Radiation hormesis: data and interpretations

Mechanism underlying the rapid growth of Phalaenopsis equestris induced by 60Co-γ-ray irradiation

Gamma (γ)-ray irradiation is one of the important modern breeding methods. Gamma-ray irradiation can affect the growth rate and other characteristics of plants. Plant growth rate is crucial for plants. In horticultural crops, the growth rate of plants is closely related to the growth of leaves and flowering time, both of which have important ornamental value. In this study, 60Co-γ-ray was used to treat P. equestris plants. After irradiation, the plant's leaf growth rate increased, and sugar content and antioxidant enzyme activity increased. Therefore, we used RNA-seq technology to analyze the differential gene expression and pathways of control leaves and irradiated leaves. Through transcriptome analysis, we investigated the reasons for the rapid growth of P. equestris leaves after irradiation. In the analysis, genes related to cell wall relaxation and glucose metabolism showed differential expression. In addition, the expression level of genes encoding ROS scavenging enzyme synthesis regulatory genes increased after irradiation. We identified two genes related to P. equestris leaf growth using VIGS technology: PeNGA and PeEXPA10. The expression of PeEXPA10, a gene related to cell wall expansion, was down-regulated, cell wall expansion ability decreased, cell size decreased, and leaf growth rate slowed down. The TCP-NGATHA (NGA) molecular regulatory module plays a crucial role in cell proliferation. When the expression of the PeNGA gene decreases, the leaf growth rate increases, and the number of cells increases. After irradiation, PeNGA and PeEXPA10 affect the growth of P. equestris leaves by influencing cell proliferation and cell expansion, respectively. In addition, many genes in the plant hormone signaling pathway show differential expression after irradiation, indicating the crucial role of plant hormones in plant leaf growth. This provides a theoretical basis for future research on leaf development and biological breeding.

Role of low-dose radiation in senescence and aging: A beneficial perspective

Cellular senescence refers to the permanent arrest of cell cycle caused by intrinsic and/or extrinsic stressors including oncogene activation, irradiation, DNA damage, oxidative stress, and certain cytokines (including senescence associated secretory phenotype). Cellular senescence is an important factor in aging. Accumulation of senescent cells has been implicated in the causation of various age-related organ disorders, tissue dysfunction, and chronic diseases. It is widely accepted that the biological effects triggered by low-dose radiation (LDR) are different from those caused by high-dose radiation. Experimental evidence suggests that LDR may promote growth and development, enhance longevity, induce embryo production, and delay the progression of chronic diseases. The underlying mechanisms of these effects include modulation of immune response, stimulation of hematopoietic system, antioxidative effect, reduced DNA damage and improved ability for DNA damage repair. In this review, we discuss the possible mechanisms by which LDR prevents senescence and aging from the perspectives of inhibiting cellular senescence and promoting the removal of senescent cells. We review a wide broad of evidence about the beneficial impact of LDR in senescence and aging models (including cardiovascular diseases, neurological diseases, arthritis and osteoporosis, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis) to highlight the potential value of LDR in preventing aging and age-related diseases. However, there is no consensus on the effect of LDR on human health, and several important aspects require further investigation.

An evaluation of DNA double strand break formation and excreted guanine species post whole body PET/CT procedure

Ionizing radiation-induced oxidation and formation of deoxyribonucleic acid (DNA) double strand breaks (DSBs) are considered the exemplar of genetic lesions. Guanine bases are most prone to be oxidized when DNA and Ribonucleic acid (RNA) are damaged. The repair processes that are initiated to correct this damage release multiple oxidized guanine species into the urine. Hence, the excretion of guanine species can be related with the total repair process. Our study quantified the total DSBs formation and the amount of guanine species in urine to understand the DNA break and repair process after whole body (WB) exposure to 18F-FDG positron emission tomography/computed tomography (PET/CT). A total of 37 human participants were included with control and test groups and the average radiation dose was 27.50 ± 2.91 mSv. γ-H2AX foci assay in the collected blood samples was performed to assess the DSBs, and excreted guanine species in urine were analyzed by a competitive ELISA method. We observed a significant increase of DNA damage that correlated well with the increasing dose (p-value 0.009) and body weight (p-value 0.05). In the test group, excreted guanine species in urine sample significantly increased (from 24.29 ± 5.82 to 33.66 ± 7.20 mg/mmol creatinine). A minimum (r2 = 0.0488) correlation was observed between DSBs formation and excreted guanine species. A significant difference of DNA damage and 8-OHdG formation was seen in the test group compared to controls. Larger population studies are needed to confirm these observations, describe the fine-scale timing of changes in the biomarker levels after exposure, and further clarify any potential risks to patients from PET/CT procedures.

Use of Photobioreactors in Regenerative Life Support Systems for Human Space Exploration

There are still many challenges to overcome for human space exploration beyond low Earth orbit (LEO) (e.g., to the Moon) and for long-term missions (e.g., to Mars). One of the biggest problems is the reliable air, water and food supply for the crew. Bioregenerative life support systems (BLSS) aim to overcome these challenges using bioreactors for waste treatment, air and water revitalization as well as food production. In this review we focus on the microbial photosynthetic bioprocess and photobioreactors in space, which allow removal of toxic carbon dioxide (CO2) and production of oxygen (O2) and edible biomass. This paper gives an overview of the conducted space experiments in LEO with photobioreactors and the precursor work (on ground and in space) for BLSS projects over the last 30 years. We discuss the different hardware approaches as well as the organisms tested for these bioreactors. Even though a lot of experiments showed successful biological air revitalization on ground, the transfer to the space environment is far from trivial. For example, gas-liquid transfer phenomena are different under microgravity conditions which inevitably can affect the cultivation process and the oxygen production. In this review, we also highlight the missing expertise in this research field to pave the way for future space photobioreactor development and we point to future experiments needed to master the challenge of a fully functional BLSS.

An update in toxicology of ageing

The field of ageing research has been rapidly advancing in recent decades and it had provided insight into the complexity of ageing phenomenon. However, as the organism-environment interaction appears to significantly affect the organismal pace of ageing, the systematic approach for gerontogenic risk assessment of environmental factors has yet to be established. This puts demand on development of effective biomarker of ageing, as a relevant tool to quantify effects of gerontogenic exposures, contingent on multidisciplinary research approach. Here we review the current knowledge regarding the main endogenous gerontogenic pathways involved in acceleration of ageing through environmental exposures. These include inflammatory and oxidative stress-triggered processes, dysregulation of maintenance of cellular anabolism and catabolism and loss of protein homeostasis. The most effective biomarkers showing specificity and relevancy to ageing phenotypes are summarized, as well. The crucial part of this review was dedicated to the comprehensive overview of environmental gerontogens including various types of radiation, certain types of pesticides, heavy metals, drugs and addictive substances, unhealthy dietary patterns, and sedentary life as well as psychosocial stress. The reported effects in vitro and in vivo of both recognized and potential gerontogens are described with respect to the up-to-date knowledge in geroscience. Finally, hormetic and ageing decelerating effects of environmental factors are briefly discussed, as well.

