ATR-dependent radiation-induced γH2AX foci in bystander primary human astrocytes and glioma cells

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.

Evidence of an adaptive response targeting DNA nonhomologous end joining and its transmission to bystander cells

Adaptive response (AR) is a term describing resistance to ionizing radiation-induced killing or formation of aberrant chromosomes that is mediated by pre-exposure to low ionizing radiation doses. The mechanism of AR remains elusive. Because cell killing and chromosome aberration formation derive from erroneous processing of DNA double-strand breaks (DSB), AR may reflect a modulation of DSB processing by nonhomologous end joining (NHEJ) or homologous recombination repair. Here, we use plasmid end-joining assays to quantify modulations induced by low ionizing radiation doses to NHEJ, the dominant pathway of DSB repair in higher eukaryotes, and investigate propagation of this response through medium transfer to nonirradiated bystander cells. Mouse embryo fibroblasts were conditioned with 10 to 1000 mGy and NHEJ quantified at different times thereafter by challenging with reporter plasmids containing a DSB. We show robust increases in NHEJ efficiency in mouse embryo fibroblasts exposed to ionizing radiation >100 mGy, irrespective of reporter plasmid used. Human tumor cells also show AR of similar magnitude that is compromised by caffeine, an inhibitor of DNA damage signaling acting by inhibiting ATM, ATR, and DNA-PKcs. Growth medium from pre-irradiated cells induces a caffeine-sensitive AR in nonirradiated cells, similar in magnitude to that seen in irradiated cells. In bystander cells, γH2AX foci are specifically detected in late S-G(2) phase and are associated with Rad51 foci that signify the function of homologous recombination repair, possibly on DNA replication-mediated DSBs. The results point to enhanced NHEJ as a mechanism of AR and suggest that AR may be transmitted to bystander cells through factors generating replication-mediated DSBs.

Non-targeted effects of ionising radiation and radiotherapy

Modern radiobiology is undergoing rapid change due to new discoveries contradicting the target concept which is currently used to predict dose-response relationships. Thus relatively recently discovered radiation-induced bystander effects (RIBEs), that include additional death, mutation and radio-adaptation in non-irradiated cells, change our understanding of the target concept and broadens its boundaries. This can be significant from a radioprotection point of view and also has the potential to reassess radiation damage models currently used in radiotherapy. This article reviews briefly the general concepts of RIBEs such as the proposed underlying mechanisms of signal induction and propagation, experimental approaches and biological end points used to investigate these phenomena. It also summarises several mathematical models currently proposed in an attempt to quantify RIBE. The main emphasis of this article is to review and highlight the potential impact of the bystander phenomena in radiotherapy.

Tumors induce complex DNA damage in distant proliferative tissues in vivo

That tumors cause changes in surrounding tissues is well documented, but whether they also affect distant tissues is uncertain. Such knowledge may be important in understanding the relationship between cancer and overall patient health. To address this question, we examined tissues distant to sites of implanted tumors for genomic damage using cohorts of C57BL/6 and BALB/c mice with early-stage subcutaneous syngeneic grafts, specifically, B16 melanoma, MO5076 sarcoma, and COLON26 carcinoma. Here we report that levels of two serious types of DNA damage, double-strand breaks (DSBs) measured by γ-H2AX focus formation and oxidatively induced non-DSB clustered DNA lesions (OCDLs), were elevated in tissues distant from the tumor site in tumor-bearing mice compared with their age- and sex-matched controls. Most affected were crypts in the gastrointestinal tract organs and skin, both highly proliferative tissues. Further investigation revealed that, compared with controls, tumor-bearing mice contained elevated amounts of activated macrophages in the distant gastrointestinal tissues, as well as elevated serum levels of several cytokines. One of these cytokines, CCL2/MCP-1, has been linked to several inflammation-related conditions and macrophage recruitment, and strikingly, CCL2-deficient mice lacked increased levels of DSBs and OCDLs in tissues distant from implanted tumors. Thus, this study is unique in being a direct demonstration that the presence of a tumor may induce a chronic inflammatory response in vivo, leading to increased systemic levels of DNA damage. Importantly, these findings suggest that tumors may have more profound effects on their hosts than heretofore expected.

Induction of bystander response in human glioma cells using high-energy electrons: a role for TGF-beta1

We examined bystander cell death produced in T98G cells by exposure to irradiated cell conditioned medium (ICCM) produced by high-energy 20 MeV electrons at a dose rate of 10 Gy min(-1) and doses up to 20 Gy. ICCM induced a bystander response in T98G glioma cells, reducing recipient cell survival by more than 25% below controls at 5 and 10 Gy. Higher doses increased survival to near control levels. ICCM was analyzed for the presence of transforming growth factor alpha (TGF-alpha) and transforming growth factor beta1 (TGF-beta1). Monoclonal antibodies for TGF-alpha (mAb TGF-alpha) and TGF-beta1 (mAb TGF-beta1) were added to the ICCM to neutralize any potential effect of the cytokines. The results indicate that TGF-alpha was not present in the ICCM and addition of mAb TGF-alpha to the ICCM had no effect on bystander cell survival. No active TGF-beta1 was present in the ICCM; however, addition of mAb TGF-beta1 completely abolished bystander death of reporter cells at all doses. These results indicate that bystander cell death can be induced in T98G glioma if a large enough radiation stress is applied and that TGF-beta1 plays a downstream role in this response.

