Effects of heavy ions and energetic protons on normal human fibroblasts

At the low particle fluences of radiation to which astronauts are exposed in space, "non-targeted" effects such as the bystander response may have increased significance. The radiation-induced bystander effect is the occurrence of biological responses in unirradiated cells near to or sharing medium with cells traversed by radiation. The objectives of this study were to establish the responses of AG01522 diploid human fibroblasts after exposure to several heavy ions and energetic protons, as compared to X-rays, and to obtain initial information on the bystander effect in terms of cell clonogenic survival after Fe ion irradiation. Using a clonogenic survival assay, relative biological effectiveness (RBE) values at 10% survival were 2.5, 2.3, 1.0 and 1.2 for 1 GeV/amu Fe, 1 GeV/amu Ti, 290 MeV/amu C and 1 GeV/amu protons, respectively, compared to 250 kVp X-rays. For induction of micronuclei (MN), compared to the low LET protons, Fe and Ti are very effective inducers of damage, although C ions are similar to protons. Using a transwell insert system in which irradiated and unirradiated bystander cells share medium but are not touching each other, it was found that clonogenic survival in unirradiated bystander cells was decreased when irradiated cells were exposed to Fe ions or X-rays. The magnitude of the decrease in bystander survival was similar with both radiation types, reaching a plateau of about 80% survival at doses of about 0.5 Gy or larger.

Targeted irradiation of shoot apical meristem of Arabidopsis embryos induces long-distance bystander/abscopal effects

Bystander effects induced by low-dose ionizing radiation have been shown to occur widely in many cell types and may have a significant impact on radiation risk assessment. Although the region of radiation damage is known to be much greater than the initial target volume irradiated, it remains to be seen whether this response is limited to the specific organ irradiated, spans a limited region of the body, or even covers the whole body of the target. To determine whether long-distance bystander/abscopal effects exist in whole organisms and to clarify the problem of intercellular communication, in the present study a specific cell group, the shoot apical meristem in Arabidopsis embryo, was irradiated with a defined number of protons and examined for root development postirradiation. The results showed that after direct damage to the shoot apical meristem from ion traversals, root hair differentiation, primary root elongation and lateral root initiation were all inhibited significantly in postembryonic development, suggesting that radiation-induced long-distance bystander/abscopal responses might exist in the whole organism. To further scrutinize the mechanism(s) underlying these inhibitory effects, a DR5-GUS transgenic Arabidopsis was used. The results showed that accumulation of the reporter GUS gene transcript in irradiated shoot apical meristem embryos decreased in the postembryonic development. Treatment with either 2,4-dichlorophenoxyacetic acid, a synthetic plant auxin, or DMSO, a effective reactive oxygen species (ROS) scavenger, could rescue the reporter GUS enzyme accumulation and the length of primary root in irradiated shoot apical meristem embryos, indicating that ROS or probably the ROS related auxin and auxin-dependent transcription process may be involved in radiation-induced long-distance bystander/abscopal effects.

Quantifying a bystander response following microbeam irradiation using single-cell RT-PCR analyses

OBJECTIVE

There is growing recognition that the effects of ionizing radiation may extend to more than those cells that directly suffer damage to DNA in the cell nucleus. Data from several investigators have indicated that cells neighboring those that are irradiated also demonstrate several responses seen in hit cells--the so-called bystander effect. The microbeam facility at the Center for Radiological Research is particularly well suited for the study of this bystander effect, since it has the ability to place known numbers of charged particles (protons or alpha-particles at LETs from 20 to 180 KeV/microm) at defined positions relative to individual cells. That is, some known fraction of cells in a population can be irradiated through the nucleus, or the cytoplasm or even adjacent to cells through the media. Therefore, using the microbeam it is possible to examine individual cell responses in both hit and nonhit cells in the same population.

