EXPLAINING DIFFERENCES IN SENSITIVITY TO KILLING BY IONIZING-RADIATION BETWEEN HUMAN LYMPHOID-CELL LINES

We surveyed five human hematopoietic cell lines (HSB-2, MOLT-4, Reh, C EM, and HL-60) to determine whether any simple correlates with sensiti vity to killing by gamma-irradiation might be revealed. The clonogenic survival gamma-ray dose-response curves for these cell lines cover a wide range of sensitivities. Consistent with previous results for muri ne hematopoietic cell lines, there was a clear correlation between the rapidity with which irradiation induced apoptosis and clonogenic radi osensitivity of a cell line, although the relationship between timing of apoptosis and radiosensitivity differed between human and murine ce ll lines. Flow cytometric determination of cell cycle distribution aft er irradiation showed that differences between human hematopoietic cel l lines, in the rate of induction of apoptosis, were generally related to the functioning of cell cycle checkpoints. Whereas the rapidly dyi ng and radiosensitive HSB-2 cell line underwent apoptosis from differe nt points in the cell cycle, the more slowly dying cell lines showed a variety of cell cycle arrest profiles and initiated apoptosis after a ccumulation of cells in the G(2) phase, The lag-phase between arrest i n G(2) and induction of apoptosis was comparable for MOLT-4, Reh, and GEM; however, HL-60 cells showed a markedly longer G(2) arrest that co rrelated with their greater radioresistance, The results suggest that the total length of time available for DNA damage repair (irrespective of whether this time accrues as blockage in G(1), S, or G(2)), prior to potential activation of apoptosis, is a critical determinant of rad iosensitivity in human hematopoietic cell lines. Comparison of the p53 status of these cell lines suggested that mutations in the TP53 gene are contributing to the delay of induction of apoptosis seen in the mo re radioresistant cell lines, The sensitivity of MOLT-4 and HL-60 cell s to killing by DNA-associated I-125 decays was determined and was fou nd to correlate with the relative sensitivity of these Lines to gamma- irradiation. The highly localized deposition of energy by I-125 decays argues that DNA damage is a potent initiator of apoptosis in these ce ll lines. The results presented suggest that differences in the radios ensitivity of the cell lines examined reflect differences in the rapid ity of induction of apoptosis and that radiation-induced cell death in hematopoietic cells can be explained as a response to DNA damage.