mTOR kinase inhibitor pp242 causes mitophagy terminated by apoptotic cell death in E1A-Ras transformed cells

Abstract

mTOR is a critical target for controlling cell cycle progression, senescence and cell death in mammalian cancer cells. Here we studied the role of mTOR-dependent autophagy in implementating the antiprolifrative effect of mTORC1-specific inhibitor rapamycin and ATP-competitive mTOR kinase inhibitor pp242. We carried out a comprehensive analysis of pp242- and rapamycin-induced autophagy in ERas tumor cells. Rapamycin exerts cytostatic effect on ERas tumor cells, thus causing a temporary and reversible cell cycle arrest, activation of non-selective autophagy not accompanied by cell death. The rapamycin-treated cells are able to continue proliferation after drug removal. The ATP-competitive mTORC1/mTORC2 kinase inhibitor pp242 is highly cytotoxic by suppressing the function of mTORC1-4EBP1 axis and mTORC1-dependent phosphorylation of mTORC1 target - ULK1-Ser757 (Atg1). In contrast to rapamycin, pp242 activates the selective autophagy targeting mitochondria (mitophagy). The pp242-induced mitophagy is accompanied by accumulation of LC3 and conversion of LC3-I form to LC3-II. However reduced degradation of p62/SQSTM indicates abnormal flux of autophagic process. According to transmission electron microscopy data, short-term pp242-treated ERas cells exhibit numerous heavily damaged mitochondria, which are included in single membrane-bound autophagic/autolysophagic vacuoles (mitophagy). Despite the lack of typical for apoptosis features, ERas-treated cells with induced mitophagy revealed the activation of caspase 3, 9 and nucleosomal DNA fragmentation. Thus, pp242 activates autophagy with suppressed later stages, leading to impaired recycling and accumulation of dysfunctional mitochondria and cell death. Better understanding of how autophagy determines the fate of a cell - survival or cell death, can help to development of new strategy for cancer therapy.