Repair of radiation-induced DNA damage in thermotolerant and nonthermotolerant HeLa cells

The effect of heat exposure on the repair of radiation-induced DNA damage which inhibits the ability of nuclear DNA to undergo supercoiling changes was studied using the fluorescent halo assay in thermotolerant and nonthermotolerant (normal) cells. The assay utilizes an intercalating, fluorescent dye to unwind and rewind endogenous DNA supercoils. When HeLa cells are exposed to 17.3 Gy radiation the ability of DNA to be rewound into supercoils is completely inhibited. However, the ability of DNA to rewind is 70% restored by 30 min after irradiation. Both thermotolerant and normal cells exposed to 45 degrees C for 30 min prior to irradiation had a rewinding ability intermediate between control and unheated cells, but there was no restoration of rewinding ability up to 3 h postirradiation. Thus, when irradiation immediately followed heating, there was no difference between thermotolerant and normal cells. However, when various time intervals were imposed between heating and irradiation, a difference in the ability of the cells to recover from heat-induced alterations became apparent. In normal cells after 6 h of postheat incubation the cells' ability to restore DNA supercoiling was approximately the same as that of control cells, while in thermotolerant cells only 2 h was required to repair the ability to restore supercoiling at the same rate. The rate of repair of DNA remained correlated with relative nuclear protein content as measured by fluorescein isothiocyanate staining in both thermotolerant and normal cells, indicating a possible relationship between the two.