Mechanisms of amelioration of lipid-induced insulin resistance: role of AMP-activated protein kinase

Insulin resistance is an early marker of Type II diabetes. Excessive lipid accumulation in muscle and liver leads to insulin resistance, and lowering tissue lipids causes an enhancement of insulin action.The enzyme AMP-activated protein kinase (AMPK) is activated when cellular energy levels are compromised, such as during exercise; this enhances fuel oxidation and inhibits energy consuming processes.The hypothesis in this thesis was that activating AMPK in a lipid-induced insulin resistant state leads to tissue lipid reduction and improved insulin sensitivity.Insulin resistant high-fat fed (HF-) rats were administered 5-aminoimidazole-4-carboxamide-1-ß-D-ribofuranoside (AICAR), a specific AMPK activator. During an euglycaemic hyperinsulinaemic clamp performed 24h later, HF-rats showed increased whole body, muscle and liver insulin action, independent of changes in PKB-phosphorylation. The liver had reduced triglycerides, malonyl-CoA and increased IkB-a content. A lowering of muscle malonyl-CoA was consistent with conditions favouring increased lipid utilisation. Normal, chow-fed rats also showed improved insulin action post-AICAR.Further studies showed that basal glucose uptake was not increased 24h after AICAR, suggesting that AMPK activation had caused an increase in insulin sensitivity. Diacylglycerols and triglycerides, but not ceramides, were reduced in the liver of AICAR treated HF-rats, suggesting lipid reduction as a likely mediator of enhanced liver insulin action. These lipid species were not reduced in muscle.AICAR administration to HF-rats lowered plasma glucose and fatty acids (FA) acutely, probably due to increased muscle glucose uptake and FA oxidation. Glycogen was reduced in liver and increased in muscle, suggesting glucose mobilisation from liver to muscle. Adrenergic blockade excluded the sympathetic nervous system in the acute AICAR effects. AMPK was activated in white muscle and liver of HF-rats immediately after AICAR, the same tissues that exhibited later improved insulin sensitivity. Tracer technologies used to investigate glucose and lipid fluxes showed that AMPK activation in white muscle simultaneously increased both glucose and FA uptake and their metabolism, with glucose also being stored as glycogen. The liver showed lower lipid synthesis, consistent with reduced liver lipid accumulation observed 24h post-AICAR.In conclusion, these results suggest that activation of AMPK leads to selective tissue lipid reduction and improved insulin action, and is a potential target for the treatment of insulin resistance and type II diabetes