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

ATR signaling controls the bystander responses of human chondrosarcoma cells by promoting RAD51-dependent DNA repair

PURPOSE

Radiation-induced bystander effect (RIBE) frequently is seen as DNA damage in unirradiated bystander cells, but the repair processes initiated in response to that DNA damage are not well understood. RIBE-mediated formation of micronuclei (MN), a biomarker of persistent DNA damage, was previously observed in bystander normal fibroblast (AG01522) cells, but not in bystander human chondrosarcoma (HTB94) cells. The molecular mechanisms causing this disparity are not clear. Herein, we investigate the role of DNA repair in the bystander responses of the two cell lines.

METHODS

Cells were irradiated with X-rays and immediately co-cultured with un-irradiated cells using a trans-well insert system in which they share the same medium. The activation of DNA damage response (DDR) proteins was detected by immunofluorescence staining or Western blotting. MN formation was examined by the cytokinesis-block MN assay, which is a robust method to detect persistent DNA damage.

RESULTS

Immunofluorescent foci of γH2AX and 53BP1, biomarkers of DNA damage and repair, revealed a greater capacity for DNA repair in HTB94 cells than in AG01522 cells in both irradiated and bystander populations. Autophosphorylation of ATR at the threonine 1989 site was expressed at a greater level in HTB94 cells compared to AG01522 cells at the baseline and in response to hydroxyurea treatment or exposure to 1 Gy of X-rays. An inhibitor of ATR, but not of ATM, promoted MN formation in bystander HTB94 cells. In contrast, no effect of either inhibitor was observed in bystander AG01522 cells, indicating that ATR signaling might be a pivotal pathway to preventing the MN formation in bystander HTB94 cells. Supporting this idea, we found an ATR-dependent increase in the fractions of bystander HTB94 cells with pRPA2 S33 and RAD51 foci. A blocker of RAD51 facilitated MN formation in bystander HTB94 cells.

CONCLUSION

Our results indicate that HTB94 cells were likely more efficient in DNA repair than AG01522 cells, specifically via ATR signaling, which inhibited the bystander signal-induced MN formation. This study highlights the significance of DNA repair efficiency in bystander cell responses.

Low-Dose Non-Targeted Effects and Mitochondrial Control

Non-targeted effects (NTE) have been generally regarded as a low-dose ionizing radiation (IR) phenomenon. Recently, regarding long distant abscopal effects have also been observed at high doses of IR) relevant to antitumor radiation therapy. IR is inducing NTE involving intracellular and extracellular signaling, which may lead to short-ranging bystander effects and distant long-ranging extracellular signaling abscopal effects. Internal and "spontaneous" cellular stress is mostly due to metabolic oxidative stress involving mitochondrial energy production (ATP) through oxidative phosphorylation and/or anaerobic pathways accompanied by the leakage of O₂^- and other radicals from mitochondria during normal or increased cellular energy requirements or to mitochondrial dysfunction. Among external stressors, ionizing radiation (IR) has been shown to very rapidly perturb mitochondrial functions, leading to increased energy supply demands and to ROS/NOS production. Depending on the dose, this affects all types of cell constituents, including DNA, RNA, amino acids, proteins, and membranes, perturbing normal inner cell organization and function, and forcing cells to reorganize the intracellular metabolism and the network of organelles. The reorganization implies intracellular cytoplasmic-nuclear shuttling of important proteins, activation of autophagy, and mitophagy, as well as induction of cell cycle arrest, DNA repair, apoptosis, and senescence. It also includes reprogramming of mitochondrial metabolism as well as genetic and epigenetic control of the expression of genes and proteins in order to ensure cell and tissue survival. At low doses of IR, directly irradiated cells may already exert non-targeted effects (NTE) involving the release of molecular mediators, such as radicals, cytokines, DNA fragments, small RNAs, and proteins (sometimes in the form of extracellular vehicles or exosomes), which can induce damage of unirradiated neighboring bystander or distant (abscopal) cells as well as immune responses. Such non-targeted effects (NTE) are contributing to low-dose phenomena, such as hormesis, adaptive responses, low-dose hypersensitivity, and genomic instability, and they are also promoting suppression and/or activation of immune cells. All of these are parts of the main defense systems of cells and tissues, including IR-induced innate and adaptive immune responses. The present review is focused on the prominent role of mitochondria in these processes, which are determinants of cell survival and anti-tumor RT.

Radiation-induced bystander effect and its clinical implications

For many years, targeted DNA damage caused by radiation has been considered the main cause of various biological effects. Based on this paradigm, any small amount of radiation is harmful to the organism. Epidemiological studies of Japanese atomic bomb survivors have proposed the linear-non-threshold model as the dominant standard in the field of radiation protection. However, there is increasing evidence that the linear-non-threshold model is not fully applicable to the biological effects caused by low dose radiation, and theories related to low dose radiation require further investigation. In addition to the cell damage caused by direct exposure, non-targeted effects, which are sometimes referred to as bystander effects, abscopal effects, genetic instability, etc., are another kind of significant effect related to low dose radiation. An understanding of this phenomenon is crucial for both basic biomedical research and clinical application. This article reviews recent studies on the bystander effect and summarizes the key findings in the field. Additionally, it offers a cross-sectional comparison of bystander effects caused by various radiation sources in different cell types, as well as an in-depth analysis of studies on the potential biological mechanisms of bystander effects. This review aims to present valuable information and provide new insights on the bystander effect to enlighten both radiobiologists and clinical radiologists searching for new ways to improve clinical treatments.

Phase separation of p53 induced by its unstructured basic region and prevented by oncogenic mutations in tetramerization domain

Liquid-liquid phase separation (LLPS) drives the formation of extensive membrane-less compartments to regulate various cellular biological activities both physiologically and pathologically. It has been widely accepted that LLPS is closely related to amyloid diseases and increasing reports have linked this phenomenon to cancers. Mutations of tumor suppressor protein p53 exist in more than half of malignant tumors, making the protein vitally important in cancer research. Recently, p53 was reported to undergo phase separation, which may regulate the function of p53. The molecular mechanism of p53 phase separation and how this process relates to cancer remains largely unclear. Herein, we find that the disordered unstructured basic region (UBR) plays a crucial role in p53 LLPS, driven by electrostatic and hydrophobic interactions. Mutations in the tetramerization domain (TD) disrupt p53 phase separation by preventing the tetramer formation. Furthermore, our results have revealed that, in response to DNA damage in cell, the wild type (WT) p53 undergoes LLPS, while LLPS in oncogenic mutations is diminished or eliminated. The expression of the target gene of p53 decreased significantly with the mutations and cell survival increased with the mutations. Thus, we propose a novel mechanism of p53 carcinogenesis, whereby oncogenic mutations in TD impair the formation of p53 condensates, decreasing the activation of target genes and promoting cancer progression. This study helps to understand the behavior and function of p53 in a different aspect and may provide insights into cancer therapies targeting p53.