Trans-Generational Impacts of Paternal Irradiation in a Cricket: Damage, Life-History Features and Hormesis in F1 Offspring

Animals exposed to significant stress express multi-modal responses to buffer negative impacts. Trans-generational impacts have been mainly studied in maternal lines, with paternal lines having received less attention. Here, we assessed paternal generational effects using irradiated male crickets (Acheta domesticus), and their F1 offspring (irradiated males mated to unirradiated females). Paternal transmission of radiation impacts emerged in multiple life history traits when compared to controls. Irradiated males and their F1 offspring expressed hormetic responses in survivorship and median longevity at mid-range doses. For F0 males, 7 Gy & 10 Gy doses extended F0 longevity by 39% and 34.2% respectively. F1 offspring of 7 Gy and 10 Gy sires had median lifespans 71.3% and 110.9% longer, respectively. Survivorship for both F0 7 Gy (p < 0.0001) and 10 Gy (p = 0.0055) males and F1 7 Gy and 10 Gy (p < 0.0001) offspring significantly surpassed that of controls. Irradiated F0 males and F1 offspring had significantly reduced growth rates. For F0 males, significant reductions were evident in 4Gy-12 Gy males and F1 offspring in 4 Gy (p < 0.0001), 7 Gy (p < 0.0001), and 10 Gy (p = 0.017). Our results indicate paternal effects; that irradiation directly impacted males but also mediated diverse alterations in the life history features (particularly longevity and survivorship) of F1 offspring.

Low-dose radiation therapy for coronavirus disease-2019 pneumonia: Is it time to look beyond apprehensions?

Coronavirus disease-2019 (COVID-19) has become a global health crisis. Mortality associated with COVID-19 is characterized mainly by acute respiratory distress syndrome (ARDS), sepsis, pneumonia, and respiratory failure. The pathogenesis of the disease is known to be associated with pro-inflammatory processes after virus infection. Hence, various therapeutic strategies are being developed to control the inflammation and cytokine storm in COVID-19 patients. Recently, low-dose radiation therapy (LDRT) has been suggested for the treatment of pneumonia/ADRS in COVID-19 patients through irradiation of lungs by gamma/X-ray. In this direction, a few clinical trials have also been initiated. However, a few recent publications have raised some concerns regarding LDRT, especially about possibilities of activation/aggressiveness of virus (severe acute respiratory syndrome coronavirus 2 in case of COVID-19), lung injury and risk of second cancer after low-dose therapy. The present manuscript is an attempt to analyze these apprehensions based on cited references and other available literature, including some from our laboratory. At this point, LDRT may be not the first line of therapy. However, based on existing anti-inflammatory evidence of LDRT, it needs encouragement as an adjuvant therapy and for more multi-centric clinical trials. In addition, it would be worth combining LDRT with other anti-inflammatory therapies, which would open avenues for multi-modal therapy of pneumonia/ARDS in COVID-19 patients. The mode of irradiation (local lung irradiation or whole-body irradiation) and the window period after infection of the virus, need to be optimized using suitable animal studies for effective clinical outcomes of LDRT. However, considering ample evidence, it is time to look beyond the apprehensions if a low dose of radiation could be exploited for better management of COVID-19 patients.

IL-2 and IL-2R gene polymorphisms and immune function in people residing in areas with high background radiation, Yangjiang, China

PURPOSE

Long-term exposure to low dose radiation may trigger immune response and stimulate hormesis. Interleukin-2 (IL-2) and interleukin-2 receptor (IL-2R) play a crucial role in immune function. We aimed to explore the possible association of IL-2 and IL-2R gene polymorphisms with low dose radiation exposure, as well as the relationship with IL-2 gene expression in people residing in areas with a high background radiation in Yangjiang, China.

MATERIALS AND METHODS

We recruited and assigned 54 native men residing in Yangxi County, Yangjiang city to the high natural background radiation (HNBR) group, and 53 native men residing in Hengpi County, Enping city to the control area (CA) group. All the participants wore a thermoluminescent dosimeter (TLD) for 90 days, and answered questionnaires. The serum levels of IL2, IL4, IL5, sIL2R, and tumor growth factor (TGF), and expression levels of IL2RA, IL2RB, IL2RG, and IL2 were also analyzed. Additionally, we tested 10 polymorphic loci associated with the IL-2 gene.

RESULTS

The annual effective radiation doses in the HNBR and CA groups were 6.24 mSv y^-1 and 1.95 mSv y^-1, respectively. After adjusting for potential confounding factors, the serum levels of IL-2 and IL-5 were higher in the HNBR group than the CA group (p < .05), while the serum level of TGFβ was lower in the HNBR group (p < .05). The IL-2 gene mRNA expression level was higher in the HNBR group than the CA group (p < .05). The IL-2RB rs76206423 AA allele showed significant variations in the HNBR group (p = .0381).

CONCLUSIONS

Long-term exposure to low dose radiation may enhance immune function, and IL-2RB rs76206423 may be related to the expression of IL-2 by other coding variants. Moreover, our data provide a better understanding of the molecular mechanism of the immune response to low dose radiation.

Construction and Preclinical Evaluation of 211At Labeled Anti-mesothelin Antibodies as Potential Targeted Alpha Therapy Drugs

Targeted alpha therapy (TAT) is a promising tumor therapy that can specifically transport α particle to the vicinity of tumor cells while the normal cells are only slightly irradiated. Mesothelin is a highly promising molecular signature for many types of solid tumors including malignant mesothelioma, pancreatic cancer, ovarian cancer and lung adenocarcinoma etc., while the expression in normal human tissues are limited, thus making mesothelin a promising antigen for TAT. Previously we developed a theoretical model that could predict and optimize in vitro screening of potential TAT drugs. The aim of the study is construction and preclinical evaluation of 211At labeled anti-mesothelin antibodies as potential TAT drugs. Mesothelin expression of two tumor cell lines were confirmed by flow cytometry, and their radiosensitivities were also evaluated. We used two kinds of anti-mesothelin antibodies, ET210-6 and ET210-28, to construct TAT drugs. Then, radiochemical purity, stability in vitro, affinity of the conjugates and mesothelin expression level were assessed. The specific killing of mesothelin-positive cancer cells treated by 211At-ET210-28 and 211At-ET210-6 were studied via Cell Counting Kit-8 assay and colony formation assay. 211At-ET210-28 and 211At-ET210-6 revealed excellent affinity and stability in both phosphate buffer saline and fetal bovine serum environment. Radiolabeled antibody conjugates bound specifically to mesothelin-positive cells in vitro. Both 211At-ET210-28 and 211At-ET210-6 could specifically kill mesothelin-positive cells with negligible damages to mesothelin-negative cells. Our findings provide initial proof-of-concept for the potential use of 211At labeled ET210-28/ET210-6 anti-mesothelin antibody in specific killings of mesothelin-positive tumor cells.

Low dose or low dose rate ionizing radiation-induced health effect in the human

The extensive literature review on human epidemiological studies suggests that low dose ionizing radiation (LDIR) (≤100 mSv) or low dose rate ionizing radiation (LDRIR) (<6mSv/H) exposure could induce either negative or positive health effects. These changes may depend on genetic background, age (prenatal day for embryo), sex, nature of radiation exposure, i.e., acute or chronic irradiation, radiation sources (such as atomic bomb attack, fallout from nuclear weapon test, nuclear power plant accidents, ⁶⁰Co-contaminated building, space radiation, high background radiation, medical examinations or procedures) and radionuclide components and human epidemiological experimental designs. Epidemiological and clinical studies show that LDIR or LDRIR exposure may induce cancer, congenital abnormalities, cardiovascular and cerebrovascular diseases, cognitive and other neuropsychiatric disorders, cataracts and other eye and somatic pathology (endocrine, bronchopulmonary, digestive, etc). LDIR or LDRIR exposure may also reduce mutation and cancer mortality rates. So far, the mechanisms of LDIR- or LDRIR -induced health effect are poorly understood. Further extensive studies are still needed to clarify under what circumstances, LDIR or LDRIR exposure may induce positive or negative effects, which may facilitate development of new therapeutic approaches to prevent or treat the radiation-induced human diseases or enhance radiation-induced positive health effect.

