Inhibitory effect of cathepsin K inhibitor (ODN‑MK‑0822) on invasion, migration and adhesion of human breast cancer cells in vitro

Yaongamphi Vashum1 · Riya Premsingh3 · Amuthavalli Kottaiswamy2 · Mathangi Soma2 · Abirami Padmanaban2 ·
Parkavi Kalaiselvan4 · Shila Samuel2

Received: 16 June 2020 / Accepted: 23 October 2020 © Springer Nature B.V. 2020

Abstract
Approximately 90% of patients with advanced breast cancer develop bone metastases; an event that results in severe decrease of quality of life and a drastic deterioration in prognosis. Therefore, to increase the survival of breast cancer patients, the development of new therapeutic strategies to impair metastatic process and skeletal complications is critical. Previous stud- ies on the role of cathepsin K (CTSK) in metastatic spreading led to several strategies for inhibition of this molecule such as MIV-711 (Medivir), balicatib and odanacatib (ODN) which were on trial in the past. The present study intended to assess the anti-metastatic efficacy of ODN in breast cancer cells. Human breast cancer cell lines MDA-MB-231 were treated with different concentrations of ODN and performed invasion, adhesion and migration assays and, RT-PCR and western blot to evaluate the effect of ODN on the metastatic potential of breast cancer cells. ODN markedly decreased wound healing cell migration, invasion and adhesion at a dose dependent manner. ODN inhibits cell invasion by decreasing the matrix metalloproteinase (MMP-9) with the upregulation of TIMP-1 expression. ODN effectively inhibited the phosphorylation of extracellular signal-regulated kinase (ERK), p38, and c-Jun N-terminal Kinase (JNK), and blocked the expression of β-integrins and FAK proteins. ODN also significantly inhibited PI3K downstream targets Rac1, Cdc42, paxillin and Src which are critical for cell adhesion, migration and cytoskeletal reorganization. ODN exerts anti-metastatic action through inhibition of signaling pathway for MMP-9, PI3K and MAPK. This indicates potential therapeutic effects of ODN in the treatment of metastatic breast cancer.

Keywords Cathepsin K · Odanacatib · Breast cancer · Migration · Invasion · Adhesion

 

Introduction

Metastatic breast cancer tends to spread throughout the body but mainly spreads to bone, lungs, liver, brain and regional lymph nodes. Approximately 70% of breast can- cer death is attributable to skeletal metastasis [1]. Although
the advanced therapy is available in the treatment of breast cancer, the incidence and mortality rates continue to rise due to low efficacy, severe side effects and lack of treatment access [2, 3]. Therefore, the alternate therapies are needed to increase the current therapies.
Cathepsin K, a cysteine protease with strong colla- genolytic and elastolytic activity produced by osteoclasts, appeared to be overexpressed in various types of cancers.

*
[email protected]
Data accumulated over the past few years on CTSK expres- sion in various cancers suggest both its critical role in