The intercellular communications mediating radiation-induced bystander effects and their relevance to environmental, occupational, and therapeutic exposures

PURPOSE

The assumption that traversal of the cell nucleus by ionizing radiation is a prerequisite to induce genetic damage, or other important biological responses, has been challenged by studies showing that oxidative alterations extend beyond the irradiated cells and occur also in neighboring bystander cells. Cells and tissues outside the radiation field experience significant biochemical and phenotypic changes that are often similar to those observed in the irradiated cells and tissues. With relevance to the assessment of long-term health risks of occupational, environmental and clinical exposures, measurable genetic, epigenetic, and metabolic changes have been also detected in the progeny of bystander cells. How the oxidative damage spreads from the irradiated cells to their neighboring bystander cells has been under intense investigation. Following a brief summary of the trends in radiobiology leading to this paradigm shift in the field, we review key findings of bystander effects induced by low and high doses of various types of radiation that differ in their biophysical characteristics. While notable mechanistic insights continue to emerge, here the focus is on the many means of intercellular communication that mediate these effects, namely junctional channels, secreted molecules and extracellular vesicles, and immune pathways.

CONCLUSIONS

The insights gained by studying radiation bystander effects are leading to a basic understanding of the intercellular communications that occur under mild and severe oxidative stress in both normal and cancerous tissues. Understanding the mechanisms underlying these communications will likely contribute to reducing the uncertainty of predicting adverse health effects following exposure to low dose/low fluence ionizing radiation, guide novel interventions that mitigate adverse out-of-field effects, and contribute to better outcomes of radiotherapeutic treatments of cancer. In this review, we highlight novel routes of intercellular communication for investigation, and raise the rationale for reconsidering classification of bystander responses, abscopal effects, and expression of genomic instability as non-targeted effects of radiation.

LncRNA MT1DP promotes cadmium-induced DNA replication stress by inhibiting chromatin recruitment of SMARCAL1

Cadmium (Cd) is a well-known carcinogenic metal and widespread environmental pollutant. The effect of Cd-induced carcinogenesis is partly due to accumulated DNA damage and chromosomal aberrations, but the exact mechanisms underlying the genotoxicity of Cd have not been clearly understood. Here, we found that one long non-coding RNA MT1DP is participated in Cd-induced DNA damage and replication stress. Through analyzing the residents from Cd-contaminated area in Southern China, we found that blood DNA repair genes are down-regulated in individuals with high urine Cd values compared to those with low urine Cd values, which contrast to the blood MT1DP levels. Through in vitro experiments, we found that MT1DP promotes Cd-induced DNA damage response, genome instability and replication fork stalling. Mechanically, upon Cd treatment, ATR is activated to enhance HIF-1α expression, which in turn promotes the transcription level of MT1DP. Subsequently MT1DP is recruited on the chromatin and binds to SMARCAL1 to competitive inhibit latter's interaction with RPA complexes, finally leading to increased replication stress and DNA damage. In summary, this study provides clear evidence for the role of epigenetic regulation on the genotoxic effect of Cd, and MT1DP-mediated replication stress may represent a novel mechanism for Cd-induced carcinogenesis.

Role of Mitochondria in Radiation Responses: Epigenetic, Metabolic, and Signaling Impacts

Until recently, radiation effects have been considered to be mainly due to nuclear DNA damage and their management by repair mechanisms. However, molecular biology studies reveal that the outcomes of exposures to ionizing radiation (IR) highly depend on activation and regulation through other molecular components of organelles that determine cell survival and proliferation capacities. As typical epigenetic-regulated organelles and central power stations of cells, mitochondria play an important pivotal role in those responses. They direct cellular metabolism, energy supply and homeostasis as well as radiation-induced signaling, cell death, and immunological responses. This review is focused on how energy, dose and quality of IR affect mitochondria-dependent epigenetic and functional control at the cellular and tissue level. Low-dose radiation effects on mitochondria appear to be associated with epigenetic and non-targeted effects involved in genomic instability and adaptive responses, whereas high-dose radiation effects (>1 Gy) concern therapeutic effects of radiation and long-term outcomes involving mitochondria-mediated innate and adaptive immune responses. Both effects depend on radiation quality. For example, the increased efficacy of high linear energy transfer particle radiotherapy, e.g., C-ion radiotherapy, relies on the reduction of anastasis, enhanced mitochondria-mediated apoptosis and immunogenic (antitumor) responses.

ATM: Main Features, Signaling Pathways, and Its Diverse Roles in DNA Damage Response, Tumor Suppression, and Cancer Development

ATM is among of the most critical initiators and coordinators of the DNA-damage response. ATM canonical and non-canonical signaling pathways involve hundreds of downstream targets that control many important cellular processes such as DNA damage repair, apoptosis, cell cycle arrest, metabolism, proliferation, oxidative sensing, among others. Of note, ATM is often considered a major tumor suppressor because of its ability to induce apoptosis and cell cycle arrest. However, in some advanced stage tumor cells, ATM signaling is increased and confers remarkable advantages for cancer cell survival, resistance to radiation and chemotherapy, biosynthesis, proliferation, and metastasis. This review focuses on addressing major characteristics, signaling pathways and especially the diverse roles of ATM in cellular homeostasis and cancer development.

External modulators and redox homeostasis: Scenario in radiation-induced bystander cells

Redox homeostasis is imperative to maintain normal physiologic and metabolic functions. Radiotherapy disturbs this balance and induces genomic instability in diseased cells. However, radiation-induced effects propagate beyond the targeted cells, affecting the adjacent non-targeted cells (bystander effects). The cellular impact of radiation, thus, encompasses both targeted and non-targeted effects. Use of external modulators along with radiation can increase radio-therapeutic efficiency. The modulators' classification as protectors or sensitizers depends on interactions with damaged DNA molecules. Thus, it is necessary to realize the functions of various radio-sensitizers or radio-protectors in both irradiated and bystander cells. This review focuses on some modulators of radiation-induced bystander effects (RIBE) and their action mechanisms. Knowledge about the underlying signaling cross-talk may promote selective sensitization of radiation-targeted cells and protection of bystander cells.

The Radiobiology of Radiopharmaceuticals

Radiopharmaceutical therapy or targeted radionuclide therapy (TRT) is a well-established class of cancer therapeutics that includes a growing number of FDA-approved drugs and a promising pipeline of experimental therapeutics. Radiobiology is fundamental to a mechanistic understanding of the therapeutic capacity of these agents and their potential toxicities. However, the field of radiobiology has historically focused on external beam radiation. Critical differences exist between TRT and external beam radiotherapy with respect to dosimetry, dose rate, linear energy transfer, duration of treatment delivery, fractionation, range, and target volume. These distinctions simultaneously make it difficult to extrapolate from the radiobiology of external beam radiation to that of TRT and pose considerable challenges for preclinical and clinical studies investigating TRT. Here, we discuss these challenges and explore the current understanding of the radiobiology of radiopharmaceuticals.

Interplay between DNA replication stress, chromatin dynamics and DNA-damage response for the maintenance of genome stability

DNA replication stress is a major source of DNA damage, including double-stranded breaks that promote DNA damage response (DDR) signaling. Inefficient repair of such lesions can affect genome integrity. During DNA replication different factors act on chromatin remodeling in a coordinated way. While recent studies have highlighted individual molecular mechanisms of interaction, less is known about the orchestration of chromatin changes under replication stress. In this review we attempt to explore the complex relationship between DNA replication stress, DDR and genome integrity in mammalian cells, taking into account the role of chromatin disposition as an important modulator of DNA repair. Recent data on chromatin restoration and epigenetic re-establishment after DNA replication stress are reviewed.

