Effects of GPR81 agonist on insulin resistance in rats with nonalcoholic fatty liver disease

Abstract

Objective: To investigate the effects of NOD-like receptor protein 3 (NLRP3) signaling pathway on insulin resistance and the intervention of lactic acid receptor G protein-coupled receptor 81 (GPR81) agonist in nonalcoholic fatty liver disease (NAFLD) rats. Methods: Thirty SD male rats were randomly divided into three groups: control group, NAFLD group and GPR81 agonist group, with 10 rats in each group. Nonalcoholic fatty liver rat model was established by high fat diet. The rats in GPR81 agonist group were injected intraperitoneally with GPR81 specific agonist lactate (50 nmol/L) on the basis of nonalcoholic fatty liver model once a week, and the other two groups were injected with the same amount of normal saline for 12 weeks. The levels of liver biochemical indexes, fasting blood glucose, insulin and inflammatory factors in liver homogenate were measured, and the histopathological morphology of liver in each group was observed. The protein expressions of NLRP3, apoptosis-associated speck-like protein containing a CARD (ASC), cysteinyl aspartate specific proteinase-1 (caspase-1), insulin receptor substrate 1 (IRS-1), Tyr465-IRS-1, Ser636-IRS-1, glucose transporter 4 (GLUT4) in liver tissue were detected by Western blot. The mRNA expression levels of NLRP3, ASC, caspase-1, IRS-1 and GLUT4 in liver tissue were detected by qRT-PCR. Results: Compared with the control group, the serum levels of triglyceride (TG), alanine aminotransfease (ALT), aspartate aminotransfease (AST), fasting plasma glucose (FPG), fasting insulin (FINS) and homeostasis model assessment of insulin resistance (HOMA-IR) of NAFLD group were increased significantly (P＜ 0.05). The results of liver histopathology showed that in NAFLD group, there were obvious fatty changes in liver tissue, fat droplets in hepatocytes and inflammatory cell infiltration. And the mRNA and protein expressions of NLRP3, ASC, caspase-1 and the protein expression of Ser636-IRS-1 in NAFLD group were increased significantly, and the contents of interleukin 1β (IL-1β) and interleukin 18 (IL-18) in liver and serum were increased, while the mRNA and protein expressions of IRS-1 and GLUT4 and Tyr465-IRS-1 were decreased significantly (P＜0.05). Compared with NAFLD group, the above indexes of GPR81 agonist group were all significantly improved. Conclusion: The activation of NLRP3 signaling pathway mediates the production of inflammatory factors and promotes the development of NAFLD. GPR81 agonist may be a potential treatment for NAFLD.