The pseudo-caspase FLIP(L) regulates cell fate following p53 activation

Andrea Lees, Alexander J McIntyre, Nyree T Crawford, Fiammetta Falcone, Christopher McCann, Caitriona Holohan, Gerard P Quinn, Jamie Z Roberts, Tamas Sessler, Peter F Gallagher, Gemma M A Gregg, Katherine McAllister, Kirsty M McLaughlin, Wendy L Allen, Laurence J Egan, Aideen E Ryan, Melissa J Labonte-Wilson, Philip D Dunne, Mark Wappett, Vicky M CoylePatrick G Johnston, Emma M Kerr, Daniel B Longley, Simon S McDade

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p53 is the most frequently mutated, well-studied tumor-suppressor gene, yet the molecular basis of the switch from p53-induced cell-cycle arrest to apoptosis remains poorly understood. Using a combination of transcriptomics and functional genomics, we unexpectedly identified a nodal role for the caspase-8 paralog and only human pseudo-caspase, FLIP(L), in regulating this switch. Moreover, we identify FLIP(L) as a direct p53 transcriptional target gene that is rapidly up-regulated in response to Nutlin-3A, an MDM2 inhibitor that potently activates p53. Genetically or pharmacologically inhibiting expression of FLIP(L) using siRNA or entinostat (a clinically relevant class-I HDAC inhibitor) efficiently promoted apoptosis in colorectal cancer cells in response to Nutlin-3A, which otherwise predominantly induced cell-cycle arrest. Enhanced apoptosis was also observed when entinostat was combined with clinically relevant, p53-activating chemotherapy in vitro, and this translated into enhanced in vivo efficacy. Mechanistically, FLIP(L) inhibited p53-induced apoptosis by blocking activation of caspase-8 by the TRAIL-R2/DR5 death receptor; notably, this activation was not dependent on receptor engagement by its ligand, TRAIL. In the absence of caspase-8, another of its paralogs, caspase-10 (also transcriptionally up-regulated by p53), induced apoptosis in Nutlin-3A-treated, FLIP(L)-depleted cells, albeit to a lesser extent than in caspase-8-proficient cells. FLIP(L) depletion also modulated transcription of canonical p53 target genes, suppressing p53-induced expression of the cell-cycle regulator p21 and enhancing p53-induced up-regulation of proapoptotic PUMA. Thus, even in the absence of caspase-8/10, FLIP(L) silencing promoted p53-induced apoptosis by enhancing PUMA expression. Thus, we report unexpected, therapeutically relevant roles for FLIP(L) in determining cell fate following p53 activation.

Original languageEnglish
Pages (from-to)17808–17819
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number30
Early online date13 Jul 2020
Publication statusPublished - 28 Jul 2020


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