Interrelationships amongst radiation-induced genomic instability, bystander effects, and the adaptive response

Cell division cycle 25 homolog c effects on low-dose hyper-radiosensitivity and induced radioresistance at elevated dosage in A549 cells

The underlying mechanisms behind both low-dose hyper-radiosensitivity (HRS) and induced radioresistance (IRR), generally occurring at elevated radiation levels, remain unclear; however, elucidation of the relationship between cell cycle division 25 homolog c (Cdc25c) phosphatase and HRS/IRR may provide important insights into this process. Two cell lines with disparate HRS status, A549 and SiHa cells, were selected as cell models for comparison of dose-dependent Cdc25c phosphatase expression subsequent to low-dose irradiation. Knockdown of Cdc25c in A549 cells was mediated by transfection with a pGCsi-RAN-U6neo vector containing hairpin siRNA sequences. S216-phosphorylated Cdc25c protein [p-Cdc25c (Ser216)], cell survival and mitotic ratio were measured by western blot, colony-forming assay and histone H3 phosphorylation analysis. Variant p-Cdc25c (Ser216) expression was observed in the two cell lines after irradiation. The p-Cdc25c (Ser216) expression noted in SiHa cells after administration of 0-1 Gy radiation was similar to the radioresistance model; however, in A549 cells, the dose response for the phosphorylation of the Cdc25c Ser216 residue overlapped the level required to overcome the HRS response. Furthermore, Cdc25c repression prior to low-dose radiation induced more distinct HRS and prevented the development of IRR. The dose required to overcome the HRS response coincided with the effect of early G2-phase checkpoint arrest in A549 cells (approximately 0.3 Gy), and Cdc25c knockdown in A549 cells (approximately 0.5 Gy) corresponded to the phosphorylation of the Cdc25c Ser216 residue. Resultant data confirmed that dose-dependent Cdc25c phosphatase does effectively act as an early G2-phase checkpoint, thus indicating mechanistic importance in the HRS to IRR transition in A549 cells.

Possible role of exosomes containing RNA in mediating nontargeted effect of ionizing radiation

Communication between irradiated and un-irradiated (bystander) cells can cause damage in cells that are not directly targeted by ionizing radiation, a process known as the bystander effect. Bystander effects can also lead to chromosomal/genomic instability within the progeny of bystander cells, similar to the progeny of directly irradiated cells. The factors that mediate this cellular communication can be transferred between cells via gap junctions or released into the extracellular media following irradiation, but their nature has not been fully characterized. In this study we tested the hypothesis that the bystander effect mediator contains an RNA molecule that may be carried by exosomes. MCF7 cells were irradiated with 2 Gy of X rays and the extracellular media was harvested. RNase treatment abrogated the ability of the media to induce early and late chromosomal damage in bystander cells. Furthermore, treatment of bystander cells with exosomes isolated from this media increased the levels of genomic damage. These results suggest that the bystander effect, and genomic instability, are at least in part mediated by exosomes and implicate a role for RNA.

Dependence of adaptive response and its bystander transmission on the genetic background of tested cells

PURPOSE

Radiation-induced adaptive response (AR) is a phenomenon of increased radioresistance mediated by a low priming dose of ionizing radiation (IR) applied prior to a higher challenging dose. We have previously shown that in mouse-embryo fibroblasts (MEF) and human A549 cells, AR is associated with enhanced repair of DNA double-strand breaks (DSB) by the DNA-PK-dependent pathway of non-homologous end-joining (D-NHEJ). Importantly, AR was 'transmitted' to non-irradiated bystander cells through transfer of medium from cells that had received a priming dose of IR. Here, we examine the influence of the genetic background in these responses.

MATERIALS AND METHODS

Two plasmid-based assays specifically designed to measure the efficiency of NHEJ and HRR (homologous recombination repair) were deployed. MEF and the primary human fibroblast cell lines HF12 and HF19 were exposed to 10 mGy to 5 Gy X-rays. Bystander effects were investigated using the medium-transfer technique.

RESULTS

In contrast to MEF, which induce robust AR to NHEJ, even as a bystander response, human fibroblasts fail to develop such phenomena.

CONCLUSIONS

The development of AR is cell-type-specific. The same holds true for the development of AR as a bystander effect. Better understanding of the underlying mechanisms will help to understand the molecular basis of these differences in response.

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.

Biological complexities in radiation carcinogenesis and cancer radiotherapy: impact of new biological paradigms

Although radiation carcinogenesis has been shown both experimentally and epidemiologically, the use of ionizing radiation is also one of the major modalities in cancer treatment. Various known cellular and molecular events are involved in carcinogenesis. Apart from the known phenomena, there could be implications for carcinogenesis and cancer prevention due to other biological processes such as the bystander effect, the abscopal effect, intrinsic radiosensitivity and radioadaptation. Bystander effects have consequences for mutation initiated cancer paradigms of radiation carcinogenesis, which provide the mechanistic justification for low-dose risk estimates. The abscopal effect is potentially important for tumor control and is mediated through cytokines and/or the immune system (mainly cell-mediated immunity). It results from loss of growth and stimulatory and/or immunosuppressive factors from the tumor. Intrinsic radiosensitivity is a feature of some cancer prone chromosomal breakage syndromes such as ataxia telangectiasia. Radiosensitivity is manifested as higher chromosomal aberrations and DNA repair impairment is now known as a good biomarker for breast cancer screening and prediction of prognosis. However, it is not yet known whether this effect is good or bad for those receiving radiation or radiomimetic agents for treatment. Radiation hormesis is another major concern for carcinogenesis. This process which protects cells from higher doses of radiation or radio mimic chemicals, may lead to the escape of cells from mitotic death or apoptosis and put cells with a lower amount of damage into the process of cancer induction. Therefore, any of these biological phenomena could have impact on another process giving rise to genome instability of cells which are not in the field of radiation but still receiving a lower amount of radiation. For prevention of radiation induced carcinogenesis or risk assessment as well as for successful radiation therapy, all these phenomena should be taken into account.

