Phosphorylation of histone H2AX as a measure of radiosensitivity

Nitric oxide and acid induce double-strand DNA breaks in Barrett's esophagus carcinogenesis via distinct mechanisms

BACKGROUND & AIMS

The luminal microenvironment including acid and nitric oxide (NO) has been implicated in Barrett's esophagus carcinogenesis. We investigated the ability of acid and NO to induce DNA damage in esophageal cells.

METHODS

Transformed and primary Barrett's esophagus and adenocarcinoma cells were exposed to either acid, (pH 3.5), +/- antioxidant or NO from a donor or generated by acidification of nitrite in the presence of ascorbate +/- NO scavenger. Phosphorylation of histone H2AX and the neutral comet assay were used to detect DNA double-strand breaks (DSBs). Intracellular levels of reactive oxygen species and NO were detected with fluorescent dyes. Mitochondrial viability was measured with a rhodamine dye. Long-term survival was assessed by clonogenic assay.

RESULTS

Exposure to acid (pH 3.5) for > or =15 minutes induced DSBs in all cell lines (P < .05). There was a concomitant increase in intracellular reactive oxygen species in the absence of mitochondrial damage, and pretreatment with antioxidants inhibited DNA damage. Exposure to physiologic concentrations of NO produced from the NO donor or acidification of salivary nitrite induced DSBs in a dose- (>25 micromol/L) and cell-dependent manner (adenocarcinoma >Barrett's esophagus, P < .05). This occurred preferentially in S-phase cells consistent with stalled replication forks and was blocked with a NO scavenger. NO also induced DSBs in primary Barrett's esophagus cells treated ex vivo. Cells were able to survive when exposed to acid and NO.

CONCLUSIONS

Both acid and NO have the potential to generate DSBs in the esophagus and via distinct mechanisms.

Identification and biological evaluation of a novel and potent small molecule radiation sensitizer via an unbiased screen of a chemical library

For patients with solid tumors, the tolerance of surrounding tissues often limits the dose of radiation that can be delivered. Thus, agents that preferentially increase the cytotoxic effects of radiation toward tumor cells would significantly alter the therapeutic ratio and improve patient survival. Using a high-throughput, unbiased screening approach, we have identified 4'-bromo-3'-nitropropiophenone (NS-123) as a radiosensitizer of human glioma cells in vitro and in vivo. NS-123 radiosensitized U251 glioma cells in a dose-dependent and time-dependent manner, with dose enhancement ratios ranging from 1.3 to 2.0. HT-29 colorectal carcinoma and A549 lung adenocarcinoma cells were also radiosensitized by NS-123 in vitro, whereas NS-123 did not increase the radiation sensitivity of normal human astrocytes or developmental abnormalities or lethality of irradiated Zebrafish embryos. In a novel xenograft model of U251 cells implanted into Zebrafish embryos, NS-123 enhanced the tumor growth-inhibitory effects of ionizing radiation (IR) with no apparent effect on embryo development. Similar results were obtained using a mouse tumor xenograft model in which NS-123 sensitized U251 tumors to IR while exhibiting no overt toxicity. In vitro pretreatment with NS-123 resulted in accumulation of unrepaired IR-induced DNA strand breaks and prolonged phosphorylation of the surrogate markers of DNA damage H2AX, ataxia telangiectasia mutated protein, DNA-dependent protein kinase, and CHK2 after IR, suggesting that NS-123 inhibits a critical step in the DNA repair pathway. These results show the potential of this cell-based, high-throughput screening method to identify novel radiosensitizers and suggest that NS-123 and similar nitrophenol compounds may be effective in antiglioma modalities.

Cytometry of ATM activation and histone H2AX phosphorylation to estimate extent of DNA damage induced by exogenous agents

This review covers the topic of cytometric assessment of activation of Ataxia telangiectasia mutated (ATM) protein kinase and histone H2AX phosphorylation on Ser139 in response to DNA damage, particularly the damage that involves formation of DNA double-strand breaks. Briefly described are molecular mechanisms associated with activation of ATM and the downstream events that lead to recruitment of DNA repair machinery, engagement of cell cycle checkpoints, and activation of apoptotic pathway. Examples of multiparameter analysis of ATM activation and H2AX phosphorylation vis-a-vis cell cycle phase position and induction of apoptosis that employ flow- and laser scanning-cytometry are provided. They include cells treated with a variety of exogenous genotoxic agents, such as ionizing and UV radiation, DNA topoisomerase I (topotecan) and II (mitoxantrone, etoposide) inhibitors, nitric oxide-releasing aspirin, DNA replication inhibitors (aphidicolin, hydroxyurea, thymidine), and complex environmental carcinogens such as present in tobacco smoke. Also presented is an approach to identify DNA replicating (BrdU incorporating) cells based on selective photolysis of DNA that triggers H2AX phosphorylation. Listed are strategies to distinguish ATM activation and H2AX phosphorylation induced by primary DNA damage by genotoxic agents from those effects triggered by DNA fragmentation that takes place during apoptosis. While we review most published data, recent new findings also are included. Examples of multivariate analysis of ATM activation and H2AX phosphorylation presented in this review illustrate the advantages of cytometric flow- and image-analysis of these events in terms of offering a sensitive and valuable tool in studies of factors that induce DNA damage and/or affect DNA repair and allow one to explore the linkage between DNA damage, cell cycle checkpoints and initiation of apoptosis.

