Calcium Fluxes Modulate the Radiation-Induced Bystander Responses in Targeted Glioma and Fibroblast Cells

Abscopal Effects, Clastogenic Effects and Bystander Effects: 70 Years of Non-Targeted Effects of Radiation

In vitro and in vivo observations accumulated over several decades have firmly shown that the biological effects of ionizing radiation can spread from irradiated cells/tissues to non-targeted cells/tissues. Redox-modulated intercellular communication mechanisms that include a role for secreted factors and gap junctions, can mediate these non-targeted effects. Clearly, the expression of such effects and their transmission to progeny cells has implications for issues related to radiation protection. Their elucidation is also relevant towards enhancing the efficacy of cancer radiotherapy and reducing its impact on the development of normal tissue toxicities. In addition, the study of non-targeted effects is pertinent to our basic understanding of intercellular communications under conditions of oxidative stress. This review will trace the history of non-targeted effects of radiation starting with early reports of abscopal effects which described radiation induced effects in tissues distant from the site of radiation exposure. A related effect involved the production of clastogenic factors in plasma following irradiation which can induce chromosome damage in unirradiated cells. Despite these early reports suggesting non-targeted effects of radiation, the classical paradigm that a direct deposition of energy in the nucleus was required still dominated. This paradigm was challenged by papers describing radiation induced bystander effects. This review will cover mechanisms of radiation-induced bystander effects and the potential impacts on radiation protection and radiation therapy.

Factors Influencing Effects of Low-dose Radiation Exposure

ABSTRACT

It is now well accepted that the mechanisms induced by low-dose exposures to ionizing radiation (LDR) are different from those occurring after high-dose exposures. However, the downstream effects of these mechanisms are unclear as are the quantitative relationships between exposure, effect, harm, and risk. In this paper, we will discuss the mechanisms known to be important with an overall emphasis on how so-called "non-targeted effects" (NTE) communicate and coordinate responses to LDR. Targeted deposition of ionizing radiation energy in cells causing DNA damage is still regarded as the dominant trigger leading to all downstream events whether targeted or non-targeted. We regard this as an over-simplification dating back to formal target theory. It ignores that last 100 y of biological research into stress responses and signaling mechanisms in organisms exposed to toxic substances, including ionizing radiation. We will provide evidence for situations where energy deposition in cellular targets alone cannot be plausible as a mechanism for LDR effects. An example is where the energy deposition takes place in an organism not receiving the radiation dose. We will also discuss how effects after LDR depend more on dose rate and radiation quality rather than actual dose, which appears rather irrelevant. Finally, we will use recent evidence from studies of cataract and melanoma induction to suggest that after LDR, post-translational effects, such as protein misfolding or defects in energy metabolism or mitochondrial function, may dominate the etiology and progression of the disease. A focus on such novel pathways may open the way to successful prophylaxis and development of new biomarkers for better risk assessment after low dose exposures.

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.

X-rays-Induced Bystander Effect Consists in the Formation of DNA Breaks in a Calcium-Dependent Manner: Influence of the Experimental Procedure and the Individual Factor

Radiation-induced bystander effects (RIBE) describe the biological events occurring in non-targeted cells in the vicinity of irradiated ones. Various experimental procedures have been used to investigate RIBE. Interestingly, most micro-irradiation experiments have been performed with alpha particles, whereas most medium transfers have been done with X-rays. With their high fluence, synchrotron X-rays represent a real opportunity to study RIBE by applying these two approaches with the same radiation type. The RIBE induced in human fibroblasts by the medium transfer approach resulted in a generation of DNA double-strand breaks (DSB) occurring from 10 min to 4 h post-irradiation. Such RIBE was found to be dependent on dose and on the number of donor cells. The RIBE induced with the micro-irradiation approach produced DSB with the same temporal occurrence. Culture media containing high concentrations of phosphates were found to inhibit RIBE, while media rich in calcium increased it. The contribution of the RIBE to the biological dose was evaluated after synchrotron X-rays, media transfer, micro-irradiation, and 6 MeV photon irradiation mimicking a standard radiotherapy session: the RIBE may represent less than 1%, about 5%, and about 20% of the initial dose, respectively. However, RIBE may result in beneficial or otherwise deleterious effects in surrounding tissues according to their radiosensitivity status and their capacity to release Ca2+ ions in response to radiation.

Radiation-Induced Bystander Effect and Cytoplasmic Irradiation Studies with Microbeams

Simple Summary

Microbeams are useful tools in studies on non-target effects, such as the radiation-induced bystander effect, and responses related to cytoplasmic irradiation. A micrometer or even sub-micrometer-level beam size enables the precise delivery of radiation energy to a specific target. Here we summarize the observations of the bystander effect and the cytoplasmic irradiation-related effect using different kinds of microbeam irradiators as well as discuss the cellular and molecular mechanisms that are involved in these responses. Non-target effects may increase the detrimental effect caused by radiation, so a more comprehensive knowledge of the process will enable better evaluation of the damage resulting from irradiation.

Abstract

Although direct damage to nuclear DNA is considered as the major contributing event that leads to radiation-induced effects, accumulating evidence in the past two decades has shown that non-target events, in which cells are not directly irradiated but receive signals from the irradiated cells, or cells irradiated at extranuclear targets, may also contribute to the biological consequences of exposure to ionizing radiation. With a beam diameter at the micrometer or sub-micrometer level, microbeams can precisely deliver radiation, without damaging the surrounding area, or deposit the radiation energy at specific sub-cellular locations within a cell. Such unique features cannot be achieved by other kinds of radiation settings, hence making a microbeam irradiator useful in studies of a radiation-induced bystander effect (RIBE) and cytoplasmic irradiation. Here, studies on RIBE and different responses to cytoplasmic irradiation using microbeams are summarized. Possible mechanisms related to the bystander effect, which include gap-junction intercellular communications and soluble signal molecules as well as factors involved in cytoplasmic irradiation-induced events, are also discussed.

