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

Our current knowledge of the mechanisms underlying the induction of bystander effects by low doses of high or low LET ionizing radiation is reviewed. The question of what actually constitutes a protective effect is discussed in the context of adaptive (often referred to as hormetic or protective) responses. Finally the review considers critically, how bystander effects may be related to observed adaptive responses or other seemingly protective effects of low doses exposures. Bystander effects induce responses at the tissue level, which are similar to generalized stress responses. Most of the work involving low LET radiation exposure discussed in the existing literature measures a death response. Since many cell populations carry damaged cells without being exposed to radiation (so-called "background damage"), it is possible that low doses exposures cause removal of cells carrying potentially problematic lesions, prior to exposure to radiation. This mechanism could lead to the production of "U-shaped" or hormetic dose-response curves. The level of adverse, adaptive or apparently beneficial response will be related to the background damage carried by the original cell population, the level of organization at which damage or harm are scored and the precise definition of "harm". This model may be important when attempting to predict the consequences of mixed exposures involving low doses of radiation and other environmental stressors.     

A monomer-dimer model explains the results of radiation inactivation: binding characteristics of insulin receptor purified from human placenta

The technique of radiation inactivation has been used on highly purified human placental insulin receptor in order to determine the functional molecular size responsible for the insulin binding and to evaluate the "affinity regulator" hypothesis, which has been proposed to explain the increase in specific insulin binding to rat liver membranes observed at low radiation doses [Harmon, J. T., Hedo, J. A., & Kahn, C. R. (1983) J. Biol. Chem. 258, 6875-6881]. Three different types of inactivation curves were observed: (1) biphasic with an enhanced binding activity after exposure to low radiation doses, (2) nonlinear with no change in binding activity after exposure to low radiation doses, and (3) linear with a loss in the binding activity with increasing radiation exposures. A monomer-dimer model was the simplest model that best described the three types of radiation inactivation curves observed. The model predicts that an increase in insulin binding activity would result after exposure to low radiation doses when the initial dimer/monomer ratio is equal to or greater than 1 and a monomer is more active than a dimer. The monomer size of the binding activity was estimated to be 227,000 daltons by this model. This value most likely reflects the size of the monomeric alpha beta form. To substantiate this model, the purified receptor was fractionated by Sepharose CL-6B chromatography. The insulin binding profile of this column indicated two peaks.(ABSTRACT TRUNCATED AT 250 WORDS)     

Relationship between radiation-induced low-dose hypersensitivity and the bystander effect

Recent advances in our knowledge of the biological effects of low doses of ionizing radiation have shown two unexpected phenomena: a "bystander effect" that can be demonstrated at low doses as a transferable factor(s) causing radiobiological effects in unexposed cells, and low-dose hyper-radiosensitivity and increased radioresistance that can be demonstrated collectively as a change in the dose-effect relationship, occurring around 0.5-1 Gy of low-LET radiation. In both cases, the effect of very low doses is greater than would be predicted by conventional DNA strand break/repair-based radiobiology. This paper addresses the question of whether the two phenomena have similar or exclusive mechanisms. Cells of 13 cell lines were tested using established protocols for expression of both hyper-radiosensitivity/increased radioresistance and a bystander response. Both were measured using clonogenicity as an end point. The results showed considerable variation in the expression of both phenomena and suggested that cell lines with a large bystander effect do not show hyper-radiosensitivity. The reverse was also true. This inverse relationship was not clearly related to the TP53 status or malignancy of the cell line. There was an indication that cell lines that have a radiation dose-response curve with a wide shoulder show hyper-radiosensitivity/increased radioresistance and no bystander effect. The results may suggest new approaches to understanding the factors that control cell death or the sectoring of survival at low radiation doses.     

The absence of the facts connected with the genomic instability after the irradiation in low doses by radiation with low LET

In the review which is a brief account of more complete document (Koterov A.N. // Int. J. Low Radiat. 2005. V. 1. No. 4. P. 376-451) the data of world researches devoted to a phenomenon of radiation-induced genomic instability (RIGI) are considered. The purpose of the review is the definition of the bottom limit of radiation doses which induced of RIGI in experiments at different methodical approaches (irradiation in vitro, in vivo, in utero, bystander effect and transgeneration effects of radiation). The action only radiation with low LET is examined. Among several hundreds works wasn't revealed any fact, when RIGI induced by low doses irradiation (up to 0.2 Gy) for normal cells and for organism left from maternal womb. Six exceptions are revealed which are named as "apparent" so in all cases the abnormal, unstable, defective objects or ambiguous final parameter were used. Thus, RIGI at low doses of radiation with low LET is a myth.     

