It's time for a new low-dose-radiation risk assessment paradigm--one that acknowledges hormesis

Low dose ionizing radiation and the immune response: what is the role of non-targeted effects?

OBJECTIVES

This review aims to trace the historical narrative surrounding the low dose effects of radiation on the immune system and how our understanding has changed from the beginning of the 20th century to now. The particular focus is on the non-targeted effects (NTEs) of low dose ionizing radiation (LDIR) which are effects that occur when irradiated cells emit signals that cause effects in the nearby or distant non-irradiated cells known as radiation induced bystander effect (RIBE). Moreover, radiation induced genomic instability (RIGI) and abscopal effect (AE) also regarded as NTE. This was prompted by our recent discovery that ultraviolet A (UVA) photons are emitted by the irradiated cells and that these photons can trigger NTE such as the RIBE in unirradiated recipients of these photons. Given the well-known association between UV radiation and the immune response, where these biophotons may pose as bystander signals potentiating processes in deep tissues as a consequence of LDIR, it is timely to review the field with a fresh lens. Various pathways and immune components that contribute to the beneficial and adverse types of modulation induced by LDR will also be revisited.

CONCLUSION

There is limited evidence for LDIR induced immune effects by way of a non-targeted mechanism in biological tissue. The literature examining low to medium dose effects of ionizing radiation on the immune system and its components is complex and controversial. Early work was compromised by lack of good dosimetry while later work mainly looks at the involvement of immune response in radiotherapy. There is a lack of research in the LDIR/NTE field focusing on immune response although bone marrow stem cells and lineages were critical in the identification and characterization of NTE where effects like RIGI and RIBE were heavily researched. This may be in part, a result of the difficulty of isolating NTE in whole organisms which are essential for good immune response studies. Models involving inter organism transmission of NTE are a promising route to overcome these issues.

Environmental toxicology and ecotoxicology: How clean is clean? Rethinking dose-response analysis

Global agendas for sustaining clean environments target remediation of multimedia contaminants, but how clean is clean? Environmental Toxicology and Ecotoxicology focus on issues concerning "clean". However, the models used to assess the effects of environmental multimedia on individual living organisms and communities or populations in Environmental Toxicology and Ecotoxicology may fail to provide reliable estimates for risk assessment and optimize health. Recent developments in low-dose effects research provide a novel means in Environmental Toxicology and Ecotoxicology to improve the quality of hazard and risk assessment.

The LNT model for cancer induction is not supported by radiobiological data

The hallmarks of cancer have been the focus of much research and have influenced the development of risk models for radiation-induced cancer. However, natural defenses against cancer, which constitute the hallmarks of cancer prevention, have largely been neglected in developing cancer risk models. These natural defenses are enhanced by low doses and dose rates of ionizing radiation, which has aided in the continuation of human life over many generations. Our natural defenses operate at the molecular, cellular, tissue, and whole-body levels and include epigenetically regulated (epiregulated) DNA damage repair and antioxidant production, selective p53-independent apoptosis of aberrant cells (e.g. neoplastically transformed and tumor cells), suppression of cancer-promoting inflammation, and anticancer immunity (both innate and adaptive components). This publication reviews the scientific bases for the indicated cancer-preventing natural defenses and evaluates their implication for assessing cancer risk after exposure to low radiation doses and dose rates. Based on the extensive radiobiological evidence reviewed, it is concluded that the linear-no-threshold (LNT) model (which ignores natural defenses against cancer), as it relates to cancer risk from ionizing radiation, is highly implausible. Plausible models include dose-threshold and hormetic models. More research is needed to establish when a given model (threshold, hormetic, or other) applies to a given low-dose-radiation exposure scenario.

Hormetic effects of curcumin: What is the evidence?

Curcumin (diferuloylmethane), a component of the yellow powder prepared from the roots of Curcuma longa or Zingiberaceae (known as turmeric) is not only widely used to color and flavor food but also used as a pharmaceutical agent. Curcumin demonstrates anti-inflammatory, anticarcinogenic, antiaging, and antioxidant activity, as well as efficacy in wound healing. Notably, curcumin is a hormetic agent (hormetin), as it is stimulatory at low doses and inhibitory at high doses. Hormesis by curcumin could be also a particular function at low doses (i.e., antioxidant behavior) and another function at high dose (i.e., induction of autophagy and cell death). Recent findings suggest that curcumin exhibits biphasic dose-responses on cells, with low doses having stronger effects than high doses; examples being activation of the mitogen-activated protein kinase signaling pathway or antioxidant activity. This indicates that many effects induced by curcumin are dependent on dose and some effects might be greater at lower doses, indicative of a hormetic response. Despite the consistent occurrence of hormetic responses of curcumin in a wide range of biomedical models, epidemiological and clinical trials are needed to assess the nature of curcumin's dose-response in humans. Fortunately, more than one hundred clinical trials with curcumin and curcumin derivatives are ongoing. In this review, we provide the first comprehensive analysis supportive of the hormetic behavior of curcumin and curcumin derivatives.