An Emerging Role for Calcium Signaling in Cancer-Associated Fibroblasts

Tumors exist in a complex milieu where interaction with their associated microenvironment significantly contributes to disease progression. Cancer-associated fibroblasts (CAFs) are the primary component of the tumor microenvironment and participate in complex bidirectional communication with tumor cells. CAFs support the development of various hallmarks of cancer through diverse processes, including direct cell-cell contact, paracrine signaling, and remodeling and deposition of the extracellular matrix. Calcium signaling is a key second messenger in intra- and inter-cellular signaling pathways that contributes to cancer progression; however, the links between calcium signaling and CAFs are less well-explored. In this review, we put into context the role of calcium signaling in interactions between cancer cells and CAFs, with a focus on migration, proliferation, chemoresistance, and genetic instability.

Non-targeted effects of radiation: a personal perspective on the role of exosomes in an evolving paradigm

PURPOSE

Radiation-induced non-targeted effects (NTE) have implications in a variety of areas relevant to radiation biology. Here we evaluate the various cargo associated with exosomal signalling and how they work synergistically to initiate and propagate the non-targeted effects including Genomic Instability and Bystander Effects.

CONCLUSIONS

Extra cellular vesicles, in particular exosomes, have been shown to carry bystander signals. Exosome cargo may contain nucleic acids, both DNA and RNA, as well as proteins, lipids and metabolites. These cargo molecules have all been considered as potential mediators of NTE. A review of current literature shows mounting evidence of a role for ionizing radiation in modulating both the numbers of exosomes released from affected cells as well as the content of their cargo, and that these exosomes can instigate functional changes in recipient cells. However, there are significant gaps in our understanding, particularly regarding modified exosome cargo after radiation exposure and the functional changes induced in recipient cells.

Redox interactions-induced cardiac toxicity in cancer therapy

Cancer patients undergoing radiotherapy, chemotherapy, or targeted cancer therapy are exposed to the risk of several side effects because of the heavy production of ROS by ionizing radiation or some chemotherapy drugs. Damages to DNA, mitochondria, membrane and other organelles within normal tissue cells such as cardiomyocytes and endothelial cells lead to the release of some toxins which are associated with triggering inflammatory cells to release several types of cytokines, chemokines, ROS, and RNS. The release of some molecules following radiotherapy or chemotherapy stimulates reduction/oxidation (redox) reactions. Redox reactions cause remarkable changes in the level of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Excessive production of ROS and RNS or suppression of antioxidant defense enzymes leads to damage to critical macromolecules, which may continue for long times. Increased levels of some cytokines and oxidative injury are hallmarks of heart injury following cancer therapy. Redox reactions may be involved in several heart disorders such as fibrosis, cardiomyopathy, and endothelium injury. In the current review, we explain the cellular and molecular mechanisms of redox interactions following radiotherapy, chemotherapy, and targeted cancer therapy. Afterward, we explain the evidence of the involvement of redox reactions in heart diseases.

Low dose ionizing radiation and the immune response: what is the role of non-targeted effects?

OBJECTIVES

This review aims to trace the historical narrative surrounding the low dose effects of radiation on the immune system and how our understanding has changed from the beginning of the 20th century to now. The particular focus is on the non-targeted effects (NTEs) of low dose ionizing radiation (LDIR) which are effects that occur when irradiated cells emit signals that cause effects in the nearby or distant non-irradiated cells known as radiation induced bystander effect (RIBE). Moreover, radiation induced genomic instability (RIGI) and abscopal effect (AE) also regarded as NTE. This was prompted by our recent discovery that ultraviolet A (UVA) photons are emitted by the irradiated cells and that these photons can trigger NTE such as the RIBE in unirradiated recipients of these photons. Given the well-known association between UV radiation and the immune response, where these biophotons may pose as bystander signals potentiating processes in deep tissues as a consequence of LDIR, it is timely to review the field with a fresh lens. Various pathways and immune components that contribute to the beneficial and adverse types of modulation induced by LDR will also be revisited.

CONCLUSION

There is limited evidence for LDIR induced immune effects by way of a non-targeted mechanism in biological tissue. The literature examining low to medium dose effects of ionizing radiation on the immune system and its components is complex and controversial. Early work was compromised by lack of good dosimetry while later work mainly looks at the involvement of immune response in radiotherapy. There is a lack of research in the LDIR/NTE field focusing on immune response although bone marrow stem cells and lineages were critical in the identification and characterization of NTE where effects like RIGI and RIBE were heavily researched. This may be in part, a result of the difficulty of isolating NTE in whole organisms which are essential for good immune response studies. Models involving inter organism transmission of NTE are a promising route to overcome these issues.

Radiation exposure and mitochondrial insufficiency in chronic fatigue and immune dysfunction syndrome

Chronic fatigue and Immune Dysfunction Syndrome (CFIDS) is a heterogeneous disease that may be promoted by various environmental stressors, including viral infection, toxin uptake, and ionizing radiation exposure. Previous studies have identified mitochondrial dysfunction in CFIDS patients, including modulation of mitochondrial respiratory chain activity, deletions in the mitochondrial genome, and upregulation of reactive oxygen species (ROS). This paper focuses on radiation effects and hypothesizes that CFIDS is primarily caused by stressor-induced mitochondrial metabolic insufficiency, which results in decreased energy production and anabolic metabolites required for normal cellular metabolism. Furthermore, tissues neighbouring or distant from directly perturbed tissues compensate for this dysfunction, which causes symptoms associated with CFIDS. This hypothesis is justified by reviewing the links between radiation exposure and CFIDS, cancer, immune dysfunction, and induction of oxidative stress. Moreover, the relevance of mitochondria in cellular responses to radiation and metabolism are discussed and putative mitochondrial biomarkers for CFIDS are introduced. Implications for diagnosis are then described, including a potential urine assay and PCR test for mitochondrial genome mutations. Finally, future research needs are offered with an emphasis on where rapid progress may be made to assist the afflicted.

