If bystander effects for apoptosis occur in spleen after low-dose irradiation in vivo then the magnitude of the effect falls within the range of normal homeostatic apoptosis

What's Changed in 75 Years of RadRes? - An Australian Perspective on Selected Topics

Several scientific themes are reviewed in the context of the 75-year period relevant to this special platinum issue of Radiation Research. Two criteria have been considered in selecting the scientific themes. One is the exposure of the associated research activity in the annual meetings of the Radiation Research Society (RRS) and in the publications of the Society's Journal, thus reflecting the interest of members of RRS. The second criteria is a focus on contributions from Australian members of RRS. The first theme is the contribution of radiobiology to radiation oncology, featuring two prominent Australian radiation oncologists, the late Rod Withers and his younger colleague, Lester Peters. Two other themes are also linked to radiation oncology; preclinical research aimed at developing experimental radiotherapy modalities, namely microbeam radiotherapy (MRT) and Auger endoradiotherapy. The latter has a long history, in contrast to MRT, especially in Australia, given that the associated medical beamline at the Australian Synchrotron in Melbourne only opened in 2011. Another theme is DNA repair, which has a trajectory parallel to the 75-year period of interest, given the birth of molecular biology in the 1950s. The low-dose radiobiology theme has a similar timeline, predominantly prompted by the nuclear era, which is also connected to the radioprotector theme, although radioprotectors also have a long-established potential utility in cancer radiotherapy. Finally, two themes are associated with biodosimetry. One is the micronucleus assay, highlighting the pioneering contribution from Michael Fenech in Adelaide, South Australia, and the other is the γ-H2AX assay and its widespread clinical applications.

Temporal Responses to X-Radiation Exposure in Spleen in the pKZ1 Mouse Recombination Assay

The in vivo mouse transgenic pKZ1 chromosomal inversion assay is a sensitive assay that responds to very low doses of DNA-damaging agents. pKZ1 inversions are measured as the frequency of cells expressing E. coli β-galactosidase protein, which can only be produced from an inverted pKZ1 transgene. In previous studies we reported that a single whole-body low dose of 0.01 mGy X rays alone caused an increase in pKZ1 chromosomal inversions in spleen when analyzed 3 days postirradiation, and yet this same dose could protect from high-dose-induced inversions when delivered as a conditioning dose 4 h before or after a 1 Gy challenge dose. In an attempt to explain these results, we performed temporal studies over a wide radiation dose range to determine if the inversion response was temporally different at different doses. pKZ1 mice were irradiated with a single whole-body X-ray dose of 0.01 mGy, 1 mGy or 1 Gy, and spleen sections were then analyzed for pKZ1 inversions at 7 h, 1 day or 7 days after exposure. No change in inversion frequency was observed at the 7 h time point at any dose. At day 1, an increase in inversions was observed in response to the 0.01 mGy dose, whereas a decrease in inversions below sham-treated frequency was observed for the 1 mGy dose. Inversion frequency for both doses returned to sham-treated inversion frequency by day 7. To our knowledge, this is the first reported study to examine the temporal nature of a radiation response spanning a wide dose range, including doses relevant to occupational exposure, and the results are dynamic and dose specific. The results suggest that inversions induced after low-dose irradiation are removed by homeostatic mechanisms within a short time frame, and underscore the importance of studying responses over a period of time when interpreting radiation effects.

Use of synchrotron medical microbeam irradiation to investigate radiation-induced bystander and abscopal effects in vivo

The question of whether bystander and abscopal effects are the same is unclear. Our experimental system enables us to address this question by allowing irradiated organisms to partner with unexposed individuals. Organs from both animals and appropriate sham and scatter dose controls are tested for expression of several endpoints such as calcium flux, role of 5HT, reporter assay cell death and proteomic profile. The results show that membrane related functions of calcium and 5HT are critical for true bystander effect expression. Our original inter-animal experiments used fish species whole body irradiated with low doses of X-rays, which prevented us from addressing the abscopal effect question. Data which are much more relevant in radiotherapy are now available for rats which received high dose local irradiation to the implanted right brain glioma. The data were generated using quasi-parallel microbeams at the biomedical beamline at the European Synchrotron Radiation Facility in Grenoble France. This means we can directly compare abscopal and "true" bystander effects in a rodent tumour model. Analysis of right brain hemisphere, left brain and urinary bladder in the directly irradiated animals and their unirradiated partners strongly suggests that bystander effects (in partner animals) are not the same as abscopal effects (in the irradiated animal). Furthermore, the presence of a tumour in the right brain alters the magnitude of both abscopal and bystander effects in the tissues from the directly irradiated animal and in the unirradiated partners which did not contain tumours, meaning the type of signal was different.

Mechanisms and biological importance of photon-induced bystander responses: do they have an impact on low-dose radiation responses

Elucidating the biological effect of low linear energy transfer (LET), low-dose and/or low-dose-rate ionizing radiation is essential in ensuring radiation safety. Over the past two decades, non-targeted effects, which are not only a direct consequence of radiation-induced initial lesions produced in cellular DNA but also of intra- and inter-cellular communications involving both targeted and non-targeted cells, have been reported and are currently defining a new paradigm in radiation biology. These effects include radiation-induced adaptive response, low-dose hypersensitivity, genomic instability, and radiation-induced bystander response (RIBR). RIBR is generally defined as a cellular response that is induced in non-irradiated cells that receive bystander signals from directly irradiated cells. RIBR could thus play an important biological role in low-dose irradiation conditions. However, this suggestion was mainly based on findings obtained using high-LET charged-particle radiations. The human population (especially the Japanese, who are exposed to lower doses of radon than the world average) is more frequently exposed to low-LET photons (X-rays or γ-rays) than to high-LET charged-particle radiation on a daily basis. There are currently a growing number of reports describing a distinguishing feature between photon-induced bystander response and high-LET RIBR. In particular, photon-induced bystander response is strongly influenced by irradiation dose, the irradiated region of the targeted cells, and p53 status. The present review focuses on the photon-induced bystander response, and discusses its impact on the low-dose radiation effect.

