NF-kappa B mediates the protein loss induced by TNF-alpha in differentiated skeletal muscle myotubes

Nuclear factor-kappa B (NF kappa B) regulates the transcription of a variet y of genes involved in immune responses, cell growth, and cell death. Howev er, the role of NF-kappa B in muscle biology is poorly understood. We recen tly reported that tumor necrosis factor-alpha (TNF-alpha) rapidly activates NF-kappa B in differentiated skeletal muscle myotubes and that TNF-alpha a cts directly on the muscle cell to induce protein degradation. In the prese nt study, we ask whether NF-kappa B mediates the protein loss induced by TN F-alpha. We addressed this problem by creating stable, transdominant negati ve muscle cell lines. C2C12 myoblasts were transfected with viral plasmid c onstructs that induce overexpression of mutant I-kappa B alpha proteins tha t are insensitive to degradation via the ubiquitin-proteasome pathway. Thes e mutant proteins selectively inhibit NF-kappa B 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-cha in content during 72 h of exposure to TNF-alpha. In contrast, total protein and fast-type myosin heavy-chain levels were unaltered by TNF-alpha in the transdominant negative cell lines. TNF-alpha did not induce apoptosis in a ny cell line, as assessed by DNA ladder and annexin V assays. These data in dicate that NF-kappa B is an essential mediator of TNF-alpha-induced catabo lism in differentiated muscle cells.