Further characterization of tumor-promoter-mediated activation of protein kinase C

Tumor promoting phorbol esters are able to activate Ca2+-sensitive, phospholipid-dependent protein kinase (protein kinase C) in a reconstituted system. Indol alkaloid teleocidin, a tumor promoter, has been found to be as potent as tumor promoters from the series of phorbol esters and mezerein in activating the mouse brain enzyme. Chemically unrelated tumor promoters such as tetrachlorodibenzo-p-dioxin, anthralin and phenobarbital are devoid of effect. Diacylglycerol 1,2 diolein strongly activated the enzyme whereas 1,3 diolein like 1,2 distearin were poor activators and 1,3 distearin was inactive. Although tumor-promoter-or diacylglycerol-mediated activation of protein kinase C was observed in the presence of 0.5mM EGTA, the reaction requires traces of Ca2+. Tumor promoters did not prevent inhibitory action of antipsychotic phenothiazines and local anesthetics but appear to increase IC50 of these drugs.