FZD2 Promotes TGF-β-induced Epithelial-to-Mesenchymal Transition in Breast Cancer via Activating Notch Signaling Pathway

DOI: https://doi.org/10.21203/rs.3.rs-63207/v2


Background Breast cancer (BC) is one of the commonest female cancers, which is characterized with high incidence. Although treatments have been improved, the prognosis of BC patients in advanced stages remains unsatisfactory. Thus, exploration of the molecular mechanisms underneath BC progression is necessary to find novel therapeutic methods. Frizzled class receptor (FZD2) belongs to Frizzled family, which has been proven to promote cell growth and invasion in various human cancers. The purpose of our study was to detect the functions of FZD2 and explore its mechanism in BC. Methods The level of FZD2 was measured in BC tissues by quantitative realtime polymerase chain reaction (qRT-PCR), western blot, immunohistochemistry (IHC) respectively. Cell Counting Kit-8 (CCK-8), standard colony formation, transwell aasays, wound healing and flow cytometry experiments were adopted separately to test cell viability, invasion, migration, apoptosis and cell cycle distribution. Epithelial-mesenchymal transition (EMT) biomarker were determined by using Immunofluorescence assay. Xenograft tumorigenicity assay was performed to assess the effect of FZD2 on tumor growth in vivo . Results We determined that FZD2 mRNA and protein expression was abundant in BC tissues. Moreover, high level of FZD2 had significant correlation with poor prognosis. In vitro functional assays revealed that silencing of FZD2 had suppressive effects on BC cell growth, migration and invasion. Animal study further demonstrated that FZD2 silencing inhibited BC cell growth in vivo. In addition, FZD2 induced EMT in BC cells in a transforming growth factor (TGF)-β1-dependent manner. Mechanistically, knockdown of FZD2 led to the inactivation of Notch signaling pathway. Conclusion Based on all these data, we concluded that FZD2 facilitates BC progression and promotes TGF-β1-inudced EMT process through activating Notch signaling pathway.


Breast cancer (BC) is considered to be one of the commonest life-threatening cancer types among women. As reported in 2019, BC accounts for 30% among all new female cancer diagnoses [1]. Local recurrence and distant metastasis are the main reasons for the high mortality of BC patients [2]. Although various treatments have been developed for BC patients, the effective therapeutic targets remain limited. Therefore, exploring the molecular mechanism underneath BC progression is of great significance to develop novel therapeutic strategies.

Frizzled family proteins (FZDs, including FZD1-FZD10) function as cell surface receptors of WNT signaling pathway. Each member of FZDs can activate WNT signaling pathway through interacting with different WNT proteins [3]. As previously reported, FZD2 is dysregulated in various cancer types, such as oral squamous cell carcinoma [4]{Asano, 2017 #3793}, gastric cancer [5], endometrial cancer [6], hepatocellular carcinoma [7] and tongue squamous cell carcinoma [8]. There is also a study reported that FZD2 suppresses tumor growth in salivary adenoid cystic carcinoma [9]. In addition, non-canonical FZD2 pathway has been recognized to be a regulator for epithelial-to-mesenchymal transition (EMT) and migration [10]. Nevertheless, the mechanism of FZD2 underneath BC progression remains largely unknown.

EMT process is relevant to metastasis and chemotherapy resistance in human cancers [11]. In EMT process, epithelial cells undergo a variety of biochemical alterations and thus change to mesenchymal phenotypes [12]. EMT is closely correlated with the development of human cancers. For example, EMT progress in bladder cancer leads to the high tumor grades and stages [13]. Alterations on the levels of epithelial or mesenchymal markers contribute to the change of EMT process [14]. The expression of EMT markers is tightly associated with the disease progression of SCLCs [15]. Importantly, expression changes of EMT markers are also correlated with BC progression [16].

TGF-β family is a group of cytokines that can act on EMT process in cancer progression [17]. TGF-β1 promotes the acquisition of a mesenchymal phenotype and thus promotes cell invasion and migration [18]. Notch signaling pathway is primarily functions by controlling cell fate decisions, differentiation, and proliferation [19]. According to previous studies, Notch signaling pathway can also involve in EMT process in several different cancer types, such as lung cancer, pancreatic cancer and breast cancer [20-22].

