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Title: Elucidating the mechanisms of synergy between p53 agonists and histone deacetylase inhibitors in wild-type p53 colorectal cancer
Author: McIntyre, Alexander
Awarding Body: Queen's University Belfast
Current Institution: Queen's University Belfast
Date of Award: 2017
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The tumour-suppressive functions of the p53 transcription factor are compromised in 50% of colorectal cancers (CRCs) via mutation. In colorectal tumours harbouring wild-type p53, its function is circumvented through a variety of other mechanisms including over-expression of negative regulators MDM2 and Wip1. Recent successes have been made in the design of small-molecule inhibitors of the p53-MDM2 interaction such as Nutlins, which inhibit WTp53 ubiquitination. WTp53 protein stabilisation promotes cell cycle arrest but is however insufficient to induce cell death, suggesting the existence of tumour adaptations which prevent full activation of the pro-apoptotic functions of the p53 protein. Recent data demonstrated that apoptosis may be enhanced as a result of combining Histone Deacetylase inhibitors (HDACi) with DNA-damaging agents. It was hypothesised that such phenotypes are a result of full WTp53 reactivation. We reveal that HDAC inhibition synergistically induces apoptosis when combined with direct/indirect p53 agonists. Subsequently, cell death resulting from combination of p53 agonists with HDACi was found to be dependent on both the DNA-binding activities and acetylation of the p53 protein. Genome-wide analyses also revealed that addition of Class I HDACi Entinostat enhanced the number of transcripts significantly up/downregulated by p53 stabilisation. Surprisingly, few pro-apoptotic genes were synergistically induced by this co-treatment. In contrast, addition of Entinostat was found to attenuate transcription of several anti-apoptotic proteins by p53, notably the caspase-inhibitory protein FLIPL. Preventing FLIPL up-regulation with siRNA resulted in a synergistic increase in apoptosis, while the cooperation between p53 agonists and Entinostat was abrogated in cells over-expressing FLIP transgenes. These results uncover an ‘induced addiction’, whereby WTp53 upregulates FLIPL to increase the apoptotic signalling threshold in the presence of p53-primed cell death effectors. In summary, this work identifies a critical resistance mechanism acquired by colorectal tumours to inhibit apoptosis and promote alternative pro-survival functions of WTp53.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available