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Introduction

Breast cancer is a malignant tumor with a high mortality rate (1), which can be attributed primarily to invasion and metastasis. One of the primary approaches to treating breast cancer metastasis has been the development of effective anti-invasive agents (2,3). The initial steps of metastasis include cellular invasion through the degradation of the extracellular matrix (ECM), followed by the migration of cancer cells to other organs through the surrounding tissues (4,5). The ECM comprises collagens, laminins, glycoproteins, proteoglycans/glycosaminoglycans and fibronectin (6). It is degraded by extracellular proteases, of which MMPs play an important role in breast cancer (4,5).

Matriptases are members of the type II transmembrane serine protease (TTSP) family and are expressed in the epithelial compartments of all tissue types (7). Matriptase dysregulation is involved in various epithelial carcinomas, such as breast, prostate, colon, ovarian, uterine, cervical and skin cancers, where it is reportedly upregulated (8,9). Matriptase was first discovered in breast cancer cell lines, where it is highly expressed (10,11); however, despite its importance in breast cancer (12,13), the mechanism underlying its effects on breast cancer metastasis is unclear.

Protease-activated receptor-2 (PAR-2), a G protein-coupled receptor 11, induces various intracellular signaling pathways by activating endogenous serine proteinases, including matriptase (14–17). Previous studies have shown that matriptases are important activators of PAR-2 (16,18). When PAR-2 binds to a G protein, it produces diacylglycerol (DAG) and activates canonical phospholipase C (PLC)/Ca2+/protein kinase C (PKC) signaling or extracellular signal-regulated kinase-1/2 (19–21). Furthermore, PAR-2 levels are elevated in breast cancer, which plays a key role in regulating cellular migration by MAPKs (22,23).

MMPs are a family of zinc-dependent endopeptidases that consist of six subclasses: Collagenases, stromelysins, gelatinases, matrilysins, membrane-associated MMP and other MMPs (24). MMP-9 is involved in cancer cell infiltration and is directly associated with poor patient prognosis and the metastasis of breast cancer (25,26). Therefore, the regulation of signaling pathways to inhibit MMP-9 expression may play an important role in the treatment of various malignancies, including breast cancer (27–31). The expression of MMP-9 is induced by various stimuli, including cytokines, growth factors and 12-O-tetradecanoylphorbol-13-acetate (TPA) [10] (32–36). In particular, TPA is known to stimulate MMP expression by activating PKC in breast cancer cells (27,28,33). Furthermore, several studies have indicated that TPA activates PKC by activating PLC. In breast cancer invasion, TPA-induced MMP-9 expression is known to be induced by activation of NF-κB and activator protein-1 (AP-1) (37,38), transcription factors whose expression is regulated by MAPKs (39,40).

In the present study, the regulatory role of matriptase in TPA-induced MMP-9 expression, as well as invasion and migration, were investigated using MCF-7 breast cancer cells. Furthermore, to confirm the signaling mechanism of matriptase, the association between PAR2 and PLC/PKC was investigated. These results may provide a potential strategy for the treatment of breast cancer metastasis.

Materials and methods

Cell lines and culture

The human MCF-7 breast cancer cell line was purchased from the American Type Culture Collection. The cells were cultured in high-glucose Dulbecco's Modified Eagle's Medium (DMEM) containing 10% fetal bovine serum (Gibco; Thermo Fisher Scientific, Inc.) and 1% antibiotics (antibiotic-antimycotic, 100X; Gibco; Thermo Fisher Scientific, Inc.), and maintained in a humidified incubator at 37°C (5% CO2).

Reagents

TPA (cat. no. P1585) and DMSO were obtained from Sigma-Aldrich (Merck KGaA). Matrigel was acquired from Corning, Inc. The PAR-2 antagonist (GB83) was purchased from Axon Medchem LLC, and BAPTA-AM was obtained from Invitrogen (Thermo Fisher Scientific, Inc.).

