Low-dose radiation hypersensitivity is associated with p53-dependent apoptosis.

Exposure to environmental radiation and the application of new clinical modalities, such as radioimmunotherapy, have heightened the need to understand cellular responses to low dose and low-dose rate ionizing radiation. Many tumor cell lines have been observed to exhibit a hypersensitivity to radiation doses <50 cGy, which manifests as a significant deviation from the clonogenic survival response predicted by a linear-quadratic fit to higher doses. However, the underlying processes for this phenomenon remain unclear. Using a gel microdrop/flow cytometry assay to monitor single cell proliferation at early times postirradiation, we examined the response of human A549 lung carcinoma, T98G glioma, and MCF7 breast carcinoma cell lines exposed to gamma radiation doses from 0 to 200 cGy delivered at 0.18 and 22 cGy/min. The A549 and T98G cells, but not MCF7 cells, showed the marked hypersensitivity at doses <50 cGy. To further characterize the low-dose hypersensitivity, we examined the influence of low-dose radiation on cell cycle status and apoptosis by assays for active caspase-3 and phosphatidylserine translocation (Annexin V binding). We observed that caspase-3 activation and Annexin V binding mirrored the proliferation curves for the cell lines. Furthermore, the low-dose hypersensitivity and Annexin V binding to irradiated A549 and T98G cells were eliminated by treating the cells with pifithrin, an inhibitor of p53. When p53-inactive cell lines (2800T skin fibroblasts and HCT116 colorectal carcinoma cells) were examined for similar patterns, we found that there was no hyperradiosensitivity and apoptosis was not detectable by Annexin V or caspase-3 assays. Our data therefore suggest that low-dose hypersensitivity is associated with p53-dependent apoptosis.