Abstract 269: Reactivation of mutant p53 in human prostate cancer cells as a critical mechanism for inhibition of tumor growth by phenethyl isothiocyanate

Mutations in the p53 gene occur in a wide variety of human cancers at remarkably high frequencies. Prostate cancer (PCa), the most commonly diagnosed cancer and the second leading cause of cancer-related death in men in the United States, show a significantly high frequency of p53 mutations (~30%-70%). However, the role of mutant p53 as a target for the treatment or prevention of PCa remained to be investigated. In this study, we showed that cruciferous vegetable-derived phenethyl isothiocyanate (PEITC) can reactivate p53 mutants, irrespective of its site of mutation, in PCa cells in vitro. Importantly, this study showed that dietary supplementation of PEITC can significantly inhibit DU145 p53 mutant xenograft tumor in a nu/nu Balb/c mice. Interestingly, the animals in the PEITC-treated group showed a significant decrease in the mutant p53 expression levels as compared to the animals in the control group. DU145 cells harboring p53V274F/P223L mutant display maximum sensitivity to PEITC-induced growth inhibition as compared to the wild-type p53 LNCaP and p53 null PC-3 cells. PEITC treatment also caused a significant decrease in the p53 expression level in DU145 cells. No significant inhibition of cell proliferation was detected for the normal prostate epithelial RWPE1 cells treated with PEITC. Mechanistic studies revealed that PEITC induces a conformation change and restores the transactivation functions to the p53 mutant. Accordingly, in PEITC-treated cells the reactivated p53 mutant(s) induces apoptosis by activating canonical wild-type p53 targets, inducing a G1 and S phase delay and by phosphorylating ATM. PEITC treatment of isogenic PC-3 p53 null cells revealed that PC-3 cells expressing p53P223L mutant exhibit maximum inhibition of cell proliferation, suggesting that PEITC acts in a p53P223L mutant-dependent manner. Furthermore, PEITC treatment restores the transactivation functions to p53P223L mutant as shown by the activation of p53 regulated downstream target genes. Interestingly p73, a member of the p53 gene family that shares high functional homology and regulate cell-growth/ cell-death pathways in a manner similar to p53, does not play a role in p53-mediated pathway activation in PEITC treated cells. We further showed that PEITC inhibits cell proliferation and induces apoptosis in prostate cancer cells with hotspot mutants R248W (VCaP) and R175H (LAPC-4). Importantly, PEITC can induce a conformation change, reactivates p53 mutants and induces G1 and S-phase delays in these cells. These results demonstrate that PEITC can reactivate mutant p53, irrespective of the site of p53 mutation, in human prostate cancer cells. These studies provide the first example of targeting mutant p53 in prostate cancer cells by a dietary-related compound that may have impact on the prevention and treatment strategies of this disease. Citation Format: Monika Aggarwal, Rahul Saxena, Nasir Asif, Elizabeth Sinclair, Judy Tan, Idalia Cruz, Deborah Berry, Bhaskar Kallakury, Quynhchi Pham, Thomas Ty Wang, Fung-Lung Chung. Reactivation of mutant p53 in human prostate cancer cells as a critical mechanism for inhibition of tumor growth by phenethyl isothiocyanate [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 269.