TRAF6 negatively regulates TNFa-induced NF-kB activation

TNF receptor-associated factor 6 (TRAF6) is an essential adaptor protein for the Interleukin-1 (IL-1) signaling pathway; however, its role in the signaling of another proinflammatory cytokine, tumor necrosis factor a (TNFa, has not been explored. Interestingly, we observed that TNFa-induced expression of IL-6, CXCL1 and granulocyte macrophage colony stimulating factor (GM-CSF) were significantly enhanced in TRAF6-deficient MEFs. Compared to those observed in wild-type MEFs, TNFa-induced IkB kinase (IKK) activation and IkBa degradation were enhanced in TRAF6-deficient MEFs. Also, TNFa-induced DNA binding activity and transcriptional activation of nuclear factor kappaB (NF-kB) were also augmented in TRAF6-deficient MEFs. On the other hand, TRAF6 deficiency did not affect the TNFa-induced activation of mitogen-activated protein (MAP) kinases, ERK, JNK, and p38. Moreover, the reintroduction of exogenous TRAF6 into TRAF6-deficient MEFs clearly suppressed TNFa-induced IKK activation, NF-kB activation and subsequent cytokine expression. In contrast, both the deletion mutant (N) and the point mutant (C70A) of TRAF6, which is defective in its ubiquitin ligase activity, failed to repress TNFa-induced IKK activation, NF-kB activation and cytokine production. Thus, these data suggest that TRAF6 negatively regulates TNFa-induced NF-kB activation through its ubiquitin ligase activity.