Background
Melanoma is a highly aggressive skin cancer originating from melanocytes that is dramatically increasing in incidence worldwide [
1],[
2]. Several molecular alterations associated with melanoma have been identified and have provided new targets for therapy. In particular, gene mutations in the RAS/RAF/MEK/ERK or mitogen-activated protein kinase (MAPK) pathway have been found to be highly prevalent in melanomas. For example, around 40 different mutations in the B-RAF gene, encoding for a serine-threonine kinase, have been identified. The most frequent activating mutation (V600E-B-RAF) has been found in exon 15 and results in the substitution of valine for glutamic acid at position 600 of the BRAF protein [
3],[
4]. This mutation represents around 90% of BRAF mutations described in melanoma and occurs in approximately 50% of melanoma cases [
5]. Melanomas carrying a B-RAF V600E mutation constitutively activate the MAPK pathway, promoting cell proliferation and preventing apoptosis [
6]. Mutations in the NRAS gene have also been observed in approximately 15-20% of melanomas [
7]-[
9]. In addition, molecular alterations in the c-KIT and GNαQ genes have been described, primarily in mucosal and uveal melanoma subtypes, respectively [
10].
B-RAF (V600E) inhibitors (e.g. vemurafenib, dabrafenib) [
11] were the first targeted therapy in melanoma and have been associated with significant improvements in both progression-free survival and overall survival compared with chemotherapy in patients with B-RAF mutated metastatic melanoma [
12]-[
16]. However, although B-RAF inhibitor therapy produced tumor responses in the majority of patients and prolonged median survival, responses were largely partial, and clinical evidence of tumor resistance typically developed at a median of 5–7 months [
17],[
18]. The different resistance mechanisms following B-RAF inhibitor treatment are a consequence of the activation of alternative pathways, which can be classed as MAPK-dependent [
19] or MAPK-independent [
20],[
21]. Acquired resistance to B-RAF inhibitors has been reported to primarily result from MAPK reactivation, driven by secondary mutations in NRAS and MEK1 genes in a subset of melanoma patients [
19]. Based on these data, the combination of a B-RAF inhibitor with an inhibitor of the MAPK pathway downstream from BRAF, such as a MEK inhibitor (e.g. trametinib), has been proposed in order to overcome acquired B-RAF inhibitor resistance.
It has been observed that MEK inhibition leads to decreased cell signaling and proliferation in cancer cells [
22],[
23]. In particular, human melanoma cell lines with N-RAS and B-RAF mutations seem to be more sensitive to MEK inhibitor treatment than melanoma cell lines with a B-RAF-mutation only [
24]. While MEK inhibitor treatment is associated with improved response rate, progression free-survival and overall survival in patients with B-RAF mutant metastatic melanoma, combined B-RAF and MEK inhibitor treatment seems to provide greater improvements in progression-free survival and overall survival compared with B-RAF inhibitor monotherapy [
25]. The observation that accelerated growth of lesions harboring H-RAS mutations following exposure to B-RAF inhibitors was blocked by subsequent combined B-RAF/MEK inhibitor treatment is of particular note [
26], and further supports the efficacy of MEK inhibitors and their potential role in metastatic melanoma [
27],[
28].
Aurora kinase A (AurkA), one of the key regulators of M phase progression, has been shown to be expressed at high levels in melanoma [
29]. AurkA is a member of a serine/threonine kinase family consisting of three classes (Aurk A, B, and C) that are essential components of the mitotic pathway [
30]. They ensure proper chromosome assembly, formation of the mitotic spindle, and cytokinesis. Over-expression of these kinases has been observed in several tumor types, including colon, breast, prostate, pancreas, thyroid, and head and neck, and is associated with advanced clinical stage and poor prognosis [
31]-[
33]. AurkA inhibition has been shown to limit tumor growth, impair mitosis and induce senescence in melanoma, suggesting a potential role in the treatment of these tumors [
34].
In this study, we assessed the anti-tumor effects of an Aurk A inhibitor, B-RAF inhibitor and MEK inhibitor as single agents and in various combinations in a B-RAF (V600E) mutated human melanoma cell line and a three-dimensional (3D) human skin reconstruction model in order to provide a basis for further development of novel therapeutic strategies in the treatment of melanoma.
Methods
Inhibitor drugs
AurkA inhibitor (MLN8054) was purchased from Selleck Chemicals (Munich, Germany), while GSK2118436 (B-RAF inhibitor) and GSK1120212 (MEK inhibitor) were kindly provided by Glaxo SmithKline (London, UK). The B-RAF and MEK inhibitors were tested at a concentration of 30 nM in all experiments while the AurkA inhibitor was tested at a concentration of 1 μM concentration.
Cell lines and reagents
A human melanoma cell line with the B-RAF (V600E) mutation (A375mel) was kindly provided by Dr. M. Bettinotti (NIH, Bethesda, USA). The cells were cultured in RPMI 1640 medium (Lonza, Milan, Italy), supplemented with 3 mM L-glutamine (Invitrogen-Gibco®/Life Technologies, Monza, Italy), 2% penicillin/streptomycin and 10% fetal bovine serum (FBS). All cultures were incubated at 37°C in a humidified 5% CO2 atmosphere.
