Preclinical Evaluation of the ATR Inhibitor BAY 1895344 as a Radiosensitizer for Head and Neck Squamous Cell Carcinoma

PURPOSE

Despite aggressive multimodal treatment that typically includes definitive or adjuvant radiation therapy (RT), locoregional recurrence rates approach 50% for patients with locally advanced human papillomavirus (HPV)-negative head and neck squamous cell carcinoma (HNSCC). Thus, more effective therapeutics are needed to improve patient outcomes. We evaluated the radiosensitizing effects of ataxia telangiectasia and RAD3-related (ATR) inhibitor (ATRi) BAY 1895344 in preclinical models of HNSCC.

METHODS AND MATERIALS

Murine and human HPV-negative HNSCC cells (MOC2, MOC1, JHU-012) were treated with vehicle or ATRi with or without 4 Gy. Checkpoint kinase 1 phosphorylation and DNA damage (γH2AX) were evaluated by Western blot, and ATRi half-maximal inhibitory concentration was determined by MTT assay for HNSCC cells and immortalized murine oral keratinocytes. In vitro radiosensitization was tested by clonogenic assay. Cell cycle distribution and mitotic catastrophe were evaluated by flow cytometry. Mitotic aberrations were quantified by fluorescent microscopy. Tumor growth delay and survival were assessed in mice bearing MOC2 or JHU-012 transplant tumors treated with vehicle, ATRi, RT (10 Gy × 1 or 8 Gy × 3), or combined ATRi + RT.

RESULTS

ATRi caused dose-dependent reduction in checkpoint kinase 1 phosphorylation at 1 hour post-RT (4 Gy) and dose-dependent increase in γH2AX at 18 hours post-RT. Addition of RT to ATRi led to decreased BAY 1895344 half-maximal inhibitory concentration in HNSCC cell lines but not in normal tissue surrogate immortalized murine oral keratinocytes. Clonogenic assays demonstrated radiosensitization in the HNSCC cell lines. ATRi abrogated the RT-induced G2/M checkpoint, leading to mitosis with unrepaired DNA damage and increased mitotic aberrations (multinucleated cells, micronuclei, nuclear buds, nucleoplasmic bridges). ATRi and RT significantly delayed tumor growth in MOC2 and JHU-012 in vivo models, with improved overall survival in the MOC2 model.

CONCLUSIONS

These findings demonstrated that BAY 1895344 increased in vitro and in vivo radiosensitivity in HPV-negative HNSCC preclinical models, suggesting therapeutic potential warranting evaluation in clinical trials for patients with locally advanced or recurrent HPV-negative HNSCC.

WEE1 kinase inhibitor MK-1775 sensitizes oral tongue squamous cell carcinoma cells to radiation irrespective of TP53 status

OBJECTIVE

Oral tongue squamous cell carcinoma (OTSCC) frequently harbors non-functional p53 and depends on G2/M checkpoint mediated by WEE1. WEE1 suppression has been identified as a promising anti-tumor strategy. This study investigated the capacity of WEE1 kinase inhibitor (MK-1775) and its underlying mechanisms in enhancing radiation responses of OTSCC cells in vitro.

MATERIALS AND METHODS

WEE1 kinase expression and its downstream target (CDK1) were investigated in OTSCC versus normal oral tissue. A synergistic combination of MK-1775 with radiation on OTSCC cell lines with different p53 statuses was assessed by viability assay. The radio-sensitizing effects of MK-1775 on apoptosis, cell cycle, DNA damage, and mitotic entry were also determined.

RESULTS

Irradiation enhanced CDK1 expression in all tested cell lines, though the effect was far more pronounced in p53 mutated cell lines. MK-1775 exhibited inhibitory effects against the survival of all cell lines and enhanced their response to the radiation. These effects were strongly elicited by induction of apoptosis and lethal mitosis, but less likely by abrogation of radiation-induced G2 arrest.

CONCLUSION

These results demonstrate the efficacy of MK-1775 in enhancing the radiation effect on OTSCC in vitro associated with a significant apoptotic death rate, identifying WEE1 inhibitor as a potent radiosensitizer in OTSCC irrespective of p53 mutational status.

Synergistic Tumoricidal Effects of Alpha-Lipoic Acid and Radiotherapy on Human Breast Cancer Cells via HMGB1

PURPOSE

Radiotherapy (RT) is one of main strategies of cancer treatment. However, some cancer cells are resistant to radiation-induced cell death, including apoptosis. Therefore, alternative approaches targeting different anti-tumor mechanisms such as cell senescence are required. This study aimed to investigate the synergistic effect of alpha-lipoic acid (ALA) on radiation-induced cell death and senescence in MDA-MB-231 human breast cancer cells.

MATERIALS AND METHODS

The cells were divided into four groups depending on the cell treatment (control, ALA, RT, and ALA+RT). Cells were analyzed for morphology, apoptotic cell death, mitochondrial reactive oxygen species, membrane potential, cellular senescence, and cell cycle.

RESULTS

Our data showed that ALA significantly promoted apoptotic cell death when combined with RT, as reflected by Annexin V staining, expression of apoptosis-related factors, mitochondrial damages as well as cell morphological changes and reduction of cell numbers. In addition, ALA significantly enhanced radiation-induced cellular senescence, which was shown by increased HMGB1 expression in the cytosol fraction compared to the control, increased p53 expression compared to the control, activation of p38 as well as nuclear factor кB, and G2/M cell cycle arrest.

CONCLUSION

The current study is the first report showing a new mode of action (senescence induction) of ALA beyond apoptotic cell death in MDA-MB-231 cancer cells known to be resistant to RT.

Coralyne Radiosensitizes A549 Cells by Upregulation of CDKN1A Expression to Attenuate Radiation Induced G2/M Block of the Cell Cycle

Coralyne is a synthetic analog of berberine related to protoberberine-isoquinoline alkaloids. Isoquinoline derivatives and analogs are renowned as potent radiosensitizers with potential medical application. In the present study, we investigated the effect of coralyne on the cell death, cytoskeletal changes and cell cycle progression of irradiated A549 cells. A clonogenic assay revealed that coralyne pretreatment decreased the viability of A549 cells in a time- and dose-dependent manner. Moreover, exposure to coralyne and ionizing radiation (IR) markedly altered the filamentous actin cytoskeletal architecture and integrin-β binding sites of A549 cells. Treatment with 1–25 µM coralyne in combination with 2 Gy of IR significantly reduced the percentage of cells in G2/M phase compared with 2 Gy IR alone. These results indicate that coralyne is a potent radiosensitizing agent that may find an application in medicine.

SPRTN protease-cleaved MRE11 decreases DNA repair and radiosensitises cancer cells

The human MRE11/RAD50/NBS1 (MRN) complex plays a crucial role in sensing and repairing DNA DSB. MRE11 possesses dual 3'-5' exonuclease and endonuclease activity and forms the core of the multifunctional MRN complex. We previously identified a C-terminally truncated form of MRE11 (TR-MRE11) associated with post-translational MRE11 degradation. Here we identified SPRTN as the essential protease for the formation of TR-MRE11 and characterised the role of this MRE11 form in its DNA damage response (DDR). Using tandem mass spectrometry and site-directed mutagenesis, the SPRTN-dependent cleavage site for MRE11 was identified between 559 and 580 amino acids. Despite the intact interaction of TR-MRE11 with its constitutive core complex proteins RAD50 and NBS1, both nuclease activities of truncated MRE11 were dramatically reduced due to its deficient binding to DNA. Furthermore, lack of the MRE11 C-terminal decreased HR repair efficiency, very likely due to abolished recruitment of TR-MRE11 to the sites of DNA damage, which consequently led to increased cellular radiosensitivity. The presence of this DNA repair-defective TR-MRE11 could explain our previous finding that the high MRE11 protein expression by immunohistochemistry correlates with improved survival following radical radiotherapy in bladder cancer patients.

Targeting Aurora B kinase with Tanshinone IIA suppresses tumor growth and overcomes radioresistance

Aurora B kinase is aberrantly overexpressed in various tumors and shown to be a promising target for anti-cancer therapy. In human oral squamous cell carcinoma (OSCC), the high protein level of Aurora B is required for maintaining of malignant phenotypes, including in vitro cell growth, colony formation, and in vivo tumor development. By molecular modeling screening of 74 commercially available natural products, we identified that Tanshinone IIA (Tan IIA), as a potential Aurora B kinase inhibitor. The in silico docking study indicates that Tan IIA docks into the ATP-binding pocket of Aurora B, which is further confirmed by in vitro kinase assay, ex vivo pull-down, and ATP competitive binding assay. Tan IIA exhibited a significant anti-tumor effect on OSCC cells both in vitro and in vivo, including reduction of Aurora B and histone H3 phosphorylation, induction of G2/M cell cycle arrest, increase the population of polyploid cells, and promotion of apoptosis. The in vivo mouse model revealed that Tan IIA delayed tumor growth of OSCC cells. Tan IIA alone or in combination with radiation overcame radioresistance in OSCC xenograft tumors. Taken together, our data indicate that Tan IIA is an Aurora B kinase inhibitor with therapeutic potentials for cancer treatment.

