ATM-Dependent Hyper-Radiosensitivity in Mammalian Cells Irradiated by Heavy Ions

Potential Benefits of Combining Proton or Carbon Ion Therapy with DNA Damage Repair Inhibitors

The use of charged particle radiotherapy is currently increasing, but combination therapy with DNA repair inhibitors remains to be exploited in the clinic. The high-linear energy transfer (LET) radiation delivered by charged particles causes clustered DNA damage, which is particularly effective in destroying cancer cells. Whether the DNA damage response to this type of damage is different from that elicited in response to low-LET radiation, and if and how it can be targeted to increase treatment efficacy, is not fully understood. Although several preclinical studies have reported radiosensitizing effects when proton or carbon ion irradiation is combined with inhibitors of, e.g., PARP, ATR, ATM, or DNA-PKcs, further exploration is required to determine the most effective treatments. Here, we examine what is known about repair pathway choice in response to high- versus low-LET irradiation, and we discuss the effects of inhibitors of these pathways when combined with protons and carbon ions. Additionally, we explore the potential effects of DNA repair inhibitors on antitumor immune signaling upon proton and carbon ion irradiation. Due to the reduced effect on healthy tissue and better immune preservation, particle therapy may be particularly well suited for combination with DNA repair inhibitors.

TP53 and the Ultimate Biological Optimization Steps of Curative Radiation Oncology

The new biological interaction cross-section-based repairable-homologically repairable (RHR) damage formulation for radiation-induced cellular inactivation, repair, misrepair, and apoptosis was applied to optimize radiation therapy. This new formulation implies renewed thinking about biologically optimized radiation therapy, suggesting that most TP53 intact normal tissues are low-dose hypersensitive (LDHS) and low-dose apoptotic (LDA). This generates a fractionation window in LDHS normal tissues, indicating that the maximum dose to organs at risk should be ≤2.3 Gy/Fr, preferably of low LET. This calls for biologically optimized treatments using a few high tumor dose-intensity-modulated light ion beams, thereby avoiding secondary cancer risks and generating a real tumor cure without a caspase-3-induced accelerated tumor cell repopulation. Light ions with the lowest possible LET in normal tissues and high LET only in the tumor imply the use of the lightest ions, from lithium to boron. The high microscopic heterogeneity in the tumor will cause local microscopic cold spots; thus, in the last week of curative ion therapy, when there are few remaining viable tumor clonogens randomly spread in the target volume, the patient should preferably receive the last 10 GyE via low LET, ensuring perfect tumor coverage, a high cure probability, and a reduced risk for adverse normal tissue reactions. Interestingly, such an approach would also ensure a steeper rise in tumor cure probability and a higher complication-free cure, as the few remaining clonogens are often fairly well oxygenated, eliminating a shallower tumor response due to inherent ion beam heterogeneity. With the improved fractionation proposal, these approaches may improve the complication-free cure probability by about 10-25% or even more.

Datasets of in vitro clonogenic assays showing low dose hyper-radiosensitivity and induced radioresistance

Low dose hyper-radiosensitivity and induced radioresistance are primarily observed in surviving fractions of cell populations exposed to ionizing radiation, plotted as the function of absorbed dose. Several biophysical models have been developed to quantitatively describe these phenomena. However, there is a lack of raw, openly available experimental data to support the development and validation of quantitative models. The aim of this study was to set up a database of experimental data from the public literature. Using Google Scholar search, 46 publications with 101 datasets on the dose-dependence of surviving fractions, with clear evidence of low dose hyper-radiosensitivity, were identified. Surviving fractions, their uncertainties, and the corresponding absorbed doses were digitized from graphs of the publications. The characteristics of the cell line and the irradiation were also recorded, along with the parameters of the linear-quadratic model and/or the induced repair model if they were provided. The database is available in STORE^DB, and can be used for meta-analysis, for comparison with new experiments, and for development and validation of biophysical models.

Measurement(s)	surviving fraction of cells
Technology Type(s)	optical microscopy
Factor Type(s)	absorbed dose
Sample Characteristic - Organism	Homo sapiens • Chinese hamster • Rattus sp.
Sample Characteristic - Environment	cell culture

Low-Dose-Rate Radiation-Induced Secretion of TGF-β3 Together with an Activator in Small Extracellular Vesicles Modifies Low-Dose Hyper-Radiosensitivity through ALK1 Binding

Hyper-radiosensitivity (HRS) is the increased sensitivity to low doses of ionizing radiation observed in most cell lines. We previously demonstrated that HRS is permanently abolished in cells irradiated at a low dose rate (LDR), in a mechanism dependent on transforming growth factor β3 (TGF-β3). In this study, we aimed to elucidate the activation and receptor binding of TGF-β3 in this mechanism. T-47D cells were pretreated with inhibitors of potential receptors and activators of TGF-β3, along with addition of small extracellular vesicles (sEVs) from LDR primed cells, before their radiosensitivity was assessed by the clonogenic assay. The protein content of sEVs from LDR primed cells was analyzed with mass spectrometry. Our results show that sEVs contain TGF-β3 regardless of priming status, but only sEVs from LDR primed cells remove HRS in reporter cells. Inhibition of the matrix metalloproteinase (MMP) family prevents removal of HRS, suggesting an MMP-dependent activation of TGF-β3 in the LDR primed cells. We demonstrate a functional interaction between TGF-β3 and activin receptor like kinase 1 (ALK1) by showing that TGF-β3 removes HRS through ALK1 binding, independent of ALK5 and TGF-βRII. These results are an important contribution to a more comprehensive understanding of the mechanism behind TGF-β3 mediated removal of HRS.

