Abstract
Astronauts are known to receive approximately 1 mGy/day of ionizing radiation (IR) in the space environment. Although protons are the most prevalent radiation in space, low levels of heavy ion particles become important due to their severe biological effects. We irradiated normal human fibroblasts (HFL III) with carbon ions (290MeV/n, 70 keV/um) as well as gamma-rays at 1 mGy/6-8 h once and the cell growth was observed up to five months by continuous culturing. The carbon irradiated cells went to senescence much earlier than non-irradiated control cells, while there was no growth disadvantage in gamma-irradiated cells under the same conditions. Since not all the cells actually were hit by carbon irradiation at this dose level, this phenomenon seems to be a by-stander effect. DNA double strand break (repair) markers (gammaH2AX and phosphorylated DNA-PKcs) were studied by immuno-staining method in these cells. The numbers of foci for these markers increased several passages before senescence and the increase was more pronounced in carbon irradiated cells than control or gamma-irradiated cells. Of special interest, the number of foci was significantly reduced after cells became fully senescent. Studies with non-homologous end joining (NHEJ) deficient cells (180BR) showed accelerated senescence significantly sooner than normal cells. Our studies indicate that a single low dose heavy ion irradiation can affect the genome stability many weeks after irradiation, and that the protein(s) necessary for NHEJ appear to be involved in the cell senescence process.