Non-targeted effects as a paradigm breaking evidence

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

Role of oxidatively induced DNA lesions in human pathogenesis

Genome stability is essential for maintaining cellular and organismal homeostasis, but it is subject to many threats. One ubiquitous threat is from a class of compounds known as reactive oxygen species (ROS), which can indiscriminately react with many cellular biomolecules including proteins, lipids, and DNA to produce a variety of oxidative lesions. These DNA oxidation products are a direct risk to genome stability, and of particular importance are oxidative clustered DNA lesions (OCDLs), defined as two or more oxidative lesions present within 10 bp of each other. ROS can be produced by exposure of cells to exogenous environmental agents including ionizing radiation, light, chemicals, and metals. In addition, they are produced by cellular metabolism including mitochondrial ATP generation. However, ROS also serve a variety of critical cellular functions and optimal ROS levels are maintained by multiple cellular antioxidant defenses. Oxidative DNA lesions can be efficiently repaired by base excision repair or nucleotide excision repair. If ROS levels increase beyond the capacity of its antioxidant defenses, the cell's DNA repair capacity can become overwhelmed, leading to the accumulation of oxidative DNA damage products including OCDLs, which are more difficult to repair than individual isolated DNA damage products. Here we focus on the induction and repair of OCDLs and other oxidatively induced DNA lesions. If unrepaired, these lesions can lead to the formation of mutations, DNA DSBs, and chromosome abnormalities. We discuss the roles of these lesions in human pathologies including aging and cancer, and in bystander effects.

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.

Cohesin promotes the repair of ionizing radiation-induced DNA double-strand breaks in replicated chromatin

The cohesin protein complex holds sister chromatids together after synthesis until mitosis. It also contributes to post-replicative DNA repair in yeast and higher eukaryotes and accumulates at sites of laser-induced damage in human cells. Our goal was to determine whether the cohesin subunits SMC1 and Rad21 contribute to DNA double-strand break repair in X-irradiated human cells in the G2 phase of the cell cycle. RNA interference-mediated depletion of SMC1 sensitized HeLa cells to X-rays. Repair of radiation-induced DNA double-strand breaks, measured by gammaH2AX/53BP1 foci analysis, was slower in SMC1- or Rad21-depleted cells than in controls in G2 but not in G1. Inhibition of the DNA damage kinase DNA-PK, but not ATM, further inhibited foci loss in cohesin-depleted cells in G2. SMC1 depletion had no effect on DNA single-strand break repair in either G1 or late S/G2. Rad21 and SMC1 were recruited to sites of X-ray-induced DNA damage in G2-phase cells, but not in G1, and only when DNA damage was concentrated in subnuclear stripes, generated by partially shielded ultrasoft X-rays. Our results suggest that the cohesin complex contributes to cell survival by promoting the repair of radiation-induced DNA double-strand breaks in G2-phase cells in an ATM-dependent pathway.

Membrane-dependent bystander effect contributes to amplification of the response to alpha-particle irradiation in targeted and nontargeted cells

PURPOSE

Free radicals are believed to play an active role in the bystander response. This study investigated their origin as well as their temporal and spatial impacts in the bystander effect.

METHODS AND MATERIALS

We employed a precise alpha-particle microbeam to target a small fraction of subconfluent osteoblastic cells (MC3T3-E1). gammaH2AX-53BP1 foci, oxidative metabolism changes, and micronuclei induction in targeted and bystander cells were assessed.

RESULTS

Cellular membranes and mitochondria were identified as two distinct reactive oxygen species producers. The global oxidative stress observed after irradiation was significantly attenuated after cells were treated with filipin, evidence for the primal role of membrane in the bystander effect. To determine the membrane's impact at a cellular level, micronuclei yield was measured when various fractions of the cell population were individually targeted while the dose per cell remained constant. Induction of micronuclei increased in bystander cells as well as in targeted cells and was attenuated by filipin treatment, demonstrating a role for bystander signals between irradiated cells in an autocrine/paracrine manner.

CONCLUSIONS

A complex interaction of direct irradiation and bystander signals leads to a membrane-dependent amplification of cell responses that could influence therapeutic outcomes in tissues exposed to low doses or to environmental exposure.