Relationship between phosphorylated histone H2AX formation and cell survival in human microvascular endothelial cells (HMEC) as a function of ionizing radiation exposure in the presence or absence of thiol-containing drugs

Human microvascular endothelial cells (HMEC) were exposed to ionizing radiation at doses ranging from 0 to 16 Gy in either the presence or absence of the active thiol forms of amifostine (WR1065), phosphonol (WR255591), N-acetyl-l-cysteine (NAC), captopril or mesna. Each of these clinically relevant thiols, administered to HMEC at a dose of 4 mM for 30 min prior to irradiation, is known to exhibit antioxidant properties. The purpose of this investigation was to determine the relationship(s), if any, between the frequency of radiation-induced histone H2AX phosphorylation at serine 139 (gamma-H2AX) in cells and subsequent survival, as assessed by colony-forming ability, in exposed cell populations as a function of the presence or absence of each of the five thiol compounds during irradiation. gamma-H2AX formation in irradiated cells, as a function of relative DNA content, was quantified by bivariant flow cytometry analysis with FITC-conjugated gamma-H2AX antibody and nuclear DAPI staining. gamma-H2AX formation in cells was measured as the relative fold increase as a function of the treatment conditions. The frequency of gamma-H2AX-positive cells increased with increasing dose of radiation followed by a dose- and time-dependent decay. The most robust response for gamma-H2AX formation occurred 1 h after irradiation with their relative frequencies decreasing as a function of time 4 and 24 h later. To assess the effects of the various thiols on gamma-H2AX formation, all measurements were made 1 h after irradiation. WR1065 was not only effective in protecting HMEC against gamma-H2AX formation across the entire dose range of radiation exposures used, but it was also significantly more cytoprotective than either its prodrug (WR2721) or disulfide (WR33278) analogue. WR1065 had no significant effect on gamma-H2AX formation when administered immediately or up to 30 min after radiation exposure. An inhibitory effect against gamma-H2AX formation induced by 8 Gy of radiation was expressed by each of the thiols tested. NAC, captopril and mesna were equally effective in reducing the frequency of gamma-H2AX formation, with both WR1065 and WR255591 exhibiting a slightly more robust protective effect. Each of the five thiols was effective in reducing the frequency of gamma-H2AX-positive cells across all phases of the cell cycle. In contrast to the relative ability of each of these thiols to inhibit gamma-H2AX formation after irradiation, NAC, captopril and mesna afforded no protection to HMEC as determined using a colony-forming survival assay. Only WR1065 and WR255591 were effective in reducing the frequencies of radiation-induced gamma-H2AX-positive cells as well as protecting against cell death. These results suggest that the use of gamma-H2AX as a biomarker for screening the efficacy of novel antioxidant radioprotective compounds is highly problematic since their formation and disappearance may be linked to processes beyond simply the formation and repair of radiation-induced DSBs.

A phase 1 clinical-laboratory study of clofarabine followed by cyclophosphamide for adults with refractory acute leukemias

Clofarabine has shown impressive response rates in patients with acute leukemias. In vitro investigations with clofarabine in combination with cyclophosphamide in primary cells have demonstrated synergistic cytotoxicity and inhibition of DNA repair. Based on these clinical and laboratory observations, we designed a mechanism-based combination protocol with clofarabine and cyclophosphamide for patients with relapsed acute leukemias. Eighteen patients were treated with cyclophosphamide (200 mg/m(2)) alone on day 0 and with clofarabine plus cyclophosphamide on day 1. Clinical responses, toxicity, DNA damage measured as H2AX phosphorylation, and accumulation of clofarabine triphosphate (TP) were analyzed. At dose level 1 (20 mg/m(2) clofarabine + cyclophosphamide, 6 patients) and dose level 0 (10 mg/m(2) clofarabine + cyclophosphamide, 12 patients) overall response rates were 50% and 30%, respectively, with responses in 4 (67%) of 6 patients with refractory acute lymphoblastic leukemia. Dose-limiting toxicity occurred at dose level 1 with prolonged marrow aplasia. Four (22%) patients died from prolonged aplasia (1), fungal pneumonia (1), or multiorgan failure (2). In 12 of 13 patient samples, increased DNA damage (gammaH2AX) was observed with clofarabine and cyclophosphamide compared with cyclophosphamide alone. In conclusion, pharmacodynamic end points along with clinical results suggest usefulness of this combination strategy, whereas toxicity data suggest reduction in chemotherapeutic intensity. This clinical trial is registered with the National Cancer Institute's PDQ at www.clinicaltrials.gov as no. JHOC-J0561.

In vivo efficacy of a novel histone deacetylase inhibitor in combination with radiation for the treatment of gliomas

Histone modification has emerged as a promising approach to cancer therapy. We explored the in vivo efficacy of a butyric acid derivative, pivaloyloxymethyl butyrate (AN-9), for the treatment of gliomas. Relative to control and single-modality treatments, the combination of AN-9 and radiation significantly inhibited tumor growth and prolonged time to failure in mice bearing glioma xenografts. The enhanced response to radiation was accompanied by inhibition of cellular proliferation and by increased phosphorylation of H2AX, implicating DNA double-strand breaks in the antineoplastic effects of AN-9 and radiation. The data suggest that AN-9 in combination with radiation may be an effective therapy for malignant gliomas.

An optimized method for detecting gamma-H2AX in blood cells reveals a significant interindividual variation in the gamma-H2AX response among humans

Phosphorylation of histone H2AX on serine 139 (gamma-H2AX, γH2AX) occurs at sites flanking DNA double-strand breaks (DSBs) and can provide a measure of the number of DSBs within a cell. Here we describe a rapid and simple flow-cytometry-based method, optimized to measure gamma-H2AX in non-fixed peripheral blood cells. No DSB induced signal was observed in H2AX−/− cells indicating that our FACS method specifically recognized gamma-H2AX accumulation. The gamma-H2AX assay was capable of detecting DNA damage at levels 100-fold below the detection limit of the alkaline comet assay. The gamma-H2AX signal was quantitative with a linear increase of the gamma-H2AX signal over two orders of magnitude. We found that all nucleated blood cell types examined, including the short-lived neutrophils induce gamma-H2AX in response to DSBs. Interindividual difference in the gamma-H2AX signal in response to ionizing radiation and the DSB-inducing drug calicheamicin was almost 2-fold in blood cells from patients, indicating that the amount of gamma-H2AX produced in response to a given dose of radiation varies significantly in the human population. This simple method could be used to monitor response to radiation or DNA-damaging drugs.