Hormesis and radiation safety norms: Comments for an update

Hormesis can be explained by evolutionary adaptation to the current level of a factor present in the natural environment or to some average from the past. This pertains also to ionizing radiation as the natural background has been decreasing during the time of the life existence. DNA damage and repair are normally in a dynamic balance. The conservative nature of the DNA repair suggests that cells may have retained some capability to repair damage from higher radiation levels than that existing today. According to this concept, the harm caused by radioactive contamination would tend to zero with a dose rate tending to a wide range level of the natural radiation background. Existing evidence in favor of hormesis is substantial, experimental data being partly at variance with results of epidemiological studies. Potential bias, systematic errors, and motives to exaggerate risks from low-dose low-rate ionizing radiation are discussed here. In conclusion, current radiation safety norms are exceedingly restrictive and should be revised on the basis of scientific evidence. Elevation of the limits must be accompanied by measures guaranteeing their observance.

Vive la radiorésistance!: converging research in radiobiology and biogerontology to enhance human radioresistance for deep space exploration and colonization

While many efforts have been made to pave the way toward human space colonization, little consideration has been given to the methods of protecting spacefarers against harsh cosmic and local radioactive environments and the high costs associated with protection from the deleterious physiological effects of exposure to high-Linear energy transfer (high-LET) radiation. Herein, we lay the foundations of a roadmap toward enhancing human radioresistance for the purposes of deep space colonization and exploration. We outline future research directions toward the goal of enhancing human radioresistance, including upregulation of endogenous repair and radioprotective mechanisms, possible leeways into gene therapy in order to enhance radioresistance via the translation of exogenous and engineered DNA repair and radioprotective mechanisms, the substitution of organic molecules with fortified isoforms, and methods of slowing metabolic activity while preserving cognitive function. We conclude by presenting the known associations between radioresistance and longevity, and articulating the position that enhancing human radioresistance is likely to extend the healthspan of human spacefarers as well.

Cancer immunotherapy: how low-level ionizing radiation can play a key role

The cancer immunoediting hypothesis assumes that the immune system guards the host against the incipient cancer, but also "edits" the immunogenicity of surviving neoplastic cells and supports remodeling of tumor microenvironment towards an immunosuppressive and pro-neoplastic state. Local irradiation of tumors during standard radiotherapy, by killing neoplastic cells and generating inflammation, stimulates anti-cancer immunity and/or partially reverses cancer-promoting immunosuppression. These effects are induced by moderate (0.1-2.0 Gy) or high (>2 Gy) doses of ionizing radiation which can also harm normal tissues, impede immune functions, and increase the risk of secondary neoplasms. In contrast, such complications do not occur with exposures to low doses (≤0.1 Gy for acute irradiation or ≤0.1 mGy/min dose rate for chronic exposures) of low-LET ionizing radiation. Furthermore, considerable evidence indicates that such low-level radiation (LLR) exposures retard the development of neoplasms in humans and experimental animals. Here, we review immunosuppressive mechanisms induced by growing tumors as well as immunomodulatory effects of LLR evidently or likely associated with cancer-inhibiting outcomes of such exposures. We also offer suggestions how LLR may restore and/or stimulate effective anti-tumor immunity during the more advanced stages of carcinogenesis. We postulate that, based on epidemiological and experimental data amassed over the last few decades, whole- or half-body irradiations with LLR should be systematically examined for its potential to be a viable immunotherapeutic treatment option for patients with systemic cancer.

Molecular Breast Imaging for Screening in Dense Breasts: State of the Art and Future Directions

OBJECTIVE

The purposes of this review are to discuss the motivation for supplemental screening, to address molecular breast imaging (MBI) radiation dose concerns, and to provide an updated guide to current MBI technology, clinical protocols, and screening performance. Future directions of MBI are also discussed.

CONCLUSION

MBI offers detection of mammographically occult cancers in women with dense breasts. Although MBI has been under investigation for nearly 15 years, it has yet to gain widespread adoption in breast screening.