Ionizing radiation-induced bystander effects, potential targets for modulation of radiotherapy

Cells exposed to ionizing radiation show DNA damage, apoptosis, chromosomal aberrations or increased mutation frequency and for a long time it was generally accepted that these effects resulted from ionization of cell structures and the action of reactive oxygen species formed by water radiolysis. In the last few years, however, it has appeared that cells exposed to ionizing radiation and other genotoxic agents can release signals that induce very similar effects in non-targeted neighboring cells, phenomena known as bystander effects. These signals are transmitted to the neighboring non-hit cells by intercellular gap-junction communication or are released outside the cell, in the case of cultured cells into the medium. The signaling is mutual, and irradiated cells can also receive signals from non-irradiated neighbors. Most experiments show a decrease in survival of unirradiated bystander cells, but some studies of the influence of unirradiated or low dose-irradiated cells on those irradiated with higher doses show that intercellular bystander signaling can also increase the survival of irradiated cell populations. In the last few years, communication between irradiated and non-irradiated cells has attracted interest in many studies as a possible target for modulation of radiotherapy. Understanding the mechanisms underlying bystander effects is important for radiation risk assessment and for evaluation of protocols for cancer radiotherapy. In this review we describe different aspects of ionizing radiation-induced bystander effects: experimental examples, types of DNA damage, situations in vivo, and their possible role in adaptive response to irradiation, and we discuss their possible significance for anticancer therapy.

New molecular targets in radiotherapy: DNA damage signalling and repair in targeted and non-targeted cells

Ionising radiation plays a key role in therapy due to its ability to directly induce DNA damage, in particular DNA double-strand breaks leading to cell death. Cells have multiple repair pathways which attempt to maintain genomic stability. DNA repair proteins have become key targets for therapy, using small molecule inhibitors, in combination with radiation and or chemotherapeutic agents as a means of enhancing cell killing. Significant advances in our understanding of the response of cells to radiation exposures has come from the observation of non-targeted effects where cells respond via mechanisms other than those which are a direct consequence of energy-dependent DNA damage. Typical of these is bystander signalling where cells respond to the fact that their neighbours have been irradiated. Bystander cells show a DNA damage response which is distinct from directly irradiated cells. In bystander cells, ATM- and Rad3-related (ATR) protein kinase-dependent signalling in response to stalled replication forks is an early event in the DNA damage response. The ATM protein kinase is activated downstream of ATR in bystander cells. This offers the potential for differential approaches for the modulation of bystander and direct effects with repair inhibitors which may impact on the response of tumours and on the protection of normal tissues during radiotherapy.

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.

Intercellular communication of cellular stress monitored by gamma-H2AX induction

When cells are exposed to ionizing radiation (IR), unexposed cells that share media with damaged cells exhibit similar effects to irradiated cells including increased levels of DNA double-strand breaks (DSBs). Hypothesizing that this effect, known as the radiation-induced bystander effect, may be a specific instance of communication between damaged and undamaged cells regardless of damage source, we demonstrated that exposure of target cells to non-IR induces bystander damage in non-targeted cells as measured by gamma-H2AX and 53BP1 focal formation. Initially, bystander damage was found primarily in S-phase cells, but at later times, non-S-phase cells were also affected. In addition, media from undamaged malignant and senescent cells also was found to induce DSBs in primary cultures. Media conditioned on cells targeted with either ionizing or non-IR as well as on undamaged malignant and senescent cells contained elevated levels of several cytokines. One of these, transforming growth factor beta (TGF-beta), and nitric oxide (NO) were found to elevate numbers of gamma-H2AX/53BP1 foci in normal cell cultures similar to levels found in bystander cells, and this elevation was abrogated by NO synthase inhibitors, TGF-beta blocking antibody and antioxidants. These findings support the hypothesis that damage in bystander cells results from their exposure to cytokines or reactive compounds released from stressed cells, regardless of damage source. These results have implications for oncogenesis in that they indicate that damaged normal cells or undamaged tumor cells may induce genomic instability, leading to an increased risk of oncogenic transformation in other cells with which they share media or contact directly.

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.

Radiation and chemotherapy bystander effects induce early genomic instability events: telomere shortening and bridge formation coupled with mitochondrial dysfunction

The bridge breakage fusion cycle is a chromosomal instability mechanism responsible for genomic changes. Radiation bystander effects induce genomic instability; however, the mechanism driving this instability is unknown. We examined if radiation and chemotherapy bystander effects induce early genomic instability events such as telomere shortening and bridge formation using a human colon cancer explant model. We assessed telomere lengths, bridge formations, mitochondrial membrane potential and levels of reactive oxygen species in bystander cells exposed to medium from irradiated and chemotherapy-treated explant tissues. Bystander cells exposed to media from 2Gy, 5Gy, FOLFOX treated tumor and matching normal tissue showed a significant reduction in telomere lengths (all p values <0.018) and an increase in bridge formations (all p values <0.017) compared to bystander cells treated with media from unirradiated tissue (0Gy) at 24h. There was no significant difference between 2Gy and 5Gy treatments, or between effects elicited by tumor versus matched normal tissue. Bystander cells exposed to media from 2Gy irradiated tumor tissue showed significant depolarisation of the mitochondrial membrane potential (p=0.012) and an increase in reactive oxygen species levels. We also used bystander cells overexpressing a mitochondrial antioxidant manganese superoxide dismutase (MnSOD) to examine if this antioxidant could rescue the mitochondrial changes and subsequently influence nuclear instability events. In MnSOD cells, ROS levels were reduced (p=0.02) and mitochondrial membrane potential increased (p=0.04). These events were coupled with a decrease in percentage of cells with anaphase bridges and a decrease in the number of cells undergoing telomere length shortening (p values 0.01 and 0.028 respectively). We demonstrate that radiation and chemotherapy bystander responses induce early genomic instability coupled with defects in mitochondrial function. Restoring mitochondrial function through overexpression of MnSOD significantly rescues nuclear instability events; anaphase bridges and telomere length shortening.

