The Importance of Nitric Oxide in the Cytokine-Induced Inhibition of Glucose Formation by Cultured Hepatocytes Incubated with Insulin, Dexamethasone, and Glucagon

Abstract

Culturing hepatocytes with a combination of tumor necrosis factor α, interferon γ, and interleukin 1β plus lipopolysaccharide resulted in an induction of nitric oxide synthase and concomitant inhibition of both hepatic gluconeogenesis and glycogenolysis. The inhibition of gluconeogenesis was evident both under basal conditions and in cells stimulated acutely with glucagon. The stimulation of glycogen mobilization by glucagon was largely prevented by the presence of the cytokines. Chronic 24-h treatment of the cells with glucagon attenuated the cytokine response on both glucose output and NO formation in the dexamethasone-treated cells. This effect was antagonized by insulin. Inclusion of 1 mMNG-nitro-l-arginine methyl ester or 0.5 mMNG-monomethyl-l-arginine in the incubation abolished the increase in NO−2plus NO−3induced by the cytokine mixture and partially reversed the inhibitory effects on glucose mobilization in the presence of either insulin or glucagon, confirming the involvement of NO. In contrast the NO synthase inhibitors had little effect on either gluconeogenesis or glycogenolysis in the presence of dexamethasone alone, indicating that NO is only partially responsible for the inhibitory action of the cytokines, and the extent of its involvement depends upon the influence of other hormonal factors on the pathways. The antioxidant trolox also suppressed the inhibition of glucose release by the cytokines under conditions where nitric oxide synthase inhibitors were ineffective, suggesting that both reactive oxygen intermediates and NO may act as mediators, the relative importance of each depending upon the metabolic status of the cell.