Non-targeted radiation effects in vivo: a critical glance of the future in radiobiology

Radiation-induced bystander effects (RIBE), demonstrate the induction of biological non-targeted effects in cells which have not directly hit by radiation or by free radicals produced by ionization events. Although RIBE have been demonstrated using a variety of biological endpoints the mechanism(s) of this phenomenon still remain unclear. The controversial results of the in vitro RIBE and the evidence of non-targeted effects in various in vivo systems are discussed. The experimental evidence on RIBE, indicate that a more analytical and mechanistic in depth approach is needed to secure an answer to one of the most intriguing questions in radiobiology.

False-positive TUNEL staining observed in SV40 based transgenic murine prostate cancer models

The TRAMP (Transgenic Adenocarcinoma of the Mouse Prostate) and LADY (Probasin-large T antigen transgenic mouse) mice are widely used autochthonous models of prostate cancer. Both models utilise probasin promoters to direct androgen-regulated expression of oncogenic SV40 specifically to epithelial cells of the mouse prostate. The oncogenic processes and phenotypes which result mimic many features of human prostate cancer, making these transgenic mouse models useful experimental systems. The terminal deoxynucleotidyl transferase (Tdt)-mediated dUTP in situ nick end labelling (TUNEL) assay is a commonly used method for the detection of cells undergoing apoptosis. In this study, we demonstrate false-positive TUNEL staining in frozen prostate tissue from TRAMP and LADY mice, which was not observed in non-transgenic control animals and is not due to non-specific binding of labelled-dUTP substrate. The false-positive signal co-localised with large SV40 T-antigen expression. False-positive signal was apparent using multiple commercial TUNEL kits with different detection systems. These results caution against the use of the TUNEL assay for detection of apoptosis in frozen prostate tissue of large T-antigen based autochthonous transgenic models of prostate cancer.

Bystander apoptosis in human cells mediated by irradiated blood plasma

Following exposure to high doses of ionizing radiation, due to an accident or during radiotherapy, bystander signalling poses a potential hazard to unirradiated cells and tissues. This process can be mediated by factors circulating in blood plasma. Thus, we assessed the ability of plasma taken from in vitro irradiated human blood to produce a direct cytotoxic effect, by inducing apoptosis in primary human peripheral blood mononuclear cells (PBM), which mainly comprised G(0)-stage lymphocytes. Plasma was collected from healthy donors' blood irradiated in vitro to 0-40Gy acute γ-rays. Reporter PBM were separated from unirradiated blood with Histopaque and held in medium with the test plasma for 24h at 37°C. Additionally, plasma from in vitro irradiated and unirradiated blood was tested against PBM collected from blood given 4Gy. Apoptosis in reporter PBM was measured by the Annexin V test using flow cytometry. Plasma collected from unirradiated and irradiated blood did not produce any apoptotic response above the control level in unirradiated reporter PBM. Surprisingly, plasma from irradiated blood caused a dose-dependent reduction of apoptosis in irradiated reporter PBM. The yields of radiation-induced cell death in irradiated reporter PBM (after subtracting the respective values in unirradiated reporter PBM) were 22.2±1.8% in plasma-free cultures, 21.6±1.1% in cultures treated with plasma from unirradiated blood, 20.2±1.4% in cultures with plasma from blood given 2-4Gy and 16.7±3.2% in cultures with plasma from blood given 6-10Gy. These results suggested that irradiated blood plasma did not cause a radiation-induced bystander cell-killing effect. Instead, a reduction of apoptosis in irradiated reporter cells cultured with irradiated blood plasma has implications concerning oncogenic risk from mutated cells surviving after high dose in vivo irradiation (e.g. radiotherapy) and requires further study.

Bystander-type effects mediated by long-lived inflammatory signaling in irradiated bone marrow

Radiation-induced bystander and abscopal effects, in which DNA damage is produced in nonirradiated cells as a consequence of communication with irradiated cells, indicate mechanisms of inducing damage and cell death additional to the conventional model of deposition of energy in the cell nucleus at the time of irradiation. In this study we show that signals generated in vivo in the bone marrow of mice irradiated with 4 Gy γ rays 18 h to 15 months previously are able to induce DNA damage and apoptosis in nonirradiated bone marrow cells but that comparable signals are not detected at earlier times postirradiation or at doses below 100 mGy. Bone marrow cells of both CBA/Ca and C57BL/6 genotypes exhibit responses to signals produced by either irradiated CBA/Ca or C57BL/6 mice, and the responses are mediated by the cytokines FasL and TNF-α converging on a COX-2-dependent pathway. The findings are consistent with indirect inflammatory signaling induced as a response to the initial radiation damage rather than to direct signaling between irradiated and nonirradiated cells. The findings also demonstrate the importance of studying tissue responses when considering the mechanisms underlying the consequences of radiation exposures.

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.