- Open Access
The c-Met receptor tyrosine kinase inhibitor MP470 radiosensitizes glioblastoma cells
© Welsh et al; licensee BioMed Central Ltd. 2009
Received: 27 August 2009
Accepted: 22 December 2009
Published: 22 December 2009
Glioblastoma multiforme (GBM) is resistant to current cytotoxic therapies, in part because of enhanced DNA repair. Activation of the receptor tyrosine kinase c-Met has been shown to protect cancer cells from DNA damage. We hypothesized that inhibiting c-Met would decrease this protection and thus sensitize resistant tumor cells to the effects of radiation therapy.
Materials and methods
Eight human GBM cell lines were screened for radiosensitivity to the small-molecule c-Met inhibitor MP470 with colony-count assays. Double-strand (ds) DNA breaks was quantified by using antibodies to gamma H2AX. Western blotting demonstrate expression of RAD51, glycogen synthase kinase (GSK)-3β, and other proteins. A murine xenograft tumor flank model was used for in vivo radiosensitization studies.
MP470 reduced c-Met phosphorylation and enhanced radiation-induced cell kill by 0.4 logs in SF767 cells. Cells pretreated with MP470 had more ds DNA damage than cells treated with radiation alone. Mechanistically, MP470 was shown to inhibit dsDNA break repair and increase apoptosis. MP470 influences various survival and DNA repair related proteins such as pAKT, RAD51 and GSK3β. In vivo, the addition of MP470 to radiation resulted in a tumor-growth-delay enhancement ratio of 2.9 over radiation alone and extended survival time.
GBM is a disease site where radiation is often used to address both macroscopic and microscopic disease. Despite attempts at dose escalation outcomes remain poor. MP470, a potent small-molecule tyrosine kinase inhibitor of c-Met, radiosensitized several GBM cell lines both in vitro and in vivo, and may help to improve outcomes for patients with GBM.
The management of malignant gliomas continues to pose a difficult therapeutic challenge. Use of radiation therapy and chemotherapy after maximal tumor debulking can improve both local control and survival for some patients with malignant gliomas . Unfortunately, despite adjuvant therapy nearly all patients with glioblastoma multiforme (GBM) will eventually develop tumor recurrence and die of the disease. Patterns-of-failure studies conducted after primary therapy for GBM have shown that 75%-90% of patients experience tumor recurrence within 2 cm of the resection margin . Attempts to increase the local radiation dose have not led to any meaningful improvement in survival when tested in randomized trials . The most likely cause of recurrence is believed to be intrinsic radioresistance, mediated in part by efficient DNA repair. This suggests that interventions aimed at modifying cellular resistance to radiation or chemotherapy may confer a survival benefit.
Hepatocyte growth factor (HGF) is a multifunctional heterodimeric protein typically produced by mesenchymal cells. Its pleiotropic activities are mediated through its cellular receptor, a transmembrane tyrosine kinase encoded by the proto-oncogene c-Met. In malignant cells, HGF has been shown to protect cells from death induced by a variety of DNA-damaging agents, including radiation and topoisomerase inhibitors . Interestingly HGF/SF not only blocked DNA damage-induced apoptosis but also enhanced the rate of repair of DNA strand breaks . HGF also functions as an autocrine or paracrine growth factor and activates a program of cell dissociation and motility coupled with increased protease production that has been shown to promote cellular invasion [6, 7]. HGF and c-Met are co-expressed and often overexpressed in a broad spectrum of human solid tumors including lung, breast, and brain malignancies [7, 8]. Therefore, the overexpression of c-Met by GBM cells suggests that blocking HGF or its receptor c-Met might be an attractive strategy when combined with conventional treatment for the treatment of GBM. A recent review of this approach indicates that several novel inhibitors of the tyrosine kinase activity of c-Met have been developed and tested as a single agent or in combination with cytoxic chemotherapy . Although it has previously been shown that targeting HGF or c-Met expression using ribozyme radiosensitizers in GBM cells in vitro and xenograft tumor in vivo , demonstration of clinically useful inhibitors of the tyrosine kinase activity of c-Met combined with radiation have not been previously tested in GBM models.
In the work presented here, a novel inhibitor of c-Met tyrosine kinase, MP470 , was tested for its ability to radiosensitize GBM cells both in vitro and in vivo.