Hypersensitivity and Induced Radioresistance in Chinese Hamster Cells Exposed to Radiations with Different LET Values

We study the impact of radiation LET on manifestation of HRS/IRR response in Chinese hamster cells ovary cells exposed to radiations used in radiotherapy. Earlier we have investigated this response to carbon ions (455 MeV/amu) in the pristine Bragg curve plateau and behind the Bragg peak, 60Co γ-rays, and 14.5 MeV neutrons. Now we present results of cytogenetic metaphase analysis in plateau-phase CHO-K1 cells irradiated with scanning beam protons (83 MeV) at doses < 1 Gy and additional data for 14.5 MeV neutrons. Dose curves for frequency of total chromosome aberrations (CA, protons), paired fragments (protons, neutrons), aberrant cells (neutrons) had typical HRS/IRR structure: HRS region (up to 0.1 and 0.15 Gy), IRR region (0.1−0.6 Gy and 0.15−0.35 Gy) for protons and neutrons, respectively, and regular dose dependence. Taken together with previous results, the data show that LET increase shifts the HRS upper border (from 0.08−0.1 Gy for γ-rays, protons and plateau carbons to 0.12−0.15 Gy for “tail” carbons and neutrons). The IRR regions shortens (0.52−0.4 γ-rays and protons, 0.25 plateau carbons, 0.2 Gy “tail” carbons and neutrons). CA level of IRR increases by 1.5−2.5 times for carbons as compared to γ-rays and protons. Outside HRS/IRR the yield of CA also enhanced with LET increase. The results obtained for different LET radiations suggest that CHO-K1 cells with G1-like CA manifested the general feature of the HRS/IRR phenomena.

Hydroxychloroquine synergizes with the PI3K inhibitor BKM120 to exhibit antitumor efficacy independent of autophagy

BACKGROUND

The critical role of phosphoinositide 3-kinase (PI3K) activation in tumor cell biology has prompted massive efforts to develop PI3K inhibitors (PI3Kis) for cancer therapy. However, recent results from clinical trials have shown only a modest therapeutic efficacy of single-agent PI3Kis in solid tumors. Targeting autophagy has controversial context-dependent effects in cancer treatment. As a FDA-approved lysosomotropic agent, hydroxychloroquine (HCQ) has been well tested as an autophagy inhibitor in preclinical models. Here, we elucidated the novel mechanism of HCQ alone or in combination with PI3Ki BKM120 in the treatment of cancer.

METHODS

The antitumor effects of HCQ and BKM120 on three different types of tumor cells were assessed by in vitro PrestoBlue assay, colony formation assay and in vivo zebrafish and nude mouse xenograft models. The involved molecular mechanisms were investigated by MDC staining, LC3 puncta formation assay, immunofluorescent assay, flow cytometric analysis of apoptosis and ROS, qRT-PCR, Western blot, comet assay, homologous recombination (HR) assay and immunohistochemical staining.

RESULTS

HCQ significantly sensitized cancer cells to BKM120 in vitro and in vivo. Interestingly, the sensitization mediated by HCQ could not be phenocopied by treatment with other autophagy inhibitors (Spautin-1, 3-MA and bafilomycin A1) or knockdown of the essential autophagy genes Atg5/Atg7, suggesting that the sensitizing effect might be mediated independent of autophagy status. Mechanistically, HCQ induced ROS production and activated the transcription factor NRF2. In contrast, BKM120 prevented the elimination of ROS by inactivation of NRF2, leading to accumulation of DNA damage. In addition, HCQ activated ATM to enhance HR repair, a high-fidelity repair for DNA double-strand breaks (DSBs) in cells, while BKM120 inhibited HR repair by blocking the phosphorylation of ATM and the expression of BRCA1/2 and Rad51.

CONCLUSIONS

Our study revealed that HCQ and BKM120 synergistically increased DSBs in tumor cells and therefore augmented apoptosis, resulting in enhanced antitumor efficacy. Our findings provide a new insight into how HCQ exhibits antitumor efficacy and synergizes with PI3Ki BKM120, and warn that one should consider the "off target" effects of HCQ when used as autophagy inhibitor in the clinical treatment of cancer.

