Radiation-induced bystander effects and adaptive responses--the Yin and Yang of low dose radiobiology?

The scientific nature of the linear no-threshold (LNT) model used in the system of radiological protection

During the first half of the 20th century, it was commonly assumed that radiation-induced health effects occur only when the dose exceeds a certain threshold. This idea was discarded for stochastic effects when more knowledge was gained about the mechanisms of radiation-induced cancer. Currently, a key tenet of the international system of radiological protection is the linear no-threshold (LNT) model where the risk of radiation-induced cancer is believed to be directly proportional to the dose received, even at dose levels where the effects cannot be proven directly. The validity of the LNT approach has been questioned on the basis of a claim that only conclusions that can be verified experimentally or epidemiologically are scientific and LNT should, thus, be discarded because the system of radiological protection must be based on solid science. The aim of this publication is to demonstrate that the LNT concept can be tested in principle and fulfils the criteria of a scientific hypothesis. The fact that the system of radiological protection is also based on ethics does not render it unscientific either. One of the fundamental ethical concepts underlying the LNT model is the precautionary principle. We explain why it is the best approach, based on science and ethics (as well as practical experience), in situations of prevailing uncertainty.

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.

Bystander Effects in Spatially Fractionated Radiation Therapy: From Molecule To Organism To Clinical Implications

The standard of care for radiation therapy is numerous, low-dose fractions that are distributed homogeneously throughout the tumor. An alternative strategy under scrutiny is to apply spatially fractionated radiotherapy (high and low doses throughout the tumor) in one or several fractions, either alone or followed by conventional radiation fractionation . Spatial fractionation allows for significant sparing of normal tissue, and the regions of tumor or normal tissue that received sublethal doses can give rise to beneficial bystander effects in both cases. Bystander effects are broadly defined as biological responses that are significantly greater than would be anticipated based on the radiation dose received. Typically these effects are initiated by diffusion of reactive oxygen species and secretion of various cytokines. As demonstrated in the literature, spatial fractionation related bystander effects can occur locally from cell to cell and in what are known as "cohort effects," which tend to take the form of restructuring of the vasculature, enhanced immune infiltration, and development of immunological memory. Other bystander effects can take place at distant sites in what are known as "abscopal effects." While these events are rare, they are mediated by the immune system and can result in the eradication of secondary and metastatic disease. Currently, due to the complexity and variability of these bystander effects, they are not thoroughly understood, but as knowledge improves they may present significant opportunities for improved clinical outcomes.

Effects of exposing Japanese quail eggs to a low dose of gamma radiation and in ovo feeding by two sources of trace elements on embryonic development activities

The present study investigated the influence of exposing quail eggs to low-dose gamma radiation (GR) and in ovo feeding with 2 sources of a mixture of trace elements (Zn, Fe, and Cu), including sulfate (TES) and loaded with montmorillonite (TEM), on embryonic development activities and prehatch quality. A total of 960 eggs on the seventh day of incubation were randomly divided into 6 groups (160 eggs/group) with 4 replicate of 40 eggs in each. A 3 × 2 factorial arrangement experiment was performed and included 3 sources in ovo feeding with a mixture of trace elements (Zn, Fe, and Cu), including 0 mg/egg, 50 mg TES/egg, and 50 mg TEM/egg with egg irradiation using 0 and 0.2 Gy from GR. Eggs injected with 50 mg TEM/egg and exposed to 0.2 Gy from GR (TEM/GR) was significantly (P ≤ 0.05 and 0.01) higher in hatchability, hatch body weight, and relative organ weight (liver, gizzard, proventriculus, heart, and intestine). The obtained results indicated significant (P ≤ 0.05) decreased in the serum concentration of malondialdehyde (MDA) in TEM/GR group. There was significant (P ≤ 0.05) increased of catalase (CAT) activity and the concentrations of growth hormone (GH) and insulin-like growth factor-1 (IGF-1) in TEM/GR group; however; total antioxidant capacity (T-AOC) was significant (P ≤ 0.05) increased in CT/GR group. Serum concentrations of immunoglobulin M (IgM) (P ≤ 0.05) and tumor necrosis factor-alpha (TNF-α) were increased in the TEM/CR group; the concentration of transforming growth factor beta (TGF-β) significant (P ≤ 0.05) increased in the TEM/GR group; and interleukins (IL6 and IL10) showed no significant differences among the groups. Our results showed increase in thyroxine and myostatin concentrations with TES/CR and CT/GR of our study groups, respectively. The relative mRNA expression levels of the GH, IGF-1, and Fas cell surface death receptor (FAS) genes were significantly (P ≤ 0.05 and 0.01) upregulated in the liver tissue of the TEM/GR group compared with the other groups. In conclusion, TEM/GR was the best treatment for improving prehatch quality, increasing serum antioxidant enzyme activities, and promoting the expression of growth and immune genes in fertilized quail eggs.

Radiology's Ionising Radiation Paradox: Weighing the Indispensable Against the Detrimental in Medical Imaging

Ionising radiation stands as an indispensable protagonist in the narrative of medical imaging, underpinning diagnostic evaluations and therapeutic interventions across an array of medical conditions. However, this protagonist poses a paradox - its inestimable service to medicine coexists with an undercurrent of potential health risks, primarily DNA damage and subsequent oncogenesis. The narrative of this comprehensive review unfurls around this intricate enigma, delicately balancing the indispensable diagnostic utility against the non-negotiable commitment to patient safety. In this critical discourse, the intricacies of ionising radiation are dissected, illuminating not only its sources but also the associated biological and health hazards. The exploration delves into the labyrinth of strategies currently deployed to minimise exposure and safeguard patients. By casting light on the scientific nuances of X-rays, computed tomography (CT), and nuclear medicine, it traverses the complex terrain of radiation use in radiology, to promote safer medical imaging practices, and to facilitate an ongoing dialogue about diagnostic necessity and risk. Through a rigorous examination, the pivotal relationship between radiation dose and dose response is elucidated, unravelling the mechanisms of radiation injury and distinguishing between deterministic and stochastic effects. Moreover, protection strategies are illuminated, demystifying concepts such as justification, optimisation, the As Low As Reasonably Achievable (ALARA) principle, dose and diagnostic reference levels, along with administrative and regulatory approaches. With an eye on the horizon, promising avenues of future research are discussed. These encompass low-radiation imaging techniques, long-term risk assessment in large patient cohorts, and the transformative potential of artificial intelligence in dose optimisation. This exploration of the nuanced complexities of radiation use in radiology aims to foster a collaborative impetus towards safer medical imaging practices. It underscores the need for an ongoing dialogue around diagnostic necessity and risk, thereby advocating for a continual reassessment in the narrative of medical imaging.

