Quiescence in S-phase and G1 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 intermediate between G1 and G2 that nevertheless do not synthesize DNA have been previously observed in human melanoma cells exposed to radiation and/or hyperthermia. This phenomenon has now been studied in more detail comparing six human tumour cell lines of different p53 status and thus different cell-cycle checkpoint control.

Materials And Methods: Two melanoma (Be11, MeWo), two squamous carcinoma (4197, 4451) and two glioma (EA14, U87) cell lines were used. Changes in the cell-cycle distribution after treatment were studied using two-parameter flow cytometry in order to measure DNA content and BrdU incorporation simultaneously.

Results: The fraction of unlabelled cells in the S-phase compartment was determined at daily intervals after treatment. Only background levels of such cells were seen in three of the cell lines (Be11, 4197, EA14). With the other three cell lines (MeWo, 4451, U87) we observed a time- and dose-dependent increase: a few days after treatment up to 20% of all cells did not incorporate BrdU. It is interesting to note that Bell, 4197 and EA14 are p53 wild-types and show a G1 block of several hours after irradiation and/or hyperthermia, while MeWo and 4451 are p53 mutants unable to exhibit such a delay, 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. Radiation-induced apoptosis does not seem to be involved here, as it is not unequivocally correlated with the induction of a G1 block or with p53 status.