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.

H2AX is required for cell cycle arrest via the p53/p21 pathway

Phosphorylation of H2AX (gammaH2AX) is an early sign of DNA damage induced by replication stalling. However, the role of H2AX in the repair of this type of DNA damage is still unclear. In this study, we used an inactivated adeno-associated virus (AAV) to induce a stalled replication fork signal and investigate the function of gammaH2AX. The cellular response to AAV provides a unique model to study gammaH2AX function, because the infection causes pannuclear H2AX phosphorylation without any signs of damage to the host genome. We found that pannuclear gammaH2AX formation is a result of ATR overactivation and diffusion but is independent of ATM. The inhibition of H2AX with RNA interference or the use of H2AX-deficient cells showed that gammaH2AX is dispensable for the formation and maintenance of DNA repair foci induced by stalled replication. However, in the absence of H2AX, the AAV-containing cells showed proteosome-dependent degradation of p21, followed by caspase-dependent mitotic catastrophe. In contrast, H2AX-proficient cells as well as H2AX-complemented H2AX(-/-) cells reacted by increasing p21 levels and arresting the cell cycle. The results establish a new role for H2AX in the p53/p21 pathway and indicate that H2AX is required for p21-induced cell cycle arrest after replication stalling.

A new paradigm in radioadaptive response developing from microbeam research

A classic paradigm in radiation biology asserts that all radiation effects on cells, tissues and organisms are due to the direct action of radiation on living tissue. Using this model, possible risks from exposure to low dose ionizing radiation (below 100 mSv) are estimated by extrapolating from data obtained after exposure to higher doses of radiation, using a linear non-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. These important responses to low dose/low dose-rate radiation are the radiation-induced adaptive response, the bystander response, low-dose hypersensitivity, and genomic instability. The mechanisms underlying these responses often involve biochemical and molecular signals generated in response to targeted and non-targeted events. In order to define and understand the bystander response to provide a basis for the understanding of non-targeted events and to elucidate the mechanisms involved, recent sophisticated research has been conducted with X-ray microbeams and charged heavy particle microbeams, and these studies have produced many new observations. Based on these observations, associations have been suggested to exist between the radioadaptive and bystander responses. The present review focuses on these two phenomena, and summarizes observations supporting their existence, and discusses the linkage between them in light of recent results obtained from experiments utilizing microbeams.

Consequences of cytoplasmic irradiation: studies from microbeam

The prevailing dogma for radiation biology is that genotoxic effects of ionizing radiation such as mutations and carcinogenesis are attributed mainly to direct damage to the nucleus. However, with the development of microbeam that can target precise positions inside the cells, accumulating evidences have shown that energy deposit by radiation in nuclear DNA is not required to trigger the damage, extra-nuclear or extra-cellular radiation could induce the similar biological effects as well. This review will summarize the biological responses after cytoplasm irradiated by microbeam, and the possible mechanisms involved in cytoplasmic irradiation.

Bystander effects induced by continuous low-dose-rate 125I seeds potentiate the killing action of irradiation on human lung cancer cells in vitro

PURPOSE

To investigate bystander effects of low-dose-rate (LDR) (125)I seed irradiation on human lung cancer cells in vitro.

METHODS AND MATERIALS

A549 and NCI-H446 cell lines of differing radiosensitivity were directly exposed to LDR (125)I seeds irradiation for 2 or 4 Gy and then cocultured with nonirradiated cells for 24 hours. Induction of micronucleus (MN), gammaH2AX foci, and apoptosis were assayed.

RESULTS

After 2 and 4 Gy irradiation, micronucleus formation rate (MFR) and apoptotic rate of A549 and NCI-H446 cells were increased, and the MFR and apoptotic rate of NCI-H446 cells was 2.1-2.8 times higher than that of A549 cells. After coculturing nonirradiated bystander cells with (125)I seed irradiated cells for 24 hours, MFR and the mean number of gammaH2AX foci/cells of bystander A549 and NCI-H446 cells were similar and significantly higher than those of control (p <0.05), although they did not increase with irradiation dose. However, the proportion of bystander NCI-H446 cells with MN numbers >/=3 and gammaH2AX foci numbers 15-19 and 20-24 was higher than that of bystander A549 cells. In addition, dimethyl sulfoxide (DMSO) treatment could completely suppress the bystander MN of NCI-H446 cells, but it suppressed only partly the bystander MN of A549 cells, indicating that reactive oxygen species are involved in the bystander response to NCI-H446 cells, but other signaling factors may contribute to the bystander response of A549 cells.