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

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 bystander effects, genomic instability, and adaptive responses. All three have been observed at very low doses, and at time points far removed from the initial radiation exposure, and are extremely relevant for linear extrapolation to low doses; the adaptive response is particularly relevant when exposure is spread over a period of time. These are precisely the circumstances that are most relevant to understanding cancer risk associated with environmental and occupational radiation exposures. This review will provide a synthesis of the known, and proposed, interrelationships amongst low-dose cellular responses to radiation. It also will examine the potential importance of non-targeted cellular responses to ionizing radiation in setting acceptable exposure limits especially to low-LET radiations.     

Phosphoproteomics profiling of human skin fibroblast cells reveals pathways and proteins affected by low doses of ionizing radiation

Background

High doses of ionizing radiation result in biological damage; however, the precise relationships between long-term health effects, including cancer, and low-dose exposures remain poorly understood and are currently extrapolated using high-dose exposure data. Identifying the signaling pathways and individual proteins affected at the post-translational level by radiation should shed valuable insight into the molecular mechanisms that regulate dose-dependent responses to radiation.

Principal Findings

We have identified 7117 unique phosphopeptides (2566 phosphoproteins) from control and irradiated (2 and 50 cGy) primary human skin fibroblasts 1 h post-exposure. Semi-quantitative label-free analyses were performed to identify phosphopeptides that are apparently altered by radiation exposure. This screen identified phosphorylation sites on proteins with known roles in radiation responses including TP53BP1 as well as previously unidentified radiation-responsive proteins such as the candidate tumor suppressor SASH1. Bioinformatic analyses suggest that low and high doses of radiation affect both overlapping and unique biological processes and suggest a role for MAP kinase and protein kinase A (PKA) signaling in the radiation response as well as differential regulation of p53 networks at low and high doses of radiation.

Conclusions

Our results represent the most comprehensive analysis of the phosphoproteomes of human primary fibroblasts exposed to multiple doses of ionizing radiation published to date and provide a basis for the systems-level identification of biological processes, molecular pathways and individual proteins regulated in a dose dependent manner by ionizing radiation. Further study of these modified proteins and affected networks should help to define the molecular mechanisms that regulate biological responses to radiation at different radiation doses and elucidate the impact of low-dose radiation exposure on human health.     

The low-dose problem in radiobiology

Three aspects of the problem of "low doses" in radiobiology are discussed, they are: (1) universality of the stimulatory effect of penetrating radiation on biological objects, (2) the doses used, and (3) the damages induced by irradiation with these doses.     

Neutron RBEs for mouse skin at low doses per fraction

The effect of low doses of 240 kVp X rays or of 3 MeV neutrons has been investigated using skin reactions on mouse feet as the biological system. Eight or nine repeated small doses of radiation were used, followed by graded "top-up" doses to bring the reactions into a detectable range. By comparing dose-response curves, the RBE has been determined for neutron doses per fraction ranging from 0.25-1.0 Gy. The data are consistent with a limiting RBE of between 7 and 10 at very low doses. A review of other published RBE values for low doses per fraction shows a wide range of RBEs . Very few studies show a plateau value for the RBE. These findings are more consistent with dose-response data that fit a linear-quadratic model than with a multitarget single-hit model.     

High radiopathogenic children's loads caused by "low doses" of technogenic chronic radiation

To study influence of "low radiodoses" on their early effects, the method to monitor peculiarities for human biochemphysic pathologies caused by technogenic chronic doses divided by natural constant radiation background doses (relative radiation loads as their ratios, RRL) has been developed. High radiolabile coupled system of harmful catabolites of the initiated lipoperoxic cascade with essential radioprotectors (vitamins A and E as natural antioxidants) as pathogenic significant test-objects were monitored. For exposed children low radiation technogenic doses were high enough as RRL to disturb and to dysregulate both systems. For evaluation of individual radiosensitivity significances of age for children especially, as of the biochemical status for both systems are shown. The approach permits to investigate pathologies of studied systems as primary factors for mechanisms of related radiogenic known somatic health secondary consequences and to use it for some important practical applications.     

The risk of chronic myeloid leukemia: can the dose-response curve be U-shaped?

Chronic myeloid leukemia (CML) is caused by a BCR-ABL chromosome translocation in a primitive hematopoietic stem cell. The number of hematopoietic stem cells in the body is thus a major factor in CML risk. Evidence suggests that the number of hematopoietic stem cells in the body is only loosely regulated, having a broad "dead-band" of physiologically acceptable values. The existence of a dead-band is important, because it would imply that low levels of hematopoietic stem cell killing can be permanent; i.e., it would imply that low doses of ionizing radiation can cause permanent reductions in the total number of CML target cells and thus permanent reductions in the subsequent risk of spontaneous CML. Such reductions in risk could be substantial if hematopoietic stem cells are also hypersensitive to radiation killing at low dose. Our calculations indicate that, due to dead-band hematopoietic stem cell control, if hematopoietic stem cells are as hypersensitive to killing at low doses as epithelial cells, reductions in the spontaneous CML risk could exceed the low-dose risks of induced CML; i.e., the net lifetime CML risk could have a U-shaped dose-response curve.     

