CFTR induces the expression of DRA along with Cl-/HCO3- exchange activity in tracheal epithelial cells

Thickening of airway mucus and lung dysfunction in cystic fibrosis (CF) res ults, at least in part, from abnormal secretion of Cl- and HCO3- across the tracheal epithelium. The mechanism of the defect in HCO3- secretion is ill defined; however, a lack of apical Cl-/HCO3- exchange may exist in CF. To test this hypothesis, we examined the expression of Cl-/HCO3- exchangers in tracheal epithelial cells exhibiting physiological features prototypical o f cystic fibrosis [CFT-1 cells, lacking a functional cystic fibrosis transm embrane conductance regulator (CFTR)] or normal trachea (CFT-1 cells transf ected with functional wild-type CFTR, termed CFT-WT). Cells were grown on c overslips and were loaded with the pH-sensitive dye 2',7'-bis(2-carboxyethy l)-5(6)-carboxyfluorescein, and intracellular pH was monitored. Cl-/HCO3- e xchange activity increased by similar to 300% in cells transfected with fun ctional CFTR, with activities increasing from 0.034 pH/min in CFT-1 cells t o 0.11 in CFT-WT cells (P < 0.001, n = 8). This activity was significantly inhibited by DIDS. The mRNA expression of the ubiquitous basolateral AE-2 C l-/HCO3- exchanger remained unchanged. However, mRNA encoding DRA, recently shown to be a Cl-/HCO3- exchanger (Melvin JE, Park K, Richardson L, Schult heis PJ, and Shull GE. J Biol Chem 274: 22855-22861, 1999.) was abundantly expressed in cells expressing functional CFTR but not in cells that lacked CFTR or that expressed mutant CFTR. In conclusion, CFTR induces the mRNA ex pression of "downregulated in adenoma" (DRA) and, as a result, upregulates the apical Cl-/HCO3- exchanger activity in tracheal cells. We propose that the tracheal HCO3- secretion defect in patients with CF is partly due to th e downregulation of the apical Cl-/HCO3- exchange activity mediated by DRA.