Role of gap junctional intercellular communication in radiation-induced bystander effects in human fibroblasts

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.

Impact of the redox environment on propagation of radiation bystander effects: The modulating effect of oxidative metabolism and oxygen partial pressure

Redox modulated pathways play important roles in out-of-field effects of ionizing radiation. We investigated how the redox environment impacts the magnitude of propagation of stressful effects from irradiated to bystander cells. Normal human fibroblasts that have incorporated [³H]-thymidine were intimately co-cultured with bystander cells in a strategy that allowed isolation of bystander cells with high purity. The antioxidant glutathione peroxidase (GPX) was maintained either at wild-type conditions or overexpressed in the bystanders. Following 24 h of coculture, levels of stress-responsive p21^Waf1, p-Hdm2, and connexin43 proteins were increased in bystander cells expressing wild-type GPX relative to respective controls. These levels were significantly attenuated when GPX was ectopically overexpressed, demonstrating by direct approach the involvement of a regulator of intracellular redox homeostasis. Evidence of participation of pro-oxidant compounds was generated by exposing confluent cell cultures to low fluences of 3.7 MeV α particles in presence or absence of t-butyl hydroperoxide. By 3 h post-exposure to fluences wherein only ∼2% of cells are traversed through the nucleus by a particle track, increases in chromosomal damage were greater than expected in absence of the drug (p < 0.001) and further enhanced in its presence (p < 0.05). While maintenance and irradiation of cell cultures at low oxygen pressure (pO₂ 3.8 mm Hg) to mimic in vivo still supported the participation of bystander cells in responses assessed by chromosomal damage and stress-responsive protein levels (p < 0.001), the effects were attenuated compared to ambient pO₂ (155 mm Hg) (p < 0.05). Together, the results show that bystander effects are attenuated at below ambient pO₂ and when metabolic oxidative stress is reduced but increased when the basal redox environment tilts towards oxidizing conditions. They are consistent with bystander effects being independent of radiation dose rate.

Evidence for Recombinant GRP78, CALR, PDIA3 and GPI as Mediators of Genetic Instability in Human CD34+ Cells

Soluble factors released from irradiated human mesenchymal stromal cells (MSC) may induce genetic instability in human CD34+ cells, potentially mediating hematologic disorders. Recently, we identified four key proteins in the secretome of X-ray-irradiated MSC, among them three endoplasmic reticulum proteins, the 78 kDa glucose-related protein (GRP78), calreticulin (CALR), and protein disulfide-isomerase A3 (PDIA3), as well as the glycolytic enzyme glucose-6-phosphate isomerase (GPI). Here, we demonstrate that exposition of CD34+ cells to recombinant GRP78, CALR, PDIA3 and GPI induces substantial genetic instability. Increased numbers of γH2AX foci (p < 0.0001), centrosome anomalies (p = 0.1000) and aberrant metaphases (p = 0.0022) were detected in CD34+ cells upon incubation with these factors. Specifically, γH2AX foci were found to be induced 4−5-fold in response to any individual of the four factors, and centrosome anomalies by 3−4 fold compared to control medium, which contained none of the recombinant proteins. Aberrant metaphases, not seen in the context of control medium, were detected to a similar extent than centrosome anomalies across the four factors. Notably, the strongest effects were observed when all four factors were collectively provided. In summary, our data suggest that specific components of the secretome from irradiated MSC act as mediators of genetic instability in CD34+ cells, thereby possibly contributing to the pathogenesis of radiation-induced hematologic disorders beyond direct radiation-evoked DNA strand breaks.

Proteins Marking the Sequence of Genotoxic Signaling from Irradiated Mesenchymal Stromal Cells to CD34+ Cells

Non-targeted effects (NTE) of ionizing radiation may initiate myeloid neoplasms (MN). Here, protein mediators (I) in irradiated human mesenchymal stromal cells (MSC) as the NTE source, (II) in MSC conditioned supernatant and (III) in human bone marrow CD34+ cells undergoing genotoxic NTE were investigated. Healthy sublethal irradiated MSC showed significantly increased levels of reactive oxygen species. These cells responded by increasing intracellular abundance of proteins involved in proteasomal degradation, protein translation, cytoskeleton dynamics, nucleocytoplasmic shuttling, and those with antioxidant activity. Among the increased proteins were THY1 and GNA11/14, which are signaling proteins with hitherto unknown functions in the radiation response and NTE. In the corresponding MSC conditioned medium, the three chaperones GRP78, CALR, and PDIA3 were increased. Together with GPI, these were the only four altered proteins, which were associated with the observed genotoxic NTE. Healthy CD34+ cells cultured in MSC conditioned medium suffered from more than a six-fold increase in γH2AX focal staining, indicative for DNA double-strand breaks, as well as numerical and structural chromosomal aberrations within three days. At this stage, five proteins were altered, among them IQGAP1, HMGB1, and PA2G4, which are involved in malign development. In summary, our data provide novel insights into three sequential steps of genotoxic signaling from irradiated MSC to CD34+ cells, implicating that induced NTE might initiate the development of MN.

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.

Radiation-Induced Bystander Effect: Loss of Radioprotective Capacity of Rosmarinic Acid In Vivo and In Vitro

In radiation oncology, the modulation of the bystander effect is a target both for the destruction of tumor cells and to protect healthy cells. With this objective, we determine whether the radioprotective capacity of rosmarinic acid (RA) can affect the intensity of these effects. Genoprotective capacity was obtained by determining the micronuclei frequencies in in vivo and in vitro assays and the cell survival was determined by the (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay) (MTT) assay in three cell lines (PNT2, TRAMPC1 and B16F10), both in direct exposure to X-rays and after the production of radiation-induced bystander effect. The administration of RA in irradiated cells produced a decrease in the frequency of micronuclei both in vivo and in vitro, and an increase in cell survival, as expression of its radioprotective effect (p < 0.001) attributable to its ability to scavenge radio-induced free radicals (ROS). However, RA does not achieve any modification in the animals receiving serum or in the cultures treated with the irradiated medium, which expresses an absence of radioprotective capacity. The results suggest that ROS participates in the formation of signals in directly irradiated cells, but only certain subtypes of ROS, the cytotoxic products of lipid peroxidation, participate in the creation of lesions in recipient cells.

