Effects of heavy ions and energetic protons on normal human fibroblasts

At the low particle fluences of radiation to which astronauts are exposed in space, "non-targeted" effects such as the bystander response may have increased significance. The radiation-induced bystander effect is the occurrence of biological responses in unirradiated cells near to or sharing medium with cells traversed by radiation. The objectives of this study were to establish the responses of AG01522 diploid human fibroblasts after exposure to several heavy ions and energetic protons, as compared to X-rays, and to obtain initial information on the bystander effect in terms of cell clonogenic survival after Fe ion irradiation. Using a clonogenic survival assay, relative biological effectiveness (RBE) values at 10% survival were 2.5, 2.3, 1.0 and 1.2 for 1 GeV/amu Fe, 1 GeV/amu Ti, 290 MeV/amu C and 1 GeV/amu protons, respectively, compared to 250 kVp X-rays. For induction of micronuclei (MN), compared to the low LET protons, Fe and Ti are very effective inducers of damage, although C ions are similar to protons. Using a transwell insert system in which irradiated and unirradiated bystander cells share medium but are not touching each other, it was found that clonogenic survival in unirradiated bystander cells was decreased when irradiated cells were exposed to Fe ions or X-rays. The magnitude of the decrease in bystander survival was similar with both radiation types, reaching a plateau of about 80% survival at doses of about 0.5 Gy or larger.     

Redox changes induced in hippocampal precursor cells by heavy ion irradiation

Hippocampal precursors retain the capacity to proliferate and differentiate throughout life, and their progeny, immature neurons, can undergo neurogenesis, a process believed to be important in maintaining the cognitive health of an organism. A variety of stresses including irradiation have been shown to deplete neural precursor cells, an effect that inhibits neurogenesis and is associated with the onset of cognitive impairments. Our past work has shown that neural precursor cells exposed to X-rays or protons exhibit a prolonged increase in oxidative stress, a factor we hypothesize to be critical in regulating the function of these cells after irradiation and other stresses. Here we report that irradiation of hippocampal precursor cells with high-linear energy transfer (LET) 1 GeV/nucleon ⁵⁶Fe ions leads to significantly higher levels of oxidative stress when compared to lower LET radiations (X-rays, protons). Irradiation with 1 Gy of ⁵⁶Fe ions elicits twofold to fivefold higher levels of reactive oxygen species (ROS) compared to unirradiated controls, and at lower doses (≤1 Gy) neural precursors exhibit a linear dose response 6 h after heavy ion exposure. The use of the antioxidant lipoic acid (LA) was able to reduce ROS levels below background levels when added before or after ⁵⁶Fe ion irradiation. These results conclusively show that low doses of ⁵⁶Fe ions can elicit significant levels of oxidative stress in neural precursor cells. Given the prevalence of heavy ions in space and the duration of interplanetary travel, these data suggest that astronauts are at risk for developing cognitive decrements. However, our results also indicate that antioxidants delivered before as radioprotective agents or after as mitigating agents hold promise as effective countermeasures for ameliorating certain adverse effects of heavy ion exposure to the CNS.     

Effects of 28Si Ions, 56Fe Ions,and Protons on the Induction of Murine Acute Myeloid Leukemia and Hepatocellular Carcinoma

Estimates of cancer risks posed to space-flight crews by exposure to high atomic number, high-energy (HZE) ions are subject to considerable uncertainty because epidemiological data do not exist for human populations exposed to similar radiation qualities. We assessed the carcinogenic effects of 300 MeV/n ²⁸Si or 600 MeV/n ⁵⁶Fe ions in a mouse model for radiation-induced acute myeloid leukemia and hepatocellular carcinoma. C3H/HeNCrl mice were irradiated with 0.1, 0.2, 0.4, or 1 Gy of 300 MeV/n ²⁸Si ions, 600 MeV/n ⁵⁶Fe ions or 1 or 2 Gy of protons simulating the 1972 solar particle event (1972SPE) at the NASA Space Radiation Laboratory. Additional mice were irradiated with ¹³⁷Cs gamma rays at doses of 1, 2, or 3 Gy. All groups were followed until they were moribund or reached 800 days of age. We found that ²⁸Si or ⁵⁶Fe ions do not appear to be substantially more effective than gamma rays for the induction of acute myeloid leukemia. However, ²⁸Si or ⁵⁶Fe ion irradiated mice had a much higher incidence of hepatocellular carcinoma than gamma ray irradiated or proton irradiated mice. These data demonstrate a clear difference in the effects of these HZE ions on the induction of leukemia compared to solid tumors, suggesting potentially different mechanisms of tumorigenesis. Also seen in this study was an increase in metastatic hepatocellular carcinoma in the ²⁸Si and ⁵⁶Fe ion irradiated mice compared with those exposed to gamma rays or 1972SPE protons, a finding with important implications for setting radiation exposure limits for space-flight crew members.     

