Medium-mediated intercellular communication is involved in bystander responses of X-ray-irradiated normal human fibroblasts

DNA damage and repair kinetics after microbeam radiation therapy emulation in living cells using monoenergetic synchrotron X-ray microbeams

A novel synchrotron-based approach, known as microbeam radiation therapy (MRT), currently shows considerable promise in increased tumour control and reduced normal tissue damage compared with conventional radiotherapy. Different microbeam widths and separations were investigated using a controlled cell culture system and monoenergetic (5.35 keV) synchrotron X-rays in order to gain further insight into the underlying cellular response to MRT. DNA damage and repair was measured using fluorescent antibodies against phosphorylated histone H2AX, which also allowed us to verify the exact location of the microbeam path. Beam dimensions that reproduced promising MRT strategies were used to identify useful methods to study the underpinnings of MRT. These studies include the investigation of different spatial configurations on bystander effects. γH2AX foci number were robustly induced in directly hit cells and considerable DNA double-strand break repair occurred by 12 h post-10 Gy irradiation; however, many cells had some γH2AX foci at the 12 h time point. γH2AX foci at later time points did not directly correspond with the targeted regions suggesting cell movement or bystander effects as a potential mechanism for MRT effectiveness. Partial irradiation of single nuclei was also investigated and in most cases γH2AX foci were not observed outside the field of irradiation within 1 h after irradiation indicating very little chromatin movement in this time frame. These studies contribute to the understanding of the fundamental radiation biology relating to the MRT response, a potential new therapy for cancer patients.

Activation of interferon-stimulated genes by gamma-ray irradiation independently of the ataxia telangiectasia mutated-p53 pathway

The ataxia telangiectasia mutated (ATM)-p53 pathway is a well-known main signal transduction pathway for cellular responses, which is activated by γ-ray irradiation. Microarray analysis showed changes in the expressions of IFN-stimulated genes (ISG) in γ-ray-irradiated Balb/cA/Atm-deficient mouse embryonic fibroblasts (MEF) (ATM-KO), indicating that another pathway for cellular responses besides the ATM-p53 pathway was activated by γ-ray irradiation. The basal expression levels of Irf7 and Stat1 in ATM-KO and p53-deficient MEFs (p53-KO) were higher than those in Atm-wild-type MEFs (ATM-WT) and p53-wild-type MEFs (p53-WT), respectively. Irradiation stimulated the expressions of Irf7 and Stat1 in ATM-KO, p53-KO, ATM-WT, and p53-WT, indicating that upregulation of Irf7 and Stat1 expressions by irradiation did not depend on the ATM-p53 pathway. When conditioned medium (CM) obtained from irradiated ATM-WT or ATM-KO was added to nonirradiated MEFs, the expressions of Irf7 and Stat1 increased. We predicted that gene activation in nonirradiated MEFs was caused by IFN-α/β. Unexpectedly, significant amount of IFN-α/β could not be detected in the CM from irradiated ATM-WT or ATM-KO. Meanwhile, increased expression of Ccl5 (RANTES) protein was detected in the CM from irradiated MEFs. These results indicate that ISGs were activated by γ-ray irradiation independently of the ATM-p53 pathway and gene activation was caused by radiation-induced soluble factors.

Oxidative metabolism involved in non-targeted effects induced by proton radiation in intact Arabidopsis seeds

Non-targeted effects induced by ionizing radiation have been demonstrated both in vitro and in vivo. Previously, we have also demonstrated the existence of non-targeted effects in intact Arabidopsis seeds following low-energy heavy-ion radiation. In the present study, 6.5 MeV protons with 8 × 10(11) ions/cm(2) and 2 × 10(11) ions/cm(2) fluence respectively were used to irradiate non-shielded or partial-shielded Arabidopsis seeds to further explore the mechanisms which regulate in vivo non-targeted effects and to investigate the difference between damage caused by non-targeted effects and direct irradiation. Results showed that excess reactive oxygen species (ROS) are present in the non-irradiated part of the partially irradiated samples, indicating that in vivo non-targeted effects can promote the generation of excess metabolic ROS in the non-irradiated shoot apical meristem/root apical meristem cells. Furthermore, pretreatment with 0.5% ROS scavenger dimethyl sulfoxide (DMSO) or 0.02 mM reactive nitrogen species (RNS) scavenger 2-4-carboxyphenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) significantly suppresses the non-targeted effects in the partially irradiated samples, while in the whole-body irradiated samples, the cPTIO pretreatment has no effect. On the other hand using antioxidant enzyme assays, superoxide dismutase activity was found to increase for partial irradiated samples and decrease for the whole-body exposed seeds. Taken together, these results implicate that damage caused by non-targeted effects is different from that induced by direct irradiation in vivo. Metabolic products such as ROS and RNS are involved in the in vivo non-targeted effects.

