Suppression of Thymic Lymphoma Induction by Life-Long Low-Dose-Rate Irradiation Accompanied by Immune Activation in C57BL/6 Mice

Low-dose whole-body irradiation induced radioadaptive response in C57BL/6 mice

Radioadaptive survival responses after relatively low doses of radiation were investigated in C57BL/6 mice. The 8-week-old mice received whole-body mid-lethal challenging irradiation (5.9 Gy) at various intervals after conditioning whole-body irradiation with 50-400 mGy. Thereafter, survival of the mice was observed for 30 days. The mice receiving 400 mGy at 6 h before the challenging dose had a lower survival rate than the control group, but it was not observed when the conditioning 400-mGy irradiation was given 24 h beforehand. The conditioning doses of 100 and 200 mGy did not influence the survival of mice after the challenging dose. The mice receiving 50 mGy at 1 day, 3 days or 1 week before the challenging dose had a higher survival rate than the control, although this adaptive response was not observed when 50 mGy was given 6 h, 12 h, 3.5 weeks, or 5 weeks beforehand. When 50 mGy was given 2 weeks before the challenging dose, the adaptive response was observed in an experiment in which the mice were caged in our laboratory at the age of 5 weeks, whereas it was not observed in another experiment in which the mice were caged at 3 weeks. This study confirmed the presence of radioadaptive survival responses at the dose of 50 mGy given relatively shortly before the challenging dose.

Requirements for identification of low dose and non-linear mutagenic responses to ionising radiation

Cancer results from multiple changes in gene expression that can occur both genetically and epigenetically. High doses of radiation can lead to mutations and cancer. At high doses the number of mutations caused by radiation is essentially linear with dose. Low dose radiation induced protective responses observed for cancer in vivo and cellular transformation in vitro would predict that hormetic responses would also be observed in mutation assays. Although there are a large number of different mutation assays available, very few are able to detect changes in mutation frequency in response to very low doses of DNA damaging agents. The easiest way to cope with this lack of data in the low dose range is to invoke a linear-no-threshold model for risk assessment. The reasons for the lack of data are discussed. In order to identify hormetic mutation responses, assays need to have a spontaneous frequency that is high enough to enable a reduction below spontaneous frequency to be detected in a feasible number of scored cells and also need to be able to identify both genetic and epigenetic changes. The pKZ1 chromosomal inversion assay fits the criteria for detecting hormetic responses to low dose radiation.

Amelioration of type II diabetes in db/db mice by continuous low-dose-rate gamma irradiation

Low-dose-rate radiation modulates various biological responses including carcinogenesis, immunological responses and diabetes. We found that continuous irradiation with low-dose-rate gamma rays ameliorated type II diabetes in db/db mice, diabetic mice that lack leptin receptors. Whole-body exposure of db/db mice to low dose-rate gamma radiation improved glucose clearance without affecting the response to insulin. Histological studies suggested that degeneration of pancreatic islets was significantly suppressed by the radiation. Insulin secretion in response to glucose loading was increased significantly in the irradiated mice. These results suggest that low-dose-rate gamma radiation ameliorates type II diabetes by maintaining insulin secretion, which gradually decreases during the progression of diabetes due to degeneration of pancreatic islets. We also inferred that protection from oxidative damage is involved in the anti-diabetic effect of low-dose-rate gamma rays because expression and activity of pancreatic superoxide dismutase were significantly elevated by low-dose-rate gamma radiation.

Radioadaptive response revisited

Radiation-induced adaptive response belongs to the group of non-targeted effects that do not require direct exposure of the cell nucleus by radiation. It is described as the reduced damaging effect of a challenging radiation dose when induced by a previous low priming dose. Adaptive responses have been observed in vitro and in vivo using various indicators of cellular damage, such as cell lethality, chromosomal aberrations, mutation induction, radiosensitivity, and DNA repair. Adaptive response can be divided into three successive biological phenomena, the intracellular response, the extracellular signal, and the maintenance. The intracellular response leading to adaptation of a single cell is a complex biological process including induction or suppression of gene groups. An extracellular signal, the nature of which is unknown, may be sent by the affected cell to neighbouring cells causing them to adapt as well. This occurs either by a release of diffusible signalling molecules or by gap-junction intercellular communication. Adaptive response can be maintained for periods ranging from of a few hours to several months. Constantly increased levels of reactive oxygen species (ROS) or nitric oxide (NO) have been observed in adapted cells and both factors may play a role in the maintenance process. Although adaptive response seems to function by an on/off principle, it is a phenomenon showing a high degree of inter- and intraindividual variability. It remains to be seen to what extent adaptive response is functional in humans at relevant dose and dose-rate exposures. A better understanding of adaptive response and other non-targeted effects is needed before they can be confirmed as risk estimate factors for the human population at low levels of ionising radiation.

