Low dose, low-LET ionizing radiation-induced radioadaptation and associated early responses in unirradiated cells

Effects of Gamma Radiation on FcεRI and TLR-Mediated Mast Cell Activation

Ionizing gamma radiation has several therapeutic indications including bone marrow transplantation and tumor ablation. Among immune cells, susceptibility of lymphocytes to gamma radiation is well known. However, there is little information on the effects of gamma radiation on mast cells, which are important in both innate and acquired immunity. Previous studies have suggested that mast cells may release histamine in response to high doses of gamma radiation, whereas other reports suggest that mast cells are relatively radioresistant. No strong link has been established between gamma radiation and its effect on mast cell survival and activation. We examined both human and murine mast cell survival and activation, including mechanisms related to innate and acquired immune responses following gamma radiation. Data revealed that human and murine mast cells were resistant to gamma radiation-induced cytotoxicity and, importantly, that irradiation did not directly induce beta-hexosaminidase release. Instead, a transient attenuation of IgE-mediated beta-hexosaminidase release and cytokine production was observed which appeared to be the result of reactive oxygen species formation after irradiation. Mast cells retained the ability to phagocytose Escherichia coli particles and respond to TLR ligands as measured by cytokine production after irradiation. In vivo, there was no decrease in mast cell numbers in skin of irradiated mice. Additionally, mast cells retained the ability to respond to Ag in vivo as measured by passive cutaneous anaphylaxis in mice after irradiation. Mast cells are thus resistant to the cytotoxic effects and alterations in function after irradiation and, despite a transient inhibition, ultimately respond to innate and acquired immune activation signals.

Radiation induced bystander signals are independent of DNA damage and DNA repair capacity of the irradiated cells

Evidence is accumulating that irradiated cells produce signals, which interact with non-exposed cells in the same population. Here, we analysed the mechanism for bystander signal arising in wild-type CHO cells and repair deficient varients, focussing on the relationship between DNA repair capacity and bystander signal arising in irradiated cells. In order to investigate the bystander effect, we carried out medium transfer experiments after X-irradiation where micronuclei were scored in non-targeted DSB repair deficient xrs5 cells. When conditioned medium from irradiated cells was transferred to unirradiated xrs5 cells, the level of induction was independent of whether the medium came from irradiated wild-type, ssb or dsb repair deficient cells. This result suggests that the activation of a bystander signal is independent of the DNA repair capacity of the irradiated cells. Also, pre-treatment of the irradiated cells with 0.5% DMSO, which suppresses micronuclei induction in CHO but not in xrs5 cells, suppressed bystander effects completely in both conditioned media, suggesting that DMSO is effective for suppression of bystander signal arising independently of DNA damage in irradiated cells. Overall the work presented here adds to the understanding that it is the repair phenotype of the cells receiving bystander signals, which determines overall response rather than that of the cell producing the bystander signal.

Modulation of radiation responses by pre-exposure to irradiated cell conditioned medium

The aim of this study was to investigate whether exposure of HPV-G cells to irradiated cell conditioned medium (ICCM) could induce an adaptive response if the cells were subsequently challenged with a higher ICCM dose. Clonogenic survival and major steps in the cascade leading to apoptosis, such as calcium influx and loss of mitochondrial membrane potential, were examined to determine whether these events could be modified by giving a priming dose of ICCM before the challenge dose. Clonogenic survival data indicated an ICCM-induced adaptive response in HPV-G cells "primed" with 5 mGy or 0.5 Gy ICCM for 24 h and then exposed to 0.5 Gy or 5 Gy ICCM. Reactive oxygen species (ROS) were found to be involved in the bystander-induced cell death. Calcium fluxes varied in magnitude across the exposed cell population, and a significant number of the primed HPV-G cells did not respond to the challenge ICCM dose. No significant loss of mitochondrial membrane potential was observed when HPV-G cells were exposed to 0.5 Gy ICCM for 24 h followed by exposure to 5 Gy ICCM for 6 h. Exposure of HPV-G cells to 5 mGy ICCM for 24 h followed by exposure to 0.5 Gy ICCM for 18 h caused a significant increase in mitochondrial mass and a change in mitochondrial location, events associated with the perpetuation of genomic instability. This study has shown that a priming dose of ICCM has the ability to induce an adaptive response in HPV-G cells subsequently exposed to a challenge dose of ICCM.

Tissue-sparing effect of x-ray microplanar beams particularly in the CNS: is a bystander effect involved?

OBJECTIVE

Normal tissues, including the central nervous system, tolerate single exposures to narrow planes of synchrotron-generated x-rays (microplanar beams; microbeams) up to several hundred Gy. The repairs apparently involve the microvasculature and the glial system. We evaluate a hypothesis on the involvement of bystander effects in these repairs.

METHODS

Confluent cultures of bovine aortic endothelial cells were irradiated with three parallel 27-microm microbeams at 24 Gy. Rats' spinal cords were transaxially irradiated with a single microplanar beam, 270 microm thick, at 750 Gy; the dose distribution in tissue was calculated.

RESULTS

Within 6 hours following irradiation of the cell culture the hit cells died, apparently by apoptosis, were lost, and the confluency was maintained. The spinal cord study revealed a loss of oligodendrocytes, astrocytes, and myelin in 2 weeks, but by 3 months repopulation and remyelination was nearly complete. Monte Carlo simulations showed that the microbeam dose fell from the peak's 80% to 20% in 9 microm.

