Modulation of CFTR chloride channels by calyculin A and genistein.

Modulation of the cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channel by calyculin A and genistein was studied in Hi-5 insect cells infected with baculovirus containing the wild-type CFTR cDNA. In cell-attached patches, CFTR channel activity was not observed until stimulated by forskolin in 90% of the cells, suggesting a low level of basal adenosine 3',5'-cyclic monophosphate activity. Calyculin A, a specific inhibitor of phosphatases 1 and 2A, increased forskolin-induced CFTR activity by 17.2-fold. CFTR channel currents did not deactivate completely after forskolin was withdrawn in the continued presence of calyculin A. Genistein enhanced forskolin-induced CFTR activity by 44.9-fold but could neither activate the CFTR by itself nor prevent complete deactivation on removal of forskolin. Genistein together with calyculin A could adequately prevent deactivation of CFTR currents. Noise analysis of the macroscopic CFTR currents revealed significant differences in the mean current-variance-relationship and the corner frequency of the noise spectra between currents activated by forskolin plus genistein and those activated by forskolin plus calyculin A. Furthermore, genistein enhanced CFTR activity induced by saturating concentrations of forskolin and calyculin A. Our results suggest that genistein and calyculin A modulate the CFTR by different mechanisms and that genistein might inhibit calyculin A-insensitive dephosphorylation of the CFTR.