Background
FOXP3 has been studied in several types of cancer cells [
1‐
7]. Most of these studies have shown the expression of FOXP3 is correlated with unfavourable prognosis, although there are some reports indicating the opposite role of FOXP3 [
8,
9]. In in vitro and in vivo studies, FOXP3 has been widely reported as a suppressor gene in breast cancer [
7,
10‐
12] and prostate cancer [
3].
In lung cancer, Dimitrakopoulos et al. found that FOXP3 expression was correlated with lymph node metastasis [
13]. Fu et al. reported that FOXP3 expression was associated with TNM stage and lymph node metastasis [
14]. Tao et al. claimed FOXP3 alone had no prognostic value but a favourable prognostic value only when it was combined with T regulatory lymphocyte (Treg) counts [
15]. Li et al. observed that FOXP3 was expressed in lung adenocarcinoma [
16]. However, the function of FOXP3 in non-small cell lung cancer (NSCLC) has not been studied and the impact of FOXP3 on the Wnt/β-catenin pathway in cancers is unknown, Here, using both in vivo, in vitro models, we have demonstrated that FOXP3 is functional as an oncogenic molecule in NSCLC and have, for the first time, demonstrated that FOXP3 can act as a co-activator to facilitate the Wnt-catenin signaling pathway, inducing EMT and tumor growth and metastasis in NSCLC.
Methods
Cell lines and cultures
Human NSCLC cell lines, A549 and NCI-H460, were obtained from the American Type Culture Collection (ATCC, Manassas, VA), and cultured in RPMI-1640 medium. Human embryonic kidney epithelial cell line 293T cell line was purchased from Invitrogen, and cultured in DMEM medium. Lipofectamine 2000 (Invitrogen, Carlsbad, CA) was used to transfect cells with plasmid DNA. Total RNA and proteins were extracted from 80% confluent cells in culture dishes.
NSCLC patient samples and immunohistochemistry
One hundred and six patients with histologically-confirmed NSCLC who underwent surgery at Prince of Wales Hospital between 2001 and 2007 with complete clinicopathologic characteristics (Additional file
1: Table S1) and follow-up data were enrolled in the study. Histologic type was determined according to the 2015 WHO classification [
17]. Immunohistochemistry was performed on formalin-fixed paraffin sections according to standard protocols using two types of primary antibodies from Santa Cruz and Abcam, The tumor FOXP3 staining intensities were scored using the immunoreactive score (IRS) method [
4] by a pathologist and an investigator. Tumor infiltrating FOXP3
+ regulatory T-cells (Treg cells) were also counted at the same time. Human studies were approved by the Joint CUHK-NTEC Clinical Research Ethics committee.
Western blot
The anti-E-Cadherin (CellSignaling, Danvers, MA, 1:2000), anti-N-Cadherin (Cellsignaling, 1:2000), anti-Vimentin (Abcam, 1:1000), anti-Snail (Cellsignaling, 1:2000), anti-Slug (Abcam, 1:2000), anti-MMP9 (Cellsignaling, 1:2000), anti-β-actin (Santa Cruz, 1:3000), anti-c-Myc (Santa Cruz, 1:1000), anti-Cyclin D1 (Santa Cruz, 1:1000), anti-TCF4 (Santa Cruz, 1:1000; Cellsignaling, 1:2000)), anti-LaminB1 (Cellsignaling, 1:2000), anti-FOXP3 (Abcam 1:3000, Cellsignaling 1:2000), anti-β-catenin (Cayman, Michigan, 1:5000), anti-GFP (Santa Cruz, 1:4000) were used as the primary antibodies. A 1:3000–5000 dilution of the HRP-linked anti-IgG (Santa Cruz) was used as the secondary antibody.
Quantitative real-time PCR
Genes of interest were detected using SYBR® Premix Ex Taq ™ (Takara, Dalian, China) on Quantstudio™ 12k Flex Real-time PCR system (Applied Biosystems, Foster City, CA). The primer sequences are listed in Additional file
1: Table S2.