Materials and methods
All of the human GBM cell lines tested (SF763, SF268, SF295, SF126, SF188, SF767, U-87, and SF210) were obtained from the University of California, San Francisco, and maintained in Dulbecco's Modified Eagle Medium supplemented with 10% fetal calf serum and 1% penicillin-streptomycin . Cells were incubated at 37°C in a 5% CO2 incubator. MP470 (SuperGen, Dublin, CA) was stored in the dark at 4°C until use, when it was dissolved in dimethyl sulfoxide and used at a final concentration of 5.0-10 μ M. The drug was added to cells 1 hour before irradiation unless otherwise specified. Control cells were treated with equal volumes of dimethylsulfoxide. A cobalt-60 teletherapy unit (Atomic Energy of Canada Limited Theratron-80) was used to irradiate the GBM cells at a dose rate of 2 Gy/min.
Cell proliferation assay
The cytotoxicity of MP470 was assessed in vitro in all eight cell lines by using an MTS assay performed in a 96-well plate format. Cells were plated with a multichannel pipetter and MP470 (to a final concentration of 5 μ M) was added to triplicate wells 24-48 hours later, after which the plates were incubated for up to 4 days. The MTS assay was done with a CellTiter 96 AQueous Non-Radioactive Cell Proliferation Assay kit as per the manufactures recommendations (Cat #G5421, Promega, Madison, WI). The IC50 (the concentration that inhibited proliferation in 50% of the samples) was determined from standard curves.
In vitro clonogenic assay
The eight human GBM cell lines were cultured as described above, harvested, counted, and seeded onto 60-mm petri dishes at specific cell densities. MP470 (5 μ M) was added 1 hour before the cells were irradiated with single doses ranging from 2 to 8 Gy, after which the cells were returned to a 37°C incubator and cultured for 14 days in the presence of the MP470 before fixation. Cells were fixed for 5 minutes with 3:1 methanol: acetic acid solution and stained for 5 minutes with 0.5% crystal violet (Sigma, St. Louis, MO) in methanol. Colonies were counted with a Colcount automated colony counter (Optronix, Milton Port, Oxford, UK) using the discrete colony mode. The surviving fraction was calculated as (mean colony counts)/(cells plated) × (plating efficiency), where plating efficiency was defined as (mean colony counts)/(cells plated for unirradiated controls). All experiments were done in duplicate in 3 independent experiments and averaged data points represent the means ± standard deviations (SD).
Near-confluent SF767 cells were pretreated with 5 μ M MP470 irradiated (8 Gy), and analyzed 4 hours later as follows. Briefly, after pretreatment with MP470 for 5 hours, cells were suspended in phosphate-buffered saline (PBS, made by dissolving 8 g NaCl, 0.2 g KCl, 1.44 g Na2HPO4, and 0.24 g KH2PO4 in 800 mL of H2O) (pH 7.2) containing acridine orange (1 μ g·mL-1) and RNAse A and then co-stained with 1 μ g·mL-1 ethidium bromide (all reagents from Sigma); cells were then washed and examined under a fluorescence microscope (×150, Olympus, Center Valley, PA; exciting filter BG12, barrier filter 0-5.30). For quantitative analyses, 200 cells were counted and the percentages of necrotic and apoptotic cells calculated.
Double-stranded (ds) DNA breaks lead to the formation of γ-H2AX, a unique histone complex. We used a γ-H2AX antibody (Upstate Cat #05-636, Millipore, Billerica, MA) to visualize dsDNA breaks as follows. Cells were plated in chamber slides, grown for 48 hours, and treated with 5 μ M MP470, one hour later, the cells were irradiated with 4 Gy and processed either 1 hour or 8 hours later. Cells were first fixed in 4% paraformaldehyde and incubated with the primary antibody against γ-H2AX. The primary antibody was then washed off, and a secondary antibody conjugated to fluorescein isothiocyanate (FITC) was added to the slides. DNA damage was visualized by using confocal microscopy. Median intensity of each cell was calculated using Photoshop and a 2 sided t-test was used to calculate the difference.
dsDNA breaks were visualized by using a neutral comet assay (Cat #4250-050-K, Trevigen, Gaithersburg, MD). Cells were plated on 10-cm BD Falcon Cell Culture Plates (BD, Franklin Lakes, NJ), incubated for 2 days, treated with 10 μ M MP470 or dimethylsulfoxide (control) for 1 hour, and then irradiated with 8 Gy. Cells were then trypsinized, placed on glass slides, and subjected to electrophoresis according to the manufacturer's instructions. dsDNA breaks were measured by olive tail movement, (OTM), defined as (tail length) × (the fraction of total DNA in the tail). OTM values were calculated with TriTek Comet Score V 1.5 software (The TriTek Corporation, Sumerduck, VA). Data points represent means ± SDs from triplicate experiments.
