relationship between KCNE2 gene disruption and lung dysfunction.-Zhou, L., Köhncke, C., Hu, Z., Roepke, T. K., Abbott, G. W. The KCNE2 potassium channel β subunit is required for normal lung function and resilience to ischemia and reperfusion injury.

The ΔF508-CFTR mutation inhibits wild-type CFTR processing and function when co-expressed in human airway epithelia and in mouse nasal mucosa

BACKGROUND

Rescue or correction of CFTR function in native epithelia is the ultimate goal of CF therapeutics development. Wild-type (WT) CFTR introduction and replacement is also of particular interest. Such therapies may be complicated by possible CFTR self-assembly into an oligomer or multimer.

RESULTS

Surprisingly, functional CFTR assays in native airway epithelia showed that the most common CFTR mutant, ΔF508-CFTR (ΔF-CFTR), inhibits WT-CFTR when both forms are co-expressed. To examine more mechanistically, both forms of CFTR were transfected transiently in varying amounts into IB3-1 CF human airway epithelial cells and HEK-293 human embryonic kidney cells null for endogenous CFTR protein expression. Increasing amounts of ΔF-CFTR inhibited WT-CFTR protein processing and function in CF human airway epithelial cells but not in heterologous HEK-293 cells. Stably expressed ΔF-CFTR in clones of the non-CF human airway epithelial cell line, CALU-3, also showed reduction in cAMP-stimulated anion secretion and in WT-CFTR processing. An ultimate test of this dominant negative-like effect of ΔF-CFTR on WT-CFTR was the parallel study of two different CF mouse models: the ΔF-CFTR mouse and the bitransgenic CFTR mouse corrected in the gut but null in the lung and airways. WT/ΔF heterozygotes had an intermediate phenotype with regard to CFTR agonist responses in in vivo nasal potential difference (NPD) recordings and in Ussing chamber recordings of short-circuit current (ISC) in vitro on primary tracheal epithelial cells isolated from the same mice. In contrast, CFTR bitransgenic +/- heterozygotes had no difference in their responses versus +/+ wild-type mice.

CONCLUSIONS

Taken altogether, these data suggest that ΔF-CFTR and WT-CFTR co-assemble into an oligomeric macromolecular complex in native epithelia and share protein processing machinery and regulation at the level of the endoplasmic reticulum (ER). As a consequence, ΔF-CFTR slows WT-CFTR protein processing and limits its expression and function in the apical membrane of native airway epithelia. Implications of these data for the relative health of CF heterozygous carriers, for CFTR protein processing in native airway epithelia, and for the relative efficacy of different CF therapeutic approaches is significant and is discussed.

Anion secretion by a model epithelium: more lessons from Calu-3

Anion transport drives fluid into the airways and is essential for humidifying inspired air and supplying surface liquid for mucociliary transport. Despite the importance of airway secretion in diseases such as cystic fibrosis, the cellular mechanisms remain poorly understood, in part due to the small size and complicated structure of the submucosal glands that produce most of the fluid. The Calu-3 human lung adenocarcinoma cell line has become a popular model for studying airway secretion because it can be cultured as a flat sheet, expresses the cystic fibrosis transmembrane conductance regulator and several acinar cell markers, forms polarized monolayers with tight junctions, has robust cAMP-stimulated anion transport, and responds to secretagogues that regulate the glands in vivo. However, some properties of Calu-3 cells are less consistent with those of native tissue. In particular, Calu-3 monolayers do not secrete chloride when stimulated by forskolin under short-circuit conditions. Bicarbonate ions are thought to carry the short-circuit current (I(sc)) and the drive secretion of alkaline fluid, in contrast to the neutral pH secretions that are produced by submucosal glands. Calu-3 cells also have abnormal chromosomes and characteristics of both serous and mucus cells. In this article, we discuss Calu-3 as a model in light of our ongoing studies, which suggest that Calu-3 monolayers resemble submucosal glands more closely than was previously thought. For example, we find that net HCO(3)(-) flux fully accounts for I(sc) as previously suggested but Cl(-) is the main anion transported under physiological conditions. A novel, HCO(3)(-) -dependent mechanism of Cl(-) transport is emerging which may explain secretion by Calu-3 and perhaps other epithelial cells.

