Introduction
Cadherins are a family of transmembrane proteins that, together with their associated intracellular catenins, have important functions in cell-cell adhesion. Different cell types express different members of the cadherin family. Epithelial (E)-cadherin is a key component of adherens junctions in epithelial cells and functions as a suppressor of tumor growth and invasion. Perturbation of its function leads to an invasive phenotype in many tumors [
1‐
3]. Neural (N)-cadherin is expressed in neural tissues and fibroblasts, where it mediates a less stable and more dynamic form of cell-cell adhesion [
1‐
4]. Vascular endothelial (VE)-cadherin is the primary component of endothelial cell adherens junctions and has an important function in regulating vascular permeability and angiogenesis [
5]. Because of the important role played by cadherins in cell recognition, adhesion, and signaling, modulation of their function and expression has significant implications for the progression of tumors [
1,
6‐
10]. For instance, a switch from E-cadherin to N-cadherin expression contributes to increased tumor cell migration, invasion and metastasis [
8‐
10]. Aberrant expression of VE-cadherin was first detected in aggressive melanoma cells and in some cases of sarcoma [
11‐
13]. A recent study from our group has revealed that VE-cadherin is expressed aberrantly in a subset of tumor cells in human breast cancer [
7]. In a mouse mammary carcinoma model, VE-cadherin expression was induced in cancer cells that had undergone epithelial-mesenchymal transition (EMT). Functional experiments showed that VE-cadherin promotes malignant tumor cell proliferation and invasion by enhancing the protumorigenic transforming growth factor-beta (TGF-β) pathway. However, the functional interaction between VE-cadherin and N-cadherin during tumor progression is poorly characterized to date.
EMT was first described by Elizabeth Hay in the 1980s as a central process in early embryonic morphogenesis [
14]. The initial step of EMT includes the loss of epithelial markers such as E-cadherin via its transcriptional repression and the gain of mesenchymal markers such as vimentin. As a consequence, the cadherin-binding partner β-catenin can dissociate from the E-cadherin complex at the plasma membrane and translocate to the nucleus where it participates in EMT signaling and activates genes involved in tumor progression [
15]. Epithelial cells then lose their typical baso-apical polarization as cell-cell junctions disassemble. Additionally, the cytoskeleton undergoes dynamic cortical actin remodeling and gains the front-rear polarization that facilitates cell movement [
16]. Finally, cell-matrix adhesion changes as proteolytic enzymes such as matrix metalloproteases are activated [
17,
18]. The transition from an epithelial to mesenchymal phenotype is reversible; for example, several rounds of EMT and mesenchymal-epithelial transition (MET) occur during development as cells differentiate and the complex three dimensional structure of internal organs forms [
19]. There is increasing evidence that EMT also facilitates the dissemination of tumor cells to form distant metastasis [
20]. Various publications have described a switch between the epithelial and mesenchymal phenotypes through EMT and MET in models of colorectal [
21], bladder [
22], ovarian [
23] and breast cancer [
24]. These findings indicate that the phenotypic conversion of tumor cells in the metastatic cascade is multifaceted, with EMT being critical for the initial transformation from benign to invasive carcinoma and the spreading of tumor cells, but MET occurring at the site of metastatic colonization [
6].
The mouse mammary carcinoma model that we have previously used to study the expression of cadherins [
7] utilizes tumor cell lines that represent different stages of tumor progression: Ep5 cells are tumorigenic mammary epithelial cells transformed by the v-Ha-Ras oncogene, whereas Ep5ExTu cells, isolated from Ep5 cell tumors grown in mice, have undergone EMT
in vivo and present a mesenchymal, invasive and angiogenic phenotype [
25,
26]. We observed that VE-cadherin expression is induced in these murine breast cancer cells during (TGF-β-mediated) EMT [
7]. On the other hand, E-cadherin expression was downregulated, and N-cadherin levels remained unchanged. Silencing VE-cadherin expression inhibited tumor cell proliferation and invasion
in vitro, and experimental tumor growth in mice. However, the role of N-cadherin and its potential interaction with VE-cadherin in this model is unclear. Here, we investigate the influence of N-cadherin on EMT and tumor progression in Ep5ExTu cells. Silencing N-cadherin significantly decreased VE-cadherin expression and stimulated Ep5ExTu cells to re-express E-cadherin at the cell surface. This promoted localization of β-catenin at the plasma membrane and induced the cells to undergo MET. Efficient silencing of N-cadherin expression in Ep5ExTu cells consistently inhibited tumor growth, and complete tumor regression was even seen in some cases. Taken together, these results reveal a novel interplay between classical cadherins in breast cancer progression.
