GFP-SUMO or untransfected MIN6 cells showed basal expression of Chop, whereas cells expressing GFP-tagged misfolded insulin mutant C96Y (Akita) showed enhanced Chop gene expression (29). This indicates that ER stress is unlikely the reason for attenuated insulin gene expression in MIN6 cells expressing GFP-SUMO. Partial Knock Down of Ubc-9 Results http://www.selleckchem.com/products/ABT-888.html in Improved Insulin Secretion Next, we tested whether downregulation of the SUMO pathway by partial knock down of the SUMO-conjugating enzyme Ubc-9 improves agonist-induced insulin secretion in mouse islets. Islet cells cultured in 5 or 16 mM glucose were transduced with retroviral particles expressing shRNA against Ubc-9, and exendin-4-stimulated insulin secretion was quantified.
No significant change in secretion between sh-RNA-Ubc-9 transduced and control cells was observed in cells maintained in low glucose, whereas cells in high glucose expressing Ubc-9 shRNA showed significant increase in insulin secretion compared with the scramble transduced control. Thus, partial inhibition of the SUMO pathway is shown to rescue glucose-induced reduction in exendin-4-stimulated insulin secretion. DISCUSSION The incretin pathway has important pancreatic and extrapancreatic effects but is impaired in type 2 diabetes. GLP-1R gene expression is downregulated in hyperglycemia, contributing to reduced ��-cell incretin responses in a diabetic rodent model (37). In addition, islets from type 2 diabetic patients also showed diminished incretin responsiveness (13, 16, 25, 31, 33).
Even though protein kinase C has been implicated in the reduced gene expression of incretin receptors in hyperglycemia (37), other mechanisms are likely to contribute to the lack of incretin responsiveness. In this report, we show elevated mRNA expression of three isoforms of SUMO and the SUMO-conjugating enzyme Ubc-9 on exposure of pancreatic islets to high-glucose conditions. Enhanced expression of SUMO-1 leads to downregulation of GLP-1 signaling when measured as a function of cAMP generation. In addition, elevated expression of SUMO-1 also causes reduction in total insulin content and GLP-1R agonist-stimulated insulin secretion. The cellular conditions that cause an increase in sumoylation in pancreatic ��-cells have not been well studied. The SUMO conjugation/deconjugation equilibrium is modified under various cellular stress conditions in other cell types (21, 36).
SUMO-1 expression is upregulated in hypoxia, resulting in enhanced sumoylation of target proteins such as cAMP-response element-binding protein and hypoxia-inducible factor-1�� (1, 4). GSK-3 Similarly, we found that RNA transcripts for SUMO and the SUMO-conjugating enzyme Ubc-9 are upregulated in mouse islets maintained in high glucose, indicating that expression of the SUMO pathway is upregulated when islets are exposed to high-glucose concentrations.