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.
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.
Casas
Sílvia
S
Endocrinology and Diabetes Unit, Laboratory of Experimental Diabetes, Institut d'Investigacions Biomèdiques August Pi i Sunyer, Hospital Clinic and University of Barcelona, Barcelona, Spain.
Gomis
Ramon
R
Gribble
Fiona M
FM
Altirriba
Jordi
J
Knuutila
Sakari
S
Novials
Anna
A
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*
D000682:Amyloid; D058228:Islet Amyloid Polypeptide; D025801:Ubiquitin; D012313:RNA; D046988:Proteasome Endopeptidase Complex; D002118:Calcium