The effect of non-targeted cellular mechanisms on lung cancer risk for chronic, low level radon exposures

PURPOSE

The goal of the present study was to investigate the effect of non-targeted mechanisms on the shape of the lung cancer risk function at chronic, low level radon exposures relative to direct cellular radiation effects. This includes detrimental and protective bystander effects, radio-adaptive bystander response, genomic instability and induction of apoptosis by surrounding cells.

METHODS

To quantify the dependence of these mechanisms on dose, analytical functions were derived from the experimental evidence presently available. Alpha particle intersections of bronchial target cells during a given exposure period were simulated by a Transformation Frequency-Tissue Response (TF-TR) model, formulated in terms of cellular hits within the cycle time of the cell and then integrated over the whole exposure period.

RESULTS

In general, non-targeted effects like genomic instability and bystander effects amplify the biological effectiveness of a given radiation dose, while induction of apoptosis and adaptive response will decrease the risk values. While these observations are related to the absolute number of lung cancer cases, normalization to the epidemiologically observed risk at 0.675 Gy suggests that the effect of such mechanisms on the shape of the dose-response relationship may be different. Indeed, genomic instability and adaptive response cause a substantial reduction of the risk at low doses, while induction of apoptosis and detrimental bystander effects slightly increase the risk.

CONCLUSIONS

Predictions of lung cancer risk, including these mechanisms, exhibit a distinct sublinear dose-response relationship at low exposures, particularly for very low exposure rates. However, the relatively large error bars of the epidemiological data do not currently allow the prediction of a statistically significant deviation from the Linear - No Threshold (LNT) assumption.

Repeated exposure of human fibroblasts to ionizing radiation reveals an adaptive response that is not mediated by interleukin-6 or TGF-β

Exposing cells to a low dose can protect them against a subsequent higher exposure. This phenomenon is known as adaptive response and is frequently observed in a variety of cells. Even though similarities are suspected with other non-targeted effects, such as bystander effects, the exact mechanism behind adaptive response is not fully clarified. In this study human primary fibroblasts were tested for their response to ionizing radiation (IR) after administrating a low priming dose (0.1-0.5Gy). Both the abundance of γH2AX as a marker for double-stranded breaks and the levels of cytokines, secreted in the medium, were monitored in time. Upon challenge, IR-primed cells showed modified γH2AX spot size distributions and altered repair kinetics, consistent with an adaptive response. In addition, 24h after priming with IR, four cytokines were significantly upregulated in the medium - GM-CSF (1.33×); IL6 (4.24×); IL8 (1.33×); TGF-β (1.46×). In order to mimick the protective effect of IR priming, we primed the cells with either IL6 or TGF-β. This did not elicit an altered γH2AX response as observed in IR-primed cells, indicating that the adaptive response in these primary fibroblasts is regulated in an IL-6 and TGF-β independent manner.

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.

Differential expression of oxidative stress and extracellular matrix remodeling genes in low- or high-dose-rate photon-irradiated skin

Changes in the expression of genes implicated in oxidative stress and in extracellular matrix (ECM) remodeling and selected protein expression profiles in mouse skin were examined after exposure to low-dose-rate or high-dose-rate photon irradiation. ICR mice received whole-body γ rays to total doses of 0, 0.25, 0.5 and 1 Gy at dose rates of 50 cGy/h or 50 cGy/min. Skin tissues were harvested for characterization at 4 h after irradiation. For oxidative stress after low-dose-rate exposure, 0.25, 0.5 and 1 Gy significantly altered 27, 23 and 25 genes, respectively, among 84 genes assessed (P < 0.05). At doses as low as 0.25 Gy, many genes responsible for regulating the production of reactive oxygen species (ROS) were significantly altered, with changes >2-fold compared to 0 Gy. For an ECM profile, 18-20 out of 84 genes were significantly up- or downregulated after low-dose-rate exposure. After high-dose-rate irradiation, of 84 genes associated with oxidative stress, 16, 22 and 22 genes were significantly affected after 0.25, 0.5 and 1 Gy, respectively. Compared to low-dose-rate radiation, high-dose-rate exposure resulted in different ECM gene expression profiles. The most striking changes after low-dose-rate or high-dose-rate exposure on ECM profiles were on genes encoding matrix metalloproteinases (MMPs), e.g., Mmp2 and Mmp15 for low dose rate and Mmp9 and Mmp11 for high dose rate. Immunostaining for MMP-2 and MMP-9 proteins showed radiation dose rate-dependent differences. These data revealed that exposure to low total doses with low-dose-rate or high-dose-rate photon radiation induced oxidative stress and ECM-associated alterations in gene expression profiles. The expression of many genes was differentially regulated by different total dose and/or dose-rate regimens.