METHOD AND RESULTS

Alterations in the cyclin-dependent kinase inhibitor CDKN1a (p21/Cip1/WAF1) were quantified at the mRNA level in single normal human fibroblasts following precise delivery of 0 or 10 alpha-particles per cell at 90 KeV/microm to 50% of cells in a population. Semiquantitative RT-PCR of individual hit cells demonstrated increases in the levels of CDKN1A message that followed the kinetics previously described for irradiated populations. Furthermore, nonhit bystander cells also showed increased (though lesser) levels of CDKN1a message.

CONCLUSION

Data presented here demonstrate the power of this approach, which combines the ability of the microbeam to irradiate specific cells in a population and the ability to quantify the response to the irradiation in individual targeted and bystander cells.

Role of epigenetic effectors in maintenance of the long-term persistent bystander effect in spleen in vivo

Radiation therapy is a primary treatment modality for brain tumors, yet it has been linked to the increased incidence of secondary, post-radiation therapy cancers. These cancers are thought to be linked to indirect radiation-induced bystander effect. Bystander effect occurs when irradiated cells communicate damage to nearby, non-irradiated 'bystander' cells, ultimately contributing to genome destabilization in the non-exposed cells. Recent evidence suggests that bystander effect may be epigenetic in nature; however, characterization of epigenetic mechanisms involved in bystander effect generation and its long-term persistence has yet to be defined. To investigate the possibility that localized X-ray irradiation induces persistent bystander effects in distant tissue, we monitored the induction of epigenetic changes (i.e. alterations in DNA methylation, histone methylation and microRNA (miRNA) expression) in the rat spleen tissue 24 h and 7 months after localized cranial exposure to 20 Gy of X-rays. We found that localized cranial radiation exposure led to the induction of bystander effect in lead-shielded, distant spleen tissue. Specifically, this exposure caused the profound epigenetic dysregulation in the bystander spleen tissue that manifested as a significant loss of global DNA methylation, alterations in methylation of long interspersed nucleotide element-1 (LINE-1) retrotransposable elements and down-regulation of DNA methyltransferases and methyl-binding protein methyl CpG binding protein 2 (MeCP2). Further, irradiation significantly altered expression of miR-194, a miRNA putatively targeting both DNA methyltransferase-3a and MeCP2. This study is the first to report conclusive evidence of the long-term persistence of bystander effects in radiation carcinogenesis target organ (spleen) upon localized distant exposure using the doses comparable with those used for clinical brain tumor treatments.

[The DNA fragments obtained from the culture media exposed to adaptive doses of the ionizing radiation as factors of stress signaling between lymphocytes and bystander cells]

At the initial stages of an adaptive response the transposition of the homologous chromosome loci from the peripheral parts of the nucleus and their approach happens. It is necessary for the repair of DNA double strand breaks in the process of the homologous recombination. Was shown that the chromosome loci transposition and accompanied by the nucleolus activities took place first in the irradiated (X-rays, 10 cGy) G0-lymphocytes, and then in the intact (bystander) cells incubated in the growth medium of irradiated lymphocytes. If there is a bystander effect the quantity of irradiated cells may be three order less than the bystander cells that affirms the great capacity of stress-signalization system. Moreover, the DNA fragments (the factors of stress signaling) were obtained from the growth medium supernatant of the irradiated and of the intact lymphocytes. In other independent experiments they were inoculated into the growth medium of recipient cells. Was demonstrated that there is loci transposition of homologous chromosomes loci and of nucleus activity after introducing the DNA fragments of irradiated cells. After introducing the DNA fragments of non-irradiated cells the both effects were not observed. In the work the characteristics of the obtained factors and the possible ways of stress signaling between the irradiated and the bystander lymphocytes were discussed.

Radiation-induced bystander effect: Activation of signaling molecules in K562 erythroleukemia cells

Gap junction independent signaling mechanism was investigated using K562 human erythroleukemia cells. They were exposed to 2, 5, or 10 Gy of (60)Co gamma irradiation, the medium isolated 20 min post-irradiation and added to fresh cells. Evidence of radiation-induced bystander effect was observed wherein there was activation of p21, nuclear factor-kappaB (NF-kappaB), Bax, Bcl-2 and cleavage of poly(ADP-ribose) polymerase in bystander cells. The study implicates the involvement of signaling molecules released into the medium and factors like stable free radicals that are generated in the surrounding medium. The response elicited appears to be primarily via NF-kappaB and p21 activation.

