Low-dose hyper-radiosensitivity (HRS) is the phenomenon whereby cells exposed to radiation doses of less than ∼0.5 Gy exhibit increased cell killing relative to that predicted from back-extrapolating high-dose survival data using a linear-quadratic model. While the exact mechanism remains to be elucidated, the involvement of several molecular repair pathways has been documented. These processes in turn are also associated with the response of cells to O6-methylguanine (O6MeG) lesions. We propose a model in which the level of low-dose cell killing is determined by the efficiency of both pre-replicative repair by the DNA repair enzyme O6-methylguanine methyltransferase (MGMT) and post-replicative repair by the DNA mismatch repair (MMR) system. We therefore hypothesized that the response of cells to low doses of radiation is dependent on the expression status of MGMT and MMR proteins. MMR (MSH2, MSH6, MLH1, PMS1, PMS2) and MGMT protein expression signatures were determined in a panel of normal (PWR1E, RWPE1) and malignant (22RV1, DU145, PC3) prostate cell lines and correlated with clonogenic survival and cell cycle analysis. PC3 and RWPE1 cells (HRS positive) were associated with MGMT and MMR proficiency, whereas HRS negative cell lines lacked expression of at least one (MGMT or MMR) protein. MGMT inactivation had no significant effect on cell survival. These results indicate a possible role for MMR-dependent processing of damage produced by low doses of radiation.

To assist in screening existing drugs for use as potential radioprotectors, we used a human unbiased 16,560 short interfering RNA (siRNA) library targeting the druggable genome. We performed a synthetic protection screen that was designed to identify genes that, when silenced, protected human glioblastoma T98G cells from γ-radiation-induced cell death. We identified 116 candidate protective genes, then identified 10 small molecule inhibitors of 13 of these candidate gene products and tested their radioprotective effects. Glyburide, a clinically used second-generation hypoglycemic drug, effectively decreased radiation-induced cell death in several cell lines including T98G, glioblastoma U-87 MG, and normal lung epithelial BEAS-2B and in primary cultures of astrocytes. Glyburide significantly increased the survival of 32D cl3 murine hematopoietic progenitor cells when administrated before irradiation. Glyburide was radioprotective in vivo (90% of C57BL/6NHsd female mice pretreated with 10 mg/kg glyburide survived 9.5 Gy total-body irradiation compared to 42% of irradiated controls, P  =  0.0249). These results demonstrate the power of unbiased siRNA synthetic protection screening with a druggable genome library to identify new radioprotectors.

Space travel imposes risks to human health, in large part by the increased radiation levels compared to those on Earth. To understand the effects of space radiation on humans, it is important to determine the underlying cellular mechanisms. While general dosimetry describes average radiation levels accurately, it says little about the actual physiological impact and does not provide biological information about individual cellular events. In addition, there is no information about the nature and magnitude of a systemic response through extra- and intercellular communication. To assess the stress response in human fibroblasts that were sent into space with the Foton-M3 mission, we have developed a pluralistic setup to measure DNA damage and inflammation response by combining global and local dosimetry, image cytometry and multiplex array technology, thereby maximizing the scientific output. We were able to demonstrate a significant increase in DNA double-strand breaks, determined by a twofold increase of the γ-H2AX signal at the level of the single cell and a threefold up-regulation of the soluble signal proteins CCL5, IL-6, IL-8, β-2 microglobulin and EN-RAGE, which are key players in the process of inflammation, in the growth medium.

