Sulfur mustard induces apoptosis in lung epithelial cells via a caspase amplification loop

Sulfur mustard (SM [bis-(2-chloroethyl) sulfide]) is a chemical warfare agent that causes skin blisters presumably due to DNA alkylation and cross-links. We recently showed that SM also induces apoptotic death in cultured normal human bronchial/tracheal epithelial (NHBE) cells and small airway epithelial cells (SAEC) in vitro. In this process, caspases-8 and -3, but not caspase-9, were strongly activated; this suggests a death receptor pathway for apoptosis. We now show that rat lungs were induced to undergo apoptosis in vivo following exposure of rats to SM by inhalation. Further study of the mechanism of apoptosis due to SM was performed with cultured NHBE cells and SAEC using tetrapeptide inhibitors of caspases-3, and -8. Inhibition of caspase-8 drastically reduced the activation of caspase-3 and almost eliminated that of caspase-9. Moreover, caspase-3 inhibition markedly reduced the activation of caspase-8 and also almost completely inhibited activation of caspase-9. These results suggest a death receptor pathway of apoptosis that utilizes a feedback amplification mechanism involving an activated death receptor complex that leads to the activation of caspase-9 via a caspase-3 pathway. These results may be important for the design of inhibitors of these pathways for therapeutic intervention to attenuate SM injury in respiratory tract lesions.