Cells were plated on 10-cm petri dishes and grown for 24-48 hours. MP470 was then added at a concentration of 10 μ M (in 100% dimethlysulfoxide) for maximum inhibition. Cells were incubated with the MP470 for 24 hours (unless otherwise specified) before being irradiated with 4 Gy. After irradiation, cells were lysed on the plates by adding 350 μ L of sodium dodecyl sulfate lysis buffer (20 mM HEPES [pH 7.9], 400 mM NaCl, 1 mM EDTA, 1 mM EGTA, 1% NP40, 1 mM DTT, 1 mM phenylmethylsulfonyl fluoride, 1 mg/mL aprotinin, 1 mg/mL leupeptin, 250 mg/mL benzamide, 50 mM NaF, and 1 mM NaO3V4). The lysate was transferred to a 1.5-mL microcentrifuge tube, boiled for 5 minutes with intermittent vortexing, and then centrifuged for 5 minutes at 10,000 rpm, after which the supernatant was transferred to a new microcentrifuge tube. Lysates were subjected to electrophoresis on 10%-20% HCl pre-poured gels (Cat #161-1108, Bio-Rad, Hercules, CA). The proteins were then transferred to nitrocellulose paper and probed with the appropriate antibodies under the conditions recommended by the suppliers. The following antibodies were used Phospho-AKT (Ser 473 Cat, #4058), glycogen synthase kinase (GSK)-3β with Phospho-GSK-3β (Ser9, #9336) Cell Signaling Technology, Danvers, MA), RAD51 H-92 (Santa Cruz Biotechnology, Santa Cruz, CA) and c-Met phosphospecific Anti-c-Met [pY1003] (Cell Signaling Technology) .
siRNA to c-Met (sc-29397) and control siRNA (sc-37007) were purchased from Santa Cruz Biotechnology. The transfection reagent Lipofectamine was from Invitrogen (Carlsbad, CA). U87 cells were grown to 70% confluence and transfected with siRNA at a final concentration of 100 nM. Seventy-two hours later, the cells were lysed for western blotting analysis as described above.
To create subcutaneous tumors, cells (4 × 106 per animal) were implanted in the flanks of 32 outbred athymic nude mice, 8 per arm (Taconic, Germantown, NY). U87 cells were chosen for their high level of c-Met expression and ability to rapidly produce tumors. Twenty-five days after the cells were injected, animals were pair-matched (based on tumor volume) and assigned to one of four treatment groups: control; MP470 (60 mg/kg) alone; radiation alone (2 Gy per day × 10 days); and MP470 + radiation. MP470 was delivered daily by gavage at a dose of 60 mg/kg in peanut oil starting on day 25 for 14 consecutive days. Radiation was started on day 27 and consisted of 2 Gy per day delivered to the tumor by a cobalt-60 irradiator (Theratron-80).
Radiation was delivered daily, 5 days per week for 2 weeks, at 1 hour after the MP470 treatment. The total cumulative dose delivered to the tumor was thus 20 Gy. Animals were euthanized by CO2 asphyxiation when the tumor volume reached 2000 mm3, as required by our institutional animal care and use committee protocol #07-029. All remaining animals were euthanized on day 48. Tumors were measured with calipers every 5 days and the volume calculated according to the formula (a2 - b/2), where a is the smallest diameter and b is the largest diameter of the tumor.
Tumor growth delay was expressed in absolute and normalized terms as follows. Absolute growth delay (AGD) was defined as the number of days for tumors in the radiation-only and the MP470 + radiation groups to reach 1,500 mm3 minus the number of days for tumors in the control group to reach the same size. Normalized growth delay (NGD) was calculated as the number of days for tumors in the combined-therapy group to reach 1,500 mm3 minus the number of days for tumors in the MP470-only group to reach 1,500 mm3. The enhancement factor was then determined by dividing the NGD for the group receiving MP470 plus radiation by the AGD for the group given radiation alone. All statistical analyses were carried out with Stata 9.2 for Windows (College Station, TX), and P values < 0.05 were considered significant.