1Department of Biochemistry, Armed Forces Medical College, Pune, India
2Department of Biochemistry, VRR Institute of Biomedical Science (Affiliated to University of Madras), Chennai, India
3Department of Biochemistry and Chemical Biology, Indian Institute of Science Education & Research (IISER), Pune, India
4Department of Medicine and Surgery, Chettinad Hospital and Research Institute, Chennai, India
tumour progression and its potential diagnostic and prog- nostic effects. Previous studies have shown that the CTSK expression level is associated with higher breast cancer stages and negative estrogen receptor status, two features associated with poor prognosis in patients [4, 5] and lev- els of CTSK in bone metastasis are higher than in primary tumour or non-osseous metastasis of the same patient [6, 7].
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Hence, CTSK inhibition can be a viable anti-cancer strat- egy for the treatment and prevention of metastasis of breast cancer.
The metastatic process involves losing its adherence of cells from the basement membrane, thereby amplifying cell motility; invasion through extracellular matrix and adhesion of individual cancer cells to the epithelial tissue and base- ment membrane at the secondary site. Metastasis processes are mainly mediated by degradation functions of proteases. MMP-9, is strongly connected with destructive tumour and links with poor prognosis in various cancers [8]. Recent study indicates that CTSK cleaves and activates MMP-9 in an acidic environment [9]. Activated MMP-9 releases TGF-β in the latent state, allowing it to become active and initiates metastasis [10].
In the course of cell migration, Rac, Cdc42 and Rho GTPases members of the Rho subfamily of Ras-related (small) GTP-binding proteins were found to be greatly involved in the initial step of the process defining the site of formation of cell protrusion [11]. Other molecules important for cell polarization are phosphoinositide 3-kinases (PI3Ks) which regulate the level of phosphoinositides, PIP2 and PIP3 which serve to amplify small chemoattractant gradient face by the cell and to maintain positive feedback loops for the activity of Cdc42 and Rac GTPases [11]. It is well-docu- mented that PI3K signaling pathway is actively involved in the process of tumour migration and progression [12].
Cell adhesion, another critical component of metasta- sis process, is the process at which the, cell signal is ini- tiated allowing cells to maintain their immediate setting. Adhesion and spreading of the tumour cells are mediated by transmembrane proteins, mostly integrins. Integrins are α/β- two polypeptide chains membrane proteins that enable the anchorage of cells to the components of ECM [13]. The connection of cells to certain ECM proteins results in the group of integrin, with course arrangement of central attach- ment [14].
FAK (focal adhesion kinase) is a cytoplasmic tyrosine kinase which is crucial for integrin mediated signaling and different cell surface receptors signaling cascade. Binding of β subunit of integrins and FAK results in kinase autophos- phorylation at Y397 of FAK [15]. Subsequently, the src upon binding of its SH2 domain to FAK pY397 leads to its domain activation. This causes the phosphorylation of many parts of focal complexes like FAK, p130Cas and paxil- lin [16]. Activated paxillin binds to integrin β1 cytoplasmic tails, as well as the α4 cytoplasmic tail and forms paxillin/
FAK/Src complex that is crucial for focal adhesion [17]. The exact mechanism behind FAK over expression in cancer is still unknown. However, it is proven that the over expression of FAK facilitates cancer progression through the regulation of cell survival and proliferation [18].

Odanacatib (ODN) was an investigational agent previ- ously in the development for osteoporosis and bone metas- tasis. For this study, we have chosen ODN as it is considered to be a highly selective CTSK inhibitor [19]. Previous stud- ies have documented the inhibition of CTSK by ODN; it possesses variety of biological activities like anti-metastatic effects on oral squamous breast cancer [20], anti-parasite activities [21], anti-osteoporotic activities [22, 23], anti-ath- erosclerotic activities and anti-cardiomyocyte hypertrophy activities via inhibition of ERK1/2 signaling [24]. Addi- tional studies have shown ODN significantly reduced cell viability in various types of oral squamous cell carcinoma cell lines such as Ca9-22, OSC20, HSC2, HSC3, HSC4 and SAS and breast cancer, osteosarcoma and colon cancer cell lines [25, 26]. Despite mounting evidence of ODN as an anti-resorptive and anti-metastatic agent, the ability of ODN to inhibit the migration, invasion, and adhesion in metastatic cancer cells has not been reported, and little is known about the mechanism responsible for those effects. In this study, we studied the effect of ODN on the motility, invasion, and adhesion of the highly metastatic human breast cancer cell line MDA-MB-231, as well as explored the underlying molecular mechanisms.
Materials and methods

Reagents and antibodies

Primary antibodies for JNK, p-JNK, NFkB, p38, p-p38, MMP-9, p-ERK, Integrin β1, total Src-Tyr-416, FAK- Tyr-397, active FAK and GAPDH were obtained from Cell Signaling Tech. Inc. USA. Cathepsin K was purchased from Santa Cruz; USA. Secondary antibodies (anti-rabbit, anti- mouse and anti-goat) were purchased from Sigma-Aldrich Co, (USA). Odnacatib (Cat No. A3014, purity ≥ 95%) was obtained from Santa Cruz Biotechnology, Inc., USA.

