Gene expression in human skeletal muscle : effects of activity, fibre type and inheritance for diabetes

Abstract

One fundamental underlying risk factor for the development of non-insulin dependent (type 2) diabetes mellitus is insulin resistance, which 80-90% of all type 2 diabetic patients suffer from. Risk factors for insulin resistance are for example obesity, surgery, severe illness, spinal cord injury, and pregnancy. Too high energy intake and physical inactivity are direct causes of the increasing prevalence of overweight and obesity. Exercise prevents or delays the onset of type 2 diabetes. A series of beneficial effects of exercise are seen e.g. increased muscle mass, increased oxygen uptake, increased insulin sensitivity and glucose transport. Skeletal muscle represents 55% of the total body weight and is the major organ for insulinstimulated glucose uptake. The increased uptake of glucose during exercise is mediated via alternative, insulin-independent mechanisms e.g. through activated protein kinase (AMPK).

A cohort of healthy glucose tolerant first-degree relatives of type 2 diabetic patients was studied to determine whether early metabolic and molecular defects contribute to insulin resistance in skeletal muscle. Skeletal muscle strips from relatives and control subjects were incubated in vitro in the absence or presence of increasing concentrations of insulin. Glucose transport, Akt phosphorylation, AS 160 phosphorylation, and GLUT4 expression were assessed. Insulin-stimulated glucose transport at a maximal insulin concentration was reduced in the relatives. Insulin increased Akt and AS 160 phosphorylation in a dose-dependent manner, with similar responses between the groups. Gene expression of some of the key transcriptional factors and co-regulators of mitochondrial biogenesis PGC-1alpha, PGC-1beta, PPARdelta, NRF-1 and UCP-3 was assessed. All these genes showed similar expression when corrected to either Pactin or GAPDH, indicating that disturbances in mitochondrial function do not precede development of insulin resistance and type 2 diabetes. PPARalpha, PPARdelta, PGC-1alpha, PGC-1beta, calcineurin Aalpha and calcineurin Abeta have also been implicated in the regulation of skeletal muscle fibre type distribution, as well as mitochondrial biogenesis. mRNA levels of these genes was determined in skeletal muscle biopsies from elite athletes with a significantly (P<0.05) higher portion of type I fibres compared to sedentary controls. mRNA expression of these genes was also assessed in spinal cord injured subjects who have lost almost all of their type 1 fibres. PPARalpha, PPARdelta, PGC-1alpha, PGC-1beta mRNA correlated to fibre types in skeletal muscle, whereas calcineurin mRNA expression was unrelated.

Finally, gene expression pattern of AMPK alpha, beta and gamma isoforms in skeletal muscle from ablebodied subjects and spinal cord injured individuals was established. The effect of an eight week training study was determined on gene expression in spinal cord injured subjects after cycling seven days a week. Expression of AMPK also differed depending on time since injury.

In summary, this thesis work has shown that exercise promotes a more oxidative phenotype in skeletal muscle, with increased glucose uptake. This is also seen in skeletal muscle that has been unused for several years. Exercised muscle, with a more oxidative phenotype has increased expression of several genes important for mitochondrial biogenesis and function. No alteration in mRNA expression of these genes is noted in skeletal muscle from subjects with a genetic predisposition for metabolic disease prior to onset of clinical symptoms.