CONCLUSIONS

In both studies the repair processes could have involved "beneficial" bystander effects leading to tissue restoration, most likely through the release of growth factors, such as cytokines, and the initiation of cell-signaling cascades. In cell culture these events could have promoted fast disappearance of the hit cells and fast structural response of the surviving neighboring cells, while in the spinal cord study similar events could have been promoting angiogenesis to replace damaged capillary blood vessels, and proliferation, migration, and differentiation of the progenitor glial cells to produce new, mature, and functional glial cells.

A role for bioelectric effects in the induction of bystander signals by ionizing radiation?

The induction of "bystander effects" i.e. effects in cells which have not received an ionizing radiation track, is now accepted but the mechanisms are not completely clear. Bystander effects following high and low LET radiation exposure are accepted but mechanisms are still not understood. There is some evidence for a physical component to the signal. This paper tests the hypothesis that bioelectric or biomagnetic phenomena are involved. Human immortalized skin keratinocytes and primary explants of mouse bladder and fish skin, were exposed directly to ionizing radiation or treated in a variety of bystander protocols. Exposure of cells was conducted by shielding one group of flasks using lead, to reduce the dose below the threshold of 2mGy (60)Cobalt gamma rays established for the bystander effect. The endpoint for the bystander effect in the reporter system used was reduction in cloning efficiency (RCE). The magnitude of the RCE was similar in shielded and unshielded flasks. When cells were placed in a Faraday cage the magnitude of the RCE was less but not eliminated. The results suggest that liquid media or cell-cell contact transmission of bystander factors may be only part of the bystander mechanism. Bioelectric or bio magnetic fields may have a role to play. To test this further, cells were placed in a Magnetic Resonance Imaging (MRI) machine for 10 min using a typical head scan protocol. This treatment also induced a bystander response. Apart from the obvious clinical relevance, the MRI results further suggest that bystander effects may be produced by non-ionizing exposures. It is concluded that bioelectric or magnetic effects may be involved in producing bystander signaling cascades commonly seen following ionizing radiation exposure.

Radioadaptive response revisited

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

Vanguards of paradigm shift in radiation biology: radiation-induced adaptive and bystander responses

The risks of exposure to low dose ionizing radiation (below 100 mSv) are estimated by extrapolating from data obtained after exposure to high dose radiation, using a linear no-threshold model (LNT model). However, the validity of using this dose-response model is controversial because evidence accumulated over the past decade has indicated that living organisms, including humans, respond differently to low dose/low dose-rate radiation than they do to high dose/high dose-rate radiation. In other words, there are accumulated findings which cannot be explained by the classical "target theory" of radiation biology. The radioadaptive response, radiation-induced bystander effects, low-dose radio-hypersensitivity, and genomic instability are specifically observed in response to low dose/low dose-rate radiation, and the mechanisms underlying these responses often involve biochemical/molecular signals that respond to targeted and non-targeted events. Recently, correlations between the radioadaptive and bystander responses have been increasingly reported. The present review focuses on the latter two phenomena by summarizing observations supporting their existence, and discussing the linkage between them from the aspect of production of reactive oxygen and nitrogen species.

Low-dose irradiation of nontransformed cells stimulates the selective removal of precancerous cells via intercellular induction of apoptosis

An important stage in tumorigenesis is the ability of a precancerous cell to escape natural anticancer signals imposed on it by neighboring cells and its microenvironment. We have previously characterized a system of intercellular induction of apoptosis whereby nontransformed cells selectively remove transformed cells from coculture via cytokine and reactive oxygen/nitrogen species (ROS/RNS) signaling. We report that irradiation of nontransformed cells with low doses of either high linear energy transfer (LET) alpha-particles or low-LET gamma-rays leads to stimulation of intercellular induction of apoptosis. The use of scavengers and inhibitors confirms the involvement of ROS/RNS signaling and of the importance of transformed cell NADPH oxidase in the selectivity of the system. Doses as low as 2-mGy gamma-rays and 0.29-mGy alpha-particles were sufficient to produce an observable increase in transformed cell apoptosis. This radiation-stimulated effect saturates at very low doses (50 mGy for gamma-rays and 25 mGy for alpha-particles). The use of transforming growth factor-beta (TGF-beta) neutralizing antibody confirms a role for the cytokine in the radiation-induced signaling. The system may represent a natural anticancer mechanism stimulated by extremely low doses of ionizing radiation.

Radiation-induced bystander effects and adaptive response in murine lymphocytes

PURPOSE

To study the bystander effects of gamma-radiation in murine lymphocytes using irradiated conditioned medium (ICM) generated from irradiated lymphocytes.

METHODS

Proliferation response of unirradiated lymphocytes to mitogen concanavalin A (con A) in presence of ICM, collected from gamma-irradiated lymphocytes (60Co source; 0.35 Gy/min; 0.1-1 Gy), was studied by 3H-thymidine incorporation and also by dye dilution using carboxyfluorescein succinimidyl ester (CFSE). Expression of proliferation markers, interleukin 2 receptor alpha chain (CD25) and cyclin D in ICM treated lymphocytes was analyzed by labeling with specific antibodies. Intracellular reactive oxygen species (ROS) and apoptosis were estimated by flow cytometry using dichlorodihydrofluorescein diacetate (H2DCFDA) and propidium iodide, respectively. Nitric oxide (NO) was measured using Griess reagent.

