Berberine: For Glucose-Lowering Activity And Molecular Mechanism

Blood glucose elevates in clinic before diabetic conditions and is by far one of the most important risk factors for the onset of diabetes.Lowering blood glucose significantly improves insulin sensitivity and decreases the mortality of diabetes.Insulin receptor(InsR) is a transmembrane receptor located on cell surface.Binding of insulin to InsR increases glucose uptake and glycogen synthesis.Up-regulation of InsR expression is considered to be one of the most effective means to lower blood glucose.Currently,the drugs widely-used to treat type 2 diabetes in clinic are Sulfonylureas,biguanide,α-Glucosidase Inhibitor and Thiazolidinedione. Although these drugs have good effects in blood glucose,a small portion of patients does not tolerate the adverse effects such as hepatotoxicity,hypoglycemia and edema et al,and the therapeutic effects are not satisfactory in some of the patients.Therefore,finding of new mechanism drugs to treat diabetes is highly desirable.Berberine(BBR),an alkaloid from herbs(such as Coptis chinensis),is known to lower blood glucose and has a well-defined chemical structure.It has been used in China to treat diarrhea for decades.The clinical effectiveness and safety of BBR has been widely accepted.To learn the glucose-lowering mechanism of BBR and explore the possibility of using BBR as a new drug to treat diabetes in the future,we have designed a series of experiments and the results are presented in the report.BBR up-regulates the expression of InsR mRNA in the human liver cell lines Bel-7402 and HepG2.The effect of BBR is in a time-and dose-dependent fashion.We found that InsR expression was up-regulated 4 hrs after BBR treatment and lasted for at least 24 hrs.BBR also up-regulates InsR mRNA expression in other types of human cells,such as pancreas cell SW1990,colon cell HCT116,human embryo kidney cell 293T and lymphocyte CEM.The InsR protein level on the cell surface was accordingly increased.BBR treatment didn't alter the half life of InsR mRNA.To learn if BBR enhances the transcriptional activity of InsR promoter,we used a plasmid containing the InsR promoter sequence and Luc reporter to transfect HepG2 cells,followed by BBR treatment.We found that BBR largely increased the activity of InsR gene promoter.These results indicated that BBR up-regulates InsR expression at the transcriptional level by promoting the transcriptional activity of InsR gene promoter. Glucose concentration in the culture medium was largely decreased after BBR treatment in vitro.The increase of glucose consumption by BBR is mediated through the insulin signal way,because remove of insulin in the culture circumstances or blockage of the activity of PI3K abolishes or decreases the hypoglycemic effect of BBR.BBR activates cellular PKC activity in a time- and dose-dependent manner,and increases the activity of InsR promoter through a PKC-mediated mechanism.Activation of PKC by BBR occurs before the increase of InsR expression.PKC activation is essential,because blocking PKC pathway by Calphostin C abolishes the effect of BBR on InsR up-regulation. Furthermore,PKCμ(PKD1),one of the subtypes of PKCs,was identified to be responsible for BBR's activity on InsR.We have then studied the hypoglycemic effect of BBR in hyperglycaemia Wistar rats. Treating the rats with 75 mg/kg/d and 150 mg/kg/d of BBR(orally) for 15 days reduced blood glucose and up-regulated InsR mRNA in liver and muscle by 1.8 and 2.25 folds,respectively. The insulin sensitivity increased by 2 folds and the activity of PKC in the rat livers also elevated.BBR effectively reduced blood glucose in the type 2 diabetic KK-Ay mice.The hypoglycemic effect of BBR required the presence of insulin in the blood,because the therapeutic effect of BBR vanished in the NOD/LtJ mice that were null of blood insulin.Fifty type 2 diabetic patients were then orally treated with BBR(0.5g,bid,for 2 months,). The treatment effectively lowered fasting blood glucose,haemoglobin A1c and triglyceride by 25.9%,18.1%and 17.6%,respectively.BBR also lowered blood insulin in these patients.InsR expressed on the peripheral blood lymphocytes elevated after BBR treatment.Liver function was improved.Side-effects in the BBR treated patients were not observed.In conclusion,we consider BBR to be a promising glucose-lowering agent,with good safety and a new mechanism distinctive from that of the known diabetes drugs.