Analysis of the common deletions in the mitochondrial DNA is a sensitive biomarker detecting direct and non-targeted cellular effects of low dose ionizing radiation

One of the key issues of current radiation research is the biological effect of low doses. Unfortunately, low dose science is hampered by the unavailability of easily performable, reliable and sensitive quantitative biomarkers suitable detecting low frequency alterations in irradiated cells. We applied a quantitative real time polymerase chain reaction (qRT-PCR) based protocol detecting common deletions (CD) in the mitochondrial genome to assess direct and non-targeted effects of radiation in human fibroblasts. In directly irradiated (IR) cells CD increased with dose and was higher in radiosensitive cells. Investigating conditioned medium-mediated bystander effects we demonstrated that low and high (0.1 and 2Gy) doses induced similar levels of bystander responses and found individual differences in human fibroblasts. The bystander response was not related to the radiosensitivity of the cells. The importance of signal sending donor and signal receiving target cells was investigated by placing conditioned medium from a bystander response positive cell line (F11-hTERT) to bystander negative cells (S1-hTERT) and vice versa. The data indicated that signal sending cells are more important in the medium-mediated bystander effect than recipients. Finally, we followed long term effects in immortalized radiation sensitive (S1-hTERT) and normal (F11-hTERT) fibroblasts up to 63 days after IR. In F11-hTERT cells CD level was increased until 35 days after IR then reduced back to control level by day 49. In S1-hTERT cells the increased CD level was also normalized by day 42, however a second wave of increased CD incidence appeared by day 49 which was maintained up to day 63 after IR. This second CD wave might be the indication of radiation-induced instability in the mitochondrial genome of S1-hTERT cells. The data demonstrated that measuring CD in mtDNA by qRT-PCR is a reliable and sensitive biomarker to estimate radiation-induced direct and non-targeted effects.

[Radiation-induced "bystander effect" revealed by means of adaptive response in cocultured lymphocytes from humans of different genders]

The "bystander effect" was investigated in mixed cultures of lymphocytes from humans of opposite genders. Development of the adaptive response (AR) in non-irradiated female/male cells was estimated after adaptive pretreatment of opposite gender lymphocytes, chromosome aberrations being evaluated. Experiments were performed using two schedules of adaptive (0.05 Gy) and challenging (1 Gy) irradiations: G0-G1 and G1-G1. The results obtained indicate the development of a mediated adaptive response ("bystander effect") in the lymphocytes neighboring pre-irradiated cells, as well as the influence of a time scheme of adapting and challenging irradiations on the amount of induced chromosome aberrations in mixed cultures and a possible dependence of the adaptive response intensity on the donor gender.

Radiation-induced bystander effect in healthy G(o) human lymphocytes: biological and clinical significance

To study the bystander effects, G(0) human peripheral blood lymphocytes were X-irradiated with 0.1, 0.5 and 3 Gy. After 24h, cell-free conditioned media from irradiated cultures were transferred to unexposed lymphocytes. Following 48 h of medium transfer, viability, induction of apoptosis, telomere shortening, reactive oxygen species (ROS) levels and micronuclei (after stimulation) were analyzed. A statistically significant decrement in cell viability, concomitant with the loss of mitochondrial membrane potential, telomere shortening, increases in hydrogen peroxide (H(2)O(2)) and superoxide anion (O(2)(-)) with depletion of intracellular glutathione (GSH) level, and higher frequencies of micronuclei, were observed in bystander lymphocytes incubated with medium from 0.5 and 3 Gy irradiated samples, compared to lymphocytes unexposed. Furthermore, no statistically significant difference between the response to 0.5 and 3 Gy of irradiation in bystander lymphocytes, was found. However, when lymphocytes were irradiated with 0.1 Gy, no bystander effect with regard to viability, apoptosis, telomere length, and micronuclei was observed, although a high production of ROS level persisted. Radiation in the presence of the radical scavenger dimethyl sulfoxide (DMSO) suppressed oxidative stress induced by 3 Gy of X-rays with the effective elimination of bystander effects, suggesting a correlation between ROS and bystander signal formation in irradiated cells. The data propose that bystander effect might be mostly due to the reactions of radiation induced free radicals on DNA, with the existence of a threshold at which the bystander signal is not operative (0.1 Gy dose of X-rays). Our results may have clinical implications for health risk associated with radiation exposure.

