Diabetes, obesity and exercise in skeletal muscle : effects on gene expression and DNA methylation

Sammanfattning: Type 2 diabetes, obesity and depression are growing concerns for human health. Physical exercise is a known protective factor against these disorders, although the underlying mechanisms are incompletely understood. The studies in this thesis aim to increase the understanding of mechanisms controlling gene expression and DNA methylation in the context of type 2 diabetes, obesity and exercise. TWIST1 and TWIST2 proteins play an important role in embryonic muscle development, inflammation and tumor metabolism. We demonstrated that Twist1 or Twist2 overexpression in mature skeletal muscle favors glycolysis and increases the expression of pro-inflammatory cytokines. Gene expression of TWIST1 and TWIST2 is unaltered by obesity, type 2 diabetes or exercise training. Decreased circulating kynurenine levels are associated with resistance to depression. Kynurenine is transformed into kynurenic acid by kynurenine aminotransferases (KATs). Exercise training and PGC1α induce expression of KATs in skeletal muscle. We report that a single bout of exercise acutely decreased plasma kynurenine, while concomitantly increasing kynurenic acid in both type 2 diabetic and healthy subjects. Exercise-induced changes in kynurenine metabolism were independent of mRNA expression of the KATs. Kynurenine levels correlated with body mass index, suggesting kynurenine metabolism may link obesity and depression. Exercise and diet affect skeletal muscle insulin sensitivity and DNA methylation. Using genome-wide approaches, we unraveled the effect of exercise on the skeletal muscle methylome. Training and high-fat diet, but not in vitro contraction, lead to epigenetic changes in the promoter of Sprouty RTK Signaling Antagonist 1 (Spry1), a gene involved in muscle stem cell quiescence. We found DNA methylation of Spry1 increased binding of nuclear proteins to the promoter. Insulin is a metabolic and growth promoting hormone. Using genome-wide approaches, we unraveled the effect of insulin on the skeletal muscle methylome. We observed that insulin treatment of skeletal muscle in vitro increased DNA methylation of the death-associated protein Kinase 3 (DAPK3). Conversely, DAPK3 DNA methylation was reduced in type 2 diabetic subjects compared to controls. A glucose challenge further decreased DAPK3 methylation suggesting that additional factors in the systemic milieu may affect DAPK3 DNA methylation. Collectively, our results indicate that TWIST proteins affect skeletal muscle metabolism and inflammation. We provide a potential mechanism for the anti-depressive effects of exercise and shed new light on the complex interplay between metabolic conditions, skeletal muscle and DNA methylation. We provide a new insight in the protective effect of exercise or the pathophysiology of type 2 diabetes and obesity, opening opportunities for improvements in the management and treatment of metabolic diseases.

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