Low dose radiation and intercellular induction of apoptosis: potential implications for the control of oncogenesis

Lung cancer mortality (1950-1999) among Eldorado uranium workers: a comparison of models of carcinogenesis and empirical excess risk models

Lung cancer mortality after exposure to radon decay products (RDP) among 16,236 male Eldorado uranium workers was analyzed. Male workers from the Beaverlodge and Port Radium uranium mines and the Port Hope radium and uranium refinery and processing facility who were first employed between 1932 and 1980 were followed up from 1950 to 1999. A total of 618 lung cancer deaths were observed. The analysis compared the results of the biologically-based two-stage clonal expansion (TSCE) model to the empirical excess risk model. The spontaneous clonal expansion rate of pre-malignant cells was reduced at older ages under the assumptions of the TSCE model. Exposure to RDP was associated with increase in the clonal expansion rate during exposure but not afterwards. The increase was stronger for lower exposure rates. A radiation-induced bystander effect could be a possible explanation for such an exposure response. Results on excess risks were compared to a linear dose-response parametric excess risk model with attained age, time since exposure and dose rate as effect modifiers. In all models the excess relative risk decreased with increasing attained age, increasing time since exposure and increasing exposure rate. Large model uncertainties were found in particular for small exposure rates.

The new radiobiology: returning to our roots

In 2005, two expert advisory bodies examined the evidence on the effects of low doses of ionizing radiation. The U.S. National Research Council concluded that current scientific evidence is consistent with the linear no-threshold dose-response relationship (NRCNA 2005) while the French National Academies of Science and Medicine concluded the opposite (Aurengo et al. 2005). These contradictory conclusions may stem in part from an emphasis on epidemiological data (a "top down" approach) versus an emphasis on biological mechanisms (a "bottom up" approach). In this paper, the strengths and limitations of the top down and bottom up approaches are discussed, and proposals for strengthening and reconciling them are suggested. The past seven years since these two reports were published have yielded increasing evidence of nonlinear responses of biological systems to low radiation doses delivered at low dose-rates. This growing body of evidence is casting ever more doubt on the extrapolation of risks observed at high doses and dose-rates to estimate risks associated with typical environmental and occupational exposures. This paper compares current evidence on low dose, low dose-rate effects against objective criteria of causation. Finally, some questions for a post-LNT world are posed.

Upsides and downsides of reactive oxygen species for cancer: the roles of reactive oxygen species in tumorigenesis, prevention, and therapy

SIGNIFICANCE

Extensive research during the last quarter century has revealed that reactive oxygen species (ROS) produced in the body, primarily by the mitochondria, play a major role in various cell-signaling pathways. Most risk factors associated with chronic diseases (e.g., cancer), such as stress, tobacco, environmental pollutants, radiation, viral infection, diet, and bacterial infection, interact with cells through the generation of ROS.

RECENT ADVANCES

ROS, in turn, activate various transcription factors (e.g., nuclear factor kappa-light-chain-enhancer of activated B cells [NF-κB], activator protein-1, hypoxia-inducible factor-1α, and signal transducer and activator of transcription 3), resulting in the expression of proteins that control inflammation, cellular transformation, tumor cell survival, tumor cell proliferation and invasion, angiogenesis, and metastasis. Paradoxically, ROS also control the expression of various tumor suppressor genes (p53, Rb, and PTEN). Similarly, γ-radiation and various chemotherapeutic agents used to treat cancer mediate their effects through the production of ROS. Interestingly, ROS have also been implicated in the chemopreventive and anti-tumor action of nutraceuticals derived from fruits, vegetables, spices, and other natural products used in traditional medicine.

CRITICAL ISSUES

These statements suggest both "upside" (cancer-suppressing) and "downside" (cancer-promoting) actions of the ROS. Thus, similar to tumor necrosis factor-α, inflammation, and NF-κB, ROS act as a double-edged sword. This paradox provides a great challenge for researchers whose aim is to exploit ROS stress for the development of cancer therapies.

FUTURE DIRECTIONS

the various mechanisms by which ROS mediate paradoxical effects are discussed in this article. The outstanding questions and future directions raised by our current understanding are discussed.

Dependence of adaptive response and its bystander transmission on the genetic background of tested cells

PURPOSE

Radiation-induced adaptive response (AR) is a phenomenon of increased radioresistance mediated by a low priming dose of ionizing radiation (IR) applied prior to a higher challenging dose. We have previously shown that in mouse-embryo fibroblasts (MEF) and human A549 cells, AR is associated with enhanced repair of DNA double-strand breaks (DSB) by the DNA-PK-dependent pathway of non-homologous end-joining (D-NHEJ). Importantly, AR was 'transmitted' to non-irradiated bystander cells through transfer of medium from cells that had received a priming dose of IR. Here, we examine the influence of the genetic background in these responses.

MATERIALS AND METHODS

Two plasmid-based assays specifically designed to measure the efficiency of NHEJ and HRR (homologous recombination repair) were deployed. MEF and the primary human fibroblast cell lines HF12 and HF19 were exposed to 10 mGy to 5 Gy X-rays. Bystander effects were investigated using the medium-transfer technique.