Blood Gene Expression Profile Study Revealed the Activation of Apoptosis and p53 Signaling Pathway May Be the Potential Molecular Mechanisms of Ionizing Radiation Damage and Radiation-Induced Bystander Effects

Radiotherapy is an effective treatment for local solid tumors, but the mechanism of damage to human body caused by radiation therapy needs further study. In this study, gene expression profiles of human peripheral blood samples exposed to different doses and rates of ionizing radiation (IR) were used for bioinformatics analysis to investigate the mechanism of IR damage and radiation-induced bystander effect (RIBE). Differentially expressed genes analysis, weighted gene correlation network analysis, functional enrichment analysis, hypergeometric test, gene set enrichment analysis, and gene set variation analysis were applied to analyze the data. Moreover, receiver operating characteristic curve analysis was performed to identify core genes of IR damage. Weighted gene correlation network analysis identified 3 modules associated with IR damage, 2 were positively correlated and 1 was negatively correlated. The analysis showed that the positively correlated modules were significantly involved in apoptosis and p53 signaling pathway, and ESR1, ATM, and MYC were potential transcription factors regulating these modules. Thus, the study suggested that apoptosis and p53 signaling pathway may be the potential molecular mechanisms of IR damage and RIBE, which could be driven by ESR1, ATM, and MYC.

The dialogue between died and viable cells: in vitro and in vivo bystander effects and 1H-NMR-based metabolic profiling of soluble factors

The bystander effect (BE) is an important biological phenomenon that induces damages in distant and not directly affected by a chemical/physical stress cells. This effect, well known in ionizing radiation treatment, relies on reactive signals released by exposed cells and transmitted via cell–cell interaction or culture medium. In this study, cycloheximide (CHX)-induced apoptotic U937 cells and untreated THP-1 cells were chosen to investigate the chemical-induced BE. The effects of apoptotic U937 cells culture medium, Conditioned Medium (CM), on THP-1 cells were evaluated by morphological and immunohistochemical analysis performed by light microscopy; 1D 1H and 2D J-resolved (JRES) NMR metabolomic analysis has been used to characterize the molecules involved in the BE. In summary, this study indicates that: CM of CHX-treated U937 cells induces a time-dependent induction of toxicity, probably apoptotic cell death, and macrophagic differentiation in THP-1 cells; CM contains different metabolites respect fresh culture medium; CM recruits in vivo activated fibroblasts, endothelial cells, macrophages and mononuclear inflammatory cells in rat calf muscles. These data suggest that CHX exposed cells could cause BE through the release, during the apoptotic process, of soluble factors into the medium that could be exploited in anticancer protocols.

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.

Biological Entanglement-Like Effect After Communication of Fish Prior to X-Ray Exposure

The phenomenon by which irradiated organisms including cells in vitro communicate with unirradiated neighbors is well established in biology as the radiation-induced bystander effect (RIBE). Generally, the purpose of this communication is thought to be protective and adaptive, reflecting a highly conserved evolutionary mechanism enabling rapid adjustment to stressors in the environment. Stressors known to induce the effect were recently shown to include chemicals and even pathological agents. The mechanism is unknown but our group has evidence that physical signals such as biophotons acting on cellular photoreceptors may be implicated. This raises the question of whether quantum biological processes may occur as have been demonstrated in plant photosynthesis. To test this hypothesis, we decided to see whether any form of entanglement was operational in the system. Fish from 2 completely separate locations were allowed to meet for 2 hours either before or after which fish from 1 location only (group A fish) were irradiated. The results confirm RIBE signal production in both skin and gill of fish, meeting both before and after irradiation of group A fish. The proteomic analysis revealed that direct irradiation resulted in pro-tumorigenic proteomic responses in rainbow trout. However, communication from these irradiated fish, both before and after they had been exposed to a 0.5 Gy X-ray dose, resulted in largely beneficial proteomic responses in completely nonirradiated trout. The results suggest that some form of anticipation of a stressor may occur leading to a preconditioning effect or temporally displaced awareness after the fish become entangled.

Bystander signaling via oxidative metabolism

The radiation-induced bystander effect (RIBE) is the initiation of biological end points in cells (bystander cells) that are not directly traversed by an incident-radiation track, but are in close proximity to cells that are receiving the radiation. RIBE has been indicted of causing DNA damage via oxidative stress, besides causing direct damage, inducing tumorigenesis, producing micronuclei, and causing apoptosis. RIBE is regulated by signaling proteins that are either endogenous or secreted by cells as a means of communication between cells, and can activate intracellular or intercellular oxidative metabolism that can further trigger signaling pathways of inflammation. Bystander signals can pass through gap junctions in attached cell lines, while the suspended cell lines transmit these signals via hormones and soluble proteins. This review provides the background information on how reactive oxygen species (ROS) act as bystander signals. Although ROS have a very short half-life and have a nanometer-scale sphere of influence, the wide variety of ROS produced via various sources can exert a cumulative effect, not only in forming DNA adducts but also setting up signaling pathways of inflammation, apoptosis, cell-cycle arrest, aging, and even tumorigenesis. This review outlines the sources of the bystander effect linked to ROS in a cell, and provides methods of investigation for researchers who would like to pursue this field of science.

Understanding low radiation background biology through controlled evolution experiments

Biological experiments conducted in underground laboratories over the last decade have shown that life can respond to relatively small changes in the radiation background in unconventional ways. Rapid changes in cell growth, indicative of hormetic behaviour and long-term inheritable changes in antioxidant regulation have been observed in response to changes in the radiation background that should be almost undetectable to cells. Here, we summarize the recent body of underground experiments conducted to date, and outline potential mechanisms (such as cell signalling, DNA repair and antioxidant regulation) that could mediate the response of cells to low radiation backgrounds. We highlight how multigenerational studies drawing on methods well established in studying evolutionary biology are well suited for elucidating these mechanisms, especially given these changes may be mediated by epigenetic pathways. Controlled evolution experiments with model organisms, conducted in underground laboratories, can highlight the short- and long-term differences in how extremely low-dose radiation environments affect living systems, shining light on the extent to which epimutations caused by the radiation background propagate through the population. Such studies can provide a baseline for understanding the evolutionary responses of microorganisms to ionizing radiation, and provide clues for understanding the higher radiation environments around uranium mines and nuclear disaster zones, as well as those inside nuclear reactors.

