Different Effects Of DHA And EPA On Insulin Resistance In Mice And The Underlying Mechanism Mediated By GPR120/PPAR?

Background and objectiveInsulin resistance(IR)has been regarded as the key target for the prevention and treatment of non-communicable diseases(NCDs).Recently,eicosapentaenoic(EPA)and docosahexaenoic(DHA)have attracted great attention due to their regulations on IR.To the best of our knowledge,research specifically designed to explore the difference between DHA and EPA on IR is limited.Meanwhile,whether DHA or EPA could regulate IR by directly regulating G protein-coupled receptor 120(GPR120)and then acting at peroxisome proliferator-activated receptor gamma(PPARy)is unclear.The present study therefore aimed to observe and compare the effect of different doses of DHA and EPA on high fat diet-induced IR mice.Meanwhile,whether DHA and EPA differently regulate GPR120/PPAR? was tested to find a preliminary explanation for their distinct actions on IR.MethodsC57BL/6J mice were fed a high-fat diet for 16 weeks to induce IR,after which randomly assigned to different interventions for 12 weeks including DHA or EPA at concentrations of 1%,2%and 4%(w/w).Food intake and body weight were recorded every day during the research.Glucose tolerance was evaluated by OGTT.Mice were sacrificed after 8 hours of fasting to collect samples.Fasting blood glucose,glycosylated serum protein,TG,TC,LDL-c and HDL-c were detected with commercial kits.Serum insulin,TNF-?,IL-6,IL-10,adiponectin and leptin were detected by ELISA.Visceral organs,epididymis,perirenal and intestinal fat were weighed.Peripheral adipose tissue was used for H&E staining and immunohistochemical observation of GLUT4 translocation.In this research,GPR120,PPAR?,IRS1,PI3K,Akt,p-Akt,GLUT4,MCP-1 and IL-6 were detected using western blot and RT-qPCR respectively.CD11c,CD206,leptin,UCP-1,HSL,ATGL and LPL werer determined using RT-qPCR.The insulin resistance index HOMA-IR and the insulin sensitivity index ISI were calculated.The correlations among GPR120,PPAR? and insulin signaling pathway related molecules were analyzed.ResultsHigh fat diet-induced insulin resistance mice:1.Mice fed with high fat diet were obese,accompanied with inflammation,impaired glucose tolerance and insulin resistance.Therefore,the insulin resistance mice model was successfully established.2.The protein expressions of IRS1,PI3K,Akt,GLUT4 decreased,and p-Akt decreased among mice of the high-fat group.EPA and DHA interventions:1.The state of inflammation was ameliorated in the intervention groups.2.In the intervention groups,the mRNA expressions of ATGL and LPL,the key enzymes of lipid metabolism,were increased,but EPA treatment reduced the mRNA expression of HSL,while DHA had no effect on it.3.The levels of serum TG,TC and LDL-c of the mice in the intervention groups all decreased,4%DHA treatment decreased most.4.Insulin resistance of mice in the intervention group was improved,and fasting blood glucose,HOMA-IR and AUC of OGTT were decreased most in the 4%DHA group.5.The protein expression levels of IRS1,PI3K,Akt and GLUT4 increased in the intervention groups,and the expression levels of IRS1 and Akt in the DHA intervention group were higher than those in the EPA intervention group,with the highest increase in the 4%DHA group.6.DHA intervention group showed an increase in p-Akt,while EPA group showed no significant increase,and 4%DHA group showed obvious increase in p-Akt.7.DHA-stimulated mRNA expressions of GPR120 were positively correlated with the expressionss of PPARy,IRS1 and GLUT4,and EPA-stimulated mRNA expressions of GPR120 were positively correlated with the expressionss of PPARy,IRS1 and GLUT4.DHA-stimulated mRNA expressions of PPARy were positively correlated with the expressionss of GPR120,IRS1 and GLUT4,and EPA-stimulated mRNA expressions of PPARy were positively correlated with the expressionss of GPR120 and GLUT4.Conclusion:1.High-fat diet successfully induced insulin resistance in mice.2.Both DHA and EPA improved insulin resistance,and DHA has a stronger effect,with 4%DHA showing the best function.The difference between EPA and DHA was as follows:(1)DHA up-regulated the mRNA expression of ATGL and LPL better than EPA.Meanwhile,DHA had no effect on HSL mRNA expression,while EPA inhibited it.(2)DHA exerted a stronger effect than EPA in improving glucose tolerance,serum LDL-c and cardiovascular disease risk index in mice.3.DHA might regulate the insulin signaling pathway through GPR120-PPAR? signaling pathway,EPA might regulate the insulin signaling pathway through GPR120.DHA showed a stronger effect:(1)DHA exerted a more powerful capacity of upregulating the protein expressions of IRS1 and Akt.(2)DHA increased the phosphorylation of Akt,while EPA was not obvious.