Relationship between DNA double-strand break rejoining and cell survival after exposure to ionizing radiation in human fibroblast strains with differing ATM/p53 status: implications for evaluation of clinical radiosensitivity

PURPOSE

To better understand the impact of defects in the DNA damage-surveillance network on the various cell-based assays used for the prediction of patient radiosensitivity.

METHODS AND MATERIALS

We examined noncancerous human fibroblast strains from individuals with ataxia telangiectasia (ataxia telangiectasia mutated [ATM] deficient) or Li-Fraumeni syndrome (p53 deficient) using the neutral comet, H2AX phosphorylation, and clonogenic survival assays.

RESULTS

Using the comet assay, we found that, compared with normal fibroblasts, cells lacking either ATM or p53 function exhibited a reduced rate of double-strand break (DSB) rejoining early (< or =4 h) after exposure to 8 Gy of gamma-radiation and also exhibited high levels of unrejoined DSBs later after irradiation. ATM-deficient and p53-deficient fibroblasts also exhibited abnormally increased levels of phosphorylated H2AX (gamma-H2AX) at later intervals after irradiation. In the clonogenic assay, ATM-deficient cells exhibited marked radiosensitivity and p53-deficient cells had varying degrees of radioresistance compared with normal fibroblasts.

CONCLUSION

Regardless of whether ataxia telangiectasia and Li-Fraumeni syndrome fibroblasts are DSB-repair deficient per se, it is apparent that p53 and ATM defects greatly influence the cellular phenotype as evidenced by the neutral comet and gamma-H2AX assays. Our data suggest that the gamma-H2AX levels observed at later intervals after irradiation may represent a reliable measure of the overall DSB rejoining capabilities of human fibroblasts. However, it appears that using this parameter as a predictor of radiosensitivity without knowledge of the cells' p53 status could lead to incorrect conclusions.

Inhibition of hsp90 compromises the DNA damage response to radiation

Inhibitors of the molecular chaperone Hsp90 have been shown to enhance tumor cell radiosensitivity. To begin to address the mechanism responsible, we have determined the effect of the Hsp90 inhibitor 17-(dimethylaminoethylamino)-17-demethoxygeldanamycin (17DMAG) on the DNA damage response to radiation. Exposure of MiaPaCa tumor cells to 17DMAG, which results in radiosensitization, inhibited the repair of DNA double-strand breaks according to gammaH2AX foci dispersal and the neutral comet assay. This repair inhibition was associated with reduced DNA-PK catalytic subunit (DNA-PKcs) phosphorylation after irradiation and a disruption of DNA-PKcs/ErbB1 interaction. These data suggest that the previously established 17DMAG-mediated reduction in ErbB1 activity reduces its interaction with DNA-PKcs and thus accounts for the attenuation of radiation-induced DNA-PK activation. 17DMAG was also found to abrogate the activation of the G(2)- and S-phase cell cycle checkpoints. Associated with these events was a reduction in radiation-induced ataxia-telangiectasia mutated (ATM) activation and foci formation in 17DMAG-treated cells. Although no interaction between ATM and Hsp90 was detected, Hsp90 was found to interact with the MRE11/Rad50/NBS1 (MRN) complex. 17DMAG exposure reduced the ability of the MRN components to form nuclear foci after irradiation. Moreover, 17DMAG exposure reduced the interaction between NBS1 and ATM, although no degradation of the MRN complex was detected. These results suggest that the diminished radiation-induced activation of ATM in 17DMAG-treated cells was the result of a compromise in the function of the MRN complex. These data indicate that Hsp90 can contribute to the DNA damage response to radiation affecting both DNA repair and cell cycle checkpoint activation.

Phosphorylated histone H2AX in relation to cell survival in tumor cells and xenografts exposed to single and fractionated doses of X-rays

BACKGROUND AND PURPOSE

Human tumor cell lines grown as monolayers or xenograft tumors were exposed to single or multiple fractions of X-rays and the ability to use residual gammaH2AX to identify radiosensitive cells was assessed.

MATERIALS AND METHODS

Twenty-four hour after exposure to single or daily fractions of X-rays, human tumor cells from monolayers or xenografts were analyzed for clonogenic surviving fraction. Cells were also fixed and labeled with anti-gammaH2AX antibodies for analysis by flow and image cytometry. The relative amount of residual gammaH2AX and the percentage of cells with <3 foci were compared with the clonogenic surviving fraction measured for the same population.

RESULTS

The fraction of gammaH2AX remaining 24h after X-irradiation relative to peak levels 1h after exposure was correlated with radiosensitivity (SF2) for 18 human tumor cell lines. The fraction of SiHa, C33A and WiDr cells with <3 gammaH2AX foci was predictive of clonogenic surviving fraction for both monolayer cells exposed to either single doses or up to 5 fractions. Similar results were obtained using cells from xenograft tumors of irradiated mice.

CONCLUSION

The percentage of tumor cells that retain gammaH2AX foci 24h after single or fractionated doses appears to be a useful measure of cellular radiosensitivity that is potentially applicable in the clinic.

