Generation of
reactive oxygen species (ROS) and activation of
caspase cascade are both indispensable in Fas-mediated apoptotic signaling. Although ROS was presumed to affect the activity of the
caspase cascade on the basis of findings that
antioxidants inhibited the activation of
caspases and that the stimulation of ROS by itself activated
caspases, the mechanism by which these cellular events are integrated in Fas signaling is presently unclear. In this study, using human
T cell leukemia Jurkat cells as well as an in vitro reconstitution system, we demonstrate that ROS are required for the formation of
apoptosome. We first showed that ROS derived from mitochondrial permeability transition positively regulated the apoptotic events downstream of mitochondrial permeability transition. Then, we revealed that
apoptosome formation in Fas-stimulated Jurkat cells was clearly inhibited by
N-acetyl-L-cysteine and
manganese superoxide dismutase by using both the immunoprecipitation and size-exclusion chromatography methods. To confirm these in vivo findings, we next used an in vitro reconstitution system in which in vitro-translated
apoptotic protease-activating factor 1 (Apaf-1),
procaspase-9, and
cytochrome c purified from human placenta were activated by dATP to form
apoptosome; the formation of
apoptosome was markedly inhibited by reducing
reagents such as DTT or
reduced glutathione (GSH), whereas
hydrogen peroxide prevented this inhibition. We also found that
apoptosome formation was substantially impaired by GSH-pretreated Apaf-1, but not GSH-pretreated
procaspase-9 or GSH-pretreated
cytochrome c. Collectively, these results suggest that ROS plays an essential role in
apoptosome formation by oxidizing Apaf-1 and the subsequent activation of
caspase-9 and -3.