ABSTRACT
A phenomenon in which exposure to a low adapting dose of radiation makes cells more resistant to the effects of a subsequent high dose exposure is termed radio-adaptive response. Adaptive response could hypothetically reduce the risk of late adverse effects of chronic or acute radiation exposures in humans. Understanding the underlying mechanisms of such responses is of relevance for radiation protection as well as for the clinical applications of radiation in medicine. However, due to the variability of responses depending on the model system and radiation condition, there is a need to further study under what conditions adaptive response can be induced. In this study, we analyzed if there is a dose rate dependence for the adapting dose, assuming that the adapting dose induces DNA response/repair pathways that are dose rate dependent. MCF-10A cells were exposed to a 50mGy adapting dose administered acutely (0.40Gy/min) or chronically (1.4mGy/h or 4.1mGy/h) and then irradiated by high acute challenging doses. The endpoints of study include clonogenic cell survival and mutation frequency at X-linked hprt locus. In another series of experiment, cells were exposed to 100mGy and 1Gy at different dose rates (acutely and chronically) and then the mutation frequencies were studied. Adaptive response was absent at the level of clonogenic survival. The mutation frequencies were significantly decreased in the cells pre-exposed to 50mGy at 1.4mGy/h followed by 1Gy acute exposure as challenging dose. Importantly, at single dose exposures (1 Gy or 100mGy), no differences at the level of mutation were found comparing different dose rates.
Subject(s)
Adaptation, Physiological/radiation effects , DNA Damage , DNA Repair/radiation effects , Gamma Rays/adverse effects , Mutation Rate , Adaptation, Physiological/genetics , Cell Line, Tumor , Cell Survival/genetics , Cell Survival/radiation effects , DNA Repair/genetics , Dose-Response Relationship, Radiation , Humans , Radiation Tolerance/radiation effects , Time FactorsABSTRACT
The aim of the present study was to analyse the dose rate effect of gamma radiation at the level of mutations, chromosomal aberrations, and cell growth in TK6 cells with normal as well as reduced levels of hMTH1 protein. TK6 cells were exposed to gamma radiation at dose rates ranging from 1.4 to 30.0 mGy/h (chronic exposure) as well as 24 Gy/h (acute exposure). Cell growth, frequency of thymidine kinase mutants, and of chromosomal aberrations in painted chromosomes 2, 8, and 14 were analysed. A decline in cell growth and an increase in unstable-type chromosomal aberrations with increasing dose rate were observed in both cell lines. A dose rate effect was not seen on mutations or stable-type chromosomal aberrations in any of the two cell lines. Reduction in the hMTH1 protein does not influence the sensitivity of TK6 cells to gamma radiation. This result fits well with data of others generated with the same cell line.
Subject(s)
Chromosome Aberrations/radiation effects , DNA Repair Enzymes/genetics , Gamma Rays/adverse effects , Mutation/radiation effects , Phosphoric Monoester Hydrolases/genetics , Radiation Dosage , Transfection , Cell Line , Cell Proliferation/radiation effects , Cell Survival/radiation effects , Clone Cells/cytology , Clone Cells/radiation effects , Dose-Response Relationship, Radiation , HumansABSTRACT
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°C and subsequently exposed to 1 Gy of γ- 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 (γH2AX) 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 γH2AX 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.
