Modulation of insulin effects on phosphorylation of protein kinase B and glycogen synthesis by tumor necrosis factor-α in HepG2 cells

The effects of tumor necrosis factor-alpha (TNF-alpha) on insulin-induced phosphorylation of protein kinase B-alpha (PKB-alpha) and downstream enzyme glycogen synthase kinase-3beta (GSK-3beta) was examined in HepG2 liver cells. The exogenous treatment of HepG2 cells with TNF-alpha for 1 h caused phosphorylation of Ser(473) and Thr(308) residues of PKB-alpha. The maximal phosphorylation ( similar to 4-fold) was obtained with 1 ng/ml TNF-alpha. and no further increase was observed with higher concentrations of this cytokine. The cells pretreated with TNF-alpha for 1 h followed by incubation with insulin (10 nM) showed near additive effect on phosphorylation of PKB-alpha and downstream enzyme GSK-3beta. The long-term (4, 8, 24 h) exogenous treatment of cells with optimal (1 ng/ml) concentration of TNF-alpha also caused phosphorylation of PKB-alpha, albeit to a lesser degree. However, long-term pretreatments of cells with TNF-alpha reduced insulin-stimulated phosphorylation of PKB-alpha and GSK-3beta. Short- and long-term preincubation of HepG2 cells with TNF-a also resulted in parallel changes in glycogen synthesis in the presence of insulin. In fact, long-term preincubation with TNF-alpha completely abolished the insulin-induced glycogen synthesis. These results suggest that short-term exposure to TNF-alpha augments insulin effects whereas long-term exposure causes insulin resistance in HepG2 cells. (C) 2004 Elsevier B.V All rights reserved.