Stem cell niches and other factors that influence the sensitivity of bone marrow to radiation-induced bone cancer and leukaemia in children and adults

PURPOSE

This paper reviews and reassesses the internationally accepted niches or 'targets' in bone marrow that are sensitive to the induction of leukaemia and primary bone cancer by radiation.

CONCLUSIONS

The hypoxic conditions of the 10 μm thick endosteal/osteoblastic niche where preleukemic stem cells and hematopoietic stem cells (HSC) reside provides a radioprotective microenvironment that is 2- to 3-fold less radiosensitive than vascular niches. This supports partitioning the whole marrow target between the low haematological cancer risk of irradiating HSC in the endosteum and the vascular niches within central marrow. There is a greater risk of induced bone cancer when irradiating a 50 μm thick peripheral marrow adjacent to the remodelling/reforming portion of the trabecular bone surface, rather than marrow next to the quiescent bone surface. This choice of partitioned bone cancer target is substantiated by the greater radiosensitivity of: (i) Bone with high remodelling rates, (ii) the young, (iii) individuals with hypermetabolic benign diseases of bone, and (iv) the epidemiology of alpha-emitting exposures. Evidence is given to show that the absence of excess bone-cancer in atomic-bomb survivors may be partially related to the extremely low prevalence among Japanese of Paget's disease of bone. Radiation-induced fibrosis and the wound healing response may be implicated in not only radiogenic bone cancers but also leukaemia. A novel biological mechanism for adaptive response, and possibility of dynamic targets, is advocated whereby stem cells migrate from vascular niches to stress-mitigated, hypoxic niches.

Long-term consequences of radiation-induced bystander effects depend on radiation quality and dose and correlate with oxidative stress

Widespread evidence indicates that exposure of cell populations to ionizing radiation results in significant biological changes in both the irradiated and nonirradiated bystander cells in the population. We investigated the role of radiation quality, or linear energy transfer (LET), and radiation dose in the propagation of stressful effects in the progeny of bystander cells. Confluent normal human cell cultures were exposed to low or high doses of 1GeV/u iron ions (LET ∼ 151 keV/µm), 600 MeV/u silicon ions (LET ∼ 51 keV/µm), or 1 GeV protons (LET ∼ 0.2 keV/µm). Within minutes after irradiation, the cells were trypsinized and co-cultured with nonirradiated cells for 5 h. During this time, irradiated and nonirradiated cells were grown on either side of an insert with 3-µm pores. Nonirradiated cells were then harvested and allowed to grow for 20 generations. Relative to controls, the progeny of bystander cells that were co-cultured with cells irradiated with iron or silicon ions, but not protons, exhibited reduced cloning efficiency and harbored higher levels of chromosomal damage, protein oxidation and lipid peroxidation. This correlated with decreased activity of antioxidant enzymes, inactivation of the redox-sensitive metabolic enzyme aconitase, and altered translation of proteins encoded by mitochondrial DNA. Together, the results demonstrate that the long-term consequences of the induced nontargeted effects greatly depend on the quality and dose of the radiation and involve persistent oxidative stress due to induced perturbations in oxidative metabolism. They are relevant to estimates of health risks from exposures to space radiation and the emergence of second malignancies after radiotherapy.

Direct and bystander effects induced by scattered radiation generated during penetration of radiation inside a water-phantom

In this study, the dose distribution of photon (6 MV) and electron (22 MeV) radiation in a water-phantom was compared with the frequency of apoptotic and micronucleated cells of two human cell lines (BEAS-2B normal bronchial epithelial cells and A549 lung cancer epithelial cells). Formation of micronuclei and apoptotic-like bodies was evaluated by the cytokinesis-block micronucleus test. Measurements were performed for five different phantom depths (3-20 cm). Irradiated cells were placed in a water-phantom in three variants: directly on the axis in the beam, under shielding (only in photon radiation) and outside the beam field. The results reveal a discrepancy between the distribution of physical dose at different depths of the water-phantom and biological effects. This discrepancy is of special significance in case of cells irradiated at a greater depth or placed outside the field and under shield during the exposure to radiation. The frequency of cytogenetic damage was higher than the expected value based on the physical dose received at different depths. Cells placed outside the beam axis were exposed to scattered radiation at very low doses, so we tested if bystander effects could have had a role in the observed discrepancy between physical radiation dose and biological response. We explored this question by use of a medium-transfer technique in which medium (ICM-irradiation conditioned medium) from irradiated cells was transferred to non-irradiated (bystander) cells. The results indicate that when cells were incubated in ICM transferred from cells irradiated at bigger depths or from cells exposed outside the radiation field, the number of apoptotic and micronucleated cells was similar to that after direct irradiation. This suggests that these damages are caused by factors released by irradiated cells into the medium rather than being induced directly in DNA by X-rays. Evaluation of biological effects of scattered radiation appears useful for clinical practice.

