Upgrading of the Gray Laboratory soft X ray microprobe and V79 survival measurements following irradiation of one or all cells with a CK X ray beam of different size

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

PURPOSE

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

CONCLUSIONS

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

Radiation-induced bystander effects: what are they, and how relevant are they to human radiation exposures?

The term radiation-induced bystander effect is used to describe radiation-induced biological changes that manifest in unirradiated cells remaining within an irradiated cell population. Despite their failure to fit into the framework of classical radiobiology, radiation-induced bystander effects have entered the mainstream and have become established in the radiobiology vocabulary as a bona fide radiation response. However, there is still no consensus on a precise definition of radiation-induced bystander effects, which currently encompasses a number of distinct signal-mediated effects. These effects are classified here into three classes: bystander effects, abscopal effects and cohort effects. In this review, the data have been evaluated to define, where possible, various features specific to radiation-induced bystander effects, including their timing, range, potency and dependence on dose, dose rate, radiation quality and cell type. The weight of evidence supporting these defining features is discussed in the context of bystander experimental systems that closely replicate realistic human exposure scenarios. Whether the manifestation of bystander effects in vivo is intrinsically limited to particular radiation exposure scenarios is considered. The conditions under which radiation-induced bystander effects are induced in vivo will ultimately determine their impact on radiation-induced carcinogenic risk.

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.

An ultrasoft X-ray multi-microbeam irradiation system for studies of DNA damage responses by fixed- and live-cell fluorescence microscopy

Localized induction of DNA damage is a valuable tool for studying cellular DNA damage responses. In recent decades, methods have been developed to generate DNA damage using radiation of various types, including photons and charged particles. Here we describe a simple ultrasoft X-ray multi-microbeam system for high dose-rate, localized induction of DNA strand breaks in cells at spatially and geometrically adjustable sites. Our system can be combined with fixed- and live-cell microscopy to study responses of cells to DNA damage.

Histone H2AX Phosphorylation in Normal Human Cells Irradiated with Focused Ultrasoft X Rays: Evidence for Chromatin Movement during Repair

DNA repair within the cell nucleus is a dynamic process involving a close interaction between repair proteins and chromatin structure. Recent studies have indicated a quantitative relationship between DNA double-strand break induction and histone H2AX phosphorylation. The dynamics of this process within individual cell nuclei is unknown. To address this, we have used a novel focused ultrasoft X-ray microprobe that is capable of inducing localized DNA damage within a subnuclear area of intact cells with a 2.5-microm-diameter beam spot. The present investigation was undertaken to explore the influence of focused irradiation of individual nuclei with 1.49 keV characteristic aluminum K-shell (AlK) X rays on H2AX phosphorylation in normal human cells. Immunofluorescence analyses revealed that significant diffusion of the initial spots of clustered foci of phosphorylated H2AX occurred in a time-dependent fashion after exposure to AlK X rays. Irradiation under cooled conditions resulted in a reduction in the size of spots of clustered foci of phosphorylated H2AX as well as of individual phosphorylated H2AX foci. These findings strongly suggest that diffusion of the chromatin microenvironment occurs during the repair of DNA damage. We also found that AlK ultrasoft X rays (71 foci per gray) were 2.2-fold more effective at the initial formation of phosphorylated H2AX foci than with conventional X rays (32 foci per gray), and that the time required to eliminate 50% of the initial number of foci was 3.4-fold longer in AlK-irradiated cells than that in cells exposed to conventional X rays. For conventional X rays, we also report significant accumulation of larger-sized foci at longer times after irradiation.

A Variable-Energy Electron Microbeam: A Unique Modality for Targeted Low-LET Radiation

We have designed and constructed a low-cost, variable-energy low-LET electron microbeam that uses energetic electrons to mimic radiation damage produced by gamma and X rays. The microbeam can access lower regions of the LET spectrum, similar to conventional X-ray or 60Co gamma-ray sources. The device has two operating modes, as a conventional microbeam targeting single cells or subpopulations of cells or as a pseudo broad-beam source allowing for direct comparison with conventional sources. By varying the incident electron energy, the target cells can be selectively exposed to different parts of the energetic electron tracks, including the track ends.

Evidence for induction of DNA double strand breaks in the bystander response to targeted soft X-rays in CHO cells

This study investigated the role of DNA double strand breaks and DNA base damage in radiation-induced bystander responses in Chinese hamster ovary (CHO) cell lines. Two CHO repair-deficient clones, xrs5 (DNA double strand break repair-deficient) and EM9 (DNA base excision repair-deficient) were used in addition to the wild type (CHO). The Gray Cancer Institute ultrasoft X-ray microprobe is a powerful tool for investigating the bystander response, because it permits the irradiation of only a single nucleus of a cell, as reported previously. In order to investigate the bystander effect in each repair-deficient cell line, we irradiated a single cell within a population and scored the formation of micronuclei. When a single nucleus in the population was targeted with 1 Gy, elevated numbers of micronuclei were induced in the neighbouring unirradiated cells in the EM9 and xrs5 cell lines, whereas induction was not observed in CHO. The induction of micronuclei in xrs5 was significantly higher than that in EM9. Under these conditions, the surviving fraction in the neighbouring cells was significantly lower in xrs5 than in the other cell lines, showing a higher cell killing effect in xrs5. To confirm that bystander factors secreted from irradiated cells caused these effects, we carried out medium transfer experiments using conventional X-irradiation. Medium conditioned for 24 h with irradiated cells was transferred to unirradiated cells and elevated induction of micronuclei was observed in xrs5. These results suggest that DNA double strand breaks rather than base damage are caused by factors secreted in the medium from irradiated cells.

Bystander responses induced by low LET radiation

Radiation-induced bystander responses are observed when cells respond to their neighbours being irradiated. Considerable evidence is now available regarding the importance of these responses in cell and tissue models. Most studies have utilized two approaches where either a media-transferable factor has been assessed or cells have been exposed to low fluences of charged particles, where only a few percent are exposed. The development of microbeams has allowed nontargeted responses such as bystander effects to be more carefully analysed. As well as charged particle microbeams, X-ray microprobes have been developed, and several groups are also developing electron microbeams. Using the Gray Cancer Institute soft X-ray microprobe, it has been possible to follow the response of individual cells to targeted low doses of carbon-characteristic soft X-rays. Studies in human fibroblasts have shown evidence of a significant radiation quality-dependent bystander effect, measured as chromosomal damage in the form of micronuclei which is radiation quality dependent. Other studies show that even under conditions when only a single cell is targeted with soft X-rays, significant bystander-mediated cell killing is observed. The observation of bystander responses with low LET radiation suggests that these may be important in understanding radiation risk from background levels of radiation, where cells observe only single electron track traversals. Also, the indirect evidence for these responses in vivo indicates that they may have a role to play in current radiotherapy approaches and future novel strategies involving modulating nontargeted responses.