Calcium-stimulated Cl- secretion in Calu-3 human airway cells requires CFTR

Epithelial Chloride Transport by CFTR Requires TMEM16A

Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) is the secretory chloride/bicarbonate channel in airways and intestine that is activated through ATP binding and phosphorylation by protein kinase A, but fails to operate in cystic fibrosis (CF). TMEM16A (also known as anoctamin 1, ANO1) is thought to function as the Ca²⁺ activated secretory chloride channel independent of CFTR. Here we report that tissue specific knockout of the TMEM16A gene in mouse intestine and airways not only eliminates Ca²⁺-activated Cl⁻ currents, but unexpectedly also abrogates CFTR-mediated Cl⁻ secretion and completely abolishes cAMP-activated whole cell currents. The data demonstrate fundamentally new roles of TMEM16A in differentiated epithelial cells: TMEM16A provides a mechanism for enhanced ER Ca²⁺ store release, possibly engaging Store Operated cAMP Signaling (SOcAMPS) and activating Ca²⁺ regulated adenylyl cyclases. TMEM16A is shown to be essential for proper activation and membrane expression of CFTR. This intimate regulatory relationship is the cause for the functional overlap of CFTR and Ca²⁺-dependent chloride transport.

CK2 is a key regulator of SLC4A2-mediated Cl-/HCO3- exchange in human airway epithelia

Transepithelial bicarbonate secretion by human airway submucosal glands and surface epithelial cells is crucial to maintain the pH-sensitive innate defence mechanisms of the lung. cAMP agonists stimulate HCO₃^- secretion via coordinated increases in basolateral HCO₃^- influx and accumulation, as well as CFTR-dependent HCO₃^- efflux at the luminal membrane of airway epithelial cells. Here, we investigated the regulation of a basolateral located, DIDS-sensitive, Cl^-/HCO₃^- exchanger, anion exchanger 2 (AE2; SLC4A2) which is postulated to act as an acid loader, and therefore potential regulator of HCO₃^- secretion, in human airway epithelial cells. Using intracellular pH measurements performed on Calu-3 cells, we demonstrate that the activity of the basolateral Cl^-/HCO₃^- exchanger was significantly downregulated by cAMP agonists, via a PKA-independent mechanism and also required Ca²⁺ and calmodulin under resting conditions. AE2 contains potential phosphorylation sites by a calmodulin substrate, protein kinase CK2, and we demonstrated that AE2 activity was reduced in the presence of CK2 inhibition. Moreover, CK2 inhibition abolished the activity of AE2 in primary human nasal epithelia. Studies performed on mouse AE2 transfected into HEK-293T cells confirmed almost identical Ca²⁺/calmodulin and CK2 regulation to that observed in Calu-3 and primary human nasal cells. Furthermore, mouse AE2 activity was reduced by genetic knockout of CK2, an effect which was rescued by exogenous CK2 expression. Together, these findings are the first to demonstrate that CK2 is a key regulator of Cl^--dependent HCO₃^- export at the serosal membrane of human airway epithelial cells.

Bioelectric and Morphological Response of Liquid-Covered Human Airway Epithelial Calu-3 Cell Monolayer to Periodic Deposition of Colloidal 3-Mercaptopropionic-Acid Coated CdSe-CdS/ZnS Core-Multishell Quantum Dots

Lung epithelial cells are extensively exposed to nanoparticles present in the modern urban environment. Nanoparticles, including colloidal quantum dots (QDs), are also considered to be potentially useful carriers for the delivery of drugs into the body. It is therefore important to understand the ways of distribution and the effects of the various types of nanoparticles in the lung epithelium. We use a model system of liquid-covered human airway epithelial Calu-3 cell cultures to study the immediate and long-term effects of repeated deposition of colloidal 3-mercaptopropionic-acid coated CdSe-CdS/ZnS core-multishell QDs on the lung epithelial cell surface. By live confocal microscope imaging and by QD fluorescence measurements we show that the QD permeation through the mature epithelial monolayers is very limited. At the time of QD deposition, the transepithelial electrical resistance (TEER) of the epithelial monolayers transiently decreased, with the decrement being proportional to the QD dose. Repeated QD deposition, once every six days for two months, lead to accumulation of only small amounts of the QDs in the cell monolayer. However, it did not induce any noticeable changes in the long-term TEER and the molecular morphology of the cells. The colloidal 3-mercaptopropionic-acid coated CdSe-CdS/ZnS core-multishell QDs could therefore be potentially used for the delivery of drugs intended for the surface of the lung epithelia during limited treatment periods.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

From synthesis to function via iterative assembly of N-methyliminodiacetic acid boronate building blocks

The study and optimization of small molecule function is often impeded by the time-intensive and specialist-dependent process that is typically used to make such compounds. In contrast, general and automated platforms have been developed for making peptides, oligonucleotides, and increasingly oligosaccharides, where synthesis is simplified to iterative applications of the same reactions. Inspired by the way natural products are biosynthesized via the iterative assembly of a defined set of building blocks, we developed a platform for small molecule synthesis involving the iterative coupling of haloboronic acids protected as the corresponding N-methyliminodiacetic acid (MIDA) boronates. Here we summarize our efforts thus far to develop this platform into a generalized and automated approach for small molecule synthesis. We and others have employed this approach to access many polyene-based compounds, including the polyene motifs found in >75% of all polyene natural products. This platform further allowed us to derivatize amphotericin B, the powerful and resistance-evasive but also highly toxic last line of defense in treating systemic fungal infections, and thereby understand its mechanism of action. This synthesis-enabled mechanistic understanding has led us to develop less toxic derivatives currently under evaluation as improved antifungal agents. To access more Csp(3)-containing small molecules, we gained a stereocontrolled entry into chiral, non-racemic α-boryl aldehydes through the discovery of a chiral derivative of MIDA. These α-boryl aldehydes are versatile intermediates for the synthesis of many Csp(3) boronate building blocks that are otherwise difficult to access. In addition, we demonstrated the utility of these types of building blocks in accessing pharmaceutically relevant targets via an iterative Csp(3) cross-coupling cycle. We have further expanded the scope of the platform to include stereochemically complex macrocyclic and polycyclic molecules using a linear-to-cyclized strategy, in which Csp(3) boronate building blocks are iteratively assembled into linear precursors that are then cyclized into the cyclic frameworks found in many natural products and natural product-like structures. Enabled by the serendipitous discovery of a catch-and-release protocol for generally purifying MIDA boronate intermediates, the platform has been automated. The synthesis of 14 distinct classes of small molecules, including pharmaceuticals, materials components, and polycyclic natural products, has been achieved using this new synthesis machine. It is anticipated that the scope of small molecules accessible by this platform will continue to expand via further developments in building block synthesis, Csp(3) cross-coupling methodologies, and cyclization strategies. Achieving these goals will enable the more generalized synthesis of small molecules and thereby help shift the rate-limiting step in small molecule science from synthesis to function.

Impaired mucus detachment disrupts mucociliary transport in a piglet model of cystic fibrosis

Lung disease in people with cystic fibrosis (CF) is initiated by defective host defense that predisposes airways to bacterial infection. Advanced CF is characterized by a deficit in mucociliary transport (MCT), a process that traps and propels bacteria out of the lungs, but whether this deficit occurs first or is secondary to airway remodeling has been unclear. To assess MCT, we tracked movement of radiodense microdisks in airways of newborn piglets with CF. Cholinergic stimulation, which elicits mucus secretion, substantially reduced microdisk movement. Impaired MCT was not due to periciliary liquid depletion; rather, CF submucosal glands secreted mucus strands that remained tethered to gland ducts. Inhibiting anion secretion in non-CF airways replicated CF abnormalities. Thus, impaired MCT is a primary defect in CF, suggesting that submucosal glands and tethered mucus may be targets for early CF treatment.

