Low doses of alpha particles do not induce sister chromatid exchanges in bystander Chinese hamster cells defective in homologous recombination

In Vitro Assessment of the Genotoxic Hazard of Novel Hydroxamic Acid- and Benzamide-Type Histone Deacetylase Inhibitors (HDACi)

Histone deacetylase inhibitors (HDACi) are already approved for the therapy of leukemias. Since they are also emerging candidate compounds for the treatment of non-malignant diseases, HDACi with a wide therapeutic window and low hazard potential are desirable. Here, we investigated a panel of 12 novel hydroxamic acid- and benzamide-type HDACi employing non-malignant V79 hamster cells as toxicology guideline-conform in vitro model. HDACi causing a ≥10-fold preferential cytotoxicity in malignant neuroblastoma over non-malignant V79 cells were selected for further genotoxic hazard analysis, including vorinostat and entinostat for control. All HDACi selected, (i.e., KSK64, TOK77, DDK137 and MPK77) were clastogenic and evoked DNA strand breaks in non-malignant V79 cells as demonstrated by micronucleus and comet assays, histone H2AX foci formation analyses (γH2AX), DNA damage response (DDR) assays as well as employing DNA double-strand break (DSB) repair-defective VC8 hamster cells. Genetic instability induced by hydroxamic acid-type HDACi seems to be independent of bulky DNA adduct formation as concluded from the analysis of nucleotide excision repair (NER) deficient mutants. Summarizing, KSK64 revealed the highest genotoxic hazard and DDR stimulating potential, while TOK77 and MPK77 showed the lowest DNA damaging capacity. Therefore, these compounds are suggested as the most promising novel candidate HDACi for subsequent pre-clinical in vivo studies.

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

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

A historical reflection on our understanding of radiation-induced DNA double strand break repair in somatic mammalian cells; interfacing the past with the present

Purpose: The International Journal of Radiation Biology (IJRB) is celebrating 60 years of publishing in 2019. IJRB has made an enormous contribution to publishing papers that have enhanced our understanding of the DNA damage response (DDR) activated following exposure to ionizing radiation (IR). The IR-induced DDR field has a rich history but many outstanding papers pass unread by young scientists overwhelmed by the current literature. We provide a historical reflection on key advances in the DDR field and interface them with current knowledge. Conclusions: DNA double strand breaks (DSBs) were identified as the major biological lesion induced by IR. But early studies on cells from IR-sensitive ataxia telangiectasia patients showed that DSB repair was not sufficient to prevent IR hypersensitivity. Subsequently, the ATM-dependent signal transduction process was revealed, with the breadth of the response being slowly unearthed. Early studies demonstrated at least two processes of DSB repair and revealed that mis-repair causes translocation formation. Recent studies, however, are unraveling more complexity in the repair process, including the specific processing of DSBs within transcriptionally active regions, and the significance of the chromatin environment. Despite the quality of these early and current studies, many questions remain to be addressed.

Coordination of the Ser2056 and Thr2609 Clusters of DNA-PKcs in Regulating Gamma Rays and Extremely Low Fluencies of Alpha-Particle Irradiation to G0/G1 Phase Cells

The catalytic subunit of DNA dependent protein kinase (DNA-PKcs) and its kinase activity are critical for mediation of non-homologous end-joining (NHEJ) of DNA double-strand breaks (DSB) in mammalian cells after gamma-ray irradiation. Additionally, DNA-PKcs phosphorylations at the T2609 cluster and the S2056 cluster also affect DSB repair and cellular sensitivity to gamma radiation. Previously we reported that phosphorylations within these two regions affect not only NHEJ but also homologous recombination repair (HRR) dependent DSB repair. In this study, we further examine phenotypic effects on cells bearing various combinations of mutations within either or both regions. Effects studied included cell killing as well as chromosomal aberration induction after 0.5-8 Gy gamma-ray irradiation delivered to synchronized cells during the G₀/G₁ phase of the cell cycle. Blocking phosphorylation within the T2609 cluster was most critical regarding sensitization and depended on the number of available phosphorylation sites. It was also especially interesting that only one substitution of alanine in each of the two clusters separately abolished the restoration of wild-type sensitivity by DNA-PKcs. Similar patterns were seen for induction of chromosomal aberrations, reflecting their connection to cell killing. To study possible change in coordination between HRR and NHEJ directed repair in these DNA-PKcs mutant cell lines, we compared the induction of sister chromatid exchanges (SCEs) by very low fluencies of alpha particles with mutant cells defective in the HRR pathway that is required for induction of SCEs. Levels of true SCEs induced by very low fluence of alpha-particle irradiation normally seen in wild-type cells were only slightly decreased in the S2056 cluster mutants, but were completely abolished in the T2609 cluster mutants and were indistinguishable from levels seen in HRR deficient cells. Again, a single substitution in the S2056 together with a single substitution in the T2609 cluster abolished SCE formation and thus also effectively interferes with HRR.