On various possibilities in pulsed radiation biochemistry and chemistry

Several experiments are described that relate to the application of new regimes of radiation action on enzymes in vitro and some other materials. These regimes have recently come into practice due to the appearance of a new generation of devices with very short high-energy pulses of ionizing radiation. It is shown that the term flash radiation biochemistry in its perfect sense has to be used at the condition of the overlapping individual effective interaction microvolumes (e.g. spurs and blobs) realized during a time interval (radiation pulse duration) that is low compared with the corresponding physical-chemical process. In this situation a number of unexpected effects occur at very low absolute doses. These processes are analyzed in terms of their non-stationary and non-diffusive developments.     

What can epidemiology tell us about risks at low doses?

Puskin, J. S. What Can Epidemiology Tell Us about Risks at Low Doses? Radiat. Res. 169, 122-124 (2008). Limitations on statistical power preclude direct detection and quantification of radiogenic cancer risks at very low (environmental) levels of low-LET radiation through epidemiological studies. Given this limitation and our incomplete understanding of cellular processes leading to radiation carcinogenesis, an "effective threshold" in the dose range of interest for radiation protection cannot yet be ruled out. Ongoing epidemiological studies of chronically exposed individuals receiving very low daily doses of radiation can be used, however, together with radiobiological data, to critically test whether such a threshold is plausible.     

Vicia faba chromosome damage induced by low doses of gamma radiation

The rate of radiation damage to chromosomes by low doses of gamma rays (0.01-0.30 Gy) was studied in the root tips ofVicia faba. As criteria of the effect of ionizing radiation, the frequency of sister chromatid exchanges (SCEs), incidence of chromosomal aberrations and the number of micronuclei were evaluated and compared in irradiated cells. The results obtained confirmed that the analysis of SCEs did not represent an efficient indicator of radiation damage to chromosomes. On the contrary, the formation of chromosomal aberrations and micronuclei was effectively stimulated by low radiation doses, there being linear dose-effect relationships in the low doses region used.     

Radioresponsiveness at low doses: hyper-radiosensitivity and increased radioresistance in mammalian cells

The rationale for and importance of research on effects after radiation at "low doses" are outlined. Such basic radiobiological studies on induction of repair enzymes, protective mechanisms, priming, and hypersensitivity are certainly all relevant to treatment of cancer (see Section 1, Studies at low doses - relevance to cancer treatment). Included are examples from many groups, using various endpoints to address the possibility of an induced resistance, which has been compared to the adaptive response [M.C. Joiner, P. Lambin, E.P. Malaise, T. Robson, J.E. Arrand, K.A. Skov, B. Marples, Hypersensitivity to very low single radiation doses: its relationship to the adaptive response and induced radioresistance, Mutat. Res. 358 (1996) 171-183.]. This is not intended to be an exhaustive review--rather a re-introduction of concepts such as priming and a short survey of molecular approaches to understanding induced resistance. New data on the response of HT29 cells after treatment (priming) with co-cultured activated neutrophils are included, with protection against X-rays (S1). Analysis of previously published results in various cells lines in terms of increased radioresistance (IRR)/intrinsic sensitivity are presented which complement a study on human tumour lines [P. Lambin, E.P. Malaise, M.C. Joiner, Might intrinsic radioresistance of human tumour cells be induced by radiation?, Int. Radiat. Biol. 69 (1996) 279-290].It is not feasible to extrapolate to low doses from studies at high doses. The biological responses probably vary with dose, LET, and have variable time frames. The above approaches may lead to new types of treatment, or additional means to assess radioresponsiveness of tumours. Studies in many areas of biology would benefit from considerations of different dose regions, as the biological responses vary with dose. There may also be some implications in the fields of radiation protection and carcinogenesis, and the extensions of concepts of hyper-radiosensitivity (HRS)/IRR extended to radiation exposure are considered in Section 2, Possible relevance of IRR concepts to radiation exposure (space). More knowledge on inducible responses could open new approaches for protection and means to assess genetic predisposition. Many endpoints are used currently--clonogenic survival, mutagenesis, chromosome aberrations and more direct--proteins/genes/functions/repair/signals, as well as different biological systems. Because of scant knowledge of the relevant aspects at low doses, such as inducible/protective mechanisms, threshold, priming, dose-rate effects, LET within one system, it is still too early to draw conclusions in the area of radiation exposure. Technological advances may permit much needed studies at low doses in the areas of both treatment and protection.     

