DNA damage recognition proteins localize along heavy ion induced tracks in the cell nucleus

Induction of Chromosomal Aberrations after Exposure to the Auger Electron Emitter Iodine-125, the β--emitter Tritium and Cesium-137 γ rays

High-LET-type cell survival curves have been observed in cells that were allowed to incorporate 125I-UdR into their DNA. Incorporation of tritiated thymidine into the DNA of cells has also been shown to result in an increase in relative biological effectiveness in cell survival experiments, but the increase is smaller than observed after incorporation of 125I-UdR. These findings are explained in the literature by the overall complexity of the induced DNA damage resulting from energies of the ejected electron(s) during the decay of 3H and 125I. Chromosomal aberrations (CA) are defined as morphological or structural changes of one or more chromosomes, and can be induced by ionizing radiation. Whether the number of CA is associated with the linear energy transfer (LET) of the radiation and/or the actual complexity of the induced DNA double-strand breaks (DSB) remains elusive. In this study, we investigated whether DNA lesions induced at different cell cycle stages and by different radiation types [Auger-electrons (125I), β- particles (3H), or γ radiation (137Cs)] have an impact on the number of CA induced after induction of the same number of DSB as determined by the γ-H2AX foci assay. Cells were synchronized and pulse-labeled in S phase with low activities of 125I-UdR or tritiated thymidine. For decay accumulation, cells were cryopreserved either after pulse-labeling in S phase or after progression to G2/M or G1 phase. Experiments with γ irradiation (137Cs) were performed with synchronized and cryopreserved cells in S, G2/M or G1 phase. After thawing, a CA assay was performed. All experiments were performed after a similar number of DSB were induced. CA induction after 125I-UdR was incorporated was 2.9-fold and 1.7-fold greater compared to exposure to γ radiation and radiation from incorporated tritiated thymidine, respectively, when measured in G2/M cells. In addition, measurement of CA in G2/M cells after incorporation of 125I-UdR was 2.5-fold greater when compared to cells in G1 phase. In contrast, no differences were observed between the three radiation qualities with respect to exposure after cryopreservation in S or G1 phase. The data indicate that the 3D organization of replicated DNA in G2/M cells seems to be more sensitive to induction of more complex DNA lesions compared to the DNA architecture in S or G1 cells. Whether this is due to the DNA organization itself or differences in DNA repair capability remains unclear.

Effects of Differing Underlying Assumptions in In Silico Models on Predictions of DNA Damage and Repair

The induction and repair of DNA double-strand breaks (DSBs) are critical factors in the treatment of cancer by radiotherapy. To investigate the relationship between incident radiation and cell death through DSB induction many in silico models have been developed. These models produce and use custom formats of data, specific to the investigative aims of the researchers, and often focus on particular pairings of damage and repair models. In this work we use a standard format for reporting DNA damage to evaluate combinations of different, independently developed, models. We demonstrate the capacity of such inter-comparison to determine the sensitivity of models to both known and implicit assumptions. Specifically, we report on the impact of differences in assumptions regarding patterns of DNA damage induction on predicted initial DSB yield, and the subsequent effects this has on derived DNA repair models. The observed differences highlight the importance of considering initial DNA damage on the scale of nanometres rather than micrometres. We show that the differences in DNA damage models result in subsequent repair models assuming significantly different rates of random DSB end diffusion to compensate. This in turn leads to disagreement on the mechanisms responsible for different biological endpoints, particularly when different damage and repair models are combined, demonstrating the importance of inter-model comparisons to explore underlying model assumptions.

Applications of nanodosimetry in particle therapy planning and beyond

This topical review summarizes underlying concepts of nanodosimetry. It describes the development and current status of nanodosimetric detector technology. It also gives an overview of Monte Carlo track structure simulations that can provide nanodosimetric parameters for treatment planning of proton and ion therapy. Classical and modern radiobiological assays that can be used to demonstrate the relationship between the frequency and complexity of DNA lesion clusters and nanodosimetric parameters are reviewed. At the end of the review, existing approaches of treatment planning based on relative biological effectiveness (RBE) models or dose-averaged linear energy transfer are contrasted with an RBE-independent approach based on nandosimetric parameters. Beyond treatment planning, nanodosimetry is also expected to have applications and give new insights into radiation protection dosimetry.

DNA damage interactions on both nanometer and micrometer scale determine overall cellular damage

DNA double strand breaks (DSB) play a pivotal role for cellular damage, which is a hazard encountered in toxicology and radiation protection, but also exploited e.g. in eradicating tumors in radiation therapy. It is still debated whether and in how far clustering of such DNA lesions leads to an enhanced severity of induced damage. Here we investigate - using focused spots of ionizing radiation as damaging agent - the spatial extension of DNA lesion patterns causing cell inactivation. We find that clustering of DNA damage on both the nm and µm scale leads to enhanced inactivation compared to more homogeneous lesion distributions. A biophysical model interprets these observations in terms of enhanced DSB production and DSB interaction, respectively. We decompose the overall effects quantitatively into contributions from these lesion formation processes, concluding that both processes coexist and need to be considered for determining the resulting damage on the cellular level.

