Melanoma is a highly aggressive disease that is difficult to treat owing to rapid tumor growth, apoptotic resistance and high metastatic potential. The MET proto-oncogene (MET) tyrosine kinase receptor promotes many of these cellular processes, but while MET is often overexpressed in melanoma, the mechanism driving this overexpression is unknown. As the MET gene is rarely mutated or amplified in melanoma, MET overexpression may be driven to increased activation through promoter elements. In this report, we find that transcription factors PAX3 and ETS1 directly interact to synergistically activate MET expression. Inhibition of PAX3 and ETS1 expression in melanoma cells leads to a significant reduction of MET receptor levels. The 300-bp 5′ proximal MET promoter contains a PAX3 response element and two ETS1 consensus motifs. Although ETS1 can moderately activate both of these sites without cofactors, robust MET promoter activation of the first site is PAX dependent and requires the presence of PAX3, whereas the second site is PAX independent. The induction of MET by ETS1 via this second site is enhanced by hepatocyte growth factor-dependent ETS1 activation, thereby MET indirectly promotes its own expression. We further find that expression of a dominant-negative ETS1 reduces the ability of melanoma cells to grow both in culture and in vivo. Thus, we discover a pathway where ETS1 advances melanoma through the expression of MET via PAX-dependent and -independent mechanisms.
ETS1 is a member of the ETS family of transcription factors, defined by the presence of a winged helix-turn-helix DNA-binding domain.1 Expression of ETS1 is absent or low in melanocytes of normal skin or within simple nevus structures (lentigo simplex); however, ETS1 is overexpressed in melanoma in situ and in invasive and metastatic primary tissues, as well as in melanoma cell lines.2, 3 Although the role of ETS1 in melanoma is unclear, its major functionality likely lies in transcriptional regulation. Evidence supports that ETS1 promotes cell survival, tumor progression and invasion. ETS1 may act as either a pro- or anti-apoptotic factor depending on the cell type. In melanoma, ETS1 has an anti-apoptotic role, at least partially due to upregulation of MCL1.4 In terms of tumor invasion and progression, inhibition of ETS1 leads to a decrease in expression of uPA, MMP1, MMP3 and integrin-β3.3 In addition, ETS1 directly activates the integrin-αv promoter.5
There are several lines of evidence supporting that ETS1 is upstream of MET proto-oncogene (MET), a receptor tyrosine kinase that promotes melanoma cell growth and survival.6, 7, 8 An increase in ETS1 protein levels raises MET levels, whereas inhibition of ETS1 decreases MET receptor expression.9, 10, 11, 12 In addition, in esophageal cancer, levels of MET and ETS1 protein correlate significantly.13 Although in silico studies predict that ETS1 is directly upstream of the MET promoter,9 this has not been proven definitively through experimentation in any cell type.
We previously identified the transcription factor PAX3 as an upstream regulator of MET in melanoma.14 During normal melanocyte development, PAX3 is necessary for the regulation of genes involved in cell type specification while maintaining an undifferentiated state, proliferation and migration (reviewed in Kubic et al.15). These characteristics are mirrored in melanoma, where our group and others find PAX3 expression.16, 17, 18, 19 Along with MET, PAX3 mediates its cellular effects in melanoma through the regulation of downstream targets, such as BRN2 and TBX2.20, 21 However, PAX3 is a weak transcription factor on its own, and often recruits other factors to synergistically regulate gene expression.
Here, we discover a pathway for promoting MET receptor expression by the transcription factors ETS1 and PAX3. We find that both transcription factors directly interact and synergistically drive MET expression by binding to promoter enhancer elements. The MET promoter contains two ETS1 sites, and activation through these two elements is enhanced by different mechanisms that are either PAX3- or hepatocyte growth factor (HGF) dependent. Our data support a model for an oncogenic pathway where PAX3 and ETS1 drive MET expression, and this pathway is further driven in a feed-forward manner through the ligand for MET, HGF.
