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

Serous cells secrete Cl- and HCO3- and play an important role in airway function. Recent studies suggest that a Cl-/HCO3- 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-/HCO3-/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 mu M) and 4,4'-dinitrostilbene-2,2'-disulfonic acid [DNDS (500 mu M)], inhibitors of AEs. In the absence of Na+ in the bath solutions, to eliminate the contributions of the Na+/HCO3- 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.