CONCLUSIONS

Continuous LDR irradiation of (125)I seeds could induce bystander effects, which potentiate the killing action on tumor cells and compensate for the influence of nonuniform distribution of radiation dosage on therapeutic outcomes.

The role of nonhomologous end joining and homologous recombination in the clonogenic bystander effects of mammalian cells after exposure to counted 10 MeV protons and 4.5 MeV alpha-particles of the PTB microbeam

We have studied the dependence of clonogenic bystander effects on defects in the pathways of DNA double-strand break (DSB) repair and on linear energy transfer (LET). The single-ion microbeam of the Physikalisch-Technische Bundesanstalt (PTB) was used to irradiate parental Chinese hamster ovary cells or derivatives deficient in nonhomologous end joining (NHEJ) or homologous recombination (HR) in the G1-phase of the cell cycle. Cell nuclei were targeted with 10 MeV protons (LET = 4.7 keV/microm) or 4.5 MeV alpha-particles (LET = 100 keV/microm). During exposure, the cells were confluent, allowing signal transfer through both gap junctions and diffusion. When all cell nuclei were targeted with 10 MeV protons, approximately exponential survival curves were obtained for all three cell lines. When only 10% of all cell nuclei were targeted, a significant bystander effect was observed for parental and HR-deficient cells, but not for NHEJ-deficient cells. For all three cell lines, the survival data after exposure of all cell nuclei to 4.5 MeV alpha-particles could be fitted by exponential curves. When only 10% of all cell nuclei were targeted, significant bystander effects were obtained for parental and HR-deficient cells, whereas for NHEJ-deficient cells a small, but significant, bystander effect was observed only at higher doses. The data suggest that bystander cell killing is a consequence of un- or misrejoined DSB which occur in bystander cells during the S-phase as a result of the processing of oxidative bistranded DNA lesions. The relative contributions of NHEJ and HR to the repairing of DSB in the late S/G2-phase may affect clonogenic bystander effects.

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.

Estrogen enhanced cell-cell signalling in breast cancer cells exposed to targeted irradiation

Background

Radiation-induced bystander responses, where cells respond to their neighbours being irradiated are being extensively studied. Although evidence shows that bystander responses can be induced in many types of cells, it is not known whether there is a radiation-induced bystander effect in breast cancer cells, where the radiosensitivity may be dependent on the role of the cellular estrogen receptor (ER). This study investigated radiation-induced bystander responses in estrogen receptor-positive MCF-7 and estrogen receptor-negative MDA-MB-231 breast cancer cells.

Methods

The influence of estrogen and anti-estrogen treatments on the bystander response was determined by individually irradiating a fraction of cells within the population with a precise number of helium-3 using a charged particle microbeam. Damage was scored as chromosomal damage measured as micronucleus formation.

Results

A bystander response measured as increased yield of micronucleated cells was triggered in both MCF-7 and MDA-MB-231 cells. The contribution of the bystander response to total cell damage in MCF-7 cells was higher than that in MDA-MB-231 cells although the radiosensitivity of MDA-MB-231 was higher than MCF-7. Treatment of cells with 17β-estradiol (E2) increased the radiosensitivity and the bystander response in MCF-7 cells, and the effect was diminished by anti-estrogen tamoxifen (TAM). E2 also increased the level of intracellular reactive oxygen species (ROS) in MCF-7 cells in the absence of radiation. In contrast, E2 and TAM had no influence on the bystander response and ROS levels in MDA-MB-231 cells. Moreover, the treatment of MCF-7 cells with antioxidants eliminated both the E2-induced ROS increase and E2-enhanced bystander response triggered by the microbeam irradiation, which indicates that ROS are involved in the E2-enhanced bystander micronuclei formation after microbeam irradiation.

Conclusion

The observation of bystander responses in breast tumour cells may offer new potential targets for radiation-based therapies in the treatment of breast cancer.

HIV-1 Tat and AIDS-associated cancer: targeting the cellular anti-cancer barrier?

The acquired immunodeficiency syndrome (AIDS) is accompanied by a significant increase in the incidence of neoplasms. Several causative agents have been proposed for this phenomenon. These include immunodeficiency and oncogenic DNA viruses and the HIV-1 protein Tat. Cancer in general is closely linked to genomic instability and DNA repair mechanisms. The latter maintains genomic stability and serves as a cellular anti-cancer barrier. Defects in DNA repair pathway are associated with carcinogenesis.

This review focuses on newly discovered connections of the HIV-1 protein Tat, as well as cellular co-factors of Tat, to double-strand break DNA repair. We propose that the Tat-induced DNA repair deficiencies may play a significant role in the development of AIDS-associated cancer.

