Background
Hepatocellular carcinoma (HCC), one of the most commonly diagnosed human malignancies, is the third top cause of cancer-related mortalities worldwide [
1]. Despite continuous advances in diagnosis and treatments over decades, the prognosis of HCC patients remains poor due to rapid progression and high incidence of tumor recurrence [
2]. It is extremely urgent to illuminate the molecular mechanisms underlying HCC progression as well as identify effective therapeutic strategies against HCC progression.
γ-Secretase, a transmembranous multiprotein enzyme complex, consists of four core subunits: presenilin-1, nicastrin (NCSTN), anterior pharynxdefective phenotype-1 and the presenilin enhancer-2 [
3]. The γ-Secretase complex functions as a protease to cleave and activate various transmembrane proteins [
4]. Evidence is mounting that NCSTN is essential for the intracellular trafficking and stability of γ-Secretase, as well as recognition of γ-Secretase substrates [
5]. As a transmembrane protein, NCSTN contains extracellular and transmembrane domains, which are identified to be the functional sites for recruitment of γ-Secretase substrates [
6]. Previous studies have examined the upregulated expression level of NCSTN in breast cancer and demonstrated the oncogenic role of NCSTN in vivo and in vitro assays [
7,
8]. NCSTN overexpression regulated the properties of breast cancer stem cells and induced epithelial-mesenchymal transition (EMT) via cleavage of Notch1 [
8]. Besides, it has been revealed that NCSTN was correlative to response to chemotherapy for colon cancer [
9]. Additionally, the specific monoclonal antibodies against NCSTN were evaluated in cellular and pre-clinical assays, which suggested a promising role of targeting NCSTN for the treatment of invasive triple negative breast cancer [
10]. However, no study till now has examined the expression of NCSTN and its biological relevance in tumor progression in HCC.
In the present study, we found that NCSTN was up-regulated in HCC and could serve as an independent prognostic factor for HCC patients. By stable silencing and overexpression of NCSTN, we addressed the crucial effects of NCSTN on proliferative and invasive properties of HCC cells in vitro and vivo. We revealed that increased expression of NCSTN promoted the invasive capacity of HCC cells through activation of β-catenin, which subsequently induced EMT process. Further investigation demonstrated NCSTN-mediated intramembranous cleavage of Notch 1 and activation of AKT signaling as key regulators for malignant phenotype of HCC cells. Collectively, our findings indicate that NCSTN promotes HCC cell growth and metastasis through β-catenin activation in a Notch1/AKT dependent manner, and may be characterized as a promising target for HCC therapeutic strategies.
Materials and methods
Study population and specimens
A total of 245 samples were obtained from surgically treated HCC patients who had not receive preoperative treatments at West China Hospital (Chengdu, China) from 2009 to 2013. Of them, 108 samples contained HCC and paired adjacent tissues, were used for investigating the expression difference between tumor and adjacent tissues. The total 245 samples were used for quantification of NCSTN expression and analysis of their correlation with patient outcomes after hepatectomy. This study has been approved by the ethics committee of West China Hospital.
Tissue microarray and immunohistochemistry (IHC)
The tissue microarrays and IHC analysis were performed as we previously described [
11]. Two independent pathologists evaluated the staining. Quantitative analyses were performed by summing the staining intensity score (0, negative; 1, weak; 2, moderate; 3, strong) of ten randomly selected fields. The cut-off value for classification of patients was 150 points. The antibodies for IHC were shown in Table S1 (Additional file
1).
Western blotting (WB)
Tissues and cells were lysed in RIPA buffer, which was supplemented with protease and phosphatase inhibitor (Thermo Fisher Scientific, CA, USA), and then quantified by using BCA protein assay kit (Beyotime Biotechnology, China). The WB assay was conducted as we previously described [
12]. The intensity of signals was scanned using ChemiDoc MP Imager System (Bio-Rad, California, USA). The primary antibodies for WB were shown in Table S1 (Additional file
1).
The Cancer genome atlas (TCGA) data analysis
High-throughput RNA-sequencing data of 371 samples and 50 matched adjacent samples were downloaded from the TCGA dataset. Of them, 7 were confirmed as HCC plus intrahepatic cholangiocarcinoma (ICC) by histology and were excluded. Therefore, 364 HCC patients were finally included in this study (Additional file
3). The software of X-tile (version 3.6.0, Yale University School of Medicine) was used for identification of optimal cut-off value for NCSTN based on the association between mRNA expression level and overall survival (OS).
