1 Introduction
Hepatocellular carcinoma (HCC) is one of the most common malignancies and one of the most frequent causes of cancer-related death in the world [
1]. Despite significant improvements in both diagnostic and therapeutic modalities, metastasis is still a major contributor of treatment failure and death [
2‐
4]. The capacity of cancer cells to metastasize to distant sites is controlled by complicated cellular processes involving microenvironmental changes and increasing cell migration and invasion abilities, as well as multiple genetic events and alterations in regulatory factors [
5]. Nevertheless, despite the progress that has been made in elucidating the molecular mechanisms underlying HCC metastasis [
6,
7], its relapse rates remain high after resection, and the relapses nearly always originate from metastases [
8]. Failures to combat HCC invasion and metastasis have become major obstacles to improvements in the survival and quality of life of HCC patients [
9].
CD147, also known as extracellular matrix metalloproteinase inducer (EMMPRIN), is a multifunctional cell adhesion molecule that plays important roles in both normal physiological and pathological conditions, including reproduction, development, immunological responses, infectious diseases and malignant tumors [
10]. CD147 has been found to be overexpressed in a broad range of human malignant tumors including HCC [
11] and has been implicated in various aspects of tumor progression, in particular HCC metastasis [
12]. We and others have previously shown that CD147 can promote the migration, invasion, proliferation and survival of tumor cells [
13‐
17]. In addition, overexpression of CD147 in tumor cells as well as in serum has been recognized as an unfavourable prognostic factor [
18,
19].
CD147 functions through multiple molecular mechanisms. CD147 is best known as a potent inducer of extracellular matrix metalloproteinases (MMPs) and has been found to induce the production of MMPs by tumor cells as well as mesenchymal cells [
20]. CD147 has also been found to be involved in epithelial mesenchymal transition (EMT), cytoskeleton rearrangement and the formation of lamellipodia and invadopodia [
21,
22]. CD147 serves as a receptor for several molecules through trans-recognition using its extracellular Ig domain [
10]. A typical ligand for CD147 is Cyclophilin A (CyPA). CyPA is secreted by endothelial cells within the bone marrow and attracts myeloma cells that strongly express CD147 [
23]. As yet, however, it is far from clear how, upon receiving a signal through trans-recognition, this signal is transduced and whether the intracellular domain of CD147 is participating in this process.
Post-translational modifications (PTMs) such as glycosylation and phosphorylation can fine-tune the cellular functions of proteins. Uncovering the relationships between PTMs and functional changes is critical for our understanding of the molecular mechanisms underlying particular cellular processes. Previously, we found that CD147 purified from human lung cancer tissue was N-glycosylated and contained a series of high-mannose and complex-type N-linked glycan structures. Subsequent mutation analysis revealed that N-glycosylation was crucial for CD147 protein folding and MMP induction [
24]. Protein phosphorylation plays a central role in cellular signaling and is employed by cells to transiently alter protein localization, conformation and interaction with other proteins. When deregulated, it may also be involved in disease processes, notably cancer. Phosphoproteome analysis has revealed that CD147 can be phosphorylated at Ser246 and/or Ser252 in various human tissues and its derived cell lines, including those from muscle [
25], liver [
26,
27], B cell non-Hodgkin lymphoma [
28] and lung cancer [
29], indicating that phosphorylation is an important form of CD147 post translational modification. The function of CD147 phosphorylation in both normal physiological and pathological conditions is as yet, however, unknown.
Here, we show that CD147 is phosphorylated in primary HCC tissues and derived cell lines, with major phosphorylation sites at S246 and S252 in HCC tissues and Huh-7 cells, respectively. Abolishing CD147 phosphorylation by mutating S246 and S252 (S246A/S252A) led to expression alteration of a set of genes related to extracellular matrix (ECM) remodeling and cell migration and invasion enhancement via STAT3 and Akt signaling. Moreover, we found that the phosphorylation level of CD147 was dramatically decreased in HCCs with distant metastases and that low CD147 phosphorylation levels were associated with high serum AFP levels, disease recurrence and a poor overall survival, suggesting that the aberrantly hypo-phosphorylated form of CD147 may serve as a valuable biomarker for prognosis assessment and the development of novel therapeutic modalities directed against HCC metastasis.