The Role of the Nuclear Factor κB Pathway in the Cellular Response to Low and High Linear Energy Transfer Radiation

Astronauts are exposed to considerable doses of space radiation during long-term space missions. As complete shielding of the highly energetic particles is impracticable, the cellular response to space-relevant radiation qualities has to be understood in order to develop countermeasures and to reduce radiation risk uncertainties. The transcription factor Nuclear Factor κB (NF-κB) plays a fundamental role in the immune response and in the pathogenesis of many diseases. We have previously shown that heavy ions with a linear energy transfer (LET) of 100–300 keV/µm have a nine times higher potential to activate NF-κB compared to low-LET X-rays. Here, chemical inhibitor studies using human embryonic kidney cells (HEK) showed that the DNA damage sensor Ataxia telangiectasia mutated (ATM) and the proteasome were essential for NF-κB activation in response to X-rays and heavy ions. NF-κB’s role in cellular radiation response was determined by stable knock-down of the NF-κB subunit RelA. Transfection of a RelA short-hairpin RNA plasmid resulted in higher sensitivity towards X-rays, but not towards heavy ions. Reverse Transcriptase real-time quantitative PCR (RT-qPCR) showed that after exposure to X-rays and heavy ions, NF-κB predominantly upregulates genes involved in intercellular communication processes. This process is strictly NF-κB dependent as the response is completely absent in RelA knock-down cells. NF-κB’s role in the cellular radiation response depends on the radiation quality.

Comparative gene expression analysis after exposure to 123I-iododeoxyuridine, γ- and α-radiation-potential biomarkers for the discrimination of radiation qualities

Gene expression analysis was carried out in Jurkat cells in order to identify candidate genes showing significant gene expression alterations allowing robust discrimination of the Auger emitter 123I, incorporated into the DNA as 123I-iododeoxyuridine (123IUdR), from α- and γ-radiation. The γ-H2AX foci assay was used to determine equi-effect doses or activity, and gene expression analysis was carried out at similar levels of foci induction. Comparative gene expression analysis was performed employing whole human genome DNA microarrays. Candidate genes had to show significant expression changes and no altered gene regulation or opposite regulation after exposure to the radiation quality to be compared. The gene expression of all candidate genes was validated by quantitative real-time PCR. The functional categorization of significantly deregulated genes revealed that chromatin organization and apoptosis were generally affected. After exposure to 123IUdR, α-particles and γ-rays, at equi-effect doses/activity, 155, 316 and 982 genes were exclusively regulated, respectively. Applying the stringent requirements for candidate genes, four (PPP1R14C, TNFAIP8L1, DNAJC1 and PRTFDC1), one (KLF10) and one (TNFAIP8L1) gene(s) were identified, respectively allowing reliable discrimination between γ- and 123IUdR exposure, γ- and α-radiation, and α- and 123IUdR exposure, respectively. The Auger emitter 123I induced specific gene expression patterns in Jurkat cells when compared with γ- and α-irradiation, suggesting a unique cellular response after 123IUdR exposure. Gene expression analysis might be an effective tool for identifying biomarkers for discriminating different radiation qualities and, furthermore, might help to explain the varying biological effectiveness at the mechanistic level.

pATM and γH2AX are effective radiation biomarkers in assessing the radiosensitivity of 12C6+ in human tumor cells

Background

Tumour radiosensitivity would be particularly useful in optimizing the radiation dose during radiotherapy. The aim of the current study was to evaluate the potential value of phosphorylated H2AX (γH2AX) and ATM (pATM) in assessing ¹²C⁶⁺ radiosensitivity of tumour cells.

Methods

Human cervical carcinoma HeLa cells, hepatoma HepG2 cells, and mucoepidermoid carcinoma MEC-1 cells were irradiated with different doses of ¹²C⁶⁺. The survival fraction was assayed with clonogenic survival method and the foci of γH2AX and pATM was visualized using immunocytochemical methods. Flow cytometry was used to assay γH2AX, pATM and the cell cycle.

Results

The survival fraction decreased immediately in dose-dependent manner, but in turn, significantly increased during 24 h after ¹²C⁶⁺ irradiation. Both γH2AX and pATM foci accumulated linearly with doses and with a maximum induction at 0.5 h for γH2AX and 0.5 or 4 h for pATM, respectively, and a fraction foci kept for 24 h. The expression of γH2AX and pATM was in relation to cell cycle. The G0/G1 phase cells had the highest expression of γH2AX after 0.5 h irradiation and then decreased to a lower level at 24 h after irradiation. An obvious increase of pATM in G2/M phase was shown after 24 h of 2 and 4 Gy irradiation. The significant G2/M phase arrest was shown. There was a close relationship between the clonogenic survival and γH2AX and pATM expression both in timing and dose in response to ¹²C⁶⁺.

