Background
Breast cancer is one of the most frequently diagnosed cancers and the leading cause of cancer death in females worldwide. It accounts for 23% of total cancer cases and 14% of cancer deaths [
1]. Although there have been substantial advances in the treatment of localized malignancies, metastatic breast cancer still lacks effective treatment and remains the primary cause of breast cancer mortality [
2]. Mortality is almost invariably due to metastasis [
2], which is a complex process involving a succession of cell biological events [
3].
The process of epithelial–mesenchymal transition (EMT) plays a key role in tumor metastasis [
4]. Many factors can stimulate the EMT process, during which epithelial cells lose their polarity and cell–cell contacts, acquire a migratory mesenchymal phenotype, and increase resistance to chemotherapy and radiotherapy [
5,
6]. The process of EMT coincides with the loss of epithelial markers, such as E-cadherin and occludin, and acquisition of mesenchymal markers, such as N-cadherin and vimentin [
7]. E-cadherin is a repressor of cancer invasion and EMT induction [
8,
9]. A group of transcription factors has been demonstrated to be capable of orchestrating EMT in cancer progression. Snail, Slug, and ZEB2/SIP1 are direct transcriptional repressors of E-cadherin, whereas Twist and ZEB1 act indirectly on E-cadherin [
10].
The zinc-finger transcription factors Slug (Snail2) and Snail (Snail1) belong to the Snail superfamily [
11]. Slug and Snail are two known important transcription factors that initiate EMT through downregulation of E-cadherin expression in breast cancer, and their expression has been shown to be regulated by Notch signaling [
12]. Interestingly, Slug expression presents a much stronger correlation with loss of E-cadherin in breast cancer cell lines than Snail expression, indicating that Slug is a likely
in vivo repressor of E-cadherin expression in breast cancer [
13].
Accumulating evidence demonstrates that Notch signaling regulates many physiological processes, including cell fate determination in the process of embryonic development, tissue maturity, tumor cell proliferation, cancer stem cell maintenance, EMT, and chemoresistance [
14,
15]. Notch receptors and ligands are single-pass transmembrane proteins that regulate cell fate via cell–cell contact [
16,
17]. Human Notch families have four receptors (Notch1–4) and five ligands (Delta-like-1, Delta-like-3, Delta-like-4, Jagged1, and Jagged2) [
14]. Notch signaling is activated by ligand–receptor interactions between neighboring cells, promoting γ-secretase-dependent cleavage of the Notch receptor and releasing the Notch intracellular domain (NICD) into the nucleus, where the NICD binds to the transcription factor CSL, resulting in activation of the pathway [
16,
18]. The NICD/CSL complex causes the expression of target genes, such as those of the Hairy enhancer of spit (Hes) family.
The Notch signaling pathway is dysregulated in many human malignancies. Overexpression of Notch receptors and their ligands has been found in cervical, colon, head and neck, lung, and renal carcinoma; pancreatic and breast cancer; as well as acute myeloid, Hodgkin, and large-cell lymphomas [
19-
21]. The first evidence that Notch receptors are breast oncogenes was provided in mouse studies in which active forms of Notch1 or Notch4 formed spontaneous murine mammary tumors
in vivo [
22]. Moreover, overexpression of Notch1 and/or its ligand Jagged1 is related to the poorest overall patient survival in human breast cancer [
23-
25]. Accumulating evidence indicates that Notch1 cross-talk with other major cell growth and apoptotic regulatory pathways regulates the activity of transcription factors, such as nuclear factor kappa B (NF-κB) [
26]. However, the role of Notch signaling in regulating EMT remains largely unknown.