Production of lentivirus for FOXP3 overexpression and knockdown
The full-length ORF (open reading sequence) of human FOXP3 gene (NM_014009.3) was PCR amplified from pcDNA3.1-FOXP3 and subcloned into the self-inactivating lentiviral vector PHIV-EGFP (Addgene) with an EF1-alpha promoter, an internal ribosome entry site (IRES) and EGFP. Lentiviral preparations were generated by transient transfection of HEK-293T cells by using pHIV-EGFP-FOXP3 (10 μg), pRSV-Rev. (2.2 μg), pMDLg/pRRE (4.72 μg), pMD2.G (3.08 μg).
FOXP3 shRNA was selected from the RNAi Consortium library (
www.broadinstitute.org/rnai/public), which contains shRNAs against 15,000 human genes. We selected 5 highly scored target sequence against FOXP3 (NM_014009.3) mRNA sequence from the database: CCTCCACAACATGGACTACTT; CACACGCATGTTTGCCTTCTT; CTGAGTCTGCACAAGTGCTTT; TCCTACCCACTGCTGGCAAAT; TGTCCCTCACTCAACACAAAC. The corresponding oligoes generated by Invitrogen were subcloned into pLKO.1 (Addgene). Lentiviral preparations were generated as above except for that the transfection cocktail was replaced with pLKO.1 (6 μg), psPAX2 (4.5 μg) and pMD2.G (1.5 μg).
Cell viability assay
Cell viability was determined by the MTT assay (ThermoFisher, Waltham, MA), and the absorbance was measured at 570 nm.
One thousand cells were seeded and cultured for 10–14 days. Colonies (⩾50 cells/colony) were counted.
Soft agar assay
Difco™ Agar (BD Biosciences, Erembodegem, Belgium) was used in the assay, 1000 cells per well were added in top agar and were cultured for 2 weeks. Colonies (⩾50 cells/colony) were counted.
Wound healing assay
Scratch was made by a 200 μl pipette tip after cells were grown to 80% confluence. Cells were then incubated in medium containing 5% FBS. Gap size was measured 24 h later.
Cell invasion assay
2.5 × 104 cells were seeded in upper chamber with 300 μl medium containing 5% FBS. The lower chamber was filled with 1200 μl medium containing 10% FBS acting as chemotactic factors. After the incubation of 24–48 h, the migrated cells to the underside of the membrane were fixed and stained with 0.1% crystal violet.
Immunofluorescence assay
The anti-E-Cadherin (Cellsignaling, 1:200) and anti-FOXP3 (Abcam, 1:100) were used as the primary antibodies. The cells were incubated with Alexa Fluor® 594 dye conjugated secondary antibody (ThermoFisher). The nucleus was stained by DAPI (ThermoFisher).
Dual-luciferase reporter assay
HEK293T cells were plated in 24-well plates and co-transfected with various plasmids as indicated in the figures. Cells were collected 24 h after transfection, and luciferase activities were analyzed by the dual-luciferase reporter assay kit (Promega, Madison, WI). Reporter activity was normalized to the control Renilla.
Gene expression microarrays and data analysis
Total RNA was extracted from A549-FOXP3 and A549-Control using TRIzol reagent (Invitrogen). The marked cRNAs were hybridized with the Agilent human whole genome gene expression Microarray (Agilent Technologies, Santa Clara, CA). Gene expression levels were standardized by the level of GAPDH. Differentially expressed genes were screened by the threshold of 2.0 fold-change and
p value that was more than 0.05. Student’s t test was adopted for statistical analysis. Pathway analysis and Gene Ontology (GO) analysis were applied to determine the functions of those differentially expressed mRNAs by GO (
www.geneontology.gov) [
18] and the KEGG (Kyto Encyclopedia of Genes and Genomes) pathway database (
http://www.genome.jp/kegg/pathway.html).