Conclusions

The rate of γH2AX and pATM formation and loss may be an important factor in the response of cells to ¹²C⁶⁺. pATM and γH2AX are effective radiation biomarkers in assessing the radiosensitivity of ¹²C⁶⁺ in human tumor cells.

Detection of DNA damage by space radiation in human fibroblasts flown on the International Space Station

Although charged particles in space have been detected with radiation detectors on board spacecraft since the discovery of the Van Allen Belts, reports on the effects of direct exposure to space radiation in biological systems have been limited. Measurement of biological effects of space radiation is challenging due to the low dose and low dose rate nature of the radiation environment, and due to the difficulty in distinguishing the radiation effects from microgravity and other space environmental factors. In astronauts, only a few changes, such as increased chromosome aberrations in their lymphocytes and early onset of cataracts, are attributed primarily to their exposure to space radiation. In this study, cultured human fibroblasts were flown on the International Space Station (ISS). Cells were kept at 37°C in space for 14 days before being fixed for analysis of DNA damage with the γ-H2AX assay. The 3-dimensional γ-H2AX foci were captured with a laser confocal microscope. Quantitative analysis revealed several foci that were larger and displayed a track pattern only in the Day 14 flight samples. To confirm that the foci data from the flight study was actually induced from space radiation exposure, cultured human fibroblasts were exposed to low dose rate γ rays at 37°C. Cells exposed to chronic γ rays showed similar foci size distribution in comparison to the non-exposed controls. The cells were also exposed to low- and high-LET protons, and high-LET Fe ions on the ground. Our results suggest that in G1 human fibroblasts under the normal culture condition, only a small fraction of large size foci can be attributed to high-LET radiation in space.

Life science experiments performed in space in the ISS/Kibo facility and future research plans

Over the past several years, current techniques in molecular biology have been used to perform experiments in space, focusing on the nature and effects of space radiation. In the Japanese 'Kibo' facility in the International Space Station (ISS), the Japan Aerospace Exploration Agency (JAXA) has performed five life science experiments since 2009, and two additional experiments are currently in progress. The first life science experiment in space was the 'Rad Gene' project, which utilized two human cultured lymphoblastoid cell lines containing a mutated P53 : gene (m P53 : ) and a parental wild-type P53 : gene (wt P53 : ) respectively. Four parameters were examined: (i) detecting space radiation-induced DSBs by observing γH2AX foci; (ii) observing P53 : -dependent gene expression during space flight; (iii) observing P53 : -dependent gene expression after space flight; and (iv) observing the adaptive response in the two cell lines containing the mutated and wild type P53 : genes after exposure to space radiation. These observations were completed and have been reported, and this paper is a review of these experiments. In addition, recent new information from space-based experiments involving radiation biology is presented here. These experiments involve human cultured cells, silkworm eggs, mouse embryonic stem cells and mouse eggs in various experiments designed by other principal investigators in the ISS/Kibo. The progress of Japanese science groups involved in these space experiments together with JAXA are also discussed here. The Japanese Society for Biological Sciences in Space (JSBSS), the Utilization Committee of Space Environment Science (UCSES) and the Science Council of Japan (ACJ) have supported these new projects and new experimental facilities in ISS/Kibo. Currently, these organizations are proposing new experiments for the ISS through 2024.

Establishment of a γ-H2AX foci-based assay to determine biological dose of radon to red bone marrow in rats

The biodosimetric information is critical for assessment of cancer risk in populations exposed to high radon. However, no tools are available for biological dose estimation following radon exposure. Here, we established a γ-H2AX foci-based assay to determine biological dose to red bone marrow (RBM) in radon-inhaled rats. After 1–3 h of in vitro radon exposure, a specific pattern of γ-H2AX foci, linear tracks with individual p-ATM and p-DNA-PKcs foci, was observed, and the yield of γ-H2AX foci and its linear tracks displayed a linear dose-response manner in both rat peripheral blood lymphocytes (PBLs) and bone-marrow lymphocytes (BMLs). When the cumulative doses of radon inhaled by rats reached 14, 30 and 60 working level months (WLM), the yields of three types of foci markedly increased in both PBLs and BMLs, and γ-H2AX foci-based dose estimates to RBM were 0.97, 2.06 and 3.94 mGy, respectively. Notably, BMLs displayed a more profound increase of three types of foci than PBLs, and the absorbed dose ratio between BMLs and PBLs was similar between rats exposed to 30 and 60 WLM of radon. Taken together, γ-H2AX foci quantitation in PBLs is able to estimate RBM-absorbed doses with the dose-response curve of γ-H2AX foci after in vitro radon exposure and the ratio of RBM- to PBL-absorbed doses in rats following radon exposure.

