Hypoglycemic Effects And Mechanism Of Eicosapentaenoic Acid Riched In Phosphatidylcholine On STZ-induced Diabetic Rats

Sea cucumber is an important marine food and medicinal resource which is wellknown for its plentiful bioactive components in the body wall, such aspolysaccharides, polypeptide, saponins, cerebrosides, gangliosides andEPA/DHA-phospholipid. Chemical analysis showed that the contents andcompositions of the total lipids in sea cucumber are different in various species frommultiple areas. The total lipids were approximately2.0%in the body of tropical seacucumber, in which phospholipids, cholesterol, and triglycerides account for31.5%-35.0%,0.8%-1.2%, and10.9%-20.7%, respectively. While in the sea cucumberfrom frigid regions, the total lipids were approximately13.5%, containing35.1%-41.3%phospholipids,0.9%-1.3%cholesterol, and12.2%-25.5%triglycerides. Additionally, the analysis on the compositions of the phospholipid fromthe sea cucumbers in different regions showed that the main phospholipids isphosphatidyl ethanolamine in the tropical species and it is phosphatidylcholine in thefrigid individuals. In this study, EPA-PC was extracted and purified fromApostichopus japonicus, and its hypoglycemic effects and mechanisms wereinvestigated in diabetic rats induced by intraperitoneal injection of streptozotocin(STZ). The main results are shown as follows.There was70.2%PC in the total lipid isolated from the body wall Apostichopusjaponicus, and EPA was30.1%in PC. When treated with EPA-CP for8weeks,EPA-PC could remarkably reduce the fasting blood glucose level(P<0.05), improveoral glucose tolerance(P<0.05), decrease the contents of glycosylated hemoglobin andglycosylated serum protein(P<0.01), promote the secretion of insulin and itsinsulinogenic index or β-cell function index (P<0.01), and increase the contents ofhepatic and skeletal muscle glycogen(P<0.01). The microstructure andsubmicroscopic structure of islets of langerhans in diabetic rats were observed byusing optical microscope and transmission electron microscope, respectively. Results showed that EPA-PC could alleviate the injury of diabetic rats induced by STZ, andthe morphology of islets in the dosage group renewed to that in the normal controlgroup, in which connective tissue and basement membrane were legibled. By usingtransmission electron microscope, the β-cells with structural rules nucleus, obviousnucleolus, smooth nuclear membranes, luxuriant zymogen granule, and so forth wereobserved. These indicated that EPA-PC could significantly reduce blood glucose levelvia repairing theβ-cells, promoting insulin secretion, and increasing glycogensynthesis diabetic rats.To investigate the mechanism of EPA-PC on hypoglycemic effects, the presentstudy analysed the mRNA and protein expression levels of the pivotal genes inPI3K/PKB glucose uptake pathway and glycogen synthesis pathway mediated byinsulin. Results showed that EPA-PC could not only significantly increase the mRNAand protein expression levels of InsR、IRS-1/2、PI3K and Akt2in liver, skeletalmuscle, and adipose tissues, but also enhance the phosphorylation of InsR-βTyr612、PI3K-P85and Akt-Ser473. GLUT4is the key protein in the glucose translocation,which can promote glucose transposition, uptake, and utilization. GSK-3β is therate-limiting enzyme, which high expression can lower the downstream gene GSactivities and inhibit glycogen synthesis. The experiment showed that EAP-PC fromsea cucumber could remarkably up-regulate the mRNA and protein expression levelof GLUT4in skeletal muscle and adipose tissues, and simultaneously dramaticallyinhibit the mRNA and protein expression level of GSK-3β and promote GS mRNAexpression in liver and skeletal muscle. These results indicated that EPA-PC from seacucumber could increase the capability of glucose uptake and glycogen synthesis viaregulating PI3K/PKB signal pathway mediated by insulin in diabetic rats.Hyperglycemia is the primary pathogenesis of diabetic nephropathy, whichcauses the serious injoury of glomerulus and renal tubule by stimulated advancedglycosylation end products. Therefore, the current study further investigated the effetsof EPA-PC on protecting kidney in diabetic rats. The results showed that EPA-PCcould remarkably decrease the level of24h urine volume and renal index（P<0.01, P<0.05）, reduce the concentration of BUN and Cr in plasma and urine（P<0.05，P<0.01）, and lower the excretion of total protein, mAlb, UA, and NAG. PPAR-γ, TβRⅡ, and CTGF are the main cytokines mediated kidney fibrosis in the early stage ofdiabetic nephropathy, and PPAR-γ can inhibite tissue proliferation. Moreover, thechanges of kidney morphology in diabetic rats were observed by optical mieroscope,while mRNA expression levels of TGF-β1, TβRⅡ, CTGF and PPARγ were assessedby RT-PCR. Results showed that EPA-PC could signifivantly inhibite the glomerularhypertrophy, down-regulate the mRNA expression levels of TGF-β1, TβRⅡandCTGF, and up-regulate PPARγ expression level (P<0.01).To the best of our knowledge, the present study systematically investigated thehypoglycemic and kidney protection effects of EPA-PC from sea cucumber, anddiscussed their mechanisms, respectively. The study provides theoretical foundationand scientific evidence for the high-value utilization of low-value sea cucumber andapplication of EPA-PC form sea cucumber on the medical health and functional foodterritories.