Molecular Mechanisms By Which Cflip Overexpression Regulates Trail-Induced Nf-Kappa B Activation And Lymphoma Survival

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Although recombinant TRAIL and agonistic antibodies directed against TRAILR1/2 are considered potential anti-cancer agents, their anti-cancer response in clinical trials was poor with the vast majority of patients showing no remission. In addition, TRAIL has been found to act as tumor promoters in certain contexts, increasing metastasis in apoptosis-resistant cancer cells by activating NF-κB. Therefore, defining the mechanism that permits TRAIL to activate NF-κB is critical for the development of strategies that maximize the potential effectiveness of TRAIL in clinical applications. We found that cFLIP-mediated and caspase-8-dependent cleavage of RIP1 at two sites, D180 in the kinase domain (KD) and D324 in the intermediate domain (ID), determines the activation state of the IKK/NF-κB pathway in response to TRAIL treatment. In apoptosis sensitive cells, caspase-8 cleaves RIP1 at KD and ID domains immediately after its recruitment to the receptor complex, impairing IKK recruitment and activation. In cFLIP-overexpressing cells, cFLIP restricts caspase-8 activity and RIP1 cleavage, generating a KD-cleaved RIP1 fragment that is capable of activating the NF-κB but not the apoptotic pathway. Of note, cFLIP expression, caspase-8 activity and RIP1 cleavage in KD per se are not directly involved in IKK activation; instead, they suppress RIP1-dependent activation of the apoptotic and necrotic cascades, and thereby ensure the integrity of the NF-κB pathway. Moreover, inflammatory ligands such as CD40L deplete cytoplasmic pool of TRAF2 and cIAP1/2, and promote RIP1 cleavage in lymphomas. Inhibition of RIP1 cleavage in KD suppresses NF-κB activation, and significantly increases TRAIL-induced cell death even in cFLIP-overexpressing lymphomas. Importantly, a portion of RIP1 is constitutively cleaved in many types of human and mouse B-cell lymphomas. These data suggest that, in addition to TRAF2- and cIAP1/2-mediated ubiquitination, cFLIP-regulated and caspase-8-dependent limited cleavage of RIP1 is a new layer of mechanism that promotes NF-κB activation and cancer cell survival. Citation Format: Hasem Habelhah, Lauren Workman, Laiqun Zhang. Molecular mechanisms by which cFLIP overexpression regulates TRAIL-induced NF-κB activation and lymphoma survival. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 2281. doi:10.1158/1538-7445.AM2014-2281