Inhibition of ATM Induces Hypersensitivity to Proton Irradiation by Upregulating Toxic End Joining

Proton Bragg peak irradiation has a higher ionizing density than conventional photon irradiation or the entrance of the proton beam profile. Whether targeting the DNA damage response (DDR) could enhance vulnerability to the distinct pattern of damage induced by proton Bragg peak irradiation is currently unknown. Here, we performed genetic or pharmacologic manipulation of key DDR elements and evaluated DNA damage signaling, DNA repair, and tumor control in cell lines and xenografts treated with the same physical dose across a radiotherapy linear energy transfer spectrum. Radiotherapy consisted of 6 MV photons and the entrance beam or Bragg peak of a 76.8 MeV spot scanning proton beam. More complex DNA double-strand breaks (DSB) induced by Bragg peak proton irradiation preferentially underwent resection and engaged homologous recombination (HR) machinery. Unexpectedly, the ataxia-telangiectasia mutated (ATM) inhibitor, AZD0156, but not an inhibitor of ATM and Rad3-related, rendered cells hypersensitive to more densely ionizing proton Bragg peak irradiation. ATM inhibition blocked resection and shunted more DSBs to processing by toxic ligation through nonhomologous end-joining, whereas loss of DNA ligation via XRCC4 or Lig4 knockdown rescued resection and abolished the enhanced Bragg peak cell killing. Proton Bragg peak monotherapy selectively sensitized cell lines and tumor xenografts with inherent HR defects, and the repair defect induced by ATM inhibitor coadministration showed enhanced efficacy in HR-proficient models. In summary, inherent defects in HR or administration of an ATM inhibitor in HR-proficient tumors selectively enhances the relative biological effectiveness of proton Bragg peak irradiation. SIGNIFICANCE: Coadministration of an ATM inhibitor rewires DNA repair machinery to render cancer cells uniquely hypersensitive to DNA damage induced by the proton Bragg peak, which is characterized by higher density ionization.See related commentary by Nickoloff, p. 3156.

The Role and Mechanism of ATM-Mediated Autophagy in the Transition From Hyper-Radiosensitivity to Induced Radioresistance in Lung Cancer Under Low-Dose Radiation

Objective: This study aimed to investigate the effect of ataxia telangiectasia mutated (ATM)–mediated autophagy on the radiosensitivity of lung cancer cells under low-dose radiation and to further investigate the role of ATM and its specific mechanism in the transition from hyper-radiosensitivity (HRS) to induced radioresistance (IRR).

Methods: The changes in the HRS/IRR phenomenon in A549 and H460 cells were verified by colony formation assay. Changes to ATM phosphorylation and cell autophagy in A549 and H460 cells under different low doses of radiation were examined by western blot, polymerase chain reaction (PCR), and electron microscopy. ATM expression was knocked down by short interfering RNA (siRNA) transfection, and ATM-regulated molecules related to autophagy pathways were screened by transcriptome sequencing analysis. The detection results were verified by PCR and western blot. The differential metabolites were screened by transcriptome sequencing and verified by colony formation assay and western blot. The nude mouse xenograft model was used to verify the results of the cell experiments.

Results: (1) A549 cells with high expression of ATM showed positive HRS/IRR, whereas H460 cells with low expression of ATM showed negative HRS/IRR. After the expression of ATM decreased, the HRS phenomenon in A549 cells increased, and the radiosensitivity of H460 cells also increased. This phenomenon was associated with the increase in the autophagy-related molecules phosphorylated c-Jun N-terminal kinase (p-JNK) and autophagy/Beclin 1 regulator 1 (AMBRA1). (2) DL-Norvaline, a product of carbon metabolism in cells, inhibited autophagy in A549 cells under low-dose radiation. DL-Norvaline increased the expression levels of ATM, JNK, and AMBRA1 in A549 cells. (3) Mouse experiments confirmed the regulatory role of ATM in autophagy and metabolism and its function in HRS/IRR.

Conclusion: ATM may influence autophagy through p-JNK and AMBRA1 to participate in the regulation of the HRS/IRR phenomenon. Autophagy interacts with the cellular carbon metabolite DL-Norvaline to participate in regulating the low-dose radiosensitivity of cells.

Influence of Individual Radiosensitivity on the Hormesis Phenomenon: Toward a Mechanistic Explanation Based on the Nucleoshuttling of ATM Protein

Hormesis is a low-dose phenomenon that has been reported to occur, to different extents, in animals, plants, and microorganisms. However, a review of the literature shows that only a few reports describe it in humans. Also, the diversity of experimental protocols and cellular models used makes deciphering the mechanisms of hormesis difficult. In humans, hormesis mostly appears in the 20 to 75 mGy dose range and in nontransformed, radioresistant cells. In a previous paper by Devic et al, a biological interpretation of the adaptive response (AR) phenomenon was proposed using our model that is based on the radiation-induced nucleoshuttling of the ATM protein (the RIANS model). Here, we showed that the 20 to 75 mGy dose range corresponds to a maximum amount of ATM monomers diffusing into the nucleus, while no DNA double-strand breaks is produced by radiation. These ATM monomers are suggested to help in recognizing and repairing spontaneous DNA breaks accumulated in cells and contribute to reductions in genomic instability and aging. The RIANS model also permitted the biological interpretation of hypersensitivity to low doses (HRS)-another low-dose phenomenon. Hence, for the first time to our knowledge, hormesis, AR, and HRS can be explained using the same unified molecular model.

