We have investigated the effects of TNF- on the production of extracellular matrix-degrading proteases in skeletal muscles. Using microarray, quantitative PCR, and zymography, we found that TNF- drastically increases the expression and production of MMP-9 from myotubes. In vivo administration of TNF- in mice also increased the transcript level of MMP-9 in skeletal muscle tissues. Although TNF- activated all the three MAPKs (i.e. ERK1/2, JNK, and p38), inhibition of ERK1/2 or p38 but not JNK blunted the TNF--induced production of MMP-9 from C2C12 myotubes. Inhibition of Akt also inhibited the TNF--induced production of MMP-9. TNF- increased the activation of NF-B transcription factor with a concomitant activation of IB kinase and phosphorylation of IB protein. TNF- also activated AP-1 but not SP-1 transcription factor in myotubes. Overexpression of a dominant-negative inhibitor of NF-B or AP-1 blocked the TNF--induced expression of MMP-9 in C2C12 myotubes. Similarly, point mutations in the AP-1 or NF-B binding sites in MMP-9 promoter inhibited the TNF--induced expression of a reporter gene. TNF- increased the activity of TGF--activating kinase-1 (TAK1). Furthermore, overexpression of a dominant-negative mutant of TAK1 blocked the TNF--induced expression of MMP-9 and activation of NF-B and AP-1. Our results also suggest that TNF- induces MMP-9 expression in C2C12 myotubes through the recruitment of TRAF-2, FADD, and TRADD but not NIK or TRAF-6 proteins. We conclude that TAK1-mediated pathways are involved in TNF--induced MMP-9 production in C2C12 myotubes. The study unveils a novel mechanism by which TNF-a could promote skeletal muscle destruction in pathological conditions.