Materials and methods
Cell culture
Ep5 and Ep5ExTu cells were cultured as described [
26] in Dulbecco's modified Eagle's medium (DMEM-F12; Lonza, Basel, Switzerland) supplemented with 15% fetal calf serum (FCS). 293T cells were kept in DMEM Glutamax (Gibco, Darmstadt, Germany) supplemented with 10% FCS.
Generation of VE-cadherin or N-cadherin-silenced Ep5ExTu cells
Oligonucleotides (Eurogentec, Seraing, Belgium) encoding small interfering RNA (siRNA) molecules specific for mouse VE-cadherin (5'-GUCUCUGAGU ACUUCCUUA-3') or N-cadherin (5'-GGAUGUGCAG GAAGGACAG-3' and 5'-UGUCAAUGGG GUUCUCCAC-3') were designed and verified to be specific for each cadherin by a Blast search (National Center for Biotechnology Information, Bethesda, MD, USA) against the mouse genome. A scrambled oligonucleotide sequence without significant homology to murine sequences (5'-AGUCGCUUAG AAACGAGAA-3') was used as a control. These oligonucleotides were then cloned into the lentiviral vector, pLVTHM, according to the guidelines provided by Tronolab (Laboratory of Virology and Genetics, École Polytechnique Fédérale de Lausanne, Switzerland). Viral particles were produced by transient co-transfection of 293T cells with the recombinant pLVTHM lentivector constructs, the packaging vector psPAX2 and the envelope vector pMD2.G. Ep5ExTu cells were then transduced with the lentiviral particles contained in supernatants of transfected 293T cells, and stable cell lines were selected by fluorescence-activated cell sorting (FACS) on the basis of green fluorescent protein (GFP) expression. Clones of Ep5ExTu cells expressing Sh-VE-cadherin and Sh-N-cadherin were expanded and the expression of VE-cadherin and N-cadherin was monitored by immunoblot. Experiments were approved by the Sächsisches Staatsministerium für Umwelt und Landwirtschaft, Dresden, Germany (Re: 55-8811.72/69).
Silencing VE-cadherin or N-cadherin in human breast cancer cell lines
Human SUM 149 cells were cultured in DMEM-F12 supplemented with 15% FCS. Silencing of human N-cadherin or human VE-cadherin was performed by using SMARTpools (Dharmacon, Lafayette, CO, USA). Cells (1-2 × 105) were seeded in 2 ml DMEM, 15% FCS in 6-well plates 24 h before transfection. The medium was replaced by 2 ml Opti-MEM I (Invitrogen, Karlsruhe, Germany) 1 h before transfection. 100 pmol siRNA was mixed with 500 μl Lipofectamine 2000 (Invitrogen) diluted in a final volume of 1 ml Opti-MEM I and incubated for 30 min at room temperature to allow the formation of complexes. For transfection, the medium was removed and the DNA-Lipofectamine mixture was added to the cells, which were then incubated at 37°C. 1 ml DMEM, 15% FCS was added 6 h after transfection and the cells were cultivated for another 24 h before analysis.
Generation of Sh-N-cadherin cell lines stably expressing VE-cadherin
Mouse cDNA encoding VE-cadherin (kindly provided by Prof. D. Vestweber, Münster, Germany) was cloned into the P6NST50 vector (kindly provided by Prof. D. Lindemann, Dresden, Germany). Ep5ExTu (Sh-N-cad2) cells were transduced with the VE-cadherin-encoding virus particles. Cells stably expressing VE-cadherin were then selected by FACS on the basis of their GFP expression.
Cell proliferation
Ep5ExTu cells (105) were plated and labeled with bromodeoxyuridine (BrdU) in 96-well plates in DMEM-F12 supplemented with 15% FCS. After 24 h, cell proliferation was quantified using a colorimetric immunoassay based on BrdU incorporation according to manufacturer's instructions (Roche, Mannheim, Germany). The amount of BrdU incorporated into the cells was measured by using an ELISA plate reader (Plus MS2 Reader, Titertek, Huntsville, AL, USA). For each independent experiment, six wells per condition were used.