In vitro effects of Trastuzumab Emtansine (T-DM1) and concurrent irradiation on HER2-positive breast cancer cells

BACKGROUND

To determine the effects of concurrent irradiation and T-DM1 on HER2-positive breast cancer cell lines.

METHODS

Five human breast cancer cell lines (in vitro study) presenting various levels of HER2 expression were used to determine the potential therapeutic effect of T-DM1 combined with radiation. The toxicity of T-DM1 was assessed using viability assay and cell cycle analysis was performed by flow cytometry after BrdU incorporation. HER2 cells were irradiated at different dose levels after exposure to T-DM1. Survival curves were determined by cell survival assays (after 5 population doubling times).

RESULTS

The results revealed that T-DM1 induced significant lethality due to the intracellular action of DM1 on the cell cycle with significant G2/M phase blocking. Even after a short time incubation, the potency of T-DM1 was maintained and even enhanced over time, with a higher rate of cell death. After irradiation alone, the D₁₀ (dose required to achieve 10% cell survival) was significantly higher for high HER2-expressing cell lines than for low HER2-expressing cells, with a linearly increasing relationship. In combination with irradiation, using conditions that allow cell survival, T-DM1 does not induce a radiosensitivity.

CONCLUSIONS

Although there is a linear correlation between intrinsic HER2 expression and radioresistance, the results indicated that T-DM1 is not a radiation-sensitizer under the experimental conditions of this study that allowed cell survival. However, further investigations are needed, in particular in vivo studies before reaching a final conclusion.

Radiosensitization of NSCLC cells to X-rays and carbon ions by the CHK1/CHK2 inhibitor AZD7762, Honokiol and Tunicamycin

Although radiotherapy, especially carbon-ion radiotherapy, is an effective treatment modality against non-small-cell lung cancer (NSCLC), studies using radiation combined with sensitizer for improving the efficacy of radiotherapy are still needed. In this work, we aimed to investigate in NSCLC A549 and H1299 cell lines the effects of different linear energy transfer (LET) radiations combined with diverse sensitizing compounds. Cells pretreated with the CHK1/CHK2 inhibitor AZD7762, Honokiol or Tunicamycin were irradiated with low-LET X-rays and high-LET carbon ions. Cell survival was assessed using the clonogenic cell survival assay. Cell cycle distribution and apoptosis were measured with flow cytometry, and DNA double strand break (DSB) and repair were detected using γ-H2AX immunofluorescence staining. Our results revealed that AZD7762, Honokiol and Tunicamycin demonstrated low cytotoxicity to NSCLC cells and a pronounced radiosensitizing effect on NSCLC cells exposed to carbon ions than X-rays. Unrepaired DNA DSB damages, the abrogation of G2/M arrest induced by irradiation, and finally apoptotic cell death were the main causes of the radiosensitizing effect. Thus, our data suggest that high-LET carbon ion combined with these compounds may be a potentially effective therapeutic strategy for locally advanced NSCLC.

Selective Killing of RAS-Malignant Tissues by Exploiting Oncogene-Induced DNA Damage

Several oncogenes induce untimely entry into S phase and alter replication timing and progression, thereby generating replicative stress, a well-known source of genomic instability and a hallmark of cancer. Using an epithelial model in Drosophila, we show that the RAS oncogene, which triggers G1/S transition, induces DNA damage and, at the same time, silences the DNA damage response pathway. RAS compromises ATR-mediated phosphorylation of the histone variant H2Av and ATR-mediated cell-cycle arrest in G2 and blocks, through ERK, Dp53-dependent induction of cell death. We found that ERK is also activated in normal tissues by an exogenous source of damage and that this activation is necessary to dampen the pro-apoptotic role of Dp53. We exploit the pro-survival role of ERK activation upon endogenous and exogenous sources of DNA damage to present evidence that its genetic or chemical inhibition can be used as a therapeutic opportunity to selectively eliminate RAS-malignant tissues.

Different Mechanisms Underlie the Metabolic Response of GBM Stem-Like Cells to Ionizing Radiation: Biological and MRS Studies on Effects of Photons and Carbon Ions

Glioblastoma multiforme (GBM) is a malignant primary brain tumor with very poor prognosis, high recurrence rate, and failure of chemo-radiotherapy, mainly due to a small fraction of cells with stem-like properties (GSCs). To study the mechanisms of GSCs resistance to radiation, two GSC lines, named line #1 and line #83, with different metabolic patterns and clinical outcome, were irradiated with photon beams and carbon ions and assessed by ¹H Magnetic Resonance Spectroscopy (MRS). Both irradiation modalities induced early cytotoxic effects in line #1 with small effects on cell cycle, whereas a proliferative G2/M cytostatic block was observed in line #83. MR spectroscopy signals from mobile lipids (ML) increased in spectra of line #1 after photon and C-ion irradiation with effects on lipid unsaturation level, whereas no effects were detected in line #83 spectra. Gamma-Aminobutyric Acid (GABA), glutamic acid (glu) and Phosphocreatine (pCr) signals showed a significant variation only for line #1 after carbon ion irradiation. Glucose (glc) level and lactate (Lac) extrusion behaved differently in the two lines. Our findings suggest that the differences in irradiation response of GSCs #1 and #83 lines are likely attributable to their different metabolic fingerprint rather than to the different radiation types.

Wee1 Kinase Inhibitor AZD1775 Effectively Sensitizes Esophageal Cancer to Radiotherapy

PURPOSE

Esophageal cancer is a deadly malignancy with a 5-year survival rate of only 5% to 20%, which has remained unchanged for decades. Esophageal cancer possesses a high frequency of TP53 mutations leading to dysfunctional G1 cell-cycle checkpoint, which likely makes esophageal cancer cells highly reliant upon G2-M checkpoint for adaptation to DNA replication stress and DNA damage after radiation. We aim to explore whether targeting Wee1 kinase to abolish G2-M checkpoint sensitizes esophageal cancer cells to radiotherapy.

EXPERIMENTAL DESIGN

Cell viability was assessed by cytotoxicity and colony-forming assays, cell-cycle distribution was analyzed by flow cytometry, and mitotic catastrophe was assessed by immunofluorescence staining. Human esophageal cancer xenografts were generated to explore the radiosensitizing effect of AZD1775 in vivo.

RESULTS

The IC50 concentrations of AZD1775 on esophageal cancer cell lines were between 300 and 600 nmol/L. AZD1775 (100 nmol/L) as monotherapy did not alter the viability of esophageal cancer cells, but significantly radiosensitized esophageal cancer cells. AZD1775 significantly abrogated radiation-induced G2-M phase arrest and attenuation of p-CDK1-Y15. Moreover, AZD1775 increased radiation-induced mitotic catastrophe, which was accompanied by increased γH2AX levels, and subsequently reduced survival after radiation. Importantly, AZD1775 in combination with radiotherapy resulted in marked tumor regression of esophageal cancer tumor xenografts.

CONCLUSIONS

Abrogation of G2-M checkpoint by targeting Wee1 kinase with AZD1775 sensitizes esophageal cancer cells to radiotherapy in vitro and in mouse xenografts. Our findings suggest that inhibition of Wee1 by AZD1775 is an effective strategy for radiosensitization in esophageal cancer and warrants clinical testing.

Oxidation resistance 1 prevents genome instability through maintenance of G2/M arrest in gamma-ray-irradiated cells

Human oxidation resistance 1 (OXR1) was identified as a protein that decreases genomic mutations in Escherichia coli caused by oxidative DNA damage. However, the mechanism by which OXR1 defends against genome instability has not been elucidated. To clarify how OXR1 maintains genome stability, the effects of OXR1-depletion on genome stability were investigated in OXR1-depleted HeLa cells using gamma-rays (γ-rays). The OXR1-depleted cells had higher levels of superoxide and micronucleus (MN) formation than control cells after irradiation. OXR1-overexpression alleviated the increases in reactive oxygen species (ROS) level and MN formation after irradiation. The increased MN formation in irradiated OXR1-depleted cells was partially attenuated by the ROS inhibitor N-acetyl-L-cysteine, suggesting that OXR1-depeletion increases ROS-dependent genome instability. We also found that OXR1-depletion shortened the duration of γ-ray-induced G2/M arrest. In the presence of the cell cycle checkpoint inhibitor caffeine, the level of MN formed after irradiation was similar between control and OXR1-depleted cells, demonstrating that OXR1-depletion accelerates MN formation through abrogation of G2/M arrest. In OXR1-depleted cells, the level of cyclin D1 protein expression was increased. Here we report that OXR1 prevents genome instability by cell cycle regulation as well as oxidative stress defense.

Protective effects of SND1 in retinal photoreceptor cell damage induced by ionizing radiation

Staphylococcal nuclease and tudor domain containing 1 (SND1) has multiple functions in a variety of cellular processes. Here, we assessed the effects of SND1 in cellular DNA damage after ionizing radiation (IR). Knocking down SND1 in the mouse-derived photoreceptor 661 W cell line markedly inhibited cell proliferation and increased apoptosis after IR treatment. After DNA damage, SND1 induced Ataxia telangiectasia mutated kinase (ATM) signaling to launch DNA repair. Defects of SND1 were associated with missing response to DNA damage signal to cell cycle checkpoints or DNA repair. The current findings reveal SND1 as a new regulatory factor in DNA damage response.

