title: Markers of autophagy are adapted to hyperglycaemia in skeletal muscle in type 2 diabetes.
abstract: OBJECTIVE
  Autophagy is a catabolic process that maintains cellular homeostasis by degradation of protein aggregates and selective removal of damaged organelles, e.g. mitochondria (mitophagy). Insulin resistance in skeletal muscle has been linked to mitochondrial dysfunction and altered protein metabolism. Here, we investigated whether abnormalities in autophagy are present in human muscle in obesity and type 2 diabetes.
  
  
  METHODS
  Using a case-control design, skeletal muscle biopsies obtained in the basal and insulin-stimulated states from patients with type 2 diabetes during both euglycaemia and hyperglycaemia, and from glucose-tolerant lean and obese individuals during euglycaemia, were used for analysis of mRNA levels, protein abundance and phosphorylation of autophagy-related proteins.
  
  
  RESULTS
  Muscle transcript levels of autophagy-related genes (ULK1, BECN1, PIK3C3, ATG5, ATG7, ATG12, GABARAPL1, MAP1LC3B, SQSTM1, TP53INP2 and FOXO3A [also known as FOXO3]), including some specific for mitophagy (BNIP3, BNIP3L and MUL1), and protein abundance of autophagy-related gene (ATG)7 and Bcl-2/adenovirus E1B 19-kDa-interacting protein 3 (BNIP3), as well as content and phosphorylation of forkhead box O3A (FOXO3A) were similar among the groups. Insulin reduced lipidation of microtubule-associated protein light chain 3 (LC3)B-I to LC3B-II, a marker of autophagosome formation, with no effect on p62/sequestosome 1 (SQSTM1) content in muscle of lean and obese individuals. In diabetic patients, insulin action on LC3B was absent and p62/SQSTM1 content increased when studied under euglycaemia, whereas the responses of LC3B and p62/SQSTM1 to insulin were normalised during hyperglycaemia.
  
  
  CONCLUSIONS
  Our results demonstrate that the levels of autophagy-related genes and proteins in muscle are normal in obesity and type 2 diabetes. This suggests that muscle autophagy in type 2 diabetes has adapted to hyperglycaemia, which may contribute to preserve muscle mass.
authors.affiliation: The Section of Molecular Diabetes & Metabolism, Institute of Clinical Research and Institute of Molecular Medicine, University of Southern Denmark, Odense, Denmark.
authors.forename: Rikke
authors.initials: R
authors.lastname: Kruse
authors.forename: Birgitte F
authors.initials: BF
authors.lastname: Vind
authors.forename: Stine J
authors.initials: SJ
authors.lastname: Petersson
authors.forename: Jonas M
authors.initials: JM
authors.lastname: Kristensen
authors.forename: Kurt
authors.initials: K
authors.lastname: Højlund
mesh_terms: D051017:Apoptosis Regulatory Proteins / Q000378:metabolism; D001343:Autophagy; D001706:Biopsy; D016022:Case-Control Studies; D003924:Diabetes Mellitus, Type 2 / Q000276:immunology / Q000378:metabolism*; D005260:Female; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors / Q000378:metabolism; D015309:Glucose Clamp Technique; D005951:Glucose Tolerance Test; D006706:Homeostasis; D006801:Humans; D006943:Hyperglycemia / Q000276:immunology / Q000378:metabolism*; D007333:Insulin Resistance; D008297:Male; D008869:Microtubule-Associated Proteins / Q000378:metabolism; D008875:Middle Aged; D008928:Mitochondria / Q000378:metabolism; D018482:Muscle, Skeletal / Q000276:immunology / Q000378:metabolism*; D009765:Obesity / Q000276:immunology; D010766:Phosphorylation; D012333:RNA, Messenger / Q000378:metabolism; D058570:TOR Serine-Threonine Kinases / Q000378:metabolism
chemical_list: D051017:Apoptosis Regulatory Proteins; C499517:FOXO3 protein, human; D000071316:Forkhead Box Protein O3; D051858:Forkhead Transcription Factors; D008869:Microtubule-Associated Proteins; D012333:RNA, Messenger; C546842:MTOR protein, human; D058570:TOR Serine-Threonine Kinases