Abstract 1173: AKT-mTOR pathway mediates mutant p53 gain-of-function by inhibiting autophagy

Abstract

Abstract Nearly half of human cancers harbor p53 mutations, which are predominantly through missense mutations that result in accumulation of mutant p53 proteins in cancer cells. Persistent evidence supports that the mutant p53 proteins not only lose tumor suppressor activity, but also acquire gain-of-function abilities to promote carcinogenesis, metastasis, tumor recurrence and chemoresistance. Indeed, knock-in mouse models bearing the mutant p53R172H (equivalent to human R175H mutation) at physiological level in vivo display gain-of-function phenotypes, such as enhanced cell transformation potentials and tumor metastasis. However, the molecular mechanism of mutated p53R172H conferring gain-of-function remains poorly understood. In this study, utilizing mouse embryonic fibroblasts (MEFs) and lung adenocarcinoma cells (93-1) derived from p53R172H knock-in mice, we observed that AKT was preferentially activated in p53R172H mutant cells over p53-deficient cells. This activation of AKT allows p53R172H mutant cells to bypass contact inhibition, grow in an anchorage-independent manner and form tumors after subcutaneous injection into SCID mice. In contrast, expression of dominant negative AKT or shRNA-mediated AKT knockdown in p53R172H mutant cells inhibited colony formation in soft-agar and suppressed tumor growth in mouse xenograft tumor model. To directly dissect the essential role of AKT in p53R172H mutant-initiated tumorigenesis in vivo, we bred AKT1 knockout mice to p53R172H mutant knock-in mice. Loss of AKT1 significantly reduced the incidence of tumor development and extended the life span from around 3 months in AKT1-proficient and p53R172H mutant mice (AKT1+/+; p53R172H/R172H) to about 12 months in AKT1-deficient and p53R172H mutant mice (AKT1−/−; p53R172H/R172H). To understand how the activated AKT conferred mutant p53 gain-of-function, we demonstrated that p53R172H mutant activated AKT-mTOR signaling pathway that led to an inhibition of autophagy both in vitro and in vivo. Furthermore, induction of autophagy in p53R172H mutant lung cancer cells through suppressing AKT-mTOR pathway pharmacologically by the dual kinase inhibitor NVP-BEZ235 treatment or genetically by RNAi-mediated knockdown of mTOR dramatically inhibited p53R172H mutant-induced colony formation in soft agar and tumor growth in SCID mice. Our results support that AKT plays a crucial role in mediating mutant p53 gain-of-function phenotypes in vivo through the inhibition of autophagy by the activated AKT-mTOR signaling pathway. These findings provide the proof-of-principle that targeting the AKT-mTOR-autophagy pathway might have promising therapeutic benefits for cancer patients carrying such p53 mutations. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 1173. doi:1538-7445.AM2012-1173