Gene expression profiles were examined using cDNA microarray technology in human thyroid epithelial (Htori-3) cells exposed to a low, non-toxic dose (10 cGy) of radiation from HZE particles in the form of iron ions in the absence or presence of selenomethionine (SeM). A total of 215 genes were differentially regulated 2 h after exposure to a 10-cGy dose of iron-ion radiation. In the microarray analysis, SeM had profound effects on the radiation-induced expression of several specific genes, which includes PLAU, IGFBP3, FOLR1, B4GALT1 and COL1A1. Of particular interest to us was a gene cluster, “secreted proteins”, that was up-regulated after radiation exposure. Seven up-regulated genes of this gene cluster fall within the chemokine/cytokine gene cluster, namely, CXCL1, CXCL2, IL6, IL11, IL8, IL24 and TGFβ2. In microarray studies, the radiation-induced up-regulated expression of some these genes encoding cytokine/chemokine proteins was significantly decreased by SeM treatment. For IL8, TGFβ2, CXCL1 and CXCL2, these observations were validated by qPCR techniques. It is concluded that SeM can regulate ionizing radiation-induced gene expression and may serve as an effective countermeasure for some of the acute inflammatory/immune responses induced by low-dose HZE-particle radiation.

The aim of this study was to investigate the influence of 50 Hz magnetic or static magnetic fields of 0.5 mT on subsets of human CD4 T cells in terms of cytokine release/content, cell proliferation and intracellular free calcium concentration. CD4 T cells can be divided into different subsets on the basis of surface marker expression, such as CD45, and T cells can be divided into naive (CD45RA ) and memory (CD45RA⁻) cells. In this study, the effects of magnetic fields after 24 and 48 h of cell culture were analyzed. We found that the CD4 CD45RA⁻ T subset were more sensitive after 2 h of exposure. Decreases in the release/content of IFN-γ, in cell proliferation and in intracellular free calcium concentrations were observed in exposed CD4 CD45RA⁻ T cells compared to CD4 CD45RA T cells. The results suggest that exposure to the magnetic fields induces a delay in the response to stimulants and that modifications are rapidly reversible, at least after a short exposure.

Photodynamic therapy (PDT) produces singlet oxygen and reactive oxygen species (ROS) that damage tumor cells and the vasculature. The resulting effect is a balance between photo-oxidations through primary or secondary ROS and scavenging activity. Sensitizers are distributed in the extracellular space before and during cell sensitization, suggesting that PDT could act directly on cell structures and on extracellular compartments, including sera. In this study we endeavored to determine whether the application of PDT to culture medium could affect cell survival. Culture medium [RPMI 1640 supplemented with fetal calf serum (FCS)] was incubated with Rose Bengal and irradiated before being added to cells for various contact times as a replacement for untreated medium. Cells were then kept in darkness until the survival assay. Treated medium reduced cell survival by up to 40% after 30 min of contact for 10 µg/ml of Rose Bengal and 20 J/cm². Rose Bengal or m-THPC alone or irradiated in water had no effect. This effect was dependent on the doses of Rose Bengal and light and decreased when FCS was replaced by human serum mixed with FCS. The reduction in survival observed with treated medium was more pronounced when the cell doubling time was shorter. Analysis of ROS or peroxide production in treated medium by DCFH added at the end of irradiation of Rose Bengal in serum-containing medium revealed a long-lasting oxidizing activity. Our findings support the hypothesis of an ROS- or peroxide-mediated, PDT-induced, long-lasting cell toxicity.

The murine Chk2 kinase is activated after exposure to ionizing radiation and is necessary for p53-dependent apoptosis, but the role Chk2 plays in determining genomic stability is poorly understood. By analyzing the sensitivity of Chk2-deficient murine and human cells to a range of DNA-damaging agents, we show that Chk2 deficiency results in resistance to agents that generate double-strand breaks but not to other forms of damage. Surprisingly, the absence of Chk2 results in increased sensitivity to UV-radiation-induced DNA damage. Defective apoptosis after radiation-induced DNA damage may result in genomic instability; therefore, the consequences of Chk2 deficiency on genomic instability were assayed using an in vitro screen. Gene amplification was not detected in untreated Chk2^−/− cells, but the rate of gene amplification after irradiation was elevated and was similar to that found in p53 compromised cells. A synergistic increase in genomic instability was seen after disruption of both Chk2 and p53 function, indicating that the two proteins have non-redundant roles in regulating genome stability after irradiation. The data demonstrate that Chk2 functions to maintain genome integrity after radiation-induced damage and has important implications for the use of Chk2 inhibitors as adjuvant cancer therapy.

