Discovery of novel DNA-damaging agents through phenotypic screening for DNA double-strand break.

DNA double-strand breaks (DSBs) seriously damage DNA and promote genomic instability that can lead to cell death. They are the source of conditions such as carcinogenesis and aging, but also have important applications in cancer therapy. Therefore, rapid detection and quantification of DSBs in cells are necessary for identifying carcinogenic and anticancer factors. In this study, we detected DSBs using a flow cytometry-based high-throughput method to analyze γH2AX intensity. We screened a chemical library containing 9600 compounds and detected multiple DNA-damaging compounds, although we could not identify mechanisms of action through this procedure. Thus, we also profiled a representative compound with the highest DSB potential, DNA-damaging agent-1 (DDA-1), using a bioinformatics-based method we termed "molecular profiling." Prediction and verification analysis revealed DDA-1 as a potential inhibitor of topoisomerase IIα, different from known inhibitors such as etoposide and doxorubicin. Additional investigation of DDA-1 analogs and xenograft models suggested that DDA-1 is a potential anticancer drug. In conclusion, our findings established that combining high-throughput DSB detection and molecular profiling to undertake phenotypic analysis is a viable method for efficient identification of novel DNA-damaging compounds for clinical applications.

Functional evaluation of BRCA1/2 variants of unknown significance with homologous recombination assay and integrative in silico prediction model.

Numerous variants of unknown significance (VUSs) exist in hereditary breast and ovarian cancers. Although multiple methods have been developed to assess the significance of BRCA1/2 variants, functional discrepancies among these approaches remain. Therefore, a comprehensive functional evaluation system for these variants should be established. We performed conventional homologous recombination (HR) assays for 50 BRCA1 and 108 BRCA2 VUSs and complementarily predicted VUSs using a statistical logistic regression prediction model that integrated six in silico functional prediction tools. BRCA1/2 VUSs were classified according to the results of the integrative in vitro and in silico analyses. Using HR assays, we identified 10 BRCA1 and 4 BRCA2 VUSs as low-functional pathogenic variants. For in silico prediction, the statistical prediction model showed high accuracy for both BRCA1 and BRCA2 compared with each in silico prediction tool individually and predicted nine BRCA1 and seven BRCA2 variants to be pathogenic. Integrative functional evaluation in this study and the American College of Medical Genetics and Genomics and the Association for Molecular Pathology (ACMG/AMP) guidelines strongly suggested that seven BRCA1 variants (p.Glu272Gly, p.Lys1095Glu, p.Val1653Leu, p.Thr1681Pro, p.Phe1761Val, p.Thr1773Ile, and p.Gly1803Ser) and four BRCA2 variants (p.Trp31Gly, p.Ser2616Phe, p.Tyr2660Cys, and p.Leu2792Arg) were pathogenic. This study demonstrates that integrative evaluation using conventional HR assays and optimized in silico prediction comprehensively classified the significance of BRCA VUSs for future clinical applications.

CDK1 inhibitor controls G2/M phase transition and reverses DNA damage sensitivity.

CDK1 plays key roles in cell cycle progression through the G2/M phase transition and activation of homologous recombination (HR) DNA repair pathway. Accordingly, various CDK1 inhibitors have been developed for cancer therapy that induce prolonged G2 arrest and/or sensitize cells to DNA damaging agents in tumor cells, resulting in cell death. However, CDK1 inhibition can induce resistance to DNA damage in certain conditions. The mechanism of different DNA damage sensitivity is not completely understood. We performed immunofluorescence and flow cytometry analysis to investigate DNA damage responses in human tumor cells during low and high dose treatments with RO-3306, a selective CDK1 inhibitor. This comparative investigation demonstrated that RO-3306-induced G2 arrest prevented cells with DNA double-strand breaks from transitioning into the M-phase and that the cells maintained their DNA repair capacity in G2-phase, even under RO-3306 dose-dependent DNA repair inhibition. These findings reveal that CDK1 inhibitor-induced DNA repair inhibition and cell cycle control, which regulate each other during the G2/M phase transition determine the cellular sensitivity to DNA damage, providing insight useful for developing clinical strategies targeting CDK1 inhibition in tumor cells.

