Combined Electron Radiation and Hyperthermia. Repair of DNA Strand Breaks in NHIK 3025 Cells Irradiated and Incubated at 37, 42.5, or 45°C

Abstract

Induction of DNA strand breaks by a short electron pulse (18.5 Gy[Gray]) and the subsequent strand-break rejoining were investigated at hyperthermia (42.5 and 45.degree. C) and at 37.degree. C during irradiation and repair. The [NHIK 3025 human cervical carcinoma] cells were irradiated immediately after 2.5 min equilibration (i.e, from 37 to 42.5 or 45.degree. C) to investigate the effect of short-duration hyperthermia on radiation damage and subsequent repair. Due to a high radiation dose rate and a rapid lysis technique, the cells could be kept at the actual temperature during irradiation and repair, and the strand-break frequency could be measured only seconds after irradiation. At all temperatures, a constant or possible increase in the initial number of breaks was observed during the first 7 s after the electron pulse. At 37.degree. C, strand-break rejoining was nearly complete within 1 h. Hyperthermia at 42.5.degree. C had only minor influence on the net rate of strand-break rejoining. At 45.degree. C, 50% of the breaks remained after 1 h. Subsequent incubation for 23 h at 37.degree. C reduced by half the number of breaks remaining at 1 h in irradiated samples. Unirradiated samples exposed to the same heat treatment showed a significant increase in the number of DNA strand breaks. Heat treatment at 45.degree. C may lead to a combined effect of reduced rejoining capacity and formation of breaks after the electron pulse, which in turn may be responsible for increased cell death when both modalities are used.