Iron-mediated lipid peroxidation and lipid raft disruption in low-dose silica-induced macrophage cytokine production
Silica inhalation can induce respiratory disease. Iron is suspected of playing an important role in silica-mediated respiratory toxicity, but unambiguously determining its role has been hampered by incomplete characterization, use of high particle doses and lack of understanding of pro-inflammatory mechanisms. In this study, we investigated a novel hypothesis for the mechanism of the silica particle-induced increase in cytokine production. We studied the role of iron in lipid peroxidation-dependent transcription of cytokines in macrophages by ground natural silica particles at low sub-lethal doses. Particle size, size distribution, surface area and structure were determined using electron microscopy, nitrogen adsorption and x-ray diffraction. Iron impurity concentrations before and after acid treatment was determined by energy-dispersive x-ray and inductively coupled plasma mass spectroscopies. At a low non-cytotoxic dose (1 μg/ml) of 2 μm silica, the presence of iron significantly increased superoxide (O2.-), lipid peroxidation, lipid raft disruption and cytokine production in macrophages. Iron chelators, deferoxamine mesylate and diethylenetriaminepentaacetic acid were found to abrogate O2.- production and inhibit lipid peroxidation, raft disruption and cytokine induction. Tricychodecan-9-yl-xanthate, a competitive inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), which is an upstream participant in NF-κB activation, and manganese (III) tetrakis (N-ethylpyridinium-2-yl) porphyrin, a superoxide dismutase and catalase mimic, blocked silica-stimulated cytokine production. We propose a pathway of iron-induced lipid peroxidation disrupting lipid rafts and signaling for the production of cytokines through PC-PLC in silica-exposed macrophages.