PAX3, ETS1 and MET are expressed in melanoma cell lines and tumors
To determine the presence of PAX3, ETS1 and MET proteins in human melanoma cell lines, a panel of seven independent lines was analyzed (Figure 1a). All cell lines expressed these three proteins to varying degrees. ETS1 contains a Ras-responsive site at threonine 38 (T38), and phosphorylation of this epitope strongly increases the protein’s transcriptional activity.22, 23, 24, 25 The phosphorylation status of T38 in ETS1 was measured in the melanoma cell line panel (Figure 1b). In comparison with calf intestinal phosphatase controls or samples that were ETS1 negative, phosphorylated ETS1 (pETS1) levels are considered high for A375, SKMEL-5 and SKMEL-23 (P<0.0005), and significant for mel-537 (P<0.05)(n=3). The pETS1 levels are considered undetectable for mel-888 (P=0.051) and SKMEL28 (P=0.234) cells.
Expression of PAX3, ETS1, MET, pETS1 and phosphorylated-MET (active form of the MET receptor, pMET) was measured in 20 superficial spreading melanoma primary tissue samples. Representative results for PAX3 and MET, or ETS1 and MET immunofluorescence and co-expression are shown in Figures 1c–j and m. PAX3 and ETS1 expression is predominantly nuclear, whereas MET is principally located in the membrane with focal regions of pan-cellular expression. The majority of samples express PAX3 and ETS1. No melanoma samples in the panel were both PAX3 and ETS1 negative. All 20 samples expressed MET receptor. Half of the samples expressed pETS1, whereas 6/20 samples presented with pMET (Figures 1k–m).
The MET proximal promoter contains potential PAX3- and ETS1-binding sites
The transcriptional start site of the MET gene has been determined, and potential ETS-family-binding sites have been predicted through in silico analysis.9, 26 Although ETS family members all bind to a core sequence of GGA(A/T),27 each factor has longer high-affinity DNA-binding sites. High-affinity recognition motifs for ETS1 have been experimentally determined to be C(C/A)GGA(A/T)G(T/C),28 AC(C/A)GGA(A/T)(G/A)29 and (G/C)CGGAAGT.30 Combining these studies produces a high-affinity consensus site of C(A/C)GGA(A/T)(G/A) or CMGGAWR. Within a short segment of the MET promoter (297 bp 5′ proximal to the transcriptional start site, and 25 bp of the 5′ untranslated region), there is one CMGGAWR ETS1 consensus site located −85 to −79 from the transcriptional start (Figure 2a). Although ETS1 binds to CCGGAA with 100-fold higher affinity than to CCGGAG, ETS1 recruitment to CCGGAG is enhanced 1000-fold when ETS1 is in complex with PAX5.31, 32 In contrast, converting this CCGGAG-ETS/PAX site to an optimum site for independent ETS1 binding (CMGGAWR) reduces the ability of PAX5 to interact with DNA.33, 34 A CCGGAG-ETS/PAX site is located in the proximal MET promoter at −241 to −236 (reverse strand, Figure 2a). This ETS/PAX site is adjacent to a previously characterized PAX3 site.14, 35
PAX3 and ETS1 activate the proximal MET promoter
PAX3 drives expression of MET in melanoma and in muscle cells.14, 35 To test the ability of PAX3 and ETS1 to activate the MET promoter alone or together, a DNA fragment containing 297 base pairs of the MET 5′ proximal promoter and 28 base pairs of the 5′ untranslated region was sub-cloned into the reporter vector pGL2-Luciferase (Figures 2a and b). Separately, PAX3 or ETS1 drive reporter expression 2.9±0.1-fold and 4.0±0.4-fold over light levels of the reporter alone (Figure 3a, set 1). Together, PAX3 and ETS1 activated the MET promoter synergistically 17.0±2.8-fold (P<0.005).