Epinephrine stimulation of anion secretion in the Calu-3 serous cell model

Calu-3 is a well-differentiated human bronchial cell line with the characteristics of the serous cells of airway submucosal glands. The submucosal glands play a major role in mucociliary clearance because they secrete electrolytes that facilitate airway hydration. Given the significance of both long- and short-term β-adrenergic receptor agonists in the treatment of respiratory diseases, it is important to determine the role of these receptors and their ligands in normal physiological function. The present studies were designed to characterize the effect of epinephrine, the naturally occurring β-adrenergic receptor agonist, on electrolyte transport of the airway serous cells. Interestingly, epinephrine stimulated two anion secretory channels, the cystic fibrosis transmembrane conductance regulator and a Ca(2+)-activated Cl(-) channel, with the characteristics of transmembrane protein 16A, thereby potentially altering mucociliary clearance via multiple channels. Consistent with the dual channel activation, epinephrine treatment resulted in increases in both intracellular cAMP and Ca(2+). Furthermore, the present results extend previous reports indicating that the two anion channels are functionally linked.

Thapsigargin blocks Pseudomonas aeruginosa homoserine lactone-induced apoptosis in airway epithelia

Pseudomonas aeruginosa secretes N-(3-oxododecanoyl)-homoserine lactone (C12) as a quorum-sensing molecule to regulate gene expression. Micromolar concentrations are found in the airway surface liquid of infected lungs. Exposure of the airway surface to C12 caused a loss of transepithelial resistance within 1 h that was accompanied by disassembly of tight junctions, as indicated by relocation of the tight junction protein zonula occludens 1 from the apical to the basolateral pole and into the cytosol of polarized human airway epithelial cell cultures (Calu-3 and primary tracheal epithelial cells). These effects were blocked by carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone, a pan-caspase blocker, indicating that tight junction disassembly was an early event in C12-triggered apoptosis. Short-duration (10 min) pretreatment of airway epithelial (Calu-3 and JME) cells with 1 μM thapsigargin (Tg), an inhibitor of Ca(2+) uptake into the endoplasmic reticulum (ER), was found to be protective against the C12-induced airway epithelial barrier breakdown and also against other apoptosis-related effects, including shrinkage and fragmentation of nuclei, activation of caspase 3/7 (the executioner caspase in apoptosis), release of ER-targeted redox-sensitive green fluorescent protein into the cytosol, and depolarization of mitochondrial membrane potential. Pretreatment of Calu-3 airway cell monolayers with BAPTA-AM [to buffer cytosolic Ca(2+) concentration (Cacyto)] or Ca(2+)-free solution + BAPTA-AM reduced C12 activation of apoptotic events, suggesting that C12-triggered apoptosis may involve Ca(2+). Because C12 and Tg reduced Ca(2+) concentration in the ER and increased Cacyto, while Tg increased mitochondrial Ca(2+) concentration (Camito) and C12 reduced Camito, it is proposed that Tg may reduce C12-induced apoptosis in host cells not by raising Cacyto, but by preventing C12-induced decreases in Camito.

Nongenomic effects of fluticasone propionate and budesonide on human airway anion secretion

This study investigated the physiological effects of inhaled corticosteroids, which are used widely to treat asthma. The application of fluticasone propionate (FP, 100 μM) induced sustained increases in the short-circuit current (I(SC)) in human airway Calu-3 epithelial cells. The FP-induced I(SC) was prevented by the presence of H89 (10 μM, a protein kinase A inhibitor) and SQ22536 (100 μM, an adenylate cyclase inhibitor). The FP-induced responses involved bumetanide (a Na(+)-K(+)-2Cl(-) cotransporter inhibitor)-sensitive and 4,4'-dinitrostilbene-2,2'-disulfonic acid (an inhibitor of HCO(3)(-)-dependent anion transporters)-sensitive components, both of which reflect basolateral anion transport. Further, FP augmented apical membrane Cl(-) current (I(Cl)), reflecting cystic fibrosis transmembrane conductance regulator (CFTR)-mediated conductance, in the nystatin-permeabilized monolayer. In I(SC) and I(Cl) responses, FP failed to enhance the responses to forskolin (10 μM, an adenylate cyclase activator). Nevertheless, we found that FP synergistically increased cytosolic cAMP concentrations in combination with forskolin. All these effects of FP were reproduced with the use of budesonide. Collectively, inhaled corticosteroids such as FP and budesonide stimulate CFTR-mediated anion transport through adenylate cyclase-mediated mechanisms in a nongenomic fashion, thus sharing elements of a common pathway with forskolin. However, the corticosteroids cooperate with forskolin for synergistic cAMP production, suggesting that the corticosteroids and forskolin do not compete with each other to exert their effects on adenylate cyclase. Considering that such synergism was also observed in the FP/salmeterol combination, these nongenomic aspects may play therapeutic roles in mucus congestive airway diseases, in addition to genomic aspects that are generally recognized.

Bicarbonate-dependent chloride transport drives fluid secretion by the human airway epithelial cell line Calu-3

Anion and fluid secretion are both defective in cystic fibrosis (CF); however, the transport mechanisms are not well understood. In this study, Cl(-) and HCO(3)(-) secretion was measured using genetically matched CF transmembrane conductance regulator (CFTR)-deficient and CFTR-expressing cell lines derived from the human airway epithelial cell line Calu-3. Forskolin stimulated the short-circuit current (I(sc)) across voltage-clamped monolayers, and also increased the equivalent short-circuit current (I(eq)) calculated under open-circuit conditions. I(sc) was equivalent to the HCO(3)(-) net flux measured using the pH-stat technique, whereas I(eq) was the sum of the Cl(-) and HCO(3)(-) net fluxes. I(eq) and HCO(3)(-) fluxes were increased by bafilomycin and ZnCl(2), suggesting that some secreted HCO(3)(-) is neutralized by parallel electrogenic H(+) secretion. I(eq) and fluid secretion were dependent on the presence of both Na(+) and HCO(3)(-). The carbonic anhydrase inhibitor acetazolamide abolished forskolin stimulation of I(eq) and HCO(3)(-) secretion, suggesting that HCO(3)(-) transport under these conditions requires catalysed synthesis of carbonic acid. Cl(-) was the predominant anion in secretions under all conditions studied and thus drives most of the fluid transport. Nevertheless, 50-70% of Cl(-) and fluid transport was bumetanide-insensitive, suggesting basolateral Cl(-) loading by a sodium-potassium-chloride cotransporter 1 (NKCC1)-independent mechanism. Imposing a transepithelial HCO(3)(-) gradient across basolaterally permeabilized Calu-3 cells sustained a forskolin-stimulated current, which was sensitive to CFTR inhibitors and drastically reduced in CFTR-deficient cells. Net HCO(3)(-) secretion was increased by bilateral Cl(-) removal and therefore did not require apical Cl(-)/HCO(3)(-) exchange. The results suggest a model in which most HCO(3)(-) is recycled basolaterally by exchange with Cl(-), and the resulting HCO(3)(-)-dependent Cl(-) transport provides an osmotic driving force for fluid secretion.

