Exendin-4 protects hypoxic islets from oxidative stress and improves islet transplantation outcome.

Oxidative stress produced during pancreatic islet isolation leads to significant ß-cell damage. Homeostatic cytokines secreted subsequently to islet transplantation damage ß-cells by generating oxygen free radicals. In this study, exendin-4, a glucagon-like peptide-1 analog improved islet transplantation outcome by increasing the survival of diabetic recipient mice from 58% to 100%. We hypothesized that this beneficial effect was due to the ability of exendin-4 to reduce oxidative stress. Further experiments showed that it significantly reduced the apoptotic rate of cultured ß-cells subjected to hypoxia or to IL-1ß. Reduction of apoptotic events was confirmed in pancreatic islet grafts of exendin-4-treated mice. Exendin-4 enhanced Akt phosphorylation of ß-cells and insulin released from them. It even augmented insulin secretion from islets cultivated at hypoxic conditions. Exposure to hypoxia led to a decrease in the activation of Akt, which was reversed when ß-cells were pretreated with exendin-4. Moreover, exendin-4 increased the activity of redox enzymes in a hypoxia-treated ß-cell line and reduced reactive oxygen species production in isolated pancreatic islets. Recovery from diabetes in mice transplanted with hypoxic islets was more efficient when they received exendin-4. In conclusion, exendin-4 rescued islets from oxidative stress caused by hypoxia or due to cytokine exposure. It improved the outcome of syngenic and xenogenic islet transplantation.

Improved intraportal islet transplantation outcome by systemic IKK-beta inhibition: NF-κB activity in pancreatic islets depends on oxygen availability.

Intraportal islet transplantation suffers from low efficiency caused by substantial islet mass loss after transplantation. How this process is regulated is still unclear. Here, we show that NF-κB activation was detectable in islet grafts shortly after transplantation of porcine islets to diabetic NMRI nu/nu mice, and systemic NF-κB inhibition in transplanted animals significantly prolonged islet graft survival. Proinflammatory cytokines alone did not cause evident cell death in pancreatic islet within 24 h, while the combination of cytokines with hypoxia resulted in a strong induction of cell death that could be blocked dose-dependently by a selective IKK-ß inhibitor. Under hypoxia, NF-κB activity impaired expression of antiapoptotic gene BCL-xL, c-FLIP and survivin. NF-κB activation in isolated islets was reduced by hypoxia in a time-dependent manner, accordingly, NF-κB activation in transplanted islets diminished by time. Our data indicate that, while NF-κB has an antiapoptotic role under normoxia, low oxygen conditions decrease its activity and transform it to a proapoptotic transcription factor in pancreatic islets. We conclude that NF-κB inhibition represents a potential strategy to improve islet transplantation efficiency.