Many extracellular signals elicit Ca2+ mobilization and diacylglycerol formation in their target cells. Diacylglycerol is derived from the receptor‐linked phosphoinositide turnover and serves as a second messenger for the activation of protein kinase C in the presence of Ca2+ and phosphatidylserine. Unique diacylglycerols such as 1‐oleoyl‐2‐acetyl‐glycerol, which activate intracellular protein kinase C when added to intact cells, have been synthesized. Tumor‐promoting phorbol esters substitute for such diacylglycerols and directly activate protein kinase C in both intact cell and cell‐free systems. Under appropriate conditions, the synthetic diacylglycerols and phorbol esters induce protein kinase C activation without Ca2+ mobilization, whereas Ca2+ ionophore A23187 induces Ca2+ mobilization without protein kinase C activation. Using these substances, we have obtained evidence that both protein kinase C and Ca2+ are involved in and play a synergistic role in exocytosis, cell division, and other cellular functions. In this article, the role of protein kinase C in transmembrane signaling is discussed.
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