Long-term consequences of radiation-induced bystander effects depend on radiation quality and dose and correlate with oxidative stress

Widespread evidence indicates that exposure of cell populations to ionizing radiation results in significant biological changes in both the irradiated and nonirradiated bystander cells in the population. We investigated the role of radiation quality, or linear energy transfer (LET), and radiation dose in the propagation of stressful effects in the progeny of bystander cells. Confluent normal human cell cultures were exposed to low or high doses of 1GeV/u iron ions (LET ∼ 151 keV/µm), 600 MeV/u silicon ions (LET ∼ 51 keV/µm), or 1 GeV protons (LET ∼ 0.2 keV/µm). Within minutes after irradiation, the cells were trypsinized and co-cultured with nonirradiated cells for 5 h. During this time, irradiated and nonirradiated cells were grown on either side of an insert with 3-µm pores. Nonirradiated cells were then harvested and allowed to grow for 20 generations. Relative to controls, the progeny of bystander cells that were co-cultured with cells irradiated with iron or silicon ions, but not protons, exhibited reduced cloning efficiency and harbored higher levels of chromosomal damage, protein oxidation and lipid peroxidation. This correlated with decreased activity of antioxidant enzymes, inactivation of the redox-sensitive metabolic enzyme aconitase, and altered translation of proteins encoded by mitochondrial DNA. Together, the results demonstrate that the long-term consequences of the induced nontargeted effects greatly depend on the quality and dose of the radiation and involve persistent oxidative stress due to induced perturbations in oxidative metabolism. They are relevant to estimates of health risks from exposures to space radiation and the emergence of second malignancies after radiotherapy.

No evidence for a low linear energy transfer adaptive response in irradiated RKO cells

It has become increasingly evident from reports in the literature that there are many confounding factors capable of modulating radiation-induced non-targeted responses, such as the bystander effect and the adaptive response. In this paper, we examine recent data which suggest that the observation of non-targeted responses may not be universally observable for differing radiation qualities. We have conducted a study of the adaptive response following low-linear energy transfer exposures for human colon carcinoma cells and failed to observe adaption for the endpoints of clonogenic survival or micronucleus formation.

Targeted and non-targeted effects from combinations of low doses of energetic protons and iron ions in human fibroblasts

PURPOSE

In space, astronauts are exposed to mixed radiation fields consisting of energetic protons and high atomic number, high energy (HZE) particles at low dose rates. Therefore, it is critical to understand effects of combinations of low doses of different radiation types at the cellular level.

MATERIALS AND METHODS

AG01522 normal human skin fibroblasts and a transwell insert co-culture system were used. Irradiations used were 1 GeV/amu (gigaelectron volt/atomic mass unit) protons and 1 GeV/amu iron (Fe) ions. DNA damage was measured as micronucleus (MN) formation and p53 binding protein 1 (53BP1) foci induction.

RESULTS

The same magnitude of DNA damage was induced in cells sequentially exposed to 1 cGy protons and 1 cGy Fe ions as in cells irradiated with either protons or Fe ions alone. The same magnitude of DNA damage was also observed in non-irradiated bystander cells sharing medium with cells irradiated with either 1 cGy protons or iron ions or protons plus iron ions. However, when the 'bystander' cells were exposed to 1 cGy protons up to 3 h before co-culture with Fe ion-irradiated cells, no DNA damage in the 'bystander' cells was observed.

CONCLUSIONS

These data provide the first evidence of interactions between targeted and non-targeted DNA damage caused by dual exposure to low doses of energetic protons and iron ions.

Radiation-induced bystander effects in cultured human stem cells

Background

The radiation-induced “bystander effect” (RIBE) was shown to occur in a number of experimental systems both in vitro and in vivo as a result of exposure to ionizing radiation (IR). RIBE manifests itself by intercellular communication from irradiated cells to non-irradiated cells which may cause DNA damage and eventual death in these bystander cells. It is known that human stem cells (hSC) are ultimately involved in numerous crucial biological processes such as embryologic development; maintenance of normal homeostasis; aging; and aging-related pathologies such as cancerogenesis and other diseases. However, very little is known about radiation-induced bystander effect in hSC. To mechanistically interrogate RIBE responses and to gain novel insights into RIBE specifically in hSC compartment, both medium transfer and cell co-culture bystander protocols were employed.

Methodology/Principal Findings

Human bone-marrow mesenchymal stem cells (hMSC) and embryonic stem cells (hESC) were irradiated with doses 0.2 Gy, 2 Gy and 10 Gy of X-rays, allowed to recover either for 1 hr or 24 hr. Then conditioned medium was collected and transferred to non-irradiated hSC for time course studies. In addition, irradiated hMSC were labeled with a vital CMRA dye and co-cultured with non-irradiated bystander hMSC. The medium transfer data showed no evidence for RIBE either in hMSC and hESC by the criteria of induction of DNA damage and for apoptotic cell death compared to non-irradiated cells (p>0.05). A lack of robust RIBE was also demonstrated in hMSC co-cultured with irradiated cells (p>0.05).