Radiation-induced bystander effects induce radioadaptive response by low-dose radiation

When normal human fibroblast cells (MRC-5) received a priming irradiation of 3-20 mGy 4 h prior to irradiation with 1000 mGy, the number of DNA double-stranded breaks (DSBs) decreased significantly to 18.2-18.7 per cell compared with 21 per cell when there was no priming irradiation. This result indicates that a priming irradiation of 3-20 mGy induces a radioadaptive response in MRC-5. The authors' previous study had indicated that DSBs induced by ≤ 20 mGy are due to a radiation-induced bystander effect. These findings suggest that radiation-induced bystander effects might contribute to induction of the radioadaptive response. To test this hypothesis, MRC-5 were suspended in lindane, an inhibitor of radiation-induced bystander effects, which was added to the medium for the priming irradiation of 3-20 mGy. Lindane inhibited the protective effect of priming irradiation on DSBs caused by subsequent irradiation with 1000 mGy. Thus, radiation-induced bystander effects may play a role in radioadaptive responses.

An extracellular DNA mediated bystander effect produced from low dose irradiated endothelial cells

The human umbilical vein endothelial cells culture was exposed to X-ray radiation in a low dose of 10cGy. The fragments of extracellular genomic DNA (ecDNA(R)) were isolated from the culture medium after the short-term incubation. A culture medium of unirradiated endothelial cells was then supplemented with ecDNA(R), followed by analysing the cells along the series of parameters (bystander effect). The exposed cells and bystander endotheliocytes showed similar response to low doses: approximation of the 1q12 loci of chromosome 1 and their transposition into the cellular nucleus, change in shape of the endotheliocytic nucleus, activation of the nucleolus organizing regions (NORs), actin polymerization, and an elevated level of DNA double-stranded breaks. Following blockade of TLR9 receptors with oligonucleotide-inhibitor or chloroquine in the bystander cells these effects - except of activation of NORs - on exposure to ecDNA(R) disappeared, with no bystander response thus observed. The presence of the radiation-induced apoptosis in the bystander effect being studied suggests a possibility for radiation-modified ecDNA fragments (i.e., stress signaling factors) to be released into the culture medium, whereas inhibition of TLR9 suggests the binding these ligands to the recipient cells. A similar DNA-signaling pathway in the bystander effect we previously described for human lymphocytes. Integrity of data makes it possible to suppose that a similar signaling mechanism which we demonstrated for lymphocytes (humoral system) might also be mediated in a monolayer culture of cells (cellular tissue) after the development of the bystander effect in them and transfer of stress signaling factors (ecDNA(R)) through the culture medium.

Dosimetry of a 238Pu-based alpha-particle irradiator and its biological application in a study of the bystander effect

A better understanding of the non-targeted (bystander) effects of radiation may have important implications with regards to radiation risk assessment, radiation protection, and targeted cancer therapy. In the present study, the direct and bystander effects of α-particle irradiation in immortalized human fibroblasts (F11hTERT) and breast cancer cells (MCF-7) was investigated. To ensure a more accurate dose delivery to these different cell lines, an existing 238Pu α-particle irradiator was improved by the addition of a collimator and the development of an analytical equation for calculation of the radiation dose to cells. The mean dose rate and α-particle fluence were calculated for each cell line by taking into consideration the size of their nuclei. Bystander effect experiments were performed by transferring medium from irradiated to unirradiated cells and by measuring micronucleus formation in the cells. Both the immortalized human fibroblasts and the breast cancer cells displayed a bystander effect. In conclusion, the broad-beam α-particle irradiator improved in this study represents a useful tool in the investigation of direct and non-targeted effects of α-particle radiation.

Communicating non-targeted effects of ionizing radiation to achieve adaptive homeostasis in tissues

Non-targeted effects, i.e., those responses in cells or tissues that were not subject to energy deposition events after localized exposure to ionizing radiation, are well established. While they are not a universal phenotype, when they do occur they can be associated with subsequent tissue or whole body responses. Here it is argued that non-targeted effects are a tissue level response to restore equilibrium within an organ system, and thus restore tissue homeostasis. This "adaptive homeostasis" has evolved in response to a variety of environmental and other such stresses an individual is exposed to in their lifetime. These non-targeted effects are not likely to impact significantly on estimates of potential risks associated with radiation exposure because they are presumably "built into" current risk estimates. However, they could have implications for radiation carcinogenesis, by driving processes in targeted and non-targeted cells that could eliminate transformed cells or transform cells from a normal phenotype to a phenotype associated with malignancy within a tissue.

