Iron sequestration by macrophages decreases the potential for extracellular hydroxyl radical formation

Alveolar macrophages (AM) from smokers contain a much higher quantity of intracellular iron than AM from nonsmokers. Since some forms of iron will catalyze the formation of hydroxyl radical (.OH) from superoxide and hydrogen peroxide, the ability of AM derived from smokers and nonsmokers to generate .OH was assessed. No detectable .OH was produced by AM from either source, suggesting that iron sequestration by AM may limit the potential for .OH-mediated lung injury. Consistent with this hypothesis, the ability of bronchoalveolar lavage fluid (BAL) from smokers and nonsmokers to act as an .OH catalyst decreased after exposure to AM. We found that, like AM, human monocyte-derived macrophages (MDM) have the ability to acquire large quantities of iron from small low molecular weight iron chelates as well as decrease the ability of BAL to act as a .OH catalyst. When MDM or AM were exposed to the iron chelates or BAL they were then able to generate .OH after phorbol myristate acetate stimulation. However, when acutely iron-loaded or BAL-exposed MDM were placed in culture, their ability to produce .OH decreased with time to the level of non-iron-exposed controls. This process correlated with iron translocation from the plasma membrane to the cytosol as well as a 3-9-fold increase in cellular ferritin. No increase in antioxidant enzyme levels or induction of the heat shock response was observed. Iron sequestration by macrophages may protect nearby cells from exposure to potentially cytotoxic iron-catalyzed oxidants such as .OH.