Extracellular vesicles released following heat stress induce bystander effect in unstressed populations

Cells naïve to stress can display the effects of stress, such as DNA damage and apoptosis, when they are exposed to signals from stressed cells; this phenomenon is known as the bystander effect. We previously showed that bystander effect induced by ionising radiation are mediated by extracellular vesicles (EVs). Bystander effect can also be induced by other types of stress, including heat shock, but it is unclear whether EVs are involved. Here we show that EVs released from heat shocked cells are also able to induce bystander damage in unstressed populations. Naïve cells treated with media conditioned by heat shocked cells showed higher levels of DNA damage and apoptosis than cells treated with media from control cells. Treating naïve cells with EVs derived from media conditioned by heat shocked cells also induced a bystander effect when compared to control, with DNA damage and apoptosis increasing whilst the level of cell viability was reduced. We demonstrate that treatment of naïve cells with heat shocked cell-derived EVs leads to greater invasiveness in a trans-well Matrigel assay. Finally, we show that naïve cells treated with EVs from heat-shocked cells are more likely to survive a subsequent heat shock, suggesting that these EVs mediate an adaptive response. We propose that EVs released following stress mediate an intercellular response that leads to apparent stress in neighbouring cells but also greater robustness in the face of a subsequent insult.

Irradiated human endothelial progenitor cells induce bystander killing in human non-small cell lung and pancreatic cancer cells

Purpose To investigate whether irradiated human endothelial progenitor cells (hEPC) could induce bystander killing in the A549 non-small cell lung cancer (NSCLC) cells and help explain the improved radiation-induced tumor cures observed in A549 tumor xenografts co-injected with hEPC. Materials and methods We investigated whether co-injection of CBM3 hEPC with A549 NSCLC cells would alter tumor xenograft growth rate or tumor cure after a single dose of 0 or 5 Gy of X-rays. We then utilized dual chamber Transwell dishes, to test whether medium from irradiated CBM3 and CBM4 hEPC would induce bystander cell killing in A549 cells, and as an additional control, in human pancreatic cancer MIA PaCa-2 cells. The CBM3 and CBM4 hEPC were plated into the upper Transwell chamber and the A549 or MIA PaCa-2 cells were plated in the lower Transwell chamber. The top inserts with the CBM3 or CBM4 hEPC cells were subsequently removed, irradiated, and then placed back into the Transwell dish for 3 h to allow for diffusion of any potential bystander factors from the irradiated hEPC in the upper chamber through the permeable membrane to the unirradiated cancer cells in the lower chamber. After the 3 h incubation, the cancer cells were re-plated for clonogenic survival. Results We found that co-injection of CBM3 hEPC with A549 NSCLC cells significantly increased the tumor growth rate compared to A549 cells alone, but paradoxically also increased A549 tumor cure after a single dose of 5 Gy of X-rays (p < 0.05). We hypothesized that irradiated hEPC may be inducing bystander killing in the A549 NSCLC cells in tumor xenografts, thus improving tumor cure. Bystander studies clearly showed that exposure to the medium from irradiated CBM3 and CBM4 hEPC induced significant bystander killing and decreased the surviving fraction of A549 and MIA PaCa-2 cells to 0.46 (46%) ± 0.22 and 0.74 ± 0.07 (74%) respectively (p < 0.005, p < 0.0001). In addition, antibody depletion studies demonstrated that the bystander killing induced in both A549 and MIA PaCa-2 cells was mediated by the cytokines TNF-α and TGF-β (p < 0.05). Conclusions These data provide evidence that irradiated hEPC can induce strong bystander killing in A549 and MIA PaCa-2 human cancer cells and that this bystander killing is mediated by the cytokines TNF-α and TGF-β.

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.

Microbial influences on hormesis, oncogenesis, and therapy: A review of the literature

Utilization of environmental stimuli for growth is the main factor contributing to the evolution of prokaryotes and eukaryotes, independently and mutualistically. Epigenetics describes an organism's ability to vary expression of certain genes based on their environmental stimuli. The diverse degree of dose-dependent responses based on their variances in expressed genetic profiles makes it difficult to ascertain whether hormesis or oncogenesis has or is occurring. In the medical field this is shown where survival curves used in determining radiotherapeutic doses have substantial uncertainties, some as large as 50% (Barendsen, 1990). Many in-vitro radiobiological studies have been limited by not taking into consideration the innate presence of microbes in biological systems, which have either grown symbiotically or pathogenically. Present in-vitro studies neglect to take into consideration the varied responses that commensal and opportunistic pathogens will have when exposed to the same stimuli and how such responses could act as stimuli for their macro/microenvironment. As a result many theories such as radiation carcinogenesis explain microscopic events but fail to describe macroscopic events (Cohen, 1995). As such, this review shows how microorganisms have the ability to perturb risks of cancer and enhance hormesis after irradiation. It will also look at bacterial significance in the microenvironment of the tumor before and during treatment. In addition, bacterial systemic communication after irradiation and the host's immune responses to infection could explain many of the phenomena associated with bystander effects. Therefore, the present literature review considers the paradigms of hormesis and oncogenesis in order to find a rationale that ties them all together. This relationship was thus characterized to be the microbiome.

Dependence of the bystander effect for micronucleus formation on dose of heavy-ion radiation in normal human fibroblasts

Ionising radiation-induced bystander effects are well recognised, but its dependence on dose or linear energy transfer (LET) is still a matter of debate. To test this, 49 sites in confluent cultures of AG01522D normal human fibroblasts were targeted with microbeams of carbon (103 keV µm(-1)), neon (375 keV µm(-1)) and argon ions (1260 keV µm(-1)) and evaluated for the bystander-induced formation of micronucleus that is a kind of a chromosome aberration. Targeted exposure to neon and argon ions significantly increased the micronucleus frequency in bystander cells to the similar extent irrespective of the particle numbers per site of 1-6. In contrast, the bystander micronucleus frequency increased with increasing the number of carbon-ion particles in a range between 1 and 3 particles per site and was similar in a range between 3 and 8 particles per site. These results suggest that the bystander effect of heavy ions for micronucleus formation depends on dose.