The Nucleoshuttling of the ATM Protein: A Unified Model to Describe the Individual Response to High- and Low-Dose of Radiation?

The evaluation of radiation-induced (RI) risks is of medical, scientific, and societal interest. However, despite considerable efforts, there is neither consensual mechanistic models nor predictive assays for describing the three major RI effects, namely radiosensitivity, radiosusceptibility, and radiodegeneration. Interestingly, the ataxia telangiectasia mutated (ATM) protein is a major stress response factor involved in the DNA repair and signaling that appears upstream most of pathways involved in the three precited RI effects. The rate of the RI ATM nucleoshuttling (RIANS) was shown to be a good predictor of radiosensitivity. In the frame of the RIANS model, irradiation triggers the monomerization of cytoplasmic ATM dimers, which allows ATM monomers to diffuse in nucleus. The nuclear ATM monomers phosphorylate the H2AX histones, which triggers the recognition of DNA double-strand breaks and their repair. The RIANS model has made it possible to define three subgroups of radiosensitivity and provided a relevant explanation for the radiosensitivity observed in syndromes caused by mutated cytoplasmic proteins. Interestingly, hyper-radiosensitivity to a low dose and adaptive response phenomena may be also explained by the RIANS model. In this review, the relevance of the RIANS model to describe several features of the individual response to radiation was discussed.

DNA breaks induced by iodine-containing contrast medium in radiodiagnostics: a problem of tungsten?

Iodine-containing contrast media (ICM) are extensively used to improve image quality and information content in x-ray-based examinations, particularly in computed tomography (CT). In parallel, there is increasing evidence that the use of ICM during CT sessions is associated with deoxyribonucleic acid (DNA) breaks that may influence the estimation of the risks linked to x-ray exposure. Why has iodine been preferred to any other heavy elements to enhance contrast in radiodiagnostics? How to understand such DNA breaks effect? We searched for the answers in the early times of x-ray medical use. It appeared that the maximal ratio between the relative iodine and water mass energy absorption coefficients is reached in the range of 40–60 keV, which defines the energy range in which the dose is preferentially absorbed by ICM. This range does not correspond to the K-edge of iodine but to that of tungsten, the major component of the x-ray tube anode of CT scanners. At such energy, radiolysis of the ICM produces sodium or potassium iodide that prevents a normal DNA breaks repair and influences the individual response to x-ray low-dose. Both contrast enhancement and DNA breaks effect may therefore be caused by tungsten of the anodes of x-ray tubes.

Influence of Individual Radiosensitivity on the Adaptive Response Phenomenon: Toward a Mechanistic Explanation Based on the Nucleo-Shuttling of ATM Protein

The adaptive response (AR) phenomenon generally describes a protective effect caused by a "priming" low dose (d_AR) delivered after a period of time (Δt_AR) before a higher "challenging" dose (D_AR). The AR is currently observed in human cells if d_AR, Δt_AR, and D_AR belong to (0.001-0.5 Gy), (2-24 hours), (0.1-5 Gy), respectively. In order to investigate the molecular mechanisms specific to AR in human cells, we have systematically reviewed the experimental AR protocols, the cellular models, and the biological endpoints used from the 1980s. The AR appears to be preferentially observed in radiosensitive cells and is strongly dependent on individual radiosensitivity. To date, the model of the nucleo-shuttling of the ATM protein provides a relevant mechanistic explanation of the AR molecular and cellular events. Indeed, the priming dose d_AR may result in the diffusion of a significant amount of active ATM monomers in the nucleus. These ATM monomers, added to those induced directly by the challenging dose D_AR, may increase the efficiency of the response to D_AR by a better ATM-dependent DNA damage recognition. Such mechanistic model would also explain why AR is not observed in radioresistant or hyperradiosensitive cells. Further investigations at low dose are needed to consolidate our hypotheses.

Carcinogenesis Induced by Low-dose Radiation

Background

Although the effects of high dose radiation on human cells and tissues are relatively well defined, there is no consensus regarding the effects of low and very low radiation doses on the organism. Ionizing radiation has been shown to induce gene mutations and chromosome aberrations which are known to be involved in the process of carcinogenesis. The induction of secondary cancers is a challenging long-term side effect in oncologic patients treated with radiation. Medical sources of radiation like intensity modulated radiotherapy used in cancer treatment and computed tomography used in diagnostics, deliver very low doses of radiation to large volumes of healthy tissue, which might contribute to increased cancer rates in long surviving patients and in the general population. Research shows that because of the phenomena characteristic for low dose radiation the risk of cancer induction from exposure of healthy tissues to low dose radiation can be greater than the risk calculated from linear no-threshold model. Epidemiological data collected from radiation workers and atomic bomb survivors confirms that exposure to low dose radiation can contribute to increased cancer risk and also that the risk might correlate with the age at exposure.

