Biochemical characteristics of apoptosis in two human leukaemic cell lines.

Apoptosis is a form of cell death in which the cell actively participates. Apoptosis was induced in two human leukaemic cell lines, U937 and HL-60, by incubation with a diverse array of chemical agents. Cell death was assessed by gel electrophoresis, light microscopy and flow cytometry. It was demonstrated that apoptosis involved the formation of large kilobase pair DNA fragments (20-50, 145-245 and 580 kilobase pairs) prior to, or accompanying, internucleosomal cleavage. Degradation of DNA to large kilobase pair sizes also occurred in some forms of necrosis. These fragments were similar, but not identical, to those generated during apoptosis. The identity of the endonuclease(s) responsible for DNA cleavage to large kilobase pair fragments is as yet unknown. One suggestion is that topoisomerase II might be involved. Using an HL-60 subclone with reduced topoisomerase II expression, it was shown that topoisomerase II was not necessary for the formation of large kilobase pair DNA fragments and oligonucleosomal fragments, except where topoisomerase II inhibition was the stimulus for apoptosis. However, topoisomerase II did appear to facilitate 20-50 kilobase pair-to-oligonucleosome conversion, and in one case, it was an absolute requirement for this conversion. Specific proteases have been implicated in the process of apoptosis. The effect of three protease inhibitors was investigated in U937 and HL-60 cells. Aproposed inhibitor of the protease, calpain, had little effect. In contrast, two serine protease inhibitors inhibited the formation of oligonucleosomal fragments. One of these also inhibited large kilobase pair fragments. The other induced apoptotic fragmentation of DNA to large kilobase pair fragments. The results suggest that different proteases may suppress or promote apoptosis within the same cell.