Combining carbon ion irradiation and non-homologous end-joining repair inhibitor NU7026 efficiently kills cancer cells

Background

Our previous data demonstrated that targeting non-homologous end-joining repair (NHEJR) yields a higher radiosensitivity than targeting homologous recombination repair (HRR) to heavy ions using DNA repair gene knockouts (KO) in mouse embryonic fibroblast (MEF). In this study, we determined if combining the use of an NHEJR inhibitor with carbon (C) ion irradiation was more efficient in killing human cancer cells compared with only targeting a HRR inhibitor.

Methods

The TP53-null human non-small cell lung cancer cell line H1299 was used for testing the radiosensitizing effect of NHEJR-related DNA-dependent protein kinase (DNA-PK) inhibitor NU7026, HRR-related Rad51 inhibitor B02, or both to C ion irradiation using colony forming assays. The mechanism underlying the inhibitor radiosensitization was determined by flow cytometry after H2AX phosphorylation staining. HRR-related Rad54-KO, NHEJR-related Lig4-KO, and wild-type TP53-KO MEF were also included to confirm the suppressing effect specificity of these inhibitors.

Results

NU7026 showed significant sensitizing effect to C ion irradiation in a concentration-dependent manner. In contrast, B02 showed a slight sensitizing effect to C ion irradiation. The addition of NU7026 significantly increased H2AX phosphorylation after C ion and x-ray irradiations in H1299 cells, but not B02. NU7026 had no effect on radiosensitivity to Lig4-KO MEF and B02 had no effect on radiosensitivity to Rad54-KO MEF in both irradiations.

Conclusion

These results suggest that inhibitors targeting the NHEJR pathway could significantly enhance radiosensitivity of human cancer cells to C ion irradiation, rather than targeting the HRR pathway.

Electronic supplementary material

The online version of this article (doi:10.1186/s13014-015-0536-z) contains supplementary material, which is available to authorized users.

Nonhomologous end-joining repair plays a more important role than homologous recombination repair in defining radiosensitivity after exposure to high-LET radiation

DNA double-strand breaks (DSBs) induced by ionizing radiation pose a major threat to cell survival. The cell can respond to the presence of DSBs through two major repair pathways: homologous recombination (HR) and nonhomologous end joining (NHEJ). Higher levels of cell death are induced by high-linear energy transfer (LET) radiation when compared to low-LET radiation, even at the same physical doses, due to less effective and efficient DNA repair. To clarify whether high-LET radiation inhibits all repair pathways or specifically one repair pathway, studies were designed to examine the effects of radiation with different LET values on DNA DSB repair and radiosensitivity. Embryonic fibroblasts bearing repair gene (NHEJ-related Lig4 and/or HR-related Rad54) knockouts (KO) were used and their responses were compared to wild-type cells. The cells were exposed to X rays, spread-out Bragg peak (SOBP) carbon ion beams as well as with carbon, iron, neon and argon ions. Cell survival was measured with colony-forming assays. The sensitization enhancement ratio (SER) values were calculated using the 10% survival dose of wild-type cells and repair-deficient cells. Cellular radiosensitivity was listed in descending order: double-KO cells > Lig4-KO cells > Rad54-KO cells > wild-type cells. Although Rad54-KO cells had an almost constant SER value, Lig4-KO cells showed a high-SER value when compared to Rad54-KO cells, even with increasing LET values. These results suggest that with carbon-ion therapy, targeting NHEJ repair yields higher radiosensitivity than targeting homologous recombination repair.

Alpha particle induced DNA damage and repair in normal cultured thyrocytes of different proliferation status

Childhood exposure to ionizing radiation increases the risk of developing thyroid cancer later in life and this is suggested to be due to higher proliferation of the young thyroid. The interest of using high-LET alpha particles from Astatine-211 ((211)At), concentrated in the thyroid by the same mechanism as (131)I [1], in cancer treatment has increased during recent years because of its high efficiency in inducing biological damage and beneficial dose distribution when compared to low-LET radiation. Most knowledge of the DNA damage response in thyroid is from studies using low-LET irradiation and much less is known of high-LET irradiation. In this paper we investigated the DNA damage response and biological consequences to photons from Cobolt-60 ((60)Co) and alpha particles from (211)At in normal primary thyrocytes of different cell cycle status. For both radiation qualities the intensity levels of γH2AX decreased during the first 24h in both cycling and stationary cultures and complete repair was seen in all cultures but cycling cells exposed to (211)At. Compared to stationary cells alpha particles were more harmful for cycling cultures, an effect also seen at the pChk2 levels. Increasing ratios of micronuclei per cell nuclei were seen up to 1Gy (211)At. We found that primary thyrocytes were much more sensitive to alpha particle exposure compared with low-LET photons. Calculations of the relative biological effectiveness yielded higher RBE for cycling cells compared with stationary cultures at a modest level of damage, clearly demonstrating that cell cycle status influences the relative effectiveness of alpha particles.