2 Materials and methods
2.1 Reagents
TRIzol was purchased from Sigma (St Louis, Missouri, USA). Antibodies directed against phospho-serine (ab9392) and α-tubulin (ab80779) were purchased from Abcam (Cambridge, MA, USA); Antibodies directed against HA tag (0906–1), STAT3 (ET1605–45), p-STAT3 (ET1603–40), Akt (ET1609–47), p-Akt (ET1607–73), c-Jun (ET1608–3), p-c-Jun (ET1608–4) and p38 (ET1602–26) were obtain from HuaAn Biotechnology (Hangzhou, China); goat anti-mouse IgG antibody (31430), goat anti‐rabbit IgG antibody (31460), anti‐p‐p38 antibody (44‐684G) and anti‐NEK6 antibody (MA5–24947) were purchased from Thermo Fisher Scientifc (Waltham, MA, USA); CD147-specific antibody was produced by our lab.
2.2 Cell lines and culture conditions
Human hepatocellular carcinoma (HCC) cell line Huh-7 was obtained from the Japanese Collection of Research Bioresources (JCRB, Osaka, Japan). HepG2 and SMMC-7721 cells were obtained from the Chinese Academy of Medical Sciences (Shanghai, China). A Huh-7 CD147-KO (Huh-7 CD147
−/−) cell line was generated using a CRISPR/Cas9 system as previously reported [
14]. All cells were cultured at 37 °C, 5% CO
2, in RPMI-1640 medium supplemented with 10% fetal bovine serum (FBS). All cells have been authenticated using short tandem repeat profiling.
2.3 Tissue specimens and immunocytochemistry
76 HCC tissue specimens were collected from the Department of Pathology, Eastern Hepatobiliary Surgery Hospital, which is affiliated with the Second Military Medical University, from 2008 to 2012 and were histological confirmed by staining with hematoxylin and eosin (HE). All patients provided written informed consent, and the study was approved by the hospital Ethics Committee
Immunohistochemical (IHC) staining was performed on 5 μm tissue sections. To this end, paraffin sections were dewaxed, followed by antigen retrieval with 10 μM citrate buffer at pH 6.0. The deparaffinized sections were treated with methanol containing 3% hydrogen peroxide for 15 min. After washing with PBS, the sections were incubated with blocking serum for 30 min. Then, the sections were incubated with anti-CD147 antibody at 4 °C overnight. Following incubation, immunoperoxidase staining was conducted using a streptavidin-peroxidase kit (Zhongshan Jinqiao Co., Beijing, China) and the sections were treated with 3,3′-diaminobenzidine (Zhongshan Jinqiao Co., Beijing, China) to detect the target proteins. Hematoxylin was used to counterstain the nuclei. The expression levels were independently evaluated by two senior pathologists according to the proportion and intensity of positive cells. The following criteria were used to score each specimen: 0 (no staining), 1 (any percentage with weak intensity or < 30% with intermediate intensity), 2 (> 30% with intermediate intensity or < 50% with strong intensity) or 3 (> 50% with strong intensity).
2.4 Immunofluorescence assay
Immunofluorescence was performed as described previously [
13]. Briefly, cells were harvested and allowed to attach for 24 h to fibronectin pre-coated cell culture dishes with glass bottoms (NEST Biotechnology Co., LTD.). After washing twice with PBS, the cells were fixed with paraformaldehyde in PBS, permeabilized with 0.1% Triton X-100, and blocked with 1% BSA in PBS for 1 h. The resulting cells were first incubated with the indicated antibodies for 1 h, washed twice with PBS, and then incubated with Alexa 488-phalloidin solution and the corresponding FITC-conjugated secondary antibodies for 30 min in the dark. Cell nuclei were stained with DAPI (Vector Labs). After washing, the cells were visualized using an A1R-A1 confocal laser microscope (Nikon, Japan).
