Caspase cascade of apoptosis induced by satratoxin G in HL-60 cells: possible involvement of caspase-2 in direct activation of caspase-3

Satratoxins, which are produced Stachybotrys species, have been recognized as potential agents associated with sick building syndromes. In spite of the potential importance of satratoxins, the molecular mechanism induced by them remains unknown. Recently, we have found that satratoxin G induced potent apoptosis in HL-60 human promyelotic leukemia cells and both caspase-8 and caspase-9 are activator of caspase-3. However, the detail activation profile of caspases and role of the other caspases including caspase-6, caspase-7 and a novel initiator caspase, caspase-2 remain to be clear. Here we report detail caspase cascade in satratoxin G-induced apoptosis in HL-60 cells. Caspase assay with DEVD-AMC, a fluorogenic substrate, and western blot analysis of caspase-7 suggested that caspase-3 was involved, but casapase-7 was not directly involved in satratoxin-induced apoptosis. We investigated in-depth time courses of caspase activation. Caspase-3 was activated after 1 hour from satratoxin treatment. Activation of caspase-2 was observed apparently as early as 1 h. Caspase-9 was also activated significantly at the 1 hour time point. Caspase-6 and caspase-8 were activated at the 3-hour time point. Western blot analysis showed that cleavage of bid and activation of caspase-12 were not involved in satratoxin-mediated apoptosis. These findings suggest that the initial activators of caspase-3 are caspase-2 and casapse-9 in satatoxin-induced apoptosis. Because caspase-8 and casapse-9 were activated independently, caspase-8 is also involved in activation of caspase-3 in the late stage of the apoptosis. Moreover, caspase-6 appears to be associate with activation of caspase-8. Both caspase-7 and caspase-12 may be not potential caspases in the apoptosis.