The incidence rate of nonalcoholic fatty liver disease is increasing year by year, but there is still no effective cure. Clinical and basic studies show that a type of hypoglycemic drug, namely glucagon-like peptide-1 (GLP-1) receptor agonists can improve liver lipid deposition, but the specific mechanism is unknown.

To explore the mechanism of GLP-1 receptor agonists improving liver lipid deposition in a rat model of insulin resistance induced by high-fructose diet.

This experiment was carried out from January to April 2016. Thirty-six Wistar rats were randomly divided into a control group (ND) receiving a normal diet and a model group receiving a high-fructose diet. After 8 weeks, a hyperinsulinemic-euglycemic clamp test was performed in the model group to verify the formation of insulin resistance. The rats in the model group were further randomized into a high-fructose (HFD) subgroup and a high fructose with exenatide (HFD+Ex) subgroup. The changes of glucose and lipid levels, insulin resistance, liver lipid deposition, the expression and nuclear translocation of β-catenin and lipid synthesis pathway related factors were observed in HFD+Ex subgroup at four weeks after receiving subcutaneous abdominal injection of exenatide injection. Further changes in cell lipid deposition and lipid synthesis pathway related factors were observed after inhibiting the expression of β-catenin with small interfering RNA (siRNA) by transfection techniques in HepG2 cells. HepG2 cells were treated with 25 mmol/L fructose, 100 nmol/L exendin-4, and non-transfected HepG2 cells were used as controls. ALL of the cells were divided into normal control group (Con) , high-fructose (HF) group, high fructose with exendin-4 (HF+Ex4) group, high fructose with exendin-4 and control siRNA (HF+Ex4+Si-control) group, and high fructose with exendin-4 and β-catenin siRNA (HF+Ex4+Si-β-catenin) group. After the experiment, the rats' weight and liver index, serum concentrations of triglyceride (TG) , total cholesterol (TC) , free fatty acid (FFA) , alanine aminotransferase (ALT) , aspartate aminotransferase (AST) , fasting blood glucose (FBG) , fasting insulin (FINS) and liver TG concentration were measured, and the area under the plasma glucose curve (AUC_glu) , and glucose infusion rate (GIR) were calculated, and lipid droplets in liver tissues were observed using Oil Red O staining. The protein expression levels of sterol regulatory element binding protein 1 (SREBP-1) and the key enzymes for downstream lipid synthesis, fatty acid synthase (FAS) , acetyl coenzyme A carboxylase (ACC) , stearoyl-CoA desaturase 1 (SCD-1) and β-catenin of liver tissues and HepG2 cells were also measured.

(1) After 8-week high-fructose feeding, the model group had significantly higher weight, liver index and liver TG concentration, and lower GIR than the ND group (P<0.05) . After 4 weeks of drug intervention, HFD subgroup demonstrated higher weight, liver index, TG, FFA, ALT, FBG, FINS and AUC_glu, and lower GIR than the ND group (P<0.05) . HFD+Ex subgroup showed lower weight, liver index, FFA, ALT, FBG, FINS, and AUC_glu, and higher GIR than HFD subgroup (P<0.05) . (2) Compared with ND group, HFD subgroup demonstrated higher concentration of TG in the liver (P<0.05) , and a large number of red lipid droplets in liver cells. HFD+Ex subgroup had lower concentration of TG in the liver (P<0.05) and reduced red lipid droplets in liver cells compared with HFD subgroup. (3) Compared with ND group, the expression of SREBP-1, FAS, SCD-1 and ACC in liver of rats in HFD subgroup increased (P<0.05) . Compared with HFD subgroup, the protein expression of SREBP-1, FAS, SCD-1 and ACC in HFD+Ex subgroup decreased (P<0.05) . (4) Compared with ND group, the expression levels of total protein and nuclear protein of β-catenin in liver of rats in HFD subgroup were significantly decreased (P<0.05) . Compared with HFD subgroup, the expression levels of total protein and nuclear protein of β-catenin increased in HFD+Ex subgroup (P<0.05) . (5) Compared the HepG2 cells treated with HF, HF+Ex4 group had higher expression levels of total protein and nuclear protein of β-catenin and lower levels of serum TG, and so did HF+Ex4+Si-control group (P<0.05) . Compared with HF+Ex4 group, HF+Ex4+Si-β-catenin group had down-regulated expression of total protein and nuclear protein of β-catenin (P<0.05) . The levels of serum TG of HepG2 cells in HF+Ex4+Si-β-catenin group was higher than that in HF+Ex4 group (P<0.05) . (6) HF+Ex4 group had lower protein expression levels of SREBP-1, ACC, FAS, and SCD-1 of HepG2 cells than HF group, and so did the HF+Ex4+Si-control group (P<0.05) . The protein expression levels of SREBP-1, ACC, FAS, and SCD-1 of HepG2 cells in HF+Ex4+Si-β-catenin group were higher than those in HF+Ex4 group (P<0.05) .

GLP-1 receptor agonists may regulate β-catenin expression to improve liver lipid deposition in rats with insulin resistance, which are potential new drugs for nonalcoholic fatty liver disease. β-catenin may be an important target for drug treatment.