Background
Metastatic melanoma is notoriously resistant to cytotoxic chemotherapy. Commonly used agents such as dacarbazine and temozolomide yield poor response rates of less than 20% [
1] and combination regimes have not been proven superior over single agents [
2]. Therefore novel, more efficacious treatment strategies are urgently needed for melanoma.
Sorafenib (BAY43-9006) inhibits vascular endothelial growth factor receptor (VEGFR) and Raf kinase, but also has activity against c-kit and platelet derived growth factor receptor beta (PDGFR-β). Activating B-Raf mutations are detected in greater than 60% of malignant melanomas [
3] and sorafenib inhibits the growth of melanoma cells carrying B-Raf mutations. Sorafenib has shown little activity as a single agent in the treatment of malignant melanoma, irrespective of B-Raf status [
4], however in combination with carboplatin it has shown promising clinical activity [
5] and is presently being tested in several clinical trials in melanoma either alone or in combination with other agents
http://www.clinicaltrials.gov.
Src kinase regulates key pathways in metastasis including cell adhesion, invasion and motility [
6] and members of the Src family have been implicated in melanoma progression [
7‐
11]. Both Src and Yes are reported to be elevated in melanoma cells compared to normal melanocytes [
7,
12]. Dasatinib, a multi-target tyrosine kinase inhibitor, targets Src kinase, in addition to BCR-Abl, c-KIT, PDGFR and ephrin-A receptor kinases. It is the most potent Src kinase inhibitor currently in clinical development with an IC
50 of 0.5 nM for Src kinase (IC
50 of < 30 nM for the other targets) [
13]. Dasatinib has shown preclinical activity in prostate cancer [
14], triple negative breast cancer [
15] and colon cancer cells.
Due to the deficiency of effective treatment options for advanced melanoma and the reported relationship between Src kinase and melanoma progression, we examined the preclinical activity of Src inhibition, using dasatinib, alone and in combination with temozolomide in metastatic melanoma cell lines.
Methods
Cells and reagents
Lox-IMVI, Malme-3M, Sk-Mel-5, and Sk-Mel-28 were obtained from the Department of Developmental Therapeutics, National Cancer Institute (NCI) and HT144 from the American Tissue Culture Centre (ATCC). Cell lines were grown at 37°C with 5% CO2 in RPMI medium with 10% FCS (Gibco) except HT144 which was grown in McCoys 5A (Sigma-Aldrich) with 10% FCS. Stock solutions of temozolomide (9.7 mM), (Department of Developmental Therapeutics, National Cancer Institute), epirubicin (3.45 mM), taxotere (11.6 μM) (Dept of Pharmacy, St. Vincent's University Hospital), dasatinib (10 mM), sorafenib (10 mM) (Sequoia Research Products) and imatinib (16.9 mM) (Novartis) were prepared in dimethyl sulfoxide (Sigma-Aldrich).
500 μL RIPA buffer with 1 × protease inhibitors, 2 mM PMSF and 1 mM sodium orthovanadate (Sigma-Aldrich) was added to cells and incubated on ice for 20 minutes. Following centrifugation at 10,000 rpm for 5 minutes at 4°C the resulting lysate was stored at -80°C. Protein quantification was performed using the Bicinchoninic acid (BCA) assay (Pierce). 40 μg of protein in sample buffer was heated to 95°C for 5 minutes and proteins were separated on 7.5 or 10% gels (Cambrex). The protein was transferred to Hybond-ECL nitrocellulose membrane (Amersham Biosciences). The membrane was blocked with blocking solution (PBS + 0.1% Tween + 5% skimmed milk powder (BioRad)) at room temperature for 1 hour, then incubated overnight at 4°C with 1 μg/ml primary antibody (mouse anti-Epha2, Millipore; mouse anti-Src kinase, Upstate Cell Signalling Solutions; rabbit anti-phospho-Src py 418, Biosource Europe; mouse anti-FAK kinase BD Biosciences; rabbit anti-FAK py 861 and py 397, Invitrogen; mouse anti-tubulin, Sigma-Aldrich) in blocking solution. The membrane was washed three times with PBS-Tween, then incubated at room temperature with anti-mouse secondary antibody (Sigma-Aldrich) at 1:1000 dilution or anti-rabbit secondary antibody (Pierce) at 1:3000 dilution) in blocking solution for 1 hour. The membrane was washed three times with PBS-Tween followed by one PBS wash. Detection was performed using Luminol (Santa Cruz Biotechnology). For detection of phosphorylated EphA2, EphA2 was immunoprecipitated from 500 μg of protein using EphA2 antibody (Millipore) and immunoblotted with a mouse anti-phosphotyrosine antibody (Upstate Cell Signalling Solutions).
Proliferation assay
Proliferation was measured using an acid phosphatase assay. 1 × 10
3 cells/well were seeded in 96-well plates, apart from HT144 and Malme-3M which were seeded at 2 × 10
3 cells/well. Plates were incubated overnight at 37°C followed by addition of drug at the appropriate concentrations and incubated for a further 5 days until wells were 80% to 90% confluent. All media was removed and the wells were washed once with PBS. Paranitrophenol phosphate substrate (0.263 g of PNP in 100 ml sodium acetate buffer) was added to each well and incubated at 37°C for 2 hours. 50 μl of 1 M NaOH was added and the absorbance was read at 405 nM (reference – 620 nM), as previously described [
16].