Bisdemethoxycurcumin Enhances the Sensitivity of Non-small Cell Lung Cancer Cells to Icotinib via Dual Induction of Autophagy and Apoptosis

Non-small cell lung cancer (NSCLC) with epidermal growth factor receptor (EGFR) wild-type is intrinsic resistance to EGFR-tyrosine kinase inhibitors (TKIs). In this study, we assessed whether the combination of bisdemethoxycurcumin (BDMC) and icotinib could surmount primary EGFR-TKI resistance in NSCLC cells and investigated its molecular mechanism. Results showed that the combination of BDMC and icotinib produced potently synergistic growth inhibitory effect on primary EGFR-TKI-resistant NSCLC cell lines H460 (EGFR wild-type and K-ras mutation) and H1781 (EGFR wild-type and Her2 mutation). Compared with BDMC or icotinib alone, the two drug combination induced more significant apoptosis and autophagy via suppressing EGFR activity and interaction of Sp1 and HDCA1/HDCA2, which was accompanied by accumulation of reactive oxygen species (ROS), induction of DNA damage, and inhibition of cell migration and invasion. ROS inhibitor (NAC) and autophagy inhibitors (CQ or 3-MA) partially reversed BDMC plus icotinib-induced growth inhibitory effect on the NSCLC cells. Meanwhile, co-treatment with NAC attenuated the two drug combination-induced autophagy, apoptosis, DNA damage and decrease of cell migration and invasion ability. Also, 3-MA or CQ can abate the combination treatment-induced apoptosis and DNA damage, suggesting that there is crosstalk between different signaling pathways in the effect produced by the combination treatment. Our data indicate that BMDC has the potential to improve the treatment of primary EGFR-TKI resistant NISCLC that cannot be controlled with single-target agent, such as icotinib.

Enhancing the Bystander and Abscopal Effects to Improve Radiotherapy Outcomes

In this paper, we summarize published articles and experiences related to the attempt to improve radiotherapy outcomes and, thus, to personalize the radiation treatment according to the individual characteristics of each patient. The evolution of ideas and the study of successively published data have led us to envisage new biophysical models for the interpretation of tumor and healthy normal tissue response to radiation. In the development of the model, we have shown that when mesenchymal stem cells (MSCs) and radiotherapy are administered simultaneously in experimental radiotherapy on xenotumors implanted in a murine model, the results of the treatment show the existence of a synergic mechanism that is able to enhance the local and systemic actions of the radiation both on the treated tumor and on its possible metastasis. We are convinced that, due to the physical hallmarks that characterize the neoplastic tissues, the physical-chemical tropism of MSCs, and the widespread functions of macromolecules, proteins, and exosomes released from activated MSCs, the combination of radiotherapy plus MSCs used intratumorally has the effect of counteracting the pro-tumorigenic and pro-metastatic signals that contribute to the growth, spread, and resistance of the tumor cells. Therefore, we have concluded that MSCs are appropriate for therapeutic use in a clinical trial for rectal cancer combined with radiotherapy, which we are going to start in the near future.

ATM controls DNA repair and mitochondria transfer between neighboring cells

Intercellular communication is essential for multicellular tissue vitality and homeostasis. We show that healthy cells message protective signals through direct cell–cell connections to adjacent DNA–damaged cells in a microtubule–dependent manner. In DNA–damaged cells, mitochondria restoration is facilitated by fusion with undamaged mitochondria from healthy cells and their DNA damage repair is optimized in presence of healthy cells. Both, mitochondria transfer and intercellular signaling for an enhanced DNA damage response are critically regulated by the activity of the DNA repair protein ataxia telangiectasia mutated (ATM). These healthy–to–damaged prosurvival processes sustain normal tissue integrity and may be exploitable for overcoming resistance to therapy in diseases such as cancer.

Both irradiated and bystander effects link with DNA repair capacity and the linear energy transfer

AIMS

In comparison with a low linear energy transfer (LET) radiation, a high-LET radiation induces more complex DNA damage. This study wonders whether radiation-induced bystander effect (RIBE) is dependent of LET.

MATERIALS AND METHODS

Chinese hamster ovary CHO-9 cells and its subline EM-C11 cells (SSB repair deficient) and XR-C1 cells (DSB repair deficient) were irradiated by γ-rays, α-particles, or carbon ions with different LETs of 13, 30 and 70 keV/μm. Cell proliferation, cell death, DNA damage, cell cycle distribution and some protein expressions were measured with the cell counting kit-8 (CCK-8), colony formation, micronuclei (MN), flow cytometry and western blot, respectively.

KEY FINDINGS

A series of cell responses were induced by these radiations in a LET-dependent manner, including proliferation inhibition, cell death, MN induction, G₂/M phase arrest and the expression of γH2AX protein. These cell injuries were also depended on DNA repair capacity, and XR-C1 cells were the most sensitive to each radiation. Furthermore, when the cells were treated with the conditioned medium (CM) collected from irradiated CHO-9 cells, the MN induction and cell death response in the bystander cells of EM-C11 or XR-C1 increased along with LET of irradiation, and the bystander damage was easier to be induced in EM-C11 and XR-C1 cells than that in CHO-9 cells.

SIGNIFICANCE

Both cellular DNA repair capacity and the LET value of radiation could deeply influence damage extents of not only the irradiated cells but also the bystander cells.

A historical reflection on our understanding of radiation-induced DNA double strand break repair in somatic mammalian cells; interfacing the past with the present

Purpose: The International Journal of Radiation Biology (IJRB) is celebrating 60 years of publishing in 2019. IJRB has made an enormous contribution to publishing papers that have enhanced our understanding of the DNA damage response (DDR) activated following exposure to ionizing radiation (IR). The IR-induced DDR field has a rich history but many outstanding papers pass unread by young scientists overwhelmed by the current literature. We provide a historical reflection on key advances in the DDR field and interface them with current knowledge. Conclusions: DNA double strand breaks (DSBs) were identified as the major biological lesion induced by IR. But early studies on cells from IR-sensitive ataxia telangiectasia patients showed that DSB repair was not sufficient to prevent IR hypersensitivity. Subsequently, the ATM-dependent signal transduction process was revealed, with the breadth of the response being slowly unearthed. Early studies demonstrated at least two processes of DSB repair and revealed that mis-repair causes translocation formation. Recent studies, however, are unraveling more complexity in the repair process, including the specific processing of DSBs within transcriptionally active regions, and the significance of the chromatin environment. Despite the quality of these early and current studies, many questions remain to be addressed.

Targeted and Off-Target (Bystander and Abscopal) Effects of Radiation Therapy: Redox Mechanisms and Risk/Benefit Analysis

SIGNIFICANCE

Radiation therapy (from external beams to unsealed and sealed radionuclide sources) takes advantage of the detrimental effects of the clustered production of radicals and reactive oxygen species (ROS). Research has mainly focused on the interaction of radiation with water, which is the major constituent of living beings, and with nuclear DNA, which contains the genetic information. This led to the so-called target theory according to which cells have to be hit by ionizing particles to elicit an important biological response, including cell death. In cancer therapy, the Poisson law and linear quadratic mathematical models have been used to describe the probability of hits per cell as a function of the radiation dose. Recent Advances: However, in the last 20 years, many studies have shown that radiation generates "danger" signals that propagate from irradiated to nonirradiated cells, leading to bystander and other off-target effects.

CRITICAL ISSUES

Like for targeted effects, redox mechanisms play a key role also in off-target effects through transmission of ROS and reactive nitrogen species (RNS), and also of cytokines, ATP, and extracellular DNA. Particularly, nuclear factor kappa B is essential for triggering self-sustained production of ROS and RNS, thus making the bystander response similar to inflammation. In some therapeutic cases, this phenomenon is associated with recruitment of immune cells that are involved in distant irradiation effects (called "away-from-target" i.e., abscopal effects).

