Radiosensitization by the novel DNA intercalating agent vosaroxin
© Gordon et al; licensee BioMed Central Ltd. 2012
Received: 28 November 2011
Accepted: 27 February 2012
Published: 27 February 2012
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© Gordon et al; licensee BioMed Central Ltd. 2012
Received: 28 November 2011
Accepted: 27 February 2012
Published: 27 February 2012
Vosaroxin is a first in class naphthyridine analog structurally related to quinolone antibacterials, that intercalates DNA and inhibits topoisomerase II. Vosaroxin is not a P-glycoprotein receptor substrate and its activity is independent of p53, thus evading common drug resistance mechanisms. To evaluate vosaroxin as a clinically applicable radiation sensitizer, we investigated its effects on tumor cell radiosensitivity in vitro and in vivo.
Vosaroxin's effect on post-irradiation sensitivity of U251, DU145, and MiaPaca-2 cells was assessed by clonogenic assay. Subsequent mechanistic and in vivo studies were performed with U251 cells. Cell cycle distribution and G2 checkpoint integrity was analyzed by flow cytometry. DNA damage and repair was evaluated by a high throughput gamma-H2AX assay. Apoptosis was assessed by flow cytometry. Mitotic catastrophe was assessed by microscopic evidence of fragmented nuclei by immunofluorescence. In vivo radiosensitization was measured by subcutaneous tumor growth delay.
50-100 nmol/L treatment with vosaroxin resulted in radiosensitization of all 3 cell lines tested with a dose enhancement factor of 1.20 to 1.51 measured at a surviving fraction of 0.1. The maximal dose enhancement was seen in U251 cells treated with 75 nmol/L vosaroxin (DEF 1.51). Vosaroxin exposure did not change cell cycle distribution prior to irradiation nor alter G2 checkpoint integrity after irradiation. No difference was seen in the apoptotic fraction regardless of drug or radiation treatment. The number of cells in mitotic catastrophe was significantly greater in irradiated cells treated with vosaroxin than cells receiving radiation only at 72 hr (p = 0.009). Vosaroxin alone did not significantly increase mitotic catastrophe over control (p = 0.53). Cells treated with vosaroxin and radiation maintained significantly higher gamma-H2AX levels than cells treated with vehicle control (p = 0.014), vosaroxin (p = 0.042), or radiation alone (p = 0.039) after 24 hr. In vivo tumor growth delay was 1.5 days for vosaroxin alone (IV 10 mg/kg), 1.0 days for radiation (3 Gy) alone, and 8.6 days for the group treated with vosaroxin 4 hours prior to radiation.
Vosaroxin enhanced tumor cell radiosensitivity in vitro and in vivo. The mechanism appears to be related to inhibition of DNA repair and increased mitotic catastrophe.
Topoisomerase II inhibitors are a diverse class of anti-cancer drugs that include the anthracyclines (doxorubicin and daunorubicin), etoposide and quinolones. Anthracyclines have effective and broad-spectrum anti-tumor activity but their clinical utility is frequently limited by systemic toxicity (i.e. cardiotoxicity with doxorubicin) or drug resistance (frequently mediated by p-glycoprotein). Several topoisomerase II inhibitors are known to potentiate the effects of radiation on tumor cells although the mechanisms of radiation sensitization remain an area of research[2–4].
DU145 prostate carcinoma cells, MiaPaCa-2 pancreatic carcinoma cells, and U251 glioblastoma cells obtained from the ATCC were used for clonogenic assay experiments and U251 cells were used in subsequent experiments to investigate the mechanisms of radiosensitization. Cells were grown in DMEM (Invitrogen) supplemented with 10% fetal bovine serum and maintained at 37°C in a 5% CO2 atmosphere. Vosaroxin, provided by Sunesis, was reconstituted in DMSO (10 mmol/L) and stored in the dark at room temperature. For all studies, the working concentration of DMSO was < 0.1%. Cultures were irradiated at a dose rate of 2.28 Gy/min by a Pantak X-ray source.
Cultures were trypsinized to generate a single-cell suspension. A specified number of cells were seeded into each well of six-well tissue culture plates. Cells were allowed to attach for 6 hours followed by treatment with vehicle control or vosaroxin and were irradiated 16 hours later. Ten to fourteen days after seeding the cells, colonies were stained with crystal violet and the number of colonies containing at least 50 cells was determined. The surviving fractions were calculated and survival curves generated after normalizing for cytotoxicity from vosaroxin treatment alone.