Invasion assays
Invasion and migration assays were performed as previously described [
17], using 1 × 10
5 cells in matrigel-coated 24-well invasion inserts for invasion assays and uncoated inserts for migration assays. Cells were incubated for 6 hours before dasatinib treatment to allow cells to attach and then incubated at 37°C with dasatinib at varying concentrations for 24 hours. Cells were stained with crystal violet and the number of invading/migrating cells was estimated by counting 10 fields of view at 200 × magnification. The average count was multiplied by the conversion factor 140 (growth area of membrane divided by field of view area, viewed at 200 × magnification) to determine the total number of invading/migrating cells. All assays were performed in triplicate.
2.5 × 104 cells were seeded per well in 24-well plates and incubated overnight at 37°C, followed by addition of drug at the appropriate concentrations. After 72 hours, media was collected and the wells washed once with PBS. Cells were trypsinised and added to the media collected for each sample. Cells were centrifuged at 300 × g for 5 minutes and the media was aspirated. 150 μl of PBS was added, the pellet re-suspended and the total volume transferred to a round bottomed 96 well plate. 50 μL of 4% para-formaldehyde was added to the wells and mixed. Cells were incubated at 4°C for 60 minutes. The plate was centrifuged at 300 × g for 5 minutes and the supernatant aspirated leaving approximately 15 μL in each well. The remaining volume was used to resuspend the cells and 200 μL of ice cold 70% ethanol was added to the cells. The plates were then stored at -20°C for 2 hours. After fixing the cells were stained according to the protocol for the TUNEL assay (Guava Technologies). Cells were analysed on the Guava EasyCyte (Guava Technologies). Positive and negative controls were performed with each assay.
Cell cycle assays
2.5 × 104 cells were seeded per well in 24-well plates and incubated overnight at 37°C. After 24 hours cells were synchronised by removing the media and replacing it with serum free medium (SFM) for a further 24 hours. SFM was removed and the cells incubated for a further 6 hours in media containing serum before the drug was added at the appropriate concentrations. Plates were then incubated at 37°C for a further 24 hours. Media was collected and the wells washed once with PBS. Cells were trypsinised and added to the media collected for each sample. Cells were centrifuged at 300 × g for 5 minutes and the media was aspirated. 150 μl of PBS was added, the pellet re-suspended and the total volume transferred to a round bottomed 96 well plate. The plate was centrifuged at 300 × g for 5 minutes and the supernatant aspirated leaving approximately 15 μL in each well. The remaining volume was used to resuspend the cells and 200 μL of ice cold 70% ethanol was added. The plates were then stored at -20°C for 2 hours. After fixing the cells were stained according to the protocol for the Guava Cell Cycle assay (Guava Technologies). Cells were analysed on the Guava EasyCyte and the data was analysed using Modfit LT software (Verity).
Statistical analysis
IC50 values were calculated using CalcuSyn software (BioSoft). For Lox-IMVI, combination index (CI) values were calculated using CalcuSyn software. A CI value of < 1 is considered synergistic, 1 is considered additive and > 1 is considered antagonistic. CI values were not calculated for the other cell lines, as dasatinib did not achieve 50% inhibition of growth at concentrations up to 1 μM. The Student's t test was used to compare temozolomide IC50s alone and in combination with dasatinib, migration/invasion assays and cell cycle assays P < 0.05 was considered statistically significant. ANOVA one way analysis was performed to compare dasatinib alone, taxotere/epirubicin alone and the combination. P < 0.05 was considered statistically significant.
Discussion
We have evaluated the effects of dasatinib, a multi-targeted tyrosine kinase inhibitor, in human melanoma cell lines [
6]. In a previous study in breast cancer cell lines, sensitivity to dasatinib was characterised as greater than 60% inhibition, moderate sensitivity as 40–59% inhibition and resistance as less than 40% inhibition in response to 1 μM dasatinib [
15] (assuming higher concentrations would not be achievable
in vivo) [
15]. Therefore, Lox-IMVI can be classified as being highly sensitive to dasatinib, HT144 moderately sensitive and the remaining three cell lines are resistant, although Malme-3M shows some sensitivity.
Sorafenib which is currently in clinical trials for advanced melanoma, has shown little activity when tested alone but shows promising results when tested in combination with chemotherapy [
5]. In the five cell lines tested in this study, which are B-Raf mutated
http://www.sanger.ac.uk/genetics/CGP/cosmic/, the IC
50 for sorafenib was above 1 μM in each case. These results suggest that dasatinib-sensitive melanoma cells are more sensitive to dasatinib than to sorafenib
in vitro.
