Phorbol esters, phospholipase C, and growth factors rapidly stimulate the phosphorylation of a Mr 80,000 protein in intact quiescent 3T3 cells.

Addition of biologically active phorbol esters to intact quiescent 3T3 mouse cells stimulates an extremely rapid (detectable within seconds) phosphorylation of a Mr 80,000 cellular protein (termed "80k"). Phorbol 12,13-dibutyrate enhances 80k phosphorylation in a dose-dependent manner; half-maximal effect is obtained at 32 nM. The possibility that this phosphorylation is related to the activation of Ca2+-activated phospholipid-dependent protein kinase is suggested by the fact that phospholipid breakdown induced by exogenous treatment of the cells with phospholipase C from Clostridium perfringens or with platelet-derived growth factor, which is a potent activator of endogenous phospholipase C activity, also causes a rapid enhancement of 80k phosphorylation. Moreover, prolonged pretreatment of the cells with phorbol 12,13-dibutyrate, which leads to a marked decrease in the number of specific phorbol ester binding sites, prevents the phosphorylation of 80k stimulated by phorbol esters, phospholipase C, and platelet-derived growth factor. These findings provide evidence obtained with intact cells that implicate the stimulation of Ca2+-activated phospholipid-dependent protein kinase in the action of phorbol esters and other growth factors.