Evaluation of the cell cycle and G2-checkpoint integrity was performed by flow cytometry. Cells grown in 100 mm2 culture dishes were exposed to vehicle control or vosaroxin for 16 hours prior to administration of 2 Gy or sham radiation. Cells were trypsinized 1, 3, 6, and 24 hours later, then fixed and stained per manufacturer's instructions with Cell Cycle Reagent and analyzed on an EasyCyte Plus flow cytometer (Guava Technologies, Hayward, CA). G2-checkpoint integrity was evaluated as previously reported by Xu, et al. [8, 9] utilizing rabbit polycolonal antibody against phospho-H3 histone (Millipore) followed by staining with a goat anti-rabbit-FITC conjugated secondary antibody (Jackson ImmunoResearch) to distinguish mitotic cells.
Apoptotic fraction was evaluated by flow cytometry using the Guava Nexin assay. Cells grown in 100 mm2 culture dishes were exposed to vehicle control or vosaroxin, for 16 hours prior to administration of 2 Gy or sham radiation. Cells were trypsinized and stained 24 and 72 hours after radiation per manufacturer's instructions with Nexin Reagent to assess annexin V-PE as a marker of cells in early apoptosis and 7-AAD as an indicator of late apoptosis (Guava Technologies, Hayward, CA). Analysis was performed on an EasyCyte Plus flow cytometer. Positive controls were treated 24 hours with 0.5 μM staurosporine (Sigma).
The presence of fragmented nuclei was used to define cells undergoing mitotic catastrophe. Cells were grown on 4-well chamber slides. Cells were fixed with methanol for 15 minutes at -20°C, blocked with 1% BSA, and stained overnight at 4°C with mouse anti-α-tubulin antibody (Sigma) followed by staining with goat anti-mouse-Texas Red antibody (Jackson ImmunoResearch) 2 hours at room temperature. Nuclei were counterstained with DAPI (Sigma). Coverslips were mounted with VectaShield anti-fade solution (Vector Labs) and were visualized on a Leica DMRXA fluorescent microscope with a 20x objective (Wetzlar, Germany). Digital images were captured by a MicroPublisher 3.3 camera (QImaging, Surrey, Canada) and overlaid in Adobe Photoshop CS (San Jose, CA). Normal cells and cells in mitotic catastrophe were manually counted with the presence of nuclei fragmented with ≥ 2 lobes as the criteria for defining cells undergoing mitotic catastrophe. For each treatment condition 150-200 cells were scored.
A previously validated electrochemiluminescent assay was used to measure gamma-H2AX levels . Cells were grown and treated on 150 mm2 plates. Cells were exposed to vehicle control or vosaroxin, for 16 hours prior to administration of 2 Gy or sham radiation. 1, 6, and 24 hours later, cells were harvested and scraped into PBS, washed, and pelleted by centrifugation at 100 relative centrifugal force for 10 minutes. The PBS supernatant was discarded and the cell pellet was frozen at -80°C overnight. Cells were resuspended in lysis buffer of NaCl (500 mM), EDTA (2 mM), Triton X-100 (1%), sodium deoxycholate (1%), SDS (1%), Tris HCl (50 mM), NaF (10 mM), phosphatase and protease inhibitors (1 ×), and PMSF (2 mM). Proteins were solubilized by sonication and protein concentration was determined by Bradford assay. 5 μg of protein from each cell lysate was coated onto a 96-well plates and left overnight. Wells were blocked with 3% blocking solution, washed, and a sulfo-ester tag conjugated phospho-H2AX (Abcam) detection antibody was added in 1% blocking solution (1 μg/ml). Wells were washed thrice with TBS. A read buffer was added before analysis in an electrochemiluminescent imager (Meso Scale Discovery).