Effects of Cordyceps sinensis, Cordyceps militaris and their isolated compounds on ion transport in Calu-3 human airway epithelial cells

AIM OF THE STUDY

The traditional Chinese medicine Cordyceps sinensis (CS) (Clavicipitaceae) improves pulmonary function and is used to treat respiratory disease. Here, we compare the efficacy and mechanisms of action of Cordyceps sinensis and Cordyceps militaris (CM) (Clavicipitaceae) in Calu-3 human airway epithelial monolayer model.

MATERIAL AND METHODS

The extracts of Cordyceps sinensis and Cordyceps militaris, as well as their isolated compounds, cordycepin and adenosine, stimulated ion transport in a dose-dependent manner in Calu-3 monolayers. In subsequent experiments, transport inhibitor bumetanide and carbonic anhydrase inhibitor acetazolamide were added after Cordyceps sinensis and Cordyceps militaris extracts to determine their effects on Cl- and HCO3- movement.

RESULTS

The results suggested that Cordyceps sinensis and Cordyceps militaris extracts may affect the anion movement from the basolateral to apical compartments in the airway epithelia.

CONCLUSIONS

Basolateral Na+-K+-2Cl- cotransporter and apical cAMP-dependent cystic fibrosis transmembrane conductance regulator Cl- channel are involved in the process. The results provide the first evidence for the pharmacological mechanism of Cordyceps sinensis and Cordyceps militaris on respiratory tract.

The distribution of intermediate-conductance, calcium-activated, potassium (IK) channels in epithelial cells

Intermediate-conductance, calcium-activated, potassium (IK) channels were first identified by their roles in cell volume regulation, and were later shown to be involved in control of proliferation of lymphocytes and to provide a K+ current for epithelial secretory activity. Until now, there has been no systematic investigation of IK channel localization within different epithelia. IK channel immunoreactivity was present in most epithelia, where it occurred in surface membranes of epithelial cells. It was found in all stratified epithelia, including skin, cornea, oral mucosa, vaginal mucosa, urothelium and the oesophageal lining. It occurred in the ducts of fluid-secreting glands, the salivary glands, lacrimal glands and pancreas, and in the respiratory epithelium. A low level of expression was seen in serous acinar cells. It was also found in other epithelia with fluid-exchange properties, the choroid plexus epithelium, the ependyma, visceral pleura and peritoneum, bile ducts and intestinal lining epithelium. However, there was little or no expression in vascular endothelial cells, kidney tubules or collecting ducts, lung alveoli, or in sebaceous glands. It is concluded that the channel is present in surface epithelia (e.g. skin) where it has a cell-protective role against osmotic challenge, and in epithelia where there is anion secretion that is facilitated by a K+ current-dependent hyperpolarization. It was also in some epithelial cells where its roles are as yet unknown.

Discovery of pyrrolo[2,3-b]pyrazines derivatives as submicromolar affinity activators of wild type, G551D, and F508del cystic fibrosis transmembrane conductance regulator chloride channels

The cystic fibrosis transmembrane conductance regulator (CFTR) represents the main Cl(-) channel in the apical membrane of epithelial cells for cAMP-dependent Cl(-) secretion. Here we report on the synthesis and screening of a small library of 6-phenylpyrrolo[2,3-b]pyrazines (named RP derivatives) evaluated as activators of wild-type CFTR, G551D-CFTR, and F508del-CFTR Cl(-) channels. Iodide efflux and whole-cell patch-clamp recordings analysis identified RP107 [7-n-butyl-6-(4-hydroxyphenyl)[5H]-pyrrolo[2,3-b]pyrazine] as a submicromolar activator of wild-type (WT)-CFTR [human airway epithelial Calu-3 and WT-CFTR-Chinese hamster ovary (CHO) cells], G551D-CFTR (G551D-CFTR-CHO cells), and F508del-CFTR (in temperature-corrected human airway epithelial F508del/F508del CF15 cells). The structural analog RP108 [7-n-butyl-6-(4-chlorophenyl)[5H]pyrrolo[2,3-b]pyrazine], contrary to RP107, was a less potent activator only at micromolar concentrations. RP107 and RP108 did not have any effect on the cellular cAMP level. Activation was potentiated by low concentration of forskolin and inhibited by glibenclamide and CFTR(inh)-172 [3-[(3-trifluoromethyl)phenyl]-5-[(4-carboxyphenyl-)methylene]-2-thioxo-4-thiazolidinone]but not by calixarene or DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid). Finally, we found significant stimulation of short circuit current (I(sc)) by RP107 (EC(50) = 89 nM) and RP108 (EC(50) = 103 microM) on colon of Cftr(+)(/)(+) but not of Cftr(-/-) mice mounted in Ussing chamber. Stimulation of I(sc) was inhibited by glibenclamide but not affected by DIDS. These results show that RP107 stimulates wild-type CFTR and mutated CFTR, with submicromolar affinity by a cAMP-independent mechanism. Our preliminary structure-activity relationship study identified 4-hydroxyphenyl and 7-n-butyl as determinants required for activation of CFTR. The potency of these agents indicates that compounds in this class may be of therapeutic benefit in CFTR-related diseases, including cystic fibrosis.

