Continuous synthesis of two protein-kinase-C-related proteins after down-regulation by phorbol esters.

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Abstract
The phorbol 12-myristate 13-acetate (PMA)-dependent down-regulation of immunoprecipitable protein kinase C was studied in human breast cancer cell lines that display different growth inhibitions toward the tumor promoter. PMA induces translocation of [35S]methionine-prelabeled cytosolic protein kinase C to membranes, followed by complete degradation of the enzyme (t1/2, 2 hr). PMA does not affect the protein kinase C synthesis; 20-80% of total protein kinase C of control cells was still immunoprecipitable as membrane-bound 74- and 80-kDa protein kinase C-related polypeptides if cells were allowed to incorporate [35S]methionine during PMA exposure for > 6 hr. These two proteins lack protein kinase activity and phorbol ester binding but reveal V8 peptide patterns identical to the active forms of protein kinase C (77/80 kDa) of PMA-untreated cells. The amounts of the immunoprecipitable membrane-bound 80-kDa protein kinase C-related polypeptide synthesized during the prolonged PMA treatment appear to inversely correlate with the extent of PMA-mediated growth inhibition of the respective human breast cancer cell line. These data suggest that after homologous down-regulation, functional protein kinase C (77/80 kDa) is replaced by a population of membrane-associated but enzymatically inactive protein kinase C-related polypeptides (74/80 kDa).