A critical evaluation of the NCRP COMMENTARY 27 endorsement of the linear no-threshold model of radiation effects

Regulatory policy to protect the public and the environment from radiation is universally based on the linear, no-threshold model (LNT) of radiation effects. This model has been controversial since its inception over nine decades ago, and remains so to this day, but it has proved remarkably resistant to challenge from the scientific community. The LNT model has been repeatedly endorsed by expert advisory bodies, and regulatory agencies in turn adopt policies that reflect this advice. Unfortunately, these endorsements rest on a foundation of institutional inertia and numerous logical fallacies. These include most significantly setting the LNT as the null hypothesis, and shifting the burden of proof onto LNT skeptics. Other examples include arbitrary exclusion of alternative hypotheses, ignoring criticisms of the LNT, cherry-picking evidence, and making policy judgements without foundation. This paper presents an evaluation of the National Council on Radiation Protection and Measurements' (NCRP) Commentary 27, which concluded that recent epidemiological studies are compatible with the continued use of the LNT model for radiation protection. While this report will likely provide political cover for regulators' continued reliance on the LNT, it is a missed opportunity to advance the scientific discussion of the effects of low dose, low dose-rate radiation exposure. Due to its Congressionally chartered mission, no organization is better positioned than the NCRP to move this debate forward, and recommendations for doing so in future reviews are provided.

Overview of Biological, Epidemiological, and Clinical Evidence of Radiation Hormesis

The effects of low-dose radiation are being increasingly investigated in biological, epidemiological, and clinical studies. Many recent studies have indicated the beneficial effects of low doses of radiation, whereas some studies have suggested harmful effects even at low doses. This review article introduces various studies reporting both the beneficial and harmful effects of low-dose radiation, with a critique on the extent to which respective studies are reliable. Epidemiological studies are inherently associated with large biases, and it should be evaluated whether the observed differences are due to radiation or other confounding factors. On the other hand, well-controlled laboratory studies may be more appropriate to evaluate the effects of low-dose radiation. Since the number of such laboratory studies is steadily increasing, it will be concluded in the near future whether low-dose radiation is harmful or beneficial and whether the linear-no-threshold (LNT) theory is appropriate. Many recent biological studies have suggested the induction of biopositive responses such as increases in immunity and antioxidants by low-dose radiation. Based on recent as well as classical studies, the LNT theory may be out of date, and low-dose radiation may have beneficial effects depending on the conditions; otherwise, it may have no effects.

Delayed effects of accelerated heavy ions on the induction of HPRT mutations in V79 hamster cells

Fundamental research on the harmful effects of ionizing radiation on living cells continues to be of great interest. Recently, priority has been given to the study of high-charge and high-energy (HZE) ions that comprise a substantial part of the galactic cosmic ray (GCR) spectra that would be encountered during long-term space flights. Moreover, predictions of the delayed genetic effects of high linear energy transfer (LET) exposure is becoming more important as heavy ion therapy use is increasing. This work focuses mainly on the basic research on the delayed effects of HZE ions on V79 Chinese hamster cells, with emphasis on the induction of HPRT mutations after prolonged expression times (ET). The research was conducted under various irradiation conditions with accelerated ions ¹⁸O (E=35.2MeV/n), ²⁰Ne (E=47.7MeV/n and 51.8MeV/n), and ¹¹B (E=32.4MeV/n), with LET in the range from 49 to 149 keV/μm and with ⁶⁰Co γ-rays. The HPRT mutant fractions (MF) were detected in irradiated cells in regular intervals during every cell culture recultivation (every 3days) up to approximately 40days (70-80 generations) after irradiation. The MF maximum was reached at different ET depending on ionizing radiation characteristics. The position of the maximum was shifting towards longer ET with increasing LET. We speculate that the delayed mutations are created de novo and that they are the manifestation of genomic instability. Although the exact mechanisms involved in genomic instability initiation are yet to be identified, we hypothesize that differences in induction of delayed mutations by radiations with various LET values are related to variations in energy deposition along the particle track. A dose dependence of mutation yield is discussed as well.

A critique of the use of hormesis in risk assessment

There are severe problems and limitations with the use of hormesis as the principal dose-response default assumption in risk assessment. These problems and limitations include: (a) unknown prevalence of hormetic doseresponse curves; (b) random chance occurrence of hormesis and the shortage of data on the repeatability of hormesis; (c) unknown degree of generalizability of hormesis; (d) there are dose-response curves that are not hormetic, therefore hormesis cannot be universally generalized; (e) problems of post hoc rather than a priori hypothesis testing; (f) a possible large problem of ‘false positive’ hormetic data sets which have not been extensively replicated; (g) the ‘mechanism of hormesis’ is not understood at a rigorous scientific level; (h) in some cases hormesis may merely be the overall sum of many different mechanisms and many different dose-response curves - some beneficial and some toxic. For all of these reasons, hormesis should not now be used as the principal dose-response default assumption in risk assessment. At this point, it appears that hormesis is a long way away from common scientific acceptance and wide utility in biomedicine and use as the principal default assumption in a risk assessment process charged with ensuring public health protection.

Hormetic and regulatory effects of lipid peroxidation mediators in pancreatic beta cells

Nutrient sensing mechanisms of carbohydrates, amino acids and lipids operate distinct pathways that are essential for the adaptation to varying metabolic conditions. The role of nutrient-induced biosynthesis of hormones is paramount for attaining metabolic homeostasis in the organism. Nutrient overload attenuate key metabolic cellular functions and interfere with hormonal-regulated inter- and intra-organ communication, which may ultimately lead to metabolic derangements. Hyperglycemia and high levels of saturated free fatty acids induce excessive production of oxygen free radicals in tissues and cells. This phenomenon, which is accentuated in both type-1 and type-2 diabetic patients, has been associated with the development of impaired glucose tolerance and the etiology of peripheral complications. However, low levels of the same free radicals also induce hormetic responses that protect cells against deleterious effects of the same radicals. Of interest is the role of hydroxyl radicals in initiating peroxidation of polyunsaturated fatty acids (PUFA) and generation of α,β-unsaturated reactive 4-hydroxyalkenals that avidly form covalent adducts with nucleophilic moieties in proteins, phospholipids and nucleic acids. Numerous studies have linked the lipid peroxidation product 4-hydroxy-2E-nonenal (4-HNE) to different pathological and cytotoxic processes. Similarly, two other members of the family, 4-hydroxyl-2E-hexenal (4-HHE) and 4-hydroxy-2E,6Z-dodecadienal (4-HDDE), have also been identified as potential cytotoxic agents. It has been suggested that 4-HNE-induced modifications in macromolecules in cells may alter their cellular functions and modify signaling properties. Yet, it has also been acknowledged that these bioactive aldehydes also function as signaling molecules that directly modify cell functions in a hormetic fashion to enable cells adapt to various stressful stimuli. Recent studies have shown that 4-HNE and 4-HDDE, which activate peroxisome proliferator-activated receptor δ (PPARδ) in vascular endothelial cells and insulin secreting beta cells, promote such adaptive responses to ameliorate detrimental effects of high glucose and diabetes-like conditions. In addition, due to the electrophilic nature of these reactive aldehydes they form covalent adducts with electronegative moieties in proteins, phosphatidylethanolamine and nucleotides. Normally these non-enzymatic modifications are maintained below the cytotoxic range due to efficient cellular neutralization processes of 4-hydroxyalkenals. The major neutralizing enzymes include fatty aldehyde dehydrogenase (FALDH), aldose reductase (AR) and alcohol dehydrogenase (ADH), which transform the aldehyde to the corresponding carboxylic acid or alcohols, respectively, or by biding to the thiol group in glutathione (GSH) by the action of glutathione-S-transferase (GST). This review describes the hormetic and cytotoxic roles of oxygen free radicals and 4-hydroxyalkenals in beta cells exposed to nutritional challenges and the cellular mechanisms they employ to maintain their level at functional range below the cytotoxic threshold.