Furthermore, dasatinib in combination with temozolomide significantly improved response in HT144 and Lox-IMVI compared to either drug alone. In Malme-3M cells, there was a small but significant improvement in response compared to temozolomide alone. In the dasatinib-resistant cell line Sk-Mel-28, the combination was slightly better than temozolomide alone although the difference was not significant. Therefore the combination of dasatinib with temozolomide may improve response in some melanoma patients. In dasatinib resistant tumours, the addition of dasatinib would not impact on sensitivity to temozolomide but may help to prevent further tumour spread by inhibiting melanoma cell migration and invasion, as we observed in dasatinib-resistant Sk-Mel-28 cells.
Studies in lung cancer [
18], head and neck squamous cell carcinoma [
19] and malignant pleural mesothelioma [
20] showed that dasatinib induces both cell cycle arrest and apoptosis. In Lox-IMVI, the most sensitive cell line, treatment with dasatinib induced both apoptosis and cell cycle arrest. In the other dasatinib responsive cell lines, HT144 and Malme-3M, dasatinib induced either cell cycle arrest or apoptosis respectively. Therefore, optimal response to dasatinib in melanoma cells may require efficient induction of both cell cycle arrest and apoptosis.
Imatinib targets Bcr-Abl, c-Kit and PDGFR. Previous studies identified that c-kit expression was reduced with melanoma progression and trials testing imatinib as a single agent showed no benefit in the clinical setting [
21,
22]. However recent studies have identified a group of chronic sun damaged patients who maintain c-kit expression despite melanoma progression [
23] and as a result clinical trials have been undertaken to target c-kit with imatinib in this population [
21].
Imatinib however does not inhibit the growth of either HT144 or Lox-IMVI cells. Thus sensitivity of melanoma cell lines to dasatinib may be due to targeting Src kinase or EphA receptors, which are not targeted by imatinib. Differences in the level or phosphorylation of Src kinase do not appear to predict sensitivity to dasatinib in the melanoma panel. Similar to preclinical studies in other solid tumour types [
20], phosphorylation of Src was reduced in dasatinib sensitive cell lines, whereas in the dasatinib resistant cell lines Sk-Mel-28 and Sk-Mel-5, phospho-Src was either unchanged or slightly increased, in response to dasatinib treatment. Thus inhibition of Src phosphorylation may be an appropriate marker of response to dasatinib. Serrels
et al [
24] showed that inhibition of phospho-Src in peripheral blood mononuclear cells correlated with inhibition of phospho-Src in colon tumours. Measuring changes in phospho-Src in peripheral blood mononuclear cells may therefore serve as a surrogate marker for response to dasatinib in the clinic [
25].
Previous studies have shown that dasatinib treatment did not reduce phosphorylation of FAK at Tyr397, an autophosphorylation site required for recruitment of Src kinase which in turn phosphorylates FAK at Tyr576, Tyr577, and Tyr861 [
24]. Phosphorylation at these sites is important for FAK downstream signalling [
26]. Dasatinib reduced the level of FAK phosphorylation at Tyr861 in all of the melanoma cell lines and therefore does not appear to be associated with inhibition of proliferation but may play a role in inhibition of migration and invasion in melanoma cells. In colon cancer cells, reduced phosphorylation of FAK at tyrosine 861 was implicated in dasatinib-mediated inhibition of migration and invasion [
24]. Recently enzyme assays have shown that dasatinib is a potent inhibitor of several additional kinases, including FAK (IC
50 = 0.2 nM) [
27]. Therefore, dasatinib may directly target FAK, independently of Src, resulting in inhibition of migration/invasion without inhibition of proliferation, as was observed in Sk-Mel-28 cells.
Other dasatinib preclinical studies did not examine the role of EphA receptors in response to dasatinib. EphA2 has been identified as a potential dasatinib sensitivity biomarker [
28]. Interestingly EphA2 levels were significantly higher in the three dasatinib sensitive cell lines than in the two resistant cell lines. Although the number of cell lines is small, this suggests that EphA2 expression may predict response to dasatinib treatment and warrants further investigation in a larger panel of cell lines. Dasatinib treatment for 6 hours had no effect on phosphorylation of EphA2. However, in Lox-IMVI, phosphorylation of EphA2 was transiently decreased at 30 minutes, but was restored by 2 hours. EphA2 activity may also be altered by decreased phosphorylation of Src and FAK, which form a complex with EphA2 [
29]. Dasatinib may also target other members of the Ephrin receptor family such as EphB4 [
27]. Further research is required to elucidate the role of Ephrin receptors in response to dasatinib treatment in melanoma and other solid tumours.
The
in vitro effects of dasatinib in melanoma cell lines observed in this study provide strong evidence for evaluation of dasatinib in clinical trials in melanoma patients. Two clinical trials of dasatinib in melanoma are currently underway, including a phase I/II study of dasatinib in combination with dacarbazine
http://www.clinicaltrials.gov.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
AJE contributed to the design of the study and carried out the proliferation assays, TUNEL assays, cell cycle assays, Western blotting and statistical analysis. JC and MC contributed to the interpretation of the data. NOD conceived the study, supervised the research, and participated in interpretation of the data and drafting the manuscript. All authors read and approved the final manuscript.