In the current study, we found that Notch1 knockdown in breast cancer cells suppressed the EMT process, tumor growth, migration, and invasion using in vitro and in vivo models. Jagged1-mediated Notch signaling activation was able to activate the EMT process and increase migration and invasion in breast cancer mainly though upregulation of N1ICD, rather than Notch2 NICD (N2ICD), Notch3 NICD (N3ICD), or Notch4 NICD (N4ICD). Moreover, we revealed that Notch1 signaling played a vital role in regulating EMT mainly in a Slug-dependent manner. Our findings indicate that Notch1 signaling is a promising therapeutic target for preventing breast cancer progression.
Discussion
In the current study, our data showed that inhibition of Notch1 reversed the EMT process both
in vitro and
in vivo, and inhibited migration and invasion in breast cancer cells. We found a positive association between Notch1 signaling and breast cancer invasion and progression, thus presenting a potential oncogenic role. Moreover, our study identified that EMT induced by Jagged1 is mainly through Notch1-induced Notch signaling activation and that Slug is an important target gene of Notch1 signaling in regulating the EMT process. Notch signaling, which plays a critical role in tumor pathology and progression, is frequently observed in breast cancer and other solid tumors [
28]. Recently, several studies have revealed that overexpression of Notch1 and/or Jagged1 indicates a poor prognosis for breast cancer patients. Furthermore, the Notch signaling pathway is related to a number of protumorigenic activities in breast cancer cell lines and can cause mammary hyperplasia and carcinogenesis in mice [
29-
32]. A study on human breast cancer has shown that the protein level of Numb, which is a negative regulator of Notch signaling, is reduced in 50% of human breast tumors [
33]. Notch1 has also been found to be a downstream effector of the oncogenic gene Ras in human mammary tumorigenesis [
34]. Although there is abundant evidence demonstrating that the Notch signaling pathway is closely associated with human breast cancer development, our study has provided novel insights into the role of Notch1 signaling in regulating EMT and invasion mainly in a Slug-dependent manner.
Invasion and metastasis are two of the most important features of malignant tumors, the main cause of cancer-related death, and the most difficult problem in clinical treatment. EMT is a key step in tumor invasion and metastasis. Our study results emphasize the key role of the Notch1 signaling pathway in regulating EMT via the transcription factor Slug during cancer cell invasion and metastasis, using both
in vitro and
in vivo models. Our data showed that the expression of the Notch signaling downstream genes Hes1 and Hey1 decreased in breast cancer cells with Notch1 inhibition. In contrast, the expression levels of Hes1 and Hey1 were significantly increased during Notch signaling pathway activation induced by Jagged1. These data indicate that both Hes1 and Hey1 are sensitive Notch target genes for the Notch signaling pathway. While previous studies have shown that both Hes1 and Hey1 are sensitive Notch target genes,
Hey is more sensitive than
Hes gene for Notch pathway inhibition in breast cancer [
35]. Recently, a few other Notch target genes have been found, including NF-κB, cyclinD1, c-myc, p21, p27, Akt, mTOR, VEGF, etc. [
20,
21,
36]. In this study, our data also showed that NF-κB p65 expression was dramatically downregulated by Notch1 knockdown in breast cancer cells. Similarly, a recent report has revealed that Notch signaling pathway inhibition results in decreased NF-κB in pancreatic cancer cells, suggesting a molecular link or cross-talk between Notch and NF-κB; furthermore, activation of these signaling pathways is associated with acquisition of the EMT phenotype [
37].