Conclusions/Significance

These data indicate that hSC might not be susceptible to damaging effects of RIBE signaling compared to differentiated adult human somatic cells as shown previously. This finding could have profound implications in a field of radiation biology/oncology, in evaluating radiation risk of IR exposures, and for the safety and efficacy of hSC regenerative-based therapies.

Investigation of adaptive responses in bystander cells in 3D cultures containing tritium-labeled and unlabeled normal human fibroblasts

The study of radiation-induced bystander effects in normal human cells maintained in three-dimensional (3D) architecture provides more in vivo-like conditions and is relevant to human risk assessment. Linear energy transfer, dose and dose rate have been considered as critical factors in propagating radiation-induced effects. This investigation uses an in vitro 3D tissue culture model in which normal AG1522 human fibroblasts are grown in a carbon scaffold to investigate induction of a G(1) arrest in bystander cells that neighbor radiolabeled cells. Cell cultures were co-pulse-labeled with [(3)H]deoxycytidine ((3)HdC) to selectively irradiate a minor fraction of cells with 1-5 keV/microm beta particles and bromodeoxyuridine (BrdU) to identify the radiolabeled cells using immunofluorescence. The induction of a G(1) arrest was measured specifically in unlabeled cells (i.e. bystander cells) using a flow cytometry-based version of the cumulative labeling index assay. To investigate the relationship between bystander effects and adaptive responses, cells were challenged with an acute 4 Gy gamma-radiation dose after they had been kept under the bystander conditions described above for several hours, and the regulation of the radiation-induced G(1) arrest was measured selectively in bystander cells. When the average dose rate in (3)HdC-labeled cells (<16% of population) was 0.04-0.37 Gy/h (average accumulated dose 0.14-10 Gy), no statistically significant stressful bystander effects or adaptive bystander effects were observed as measured by magnitude of the G(1) arrest, micronucleus formation, or changes in mitochondrial membrane potential. Higher dose rates and/or higher LET may be required to observe stressful bystander effects in this experimental system, whereas lower dose rates and challenge doses may be required to detect adaptive bystander responses.

Effects of ionizing radiation on cell-to-cell communication

Cell-to-cell signaling has become a significant issue in radiation biology due to experimental evidence, accumulated primarily since the early 1990s, of radiation-induced bystander effects. Several candidate mediators involved in cell-to-cell communication have been investigated and proposed as being responsible for this phenomenon, but the current investigation techniques (both theoretical and experimental) of the mechanisms involved, due to the particular set-up of each experiment, result in experimental data that often are not directly comparable. In this study, a comprehensive approach was adopted to describe cell-to-cell communication (focusing on cytokine signaling) and its modulation by external agents such as ionizing radiation. The aim was also to provide integrated theoretical instruments and experimental data to help in understanding the peculiarities of in vitro experiments. Theoretical/modeling activities were integrated with experimental measurements by (1) redesigning a cybernetic model (proposed in its original form in the 1950s) to frame cell-to-cell communication processes, (2) implementing and developing a mathematical model, and (3) designing and carrying out experiments to quantify key parameters involved in intercellular signaling (focusing as a pilot study on the release and decay of IL-6 molecules and their modulation by radiation). This formalization provides an interpretative framework for understanding the intercellular signaling and in particular for focusing on the study of cell-to-cell communication in a "step-by-step" approach. Under this model, the complex phenomenon of signal transmission was reduced where possible into independent processes to investigate them separately, providing an evaluation of the role of cell communication to guarantee and maintain the robustness of the in vitro experimental systems against the effects of perturbations.

Medium-mediated DNA repair response after ionizing radiation is correlated with the increase of specific cytokines in human fibroblasts

Radiation induced bystander effects, either protective or adverse, have been identified in a variety of cells and for different endpoints. They are thought to arise from communication between cells through direct cell-cell contacts and via transmissible molecules secreted into the medium by targeted cells. We have investigated medium-mediated damage response in human dermal fibroblasts (HDF) after exposure to ionizing irradiation. We show that HDF experience an elevated level of double stranded DNA damage repair response when incubated with conditioned growth medium of irradiated cells. The magnitude of this response is much lower than observed for directly irradiated cells and is proportional to the radiation dose, as is its persistence across time. Since secretion of cytokines is one of the possible pathways linking targeted and non-targeted cells a multiplex analysis was performed. Four cytokines - IL6, IL8, MCP-1 and RANTES - were identified in the growth medium of irradiated cells after exposure to X-rays (2Gy). These cytokines were significantly upregulated and each cytokine showed differential upregulation kinetics. Finally we performed a functional analysis to see if IL6 and MCP-1 could induce gammaH2AX foci formation. IL6 caused a significant increase in spot occupancy compared to controls. Although only indicative MCP-1 appears to have the opposite effect as it caused a drop in spot occupancy. The combined addition of these two cytokines produced no significant response was observed. Both IL6 and MCP-1 have an effect on the gammaH2AX spot occupancy possibly linking these cytokines to the bystander response.

