The role of mitochondrial function in gold nanoparticle mediated radiosensitisation
© Taggart et al.; licensee Springer. 2014
Received: 27 February 2014
Accepted: 30 June 2014
Published: 16 September 2014
Gold nanoparticles (GNPs), have been demonstrated as effective preclinical radiosensitising agents in a range of cell models and radiation sources. These studies have also highlighted difficulty in predicted cellular radiobiological responses mediated by GNPs, based on physical assumptions alone, and therefore suggest a significant underlying biological component of response. This study aimed to determine the role of mitochondrial function in GNP radiosensitisation. Using assays of DNA damage and mitochondrial function through levels of oxidation and loss of membrane potential, we demonstrate a potential role of mitochondria as a central biological mechanism of GNP mediated radiosensitisation.
KeywordsGold nanoparticles Radiosensitisation Radiation Mitochondria Oxidative stress
The application of radiobiological principles in clinical oncology aims to describe the relationship between absorbed dose and the resulting biological responses of tumour and normal tissues (Hall & Giaccia ). Central to the development of novel clinic approaches is improvement in the differential responses between normal and tumour tissue at a fixed dose, termed the therapeutic ratio. Improvements in the therapeutic ratio of radiotherapy have been driven by developments in both radiation biology and radiation physics which have translated into significant advances in targeted dose delivery, radiological imaging and biological effectiveness.
Since the pioneering attempts of Denekamp and colleagues in the mid-1970s to sensitize hypoxic tumour cells (Fowler et al. ), much effort has focussed on increasing tumour cell sensitivity to the biological effects of ionising radiation (Wardman ). In the nanotechnology field, gold nanoparticles (GNPs) have been extensively investigated as radiosensitisers, reviewed by our laboratory (Butterworth et al. ); and have recently shown efficacy under hypoxic conditions (Jain et al. ). GNPs are applicable as radiosensitsers due to their high atomic number (Z = 79) which results in preferential mass energy absorption compared to soft tissue (Hubbell & Seltzer ). Additionally, GNPs are relatively easy to synthesize in a range of sizes, can be readily functionalised, and have been shown to passively accumulate in tumours through the enhance permeability and retention effect (EPR) (Maeda et al. ).
Calculations of X-ray dose enhancement factors based on physical absorption characteristics have predicted enhancements of between 1.2 and 5 depending on the GNP concentration and beam energy, with the greatest effect predicted at kilovoltage energies (Cho ; McMahon et al. ). Despite these predictions radiosensitisation of cells exposed to GNPs and irradiated with megavoltage energies has been shown suggesting additional processes in the radiosensitising effect of GNPs (Chithrani et al. ; Jain et al. ). In addition to possible biological mechanisms, one factor which may contribute to these effects is localised energy deposition around GNPs. Following ionisation of gold atoms, large numbers of low-energy electrons are generated through Auger cascades which deposit their energy at high density within a small radius around the GNP, leading to high localised doses. These high, inhomogeneous doses generated in close proximity to the nanoparticle surface are known to have significantly increased biological effectiveness with analysis of nanoscale dose distributions around GNPs using the Local Effect Model (McMahon et al. [2011a]; McMahon et al. [2011b]) suggesting this may contribute to the observed radiosensitising effects of GNPs.
Of the wide ranging studies describing the biological effects of GNPs, several have reported elevated levels of reactive oxygen species for GNPs of differing size, shape and surface functionalization (Pan et al. ; Chompoosor et al. ; Li et al. ; Piryazev et al. ; Mateo et al. ). Comparatively few reports have demonstrated a role for ROS or the involvement of mitochondria as mechanism of GNP radiosensitisation (Geng et al. ). The current study builds on previous data from our laboratory demonstrating radiosensitising effects of 1.9 nm Aurovist GNPs at kilovoltage energies (Butterworth et al. ) as a result of significantly elevated levels of DNA damage which may be a direct result of impaired mitochondrial functional manifested by increased oxidation and loss of membrane potential.
Materials and methods
All cell lines were obtained from Cancer Research UK. The human breast cancer cell line, MDA-MB-231 was maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% foetal bovine serum and 50 μg/ml penicillin/streptomycin. The human prostate cell line, DU-145 was maintained in RPMI-1640 medium with 10% foetal bovine serum and 50 μg/ml penicillin/streptomycin. The human glioma cell line, T98G was maintained in EMEM supplemented with 10% foetal bovine serum and 50 μg/ml penicillin/streptomycin.