To summarize, this study focused on the functions of FZD2 in BC progression and its regulatory effects on TGF-β1-inudced EMT and Notch signaling pathway.

Materials And Methods

Tissue samples

Primary invasive ductal carcinomas tissues and adjacent normal tissues were obtained from female patients at the Jinling Hospital, affiliated with the Medical School of Nanjing University. All participants didn’t receive neoadjuvant therapies before the operation. A total of 147 female patients participated in this study. The written informed consent had been collected from patients before the study. The investigation was approved by the ethics committee of Jinling Hospital.

Cell culture and treatment

SK-BR-3, MCF-7 and MDA-MB-231 cell lines used in this study were obtained from the Type Culture Collection of the Chinese Academy of Sciences (Shanghai, China). MCF-10A (normal human mammary epithelial cell line) purchased from KeyGEN Biotech Company (Nanjing, China) was used as the control cell line. All cell lines were grown in Dulbecco’s Modified Eagle’s medium (DMEM, KeyGEN) in which 10% fetal bovine serum (A3160801, Gibco) and 1% penicillin-streptomycin were added. Cell culture was accomplished in a humidified incubator containing 5% CO2 at 37 °C. Cell passage was performed when the confluence reached to 90%. Reagents used in this study, including TGF-β1 and TGF-β type I Receptor inhibitor SB431542, were separately purchased from PeproTech (#100-21, USA) and MedChemExpress (HY-10431, USA).

siRNA Transfection

siRNAs targeting FZD2 were designed and synthesized by Ribobio (Guangzhou, China). . For the silencing of FZD2, siRNAs were transfected into SK-BR-3 and MDA-MB-231 cells in accordance with the instruction manual for Lipofectamine 3000 (Invitrogen, #L3000015, Carlsbad, CA, USA). Plasmid amplification was accomplished through transformation and pumping. The levels of FZD2 mRNA and protein were separately checked by qRT-PCR and western blot. Sequences for all siRNAs are as follows:







RNA extraction and qRT-PCR

Total RNA was extracted from tissues with RNA kit (Promega), whereas those extracted from cells were accomplished with TRIzol (Invitrogen). RNA was reverse transcribed into cDNA using PrimeScript RT Master Mix (Perfect Real Time, TaKaRa, Shiga, Japan) and then analyzed by qRT-PCR with SYBR Green Master Mix on an Applied biosystem 7500 machine (USA). 2−ΔΔCT method was used to calculate relative mRNA expression by normalizing to GAPDH. Sequences for all primers used for qRT-PCR were provided in Table 1.

Western blot

RIPA buffer (Beyotime) was used for extraction of total protein from tissues and cells. BCA Protein Assay Kit (KeyGen Biotech, Nanjing, China) was applied to measure the protein concentration. After resolved on SDS-PAGE, the protein lysates were transferred onto a polyvinylidene fluoride (PVDF) membrane (Immobilon, USA). After blocked in 5% non-fat milk, the membranes were incubated with primary antibodies at 4°C overnight. After washing, the membranes were incubated with a secondary antibody against rabbit (1: 5000, Abcam) at 37°C for 1 h. Primary antibodies used in this experiment were shown in Table 2. The blots were detected by the ECL (NCM Biotech, China) method.

Immunohistochemistry (IHC)

hesFor IHC staining, sections were dewaxed with xylene and rehydrated in ethanol. To avoid non-specific staining, samples were washed with PBS and blocked with 5% BSA at room temperature for 30 min. After washing, sections were incubated with primary antibodies against FZD2 (1:50, Abcam), TGF-β1 (1:500, Proteintech), Ki-67 (1:10000, Proteintech) at 4 °C overnight. Afterwards, sections were incubated with secondary antibody kit SP0031 and/or SP0021 (Solarbio, China) in accordance with the instruction manual. Images were taken under a microscope (Olympus CX41, Japan).

