Mechanism-based discovery of new anti-diabetic drugs from the natural products in traditional Chinese medicine

Insulin resistance is a fundamental metabolic defect of type 2 diabetes. Excessive lipid accumulation in the muscle and liver is the most common cause of insulin resistance. Hence, reducing lipid levels in these tissues can potentially reverse insulin resistance as an effective approach for the treatment of type 2 diabetes. Although there are a number of anti-diabetic drugs in clinical use, the long-term effects of the current medications are still unsatisfactory. Natural products have been recently rejuvenated as an important source for the discovery of anti-diabetic drugs due to their rich source and chemical diversity. Traditional Chinese Medicine has been used for centuries to treat diseases including diabetes. In order to identify the compounds with potential anti-diabetic properties by reducing lipid accumulation, a novel biochemical screening assay was developed using 3T3-L1 adipocytes and triglyceride content was assessed as readout of cellular lipids level. By using this assay, this thesis has screened >200 candidates selected from Traditional Chinese Medicine and identified 76 hits from >10 different classes in terms of triglyceride reduction. In the first animal study, the efficacy of albiflorin was tested in insulin resistant mice induced by high-fat cholesterol diet. Albiflorin significantly reduced triglyceride/cholesterol levels in the liver of high-fat cholesterol fed mice. Interestingly, further examinations of liver samples showed significant reduction of pro-inflammatory cytokine TNFα by albiflorin treatment. These results suggest albiflorin may have potential therapeutics for hepatic steatosis and associated liver metabolic conditions. To further test the effects of albiflorin on insulin resistance, this thesis examined its efficacy in a more established insulin resistant mice model induced by high-fat diet. Albiflorin significantly improved glucose intolerance in insulin resistant mice. However, further studies are required to confirm its therapeutic potential for insulin resistance. In the second animal study, this thesis investigated the effects oleanolic acid (OA) on hyperglycaemia in type 2 diabetic mice model. OA-treated mice effectively reduced hyperlipidaemia and reversed hyperglycaemia with liver as major target tissue. Moreover, this anti-hyperglycaemia effect was sustained beyond the treatment period. My subsequent studies focused on the mechanisms for the persistent effective maintenance of the corrected hyperglycaemia. By comparing the changes of key regulators in liver during and post-OA administration, this thesis found that the OA-induced changes in FoxO1, G6Pase, HAT1 and HDACs persisted in post-OA treatment where the increased phosphorylation of AMPK, SIRT1 and the reduced liver triglyceride had subsided. Further studies indicated that the anti-hyperglycaemic effects observed in post-OA treatment may result from persistent acetylation of FoxO1 to suppress hepatic gluconeogenic pathway. In summary, the results of my PhD project indicate using targeted screening approach for the intracellular lipid-lowering efficacy is an effective phenotypic screening tool, which is capable of identifying potential anti-obese and anti-diabetic compounds via different metabolisms. The present study highlights that mechanism-based discovery of compounds derived from our unique natural products library in Traditional Chinese Medicine have potential for the treatment of type 2 diabetes by reversing lipid-induced insulin resistance