Homology-directed repair is required for the development of radioresistance during S phase: interplay between double-strand break repair and checkpoint response

A systematic review and meta-analysis of the genotoxic and cytotoxic effects on oral epithelium induced by cone beam computed tomography

OBJECTIVE

To evaluate the occurrence of genotoxic and cytotoxic effects in oral epithelium after exposure of patients to cone beam computed tomography (CBCT).

METHODS

A systematic review (SR) was conducted following the PECO (Population, Exposure, Comparison, Outcome) criteria. The study was registered in the International Prospective Register of Systematic Reviews (PROSPERO). A search was performed on the PubMed, Scopus, ScienceDirect, and Google Scholar databases. Effect size and heterogeneity of data were evaluated statistically. The Joanna Briggs Institute questionnaire for observational studies was utilized to assess the risk of bias. The GRADE tool was applied for the assessment of the quality of evidence. Begg's funnel plot was used to evaluate publication bias.

RESULTS

In total, 10 full-text articles were included in the SR, with 6 of them in the meta-analysis. The SR showed a significant increase in micronuclei after exposure, with a large effect size of 1.03. For genotoxicity, the tau2 for heterogeneity was 0.96, the chi-squared test for heterogeneity P < .00001, the I2 statistics for random effects was 91%, and the overall effect for Z value was 2.46 (P = .01). The risk of bias was low, the quality of evidence was strong, and publication bias was absent.

CONCLUSION

CBCT can cause genotoxicity in the oral epithelium with a large effect size. The measure of cytotoxicity after CBCT exposure was not possible due to the lack of homogeneity of the included articles.

Radiosensitizing effects of heparinized magnetic iron oxide nanoparticles in colon cancer

The combination of radiation treatment and chemotherapy is currently the standard for management of cancer patients. However, safe doses do not often provide effective therapy, then pre-treated patients are forced to repeat treatment with often already increased tumor resistance to drugs and irradiation. One of the solutions we suggest is to improve primary course of radiation treatment via enhancing radiosensitivity of tumors by magnetic-guided iron oxide nanoparticles (magnetite). We obtained spherical heparinized iron oxide nanoparticles (hIONPs, ∼20 nm), characterized it by TEM, Infrared spectroscopy and DLS. Then hIONPs cytotoxicity was assessed for colon cancer cells (XTT assay) and cellular uptake of nanoparticles was analyzed with X-ray fluorescence. Combination of ionizing radiation (IR) and hIONPs in vitro caused an increase of G2/M arrest of cell cycle, mitotic errors and decrease in survival (compared with samples exposed to IR and hIONPs separately). The promising results were shown for magnetic-guided hIONPs in CT26-grafted BALB/C mice: the combination of intravenously administrated hIONPs and IR showed 20,8% T/C ratio (related to non-treated mice), while single radiation had no shown significant decrease in tumor growth (72,4%). Non-guided by magnets hIONPs with IR showed 57,9% of T/C. This indicates that ultra-small size and biocompatible molecule are not the key to successful nano-drug design, in each case, delivery technologies need to be improved when transferred to in vivo model.

Alpha Particle-Emitting Radiopharmaceuticals as Cancer Therapy: Biological Basis, Current Status, and Future Outlook for Therapeutics Discovery

Critical advances in radionuclide therapy have led to encouraging new options for cancer treatment through the pairing of clinically useful radiation-emitting radionuclides and innovative pharmaceutical discovery. Of the various subatomic particles used in therapeutic radiopharmaceuticals, alpha (α) particles show great promise owing to their relatively large size, delivered energy, finite pathlength, and resulting ionization density. This review discusses the therapeutic benefits of α-emitting radiopharmaceuticals and their pairing with appropriate diagnostics, resulting in innovative "theranostic" platforms. Herein, the current landscape of α particle-emitting radionuclides is described with an emphasis on their use in theranostic development for cancer treatment. Commonly studied radionuclides are introduced and recent efforts towards their production for research and clinical use are described. The growing popularity of these radionuclides is explained through summarizing the biological effects of α radiation on cancer cells, which include DNA damage, activation of discrete cell death programs, and downstream immune responses. Examples of efficient α-theranostic design are described with an emphasis on strategies that lead to cellular internalization and the targeting of proteins involved in therapeutic resistance. Historical barriers to the clinical deployment of α-theranostic radiopharmaceuticals are also discussed. Recent progress towards addressing these challenges is presented along with examples of incorporating α-particle therapy in pharmaceutical platforms that can be easily converted into diagnostic counterparts.

Iron Oxide Nanoparticles in Cancer Treatment: Cell Responses and the Potency to Improve Radiosensitivity

The main concept of radiosensitization is making the tumor tissue more responsive to ionizing radiation, which leads to an increase in the potency of radiation therapy and allows for decreasing radiation dose and the concomitant side effects. Radiosensitization by metal oxide nanoparticles is widely discussed, but the range of mechanisms studied is not sufficiently codified and often does not reflect the ability of nanocarriers to have a specific impact on cells. This review is focused on the magnetic iron oxide nanoparticles while they occupied a special niche among the prospective radiosensitizers due to unique physicochemical characteristics and reactivity. We collected data about the possible molecular mechanisms underlying the radiosensitizing effects of iron oxide nanoparticles (IONPs) and the main approaches to increase their therapeutic efficacy by variable modifications.

Transcriptomic Profiling and Pathway Analysis of Mesenchymal Stem Cells Following Low Dose-Rate Radiation Exposure

Low dose-rate radiation exposure can occur in medical imaging, as background from environmental or industrial radiation, and is a hazard of space travel. In contrast with high dose-rate radiation exposure that can induce acute life-threatening syndromes, chronic low-dose radiation is associated with Chronic Radiation Syndrome (CRS), which can alter environmental sensitivity. Secondary effects of chronic low dose-rate radiation exposure include circulatory, digestive, cardiovascular, and neurological diseases, as well as cancer. Here, we investigated 1-2 Gy, 0.66 cGy/h, ⁶⁰Co radiation effects on primary human mesenchymal stem cells (hMSC). There was no significant induction of apoptosis or DNA damage, and cells continued to proliferate. Gene ontology (GO) analysis of transcriptome changes revealed alterations in pathways related to cellular metabolism (cholesterol, fatty acid, and glucose metabolism), extracellular matrix modification and cell adhesion/migration, and regulation of vasoconstriction and inflammation. Interestingly, there was increased hypoxia signaling and increased activation of pathways regulated by iron deficiency, but Nrf2 and related genes were reduced. The data were validated in hMSC and human lung microvascular endothelial cells using targeted qPCR and Western blotting. Notably absent in the GO analysis were alteration pathways for DNA damage response, cell cycle inhibition, senescence, and pro-inflammatory response that we previously observed for high dose-rate radiation exposure. Our findings suggest that cellular gene transcription response to low dose-rate ionizing radiation is fundamentally different compared to high-dose-rate exposure. We hypothesize that cellular response to hypoxia and iron deficiency are driving processes, upstream of the other pathway regulation.

