Osteopontin Protects the Islets and β-Cells from Interleukin-1 β-Mediated Cytotoxicity through Negative Feedback Regulation of Nitric Oxide

Abstract

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-1β in freshly isolated rat islets improved their glucose stimulated insulin secretion dose-dependently and inhibited IL-1β-induced NO production in an arginine-glycine-aspartate-dependent manner. Transient transfection of OPN gene in RINm5F β-cells fully prevented the toxic effect of IL-1β at concentrations that reduced the viability by 50% over 3 d. OPN prevention of IL-1β-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-1β-induced nuclear factor-κB activity was significantly reduced. Islets exposed to IL-1β revealed a naturally occurring early up-regulated OPN transcription. OPN promoter activity was increased in the presence of IL-1β, IL-1β-induced NO, and an inducer of NO synthesis. These data suggest the presence of a cross talk between the IL-1β and OPN pathways and a unique trans-regulatory mechanism in which IL-1β-induced NO synthesis feedback regulates itself through up-regulation 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.