Acute treatment with TNF-α attenuates insulin-stimulated protein synthesis in cultures of C₂C₁₂ myotubes through a MEK1-sensitive mechanism

Insulin and TNF-α exert opposing effects on skeletal muscle protein synthesis that are mediated in part by the rapamycin-sensitive mammalian target of rapamycin (mTOR) pathway and the PD-98059-sensitive, extracellular signal-regulated kinase (ERK) 1/2 pathway. The present study examined the separate and combined effects of insulin (INS), TNF, PD-98059, or dnMEK1 adenovirus on the translational control of protein synthesis in C2C12 myotubes. Cultures were treated with INS, TNF, PD-98059, dnMEK1, or a combination of INS + TNF with PD-98059 or dnMEK1. INS stimulated protein synthesis, enhanced eIF4E·eIF4G association, and eIF4G phosphorylation and repressed eIF4E·4E-BP1 association vs. control. INS also promoted phosphorylation of ERK1/2, S6K1, and 4E-BP1 and dephosphorylation of eIF4E. TNF alone did not have an effect on protein synthesis (vs. control), eIF4E·eIF4G association, or the phosphorylation of eIF4G, S6K1, or 4E-BP1, although it transiently increased ERK1/2 and eIF4E phosphorylation. When myotubes were treated with TNF + INS, the cytokine blocked the insulin-induced stimulation of protein synthesis. This appeared to be due to an attenuation of insulin-stimulated eIF4E·eIF4G association, because other stimulatory effects of INS, e.g., phosphorylation of ERK1/2, 4E-BP1, S6K1, eIF4G, and eIF4E and eIF4E·4E-BP1 association, were unaffected. Finally, treatment of myotubes with PD-98059 or dnMEK1 adenovirus before TNF + INS addition resulted in a derepression of protein synthesis and the association of eIF4G with eIF4E. These findings suggest that TNF abrogates insulin-induced stimulation of protein synthesis in myotubes through a decrease in eIF4F complex assembly independently of S6K1 and 4E-BP1 signaling and dependently on a MEK1-sensitive signaling pathway.