Protease-activated receptor regulation of Cl- secretion in Calu-3 cells requires prostaglandin release and CFTR activation

Human lung epithelial (Calu-3) cells were used to investigate the effects of protease-activated receptor (PAR) stimulation on Cl(-) secretion. Quantitative RT-PCR (QRT-PCR) showed that Calu-3 cells express PAR-1, -2, and -3 receptor mRNAs, with PAR-2 mRNA in greatest abundance. Addition of either thrombin or the PAR-2 agonist peptide SLIGRL to the basolateral solution of monolayers mounted in Ussing chambers produced a rapid increase in short-circuit current (I(sc): thrombin, 21 +/- 2 microA; SLIGRL, 83 +/- 22 microA), which returned to baseline within 5 min after stimulation. Pretreatment of monolayers with the cell-permeant Ca(2+)-chelating agent BAPTA-AM (50 microM) abolished the increase in I(sc) produced by SLIGRL. When monolayers were treated with the cyclooxygenase inhibitor indomethacin (10 microM), nearly complete inhibition of both the thrombin- and SLIGRL-stimulated I(sc) was observed. In addition, basolateral treatment with the PGE(2) receptor antagonist AH-6809 (25 microM) significantly inhibited the effects of SLIGRL on I(sc). QRT-PCR revealed that Calu-3 cells express mRNAs for CFTR, the Ca(2+)-activated KCNN4 K(+) channel, and the KCNQ1 K(+) channel subunit, which, in association with KCNE3, is known to be regulated by cAMP. Stimulation with SLIGRL produced an increase in apical Cl(-) conductance that was blocked in cells expressing short hairpin RNAs designed to target CFTR. These results support the conclusion that PAR stimulation of Cl(-) secretion occurs by an indirect mechanism involving the synthesis and release of prostaglandins. In addition, PAR-stimulated Cl(-) secretion requires activation of CFTR and at least two distinct K(+) channels located in the basolateral membrane.

The KCNQ1 potassium channel: from gene to physiological function

The voltage-gated KCNQ1 (KvLQT1, Kv7.1) potassium channel plays a crucial role in shaping the cardiac action potential as well as in controlling the water and salt homeostasis in several epithelial tissues. KCNQ1 channels in these tissues are tightly regulated by auxiliary proteins and accessory factors, capable of modulating the properties of the channel complexes. This paper reviews the current knowledge about the KCNQ1 channel with a major focus on interacting proteins and physiological functions.

Two-pore-domain potassium channels support anion secretion from human airway Calu-3 epithelial cells

Potassium channels are required for the absorption and secretion of fluids and electrolytes in epithelia. Calu-3 cells possess a secretory phenotype, and are a model human airway submucosal gland serous cell. Short-circuit current (I(sc)) recordings from Calu-3 cells indicated that basal anion secretion was reduced by apical application of the K+ channel inhibitors bupivicaine, lidocaine, clofilium, and quinidine. Application of riluzole resulted in a large increase in I(sc), inhibited by apical application of either bupivicane or the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel blocker DPC. These results suggested that one or more members of the two-pore-domain K+ (K(2P)) channel family could influence anion secretion. Using RT-PCR, we found that Calu-3 cells express mRNA transcripts for TASK-2 (KCNK5), TWIK-1 (KCNK1), TWIK-2 (KCNK6) and TREK-1 (KCNK2). TASK-2, TWIK-2 and TREK-1 protein were detected by Western blotting, while immunolocalization of polarized cells confirmed protein expression of TREK-1 and TWIK-2 at the plasma cell membrane. TASK-2 protein staining was localized to intracellular vesicles, located beneath the apical membrane. While the pro-secretory role of basolateral K+ channels is well established, we suggest that apically located K2P channels, not previously described in airway epithelial cells, also play an important role in controlling the rate of transepithelial anion secretion.