FUTURE DIRECTIONS

Determining the contribution of targeted and off-target effects in the clinic is still challenging. This has important consequences not only in radiotherapy but also possibly in diagnostic procedures and in radiation protection.

Radiation, inflammation and the immune response in cancer

Radiation is an important component of cancer treatment with more than half of all patients receive radiotherapy during their cancer experience. While the impact of radiation on tumour morphology is routinely examined in the pre-clinical and clinical setting, the impact of radiation on the tumour microenvironment and more specifically the inflammatory/immune response is less well characterised. Inflammation is a key contributor to short- and long-term cancer eradication, with significant tumour and normal tissue consequences. Therefore, the role of radiation in modulating the inflammatory response is highly topical given the current wave of targeted and immuno-therapeutic treatments for cancer. This review provides a general overview of how radiation modulates the inflammatory and immune response—(i) how radiation induces the inflammatory/immune system, (ii) the cellular changes that take place, (iii) how radiation dose delivery affects the immune response, and (iv) a discussion on research directions to improve patient survival, reduce side effects, improve quality of life, and reduce financial costs in the immediate future. Harnessing the benefits of radiation on the immune response will enhance its maximal therapeutic benefit and reduce radiation-induced toxicity.

Extracellular vesicles transfer nuclear Abl-dependent and radiation-induced miR-34c into unirradiated cells to cause bystander effects

Ionizing radiation (IR) not only activates DNA damage response (DDR) in irradiated cells but also induces bystander effects (BE) in cells not directly targeted by radiation. How DDR pathways activated in irradiated cells stimulate BE is not well understood. We show here that extracellular vesicles secreted by irradiated cells (EV-IR), but not those from unirradiated controls (EV-C), inhibit colony formation in unirradiated cells by inducing reactive oxygen species (ROS). We found that µEV-IR from Abl nuclear localization signal-mutated ( Abl-µNLS) cells could not induce ROS, but expression of wild-type Abl restored that activity. Because nuclear Abl stimulates miR-34c biogenesis, we measured miR-34c in EV and found that its levels correlated with the ROS-inducing activity of EV. We then showed that EV from miR-34c minigene-transfected, but unirradiated cells induced ROS; and transfection with miR-34c-mimic, without radiation or EV addition, also induced ROS. Furthermore, EV-IR from miR34-family triple-knockout cells could not induce ROS, whereas EV-IR from wild-type cells could cause miR-34c increase and ROS induction in the miR-34 triple-knockout cells. These results establish a novel role for extracellular vesicles in transferring nuclear Abl-dependent and radiation-induced miR-34c into unirradiated cells to cause bystander oxidative stress.

Intercellular communication of DNA damage and oxidative status underpin bystander effects

PURPOSE

A well-known phenomenon in the field of radiation biology is that cells exposed to ionizing radiation (IR) (targeted cells) can induce in non-irradiated (non-targeted), bystander cells effects reminiscent of DNA damage responses (DDR) normally expected, exclusively in targeted cells. These phenomena are collectively referred to as radiation-induced bystander effects (RIBE) and have different manifestations depending on the endpoint studied. Although it is now recognized that RIBE reflects to a considerable extent communication by the targeted cells to undamaged cells of their damaged status, the molecular underpinnings of this communication and its significance for the organism are only partly understood. In particular, it remains unknown why and how targeted cells induce DNA damage in non-targeted, bystander cells threatening their genomic stability and risking thus their transformation to cancer cells. Here, we outline observations hinting to possible sources of artifacts in experiments designed to detect RIBE and summarize a model according to which targeted cells modulate their redox status as part of their overall response to IR and use this modified redox status as a source to generate signals that are transmitted to non-irradiated cells of the organism.

MATERIAL AND METHODS

A synthesis of published evidence is presented.

RESULTS

Depending on type, RIBE signals may be transmitted through various forms of direct intercellular contact, through molecules acting locally in a paracrine fashion, or through molecules acting remotely in an endocrine fashion. We reason that DNA damage generated in bystander cells is unlikely to manifest the clustered character exhibited in directly exposed cells and postulate that RIBE will depend on complications generated when simpler forms of damage encounter the DNA replication fork.

CONCLUSIONS

We suggest that RIBE result from intercellular communication mechanisms designed to spread within tissues, or the organism, alarm signals of DNA damage inflicted in subsets of the constituent cells. This response likely evolved to protect organisms by appropriately modulating stress response, repair or apoptosis, and may in some instances also cause adverse effects, e.g. as collateral damage.

Rescue Effect Inherited in Colony Formation Assays Affects Radiation Response

It is well known that nonirradiated cells can exhibit radiation damage (bystander effect), and recent findings have shown that nonirradiated cells may help protect irradiated cells (rescue effect). These findings call into question the traditional view of radiation response: cells cannot be envisioned as isolated units. Here, we investigated traditional colony formation assays to determine if they also comprise cellular communication affecting the radiation response, using colony formation assays with varying numbers of cells, modulated beam irradiation and media transfer. Our findings showed that surviving fraction gradually increased with increasing number of irradiated cells. Specifically, for DU-145 human prostate cancer cells, surviving fraction increased 1.9-to-4.1-fold after 5-12 Gy irradiation; and for MM576 human melanoma cells, surviving fraction increased 1.9-fold after 5 Gy irradiation. Furthermore, increased surviving fraction was evident after modulated beam irradiation, where irradiated cells could communicate with nonirradiated cells. Media from dense cell culture also increased surviving fraction. The results suggest that traditional colony formation assays comprise unavoidable cellular communication affecting radiation outcome and the shape of the survival curve. We also propose that the increased in-field surviving fraction after modulated beam irradiation is due to the same effect.

The influence of non-DNA-targeted effects on carbon ion-induced low-dose hyper-radiosensitivity in MRC-5 cells

Low-dose hyper-radiosensitivity (LDHRS) is a hot topic in normal tissue radiation protection. However, the primary causes for LDHRS still remain unclear. In this study, the impact of non-DNA-targeted effects (NTEs) on high-LET radiation-induced LDHRS was investigated. Human normal lung fibroblast MRC-5 cells were irradiated with high-LET carbon ions, and low-dose biological effects (in terms of various bio-endpoints, including colony formation, DNA damage and micronuclei formation) were detected under conditions with and without gap junctional intercellular communication (GJIC) inhibition. LDHRS was observed when the radiation dose was <0.2 Gy for all bio-endpoints under investigation, but vanished when the GJIC was suppressed. Based on the probability of cells being hit and micro-dose per cell calculation, we deduced that the LDHRS phenomenon came from the combined action of direct hits and NTEs. We concluded that GJIC definitely plays an important role in cytotoxic substance spreading in high-LET carbon ion-induced LDHRS.