Conclusions

Understanding the molecular mechanisms of response to low dose radiation is crucial for the proper evaluation of risks and benefits that stem from these exposures and should be considered in the radiotherapy treatment planning and in determining the allowed occupational exposures.

Biological determinants of radioresistance and their remediation in pancreatic cancer

Despite recent advances in radiotherapy, a majority of patients diagnosed with pancreatic cancer (PC) do not achieve objective responses due to the existence of intrinsic and acquired radioresistance. Identification of molecular mechanisms that compromise the efficacy of radiation therapy and targeting these pathways is paramount for improving radiation response in PC patients. In this review, we have summarized molecular mechanisms associated with the radio-resistant phenotype of PC. Briefly, we discuss the reversible and irreversible biological consequences of radiotherapy, such as DNA damage and DNA repair, mechanisms of cancer cell survival and radiation-induced apoptosis following radiotherapy. We further describe various small molecule inhibitors and molecular targeting agents currently being tested in preclinical and clinical studies as potential radiosensitizers for PC. Notably, we draw attention towards the confounding effects of cancer stem cells, immune system, and the tumor microenvironment in the context of PC radioresistance and radiosensitization. Finally, we discuss the need for examining selective radioprotectors in light of the emerging evidence on radiation toxicity to non-target tissue associated with PC radiotherapy.

Fractionated exposure to low doses of ionizing radiation results in accumulation of DNA damage in mouse spleen tissue and activation of apoptosis in a p53/Atm-independent manner

PURPOSE

While the effects of high doses of ionizing radiation (IR) are relatively well characterized, the molecular mechanisms underlying cellular responses to prolonged exposure to low doses of radiation remain largely under-investigated.

MATERIALS AND METHODS

Here, we addressed the DNA damage and apoptotic response in the spleen tissue of C57BL/6 male mice after fractionated exposure to X-rays within the 0.1-0.5 Gy dose range.

RESULTS

The response to initial exposure to 0.1 Gy of IR was characterized by increased DNA damage and elevated levels of apoptosis. Subsequent exposures (cumulative doses of 0.2 and 0.3 Gy) resulted in adaptive response-like changes, represented as increased proliferation and apoptotic response. Cumulative doses of 0.4 and 0.5 Gy were characterized by accumulation of DNA damage and reactivation of apoptosis and apoptosis-related proteins. Additionally, spleen cells with irreversible damage caused by radiation can undergo apoptosis via activation of p38, which does not necessarily involve the Atm/p53 pathway.

CONCLUSIONS

Fractionated exposure to low doses of X-rays resulted in accumulation of DNA damage in the murine spleen and induction of apoptotic response in p53/Atm-independent manner. Further studies are needed to understand the outcomes and molecular mechanisms underlying cellular responses and early induction of p38 in response to prolonged exposure to IR.

Individual variations in dose response for spatial memory learning among outbred wistar rats exposed from 5 to 20 cGy of (56) Fe particles

Exposures of brain tissue to ionizing radiation can lead to persistent deficits in cognitive functions and behaviors. However, little is known about the quantitative relationships between exposure dose and neurological risks, especially for lower doses and among genetically diverse individuals. We investigated the dose relationship for spatial memory learning among genetically outbred male Wistar rats exposed to graded doses of (56) Fe particles (sham, 5, 10, 15, and 20 cGy; 1 GeV/n). Spatial memory learning was assessed on a Barnes maze using REL3 ratios measured at three months after exposure. Irradiated animals showed dose-dependent declines in spatial memory learning that were fit by a linear regression (P for slope <0.0002). The irradiated animals showed significantly impaired learning at 10 cGy exposures, no detectable learning between 10 and 15 cGy, and worsened performances between 15 and 20 cGy. The proportions of poor learners and the magnitude of their impairment were fit by linear regressions with doubling doses of ∼10 cGy. In contrast, there were no detectable deficits in learning among the good learners in this dose range. Our findings suggest that genetically diverse individuals can vary substantially in their spatial memory learning, and that exposures at low doses appear to preferentially impact poor learners. This hypothesis invites future investigations of the genetic and physiological mechanisms of inter-individual variations in brain function related to spatial memory learning after low-dose HZE radiation exposures and to determine whether it also applies to physical trauma to brain tissue and exposures to chemical neurotoxicants. Environ. Mol. Mutagen. 57:331-340, 2016. © 2016 Wiley Periodicals, Inc.