Evaluation of staining of tissue slides

After immunostaining, FZD2 in different tissues was evaluated by a semi-quantitative immunoreactivity scoring system (IRS). The indexes of Immunostaining intensity were separated as 0 (no immunostaining), 1 (weak), 2 (moderate) and 3 (strong). The scores for immunoreactive cells were separately defined as 0 (no immunoreactive cells), 1 (less than10%), 2 (between 10% and 50%), 3 (between 51% and 80%) and 4 (more than 80%). The IRS index for each case ranges from 0 to 12. FZD2 was considered to be upregulated in cases with 6 or higher IRS score, whereas FZD2 was considered to be downregulated in those with IRS score less than 6.

Cell counting kit 8 (CCK-8) assay

The transfected cells were plated into 96-well plates at 6-8 × 103 cells per well in 100 μL cell suspension. After added CCK-8 solution (10 μL) (Dojindo, Kumamoto, Japan) at six different time points (0 h, 24 h, 48 h, 72 h, 96 h, 120 h), cells were incubated at 37°C for 2 h. Finally, a microplate reader was applied to measure the absorbance at 450 nm (OD 450 nm).

Colony formation assay

The transfected cells were seeded in 6-well plates at 3 × 103 cells per well and cultured in DMEM containing 10% FBS. At day 14, cells were fixed with methanol for 10 min and stained by 0.1% crystal violet for 15 min. After captured the pictures, the number of colonies was recorded manually.

Cell migration and invasion assays

Matrigel-coated 24-well Transwell chambers (Corning Incorporated, Corning, NY, USA) and non-coated Transwell chambers (#353097; BD Biosciences, USA) were separately used for cell invasion assay and cell migration assay. Twenty-four hours after transfection, the cells were plated into the upper compartment containing serum-free DMEM at a density of 2× 104 cells per well. The lower chamber was added with DMEM containing 20% FBS. Forty-eight hours later, the cells in the upper chamber were removed with cotton swabs, while the cells on the lower surface were fixed with 4% paraform-aldehyde for 10 min and stained with 0.1% crystal violet for 15 min. The results were obtained using an inverted microscope (Olympus, Japan).

Wound healing assay

Twenty-four hours after siRNA transfection, cells were seeded in 6-well plates at a density of 1×106 cells each well. After cell attachment, a scratch was created with a 200 μL pipette tip in the cell layer. Cells that had been scraped off were washed away with PBS. Forty-eight hours later, the distance of wound was measured by observing images at 0 h and 48 h.

Flow cytometry analysis of cell cycle distribution

Twenty-four hours after siRNA transfection, cells were plated into 6-well plates at a density of 1 × 106 cells per well. Next, cells were rinsed in prepared phosphate buffer saline (PBS). After incubated with the propidium iodide (KeyGEN, China), flow cytometry was applied to detect the cell population at different phase (G0/G1, S and G2/M). Percent of cells in different phases was measured by BD FACSCantoII (BD Biosciences, USA).

Flow cytometry analysis of cell apoptosis

Twenty-four hours after siRNA transfection, EDTA-free trypsin (Beyotime, China) was used to digest the cells, and 1 × 106 cells/ml were counted. Cell apoptosis assay was processed under the FITC Annexin V Apoptosis Detection Kit (BD Biosciences), cell apoptosis was detected by BD FACSCanto II (BD Biosciences).

Immunofluorescence microscopy

Cells were fixed in parafolmaldehyde (4%) for 20 min and permeabilized with 0.5% Triton X-100 in PBS for 10 min. Cells were blocked with PBS/BSA 5% for 30 min. The membranes were incubated with primary antibody against E-cadherin and Vimentin (1: 50, proteintech) at 4°C overnight. After washing, the membranes were incubated with green-fluorescence conjugated Affinipure antibody (Coralite 488, diluted 1:100, Proteintech) or Red-fluorescence conjugated Affinipure antibody (Coralite 594, diluted 1:100, Proteintech). After washing, DAPI (Sigma, Cat# D-9542) was used for the nuclei staining. Images were visualized using a Nikon ECLIPSE NI (Japan) microscope.

