Abstract TWEAK, TNF-like weak inducer of apoptosis, is a relatively recently identified proinflammatory cytokine that functions through binding to Fn14 receptor in target cells. Although TWEAK has been shown to modulate several biological responses, the TWEAK-induced signaling pathways remain poorly understood. In this study, we tested the hypothesis that TAK1 (TGF-β-activated kinase 1) is involved in TWEAK-induced activation of NF-κB and MAPK and expression of proinflammatory protein. TWEAK increased the phosphorylation and kinase activity of TAK1 in cultured myoblast and fibroblast cells. The activation of NF-κB was significantly inhibited in TAK1-deficient (TAK1−/−) mouse embryonic fibroblasts (MEF) compared with wild-type MEF. Deficiency of TAK1 also inhibited the TWEAK-induced activation of IκB kinase and the phosphorylation and degradation of IκBα protein. However, there was no difference in the levels of p100 protein in TWEAK-treated wild-type and TAK1−/− MEF. Furthermore, TWEAK-induced transcriptional activation of NF-κB was significantly reduced in TAK1−/− MEF and in C2C12 myoblasts transfected with a dominant-negative TAK1 or TAK1 short interfering RNA. TAK1 was also required for the activation of AP-1 in response to TWEAK. Activation of JNK1 and p38 MAPK, but not ERK1/2 or Akt kinase, was significantly inhibited in TAK1−/− MEF compared with wild-type MEF upon treatment with TWEAK. TWEAK-induced expression of proinflammatory genes such as MMP-9, CCL-2, and VCAM-1 was also reduced in TAK1−/− MEF compared with wild-type MEF. Furthermore, the activation of NF-κB and the expression of MMP-9 in response to TWEAK involved the upstream activation of Akt kinase. Collectively, our study demonstrates that TAK1 and Akt are the important components of TWEAK-induced proinflammatory signaling and gene expression.