HZE radiation effects for hereditary renal carcinomas.

Eker rat known as a model of hereditary renal carcinoma (RC) is an example of Mendelian dominantly inherited predisposition to a specific cancer in experimental animals. We investigate the effects of simulated space radiation on carcinogenesis using HIMAC. We estimated RBE from the Eker rats exposed to the heavy-ions, C (290 MeV/u) and Fe (500 MeV/u) ions, comparing to the effects of X-ray irradiation. Pregnant rats were exposed to C and Fe ions and X-rays with a single dose of 1 Gy, 2 Gy, 3 Gy on day 19 of gestation. The offspring were sacrificed at 8 weeks of age. We evaluated organ weights and tumor genesis. The weights of thymus, lung, liver, spleen were found to be no difference from the control at 1 Gy irradiation but 50% decrease at 3 Gy irradiation. We found in the irradiated animal that kidney, brain and testis were very sensitive organs of which the weight decreased to approximately 80% at 1 Gy and to 40% at 3 Gy irradiations. Based on the dose-response relationship of the radiation-induced carcinoma, averaged RBE ware calculated to be 1.1 for C-ion, 1.6 for Fe-ion.     

Lung tumour induction in mice after uniform and non-uniform external thoracic X-irradiation

The aim of this study was to investigate the validity of the ICRP procedure of using average tissue/organ dose in estimating carcinogenic risk. It has been suggested that highly non-uniform exposure ('hot spots') is much more carcinogenic than an equivalent dose delivered uniformly. In a series of experiments, mice were irradiated with X-rays either uniformly to the thorax or non-uniformly with 72 1-mm microbeams which irradiated approximately 20 per cent of the total lung volume. Two experiments involving uniform irradiation showed a peaked tumour incidence curve with a maximum at 5 Gy. The first 'microbeam' study also produced a pronounced peak in the dose response with a maximum tumour incidence at 1 Gy average lung dose or 5 Gy to the irradiated lung tissue. This implied the use of average tissue dose might underestimate the carcinogenic hazard of non-uniform exposure. Later, more extensive, microbeam experiments failed to replicate this finding. The results were nearly similar to those for uniform irradiation, with a slight increase in tumour incidence from 2.5-5.0 Gy average lung dose. These results imply that for these irradiation conditions the ICRP dose averaging procedure remains valid.     

Isolation and characterization of highly radioresistant malignant hamster fibroblasts that survive acute gamma irradiation with 20 Gy

To study the acquired radioresistance of tumor cells, a model system of two cell lines, Djungarian hamster fibroblasts (DH-TK-) and their radioresistant progeny, was established. The progeny of irradiated cells were isolated by treating the parental cell monolayer with a single dose of 20 Gy (PIC-20). The genetic and morphological features, clonogenic ability, radiosensitivity, cell growth kinetics, ability to grow in methylcellulose, and tumorigenicity of these cell lines were compared. The plating efficiency of PIC-20 cells exceeded that of DH-TK- cells. The progeny of irradiated cells were more radioresistant than parental cells. The average D0 for PIC-20 cells was 7.4 +/- 0.2 Gy, which is three times higher than that for parental cells (2.5 +/- 0.1 Gy). Progeny cell survival in methylcellulose after irradiation with a dose of 10 Gy was 15 times higher than that of DH-TK- cells. In contrast to parental cells, the progeny of irradiated cells showed fast and effective repopulation after irradiation with doses of 12.5 and 15 Gy. The tumor formation ability of irradiated progeny cells was higher than that of parental cells; after 15 Gy irradiation, PIC-20 cells produced tumors as large as unirradiated progeny of irradiated cells, whereas the tumor development of DH-TK- cells diminished by 70%. High radioresistance of progeny of irradiated cells was reproduced during the long period of cultivation (more than 80 passages). The stability of the radioresistant phenotype of PIC-20 cells allows us to investigate the possible mechanisms of acquired tumor radioresistance.     