Whole body exposure to low-dose gamma radiation promotes kidney antioxidant status in Balb/c mice

We examined the effect of whole body low-dose gamma-irradiation on the status of the antioxidant defense system in the rodent kidneys at different time intervals. Young male Balb/c mice were exposed to whole body radiation from a (60)Co source at doses of 10, 25 and 50 cGy (48.78 cGy/min). Antioxidant status and lipid peroxidation were estimated in the kidneys at 4, 12 and 24 h after irradiation. Lipid peroxidation increased between 33% and 49% and reduced glutathione between 12% and 47% at 12 h at different radiation doses. Reduced glutathione level remained significantly (p < 0.05) elevated even at 24 h after irradiation to 25 cGy. Superoxide dismutase activity also increased by 37% at 12 h on exposure of animals to all the doses up to 50 cGy. Catalase activity increased significantly at 12 h on exposure to 10 cGy and 50 cGy. Interestingly, glutathione peroxidase activity increased by 31% at 4 h and subsequently returned to control levels at 24 h after exposure to 50 cGy. Glutathione reductase activity increased by 10-12% at 12 h after exposure to 25 cGy and 50 cGy. The results suggest that the whole body exposure of animals to gamma radiation stimulates the antioxidant defense system in the kidneys within 4 to 24 h after irradiation, at doses of 25 cGy and 50 cGy.

[Biological responses to low dose radiation--hormesis and adaptive responses]

"Hormesis" is defined, originally in the field of toxicology, as a phenomenon in which a harmful substance gives stimulating effects to living organisms when the quantity is small. The concept was extended and applied to ionizing radiation, high doses of which are harmful. Although radiation has been thought to be, based on findings in high dose ranges, harmful no matter low the dose is, recent investigation revealed that living organisms possess the ability to respond to low-dose radiation in very sophisticated ways. A good example of such responses is the so-called radiation adaptive response, a process in which acquired radioresistance is induced by low-dose radiation given in advance. The stimulation of certain bioprotective functions, including antioxidative capacity, DNA repair functions, apoptosis, and immune functions are thought to underly the adaptive response. The adaptive response is effective for chromosome induction, acute death, and tumorigenesis induced by high doses of radiation. Radiation hormesis and adaptive response provide a new scope in the risk assessment and medical application of ionizing radiation.

Radiation-induced neoplastic transformation in vitro, hormesis and risk assessment

Dose-response curves for various low-LET radiation sources have consistently been demonstrated to be J-shaped for the cancer-relevant endpoint of neoplastic transformation in vitro. Most of these studies have been performed where the radiation has been delivered at intermediate to high dose-rates (30-3000 mGy/min), where the threshold dose for induction of neoplastic transformation is around 100-200 mGy. Below these doses, the transformation frequency is less than that seen spontaneously, indicative of a hormetic effect. More recently, data have been obtained for low dose rates (<0.5 mGy/min) of low-LET radiation, and again hormetic effects are apparent but with threshold doses now being >1000 mGy. Similar trends have been reported in animal experiments as well as in human epidemiologic studies. Indeed, the relative risks for induction of neoplastic transformation in vitro in the dose range 1 to 1000 mGy agree well with those for incidence of radiation-induced breast cancer and leukemia in humans. These findings support the notion that the endpoint of neoplastic transformation in vitro is a plausible endpoint to not only study mechanisms involved in response to low doses of radiation, but also to provide information of potential importance to risk assessment.

Radiation hormesis: the good, the bad, and the ugly

Three aspects of hormesis with low doses of ionizing radiation are presented: the good, the bad, and the ugly. The good is acceptance by France, Japan, and China of the thousands of studies showing stimulation and/or benefit, with no harm, from low dose irradiation. This includes thousands of people who live in good health with high background radiation. The bad is the nonacceptance of radiation hormesis by the U. S. and most other governments; their linear no threshold (LNT) concept promulgates fear of all radiation and produces laws which have no basis in mammalian physiology. The LNT concept leads to poor health, unreasonable medicine and oppressed industries. The ugly is decades of deception by medical and radiation committees which refuse to consider valid evidence of radiation hormesis in cancer, other diseases, and health. Specific examples are provided for the good, the bad, and the ugly in radiation hormesis.

Non-linear chromosomal inversion response in prostate after low dose X-radiation exposure