1.9 nm AurovistTM particles were purchased from Nanoprobes Inc. (NY, USA) and re-suspended in sterile water. 1.9 nm AurovistTM are spherical particles with a proprietary thiol coating (Coulter et al. ). Cells were treated at a concentration of 500 μg/ml for 24 hours unless otherwise indicated. This concentration of 500 μg/ml and time point of 24 hours was chosen as a result of previous work within the group showing that these conditions allow for optimal cell uptake of GNPs (Coulter et al. ).
Cells were irradiated with 225 kVp X-rays produced using an X-Rad 225 X-ray generator (Precision, X-ray Inc, USA). All quoted doses are the absorbed dose from this source in water.
Clonogenic cell survival assay
Sub-confluent cells were removed from flasks using a solution of 0.25% Trypsin and 1 mM EDTA, they were counted using a Coulter counter and re-seeded into six well plates at a density of 1.5 x 105 cells per well. Cells were left to attach for 4–6 hours and treated with gold nanoparticles for 24 hours. Cells were then irradiated, trypsinised and counted, then seeded into T25 flasks and left to proliferate for 7–9 days. For MDA-MB-231, DU145 and T98G cell lines 500 cells were seeded per treatment for 0 Gy and 2 Gy doses, 1,000 cells for 4 Gy and 2,000 cells for 8 Gy. MDA-MB-231, DU-145 and T98G cells had plating efficiencies of approximately 50%. Surviving fraction was calculated by dividing the number of surviving colonies in the irradiated samples by the number of surviving colonies in the non-irradiated controls for each treatment. Dose enhancement factor (DEF) is defined here as the ratio of doses which lead to equal levels of cell survival with and without GNPs. DEFs can vary with delivered dose, and are quoted with reference to the dose delivered to cells in the absence of GNPs.
Cells were seeded onto sterile 16 mm2 coverslips placed in six well plates at a density of 1 x 105 cells per well. Cells were left to attach for 4–6 hours before treatment. After incubation with GNPs cells were irradiated with 2 Gy and fixed 1 hour or 24 hours post irradiation with a 50% acetone/50% methanol solution for 10 minutes. Cells were then permeabilised with 0.5% Triton X-100 and PBS solution for 10 minutes before being incubated with a blocking buffer of 0.2% milk, 5% Horse serum, 0.1% Triton X-100 in PBS for 1 hour at room temperature. Coverslips were then incubated with 53BP1 antibody (Novus Biologicals, Colorado, USA) at a dilution of 1:1000 in blocking buffer for 1 hour at room temperature. They were then rinsed three times with washing buffer, 0.1% Triton X-100 in PBS before being incubated with Alexa Fluor 488 Goat anti Rabbit secondary antibody (Invitrogen Molecular Probes, Oregon, USA) at a dilution of 1:1000 in blocking buffer for one hour at room temperature. Coverslips were rinsed three times in washing buffer and then mounted onto glass microscope slides with 5 μl of Vectashield mounting media (Vector Labs Ltd, UK) and sealed with nail varnish. Foci were viewed and counted manually on a Zeiss Axiovert 200 M fluorescent microscope.
Mitochondrial membrane polarisation measurement
Cells were seeded into 12 well plates at a density of 1 × 105 cells per well and left to attach for 4–6 hours before treatment. 25 nM Tetramethylrhodamine ethyl ester perchlorate (TMRE) (Sigma-Aldrich) was added to each well and incubated for 15 minutes at 37°C. Media was then transferred to 15 ml centrifuge tubes and placed on ice. Cells were detached using 0.25% trypsin and 1 mM EDTA and the cell solution was then transferred to the corresponding 15 ml tube left on ice. Cells were then pelleted by centrifugation at 2000 rpm at 4°C for 5 minutes. Media was removed and cell pellets were resuspended in 300 μl of PBS and TMRE fluorescence was analysed immediately using a FACSCalibur flow cytometer with an air-cooled argon-ion 15 milliwat 488 nm laser and 585 nm detector and CELL-Quest software (BD biosciences) 1 x 104 cells were analysed per sample.