Comparison of the Medical Uses and Cellular Effects of High and Low Linear Energy Transfer Radiation

Exposure to ionizing radiation can occur during medical treatments, from naturally occurring sources in the environment, or as the result of a nuclear accident or thermonuclear war. The severity of cellular damage from ionizing radiation exposure is dependent upon a number of factors including the absorbed radiation dose of the exposure (energy absorbed per unit mass of the exposure), dose rate, area and volume of tissue exposed, type of radiation (e.g., X-rays, high-energy gamma rays, protons, or neutrons) and linear energy transfer. While the dose, the dose rate, and dose distribution in tissue are aspects of a radiation exposure that can be varied experimentally or in medical treatments, the LET and eV are inherent characteristics of the type of radiation. High-LET radiation deposits a higher concentration of energy in a shorter distance when traversing tissue compared with low-LET radiation. The different biological effects of high and low LET with similar energies have been documented in vivo in animal models and in cultured cells. High-LET results in intense macromolecular damage and more cell death. Findings indicate that while both low- and high-LET radiation activate non-homologous end-joining DNA repair activity, efficient repair of high-LET radiation requires the homologous recombination repair pathway. Low- and high-LET radiation activate p53 transcription factor activity in most cells, but high LET activates NF-kB transcription factor at lower radiation doses than low-LET radiation. Here we review the development, uses, and current understanding of the cellular effects of low- and high-LET radiation exposure.

Induction and assessment of persistent radioresistance in murine leukocytes in vivo

The aim of the present study was to investigate whether weekly exposure to gamma rays causes a persistent increase in the number of radioresistant leukocytes in mice in vivo. Using the comet assay, 1 Gy radiation exposure decreased the percentage of leukocytes with less than 5% DNA in the tail (<5% DNAT), and we propose that radioresistance induction might increase the number of cells with <5% DNAT after radiation exposure. We exposed mice to 1 Gy gamma rays weekly for four weeks or 2 Gy per week for nine weeks. We observed a significant increase in cells with <5% DNAT after the third week and up to nine weeks of exposure. We exposed animals to gradually increasing radiation doses and finally challenged the lymphocytes with 1 Gy radiation both in vivo and in vitro. We observed increased radioresistance in vitro, providing evidence that a cellular process is involved. However, more radioresistance was observed in vivo than in vitro, suggesting a physiological effect. Cells challenged in vitro were maintained on ice during and after exposure, which likely caused a reduction in DNA repair. Radioresistance induction likely arose from mutation selection in stem cells because leukocytes are unable to proliferate in peripheral blood.

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First evidence of cell radioresistance induced in vivo in mice.

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Leukocyte precursor cells in vivo a model for study radioresistance induction.

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Irradiation-division cycles in vivo cause long-lasting cellular radioresistance.

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Increase of <5% DNA at tail after irradiation an index of cell radioresistance.

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Course of radioresistance caused by mutation-selection differ from adaptive response.

Radiation-Induced Heart Disease

Cancer incidence and survivorship have had a rising tendency over the last two decades due to better treatment modalities. One of these is radiation therapy (RT), which is used in 20-55% of cancer patients, and its basic principle consists of inhibiting proliferation or inducing apoptosis of cancer cells. Classically, photon beam RT has been the mainstay therapy for these patients, but, in the last decade, proton beam has been introduced as a new option. This newer method focuses more on the tumor and affects less of the surrounding normal tissue, i.e., the heart. Radiation to the heart is a common complication of RT, especially in patients with lymphoma, breast, lung, and esophageal cancer. The pathophysiology is due to changes in the microvascular and macrovascular milieu that can promote accelerated atherosclerosis and/or induce fibrosis of the myocardium, pericardium, and valves. These complications occur days, weeks, or years after RT and the risk factors associated are high radiation doses (>30 Gy), concomitant chemotherapy (primarily anthracyclines), age, history of heart disease, and the presence of cardiovascular risk factors. The understanding of these mechanisms and risk factors by physicians can lead to a tailored assessment and monitorization of these patients with the objective of early detection or prevention of radiation-induced heart disease. Echocardiography is a noninvasive method which provides a comprehensive evaluation of the pericardium, valves, myocardium, and coronaries, making it the first imaging tool in most cases; however, other modalities, such as computed tomography, nuclear medicine, or cardiac magnetic resonance, can provide additional value.

Transcriptomic profiling and pathway analysis of cultured human lung microvascular endothelial cells following ionizing radiation exposure

The vascular system is sensitive to radiation injury, and vascular damage is believed to play a key role in delayed tissue injury such as pulmonary fibrosis. However, the response of endothelial cells to radiation is not completely understood. We examined the response of primary human lung microvascular endothelial cells (HLMVEC) to 10 Gy (1.15 Gy/min) X-irradiation. HLMVEC underwent senescence (80-85%) with no significant necrosis or apoptosis. Targeted RT-qPCR showed increased expression of genes CDKN1A and MDM2 (10-120 min). Western blotting showed upregulation of p2/waf1, MDM2, ATM, and Akt phosphorylation (15 min-72 h). Low levels of apoptosis at 24-72 h were identified using nuclear morphology. To identify novel pathway regulation, RNA-seq was performed on mRNA using time points from 2 to 24 h post-irradiation. Gene ontology and pathway analysis revealed increased cell cycle inhibition, DNA damage response, pro- and anti- apoptosis, and pro-senescence gene expression. Based on published literature on inflammation and endothelial-to-mesenchymal transition (EndMT) pathway genes, we identified increased expression of pro-inflammatory genes and EndMT-associated genes by 24 h. Together our data reveal a time course of integrated gene expression and protein activation leading from early DNA damage response and cell cycle arrest to senescence, pro-inflammatory gene expression, and endothelial-to-mesenchymal transition.

Expanding roles of cell cycle checkpoint inhibitors in radiation oncology

PURPOSE

Radiation-induced activation of cell cycle checkpoints have been of long-standing interest. The WEE1, CHK1 and ATR kinases are key factors in cell cycle checkpoint regulation and are essential for the S and G2 checkpoints. Here, we review the rationale for why inhibitors of WEE1, CHK1 and ATR could be beneficial in combination with radiation.

