The cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP-activated Cl- channel critically important in Cl- secreting epithelia. Mutations in the CFTR gene, such as Delta(CFTR)-C-F508 leads to cystic fibrosis, a severe disease with defective Cl- secretion. CFTR is stimulated by the serum and glucocorticoid-inducible kinase SGK1. The SGK1 dependent regulation of several carriers and channels involves the phosphatidylinositol-3-phosphate-5-kinase PIKfyve, which similarly mediates the regulation of glucose carriers by PKB/Akt. The present study was thus performed to elucidate whether PKB/Akt and PIKfyve are regulators of CFTR. To this end CFTR or (CFTR)-C-Delta F508 were expressed in Xenopus oocytes alone or together with PKB, PIKfyve or the SGK1/PKB resistant mutant (S318A)PIKfyve, and the current generated by cAMP upregulation with 10 mu M forskolin + 1 mM IBMX determined utilizing dual electrode voltage clamp. As a result, forskolin/IBMX treatment triggered a current (I-cAMP) in CFTR-expressing Xenopus oocytes, but not in oocytes expressing (CFTR)-C-Delta F508. Coexpression of PKB/Akt and PIKfyve, but not of (S318A)PIKfyve, stimulated I-cAMP in CFTR-expressing (approximate to 2- to 3-fold) but not in (CFTR)-C-Delta F508-expressing or water injected Xenopus oocytes. Immunohistochemistry revealed that the coexpression of PIKfyve, but not of (S318A)PIKfyve, enhanced the CFTR protein abundance but not the (CFTR)-C-Delta F508 protein abundance in CFTR or (CFTR)-C-Delta F508-expressing oocytes. The present observations reveal a novel powerful regulator of intact but not of defective CFTR. (C) 2009 Elsevier Inc. All rights reserved.