An accelerator-based neutron microbeam system for studies of radiation effects

A novel neutron microbeam is being developed at the Radiological Research Accelerator Facility (RARAF) of Columbia University. The RARAF microbeam facility has been used for studies of radiation bystander effects in mammalian cells for many years. Now a prototype neutron microbeam is being developed that can be used for bystander effect studies. The neutron microbeam design here is based on the existing charged particle microbeam technology at the RARAF. The principle of the neutron microbeam is to use the proton beam with a micrometre-sized diameter impinging on a very thin lithium fluoride target system. From the kinematics of the ⁷Li(p,n)⁷Be reaction near the threshold of 1.881 MeV, the neutron beam is confined within a narrow, forward solid angle. Calculations show that the neutron spot using a target with a 17-µm thick gold backing foil will be <20 µm in diameter for cells attached to a 3.8-µm thick propylene-bottomed cell dish in contact with the target backing. The neutron flux will roughly be 2000 per second based on the current beam setup at the RARAF singleton accelerator. The dose rate will be about 200 mGy min⁻¹. The principle of this neutron microbeam system has been preliminarily tested at the RARAF using a collimated proton beam. The imaging of the neutron beam was performed using novel fluorescent nuclear track detector technology based on Mg-doped luminescent aluminum oxide single crystals and confocal laser scanning fluorescent microscopy.

Mechanism of protection of bystander cells by exogenous carbon monoxide: impaired response to damage signal of radiation-induced bystander effect

A protective effect of exogenous carbon monoxide (CO), generated by CO releasing molecule ticarbonyldichlororuthenium (II) dimer (CORM-2), on the bystander cells from the toxicity of radiation-induced bystander effect (RIBE) was revealed in our previous study. In the present work, a possible mechanism of this CO effect was investigated. The results from medium transfer experiments showed that α-particle irradiated Chinese hamster ovary (CHO) cells would release nitric oxide (NO), which was detected with specific NO fluorescence probe, to induce p53 binding protein 1 (BP1) formation in the cell population receiving the medium, and the release peak was found to be at 1h post irradiation. Treating the irradiated or bystander cells separately with CO (CORM-2) demonstrated that CO was effective in the bystander cells but not the irradiated cells. Measurements of NO production and release with a specific NO fluorescence probe also showed that CO treatment did not affect the production and release of NO by irradiated cells. Protection of CO on cells to peroxynitrite, an oxidizing free radical from NO, suggested that CO might protect bystander cells via impaired response of bystander cells to NO, a RIBE signal in our research system.

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.

No evidence for a low linear energy transfer adaptive response in irradiated RKO cells

It has become increasingly evident from reports in the literature that there are many confounding factors capable of modulating radiation-induced non-targeted responses, such as the bystander effect and the adaptive response. In this paper, we examine recent data which suggest that the observation of non-targeted responses may not be universally observable for differing radiation qualities. We have conducted a study of the adaptive response following low-linear energy transfer exposures for human colon carcinoma cells and failed to observe adaption for the endpoints of clonogenic survival or micronucleus formation.

Radiation-induced perturbation of cell-to-cell signalling and communication

The investigation of the bystander phenomena (i.e. the induction of damage in cells not directly traversed by radiation) is strictly related to the study of the mechanisms of intercellular communication and of the perturbative effects of radiation. A new possible way to try to solve the bystander puzzle is through a 'systems radiation biology' approach with the total integration of experimental and theoretical activities. In particular, this contribution will focus on: (1) 'ad hoc' experiments designed to quantify key parameters involved in intercellular signalling (focusing, as a pilot study, on release, decay and internalization of interleukine-6 molecules, their modulation by radiation, and possible differences between in vivo/in vitro behaviour); (2) the implementation and the development of two different modelling approaches: a stochastic model (based on a Monte Carlo code) that takes account of the local mechanisms of release and internalization of signalling molecules (e.g. cytokines) and an analytical model where signal molecules are treated as a population and their temporal behaviour is described by differential equations. This approach provided instruments to investigate the complex phenomena of signal transmission and the role of cell communication to guarantee (maintain) the robustness of the in vitro experimental systems against the effects of perturbations.

