A possible mechanism for hyperthermic radiosensitization mediated through hyperthermic lability of Ku subunits in DNA-dependent protein kinase.

DNA-dependent protein kinase (DNA-PK), composed of p470 catalytic subunit and p85/p70 heterodimer of Ku autoantigen, is considered a critical enzyme in DNA double-strand break repair. We purified DNA-PK from human leukaemic MOLT-4 cells by successive column chromatography and separated into p470 and Ku subunits by ultracentrifugation in glycerol gradient. We studied hyperthermic stability of DNA-PK holoenzyme and its separated subunits to test a possible role of DNA-PK in hyperthermic radiosensitization. DNA-PK was found to lose its activity rapidly at hyperthermic 44 degrees C, and further, Ku subunits instead of p470 catalytic subunits were found to be sensitive to hyperthermia. These results indicate a possibility that hyperthermic radiosensitization is mediated through the heat lability of Ku subunits of DNA-PK, impairing repair of radiation-induced double-strand break of DNA.