Osteopontin protects the islets and β-cells from interleukin-1 β-mediated cytotoxicity through negative feedback regulation of nitric oxide

Osteopontin (OPN), a phosphorylated glycoprotein that binds to an integrin-binding motif, has been shown to regulate nitric oxide ( NO) production via inhibition of induced NO synthase ( iNOS) synthesis. In the transplanted islets, iNOS and toxic amounts of NO are produced as a result of islets infiltration with inflammatory cells and production of proinflammatory cytokines. Here, we demonstrate that addition of OPN before IL-1 beta in freshly isolated rat islets improved their glucose stimulated insulin secretion dose-dependently and inhibited IL-1 beta-induced NO production in an arginine-glycine-aspartate-dependent manner. Transient transfection of OPN gene in RINm5F beta-cells fully prevented the toxic effect of IL-1 beta at concentrations that reduced the viability by 50% over 3 d. OPN prevention of IL-1 beta-induced toxicity was accompanied by inhibited transcription of iNOS by 80%, resulting in 50% decreased formation of the toxic NO. In OPN-transfected cells, the IL-1 beta-induced nuclear factor-kappa B activity was significantly reduced. Islets exposed to IL-1 beta revealed a naturally occurring early up-regulated OPN transcription. OPN promoter activity was increased in the presence of IL-1 beta, IL-1 beta-induced NO, and an inducer of NO synthesis. These data suggest the presence of a cross talk between the IL-1 beta and OPN pathways and a unique trans-regulatory mechanism in which IL-1 beta-induced NO synthesis feedback regulates itself through upregulation of OPN gene transcription. Our data also suggest that influencing OPN expression represents an approach for affecting cytokine-induced signal transduction to prevent or reduce activation of the cascade of downstream devastating effects after islet transplantation.