CONCLUSIONS

Combined treatment with radiation and inhibitors of these kinases results in checkpoint abrogation and subsequent mitotic catastrophe. This might selectively radiosensitize tumor cells, as they often lack the p53-dependent G1 checkpoint and therefore rely more on the G2 checkpoint to repair DNA damage. Further affecting the repair of radiation damage, inhibition of WEE1, CHK1 or ATR also specifically suppresses the homologous recombination repair pathway. Moreover, inhibition of these kinases can induce massive replication stress during S phase of the cell cycle, likely contributing to eliminate radioresistant S phase cells. Intriguingly, recent findings suggest that cell cycle checkpoint inhibitors in combination with radiation can also enhance anti-tumor immune effects. Altogether, the expanding knowledge about the functional roles of WEE1, CHK1 and ATR inhibitors support that they are promising candidates for use in combination with radiation treatment.

Modern Radiotherapy and Risk of Cardiotoxicity

Despite the advancements of modern radiotherapy, radiation-induced heart disease remains a common cause of morbidity and mortality amongst cancer survivors. This review outlines the basic mechanism, clinical presentation, risk stratification, early detection, possible mitigation, and treatment of this condition.

Homologous recombination enhances radioresistance in hypopharyngeal cancer cell line by targeting DNA damage response

BACKGROUND

Radiotherapy is a central treatment option for hypopharyngeal squamous cell carcinoma, but the prognoses of patients treated with radiotherapy only are not satisfactory due to radioresistance. The underlying molecular mechanisms remain largely elusive, and mechanism-derived predictive markers of radioresistance are currently unavailable.

METHODS

In this study, we first established a specifically radioresistant FaDu cell line by repeated exposure to ionizing radiation with a total dose of 60 Gy (FaDu-RR). The validation of FaDu-RR cells was performed by clonogenic cell survival assay and cell proliferation assay. Microarrays and bioinformatics were analyzed to determine the differentially expressed mRNAs and their functions. DNA-repair capabilities were tested by cell cycle analysis and comet assay. The expressions of four key proteins in homologous recombination pathways, including BRCA1, BRCA2, RPA1, and Rad51, were detected both in FaDu-RR cells and radioresistant xenograft.

RESULTS

We established the specifically radioresistant FaDu cell line. Through microarrays and bioinformatics, homologous recombination pathways were suggested to play important roles in radioresistant mechanisms. High expression levels of key proteins in homologous recombination pathways were then detected both in FaDu-RR cells and radioresistant xenograft. Silencing RPA1 could reduce the radioresistance of FaDu-RR cells.

CONCLUSION

Our results provided strong evidence that homologous recombination enhances the radioresistance in hypopharyngeal carcinoma. Proteins in homologous recombination pathways may be potential biomarkers to predict hypopharyngeal carcinoma response to radiotherapy, establishing a basis for their utility in clinical practice.

Delayed Captopril Administration Mitigates Hematopoietic Injury in a Murine Model of Total Body Irradiation

The increasing potential for accidental radiation exposure from either nuclear accidents or terrorist activities has escalated the need for radiation countermeasure development. We previously showed that a 30-day course of high-dose captopril, an ACE inhibitor, initiated 1–4 h after total body irradiation (TBI), improved Hematopoietic Acute Radiation Syndrome (H-ARS) and increased survival in mice. However, because of the time likely required for the deployment of a stockpiled radiation countermeasure to a radiation mass casualty site, there is a need for therapies that can be administered 24–48 hours after initial exposure. Using C57BL/6 mice exposed to an LD_50-80/30 of ⁶⁰Co TBI (7.75–7.9 Gy, 0.615 Gy/min), we show that low-dose captopril administration, initiated as late as 48 h post-TBI and continued for 14 days, significantly enhanced overall survival similarly to high-dose, rapid administration. Captopril treatment did not affect radiation-induced cell cycle arrest genes or the immediate loss of hematopoietic precursors. Reduced mortality was associated with the recovery of bone marrow cellularity and mature blood cell recovery at 21–30 days post-irradiation. Captopril reduced radiation-induced cytokines EPO, G-CSF, and SAA in the plasma. Finally, delayed captopril administration mitigated brain micro-hemorrhage at 21 days post-irradiation. These data indicate that low dose captopril administered as late as 48 h post-TBI for only two weeks improves survival that is associated with hematopoietic recovery and reduced inflammatory response. These data suggest that captopril may be an ideal countermeasure to mitigate H-ARS following accidental radiation exposure.

Autocrine secretions enhance radioresistance in an exosome‑independent manner in NSCLC cells

Radiotherapy resistance in patient with non‑small cell lung cancer (NSCLC) reduces patient survival and remains a significant challenge for the treatment of NSCLC. Radiation resistance has been demonstrated to be affected by secreted factors, yet it remains unclear how autocrine secretions affect the radioresistance of NSCLC cells. In the present study, the NSCLC cell line, NCI‑H460, was irradiated with γ‑rays (4 Gy) and then cultured in medium from H460 cells or normal medium to examine the potential influence of cell secretions on the radiation resistance of H460 cells. Cell viability, accumulation of reactive oxygen species and DNA repair capacity were all markedly improved in the irradiated H460 cells that were cultured in conditioned medium (CM), compared with those cells cultured in normal medium. In addition, G2/M cell cycle arrest and upregulation of homologous recombination repair proteins were observed in the CM‑treated cells, while exosomes secreted by H460 cells had no influence on the radiation resistance of H460 cells. Taken together, these results indicate that autocrine secretions enhance the radiation resistance of γ‑irradiated H460 cells and that these secretions mainly affect the DNA repair process.

Pharmacology of natural radioprotectors

Radiotherapy is one of the most efficient ways to treat cancer. However, deleterious effects, such as acute and chronic toxicities that reduce the quality of life, may result. Naturally occurring compounds have been shown to be non-toxic over wide dose ranges and are inexpensive and effective. Additionally, pharmacological strategies have been developed that use radioprotectors to inhibit radiation-induced toxicities. Currently available radioprotectors have several limitations, including toxicity. In this review, we present the mechanisms of proven radioprotectors, ranging from free radical scavenging (the best-known mechanism of radioprotection) to molecular-based radioprotection (e.g., upregulating expression of heat shock proteins). Finally, we discuss naturally occurring compounds with radioprotective properties in the context of these mechanisms.

Mechanisms of Endogenous Neuroprotective Effects of Astrocytes in Brain Injury

Astrocytes, once believed to serve only as "glue" for the structural support of neurons, have been demonstrated to serve critical functions for the maintenance and protection of neurons, especially under conditions of acute or chronic injury. There are at least seven distinct mechanisms by which astrocytes protect neurons from damage; these are (1) protection against glutamate toxicity, (2) protection against redox stress, (3) mediation of mitochondrial repair mechanisms, (4) protection against glucose-induced metabolic stress, (5) protection against iron toxicity, (6) modulation of the immune response in the brain, and (7) maintenance of tissue homeostasis in the presence of DNA damage. Astrocytes support these critical functions through specialized responses to stress or toxic conditions. The detoxifying activities of astrocytes are essential for maintenance of the microenvironment surrounding neurons and in whole tissue homeostasis. Improved understanding of the mechanisms by which astrocytes protect the brain could lead to the development of novel targets for the development of neuroprotective strategies.