Radiation-induced bystander effects in malignant trophoblast cells are independent from gap junctional communication

It is controversially discussed that irradiation induces bystander effects via gap junction channels and/or diffusible cellular factors such as nitric oxide or cytokines excreted from the cells into the environment. But up to now the molecular mechanism leading to a bystander response is not well understood. To discriminate between both mechanisms of bystander response, (i) mediated by gap junctional communication and/or (ii) mediated by diffusible molecules, we used non-communicating Jeg3 malignant trophoblast cells transfected with inducible gap junction proteins, connexin43 and connexin26, respectively, based on the Tet-On system. We co-cultivated X-ray irradiated and non-irradiated bystander Jeg3 cells for 4 h, separated both cell populations by flow cytometry and evaluated the expression of activated p53 by Western blot analysis. The experimental design was proven with communicating versus non-communicating Jeg3 cells. Interestingly, our results revealed a bystander effect which was independent from gap junctional communication properties and the connexin isoform expressed. Therefore, it seems more likely that the bystander effect is not mediated via gap junction channels but rather by paracrine mechanisms via excreted molecules in Jeg3 cells.

Radiation-induced bystander effects and adaptive response in murine lymphocytes

PURPOSE

To study the bystander effects of gamma-radiation in murine lymphocytes using irradiated conditioned medium (ICM) generated from irradiated lymphocytes.

METHODS

Proliferation response of unirradiated lymphocytes to mitogen concanavalin A (con A) in presence of ICM, collected from gamma-irradiated lymphocytes (60Co source; 0.35 Gy/min; 0.1-1 Gy), was studied by 3H-thymidine incorporation and also by dye dilution using carboxyfluorescein succinimidyl ester (CFSE). Expression of proliferation markers, interleukin 2 receptor alpha chain (CD25) and cyclin D in ICM treated lymphocytes was analyzed by labeling with specific antibodies. Intracellular reactive oxygen species (ROS) and apoptosis were estimated by flow cytometry using dichlorodihydrofluorescein diacetate (H2DCFDA) and propidium iodide, respectively. Nitric oxide (NO) was measured using Griess reagent.

RESULTS

Proliferation response to con A in unirradiated lymphocytes was enhanced in the presence of ICM with maximum enhancement observed in the presence of 0.5 Gy ICM. Augmentation of proliferation in the presence of ICM was accompanied by an increase in CD25 and cyclin D expression, enhanced ROS and NO generation. ICM pretreated lymphocytes showed adaptive response to radiation which was not abrogated by wortmannin, a phosphatidyl inositol 3-kinase (PI3K) inhibitor.

CONCLUSION

Soluble factors released from irradiated lymphocytes initiate a signaling cascade in unirradiated lymphocytes resulting in increased response to mitogen and radioresistance which may have an important role in radiation-induced immunomodulation.

Simulation of radiation damage to lung cells after exposure to radon decay products

OBJECTIVES

Exposure to radon and radon decay products represents one of the greatest risks of ionizing radiation from natural sources in some residential and working areas. Recently increasing attention has been paid to accurate estimations of this health risk by using various models.

METHODS

In the presented study, a bystander model was used to predict biological effects of radon products on lung tissue target cells. The model considers radiation response as a superposition of the direct alpha particles hit effect and the bystander (cells communication) effect. Energy deposition in the lung tissue and in the air gap was calculated using the Bethe-Bloch equation. The exponential distribution of radon progenies in the mucous layer of smokers and non-smokers was evaluated.

