Hyperthermia alters the interaction of proteins of the Mre11 complex in irradiated cells.

Radiosensitization of mammalian cells by heat is believed to involve the inhibition of repair of DNA double-strand breaks (DSBs). The Mre11 complex (composed of Mre11, Rad50, and Nbs1) is involved in DSB repair and forms foci at sites of radiation-induced DSBs. Heat induces the translocation of a significant amount of Mre11, Rad50, and Nbs1 from the nucleus to the cytoplasm, but little is known about how heat affects the integrity of the proteins still remaining in nuclei, or alters kinetics of formation/disappearance of DNA repair foci in heated, irradiated cells. Here, we show that hyperthermia alters the interaction between proteins of the Mre11 complex in irradiated human melanoma cells and inhibits the formation of repair foci. At various times after X-irradiation and/or heating (2 h at 41.5 or 42.5 °C), the cells were fixed and stained for Mre11, Rad50, and Nbs1. Colocalization of proteins and formation and disappearance of nuclear foci in heated and/or irradiated cells, determined using confocal microscopy, were compared. In heated, irradiated cells, focus formation was inhibited for 2-8 h, and colocalization of the proteins of the Mre11 complex was reduced for 12-24 h post-treatment. Colocalization was recovered in irradiated cells within 24 h after heating at 41.5 °C, but was inhibited longer after heating at 42.5 °C. The decreased colocalization in heated, irradiated cells suggests that there is a decrease in protein interaction, and Mre11 complexes in nuclei disassemble after heating. Such changes could be involved, at least in part, in heat radiosensitization and inhibition of DSB repair. Also, the kinetics of disassembly and reassembly of Mre11 complexes appears to be dependent upon treatment temperature.