The effect of microdosimetric 12C6+ heavy ion irradiation and Mg2+ on canthaxanthin production in a novel strain of Dietzia natronolimnaea

Background

Dietzia natronolimnaea is one of the most important bacterial bioresources for high efficiency canthaxanthin production. It produces the robust and stable pigment canthaxanthin, which is of special interest for the development of integrated biorefineries. Mutagenesis employing ¹²C⁶⁺ irradiation is a novel technique commonly used to improve microorganism productivity. This study presents a promising route to obtaining the highest feasible levels of biomass dry weight (BDW), and total canthaxanthin by using a microdosimetric model of ¹²C⁶⁺ irradiation mutation in combination with the optimization of nutrient medium components.

Results

This work characterized the rate of both lethal and non-lethal dose mutations for ¹²C⁶⁺ irradiation and the microdosimetric kinetic model using the model organism, D. natronolimnaea svgcc1.2736. Irradiation with ¹²C⁶⁺ ions resulted in enhanced production of canthaxanthin, and is therefore an effective method for strain improvement of D. natronolimnaea svgcc1.2736. Based on these results an optimal dose of 0.5–4.5 Gy, Linear energy transfer (LET) of 80 keV μm^-1and energy of 60 MeV u^-1 for ¹²C⁶⁺ irradiation are ideal for optimum and specific production of canthaxanthin in the bacterium. Second-order empirical calculations displaying high R-squared (0.996) values between the responses and independent variables were derived from validation experiments using response surface methodology. The highest canthaxanthin yield (8.14 mg) was obtained with an optimized growth medium containing 21.5 g L^-1 D-glucose, 23.5 g L^-1 mannose and 25 ppm Mg²⁺ in 1 L with an irradiation dose of 4.5 Gy.

Conclusions

The microdosimetric ¹²C⁶⁺ irradiation model was an effective mutagenic technique for the strain improvement of D. natronolimnaea svgcc1.2736 specifically for enhanced canthaxanthin production. At the very least, random mutagenesis methods using ¹²C⁶⁺ions can be used as a first step in a combined approach with long-term continuous fermentation processes. Central composite design-response surface methodologies (CCD-RSM) were carried out to optimize the conditions for canthaxanthin yield. It was discovered D-glucose, Mg²⁺ and mannose have significant influence on canthaxanthin biosynthesis and growth of the mutant strain.

The potential value of the neutral comet assay and γH2AX foci assay in assessing the radiosensitivity of carbon beam in human tumor cell lines

Background

Carbon ions (¹²C⁶⁺) are high linear energy transfer (LET) radiation characterized by higher relative biological effectiveness than low LET radiation. The assessment of tumour radiosensitivity would be particularly useful in optimizing the radiation dose during radiotherapy. The aim of the current study was to evaluate the potential value of the neutral comet assay and γH2AX foci assay in assessing ¹²C⁶⁺ radiosensitivity of tumour cells.

Materials and methods

The doses of ¹²C⁶⁺ and X-rays used in the present study were 2 and 4 Gy. The survival fraction, DNA double-strand breaks (DSB) and repair kinetics of DSB were assayed with clonogenic survival, neutral comet assay and γH2AX foci assay in human cervical carcinoma HeLa cells, hepatoma HepG2 cells, and mucoepidermoid carcinoma MEC-1 cells at the time points of 0.5, 4, 16 and 24 h after ¹²C⁶⁺ and X-rays irradiation.

Results

The survival fraction for 12C6+ irradiation was much more inhibited than for X-rays (p < 0.05) in all three tumour cell lines tested. Substantial amounts of residual damage, assessed by the neutral comet assay, were present after irradiation (p < 0.05). The highest residual damage was observed at 0.5 or 4 h, both for ¹²C⁶⁺ and X-ray irradiation. However, the residual damage in HeLa and MEC-1 cells was higher for ¹²C⁶⁺ than X-rays (p < 0.05). The strongest induction of γH2AX foci was observed after 30 min, for all three tumour cell lines (p < 0.01). The franction of γH2AX foci persisted for at least 24 h after ¹²C6+ irradiation; in HeLa cells and MEC-1 was higher than after X-ray irradiation (p < 0.05). The correlation coefficients between the clonogenic survival, neutral comet assay and γH2AX foci assay were not statistically significant, except for some tumour cells at individual irradiation doses and types.

Conclusions

Our study demonstrated that the neutral comet assay and γ-H2AX foci assay could be used to assess the radiosensitivity of ¹²C⁶⁺ in human tumour cells.