Retention of tumorigenicity in radioresistant progeny of cells surviving exposure to high doses of gamma irradiation

The cell tumorigenic ability and the cell clonogenicity in semi-solid medium of highly radioresistant variant cell line, PIC-20 (the progeny of djungarian hamster fibroblast cell line DX-TK- surviving acute exposure to 20 Gy of gamma-irradiation), were examined. In the absence of additional radiation, no differences between tested features of non-irradiated PIC-20 cells and parental DX-TK- cells were observed. On the contrary, after gamma-irradiation with high doses the essential differences in the properties of the examined cell lines were revealed. After exposure to 10 Gy the surviving fraction of PIC-20 cells was 20 times higher than that of the parental cells. Both irradiated and non-irradiated PIC-20 cells produced colonies of similar size. It is revealed that even after irradiation with doses of 5, 10 or 15 Gy, the PIC-20 cells kept their tumorigenicity as high as non-irradiated ones. In all these cases the 90-100% of animals had the tumour, with the average latent period of tumour appearance after inoculation being the same both for irradiated and non-irradiated PIC-20 cells. After irradiation of parental DX-TK- cells with the highest dose of 15 Gy, the amount animals with tumour decreased by 70% and the average latent period of tumour appearance increased fivefold as compared with that for non-irradiated DX-TK- cells. The data obtained indicate that PIC-20 is highly radioresistant cells, which are able to proliferate both in semi-solid medium and in an animal organism even after radiation exposure to high doses.     

Long-term consequences of radiation-induced bystander effects depend on radiation quality and dose and correlate with oxidative stress

Widespread evidence indicates that exposure of cell populations to ionizing radiation results in significant biological changes in both the irradiated and nonirradiated bystander cells in the population. We investigated the role of radiation quality, or linear energy transfer (LET), and radiation dose in the propagation of stressful effects in the progeny of bystander cells. Confluent normal human cell cultures were exposed to low or high doses of 1GeV/u iron ions (LET ～ 151 keV/μm), 600 MeV/u silicon ions (LET ～ 51 keV/μm), or 1 GeV protons (LET ～ 0.2 keV/μm). Within minutes after irradiation, the cells were trypsinized and co-cultured with nonirradiated cells for 5 h. During this time, irradiated and nonirradiated cells were grown on either side of an insert with 3-μm pores. Nonirradiated cells were then harvested and allowed to grow for 20 generations. Relative to controls, the progeny of bystander cells that were co-cultured with cells irradiated with iron or silicon ions, but not protons, exhibited reduced cloning efficiency and harbored higher levels of chromosomal damage, protein oxidation and lipid peroxidation. This correlated with decreased activity of antioxidant enzymes, inactivation of the redox-sensitive metabolic enzyme aconitase, and altered translation of proteins encoded by mitochondrial DNA. Together, the results demonstrate that the long-term consequences of the induced nontargeted effects greatly depend on the quality and dose of the radiation and involve persistent oxidative stress due to induced perturbations in oxidative metabolism. They are relevant to estimates of health risks from exposures to space radiation and the emergence of second malignancies after radiotherapy.     

