BASAL EXPRESSION OF THE CYSTIC-FIBROSIS TRANSMEMBRANE CONDUCTANCE REGULATOR GENE IS DEPENDENT ON PROTEIN-KINASE-A ACTIVITY

The cystic fibrosis transmembrane conductance regulator (CFTR) functio ns as a Cl- channel that becomes activated after phosphorylation by cA MP-dependent protein kinase (PKA). We demonstrate that PKA also plays a crucial role in maintaining basal expression of the CFTR gene in the human colon carcinoma cell line T84. Inhibition of PKA activity by ex pression of a dominant-negative regulatory subunit or treatment with t he PKA-selective inhibitor N-[2-(p-bromocinnamylamino)ethyl] -5-isoqui nolinesulfonamide (H-89) caused a complete suppression of CFTR gene ex pression without affecting other constitutively active genes. Basal ex pression of a 2.2-kb region of the CFTR promoter linked to a luciferas e reporter gene (CFTR-luc) exhibited the same dependence on PKA. The a bility of cAMP to induce CFTR over basal levels is cell-type specific. In T84 cells, both the endogenous CFTR gene and CFTR-luc exhibited on ly a modest inducibility (approximate to 2-fold), whereas in the human choriocarcinoma cell line JEG-3, CFTR-luc could be induced at least 4 -fold. A variant cAMP-response element is present at position -48 to - 41 in the CFTR promoter, and mutation of this sequence blocks basal ex pression. We conclude that cAMP, acting through PKA, is an essential r egulator of basal CFTR gene expression and may mediate an induction of CFTR in responsive cell types.