Maintenance of cell lines

MDA-MB-231 cell lines were purchased from National Center for Cell Science (NCCS), Pune, India. The cells were maintained in plastic tissue culture T25 flasks in the presence of Dulbecco’s Modified Eagle’s Medium (DMEM) (Gibco, Grand Island, New York, USA) and grown in a humidified incubator at 37 °C with 5% CO2. The medium was supple- mented with 10% fetal bovine serum (FBS) (Gibco, Grand Island, New York, USA) and antibiotics (penicillin 50 IU/
mL, streptomycin 3.5 μg/mL and gentamycin 2.5 μg/mL) (Gibco, Grand Island, New York, USA).
Cell viability assay

Cells seeded in 96 well plates at a density of 1 × 104 cells/
well in 200 μL of DMEM media, were incubated with dif- ferent concentrations of ODN for 48 h in 96 well plates and were incubated at 37 °C in a humidified mixture of 5% CO2 and 95% air in an incubator. The cell proliferation was determined using stock solution of compounds which was initially dissolved in DMSO and further diluted with fresh complete medium. 25 µl of MTT reagent (5 mg/ml in PBS) was added to each well and incubated at 37 °C for 3 h. At the end of the incubation period, the supernatant was removed by tilting the plate completely without disturbing cell layer and added 150 µl of DMSO to each well. After 15 min of shaking, the readings were recorded at 590 nm on a micro- plate reader (Biotek, USA).

Transwell assay

Invasion and migration of MDA-MB-231 cells were deter- mined in transwell chambers (8 mm pore size) according to previous method with little modification [27]. The upper chambers of the transwell membranes were coated with matrigel (BD Biosciences) for the detection of migration/
invasion. The lower chamber was filled with 500 µl DMEM and 10% FBS. Cells were treated with various concentra- tions of ODN for 24 h. After treatment, the cells were sus- pended in serum-free media and reseeded at a density of 1 × 105 cells per well. After 24 h of incubation, the migrating cells on the underside of the filter were washed with PBS and fixed with 3.5% formaldehyde. Then, the cells were per- meabilized by 100% methanol for 20mins and methanol was removed by washing with PBS twice. The cells were then stained with 10% giemsa stain (pH 6.8) for 10 min. The cells from at least 5 separate random fields were counted under the microscope after washed with PBS twice.

Cell adhesion assay

Cell adhesion assay was performed as previously reported [28]. MDA-MB-231 cells with or without ODN were incu- bated for 24 h. In culture media, the cells were collected, counted, and resuspended. The cells were then transferred to a precoated 96-well platform matrigel (100 μg/ml). The medium was discarded after incubation at 37 °C for 4 h, and washed with PBS to eliminate the non-adherent cells. Some wells were then filled with 4% formaldehyde (w/v) and stained with 0.2% crystal violet and photographed. The other wells were quantified by MTT assay to determine the number of adherent cells.

Western blotting

The cells were treated with ODN (0–5.5 μM/mL), washed with cold PBS and lysed with a cold RIPA buffer and fresh protease inhibitor mixture (Cat No. P8340, Sigma Aldrich). The cell lysate was then centrifuged for 20 min at 4 °C at 14,000 rpm. The supernatant was obtained, and the concen- tration of the protein was calculated using BCA method (Cat No. 23227, Pierce. USA). The protein sample was denatured in sample buffer, then separated on 12% SDS-PAGE, and transferred to polyvinylidene difluoride membranes. The blots were later blocked with Tris-buffered saline contain- ing 5% of non-fat milk for 2 h at room temperature. The blots were then washed with TBST three times and incubated with 1:1000 dilution of specific primary antibodies over night at
4°C. The next day, the blot was incubated for an hour at room temperature with 1:1000 dilution of secondary anti- bodies. Later, the blot was analyzed with the ECL detection reagent and the study of densitometry was performed using ImageJ software. GAPDH was used as an internal control.