RNA isolation and reverse transcription-PCR analysis
Ep5ExTu cells (2 × 10
5) were seeded in 4 ml DMEM-F12 supplemented with 15% FCS in 6 cm dishes 24 h before RNA isolation. Total RNA was isolated from cell lysates using a universal RNA Purification Kit according to the manufacturer's protocol (Roboklon, Berlin, Germany). Aliquots of 3 μg of total RNA were reverse transcribed using Superscript II (Invitrogen, Karlsruhe, Germany) and random hexameric primers (Roche, Mannheim, Germany). The sequences of the PCR primers used are shown in Additional file
1. The intensity of the PCR bands was quantified using Bio-Rad densitometer and Quantity One analysis software (Hercules, CA, USA). qRT-PCR analysis for human VE-cadherin and N-cadherin was performed following reverse transcription of total RNA using a Reverse Transcriptase Core Kit (Eurogentec, Seraing, Belgium), by real-time PCR (Mastercycler ep Realplex; Eppendorf, Hamburg, Germany) using QuantiFast SYBR Green PCR Kit (Qiagen, Valencia, CA, USA). All reactions were run in duplicates and Ct values were normalized against the GAPDH gene, using the delta-delta-Ct method. Primer sequences used were: VE-cadherin (CDH5), (forward) 5'-CGT GAG CAT CCA GGC AGT GGT AGC-3', (reverse) 5'-GAG CCG CCG CCG CAG GAA G-3'; N-cadherin (CDH2) (forward) 5'-CCA CCT TAA AAT CTG CAG GC-3', (reverse) 5'-GTG CAT GAA GGA CAG CCT CT-3'; GAPDH, (forward) 5'-CTC CTC TGA CTT CAA CAG CGA CA-3', (reverse) 5'-GAG GGT CTC TCT CTT CCT CTT GT-3'.
Immunoblot analysis
Immunoblot analysis was performed as described previously [
7,
26,
27]. The primary antibodies used were anti-VE-cadherin (R&D Systems, Wiesbaden, Germany), anti-N-cadherin and anti-β-actin (Sigma-Aldrich, Munich, Germany). The secondary antibodies were horseradish peroxidase-conjugated anti-rabbit immunoglobulin G (IgG) (Novus Biologicals, Littleton, CO, USA), anti-goat IgG (Jackson Immunoresearch, Soham, UK) and anti-mouse IgG (Cell Signaling Technology, Frankfurt, Germany). Band intensity was quantified using Quantity One analysis software (Bio-Rad, Hercules, CA, USA). Antibodies used for detection of human VE-cadherin, N-cadherin and E-cadherin were goat anti-VE-cadherin (Santa Cruz Biotechnology, Santa Cruz, CA, USA), mouse anti-N-cadherin (BD Biosciences, Bedford, MA, USA), mouse anti-E-cadherin (BD Biosciences, Bedford, MA, USA). Membranes were incubated with secondary antibody conjugated with IRDye Infrared Dyes (IRDYE 800CW donkey anti-goat, IRDYE 800CW donkey anti-mouse), and bands were revealed with a LI-COR scanner (LI-COR Biosciences, Lincoln, NE, USA).
Immunofluorescence staining of cells
Cells (2 × 10
5) were seeded on glass coverslips. After 24 h, cells were fixed as described [
7] and stained with the following primary antibodies: anti-VE-cadherin (R&D Systems, Wiesbaden, Germany), anti-N-cadherin (BD Biosciences, Bedford, MA, USA), anti-E-cadherin (Sigma-Aldrich, Munich, Germany), anti-β-catenin (Cell Signaling Technology, Frankfurt, Germany) and anti-Vimentin (Sigma-Aldrich, Munich, Germany). The secondary antibodies used were goat anti-rat Alexa 594, chicken anti-rabbit Alexa 594 and rabbit anti-goat 594 (Molecular Probes, Leiden, The Netherlands). Antibodies used for detection of human VE-cadherin, N-cadherin and E-cadherin were: goat anti-VE-cadherin (Santa Cruz Biotechnology, Santa Cruz, CA, USA), mouse anti-N-cadherin (BD Biosciences, Bedford, MA, USA), mouse anti-E-cadherin (BD Biosciences, Bedford, MA, USA).
Tumor experiments
Ep5ExTu cells were cultured in DMEM-F12 supplemented with 15% FCS. Cells were then trypsinized, rinsed twice in 5 ml PBS and resuspended in PBS at a concentration of 1 × 10
6 cells/ml. Tumor experiments were performed as described previously [
27] with eight- to twelve-week-old female BALB/c mice (Taconic, Ejby, Denmark). All tumors were measured with calipers every two to three days and the volume of each measurement was calculated as: (width
2 × length)/2. Tumors were collected 10 or 15 days after injection, embedded in Tissue Tek (Sakura Finetek, Staufen, Germany) and frozen on dry ice. Animal experimentation was approved by the Landesdirektion Dresden, Germany (Re: 24-9168.11-1/2009-19).
