Abstract 2409: FLIP(L) determines colon cancer cell fate following p53 activation
Authors: Lees Andrea , McIntyre Alexander J. McIntyre J. , Crawford Nyree T. , Falcone Fiammetta , McCann Chris , Quinn Gerard P. , Roberts Jamie Z. , Sessler Thomas , Gallagher Peter F. , Gregg Gemma M. , McAllister Katherine , McLaughlin Kirsty M. , Allen Wendy L. , Holohan Caitriona , Egan Laurence J. , Ryan Aideen E. , Labonte-Wilson Melissa , Dunne Phillip D. , Wappett Mark , Coye Vicky M. , Johnston Patrick G. , Kerr Emma M. , Longley Daniel B. , McDade Simon S. 
Published: September 2020

Abstract
Background: How p53 differentially activates cell cycle arrest versus cell death remains poorly understood. Understanding the mechanisms governing the switch from p53-induced cell-fates is important for optimising efficacy of p53-activating therapies, such as DNA damaging chemotherapy and radiotherapy. HDAC inhibitors (HDACi) have potential to enhance p53 induced cell death through enhancing p53 activation and altering regulation of other cell death regulatory proteins.
Methods: We utilised a panel of matched p53 wild-type and deficient colorectal cancer cell line models to the potential for HDACi to augment cell death induced by direct and indirect p53 activating agents. A number of molecular (Western blot, RT-PCR), phenotypic (cell death) and functional genomic (RNA-seq, CRISPR, ChIP-seq) analyses were used to investigate the importance of p53 and its downstream transcriptional programs.
Results: Here we report that upregulation of canonical pro-apoptotic p53 target genes in colon cancer cells imposes critical dependence on the long splice form of the caspase-8 regulator FLIP (FLIP(L)), which we identify as a direct p53 transcriptional target. Inhibiting FLIP(L) expression with siRNA or Class-I HDAC inhibitors promotes apoptosis in response to p53 activation by the MDM2 inhibitor Nutlin-3A, which otherwise predominantly induces cell-cycle arrest. When FLIP(L) upregulation is inhibited, apoptosis is induced in response to p53 activation via a ligand-independent TRAIL-R2/caspase-8 complex, which is distinct from the ligand-dependent DISC. Notably, FLIP(L) depletion inhibits p53-induced expression of the cell cycle regulator p21 and enhances p53-mediated upregulation of PUMA, with the latter activating mitochondrial-mediated apoptosis in FLIP(L)-depleted, Nutlin-3A-treated cells lacking TRAIL-R2/caspase-8.
Conclusion: Acute p53-mediated transcriptional upregulation of FLIP(L) plays an unexpected nodal role in determining cell fate following p53 activation. This is mediated through two previously undescribed mechanisms, preventing apoptosis by a ligand-independent TRAIL-R2 complex and by suppressing expression of pro-apoptotic PUMA. Which, importantly imposes a critical dependence on FLIP(L) which can be overcome through combinations with class-I HDAC inhibitors such as Entinostat.
Citation Format: Andrea Lees, Alexander J. McIntyre J. McIntyre, Nyree T. Crawford, Fiammetta Falcone, Chris McCann, Gerard P. Quinn, Jamie Z. Roberts, Thomas Sessler, Peter F. Gallagher, Gemma M. Gregg, Katherine McAllister, Kirsty M. McLaughlin, Wendy L. Allen, Caitriona Holohan, Laurence J. Egan, Aideen E. Ryan, Melissa Labonte-Wilson, Phillip D. Dunne, Mark Wappett, Vicky M. Coye, Patrick G. Johnston, Emma M. Kerr, Daniel B. Longley, Simon S. McDade. FLIP(L) determines colon cancer cell fate following p53 activation [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2409.