Cell proliferation
Cell proliferation was assessed using E-16-well plates and the xCELLigence technology (Acea Bioscience, San Diego, CA, USA, distributed by Roche Diagnostics) [
35]. Briefly, cells (1 × 10
3 A375 cells/well) were seeded in E-16-well plates in complete medium and grown for 48 hours. Inhibitor drugs were then added as single agents or in different combinations (B-RAF inhibitor [GSK2118436] plus MEK inhibitor [GSK1120212], BRAF inhibitor plus AurkA inhibitor [MLN8054], MEK inhibitor plus AurkA inhibitor [MLN8054] or triple-combination of all three drugs) and the cell growth was monitored for an additional 72 hours. Microelectrodes, placed on the bottom of plates, were used to detect impedance changes proportional to the number of adherent cells and expressed as the cell index. The impedance value of each well was automatically monitored by the xCELLigence system and expressed as a cell index value. Doubling times for each cell line were calculated from the cell growth curve during the exponential phase. The experiments were conducted in triplicate and repeated twice.
Western blot analysis
Western blot was performed according to standard procedures. Mouse monoclonal antibody against p53 (DO-1; diluted 1:1000) (Santa Cruz Biotechnology, Inc. Dallas, TX, USA), rabbit antibody monoclonal to c-Myc (1:5000; Abcam, Cambridge, UK), and rabbit polyclonal antibody against β-actin (1:1000, Cell Signaling Technology, Danvers, MA, USA) were used. Detection was achieved by HRP-conjugated anti-mouse (Biorad, Hercules, CA, USA, 1:2,000) or HRP-conjugated anti-rabbit (Cell Signaling Technology, 1:1000) antibodies. Immune complexes were visualized by an enhanced chemiluminescence system (ECL Advance™, Amersham Pharmacia Biotech, Piscataway, NJ, USA). Actin was used as a loading control. The image analysis was performed by ImageJ software (
http://rsbweb.nih.gov/ij/). Results represent the means (±SEM) of three independent experiments performed in triplicate.
P-values were determined by using t-tests and a value ≤0.005 is reported (in figures with the symbol ***).
3D human skin reconstruction model
A 3D culture system of a differentiated and full-thickness skin reconstruction model of A375 melanoma cells was purchased from MatTek (Ashland, MA, USA). The 3D tissues were fed through the basolateral (bottom) surface, and incubated in duplicate with serum-free medium containing DMSO 0.2% as control, B-RAF inhibitor plus MEK inhibitor, MEK inhibitor plus AurkA inhibitor or a triple-combination of all three drugs.
The medium was replenished every other day, and the 3D tissues were collected on days 0, 5, 9 and 12 from the beginning of treatment and fixed with 4% paraformaldehyde. Tissues were paraffin-embedded, serial-sectioned, deparaffinized in xylene and rehydrated through graded decreasing concentrations of alcohol. Finally, sections were stained by hematoxylin and eosin (H&E) for cell morphology analysis. For immunohistochemistry, antigen retrieval was carried out in citrate buffer (pH6) by heating in a pressure cooker. The sections were then stained with antibodies specific for the detection of S-100 (rabbit polyclonal; Dako, Milan, Italy) and Ki-67 (Mib-1clone; Dako). Appropriate positive and negative controls were included for each antibody test. Serial sections, stained with H&E, S100 and Ki-67 were observed under a 10x/0.30 NA and 40x/0.75 NA objective lens, using a DM6000B upright microscope (Leica Microsystems, Milan, Italy). Images were captured using a high-resolution Leica DFC480 camera and digitally transferred by the Leica Application Suite 4.0 software. AdobePhotoshop CS5 software was used for image analysis. Ki-67 and S-100 positively stained cells were counted in six different fields on each slide (observed under a 40x/0.75 NA objective lens using the Leica DM6000B microscope) and their quantification was performed on digital images with the ImageJ program and AdobePhotoshop CS5. Results were expressed as percentages with respect to the controls.
Conclusions
Our findings provide new prospects for melanoma research, suggesting that combined B-RAF/AurkA inhibition might offer a therapeutic alternative to B-RAF/MEK inhibition for B-RAF mutated melanomas, while a MEK/AurkA inhibitor combination could represent a possible option for patients without B-RAF mutations. Moreover, for the first time, we have shown that a triple drug combination comprising of inhibitors of B-RAF, MEK and AurkA offers increased efficacy against melanoma cell growth and might be considered as a potential treatment strategy for enhancing clinical response in melanoma. However, although this triple drug combination was more effective as anti-tumor therapy at the epidermal/dermal junction, it seemed to leave individual S-100 and Ki-67 positively stained spindle-shaped melanoma cells alive and proliferating in the dermal stratum, which may result in drug resistance and disease recurrence. Molecular characterization of these dermal cells may be critical in providing additional tools for the development of novel therapeutic strategies.
Competing interests
All authors have disclosed any financial competing interests.
PAA has/had a consultant/advisory role for Bristol Myers-Squibb, Roche-Genentech, Merck Sharp & Dohme, GSK, Ventana, and Novartis. He received research funds from Bristol Myers-Squibb, Roche-Genentech, and Ventana. He also received honoraria from Bristol Myers-Squibb, Roche-Genentech, and GSK.
Authors’ contributions
MLM, MVC and FF carried out the cell proliferation assays. RM carried out the molecular studies by western blot analysis and helped to draft the manuscript. RT fixed the 3D tissues, which were then paraffin embedded and serial sectioned for immunohistochemical analysis, and performed the photos of the immunostained tissue samples. GB. ErC, performed 3D tissue immunostaining. PA, NM, GPal, GC and GPir participated in the design of the study. EmC conceived of the study, and participated in its design and coordination and drafted the manuscript. She also cultured and treated the 3D-tissues. All authors read and approved the final manuscript.