β-cell dysfunctional ERAD/ubiquitin/proteasome system in type 2 diabetes mediated by islet amyloid polypeptide-induced UCH-L1 deficiency.
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.
Costes
Safia
S
Larry Hillblom Islet Research Center, University of California, Los Angeles, USA. scostes@mednet.ucla.edu
Huang
Chang-jiang
CJ
Gurlo
Tatyana
T
Daval
Marie
M
Matveyenko
Aleksey V
AV
Rizza
Robert A
RA
Butler
Alexandra E
AE
Butler
Peter C
PC
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
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