Interaction of solar protons and galactic cosmic radiation with the atmosphere and other materials produces high-energy secondary neutrons from below 1 to 1000 MeV and higher. Although secondary neutrons may provide an appreciable component of the radiation dose equivalent received by space and high-altitude air travelers, the biological effects remain poorly defined, particularly in vivo in intact organisms. Here we describe the acute response of Japanese medaka (Oryzias latipes) embryos to a beam of high-energy spallation neutrons that mimics the energy spectrum of secondary neutrons encountered aboard spacecraft and high-altitude aircraft. To determine RBE, embryos were exposed to 0–0.5 Gy of high-energy neutron radiation or 0–15 Gy of reference γ radiation. The radiation response was measured by imaging apoptotic cells in situ in defined volumes of the embryo, an assay that provides a quantifiable, linear dose response. The slope of the dose response in the developing head, relative to reference γ radiation, indicates an RBE of 24.9 (95% CI 13.6–40.7). A higher RBE of 48.1 (95% CI 30.0–66.4) was obtained based on overall survival. A separate analysis of apoptosis in muscle showed an overall nonlinear response, with the greatest effects at doses of less than 0.3 Gy. Results of this experiment indicate that medaka are a useful model for investigating biological damage associated with high-energy neutron exposure.

Gamma radiation is known to induce cell death in several organs. This damage is associated with endonuclease-mediated DNA fragmentation; however, the enzyme that produces the latter and is likely to cause cell death is unknown. To determine whether the most abundant cytotoxic endonuclease DNase I mediates γ-radiation-induced tissue injury, we used DNase I knockout mice and zinc chelate of 3,5-diisopropylsalicylic acid (Zn-DIPS), which, as we show, has DNase I inhibiting activity in vitro. The study demonstrated for the first time that inactivation or inhibition of DNase I ameliorates radiation injury to the white pulp of spleen, intestine villi and bone marrow as measured using a quantitative TUNEL assay. The spleen and intestine of DNase I knockout mice were additionally protected from radiation by Zn-DIPS, perhaps due to the broad radioprotective effect of the zinc ions. Surprisingly, the main DNase I-producing tissues such as the salivary glands, pancreas and kidney showed no effect of DNase I inactivation. Another unexpected observation was that even without irradiation, DNA fragmentation and cell death were significantly lower in the intestine of DNase I knockout mice than in wild-type mice. This points to the physiological role of DNase I in normal cell death in the intestinal epithelium. In conclusion, our results suggested that DNase I-mediated mechanism of DNA damage and subsequent tissue injury are essential in γ-radiation-induced cell death in radiosensitive organs.

In boron neutron capture therapy, the absorbed dose from the ¹⁰B(n,α)⁷Li reaction depends on the ¹⁰B concentration and ¹⁰B distribution in the irradiated volume. Thus compounds used in BNCT should have tumor-specific uptake and low accumulation in normal tissues. This study compares in a mouse model the ¹⁰B uptake in different organs as delivered by l-para-boronophenylalanine (BPA, 700 mg/kg body weight, i.p.) and/or sodium mercaptoundecahydro-closo-dodecaborate (BSH, 200 mg/kg body weight, i.p). After BSH injection, the ¹⁰B concentration was high in kidneys (20 ± 12 µg/g) and liver (20 ± 12 µg/g) but was low in brain (1.0 ± 0.8 µg/g) and muscle (1.9 ± 1.2 µg/g). After BPA injection, the ¹⁰B concentration was high in kidneys (38 ± 25 µg/g) and spleen (17 ± 8 µg/g) but low in brain (5 ± 3 µg/g). After combined BPA and BSH injection, the effect on the absolute ¹⁰B concentration was additive in all organs. The ratio of the ¹⁰B concentrations in tissues and blood differed significantly for the two compounds depending on the compound combination, which implies a different uptake profile for normal organs.

