Recently, the relationship between inflammation and cancer has become more widely accepted. The pro-inflammatory cytokine, TNF-α has been known to play crucial roles in tumorigenesis and tumor progression, regulating a cascade of cytokines, chemokines, adhesion molecules, extracellular proteases and pro-angiogenic molecules. We have recently established a TNF-α-promoted metastasis model, in which the ability to metastasize to the lung was enhanced by stimulation of cultured colon 26 cells with TNF-α before intravenous inoculation. To investigate intracellular events in metastatic cascades of TNF-α-treated cancer cells, we have focused on the stress signaling pathways to JNK and p38, these are reported to be related to metastatic properties including migration, invasion, and expression of MMPs and uPA in some carcinomas. Treatment with their specific inhibitor, SP600125 (JNK) or SB203580 (p38), in vitro completely suppressed TNF-α induced migration to fibronectin in a dose-dependent manner. In addition, these inhibitors completely abrogated the increased pulmonary metastasis of TNF-α-treated cells. There are several MAP3Ks regulating JNK/p38 signaling pathways such as TAK1, ASK1, MEKK1 and MLK3. Recently, we have shown that knockdown of endogenous TAK1 by RNA interference significantly blocked the TNF-α-induced, rapid activation of JNK/p38 in HeLa cells, in which phosphorylation at Thr-187 is involved in the TNF-α-induced activation of TAK1. Therefore, we focused on TAK1 to characterize the functional significance of JNK/p38 activation and cytokine-induced metastasis of murine colon cancer cells. Activation of endogenous TAK1 in colon 26 cells was induced rapidly by TNF-α, and phosphorylation of its activator protein TAB1 and TAB2 was also detected. Co-transfection of TAK1 and TAB1 stimulated activation of JNK and p38 MAPKs, which led to activation of the transcription factor AP-1. Twenty-four hours after transfection, 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 suppressed the TNF-α-induced JNK/p38 activation, migration and pulmonary metastasis. These results indicate that TAK1-mediated stress signaling pathways in cancer cells are essential for TNF-α-promoted metastasis.

[Proc Amer Assoc Cancer Res, Volume 47, 2006]