Radioprotective effect of hypothermia on cells - a multiparametric approach to delineate the mechanisms

Purpose: Low temperature (hypothermia) during irradiation of cells has been reported to have a radioprotective effect. The mechanisms are not fully understood. This study further investigates the possible mechanisms behind hypothermia-mediated radioprotection. Materials and methods: Human lymphoblastoid TK6 cells were incubated for 20 min at 0.8 or 37 degrees C and subsequently exposed to 1 Gy of gamma- or X-rays. The influence of ataxia telangiectasia mutated (ATM)-mediated double-strand break signalling and histone deacetylase-dependent chromatin condensation was investigated using the micronucleus assay. Furthermore, the effect of hypothermia was investigated at the level of phosphorylated histone 2AX (gamma H2AX) foci, clonogenic cell survival and micronuclei in sequentially-harvested cells. Results: The radioprotective effect of hypothermia (called the temperature effect [TE]) was evident only at the level of micronuclei at a single fixation time, was not influenced by the inhibition of ATM kinase activity and completely abolished by the histone deacetylase inhibition. No TE was seen at the level of gamma H2AX foci and cell survival. Conclusions: We suggest that low temperature during irradiation can induce a temporary cell cycle shift, which could lead to a reduced micronucleus frequency. Future experiments focused on cell cycle progression are needed to confirm this hypothesis.