Role of Pro-inflammatory Cytokines in Radiation-Induced Genomic Instability in Human Bronchial Epithelial Cells

Inflammatory cytokines have been implicated in the regulation of radiation-induced genomic instability in the hematopoietic system and have also been shown to induce chronic DNA damage responses in radiation-induced senescence. We have previously shown that human bronchial epithelial cells (HBEC3-KT) have increased genomic instability and IL-8 production persisting at day 7 after exposure to high-LET (600 MeV/nucleon (56)Fe ions) compared to low-LET (320 keV X rays) radiation. Thus, we investigated whether IL-8 induction is part of a broader pro-inflammatory response produced by the epithelial cells in response to damage, which influences genomic instability measured by increased micronuclei and DNA repair foci frequencies. We found that exposure to radiation induced the release of multiple inflammatory cytokines into the media, including GM-CSF, GROα, IL-1α, IL-8 and the inflammation modulator, IL-1 receptor antagonist (IL-1RA). Our results suggest that this is an IL-1α-driven response, because an identical signature was induced by the addition of recombinant IL-1α to nonirradiated cells and functional interference with recombinant IL-1RA (Anakinra) or anti-IL-1α function-blocking antibody, decreased IL-8 production induced by radiation exposure. However, genomic instability was not influenced by this pathway as addition of recombinant IL-1α to naive or irradiated cells or the presence of IL-1 RA under the same conditions as those that interfered with the function of IL-8, did not affect micronuclei or DNA repair foci frequencies measured at day 7 after exposure. While dose-response studies revealed that genomic instability and IL-8 production are the consequences of targeted effects, experiments employing a co-culture transwell system revealed the propagation of pro-inflammatory responses but not genomic instability from irradiated to nonirradiated cells. Collectively, these results point to a cell-autonomous mechanism sustaining radiation-induced genomic instability in this model system and suggest that while molecules associated with these mechanisms could be markers for persisting damage, they reflect two different outcomes.

Nonenzymatic role for WRN in preserving nascent DNA strands after replication stress

WRN, the protein defective in Werner syndrome (WS), is a multifunctional nuclease involved in DNA damage repair, replication, and genome stability maintenance. It was assumed that the nuclease activities of WRN were critical for these functions. Here, we report a nonenzymatic role for WRN in preserving nascent DNA strands following replication stress. We found that lack of WRN led to shortening of nascent DNA strands after replication stress. Furthermore, we discovered that the exonuclease activity of MRE11 was responsible for the shortening of newly replicated DNA in the absence of WRN. Mechanistically, the N-terminal FHA domain of NBS1 recruits WRN to replication-associated DNA double-stranded breaks to stabilize Rad51 and to limit the nuclease activity of its C-terminal binding partner MRE11. Thus, this previously unrecognized nonenzymatic function of WRN in the stabilization of nascent DNA strands sheds light on the molecular reason for the origin of genome instability in WS individuals.

