Pyruvate dehydrogenase PDH constitutes the only mammalian pathway for irreversible conversion of pyruvate to acetyl-CoA thus providing the vital link between glycolytic energy production, the TCA cycle, and oxidative phosphorylation. Because the PDC controls the conversion of pyruvate it occupies a central position in relation to the control of mitochondrial energy production and cellular substrate metabolism. Suppression and activation of PDH becomes essential in situations where glucose availability and/or use changes with swift and appropriate regulation of the complex to maintain energy homeostasis in response to varying nutritional and metabolically challenging states.
The aim of the present thesis was investigate the fasting- and exercise induced regulation of (PDH) to test the hypotheses that 1) Skeletal muscle IL-6 contributes to the regulation of PDH in mouse skeletal muscle at rest as well as during prolonged exercise and that this is associated with IL-6 mediated regulation of AMPK. 2) Skeletal muscle IL-6 affects short-term and prolonged fasting-induced PDH regulation and substrate utilization in mice. 3) Lack of muscle PGC-1α affects the time course of the switch in substrate utilization during the transition from the fed to the fasted state affecting regulation of fasting-induced changes in PDHa activity, PDH phosphorylation and PDH acetylation in skeletal muscle. 4) Differences in substrate utilization between trained and untrained individuals are associated with differences in adaptive changes and regulation of PDH in skeletal muscle.
Study I demonstrated that that lack of skeletal muscle IL-6 led to elevated PDHa activity, both at rest and during exercise, without significant changes in PDHa activity during prolonged exercise. IL-6 MKO mice had an overall higher RER during exercise than controls, but maintained the ability to reduce RER during prolonged exercise. Lack of muscle IL-6 did not affect AMPK activation during exercise. Together this indicates that muscle IL-6 influences substrate utilization in skeletal muscle through effects on PDH, but is not required for the shift in substrate use during prolonged exercise.
Study II demonstrated that lack of skeletal muscle IL-6 resulted in elevated resting RER in the fed state, while the RER in the fasted state was similar in IL-6 MKO and Control. Furthermore, skeletal muscle PDHa activity was higher and PDH phosphorylation lower in IL-6 MKO than Control mice in the fasted state, but knockout of muscle IL-6 did not prevent the ability to regulate skeletal muscle PDH in response to fasting. Taken together this indicates that skeletal muscle IL-6 regulates substrate utilization at rest potentially through effects on skeletal muscle PDH, whereas muscle IL-6 is not required for fasting-induced substrate switch and skeletal muscle PDH regulation in mice.
Study III demonstrated that lack of muscle PGC-1α did not affect the switch from carbohydrate to predominant fat utilization in the transition from the fed to the fasted state. Fasting-induced down-regulation of PDHa activity in skeletal muscle of control mice was associated with increased phosphorylation of all four known sites in PDH-E1α as well as with increased PDK4 and SIRT3 protein without changes in total acetylation of PDH-E1α. Lack of muscle PGC-1α reduced PDH-E1α, PDK1, 2, 4, PDP1, and SIRT3 protein content as well as increased total lysine PDH-E1α acetylation in the fed state. Knockout of muscle PGC-1α did not influence the fasting-induced increase in PDH-E1α phosphorylation, but prevented the fasting-induced increase in SIRT3 protein.
Study IV demonstrated that exercising at the same relative intensity at steady state and with a short-term increase in intensity, elicited similar PDH activation in skeletal muscle from untrained and trained subjects. Skeletal muscle PDHa activity was higher in trained than untrained humans at exhaustion providing a contributing mechanism for the augmented capacity for carbohydrate oxidation in the trained state. This reflects a well-controlled tight regulation of PDH together with the ability to rely on fat oxidation at a higher exercise intensity in the trained than untrained state. This metabolic response was associated with higher PDHE1α content, PDH phosphorylation and PDH acetylation as well as protein content of PDH regulators.
In conclusion, IL-6 appears to exert dampening effects on skeletal muscle PDHa activity, both during fasting and prolonged exercise. Furthermore, both IL-6 and PGC-1α are dispensable for maintaining short-term metabolic flexibility, as muscle IL-6 was not necessary for exercise-induced switches in substrate utilization and neither lack of skeletal muscle IL-6 or PGC-1α affected fasting–induced switch to fat oxidation. Lack of muscle PGC-1α did however blunt the fasting-induced increase in selected mitochondrial proteins. Lastly, increased oxidative capacity leads to exercise-induced skeletal muscle PDH activation that is closely matched to the relative exercise intensity at submaximal exercise, while reaching a higher level at maximal exercise in trained individuals. These responses are associated with increased PDH phosphorylation, acetylation and content of covalent regulators.