Cell culture and reagents
Human HCC cell lines Hep3B, Huh7, HepG2 and HCCLM3 were purchased from Cell Bank of Shanghai Institutes for Biological Science, Chinese Academy of Science (Shanghai, China), SNU449 and SNU387 were purchased from American Type Culture Collection (ATCC, Manassas, VA, USA). Cells were cultivated in recommended medium, supplemented with 10% FBS, 100 μg/mL streptomycin and 100 U/mL penicillin, at 37 °C with 5% CO2. The Notch inhibitor FLI-06 and AKT inhibitor MK-2206 2HCl were obtained from Selleck (Houston, TX, USA).
Cell transfection
Lentiviral vector containing human NCSTN sequence and lentiviral particles containing shNCSTN-1 and shNCSTN-2 were synthesized by GenePharma (Shanghai, China). The specific small interfering RNA (siRNA) oligonucleotides targeting β-catenin and Zeb1 were synthesised by Viewsolid Biotech (Beijing, China). 1 × 10
6 cells were seeded in 6-well plate and transfected with specific siRNA using GenmuteTM Reagent (SignaGen Laboratories, Maryland, USA) according to the manufacturer’s instructions, and then harvested 48 h post transfection. The sequences of siRNA and shRNA used in this study were showed in Table S3 (Additional file
1).
In vitro cell proliferation and cell cycle assays
2 × 103 cells within 100 μl medium were seeded in triplicate wells of 96-well plate and observed for 120 h. Each well was co-cultured with 10 μl Cell Counting Kit-8 (CCK-8) (Beyotime Biotechnology, Shanghai, China) and incubated at 37 °C for 2 h. The absorbance was detected at 450 nm by using the EonTM Microplate Reader (BioTek, VT, USA). 1 × 103 cells were seeded in triplicate wells of 6-well plate and cultured for 2 weeks. After fixed with 4% paraformaldehyde for 20 min, the colonies were stained using 0.1% crystal violet and counted. According to the manufacturer’s instructions of Cell Cycle Kit (4A Biotech, Beijing, China), cells were fixed using 95% cold ethanol, followed by incubation with propidium iodide at 37 °C for 30 min and analyzed by the CytoFLEX Research Flow Cytometer (Beckman Coulter, CA, USA).
In vitro wound-healing assay
Cells were plated in triplicate into 6-well plates. A standard 10 μl pipette tip was used to scratch wound when the cells reached a density of 95%. Subsequently, the cells were cultured in FBS-free medium. After 24 or 48 h, the wound closure was captured by a microscope and calculated using the software of Image J (National Institutes of Health, Bethesda, MD, USA).
In vitro transwell migration and matrigel invasion assay
Transwell chambers (8.0 μm pore size, Corning Costar, Kennebunk, USA) were applied in migration and matrigel invasion assays. For migration assay, 3 × 104 cells were suspended in FBS-free DMED medium and then seeded into upper chamber. For matrigel invasion assay, 5 × 104 cells were suspended in FBS-free medium and seeded into upper chamber with matrigel coating. DMEM medium containing 10% FBS were added to the lower chamber. After cultivation for 24 or 48 h, cells which migrated or invaded onto the lower surface of lower chambers were fixed using 4% paraformaldehyde, followed by stained using 0.1% crystal violet. Cells of 5 randomly selected fields were calculated and counted using Image J.
In vivo xenograft assay
The in vivo animal experiment was authorized by the Animal Ethic Review Committees of the West China Hospital. 5-week old male BALB/c nude mice were obtained from HFK BIOSCIENCE (Beijing, China) and used for in vivo assays. 1 × 106 cells (HCCLM3) or 2 × 106 cells (Hep3B) were suspended using 100 μl PBS and subsequently implanted subcutaneously into right axillas of mice (5 mice for each group). Measurement of tumor size was performed weekly by using the formula of length × width2 × 0.52. The mice were sacrificed at 4 or 6 weeks after implantation, followed by measure of tumor weight. For liver orthotopic-implanted HCC models, 1 × 106 cells were implanted into left lobe after mice were anesthetized. Mice were sacrificed 4 weeks after implantation and the livers were scanned using the IVIS@ Lumina II system (Caliper Life Sciences, Hopkinton, MA, USA). 2 × 106 cells were used for construction of lung metastasis models by injection into tail veins and the mice were sacrificed at the time of 6 weeks post implantation and the lungs were scanned using the IVIS@ Lumina II system. The metastatic foci were confirmed by hematoxylin and eosin staining.