RNA extraction and RT‑PCR

Total RNA was isolated from the cells using Trizol reagent (Invitrogen, Carlsbad, CA, USA) according to the manufac- turer instructions. cDNA was synthesized from 1 µg of RNA using IScript cDNA Synthesis Kit (Bio-Rad Laboratories, Hercules, CA, USA). PCR amplifications were performed as follows: 35 cycles for MMP-9, TIMP-1 and CTSK (95 °C for 30 s, 58 °C for 2 min, and 72 °C for 1 min), 40 cycles for c-Fos, c-Jun (95 °C for 10 s, 55 °C for 30 s, and 72 °C for 30 s), 35 cycles for E-cadherin (94 °C for 5 min, 55 °C for 1 min, and 74 °C for 10 min) and 30 cycles for β-actin (94 °C for 30 s, 64 °C for 45 s, and 72 °C for 1 min). β-actin was used as an endogenous control. The PCR products were then processed and coated with ethidium bromide in 1% agarose gel. A 100 bp ladder (Fermentas) verified the size of the amplification materials. The relative levels of mRNA expres- sion were obtained by normalizing to the expression of β-actin. Both experiments were replicated 3 times. Primer sequences used for PCR were: β-actin Forward:5′TCACCC ACACTGTGCCCATCTACGA-3′Reverse:5′-CAGCGG AACCGCTCATTGCCAATGG-3, MMP-9 Forward:5′TTG ACAGCGACAAGAAGTGG-3, Reverse:5′-CCCTCAGTG AAGCGGTACAT-3, CTSK Forward: 5′-CCGCAGTAA TGACACCCTTT-3′ Reverse:5′-AAGGCATTGGTCATG TAGCC-3, c Fos Forward:5′-ATGATGTTCTCGGGTTTC AA-3′Reverse:5′-TGACATGGTCTTCACCACTC3, c Jun Forward:5′ACTCGGACCTTCTCACGTCG-3′ Reverse:5- TAGACCGGAGGCTCACTGTG-3, E cadherin Forward:5′- TAGACCGGAGGCTCACTGTG-3′ Reverse:5′- GGGTGT CGAGGGAAAAATAGG -3, TIMP 1 Forward:5′-ACAACC
GCAGCGAGGAGT-3′ Reverse:5′-AGGTGACGGGAC TGGAAGC-3′.

Statistical analysis

Data are presented as mean ± standard deviation (SD). Each value is the mean of at least three separate experiments. Dif- ferences between groups were assessed by one way ANOVA and Dunnett’s multiple comparison tests for intergroup com- parison using SPSS software. p < 0.05 was considered to be statistically significant.

Results

ODN decreases the cell viability of MDA‑MB‑231 cells

To determine the non-cytotoxic concentration of ODN, MDA-MB231 cells were exposed to various concentra- tions of ODN (0 to 100μM) for either 24 h or 48 h, and then the cell viability was evaluated using MTT assay (Fig. 1D). ODN did not show any cytotoxic effect on MDA- MB-231 cells within the range of 0.01 to 1 μM, but ODN slightly decreased the cell viability at higher concentrations (2.5–10 μM). Therefore, the concentrations of ODN lower than 5.5 μM were chosen and used in this study. A concen- tration range of ODN 1–5.5 μM was selected for all subse- quent experiments.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 1 Inhibition of Invasion, Migration and Adhesion by ODN in MDA-MB-231 cells. ODN inhibits metastasis of the MDA-MB-231 human breast cancer cells. A Boyden chamber transwell assay indi- cating that ODN inhibits the invasion of MDA-MB-231 cells. B Boyden chamber transwell assay indicating that ODN inhibits the migration of MDA-MB-231 cells. C Effects of ODN on adhesion of MDA-MB-231 cells to matrigel. Data are presented as Mean ± SD

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
of three independent experiments and expressed as percentage com- paring to the control. D Effect of ODN on the proliferation of MDA- MB-231 cells was assessed by MTT assay. E Effects of various ODN concentrations on CTSK protein expression. Cells were exposed to various concentrations of ODN. Data represented are Mean ± SD (n = 3)
Inhibition of CTSK by ODN suppresses invasion, migration and adhesion of MDA‑MB‑231 human breast cancer cell line

The present study indicates a dose-dependent effect of ODN on CTSK protein expression in breast cancer cells. As determined by blot, the cells treated with ODN showed a significant reduction in the expression level of CTSK com- pared to the control (Fig. 1E). The important characteristic of metastasis is the migratory and invasive ability of tumour cells. To further investigate the pharmacological activity of ODN against cancer metastasis, we first examined the effect of ODN on cell migration and invasion in MDA-MB-231 cells. Cells migrating to the underside in the invasion assay were reduced to 40% and 22% of the control (Fig. 1A) in the MDA-MB-231 cells treated with and without ODN, respec- tively. Cells that crossed the matrigel in the migration assay were similarly reduced to 12% and 7% of the control in the MDA-MB-231 cells treated with or without ODN respec- tively (Fig. 1B). The effect of ODN on cell adhesion to ECM gel component was examined. ODN suppressed the cancer

cell adhesion to ECM gel in a dose-dependent manner. For instance, at 5.5 μM, ODN inhibited 70% of adhesion com- pared to ECM gel control group (Fig. 1C).