Immunofluorescence staining of frozen tumor sections
Eight μm frozen sections were cut and air dried. Sections were then fixed in 100% acetone for 10 min at -20°C and air dried. Rehydrated sections were stained with antibodies as described for immunofluorescence staining of cells.
Immunohistochemistry on human tumor tissue microarray
The tissue microarray included formalin-fixed, paraffin-embedded probes of 84 invasive breast cancers. The clinicopathological features are summarized in Additional file
2. Specimens were dewaxed, and immunohistochemical staining was performed using an automated immunostainer according to the manufacturer's protocol (Benchmark; Ventana Medical Systems, Tucson, AZ, USA) as described previously [
7]. The primary antibodies were anti-human VE-cadherin and anti-human N-cadherin (Polyclonal; Abcam, Cambridge, UK). The signal was amplified using the VENTANA amplification kit (Benchmark; Ventana Medical Systems, Tucson, AZ, USA) and visualized using avidin-biotin labeling and 3, 3'-diaminobenzidine. Slides were counterstained with hematoxylin. Evaluation of the staining was performed separately for nuclear, cytoplasmic and membrane-associated expression in a semi-quantitative manner. Expression was considered as positive if at least 1% of the tumor cells were stained. Chi-squared test was used for statistical evaluation of the results, and a
P value < 0.05 was considered as statistically significant. The study was approved by the Ethics Committee (Ethikkommisssion) of the Faculty of Medicine of the University of Dresden (Re: EK59032007).
Discussion
Breast cancer is one the leading causes of death due to cancer worldwide. Although the genetic defects underlying breast carcinogenesis have been extensively studied, important signaling pathways involved in the progression of this specific tumor type are still poorly characterized. The loss of E-cadherin and concomitant gain of N-cadherin expression is known to promote EMT and carcinoma progression. Our previous observation that endothelial cell-selective VE-cadherin is expressed aberrantly in breast cancer cells and promotes their proliferation both
in vitro and
in vivo [
7] led us to analyze the specific roles of these cadherins as well as their interplay in experimental breast cancer in more detail. Here, we show that N-cadherin silencing in murine breast cancer cells suppresses tumor growth by upregulating E-cadherin, repressing EMT regulators, and reversing the invasive mesenchymal phenotype to epithelial phenotype. Although both N-cadherin and VE-cadherin promote tumor growth, their influence on E-cadherin expression in mesenchymal tumor cells is divergent: whereas N-cadherin is capable of repressing E-cadherin expression in Ep5ExTu cells, VE-cadherin has no effect on its expression levels [
7]. Moreover, N-cadherin is required for maintaining VE-cadherin expression, but not vice versa. The regulation of VE-cadherin expression by N-cadherin is a novel mechanism of tumor progression in breast cancer and shows that N-cadherin both inhibits the expression of E-cadherin and stimulates the expression of VE-cadherin.
The downregulation of VE-cadherin in the N-cadherin-deficient Ep5ExTu cells shows that N-cadherin is required (although not necessarily sufficient) for VE-cadherin expression in aggressive carcinoma cells. Regulation of VE-cadherin by N-cadherin was already described before in (nonmalignant) human umbilical vein endothelial cells (HUVEC) [
40], however, evidence for direct regulation of VE-cadherin by N-cadherin is lacking, and the precise mechanisms involved in this regulation remain to be determined. The ability of N-cadherin to regulate VE-cadherin was nonreciprocal because VE-cadherin silencing had no effect on N-cadherin expression. However, as described also for other cell types [
28,
29], VE-cadherin expression in Ep5ExTu cells affected the localization of N-cadherin protein. In control Ep5ExTu cells, which express both cadherins, N-cadherin displayed a nonjunctional distribution whereas in Sh-VE-cadherin knockdown cell lines, N-cadherin was enriched at cell-cell junctions. Whether VE-cadherin expression can influence signaling pathways regulated by N-cadherin as a consequence of excluding it from cell contacts remains to be determined.
