CFTR-Adenylyl Cyclase I Association Responsible for UTP Activation of CFTR in Well-Differentiated Primary Human Bronchial Cell Cultures

Abstract

Chloride secretion by airway epithelial cells is defective in cystic fibrosis (CF). The conventional paradigm is that CFTR is activated through cAMP and protein kinase A (PKA), whereas the Ca²⁺-activated chloride channel (CaCC) is activated by Ca²⁺agonists like UTP. We found that most chloride current elicited by Ca²⁺agonists in primary cultures of human bronchial epithelial cells is mediated by CFTR by a mechanism involving Ca²⁺activation of adenylyl cyclase I (AC1) and cAMP/PKA signaling. Use of selective inhibitors showed that Ca²⁺agonists produced more chloride secretion from CFTR than from CaCC. CFTR-dependent chloride secretion was reduced by PKA inhibition and was absent in CF cell cultures. Ca²⁺agonists produced cAMP elevation, which was blocked by adenylyl cyclase inhibition. AC1, a Ca²⁺/calmodulin-stimulated adenylyl cyclase, colocalized with CFTR in the cell apical membrane. RNAi knockdown of AC1 selectively reduced UTP-induced cAMP elevation and chloride secretion. These results, together with correlations between cAMP and chloride current, suggest that compartmentalized AC1–CFTR association is responsible for Ca²⁺/cAMP cross-talk. We further conclude that CFTR is the principal chloride secretory pathway in non-CF airways for both cAMP and Ca²⁺agonists, providing a novel mechanism to link CFTR dysfunction to CF lung disease.