Effects of troglitazone on skeletal muscle and liver protein tyrosine phosphatase activity in insulin-resistant otsuka long-evans tokushima fatty rats

Effect of a novel proteoglycan PTP1B inhibitor from Ganoderma lucidum on the amelioration of hyperglycaemia and dyslipidaemia in db/db mice

Protein tyrosine phosphatase 1B (PTP1B) is implicated in the negative regulation of the insulin signalling pathway by dephosphorylating the insulin receptor (IR) and IR substrates. Ganoderma lucidum has traditionally been used for the treatment of diabetes in Chinese medicine; however, its anti-diabetic potency and mechanism in vivo is still unclear. Our previously published study reported a novel proteoglycan PTP1B inhibitor, named Fudan-Yueyang-Ganoderma lucidum (FYGL) from G. lucidum, with a half-maximal inhibitory concentration (IC₅₀) value of 5·12 (sem 0·05) μg/ml, a protein:polyglycan ratio of 17:77 and 78 % glucose in polysaccharide, and dominant amino acid residues of aspartic acid, glycine, glutamic acid, alanine, serine and threonine in protein. FYGL is capable of decreasing plasma glucose in streptozotocin-induced diabetic mice with a high safety of median lethal dose (LD₅₀) of 6 g/kg. In the present study, C57BL/6 db/db diabetic mice were trialed further using FYGL as well as metformin for comparison. Oral treatment with FYGL in db/db diabetic mice for 4 weeks significantly (P < 0·01 or 0·05) decreased the fasting plasma glucose level, serum insulin concentration and the homeostasis model assessment of insulin resistance. FYGL also controlled the biochemistry indices relative to type 2 diabetes-accompanied lipidaemic disorders. Pharmacology research suggests that FYGL decreases the plasma glucose level by the mechanism of inhibiting PTP1B expression and activity, consequently, regulating the tyrosine phosphorylation level of the IR β-subunit and the level of hepatic glycogen, thus resulting in the improvement of insulin sensitivity. Therefore, FYGL is promising as an insulin sensitiser for the therapy of type 2 diabetes and accompanied dyslipidaemia.

Hypoglycemic effect of Astragalus polysaccharide and its effect on PTP1B

AIM

To examine the effects of Astragalus polysaccharide (APS), a component of an aqueous extract of Astragalus membranaceus roots, on protein tyrosine phosphatase 1B (PTP1B), a negative regulator of insulin-receptor (IR) signal transduction, and its potential role in the amelioration of insulin resistance.

METHODS

Ten-week-old fat-fed streptozotocin (STZ)-treated rats, an animal model of type II diabetes mellitus (TIIDM), were treated with APS (400 mg/kg p.o.) for 5 weeks. Insulin sensitivity was identified by the insulin-tolerance test. Further analyses on the possible changes in insulin signaling occurring in skeletal muscle and liver were performed by immunoprecipitation or Western blotting. PTP1B activity was measured by an assay kit.

RESULTS

The diabetic rats responded to APS with a significant decrease in body weight, plasma glucose, and improved insulin sensitivity. The activity and expression of PTP1B were elevated in the skeletal muscle and liver of TIIDM rats. Thus the insulin signaling in target tissues was diminished. APS reduced both PTP1B protein level and activity in the muscle, but not in the liver of TIIDM rats. Insulin-induced tyrosine phosphorylation of the IR beta-subunit and insulin receptor substrate-1 (IRS-1) were increased in the muscle, but not in the liver of APS-treated TIIDM rats. There was no change in the activity or expression of PTP1B in APS-treated normal rats, and blood insulin levels did not change in TIIDM rats after treatment with APS.

CONCLUSION

APS enables insulin-sensitizing and hypoglycemic activity at least in part by decreasing the elevated expression and activity of PTP1B in the skeletal muscles of TIIDM rats.