Caspase-8 mediates caspase-3 activation and cytochrome c release during singlet oxygen-induced apoptosis of HL-60 cells

We reported previously that singlet oxygen, generated by irradiation of ros e bengal with visible light, induced apoptosis in human promyelocytic leuke mia HL-60 cells. However, the mechanism of apoptosis caused by this reactiv e oxygen species is unclear. In this study, are demonstrate that singlet ox ygen induced caspase-3 activation and Z-DEVD-FMK, a caspase-3 inhibitor, bl ocked apoptosis induction, while caspase-1 activity was not detectable and the caspase-1 inhibitor Z-YVAD-FMK had a very limited effect on apoptosis. This suggests that the activation of caspase-3 by singlet oxygen is essenti al for the commitment of cells to undergo apoptosis. Further studies showed that singlet oxygen induced an increase in caspase-8 activity and a reduct ion in mitochondrial cytochrome c. Time course analysis indicated that the cleavage of caspase-8 precedes that of caspase-3. In addition, blockade of caspase-8 by Z-IETD-FMK inhibited cleavage of pro-caspase-3 and prevented l oss of mitochondrial cytochrome c, These results suggest that caspase-8 med iates caspase-3 activation and cytochrome c release during singlet oxygen-i nduced apoptosis in HL-60 cells, (C) 1999 Academic Press.