RESULTS

Proliferation response to con A in unirradiated lymphocytes was enhanced in the presence of ICM with maximum enhancement observed in the presence of 0.5 Gy ICM. Augmentation of proliferation in the presence of ICM was accompanied by an increase in CD25 and cyclin D expression, enhanced ROS and NO generation. ICM pretreated lymphocytes showed adaptive response to radiation which was not abrogated by wortmannin, a phosphatidyl inositol 3-kinase (PI3K) inhibitor.

CONCLUSION

Soluble factors released from irradiated lymphocytes initiate a signaling cascade in unirradiated lymphocytes resulting in increased response to mitogen and radioresistance which may have an important role in radiation-induced immunomodulation.

Variability: the common factor linking low dose-induced genomic instability, adaptation and bystander effects

The characteristics of low dose radiation-induced genomic instability, adaptive responses, and bystander effects were compared in order to probe possible underlying mechanisms, and develop models for predicting response to in vivo low dose radiation exposures. While there are some features that are common to all three (e.g., absence of a true dose-response, the multiple endpoints affected by each), other characteristics appear to distinguish one from the other (e.g., TP53 involvement, LET response, influence of DNA repair). Each of the responses is also highly variable; not all cell and tissue models show the same response and there is much interindividual variation in response. Most of these studies have employed in vitro cell culture or tissue explant models, and understanding underlying mechanisms and the biological significance of these low dose-responses will require study of tissue-specific in vivo endpoints. The in vitro studies strongly suggest that modeling low dose radiation effects will be a complex process, and will likely require separate study of each of these low dose phenomena. Knowledge of instability responses, for example, may not aid in predicting other low dose effects in the same tissue.

Non-targeted bystander effects induced by ionizing radiation

Radiation-induced bystander effects refer to those responses occurring in cells that were not subject to energy deposition events following ionizing radiation. These bystander cells may have been neighbors of irradiated cells, or physically separated but subject to soluble secreted signals from irradiated cells. Bystander effects have been observed in vitro and in vivo and for various radiation qualities. In tribute to an old friend and colleague, Anthony V. Carrano, who would have said "well what are the critical questions that should be addressed, and so what?", we review the evidence for non-targeted radiation-induced bystander effects with emphasis on prevailing questions in this rapidly developing research field, and the potential significance of bystander effects in evaluating the detrimental health effects of radiation exposure.

Fractionated X-radiation treatment can elicit an inducible-like radioprotective response that is not dependent on the intrinsic cellular X-radiation resistance/sensitivity

Inducible responses are well documented to play a role in the radiation response of cells. However, it is not known whether clinically relevant fractionated X-radiation treatment could elicit an inducible-like radioprotective response and whether there is a direct correlation between the inducible radiation response phenomenon and the intrinsic radiation response of the cell. Therefore, the purpose of this study was to determine whether closely related human colorectal tumor (HCT116) clones treated with fractionated X rays could elicit an inducible-like radiation response to a subsequent acute (i.e. single) X-ray challenge, and whether the magnitude of the inducible-like response correlates with the intrinsic X-ray resistance of the responding clones. After fractionated X irradiation, only the radiosensitive clone showed enhanced clonogenic survival with a subsequent acute X-ray exposure. Cell cycle changes or the selection of subclones with increased intrinsic radiation resistance induced by the fractionated X rays were excluded as the basis of this enhanced tolerance, suggesting the presence of an inducible-like radioprotective response. Using the comet assay, we found similar amounts of intrinsic DNA damage among the clones after acute X irradiation. Our findings demonstrate that fractionated X-ray treatment can elicit an inducible-like radioprotective response and represent the first evidence that this response is independent of the intrinsic radiation resistance/sensitivity of the responding cells.

Extremely Low Priming Doses of X Radiation Induce an Adaptive Response for Chromosomal Inversions in pKZ1 Mouse Prostate

An adaptive response is a response to a stress such as radiation exposure that results in a lower than expected biological response. We describe an adaptive response to X radiation in mouse prostate using the pKZ1 chromosomal inversion assay. pKZ1 mice were treated with a priming dose of 0.001, 0.01, 1 or 10 mGy followed 4 h later by a 1000-mGy challenge dose. All priming doses caused a similar reduction in inversions compared to the 1000-mGy group, supporting the hypothesis that the adaptive response is the result of an on/off mechanism. The adaptive response was induced by a priming dose of 0.001 mGy, which is three orders of magnitude lower than has been reported previously. The adaptive responses completely protected against the inversions that would have been induced by a single 1000-mGy dose as well as against a proportion of spontaneous background inversions. The distribution of inversions across prostate gland cross sections after priming plus challenge irradiation suggested that adaptive responses were predominantly due to reduced low-dose radiation-induced inversions rather than to reduced high-dose radiation-induced inversions. This study used radiation doses relevant to human exposure.

Radiation-induced bystander and adaptive responses in cell and tissue models

The use of microbeam approaches has been a major advance in probing the relevance of bystander and adaptive responses in cell and tissue models. Our own studies at the Gray Cancer Institute have used both a charged particle microbeam, producing protons and helium ions and a soft X-ray microprobe, delivering focused carbon-K, aluminium-K and titanium-K soft X-rays. Using these techniques we have been able to build up a comprehensive picture of the underlying differences between bystander responses and direct effects in cell and tissue-like models. What is now clear is that bystander dose-response relationships, the underlying mechanisms of action and the targets involved are not the same as those observed for direct irradiation of DNA in the nucleus. Our recent studies have shown bystander responses even when radiation is deposited away from the nucleus in cytoplasmic targets. Also the interaction between bystander and adaptive responses may be a complex one related to dose, number of cells targeted and time interval.