RESULTS

The excess relative risk value of lung cancer occurrence per unit exposure obtained in our study was ERR/WLM=0.0047 for smokers and ERR/WLM=0.0171 (taking into account the environment in radioactive ore mines) for non-smokers. Other published results give the average excess relative risk values per unit exposure in the dwellings for smokers ERR=0.050 per 100 Bq.m(-3) and for non-smokers ERR=0.198 per 100 B.qm(-3). Results presented in this study are in good agreement with the published epidemiological data on lung cancer incidence for mines and residential areas.

CONCLUSIONS

The bystander model is suitable for radon risk prediction in dwellings and at workplaces (residential and working areas).

Lauriston S. Taylor lecture: nontargeted effects of radiation: implications for low-dose exposures

Traditional thinking has been that the biological effects of ionizing radiation occur in irradiated cells as a consequence of the DNA damage they incur. This implies that (1) biological effects occur only in irradiated cells, (2) radiation traversal through the nucleus of the cell is a prerequisite to produce a biological response, and (3) DNA is the target molecule in the cell. Evidence has been emerging, however, for non-DNA targeted effects of radiation; that is, effects including mutations, chromosomal aberrations, and changes in gene expression that occur in cells that in themselves receive no radiation exposure. Two of these phenomena will be described in this paper. The first is radiation-induced genomic instability whereby biological effects, including elevated frequencies of mutations and chromosomal aberrations, arise in the distant descendants of irradiated cells. The second phenomenon has been termed the "bystander effect," whereby in a mixed population of irradiated and nonirradiated cells, biological effects arise in those cells that receive no radiation exposure. The damage signals are transmitted from cell to cell through gap junction channels, and the genetic effects observed in bystander cells appear to result from an upregulation of oxidative stress. The possible influence of these nontargeted effects of radiation on the response to low-dose exposures is discussed.

The Release of Bystander Factor(s) from Tissue Explant Cultures of Rainbow Trout (Onchorhynchus mykiss) after Exposure to γ Radiation

The bystander response has been documented in cell lines and cell cultures derived from aquatic species over the past several years. However, little work has been undertaken to identify a similar bystander response in tissue explant cultures from fish. In this study, indirect effects of ionizing gamma radiation on tissue explant cultures of fish were investigated. Tissue explants in culture were exposed to 0.5 Gy and 5 Gy gamma radiation from a 60Co teletherapy unit. A bystander response in Epithelioma papulosum cyprini (EPC) cells exposed to gamma-irradiated tissue conditioned medium from rainbow trout explants was investigated, and the effects on cell survival were quantified by the clonogenic survival assay. Dichlorofluorescein and rhodamine 123 fluorescent dyes were used to identify alterations in reactive oxygen species (ROS) and mitochondrial membrane potential (MMP), respectively. Results indicate a different response for the three tissue types investigated. Clonogenic assay results vary from a decrease in cell survival (gill) to no effect (skin) to a stimulatory effect (spleen). Results from fluorescence assays of ROS and MMP show similarities to clonogenic assay results. This study identifies a useful model for further studies relating to the bystander effect in aquatic organisms in vivo and ex vivo.

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

Bystander responses have been reported to be a major determinant of the response of cells to radiation exposure at low doses, including those of relevance to therapy. This study investigated the role of changes in calcium levels in bystander responses leading to chromosomal damage in nonirradiated T98G glioma cells and AG01522 fibroblasts that had been either exposed to conditioned medium from irradiated cells or co-cultured with a population where a fraction of cells were individually targeted through the nucleus or cytoplasm with a precise number of microbeam helium-3 particles. After the recipient cells were treated with conditioned medium from T98G or AG01522 cells that had been irradiated through either nucleus or cytoplasm, rapid calcium fluxes were monitored in the nonirradiated recipient cells. Their characteristics were dependent on the source of the conditioned medium but had no dependence on radiation dose. When recipient cells were co-cultured with an irradiated population of either T98G or AG01522 cells, micronuclei were induced in the nonirradiated cells, but this response was eliminated by treating the cells with calcicludine (CaC), a potent blocker of Ca(2+) channels. Moreover, both the calcium fluxes and the bystander effect were inhibited when the irradiated T98G cells were treated with aminoguanidine, an inhibitor of nitric oxide synthase (NOS), and when the irradiated AG01522 cells were treated with DMSO, a scavenger of reactive oxygen species (ROS), which indicates that NO and ROS were involved in the bystander responses generated from irradiated T98G and AG01522 cells, respectively. Our findings indicate that calcium signaling may be an early response in radiation-induced bystander effects leading to chromosome damage.