Western blot analysis

MCF-7 cells (5×107) were transfected with matriptase siRNA for 24 h. Additionally, cells (7×105) were treated with BAPTA-AM or GB83 for 1 h, and then incubated with TPA for 24 h at 37°C. Total protein was extracted from cells using RIPA lysis buffer (Thermo Fisher Scientific, Inc.) containing protease and phosphatase inhibitors (Calbiochem; Merck KGaA). The lysates were centrifuged at 16,000 × g for 10 min at 4°C, and the protein concentrations were evaluated using the BioSpec-nano spectrophotometer (Shimadzu Corporation). The samples (20 µg) were separated by 10% SDS-PAGE and then transferred to Hybond™ polyvinylidene fluoride membranes (Cytiva). The membranes were blocked with 5% BSA (bovine serum albumin) or 5% skim milk buffers for 2 h at 4°C, and then incubated with the following primary antibodies (all 1:2,500) overnight at 4°C: Anti-β-actin (cat. no. A5441; Sigma-Aldrich; Merck KGaA); JNK (cat. no. 9252), p38 (cat. no. 9212), ERK (cat. no. 9102), IκB kinase α (IKKα; cat. no. 2682), IKKβ (cat. no. 2678), phosphorylated forms of PLCγ2 (cat. no. 3874), JNK (cat. no. 9251), p38 (cat. no. 9211), ERK (cat. no. 9101), c-Jun, IκBα (cat. no. 2859) and IKKαβ (cat. no. 2697) (all Cell Signaling Technology, Inc.). PLCγ2 (cat. no. SC-5283), p50 (cat. no. SC-7178), IκBα (cat. no. SC-371), MMP-9 (cat. no. SC-12759) and proliferating cell nuclear antigen (cat. no. SC-7907) (all Santa Cruz Biotechnology, Inc.). PKCα (cat. no. ab32376), PKCβ (cat. no. ab32026), PKCδ (cat. no. ab182126) and anti-sodium ATPase plasma membrane loading control (cat. no. ab76020) (all Abcam). Matriptase-specific antibodies were obtained from R&D Systems (cat. no. MAB3946). The blots were washed in TBS with 0.2% Tween-20 and then incubated with secondary HRP (horseradish peroxidase)-conjugated anti-mouse (cat. no. SC-2005) or anti-rabbit (cat. no. SC-2004) antibodies (1:2,500; both Santa Cruz Biotechnology, Inc.) for 1 h at 4°C. Immunoreactive bands were detected using Luminol HRP Substrate Reagent (EMD Millipore) with a Mini HD6 Image Analyzer and Alliance 1D (UVItec Cambridge; Cleaver Scientific Ltd.). Immunoreactive bands were quantified using ImageJ software (Version 1.53k; National Institutes of Health).

RNA isolation and reverse transcription-quantitative (RT-q) PCR

RT-qPCR was performed using the StepOnePlus™ Real-time PCR System and SYBR-Green PCR Master Mix (Applied Biosystems; Thermo Fisher Scientific, Inc.). Total RNA was isolated from MCF-7 cells using TRIzol® reagent (Invitrogen; Thermo Fisher Scientific, Inc.) according to the manufacturer's instructions. RNA concentration and purity were determined by absorbance at 260/280 nm. Complementary DNA was synthesized from 1 µg total RNA using the PrimeScript™ RT Reagent Kit (Takara Bio, Inc.) according to the manufacturer's instructions. The primers were: MMP-9 forward, 5′-CCTGGAGACCTGAGAACCAATCT-3′ and reverse, 5′-CCACCCGAGTGTAACCATAGC-3′; and GAPDH forward, 5′-ATGGAAATCCCATCACCATCTT-3′ and reverse, 5′-CGCCCCACTTGATTTTGG-3′. mRNA expression levels were normalized to those of GAPDH. The qPCR cycling conditions were as follows: Initial denaturation at 95°C for 10 min, 40 cycles of 95°C for 15 sec and 60°C for 1 min, followed by a melting curve ranging from 95°C for 15 sec, 60°C for 1 min, to 95°C for 15 sec. Relative quantitation was performed using the comparative 2−∆∆Cq method (41).