Radiation-induced bystander effects: evidence for an adaptive response to low dose exposures?

This paper reviews our current knowledge of the mechanisms underlying the induction of bystander effects by low dose, low-LET ionizing radiation and discusses how they may be related to observed adaptive responses or other protective effects of low dose exposures. Bystander effects appear to be the result of a generalized stress response in tissues or cells. The signals may be produced by all exposed cells, but the response appears to require a quorum in order to be expressed. The major response involving low LET radiation exposure discussed in the existing literature is a death response. This has many characteristics of apoptosis but is p53 independent. While a death response might appear to be adverse, the position is argued in this paper that it is in fact protective and removes damaged cells from the population. Since many cell populations carry damaged cells without being exposed to radiation, so called "background damage", it is possible that low doses exposures cause removal of cells damaged by agents other than the test dose of radiation. This mechanism would lead to the production of "U-shaped" dose response curves. In this scenario, the level of "adaptive" or beneficial response will be related to the background damage carried by the cell population. This model may be important when attempting to predict the consequences of mixed exposures involving radiation and other environmental stressors.

Bystander effect: biological endpoints and microarray analysis

In cell populations exposed to ionizing radiation, the biological effects occur in a much larger proportion of cells than are estimated to be traversed by radiation. It has been suggested that irradiated cells are capable of providing signals to the neighboring unirradiated cells resulting in damage to these cells. This phenomenon is termed the bystander effect. The bystander effect induces persistent, long-term, transmissible changes that result in delayed death and neoplastic transformation. Because the bystander effect is relevant to carcinogenesis, it could have significant implications for risk estimation for radiation exposure. The nature of the bystander effect signal and how it impacts the unirradiated cells remains to be elucidated. Examination of the changes in gene expression could provide clues to understanding the bystander effect and could define the signaling pathways involved in sustaining damage to these cells. The microarray technology serves as a tool to gain insight into the molecular pathways leading to bystander effect. Using medium from irradiated normal human diploid lung fibroblasts as a model system we examined gene expression alterations in bystander cells. The microarray data revealed that the radiation-induced gene expression profile in irradiated cells is different from unirradiated bystander cells suggesting that the pathways leading to biological effects in the bystander cells are different from the directly irradiated cells. The genes known to be responsive to ionizing radiation were observed in irradiated cells. Several genes were upregulated in cells receiving media from irradiated cells. Surprisingly no genes were found to be downregulated in these cells. A number of genes belonging to extracellular signaling, growth factors and several receptors were identified in bystander cells. Interestingly 15 genes involved in the cell communication processes were found to be upregulated. The induction of receptors and the cell communication processes in bystander cells receiving media from irradiated cells supports the active involvement of these processes in inducing bystander effect.

Untargeted effects of ionizing radiation: implications for radiation pathology

The dogma that genetic alterations are restricted to directly irradiated cells has been challenged by observations in which effects of ionizing radiation, characteristically associated with the consequences of energy deposition in the cell nucleus, arise in non-irradiated cells. These, so called, untargeted effects are demonstrated in cells that have received damaging signals produced by irradiated cells (radiation-induced bystander effects) or that are the descendants of irradiated cells (radiation-induced genomic instability). Radiation-induced genomic instability is characterized by a number of delayed adverse responses including chromosomal abnormalities, gene mutations and cell death. Similar effects, as well as responses that may be regarded as protective, have been attributed to bystander mechanisms. Whilst the majority of studies to date have used in vitro systems, some adverse non-targeted effects have been demonstrated in vivo. However, at least for haemopoietic tissues, radiation-induced genomic instability in vivo may not necessarily be a reflection of genomically unstable cells. Rather the damage may reflect responses to ongoing production of damaging signals; i.e. bystander responses, but not in the sense used to describe the rapidly induced effects resulting from direct interaction of irradiated and non-irradiated cells. The findings are consistent with a delayed and long-lived tissue reaction to radiation injury characteristic of an inflammatory response with the potential for persisting bystander-mediated damage. An important implication of the findings is that contrary to conventional radiobiological dogma and interpretation of epidemiologically-based risk estimates, ionizing radiation may contribute to malignancy and particularly childhood leukaemia by promoting initiated cells rather than being the initiating agent. Untargeted mechanisms may also contribute to other pathological consequences.

Neoplastic transformation in vitro by low doses of ionizing radiation: role of adaptive response and bystander effects

The shape of the dose-response curve for cancer induction by low doses of ionizing radiation is of critical importance to the assessment of cancer risk at such doses. Epidemiologic analyses are limited by sensitivity to doses typically greater than 50-100 mGy for low LET radiation. Laboratory studies allow for the examination of lower doses using cancer-relevant endpoints. One such endpoint is neoplastic transformation in vitro. It is known that this endpoint is responsive to both adaptive response and bystander effects. The relative balance of these processes is likely to play an important role in determining the shape of the dose-response curve at low doses. A factor that may influence this balance is cell density at time of irradiation. The findings reported in this paper indicate that the transformation suppressive effect of low doses previously seen following irradiation of sub-confluent cultures, and attributed to an adaptive response, is reduced for irradiated confluent cultures. However, even under these conditions designed to optimize the role of bystander effects the data do not fit a linear no-threshold model and are still consistent with the notion of a threshold dose for neoplastic transformation in vitro by low LET radiation.