[Dependence of mice fibrobslasts radiosensitivity on the exposure conditions]

In the scientific literature there is lengthy discussion concerning utilization of low-dose hypersensitivity and bystander in radiation therapy which inspired us to investigate these effects using cells of mice fibroblasts C3H10T1/2. Irradiated were monolayers of fibroblasts cells cultivated on wall of plastic vials. To study the bystander effect, the therapeutic proton beam with the onset energy of 150 Me V was directed at the whole wall (25 cr2) or only the central area of 1 cv(2). In an hour after irradiation the cells were dispersed in 0.25 % tripsin solution and inoculated in vials for survivability analysis. In both cases survivability of the cells was essentially equal following irradiation by 0.5 Gy, 2 Gy and 5 Gy. The same observation was made also after fractionated irradiation by the total doses of 10 and 20 Gy (2 Gy per a day, 5 times a week). In these experiments, each time another area (1 cm(2)) of the vial wall was subjected to irradiation. Three 0.4 Gy fractionated irradiations of cells C3H10OT1/2 per a day (5 times a week) by gamma-source 60Co with an interval of 3 hours showed that the total dose of 6 Gy and 12 Gy gathered by this protocol had the strongest lethal effect on fibroblast cells as compared with the daily one-time irradiation by 1.2 Gy. According to these results, detail studies of the low-dose hypersensitivity and bystander effects may come up with their effective utilization in radiation therapy.

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

Radiotherapy is an important treatment for patients suffering from high-grade malignant gliomas. Non-targeted (bystander) effects may influence these cells' response to radiation and the investigation of these effects may therefore provide new insights into mechanisms of radiosensitivity and responses to radiotherapy as well as define new targets for therapeutic approaches. Normal primary human astrocytes (NHA) and T98G glioma cells were irradiated with helium ions using the Gray Cancer Institute microbeam facility targeting individual cells. Irradiated NHA and T98G glioma cells generated signals that induced gammaH2AX foci in neighbouring non-targeted bystander cells up to 48 h after irradiation. gammaH2AX bystander foci were also observed in co-cultures targeting either NHA or T98G cells and in medium transfer experiments. Dimethyl sulphoxide, Filipin and anti-transforming growth factor (TGF)-beta 1 could suppress gammaH2AX foci in bystander cells, confirming that reactive oxygen species (ROS) and membrane-mediated signals are involved in the bystander signalling pathways. Also, TGF-beta 1 induced gammaH2AX in an ROS-dependent manner similar to bystander foci. ROS and membrane signalling-dependent differences in bystander foci induction between T98G glioma cells and normal human astrocytes have been observed. Inhibition of ataxia telangiectasia mutated (ATM) protein and DNA-PK could not suppress the induction of bystander gammaH2AX foci whereas the mutation of ATM- and rad3-related (ATR) abrogated bystander foci induction. Furthermore, ATR-dependent bystander foci induction was restricted to S-phase cells. These observations may provide additional therapeutic targets for the exploitation of the bystander effect.

Design, spectrum measurements and simulations for a 238Pu α-particle irradiator for bystander effect and genomic instability experiments

Design, spectrum measurements and simulations for an alpha-particle irradiator for bystander effect and genomic instability experiments are presented. Measured alpha-particle energy spectra were used to confirm the characteristics of the source of the irradiator specified by the manufacturer of the source. The spectra were measured in vacuum with a high-resolution spectrometer and simulated with an AASI Monte Carlo code. As a next step, we simulated alpha-particle energy spectra at the target plane of the irradiator for three different source-to-target distances. In these simulations, helium was used as the medium between the source and the exit window of the irradiator; its pressure and temperature corresponded to those of the ambient air. Mean energies and full-widths at half-maximum (FWHM) were calculated for the three different helium gas tracks.

