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 TNF alpha-induced activation of the c-Jun N-terminal kinase and nuclear factor kappa B (NF-kappa B) pathways. Currently, TNF alpha-induced NF-kappa B 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 I kappa B kinase (IKK) activity and elevated expression of NF-kappa B-dependent genes in unstimulated conditions. Although TNF alpha-induced receptor-interacting protein 1 ubiquitination is indeed impaired in T2/5 DKO cells, TNF alpha stimulation further increases IKK activity in these cells, resulting in significantly elevated expression of NF-kappa B 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 TNF alpha-induced IKK activation to a level comparable with that seen in wild-type cells. This suggests that TNF alpha can activate IKK in the absence of TRAF2 and TRAF5 expression and receptor-interacting protein I ubiquitination. In addition, both the basal and TNF alpha-induced expression of anti-apoptotic proteins are normal in T2/5 DKO cells, yet these DKO cells remain sensitive to TNF alpha-induced cell death, due to the impaired recruitment of antiapoptotic 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 TNF alpha-induced cell death by recruiting antiapoptotic proteins to the TNFR1 complex rather than by activating the NF-kappa B pathway. (C) 2009 Elsevier Ltd. All rights reserved.