Lack of hyper-radiosensitivity and induced radioresistance and of bystander effect in V79 cells after proton irradiation of different energies

A huge body of evidence about the hyper-radiosensitivity and induced radioresistance (HRS/IRR) phenomena and the bystander effect (BE) is reported in the literature, in many cell types and in terms of various biological endpoints, after high- and low-linear energy transfer irradiation. However, the mechanisms underlying these effects together with their inter-relationship, and the correlation of HRS/IRR and BE phenomena with radiation quality are not yet well established and elucidated. To study these phenomena, the radiation response of V79 cells has been evaluated in terms of cell survival after irradiation with broad beams of 7.7- and 28.5-keV μm(-1) protons. HRS/IRR has been investigated also in terms of micronuclei and chromosomal aberration induction. The presence of BE has been investigated with a 'partial shielding irradiation' system, which prevents the irradiation of 35 % (on average) of the cell population. No clear evidence of HRS/IRR, nor of a significant BE response, can be identified in the low-dose region of V79 dose-response curves after proton irradiation of different energies.

Track structure calculations on hypothetical subcellular targets for the release of cell-killing signals in bystander experiments with medium transfer

Track structure studies using PARTRAC have been performed with the aim to investigate the possibility of revealing information on initiating targets and mechanisms of bystander effects mediated by signals released into the culture medium. Dependences on radiation dose have been assessed for alternative signal emission scenarios, defined by required energy deposits in a number of subcellular targets, mimicking e.g. mitochondria as hypothetical targets for the release of signals. The simulation results agree with target theory, and elucidate the characteristic dose for signal emission as a function of target topology, size and activation energy. The observed dose dependence of bystander cell kill in medium transfer experiments is not as steep as predicted by the considered simple signal emission scenarios with a single or even multiple hits to the hypothetical targets. This has been resolved by accounting for variations in cellular characteristics among the irradiated cells.

H2AX phosphorylation in response to DNA double-strand break formation during bystander signalling: effect of microRNA knockdown

Upon DNA double-strand break (DSB) formation, hundreds of H2AX molecules in the chromatin flanking the break site are phosphorylated on serine residue 139, termed gamma-H2AX, so that virtually every DSB site in a nucleus can be visualised within 10 min of its formation using an antibody to gamma-H2AX. One application of this sensitive assay is to examine the induction of DNA double-strand damage in subtle non-targeted cellular effects such as the bystander effect. Here whether microRNA (miRNA) serve as a primary signalling mechanism for bystander effect propagation by comparing matched human colon carcinoma cell lines with wild-type or depleted levels of mature miRNAs was investigated. No major differences were found in the levels of induced gamma-H2AX foci in the tested cell lines, indicating that though miRNAs play a role in bystander effect manifestation, they appear not to be the primary bystander signalling molecules in the formation of bystander effect-induced DSBs.

Molecular and cellular radiobiological effects of Auger emitting radionuclides

Although the general radiobiologic principles underlying external beam therapy and radionuclide therapy are similar, significant differences in the biophysical and radiobiologic effects from the two types of radiation continue to accumulate. Here, I will address the unique features that distinguish the molecular and cellular radiobiological effects of Auger electron-emitting radionuclides consequent to (1) the physical characteristics of the decaying atom and its subcellular localisation, (2) DNA topology and (3) the bystander effect. Based on these experimental findings, I postulate that the ability of track structural simulations as primary tools in modelling DNA damage and cellular survival at the molecular level would be greatly enhanced when these contributions are factored in.

Radiation-induced bystander effects in cultured human stem cells

Background

The radiation-induced “bystander effect” (RIBE) was shown to occur in a number of experimental systems both in vitro and in vivo as a result of exposure to ionizing radiation (IR). RIBE manifests itself by intercellular communication from irradiated cells to non-irradiated cells which may cause DNA damage and eventual death in these bystander cells. It is known that human stem cells (hSC) are ultimately involved in numerous crucial biological processes such as embryologic development; maintenance of normal homeostasis; aging; and aging-related pathologies such as cancerogenesis and other diseases. However, very little is known about radiation-induced bystander effect in hSC. To mechanistically interrogate RIBE responses and to gain novel insights into RIBE specifically in hSC compartment, both medium transfer and cell co-culture bystander protocols were employed.

