Objective: To investigate the effect of early insulin therapy on the nuclear factor kappaB (NF-kappaB) pathway and inflammatory cytokine responses in skeletal muscle in type 2 diabetes mellitus (DM).
Methods: SD rats underwent intraperitoneal injection of streptozotocin to establish DM models and then divided into 5 groups: early un-treated group, early gliclazide treated group (receiving gliclazide since the third day after blood glucose increase for 3 weeks for 3 weeks), early insulin treated group (receiving insulin since the third day after blood glucose increase for 3 weeks for 3 weeks), late un-treated group, and late insulin treated group (receiving insulin since the fourth week after blood glucose increase for 3 weeks). By the end of treatment the rats were killed. Homogenate of skeletal muscle was made. The NF-kappaB P65 DNA binding was assayed by ELISA-based assay kit. Real time PCR was used to detect the mRNA expression levels of the gene of the cytokines: glucose transporter 4 (Glut4), inhibitor kappaB (IkappaBalpha), IL-1beta, IL-6, and tumor necrosis factor (TNF)-alpha. And Glut4 and IkappaBalpha protein expression levels were assayed by Western blotting.
Results: The Glut4 mRNA level in the skeletal muscle of the untreated DM rats decreased by 59% and the Glut4 protein level in the muscle cell membrane decreased by 69%. Insulin treatment and gliclazide treatment increased the Glut4 mRNA expression by 17% and 13% respectively, increased the Glut4 protein expression in cell membrane by 23% and 10% respectively, and decreased the Glut4 protein expression in the cytoplasm. In the DM rats the IkappaBalpha protein expression in the skeletal muscle was significantly lower (P < 0.05) and the NF-kappaB P65 DNA binding activity increased, and TNF-alpha, IL-1B, and IL-6 expression levels were up-regulated in comparison with the normal control group. Early treatment of insulin and gliclazide increased the IkappaBalpha protein expression, decreased the NF-kappaB P65 DNA binding activity and the TNF-alpha expression in the skeletal muscle.
Conclusion: Early insulin treatment inhibits the NFkappaB activity and inflammatory cytokine responses in skeletal muscle that are involved in the amelioration of insulin resistance in type 2 DM. Such results may be due to indirect antiinflammatory effects of insulin relieving glucotoxicity and lipotoxicity in peripheral tissues.