The influence of non-DNA-targeted effects on carbon ion-induced low-dose hyper-radiosensitivity in MRC-5 cells

Low-dose hyper-radiosensitivity (LDHRS) is a hot topic in normal tissue radiation protection. However, the primary causes for LDHRS still remain unclear. In this study, the impact of non-DNA-targeted effects (NTEs) on high-LET radiation-induced LDHRS was investigated. Human normal lung fibroblast MRC-5 cells were irradiated with high-LET carbon ions, and low-dose biological effects (in terms of various bio-endpoints, including colony formation, DNA damage and micronuclei formation) were detected under conditions with and without gap junctional intercellular communication (GJIC) inhibition. LDHRS was observed when the radiation dose was <0.2 Gy for all bio-endpoints under investigation, but vanished when the GJIC was suppressed. Based on the probability of cells being hit and micro-dose per cell calculation, we deduced that the LDHRS phenomenon came from the combined action of direct hits and NTEs. We concluded that GJIC definitely plays an important role in cytotoxic substance spreading in high-LET carbon ion-induced LDHRS.

ATR signaling cooperates with ATM in the mechanism of low dose hypersensitivity induced by carbon ion beam

Little work has been done on the mechanism of low dose hyper-radiosensitivity (HRS) and later appeared radioresistance (termed induced radioresistance (IRR)) after irradiation with medium and high linear energy transfer (LET) particles. The aim of this study was to find out whether ATR pathway is involved in the mechanism of HRS induced by high LET radiation. GM0639 cells and two ATM deficient/mutant cells, AT5BIVA and AT2KY were irradiated by carbon ion beam. Thymidine block technique was developed to enrich the G2-phase population. Radiation induced early G2/M checkpoint was quantitatively assess with dual-parameter flow cytometry by detecting the cells positive for phospho-histone H3. The involvement of ATR pathway in HRS/IRR response was detected with pretreatment of specific inhibitors prior to carbon ion beam. The link between the early G2/M checkpoint and HRS/IRR under carbon ion beam was first confirmed in GM0639 cells, through the enrichment of cell population in G2-phase or with Aurora kinase inhibitor that attenuates the transition from G2 to M phase. Interestingly, the early G2/M arrest could still be observed in ATM deficient/mutant cells with an effect of ATR signaling, which was discovered to function in an LET-dependent manner, even as low as 0.2Gy for carbon ion radiation. The involvement of ATR pathway in heavy particles induced HRS/IRR was determined with the specific ATR inhibitor in GM0639 cells, which affected the HRS/IRR occurrence similarly as ATM inhibitor. These data demonstrate that ATR pathway may cooperate with ATM in the mechanism of low dose hypersensitivity induced by carbon ion beam.

[The low doses of radiation: Towards a new reading of the risk assessment]

From Hiroshima bomb explosion data, the risk of radiation-induced cancer is significant from 100 mSv for a population considered as uniform and radioresistant. However, the recent radiobiological data bring some new elements that highlight some features that were not taken into account: the individual factor, the dose rate and the repeated dose effect. The objective evaluation of the cancer risk due to doses lower than 100 mSv is conditioned by high levels of measurability and statistical significance. However, it appears that methodological rigor is not systematically applied in all the papers. Furthermore, unclear communication in press often leads to some announcement effects, which does not improve the readability of the issue. This papers aims to better understand the complexity of the low-dose-specific phenomena as a whole, by confronting the recent biological data with epidemiological data.

The complexity of DNA double strand break is a crucial factor for activating ATR signaling pathway for G2/M checkpoint regulation regardless of ATM function

DNA double strand break (DSB) repair pathway choice following ionizing radiation (IR) is currently an appealing research topic, which is still largely unclear. Our recent paper indicated that the complexity of DSBs is a critical factor that enhances DNA end resection. It has been well accepted that the RPA-coated single strand DNA produced by resection is a signaling structure for ATR activation. Therefore, taking advantage of high linear energy transfer (LET) radiation to effectively produce complex DSBs, we investigated how the complexity of DSB influences the function of ATR pathway on the G2/M checkpoint regulation. Human skin fibroblast cells with or without ATM were irradiated with X rays or heavy ion particles, and dual-parameter flow cytometry was used to quantitatively assess the mitotic entry at early period post radiation by detecting the cells positive for phosphor histone H3. In ATM-deficient cells, ATR pathway played a pivotal role and functioned in a dose- and LET-dependent way to regulate the early G2/M arrest even as low as 0.2Gy for heavy ion radiation, which indicated that ATR pathway could be rapidly activated and functioned in an ATM-independent, but DSB complexity-dependent manner following exposure to IR. Furthermore, ATR pathway also functioned more efficiently in ATM-proficient cells to block G2 to M transition at early period of particle radiation exposure. Accordingly, in contrast to ATM inhibitor, ATR inhibitor had a more effective radiosensitizing effect on survival fraction following heavy ion beams as compared with X ray radiation. Taken together, our results reveal that the complexity of DSBs is a crucial factor for the activation of ATR pathway for G2/M checkpoint regulation, and ATM-dependent end resection is not essential for the activation.