Methodology/Principal Findings

Human bone-marrow mesenchymal stem cells (hMSC) and embryonic stem cells (hESC) were irradiated with doses 0.2 Gy, 2 Gy and 10 Gy of X-rays, allowed to recover either for 1 hr or 24 hr. Then conditioned medium was collected and transferred to non-irradiated hSC for time course studies. In addition, irradiated hMSC were labeled with a vital CMRA dye and co-cultured with non-irradiated bystander hMSC. The medium transfer data showed no evidence for RIBE either in hMSC and hESC by the criteria of induction of DNA damage and for apoptotic cell death compared to non-irradiated cells (p>0.05). A lack of robust RIBE was also demonstrated in hMSC co-cultured with irradiated cells (p>0.05).

Conclusions/Significance

These data indicate that hSC might not be susceptible to damaging effects of RIBE signaling compared to differentiated adult human somatic cells as shown previously. This finding could have profound implications in a field of radiation biology/oncology, in evaluating radiation risk of IR exposures, and for the safety and efficacy of hSC regenerative-based therapies.

Probability of bystander effect induced by alpha-particles emitted by radon progeny using the analytical model of tracheobronchial tree

Radiation-induced biological bystander effects have become a phenomenon associated with the interaction of radiation with cells. There is a need to include the influence of biological effects in the dosimetry of the human lung. With this aim, the purpose of this work is to calculate the probability of bystander effect induced by alpha-particle radiation on sensitive cells of the human lung. Probability was calculated by applying the analytical model cylinder bifurcation, which was created to simulate the geometry of the human lung with the geometric distribution of cell nuclei in the airway wall of the tracheobronchial tree. This analytical model of the human tracheobronchial tree represents the extension of the ICRP 66 model, and follows it as much as possible. Reported probabilities are calculated for various targets and alpha-particle energies. Probability of bystander effect has been calculated for alpha particles with 6 and 7.69 MeV energies, which are emitted in the (222)Rn chain. The application of these results may enhance current dose risk estimation approaches in the sense of the inclusion of the influence of the biological effects.

A mathematical framework for separating the direct and bystander components of cellular radiation response

A mathematical model for fractional tumor cell survival was developed incorporating components of cell killing due to direct radiation interactions and bystander signals resulting from non-local dose deposition.

MATERIAL AND METHODS

Three possible mechanisms for signal production were tested by fitting predictions to available experimental results for tumor cells (non-small cell lung cancer NCI-H460 and melanoma MM576) exposed to gradient x-ray fields. The parameter fitting allowed estimation of the contribution of bystander signaling to cell death (20-50% for all models). Separation of the two components of cell killing allowed determination of the α and β parameters of the linear-quadratic model both with and without the presence of bystander signaling.

RESULTS AND DISCUSSION

For both cell lines, cell death from bystander signaling and direct radiation interactions were comparable. For NCI-H460 cells, the values for α and β were 0.18 Gy⁻¹ and 0.10 Gy⁻² respectively when direct and bystander effects were combined, and 0.053 Gy⁻¹ and 0.061 Gy⁻² respectively when the signaling component was removed. For MM576, the corresponding respective values were 0.09 Gy⁻¹ and 0.011 Gy⁻² for the combined response, and 0.014 Gy⁻¹ and 0.002 Gy⁻² for the isolated direct radiation response. The bystander component in cell death was found to be significant and should not be ignored. Further experimental evidence is required to determine how these results translate to the in vivo situation where tumor control probability (TCP) models that currently assume cellular independence may need to be revised.

A review of in vitro experimental evidence for the effect of spatial and temporal modulation of radiation dose on response

BACKGROUND

Intensity modulated radiation therapy introduces strong spatial and temporal modulation of the dose delivery that may have therapeutic benefits, as yet unrealized.

MATERIAL AND METHODS

Experimental evidence for spatial and temporal modulation affecting the cell survival following in vitro irradiation has been derived using clonogenic assays.