Dichotomous effects of in vivo and in vitro ionizing radiation exposure on lymphatic function

On the one hand, lymphatic dysfunction induces interstitial edema and inflammation. On the other hand, the formation of edema and inflammation induce lymphatic dysfunction. However, informed by the earlier reports of undetected apoptosis of irradiated lymphatic endothelial cells (LECs) in vivo, lymphatic vessels are commonly considered inconsequential to ionizing radiation (IR)-induced inflammatory injury to normal tissues. Primarily because of the lack of understanding of the acute effects of IR exposure on lymphatic function, acute edema and inflammation, common sequelae of IR exposure, have been ascribed solely to blood vessel damage. Therefore, in the present study, the lymphatic acute responses to IR exposure were quantified to evaluate the hypothesis that IR exposure impairs lymphatic pumping. Rat mesenteric lymphatic vessels were irradiated in vivo or in vitro, and changes in pumping were quantified in isolated vessels in vitro. Compared with sham-treated vessels, pumping was lowered in lymphatic vessels irradiated in vivo but increased in vessels irradiated in vitro. Furthermore, unlike in blood vessels, the acute effects of IR exposure in lymphatic vessels were not mediated by nitric oxide-dependent pathways in either in vivo or in vitro irradiated vessels. After cyclooxygenase blockade, pumping was partially restored in lymphatic vessels irradiated in vitro but not in vessels irradiated in vivo. Taken together, these findings demonstrated that lymphatic vessels are radiosensitive and LEC apoptosis alone may not account for all the effects of IR exposure on the lymphatic system.NEW & NOTEWORTHY Earlier studies leading to the common belief that lymphatic vessels are radioresistant either did not characterize lymphatic pumping, deemed necessary for the resolution of edema and inflammation, or did it in vivo. By characterizing pumping in vitro, the present study, for the first time, demonstrated that lymphatic pumping was impaired in vessels irradiated in vivo and enhanced in vessels irradiated in vitro. Furthermore, the pathways implicated in ionizing radiation-induced blood vessel damage did not mediate lymphatic responses.

Radiobiological and social considerations following a radiological terrorist attack; mechanisms, detection and mitigation: review of new research developments

PURPOSE

This review focuses on recent research in understanding the different aspects of what society should expect from a radiological attack. Although some scenarios of a radiologic event can be impossible to be prepared for, the effort put toward educating and better preparing for these types of events can help minimize some of the issues. The different areas discussed in this review include radioisotopes of concern, detection of radiation dose, biological effects of ionizing radiation exposures, low dose effects, targeted and non-targeted effects (NTE), psychological effects, mitigations, with a brief mention of other considerations such as medical preparedness, communication, policy implications and ethical issues. This review also discusses solutions to rectify the issues faced at hand that may come up in the event of a radiologic terrorist attack.

CONCLUSIONS

A review of recent work in the area shows that a multi-layered and interdisciplinary approach is needed to prepare for a radiological terrorist attack. As well as medical preparedness, the approach needs to include sociological and psychological planning as well as an understanding of ethical issues. Since the likely 'dirty bomb' scenarios may involve low dose exposures to high numbers of people, a much better theoretical and practical understanding of low dose radiobiology and the development of robust low dose exposure biomarkers is needed as part of an integrated plan.

Ceramide-Enriched Membrane Domains Contribute to Targeted and Nontargeted Effects of Radiation through Modulation of PI3K/AKT Signaling in HNSCC Cells

We investigated the potential involvement of ceramide-enriched membrane domains in radiation-induced targeted and nontargeted effects using head and neck squamous cell carcinoma with opposite radiosensitivities. In radiosensitive SCC61 cells, the proportion of targeted effects was 34% and nontargeted effects killed 32% of cells. In contrast, only targeted effects (30%) are involved in the overall death of radioresistant SQ20B cells. We then demonstrated in SCC61 cells that nontargeted cell response was driven by the formation of the radiation-induced ceramide-enriched domain. By contrast, the existence of these platforms in SQ20B cells confers a permissive region for phosphatidylinositol-3-kinase (PI3K)/AKT activation. The disruption of lipid raft results in strong inhibition of PI3K/AKT signaling, leading to radiosensitization and apparition of nontargeted effects. These results suggest that ceramide-enriched platforms play a significant role in targeted and nontargeted effects during radiotherapy and that drugs modulating cholesterol levels may be a good alternative for improving radiotherapy effectiveness.

Multiple Stressor Effects of Radon and Phthalates in Children: Background Information and Future Research

The present paper reviews available background information for studying multiple stressor effects of radon (222Rn) and phthalates in children and provides insights on future directions. In realistic situations, living organisms are collectively subjected to many environmental stressors, with the resultant effects being referred to as multiple stressor effects. Radon is a naturally occurring radioactive gas that can lead to lung cancers. On the other hand, phthalates are semi-volatile organic compounds widely applied as plasticizers to provide flexibility to plastic in consumer products. Links of phthalates to various health effects have been reported, including allergy and asthma. In the present review, the focus on indoor contaminants was due to their higher concentrations and to the higher indoor occupancy factor, while the focus on the pediatric population was due to their inherent sensitivity and their spending more time close to the floor. Two main future directions in studying multiple stressor effects of radon and phthalates in children were proposed. The first one was on computational modeling and micro-dosimetric studies, and the second one was on biological studies. In particular, dose-response relationship and effect-specific models for combined exposures to radon and phthalates would be necessary. The ideas and methodology behind such proposed research work are also applicable to studies on multiple stressor effects of collective exposures to other significant airborne contaminants, and to population groups other than children.

