Chrysotile asbestos causes AEC apoptosis via the caspase activation in vitro and in vivo

To determine whether alveolar epithelial cell (AEC) apoptosis via caspase activation is involved in asbestos-induced lung injury, we examined apoptosis, caspase-3 and-9 activation using chrysotile asbestos exposure models in vitro and in vivo. Apoptotic cells were assessed in A549 cells with terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL) method after 48 hours exposure of chrysotile asbestos (5 to 100 ug/cm2). Asbestos exposure induced a dose-dependent increase of apoptotic cells, and both pretreatment with Z-LEHD-FMK (caspase-3 inhibitor) or Z-DEMK-FMK (caspase-9 inhibitor) significantly suppressed asbestos-induced apoptosis. Expression of cleaved caspase-3 and-9 increased significantly from 18 to at least 48 hours after asbestos exposure. In vivo study, either 1 or 2 mg of chrysotile asbestos were administered into rat lungs intratracheally, and the lungs were obtained 3 days, 1 and 2 weeks, 1, 3 and 6 months after the administration. Asbestos exposure increased the number of apoptotic cells and the activation of cleaved caspase-3 and -9 most at 3 days in a dose dependent manner, and continued to increase them until at least 6 months after asbestos exposure. Apoptotic cells and cleaved caspase-3 and -9 positive cells were mainly observed in AECs. These findings suggest that AEC apoptosis via caspase-3 and -9 activation is involved in asbestos-induced lung injury.