ODN attenuates the gene expression of CTSK and MMP‑9 and upregulates TIMP‑1 expression

We further investigated the role of proteolytic enzymes in the cellular response to ODN. The mRNA levels of MMP-9, CTSK and TIMP-1 were evaluated in MDA-MB-231 cells. RT-PCR showed that the gene expression levels of CTSK and MMP-9 were decreased in a concentration-dependent manner by ODN treatment (p < 0.001; Fig. 2). The levels of TIMP-1 were increased, and they were significantly altered after ODN treatment (p < 0.001; Fig. 2).

ODN inhibits the expressions of AP‑1 subunits, c‑Fos and c‑Jun in MDA‑MB‑231 cells

To understand the possible mechanisms involved in MMP-9 inhibition action, we further examined the effect of ODN on

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Fig.2 Effects of ODN on mRNA expression of metastatic genes in MDA-MB-231 cells. RT-PCR analysis of metastatic genes, CTSK, MMP-9, TIMP-1, c-Jun, c-Fos and E-cadherin were determined and normalized to β-actin. Representative RT-PCR gel image and relative

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

densitometric analysis in histograms are shown. Triplicates of each treatment group were used in each independent experiment. Results are expressed as the Mean ± SD. *p < 0.05, #p < 0.01, $p < 0.001, ns non-significance compared with control
the AP-1 gene. According to previous studies, the promoter region of the MMP-9 gene facilitates binding sites for both nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) which are essential for MMP-9 expression [28, 29]. c-Jun is one of the main components of transcription factor AP-1 and also serves as the target for ERK, JNK and p38 kinase [15, 30]. Therefore, we next determined whether c-Jun and c-Fos mRNA could be inhibited by ODN. ODN significantly impaired c-Jun gene expression in a concentration-depend- ent manner (Fig. 2). As shown in Fig. 2, the incubation of MDA-MB-231 with various concentrations of ODN caused a dramatic reduction in the level of AP-1 subunit, c-Fos and c-Jun (p < 0.001) in a dose dependent manner.

ODN promotes cell‑cell aggregation and induces E‑cadherin upregulation in MDA‑MB‑231 cells

One important step in the development of various cancers is the loss of E-cadherin gene expression or inactivation of E-cadherin mediated cell-cell adhesion. Cathepsin K cleaves

and down regulates E-cadherin expression, which abrogates detaching cells and correlates with enhanced cell migration and invasiveness. The present study results indicated that ODN significantly increased the ability of cells to form cell aggregates visibly as early as 30 min post treatment (Fig not shown). We further examined the mRNA expres- sion of E-cadherin in MDA-MB-231 cells in response to ODN exposure. As shown in Fig. 2, ODN induced a signifi- cant increase on the mRNA expression level of E-cadherin (p < 0.001) when compared to untreated cells.

ODN inhibits the activation of MAPKs and NF‑κB pathway in MDA‑MB‑231 cells

NF-κB is another major transcription factor that regulates MMP-9 expression [29]. To understand the involvement of NFκB protein in the mechanism of ODN action, the can- cer cells were exposed to various concentrations of ODN. NFκB levels were gradually decreased in a concentra- tion-dependent manner (Fig. 3). Since mitogen-activated

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Fig. 3 ODN inhibits the activation of MAPKs and NF-κB pathway, Integrin β1-FAK expression in MDA-MB-231 cells. Cells were treated with varying concentrations of ODN. The expressions of JNK, pJNK, NFκB, MMP-9, p38, p-p38, and p-ERK are shown Inte- grin β1, total and active FAK (Tyr-397) in cell lysates were analyzed

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

by Immuno blotting. GAPDH was used as an internal control. Rep- resentative blot image and relative densitometric analysis in histo- grams are shown. Results are expressed as the Mean ± SD. *p < 0.05, #p < 0.01, $p < 0.001, nsnon-significance compared with control
protein kinases (MAPKs) are critical for the expression of MMPs, we assessed the effect of ODN on the MAPK signaling pathway, by exposing the cells to different con- centrations of ODN. ODN specifically inhibited the level of p38 (p < 0.001), p-ERK1/2 (p < 0.001) and p-JNK (p < 0.01) (Fig. 3) whereas, least or no significant differ- ence in the expression of JNK was observed as compared to untreated group. Thus, we suggest that the inhibition of MAPK signaling pathway is essential for ODN to shunt MMP-9 expression.
ODN inhibits integrin β1‑FAK expression

