Reg3α Overexpression Protects Pancreatic ß Cells from Cytokine-Induced Damage and Improves Islet Transplant Outcome.

The process of islet transplantation for treating type 1 diabetes has been limited by the high level of graft failure. This may be overcome by locally delivering trophic factors to enhance engraftment. Regenerating islet-derived protein 3α (Reg3α) is a pancreatic secretory protein which functions as an antimicrobial peptide in control of inflammation and cell proliferation. In this study, to investigate whether Reg3α could improve islet engraftment, a marginal mass of syngeneic islets pretransduced with adenoviruses expressing Reg3α or control EGFP were transplanted under the renal capsule of streptozotocin-induced diabetic mice. Mice receiving islets with elevated Reg3α production exhibited significantly lower blood glucose levels (9.057 ± 0.59 mmol/L versus 13.48 ± 0.35 mmol/L, P < 0.05) and improved glucose-stimulated insulin secretion (1.80 ± 0.17 ng/mL versus 1.16 ± 0.16 ng/mL, P < 0.05) compared with the control group. The decline of apoptotic events (0.57% ± 0.15% versus 1.06% ± 0.07%, P < 0.05) and increased ß-cell proliferation (0.70% ± 0.10% versus 0.36% ± 0.14%, P < 0.05) were confirmed in islet grafts overexpressing Reg3α by morphometric analysis. Further experiments showed that Reg3α production dramatically protected cultured islets and pancreatic ß cells from cytokine-induced apoptosis and the impairment of glucose-stimulated insulin secretion. Moreover, exposure to cytokines led to the activation of MAPKs in pancreatic ß cells, which was reversed by Reg3α overexpression in contrast to control group. These results strongly suggest that Reg3α could enhance islet engraftments through its cytoprotective effect and advance the therapeutic efficacy of islet transplantation.

Luteolin prevents uric acid-induced pancreatic ß-cell dysfunction.

Elevated uric acid causes direct injury to pancreatic ß-cells. In this study, we examined the effects of luteolin, an important antioxidant, on uric acid-induced ß-cell dysfunction. We first evaluated the effect of luteolin on nitric oxide (NO) formation in uric acid-stimulated Min6 cells using the Griess method. Next, we performed transient transfection and reporter assays to measure transcriptional activity of nuclear factor (NF)-κB. Western blotting assays were also performed to assess the effect of luteolin on the expression of MafA and inducible NO synthase (iNOS) in uric acid-treated cells. Finally, we evaluated the effect of luteolin on uric acid-induced inhibition of glucose-stimulated insulin secretion (GSIS) in Min6 cells and freshly isolated mouse pancreatic islets. We found that luteolin significantly inhibited uric acid-induced NO production, which was well correlated with reduced expression of iNOS mRNA and protein. Furthermore, decreased activity of NF-κB was implicated in inhibition by luteolin of increased iNOS expression induced by uric acid. Besides, luteolin significantly increased MafA expression in Min6 cells exposed to uric acid, which was reversed by overexpression of iNOS. Moreover, luteolin prevented uric acid-induced inhibition of GSIS in both Min6 cells and mouse islets. In conclusion, luteolin protects pancreatic ß-cells from uric acid-induced dysfunction and may confer benefit on the protection of pancreatic ß-cells in hyperuricemia-associated diabetes.

Hyperuricemia causes pancreatic ß-cell death and dysfunction through NF-κB signaling pathway.

Accumulating clinical evidence suggests that hyperuricemia is associated with an increased risk of type 2 diabetes. However, it is still unclear whether elevated levels of uric acid can cause direct injury of pancreatic ß-cells. In this study, we examined the effects of uric acid on ß-cell viability and function. Uric acid solution or normal saline was administered intraperitoneally to mice daily for 4 weeks. Uric acid-treated mice exhibited significantly impaired glucose tolerance and lower insulin levels in response to glucose challenge than did control mice. However, there were no significant differences in insulin sensitivity between the two groups. In comparison to the islets in control mice, the islets in the uric acid-treated mice were markedly smaller in size and contained less insulin. Treatment of ß-cells in vitro with uric acid activated the NF-κB signaling pathway through IκBα phosphorylation, resulting in upregulated inducible nitric oxide synthase (iNOS) expression and excessive nitric oxide (NO) production. Uric acid treatment also increased apoptosis and downregulated Bcl-2 expression in Min6 cells. In addition, a reduction in insulin secretion under glucose challenge was observed in the uric acid-treated mouse islets. These deleterious effects of uric acid on pancreatic ß-cells were attenuated by benzbromarone, an inhibitor of uric acid transporters, NOS inhibitor L-NMMA, and Bay 11-7082, an NF-κB inhibitor. Further investigation indicated that uric acid suppressed levels of MafA protein through enhancing its degradation. Collectively, our data suggested that an elevated level of uric acid causes ß-cell injury via the NF-κB-iNOS-NO signaling axis.