There is growing evidence indicating that EMT is a reversible process in cancer cells. Recently, it was hypothesized that tumor cells in metastatic sites can undergo re-differentiation and undergo MET [
1,
3,
6]. This transition could allow metastatic cells to adapt to a new microenvironment. Re-expression of E-cadherin is a critical component of the MET [
41,
42]. However, little is known about the exact mechanism and biological or clinical significance of MET in cancer. Islam
et al. reported that blocking N-cadherin expression upregulates E-cadherin expression in squamous epithelial cells [
10]. Interestingly, we observed that N-cadherin silencing promoted multiple aspects of MET in Ep5ExTu cells in a concentration-dependent manner, including morphological changes, increased levels of E-cadherin and decreased levels of mesenchymal markers. In contrast, VE-cadherin silencing led only to a weaker induction of epithelial markers and had no effect on E-cadherin expression, indicating that MET is activated more efficiently by N-cadherin silencing than by VE-cadherin silencing. This difference might be explained by the difference in β-catenin localization in Sh-N-cadherin and Sh-VE-cadherin cell lines. Interestingly, in Sh-N-cadherin cell lines (Sh-Ncad1.1 and Sh-Ncad2) that displayed the most efficient N-cadherin downregulation, β-catenin was localized at the cell membrane like in the epithelial Ep5 cell line. As reported by other groups, alteration of β-catenin localization alone can be sufficient for the suppression of an invasive phenotype [
24,
43].
The intermediate filament vimentin is an important marker of EMT and its expression is related to the adhesion and migration properties of tumor cells [
44]. A previous study using human breast cancer cells showed that accumulation of cytoplasmic or nuclear β-catenin and vimentin expression coincide [
36]. Additionally, β-catenin can directly transactivate vimentin expression through its binding to the T cell factor (TCF)/lymphoid enhancer factor (LEF) 1 transcription factor family. Vimentin expression was consistently downregulated in mammary carcinoma cell lines in which β-catenin was localized at the plasma membrane (Sh-Ncad1.1 and Sh-Ncad2), but vimentin levels remained unchanged in lines that that showed cytoplasmic and/or nuclear distribution of β-catenin (Sh-Ncad1.2 and Sh-VEcad1).
Several transcriptional regulators are known to repress E-cadherin expression and thereby induce EMT. Among these, we analyzed the expression level of Snail and SIP1, which emerged as key factors regulating E-cadherin expression [
45]. Whereas the level of Snail was downregulated in all Sh-N-cadherin cell lines, the level of SIP1 was decreased only in the Sh-Ncad1.1 and Sh-Ncad2 cell lines, which expressed higher E-cadherin levels. This result therefore suggests that the re-expression of E-cadherin is stimulated more efficiently if the expression of both transcriptional repressors of E-cadherin is decreased.
Deregulation of E-cadherin in breast cancer correlates with higher tumor grade and metastatic tumor cell behavior [
46,
47]. Also in other cell types and in animal models, E-cadherin has been shown to act as a suppressor of tumor growth and invasion [
48]. In our study, suppressing N-cadherin significantly reduced Ep5ExTu tumor growth. Remarkably, Sh-Ncad1.1 and Sh-Ncad2 cell lines hardly grew
in vivo, and mice injected with either cell line were often tumor-free 14 days after inoculation. Histological analysis of tumor sections isolated at day 10 post injection confirmed the re-expression of E-cadherin and downregulation of vimentin in Sh-Ncad2 tumors
in vivo. Since N-cadherin silencing did not change the proliferation rate of Sh-Ncad1.1 and Sh-Ncad2
in vitro, it is likely that the phenotypic reversion of these cell lines, along with E-cadherin expression and associated β-catenin, leads to the inhibition of tumor growth
in vivo. In contrast, moderate suppression of N-cadherin in Sh-Ncad1.2, which greatly inhibits VE-cadherin expression, resulted in a growth rate similar to the Sh-VEcad2 cell line. This growth inhibition correlates well with the decrease in cell proliferation observed for both cell lines
in vitro. It is therefore possible that the growth inhibition of the Sh-Ncad1.2 cell line is caused primarily by the strong VE-cadherin suppression.
Does the introduction of VE-cadherin in N-cadherin (and consequently VE-cadherin) deficient cells restore tumor growth? The forced expression of VE-cadherin in the Sh-Ncad2 cells (that had undergone MET) did not change their epithelial phenotype, as indicated by unchanged E-cadherin and vimentin expression levels. Additionally, junctional localization of E-cadherin was preserved in VE-cadherin re-expressing cell lines. In line with the unaltered epithelial phenotype, forced expression of VE-cadherin failed to evoke a significant difference in tumor growth; the VE-cadherin-expressing and control cells had similar growth rates in vivo. These results suggest that VE-cadherin expression, at least in the presence of E-cadherin, is not sufficient to promote tumor progression.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
MR participated in the design of the project, performed the majority of in vitro experiments, analyzed the experimental tumors, and drafted the manuscript. KF helped to obtain human breast tumors and conducted tumor histology analysis. BW performed tumor experiments. AKu and AKe helped to perform in vitro experiments and analyses of experimental tumors. ML and GBa participated in the design of the project. HS helped to analyze expression of VE-cadherin in human cells. GB conceived the study, participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.