Role of nitric oxide in the radiation-induced bystander effect

Cells that are not irradiated but are affected by “stress signal factors” released from irradiated cells are called bystander cells. These cells, as well as directly irradiated ones, express DNA damage-related proteins and display excess DNA damage, chromosome aberrations, mutations, and malignant transformation. This phenomenon has been studied widely in the past 20 years, since its first description by Nagasawa and Little in 1992, and is known as the radiation-induced bystander effect (RIBE). Several factors have been identified as playing a role in the bystander response. This review will focus on one of them, nitric oxide (NO), and its role in the stimulation and propagation of RIBE. The hydrophobic properties of NO, which permit its diffusion through the cytoplasm and plasma membranes, allow this signaling molecule to easily spread from irradiated cells to bystander cells without the involvement of gap junction intercellular communication. NO produced in irradiated tissues mediates cellular regulation through posttranslational modification of a number of regulatory proteins. The best studied of these modifications are S-nitrosylation (reversible oxidation of cysteine) and tyrosine nitration. These modifications can up- or down-regulate the functions of many proteins modulating different NO-dependent effects. These NO-dependent effects include the stimulation of genomic instability (GI) and the accumulation of DNA errors in bystander cells without direct DNA damage.

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Ionizing radiation stimulates generation of nitric oxide (NO).

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NO stimulates genomic instability by inhibiting BRCA1 protein expression.

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NO can diffuse and stimulate genomic instability in the bystander cells.

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Propagation of NO from cell-to-cell creates a “mutator fields”.

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Definition of the “mutator filed” is proposed.

Compartmental stress responses correlate with cell survival in bystander effects induced by the DNA damage agent, bleomycin

Physical or chemical stress applied to a cell system trigger a signal cascade that is transmitted to the neighboring cell population in a process known as bystander effect. Despite its wide occurrence in biological systems this phenomenon is mainly documented in cancer treatments. Thus understanding whether the bystander effect acts as an adaptive priming element for the neighboring cells or a sensitization factor is critical in designing treatment strategies. Here we characterize the bystander effects induced by bleomycin, a DNA-damaging agent, and compartmental stress responses associated with this phenomenon. Mouse fibroblasts were treated with increasing concentrations of bleomycin and assessed for DNA damage, cell death and induction of compartmental stress response (endoplasmic reticulum, mitochondrial and cytoplasmic stress). Preconditioned media were used to analyze bystander damage using the same end-points. Bleomycin induced bystander response was reflected primarily in increased DNA damage. This was dependent on the concentration of bleomycin and time of media conditioning. Interestingly, we found that ROS but not NO are involved in the transmission of the bystander effect. Consistent transcriptional down-regulation of the stress response factors tested (i.e. BiP, mtHsp60, Hsp70) occurred in the direct effect indicating that bleomycin might induce an arrest of transcription correlated with decreased survival. We observed the opposite trend in the bystander effect, with specific stress markers appearing increased and correlated with increased survival. These data shed new light on the potential role of stress pathways activation in bystander effects and their putative impact on the pro-survival pro-death balance.

Dynamics of cellular responses to radiation

Understanding the consequences of exposure to low dose ionizing radiation is an important public health concern. While the risk of low dose radiation has been estimated by extrapolation from data at higher doses according to the linear non-threshold model, it has become clear that cellular responses can be very different at low compared to high radiation doses. Important phenomena in this respect include radioadaptive responses as well as low-dose hyper-radiosensitivity (HRS) and increased radioresistance (IRR). With radioadaptive responses, low dose exposure can protect against subsequent challenges, and two mechanisms have been suggested: an intracellular mechanism, inducing cellular changes as a result of the priming radiation, and induction of a protected state by inter-cellular communication. We use mathematical models to examine the effect of these mechanisms on cellular responses to low dose radiation. We find that the intracellular mechanism can account for the occurrence of radioadaptive responses. Interestingly, the same mechanism can also explain the existence of the HRS and IRR phenomena, and successfully describe experimentally observed dose-response relationships for a variety of cell types. This indicates that different, seemingly unrelated, low dose phenomena might be connected and driven by common core processes. With respect to the inter-cellular communication mechanism, we find that it can also account for the occurrence of radioadaptive responses, indicating redundancy in this respect. The model, however, also suggests that the communication mechanism can be vital for the long term survival of cell populations that are continuously exposed to relatively low levels of radiation, which cannot be achieved with the intracellular mechanism in our model. Experimental tests to address our model predictions are proposed.

The effect of low-dose radiation on cells and tissues is a public health concern, because the human population is exposed to low-dose ionizing radiation coming from a variety of sources, such as cosmic rays, soil radioactivity, environmental contaminations, and various medical procedures. At low doses of radiation, phenomena are observed that do not occur at higher doses, such as radioadaptive responses as well as low-dose hyper-radiosensitivity (HRS) and increased radioresistance (IRR), which are so far not fully understood. Each of these phenomena have been investigated separately, and specific mechanisms have been suggested to explain them. Using mathematical models that are successfully fitted to experimental data under a variety of conditions, we show that a set of basic and documented assumptions about cellular responses to low-dose radiation can explain all three low-dose phenomena, indicating that they are inter-related. According to the model, these phenomena are brought about by the multi-factorial interactions that underlie the population dynamics of the cells involved, and this provides a new framework to understand these responses, and to evaluate the risk to human health posed by exposure to low-dose radiation.