Exposure to heavy ion radiation induces persistent oxidative stress in mouse intestine

Ionizing radiation-induced oxidative stress is attributed to generation of reactive oxygen species (ROS) due to radiolysis of water molecules and is short lived. Persistent oxidative stress has also been observed after radiation exposure and is implicated in the late effects of radiation. The goal of this study was to determine if long-term oxidative stress in freshly isolated mouse intestinal epithelial cells (IEC) is dependent on radiation quality at a dose relevant to fractionated radiotherapy. Mice (C57BL/6J; 6 to 8 weeks; female) were irradiated with 2 Gy of γ-rays, a low-linear energy transfer (LET) radiation, and intestinal tissues and IEC were collected 1 year after radiation exposure. Intracellular ROS, mitochondrial function, and antioxidant activity in IEC were studied by flow cytometry and biochemical assays. Oxidative DNA damage, cell death, and mitogenic activity in IEC were assessed by immunohistochemistry. Effects of γ radiation were compared to ⁵⁶Fe radiation (iso-toxic dose: 1.6 Gy; energy: 1000 MeV/nucleon; LET: 148 keV/µm), we used as representative of high-LET radiation, since it's one of the important sources of high Z and high energy (HZE) radiation in cosmic rays. Radiation quality affected the level of persistent oxidative stress with higher elevation of intracellular ROS and mitochondrial superoxide in high-LET ⁵⁶Fe radiation compared to unirradiated controls and γ radiation. NADPH oxidase activity, mitochondrial membrane damage, and loss of mitochondrial membrane potential were greater in ⁵⁶Fe-irradiated mice. Compared to γ radiation oxidative DNA damage was higher, cell death ratio was unchanged, and mitotic activity was increased after ⁵⁶Fe radiation. Taken together our results indicate that long-term functional dysregulation of mitochondria and increased NADPH oxidase activity are major contributing factors towards heavy ion radiation-induced persistent oxidative stress in IEC with potential for neoplastic transformation.

Repair of DNA damage induced by accelerated heavy ions--a mini review

Increasing use of heavy ions for cancer therapy and concerns from exposure to heavy charged particles in space necessitate the study of the basic biological mechanisms associated with exposure to heavy ions. As the most critical damage induced by ionizing radiation is DNA double strand break (DSB), this review focuses on DSBs induced by heavy ions and their repair processes. Compared with X- or gamma-rays, high-linear energy transfer (LET) heavy ion radiation induces more complex DNA damage, categorized into DSBs and non-DSB oxidative clustered DNA lesions (OCDL). This complexity makes the DNA repair process more difficult, partially due to retarded enzymatic activities, leading to increased chromosome aberrations and cell death. In general, the repair process following heavy ion exposure is LET-dependent, but with nonhomologous end joining defective cells, this trend is less emphasized. The variation in cell survival levels throughout the cell cycle is less prominent in cells exposed to high-LET heavy ions when compared with low LET, but this mechanism has not been well understood until recently. Involvement of several DSB repair proteins is suggested to underlie this interesting phenomenon. Recent improvements in radiation-induced foci studies combined with high-LET heavy ion exposure could provide a useful opportunity for more in depth study of DSB repair processes. Accelerated heavy ions have become valuable tools to investigate the molecular mechanisms underlying repair of DNA DSBs, the most crucial form of DNA damage induced by radiation and various chemotherapeutic agents.

Comparison of RBE values of high-LET α-particles for the induction of DNA-DSBs, chromosome aberrations and cell reproductive death

Background

Various types of radiation effects in mammalian cells have been studied with the aim to predict the radiosensitivity of tumours and normal tissues, e.g. DNA double strand breaks (DSB), chromosome aberrations and cell reproductive inactivation. However, variation in correlations with clinical results has reduced general application. An additional type of information is required for the increasing application of high-LET radiation in cancer therapy: the Relative Biological Effectiveness (RBE) for effects in tumours and normal tissues. Relevant information on RBE values might be derived from studies on cells in culture.

Methods

To evaluate relationships between DNA-DSB, chromosome aberrations and the clinically most relevant effect of cell reproductive death, for ionizing radiations of different LET, dose-effect relationships were determined for the induction of these effects in cultured SW-1573 cells irradiated with gamma-rays from a Cs-137 source or with α-particles from an Am-241 source. RBE values were derived for these effects. Ionizing radiation induced foci (IRIF) of DNA repair related proteins, indicative of DSB, were assessed by counting gamma-H2AX foci. Chromosome aberration frequencies were determined by scoring fragments and translocations using premature chromosome condensation. Cell survival was measured by colony formation assay. Analysis of dose-effect relations was based on the linear-quadratic model.