The concept of the effective dose a proposal for the combination of organ doses

Irradiation of the human body by external or internal sources leads mostly to a simultaneous exposure of several organs. However, so far no clear and consistent recommendations for the combination of organ doses and the assessment of an exposure limit under such irradiation conditions are available. Following a proposal described in ICRP-publication 14 one possible concept for the combination of organ doses is discussed in this paper. This concept is based on the assumption that at low doses the total radiation detriment to the exposed person is given by the sum of radiation detriments to the single organs. Taking into account a linear dose-risk relationship, the sum of weighted organ doses leads to the definition of an "Effective Dose". The applicability and consequences of this "Effective Dose Concept" are discussed especially with regard to the assessment of the maximum permissible intake of radionuclides into the human body and the combination of external and internal exposure.     

Neoplastic transformation in vitro by low doses of ionizing radiation: role of adaptive response and bystander effects

The shape of the dose-response curve for cancer induction by low doses of ionizing radiation is of critical importance to the assessment of cancer risk at such doses. Epidemiologic analyses are limited by sensitivity to doses typically greater than 50-100 mGy for low LET radiation. Laboratory studies allow for the examination of lower doses using cancer-relevant endpoints. One such endpoint is neoplastic transformation in vitro. It is known that this endpoint is responsive to both adaptive response and bystander effects. The relative balance of these processes is likely to play an important role in determining the shape of the dose-response curve at low doses. A factor that may influence this balance is cell density at time of irradiation. The findings reported in this paper indicate that the transformation suppressive effect of low doses previously seen following irradiation of sub-confluent cultures, and attributed to an adaptive response, is reduced for irradiated confluent cultures. However, even under these conditions designed to optimize the role of bystander effects the data do not fit a linear no-threshold model and are still consistent with the notion of a threshold dose for neoplastic transformation in vitro by low LET radiation.     

An apparent correlation between the inhibition of induced ornithine decarboxylase activity by γ radiation and the capacity for DNA repair synthesis in normal and ataxia telangiectasia human fibroblasts: No correlation with cell survival

Summary Exposure of normal human fibroblasts (F107) in stationary phase to radiation inhibited the appearance of induced ornithine decarboxylase (ODC) activity. Skin fibroblasts derived from two ataxia telangiectasia (AT) patients (F184 and F182) displayed a similar response. The level of DNA repair synthesis was also similar in the three cell strains. Fibroblasts from another apparently normal donor (F196) were very sensitive to inhibition of induced ODC activity by radiation and were also deficient in radiation-induced DNA repair synthesis. However, the two strains derived from normal donors displayed the same degree of cellular sensitivity towards X-rays, whereas the two AT strains showed the typical hypersensitivity to the cytotoxic effect of X-irradiation. The results suggest a possible correlation between the inhibition of induced ODC activity by radiation and the extent of DNA repair synthesis at high radiation doses, but there is no correlation between these two parameters and cellular survival at low radiation doses.     

Exposure to low dose of gamma radiation enhances the excision repair in Saccharomyces cerevisiae

The effect of low doses of ionizing and nonionizing radiation on the radiation response of yeast Saccharomyces cerevisiae toward ionizing and nonionizing radiation was studied. The wild-type strain D273-10B on exposure to 54 Gy gamma radiation (resulting in about 10% cell killing) showed enhanced resistance to subsequent exposure to UV radiation. This induced UV resistance increased with the incubation time between the initial gamma radiation stress and the UV irradiation. Exposure to low doses of UV light on the other hand showed no change in gamma or UV radiation response of this strain. The strains carrying a mutation at rad52 behaved in a way similar to the wild type, but with slightly reduced induced response. In contrast to this, the rad3 mutants, defective in excision repair, showed no induced UV resistance. Removal of UV-induced pyrimidine dimers in wild-type yeast DNA after UV irradiation was examined by analyzing the sites recognized by UV endonuclease from Micrococcus luteus. The samples that were exposed to low doses of gamma radiation before UV irradiation were able to repair the pyrimidine dimers more efficiently than the samples in which low gamma irradiation was omitted. The nature of enhanced repair was studied by scoring the frequency of induced gene conversion and reverse mutation at trp and ilv loci respectively in strain D7, which showed similar enhanced UV resistance induced by low-dose gamma irradiation. The induced repair was found to be essentially error-free. These results suggest that irradiation of strain D273-10B with low doses of gamma radiation enhances its capability for excision repair of UV-induced pyrimidine dimers.