Evidence of bystander effect induced by radiofrequency radiation in a human neuroblastoma cell line

In previous studies we demonstrated that radiofrequency (RF) electromagnetic fields (EMF) is able to reduce DNA damage induced by a subsequent treatment with genotoxic agents, resembling the adaptive response, a phenomenon well known in radiobiology. In this study we report on the capability of the culture medium from SH-SY5Y neuroblastoma cells exposed to 1950 MHz to elicit, in recipient non-exposed cells, a reduction of menadione-induced DNA damage (P < 0.05; comet assay), indicating the capability of non-ionizing radiation to elicit a bystander effect. A comparable reduction was also detected in cultures directly exposed to the same EMF conditions (P < 0.05), confirming the adaptive response. In the same exposure conditions, we also evidenced an increase of heat shock protein 70 (hsp70) in culture medium of cells exposed to RF with respect to sham exposed ones (P < 0.05; western blot analysis), while no differences were detected in the intracellular content of hsp70. On the whole, our results evidence a protective effect of RF against menadione-induced DNA damage in directly and non-directly exposed cells, and suggest hsp70 pathway to be investigated as one of the potential candidate underpinning the interaction between RF exposure and biological systems.

HMGB1 knockdown increases the radiosensitivity of esophageal squamous cell carcinoma by regulating the expression of molecules involved in DNA repair

Radiotherapy is an effective therapeutic strategy in esophageal squamous cell carcinoma (ESCC). However, acquired radioresistance of cancer cells leads to radiotherapy failure. The present study aimed to investigate the mechanisms of the effect of high mobility group box 1 (HMGB1) on the radiation sensitivity of ESCC. Small interfering RNA (si) transfection was used to generate three groups of TE-1 cells (TE-1, negative control and TE-1+siHMGB1), and the protein expression levels of HMGB1 in TE-1 cells were detected by western blotting. These groups of TE-1 cells were irradiated with different doses (0, 2, 4, 6 and 8 Gy) of X-rays after transfection. Subsequently, the viability of TE-1 cells was detected using an MTT assay, and the survival fraction of TE-1 cells was observed using a colony formation assay. The apoptotic rate, reactive oxygen species (ROS) content and levels of phosphorylated (p)-histone H2AX at S139 (p-γH2AX) of the cells were detected by flow cytometry. The alterations in mRNA expression levels of nicotinamide adenine nucleotide phosphate oxidase (NOX)1 and NOX5 were detected by reverse transcription-quantitative PCR, while the changes in protein levels of caspase-3, poly(ADP-ribose) polymerase, p-p38, p-ERK1/2 and p-JNK were detected by western blotting. The results revealed that HMGB1 knockdown significantly decreased cell viability, and the apoptosis rate of TE-1 cells transfected with siHMGB1 combined with radiation treatment was increased compared with that in cells with either siHMGB1 transfection or radiation treatment alone. HMGB1 knockdown increased nicotinamide adenine nucleotide phosphate oxidase-mediated ROS production and induced DNA damage via the MAPK signaling pathway, which may promote apoptosis and radiosensitivity after radiation in TE-1 cells. In conclusion, targeting HMGB1 may represent a promising strategy to increase the efficacy of radiation therapy for ESCC.

Genotoxic Bystander Signals from Irradiated Human Mesenchymal Stromal Cells Mainly Localize in the 10-100 kDa Fraction of Conditioned Medium

Genotoxic bystander signals released from irradiated human mesenchymal stromal cells (MSC) may induce radiation-induced bystander effects (RIBEs) in human hematopoietic stem and progenitor cells (HSPC), potentially causing leukemic transformation. Although the source of bystander signals is evident, the identification and characterization of these signals is challenging. Here, RIBEs were analyzed in human CD34+ cells cultured in distinct molecular size fractions of medium, conditioned by 2 Gy irradiated human MSC. Specifically, γH2AX foci (as a marker of DNA double-strand breaks) and chromosomal instability were evaluated in CD34+ cells grown in approximate (I) < 10 kDa, (II) 10–100 kDa and (III) > 100 kDa fractions of MSC conditioned medium and un-/fractionated control medium, respectively. Hitherto, significantly increased numbers of γH2AX foci (p = 0.0286) and aberrant metaphases (p = 0.0022) were detected in CD34+ cells grown in the (II) 10–100 kDa fraction (0.67 ± 0.10 γH2AX foci per CD34+ cell ∨ 3.8 ± 0.3 aberrant metaphases per CD34+ cell sample; mean ± SEM) when compared to (I) < 10 kDa (0.19 ± 0.01 ∨ 0.3 ± 0.2) or (III) > 100 kDa fractions (0.23 ± 0.04 ∨ 0.4 ± 0.4) or un-/fractionated control medium (0.12 ± 0.01 ∨ 0.1 ± 0.1). Furthermore, RIBEs disappeared after heat inactivation of medium at 75 °C. Taken together, our data suggest that RIBEs are mainly mediated by the heat-sensitive (II) 10–100 kDa fraction of MSC conditioned medium. We postulate proteins as RIBE mediators and in-depth proteome analyses to identify key bystander signals, which define targets for the development of next-generation anti-leukemic drugs.

