Induction of Activator Protein-1 through Reactive Oxygen Species by Crystalline Silica in JB6 Cells

We reported previously that freshly fractured silica (FFSi) induces activator protein-1 (AP-1) activation through extracellular signal-regulated protein kinases (ERKs) and p38 kinase pathways. In the present study, the biologic activities of FFSi and aged silica (ASi) were compared by measuring their effects on the AP-1 activation and phosphorylation of ERKs and p38 kinase. The roles of reactive oxygen species (ROS) in this silica-induced AP-1 activation were also investigated. We found that FFSi-induced AP-1 activation was four times higher than that of ASi in JB6 cells. FFSi also caused greater phosphorylation of ERKs and p38 kinase than ASi. FFSi generated more ROS than ASi when incubated with the cells as measured by electron spin resonance (ESR). Studies using ROS-sensitive dyes and oxygen consumption support the conclusion that ROS are generated by silica-treated cells. N-Acetylcysteine (an anti-oxidant) and polyvinyl pyridine-N-oxide (an agent that binds to Si-OH groups on silica surfaces) decreased AP-1 activation and phosphorylation of ERKs and p38 kinase. Catalase inhibited phosphorylation of ERKs and p38 kinase, as well as AP-1 activation induced by FFSi, suggesting the involvement of H ₂O₂ in the mechanism of silica-induced AP-1 activation. Sodium formate (an OH scavenger) had no influence on silica-induced MAPKs or AP-1 activation. Superoxide dismutase enhanced both AP-1 and MAPKs activation, indicating that H₂O₂, but not O₂^.-, may play a critical role in silica-induced AP-1 activation. These studies indicate that freshly ground silica is more biologically active than aged silica and that ROS, in particular H₂O₂, play a significant role in silica-induced AP-1 activation.