Hepatocellular carcinoma (HCC) is the most frequent primary liver cancer and occurs mainly in patients with liver cirrhosis. The mammalian target of rapamycin (mTOR) signaling pathway is involved in many hallmarks of cancer including cell growth, metabolism re-programming, proliferation and inhibition of apoptosis. The mTOR pathway is upregulated in HCC tissue samples as compared with the surrounding liver cirrhotic tissue. In addition, the activation of mTOR is more intense in the tumor edge, thus reinforcing its role in HCC proliferation and spreading. The inhibition of the mTOR pathway by currently available pharmacological compounds (i.e., sirolimus or everolimus) is able to hamper tumor progression both in vitro and in animal models. The use of mTOR inhibitors alone or in combination with other therapies is a very attractive approach, which has been extensively investigated in humans. However, results are contradictory and there is no solid evidence suggesting a true benefit in clinical practice. As a result, neither sirolimus nor everolimus are currently approved to treat HCC or to prevent tumor recurrence after curative surgery. In the present comprehensive review, we analyzed the most recent scientific evidence while providing some insights to understand the gap between experimental and clinical studies.

About half of the mitochondrial DNA (mtDNA) mutations causing diseases in humans occur in tRNA genes. Particularly intriguing are those pathogenic tRNA mutations than can reach homoplasmy and yet show very different penetrance among patients. These mutations are scarce and, in addition to their obvious interest for understanding human pathology, they can be excellent experimental examples to model evolution and fixation of mitochondrial tRNA mutations. To date, the only source of this type of mutations is human patients. We report here the generation and characterization of the first mitochondrial tRNA pathological mutation in mouse cells, an m.3739G>A transition in the mitochondrial mt-Ti gene. This mutation recapitulates the molecular hallmarks of a disease-causing mutation described in humans, an m.4290T>C transition affecting also the human mt-Ti gene. We could determine that the pathogenic molecular mechanism, induced by both the mouse and the human mutations, is a high frequency of abnormal folding of the tRNA^Ile that cannot be charged with isoleucine. We demonstrate that the cells harboring the mouse or human mutant tRNA have exacerbated mitochondrial biogenesis triggered by an increase in mitochondrial ROS production as a compensatory response. We propose that both the nature of the pathogenic mechanism combined with the existence of a compensatory mechanism can explain the penetrance pattern of this mutation. This particular behavior can allow a scenario for the evolution of mitochondrial tRNAs in which the fixation of two alleles that are individually deleterious can proceed in two steps and not require the simultaneous mutation of both. Mitochondrial DNA (mtDNA) encodes for 13 proteins that are subunits of the Oxidative Phosphorylation system. These proteins are synthesized within the mitochondria by its own set of ribosomes. In mammals the rRNAs and all the tRNAs required for the mitoribosomes to work are also encoded by the mtDNA. Surprisingly, half of mtDNA mutations causing diseases occur in tRNA genes while they represent only 10% of the total sequence. Moreover, these pathological tRNA mutations can occur in homoplasmic form (all the copies of the mtDNA are mutant). It is puzzling that, with these homoplasmic mutations, some individuals suffer a severe disease while others are healthy. Recently we have described that common variants of mtDNA in mouse induce differences in the performance of the Oxidative Phosphorylation capacity but that the cell is capable to sense these differences and adapt the biogenesis of the OXPHOS system to compensate for them. We also showed that this mechanism was triggered by differences in the basal level of ROS. It was considered that this adaptation mechanism was of no relevance in pathological mutations. The implication of the ROS-mediated mitochondrial biogenesis in modulating the consequences of mild pathogenic mutations and its significance for disease penetrance and mitochondrial tRNA evolution are underscored.