RESULTS

In contrast to MEF, which induce robust AR to NHEJ, even as a bystander response, human fibroblasts fail to develop such phenomena.

CONCLUSIONS

The development of AR is cell-type-specific. The same holds true for the development of AR as a bystander effect. Better understanding of the underlying mechanisms will help to understand the molecular basis of these differences in response.

Confocal microscopy and image analysis indicates a region-specific relation between active caspases and cytoplasm in ejaculated and epididymal sperm

Previously, it was suggested a relation between the presence of apoptosis markers with cytoplasm in mammalian sperm. In this work, flow cytometry, confocal microscopy and image analysis were used to analyze the relationship between active caspase-3 and -7 and intracellular esterases expression in ejaculated or epididymal ram sperm. Sperm obtained from ejaculates from the caput, corpus, or cauda of the epididymis were treated with an inhibitor of active caspase-3 and -7 and a marker of cytoplasmic esterases. Additionally, ejaculated sperm were incubated for one, two, or three hours before evaluation for active caspases. Sperm subpopulations positive for active caspases and/or intracellular esterases were detected by flow cytometry and confocal microscopy; however, image analysis of confocal images showed that the correlation between active caspases and cytoplasmic esterases in sperm is region-specific. Lower values of Spearman correlation coefficients were found when whole sperm or head sperm was analyzed; however, a high correlation was observed for midpiece sperm. Incubation of sperm for two or three hours promoted the autoactivation of caspases. It has been suggested that the presence of apoptotic markers in sperm are related with a process of abortive apoptosis and with errors during spermiogenesis. Our results permit us suggest that the origin of the relationship between active caspases and cytoplasmic esterases is due to differentiation errors occurring during spermiogenesis because the percentages of sperm with active caspases are not different in the caput, corpus, or cauda of the epididymis. In this study we demonstrate that existing sperm subpopulations can express active caspases and intracellular esterases and that the correlation between these molecules is high in midpiece sperm.

Computational modeling of cellular effects post-irradiation with low- and high-let particles and different absorbed doses

The use of computational methods to improve the understanding of biological responses to various types of radiation is an approach where multiple parameters can be modelled and a variety of data is generated. This study compares cellular effects modelled for low absorbed doses against high absorbed doses. The authors hypothesized that low and high absorbed doses would contribute to cell killing via different mechanisms, potentially impacting on targeted tumour radiotherapy outcomes. Cellular kinetics following irradiation with selective low- and high-linear energy transfer (LET) particles were investigated using the Virtual Cell (VC) radiobiology algorithm. Two different cell types were assessed using the VC radiobiology algorithm: human fibroblasts and human crypt cells. The results showed that at lower doses (0.01 to 0.2 Gy), all radiation sources used were equally able to induce cell death (p>0.05, ANOVA). On the other hand, at higher doses (1.0 to 8.0 Gy), the radiation response was LET and dose dependent (p<0.05, ANOVA). The data obtained suggests that the computational methods used might provide some insight into the cellular effects following irradiation. The results also suggest that it may be necessary to re-evaluate cellular radiation-induced effects, particularly at low doses that could affect therapeutic effectiveness.

Mechanistic modelling suggests that the size of preneoplastic lesions is limited by intercellular induction of apoptosis in oncogenically transformed cells

Selective removal of oncogenically transformed cells by apoptosis induced via signalling by surrounding cells has been suggested to represent a natural anticarcinogenic process. To investigate its potential effect in detail, a mechanistic model of this process is proposed. The model is calibrated against in vitro data on apoptosis triggered in transformed cells by defined external inducers as well as through signalling by normal cells under coculture conditions. The model predicts that intercellular induction of apoptosis is capable of balancing the proliferation of oncogenically transformed cells and limiting the size of their populations over long times, even if their proliferation per se were unlimited. Experimental research is desired to verify whether the predicted stable population of transformed cells corresponds to a kind of dormancy during early-stage carcinogenesis (dormant preneoplastic lesions), and how this process relates to other anticarcinogenic mechanisms taking place under in vivo conditions.

Identification of proteins secreted into the medium by human lymphocytes irradiated in vitro with or without adaptive environments

There is increasing evidence to support the hypothesis of adaptive response, a phenomenon in which protection arises from a low-dose radiation (<0.1 Gy) against damage induced by subsequent exposure to high-dose radiation. The molecular mechanisms underlying such protection are poorly understood. The goal of this study was to fill this knowledge gap. Mass spectrometry-based proteomics was used to characterize global protein expression profiles in the medium collected from human lymphocyte cultures given sham irradiation (0 Gy) or a priming low dose of 0.03 Gy 137Cs γ rays 4 h prior to a challenging dose of 1 Gy 137Cs γ rays. Adaptive response was determined by decreased micronucleus frequencies in lymphocytes receiving low dose irradiation prior to high dose irradiation compared to those receiving only high dose irradiation. Adaptive response was found in these experiments. Proteomic analysis of media revealed: (a) 55 proteins with similar abundance in both groups; (b) 23 proteins in both groups, but 7 of them were high abundance in medium with adaptive environment, while 16 high abundance proteins were in medium without adaptive environment; (c) 17 proteins in medium with adaptive environment only; and (d) 8 proteins in medium without adaptive environment only. The results provide a foundation for improving understanding of the molecular mechanisms associated with the beneficial effects of low dose radiation that, in turn, will have an important impact on radiation risk estimation. Hence, these studies are highly relevant to radiation protection due to an increased use of low dose radiation in daily life (e.g., medical diagnosis or airport safety) or an unavoidable exposure to low level background radiation.

