Inhibition of autophagic proteolysis by inhibitors of phosphoinositide 3-kinase can interfere with the regulation of glycogen synthesis in isolated hepatocytes

Abstract

Amino acid-induced cell swelling stimulates conversion of glucose into glycogen in isolated hepatocytes. Activation of glycogen synthase (GS) phosphatase, caused by the fall in intracellular chloride accompanying regulatory volume decrease, and activation of phosphoinositide 3-kinase (PI 3-kinase), induced by cell swelling, have been proposed as underlying mechanisms. Because PI 3-kinase controls autophagic proteolysis, we examined the possibility that PI 3-kinase inhibitors interfere with glycogen production due to their anti-proteolytic action. The PI 3-kinase inhibitor wortmannin inhibited endogenous proteolysis, the production of glycogen from glucose and the activity of active (dephosphorylated) GS (GSa) in the absence of added amino acids. The stimulation by amino acids of glycogen production and of GSa was only slightly affected by wortmannin. These effects of wortmannin could be mimicked by proteinase inhibitors. A combination of leucine, phenylalanine and tyrosine, which we showed previously to stimulate PI 3-kinase-dependent phosphorylation of ribosomal protein S6, did not stimulate glycogen production from glucose. In contrast with wortmannin, LY294002, another PI 3-kinase inhibitor, strongly inhibited both glycogen synthesis and GSa activity, irrespective of the presence of amino acids. Inhibition of glycogen synthesis by LY294002 could be ascribed in part to increased glycogenolysis and glycolysis. It is concluded that, in hepatocytes, activation of PI 3-kinase may not be responsible for the stimulation of glycogen synthesis by amino acids; LY294002 inhibits glycogen synthesis and stimulates glycogen breakdown by a mechanism that is unrelated to its action as an inhibitor of PI 3-kinase.