Duodenal secretion in humans mediated by the EP4 receptor subtype

AIM

Assessment of functional EP receptor subtypes involved in PGE2-induced secretion in human duodenum. The spectrum of activities by PGE2 in mammals, including cytoprotective bicarbonate secretion in duodenum, is mediated through four G protein-coupled receptor subtypes (EP1-EP4).

METHODS

Biopsies from the second part of duodenum from patients undergoing endoscopy were mounted in modified Ussing chambers. Basal and stimulated short circuit current (SCC) and slope conductance (SG) were measured. Dose-response relations for PGE2 and subtype receptors EP1/EP3 (sulprostone), EP2 (butaprost), and EP4 (1-OH PGE1) were assessed by cumulated doses of single agonists.

RESULTS

PGE2 caused a dose-dependent increase in SCC, maximum 101 +/- 20 microA cm(-2) with an EC50 of 35.6 +/- 5.8 nm (n = 3). Likewise 1-OH PGE1 caused a dose-dependent SCC increase, maximum 63.3 +/- 28.6 microA cm(-2) with an EC50 of 56.7 +/- 7.2 nm (n = 3). 1-OH PGE1 at 500 nm increased SCC by 18.0 +/- 3.0 microA cm(-2) (n = 10) and SG by 2.9 +/- 0.4 mS cm(-2) (n = 6). Sulprostone (n = 6) and butaprost (n = 6) had no effects on SCC or SG. SCC was inhibited 31.4 +/- 13.2% (n = 10) by bumetanide (25 microM serosa) and 18.6 +/- 5.8% (n = 10) by acetazolamide (250 microM lumen). Diphenylamine-2-carboxylate (DPC) (250 microM mucosa) and SITS (10 microM mucosa) had almost no effect.

CONCLUSIONS

Effects of PGE2 on secretion in the second part of human duodenum is mediated through the EP4 receptor and not through EP1, EP2, or EP3.

KCNE3 truncation mutants reveal a bipartite modulation of KCNQ1 K+ channels

The five KCNE genes encode a family of type I transmembrane peptides that assemble with KCNQ1 and other voltage-gated K⁺ channels, resulting in potassium conducting complexes with varied channel-gating properties. It has been recently proposed that a triplet of amino acids within the transmembrane domain of KCNE1 and KCNE3 confers modulation specificity to the peptide, since swapping of these three residues essentially converts the recipient KCNE into the donor (Melman, Y.F., A. Domenech, S. de la Luna, and T.V. McDonald. 2001. J. Biol. Chem. 276:6439–6444). However, these results are in stark contrast with earlier KCNE1 deletion studies, which demonstrated that a COOH-terminal region, highly conserved between KCNE1 and KCNE3, was responsible for KCNE1 modulation of KCNQ1 (Tapper, A.R., and A.L. George. 2000 J. Gen. Physiol. 116:379–389.). To ascertain whether KCNE3 peptides behave similarly to KCNE1, we examined a panel of NH₂- and COOH-terminal KCNE3 truncation mutants to directly determine the regions required for assembly with and modulation of KCNQ1 channels. Truncations lacking the majority of their NH₂ terminus, COOH terminus, or mutants harboring both truncations gave rise to KCNQ1 channel complexes with basal activation, a hallmark of KCNE3 modulation. These results demonstrate that the KCNE3 transmembrane domain is sufficient for assembly with and modulation of KCNQ1 channels and suggests a bipartite model for KCNQ1 modulation by KCNE1 and KCNE3 subunits. In this model, the KCNE3 transmembrane domain is active in modulation and overrides the COOH terminus' contribution, whereas the KCNE1 transmembrane domain is passive and reveals COOH-terminal modulation of KCNQ1 channels. We furthermore test the validity of this model by using the active KCNE3 transmembrane domain to functionally rescue a nonconducting, yet assembly and trafficking competent, long QT mutation located in the conserved COOH-terminal region of KCNE1.

4-Chlorobenzo[F]isoquinoline (CBIQ), a novel activator of CFTR and ΔF508 CFTR

4-Chlorobenzo[F]isoquinoline (CBIQ) is a novel compound, here shown to activate both CFTR (cystic fibrosis transmembrane conductance regulator) Cl- ion channels and KCNN4, intermediate conductance, calcium-sensitive K+-channels, present in transporting epithelia by the use of heterologous expression systems. Earlier studies with other benzoquinolines, namely 7,8- and 5,6 benzoquinoline, showed they too could activate CFTR and KCNN4, but the evidence was only indirect. However this study also shows that CBIQ can also activate DeltaF508 CFTR, the most common mutant form of CFTR present in approximately 75% of patients with cystic fibrosis. This property is not shared with the other benzoquinolines. As activation of CFTR and KCNN4 work in unison to promote epithelial chloride secretion, CBIQ is a new chemical scaffold for developing agents that may be useful in cystic fibrosis.