Four to six week old, female, athymic NCr nu/nu, nude mice (NCI Animal Production Program, Frederick, MD) were used for all in vivo studies. Animals were caged in groups of 5 or less and were fed animal chow and water ad libitum. A single cell suspension (10 × 106) of U251 cells was implanted on the lateral aspect of the rear leg. When tumors reached ≈ 100 mm3 ([L × W 2 ]/2) animals were randomized to four groups including untreated controls, vosaroxin (10 mg/kg IV into tail vein), irradiation alone (3 Gy), or vosaroxin + irradiation (10 mg/kg IV into tail vein + 3 Gy). Irradiation of tumors took place 4 hours after treatment with vosaroxin based on suspected drug metabolism in vivo, with animals restrained in lead jigs custom made by the Radiation Biology Branch of the National Cancer Institute. Tumors were measured three times per week until they reached ≥ 1000 mm3. Specific tumor growth delay was calculated for individual animals. The growth delay for the group was calculated as the mean number of days to reach 1,000 mm3 in the treatment group minus the mean number of days for the control group to reach the same size. Group means ± standard error are reported. Each experimental group contained 6 animals. All animal studies were conducted in accordance with the principles and procedures outlined in the NIH Guide for the Care and Use of Animals.
In vitro studies were subject to three independent experiments. Data is presented as mean ± SE. A Student's t test was used to compare sample means with a p value of < 0.05 considered significant.
To determine the effects of vosaroxin on the radiosensitivity of tumor cells, a clonogenic assay was performed. The previously reported average IC50 value for vosaroxin alone in 20 cancer cell lines was 322 nmol/L . As drug exposure times are longer during clonogenic survival studies with radiation, concentrations from 50 nmol/L to 100 nmol/L of vosaroxin were used.
To provide insight into the mechanism of radiosensitization, we then determined the mode of cell death. At micromolar doses, vosaroxin has been shown to cause apoptosis in several cancer cell lines [5, 11]. To determine if the 75 nM concentration of vosaroxin used in this study was inducing apoptosis after radiation, flow cytometry was performed. Less than 2% of cells in all treatment groups were apoptotic at 24 and 72 hours with no difference seen between untreated controls and cells treated with 75 nmol/L vosaroxin, 2 Gy, or the combination of 2 Gy and 75 nmol/L vosaroxin (data not shown).
This study demonstrated the enhanced radiosensitivity of human tumor cells after exposure to vosaroxin, an agent that acts as a DNA intercalator and an inhibitor of topoisomerase II. The potential of topoisomerase II as a target for radiosensitization has been previously suggested in studies with other agents in experimental tumor models.[2–4] Etoposide is a non-intercalating topoisomerase II inhibitor that enhances radiosensitivity due to effects on radiation repair and cell cycle [2, 12]. The role of cell cycle in radiosensitization by topoisomerase inhibitors is complex. The mechanisms of radiation sensitization are important considerations because they influence the maximal cytotoxicity achieved in a time and sequence specific manner . Radiation causes cells to arrest in G2/M phase which is when topoisomerase II typically functions as an important repair enzyme. Conversely, topoisomerase II inhibitors can cause G2 arrest which places cells in a relatively radiosensitive phase of the cell cycle . However, in this study, G2 arrest was not seen at the doses of vosaroxin used. Therefore, this does not appear to be the mechanism of radiosensitization in this study.
The data presented here indicate that there was no significant initial increase in DNA damage based on gamma-H2AX levels at early time points after radiation suggesting no increase in the number of DNA-DSBs. However, at 24 hours after radiation, there was increased gamma-H2AX foci retention suggesting vosaroxin inhibits the repair of radiation induced DNA damage in U251 cells. Mechanistic conclusions made here are based only on one cell line and the mechanism of action of vosaroxin radiosensitization may differ among cell types. In addition, the potential for normal tissue radiosensitization will need to be considered in future work. A previous study evaluating the topoisomerase II inhibitors amrubicin and amrubicinol in lung adenocarcinoma showed similar enhancement of radiosensitivity to the results reported here with enhancement ratios of 1.38 and 1.57. Similar to our findings, this study showed no increase in apoptosis when cells were irradiated and treated with topoisomerase inhibitors. The study found an increased proportion of necrotic cells after radiation and drug treatment with sub-additive increases in percent necrosis in the combination treated cells .
This data provides support for further evaluation of vosaroxin as a radiation sensitizer. As the first study to evaluate the radiation sensitizing properties of vosaroxin, this provides a basis for additional preclinical exploration of the radiosensitizing properties of vosaroxin. More thorough investigation and understanding of the specific molecular mechanisms leading to radiosensitization is warranted as these pathways were not identified in this study.
This research was supported in part by the Intramural Research Program of the National Institutes of Health, National Cancer Institute.