Xenograft tumorigenicity assay

For animal study, female BALB/c nude mice (4 weeks old) were bought from the Comparative Medicine Department of Jinling Hospital. Mice were randomly divided into two groups (n=6 per group) after the tumor volume reached 100mm3. si-FZD2-2 and si-NC(Ribobio, China) was administered by intratumoral injection at a dose of 30 μL/piece, administered once every 3 days and continuously for 14 times. Forty days after injection, all mice were killed and the tumors were removed. Tumor tissues were embedded in paraffin for further analyses. Animal study was conducted followed the protocol of the animal ethics committee of Jinling Hospital.

TUNEL assay

Apoptosis in tumor tissues removed from nude mice was detected by using the TUNEL assay kit (Roche, USA) according to the manufacturer’s guidelines.

Statistical analysis

Statistical significance of differences was analyzed with two-tailed student’s t test (between two groups) or one-way/two-way ANOVA (among multiple groups). All experimental data were obtained from three or more independent experiments and shown as mean ± SD (standard deviation). The correlation between FZD2 expression and the clinicopathological characteristics of BC patients was analyzed using the Chi-squared test. SPSS v17.0 (SPSS Inc., Chicago, IL, USA) and Prism v8.0 (GraphPad, San Diego, CA, USA) were utilized for statistical analyses. Data were statistically significant when p values less than 0.05.


FZD2 is significantly upregulated in BC tissues and associated with poor prognosis

According to UALCAN [23] dataset, FZD2 expression is associated with various tumors (Figure 1A). Based on TCGA dataset, the prognostic value of FZD2 in 701 BC patients was analyzed (Figure 1B). After analysis, FZD2 overexpression was identified to be correlated with worse distant metastasis free survival (DMFS) indicating the potential involvement of FZD2 in cancer progression. Searching from ONCOMINE database(http://www.oncomine.org/resource/login.html),we identified the upregulation of FZD2 in BC tissues (Figure 1C).Then, we performed qRT-PCR to examine FZD2 mRNA expression in 42 BC tissues relative to adjacent normal tissues. Not surprisingly, FZD2 presented a higher expression in BC tissues (Figure 1D). The protein level of FZD2 exhibited the same tendency in BC tissues, as evidenced by western blot analysis (Figure 1F). IHC revealed that FZD2 was mainly expressed in the cytoplasm (Figure 1E). Weak staining (+), moderate staining (++) and strong staining (+++) were separately 28 cases, 43 cases and 34 cases. Simultaneously, we found that patients with higher FZD2 expression exhibited significantly poorer OS and DMFS than those with lower FZD2 level (Figure 1G). Finally, we divided the 105 patients into high and low level of FZD2 expression groups and analyzed the relationship between FZD2 expression and clinical characteristics (Table 3). Overexpression of FZD2 had significant correlations with TNM stages (p = 0.006), lymph-node metastasis (p = 0.024) and organ metastasis(p=0). Overall, highly expressed FZD2 in BC patient samples is correlated with poor prognosis.

FZD2 promotes BC cell growth, migration and invasion, while induces cell apoptosis in vitro

     Furthermore, we investigated FZD2 expression in BC cells through comparing with the normal MCF-10A cell. As expected, FZD2 was upregulated in three BC cells (Figure 2A), among which SK-BR-3 and MDA-MB-231 presented the highest FZD2 level (p < 0.01). We thus selected these two cells for loss-of-function experiments. At first, silencing FZD2 was confirmed in SK-BR-3 and MDA-MB-231 cells through qRT-PCR and western blot analyses (Figure 2B). Since si-FZD2-1 and siFZD2-2 resulted in the most significant knockdown effect on FZD2, we used these two siRNAs for subsequent experiments. FZD2 silencing dramatically reduced cell proliferation (Figure 2C-D). Moreover, cells were arrested at G2/M phase after FZD2 knockdown (Figure 2E). Therefore, we confirmed that FZD2 promotes in vitro cell growth in BC. As for apoptosis, flow cytometry analysis and western blot were conducted in FZD2-silenced BC cells. Then, we observed an enhanced apoptosis rate in cells with FZD2 silencing (Figure 2F). Consistently, the levels of Bax and cleaved caspase 3 were increased, whereas the level of Bcl-2 was decreased in FZD2-downregulated BC cells (Figure 2G). The mobility of BC cells was evaluated after FZD2 depletion. Intriguingly, knockdown of FZD2 weakened the migratory and invasive ability of BC cells (Figure 2H-I). Accordingly, we identified the important role of FZD2 in regulating BC cell migration, invasion and apoptosis in vitro.