Small interfering RNA (siRNA) transfection and preparation of cytosolic and nuclear protein extracts

MCF-7 cells were transfected with 100 pmol matriptase siRNA or negative control siRNA (Shanghai GenePharma Co., Ltd.) using Lipofectamine® RNAiMAX reagent (Invitrogen; Thermo Fisher Scientific, Inc.) for 24 h at 37°C (5% CO2), and then incubated with TPA for 3 h at 37°C. The sequences of human siRNA were as follows: Matriptase siRNA, 5′-GUGUCCAGAAGGUCUUCAATT-3′ (sense) and 5′-UUGAAGACCUUCUGGACACTT (antisense); control siRNA, 5′-UUCUCCGAACGUGUCACGUTT-3′ (sense) and 5′-ACGUGACACGUUCGGAGAATT-3′ (antisense). Cytoplasmic and nuclear extracts were prepared from the cells using NE-PER Nuclear and Cytoplasmic Extraction Reagents (Thermo Fisher Scientific, Inc.) according to the manufacturer's protocol.

Dual-luciferase reporter assay

Cells transfected with matriptase siRNA were then transfected with the NF-κB/AP-1 luciferase reporter plasmid (Agilent Technologies, Inc.) using Lipofectamine® 2000 (Invitrogen; Thermo Fisher Scientific, Inc.) according to the manufacturer's instructions. At 24 h post transfection, the cells were treated with 20 nM TPA for 4 h at 37°C. Whole-cell lysates were prepared, and luciferase activity was measured using the Dual-luciferase Reporter Assay Kit (Promega Corporation) and Lumat LB 9507 Luminometer (Berthold Technologies GmbH & Co.KG). Relative Firefly luciferase activity was normalized to Renilla luciferase activity.

Membrane fractionation

MCF-7 cells (5×107) were transfected with matriptase siRNA for 24 h. Additionally, 7×105 cells were treated with BAPTA-AM or GB83 for 1 h and then incubated with TPA for 1 h at 37°C. The cells were mixed with homogenization buffer (20 mM Tris-HCl, 2 mM EDTA, 5 mM EGTA, 5 mM DTT and protease inhibitor; pH 7.5) and homogenized using a sonicator (5 times for 10 sec, each at 10% amplitude) and incubated on ice for 30 min. To separate the soluble (cytosolic) and pellet (membrane) fractions, the cell lysate was centrifuged at 16,000 × g for 15 min at 4°C. The pellet fraction was incubated in a solubilization buffer (homogenization buffer containing 1% NP-40) for 30 min on ice, and then centrifuged at 16,000 × g for 15 min at 4°C.

Cellular invasion and migration assays

The invasion assay was carried out in 24-well chambers (pore size, 8 µm) coated with 20 µl Matrigel (diluted in DMEM) for 30 min at 37°C; Matrigel Basement Membrane Matrix (Corning, Inc.) was rehydrated in 0.5 ml DMEM for 2 h immediately prior to experimentation. The top chamber was seeded with medium (0.5 ml; 10% FBS and 1% antibiotics) with 3×105 resuspended cells transfected with matriptase siRNA, while the lower chamber was filled with medium containing TPA alone or combined with GB83. A migration assay was performed using chambers without Matrigel. Cells transfected with control and matriptase siRNA were added to the upper chamber and medium with TPA alone or with GB83 was added to the bottom chamber. Cells were allowed to invade/migrate to the lower membrane for 24 h (37°C). After incubation, the cells on the upper membrane surface were removed with cotton swabs. The migrated/invasive cells were fixed with formaldehyde solution (3.6%) for 10 min, stained with crystal violet for 20 min (both at room temperature), and counted in five random fields per chamber at ×10 magnification, using a Leica DM ILLED inverted microscope (Leica Microsystems).

Statistical analysis

Data are presented as the mean ± SD of ≥3 independent experiments. Statistical analysis was performed using ANOVA with Scheffe's post hoc test (SAS software, version 9.3: SAS Institute Inc.), and P<0.05 was considered to indicate a statistically significant difference.

Results

Downregulation of matriptase suppresses TPA-induced MMP-9 expression in MCF-7 breast cancer cells

Western blotting and RT-qPCR were used to determine the effect of matriptase on TPA-induced MMP-9 expression. Intracellular matriptase expression was suppressed by transfection with matriptase siRNA (Fig. 1A), which inhibited the protein/mRNA levels of TPA-induced MMP-9 (Fig. 1B and C). These results suggested that matriptase was involved in TPA-induced MMP-9 expression.