Leucine zipper and ICAT domain containing (LZIC) protein regulates cell cycle transitions in response to ionizing radiation

Common hallmarks of cancer include the dysregulation of cell cycle progression and the acquisition of genome instability. In tumors, G1 cell cycle checkpoint induction is often lost. This increases the reliance on a functional G2/M checkpoint to prevent progression through mitosis with damaged DNA, avoiding the introduction of potentially aberrant genetic alterations. Treatment of tumors with ionizing radiation (IR) utilizes this dependence on the G2/M checkpoint. Therefore, identification of factors which regulate this process could yield important biomarkers for refining this widely used cancer therapy. Leucine zipper and ICAT domain containing (LZIC) downregulation has been associated with the development of IR-induced tumors. However, despite LZIC being highly conserved, it has no known molecular function. We demonstrate that LZIC knockout (KO) cell lines show a dysregulated G2/M cell cycle checkpoint following IR treatment. In addition, we show that LZIC deficient cells competently activate the G1 and early G2/M checkpoint but fail to maintain the late G2/M checkpoint after IR exposure. Specifically, this defect was found to occur downstream of PIKK signaling. The LZIC KO cells demonstrated severe aneuploidy indicative of genomic instability. In addition, analysis of data from cancer patient databases uncovered a strong correlation between LZIC expression and poor prognosis in several cancers. Our findings suggest that LZIC is functionally involved in cellular response to IR, and its expression level could serve as a biomarker for patient stratification in clinical cancer practice.

Increased Chromosome Aberrations in Cells Exposed Simultaneously to Simulated Microgravity and Radiation

Space radiation and microgravity (μG) are two major environmental stressors for humans in space travel. One of the fundamental questions in space biology research is whether the combined effects of μG and exposure to cosmic radiation are interactive. While studies addressing this question have been carried out for half a century in space or using simulated μG on the ground, the reported results are ambiguous. For the assessment and management of human health risks in future Moon and Mars missions, it is necessary to obtain more basic data on the molecular and cellular responses to the combined effects of radiation and µG. Recently we incorporated a μG⁻irradiation system consisting of a 3D clinostat synchronized to a carbon-ion or X-ray irradiation system. Our new experimental setup allows us to avoid stopping clinostat rotation during irradiation, which was required in all other previous experiments. Using this system, human fibroblasts were exposed to X-rays or carbon ions under the simulated μG condition, and chromosomes were collected with the premature chromosome condensation method in the first mitosis. Chromosome aberrations (CA) were quantified by the 3-color fluorescent in situ hybridization (FISH) method. Cells exposed to irradiation under the simulated μG condition showed a higher frequency of both simple and complex types of CA compared to cells irradiated under the static condition by either X-rays or carbon ions.

RPA1 downregulation enhances nasopharyngeal cancer radiosensitivity via blocking RAD51 to the DNA damage site

BACKGROUND/AIM

Nasopharyngeal cancer (NPC) has a high local recurrence rate due to its resistance to ionizing radiation (IR). Replication protein A1 (RPA1) is one of the main elements in the homologous repair (HR) pathway, which is closely associated with the repair of DNA double strand breaks (DDBs). Studies on the relationship between RPA1 and the radiosensitivity of NPC are substantially limited. It was hypothesized that RPA1 plays a crucial role in predicting the radiosensitivity of NPC.

METHODS

The protein expression of RPA1 in 182 patients with NPC in the complete response (CR) and non-complete response (nCR) groups was evaluated using immunohistochemistry. Then, univariate and multivariate analysis were performed using SPSS software vision 22 to determine the relationship between the expression of RPA1 and the clinicopathological features. In addition, the mRNA expression of RPA1 was tested in 24 fresh samples using qRT-PCR. RPA1 was silenced in CNE-2R cell lines combined with IR to measure the radiosensitivity, proliferation, DNA damage repair and cell cycle of CNE-2R cells. Xenograft models in nude mice were used to determine the effect of RPA1 on tumor growth after IR. Immunoblotting and immunofluorescence staining were performed to identify proteins that interacted with RPA1. All statistical tests were two-sided.

RESULTS

RPA1 protein was overexpressed in NPC patients with nCR (65.31%), and was an independent predictor of radiosensitivity (HR: 3.755, 95% CI: 1.990-7.085), in addition to Epstein-Barr virus (EBV; HR: 3.984; 95% CI: 1.524-10.410). The silencing of RPA1 increased the radiosensitivity of CNE-2R cells, blocked the repair of DNA, impaired cell proliferation, and contributed to G2/M cell cycle arrest. Furthermore, the xenograft models in nude mice revealed that silencing RPA1 combined with irradiation significantly retarded the growth of tumors. Moreover, the knockdown of RPA1 decreased Rad51 collection to the damage site and prolonged the time of DNA repair.

CONCLUSION

RPA1 protein is frequently overexpressed in NPC patients with nCR. The silencing of RPA1 enhanced the radiosensitivity of CNE-2R cells. These present findings reveal that RPA1 is a potential biomarker for predicting the radiosensitivity in NPC.

Modeling the role of microRNA-449a in the regulation of the G2/M cell cycle checkpoint in prostate LNCaP cells under ionizing radiation

Recent studies showed that induced microRNA-449a (miR-449a) enhances a G2/M cell cycle checkpoint arrest in prostate cancer (LNCaP) and lung adenocarcinoma cell lines. In the case of LNCaP cells, upregulated miR-449a directly downregulates c-Myc that is required to induce the cell cycle regulators Cdc25A and Cdc2/CyclinB whose inactivation blocks G2 to M phase transition. However, the molecular mechanisms involved are yet unclear, although in other prostate cancer cells the interactions among p53, miR-449a and Sirt-1 can affect the induction of the G2/M arrest. In order to clarify these molecular mechanisms, in this work we propose a boolean model of the G2/M checkpoint arrest regulation contemplating the influence of miR-449a. The model shows that the cell fate determination between two cellular phenotypes: G2/M-Arrest for DNA repair and G2/M-induced apoptosis is stochastic and influenced by miR-449a state of activation. The results were compared with experimental data available presenting agreement. We also found that several feedback loops are involved in this cell fate regulation and we indicate, through in silico gain or loss of function perturbations of genes, which of these feedback loops are more efficient to favor a specific phenotype.

DNA repair and cell cycle checkpoint defects in a mouse model of 'BRCAness' are partially rescued by 53BP1 deletion

'BRCAness' is a term used to describe cancer cells that behave similarly to tumors with BRCA1 or BRCA2 mutations. The BRCAness phenotype is associated with hypersensitivity to chemotherapy agents including PARP inhibitors, which are a promising class of recently-licensed anti-cancer treatments. This hypersensitivity arises because of a deficiency in the homologous recombination (HR) pathway for DNA double-strand break repair. To gain further insight into how genetic modifiers of HR contribute to the BRCAness phenotype, we created a new mouse model of BRCAness by generating mice that are deficient in BLM helicase and the Exo1 exonuclease, which are involved in the early stages of HR. We find that cells lacking BLM and Exo1 exhibit a BRCAness phenotype, with diminished HR, and hypersensitivity to PARP inhibitors. We further tested how 53BP1, an important regulator of HR, affects repair efficiency in our BRCAness model. We find that deletion of 53BP1 can relieve several of the repair deficiencies observed in cells lacking BLM and Exo1, just as it does in cells lacking BRCA1. These results substantiate the importance of BRCAness as a concept for classification of cancer cases, and further clarify the role of 53BP1 in regulation of DNA repair pathway choice in mammalian cells.

Toll-like receptor 5 agonist CBLB502 induces radioprotective effects in vitro

CBLB502 derived from Salmonella flagellin is a novel agonist of Toll-like receptor 5 (TLR5). It has been shown that CBLB502 can exert high radioprotective efficacy on mice and primates from both GI and hematopoietic syndromes during whole-body irradiation with low toxicity and immunogenicity. However, no effective system has been used to investigate the protective effect of CBLB502 against irradiation and the related mechanism in vitro. In this study, we investigated the radioprotective properties of CBLB502 in HEK293-N-T cells constitutively expressing human TLR5 and NF-κB-dependent luciferase. HEK293-N-T cells were treated with different doses of CBLB502 prior to 60Co-γ ray irradiation. After irradiation, cell viability was real-time measured for 4 days by using the real-time cell analysis system. We found that CBLB502 was capable of efficiently maintaining the survival rate of irradiated HEK293-N-T cells. Then apoptotic cell death and cell cycle were detected by flow cytometry. The results showed that CBLB502 pre-treatment could reduce the apoptosis and promote the recovery of irradiated HEK293-N-T cells from G2-phase arrest in a dose-dependent manner. Our data indicated that CBLB502 has a direct radioprotective effect in vitro via anti-apoptosis and promotes cell cycle recovery. The method developed here could be an effective in vitro system to screen other TLR5-target radioprotectants like CBLB502.

