Reciprocal effects of the protein kinase C inhibitors staurosporine and H-7 on the regulation of glycogen synthase and phosphorylase in the primary culture of hepatocytes

Abstract

The effects of the protein kinase C inhibitors staurosporine and H-7 [1-(5-isoquinolinylsulfonyl)-2-methylpiperazine] on glucose-induced regulation of glycogen synthase and phosphorylase activities were investigated in the primary culture of hepatocytes. Glycogen synthesis as measured by the incorporation of [ 14C]glucose into glycogen was enhanced up to 78% ( P < .001) by 100 nmol/L staurosporine. In contrast, H-7 inhibited glycogen synthesis in a dose-dependent manner, with an IC 50 value of 70 μmol/L. Activation of glycogen synthase by 30 mmol/L glucose was enhanced significantly ( P < .02 and less) by staurosporine at 20 nmol/L and higher concentrations whereas the activity of this enzyme was inhibited by H-7 (IC 50 = 50 μmol/L). The inactivation of phosphorylase by glucose was significantly greater when staurosporine was included in the medium. However, H-7 increased the phosphorylase activity ratio by 1.5- to 2.5-fold at concentrations of 20 to 100 μmol/L. The time course of synthase activation and phosphorylase inactivation showed that the effect of glucose was enhanced by staurosporine and inhibited by H-7. These novel reciprocal effects of protein kinase C inhibitors were also observed at different concentrations of glucose. The effects of H-8, a compound with structural resemblance to H-7 and an inhibitor of protein kinase A, were similar to those of staurosporine but not to those of H-7. Staurosporine blocked the effects of vasopressin and 4β-phorbol 12β-myristate 13α-acetate (PMA), whereas H-7 in combination with these protein kinase C activators acted in the same direction. The effects of staurosporine, a relatively more specific inhibitor of protein kinase C, indicated that this enzyme plays a role in the regulation of glycogen metabolism in liver. However, H-7, which is known to have protein kinase C-independent effects in intact cells, seems to alter the activities of glycogen synthase and phosphorylase by a different mechanism.