Involvement of purinergic signaling in cellular response to gamma radiation

Recent studies have suggested a bystander effect in nonirradiated cells adjacent to irradiated cells; however, the mechanism is poorly understood. In this study, we investigated the involvement of both extracellular nucleotides and activation of P2 receptors in cellular responses to gamma radiation using human HaCaT keratinocytes. The concentration of ATP in culture medium was increased after gamma irradiation (0.1-1.0 Gy), suggesting that radiation induces ATP release from cells. Intracellular Ca(2+) concentration was elevated when conditioned medium from irradiated cells was transferred to nonirradiated cells, and this elevation was suppressed by apyrase (ecto-nucleotidase), indicating the involvement of extracellular nucleotides in this event. Further, we examined the activation of ERK1/2 by gamma radiation and nucleotides (ATP and UTP). Both gamma radiation and nucleotides induced activation of ERK1/2. Next, the effect of inhibitors of P2 receptors on radiation-induced activation of ERK1/2 was examined. The activation of ERK1/2 was blocked by suramin (P2Y inhibitor), MRS2578 (P2Y(6) antagonist) and apyrase. These results suggest that both released nucleotides and activation of P2Y receptors are involved in gamma-radiation-induced activation of ERK1/2. We conclude that ionizing radiation induces release of nucleotides from cells, leading to activation of P2Y receptors, which in turn would result in a variety of biological effects.

Cytogenetic damage in cells exposed to ionizing radiation under conditions of a changing dose rate

The current international paradigm on the biological effects of radiation is based mainly on the effects of dose with some consideration for the dose rate. No allowance has been made for the potential influence of a changing dose rate (second derivative of dose), and the biological effects of exposing cells to changing dose rates have never been analyzed. This paper provides evidence that radiation effects in cells may depend on temporal changes in the dose rate. In these experiments, cells were moved toward or away from an X-ray source. The speed of movement, the time of irradiation, and the temperature during exposure were controlled. Here we report the results of the first experiments with TK6 cells that were exposed at a constant dose rate, at an increasing dose rate, or at a decreasing dose rate. The average dose rate and the total dose were same for all samples. Micronuclei were scored as the end point. The results show that the level of cytogenetic damage was higher in cells exposed to a decreasing dose rate compared to both an increasing and a constant dose rate. This finding may suggest that the second derivative of dose may influence radiation risk estimates, and the results should trigger further studies on this issue.

Lack of evidence for low-LET radiation induced bystander response in normal human fibroblasts and colon carcinoma cells

PURPOSE

To investigate radiation-induced bystander responses and to determine the role of gap junction intercellular communication and the radiation environment in propagating this response.

MATERIALS AND METHODS

We used medium transfer and targeted irradiation to examine radiation-induced bystander effects in primary human fibroblast (AG01522) and human colon carcinoma (RKO36) cells. We examined the effect of variables such as gap junction intercellular communication, linear energy transfer (LET), and the role of the radiation environment in non-targeted responses. Endpoints included clonogenic survival, micronucleus formation and foci formation at histone 2AX over doses ranging from 10-100 cGy.

RESULTS

The results showed no evidence of a low-LET radiation-induced bystander response for the endpoints of clonogenic survival and induction of DNA damage. Nor did we see evidence of a high-LET, Fe ion radiation (1 GeV/n) induced bystander effect. However, direct comparison for 3.2 MeV alpha-particle exposures showed a statistically significant medium transfer bystander effect for this high-LET radiation.

CONCLUSIONS

From our results, it is evident that there are many confounding factors influencing bystander responses as reported in the literature. Our observations reflect the inherent variability in biological systems and the difficulties in extrapolating from in vitro models to radiation risks in humans.

Tracking genomic instability within irradiated and bystander populations

Over the past two decades, our understanding of radiation biology has undergone a fundamental shift in paradigms away from deterministic ‘hit-effect’ relationships and towards complex ongoing ‘cellular responses’. These responses include now familiar, but still poorly understood, phenomena associated with radiation exposure such as genomic instability and bystander effects. Although these responses share some common features (e.g. they occur at high frequency following very low doses, are heterogeneous in their induction and are observed at time points far removed from the initial radiation exposure), the precise relationship between genomic instability and bystander effects remains to be elucidated. This review will provide a synthesis of the known, and proposed, interrelationships among irradiated and bystander cellular responses to radiation. It also discusses our current experimental approach for gaining a clearer understanding of the relationship between damage induction and long-term effects in both irradiated and bystander cells.