Nuclear and cytoplasmic protein extraction
Cells were resuspended in 600 μl ice-cold Buffer I (1.5 mM MgCl2, 10 mM HEPES, 10 mM KCl, and protease inhibitor cocktail, pH 8.0), incubated on ice for 15 min and rotated once every 5 min. Then 10% Nonidet P-40 was added to a final 1% concentration. After a 10-s slight vortex, cells were centrifuged at 14,000 rpm for 3 min. Then the supernatants were collected as the cytoplasmic protein. The pellets were resuspended in 220 μl ice-cold Buffer II (420 mM NaCl, 20 mM HEPES, 0.2 mM EDTA, 1.5 mM MgCl2, 25% glycerol, and protease inhibitor cocktail, pH 8.0) and incubated on ice for 30 min. Then samples were centrifuged and the supernatants were transferred to new tubes as the nuclear fraction which was stored at −80 °C for later use.
Co-immunoprecipitation assay
HEK-293T cells were co-transfected with the indicated plasmids with lipofectamine 2000 (Invitrogen), and the nuclear and cytoplasmic proteins were extracted as previously described [
19,
20]. Three kinds of beads were used in this study for Co-IP assay: anti-FLAG M2 Magnetic Beads (Sigma-Aldrich, St Louis, MO); Pierce Anti-c-Myc Magnetic Beads (ThermoFisher); Protein A/G PLUS-Agarose (Santa Cruz). Briefly, the protein extracts were incubated with the equilibrated beads at 4 °C overnight with gentle mixing to capture the FLAG fusion proteins or Myc fusion proteins or specific antibody captured proteins. The magnetic beads or agarose beads were collected by placing the tube in the appropriate magnetic separator or by centrifuging. The beads were washed with TBS buffer to remove all of the non-specifically bounded proteins. The bounded fusion proteins were eluted from the beads with corresponding elution buffer for western blot analysis.
In vivo tumor xenograft assays and metastasis assays
2 × 106 A549-FOXP3 and A549-Control cells were separately subcutaneously inoculated into the left and right flank in the dorsal of the nude mice for in vivo xenograft assay. Tumor size was measured every 3 days for 18 days. The tumor volume (V) was calculated by the formula (length × width × width)/2. The tumors were excised and embedded in paraffin. For lung metastasis formation, 5 × 105 A549-FOXP3 and A549-Control cells were injected into the lateral tail vein of the nude mice. Mice were euthanized 9 weeks after injection, and the lung, liver and spleen of each mice were subjected to formaldehyde fixation and followed by H&E staining. All experimental procedures were approved by the Animal Ethics Committee of the Chinese University of Hong Kong.
Statistics
Continuous data were expressed as the median and range, discrete variables were presented as absolute values with relative frequencies. The independent Student’s t test was used to compare colony formation and gene expression between two groups. Paired t-test was used to compare the expression levels of FOXP3 in tumor tissues and adjacent normal tissues. Repeated Measures ANOVA was used to compare the tumor growth rate between two groups in the in vivo assay. The clinicopathologic features were compared using Pearson’s chi-squared test or Fisher’s exact test. Kaplan-Meier plots were used for OS and DFS rates, then compared with the log-rank test. Univariable or multivariable Cox proportional hazard regression was performed to evaluate the predictive values of FOXP3 and other clinicopathologic features. A two-tailed p value less than 0.05 was considered statistical significance. All the tests were performed by SPSS version 16.0.
Discussion
In this study, we found that tumor FOXP3 expression was frequently upregulated in NSCLC tissues, and in most cases showed a mixture of cytoplasmic and nuclear expression. However, tumor FOXP3 expression was not statistically correlated with any clinicopathologic features, though advanced pathologic stage (III-IV) seemed to be associated with high FOXP3 expression. Tumor FOXP3 has been claimed as an independent prognostic indicator for tongue squamous cell carcinoma [
2] and breast cancer [
29]. However, to our best of knowledge it has not been reported as an independent prognostic factor in NSCLC. In this study, we found that high tumor FOXP3 expression was significantly correlated with worse overall survival and recurrence-free survival of the patients, and showed independent prognostic value in NSCLC. FOXP3 expression in Treg cells could also be seen in NSCLC specimens. Although Treg cells in NSCLC were reported with poor prognosis in some publications [
30,
31]. However, we did not find its prognostic value in NSCLC. Tumor FOXP3 expression has a positive correlation with Treg cell counts in our NSCLC specimens. Thus we speculate that tumor FOXP3 may be a confounding factor to affect the prognostic value in NSCLC.