Retrospective biodosimetry techniques: Focus on cytogenetics assays for individuals exposed to ionizing radiation

In the absence of physical data, biodosimetry tools are required for fast dose and risk assessment in the event of radiological or nuclear mass accidents or attacks to triage exposed humans and take immediate medical countermeasures. Biodosimetry tools have mostly been developed for retrospective dose assessment and the follow-up of victims of irradiation. Among them, cytogenetics analyses, to reveal chromosome damage, are the most developed and allow the determination of doses from blood samples as low as 100 mGy. Various cytogenetic tests have already allowed retrospective dose assessment of Chernobyl liquidators and military personnel exposed to nuclear tests after decades. In this review, we discuss the properties of various biodosimetry techniques, such as their sensitivity and limitations as a function of the time from exposure, using multiple examples of nuclear catastrophes or working exposure. Among them, chromosome FISH hybridization, which reveals chromosome translocations, is the most reliable due to the persistence of translocations for decades, whereas dicentric chromosome and micronuclei assays allow rapid and accurate dose assessment a short time after exposure. Both need to be adjusted through mathematical algorithms for retrospective analyses, accounting for the time since exposure and the victims' age. The goal for the future will be to better model chromosome damage, reduce the time to result, and develop new complementary biodosimetry approaches, such as mutation signatures.

Towards a New Concept of Low Dose

When people discuss the risks associated with low doses of ionizing radiation, central to the discussion is the definition of a low dose and the nature of harm. Standard answers such as "doses below 0.1 Gy are low" or "cancer is the most sensitive measure of harm" obscure the complexity within these seemingly simple questions. This paper will discuss some of the complex issues involved in determining risks to human and nonhuman species from low-dose exposures. Central to this discussion will be the role of communicable responses to all stressors (often referred to as bystander responses), which include recently discovered epigenetic and nontargeted mechanisms. There is a growing consensus that low-dose exposure to radiation is but one of many stressors to impact populations. Many of these stressors trigger responses that are generic and not unique to radiation. The lack of a unique radiation signature makes absolute definition of radiation risk difficult. This paper examines a possible new way of defining low dose based on the systemic response to the radiation. Many factors will influence this systemic response and, because it is inherently variable, it is difficult to predict and so makes low-dose responses very uncertain. Rather than seeking to reduce uncertainty, it might be valuable to accept the variability in outcomes, which arise from the complexity and multifactorial nature of responses to stressors.

Relevance of Non-Targeted Effects for Radiotherapy and Diagnostic Radiology; A Historical and Conceptual Analysis of Key Players

Non-targeted effects (NTE) such as bystander effects or genomic instability have been known for many years but their significance for radiotherapy or medical diagnostic radiology are far from clear. Central to the issue are reported differences in the response of normal and tumour tissues to signals from directly irradiated cells. This review will discuss possible mechanisms and implications of these different responses and will then discuss possible new therapeutic avenues suggested by the analysis. Finally, the importance of NTE for diagnostic radiology and nuclear medicine which stems from the dominance of NTE in the low-dose region of the dose–response curve will be presented. Areas such as second cancer induction and microenvironment plasticity will be discussed.

Adaptive responses to low doses of radiation or chemicals: their cellular and molecular mechanisms

This article reviews the current knowledge on the mechanisms of adaptive response to low doses of ionizing radiation or chemical exposure. A better knowledge of these mechanisms is needed to improve our understanding of health risks at low levels of environmental or occupational exposure and their involvement in cancer or non-cancer diseases. This response is orchestrated through a multifaceted cellular program involving the concerted action of diverse stress response pathways. These evolutionary highly conserved defense mechanisms determine the cellular response to chemical and physical aggression. They include DNA damage repair (p53, ATM, PARP pathways), antioxidant response (Nrf2 pathway), immune/inflammatory response (NF-κB pathway), cell survival/death pathway (apoptosis), endoplasmic response to stress (UPR response), and other cytoprotective processes including autophagy, cell cycle regulation, and the unfolded protein response. The coordinated action of these processes induced by low-dose radiation or chemicals produces biological effects that are currently estimated with the linear non-threshold model. These effects are controversial. They are difficult to detect because of their low magnitude, the scarcity of events in humans, and the difficulty of corroborating associations over the long term. Improving our understanding of these biological consequences should help humans and their environment by enabling better risk estimates, the revision of radiation protection standards, and possible therapeutic advances.

Effect of gamma radiation on the production of bystander signals from three earthworm species irradiated in vivo

The effect of gamma radiation delivered over 24 h on the induction of bystander signals of three earthworm species exposed in vivo was investigated: A. chlorotica, A. caliginosa, and E. tetraedra. Worms were exposed to external gamma irradiation (Co-60 source) for 24 h and samples of head, body, and clitellum were dissected from exposed and control worms and placed in culture medium for 24 h at 19 C. The harvested medium was filtered and assayed for expression of bystander signals using both clonogenic and mitochondrial reporter assays. Different responses were observed in the different species and in the different tissues. A. chlorotica worm-treated reporters show insignificant mitochondrial response for all sections, yet a significant clonogenic reduction in survival for body sections. A. caliginosa worm-treated reporters show a significant mitochondrial response for some sections and insignificant mitochondrial response and insignificant reduction in clonogenic survival for the rest. E. tetraedra worms from a control site show significant evidence of bystander signalling, measured by mitochondrial response in reporter cells, for all sections while those harvested from a contaminated site show insignificant changes in baseline signalling when exposed to the challenge dose. In vivo exposure of earthworm species shows evidence of bystander signalling using two different reporter assays. This effect varied between the different species and tissues. There is also evidence of attenuated bystander signalling in worms harvested from a site contaminated with radiation.

Old Data-New Concepts: Integrating "Indirect Effects" Into Radiation Protection

PURPOSE

To address the following key question, what are the consequences of nontargeted and delayed effects for linear nonthreshold models of radiation risk? This paper considers low-dose "indirect" or nontargeted effects and how they might impact radiation protection, particularly at the level of the environment. Nontargeted effects refer to effects in cells, tissues, or organisms that were not targeted by irradiation and that did not receive direct energy deposition. They include genomic instability and lethal mutations in progeny of irradiated cells and bystander effects in neighboring cells, tissues, or organisms. Low-dose hypersensitivity and adaptive responses are sometimes included under the nontargeted effects umbrella, but these are not considered in this paper. Some concepts emerging in the nontargeted effects field that could be important include historic dose. This suggests that the initial exposure to radiation initiates the instability phenotype which is passed to progeny leading to a transgenerational radiation-response phenotype, which suggests that the system response rather than the individual response is critical in determining outcome.

CONCLUSION

Nontargeted effects need to be considered, and modeling, experimental, and epidemiological approaches could all be used to determine the impact of nontargeted effects on the currently used linear nonthreshold model in radiation protection.

