Decreased secretion of tumor necrosis factor-α attenuates macrophages-induced insulin resistance in skeletal muscle

Aims: Macrophages, as an important member of immune system, engulf and digest pathogens in innate immunity and help initiate adaptive immunity. However, macrophages also involve in occurrence and development of many diseases, such as obesity and type 2 diabetes. Here, we aimed to reveal how activated macrophages cause insulin resistance in skeletal muscle in vitro through simulating body environment. Main methods: We established RAW264.7 macrophages and C2C12 myotubes co-incubation model in vitro using Transwell filter to simulate body environment and investigated effects of RAW264.7 cells on insulin-regulated glucose metabolism in C2C12 myotubes. Immunofluorescence, Immunoblot and glucose uptake tests were used to assess metabolic changes in C2C12 myotubes. ELISA test detected secretions of tumor necrosis factor alpha (TNF-alpha) and interleukin-6 (IL-6) from RAW264.7 cells. In addition, RNA interference and inhibitor treatment were used. Key findings: Activated RAW264.7 cells attenuated insulin response in C2C12 myotubes. Activated RAW264.7 cells secreted a lot of TNF-alpha and IL-6. We found that TNF alpha, but not IL-6, caused insulin resistance of skeletal muscle in a dose-dependent manner. The results further indicated that activation of TNF-alpha downstream proteins, inhibitor of nuclear factor kappa-B kinase (IKK) and the jun-N-terminal kinase 1 (JNK1) led to phosphorylation of insulin receptor substrate 1 (IRS-1) at Ser residues and insulin resistance in C2C12 myotubes. Significance: Our research provided further and direct demonstration on activated macrophage-induced insulin resistance in skeletal muscle, suggesting TNF-alpha might become a therapeutic target to ameliorate and treat type 2 diabetes.