Szumiel, I. Intrinsic Radiation Sensitivity: Cellular Signaling is the Key. Radiat. Res. 169, 249–258 (2008).

The concept that the balance between DNA damage and repair determines intrinsic radiation sensitivity has dominated radiobiology for several decades. There is undeniably a cause– effect relationship between radiation-induced molecular alterations in the genomic DNA and cellular consequences. In the last decade, however, it has become obvious that the chromatin context affects the fate of damaged DNA and that cellular signaling is an important factor in defining intrinsic radiation sensitivity. Damaged DNA is the site of signal generation; however, alternative signaling at the plasma membrane is triggered: Reactive oxygen species (ROS) inactivate phosphatases and consequently cause activation of kinases localized at the plasma membrane; this includes ligand-independent activation of receptor kinases. Cells with an apparently functional DNA repair system may show increased radiation sensitivity due to deficiencies in specific kinases essential for repair activation and checkpoint control. Other signals that determine intrinsic radiosensitivity may affect proneness to apoptosis, the balance between DNA damage fixation and repair, and the translocation of proteins participating in the response to ionizing radiation. Interplay between the various signals decides the extent to which the repair of radiation-inflicted damage is supported or limited; in some cell types, this includes DNA-damage-independent processes guided by plasma membrane-generated signaling. Cellular signaling in the context of specific subcellular structures is the key to understanding how the molecular effects of radiation are expressed as biological consequences in various cell types. A systems approach should bring us closer to this end.

Kimmel, R. R., Agnani, S., Yang, Y., Jordan, R. and Schwartz, J. L. DNA Copy-Number Instability in Low-Dose Gamma-Irradiated TK6 Lymphoblastoid Clones. Radiat. Res. 169, 259–269 (2008).

Genomic instability that might occur early during low-dose, fractionated radiation exposures may be traceable in radiogenic compared to spontaneous cancers. Using a human 18K cDNA microarray-based comparative genome hybridization protocol, we measured changes in DNA copy number at over 14,000 loci in nine low-dose ¹³⁷Cs γ-irradiated (acute exposure to 10 cGy/day × 21 days) and nine unirradiated TK6 clones and estimated locus-specific copy-number differences between them. Radiation induced copy-number hypervariability at thousands of loci across all chromosomes, with a sevenfold increase in low-level, randomly positioned DNA gains. Recurrent gains at 40 loci occurred among irradiated clones and were distributed nonrandomly across the genome, with the highest densities in 3q, 13q and 20q at sites that were hypodiploid without irradiation. Another nonrandomly distributed set of 94 loci exhibited relative recurrent gains from a hypodiploid state to a diploid state, suggesting hemizygous-to-homozygous transitions. Frequently recurring losses at 57 loci were concentrated on the single X-chromosome but were sparsely distributed at 0–2 loci per autosome. These results suggest induced mitotic homologous recombination as a possible mechanism of low-dose radiation-induced genomic instability. Genomic instability induced in TK6 cells resembled that seen in radiogenic tumors and suggests a way that radiation could induce genomic instability in preneoplastic cells.

Valbonesi, P., Franzellitti, S., Piano, A., Contin, A., Biondi, C. and Fabbri, E. Evaluation of HSP70 Expression and DNA Damage in Cells of a Human Trophoblast Cell Line Exposed to 1.8 GHz Amplitude-Modulated Radiofrequency Fields. Radiat. Res. 169, 270–279 (2008).

The aim of this study was to determine whether high-frequency electromagnetic fields (EMFs) could induce cellular effects. The human trophoblast cell line HTR-8/SVneo was used as a model to evaluate the expression of proteins (HSP70 and HSC70) and genes (HSP70A, B, C and HSC70) of the HSP70 family and the primary DNA damage response after nonthermal exposure to pulse-modulated 1817 MHz sinusoidal waves (GSM-217 Hz; 1 h; SAR of 2 W/kg). HSP70 expression was significantly enhanced by heat, which was applied as the prototypical stimulus. The HSP70A, B and C transcripts were differentially expressed under basal conditions, and they were all significantly induced above basal levels by thermal stress. Conversely, HSC70 protein and gene expression was not influenced by heat. Exposing HTR-8/SVneo cells to high-frequency EMFs did not change either HSP70 or HSC70 protein or gene expression. A significant increase in DNA strand breaks was caused by exposure to H₂O₂, which was used as a positive stimulus; however, no effect was observed after exposure of cells to high-frequency EMFs. Overall, no evidence was found that a 1-h exposure to GSM-217 Hz induced a HSP70-mediated stress response or primary DNA damage in HTR-8/SVneo cells. Nevertheless, further investigations on trophoblast cell responses after exposure to GSM signals of different types and durations are needed.

