Transforming growth factor-beta 1 (TGF-beta1) signaling in tumor cells has been implicated in tumor angiogenesis and metastasis by regulating matrix proteolysis. Although MMP-9/gelatinase-B is an important component of these TGF-beta1 responses, the mechanism of its regulation is not well understood. Here, we present evidence that TGF-beta-activated protein kinase 1 (TAK1) is critical for TGF-beta regulation of MMP-9 and the metastatic potential of breast cancer cell line MDA-MB-231. We found that suppression of TAK1 signaling by dominant-negative (dn) TAK1 or RNA interference (siRNA) reduces expression of MMP-9 and tumor cell invasion, without growth inhibition in cell culture. The orthotopic xenograft studies in SCID mice showed that suppression of TAK1 signaling by dn-TAK1 reduces tumor growth and formation of lung metastases. Dn-TAK1 reduced the proliferation Ki-67 index and neovasculature of orthotopic xenografts. TAK1-mediated regulation of MMP-9 involves NF-kappaB signaling. Dn-TAK1 reduces NF-kappaB transcriptional response and inhibition of NF-kappaB reduces expression of MMP-9 and activity of the MMP-9 promoter reporter. Together, these findings suggest that TAK1 contributes to TGF-beta1-mediated tumor angiogenesis and metastasis via a mechanism involving the TAK1-NF-kappaB-MMP-9 pathway.