Inter-Relationship between Low-Dose Hyper-Radiosensitivity and Radiation-Induced Bystander Effects in the Human T98G Glioma and the Epithelial HaCaT Cell Line

Over the past several years, investigations in both low-dose hyper-radiosensitivity and increased radioresistance have been a focus of radiation oncology and biology research, since both conditions occur primarily in tumor cell lines. There has been significant progress in elucidating their signaling pathways, however uncertainties exist when they are studied together with radiation-induced bystander effects. Therefore, the aim of this work was to further investigate this relationship using the T98G glioma and HaCaT cell lines. T98G glioma cells have demonstrated a strong transition from hyper-radiosensitivity to induced radioresistance, and HaCaT cells do not show low-dose hypersensitivity. Both cell lines were paired using a mix-and-match protocol, which involved growing nonirradiated cells in culture media from irradiated cells and covering all possible combinations between them. The end points analyzed were clonogenic cell survival and live calcium measurements through the cellular membrane. Our data demonstrated that T98G cells produced bystander signals that decreased the survival of both reporter T98G and HaCaT cells. The bystander effect occurred only when T98G cells were exposed to doses below 1 Gy, which was corroborated by the induction of calcium fluxes. However, when bystander signals originated from HaCaT cells, the survival fraction increased in reporter T98G cells while it decreased in HaCaT cells. Moreover, the corresponding calcium data showed no calcium fluxes in T98G cells, while HaCaT cells displayed a biphasic calcium profile. In conclusion, our findings indicate a possible link between low-dose hyper-radiosensitivity and bystander effects. This relationship varies depending on which cell line functions as the source of bystander signals. This further suggests that the bystander mechanisms are more complex than previously expected and caution should be taken when extrapolating bystander results across all cell lines and all radiation doses.

Oxidative Stress and Carbonyl Lesions in Ulcerative Colitis and Associated Colorectal Cancer

Oxidative stress has long been known as a pathogenic factor of ulcerative colitis (UC) and colitis-associated colorectal cancer (CAC), but the effects of secondary carbonyl lesions receive less emphasis. In inflammatory conditions, reactive oxygen species (ROS), such as superoxide anion free radical (O2 (∙-)), hydrogen peroxide (H2O2), and hydroxyl radical (HO(∙)), are produced at high levels and accumulated to cause oxidative stress (OS). In oxidative status, accumulated ROS can cause protein dysfunction and DNA damage, leading to gene mutations and cell death. Accumulated ROS could also act as chemical messengers to activate signaling pathways, such as NF-κB and p38 MAPK, to affect cell proliferation, differentiation, and apoptosis. More importantly, electrophilic carbonyl compounds produced by lipid peroxidation may function as secondary pathogenic factors, causing further protein and membrane lesions. This may in turn exaggerate oxidative stress, forming a vicious cycle. Electrophilic carbonyls could also cause DNA mutations and breaks, driving malignant progression of UC. The secondary lesions caused by carbonyl compounds may be exceptionally important in the case of host carbonyl defensive system deficit, such as aldo-keto reductase 1B10 deficiency. This review article updates the current understanding of oxidative stress and carbonyl lesions in the development and progression of UC and CAC.

G(2)-M phase-correlative bystander effects are co-mediated by DNA-PKcs and ATM after carbon ion irradiation

Accumulated evidence has shown that radiation-induced bystander effect (RIBE) may have significant implications to the efficiency of radiotherapy. Although cellular radiosensitivity relies on cell cycle status, it is largely unknown how about the relationship between RIBE and cell cycle distribution, much less the underlying mechanism. In the present study, the lung cancer A549 cells were synchronized into different cell cycle phases of G1, S and G2/M and irradiated with high linear energy transfer (LET) carbon ions. By treating nonirradiated cells with the conditioned medium from these irradiated cells, it was found that the G2-M phase cells had the largest contribution to RIBE. Meanwhile, the activity of DNA-PKcs but not ATM was increased in the synchronized G2-M phase cells in spite of both of them were activated in the asynchronous cells after carbon ion irradiation. When the G2-M phased cells were transferred with DNA-PKcs siRNA and ATM siRNA individually or treated with an inhibitor of either DNA-PKcs or ATM before carbon ion irradiation, the RIBE was effectively diminished. These results provide new evidence linking cell cycle to bystander responses and demonstrate that DNA-PKcs and ATM are two associated factors in co-regulating G2-M phase-related bystander effects.

Role of ATM in bystander signaling between human monocytes and lung adenocarcinoma cells

The response of a cell or tissue to ionizing radiation is mediated by direct damage to cellular components and indirect damage mediated by radiolysis of water. Radiation affects both irradiated cells and the surrounding cells and tissues. The radiation-induced bystander effect is defined by the presence of biological effects in cells that were not themselves in the field of irradiation. To establish the contribution of the bystander effect in the survival of the neighboring cells, lung carcinoma A549 cells were exposed to gamma-irradiation, 2Gy. The medium from the irradiated cells was transferred to non-irradiated A549 cells. Irradiated A549 cells as well as non-irradiated A549 cells cultured in the presence of medium from irradiated cells showed decrease in survival and increase in γ-H2AX and p-ATM foci, indicating a bystander effect. Bystander signaling was also observed between different cell types. Phorbol-12-myristate-13-acetate (PMA)-stimulated and gamma-irradiated U937 (human monocyte) cells induced a bystander response in non-irradiated A549 (lung carcinoma) cells as shown by decreased survival and increased γ-H2AX and p-ATM foci. Non-stimulated and/or irradiated U937 cells did not induce such effects in non-irradiated A549 cells. Since ATM protein was activated in irradiated cells as well as bystander cells, it was of interest to understand its role in bystander effect. Suppression of ATM with siRNA in A549 cells completely inhibited bystander effect in bystander A549 cells. On the other hand suppression of ATM with siRNA in PMA stimulated U937 cells caused only a partial inhibition of bystander effect in bystander A549 cells. These results indicate that apart from ATM, some additional factor may be involved in bystander effect between different cell types.

Non-targeted effects of radiation exposure: recent advances and implications

The target theory of radiation-induced effects has been challenged by numerous studies, which indicate that in addition to biological effects resulting from direct DNA damage within the cell, a variety of non-DNA targeted effects (NTE) may make important contributions to the overall outcome. Ionising radiation induces complex, global cellular responses, such as genomic instability (GI) in both irradiated and never-irradiated 'bystander' cells that receive molecular signals produced by irradiated cells. GI is a well-known feature of many cancers, increasing the probability of cells to acquire the 'hallmarks of cancer' during the development of tumours. Although epidemiological data include contributions of both direct and NTE, they lack (i) statistical power at low dose where differences in dose response for NTE and direct effects are likely to be more important and (ii) heterogeneity of non-targeted responses due to genetic variability between individuals. In this article, NTE focussing on GI and bystander effects were critically examined, the specific principles of NTE were discussed and the potential influence on human health risk assessment from low-dose radiation was considered.

DNA damage foci: Meaning and significance

The discovery of DNA damage response proteins such as γH2AX, ATM, 53BP1, RAD51, and the MRE11/RAD50/NBS1 complex, that accumulate and/or are modified in the vicinity of a chromosomal DNA double-strand break to form microscopically visible, subnuclear foci, has revolutionized the detection of these lesions and has enabled studies of the cellular machinery that contributes to their repair. Double-strand breaks are induced directly by a number of physical and chemical agents, including ionizing radiation and radiomimetic drugs, but can also arise as secondary lesions during replication and DNA repair following exposure to a wide range of genotoxins. Here we aim to review the biological meaning and significance of DNA damage foci, looking specifically at a range of different settings in which such markers of DNA damage and repair are being studied and interpreted.

