DEVELOPMENT OF APOPTOSIS AND POLYPLOIDY IN HUMAN LYMPHOBLAST CELLS ASA FUNCTION OF POSITION IN THE CELL-CYCLE AT THE TIME OF IRRADIATION

The TK6 and WI-L2-NS human lymphoblast cell lines are derived from the same donor, but WI-L2-NS cells are more resistant to killing by ioniz ing radiation and also contain a mutation in the coding region of the p53 gene which delays the appearance of apoptosis. To determine whethe r the difference in radiosensitivity is a result of the slower rate of appearance of apoptosis (referred to as the rate of apoptosis) or to other factors related to the function of p53 such as the G(1)-phase ch eckpoint, we have examined the clonogenicity and rates of apoptosis in synchronous populations of these two cell lines as a function of time after exposure to X rays. The greater radiosensitivity of G(1)-phase TK6 cells compared to G(1)-phase WI-L2-NS cells appeared adequate to e xplain the difference in radiosensitivity of asynchronous cultures. Ho wever, TK6 cells irradiated in G(1) phase underwent apoptosis about th ree times more slowly than cells irradiated in other phases of the cyc le, cautioning against equating more rapid apoptosis with greater radi osensitivity. The slower rate of apoptosis of G(1)-phase TK6 cells cou ld not be explained by a radiation-induced block in G(1) phase since t here was only a short delay in exit of cells from G(1) phase. Giant ce lls (primarily polyploid) were formed after irradiation of WI-L2-NS bu t not TK6 cells, and fewer giant cells were observed 3-4 days after ir radiation in WI-L2-NS cells irradiated in S phase than cells irradiate d in other phases. Giant cells were lost from the population through a poptosis which occurred in a synchronous fashion in the multiple nucle i. These results highlight interesting differences in the pattern of c ell death between the two cell lines and suggest that the response of TK6 cells in G(1) phase, which has the slowest rate of appearance of a poptosis in spite of the fact that it is the most radiosensitive phase for clonogenic survival, may be largely responsible for the reduced s houlder on the radiation survival curve for TK6 cells compared with WI -L2-NS cells. (C) 1996 by Radiation Research Society