RNA extraction and quantitative real-time PCR
Total RNAs of HCC cells were extracted by using Cell Total RNA Isolation Kit (Foregene, Chengdu, China) in accordance with the manufacturer’s instructions. The first-strand complementary DNA was synthesized using HiScript II Reverse Transcriptase (Vazyme, Nanjing, China). ChamQ™ SYBR@ qPCR Master Mix (Vazyme Biotech, Nanjing, China) was utilized for real-time PCR. All the reactions were performed in triplicate and the relative mRNA expression levels were quantitated using the 2
-ΔΔCT method. U6 was utilized as endogenous control. Primers used in this study were summarized in Table S2 (Additional file
1).
Immunofluorescence (IF) analysis
3 × 103 cells were plated on cover slips in 24-well plates. The cells were fixed using 4% paraformaldehyde at room temperature for 20 min, premeabilized using 0.2% Triton X-100 in PBS for 10 min and then blocked with 5% BSA at room temperature for 1 h, followed by incubation of primary antibodies at 4 °C overnight. Then the cells were washed using PBST (0.1% Tween-20 in PBS) for tree times and incubated with appropriate fluorophore-conjugated secondary antibodies (1:1000, Thermo Fisher Scientific, CA, USA). After incubation with DAPI (Kaiji, Nanjing, China), the cover slips were mounted on slides and scanned using AX10 imager A2 microscope (Carl Zeiss MicroImaging).
Subcellular protein fractionation
Cytoplasmic and nuclear protein fractions were extracted by using the NE-PER™ Nuclear and Cytoplasmic Extraction Reagents (Thermo Fisher Scientific, CA, USA) according to its protocol. The extracted protein samples were subsequently analyzed by WB assay. The β-Tubulin and Histone H3 were used as cytoplasmic and nuclear endogenous control, respectively.
Luciferase reporter assay
For TOP/FOP Flash assay, 1 × 104 cells were seeded in triplicate in 24-well plate 24 h prior to transfection. They were co-transfected with the pRL-CMV plasmid (Renilla luciferase, Promega) plus either TOP-Flash or FOP-Flash plasmid (Upstate) according to the instructions of Lipofectamine 3000 (Invitrogen, Carlsbad, CA, USA). Cells were cultured for 48 h and then the luciferase activity were evaluated using Duo-Luciferase HS Assay Kit (GeneCopoeia, CA, USA) in accordance with the manufacturer’s instructions. The results were shown as TOP/FOP Flash activity. For Zeb1 promoter activity luciferase reporter assay, pRP-Zeb1 were transfected into indicated cells. Firefly and renilla luciferase activities were detected after 48 h. The relative luciferase activity was shown as firefly/renilla luciferase activity.
Chromatin immunoprecipitation assay
Indicated cells were cross-linked using 1% paraformaldehyde and quenched by glycine solution [
13]. The chromatin immunoprecipitation (ChIP) assay was performed using the Pierce Magnetic ChIP Kit (Thermo Fisher Scientific), in accordance with the manufacturer’s instructions. Anti–β-catenin antibody and normal IgG (Millipore) were used for immunoprecipitation. ChIP-enriched DNA samples were quantified by real-time PCR to determine the special binding sites (BS) of the Zeb1 promoter region. The sequences of primers used for real-time PCR were summarized in Table S2 (Additional file
1).
Co-immunoprecipitation (co-IP)
Co-IP was conducted using Pierce Crosslink Magnetic IP/Co-IP Kit in accordance with the manufacturer’s instructions (Thermo Fisher Scientific, CA, USA). Briefly, cell lysates were incubated with protein A/G magnetic beads which were previously bind to primary antibodies, then dissociated the bound antigens from antibody-crosslinked beads by eluting in a low-pH elution buffer. The eluent was detected by WB analysis.