Gamma radiation at a human relevant low dose rate is genotoxic in mice

Even today, 70 years after Hiroshima and accidents like in Chernobyl and Fukushima, we still have limited knowledge about the health effects of low dose rate (LDR) radiation. Despite their human relevance after occupational and accidental exposure, only few animal studies on the genotoxic effects of chronic LDR radiation have been performed. Selenium (Se) is involved in oxidative stress defence, protecting DNA and other biomolecules from reactive oxygen species (ROS). It is hypothesised that Se deficiency, as it occurs in several parts of the world, may aggravate harmful effects of ROS-inducing stressors such as ionising radiation. We performed a study in the newly established LDR-facility Figaro on the combined effects of Se deprivation and LDR γ exposure in DNA repair knockout mice (Ogg1^−/−) and control animals (Ogg1^+/−). Genotoxic effects were seen after continuous radiation (1.4 mGy/h) for 45 days. Chromosomal damage (micronucleus), phenotypic mutations (Pig-a gene mutation of RBC^CD24−) and DNA lesions (single strand breaks/alkali labile sites) were significantly increased in blood cells of irradiated animals, covering three types of genotoxic activity. This study demonstrates that chronic LDR γ radiation is genotoxic in an exposure scenario realistic for humans, supporting the hypothesis that even LDR γ radiation may induce cancer.

Low-dose radiation may be a novel approach to enhance the effectiveness of cancer therapeutics

It has been generally accepted that both natural and man-made sources of ionizing radiation contribute to human exposure and consequently pose a possible risk to human health. However, accumulating evidence has shown that the biological effects of low-dose radiation (LDR) are different from those of high-dose radiation. LDR can stimulate proliferation of normal cells and activate their defense systems, while these biological effects are not observed in some cancer cell types. Although there is still no concordance on this matter, the fact that LDR has the potential to enhance the effects of cancer therapeutics and reduce the toxic side effects of anti-cancer therapy has garnered significant interest. Here, we provide an overview of the current knowledge regarding the experimental data detailing the different responses of normal and cancer tissues to LDR, the underlying mechanisms, and its significance in clinical application.

A Bayesian Semiparametric Model for Radiation Dose-Response Estimation

In evaluating the risk of exposure to health hazards, characterizing the dose-response relationship and estimating acceptable exposure levels are the primary goals. In analyses of health risks associated with exposure to ionizing radiation, while there is a clear agreement that moderate to high radiation doses cause harmful effects in humans, little has been known about the possible biological effects at low doses, for example, below 0.1 Gy, which is the dose range relevant to most radiation exposures of concern today. A conventional approach to radiation dose-response estimation based on simple parametric forms, such as the linear nonthreshold model, can be misleading in evaluating the risk and, in particular, its uncertainty at low doses. As an alternative approach, we consider a Bayesian semiparametric model that has a connected piece-wise-linear dose-response function with prior distributions having an autoregressive structure among the random slope coefficients defined over closely spaced dose categories. With a simulation study and application to analysis of cancer incidence data among Japanese atomic bomb survivors, we show that this approach can produce smooth and flexible dose-response estimation while reasonably handling the risk uncertainty at low doses and elsewhere. With relatively few assumptions and modeling options to be made by the analyst, the method can be particularly useful in assessing risks associated with low-dose radiation exposures.

The risk of radiation-induced breast cancers due to biennial mammographic screening in women aged 50-69 years is minimal

BACKGROUND

The main aim of mammographic screening is to reduce the mortality from breast cancer. However, use of ionizing radiation is considered a potential harm due to the possible risk of inducing cancer in healthy women.

PURPOSE

To estimate the potential number of radiation-induced breast cancers, radiation-induced breast cancer deaths, and lives saved due to implementation of organized mammographic screening as performed in Norway.

MATERIAL AND METHODS

We used a previously published excess absolute risk model which assumes a linear no-threshold dose-response. The estimates were calculated for 100,000 women aged 50-69 years, a screening interval of 2 years, and with an assumed follow-up until the age of 85 or 105 years. Radiation doses of 0.7, 2.5, and 5.7 mGy per screening examination, a latency time of 5 or 10 years, and a dose and dose-rate effectiveness factor (DDREF) of 1 or 2 were applied.

RESULTS

The total lifetime risk of radiation-induced breast cancers per 100,000 women was 10 (95% CI: 4-25) if the women were followed from the ages of 50 to 85 years, for a dose of 2.5 mGy, a latency time of 10 years, and a DDREF of 1. For the same parameter values the number of radiation-induced breast cancer death was 1 (95% CI: 0-2). The assumed number of lives saved is approximately 350.

CONCLUSION

The risk of radiation-induced breast cancer and breast cancer death due to mammographic screening is minimal. Women should not be discouraged from attending screening due to fear of radiation-induced breast cancer death.