Vorinostat, a histone deacetylase inhibitor, enhances the response of human tumor cells to ionizing radiation through prolongation of gamma-H2AX foci

Vorinostat (suberoylanilide hydroxamic acid) is the prototype of a family of hybrid polar compounds that can induce growth arrest in transformed cells and shows promise for the treatment of cancer. Vorinostat specifically binds to and inhibits the activity of histone deacetylases resulting in acetylation of nucleosomal histones and an activation of gene transcription. Because histone deacetylases modulate chromatin structure and gene expression, both of which can influence radioresponse, this study was designed to examine the capacity of Vorinostat to influence radiation response in human tumor cells and investigate the mechanism underlying these interactions. Vorinostat induced hyperacetylation of histone H4 in a dose-dependent manner. We tested its ability to radiosensitize three human tumor cell lines (A375, MeWo, and A549) using clonogenic cell survival assays. Clonogenic cell survival assay showed that Vorinostat significantly radiosensitized all three tumor cell lines, substantially reducing the surviving fraction at 2 Gy. We examined potential mechanisms that may contribute to the enhanced radiation response induced by Vorinostat. Vorinostat and radiation alone did not induce apoptosis in the melanoma cell line. However, enhanced apoptosis was observed when cells were exposed to both Vorinostat and radiation, suggesting that Vorinostat renders tumor cells more susceptible to radiation-induced apoptosis. Results from DNA damage repair analysis in cultured A375 cells showed that Vorinostat had a strong inhibitory effect on the nonhomologous end joining pathway after radiation. A detailed examination of the involvement of the DNA repair pathway following Vorinostat treatment showed that Vorinostat reduced the expression of the repair-related genes Ku70, Ku80, and Rad50 in A375 cells as detected by Western blot analysis. We also examined gamma-H2AX phosphorylation as a predictive marker of radiotherapy response to Vorinostat and observed that the combination of Vorinostat and radiation caused a prolongation of expression of DNA repair proteins such as gamma-H2AX. Overall, we conclude that Vorinostat enhances tumor radioresponse by multiple mechanisms that may involve antiproliferative growth inhibition and effects on DNA repair after exposure to radiation.

Preclinical evaluation of a potent novel DNA-dependent protein kinase inhibitor NU7441

DNA double-strand breaks (DSB) are the most cytotoxic lesions induced by ionizing radiation and topoisomerase II poisons, such as etoposide and doxorubicin. A major pathway for the repair of DSB is nonhomologous end joining, which requires DNA-dependent protein kinase (DNA-PK) activity. We investigated the therapeutic use of a potent, specific DNA-PK inhibitor (NU7441) in models of human cancer. We measured chemosensitization by NU7441 of topoisomerase II poisons and radiosensitization in cells deficient and proficient in DNA-PK(CS) (V3 and V3-YAC) and p53 wild type (LoVo) and p53 mutant (SW620) human colon cancer cell lines by clonogenic survival assay. Effects of NU7441 on DSB repair and cell cycle arrest were measured by gammaH2AX foci and flow cytometry. Tissue distribution of NU7441 and potentiation of etoposide activity were determined in mice bearing SW620 tumors. NU7441 increased the cytotoxicity of ionizing radiation and etoposide in SW620, LoVo, and V3-YAC cells but not in V3 cells, confirming that potentiation was due to DNA-PK inhibition. NU7441 substantially retarded the repair of ionizing radiation-induced and etoposide-induced DSB. NU7441 appreciably increased G(2)-M accumulation induced by ionizing radiation, etoposide, and doxorubicin in both SW620 and LoVo cells. In mice bearing SW620 xenografts, NU7441 concentrations in the tumor necessary for chemopotentiation in vitro were maintained for at least 4 hours at nontoxic doses. NU7441 increased etoposide-induced tumor growth delay 2-fold without exacerbating etoposide toxicity to unacceptable levels. In conclusion, NU7441 shows sufficient proof of principle through in vitro and in vivo chemosensitization and radiosensitization to justify further development of DNA-PK inhibitors for clinical use.

Low-Dose Hyper-radiosensitivity is not Caused by a Failure to Recognize DNA Double-Strand Breaks

One of the earliest cellular responses to radiation-induced DNA damage is the phosphorylation of the histone variant H2AX (gamma-H2AX). gamma-H2AX facilitates the local concentration and focus formation of numerous repair-related proteins within the vicinity of DNA DSBs. Previously, we have shown that low-dose hyper-radiosensitivity (HRS), the excessive sensitivity of mammalian cells to very low doses of ionizing radiation, is a response specific to G(2)-phase cells and is attributed to evasion of an ATM-dependent G(2)-phase cell cycle checkpoint. To further define the mechanism of low-dose hyper-radiosensitivity, we investigated the relationship between the recognition of radiation-induced DNA double-strand breaks as defined by gamma-H2AX staining and the incidence of HRS in three pairs of isogenic cell lines with known differences in radiosensitivity and DNA repair functionality (disparate RAS, ATM or DNA-PKcs status). Marked differences between the six cell lines in cell survival were observed after high-dose exposures (>1 Gy) reflective of the DNA repair capabilities of the individual six cell lines. In contrast, the absence of functional ATM or DNA-PK activity did not affect cell survival outcome below 0.2 Gy, supporting the concept that HRS is a measure of radiation sensitivity in the absence of fully functional repair. No relationship was evident between the initial numbers of DNA DSBs scored immediately after either low- or high-dose radiation exposure with cell survival for any of the cell lines, indicating that the prevalence of HRS is not related to recognition of DNA DSBs. However, residual DNA DSB damage as indicated by the persistence of gamma-H2AX foci 4 h after exposure was significantly correlated with cell survival after exposure to 2 Gy. This observation suggests that the persistence of gamma-H2AX foci could be adopted as a surrogate assay of cellular radiosensitivity to predict clinical radiation responsiveness.