RESULTS AND DISCUSSION

The experimental results show that the survival status of a cell is strongly influenced by the spatial dose modulation. The classical bystander effect of decreased survival has now been supplemented by observations of increased survival, which may result from the same or different signaling mechanisms. Temporal dose modulation experiments show that dose protraction significantly increases cell survival. An appropriate choice of temporal dose modulation pattern enables cell death to be maximized or minimized for a constant dose and delivery time.

CONCLUSION

Bystander effects challenge the assumption that outcome is solely dependent on local dose. Intra-fractional temporal modulation via protracted treatments and time varying dose delivery both affect the cell survival. The presence of bystander and temporal effects emphasize the need for a mathematical framework which incorporates their influence on cell survival.

Some considerations for the study of TGFbeta in medium of irradiated T98G cells: activation, release and consumption

BACKGROUND

Transforming growth factor β1 (TGFβ1) has been proposed as a candidate for the transmission of radiation-induced bystander signals.

AIM

To assess the influence that the presence of latent TGFβ in the medium may have on the modulation of TGFβ1 release and on its receptor (TGFβR2) expression after irradiation of glioblastoma cells or after treatment with medium collected from γ-irradiated cells.

MATERIALS AND METHODS

T98G cells cultured with a complete medium or a serum-free medium were irradiated with 0.25 and 1 Gy and the concentration of total TGFβ1 was measured.

RESULTS AND CONCLUSIONS

Irradiation of cells growing with a complete medium (i.e. a medium containing latent TGFβ1, LTGFβ1) caused a consistent dose-dependent decrease of the TGFβ1 available in the medium. When LTGFβ1 was not available in the medium (i.e. a medium without serum supplement), the levels of TGFβ1 increased significantly. Changes in the pattern of expression of TGFβR2 were evident only when a serum-free medium was used.

Heavy ion irradiation induces autophagy in irradiated C2C12 myoblasts and their bystander cells

Autophagy is one of the major processes involved in the degradation of intracellular materials. Here, we examined the potential impact of heavy ion irradiation on the induction of autophagy in irradiated C2C12 mouse myoblasts and their non-targeted bystander cells. In irradiated cells, ultrastructural analysis revealed the accumulation of autophagic structures at various stages of autophagy (i.e. phagophores, autophagosomes and autolysosomes) within 20 min after irradiation. Multivesicular bodies (MVBs) and autolysosomes containing MVBs (amphisomes) were also observed. Heavy ion irradiation increased the staining of microtubule-associated protein 1 light chain 3 and LysoTracker Red (LTR). Such enhanced staining was suppressed by an autophagy inhibitor 3-methyladenine. In addition to irradiated cells, bystander cells were also positive with LTR staining. Altogether, these results suggest that heavy ion irradiation induces autophagy not only in irradiated myoblasts but also in their bystander cells.

Non-targeted effects as a paradigm breaking evidence

The finding that mammalian cells and tissues and whole organisms react differently at high than at low doses of ionizing radiation questions the scientific validity of the linear no-threshold concept for low-dose exposures. Indeed, the classical paradigm of radiobiology was based on the concept that all radiation effects on living matter are due to the direct action of radiation. Meanwhile, the discovery of non-targeted and delayed radiation effects has challenged this concept, and one might ask whether a new paradigm has to be developed to provide more realistic protection against low radiation doses. The present overview summarizes recent findings on the low-dose radiation-induced bystander effect, genomic instability, radiation hypersensitivity, hormesis, radioadaptive and transgenerational responses. For these, some common features can be recognized. Most of these phenomena include (1) intra- and intercellular signaling, involving reactive oxygen species (ROS). This signaling may be transient or persistent, and may involve the release of cytokines (bystander effect, genomic instability) or epigenetic changes (translesional responses), (2) a large variability of responses depending on the type of radiation, genotype (DNA repair capacity) and physiological state of the cells and tissues. Many more parameters are involved in responses at low doses than at high doses, and different pathways are activated. At low doses, non-linear responses are obtained that are not compatible with the LNT concept. At present, more work is needed to identify the essential parameters involved and to provide a basis for proper modelling of low-dose radiation health effects for radiation protection purposes.