Cytogenetic characterization of low-dose hyper-radiosensitivity in Cobalt-60 irradiated human lymphoblastoid cells

The dose-effect relationships of cells exposed to ionizing radiation are frequently described by linear quadratic (LQ) models over an extended dose range. However, many mammalian cell lines, when acutely irradiated in G2 at doses ≤0.3Gy, show hyper-radiosensitivity (HRS) as measured by reduced clonogenic cell survival, thereby indicating greater cell lethality than is predicted by extrapolation from high-dose responses. We therefore hypothesized that the cytogenetic response in G2 cells to low doses would also be steeper than predicted by LQ extrapolation from high doses. We tested our hypothesis by exposing four normal human lymphoblastoid cell lines to 0-400cGy of Cobalt-60 gamma radiation. The cytokinesis block micronucleus assay was used to determine the frequencies of micronuclei and nucleoplasmic bridges. To characterize the dependence of the cytogenetic damage on dose, univariate and multivariate regression analyses were used to compare the responses in the low- (HRS) and high-dose response regions. Our data indicate that the slope of the response for all four cell lines at ≤20cGy during G2 is greater than predicted by an LQ extrapolation from the high-dose responses for both micronuclei and bridges. These results suggest that the biological consequences of low-dose exposures could be underestimated and may not provide accurate risk assessments following such exposures.

Radiation-hormesis phenotypes, the related mechanisms and implications for disease prevention and therapy

Humans are continuously exposed to ionizing radiation throughout life from natural sources that include cosmic, solar, and terrestrial. Much harsher natural radiation and chemical environments existed during our planet's early years. Mammals survived the harsher environments via evolutionarily-conserved gifts ̶ a continuously evolving system of stress-induced natural protective measures (i.e., activated natural protection [ANP]). The current protective system is differentially activated by stochastic (i.e., variable) low-radiation-dose thresholds and when optimally activated in mammals includes antioxidants, DNA damage repair, p53-related apoptosis of severely-damaged cells, reactive-oxygen-species (ROS)/reactive-nitrogen-species (RNS)- and cytokine-regulated auxiliary apoptosis that selectively removes aberrant cells (e.g., precancerous cells), suppression of disease promoting inflammation, and immunity against cancer cells. The intercellular-signaling-based protective system is regulated at least in part via epigenetic reprogramming of adaptive-response genes. When the system is optimally activated, it protects against cancer and some other diseases, thereby leading to hormetic phenotypes (e.g., reduced disease incidence to below the baseline level; reduced pain from inflammation-related problems). Here, some expressed radiation hormesis phenotypes and related mechanisms are discussed along with their implications for disease prevention and therapy.

Remedy for radiation fear - discard the politicized science

Seeking a remedy for the radiation fear in Japan, the author re-examined an article on radiation hormesis. It describes the background for this fear and evidence in the first UNSCEAR report of a reduction in leukemia of the Hiroshima survivors in the low dose zone. The data are plotted and dose-response models are drawn. While UNSCEAR suggested the extra leukemia incidence is proportional to radiation dose, the data are consistent with a hormetic J-shape and a threshold at about 100 rem (1 Sv). UNSCEAR data on lifespan reduction of mammals exposed continuously to gamma rays indicate a 2 gray/year threshold. This contradicts the conceptual basis for radiation protection and risk determination established in 1956-58. In this paper, beneficial effects and thresholds for harmful effects are discussed, and the biological mechanism is explained. The key point: the rate of DNA damage (double-strand breaks) caused by background radiation is 1000 times less than the endogenous (spontaneous) rate. It is the effect of radiation on an organism's very powerful adaptive protection systems that determines the dose-response characteristic. Low radiation up-regulates the protection systems, while high radiation impairs these systems. The remedy for radiation fear is to expose and discard the politicized science.

Novel acyclic nucleotide analogues GS-343074 and GS-424044 demonstrate antiproliferative and pro-apoptotic activity in canine neoplastic cell lines

GS-9219, a novel prodrug of the nucleotide analogue 9-(2-phosphonylmethoxyethyl) guanine (PMEG) has significant activity as monotherapy in dogs with non-Hodgkin's lymphoma. Phase I trials have been initiated in humans based on the encouraging activity observed in canine lymphoma. Two new analogues of GS-9219 (GS-343074 and GS-424044) were recently produced for evaluation as potential novel antineoplastic agents against solid tumours. As a preclinical step, effect of GS-343074 and GS-424044 were evaluated against ten canine cancer cell lines for antiproliferative effect. Both analogues displayed antiproliferative activity against multiple canine cancer cell lines, although GS-343074 was more potent and of broader spectrum compared to GS-424044. Flow cytometric analysis of cells that experienced growth inhibition support apoptotic death as a mechanism of action for both analogues. On the basis of in vitro results described here, GS-343074 and GS-424044 show promise as novel anticancer agents in canine cancer.