Low-Dose Ionizing Radiation Affects Mesenchymal Stem Cells via Extracellular Oxidized Cell-Free DNA: A Possible Mediator of Bystander Effect and Adaptive Response

We have hypothesized that the adaptive response to low doses of ionizing radiation (IR) is mediated by oxidized cell-free DNA (cfDNA) fragments. Here, we summarize our experimental evidence for this model. Studies involving measurements of ROS, expression of the NOX (superoxide radical production), induction of apoptosis and DNA double-strand breaks, antiapoptotic gene expression and cell cycle inhibition confirm this hypothesis. We have demonstrated that treatment of mesenchymal stem cells (MSCs) with low doses of IR (10 cGy) leads to cell death of part of cell population and release of oxidized cfDNA. cfDNA has the ability to penetrate into the cytoplasm of other cells. Oxidized cfDNA, like low doses of IR, induces oxidative stress, ROS production, ROS-induced oxidative modifications of nuclear DNA, DNA breaks, arrest of the cell cycle, activation of DNA reparation and antioxidant response, and inhibition of apoptosis. The MSCs pretreated with low dose of irradiation or oxidized cfDNA were equally effective in induction of adaptive response to challenge further dose of radiation. Our studies suggest that oxidized cfDNA is a signaling molecule in the stress signaling that mediates radiation-induced bystander effects and that it is an important component of the development of radioadaptive responses to low doses of IR.

Zebrafish as an In Vivo Model to Assess Epigenetic Effects of Ionizing Radiation

Exposure to ionizing radiations (IRs) is ubiquitous in our environment and can be categorized into “targeted” effects and “non-targeted” effects. In addition to inducing deoxyribonucleic acid (DNA) damage, IR exposure leads to epigenetic alterations that do not alter DNA sequence. Using an appropriate model to study the biological effects of radiation is crucial to better understand IR responses as well as to develop new strategies to alleviate exposure to IR. Zebrafish, Danio rerio, is a scientific model organism that has yielded scientific advances in several fields and recent studies show the usefulness of this vertebrate model in radiation biology. This review briefly describes both “targeted” and “non-targeted” effects, describes the findings in radiation biology using zebrafish as a model and highlights the potential of zebrafish to assess the epigenetic effects of IR, including DNA methylation, histone modifications and miRNA expression. Other in vivo models are included to compare observations made with zebrafish, or to illustrate the feasibility of in vivo models when the use of zebrafish was unavailable. Finally, tools to study epigenetic modifications in zebrafish, including changes in genome-wide DNA methylation, histone modifications and miRNA expression, are also described in this review.

The effects of chronic, low doses of Ra-226 on cultured fish and human cells

PURPOSE

To determine the chronic low-dose radiation effects caused by α-particle radiation from (226)Ra over multiple cell generations in CHSE/F fish cells and HaCaT human cells.

METHODS

CHSE/F cells and HaCaT cells were cultured in medium containing (226)Ra to deliver the chronic low-dose α-particle radiation. Clonogenic assay was used to test the clonogenic survival fractions of cells with or without being exposed to radiation from (226)Ra.

RESULTS

The chronic low-dose radiation from (226)Ra does have effects on the clonogenic survival of CHSE/F cells and HaCaT cells. When CHSE/F cells were cultured in (226)Ra-medium over 9 passages for about 134 days, the clonogenic surviving fractions for cells irradiated at dose rates ranging from 0.00066 to 0.66mGy/d were significantly lower than that of cells sham irradiated. For HaCaT cells grown in medium containing the same range of (226)Ra activity, the clonogenic surviving fraction decreased at first and reached the lowest value at about 42 days (8 passages). After that, the clonogenic survival began to increase, and was significantly higher than that of control cells by the end of the experimental period.

CONCLUSION

The chronic, low-dose high LET radiation from (226)Ra can influence the clonogenic survival of irradiated cells. CHSE/F cells were sensitized by the radiation, and HaCaT cells were initially sensitized but later appeared to be adapted. The results could have implications for determining risk from chronic versus acute exposures to radium.

Impact of fractionation on out-of-field survival and DNA damage responses following exposure to intensity modulated radiation fields

To limit toxicity to normal tissues adjacent to the target tumour volume, radiotherapy is delivered using fractionated regimes whereby the total prescribed dose is given as a series of sequential smaller doses separated by specific time intervals. The impact of fractionation on out-of-field survival and DNA damage responses was determined in AGO-1522 primary human fibroblasts and MCF-7 breast tumour cells using uniform and modulated exposures delivered using a 225 kVp x-ray source. Responses to fractionated schedules (two equal fractions delivered with time intervals from 4 h to 48 h) were compared to those following acute exposures. Cell survival and DNA damage repair measurements indicate that cellular responses to fractionated non-uniform exposures differ from those seen in uniform exposures for the investigated cell lines. Specifically, there is a consistent lack of repair observed in the out-of-field populations during intervals between fractions, confirming the importance of cell signalling to out-of-field responses in a fractionated radiation schedule, and this needs to be confirmed for a wider range of cell lines and conditions.

Oxidized extracellular DNA as a stress signal in human cells

The term "cell-free DNA" (cfDNA) was recently coined for DNA fragments from plasma/serum, while DNA present in in vitro cell culture media is known as extracellular DNA (ecDNA). Under oxidative stress conditions, the levels of oxidative modification of cellular DNA and the rate of cell death increase. Dying cells release their damaged DNA, thus, contributing oxidized DNA fragments to the pool of cfDNA/ecDNA. Oxidized cell-free DNA could serve as a stress signal that promotes irradiation-induced bystander effect. Evidence points to TLR9 as a possible candidate for oxidized DNA sensor. An exposure to oxidized ecDNA stimulates a synthesis of reactive oxygen species (ROS) that evokes an adaptive response that includes transposition of the homologous loci within the nucleus, polymerization and the formation of the stress fibers of the actin, as well as activation of the ribosomal gene expression, and nuclear translocation of NF-E2 related factor-2 (NRF2) that, in turn, mediates induction of phase II detoxifying and antioxidant enzymes. In conclusion, the oxidized DNA is a stress signal released in response to oxidative stress in the cultured cells and, possibly, in the human body; in particular, it might contribute to systemic abscopal effects of localized irradiation treatments.

Radiation hormesis: Autophagy and other cellular mechanisms

PURPOSE

To review the cellular mechanisms of hormetic effects induced by low dose and low dose rate ionising radiation in model systems, and to call attention to the possible role of autophagy in some hormetic effects.