Evolution of Carbon Ion Radiotherapy at the National Institute of Radiological Sciences in Japan

Charged particles can achieve better dose distribution and higher biological effectiveness compared to photon radiotherapy. Carbon ions are considered an optimal candidate for cancer treatment using particles. The National Institute of Radiological Sciences (NIRS) in Chiba, Japan was the first radiotherapy hospital dedicated for carbon ion treatments in the world. Since its establishment in 1994, the NIRS has pioneered this therapy with more than 69 clinical trials so far, and hundreds of ancillary projects in physics and radiobiology. In this review, we will discuss the evolution of carbon ion radiotherapy at the NIRS and some of the current and future projects in the field.

Scaffold attachment factor A (SAF-A) and Ku temporally regulate repair of radiation-induced clustered genome lesions

Ionizing radiation (IR) induces highly cytotoxic double-strand breaks (DSBs) and also clustered oxidized bases in mammalian genomes. Base excision repair (BER) of bi-stranded oxidized bases could generate additional DSBs as repair intermediates in the vicinity of direct DSBs, leading to loss of DNA fragments. This could be avoided if DSB repair via DNA-PK-mediated nonhomologous end joining (NHEJ) precedes BER initiated by NEIL1 and other DNA glycosylases (DGs). Here we show that DNA-PK subunit Ku inhibits DGs via direct interaction. The scaffold attachment factor (SAF)-A, (also called hnRNP-U), phosphorylated at Ser59 by DNA-PK early after IR treatment, is linked to transient release of chromatin-bound NEIL1, thus preventing BER. SAF-A is subsequently dephosphorylated. Ku inhibition of DGs in vitro is relieved by unphosphorylated SAF-A, but not by the phosphomimetic Asp59 mutant. We thus propose that SAF-A, in concert with Ku, temporally regulates base damage repair in irradiated cell genome.

Carbon Ion Radiotherapy: A Review of Clinical Experiences and Preclinical Research, with an Emphasis on DNA Damage/Repair

Compared to conventional photon-based external beam radiation (PhXRT), carbon ion radiotherapy (CIRT) has superior dose distribution, higher linear energy transfer (LET), and a higher relative biological effectiveness (RBE). This enhanced RBE is driven by a unique DNA damage signature characterized by clustered lesions that overwhelm the DNA repair capacity of malignant cells. These physical and radiobiological characteristics imbue heavy ions with potent tumoricidal capacity, while having the potential for simultaneously maximally sparing normal tissues. Thus, CIRT could potentially be used to treat some of the most difficult to treat tumors, including those that are hypoxic, radio-resistant, or deep-seated. Clinical data, mostly from Japan and Germany, are promising, with favorable oncologic outcomes and acceptable toxicity. In this manuscript, we review the physical and biological rationales for CIRT, with an emphasis on DNA damage and repair, as well as providing a comprehensive overview of the translational and clinical data using CIRT.

Erythropoietin Regulation by Angiotensin II

The renin-angiotensin system (RAS) is a key regulator of blood pressure and blood volume homeostasis. The RAS is primarily comprised of the precursor protein angiotensinogen and the two proteases, renin and angiotensin-converting enzyme (ACE). Angiotensin I (Ang I) is derived from angiotensinogen by renin, but appears to have no biological activity. In contrast, angiotensin II (Ang II) that has a variety of biological functions in the cells is converted from Ang I through removal of two-C-terminal residues by ACE. The physiological effects of Ang II are due to Ang II signaling through specific receptor binding, resulting in muscle contraction leading to increased blood pressure and volume. To modulate RAS, three classes of drugs have been developed: (1) renin inhibitors to prevent angiotensinogen conversion to Ang I, (2) ACE inhibitors, to prevent Ang I processing to Ang II and (3) angiotensin receptor blockers, to inhibit Ang II signaling through its receptor. Studies using the RAS inhibitors and Ang II demonstrated that RAS signaling mediates actions of Ang II in the regulation of proliferation and differentiation of specific hematopoietic cell types, especially in the red blood cell lineage. Accumulating evidence indicates that RAS regulates EPO, an essential mediator of red cell production, for human anemia and erythropoiesis in vivo and in vitro. The regulation of EPO expression by Ang II may be responsible for maintaining red blood cell homeostasis. This review highlights the biological roles of RAS for blood cell and EPO homeostasis through Ang II signaling. The molecular mechanism for Ang II-induced EPO production of the cell or tissue type-specific expression is discussed.

S-phase-specific radiosensitization by gemcitabine for therapeutic carbon ion exposure in vitro

Densely ionizing charged particle irradiation offers physical as well as biological advantages compared with photon irradiation. Radiobiological data for the combination of such particle irradiation (i.e. therapeutic carbon ions) with commonly used chemotherapeutics are still limited. Recent in vitro results indicate a general prevalence of additive cytotoxic effects in combined treatments, but an extension of established multimodal treatment regimens with photons to the inclusion of particle therapy needs to evaluate possible peculiarities of using high linear energy transfer (LET) radiation. The present study investigates the effect of combined radiochemotherapy using gemcitabine and high-LET irradiation with therapeutic carbon ions. In particular, the earlier observation of S-phase specific radiosensitization with photon irradiation should be evaluated with carbon ions. In the absence of the drug gemcitabine, carbon ion irradiation produced the typical survival behavior seen with X-rays-increased relative biological efficiency, and depletion of the survival curve's shoulder. By means of serum deprivation and subsequent replenishment, ∼70% S-phase content of the cell population was achieved, and such preparations showed radioresistance in both treatment arms-,photon and carbon ion irradiation. Combined modality treatment with gemcitabine caused significant reduction of clonogenic survival especially for the S-phase cells. WIDR cells exhibited S-phase-specific radioresistance with high-LET irradiation, although this was less pronounced than for X-ray exposure. The combined treatment with therapeutic carbon ions and gemcitabine caused the resistance phenomenon to disappear phenotypically.

BRCA1 Mutation: A Predictive Marker for Radiation Therapy?

