TREM2 Regulates Obesity-induced Insulin Resistance Via Adipose Tissue Remodeling In Mice Of High-fat Feeding

Part Ⅰ Dynamic changes of TREM2 expression levels in adipose tissue in mice of high-fat feedingAims:Trigger receptor expressed on myeloid cells 2(TREM2),which was found expressed on myeloid cells as macrophages at first,regulating multiple cell functions.Recently it was discovered that TREM2 also expressed on mature adipocytes.In adipose tissue of animal models of obesity,TREM2 gene was up-regulated.Adipose tissue remodeling plays a significant role during obesity-induced insulin resistance via manipulating number and function of adipocytes and macrophages.This part of study aims to investigate the dynamic changes of TREM2 expression levels of adipocytes and macrophages of epididymal adipose tissue(EAT)in mice of high-fat feeding.Materials and methods:Wild type(WT)mice of C57BL/6 were randomly divided into two groups and fed with a high-fat diet(HFD)or a controlled-fat diet(CFD)respectively for 16 weeks.EAT was collected after 0,4,8,12 and 16 weeks of HFD feeding.After isolation and purification,TREM2 expression levels of adipocytes and macrophages from EAT were measured with RT-PCR.Results:After 12 weeks of feeding,HFD mice demonstrated with obesity,adipose tissue remodeling and insulin resistance.TREM2 expression levels of adipocytes and macrophages from EAT did not change with time of feeding in CFD mice.In HFD mice,both adipocytes and macrophages up-regulated TREM2 expression levels after 12 weeks of feeding.Conclusions:These results indicated that TREM2 expression was up-regulated in adipocytes and macrophages in mice of high-fat feeding.In addition,TREM2 may regulate adipose tissue remodeling during pathogenesis of obesity-induced insulin resistance.Part Ⅱ The role and mechanism of TREM2 in obesity-induced insulin resistance in mice of high-fat feedingAims:Expression levels of TREM2 of adipocytes and macrophages were up-regulated in EAT in mice of high-fat feeding,indicating that TREM2 may regulate adipose tissue remodeling during pathogenesis of obesity.However,the effect of TREM2 on adipose tissue remodeling remains elusive.This part of study aims to investigate the role and underlying mechanism of TREM2 on adipose tissue remodeling and obesity-induced insulin resistance via challenging TREM2 knock out(TREM2-/-)mice with high-fat feeding.Materials and methods:Wild type(WT)and TREM2-/-mice were fed with a high-fat diet(HFD)for 12 weeks.Body weight and food consumption was monitored during HFD feeding.Epididymal adipose tissue(EAT)was collected and examined for adipose tissue remodeling;while livers were harvested to determine hepatic steatosis.Serum metabolic profiles as fasting blood glucose,free fatty acid,triglyceride and total cholesterol were measured to analyze metabolic disturbances.Insulin resistance status was determined with glucose tolerance test(GTT)and insulin tolerance test(ITT).Moreover,insulin signaling pathway of EAT was analyzed.Adipocyte hyperplasia,adipocyte hypertrophy and adipocyte death was determined via histology analysis.F4/80 staining and flow cytometry were used for determining macrophage from EAT.Adipose tissue derived MCP-1 was measured with RT-PCR and western blotting;while circulating MCP-1 levels were measured with ELISA.In addition,cytokines were determined in macrophage and total adipose tissue to verify inflammatory status.Results:TREM2 deficiency did not change bodyweight,adipose tissue remodeling,hepatic steatosis and insulin resistance in mice of controlled-fat diet(CFD).After 12 weeks of HFD feeding,TREM2-/-mice demonstrated with increased bodyweight,promoted hepatic steatosis,deteriorated insulin resistance and decreased EAT mass.Moreover,EAT from TREM2-/-mice exhibit with suppressed adipocyte hyperplasia,promoted adipocyte hypertrophy,and deteriorated adipocyte death.Besides,F4/80+CDllc+macrophages from EAT was reduced due to decreased expression of adipocyte derived MCP-1.Moreover,F4/80+CD11c+macrophages from TREM2-/-mice cannot form crown-like structures(CLS)to isolate and clear dead adipocytes.In addition,macrophages from adipose tissue of TREM2-/-mice released more cytokines,and EAT of TREM2-/-mice demonstrated with magnified inflammation.Conclusions:These results indicated that TREM2 protects mice of HFD feeding from obesity,hepatic steatosis and insulin resistance via manipulating adipose tissue remodeling in three ways.1.TREM2 promotes adipocyte hyperplasia while suppressing adipocyte hypertrophy and adipocyte death.2.TREM2 up-regulates adipocyte MCP-1 expression to recruit F4/80+CDllc+macrophage infiltration to isolate and clear dead adipocytes.3.TREM2 decreases pro-inflammatory cytokines released from macrophages.