Bystander effects induced by chemicals and ionizing radiation: evaluation of changes in gene expression of downstream MAPK targets

Radiation-induced bystander effects have been evaluated extensively, including the involvement of the mitogen-activated protein kinase (MAPK) pathways. However, few studies have examined the ability of chemicals to induce bystander effects, and the molecular mechanisms involved in chemical bystander effects have not been investigated. We have previously demonstrated the ability of mitomycin C (MMC) and phleomycin (PHL) to induce bystander effects in normal human lymphoblastoid cells. Here, we demonstrate changes in the expression of MAPK target genes following bystander exposure to MMC or PHL or ionizing radiation. The expression changes of 18 genes, which code for proteins that are downstream targets of MAPK proteins, were evaluated at various time points following direct or bystander exposure to MMC, PHL and ionizing radiation. The 18 genes were analysed as groups belonging to one of the seven possible combinations of the three MAPK pathways. We observed statistically significant changes in expression of several genes following exposure to each agent. However, when the expression changes were analysed in the bystander cells alone, significant increases in expression of MAPK target genes were observed for MMC- and radiation-induced bystander effects but not for PHL. PHL is an acknowledged radiomimetic agent; however, in the present study, PHL responses did not resemble those of radiation. These results provide evidence for bystander-induced changes in MAPK proteins and downstream targets and suggest that the bystander effects are a part of a general stress response.

Exogenous carbon monoxide protects the bystander Chinese hamster ovary cells in mixed coculture system after alpha-particle irradiation

In the present work, the inhibitory effect of carbon monoxide (CO), generated by tricarbonyldichlororuthenium (II) dimer [CO-releasing molecule (CORM-2)], on the toxicity of radiation-induced bystander effect (RIBE) after alpha-particle irradiation was studied in a mixed coculture system. CO (CORM-2) treatment showed a significant inhibitory effect to the formation of p53 binding protein 1 (BP1) and micronuclei (MN) induced by RIBE in a concentration-dependent manner, but in the directly irradiated cell population no distinct decreases of BP1 and MN formation were observed. In this mixed coculture system, nitric oxide (NO) or superoxide anion (O2(*-)) was also proved to mediate the transduction of RIBE by using a NO synthase inhibitor or NADPH-oxidase-specific inhibitor treatment. The elevated O2(*-) was attenuated by CO (CORM-2) treatment in the bystander cells as measured by hydroethidine staining and fluorescence assessment. The exogenous NO (sper) or O2(*-) (H2O2) was used to mimic NO/O(2)-mediated RIBE, and CO (CORM-2) treatment also showed a protective effect to cells against the toxicity of these exogenous factors. Considering the inhibitory effect of CO on RIBE and the wide use of CO in therapy of diseases, it is hoped that a low concentration of CO can protect normal tissues against RIBE during radiotherapy.

γ-H2AX Foci as a Biomarker for Patient X-Ray Exposure in Pediatric Cardiac Catheterization

Background— A better knowledge of patient x-ray dose and the associated radiation risk in pediatric interventional cardiology is warranted in view of the extensive use of x-rays and the higher radiosensitivity of children. In the present study, γ-H2AX foci were used as a biomarker for radiation-induced effects. Patient-specific dose was assessed and radiation risks were estimated according to the linear-no-threshold model, commonly used in radiation protection, and the γ-H2AX foci data.

Methods and Results— In 49 pediatric patients (median age, 0.75 years) with congenital heart disease who underwent cardiac catheterization procedures, blood samples were taken before and shortly after the procedure. γ-H2AX foci were determined in peripheral blood T lymphocytes. In each patient, a net increase in γ-H2AX foci, representing DNA double-strand breaks induced by interventional x-rays, was observed. In addition, a patient-specific Monte Carlo simulation of the procedure was performed, resulting in individual blood, organ, and tissue doses. Plotting of γ-H2AX foci versus blood dose indicated a low-dose hypersensitivity. Median effective doses calculated according to the International Commission on Radiological Protection 60 and 103 publications are 5.6 and 6.4 mSv, respectively. The lifetime-attributable risk of cancer mortality was calculated from the linear-no-threshold model and the γ-H2AX foci data. This resulted in lifetime-attributable risk values of 1% and 4%, respectively, for the patient population under study.

Conclusions— γ-H2AX foci as a biomarker for DNA damage indicate that radiation risk estimates according to the linear-no-threshold hypothesis are possibly underestimates. Great care should be taken to minimize and optimize patient radiation exposure.

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.

Non-targeted effects of ionizing radiation: implications for risk assessment and the radiation dose response profile

Radiation risks at low doses remain a hotly debated topic. Recent experimental advances in our understanding of effects occurring in the progeny of irradiated cells, and/or the non-irradiated neighbors of irradiated cells (i.e., non-targeted effects associated with exposure to ionizing radiation), have influenced this debate. The goal of this document is to summarize the current status of this debate and speculate on the potential impact of non-targeted effects on radiation risk assessment and the radiation dose response profile.

