Radiation-Induced Division Delay in Chinese Hamster Ovary Fibroblast and Carcinoma Cells: Dose Effect and Ploidy

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Abstract
The mitotic selection procedure for cell cycle analysis was utilized to investigate the G2 transition point for and the duration of radiation-induced division delay in diploid and tetraploid Chinese hamster ovary (CHO) fibroblasts and in Chinese hamster ovarian carcinoma [OvCa] cells. The location of the radiation-induced division delay transition point, i.e., the last point in the cell cycle at which X irradiation can block progression, was dose independent at high doses (approximately 1.5 Gy and above) and located approximately 42 min before division. At lower doses, as more cells were refractory to delay, only an estimate of the point of blockade was possible; but the G2 transition point appeared to be earlier in the cell cycle, e.g., 54 min at 0.25 Gy. The duration of radiation-induced division delay was dose-dependent. At low doses, there was a nonlinear rapid increase in delay up to approximately 1.0 Gy and from 1.0-6.0 Gy there was a linear dose response of approximately 50 min/Gy as determined from over 80 experiments conducted over 3 yr. This response is consistent with a sensitive population of cells in late G2 that define the location of the transition point and the length of division delay. There was no difference observed in the dose response for radiation-induced division delay between the pseudotetraploid cell line of CHO (TD = 13 h, .hivin.N = 40 chromosomes, delay = 61 .+-. 2 min/Gy) and the pseudodiploid parent strain (TD = 13 h, .hivin.N = 21 chromosomes, delay .+-. 2 min/Gy). In the cell line derived from a spontaneous Chinese hamster OvCa (Td = 11.5 h, .hivin.N = 40 chromosomes), the division delay was 39 .+-. 4 min/Gy. Radiation-induced division delay is independent of chromosome ploidy, but can show intraspecies cell line specificity.