In the present study, we show that aberrant activation of lipogenesis is a dominant oncogenic event in human HCC. Importantly, no significant differences were detected in the extent of de novo
lipogenesis with regard to HCC etiology, suggesting that exacerbated lipogenesis is a general molecular phenomenon in hepatocarcinogenesis. Indeed, previous reports demonstrated that both hepatitis B and C viruses are able to induce FASN expression,23–25
and that overexpression of FASN is a typical feature of another predisposing condition for liver cancer, the alcoholic steatohepatitis.26
Furthermore, strong upregulation of FASN characterizes a rat model of insulin-induced hepatocarcinogenesis,27
which resembles the occurrence of HCC in people affected by type II diabetes mellitus and/or metabolic syndrome, two clinical conditions associated with an increased risk of liver cancer development.28
The highest levels of lipogenic proteins were detected in HCC characterized by an aggressive phenotype, supporting an important prognostic role for de novo
lipogenesis in HCC, as observed in many other epithelial cancer types4–6
and in accordance with the recent finding that SREBP1 levels correlate with HCC proliferation and patient’s prognosis.14
Previous reports indicate that signaling cascades driven by AKT/mTOR, MAPK, and AMPK regulate de novo
Our present investigation clearly demonstrates that the AKT/mTORC1/RPS6 axis is the major regulator of the lipogenic phenotype in liver cancer. Indeed, forced overexpression of activated Akt resulted in induction of lipid biosynthesis and upregulation of lipogenic proteins in in vitro
cultured HCC cell lines and in vivo
, and AKT suppression by either specific siRNAs or transfection of a AKT dominant negative form was associated with decreased lipogenesis and down-regulation of lipogenic proteins. Equivalent results were obtained by treatment of AKT-transfected cells with the mTORC1 inhibitor, Rapamycin, as well as by siRNA-mediated inactivation of Raptor and RPS6. Also, we found that modulation of the MAPK cascade, another putative pathway responsible for lipogenesis,4–6
had no effect on either lipogenesis or levels of the lipogenic proteins (data not shown).
Significantly, the present findings indicate that the AKT/mTORC1 pathway induces lipogenesis both via transcriptional and post-transcriptional mechanisms. The pro-lipogenic transcriptional activity played by AKT is in accordance with a previous report in human retinoic pigment epithelial and osteosarcoma cell lines.29
Nevertheless, we show that AKT promotes de novo
lipogenesis by post-transcriptional mechanisms as well, namely by impeding proteasomal degradation of SREBP1, SREBP2, and FASN. Suppression of SREBP1 and SREBP2 ubiquitination was achieved by AKT through its ability of inhibiting GSK-3β, which primes SREBP1 and SREBP2 for phosphorylation-dependent proteolysis,19
while degradation of FASN was impaired through transcriptional upregulation of the USP2a deubiquitinase. To the best of our knowledge, this is the first report showing that AKT induces USP2a in cancer.
Moreover, our results indicate that induction of lipogenic proteins is an important effector pathway of the AKT/mTORC1 axis in human HCC. On one hand, we showed that modulation of SREBP1, FASN, ACAC, ACLY, or SCD1 was able to significantly affect both AKT activation and the levels of the other lipogenic proteins, implying the presence of complex, positive feedback loops reinforcing AKT activation and sustaining overexpression of the lipogenic proteins in liver cancer (Supplementary Figure 14
). Further studies are needed to define the mechanisms whereby lipogenic proteins promote each other’s upregulation and AKT activation. On the other hand, we found that AKT-dependent proliferation and resistance to apoptosis were markedly reduced when AKT overexpression was accompanied by selective inactivation of SREBP1, FASN, ACLY, ACAC, or SCD1 in vitro
. Of note, the finding that suppression of lipogenic proteins exerted a strong growth restraint only in AKT-overexpressing cells, but not in the non-transfected counterparts, envisages the possibility that treatment with inhibitors of de novo
lipogenesis might specifically target cells characterized by activation of the AKT/mTOR pathway in liver cancer.
Finally, we demonstrated that overexpression of AKT alone is sufficient to induce hepatocarcinogenesis in the mouse, suggesting that AKT deregulation has a pivotal role in human liver cancer. Our results are consistent with published data using mice harboring a deletion of Pten, a tumor suppressor gene that negatively regulates the Akt/mTOR pathway, since liver specific inactivation of Pten leads to steatosis and, eventually, HCC development.30,31
Consistent with the pivotal role of Akt pathway in HCC pathogenesis, levels of activated AKT are almost ubiquitously upregulated in human HCC.32
Therefore, the AKT overexpressing mouse might represent a valuable model both to investigate the molecular mechanisms responsible for AKT-induced hepatocarcinogenesis and to evaluate the effect of suppressing the AKT/mTOR pathway and its lipogenic effectors in human HCC treatment.