Previous studies have demonstrated that Jagged1-induced Notch signaling activation induces EMT through Slug-mediated repression of E-cadherin [
35]. A recent study has shown that Notch1 mediates esophageal carcinoma cell invasion and metastasis by inducing EMT through upregulation of Snail [
38]. Likewise, our study showed that the EMT process was closely related to Notch1 signaling in breast cancer cells. Notch1 silencing could reverse the EMT process and lead to MET. The MET program is characterized by increased E-cadherin and occludin expression, and decreased N-cadherin, vimentin, and nuclear β-catenin, which leads to the acquisition of an epithelial phenotype. Meanwhile, breast cancer cell migration and invasion abilities were inhibited by Notch1 interference. Furthermore, our data showed that the EMT process was largely driven by Jagged1-induced Notch signaling activation. This EMT program displayed a significant reduction of E-cadherin and occludin expression, and a marked increase of N-cadherin, vimentin, and nuclear β-catenin expression, resulting in cancer cells evolving into a highly invasive and mesenchymal phenotype. A recent study has reported that EMT results in a loss of E-cadherin, which impairs cell–cell adhesion and allows nuclear localization of β-catenin [
39]. Apparently, our findings are in concordance with this study. Notably, culturing breast cancer cells with soluble Jagged1 protein induced EMT and increased their migration and invasion, which could be partially abolished by Notch1 knockdown. These data indicate that the Jagged1–Notch1 axis plays an important role in promoting EMT and invasion in breast cancer. In addition, Chen
et al. have shown that inhibition of Notch signaling abrogates the downregulation of E-cadherin and increases migration and invasion under hypoxic conditions in breast cancer [
12]. Interestingly, our results showed that Jagged1 could not rescue the changes caused by downregulation of Notch1. Jagged1 ligand-triggered Notch activation is mainly mediated by the interaction between Notch ligands and receptors. Our data also showed that Jagged1 could cause a significant upregulation of N1ICD and less upregulation of N4ICD. Our study provides noteworthy evidence that the Jagged1 ligand can interact with Notch1 to activate Notch signaling and promote EMT.
Slug and Snail are associated with EMT during both embryonic development and cancer metastasis [
40,
41]. Our data showed that Notch1 knockdown mainly decreased Slug expression, compared with the other transcription factors. Downregulation of Slug resulted in a significant increase of E-cadherin expression and a prominent decrease of the vimentin level, and weakened the migration and invasion capacities in breast cancer cells. Importantly, the Notch1-reversed EMT was restored by the overexpression of Slug. Furthermore, Notch1 was found to be more effective at downregulating the Slug expression levels than Hes1 and Hey1. This result led us to investigate whether Slug expression was directly regulated by Notch1 at the transcriptional level. The luciferase reporter assay further demonstrated that Notch1/NICD positively regulated Slug expression by enhancing Slug promoter activity. In light of our studies, Notch1 might downregulate Slug to maintain the epithelial phenotype and inhibit the migration and invasion behavior of breast cancer cells. The Notch1–Slug axis might play an important role in breast cancer progression. However, the detailed mechanism by which the Notch1–Slug axis regulates EMT remains unclear, and much work needs to be done in the future.
In our experiments, we also observed that Notch signaling activation induced by Jagged1 partially inhibited the upregulation of E-cadherin and the downregulation of vimentin caused by Slug interference. These data suggest that Notch signaling could bypass Slug to trigger some other factors to regulate EMT. Leong
et al. have reported that Notch signaling activation targets Slug, not Snail or Twist, to suppress E-cadherin and initiate the EMT process [
35]. However, Chen
et al. have shown that Notch signaling activation under hypoxic conditions could increase the expression of Slug and Snail to initiate EMT [
12].
Taken together, our observations imply that Jagged1-triggered Notch signaling activation is mainly dependent on Notch1. Slug serves as a mediator, which contributes to the enhanced migration and invasion in breast cancer cells. EMT is initiated mainly though the Notch1–Slug signaling axis. Thus, Slug plays a key role in Notch1 signaling that modulates EMT and metastasis in breast cancer.
Materials and methods
Cell culture
The human breast cancer cell lines (MCF-7, MDA-MB-231, SK-BR-3, T47D, and ZR-75-1) used in this study were purchased from the Cell Bank of Type Culture Collection of the Chinese Academy of Sciences (Shanghai, China). Human mammary epithelial cells (HMECs) and non-tumorigenic MCF-10A cells were obtained from the American Type Culture Collection. The human breast cancer cells were cultured in DMEM, RPMI-1640, or McCoy’s 5A medium, respectively, according to the recommended culture method. All media were supplemented with 10% fetal bovine serum (FBS; Gibco, Grand Island, NY, USA) and 1% penicillin-streptomycin. All cells were grown at 37°C in an atmosphere of 5% CO
2. The culture methods used for HMECs and MCF-10A cells were as described previously [
42].