Figure 1. Effect of matriptase on the TPA-mediated expression of MMP-9 in MCF-7 cells. (A) MCF-7 cells were transfected with control and matriptase siRNAs for 24 and 48 h, and matriptase expression was confirmed by western blot analysis. (B) Transfected MCF-7 cells were treated with TPA for 24 h, and the levels of MMP-9 protein expression were detected by western blotting. (C) Total RNA was isolated from MCF-7 cells and MMP-9 mRNA levels were detected using reverse transcription-quantitative PCR. Data represent the mean ± SEM of three independent experiments. #P<0.01 vs. untreated control; *P<0.01 vs. TPA. TPA, 12-O-tetradecanoylphorbol-13-acetate; siRNA, small interfering RNA.

Downregulation of matriptase reduces PLCγ2 phosphorylation in MCF-7 breast cancer cells

Activation of PKC isozymes is mediated by DAG and Ca2+ (42). Therefore, intracellular calcium levels are important for MMP-9 expression and cellular metastasis through TPA-mediated PKC activation. In the present study, it was confirmed that an intracellular calcium chelator (BAPTA-AM) inhibited TPA-induced PKC activation (Fig. 2A) and MMP-9 expression (Fig. 2B). In addition, matriptase-knockdown suppressed PLCγ2 phosphorylation (Fig. 2C) in MCF-7 cells. These findings indicated that intracellular calcium levels are important for PKC-mediated MMP-9 expression, and that matriptase downregulation inhibited MMP expression and metastatic ability by regulating PLCγ2-mediated intracellular calcium levels.

Figure 2. Effect of BAPTA-AM on TPA-induced PKC activation and MMP-9 expression, and the effect of matriptase on PLCγ2 phosphorylation, in MCF-7 cells. MCF-7 cells were treated with BAPTA-AM (20 µM) and TPA for (A) 40 min (to evaluate PKC activation) or (B) 24 h (to analyze MMP-9 expression). PKC isozyme and MMP-9 protein levels were detected by western blotting in MCF-7 cells. (C) MCF-7 cells were transfected with control and matriptase siRNAs for 24 h, after which PLCγ2 activation/levels of p-PLCγ2 were detected using western blotting. Data represent the mean ± SEM of three independent experiments. #P<0.01 vs. control siRNA-treated. TPA, 12-O-tetradecanoylphorbol-13-acetate; PKC, protein kinase C; siRNA, small interfering RNA; PLC, phospholipase C; p-, phosphorylated; Na K ATPase, sodium/potassium ATPase.

Matriptase regulates TPA-induced PKC activation, as well as the MAPK and IKK signaling pathways in MCF-7 breast cancer cells

Previous studies have shown that PKC and the MAPK and IKK signaling pathways are involved in the expression of MMP-9 induced by TPA (38). In the present study, the effects of matriptase on PKC and the MAPK and IKK signaling pathways were confirmed. To determine whether matriptase affects PKC activation in TPA-induced MCF-7 breast cancer cells, membrane translocation levels of PKCα, PKCβ and PKCδ were evaluated. As shown in Fig. 3A, matriptase-knockdown attenuated TPA-mediated PKC membrane translocation. Furthermore, matriptase-knockdown reduced MAPK phosphorylation (p38, ERK and JNK) at 30 min post-TPA treatment (Fig. 3B), confirming the effect of matriptase on MAPK activation by TPA. In addition, matriptase-knockdown suppressed p-IKKαβ and p-IκBα levels and the degradation of IκBα in the cytoplasmic fraction, which confirms the role of matriptase on the NF-κB signal transduction cascade (Fig. 3C). These findings suggested that matriptase is involved in the activation of PKC and the MAPK and IKK signaling pathways through TPA-induced expression of MMP-9 in MCF-7 breast cancer cells.

Figure 3. Effect of matriptase on TPA-induced PKC activation and MAPK signaling in MCF-7 cells. (A) MCF-7 cells were transfected with control and matriptase siRNAs for 24 h, followed by incubation with 20 nM TPA for 40 min. PKC isozyme levels in cell membrane fractions were analyzed by western blotting. (B) Transfected cells were treated with 20 nM TPA for 30 min and cell lysates were prepared for western blotting to evaluate the MAPK signaling pathway. (C) Additionally, transfected cells were treated with TPA, and after 4 h of incubation, cytoplasmic lysates were prepared; expression of upstream signaling molecules NF-κB, p-IκBα, IκBα, p-IKKαβ, IKKα and IKKβ were than analyzed via western blotting. TPA, 12-O-tetradecanoylphorbol-13-acetate; PKC, protein kinase C; siRNA, small interfering RNA; p-, phosphorylated; Na K ATPase, sodium/potassium ATPase.

