p53 is the most frequently mutated tumor suppressor gene in human cancers. Unlike other tumor-suppressor genes, p53 mutations mainly occur as missense mutations within the DNA-binding domain, leading to the expression of full-length mutant p53 protein. Mutant p53 proteins not only lose their tumor suppressor function, but may also gain new oncogenic function and promote tumorigenesis through transactivation of new target genes. By using an RNA interference (RNAi) approach, we show that silencing of endogenous p53 R273H contact mutant in MDA-MB-468 cells induced massive apoptosis while no apoptosis was observed in the silencing of the p53 R175H structural mutant in SKBR3 cells. Further analysis shows that the apoptotic effects of mutant p53 knock-down involved activation of caspase-9 and mitochondria depolarisation, suggesting a role for mutant p53 in suppressing the intrinsic apoptotic pathway. Indeed, a BCL-2 family member, BCL-2 modifying factor (BMF), was reproducibly up-regulated following knock-down of mutant p53 in MDA-MB-468 cells while no significant changes was observed in other BCL2 family members. Furthermore, we show that p53 R273H contact mutant, but not p53 R175H conformation mutant, suppressed anoikis by down-regulation of BMF expression. Anoikis is a form of apoptosis induced by cell detachment and is a major mechanism to prevent metastasis. Further analyses using microarray gene profiling and connectivity map analysis show that silencing of mutant p53 in MDA-MB-468 cells generates gene expression signatures characteristic of PI3K inhibitors, suggesting that apoptotic effects of mutant p53 knock-down involved dephosphorylation of AKT, a direct substrate of PI3K. Ectopic expression of the activated form of AKT in MDA-MB-468 completely abrogated the apoptotic effects of mutant p53 knock-down. In summary, our results demonstrate that R273H mutant p53 promotes anoikis resistance in breast cancer cells that express it through suppression of BMF expression by the constitutively activated AKT pathway.
Citation Format: Boon Shing Tan, Kai Hung Tiong, Ivan Kok Seng Yap, Rozita Rosli, Soon-Keng Cheong, Chee-Onn Leong. p53 mutant regulates a network of cellular anoikis through activation of AKT and suppression of BMF. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 1551. doi:10.1158/1538-7445.AM2014-1551