Abstract 4385: p53 activation induces a targetable dependence on FLIPL

Abstract

Abstract Background The tumor suppressive functions of the p53 transcription factor are inactivated via mutations or suppressed through non-mutational mechanisms in almost all cancer cells. A better understanding of the mechanisms through which p53 differentially regulates cell cycle arrest and cell death is important to maximize benefits from wild-type p53-dependent therapeutic strategies Methods A panel of matched p53 wild-type and deficient colorectal cancer cell line models were studied, using Nutlin-3A and Oxaliplatin as direct and indirect p53 activating agents respectively. A number of molecular (Western blot, RT-PCR), phenotypic (cell death) and genomic analyses were used to investigate the importance of p53 and its downstream transcriptional programs. Results Here, we report that activation of pro-apoptotic p53 targets in colorectal cancer cells imposes a critical targetable dependence on the long splice form of the caspase-8 regulator FLIP (FLIPL) for survival. p53 binds the promoter of the FLIP gene (CFLAR) and upregulates FLIPL expression in response to the p53 agonist Nutlin-3A in a manner dependent on HDAC1/2/3 activity. As such, preventing FLIPL upregulation with the clinically relevant HDAC1/2/3-selective inhibitor Entinostat promotes apoptosis induction in response to Nutlin-3A (or p53-activating chemotherapy), which otherwise predominantly induces growth arrest despite upregulating a range of pro-apoptotic target genes. Cell death in response to Nutlin-3A in FLIPL-depleted cells is primarily mediated via caspase-8. However, in the absence of caspase-8, apoptosis is delayed, but not prevented and is mediated via caspase-10. Of note, the cell death induced in both caspase-8-proficient and -deficient cells is mediated via TRAIL-R2 in a ligand-independent manner. Conclusion In summary, this work has uncovered novel, clinically-relevant biology, in which p53-mediated upregulation of FLIPL primes cells for TRAIL-R2-mediated apoptosis and identifies FLIPL as a key target for overcoming resistance to p53-stabilising agents in p53 wild-type cancers. Moreover, we show the potential of combining Nutlin-3A (or other p53 activating chemotherapies) with the clinically relevant Class I HDAC inhibitor Entinostat for the treatment of p53 wild-type CRC, and identify FLIPL as a critical p53-induced signaling node, the inhibition of which is necessary to promote Nutlin-3A-induced apoptosis. Citation Format: Alexander McIntyre, Andrea Lees, Fiammetta Falcone, Gemma Gregg, Sessler Tamas, Gerard Quinn, Nyree Crawford, Darragh McArt, Phillip Dunne, Mark Lawler, Longley B. Daniel, Simon S. McDade. p53 activation induces a targetable dependence on FLIPL [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4385.