Mitochondria-dependent signalling pathway are involved in the early process of radiation-induced bystander effects

Bystander effects induced by cytoplasmic irradiation have been reported recently. However, the mechanism(s) underlying, such as the functional role of mitochondria, is not clear. In the present study, we used either mtDNA-depleted (ρ⁰) A_L or normal (ρ⁺) A_L cells as irradiated donor cells and normal human skin fibroblasts as receptor cells in a series of medium transfer experiments to investigate the mitochondria-related signal process. Our results indicated that mtDNA-depleted cells or normal A_L cells treated with mitochondrial respiratory chain function inhibitors had an attenuated γ-H2AX induction, which indicates that mitochondria play a functional role in bystander effects. Moreover, it was found that treatment of normal A_L donor cells with specific inhibitors of NOS, or inhibitor of mitochondrial calcium uptake (ruthenium red) significantly decreased γ-H2AX induction and that radiation could stimulate cellular NO and O₂^•− production in irradiated ρ⁺ A_L cells, but not in ρ⁰ A_L cells. These observations, together with the findings that ruthenium red treatment significantly reduced the NO and O₂^•− levels in irradiated ρ⁺ A_L cells, suggest that radiation-induced NO derived from mitochondria might be an intracellular bystander factor and calcium-dependent mitochondrial NOS might play an essential role in the process.

Expression profiles are different in carbon ion-irradiated normal human fibroblasts and their bystander cells

Evidence has accumulated that ionizing radiation induces biological effects in non-irradiated bystander cells having received signals from directly irradiated cells; however, energetic heavy ion-induced bystander response is incompletely characterized. Here we performed microarray analysis of irradiated and bystander fibroblasts in confluent cultures. To see the effects in bystander cells, each of 1, 5 and 25 sites was targeted with 10 particles of carbon ions (18.3 MeV/u, 103 keV/microm) using microbeams, where particles traversed 0.00026, 0.0013 and 0.0066% of cells, respectively. diated cells, cultures were exposed to 10% survival dose (D), 0.1D and 0.01D of corresponding broadbeams (108 keV/microm). Irrespective of the target numbers (1, 5 or 25 sites) and the time (2 or 6h postirradiation), similar expression changes were observed in bystander cells. Among 874 probes that showed more than 1.5-fold changes in bystander cells, 25% were upregulated and the remainder downregulated. These included genes related to cell communication (PIK3C2A, GNA13, FN1, ANXA1 and IL1RAP), stress response (RAD23B, ATF4 and EIF2AK4) and cell cycle (MYCN, RBBP4 and NEUROG1). Pathway analysis revealed serial bystander activation of G protein/PI-3 kinase pathways. Instead, genes related to cell cycle or death (CDKN1A, GADD45A, NOTCH1 and BCL2L1), and cell communication (IL1B, TCF7 and ID1) were upregulated in irradiated cells, but not in bystander cells. Our results indicate different expression profiles in irradiated and bystander cells, and imply that intercellular signaling between irradiated and bystander cells activate intracellular signaling, leading to the transcriptional stress response in bystander cells.

Epigenetic changes and nontargeted radiation effects--is there a link?

It is now well accepted that the effects of ionizing radiation (IR) exposure can be noticed far beyond the borders of the directly irradiated tissue. IR can affect neighboring cells in the proximity, giving rise to a bystander effect. IR effects can also span several generations and influence the progeny of exposed parents, leading to transgeneration effects. Bystander and transgeneration IR effects are linked to the phenomenon of the IR-induced genome instability that manifests itself as chromosome aberrations, gene mutations, late cell death, and aneuploidy. While the occurrence of the above-mentioned phenomena is well documented, the exact mechanisms that lead to their development have still to be delineated. Evidence suggests that the IR-induced genome instability, bystander, and transgeneration effects may be epigenetically mediated. The epigenetic changes encompass DNA methylation, histone modification, and RNA-associated silencing. Recent studies demonstrated that IR exposure alters epigenetic parameters in the directly exposed tissues and in the distant bystander tissues. Transgeneration radiation effects were also proposed to be of an epigenetic nature. We will discuss the role of the epigenetic mechanisms in radiation responses, bystander effects, and transgeneration effects.

Effect of dose rate on the radiation-induced bystander response

Radiation-induced biological bystander effects have become a well-established phenomenon associated with the interaction of radiation with cells. These so-called bystander effects have been seen across a variety of end points for both high and low linear energy transfer (LET) radiations, utilizing a variety of dose rates and radiation sources. In this study, the effect of dose rate and different low LET sources on the bystander cell survival fraction (SF) was examined. The cell line investigated was the human keratinocyte HPV-G. The bystander response was measured via clonogenic assay after medium transfer protocol. Cells were irradiated using (60)Co gamma-rays and 20 MeV electrons at doses of 0.5, 5 and 10 Gy with varying dose rates. Both gamma and electron irradiation decreased recipient SF at 0.5 Gy and 5 Gy, respectively. Subsequent recovery of the SF to control levels for 10 Gy was observed. There was no dose rate dependence for (60)Co irradiation. A significant difference in the survival fraction was observed for electron irradiation at 10 Gy and a high dose rate. Furthermore, survival fractions were compared between (60)Co and 20 MeV electron irradiations. This showed a significant increase in the survival fraction 'recovery' at 10 Gy for a (60)Co dose rate of 1.1 Gy min(-1) compared to 20 MeV electrons at 1.0 Gy min(-1). No such difference was observed when comparing at higher dose rates. Lastly, increases in survival fraction at 10 Gy were abolished and the SF decreased by the plating of increased numbers of recipient cells. Such evidence may help gain insight into the nature and mechanism(s) surrounding bystander signal production, how these phenomena are tested and their eventual application in a clinical setting.