Phosphorylated histone H2AX foci persist on rejoined mitotic chromosomes in normal human diploid cells exposed to ionizing radiation

Histone H2AX is phosphorylated and forms foci in response to exposure to ionizing radiation. It has been thought that phosphorylated histone H2AX foci reflect unrepaired DNA double-strand breaks; however, we report here the localization of phosphorylated histone H2AX foci at the site of rejoined DNA double-strand breaks. We observed that phosphorylated histone H2AX foci remained even 96 h after exposure to X rays in interphase cells. To clarify the localization of residual phosphorylated histone H2AX foci, we examined localization of focus formation on mitotic chromosomes irradiated with X rays. We found that phosphorylated histone H2AX foci were located not only on chromosomal fragments but also on intact metaphase chromosomes without fragments. In anaphase cells, chromosomal bridges, which resulted from illegitimate rejoining of DNA broken ends, had phosphorylated histone H2AX foci. These foci were detected as individual small spots 30 min after X irradiation, but foci detected 20 or 96 h after X irradiation were clustered along the chromosomal bridges. These results indicate that phosphorylated histone H2AX foci persist if DNA breaks are rejoined. It is suggested that "residual" foci indicate an aberrant chromatin structure by illegitimate rejoining but not a DNA double-strand break itself.

Radiation-induced H2AX phosphorylation and neural precursor apoptosis in the developing brain of mice

We showed that gamma irradiation of the developing mouse brain with 2 Gy induced a massive apoptosis of neural precursors but not of neurons within 24 h. Successive phosphorylation and dephosphorylation of histone H2AX have been linked to DNA breaks and repair. Similar numbers of nuclear foci of phosphorylated H2AX (gamma-H2AX) were found 1 h postirradiation in neural precursors and in neurons, suggesting that differences in radiosensitivity were not related to variations in the numbers of DNA double-strand breaks induced by radiation. Surviving neural precursors like neurons totally lost gamma-H2AX within 24 h after irradiation, but they had a slower kinetics of loss of gamma-H2AX foci. This suggests that the DNA repair machinery processed damage more slowly in these neural precursors in relation to their greater radiosensitivity. We also found a bright and diffuse gamma-H2AX staining of nuclei of cells at an early stage of apoptosis, whereas cells at later stages of apoptosis were unstained. This was probably related to phosphorylation and subsequent degradation of H2AX in the course of DNA fragmentation during apoptosis. Detection of gamma-H2AX-bright nuclei may thus be a useful marker of neural cells at an early stage of apoptosis.

Radiation induced DNA damage and damage repair in human tumor and fibroblast cell lines assessed by histone H2AX phosphorylation

PURPOSE

To analyze the radiation-induced levels of gammaH2AX and its decay kinetics in 10 human cell lines covering a wide range of cellular radiosensitivity (SF2, 0.06-0.63).

METHODS AND MATERIALS

Five tumor cell lines included Colo-800 melanoma, two glioblastoma (MO59J and MO59K), fibrosarcoma HT 1080, and breast carcinoma MCF7. Five primary skin fibroblasts lines included two normal strains, an ataxia telangiectasia strain, and two fibroblast strains from breast cancer patients with an adverse early skin reaction to radiotherapy. Cellular radiosensitivity was assessed by colony-forming test. Deoxyribonucleic acid damage and repair were analyzed according to nuclear gammaH2AX foci intensity, with digital image analysis.

RESULTS

The cell lines tested showed a wide degree of variation in the background intensity of immunostained nuclear histone gammaH2AX, which was higher for the tumor cell lines compared with the fibroblast strains. It was not possible to predict clonogenic cell survival (SF2) for the 10 cell lines studied from the radiation-induced gammaH2AX intensity. In addition, the slopes of the dose-response (0-4 Gy) curves, the rates of gammaH2AX disappearance, and its residual expression (<or=18 h after irradiation) did not correlate with SF2 values.

CONCLUSIONS

The results from 10 cell lines showed that measurements of immunofluorescence intensity by digital image analysis of phosphorylated histone H2AX as a surrogate marker of DNA double-strand breaks did not allow reliable ranking of cell strains according to their clonogenic survival after irradiation.

Modulation of cellular radiation responses by histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors are emerging as a new class of targeted cancer chemotherapeutics. Several HDAC inhibitors are currently in clinical trials and promising anticancer effects at well-tolerated doses have been observed for both hematologic and solid cancers. HDAC inhibitors have been shown to induce cell-cycle and growth arrest, differentiation and in certain cases apoptosis in cell cultures and in vivo. However, it is known that these compounds induce varying responses in different cells and biological settings, and identifying their precise mechanisms of action is an area of great interest. Important findings are continually expanding our understanding of the cellular effects of HDAC inhibitors and recent studies will be briefly outlined in this review. In addition to their intrinsic anticancer properties, numerous studies have demonstrated that HDAC inhibitors can modulate cellular responses to other cytotoxic modalities including ionizing radiation, ultraviolet radiation and chemotherapeutic drugs. Hence, there is a growing interest in potential clinical use of HDAC inhibitors in combination with conventional cancer therapies. In this review, the interaction of HDAC inhibitors with other anticancer agents is discussed. The focus of the article is on the different mechanisms by which HDAC inhibitors enhance the sensitivity of cells to the effects of ionizing radiation.