RESULTS AND CONCLUSIONS

Very low radiation doses stimulate cell proliferation by changing the equilibrium between the phosphorylated and dephosphorylated forms of growth factor receptors. Radioadaptation is induced by various weak stress stimuli and depends on signalling events that ultimately decrease the molecular damage expression at the cellular level upon subsequent exposure to a moderate radiation dose. Ageing and cancer result from oxidative damage under oxidative stress conditions; nevertheless, ROS are also prominent inducers of autophagy, a cellular process that has been shown to be related both to ageing retardation and cancer prevention. A balance between the signalling functions and damaging effects of ROS seems to be the most important factor that decides the fate of the mammalian cell when under oxidative stress conditions, after exposure to ionising radiation. Not enough is yet known on the pre-requirements for maintaining such a balance. Given the present stage of investigation into radiation hormesis, the application of the conclusions from experiments on model systems to the radiation protection regulations would not be justified.

Checking the foundation: recent radiobiology and the linear no-threshold theory

The linear no-threshold (LNT) theory has been adopted as the foundation of radiation protection standards and risk estimation for several decades. The "microdosimetric argument" has been offered in support of the LNT theory. This argument postulates that energy is deposited in critical cellular targets by radiation in a linear fashion across all doses down to zero, and that this in turn implies a linear relationship between dose and biological effect across all doses. This paper examines whether the microdosimetric argument holds at the lowest levels of biological organization following low dose, low dose-rate exposures to ionizing radiation. The assumptions of the microdosimetric argument are evaluated in light of recent radiobiological studies on radiation damage in biological molecules and cellular and tissue level responses to radiation damage. There is strong evidence that radiation initially deposits energy in biological molecules (e.g., DNA) in a linear fashion, and that this energy deposition results in various forms of prompt DNA damage that may be produced in a pattern that is distinct from endogenous (e.g., oxidative) damage. However, a large and rapidly growing body of radiobiological evidence indicates that cell and tissue level responses to this damage, particularly at low doses and/or dose-rates, are nonlinear and may exhibit thresholds. To the extent that responses observed at lower levels of biological organization in vitro are predictive of carcinogenesis observed in vivo, this evidence directly contradicts the assumptions upon which the microdosimetric argument is based.

Low-dose Photon and Simulated Solar Particle Event Proton Effects on Foxp3+ T Regulatory Cells and other Leukocytes

Radiation is a major factor in the spaceflight environment that has carcinogenic potential. Astronauts on missions are continuously exposed to low-dose/low-dose-rate (LDR) radiation and may receive relatively high doses during a solar particle event (SPE) that consists primarily of protons. However, there are very few reports in which LDR photons were combined with protons. In this study, C57BL/6 mice were exposed to 1.7 Gy simulated SPE (sSPE) protons over 36 h, both with and without pre-exposure to 0.01 Gray (Gy) LDR γ-rays at 0.018 cGy/h. Apoptosis in skin samples was determined by immunohistochemistry immediately post-irradiation (day 0). Spleen mass relative to body mass, white blood cells (WBC), major leukocyte populations, lymphocyte subsets (T, Th, Tc, B, NK), and CD4+ CD25+ Foxp3+ T regulatory (Treg) cells were analyzed on days 4 and 21. Apoptosis in skin samples was evident in all irradiated groups; the LDR+sSPE mice had the greatest expression of activated caspase-3. On day 4 post-irradiation, the sSPE and LDR+sSPE groups had significantly lower WBC counts in blood and spleen compared to non-irradiated controls ( p < 0.05 vs. 0 Gy). CD4+ CD25+ Foxp3+ Treg cell numbers in spleen were decreased at day 4, but proportions were increased in the sSPE and LDR+sSPE groups ( p < 0.05 vs. 0 Gy). By day 21, lymphocyte counts were still low in blood from the LDR+sSPE mice, especially due to reductions in B, NK, and CD8+ T cytotoxic cells. The data demonstrate, for the first time, that pre-exposure to LDR photons did not protect against the adverse effects of radiation mimicking a large solar storm. The increased proportion of immunosuppressive CD4+ CD25+ Foxp3+ Treg and persistent reduction in circulating lymphocytes may adversely impact immune defenses that include removal of sub-lethally damaged cells with carcinogenic potential, at least for a period of time post-irradiation.

Investigation of adaptive responses in bystander cells in 3D cultures containing tritium-labeled and unlabeled normal human fibroblasts

The study of radiation-induced bystander effects in normal human cells maintained in three-dimensional (3D) architecture provides more in vivo-like conditions and is relevant to human risk assessment. Linear energy transfer, dose and dose rate have been considered as critical factors in propagating radiation-induced effects. This investigation uses an in vitro 3D tissue culture model in which normal AG1522 human fibroblasts are grown in a carbon scaffold to investigate induction of a G(1) arrest in bystander cells that neighbor radiolabeled cells. Cell cultures were co-pulse-labeled with [(3)H]deoxycytidine ((3)HdC) to selectively irradiate a minor fraction of cells with 1-5 keV/microm beta particles and bromodeoxyuridine (BrdU) to identify the radiolabeled cells using immunofluorescence. The induction of a G(1) arrest was measured specifically in unlabeled cells (i.e. bystander cells) using a flow cytometry-based version of the cumulative labeling index assay. To investigate the relationship between bystander effects and adaptive responses, cells were challenged with an acute 4 Gy gamma-radiation dose after they had been kept under the bystander conditions described above for several hours, and the regulation of the radiation-induced G(1) arrest was measured selectively in bystander cells. When the average dose rate in (3)HdC-labeled cells (<16% of population) was 0.04-0.37 Gy/h (average accumulated dose 0.14-10 Gy), no statistically significant stressful bystander effects or adaptive bystander effects were observed as measured by magnitude of the G(1) arrest, micronucleus formation, or changes in mitochondrial membrane potential. Higher dose rates and/or higher LET may be required to observe stressful bystander effects in this experimental system, whereas lower dose rates and challenge doses may be required to detect adaptive bystander responses.