DNA repair, in particular, DNA double-strand break (DSB) repair, is essential for the survival of both normal and cancer cells. An elaborate repair mechanism has been developed in cells to efficiently repair the damaged DNA. The pathways predominately involved in DSB repair are homologous recombination and classic nonhomologous end-joining, although the alternative NHEJ pathway, a third DSB repair pathway, could also be important in certain contexts. The protein of BRCA1 encoded by the tumor suppressor gene BRCA1 regulates all DSB repair pathways. Given that DSBs represent the most biologically significant lesions induced by ionizing radiation and that impaired DSB repair leads to radiation sensitivity, it has been expected that cancer patients with BRCA1 mutations should benefit from radiation therapy. However, the clinical data have been conflicting and inconclusive. We provide an overview about the current status of the data regarding BRCA1 deficiency and radiation therapy sensitivity in both experimental models and clinical investigations. In addition, we discuss a strategy to potentiate the effects of radiation therapy by poly(ADP-ribose) polymerase inhibitors, the pharmacologic drugs being investigated as monotherapy for the treatment of patients with BRCA1/2 mutations.

NUCKS1 is a novel RAD51AP1 paralog important for homologous recombination and genome stability

NUCKS1 (nuclear casein kinase and cyclin-dependent kinase substrate 1) is a 27 kD chromosomal, vertebrate-specific protein, for which limited functional data exist. Here, we demonstrate that NUCKS1 shares extensive sequence homology with RAD51AP1 (RAD51 associated protein 1), suggesting that these two proteins are paralogs. Similar to the phenotypic effects of RAD51AP1 knockdown, we find that depletion of NUCKS1 in human cells impairs DNA repair by homologous recombination (HR) and chromosome stability. Depletion of NUCKS1 also results in greatly increased cellular sensitivity to mitomycin C (MMC), and in increased levels of spontaneous and MMC-induced chromatid breaks. NUCKS1 is critical to maintaining wild type HR capacity, and, as observed for a number of proteins involved in the HR pathway, functional loss of NUCKS1 leads to a slow down in DNA replication fork progression with a concomitant increase in the utilization of new replication origins. Interestingly, recombinant NUCKS1 shares the same DNA binding preference as RAD51AP1, but binds to DNA with reduced affinity when compared to RAD51AP1. Our results show that NUCKS1 is a chromatin-associated protein with a role in the DNA damage response and in HR, a DNA repair pathway critical for tumor suppression.

Impact of Charged Particle Exposure on Homologous DNA Double-Strand Break Repair in Human Blood-Derived Cells

Ionizing radiation generates DNA double-strand breaks (DSB) which, unless faithfully repaired, can generate chromosomal rearrangements in hematopoietic stem and/or progenitor cells (HSPC), potentially priming the cells towards a leukemic phenotype. Using an enhanced green fluorescent protein (EGFP)-based reporter system, we recently identified differences in the removal of enzyme-mediated DSB in human HSPC versus mature peripheral blood lymphocytes (PBL), particularly regarding homologous DSB repair (HR). Assessment of chromosomal breaks via premature chromosome condensation or γH2AX foci indicated similar efficiency and kinetics of radiation-induced DSB formation and rejoining in PBL and HSPC. Prolonged persistence of chromosomal breaks was observed for higher LET charged particles which are known to induce more complex DNA damage compared to X-rays. Consistent with HR deficiency in HSPC observed in our previous study, we noticed here pronounced focal accumulation of 53BP1 after X-ray and carbon ion exposure (intermediate LET) in HSPC versus PBL. For higher LET, 53BP1 foci kinetics was similarly delayed in PBL and HSPC suggesting similar failure to repair complex DNA damage. Data obtained with plasmid reporter systems revealed a dose- and LET-dependent HR increase after X-ray, carbon ion and higher LET exposure, particularly in HR-proficient immortalized and primary lymphocytes, confirming preferential use of conservative HR in PBL for intermediate LET damage repair. HR measured adjacent to the leukemia-associated MLL breakpoint cluster sequence in reporter lines revealed dose dependency of potentially leukemogenic rearrangements underscoring the risk of leukemia-induction by radiation treatment.

Irradiation Can Selectively Kill Tumor Cells while Preserving Erythrocyte Viability in a Co-Culture System

An understanding of how to safely apply intraoperative blood salvage (IBS) in cancer surgery has not yet been obtained. Here, we investigated the optimal dose of ¹³⁷Cs gamma-ray irradiation for killing human hepatocarcinoma (HepG2), gastrocarcinoma (SGC7901), and colonic carcinoma (SW620) tumor cells while preserving co-cultured erythrocytes obtained from 14 healthy adult volunteers. HepG2, SGC7901, or SW620 cells were mixed into the aliquots of erythrocytes. After the mixed cells were treated with ¹³⁷Cs gamma-ray irradiation (30, 50, and 100 Gy), tumor cells and erythrocytes were separated by density gradient centrifugation in Percoll with a density of 1.063 g/ml. The viability, clonogenicity, DNA synthesis, tumorigenicity, and apoptosis of the tumor cells were determined by MTT assay, plate colony formation, 5-ethynyl-2'-deoxyuridine (EdU) incorporation, subcutaneous xenograft implantation into immunocompromised mice, and annexin V/7-AAD staining, respectively. The ATP concentration, 2,3-DPG level, free Hb concentration, osmotic fragility, membrane phosphatidylserine externalization, blood gas variables, reactive oxygen species levels, and superoxide dismutase levels in erythrocytes were analyzed. We found that ¹³⁷Cs gamma-ray irradiation at 50 Gy effectively inhibited the viability, proliferation, and tumorigenicity of HepG2, SGC7901, and SW620 cells without markedly damaging the oxygen-carrying ability or membrane integrity or increasing the oxidative stress of erythrocytes in vitro. These results demonstrated that 50 Gy irradiation in a standard ¹³⁷Cs blood irradiator might be a safe and effective method of inactivating HepG2, SGC7901, and SW620 cells mixed with erythrocytes, which might help to safely allow IBS in cancer surgery.

New approaches to radiation protection

Radioprotectors are compounds that protect against radiation injury when given prior to radiation exposure. Mitigators can protect against radiation injury when given after exposure but before symptoms appear. Radioprotectors and mitigators can potentially improve the outcomes of radiotherapy for cancer treatment by allowing higher doses of radiation and/or reduced damage to normal tissues. Such compounds can also potentially counteract the effects of accidental exposure to radiation or deliberate exposure (e.g., nuclear reactor meltdown, dirty bomb, or nuclear bomb explosion); hence they are called radiation countermeasures. Here, we will review the general principles of radiation injury and protection and describe selected examples of radioprotectors/mitigators ranging from small-molecules to proteins to cell-based treatments. We will emphasize agents that are in more advanced stages of development.