No Evidence for the In Vivo Induction of Genomic Instability by Low Doses of CS Gamma Rays in Bone Marrow Cells of BALB/CJ and C57BL/6J Mice

In spite of extensive research, assessment of potential health risks associated with exposure to low-dose (≤ 0.1 Gy) radiation is still challenging. We evaluated the in vivo induction of genomic instability, expressed as late-occurring chromosome aberrations, in bone-marrow cells of two strains of mouse with different genetic background, i.e. the radiosensitive BALB/cJ and the radioresistant C57BL/6J strains following a whole-body exposure to varying doses of (137)Cs gamma rays (0, 0.05, 0.1, and 1.0 Gy). A total of five mice per dose per strain were sacrificed at various times post-irradiation up to 6 months for sample collections. Three-color fluorescence in situ hybridization for mouse chromosomes 1, 2, and 3 was used for the analysis of stable-aberrations in metaphase-cells. All other visible gross structural-abnormalities involving non-painted-chromosomes were also evaluated on the same metaphase-cells used for scoring the stable-aberrations of painted-chromosomes. Our new data demonstrated in bone-marrow cells from both strains that low doses of low LET-radiation (as low as 0.05 Gy) are incapable of inducing genomic instability but are capable of reducing specific aberration-types below the spontaneous rate with time post-irradiation. However, the results showed the induction of genomic instability by 1.0 Gy of (137)Cs gamma rays in the radiosensitive strain only.

Effect of site-specific bronchial radon progeny deposition on the spatial and temporal distributions of cellular responses

Inhaled short-lived radon progenies may deposit in bronchial airways and interact with the epithelium by the emission of alpha particles. Simulation of the related radiobiological effects requires the knowledge of space and time distributions of alpha particle hits and biological endpoints. Present modelling efforts include simulation of radioaerosol deposition patterns in a central bronchial airway bifurcation, modelling of human bronchial epithelium, generation of alpha particle tracks, and computation of spatio-temporal distributions of cell nucleus hits, cell killing and cell transformation events. Simulation results indicate that the preferential radionuclide deposition at carinal ridges plays an important role in the space and time evolution of the biological events. While multiple hits are generally rare for low cumulative exposures, their probability may be quite high at the carinal ridges of the airway bifurcations. Likewise, cell killing and transformation events also occur with higher probability in this area. In the case of uniform surface activities, successive hits as well as cell killing and transformation events within a restricted area (say 0.5 mm(2)) are well separated in time. However, in the case of realistic inhomogeneous deposition, they occur more frequently within the mean cycle time of cells located at the carinal ridge even at low cumulative doses. The site-specificity of radionuclide deposition impacts not only on direct, but also on non-targeted radiobiological effects due to intercellular communication. Incorporation of present results into mechanistic models of carcinogenesis may provide useful information concerning the dose-effect relationship in the low-dose range.

Low dose radiation response curves, networks and pathways in human lymphoblastoid cells exposed from 1 to 10cGy of acute gamma radiation

We investigated the low dose dependency of the transcriptional response of human cells to characterize the shape and biological functions associated with the dose-response curve and to identify common and conserved functions of low dose expressed genes across cells and tissues. Human lymphoblastoid (HL) cells from two unrelated individuals were exposed to graded doses of radiation spanning the range of 1-10cGy were analyzed by transcriptome profiling, qPCR and bioinformatics, in comparison to sham irradiated samples. A set of ∼80 genes showed consistent responses in both cell lines; these genes were associated with homeostasis mechanisms (e.g., membrane signaling, molecule transport), subcellular locations (e.g., Golgi, and endoplasmic reticulum), and involved diverse signal transduction pathways. The majority of radiation-modulated genes had plateau-like responses across 1-10cGy, some with suggestive evidence that transcription was modulated at doses below 1cGy. MYC, FOS and TP53 were the major network nodes of the low-dose-response in HL cells. Comparison our low dose expression findings in HL cells with those of prior studies in mouse brain after whole body exposure, in human keratinocyte cultures, and in endothelial cells cultures, indicates that certain components of the low dose radiation response are broadly conserved across cell types and tissues, independent of proliferation status.

Biological consequences and health risks of low-level exposure to ionizing radiation: commentary on the workshop

This paper provides an integration and discussion of the information presented at the workshop held from 2-5 May 2010 in Richland, WA, adjacent to the Pacific Northwest National Laboratory (PNNL). Consequently, this is commentary and not necessarily a consensus document. This workshop was in honor of Dr. Victor P. Bond in celebration of his numerous contributions to the radiation sciences.