Split-dose exposures versus dual ion exposure in human cell neoplastic transformation

Since radiation fields of space contain many-fold more protons than high atomic number, high energy (HZE) particles, cells in astronaut crews will experience on average several proton hits before an HZE hit. Thus radiation regimes of proton exposure before HZE particle exposure simulate space radiation exposure, and measurement of the frequency of neoplastic transformation of human primary cells to anchorage-independent growth simulates an initial step in cancer induction. Although previous investigations indicated a synergistic increase in transformation yields in the cells exposed to protons followed by HZE particles, these experiments did not differentiate between the effect of splitting of the dose into two fractions and that of changing the ion beams. To test this, we irradiated cells with split doses of either protons or HZE particles, then measured clonogenic survival and neoplastic transformation, as measured by colony formation in semi-solid soft agar medium. The data show that the split dose of 20 cGy plus 20 cGy of either H or HZE ions gave about the same effect as the 40 cGy uninterrupted dose, quite different from the effect of the mixed ion beam H + HZE irradiation. We also asked if lower proton doses than 20 cGy followed 15 min later by 20 cGy of HZE ions gave greater than additive transformation frequencies. Substantial increases in transformation levels were observed for all proton doses tested, including 1 cGy. These results point to the signal importance of protons in affecting the effect of space radiation on human cells.     

Increased frequency of spontaneous neoplastic transformation in progeny of bystander cells from cultures exposed to densely ionizing radiation

An increased risk of carcinogenesis caused by exposure to space radiation during prolonged space travel is a limiting factor for human space exploration. Typically, astronauts are exposed to low fluences of ionizing particles that target only a few cells in a tissue at any one time. The propagation of stressful effects from irradiated to neighboring bystander cells and their transmission to progeny cells would be of importance in estimates of the health risks of exposure to space radiation. With relevance to the risk of carcinogenesis, we investigated, in model C3H 10T½ mouse embryo fibroblasts (MEFs), modulation of the spontaneous frequency of neoplastic transformation in the progeny of bystander MEFs that had been in co-culture 10 population doublings earlier with MEFs exposed to moderate doses of densely ionizing iron ions (1 GeV/nucleon) or sparsely ionizing protons (1 GeV). An increase (P<0.05) in neoplastic transformation frequency, likely mediated by intercellular communication through gap junctions, was observed in the progeny of bystander cells that had been in co-culture with cells irradiated with iron ions, but not with protons.     

Relative biological effectiveness of 12C and 28Si radiation in C57BL/6J mice

Study of heavy ion radiation-induced effects on mice could provide insight into the human health risks of space radiation exposure. The purpose of the present study is to assess the relative biological effectiveness (RBE) of (12)C and (28)Si ion radiation, which has not been reported previously in the literature. Female C57BL/6J mice (n = 15) were irradiated using 4-8 Gy of (28)Si (300 MeV/nucleon energy; LET 70 keV/μm) and 5-8 Gy of (12)C (290 MeV/nucleon energy; LET 13 keV/μm) ions. Post-exposure, mice were monitored regularly, and their survival observed for 30 days. The LD(50/30) dose (the dose at which 50 % lethality occurred by 30-day post-exposure) was calculated from the survival curve and was used to determine the RBE of (28)Si and (12)C in relation to γ radiation. The LD(50/30) for (28)Si and (12)C ion is 5.17 and 7.34 Gy, respectively, and the RBE in relation to γ radiation (LD(50/30)-7.25 Gy) is 1.4 for (28)Si and 0.99 for (12)C. Determination of RBE of (28)Si and (12)C for survival in mice is not only important for space radiation risk estimate studies, but it also has implications for HZE radiation in cancer therapy.     

Effects of 1 GeV/nucleon ⁵⁶Fe particles on longevity, carcinogenesis and neuromotor ability in Atm-deficient mice

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 ??Fe particles on life span and tumorigenesis are genotype- and sex-specific. Significant effects of 1 Gy 1 GeV/nucleon ??Fe particles on incidence of non-cancer end points were seen; however, 2 Gy 1 GeV/nucleon ??Fe 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.     