The Roles of HIF-1α in Radiosensitivity and Radiation-Induced Bystander Effects Under Hypoxia

Radiation-induced bystander effects (RIBE) may have potential implications for radiotherapy, yet the radiobiological impact and underlying mechanisms in hypoxic tumor cells remain to be determined. Using two human tumor cell lines, hepatoma HepG2 cells and glioblastoma T98G cells, the present study found that under both normoxic and hypoxic conditions, increased micronucleus formation and decreased cell survival were observed in non-irradiated bystander cells which had been co-cultured with X-irradiated cells or treated with conditioned-medium harvested from X-irradiated cells. Although the radiosensitivity of hypoxic tumor cells was lower than that of aerobic cells, the yield of micronucleus induced in bystander cells under hypoxia was similar to that measured under normoxia indicating that RIBE is a more significant factor in overall radiation damage of hypoxic cells. When hypoxic cells were treated with dimethyl sulfoxide (DMSO), a scavenger of reactive oxygen species (ROS), or aminoguanidine (AG), an inhibitor of nitric oxide synthase (NOS), before and during irradiation, the bystander response was partly diminished. Furthermore, when only hypoxic bystander cells were pretreated with siRNA hypoxia-inducible factor-1α (HIF-1α), RIBE were decreased slightly but if irradiated cells were treated with siRNA HIF-1α, hypoxic RIBE decreased significantly. In addition, the expression of HIF-1α could be increased in association with other downstream effector molecules such as glucose transporter 1 (GLUT-1), vascular endothelial growth factor (VEGF), and carbonic anhydrase (CA9) in irradiated hypoxic cells. However, the expression of HIF-1α expression in bystander cells was decreased by a conditioned medium from isogenic irradiated cells. The current results showed that under hypoxic conditions, irradiated HepG2 and T98G cells showed reduced radiosensitivity by increasing the expression of HIF-1α and induced a syngeneic bystander effect by decreasing the expression of HIF-1α and regulating its downstream target genes in both the irradiated or bystander cells.

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.

Radiation-Induced Salivary Gland Dysfunction: Mechanisms, Therapeutics and Future Directions

Salivary glands sustain collateral damage following radiotherapy (RT) to treat cancers of the head and neck, leading to complications, including mucositis, xerostomia and hyposalivation. Despite salivary gland-sparing techniques and modified dosing strategies, long-term hypofunction remains a significant problem. Current therapeutic interventions provide temporary symptom relief, but do not address irreversible glandular damage. In this review, we summarize the current understanding of mechanisms involved in RT-induced hyposalivation and provide a framework for future mechanistic studies. One glaring gap in published studies investigating RT-induced mechanisms of salivary gland dysfunction concerns the effect of irradiation on adjacent non-irradiated tissue via paracrine, autocrine and direct cell-cell interactions, coined the bystander effect in other models of RT-induced damage. We hypothesize that purinergic receptor signaling involving P2 nucleotide receptors may play a key role in mediating the bystander effect. We also discuss promising new therapeutic approaches to prevent salivary gland damage due to RT.

Radiation-induced Bystander Effect (RIBE) alters mitochondrial metabolism using a human rectal cancer ex vivo explant model

Locally advanced rectal cancer is treated with neoadjuvant-chemoradiotherapy, however only 22% of patients achieve a complete response. Resistance mechanisms are poorly understood. Radiation-induced Bystander Effect (RIBE) describes the effect of radiation on neighbouring unirradiated cells. We investigated the effects of ex vivo RIBE-induction from normal and rectal cancer tissue on bystander cell metabolism, mitochondrial function and metabolomic profiling. We correlated bystander events to patient clinical characteristics.

Ex vivo RIBE-induction caused metabolic alterations in bystander cells, specifically reductions in OXPHOS following RIBE-induction in normal (p = 0.01) and cancer tissue (p = 0.03) and reduced glycolysis following RIBE-induction in cancer tissue (p = 0.01). Visceral fat area correlated with glycolysis (p = 0.02) and ATP production (p = 0.03) following exposure of cells to TCM from irradiated cancer biopsies. Leucine levels were reduced in the irradiated cancer compared to the irradiated normal secretome (p = 0.04). ROS levels were higher in cells exposed to the cancer compared to the normal secretome (p = 0.04).

RIBE-induction ex vivo causes alterations in the metabolome in normal and malignant rectal tissue along with metabolic alterations in bystander cellular metabolism. This may offer greater understanding of the effects of RIBE on metabolism, mitochondrial function and the secreted metabolome.

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RIBE induction ex vivo alters mitochondrial metabolism in bystander cells.

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Rectal cancer secretome increases ROS in bystander cells.

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Higher leucine levels in the irradiated normal rectal secretome compared to the irradiated rectal cancer secretome

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Glycolysis and ATP levels in bystander cells correlate with patient's visceral fat area.

Suppression of innate immune signaling molecule, MAVS, reduces radiation-induced bystander effect

PURPOSE

Mitochondrial antiviral signaling (MAVS) protein, located in the mitochondrial out-membrane, is necessary for IFN-beta induction and IFN-stimulated gene expression in response to external stress such as viral invasion and ionizing radiation (IR). Although the involvement of radiation induced bystander effect (RIBE) has been investigated for decades for secondary cancer risk related to radiotherapy, the underlying regulatory mechanisms remain largely unclear, especially the roles played by the immune factors such as MAVS.

MATERIAL AND METHODS

MAVS gene knockout cells using CRISPR/Cas9 technology were used as donor cells or recipient cells to assess the role of MAVS in RIBE by means of co-cultured system. The micronucleus and γH2AX foci in the recipient cells were counted to demonstrate the degree of RIBE. The reactive oxygen species (ROS) level in the recipient was measured using the fluorescent dye 2'7'-dichlorofluorescein.

RESULTS

Firstly, we found that MAVS expression level was different in A549, BEAS-2B, U937 and HepG2 cells. Cell co-culture experiments showed that MAVS participate in RIBE. Interestingly, the RIBE response was more significant in recipient cells with higher level of MAVS (i.e. A549) than that in recipient cells showing lower level of MAVS (i.e. BEAS-2B). Further, the bystander response was dramatically suppressed in MAVS-silenced A549 and BEAS-2B recipient cells. MAVS-silenced recipient cells exhibited lower level of ROS induced by IR.