Computational Methods for Analysis of Foci: Validation for Radiation-Induced γ-H2AX Foci in Human Cells

Observation and counting of gamma-H2AX foci in untreated cells as well as in cells exposed to cytotoxic agents is a widely used method for documenting the presence of double-strand breaks (DSBs) in the DNA and for analysis of their repair. Similar methods are employed to analyze formation of foci by a variety of proteins implicated in the cellular responses to DNA damage. Despite the wide application of the approach, the manual counting that is frequently used is prone to inaccuracies and investigator-related biases and artifacts. To alleviate this limitation, we developed and describe here personal computer-based algorithms, operating as utilities on available software, that allow an objective and quantitative analysis of foci from confocal images. The algorithms allow focus counting as well as size definition and correct for focus coincidence due to the overlap normally occurring with an increasing number of foci per nucleus. Furthermore, the software allows measurement of the integrated optical density (IOD) of each individual focus, which enables analysis of properties of foci as a function of time. Finally, the information generated by the above analysis algorithms can be employed to evaluate colocalization between foci formed by different proteins. A validation of the software is presented for radiation-induced gamma-H2AX foci in three widely used human cell lines and colocalization tested with RAD51 and gamma-H2AX foci. The computational methods presented extend to images generated by digital cameras.

Interstitial chromatin alteration causes persistent p53 activation involved in the radiation-induced senescence-like growth arrest

Various stresses including ionizing radiation give normal human fibroblasts a phenotype of senescence-like growth arrest (SLGA), manifested by p53-dependent irreversible G1 arrest. To determine the mechanism of persistent activation of p53, we examined phosphorylated Ataxia telangiectasia mutated (ATM) and phosphorylated histone H2AX foci formation after X-irradiation. Although the multiple tiny foci, detected soon after (<30 min) irradiation, gradually disappeared, some of these foci changed to large foci and persisted for 5 days. Large foci containing phosphorylated ATM and gamma-H2AX co-localized and foci with p53 phosphorylated at serine 15 also showed the same distribution. Interestingly, the signals obtained by telomere fluorescence in situ hybridization (FISH) assay did not co-localize with 90% of the large foci. Our results indicate that chromatin alteration in interstitial chromosomal regions is the most likely cause of continuous activation of p53, which results in the induction of SLGA by ionizing radiation.

Histone deacetylase inhibitors radiosensitize human melanoma cells by suppressing DNA repair activity

PURPOSE

Histone deacetylase (HDAC) inhibitors have emerged recently as promising anticancer agents. They arrest cells in the cell cycle and induce differentiation and cell death. The antitumor activity of HDAC inhibitors has been linked to their ability to induce gene expression through acetylation of histone and nonhistone proteins. However, it has recently been suggested that HDAC inhibitors may also enhance the activity of other cancer therapeutics, including radiotherapy. The purpose of this study was to evaluate the ability of HDAC inhibitors to radiosensitize human melanoma cells in vitro.

EXPERIMENTAL DESIGN

A panel of HDAC inhibitors that included sodium butyrate (NaB), phenylbutyrate, tributyrin, and trichostatin A were tested for their ability to radiosensitize two human melanoma cell lines (A375 and MeWo) using clonogenic cell survival assays. Apoptosis and DNA repair were measured by standard assays.

RESULTS

NaB induced hyperacetylation of histone H4 in the two melanoma cell lines and the normal human fibroblasts. NaB radiosensitized both the A375 and MeWo melanoma cell lines, substantially reducing the surviving fraction at 2 Gy (SF2), whereas it had no effect on the normal human fibroblasts. The other HDAC inhibitors, phenylbutyrate, tributyrin, and trichostatin A had significant radiosensitizing effects on both melanoma cell lines tested. NaB modestly enhanced radiation-induced apoptosis that did not correlate with survival but did correlate with functional impairment of DNA repair as determined based on the host cell reactivation assay. Moreover, NaB significantly reduced the expression of the repair-related genes Ku70 and Ku86 and DNA-dependent protein kinase catalytic subunit in melanoma cells at the protein and mRNA levels. Normal human fibroblasts showed no change in DNA repair capacity or levels of DNA repair proteins following NaB treatment. We also examined gamma-H2AX phosphorylation as a marker of radiation response to NaB and observed that compared with controls, gamma-H2AX foci persisted long after ionizing exposure in the NaB-treated cells.

CONCLUSIONS

HDAC inhibitors radiosensitize human tumor cells by affecting their ability to repair the DNA damage induced by ionizing radiation and that gamma-H2AX phosphorylation can be used as a predictive marker of radioresponse.

Roscovitine Modulates DNA Repair and Senescence: Implications for Combination Chemotherapy

PURPOSE

Treatment of tumor cells by chemotherapy activates a series of responses ranging from apoptosis to premature senescence and repair. Survival responses are characterized by inhibition of cyclin-dependent kinases. Because inhibition of cyclin-dependent kinases represents a distinctive feature of DNA damage-induced prosurvival responses, we investigated the possibility that the cyclin-dependent kinase inhibitor roscovitine modulates drug-induced responses in human adenocarcinoma cells, favoring cell survival.

EXPERIMENTAL DESIGN

Sublethal concentrations of doxorubicin were used to induce premature senescence in human adenocarcinoma cells. The effect of the cyclin-dependent kinase inhibitor roscovitine on the doxorubicin-dependent cell cycle checkpoint activation and DNA repair pathways was evaluated.

RESULTS

Roscovitine reinforces doxorubicin-dependent G(1) checkpoint in A549 and HEC1B cells leading to decreased frequency of double-strand breaks and to the preferential induction of senescence and enhanced clonogenic survival. However, in other tumor cell lines, such as HCT116 and H1299, combined treatment with doxorubicin and roscovitine increases the frequency of double-strand breaks and dramatically sensitizes to doxorubicin. This unexpected effect of roscovitine depends on a novel ability to inhibit DNA double-strand break repair processes and requires inactivation of the pRb pathway.