The use of nuclear resources for medical purposes causes considerable concern about occupational exposure. Nevertheless, little information is available regarding the effects of low-dose irradiations protracted over time. We used oligomicroarrays to identify the genes that are transcriptionally regulated by persistent exposure to extremely low doses of ionizing radiation in 28 exposed professionals (mean cumulative effective dose ± SD, 19 ± 38 mSv) compared with a matched sample of nonexposed subjects. We identified 256 modulated genes from peripheral blood mononuclear cells profiles, and the main biological processes we found were DNA packaging and mitochondrial electron transport NADH to ubiquinone. Next we investigated whether a different pattern existed when only 22 exposed subjects with accumulated doses >2.5 mSv, a threshold corresponding to the natural background radiation in Italy per year, and mean equal to 25 ± 41 mSv were used. In addition to DNA packaging and NADH dehydrogenase function, the analysis of the higher-exposed subgroup revealed a significant modulation of ion homeostasis and programmed cell death as well. The changes in gene expression that we found suggest different mechanisms from those involved in high-dose studies that may help to define new biomarkers of radiation exposure for accumulated doses below 25 mSv.

Two recent studies analyzed thyroid cancer incidence in Belarus and Ukraine during the period from 1990 to 2001, for the birth cohort 1968 to 1985, and the related ¹³¹I exposure associated with the Chernobyl accident in 1986. Contradictory age-at-exposure and time-since-exposure effect modifications of the excess relative risk (ERR) were reported. The present study identifies the choice of baseline modeling method as the reason for the conflicting results. Various quality-of-fit criteria favor a parametric baseline model to various categorical baseline models. The model with a parametric baseline results in a decrease of the ERR by a factor of about 0.2 from an age at exposure of 5 years to an age at exposure of 15 years (for a time since exposure of 12 years) and a decrease of the ERR from a time since exposure of 4 years to a time since exposure of 14 years of about 0.25 (for an age at exposure of 10 years). Central ERR estimates (of about 20 at 1 Gy for an age at exposure of 10 years and an attained age of 20 years) and their ratios for females compared to males (about 0.3) turn out to be relatively independent of the modeling. Excess absolute risk estimates are also predicted to be very similar from the different models. Risk models with parametric and categorical baselines were also applied to thyroid cancer incidence among the atomic bomb survivors. For young ages at exposure, the ERR values in the model with a parametric baseline are larger. Both data sets cover the period of 12 to 15 years since exposure. For this period, higher ERR values and a stronger age-at-exposure modification are found for the Chernobyl data set. Based on the results of the study, it is recommended to test parametric and categorical baseline models in risk analyses.

The effects of TMG [2-(α-d-glucopyranosyl) methyl-2,5,7,8-tetramethylchroman-6-ol], a water-soluble vitamin E derivative, administered after irradiation on the mortality of X-irradiated mice and on the development of tumors in the mammary and pituitary glands in rats were investigated. When TMG (650 mg/kg) was administered intraperitoneally (i.p.) to C3H mice immediately after whole-body exposure to 7 Gy radiation, the 30-day survival was significantly higher than that of the control mice. The i.p. administration of TMG at 4 h after irradiation significantly improved survival compared to that of the controls, but administration 8 h after irradiation did not have a significant effect. Subcutaneous administration of TMG immediately after irradiation also decreased mortality significantly. When dams of lactating Wister rats were exposed to 1.5 Gy of X rays at day 21 after parturition and were then treated with diethylstilbestrol as a tumor promoter, the incidence of mammary tumors and pituitary tumors was increased compared to that in the nonirradiated control group. The administration of TMG (600 mg/kg, i.p.) after irradiation significantly reduced the incidence of mammary tumors and pituitary tumors. The number of rats that were free of both mammary and pituitary gland tumors was enhanced fourfold by TMG. These results suggest that TMG is effective in preventing radiation-induced bone marrow death in mice and in reducing mammary and pituitary tumors in rats even when it is administered after irradiation.