Gridley, D. S., Coutrakon, G. B., Rizvi, A., Bayeta, E. J. M., Luo-Owen, X., Makinde, A. Y., Baqai, F., Koss, P., Slater, J. M. and Pecaut, M. J. Low-Dose Photons Modify Liver Response to Simulated Solar Particle Event Protons. Radiat. Res. 169, 280–287 (2008).

The health consequences of exposure to low-dose radiation combined with a solar particle event during space travel remain unresolved. The goal of this study was to determine whether protracted radiation exposure alters gene expression and oxidative burst capacity in the liver, an organ vital in many biological processes. C57BL/6 mice were whole-body irradiated with 2 Gy simulated solar particle event (SPE) protons over 36 h, both with and without pre-exposure to low-dose/low-dose-rate photons (⁵⁷Co, 0.049 Gy total at 0.024 cGy/h). Livers were excised immediately after irradiation (day 0) or on day 21 thereafter for analysis of 84 oxidative stress-related genes using RT-PCR; genes up or down-regulated by more than twofold were noted. On day 0, genes with increased expression were: photons, none; simulated SPE, Id1; photons simulated SPE, Bax, Id1, Snrp70. Down-regulated genes at this same time were: photons, Igfbp1; simulated SPE, Arnt2, Igfbp1, Il6, Lct, Mybl2, Ptx3. By day 21, a much greater effect was noted than on day 0. Exposure to photons simulated SPE up-regulated completely different genes than those up-regulated after either photons or the simulated SPE alone (photons, Cstb; simulated SPE, Dctn2, Khsrp, Man2b1, Snrp70; photons simulated SPE, Casp1, Col1a1, Hspcb, Il6st, Rpl28, Spnb2). There were many down-regulated genes in all irradiated groups on day 21 (photons, 13; simulated SPE, 16; photons simulated SPE, 16), with very little overlap among groups. Oxygen radical production by liver phagocytes was significantly enhanced by photons on day 21. The results demonstrate that whole-body irradiation with low-dose-rate photons, as well as time after exposure, had a great impact on liver response to a simulated solar particle event.

Millenbaugh, N. J., Roth, C., Sypniewska, R., Chan, V., Eggers, J. S., Kiel, J. L., Blystone, R. V. and Mason, P. A. Gene Expression Changes in the Skin of Rats Induced by Prolonged 35 GHz Millimeter-Wave Exposure. Radiat. Res. 169, 288–300 (2008).

To better understand the cellular and molecular responses to overexposure to millimeter waves, alterations in the gene expression profile and histology of skin after exposure to 35 GHz radiofrequency radiation were investigated. Rats were subjected to sham exposure, to 42°C environmental heat, or to 35 GHz millimeter waves at 75 mW/cm². Skin samples were collected at 6 and 24 h after exposure for Affymetrix GeneChip analysis. The skin was harvested from a separate group of rats at 3–6 h or 24–48 h after exposure for histopathology analysis. Microscopic findings observed in the dermis of rats exposed to 35 GHz millimeter waves included aggregation of neutrophils in vessels, degeneration of stromal cells, and breakdown of collagen. Changes were detected in 56 genes at 6 h and 58 genes at 24 h in the millimeter-wave-exposed rats. Genes associated with regulation of transcription, protein folding, oxidative stress, immune response, and tissue matrix turnover were affected at both times. At 24 h, more genes related to extracellular matrix structure and chemokine activity were altered. Up-regulation of Hspa1a, Timp1, S100a9, Ccl2 and Angptl4 at 24 h by 35 GHz millimeter-wave exposure was confirmed by real-time RT-PCR. These results obtained from histopathology, microarrays and RT-PCR indicate that prolonged exposure to 35 GHz millimeter waves causes thermally related stress and injury in skin while triggering repair processes involving inflammation and tissue matrix recovery.

Maier, P., Herskind, C., Fleckenstein, K., Spier, I., Laufs, S., Jens Zeller, W., Fruehauf, S. and Wenz, F. MDR1 Gene Transfer Using a Lentiviral SIN Vector Confers Radioprotection to Human CD34 Hematopoietic Progenitor Cells. Radiat. Res. 169, 301–310 (2008).

