IRON AND REACTIVE OXYGEN SPECIES IN THE ASBESTOS-INDUCED TUMOR-NECROSIS-FACTOR-ALPHA RESPONSE FROM ALVEOLAR MACROPHAGES

Free radicals and other reactive oxygen species (ROS) are important me diators in asbestos-induced lung toxicity. Asbestos fibers are thought to stimulate cells to generate ROS via iron that is present on fibrou s silicates. The pathophysiologic responses in the lung after asbestos exposure are characterized by the accumulation of macrophages at the site of fiber deposition and the release of growth factors and proinfl ammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha). W e have examined the role of iron-catalyzed ROS in asbestos induction o f TNF-alpha from rat alveolar macrophages. Treatment of alveolar macro phage cultures with asbestos stimulated dose-dependently TNF-alpha, se cretion, which was inhibited by the addition of deferoxamine, an iron chelator. Asbestos fibers, pretreated with deferoxamine to remove iron from the fibers before addition to alveolar macrophages, also signifi cantly reduced the TNF-alpha response. Consistent with the role of iro n on asbestos fibers in catalyzing hydroxyl radical generation, membra ne-permeable hydroxyl radical scavengers (tetramethylthiourea, dimethy l sulfoxide) inhibited the asbestos-induced TNF-alpha response. The as bestos-induced increase in TNF-alpha, as well as in interleukin-1 alph a, and their inhibition by tetramethylthiourea occurred at the transcr iptional level. The role of ROS in signaling TNF-alpha stimulation was confirmed by use of free radical-generating systems (hypoxanthine-xan thine oxidase, hydrogen peroxide, glucose-glucose oxidase, or ferrous plus hydrogen peroxide). These results suggest that intracellularly ge nerated ROS can stimulate TNF-alpha in alveolar macrophages and that a sbestos-induced TNF-alpha gene expression and secretion are mediated b y iron-catalyzed production of ROS.