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Purpose: To determine whether yeast cells (Saccharomyces cerevisiae) defective in damage-inducible cell-cycle arrest can invoke an adaptive response and become resistant to normally lethal doses of ionizing radiation. Materials and methods: Wild-type yeast cells, cells defective for DNA-damage-responsive G1 and G2 cell-cycle arrest (rad9 δ), and cells defective for recombinational repair of DNA damage (rad50, 51, 52) were subjected to adapting treatments of heat or radiation and subsequently exposed to normally lethal doses of radiation. Survival, as measured by colony-forming ability, was compared with non-adapted, control cells. Results: Wild-type and rad9 δ cells became more resistant to potentially lethal doses of radiation after exposure to conditions that are known to elicit the adaptive response. Further, the relative magnitude of resistance developed by the normal, wildtype and rad9 δ yeast cells was similar, with a dose modifying factor (at D[sub 1]) for radiation-induced radiation resistance of 1.3 for both strains. Dose modifying factors (at D[sub 1]) for heat-induced radiation resistance were 1.7 and 1.6 for wild-type and rad9 δ cells, respectively. In contrast, none of the recombinational repair-defective cells exhibited radiation resistance after an adapting treatment. Conclusions: The ability of yeast cells to arrest in cell-cycle gap phases did not appear to contribute significantly to radiation resistance induced by radiation or heat. Instead, it is suggested that the adaptive response was due mainly to the existence and enhancement of cellular recombinational repair capacity, which was sufficient to repair any DNA damage without the requirement of a detectable cell-cycle delay. [ABSTRACT FROM AUTHOR]

Purpose: To investigate the repair of ionizing radiation-induced DNA lesions in human skin fibroblasts in the presence of cisplatin-DNA adducts and to determine the persistence of DNA repair inhibition by cisplatin. Materials and methods: Normal human fibroblasts (AG1522) treated with cisplatin were exposed to 4 Gy 60Co gamma-radiation and assayed for repair of radiation-induced damage under growth-permissive conditions. DNA damage was measured by the fluorescence analysis of DNA unwinding (FADU) and cytokinesis-blocked micronucleus assays. Results: Rejoining of strand breaks caused by 4 Gy radiation in cells without cisplatin pre-treatment appeared to be biphasic with an initial fast component (up to 15 min of repair time) followed by a slower component, and was completed by 90 min. Cisplatin treatment (10 mu g/ml, 30 min) immediately before irradiation had no effect on the fast rejoining component, but inhibited the slow component ( p 0.01). The same cisplatin treatment 24 h prior to irradiation inhibited both slow and fast components ( p 0.01). In contrast, decreasing the cisplatin exposure to 1.0 mu g/ml for 30 min, 24 h prior to irradiation, resulted in an increased amount of strand break repair at each time point measured compared with irradiated control cells. This mild cisplatin treatment (95% survival) also resulted in a reduction of radiation-generated micronuclei indicating an adaptive response. Conclusions: Cisplatin used in combination with ionizing radiation can produce differential cellular responses depending upon the severity of the cisplatin treatment and the time interval between cisplatin and radiation exposures. [ABSTRACT FROM AUTHOR]

Abstract. Chromosomes are located within the interphase nucleus in regions called domains. Using fluorescence in situ hybridization with whole chromosome paints, a pair of homologous chromosomes can be visualized as two discrete domains and their relative spatial location determined. This study examines the effects of an ionizing radiation exposure on the relative spatial location of chromosome 7 and 21 domains in human skin fibroblasts and lung endothelial cells. The distance between homologous chromosome domains was assessed for each nucleus, before and after exposure to ionizing radiation, using conventional epifluorescence and confocal laser scanning microscopy. Results from conventional microscopy indicated that homologous chromosome domains were re-positioned closer to each other within interphase nuclei after exposure to radiation. Analysis of three-dimensional data obtained from confocal microscopy confirmed these results. In control cells, and in cells examined immediately after irradiation, 66.2% 2.1% of the homologous chromosome 21 domains within endothelial cell nuclei were located greater than 4.0 mu m apart (33.8% 1.9% were less than 4.0 mu m apart). However, when cells were examined 2 h after a 4.0 Gy gamma-ray exposure, only 30.5% 2.1% of the homologous chromosome domains were greater than 4.0 mu m apart (69.5% 2.1% were less than 4.0 mu m apart). Similar results were obtained for chromosomes 7 and 21 in skin fibroblast nuclei. The results indicate that homologous chromosome domains rearranged and became closer together within the interphase nuclei in response to ionizing radiation. The exact mechanism of this response is unknown, but it may be related to DNA repair processes. It is speculated that chromosome domains are re-positioned to permit repair of radiation-induced DNA damage. [ABSTRACT FROM AUTHOR]