CONCLUSIONS

Our results indicated that the innate immune signaling molecule MAVS in recipient cells participate in RIBE. ROS is an important factor in RIBE via MAVS pathway and MAVS may be a potential target for the precise radiotherapy and radioprotection.

Cx43 channels and signaling via IP3/Ca2+, ATP, and ROS/NO propagate radiation-induced DNA damage to non-irradiated brain microvascular endothelial cells

Radiotherapeutic treatment consists of targeted application of radiation beams to a tumor but exposure of surrounding healthy tissue is inevitable. In the brain, ionizing radiation induces breakdown of the blood-brain barrier by effects on brain microvascular endothelial cells. Damage from directly irradiated cells can be transferred to surrounding non-exposed bystander cells, known as the radiation-induced bystander effect. We investigated involvement of connexin channels and paracrine signaling in radiation-induced bystander DNA damage in brain microvascular endothelial cells exposed to focused X-rays. Irradiation caused DNA damage in the directly exposed area, which propagated over several millimeters in the bystander area. DNA damage was significantly reduced by the connexin channel-targeting peptide Gap26 and the Cx43 hemichannel blocker TAT-Gap19. ATP release, dye uptake, and patch clamp experiments showed that hemichannels opened within 5 min post irradiation in both irradiated and bystander areas. Bystander signaling involved cellular Ca²⁺ dynamics and IP₃, ATP, ROS, and NO signaling, with Ca²⁺, IP₃, and ROS as crucial propagators of DNA damage. We conclude that bystander effects are communicated by a concerted cascade involving connexin channels, and IP₃/Ca²⁺, ATP, ROS, and NO as major contributors of regenerative signal expansion.

Tidal variations of background ionizing radiation and circadian timing of the suprachiasmatic nucleus clock

Recently, correlations of different physiological processes in humans with variations in the local lunisolar gravitational tide force have been observed under highly controlled laboratory conditions. Understanding of the physical nature of this phenomenon needs a comprehensive study of its possible molecular mechanisms. One of the possible timing cues is the strong periodic variation of the emanation fields of radon-222 and its progeny produced by tidal deformations of geological environment. In the present work, we argue that this variation could induce temporal modulation of radiation-induced bystander signaling pathways associated with fundamental regulators of gene expression in the suprachiasmatic nucleus clock.

The Radiation-Induced Bystander Effect (RIBE) and its Connections with the Hallmarks of Cancer

Radiation therapy is one of the pillars of cancer treatment, with approximately one half of all cancer patients receiving it as part of their standard of care. Emerging evidence indicates that the biological effects of radiation are not limited to targeted cells. The radiation-induced bystander effect (RIBE) refers to the plethora of biological phenomena occurring in nonirradiated cells as a result of signal transmission from an irradiated cell. Experimental evidence has linked RIBE to numerous hallmarks of cancer including resisting cell death, tumor immune evasion, genomic instability, deregulated cellular energetics, tumor-promoting inflammation and sustained proliferative signaling as well as enhanced radioresistance, thus highlighting the potential role of RIBE events in patient treatment response. The mechanisms underlying RIBE events in vivo are poorly understood. However, elucidating the molecular mechanisms involved in their manifestation may reveal novel therapeutic targets to improve radiation response in cancer patients.

Relevance of Non-Targeted Effects for Radiotherapy and Diagnostic Radiology; A Historical and Conceptual Analysis of Key Players

Non-targeted effects (NTE) such as bystander effects or genomic instability have been known for many years but their significance for radiotherapy or medical diagnostic radiology are far from clear. Central to the issue are reported differences in the response of normal and tumour tissues to signals from directly irradiated cells. This review will discuss possible mechanisms and implications of these different responses and will then discuss possible new therapeutic avenues suggested by the analysis. Finally, the importance of NTE for diagnostic radiology and nuclear medicine which stems from the dominance of NTE in the low-dose region of the dose–response curve will be presented. Areas such as second cancer induction and microenvironment plasticity will be discussed.

An investigation into neutron-induced bystander effects: How low can you go?

Neutron radiation is very harmful to both individual organisms and the environment. A n understanding of all aspects of both direct and indirect effects of radiation is necessary to accurately assess the risk of neutron radiation exposure. This review seeks to review current evidence in the literature for radiation-induced bystander effects and related effects attributable to neutron radiation. It also attempts to determine if the suggested evidence in the literature is sufficient to justify claims that neutron-based radiation can cause radiation-induced bystander effects. Lastly, the present paper suggests potential directions for future research concerning neutron radiation-induced bystander effects. Data was collected from studies investigating radiation-induced bystander effects and was used to mathematically generate pooled datasets and putative trends; this was done to potentially elucidate both the appearance of a conventional trend for radiation-induced bystander effects in studies using different types of radiation. Furthermore, literature review was used to compare studies utilizing similar tissue models to determine if neutron effects follow similar trends as those produced by electromagnetic radiation. We conclude that the current understanding of neutron-attributable radiation-induced bystander effects is incomplete. Various factors such as high gamma contamination during the irradiations, unestablished thresholds for gamma effects, different cell lines, energies, and different dose rates affected our ability to confirm a relationship between neutron irradiation and RIBE, particularly in low-dose regions below 100 mGy. It was determined through meta-analysis of the data that effects attributable to neutrons do seem to exist at higher doses, while gamma effects seem likely predominant at lower dose regions. Therefore, whether neutrons can induce bystander effects at lower doses remains unclear. Further research is required to confirm these findings and various recommendations are made to assist in this effort. With these recommendations, we hope that research conducted in the future will be better equipped to explore the indirect effects of neutron radiation as they pertain to biological and ecological phenomena.