Sensitization of Radioresistant Prostate Cancer Cells by Resveratrol Isolated from Arachis hypogaea Stems

Resveratrol (RV, 3,4ʹ,5-trihydroxystilbene) is naturally produced by a wide variety of plants including grapes and peanuts (Arachis hypogaea). However, the yield of RV from peanut stem and its potential radiosensitizing effects in prostate cancer (PCa) have not been well investigated. In this study, we characterized RV in peanut stem extract (PSE) for the first time and showed that both RV and PSE dose-dependently induced cell death in DOC-2/DAB2 interactive protein (DAB2IP)-deficient PCa cells with the radioresistant phenotype. Furthermore, the combination of radiation with either RV or PSE induced the death of radioresistant PCa cells through delayed repair of radiation-induced DNA double-strand break (DSB) and prolonged G2/M arrest, which induced apoptosis. The administration of RV and PSE effectively enhanced radiation therapy in the shDAB2IP PCa xenograft mouse model. These results demonstrate the promising synergistic effect of RV and PSE combined with radiation in the treatment of radioresistant PCa.

Radiosensitization by the ATR Inhibitor AZD6738 through Generation of Acentric Micronuclei

AZD6738 is an orally active ATR inhibitor (ATRi) currently in phase I clinical trials. We found in vitro growth inhibitory activity of this ATRi in a panel of human cancer cell lines. We demonstrated radiosensitization by AZD6738 to single radiation fractions in multiple cancer cell lines independent of both p53 and BRCA2 status by the clonogenic assay. Radiosensitization by AZD6738 to clinically relevant doses of fractionated radiation was demonstrated in vitro using a 3D tumor spheroid model and, in vivo, AZD6738 radiosensitized by abrogating the radiation-induced G2 cell-cycle checkpoint and inhibiting homologous recombination. Mitosis with damaged DNA resulted in mitotic catastrophe as measured by micronucleus formation by live-cell fluorescent-ubiquitination cell-cycle imaging of cell-cycle progression and nuclear morphology. Induction of micronuclei was significantly more prominent for AZD6738 compared with inhibition of the downstream kinase CHK1 alone at isoeffective doses. Micronuclei were characterized as acentric chromosomal fragments, which displayed characteristics of increased DNA damage and cell-cycle dyssynchrony when compared with the primary nucleus. Mol Cancer Ther; 16(1); 25-34. ©2016 AACR.

Biodosimetry for High-Dose Exposures Based on Dicentric Analysis in Lymphocytes Released from the G2-Block by Caffeine

High-dose assessments using the conventional dicentric assay are essentially restricted to doses up to 5 Gy and only to lymphocytes that succeed to proceed to first post-exposure mitosis. Since G2-checkpoint activation facilitates DNA damage recognition and arrest of damaged cells, caffeine is used to release G2-blocked lymphocytes overcoming the mitotic index and dicentric yield saturation problems, enabling thus dicentric analysis even at high-dose exposures. Using the fluorescence in situ hybridization technique with telomere and centromere peptide nucleic acid probes, the released lymphocytes, identified as metaphases with decondensed chromosomes following 1.5 h caffeine treatment, show increased yield of dicentrics compared to that obtained in lymphocytes that reach metaphase without G2-checkpoint abrogation by caffeine. Here, a 3-h caffeine/colcemid co-treatment before harvesting at 55 h post-exposure is used so that the dicentric analysis using Giemsa staining is based predominantly on lymphocytes released from the G2-block, increasing thus dicentric yield and enabling construction of a dose-response calibration curve with improved precision of high-dose estimates.

Chitooligosaccharides promote radiosensitivity in colon cancer line SW480

AIM: To investigate the anti-proliferation and radiosensitization effect of chitooligosaccharides (COS) on human colon cancer cell line SW480.

METHODS: SW480 cells were treated with 0, 1.0, 2.0, 3.0, 4.0 and 5.0 mg/mL of COS for 48 h. CCK-8 assay was employed to obtain the cell survival ratio of SW480 cells, and the anti-proliferation curve was observed with the inhibition ratio of COS on SW480 cells. The RAY + COS group was treated with 1.0 mg/mL of COS for 48 h, while both the RAY and RAY+COS groups were exposed to X-ray at 0, 1, 2, 4, 6 and 8 Gy, respectively. Clonogenic assay was used to analyze cell viability in the two groups at 10 d after treatment, and a cell survival curve was used to analyze the sensitization ratio of COS. The RAY group was exposed to X-ray at 6 Gy, while the RAY+COS group was treated with 1.0 mg/mL of COS for 48 h in advance and exposed to X-ray at 6 Gy. Flow cytometry was employed to detect cell cycle and apoptosis rate in the non-treatment group, as well as in the RAY and RAY + COS groups after 24 h of treatment.

RESULTS: COS inhibited the proliferation of SW480 cells, and the inhibition rate positively correlated with the concentration of COS (P < 0.01). Cell viability decreased as radiation dose increased in the RAY and RAY+COS groups (P < 0.01). Cell viabilities in the RAY+COS group were lower than in the RAY group at all doses of X-ray exposure (P < 0.01), and the sensitization ratio of COS on SW480 cells was 1.39. Compared with the non-treatment group, there was a significant increase in apoptosis rate in both the RAY and RAY + COS groups; while the apoptosis rate in the RAY+COS group was significantly higher than in the RAY group (P < 0.01). In comparing these three groups, the percentage of G₂/M phase in both the RAY and RAY + COS groups significantly increased, and the percentage of the S phase and G₀/G₁ phase was downregulated. Furthermore, the percentage in the G₂/M phase was higher, and the percentage in the S phase and G₀/G₁ phase was lower in the RAY + COS group vs RAY group (P < 0.01).

CONCLUSION: COS can inhibit the proliferation of SW480 cells and enhance the radiosensitization of SW480 cells, inducing apoptosis and G₂/M phase arrest.

Targeting Rad50 sensitizes human nasopharyngeal carcinoma cells to radiotherapy

BACKGROUND

The Mre11-Rad50-Nbs1 (MRN) complex is well known for its crucial role in initiating DNA double strand breaks (DSBs) repair pathways to resistant irradiation (IR) injury and thus facilitating radioresistance which severely reduces radiocurability of nasopharyngeal cancer (NPC). Targeting native cellular MRN function would sensitize NPC cells to IR.

METHODS

A recombinant adenovirus containing a mutant Rad50 gene (Ad-RAD50) expressing Rad50 zinc hook domain but lacking the ATPase domain and the Mre11 interaction domain was constructed to disrupt native cellular MRN functions. The effects of Ad-RAD50 on the MRN functions were assessed in NPC cells lines using western blot, co-immunoprecipitation and confocal microscopy analyses. The increased radiosensitivity of transient Ad-RAD50 to IR was examined in NPC cells, including MTT assay, colony formation. The molecular mechanisms of radiosensitization were confirmed by neutral comet assay and western bolts. Nude mice subcutaneous injection, tumor growth curve and TUNEL assay were used to evaluate tumor regression and apoptosis in vivo.

RESULTS

Rad50 is remarkably upregulated in NPC cells after IR, implying the critical role of Rad50 in MRN functions. The transient expression of this mutant Rad50 decreased the levels of native cellular Rad50, Mre11 and Nbs1, weakened the interactions among these proteins, abrogated the G2/M arrest induced by DSBs and reduced the DNA repair ability in NPC cells. A combination of IR and mutant RAD50 therapy produced significant tumor cytotoxicity in vitro, with a corresponding increase in DNA damage, prevented proliferation and cell viability. Furthermore, Ad-RAD50 sensitized NPC cells to IR by causing dramatic tumor regression and inducing apoptosis in vivo.

CONCLUSION

Our findings define a novel therapeutic approach to NPC radiosensitization via targeted native cellular Rad50 disruption.

Visualisation of cell cycle modifications by X-ray irradiation of single HeLa cells using fluorescent ubiquitination-based cell cycle indicators

To explore the effects of X-ray irradiation on mammalian cell cycle dynamics, single cells using the fluorescent ubiquitination-based cell cycle indicator (Fucci) technique were tracked. HeLa cells expressing Fucci were used to visualise cell cycle modifications induced by irradiation. After cultured HeLa-Fucci cells were exposed to 5 Gy X-rays, fluorescent cell images were captured every 20 min for 48 h using a fluorescent microscope. Time dependence of the fluorescence intensity of S/G2 cells was analysed to examine the cell cycle dynamics of irradiated and non-irradiated control cells. The results showed that irradiated cells could be divided into two populations: one with similar cell cycle dynamics to that of non-irradiated cells, and another displaying a prolonged G2 phase. Based on these findings, it is proposed in this article that an underlying switch mechanism is involved in cell cycle regulation and the G2/M checkpoint of HeLa cells.