Evidence of an adaptive response targeting DNA nonhomologous end joining and its transmission to bystander cells

Adaptive response (AR) is a term describing resistance to ionizing radiation-induced killing or formation of aberrant chromosomes that is mediated by pre-exposure to low ionizing radiation doses. The mechanism of AR remains elusive. Because cell killing and chromosome aberration formation derive from erroneous processing of DNA double-strand breaks (DSB), AR may reflect a modulation of DSB processing by nonhomologous end joining (NHEJ) or homologous recombination repair. Here, we use plasmid end-joining assays to quantify modulations induced by low ionizing radiation doses to NHEJ, the dominant pathway of DSB repair in higher eukaryotes, and investigate propagation of this response through medium transfer to nonirradiated bystander cells. Mouse embryo fibroblasts were conditioned with 10 to 1000 mGy and NHEJ quantified at different times thereafter by challenging with reporter plasmids containing a DSB. We show robust increases in NHEJ efficiency in mouse embryo fibroblasts exposed to ionizing radiation >100 mGy, irrespective of reporter plasmid used. Human tumor cells also show AR of similar magnitude that is compromised by caffeine, an inhibitor of DNA damage signaling acting by inhibiting ATM, ATR, and DNA-PKcs. Growth medium from pre-irradiated cells induces a caffeine-sensitive AR in nonirradiated cells, similar in magnitude to that seen in irradiated cells. In bystander cells, γH2AX foci are specifically detected in late S-G(2) phase and are associated with Rad51 foci that signify the function of homologous recombination repair, possibly on DNA replication-mediated DSBs. The results point to enhanced NHEJ as a mechanism of AR and suggest that AR may be transmitted to bystander cells through factors generating replication-mediated DSBs.

Modeling Dose-response at Low Dose: A Systems Biology Approach for Ionization Radiation

For ionization radiation (IR) induced cancer, a linear non-threshold (LNT) model at very low doses is the default used by a number of national and international organizations and in regulatory law. This default denies any positive benefit from any level of exposure. However, experimental observations and theoretical biology have found that both linear and J-shaped IR dose-response curves can exist at those very low doses. We develop low dose J-shaped dose-response, based on systems biology, and thus justify its use regarding exposure to IR. This approach incorporates detailed, molecular and cellular descriptions of biological/toxicological mechanisms to develop a dose-response model through a set of nonlinear, differential equations describing the signaling pathways and biochemical mechanisms of cell cycle checkpoint, apoptosis, and tumor incidence due to IR. This approach yields a J-shaped dose response curve while showing where LNT behaviors are likely to occur. The results confirm the hypothesis of the J-shaped dose response curve: the main reason is that, at low-doses of IR, cells stimulate protective systems through a longer cell arrest time per unit of IR dose. We suggest that the policy implications of this approach are an increasingly correct way to deal with precautionary measures in public health.

Cellular radiosensitivity: how much better do we understand it?

PURPOSE

Ionising radiation exposure gives rise to a variety of lesions in DNA that result in genetic instability and potentially tumourigenesis or cell death. Radiation extends its effects on DNA by direct interaction or by radiolysis of H(2)O that generates free radicals or aqueous electrons capable of interacting with and causing indirect damage to DNA. While the various lesions arising in DNA after radiation exposure can contribute to the mutagenising effects of this agent, the potentially most damaging lesion is the DNA double strand break (DSB) that contributes to genome instability and/or cell death. Thus in many cases failure to recognise and/or repair this lesion determines the radiosensitivity status of the cell. DNA repair mechanisms including homologous recombination (HR) and non-homologous end-joining (NHEJ) have evolved to protect cells against DNA DSB. Mutations in proteins that constitute these repair pathways are characterised by radiosensitivity and genome instability. Defects in a number of these proteins also give rise to genetic disorders that feature not only genetic instability but also immunodeficiency, cancer predisposition, neurodegeneration and other pathologies.

CONCLUSIONS

In the past 50 years our understanding of the cellular response to radiation damage has advanced enormously with insight being gained from a wide range of approaches extending from more basic early studies to the sophisticated approaches used today. In this review we discuss our current understanding of the impact of radiation on the cell and the organism gained from the array of past and present studies and attempt to provide an explanation for what it is that determines the response to radiation.

Adaptive response in zebrafish embryos induced using microbeam protons as priming dose and X-ray photons as challenging dose

In the studies reported here, a high-linear-energy-transfer (high-LET)-radiation dose was used to induce adaptive response in zebrafish embryos in vivo. Microbeam protons were used to provide the priming dose and X-ray photons were employed to provide the challenging dose. The microbeam irradiation system (Single-Particle Irradiation System to Cell, acronym as SPICE) at the National Institute of Radiological Sciences (NIRS), Japan, was employed to control and accurately quantify the number of protons at very low doses, viz., about 100 µGy. The embryos were dechorionated at 4 h post fertilization (hpf) and irradiated at 5 hpf by microbeam protons. For each embryo, ten irradiation points were arbitrarily chosen without overlapping with one another. To each irradiation point, 5, 10 or 20 protons each with an energy of 3.4 MeV were delivered. The embryos were returned back to the incubator until 10 hpf to further receive the challenging exposure, which was achieved using 2 Gy of X-ray irradiation, and then again returned to the incubator until 24 hpf for analyses. The levels of apoptosis in zebrafish embryos at 25 hpf were quantified through terminal dUTP transferase-mediated nick end-labeling (TUNEL) assay, with the apoptotic signals captured by a confocal microscope. The results revealed that 5 to 20 protons delivered at 10 points each on the embryos, or equivalently 110 to 430 µGy, could induce radioadaptive response in the zebrafish embryos in vivo.

Ionizing radiation-induced bystander mutagenesis and adaptation: quantitative and temporal aspects

This work explores several quantitative aspects of radiation-induced bystander mutagenesis in WTK1 human lymphoblast cells. Gamma-irradiation of cells was used to generate conditioned medium containing bystander signals, and that medium was transferred onto naïve recipient cells. Kinetic studies revealed that it required up to 1h to generate sufficient signal to induce the maximal level of mutations at the thymidine kinase locus in the bystander cells receiving the conditioned medium. Furthermore, it required at least 1h of exposure to the signal in the bystander cells to induce mutations. Bystander signal was fairly stable in the medium, requiring 12-24h to diminish. Medium that contained bystander signal was rendered ineffective by a 4-fold dilution; in contrast a greater than 20-fold decrease in the cell number irradiated to generate a bystander signal was needed to eliminate bystander-induced mutagenesis. This suggested some sort of feedback inhibition by bystander signal that prevented the signaling cells from releasing more signal. Finally, an ionizing radiation-induced adaptive response was shown to be effective in reducing bystander mutagenesis; in addition, low levels of exposure to bystander signal in the transferred medium induced adaptation that was effective in reducing mutations induced by subsequent gamma-ray exposures.

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.