Functional effects of nanoparticle exposure on Calu-3 airway epithelial cells

High concentrations of manufactured carbon nanoparticles (CNP) are known to cause oxidative stress, inflammatory responses and granuloma formation in respiratory epithelia. To examine the effects of lower, more physiologically relevant concentrations, the human airway epithelial cell line, Calu-3, was used to evaluate potential alterations in transepithelial permeability and cellular function of airway epithelia after exposure to environmentally realistic concentrations of carbon nanoparticles. Three common carbon nanoparticles, fullerenes, single- and multi-wall carbon nanotubes (SWCNT, MWCNT) were used in these experiments. Electrophysiological measurements were performed to assay transepithelial electrical resistance (TEER) and epinephrine-stimulated chloride (Cl(-)) ion secretion of epithelial cell monolayers that had been exposed to nanoparticles for three different times (1 h, 24 h and 48 h) and over a 7 log unit range of concentrations. Fullerenes did not have any effect on the TEER or stimulated ion transport. However, the carbon nanotubes (CNT) significantly decreased TEER and inhibited epinephrine-stimulated Cl(-) secretion. The changes were time dependent and at more chronic exposures caused functional effects which were evident at concentrations substantially lower than have been previously examined. The functional changes manifested in response to physiologically relevant exposures would inhibit mucociliary clearance mechanisms and compromise the barrier function of airway epithelia.

Small conductance potassium channels drive ATP-activated chloride secretion in the oviduct

The molecular mechanisms controlling fluid secretion within the oviduct have yet to be determined. As in other epithelia, both secretory and absorptive pathways are likely to work in tandem to drive appropriate ionic movement to support fluid movement across the oviduct epithelium. This study explored the role of potassium channels in basolateral extracellular ATP (ATP(e))-stimulated ion transport in bovine oviduct epithelium using the Ussing chamber short-circuit current (I(SC)) technique. Basal I(SC) in bovine oviduct epithelium comprises both chloride secretion and sodium absorption and was inhibited by treatment with basolateral K(+) channel inhibitors tetrapentlyammonium chloride (TPeA) or BaCl(2). Similarly, ATP-stimulated chloride secretion was significantly attenuated by pretreatment with BaCl(2,) tetraethylammonium (TEA), tolbutamide, and TPeA. Basolateral K(+) current, isolated using nystatin-perforation technique, was rapidly activated by ATP(e), and pretreatment of monolayers with thapsigargin or TPeA abolished this ATP-stimulated K(+) current. To further investigate the type of K(+) channel involved in the ATP response in the bovine oviduct, a number of specific Ca(2+)-activated K(+) channel inhibitors were tested on the ATP-induced ΔI(SC) in intact monolayers. Charbydotoxin, (high conductance and intermediate conductance inhibitor), or paxilline, (high conductance inhibitor) did not significantly alter the ATP(e) response. However, pretreatment with the small conductance inhibitor apamin resulted in a 60% reduction in the response to ATP(e). The presence of small conductance family member KCNN3 was confirmed by RT-PCR and immunohistochemistry. Measurements of intracellular calcium using Fura-2 spectrofluorescence imaging revealed the ability of ATP(e) to increase intracellular calcium in a phospholipase C-inositol 1,4,5-trisphosphate pathway-sensitive manner. In conclusion, these results provide strong evidence that purinergic activation of a calcium-dependent, apamin-sensitive potassium conductance is essential to promote chloride secretion and thus fluid formation in the oviduct.

Capsaicinoids regulate airway anion transporters through Rho kinase- and cyclic AMP-dependent mechanisms

To investigate the effects of capsaicinoids on airway anion transporters, we recorded and analyzed transepithelial currents in human airway epithelial Calu-3 cells. Application of capsaicin (100 μM) attenuated vectorial anion transport, estimated as short-circuit currents (I(SC)), before and after stimulation by forskolin (10 μM) with concomitant reduction of cytosolic cyclic AMP (cAMP) levels. The capsaicin-induced inhibition of I(SC) was also observed in the response to 8-bromo-cAMP (1 mM, a cell-permeable cAMP analog) and 3-isobutyl-1-methylxanthine (1 mM, an inhibitor of phosphodiesterases). The capsaicin-induced inhibition of I(SC) was attributed to suppression of bumetanide (an inhibitor of the basolateral Na(+)-K(+)-2 Cl(-) cotransporter 1)- and 4,4'-dinitrostilbene-2,2'-disulfonic acid (an inhibitor of basolateral HCO(3)(-)-dependent anion transporters)-sensitive components, which reflect anion uptake via basolateral cAMP-dependent anion transporters. In contrast, capsaicin potentiated apical Cl(-) conductance, which reflects conductivity through the cystic fibrosis transmembrane conductance regulator, a cAMP-regulated Cl(-) channel. All these paradoxical effects of capsaicin were mimicked by capsazepine. Forskolin application also increased phosphorylated myosin phosphatase target subunit 1, and the phosphorylation was prevented by capsaicin and capsazepine, suggesting that these capsaicinoids assume aspects of Rho kinase inhibitors. We also found that the increments in apical Cl(-) conductance were caused by conventional Rho kinase inhibitors, Y-27632 (20 μM) and HA-1077 (20 μM), with selective inhibition of basolateral Na(+)-K(+)-2 Cl(-) cotransporter 1. Collectively, capsaicinoids inhibit cAMP-mediated anion transport through down-regulation of basolateral anion uptake, paradoxically accompanied by up-regulation of apical cystic fibrosis transmembrane conductance regulator-mediated anion conductance. The latter is mediated by inhibition of Rho-kinase, which is believed to interact with actin cytoskeleton.

Mechanisms of Ca2+-stimulated fluid secretion by porcine bronchial submucosal gland serous acinar cells

The serous acini of airway submucosal glands are important for fluid secretion in the lung. Serous cells are also sites of expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel. However, the mechanisms of serous cell fluid secretion remain poorly defined. In this study, serous acinar cells were isolated from porcine bronchi and studied using optical techniques previously used to examine fluid secretion in rat parotid and murine nasal acinar cells. When stimulated with the cholinergic agonist carbachol, porcine serous cells shrank by approximately 20% (observed via DIC microscopy) after a profound elevation of intracellular [Ca(2+)] ([Ca(2+)](i); measured by simultaneous fura 2 fluorescence imaging). Upon removal of agonist and relaxation of [Ca(2+)](i) to resting levels, cells swelled back to resting volume. Similar results were observed during stimulation with histamine and ATP, and elevation of [Ca(2+)](i) was found to be necessary and sufficient to activate shrinkage. Cell volume changes were associated with changes in [Cl(-)](i) (measured using SPQ fluorescence), suggesting that shrinkage and swelling are caused by loss and gain of intracellular solute content, respectively, likely reflecting changes in the secretory state of the cells. Shrinkage was inhibited by niflumic acid but not by GlyH-101, suggesting Ca(2+)-activated secretion is mediated by alternative non-CFTR Cl(-) channels, possibly including Ano1 (TMEM16A), expressed on the apical membrane of porcine serous cells. Optimal cell swelling/solute uptake required activity of the Na(+)K(+)2Cl(-) cotransporter and Na(+)/H(+) exchanger, both of which are expressed on the basolateral membrane of serous acini and likely contribute to sustaining transepithelial secretion.