Modeling DNA double-strand break repair kinetics as an epiregulated cell-community-wide (epicellcom) response to radiation stress

The multicellular signaling model (MULTISIG1) was recently introduced to simulate the kinetics of repair of DNA double-strand breaks (DSBs) that were induced in confluent (non-dividing) cultured cells by a very low radiation dose where at most a single induced DSB would be expected in a given cell nucleus. The repair kinetics was modeled as representing what is now called an epigenetically-regulated (epiregulated) cell-community-wide (epicellcom) response to radiation stress. DSB repair initiation is assumed to require a threshold number of cells with DSBs participating in intercellular stress-response signaling. The MULTISIG1 model is extended in this study to apply to moderate doses where several DSBs can occur on the same DNA molecule. The repair of multiple breaks on the same molecule is treated as sequential stochastic events. For cells of differing genetic characteristics and epigenetic statuses, relationships are provided for evaluating the relative susceptibility (RS) for DSB induction, relative repair capacity (RRC) for DSB repair, and relative epiapoptosis capacity (REC), for epigenetically regulated apoptosis. The modified MULTISIG1 model is used to characterize the expected repair kinetics for confluent, human lung fibroblasts (MRC-5 line) briefly exposed in vitro to 90-kV x-rays. Possible application of the model to biological dosimetry is also discussed.

Modelling of intercellular induction of apoptosis in oncogenic transformed cells and radiation effects on the phenomenon

The removal of transformed cells via induction of apoptosis through intercellular signalling by surrounding cells is supposed to represent an important control mechanism limiting carcinogenesis. Low doses of radiation influence the efficiency of this anti-carcinogenesis process, indicating possible beneficial effects of low doses of radiation mediated by intercellular communication ('non-targeted effects'). To quantitatively understand the signalling system involved and the effects of radiation and to assess the role of this phenomenon in radiation-induced carcinogenesis, multi-scale modelling studies have been started. The proposed kinetic model takes into account (i) triggering of the effector function in cells in the vicinity of transformed cells, (ii) intercellular signalling between effector and transformed cells and (iii) execution of apoptosis in attacked cells. The systems model without radiation perturbance is reviewed. First results accounting for radiation-induced modulations of the signalling schemes are presented.

Track structure calculations on hypothetical subcellular targets for the release of cell-killing signals in bystander experiments with medium transfer

Track structure studies using PARTRAC have been performed with the aim to investigate the possibility of revealing information on initiating targets and mechanisms of bystander effects mediated by signals released into the culture medium. Dependences on radiation dose have been assessed for alternative signal emission scenarios, defined by required energy deposits in a number of subcellular targets, mimicking e.g. mitochondria as hypothetical targets for the release of signals. The simulation results agree with target theory, and elucidate the characteristic dose for signal emission as a function of target topology, size and activation energy. The observed dose dependence of bystander cell kill in medium transfer experiments is not as steep as predicted by the considered simple signal emission scenarios with a single or even multiple hits to the hypothetical targets. This has been resolved by accounting for variations in cellular characteristics among the irradiated cells.

Checking the foundation: recent radiobiology and the linear no-threshold theory

The linear no-threshold (LNT) theory has been adopted as the foundation of radiation protection standards and risk estimation for several decades. The "microdosimetric argument" has been offered in support of the LNT theory. This argument postulates that energy is deposited in critical cellular targets by radiation in a linear fashion across all doses down to zero, and that this in turn implies a linear relationship between dose and biological effect across all doses. This paper examines whether the microdosimetric argument holds at the lowest levels of biological organization following low dose, low dose-rate exposures to ionizing radiation. The assumptions of the microdosimetric argument are evaluated in light of recent radiobiological studies on radiation damage in biological molecules and cellular and tissue level responses to radiation damage. There is strong evidence that radiation initially deposits energy in biological molecules (e.g., DNA) in a linear fashion, and that this energy deposition results in various forms of prompt DNA damage that may be produced in a pattern that is distinct from endogenous (e.g., oxidative) damage. However, a large and rapidly growing body of radiobiological evidence indicates that cell and tissue level responses to this damage, particularly at low doses and/or dose-rates, are nonlinear and may exhibit thresholds. To the extent that responses observed at lower levels of biological organization in vitro are predictive of carcinogenesis observed in vivo, this evidence directly contradicts the assumptions upon which the microdosimetric argument is based.

[IMRT combined to IGRT: increase of the irradiated volume. Consequences?]

Image-guided radiotherapy (IGRT) combined or not with intensity-modulated radiation therapy (IMRT) are new and very useful techniques. However, these new techniques are responsible of irradiation at low dose in large volumes. The control of alignment, realignment of the patient and target positioning in external beam radiotherapy are increasingly performed by radiological imaging devices. The management of this medical imaging depends on the practice of each radiotherapy centre. The physical doses due to the IGRT are however quantifiable and traceable. In one hand, these doses appear justified for a better targeting and could be considered negligible in the context of radiotherapy. On the other hand, the potential impact of these low doses should deserve the consideration of professionals. It appears important therefore to report and consider not only doses in target volumes and in "standard" organs at risk, but also the volume of all tissue receiving low doses of radiation. The recent development of IMRT launches the same issue concerning the effects of low doses of radiation. Indeed, IMRT increases the volume of healthy tissue exposed to radiation. At low dose (<100mGy), many parameters have to be considered for health risk estimations: the induction of genes and activation of proteins, bystander effect, radio-adaptation, the specific low-dose radio-hypersensitivity and individual radiation sensitivity. With the exception of the latter, the contribution of these parameters is generally protective in terms of carcinogenesis. An analysis of secondary cancers arising out of field appears to confirm such notion. The risk of secondary tumours is not well known in these conditions of treatment associating IMRT and IGRT. It is therefore recommended that the dose due to imaging during therapeutic irradiation be reported.