Radioadaptive response induced by alpha-particle-induced stress communicated in vivo between zebrafish embryos

We report data demonstrating that zebrafish embryos irradiated by alpha particles can release a stress signal into the water, which can be communicated to the unirradiated zebrafish embryos sharing the same water medium and thereby inducing a radioadaptive response in these unirradiated zebrafish embryos. The effects of radiation on the whole embryos were studied through quantification of apoptotic signals at 24 h post fertilization through staining with the vital dye acridine orange, followed by counting the stained cells under a microscope. In these experiments, dechorionated embryos were irradiated and then partnered with two other groups of unirradiated embryos, namely the bystander group (no more further treatments) and adaptive group (subjected to a further challenging dose) of embryos. The adaptive group of embryos were then separately further irradiated with a challenging dose. The results show that the number of apoptotic signals for the adaptive group is smaller than that for the corresponding control group, while that for the bystander group is larger than that for the corresponding control group. These suggest that the stress communicated in vivo between the irradiated zebrafish embryos and those unirradiated embryos sharing the same medium will induce radioadaptive response in the unirradiated embryos.

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.

Low-dose radiation-induced responses: focusing on epigenetic regulation

PURPOSE

With the widespread use of ionising radiation, the risks of low-dose radiation have been increasingly highlighted for special attention. This review introduces the potential role of epigenetic elements in the regulation of the effects of low-dose radiation.

MATERIALS AND METHODS

The related literature has been analysed according to the topics of DNA methylation, histone modifications, chromatin remodelling and non-coding RNA modulation in low-dose radiation responses.

RESULTS

DNA methylation and radiation can reciprocally regulate effects, especially in the low-dose radiation area. The relationship between histone methylation and radiation mainly exists in the high-dose radiation area; histone deacetylase inhibitors show a promising application to enhance radiation sensitivity, both in the low-dose and high-dose areas; phosphorylated histone 2 AX (H2AX) shows a low sensitivity with 1-15 Gy irradiation as compared with lower dose radiation; and histone ubiquitination plays an important role in DNA damage repair mechanisms. Moreover, chromatin remodelling has an integral role in the repair of DNA double-strand breaks and the response of chromatin to ionising radiation. Finally, the effect of radiation on microRNA expression seems to vary according to cell type, radiation dose, and post-irradiation time point.

CONCLUSION

Small advances have been made in the understanding of epigenetic regulation of low-dose radiation responses. Many questions and blind spots deserve to be investigated. Many new epigenetic elements will be identified in low-dose radiation responses, which may give new insights into the mechanisms of radiation response and their exploitation in radiotherapy.

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.

Comparison of mortality and incidence solid cancer risk after radiation exposure in the Techa River Cohort

In the present paper, analysis of solid cancer mortality and incidence risk after radiation exposure in the Techa River Cohort in the Southern Urals region of Russia is described. Residents along the Techa River received protracted exposure to ionizing radiation in the 1950s due to the releases of radioactive materials from the Mayak Production Association. The current follow-up through December 2003 includes individuals exposed on the Techa riverside within the Chelyabinsk and Kurgan oblasts using mortality data, and within the Chelyabinsk oblast using incidence data. The analysis was performed by means of the biologically based two-stage clonal expansion (TSCE) model and conventional excess relative risk models. For the mortality and incidence cohorts, central estimates of the excess relative risk per dose of 0.85 Gy(-1) (95% CI 0.36; 1.38) and 0.91 Gy(-1) (95% CI 0.35; 1.52) were found, respectively. For both the mortality and incidence cohorts, the best description of the radiation risk was achieved with the same TSCE model including a lifelong radiation effect on the promotion rate of initiated cells. An increase in the excess risk with attained age was observed, whereas no significant change of risk with age at exposure was seen. Direct comparison of the mortality and incidence cohorts showed that the excess relative risk estimates agreed very well in both cohorts, as did the excess absolute risk and the hazard after correction for the different background rates.

Investigation of bystander effects in hybrid cells by means of cell fusion and premature chromosome condensation induction

The established dogma in radiation sciences that underlies radiation protection and therapeutic applications is that radiation effects require induction of DNA damage only in cells that are directly hit by the radiation. However, extensive work during the last decade demonstrates that DNA damage responses can be detected in cells that are only bystanders. Such effects include cell killing and responses associated with DNA and chromosome damage. Here, we developed a strategy for investigating bystander effects on chromosomal integrity by premature chromosome condensation using hybrid cell formation between nontargeted human lymphocytes and targeted CHO cells or vice versa. We reasoned that signaling molecules generated in the targeted component of the hybrid will transfer to the nontargeted cell, inducing damage detectable at the chromosomal level. The results indicate that bystander cytogenetic effects between CHO and human lymphocytes cannot be detected under the experimental conditions used. This may be due either to the lack of communication of such responses between the components of the hybrid or to their abrogation by the experimental manipulations. These observations and the methodology developed should be useful in the further development of protocols for investigating bystander responses and for elucidating the underlying mechanisms.

