It has been hypothesized that ionizing radiation-induced disruptions in mitochondrial O₂ metabolism lead to persistent heritable increases in steady-state levels of intracellular superoxide (O₂^{•U 2212}) and hydrogen peroxide (H₂O₂) that contribute to the biological effects of radiation. Hamster fibroblasts (B9 cells) expressing a mutation in the gene coding for the mitochondrial electron transport chain protein succinate dehydrogenase subunit C (SDHC) demonstrate increases in steady-state levels of O₂^•− and H₂O₂. When B9 cells were exposed to low-dose/low-LET radiation (5–50 cGy), they displayed significantly increased clonogenic cell killing compared with parental cells. Clones derived from B9 cells overexpressing a wild-type human SDHC (T4, T8) demonstrated significantly increased surviving fractions after exposure to 5–50 cGy relative to B9 vector controls. In addition, pretreatment with polyethylene glycol-conjugated CuZn superoxide dismutase and catalase as well as adenoviral-mediated overexpression of MnSOD and/or mitochondria-targeted catalase resulted in significantly increased survival of B9 cells exposed to 10 cGy ionizing radiation relative to vector controls. Adenoviral-mediated overexpression of either MnSOD or mitochondria-targeted catalase alone was equally as effective as when both were combined. These results show that mammalian cells over expressing mutations in SDHC demonstrate low-dose/low-LET radiation sensitization that is mediated by increased levels of O₂^•− and H₂O₂. These results also support the hypothesis that mitochondrial O₂^•− and H₂O₂ originating from SDH are capable of playing a role in low-dose ionizing radiation-induced biological responses.

Cells without intact mitochondrial DNA have been shown to lack the bystander effect, which is an energy-dependent process. We hypothesized that cells harboring mutations in mitochondrial genes responsible for ATP synthesis would show a decreased bystander effect compared to normal cells. Radiation-induced bystander effects were analyzed in two normal and four mitochondrial mutant human lymphoblastoid cells. Medium from previously irradiated cells (conditioned medium) was transferred to unirradiated cells from the respective cell lines and evaluated for the bystander effect using the cytokinesis-block micronucleus assay. Unlike normal cells that were used as a control, mitochondrial mutant cells neither generated nor responded to the bystander signals. The bystander effect was inhibited in normal cells by adding the mitochondrial inhibitors rotenone and oligomycin to the culture medium. Time-controlled blocking of the bystander effect by inhibitors was found to occur either for prolonged exposure to the inhibitor prior to irradiation with an immediate and subsequent removal of the inhibitors or immediate post-application of the inhibitor. Adding the inhibitors just prior to irradiation and removing them immediately after irradiation was uneventful. Fully functional mitochondrial metabolic capability may therefore be essential for the bystander effect.

In the event of a radiation accident or attack, it will be imperative to quickly assess the amount of radiation exposure to accurately triage victims for appropriate care. RNA-based radiation dosimetry assays offer the potential to rapidly screen thousands of individuals in an efficient and cost-effective manner. However, prior to the development of these assays, it will be critical to identify those genes that will be most useful to delineate different radiation doses. Using global expression profiling, we examined expression changes in nonimmortalized T cells across a wide range of doses (0.15–12 Gy). Because many radiation responses are highly dependent on time, expression changes were examined at three different times (3, 8, and 24 h). Analyses identified 61, 512 and 1310 genes with significant linear dose-dependent expression changes at 3, 8 and 24 h, respectively. Using a stepwise regression procedure, a model was developed to estimate in vitro radiation exposures using the expression of three genes (CDKN1A, PSRC1 and TNFSF4) and validated in an independent test set with 86% accuracy. These findings suggest that RNA-based expression assays for a small subset of genes can be employed to develop clinical biodosimetry assays to be used in assessments of radiation exposure and toxicity.