Comparison of direct and bystander effects induced by ionizing radiation in eight fish cell lines

PURPOSE

To determine bystander and direct effects of ionizing radiation on eight fish cell lines.

MATERIALS AND METHODS

Fish cell lines were irradiated at a range of doses from 0.5 - 5 Gy. The Irradiated Cell Conditioned Medium (ICCM) was then harvested and placed onto a HPV-G, reporter cell line as well as onto autologous fish cell lines. Cloning efficiency (CE) was the end point used. The HPV-G reporter cell line was chosen because this cell line is capable of transmitting and producing the bystander effect.

RESULTS

Four of the eight fish cell lines were clonogenic. These, with the exception of RTG-2 cells, showed increased CE when ICCM was tested on unirradiated autologous cells or on HPV-G cells. ICCM from RTG-2 cells reduced survival. The non-clonogenic cells ICCM tested on HPV-G all showed increased CE.

CONCLUSIONS

The results show that both bystander signal production and cellular response varies depending on the cell line and that in general signals from established fish cells do not produce death inducing bystander effects. Thus, the comparison of the effect from fish cell ICCM on autologous cells or HPV-G human cells allowed us to separate signal production from response. In almost all cases, for both non-clonogenic and clonogenic fish cell lines, the HPV-G recipient cell line showed an increase in percent survival compared to controls while the clonogenic fish cell lines do not appear to respond.

The hormesis concept and risk assessment: are there unique ethical and policy considerations?

The hormesis concept holds that low doses of toxic substances and radiation elicit modest biological responses opposite to those caused by higher doses of the same agents. This concept stands in contrast to the prevailing views that a threshold model predicts most responses to toxicants at low doses and that linear extrapolation best predicts mutagenic and carcinogenic responses. Beyond the scientific considerations, there has been concern that inclusion of the hormesis model in risk assessment would raise complex ethical questions, pose serious challenges for policy makers, and threaten public safety. This article briefly reviews the growing evidence for hormesis and offers a perspective on the related ethical and societal issues. Complexities stem from the nature of biphasic curves, in which biological responses fall both above and below background levels. The monotonic responses of the threshold and linear models lend themselves to a single policy objective--avoiding harm associated with exposures. The biphasic responses of the hormesis model, however, suggest the possibility of accruing benefit as well as avoiding harm. The prospect of applying the hormesis model to public health policy is impeded by insufficient ability to identify the hormetic and toxic zones with precision. Moreover, heterogeneity among individuals in susceptibility to toxicants suggests that benefit and risk may be distributed unequally in the population. The potential shift in policy objectives associated with hormesis is considered relative to the difficulty of balancing the ethical principles of nonmaleficence and beneficence while maintaining a higher priority on the former.

A perspective on the scientific, philosophical, and policy dimensions of hormesis

The hormesis concept has broad implications for biology and the biomedical sciences. This perspective on hormesis concentrates on toxicology and toxicological risk assessment and secondarily explores observations from other fields. It considers the varied manifestations of hormesis in the context of a broad family of biological stress responses. Evidence for hormesis is reviewed, and the hormesis model is contrasted with more widely accepted dose-response models in toxicology: a linear nonthreshold (LNT) model for mutagenesis and carcinogenesis, and a threshold model for most other toxicologic effects. Scientific, philosophical, and political objections to the hormesis concept are explored, and complications in the hormesis concept are analyzed. The review concludes with a perspective on the current state of hormesis and challenges that the hormesis model poses for risk assessment.

Effects of hTERT on genomic instability caused by either metal or radiation or combined exposure

Genomic instability is considered to be an important component in carcinogenesis. It can be caused by low-dose exposure to agents, which appear to act through induction of stress-response pathways related to oxidative stress. These agents have been studied mostly in the radiation field but evidence is accumulating that chemicals, especially heavy metals such as Cr (VI), can also act in the same manner. Previous work showed that metal ions could initiate long-term genomic instability in human primary fibroblasts and this phenomenon was regulated by telomerase. The aim of this study was to examine the difference in clonogenic survival and cytogenetic damage after exposure to Cr (VI) and radiation both singly and in combination in normal human fibroblasts (hTERT- cells) and engineered human fibroblasts, infected with a retrovirus carrying a cDNA encoding hTERT, which rendered these cells telomerase positive and replicatively immortal (hTERT+ cells). Cr (VI) induced genomic instability in hTERT- cells but not in hTERT+ cells, whereas radiation induced genomic instability in hTERT+ cells and to a lesser extent in hTERT- cells. Combined exposure caused genomic instability in both types of cells. However, this genomic instability was more pronounced in hTERT- cells after radiation followed by Cr (VI) and more pronounced in hTERT+ cells after Cr (VI) followed by radiation. Moreover, the biological effects provoked by combined exposure of Cr (VI) and radiation also led to a synergistic action in both types of cells, compared to either Cr (VI) treatment only or radiation exposure only. This study suggests that telomerase can prevent genomic instability caused by Cr (VI), but not by radiation. Furthermore, genomic instability may be prevented by telomerase when cells are exposed to radiation and then Cr (VI) but not after exposure to Cr (VI) and then radiation.

Radiation-induced adaptive response in fish cell lines

There is considerable interest at present in low-dose radiation effects in non-human species. In this study gamma radiation-induced adaptive response, a low-dose radiation effect, was examined in three fish cell lines, (CHSE-214 (Chinook salmon), RTG-2 (rainbow trout) and ZEB-2J (zebrafish)). Cell survival after exposure to direct radiation with or without a 0.1 Gy priming dose, was determined using the colony forming assay for each cell line. Additionally, the occurrence of a bystander effect was examined by measuring the effect of irradiated cell culture medium from the fish cell lines on unexposed reporter cells. A non-linear dose response was observed for all cell lines. ZEB-2J cells were very sensitive to low doses and a hyper-radiosensitive (HRS) response was observed for doses <0.5 Gy. A typical protective adaptive response was not detected in any of the three fish cell lines tested. Rather, it was found that pre-exposure of these cells to 0.1 Gy radiation sensitized the cells to subsequent high doses. In CHSE-214 cells, increased sensitivity to subsequent high doses of radiation was observed when the priming and challenge doses were separated by 4 h; however, this sensitizing effect was no longer present when the interval between doses was greater than 8 h. Additionally, a "protective" bystander response was observed in these cell lines; exposure to irradiated medium from fish cells caused increased cloning efficiency in unirradiated reporter cells. The data confirm previous conclusions for mammalian cells that the adaptive response and bystander effect are inversely correlated and contrary to expectations probably have different underlying mechanisms.