Medium-mediated DNA repair response after ionizing radiation is correlated with the increase of specific cytokines in human fibroblasts

Radiation induced bystander effects, either protective or adverse, have been identified in a variety of cells and for different endpoints. They are thought to arise from communication between cells through direct cell-cell contacts and via transmissible molecules secreted into the medium by targeted cells. We have investigated medium-mediated damage response in human dermal fibroblasts (HDF) after exposure to ionizing irradiation. We show that HDF experience an elevated level of double stranded DNA damage repair response when incubated with conditioned growth medium of irradiated cells. The magnitude of this response is much lower than observed for directly irradiated cells and is proportional to the radiation dose, as is its persistence across time. Since secretion of cytokines is one of the possible pathways linking targeted and non-targeted cells a multiplex analysis was performed. Four cytokines - IL6, IL8, MCP-1 and RANTES - were identified in the growth medium of irradiated cells after exposure to X-rays (2Gy). These cytokines were significantly upregulated and each cytokine showed differential upregulation kinetics. Finally we performed a functional analysis to see if IL6 and MCP-1 could induce gammaH2AX foci formation. IL6 caused a significant increase in spot occupancy compared to controls. Although only indicative MCP-1 appears to have the opposite effect as it caused a drop in spot occupancy. The combined addition of these two cytokines produced no significant response was observed. Both IL6 and MCP-1 have an effect on the gammaH2AX spot occupancy possibly linking these cytokines to the bystander response.

Delayed chromosomal instability in lymphocytes of cancer patients after radiotherapy

PURPOSE

To assess possible delayed chromosomal instability (DCI) expressed as elevated chromatid breakage in cells containing previously formed chromosome type aberrations in cultured blood lymphocytes of cancer patients after radiotherapy (RT).

MATERIALS AND METHODS

Twenty patients treated for uterine cancer with external Co(60) RT, without chemotherapy, were selected. Blood was taken before, 1-2 days after RT and one year later. Lymphocytes were cultured for 50 and 100 h. Metaphases were stained with fluorescence-plus-Giemsa and analysed for chromosome and chromatid aberrations in 1st (M1) and 3rd plus later (M3+) mitoses.

RESULTS

RT caused a significant increase of radiation-specific chromosome aberrations in patients' lymphocytes together with DCI, which was observed as an excessive yield of cells containing both chromosome and chromatid aberrations (defined as C(acs&act)). This DCI passed successfully through mitoses in vitro, and at the end of RT a mean yield of 'extra' C(acs&act) was 3 x 10(-3) x cell(-1) amongst either M1 or M3+ cells. At the end of RT and one year later DCI in M1 lymphocytes appeared at random amongst patients, but some inter-individual variation was found for DCI presence in M3+ cells at both post-irradiation samplings. As time passed, the mean yield of lymphocytes exhibiting DCI decreased in vivo and one year after RT reached the pre-treatment level of 1 x 10(-3) x cell(-1).

CONCLUSIONS

DCI was demonstrated in descendants of human lymphocytes after therapeutic irradiation. The effect diminished one year later, suggesting that the progeny of patients' irradiated stem cells did not produce new daughter lymphocytes exhibiting DCI during the studied post-irradiation period.

Tracking genomic instability within irradiated and bystander populations

Over the past two decades, our understanding of radiation biology has undergone a fundamental shift in paradigms away from deterministic ‘hit-effect’ relationships and towards complex ongoing ‘cellular responses’. These responses include now familiar, but still poorly understood, phenomena associated with radiation exposure such as genomic instability and bystander effects. Although these responses share some common features (e.g. they occur at high frequency following very low doses, are heterogeneous in their induction and are observed at time points far removed from the initial radiation exposure), the precise relationship between genomic instability and bystander effects remains to be elucidated. This review will provide a synthesis of the known, and proposed, interrelationships among irradiated and bystander cellular responses to radiation. It also discusses our current experimental approach for gaining a clearer understanding of the relationship between damage induction and long-term effects in both irradiated and bystander cells.

Proteomic alterations in progeny of irradiated human liver cells

This study was designed to characterize the differential protein expression in the progeny of human liver cells surviving exposure to ionizing radiation. The progeny of irradiated cells were derived from a human liver cell line exposed to 0, 2, 4, or 6 Gy of (60)Co gamma-irradiation. Total protein of the cells was extracted by two-dimensional electrophoresis (2-DE) and analyzed with ImageMaster 2D Platinum software. In total, 42 differentially expressed proteins from the progeny of irradiated cells were screened, of which 17 were identified by matrix assistant laser desorption ion-top flight-mass spectrometry (MALDI-TOF-MS) analysis. There were 4 upregulated and 13 downregulated proteins detected. The upregulated expression of two proteins, mitochondrial heat-shock 60-kD protein (HSP60) and globin transcription factor 1 (GATA-1), was further confirmed by immunoblotting. Database search revealed that these differentially expressed proteins may function in cell cycle regulation, cytoskeleton maintenance, stress response, and tumor metastasis, indicating an effect of radiation-induced genomic instability (RIGI) in the progeny of irradiated cells. Analysis on functional roles of the screened proteins may provide insight into further mechanistic investigations underlying molecular events induced by RIGI.

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.

