Iron uptake promotes hyperoxic injury to alveolar macrophages

Iron uptake by cells may increase the intracellular pool of prooxidant iron prior to storage of iron within ferritin. Because hyperoxia is toxic to al veolar macrophages (AM) via mechanisms involving oxidant stress, we hypothe sized that iron uptake by AM might promote hyperoxia-induced injury. To ass ess this hypothesis, we cultured AM recovered from healthy volunteers under conditions of normoxia or hyperoxia (60% or 95% oxygen) in media of varyin g iron content, including control media (3 mu M iron) and media supplemente d with iron (FeCl3; total iron 10, 20, or 40 mu M). AM injury was assessed by measuring release of lactate dehydrogenase (LDH), phagocytic activity fo r yeast, and cytosolic concentrations of calcium ([Ca2+](i)) as determined by ratio image analysis of AM loaded with the fluorescent calcium probe ind o-1. There was dose-dependent accumulation of iron and ferritin synthesis i n AM exposed to iron-supplemented media. Exposure of AM to hyperoxia (60% a nd 95% oxygen, 18 h) in control media increased LDH release and impaired ph agocytic activity for yeast; however, similar hyperoxic exposures in iron-s upplemented media significantly increased the cells' LDH release and decrea sed phagocytosis, Exposure to 95% oxygen increased the [Ca2+](i) of AM over 18 h, but similar exposure in iron-supplemented media induced greater incr eases in [Ca2+](i). As compared with exposure to normoxia, exposure to hype roxia (60% and 95% oxygen) also decreased iron uptake and, to a greater ext ent, ferritin synthesis by AM in iron-supplemented media. These data sugges t that: (1) iron uptake promotes hyperoxic injury to AM; and (2) hyperoxia impairs the capacity of AM to sequester iron in ferritin.