Protective role of RAD50 on chromatin bridges during abnormal cytokinesis

Faithful chromosome segregation is required for preserving genomic integrity. Failure of this process may entail chromatin bridges preventing normal cytokinesis. To test whether RAD50, a protein normally involved in DNA double-strand break repair, is involved in abnormal cytokinesis and formation of chromatin bridges, we used immunocytochemical and protein interaction assays. RAD50 localizes to chromatin bridges during aberrant cytokinesis and subsequent stages of the cell cycle, either decorating the entire bridge or focally accumulating at the midbody zone. Ionizing radiation led to an ∼4-fold increase in the rate of chromatin bridges in an ataxia telangiectatica mutated (ATM)-dependent manner in human RAD50-proficient fibroblasts but not in RAD50-deficient cells. Cells with a RAD50-positive chromatin bridge were able to continue cell cycling and to progress through S phase (44%), whereas RAD50 knockdown caused a deficiency in chromatin bridges as well as an ∼4-fold prolonged duration of mitosis. RAD50 colocalized and directly interacted with Aurora B kinase and phospho-histone H3, and Aurora B kinase inhibition led to a deficiency in RAD50-positive bridges. Based on these observations, we propose that RAD50 is a crucial factor for the stabilization and shielding of chromatin bridges. Our study provides evidence for a hitherto unknown role of RAD50 in abnormal cytokinesis.

Mechanisms of radiation toxicity in transformed and non-transformed cells

Radiation damage to biological systems is determined by the type of radiation, the total dosage of exposure, the dose rate, and the region of the body exposed. Three modes of cell death—necrosis, apoptosis, and autophagy—as well as accelerated senescence have been demonstrated to occur in vitro and in vivo in response to radiation in cancer cells as well as in normal cells. The basis for cellular selection for each mode depends on various factors including the specific cell type involved, the dose of radiation absorbed by the cell, and whether it is proliferating and/or transformed. Here we review the signaling mechanisms activated by radiation for the induction of toxicity in transformed and normal cells. Understanding the molecular mechanisms of radiation toxicity is critical for the development of radiation countermeasures as well as for the improvement of clinical radiation in cancer treatment.

Enhanced hematopoietic protection from radiation by the combination of genistein and captopril

The hematopoietic system is sensitive to radiation injury, and mortality can occur due to blood cell deficiency and stem cell loss. Genistein and the angiotensin converting enzyme (ACE) inhibitor captopril are two agents shown to protect the hematopoietic system from radiation injury. In this study we examined the combination of genistein with captopril for reduction of radiation-induced mortality from hematopoietic damage and the mechanisms of radiation protection. C57BL/6J mice were exposed to 8.25Gy (60)Co total body irradiation (TBI) to evaluate the effects of genistein and captopril alone and in combination on survival, blood cell recovery, hematopoietic progenitor cell recovery, DNA damage, and erythropoietin production. 8.25Gy TBI resulted in 0% survival after 30days in untreated mice. A single subcutaneous injection of genistein administered 24h before TBI resulted in 72% survival. Administration of captopril in the drinking water, from 1h through 30days postirradiation, increased survival to 55%. Genistein plus captopril increased survival to 95%. Enhanced survival was reflected in a reduction of radiation-induced anemia, improved recovery of nucleated bone marrow cells, splenocytes and circulating red blood cells. The drug combination enhanced early recovery of marrow progenitors: erythroid (CFU-E and BFU-E), and myeloid (CFU-GEMM, CFU-GM and CFU-M). Genistein alone and genistein plus captopril protected hematopoietic progenitor cells from radiation-induced micronuclei, while captopril had no effect. Captopril alone and genistein plus captopril, but not genistein alone, suppressed radiation-induced erythropoietin production. These data suggest that genistein and captopril protect the hematopoietic system from radiation injury via independent mechanisms.

Homologous recombination mediates S-phase-dependent radioresistance in cells deficient in DNA polymerase eta

DNA polymerase eta (pol η) is the only DNA polymerase causally linked to carcinogenesis in humans. Inherited deficiency of pol η in the variant form of xeroderma pigmentosum (XPV) predisposes to UV-light-induced skin cancer. Pol η-deficient cells demonstrate increased sensitivity to cisplatin and oxaliplatin chemotherapy. We have found that XP30R0 fibroblasts derived from a patient with XPV are more resistant to cell kill by ionising radiation (IR) than the same cells complemented with wild-type pol η. This phenomenon has been confirmed in Burkitt's lymphoma cells, which either expressed wild-type pol η or harboured a pol η deletion. Pol η deficiency was associated with accumulation of cells in S-phase, which persisted after IR. Cells deficient in pol η demonstrated increased homologous recombination (HR)-directed repair of double strand breaks created by IR. Depletion of the HR protein, X-ray repair cross-complementing protein 3 (XRCC3), abrogated the radioresistance observed in pol η-deficient cells as compared with pol η-complemented cells. These findings suggest that HR mediates S-phase-dependent radioresistance associated with pol η deficiency. We propose that pol η protein levels in tumours may potentially be used to identify patients who require treatment with chemo-radiotherapy rather than radiotherapy alone for adequate tumour control.

Recognition, signaling, and repair of DNA double-strand breaks produced by ionizing radiation in mammalian cells: the molecular choreography

The faithful maintenance of chromosome continuity in human cells during DNA replication and repair is critical for preventing the conversion of normal diploid cells to an oncogenic state. The evolution of higher eukaryotic cells endowed them with a large genetic investment in the molecular machinery that ensures chromosome stability. In mammalian and other vertebrate cells, the elimination of double-strand breaks with minimal nucleotide sequence change involves the spatiotemporal orchestration of a seemingly endless number of proteins ranging in their action from the nucleotide level to nucleosome organization and chromosome architecture. DNA DSBs trigger a myriad of post-translational modifications that alter catalytic activities and the specificity of protein interactions: phosphorylation, acetylation, methylation, ubiquitylation, and SUMOylation, followed by the reversal of these changes as repair is completed. "Superfluous" protein recruitment to damage sites, functional redundancy, and alternative pathways ensure that DSB repair is extremely efficient, both quantitatively and qualitatively. This review strives to integrate the information about the molecular mechanisms of DSB repair that has emerged over the last two decades with a focus on DSBs produced by the prototype agent ionizing radiation (IR). The exponential growth of molecular studies, heavily driven by RNA knockdown technology, now reveals an outline of how many key protein players in genome stability and cancer biology perform their interwoven tasks, e.g. ATM, ATR, DNA-PK, Chk1, Chk2, PARP1/2/3, 53BP1, BRCA1, BRCA2, BLM, RAD51, and the MRE11-RAD50-NBS1 complex. Thus, the nature of the intricate coordination of repair processes with cell cycle progression is becoming apparent. This review also links molecular abnormalities to cellular pathology as much a possible and provides a framework of temporal relationships.