Biological considerations when comparing proton therapy with photon therapy

Owing to the limited availability of data on the outcome of proton therapy, treatments are generally optimized based on broadly available data on photon-based treatments. However, the microscopic pattern of energy deposition of protons differs from that of photons, leading to a different biological effect. Consequently, proton therapy needs a correction factor (relative biological effectiveness) to relate proton doses to photon doses, and currently, a generic value is used. Moreover, the macroscopic distribution of dose in proton therapy differs compared with photon treatments. Although this may offer new opportunities to reduce dose to normal tissues, it raises the question whether data obtained from photon-based treatments offer sufficient information on dose-volume effects to optimally use unique features of protons. In addition, there are potential differences in late effects due to low doses of secondary radiation outside the volume irradiated by the primary beam. This article discusses the controversies associated with these 3 issues when comparing proton and photon therapy.

Systems Cancer Biology and the Controlling Mechanisms for the J-Shaped Cancer Dose Response: Towards Relaxing the LNT Hypothesis

The hormesis phenomena or J-shaped dose response have been accepted as a common phenomenon regardless of the involved biological model, endpoint measured and chemical class/physical stressor. This paper first introduced a mathematical dose response model based on systems biology approach. It links molecular-level cell cycle checkpoint control information to clonal growth cancer model to predict the possible shapes of the dose response curves of Ionizing Radiation (IR) induced tumor transformation frequency. J-shaped dose response curves have been captured with consideration of cell cycle checkpoint control mechanisms. The simulation results indicate the shape of the dose response curve relates to the behavior of the saddle-node points of the model in the bifurcation diagram. A simplified version of the model in previous work of the authors was used mathematically to analyze behaviors relating to the saddle-node points for the J-shaped dose response curve. It indicates that low-linear energy transfer (LET) is more likely to have a J-shaped dose response curve. This result emphasizes the significance of systems biology approach, which encourages collaboration of multidiscipline of biologists, toxicologists and mathematicians, to illustrate complex cancer-related events, and confirm the biphasic dose-response at low doses.

Adaptation of the black yeast Wangiella dermatitidis to ionizing radiation: molecular and cellular mechanisms

Observations of enhanced growth of melanized fungi under low-dose ionizing radiation in the laboratory and in the damaged Chernobyl nuclear reactor suggest they have adapted the ability to survive or even benefit from exposure to ionizing radiation. However, the cellular and molecular mechanism of fungal responses to such radiation remains poorly understood. Using the black yeast Wangiella dermatitidis as a model, we confirmed that ionizing radiation enhanced cell growth by increasing cell division and cell size. Using RNA-seq technology, we compared the transcriptomic profiles of the wild type and the melanin-deficient wdpks1 mutant under irradiation and non-irradiation conditions. It was found that more than 3000 genes were differentially expressed when these two strains were constantly exposed to a low dose of ionizing radiation and that half were regulated at least two fold in either direction. Functional analysis indicated that many genes for amino acid and carbohydrate metabolism and cell cycle progression were down-regulated and that a number of antioxidant genes and genes affecting membrane fluidity were up-regulated in both irradiated strains. However, the expression of ribosomal biogenesis genes was significantly up-regulated in the irradiated wild-type strain but not in the irradiated wdpks1 mutant, implying that melanin might help to contribute radiation energy for protein translation. Furthermore, we demonstrated that long-term exposure to low doses of radiation significantly increased survivability of both the wild-type and the wdpks1 mutant, which was correlated with reduced levels of reactive oxygen species (ROS), increased production of carotenoid and induced expression of genes encoding translesion DNA synthesis. Our results represent the first functional genomic study of how melanized fungal cells respond to low dose ionizing radiation and provide clues for the identification of biological processes, molecular pathways and individual genes regulated by radiation.

Assessment of the risk for developing a second malignancy from scattered and secondary radiation in radiation therapy

With the average age of radiation therapy patients decreasing and the advent of more complex treatment options comes the concern that the incidences of radiation-induced cancer might increase in the future. The carcinogenic effects of radiation are not well understood for the entire dose range experienced in radiation therapy. Longer epidemiologic studies are needed to improve current risk models and reduce uncertainties of current risk model parameters. On the other hand, risk estimations are needed today to judge the risks versus benefits of modern radiation therapy techniques. This paper describes the current state-of-the-art in risk modeling for radiation-induced malignancies in radiation therapy, distinguishing between two volumes: first, the organs within the main radiation field receiving low or intermediate doses (typically between 0.1 and 50 Gy); and second, the organs far away from the treatment volume receiving low doses mainly due to scattered and secondary radiation (typically below 0.1 Gy). The dosimetry as well as the risk model formalisms are outlined. Furthermore, example calculations and results are presented for intensity-modulated photon therapy versus proton therapy.

A meta-analysis of evidence for hormesis in animal radiation carcinogenesis, including a discussion of potential pitfalls in statistical analyses to detect hormesis

A database containing 800 datasets on the incidence of specific tumor types from 262 radiation carcinogenicity experiments identified in a comprehensive literature search through September 2000 was analyzed for evidence of hormesis. This database includes lifetime studies of tumorigenic responses in mice, rats, and dogs to exposures to alpha, beta, gamma, neutron, or x-ray radiation. A J-shaped dose response, in the form of a significant decreased response at some low dose followed by a significant increased response at a higher dose, was found in only four datasets from three experiments. Three of these datasets involved the same control animals and two also shared dosed animals; the J shape in the fourth dataset appeared to be the result of an outlier within an otherwise monotonic dose response. A meta-analysis was conducted to determine whether there was an excess of dose groups with decreases in tumor response below that in controls at doses below no-observed-effect levels (NOELs) in individual datasets. Because the probability of a decreased response is generally not equal to the probability of an increased response even in the null case, the meta-analysis focused on comparing the number of statistically significant diminished responses to the number expected, assuming no dose effect below the NOEL. Only 54 dose groups out of the total of 2579 in the database had doses below the dataset-specific NOEL and that satisfied an a priori criterion for sufficient power to detect a reduced response. Among these 54, a liberal criterion for defining a significant decreases identified 15 such decreases, versus 54 × 0.2 = 10.8 expected. The excess in significant reductions was accounted for almost entirely by the excess from neutron experiments (10 observed, 6.2 expected). Nine of these 10 dose groups involved only 2 distinct control groups, and 2 pairs from the 10 even shared dosed animals. Given this high degree of overlap, this small excess did not appear remarkable, although the overlap prevented a formal statistical analysis. A comprehensive post hoc evaluation using a range of NOEL definitions and alternative ways of restricting the data entering the analysis did not produce materially different results. A second meta-analysis found that, in every possible low dose range ([0, d] for every dose, d) of each of the radiation types, the number of dose groups with significantly increased tumorigenic responses was either close to or exceeded the number showing significantly reduced responses. This meta-analysis was considered to be the more definitive one. Not only did it take dose into account by looking for consistent evidence of hormesis throughout defined low-dose ranges, it was also potentially less susceptible to limitations in experimental protocols that would cause individual animals to respond in a non-independent fashion. Overall, this study found little evidence in a comprehensive animal radiation database to support the hormesis hypothesis. However, the ability of the database to detect a hormetic effect was limited both by the small number of dose groups with doses below the range where positive effects have been found in epidemiological studies (≤ 0.1 Gy) and by the limited power of many of these dose groups for detecting a decrease in response.