Integrins are the most prevalent and well characterized cell surface receptors of various proteins of the extracellular matrix (ECM) involved in cancer metastasis [15]. Interac- tion of β subunit of integrins and FAK, a key mediator of integrins signaling causes kinase autophosphorylation at Y397 of FAK where Src gets activated and phosphorylates

several components of focal complexes molecules; this activation promotes cell motility, survival and prolifera- tion facilitating the passage of the cancer cells to second- ary sites [30]. ODN significantly suppressed the activation of p-FAK (p < 0.001), FAK (p < 0.05) in a dose-dependent manner. ODN also significantly suppressed the activation of integrin β1 in a dose-dependent manner (p < 0.001) (Fig. 3).

Effect of ODN on the expression of Rho GTPase proteins

PI3K signaling downstream target proteins, Rac1 and Cdc42 are critical for modulating the cell dynamic structural changes during cell migration. Decrease in protein levels of Rac1 and Cdc42 was observed after ODN treatment which was statistically significant (Fig. 4) (p < 0.001). This finding strongly indicates that ODN inhibits cell migration through suppression of Rac1 and Cdc42 expression.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
Fig. 4 Effect of ODN on the expression of Rho GTPase proteins, paxicilin and src expression in MDA-MB-231 cells. Protein expres- sion of Rac1, Cdc42, paxicillin, total and active src on cells treated with ODN was analysed using western blot. GAPDH was used as an

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

internal control. Representative blot image and relative densitomet- ric analysis in histograms are shown. Results are expressed as the Mean ± SD. *p < 0.05, #p < 0.01, $p < 0.001, nsnon-significance com- pared with control
ODN inhibits paxicilin and src expression in MDA‑MB‑231 cells

The activated complex FAK/Src and the phosphorylated protein such as paxicilin at Tyr 118 are essential for cell adhesion, migration and cytoskeletal reorganization [20]. Using western blot analysis, we assessed the protein levels of paxillin and src. Our result showed that the levels of total paxillin and src after ODN treatment showed no significant changes; however, it was observed that ODN down regulated the expression of active p-src (p < 0.001) (Fig. 4).
Discussion

At the advanced stage, the breast cancer cells become more aggressive with rapid growth, migration and potential for invasion of the cells. Controlling or inhibiting this process may suppress cancer metastasis and improve treatment. ODN, and other CTSK inhibitor named as compounds 3, 4,
5and 21 have been shown to have anti-metastatic effects in human OSCC cell lines, HCT116 and 143B cancer cell lines [21, 26]. So far, the effects of CTSK inhibition on invasion, migration, and adhesion of oral squamous cell carcinoma cell lines have been recorded in only one study [15]. The molecular mechanisms behind its effects on cell migration, adhesion and the process of invasion have not been studied.
In this study, the effect of ODN on breast cancer and the mechanisms involved in inhibition of cancer metastasis process are investigated in MBA-MB-231 breast cancer cell lines in vitro. Our results revealed that the anti-metastatic activities of ODN were mediated through several processes. As shown in the Fig. 1E, the cells treated with ODN showed a significant reduction in the expression level of CTSK as compared to the control in a dose-dependent manner. ODN significantly suppressed the invasiveness of MBA-MB-231 cells by suppressing the expression level of MMP-9 through inhibiting AP-1 and NF-κB transcription factors (Fig. 2 and 3). It is a well-known fact that MMP-9 is critical for degra- dation of ECM for the tumour cell to invade [31]. Type IV collagen, which is the main component of basement mem- brane is degraded by the action of proteases such as MMP- 9, CTSK and it is considered as an ideal marker of tumour metastasis [32, 33].
The inhibitory effects of ODN on MMP-9 expression provide persuasive justification for its suppression of inva- siveness of MDA-MB-231 cells, which were determined in the cell invasion assay (Fig. 1A). It is considered that, at the transcriptional level, MMP-9 is totally regulated [34]. MMP-9 promoter region facilitates the binding site for NF-κB and two for AP-1 subunits which is essential for MMP-9 expression [35–37]. Any alteration in the promoter region can inhibit or diminish the expression of MMP-9 as

