89 - Delayed Nrf2-Regulated Antioxidant Gene Induction in Response to Exposure of Iron-Doped Silica Nanoparticles

Abstract

Studies have demonstrated that airborne nanoparticulate matter causes oxidative stress, inflammation, and induces an Nrf2-regulated adaptive response. However, the contribution of particulate components to these biologic effects and how Nrf2 signaling is activated need to be further clarified. In this study, we focused on silica and iron, two common components of airborne nanoparticles, and investigate how silica and iron-doped silica nanoparticles activated Nrf2-regulated antioxidant genes (GCLC, GCLM, HO-1 and NQO1) in human macrophages (differentiated from THP-1 monocytes). We found that expression of pro-inflammatory cytokines was significantly increased by only iron-doped silica at 6 h, while the expression of Nrf2-regulated antioxidant genes was unchanged except for GCLC, which was reduced at this time point. At 18 h however, the mRNA level of all antioxidant genes was significantly increased, indicating a delayed induction. Silica particles alone showed little effect on cytokine or antioxidant gene induction. The nuclear translocation of Nrf2 occurred later than that of NF-κB p65 protein and cytokine induction, further indicating a delayed response of Nrf2 signaling. Pretreatment of cells with NF-κB inhibitor SN50, which inhibited cytokine induction, abolished antioxidant gene induction by iron-doped silica at 18 h, suggesting that NF-κB plays a key role in the regulation of Nrf2-mediated gene expression. Consistently, SN50 also inhibited the nuclear translocation of Nrf2 caused by iron-doped silica. In conclusion, these data indicated that iron played a significant role in the biologic response to airborne nanoparticles in macrophages and that iron-doped silica induced a delayed response of Nrf2-regulated antioxidant genes, possibly through a NF-κB-cytokine-NOX-oxidants-Nrf2 pathway. Supported by ES023864 from the National Institutes of Health.