Results

Our results show that, although both investigated radiation types induce similar numbers of IRIF per absorbed dose, only a small fraction of the DSB induced by the low-LET gamma-rays result in chromosome rearrangements and cell reproductive death, while this fraction is considerably enhanced for the high-LET alpha-radiation. Calculated RBE values derived for the linear components of dose-effect relations for gamma-H2AX foci, cell reproductive death, chromosome fragments and colour junctions are 1.0 ± 0.3, 14.7 ± 5.1, 15.3 ± 5.9 and 13.3 ± 6.0 respectively.

Conclusions

These results indicate that RBE values for IRIF (DNA-DSB) induction provide little valid information on other biologically-relevant end points in cells exposed to high-LET radiations. Furthermore, the RBE values for the induction of the two types of chromosome aberrations are similar to those established for cell reproductive death. This suggests that assays of these aberrations might yield relevant information on the biological effectiveness in high-LET radiotherapy.

Persistence of γ-H2AX and 53BP1 foci in proliferating and non-proliferating human mammary epithelial cells after exposure to γ-rays or iron ions

PURPOSE

To investigate γ-H2AX (phosphorylated histone H2AX) and 53BP1 (tumour protein 53 binding protein No. 1) foci formation and removal in proliferating and non-proliferating human mammary epithelial cells (HMEC) after exposure to sparsely and densely ionising radiation under different cell culture conditions.

MATERIAL AND METHODS

HMEC cells were grown either as monolayers (2D) or in extracellular matrix to allow the formation of acinar structures in vitro (3D). Foci numbers were quantified by image analysis at various time points after exposure.

RESULTS

Our results reveal that in non-proliferating cells under 2D and 3D cell culture conditions, iron-ion induced γ-H2AX foci were still present at 72 h after exposure, although 53BP1 foci returned to control levels at 48 h. In contrast in proliferating HMEC, both γ-H2AX and 53BP1 foci decreased to control levels during the 24-48 h time interval after irradiation under 2D conditions. Foci numbers decreased faster after γ-ray irradiation and returned to control levels by 12 h regardless of marker, cell proliferation status, and cell culture condition.

CONCLUSIONS

The disappearance of radiation-induced γ-H2AX and 53BP1 foci in HMEC has different dynamics that depend on radiation quality and proliferation status. Notably, the general patterns do not depend on the cell culture condition (2D versus 3D). We speculate that the persistent γ-H2AX foci in iron-ion irradiated non-proliferating cells could be due to limited availability of double-strand break (DSB) repair pathways in G0/G1-phase, or that repair of complex DSB requires replication or chromatin remodelling.

Evaluation of the efficacy of radiation-modifying compounds using γH2AX as a molecular marker of DNA double-strand breaks

Radiation therapy is a widely used therapeutic approach for cancer. To improve the efficacy of radiotherapy there is an intense interest in combining this modality with two broad classes of compounds, radiosensitizers and radioprotectors. These either enhance tumour-killing efficacy or mitigate damage to surrounding non-malignant tissue, respectively. Radiation exposure often results in the formation of DNA double-strand breaks, which are marked by the induction of H2AX phosphorylation to generate γH2AX. In addition to its essential role in DDR signalling and coordination of double-strand break repair, the ability to visualize and quantitate γH2AX foci using immunofluorescence microscopy techniques enables it to be exploited as an indicator of therapeutic efficacy in a range of cell types and tissues. This review will explore the emerging applicability of γH2AX as a marker for monitoring the effectiveness of radiation-modifying compounds.

Visualization of heavy ion tracks by labeling 3'-OH termini of induced DNA strand breaks

African green monkey kidney cells, CV-1, were irradiated with Carbon ions (LET: 735 keV/µm Argon ions (LET: 3,000 keV/µm) to visualize ion tracks through the cell nucleus by labeling the 3'-OH termini result of DNA strand breaks. The 3'-OH termini of DNA were labeled with BrdU-triphosphate catalyzed by TdT. This method of TUNEL (TdT-mediated dUTP Nick End labeling) is based on the specific binding of TdT to 3'-OH termini of DNA. Subsequent immuno-fluorescent staining with the primary monoclonal antibody against BrdU, followed by a secondary antibody of Alexa Fluor 488, was performed to visualize the BrdU labeled DNA termini. Images of the cell nuclei were acquired by confocal laser microscopy. When cell monolayers were irradiated perpendicularly with argon ions, induced DSBs in cell nuclei were identifiable as fluorescent spots. In another irradiation setup, when cells were irradiated at a small angle with incident argon ions, DNA strand breaks were detected as fluorescent stripes across the cell nucleus. These results demonstrate the induction of 3'-OH termini at sites of DNA strand breaks along Argon ion tracks.