In vitro and in vivo assays demonstrated the oncogenic role of FOXP3 in NSCLC. Ectopic expression of FOXP3 contributed to tumor growth and metastasis in NSCLC cells (A549 and H460) by promoting cell proliferation, migration and invasion as evidenced by cell viability assay, colony formation assay, soft agar assay, wound healing assay and transwell assay. The promotion of tumor growth and metastasis by FOXP3 was further confirmed in subcutaneous xenograft tumor mouse model and tail-vein-injection metastatic mouse model. On the other hand, lentivirus-mediated knockdown of FOXP3 significantly inhibited cell proliferation and clonogenicity. Moreover, we observed that ectopic expression of FOXP3 in A549 and H460 led to downregulation of E-Cadherin and upregulation of N-Cadherin, vimentin, Snail, Slug and MMP9, as well as the mesenchymal specific morphology changes: spindle shape, loss of cell-cell contact and cell scattering. All these changes in biomarkers and morphology suggest a transition of epithelial cells to mesenchymal cells. This transition was further confirmed by microarray analysis of biological process and cellular component. On the other hand, the knockdown of FOXP3 impairs its ability in downregulating E-Cadherin and upregulating N-Cadherin, Snail, Slug and MMP9. This is the first report on the role of FOXP3 in EMT induction.
FOXP3 has been reported as a suppressor gene in breast cancer [
7,
10‐
12] and prostate cancer [
3] via repressing the expression of oncogene such as HER2, SKP2, p21, LATS2 and c-Myc. Using qPCR method, we did not see the similar changes of these molecules in NSCLC cells when FOXP3 was overexpressed (data not shown). Conversely, the pathway analysis of the gene expression profiling indicated an activated HER2 signaling pathway in FOXP3-overexpression NSCLC cells. The findings of Western blot and qPCR verified the upregulation of c-Myc in FOXP3-overexpression NSCLC cells. EMT is an important step in the progression of cancer metastasis and invasion [
32], and also contribute to the increase in resistance to pro-apoptotic and chemotherapeutic drugs and the induction of cancer cell stemness [
33]. Our results have suggested that FOXP3 can function as an oncogene in NSCLC that has a different genetic background from breast cancer and prostate cancers. Our finding is supported by the studies reported by others, demonstrating that the overexpression of FOXP3 decreases mouse Lewis lung cancer cell sensitivity to chemotherapy and that the ectopic expression of FOXP3 promotes cell growth, migration and invasion in lung adenocarcinoma [
16]. In some other types of cancers [
34,
35], FOXP3 has also been shown to promote cancer growth, migration and invasion.
We further elucidated the downstream signaling pathway responsible for FOXP3 oncogenic function in NSCLC and found that the FOXP3-mediated tumor growth and metastasis could be, at least partly, attributed to the activation of Wnt/β-catenin signaling, which is critical for the initiation and progression of NSCLC [
36,
37]. The FOXP3-induced Wnt/β-catenin signaling in A549 and H460 was evidenced by the high luciferase activity of Topflash reporter, and the upregulation of Wnt signaling target gene (c-Myc and Cyclin D1) expression. Moreover, the knockdown of FOXP3 impaired the Topflash reporter luciferase activity. The activation of Wnt/β-catenin signaling pathway can also contribute to the induction of EMT via stimulating several EMT-related transcription factors, such as Snail, Slug, Twist, ZEB1, ZEB2 and E47 [
38].
FOXP3 has constantly been regarded as a transcriptional factor functioned in cancer cells [
11,
39‐
41]. However its role in the regulation of Wnt/β-catenin signaling pathway in human cancers is unknown. Our Co-IP result indicated that FOXP3 could interact with β-catenin and TCF4 respectively and reciprocally, and enhance the function of β-catenin and TCF4 in the nucleus.