To test the importance of the putative PAX and ETS sites, METpm was utilized as a template to create reporter constructs containing single, double or triple mutations in these sites (Figure 2b). The constructs were transfected into HEK-293T cells in the presence or absence of PAX3 and/or ETS1 expression constructs. Any mutation of the PAX site alone (Figure 3a, set 2) or in concert with ETS mutations (sets 6, 7 and 8) completely abrogated the ability of PAX3 to drive reporter expression (P<0.005). Mutation of the PAX site did not disrupt the ability of ETS1 to drive reporter expression, but the synergistic activation was abolished. Mutation of either ETS site did not block the ability of ETS1 to activate the MET promoter (gray bars, set 3, 3.6±0.8-fold; set 4, 2.5±0.2-fold). Mutation of both sites reduced the ETS1 activation significantly (set 5 gray bar, 1.8±0.2-fold, P<0.005). Synergistic activation of the MET promoter by PAX3 and ETS1 was lost when the E1 site was mutated (Figure 3a, sets 3 and 5, black bar, P<0.005), or loss of the functional PAX site (sets 2, 6, 7 and 8, black bars, P<0.0005). However, if both the PAX and E1 sites were intact (Figure 3a, sets 1 and 4, black bars), PAX3 and ETS1 activated the MET promoter synergistically (P<0.0005). Electrophoretic mobility shift assays were performed to determine whether these factors bind directly to the MET promoter. Utilizing radiolabeled oligonucleotide probes containing the P and E1 sites (Figure 3b), a slower-migrating band was seen in electrophoretic mobility shift assays, with the addition of PAX3 protein (Figure 3c, arrows B and C, lane 3), but not with ETS1 alone (lane 5). A slower-migrating band was also seen when ETS1 was added to a probe containing the E2 site (Figures 3b and c arrows A and C, lane 9). All slower-migrating bands were significantly reduced with the addition of 100-fold excess cold probe (lanes 6 and 10). Our findings support that PAX3 and ETS1 bind to and activate the MET promoter through specific enhancer sites.
PAX3 and ETS1 directly interact and promote MET expression in melanoma cells
PAX3 and ETS1 are co-expressed in melanoma and activate the MET promoter more robustly together than either factor alone (Figures 1 and 3a). Immunoprecipitation assays were performed to determine whether these proteins interact within melanoma cells. Immunoprecipitation using an anti-ETS1 antibody yielded a band when probed for PAX3 by western analysis in A375 and mel-624 melanoma cells (Figure 4a, lanes 3 and 6). No PAX3 was detected with beads alone or with a non-specific immunoglobulin G antibody. To determine whether ETS1 and PAX3 directly interact, in vitro immunoprecipitation experiments were preformed utilizing recombinant ETS1 and PAX3 proteins (Figure 4). PAX3 and ETS1 directly interacted only when both proteins were present. We find that PAX3 and ETS1 directly interact in A375 and mel-624 melanoma cells.
To determine whether endogenous PAX3 and ETS1 bind to the MET promoter, chromatin immunoprecipition assays were performed on melanoma cell lysates, as well as cells that do not express MET (3T3 cells). The MET promoter sequence was amplified from DNA precipitated with proteins bound to antibodies specific for PAX3 or ETS1 (Figure 4c, lanes 1, 2, 6 and 7), but not with control immunoglobulin G antibody (lane 3 and 8) in both A375 and mel-624 melanoma cells. These bands were absent when 3T3 cell lysates were utilized. These experiments support that PAX3 and ETS1 are located on the MET promoter in melanoma cells.
To determine whether the PAX3 and ETS sites discovered in the MET promoter are active in melanoma cells, METpm vector or constructs with engineered mutations in the PAX, E1 and E2 sites (Figure 2b) were transfected into A375 and mel-624 melanoma cells (Figure 4d). Mutation of any of the sites resulted in a significant decrease (P<0.005) in reporter activity in both cell lines. Mutation of all three sites resulted in the most marked decrease in luciferase activity, 11.78±3.98% (A375, P<0.0005) or 19.75±5.74% (mel-624, P<0.0005). These findings demonstrate that these binding sites within the MET promoter are functional in melanoma cells.
Blocking PAX3 and ETS1 expression in melanoma cells lead to a reduction of MET levels. Inhibiting PAX3 alone reduced MET expression in some, but not all, melanoma cell lines.14 Reduction of PAX3 or ETS1 levels by less than half of the control cells did not significantly decrease MET levels in mel-624 cells (Figure 4e). Reduction of both PAX3 and ETS1 leads to a significant loss (P<0.02) of MET expression to 30.0±14.7% of control levels (Figure 4f). These data demonstrate that expression of MET is at least partially dependent on PAX3 and ETS1 in mel-624 cells.
ETS1 synergistically activates the MET promoter with either PAX3 or HGF
ETS1 may be activated by HGF via the RAS signaling pathway.22 This supports a positive feed-forward loop model wherein MET is activated by HGF, which indirectly activates ETS1 and consequently its own promoter. The ability of HGF to synergistically activate the MET promoter was tested in cells expressing the MET receptor (HEK-293T) and cells lacking this protein (3T3) (Figure 5a). In HEK-293T cells, HGF synergistically activated the MET reporter construct significantly (16.9±3.7-fold, P<0.005) at levels parallel to ETS1/PAX3 activation (Figure 5b). In the MET-deficient 3T3 cells, the ability of ETS1 to activate the MET promoter was similar without (2.2±0.3-fold) or with (2.2±0.7-fold) the addition of HGF (Figure 5c). The addition of HGF did not significantly increase luciferase levels (P=0.48). ETS1 and PAX3 still yielded a significant synergistic activation of the MET promoter in the 3T3 cells (P<0.005).
