Oleanoli Acid Supplement Attenuates Liquid Fructose-induce Adipose Tissue Insulin Resistance Through The Insulin Receptor Substrate-1/phosphatidylinositol3-kinase/akt Signaling Pathway In Rats

Objective： Molecular mechanisms of triterpenoid oleanolic acid（OA）improve liquid fructose-induced adipose tissue insulin resistance inrats.Methods：24rats were randomly divided into4groups (n=6pergroup)，administered for10weeks.(1) control group: free access towater,(2) fructose control: free access to10%fructose solution (w/v,preparation every day),(3)low concentration group: fructose solution+OA5mg/kg，(4) high concentration group: fructose solution+OA25mg/kg，Animals in oleanolic acid-treated groups were administered by oleanolicacid5or25mg/kg for10weeks, respectively. General condition andhistopathological changes of epididymal white adipose tissue (WAT) inrats were observed. Here, plasma concentrations of triglycerides， cholesterol and glucose were detected. Simultaneously, plasmaconcentrations of glucose, insulin and NEFA were determined usingenzymatic methods or by ELISA.The homeostasis model assessment ofinsulin resistance (HOMA-IR) index and adipo-IR index was calculated.The hepatic mRNA expression of IR、IRS-1、IRS-2、PI3K、Akt、TNF-α、PPAR-γ、adiponectin、SREBP-1c、FAS、ACC-1、SCD-1by Real-TimePCR.Whole protein,cytoplasma protein and nuclear pootein of IRS-1、pIRS-1、Akt、pAkt、Akt/pAkt、IRS-2、PI3K were demonstrated byWestern Blot analysis.Results：1. Morphological changes of WAT, epididymal WAT weight, the ratio ofepididymal WAT weight to body weight and adipocyte size wereincreased in fructose contral, whereas adipocyte number was decreased（ineach microscopic field）. Treatment with oleanolic acid (5and25mg/kg) unchanged epididymal WAT weight, the ratio of epididymalWAT weight to body weight, adipocyte size and adipocyte number.2. The detection of HOMA-IR,fructose feeding increased basal plasmaglucose and insulin concentrations. These increases contributed to anincrease in the HOMA-IR index. Fructose also increased plasmaglucose concentration at20min after oral glucose challenge. However,the AUC of plasma glucose concentrations during OGTT remained unchanged. Oleanolic acid treatment at the dosage of25mg/kgsuppressed the increased plasma insulin concentration at baseline andthe HOMA-IR index, and plasma glucose concentration at20min afteroral glucose administration. However, it did not affect plasma glucoseconcentration at baseline and the AUC during OGTT. Oleanolic acid at5mg/kg showed minimal effect on these variables.3. The detection of biochemical indicators, fructose feeding increased basalplasma concentrations of total cholesterol, triglyceride and NEFA. Theplasma NEFA concentrations at20,60and120min after oral glucoseadministration and the AUC were higher in fructose control than thoseof the corresponding water control. The Adipo-IR index was alsoincreased by fructose feeding. Treatment with oleanolic acid at25mg/kg lowered plasma NEFA concentration at60min after glucoseloading and the AUC. It also decreased the Adipo-IR index. However,this treatment did not significantly affect total cholesterol, triglycerideand NEFA. Oleanolic acid at5mg/kg was without significant effect onthese parameters.4. The expression of gene and protein associated with adipose tissue insulinresistance in rats, treatment with oleanolic acid upregulated adiposemRNA expression of insulin receptor, IRS-1, PI3K and Akt.Furthermore, it increased total IRS-1protein expression, attenuated fructose-stimulated pIRS-1protein expression and restored the ratio ofpIRS-1to total IRS-1protein expression to the level of water controlgroup. Although Akt and pAkt protein expression levels were notaltered significantly, the ratio of pAkt protein to Akt protein wasincreased significantly. Moreover,fructose-induced decrease in theratio of pAkt protein/Akt protein to plasma insulin concentration wasrestored to similar level to water control group. However, oleanolic acidaffected neither IRS-2, TNF-α, PPAR-γ, adiponectin, SREBP-1c, FAS,ACC-1, nor SCD-1.Conclusions1. Oleanolic acid supplement attenuates fructose-induced adiposetissue insulin resistance.2. IRS-1/PI3K/Akt signaling pathway is important molecular target inthe mechanism which oleanolic acid supplement attenuatesfructose-induced adipose tissue insulin resistance.