Cytotoxicity and radiosensitization
MP470 inhibits GSK3β and induces apoptosis
Influence of MP470 on repair of dsDNA breaks
In vivo radiosensitization studies
MP470-induced tumor growth delay in U87 xenografts
No. of Mice Per Group
Tumor Growth Period, days*
Absolute Growth Delay†
Normalized Growth Delay‡
9.7 ± 1.01
8.6 ± 1.1
0 ± 1.1
15.8 ± 2.3
6.1 ± 2.3
MP470 + Radiation
27.4 ± 2.8
17.7 ± 2.8
17.7 ± 2.8
The small molecule MP470 is a potent c-Met antagonist that is cytotoxic to a variety of cell lines in vitro. In this report, we demonstrated that concurrent inhibition of c-Met in combination with irradiation led to both reduced dsDNA repair and enhanced apoptosis in GBM. Our in vitro findings were supported by our in vivo observations using a xenograft model in nude mice. In this model, MP470 by itself, at a dose of 60 mg/kg, had no effect on tumor size or survival; radiation by itself was somewhat more effective in reducing tumor volume and improving survival; but the combination of radiation plus MP470 produced the best response in terms of both local control and survival.
High-grade glial neoplasms of the brain continue to be one of the most challenging malignancies to treat, and their poor prognosis has improved only marginally over the past four decades (survival rates of 10%-20% at 2 years) . Postoperative radiation provides a clear survival advantage for patients with gliomas, yet the majority of disease recurrences present within 2 cm of the postoperative bed-the very place targeted by the radiation. Unfortunately, attempts to escalate treatment doses to the tumor bed have provided only modest benefit . To better understand why requires evaluating the cellular and molecular interactions in the resistant tumor cells. The pathway to malignancy consists of multiple genetic mutations, often in key regulators of the cell cycle or DNA repair process. These alterations allow cancer cells to not only divide unchecked, but also to repair DNA damage at an accelerated or more efficient rate. One of the genes implicated in homologous recombination repair of dsDNA damage is RAD51. Prior work from our lab has demonstrated that RAD51 expression levels at the time of initial surgical resection are an independent prognosticator of survival for GBM patients receiving radiation . In the present paper, we evaluated whether MP470 could influence RAD51 expression in GBM tumors cell and found that pretreatment with MP470 inhibited XRT induced expression of RAD51. This compliments our prior GBM tissue microarray findings that 70% of recurrent GBM tumors, treated with XRT, were found to have elevated RAD51 at the time of recurrence . Paradoxically, this suggests that the ability of malignant cells to repair dsDNA damage can be enhanced by the very agents used to treat malignancies. The stimulation of RAD51 by radiation may explain why current therapies temporarily improve local control but fail to offer definitive cures. Clearly, substantial improvements in local control and survival of patients with GBM will require targeting (inhibiting) the molecular machinery that mediates the development of resistance.
To our knowledge, this is the first demonstration that MP470, an orally available c-Met antagonist, causes radiosensitization of several GBM cell lines. We have shown evidence that supports a mechanism of action consistent with a decrease in dsDNA break repair, along with enhanced radiation-induced apoptosis. Other investigators have shown that c-Met inhibition can enhance radiation-induced tumor cell death in vitro using a retrovirally based approach that would not be a clinically viable option, although it did serve as an important proof of concept . This stands in contrast to MP470, which is well tolerated in animals, with no observable adverse effects from daily administration of 2,000 mg/kg to rats and 240 mg/kg to dogs . This initial work on MP470 provided the foundation to support a phase I trial, to establish the maximum tolerated dose of MP470 in humans. Our work reported here suggests that c-Met inhibition can offer therapeutically relevant radiosensitization and potentially improve the therapeutic ratio in radiation-resistant tumors such as GBM.
This work was supported by a grant from SuperGen and we would like to thank their support of our work. We thank the members of the Experimental Mouse Shared Services at the Arizona Cancer Center, along with the University of California San Francisco's generosity in sharing their GBM cell lines and Dr. Raymond Meyn, (M.D. Anderson Cancer Center) for his assistance in publication preparation.
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