Protein kinase C mediates down-regulation of cystic fibrosis transmembrane conductance regulator levels in epithelial cells

Expression of the cystic fibrosis transmembrane conductance regulator (CFTR) in epithelial cells is known to be down-regulated by the action of phorbol myristate acetate (PMA). We show here that in addition to suppressing the rate of transcription of the CFTR gene, PMA treatment stimulates degradation of the CFTR protein. HT-29 colon epithelial cells and the CFTR-transfected pancreatic cells PLJ-4.7 lost 55-80% of their CFTR protein after 3-6 h of treatment with 100 nM PMA, as analyzed by quantitative Western blotting. In contrast to PMA, actinomycin D and cycloheximide reduced the CFTR protein content by 19 and 9% in HT-29 cells and by 22 and 40% in PLJ-4.7 cells, respectively, while inhibiting total cellular RNA and protein synthesis by over 80%. The PMA-induced loss of CFTR was partially reversed by the protein kinase C inhibitor GF109203X. The PMA-induced degradation of CFTR may represent a regulatory pathway for terminating CFTR-mediated chloride and mucin secretion.