| Metabolic syndrome(MetS)is a complex metabolic disorder that results from the increasing prevalence of obesity,which includes abnormal adipose deposition and function,dyslipidemia,and hyperglycemia.Insulin resistance(IR)is considered to be an important causative factor of MetS and the main pathologic characteristic,as well as the most accepted theory explaining the pathophysiology of the disease.IR is also a metabolic disorder characterized by reduced sensitivity to the action of insulin on tissues and efficiency of insulin signaling for blood glucose regulation.Insulin is the hormone responsible for the maintenance of normal blood glucose level by promoting glucose consumption processes while inhibiting glucose production ones and exerts a broad spectrum of anabolic effects in multiple tissues,such as pancreas,liver,skeletal muscle,adipose,brain,and so on.However,the development of insulin resistance inhibits insulin’s ability to regulate blood glucose,leading to increase the risk of MetS.The risk of metabolic abnormality such as obesity and high blood glucose caused by a high-fat diet increases year by year.Regulation of IR is one of the important approaches to reduce the risk of diet-induced metabolic disorder in daily life.Pomegranate(Punica granatum L.)is a popular fruit and it is well known that it is a dietary source of diverse bioactive phytochemicals.One study has reported that pomegranate peel extract has higher concentrations of total polyphenols,flavonoids,and tannins than pomegranate juice or seeds.As a rich source of polyphenols,several studies have shown that pomegranate has anti-oxidant,anti-inflammatory,anti-diabetic,anti-obesity,protecting liver effects in vivo and in vitro.However,the improvement of insulin resistance and the molecular mechanism remain unclear.Therefore,it is necessary to further study the improvement effect and molecular mechanism of Pomegranate Peel Polyphenols(PPPs)and its main components on insulin resistance.In this study,the effects of PPPs and its main components on insulin resistance were studied in vitro and in vivo.Firstly,the HepG2 human hepatoma cell line was induced by high glucose to establish insulin resistance model in vitro,and the effects of PPPs and its main component,punicalagin(PC),on glucose uptake and glycogen synthesis were studied.Furthermore,the mechanism of improving insulin resistance was revealed through insulin signal transduction pathway.At the same time,a MetS model of SD rats induced by high glucose and high lipid was established,and the regulation of PPPs in improving insulin resistance in MetS rats was studied through in vivo experiments in experimental animals.The results of the study are as follows:(1)In this paper,the purified polyphenols extracted from pomegranate peel were detected and analyzed,and the total polyphenols content was up to 80.07%.Then,HPLC was used to identify the components of the extracted PPPs.A total of 7 components were detected,including gallic acid,punicalagin-α and punicalagin-β,catechin,chlorogenic acid,caffeic acid,epicatechin and ellagic acid.Among them,the content of punicalagin was the highest,reaching 38.9%.(2)Results showed that in vitro PPPs significantly up regulated Ins R,IRS-1,GLUT2 and GLUT4 mRNA expression,down regulated PPARγ and AKT mRNA expression.The protein expression of Ins R,p-IRS-1,PI3 K,p-AKT,GLUT2 and p-GSK-3β were significantly increased.The results showed that PPPs can improve the transmission of insulin signal by insulin signal transduction pathway,promote the absorption and utilization of glucose by HepG2 cells,increase glucose intake and glycogen content,restore the ability of HepG2 cells regulated by insulin,enhance insulin sensitivity and alleviate IR.Among them,the effect of PC on regulating PPARγ,AKT and GLUT2 mRNA expression and the protein expression of Ins R,p-IRS-1,PI3 K,p-AKT and GLUT2 is consistent with that of PPPs.In additon,PC can also promote the protein expression of PPARγ,which has good biological activity to improve IR.PC is the major active compent of PPPs in improving the insulin resistance via Ins R/IRS-1/PI3K/AKT signal pathway in IR cells.(3)A MetS model was established successfullyby feeding SD rats with high sugar and high fat diet for a long time.The rats were gavaged with PPPs at 150 mg/kg BW/d and 300 m/kg BW/d.In the control group,rosiglitazone was given intragastric concentration of 1 mg/kg BW/d.At the same time,PPPs control group was set up and gavage concentration was 300 m/kg BW/d.After 12 weeks of feeding and intragastric treatment,MetS rats model were successfully established.The results showed that PPPs could significantly reduce the weight of rats and the risk of obesity.It can significantly reduce liver weight index and serum ALT and AST contents,and protect liver function.It can significantly reduce the contents of TC and TG in serum,reduce the number of lipid droplets in liver pathological sections,and improve lipid metabolism.FBS and FINS contents were significantly reduced and blood glucose metabolism was restored in rats.HOMA-IR index was significantly decreased,ISI and HOMA-β index were significantly increased,and insulin resistance status of rats was alleviated.(4)The results of MetS rat liver showed that PPPs significantly up regulated Ins R,IRS-1,PPARγ,AKT,GLUT2,GLUT4 and GSK-3β mRNA expression in insulin signaling pathway.The protein expression of Ins R,p-IRS-1,PPARγ,PI3 K,p-AKT,GLUT2,GLUT4,p-GSK-3β were significantly increased.The results showed that PPPs can improve the sensitivity of MetS rat liver to insulin,reduce the glucose content and increase the glycogen content in liver tissue,so as to reduce the blood glucose content and alleviate the IR state in MetS rats.(5)The results of MetS rat muscle tissue showed that the action mode of PPPs in improving the IR of MetS rat skeletal muscle tissue was basically consistent with that of liver tissue.PPPs significantly up regulated Ins R,IRS-1,PPARγ,PI3 K,GLUT1 and GLUT4 mRNA expression in insulin signaling pathway.The protein expression of Ins R,p-IRS-1,PPARγ,PI3 K,p-AKT,GLUT1,GLUT4 and p-GSK-3β were significantly increased.The results showed that PPPs can improve the sensitivity of MetS rat muslce to insulin,reduce the glucose content and increase the glycogen content in liver tissue,so as to alleviate the IR state in MetS rats.(6)In vivo,the results of PPPs control group showed that gavage of 300 mg/kg BW/d PPPs had no significant effect on body weight,liver function,blood lipid and blood glucose,and no significant effect on histopathological sections of liver tissue and skeletal muscle.Moreover,PPPs can maintain the content of glucose and glycogen in rat liver and muscle tissue by promoting the mRNA or protein expression of IRS-1,PPARγ,PI3 K,AKT,GLUTs and GSK-3β in insulin signaling pathway,improve the transport rate of glucose in liver and muscle tissue,promote the metabolism of glucose,and maintain the homeostasis of glucose environment in rats.PPPs has certain safety in use.In conclusion,both in vitro and in vivo experiments have confirmed that pomegranate peel polyphenols can significantly improve insulin resistance.PPPs can regulate the expressions of mRNA and protein of Ins R,IRS-1,PPARγ,PI3 K,AKT,GLUTs and GSK-3β,so as to promote glucose absorption and glycogen synthesis,increase insulin sensitivity and alleviate IR status. |
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