An alpha-particle irradiator for radiobiological research and its implementation for bystander effect studies

An experimental system based on an improved version of an existing alpha-particle irradiator has been developed for radiobiological studies, in particular those investigating bystander effects. It consists of a 20-mm-diameter stainless steel chamber that can be equipped alternatively with 244Cm or 241Am sources of different activities. Mylar-based petri dishes 56 mm in diameter were specially designed to house adaptors for permeable membrane inserts that reproduce the geometry of commercial cell culture insert companion plates. Characterization of the radiation field at the cell level was performed by experimental measurements and calculations. The average incident LET was about 122 keV/microm for 244Cm and about 125 keV/microm for 241Am. Dose rates at the chosen source-sample distance were 2.8 and 88.6 mGy/min, respectively. These low dose rates are suitable for our planned experiments on low-dose effects. For both sources, the uniformity of the alpha-particle dose was better than +/-7%, and the photon dose calculated at the cell entrance was negligible compared to the alpha-particle dose. The irradiator is small enough to be inserted into a cell incubator for irradiation under physiological conditions or into a refrigerator to prevent metabolic processes during irradiation. Benchmark experiments using the 241Am source to examine DNA double-strand breaks in directly hit and bystander primary human fibroblasts have shown that the irradiator can be used successfully for bystander effect studies.

Breast cancer risk among Swedish hemangioma patients and possible consequences of radiation-induced genomic instability

Breast cancer incidence among 17,158 female Swedish hemangioma patients was analyzed with empirical excess relative risk models and with a biologically-based model of carcinogenesis. The patients were treated in infancy mainly by external application of radium-226. The mean and median absorbed doses to the breast were 0.29 and 0.04Gy, and a total of 678 breast cancer cases have been observed. Both models agree very well in the risk estimates with an excess relative risk and excess absolute risk at the age of 50 years, about the mean age of breast cancer incidence, of 0.25Gy(-1)(95% CI 0.14; 0.37) and 30.7 (10(5) BYR Gy)(-1) (95% CI 16.9; 42.8), respectively. Models incorporating effects of radiation-induced genomic instability were developed and applied to the hemangioma cohort. The biologically-based description of the radiation risk was significantly improved with a model of genomic instability at an early stage of carcinogenesis.

Estimation of ionizing radiation impact on natural Vicia cracca populations inhabiting areas contaminated with uranium mill tailings and radium production wastes

Industrial areas in proximity to the Vodny settlement in the Komi Republic, Russia, have been contaminated by uranium mill tailings and radium production wastes. These areas, exhibiting high activity concentrations of naturally occurring radionuclides in soils, constitute a field laboratory where the effects of combined chronic exposures to alpha-, beta- and gamma-emitting radionuclides on natural plant populations can be studied. The aim of the present work was to determine dose-effect relationships and the range of doses that cause biological effects in natural Vicia cracca L. populations inhabiting the study area. The studied plant species is native to the area and is found ubiquitously. Soil and vegetation samples were taken at a reference location and six contaminated sites characterized by distinct floodplain depositional units with different enhanced levels of naturally occurring radionuclides. A large fraction of the dose at the study sites (including the reference location) was attributable to internal irradiation and (226)Ra was found to be an important contributor to this component of dose. The relationship between the frequency of chromosome aberrations in seedlings' root tip cells and the absorbed dose was found to be quadratic. An exponential model provided the best result in describing the empirical dependence between the absorbed dose and both the germination capacity of seeds and the survival rate of sprouts of V. cracca. For V. cracca plants inhabiting areas contaminated with uranium mill tailings and radium production wastes, a weighted absorbed dose of 0.2 Gy (weighting factor for alpha particles=5) during the vegetation period could be considered to be a level below which no increase in genetic variability and decrease in reproductive capacity might be observed above background.

Investigation of non-linear adaptive responses and split dose recovery induced by ionizing radiation in three human epithelial derived cell lines

Two almost completely exclusive fields in radiobiology deal with splitting doses of radiation and comparing the effect to a similar total dose given in one exposure. In radiotherapy, dose "fractionation" is used to "spare" normal tissue and in the low dose field, the adaptive response is well documented as a phenomenon where a small "priming" dose administered before the larger "challenge" dose reduces the effect of the large dose. There have been very few studies where these fields overlap, thus it is not possible to ascertain whether common or distinct mechanisms underlie both phenomena but this is certainly an interesting question and relevant to our understanding of high and low dose radiobiology. This paper presents data for three human cell lines with varying p53 status and radiation responses, treated at a range of times between first and second dose and for 3 different first doses (0.1, 0.5 and 2Gy). The data show that time between doses is critical. Protective (adaptive) effects were seen in each cell line but most prominently in the malignant HT 29 cell line. Surprisingly none of the cell lines showed pronounced split dose recovery. This suggests different mechanisms may underlie the two phenomena.

Radiation-induced genomic instability in repair deficient mutants of Chinese hamster cells

PURPOSE

To determine the role of single (SSB) and double strand break (DSB) repair in the induction and propagation of radiation-induced instability.

MATERIALS AND METHODS

Two defined hamster cell lines with known DNA repair deficiencies in DSB repair (XR-C1) and base excision repair (EM-C11) and the parental wild-type line (CHO-9) were used. The rate of micronucleus formation, apoptosis and survival were measured at 0, 7 and 14 days after X-ray radiation.

RESULTS

An enhanced rate of production of damaged cells was observed in wild type and the repair deficient mutants after irradiation. This was cell type, dose and time-dependent. All cells demonstrated delayed death up to day 14 after irradiation along with an elevated apoptosis frequency. The yield of micronuclei was not significantly increased in the wild-type cells, but was in the mutant cells, over the dose and time range studied. For all three endpoints the increase in damage was most pronounced in the SSB deficient cell line.

CONCLUSIONS

SSB and/or oxidized base damage play a major role, rather than DSB, in radiation induced instability.