Radiobiological Characterization of Canine Malignant Melanoma Cell Lines with Different Types of Ionizing Radiation and Efficacy Evaluation with Cytotoxic Agents.

Canine malignant melanoma (CMM) is a locally and systemically aggressive cancer that shares many biological and clinical characteristics with human mucosal melanoma. Hypofractionated radiation protocols have been used to treat CMM but little is known about its radiation biology. This pilot study is designed to investigate response of CMM cell lines to various ionizing radiations and cytotoxic agents to better understand this canine cancer. Four CMM cell lines were evaluated by clonogenic survival assay under aerobic and hypoxic conditions and parameters such as alpha beta (α/ß) ratio, oxygen enhancement ratio (OER), and relative biological effectiveness (RBE) were calculated after 137Cs, 6 megavoltage (MV) photon, or carbon ion irradiation. Six cytotoxic agents (cisplatin, camptothecin, mitomycin C, bleomycin, methtyl methanesulfonate and etoposide) were also assessed for their efficacy. Under aerobic condition with 6 MV photon, the α/ß ratio of the four cell lines ranged from 0.3 to >100, indicating a wide variation of cellular sensitivity. The ratio increased under hypoxic condition compared to aerobic condition and this was more dramatic in 137Cs and 6 MV photon treatments. OER of carbon was lower than 137Cs at D10 in 3 of the 4 cell lines. The RBE values generally increased with the increase of LET. Different cell lines showed sensitivity/resistance to different cytotoxic agents. This study revealed that CMM has a wide range of radiosensitivity and that hypoxia can reduce it, indicating that widely used hypofractionated protocols may not be optimal for all CMM patients. Several cytotoxic agents that have never been clinically assessed can improve treatment outcome.

Crosstalk of DNA double-strand break repair pathways in poly(ADP-ribose) polymerase inhibitor treatment of breast cancer susceptibility gene 1/2-mutated cancer.

Germline mutations in breast cancer susceptibility gene 1 or 2 (BRCA1 or BRCA2) significantly increase cancer risk in hereditary breast and ovarian cancer syndrome (HBOC). Both genes function in the homologous recombination (HR) pathway of the DNA double-strand break (DSB) repair process. Therefore, the DNA-repair defect characteristic of cancer cells brings about a therapeutic advantage for poly(ADP-ribose) polymerase (PARP) inhibitor-induced synthetic lethality. PARP inhibitor-based therapeutics initially cause cancer lethality but acquired resistance mechanisms have been found and need to be elucidated. In particular, it is essential to understand in detail the mechanism of DNA damage and repair to PARP inhibitor treatment. Further investigations have shown the roles of BRCA1/2 and its associations to other molecules in the DSB repair system. Notably, the repair pathway chosen in BRCA1-deficient cells could be entirely different from that in BRCA2-deficient cells after PARP inhibitor treatment. The present review describes synthetic lethality and acquired resistance mechanisms to PARP inhibitor through the DSB repair pathway and subsequent repair process. In addition, recent knowledge of resistance mechanisms is discussed. Our model should contribute to the development of novel therapeutic strategies.

DNA Repair Deficient Chinese Hamster Ovary Cells Exhibiting Differential Sensitivity to Charged Particle Radiation under Aerobic and Hypoxic Conditions.

It has been well established that hypoxia significantly increases both cellular and tumor resistance to ionizing radiation. Hypoxia associated radiation resistance has been known for some time but there has been limited success in sensitizing cells to radiation under hypoxic conditions. These studies show that, when irradiated with low linear energy transfer (LET) gamma-rays, poly (ADP-ribose), polymerase (PARP), Fanconi Anemia (FANC), and mutant Chinese Hamster Ovary (CHO) cells respond similarly to the non-homologous end joining (NHEJ) and the homologous recombination (HR) repair mutant CHO cells. Comparable results were observed in cells exposed to 13 keV/µm carbon ions. However, when irradiated with higher LET spread out Bragg peak (SOBP) carbon ions, we observed a decrease in the oxygen enhancement ratio (OER) in all the DNA of repair mutant cell lines. Interestingly, PARP mutant cells were observed as having the largest decrease in OER. Finally, these studies show a significant increase in the relative biological effectiveness (RBE) of high LET SOBP carbon and iron ions in HR and PARP mutants. There was also an increase in the RBE of NHEJ mutants when irradiated to SOBP carbon and iron ions. However, this increase was lower than in other mutant cell lines. These findings indicate that high LET radiation produces unique types of DNA damage under hypoxic conditions and PARP and HR repair pathways play a role in repairing this damage.

