title: Mitochondrial Respiration in Insulin-Producing β-Cells: General Characteristics and Adaptive Effects of Hypoxia.
abstract: OBJECTIVE
  To provide novel insights on mitochondrial respiration in β-cells and the adaptive effects of hypoxia.
  
  
  METHODS
  Insulin-producing INS-1 832/13 cells were exposed to 18 hours of hypoxia followed by 20-22 hours re-oxygenation. Mitochondrial respiration was measured by high-resolution respirometry in both intact and permeabilized cells, in the latter after establishing three functional substrate-uncoupler-inhibitor titration (SUIT) protocols. Concomitant measurements included proteins of mitochondrial complexes (Western blotting), ATP and insulin secretion.
  
  
  RESULTS
  Intact cells exhibited a high degree of intrinsic uncoupling, comprising about 50% of oxygen consumption in the basal respiratory state. Hypoxia followed by re-oxygenation increased maximal overall respiration. Exploratory experiments in peremabilized cells could not show induction of respiration by malate or pyruvate as reducing substrates, thus glutamate and succinate were used as mitochondrial substrates in SUIT protocols. Permeabilized cells displayed a high capacity for oxidative phosphorylation for both complex I- and II-linked substrates in relation to maximum capacity of electron transfer. Previous hypoxia decreased phosphorylation control of complex I-linked respiration, but not in complex II-linked respiration. Coupling control ratios showed increased coupling efficiency for both complex I- and II-linked substrates in hypoxia-exposed cells. Respiratory rates overall were increased. Also previous hypoxia increased proteins of mitochondrial complexes I and II (Western blotting) in INS-1 cells as well as in rat and human islets. Mitochondrial effects were accompanied by unchanged levels of ATP, increased basal and preserved glucose-induced insulin secretion.
  
  
  CONCLUSIONS
  Exposure of INS-1 832/13 cells to hypoxia, followed by a re-oxygenation period increases substrate-stimulated respiratory capacity and coupling efficiency. Such effects are accompanied by up-regulation of mitochondrial complexes also in pancreatic islets, highlighting adaptive capacities of possible importance in an islet transplantation setting. Results also indicate idiosyncrasies of β-cells that do not respire in response to a standard inclusion of malate in SUIT protocols.
authors.affiliation: Department of Cancer Research and Molecular Medicine, Faculty of Medicine, Norwegian University of Science and Technology (NTNU), Trondheim, Norway.
authors.forename: Ingrid K
authors.initials: IK
authors.lastname: Hals
authors.affiliation: Department of Cancer Research and Molecular Medicine, Faculty of Medicine, Norwegian University of Science and Technology (NTNU), Trondheim, Norway.
authors.forename: Simon Gustafson
authors.initials: SG
authors.lastname: Bruerberg
authors.affiliation: Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Zuheng
authors.initials: Z
authors.lastname: Ma
authors.affiliation: Department of Transplantation Medicine and Institute for Surgical Research, Oslo University Hospital, Oslo, Norway.
authors.forename: Hanne
authors.initials: H
authors.lastname: Scholz
authors.affiliation: Department of Molecular Medicine and Surgery, Karolinska Institutet, Stockholm, Sweden.
authors.forename: Anneli
authors.initials: A
authors.lastname: Björklund
authors.affiliation: Department of Cancer Research and Molecular Medicine, Faculty of Medicine, Norwegian University of Science and Technology (NTNU), Trondheim, Norway; Department of Endocrinology, St Olav University Hospital, Trondheim, Norway.
authors.forename: Valdemar
authors.initials: V
authors.lastname: Grill
mesh_terms: D000220:Adaptation, Biological; D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D016923:Cell Death; D002460:Cell Line; D049109:Cell Proliferation; D019069:Cell Respiration; D042967:Electron Transport Complex I / Q000378:metabolism; D042963:Electron Transport Complex II / Q000378:metabolism; D006801:Humans; D000860:Hypoxia / Q000378:metabolism; D007328:Insulin / Q000378:metabolism; D050417:Insulin-Secreting Cells / Q000378:metabolism*; D008293:Malates / Q000378:metabolism; D008928:Mitochondria / Q000378:metabolism*; D010101:Oxygen Consumption; D010539:Permeability; D051381:Rats; D013379:Substrate Specificity
chemical_list: D007328:Insulin; D008293:Malates; C030298:malic acid; D000255:Adenosine Triphosphate; D042963:Electron Transport Complex II; D042967:Electron Transport Complex I