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.

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 p21Waf1, 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.

From Energy Deposition of Ionizing Radiation to Cell Damage Signaling: Benchmarking Simulations by Measured Yields of Initial DNA Damage after Ion Microbeam Irradiation.

Advances in accelerator technology, which have enabled conforming radiotherapy with charged hadronic species, have brought benefits as well as potential new risks to patients. To better understand the effects of ionizing radiation on tumor and surrounding tissue, it is important to investigate and quantify the relationship between energy deposition at the nanometric scale and the initial biological events. Monte Carlo track structure simulation codes provide a powerful tool for investigating this relationship; however, their success and reliability are dependent on their improvement and development accordingly to the dedicated biological data to which they are challenged. For this aim, a microbeam facility that allows for fluence control, down to one ion per cell nucleus, was used to evaluate relative frequencies of DNA damage after interaction between the incoming ion and DNA according to radiation quality. Primary human cells were exposed to alpha particles of three different energies with respective linear energy transfers (LETs) of approximately 36, 85 or 170 keV·µm-1 at the cells' center position, or to protons (19 keV·µm-1). Statistical evaluation of nuclear foci formation (53BP1/γ-H2AX), observed using immunofluorescence and related to a particle traversal, was undertaken in a large population of cell nuclei. The biological results were adjusted to consider the factors that drive the experimental uncertainties, then challenged with results using Geant4-DNA code modeling of the ionizing particle interactions on a virtual phantom of the cell nucleus with the same mean geometry and DNA density as the cells used in our experiments. Both results showed an increase of relative frequencies of foci (or simulated DNA damage) in cell nuclei as a function of increasing LET of the traversing particles, reaching a quasi-plateau when the LET exceeded 80-90 keV·µm-1. For the LET of an alpha particle ranging from 80-90 to 170 keV·µm-1, 10-30% of the particle hits did not lead to DNA damage inducing 53BP1 or γ-H2AX foci formation.

RENEB accident simulation exercise.

PURPOSE: The RENEB accident exercise was carried out in order to train the RENEB participants in coordinating and managing potentially large data sets that would be generated in case of a major radiological event. MATERIALS AND METHODS: Each participant was offered the possibility to activate the network by sending an alerting email about a simulated radiation emergency. The same participant had to collect, compile and report capacity, triage categorization and exposure scenario results obtained from all other participants. The exercise was performed over 27 weeks and involved the network consisting of 28 institutes: 21 RENEB members, four candidates and three non-RENEB partners. RESULTS: The duration of a single exercise never exceeded 10 days, while the response from the assisting laboratories never came later than within half a day. During each week of the exercise, around 4500 samples were reported by all service laboratories (SL) to be examined and 54 scenarios were coherently estimated by all laboratories (the standard deviation from the mean of all SL answers for a given scenario category and a set of data was not larger than 3 patient codes). CONCLUSIONS: Each participant received training in both the role of a reference laboratory (activating the network) and of a service laboratory (responding to an activation request). The procedures in the case of radiological event were successfully established and tested.

Health risks of space exploration: targeted and nontargeted oxidative injury by high-charge and high-energy particles.

SIGNIFICANCE: During deep space travel, astronauts are often exposed to high atomic number (Z) and high-energy (E) (high charge and high energy [HZE]) particles. On interaction with cells, these particles cause severe oxidative injury and result in unique biological responses. When cell populations are exposed to low fluences of HZE particles, a significant fraction of the cells are not traversed by a primary radiation track, and yet, oxidative stress induced in the targeted cells may spread to nearby bystander cells. The long-term effects are more complex because the oxidative effects persist in progeny of the targeted and affected bystander cells, which promote genomic instability and may increase the risk of age-related cancer and degenerative diseases. RECENT ADVANCES: Greater understanding of the spatial and temporal features of reactive oxygen species bursts along the tracks of HZE particles, and the availability of facilities that can simulate exposure to space radiations have supported the characterization of oxidative stress from targeted and nontargeted effects. CRITICAL ISSUES: The significance of secondary radiations generated from the interaction of the primary HZE particles with biological material and the mitigating effects of antioxidants on various cellular injuries are central to understanding nontargeted effects and alleviating tissue injury. FUTURE DIRECTIONS: Elucidation of the mechanisms underlying the cellular responses to HZE particles, particularly under reduced gravity and situations of exposure to additional radiations, such as protons, should be useful in reducing the uncertainty associated with current models for predicting long-term health risks of space radiation. These studies are also relevant to hadron therapy of cancer.

Nontargeted stressful effects in normal human fibroblast cultures exposed to low fluences of high charge, high energy (HZE) particles: kinetics of biologic responses and significance of secondary radiations.

The induction of nontargeted stressful effects in cell populations exposed to low fluences of high charge (Z) and high energy (E) particles is relevant to estimates of the health risks of space radiation. We investigated the up-regulation of stress markers in confluent normal human fibroblast cultures exposed to 1,000 MeV/u iron ions [linear energy transfer (LET) ∼151 keV/µm] or 600 MeV/u silicon ions (LET ∼50 keV/µm) at mean absorbed doses as low as 0.2 cGy, wherein 1-3% of the cells were targeted through the nucleus by a primary particle. Within 24 h postirradiation, significant increases in the levels of phospho-TP53 (serine 15), p21(Waf1) (CDKN1A), HDM2, phospho-ERK1/2, protein carbonylation and lipid peroxidation were detected, which suggested participation in the stress response of cells not targeted by primary particles. This was supported by in situ studies that indicated greater increases in 53BP1 foci formation, a marker of DNA damage. than expected from the number of primary particle traversals. The effect was expressed as early as 15 min after exposure, peaked at 1 h and decreased by 24 h. A similar tendency occurred after exposure of the cell cultures to 0.2 cGy of 3.7 MeV α particles (LET ∼109 keV/µm) that targets ∼1.6% of nuclei, but not after 0.2 cGy from 290 MeV/u carbon ions (LET ∼13 keV/µm) by which, on average, ∼13% of the nuclei were hit, which highlights the importance of radiation quality in the induced effect. Simulations with the FLUKA multi-particle transport code revealed that fragmentation products, other than electrons, in cell cultures exposed to HZE particles comprise <1% of the absorbed dose. Further, the radial spread of dose due to secondary heavy ion fragments is confined to approximately 10-20 µm. Thus, the latter are unlikely to significantly contribute to stressful effects in cells not targeted by primary HZE particles.

Broadcasting in the airways: the fifth anniversary of the Radiation Research Podcast (1).

The Radiation Research Podcast was funded just over five years ago by a few Radiation Research Society members. To date, the volunteers running the podcast have produced and published online, open access, over 70 audio interviews. The program includes monthly interviews with authors of articles, award winners, and other recordings at conferences, such as round table discussions. We here present an overview of the podcast, from its creation to its fifth birthday, to explain how it is working, how the featured interviews are scheduled, and what future directions are taken. So, stay tuned!