Neoplastic transformation in vitro induced by low doses of 232 MeV protons

The aim was to define the dose--response curve for high-energy proton-induced neoplastic transformation in vitro. The HeLa x skin fibroblast human hybrid cell assay was used to determine the frequency of neoplastic transformation following doses of 232 MeV protons (mean linear energy transfer, LET=0.44 keV microm(-1)) in the range 5-600 mGy. Proton irradiations were carried out at the Loma Linda University Proton Treatment Facility, CA, USA. The data indicate no evidence for induction of transformation below a dose of 100 mGy. At doses of 5 and 50 mGy, there is evidence for a possible suppression of transformation frequencies below that for spontaneous transformation. The shape of the dose--response curve for high-energy proton-induced transformation of the human hybrid cell line CGL1 does not follow a linear no-threshold model and shows evidence for a threshold as well as for possible suppression of transformation at doses <100 mGy, similar to that seen for other low-LET radiations.

Radiation-induced bystander effects and adaptive responses--the Yin and Yang of low dose radiobiology?

Our current knowledge of the mechanisms underlying the induction of bystander effects by low doses of high or low LET ionizing radiation is reviewed. The question of what actually constitutes a protective effect is discussed in the context of adaptive (often referred to as hormetic or protective) responses. Finally the review considers critically, how bystander effects may be related to observed adaptive responses or other seemingly protective effects of low doses exposures. Bystander effects induce responses at the tissue level, which are similar to generalized stress responses. Most of the work involving low LET radiation exposure discussed in the existing literature measures a death response. Since many cell populations carry damaged cells without being exposed to radiation (so-called "background damage"), it is possible that low doses exposures cause removal of cells carrying potentially problematic lesions, prior to exposure to radiation. This mechanism could lead to the production of "U-shaped" or hormetic dose-response curves. The level of adverse, adaptive or apparently beneficial response will be related to the background damage carried by the original cell population, the level of organization at which damage or harm are scored and the precise definition of "harm". This model may be important when attempting to predict the consequences of mixed exposures involving low doses of radiation and other environmental stressors.

Radiation-induced adaptive responses and bystander effects

A classical paradigm [correction of paradym] of radiation biology asserts that all radiation effects on cells, tissues and organisms are due to the direct action of radiation. However, there has been a recent growth of interest in the indirect actions of radiation including the radiation-induced adaptive response, the bystander effect, low-dose hypersensitivity, and genomic instability, which are specific modes of stress exhibited in response to low-dose/low-dose rate radiation. This review focuses on the radiation-induced bystander effect and the adaptive response, provides a description of the two phenomena, and discusses the contribution of the former to the latter.

Radiation-induced bystander effects: are they good, bad or both?

Our current knowledge of the mechanisms underlying the induction of bystander effects by low dose-low linear-energy-transfer ionising radiation is reviewed, and the question of how bystander effects may be related to observed adaptive responses, systemic genomic instability or other effects of low doses exposures is considered. Bystander effects appear to be the result of a generalised stress response in tissues or cells. The signals may be produced by all exposed cells but the response may require a quoram in order to be expressed. The major response involving low LET radiation exposure discussed in the existing literature is a death response, which has many characteristics of apoptosis but may be detected in cell lines without p53 expression. While a death response might appear to be adverse, it can in fact be protective and remove damaged cells from the population. Since many cell populations carry damaged cells without being exposed to radiation ('background damage') low doses exposures might cause removal of cells damaged by agents other than the test dose of radiation, which would lead to the production of 'u- or n-shaped' dose-response curves. The level of harmful or beneficial response would then be related to the background damage carried by the cell population and the genetic programme determining response to damage. This model may be important when attempting to predict the consequences of mixed exposures involving radiation and other environmental stressors.

A biological-based model that links genomic instability, bystander effects, and adaptive response

This paper links genomic instability, bystander effects, and adaptive response in mammalian cell communities via a novel biological-based, dose-response model called NEOTRANS3. The model is an extension of the NEOTRANS2 model that addressed stochastic effects (genomic instability, mutations, and neoplastic transformation) associated with brief exposure to low radiation doses. With both models, ionizing radiation produces DNA damage in cells that can be associated with varying degrees of genomic instability. Cells with persistent problematic instability (PPI) are mutants that arise via misrepair of DNA damage. Progeny of PPI cells also have PPI and can undergo spontaneous neoplastic transformation. Unlike NEOTRANS2, with NEOTRANS3 newly induced mutant PPI cells and their neoplastically transformed progeny can be suppressed via our previously introduced protective apoptosis-mediated (PAM) process, which can be activated by low linear energy transfer (LET) radiation. However, with NEOTRANS3 (which like NEOTRANS2 involves cross-talk between nongenomically compromised [e.g., nontransformed, nonmutants] and genomically compromised [e.g., mutants, transformants, etc.] cells), it is assumed that PAM is only activated over a relatively narrow, dose-rate-dependent interval (D(PAM),D(off)); where D(PAM) is a small stochastic activation threshold, and D(off) is the stochastic dose above which PAM does not occur. PAM cooperates with activated normal DNA repair and with activated normal apoptosis in guarding against genomic instability. Normal repair involves both error-free repair and misrepair components. Normal apoptosis and the error-free component of normal repair protect mammals by preventing the occurrence of mutant cells. PAM selectively removes mutant cells arising via the misrepair component of normal repair, selectively removes existing neoplastically transformed cells, and probably selectively removes other genomically compromised cells when it is activated. PAM likely involves multiple pathways to apoptosis, with the selected pathway depending on the type of cell to be removed, its cellular environment, and on the nature of the genomic damage.

