Inhibitory effects of ursolic acid on hepatic polyol pathway and glucose production in streptozotocin-induced diabetic mice

Abstract

The effects of ursolic acid on the polyol pathway and glucose homeostasis–related metabolism were examined in the livers of streptozotocin (STZ)-induced diabetic mice fed a high-fat (37% calories from fat) diet for 4 weeks. Male mice were divided into nondiabetic, diabetic control, and diabetic–ursolic acid (0.05% wt/wt) groups. Diabetes was induced by the injection of STZ (200 mg/kg body weight, intraperitoneally). Although an ursolic acid supplement lowered the blood glucose level, it did not affect the plasma leptin and adiponectin levels. The present study shows that the blood glucose levels have a positive correlation with the hepatic sorbitol dehydrogenase activities ( r = 0.39, P < .05). Ursolic acid significantly inhibited sorbitol dehydrogenase activity as well as aldose reductase activity in the liver. The supplementation of ursolic acid significantly increased glucokinase activity, while decreasing glucose-6-phosphatase activity in the livers of STZ-induced diabetic mice. Ursolic acid significantly elevated the hepatic glycogen content compared with the diabetic control group. Supplementation with ursolic acid significantly lowered the plasma total cholesterol, free fatty acid, and triglyceride concentrations compared with the diabetic control group, whereas it normalized hepatic triglyceride concentration. A negative correlation was found between the hepatic triglyceride concentration and blood glucose levels ( r = −0.50, P < .01) in regard to insulin-dependent diabetic mice. The hepatic fatty acid synthase activity was significantly lower in the ursolic acid group than in the diabetic control group, whereas hepatic fatty acid β-oxidation and carnitine palmitoyltransferase activities were significantly higher. These results indicate that ursolic acid may be beneficial in preventing diabetic complications by improving the polyol pathway as well as the lipid metabolism and that it can function as a potential modulator of hepatic glucose production, which is partly mediated by up-regulating glucose utilization and glycogen storage and down-regulating glyconeogenesis in the liver.