Genetics and genomics of radiotherapy toxicity: towards prediction

Radiotherapy is involved in many curative treatments of cancer; millions of survivors live with the consequences of treatment, and toxicity in a minority limits the radiation doses that can be safely prescribed to the majority. Radiogenomics is the whole genome application of radiogenetics, which studies the influence of genetic variation on radiation response. Work in the area focuses on uncovering the underlying genetic causes of individual variation in sensitivity to radiation, which is important for effective, safe treatment. In this review, we highlight recent advances in radiotherapy and discuss results from four genome-wide studies of radiotoxicity.

The role of homologous recombination in radiation-induced double-strand break repair

DNA double-strand breaks (DSBs) represent the most biologically significant lesions induced by ionizing radiation (IR). HR is the predominant pathway for repairing one-ended DSBs arising in S-phase when the replication fork encounters single-stranded breaks or base damages. Here, we discuss recent findings that two-ended DSBs directly induced by X- or γ-rays in late S- or G2-phase are repaired predominantly by NHEJ, with HR only repairing a sub-fraction of such DSBs. This sub-fraction represents DSBs which localize to heterochromatic DNA regions and, which in control cells, are repaired with slow kinetics over many hours post irradiation. The observation that defined DSB populations are repaired by either NHEJ or HR suggests an assignment of specific tasks for each of the two processes. Furthermore, heavy ion induced complex DSBs, which are in general more slowly repaired than X- or γ-ray induced breaks, are nearly always repaired by HR independent of chromatin localization suggesting that the speed of repair is an important factor determining the DSB repair pathway usage. Finally, NHEJ and HR can, under certain conditions, also compensate for each other such that DSBs normally repaired by one pathway can undergo repair by the other if genetic failures necessitate the pathway switch.

Oligo/polynucleotide-based gene modification: strategies and therapeutic potential

Oligonucleotide- and polynucleotide-based gene modification strategies were developed as an alternative to transgene-based and classical gene targeting-based gene therapy approaches for treatment of genetic disorders. Unlike the transgene-based strategies, oligo/polynucleotide gene targeting approaches maintain gene integrity and the relationship between the protein coding and gene-specific regulatory sequences. Oligo/polynucleotide-based gene modification also has several advantages over classical vector-based homologous recombination approaches. These include essentially complete homology to the target sequence and the potential to rapidly engineer patient-specific oligo/polynucleotide gene modification reagents. Several oligo/polynucleotide-based approaches have been shown to successfully mediate sequence-specific modification of genomic DNA in mammalian cells. The strategies involve the use of polynucleotide small DNA fragments, triplex-forming oligonucleotides, and single-stranded oligodeoxynucleotides to mediate homologous exchange. The primary focus of this review will be on the mechanistic aspects of the small fragment homologous replacement, triplex-forming oligonucleotide-mediated, and single-stranded oligodeoxynucleotide-mediated gene modification strategies as it relates to their therapeutic potential.

Hepatocyte growth factor in lung repair and pulmonary fibrosis

Pulmonary remodeling is characterized by the permanent and progressive loss of the normal alveolar architecture, especially the loss of alveolar epithelial and endothelial cells, persistent proliferation of activated fibroblasts, or myofibroblasts, and alteration of extracellular matrix. Hepatocyte growth factor (HGF) is a pleiotropic factor, which induces cellular motility, survival, proliferation, and morphogenesis, depending upon the cell type. In the adult, HGF has been demonstrated to play a critical role in tissue repair, including in the lung. Administration of HGF protein or ectopic expression of HGF has been demonstrated in animal models of pulmonary fibrosis to induce normal tissue repair and to prevent fibrotic remodeling. HGF-induced inhibition of fibrotic remodeling may occur via multiple direct and indirect mechanisms including the induction of cell survival and proliferation of pulmonary epithelial and endothelial cells, and the reduction of myofibroblast accumulation.

Mechanism of cluster DNA damage repair in response to high-atomic number and energy particles radiation

Low-linear energy transfer (LET) radiation (i.e., γ- and X-rays) induces DNA double-strand breaks (DSBs) that are rapidly repaired (rejoined). In contrast, DNA damage induced by the dense ionizing track of high-atomic number and energy (HZE) particles is slowly repaired or is irreparable. These unrepaired and/or misrepaired DNA lesions may contribute to the observed higher relative biological effectiveness for cell killing, chromosomal aberrations, mutagenesis, and carcinogenesis in HZE particle irradiated cells compared to those treated with low-LET radiation. The types of DNA lesions induced by HZE particles have been characterized in vitro and usually consist of two or more closely spaced strand breaks, abasic sites, or oxidized bases on opposing strands. It is unclear why these lesions are difficult to repair. In this review, we highlight the potential of a new technology allowing direct visualization of different types of DNA lesions in human cells and document the emerging significance of live-cell imaging for elucidation of the spatio-temporal characterization of complex DNA damage. We focus on the recent insights into the molecular pathways that participate in the repair of HZE particle-induced DSBs. We also discuss recent advances in our understanding of how different end-processing nucleases aid in repair of DSBs with complicated ends generated by HZE particles. Understanding the mechanism underlying the repair of DNA damage induced by HZE particles will have important implications for estimating the risks to human health associated with HZE particle exposure.

Differential role of DNA-PKcs phosphorylations and kinase activity in radiosensitivity and chromosomal instability

The catalytic subunit of DNA-dependent protein kinase (DNA-PKcs) is the key functional element in the DNA-PK complex that drives nonhomologous end joining (NHEJ), the predominant DNA double-strand break (DSB) repair mechanism operating to rejoin such breaks in mammalian cells after exposure to ionizing radiation. It has been reported that DNA-PKcs phosphorylation and kinase activity are critical determinants of radiosensitivity, based on responses reported after irradiation of asynchronously dividing populations of various mutant cell lines. In the present study, the relative radiosensitivity to cell killing as well as chromosomal instability of 13 DNA-PKcs site-directed mutant cell lines (defective at phosphorylation sites or kinase activity) were examined after exposure of synchronized G(1) cells to (137)Cs γ rays. DNA-PKcs mutant cells defective in phosphorylation at multiple sites within the T2609 cluster or within the PI3K domain displayed extreme radiosensitivity. Cells defective at the S2056 cluster or T2609 single site alone were only mildly radiosensitive, but cells defective at even one site in both the S2056 and T2609 clusters were maximally radiosensitive. Thus a synergism between the capacity for phosphorylation at the S2056 and T2609 clusters was found to be critical for induction of radiosensitivity.

