cAMP- and Ca²⁺-independent activation of cystic fibrosis transmembrane conductance regulator channels by phenylimidazothiazole drugs

Patch-clamp, iodide efflux, and biochemical techniques were used to evaluate the ability of phenylimidazothiazoles to open normal and mutated cystic fibrosis transmembrane conductance regulator (CFTR) chloride channels and to investigate the mechanism of activation. As reported previously for bromotetramisole, levamisole activated wild-type CFTR channels stably expressed in Chinese hamster ovary cells in the absence of other secretagogues and without elevating intracellular cAMP or calcium. The protein kinase A (PKA) inhibitor N-(2-(p-bromocinnamylamino)ethyl)-5- isoquinolinesulfonamide abolished activation by forskolin but only partially inhibited stimulation by levamisole, suggesting the involvement of other kinases. CFTR channels bearing mutations at multiple phosphorylation sites, in the membrane domains, and in the first nucleotide binding domain (including the disease-causing mutations G551D and ΔF508) all responded to phenylimidazothiazoles. Moreover, levamisole and bromotetramisole increased the activity of wild-type and mutant channels already exposed to PKA + MgATP, consistent with the inhibition of a constitutive, membrane-associated phosphatase activity. We conclude that phenylimidazothiazole drugs can open normal and mutated CFTR channels by stabilization of phosphoforms of CFTR that are produced by basal activity of PKA and alternative protein kinases. If similar stimulation is observed in humans in vivo, phenylimidazothiazoles may be useful in the development of pharmacological therapies for cystic fibrosis.