Differential effects of p53 on bystander phenotypes induced by gamma ray and high LET heavy ion radiation

High LET particle irradiation has several potential advantages over γ-rays such as p53-independent response. The purpose of this work is to disclose the effect of p53 on the bystander effect induced by different LET irradiations and underlying mechanism. Lymphocyte cells of TK6 (wild type p53) and HMy2.CIR (mutated p53) were exposed to either low or high LET irradiation, then their mitochondrial dysfunction and ROS generation were detected. The micronuclei (MN) induction in HL-7702 hepatocytes co-cultured with irradiated lymphocytes was also measured. It was found that the mitochondrial dysfunction, p66(Shc) activation, and intracellular ROS were enhanced in TK6 but not in HMy2.CIR cells after γ-ray irradiation, but all of them were increased in both cell lines after carbon and iron irradiation. Consistently, the bystander effect of MN formation in HL-7702 cells was only triggered by γ-irradiated TK6 cells but not by γ-irradiated HMy2.CIR cells. But this bystander effect was induced by both lymphocyte cell lines after heavy ion irradiation. PFT-μ, an inhibitor of p53, only partly inhibited ROS generation and bystander effect induced by 30 keV/μm carbon-irradiated TK6 cells but failed to suppress the bystander effect induced by the TK6 cells irradiated with either 70 keV/μm carbon or 180 keV/μm iron. The mitochondrial inhibitors of rotenone and oligomycin eliminated heavy ion induced ROS generation in TK6 and HMy2.CIR cells and hence diminished the bystander effect on HL-7702 cells. These results clearly demonstrate that the bystander effect is p53-dependent for low LET irradiation, but it is p53-independent for high LET irradiation which may be because of p53-independent ROS generation due to mitochondrial dysfunction.

Low doses of gamma-irradiation induce an early bystander effect in zebrafish cells which is sufficient to radioprotect cells

The term “bystander effect” is used to describe an effect in which cells that have not been exposed to radiation are affected by irradiated cells though various intracellular signaling mechanisms. In this study we analyzed the kinetics and mechanisms of bystander effect and radioadaptation in embryonic zebrafish cells (ZF4) exposed to chronic low dose of gamma rays. ZF4 cells were irradiated for 4 hours with total doses of gamma irradiation ranging from 0.01–0.1 Gy. In two experimental conditions, the transfer of irradiated cells or culture medium from irradiated cells results in the occurrence of DNA double strand breaks in non-irradiated cells (assessed by the number of γ-H2AX foci) that are repaired at 24 hours post-irradiation whatever the dose. At low total irradiation doses the bystander effect observed does not affect DNA repair mechanisms in targeted and bystander cells. An increase in global methylation of ZF4 cells was observed in irradiated cells and bystander cells compared to control cells. We observed that pre-irradiated cells which are then irradiated for a second time with the same doses contained significantly less γ-H2AX foci than in 24 h gamma-irradiated control cells. We also showed that bystander cells that have been in contact with the pre-irradiated cells and then irradiated alone present less γ-H2AX foci compared to the control cells. This radioadaptation effect is significantly more pronounced at the highest doses. To determine the factors involved in the early events of the bystander effect, we performed an extensive comparative proteomic study of the ZF4 secretomes upon irradiation. In the experimental conditions assayed here, we showed that the early events of bystander effect are probably not due to the secretion of specific proteins neither the oxidation of these secreted proteins. These results suggest that early bystander effect may be due probably to a combination of multiple factors.

Reciprocal bystander effect between α-irradiated macrophage and hepatocyte is mediated by cAMP through a membrane signaling pathway

Irradiated cells can induce biological effects on vicinal non-irradiated bystander cells, meanwhile the bystander cells may rescue the irradiated cells through a feedback signal stress. To elucidate the nature of this reciprocal effect, we examined the interaction between α-irradiated human macrophage cells U937 and its bystander HL-7702 hepatocyte cells using a cell co-culture system. Results showed that after 6h of cell co-culture, mitochondria depolarization corresponding to apoptosis was significantly induced in the HL-7702 cells, but the formation of micronuclei in the irradiated U937 cells was markedly decreased compared to that without cell co-culture treatment. This reciprocal effect was not observed when the cell membrane signaling pathway was blocked by filipin that inhibited cAMP transmission from bystander cells to irradiated cells. After treatment of cells with exogenous cAMP, forskolin (an activator of cAMP) or KH-7 (an inhibitor of cAMP), respectively, it was confirmed that cAMP communication from bystander cells to targeted cells could mitigate radiation damage in U739 cells, and this cAMP insufficiency in the bystander cells contributed to the enhancement of bystander apoptosis. Moreover, the bystander apoptosis in HL-7702 cells was aggravated by cAMP inhibition but it could not be evoked when p53 of HL-7702 cells was knocked down no matter of forskolin and KH-7 treatment. In conclusion, this study disclosed that cAMP could be released from bystander HL-7702 cells and compensated to α-irradiated U937 cells through a membrane signaling pathway and this cAMP communication played a profound role in regulating the reciprocal bystander effects.

Bystander effect induced by UVC radiation in Chinese hamster V79 cells

In past decades, researches on radiation-induced bystander effect mainly focused on ionizing radiation such as α-particle, β-particle, X-ray and γ-ray. But few researches have been conducted on the ability of ultraviolet (UV) radiation-induced bystander effect, and knowledge of UVC-induced bystander effect is far limited. Here, we adopted medium transfer experiment to detect whether UVC could cause bystander effect in Chinese hamster V79 cells. We determined the cell viability, apoptosis rate, chromosome aberration and ultrastructure changes, respectively. Our results showed that: (1) the viability of UVC-irradiated V79 cells declined significantly with the dosage of UVC; (2) similar to the irradiated cells, the main death type of bystander cells cultured in irradiation conditioned medium (ICMs) was also apoptosis; (3) soluble factors secreted by UVC-irradiated cells could induce bystander effect in V79 cells; (4) cells treated with 4 h ICM collected from 90 mJ cm(-2) UVC-irradiated cells displayed the strongest response. Our data revealed that UVC could cause bystander effect through the medium soluble factors excreted from irradiated cells and this bystander effect was a novel quantitative and kinetic response. These findings might provide a foundation to further explore the exact soluble bystander factors and detailed mechanism underlying UVC-induced bystander effect.