BRCA1, FANCD2 and Chk1 are potential molecular targets for the modulation of a radiation-induced DNA damage response in bystander cells

Radiotherapy is an important treatment option for many human cancers. Current research is investigating the use of molecular targeted drugs in order to improve responses to radiotherapy in various cancers. The cellular response to irradiation is driven by both direct DNA damage in the targeted cell and intercellular signalling leading to a broad range of bystander effects. This study aims to elucidate radiation-induced DNA damage response signalling in bystander cells and to identify potential molecular targets to modulate the radiation induced bystander response in a therapeutic setting. Stalled replication forks in T98G bystander cells were visualised via bromodeoxyuridine (BrdU) nuclear foci detection at sites of single stranded DNA. γH2AX co-localised with these BrdU foci. BRCA1 and FANCD2 foci formed in T98G bystander cells. Using ATR mutant F02-98 hTERT and ATM deficient GM05849 fibroblasts it could be shown that ATR but not ATM was required for the recruitment of FANCD2 to sites of replication associated DNA damage in bystander cells whereas BRCA1 bystander foci were ATM-dependent. Phospho-Chk1 foci formation was observed in T98G bystander cells. Clonogenic survival assays showed moderate radiosensitisation of directly irradiated cells by the Chk1 inhibitor UCN-01 but increased radioresistance of bystander cells. This study identifies BRCA1, FANCD2 and Chk1 as potential targets for the modulation of radiation response in bystander cells. It adds to our understanding of the key molecular events propagating out-of-field effects of radiation and provides a rationale for the development of novel molecular targeted drugs for radiotherapy optimisation.

Cancer risk at low doses of ionizing radiation: artificial neural networks inference from atomic bomb survivors

Cancer risk at low doses of ionizing radiation remains poorly defined because of ambiguity in the quantitative link to doses below 0.2 Sv in atomic bomb survivors in Hiroshima and Nagasaki arising from limitations in the statistical power and information available on overall radiation dose. To deal with these difficulties, a novel nonparametric statistics based on the 'integrate-and-fire' algorithm of artificial neural networks was developed and tested in cancer databases established by the Radiation Effects Research Foundation. The analysis revealed unique features at low doses that could not be accounted for by nominal exposure dose, including (i) the presence of a threshold that varied with organ, gender and age at exposure, and (ii) a small but significant bumping increase in cancer risk at low doses in Nagasaki that probably reflects internal exposure to (239)Pu. The threshold was distinct from the canonical definition of zero effect in that it was manifested as negative excess relative risk, or suppression of background cancer rates. Such a unique tissue response at low doses of radiation exposure has been implicated in the context of the molecular basis of radiation-environment interplay in favor of recently emerging experimental evidence on DNA double-strand break repair pathway choice and its epigenetic memory by histone marking.

PF, Brogan JR, Kurimasa A, Chen DJ, Bedford JS, Chen BPC. Differential radiosensitivity phenotypes of DNA-PKcs mutations affecting NHEJ and HRR systems following irradiation with gamma-rays or very low fluences of alpha particles

We have examined cell-cycle dependence of chromosomal aberration induction and cell killing after high or low dose-rate γ irradiation in cells bearing DNA-PKcs mutations in the S2056 cluster, the T2609 cluster, or the kinase domain. We also compared sister chromatid exchanges (SCE) production by very low fluences of α-particles in DNA-PKcs mutant cells, and in homologous recombination repair (HRR) mutant cells including Rad51C, Rad51D, and Fancg/xrcc9. Generally, chromosomal aberrations and cell killing by γ-rays were similarly affected by mutations in DNA-PKcs, and these mutant cells were more sensitive in G1 than in S/G2 phase. In G1-irradiated DNA-PKcs mutant cells, both chromosome- and chromatid-type breaks and exchanges were in excess than wild-type cells. For cells irradiated in late S/G2 phase, mutant cells showed very high yields of chromatid breaks compared to wild-type cells. Few exchanges were seen in DNA-PKcs-null, Ku80-null, or DNA-PKcs kinase dead mutants, but exchanges in excess were detected in the S2506 or T2609 cluster mutants. SCE induction by very low doses of α-particles is resulted from bystander effects in cells not traversed by α-particles. SCE seen in wild-type cells was completely abolished in Rad51C- or Rad51D-deficient cells, but near normal in Fancg/xrcc9 cells. In marked contrast, very high levels of SCEs were observed in DNA-PKcs-null, DNA-PKcs kinase-dead and Ku80-null mutants. SCE induction was also abolished in T2609 cluster mutant cells, but was only slightly reduced in the S2056 cluster mutant cells. Since both non-homologous end-joining (NHEJ) and HRR systems utilize initial DNA lesions as a substrate, these results suggest the possibility of a competitive interference phenomenon operating between NHEJ and at least the Rad51C/D components of HRR; the level of interaction between damaged DNA and a particular DNA-PK component may determine the level of interaction of such DNA with a relevant HRR component.

