Skeletal muscle myocytes undergo protein loss and reactive oxygen-mediated NF-κB activation in response to tumor necrosis factor α

Skeletal muscle atrophy and weakness are thought to be stimulated by tumor necrosis factor a (TNF-alpha) in a variety of chronic diseases. However, little is known about the direct effects of TNF-alpha on differentiated skeletal muscle cells or the signaling mechanisms involved, We have tested the effects of TNF-alpha on the mouse-derived C2C12 muscle cell line and on primary cultures from rat skeletal muscle. TNF-alpha treatment of differentiated myotubes stimulated time- and concentration-dependent reductions in total protein content and loss of adult myosin heavy chain (MHCf) content; these changes were evident at low TNF-alpha concentrations (1-3 ng/ml) that did not alter muscle DNA content and were not associated with a decrease in MHCf synthesis. TNF-alpha activated binding of nuclear factor KB (NF-kappa B) to its targeted DNA sequence and stimulated degradation of I-kappa B alpha, an NF-kappa B inhibitory protein. TNF-alpha stimulated total ubiquitin conjugation whereas a 26S proteasome inhibitor (MG132 10-40 mu M) blocked TNF-alpha activation of NF-kappa B, Catalase 1 kU/ml inhibited NF-kappa B activation by TNF-alpha; exogenous hydrogen peroxide 200 mu M activated NF-kappa B and stimulated I-kappa B alpha degradation. These data demonstrate that TNF-alpha directly induces skeletal muscle protein loss, that NF-kappa B is rapidly activated by TNF-alpha in differentiated skeletal muscle cells, and that TNF-alpha/NF-kappa B signaling in skeletal muscle is regulated by endogenous reactive oxygen species.-Li, Y.-P., Schwartz, R. J., Waddell, I. D., Holloway, B. R., Reid, M. B. Skeletal muscle myocytes undergo protein loss and reactive oxygen-mediated NF-kappa B activation in response to tumor necrosis factor alpha.