Risk of lung cancer mortality in relation to lung doses among French uranium miners: follow-up 1956-1999

The aim of this study was to assess the risk of lung cancer death associated with cumulative lung doses from exposure to α-particle emitters, including radon gas, radon short-lived progeny, and long-lived radionuclides, and to external γ rays among French uranium miners. The French "post-55" sub-cohort included 3,377 uranium miners hired from 1956, followed up through the end of 1999, and contributing to 89,405 person-years. Lung doses were calculated with the ICRP Human Respiratory Tract Model (Publication 66) for 3,271 exposed miners. The mean "absorbed lung dose" due to α-particle radiation was 78 mGy, and that due to the contribution from other types of radiation (γ and β-particle radiation) was 56 mGy. Radon short-lived progeny accounted for 97% of the α-particle absorbed dose. Out of the 627 deaths, the cause of death was identified for 97.4%, and 66 cases were due to lung cancer. A significant excess relative risk (ERR) of lung cancer death was associated with the total absorbed lung dose (ERR/Gy = 2.94, 95% CI 0.80, 7.53) and the α-particle absorbed dose (4.48, 95% CI 1.27, 10.89). Assuming a value of 20 for the relative biological effectiveness (RBE) of α particles for lung cancer induction, the ERR/Gy-Eq for the total weighted lung dose was 0.22 (95% CI: 0.06, 0.53).     

Survivin expressions in human hepatoma HepG2 cells exposed to ionizing radiation of different LET

Survivin is a member of the inhibitors of apoptosis (IAP) protein family that interferes with post-mitochondrial events including activation of caspases. To examine the regulation of survivin expression in response to irradiation with different linear energy transfer (LET), human hepatoma HepG2 cells were irradiated in vitro with X-rays and carbon ions. Cellular sensitivities to low- and high-LET radiation were determined by colony formation. Survivin expression at mRNA and protein level were measured with RT-PCR and Western blot analyses, respectively. Radiation-induced cell cycle arrest and apoptosis were investigated with flow cytometry. We found that low-LET X-rays induced dose-dependent increases in survivin expression. After exposure to high-LET carbon ions, survivin expression gradually increased from 0 to 4 Gy, and then declined at 6 Gy. More pronounced survivin expression, stronger G(2)/M phase arrest was observed after exposure to carbon ions in comparison with X-rays at doses from 0 to 4 Gy. These observations indicate that there is a differential survivin expression in response to different LET radiations and the cycle arrest mechanism may be associated with it. In addition, our data on induction of apoptosis are compatible with the assumption that survivin expression induced by low-LET X-rays radiation may play a critical role in inhibiting apoptosis. However, after irradiation with ions, an anti-apoptotic function of survivin is not evident, possibly because of the serious damage produced by densely ionizing radiation.     

Radiation-induced epigenetic alterations after low and high LET irradiations

Epigenetics, including DNA methylation and microRNA (miRNA) expression, could be the missing link in understanding radiation-induced genomic instability (RIGI). This study tests the hypothesis that irradiation induces epigenetic aberrations, which could eventually lead to RIGI, and that the epigenetic aberrations induced by low linear energy transfer (LET) irradiation are different than those induced by high LET irradiations. GM10115 cells were irradiated with low LET X-rays and high LET iron (Fe) ions and evaluated for DNA damage, cell survival and chromosomal instability. The cells were also evaluated for specific locus methylation of nuclear factor-kappa B (NFκB), tumor suppressor in lung cancer 1 (TSLC1) and cadherin 1 (CDH1) gene promoter regions, long interspersed nuclear element 1 (LINE-1) and Alu repeat element methylation, CpG and non-CpG global methylation and miRNA expression levels. Irradiated cells showed increased micronucleus induction and cell killing immediately following exposure, but were chromosomally stable at delayed times post-irradiation. At this same delayed time, alterations in repeat element and global DNA methylation and miRNA expression were observed. Analyses of DNA methylation predominantly showed hypomethylation, however hypermethylation was also observed. We demonstrate that miRNA expression levels can be altered after X-ray irradiation and that these miRNA are involved in chromatin remodeling and DNA methylation. A higher incidence of epigenetic changes was observed after exposure to X-rays than Fe ions even though Fe ions elicited more chromosomal damage and cell killing. This distinction is apparent at miRNA analyses at which only three miRNA involved in two major pathways were altered after high LET irradiations while six miRNA involved in five major pathways were altered after low LET irradiations. This study also shows that the irradiated cells acquire epigenetic changes suggesting that epigenetic aberrations may arise in the cell without initiating chromosomal instability.     