Gap junction communication and the propagation of bystander effects induced by microbeam irradiation in human fibroblast cultures: the impact of radiation quality

Understanding the mechanisms underlying the bystander effects of low doses/low fluences of low- or high-linear energy transfer (LET) radiation is relevant to radiotherapy and radiation protection. Here, we investigated the role of gap-junction intercellular communication (GJIC) in the propagation of stressful effects in confluent normal human fibroblast cultures wherein only 0.036-0.144% of cells in the population were traversed by primary radiation tracks. Confluent cells were exposed to graded doses from monochromatic 5.35 keV X ray (LET ~6 keV/μm), 18.3 MeV/u carbon ion (LET ~103 keV/μm), 13 MeV/u neon ion (LET ~380 keV/μm) or 11.5 MeV/u argon ion (LET ~1,260 keV/μm) microbeams in the presence or absence of 18-α-glycyrrhetinic acid (AGA), an inhibitor of GJIC. After 4 h incubation at 37°C, the cells were subcultured and assayed for micronucleus (MN) formation. Micronuclei were induced in a greater fraction of cells than expected based on the fraction of cells targeted by primary radiation, and the effect occurred in a dose-dependent manner with any of the radiation sources. Interestingly, MN formation for the heavy-ion microbeam irradiation in the absence of AGA was higher than in its presence at high mean absorbed doses. In contrast, there were no significant differences in cell cultures exposed to X-ray microbeam irradiation in presence or absence of AGA. This showed that the inhibition of GJIC depressed the enhancement of MN formation in bystander cells from cultures exposed to high-LET radiation but not low-LET radiation. Bystander cells recipient of growth medium harvested from 5.35 keV X-irradiated cultures experienced stress manifested in the form of excess micronucleus formation. Together, the results support the involvement of both junctional communication and secreted factor(s) in the propagation of radiation-induced stress to bystander cells. They highlight the important role of radiation quality and dose in the observed effects.

Alpha particle-induced bystander effect is mediated by ROS via a p53-dependent SCO2 pathway in hepatoma cells

PURPOSE

The radiation-induced bystander effect (RIBE) has important implications for the efficiency of radiotherapy but the underlying role of cellular metabolism is widely unknown. The roles of synthesis of cytochrome c oxidase 2 (SCO2), a key effector for respiratory chain, and related signaling factors in α-particle-induced bystander damage were currently investigated in a liver cell co-culture system.

MATERIALS AND METHODS

Human hepatoma cells of HepG2 with wild-type p53 (wtp53) and Hep3B (p53 null) were irradiated with 0.4 Gy of α-particles and co-cultured with non-irradiated normal liver cells HL-7702 for 6 h, then the incidence of micronucleus (MN) in the bystander HL-7702 cells was analyzed. The expressions of total P53, phospho-P53 (p-P53), SCO2, and reactive oxygen species (ROS) in the irradiated hepatoma cells were detected. In some experiments, the hepatoma cells were respectively treated with p53 siRNA, SCO2 siRNA, or dimethyl sulfoxide (DMSO) before irradiation.

RESULTS

Bystander damage in HL-7702 cells was induced by α-irradiated HepG2 cells but not by α-irradiated Hep3B cells, and this bystander effect was diminished when the irradiated HepG2 cells were pretreated with p53 siRNA, SCO2 siRNA, or DMSO. Meanwhile, the expressions of p-P53 protein and SCO2 mRNA, the activity of SCO2 protein, and intracellular ROS were all increased in the irradiated HepG2 cells but not Hep3B cells and these expressions were eliminated by p53 siRNA treatment. Moreover, the radiation-enhanced expressions of SCO2 and ROS were inhibited by SCO2 siRNA.

CONCLUSION

α-particle-induced bystander effect was regulated by p53 and its downstream SCO2 in the irradiated hepatoma cells, and ROS generation could be an early event for triggering this bystander response.

Dose calculations for [(131)i] meta-iodobenzylguanidine-induced bystander effects

Targeted radiotherapy is a potentially useful treatment for some cancers and may be potentiated by bystander effects. However, without estimation of absorbed dose, it is difficult to compare the effects with conventional external radiation treatment.

METHODS

Using the Vynckier - Wambersie dose point kernel, a model for dose rate evaluation was created allowing for calculation of absorbed dose values to two cell lines transfected with the noradrenaline transporter (NAT) gene and treated with [(131)I]MIBG.

RESULTS

The mean doses required to decrease surviving fractions of UVW/NAT and EJ138/NAT cells, which received medium from [(131)I]MIBG-treated cells, to 25 - 30% were 1.6 and 1.7 Gy respectively. The maximum mean dose rates achieved during [(131)I]MIBG treatment were 0.09 - 0.75 Gy/h for UVW/NAT and 0.07 - 0.78 Gy/h for EJ138/NAT. These were significantly lower than the external beam gamma radiation dose rate of 15 Gy/h. In the case of control lines which were incapable of [(131)I]MIBG uptake the mean absorbed doses following radiopharmaceutical were 0.03 - 0.23 Gy for UVW and 0.03 - 0.32 Gy for EJ138.

CONCLUSION

[(131)I]MIBG treatment for ICCM production elicited a bystander dose-response profile similar to that generated by external beam gamma irradiation but with significantly greater cell death.

Epigenetics in radiation biology: a new research frontier

The number of people that receive exposure to ionizing radiation (IR) via occupational, diagnostic, or treatment-related modalities is progressively rising. It is now accepted that the negative consequences of radiation exposure are not isolated to exposed cells or individuals. Exposure to IR can induce genome instability in the germline, and is further associated with transgenerational genomic instability in the offspring of exposed males. The exact molecular mechanisms of transgenerational genome instability have yet to be elucidated, although there is support for it being an epigenetically induced phenomenon. This review is centered on the long-term biological effects associated with IR exposure, mainly focusing on the epigenetic mechanisms (DNA methylation and small RNAs) involved in the molecular etiology of IR-induced genome instability, bystander and transgenerational effects. Here, we present evidence that IR-mediated effects are maintained by epigenetic mechanisms, and demonstrate how a novel, male germline-specific, small RNA pathway is posited to play a major role in the epigenetic inheritance of genome instability.