2.5 In situ proximity ligation assay
In situ proximity ligation assay (PLA) experiments were performed using reagents and instructions provided by a commercially available kit (Duolink In Situ Detection Reagents Red) from Sigma-Aldrich (St Louis, MI, USA). Briefly, cells were seeded into dishes with glass bottoms (NEST Biotechnology Co., LTD.). After washing twice with PBS, the cells were fixed in paraformaldehyde for 15 min at room temperature and blocked with Blocking Solution for 1 h at 37 °C. Next, the cells were rinsed twice with PBS/0.1% Tween 20 (PBST) after which primary mouse anti-CD147 (HAb18, prepared by our laboratory, 1:1000) and rabbit anti-phospho-serine (Abcam, ab9392, 1:100) antibodies in Duolink In Situ Antibody Diluent were applied and incubated overnight at 4 °C. Next, the cells were rinsed three times with Wash Buffer A. Secondary probes (anti-mouse-PLUS and anti-rabbit-MINUS, conjugated to oligonucleotides) were diluted to final concentrations of 1:5 in antibody diluent. The secondary probe mix was added to each sample, incubated for 1 h at 37 °C and washed with Wash Buffer A, after which 40 μl ligation solution was added. The dishes were incubated for 30 min at 37 °C. After washing with Wash Buffer A, 40 μl amplification solution was added and incubated for 100 min at 37 °C. Next, the cells were rinsed three times with Wash Buffer B. Before detection, 50 μl Duolink In Situ Mounting Medium with DAPI was added to each sample. Images were captured using a fluorescence microscope, and PLA signals were analyzed using the Duolink ImageTool software (Sigma-Aldrich). Formalin-fixed paraffin-embedded (FFPE) HCC tissue samples were prepared as described under 2.3 prior to the blocking step. Next, the assay was performed according to the manufacturer’s protocol.
2.6 Transfection and generation of stable cell lines
One day prior to transfection, 4 × 105 cells were seeded per well in a 12-well plate in complete medium. Subsequent transfection was carried out using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. After transfection, the cells were subjected to selection in 100 μg/ml G418 for 2 weeks. Antibiotic resistant colonies were subsequently picked, pooled, and expanded for further analysis under selective conditions.
2.7 Co-immunoprecipitation assay
Co-immunoprecipitation (co-IP) was performed using a Pierce™ Co-IP Kit (Thermo Fisher Scientifc, MA, USA) according to the manufacturer’s protocol. For each co-IP assay 10 μg affinity-purified antibody was used. Cell lysates were incubated with gentle rocking overnight at 4 °C. The eluted samples were analyzed by Western blotting using antibodies as indicated.
2.8 Western blotting
Western blotting was performed as described previously [
13]. Briefly, equal amounts of protein were separated by denaturing SDS-PAGE and transfered to polyvinylidene fluoride (PVDF) microporous membranes (Millipore, Boston, MA). Next, the resulting blots were blocked with 5% nonfat milk in TBS/0.5‰ Tween (TBS-T). The primary antibodies were diluted in TBS-T, and the blots were incubated with these antibodies overnight at 4 °C followed by washing in TBS-T and incubation with HRP-conjugated secondary antibodies for 1 h at room temperature. Signal detection was conducted using a ChemiDoc™ Touch Imaging System and analyzed using Image Lab™ Software (Bio-Rad, CA, USA).
2.9 Scratch wound healing assay
In vitro scratch wound healing assays were performed as described previously [
13]. Briefly, 24 h after treatment, the cells were harvested, seeded in 12-well plates and grown until confluence. Next, a pipette was used to scratch (‘wound’) the monolayer after which the remaining cells were washed with serum-free medium. Subsequently, photomicrographs were taken at various time points.
2.10 Transwell invasion assay
Chambers with polycarbonate filters with a 8 μm nominal pore size (Millipore, Boston, MA) coated on the upper side with Matrigel (BD Bioscience, San Jose, CA) were used to assess cell invasiveness. The chambers were placed into a 24-well plate. Cells were trypsinized, resuspended in serum-free medium, and seeded at a density of 1 × 105 cells per well in the upper chambers. The lower chambers were filled with 500 μl RPMI-1640 containing 10% FBS. After 24 h, the chambers were moved to a fresh 24-well plate and stained with 0.2% crystal violet for 20 min. The number of cells that had attached to the lower surface was counted under a light microscope and statistically analyzed.
2.11 RNA interference assay
Cells were transfected with siRNAs using Lipofectamine 2000 (Invitrogen, Carlsbad, CA, USA) according to the manufacturer’s instructions. siRNAs targeting NEK6 were designed and synthesized by Shanghai GenePharma Co. (Shanghai, China). The siRNA sequences used are depicted in Table
S1.