Matriptase-knockdown decreases TPA-mediated activation of NF-κB and AP-1 in MCF-7 breast cancer cells

To elucidate the mechanism by which matriptase inhibits TPA-induced MMP-9 expression, the effect of matriptase siRNA on TPA-induced NF-κB and AP-1 activation was evaluated using a luciferase reporter assay. First, western blot analysis was used to confirm that matriptase-knockdown suppressed p50 levels in the nuclear fraction (Fig. 4A). Furthermore, TPA induced the phosphorylation of c-Jun, a major subunit of AP-1, and matriptase-knockdown inhibited the phosphorylation of c-Jun (Fig. 4A). Also, matriptase siRNA treatment inhibited TPA-stimulated NF-κB/AP-1 binding in a luciferase assay (Fig. 4B and C). These findings demonstrate that matriptase regulated the expression of MMP-9 induced by TPA through the NF-κB and AP-1 pathways in MCF-7 breast cancer cells.

Figure 4. Effect of matriptase on TPA-induced NF-κB and AP-1 activation in MCF-7 cells. (A) MCF-7 cells were transfected with control and matriptase siRNAs for 24 h, followed by incubation with 20 nM TPA. After 4 h of incubation, western blot analysis was performed to determine the nuclear levels of p50 and AP-1 (p-c-Jun) subunits. (B) NF-κB-Luc or (C) AP-1-Luc reporter and the Renilla luciferase thymidine kinase reporter vector were co-transfected into MCF-7 cells. Cells transfected with matriptase siRNA for 24 h with TPA and promoter activity of NF-κB and AP-1 was measured with dual-luciferase reporter assays. Data represent the mean ± SEM of three independent experiments. #P<0.01 vs. untreated control; *P<0.01 vs. TPA. TPA, 12-O-tetradecanoylphorbol-13-acetate; siRNA, small interfering RNA; p-, phosphorylated; PCNA, proliferating cell nuclear antigen; AP-1, activator protein-1.

Matriptase-knockdown inhibits TPA-mediated migration and invasiveness of MCF-7 breast cancer cells

In previous study, upregulation of MMP-9 has been associated with the induction of cancer cell metastasis, including breast cancer (43). Therefore, the inhibitory effect of matriptase siRNA on the metastatic efficacy of MCF-7 cells was investigated using invasion (Fig. 5A) and migration (Fig. 5B) assays. TPA-induced invasiveness and migration were significantly reduced in cells treated with matriptase siRNA, compared with control siRNA- and TPA-treated cells.

Figure 5. Effect of matriptase on the TPA-mediated invasiveness and migration of MCF-7 cells. (A) MCF-7 cells (3×105) were transfected with control and matriptase siRNAs, and seeded into the upper chamber of a Matrigel-coated Transwell insert; TPA was added to the lower chamber. (B) Transfected cell lysates were subjected to migration analyses. At 24 h post-incubation, invasive and migratory cells were stained and counted; magnification, ×10. Data represent the mean ± SEM of three independent experiments. #P<0.05 vs. untreated control; *P<0.05 vs. control + TPA. TPA, 12-O-tetradecanoylphorbol-13-acetate; siRNA, small interfering RNA.

Inhibition of PAR-2 suppresses TPA-induced PKC activation, MMP expression, as well as invasiveness and migration, in MCF-7 breast cancer cells

Matriptase induces PAR-2 activation (15); therefore, to investigate the effect of PAR-2-mediated breast cancer invasiveness, the effect of a PAR-2 inhibitor (GB83) on PKC activation and MMP-9 expression was evaluated in TPA-treated MCF-7 cells. GB83 (10 µM) was found to inhibit the expression of TPA-induced MMP-9 (Fig. 6A). It also attenuated TPA-mediated translocation of PKC to the membrane (Fig. 6B). Furthermore, invasion and migration assays revealed the inhibitory effect of GB83 on the metastatic properties of MCF-7 cells (Fig. 6C). These results suggested that PAR-2 was involved in PKC-mediated MMP-9 expression and metastatic ability in MCF-7 breast cancer cells.