Review and evaluation of updated research on the health effects associated with low-dose ionising radiation

While radiation health risks at low doses have traditionally been estimated from high-dose studies, we have reviewed recent literature and concluded that the mechanisms of action for many biological endpoints may be different at low doses from those observed at high doses; that acute doses <100 mSv may be too small to allow epidemiological detection of excess cancers given the background of naturally occurring cancers; that low-dose radiation research should use holistic approaches such as systems-based methods to develop models that define the shape of the dose-response relationship; and that these results should be combined with the latest epidemiology to produce a comprehensive understanding of radiation effects that addresses both damage, likely with a linear effect, and response, possibly with non-linear consequences. Continued research is needed to understand how radiobiology and epidemiology advances should be used to effectively model radiation worker risks.

Non-targeted effects as a paradigm breaking evidence

The finding that mammalian cells and tissues and whole organisms react differently at high than at low doses of ionizing radiation questions the scientific validity of the linear no-threshold concept for low-dose exposures. Indeed, the classical paradigm of radiobiology was based on the concept that all radiation effects on living matter are due to the direct action of radiation. Meanwhile, the discovery of non-targeted and delayed radiation effects has challenged this concept, and one might ask whether a new paradigm has to be developed to provide more realistic protection against low radiation doses. The present overview summarizes recent findings on the low-dose radiation-induced bystander effect, genomic instability, radiation hypersensitivity, hormesis, radioadaptive and transgenerational responses. For these, some common features can be recognized. Most of these phenomena include (1) intra- and intercellular signaling, involving reactive oxygen species (ROS). This signaling may be transient or persistent, and may involve the release of cytokines (bystander effect, genomic instability) or epigenetic changes (translesional responses), (2) a large variability of responses depending on the type of radiation, genotype (DNA repair capacity) and physiological state of the cells and tissues. Many more parameters are involved in responses at low doses than at high doses, and different pathways are activated. At low doses, non-linear responses are obtained that are not compatible with the LNT concept. At present, more work is needed to identify the essential parameters involved and to provide a basis for proper modelling of low-dose radiation health effects for radiation protection purposes.

Possible expressions of radiation-induced genomic instability, bystander effects or low-dose hypersensitivity in cancer epidemiology

Recent publications on the integration of radiobiological effects in the two-step clonal expansion (TSCE) model of carcinogenesis and applications to radioepidemiological data are reviewed and updated. First, a model version with radiation-induced genomic instability was shown to be a possible explanation for the age dependence of the radiation-induced cancer mortality in the Techa River Cohort. Second, it is demonstrated that inclusion of a bystander effect with a dose threshold allows an improved description of the lung cancer mortality risk for the Mayak workers cohort due to incorporation of plutonium. The threshold for the annual lung dose is estimated to 12 (90%CI: 4; 14)mGy/year. This threshold applies to the initiation of preneoplastic cells and to hyperplastic growth. There is, however, no evidence for a threshold for the effects of gamma radiation. Third, models with radiation-induced cell inactivation tend to predict lower cancer risks among the atomic bomb survivors with exposure at young age than conventionally used empirical models. Also, risks after exposures with doses in the order of 100mGy are predicted to be higher in models with low-dose hypersensitivity than in models with conventional cell survival curves. In the reviewed literature, models of carcinogenesis tend to describe radioepidemiological data better than conventionally used empirical models.

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.

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.

The role of dose-rate on risk from internally-deposited radionuclides and the potential need to separate dose-rate effectiveness factor (DREF) from the dose and dose-rate effectiveness factor (DDREF)

In 1980, National Council on Radiation Protection and Measurements suggested the term dose-rate effectiveness factor (DREF) to describe the reduction of effectiveness of protracted radiation in producing biological damage and risk. A nonlinear decrease in damage was also noted following low total doses. The International Commission on Radiological Protection therefore combined the influence of low dose and low dose-rate and assigned a single value of 2.0 for a dose and dose-rate effectiveness factor (DDREF) to be applied for estimating risk for both low total dose and low dose-rate exposures. This paper re-evaluates one extensive data set on inhaled radionuclides in dogs which suggests that there may be a need to separate these factors (DREF and DDREF) for larger protracted doses from internally-deposited radioactive materials. Extensive recent research on the mechanisms of action of both low dose and low dose-rate radiation exposure at the molecular, cellular, and animal level of biological organization suggest that the influence of protraction of radiation may be large and variable, due to adaptive and protective responses, following very low doses and dose-rate exposures. Important observations in this paper in dogs exposed by inhalation to beta-gamma emitting radionuclides include (1) discontinuities in the data sets as a function of both dose and dose-rate suggesting shifts in mechanisms of action following high doses from protracted exposure away from those postulated for cancer from low total doses; (2) no increase in non-neoplastic disease, cancer frequency, or life-shortening following low dose-rate exposures to high total lung doses (up to 25 Gy); (3) all dogs that received doses below 25 Gy were combined and a decrease in the frequency of lung cancer in these exposed animals relative to the controls was noted, while very large doses from all radionuclides studied resulted in very marked increases in lung cancer; (4) a significant increase in hemangiosarcoma in the heart and tracheobronchial lymph nodes was observed after very high doses; (5) in this paper the DREF for lung cancer in dogs relative to single acute radiation exposure was as high as 35; and (6) the amount of life-shortening increased per unit dose as a function of the half-life with (90)Y being eight times as effective per unit of dose as (90)Sr. Such information suggests that there may be a need to assign different values for DDREF and DREF, especially in situations where there are large nonuniform total doses delivered by internally-deposited radionuclides. This is extremely important since the risk from radiation exposure from internally-deposited radionuclides in the lungs following nuclear fallout, accidents and terrorist activities may be much less than currently projected.

