title: Impairment of the ubiquitin-proteasome pathway is a downstream endoplasmic reticulum stress response induced by extracellular human islet amyloid polypeptide and contributes to pancreatic beta-cell apoptosis.
abstract: OBJECTIVE
  Human islet amyloid polypeptide (hIAPP) aggregation plays a major role in the development of islet amyloidosis in type 2 diabetes. It is known that extracellular hIAPP oligomers are toxic to pancreatic beta-cells and associated with apoptosis. We therefore investigated the molecular mechanism by which extracellular hIAPP mediates pancreatic beta-cell apoptosis.
  
  
  METHODS
  MIN6 cells and primary cultures of human pancreatic islets were treated with freshly dissolved hIAPP peptide. Morphology of the cultures was evaluated by electron microscopy. Gene expression was analyzed by microarray, RT-PCR, and immunoblot. Calcium levels were measured in fura-2-loaded cells. Apoptosis was quantified by cytometry.
  
  
  RESULTS
  Increased expression of several heat shock proteins and activation of the spliced form of XBP-1, a transcription factor for overexpression of chaperones during endoplasmic reticulum (ER) stress, were detected together with morphological evidence of ER dysfunction. Intracellular calcium overload was detected in association with this process. Moreover, reduction in the proteasome activity, which was detected over time, contributed to the intracellular accumulation of ubiquitinated proteins, leading to a functional suppression of the ubiquitin-proteasome pathway. In addition, impairment of the proteasome function contributed to apoptosis, while, despite the presence of hIAPP, cell viability improved when a proteasome activator was overexpressed. The key cytotoxic events induced by extracellular hIAPP were also observed in treated human islets.
  
  
  CONCLUSIONS
  Our data suggest that ER stress responses are intracellular signaling mechanisms induced by extracellular hIAPP aggregation and that impairment of the ubiquitin-proteasome pathway is implicated in ER stress-mediated pancreatic beta-cell apoptosis.
authors.affiliation: Endocrinology and Diabetes Unit, Laboratory of Experimental Diabetes, Institut d'Investigacions Biomèdiques August Pi i Sunyer, Hospital Clinic and University of Barcelona, Barcelona, Spain.
authors.forename: Sílvia
authors.initials: S
authors.lastname: Casas
authors.forename: Ramon
authors.initials: R
authors.lastname: Gomis
authors.forename: Fiona M
authors.initials: FM
authors.lastname: Gribble
authors.forename: Jordi
authors.initials: J
authors.lastname: Altirriba
authors.forename: Sakari
authors.initials: S
authors.lastname: Knuutila
authors.forename: Anna
authors.initials: A
authors.lastname: Novials
mesh_terms: D000328:Adult; D000682:Amyloid / Q000502:physiology*; D002102:Cadaver; D002118:Calcium / Q000502:physiology; D002460:Cell Line; D002470:Cell Survival / Q000502:physiology; D003924:Diabetes Mellitus, Type 2 / Q000473:pathology; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000502:physiology*; D005786:Gene Expression Regulation; D006801:Humans; D050417:Insulin-Secreting Cells / Q000502:physiology*; D058228:Islet Amyloid Polypeptide; D007515:Islets of Langerhans / Q000473:pathology / Q000648:ultrastructure; D008875:Middle Aged; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D012313:RNA / Q000235:genetics / Q000302:isolation & purification; D013312:Stress, Physiological; D014019:Tissue Donors; D025801:Ubiquitin / Q000378:metabolism*
chemical_list: D000682:Amyloid; D058228:Islet Amyloid Polypeptide; D025801:Ubiquitin; D012313:RNA; D046988:Proteasome Endopeptidase Complex; D002118:Calcium