p53 activation induces targetable dependence on FLIPL.
Session type: Poster / e-Poster / Silent Theatre session
The mechanisms through which the tumour suppressor p53 differentiates between activating cell cycle arrest or apoptosis and the underlying transcriptional programs responsible for this decision-making process remain poorly understood. This work focuses on identifying mechanisms through which tumours retaining wild-type p53 evade apoptosis.
A panel of matched p53 wild-type and deficient colorectal cancer cell lines were studied using Nutlin-3A as a direct p53 activating agent. The HDAC dependency of the p53 response was assessed using the clinically relevant Class-I selective HDAC inhibitor, Entinostat. Molecular (Western blot, RT-PCR), phenotypic (Flow cytometry, High Content Microscopy) and genomic analyses were used to investigate the role of p53 and its downstream transcriptional programs.
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 Entinostat promotes apoptosis in response to Nutlin-3A, which otherwise predominantly induces growth arrest despite upregulation 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.
These results uncover a p53-regulated apoptosis priming event in which key pro-apoptotic genes are up-regulated in response to p53 stabilisation. However, immediate commitment to apoptosis is prevented by the concomitant p53-mediated up-regulation of FLIPL.From a cancer therapeutics point of view, this work uncovers a novel mechanism for the synergy previously reported between DNA damage and HDACi, while identifying FLIPLas a key target for overcoming resistance to p53-stabilising agents in cancers harbouring the WTp53 allele.