TRAF2 Ring Domain Plays An Essential Role in Inhibition of TNFα-Induced Cell Death Independent of NF-kB Activation.

Abstract 3621

Poster Board III-557

The proinflammatory cytokine TNFα regulates immune responses, inflammation and programmed cell death by activating the JNK/c-Jun and IKK/NF-κB signaling pathways. TRAF2 and RIP1 play critical roles in TNFα-induced activation of JNK and IKK, and inhibition of apoptosis. In the field, TRAF2 RING domain-mediated RIP1 ubiquitination is believed to play an essential role in TNFα-induced IKK activation, and the RING domain-deleted TRAF2 (TRAF2-ΔN) has been widely used as a dominant negative to block TNFα-induced JNK and IKK activation in transient overexpression systems. Here, we report that stable expression of TRAF2-Δ[N in TRAF2 and TRAF5 double knockout (T2/5 DKO) cells at a physiological level almost completely restores TNFα-induced IKK activation, but does not restore RIP1 ubiquitination. In addition, stable expression of TRAF2-ΔN in T2/5 DKO cells efficiently inhibited TNFα-induced later-phase prolonged JNK activation, but failed to inhibit TNFα-induced cell death. The basal and inducible expression of antiapoptotic proteins in TRAF2-ΔN-expressing cells is totally normal, yet these cells are still sensitive to TNFα-induced cell death due to the inefficient recruitment of cIAP1 to the TNFR1 complex. Moreover, stable expression of TRAF2-WT, but not TRAF2-ΔN, in T2/5 DKO cells suppressed constitutive p100 processing. On the other hand, TRAF2-ΔN-expressing cells exhibited significant resistance to oxidative stress-induced cell death compared to TRAF2-WT-expressing cells. These data suggest that: i) the TRAF2 RING domain plays a critical role in the inhibition of cell death induced by TNFα, and is essential for suppression of p100 processing in unstimulated cells; ii) RIP1 polyubiquitination is not required for TNFα-induced IKK activation, but may play a role in inhibition of TNFα-induced cell death; and iii) prolonged JNK activation has no obligate role in TNFα-induced cell death.