The complex interactions between radiation induced non-targeted effects and cancer

Radiation induced non-targeted effects have been widely investigated in the last two decades for their potential impact on low dose radiation risk. In this paper we will give an overview of the most relevant aspects related to these effects, starting from the definition of the low dose scenarios. We will underline the role of radiation quality, both in terms of mechanisms of interaction with the biological matter and for the importance of charged particles as powerful tools for low dose effects investigation. We will focus on cell communication, representing a common feature of non-targeted effects, giving also an overview of cancer models that have explicitly considered such effects.

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.

Essential roles of BCCIP in mouse embryonic development and structural stability of chromosomes

BCCIP is a BRCA2- and CDKN1A(p21)-interacting protein that has been implicated in the maintenance of genomic integrity. To understand the in vivo functions of BCCIP, we generated a conditional BCCIP knockdown transgenic mouse model using Cre-LoxP mediated RNA interference. The BCCIP knockdown embryos displayed impaired cellular proliferation and apoptosis at day E7.5. Consistent with these results, the in vitro proliferation of blastocysts and mouse embryonic fibroblasts (MEFs) of BCCIP knockdown mice were impaired considerably. The BCCIP deficient mouse embryos die before E11.5 day. Deletion of the p53 gene could not rescue the embryonic lethality due to BCCIP deficiency, but partially rescues the growth delay of mouse embryonic fibroblasts in vitro. To further understand the cause of development and proliferation defects in BCCIP-deficient mice, MEFs were subjected to chromosome stability analysis. The BCCIP-deficient MEFs displayed significant spontaneous chromosome structural alterations associated with replication stress, including a 3.5-fold induction of chromatid breaks. Remarkably, the BCCIP-deficient MEFs had a ∼20-fold increase in sister chromatid union (SCU), yet the induction of sister chromatid exchanges (SCE) was modestly at 1.5 fold. SCU is a unique type of chromatid aberration that may give rise to chromatin bridges between daughter nuclei in anaphase. In addition, the BCCIP-deficient MEFs have reduced repair of irradiation-induced DNA damage and reductions of Rad51 protein and nuclear foci. Our data suggest a unique function of BCCIP, not only in repair of DNA damage, but also in resolving stalled replication forks and prevention of replication stress. In addition, BCCIP deficiency causes excessive spontaneous chromatin bridges via the formation of SCU, which can subsequently impair chromosome segregations in mitosis and cell division.

BCCIP is a BRCA2- and p21-interacting protein. Studies with cell culture systems have suggested an essential role of BCCIP gene in homologous recombination and suppression of replication stress and have suggested that BCCIP defects causes mitotic errors. However, the in vivo function(s) of BCCIP and the mechanistic links between BCCIP's role in suppression of replication stress and mitotic errors are largely unknown. We generated transgenic mouse lines that conditionally express shRNA against the BCCIP, and we found an essential role of BCCIP in embryo development. We demonstrate that BCCIP deficiency causes the formation of a unique type of structural abnormality of chromosomes called sister chromatid union (SCU). It has been noted in the past that impaired homologous recombination and resolution of stalled replication forks can have detrimental consequences in mitosis. However, the physical evidence for this link has not been fully identified. SCU is the product of ligation between sister chromatids, likely formed as a result of unsuccessful attempt(s) to resolve stalled replication forks. Because the SCU will progress into chromatin bridges at anaphase, resulting in mitosis errors, it likely constitutes one of the physical links between S-phase replication stress and mitotic errors.

