title: Regulation of autophagy in human skeletal muscle: effects of exercise, exercise training and insulin stimulation.
abstract: Regulation of autophagy in human muscle in many aspects differs from the majority of previous reports based on studies in cell systems and rodent muscle. An acute bout of exercise and insulin stimulation reduce human muscle autophagosome content. An acute bout of exercise regulates autophagy by a local contraction-induced mechanism. Exercise training increases the capacity for formation of autophagosomes in human muscle. AMPK activation during exercise seems insufficient to regulate autophagosome content in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin. Studies in rodent muscle suggest that autophagy is regulated by acute exercise, exercise training and insulin stimulation. However, little is known about the regulation of autophagy in human skeletal muscle. Here we investigate the autophagic response to acute one-legged exercise, one-legged exercise training and subsequent insulin stimulation in exercised and non-exercised human muscle. Acute one-legged exercise decreased (P<0.01) lipidation of microtubule-associated protein 1A/1B-light chain 3 (LC3) (∼ 50%) and the LC3-II/LC3-I ratio (∼ 60%) indicating that content of autophagosomes decreases with exercise in human muscle. The decrease in LC3-II/LC3-I ratio did not correlate with activation of 5'AMP activated protein kinase (AMPK) trimer complexes in human muscle. Consistently, pharmacological AMPK activation with 5-aminoimidazole-4-carboxamide riboside (AICAR) in mouse muscle did not affect the LC3-II/LC3-I ratio. Four hours after exercise, insulin further reduced (P<0.01) the LC3-II/LC3-I ratio (∼ 80%) in muscle of the exercised and non-exercised leg in humans. This coincided with increased Ser-757 phosphorylation of Unc51 like kinase 1 (ULK1), which is suggested as a mammalian target of rapamycin complex 1 (mTORC1) target. Accordingly, inhibition of mTOR signalling in mouse muscle prevented the ability of insulin to reduce the LC3-II/LC3-I ratio. In response to 3 weeks of one-legged exercise training, the LC3-II/LC3-I ratio decreased (P<0.05) in both trained and untrained muscle and this change was largely driven by an increase in LC3-I content. Taken together, acute exercise and insulin stimulation reduce muscle autophagosome content, while exercise training may increase the capacity for formation of autophagosomes in muscle. Moreover, AMPK activation during exercise may not be sufficient to regulate autophagy in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin.
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Andreas M
authors.initials: AM
authors.lastname: Fritzen
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Agnete B
authors.initials: AB
authors.lastname: Madsen
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Maximilian
authors.initials: M
authors.lastname: Kleinert
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.|The Novo Nordisk Foundation Center for Basic Metabolic Research, Section of Integrative Physiology, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Jonas T
authors.initials: JT
authors.lastname: Treebak
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Lundsgaard
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Jensen
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Erik A
authors.initials: EA
authors.lastname: Richter
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Jørgen
authors.initials: J
authors.lastname: Wojtaszewski
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Bente
authors.initials: B
authors.lastname: Kiens
authors.affiliation: Section of Molecular Physiology, Department of Nutrition, Exercise and Sports, the August Krogh Centre, Faculty of Science, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Christian
authors.initials: C
authors.lastname: Frøsig
mesh_terms: D055372:AMP-Activated Protein Kinases / Q000502:physiology; D000328:Adult; D000818:Animals; D001343:Autophagy / Q000502:physiology*; D002478:Cells, Cultured; D015444:Exercise / Q000502:physiology*; D005260:Female; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D008297:Male; D008810:Mice, Inbred C57BL; D018485:Muscle Fibers, Skeletal / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D017208:Rats, Wistar; D055815:Young Adult
chemical_list: D007328:Insulin; D055372:AMP-Activated Protein Kinases