Direct and indirect effects of exposure to 900 MHz GSM radiofrequency electromagnetic fields on CHO cell line: Evidence of bystander effect by non-ionizing radiation

INTRODUCTION

The rapid rise in global concerns about the adverse health effects of exposure to radiofrequency radiation (RFR) generated by common devices such as mobile phones has prompted scientists to further investigate the biological effects of these environmental exposures. Non-targeted effects (NTEs) are responses which do not need a direct exposure to be expressed and are particularly significant at low energy radiations. Although NTEs of ionizing radiation are well documented, there are scarcely any studies on non-targeted responses such as bystander effect (BE) after exposure to non-ionizing radiation. The main goal of this research is to study possible RFR-induced BE.

MATERIAL AND METHODS

Chinese hamster ovary cells were exposed to 900 MHz GSM RFR at an average specific absorption rate (SAR) of 2 W/kg for 4, 12 and 24 hours (h). To generate a uniformly distributed electromagnetic field and avoid extraneous RF exposures a cavity was desined and used. Cell membrane permeability, cell redox activity, metabolic and mitotic cell death and DNA damages were analyzed. Then the most effective exposure durations and statistically significant altered parameters were chosen to assess the induction of BE through medium transfer procedure. Furthermore, intra and extra cellular reactive oxygen species (ROS) levels were measured to assess the molecular mechanism of BE induced by non-ionizing radiation.

RESULTS

No statistically significant alteration was found in cell membrane permeability, cell redox activity, metabolic cell activity and micronuclei (MN) frequency in the cells directly exposed to RFR for 4, 12, or 24 h. However, RFR exposure for 24 h caused a statistically significant decrease in clonogenic ability as well as a statistically significant increase in olive moment in both directly exposed and bystander cells which received media from RFR-exposed cells (conditioned culture medium; CCM). Exposure to RFR also statistically significant elevated both intra and extra cellular levels of ROS.

CONCLUSION

Our observation clearly indicated the induction of BE in cells treated with CCM. To our knowledge, this is the first report that a non-ionizing radiation (900 MHz GSM RFR) can induce bystander effect. As reported for ionizing radiation, our results proposed that ROS can be a potential molecule in indirect effect of RFR. On the other hand, we found the importance of ROS in direct effect of RFR but in different ways.

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.

Reactive oxygen species and nitric oxide signaling in bystander cells

It is now well accepted that radiation induced bystander effects can occur in cells exposed to media from irradiated cells. The aim of this study was to follow the bystander cells in real time following addition of media from irradiated cells and to determine the effect of inhibiting these signals. A human keratinocyte cell line, HaCaT cells, was irradiated (0.005, 0.05 and 0.5 Gy) with γ irradiation, conditioned medium was harvested after one hour and added to recipient bystander cells. Reactive oxygen species, nitric oxide, Glutathione levels, caspase activation, cytotoxicity and cell viability was measured after the addition of irradiated cell conditioned media to bystander cells. Reactive oxygen species and nitric oxide levels in bystander cells treated with 0.5Gy ICCM were analysed in real time using time lapse fluorescence microscopy. The levels of reactive oxygen species were also measured in real time after the addition of extracellular signal-regulated kinase and c-Jun amino-terminal kinase pathway inhibitors. ROS and glutathione levels were observed to increase after the addition of irradiated cell conditioned media (0.005, 0.05 and 0.5 Gy ICCM). Caspase activation was found to increase 4 hours after irradiated cell conditioned media treatment (0.005, 0.05 and 0.5 Gy ICCM) and this increase was observed up to 8 hours and there after a reduction in caspase activation was observed. A decrease in cell viability was observed but no major change in cytotoxicity was found in HaCaT cells after treatment with irradiated cell conditioned media (0.005, 0.05 and 0.5 Gy ICCM). This study involved the identification of key signaling molecules such as reactive oxygen species, nitric oxide, glutathione and caspases generated in bystander cells. These results suggest a clear connection between reactive oxygen species and cell survival pathways with persistent production of reactive oxygen species and nitric oxide in bystander cells following exposure to irradiated cell conditioned media.

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.

The Impact of Hypoxia on Out-of-Field Cell Survival after Exposure to Modulated Radiation Fields

Advanced radiotherapy techniques such as intensity modulated radiation therapy achieve highly conformal dose distributions within target tumor volumes through the sequential delivery of multiple spatially and temporally modulated radiation fields and have been shown to influence radiobiological response. The goals of this study were to determine the effect of hypoxia on the cell survival responses of different cell models (H460, DU145, A549, MDA231 and FADU) to modulated fields and to characterize the time dependency of signaling under oxic conditions, following reoxygenation and after prolonged hypoxia. Hypoxia was induced by incubating cells at 95% nitrogen and 5% carbon dioxide for 4 h prior to irradiation. The out-of-field response in MDA231 cells was oxygen dependent and therefore selected for co-culture studies to determine the signaling kinetics at different time intervals after irradiation under oxic and hypoxic conditions. Under both oxic and hypoxic conditions, significant increases in cell survival were observed in-field with significant decreases in survival observed out-of-field (P < 0.05), which were dependent on intercellular communication. The in-field response of MDA231 cells showed no significant time dependency up to 24 h postirradiation, while out-of-field survival decreased significantly during the first 6 h postirradiation (P < 0.05). While in-field responses were oxygen dependent, out-of-field effects were observed to be independent of oxygen, with similar or greater cell killing under hypoxic conditions. This study provides further understanding of intercellular signaling under hypoxic conditions and highlights the need for further refinement of established radiobiological models for future applications in advanced radiotherapies.

