Activation of membrane protein kinase C by glucagon and Ca2+-mobilizing hormones in cultured rat hepatocytes. Role of phosphatidylinositol and phosphatidylcholine hydrolysis

Abstract

We found that glucagon stimulated membrane protein kinase C (PKC) activity and phosphatidylcholine hydrolysis in 24 h-cultured rat hepatocytes. Phorbol myristate acetate, 8-bromo cyclic AMP, vasopressin, noradrenaline and the Ca2+ ionophore A23187 also stimulated membrane PKC activity. However, only vasopressin and noradrenaline stimulated inositol phosphate accumulation, whereas all agonists stimulated the rate of release of water-soluble choline metabolites into the medium. Choline, and to a much lesser extent phosphocholine, were released, suggesting predominantly phospholipase D activation. This was supported by the finding that the accumulation of phosphatidate and diacylglycerol was enhanced by the agents in [3H]myristate-labelled hepatocytes, as was [32P]phosphatidylethanol formation. Since the time courses for the release of choline into the medium and the accumulation of phosphatidate and diacylglycerol caused by vasopressin and glucagon were similar, the more rapid activation of PKC by vasopressin probably reflects diacylglycerol formation from phosphoinositide breakdown. The inability of glucagon to stimulate inositol phosphate production was not due to the prolonged culture, since similar results were obtained in 4 h cultures. We conclude that the stimulation of membrane PKC activity by glucagon correlates with accumulation of diacylglycerol and phosphatidate derived from the hydrolysis of phosphatidylcholine.

Calcium: its modulation in liver by cross-talk between the actions of glucagon and calcium-mobilizing agonists

“…EGF does not increase Ins(1,4,5)P3 formation, and pertussis toxin treatment (which blocks the action of G proteins) of rat hepatocytes abolished EGF-and vasopressin-induced Ca2+ inflow. Also relevant are the reports showing that 4fl-phorbol 12-myristate 13-acetate treatment of hepatocytes, which leads to activation of protein kinase C, attenuates the Ca2+-mobilizing action of glucagon (Staddon and Hansford, 1986; see also Garcia-Sainz et al, 1985;Staddon and Hansford, 1989;Pittner and Fain, 1991). Whether some of the signals being generated here result from hormone-induced phosphatidylcholine breakdown (Exton, 1990) remains an interesting possibility.…”

Effects of phorbol ester‐sensitive PKC (c/nPKC) activation on the production of adiponectin in 3T3‐L1 adipocytes

“…Our previous study (13) showed that activation of atypical PKC (aPKC) led to NF-jB activation and a decrease of PPARc expression. Considering the fact that various hormones, including insulin, angiotensin II, glucagons, parathyroid hormone, endothelin-1, thrombin, and vitamin D3, and high glucose concentration, activate c/nPKC (14)(15)(16)(17)(18)(19), evaluating the role of c/nPKC on gene expression in adipocyte may provide a new understanding of the regulation of insulin sensitivity.…”

Diabetes induces selective alterations in the expression of protein kinase C isoforms in hepatocytes

“…Such cAMP-independent actions of glucagon appear to be attributable to its ability to activate lipid signalling pathways leading to the production of DAG [19][20][21][22], elevation of Ca 2+ levels and the activation of PKC [22,23]. The recent molecular cloning of the glucagon receptor indicates that a single receptor, able to couple to more than one signalling system, is responsible for these actions [24].…”