Activation of mTORC1 by Free Fatty Acids Suppresses LAMP2 and Autophagy Function via ER Stress in Alcohol-Related Liver Disease

Abstract

Alcohol-related liver disease (ALD) is characterized by accumulation of hepatic free fatty acids (FFAs) and liver injury. The present study aimed to investigate if mechanistic target of rapamycin complex 1 (mTORC1) plays a role in FFA-induced organelle dysfunction, thereby contributing to the development of ALD. Cell studies were conducted to define the causal role and underlying mechanism of FFA-activated mTORC1 signaling in hepatocellular cell injury. C57BL/6J wild-type mice were subjected to chronic alcohol feeding with or without rapamycin to inhibit mTORC1 activation. We revealed that palmitic acid (PA)-induced ER stress and suppression of LAMP2 and autophagy flux were mTORC1-dependent as rapamycin reversed such deleterious effects. C/EBP homologous protein (CHOP) was downstream of ATF4 which partially modulated LAMP2. Supplementation with rapamycin to alcohol-fed mice attenuated mTORC1 activation and ER stress, restored LAMP2 protein, and improved autophagy, leading to amelioration of alcohol-induced liver injury. Induction of mTORC1 signaling and CHOP were also detected in the liver of patients with severe alcoholic hepatitis. This study demonstrates that hepatic FFAs play a crucial role in the pathogenesis of ALD by activating mTORC1 signaling, thereby inducing ER stress and suppressing LAMP2-autophagy flux pathway, which represents an important mechanism of FFA-induced hepatocellular injury.

Free fatty acid treatment of mouse preimplantation embryos demonstrates contrasting effects of palmitic acid and oleic acid on autophagy

“…We observed that lysotracker staining and thus lysosomal activity in PA-treated embryos was significantly lowered, indicating that autophagosome degradation was disrupted. This interruption in lysosomal activity is not limited to preimplantation embryos, as PA treatment also significantly lowered the hydrolase activity of lysosomes in INS1 β-cells ( 50 ) as well as lowered LAMP2 levels in mouse hepatocytes ( 48 ). On the other hand, the addition of OA in PA treatment restored normal levels of autophagosome formation.…”

“…Exposure of HepG2 cells to PA revealed that autophagy is induced by the protein kinase C (PKC) pathway as the high level of DAG presence from PA exposure elevated PKC-α activation ( 47 ). Another study of mouse hepatocytes suggested that autophagy was dependent on the mammalian target of rapamycin (mTOR) pathway, where PA exposure elevated ER stress and reduced lysosomal-associated membrane protein 2 (LAMP2) expression, thus impairing the autophagic mechanisms ( 48 ). Further investigation is required to determine if either PKC or mTOR pathway is implicated in mediating PA treatment effects on mouse preimplantation embryos.…”

4-O-Methylascochlorin-Mediated BNIP-3 Expression Controls the Balance of Apoptosis and Autophagy in Cervical Carcinoma Cells

“…When cells are transfected with GFP-RFP-LC3, yellow puncta indicate autophagosome and red puncta indicate autolysosome [ 25 ]. GFP is sensitive to acidic PH in the lysosome and GFP fluorescence signals is quenched in the autolysosome [ 26 , 27 , 28 ].…”

Regulation of Autophagy by the Glycogen Synthase Kinase-3 (GSK-3) Signaling Pathway

“…Studies have shown that repression of mTORC1 by hayatine, an mTORC1 inhibitor, can enhance autophagy flux in tumor cells [39]. Other studies have shown that activation of mTORC1 by palmitic acid (PA) impaired autophagy flux and repressed lysosomal associated membrane protein-2 (LAMP2) expression in mouse Hepa-1c1c7 cells [40]. Furthermore, mTORC1 inhibition by rapamycin showed an induction of autophagy in C57BL/6J mice fed with an alcohol-containing diet for 8 weeks [40].…”

“…Other studies have shown that activation of mTORC1 by palmitic acid (PA) impaired autophagy flux and repressed lysosomal associated membrane protein-2 (LAMP2) expression in mouse Hepa-1c1c7 cells [40]. Furthermore, mTORC1 inhibition by rapamycin showed an induction of autophagy in C57BL/6J mice fed with an alcohol-containing diet for 8 weeks [40]. mTORC1 hyperactivation was reported in the isolated islet of type 2 diabetes patients and type 2 diabetes animal models [41].…”