TRAF2 Suppresses Basal IKK Activity in Resting Cells and TNFα Can Activate IKK in TRAF2 and TRAF5 Double Knockout Cells

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) and TRAF5 are adapter proteins involved in TNFα-induced activation of the c-Jun N-terminal kinase and nuclear factor κB (NF-κB) pathways. Currently, TNFα-induced NF-κB activation is believed to be impaired in TRAF2 and TRAF5 double knockout (T2/5 DKO) cells. Here, we report instead that T2/5 DKO cells exhibit high basal IκB kinase (IKK) activity and elevated expression of NF-κB-dependent genes in unstimulated conditions. Although TNFα-induced receptor-interacting protein 1 ubiquitination is indeed impaired in T2/5 DKO cells, TNFα stimulation further increases IKK activity in these cells, resulting in significantly elevated expression of NF-κB target genes to a level higher than that in wild-type cells. Inhibition of NIK in T2/5 DKO cells attenuates basal IKK activity and restores robust TNFα-induced IKK activation to a level comparable with that seen in wild-type cells. This suggests that TNFα can activate IKK in the absence of TRAF2 and TRAF5 expression and receptor-interacting protein 1 ubiquitination. In addition, both the basal and TNFα-induced expression of anti-apoptotic proteins are normal in T2/5 DKO cells, yet these DKO cells remain sensitive to TNFα-induced cell death, due to the impaired recruitment of anti-apoptotic proteins to the TNFR1 complex in the absence of TRAF2. Thus, our data demonstrate that TRAF2 negatively regulates basal IKK activity in resting cells and inhibits TNFα-induced cell death by recruiting anti-apoptotic proteins to the TNFR1 complex rather than by activating the NF-κB pathway.