HGF activates ETS1 by a RAS signaling pathway, leading to phosphorylation of T38 within the ETS1 Pointed domain (Figure 5d).22, 23, 24, 25 Mutation of T38 blocks the ability of the RAS pathway to phosphorylate ETS1. HEK-293T cells were transfected with ETS1 or a mutant form of ETS1, ETS1(T38A) (Figure 5d) and were expressed with or without HGF or PAX3 (Figure 5e). Both versions of ETS1 activated the MET promoter to similar levels (ETS1, 2.8±1.1-fold; ETS1(T38A), 3.1±0.4-fold). Both proteins synergistically activated the MET promoter with PAX3, whereas only the wild-type protein was able to have the same effect with HGF (all groups P<0.005 except for ETS1(T38A)+HGF, P=0.172). To verify that the ETS1(T38A) protein is not phosphorylated by HGF, protein levels of MET, ERK and ETS1 were measured in transfected HEK-293T cells. Although all cell sets responded to HGF through the phosphorylation and activation of MET and ERK, the wild-type ETS1 protein, but not the mutant, was phosphorylated (Figure 5f). Here, we find that exogenous wild-type ETS1 protein is phosphorylated concurrently to ERK activation due to HGF stimulation, and is able to promote MET expression when the MET receptor is present.
To determine whether HGF activates endogenous ETS1 protein within melanoma cells, HGF was added to a panel of six melanoma cell lines, and MET and ETS1 protein levels were measured. Levels of pETS1 were significantly higher in 5/6 cell lines after HGF treatment in comparison with untreated cells (Figures 5g and h, P<0.05). SKMEL-28 cells also had an increase in pETS1 levels post HGF treatment, but not to a significant level (P<0.218). The increase in pETS1 levels corresponded to a parallel rise in pMET levels. To determine whether ETS1 phosphorylation levels are dependent on MEK and ERK activity, HGF-induced melanoma cells were treated with a specific inhibitor of MEK, PD184352. Levels of pETS1 decreased in A375 and mel-624 cells in the presence of PD184352, but not in untreated or mock-treated cells (Figure 5i). These data support a model where MET receptor activation via HGF activates ETS1 and consequently its own promoter.
The proximal MET promoter contains two ETS-binding sites that function synergistically with PAX3 or through HGF activation
To determine whether the E1 and E2 enhancers where necessary for ETS1-dependent synergistic activation of METpm with PAX3 or HGF, MET promoter reporter and ETS1 expression constructs were transfected into HEK-293T cells in the presence or absence of PAX3 or HGF. Reporter constructs contained intact or mutated E1 and E2 sites (Figures 2 and 6a). ETS1 was able to synergistically activate METpm vector, with the addition of either PAX3 or HGF (Figures 5b and 6a, set 1). A mutation in the E1 site resulted in a loss of synergistic activation with PAX3, but not with HGF treatment (Figure 6a, set 2). Conversely, mutation of the E2 site did not significantly effect ETS1–PAX3 activation of the MET reporter vector, but the robust activation of the reporter after HGF treatment was lost (Figure 6a, set 3). Loss of both E1 and E2 sites abrogated both the ETS1–PAX3 and ETS1–HGF activation (Figure 6a, set 4). These data support a model of two distinct ETS-binding sites in the MET promoter (Figure 6b). The E1 site, along with an adjacent PAX site, comprises an ‘ETS–PAX enhancer.’ Optimum activation of this site requires the presence of both PAX3 and ETS1. The other site, E2, contains the ideal binding sequence for independent ETS1 binding (CCGGAWR). The ability of ETS1 to drive expression from this ‘ETS–pETS1 enhancer’ is heightened by ETS1 phosphorylation as a result of HGF signaling.