PKA mediates constitutive activation of CFTR in human sweat duct

The cystic fibrosis transmembrane conductance regulator (CFTR) Cl⁻ channels are constitutively activated in sweat ducts. Since phosphorylation-dependent and -independent mechanisms can activate CFTR, we sought to determine the actual mechanism responsible for constitutive activation of these channels in vivo. We show that the constitutively activated CFTR Cl⁻ conductance (gCFTR) in the apical membrane is completely deactivated following α-toxin permeabilization of the basolateral membrane. We investigated whether such inhibition of gCFTR following permeabilization is due to the loss of cytoplasmic glutamate or due to dephosphorylation of CFTR by an endogenous phosphatase in the absence of kinase activity (due to the loss of kinase agonist cAMP, cGMP or GTP through α-toxin pores). In order to distinguish between these two possibilities, we examined the effect of inhibiting the endogenous phosphatase activity with okadaic acid (10⁻⁸ M) on the permeabilization-induced deactivation of gCFTR. We show that okadaic acid (1) inhibits an endogenous phosphatase responsible for dephosphorylating cAMP but not cGMP or G protein-activated CFTR and (2) prevents deactivation of CFTR following permeabilization of the basolateral membrane. These results indicate that distinctly different phosphatases may be responsible for dephosphorylating different kinase-specific sites on CFTR. We conclude that the phosphorylation by PKA alone appears to be primarily responsible for constitutive activation of gCFTR in vivo.

Eicosanoid release is increased by membrane destabilization and CFTR inhibition in Calu-3 cells

The antiinflammatory protein annexin-1 (ANXA1) and the adaptor S100A10 (p11), inhibit cytosolic phospholipase A2 (cPLA2α) by direct interaction. Since the latter is responsible for the cleavage of arachidonic acid at membrane phospholipids, all three proteins modulate eicosanoid production. We have previously shown the association of ANXA1 expression with that of CFTR, the multifactorial protein mutated in cystic fibrosis. This could in part account for the abnormal inflammatory status characteristic of this disease. We postulated that CFTR participates in the regulation of eicosanoid release by direct interaction with a complex containing ANXA1, p11 and cPLA2α. We first analyzed by plasmon surface resonance the in vitro binding of CFTR to the three proteins. A significant interaction between p11 and the NBD1 domain of CFTR was found. We observed in Calu-3 cells a rapid and partial redistribution of all four proteins in detergent resistant membranes (DRM) induced by TNF-α. This was concomitant with increased IL-8 synthesis and cPLA2α activation, ultimately resulting in eicosanoid (PGE2 and LTB4) overproduction. DRM destabilizing agent methyl-β-cyclodextrin induced further cPLA2α activation and eicosanoid release, but inhibited IL-8 synthesis. We tested in parallel the effect of short exposure of cells to CFTR inhibitors Inh172 and Gly-101. Both inhibitors induced a rapid increase in eicosanoid production. Longer exposure to Inh172 did not increase further eicosanoid release, but inhibited TNF-α-induced relocalization to DRM. These results show that (i) CFTR may form a complex with cPLA2α and ANXA1 via interaction with p11, (ii) CFTR inhibition and DRM disruption induce eicosanoid synthesis, and (iii) suggest that the putative cPLA2/ANXA1/p11/CFTR complex may participate in the modulation of the TNF-α-induced production of eicosanoids, pointing to the importance of membrane composition and CFTR function in the regulation of inflammation mediator synthesis.

A mathematical model of calcium-induced fluid secretion in airway epithelium

Regulation of periciliary liquid (PCL) depth is of central importance to mucociliary clearance by the airway epithelium. Without adequate hydration mucociliary transport would cease, leading to build up of mucus in the airways, and impairing the clearance of any trapped inhaled particulates. Airway epithelial cells are known to release ATP under a number of stress conditions. Cell surface receptors bind ATP and trigger an intracellular calcium response which regulates the gating of specific ion channels on the apical and basolateral cell membranes. This shifts the electrochemical balance, resulting in the accumulation of Na(+) and Cl(-) in the periciliary liquid, and providing an osmotic driving force for water flux. In this study, we present a mathematical model of a single airway epithelial cell which describes the fluid secretion elicited after a rise in intracellular calcium. The model provides a basis to quantitatively analyse the influence of intracellular calcium signalling on fluid movement. The model demonstrates behaviour consistent with a number of experimental data on manipulating periciliary liquid volume and tonicity, and provides a quantitative basis for analysing the role of the different membrane ion channels in determining water flux following different physiological stimuli.

Optical imaging of Ca2+-evoked fluid secretion by murine nasal submucosal gland serous acinar cells

Airway submucosal glands are sites of high expression of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and contribute to fluid homeostasis in the lung. However, the molecular mechanisms of gland ion and fluid transport are poorly defined. Here, submucosal gland serous acinar cells were isolated from murine airway, identified by immunofluorescence and gene expression profiling, and used in physiological studies. Stimulation of isolated acinar cells with carbachol (CCh), histamine or ATP was associated with marked decreases in cell volume (20 +/- 2% within 62 +/- 5 s) that were tightly correlated with increases in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) as revealed by simultaneous DIC and fluorescent indicator dye microscopy. Simultaneous imaging of cell volume and the Cl(-)-sensitive fluorophore SPQ indicated that the 20% shrinkage was associated with a fall of [Cl(-)](i) from 65 mm to 28 mm, reflecting loss of 67% of cell Cl(-) content, accompanied by parallel efflux of K(+). Upon agonist removal, [Ca(2+)](i) relaxed and the cells swelled back to resting volume via a bumetanide-sensitive Cl(-) influx pathway, likely to be NKCC1. Accordingly, agonist-induced serous acinar cell shrinkage and swelling are caused by activation of solute efflux and influx pathways, respectively, and cell volume reflects the secretory state of these cells. In contrast, elevation of cAMP failed to elicit detectible volume responses, or enhance those induced by submaximal [CCh], because the magnitude of the changes were likely to be below the threshold of detection using optical imaging. Finally, when stimulated with cholinergic or cAMP agonists, cells from mice that lacked CFTR, as well as wild-type cells treated with a CFTR inhibitor, exhibited identical rates and magnitudes of shrinkage and Cl(-) efflux compared with control cells. These results provide insights into the molecular mechanisms of salt and water secretion by lung submucosal glands, and they suggest that while murine submucosal gland fluid secretion in response to cholinergic stimulation can originate from CFTR-expressing serous acinar cells, it is not dependent upon CFTR function.

Cystic fibrosis transmembrane conductance regulator is vital to sperm fertilizing capacity and male fertility

Cystic fibrosis transmembrane conductance regulator (CFTR) is an anion channel, mutations of which cause cystic fibrosis, a disease characterized by defective Cl(-) and HCO(3)(-) transport. Although >95% of all CF male patients are infertile because of congenital bilateral absence of the vas deferens (CBAVD), the question whether CFTR mutations are involved in other forms of male infertility is under intense debates. Here we report that CFTR is detected in both human and mouse sperm. CFTR inhibitor or antibody significantly reduces the sperm capacitation, and the associated HCO(3)(-)-dependent events, including increases in intracellular pH, cAMP production and membrane hyperpolarization. The fertilizing capacity of the sperm obtained from heterozygous CFTR mutant mice is also significantly lower compared with that of the wild-type. These results suggest that CFTR in sperm may be involved in the transport of HCO(3)(-) important for sperm capacitation and that CFTR mutations with impaired CFTR function may lead to reduced sperm fertilizing capacity and male infertility other than CBAVD.

Chloride transporting capability of Calu-3 epithelia following persistent knockdown of the cystic fibrosis transmembrane conductance regulator, CFTR

BACKGROUND AND PURPOSE

Calu-3 cells are derived from serous cells of human lung submucosal glands, a prime target for therapy in cystic fibrosis (CF). Calu-3 cells can be cultured to form epithelia capable of transepithelial transport of chloride. A CF Calu-3 cell is not available.

EXPERIMENTAL APPROACH

A retroviral vector was used to cause persistent down regulation of CFTR using siRNA methodology, in Calu-3 cells. A Calu-3 cell line with CFTR content less than 5% of the original line has been established. Epithelia grown using the modified cells have been used in comparative studies of transporting capability.