Mitochondrial oxidation detection
Mitochondrial oxidation was measured using Nonyl-Acridine Orange (NAO) (cat no A-1372, Molecular Probes, Invitrogen, NY). 1 × 105 cells were seeded into 12 well plates and left to attach for 4–6 hours before being treated accordingly. At the end of treatment, media was removed from cells and transferred to 15 ml centrifuge tubes on ice. Cells were detached using 0.25% Tryspin/1 mM EDTA solution and added to corresponding tubes containing media. Cells were then pelleted by centrifugation at 2000 rpm at 4°C for 5 minutes. Media was removed and cell pellets were resuspended in 300 μl of 0.1% BSA-PBS solution containing 25 ng/ml NAO and left to incubate at 37° for 10 minutes. Cells were placed on ice post-incubation and analysed immediately using FACSCalibur flow cytometer with an air-cooled argon-ion 15 milliwatt 488 nm laser and 585 nm detector and CELL-Quest software (BD biosciences). 1 × 104 cells were analysed per sample.
Radiosensitising effects of 1.9 nm GNPs
Summary of dose enhancement factors (DEF) ± uncertainties for the cell lines investigated when irradiated at 2 Gy, 4 Gy and 8 Gy after treatment with 1.9 nm gold nanoparticles
1.23 ± 0.14
1.01 ± 0.20
1.90 ± 0.22
1.20 ± 0.06
1.06 ± 0.10
1.57 ± 0.08
1.17 ± 0.02
1.1 ± 0.04
1.35 ± 0.03
Significant radiosensitising effects were observed in both MDA-MB-231 and T98G cell lines with 1.9 nm GNPs but not DU-145 cells as shown in Figure 1. T98G glioma cells show the greatest amount of cell death enhancement with a DEF of 1.90 ± 0.22 at 2 Gy with 1.9 nm GNPs. MDA-MB-231 cells also show increased cell kill with GNPs with a lower DEF of 1.23 ± 0.14 at 2 Gy compared to T98G cells. DU-145 cells show virtually no change in cell survival across all doses investigated. It should also be noted that in the T98G cell line, GNP DEFs appear to decrease with increasing dose; at 8 Gy the DEF decreased to 1.35 ± 0.03, suggesting GNPs are not solely acting as a dose modifying agent as DEFs would be expected to be uniform across all doses in this case.
GNP induced changes in DNA damage
Furthermore, the distribution of foci numbers per cell was analysed in Figure 3B in order to determine if there was an overall increase in the levels of DNA damage across the population or if a subset of the population with a significant increase in DNA damage was driving the increase in average foci number. MDA-MB-231 and DU-145 cells both show a slight shift in a population subset with a peak of increased DNA damage when cells are treated with GNPs, which is further amplified with irradiation. T98G cells also show a slight peak shift towards additional damage upon nanoparticle treatment, but not in the presence of radiation.
GNP induced changes in mitochondrial membrane polarisation
GNP induced changes in mitochondrial membrane oxidation
Classical approaches used to radiosensitise cells have included radiation induced activation of prodrugs, suppression of intracellular thiols, inhibition of DNA repair and oxygen mimetics (Wardman ). Nitrobenzenes, nitrofurans and nitroimidazoles have been used to radiosensitise hypoxic cells with their radiosensiting ability attributed to their high electron affinity (Adams & Cooke ). These compounds are generally activated by reduction in hypoxic conditions and work in a similar way to oxygen by causing DNA double strand breaks in the presence of irradiation as a result of the fixation of free radical damage (Katz et al. ). Despite extensive preclinical research and promising evidence, hypoxic radiosensitisers have failed to reach their full potential in the clinic (Bischoff et al. ).
The concept of targeting repair DNA stems from the central dogma underpinning radiotherapy, which is to induce complex DNA damage lesions which are difficult to repair resulting in cell death. Cisplatin and 5-fluorouracil exemplify radiosensitisers in clinical use, acting by interfering with DNA synthesis, however, their precise mechanism of action in radiosensitisation is not fully understood (Katz et al. ).
Similarly, whilst GNPs have been demonstrated as effective radiosensitisers at a range of photon energies, there is insufficient explanation of their underlying biological mechanism of action (Butterworth et al. ). In this study we further validate previous reports from our laboratory showing significant radiosensitising effects of GNPs at 225 kVp (Butterworth et al. ). Analysis of DNA damage foci distributions from Figure 3B compared to foci scores in Figures 2 and 3A, shows the increased DNA damage following treatment with GNPs alone appears to be a result of a small shift in the observed levels of DNA damage within the whole cell population. In contrast, the increased levels of DNA damage seen after irradiation with GNPs appeared to be a result of a cell population subset with greatly amplified levels of DNA damage rather than the whole population. This is particularly obvious in MDA-MB-231 cells and can be seen at 1 and 24 hours post irradiation. This could be a result of the induction of oxidative stress which has previously been observed in our laboratory for the same GNPs (Butterworth et al. ).
