Stine Ringholm Jørgensen:
PGC-1alpha in exercise- and exercise training-induced metabolic adaptations

Date: 01-05-2013    Supervisor: Henriette Pilegaard

The aim of the present thesis was to investigate the hypotheses that 1) bed rest reduces metabolic and angiogenic proteins and changes microRNA (miRNA) content as well as alters exercise-induced mRNA responses in human skeletal muscle, 2) Peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α is required for exercise-, exercise training- and fasting-induced mRNA and protein responses, respectively, of metabolic, angiogenic and gluconeogenic proteins in liver and adipose tissue in mice, 3) PGC-1α is required for both exercise training and resveratrol mediated prevention of age-associated decreases in oxidative and angiogenic proteins in mouse skeletal muscle.

Study I demonstrates that only 7 days of bed rest reduced leg muscle mass, mitochondrial enzyme activities, mitochondrial (mt)DNA/nuclear (n)DNA content, protein content of oxidative proteins and miRNA content in human skeletal muscle. Furthermore the physical inactivity abolished the exercise-induced mRNA response of PGC-1α and vascular endothelial growth factor (VEGF) in skeletal muscle that was present before bed rest. This indicates that just 7 days of physical inactivity reduces the metabolic capacity of human skeletal muscle and interferes with the exercise-induced adaptive response in human skeletal muscle.

Study II demonstrates that mouse liver glucose-6-phosphatase (G6Pase) mRNA content increased in recovery from acute exercise in both wildtype (WT) and PGC-1α knockout (KO) mice, while phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate carboxylase mRNA content did not change in either genotype. Exercise training increased PEPCK protein content in both WT and PGC-1α KO mice. In addition, the mRNA and protein content of cytochrome (Cyt) c and cytochrome c oxidase (COX) subunit I increased in response to acute exercise and exercise training, respectively, in WT mice while there was no change in PGC-1α KO mice. Furthermore, fasting increased G6Pase and PEPCK mRNA content in both WT and PGC-1α KO mice. This implies that exercise- and exercise training-induced improvements in hepatic oxidative capacity, but not regulation of gluconeogenesis, requires PGC-1α, while fasting-induced regulation of gluconeogenic capacity does not require PGC-1α.

Study III demonstrates the time course of an exercise-induced uncoupling protein (UCP)1 mRNA response in WT mouse epididymal (eWAT) and inguinal (iWAT) white adipose tissue, and that these responses were absent in PGC-1α KO mice. Furthermore, while UCP1 protein content was undetectable in eWAT, exercise training increased UCP1 protein content in iWAT of WT, but not PGC-1α KO mice. This training response was associated with increased COX subunit IV and cluster of differentiation (CD)31 protein content in WT, but not in PGC-1α KO mice. This shows that exercise training increases UCP1, COXIV and CD31 protein in mouse iWAT, likely as a cumulative effect of transient increases in mRNA expression after each exercise bout, and that PGC-1α is required for these adaptations.

Study IV demonstrates that citrate synthase (CS) activity and mtDNA/nDNA content decreased with age in skeletal muscle of WT mice. CS activity, mtDNA/nDNA content, pyruvate dehydrogenase-E1α and VEGF protein content increased with lifelong exercise training in WT mice but not in PGC-1α KO mice. In contrast, lifelong resveratrol supplementation had no significant effect on these proteins in either genotype. This indicates that lifelong exercise training, but not resveratrol supplementation alone increases the oxidative capacity of skeletal muscle and that combining resveratrol with exercise training does not elicit more marked improvement in oxidative capacity than exercise alone.

In conclusion, just 7 days of physical inactivity reduces metabolic capacity and abolished exercise-induced responsiveness of human skeletal muscle. PGC-1α is required for exercise-induced mRNA responses and exercise training-induced protein changes in oxidative and/or angiogenic proteins in mouse liver and adipose tissue, while fasting-induced regulation of gluconeogenic capacity does not. In addition, resveratrol supplementation seems to have minor effects on the content of oxidative and angiogenic proteins in mouse skeletal muscle compared with exercise training.