Knowledge on the mechanisms of acid and base secretion in airways has progressed recently. The aim of this review is to summarize the known mechanisms of airway surface liquid (ASL) pH regulation and their implication in lung diseases. Normal ASL is slightly acidic relative to the interstitium, and defects in ASL pH regulation are associated with various respiratory diseases, such as cystic fibrosis. Basolateral bicarbonate (HCO₃⁻) entry occurs via the electrogenic, coupled transport of sodium (Na⁺) and HCO₃⁻, and, together with carbonic anhydrase enzymatic activity, provides HCO₃⁻ for apical secretion. The latter mainly involves CFTR, the apical chloride/bicarbonate exchanger pendrin and paracellular transport. Proton (H⁺) secretion into ASL is crucial to maintain its relative acidity compared to the blood. This is enabled by H⁺ apical secretion, mainly involving H⁺/K⁺ ATPase and vacuolar H⁺-ATPase that carry H⁺ against the electrochemical potential gradient. Paracellular HCO₃⁻ transport, the direction of which depends on the ASL pH value, acts as an ASL protective buffering mechanism. How the transepithelial transport of H⁺ and HCO₃⁻ is coordinated to tightly regulate ASL pH remains poorly understood, and should be the focus of new studies.

Proton secretion mediated by ATP12A protein on the surface of the airway epithelium may contribute to cystic fibrosis (CF) lung disease by favoring bacterial infection and airway obstruction. We studied ATP12A in fresh bronchial samples and in cultured epithelial cells. In vivo, ATP12A expression was found almost exclusively at the apical side of nonciliated cells of airway epithelium and in submucosal glands, with much higher expression in CF samples. This could be due to bacterial infection and inflammation, since treating cultured cells with bacterial supernatants or with IL-4 (a cytokine that induces goblet cell hyperplasia) increased the expression of ATP12A in nonciliated cells. This observation was associated with upregulation and translocation of ATP1B1 protein from the basal to apical epithelial side, where it colocalizes with ATP12A. ATP12A function was evaluated by measuring the pH of the apical fluid in cultured epithelia. Under resting conditions, CF epithelia showed more acidic values. This abnormality was minimized by inhibiting ATP12A with ouabain. Following treatment with IL-4, ATP12A function was markedly increased, as indicated by strong acidification occurring under bicarbonate-free conditions. Our study reveals potentially novel aspects of ATP12A and remarks its importance as a possible therapeutic target in CF and other respiratory diseases.

The chloroquine resistance transporter of the human malaria parasite Plasmodium falciparum, PfCRT, is an important determinant of resistance to several quinoline and quinoline-like antimalarial drugs. PfCRT also plays an essential role in the physiology of the parasite during development inside erythrocytes. However, the function of this transporter besides its role in drug resistance is still unclear. Using electrophysiological and flux experiments conducted on PfCRT-expressing Xenopus laevis oocytes, we show here that both wild-type PfCRT and a PfCRT variant associated with chloroquine resistance transport both ferrous and ferric iron, albeit with different kinetics. In particular, we found that the ability to transport ferrous iron is reduced by the specific polymorphisms acquired by the PfCRT variant as a result of chloroquine selection. We further show that iron and chloroquine transport via PfCRT is electrogenic. If these findings in the Xenopus model extend to P. falciparum in vivo, our data suggest that PfCRT might play a role in iron homeostasis, which is essential for the parasite's development in erythrocytes.

The fluid covering the surface of airway epithelia represents a first barrier against pathogens. The chemical and physical properties of the airway surface fluid are controlled by the activity of ion channels and transporters. In cystic fibrosis (CF), loss of CFTR chloride channel function causes airway surface dehydration, bacterial infection, and inflammation. We investigated the effects of IL-17A plus TNF-α, 2 cytokines with relevant roles in CF and other chronic lung diseases. Transcriptome analysis revealed a profound change with upregulation of several genes involved in ion transport, antibacterial defense, and neutrophil recruitment. At the functional level, bronchial epithelia treated in vitro with the cytokine combination showed upregulation of ENaC channel, ATP12A proton pump, ADRB2 β-adrenergic receptor, and SLC26A4 anion exchanger. The overall result of IL-17A/TNF-α treatment was hyperviscosity of the airway surface, as demonstrated by fluorescence recovery after photobleaching (FRAP) experiments. Importantly, stimulation with a β-adrenergic agonist switched airway surface to a low-viscosity state in non-CF but not in CF epithelia. Our study suggests that CF lung disease is sustained by a vicious cycle in which epithelia cannot exit from the hyperviscous state, thus perpetuating the proinflammatory airway surface condition.

BACKGROUND AND PURPOSE The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-dependent chloride channel in the plasma membrane of epithelia whose mutation is the cause of the genetic disease cystic fibrosis (CF). The most frequent CFTR mutation is deletion of Phe⁵⁰⁸ and this mutant protein (delF508CFTR) does not readily translocate to the plasma membrane and is rapidly degraded within the cell. We hypothesized that treating epithelial cells with resveratrol, a natural polyphenolic, phyto-ooestrogenic compound from grapes, could modulate both the expression and localization of CFTR. EXPERIMENTAL APPROACH Cells endogenously expressing CFTR (MDCK1 and CAPAN1 cells) or delF508CFTR (CFPAC1 and airway epithelial cells, deriving from human bronchial biopsies) were treated with resveratrol for 2 or 18 h. The effect of this treatment on CFTR and delF508CFTR expression and localization was evaluated using RT-PCR, Western blot and immunocytochemistry. Halide efflux was measured with a fluorescent dye and with halide-sensitive electrodes. Production of interleukin-8 by these cells was assayed by ELISA. KEY RESULTS Resveratrol treatment increased CFTR expression or maturation in immunoblotting experiments in MDCK1 cells or in CFPAC1 cells. Indirect immunofluorescence experiments showed a shift of delF508CFTR localization towards the (peri)-membrane area in CFPAC1 cells and in human airway epithelial cells. A cAMP-dependent increase in membrane permeability to halide was detected in resveratrol-treated CFPAC1 cells, and was inhibited by a selective inhibitor of CFTR. CONCLUSION AND IMPLICATIONS These results show that resveratrol modulated CFTR expression and localization and could rescue cAMP-dependent chloride transport in delF508CFTR cells.

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