To further determine the biological mechanism of GNP mediated radiosensitisation, this study considered the mitochondria as an extra-nuclear target for GNPs within the cell. Mitochondria have multiple roles in important cellular functions, including the production of adenosine triphosphate (ATP), cell signalling, cell growth, cell cycle progression and cell death (Raimundo ). In this study we clearly demonstrate GNPs to have a significant impact on mitochondrial function, manifested by oxidation of the mitochondrial membrane protein, cardiolipin and cell specific disruption of mitochondrial membrane potential. Although these effects could be driven by direct physical interaction with mitochondrial proteins and enzymes, this study supports an indirect interaction of GNPs with mitochondria, triggered by whole cell chemical processes such as oxidative stress. Additional experimental studies are required to further elucidate the precise mechanism of interaction.
Mitochondrial membrane depolarisation can be caused by the presence of free radicals, high intracellular calcium concentrations or stress of the endoplasmic reticulum (Gunter & Pfeiffer ; Deniaud et al. ). Considering the various reports of GNPs causing the induction of ROS and specifically the GNPs used in our experiments, it is likely that elevated ROS result in mitochondrial depolarisation (Butterworth et al. ). Mitochondria and mitochondrial function can be downstream targets of oxidative stress which impairs their function, and they themselves can produce reactive oxygen species and induce oxidative stress in the cell (Zorov et al. ). The effect of GNPs on mitochondrial processes could be a direct contributor to the DNA damage seen upon exposure to gold nanoparticles, as mitochondria have been shown to play a role in the induction of DNA damage (Tartier et al. ).
Oxidative stress and mitochondrial depolarisation are often significant cellular events preceding the induction of cell death, particularly by apoptosis. A key step in the initiation of the intrinsic apoptotic pathway is the oxidation of cardiolipin, which is assessed in this study by measuring the binding of the fluorescent compound NAO through flow cytometry. The oxidation of cardiolipin releases cytochrome c into the cytosol initiating apoptosis; this has been described as critical point in apoptotic signalling beyond which the cell is terminally committed to die (Jiang et al. ). Significant loss of fluorescence from nonyl-acridine orange in both MDA-MB-231 and T98G cells indicates oxidation of cardiolipin. Some loss of NAO fluorescence was also observed in DU-145 cells however, the level was not statistically significant.
1.9 nm gold nanoparticles are effective radiosensitisers showing significant decreases cell survival. In the absence of ionising radiation, GNPs have effects on DNA damage levels as well as mitochondrial function. These cell specific responses to GNPs have the potential to provide a biological mechanism for the sensitisation of cells to the effects of ionising radiation. This mitochondria mediated enhancement in cell death may in part explain the disparities between predicted physical dose enhancement and observed biological effect.
LET designed, performed and analysed laboratory experiments, and with KTB drafted the manuscript. SJM provided statistical support and intellectual contribution. FJC, KMP and KTB proposed experimental objectives and supervision. All authors made extensive intellectual contributions to the work and in reviewing of the manuscript. All authors read and approved the final manuscript.
Dose enhancement factor
Reactive oxygen species
Tetramethylrhodamine ethyl ester perchlorate
The authors are grateful to the EU Marie-Curie Training Network ARGENT (608163) for supporting their work. LET is supported by a Department for Employment and Learning Northern Ireland Studentship.