Notch signaling activation by using immobilized recombinant Notch ligands
The Notch signaling activation assay was performed as described previously [
43]. Cell culture plates were coated with 50 μg/ml Protein G (Zymed, USA) in phosphate-buffered saline (PBS) at room temperature overnight. The plates were washed with PBS three times and blocked with 10 mg/ml bovine serum albumin (BSA) in PBS for 2 h at room temperature. The blocked plates were then washed with PBS and incubated with recombinant Jagged1-FC chimera (R&D Systems, USA) at a concentration of 3 μg/ml in 0.1% BSA/PBS for 3 h at room temperature. After washing three times with PBS, the cells were immediately seeded in the coated plates using the culture medium as described above. The schematic diagram of this Notch signaling activation method is shown in Additional file
1: Figure S4.
Notch1 shRNA lentivirus transfection
Notch1 shRNA (shNotch1) and negative control (shNC) in eukaryotic GV248 lentiviral vectors were purchased from GeneChem Co., Ltd. (Shanghai, China). The target sequence for Notch1 shRNA was GTCCAGGAAACAACTGCAA. The cells were seeded at 1 × 103 cells/well into 96-well plates at 24 h prior to transfection. When the cells grew to 30–70% confluence, transfection was carried out by using lentiviral particles (MCF-7 multiplicity of infection (MOI) = 20; MDA-MB-231 MOI = 10), polybrene (5 μg/ml), and enhanced infection solution (GeneChem Co., Ltd., Shanghai, China), according to the manufacturer’s protocol. At 12 h post-transfection, virus-containing medium was replaced with complete medium. At 96 h post-transfection, all cells were selected by puromycin (Merck, USA) at a final concentration of 5 μg/ml (MCF-7) or 4 μg/ml (MDA-MB-231) for 10 days. Then, the cells were maintained in 2.5 μg/ml (MCF-7) or 2 μg/ml (MDA-MB-231) puromycin. To generate stable transfected cells, 100 transfected cells were seeded into a 10-cm Petri dish, and the medium was changed three times per week. After 3 weeks, puromycin-resistant colonies were isolated and seeded into 96-well plates for further study. The 954-bp coding sequence of Slug was amplified by PCR from the cDNAs of MDA-MB-231 cells and subcloned into pcDNA3.1 (+) by EcoR V and EcoR I (Shanghai Genechem Co., Ltd, China).
RNA interference
The siRNAs for Slug (5′-GCAUUUGCAGACAGGUCAAdTdT-3′, 5′-UUGACCUGUCUGCAAAUGCdTdT-3′), Hey1 (5′-CAGUUUGUCUGAGCUGAGATT-3′, 5′-UCUCAGCUCAGACAAACUGTT-3′), Hes1 (5′-AGGCUGGAGAGGCGGCUAATT-3′, 5′-UUAGCCGCCUCUCCAGCCUTT-3′), and negative control (NC: 5′-UUCUCCGAACGUGUCACGUTT-3′, 5′-ACGUGACACGUUCGGAGAATT-3′) were synthesized by GenePharm (Shanghai, China). Cells (5 × 104 per well) were seeded in six-well plates 24 h prior to transfection and transfected with 80 pM siRNA using TurboFectTM siRNA Transfection Reagent (Fermentas, Beijing, China), according to the manufacturer’s protocol.