Regulatory-Science: Biphasic Cancer Models or the LNT-Not Just a Matter of Biology!

There is no doubt that prudence and risk aversion must guide public decisions when the associated adverse outcomes are either serious or irreversible. With any carcinogen, the levels of risk and needed protection before and after an event occurs, are determined by dose-response models. Regulatory law should not crowd out the actual beneficial effects from low dose exposures-when demonstrable-that are inevitably lost when it adopts the linear non-threshold (LNT) as its causal model. Because regulating exposures requires planning and developing protective measures for future acute and chronic exposures, public management decisions should be based on minimizing costs and harmful exposures. We address the direct and indirect effects of causation when the danger consists of exposure to very low levels of carcinogens and toxicants. The societal consequences of a policy can be deleterious when that policy is based on a risk assumed by the LNT, in cases where low exposures are actually beneficial. Our work develops the science and the law of causal risk modeling: both are interwoven. We suggest how their relevant characteristics differ, but do not attempt to keep them separated; as we demonstrate, this union, however unsatisfactory, cannot be severed.

Use of threshold and mode of action in risk assessment

Under current guidelines, exposure guidelines for toxicants are determined by following one of two different tracks depending on whether the toxicant's mode of action (MOA) is believed to involve an exposure threshold. Although not denying the existence of thresholds, this paper points out problems with how the threshold concept and MOA is used in risk assessment. Thresholds are frequently described using imprecise terms that imply some unspecified increase in risk, which robs them of any meaning (any reasonable dose response will satisfy such a definition) and tacitly implies a value judgment about how large a risk is acceptable. MOA is generally used only to inform a threshold's existence and not its value. Often MOA is used only to conclude that the adverse effect requires an upstream cellular or biochemical response for which a threshold is simply assumed. Data to inform MOA often come from animals, which complicates evaluation of the role of human variation in genetic and environmental conditions, and the possible interaction of the toxicant with processes already producing background toxicity in humans. In response to these and other problems with the current two-track approach, this paper proposes a modified point of departure/safety factor approach to setting exposure guidelines for all toxicants. MOA and the severity of the toxic effect would be addressed using safety factors calculated from guidelines established by consensus and based on scientific judgment. The method normally would not involve quantifying low-dose risk, and would not require a threshold determination, although MOA information regarding the likelihood of a threshold could be used in setting safety factors.

Nutrition, hormetic stress and health

Hormesis defines an effect where exposure to a low dose of a toxic agent results in a beneficial response. It has been described in organisms exposed to low-dose radiation, heat stress, and chemicals. The effect is characterised by a J-shaped dose-response as opposed to a linear dose-response. Confirmation of the general phenomena of hormesis has proved difficult due to the lack of appropriate methodology and the absence of well-defined mechanisms to support the experimental observations. In the nutritional field there are few reports of its existence. The clearest illustration of the effect is seen in animals that are energy restricted when there is a clear benefit in the reduction of age-related disease, and an extension of maximum lifespan. DNA microarray experiments have shown that there is a down regulation of the stress-response genes that are up regulated through the ageing process. Electrophilic phytochemicals, that have been shown to have beneficial health effects at low doses, up regulate the antioxidant-electrophile response element. This probably occurs through an alteration in the redox state of the target cells which causes activation of protein kinases, the activation of the Nrf2 transcription factor and the up regulation of the phase II enzymes, similar to responses that occur under mild chemical stress. This situation might enable organisms to adapt to stress such that the effects of a subsequent exposure to a harmful challenge are reduced. There may be a permanent alteration in cellular homeostasis, or redox state, if the low level exposure is maintained. It remains to be proven if such a situation occurs in response to chronic low-dose exposure to dietary phytochemicals such that the target cells are better able to respond to a subsequent stress challenge.

Modification in the expression of Mre11/Rad50/Nbs1 complex in low dose irradiated human lymphocytes

Despite the fact that high doses of radiation are detrimental, low dose radiation (LDR) often protects the organism against a subsequent exposure of lethal doses of radiation. Present study was undertaken to understand the role of Mre11, Rad50 and Nbs1 genes in the low dose radio-adapted human peripheral blood mononuclear cells (PBMCs). Optimum time interval between low dose (0.07 Gy) and high dose (5.0 Gy) of (60)Co-gamma-radiation was observed to be 5.0 hours, at which PBMCs showed maximum LDR induced resistance (RIR). At cytogenetic level, micronuclei frequency was found to be reduced in LDR pre-irradiated PBMCs subsequently exposed to high dose radiation (HDR) as compared to controls. At transcriptional level, with reference to sham-irradiated cells significantly (p< or =0.05) altered expression of Mre11, Rad50 and Nbs1 genes was observed in low dose irradiated cells. At protein level, Mre11, Rad50 and Nbs1 were enhanced significantly (p< or =0.05) in low dose pre-irradiated cells subsequently exposed to high dose of radiation as compared to only high dose irradiated cells. Transcriptional as well as translational modulation in the expression of MRN complex components upon low dose irradiation may confer its participation in repair pathways, resulting in induced resistance.

The stimulatory effects of topical application of radioactive lantern mantle powder on wound healing

Some people in different parts of Iran use burned mantles as a wound healing medicine. To perform surface area measurement, twenty rats were divided randomly into two groups of 10 animals each. The 1st group received topical burned radioactive lantern mantle powder at 1st-3rd day after making excision wounds. The 2nd group received non-radioactive lantern mantle powder. For histological study, 36 male rats randomly divided into two groups of 18 animals each. Full thickness excision wound (314+/-31.4 mm(2)) was made on the dorsal neck in all animals after inducing general anesthesia. For the first 3 days, cases received topical application of the radioactive lantern mantle powder. Finally, to measure the tensile strength, an incision was made on the dorsal neck of the rats. Surface area measurement of the wounds showed a progressive surface reduction in both groups. Histological study showed a significant statistically difference between cases and controls with respect to fibrinoid necrosis and neutrophilic exudate at the days 3 and 14. Considering the existence of granulation tissue, a significant difference was observed between case and control groups at days 3 and 7. Tensile strength study showed no significant difference between the cases and controls until 30 days after excision.