2.12 Apoptosis and cell cycle assays
The apoptosis and cell cycle rates of Huh-7 CD147-KO cells transfected with wildtype CD147 or S246A/S252A mutants were assessed using an Annexin V-FITC/propidium iodide (PI) apoptosis detection kit and a cell cycle detection kit, respectively (KeyGEN Biotech, Nanjing, China). Quantification of PI and FITC signals was performed using a fluorescence activated cell sorter FACSAria (BD Bioscience, San Jose, CA) system.
2.13 qRT-PCR
Total RNA was extracted using TRIzol reagents (OMEGA Bio-Tek, Norcross, GA, USA). Reverse transcription was performed using a PrimeScript RT reagent kit (TaKaRa Biotechnology, Japan). All primers were synthesized by BGI (BGI, Shenzhen, China) and their sequences are listed in Table
S1. Quantitative real-time PCR (qRT-PCR) was performed using a SYBR Premix Ex Taq II Kit (TaKaRa Biotechnology, Japan).
2.14 RNA sequencing
Total RNA was extracted using Trizol (Tiangen, Beijing) and assessed for quantity and quality using an Agilent 2100 BioAnalyzer (Agilent Technologies, Santa Clara, CA, USA) and a Qubit Fluorometer (Invitrogen, Carlsbad, CA, USA), respectively. Total RNA samples that met the following requirements were used in subsequent experiments: RNA integrity number (RIN) > 7.0 and a 28S:18S ratio > 1.8. Sequence libraries were generated and sequenced by CapitalBio Technology (Beijing, China). A NEB Next Ultra RNA Library Prep Kit for Illumina (NEB, Ipswich, MA, USA) was used to construct the libraries for sequencing. A NEB Next Poly(A) mRNA Magnetic Isolation Module (NEB, Ipswich, MA, USA) kit was used to enrich poly(A) tailed mRNA molecules from 1 μg total RNA. The mRNA was fragmented into ∼200 base pair pieces. First-strand cDNA was synthesized from the mRNA fragments using reverse transcriptase and random hexamer primers, after which second-strand cDNA was synthesized using DNA polymerase I and RNase H. The ends of the cDNA fragments were subjected to an end repair process including the addition of a single “A” base, followed by ligation of adapters. The resulting products were purified and enriched by PCR to amplify the library DNA. The final libraries were quantified using a KAPA Library Quantification kit (KAPA Biosystems, South Africa) and an Agilent 2100 Bioanalyzer. After qRT-PCR validation, the libraries were subjected to paired-end sequencing with a pair-end 150-base pair reading length on an Illumina HiSeq sequencer (Illumina) [
30].
The sequencing quality was assessed using FastQC (Version 0.11.5) after which low quality data were filtered using NGSQC (v0.4). The clean reads were subsequently aligned to the reference genome using HISAT2 (Johns Hopkins University, USA) with default parameters [
31]. The processed reads from each sample were aligned using HISAT (Johns Hopkins University, USA) against the corresponding human reference genome. Gene expression analyses were performed using Cuffquant and Cuffnorm (Cufflinks 2.2.1). Cuffdiff was used to analyze the differentially expressed genes (DEGs) between samples. The standardization method of Cuffdiff is geometric, with per-condition and pooled as discrete model [
32]. Thousands independent statistical hypothesis-driven tests were conducted on DEGs, separately, after which a
p value was obtained that was corrected using a FDR method. This
p value was used to perform significance analysis. The parameters for classifying significantly DEGs was ≥ 2-fold difference (|log2FC| ≥ 1, FC: fold change of expression) in transcript abundance. By searching the ENSEMBL, NCBI, UniProt, GO and KEGG databases, BLAST (Basic Local Alignment Search Tool) alignment was performed to determine the functional annotation of the DEGs. The best matches were selected to annotate the DEGs. Finally, a KEGG pathway enrichment analysis was performed for the DEGs using KOBAS 3.0 software (Available online:
http://kobas.cbi.pku.edu.cn).
