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

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Abstract
Thickening of airway mucus and lung dysfunction in cystic fibrosis (CF) results, 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 of cystic fibrosis [CFT-1 cells, lacking a functional cystic fibrosis transmembrane conductance regulator (CFTR)] or normal trachea (CFT-1 cells transfected with functional wild-type CFTR, termed CFT-WT). Cells were grown on coverslips and were loaded with the pH-sensitive dye 2′,7′-bis(2-carboxyethyl)-5(6)-carboxyfluorescein, and intracellular pH was monitored. Cl−/HCO3 −exchange activity increased by ∼300% in cells transfected with functional CFTR, with activities increasing from 0.034 pH/min in CFT-1 cells to 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 Cl−/HCO3 − exchanger remained unchanged. However, mRNA encoding DRA, recently shown to be a Cl−/HCO3 − exchanger (Melvin JE, Park K, Richardson L, Schultheis 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 expression 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 the downregulation of the apical Cl−/HCO3 −exchange activity mediated by DRA.