[The role of protein kinase C in cell surface signal transduction and tumor promotion].

Information from certain extracellular signals, including a group of peptide hormones, some neurotransmitters and many other biologically active substances flows from the cell surface into the cell interior by means of two mechanisms, Ca2+ mobilization and protein kinase C activation. These two signal pathways are activated by the receptor-mediated hydrolysis of phosphatidylinositol-4, 5-bisphosphate. Both protein kinase C activation and Ca2+ mobilization are essential for short-term responses as well as long-term responses. However, additional receptor occupation by growth factor is necessary to induce full activation of cell proliferation, and the signal pathway through protein kinase C appears to be separate from and synergistic to that via growth factors. Several functional proteins in many tissues have been reported to serve as substrates of protein kinase C. The phosphorylation of some of these proteins is apparently related to down-regulation or negative feedback control of cellular function activation. It is possible that protein kinase C has a dual action in the positive as well as the negative phase of regulation depending on the function of each target substrate protein. This article summarizes the possible roles of this unique protein kinase system.