Damaging and protective cell signalling in the untargeted effects of ionizing radiation

The major adverse consequences of radiation exposures are attributed to DNA damage in irradiated cells that has not been correctly restored by metabolic repair processes. However, the dogma that genetic alterations are restricted to directly irradiated cells has been challenged by observations in which effects of ionizing radiation arise in non-irradiated cells. These, so called, untargeted effects are demonstrated in cells that are the descendants of irradiated cells either directly or via media transfer (radiation-induced genomic instability) or in cells that have communicated with irradiated cells (radiation-induced bystander effects). Radiation-induced genomic instability is characterized by a number of delayed responses including chromosomal abnormalities, gene mutations and cell death. Bystander effects include increases or decreases in damage-inducible and stress-related proteins, increases or decreases in reactive oxygen and nitrogen species, cell death or cell proliferation, cell differentiation, radioadaptation, induction of mutations and chromosome aberrations and chromosomal instability. The phenotypic expression of untargeted effects and the potential consequences of these effects in tissues reflect a balance between the type of bystander signals produced and the responses of cell populations to such signals, both of which may be significantly influenced by cell type and genotype. Thus, in addition to targeted effects of damage induced directly in cells by irradiation, a variety of untargeted effects may also make important short-term and long-term contributions to determining overall outcome after radiation exposures.

An examination of radiation hormesis mechanisms using a multistage carcinogenesis model

A multistage cancer model that describes the putative rate-limiting steps in carcinogenesis is developed and used to investigate the potential impact on cumulative lung cancer incidence of the hormesis mechanisms suggested by Feinendegen and Pollycove. In the model, radiation and endogenous processes damage the DNA of target cells in the lung. Some fraction of the misrepaired or unrepaired DNA damage induces genomic instability and, ultimately, leads to the accumulation of malignant cells. The model explicitly accounts for cell birth and death processes, the clonal expansion of initiated cells, malignant conversion, and a lag period for tumor formation. Radioprotective mechanisms are incorporated into the model by postulating dose and dose-rate-dependent radical scavenging. The accuracy of DNA damage repair also depends on dose and dose rate. As currently formulated, the model is most applicable to low-linear-energy-transfer (LET) radiation delivered at low dose rates. Sensitivity studies are conducted to identify critical model inputs and to help define the shapes of the cumulative lung cancer incidence curves that may arise when dose and dose-rate-dependent cellular defense mechanisms are incorporated into a multistage cancer model. For lung cancer, both linear no-threshold (LNT-), and non-LNT-shaped responses can be obtained. If experiments demonstrate that the effects of DNA damage repair and radical scavenging are enhanced at least three-fold under low-dose conditions, our studies would support the existence of U-shaped responses. The overall fidelity of the DNA damage repair process may have a large impact on the cumulative incidence of lung cancer. The reported studies also highlight the need to know whether or not (or to what extent) multiply damaged DNA sites are formed by endogenous processes. Model inputs that give rise to U-shaped responses are consistent with an effective cumulative lung cancer incidence threshold that may be as high as 300 mGy (4 mGy per year for 75 years) for low-LET radiation.

Commentary on radiation-induced bystander effects

The paradigm of genetic alterations being restricted to direct DNA damage after exposure to ionizing radiation has been challenged by observations in which effects of ionizing radiation arise in cells that in themselves receive no radiation exposure. These effects are demonstrated in cells that are the descendants of irradiated cells (radiation-induced genomic instability) or in cells that are in contact with irradiated cells or receive certain signals from irradiated cells (radiation-induced bystander effects). Bystander signals may be transmitted either by direct intercellular communication through gap junctions, or by diffusible factors, such as cytokines released from irradiated cells. In both phenomena, the untargeted effects of ionizing radiation appear to be associated with free radical-mediated processes. There is evidence that radiation-induced genomic instability may be a consequence of, and in some cell systems may also produce, bystander interactions involving intercellular signalling, production of cytokines and free radical generation. These processes are also features of inflammatory responses that are known to have the potential for both bystander-mediated and persisting damage as well as for conferring a predisposition to malignancy. Thus, radiation-induced genomic instability and untargeted bystander effects may reflect interrelated aspects of inflammatory type responses to radiation-induced stress and injury and contribute to the variety of the pathological consequences of radiation exposures.

The bystander effect: recent developments and implications for understanding the dose response

The bystander effect refers to the biological response of a cell resulting from an event in an adjacent or nearby cell. Such effects depend on intercellular communication and amplify the consequences of the original event. These responses are of particular interest in the assessment of ionizing radiation risk because at public or occupational exposure levels not every cell receives a radiation track. Current radiation protection regulations and practices are based on the assumption of a linear increase in risk with dose, including low doses where not all cells are hit. Mechanisms that amplify biological effects are inconsistent with these assumptions. Evidence suggests that there are two different bystander effects in mammalian cells. In one type, a radiation track in one cell leads to damaging, mutagenic, and sometimes lethal events in adjacent, unhit cells. In the other type, a radiation track in one cell leads to an adaptive response in bystander cells, increasing resistance to spontaneous or radiation-induced events. This paper describes some of the data for radiation-induced bystander effects in vitro and correlates that data with in vitro and in vivo observations of risk at low doses. The data suggest that protective effects, including beneficial bystander effects, outweigh detrimental effects at doses below about 100 mGy, but that the reverse is true above this threshold.

