Translocation of MRE11 from the nucleus to the cytoplasm as a mechanism ofradiosensitization by heat

Hyperthermia sensitizes mammalian cells to ionizing radiation, presumably b y inhibiting the repair of radiation-induced double-strand breaks (DSBs), H owever, the mechanism by which heat inhibits DSB repair is unclear. The nuc lear protein MRE11 is a component of a multi-protein complex involved in no nhomologous end joining (NHEJ) of radiation-induced DSBs, Using one-dimensi onal sodium dodecylsulfate polyacrylamide gel electrophoresis and Western b lotting, we found that MRE11 is translocated from the nucleus to the cytopl asm when human U-1 melanoma or HeLa cells are heated for 15 min at 45.5 deg reesC or when cells are heated after irradiation with 12 Gy of X rays. No s uch translocation is observed in unheated irradiated cells. The kinetics of migration of MRE11 to the cytoplasm was dependent upon whether the heated cells were irradiated, while the magnitude of redistribution of MRE11 was d ependent upon post-treatment incubation time at 37 degreesC, Cytoplasmic MR E11 content reached a maximum 2-4 h after heating; the increase was not due to new protein synthesis. Partial recovery of nuclear MRE11 content was ob served when heated cells or heated irradiated cells were incubated for up t o 7 h at 37 degreesC after treatment. Western blotting results showing tran slocation of MRE11 from the nucleus to the cytoplasm after heating and irra diation were confirmed using confocal microscopy and immunofluorescence sta ining of fixed cells. Our data suggest that radiosensitization by heat may be caused, at least in part, by translocation of the DNA repair protein MRE 11 from the nucleus to the cytoplasm. (C) 2001 by Radiation Research Societ y.