Ionizing radiation-induced bystander mutagenesis and adaptation: quantitative and temporal aspects

This work explores several quantitative aspects of radiation-induced bystander mutagenesis in WTK1 human lymphoblast cells. Gamma-irradiation of cells was used to generate conditioned medium containing bystander signals, and that medium was transferred onto naïve recipient cells. Kinetic studies revealed that it required up to 1h to generate sufficient signal to induce the maximal level of mutations at the thymidine kinase locus in the bystander cells receiving the conditioned medium. Furthermore, it required at least 1h of exposure to the signal in the bystander cells to induce mutations. Bystander signal was fairly stable in the medium, requiring 12-24h to diminish. Medium that contained bystander signal was rendered ineffective by a 4-fold dilution; in contrast a greater than 20-fold decrease in the cell number irradiated to generate a bystander signal was needed to eliminate bystander-induced mutagenesis. This suggested some sort of feedback inhibition by bystander signal that prevented the signaling cells from releasing more signal. Finally, an ionizing radiation-induced adaptive response was shown to be effective in reducing bystander mutagenesis; in addition, low levels of exposure to bystander signal in the transferred medium induced adaptation that was effective in reducing mutations induced by subsequent gamma-ray exposures.

Bystander effects induced by diffusing mediators after photodynamic stress

The bystander effect, whereby cells that are not traversed by ionizing radiation exhibit various responses when in proximity to irradiated cells, is well documented in the field of radiation biology, Here we demonstrate that considerable bystander responses are also observed after photodynamic stress using the membrane-localizing dye deuteroporphyrin (DP). Using cells of a WTK1 human lymphoblastoid cell line in suspension and a transwell insert system that precludes contact between targeted and bystander cells, we have shown that the bystander signaling is mediated by diffusing species. The extranuclear localization of the photosensitizer used suggests that primary DNA damage is not the trigger for initiating these bystander responses, which include elevated oxidative stress, DNA damage (micronucleus formation), mutagenesis and decreased clonogenic survival. In addition, oxidative stress in the bystander population was reduced by the presence of the membrane antioxidant vitamin E in the targeted cells, suggesting that lipid peroxidation may play a key role in mediating these bystander effects. The fluence responses for these bystander effects are non-linear, with larger effects seen at lower fluences and toxicity to the target cell population. Hence, when considering outcomes of photodynamic action in cells and tissue, bystander effects may be significant, especially at sublethal fluences.

Real-time imaging of novel spatial and temporal responses to photodynamic stress

Cells subjected to various forms of stress have been shown to induce bystander responses in nontargeted cells, thus extending the stress response to a larger population. However, the mechanism(s) of bystander responses remains to be clearly identified, particularly for photodynamic stress. Oxidative stress and cell viability were studied on the spatial and temporal levels after photodynamic targeting of a subpopulation of EMT6 murine mammary cancer cells in a multiwell plate by computerized time-lapse fluorescence microscopy. In the targeted population a dose-dependent loss of cell viability was observed in accordance with increased oxidative stress. This was accompanied by increased oxidative stress in bystander populations but on different time scales, reaching a maximum more rapidly in targeted cells. Treatment with extracellular catalase, or the NADPH oxidase inhibitor diphenyleneiodinium, decreased production of reactive oxygen species (ROS) in both populations. These effects are ascribed to photodynamic activation of NADPH-oxidase in the targeted cells, resulting in a rapid burst of ROS formation with hydrogen peroxide acting as the signaling molecule responsible for initiation of these photodynamic bystander responses. The consequences of increased oxidative stress in bystander cells should be considered in the overall framework of photodynamic stress.

No evidence for DNA and early cytogenetic damage in bystander cells after heavy-ion microirradiation at two facilities

The occurrence of bystander effects has challenged the evaluation of risk for heavy ions, mainly in the context of space exploration and the increasing application of carbon ions in radiotherapy. In the present study, we addressed whether heavy-ion-induced DNA and cytogenetic damage is detectable in bystander cells. The formation of gamma-H2AX foci, sister chromatid exchanges and micronuclei were used as markers of damage to DNA. Normal human fibroblasts were exposed to low fluences of carbon and uranium ions, and alternatively single cells were targeted with heavy ions using the GSI microbeam. We did not observe a significant increase in the bystander formation of gamma-H2AX foci, sister chromatid exchanges or micronuclei. In addition, we performed for the first time parallel experiments at two microbeam facilities (GSI, JAEA) using the same cell line, culture conditions and irradiation protocols. No significant enhancement of the micronucleus frequencies in bystander cells was detected after targeted carbon-ion irradiation, confirming the results. Details regarding the history, culture conditions or support of the cells might be affecting the detection of bystander effects. On the other hand, the potential X-ray- and heavy-ion-induced bystander effects investigated herein clearly do not exceed the experimental error and thus are either lacking or are less pronounced than the effects reported in the literature for similar end points after alpha-particle and X-ray exposure.

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.

Role of DNA-PKcs in the bystander effect after low- or high-LET irradiation

PURPOSE

To investigate the role of the catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) in the medium-mediated bystander effect for chromosomal aberrations induced by low-linear energy transfer (LET) X-rays and high-LET heavy ions in normal human fibroblast cells.