Does scientific evidence support a change from the LNT model for low-dose radiation risk extrapolation?

The linear no-threshold (LNT) model has been widely used to establish international rules and standards in radiation protection. It is based on the notion that the physical energy deposition of ionizing radiation (IR) increases carcinogenic risk linearly with increasing dose (i.e., the carcinogenic effectiveness remains constant irrespective of dose) and, within a factor of two, also with dose-rate. However, recent findings have strongly put into question the LNT concept and its scientific validity, especially for very low doses and dose-rates. Low-dose effects are more difficult to ascertain than high-dose effects. Epidemiological studies usually lack sufficient statistical power to determine health risks from very low-dose exposures. In this situation, studies of the fundamental mechanisms involved help to understand and assess short- and long-term effects of low-dose IR and to evaluate low-dose radiation risks. Several lines of evidence demonstrate that low-dose and low dose-rate effects are generally lower than expected from high-dose exposures. DNA damage signaling, cell cycle checkpoint activation, DNA repair, gene and protein expression, apoptosis, and cell transformation differ qualitatively and quantitatively at high- and low-dose IR exposures, and most animal and epidemiological data support this conclusion. Thus, LNT appears to be scientifically invalid in the low-dose range.

Substance P accelerates intestinal tissue regeneration after gamma-irradiation-induced damage

Radiation therapy causes varying degrees of damage to biological systems. Many groups are investigating the mechanism underlying radiation-induced cellular damage but there are limited therapeutic solutions for affected patients. Recent studies show that substance P (SP) participates in cell proliferation. In the present study, we characterized the mechanism underlying SP-induced cellular signaling in radiation-induced damage of the intestine. Exposure of Caco-2 cells to SP increases cell proliferation and Erk phosphorylation in a time- and dose-dependent manner. The proliferation of cells exposed to gamma-irradiation is also stimulated by exposure to SP, a phenomenon that may result from inhibition of apoptosis because SP activates Akt and inhibits the cleavage of caspase-3. The effect of SP on cell proliferation and protection was confirmed by investigations in mice. Proliferating cell nuclear antigen staining shows that cell proliferation in radiation-damaged mouse intestine increases significantly upon exposure to SP. Furthermore, terminal deoxynucleotidyl transferase-mediated dUTP-fluorescein nick end labeling assay reveals fewer cells stained in SP-treated mice compared with untreated controls. These findings show the potential for SP-induced acceleration of intestinal wound healing and reveal that the mechanism underlying this process involves activation of Erk and Akt and inhibition of caspase-3 cleavage.

Assessing cancer risks of low-dose radiation

Ionizing radiation is considered a non-threshold carcinogen. However, quantifying the risk of the more commonly encountered low and/or protracted radiation exposures remains problematic and subject to uncertainty. Therefore, a major challenge lies in providing a sound mechanistic understanding of low-dose radiation carcinogenesis. This Perspective article considers whether differences exist between the effects mediated by high- and low-dose radiation exposure and how this affects the assessment of low-dose cancer risk.

Propagation distance of the alpha-particle-induced bystander effect: the role of nuclear traversal and gap junction communication

When cell populations are exposed to low-dose alpha-particle radiation, a significant fraction of the cells will not be traversed by a radiation track. However, stressful effects occur in both irradiated and bystander cells in the population. Characterizing these effects, and investigating their underlying mechanism(s), is critical to understanding human health risks associated with exposure to alpha particles. To this end, confluent normal human fibroblast cultures were grown on polyethylene terephthalate foil grafted to an ultrathin solid-state nuclear track detector and exposed under non-perturbing conditions to low-fluence alpha particles from a broadbeam irradiator. Irradiated and affected bystander cells were localized with micrometer precision. The stress-responsive protein p21(Waf1) (also known as CDKN1A) was induced in bystander cells within a 100-microm radius from an irradiated cell. The mean propagation distance ranged from 20 to 40 microm around the intranuclear alpha-particle impact point, which corresponds to a set of approximately 30 cells. Nuclear traversal, induced DNA damage, and gap junction communication were critical contributors to propagation of this stressful effect. The strategy described here may be ideal to investigate the size of radiation-affected target and the relative contribution of different cellular organelles to bystander effects induced by energetic particles, which is relevant to radioprotection and cancer radiotherapy.