Western blot analysis and antibodies
The cells were harvested, and total protein was extracted from the stable cell lines. Nuclear protein was isolated according to the manufacturer’s instructions (Pioneer Biotechnology, Inc.). Equal amounts of protein (150 μg) were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (10% SDS-PAGE) and then transferred onto a polyvinylidene difluoride (PVDF) membrane (Roche). The immunoblots were incubated in 5% (w/v) skim milk powder dissolved in TBST (10 mM Tris–HCl, pH 8.0, 150 mM NaCl, and 0.05% Tween-20) for 2 h at room temperature. Next, the blots were probed first with specific antibodies and then with the appropriate secondary antibodies.β-Actin or LaminB1 was used as a control. Signals were quantified using Image-Pro Plus 6.0 software (Media Cybernetics).
Antibodies were purchased from the following sources: anti-Notch1 antibody (Cell Signaling Technology, Boston, MA, USA), anti-NF-κB65 antibody (Cell Signaling Technology), anti-Hey1 antibody (Abcam, Cambridge, MA, USA), anti-Hes1 antibody (Cell Signaling Technology), anti-E-cadherin antibody (Cell Signaling Technology), anti-N-cadherin antibody (Abcam), anti-vimentin antibody (Cell Signaling Technology), anti-occludin antibody (Proteintech Group Inc., USA), anti-Slug antibody (Abcam), anti-β-catenin antibody (Cell Signaling Technology), anti-Snail (Proteintech Group Inc.), anti-Twist antibody (Proteintech Group Inc.), anti-ZEB1 (Cell Signaling Technology), anti-ZEB2 (Abcam), anti-β-actin antibody (Santa Cruz Biotechnology, Santa Cruz, CA, USA), anti-LaminB1 antibody (Santa Cruz Biotechnology), anti-N2ICD (Santa Cruz Biotechnology), anti-N3ICD (Santa Cruz Biotechnology), and anti-N4ICD (Santa Cruz Biotechnology).
RNA extraction and quantitative real-time PCR
Total RNA was isolated from breast cancer cells using TRIzol reagent (Invitrogen, CA, USA). cDNA was synthesized by using a PrimeScript RT reagent kit (Fermentas, Beijing, China). The real-time PCR was carried out using a SYBR Green PCR Kit (TaKaRa, Dalian, China), according to the manufacturer’s instructions. GAPDH was used as an internal control, and the relative expression levels were assessed using the ΔΔCt method. The sequences of the primers for real-time PCR are listed in Additional file
1: Table S1.
In vitro cell migration and invasion assays
In the cell migration assay, equal numbers of cells (5 × 104 cells/well) were suspended in the top compartment of a 24-well chamber (Millipore Co., Billerica, MA, USA) in 300 μl of DMEM (MCF-7 cells) or RPMI 1640 (MDA-MB-231 cells) containing 0.1% BSA. Then, 600 μl of DMEM containing 15% FBS or RPMI 1640 containing 10% FBS was added into the bottom compartment. The cells were incubated for 24 h. Then, the nonmigrated cells were removed from the membrane of the top compartment with a cotton swab, and the cells that had migrated through the membrane were fixed and stained in 4% paraformaldehyde and 0.01% crystal violet solution. The numbers of cells that migrated were determined from six random high-power fields (HPFs) visualized at 100× magnification, and means were obtained for statistical analysis.
The invasion assays were performed in a similar manner as the migration assays, except that the cells were placed in the upper compartment with a Matrigel (BD Biosciences, San Jose, CA, USA)-coated membrane, as previously described [
44]. The numbers of invading cells were quantified from six random HPFs visualized at 100× magnification.
Immunofluorescence
A total of 2 × 104 cells per well were grown on glass coverslips in a 24-well plate overnight. The next day, when the cells were 50–70% confluent, they were washed twice with PBS, then fixed in 4% paraformaldehyde solution, and permeabilized in 0.03% Triton X-100 (Sigma) in PBS for 20 min. The cells were then washed three times (5 min each time) with PBS and blocked with 5% BSA in PBS for 1 h at room temperature. The cells on the coverslips were incubated in a humidified box with the respective primary antibodies at a 1:100 dilution overnight at 4°C. Next, the cells were washed three times (5 min each time) in PBS and incubated for 1 h with CY3-conjugated secondary antibodies at a 1:50 dilution (CWBIO, Beijing, China) at room temperature in the dark. Finally, the cells were washed three times in PBS and incubated with 1 μg/ml 4, 6-diamidino-2-phenylindole (DAPI, Roche) for 5 min at room temperature in the dark. Then, the slides were washed extensively with PBS and observed with an immunofluorescence microscope (Nikon, Japan) with identical exposure times at 400× magnification.