The effects of low doses of ionizing radiation on cellular development in the nervous system are presently unclear. The focus of the present study was to examine low-dose γ-radiation-induced effects on the differentiation of neuronal cells and on the development of neural stem cells to glial cells. Human neuroblastoma SH-SY5Y cells were exposed to ¹³⁷Cs γ rays at different stages of retinoic acid-induced neuronal differentiation, and neurite formation was determined 6 days after exposure. When SH-SY5Y cells were exposed to low-dose-rate γ rays at the onset of differentiation, the number of neurites formed per cell was significantly less after exposure to either 10, 30 or 100 mGy compared to control cells. Exposure to 10 and 30 mGy attenuated differentiation of immature C17.2 mouse-derived neural stem cells to glial cells, as verified by the diminished expression of glial fibrillary acidic protein. Proteomic analysis of the neuroblastoma cells by 2D-PAGE after 30 mGy irradiation showed that proteins involved in neuronal development were downregulated. Proteins involved in cell cycle and proliferation were altered in both cell lines after exposure to 30 mGy; however, the rate of cell proliferation was not affected in the low-dose range. The radiation-induced attenuation of differentiation and the persistent changes in protein expression is indicative of an epigenetic rather than a cytotoxic mechanism.

The EGF receptor (EGFR) is frequently expressed in tumors of epithelial origin. Although it is well known that ionizing radiation induces activation of EGFR, the mechanism remains unknown. Recently, we reported that activation of P2Y receptors is involved in γ-radiation-induced activation of extracellular signal-regulated kinase1/2 (ERK1/2), which is dependent on activation of EGFR. Here we focused on the mechanism of activation of EGFR in response to low-dose γ radiation, mainly in terms of the activation-associated formation of EGFR foci in A549 cells. Irradiation of cells with 0.1 Gy γ rays induced biphasic phosphorylation of EGFR and ERK1/2 as well as biphasic formation of EGFR foci. The radiation-induced focus formation of EGFR was abolished by ecto-nucleotidase, P2Y receptor antagonists and knockdown of P2Y6 receptor, suggesting the involvement of extracellular nucleotides and activation of P2Y6 receptors in this process. Further, a disintegrin and metalloprotease 17 (ADAM17) is expressed in A549 cells and an ADAM17 inhibitor significantly blocked the radiation-induced focus formation of EGFR. We conclude that activation of both P2Y6 receptors and ADAM17 mediates the low-dose γ-radiation-induced activation of EGFR, as evaluated in terms of focus formation, in A549 cells.

X rays of 26–30 kVp are routinely used for mammography screening. For radioprotection purposes, a quality factor (Q) of 1 is assumed for all photon energies, but it is thought that the relative biological effectiveness (RBE) increases as the photon energy decreases. The analysis of radiation-induced chromosome aberrations is one of the most widely used methods to study the interaction between radiation and DNA. Here we present a FISH study on metaphases from peripheral blood samples irradiated with three different X-ray energies (30, 80 and 120 kVp). The study comprises two FISH approaches: one using pantelomeric and pancentromeric probes to evaluate the induction of incomplete chromosome aberrations and the other using mFISH to evaluate the induction of complex chromosome aberrations. The results indicate that exposure to 30 kVp X rays resulted in a modest increase in the induction of incomplete elements and complex aberrations compared to 80 and 120 kVp X rays.

This paper considers the dose–effect relationship for unstable chromosome aberration yields in human lymphocytes in very low-dose range. Data are presented for ⁶⁰Co γ-ray doses of 0, 10, 20, 40 and 1000 mGy. More than 5,000 metaphases were scored for each data point at the very low doses, and each cell was double-checked using a semi-automated metaphase finding/relocation system. Aberration yields of dicentrics plus centric rings followed an excellent linear dose response down to zero dose; the yields were significantly above the control frequency from 20 mGy.

mTOR (mammalian target of rapamycin) signaling plays a key role in the development of many tumor types. Therefore, mTOR is an attractive target for cancer therapeutics. Although mTOR inhibitors are thought to have radiosensitization activity, the molecular bases remain largely unknown. Here we show that treating MCF7 breast cancer cells with rapamycin (an mTOR inhibitor) results in significant suppression of homologous recombination (HR) and nonhomologous end joining (NHEJ), two major mechanisms required for repairing ionizing radiation-induced DNA DSBs. We observed that rapamycin impaired recruitment of BRCA1 and Rad51 to DNA repair foci, both essential for HR. Moreover, consistent with the suppressive role of rapamycin on both HR and NHEJ, persistent radiation-induced DSBs were detected in cells pretreated with rapamycin. Furthermore, the frequency of chromosome and chromatid breaks was increased in cells treated with rapamycin before and after irradiation. Thus our results show that radiosensitization by mTOR inhibitors occurs via disruption of the major two DNA DSB repair pathways.

