Soluble iron modulates iron oxide particle-induced inflammatory responses via prostaglandin E2 synthesis: In vitro and in vivo studies

Abstract

Ambient particulate matter (PM)-associated metals have been shown to play an important role in cardiopulmonary health outcomes. To study the modulation of PM-induced inflammation by leached off metals, we investigated intracellular solubility of radio-labeled iron oxide (⁵⁹Fe₂O₃) particles of 0.5 and 1.5 μm geometric mean diameter. Fe₂O₃ particles were examined for the induction of the release of interleukin 6 (IL-6) as pro-inflammatory and prostaglandin E₂ (PGE₂) as anti-inflammatory markers in cultured alveolar macrophages (AM) from Wistar Kyoto (WKY) rats. In addition, we exposed male WKY rats to monodispersed Fe₂O₃ particles by intratracheal instillation (1.3 or 4.0 mg/kg body weight) to examine in vivo inflammation. Particles of both sizes are insoluble extracellularly in the media but moderately soluble in AM with an intracellular dissolution rate of 0.0037 ± 0.0014 d^-1 for 0.5 μm and 0.0016 ± 0.0012 d^-1 for 1.5 μm ⁵⁹Fe₂O₃ particles. AM exposed in vitro to 1.5 μm particles (10 μg/mL) for 24 h increased IL-6 release (1.8-fold; p < 0.05) and also PGE₂ synthesis (1.9-fold; p < 0.01). By contrast, 0.5 μm particles did not enhance IL-6 release but strongly increased PGE₂ synthesis (2.5-fold, p < 0.005). Inhibition of PGE₂ synthesis by indomethacin caused a pro-inflammatory phenotype as noted by increased IL-6 release from AM exposed to 0.5 μm particles (up to 3-fold; p < 0.005). In the rat lungs, 1.5 but not 0.5 μm particles (4.0 mg/kg) induced neutrophil influx and increased vascular permeability. Fe₂O₃ particle-induced neutrophilic inflammatory response in vivo and pro-inflammatory cytokine release in vitro might be modulated by intracellular soluble iron via PGE₂ synthesis. The suppressive effect of intracellular released soluble iron on particle-induced inflammation has implications on how ambient PM-associated but soluble metals influence pulmonary toxicity of ambient PM.