Expression of a dominant-negative ETS1 protein inhibits MET induction and melanoma cell growth
To determine whether disruption of normal ETS1 function would affect MET expression and melanoma cell growth, cells were transfected with a dominant-negative ETS1 expression construct (DN-ETS1). A truncated ETS1 protein containing only the C-terminal DNA-binding ETS domain acts as a DN against wild-type activity by competitive DNA binding (Figure 7a).36, 37, 38 The presence of DN-ETS1 significantly (P<0.005) inhibits activation of MET by PAX3 and ETS1 (Figure 7b). In addition, melanoma cells demonstrated significant inhibition of growth both in cellulo and in vivo (Figures 7c–e).
A375 and mel-624 melanoma cells, transfected with either a control green fluorescent protein (GFP)-expressing vector or a dual DN-ETS1/GFP-expressing construct, demonstrated a significantly (P<0.005) attenuated growth at 48 h post transfection, when DN-ETS1 was present (Figure 7d). The transfected A375 cells, when transplanted into athymic Nu/Nu mice, grew tumors for all GFP-transfected cells 10 days post transplantation, but only 2/6 times when the DN-ETS1 was present (Figure 7e).
In summary, ETS1 drives expression of MET in melanoma cells, either through a PAX-dependent enhancer, or a feed-forward PAX-independent mechanism via ETS1 phosphorylation and activation downstream of HGF.
Although it is evident that MET overexpression drives tumor proliferation, survival and progression in melanoma, the mechanism for this overexpression is unknown. Our group and others have previously found PAX3, MITF and SOX10 upstream of MET.14, 39, 40 Although PAX3 and MITF are able to promote MET expression independently, SOX10 alone was unable to drive expression from the proximal promoter element utilized in this study. However, SOX10 was able to function as a cofactor with either PAX3 or MITF. Although PAX3 is able to drive expression and bind to the MET promoter in a number of different melanoma cell lines, inhibition of PAX3 led to a significant reduction of MET in SK-MEL23 and SK-MEL28, but not in A375 cells14 (Figure 4e). These earlier studies suggest that PAX3 is a regulator of MET in melanoma cells, but that the mechanism for activation was not the same for all melanoma cells.
Here, we identify a pathway wherein transcription factors ETS1 and PAX3 promote excess MET protein expression. We find that PAX3 and ETS1 are commonly expressed in melanoma (Figure 1), efficient at driving MET promoter expression (Figure 3a) and bind directly to the MET promoter (Figures 3c and 4b). The PAX and ETS enhancer sites are important for promoter expression in melanoma, and inhibition of these factors lead to a reduction of MET levels (Figure 4). In this report, we discover that ETS1 drives MET expression in melanoma, and ETS1 activity is increased multifold with the addition of HGF or PAX3.
Our group and others find that PAX3 is expressed in melanoma, where it actively promotes migration and invasiveness.20 These qualities are the most deadly in melanoma, and they are reacquired during resistance to the small-molecule BRAF inhibitor, vemurafenib. As PAX3 promotes migration and metastasis, it may be an active player in vemurafenib resistance. Indeed, recent reports find that PAX3 is overexpressed in vemurafenib-resistant cell lines, and PAX3 inhibition restores drug sensitivity in these cells.21 One major mechanism for melanoma progression and drug resistance may be through PAX3-dependent MET expression.
An added mechanism of PAX3-related resistance may be through its interaction with ETS1, a protein activated through pathways downstream of BRAF. Our findings suggest that ETS1 activates MET both in a PAX3-dependent and -independent manner (Figures 5 and 6). This is the first report to find that ETS1 directly interacts with the transcription factor PAX3, and that these factors synergistically activate the expression of MET (Figures 3, 4 and 6). ETS1 also binds to the PAX3-related protein PAX5, and the PAX5 epitopes that directly bind to ETS1 are also found in PAX3.31, 34, 41 It is likely that ETS1 drives the expression of many genes together with PAX factors. In this report, we find the first example of a PAX3–ETS1 regulated gene in melanoma, the tyrosine kinase receptor MET.
We find that ETS1 promotes melanoma progression, and the implementation of a DN-ETS1 protein inhibits MET promoter expression and melanoma cell growth (Figure 7). This DN-ETS1 protein also inhibits tumor growth, invasion and migration in other tumor models.37, 42 DN-ETS1 has also been previously expressed in melanoma cells to study in vivo invasiveness.43 This group found that DN-ETS1 enhances invasion and metastasis in one overexpressing clone (TM4), but not in another (TM5). Our findings, coupled to these prior discoveries, support that ETS1 may have differential roles in melanoma progression in terms of growth and metastasis. This falls in line with recent findings for MITF and BRN2, where both factors promote melanoma progression, but higher levels of each protein drives either cellular proliferation or migration.44, 45 The precise role of ETS1 in melanoma initiation, establishment and metastasis still needs to be elucidated.
