Radiosensitivity of confluent density-inhibited cells

Role for DNA polymerase beta in response to ionizing radiation

Evidence for a role of DNA polymerase beta in determining radiosensitivity is conflicting. In vitro assays show an involvement of DNA polymerase beta in single strand break repair and base excision repair of oxidative damages, both products of ionizing radiation. Nevertheless the lack of DNA polymerase beta has been shown to have no effect on radiosensitivity. Here we show that mouse embryonic fibroblasts deficient in DNA polymerase beta are considerably more sensitive to ionizing radiation than wild-type cells, but only when confluent. The inhibitor methoxyamine renders abasic sites refractory to the dRP lyase activity of DNA polymerase beta. Methoxyamine did not significantly change radiosensitivity of wild-type fibroblasts in log phase. However, DNA polymerase beta deficient cells in log phase were radiosensitized by methoxyamine. Alkaline comet assays confirmed repair inhibition of ionizing radiation induced damage by methoxyamine in these cells, indicating both the existence of a polymerase beta-dependent long patch pathway and the involvement of another methoxyamine sensitive process, implying the participation of a second short patch polymerase(s) other than DNA polymerase beta. This is the first evidence of a role for DNA polymerase beta in radiosensitivity in vivo.

Increased radioresistance of cells in cultured multicell spheroids. I. Dependence on cellular interaction

Monolayers of six different cell lines were investigated with respect to ionic coupling using micro-electrode techniques. In parallel, survival after Co-gamma-irradiation of monolayer- and spheroid cultures of these lines was compared. It was found that spheroids of coupled cell lines were more radioresistant than monolayers ("contact effect"). however, cell coupling did not enhance the survival of monolayers over single cells. This suggests that the contact effect is a "tissue phenomenon" requiring cellular interaction but is expressed only under conditions of three-dimensional growth.

The radiobiological response of the thyroid. Part II: Response of sheep thyroid cell in vitro to single doses of X rays

The response of sheep thyroid cells in culture to single doses of X or gamma rays is described. In the absence of cellular proliferation the cells are unusually radioresistant, showing little sign of interphase death at doses up to 9 krad. The follicular morphology characteristic of thyroid cells in vivo is also very radioresistant. Iodide trapping is reduced to 50% of the control value by doses of the order of 2 krad. When proliferation is induced the cells may be assayed for postirradiation survival using a clonogenic endpoint. The survival curves are sigmoid with a Do of 410 rad and a very low extrapolation number.

Cytoplasmic effects of x-irradiation on cultured cells in a nondividing stage. 1. Establishment of an experimental model

The investigation was initiated with the aim of establishing a suitable experimental model with respect to mode of radiation and radiation dose for elucidating morphologically the sequential development of radiation induced damage of interphase cells (human glia cells in vitro). Adequately defined and reproducible cellular changes were obtained using X-radiation generated by an 8 MeV linear accelerator at a dose of 20,000 rad. The cellular alterations were studied in the light microscope and by scanning and transmission electron microscopy. The most conspicuous changes--first appreciable about 5 hours after irradiation--occurred in the lysosomal vacuome, and the plasma membrane and associated structures.