Tumor radiotherapy with large-field irradiation results in an increase in apoptosis of the radiosensitive hematopoietic stem cells (CD34 ). The aim of this study was to demonstrate the radioprotective potential of MDR1 overexpression in human CD34 cells using a lentiviral self-inactivating vector. Transduced human undifferentiated CD34 cells were irradiated with 0–8 Gy and held in liquid culture under myeloid-specific maturation conditions. After 12 days, MDR1 expression was determined by the rhodamine efflux assay. The proportion of MDR1-positive cells in cells from four human donors increased with increasing radiation dose (up to a 14-fold increase at 8 Gy). Determination of expression of myeloid-specific surface marker proteins revealed that myeloid differentiation was not affected by transduction and MDR1 overexpression. Irradiation after myeloid differentiation also led to an increase of MDR1-positive cells with escalating radiation doses (e.g. 12.5–16% from 0–8 Gy). Most importantly, fractionated irradiation (3 × 2 Gy; 24-h intervals) of MDR1-transduced CD34 cells resulted in an increase in MDR1-positive cells (e.g. 3–8% from 0–3 × 2 Gy). Our results clearly support a radioprotective effect of lentiviral MDR1 overexpression in human CD34 cells. Thus enhancing repopulation by surviving stem cells may increase the radiation tolerance of the hematopoietic system, which will contribute to widening the therapeutic index in radiotherapy.

Elmore, E., Lao, X-Y., Kapadia, R., Giedzinski, E., Limoli, C. and Redpath, J. L. Low Doses of Very Low-Dose-Rate Low-LET Radiation Suppress Radiation-Induced Neoplastic Transformation In Vitro and Induce an Adaptive Response. Radiat. Res. 169, 311–318 (2008).

The purpose of this study was to determine whether adaptation against neoplastic transformation could be induced by exposure to very low-dose-rate low-LET radiation. HeLa × skin fibroblast human hybrid cells were irradiated with ∼30 kVp photons from an array of ¹²⁵I seeds. The initial dose rate was 4 mGy/day. Cell samples were taken at four intervals at various times over a period of 88 days and assayed for neoplastic transformation and the presence of reactive oxygen species (ROS). The dose rate at the end of this treatment period was 1.4 mGy/day. Transformation frequencies and ROS levels were compared to those of parallel unirradiated controls. At the end of 3 months and an accumulated dose of 216 mGy, cells treated with very low-dose-rate radiation were exposed to a high-dose-rate 3-Gy challenge dose of ¹³⁷Cs γ rays, and the effects compared with the effect of 3 Gy on a parallel culture of previously unirradiated cells. Cells exposed to very low-dose-rate radiation exhibited a trend toward a reduction in neoplastic transformation frequency compared to the unirradiated controls. This reduction seemed to diminish with time, indicating that the dose rate, rather than accumulated dose, may be the more important factor in eliciting an adaptive response. This pattern was in general paralleled by a reduction of ROS present in the irradiated cultures compared to controls. The very low-dose-rate-treated cells were less sensitive to the high challenge dose than unirradiated controls, suggesting the induction of an adaptive response. Since there was a suggestion of a dose-rate threshold for induction suppression, a second experiment was run with a fresh batch of cells at an initial dose rate of 1 mGy/day. These cells were allowed to accumulate 40 mGy over 46 days (average dose rate = 0.87 mGy/day), and there was no evidence for suppression of transformation frequency compared to parallel unirradiated controls. It is concluded that doses of less than 100 mGy delivered at very low dose rates in the range 1 to 4 mGy/day can induce an adaptive response against neoplastic transformation in vitro. When the dose rate drops below ∼1 mGy/day, this suppression is apparently lost, suggesting a possible dose-rate-dependent threshold for this process.

Rao, V. S., Titushkin, I. A., Moros, E. G., Pickard, W. F., Thatte, H. S. and Cho, M. R. Nonthermal Effects of Radiofrequency-Field Exposure on Calcium Dynamics in Stem Cell-Derived Neuronal Cells: Elucidation of Calcium Pathways. Radiat. Res. 169, 319–329 (2008).

Intracellular Ca² spikes trigger cell proliferation, differentiation and cytoskeletal reorganization. In addition to Ca² spiking that can be initiated by a ligand binding to its receptor, exposure to electromagnetic stimuli has also been shown to alter Ca² dynamics. Using neuronal cells differentiated from a mouse embryonic stem cell line and a custom-built, frequency-tunable applicator, we examined in real time the altered Ca² dynamics and observed increases in the cytosolic Ca² in response to nonthermal radiofrequency (RF)-radiation exposure of cells from 700 to 1100 MHz. While about 60% of control cells (not exposed to RF radiation) were observed to exhibit about five spontaneous Ca² spikes per cell in 60 min, exposure of cells to an 800 MHz, 0.5 W/kg RF radiation, for example, significantly increased the number of Ca² spikes to 15.7 ± 0.8 (P < 0.05). The increase in the Ca² spiking activities was dependent on the frequency but not on the SAR between 0.5 to 5 W/kg. Using pharmacological agents, it was found that both the N-type Ca² channels and phospholipase C enzymes appear to be involved in mediating increased Ca² spiking. Interestingly, microfilament disruption also prevented the Ca² spikes. Regulation of Ca² dynamics by external physical stimulation such as RF radiation may provide a noninvasive and useful tool for modulating the Ca² -dependent cellular and molecular activities of cells seeded in a 3D environment for which only a few techniques are currently available to influence the cells.