Somatic intrachromosomal recombination can result in inversions and deletions in DNA, which are important mutations in cancer. The pKZ1 chromosomal inversion assay is a sensitive assay for studying the effects of DNA damaging agents using chromosomal inversion as a mutation end-point. We have previously demonstrated that the chromosomal inversion response in pKZ1 spleen after single low doses of X-radiation exposure does not follow the linear no-threshold dose-response model. Here, we optimised a chromosomal inversion screening method to study the effect of low dose X-radiation exposure in pKZ1 prostatic tissue. In the present study, a significant induction in inversions was observed after ultra-low doses of 0.005-0.01 mGy or after a high dose of 1000 mGy, whereas a reduction in inversions to below the sham-treated frequency was observed between 1 and 10 mGy exposure. This is the first report of a reduction to below endogenous frequency for any mutation end-point in prostate. In addition, the doses of radiation studied were at least three orders of magnitude lower than have been reported in other mutation assays in prostate in vivo or in vitro. In sham-treated pKZ1 controls and in pKZ1 mice treated with low doses of 1-10 mGy the number of inversions/gland cross-section rarely exceeded three. Up to 4 and 7 inversions were observed in individual prostatic gland cross-sections after doses < or =0.02 mGy and after 1000 mGy, respectively. The number of inversions identified in individual cross-sections of prostatic glands of untreated mice and all treated mice other than the 1000 mGy treatment group followed a Poisson distribution. The dose-response curves and fold changes observed after all radiation doses studied were similar in spleen and prostate. These results suggest that the pKZ1 assay is measuring a fundamental response to DNA damage after low dose X-radiation exposure which is independent of tissue type.

Health risks of low photon energy imaging

The major health risk associated with low photon energy imaging is thought to be the induction of cancer as a consequence of the radiation exposure and this is the focus of this paper. Low photon energy imaging typically involves exposure to a low dose (<50 mGy) of low linear energy transfer (LET) radiation delivered at high dose-rate. Since epidemiologic data cannot provide an accurate assessment of risk at the doses used in imaging, risk estimates are currently made by fitting a linear response to intermediate and high dose data for cancer induction in radiation-exposed human populations. This method assumes a linear no-threshold (LNT) response and implies that no dose of radiation is safe. This assumption is not borne out by many laboratory studies of cancer-related endpoints that would suggest that the risk at low doses is much less than would be estimated from linear extrapolation from intermediate to high doses. It is also well recognised that the dose-response from many epidemiologic studies could equally well be fit by threshold models. Through the study of radiation-induced neoplastic transformation in vitro J-shaped dose-response curves for a variety of low LET radiations, including those used in low photon energy imaging, have been demonstrated. The relative risks calculated from this data compare remarkably well with those for breast cancer and leukemia incidence in radiation-exposed populations. From this it is concluded that the LNT hypothesis is likely to overestimate the risk of cancer induction by low photon energy imaging, at least for certain tumors.

Dose-Rate Effect on the Induction ofHPRTMutants in Human G0Lymphocytes ExposedIn Vitroto Gamma Radiation

The influence of dose rate on expression time, cell survival and mutant frequency at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) locus was evaluated in human G(0) peripheral blood lymphocytes exposed in vitro to gamma rays at low (0.0014 Gy/min) and high (0.85 Gy/min) dose rates. A cloning assay performed on different days of postirradiation incubation indicated an 8-day maximum expression period for the induction of HPRT mutants at both high and low dose rates. Cell survival increased markedly with decreasing dose rate, yielding D(0) values of 3.04 Gy and 1.3 Gy at low and high dose rates, respectively. The D(0) of 3.04 Gy obtained at low dose rate could be attributed to the repair of sublethal DNA damage taking place during prolonged exposure to low-LET radiation. Regression analysis of the mutant frequency yielded slopes of 12.35 x 10(-6) and 3.66 x 10(-6) mutants per gray at high and low dose rate, respectively. A dose and dose-rate effectiveness factor of 3.4 indicated a marked dose-rate effect on the induced HPRT mutant frequency. The results indicate that information obtained from in vitro measurements of dose-rate effects in human G(0) lymphocytes may be a useful parameter for risk estimation in radiation protection.

Further study of prolongation of life span associated with immunological modification by chronic low-dose-rate irradiation in MRL-lpr/lpr mice: effects of whole-life irradiation

MRL-lpr/lpr mice carry a deletion in the apoptosis-regulating Fas gene that markedly shortens life due to multiple severe diseases. In our previous study (Radiat. Res. 161, 168- 173, 2004), chronic low-dose-rate gamma irradiation of mice at 0.35 or 1.2 mGy/h for 5 weeks markedly prolonged the life span, accompanied by immunological activation. This report shows that extension of the irradiation period to the entire life of the mice at the same dose rates improved survival further. The 50% survival time for untreated mice, 134 days, was prolonged to 502 days by 1.2 mGy/h life-long irradiation. Also obtained were a time course and a radiation dose-rate response for the activation of the immune system as indicated by a significant increase in CD4+ CD8+ T cells in the thymus and CD8+ T cells in the spleen and also by a significant decrease in CD3+ CD45R/B220+ cells and CD45R/B220+ CD40+ cells in the spleen. Drastic ameliorations of multiple severe diseases, i.e. total-body lymphadenopathy, splenomegaly and serious autoimmune diseases including proteinuria, and kidney and brain-central nervous system syndromes, were found in parallel with these immunological activations, with lifelong low-dose-rate irradiation being more effective than 5-week irradiation at low dose rates.