TNF-alpha impairs insulin signaling and insulin stimulation of glucose uptake in C2C12 muscle cells.
Physiological stressors such as sepsis and tissue damage initiate an acute immune response and cause transient systemic insulin resistance. This study was conducted to determine whether tumor necrosis factor-alpha (TNF-alpha), a cytokine produced by immune cells during skeletal muscle damage, decreases insulin responsiveness at the cellular level. To examine the molecular mechanisms associated with TNF-alpha and insulin action, we measured insulin receptor substrate (IRS)-1- and IRS-2-mediated phosphatidylinositol 3-kinase (PI 3-kinase) activation, IRS-1-PI 3-kinase binding, IRS-1 tyrosine phosphorylation, and the phosphorylation of two mitogen-activated protein kinases (MAPK, known as p42(MAPK) and p44(MAPK)) in cultured C2C12 myotubes. Furthermore, we determined the effects of TNF-alpha on insulin-stimulated 2-deoxyglucose (2-DG) uptake. We observed that TNF-alpha impaired insulin stimulation of IRS-1- and IRS-2-mediated PI 3-kinase activation by 54 and 55% (P < 0.05), respectively. In addition, TNF-alpha decreased insulin-stimulated IRS-1 tyrosine phosphorylation by 40% (P < 0.05). Furthermore, TNF-alpha repressed insulin-induced p42(MAPK) and p44(MAPK) tyrosine phosphorylation by 81% (P < 0.01). TNF-alpha impairment of insulin signaling activation was accompanied by a decrease (P < 0.05) in 2-DG uptake in the muscle cells (60 +/- 4 vs. 44 +/- 6 pmol. min-1. mg-1). These data suggest that increases in TNF-alpha may cause insulin resistance in skeletal muscle by inhibiting IRS-1- and IRS-2-mediated PI 3-kinase activation as well as p42(MAPK) and p44(MAPK) tyrosine phosphorylation, leading to impaired insulin-stimulated glucose uptake.