Activation of Multiple Interleukin-1β Converting Enzyme Homologues in Cytosol and Nuclei of HL-60 Cells during Etoposide-induced Apoptosis

Abstract Recent genetic and biochemical studies have implicated cysteine-dependent aspartate-directed proteases (caspases) in the active phase of apoptosis. In the present study, three complementary techniques were utilized to follow caspase activation during the course of etoposide-induced apoptosis in HL-60 human leukemia cells. Immunoblotting revealed that levels of procaspase-2 did not change during etoposide-induced apoptosis, whereas levels of procaspase-3 diminished markedly 2-3 h after etoposide addition. At the same time, cytosolic peptidase activities that cleaved DEVD-aminotrifluoromethylcoumarin and VEID-aminomethylcoumarin increased 100- and 20-fold, respectively; but there was only a 1.5-fold increase in YVAD-aminotrifluoromethylcoumarin cleavage activity. Affinity labeling with N-(Nα-benzyloxycarbonylglutamyl-Nϵ-biotinyllysyl)aspartic acid [(2,6-dimethylbenzoyl)oxy]methyl ketone indicated that multiple active caspase species sequentially appeared in the cytosol during the first 6 h after the addition of etoposide. Analysis on one- and two-dimensional gels revealed that two species comigrated with caspase-6 and three comigrated with active caspase-3 species, suggesting that several splice or modification variants of these enzymes are active during apoptosis. Polypeptides that comigrate with the cytosolic caspases were also labeled in nuclei of apoptotic HL-60 cells. These results not only indicate that etoposide-induced apoptosis in HL-60 cells is accompanied by the selective activation of multiple caspases in cytosol and nuclei, but also suggest that other caspase precursors such as procaspase-2 are present but not activated during apoptosis.