Inhibitory and stimulatory bystander effects are differentially induced by Iodine-125 and Iodine-123

The bystander effect, originating from cells irradiated in vitro, describes responses of surrounding cells not targeted by the radiation. Previously we demonstrated that the subcutaneous injection into nude mice of human adenocarcinoma LS174T cells lethally irradiated by Auger electrons from the decay of DNA-incorporated (125)I inhibits growth of co-injected LS174T cells (inhibitory bystander effect; Proc. Natl. Acad. Sci. USA 99, 13765-13770, 2002). We have repeated these studies using cells exposed to lethal doses of (123)I, an Auger electron emitter whose emission spectrum is identical to that of (125)I, and report herein that the decay of (123)I within tumor cell DNA stimulates the proliferation of neighboring unlabeled tumor cells growing subcutaneously in nude mice (stimulatory bystander effect). Similar inhibitory bystander effects ((125)I) and stimulatory bystander effects ((123)I) are obtained in vitro. Moreover, supernatants from cultures with (125)I-labeled cells are positive for tissue inhibitors of metalloproteinases (TIMP1 and TIMP2), and those from cultures with (123)I-labeled cells are positive for angiogenin. These findings call for the re-evaluation of current dosimetric approaches for the estimation of dose-response relationships in individuals after radiopharmaceutical administration or radiocontamination and demonstrate a need to adjust all "calculated" dose estimates by a dose modification factor (DMF), a radionuclide-specific constant that factors in hitherto not-so-well recognized biophysical processes.

Can promotion of initiated cells be explained by excess replacement of radiation-inactivated neighbor cells?

Recently, the observed promotion in the clonal expansion of a two-stage cancer model was attributed to a small excess replacement probability for the initiated cells. The proposed mechanism of excess replacement was evaluated for single intermediate cells surrounded by normal cells. This paper investigates this mechanism further using the same biological parameters. If the formation of clones of intermediate cells is taken into account in a quantitative analysis of the proposed mechanism, it turns out that (1) for the initial strong increase of the promotional effect with exposure, a much larger and unlikely excess replacement probability is needed, and (2) the leveling of the promotional effect for high exposures cannot be explained by multiple normal neighbors of an intermediate cell being inactivated within one cell cycle, as it had been suggested. Perhaps these discrepancies could be partly resolved by a re-scaling of the original parameters, but this should be investigated further.

A Dose Threshold for a Medium Transfer Bystander Effect for a Human Skin Cell Line

The existence of radiation-induced bystander effects mediated by diffusible factors is now accepted, but the mechanisms and precise behavior at low doses remain unclear. We exposed cells to gamma-ray doses in the range 0.04 mGy-5 Gy, harvested the culture medium, and transferred it to unirradiated reporter cells. Calcium fluxes and clonogenic survival were measured in the recipients. We show evidence for a dose threshold around 2 mGy for the human skin cell line used with a suggestion of increased survival below that dose. Similar experiments using direct gamma irradiation showed no reduction in survival until the dose exceeded 7 mGy. Preliminary data for neutrons where the gamma-ray dose was kept below the bystander threshold do not show a significant bystander effect in the dose range 1-33 mGy. A lack of a bystander response with neutrons occurred at around 1 Gy, where significant cell killing from direct irradiation was observed. The result may have implications for understanding the role of bystander effects at low doses.

