The target for radiation-induced division delay

Abstract

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 125IUdR and the cell kinetics monitored by counting the mitotic cells selected every 10 min. Our results showed that 125I. had to be incorporated into DNA to cause a perturbation of cell progression; unlabeled G2 cells were unperturbed. As 125I-labeled cells entered the mitotic selection window, the normalized yield of mitotic cells (number of 125I-labeled mitotic cells expressed as a fraction of cells harvested from control flasks) decreased to a level inversely proportional to the incorporated 125IUdR and remained at this level for the duration of the experiment (4-5 hr). To evaluate the mechanism of 125I-induced division delay, 125IUdR-labeled cells were permitted to accumulate 125I 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 125I decays only during the S phase did not experience enhanced delay. In contrast, the yield of mitotic cells was reduced in cells which accumulated 125I 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.