KEY RESULTS

All aspects of cAMP activated chloride secretion were attenuated in the epithelia with reduced CFTR content. However transporting capability was reduced less than the CFTR content. From studies with the CFTR channel inhibitor, GlyH-101, it was concluded that wild type Calu-3 cells have a reserve of CFTR channels not located in the membrane, but available for replacement, while in the modified Calu-3 cell line there was little or no reserve. Lubiprostone, a putative ClC-2 activator, increased transepithelial chloride secretion in both modified and wild type Calu-3 epithelia. Modified Calu-3 epithelia with the residual CFTR currents blocked with GlyH-101 responded equally well to lubiprostone as those without the blocking agent.

CONCLUSIONS AND IMPLICATIONS

It appears that lubiprostone is capable of stimulating a non-CFTR dependent transepithelial chloride secretion in Calu-3 monolayers, with obvious implications for CF therapy. Cell lines, however, do not always reflect the behaviour of the native tissue with integrity.

A Ba2+-resistant, acid-sensitive K+ conductance in Na+-absorbing H441 human airway epithelial cells

By analysis of whole cell membrane currents in Na(+)-absorbing H441 human airway epithelial cells, we have identified a K(+) conductance (G(K)) resistant to Ba(2+) but sensitive to bupivacaine or extracellular acidification. In polarized H441 monolayers, we have demonstrated that bupivacaine, lidocaine, and quinidine inhibit basolateral membrane K(+) current (I(Bl)) whereas Ba(2+) has only a weak inhibitory effect. I(Bl) was also inhibited by basolateral acidification, and, although subsequent addition of bupivacaine caused a further fall in I(Bl), acidification had no effect after bupivacaine, demonstrating that cells grown under these conditions express at least two different bupivacaine-sensitive K(+) channels, only one of which is acid sensitive. Basolateral acidification also inhibited short-circuit current (I(SC)), and basolateral bupivacaine, lidocaine, quinidine, and Ba(2+) inhibited I(SC) at concentrations similar to those needed to inhibit I(Bl), suggesting that the K(+) channels underlying I(Bl) are part of the absorptive mechanism. Analyses using RT-PCR showed that mRNA encoding several two-pore domain K(+) (K2P) channels was detected in cells grown under standard conditions (TWIK-1, TREK-1, TASK-2, TWIK-2, KCNK-7, TASK-3, TREK-2, THIK-1, and TALK-2). We therefore suggest that K2P channels underlie G(K) in unstimulated cells and so maintain the driving force for Na(+) absorption. Since this ion transport process is vital to lung function, K2P channels thus play an important but previously undocumented role in pulmonary physiology.

Activation of chloride transport in CF airway epithelial cell lines and primary CF nasal epithelial cells by S-nitrosoglutathione

Background

It has been suggested that low μM concentrations of S-nitrosoglutathione (GSNO), an endogenous bronchodilator, may promote maturation of the defective cystic fibrosis (CF) transmembrane conductance regulator (CFTR). Because nitric oxide (NO) and GSNO levels appear to be low in the CF airway, there is an interest in the possibility that GSNO replacement could be of therapeutic benefit in CF.

Methods

The effect of GSNO on chloride (Cl^-) transport was investigated in primary nasal epithelial cells obtained from CF patients homozygous for the delF508 mutation, as well as in two CF cell lines (CFBE and CFSME), using both a fluorescent Cl^- indicator and X-ray microanalysis. Maturation of delF508 CFTR was determined by immunoblotting.

Results

Treatment with 60 μM GSNO for 4 hours increased cAMP-induced chloride efflux in nasal epithelial cells from 18 out of 21 CF patients, but did not significantly affect Cl^- efflux in cells from healthy controls. This Cl^- efflux was confirmed by measurements with a fluorescent Cl^- indicator in the CFBE and CFSME cell lines. The effect of GSNO on Cl^- efflux in CFBE cells could be inhibited both by a specific thiazolidinone CFTR inhibitor (CFTR_inh-172) and by 4,4'-diisothiocyanatodihydrostilbene-2,2'-disulfonic acid (H₂DIDS). X-ray microanalysis showed that, following 4 hours incubation with 60 μM GSNO, cAMP agonists caused a decrease in the cellular Cl^- concentration in CFBE cells, corresponding to Cl^- efflux. GSNO exposure resulted in an increase in the protein expression and maturation, as shown by immunoblot analysis. GSNO did not increase the cytosolic Ca²⁺ concentration in cultured airway epithelial cells.

Conclusion

Previous studies have suggested that treatment with GSNO promotes maturation of delF508-CFTR, consistent with our results in this study. Here we show that GSNO increases chloride efflux, both in the two CF cell lines and in primary nasal epithelial cells from delF508-CF patients. This effect is at least in part mediated by CFTR. GSNO may be a candidate for pharmacological treatment of the defective chloride transport in CF epithelial cells.

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.

Ion transport regulated by protease-activated receptor 2 in human airway Calu-3 epithelia

We examined the mechanisms underlying anion secretion mediated by protease-activated receptor 2 (PAR2) and its role in the regulation of ion transport, using polarized human airway Calu-3 cells. PAR2 stimulation by trypsin and a PAR2-activating peptide (PAR2AP), especially from the basolateral aspect, caused transient Cl(-) secretion due to cytosolic Ca(2+) mobilization. Antagonists of PI-PLC (U73122, ET-18-OCH(3)) and inositol 1,4,5-triphosphate (xestospongin C (Xest C)) were without effect on the PAR2AP-mediated Cl(-) secretion, whereas it was attenuated by D609 (a PC-PLC inhibitor) and phorbol 12-myristate 13 acetate (PMA, a PKC activator). Even 30 min after removal of PAR2AP after a 10-min-exposure, cells were still poorly responsive to PAR2 stimulation, but the reduced responsiveness was upregulated by a PKC inhibitor, GF109203X (GFX). Pretreatment with PAR2AP did not affect responses to anion secretagogues, such as isoproterenol, forskolin, thapsigargin, 1-ethyl-2-benzimdazolinone, and adenosine, but ATP-induced responses were significantly reduced. Nystatin permeabilization studies revealed that the presence of PAR2AP prevented ATP-induced increments in basolateral membrane K(+) conductance without affecting apical membrane Cl(-) conductance. ATP-elicited Ca(2+) mobilization, which was sensitive to D609 and PMA, was inhibited by the pretreatment with PAR2AP, and this inhibition was blunted by the presence of GFX. Collectively, stimulation of PAR2 generates a brief response of Cl(-) secretion through PC-PLC-mediated pathway, followed by not only auto-desensitization of PAR2 itself but also cross-desensitization of a PC-PLC-coupled purinoceptor. The two types of desensitization seem likely to have PKC-mediated downregulation of PC-PLC in common.

Ambroxol-induced modification of ion transport in human airway Calu-3 epithelia

Ambroxol is often used as a mucolytic agent in various lung diseases. However, it is unclear how ambroxol acts on bronchial epithelial cells. To clarify the action of ambroxol, we studied the effects of ambroxol on the ion transport in human Calu-3 cells, a human submucosal serous cell line, measuring the transepithelial short-circuit current and conductance across monolayers of Calu-3 cells. Ambroxol of 100 microM diminished the terbutaline (a beta2-adrenergic agonist)-stimulated Cl-/HCO3(-)-dependent secretion without any decreases in the conductance of cystic fibrosis transmembrane conductance regulator (CFTR) channel locating on the apical membrane. On the other hand, under the basal (unstimulated) condition ambroxol increased the Cl(-)-dependent secretion with no significant change in the apical CFTR channel conductance and decreased the HCO3- secretion associated with a decrease in the apical CFTR channel conductance. Ambroxol had no major action on the epithelial Na+ channel (ENaC) or the ENaC-mediated Na+ absorption. These results indicate that in Calu-3 cells: (1) under the basal (unstimulated) condition ambroxol increases Cl- secretion by stimulating the entry step of Cl- and decreases HCO3- secretion by diminishing the activity of the CFTR channel and/or the Na+/HCO3(-)-dependent cotransporter, (2) under the adrenergic agonist-stimulated condition, ambroxol decreases Cl- secretion by acting on the Cl-/HCO3- exchanger, and (3) ambroxol has a more powerful action than the adrenergic agonist on the Cl-/HCO3- exchanger, leading fluid secretion to a moderately stimulated level from a hyper-stimulated level.