Radiation-induced biologic bystander effect elicited in vitro by targeted radiopharmaceuticals labeled with alpha-, beta-, and auger electron-emitting radionuclides

Recent studies have shown that indirect effects of ionizing radiation may contribute significantly to the effectiveness of radiotherapy by sterilizing malignant cells that are not directly hit by the radiation. However, there have been few investigations of the importance of indirect effects in targeted radionuclide treatment. Our purpose was to compare the induction of bystander effects by external beam gamma-radiation with those resultant from exposure to 3 radiohaloanalogs of metaiodobenzylguanidine (MIBG): (131)I-MIBG (low-linear-energy-transfer [LET] beta-emitter), (123)I-MIBG (potentially high-LET Auger electron emitter), and meta-(211)At-astatobenzylguanidine ((211)At-MABG) (high-LET alpha-emitter).

METHODS

Two human tumor cell lines-UVW (glioma) and EJ138 (transitional cell carcinoma of bladder)-were transfected with the noradrenaline transporter (NAT) gene to enable active uptake of MIBG. Medium from cells that accumulated the radiopharmaceuticals or were treated with external beam radiation was transferred to cells that had not been exposed to radioactivity, and clonogenic survival was determined in donor and recipient cultures.

RESULTS

Over the dose range 0-9 Gy of external beam radiation of donor cells, 2 Gy caused 30%-40% clonogenic cell kill in recipient cultures. This potency was maintained but not increased by higher dosage. In contrast, no corresponding saturation of bystander cell kill was observed after treatment with a range of activity concentrations of (131)I-MIBG, which resulted in up to 97% death of donor cells. Cellular uptake of (123)I-MIBG and (211)At-MABG induced increasing recipient cell kill up to levels that resulted in direct kill of 35%-70% of clonogens. Thereafter, the administration of higher activity concentrations of these high-LET emitters was inversely related to the kill of recipient cells. Over the range of activity concentrations examined, neither direct nor indirect kill was observed in cultures of cells not expressing the NAT and, thus, incapable of active uptake of MIBG.

CONCLUSION

Potent toxins are generated specifically by cells that concentrate radiohalogenated MIBG. These may be LET dependent and distinct from those elicited by conventional radiotherapy.

The involvement of calcium and MAP kinase signaling pathways in the production of radiation-induced bystander effects

Much evidence now exists regarding radiation-induced bystander effects, but the mechanisms involved in the transduction of the signal are still unclear. The mitogen-activated protein kinase (MAPK) pathways have been linked to growth factor-mediated regulation of cellular events such as proliferation, senescence, differentiation and apoptosis. Activation of multiple MAPK pathways such as the ERK, JNK and p38 pathways have been shown to occur after exposure of cells to radiation and a variety of other toxic stresses. Previous studies have shown oxidative stress and calcium signaling to be important in radiation-induced bystander effects. The aim of the present study was to investigate MAPK signaling pathways in bystander cells exposed to irradiated cell conditioned medium (ICCM) and the role of oxidative metabolism and calcium signaling in the induction of bystander responses. Human keratinocytes (HPV-G cell line) were irradiated (0.005-5 Gy) using a cobalt-60 teletherapy unit. The medium was harvested 1 h postirradiation and transferred to recipient HPV-G cells. Phosphorylated forms of p38, JNK and ERK were studied by immunofluorescence 30 min-24 h after exposure to ICCM. Inhibitors of the ERK pathway (PD98059 and U0126), the JNK pathway (SP600125), and the p38 pathway (SB203580) were used to investigate whether bystander-induced cell death could be blocked. Cells were also incubated with ICCM in the presence of superoxide dismutase, catalase, EGTA, verapamil, nifedipine and thapsigargin to investigate whether bystander effects could be inhibited because of the known effects on calcium homeostasis. Activated forms of JNK and ERK proteins were observed after exposure to ICCM. Inhibition of the ERK pathway appeared to increase bystander-induced apoptosis, while inhibition of the JNK pathway appeared to decrease apoptosis. In addition, reactive oxygen species, such as superoxide and hydrogen peroxide, and calcium signaling were found to be important modulators of bystander responses. Further investigations of these signaling pathways may aid in the identification of novel therapeutic targets.