In our report, we discover a pathway for MET to promote its own expression by HGF-dependent induction of ETS1 (Figures 5 and 6). HGF signals through the MET receptor, which in turn activates ETS1 downstream of the MAPK pathway. This has potential long-term clinical implications as well, as HGF has been found to be a major driver of resistance against mutant BRAF inhibitors in melanoma therapy.46 This resistance may be supported, at least in part, by ETS1 activity. It is not known whether the consequential phosphorylation and activation of ETS1 contributes to the progression of this tumor, or how dependent the cells are on ETS1 activity. Our report further supports that ETS1, along with PAX3, drives the expression of oncogenic genes that promote tumor progression.
Materials and methods
Human melanoma lines (A375, mel-537, mel-624, mel-888, SKMEL-5, SKMEL-23 and SKMEL-28), HEK-293T and 3T3 cells (ATCC, Manassas, VA, USA, and University of Chicago Comprehensive Cancer Center Core Facilities) were cultured in Dulbecco's modified Eagle's medium/10% fetal bovine serum (Sigma-Aldrich, St Louis, MO, USA). Morphology, growth curve analysis and melanoma-marker testing verified melanoma cell identity. All cells were negative for the presence of mycoplasma. For HGF treatment, cells were grown in serum-free media, supplemented with 0.5% bovine serum albumin and 30 ng/ml HGF (EMD Millipore, Temecula, CA, USA) for 7.5–10 min prior to cell collection and analysis. For PD184352 (Sigma-Aldrich) treatment, 1 μM was added for the length of the experiment.
Primary antibodies utilized for western analysis, immunohistochemistry, chromatin immunoprecipition and immunoprecipitation were against PAX3 (University of Iowa Hybridoma Bank, Iowa City, IA, USA), ETS1 (Santa-Cruz Biotechnology, Santa-Cruz, CA, USA), 1:200 pETS1 (Sigma-Aldrich), ETS1/2 (Santa-Cruz Biotechnology), MET (Cell Signaling Technology, Danvers, MA, USA), pMET (1:500, Invitrogen Co., Carlsbad, CA, USA) or vinculin (Sigma-Aldrich) antibody.
Cells were lysed in radioimmunoprecipitation assay buffer and 50 μg total protein was separated on 4–12% Bis-Tris gels, transferred to nitrocellulose membranes and then probed with 1:200 Pax3, 1:500 ETS1, 1:200 pETS1, 1:200 ETS1/2, 1:6000 MET or 1:1 000 000 vinculin antibody. Control lysates to serve as negative controls for phosphorylated protein were treated with calf intestinal phosphatase (New England BioLabs, Ipswich, MA, USA) at 0.54 units/μg of total protein at 37 °C for 1 h.
Immunofluorescence analysis of PAX3, ETS1 and MET expression
Archival paraffin-embedded primary melanoma specimens were obtained through the University of Chicago Hospital Dermatology tissue bank following approved IRB and Clinical Trials Committee protocols. For antigen retrieval, 5-μm tissue sections were boiled in EDTA buffer (pH 8.5). Samples were blocked in 1% normal goat serum and then incubated with primary antibodies PAX3 (1:250) and ETS1 (1:500), pETS1 (1:500), MET (1:1000) or pMET (1:500). Samples were washed in phosphate-buffered saline buffer, and incubated with goat anti-rabbit fluorescein or goat anti-mouse dyl647 secondary antibody (1:1000, Pierce Biotechnology/Thermo Scientific, Rockford, IL, USA). Staining was scored as ‘positive’ or ‘negative’ with a positive score ⩾25% of the tumor tissue expressing the antigen.