Local infection with oilseed rape mosaic virus promotes genetic rearrangements in systemic Arabidopsis tissue

We have previously shown that local infection of tobacco plants with tobacco mosaic virus (TMV) or oilseed rape mosaic virus (ORMV) results in a systemic increase in the homologous recombination frequency (HRF). Here, we analyzed what other changes in the genome are triggered by pathogen infection. For the analysis of HRF, mutation frequency (MF) and microsatellite instability (MI), we used three different transgenic Arabidopsis lines carrying β-glucuronidase (GUS)-based substrates in their genome. We found that local infection of Arabidopsis with ORMV resulted in an increase of all three frequencies, albeit to differing degrees. The most prominent increase was observed in microsatellite instability. The increase in HRF was the lowest, although still statistically significant. The analysis of methylation of the 35S promoter and transgene expression showed that the greater instability of the transgene was not attributed to these changes. Strand breaks brought about a significant increase in non-treated tissues of infected plants. The expression of genes associated with various repair processes, such as KU70, RAD51, MSH2, DNA POL α and DNA POL δ, was also increased. To summarize, our data demonstrate that local ORMV infection destabilizes the genome in systemic tissues of Arabidopsis plants in various ways resulting in large rearrangements, point mutations and microsatellite instability.

Polymorphisms in RAD51, XRCC2 and XRCC3 genes of the homologous recombination repair in colorectal cancer--a case control study

XRCC2 and XRCC3 proteins are structurally and functionally related to RAD51 which play an important role in the homologous recombination, the process frequently involved in cancer transformation. In our previous work we show that the 135G>C polymorphism (rs1801320) of the RAD51 gene can modify the effect of the Thr241Met polymorphism (rs861539) of the XRCC3 gene. We tested the association between the 135G>C polymorphism of the RAD51 gene, the Thr241Met polymorphism of the XRCC3 gene and the Arg188His polymorphism (rs3218536) of the XRCC2 gene and colorectal cancer risk and clinicopathological parameters. Polymorphisms were evaluated by restriction fragment length polymorphism polymerase chain reaction (RFLP-PCR) in 100 patients with invasive adenocarcinoma of the colon and in 100 sex, age and ethnicity matched cancer–free controls. We stratified the patients by genotypes, tumour Duke’s and TNM stage and calculated the linkage of each genotype with each stratum. Carriers of Arg188Arg/Me241tMet, His188His/Thr241Thr and His188His/G135G genotypes had an increased risk of colorectal cancer occurrence (OR 5.70, 95% CI 1.10–29.5; OR 12.4, 95% CI 1.63–94.9; OR 5.88, 95% CI 1.21–28.5, respectively). The C135C genotype decreased the risk of colorectal cancer singly (OR 0.06, 95% CI 0.02–0.22) as well as in combination with other two polymorphisms. TNM and Duke’s staging were not related to any of these polymorphisms. Our results suggest that the 135G>C polymorphism of the RAD51 gene can be an independent marker of colorectal cancer risk. The Thr241Met polymorphism of the XRCC3 gene and the Arg188His polymorphism of the XRCC2 gene can modify the risk of colorectal cancer.

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.

Genomic instability after targeted irradiation of human lymphocytes: evidence for inter-individual differences under bystander conditions

Environmental (222)radon exposure is a human health concern, and many studies demonstrate that very low doses of high LET alpha-particle irradiation initiate deleterious genetic consequences in both irradiated and non-irradiated bystander cells. One consequence, radiation-induced genomic instability (RIGI), is a hallmark of tumorigenesis and is often assessed by measuring delayed chromosomal aberrations. We utilised a technique that facilitates transient immobilization of primary lymphocytes for targeted microbeam irradiation and have reported that environmentally relevant doses, e.g. a single (3)He(2+) particle traversal to a single cell, are sufficient to induce RIGI. Herein we sought to determine differences in radiation response in lymphocytes isolated from five healthy male donors. Primary lymphocytes were irradiated with a single particle per cell nucleus. We found evidence for inter-individual variation in radiation response (RIGI, measured as delayed chromosome aberrations). Although this was not highly significant, it was possibly masked by high levels of intra-individual variation. While there are many studies showing a link between genetic predisposition and RIGI, there are few studies linking genetic background with bystander effects in normal human lymphocytes. In an attempt to investigate inter-individual variation in the induction of bystander effects, primary lymphocytes were irradiated with a single particle under conditions where fractions of the population were traversed. We showed a marked genotype-dependent bystander response in one donor after exposure to 15% of the population. The findings may also be regarded as a radiation-induced genotype-dependent bystander effect triggering an instability phenotype.