MATERIALS AND METHODS

The recipient cells were treated for 12 h with conditioned medium, which was harvested from donor cells at 24 h after exposure to 10 Gy of soft X-rays (5 keV/microm) and 20Ne ions (437 keV/microm), followed by analyses of chromosome aberrations in recipient cells with premature chromosome condensation methods. To examine the role of DNA-PKcs and nitric oxide (NO), cells were treated with its inhibitor LY294002 (LY) and its scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (c-PTIO), respectively.

RESULTS

Increased frequency of chromosome aberrations in recipient cells treated with conditioned medium from irradiated but not from un-irradiated donor cells was observed which was independent of radiation type. Bystander induction of chromosome aberrations in recipient cells was mitigated when donor cells were treated with LY before irradiation and with c-PTIO after irradiation, and was enhanced when recipient cells were treated with LY before treatment of recipient cells with conditioned medium from irradiated donor cells.

CONCLUSION

Irradiated normal human cells secrete NO and other molecules which in turn transmit radiation signals to unirradiated bystander cells, leading to the induction of bystander chromosome aberrations partially repairable by DNA-PKcs-mediated DNA damage repair machinery, such as non-homologous end-joining repair pathways.

Radiation-induced bystander signalling in cancer therapy

Our understanding of how radiation kills normal and tumour cells has been based on an intimate knowledge of the direct induction of DNA damage and its cellular consequences. What has become clear is that, as well as responses to direct DNA damage, cell-cell signalling -- known as the bystander effect -- mediated through gap junctions and inflammatory responses may have an important role in the response of cells and tissues to radiation exposure and also chemotherapy agents. This Review outlines the key aspects of radiation-induced intercellular signalling and assesses its relevance for existing and future radiation-based therapies.

Chemical induction of the bystander effect in normal human lymphoblastoid cells

Many studies investigating the bystander effect have used ionizing radiation to evaluate this phenomenon, whereas very few have determined whether genotoxic chemicals are also capable of inducing this effect. Here, we show that two such chemicals, mitomycin C, a bifunctional alkylating agent and phleomycin, a glycopeptide antibiotic of the bleomycin family, cause normal human B lymphoblastoid cells to produce media soluble factors that induce a bystander effect in unexposed cells. Ionizing radiation was used in parallel experiments to verify the existence of the bystander effect in these cells. Micronuclei in Cytochalasin B-blocked binucleated cells were used as the endpoint. Conditioned media obtained from cells exposed to mitomycin C induced a 1.5-3 fold increase, while conditioned media from phleomycin induced a 1.5-4 fold increase, and conditioned media from irradiated cells induced a 2-8 fold increase in micronuclei. We conclude that the bystander effect is not restricted to ionizing radiation, suggesting it may be a part of a general cellular stress response.

Advances in radiobiological studies using a microbeam

Recent developments in microbeam technology have made drastic improvements in particle delivery, focusing, image processing and precision to allow for rapid advances in our knowledge in radiation biology. The unequivocal demonstration that targeted cytoplasmic irradiation results in mutations in the nuclei of hit cells and the presence of non-targeted effects, all made possible using a charged particle microbeam, results in a paradigm shift in our basic understanding of the target theory and other radiation-induced low dose effects. The demonstration of a bystander effect in 3D human tissue and whole organisms have shown the potential relevance of the non-targeted response in human health. The demonstration of delayed mutations in the progeny of bystander cells suggest that genomic instability induced following ionizing radiation exposure is not dependent on direct damage to cell nucleus. The identification of specific signaling pathways provides mechanistic insight on the nature of the bystander process.

Different involvement of radical species in irradiated and bystander cells

PURPOSE

To examine whether nitric oxide (NO) and other radical species are involved in radiation-induced bystander effects in normal human fibroblasts.

MATERIALS AND METHODS

Bystander effects were modeled by co-culture of non-irradiated cells with X-irradiated cells, and induction levels of micronuclei in co-cultured non-irradiated cells were examined. Three types of radical scavenger, 2-(4-carboxyphenyl)-4,4,5,5- tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO), dimethylsulfoxide (DMSO) and ascorbic acid phosphoric ester magnesium salt (APM), were used to discover which types of radicals are involved in bystander responses.

RESULTS

When irradiated cells were treated with c-PTIO, known to be an NO scavenger, the induction of micronuclei in non-irradiated bystander cells was suppressed. On the other hand, bystander effects were most effectively suppressed when non-irradiated bystander cells were treated with ascorbic acid, known to be a scavenger of long lived radicals.

CONCLUSION

These results suggest that NO participates in bystander signal formation in irradiated cells but not in bystander cells that are receiving bystander signals.