Figure 6. Effect of the PAR-2 inhibitor (GB83) on TPA-induced PKC activation, MMP-9 expression and invasiveness/migration in MCF-7 cells. MCF-7 cells were treated with GB83 and TPA for 24 h (to analyze MMP-9 expression and invasion/migration) or 40 min (to evaluate PKC activation). Western blot analysis was used to detect (A) MMP-9 and (B) PKC isozyme levels in cell membrane fractions. (C) GB83 inhibited TPA-induced invasiveness and migration of MCF-7 cells. Data represent the mean ± SEM of three independent experiments. #P<0.05 vs. untreated control; *P<0.05 vs. control + TPA. TPA, 12-O-tetradecanoylphorbol-13-acetate; PKC, protein kinase C; Na K ATPase, sodium/potassium ATPase.

Discussion

Breast cancer is a malignant tumor and the leading cause of mortality in women worldwide (2). The majority of breast cancer deaths result from metastasis to the bone, lung, liver, brain and kidney (1). The molecular mechanisms underlying cancer cell invasiveness and migration are complex; the initial event that provides biochemical and mechanical barriers to cancer cell migration is the proteolytic degradation of the ECM (4,44), which requires the activation and expression of MMPs, known to play a major role in breast cancer (43,44). Among the MMPs, MMP-9 activation is associated with tumor progression and invasion (45,46). Therefore, inhibition of the regulatory pathway involved in MMP-9 expression may be an important therapeutic strategy for preventing breast cancer metastasis. In the present study, matriptase was proposed as a signaling protein for inhibiting cellular metastasis through the regulation of MMP-9. Matriptase was first reported in 1993 to have novel gelatinolytic activity in breast cancer cells (47). Matriptase is one of the most well studied members of the TTSP family and is expressed in the epithelial compartments of all tissue types (7,48,49), where its dysregulation is associated with numerous types of cancer and poor patient outcomes therein (8,9). Furthermore, several studies have demonstrated that matriptase is highly expressed in MCF-7 breast cancer cells (10,11); in particular, matriptase is upregulated in breast cancer and increases the proliferation and invasiveness of breast cancer cells (11,50). A previous study demonstrated that inhibiting matriptase suppresses breast cancer progression using in vivo, ex vivo and in vitro approaches (50). However, the role and signaling mechanisms of matriptase in breast cancer metastasis were previously unclear. Therefore, the aim of the present study was to identify the regulatory role of matriptase in TPA-induced MMP-9 expression and invasion/migration in MCF-7 breast cancer cells. The findings show that inhibition of matriptase expression inhibited TPA-induced increases in MMP-9 expression, cellular invasiveness and migration (Fig. 1 and 5).

The present study demonstrated the role of matriptase in breast cancer metastasis by identifying its effects on MCF-7 breast cancer cell invasiveness, as well as the underlying mechanisms. Matriptase mediates multiple intracellular signaling pathways by cleaving the activation site of PAR-2, a G protein-coupled receptor (16,17). PAR-2 signaling produces DAG and activates the PKC-mediated NF-κB signaling pathway (19,22). Furthermore, the binding of PAR-2 to G protein induces canonical PLC/Ca2+/PKC signaling (19). Moreover, activation of PAR-2-induced MAPK signaling plays an important role in regulating the migration of breast cancer cells (22,23). These findings suggest that the PAR2-mediated signaling pathway is important in breast cancer cell metastasis. The current study results confirmed that that inhibition of PAR-2 in MCF-7 cells suppressed PKC activation, MMP-9 expression and cellular invasiveness (Fig. 6). In addition, inhibition of matriptase regulated MMP-9 expression and invasiveness mediated by Par-2/PLCγ2/PKC or Par-2/MAPK.