Lehnert, A., Dörr, W., Lessmann, E. and Pawelke, J. RBE of 10 kV X Rays Determined for the Human Mammary Epithelial Cell Line MCF-12A. Radiat. Res. 169, 330–336 (2008).

The dependence of relative biological effectiveness (RBE) on photon energy is a topic of extensive discussions. The increasing amount of in vitro data in the low-energy region indicates this to be a complex dependence that is influenced by the end point and cell line studied. In the present investigation, the RBE of 10 kV X rays (W anode) was determined relative to 200 kV X rays (W anode, 0.5 mm copper filter) for cell survival in the dose range 1–10 Gy and for induction of micronuclei in the range 0.5–3.6 Gy for MCF-12A human mammary epithelial cells. The RBE for cell survival was found to increase with decreasing dose, being 1.21 ± 0.03 at 10% survival. Considerably higher values were obtained for micronucleus induction, where the RBE_M obtained from the ratio of the linear coefficients of the dose–effect curves was 2.6 ± 0.4 for the fraction of binucleated cells with micronuclei and 4.1 ± 1.0 for the number of micronuclei per binucleated cell. These values, together with our previous data, support a monotonic increase in RBE with decreasing photon energy down to the mean energy of 7.3 keV used in the present study.

Djeridane, Y., Touitou, Y. and de Seze, R. Influence of Electromagnetic Fields Emitted by GSM-900 Cellular Telephones on the Circadian Patterns of Gonadal, Adrenal and Pituitary Hormones in Men. Radiat. Res. 169, 337–343 (2008).

The potential health risks of radiofrequency electromagnetic fields (RF EMFs) emitted by mobile phones are currently of considerable public interest. The present study investigated the effect of exposure to 900 MHz GSM radiofrequency radiation on steroid (cortisol and testosterone) and pituitary (thyroid-stimulating hormone, growth hormone, prolactin and adrenocorticotropin) hormone levels in 20 healthy male volunteers. Each subject was exposed to RF EMFs through the use of a cellular phone for 2 h/day, 5 days/ week, for 4 weeks. Blood samples were collected hourly during the night and every 3 h during the day. Four sampling sessions were performed at 15-day intervals: before the beginning of the exposure period, at the middle and the end of the exposure period, and 15 days later. Parameters evaluated included the maximum serum concentration, the time of this maximum, and the area under the curve for hormone circadian patterns. Each individual's pre-exposure hormone concentration was used as his control. All hormone concentrations remained within normal physiological ranges. The circadian profiles of prolactin, thyroid-stimulating hormone, adrenocorticotropin and testosterone were not disrupted by RF EMFs emitted by mobile phones. For growth hormone and cortisol, there were significant decreases of about 28% and 12%, respectively, in the maximum levels when comparing the 2-week (for growth hormone and cortisol) and 4-week (for growth hormone) exposure periods to the pre-exposure period, but no difference persisted in the postexposure period. Our data show that the 900 MHz EMF exposure, at least under our experimental conditions, does not appear to affect endocrine functions in men.

Turnbull, D. J. and Parisi, A. V. Optimizing Solar UV-Radiation Exposures for Vitamin D₃: Comparing Global and Diffuse Spectral UV Radiation. Radiat. Res. 169, 344–349 (2008).

Currently, there is a major gap in the knowledge that is needed to optimize the beneficial effects related to ultraviolet (UV) radiation at wavelengths that induce vitamin D₃ synthesis (UV_D3) compared to reducing the biologically damaging overexposure to UV radiation. The aim of this study was to investigate the use of diffuse (radiation that is scattered from all directions) UV radiation to optimize exposures to UV_D3 radiation and maximize the reduction of exposure to UVA radiation. Data on global and diffuse solar UV-radiation spectra were collected at 10-min intervals in the Southern Hemisphere in the late spring and summer from 1 November 2006 to 28 February 2007. For a solar zenith angle (SZA) of approximately 5°, the observed maximum UV_D3 irradiances were 0.80 W/m² and 0.46 W/m² for global and diffuse UV radiation, respectively. The observed maximum UVA irradiances were 79.0 W/m² and 36.2 W/m² for global and diffuse UV radiation, respectively. For diffuse UV radiation, the maximum ratio of vitamin D₃ to UVA radiation was 1.75% at a SZA of approximately 10°, whereas the maximum ratio for global UV was 1.27% at 10°. For SZAs of 25° and less, more UV radiation is in the wavelength region contributing to vitamin D₃ synthesis (UV_D3) than in the UVA region for diffuse UV radiation than for global UV radiation.

