Molecular mechanisms governing contraction-induced metabolic responses and skeletal muscle reprogramming

Abstract

Physical exercise enhances skeletal muscle responsiveness to insulin and regulates metabolism by an insulin-independent mechanism. Elucidation of contraction-mediated molecular mechanisms is imperative for a better understanding of skeletal muscle metabolism and function, and may lead to the identification or validation of possible drug targets for the prevention or treatment of metabolic disorders. This thesis focuses on the role of AMPK and Interleukin (IL)-6 in skeletal muscle metabolism, because AMPK activity and skeletal muscle IL-6 release are increased during skeletal muscle contraction.

Contraction-mediated AMPK activity in white glycolytic extensor digitorum longus (EDL) muscle was inversely coupled to skeletal muscle glycogen content in wild-type and transgenic mice expressing a mutated form of the AMPKgamma3 isoform (Tg-Prkag3225Q), but not AMPKgamma3 knockout (KO) mice, highlighting a role for the AMPKgamma3 isoform in energy sensing during muscle contraction. Isolated skeletal muscle from Tg-Prkag3225Q and AMPKgamma3 KO mice were fatigue-resistant and fatigue-prone, respectively; and work performance was coupled to glycogen content in all mouse models, highlighting a role for AMPKgamma3 in skeletal muscle ergogenics by controlling glycogen levels. Hypoxia-mediated glucose transport was partly reduced in skeletal muscle from AMPKgamma3 KO mice. AICAR and contraction-mediated phosphorylation of the Akt substrate, AS160, was dependent on functional AMPK signaling, providing direct genetic evidence that AS160 is a phosphorylation target of AMPK.

IL-6 is released from contracting human skeletal muscle. We provide evidence that IL-6 release is greater from oxidative, compared to glycolytic skeletal muscle. Basal IL-6 release was increased from oxidative soleus muscle of AMPKá1 KO and AMPKá2 kinasedead mice. Thus, we provide evidence for a role of AMPK in the basal regulation of IL-6 release from isolated oxidative skeletal muscle. Furthermore, AICAR-mediated suppression of basal IL-6 mRNA production and release was independent of functional AMPK signaling. Autocrine mechanisms may play a role in basal IL-6 release from isolated skeletal muscle.

IL-6 concentrations in the contracting skeletal muscle may exceed serum levels. We therefore investigated the direct effect of IL-6 on human skeletal muscle by exposing primary human skeletal muscle cells and isolated human skeletal muscle strips to IL-6. IL-6-exposure directly influences glucose metabolism, as determined by increased glucose transport and glucose incorporation into glycogen. In primary human skeletal muscle cells, IL-6-exposure activated components of the canonical insulin signaling cascade. Glucose incorporation into glycogen was sensitive to phosphatidylinositol (PI) 3-kinase inhibition. In contrast, IL-6-exposure was without effect on insulin signaling in isolated human skeletal muscle, and increased glucose metabolism was observed, concomitant with a trend for increased phosphorylation of AMPK. In primary human muscle cells, the IL-6-mediated enhancement of fatty acid oxidation was attenuated by silencing AMPKá isoforms. Long-term IL-6-exposure of primary myotubes enhanced growth and differentiation and increased the expression of genes involved in muscle metabolism.

In conclusion, this thesis work provides evidence that AMPK and IL-6 are central players in the regulation of contraction-mediated effects on metabolic responses in skeletal muscle. In addition to the involvement in acute regulation of metabolism, IL-6 may also participate in skeletal muscle adaptation to exercise.