PHOSPHATASE INHIBITORS ACTIVATE NORMAL AND DEFECTIVE CFTR CHLORIDE CHANNELS

The cystic fibrosis transmembrane conductance regulator (CFTR) chlorid e channel is regulated by phosphorylation and dephosphorylation at mul tiple sites. Although activation by protein kinases has been studied i n some detail, the dephosphorylation step has received little attentio n. This report examines the mechanisms responsible for the dephosphory lation and spontaneous deactivation (''rundown'') of CFTR chloride cha nnels excised from transfected Chinese hamster ovary (CHO) and human a irway epithelial cells. We report that the alkaline phosphatase inhibi tors bromotetramisole, 3-isobutyl-1-methylxanthine, theophylline, and vanadate slow the rundown of CFTR channel activity in excised membrane patches and reduce dephosphorylation of CFTR protein in isolated memb ranes. It was also found that in unstimulated cells, CFTR channels can be activated by exposure to phosphatase inhibitors alone. Most import antly, exposure of mammalian cells to phosphatase inhibitors alone act ivates CFTR channels that have disease-causing mutations, provided the mutant channels are present in the plasma membrane (R117H, G551D, and Delta F508 after cooling). These results suggest that CFTR dephosphor ylation is dynamic and that membrane-associated phosphatase activity m ay be a potential therapeutic target for the treatment of cystic fibro sis.