Modification in the expression of Mre11/Rad50/Nbs1 complex in low dose irradiated human lymphocytes

Despite the fact that high doses of radiation are detrimental, low dose radiation (LDR) often protects the organism against a subsequent exposure of lethal doses of radiation. Present study was undertaken to understand the role of Mre11, Rad50 and Nbs1 genes in the low dose radio-adapted human peripheral blood mononuclear cells (PBMCs). Optimum time interval between low dose (0.07 Gy) and high dose (5.0 Gy) of (60)Co-gamma-radiation was observed to be 5.0 hours, at which PBMCs showed maximum LDR induced resistance (RIR). At cytogenetic level, micronuclei frequency was found to be reduced in LDR pre-irradiated PBMCs subsequently exposed to high dose radiation (HDR) as compared to controls. At transcriptional level, with reference to sham-irradiated cells significantly (p< or =0.05) altered expression of Mre11, Rad50 and Nbs1 genes was observed in low dose irradiated cells. At protein level, Mre11, Rad50 and Nbs1 were enhanced significantly (p< or =0.05) in low dose pre-irradiated cells subsequently exposed to high dose of radiation as compared to only high dose irradiated cells. Transcriptional as well as translational modulation in the expression of MRN complex components upon low dose irradiation may confer its participation in repair pathways, resulting in induced resistance.

A new paradigm in radioadaptive response developing from microbeam research

A classic paradigm in radiation biology asserts that all radiation effects on cells, tissues and organisms are due to the direct action of radiation on living tissue. Using this model, possible risks from exposure to low dose ionizing radiation (below 100 mSv) are estimated by extrapolating from data obtained after exposure to higher doses of radiation, using a linear non-threshold model (LNT model). However, the validity of using this dose-response model is controversial because evidence accumulated over the past decade has indicated that living organisms, including humans, respond differently to low dose/low dose-rate radiation than they do to high dose/high dose-rate radiation. These important responses to low dose/low dose-rate radiation are the radiation-induced adaptive response, the bystander response, low-dose hypersensitivity, and genomic instability. The mechanisms underlying these responses often involve biochemical and molecular signals generated in response to targeted and non-targeted events. In order to define and understand the bystander response to provide a basis for the understanding of non-targeted events and to elucidate the mechanisms involved, recent sophisticated research has been conducted with X-ray microbeams and charged heavy particle microbeams, and these studies have produced many new observations. Based on these observations, associations have been suggested to exist between the radioadaptive and bystander responses. The present review focuses on these two phenomena, and summarizes observations supporting their existence, and discusses the linkage between them in light of recent results obtained from experiments utilizing microbeams.

Recent insights into the biological action of heavy-ion radiation

Biological effectiveness varies with the linear energy transfer (LET) of ionizing radiation. During cancer therapy or long-term interplanetary manned explorations, humans are exposed to high-LET energetic heavy ions that inactivate cells more effectively than low-LET photons like X-rays and gamma-rays. Recent biological studies have illustrated that heavy ions overcome tumor radioresistance caused by Bcl-2 overexpression, p53 mutations and intratumor hypoxia, and possess antiangiogenic and antimetastatic potential. Compared with heavy ions alone, the combination with chemical agents (a Bcl-2 inhibitor HA14-1, an anticancer drug docetaxel, and a halogenated pyrimidine analogue 5-iodo-2'-deoxyuridine) or hyperthermia further enhances tumor cell killing. Beer, its certain constituents, or melatonin ameliorate heavy ion-induced damage to normal cells. In addition to effects in cells directly targeted with heavy ions, there is mounting evidence for nontargeted biological effects in cells that have not themselves been directly irradiated. The bystander effect of heavy ions manifests itself as the loss of clonogenic potential, a transient apoptotic response, delayed p53 phosphorylation, alterations in gene expression profiles, and the elevated frequency of gene mutations, micronuclei and chromosome aberrations, which arise in nonirradiated cells having received signals from irradiated cells. Proposed mediating mechanisms involve gap junctional intercellular communication, reactive oxygen species and nitric oxide. This paper reviews briefly the current knowledge of the biological effects of heavy-ion irradiation with a focus on recent findings regarding its potential benefits for therapeutic use as well as on the bystander effect.

Radiation induction of delayed recombination in Schizosaccharomyces pombe

Ionizing radiation is known to induce delayed chromosome and gene mutations in the descendants of the irradiated tissue culture cells. Molecular mechanisms of such delayed mutations are yet to be elucidated, since high genomic complexity of mammalian cells makes it difficult to analyze. We now tested radiation induction of delayed recombination in the fission yeast Schizosaccharomyces pombe by monitoring the frequency of homologous recombination after X-irradiation. A reporter with 200 bp tandem repeats went through spontaneous recombination at a frequency of 1.0 x 10(-4), and the frequency increased dose-dependently to around 10 x 10(-4) at 500 Gy of X-irradiation. Although the repair of initial DNA damage was thought to be completed before the restart of cell division cycle, the elevation of the recombination frequency persisted for 8-10 cell generations after irradiation (delayed recombination). The delayed recombination suggests that descendants of the irradiated cells keep a memory of the initial DNA damage which upregulates recombination machinery for 8-10 generations even in the absence of DNA double-strand breaks (DSBs). Since radical scavengers were ineffective in inhibiting the delayed recombination, a memory by continuous production of DNA damaging agents such as reactive oxygen species (ROS) was excluded. Recombination was induced in trans in a reporter on chromosome III by a DNA DSB at a site on chromosome I, suggesting the untargeted nature of delayed recombination. Interestingly, Rad22 foci persisted in the X-irradiated population in parallel with the elevation of the recombination frequency. These results suggest that the epigenetic damage memory induced by DNA DSB upregulates untargeted and delayed recombination in S. pombe.