In this study we investigated the effect of the Enhanced Data rate for GSM Evolution (EDGE) signal on cells of three human brain cell lines, SH-SY5Y, U87 and CHME5, used as models of neurons, astrocytes and microglia, respectively, as well as on primary cortical neuron cultures. SXC-1800 waveguides (IT'IS-Foundation, Zürich, Switzerland) were modified for in vitro exposure to the EDGE signal radiofrequency (RF) radiation at 1800 MHz. Four exposure conditions were tested: 2 and 10 W/kg for 1 and 24 h. The production of reactive oxygen species (ROS) was measured by flow cytometry using the dichlorofluorescein diacetate (DCFH-DA) probe at the end of the 24-h exposure or 24 h after the 1-h exposure. Rotenone treatment was used as a positive control. All cells tested responded to rotenone treatment by increasing ROS production. These findings indicate that exposure to the EDGE signal does not induce oxidative stress under these test conditions, including 10 W/kg. Our results are in agreement with earlier findings that RF radiation alone does not increase ROS production.

As therapeutic uses of high-LET radiation become more prevalent and human space exploration continues to be a focus of NASA, it is important to understand the biological effects of high-LET radiation and the role of genetics in sensitivity to high-LET radiation. To study genetic susceptibility to radiation, we used mice deficient in Atm activity (AtmΔSRI). ATM is important in DNA repair, apoptosis and cell cycle regulation. Although homozygous mutations in ATM are rare, the prevalence of ATM heterozygosity is estimated to be 1% and results in an increased cancer risk. We found that the effects of 1 Gy 1 GeV/nucleon ⁵⁶Fe particles on life span and tumorigenesis are genotype- and sex-specific. Significant effects of 1 Gy 1 GeV/nucleon ⁵⁶Fe particles on incidence of non-cancer end points were seen; however, 2 Gy 1 GeV/nucleon ⁵⁶Fe particles significantly affected neuromotor ability. Our results represent an extensive investigation into the late effects of high-LET radiation exposure in a sex- and genotype-dependent manner and provide a baseline for understanding the long-term risks of high-LET radiation.

Low-energy electrons (LEE) induce single- and double-strand breaks in DNA. To investigate the mechanism of LEE-induced DNA damage, nucleotides and short oligonucleotide were irradiated with monoenergetic electrons in the solid state and the modifications were observed by chemical analyses. With 10 eV electrons and TpTpT as the target, approximately one-third of the total damage of TpTpT involves cleavage of the phosphodiester-sugar bond (C-O) and the N-glycosidic bond (C-N). Here we focus on the remaining two-thirds of the damage. The major products were observed to elute between TpT and TpTpT on the HPLC chromatogram. Of these products, three modifications were identified as XpTpT, TpXpT and TpTpX, where X  =  5,6-dihydrothymine, on the basis of comparison with standard compounds using HPLC and mass spectrometry. These results suggest that 5,6-dihydrothymine is a major product of the reaction of LEE with DNA.

We have calculated the electronic energy loss of proton and α-particle beams in dry DNA using the dielectric formalism. The electronic response of DNA is described by the MELF-GOS model, in which the outer electron excitations of the target are accounted for by a linear combination of Mermin-type energy-loss functions that accurately matches the available experimental data for DNA obtained from optical measurements, whereas the inner-shell electron excitations are modeled by the generalized oscillator strengths of the constituent atoms. Using this procedure we have calculated the stopping power and the energy-loss straggling of DNA for hydrogen- and helium-ion beams at incident energies ranging from 10 keV/nucleon to 10 MeV/nucleon. The mean excitation energy of dry DNA is found to be I  =  81.5 eV. Our present results are compared with available calculations for liquid water showing noticeable differences between these important biological materials. We have also evaluated the electron excitation probability of DNA as a function of the transferred energy by the swift projectile as well as the average energy of the target electronic excitations as a function of the projectile energy. Our results show that projectiles with energy ≲100 keV/nucleon (i.e., around the stopping-power maximum) are more suitable for producing low-energy secondary electrons in DNA, which could be very effective for the biological damage of malignant cells.