Anti-diabetic effects of oleanolic acid and matrine

摘要

Type 2 diabetes (T2D) is a metabolic disease, which has emerged as “a major threat to global development”. There is an urgent need for the development of novel drugs for this metabolic disease, as many current anti-diabetic therapeutics have treatment-limiting side effects. While the target-based high throughput screenings of large synthetic compound libraries (an approach widely used in the past three decades) dramatically increase the number of screening candidates and capacity, the pharmaceutical industry is facing a high attrition rate and unimproved new-drug output despite an increasing commitment of resources. In view of these challenges, the project presented in this thesis utilised an in vivo phenotypic screening to identify two potential compounds for the treatment of T2D, with the consideration of their oral availability and potential of chemical modification for derivative generations. Oleanolic acid (OA) and matrine (Mtr) were shown to reduce blood glucose, and plasma and/or liver triglyceride levels in high-fat streptozotocin-induced T2D mice, and were selected for further investigations. The chronic efficacy of OA on hyperglycaemia was further investigated in high-fat streptozotocin-induced T2D mice. OA administration reversed the hyperglycaemia by ~60%. Interestingly, after the cessation of OA administration, the reversed hyperglycaemia was sustained for the entire post-treatment period of the study (four weeks) despite the reoccurrence of dyslipidaemia. Examinations of insulin secretion and pancreas morphology, urine glucose loss, food intake and glucose uptake in muscle and adipose tissues, indicated that they were unlikely to be the underlying mechanism(s) of the sustained anti-hyperglycaemic effect of OA. Significant increases in the phosphorylation levels of both Akt and FoxO1 were observed in the liver of OA-treated mice. Importantly, these increases were significantly correlated with a down-regulation of glucose-6-phosphatase expression. These findings suggested that OA reversed the hyperglycaemia possibly by the suppression of hepatic glucose production via the Akt/FoxO1 axis in the liver. The therapeutic efficacy of Mtr (a safe drug for hepatic chronic viral infections and tumours) for insulin resistance and hepatic steatosis was investigated in high fat-fed mice in comparison with a commonly used anti-diabetic drug, metformin. The results showed that Mtr reduced glucose intolerance and plasma insulin level, hepatic triglyceride content and adiposity without affecting caloric intake. The reduced hepatosteatosis was attributed to a suppressed lipid synthesis pathway and increased fatty acid oxidation. Mtr neither suppressed mitochondrial respiration nor activated of AMPK in the liver. A computational docking simulation revealed HSP90 (a negative regulator of HSP72) as a potential binding target of Mtr. Consistent with the simulation results, Mtr increased the hepatic protein level of HSP72, which inversely correlated with both liver triglyceride level and glucose intolerance. These results suggested that Mtr may be used for the treatment of T2D and hepatic steatosis, and its mode of action might involve an activation of HSP72 in the liver. In summary, the results from this thesis support the notion that phenotypic screenings of natural products in combination with data mining of biological and chemical data are a viable approach for the discovery of new promising compounds for drug development.