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.

Evidence for a protective response by the gill proteome of rainbow trout exposed to X-ray induced bystander signals

The bystander effect occurs when cells which are not directly exposed to radiation, but which receive signals from irradiated cells, respond as though they were irradiated. An X-ray induced bystander effect has been demonstrated in rainbow trout gills. Therefore, a proteomic comparison was made of gills from X-ray treated trout and trout exposed to X-ray induced bystander signals. 2-D gel analysis revealed X-ray exposure increased the expression of the cancer related protein annexin II. The proteomic changes associated with the bystander effect differed from those associated with direct radiation exposure. Expression of a hemopexin-like protein, Rho GDP dissociation inhibitor (RhoGDI) and pyruvate dehydrogenase (PDH) were increased. These proteins possess protective properties against reactive oxygen damage (a component of the bystander signal), regulate epithelial polarity and prevent lactate acidosis, respectively. There was also evidence for an increase in chromosome 1 SR-like CTD-associated factor (SCAF) protein turnover, which could suggest the protective response is transcriptionally regulated. The freshwater fish gill is a polarised barrier, separating against an external hypotonic environment. Since the maintenance of epithelial polarity is vital to gill function, these bystander effect proteomic changes could collectively protect the structural, functional and intracellular integrity of gill epithelia.

Low-positive antibody titer against Helicobacter pylori cytotoxin-associated gene A (CagA) may predict future gastric cancer better than simple seropositivity against H. pylori CagA or against H. pylori

BACKGROUND

To investigate the IgG antibody titer against Helicobacter pylori CagA as a risk factor for future noncardia gastric cancer.

METHODS

A nested case-control study was done in the longitudinal cohort of atomic bomb survivors using stored sera before diagnosis (mean, 2.3 years). Enrolled were 299 cancer cases and 3 controls per case selected from cohort members matched on age, gender, city, and time and type of serum storage and countermatched on radiation dose.

RESULTS

H. pylori IgG seropositive with CagA IgG low titer was the strongest risk factor for noncardia gastric cancer [relative risk (RR), 3.9; 95% confidence interval (95% CI), 2.1-7.0; P < 0.001], especially for intestinal-type tumor (RR, 9.9, 95% CI, 3.5-27.4; P < 0.001), compared with other risk factors, H. pylori IgG seropositive with CagA IgG negative (RR, 2.2; 95% CI, 1.3-3.9; P = 0.0052), H. pylori IgG seropositive with CagA IgG high titer (RR, 2.0; 95% CI, 1.3-3.2; P = 0.0022), chronic atrophic gastritis (RR, 2.4; 95% CI, 1.8-3.3; P < 0.001), current smoking (RR, 2.3; 95% CI, 1.4-3.5; P < 0.001), or radiation dose (RR, 2.1; 95% CI, 1.2-3.1; P = 0.00193). Current smoking showed significantly higher risk for diffuse-type than intestinal-type tumors (P = 0.0372). Radiation risk was significant only for nonsmokers, all noncardia, and diffuse-type gastric cancers.

CONCLUSIONS

A low CagA IgG titer is a useful biomarker to identify a high-risk group and it also provides a clue to understanding host-pathogen interaction.

Energetic heavy ions accelerate differentiation in the descendants of irradiated normal human diploid fibroblasts

Ionizing radiation-induced genomic instability has been demonstrated in a variety of endpoints such as delayed reproductive death, chromosome instability and mutations, which occurs in the progeny of survivors many generations after the initial insult. Dependence of these effects on the linear energy transfer (LET) of the radiation is incompletely characterized; however, our previous work has shown that delayed reductions in clonogenicity can be most pronounced at LET of 108 keV/microm. To gain insight into potential cellular mechanisms involved in LET-dependent delayed loss of clonogenicity, we investigated morphological changes in colonies arising from normal human diploid fibroblasts exposed to gamma-rays or energetic carbon ions (108 keV/microm). Exposure of confluent cultures to carbon ions was 4-fold more effective at inactivating cellular clonogenic potential and produced more abortive colonies containing reduced number of cells per colony than gamma-rays. Second, colonies were assessed for clonal morphotypic heterogeneity. The yield of differentiated cells was elevated in a dose- and LET-dependent fashion in clonogenic colonies, whereas differentiated cells predominated to a comparable extent irrespective of radiation type or dose in abortive colonies. The incidence of giant or multinucleated cells was also increased but much less frequent than that of differentiated cells. Collectively, our results indicate that carbon ions facilitate differentiation more effectively than gamma-rays as a major response in the progeny of irradiated fibroblasts. Accelerated differentiation may account, at least in part, for dose- and LET-dependent delayed loss of clonogenicity in normal human diploid cells, and could be a defensive mechanism that minimizes further expansion of aberrant cells.

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.