DNA ligase IV is a potential molecular target in ACNU sensitivity

Nimustine (ACNU) is a chloroethylating agent which was the most active chemotherapy agent used for patients with high-grade gliomas until the introduction of temozolomide, which became the standard of care for patients with newly diagnosed glioblastomas in Japan. Since temozolomide was established as the standard first-line therapy for glioblastoma multiforme (GBM), ACNU has been employed as a salvage chemotherapy agent for recurrent GBM in combination with other drugs. The acting molecular mechanism in ACNU has yet to be elucidated. ACNU is a cross-linking agent which induces DNA double-strand breaks (DSBs). The work described here was intended to clarify details in repair pathways which are active in the repair of DNA DSBs induced by ACNU. DSBs are repaired through the homologous recombination (HR) and non-homologous end-joining (NHEJ) pathways. Cultured mouse embryonic fibroblasts were used which have deficiencies in DNA DSB repair genes which are involved in HR repair (X-ray repair cross-complementing group 2 [XRCC2] and radiation sensitive mutant 54 [Rad54]), and in NHEJ repair (DNA ligase IV [Lig4]). Cellular sensitivity to ACNU treatment was evaluated with colony forming assays. The most effective molecular target which correlated with ACNU cell sensitivity was Lig4. In addition, it was found that Lig4 small-interference RNA (siRNA) efficiently enhanced cell lethality which was induced by ACNU in human glioblastoma A172 cells. These findings suggest that the down-regulation of Lig4 might provide a useful tool which can be used to increase cell sensitivity in response to ACNU chemotherapy.

The foci of DNA double strand break-recognition proteins localize with gammaH2AX after heat treatment

Recently, there have been many reports concerning proteins which can recognize DNA double strand break (DSBs), and such proteins include histone H2AX phosphorylated at serine 139 (gammaH2AX), ataxia telangiectasia mutated (ATM) phospho-serine 1981, DNA-dependent protein kinase catalytic subunit (DNA-PKcs) phospho-threonine 2609, Nijmegen breakage syndrome 1 (NBS1) phospho-serine 343, checkpoint kinase 2 (CHK2), phospho-threonine 68, and structural maintenance of chromosomes 1 (SMC1) phospho-serine 966. Thus, it should be possible to follow the formation of DSBs and their repair using immunohistochemical methods with multiple antibodies to detect these proteins. When normal human fibroblasts (AG1522 cells) were exposed to 3 Gy of X-rays as a control, clearly discernable foci for these proteins were detected, and these foci localized with gammaH2AX foci. After heat treatment at 45.5 degrees C for 20 min, these proteins are partially localized with gammaH2AX foci. Here we show that there were slight differences in the localization pattern among these proteins, such as a disappearance from the nucleus (phospho-ATM) and translocation to the cytoplasm (phospho-NBS1) at 30 min after heat treatment, and some foci (phospho-DNA-PKcs and phospho-CHK2) appeared at 8 h after heat treatment. These results are discussed from perspectives of heat-induced denaturation of proteins and formation of DSBs.

Recent advances in the biology of heavy-ion cancer therapy

Superb biological effectiveness and dose conformity represent a rationale for heavy-ion therapy, which has thus far achieved good cancer controllability while sparing critical normal organs. Immediately after irradiation, heavy ions produce dense ionization along their trajectories, cause irreparable clustered DNA damage, and alter cellular ultrastructure. These ions, as a consequence, inactivate cells more effectively with less cell-cycle and oxygen dependence than conventional photons. The modes of heavy ion-induced cell death/inactivation include apoptosis, necrosis, autophagy, premature senescence, accelerated differentiation, delayed reproductive death of progeny cells, and bystander cell death. This paper briefly reviews the current knowledge of the biological aspects of heavy-ion therapy, with emphasis on the authors' recent findings. The topics include (i) repair mechanisms of heavy ion-induced DNA damage, (ii) superior effects of heavy ions on radioresistant tumor cells (intratumor quiescent cell population, TP53-mutated and BCL2-overexpressing tumors), (iii) novel capacity of heavy ions in suppressing cancer metastasis and neoangiogenesis, and (iv) potential of heavy ions to induce secondary (especially breast) cancer.

DNA ligase IV as a new molecular target for temozolomide

Temozolomide (TMZ) is a methylating agent used in chemotherapy against glioblastoma. This work was designed to clarify details in repair pathways acting to remove DNA double-strand breaks (DSBs) induced by TMZ. Cultured mouse embryonic fibroblasts were used which were deficient in DSB repair genes such as homologous recombination repair-related genes X-ray repair cross-complementing group 2 (XRCC2)and radiation sensitive mutant54 (Rad54), non-homologous end joining repair-related gene DNAligase IV (Lig4). Cell sensitivity to drug treatments was assessed using colony forming assays. The most effective molecular target which was correlated with TMZ cell sensitivity was Lig4. In addition, it was found that small interference RNAs (siRNA) for Lig4 efficiently enhanced cell lethality induced by TMZ in human glioblastoma A172 cells. These findings suggest that down regulation of Lig4 might provide a useful tool for cell sensitization during TMZ chemotherapy.