The Target for Radiation-Induced Division Delay

In an effort to elucidate the subcellular target responsible for radiation-induced division delay, Chinese hamster ovary (CHO) cells growing in monolayer cultures were pulse-labeled with 125 IUdR and the cell kinetics monitored by counting the mitotic cells selected every 10 min. Our results showed that 125 I had to be incorporated into DNA to cause a perturbation of cell progression; unlabeled G2 cells were unperturbed. As 125 I-labeled cells entered the mitotic selection window, the normalized yield of mitotic cells (number of 125 I-labeled mitotic cells expressed as a fraction of cells harvested from control flasks) decreased to a level inversely proportional to the incorporated 125 IUdR and remained at this level for the duration of the experiment (4-5 hr). To evaluate the mechanism of ^{125}{\rm I}\text{-induced}$ division delay, ^{125}{\rm IUdR}\text{-labeled}$ cells were permitted to accumulate 125 I decays either during the S phase (by cooling the monolayer to 4°C for 2 hr immediately after pulse labeling) or during both S and G2 phases (by cooling the monolayer to 4°C for 2 hr, starting 2 hr after the pulse label). The results indicated that cells which accumulated 125 I decays only during the S phase did not experience enhanced delay. In contrast, the yield of mitotic cells was reduced in cells which accumulated 125 I decays during S plus G2. Analysis of the data suggests that the target for radiation-induced mitotic delay is not the DNA, but a cell structure which comes in contact with the DNA during G2 or early M phase.