Nontoxic concentration of DNA-PK inhibitor NU7441 radio-sensitizes lung tumor cells with little effect on double strand break repair.

High-linear energy transfer (LET) heavy ions have been increasingly employed as a useful alternative to conventional photon radiotherapy. As recent studies suggested that high LET radiation mainly affects the nonhomologous end-joining (NHEJ) pathway of DNA double strand break (DSB) repair, we further investigated this concept by evaluating the combined effect of an NHEJ inhibitor (NU7441) at a non-toxic concentration and carbon ions. NU7441-treated non-small cell lung cancer (NSCLC) A549 and H1299 cells were irradiated with X-rays and carbon ions (290 MeV/n, 50 keV/µm). Cell survival was measured by clonogenic assay. DNA DSB repair, cell cycle distribution, DNA fragmentation and cellular senescence induction were studied using a flow cytometer. Senescence-associated protein p21 was detected by western blotting. In the present study, 0.3 µM of NU7441, nontoxic to both normal and tumor cells, caused a significant radio-sensitization in tumor cells exposed to X-rays and carbon ions. This concentration did not seem to cause inhibition of DNA DSB repair but induced a significant G2/M arrest, which was particularly emphasized in p53-null H1299 cells treated with NU7441 and carbon ions. In addition, the combined treatment induced more DNA fragmentation and a higher degree of senescence in H1299 cells than in A549 cells, indicating that DNA-PK inhibitor contributes to various modes of cell death in a p53-dependent manner. In summary, NSCLC cells irradiated with carbon ions were radio-sensitized by a low concentration of DNA-PK inhibitor NU7441 through a strong G2/M cell cycle arrest. Our findings may contribute to further effective radiotherapy using heavy ions.

A multimodal treatment of carbon ions irradiation, miRNA-34 and mTOR inhibitor specifically control high-grade chondrosarcoma cancer stem cells.

BACKGROUND AND PURPOSE: High-grade chondrosarcomas are chemo- and radio-resistant cartilage-forming tumors of bone that often relapse and metastase. Thus, new therapeutic strategies are urgently needed. MATERIAL AND METHODS: Chondrosarcoma cells (CH-2879) were exposed to carbon-ion irradiation, combined with miR-34 mimic and/or rapamycin administration. The effects of treatment on cancer stem cells, stemness-associated phenotype, radioresistance and tumor-initiating properties were evaluated. RESULTS: We show that high-grade chondrosarcoma cells contain a population of radioresistant cancer stem cells that can be targeted by a combination of carbon-ion therapy, miR-34 mimic administration and/or rapamycin treatment that triggers FOXO3 and miR-34 over-expression. mTOR inhibition by rapamycin triggered FOXO3 and miR-34, leading to KLF4 repression. CONCLUSION: Our results show that particle therapy combined with molecular treatments effectively controls cancer stem cells and may overcome treatment resistance of high-grade chondrosarcoma.

Sister Chromatid Exchange as a Genotoxic Stress Marker.

Sister chromatid exchange (SCE) is the phenomenon of partial DNA exchange during DNA replication. SCE detection has been developed through eliciting DNA's semiconservative replicative nature. Thymidine analogues such as 5'-bromodeoxyuridine (BrdU) and ethynyldeoxyuridine (EdU) are incorporated into the newly synthesized DNA for two cell cycles. The addition of Colcemid to the culture blocks and synchronizes cells at mitosis, and conventional cytogenetic preparations are made. Differential staining methods with Hoechst dye and Giemsa (Fluorescence Plus Giemsa staining), antibody detection against BrdU, or highly specific Click reaction to EdU, allow the newly synthesized DNA within a chromatid to be recognized. SCEs represent a point of DNA template exchange during DNA synthesis, visualized by differential chromatid staining or harlequin chromosomes. We will introduce three basic protocols in this chapter including non-fluorescence and fluorescence methods for SCE microscopic analysis. SCE is a very sensitive marker of genotoxic stress during replication.