The MET reporter construct (METpm) contains the genomic DNA fragment shown in Figure 2a and cloned as described.14 Mutations in putative PAX and ETS sites were created by site-directed mutagenesis. The sites were mutated by changing the PAX site from GTCCCGC to ACTAGTC, the E1 site from CTCCGG to CTCGAG and the E2 site from GCAGGAAG to GCTAGCAG. The PAX3 expression construct was created as described.47 The pcDNA3–ETS1 expression construct was provided by Eric Svensson (University of Chicago, Chicago, IL, USA). ETS1(T38A) was created by site-directed mutagenesis, changing codon 38 from ACT (threonine) to GCT (alanine). The pGex2T-PAX3 vector was a kind gift from Jonathan Epstein (University of Pennsylvania, Philadelphia, PA, USA). The coding sequence of ETS1 was cloned into pGex2T (GE Healthcare, Uppsala, Sweden) for overexpression in bacteria and purification using primers IndexTerm5′-AACCCGGGTATGAAGGCGGCCGTCGATCTCAA-3′ and IndexTerm5′-TTCCCGGGTCACTCGTCGGCATCTGGCTT-3′. The pcDNA3–DN-ETS1–IRES–GFP construct contains amino acids 306–441 of ETS1, which encodes the DNA-binding domain only and functions as a DN protein against wild-type ETS1 activity.36, 37, 38 This region was amplified using primers IndexTerm5′-ATAAGCTTATGGACTATGTGCGGGACCGTGCT-3′ and IndexTerm5′-ATGGATCCTCACTCGTCGGCATCTGGCT-3′ and then cloned into pcDNA3 (Invitrogen). The internal ribosomal entry site (IRES) was amplified from the pIRES-hrGFP-1a vector (Agilent Technologies, Santa Clara, CA, USA) with primers IndexTerm5′-TAGATATCCTTGGGTTACCCCCCTCTCCCT-3′ and IndexTerm5′-TACTCGAGGCGGCCGCCATTATCATCGTGTTTTTCA-3′ and then cloned into the previous construct. To add the GFP gene to this construct, primers IndexTerm5′-AAGCGGCCGCAATGGTGAGCAAGGGCGAGGAG-3′ and 5′-IndexTermAATCTAGATTACTTGTACAGCTCGTCCAT-3′ were used to amplify the GFP gene of the pEGFP-N1 vector (Clontech Laboratories Inc., Palo Alto, CA, USA). The pcDNA3–GFP vector was constructed by amplifying the EGFP gene and cloning it into pcDNA3.
Cells were transfected with MET promoter luciferase reporter constructs, an internal control beta-galactosidase-expressing construct pCMV (Clontech) and PAX3 and ETS1 expression constructs. Cells were transfected with Lipofectamine 2000 (Invitrogen), incubated for 48 h, then luciferase and beta-galactosidase levels were measured (Promega Biosciences Inc., Madison, WI, USA). For the calculation of fold induction, luciferase activity was measured in arbitrary light units, normalized against beta-galactosidase activity and divided by the measurements obtained for reporter vector alone (or with HGF, if the experimental group also had HGF treatment). All experiments were performed in at least triplicate.
Small interfering RNA inhibition
Melanoma cell line mel-624 was transfected with small interfering RNA against PAX3 (5′-IndexTermGAAACACCGUGCCGUCAGU-3′), ETS1 (IndexTerm5′-GAAAUGAUGUCUCAAGCAU-3′), both, or siScramble (100 pmol, Dharmacon, Lafayette, CO, USA) with Lipofectamine-2000 (Invitrogen) according to the manufacturer’s recommendations. Cells were transfected for two consecutive days and then the media was changed to media containing 30 ng/ml HGF (EMD Millipore) 48 h after the initial transfection.
Chromatin immunoprecipitation assays
Cells were fixed in 1% formaldehyde, quenched in 0.125 M glycine, then lysed and sonicated in SDS lysis buffer (1% SDS, 10 mM EDTA, 50 mM Tris-HCl pH 8.1). Protein–DNA complexes were incubated with antibodies against PAX3 or ETS1. Normal immunoglobulin G (Sigma-Aldrich) was used as a negative control against non-specific DNA precipitation by an antibody. PCR was performed with primers to the MET enhancer, with 5′-IndexTermTCCGCCTCTAACAATGAACTCC-3′ (F, human) or 5′-IndexTermTTGTCTGTGACAATGAGCGCC-3′ (F, mouse) and 5′-IndexTermAAGGTGAAACTTTCTAGGTGG-3′ (R). All chromatin immunoprecipition samples were tested for false-positive PCR amplification using beta-tubulin gene sequence primers. The nested primer set for this control was 5′-IndexTermAAAGGCCACTACACAGAGGG-3′ (F) and 5′-IndexTermTACCAACTGATGGACGGAGAGG-3′ (R, human) or 5′-IndexTermAACCAACTGATGGACAGACAGG-3′ (R, mouse).
