Genistein activates CFTR C1- channels via a tyrosine kinase- and protein phosphatase-independent mechanism

Previous studies have revealed an adenosine 3',5'-cyclic monophosphate (cAMP)-independent activation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl^- channels by the tyrosine kinase inhibitor genistein. To further explore its mechanism of action, we have reconstituted genistein activation of CFTR in excised inside-out membrane patches. In the presence or absence of ATP, genistein appeared unable to open silent CFTR Cl^- channels. However, on CFTR prephosphorylation by cAMP-dependent protein kinase (cAK), genistein enhanced CFTR activity by twofold, resulting from a prolonged burst duration. Genistein could also hyperactivate partially phosphorylated CFTR in the absence of cAK and therefore is different from 5'- adenylylimidodiphosphate, which required fully phosphorylated CFTR. Phosphatase-resistant thiophosphorylation likewise primed the CFTR Cl^- channel for hyperactivation by genistein in the absence of cAK. Replacement of ATP by GTP as a hydrolyzable nucleotide triphosphate for CFTR did not impair the ability of genistein to activate thiophosphorylated CFTR, despite the fact that GTP is a poor substrate for tyrosine kinases. These findings argue against a role of protein phosphatases or tyrosine kinases but suggest a more direct interaction of genistein with CFTR, possibly at the level of the second nucleotide-binding domain.