CONCLUSIONS

Roscovitine, by hindering DNA repair processes, has the potential to inhibit recovery of mildly damaged tumor cells after doxorubicin treatment and to increase the susceptibility of tumor cells to chemotherapy. However, in some tumor cells, the cell cycle inhibitory function of roscovitine prevails over the DNA repair inhibitory activity, favoring premature senescence and clonogenic growth. These data indicate a novel mechanism underlying combined chemotherapy, which may have wide application in treatment of carcinomas.

Immunofluorescence detection of clustered gamma-H2AX foci induced by HZE-particle radiation

We studied the spatial and temporal distributions of foci of the phosphorylated form of the histone protein H2AX (gamma-H2AX), which is known to be activated by double-strand breaks after irradiation of human fibroblast cells with high-energy silicon (54 keV/microm) and iron (176 keV/microm) ions. Here we present data obtained with the ion path parallel to a monolayer of human fibroblast cells that leads to gamma-H2AX aggregates in the shape of streaks stretching over several micrometers in an x/y plane, thus enabling the analysis of the fluorescence distributions along the ion trajectories. Qualitative analyses of these distributions provide insights into DNA damage processing kinetics for high charge and energy (HZE) ions, including evidence of increased clustering of DNA damage and slower processing with increasing LET.

Heterogeneity in DNA damage using the comet assay

The single-cell gel electrophoresis or "comet" assay was developed many years ago to analyze DNA damage in individual cells. It is a powerful and versatile technique that relies on microscopic visualization or imaging of DNA after single cells are embedded in agarose, lysed, and electrophoresed. In addition, the basic methodology has been extended to permit the detection of a variety of classes of DNA damage with good sensitivity in virtually any single-cell type. A unique but understudied property of the comet assay is its ability to detect and quantify cellular heterogeneity in response to DNA-damaging agents. This review outlines the considerations in producing and analyzing comet data when heterogeneity in induction of or cellular response to DNA damage is the major consideration. Examples are presented to emphasize the heterogeneity of tumor response to ionizing radiation and cytotoxic drugs.

Genetic and epigenetic features in radiation sensitivity

Recent progress especially in the field of gene identification and expression has attracted greater attention to genetic and epigenetic susceptibility to cancer, possibly enhanced by ionising radiation. It has been proposed that the occurrence and severity of the adverse reactions to radiation therapy are also influenced by such genetic susceptibility. This issue is especially important for radiation therapists since hypersensitive patients may suffer from adverse effects in normal tissues following standard radiation therapy, while normally sensitive patients could receive higher doses of radiation offering a better likelihood of cure for malignant tumours. This paper, the first of two parts, reviews the main mechanisms involved in cell response to ionising radiation. DNA repair machinery and cell signalling pathways are considered and their role in radiosensitivity is analysed. The implication of non-targeted and delayed effects in radiosensitivity is also discussed.

The Adenovirus E4orf6 Protein Inhibits DNA Double Strand Break Repair and Radiosensitizes Human Tumor Cells in an E1B-55K-independent Manner

The adenoviral protein E4orf6 has been shown to inhibit both in vitro V(D)J recombination and adenoviral DNA concatenation, two processes that rely on cellular DNA double strand break repair (DSBR) proteins. Most of the known activities of E4orf6 during adenoviral infection require its interaction with another adenoviral protein, E1B-55K. Here we report that E4orf6, stably expressed in RKO human colorectal carcinoma cells or transiently expressed by adenoviral vector in U251 human glioblastoma cells, inhibits DSBR and induces significant radiosensitization in the absence of E1B-55K. Expression of a mutant form of E4orf6 (L245P) failed to radiosensitize RKO cells. E4orf6 reduced DSBR capacity in transfected and infected cells, as measured by sublethal DNA damage repair assay and phosphorylated H2AX (gamma-H2AX) levels, respectively. Consistent with the inhibitory effect of E4orf6 on DSBR, expression of wild-type but not mutant E4orf6 reduced recovery of a transfected, replicating reporter plasmid (pSP189) in 293 cells but did not increase the mutation frequency measured in the reporter plasmid. The kinase activity of DNA-PKcs (the DNA-dependent protein kinase catalytic subunit) toward heterologous substrates was not affected by expression of E4orf6; however, autophosphorylation of DNA-PKcs at Thr-2609 following ionizing radiation was prolonged in the presence of E4orf6 when compared with control-infected cells. Our results demonstrate for the first time that E4orf6 expression hinders the cellular DNA repair process in mammalian cells in the absence of E1B-55K or other adenoviral genes and suggest that viral-mediated delivery of E4orf6, combined with localized external beam radiation, could be a useful approach for the treatment of radioresistant solid tumors such as glioblastomas.

High LET-induced H2AX phosphorylation around the Bragg curve

We investigated the spatial distribution of the induction of the phosphorylated form of the histone protein H2AX (gamma-H2AX), known to be activated by DSBs. Following irradiation of human fibroblast cells with 600 MeV/nucleon silicon and 600 MeV/nucleon iron ions we observed the formation of gamma-H2AX aggregates in the shape of streaks stretching over several micrometers in an x/y plane. Polyethylene shielding was used to achieve a Bragg curve distribution with beam geometry parallel to the monolayer of cells. We present data that highlights the formation of immunofluorescent gamma-H2AX tracks showing the ion trajectories across the Bragg peak of irradiated human fibroblast cells. Qualitative analyses of these distributions indicated potentially increased clustering of DNA damage before the Bragg peak, enhanced gamma-H2AX distribution at the peak, and provided visual evidence of high-linear energy transfer particle traversal of cells beyond the Bragg peak in agreement with one-dimensional transport approximations. Spatial assessment of gamma-H2AX fluorescence may provide direct insights into DNA damage across the Bragg curve for high charge and energy ions including the biological consequences of shielding and possible contributors to bystander effects.

