Drug-induced death of leukaemic cells after G2/M arrest: higher order DNA fragmentation as an indicator of mechanism.

Many reports have documented apoptotic death in different cell types within hours of exposure to cytotoxic drugs; lower drug concentrations may cause cell cycle arrest at G2/M and subsequent death, which has been distinguished from 'classic' apoptosis. We have analysed etoposide-induced cell death in two lymphoblastoid T-cell lines, CCRF-CEM and MOLT-4, specifically in relation to DNA cleavage as indicated by pulse-field gel and conventional electrophoresis. High (5 microM) concentration etoposide causes 50-kb cleavage of DNA that occurs at the same time as apoptotic morphology and internucleosomal cleavage. At lower concentrations (0.5-0.05 microM), sequential change may be discerned with altered gene expression being similar to that at high dose, but preceding cell cycle arrest and 50-kb cleavage. These last changes, in turn, clearly precede internucleosomal fragmentation of DNA, vital dye staining and morphological evidence cell death. The pattern of higher order fragmentation constitutes a sensitive indicator of commitment to cell death in these cells. Morphological evidence of cell death is associated with internucleosomal fragmentation in one of the lines, but the pattern of 50-kb DNA cleavage provides the clearest evidence of commonality in death processes occurring at low and high drug concentration.