Therapeutic interventions for lipid-induced insulin resistance in skeletal muscle: mechanisms of action
Creators: name: Lessard, Sarah
Publisher: name: RMIT University
Year: 2024

description: It has long been known that in addition to disruptions in glucose homeostasis, individuals with insulin resistance have a breakdown in lipid dynamics, often manifested by elevated levels of circulating fatty acids (FA) together with accumulation of lipids in insulin-sensitive tissues, including skeletal muscle. However, little is known about how common therapies used to treat insulin resistant individuals (such as Rosiglitazone and exercise training) improve skeletal muscle lipid and glucose metabolism. Thus, the primary aim of the studies undertaken for this thesis was to enhance our understanding of the mechanisms by which Rosiglitazone and exercise training improve skeletal muscle lipid metabolism and insulin sensitivity in two distinct models of insulin resistance. The first investigation determined the effect of chronic Rosiglitazone treatment on the accumulation of lipid metabolites and enzymatic regulators of lipid metabolism in the skeletal muscle of obese Zucker rats. The observation that Rosiglitazone treatment exacerbated the accumulation of muscle ceramide and diacylglycerol in skeletal muscle, while improving glucose tolerance led to the conclusion that this insulin sensitising drug improves insulin sensitivity by mechanisms other than reduction of fatty acid metabolites in this tissue. Accordingly, the second investigation sought to identify an alternative mechanism by which Rosiglitazone treatment may improve skeletal muscle insulin sensitivity. It was found that Rosiglitazone restored AMP-activated protein kinase (AMPK) á2 activity in the skeletal muscle of obese Zucker rats, providing a potential peroxisome proliferator activated receptor (PPAR) ã-independent mechanism by which this drug may mediate its insulinsensitising actions. The final experiment undertaken for this thesis determined the independent and interactive effects on Rosiglitazone and exercise training on various aspects of skeletal muscle glucose and lipid metabolism in a model of diet-induced insulin resistance, the high-fat fed rat. Exercise training, but not Rosiglitazone treatment restored skeletal muscle insulin sensitivity in high-fat fed rats. Improvements in insulin sensitivity with exercise training were associated with increased FA oxidation, increased AMPK activity and a normalisation of the expression of the Akt substrate, AS160. In contrast, Rosiglitazone treatment was associated with increased FA uptake and decreased insulin-stimulated glucose uptake in skeletal muscle. Exercise prevented the accumulation of skeletal muscle lipids in Rosiglitazone-treated animals when the two treatments were combined. In summary, the results from the studies undertaken for this thesis provide novel information regarding the mechanisms by which two insulinsensitising therapies, exercise training and Rosiglitazone treatment, act to improve glucose and lipid metabolism in skeletal muscle.It has long been known that in addition to disruptions in glucose homeostasis, individuals with insulin resistance have a breakdown in lipid dynamics, often manifested by elevated levels of circulating fatty acids (FA) together with accumulation of lipids in insulin-sensitive tissues, including skeletal muscle. However, little is known about how common therapies used to treat insulin resistant individuals (such as Rosiglitazone and exercise training) improve skeletal muscle lipid and glucose metabolism. Thus, the primary aim of the studies undertaken for this thesis was to enhance our understanding of the mechanisms by which Rosiglitazone and exercise training improve skeletal muscle lipid metabolism and insulin sensitivity in two distinct models of insulin resistance. The first investigation determined the effect of chronic Rosiglitazone treatment on the accumulation of lipid metabolites and enzymatic regulators of lipid metabolism in the skeletal muscle of obese Zucker rats. The observation that Rosiglitazone treatment exacerbated the accumulation of muscle ceramide and diacylglycerol in skeletal muscle, while improving glucose tolerance led to the conclusion that this insulin sensitising drug improves insulin sensitivity by mechanisms other than reduction of fatty acid metabolites in this tissue. Accordingly, the second investigation sought to identify an alternative mechanism by which Rosiglitazone treatment may improve skeletal muscle insulin sensitivity. It was found that Rosiglitazone restored AMP-activated protein kinase (AMPK) á2 activity in the skeletal muscle of obese Zucker rats, providing a potential peroxisome proliferator activated receptor (PPAR) ã-independent mechanism by which this drug may mediate its insulinsensitising actions. The final experiment undertaken for this thesis determined the independent and interactive effects on Rosiglitazone and exercise training on various aspects of skeletal muscle glucose and lipid metabolism in a model of diet-induced insulin resistance, the high-fat fed rat. Exercise training, but not Rosiglitazone treatment restored skeletal muscle insulin sensitivity in high-fat fed rats. Improvements in insulin sensitivity with exercise training were associated with increased FA oxidation, increased AMPK activity and a normalisation of the expression of the Akt substrate, AS160. In contrast, Rosiglitazone treatment was associated with increased FA uptake and decreased insulin-stimulated glucose uptake in skeletal muscle. Exercise prevented the accumulation of skeletal muscle lipids in Rosiglitazone-treated animals when the two treatments were combined. In summary, the results from the studies undertaken for this thesis provide novel information regarding the mechanisms by which two insulinsensitising therapies, exercise training and Rosiglitazone treatment, act to improve glucose and lipid metabolism in skeletal muscle.