Co-immunoprecipitation of PAX3 and ETS1 proteins
Proteins collected from melanoma cells were obtained via sonication in 2 × GS buffer (40 mM HEPES, 100 mM KCl, 40% glycerol and 2 mM 2-mercaptoethanol) with a protease inhibitor cocktail (Sigma-Aldrich) and 1 mM PMSF. Glutathione S-transferase (GST), GST-ETS1 and GST-PAX3 were expressed in BL21 bacteria and purified with Glutathione-Sepharose 4B beads according to the manufacturer’s protocol (GE Healthcare). Protein was incubated with either PAX3 or ETS1 antibodies for 2 h at 4 °C, then with protein A/G agarose beads (EMD Millipore) for an additional 2 h. The resulting precipitates were washed three times in lysis buffer, resolved on 10% SDS–polyacrylamide gel electrophoresis gels and evaluated by standard western blot analysis. Normal mouse immunoglobulin G antibody was used as controls.
Electrophoretic mobility shift assay
GST, GST-ETS1 or GST-PAX3 proteins were mixed in reaction buffer (10 mM Tris-HCl, pH 8.0, 150 mM KCl, 0.5 mM EDTA, 0.1% Triton-X 100, 12.5% glycerol, 0.2 mM dithiothreitol and 100 μg/ml poly(dI-dC)) for 30 min at 25 °C followed by the addition of probe for 15 min at 25 °C. Primers for probe sequences (Figure 3b) were annealed and labeled by end-filling with DNA polymerase I Large (Klenow) fragment (New England Biolabs) and then purified on Illustra ProbeQuant G-50 Micro columns (GE Healthcare). Electrophoresis was performed on 5% native gels. The gels were dried and then exposed to an autoradiography film.
In vivo tumor formation assays
Athymic Nu/Nu mice (6–7-month-old females, Charles River Laboratories, Roanoke, IL, USA) were treated and maintained in accordance to institutional guidelines. A375 cells were transfected with either the pcDNA3–GFP or pcDNA3–ETS1-DN–IRES–GFP vectors with TransIT-2020 transfection reagent (Mirus, Madison, WI, USA) according to the manufacturer’s instruction. Eighteen hours post transfection, cells were trypsinized and resuspended at 20 × 106 cells/ml in Dulbecco's modified Eagle's media containing 20% fetal bovine serum. GFP-positive cells were collected by the University of Chicago Flow Cytometry Core Facility using the FACSAria III (BD, Franklin Lakes, NJ, USA). Cell counts and viability were confirmed using hemacytometer readings and trypan blue exclusion. Sorted cells were washed once in phosphate-buffered saline and resuspended in a final volume of 15 × 106 cells/ml. Mice received subcutaneous injections of 200 μl (3 × 106 cells) in each flank (n=6, pcDNA3–GFP; n=6, pcDNA3–ETS1-DN–IRES–GFP). Tumor volume was measured using calipers, with volume calculated as (length × width2) × (π/6).
Statistical analysis and densitometry
For each experiment, three or more replicates were analyzed. Western band intensities were measured using ImageJ64 (http://rsbweb.nih.gov/ij/). Data presented as percentages or fold are normalized to a control group as stated. Data error bars represent s.e.m. for comparisons of samples within one group. One-tailed unpaired Student's T-test analysis determined significance for all graphical data, where all sample groups were compared with a control (Microsoft Excel). For in vivo tumor growth experiments, significance was determined by two-tailed Mann–Whitney U-tests. P-values of ⩽0.05 are considered significant.
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We thank Drs Ravi Salgia, Eric Svensson and Jonathan Epstein for providing material and/or intellectual support for this project. This work was supported by grants and financial support from the University of Chicago Cancer Center Pilot program (P30-CA014599), American Cancer Society (RSG-CSM-121505), Friends of Dermatology-University of Chicago, The Wendy Will Chase Foundation and the National Institutes of Health (R01CA130202 and R01AR062547).
The authors declare no conflict of interest.
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Kubic, J., Little, E., Lui, J. et al. PAX3 and ETS1 synergistically activate MET expression in melanoma cells. Oncogene 34, 4964–4974 (2015). https://doi.org/10.1038/onc.2014.420
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