Role of protein kinase C (PKC) in short-and long-term cellular responses: Inhibition of agonist-mediated calcium transients and down-regulation of PKC

Active tumor promoters such as 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or membrane-diffusible synthetic diacylglycerols such as 1,2-dioctanoyl-snglycerol (DiC₈), which specifically activate protein kinase C (PKC), inhibited the agonist-mediated rise in cytosolic calcium [(Ca²⁺)_i] in a mast cell line (PB-3c) and human platelets. TPA inhibition of agonist-mediated calcium transient in platelets was readily reversed by the PKC inhibitor staurosporine. In contrast to DiCs, only active tumor promoters induced a time- and dose-dependent translocation of cytosolic PKC to membranes as determined both enzymatically or by immunoblotting. However, the concentration of TPA required to induce a half-maximal subcellular redistribution of immunodetectable PKC activity was an order of magnitude greater than the half-maximal dose required to inhibit the intracellular rise in (Ca²⁺)_i. Thus, activation of PKC seems not to be exclusively coupled to its translocation to membranes, suggesting that translocation of PKC is mainly involved in the down-regulation of PKC. Down-regulation of immunoprecipitable PKC was studied in various human breast cancer cell lines that display differential growth inhibitory responses toward the tumor promoter. TPA induced translocation of [³⁵S]methionine-prelabeled cytosolic 80 kDa PKC to membranes followed by complete degradation of the enzyme (t = 2 h) without affecting PKC synthesis. During prolonged TPA exposure, 20-80% of total 80 kDa PKC of control cells was still synthetized as a membrane-bound 74/80 kDa PKC doublet. Although both proteins lacked PKC activity and phorbol ester binding, they revealed structural similarity with the active 80 kDa PKC form of untreated cells. The amounts of immunoprecipitable membrane-bound PKC-related poly-peptide synthesized during the prolonged TPA treatment appeared to be inversely correlated with the extent of TPA-mediated growth inhibition of the respective human breast cancer cell lines. These data suggest that following homologous down-regulation the functional 80 kDa PKC is replaced by a population of membrane-associated, but enzymatically inactive, 74 and 80 kDa PKC-related poly-peptides.