Autophagy enhances hepatocellular carcinoma progression by activation of mitochondrial β-oxidation

Several types of cancers, including hepatocellular carcinoma (HCC), show resistance to hypoxia and nutrient starvation. Autophagy is a means of providing macromolecules for energy generation under such stressed-conditions. The aim of this study was to clarify the role of autophagy in HCC development under hypoxic conditions. The expression of microtubule-associated protein 1 light chain 3 (LC3), which is a key gene involved in autophagosome formation, was evaluated in human HCC using immunohistochemistry and western blot. The relationship between LC3 and hypoxia-induced factor 1 alpha (HIF1 alpha) expression was examined using real-time PCR. In addition, human HCC cell line Huh7 was treated with pharmacological autophagy-inhibitor and inactive mutant of Atg4B (Atg4B(C74A)) under hypoxic condition to evaluate the effects of hypoxia-induced autophagy on cell survival, intracellular ATP, and mitochondrial beta-oxidation. LC3 was significantly highly expressed in HCC as compared with noncancerous tissues. LC3 expression, correlated with HIF1 alpha expression, was also significantly correlated with tumor size, and only in the context of large tumors, was an independent predictor of HCC recurrence after surgery. In addition, Huh7 treated with autophagy-inhibitor under hypoxia had lower viability, with low levels of intracellular ATP due to impaired mitochondrial beta-oxidation. Autophagy in HCC works to promote HIF1 alpha-mediated proliferation through the maintenance of intracellular ATP, depending on the activation of mitochondrial beta-oxidation. These findings demonstrated the feasibility of anti-autophagic treatment as a potential curative therapy for HCC, and improved understanding of the factors determining adaptive metabolic responses to hypoxic conditions.