Introduction to radiobiology of targeted radionuclide therapy

During the last decades, new radionuclide-based targeted therapies have emerged as efficient tools for cancer treatment. Targeted radionuclide therapies (TRTs) are based on a multidisciplinary approach that involves the cooperation of specialists in several research fields. Among them, radiobiologists investigate the biological effects of ionizing radiation, specifically the molecular and cellular mechanisms involved in the radiation response. Most of the knowledge about radiation effects concerns external beam radiation therapy (EBRT) and radiobiology has then strongly contributed to the development of this therapeutic approach. Similarly, radiobiology and dosimetry are also assumed to be ways for improving TRT, in particular in the therapy of solid tumors, which are radioresistant. However, extrapolation of EBRT radiobiology to TRT is not straightforward. Indeed, the specific physical characteristics of TRT (heterogeneous and mixed irradiation, protracted exposure, and low absorbed dose rate) differ from those of conventional EBRT (homogeneous irradiation, short exposure, and high absorbed dose rate), and consequently the response of irradiated tissues might be different. Therefore, specific TRT radiobiology needs to be explored. Determining dose-effect correlation is also a prerequisite for rigorous preclinical radiobiology studies because dosimetry provides the necessary referential to all TRT situations. It is required too for developing patient-tailored TRT in the clinic in order to estimate the best dose for tumor control, while protecting the healthy tissues, thereby improving therapeutic efficacy. Finally, it will allow to determine the relative contribution of targeted effects (assumed to be dose-related) and non-targeted effects (assumed to be non-dose-related) of ionizing radiation. However, conversely to EBRT where it is routinely used, dosimetry is still challenging in TRT. Therefore, it constitutes with radiobiology, one of the main challenges of TRT in the future.

DDR-mediated crosstalk between DNA-damaged cells and their microenvironment

The DNA damage response (DDR) is an evolutionarily conserved signaling cascade that senses and responds to double-strand DNA breaks by organizing downstream cellular events, ranging from appropriate DNA repair to cell cycle checkpoints. In higher organisms, the DDR prevents neoplastic transformation by directly protecting the information contained in the genome and by regulating cell fate decisions, like apoptosis and senescence, to ensure the removal of severely damaged cells. In addition to these well-studied cell-autonomous effects, emerging evidence now shows that the DDR signaling cascade can also function in a paracrine manner, thus influencing the biology of the surrounding cellular microenvironment. In this context, the DDR plays an emerging role in shaping the damaged tumor microenvironment through the regulation of tissue repair and local immune responses, thereby providing a promising avenue for novel therapeutic interventions. Additionally, while DDR-mediated extracellular signals can convey information to surrounding, undamaged cells, they can also feedback onto DNA-damaged cells to reinforce selected signaling pathways. Overall, these extracellular DDR signals can be subdivided into two time-specific waves: a rapid bystander effect occurring within a few hours of DNA damage; and a late, delayed, senescence-associated secretory phenotype generally requiring multiple days to establish. Here, we highlight and discuss examples of rapid and late DDR–mediated extracellular alarm signals.

SirT1 knockdown potentiates radiation-induced bystander effect through promoting c-Myc activity and thus facilitating ROS accumulation

Radiation-induced bystander effect (RIBE) has important implications for secondary cancer risk assessment during cancer radiotherapy, but the bystander signaling processes, especially under hypoxic condition, are still largely unclear. The present study found that micronuclei (MN) formation could be induced in the non-irradiated HL-7702 hepatocyte cells after being treated with the conditioned medium from irradiated hepatoma HepG2 and SK-Hep-1 cells under either normoxia or hypoxia. This bystander response was dramatically diminished or enhanced when the SirT1 gene of irradiated hepatoma cells was overexpressed or knocked down, respectively, especially under hypoxia. Meanwhile, SirT1 knockdown promoted transcriptional activity for c-Myc and facilitated ROS accumulation. But both of the increased bystander responses and ROS generation due to SirT1-knockdown were almost completely suppressed by c-Myc interference. Moreover, ROS scavenger effectively abolished the RIBE triggered by irradiated hepatoma cells even with SirT1 depletion. These findings provide new insights that SirT1 has a profound role in regulating RIBE where a c-Myc-dependent release of ROS may be involved.

Target irradiation induced bystander effects between stem-like and non stem-like cancer cells

Tumors are heterogeneous in nature and consist of multiple cell types. Among them, cancer stem-like cells (CSCs) are suggested to be the principal cause of tumor metastasis, resistance and recurrence. Therefore, understanding the behavior of CSCs in direct and indirect irradiations is crucial for clinical radiotherapy. Here, the CSCs and their counterpart non stem-like cancer cells (NSCCs) in human HT1080 fibrosarcoma cell line were sorted and labeled, then the two cell subtypes were mixed together and chosen separately to be irradiated via a proton microbeam. The radiation-induced bystander effect (RIBE) between the CSCs and NSCCs was measured by imaging 53BP1 foci, a widely used indicator for DNA double strand break (DSB). CSCs were found to be less active than NSCCs in both the generation and the response of bystander signals. Moreover, the nitric oxide (NO) scavenger c-PTIO can effectively alleviate the bystander effect in bystander NSCCs but not in bystander CSCs, indicating a difference of the two cell subtypes in NO signal response. To our knowledge, this is the first report shedding light on the RIBE between CSCs and NSCCs, which might contribute to a further understanding of the out-of-field effect in cancer radiotherapy.

BRCA1, FANCD2 and Chk1 are potential molecular targets for the modulation of a radiation-induced DNA damage response in bystander cells

Radiotherapy is an important treatment option for many human cancers. Current research is investigating the use of molecular targeted drugs in order to improve responses to radiotherapy in various cancers. The cellular response to irradiation is driven by both direct DNA damage in the targeted cell and intercellular signalling leading to a broad range of bystander effects. This study aims to elucidate radiation-induced DNA damage response signalling in bystander cells and to identify potential molecular targets to modulate the radiation induced bystander response in a therapeutic setting. Stalled replication forks in T98G bystander cells were visualised via bromodeoxyuridine (BrdU) nuclear foci detection at sites of single stranded DNA. γH2AX co-localised with these BrdU foci. BRCA1 and FANCD2 foci formed in T98G bystander cells. Using ATR mutant F02-98 hTERT and ATM deficient GM05849 fibroblasts it could be shown that ATR but not ATM was required for the recruitment of FANCD2 to sites of replication associated DNA damage in bystander cells whereas BRCA1 bystander foci were ATM-dependent. Phospho-Chk1 foci formation was observed in T98G bystander cells. Clonogenic survival assays showed moderate radiosensitisation of directly irradiated cells by the Chk1 inhibitor UCN-01 but increased radioresistance of bystander cells. This study identifies BRCA1, FANCD2 and Chk1 as potential targets for the modulation of radiation response in bystander cells. It adds to our understanding of the key molecular events propagating out-of-field effects of radiation and provides a rationale for the development of novel molecular targeted drugs for radiotherapy optimisation.

Radioprotection of targeted and bystander cells by methylproamine

Introduction

Radioprotective agents are of interest for application in radiotherapy for cancer and in public health medicine in the context of accidental radiation exposure. Methylproamine is the lead compound of a class of radioprotectors which act as DNA binding anti-oxidants, enabling the repair of transient radiation-induced oxidative DNA lesions. This study tested methylproamine for the radioprotection of both directly targeted and bystander cells.

Methods

T98G glioma cells were treated with 15 μM methylproamine and exposed to ¹³⁷Cs γ-ray/X-ray irradiation and He²⁺ microbeam irradiation. Radioprotection of directly targeted cells and bystander cells was measured by clonogenic survival or γH2AX assay.

