Hypoglycemic Effect And Mechanism Of Honokiol On Streptozotocin-induced Type2Diabetic Mice

Diabetic mellitus (DM) is recognized as a group of metabolic disorders andcharacterized by hyperglycemia and dyslipidaemia. Type1diabetes is characterizedby a lack of insulin production and type2diabetes results from the body’s ineffectiveuse of insulin, Insulin resistance in its metabolic target tissues. Type3diabetes isgestational diabetes. Type2diabetes mellitus (T2DM) is the most common form andaccounts for around90%of all diabetes worldwide. Insulin binding evokes a cascadeof phosphorylation events, beginning with the autophosphorylation of the IR onmultiple tyrosyl residues. Autophosphorylation enhances IR kinase activity and leadsto recruitment of insulin receptor substrate (IRS) proteins, followed by activation ofphosphatidylinositol3-kinase (PI3K) and downstream protein kinase AKT,subsequent translocation of the glucose transporter GLUT4and regulation of glucoseuptake. Therefore, Improving tissue sensitivity to insulin is a major clinical goal tohelp ameliorate abnormal metabolism.Protein tyrosine phosphatases (PTPs) constitute a large and structurally diversefamily of signaling enzymes that control the cellular levels of protein tyrosinephosphorylation and play an important role in the intracellular signal transductionprocess and metabolism. The protein tyrosine phosphatase1B (PTP1B) is implicatedas a negative regulators of the insulin action pathway, blocking the insulin-stimulatedtyrosine phosphorylation of the insulin receptor (IR) and thereby the insulin receptorsubstrate (IRS). Thus, developing efficient inhibitors against PTP1B is considered as atarget for type2diabetes.The principal substantial compounds in Chinese medicine Magnolia bark(Magnolia officinalis) are a variety of phenolic compounds and terpenoids. Magnoliaextract was isolated from Magnolia bark and its main component were identifiedincluding magnolol and honokiol, magnolol:honokiol=1.5-2.5:1(w/w). Honokiol andmagnolol, isomers of hydroxylated biphenolic compounds (C18H18O2, MW=266.33),are the major bioactive constituents of the Magnolia bark. In preliminary study, magnolia extract showed a efficient PTP1B inhibitory potency in vitro. Furthermore,db/db diabetic mice treated orally by0.5g/kg magnolia extract for5weeks showed anobvious decrease in fasting plasma glucose level compared with the diabetic controlmice without drug treatment. This study further screened active ingredient inhibitingPTP1B from magnolia extract. We found that honokiol showed a efficient PTP1Binhibitory potency (IC50=63.43±1.07μM) in vitro. In addition, honokiol showed amore efficient inhibitory potency against PTP1B than other PTPs in vitro. Kineticanalysis indicated that honokiol was reversible competitive inhibitors of PTP1B andthe inhibitor constant (Ki) was6.67μM. Hence, honokiol was selected to furtherinvestigate the hypoglycemic potency and mechanism in the following experimentalsection.In our further study, male Chinese Kunming mice (i. e. Mus musculus),6-8weeks old and18-22g weight were selected to induce experimengtal type2diabeticmice model. The mice were fed with a high-fat diet for4weeks and then wereintraperitoneally injected with STZ (35mg/kg). After2weeks, high-fat diet andSTZ-induced mice whose fasting glucose levels showed≥11.1mmol/L, the totalcholesterol (TC) levels in serum significantly increased, but triglyceride (TG) levelsdid not increased met the standard of type2diabetic characteristics. Ten ChineseKunming mice mice were set as normal group and thirty STZ-induced type2diabeticmice were divided into three groups with10mice in each group:200mg/kg honokiolgroup,200mg/kg metformin group and diabetes control group. The treatment of thediabetic mice with200mg/kg honokiol for8weeks showed a significant decrease infasting blood glucose level and amelioration in body weight disorder (13.14±1.32mmol/L, p <0.0001) compared with diabetic control mice without drug (33.06±0.24mmol/L). Based on the above experimental phenomenon, we investigated thehypoglycemic mechanism of honokiol. The tyrosine phosphorylation levels of the IRβ-subunit and ERK1/2significantly increased in the liver, skeletal muscles andadipose tissue of the200mg/kg honokiol-treated mice as well as the expression levelof PTP1B in skeletal muscles significantly decreased, compared with those of diabetic control mice. We speculate that the hypoglycemic mechanisms of honokiol may becaused by inhibition of the PTP1B activity, consequently, activating the signalingpathway, which also provide us a theoretical basis for honokiol on type2diabetictherapy.