Inhibitory effects of epicatechin on interleukin-1β-induced inducible nitric oxide synthase expression in RINm5F cells and rat pancreatic islets by down-regulation of NF-κB activation

Cytokines that are released by infiltrating inflammatory cells around the pancreatic islets are involved in the pathogenesis of type 1 diabetes mellitus. Specifically, interleukin-1β (IL-1β) stimulates inducible nitric oxide synthase (iNOS) expression and nitric oxide overproduction, leading to the β-cell damage. In activating this pathway, nuclear factor-κB (NF-κB) plays a crucial role, and many of the IL-1β-sensitive genes contain NF-κB binding sites in their promoter regions. We have recently shown that epicatechin, which is a flavonoid, had a protective effect on pancreatic β-cells in both streptozotocin-treated rats and islets. In the present study, the effects of epicatechin on IL-1β-induced β-cell damage were examined. RINm5F cells and islets were pretreated with epicatechin and next incubated with IL-1β. The released nitrite, iNOS protein and mRNA expression levels were then measured. IκBα protein, nuclear translocation of NF-κB, and NF-κB DNA binding activity were also determined. Following the transient transfection of an iNOS promoter into the cells, the iNOS promoter activity was measured. ATP- or d-glucose-induced insulin release was measured in RINm5F cells and islets, respectively. Epicatechin significantly reduced IL-1β-induced nitrite production, iNOS protein and mRNA expressions, and it also inhibited IL-1β-induced IκBα protein degradation, NF-κB activation, and iNOS promoter activity. Epicatechin partly restored the IL-1β-induced inhibition of insulin release. These results suggest that epicatechin inhibits the IL-1β-induced iNOS expression by down-regulating NF-κB activation, and protecting β-cells from IL-1β.