A review of dosimetry studies on external-beam radiation treatment with respect to second cancer induction

It has been long known that patients treated with ionizing radiation carry a risk of developing a second cancer in their lifetimes. Factors contributing to the recently renewed concern about the second cancer include improved cancer survival rate, younger patient population as well as emerging treatment modalities such as intensity-modulated radiation treatment (IMRT) and proton therapy that can potentially elevate secondary exposures to healthy tissues distant from the target volume. In the past 30 years, external-beam treatment technologies have evolved significantly, and a large amount of data exist but appear to be difficult to comprehend and compare. This review article aims to provide readers with an understanding of the principles and methods related to scattered doses in radiation therapy by summarizing a large collection of dosimetry and clinical studies. Basic concepts and terminology are introduced at the beginning. That is followed by a comprehensive review of dosimetry studies for external-beam treatment modalities including classical radiation therapy, 3D-conformal x-ray therapy, intensity-modulated x-ray therapy (IMRT and tomotherapy) and proton therapy. Selected clinical data on second cancer induction among radiotherapy patients are also covered. Problems in past studies and controversial issues are discussed. The needs for future studies are presented at the end.

The effects of age on radiation resistance and oxidative stress in adult Drosophila melanogaster

Drosophila melanogaster (fruit fly) is a well-established model organism for genetic studies of development and aging. We examined the effects of lethal ionizing radiation on male and female adult Drosophila of different ages, using doses of radiation from 200 to 1500 Gy. Fifty percent lethality 2 days postirradiation (LD(50/2)) in wild-type 1-day-old adult fruit flies was approximately 1238 Gy for males and 1339 Gy for females. We observed a significant age-dependent decline in the radiation resistance of both males and females. Radiation damage is postulated to occur by the generation of oxygen radicals. An age-related decline in the ability of flies to resist an agent that induces oxygen radicals, paraquat, was observed when comparing 10- and 20-day adults. Female flies are more resistant to paraquat than male flies. Oxidative stress mediated by paraquat was additive with sublethal exposures to radiation in young adults. Therefore, the ability to repair the damage caused by oxygen radicals seems to decline with the age of the flies. Because Drosophila adults are largely post-mitotic, our data suggest that adult Drosophila melanogaster can serve as an excellent model to study the factors responsible for radiation resistance in post-mitotic tissue and age-dependent changes in this resistance.

Life span alteration after irradiation in Drosophila melanogaster strains with mutations of Hsf and Hsps

The life span alteration after gamma-irradiation and/or paraquat treatment in Drosophila in wild type strain Canton-S and strains with mutations of heat shock factor (1-4 alleles) and heat shock proteins (Hsp70Ba ( 304 ), Hsp83 ( e6A ), Hsp22 ( EY09909 ), Hsp67Bb ( EY099099 )) was investigated. Chronic low-dose rate gamma-irradiation (0.017 and 0.17 cGy/h) on pre-imago stages was used as a priming dose (absorbed doses were 4 and 40 cGy). Paraquat, a free radical inducing agent, was a challenging factor (20 mM for 1 day). It was shown that chronic irradiation led to adaptive response in both sexes except homozygous males and females with mutations of Hsf ( 4 ) and Hsp70Ba ( 304 ). The gender-specific differences in stress response were discovered in wild type strain Canton-S, Hsp22 ( EY09909 ) Hsp67Bb ( EY09909 ) homozygotes and Hsp83 ( e6A ) heterozygotes: the adaptive response persisted in males, but not in females. Thus, Drosophila Hsp and Hsf mutation homozygotes did not demonstrate the adaptive response in the majority of cases, implying an important role of those genes in radiation hormesis and adaptation to stresses.

Radio-adaptive response to environmental exposures at Chernobyl

The genetic consequences resulting from environmental exposure to ionizing radiation have a significant impact on both radiation regulatory policies and the comprehension of the human health risks associated with radiation exposure. The primary objectives of the study were to assess 1) genotoxicity of exposure to radiation as a function of absorbed dose and dose rate, and 2) induction of a radio-adaptive response following a priming dose at varying dose rates. Results demonstrated that sub-acute environmental exposures of 10cGy gamma radiation resulted in indistinguishable levels of chromosomal damage as compared to controls. A radio-adaptive response was observed in all experimental groups, exposed to a subsequent acute challenge dose of 1.5 Gy, demonstrating that low dose rates of low energy transfer (LET) radiation are effective in reducing genetic damage from a subsequent acute low-LET radiation exposure. Furthermore, the data presented herein demonstrate a potential beneficial effect of sub-chronic exposure to low levels of low-LET radiation in an environmental setting and do not support the Linear No Threshold (LNT) hypothesis.

Effect of low-dose total-body irradiation on transplantability of tumor cells in syngeneic mice

The effect of pretreatment with various low doses of total-body irradiation (TBI) on tumor cell transplantability in syngeneic mice was investigated. Two cell lines, EMT6 and SCCVII, and two strains of mice, were used. First, Balb/c mice were sham-irradiated or irradiated at 200 mGy, and 6-48 h later, 1000 EMT6 cells were inoculated in the hind legs. Based on the results, 0-1500 mGy of TBI was given 6 h before inoculation of 100 or 1000 cells in the subsequent experiments. All mice were observed for 50 days after transplantation. Tumors were judged as grown when the volume of palpable nodules exceeded 200 mm(3). Tumor transplantability rate was significantly higher in the groups irradiated at 1500 mGy than in the sham-irradiated groups in both Balb/c and C3H/He mice. There were no differences in transplantability rates between the control group and the groups irradiated at various doses of 50-500 mGy. However, the mean time to tumor appearance was significantly elongated in Balb/c mice receiving TBI at 200 mGy and inoculated with 100 or 1000 EMT6 cells 6 h later. This phenomenon was also observed in Balb/c mice receiving 100 mGy TBI and inoculated with 1000 EMT6 cells. The present study might suggest that low-dose TBI to mice may delay tumor growth under certain conditions.

Radiobiology of systemic radiation therapy

Although systemic radionuclide therapy (SRT) is effective as a palliative therapy in patients with metastatic cancer, there has been limited success in expanding patterns of utilization and in bringing novel systemic radiotherapeutic agents to routine clinical use. Although there are many factors that contribute to this situation, we hypothesize that a better understanding of the radiobiology and mechanism of action of SRT will facilitate the development of future compounds and the future designs of prospective clinical trials. If these trials can be rationalized to the biological basis of the therapy, it is likely that the long-term outcome would be enhanced therapeutic efficacy. In this review, we provide perspectives of the current state of low-dose-rate (LDR) radiation research and offer linkages where appropriate with current clinical knowledge. These include the recently described phenomena of low-dose hyper-radiosensitivity-increased radioresistance (LDH-IRR), adaptive responses, and biological bystander effects. Each of these areas require a major reconsideration of existing models for radiation action and an understanding of how this knowledge will integrate into the evolution of clinical SRT practice. Validation of a role in vivo for both LDH-IRR and biological bystander effects in SRT would greatly impact the way we would assess therapeutic response to SRT, the design of clinical trials of novel SRT radiopharmaceuticals, and risk estimates for both therapeutic and diagnostic radiopharmaceuticals. We believe that the current state of research in LDR effects offers a major opportunity to the nuclear medicine community to address the basic science of clinical SRT practice, to use this new knowledge to expand the use and roles of SRT, and to facilitate the introduction of new therapeutic radiopharmaceuticals.