Kirkpatrick, J. P. and Dewhirst, M. W. Analytic Solution to Steady-State Radial Diffusion of a Substrate with First-Order Reaction in the Tissue of a Krogh's Cylinder. Radiat. Res. 169, 350–354 (2008).

It is often useful to calculate the concentration profile for a substrate undergoing reaction in the tissue surrounding a capillary. In this paper, we consider a model geometry consisting of a long straight cylinder of tissue surrounding a capillary. Substrate diffuses radially out of the capillary through the tissue, with consumption of substrate in the tissue directly proportional to substrate concentration (i.e., first-order reaction kinetics). The model is extended to include the case where a cylinder of necrotic tissue surrounds a metabolically active inner tissue cylinder. A simple analytic solution is derived, and concentration profiles are generated for various combinations of parameters. Compared to the case where substrate consumption is independent of concentration, this model predicts much more rapid depletion of substrate near the capillary interface. This can have significant implications for the calculation of the hypoxic fraction (e.g., tissue with pO₂ <0.5–5 mmHg) when tumor oxygenation is modeled. The model also permits calculation of the limiting substrate concentration for cell viability when the reaction rate constant is known and vice versa.

Naumov, S. and von Sonntag, C. The Energetics of Rearrangement and Water Elimination Reactions in the Radiolysis of the DNA Bases in Aqueous Solution (e_aq⁻ and ^·OH Attack): DFT Calculations. Radiat. Res. 169, 355–363 (2008).

DFT calculations on the relative stability of various nucleobase radicals induced by e_aq⁻ and ^·OH have been carried out for assessing the energetics of rearrangements and water elimination reactions, taking the solvent effect of water into account. Uracil and thymine radical anions are protonated fast at O2 and O4, whereby the O2-protonated anions are higher in energy (50 kJ mol⁻¹, equivalent to a 9-unit lower pK_a). The experimentally observed pK_a = 7 is thus that of the O4-protonated species. Thermodynamically favored protonation occurs slowly at C6 (driving force, thymine: 49 kJ mol⁻¹, uracil: 29 kJ mol⁻¹). The cytosine radical anion is rapidly protonated by water at N3. Final protonation at C6 is disfavored here. The kinetically favored pyrimidine C5 ^·OH adducts rearrange into the thermodynamically favored C6 ^·OH adducts (driving force, thymine: 42 kJ mol⁻¹). Very similar in energy is a water elimination that leads to the Ura-5-methyl radical. Purine ^·OH adducts at C4 and C5 (plus C2 in guanine) eliminate water in exothermic reactions, while water elimination from the C8 ^·OH adducts is endothermic. The latter open the ring en route to the FAPY products, an H transfer from the C8^·OH to N9 being the most likely process.

Naumov, S. and von Sonntag, C. Guanine-Derived Radicals: Dielectric Constant-Dependent Stability and UV/Vis Spectral Properties: A DFT Study. Radiat. Res. 169, 364–372 (2008).

Upon successive deprotonation of the guanine radical cation, various neutral radicals and radical anions can be formed. Their relative stability and UV/Vis absorption spectra have been calculated by DFT in the vacuum and in aqueous solution. Good agreement with experimental data is obtained when solvent effects are taken into account. The experimental observation that in the nucleosides deprotonation of the guanine radical cation occurs at N1 (formation of N1G^·) in water and at N2 (formation of N2G^·) in single crystals is now explained by a strong effect of the dielectric constant of the environment on their stability. While SCRF=PCM and CPCM (Gaussian 03) describe the trend, SCRF=DPCM (Gaussian 98) even shows the crossover from N2G^· to N1G^· at high dielectric constant. A crossover of the preferred deprotonation site is also given by the nucleoside itself. While for the gas phase a deprotonation at N2 is calculated to be favored over that at N1, the reverse is found for an aqueous environment (in agreement with the experiment). The radical anions of guanine, N9N1G^·− and N9N2G^·−, are very similar in energy, but a comparison of the experimental and calculated UV/Vis spectra allows us to identify the experimentally observed intermediate clearly as N9N1G^·−.