Pentamethylquercetin Ameliorates Hepatic Oxidative Stress And Insulin Resistance In MSG-induced Obese Mice

Pentamethylquercetin(PMQ) is one of the polymethoxylated flavonoids. Although previous studies have shown that PMQ could ameliorate metabolic disorders and insulin resistance in various obese mouse models, the definite mechanisms remain to be elucidated. Increased oxidative stress appears to be a deleterious factor leading to insulin resistance. The Kelch-like ECH-associated protein1(Keap1)- nuclear factor erythroid-2-related factor-2(Nrf2) pathway is the major regulator of cytoprotective responses to oxidative stress. Activation of Nrf2 has been proved to ameliorate insulin resistance induced by high-fat diet. This study was then designed to observe if keap1-Nrf2 pathway is involved in the effects of PMQ on hepatic oxidative stress and insulin resistance in monosodium glutamate-induced obese mice.Aims: To observe the effect and potential mechanisms of PMQ for ameliorating hepatic oxidative stress and insulin resistance.Methods: At the age of 2 days, neonate CD1 mice were subcutaneously injected with a solution of MSG(3mg/g/day) dissolved in saline or equipotent vehicle for 7 consecutive days via a microsyringe. After weaning(21 d), male and female mice were kept in separate cages. All male MSG mice were randomly divided into five groups(n=12) as follows: MSG model group, PMQ5, 10, 20 mg/kg groups, metformin 300mg/kg group(Met). The normal group and MSG model group were given conventional feed and regular water. The PMQ groups were fed with diets supplemented with 5, 10, 20 mg/kg/d of PMQ and regular water, while the Met group were administered with water containing 300 mg/kg/d of Met and standard chow at 8 weeks of age. Body weight, water and food consumption of each group were monitored weekly throughout the study period. Oral glucose tolerance test(OGTT) was performed at 18 th week. Body weight, waist circumference and body length were measured, and blood samples were collected from the ophthalmic venous plexus at the end of study(the 16 th week after PMQ and Met treatment). After sacrificed, the total fat pad weight, epididymal fat pad weight and liver weight were weighted. Serum and liver tissues were frozen at-80℃ until analyzed. Fasting blood glucose, triglyceride(TG), total cholesterol(TC), fasting serum insulin level were tested respectively. Pathological changes were observed by HE- and oil red o-staining. The hepatic SOD(superoxide dismutase) and MDA(malondialdehyde) levels were measured using colorimetric assay kits. The m RNA expression levels of Nrf2, Keap1 and HO-1 in the mice livers were measured by RT-PCR. Then, we performed western blot experiment to measure the protein levels of Nrf2, Keap1 and HO-1 as well as sestrin2 in the liver tissues of mice.Results:(1) Compared with control group, MSG-induced obese mice exhibited obvious obesity and glucolipid metabolic disorders including: 1) body weight, body weight gain, waist circumference, lee’s index, total fat pad weight and epididymal fat weight were strikingly increased; 2) glucose tolerance was markedly impaired and insulin resistance index was dramatically increased; 3) the levels of fasting blood glucose, fasting serum insulin, serum triglyceride and total cholesterol were significantly higher. Furthermore, MSG-induced obese mice showed hepatic steatosis accompanied with oxidative stress injuries compared with those of control mice including: 1) liver tissues morphology was obviously changed; 2) SOD content in liver tissues was significantly reduced, while MDA content in liver tissues was significantly increased; 3) the m RNA expression levels of Nrf2 and HO-1 in liver tissues were markedly lowered, whereas Keap1 m RNA level was significantly enhanced. The changing tendency of protein levels of these factors were highly in consistent with the m RNA expression variation. 4) Sestrin2 protein abundance in liver tissues of MSG mice was significantly down-regulated.(2) Compared with MSG group, PMQ treatment in a dose-dependent manner reduced body weight, body weight gain, Lee’s index, total fat pad weight and liver weight in varying degrees. PMQ significantly and dose-dependently reduced fasting blood glucose, insulin levels and HOMA-IR index and improved glucose intolerance compared to MSG group. Additionally, PMQ intervention dramatically decreased TG levels, while PMQ at a dose of 20mg/kg significantly lowered TC levels. Moreover, treatment with PMQ alleviated hepatic pathological changes, significantly elevated hepatic SOD content and decreased hepatic MDA content. Meanwhile, on m RNA and protein levels, the expression of Nrf2 and HO-1 were remarkably up-regulated and the expression of Keap1 was significantly suppressed after PMQ treatment. More importantly, PMQ supplementation increased protein abundance of Sestrin2 in liver tissues of MSG mice.Conclusion: PMQ could ameliorate metabolic disorders and attenuate the insulin resistance by upregulating Sestrin2 and enhancing endogenous Keap1-Nrf2 system against hepatic oxidative stress.