Electrophysiological and molecular identification of hepatocellular volume-activated K+ channels

Although K+ channels are essential for hepatocellular function, it is not known which channels are involved in the regulatory volume decrease (RVD) in these cells. We have used a combination of electrophysiological and molecular approaches to describe the potential candidates for these channels. The dialysis of short-term cultured rat hepatocytes with a hypotonic solution containing high K+ and low Cl- concentration caused the slow activation of an outward, time-independent current under whole-cell configuration of the patch electrode voltage clamp. The reversal potential of this current suggested that K+ was the primary charge carrier. The swelling-induced K+ current (IKvol) occurred in the absence of Ca2+ and was inhibited with 1 microM Ca2+ in the pipette solution. The activation of IKvol required both Mg2+ and ATP and an increasing concentration of Mg-ATP from 0.25 through 0.5 to 0.9 mM activated IKvol increasingly faster and to a larger extent. The KCNQ1 inhibitor chromanol 293B reversibly depressed IKvol with an IC50 of 26 microM. RT-PCR detected the expression of members of the KCNQ family from KCNQ1 to KCNQ5 and of the accessory proteins KCNE1 to KCNE3 in the rat hepatocytes, but not KCNQ2 and KCNE2 in human liver. Western blotting showed KCNE3 expression in a plasma membrane-enriched fraction from rat hepatocytes. The results suggest that KCNQ1, probably with KCNE2 or KCNE3 as its accessory unit, provides a significant fraction of IKvol in rat hepatocytes.

An inwardly rectifying potassium channel in apical membrane of Calu-3 cells

Patch clamp methods and reverse transcription-polymerase chain reaction (RT-PCR) were used to characterize an apical K+ channel in Calu-3 cells, a widely used model of human airway gland serous cells. In cell-attached and excised apical membrane patches, we found an inwardly rectifying K+ channel (Kir). The permeability ratio was PNa/PK = 0.058. In 30 patches with both cystic fibrosis transmembrane conductance regulator and Kir present, we observed 79 cystic fibrosis transmembrane conductance regulator and 58 Kir channels. The average chord conductance was 24.4 +/- 0.5 pS (n = 11), between 0 and -200 mV, and was 9.6 +/- 0.7 pS (n = 8), between 0 and 50 mV; these magnitudes and their ratio of approximately 2.5 are most similar to values for rectifying K+ channels of the Kir4.x subfamilies. We attempted to amplify transcripts for Kir4.1, Kir4.2, and Kir5.1; of these only Kir4.2 was present in Calu-3 lysates. The channel was only weakly activated by ATP and was relatively insensitive to internal pH. External Cs+ and Ba2+ blocked the channel with Kd values in the millimolar range. Quantitative modeling of Cl- secreting epithelia suggests that secretion rates will be highest and luminal K+ will rise to 16-28 mm if 11-25% of the total cellular K+ conductance is placed in the apical membrane (Cook, D. I., and Young, J. A. (1989) J. Membr. Biol. 110, 139-146). Thus, we hypothesize that the K+ channel described here optimizes the rate of secretion and is involved in K+ recycling for the recently proposed apical H+ -K+ -ATPase in Calu-3 cells.

4-Chloro-benzo[F]isoquinoline (CBIQ) activates CFTR chloride channels and KCNN4 potassium channels in Calu-3 human airway epithelial cells

