Properties of CFTR activated by the xanthine derivative X-33 in human airway Calu-3 cells

The pharmacological activation of thecystic fibrosis gene protein cystic fibrosis transmembrane conductanceregulator (CFTR) was studied in human airway epithelial Calu-3 cells,which express a high level of CFTR protein as assessed by Western blotand in vitro phosphorylation. Immunolocalization shows that CFTR islocated in the apical membrane. We performed iodide efflux, whole cell patch-clamp, and short-circuit recordings to demonstrate that the novelsynthesized xanthine derivative 3,7-dimethyl-1-isobutylxanthine (X-33)is an activator of the CFTR channel in Calu-3 cells. Whole cell currentactivated by X-33 or IBMX is linear, inhibited by glibenclamide anddiphenylamine-2-carboxylate but not by DIDS or TS-TM calix[4]arene.Intracellular cAMP was not affected by X-33. An outwardly rectifyingCl current was recorded in the absence of cAMP and X-33stimulation, inhibited by DIDS and TS-TM calix[4]arene. With the useof short-circuit recordings, X-33 and IBMX were able to stimulate alarge concentration-dependent CFTR transport that was blocked byglibenclamide but not by DIDS. Our results show that manipulating thechemical structure of xanthine derivatives offers an opportunity toidentify further specific activators of CFTR in airway cells.