High-LET ion radiolysis of water: visualization of the formation and evolution of ion tracks and relevance to the radiation-induced bystander effect

Ionizing radiation-induced bystander effects, commonly observed in cell populations exposed to high-linear energy transfer (LET) radiations, are initiated by damage to a cellular molecule which then gives rise to a toxic signal exported to neighboring cells not directly hit by radiation. A major goal in studies of this phenomenon is the identification of this initial radiation-induced lesion. Liquid water being the main constituent of biological matter, reactive species produced by water radiolysis in the cellular environment are likely to be major contributors to the induction of this lesion. In this context, the radiation track structure is of crucial importance in specifying the precise location and identity of all the radiolytic species and their subsequent signaling or damaging effects. We report here Monte Carlo track structure simulations of the radiolysis of liquid water by four different impacting ions 1H+, 4He2+, 12C6+ and 20Ne10+, with the same LET ( approximately 70 keV/ microm). The initial radial distribution profiles of the various water decomposition products (eaq(-), *OH, H*, H2 and H2O2) for the different ions considered are presented and discussed briefly in the context of track structure theory. As an example, the formation and temporal evolution of simulated 24 MeV 4He2+ ion tracks (LET approximately 26 keV/microm) are reported for each radiolytic species from 1 ps to 10 micros. The calculations reveal that the ion track structure is completely lost by approximately 1 micros.

Irradiation induces DNA damage and modulates epigenetic effectors in distant bystander tissue in vivo

Irradiated cells induce chromosomal instability in unirradiated bystander cells in vitro. Although bystander effects are thought to be linked to radiation-induced secondary cancers, almost no studies have evaluated bystander effects in vivo. Furthermore, it has been proposed that epigenetic changes mediate bystander effects, but few studies have evaluated epigenetic factors in bystander tissues in vivo. Here, we describe studies in which mice were unilaterally exposed to X-irradiation and the levels of DNA damage, DNA methylation and protein expression were evaluated in irradiated and bystander cutaneous tissue. The data show that X-ray exposure to one side of the animal body induces DNA strand breaks and causes an increase in the levels of Rad51 in unexposed bystander tissue. In terms of epigenetic changes, unilateral radiation suppresses global methylation in directly irradiated tissue, but not in bystander tissue at given time-points studied. Intriguingly, however, we observed a significant reduction in the levels of the de novo DNA methyltransferases DNMT3a and 3b and a concurrent increase in the levels of the maintenance DNA methyltransferase DNMT1 in bystander tissues. Furthermore, the levels of two methyl-binding proteins known to be involved in transcriptional silencing, MeCP2 and MBD2, were also increased in bystander tissue. Together, these results show that irradiation induces DNA damage in bystander tissue more than a centimeter away from directly irradiated tissues, and suggests that epigenetic transcriptional regulation may be involved in the etiology of radiation-induced bystander effects.

Increased radiosensitivity in cells of two human cell lines treated with bystander medium from irradiated repair-deficient cells

Radiation-induced bystander factors have been shown to be more toxic if they are from medium harvested from irradiated repair-deficient cells. The aim of this study was to test the hypothesis that the radiosensitivity of repair-proficient cells can be increased by exposing them to medium-borne factors harvested from sensitive cells and vice versa. Cells from a mismatch repair (MMR)-deficient cell line (Raji 10) with a sensitive response to radiation or the wild-type parent cell line were irradiated to 0.5 Gy gamma rays and then monitored for growth rate in their own medium or in the alternative conditioned medium. In other experiments, cells or conditioned medium were added to reporter cells (HPV-G, which are relatively sensitive keratinocytes, or highly radioresistant HT29 cells). The subsequent responses of the two cell lines to a 0.5-Gy dose of (60)Co gamma rays were measured. The results show that prior exposure of resistant cells to medium from irradiated sensitive cells reduced the clonogenic survival of the subsequently irradiated resistant cells. The reverse is also true. Measurement of the apoptosis index and BCL2 expression confirmed that the harvested medium was capable of modulating apoptosis after irradiation. This may have important applications in tumor therapy and also in the understanding of mechanisms involved in induction of adaptive responses.