Improved oxygenation promotes CFTR maturation and trafficking in MDCK monolayers

Culturing airway epithelial cells with most of the apical media removed (air-liquid interface) has been shown to enhance cystic fibrosis transmembrane conductance regulator (CFTR)-mediated Cl- secretory current. Thus we hypothesized that cellular oxygenation may modulate CFTR expression. We tested this notion using type I Madin-Darby canine kidney cells that endogenously express low levels of CFTR. Growing monolayers of these cells for 4 to 5 days with an air-liquid interface caused a 50-fold increase in forskolin-stimulated Cl- current, compared with conventional (submerged) controls. Assaying for possible changes in CFTR by immunoprecipitation and immunocytochemical localization revealed that CFTR appeared as an immature 140-kDa form intracellularly in conventional cultures. In contrast, monolayers grown with an air-liquid interface possessed more CFTR protein, accompanied by increases toward the mature 170-kDa form and apical membrane staining. Culturing submerged monolayers with 95% O-2 produced similar improvements in Cl- current and CFTR protein as air-liquid interface culture, while increasing PO2 from 2.5% to 20% in air-liquid interface cultures yielded graded enhancements. Together, our data indicate that improved cellular oxygenation can increase endogenous CFTR maturation and/or trafficking.