Total-body irradiation with high-LET particles: acute and chronic effects on the immune system

Although the immune system is highly susceptible to radiation-induced damage, consequences of high linear energy transfer (LET) radiation remain unclear. This study evaluated the effects of 0.1 gray (Gy), 0.5 Gy, and 2.0 Gy iron ion (56Fe(26)) radiation on lymphoid cells and organs of C57BL/6 mice on days 4 and 113 after whole body exposure; a group irradiated with 2.0 Gy silicon ions (28Si) was euthanized on day 113. On day 4 after 56Fe irradiation, dose-dependent decreases were noted in spleen and thymus masses and all major leukocyte populations in blood and spleen. The CD19(+) B lymphocytes were most radiosensitive and NK1.1(+) natural killer (NK) cells were most resistant. CD3(+) T cells were moderately radiosensitive and a greater loss of CD3(+)/CD8(+) T(C) cells than CD3(+)/CD4(+) T(H) cells was noted. Basal DNA synthesis was elevated on day 4, but response to mitogens and secretion of interleukin-2 and tumor necrosis factor-alpha were unaffected. Signs of anemia were noted. By day 113, high B cell numbers and low T(C) cell and monocyte percents were found in the 2.0 Gy 56Fe group; the 2.0 Gy 2)Si mice had low NK cells, decreased basal DNA synthesis, and a somewhat increased response to two mitogens. Collectively, the data show that lymphoid cells and tissues are markedly affected by high linear energy transfer (LET) radiation at relatively low doses, that some aberrations persist long after exposure, and that different consequences may be induced by various densely ionizing particles. Thus simultaneous exposure to multiple radiation sources could lead to a broader spectrum of immune dysfunction than currently anticipated.     

Radiation-induced bystander effect in healthy G(o) human lymphocytes: biological and clinical significance

To study the bystander effects, G(0) human peripheral blood lymphocytes were X-irradiated with 0.1, 0.5 and 3 Gy. After 24h, cell-free conditioned media from irradiated cultures were transferred to unexposed lymphocytes. Following 48 h of medium transfer, viability, induction of apoptosis, telomere shortening, reactive oxygen species (ROS) levels and micronuclei (after stimulation) were analyzed. A statistically significant decrement in cell viability, concomitant with the loss of mitochondrial membrane potential, telomere shortening, increases in hydrogen peroxide (H(2)O(2)) and superoxide anion (O(2)(-)) with depletion of intracellular glutathione (GSH) level, and higher frequencies of micronuclei, were observed in bystander lymphocytes incubated with medium from 0.5 and 3 Gy irradiated samples, compared to lymphocytes unexposed. Furthermore, no statistically significant difference between the response to 0.5 and 3 Gy of irradiation in bystander lymphocytes, was found. However, when lymphocytes were irradiated with 0.1 Gy, no bystander effect with regard to viability, apoptosis, telomere length, and micronuclei was observed, although a high production of ROS level persisted. Radiation in the presence of the radical scavenger dimethyl sulfoxide (DMSO) suppressed oxidative stress induced by 3 Gy of X-rays with the effective elimination of bystander effects, suggesting a correlation between ROS and bystander signal formation in irradiated cells. The data propose that bystander effect might be mostly due to the reactions of radiation induced free radicals on DNA, with the existence of a threshold at which the bystander signal is not operative (0.1 Gy dose of X-rays). Our results may have clinical implications for health risk associated with radiation exposure.     

Acute effects of iron-particle radiation on immunity. Part I: Population distributions

