RIP1 Cleavage by Caspase-8 is Essential for TRAIL-induced NF-kB Activation

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Project Summary/Abstract: TRAIL, a ligand for death receptors (DRs), is considered a potential anti-cancer agent, as it shows selective high cytotoxicity toward tumor cells and little or no toxicity against normal cells. Currently, a recombinant TRAIL and agonistic antibodies directed at DRs are in phase-II clinical trials. However, recent studies have demonstrated that many types of cancer cells possess intrinsic or acquired resistance to TRAIL. Moreover, TRAIL application has been found to activate NF-kB and enhance metastasis in apoptosis-resistant cancer cells. Gene knockout studies have demonstrated that caspase-8 activity is essential not only for TRAIL-induced cell death, but also for TRAIL- induced NF-kB activation. At present, it is believed that fully activated caspase-8 induces apoptosis whereas partially activated caspase-8 activates NF-kB. However, the caspase-8 substrates that mediate this form of NF-kB activation have not been identified. We have identified RIP1 as a caspase-8 substrate that mediates TRAIL- induced NF-kB activation, discovered that caspase-8 cleaves RIP1 at three sites, and found that this cleavage is regulated in vivo by cFLIP. In apoptosis-sensitive cells, caspase-8 cleaves RIP1 at all three sites in response to TRAIL treatment, resulting in rapid RIP1 depletion and the induction of apoptosis; in apoptosis-resistant cells, however, TRAIL induces RIP1 cleavage mainly at one site, producing a constitutively active form of RIP1 (p60RIP1n) that activates the NF-kB pathway. Notably, overexpression of cFLIP is sufficient to trigger limited RIP1 cleavage and the accumulation of p60RIP1n. Importantly, in Hodgkin's lymphoma, cFLIP is overexpressed and a portion of RIP1 is constitutively processed to p60RIP1n. These data suggest that cFLIP-regulated, caspase- 8-mediated limited cleavage of RIP1 promotes NF-kB activation, and that such cleavage occurs constitutively in certain human cancers. These findings support our central hypothesis that cFLIP overexpression restricts TRAIL- induced caspase-8 activation to a moderate level, promoting RIP1 processing to p60RIP1n and, thereby, NF-kB activation. The objective of the proposed study is to evaluate the influence of cFLIP on caspase-8-mediated RIP1 cleavage, dissect the mechanisms by which RIP1 cleavage modulates NF-kB activation in response to TRAIL stimulation, and determine the pathological role of RIP1 cleavage in cancer cell resistance to TRAIL-induced apoptosis. To achieve these objectives, we propose to carry out the following specific aims: 1) determine the role of caspase-8-mediated RIP1 cleavage in promoting TRAIL-induced NF-kB activation versus cell death; 2) characterize the mechanisms by which caspase-8-mediated RIP1 cleavage activates NF-kB and inhibits cell death; 3) assess the pathophysiological relevance of RIP1 cleavage in cancer cell resistance to TRAIL-induced apoptosis. The proposed work will define the mechanisms that underlie TRAIL-induced NF-kB activation, and guide the development of strategies to maximize the effectiveness of TRAIL as an anti-cancer agent.