title: β-cell dysfunctional ERAD/ubiquitin/proteasome system in type 2 diabetes mediated by islet amyloid polypeptide-induced UCH-L1 deficiency.
abstract: OBJECTIVE
  The islet in type 2 diabetes is characterized by β-cell apoptosis, β-cell endoplasmic reticulum stress, and islet amyloid deposits derived from islet amyloid polypeptide (IAPP). Toxic oligomers of IAPP form intracellularly in β-cells in humans with type 2 diabetes, suggesting impaired clearance of misfolded proteins. In this study, we investigated whether human-IAPP (h-IAPP) disrupts the endoplasmic reticulum-associated degradation/ubiquitin/proteasome system.
  
  
  METHODS
  We used pancreatic tissue from humans with and without type 2 diabetes, isolated islets from h-IAPP transgenic rats, isolated human islets, and INS 832/13 cells transduced with adenoviruses expressing either h-IAPP or a comparable expression of rodent-IAPP. Immunofluorescence and Western blotting were used to detect polyubiquitinated proteins and ubiquitin carboxyl-terminal hydrolase L1 (UCH-L1) protein levels. Proteasome activity was measured in isolated rat and human islets. UCH-L1 was knocked down by small-interfering RNA in INS 832/13 cells and apoptosis was evaluated.
  
  
  RESULTS
  We report accumulation of polyubiquinated proteins and UCH-L1 deficiency in β-cells of humans with type 2 diabetes. These findings were reproduced by expression of oligomeric h-IAPP but not soluble rat-IAPP. Downregulation of UCH-L1 expression and activity to reproduce that caused by h-IAPP in β-cells induced endoplasmic reticulum stress leading to apoptosis.
  
  
  CONCLUSIONS
  Our results indicate that defective protein degradation in β-cells in type 2 diabetes can, at least in part, be attributed to misfolded h-IAPP leading to UCH-L1 deficiency, which in turn further compromises β-cell viability.
authors.affiliation: Larry Hillblom Islet Research Center, University of California, Los Angeles, USA. scostes@mednet.ucla.edu
authors.forename: Safia
authors.initials: S
authors.lastname: Costes
authors.forename: Chang-jiang
authors.initials: CJ
authors.lastname: Huang
authors.forename: Tatyana
authors.initials: T
authors.lastname: Gurlo
authors.forename: Marie
authors.initials: M
authors.lastname: Daval
authors.forename: Aleksey V
authors.initials: AV
authors.lastname: Matveyenko
authors.forename: Robert A
authors.initials: RA
authors.lastname: Rizza
authors.forename: Alexandra E
authors.initials: AE
authors.lastname: Butler
authors.forename: Peter C
authors.initials: PC
authors.lastname: Butler
mesh_terms: D000818:Animals; D001344:Autopsy; D003924:Diabetes Mellitus, Type 2 / Q000188:drug therapy / Q000235:genetics* / Q000378:metabolism; D006801:Humans; D007004:Hypoglycemic Agents / Q000627:therapeutic use; D007328:Insulin / Q000627:therapeutic use; D050417:Insulin-Secreting Cells / Q000187:drug effects / Q000473:pathology / Q000502:physiology*; D058228:Islet Amyloid Polypeptide / Q000235:genetics / Q000494:pharmacology*; D009687:Nuclear Proteins / Q000187:drug effects / Q000378:metabolism; D009765:Obesity / Q000150:complications / Q000473:pathology; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism*; D051381:Rats; D055647:Rats, Transgenic; D025801:Ubiquitin / Q000187:drug effects / Q000378:metabolism*; D043222:Ubiquitin Thiolesterase / Q000172:deficiency* / Q000187:drug effects
chemical_list: D007004:Hypoglycemic Agents; D007328:Insulin; D058228:Islet Amyloid Polypeptide; D009687:Nuclear Proteins; C482860:UCHL1 protein, human; D025801:Ubiquitin; D043222:Ubiquitin Thiolesterase; D046988:Proteasome Endopeptidase Complex