Membrane-dependent bystander effect contributes to amplification of the response to alpha-particle irradiation in targeted and nontargeted cells

PURPOSE

Free radicals are believed to play an active role in the bystander response. This study investigated their origin as well as their temporal and spatial impacts in the bystander effect.

METHODS AND MATERIALS

We employed a precise alpha-particle microbeam to target a small fraction of subconfluent osteoblastic cells (MC3T3-E1). gammaH2AX-53BP1 foci, oxidative metabolism changes, and micronuclei induction in targeted and bystander cells were assessed.

RESULTS

Cellular membranes and mitochondria were identified as two distinct reactive oxygen species producers. The global oxidative stress observed after irradiation was significantly attenuated after cells were treated with filipin, evidence for the primal role of membrane in the bystander effect. To determine the membrane's impact at a cellular level, micronuclei yield was measured when various fractions of the cell population were individually targeted while the dose per cell remained constant. Induction of micronuclei increased in bystander cells as well as in targeted cells and was attenuated by filipin treatment, demonstrating a role for bystander signals between irradiated cells in an autocrine/paracrine manner.

CONCLUSIONS

A complex interaction of direct irradiation and bystander signals leads to a membrane-dependent amplification of cell responses that could influence therapeutic outcomes in tissues exposed to low doses or to environmental exposure.

New molecular targets in radiotherapy: DNA damage signalling and repair in targeted and non-targeted cells

Ionising radiation plays a key role in therapy due to its ability to directly induce DNA damage, in particular DNA double-strand breaks leading to cell death. Cells have multiple repair pathways which attempt to maintain genomic stability. DNA repair proteins have become key targets for therapy, using small molecule inhibitors, in combination with radiation and or chemotherapeutic agents as a means of enhancing cell killing. Significant advances in our understanding of the response of cells to radiation exposures has come from the observation of non-targeted effects where cells respond via mechanisms other than those which are a direct consequence of energy-dependent DNA damage. Typical of these is bystander signalling where cells respond to the fact that their neighbours have been irradiated. Bystander cells show a DNA damage response which is distinct from directly irradiated cells. In bystander cells, ATM- and Rad3-related (ATR) protein kinase-dependent signalling in response to stalled replication forks is an early event in the DNA damage response. The ATM protein kinase is activated downstream of ATR in bystander cells. This offers the potential for differential approaches for the modulation of bystander and direct effects with repair inhibitors which may impact on the response of tumours and on the protection of normal tissues during radiotherapy.

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.

DNA replication arrest in response to genotoxic stress provokes early activation of stress-activated protein kinases (SAPK/JNK)

The impact of DNA damage-induced replication blockage for early activation of stress kinases [stress-activated protein kinase (SAPK)/c-Jun N-terminal kinase (JNK)] is largely unknown. Here, we show that induction of dual phosphorylation of SAPK/JNK by the DNA polymerase inhibitor aphidicolin was not ameliorated by additional exposure to ultraviolet (UV) light, indicating that overlapping mechanisms participate in signaling to SAPK/JNK triggered by both agents. UV-induced DNA replication blockage, cyclobutane pyrimidine dimer formation and DNA strand break induction coincided with SAPK/JNK phosphorylation at early (< or =30 min) but not late (> or =2 h) time points after exposure. Genotoxin-stimulated SAPK/JNK activation was attenuated in nonproliferating cells, indicating that S phase-dependent mechanisms are involved in signaling to SAPK/JNK. Correspondingly, UV-induced phosphorylation of SAPK/JNK was higher in S-phase cells as compared with G(1)-phase cells. Activation of SAPK/JNK by genotoxins was below detection limit in nonproliferating human peripheral blood lymphocytes, whereas peripheral blood lymphocytes stimulated to proliferation displayed clear SAPK/JNK activation. UV-induced phosphorylation of SAPK/JNK was attenuated in XPC-defective cells, ameliorated in BRCA2 mutated cells and not changed in cells lacking ATM, DNA-PK, CSB, XPA, p53, ERCC1 or PARP as compared with the corresponding wild types. Based on these data, we suggest that DNA replication blockage caused by genotoxin-induced DNA damage contributes to early activation of SAPK/JNK.