Interleukin 1β and interleukin 6, but not tumor necrosis factor α, inhibit insulin-stimulated glycogen synthesis in rat hepatocytes

Recent evidence indicates that inflammatory cytokines are involved in changes of blood glucose concentrations and hepatic glucose metabolism in infectious diseases, including sepsis. However, little is known regarding how cytokines interact with glucoregulatory hormones such as insulin. The objective of the present study is to investigate if and how cytokines influence insulin-stimulated glycogen metabolism in the liver. Interleukin 1β (IL-1β) and interleukin 6 (IL-6) markedly inhibited the increase of glycogen deposition stimulated by insulin in primary rat hepatocyte cultures; however, tumor necrosis factor α had no effect. Labeling experiments revealed that both cytokines counteracted insulin action by decreasing [¹⁴C]-glucose incorporation into glycogen and by increasing [¹⁴C]-glycogen degradation. Furthermore, it was discovered that IL-1β and IL-6 inhibited glycogen synthase activity and, in contrast, accelerated glycogen phosphorylase activity. In experiments with kinase inhibitors, serine/threonine kinase inhibitor K252a blocked IL-1β- and IL-6-induced inhibitions of glycogen deposition, as well as glycogen synthase activity, whereas another kinase inhibitor staurosporine blocked only IL-6-induced inhibition. Tyrosine kinase inhibitor herbimycin A blocked only IL-1β-induced inhibition. These results indicate that IL-1β and IL-6 regulate insulin-stimulated glycogen synthesis through different pathways involving protein phosphorylation in hepatocytes. They may mediate the change of hepatic glucose metabolism under pathological and even physiological conditions by modifying insulin action in vivo.