Asbestos causes apoptosis in alveolar epithelial cells: Role of iron-induced free radicals

Asbestos causes asbestosis and malignancies by mechanisms that are not full y understood. Alveolar epithelial cell (AEC) injury by iron-induced reactiv e oxygen species (ROS) is one important mechanism. To determine whether asb estos causes apoptosis in AECs. we exposed WI-26 (human type I-like cells), A549 (human type II-like cells), and rat alveolar type II cells to amosite asbestos and assessed apoptosis by terminal deoxynucleotidyl transferase-m ediated deoxyuridine-5'-triphosphate-biotin nick end labeling (TUNEL) stain ing, nuclear morphology, annexin V staining, DNA nucleosome formation, and caspase 3 activation. In contrast to control medium and TiO2, amosite asbes tos and H2O2 each caused AEC apoptosis. A role for iron-catalyzed ROS was s uggested by the finding that asbestos-induced AEC apoptosis and caspase 3 a ctivation were each attenuated by either an iron chelator (phytic acid and deferoxamine) or a OH scavenger (dimethylthiourea, salicylate, and sodium b enzoate) but not by iron-loaded phytic acid. To determine whether asbestos causes apoptosis in vivo, rats received a single intratracheal instillation of amosite (5 mg) or normal saline solution, and apoptosis in epithelial c ells in the bronchoalveolar duct regions was assessed by TUNEL staining. On e week after exposure, amosite asbestos caused a 3-fold increase in the per centage of apoptotic cells in the bronchoalveolar duct regions as compared with control (control, 2.1% +/- 0.35%; asbestos, 7.61% +/- 0.15%; n = 3). H owever, by 4 weeks the number of apoptotic cells was similar to control. We conclude that asbestos-induced pulmonary toxicity may partly be caused by apoptosis in the lung epithelium that is mediated by iron-catalyzed ROS and caspase 3 activation.