The impact of the bystander effect on the low-dose hypersensitivity phenomenon

The aim of this study was to investigate the possible relationship between the bystander effect and the low-dose hypersensitivity/increased radio-resistance phenomenon in BJ fibroblast cells taking as response criteria different end points of radiation damage such as cell survival, chromosomal damage (as detected by using micronucleus assay) and double strand breaks (DSBs) of the DNA. Although gamma-H2AX foci were observed in confluent bystander BJ cells, our data suggest that X-irradiation does not lead to a significant rate of DSBs in bystander cells. Thus, neither bystander effect induced unstable chromosomal aberrations nor bystander effect induced DSBs are sufficiently pronounced to explain the apparent relationship between bystander effect and low-dose hypersensitivity. The experiments described here suggest that the hyper-radiosensitivity phenomenon might be related to bystander factor induced cell inactivation in the low-dose region (lower than 1 Gy).

Manganese superoxide dismutase (SOD2)-mediated delayed radioprotection induced by the free thiol form of amifostine and tumor necrosis factor alpha

RKO36 cells, a subclone of RKO colorectal carcinoma cells that have been stably transfected with the pCMV-EGFP2Xho vector, were grown to confluence and then exposed to either the radioprotector WR-1065, i.e. the active thiol form of amifostine, for 30 min at doses of 40 microM and 4 mM or the cytokine tumor necrosis factor alpha (TNFalpha, TNFA) for 30 min at a concentration of 10 ng/ml and then washed. Total protein was isolated as a function of time up to 32 h after these treatments. Both doses of WR-1065 as well as the concentration of TNFalpha used were effective in elevating intracellular levels of the antioxidant protein SOD2 (also known as MnSOD) at least 15-fold over background levels as determined by Western blot analysis, while measured SOD2 activity was elevated between 5.5- and 6.9-fold. SOD2 reached a maximal level 24 h and 20 h after WR-1065 and TNFalpha treatments, respectively. The antioxidant proteins catalase and glutathione peroxidase (GPX) were also monitored over the 32-h period. In contrast to the robust changes observed in intracellular levels of SOD2 as a function of time after exposure of cells to WR-1065, catalase levels were elevated only 2.6-fold over background as determined by Western blot analysis, while GPX activity was unaffected by WR-1065 exposure. GPX protein levels were extremely low in cells, and analysis of GPX activity using a spectrophotometric method based on the consumption of reduced NADPH also revealed no measurable change as a function of WR-1065 or TNFalpha exposure. RKO36 cells either were irradiated with X rays in the presence of either 40 microM or 4 mM WR-1065 or 10 ng/ml TNFalpha or were irradiated 24 or 20 h later, respectively, when SOD2 protein levels were most elevated. The concentrations and exposure conditions used for WR-1065 and TNFalpha were not cytotoxic and had no effect on plating efficiencies or cell survival compared to untreated controls. No protection or sensitization was observed for cells irradiated in the presence of 40 microM WR-1065 or TNFalpha. Survival was elevated 1.90-fold for cells irradiated in the presence of 4 mM WR-1065. When RKO36 cells were irradiated with 2 Gy 24 h after 40 microM or 4 mM WR-1065 and 20 h after TNFalpha treatments when SOD2 levels were the most increased, survival was elevated 1.42-, 1.48- and 1.36-fold, respectively. This increased survival represents a SOD2-mediated delayed radioprotective effect. SOD2 appears to be an important antioxidant gene whose inducible expression is an important element in adaptive cellular responses in general, and the delayed radioprotective effect in particular. It can be induced by a range of agents including cytoprotective nonprotein thiols such as WR-1065 and pleiotropic cytokines such as TNFalpha.

Radioadaptive response revisited

Radiation-induced adaptive response belongs to the group of non-targeted effects that do not require direct exposure of the cell nucleus by radiation. It is described as the reduced damaging effect of a challenging radiation dose when induced by a previous low priming dose. Adaptive responses have been observed in vitro and in vivo using various indicators of cellular damage, such as cell lethality, chromosomal aberrations, mutation induction, radiosensitivity, and DNA repair. Adaptive response can be divided into three successive biological phenomena, the intracellular response, the extracellular signal, and the maintenance. The intracellular response leading to adaptation of a single cell is a complex biological process including induction or suppression of gene groups. An extracellular signal, the nature of which is unknown, may be sent by the affected cell to neighbouring cells causing them to adapt as well. This occurs either by a release of diffusible signalling molecules or by gap-junction intercellular communication. Adaptive response can be maintained for periods ranging from of a few hours to several months. Constantly increased levels of reactive oxygen species (ROS) or nitric oxide (NO) have been observed in adapted cells and both factors may play a role in the maintenance process. Although adaptive response seems to function by an on/off principle, it is a phenomenon showing a high degree of inter- and intraindividual variability. It remains to be seen to what extent adaptive response is functional in humans at relevant dose and dose-rate exposures. A better understanding of adaptive response and other non-targeted effects is needed before they can be confirmed as risk estimate factors for the human population at low levels of ionising radiation.

Induction of chromatid-type aberrations in peripheral lymphocytes of hospital workers exposed to very low doses of radiation

Radiological personnel represent workers exposed to low cumulative doses of radiation. As their surveillance is generally based on physical dosimetry, there is little or inconclusive information on biological effects due to radiation exposure at these doses. We aimed to explore the extent of chromosomal damage in circulating lymphocytes of hospital workers (technicians, nurses and physicians) chronically exposed to a very low level of radiation using conventional and molecular cytogenetic analyses (chromosome painting with chromosomes #2, #3 and #10 as probe cocktail). Compared with controls, exposed workers displayed a significant increase in the frequency of aberrant lymphocytes (1.26+/-0.11/100 cells versus 1.63+/-0.17/100 cells). In particular, exposed technicians showed significantly higher mean values than nurses or physicians (3.68+/-1.17/100 cells versus 1.36+/-0.18/100 cells and 1.36+/-0.09/100 cells, respectively). Interestingly, we found that the chromosomal damage was prevalently expressed as chromatid-type aberrations. Chromosome painting indicated that the frequency of chromosome rearrangements (CR; translocations and dicentrics pooled together) was approximately comparable between radiological workers and the control group. Moreover, we did not detect any significant difference due to radiation exposure when CR rates were considered separately for each of the three chromosomes in the probe cocktail.

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.