FZD2 silencing suppresses BC cell growth in vivo

Animal models were established to investigate the effect of FZD2 silencing on tumor growth. Forty days after injection, we observed the smaller tumor size in si-h-FZD2 group than those in si-NC group (Figure 3A). The volume and weight of tumors in two groups presented the consistent tendencies (Figure 3B-C). IHC showed that Ki-67 and TGF-β1 positivity was lower in tumor tissues of si-h-FZD2 group (Figure 3D). Through TUNEL assays, we confirmed that FZD2 silencing induced apoptosis in vivo (Figure 3D). In the meantime, the levels of Bax and cleaved caspase 3 (pro-apoptotic proteins) were increased, whereas the level of Bcl-2 (an anti-apoptotic protein) was decreased in response to FZD2 silencing (Figure 3E). Taken together, FZD2 silencing inhibits BC cell growth in vivo.

Depletion of FZD2 reverses EMT process through inactivating Notch signaling pathway

GSEA database illustrated that FZD2 may regulate the Notch signaling pathway (Figure 4A). To analyze whether FZD2 had the potential to affect EMT process, the expression of EMT markers (E-cadherin, N-cadherin, Vimentin and Fibronectin) was assessed in FZD2-silenced cells. As presented in Figure 5B-C, the levels of E-cadherin mRNA and protein were enhanced by FZD2 knockdown, whereas N-cadherin, Vimentin and Fibronectin were remarkably downregulated after silencing of FZD2 (Figure 4B-C). Simultaneously, we measured the levels of Notch pathway-related molecules (Notch1, P21 and Hes1). Intriguingly, FZD2 depletion led to the downregulation of these three proteins (Figure 4B). At last, Immunofluorescence analysis further revealed that the level of E-cadherin was increased but the level of N-cadherin was decreased in cells with FZD2 knockdown (Figure 4D). All these data indicated that FZD2 regulates cellular processes in BC potentially through activating Notch signaling.

FZD2 positively regulates TGF1-β in BC cells through Notch signaling pathway

TGF-β signaling plays an essential role in inducing EMT in different types of tumors. According to the data of GEPIA database, TGF-β1 had positive expression correlation with FZD2 or Notch1 in BC patient samples (Figure 5A). Moreover, the level of TGF-β1 protein was decreased in FZD2-silenced cells (Figure 5B). Subsequently, the levels of Fibronetin, N-cadherin and Vimentin were significantly increased by the strengthened TGF-β1 in BC cells, whereas the levels of E-cadherin were remarkably decreased (Figure 5C-D). Meanwhile, the levels of Notch signaling pathway-related proteins were also induced after TGF-β1 treatment in BC cells (Figure 5C). In addition, TGF-β1-induced protein levels of fibronectin, N-cadherin and Vimentin were reduced again by introducing TGF-β receptor antagonist SB431542 (Figure 5E), and then the mRNA level of E-cadherin decreased by TGF-β1 was also recovered by SB431542 (Figure 5F). These findings illustrated that FZD2 promotes BC progression and involves in TGF-β-induced EMT through activating Notch signaling pathway.


Accumulating studies have demonstrated that FZDs can involve in the canonical or non‐canonical WNT pathway [24]. Notably, FZDs can also be intertwined with other signaling cascades. For example, FZD2 can function as a regulator in EMT and metastasis through Fyn/Stat3 pathway [10]. According to previous studies, FZDs can exert various biological functions in different human cancers. For example, FZD1 silencing induces a strong decrease of MDR1 expression to enhance drug resistance in neuroblastoma (NB) [25]. FZD3 protein expression has close association with the progression of colorectal cancer [26]. High FZD8 expression in human BC tissues is correlated with lymph node metastasis [27]. In our study, FZD2 was identified to be expressed in BC tissues at a significant high level. Importantly, we analyzed the significant correlation between high FZD2 level and the poor prognosis of BC patients. Functionally, FZD2 knockdown led to the inhibition on BC cell growth. Furthermore, silencing of FZD2 had suppressive effects on both migration and invasion of BC cells. Therefore, we confirmed that FZD2 exerts oncogenic role in BC.