Radiation-induced alterations of osteogenic and chondrogenic differentiation of human mesenchymal stem cells

While human mesenchymal stem cells (hMSCs), either in the bone marrow or in tumour microenvironment could be targeted by radiotherapy, their response is poorly understood. The oxic effects on radiosensitivity, cell cycle progression are largely unknown, and the radiation effects on hMSCs differentiation capacities remained unexplored. Here we analysed hMSCs viability and cell cycle progression in 21% O₂ and 3% O₂ conditions after medical X-rays irradiation. Differentiation towards osteogenesis and chondrogenesis after irradiation was evaluated through an analysis of differentiation specific genes. Finally, a 3D culture model in hypoxia was used to evaluate chondrogenesis in conditions mimicking the natural hMSCs microenvironment. The hMSCs radiosensitivity was not affected by O₂ tension. A decreased number of cells in S phase and an increase in G2/M were observed in both O₂ tensions after 16 hours but hMSCs released from the G2/M arrest and proliferated at day 7. Osteogenesis was increased after irradiation with an enhancement of mRNA expression of specific osteogenic genes (alkaline phosphatase, osteopontin). Osteoblastic differentiation was altered since matrix deposition was impaired with a decreased expression of collagen I, probably through an increase of its degradation by MMP-3. After induction in monolayers, chondrogenesis was altered after irradiation with an increase in COL1A1 and a decrease in both SOX9 and ACAN mRNA expression. After induction in a 3D culture in hypoxia, chondrogenesis was altered after irradiation with a decrease in COL2A1, ACAN and SOX9 mRNA amounts associated with a RUNX2 increase. Together with collagens I and II proteins decrease, associated to a MMP-13 expression increase, these data show a radiation-induced impairment of chondrogenesis. Finally, a radiation-induced impairment of both osteogenesis and chondrogenesis was characterised by a matrix composition alteration, through inhibition of synthesis and/or increased degradation. Alteration of osteogenesis and chondrogenesis in hMSCs could potentially explain bone/joints defects observed after radiotherapy.

Deregulation of DNA double-strand break repair in multiple myeloma: implications for genome stability

Multiple myeloma (MM) is a hematological malignancy characterized by frequent chromosome abnormalities. However, the molecular basis for this genome instability remains unknown. Since both impaired and hyperactive double strand break (DSB) repair pathways can result in DNA rearrangements, we investigated the functionality of DSB repair in MM cells. Repair kinetics of ionizing-radiation (IR)-induced DSBs was similar in MM and normal control lymphoblastoid cell lines, as revealed by the comet assay. However, four out of seven MM cell lines analyzed exhibited a subset of persistent DSBs, marked by γ-H2AX and Rad51 foci that elicited a prolonged G2/M DNA damage checkpoint activation and hypersensitivity to IR, especially in the presence of checkpoint inhibitors. An analysis of the proteins involved in DSB repair in MM cells revealed upregulation of DNA-PKcs, Artemis and XRCC4, that participate in non-homologous end joining (NHEJ), and Rad51, involved in homologous recombination (HR). Accordingly, activity of both NHEJ and HR were elevated in MM cells compared to controls, as determined by in vivo functional assays. Interestingly, levels of proteins involved in a highly mutagenic, translocation-promoting, alternative NHEJ subpathway (Alt-NHEJ) were also increased in all MM cell lines, with the Alt-NHEJ protein DNA ligase IIIα, also overexpressed in several plasma cell samples isolated from MM patients. Overactivation of the Alt-NHEJ pathway was revealed in MM cells by larger deletions and higher sequence microhomology at repair junctions, which were reduced by chemical inhibition of the pathway. Taken together, our results uncover a deregulated DSB repair in MM that might underlie the characteristic genome instability of the disease, and could be therapeutically exploited.

Biological effects of low-dose-rate irradiation of pancreatic carcinoma cells in vitro using 125I seeds

AIM: To determine the mechanism of the radiation-induced biological effects of ¹²⁵I seeds on pancreatic carcinoma cells in vitro.

METHODS: SW1990 and PANC-1 pancreatic cancer cell lines were cultured in DMEM in a suitable environment. Gray’s model of iodine-125 (¹²⁵I) seed irradiation was used. In vitro, exponential phase SW1990, and PANC-1 cells were exposed to 0, 2, 4, 6, and 8 Gy using ¹²⁵I radioactive seeds, with an initial dose rate of 12.13 cGy/h. A clonogenic survival experiment was performed to observe the ability of the cells to maintain their clonogenic capacity and to form colonies. Cell-cycle and apoptosis analyses were conducted to detect the apoptosis percentage in the SW1990 and PANC-1 cells. DNA synthesis was measured via a tritiated thymidine (³H-TdR) incorporation experiment. After continuous low-dose-rate irradiation with ¹²⁵I radioactive seeds, the survival fractions at 2 Gy (SF2), percentage apoptosis, and cell cycle phases of the SW1990 and PANC-1 pancreatic cancer cell lines were calculated and compared.

RESULTS: The survival fractions of the PANC-1 and SW1990 cells irradiated with ¹²⁵I seeds decreased exponentially as the dose increased. No significant difference in SF2 was observed between SW1990 and PANC-1 cells (0.766 ± 0.063 vs 0.729 ± 0.045, P < 0.05). The ¹²⁵I seeds induced a higher percentage of apoptosis than that observed in the control in both the SW1990 and PANC-1 cells. The rate of apoptosis increased with increasing radiation dosage. The percentage of apoptosis was slightly higher in the SW1990 cells than in the PANC-1 cells. Dose-dependent G2/M cell-cycle arrest was observed after ¹²⁵I seed irradiation, with a peak value at 6 Gy. As the dose increased, the percentage of G2/M cell cycle arrest increased in both cell lines, whereas the rate of DNA incorporation decreased. In the ³H-TdR incorporation experiment, the dosimetry results of both the SW1990 and PANC-1 cells decreased as the radiation dose increased, with a minimum at 6 Gy. There were no significant differences in the dosimetry results of the two cell lines when they were exposed to the same dose of radiation.

CONCLUSION: The pancreatic cancer cell-killing effects induced by ¹²⁵I radioactive seeds mainly occurred via apoptosis and G2/M cell cycle arrest.

RAD18 activates the G2/M checkpoint through DNA damage signaling to maintain genome integrity after ionizing radiation exposure

The ubiquitin ligase RAD18 is involved in post replication repair pathways via its recruitment to stalled replication forks, and its role in the ubiquitylation of proliferating cell nuclear antigen (PCNA). Recently, it has been reported that RAD18 is also recruited to DNA double strand break (DSB) sites, where it plays novel functions in the DNA damage response induced by ionizing radiation (IR). This new role is independent of PCNA ubiquitylation, but little is known about how RAD18 functions after IR exposure. Here, we describe a role for RAD18 in the IR-induced DNA damage signaling pathway at G2/M phase in the cell cycle. Depleting cells of RAD18 reduced the recruitment of the DNA damage signaling factors ATM, γH2AX, and 53BP1 to foci in cells at the G2/M phase after IR exposure, and attenuated activation of the G2/M checkpoint. Furthermore, depletion of RAD18 increased micronuclei formation and cell death following IR exposure, both in vitro and in vivo. Our data suggest that RAD18 can function as a mediator for DNA damage response signals to activate the G2/M checkpoint in order to maintain genome integrity and cell survival after IR exposure.

Mesenchymal stem cells are resistant to carbon ion radiotherapy

Mesenchymal stem cells (MSCs) participate in regeneration of tissues damaged by ionizing radiation. However, radiation can damage MSCs themselves. Here we show that cellular morphology, adhesion and migration abilities were not measurably altered by photon or carbon ion irradiation. The potential for differentiation was unaffected by either form of radiation, and established MSC surface markers were found to be stably expressed irrespective of radiation treatment. MSCs were able to efficiently repair DNA double strand breaks induced by both high-dose photon and carbon ion radiation. We have shown for the first time that MSCs are relatively resistant to therapeutic carbon ion radiotherapy. Additionally, this form of radiation did not markedly alter the defining stem cell properties or the expression of established surface markers in MSCs.

Abrogation of radioresistance in glioblastoma stem-like cells by inhibition of ATM kinase

Resistance to radiotherapy in glioblastoma (GBM) is an important clinical problem and several authors have attributed this to a subpopulation of GBM cancer stem cells (CSCs) which may be responsible for tumour recurrence following treatment. It is hypothesised that GBM CSCs exhibit upregulated DNA damage responses and are resistant to radiation but the current literature is conflicting. We investigated radioresistance of primary GBM cells grown in stem cell conditions (CSC) compared to paired differentiated tumour cell populations and explored the radiosensitising effects of the ATM inhibitor KU-55933. We report that GBM CSCs are radioresistant compared to paired differentiated tumour cells as measured by clonogenic assay. GBM CSC's display upregulated phosphorylated DNA damage response proteins and enhanced activation of the G2/M checkpoint following irradiation and repair DNA double strand breaks (DSBs) more efficiently than their differentiated tumour cell counterparts following radiation. Inhibition of ATM kinase by KU-55933 produced potent radiosensitisation of GBM CSCs (sensitiser enhancement ratios 2.6-3.5) and effectively abrogated the enhanced DSB repair proficiency observed in GBM CSCs at 24 h post irradiation. G2/M checkpoint activation was reduced but not abolished by KU-55933 in GBM CSCs. ATM kinase inhibition overcomes radioresistance of GBM CSCs and, in combination with conventional therapy, has potential to improve outcomes for patients with GBM.

Radiobiological characteristics of cancer stem cells from esophageal cancer cell lines

AIM: To study the cancer stem cell population in esophageal cancer cell lines KYSE-150 and TE-1 and identify whether the resulting stem-like spheroid cells display cancer stem cells and radiation resistance characteristics.