The activation of PKC is highly associated with increased invasiveness in breast cancer (51). TPA increases the invasiveness of breast cancer cells by activating MMP-9 through PKC (52,53). TPA also activates novel (δ, ε, η and θ) and conventional (α, βI, βII and γ) PKC isozymes by binding the C1 domains of these isoforms (54). The effect of TPA is similar to that of DAG, a natural activator of the PKC isoform. TPA-mediated activation of PKC involves the translocation of PKC isoforms to the plasma membrane, resulting in modulation of gene expression, proliferation, apoptosis, differentiation and malignant transformation of cancer cells (54,55).

PLCγ2 is a member of the phosphoinositide-specific PLCs and enhances PKC activation by catalyzing the degradation of phosphatidylinositol-4,5-bisphosphate in DAG and inositol-3,4,5-trisphosphate (IP3). IP3 induces an increase in intracellular calcium levels (42,55,56). A variety of cell signaling pathways act downstream of PKC isozymes, such as those of Ras/Raf/MAPK, PI3K/Akt and the transcription factors NF-κB, AP-1 and STAT-3 (57).

Our previous study demonstrated that the activation of PKCα, PKCβ and PKCδ by TPA mediates the expression and secretion of MMP-9 (58). Therefore, the current study confirmed that intracellular calcium is required for TPA-induced PKC activation and MMP-9 expression and invasiveness (Fig. 2A and B). In addition, inhibiting matriptase expression was found to reduce the expression of p-PLCγ2 (Fig. 2C). Furthermore, the study revealed that inhibition of matriptase expression reduced the TPA-induced membrane localization of PKCα, PKCβ and PKCδ in MCF-7 cells (Fig. 3A). These findings indicate that inhibiting matriptase expression modulates PKC-mediated MMP-9 expression and metastasis in MCF-7 breast cancer cells by inhibiting PLCγ2 activation, and regulating PLCγ2-mediated calcium levels.

To investigate the TPA-induced PKC downstream signaling cascade for TPA-induced MMP-9 expression, the expression of three MAPKs, and the DNA binding capacity of transcription factors, were also investigated. These MAPKs (ERK, p38 and JNK) are upstream modulators of NF-κB, and activate MMP-9 expression (59). MAPKs are expressed in MCF-7 breast cancer cells and their activation can be confirmed by analyzing their phosphorylation (60). MAPKs are required for the activation of NF-κB and AP-1, which requires IκB kinase, MAPKs and PI3K/Akt, depending on the cell type in question (39,61,62). Herein, matriptase-knockdown suppressed the phosphorylation of p38, ERK, JNK and IKK following TPA treatment (Fig. 3B and C). NF-κB and AP-1 are important for the expression of MMP-9 in MCF-7 cells, and the MMP9 gene promoter contains NF-κB and AP-1 binding sites (63). The present study revealed that inhibition of matriptase expression inhibited TPA-induced MMP-9 expression by inhibiting NF-κB and AP-1 activation in MCF-7 breast cancer cells (Fig. 4).

The primarily aim of the present study was to identify the MMP-regulated signaling mechanism for the matriptase-induced inhibition of cellular metastatic capacity. Inhibition of matriptase expression was found to attenuate TPA-induced MMP-9 expression and invasiveness by blocking NF-κB and AP-1 activation through the PAR-2/PLCγ2 and PKC/MAPK signaling pathways in MCF-7 breast cancer cells. Therefore, to the best of our knowledge, the present study is the first to demonstrated that MCF-7 breast cancer cell invasiveness is mediated by inhibiting MMP-9 expression through modulation of the PAR-2/PLCγ2-mediated PKC signaling pathway, induced by matriptase. These findings suggest that inhibiting matriptase may have potential therapeutic value in the treatment of breast cancer metastasis. Furthermore, these findings are expected to pave the way for in vivo and clinical studies to determine the efficacy of matriptase in preventing breast cancer metastasis.

Acknowledgements

Not applicable.

Funding

The present study was supported by the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology, Republic of Korea (grant nos. 2013R1A1A1059747 and 2013R1A1A2007181).

Availability of data and materials

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

Authors' contributions

HJY, YRL and JP designed the study and confirmed the authenticity of all the raw data. JMK, EMN and HKS performed the experiments. SYK and JSK analyzed the data. SHJ contributed to data analysis and interpretation, and critically revised the manuscript. YRL drafted the manuscript. All authors have read and approved the final manuscript.

Ethics approval and consent to participate

Not applicable.

Patient consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.

References