Recently, studies have revealed the involvement of EMT in promoting the malignant development of human tumors [28-30]. To date, evidence has indicated that EMT phenomenon favors the distant metastases in epithelial cancers, including breast cancer [31]. FZDs have also been reported as regulators for EMT process in human cancers. For instance, FZD4 promotes the formation of EMT phenotypes in prostate cancer [32]. SOX8- and SOX9-induced upregulation of FZD7 regulates EMT in TSCC [33] and HCC [34]. In the current study, FZD2 was verified to be a positive regulator for the EMT process of BC cells. Accumulating studies have suggested that TGF-β promotes metastasis in human cancers, including BC [35]. In the present study, we determined that FZD2 positively regulated TGF-β1 expression in BC cells. Furthermore, TGF-β1 promoted EMT process in BC cells, whereas the effect could be reversed by the suppression of TGF-β1 using SB431542. Critically, the positive expression correlation between TGF-β1 and FZD2 was verified in clinical BC samples. Hereto, we confirmed that FZD2 induces EMT in BC in a TGF-β1-dependent manner.

            Notch signaling pathway plays an important role in several human malignancies [36]. In addition, Notch signaling pathway is also known as a modulator for EMT process in several different cancer types [21, 37]. In this work, we found that FZD2 may contribute to BC development by modulating Notch signaling pathway.

In conclusion, our findings revealed that FZD2 plays an oncogenic role in BC progression. Mechanistically, FZD2 regulates BC progression and EMT process through modulating TGF-β1 signaling and Notch signaling pathway. Our research findings may help to provide an effective therapeutic target for human BC patients.


BC: Breast cancer; EMT: epithelial-mesenchymal transition; FZD: frizzled class receptor; IHC: immunohistochemistry; TGF-β1: transforming growth factor-β.


Ethics approval and consent to participate

The tissues collected from patients and animal study was approved by the ethics committee of Jinling Hospital and conducted followed the protocol of the animal ethics committee of Jinling Hospital.


Consent for publication

Not applicable.


Availability of data and materials

The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request


Competing interest

The authors declare no conflict of interest.



This study was supported in part by grants from Science and Technology Program of Nanjing (201605003) and Military Construction Project of National Clinical key Specialty (2014ZDZK002).


Author Contributions

Conceptualization, Dilihumaer Tuluhong; Data curation, Dilihumaer Tuluhong; Formal analysis, Tao Chen and Jingjie Wang; Investigation, Jingjie Wang, Hanjun Li and Wangmu Dunzhu; Methodology, Huijuan Zeng; Resources, Huijuan Zeng; Software, Tao Chen; Supervision, Qiurong Li and Shaohua Wang; Writing – original draft, Dilihumaer Tuluhong.



All authors were major contributors in the writing of the manuscript. We are grateful to all researchers of enrolled studies.


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TABLE 1 Primer sequences for qPCR primers





























FZD2, frizzled class receptor 2; TGF-β1, transforming growth factor-β1.


TABLE 2 Primary antibodies for western blot analysis



Catalog Number






















































CST, Cell signaling technology; FZD2, frizzled class receptor 2; TGF-β1, transforming growth factor-β1.


TABLE 3 The association between FZD2 expression and clinical-pathological characteristics in breast cancers

Expression of FZD2
















Tumor Size(cm)











ER Status











PR Status











Her-2 Status











Ki-67 Status (%)











Pathological grading










TNM Stage











Lymph-node metastasis










Organ metastasis












*P < .05 was considered statistically significant.

Abbreviations: ER, estrogen receptor; FZD2, frizzled class receptor 2; Her-2, human epidermal growth factor receptor 2; PR, progesterone receptor.