METHODS: A serum-free medium (SFM) suspension was used to culture esophageal cancer stem cell lines and enrich the esophageal stem-like spheres. A reverse transcription polymerase chain reaction assay was used to detect stem cell gene expression in the spheroid cells. Radiosensitivity of stem-like spheres and parental cells were evaluated by clonogenic assays. Furthermore, different cells after different doses of irradiation were tested to evaluate the change in sphere formation, cell cycle and CD44⁺CD271⁺ expression of tumor stem-like spheroid cells using flow cytometry before and after irradiation.

RESULTS: The cells were observed to generate an increased number of spheres in SFM with increasing cell passage. Radiation increased the rate of generation of stem-like spheres in both types of cells. The average survival fraction (SF₂) of the cultured KYSE-150 compared with TE-1 stem-like spheres after 2 Gy of radiation was 0.81 ± 0.03 vs 0.87 ± 0.01 (P < 0.05), while the average SF₂ of KYSE-150 compared with TE-1 parental cells was 0.69 ± 0.04 vs 0.80 ± 0.03, P < 0.05. In the esophageal parental cells, irradiation dose-dependently induced G2 arrest. Stem-like esophageal spheres were resistant to irradiation-induced G2 arrest without significant changes in the percentage population of irradiated stem-like cells. Under irradiation at 0, 4, and 8 Gy, the CD44⁺CD271⁺ cell percentage for KYSE150 parental cells was 1.08% ± 0.03% vs 1.29% ± 0.07% vs 1.11% ± 0.09%, respectively; the CD44⁺CD271⁺ cell percentage for TE1 parental cells was 1.16% ± 0.11% vs 0.97% ± 0.08% vs 1.45% ± 0.35%, respectively. The differences were not statistically significant. Under irradiation at 0, 4, and 8 Gy, the CD44⁺CD271⁺ cell percentage for KYSE-150 stem-like spheres was 35.83% ± 1.23% vs 44.9% ± 1.67% vs 57.77% ± 1.88%, respectively; the CD44⁺CD271⁺ cell percentage for TE1 stem-like spheres was 16.07% ± 0.91% vs 22.67% ± 1.12%, 16.07% ± 0.91% vs 33.27% ± 1.07%, respectively. The 4 and 8 Gy irradiated KYSE-150 and TE-1 stem-like spheres were compared with the 0 Gy irradiated group, and the differences were statistically significant (P < 0.05).

CONCLUSION: The KYSE-150 and TE-1 stem-like spheres are more radioresistant than their parental cells which may suggest that cancer stem cells are related to radioresistance.

Carbon-ion beam irradiation kills X-ray-resistant p53-null cancer cells by inducing mitotic catastrophe

Background and Purpose

To understand the mechanisms involved in the strong killing effect of carbon-ion beam irradiation on cancer cells with TP53 tumor suppressor gene deficiencies.

Materials and Methods

DNA damage responses after carbon-ion beam or X-ray irradiation in isogenic HCT116 colorectal cancer cell lines with and without TP53 (p53^+/+ and p53^-/-, respectively) were analyzed as follows: cell survival by clonogenic assay, cell death modes by morphologic observation of DAPI-stained nuclei, DNA double-strand breaks (DSBs) by immunostaining of phosphorylated H2AX (γH2AX), and cell cycle by flow cytometry and immunostaining of Ser10-phosphorylated histone H3.

Results

The p53^-/- cells were more resistant than the p53^+/+ cells to X-ray irradiation, while the sensitivities of the p53^+/+ and p53^-/- cells to carbon-ion beam irradiation were comparable. X-ray and carbon-ion beam irradiations predominantly induced apoptosis of the p53^+/+ cells but not the p53^-/- cells. In the p53^-/- cells, carbon-ion beam irradiation, but not X-ray irradiation, markedly induced mitotic catastrophe that was associated with premature mitotic entry with harboring long-retained DSBs at 24 h post-irradiation.

Conclusions

Efficient induction of mitotic catastrophe in apoptosis-resistant p53-deficient cells implies a strong cancer cell-killing effect of carbon-ion beam irradiation that is independent of the p53 status, suggesting its biological advantage over X-ray treatment.

Radiosensitisation by pharmacological ascorbate in glioblastoma multiforme cells, human glial cells, and HUVECs depends on their antioxidant and DNA repair capabilities and is not cancer specific

We previously showed that 5 mM ascorbate radiosensitized early passage radioresistant glioblastoma multiforme (GBM) cells derived from one patient tumor. Here we investigate the sensitivity of a panel of cell lines to 5 mM ascorbate and 6 Gy ionizing radiation, made up of three primary human GBM cells, three GBM cell lines, a human glial cell line, and primary human vascular endothelial cells. The response of different cells lines to ascorbate and/or radiation was determined by measuring viability, colony-forming ability, generation and repair of double-stranded DNA breaks (DSBs), cell cycle progression, antioxidant capacity and generation of reactive oxygen species. Individually, radiation and ascorbate both decreased viability and clonogenicity by inducing DNA damage, but had differential effects on cell cycle progression. Radiation led to G2/M arrest in most cells whereas ascorbate caused accumulation in S phase, which was moderately associated with poor DSB repair. While high dose ascorbate radiosensitized all cell lines in clonogenic assays, the sensitivity to radiation, high dose ascorbate, and combined treatment varied between cell lines. Normal glial cells were similar to GBM cells with respect to free radical scavenging potential and effect of treatment on DNA damage and repair, viability, and clonogenicity. Both GBM cells and normal cells coped equally poorly with oxidative stress caused by radiation and/or high dose ascorbate, dependent primarily on their antioxidant and DSB repair capacity.

Effects of Chk1 inhibition on the temporal duration of radiation-induced G2 arrest in HeLa cells

Chk1 inhibitor acts as a potent radiosensitizer in p53-deficient tumor cells by abrogating the G2/M checkpoint. However, the effects of Chk1 inhibitor on the duration of G2 arrest have not been precisely analyzed. To address this issue, we utilized a cell-cycle visualization system, fluorescent ubiquitination-based cell-cycle indicator (Fucci), to analyze the change in the first green phase duration (FGPD) after irradiation. In the Fucci system, G1 and S/G2/M cells emit red and green fluorescence, respectively; therefore, G2 arrest is reflected by an elongated FGPD. The system also allowed us to differentially analyze cells that received irradiation in the red or green phase. Cells irradiated in the green phase exhibited a significantly elongated FGPD relative to cells irradiated in the red phase. In cells irradiated in either phase, Chk1 inhibitor reduced FGPD almost to control levels. The results of this study provide the first clear information regarding the effects of Chk1 inhibition on radiation-induced G2 arrest, with special focus on the time dimension.

Alterations in gene expression during radiation-induced mitotic catastrophe in HeLa Hep2 cells

AIM

To explore kinetic changes in the gene expression profile during radiation-induced mitotic catastrophes.

MATERIALS AND METHODS

Gene expression changes were measured in HPV-infected HeLa Hep2 tumor cells following exposure to 5 Gy of ionizing radiation ((60)Co). Signaling pathways were explored and correlated to the biological responses linked to mitotic catastrophe.

RESULTS

Following irradiation a transient G2-arrest was induced. Anaphase bridge formation and centrosome hyperamplification was observed. These phenotypical changes correlated well with the observed gene expression changes. Genes with altered expression were found to be involved in mitotic processes as well as G2- and spindle assembly checkpoints. Also centrosome-associated genes displayed an increased expression.

CONCLUSION

This study elucidates specific characteristics in the altered gene expression pattern induced by irradiation, which can be correlated to the events of mitotic catastrophe in HeLa Hep2 cells. Therapeutic strategies modulating these alterations might potentiate future therapy and enhance tumor cell killing.

Mutation of serine 1333 in the ATR HEAT repeats creates a hyperactive kinase

Subcellular localization, protein interactions, and post-translational modifications regulate the DNA damage response kinases ATR, ATM, and DNA-PK. During an analysis of putative ATR phosphorylation sites, we found that a single mutation at S1333 creates a hyperactive kinase. In vitro and in cells, mutation of S1333 to alanine (S1333A-ATR) causes elevated levels of kinase activity with and without the addition of the protein activator TOPBP1. S1333 mutations to glycine, arginine, or lysine also create a hyperactive kinase, while mutation to aspartic acid decreases ATR activity. S1333A-ATR maintains the G₂ checkpoint and promotes completion of DNA replication after transient exposure to replication stress but the less active kinase, S1333D-ATR, has modest defects in both of these functions. While we find no evidence that S1333 is phosphorylated in cultured cells, our data indicate that small changes in the HEAT repeats can have large effects on kinase activity. These mutants may serve as useful tools for future studies of the ATR pathway.

Selective targeting of the G2/M cell cycle checkpoint to improve the therapeutic index of radiotherapy

Despite tremendous advances in radiotherapy techniques, allowing dose escalation to tumour tissues and sparing of organs at risk, cure rates from radiotherapy or chemoradiotherapy remain suboptimal for most cancers. In tandem with our growing understanding of tumour biology, we are beginning to appreciate that targeting the molecular response to radiation-induced DNA damage holds great promise for selective tumour radiosensitisation. In particular, approaches that inhibit cell cycle checkpoint controls offer a means of exploiting molecular differences between tumour and normal cells, thereby inducing so-called cancer-specific synthetic lethality. In this overview, we discuss cellular responses to radiation-induced damage and discuss the potential of using G2/M cell cycle checkpoint inhibitors as a means of enhancing tumour control rates.