Design and evaluation of a novel flavonoid-based radioprotective agent utilizing monoglucosyl rutin.

In this study, three novel flavonoid composite materials, created by combining an aglycone [quercetin (QUE), hesperetin (HES) or naringenin (NAR)] with monoglucosyl rutin (MGR), were designed to test for improved radioprotectivity compared with that provided by administration of MGR alone. Aglycone in the MGR-composite state was highly soluble in water, compared with aglycone alone dissolved in dimethyl sulfoxide or distilled water. The antioxidant activity of the three flavonoid composites was as high as that of MGR only. Next, the cytotoxicity test after 30 min treatment of an MGR composite showed a clear reduction in cell viability and suggested that a rapid introduction of aglycone into cells had taken place. In addition, QUE/MGR and HES/MGR composites strongly scavenged intracellular reactive oxygen species (ROS) induced by X-ray irradiation as well as MGR alone did. However, in the colony-formation assay using irradiated Chinese hamster ovary (CHO) cells, the HES/MGR composite showed a stronger radioprotective effect than MGR alone did, but the QUE/MGR composite showed no additional protective effect compared with the control. Furthermore, it was revealed that QUE and QUE/MGR composite treatment had the effect of reducing the glutathione (GSH) content in cells, and that QUE showed a stronger inhibition of PARP activity compared that of HES and NAR. Our data demonstrated that when designing a flavonoid composite as a radioprotective agent, it was necessary to select an appropriate aglycone, considering not only its antioxidant ability but also its inhibitory effect on cell recovery or DNA repair after radiation injury.

Oxygen Enhancement Ratio in Radiation-Induced Initial DSBs by an Optimized Flow Cytometry-based Gamma-H2AX Analysis in A549 Human Cancer Cells.

High-linear energy transfer (LET) heavy ions cause higher therapeutic effects than low-LET radiation due to lower dependency on oxygen concentration in tumor cell killing. The lethality after irradiation largely depends on DNA double-strand breaks (DSBs), however the detailed LET dependency for DSB induction under oxic and hypoxic conditions has not been reported. Therefore, we evaluated the oxygen enhancement ratio (OER) of heavy ion-induced DSB induction using a highly-optimized flow cytometry-based method of γ-H2AX detection. Non-small cell lung cancer (NSCLC) A549 cells were exposed to X-ray, carbon-ion and iron-ion radiations under oxic or hypoxic condition. As a DSB marker, the γ-H2AX signal was measured 1 h postirradiation and analyzed by flow cytometry. DSB slope values were calculated as DSB induction per Gy. Our method was able to detect high-LET radiation-induced DSBs even from clustered DNA damage sites. We also showed a decrease in OER value in an LET-dependent manner regardless of radiation type. In summary, we demonstrated a simple, quick and highly-optimized flow cytometry-based method of DSB analysis that detects DSBs induced by heavy-ion radiation for hypoxic and nonhypoxic cancer cells. Our study may provide a useful biological basis for heavy-ion radiotherapy.

TAS-116, a Novel Hsp90 Inhibitor, Selectively Enhances Radiosensitivity of Human Cancer Cells to X-rays and Carbon Ion Radiation.

Hsp90 inhibitors have been investigated as cancer therapeutics in monotherapy and to augment radiotherapy; however, serious adverse effects of early-generation Hsp90 inhibitors limited their development. TAS-116 is a novel Hsp90 inhibitor with lower adverse effects than other Hsp90 inhibitors, and here, we investigated the radiosensitizing effects of TAS-116 in low linear energy transfer (LET) X-ray and high LET carbon ion-irradiated human cancer cells and mouse tumor xenografts. TAS-116 decreased cell survival of both X-ray and carbon ion-irradiated human cancer cell lines (HeLa and H1299 cells), and similar to other Hsp90 inhibitors, it did not affect radiosensitivity of noncancerous human fibroblasts. TAS-116 increased the number of radiation-induced γ-H2AX foci and delayed the repair of DNA double-strand breaks (DSB). TAS-116 reduced the expression of proteins that mediate repair of DSBs by homologous recombination (RAD51) and nonhomologous end joining (Ku, DNA-PKcs), and suppressed formation of RAD51 foci and phosphorylation/activation of DNA-PKcs. TAS-116 also decreased expression of the cdc25 cell-cycle progression marker, markedly increasing G2-M arrest. Combined treatment of mouse tumor xenografts with carbon ions and TAS-116 showed promising delay in tumor growth compared with either individual treatment. These results demonstrate that TAS-116 radiosensitizes human cancer cells to both X-rays and carbon ions by inhibiting the two major DSB repair pathways, and these effects were accompanied by marked cell-cycle arrest. The promising results of combination TAS-116 + carbon ion radiotherapy of tumor xenografts justify further exploration of TAS-116 as an adjunct to radiotherapy using low or high LET radiation. Mol Cancer Ther; 16(1); 16-24. ©2016 AACR.