Radiosensitivity and Induction of Apoptosis by High LET Carbon Ion Beam and Low LET Gamma Radiation: A Comparative Study

Cancer treatment with high LET heavy ion beam, especially, carbon ion beam ((12)C), is becoming very popular over conventional radiotherapy like low LET gamma or X-ray. Combination of Poly(ADP-ribose) polymerase (PARP) inhibitor with xenotoxic drugs or conventional radiation (gamma or X-ray) is the newer approach for cancer therapy. The aim of our study was to compare the radiosensitivity and induction of apoptosis by high LET (12)C and low LET gamma radiation in HeLa and PARP-1 knocked down cells. We did comet assay to detect DNA breaks, clonogenic survival assay, and cell cycle analysis to measure recovery after DNA damage. We measured apoptotic parameters like nuclear fragmentation and caspase-3 activation. DNA damage, cell killing, and induction of apoptosis were significantly higher for (12)C than gamma radiation in HeLa. Cell killing and apoptosis were further elevated upon knocking down of PARP-1. Both (12)C and gamma induced G2/M arrest although the (12)C had greater effect. Unlike the gamma, (12)C irradiation affects DNA replication as detected by S-phase delay in cell cycle analysis. So, we conclude that high LET (12)C has greater potential over low LET gamma radiation in killing cells and radiosensitization upon PARP-1 inhibition was several folds greater for (12)C than gamma.

Chromatin structure and radiation-induced DNA damage: from structural biology to radiobiology

Genomic DNA in eukaryotic cells is basically divided into chromosomes, each consisting of a single huge nucleosomal fiber. It is now clear that chromatin structure and dynamics play a critical role in all processes involved in DNA metabolism, e.g. replication, transcription, repair and recombination. Radiation is a useful tool to study the biological effects of chromatin alterations. Conversely, radiotherapy and radiodiagnosis raise questions about the influence of chromatin integrity on clinical features and secondary effects. This review focuses on the link between DNA damage and chromatin structure at different scales, showing how a comprehensive multiscale vision is required to understand better the effect of radiations on DNA. Clinical aspects related to high- and low-dose of radiation and chromosomal instability will be discussed. At the same time, we will show that the analysis of the radiation-induced DNA damage distribution provides good insight on chromatin structure. Hence, we argue that chromatin "structuralists" and radiobiological "clinicians" would each benefit from more collaboration with the other. We hope that this focused review will help in this regard.

Is activation of the intra-S checkpoint in human fibroblasts an important factor in protection against UV-induced mutagenesis?

The ATR/CHK1-dependent intra-S checkpoint inhibits replicon initiation and replication fork progression in response to DNA damage caused by UV (UV) radiation. It has been proposed that this signaling cascade protects against UV-induced mutations by reducing the probability that damaged DNA will be replicated before it can be repaired. Normal human fibroblasts (NHF) were depleted of ATR or CHK1, or treated with the CHK1 kinase inhibitor TCS2312, and the UV-induced mutation frequency at the HPRT locus was measured. Despite clear evidence of S-phase checkpoint abrogation, neither ATR/CHK1 depletion nor CHK1 inhibition caused an increase in the UV-induced HPRT mutation frequency. These results question the premise that the UV-induced intra-S checkpoint plays a prominent role in protecting against UV-induced mutagenesis.

Low dose hypersensitivity following in vitro cell irradiation with charged particles: Is the mechanism the same as with X-ray radiation?

PURPOSE

Among the low dose effects that have been discovered during the past decade, the low dose hypersensitivity (HRS) is of prime importance. This phenomenon, compared to irradiation at higher doses used in conventional radiotherapy, enhances cell killing per unit dose at low doses and is followed by an induced radioresistance (IRR) effect. On survival fraction curves, a deviation from the linear quadratic model can be observed. HRS has mainly been studied after irradiation with sparsely ionizing radiation. Little work has been done to check its actual existence after irradiation with medium and high linear energy transfer (LET) particles. This article reviews recent studies involving HRS following irradiation of rodent and human cells with protons, alpha particles and carbon ions and assesses the applicability of a photon HRS model to charged particles.

CONCLUSION

We propose that the HRS threshold dose and the radiosensitive parameter αs may be LET and deoxyribonucleic acid (DNA) damage-clustering dependent. Combining the use of high-LET particles at low doses and chemotherapy strategies increasing the proportion of HRS-sensitive cells could become a good candidate treatment for radioresistant cancers.

Determining if low dose hyper-radiosensitivity (HRS) can be exploited to provide a therapeutic advantage: a cell line study in four glioblastoma multiforme (GBM) cell lines

PURPOSE

To determine if ultra-fractionation using repeated pulses of radiation (10 × 0.2 Gray [Gy]) would be more cytotoxic than continuously-delivered radiation to the same total dose (2 Gy) in four glioma cell lines.