- Adams GE, Cooke MS: Electron-affinic sensitization. I. A structural basis for chemical radiosensitizers in bacteria. Int J Radiat Biol Relat Stud Phys Chem Med 1969,15(5):457-471. 10.1080/09553006914550741View ArticleGoogle Scholar
- Bischoff P, Altmyer A, Dumont F: Radiosensitising agents for the radiotherapy of cancer: advances in traditional and hypoxia targeted radiosensitisers. Expert Opin Ther Pat 2009,19(5):643-662. 10.1517/13543770902824172View ArticleGoogle Scholar
- Butterworth KT, Coulter JA, Jain S, Forker J, McMahon SJ, Schettino G, Prise KM, Currell FJ, Hirst DG: Evaluation of cytotoxicity and radiation enhancement using 1.9 nm gold particles: potential application for cancer therapy. Nanotechnology 2010,21(29):295101. 10.1088/0957-4484/21/29/295101View ArticleGoogle Scholar
- Butterworth KT, McMahon SJ, Currell FJ, Prise KM: Physical basis and biological mechanisms of gold nanoparticle radiosensitization. Nanoscale 2012,4(16):4830-4838. 10.1039/c2nr31227aView ArticleGoogle Scholar
- Chithrani DB, Jelveh S, Jalali F, van Prooijen M, Allen C, Bristow RG, Hill RP, Jaffray DA: Gold nanoparticles as radiation sensitizers in cancer therapy. Radiat Res 2010,173(6):719-728. 10.1667/RR1984.1View ArticleGoogle Scholar
- Cho SH: Estimation of tumour dose enhancement due to gold nanoparticles during typical radiation treatments: a preliminary Monte Carlo study. Phys Med Biol 2005,50(15):N163-N173. 10.1088/0031-9155/50/15/N01View ArticleGoogle Scholar
- Chompoosor A, Saha K, Ghosh PS, Macarthy DJ, Miranda OR, Zhu ZJ, Arcaro KF, Rotello VM: The role of surface functionality on acute cytotoxicity, ROS generation and DNA damage by cationic gold nanoparticles. Small 2010,6(20):2246-2249. 10.1002/smll.201000463View ArticleGoogle Scholar
- Coulter JA, Jain S, Butterworth KT, Taggart LE, Dickson GR, McMahon SJ, Hyland WB, Muir MF, Trainor C, Hounsell AR, O'Sullivan JM, Schettino G, Currell FJ, Hirst DG, Prise KM: Cell type-dependent uptake, localization, and cytotoxicity of 1.9 nm gold nanoparticles. Int J Nanomedicine 2012, 7: 2673-2685. 10.2147/IJN.S31751View ArticleGoogle Scholar
- Deniaud A, Sharaf el dein O, Maillier E, Poncet D, Kroemer G, Lemaire C, Brenner C: Endoplasmic reticulum stress induces calcium-dependent permeability transition, mitochondrial outer membrane permeabilization and apoptosis. Oncogene 2008,27(3):285-299. 10.1038/sj.onc.1210638View ArticleGoogle Scholar
- Fowler JF, Adams GE, Denekamp J: Radiosensitizers of hypoxic cells in solid tumors. Cancer Treat Rev 1976,3(4):227-256. 10.1016/S0305-7372(76)80012-6View ArticleGoogle Scholar
- Geng F, Song K, Xing JZ, Yuan C, Yan S, Yang Q, Chen J, Kong B: Thio-glucose bound gold nanoparticles enhance radio-cytotoxic targeting of ovarian cancer. Nanotechnology 2011,22(28):285101. 10.1088/0957-4484/22/28/285101View ArticleGoogle Scholar
- Gunter TE, Pfeiffer DR: Mechanisms by which mitochondria transport calcium. Am J Physiol 2009,258(5 Pt 1):C755-C786.Google Scholar
- Hall EJ, Giaccia AJ: Radiobiology for the Radiologist. Lippincott, Williams and Wilkins, Philadelphia, USA; 2012.Google Scholar
- Hubbell JH, Seltzer SM: Tables of X-ray Mass Attenuation and Mass Energy Absorption Coefficients. National Institute of Standards and Technology, Gaithersberg, MD, USA; 1996.Google Scholar
- Jain S, Coulter JA, Hounsell AR, Butterworth KT, McMahon SJ, Hyland WB, Muir MF, Dickson GR, Prise KM, Currell FJ, O'Sullivan JM, Hirst DG: Cell-specific radiosensitization by gold nanoparticles at megavoltage radiation energies. Int J Radiat Oncol Biol Phys 2011,79(2):531-539. 10.1016/j.ijrobp.2010.08.044View ArticleGoogle Scholar
- Jain S, Coulter JA, Butterworth KT, Hounsell AR, McMahon SJ, Hyland WB, Muir MF, Dickson GR, Prise KM, Currell FJ, Hirst DG, O'Sullivan JM: Gold nanoparticle cellular uptake, toxicity and radiosensitisation in hypoxic conditions. Radiother Oncol 2014,110(2):342-347. 10.1016/j.radonc.2013.12.013View ArticleGoogle Scholar
