Quiescence in S-phase and G(1) arrest induced by irradiation and/or hyperthermia in six human tumour cell lines of different p53 status

Purpose: Quiescent S-phase cells, i.e. cells with a DNA content intermediat e between G(1) and G(2) that nevertheless do not synthesize DNA have been p reviously observed in human melanoma cells exposed to radiation and/or hype rthermia. This phenomenon has now been studied in more detail comparing six human tumour cell lines of different p53 status and thus different cell-cy cle checkpoint control. Materials and methods: Two melanoma (Bell, MeWo), two squamous carcinoma (4 197, 4451) and two glioma (EA14, U87) cell lines were used. Changes in the cell-cycle distribution after treatment were studied using two-parameter fl ow cytometry in order to measure DNA content and BrdU incorporation simulta neously. Results: The fraction of unlabelled cells in the S-phase compartment was de termined at daily intervals after treatment. Only background levels of such cells were seen in three of the cell lines (Bell, 4197, EA14). With the ot her three cell lines (MeWo, 4451, U87) we observed a time- and dose-depende nt increase: a few days after treatment up to 20% of all cells did not inco rporate BrdU. It is interesting to note that Bell, 4197 and EA14 are p53 wi ld-types and show a G(1) block of several hours after irradiation and/or hy perthermia, while MeWo and 4451 are p53 mutants unable to exhibit such a de lay, and U87 in spite of being a p53 wild-type has a reduced ability to do so. Conclusions: The MeWo, 4451 and U87 cell lines have less time available for the repair of DNA damage before entering into the S-phase, which leads to problems during replication and causes some kind of interphase death. Radia tion-induced apoptosis does not seem to be involved here, as it is not uneq uivocally correlated with the induction of a Gi block or with p53 status.