A Brake on Toxicity: Iron Suppresses Organic-Induced Superoxide Generation in Macrophages

Iron (Fe) is the predominant metal in brake wear particles (BWPs) and urban particulate matter, yet its role in modulating oxidative stress toxicity remains poorly understood. Here, we show that BWPs induce slight cellular superoxide (•O2-) at low doses (<10 μg mL-1) but suppress it dose-dependently at higher doses. In contrast, highway PM2.5 neither significantly induces nor suppresses cellular •O2- generation. Using Fe alone or mixed with isoprene secondary organic aerosols or 9,10-phenanthrenequinone, typical organic compounds known to induce cellular •O2- via nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation, we demonstrate that both Fe2+ and Fe3+ actively suppress baseline and organic-induced •O2- in a dose-dependent manner. Through inhibition experiments and qRT-PCR analysis, we suggest the following mechanisms for Fe suppression on cellular •O2-: exogenous Fe replenishes the intracellular labile iron pool or complexes with organics. This alleviates organic-induced iron deficiency, reduces NADPH oxidase activity, thereby reducing •O2- release. Fe simultaneously consumes •O2- via direct chemical reactions. This work reveals iron as a suppressor rather than a promoter of organic-induced •O2- generation in macrophages, operating through restoration of iron homeostasis and direct chemical quenching. These findings underscore the importance of considering both metal and organic constituents in assessing the health impacts of complex environmental mixtures.