p53 activation induces targetable dependence on FLIPL

A better understanding of how p53 differentially activates cell cycle arrest or cell death is important to maximize benefits of therapeutic strategies dependant by wild-type p53. Here, we report that activation of pro-apoptotic p53 transcriptional targets in colorectal cancer cells imposes a critical, targetable dependence on the long splice form of the caspase-8 regulator FLIP (FLIPL) for survival. Upon Nutlin-3A induced stabilisation p53 directly upregulates FLIPL expression in a manner dependent on Class-I HDAC activity. Preventing FLIPL upregulation with the clinically relevant Class-I selective inhibitor Entinostat promotes apoptosis in response to Nutlin-3A , which 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 mediated through two of p53's canonical transcriptional targets TRAIL-R2 and BAX and is caspase-8-dependent. This work uncovers novel, clinically relevant biology that identifies FLIPL as a key target for overcoming resistance to p53-activating agents. mRNA-seq analysis of treatment of HCT116 colorectal cancer cell line models isogenic for p53 treated with Nutlin-3A or Entinostat or their combination