Vascular endothelial growth factor in astroglioma stem cell biology and response to therapy

Malignant astrogliomas are among the most aggressive, highly vascular and infiltrating tumours bearing a dismal prognosis, mainly due to their resistance to current radiation treatment and chemotherapy. Efforts to identify and target the mechanisms that underlie astroglioma resistance have recently focused on candidate cancer stem cells, their biological properties, interplay with their local microenvironment or 'niche', and their role in tumour progression and recurrence. Both paracrine and autocrine regulation of astroglioma cell behaviour by locally produced cytokines such as the vascular endothelial growth factor (VEGF) are emerging as key factors that determine astroglioma cell fate. Here, we review these recent rapid advances in astroglioma research, with emphasis on the significance of VEGF in astroglioma stem-like cell biology. Furthermore, we highlight the unique DNA damage checkpoint properties of the CD133-marker-positive astroglioma stem-like cells, discuss their potential involvement in astroglioma radioresistance, and consider the implications of this new knowledge for designing combinatorial, more efficient therapeutic strategies.

Cytoplasmic irradiation induces mitochondrial-dependent 53BP1 protein relocalization in irradiated and bystander cells

The accepted paradigm for radiation effects is that direct DNA damage via energy deposition is required to trigger the downstream biological consequences. The radiation-induced bystander effect is the ability of directly irradiated cells to interact with their nonirradiated neighbors, which can then show responses similar to those of the targeted cells. p53 binding protein 1 (53BP1) forms foci at DNA double-strand break sites and is an important sensor of DNA damage. This study used an ionizing radiation microbeam approach that allowed us to irradiate specifically the nucleus or cytoplasm of a cell and quantify response in irradiated and bystander cells by studying ionizing radiation-induced foci (IRIF) formation of 53BP1 protein. Our results show that targeting only the cytoplasm of a cell is capable of eliciting 53BP1 foci in both hit and bystander cells, independently of the dose or the number of cells targeted. Therefore, direct DNA damage is not required to trigger 53BP1 IRIF. The use of common reactive oxygen species and reactive nitrogen species (RNS) inhibitors prevent the formation of 53BP1 foci in hit and bystander cells. Treatment with filipin to disrupt membrane-dependent signaling does not prevent the cytoplasmic irradiation-induced 53BP1 foci in the irradiated cells, but it does prevent signaling to bystander cells. Active mitochondrial function is required for these responses because pseudo-rho(0) cells, which lack mitochondrial DNA, could not produce a bystander signal, although they could respond to a signal from normal rho+ cells.

The Time Dependence of Bystander Responses Induced by Iron-Ion Radiation in Normal Human Skin Fibroblasts

Although bystander effects have been shown for some high-LET radiations, few studies have been done on bystander effects induced by heavy-ion radiation. In this study, using a Transwell insert co-culture system, we have demonstrated that irradiation with 1 GeV/nucleon iron ions can induce medium-mediated bystander effects in normal AG01522 human fibroblasts. When irradiated and unirradiated bystander cells were combined in shared medium immediately after irradiation, a two- to threefold increase in the percentage of bystander cells with gamma-H2AX foci occurred as early as 1 h after irradiation and lasted at least 24 h. There was a twofold increase in the formation of micronuclei in bystander cells when they were co-cultured with irradiated cells immediately or 1 or 3 h after irradiation, but there was no bystander effect when the cells were co-cultured 6 h or later after irradiation. In addition, bystander micronucleus formation was observed even when the bystander cells were co-cultured with irradiated cells for only 1 h. This indicates that the crucial signaling to bystander cells from irradiated cells occurs shortly after irradiation. Moreover, both gamma-H2AX focus formation and micronucleus formation in bystander cells were inhibited by the ROS scavengers SOD or catalase or the NO scavenger PTIO. This suggests that ROS and NO play important roles in the initiation of bystander effects. The results with iron ions were similar to those with X rays, suggesting that the bystander responses in this system are independent of LET.

Energetic heavy ions accelerate differentiation in the descendants of irradiated normal human diploid fibroblasts

