The Protective Role Of Autophagy In FFA-induced Hepatocyte Damage

Nonalcoholic fatty liver disease (NAFLD) refers to a wide spectrum of liver damage, ranging from simple steatosis to steatohepatitis, advanced fibrosis, and cirrhosis. However, the mechanisms underlying the development of NAFLD remain unclear. Therefore, searching for novel NAFLD associated molecules may enrich our knowledge about the progression and development of NAFLD.Autophagy occurs when cells need to'selfcannibalize'or degrade their constituents. Underlying'housekeeping'levels of autophagy probably occur in most normal cells to prevent the accumulation of protein aggregation and defective cellular substructures. Certain environmental cues (such as starvation, high temperature, low oxygen, hormonal stimulation) or intracellular stress (damaged organelles, accumulation of mutant proteins, microbial invasion) may activate autophagy-related signaling pathways. There are three primary forms of autophagy: chaperone-mediated autophagy, microautophagy and macroautophagy.Previously, a large number of yeast mutants (apg or atg) defective were identified in autophagy. However, many of them have mammalian homologues, suggesting that the molecular mechanism of autophagy is conserved in eukaryotes. The functions of Atg8/LC3, Atg7, and Atg6/Beclin 1 are the best characterized autophagy-related genes in mammalian cells. In addition, some others are also involved in the progression of autophagy, such as mTOR and MAPK. Recently, detecting microtubule-associated protein LC3 and its cleaved form LC3II by immunoblotting or immunofluorescence has become a widely used method for monitoring autophagy and autophagy-related processes.Recently, a number of large-scale epidemiological studies have pointed out that overweight and obesity, defined by body mass index (BMI) higher than 25, result in a substantial increase of cancer risk, especially for hepatocellular carcinoma (HCC), coupled with the progression of NAFLD. Thus, we wondered whether autophagy is activated in response to FFA stimulation and plays a role in HFD-induced liver disease. By using an HFD-fed mice model and primary hepatocytes in which the HFD-induced pathological events are well defined, we determined that HFD could induce a selective autophagy process in hepatocytes. The aim of this study was to examine the molecular mechanism of the free fatty acids (FFA)-induced autophagy.Methods: Mice were given high-fat diet. The effects of palmitate on hepatocytes and hepatic cell lines were examined.Results: Livers of HFD-feed mice exhibited an increased autophagic flux, and free fatty acid palmitate enhanced the conversion of light chain LC3-I to LC3-II, a marker of activation of autophagy, in multiple liver cell lines. Although FFAs could activate several signal transduction pathways in liver cells including ROS and MAPK, palmitate-induced autophagy was only blocked by a JNK inhibitor, SP600125, but not other modulators. Furthermore, siRNA-mediated inhibition of JNK2, but not JNK1, significantly decreased palmitate-induced LC3-II accumulation in SMMC-7721 cells. Knock down of Beclin1 and Atg5 expression inhibited LC3II accumulation which induced by palmitate treatment. Suppression of macroautophagy with pharmacological agents or small interfering RNAs significantly increased the apoptosis of hepatocyte. Conclusions: Autophagy was provoked by high-fat diet in liver of mice, and JNK2 inhibitor can reduce the FFA-induced autophagy accumulation, suggesting that FFA stimulated functional autophagy possibly through the JNK2-dependent pathway.