Detecting novel effects of exercise or AMPK activation in human skeletal muscle

Abstract

Cardiovascular and metabolic disorders are among the main causes of death today. Regular exercise can prevent and treat these chronic diseases. A molecule at the center of exercise adaptations in skeletal muscle is adenosine monophosphate-activated protein kinase (AMPK). Rapid energy turnover in cells, such as during contraction in skeletal muscle, activates AMPK. The activation of AMPK leads to inhibition of anabolic processes that consume energy and upregulation of catabolic processes that generate energy. AMPK activation increases glucose uptake into peripheral tissues. Even insulin-resistant individuals, including type 2 diabetes patients, retain the blood-glucose lowering effect of AMPK activation. There is a need to better understand how exercise provides protective benefits, and how AMPK functions at the cellular level. This thesis consists of three research papers wherein exercise and AMPK activation were used as experimental models to identify novel effects of these signals in human skeletal muscle.

The first paper explores how fasting between consecutive bouts of exercise from one day to the next enhances the adaptive response. Overlapping fasting with exercise increases AMPK signaling and expression of genes that regulate fat oxidation. In addition, the combination of exercise and fasting elicits changes in DNA promoter methylation. Fasting overnight between an evening and morning training session enhances the adaptive benefits of exercise.

The second paper investigated if AMPK activation and insulin signaling affect the focal adhesion kinase (FAK) in a differential manner in human skeletal muscle. FAK is a necessary component for insulin and growth signaling in non-human skeletal muscle models, and cancer researchers are exploring FAK inhibitors as cancer therapeutics. The new research provided herein demonstrates that insulin does not activate FAK in human skeletal muscle. However, AMPK does inhibit FAK. Furthermore, siRNA-mediated silencing of FAK in cells increases lipid oxidation. AMPK inhibits FAK, and FAK affects substrate utilization in skeletal muscle cells.

In the final paper, a bioinformatic analysis identified genes that are regulated by AMPK, including the gene for ganglioside-induced differentiation-associated protein 1 (GDAP1). GDAP1 regulates mitochondrial function in nerve cells. Silencing GDAP1 does not alter mitochondrial function or morphology. However, silencing GDAP1 does reduce lipid oxidation, non-mitochondrial respiration, and alters the expression of circadian genes. AMPK regulates GDAP1 expression, and GDAP1 alters lipid oxidation without affecting mitochondrial function in skeletal muscle.

The key findings of the thesis are; 1) combining fasting with exercise impacts the epigenetic state in muscle and induces adaptive changes that promote lipid oxidation, 2) AMPK-mediated FAK inhibition may be a therapeutic strategy to treat cells which have a reduced capacity to oxidize lipids, and 3) GDAP1 in skeletal muscle plays a role in modulating the core circadian clock and non-mitochondrial lipid oxidation.