it is shown in the previous study on HepG 2 cells [38]. Our present study indicates that ODN inhibits the expression of MMP-9 even at the transcriptional level. The possible mechanism might be through the inhibition of transcription factor nuclear factor-κB (NF-κB) and activator protein-1 (AP-1) subunit, c-Jun and cFos as shown in Fig. 2 and 3. It is therefore assumed that the effect of ODN on MMP-9 might be possibly through the suppression of transcription factor NF-κB and two AP-1 subunits, c-Jun and c-Fos. This is a significant finding which may suggest that cancer cells, if demonstrated in vivo, may modulate their microenvi- ronment through interplay with CTSK-MMP-9. However, before drawing conclusions, whether ODN inhibition of MMP-9 can control tumour progression needs to be thor- oughly investigated in appropriate animal models.
In addition, the suppressing effects of ODN might be con- tributed to c-Jun phosphorylation MAPK protein, ERK, JNK and p38. The present study showed that CTSK inhibition by ODN could suppress the progression of cancer by inhibiting MAPK cascade. ODN suppressed the level of expression of MAPK protein, ERK, JNK and p38 so that no translocation of these proteins would occur as shown in Fig. 3.
The tumour metastasis and cancer progression involve the movement of cancer cells to secondary sites. Therefore, the effect of ODN on the migration of breast cancer has been further investigated. One intracellular signaling pathway involved in cellular regulation, such as cell growth and dif- ferentiation [41], is phosphoinositide 3-kinases (PI3K). The PI3K is well-established and known to involve in regulation of cell migration and invasion [40]. The activation of cell motility and invasion is mediated by up-regulation of PI3K and signaling proteins like Cdc42 and Rac1 [42]. These proteins are known to play a role in modulating cytoskel- etal reorganization, rearrangement and thus is critical for metastasis [43]. In the current study, we found that ODN inhibits cancer cell migration by suppressing PI3K activity and also by inactivating active complex formation of Cdc42/
Rac1-PAK that regulates cell cycle progression, migration, invasion of tumours and oncogenic transformation (Fig. 4). ODN inhibitory effect on PI3K activity and its downstream protein shows clearly the potential effect of ODN on MDA- MB-231 cell migration (Fig. 2). Adhesion to cells is yet another crucial step in the cycle of cancer metastasis [44]. Cell adhesion not only provides a cell’s structural dock, but also enables a significant indication of endurance signal for the cell [45]. Failure to adhere the cells at a new site could lead to immediate apoptosis initiation [46].
Integrin-ECM is essential to facilitate signal cascades for migration, adhesion, replication or apoptosis [14]. Thus, the fate of the migrated tumour cells can be regulated by the components of the ECM. Every attachment and spreading of integrin-mediated cells is essential for cell survival and loss of adhesion causes apoptosis [47]. An altered integrin
pattern helps the cancer cells to identify variable matrices, but it can also result in altered signaling and gene expres- sion changes [48]. Previous studies have shown that the over expression of β1-integrin is associated with low survival in patients with breast cancer and functions as a progression marker for cancer [48]. In the present study, it was observed that the inhibitory effect of ODN on cell adhesion is due to its suppressive effect on the integrin molecules and its down stream targets (Fig. 3).
Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase that controls the process of cell adhesion mediated by the integrin [49]. Together with src and paxillin, it transmits extracellular signals from integrin to extracellular compart- ment by contacting several adaptor proteins [50]. This FAK scaffolding role is necessary for cell motility and hence the increased expression of FAK is linked to the progression and metastasis of cancer [51].
In addition, the auto-phosphorylation mediates the com- plete contact between FAK and Src at FAK Y397. The inter- action results in the formation of a complex of FAK/Src which later phosphorylated paxillin to allow cell adhesion and migration [52]. Considering that the two focal adhesion proteins, FAK and paxillin activation transmit signals down- stream of integrin, the present study further investigated the effects of ODN on the expression of FAK, src and paxil- lin. The study results clearly showed a significant inhibitory effects of ODN on the molecules mentioned above (Fig. 3).
We further intended to find out how FAK expression is regulated by ODN. Some studies have stated that NF-kB is involved in controlling the expression of FAK [53]. Hence, we extended our study to determine the effects of ODN on NF-kB protein expression. ODN-inhibition of CTSK has been found to substantially inhibit NF-kB expression in a dose-dependent manner (Fig. 3). This result allows us to conclude that NF-kB transcription factors could also prob- ably be involved in controlling FAK gene expression and ODN significantly improved the suppression of FAK by controlling NF-kB transcription factors.
It is well established that E-cadherin loss abrogates its adhesive qualities, by facilitating cell invasion and metas- tasis [54]. The role of E cadherin as a potent suppressor of invasion and metastasis and the loss of E-cadherin is consid- ered to be characteristic of invasive and aggressive tumour activity in cancer such as the breast [9]. Previous study indi- cated that E-cadherin is a target substrate of cathepsins such as B, L, and S [55]. The absence of cathepsin K has resulted in an elevated level of E-cadherin protein in the colon of Ctsk – / – mice, according to previous studies. Hence, the elevated level of E-cadherin in the Ctsk – / – mice colon could possibly be explained by the notion that cathepsin K can also cleave this protein [56]. The cleaved E-cadherin is thus a sign of substantial proteolytic activity and disrup- tion of adherent junctions; these characteristics are typically