1 Calu-3 cells have been used to investigate the actions of 4-chloro-benzo[F]isoquinoline (CBIQ) on short-circuit current (SCC) in monolayers, whole-cell recording from single cells and by patch clamping. 2 CBIQ caused a sustained, reversible and repeatable increase in SCC in Calu-3 monolayers with an EC50 of 4.0 microm. Simultaneous measurements of SCC and isotopic fluxes of 36Cl- showed that CBIQ caused electrogenic chloride secretion. 3 Apical membrane permeabilisation to allow recording of basolateral membrane conductance in the presence of a K+ gradient suggested that CBIQ activated the intermediate-conductance calcium-sensitive K(+)-channel (KCNN4). Permeabilisation of the basolateral membranes of epithelial monolayers in the presence of a Cl- gradient suggested that CBIQ activated the Cl(-)-channel CFTR in the apical membrane. 4 Whole-cell recording in the absence of ATP/GTP of Calu-3 cells showed that CBIQ generated an inwardly rectifying current sensitive to clotrimazole. In the presence of the nucleotides, a more complex I/V relation was found that was partially sensitive to glibenclamide. The data are consistent with the presence of both KCNN4 and CFTR in Calu-3. 5 Isolated inside-out patches from Calu-3 cells revealed clotrimazole-sensitive channels with a conductance of 12 pS at positive potentials after activation with CBIQ and demonstrating inwardly rectifying properties, consistent with the known properties of KCNN4. Cell-attached patches showed single channel events with a conductance of 7 pS and a linear I/V relation that were further activated by CBIQ by an increase in open state probability, consistent with known properties of CFTR. It is concluded that CBIQ activates CFTR and KCNN4 ion channels in Calu-3 cells.

DNA microarray analysis of gene expression in human optic nerve head astrocytes in response to hydrostatic pressure

There is clinical and experimental evidence that elevated intraocular pressure (IOP), a mechanical stress, is involved in the pathogenesis of glaucomatous optic neuropathy. The mechanism by which astrocytes in the optic nerve head (ONH) respond to changes in IOP is under study. Gene transcription by ONH astrocytes exposed either to 60 mmHg hydrostatic pressure (HP) or control ambient pressure (CP) for 6, 24, and 48 h was compared using Affymetrix GeneChip microarrays to identify HP-responsive genes. Data were normalized across arrays within each gene. A linear regression model applied to test effect of time and HP on changes in expression level identified 596 genes affected by HP over time. Using GeneSpring analysis we selected genes whose average expression level increased or decreased more than 1.5-fold at 6, 24, or 48 h. Expression of selected genes was confirmed by real-time RT-PCR; protein levels were detected by Western blot. Among the genes highly responsive to HP were those involved in signal transduction, such as Rho nucleotide exchange factors, Ras p21 protein activator, tyrosine kinases and serine threonine kinases, and genes involved in transcriptional regulation, such as c-Fos, Egr2, and Smad3. Other genes that increased expression included ATP-binding cassettes, solute carriers, and genes associated with lipid metabolism. Among the genes that decreased expression under HP were genes encoding for dual activity phosphatases, transcription factors, and enzymes involved in protein degradation. These HP-responsive genes may be important in the establishment and maintenance of the ONH astrocyte phenotype under conditions of elevated IOP in glaucoma.

Bicarbonate-dependent chloride secretion in Calu-3 epithelia in response to 7,8-benzoquinoline

Stimulation of Calu-3 epithelia with 7,8-benzoquinoline, under short circuit current conditions, produced a current increase that was completely accounted for by the net flux of chloride, measured simultaneously with 36Cl-. Nevertheless the current stimulated by 7,8-benzoquinoline was sensitive to acetazolamide, which caused up to 50 % inhibition of the stimulated current, the remainder being sensitive to the Na+-K+-2Cl- cotransport inhibitor bumetanide. The effects of acetazolamide could be mimicked by either amiloride or by the di-sodium salt of 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS) added to the basolateral side of the epithelium, but their actions were not additive. Amiloride was needed in sufficient concentration to inhibit the sodium-proton exchanger NHE1. DNDS blocks both the chloride-bicarbonate exchanger AE2 and the sodium-bicarbonate transporter NBC1. However, since 7,8-benzoquinoline activates basolateral K+ channels, causing hyperpolarisation, it is unlikely NBC1 is active after addition of 7,8-benzoquinoline. The effect of DNDS is, therefore, mainly on AE2. It is concluded that chloride enters the basolateral aspect of the cells using the Na+-K+-2Cl- cotransporter and a parallel arrangement of NHE1 with AE2, these latter two being sensitive to acetazolamide because of their association with the cytoplasmic form of carbonic anhydrase CAII. The effects of acetazolamide could be mimicked by removal of HCO3-/CO2 from the bathing medium, and furthermore showed that the NHE1-AE2 mechanism is particularly important when the transport rate is high. Thus part of the current stimulated by 7,8-benzoquinoline and inhibited by acetazolamide or HCO3-/CO2 removal can be said to represent bicarbonate-dependent chloride secretion.