The impact of mTOR, TFEB and Bid on non-alcoholic fatty liver disease and metabolic syndrome

Abstract

Non-alcoholic fatty liver disease and metabolic syndrome induced by high nutrient status have increasingly become a global health concern as it cause multiple complications. The mTOR complex is central in regulating anabolic reactions within cells under growth factors or under high nutrients stimulation. Constitutive and persistent activation of mTOR can impair cellular functions. In the first part of this study, we demonstrate a damping oscillation of mTOR activity during a long-term treatment of high fat diet. TFEB translocation and lysosomal enzyme activity also oscillate, but in an opposite direction. TFEB controls the lysosomal activity, autophagic degradation and lipid metabolism. Overexpression of wild type and mutant TFEB could inhibit NAFLD development in mice. In addition, TFEB location in nucleus inversely correlates with NAFLD severity in patients. mTOR activation under hypernutrition status suppresses TFEB translocation, inhibits lysosomal functions and autophagic degradation of lipid droplets. Inhibition of mTOR activity by rapamycin reverse the above phenotypes. Because mTOR activation also requires normal lysosomal function, the inhibition of TFEB by mTOR leads to decreased lysosomal function and mTOR downregulation. This negative feedback may explain the oscillation pattern of mTOR activation in long term high fat diet regimen and is a novel mechanism for inhibition of mTOR. In the second part of study, we report that Bid protein, previously known for its pro-apoptosis function in promoting mitochondrial permeability, plays an unexpected role in regulating fatty acid beta oxidation. Deletion of Bid in mice reprograms the body's response to hyper-nutrition caused by high fat diet, leading to the resistance to the development of obesity, liver steatosis and metabolic syndrome. These mice present a higher oxygen consumption, a lower respiratory quotient, and an increased beta-oxidation rate. Mechanistically, the high fat diet regimen triggers translocation of the full length Bid molecule to mitochondrial membrane. Genetic deletion of Bid also affects the stability of its binding protein, MTCH2 in the mitochondrial membrane. In summary, we describe in this study a mTOR-TFEB-lysosome feedback loop, which can regulate NAFLD development, and a novel Bid-mediated regulatory mechanism in beta-oxidation, which limits energy expenditure and promotes obesity development.