2.15 Statistical analysis
All experiments were performed in triplicate, and the results were expressed as mean ± SD. Statistics were evaluated using GraphPad Prism V7.0 software (GraphPad Software, La Jolla, CA). The statistical analyses were carried out using one-way ANOVA (multiple comparisons) and Student’s t test (two comparisons). Differences were deemed significant when p < 0.05. *** indicates p < 0.001, ** indicates p < 0.01, * indicates p < 0.05, and # indicates p > 0.05.
4 Discussion
Phosphorylation is an important form of post-translational modification (PTM) [
46]. Here, we uncovered a novel functional phospho-modification of the CD147 protein and confirmed its relevance for regulating its activity. Consistent with previous phospho-proteome work on other human tissues and cell lines, we show that CD147 may exhibit various degrees of phosphorylation in primary HCC tissues and derived cell lines, indicating that phosphorylation represents, next to glycosylation, a form of PTM for CD147 in these tissues and cells. Further analyses revealed that the level of phospho-CD147 in patients without metastases was higher than in those with distant metastases, indicating that low phospho-CD147 levels are relatively more potent in promoting metastasis in HCC.
CD147 is reported to exhibit two forms of PTM, glycosylation and phosphorylation. Mass spectrometry-based structural determination of N-glycans of CD147 purified from human lung cancer tissue has shown that native eukaryotic CD147 is N-glycosylated and contains a series of high-mannose and complex-type N-linked glycan structures. It has been found that glycosylated CD147 stimulates the secretion of MMPs more efficiently than non-glycosylated CD147 [
47]. Mutation analysis revealed that glycosylation at asparagine-152 is important for proper CD147 protein folding in the ER and for its stability, thereby reinforcing HCC metastasis [
48]. Here, we report that, in contrast to glycosylation, phosphorylation does not affect CD147 protein folding, nor its stability and/or translocation to the membrane. In addition, we found that phosphorylation acts as a dominant negative modification of CD147, which attenuates its promoting effects on HCC tumor cell migration and invasion, indicating that the non-phosphorylated form of CD147 is the active form that is related to tumor progression.
The transmembrane protein CD147 has a long extracellular domain and a short intracellular domain. Several studies have emphasized the critical role of the ectodomain of CD147 in promoting tumor cell proliferation, migration, invasion and metastasis, mainly by interacting with other transmembrane proteins such as integrins [
49,
50], CD44 [
51] and CD98 [
52]. Previously, we found that both the ectodomain and intracellular domain of CD147 are required for mediating the effect of CD147 on the induction of MPPs and metastasis-related processes [
53]. The potential effect of CD147 on metastasis-related processes such as adhesion and invasion in HCC cells was found to be abolished in cells expressing CD147 with a truncated C-terminal fragment. Here we provide evidence that removal of phosphorylation of the intracellular domain of CD147 enhances Akt and STAT3 signaling, which may contribute to accelerated HCC cell migration and invasion. Together, these results underscore the functional importance of the intracellular domain of CD147. The exact molecular mechanism by which the intracellular domain of CD147 affects HCC metastasis, however, still needs to be determined.
NEK6 is a serine/threonine kinase that has been identified as a homologue of the
Aspergillus nidulans protein NIMA (never in mitosis A). NEK6 increases in abundance and activity during mitosis and its activation requires phosphorylation of Serine-206 at its activation loop [
54]. Despite the critical role of NEK6 in maintaining proper anaphase progression [
55,
56], the substrates of NEK6 have remained largely undefined. Here, we found that NEK6 interacts with and phosphorylates CD147. Accordingly, we found that NEK6 silencing leads to decreased CD147 phosphorylation in Huh-7 cells, indicating that CD147 is a substrate of NEK6. On the other hand we found that CyPA, a classical ligand for CD147, binds to its extracellular domain and, by doing so, reduces its phosphorylation. Whether CD147 phosphorylation is involved in NEK6 or CyPA regulated cell functions still needs to be determined, but the evidence presented here indicates that CD147 phosphorylation is regulatory and may be involved in downstream signal transduction.
In summary, our data provide evidence for a critical role of hypo-phosphorylated CD147 in HCC progression and imply that hypo-phosphorylated CD147 may serve as a valuable prognostic biomarker and as a target for the development of novel therapeutic modalities directed against HCC metastasis.
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