4EBP1/eIF4E and p70S6K/RPS6 Axes Play Critical and Distinct Roles in Hepatocarcinogenesis Driven by AKT and N-Ras Proto-oncogenes in Mice

Concomitant expression of activated forms of v-akt murine thymoma viral oncogene homolog (AKT) and Ras in mouse liver (AKT/Ras) leads to rapid tumor development through strong activation of the mammalian target of rapamycin complex 1 (mTORC1) pathway. mTORC1 functions by regulating p70S6K/ribosomal protein S6 (RPS6) and eukaryotic translation initiation factor 4E-binding protein 1/ eukaryotic translation initiation factor 4E (4EBP1/eIF4E) cascades. How these cascades contribute to hepatocarcinogenesis remains unknown. Here, we show that inhibition of the RPS6 pathway by rapamycin effectively suppressed, whereas blockade of the 4EBP1/eIF4E cascade by 4EBP1A4, an unphosphorylatable form of 4EBP1, significantly delayed, AKT/Ras-induced hepatocarcinogenesis. Combined treatment with rapamycin and 4EBP1A4 completely inhibited AKT/Ras hepatocarcinogenesis. This strong antineoplastic effect was successfully recapitulated by ablating regulatory associated protein of mTORC1, the major subunit of mTORC1, in AKT/Ras-overexpressing livers. Furthermore, we demonstrate that overexpression of eIF4E, the proto-oncogene whose activity is specifically inhibited by 4EBP1, resulted in hepatocellular carcinoma (HCC) development in cooperation with activated Ras. Mechanistically, we identified the ectonucleoside triphosphate diphosphohydrolase 5/ adenylate kinase 1/cytidine monophosphate kinase 1 axis and the mitochondrial biogenesis pathway as targets of the 4EBP1/eIF4E cascade in AKT/Ras and Ras/eIF4E livers as well as in human HCC cell lines and tissues. Conclusions: Complete inhibition of mTORC1 is required to suppress liver cancer development induced by AKT and Ras proto-oncogenes in mice. The mTORC1 effectors, RPS6 and eIF4E, play distinct roles and are both necessary for AKT/Ras hepatocarcinogenesis. These new findings might open the way for innovative therapies against human HCC. (Hepatology 2015;61:200-213)