A Sense of Danger from Radiation1

Tissue damage caused by exposure to pathogens, chemicals and physical agents such as ionizing radiation triggers production of generic "danger" signals that mobilize the innate and acquired immune system to deal with the intrusion and effect tissue repair with the goal of maintaining the integrity of the tissue and the body. Ionizing radiation appears to do the same, but less is known about the role of "danger" signals in tissue responses to this agent. This review deals with the nature of putative "danger" signals that may be generated by exposure to ionizing radiation and their significance. There are a number of potential consequences of "danger" signaling in response to radiation exposure. "Danger" signals could mediate the pathogenesis of, or recovery from, radiation damage. They could alter intrinsic cellular radiosensitivity or initiate radioadaptive responses to subsequent exposure. They may spread outside the locally damaged site and mediate bystander or "out-of-field" radiation effects. Finally, an important aspect of classical "danger" signals is that they link initial nonspecific immune responses in a pathological site to the development of specific adaptive immunity. Interestingly, in the case of radiation, there is little evidence that "danger" signals efficiently translate radiation-induced tumor cell death into the generation of tumor-specific immunity or normal tissue damage into autoimmunity. The suggestion is that radiation-induced "danger" signals may be inadequate in this respect or that radiation interferes with the generation of specific immunity. There are many issues that need to be resolved regarding "danger" signaling after exposure to ionizing radiation. Evidence of their importance is, in some areas, scant, but the issues are worthy of consideration, if for no other reason than that manipulation of these pathways has the potential to improve the therapeutic benefit of radiation therapy. This article focuses on how normal tissues and tumors sense and respond to danger from ionizing radiation, on the nature of the signals that are sent, and on the impact on the eventual consequences of exposure.

Adaptive response in embryogenesis: V. Existence of two efficient dose-rate ranges for 0.3 Gy of priming irradiation to adapt mouse fetuses

The adaptive response is an important phenomenon in radiobiology. A study of the conditions essential for the induction of an adaptive response is of critical importance to understanding the novel biological defense mechanisms against the hazardous effects of radiation. In our previous studies, the specific dose and timing of radiation for induction of an adaptive response were studied in ICR mouse fetuses. We found that exposure of the fetuses on embryonic day 11 to a priming dose of 0.3 Gy significantly suppressed prenatal death and malformation induced by a challenging dose of radiation on embryonic day 12. Since a significant dose-rate effect has been observed in a variety of radiobiological phenomena, the effect of dose rate on the effectiveness of induction of an adaptive response by a priming dose of 0.3 Gy administered to fetuses on embryonic day 11 was investigated over the range from 0.06 to 5.0 Gy/min. The occurrence of apoptosis in limb buds, incidences of prenatal death and digital defects, and postnatal mortality induced by a challenging dose of 3.5 Gy given at 1.8 Gy/min to the fetuses on embryonic day 12 were the biological end points examined. Unexpectedly, effective induction of an adaptive response was observed within two dose-rate ranges for the same dose of priming radiation, from 0.18 to 0.98 Gy/ min and from 3.5 to 4.6 Gy/min, for reduction of the detrimental effect induced by a challenging dose of 3.5 Gy. In contrast, when the priming irradiation was delivered at a dose rate outside these two ranges, no protective effect was observed, and at some dose rates elevation of detrimental effects was observed. In general, neither a normal nor a reverse dose- rate effect was found in the dose-rate range tested. These results clearly indicated that the dose rate at which the priming irradiation was delivered played a crucial role in the induction of an adaptive response. This paper provides the first evidence for the existence of two dose-rate ranges for the same dose of priming radiation to successfully induce an adaptive response in mouse fetuses.

Expression of the oxidative base excision repair enzymes is not induced in TK6 human lymphoblastoid cells after low doses of ionizing radiation

Most of the DNA damage produced by ionizing radiation is repaired by the base excision repair (BER) pathway. To determine whether the BER genes were up-regulated by low doses of ionizing radiation, we investigated their expression in TK6 human lymphoblastoid cells by measuring mRNA levels using real-time quantitative PCR. No induction at the transcriptional level of any of the base excision repair genes, NTH1 (NTHL1), OGG1, NEIL1, NEIL2, NEIL3, APE1, POLB, or accessory protein genes, LIG3, XRCC1 or XPG, was found at gamma-radiation doses ranging from 1 cGy to 2 Gy in a 24-h period. As has been measured in other cell lines, a dose-dependent induction of CDKN1A (WAF1) mRNA levels was observed in TK6 cells in the dose range of 0.5 to 2.0 Gy. We also examined BER enzyme activity on 8-oxoguanine-, dihydrouracil- and furan-containing oligonucleotide substrates and found no increase in extracts of TK6 cells after gamma-ray doses of 0.5-2.0 Gy. These data were corroborated by Western blot analysis of APE1 and NTH1, suggesting that the BER enzymes are also not up-regulated at the post-transcriptional level after ionizing radiation exposure.