Ionizing radiation-induced genomic instability has been demonstrated in a variety of endpoints such as delayed reproductive death, chromosome instability and mutations, which occurs in the progeny of survivors many generations after the initial insult. Dependence of these effects on the linear energy transfer (LET) of the radiation is incompletely characterized; however, our previous work has shown that delayed reductions in clonogenicity can be most pronounced at LET of 108 keV/microm. To gain insight into potential cellular mechanisms involved in LET-dependent delayed loss of clonogenicity, we investigated morphological changes in colonies arising from normal human diploid fibroblasts exposed to gamma-rays or energetic carbon ions (108 keV/microm). Exposure of confluent cultures to carbon ions was 4-fold more effective at inactivating cellular clonogenic potential and produced more abortive colonies containing reduced number of cells per colony than gamma-rays. Second, colonies were assessed for clonal morphotypic heterogeneity. The yield of differentiated cells was elevated in a dose- and LET-dependent fashion in clonogenic colonies, whereas differentiated cells predominated to a comparable extent irrespective of radiation type or dose in abortive colonies. The incidence of giant or multinucleated cells was also increased but much less frequent than that of differentiated cells. Collectively, our results indicate that carbon ions facilitate differentiation more effectively than gamma-rays as a major response in the progeny of irradiated fibroblasts. Accelerated differentiation may account, at least in part, for dose- and LET-dependent delayed loss of clonogenicity in normal human diploid cells, and could be a defensive mechanism that minimizes further expansion of aberrant cells.

Role of TGF-beta1 and nitric oxide in the bystander response of irradiated glioma cells

The radiation-induced bystander effect (RIBE) increases the probability of cellular response and therefore has important implications for cancer risk assessment following low-dose irradiation and for the likelihood of secondary cancers after radiotherapy. However, our knowledge of bystander signaling factors, especially those having long half-lives, is still limited. The present study found that, when a fraction of cells within a glioblastoma population were individually irradiated with helium ions from a particle microbeam, the yield of micronuclei (MN) in the nontargeted cells was increased, but these bystander MN were eliminated by treating the cells with either aminoguanidine (an inhibitor of inducible nitric oxide (NO) synthase) or anti-transforming growth factor beta1 (anti-TGF-beta1), indicating that NO and TGF-beta1 are involved in the RIBE. Intracellular NO was detected in the bystander cells, and additional TGF-beta1 was detected in the medium from irradiated T98G cells, but it was diminished by aminoguanidine. Consistent with this, an NO donor, diethylamine nitric oxide (DEANO), induced TGF-beta1 generation in T98G cells. Conversely, treatment of cells with recombinant TGF-beta1 could also induce NO and MN in T98G cells. Treatment of T98G cells with anti-TGF-beta1 inhibited the NO production when only 1% of cells were targeted, but not when 100% of cells were targeted. Our results indicate that, downstream of radiation-induced NO, TGF-beta1 can be released from targeted T98G cells and plays a key role as a signaling factor in the RIBE by further inducing free radicals and DNA damage in the nontargeted bystander cells.

DNA double-strand breaks form in bystander cells after microbeam irradiation of three-dimensional human tissue models

The "radiation-induced bystander effect," in which irradiated cells can induce genomic instability in unirradiated neighboring cells, has important implications for cancer radiotherapy and diagnostic radiology as well as for human health in general. Although the mechanisms of this effect remain to be elucidated, we reported previously that DNA double-strand breaks (DSBs), directly measured by gamma-H2AX focus formation assay, are induced in bystander cultured cells. To overcome the deficiencies of cultured cell studies, we examined alpha-particle microbeam irradiation-induced bystander effects in human tissue models, which preserve the three-dimensional geometric arrangement and communication of cells present in tissues in vivo. In marked contrast to DNA DSB dynamics in irradiated cells, in which maximal DSB formation is seen 30 min after irradiation, the incidence of DSBs in bystander cells reached a maximum by 12 to 48 h after irradiation, gradually decreasing over the 7-day time course. At the maxima, 40% to 60% of bystander cells were affected, a 4- to 6-fold increase over controls. These increases in bystander DSB formation were followed by increased levels of apoptosis and micronucleus formation, by loss of nuclear DNA methylation, and by an increased fraction of senescent cells. These findings show the involvement of DNA DSBs in tissue bystander responses and support the notion that bystander DNA DSBs are precursors to widespread downstream effects in human tissues. Bystander cells exhibiting postirradiation signs of genomic instability may be more prone than unaffected cells to become cancerous. Thus, this study points to the importance of considering the indirect biological effects of radiation in cancer risk assessment.

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.

Intercellular and intracellular signaling pathways mediating ionizing radiation-induced bystander effects

A rapidly growing body of experimental evidence indicates that ionizing radiation induces biological effects in non-irradiated bystander cells that have received signals from adjacent or distant irradiated cells. This phenomenon, which has been termed the ionizing radiation-induced bystander effect, challenges the long-standing paradigm that radiation traversal through the nucleus of a cell is a prerequisite to elicit genetic damage or a biological response. Bystander effects have been observed in a number of experimental systems, and cells whose nucleus or cytoplasm is irradiated exert bystander responses. Bystander cells manifest a multitude of biological consequences, such as genetic and epigenetic changes, alterations in gene expression, activation of signal transduction pathways, and delayed effects in their progeny. Several mediating mechanisms have been proposed. These involve gap junction-mediated intercellular communication, secreted soluble factors, oxidative metabolism, plasma membrane-bound lipid rafts, and calcium fluxes. This paper reviews briefly the current knowledge of the bystander effect with a focus on proposed mechanisms. The potential benefit of bystander effects to cancer radiotherapy will also be discussed.