Amplification and/or activation of the c-Myc protooncogene is one of the leading genetic events along hepatocarcinogenesis. The oncogenic potential of c-Myc has been proven experimentally by the finding that its overexpression in the mouse liver triggers tumor formation. However, the molecular mechanism whereby c-Myc exerts its oncogenic activity in the liver remains poorly understood. Here, we demonstrate that the mammalian target of rapamycin complex 1 (mTORC1) cascade is activated and necessary for c-Myc dependent hepatocarcinogenesis. Specifically, we found that ablation of Raptor, the unique member of the mTORC1 complex, strongly inhibits c-Myc liver tumor formation. Also, p70S6K/ribosomal protein S6 (RPS6) and eukaryotic translation initiation factor 4E-binding protein 1/eukaryotic translation initiation factor 4E (4EBP1/eIF4E) signaling cascades downstream of mTORC1 are required for c-Myc-driven tumorigenesis. Intriguingly, microarray expression analysis revealed the upregulation of multiple amino acid transporters, including SLC1A5 and SLC7A6, leading to robust uptake of amino acids, including glutamine, into c-Myc tumor cells. Subsequent functional studies showed that amino acids are critical for activation of mTORC1, as their inhibition suppressed mTORC1 in c-Myc tumor cells. In human HCC specimens, levels of c-Myc directly correlate with those of mTORC1 activation as well as of SLC1A5 and SLC7A6. Conclusion Our current study indicates that an intact mTORC1 axis is required for c-Myc-driven hepatocarcinogenesis. Thus, targeting mTOR pathway or amino acid transporters may be an effective and novel therapeutic option for the treatment of HCC with activated c-Myc signaling.

Current evidence indicates that neoplastic nodules induced in liver of Brown Norway (BN) rats genetically resistant to hepatocarcinogenesis are not prone to evolve into hepatocellular carcinoma. We show that BN rats subjected to diethylnitrosamine/2-acetylaminofluorene/partial hepatectomy treatment with a "resistant hepatocyte" protocol displayed higher number of glutathione-S-transferase 7-7(؉) hepatocytes when compared with susceptible Fisher 344 (F344) rats, both during and at the end of 2-acetylaminofluorene treatment. However, DNA synthesis declined in BN but not F344 rats after completion of reparative growth. Upregulation of p16 INK4A , Hsp90, and Cdc37 genes; an increase in Cdc37-Cdk4 complexes; and a decrease in p16 INK4A -Cdk4 complexes occurred in preneoplastic liver, nodules, and hepatocellular carcinoma of F344 rats. These parameters did not change significantly in BN rats. E2f4 was equally expressed in the lesions of both strains, but Crm1 expression and levels of E2f4-Crm1 complex were higher in F344 rats. H uman and rodent hepatocarcinogenesis is characterized by the progressive development of foci of altered hepatocytes (FAH), neoplastic nodules, and hepatocellular carcinoma (HCC). 1,2 Epidemiological evidence suggests a polygenic predisposition for human HCC. 2 Studies on rodents allowed mapping the loci responsible for the progression of preneoplastic hepatocytes to malignancy. [3][4][5][6][7] However, the nature and temporal occurrence of the events underlying resistance to HCC remain uncertain. The evaluation of these mechanisms could help in understanding the molecular pathways affected by susceptibility genes to evaluate cancer risk and identify potentially reversible phases of carcinogenesis.Previous research has shown cell cycle deregulation in neoplastic liver lesions induced by the "resistant hepatocyte" protocol in susceptible Fisher 344 (F344) rats. [8][9][10] Lower or no changes were found in resistant Brown Norway (BN) rats in which overexpression of p16 INK4A , a well-known inhibitor of the cyclin-dependent kinase (Cdk) 4/6, occurs. Forma- PH, partial hepatectomy; BrdU, siRNA, small interfering RNA. From the

Background & Aims Cumulating evidence underlines the crucial role of aberrant lipogenesis in human hepatocellular carcinoma (HCC). Here, we investigated the oncogenic potential of fatty acid synthase (FASN), the master regulator of de novo lipogenesis, in the mouse liver. Methods FASN was overexpressed in the mouse liver, either alone or in combination with activated N-Ras, c-Met, or SCD1, via hydrodynamic injection. Activated AKT was overexpressed via hydrodynamic injection in livers of conditional FASN or Rictor knockout mice. FASN was suppressed in human hepatoma cell lines via specific small interfering RNA. Results Overexpression of FASN, either alone or in combination with other genes associated with hepatocarcinogenesis, did not induce histological liver alterations. In contrast, genetic ablation of FASN resulted in the complete inhibition of hepatocarcinogenesis in AKT-overexpressing mice. In human HCC cell lines, FASN inactivation led to a decline in cell proliferation and a rise in apoptosis, which were paralleled by a decrease in the levels of phosphorylated/activated AKT, an event controlled by the mammalian target of rapamycin complex 2 (mTORC2). Downregulation of AKT phosphorylation/activation following FASN inactivation was associated with strong inhibition of rapamycin-insensitive companion of mTOR (Rictor), the major component of mTORC2, at post-transcriptional level. Finally, genetic ablation of Rictor impaired AKT-driven hepatocarcinogenesis in mice. Conclusions FASN is not oncogenic per se in the mouse liver, but is necessary for AKT-driven hepatocarcinogenesis. Pharmacological blockade of FASN might be highly useful in the treatment of human HCC characterized by activation of the AKT pathway.

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The deregulation of the oxidative metabolism in cancer, as shown by the increased aerobic glycolysis and impaired oxidative phosphorylation (Warburg effect), is coordinated by genetic changes leading to the activation of oncogenes and the loss of oncosuppressor genes. The understanding of the metabolic deregulation of cancer cells is necessary to prevent and cure cancer. In this review, we illustrate and comment the principal metabolic and molecular variations of cancer cells, involved in their anomalous behavior, that include modifications of oxidative metabolism, the activation of oncogenes that promote glycolysis and a decrease of oxygen consumption in cancer cells, the genetic susceptibility to cancer, the molecular correlations involved in the metabolic deregulation in cancer, the defective cancer mitochondria, the relationships between the Warburg effect and tumor therapy, and recent studies that reevaluate the Warburg effect. Taken together, these observations indicate that the Warburg effect is an epiphenomenon of the transformation process essential for the development of malignancy.