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Ubiquitinated-Protein Aggregates Form in Pancreatic ß-Cells During Diabetes-Induced Oxidative Stress and Are Regulated by Autophagy

¹ Cell Biology Program, The Hospital for Sick Children, Toronto, Ontario, Canada
² Department of Physiology, University of Toronto, Ontario, Canada
³ Department of Medicine, University of Toronto, Ontario, Canada
⁴ Department of Biochemistry, University of Toronto, Ontario, Canada
⁵ Division of Cellular and Molecular Biology, Toronto General Research Institute, University Health Network, Toronto, Ontario, Canada
⁶ Department of Medical Genetics and Microbiology, University of Toronto, Ontario, Canada

Address correspondence and reprint requests to John H. Brumell, Cell Biology Program, Hospital for Sick Children, 555 University Ave., Toronto, ON, M5G 1X8 Canada. E-mail: john.brumell{at}sickkids.ca

Abbreviations: ALIS, aggresome-like induced structures; ATZ, aminotriazole; DRiP, defective ribosomal protein; ER, endoplasmic reticulum; ERAD, endoplasmic reticulum–associated degradation; 3MA, 3-methyladenine; mAb, monoclonal antibody; MTOC, microtubule organizing center; NAC, N-acetyl cysteine; Ub-product, ubiquitinated protein; UPR, unfolded protein response

Diabetes-induced oxidative stress can lead to protein misfolding and degradation by the ubiquitin-proteasome system. This study examined protein ubiquitination in pancreatic sections from Zucker diabetic fatty rats. We observed large aggregates of ubiquitinated proteins (Ub-proteins) in insulin-expressing ß-cells and surrounding acinar cells. The formation of these aggregates was also observed in INS1 832/13 ß-cells after exposure to high glucose (30 mmol/l) for 8–72 h, allowing us to further characterize this phenotype. Oxidative stress induced by aminotriazole (ATZ) was sufficient to stimulate Ub-protein aggregate formation. Furthermore, the addition of the antioxidants N-acetyl cysteine (NAC) and taurine resulted in a significant decrease in formation of Ub-protein aggregates in high glucose. Puromycin, which induces defective ribosomal product (DRiP) formation was sufficient to induce Ub-protein aggregates in INS1 832/13 cells. However, cycloheximide (which blocks translation) did not impair Ub-protein aggregate formation at high glucose levels, suggesting that long-lived proteins are targeted to these structures. Clearance of Ub-protein aggregates was observed during recovery in normal medium (11 mmol/l glucose). Despite the fact that 20S proteasome was localized to Ub-protein aggregates, epoxomicin treatment did not affect clearance, indicating that the proteasome does not degrade proteins localized to these structures. The autophagy inhibitor 3MA blocked aggregate clearance during recovery and was sufficient to induce their formation in normal medium. Together, these findings demonstrate that diabetes-induced oxidative stress induces ubiquitination and storage of proteins into cytoplasmic aggregates that do not colocalize with insulin. Autophagy, not the proteasome, plays a key role in regulating their formation and degradation. To our knowledge, this is the first demonstration that autophagy acts as a defense to cellular damage incurred during diabetes.

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