Focus small to find big - the microbeam story

PURPOSE

Even though the first ultraviolet microbeam was described by S. Tschachotin back in 1912, the development of sophisticated micro-irradiation facilities only began to flourish in the late 1980s. In this article, we highlight significant microbeam experiments, describe the latest microbeam irradiator configurations and critical discoveries made by using the microbeam apparatus.

MATERIALS AND METHODS

Modern radiological microbeams facilities are capable of producing a beam size of a few micrometers, or even tens of nanometers in size, and can deposit radiation with high precision within a cellular target. In the past three decades, a variety of microbeams has been developed to deliver a range of radiations including charged particles, X-rays, and electrons. Despite the original intention for their development to measure the effects of a single radiation track, the ability to target radiation with microbeams at sub-cellular targets has been extensively used to investigate radiation-induced biological responses within cells.

RESULTS

Studies conducted using microbeams to target specific cells in a tissue have elucidated bystander responses, and further studies have shown reactive oxygen species (ROS) and reactive nitrogen species (RNS) play critical roles in the process. The radiation-induced abscopal effect, which has a profound impact on cancer radiotherapy, further reaffirmed the importance of bystander effects. Finally, by targeting sub-cellular compartments with a microbeam, we have reported cytoplasmic-specific biological responses. Despite the common dogma that nuclear DNA is the primary target for radiation-induced cell death and carcinogenesis, studies conducted using microbeam suggested that targeted cytoplasmic irradiation induces mitochondrial dysfunction, cellular stress, and genomic instability. A more recent development in microbeam technology includes application of mouse models to visualize in vivo DNA double-strand breaks.

CONCLUSIONS

Microbeams are making important contributions towards our understanding of radiation responses in cells and tissue models.

Nocifensive Behaviors in Mice with Radiation-Induced Oral Mucositis

Oral mucositis can result in significant dysphagia, and is the most common dose-limiting acute toxicity in head and neck cancer patients receiving chemoradiotherapy. There is a critical need to determine the cellular and molecular mechanisms that underlie radiotherapy-associated discomfort in patients with mucositis. The objective was to induce oral mucositis in mice, using a clinical linear accelerator, and to quantify resultant discomfort, and characterize peripheral sensitization. A clinical linear accelerator was used to deliver ionizing radiation to the oral cavity of mice. Mucositis severity scoring, and various behavioral assays were performed to quantify bouts of orofacial wiping and scratching, bite force, gnawing behavior and burrowing activity. Calcium imaging was performed on neurons of the trigeminal ganglia. Glossitis was induced with a single fraction of at least 27 Gy. Body weight decreased and subsequently returned to baseline, in concert with development and resolution of mucositis, which was worst at day 10 and 11 postirradiation, however was resolved within another 10 days. Neither bite force, nor gnawing behavior were measurably affected. However, burrowing activity was decreased, and both facial wiping and scratching were increased while mice had visible mucositis lesions. Sensory nerves of irradiated mice were more responsive to histamine, tumor necrosis factor alpha and capsaicin. Radiation-induced glossitis is associated with hyper-reactivity of sensory neurons in the trigeminal ganglia of mice, and is accompanied by several behaviors indicative of both itch and pain. These data validate an appropriate model for cancer treatment related discomfort in humans.

Protective effect of mild endoplasmic reticulum stress on radiation-induced bystander effects in hepatocyte cells

Radiation-induced bystander effect (RIBE) has important implications for secondary cancer risk assessment during cancer radiotherapy, but the defense and self-protective mechanisms of bystander normal cells 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 cells under either normoxia or hypoxia, where the ratio of the yield of bystander MN induction to the yield of radiation-induced MN formation under hypoxia was much higher than that of normoxia. Nonetheless, thapsigargin induced endoplasmic reticulum (ER) stress and dramatically suppressed this bystander response manifested as the decrease of MN and apoptosis inductions. Meanwhile, the interference of BiP gene, a major ER chaperone, amplified the detrimental RIBE. More precisely, thapsigargin provoked ER sensor of PERK to initiate an instantaneous and moderate ER stress thus defensed the hazard form RIBE, while BiP depletion lead to persistently destroyed homeostasis of ER and exacerbated cell injury. These findings provide new insights that the mild ER stress through BiP-PERK-p-eIF2α signaling pathway has a profound role in protecting cellular damage from RIBE and hence may decrease the potential secondary cancer risk after cancer radiotherapy.

An evaluation of novel real-time technology as a tool for measurement of radiobiological and radiation-induced bystander effects

The xCELLigence real-time cell impedance system uses a non-invasive and label-free method to create a cell index that is a composite measure of cell proliferation. The aim of this study was to evaluate xCELLigence against clonogenic assay (gold standard) for measuring radiobiological effects and radiation-induced bystander effects (RIBE). A radiobiological study was conducted by irradiating EMT6.5, 4T1.2 and NMUMG cell lines with different radiation doses, while a RIBE study was done using transfer of conditioned media (CM) harvested from donor to the same type of recipient cell (EMT6.5, 4T1.2, NMUMG, HACAT and SW48). CM was harvested using two protocols which differed in the dose chosen and the exposure to the recipient cells. Results showed that xCELLigence measured a radiobiological effect which correlated with the clonogenic assay. For the RIBE study, no statistically significant differences were observed between xCELLigence or clonogenic survival in control or recipient cells incubated with CM in protocol one. However, there was a significant increase in cell index slope using CM from EMT-6.5 cells irradiated at 7.5 Gy compared with the control group under the second protocol. No other evidence of RIBE was detected by either xCELLigence or clonogenic assay. In conclusion, xCELLigence methods can measure radiobiological effects and the results correlate with clonogenic assay. We observed a lack of RIBE in all tested cell lines with the clonogenic assay; however, we observed a RIBE effect in EMT6.5 cells under one particular protocol that showed RIBE is cell type dependent, is not universally observed and can be detected in different assays.