Human cell responses to ionizing radiation are differentially affected by the expressed connexins

In multicellular organisms, intercellular communication is essential for homeostatic functions and has a major role in tissue responses to stress. Here, we describe the effects of expression of different connexins, which form gap junction channels with different permeabilities, on the responses of human cells to ionizing radiation. Exposure of confluent HeLa cell cultures to (137)Cs γ rays, 3.7 MeV α particles, 1000 MeV protons or 1000 MeV/u iron ions resulted in distinct effects when the cells expressed gap junction channels composed of either connexin26 (Cx26) or connexin32 (Cx32). Irradiated HeLa cells expressing Cx26 generally showed decreased clonogenic survival and reduced metabolic activity relative to parental cells lacking gap junction communication. In contrast, irradiated HeLa cells expressing Cx32 generally showed enhanced survival and greater metabolic activity relative to the control cells. The effects on clonogenic survival correlated more strongly with effects on metabolic activity than with DNA damage as assessed by micronucleus formation. The data also showed that the ability of a connexin to affect clonogenic survival following ionizing radiation can depend on the specific type of radiation. Together, these findings show that specific types of connexin channels are targets that may be exploited to enhance radiotherapeutic efficacy and to formulate countermeasures to the harmful effects of specific types of ionizing radiation.

Dose response of micronuclei induced by combination radiation of α-particles and γ-rays in human lymphoblast cells

Combination radiation is a real situation of both nuclear accident exposure and space radiation environment, but its biological dosimetry is still not established. This study investigated the dose-response of micronuclei (MN) induction in lymphocyte by irradiating HMy2.CIR lymphoblast cells with α-particles, γ-rays, and their combinations. Results showed that the dose-response of MN induced by γ-rays was well-fitted with the linear-quadratic model. But for α-particle irradiation, the MN induction had a biphasic phenomenon containing a low dose hypersensitivity characteristic and its dose response could be well-stimulated with a state vector model where radiation-induced bystander effect (RIBE) was involved. For the combination exposure, the dose response of MN was similar to that of α-irradiation. However, the yield of MN was closely related to the sequence of irradiations. When the cells were irradiated with α-particles at first and then γ-rays, a synergistic effect of MN induction was observed. But when the cells were irradiated with γ-rays followed by α-particles, an antagonistic effect of MN was observed in the low dose range although this combination radiation also yielded a synergistic effect at high doses. When the interval between two irradiations was extended to 4h, a cross-adaptive response against the other irradiation was induced by a low dose of γ-rays but not α-particles.

Suppression of endogenous hydrogen sulfide contributes to the radiation-induced bystander effects on hypoxic HepG2 cells

Radiation-induced bystander effects may have important implications in radiotherapy, but it is still not well known if radiation-induced bystander effects can be triggered in hypoxic tumor cells and what are the related bystander signals. Using human hepatoma cells of HepG2, the present study found that micronuclei (MN) could be induced in the nonirradiated cells after treatment with conditioned medium (CM) harvested from irradiated cells under hypoxic conditions. Bystander effects were diminished when the irradiated cells were pretreated with sodium hydrosulfide (NaHS, an exogenous H(2)S donor) (≤100 μM). However, the bystander effects were increased when the irradiated cells were pretreated with an inhibitor of cystathionine β-synthase (CBS) and cystathionine γ-lyase (CSE), the synthases of endogenous hydrogen sulfide (H(2)S). Western blotting showed that the expressions of CSE and CBS were increased in the irradiated hypoxic cells, but were reduced in the CM treated bystander cells. The ratio of Bcl-2/Bax, a molecular marker of apoptosis, decreased with CM treatment time. However, the activity of caspase-3 increased in the hypoxic bystander cells, and this could be regulated by both NaHS and the inhibitor of endogenous H(2)S. These results demonstrate that under hypoxic conditions irradiated hepatoma cells induce bystander responses by depressing the generation of endogenous H(2)S and altering Bcl-2/Bax ratios as well as caspase-3 dependent damage in the bystander cells.

Susceptibility to bystander DNA damage is influenced by replication and transcriptional activity

Direct cellular DNA damage may lead to genome destabilization in unexposed, bystander, cells sharing the same milieu with directly damaged cells by means of the bystander effect. One proposed mechanism involves double strand break (DSB) formation in S phase cells at sites of single strand lesions in the DNA of replication complexes, which has a more open structure compared with neighboring DNA. The DNA in transcription complexes also has a more open structure, and hence may be susceptible to bystander DSB formation from single strand lesions. To examine whether transcription predisposes non-replicating cells to bystander effect-induced DNA DSBs, we examined two types of primary cells that exhibit high levels of transcription in the absence of replication, rat neurons and human lymphocytes. We found that non-replicating bystander cells with high transcription rates exhibited substantial levels of DNA DSBs, as monitored by γ-H2AX foci formation. Additionally, as reported in proliferating cells, TGF-β and NO were found to mimic bystander effects in cell populations lacking DNA synthesis. These results indicate that cell vulnerability to bystander DSB damage may result from transcription as well as replication. The findings offer insights into which tissues may be vulnerable to bystander genomic destabilization in vivo.

Radiation-induced bystander effects induce radioadaptive response by low-dose radiation

When normal human fibroblast cells (MRC-5) received a priming irradiation of 3-20 mGy 4 h prior to irradiation with 1000 mGy, the number of DNA double-stranded breaks (DSBs) decreased significantly to 18.2-18.7 per cell compared with 21 per cell when there was no priming irradiation. This result indicates that a priming irradiation of 3-20 mGy induces a radioadaptive response in MRC-5. The authors' previous study had indicated that DSBs induced by ≤ 20 mGy are due to a radiation-induced bystander effect. These findings suggest that radiation-induced bystander effects might contribute to induction of the radioadaptive response. To test this hypothesis, MRC-5 were suspended in lindane, an inhibitor of radiation-induced bystander effects, which was added to the medium for the priming irradiation of 3-20 mGy. Lindane inhibited the protective effect of priming irradiation on DSBs caused by subsequent irradiation with 1000 mGy. Thus, radiation-induced bystander effects may play a role in radioadaptive responses.

Propofol depresses the cytotoxicity of X-ray irradiation through inhibition of gap junctions

BACKGROUND

General anesthetics (e.g., propofol) influence the therapeutic activity of intraoperative radiotherapy but the mechanism of the effects is largely unknown. It has been reported that propofol inhibits gap junction (GJ) function briefly, and a functional GJ enhances the efficacy of radiotherapy in some cancer cells. Yet the mechanisms underlying the inhibition of GJ function by propofol and the influence of propofol on therapeutic activity of intraoperative radiotherapy are unknown.

