We have recently established a TNF-alpha-promoted metastasis model, in which the ability to metastasize to the lung was enhanced by stimulation of cultured colon 26 cells with TNF-alpha before intravenous inoculation. To investigate intracellular events in metastatic cascades of TNF-alpha-treated cancer cells, we have focused on the stress signaling pathways to c-Jun N-terminal kinase (JNK) and p38. Treatment with a specific inhibitor, SP600125 or SB203580, in vitro suppressed TNF-alpha-induced migration and pulmonary metastasis. Activation of endogenous TAK1, a mitogen-activated protein kinase (MAP3K) regulating the JNK and p38 MAPK pathways, was induced rapidly by TNF-alpha, and co-transfection of TAK1 with its activator protein TAB1 stimulated activation of JNK and p38 MAPKs, which led to activation of the transcription factor AP-1. The activation of stress signaling pathways by TAK1 resulted in enhanced migration to fibronectin in vitro and metastasis to the lung in vivo without affecting cell proliferation in vitro and tumor growth in vivo. Moreover, knockdown of endogenous TAK1 using small interfering RNA (siRNA) suppressed the TNF-alpha-induced JNK/p38 activation, migration and pulmonary metastasis. These results indicate that TAK1-mediated stress signaling pathways in cancer cells are essential for TNF-alpha-promoted metastasis to the lung.