Luciferase reporter assays
The cDNA encoding N1ICD was subcloned into the expression vector pcDNA3.1(+) between the BamHI and XhoI sites, and subcloning was confirmed with sequencing by Shanghai Genechem Co., Ltd. The pGL3 reporter construct plasmid (−2000/+100) consisted of a 2100-bp genomic DNA fragment of the Slug promoter (Promega, Madison, WI, USA). The pGL3-Slug promoter plasmid or its negative control pGL3-basic plasmid carrying the firefly luciferase reporter were co-transfected with an internal control, pRL-TK Renilla vector (Promega), by using Lipofectamine 2000 (Invitrogen). In addition, cells were respectively transfected with 600 ng of N1ICD overexpression plasmid pcDNA3.1(+) or its negative control pcDNA3.1. Cell lysates were harvested 48 h after transfection. The firefly and renilla activities were measured by the Dual-Luciferase Reporter Assay System (Promega). Firefly luciferase activity was normalized to the Renilla luciferase activity. Each transfection was repeated three times.
Animal studies
Animal experiments were performed in accordance with the Animal Care and Use Committee guidelines of Xi’an Jiao Tong University, Shaanxi, China. The mice were divided into two groups, with four per group. The cells were resuspended in a 1:1 (v/v) mixture of culture media and Matrigel (BD Biosciences, San Jose, CA, USA), and 2 × 106 MDA-MB-231 cells were orthotopically injected into the mammary fat pads of 6-week-old female nude mice (Silaike Laboratory Animal Co., Ltd., Shanghai, China). Tumor growth was observed twice per week with calipers at the site of injection. After 38 days, the animals were observed by an IVIS imaging system (IVIS spectrum, Xenogen, CA, USA), then they were sacrificed, and the mammary tumors were isolated for immunohistochemical staining.
Immunohistochemistry
Sections of paraffin-embedded, formalin-fixed tumor tissues were deparaffinized in xylene and rehydrated in a series of graded ethanol solutions, and exposed to microwave radiation in a citrate buffer (pH = 6.0) for 15 min. Endogenous peroxidase activity was blocked with 3% H2O2 in methanol for 5 min at room temperature. The sections were washed three times with PBS and incubated with goat serum for 20 min at room temperature. Next, the sections were incubated with the primary antibodies overnight at 4°C. The sections were warmed to room temperature, then washed three times with PBS, incubated with biotinylated secondary antibodies for 30 min at room temperature, and washed again, after which immune complexes were detected with the use of a streptavidin-peroxidase complex (DAKO) and 3,3′-diaminobenzidine (DAB, DAKO). The sections were counterstained with hematoxylin, dehydrated in a graded series of alcohol solutions, and mounted in Malinol (Muto Pure Chemicals).
Statistical analysis
All data are presented as the mean ± standard deviation (SD). Data were analyzed by one-way analysis of variance (ANOVA) or two-way ANOVA as appropriate. Statistical analysis was performed with SPSS 13.0 for Windows software (SPSS Inc., Chicago, IL, USA). All statistical tests were two-sided, and P values less than 0.05 were considered statistically significant. In all figures, (*) denotes P < 0.05. All experiments were repeated independently at least three times.
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Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
SS, XAZ, XJZ, ML, and XXZ carried out all the experiments. SS, XA Z, XHZ, and SG designed the experiments and analyzed the data. SS, XAZ, SH, and YW wrote the manuscript. All authors read and approved the manuscript.