Statistical analysis
All the statistical analyses were performed by applying SPSS software (version 23.0, SPSS Inc., Chicago, IL, USA), GraphPad Prism software (version, 8.0La Jolla, CA, USA) and MedCalc software (version 15.2.2). Data was exhibited as mean ± standard deviation (SD). Student’s t-test was used to investigate continuous variables. Multiple hypothesis test was assessed by using Monte Carlo method. The Person χ2 test was applied to examine correlations. The survival curves were plotted using Kaplan-Meier method and tested by log-rank test. Subsequently, Cox proportional hazards regression model (enter method) was utilized to evaluation of potential independent prognostic factors. Potential confounders with P values less than 0.05 in univariate regression analyses were selected for multivariate regression models. A two-tailed P value < 0.05 was considered statistically significant.
Discussion
Accumulating studies have revealed that γ-Secretase played a vital role in tumorigenesis and metastasis of multiple malignancies [
17‐
19]. Recent clinical trials showed promising clinical benefits of γ-Secretase inhibitors in patients with advanced tumors [
20‐
22]. However, the γ-Secretase inhibitors inhibited the catalytic activity of Presenilin, which was responsible for several non-selective inhibition of the wide range of γ-Secretase substrates. Several phase I and II clinical trials have reported accompanying dose-limiting toxicity profiles of γ-Secretase inhibitors, primarily concerning gastrointestinal goblet cells hyperplasia [
23,
24]. Structurally, NCSTN is the only γ-Secretase component that contains a large extracellular domain, which holds the potential to serve as a drug target for monoclonal antibody (mAb) therapy. Filipovic et al. have developed mAbs against extracellular domain of NCSTN in invasive breast cancer cells, and demonstrated targeting NCSTN as a promising mode of treatment for those with few therapeutic options [
10]. Here we demonstrate that NCSTN is also a candidate therapeutic target for HCC.
In this study, we highlight the critical role of NCSTN, a core subunit of γ-Secretase, in HCC progression. Mounting evidence have revealed that EMT was of critical importance in initiation of metastasis and invasion, a binary process involving the conversion of tumor cells from epithelial to mesenchymal status, as well as acquisition of invasive capacity [
25,
26]. A series of transcriptional factors could drive EMT process and thereby tumor-initiation, metastasis, response to chemotherapy and immune evasion [
27,
28]. Our previous work has demonstrated Zeb1 as a vital activator in thioredoxin domain-containing protein 12-mediated EMT process of HCC [
29]. In the present work, we demonstrated that EMT process was dispensable for NCSTN-regulated HCC cell growth and metastasis, and Zeb1 was the key activator involved. In addition, our work also focused on the underlying molecular mechanisms of NCSTN-regulated EMT and aggressive features.
It has been widely reported that β-catenin signaling pathway played a crucial role in EMT process via nuclear translocation of β-catenin [
30,
31]. The dysregulation of β-catenin was involved in HCC progression. Additionally, CTNNB1 mutation was frequently detected in HCC tissues compared to those of adjacent liver [
32]. The data in this study identified NCSTN as the upstream regulator of β-catenin and promoted its nuclear translocation, thereby induced Zeb1-mediated EMT process, resulting in malignant phenotype. The activated β-catenin translocated into nuclear and formed a transcriptional activation complex with TCF4, then bound to Zeb1 promoter to activate its transcription [
33].
This work revealed that NCSTN overexpression activated Notch1, whereas its depletion achieved an opposite effect. The intracellular domain of Notch1 (N1ICD) was released from the membrane tether, and then induced AKT phosphorylation. These observations were consistent with the conclusions of previous studies that Notch signaling could induce an autocrine loop to activate AKT [
34]. The activated AKT promoted the phosphorylation of GSK-3β on Ser9 and thereby inactivated its activity. The inactivation of GSK-3β inhibited formation of GSK-3β/β-catenin complex and β-catenin degradation, resulting in nuclear translocation of β-catenin.
Acknowledgements
We are most grateful for Yan Wang, Jinkui Pi, Bo Su and Li Chai from Core Facility of West China Hospital, and Yang Yang, Xijing Yang from the Animal Experimental Center of West China Hospital for their technique support. We also thank Shiyang Zheng from the Third Affiliated Hospital of Guangzhou Medical University, for his assistance in diagram graphing.
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