Radiation-stimulated epigenetic reprogramming of adaptive-response genes in the lung: an evolutionary gift for mounting adaptive protection against lung cancer

Humans are continuously exposed to low-level ionizing radiation from natural sources. However, harsher radiation environments persisted during our planet's early years and mammals survived via an evolutionary gift--a system of radiation-induced natural protective measures (adaptive protection). This system includes antioxidants, DNA repair, apoptosis of severely damaged cells, epigenetically regulated apoptosis (epiapoptosis) pathways that selectively remove precancerous and other aberrant cells, and immunity against cancer. We propose a novel model in which the protective system is regulated at least in part via radiation-stress-stimulated epigenetic reprogramming (epireprogramming) of adaptive-response genes. High-dose radiation can promote epigenetically silencing of adaptive-response genes (episilencing), for example via promoter-associated DNA and/or histone methylation and/or histone deacetylation. Evidence is provided for low linear-energy-transfer (LET) radiation-activated natural protection (ANP) against high-LET alpha-radiation-induced lung cancer in plutonium-239 exposed rats and radon-progeny-exposed humans. Using a revised hormetic relative risk model for cancer induction that accounts for both epigenetic activation (epiactivation) and episilencing of genes, we demonstrate that, on average, >80% of alpha-radiation-induced rat lung cancers were prevented by chronic, low-rate gamma-ray ANP. Interestingly, lifetime exposure to residential radon at the Environmental Protection Agency's action level of 4 pCi L(-1) appears to be associated with on average a > 60% reduction in lung cancer cases, rather than an increase. We have used underlined italics to indicate newly introduced terminology.

Radiation chemistry comes before radiation biology

PURPOSE

This article seeks to illustrate some contributions of radiation chemistry to radiobiology and related science, and to draw attention to examples where radiation chemistry is central to our knowledge of specific aspects. Radiation chemistry is a mature branch of radiation science which is continually evolving and finding wider applications. This is particularly apparent in the study of the roles of free radicals in biology generally, and radiation biology specifically. The chemical viewpoint helps unite the spatial and temporal insight coming from radiation physics with the diversity of biological responses. While historically, the main application of radiation chemistry of relevance to radiation biology has been investigations of the free-radical processes leading to radiation-induced DNA damage and its chemical characterization, two features of radiation chemistry point to its wider importance. First, its emphasis on quantification and characterization at the molecular level helps provide links between DNA damage, biochemical repair processes, and mutagenicity and radiosensitivity. Second, its central pillar of chemical kinetics aids understanding of the roles of 'reactive oxygen species' in cell signalling and diverse biological effects more generally, and application of radiation chemistry in the development of drugs to enhance radiotherapy and as hypoxia-specific cytotoxins or diagnostic agents. The illustrations of the broader applications of radiation chemistry in this article focus on their relevance to radiation biology and demonstrate the importance of synergy in the radiation sciences.

CONCLUSIONS

The past contributions of radiation chemistry to radiation biology are evident, but there remains considerable potential to help advance future biological understanding using the knowledge and techniques of radiation chemistry.

Can we reduce the incidence of second primary malignancies occurring after radiotherapy? A critical review

Second primary malignancies (SPMs) occurring after oncological treatment have become a major concern during the past decade. Their incidence has long been underestimated because most patients had a short life expectancy after treatment or their follow-up was shorter than 15 years. With major improvement of long-term survival, longer follow-up, cancer registries and end-result programs, it was found that the cumulative incidence of SPM could be as high as 20% of patients treated by radiotherapy. This cumulative proportion varies with several factors, which ought to be studied more accurately. The delay between irradiation and solid tumor emergence is seldom shorter than 10 years and can be as long as half a century. Thus, inclusion in a cohort of patients with a short follow-up leads to an underestimation of the proportion of SPM caused by treatment, unless actuarial cumulative incidence is computed. The incidence varies with the tissue and organs, the age of the patient at treatment, hereditary factors, but also, and probably mainly, with dose distribution, size of the irradiated volume, dose, and dose-rate. An effort toward a reduction in their incidence is mandatory. Preliminary data suggest that SPMs are mainly observed in tissues having absorbed doses above 2 Gy (fractionated irradiation) and that their incidence increases with the dose. However, in children thyroid and breast cancers are observed following doses as low as 100 mGy, and in adults lung cancers have been reported for doses of 500 mGy, possibly due to interaction with tobacco. The dose distribution and the dose per fraction have a major impact. However, the preliminary data regarding these factors need confirmation. Dose-rates appear to be another important factor. Some data suggest that certain patients, who could be identified, have a high susceptibility to radiocancer induction. Efforts should be made to base SPM reduction on solid data and not on speculation or models built on debatable hypotheses regarding the dose-carcinogenic effect relationship. In parallel, radiation therapy philosophy must evolve, and the aim of treatment should be to deliver the minimal effective radiation therapy rather than the maximal tolerable dose.

A perspective on the scientific, philosophical, and policy dimensions of hormesis

The hormesis concept has broad implications for biology and the biomedical sciences. This perspective on hormesis concentrates on toxicology and toxicological risk assessment and secondarily explores observations from other fields. It considers the varied manifestations of hormesis in the context of a broad family of biological stress responses. Evidence for hormesis is reviewed, and the hormesis model is contrasted with more widely accepted dose-response models in toxicology: a linear nonthreshold (LNT) model for mutagenesis and carcinogenesis, and a threshold model for most other toxicologic effects. Scientific, philosophical, and political objections to the hormesis concept are explored, and complications in the hormesis concept are analyzed. The review concludes with a perspective on the current state of hormesis and challenges that the hormesis model poses for risk assessment.

