Mechanism of hepatic glycogen synthase inactivation induced by Ca²⁺-mobilizing hormones. Studies using phospholipase C and phorbol myristate acetate

Incubation of hepatocytes with the protein kinase C activator and tumour promoter 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA) produced a time- and concentration-dependent inactivation of glycogen synthase, but no change in phosphorylase. The same rate and extent of inactivation occurred in hepatocytes depleted of Ca2+ by treatment with the Ca2+ chelator EGTA. When hepatocytes were treated with the Ca2+-mobilizing hormone vasopressin (10 nM), the rate of glycogen synthase inactivation was similar to that observed with PMA (1 microM). Depletion of intracellular Ca2+ stores with EGTA abolished the ability of vasopressin to mobilize Ca2+ and activate phosphorylase without abolishing its ability to inactivate glycogen synthase and increase 1,2-diacylglycerol (DAG), the endogenous activator of protein kinase C. Protein kinase C, either in membranes or after partial purification, was shown to be activated in vitro by PMA in the presence of very low concentrations of Ca2+. Exogenous phospholipase C from Clostridium perfringens, at low concentrations, inactivated glycogen synthase and increased DAG without affecting cell Ca2+ or phosphorylase. It is proposed that the inactivation of glycogen synthase elicited by the Ca2+-mobilizing hormones is due, at least in part, to generation of DAG and activation of protein kinase C.