Results

Radioprotection of directly targeted T98G cells by methylproamine was observed for ¹³⁷Cs γ-rays and X-rays but not for He²⁺ charged particle irradiation. The effect of methylproamine on the bystander cell population was tested for both X-ray irradiation and He²⁺ ion microbeam irradiation. The X-ray bystander experiments were carried out by medium transfer from irradiated to non-irradiated cultures and three experimental designs were tested. Radioprotection was only observed when recipient cells were pretreated with the drug prior to exposure to the conditioned medium. In microbeam bystander experiments targeted and nontargeted cells were co-cultured with continuous methylproamine treatment during irradiation and postradiation incubation; radioprotection of bystander cells was observed.

Discussion and conclusion

Methylproamine protected targeted cells from DNA damage caused by γ-ray or X-ray radiation but not He²⁺ ion radiation. Protection of bystander cells was independent of the type of radiation which the donor population received.

RAD9 deficiency enhances radiation induced bystander DNA damage and transcriptomal response

BACKGROUND

Radiation induced bystander effects are an important component of the overall response of cells to irradiation and are associated with human health risks. The mechanism responsible includes intra-cellular and inter-cellular signaling by which the bystander response is propagated. However, details of the signaling mechanism are not well defined.

METHODS

We measured the bystander response of Mrad9+/+ and Mrad9-/- mouse embryonic stem cells, as well as human H1299 cells with inherent or RNA interference-mediated reduced RAD9 levels after exposure to 1 Gy α particles, by scoring chromosomal aberrations and micronuclei formation, respectively. In addition, we used microarray gene expression analyses to profile the transcriptome of directly irradiated and bystander H1299 cells.

RESULTS

We demonstrated that Mrad9 null enhances chromatid aberration frequency induced by radiation in bystander mouse embryonic stem cells. In addition, we found that H1299 cells with reduced RAD9 protein levels showed a higher frequency of radiation induced bystander micronuclei formation, compared with parental cells containing inherent levels of RAD9. The enhanced bystander response in human cells was associated with a unique transcriptomic profile. In unirradiated cells, RAD9 reduction broadly affected stress response pathways at the mRNA level; there was reduction in transcript levels corresponding to genes encoding multiple members of the UVA-MAPK and p38MAPK families, such as STAT1 and PARP1, suggesting that these signaling mechanisms may not function optimally when RAD9 is reduced. Using network analysis, we found that differential activation of the SP1 and NUPR1 transcriptional regulators was predicted in directly irradiated and bystander H1299 cells. Transcription factor prediction analysis also implied that HIF1α (Hypoxia induced factor 1 alpha) activation by protein stabilization in irradiated cells could be a negative predictor of the bystander response, suggesting that local hypoxic stress experienced by cells directly exposed to radiation may influence whether or not they will elicit a bystander response in neighboring cells.

Systemic DNA damage accumulation under in vivo tumor growth can be inhibited by the antioxidant Tempol

Recently we found that mice bearing subcutaneous non-metastatic tumors exhibited elevated levels of two types of complex DNA damage, i.e., double-strand breaks and oxidatively-induced clustered DNA lesions in various tissues throughout the body, both adjacent to and distant from the tumor site. This DNA damage was dependent on CCL2, a cytokine involved in the recruitment and activation of macrophages, suggesting that this systemic DNA damage was mediated via tumor-induced chronic inflammatory responses involving cytokines, activation of macrophages, and consequent free radical production. If free radicals are involved, then a diet containing an antioxidant may decrease the distant DNA damage. Here we repeated our standard protocol in cohorts of two syngeneic tumor-bearing C57BL/6NCr mice that were on a Tempol-supplemented diet. We show that double-strand break and oxidatively-induced clustered DNA lesion levels were considerably decreased, about two- to three fold, in the majority of tissues studied from the tumor-bearing mice fed the antioxidant Tempol compared to the control tumor-bearing mice. Similar results were also observed in nude mice suggesting that the Tempol effects are independent of functioning adaptive immunity. This is the first in vivo study demonstrating the effect of a dietary antioxidant on abscopal DNA damage in tissues distant from a localized source of genotoxic stress. These findings may be important for understanding the mechanisms of genomic instability and carcinogenesis caused by chronic stress-induced systemic DNA damage and for developing preventative strategies.

ATM signalling and cancer

ATM, the protein kinase mutated in the rare human disease ataxia telangiectasia (A-T), has been the focus of intense scrutiny over the past two decades. Initially this was because of the unusual radiosensitive phenotype of cells from A-T patients, and latterly because investigating ATM signalling has yielded valuable insights into the DNA damage response, redox signalling and cancer. With the recent explosion in genomic data, ATM alterations have been revealed both in the germline as a predisposing factor for cancer and as somatic changes in tumours themselves. Here we review these findings, as well as advances in the understanding of ATM signalling mechanisms in cancer and ATM inhibition as a strategy for cancer treatment.

Neutron exposures in human cells: bystander effect and relative biological effectiveness

Bystander effects have been observed repeatedly in mammalian cells following photon and alpha particle irradiation. However, few studies have been performed to investigate bystander effects arising from neutron irradiation. Here we asked whether neutrons also induce a bystander effect in two normal human lymphoblastoid cell lines. These cells were exposed to fast neutrons produced by targeting a near-monoenergetic 50.5 MeV proton beam at a Be target (17 MeV average neutron energy), and irradiated-cell conditioned media (ICCM) was transferred to unirradiated cells. The cytokinesis-block micronucleus assay was used to quantify genetic damage in radiation-naïve cells exposed to ICCM from cultures that received 0 (control), 0.5, 1, 1.5, 2, 3 or 4 Gy neutrons. Cells grown in ICCM from irradiated cells showed no significant increase in the frequencies of micronuclei or nucleoplasmic bridges compared to cells grown in ICCM from sham irradiated cells for either cell line. However, the neutron beam has a photon dose-contamination of 5%, which may modulate a neutron-induced bystander effect. To determine whether these low doses of contaminating photons can induce a bystander effect, cells were irradiated with cobalt-60 at doses equivalent to the percent contamination for each neutron dose. No significant increase in the frequencies of micronuclei or bridges was observed at these doses of photons for either cell line when cultured in ICCM. As expected, high doses of photons induced a clear bystander effect in both cell lines for micronuclei and bridges (p<0.0001). These data indicate that neutrons do not induce a bystander effect in these cells. Finally, neutrons had a relative biological effectiveness of 2.0 ± 0.13 for micronuclei and 5.8 ± 2.9 for bridges compared to cobalt-60. These results may be relevant to radiation therapy with fast neutrons and for regulatory agencies setting standards for neutron radiation protection and safety.