MATERIALS AND METHODS

Human T98G, U373, U87MG and U138MG cells were conventionally X-irradiated with 0.1-8 Gy and clonogenic survival assessed. Next, cells were treated with either a single dose of 2 Gy or 10 pulses of 0.2 Gy using a 3-min inter-pulse interval and DNA (Deoxyribonucleic acid) repair (pHistone H2A.X), G2-phase cell cycle checkpoint arrest (pHistone H3) and apoptosis (caspase-3) compared between the two regimens. A dose of 0.2 Gy was selected as this reflects the hyper- radiosensitivity (HRS)/increased radioresistance (IRR) transition point of the low-dose cell survival curve.

RESULTS

T98G, U87MG and U138MG exhibited distinct HRS responses and survival curves were well-described by the Induced Repair model. Despite the prolonged delivery time, ultra-fractionation (10 × 0.2 Gy) was equally effective as a single continuously-delivered 2 Gy dose. However, ultra-fractionation was more effective when given for five consecutive days to a total dose of 10 Gy. The increased effectiveness of ultra-fractionation could not be attributed directly to differences in DNA damage, repair processes or radiation-induced apoptosis.

CONCLUSIONS

Ultra-fractionation (10 × 0.2 Gy) is an effective modality for killing glioma cell lines compared with standard 2 Gy dosing when multiple days of treatment are given.

Impact of dose-rate on the low-dose hyper-radiosensitivity and induced radioresistance (HRS/IRR) response

PURPOSE

To ask whether dose-rate influences low-dose hyper- radiosensitivity and induced radioresistance (HRS/IRR) response in rat colon progressive (PRO) and regressive (REG) cells.

METHODS

Clonogenic survival was applied to tumorigenic PRO and non-tumorigenic REG cells irradiated with (60)Co γ-rays at 0.0025-500 mGy.min(-1). Both clonogenic survival and non-homologous end-joining (NHEJ) pathway involved in DNA double-strand breaks (DSB) repair assays were applied to PRO cells irradiated at 25 mGy.min(-1) with 75 kV X-rays only.

RESULTS

Irrespective of dose-rates, marked HRS/IRR responses were observed in PRO but not in REG cells. For PRO cells, the doses at which HRS and IRR responses are maximal were dependent on dose-rate; conversely exposure times during which HRS and IRR responses are maximal (t(HRSmax) and t(IRRmax)) were independent of dose-rate. The t(HRSmax) and t(IRRmax) values were 23 ± 5 s and 66 ± 7 s (mean ± standard error of the mean [SEM], n = 7), in agreement with literature data. Repair data show that t(HRSmax) may correspond to exposure time during which NHEJ is deficient while t(IRRmax) may correspond to exposure time during which NHEJ is complete.

CONCLUSION

HRS response may be maximal if exposure times are shorter than t(HRSmax) irrespective of dose, dose-rate and cellular model. Potential application of HRS response in radiotherapy is discussed.

Exposure to low dose ionising radiation: molecular and clinical consequences

This review article provides a comprehensive overview of the experimental data detailing the incidence, mechanism and significance of low dose hyper-radiosensitivity (HRS). Important discoveries gained from past and present studies are mapped and highlighted to illustrate the pathway to our current understanding of HRS and the impact of HRS on the cellular response to radiation in mammalian cells. Particular attention is paid to the balance of evidence suggesting a role for DNA repair processes in the response, evidence suggesting a role for the cell cycle checkpoint processes, and evidence investigating the clinical implications/relevance of the effect.

Caspase 8 and menin expressions are not correlated in human parathyroid tumors

Menin is lost by the sequential inactivation of both MEN1 alleles in subsets of non-hereditary endocrine tumors as well as those associated with multiple endocrine neoplasia type 1 (MEN1), an autosomal dominant hereditary cancer syndrome characterized by multiple tumors including parathyroid, pituitary and enteropancreatic endocrine tumors. Loss of menin has been reported to be associated with lowered caspase 8 expression and resistance to apoptosis in murine fibroblasts and in pancreatic islet tumors arising in heterozygous MEN1 gene knockout mice, the animal model of the human MEN1 syndrome. We confirmed by menin-knockdown experiments with specific siRNA that menin is crucial for caspase 8 expression in human culture cells while overexpression of menin did not increase caspase 8 protein over basal levels. We then examined expression of menin, caspase 8 and cyclin-dependent kinase inhibitors p27(Kip1) and p15(Ink4b) by Western blotting in human parathyroid tumors surgically resected from patients with MEN1 and those with non-hereditary primary hyperparathyroidism. The menin and p27(Kip1) expression levels were correlated with MEN1 mutation status that was confirmed by DNA analysis. The caspase 8 and p15(Ink4b) protein levels were variable among tumors, and were not correlated with menin protein levels. These findings suggest that human endocrine tumors lacking menin may not always exhibit lowered caspase 8 expression and hence may not be resistant to apoptosis-inducing therapy.