A “virtual gland” method for quantifying epithelial fluid secretion

We developed a new apparatus, the virtual gland (VG), for measuring the rate of fluid secretion ( Jv), its composition, and the transepithelial potential (TEP) in cultured epithelial cells under open circuit. The VG creates a 10-μl chamber above the apical surface of epithelial cells on a Costar filter with a small hole leading to an oil-filled reservoir. After the chamber is primed with a fluid of choice, secreted fluid is forced through the hole into the oil, where it forms a bubble that is monitored optically to determine Jv and collected for analysis. Calu-3 cells were mounted in the VG with a basolateral bath consisting of Krebs-Ringer bicarbonate buffer at 37°C. Basal Jv was 2.7 ± 0.1 μl·cm−2·h−1 ( n = 42), and TEP was −9.2 ± 0.6 mV ( n = 33); both measures were reduced to zero by ouabain ( n = 6). Jv and TEP were stimulated 64 and 59%, respectively, by 5 μM forskolin ( n = 10), 173 and 101% by 1 mM 1-ethyl-2-benzimidazolinone ( n = 5), 213 and 122% by 333 nM thapsigargin ( n = 5), and 520 and 240% by forskolin + thapsigargin ( n = 6). Basal Jv and TEP were inhibited to 82 and 63%, respectively, with 10 μM bumetanide ( n = 5), 71 and 82% with 100 μM acetazolamide ( n = 5), and 47 and 56% with 600 μM glibenclamide ( n = 4). Basal Jv and TEP were 52 and 89% of control values, respectively, after HCO3− replacement with HEPES ( n = 16). The net HCO3− concentration of the secreted fluid was close to that of the bath (25 mM), except when stimulated with forskolin or VIP, when it increased (∼80 mM). These results validate the use of the VG apparatus and provide the first direct measures of Jv in Calu-3 cells.

Acid and base secretion in the Calu-3 model of human serous cells

Submucosal glands are the primary source of airway mucus, a critical component of lung innate defenses. Airway glands are defective in cystic fibrosis (CF), showing a complete absence of secretion to vasoactive intestinal peptide or forskolin, which increase intracellular cAMP concentration. This defect is attributed to gland serous cells, which express the cystic fibrosis transmembrane conductance regulator. Calu-3 cells, which mimic many features of serous cells, secrete Cl(-) and HCO(3)(-), with HCO(3)(-) secretion predominating for forskolin stimulation and Cl(-) secretion predominating for stimuli that open basolateral K(+) channels to hyperpolarize the cells. We used pH stat and ion substitution experiments to clarify the mechanisms and consequences of these two modes of secretion. We confirm that Calu-3 cells secrete primarily HCO(3)(-) in response to forskolin. Unexpectedly, HCO(3)(-) secretion continued in response to K(+) channel openers, with Cl(-) secretion being added to it. Secretion of HCO(3)(-) from hyperpolarized cells occurs via the conversion of CO(2) to HCO(3)(-) and is reduced by approximately 50% with acetazolamide. A gap between the base equivalent current and short-circuit current was observed in all experiments and was traced to secretion of H(+) via a ouabain-sensitive, K(+)-dependent process (possibly H(+)-K(+)-ATPase), which partially neutralized the secreted HCO(3)(-). The conjoint secretion of HCO(3)(-) and H(+) may help explain the puzzling finding that mucus secreted from normal and CF glands has the same acidic pH as does mucus from glands stimulated with forskolin or ACh. It may also help explain how human airway glands produce mucus that is hypotonic.

Liquid secretion properties of airway submucosal glands

The tracheobronchial submucosal glands secrete liquid that is important for hydrating airway surfaces, supporting mucociliary transport, and serving as a fluid matrix for numerous secreted macromolecules including the gel-forming mucins. This review details the essential structural elements of airway glands and summarizes what is currently known regarding the ion transport processes responsible for producing the liquid component of gland secretion. Liquid secretion most likely arises from serous cells and is principally under neural control with muscarinic agonists, substance P, and vasoactive intestinal peptide (VIP) functioning as effective secretogogues. Liquid secretion is driven by the active transepithelial secretion of both Cl(-) and HCO(3)(-) and at least a portion of this process is mediated by the cystic fibrosis transmembrane conductance regulator (CFTR), which is highly expressed in glands. The potential role of submucosal glands in cystic fibrosis lung disease is discussed.

Cyclic ADP-ribose, a putative Ca2+-mobilizing second messenger, operates in submucosal gland acinar cells

Cyclic ADP-ribose (cADPR), a putative Ca(2+)-mobilizing second messenger, has been reported to operate in several mammalian cells. To investigate whether cADPR is involved in electrolyte secretion from airway glands, we used a patch-clamp technique, the measurement of microsomal Ca(2+) release, quantification of cellular cADPR, and RT-PCR for CD38 mRNA in human and feline tracheal glands. cADPR (>6 microM), infused into the cell via the patch pipette, caused ionic currents dependent on cellular Ca(2+). Infusions of lower concentrations (2-4 microM) of cADPR or inositol 1,4,5-trisphosphate (IP(3)) alone were without effect on the baseline current, but a combined application of cADPR and IP(3) mimicked the cellular response to low concentrations of acetylcholine (ACh). Microsomes derived from the isolated glands released Ca(2+) in response to both IP(3) and cADPR. cADPR released Ca(2+) from microsomes desensitized to IP(3) or those treated with heparin. The mRNA for CD38, an enzyme protein involved in cADPR metabolism, was detected in human tissues, including tracheal glands, and the cellular content of cADPR was increased with physiologically relevant concentrations of ACh. We conclude that cADPR, in concert with IP(3), operates in airway gland acinar cells to mobilize Ca(2+), resulting in Cl(-) secretion.

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

(1) Cultured epithelial monolayers of Calu-3 human airway cells were used to measure anion secretion in response to a number of phenanthrolines and benzoquinolines, using short-circuit current measurements. Calu-3 cells are derived from serous cells of submucosal glands of the airways and are a target for conditions in which muco-ciliary clearance is compromised. (2) Compounds studied were 5,6-benzoquinoline, 5-chloro-1,10-phenanthroline, 7,8-benzoquinoline, 5-nitro-1,10-phenanthroline, benzo[c]cinnoline and 1,10-phenanthroline, which gave EC50 values of 34, 48, 123, 235, 192 and 217 microm, respectively. Of these, 7,8-benzoquinoline was chosen for further detailed study. Concentration-response relationships for all the compounds had Hill slopes greater than 1. (3) Permeabilisation of the apical surface of epithelia with nystatin in the presence of an apical to basolateral potassium ion gradient reduced the EC50 for 7,8-benzoquinoline to 31 microm and altered the Hill slope to close to 1. (4) Using apically permeabilised epithelia it was shown that 7,8-benzoquinoline activates an intermediate-conductance calcium-sensitive potassium channel (KCNN4) and a cAMP-sensitive potassium channel (KCNQ1/KCNE3) in the basolateral epithelial membranes. (5) 7,8-Benzoquinoline was shown to increase chloride conductance of apical epithelial membranes, presumed to be by activation of the cystic fibrosis transmembrane conductance regulator. (6) 7,8-Benzoquinoline had a minor effect on cAMP accumulation in Calu-3 cells, probably by inhibition of phosphodiesterase, which may contribute to its effect on CFTR- and cAMP-sensitive potassium channels. (7) The usefulness of these novel actions in promoting secretion in airway submucosal glands is discussed.