Influence of homologous recombinational repair on cell survival and chromosomal aberration induction during the cell cycle in gamma-irradiated CHO cells

The repair of DNA double-strand breaks (DSBs) by homologous recombinational repair (HRR) underlies the high radioresistance and low mutability observed in S-phase mammalian cells. To evaluate the contributions of HRR and non-homologous end-joining (NHEJ) to overall DSB repair capacity throughout the cell cycle after gamma-irradiation, we compared HRR-deficient RAD51D-knockout 51D1 to CgRAD51D-complemented 51D1 (51D1.3) CHO cells for survival and chromosomal aberrations (CAs). Asynchronous cultures were irradiated with 150 or 300cGy and separated by cell size using centrifugal elutriation. Cell survival of each synchronous fraction ( approximately 20 fractions total from early G1 to late G2/M) was measured by colony formation. 51D1.3 cells were most resistant in S, while 51D1 cells were most resistant in early G1 (with survival and chromosome-type CA levels similar to 51D1.3) and became progressively more sensitive throughout S and G2. Both cell lines experienced significantly reduced survival from late S into G2. Metaphases were collected from every third elutriation fraction at the first post-irradiation mitosis and scored for CAs. 51D1 cells irradiated in S and G2 had approximately 2-fold higher chromatid-type CAs and a remarkable approximately 25-fold higher level of complex chromatid-type exchanges compared to 51D1.3 cells. Complex exchanges in 51D1.3 cells were only observed in G2. These results show an essential role for HRR in preventing gross chromosomal rearrangements in proliferating cells and, with our previous report of reduced survival of G2-phase NHEJ-deficient prkdc CHO cells [Hinz et al., DNA Repair 4, 782-792, 2005], imply reduced activity/efficiency of both HRR and NHEJ as cells transition from S to G2.

Expression of (NES-)hTERT in cancer cells delays cell cycle progression and increases sensitivity to genotoxic stress

Telomerase is a reverse transcriptase associated with cellular immortality through telomere maintenance. This enzyme is activated in 90% of human cancers, and inhibitors of telomerase are currently in clinical trials to counteract tumor growth. Many aspects of telomerase biology have been investigated for therapy, particularly inhibition of the enzyme, but little was done regarding its subcellular shuttling. We have recently shown that mutations in the nuclear export signal of hTERT, the catalytic component of telomerase, led to a mutant ((NES-)hTERT) that failed to immortalize cells despite nuclear localization and catalytic activity. Expression of (NES-)hTERT in primary fibroblast resulted in telomere-based premature senescence and mitochondrial dysfunction. Here we show that expression of (NES-)hTERT in LNCaP, SQ20B and HeLa cells rapidly and significantly decreases their proliferation rate and ability to form colonies in soft agar while not interfering with endogenous telomerase activity. The cancer cells showed increased DNA damage at telomeric and extra-telomeric sites, and became sensitive to ionizing radiation and hydrogen peroxide exposures. Our data show that expression of (NES-)hTERT efficiently counteracts cancer cell growth in vitro in at least two different ways, and suggest manipulation with the NES of hTERT or its subcellular shuttling as a new strategy for cancer treatment.

Homologous recombination in cancer development, treatment and development of drug resistance

Although DNA double-strand breaks (DSBs) are substrates for homologous recombination (HR) repair, it is becoming apparent that DNA lesions produced at replication forks, for instance by many anticancer drugs, are more significant substrates for HR repair. Cells defective in HR are hypersensitive to a wide variety of anticancer drugs, including those that do not produce DSBs. Several cancers have mutations in or epigenetically silenced HR genes, which explain the genetic instability that drives cancer development. There are an increasing number of reports suggesting that mutation or epigenetic silencing of HR genes explains the sensitivity of cancers to current chemotherapy treatments. Furthermore, there are also many examples of re-expression of HR genes in tumours to explain drug resistance. Emerging data suggest that there are several different subpathways of HR, which can compensate for each other. Unravelling the overlapping pathways in HR showed that BRCA1- and BRCA2-defective cells rely on the PARP protein for survival. This synthetic lethal interaction is now being exploited for selective treatment of BRCA1- and BRCA2-defective cancers with PARP inhibitors. Here, I discuss the diversity of HR and how it impacts on cancer with a particular focus on how HR can be exploited in future anticancer strategies.

Timing of captopril administration determines radiation protection or radiation sensitization in a murine model of total body irradiation

OBJECTIVE

Angiotensin II (Ang II), a potent vasoconstrictor, affects the growth and development of hematopoietic cells. Mixed findings have been reported for the effects of angiotensin-converting enzyme (ACE) inhibitors on radiation-induced injury to the hematopoietic system. We investigated the consequences of different regimens of the ACE inhibitor captopril on radiation-induced hematopoietic injury.

MATERIALS AND METHODS

C57BL/6 mice were either sham-irradiated or exposed to (60)Co total body irradiation (0.6 Gy/min). Captopril was provided in the water for different time periods relative to irradiation.

RESULTS

In untreated mice, the survival rate from 7.5 Gy was 50% at 30 days postirradiation. Captopril treatment for 7 days prior to irradiation resulted in radiosensitization with 100% lethality and a rapid decline in mature blood cells. In contrast, captopril treatment beginning 1 hour postirradiation and continuing for 30 days resulted in 100% survival, with improved recovery of mature blood cells and multilineage hematopoietic progenitors. In nonirradiated control mice, captopril biphasically modulated Lin(-) marrow progenitor cell cycling. After 2 days, captopril suppressed G(0)-G(1) transition and a greater number of cells entered a quiescent state. However, after 7 days of captopril treatment Lin(-) progenitor cell cycling increased compared to untreated control mice.

CONCLUSION

These findings suggest that ACE inhibition affects hematopoietic recovery following radiation by modulating the hematopoietic progenitor cell cycle. The timing of captopril treatment relative to radiation exposure differentially affects the viability and repopulation capacity of spared hematopoietic stem cells and, therefore, can result in either radiation protection or radiation sensitization.

Homologous recombination contributes to the repair of DNA double-strand breaks induced by high-energy iron ions

To test the contribution of homologous recombinational repair (HRR) in repairing DNA damage sites induced by high-energy iron ions, we used (1) HRR-deficient rodent cells carrying a deletion in the RAD51D gene and (2) syngeneic human cells impaired for HRR by RAD51D or RAD51 knockdown using RNA interference. We found that in response to exposure to iron ions, HRR contributed to cell survival in rodent cells and that HRR deficiency abrogated RAD51 focus formation. Complementation of the HRR defect by human RAD51D rescues both enhanced cytotoxicity and RAD51 focus formation. For human cells irradiated with iron ions, cell survival was decreased, and in p53 mutant cells, the levels of mutagenesis were increased when HRR was impaired. Human cells synchronized in S phase exhibited a more pronounced resistance to iron ions compared with cells in G(1) phase, and this increase in radioresistance was diminished by RAD51 knockdown. These results indicate a role for RAD51-mediated DNA repair (i.e. HRR) in removing a fraction of clustered lesions induced by charged-particle radiation. Our results are the first to directly show the requirement for an intact HRR pathway in human cells in ensuring DNA repair and cell survival after exposure to high-energy high-LET radiation.