Zhu, W-G., Seno, J. D., Beck, B. D. and Dynlacht, J. R. Translocation of MRE11 from the Nucleus to the Cytoplasm as a Mechanism of Radiosensitization by Heat. Radiat. Res. 156, 95-102 (2001).Hyperthermia sensitizes mammalian cells to ionizing radiation, presumably by inhibiting the repair of radiation-induced double-strand breaks (DSBs). However, the mechanism by which heat inhibits DSB repair is unclear. The nuclear protein MRE11 is a component of a multi-protein complex involved in nonhomologous end joining (NHEJ) of radiation-induced DSBs. Using one-dimensional sodium dodecylsulfate polyacrylamide gel electrophoresis and Western blotting, we found that MRE11 is translocated from the nucleus to the cytoplasm when human U-1 melanoma or HeLa cells are heated for 15 min at 45.5 degrees C or when cells are heated after irradiation with 12 Gy of X rays. No such 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 dependent upon post-treatment incubation time at 37 degrees C. Cytoplasmic MRE11 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 observed when heated cells or heated irradiated cells were incubated for up to 7 h at 37 degrees C after treatment. Western blotting results showing translocation of MRE11 from the nucleus to the cytoplasm after heating and irradiation were confirmed using confocal microscopy and immunofluorescence staining of fixed cells. Our data suggest that radiosensitization by heat may be caused, at least in part, by translocation of the DNA repair protein MRE11 from the nucleus to the cytoplasm.