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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