Effects of low oxygen levels on G2-specific cytogenetic low-dose hyper-radiosensitivity in irradiated human cells

Low-dose hyper-radiosensitivity (HRS) has been reported in normal human lymphoblastoid cell lines for exposures at20 cGy, but the cytogenetic effects of oxygen (O-2) levels in tissue culture medium on HRS have not been evaluated. We asked whether HRS was lost in G2-irradiated cells grown in atmospheres of 2.5% or 5% O-2, compared to responses by cells cultured in ambient O-2 (21%). The results indicate a loss of HRS when cells are cultured and irradiated either in 2.5% or 5% O-2. We then evaluated whether low O-2 levels either before or after exposure were responsible for the loss of HRS. For cells irradiated in 5% O-2, subsequent immediate re-oxygenation to ambient O-2 levels restored the HRS effect, while cells cultured and irradiated at ambient O-2 levels and then transferred to 5% O-2 exhibited little or no HRS, indicating that ambient O-2 levels after, but not before, radiation substantially affect the amounts of cytogenetic damage. HRS was not observed when cells were irradiated in G1. At doses of 40-400 cGy there was significantly less cytogenetic damage when cells were recovering from radiation at low O-2 levels than at ambient O-2 levels. Here we provide the first cytogenetic evidence for the loss of HRS at low O-2 levels in G2-irradiated cells; these results suggest that at low O-2 levels for all doses evaluated there is either less damage to DNA, perhaps because of lower amounts of reactive oxygen species, or that DNA damage repair pathways are activated more efficiently. Environ. Mol. Mutagen. 56:545-555, 2015. (c) Wiley Periodicals, Inc.