An immunohistochemical study of nitrotyrosine expression in pancreatic islets of cases with increasing duration of type 1 diabetes and without diabetes
The proinflammatory cytokines, interleukin-1β (IL-1β), tumor necrosis factor α (TNFα), and interferon γ (IFNγ), are cytotoxic to pancreatic islet β cells, possibly by inducing nitric oxide and/or oxygen radical production in the β cells. Peroxynitrite, the reaction product of nitric oxide and the superoxide radical, is a strong oxidant and cytotoxic mediator; therefore, we hypothesized that peroxynitrite might be a mediator of cytokine-induced islet β-cell destruction. To test this hypothesis we incubated islets isolated from human pancreata with the cytokine combination of IL-1β, TNFα, and IFNγ. We found that these cytokines induced significant increases in nitrotyrosine, a marker of peroxynitrite, in islet β cells, and the increase in nitrotyrosine preceded islet-cell destruction. Peroxynitrite mimicked the effects of cytokines on nitrotyrosine formation and islet β-cell destruction. L-NG-monomethyl arginine, an inhibitor of nitric oxide synthase, prevented cytokine-induced nitric oxide production but not hydrogen peroxide production, nitrotyrosine formation, or islet β-cell destruction. In contrast, guanidinoethyldisulphide, an inhibitor of inducible nitric oxide synthase and scavenger of peroxynitrite, prevented cytokine-induced nitric oxide and hydrogen peroxide production, nitrotyrosine formation, and islet β-cell destruction. These results suggest that cytokine-induced peroxynitrite formation is dependent upon increased generation of superoxide (measured as hydrogen peroxide) and that peroxynitrite is a mediator of cytokine-induced destruction of human pancreatic islet β cells.