39 Mechanism of Alcoholic Hepatitis: Relationship Between Leaky Gut, Endotoxemia, and Pro-Inflammatory Cytokines

Abstract

Background: ApoA5, the newest member of the apolipoprotein family, is expressed exclusively in the liver. Although plasma ApoA5 concentrations in humans are very low (114258 ng/mL) and ~1,000-fold lower than those of ApoB100 on a molar basis, human and mouse studies have clearly shown a key role of ApoA5 in plasma triglyceride (TG) homeostasis. Because ~20% of ApoA5 is secreted into the circulation and its concentration is 7-fold higher in the liver than in plasma, we hypothesize that ApoA5 plays a critical role in regulating hepatic TG metabolism and ApoA5 deficiency profoundly perturbs the balance of daily input and output of fatty acids (FA) in the liver. Methods: Male ApoA5 KO and WT mice were fed a chow diet for 12 mo. Liver pathology was examined by H&E and Sirius Red staining. Lipid droplets (LD) and endoplasmic reticulum (ER) and Golgi fractions were isolated from homogenized liver by ultracentrifugation. Gene expression and protein content were studied by qRT-PCR and Western blots. Lipidomic analysis of liver was performed by ESI-MS/MS. Results: During the 12-mo period, on chow, ApoA5 deficiency significantly reduced TG disposal by inhibiting FA β-oxidation through repressing carnitine palmitoyltransferase-1, coupled with reduced energy expenditure. In WT mice, because it is a LD-associated protein, ApoA5 guided TG-rich LD to the ER for VLDL assembly and subsequently to the Golgi apparatus for hepatic VLDL secretion through its interaction with Plin2, Cideb and ApoB100. ApoA5 deficiency reduces hepatic VLDL secretion by impairing its lipidation and maturation. As a result, accumulation of excess TG within the cytoplasm of hepatocytes induced the formation of numerous large LD, leading to liver steatosis. ApoA5 deficiency significantly increased de novo synthesis of ceramides, the major FAmetabolites, by enhancing hepatic serine palmitoyltransferase activity. Due to their hepatocellular lipotoxicity, increased ceramides induced hepatic lipoapoptosis through TNFα by stimulating both caspase-induced apoptosis and ROS-mediated mitochondrial dysfunction in ApoA5 KO, but not WT mice. These alterations triggered NASH with a main histological feature of hepatocellular ballooning in ApoA5 KO mice. Ceramide-induced hepatocyte apoptosis and activation of hepatic stellate cells promoted fibrogenesis by triggering a profibrogenic gene program, leading to liver fibrosis. Conclusions: ApoA5 deficiency in chow-fed mice over a long period leads to spontaneous development of NAFLD, gradually evolving from simple steatosis to NASH, and subsequently to liver fibrosis, which closely recapitulates many aspects of the pathogenic process of NAFLD in patients. These innovative studies will help us understand the mechanisms underlying the critical role of ApoA5 in the pathogenesis of NAFLD and provide novel targets to combat NAFLD by pharmacological intervention.