Non-targeted effects and radiation-induced carcinogenesis: a review

Exposure to ionising radiation is clearly associated with an increased risk of developing some types of cancer. However, the contribution of non-targeted effects to cancer development after exposure to ionising radiation is far less clear. The currently used cancer risk model by the international radiation protection community states that any increase in radiation exposure proportionately increases the risk of developing cancer. However, this stochastic cancer risk model does not take into account any contribution from non-targeted effects. Nor does it consider the possibility of a bystander mechanism in the induction of genomic instability. This paper reviews the available evidence to date for a possible role for non-targeted effects to contribute to cancer development after exposure to ionising radiation. An evolution in the understanding of the mechanisms driving non-targeted effects after exposure to ionising radiation is critical to determine the true contribution of non-targeted effects on the risk of developing cancer. Such an evolution will likely only be achievable through coordinated multidisciplinary teams combining several fields of study including: genomics, proteomics, cell biology, molecular epidemiology, and traditional epidemiology.

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.

Potential strategies to ameliorate risk of radiotherapy-induced second malignant neoplasms

This review is aimed at the issue of radiation-induced second malignant neoplasms (SMN), which has become an important problem with the increasing success of modern cancer radiotherapy (RT). It is imperative to avoid compromising the therapeutic ratio while addressing the challenge of SMN. The dilemma is illustrated by the role of reactive oxygen species in both the mechanisms of tumor cell kill and of radiation-induced carcinogenesis. We explore the literature focusing on three potential routes of amelioration to address this challenge. An obvious approach to avoiding compromise of the tumor response is the use of radioprotectors or mitigators that are selective for normal tissues. We also explore the opportunities to avoid protection of the tumor by topical/regional radioprotection of normal tissues, although this strategy limits the scope of protection. Finally, we explore the role of the bystander/abscopal phenomenon in radiation carcinogenesis, in association with the inflammatory response. Targeted and non-targeted effects of radiation are both linked to SMN through induction of DNA damage, genome instability and mutagenesis, but differences in the mechanisms and kinetics between targeted and non-targeted effects may provide opportunities to lessen SMN. The agents that could be employed to pursue each of these strategies are briefly reviewed. In many cases, the same agent has potential utility for more than one strategy. Although the parallel problem of chemotherapy-induced SMN shares common features, this review focuses on RT associated SMN. Also, we avoid the burgeoning literature on the endeavor to suppress cancer incidence by use of antioxidants and vitamins either as dietary strategies or supplementation.

Possible scenarios of the influence of low-dose ionizing radiation on neural functioning

Possible scenarios of the influence of ionizing radiation on neural functioning and the CNS are suggested. We argue that the radiation-induced bystander mechanisms associated with Ca(2+) flows, reactive nitrogen and oxygen species, and cytokines might lead to modulation of certain neuronal signaling pathways. The considered scenarios of conjugation of the bystander signaling and the neuronal signaling might result in modulation of certain synaptic receptors, neurogenesis, neurotransmission, channel conductance, synaptic signaling, different forms of neural plasticity, memory formation and storage, and learning. On this basis, corresponding new possible strategies for treating neurodegenerative deceases and mental disorders are proposed. The mechanisms considered might also be associated with neuronal survival and relevant to the treatment for brain injuries. At the same time, these mechanisms might be associated with detrimental effects and might facilitate the development of some neurological and psychiatric disorders.

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.

Radiation-induced bystander effect in non-irradiated glioblastoma spheroid cells

Radiation-induced bystander effects (RIBEs) are detected in cells that are not irradiated but receive signals from treated cells. The present study explored these bystander effects in a U87MG multicellular tumour spheroid model. A medium transfer technique was employed to induce the bystander effect, and colony formation assay was used to evaluate the effect. Relative changes in expression of BAX, BCL2, JNK and ERK genes were analysed using RT-PCR to investigate the RIBE mechanism. A significant decrease in plating efficiency was observed for both bystander and irradiated cells. The survival fraction was calculated for bystander cells to be 69.48% and for irradiated cells to be 34.68%. There was no change in pro-apoptotic BAX relative expression, but anti-apoptotic BCL2 showed downregulation in both irradiated and bystander cells. Pro-apoptotic JNK in bystander samples and ERK in irradiated samples were upregulated. The clonogenic survival data suggests that there was a classic RIBE in U87MG spheroids exposed to 4 Gy of X-rays, using a medium transfer technique. Changes in the expression of pro- and anti-apoptotic genes indicate involvement of both intrinsic apoptotic and MAPK pathways in inducing these effects.

The differential role of human macrophage in triggering secondary bystander effects after either gamma-ray or carbon beam irradiation

The abscopal effect could be an underlying factor in evaluating prognosis of radiotherapy. This study established an in vitro system to examine whether tumor-generated bystander signals could be transmitted by macrophages to further trigger secondary cellular responses after different irradiations, where human lung cancer NCI-H446 cells were irradiated with either γ-rays or carbon ions and co-cultured with human macrophage U937 cells, then these U937 cells were used as a bystander signal transmitter and co-cultured with human bronchial epithelial cells BEAS-2B. Results showed that U937 cells were only activated by γ-irradiated NCI-H446 cells so that the secondary injuries in BEAS-2B cells under carbon ion irradiation were weaker than γ-rays. Both TNF-α and IL-1α were involved in the γ-irradiation induced secondary bystander effect but only TNF-α contributed to the carbon ion induced response. Further assay disclosed that IL-1α but not TNF-α was largely responsible for the activation of macrophages and the formation of micronucleus in BEAS-2B cells. These data suggest that macrophages could transfer secondary bystander signals and play a key role in the secondary bystander effect of photon irradiation, while carbon ion irradiation has conspicuous advantage due to its reduced secondary injury.