Radiation hazards in children - lessons from Chernobyl, Three Mile Island and Fukushima

On March 11, 2011, Japan was hit by the Great East Japan Earthquake followed by the Fukushima Daiichi Nuclear Disaster. Firstly, this review focuses on what happened after the accidents at the Three Mile Island nuclear power station in 1979 and the Chernobyl nuclear power plant in 1986, in terms of the effects of these incidents on health. The most critical issue when considering the effects of radiation on the health of children was the increase of thyroid cancer, as clearly demonstrated among people who were children or adolescence at the time of the Chernobyl accident. Therefore, in the early days after a nuclear accident, the primary concern should be efforts to prevent the exposure of children to radioactive iodine through inhalation and ingestion, because radioactive iodine preferentially accumulates in the thyroid. In the longer term, another concern is exposure to radionuclides with long half-lives, including cesium137 and cesium134, with physical half-lives of 30 and 2 years, respectively. Secondly, fetal radiation risks and radiobiological studies on low-level radiation are briefly reviewed, with reference to the effects upon the developing brain. A fetal dose of 100 mSv may increase the risk of an effect on brain development, especially neuronal migration, based upon the results of experiments with rodents. Finally, this review proposes that research on the health effects of low level radiation should be prioritized so that accurate information on the effects of radiation can be disseminated and prevent the prevalence of unnecessary fear lacking scientific justification.

Theoretical breast cancer induction risk from thoracic spine CT in female pediatric trauma patients

OBJECTIVES

To quantify the radiation dose received during thoracic spine computed tomography (CT) versus plain radiographs as well as the theoretical risk of breast cancer induction in a pediatric trauma population.

METHODS

A retrospective evaluation of 179 female pediatric trauma patients who received CT or plain radiographs for clearance of the thoracic spine was performed. Subjects were secondarily grouped as children (0-<12 years) or adolescents (≥12-17.9 years). Radiation doses were calculated by using the ImPACT Patient Dosimetry Calculator. Excess absolute risk (EAR) of induction of breast cancer was determined by multiplying the radiation dose by breast cancer induction rates taken from the National Academy's Biological Effects of Ionizing Radiation Committee's seventh report.

RESULTS

The average radiation dose to the breast from a thoracic spine CT was 41.1 (SD 11.4) mSv and 1.8 (SD 0.9) mSv for plain radiographs. The EAR for plain radiographs was 2.7 (95% confidence interval [CI] 2.48-2.85) excess cases of breast cancer per 10 000 studies for female children and 1.4 (95% CI 1.14-1.55) for female adolescents. The breast cancer EAR for thoracic spine CT was significantly higher -79.6 (95% CI 58.6-100.5) and 45.8 (95% CI 42.0-49.6) excess cases per 10 000 scans for female children and adolescents, respectively. There was a substantially higher risk of breast cancer induction for children receiving thoracic spine CT compared with adolescents.

CONCLUSIONS

CT clearance of the thoracic spine in the pediatric trauma patient results in a high dose of radiation and an age-dependent increase in theoretical breast cancer induction.

Small γ-Ray Doses Prevent Rather than Increase Lung Tumors in Mice

We show evidence for low doses of γ rays preventing spontaneous hyperplastic foci and adenomas in the lungs of mice, presumably via activating natural anticancer defenses. The evidence partly relates to a new study we conducted whereby a small number of female A/J mice received 6 biweekly dose fractions (100 mGy per fraction) of γ rays to the total body which prevented the occurrence of spontaneous hyperplastic foci in the lung. We also analyzed data from a much earlier Oak Ridge National Laboratory study involving more than 10,000 female RFMf/Un mice whereby single γ-ray doses from 100 to 1,000 mGy prevented spontaneous lung adenomas. We point out the possibility that the decrease in lung cancer mortality observed in The National Lung Screening Trial Research Team study involving lung tumor screening using low-dose computed tomography (CT) may relate at least in part to low-dose X-rays activating the body's natural anticancer defenses (i.e., radiation hormesis). This possibility was apparently not recognized by the indicated research team.

Shifting the paradigm in radiation safety

The current radiation safety paradigm using the linear no-threshold (LNT) model is based on the premise that even the smallest amount of radiation may cause mutations increasing the risk of cancer. Autopsy studies have shown that the presence of cancer cells is not a decisive factor in the occurrence of clinical cancer. On the other hand, suppression of immune system more than doubles the cancer risk in organ transplant patients, indicating its key role in keeping occult cancers in check. Low dose radiation (LDR) elevates immune response, and so it may reduce rather than increase the risk of cancer. LNT model pays exclusive attention to DNA damage, which is not a decisive factor, and completely ignores immune system response, which is an important factor, and so is not scientifically justifiable. By not recognizing the importance of the immune system in cancer, and not exploring exercise intervention, the current paradigm may have missed an opportunity to reduce cancer deaths among atomic bomb survivors. Increased antioxidants from LDR may reduce aging-related non-cancer diseases since oxidative damage is implicated in these. A paradigm shift is warranted to reduce further casualties, reduce fear of LDR, and enable investigation of potential beneficial applications of LDR.

