title: Fasting increases human skeletal muscle net phenylalanine release and this is associated with decreased mTOR signaling.
abstract: OBJECTIVE
  Fasting is characterised by profound changes in energy metabolism including progressive loss of body proteins. The underlying mechanisms are however unknown and we therefore determined the effects of a 72-hour-fast on human skeletal muscle protein metabolism and activation of mammalian target of rapamycin (mTOR), a key regulator of cell growth.
  
  
  METHODS
  Eight healthy male volunteers were studied twice: in the postabsorptive state and following 72 hours of fasting. Regional muscle amino acid kinetics was measured in the forearm using amino acid tracers. Signaling to protein synthesis and breakdown were assessed in skeletal muscle biopsies obtained during non-insulin and insulin stimulated conditions on both examination days.
  
  
  RESULTS
  Fasting significantly increased forearm net phenylalanine release and tended to decrease phenylalanine rate of disappearance. mTOR phosphorylation was decreased by ∼50% following fasting, together with reduced downstream phosphorylation of 4EBP1, ULK1 and rpS6. In addition, the insulin stimulated increase in mTOR and rpS6 phosphorylation was significantly reduced after fasting indicating insulin resistance in this part of the signaling pathway. Autophagy initiation is in part regulated by mTOR through ULK1 and fasting increased expression of the autophagic marker LC3B-II by ∼30%. p62 is degraded during autophagy but was increased by ∼10% during fasting making interpretation of autophagic flux problematic. MAFbx and MURF1 ubiquitin ligases remained unaltered after fasting indicating no change in protesomal protein degradation.
  
  
  CONCLUSIONS
  Our results show that during fasting increased net phenylalanine release in skeletal muscle is associated to reduced mTOR activation and concomitant decreased downstream signaling to cell growth.
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Mikkel Holm
authors.initials: MH
authors.lastname: Vendelbo
authors.affiliation: Research Laboratory for Biochemical Pathology, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Andreas Buch
authors.initials: AB
authors.lastname: Møller
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Britt
authors.initials: B
authors.lastname: Christensen
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Birgitte
authors.initials: B
authors.lastname: Nellemann
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Aarhus, Denmark; Research Laboratory for Biochemical Pathology, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Berthil Frederik Forrest
authors.initials: BF
authors.lastname: Clasen
authors.affiliation: Division of Endocrinology, Endocrine Research Unit, Mayo Clinic, Rochester, Minnesota, United States of America.
authors.forename: K Sreekumaran
authors.initials: KS
authors.lastname: Nair
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Jens Otto Lunde
authors.initials: JO
authors.lastname: Jørgensen
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Aarhus, Denmark; Research Laboratory for Biochemical Pathology, Aarhus University Hospital, Aarhus, Denmark; Department of Molecular Medicine, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Niels
authors.initials: N
authors.lastname: Jessen
authors.affiliation: Department of Endocrinology and Internal Medicine, Aarhus University Hospital, Aarhus, Denmark.
authors.forename: Niels
authors.initials: N
authors.lastname: Møller
mesh_terms: D000596:Amino Acids / Q000097:blood; D000704:Analysis of Variance; D015153:Blotting, Western; D018592:Cross-Over Studies; D004734:Energy Metabolism / Q000502:physiology*; D005215:Fasting / Q000502:physiology*; D005230:Fatty Acids, Nonesterified / Q000097:blood; D008401:Gas Chromatography-Mass Spectrometry; D005934:Glucagon / Q000097:blood; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000378:metabolism*; D010649:Phenylalanine / Q000378:metabolism*; D011863:Radioimmunoassay; D015398:Signal Transduction / Q000502:physiology*; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism
chemical_list: D000596:Amino Acids; D005230:Fatty Acids, Nonesterified; D014443:Tyrosine; D010649:Phenylalanine; D005934:Glucagon; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases