In vivo evolution of tumour cells after the generation of double-strand DNA breaks.

In vitro, the ratio of single- to double-strand DNA breaks (DSB) and their absolute values determine the cell death pathway. The consequences of the generation of various numbers of DSB generated in vivo in tumour cells have been analysed in two different experimental tumour models. Synchronisation of DSB generation and control of their number have been achieved using different doses of bleomycin (BLM) and tumour cell permeabilisation by means of locally delivered electric pulses. According to BLM dose, different cell death pathways are observed. At a low therapeutic dose, a mitotic cell death pathway is detected. It is characterised by the appearance of 'atypical mitosis', TUNEL and caspase-3 positive, 24 h after the treatment, and later by the presence of typical apoptotic figures, mainly TUNEL positive but caspase-3 negative. Caspase-3 is thus an early marker of apoptosis. Mitotic cell death is also followed by lymphocytic infiltration reaction. At high doses of BLM, pseudoapoptosis is detected within a few minutes after the treatment. These cell death pathways are discussed as a function of the number of DSB generated, by comparison with previous results obtained in vitro using BLM or ionising radiation.