The Mechanism Of Hepatic Triglyceride Metabolism

Objecives:An increase in hepatic triglyceride(TG)contents usually results in Non-alcoholic fatty liver disease(NAFLD).NAFLD is characterized by hepatic steatosis,which is caused by excess TG as lipid droplets depositing in hepatocytes.NAFLD can trigger a progressive cascade of lipid disorders,ranging from hepatosteatosis to non-alcoholic steatohepatitis,liver fibrosis,cirrhosis and eventually hepatocellular carcinoma.Moreover,it tightly associated with multiple comorbid conditions,including insulin resistance and type Ⅱdiabetes.However,its molecular mechanism still remains unknown.Obesity is strongly associated with NAFLD.However,obesity is usually accompanied with systemic metabolic disorders and interacts with NAFLD as both cause and effect.Thus,our study includes two parts.The first part of our study focused on exploring the mechanisms of obesity-induced fatty liver.The second part of the study explored the mechanisms of early hepatic TG accumulation,preceding systemic metabolic disorders.Methods:In the first section: in order to identify genes that are potentially involved in dysfunctional hepatic lipid homeostasis in obesity,we performed a clustering analysis of Affymetrix arrays using livers of mice fed a long term high-fat diet(HFD).We disrupted target gene in obese mice using genetic ablation and administration of neutralizing antibody and investigated the phenotypes.We overexpressed target gene in livers of WT mice by delivering an adenoviral vector and investigated the phenotypes.In addition,Western-Blot,dual luciferase reporter,co-immunoprecipitation(Co IP)and chromatin immunoprecipitation(CHIP)assays were further applied to explore the mechanism underlying the effects of target gene in hepatic steatosis.In the second section: we employed short term HFD mice to investigate hepatic TG homeostasis.In order to identify the factors that involved in hepatic lipid homeostasis in short-term HFD mice,we performed a clustering analysis of micro RNA arrays.We overexpressed miRNA in the liver of C57BL/6 mice by delivering an adenoviral vector and investigated the phenotypes.Using bioinformatics approach and dual luciferase reporter assay,we identified the target gene of miRNA.Results:Our gene arrays showed a pronounced overexpression of OSF-2 in the liver of long term HFD mice(P<0.05).Both genetic ablation and shRNA silence of Osf-2 and in obese mice resulted in decreaced serum and liver TG,induced expression of peroxisome proliferator–activated receptor α(PPARα),a master regulator of fatty acid oxidation,and activation of the JNK signaling pathway.Conversely,overexpression of Osf-2 in livers of WT mice resulted in an opposite phenotype.With regard to the molecular mechanism,OSF-2 downregulates PPARα expression through α6β4 integrin-mediated JNK-c-jun signaling pathway,which prevent RORα binding and transcriptional activation at the PPARα promoter.Mice fed a short term HFD showed no significant changes in body weight,blood glucose,serum TG or insulin levels.However,hepatic TG contents were markedly increased.Then,we performed a clustering analysis of micro RNA arrays and found a pronounced up-regulation of miR-124 in the livers of short term HFD mice(P<0.05).Adenoviral overexpression of miR-124 in C57BL/6 mice led to accumulation of excessive triglycerides and up-regulation of lipogenic genes in the liver.We further identified tribbles homolog 3(TRB3)as a direct target of miR-124.AKT signaling,which is negatively regulated by TRB3,was enhanced by miR-124 overexpression.Conclusion:Taken together,our study identifies an Osf-2-dependent pathway that mediates obesity-induced hepatosteatosis.On the other hand,our study uncovers a novel miRNA,miR-124,that is responsible for early hepatic TG accumulation,preceding the systemic metabolic disorders.Our findings may expand our knowledge of mechanisms underlying the early and late disturbance of hepatic TG metabolism and provide a promising therapeutic target for related diseases.