Radiation‐induced changes in nucleoid halo diameters of aerobic and hypoxic SF‐126 human brain tumor cells

Nucleoid halo diameters were measured to assay changes in DNA supercoiling in human brain tumor cell line SF‐126 after irradiation under aerobic or hypoxic conditions. In unirradiated aerobic cells, a typical propidium iodide titration curve showed that with increasing concentrations of propidium iodide, the halo diameter increased and then decreased with the unwinding and subsequent rewinding of DNA supercoils. In irradiated cells, the rewinding of DNA supercoils was inhibited, resulting in an increased halo diameter, in a radiation dose‐dependent manner. To produce equal increases in halo diameter required about a threefold higher radiation dose in hypoxic cells than in aerobic cells. Quantitatively similar differences in the radiation sensitivities of hypoxic and aerobic cells were demonstrated by a colony‐forming efficiency assay. These findings suggest that the nucleoid halo assay may be used as a rapid measure of the inherent radiation sensitivity of human tumors. © 1995 Wiley‐Liss, Inc.