METHODS

The role of propofol at clinically relevant concentrations in the modulation of radiograph-induced cytotoxicity in HeLa cells transfected with connexin 32 (Cx32) plasmid was explored by manipulation of connexin expression, GJ presence, and function. GJ function, Cx32 protein level, and Cx32 mRNA expression were determined by "Parachute" dye-coupling assay, Western blotting, and reverse transcriptase-polymerase chain reaction, respectively.

RESULTS

Propofol significantly reduced radiograph-induced cytotoxicity only in the presence of functional GJ. Four-hour propofol exposure inhibited GJ function mainly by diminution of Cx32 protein levels but without influence on Cx32 mRNA expression.

CONCLUSIONS

These results suggest that propofol inhibits the function of the GJ through the reduction of Cx32 protein levels by a transcription-independent mechanism. They further indicate that propofol depresses the cytotoxicity of radiograph irradiation through inhibition of GJ activity.

Radiation-induced intercellular signaling mediated by cytochrome-c via a p53-dependent pathway in hepatoma cells

The tumor suppressor p53 has a crucial role in cellular response to DNA damage caused by ionizing radiation, but it is still unclear whether p53 can modulate radiation-induced bystander effects (RIBE). In the present work, three different hepatoma cell lines, namely HepG2 (wild p53), PLC/PRF/5 (mutation p53) and Hep3B (p53 null), were irradiated with γ-rays and then co-cultured with normal Chang liver cell (wild p53) in order to elucidate the mechanisms of RIBE. Results showed that the radiosensitivity of HepG2 cells was higher than that of PLC/PRF/5 and Hep3B cells. Only irradiated HepG2 cells, rather than irradiated PLC/PRF/5 or Hep3B cells, could induce bystander effect of micronuclei (MN) formation in the neighboring Chang liver cells. When HepG2 cells were treated with 20 μM pifithrin-α, an inhibitor of p53 function, or 5 μM cyclosporin A (CsA), an inhibitor of cytochrome-c release from mitochondria, the MN induction in bystander Chang liver cells was diminished. In fact, it was found that after irradiation, cytochrome-c was released from mitochondria into the cytoplasm only in HepG2 cells in a p53-dependent manner, but not in PLC/PRF/5 and Hep3B cells. Interestingly, when 50 μg/ml exogenous cytochrome-c was added into cell co-culture medium, RIBE was significantly triggered by irradiated PLC/PRF/5 and Hep3B cells, which previously failed to provoke a bystander effect. In addition, this exogenous cytochrome-c also partly recovered the RIBE induced by irradiated HepG2 cells even with CsA treatment. Our results provide new evidence that the RIBE can be modulated by the p53 status of irradiated hepatoma cells and that a p53-dependent release of cytochrome-c may be involved in the RIBE.

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

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

Ion beam radiobiology and cancer: time to update ourselves

High-energy protons and carbon ions exhibit an inverse dose profile allowing for increased energy deposition with penetration depth. Additionally, heavier ions like carbon beams have the advantage of a markedly increased biological effectiveness characterized by enhanced ionization density in the individual tracks of the heavy particles, where DNA damage becomes clustered and therefore more difficult to repair, but is restricted to the end of their range. These superior biophysical and biological profiles of particle beams over conventional radiotherapy permit more precise dose localization and make them highly attractive for treating anatomically complex and radioresistant malignant tumors but without increasing the severe side effects in the normal tissue. More than half a century since Wilson proposed their use in cancer therapy, the effects of particle beams have been extensively investigated and the biological complexity of particle beam irradiation begins to unfold itself. The goal of this review is to provide an as comprehensive and up-to-date summary as possible of the different radiobiological aspects of particle beams for effective application in cancer treatment.

Cell Line Specific Modulation of Connexin43 Expression after Exposure to Ionizing Radiation

Gap junctional intercellular communication plays a significant role in mediating radiation-induced bystander effects. However, the level of Cx43 itself is influenced by ionizing radiation, which could modify the bystander effect. Here we have investigated several cell lines for the modulation of Cx43 expression 24 h after irradiation with 5 Gy X-rays. The mouse endothelial cell line bEnd3 revealed a significantly elevated level of Cx43 already 15 min after exposure to X-rays, whereas human hybrid endothelial cells (EA.hy926) exhibited a transient downregulation of Cx43 mRNA. No obvious changes in the communication properties of the different cell lines could be observed after irradiation. The communication-deficient malignant human trophoblast cell line Jeg3 stably transfected with Cx43 did not reveal any induction of endogenous nor alteration in the exogenous Cx43 transcript level upon exposure to 5 Gy. Taken together, our data show a cell line specific modulation of Cx43 expression after exposure to X-rays.

Radiation response of primary human skin fibroblasts and their bystander cells after exposure to counted particles at low and high LET

PURPOSE

To investigate the dependence of bystander effects on linear energy transfer (LET) in the low dose region.

MATERIALS AND METHODS

The single-ion microbeam of the Physikalisch-Technische Bundesanstalt (PTB) was used to irradiate confluent primary human skin fibroblasts. Cells plated on a special irradiation dish were targeted with 10 MeV protons (LET 4.7 keV/microm) and 4.5 MeV a-particles (LET 100 keV/microm). During exposure, the cells were confluent allowing signal transfers through both gap junctions and diffusion.

RESULTS

For 10 MeV protons the clonogenic capability was significantly higher after exposure to 70 protons (0.31 Gy) compared with unirradiated cells. For higher doses the survival curve was exponential. Exposure of only 10% of all nuclei resulted in a similar radiation response in the low dose region. For higher doses up to 2.2 Gy no cell killing was observed. For 4.5 MeV alpha-particles an exponential survival curve was obtained. Irradiation of only 10% of all cell nuclei resulted in an survival curve as had been expected in the absence of any bystander effect.

CONCLUSION

The type and extent of bystander effects turned out to be dependent on the particles' LET and are likely to depend also on the cell line used and the techniques applied.

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

PURPOSE

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSION

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