Investigation of the relative biological effectiveness and uniform isobiological killing effects of irradiation with a clinical carbon SOBP beam on DNA repair deficient CHO cells.

Spread-out Bragg peak (SOBP) C ions have been used clinically in charged particle radiation therapy for years. An SOBP beam consists of various monoenergetic Bragg peaks; however, the biological effect of irradiation with an SOBP beam track has not been well-studied. In order to determine the clinically prospective molecular targets, radiosensitivity to the beam track in DNA repair deficient cell lines was investigated. A total of four distinct Chinese hamster ovary (CHO) cell lines, including CHO10B2 (wild-type), V3 (protein kinase DNA-activated catalytic polypeptide deficient), 51D1 (RAD51 paralog D deficient) and PADR9 [poly(ADP-ribose) polymerase (PARP) deficient], were irradiated with gamma-rays, C ions (290 MeV/n) and Fe ions (500 MeV/n), in order to compare cellular lethality. An OptiCell™ culture system was used to evaluate the lethality at distinct depths of SOBP C ions. Relative biological effectiveness (RBE) values of C ions (linear energy transfer (LET), 13 and 70 keV/µm) and Fe ions (LET, 200 keV/µm) were calculated from cell survival using colony formation assay with standard cell dishes. All cell lines exhibited similar sensitivity to 70 keV/µm C ions and 200 keV/µm Fe ions. Furthermore, V3 cells did not exhibit increased sensitivity to high LET C ions and Fe ions, compared with low LET gamma-rays and C ions, and 51D1 cells irradiated with 13 keV/µm C ions exhibited relatively high RBE values among the tested cell lines. Conversely, PADR9 cells exhibited low RBE values for 13 keV/µm C ions and high RBE values for 70 keV/µm C ions. Obtained using the OptiCell system, the survival fractions in the SOBP region were uniform for wild-type and PADR9 cells. Conversely, V3 and 51D1 cells exhibited decreased cell death in the distal region of the SOBP. These results indicated that PARP is a more effective target for clinical beam therapy, compared with the non-homologous end joining repair and homologous recombination repair pathways. PARP deficiency may be an optimal target for C ion therapy and the results of the present study may contribute to the development of a more effective heavy ion radiation therapy.

In vitro screening of radioprotective properties in the novel glucosylated flavonoids.

Novel glucosyl flavonoids are developed by the addition of glucose to naturally occurring flavonoids. Flavonoids are known antioxidants that possess radioprotective properties. In order to investigate the radioprotective properties of novel glucosyl flavonoids, in vitro DNA double-strand breaks (DSBs) analysis was carried out. In the present study, Quercetin, Naringenin, and Hesperetin groups of flavonoids included in the natural and novel glucosyl 13 flavonoids were investigated. Flavonoids were mixed with Lambda DNA, and subsequently exposed to gamma­rays. Furthermore, DNA DSB yields were visualized by gel electrophoresis. Quercetin derivatives displayed reduced DNA DSB formation at 10 µM. At a high concentration, the majority of flavonoids displayed radioprotective properties as a reduction of DSB yields. Suppression of DSB formation was confirmed via the molecular combing assay for Quercetin, and three monoglucosyl flavonoids. Glucosylation showed positive effects for radioprotection and monoglucosyl-Rutin showed superior radioprotective properties when compared to monoglucosyl-Naringin and Hesperidin. In addition, Quercetin derivatives had greater total antioxidant capacities and DPPH radical scavenging ability than other flavonoid groups. Since Quercetin, Isoquercetin, and Rutin display poor water solubility, monoglucosyl-Rutin, maltooligosyl-Isoquercetin, and maltooligosyl-Rutin may be better radioprotective agents and easily bioavailable with increased water solubility.