Health risks due to exposure to high-linear energy transfer (LET) charged particles remain unclear. The major goal of this study was to confirm and further characterize the acute effects of high-LET radiation ((56)Fe(26)) on erythrocyte, thrombocyte and leukocyte populations in three body compartments after total-body exposure. Adult female C57BL/6 mice were irradiated with total doses of 0, 0.5, 2 and 3 Gy and killed humanely 4 days later. Body and organ masses were determined and blood, spleen and bone marrow leukocytes were evaluated using a hematology analyzer and flow cytometry. Spleen and thymus (but not body, liver and lung) masses were significantly decreased in a dose-dependent manner. In general, red blood cell (RBC) counts and most other RBC parameters were depressed with increasing dose (P < 0.05); the major exception was an increase in cell size at 0.5 Gy. Platelet numbers and volume, total white blood cell counts, and all three major types of leukocytes also decreased (P < 0.05). Lymphocyte populations in blood and spleen exhibited variable degrees of susceptibility to (56)Fe-particle radiation (B > T > NK and T cytotoxic > T helper cells). In the bone marrow, leukocytes with granulocytic, lymphocytic ("dim" and "bright"), and monocytic characteristics exhibited proportional variations at the higher radiation doses in the expression of CD34 and/or Ly-6A/E. The data are discussed in relation to our previous investigations with iron ions, other forms of radiation, and space flight in this same animal model.     

HZE ⁵⁶Fe-ion irradiation induces endothelial dysfunction in rat aorta: role of xanthine oxidase

Ionizing radiation has been implicated in the development of significant cardiovascular complications. Since radiation exposure is associated with space exploration, astronauts are potentially at increased risk of accelerated cardiovascular disease. This study investigated the effect of high atomic number, high-energy (HZE) iron-ion radiation on vascular and endothelial function as a model of space radiation. Rats were exposed to a single whole-body dose of iron-ion radiation at doses of 0, 0.5 or 1 Gy. In vivo aortic stiffness and ex vivo aortic tension responses were measured 6 and 8 months after exposure as indicators of chronic vascular injury. Rats exposed to 1 Gy iron ions demonstrated significantly increased aortic stiffness, as measured by pulse wave velocity. Aortic rings from irradiated rats exhibited impaired endothelial-dependent relaxation consistent with endothelial dysfunction. Acute xanthine oxidase (XO) inhibition or reactive oxygen species (ROS) scavenging restored endothelial-dependent responses to normal. In addition, XO activity was significantly elevated in rat aorta 4 months after whole-body irradiation. Furthermore, XO inhibition, initiated immediately after radiation exposure and continued until euthanasia, completely inhibited radiation-dependent XO activation. ROS production was elevated after 1 Gy irradiation while production of nitric oxide (NO) was significantly impaired. XO inhibition restored NO and ROS production. Finally, dietary XO inhibition preserved normal endothelial function and vascular stiffness after radiation exposure. These results demonstrate that radiation induced XO-dependent ROS production and nitroso-redox imbalance, leading to chronic vascular dysfunction. As a result, XO is a potential target for radioprotection. Enhancing the understanding of vascular radiation injury could lead to the development of effective methods to ameliorate radiation-induced vascular damage.     

Radiation induced chromosomal instability in human T-lymphocytes

Chromosomal instability in proliferating mammalian cells is characterized by a persistent increase of chromosomal aberrations and rearrangements occurring de novo during successive cell generations. Recent results from many laboratories using a variety of cells and cytogenetic end points show that this phenotype can be induced by low as well as high LET irradiation. A typical feature of chromosomal instability in primary human G₀-lymphocytes exposed to γ-irradiation at both high dose rate (45 Gy h⁻¹) and low dose rate (0.024 Gy h⁻¹) is the appearance of novel aberrations in the clonal progeny of the irradiated cell, many generations after the exposure. The same phenotype was observed in lymphocytes that were allowed to recover for 5 days in G₀ after the radiation exposure, as well as in hprt-mutant T cell clones. These results demonstrate that neither the acute genotoxic stress caused by high dose rate as compared to low dose rate irradiation, nor a hypothesized conflict between mitogen induced growth stimulation and growth arrest due to radiation damage, seem to be critical conditions for the development chromosomal instability in these cells. In contrast to observations in other cells, no evidence of a persistent decrease of cloning ability was observed in the progeny of radiation-exposed human lymphocytes, and no alteration was observed in their sensitivity to a second radiation exposure. Furthermore, the frequency of CA-repeat length variation at three loci was not increased in the progeny of X-irradiated T cells as compared to non-irradiated cells, which indicates that microsatellite instability is not part of the chromosomal instability phenotype in human T-lymphocytes.