High-dose irradiation induces cell cycle arrest, apoptosis, and developmental defects during Drosophila oogenesis

Ionizing radiation (IR) treatment induces a DNA damage response, including cell cycle arrest, DNA repair, and apoptosis in metazoan somatic cells. Because little has been reported in germline cells, we performed a temporal analysis of the DNA damage response utilizing Drosophila oogenesis as a model system. Oogenesis in the adult Drosophila female begins with the generation of 16-cell cyst by four mitotic divisions of a cystoblast derived from the germline stem cells. We found that high-dose irradiation induced S and G2 arrests in these mitotically dividing germline cells in a grp/Chk1- and mnk/Chk2-dependent manner. However, the upstream kinase mei-41, Drosophila ATR ortholog, was required for the S-phase checkpoint but not for the G2 arrest. As in somatic cells, mnk/Chk2 and dp53 were required for the major cell death observed in early oogenesis when oocyte selection and meiotic recombination occurs. Similar to the unscheduled DNA double-strand breaks (DSBs) generated from defective repair during meiotic recombination, IR-induced DSBs produced developmental defects affecting the spherical morphology of meiotic chromosomes and dorsal-ventral patterning. Moreover, various morphological abnormalities in the ovary were detected after irradiation. Most of the IR-induced defects observed in oogenesis were reversible and were restored between 24 and 96 h after irradiation. These defects in oogenesis severely reduced daily egg production and the hatch rate of the embryos of irradiated female. In summary, irradiated germline cells induced DSBs, cell cycle arrest, apoptosis, and developmental defects resulting in reduction of egg production and defective embryogenesis.

[Analysis of irradiation-induced repair foci in mouse embryonic stem cells]

Somatic cells in response to DNA damage activate two important protective mechanisms: G1 checkpoint control and a program for recognizing and repairing DNA defects (DDR signaling). Both mechanisms are triggered by the activation of common sensor kinases ATM and ATR, which in turn phosphorylate downstream targets. Mouse embryonic stem cells (mESCs) lack of G1 checkpoint and undergo only temporary G2 delay after DNA damage. We have analyzed the ability of mESCs to detect DNA damage and to form repair foci after irradiation. We showed irradiation-induced activation of ATM and ATR is followed by formation of γH2AX foci co-localized with DNA repair proteins Rad51, DNA-PK and adapter protein 53BP1. Furthermore, we checked contribution of ATM/Chk2 and ATR/Chk1 cascades to cell cycle control and viability of mESCs after DNA damage. Inhibition of ATR/Chk1 cascade leads to accumulation of G1 phase cells, whereas perturbation of ATM/Chk2 activity causes no such effect. Moreover, inhibition of ATR/Chk1 activity, but not ATM/Chk2, substantially augments the killing effect of ionizing radiation on mESCs. In summary, our results indicate that mESCs are capable of recognizing DNA damage and forming repair foci, but their DDR signaling it seems to be distinct from somatic cells and tightly connected with maintaining of pluripotency and self-renewal.

Telomere-binding protein TPP1 modulates telomere homeostasis and confers radioresistance to human colorectal cancer cells

BACKGROUND

Radiotherapy is one of the major therapeutic strategies in cancer treatment. The telomere-binding protein TPP1 is an important component of the shelterin complex at mammalian telomeres. Our previous reports showed that TPP1 expression was elevated in radioresistant cells, but the exact effects and mechanisms of TPP1 on radiosensitivity is unclear.

PRINCIPAL FINDINGS

In this study, we found that elevated TPP1 expression significantly correlated with radioresistance and longer telomere length in human colorectal cancer cell lines. Moreover, TPP1 overexpression showed lengthened telomere length and a significant decrease of radiosensitivity to X-rays. TPP1 mediated radioresistance was correlated with a decreased apoptosis rate after IR exposure. Furthermore, TPP1 overexpression showed prolonged G2/M arrest mediated by ATM/ATR-Chk1 signal pathway after IR exposure. Moreover, TPP1 overexpression accelerated the repair kinetics of total DNA damage and telomere dysfunction induced by ionizing radiation.

CONCLUSIONS

We demonstrated that elevated expressions of TPP1 in human colorectal cancer cells could protect telomere from DNA damage and confer radioresistance. These results suggested that TPP1 may be a potential target in the radiotherapy of colorectal cancer.

Icaritin synergistically enhances the radiosensitivity of 4T1 breast cancer cells

Icaritin (ICT) is a hydrolytic form of icariin isolated from plants of the genus Epimedium. This study was to investigate the radiosensitization effect of icaritin and its possible underlying mechanism using murine 4T1 breast cancer cells. The combination of Icaritin at 3 µM or 6 µM with 6 or 8 Gy of ionizing radiation (IR) in the clonogenic assay yielded an ER (enhancement ratio) of 1.18 or 1.28, CI (combination index) of 0.38 or 0.19 and DRI (dose reducing index) of 2.51 or 5.07, respectively. These strongly suggest that Icaritin exerted a synergistic killing (?) effect with radiation on the tumor cells. This effect might relate with bioactivities of ICT: 1) exert an anti-proliferative effect in a dose- and time-dependent manner, which is different from IR killing effect but likely work together with the IR effect; 2) suppress the IR-induced activation of two survival paths, ERK1/2 and AKT; 3) induce the G2/M blockage, enhancing IR killing effect; and 4) synergize with IR to enhance cell apoptosis. In addition, ICT suppressed angiogenesis in chick embryo chorioallantoic membrane (CAM) assay. Taken together, ICT is a new radiosensitizer and can enhance anti-cancer effect of IR or other therapies.

Long-term selenium supplementation in HaCaT cells: importance of chemical form for antagonist (protective versus toxic) activities

The beneficial effect of selenium (Se) on cancer is known to depend on the chemical form, the dose and the duration of the supplementation. The aim of this work was to explore long term antagonist (antioxidant versus toxic) effects of an inorganic (sodium selenite, Na2SeO3) and an organic (seleno-L-methionine, SeMet) forms in human immortalized keratinocytes HaCaT cells. HaCaT cells were supplemented with Na2SeO3 or SeMet at micromolar concentrations for 144 h, followed or not by UVA radiation. Se absorption, effects of UVA radiation, cell morphology, antioxidant profile, cell cycle processing, DNA fragmentation, cell death triggered and caspase-3 activity were determined. At non-toxic doses (10 μM SeMet and 1 μM Na2SeO3), SeMet was better absorbed than Na2SeO3. The protection of HaCaT from UVA-induced cell death was observed only with SeMet despite both forms increased glutathione peroxidase-1 (GPX1) activities and selenoprotein-1 (SEPW1) transcript expression. After UVA irradiation, malondialdehyde (MDA) and SH groups were not modulated whatever Se chemical form. At toxic doses (100 μM SeMet and 5 μM Na2SeO3), Na2SeO3 and SeMet inhibited cell proliferation associated with S-G2 blockage and DNA fragmentation leading to apoptosis caspase-3 dependant. SeMet only led to hydrogen peroxide production and to a decrease in mitochondrial transmembrane potential. Our study of the effects of selenium on HaCaT cells reaffirm the necessity to take into account the chemical form in experimental and intervention studies.

Ataxia telangiectasia-mutated- and Rad3-related protein regulates the DNA damage-induced G2/M checkpoint through the Aurora A cofactor Bora protein

Polo-like kinase1 (Plk1) activation is inhibited in response to DNA damage, and this inhibition contributes to the activation of the G2/M checkpoint, although the molecular mechanism by which Plk1 is inhibited is not clear. Here we report that the DNA damage signaling pathway inhibits Plk1 activity through Bora. Following UV irradiation, ataxia telangiectasia-mutated- and Rad3-related protein phosphorylates Bora at Thr-501. The phosphorylated Thr-501 is subsequently recognized by the E3 ubiquitin ligase SCF-β-TRCP, which targets Bora for degradation. The degradation of Bora compromises Plk1 activation and contributes to DNA damage-induced G2 arrest. These findings shed new light on Plk1 regulation by the DNA damage response pathway.

MicroRNA-449a enhances radiosensitivity in CL1-0 lung adenocarcinoma cells

Lung cancer is the leading cause of cancer-related mortality worldwide. Radiotherapy is often applied for treating lung cancer, but it often fails because of the relative non-susceptibility of lung cancer cells to radiation. MicroRNAs (miRNAs) have been reported to modulate the radiosensitivity of lung cancer cells and have the potential to improve the efficacy of radiotherapy. The purpose of this study was to identify a miRNA that can adjust radiosensitivity in lung adenocarcinoma cells. Two lung adenocarcinoma cell lines (CL1-0 and CL1-5) with different metastatic ability and radiosensitivity were used. In order to understand the regulatory mechanisms of differential radiosensitivity in these isogenic tumor cells, both CL1-0 and CL1-5 were treated with 10 Gy radiation, and were harvested respectively at 0, 1, 4, and 24 h after radiation exposure. The changes in expression of miRNA upon irradiation were examined using Illumina Human microRNA BeadChips. Twenty-six miRNAs were identified as having differential expression post-irradiation in CL1-0 or CL1-5 cells. Among these miRNAs, miR-449a, which was down-regulated in CL1-0 cells at 24 h after irradiation, was chosen for further investigation. Overexpression of miR-449a in CL1-0 cells effectively increased irradiation-induced DNA damage and apoptosis, altered the cell cycle distribution and eventually led to sensitization of CL1-0 to irradiation.