Radar commentary: Use of linear no-threshold hypothesis in radiation protection regulation in the United States

Radiation protection recommendations advanced by the International Commission on Radiological Protection and National Council on Radiation Protection and Measurements, and many times adopted into regulations by the United States Nuclear Regulatory Commission, need to be based on scientifically justified assumptions and conclusions. The linear no-threshold model assigns risk to every radiation exposure above zero dose and is the current basis for setting radiation protection standards worldwide. This hypothesis is vigorously challenged by many individuals but just as vigorously defended in spite of the uncertainties surrounding health effects at low dose levels. It is clear that at radiation doses below 100 mSv, the effects, if any, are so low as to be unobservable and perhaps, therefore, unknowable. However, the linear no-threshold hypothesis is used routinely to formulate regulatory dose limits for workers and the general public and to derive stochastic radiogenic risk estimates at low doses. This note will show that while the linear no-threshold hypothesis may play a legitimate role in setting radiation protection standards and operating policies, such as establishing dose limits or as part of an operational "as low as is reasonably achievable" (ALARA) policy, it is inappropriate for use in estimating possible cancer risks associated with low-level radiation exposures. It will also demonstrate that the raising, not lowering, of current regulatory dose limits is more solidly supported by the actual observed data on radiation dose and effects. The authors submit that the misuse of the linear no-threshold model for predicting radiation effects in exposed individuals and populations should be discontinued.

Analysis of genomic instability in the offspring of fathers exposed to low doses of ionizing radiation

Transgenerational genomic instability was studied in nonirradiated children born from fathers who were irradiated with low doses of ionizing radiation while working as clean-up workers at the Chernobyl Nuclear Power Plant (liquidators) and nonirradiated mothers from nuclear families. Aberrant cell frequencies (ACFs), chromosomal type aberration frequencies, and chromatid break frequencies (CBFs) in the lymphocytes of fathers-liquidators, and their children were significantly higher when compared with the control group (P < 0.05). Individual ACFs, aberration frequencies, and CBFs were independent of the time between irradiation of the father and conception of the child (1 month to 18 years). Chromosomes were categorized into seven groups (A through G). Analysis of aberrant chromosomes within these groups showed no differences in the average frequency of aberrant chromosomes between children and fathers-liquidators. However, significant differences were observed in the average frequency of aberrant chromosomes in groups A, B, and C between children and mothers in the families of liquidators. These results suggest that low doses of radiation induce genomic instability in fathers. Moreover, low radiation doses might be responsible for individual peculiarities in transgenerational genomic instability in children (as a consequence of response to primary DNA damage). Thus, genomic instability may contribute to increased morbidity over the lifetime of these children.

Evidence for hormesis in mutagenicity dose-response relationships

This study assessed the occurrence of hormetic dose responses from three previously published data sets [1-3] with 825 chemicals in three Ames assay tester strains (i.e., TA97, TA98, TA100) with and without the S9 fraction, using a five dose protocol and semi-log dose spacing. Ninety-five (95) (11.5%) chemicals satisfied the multiple a priori entry criteria, with a total of 107 assays. Of the assays satisfying the entry criteria, 61 involved TA100, a strain that detects base-pair substitution mutations. 29.5% (18/61) satisfied the statistical evaluative criteria for hormesis, exceeding that predicted by chance by 4.0-fold (p<0.001). The remaining 46 assays involved TA97 and TA98, strains that detect frameshift mutations. Of these 46 assays, the overall responses for the lowest two doses closely approximated the control response (e.g., 101.77% of the control for TA98; 99.20% for TA97). Only 2.2% (1/46) of the assays satisfied the evaluative criteria for hormesis. In conclusion, these data support a hormetic model for TA100, whereas the responses for TA97 and TA98 are consistent with a threshold dose-response model.

Resveratrol commonly displays hormesis: occurrence and biomedical significance

Resveratrol induces hormetic dose responses in a wide range of biological models, affecting numerous endpoints of biomedical and therapeutic significance. These responses were reported for numerous human tumor cell lines affecting breast, prostate, colon, lung, uterine and leukemia. In such cases, low concentrations of resveratrol enhanced tumor cell proliferation whereas higher concentrations were inhibitory. Similar resveratrol-induced biphasic dose responses were seen with several parasitic diseases, including Leishmaniasis and trichinella. Hormetic effects were also reported in animal models for cardiovascular induced injury, gastric lesions, ischemic stroke, Alzheimer's disease and osteoporosis. In these cases, there was often a protective effect at low doses but an adverse effect at higher doses, exacerbating the disease process/incidence. This analysis indicates that many effects induced by resveratrol are dependent on dose and that opposite effects occur at low and high doses, being indicative of a hormetic dose response. Despite consistent occurrence of hormetic dose responses of resveratrol in a wide range of biomedical models, epidemiologic and clinical trials are needed to assess the nature of its dose-response in humans.