Involvement of Poly(ADP-ribose) Polymerase-1 and XRCC1/DNA Ligase III in an Alternative Route for DNA Double-strand Breaks Rejoining

The efficient repair of DNA double-strand breaks (DSBs) is critical for the maintenance of genomic integrity. In mammalian cells, the nonhomologous end-joining process that represents the predominant repair pathway relies on the DNA-dependent protein kinase (DNA-PK) and the XRCC4-DNA ligase IV complex. Nonetheless, several in vitro and in vivo results indicate that mammalian cells use more than a single end-joining mechanism. While searching for a DNA-PK-independent end-joining activity, we found that the pretreatment of DNA-PK-proficient and -deficient rodent cells with an inhibitor of the poly(ADP-ribose) polymerase-1 enzyme (PARP-1) led to increased cytotoxicity of the highly efficient DNA double-strand breaking compound calicheamicin gamma1. In addition, the repair kinetics of the DSBs induced by calicheamicin gamma1 was delayed both in PARP-1-proficient cells pretreated with the PARP-1 inhibitor and in PARP-1-deficient cells. In order to get new insights into the mechanism of an alternative route for DSBs repair, we have established a new synapsis and end-joining two-step assay in vitro, operating on DSBs with either nuclear protein extracts or recombinant proteins. We found an end-joining activity independent of the DNA-PK/XRCC4-ligase IV complex but that actually required a novel synapsis activity of PARP-1 and the ligation activity of the XRCC1-DNA ligase III complex, proteins otherwise involved in the base excision repair pathway. Taken together, these results strongly suggest that a PARP-1-dependent DSBs end-joining activity may exist in mammalian cells. We propose that this mechanism could act as an alternative route of DSBs repair that complements the DNA-PK/XRCC4/ligase IV-dependent nonhomologous end-joining.

Measurement of tumor hypoxia using single-cell methods

A growing appreciation for the importance of hypoxia in tumor progression and response to treatment has driven efforts to develop methods that could be used routinely in the clinic to identify tumors containing hypoxic cells. The ideal method would be noninvasive and could be used both before treatment to determine the presence of hypoxia and during therapy to assess tumor reoxygenation. Although this goal is being approached, there are still questions about how best to measure tumor oxygenation and whether noninvasive imaging methods can provide the necessary sensitivity. Analysis of hypoxia at the level of the individual cell can provide the following information that cannot be obtained in other ways: the degree of hypoxia, the lifetime of hypoxic cells, and the dynamic nature of hypoxia. This review will describe methods that have been used to detect hypoxia in individual cells, the relation between these measurements and patient response to treatment, and indicate where these methods can provide important additional insights into the consequences of tumor hypoxia.

Evasion of early cellular response mechanisms following low level radiation-induced DNA damage

DNA damage that is not repaired with high fidelity can lead to chromosomal aberrations or mitotic cell death. To date, it is unclear what factors control the ultimate fate of a cell receiving low levels of DNA damage (i.e. survival at the risk of increased mutation or cell death). We investigated whether DNA damage could be introduced into human cells at a level and frequency that could evade detection by cellular sensors of DNA damage. To achieve this, we exposed cells to equivalent doses of ionizing radiation delivered at either a high dose rate (HDR) or a continuous low dose rate (LDR). We observed reduced activation of the DNA damage sensor ataxia-telangiectasia mutated (ATM) and its downstream target histone H2A variant (H2AX) following LDR compared with HDR exposures in both cancerous and normal human cells. This lack of DNA damage signaling was associated with increased amounts of cell killing following LDR exposures. Increased killing by LDR radiation has been previously termed the "inverse dose rate effect," an effect for which no clear molecular processes have been described. These LDR effects could be abrogated by the preactivation of ATM or simulated in HDR-treated cells by inhibiting ATM function. These data are the first to demonstrate that DNA damage introduced at a reduced rate does not activate the DNA damage sensor ATM and that failure to activate ATM-associated repair pathways contributes to the increased lethality of continuous LDR radiation exposures. This inactivation may reflect one strategy by which cells avoid accumulating mutations as a result of error-prone DNA repair and may have a broad range of implications for carcinogenesis and, potentially, the clinical treatment of solid tumors.

DNA double strand break quantification in skin biopsies

BACKGROUND AND PURPOSE

Following induction of double strand breaks the histone H2AX is rapidly phosphorylated at regions flanking the breaks resulting in nuclear gamma H2AX foci. The purpose of this study was to use this endogenous signalling system to quantify the in vivo response to radiation in normal tissue.

PATIENTS AND METHODS

Skin biopsies were taken from prostate cancer patients undergoing radiotherapy with a curative intent. Biopsies were taken at locations corresponding to 5 different doses in the range below 1.1 Gy per fraction. Biopsies were taken from patients 30 min following the first fraction and then once again following the fraction given after the first weekend break in the treatment course. The DNA double strand breaks were visualised as gamma H2AX foci using immunohistochemistry. Images were acquired using a CCD-camera and a fluorescence microscope and the gamma H2AX foci were quantified using digital image analysis including the basic procedures of top-hat transformation, threshold setting and labelling.

RESULTS

Repeated assessments of the biopsies showed a high reproducibility in quantifying the number of foci per DNA area of the nucleated cells in epidermis. The reproducibility was equally good for the two biopsy occasions. A linear dose response was observed for the epidermis in the dose region 0-1 Gy.

CONCLUSIONS

We have established a method to measure the relative amount of DNA double strand breaks by detecting gamma H2AX foci in patients exposed to radiotherapy. The method provides a tool to study induction and repair of DNA double strand breaks and has the potential to predict individual radiosensitivity.