HBx Regulates Fatty Acid Oxidation To Promote Hepatocellular Carcinoma Survival During Metabolic Stress

Part I HBx regulates fatty acid oxidation to promote hepatocellularcarcinoma survival during metabolic stress Background & Aims:Hepatitis B virus(HBV) X protein(HBx) has been involved in the pathogenesis of HBV-induced hepatocellular carcinoma(HCC). Due to inadequate vascularization and high rate of nutrient consumption, HCC cells are chronically exposed to nutrient deficiency and continuously undergo metabolic stress during tumorigenesis. This study aims to explore the role of HBx in HCC adaptation to metabolic stress.Methods:HCC cells expressing HBx were analyzed for their responses to nutrient deprivation.Changes in intracellular redox and energy homeostasis as well as fatty acid utilization were measured during metabolic stress. Mouse xenografts and clinical HCC specimen were analyzed to examine the in vivo effects of HBx on signaling pathways regulating fatty acid oxidation(FAO).Results:HBx expression prolonged cell survival during glucose deprivation by enhancing the production of intracellular ATP and NADPH, which is essential for thae maintanence of redox and energy homeostasis. In contrast, knockdown of HBx via si RNA reversed these effects and renderred HCC cells much more sensitive to glucose deprivation. Though HBx had no direct impact on glycolysis and oxidative phosphorylation capacity of HCC cells under normal culture conditions, it activated fatty acid oxidation(FAO) in the absence of glucose, which maintained NADPH and ATP levels. Moreover, up-regulation of HBx stimulated phosphorylation of AMP-activated protein kinase(AMPK) and acetyl-Co A carboxylase(ACC) via a calcium/Ca MKK-dependent pathway under glucose limitation,which was required for the activation of FAO. On the other hand, inhibition of FAO by etomoxir(ETO) restored the sensitivity of HBx-expressing cells to glucose deficiency in vitro and restrained xenograft tumor formation in vivo. Finally, the activation of AMPK and FAO pathway by HBx expression were also observed in the xenograft tumors and HBV-associated HCC specimens in which metabolic stress frequently occur.Conclusions:HBx has a key function in NADPH and ATP maintenance via activation of FAO,which is critical for HCC cell survival under metabolic stress and can be exploited for therapeutic benefit.Part II Acetyl-Co A carboxylase α promotion of glucose-mediated fatty acid synthesis enhances survival of hepatocellular carcinoma in mice and patients Background & Aims:Nutrient deficiencies in solid tumors underlie the requirement for metabolic adaptation. Aberrantly activated de novo lipogenesis is crucial for the development and progression of hepatocellular carcinoma(HCC). However, whether de novo lipogenesis influences biologic behaviors of HCCs under conditions of metabolic stress remain obscure.Methods:The correlations of ACCα expression with lipid content, glucose uptake and cell survival rate under metabolic stress were analyzed in freshly isolated primary hepatoma cells or HCC cell lines. Co-immunoprecipitation and metabolic flux analysis were conducted to explore the molecular mechanism.Results:Primary HCC cells exhibit distinct levels of glucose-derived de novo lipogenesis,which are positively correlated with their survival responses to nutrient limitation. The enhanced lipid synthesis in HCCs is characterized by the increased expression of rate-limiting enzyme acetyl-Co A carboxylase(ACCα). ACCα-mediated fatty acid synthesis determines the content of intracellular lipid which is essential for cell survival death by means of fatty acid oxidation(FAO) during metabolic stress. ACCα forms a complex with CPT1 A and prevents its mitochondria distribution. Glucose limitation induces dissociation of this complex and facilitates the mitochondria re-localization of CPT1 A, thus providing both the substrate and enzyme storage for FAO during glucose deficiency. Interference of ACCα suppressed de novo lipid biosynthesis and renders tumor cells much more sensitive to metabolic stress in vitro. Overexpression of ACCα predicted aggressive clinicopathological features and poor prognosis of HCC. Moreover,multivariate Cox regression analysis also identified ACCα as a significant predictor of recurrence and prognosis of HCC patients.Conclusions:ACCα-mediated de novo lipid synthesis plays a critical role in the maintanence of HCCs survival under metabolic stress, whereas ACCα inhibition might be exploited as an effective anticancer therapy to target HCCs which are resistant to metabolic stress.