Cellular stress responses, the hormesis paradigm, and vitagenes: novel targets for therapeutic intervention in neurodegenerative disorders

Despite the capacity of chaperones and other homeostatic components to restore folding equilibrium, cells appear poorly adapted for chronic oxidative stress that increases in cancer and in metabolic and neurodegenerative diseases. Modulation of endogenous cellular defense mechanisms represents an innovative approach to therapeutic intervention in diseases causing chronic tissue damage, such as in neurodegeneration. This article introduces the concept of hormesis and its applications to the field of neuroprotection. It is argued that the hormetic dose response provides the central underpinning of neuroprotective responses, providing a framework for explaining the common quantitative features of their dose-response relationships, their mechanistic foundations, and their relationship to the concept of biological plasticity, as well as providing a key insight for improving the accuracy of the therapeutic dose of pharmaceutical agents within the highly heterogeneous human population. This article describes in mechanistic detail how hormetic dose responses are mediated for endogenous cellular defense pathways, including sirtuin and Nrf2 and related pathways that integrate adaptive stress responses in the prevention of neurodegenerative diseases. Particular attention is given to the emerging role of nitric oxide, carbon monoxide, and hydrogen sulfide gases in hormetic-based neuroprotection and their relationship to membrane radical dynamics and mitochondrial redox signaling.

Carcinogenesis from inhaled (239)PuO(2) in beagles: Evidence for radiation homeostasis at low doses?

From the early 1970's to the late 1980's, Pacific Northwest National Laboratory conducted life-span studies in beagle dogs on the biological effects of inhaled plutonium ((238)PuO(2), (239)PuO(2), and Pu[NO(3)](4)) to help predict risks associated with accidental intakes in workers. Years later, the purpose of the present follow-up study was to reassess the dose-response relationship for lung cancer in the PuO(2) dogs compared to controls-with particular focus on the dose-response at relatively low lung doses. A PuO(2) aerosol (2.3 mum activity-median aerodynamic diameter, 1.9 mum geometric standard deviation) was administered to six groups of 20 young (18-mo-old) beagle dogs (10 males and 10 females) by inhalation at six different activity levels, as previously described in Laboratory reports. Control dogs were sham-exposed. In dose level 1, initial pulmonary lung depositions were 130 + or - 48 Bq (3.5 + or - 1.3 nCi), corresponding to 1 Bq g lung tissue (0.029 + or - 0.001 nCi g(-1)). Groups 2 through 6 received initial lung depositions (mean values) of 760, 2,724, 10,345, 37,900, and 200,000 Bq (22, 79, 300, 1,100, and 5,800 nCi) PuO(2), respectively. For each dog, the absorbed dose to lungs was calculated from the initial lung burden and the final lung burden at time of death and lung mass, assuming a single, long-term retention function. Insoluble plutonium oxide exhibited long retention times in the lungs. Increased dose-dependent mortality due to lung cancer (bronchiolar-alveolar carcinoma, adenocarcinoma, and epidermoid carcinoma) and radiation pneumonitis (in the highest exposure group) were observed in dogs exposed to PuO(2). Calculated lung doses ranged from a few cGy (lowest exposure level) to 7,764 cGy in dogs that experienced early deaths from radiation pneumonitis. Data were regrouped by lifetime lung dose and plotted as a function of lung tumor incidence. The lung tumor incidence in controls and zero-dose exposed dogs was 18% (5/28). However, no lung tumors were observed in 16 dogs with the lowest lung doses (8 to 22 cGy, mean 14.4 + or - 7.6 cGy), and only one lung tumor was observed in the next 10 dogs with lung doses ranging from 27 to 48 cGy (mean 37.5 + or - 10.9 cGy). By least-squares analysis, a pure-quadratic function represented the overall dose-response (n = 137, r = 0.96) with no apparent dose-related threshold. Reducing this function to three linear dose-response components, we calculated risk coefficients for each. However, the incidence of lung tumors at zero dose was significantly greater than the incidence at low dose (at the p < or = 0.053 confidence level), suggesting a protective effect (radiation homeostasis) of alpha-particle radiation from PuO(2). If a threshold for lung cancer incidence exists, it will be observed in the range 15 to 40 cGy.

BELLE: an evolving legacy

This paper introduces an issue of the BELLE Newsletter that is designed to reflect on the role of BELLE in affecting how the concept of hormesis is perceived and accepted by the biomedical and toxicological communities. A brief overview of how BELLE was created is provided.

