Neither caspase-3 nor DNA fragmentation factor is required for high molecular weight DNA degradation in apoptosis

In this paper, we show that there is a two-step process of DNA fragmentation in apoptosis; DNA is first cleaved to large fragments of 50-300 kb that are subsequently cleaved to smaller oligonucleosomes in some, but not all cells. Significantly, only the first stage is considered essential for cell death since some cells, for example human MCF7 breast carcinoma cells and human NT2 neuronal cells, do not show this behavior but still display normal nuclear morphological apoptotic changes. In cells that usually produce small fragments blocking the second (internucleosomal) stage of DNA fragmentation prevents neither nuclear condensation nor apoptosis. We are beginning to understand why the extent of DNA fragmentation during apoptosis varies enormously and why it appears to be a function of the cell type not the inducer. Presumably, this reflects the content of not only endonuclease activit(ies) but also on the ability of the cells to activate caspases, particularly caspase-3, and other proteases that may be involved in endonuclease activation. Since NT2 cells activate caspase-3, but do not correctly process DFF45, other factors must also impinge on the inevitability of that process.