Persistent decrease in viability as a function of X irradiation of human bladder carcinoma cells in G1 or S phase.

Abstract

A persistent decrease in viability after treatment with a variety of mutagenic agents has been observed previously, but the dependence of the decrease on the phase of the cell cycle in which the cells are treated has not been fully explored. Synchronous human bladder carcinoma cells (EJ30-15) were obtained by mitotic selection (88-96% in or near mitosis). As monitored by microscopy and pulse labeling with [3H]dThd, approximately 98% of the cells were in G1 phase when they were irradiated after 3 h of incubation, and approximately 80% were in S phase when they were irradiated after 14 h of incubation. The initial plating efficiencies demonstrated no difference in cell survival when cells were irradiated in G1 or S phase, with normalized clonogenic survival and standard error of 60+/-6% for 3 Gy and 13+/-2% for 6 Gy. However, when the cell populations were allowed to incubate and were replated 5 to 33 days later (5.5 to 36 doublings), a difference between the populations irradiated in G1 and S phase became clear. Cells that were irradiated with 6 Gy regained and maintained the high plating efficiencies (67.9+/-3.6%) of the unirradiated populations much sooner when they were irradiated in S phase compared with irradiation in G1 phase, i.e. 11 days (12 cell doublings) for S phase compared to approximately 20 days (22 cell doublings) for G1 phase. During these periods when the plating efficiencies were increasing, the populations irradiated in G1 phase were multiplying at rates lower than those for the populations irradiated in S phase. Furthermore, after 6 Gy, more giant cells and multinucleated cells were seen in the populations irradiated in G1 phase than in the populations irradiated in S phase. These results indicate that, although the clonogenic survival was the same for cells irradiated in G1 or S phase, the residual damage in progeny of the irradiated cells persisted longer (approximately 20 days compared to 11 days) when cells were irradiated in G1 phase than when they were irradiated in S phase.