Abstract 2487: disrupting binding of p53 mutants to PEPD unleashes their tumor suppressor activities

Abstract

Abstract Background & Goals: The p53 tumor suppressor is one of the most important determinants of cell fate and is critical for tumor suppression. It functions by regulating the expression of a plethora of genes via sequence-specific DNA binding and also by transcription-independent, mitochondria-mediated mechanisms. Not surprisingly, the TP53 gene, which encodes p53, is frequently mutated in human cancer. For example, TP53 is mutated in about 75% of HER2-positive breast cancer and about 85% of triple negative breast cancer. The majority of TP53 mutations are missense mutations, changing one amino acid per mutant, most of which occur in its DNA-binding domain. p53 mutants show loss of tumor suppressor functions or gain of oncogenic functions. We recently found that peptidase D (PEPD), also known as prolidase, binds via its C-terminal sequence to the proline-rich domain (PRD) of wild type p53 and that disrupting the binding activates p53. The enzymatic activity of PEPD is not required for this function. Our objective in the present study is to determine whether PEPD also regulates p53 mutants. We focused on some of the most common oncogenic p53 mutants, including R175H, R248Q, R273H and R280K. Results: Our study shows that 1) each p53 mutant binds directly to PEPD and binds to PEPD in cells in a significant quantity, 2) disrupting their binding in cells by PEPD knockdown reactivates each p53 mutant, unleashing both transcription-dependent and -independent tumor suppressing activities, 3) after leaving PEPD, p53 mutants undergo acetylation, mono-ubiquitination and phosphorylation, which drive their refolding and reactivation, 4) Lysine 373 acetylation is essential for their reactivation, and 5) p53 mutants and p53WT show almost no difference in PEPD-binding and response to PEPD KD. Conclusions & Therapeutic Relevance: In conclusion, we have identified a cellular mechanism by which the tumor suppressor functions of p53 mutants are strongly reactivated, which is neither cell-specific nor mutant-specific. Mainly, p53 mutants bind to PEPD in cancer cells, and disrupting the complex causes posttranslational modifications of mutants freed from PEPD, which drive their refolding and unleash their tumor suppressor activities. This discovery advances our understanding of the function and regulation of p53 mutants and breaks new ground for developing novel cancer treatment strategies. Citation Format: Lu Yang. disrupting binding of p53 mutants to PEPD unleashes their tumor suppressor activities [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2487.