Assessment of targeted and non-targeted responses in cells deficient in ATM function following exposure to low and high dose X-rays

Radiation sensitivity at low and high dose exposure to X-rays was investigated by means of chromosomal aberration (CA) analysis in heterozygous ATM mutation carrier and A-T patient (biallelic ATM mutation) lymphoblastoid cell lines (LCLs). Targeted and non-targeted responses to acutely delivered irradiation were examined by applying a co-culture system that enables study of both directly irradiated cells and medium-mediated bystander effects in the same experimental setting. No indication of radiation hypersensitivity was observed at doses of 0.01 Gy or 0.1 Gy for the ATM mutation carrier LCL. The A-T patient cells also did not show low-dose response. There was significant increase in unstable CA yields for both ATM mutation carrier and A-T LCLs at 1 and 2 Gy, the A-T cells displaying more distinct dose dependency. Both chromosome and chromatid type aberrations were induced at an increased rate in the irradiated A-T cells, whereas for ATM carrier cells, only unstable chromosomal aberrations were increased above the level observed in the wild type cell line. No bystander effect could be demonstrated in any of the cell lines or doses applied. Characteristics typical for the A-T cell line were detected, i.e., high baseline frequency of CA that increased with dose. In addition, dose-dependent loss of cell viability was observed. In conclusion, CA analysis did not demonstrate low-dose (≤100 mGy) radiosensitivity in ATM mutation carrier cells or A-T patient cells. However, both cell lines showed increased radiosensitivity at high dose exposure.

The role of oxidative DNA damage in radiation induced bystander effect

Ionizing radiation (IR) has been described as a double-edged sword, since it is used for diagnostic and therapeutic medical applications, and at the same time it is a well known human mutagen and carcinogen, causing wide-ranging chromosomal aberrations. It is nowadays accepted that the detrimental effects of IR are not restricted only in the irradiated cells, but also to non-irradiated bystander or even distant cells manifesting various biological effects. This review presents the role of oxidative stress in the induction of bystander effects referring to the types of the implicated oxidative DNA lesions, the contributing intercellular and intracellular stress mediators, the way they are transmitted from irradiated to bystander cells and finally, the complex role of the bystander effect in the therapeutic efficacy of radiation treatment of cancer.

Crosstalk between telomere maintenance and radiation effects: A key player in the process of radiation-induced carcinogenesis

It is well established that ionizing radiation induces chromosomal damage, both following direct radiation exposure and via non-targeted (bystander) effects, activating DNA damage repair pathways, of which the proteins are closely linked to telomeric proteins and telomere maintenance. Long-term propagation of this radiation-induced chromosomal damage during cell proliferation results in chromosomal instability. Many studies have shown the link between radiation exposure and radiation-induced changes in oxidative stress and DNA damage repair in both targeted and non-targeted cells. However, the effect of these factors on telomeres, long established as guardians of the genome, still remains to be clarified. In this review, we will focus on what is known about how telomeres are affected by exposure to low- and high-LET ionizing radiation and during proliferation, and will discuss how telomeres may be a key player in the process of radiation-induced carcinogenesis.

The biological effect of large single doses: a possible role for non-targeted effects in cell inactivation

BACKGROUND AND PURPOSE

Novel radiotherapy techniques increasingly use very large dose fractions. It has been argued that the biological effect of large dose fractions may differ from that of conventional fraction sizes. The purpose was to study the biological effect of large single doses.

MATERIAL AND METHODS

Clonogenic cell survival of MCF7 and MDA-MB-231 cells was determined after direct X-ray irradiation, irradiation of feeder cells, or transfer of conditioned medium (CM). Cell-cycle distributions and the apoptotic sub-G1 fraction were measured by flow cytometry. Cytokines in CM were quantified by a cytokine antibody array. γH2AX foci were detected by immunofluorescence microscopy.

RESULTS

The surviving fraction of MCF7 cells irradiated in vitro with 12 Gy showed an 8.5-fold decrease (95% c.i.: 4.4-16.3; P<0.0001) when the density of irradiated cells was increased from 10 to 50×10(3) cells per flask. Part of this effect was due to a dose-dependent transferrable factor as shown in CM experiments in the dose range 5-15 Gy. While no effect on apoptosis and cell cycle distribution was observed, and no differentially expressed cytokine could be identified, the transferable factor induced prolonged expression of γH2AX DNA repair foci at 1-12 h.

CONCLUSIONS

A dose-dependent non-targeted effect on clonogenic cell survival was found in the dose range 5-15 Gy. The dependence of SF on cell numbers at high doses would represent a "cohort effect" in vivo. These results support the hypothesis that non-targeted effects may contribute to the efficacy of very large dose fractions in radiotherapy.

Bystander effects as manifestation of intercellular communication of DNA damage and of the cellular oxidative status

It is becoming increasingly clear that cells exposed to ionizing radiation (IR) and other genotoxic agents (targeted cells) can communicate their DNA damage response (DDR) status to cells that have not been directly irradiated (bystander cells). The term radiation-induced bystander effects (RIBE) describes facets of this phenomenon, but its molecular underpinnings are incompletely characterized. Consequences of DDR in bystander cells have been extensively studied and include transformation and mutation induction; micronuclei, chromosome aberration and sister chromatid exchange formation; as well as modulations in gene expression, proliferation and differentiation patterns. A fundamental question arising from such observations is why targeted cells induce DNA damage in non-targeted, bystander cells threatening thus their genomic stability and risking the induction of cancer. Here, we review and synthesize available literature to gather support for a model according to which targeted cells modulate as part of DDR their redox status and use it as a source to generate signals for neighboring cells. Such signals can be either small molecules transported to adjacent non-targeted cells via gap-junction intercellular communication (GJIC), or secreted factors that can reach remote, non-targeted cells by diffusion or through the circulation. We review evidence that such signals can induce in the recipient cell modulations of redox status similar to those seen in the originating targeted cell - occasionally though self-amplifying feedback loops. The resulting increase of oxidative stress in bystander cells induces, often in conjunction with DNA replication, the observed DDR-like responses that are at times strong enough to cause apoptosis. We reason that RIBE reflect the function of intercellular communication mechanisms designed to spread within tissues, or the entire organism, information about DNA damage inflicted to individual, constituent cells. Such responses are thought to protect the organism by enhancing repair in a community of cells and by eliminating severely damaged cells.

Combined analysis of gamma-H2AX/53BP1 foci and caspase activation in lymphocyte subsets detects recent and more remote radiation exposures

Analysis of gamma-H2AX foci in blood lymphocytes is a promising approach for rapid dose estimation to support patient triage after a radiation accident but has one major drawback: the rapid decline of foci levels post-exposure cause major uncertainties in situations where the exact timing between exposure and blood sampling is unknown. To address this issue, radiation-induced apoptosis (RIA) in lymphocytes was investigated using fluorogenic inhibitors of caspases (FLICA) as an independent biomarker for radiation exposure, which may complement the gamma-H2AX assay. Ex vivo X-irradiated peripheral blood lymphocytes from 17 volunteers showed dose- and time-dependent increases in radiation-induced apoptosis over the first 3 days after exposure, albeit with considerable interindividual variation. Comparison with gamma-H2AX and 53BP1 foci counts suggested an inverse correlation between numbers of residual foci and radiation-induced apoptosis in lymphocytes at 24 h postirradiation (P = 0.007). In T-helper (CD4), T-cytotoxic (CD8) and B-cells (CD19), some significant differences in radiation induced DSBs or apoptosis were observed, however no correlation between foci and apoptosis in lymphocyte subsets was observed at 24 h postirradiation. While gamma-H2AX and 53BP1 foci were rapidly induced and then repaired after exposure, radiation-induced apoptosis did not become apparent until 24 h after exposure. Data from six volunteers with different ex vivo doses and post-exposure times were used to test the capability of the combined assay. Results show that simultaneous analysis of gamma-H2AX and radiation-induced apoptosis may provide a rapid and more accurate triage tool in situations where the delay between exposure and blood sampling is unknown compared to gamma-H2AX alone. This combined approach may improve the accuracy of dose estimations in cases where blood sampling is performed days after the radiation exposure.