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.

Characterization of basolateral K+ channels underlying anion secretion in the human airway cell line Calu-3

Transepithelial anion secretion in many tissues depends upon the activity of basolateral channels. Using monolayers of the Calu-3 cell line, a human submucosal serous cell model mounted in an Ussing chamber apparatus, we investigated the nature of the K+ channels involved in basal, cAMP- and Ca2+-stimulated anion secretion, as reflected by the transepithelial short circuit current (I(sc)). The non-specific K+ channel inhibitor Ba2+ inhibited the basal I(sc) by either 77 or 16 % when applied directly to the basolateral or apical membranes, respectively, indicating that a basolateral K+ conductance is required for maintenance of basal anion secretion. Using the K+ channel blockers clofilium and clotrimazole, we found basal I(sc) to be sensitive to clofilium, with a small clotrimazole-sensitive component. By stimulating the cAMP and Ca2+ pathways, we determined that cAMP-stimulated anion secretion was almost entirely abolished by clofilium, but insensitive to clotrimazole. In contrast, the Ca2+-stimulated response was sensitive to both clofilium and clotrimazole. Thus, pharmacologically distinct basolateral K+ channels are differentially involved in the control of anion secretion under different conditions. Isolation of the basolateral K+ conductance in permeabilized monolayers revealed a small basal and forskolin-stimulated I(sc). Finally, using the reverse transcriptase-polymerase chain reaction, we found that Calu-3 cells express the K+ channel genes KCNN4 and KCNQ1 and the subunits KCNE2 and KCNE3. We conclude that while KCNN4 contributes to Ca2+-activated anion secretion by Calu-3 cells, basal and cAMP-activated secretion are more critically dependent on other K+ channel types, possibly involving one or more class of KCNQ1-containing channel complexes.

A(2) adenosine receptors regulate CFTR through PKA and PLA(2)

We investigated adenosine (Ado) activation of the cystic fibrosis transmembrane conductance regulator (CFTR) in vitro and in vivo. A(2B) Ado receptors were identified in Calu-3, IB-3-1, COS-7, and primary human airway cells. Ado elevated cAMP in Calu-3, IB-3-1, and COS-7 cells and activated protein kinase A-dependent halide efflux in Calu-3 cells. Ado promoted arachidonic acid release from Calu-3 cells, and phospholipase A(2) (PLA(2)) inhibition blocked Ado-activated halide efflux in Calu-3 and COS-7 cells expressing CFTR. Forskolin- and beta(2)-adrenergic receptor-stimulated efflux were not affected by the same treatment. Cytoplasmic PLA(2) (cPLA(2)) was identified in Calu-3, IB-3-1, and COS-7 cells, but cPLA(2) inhibition did not affect Ado-stimulated cAMP concentrations. In cftr(+) and cftr(-/-) mice, Ado stimulated nasal Cl(-) secretion that was CFTR dependent and sensitive to A(2) receptor and PLA(2) blockade. In COS-7 cells transiently expressing DeltaF508 CFTR, Ado activated halide efflux. Ado also activated G551D CFTR-dependent halide efflux when combined with arachidonic acid and phosphodiesterase inhibition. In conclusion, PLA(2) and protein kinase A both contribute to A(2) receptor activation of CFTR, and components of this signaling pathway can augment wild-type and mutant CFTR activity.

Mechanism of substance P-induced liquid secretion across bronchial epithelium

The present study was undertaken to identify and determine the mechanism of noncholinergic pathways for the induction of liquid secretion across airway epithelium. Excised porcine bronchi secreted substantial and significant quantities of liquid when exposed to acetylcholine, substance P, or forskolin but not to isoproterenol, norepinephrine, or phenylephrine. Bumetanide, an inhibitor of Na(+)-K(+)-2Cl(-) cotransport, reduced the liquid secretion response to substance P by 69%. Approximately two-thirds of bumetanide-insensitive liquid secretion was blocked by dimethylamiloride (DMA), a Na(+)/H(+) exchange inhibitor. Substance P responses were preserved in airways after surface epithelium removal, suggesting that secreted liquid originated from submucosal glands. The anion channel blockers diphenylamine-2-carboxylate (DPC) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) inhibited >90% of substance P-induced liquid secretion, whereas DIDS had no effect. DMA, DPC, and NPPB had greater inhibitory effects on net HCO(3)(-) secretion than on liquid secretion. Although preserved relative to liquid secretion, net HCO(3)(-) secretion was reduced by 39% in the presence of bumetanide. We conclude that substance P induces liquid secretion from bronchial submucosal glands of pigs through active transport of Cl(-) and HCO(3)(-). The pattern of responses to secretion agonists and antagonists suggests that the cystic fibrosis transmembrane conductance regulator mediates this process.

Regulation of anion secretion by nitric oxide in human airway epithelial cells

Nitric oxide (NO) is continuously produced and released in human airways, but the biological significance of this process is unknown. In this study, we have used Calu-3 cells to investigate the effects of NO on transepithelial anion secretion. An inhibitor of NO synthase, NG-nitro-L-arginine methyl ester, reduced short- circuit current (I(sc)), whereas an NO donor, S-nitrosoglutathione (GSNO), increased I(sc), with an EC50 approximately 1.2 microM. The NO-activated current was inhibited by diphenylamine-2-carboxylate, clotrimazole, and charybdotoxin. Selective permeabilization of cell membranes indicated that NO activated both apical anion channels and basolateral potassium channels. An inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, prevented activation of I(sc) by NO but not by 8-bromo-cGMP, suggesting that NO acts via a cGMP-dependent pathway. Sequential treatment of cells with forskolin and GSNO or 1-ethyl-2-benzimidazolinone and GSNO showed additive effects of these chemicals on I(sc). Interestingly, GSNO elevated intracellular Ca2+ concentration ([Ca2+]i) but had no effect on I(sc) activated by thapsigargin. These results show that NO activates transepithelial anion secretion via a cGMP-dependent pathway that involves cross talk between NO and [Ca2+]i.

Optical method for quantifying rates of mucus secretion from single submucosal glands

We describe an optical method to quantify single- gland secretion. Isolated tracheal mucosa were mounted at the air-Krebs interface and coated with oil. Gland secretions formed spherical bubbles that were digitally imaged at intervals, allowing rates of secretion to be calculated. We monitored 340 glands in 54 experiments with 12 sheep. Glands secreted basally at low rates (0.57 +/- 0.04 nl x min(-1) x gland(-1), 123 glands) in tissues up to 9 h postharvest and at lower rates for up to 3 days. Carbachol (10 microM) stimulated secretion with an early transient and a sustained or oscillating phase. Peak secretion was 15.7 +/- 1.2 nl x min(-1) x gland(-1) (60 glands); sustained secretion was 4.5 +/- 0.5 nl x min(-1) x gland(-1) (10 glands). Isoproterenol and phenylephrine (10 microM each) stimulated only small, transient responses. We confirmed that cats have a large secretory response to phenylephrine (11.6 +/- 3.7 nl x min(-1) x gland(-1), 12 glands), but pigs, sheep, and humans all have small responses (<2 nl x min(-1)m x gland(-1)). Carbachol-stimulated peak secretion was inhibited 56% by bumetanide, 67% by HCO replacement with HEPES, and 92% by both. The distribution of secretion rates was nonnormal, suggesting the existence of subpopulations of glands.