Hormesis and its place in nonmonotonic dose-response relationships: some scientific reality checks

OBJECTIVE

This analysis is a critical assessment of current hormesis literature. I discuss definitions, characterization, generalizability, mechanisms, absence of empirical data specific for hormesis hypothesis testing, and arguments that hormesis be the "default assumption" in risk assessment.

DATA SOURCES

Hormesis, a biological phenomenon typically described as low-dose stimulation from substances producing higher-dose inhibition, has recently garnered interest in several quarters. The principal sources of published materials for this analysis are the writings of certain proponents of hormesis. Surprisingly few systematic critiques of current hormesis literature exist. Limits to the phenomenon's appropriate role in risk assessment and health policy have been published.

DATA SYNTHESIS

Serious gaps in scientific understanding remain: a stable definition; generalizability, especially for humans; a clear mechanistic basis; limitations in the presence of multiple toxic end points, target organs, and mechanisms. Absence of both arms-length, consensus-driven, scientific evaluations and empirical data from studies specifically designed for hormesis testing have limited its acceptance.

CONCLUSIONS

Definition, characterization, occurrence, and mechanistic rationale for hormesis will remain speculative, absent rigorous studies done specifically for hormesis testing. Any role for hormesis in current risk assessment and regulatory policies for toxics remains to be determined.

Radiation-induced life span alteration of Drosophila lines with genotype differences

The research shows that unirradiated Drosophila lines with defects of DNA repair, antioxidant protection and apoptosis have higher speed of ageing, than wild type line. At the same time, depending on the line genotype, the irradiation results in change of life span. Mechanism of postponed effect of low dozes of ionizing radiation on the life span is elaborated. As cells with weakened protection will accumulate damages and will be exposed to ageing with the greater speed, than steady cells, their radio-induced elimination at early development stages will result in delay of age-dependent changes and will lower speed of ageing. In the subsequent irradiated generations the given somatic answer to stress (hormesis) may be replaced by negative genetic effects on the population level (shorter life span).

Secondary carcinogenesis in patients treated with radiation: a review of data on radiation-induced cancers in human, non-human primate, canine and rodent subjects

Concern for risk of radiation-induced cancer is growing with the increasing number of cancer patients surviving long term. This study examined data on radiation transformation of mammalian cells in vitro and on the risk of an increased cancer incidence after irradiation of mice, dogs, monkeys, atomic bomb survivors, occupationally exposed persons, and patients treated with radiation. Transformation of cells lines in vitro increased linearly with dose from approximately 1 to approximately 4-5 Gy. At <0.1 Gy, transformation was not increased in all studies. Dose-response relationships for cancer incidence varied with mouse strain, gender and tissue/organ. Risk of cancer in Macaca mulatta was not raised at 0.25-2.8 Gy. From the atomic bomb survivor study, risk is accepted as increasing linearly to 2 Sv for establishing exposure standards. In irradiated patients, risk of cancer increased significantly from 1 to 45 Gy (a low to a high dose level) for stomach and pancreas, but not for bladder and rectum (1-60 Gy) or kidney (1-15 Gy). Risk for several organs/tissues increased substantially at doses far above 2 Gy. There is great heterogeneity in risk of radiation-associated cancer between species, strains of a species, and organs within a species. At present, the heterogeneity between and within patient populations of virtually every parameter considered in risk estimation results in substantial uncertainty in quantification of a general risk factor. An implication of this review is that reduced risks of secondary cancer should be achieved by any technique that achieved a dose reduction down to approximately [corrected] 0.1 Gy, i.e. dose to tissues distant from the target. The proportionate gain should be greatest for dose decrement to less than 2 Gy.

New advances in radiation biology

Current understanding of risk associated with low-dose radiation exposure has for many years been embedded in the linear-no-threshold (LNT) approach, based on simple extrapolation from the Japanese atomic bomb survivors. Radiation biology research has supported the LNT approach although much of this has been limited to relatively high-dose studies. Recently, with new advances for studying effects of low-dose exposure in experimental models and advances in molecular and cellular biology, a range of new effects of biological responses to radiation has been observed. These include genomic instability, adaptive responses and bystander effects. Most have one feature in common in that they are observed at low doses and suggest significant non-linear responses. These new observations pose a significant challenge to our understanding of low-dose exposure and require further study to elucidate mechanisms and determine their relevance.

Cell mechanics and stress: from molecular details to the ‘universal cell reaction’ and hormesis

The 'universal cell reaction' (UCR), a coordinated biphasic response to external (noxious and other) stimuli observed in all living cells, was described by Nasonov and his colleagues in the mid-20th century. This work has received no attention from cell biologists in the West, but the UCR merits serious consideration. Although it is non-specific, it is likely to be underpinned by precise mechanisms and, if these mechanisms were characterized and their relationship to the UCR elucidated, then our understanding of the integration of cellular function could be improved. As a step towards identifying such mechanisms, I review some recent advances in understanding cell mechanics and the stress response and I suggest potentially testable hypotheses. There is a particular need for time-course studies of cellular responses to different stimulus doses or intensities. I also suggest a correspondence with hormesis; re-investigation of the UCR using modern biophysical and molecular-biological techniques might throw light on this much-discussed phenomenon.

Effects of low-dose irradiation on enhancement of immunity by dendritic cells

Low-doses of irradiation have been reported to have beneficial effects, particularly anti-tumor effects. In this paper, we show the effects of the low-dose irradiation on T cell activation induced by dendritic cells (DCs). DCs, which had been pre-irradiated at 0.02-1.0 Gy from a (137)Cs source, were cultured with allogeneic T cells, and the proliferation of T cells was then examined. The 0.05Gy-pre-irradiated DCs showed the highest proliferation capacity of T cells. The 0.05Gy-irradiation does not augment the expression of major histocompatibility complexes (MHCs) or costimulatory molecules on DCs, as with non-irradiated DCs or 1Gy-irradiated DCs, but does augment the production of IL-2, IL-12 and IFN-gamma DCs. These results suggest that the low-dose irradiation augments T cell-activation capacity through cytokine production by DCs, which might shift naïve helper T cells to Th1 cells.

Cancer control related to stimulation of immunity by low-dose radiation

Previous studies showed that low dose radiation (LDR) could stimulate the immune system in both animal and human populations. This paper reviews the present status of relevant research as support to the use of LDR in clinical practice for cancer prevention and treatment. It has been demonstrated that radiation-induced changes in immune activity follows an inverse J-shaped curve, i.e., low dose stimulation and high dose suppression. The stimulation of immunity by LDR concerns most anticancer parameters, including antibody formation, natural killer activity, secretion of interferon and other cytokines as well as other cellular changes. Animal studies have revealed that LDR retards tumor growth, decreases cancer metastasis, and inhibits carcinogenesis induced by high dose radiation. These effects of LDR on cancer control were found to be related to its stimulation on immunity. The experimental data may well explain the efficacy of the clinical trial of LDR in the treatment of cancer.

