CAMP-INDEPENDENT AND CA2-INDEPENDENT ACTIVATION OF CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR CHANNELS BY PHENYLIMIDAZOTHIAZOLE DRUGS()

Patch-clamp, iodide efflux, and biochemical techniques were used to ev aluate the ability of phenylimidazothiazoles to open normal and mutate d cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels and to investigate the mechanism of activation. As reported p reviously for bromotetramisole, levamisole activated wild-type CFTR ch annels stably expressed in Chinese hamster ovary cells in the absence of other secretagogues and without elevating intracellular cAMP or cal cium. The protein kinase A (PKA) inhibitor romocinnamylamino)ethyl)-5- isoquinolinesulfonamide abolished activation by forskolin but only par tially inhibited stimulation by levamisole, suggesting the involvement of other kinases. CFTR channels bearing mutations at multiple phospho rylation sites, in the membrane domains, and in the first nucleotide b inding domain (including the disease-causing mutations G551D and Delta F508) all responded to phenylimidazothiazoles. Moreover, levamisole a nd bromotetramisole increased the activity of wild-type and mutant cha nnels already exposed to PKA + MgATP, consistent with the inhibition o f a constitutive, membrane-associated phosphatase activity. We conclud e that phenylimidazothiazole drugs can open normal and mutated CFTR ch annels by stabilization of phosphoforms of CFTR that are produced by b asal activity of PKA and alternative protein kinases. If similar stimu lation is observed in humans in vivo, phenylimidazothiazoles may be us eful in the development of pharmacological therapies for cystic fibros is.