Inflammatory cytokines inhibit myogenic differentiation through activationof nuclear factor-kappa B

Muscle wasting is often associated with chronic inflammation, Because tumor necrosis factor alpha (TNF-alpha) has been implicated as a major mediator of cachexia, its effects on C2C12 myocytes were examined, TNF-alpha activat ed nuclear factor-kappaB (NF-kappaB) and interfered with the expression of muscle proteins in differentiating myoblasts, Introduction of a mutant form of inhibitory protein kappaB alpha (I kappaB alpha) restored myogenic diff erentiation in myoblasts treated with TNF-alpha or interleukin 1 beta, Conv ersely, activation of NF-kappaB by overexpression of I kappaB kinase was su fficient to block myogenesis, illustrating the causal link between NF-kappa B activation and inhibition of myogenic differentiation. The inhibitory eff ects of TNF-alpha on myogenic differentiation were reversible, indicating t hat the effects of the cytokine were not due to nonspecific toxicity, Treat ment of differentiated myotubes with TNF-alpha did not result in a striking loss of muscle-specific proteins, which shows that myogenesis was selectiv ely affected in the myoblast stage by TNF-alpha, An important finding was t hat NF-kappaB was activated to the same extent in differentiating and diffe rentiated cells, illustrating that once myocytes have differentiated they b ecome refractory to the effects of NF-kappaB activation, These results demo nstrate that inflammatory cytokines may contribute to muscle wasting throug h the inhibition of myogenic differentiation via a NF-kappaB-dependent path way.