9,10-Phenanthraquinone in diesel exhaust particles downregulates Cu,Zn-SOD and HO-1 in human pulmonary epithelial cells: Intracellular iron scavenger 1,10-phenanthroline affords protection against apoptosis

9,10-Phenanthraquinone (PQ), a major quinone contained in diesel exhaust particles and atmospheric PM2.5, undergoes one-electron reduction by flavin enzymes such as NADPH-cytochrome P450 reductase, leading to production of reactive oxygen species in vitro. We have detected an ESR signal for superoxide (O2−) and hydroxyl radicals (OH) by the spin trap method when PQ was mixed with P450 reductase, NADPH, and iron(III). When we examined the effects of PQ on A549 human pulmonary epithelial cells, PQ induced apoptosis with a LC50 of ∼7 μM. Formation of protein carbonyls was also detected in cells after treatment with PQ, suggesting that PQ induces oxidative damage. Iron chelators such as 1,10-phenanthroline (OP), desferrioxamine mesylate, and deferiprone respectively afforded protection against the toxic effects of PQ. Furthermore, treatment of A549 cells with 10-20 μM PQ for 12 h specifically down-regulated protein levels of Cu,Zn-superoxide dismutase (Cu,Zn-SOD) and heme oxygenase-1 (HO-1) by more than 50%. Pretreatment of cells with OP (10 μM) markedly reduced the down-regulation of Cu,Zn-SOD and HO-1 and protein carbonyl formation in response to PQ. The inhibitor of Cu,Zn-SOD, diethyldithiocarbamate, enhanced the toxic effects of 5 μM PQ. The present findings suggest that PQ causes iron-mediated oxidative damage that is exacerbated by the concomitant down-regulation of Cu,Zn-SOD.