Hormesis is central to toxicology, pharmacology and risk assessment

This paper summarizes numerous conceptual and experimental advances over the past two decades in the study of hormesis. Hormesis is now generally accepted as a real and reproducible biological phenomenon, being highly generalized and independent of biological model, endpoint measured and chemical class/physical stressor. The quantitative features of the hormetic dose response are generally highly consistent, regardless of the model and mechanism, and represent a quantitative index of biological plasticity at multiple levels of biological organization. The hormetic dose-response model has been demonstrated to make far more accurate predictions of responses in low dose zones than either the threshold or linear at low dose models. Numerous therapeutic agents widely used by humans are based on the hormetic dose response and its low dose stimulatory characteristics. It is expected that as low dose responses come to dominate toxicological research that risk assessment practices will incorporate hormetic concepts in the standard setting process.

Commentary on Using LNT for Radiation Protection and Risk Assessment

An article by Jerome Puskin attempts to justify the continued use of the linear no-threshold (LNT) assumption in radiation protection and risk assessment. In view of the substantial and increasing amount of data that contradicts this assumption; it is difficult to understand the reason for endorsing this unscientific behavior, which severely constrains nuclear energy projects and the use of CT scans in medicine. Many Japanese studies over the past 25 years have shown that low doses and low dose rates of radiation improve health in living organisms including humans. Recent studies on fruit flies have demonstrated that the original basis for the LNT notion is invalid. The Puskin article omits any mention of important reports from UNSCEAR, the NCRP and the French Academies of Science and Medicine, while citing an assessment of the Canadian breast cancer study that manipulated the data to obscure evidence of reduced breast cancer mortality following a low total dose. This commentary provides dose limits that are based on real human data, for both single and chronic radiation exposures.

Radiation hormesis: historical perspective and implications for low-dose cancer risk assessment

Current guidelines for limiting exposure of humans to ionizing radiation are based on the linear-no-threshold (LNT) hypothesis for radiation carcinogenesis under which cancer risk increases linearly as the radiation dose increases. With the LNT model even a very small dose could cause cancer and the model is used in establishing guidelines for limiting radiation exposure of humans. A slope change at low doses and dose rates is implemented using an empirical dose and dose rate effectiveness factor (DDREF). This imposes usually unacknowledged nonlinearity but not a threshold in the dose-response curve for cancer induction. In contrast, with the hormetic model, low doses of radiation reduce the cancer incidence while it is elevated after high doses. Based on a review of epidemiological and other data for exposure to low radiation doses and dose rates, it was found that the LNT model fails badly. Cancer risk after ordinarily encountered radiation exposure (medical X-rays, natural background radiation, etc.) is much lower than projections based on the LNT model and is often less than the risk for spontaneous cancer (a hormetic response). Understanding the mechanistic basis for hormetic responses will provide new insights about both risks and benefits from low-dose radiation exposure.

Transgenerational genomic instability in children of irradiated parents as a result of the Chernobyl Nuclear Accident

The study of families irradiated as a result of the accident at the Chernobyl Nuclear Power Plant revealed significantly increased aberrant genomes frequencies (AGFs) not only in irradiated parents (n=106, p<0.01), but also in their children born after the accident (n=159, p<0.05). This is an indicative of the phenomenon of transgenerational genomic instability. To elucidate this phenomenon, experiments were undertaken to model genomic instability by using single and fractional in vitro gamma-irradiation ((137)Cs) of peripheral blood samples from the children and their parents at doses of 0.1, 0.2 and 0.3 Gy. The spectrum and frequency of chromosome aberrations were studied in the 1st and 2nd cell generations. The average AGF was significantly increased at all doses (except 0.1 Gy) in children of irradiated parents, as compared to children born from non-irradiated parents. Amplification of cells with single-break chromosome aberrations in mitosis 2, as compared to mitosis 1, suggests the replication mechanism of realization of potential damage in DNA and the occurrence of genomic instability in succeeding cell generations.

The dose window for radiation-induced protective adaptive responses

Adaptive responses to low doses of low LET radiation occur in all organisms thus far examined, from single cell lower eukaryotes to mammals. These responses reduce the deleterious consequences of DNA damaging events, including radiation-induced or spontaneous cancer and non-cancer diseases in mice. The adaptive response in mammalian cells and mammals operates within a certain window that can be defined by upper and lower dose thresholds, typically between about 1 and 100 mGy for a single low dose rate exposure. However, these thresholds for protection are not a fixed function of total dose, but also vary with dose rate, additional radiation or non-radiation stressors, tissue type and p53 functional status. Exposures above the upper threshold are generally detrimental, while exposures below the lower threshold may or may not increase either cancer or non-cancer disease risk.