Differential regulation of Cl- transport proteins by PKC in Calu-3 cells

Cl- transport proteins expressed in a Calu-3 airway epithelial cell line were differentiated by function and regulation by protein kinase C (PKC) isotypes. mRNA expression of Cl- transporters was semiquantitated by RT-PCR after transfection with a sense or antisense oligonucleotide to the PKC isotypes that modulate the activity of the cystic fibrosis transmembrane conductance regulator [CFTR (PKC-epsilon)] or of the Na/K/2Cl (NKCC1) cotransporter (PKC-delta). Expression of NKCC1 and CFTR mRNAs and proteins was independent of antisense oligonucleotide treatment. Transport function was measured in cell monolayers grown on a plastic surface or on filter inserts. With both culture methods, the antisense oligonucleotide to PKC-epsilon decreased the amount of PKC-epsilon and reduced cAMP-dependent activation of CFTR but not alpha(1)-adrenergic activation of NKCC1. The antisense oligonucleotide to PKC-delta did not affect CFTR function but did block alpha(1)-adrenergic activation of NKCC1 and reduce PKC-delta mass. These results provide the first evidence for mRNA and protein expression of NKCC1 in Calu-3 cells and establish the differential regulation of CFTR and NKCC1 function by specific PKC isotypes at a site distal to mRNA expression and translation in airway epithelial cells.

cAMP-independent phosphorylation activation of CFTR by G proteins in native human sweat duct

It is generally believed that cAMP-dependent phosphorylation is the principle mechanism for activating cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channels. However, we showed that activating G proteins in the sweat duct stimulated CFTR Cl(-) conductance (G(Cl)) in the presence of ATP alone without cAMP. The objective of this study was to test whether the G protein stimulation of CFTR G(Cl) is independent of protein kinase A. We activated G proteins and monitored CFTR G(Cl) in basolaterally permeabilized sweat duct. Activating G proteins with guanosine 5'-O-(3-thiotriphosphate) (10-100 microM) stimulated CFTR G(Cl) in the presence of 5 mM ATP alone without cAMP. G protein activation of CFTR G(Cl) required Mg(2+) and ATP hydrolysis (5'-adenylylimidodiphosphate could not substitute for ATP). G protein activation of CFTR G(Cl) was 1) sensitive to inhibition by the kinase inhibitor staurosporine (1 microM), indicating that the activation process requires phosphorylation; 2) insensitive to the adenylate cyclase (AC) inhibitors 2',5'-dideoxyadenosine (1 mM) and SQ-22536 (100 microM); and 3) independent of Ca(2+), suggesting that Ca(2+)-dependent protein kinase C and Ca(2+)/calmodulin-dependent kinase(s) are not involved in the activation process. Activating AC with 10(-6) M forskolin plus 10(-6) M IBMX (in the presence of 5 mM ATP) did not activate CFTR, indicating that cAMP cannot accumulate sufficiently to activate CFTR in permeabilized cells. We concluded that heterotrimeric G proteins activate CFTR G(Cl) endogenously via a cAMP-independent pathway in this native absorptive epithelium.

Functional and molecular characterization of an anion exchanger in airway serous epithelial cells

Serous cells secrete Cl(-) and HCO(3)(-) and play an important role in airway function. Recent studies suggest that a Cl(-)/HCO(3)(-) anion exchanger (AE) may contribute to Cl(-) secretion by airway epithelial cells. However, the molecular identity, the cellular location, and the contribution of AEs to Cl(-) secretion in serous epithelial cells in tracheal submucosal glands are unknown. The goal of the present study was to determine the molecular identity, the cellular location, and the role of AEs in the function of serous epithelial cells. To this end, Calu-3 cells, a human airway cell line with a serous-cell phenotype, were studied by RT-PCR, immunoblot, and electrophysiological analysis to examine the role of AEs in Cl(-) secretion. In addition, the subcellular location of AE proteins was examined by immunofluorescence microscopy. Calu-3 cells expressed mRNA and protein for AE2 as determined by RT-PCR and Western blot analysis, respectively. Immunofluorescence microscopy identified AE2 in the basolateral membrane of Calu-3 cells in culture and rat tracheal serous cells in situ. In Cl(-)/HCO(3)(-)/Na(+)-containing media, the 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP)-stimulated short-circuit anion current (I(sc)) was reduced by basolateral but not by apical application of 4, 4'-diisothiocyanostilbene-2,2'-disulfonic acid (50 microM) and 4, 4'-dinitrostilbene-2,2'-disulfonic acid [DNDS (500 microM)], inhibitors of AEs. In the absence of Na(+) in the bath solutions, to eliminate the contributions of the Na(+)/HCO(3)(-) and Na(+)/K(+)/2Cl(-) cotransporters to I(sc), CPT-cAMP stimulated a small DNDS-sensitive I(sc). Taken together with previous studies, these observations suggest that a small component of cAMP-stimulated I(sc) across serous cells may be referable to Cl(-) secretion and that uptake of Cl(-) across the basolateral membrane may be mediated by AE2.

Bicarbonate and chloride secretion in Calu-3 human airway epithelial cells

Serous cells are the predominant site of cystic fibrosis transmembrane conductance regulator expression in the airways, and they make a significant contribution to the volume, composition, and consistency of the submucosal gland secretions. We have employed the human airway serous cell line Calu-3 as a model system to investigate the mechanisms of serous cell anion secretion. Forskolin-stimulated Calu-3 cells secrete HCO-3 by a Cl-offdependent, serosal Na+-dependent, serosal bumetanide-insensitive, and serosal 4,4'-dinitrostilben-2,2'-disulfonic acid (DNDS)-sensitive, electrogenic mechanism as judged by transepithelial currents, isotopic fluxes, and the results of ion substitution, pharmacology, and pH studies. Similar studies revealed that stimulation of Calu-3 cells with 1-ethyl-2-benzimidazolinone (1-EBIO), an activator of basolateral membrane Ca2+-activated K+ channels, reduced HCO-3 secretion and caused the secretion of Cl- by a bumetanide-sensitive, electrogenic mechanism. Nystatin permeabilization of Calu-3 monolayers demonstrated 1-EBIO activated a charybdotoxin- and clotrimazole- inhibited basolateral membrane K+ current. Patch-clamp studies confirmed the presence of an intermediate conductance inwardly rectified K+ channel with this pharmacological profile. We propose that hyperpolarization of the basolateral membrane voltage elicits a switch from HCO-3 secretion to Cl- secretion because the uptake of HCO-3 across the basolateral membrane is mediated by a 4,4 '-dinitrostilben-2,2'-disulfonic acid (DNDS)-sensitive Na+:HCO-3 cotransporter. Since the stoichiometry reported for Na+:HCO-3 cotransport is 1:2 or 1:3, hyperpolarization of the basolateral membrane potential by 1-EBIO would inhibit HCO-3 entry and favor the secretion of Cl-. Therefore, differential regulation of the basolateral membrane K+ conductance by secretory agonists could provide a means of stimulating HCO-3 and Cl- secretion. In this context, cystic fibrosis transmembrane conductance regulator could serve as both a HCO-3 and a Cl- channel, mediating the apical membrane exit of either anion depending on basolateral membrane anion entry mechanisms and the driving forces that prevail. If these results with Calu-3 cells accurately reflect the transport properties of native submucosal gland serous cells, then HCO-3 secretion in the human airways warrants greater attention.