Death receptor pathways mediate targeted and non-targeted effects of ionizing radiations in breast cancer cells

Delayed cell death by mitotic catastrophe is a frequent mode of solid tumor cell death after γ-irradiation, a widely used treatment of cancer. Whereas the mechanisms that underlie the early γ-irradiation-induced cell death are well documented, those that drive the delayed cell death are largely unknown. Here we show that the Fas, tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and tumor necrosis factor (TNF)-α death receptor pathways mediate the delayed cell death observed after γ-irradiation of breast cancer cells. Early after irradiation, we observe the increased expression of Fas, TRAIL-R and TNF-R that first sensitizes cells to apoptosis. Later, the increased expression of FasL, TRAIL and TNF-α permit the apoptosis engagement linked to mitotic catastrophe. Treatments with TNF-α, TRAIL or anti-Fas antibody, early after radiation exposure, induce apoptosis, whereas the neutralization of the three death receptors pathways impairs the delayed cell death. We also show for the first time that irradiated breast cancer cells excrete soluble forms of the three ligands that can induce the death of sensitive bystander cells. Overall, these results define the molecular basis of the delayed cell death of irradiated cancer cells and identify the death receptors pathways as crucial actors in apoptosis induced by targeted as well as non-targeted effects of ionizing radiation.

Mechanism of radiation-induced bystander effects: a unifying model

The radiation-induced bystander effect represents a paradigm shift in our understanding of the radiobiological effects of ionizing radiation, in that extranuclear and extracellular events may also contribute to the final biological consequences of exposure to low doses of radiation. Although radiation-induced bystander effects have been well documented in a variety of biological systems, the mechanism is not known. It is likely that multiple pathways are involved in the bystander phenomenon, and different cell types respond differently to bystander signalling. Using cDNA microarrays, a number of cellular signalling genes, including cyclooxygenase-2 (COX-2), have been shown to be causally linked to the bystander phenomenon. The observation that inhibition of the phosphorylation of extracellular signal-related kinase (ERK) suppressed the bystander response further confirmed the important role of the mitogen-activated protein kinase (MAPK) signalling cascade in the bystander process. Furthermore, cells deficient in mitochondrial DNA showed a significantly reduced response to bystander signalling, suggesting a functional role of mitochondria in the signalling process. Inhibitors of nitric oxide (NO) synthase (NOS) and mitochondrial calcium uptake provided evidence that NO and calcium signalling are part of the signalling cascade. The bystander observations imply that the relevant target for various radiobiological endpoints is larger than an individual cell. A better understanding of the cellular and molecular mechanisms of the bystander phenomenon, together with evidence of their occurrence in-vivo, will allow us to formulate a more accurate model for assessing the health effects of low doses of ionizing radiation.

ATM acts downstream of ATR in the DNA damage response signaling of bystander cells

This study identifies ataxia-telangiectasia mutated (ATM) as a further component of the complex signaling network of radiation-induced DNA damage in nontargeted bystander cells downstream of ataxia-telangiectasia and Rad3-related (ATR) and provides a rationale for molecular targeted modulation of these effects. In directly irradiated cells, ATR, ATM, and DNA-dependent protein kinase (DNA-PK) deficiency resulted in reduced cell survival as predicted by the known important role of these proteins in sensing DNA damage. A decrease in clonogenic survival was also observed in ATR/ATM/DNA-PK-proficient, nonirradiated bystander cells, but this effect was completely abrogated in ATR and ATM but not DNA-PK-deficient bystander cells. ATM activation in bystander cells was found to be dependent on ATR function. Furthermore, the induction and colocalization of ATR, 53BP1, ATM-S1981P, p21, and BRCA1 foci in nontargeted cells was shown, suggesting their involvement in bystander DNA damage signaling and providing additional potential targets for its modulation. 53BP1 bystander foci were induced in an ATR-dependent manner predominantly in S-phase cells, similar to gammaH2AX foci induction. In conclusion, these results provide a rationale for the differential modulation of targeted and nontargeted effects of radiation.

Radiobiological characterization of proton beam at the National Cancer Center in Korea

Estimation of the relative biological effectiveness (RBE) of the proton beam at the National Cancer Center Proton Therapy Center in Korea (NCCPTC) is required clinically for the treatment of cancer. The proton beam was fixed at 190 MeV with 6 cm a spread out Bragg peaks (SOBP) for which corresponds to most frequent clinical condition. The RBE was estimated from the survival of human salivary gland (HSG) cells using the traditional colonogenic and MTT assays. The HSG cells were also irradiated in a cell-stack chamber and monitored for survival to identify whether the characteristic depth-dependent survival pattern was observed. The RBE of the NCCPTC was estimated to be 1.024 +/- 0.007 and 1.049 +/- 0.028 at the middle of SOBP using colonogenic and MTT assays, respectively. Further analysis of the biological response of proton exposure revealed no difference compared to conventional X-ray treatment in western blot, and FACS analysis. The proton beam of the NCCPTC also exhibited the characteristic depth-dependent survival pattern. The estimated RBE value of NCCPTC was slightly smaller than generic RBE value of 1.1 for protons of the majority of centers. Due to the recommendation of a generic RBE of 1.1 for protons, a representative RBE value of 1.1 was assigned for clinical application for proton beams at the NCCPTC.