The PKZ1 recombination mutation assay: a sensitive assay for low dose studies

The majority of mutation studies are performed at high doses of DNA damaging agents due to the insensitivity of most mutation assays. Extrapolation using a linear no-threshold (LNT) dose response model is then used to estimate the extent of possible DNA damage at lower doses. There is increasing evidence to suggest that the LNT model may not be correct at low doses of at least some DNA damaging agents. The pKZ1 in vivo and in vitro recombination assays have proven to be very sensitive for detection of changes in chromosomal inversion in lymphoid tissue in response to low doses of DNA damaging agents. Non-linear dose response curves for chromosomal inversion as an end-point have been identified at low doses of DNA damaging agents using this assay. Here, we review the inversion results obtained to date with the pKZ1 assays and discuss their suitability for low dose studies.

p27Cip/Kip is involved in hsp25 or inducible hsp70 mediated adaptive response by low dose radiation

Thermoresistant (TR) clone of radiation-induced fibrosarcoma (RIF) cells have been reported to show adaptive response to 1 cGy of low dose radiation, and hsp25 and inducible hsp70 are involved in this process. In the present study, to further elucidate the mechanism of how hsp25 and inducible hsp70 regulate the adaptive response, hsp25 or inducible hsp70 overexpressed RIF cells were irradiated with 1 cGy and cell cycle was analyzed. Hsp25 or inducible hsp70 overexpressed cells as well as TR cells showed increase of G1 phase population after gamma-irradiation at 1 cGy, while the parent RIF cells did not. [3H]-Thymidine and BrdU incorporation also indicated that both hsp25 and inducible hsp70 were involved in G1 arrest after 1 cGy irradiation. Molecular analysis revealed upregulation of p27Cip/Kip protein in hsp25 and inducible hsp70 overexpressed cells, and cotransfection of p27Cip/Kip antisense abolished the induction of adaptive response and 1 cGy-mediated G1 arrest. The above results indicate that induction of adaptive response by hsp25 and inducible hsp70 is mediated by upregulation of p27Cip/Kip protein, resulting in low dose radiation-induced G1 arrest.

Cytoskeletal Reorganization and Altered Phagocytotic Ability in Primary Cultures of Rainbow Trout Hemopoietic Tissue Exposed to Low-Level Ionizing Radiation

It has long been known that the hematopoietic tissue of mammals is one of the most radiosensitive tissues. In vitro studies on prawns have also shown that low doses of radiation have an extremely deleterious effect on cells cultured from this animal's blood-forming tissues. This raises questions about the relative effects of radiation in animals of different species. One of the most important aquatic animals, from both an economic and an ecological point of view, is the fish. With this in mind, primary cultures of the blood-forming tissues of rainbow trout were exposed to radiation followed by a morphological comparison between control and irradiated cultures. The cultured cells were characterized as macrophages after incubation with apoptotic human polymorphonuclear leukocytes and were classified as phagocytotic leukocytes. These cells were found in two morphological forms, stretched and rounded. It was shown that there was a commensurate increase in the number of stretched cells after irradiation. Radiation was also shown to cause a dose-dependent increase in the amounts of apoptosis in these cells over time. The phagocytotic efficacy of these cells was shown to inhibited by the exposure to low doses of radiation.

In vivo formation and repair of DNA double-strand breaks after computed tomography examinations

Ionizing radiation can lead to a variety of deleterious effects in humans, most importantly to the induction of cancer. DNA double-strand breaks (DSBs) are among the most significant genetic lesions introduced by ionizing radiation that can initiate carcinogenesis. We have enumerated gamma-H2AX foci as a measure for DSBs in lymphocytes from individuals undergoing computed tomography examination of the thorax and/or the abdomen. The number of DSBs induced by computed tomography examination was found to depend linearly on the dose-length product, a radiodiagnostic unit that is proportional to both the local dose delivered and the length of the body exposed. Analysis of lymphocytes sampled up to 1 day postirradiation provided kinetics for the in vivo loss of gamma-H2AX foci that correlated with DSB repair. Interestingly, in contrast to results obtained in vitro, normal individuals repair DSBs to background levels. A patient who had previously shown severe side effects after radiotherapy displayed levels of gamma-H2AX foci at various sampling times postirradiation that were several times higher than those of normal individuals. Gamma-H2AX and pulsed-field gel electrophoresis analysis of fibroblasts obtained from this patient confirmed a substantial DSB repair defect. Additionally, these fibroblasts showed significant in vitro radiosensitivity. These data show that the in vivo induction and repair of DSBs can be assessed in individuals exposed to low radiation doses, adding a further dimension to DSB repair studies and providing the opportunity to identify repair-compromised individuals after diagnostic irradiation procedures.

Radiation biology: concepts for radiation protection

The opportunity to write a historical review of the field of radiation biology allows for the viewing of the development and maturity of a field of study, thereby being able to provide the appropriate context for the earlier years of research and its findings. The pioneering work of Muller, Sax, and McClintock, and many others, has stood the test of time. The idea that x-rays could damage the genetic material and result in interactions that could lead to gene mutations and a range of chromosomal alterations is now interpretable in terms of induced DNA damage and errors of DNA repair. The expanded idea that such genetic alterations can be induced by DNA damage that is produced by one or two tracks of ionizing radiation remains the mainstay of radiation biology. The impact of the more recent molecular approaches to unraveling the mechanism behind this simple concept has confirmed this fundamental observation. The remarkable advances have allowed for a fairly complete understanding of the specific types of DNA damage induced by ionizing radiations and the pivotal role played by the errors of repair of double-strand breaks. Given our considerably enhanced knowledge of the details of the DNA repair processes involved, misrepair is a very unlikely event. The role of potential confounders of the concept of dose-response (e.g., bystander effects, genomic instability, and adaptive responses) is taking on a growing importance to the field. The evolving need is to begin to consider mechanistically-based dose-response models for cancer risk such that any potential impact of confounders on the response at low, environmental doses can be assessed. Thus, radiation biology research has always had a focus on how best to protect human health from radiation exposures and will continue to do so.

Adaptive doses of irradiation-an approach to a new therapy concept for bladder cancer?

Radiation adaptive response in terms of induced radioresistance or hyperradiosensitivity, has been studied in HCV29 (human bladder epithelium) and RT4 (human bladder carcinoma) cell lines. After pre-irradiation doses of 0.05 Gy or 0.1 Gy, HCV29 cells showed induced radioresistance, whereas after pre-irradiation doses of 0.05 Gy, 0.1 Gy, 0.2 Gy, and 0.5 Gy, the RT4 cells clearly showed hyperradiosensitivity. On the basis of these results, an approach has been developed that may lead to a concept for a new radiotherapeutic regimen of bladder cancer that includes protection of normal cells, on the one hand, and the potential of tumor cell damage, on the other hand. These findings need to be confirmed in further studies for the benefit of the patients.

Oxidative stress, radiation-adaptive responses, and aging

Organisms living in an aerobic environment were forced to evolve effective cellular strategies to detoxify reactive oxygen species. Besides diverse antioxidant enzymes and compounds, DNA repair enzymes, and disassembly systems, which remove damaged proteins, regulation systems that control transcription, translation, and activation have also been developed. The adaptive responses, especially those to radiation, are defensive regulation mechanisms by which oxidative stress (conditioning irradiation) elicits a response against damage because of subsequent stress (challenging irradiation). Although many researchers have investigated these molecular mechanisms, they remain obscure because of their complex signaling pathways and the involvement of various proteins. This article reviews the factors concerned with radiation-adaptive response, the signaling pathways activated by conditioning irradiation, and the effects of aging on radiation-adaptive response. The proteomics approach is also introduced, which is a useful method for studying stress response in cells.