correlated with invasion and metastasis [40, 57–59]. In con- sistent with the previous findings, the present study result indicates that ODN inhibits cell migration and induces E-cadherin over expression in MDA-MB-231 cells (Fig. 2). We demonstrated that CTSK upregulation in control shows an inverse relationship with E-cadherin in MDA-MB 231 cells whereas the inhibition of CTSK was found to have pos- itive correlation with the upregulation of E-cadherin mRNA level. Taken together, these results suggest that ODN exerts its anti-metastatic effect on MDA-MB 231 by up regulating the expression of E-cadherin. Consequently, the activation or the control of tumour suppressor genes such as E-cadherin is one of the essential achievable mechanisms by which ODN exerts its anti-metastatic effect on breast cancer cells. How- ever, there has been no evidence thus far as to the specific mechanism underlying the upregulation of E-cadherin gene expression by ODN and remains to be elucidated how the drug interacts the target gene promoter region.
In conclusion, our data demonstrated for the first time that ODN inhibits the migration, invasion, and adhesion of highly metastatic MDA-MB-231 breast cancer cells by down-regulating MMP-9 and inactivating FAK as well as the MAPK and PI3K pathways. Based on our findings, we propose that ODN is a therapeutic anti-metastatic agent against MDA-MB-231 cells and could be used for the treat- ment of breast cancer metastasis. The main limitation of the present study is that we were unable to determine the expression level of cystatin C, a cathepsin K inhibitor that could have influenced the novelty of the drug. Secondly, we were unable to quantify the protein expression of cell adhesion molecule, E-cadherin expression to bring about the interconnection between CTSK inhibition and E-cadherin signaling. Future studies using neutralizing antibodies, anti- sense oligonucleotides, or inhibitors of these two proteins should help to clarify this potential association. Moreover, the additional studies are needed to explore the adverse side effects of ODN possibly by taking it with locally or bound to proteins or different structures with high target organ specificity to prevent from target side effects. In this way, the CTSK inhibitor drugs are nearer to the clinical set- ting, considering the strong association of this protease with cancer metastasis, inflammation, obesity, heart disease and metabolic disorders. CTSK inhibitor certainly can be used as mechanism-based drug to validate existing hypotheses and find novel functions/pathways for CTSK in different patholo- gies that will make it a potential therapeutic that cures dis- eases. Currently, there is no CTSK inhibitor approved drug available, however, the promising clinical outcomes with MIV-711 and robust efforts by many pharmaceutical indus- tries to develop more target specific CTSK agent would give much hope in future.
Acknowledgements The author is thankful to University Grants Com- mission (UGC), India for funding to carry out this work.

Funding This work was financially supported by University Grants Commission (UGC) for UGC: NFST India with IF no. NFST-2015–17-ST- MAN-689.

Compliance with ethical standards

Conflict of interests The authors declare that no competing or financial interest exists.
Ethical approval The authors have read and abided by the statement of the ethical standards for manuscripts submitted to this journal.
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