A prognostic signature of G(2) checkpoint function in melanoma cell lines

As DNA damage checkpoints are barriers to carcinogenesis, G(2) checkpoint function was quantified to test for override of this checkpoint during melanomagenesis. Primary melanocytes displayed an effective G(2) checkpoint response to ionizing radiation (IR)-induced DNA damage. Thirty-seven percent of melanoma cell lines displayed a significant defect in G(2) checkpoint function. Checkpoint function was melanoma subtype-specific with "epithelial-like" melanoma lines, with wild type NRAS and BRAF displaying an effective checkpoint, while lines with mutant NRAS and BRAF displayed defective checkpoint function. Expression of oncogenic B-Raf in a checkpoint-effective melanoma attenuated G(2) checkpoint function significantly but modestly. Other alterations must be needed to produce the severe attenuation of G(2) checkpoint function seen in some BRAF-mutant melanoma lines. Quantitative trait analysis tools identified mRNA species whose expression was correlated with G(2) checkpoint function in the melanoma lines. A 165 gene signature was identified with a high correlation with checkpoint function (p < 0.004) and low false discovery rate (≤ 0.077). The G(2) checkpoint gene signature predicted G(2) checkpoint function with 77-94% accuracy. The signature was enriched in lysosomal genes and contained numerous genes that are associated with regulation of chromatin structure and cell cycle progression. The core machinery of the cell cycle was not altered in checkpoint-defective lines but rather numerous mediators of core machinery function were. When applied to an independent series of primary melanomas, the predictive G(2) checkpoint signature was prognostic of distant metastasis-free survival. These results emphasize the value of expression profiling of primary melanomas for understanding melanoma biology and disease prognosis.

Endoplasmic reticulum stress sensitizes human esophageal cancer cell to radiation

AIM: To investigate the role of endoplasmic reticulum (ER) stress in cancer radiotherapy and its molecular mechanism.

METHODS: Tunicamycin (TM) was applied to induce ER stress in human esophageal cancer cell line EC109, and the radiosensitization effects were detected by acute cell death and clonogenic survival assay. Cell cycle arrest induced by TM was determined by flow cytometric analysis after the cellular DNA content was labeled with propidium iodide. Apoptosis of EC109 cells induced by TM was detected by annexin V staining and Western blotting of caspase-3 and its substrate poly ADP-ribose polymerase. Autophagic response was determined by acridine orange (AO) staining and Western blotting of microtubule-associated protein-1 light chain-3 (LC3) and autophagy related gene 5 (ATG5). In order to test the biological function of autophagy, specific inhibitor or Beclin-1 knockdown was used to inhibit autophagy, and its effect on cell apoptosis was thus detected. Additionally, involvement of the phosphatidylinositol-3 kinase (PI3K)/Akt/mammalian target of the rapamycin (mTOR) pathway was also detected by Western blotting. Finally, male nude mice inoculated subcutaneously with EC109 cells were used to confirm cell model observations.

RESULTS: Our results showed that TM treatment enhanced cell death and reduced the colony survival fraction induced by ionizing radiation (IR), which suggested an obvious radiosensitization effect of TM. Moreover, TM and IR combination treatment led to a significant increase of G2/M phase and apoptotic cells, compared with IR alone. We also observed an increase of AO positive cells, and the protein level of LC3-II and ATG5 was induced by TM treatment, which suggested an autophagic response in EC109 cells. However, inhibition of autophagy by using a chemical inhibitor or Beclin-1 silencing led to increased cell apoptosis and decreased cell viability, which suggested a cytoprotective role of autophagy in stressed EC109 cells. Furthermore, TM treatment also activated mTORC1, and in turn reduced Akt phosphorylation, which suggested the PI3K/Akt/mTOR signal pathway was involved in the TM-induced autophagic response in EC109 cells. Tumor xenograft results also showed synergistic retarded tumor growth by TM treatment and IR, as well as the involvement of the PI3K/Akt/mTOR pathway.

CONCLUSION: Our data showed that TM treatment sensitized human esophageal cancer cells to radiation via apoptosis and autophagy both in vitro and in vivo.

Restoration of IGFBP-rP1 increases radiosensitivity and chemosensitivity in hormone-refractory human prostate cancer

We previously reported the tumor-suppressive activity of insulin-like growth factor binding protein-related protein 1 (IGFBP-rP1) through induction of apoptosis in human prostate cancer cells. The aim of this study was to investigate the effects of IGFBP-rP1 for radiosensitivity and chemosensitivity in hormone-refractory human prostate PC-3 cancer cells. Five assays were performed using PC-3 cells transfected with IGFBP-rP1 (PC-3rP1) and control cells transfected with an empty vector (PC-3N): PC-3rP1 and PC-3N were compared by clonogenic survival assay, cell cycle analysis and apoptotic assay for radiosensitivity. The number of colonies of PC-3rP1 cells significantly decreased after 4 and 8 Gy of irradiation, compared with those of PC-3N in the clonogenic survival assay. After 16 hr irradiation at 8 Gy, the percentage of apoptotic cells significantly increased in PC-3rP1 compared with PC-3N. Growth of PC-3rP1 was significantly lower than that of PC-3N after docetaxel treatment both in vitro and in vivo. These results indicate that restoration of IGFBP-rP1 to PC-3 cells increases both their radiosensitivity and chemosensitivity.

[Radioresistance mechanisms of side population cells in mouse melanoma cell line B16]

As it was shown by us earlier, side population (SP) cells are more resistant to the low-LET radiation than the other part of mouse melanoma B16 cells (Matchuk et al., 2012). The aim of our research was finding some mechanisms of radioresistance, therefore we analyzed SP and nonSP cell cycle distribution, spontaneous and radiation induced DNA double-strand breaks (number of γH2AX foci) and intracellular NO concentration. The results indicate that SP cells have significantly less DNA double-strand breaks after irradiation at dose of 3 Gy than nonSP cells (24.4 vs 40.3, accordingly, P < 0.05 Mann-Whitney Ucriterion). SP cells are more quiescent compared to nonSP G1/G0 fraction is 85 vs 39%, accordingly, P < 0.01 Mann-Whitney U criterion). Most nonSP cells reside in S, G2/M phases (61%), believed to be rather radiosensitive. Thus, the difference of SP and nonSP cells radiosensitivity can be partly explained by peculiarities of cell cycle distribution. NO concentration is 1.5 times higher in SP than nonSP cells (P < 0.05 Mann-Whitney U criterion); since it is known that NO inhibits apoptosis, being one of the mechanisms of genetic stability maintenance, greater number of spontaneous DNA double-strand breaks in SP cells is unsurprising (P < 0.05 Mann-Whitney U criterion). The above-listed results explain considerably the higher resistance of SP cells to the action of low-LET radiation in comparison with other melanoma B16 cells. Further study of this question can become the basis for development of tools to target SP cells and, ultimately, more effective cancer treatment.

Cell division cycle 25 homolog c effects on low-dose hyper-radiosensitivity and induced radioresistance at elevated dosage in A549 cells

The underlying mechanisms behind both low-dose hyper-radiosensitivity (HRS) and induced radioresistance (IRR), generally occurring at elevated radiation levels, remain unclear; however, elucidation of the relationship between cell cycle division 25 homolog c (Cdc25c) phosphatase and HRS/IRR may provide important insights into this process. Two cell lines with disparate HRS status, A549 and SiHa cells, were selected as cell models for comparison of dose-dependent Cdc25c phosphatase expression subsequent to low-dose irradiation. Knockdown of Cdc25c in A549 cells was mediated by transfection with a pGCsi-RAN-U6neo vector containing hairpin siRNA sequences. S216-phosphorylated Cdc25c protein [p-Cdc25c (Ser216)], cell survival and mitotic ratio were measured by western blot, colony-forming assay and histone H3 phosphorylation analysis. Variant p-Cdc25c (Ser216) expression was observed in the two cell lines after irradiation. The p-Cdc25c (Ser216) expression noted in SiHa cells after administration of 0-1 Gy radiation was similar to the radioresistance model; however, in A549 cells, the dose response for the phosphorylation of the Cdc25c Ser216 residue overlapped the level required to overcome the HRS response. Furthermore, Cdc25c repression prior to low-dose radiation induced more distinct HRS and prevented the development of IRR. The dose required to overcome the HRS response coincided with the effect of early G2-phase checkpoint arrest in A549 cells (approximately 0.3 Gy), and Cdc25c knockdown in A549 cells (approximately 0.5 Gy) corresponded to the phosphorylation of the Cdc25c Ser216 residue. Resultant data confirmed that dose-dependent Cdc25c phosphatase does effectively act as an early G2-phase checkpoint, thus indicating mechanistic importance in the HRS to IRR transition in A549 cells.