The purine scaffold Hsp90 inhibitor PU-H71 sensitizes cancer cells to heavy ion radiation by inhibiting DNA repair by homologous recombination and non-homologous end joining.

BACKGROUND AND PURPOSE: PU-H71 is a purine-scaffold Hsp90 inhibitor developed to overcome limitations of conventional Hsp90 inhibitors. This study was designed to investigate the combined effect of PU-H71 and heavy ion irradiation on human tumor and normal cells. MATERIALS AND METHODS: The effects of PU-H71 were determined by monitoring cell survival by colony formation, and DNA double-strand break (DSB) repair by γ-H2AX foci and immuno-blotting DSB repair proteins. The mode of cell death was evaluated by sub-G1 DNA content (as an indicator for apoptosis), and mitotic catastrophe. RESULTS: PU-H71 enhanced heavy ion irradiation-induced cell death in three human cancer cell lines, but the drug did not radiosensitize normal human fibroblasts. In irradiated tumor cells, PU-H71 increased the persistence of γ-H2AX foci, and it reduced RAD51 foci and phosphorylated DNA-PKcs, key DSB repair proteins involved in homologous recombination (HR) and non-homologous end joining (NHEJ). In some tumor cell lines, PU-H71 altered the sub-G1 cell fraction and mitotic catastrophe following carbon ion irradiation. CONCLUSION: Our results demonstrate that PU-H71 sensitizes human cancer cells to heavy ion irradiation by inhibiting both HR and NHEJ DSB repair pathways. PU-H71 holds promise as a radiosensitizer for enhancing the efficacy of heavy ion radiotherapy.

VE-821, an ATR inhibitor, causes radiosensitization in human tumor cells irradiated with high LET radiation.

BACKGROUND: High linear energy transfer (LET) radiation such as carbon ion particles is successfully used for treatment of solid tumors. The reason why high LET radiation accomplishes greater tumor-killing than X-rays is still not completely understood. One factor would be the clustered or complex-type DNA damages. We previously reported that complex DNA double-strand breaks produced by high LET radiation enhanced DNA end resection, and this could lead to higher kinase activity of ATR protein recruited to RPA-coated single-stranded DNA. Although the effect of ATR inhibition on cells exposed to low LET gamma-rays has recently been reported, little is known regarding the effect of ATR inhibitor on cells treated with high LET radiation. The purpose of this study is to investigate the effects of the ATR inhibitor VE-821 in human tumor and normal cells irradiated with high LET carbon ions. FINDINGS: HeLa, U2OS, and 1BR-hTERT (normal) cells were pre-treated with 1 µM VE-821 for 1 hour and irradiated with either high LET carbon ions or X-rays. Cell survival, cell cycle distribution, cell growth, and micronuclei formation were evaluated. VE-821 caused abrogation of G2/M checkpoint and forced irradiated cells to divide into daughter cells. We also found that carbon ions caused a higher number of multiple micronuclei than X-rays, leading to decreased cell survival in tumor cells when treated with VE-821, while the survival of irradiated normal cells were not significantly affected by this inhibitor. CONCLUSIONS: ATR inhibitor would be an effective tumor radiosensitizer with carbon ion irradiation.

Monoglucosyl-rutin as a potential radioprotector in mammalian cells.

In the present study, the role of monoglucosyl­rutin as a potential radioprotector was investigated using mammalian cell culture models. Cell survival and DNA damage were assessed using colony formation, sister chromatid exchange and γH2AX assays. It was demonstrated that monoglucosyl­rutin was able to increase cell survival when exposed to ionizing radiation, possibly by decreasing the amount of base damage experienced by the cell. However, the present study also demonstrated that, despite monoglucosyl­rutin exhibiting radioprotective effects at low doses, high doses of monoglucosyl­rutin led to a decrease in plating efficiency and an increased doubling time. This effect may be due to double­strand breaks caused by high concentrations of monoglucosyl­rutin.