Induction of chromosomal instability by chronic oxidative stress

Earlier studies using GM10115 cells analyzed the capability of different DNA-damaging agents to induce genomic instability and found that acute oxidative stress was relatively inefficient at eliciting a persistent destabilization of chromosomes. To determine whether this situation would change under chronic exposure conditions, the human-hamster hybrid line GM10115 was cultured under conditions of oxidative stress. Chronic treatments consisted of 1-hour incubations using a range of hydrogen peroxide (25-200 microM) or glucose oxidase (GO; 5-50 mU/ml) concentrations that were administered once daily over 10 to 30 consecutive days. The toxicity of chronic treatments was modest (- one log kill) and consistent with the low yield of first division aberrations (<5%). However, analysis of over 180 clones and 36,000 metaphases indicated that chronic oxidative stress led to a high incidence of chromosomal instability. Treatment of cells with 100 and 200 microM hydrogen peroxide or 50 mU/ml GO was found to elicit chromosomal instability in 11%, 22%, and 19% of the clones analyzed, respectively. In contrast, control clones isolated after mock treatment did not show signs of chromosomal destabilization. These data suggest that chronic oxidative stress constitutes a biochemical mechanism capable of disrupting the genomic integrity of cells.

Interaction between radiation-induced adaptive response and bystander mutagenesis in mammalian cells

Two conflicting phenomena, the bystander effect and the adaptive response, are important in determining biological responses at low doses of radiation and have the potential to have an impact on the shape of the dose-response relationship. Using the Columbia University charged-particle microbeam and the highly sensitive AL cell mutagenic assay, we reported previously that nonirradiated cells acquired mutagenesis through direct contact with cells whose nuclei had previously been traversed with either a single or 20 alpha particles each. Here we show that pretreatment of cells with a low dose of X rays 4 h before alpha-particle irradiation significantly decreased this bystander mutagenic response. Furthermore, bystander cells showed an increase in sensitivity after a subsequent challenging dose of X rays. Results from the present study address some of the pressing issues regarding both the actual target size and the radiation dose response and can improve on our current understanding of radiation risk assessment.

Adaptive Response in Embryogenesis: IV. Protective and Detrimental Bystander Effects Induced by X Radiation in Cultured Limb Bud Cells of Fetal Mice

The radioadaptive response and the bystander effect represent important phenomena in radiobiology that have an impact on novel biological response mechanisms and risk estimates. Micromass cultures of limb bud cells provide an in vitro cellular maturation system in which the progression of cell proliferation and differentiation parallels that in vivo. This paper presents for the first time evidence for the correlation and interaction in a micromass culture system between the radioadaptive response and the bystander effect. A radioadaptive response was induced in limb bud cells of embryonic day 11 ICR mice. Conditioning irradiation of the embryonic day 11 cells with 0.3 Gy resulted in a significant protective effect against the occurrence of apoptosis, inhibition of cell proliferation, and differentiation induced by a challenging dose of 5 Gy given the next day. Both protective and detrimental bystander effects were observed; namely, irradiating 50% of the embryonic day 11 cells with 0.3 Gy led to a successful induction of the protective effect, and irradiating 70% of the embryonic day 12 cells with 5 Gy produced a detrimental effect comparable to that seen when all the cells were irradiated. Further, the bystander effect was markedly decreased by pretreatment of the cells with an inhibitor to block the gap junction-mediated intercellular communication. These results indicate that the bystander effect plays an important role in both the induction of a protective effect by the conditioning dose and the detrimental effect of the challenge irradiation. Gap junction-mediated intercellular communication was suggested to be involved in the induction of the bystander effect.

Bystander responses induced by low LET radiation

Radiation-induced bystander responses are observed when cells respond to their neighbours being irradiated. Considerable evidence is now available regarding the importance of these responses in cell and tissue models. Most studies have utilized two approaches where either a media-transferable factor has been assessed or cells have been exposed to low fluences of charged particles, where only a few percent are exposed. The development of microbeams has allowed nontargeted responses such as bystander effects to be more carefully analysed. As well as charged particle microbeams, X-ray microprobes have been developed, and several groups are also developing electron microbeams. Using the Gray Cancer Institute soft X-ray microprobe, it has been possible to follow the response of individual cells to targeted low doses of carbon-characteristic soft X-rays. Studies in human fibroblasts have shown evidence of a significant radiation quality-dependent bystander effect, measured as chromosomal damage in the form of micronuclei which is radiation quality dependent. Other studies show that even under conditions when only a single cell is targeted with soft X-rays, significant bystander-mediated cell killing is observed. The observation of bystander responses with low LET radiation suggests that these may be important in understanding radiation risk from background levels of radiation, where cells observe only single electron track traversals. Also, the indirect evidence for these responses in vivo indicates that they may have a role to play in current radiotherapy approaches and future novel strategies involving modulating nontargeted responses.

Folate deficiency and ionizing radiation cause DNA breaks in primary human lymphocytes: a comparison

DNA double-strand breaks, the most serious DNA lesion caused by ionizing radiation, are also caused by several vitamin or mineral deficiencies, such as for folate. Primary human lymphocytes were either irradiated or cultured at different levels of folate deficiency to assess cell proliferation, apoptosis, cell cycle, DNA breaks, and changes in gene expression. Both radiation and folate deficiency decreased cell proliferation and induced DNA breaks, apoptosis, and cell cycle arrest. Levels of folate deficiency commonly found resulted in effects similar to those caused by 1 Gy of radiation, a relatively high dose. Though both radiation and folate deficiency caused DNA breaks, they affected the expression of different genes. Radiation activated excision and DNA double-strand break repair genes and repressed mitochondrially encoded genes. Folate deficiency activated base and nucleotide excision repair genes and repressed folate-related genes. No DNA double-strand break repair gene was activated by folate deficiency. These findings suggest that a diet poor in folate may pose a risk of DNA damage comparable to that of a relatively high dose of radiation. Our results also suggest that research on biological effects of low-dose radiation should take into account the nutritional status of the subjects, because folate deficiency could confound the effects of low-dose radiation.