Environmental toxicity, oxidative stress and apoptosis: ménage à trois

Apoptosis is an evolutionary conserved homeostatic process involved in distinct physiological processes including organ and tissue morphogenesis, development and senescence. Its deregulation is also known to participate in the etiology of several human diseases including cancer, neurodegenerative and autoimmune disorders. Environmental stressors (cytotoxic agents, pollutants or toxicants) are well known to induce apoptotic cell death and to contribute to a variety of pathological conditions. Oxidative stress seems to be the central element in the regulation of the apoptotic pathways triggered by environmental stressors. In this work, we review the established mechanisms by which oxidative stress and environmental stressors regulate the apoptotic machinery with the aim to underscore the relevance of apoptosis as a component in environmental toxicity and human disease progression.

Evidence for radiation hormesis after in vitro exposure of human lymphocytes to low doses of ionizing radiation

Previous research has demonstrated that adding a very small gamma-ray dose to a small alpha radiation dose can completely suppress lung cancer induction by alpha radiation (a gamma-ray hormetic effect). Here we investigated the possibility of gamma-ray hormesis during low-dose neutron irradiation, since a small contribution to the total radiation dose from neutrons involves gamma rays. Using binucleated cells with micronuclei (micronucleated cells) among in vitro monoenergetic-neutron-irradiated human lymphocytes as a measure of residual damage, we investigated the influence of the small gamma-ray contribution to the dose on suppressing residual damage. We used residual damage data from previous experiments that involved neutrons with five different energies (0.22-, 0.44-, 1.5-, 5.9-, and 13.7-million electron volts [MeV]). Corresponding gamma-ray contributions to the dose were approximately 1%, 1%, 2%, 6%, and 6%, respectively. Total absorbed radiation doses were 0, 10, 50, and 100 mGy for each neutron source. We demonstrate for the first time a protective effect (reduced residual damage) of the small gamma-ray contribution to the neutron dose. Using similar data for exposure to gamma rays only, we also demonstrate a protective effect of 10 mGy (but not 50 or 100 mGy) related to reducing the frequency of micronucleated cells to below the spontaneous level.

The contribution of animal fat oxidation products to colon carcinogenesis, through modulation of TGF-beta1 signaling

It is now unanimously accepted that neoplastic cells tend to become less susceptible to the growth regulatory effects of transforming growth factor-beta1 (TGF-beta1), mainly because of reduced expression and/or activity of TGF-beta1-specific receptors, as reported for many human cancers including colon cancer. Consequently, a sustained increase of TGF-beta1 in the intestinal mucosa, like that caused by inflammatory processes and/or high dietary intake of animal fat, might become crucial for the progression of a neoplastic clone. In fact, this proapoptotic and prodifferentiating cytokine could eliminate neoplastic cells still susceptible to TGF-beta1's antiproliferative action (TGF-beta1 receptor-positive cells), indirectly favoring the expansion of TGF-beta1 resistant ones (TGF-beta1 receptors deficient or negative cells). The actual concentration of TGF-beta1 in the colonic mucosa undergoing neoplastic transformation is still debated, and the phase of the relevant carcinogenetic process in which a reduced susceptibility to this antiproliferative molecule first occurs has not been precisely established yet. However, no doubt that TGF-beta1 level and activity may be upregulated in cells of the macrophage lineage by animal fat oxidation products, such as oxysterols and aldehydes, as reviewed here. But phagocytes as well as fibroblasts constitutively express TGF-beta1 and are accumulating in tumor-associated stroma. Thus, upregulation of this cytokine system within colonic tumor-associated stroma by excess dietary intake of cholesterol and n-6 polyunsaturated fatty acids appears as a primary mechanism of cancer progression at least in neoplastic lesions of the digestive tract.

Low-dose-radiation stimulated natural chemical and biological protection against lung cancer

Research is being conducted world-wide related to chemoprevention of future lung cancer among smokers. The fact that low doses and dose rates of some sparsely ionizing forms of radiation (e.g., x rays, gamma rays, and beta radiation) stimulate transient natural chemical and biological protection against cancer in high-risk individuals is little known. The cancer preventative properties relate to radiation adaptive response (radiation hormesis) and involve stimulated protective biological signaling (a mild stress response). The biological processes associated with the protective signaling are now better understood and include: increased availability of efficient DNA double-strand break repair (p53-related and in competition with normal apoptosis), stimulated auxiliary apoptosis of aberrant cells (presumed p53-independent), and stimulated protective immune functions. This system of low-dose radiation activated natural protection (ANP) requires an individual-specific threshold level of mild stress and when invoked can efficiently prevent the occurrence of cancers as well as other genomic-instability-associated diseases. In this paper, low, essentially harmless doses of gamma rays spread over an extended period are shown via use of a biological-based, hormetic relative risk (HRR) model to be highly efficient in preventing lung cancer induction by alpha radiation from inhaled plutonium.