NF-kB mediates the protein loss induced by TNF-a in differentiated skeletal muscle myotubes

Li, Yi-Ping, and Micheal B. Reid. NF-kB mediates the protein loss induced by TNF-a in differentiated skeletal muscle myotubes. Am J Physiol Regulatory Integrative Comp Physiol 279: R1165–R1170, 2000.—Nuclear factor-kB (NFkB) regulates the transcription of a variety of genes involved in immune responses, cell growth, and cell death. However, the role of NF-kB in muscle biology is poorly understood. We recently reported that tumor necrosis factor-a (TNF-a) rapidly activates NF-kB in differentiated skeletal muscle myotubes and that TNF-a acts directly on the muscle cell to induce protein degradation. In the present study, we ask whether NF-kB mediates the protein loss induced by TNF-a. We addressed this problem by creating stable, transdominant negative muscle cell lines. C2C12 myoblasts were transfected with viral plasmid constructs that induce overexpression of mutant I-kBa proteins that are insensitive to degradation via the ubiquitin-proteasome pathway. These mutant proteins selectively inhibit NF-kB activation. We found that differentiated myotubes transfected with the empty viral vector (controls) underwent a drop in total protein content and in fast-type myosin heavy-chain content during 72 h of exposure to TNF-a. In contrast, total protein and fast-type myosin heavy-chain levels were unaltered by TNF-a in the transdominant negative cell lines. TNF-a did not induce apoptosis in any cell line, as assessed by DNA ladder and annexin V assays. These data indicate that NF-kB is an essential mediator of TNF-a-induced catabolism in differentiated muscle cells.