- Jiang J, Huang Z, Zhao Q, Feng W, Belikova NA, Kagan VE: Interplay between bax, reactive oxygen species production, and cardiolipin oxidation during apoptosis. Biochem Biophys Res Commun 2008,368(1):145-150. 10.1016/j.bbrc.2008.01.055View ArticleGoogle Scholar
- Katz D, Ito E, Liu FF: On the path to seeking novel radiosensitizers. Int J Radiat Oncol Biol Phys 2009,73(4):988-996. 10.1016/j.ijrobp.2008.12.002View ArticleGoogle Scholar
- Li JJ, Hartono D, Ong CN, Bay BH, Yung LY: Autophagy and oxidative stress associated with gold nanoparticles. Biomaterials 2010,31(23):5996-6003. 10.1016/j.biomaterials.2010.04.014View ArticleGoogle Scholar
- Maeda H, Wu J, Sawa T, Matsumura Y, Hori K: Tumor vascular permeability and the EPR effect in macromolecular therapeutics: a review. J Control Release 2000,65(1-2):271-284. 10.1016/S0168-3659(99)00248-5View ArticleGoogle Scholar
- Mateo D, Morales P, Ávalos A, Haza AI: Oxidative stress contributes to gold nanoparticle-induced cytotoxicity in human tumor cells. Toxicol Mech Methods 2014,24(3):161-172. 10.3109/15376516.2013.869783View ArticleGoogle Scholar
- McMahon SJ, Mendenhall MH, Jain S, Currell F: Radiotherapy in the presence of contrast agents: a general figure of merit and its application to gold nanoparticles. Phys Med Biol 2008,53(20):5635-5651. 10.1088/0031-9155/53/20/005View ArticleGoogle Scholar
- McMahon SJ, Hyland WB, Muir MF, Coulter JA, Jain S, Butterworth KT, Schettino G, Dickson GR, Hounsell AR, O'Sullivan JM, Prise KM, Hirst DG, Currell FJ: Nanodosimetric effects of gold nanoparticles in megavoltage radiation therapy. Radiother Oncol 2011,100(3):412-416. 10.1016/j.radonc.2011.08.026View ArticleGoogle Scholar
- McMahon SJ, Hyland WB, Muir MF, Coulter JA, Jain S, Butterworth KT, Schettino G, Dickson GR, Hounsell AR, O'Sullivan JM, Prise KM, Hirst DG, Currell FJ: Biological consequences of nanoscale energy deposition near irradiated heavy atom nanoparticles. Sci Rep 2011, 1: 18. Erratum in: Sci Rep. 2013;3:1725 10.1038/srep00018View ArticleGoogle Scholar
- Pan Y, Leifert A, Ruau D, Neuss S, Bornemann J, Schmid G, Brandau W, Simon U, Jahnen-Dechent W: Gold nanoparticles of diameter 1.4 nm trigger necrosis by oxidative stress and mitochondrial damage. Small 2009,5(18):2067-2076. 10.1002/smll.200900466View ArticleGoogle Scholar
- Piryazev AP, Azizova OA, Aseichev AV, Dudnik LB, Sergienko VI: Effect of gold nanoparticles on production of reactive oxygen species by human peripheral blood leukocytes stimulated with opsonized zymosan. Bull Exp Biol Med 2013,156(1):101-103. 10.1007/s10517-013-2288-9View ArticleGoogle Scholar
- Raimundo N: Mitochondrial pathology: stress signals from the energy factory. Trends Mol Med 2014,20(5):282-292. 10.1016/j.molmed.2014.01.005View ArticleGoogle Scholar
- Tartier L, Gilchrist S, Burdak-Rothkamm S, Folkard M, Prise KM: Cytoplasmic irradiation induces mitochondrial-dependent 53BP1 protein relocalization in irradiated and bystander cells. Cancer Res 2007,67(12):5872-5879. 10.1158/0008-5472.CAN-07-0188View ArticleGoogle Scholar
- Wang B, Matsuoka S, Carpenter PB, Elledge SJ: 53BP1, a mediator of the DNA damage checkpoint. Science 2002,298(5597):1435-1438. 10.1126/science.1076182View ArticleGoogle Scholar
- Wardman P: Chemical radiosensitizers for use in radiotherapy. Clin Oncol (R Coll Radiol) 2007,9(6):397-417. 10.1016/j.clon.2007.03.010View ArticleGoogle Scholar
- Zorov DB, Juhaszova M, Sollott SJ: Mitochondrial ROS-induced ROS release: an update and review. Biochim Biophys Acta 2006,1757(5-6):509-517. 10.1016/j.bbabio.2006.04.029View ArticleGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited.