Journal of Chemistry

Journal of Chemistry / 2016 / Article

Research Article | Open Access

Volume 2016 |Article ID 4921717 | https://doi.org/10.1155/2016/4921717

Soon Young Shin, Chang Gun Kim, Seunghyun Ahn, You Jung Jung, Dongsoo Koh, Young Han Lee, Yoongho Lim, "A Benzochalcone Derivative, (E)-1-(2-hydroxy-6-methoxyphenyl)-3-(naphthalen-2-yl)prop-2-en-1-one (DK-512), Inhibits Tumor Invasion through Inhibition of the TNFα-Induced NF-κB/MMP-9 Axis in MDA-MB-231 Breast Cancer Cells", Journal of Chemistry, vol. 2016, Article ID 4921717, 8 pages, 2016. https://doi.org/10.1155/2016/4921717

A Benzochalcone Derivative, (E)-1-(2-hydroxy-6-methoxyphenyl)-3-(naphthalen-2-yl)prop-2-en-1-one (DK-512), Inhibits Tumor Invasion through Inhibition of the TNFα-Induced NF-κB/MMP-9 Axis in MDA-MB-231 Breast Cancer Cells

Academic Editor: Joaquin Campos
Received17 Jun 2016
Accepted21 Sep 2016
Published11 Oct 2016

Abstract

Tumor invasion is a critical step in tumor metastasis. In this study, we synthesized a novel benzochalcone derivative, (E)-1-(2-hydroxy-6-methoxyphenyl)-3-(naphthalen-2-yl) prop-2-en-1-one (DK-512), and characterized its effects on tumor invasion and its mechanism of action. We found that DK-512 strongly inhibited invasion of metastatic MDA-MB-231 breast cancer cells as revealed by a three-dimensional spheroid culture system. Tumor invasion and metastasis require disruption of the extracellular matrix. Matrix metalloproteinase-9 (MMP-9) is an endopeptidase that degrades extracellular matrix components. DK-512 significantly reduced tumor necrosis factor-α- (TNFα-) induced MMP-9 mRNA expression through the inhibition of RelA nuclear factor- (NF-) κB transcription factor. As our study was assessed in vitro, further works about in vivo efficacy of DK-512 are needed to gain further insights into whether DK-512 could be utilized as a scaffold for the development of antimetastatic agents for breast cancer.

1. Introduction

Breast cancer is the most common malignancy in women and ranks second after lung cancer as a cause of cancer death in Western and Korean women [1, 2]. Metastatic breast cancer spreads to other parts of the body, such as the bones, liver, lungs, and brain [3]. Such distant metastases are the main cause of death [4].

The extracellular matrix (ECM) is a highly dynamic structure, composed of a variety of fibrous proteins and proteoglycans that are present throughout interstitial tissue. The basement membrane is a dense, fibrous, ECM-like structure that underlies epithelia and endothelia [5]. In mammary glands, the basement membrane encapsulates the glands and interacts with the luminal epithelium. The motility, invasion, and metastatic potential of epithelial tumor cells are highly correlated with the degradation of ECM in the basement membrane [5]. One enzyme involved in the degradation of ECM is matrix metalloproteinase-9 (MMP-9; also known as 92 kDa type IV collagenase or gelatinase-B), a protease that efficiently degrades denatured collagen. MMP-9 is involved in degradation of collagen IV, a major component of the basement membrane [6]. As such, upregulation of MMP-9 is associated with promotion of invasion, metastasis, and angiogenesis [7, 8]. The ubiquitous transcription factor, nuclear factor kappa B (NF-κB), plays an important role in the expression of multiple genes involved in the regulation of tumor invasion and metastasis [9, 10] and controls MMP-9 expression in a variety of cell types [1114].

Flavonoids are polyphenolic compounds consisting of a C6-C3-C6 skeleton. Chalcones (1,3-diphenyl-2-propen-1-ones), open-chain flavonoids in which the two aromatic rings are joined by a three-carbon α,β-unsaturated carbonyl system, are metabolic precursors of some flavonoids [15]. Previous studies showed that chalcones display multiple biological activities, including inhibition of cell cycle progression and induction of apoptosis, in a variety of cancer types [2, 1521]. However, the anti-invasion effects of naphtochalcone derivatives have not been well characterized.

Previously, we analyzed the structure-activity relationships of flavonoids and found that methoxy-chalcones (Figure 1(a)) efficiently inhibit NF-κB activity in HCT116 human colon cancer cells [22]. Further study showed the antitumor activity of a hydroxy-methoxy-benzochalcone with an additional benzene ring (Figure 1(b)) [21]. Based on these reports, we synthesized a novel hydroxy-methoxy-benzochalcone derivative, (E)-1-(2-hydroxy-6-methoxyphenyl)-3-(naphthalen-2-yl) prop-2-en-1-one (DK-512) (Figure 1(c)), and evaluated its biological effect on MMP-9 expression and invasion in highly metastatic MDA-MB-231 human breast cancer cells.

2. Materials and Methods

2.1. Chemical Synthesis

The procedure for synthesizing DK-512 has been described elsewhere [23].

2.2. Cells and Chemicals

MDA-MB-231 human breast carcinoma cells were obtained from the American Type Culture Collection (ATCC; Manassas, VA, USA). Cells were grown in Dulbecco’s Modified Eagle Medium (DMEM) supplemented with 10% (v/v) heat-inactivated fetal bovine serum (FBS; Cellgro, Manassas, VA, USA). Human TNFα was purchased from Sigma-Aldrich (Cat number H8916; Saint Louis, MO, USA). The firefly and Renilla Dual-Glo™ Luciferase Assay System was purchased from Promega (Madison, WI, USA). The pRL-null plasmid, which encodes Renilla luciferase, was also purchased from Promega.

2.3. Three-Dimensional Spheroid Culture and Invasion Assay

A three-dimensional (3D) invasion assay was performed using a Cultrex 3D Spheroid Cell Invasion Assay kit (Trevigen, Inc., Gaithersburg, MD, USA), as described previously [16]. Invasive spreads were visualized using an EVOS f1 fluorescence microscope (Advanced Microscopy Group, Bothell, WA, USA).

2.4. Cell Viability Assay

Cell viability was determined using a Cell Counting Kit-8 (CCK-8; Dojindo Molecular Technologies, Gaithersburg, MD, USA) according to the manufacturer’s instructions. Briefly, exponentially growing cells were treated with different concentrations (0, 1, 5, 10, and 20 μM) of DK-512. After 24 h, CCK-8 solution was added and the absorbance was measured at 450 nm after an additional 1 h using an Emax Endpoint ELISA Microplate Reader (Molecular Devices, Sunnyvale, CA, USA).

2.5. RT-PCR and Quantitative Real-Time PCR

Total RNA was extracted using Isol-RNA lysis reagent (NucleoZOL; Clontech, Mountain View, CA, USA) and the synthesis of cDNA was carried out using an iScript cDNA synthesis kit (Bio-Rad, Hercules, CA, USA). Reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR were performed as described previously [24]. A TaqMan-iQ Supermix Kit (Bio-Rad) was used with the Bio-Rad iCycler iQ thermal cycler according to the manufacturer’s instructions. The TaqMan fluorogenic probes and gene-specific PCR primers for MMP-9 and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) have been described elsewhere [24]. The relative fold changes were normalized to GAPDH mRNA in the same sample. The EC50 value was calculated using MasterPlex QT software (Hitachi Software Engineering America, South San Francisco, CA, USA).

2.6. MMP-9 Promoter Reporter Assay

The human MMP-9 promoter constructs, including wild-type pMMP9-Luc(–925/+13) and NF-κB binding site-mutated reporter pMMP9-Luc(–925/+13)mtNFκB, have been described previously [11]. Transfection of promoter reporters and luciferase reporter assays was performed as described previously [11]. Luminescence was measured using a Centro LB960 dual luminometer (Berthold Technologies, Bad Wildbad, Germany).

2.7. NF-κB-Dependent Transcriptional Activity Assay

The cis-acting 5x NFκB-Luc plasmid, containing five repeats of NF-κB binding sites, was obtained from Stratagene (La Jolla, CA, USA). MDA-MB-231 cells were transfected with 0.1 μg 5x NFκB-Luc plasmid and treated with 10 ng/mL TNFα in the absence and presence of DK-512, as described previously [25]. The luciferase activities were measured with a Centro LB960 luminometer (Berthold Technologies).

2.8. Immunoblot Analysis

Cells were lysed in 20 mM HEPES (pH 7.2), 1% (v/v) Triton X-100, 10% (v/v) glycerol, 150 mM NaCl, 10 μg/mL leupeptin, and 1 mM PMSF. Protein extracts (20 μg per sample) were separated via 10% SDS-polyacrylamide gel electrophoresis, transferred to nitrocellulose membranes, and incubated with appropriate primary and secondary antibodies. Primary antibodies against phospho-IκB (Ser32) and phospho-RelA/p65 NF-κB (Ser536) were obtained from Cell Signaling Technology (Beverly, MA, USA) and an antibody against GAPDH was obtained from Santa Cruz Biotechnology (Dallas, TX, USA). The blots were developed using an enhanced chemiluminescence detection system (GE Healthcare, Piscataway, NJ, USA). The relative protein band intensities were determined using ImageJ software.

2.9. Statistical Analysis

Statistical analysis was performed by one-way ANOVA followed by Sidak’s multiple comparisons test using GraphPad Prism version 7.0 software (GraphPad Software Inc., La Jolla, CA). A value < 0.05 was considered statistically significant.

3. Results and Discussion

3.1. Cytotoxic Effect of DK-512

We first tested the cytotoxic effect of DK-512 on MDA-MB-231 cells (Figure 2(a)). Exponentially growing cells were treated with 1 and 5 μM DK-512 for 24 h. The cell viability was not affected by DK-512 at concentrations up to 10 μM but significantly decreased to approximately 80% at 20 μM ( by Sidak’s test, ), suggesting that DK-512 is not toxic at concentrations below 10 μM.

3.2. Effect of DK-512 on Tumor Invasion

TNFα is an inflammatory cytokine that plays an important role in tumor invasion and metastasis [2630]. To determine the effect of DK-512 on the invasive capability of MDA-MB-231 cells, we used a 3D spheroid culture system (Figure 2(b)). In basal conditions, cells were aggregated in a compact multicellular spheroid. After 5 d, invasive protrusion out of the spheroid was detectable. Upon TNFα stimulation, invasive spreads into the surrounding matrix were substantially increased compared to the control spheroid. However, when the spheroids were preexposed to 5 μM DK-512, TNFα-induced invasive spreads were markedly inhibited. These data suggest that DK-512 has the ability to prevent the TNFα-induced invasive spread of breast cancer cells.

3.3. Effect of DK-512 on TNFα-Induced MMP-9 mRNA Expression

MMP-9 contributes to the invasion of epithelial tumor cells through degradation of the basement membrane [31]. TNFα is known to promote the progression of invasion and metastasis through the induction of MMP-9 in a variety of cancer cells [2630]. We also observed that MMP-9 mRNA levels accumulated within 12 h of TNFα treatment (Figure 3(a)). However, this accumulation of MMP-9 mRNA was significantly reduced by preexposure to 5 μM DK-512 ( by Sidak’s test), while GAPDH mRNA level, an internal control, was not affected (Figure 3(b)). To more precisely measure the change in MMP-9 mRNA levels due to DK-512 treatment, quantitative real-time PCR analysis was carried out [32]. Figure 3(c) shows that the MMP-9 mRNA level was increased -fold by 10 ng/mL TNFα treatment as compared to that observed in vehicle-treated control. This increase was dose-dependently reduced by -, -, -, -, -, and -fold in the presence of 0.1, 0.5, 1, 5, 10, and 20 μM DK-512, respectively (EC50 = 2.27 μM calculated by MasterPlex software). These data suggest that DK-512 inhibits TNFα-induced MMP-9 mRNA expression.

3.4. Effect of DK-512 on TNFα-Induced MMP-9 Promoter Activity

To investigate whether DK-512 inhibits MMP-9 mRNA expression at the transcriptional level, MMP-9 promoter reporter plasmid, pMMP9-Luc(–925/+13), was transfected into MDA-MB-231 cells. As shown in Figure 4(a), treatment with 5 μM DK-512 significantly reduced TNFα-induced MMP-9 gene promoter activity ( by Sidak’s test). NF-κB participates in TNFα-induced MMP-9 gene expression in various cancer cells [16, 33]. We also observed that TNFα-induced upregulation of MMP-9 promoter activity was significantly attenuated due to mutation of the NF-κB binding site in the MMP-9 gene promoter ( by Sidak’s test; Figure 4(b)). These data suggest that DK-512 may target NF-κB to prevent TNFα-induced MMP-9 expression.

3.5. Effect of DK-512 on TNFα-Induced NF-κB Activity

To investigate whether DK-512 inhibits NF-κB-dependent transcriptional activity, MDA-MB-231 cells were transfected with cis-acting reporter plasmid 5x NFκB-Luc, which contains five repeats of the NF-κB binding element. As shown in Figure 5(a), TNFα increased NF-κB-dependent transcriptional activity -fold as compared to vehicle-treated control. The increased NF-κB-dependent transcriptional activity was significantly reduced - ( by Sidak’s test) and -fold ( by Sidak’s test) by 5 and 10 μM DK-512, respectively. Thus, it seems likely that DK-512 exhibits inhibition of TNFα-induced NF-κB-dependent transcriptional activity.

To further address the effect of DK-512 on NF-κB inhibition, MDA-MB-231 cells were treated with 10 ng/mL TNFα in the presence or absence of 5 or 10 μM DK-512. Representative immunoblot (Figure 5(b)) and quantitative densitometry (Figure 5(c)) showed that DK-512 significantly reduced the phosphorylation of both IκB on serine-32 (at 5 μM, by Sidak’s test) and RelA NF-κB on serine-536 (at 5 μM, by Sidak’s test).

4. Conclusion

DK-512, a novel hydroxy-methoxy-benzochalcone derivative, displayed inhibition of tumor invasion against metastatic MDA-MB-231 cells. DK-512 inhibited TNFα-induced MMP-9 gene expression through downregulation of NF-κB-dependent transcriptional activity. These results suggest that DK-512 can be considered a potential scaffold for the development of antimetastatic agents against breast cancer cells.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (NRF-2016R1A2B4008570). This paper was supported by the KU Research Professor Program of Konkuk University.

References

  1. American Cancer Society, Cancer Facts & Figures 2015, American Cancer Society, Atlanta, Ga, USA, 2015.
  2. Korean Breast Cancer Society, Breast Cancer Facts & Figures 2015, Korean Breast Cancer Society, Seoul, Republic of Korea, 2015.
  3. M. Lacroix, “Significance, detection and markers of disseminated breast cancer cells,” Endocrine-Related Cancer, vol. 13, no. 4, pp. 1033–1067, 2006. View at: Publisher Site | Google Scholar
  4. B. Weigelt, J. L. Peterse, and L. J. Van't Veer, “Breast cancer metastasis: markers and models,” Nature Reviews Cancer, vol. 5, no. 8, pp. 591–602, 2005. View at: Publisher Site | Google Scholar
  5. R. Kalluri, “Basement membranes: structure, assembly and role in tumour angiogenesis,” Nature Reviews Cancer, vol. 3, no. 6, pp. 422–433, 2003. View at: Publisher Site | Google Scholar
  6. P. E. Van den Steen, B. Dubois, I. Nelissen, P. M. Rudd, R. A. Dwek, and G. Opdenakker, “Biochemistry and molecular biology of gelatinase B or matrix metalloproteinase-9 (MMP-9),” Critical Reviews in Biochemistry and Molecular Biology, vol. 37, no. 6, pp. 375–536, 2002. View at: Publisher Site | Google Scholar
  7. E. I. Deryugina and J. P. Quigley, “Matrix metalloproteinases and tumor metastasis,” Cancer and Metastasis Reviews, vol. 25, no. 1, pp. 9–34, 2006. View at: Publisher Site | Google Scholar
  8. N. Johansson, M. Ahonen, and V. M. Kahari, “Matrix metalloproteinases in tumor invasion,” Cellular and Molecular Life Sciences, vol. 57, no. 1, pp. 5–15, 2000. View at: Publisher Site | Google Scholar
  9. M.-A. Westhoff, S. Zhou, L. Nonnenmacher et al., “Inhibition of NF-κB signaling ablates the invasive phenotype of glioblastoma,” Molecular Cancer Research, vol. 11, no. 12, pp. 1611–1623, 2013. View at: Publisher Site | Google Scholar
  10. Y. Wang, Z. Lin, L. Sun et al., “Akt/Ezrin Tyr353/NF-κB pathway regulates EGF-induced EMT and metastasis in tongue squamous cell carcinoma,” British Journal of Cancer, vol. 110, no. 3, pp. 695–705, 2014. View at: Publisher Site | Google Scholar
  11. S. Y. Shin, J. H. Kim, A. Baker, Y. Lim, and Y. H. Lee, “Transcription factor Egr-1 is essential for maximal matrix metalloproteinase-9 transcription by tumor necrosis factor α,” Molecular Cancer Research, vol. 8, no. 4, pp. 507–519, 2010. View at: Publisher Site | Google Scholar
  12. J.-R. Yang, T.-J. Pan, H. Yang et al., “Kindlin-2 promotes invasiveness of prostate cancer cells via NF-κB-dependent upregulation of matrix metalloproteinases,” Gene, vol. 576, no. 1, part 3, pp. 571–576, 2016. View at: Publisher Site | Google Scholar
  13. J. W. Rhee, K. W. Lee, D. Kim et al., “NF-κB-dependent regulation of matrix metalloproteinase-9 gene expression by lipopolysaccharide in a macrophage cell line RAW 264.7,” Journal of Biochemistry and Molecular Biology, vol. 40, no. 1, pp. 88–94, 2007. View at: Publisher Site | Google Scholar
  14. M. Bond, A. J. Chase, A. H. Baker, and A. C. Newby, “Inhibition of transcription factor NF-κB reduces matrix metalloproteinase-1, -3 and -9 production by vascular smooth muscle cells,” Cardiovascular Research, vol. 50, no. 3, pp. 556–565, 2001. View at: Publisher Site | Google Scholar
  15. M. Das and K. Manna, “Chalcone scaffold in anticancer armamentarium: a molecular insight,” Journal of Toxicology, vol. 2016, Article ID 7651047, 14 pages, 2016. View at: Publisher Site | Google Scholar
  16. M. S. Lee, D. Koh, G. S. Kim et al., “2-Hydroxy-3,4-naphthochalcone (2H-NC) inhibits TNFα-induced tumor invasion through the downregulation of NF-κB-mediated MMP-9 gene expression,” Bioorganic and Medicinal Chemistry Letters, vol. 25, no. 1, pp. 128–132, 2015. View at: Publisher Site | Google Scholar
  17. Y. Yong, S. Y. Shin, H. Jung et al., “Investigation of 2-hydroxy-4-methoxy-2',3'-benzochalcone binding to tubulin by using NMR and in silico docking,” Journal of the Korean Society for Applied Biological Chemistry, vol. 57, no. 6, pp. 693–698, 2014. View at: Publisher Site | Google Scholar
  18. S. Y. Shin, Y. Yong, J. Lee et al., “A novel hydroxymethoxynaphthochalcone induces apoptosis through the p53-dependent caspase-mediated pathway in HCT116 human colon cancer cells,” Journal of the Korean Society for Applied Biological Chemistry, vol. 57, no. 4, pp. 413–418, 2014. View at: Publisher Site | Google Scholar
  19. J. M. Lee, M. S. Lee, D. Koh, Y. H. Lee, Y. Lim, and S. Y. Shin, “A new synthetic 2′-hydroxy-2,4,6-trimethoxy-5′,6′-naphthochalcone induces G2/M cell cycle arrest and apoptosis by disrupting the microtubular network of human colon cancer cells,” Cancer Letters, vol. 354, no. 2, pp. 348–354, 2014. View at: Publisher Site | Google Scholar
  20. S. Y. Shin, H. Yoon, D. Hwang et al., “Benzochalcones bearing pyrazoline moieties show anti-colorectal cancer activities and selective inhibitory effects on aurora kinases,” Bioorganic and Medicinal Chemistry, vol. 21, no. 22, pp. 7018–7024, 2013. View at: Publisher Site | Google Scholar
  21. S. Y. Shin, J.-H. Kim, H. Yoon et al., “Novel antimitotic activity of 2-hydroxy-4-methoxy-2′,3′-benzochalcone (HymnPro) through the inhibition of tubulin polymerization,” Journal of Agricultural and Food Chemistry, vol. 61, no. 51, pp. 12588–12597, 2013. View at: Publisher Site | Google Scholar
  22. S. Y. Shin, Y. Woo, J. Hyun et al., “Relationship between the structures of flavonoids and their NF-κB-dependent transcriptional activities,” Bioorganic and Medicinal Chemistry Letters, vol. 21, no. 20, pp. 6036–6041, 2011. View at: Publisher Site | Google Scholar
  23. D. Koh, Y. Jung, B. S. Kim, S. Ahn, and Y. Lim, “1H and 13C NMR spectral assignments of naphthalenylchalcone derivatives,” Magnetic Resonance in Chemistry, vol. 54, no. 10, pp. 842–851, 2016. View at: Google Scholar
  24. S. Y. Shin, J. M. Lee, Y. Lim, and Y. H. Lee, “Transcriptional regulation of the growth-regulated oncogene α gene by early growth response protein-1 in response to tumor necrosis factor α stimulation,” Biochimica et Biophysica Acta—Gene Regulatory Mechanisms, vol. 1829, no. 10, pp. 1066–1074, 2013. View at: Publisher Site | Google Scholar
  25. S. Y. Shin, Y. J. Jung, Y. Yong, H. J. Cho, Y. Lim, and Y. H. Lee, “Inhibition of PDGF-induced migration and TNF-α-induced ICAM-1 expression by maltotetraose from bamboo stem extract (BSE) in mouse vascular smooth muscle cells,” Molecular Nutrition & Food Research, vol. 60, no. 9, pp. 2086–2097, 2016. View at: Publisher Site | Google Scholar
  26. F. Balkwill, “Tumour necrosis factor and cancer,” Nature Reviews Cancer, vol. 9, no. 5, pp. 361–371, 2009. View at: Publisher Site | Google Scholar
  27. J. A. Joyce and J. W. Pollard, “Microenvironmental regulation of metastasis,” Nature Reviews Cancer, vol. 9, no. 4, pp. 239–252, 2009. View at: Publisher Site | Google Scholar
  28. C.-C. Lin, H.-W. Tseng, H.-L. Hsieh et al., “Tumor necrosis factor-α induces MMP-9 expression via p42/p44 MAPK, JNK, and nuclear factor-κB in A549 cells,” Toxicology and Applied Pharmacology, vol. 229, no. 3, pp. 386–398, 2008. View at: Publisher Site | Google Scholar
  29. D. Liu, X. Wang, and Z. Chen, “Tumor necrosis factor-α, a regulator and therapeutic agent on breast cancer,” Current Pharmaceutical Biotechnology, vol. 17, no. 6, pp. 486–494, 2016. View at: Publisher Site | Google Scholar
  30. V. H. Rao, R. K. Singh, D. C. Delimont et al., “Transcriptional regulation of MMP-9 expression in stromal cells of human giant cell tumor of bone by tumor necrosis factor-α,” International Journal of Oncology, vol. 14, no. 2, pp. 291–300, 1999. View at: Google Scholar
  31. N. Ramos-DeSimone, E. Hahn-Dantona, J. Sipley, H. Nagase, D. L. French, and J. P. Quigley, “Activation of matrix metalloproteinase-9 (MMP-9) via a converging plasmin/stromelysin-1 cascade enhances tumor cell invasion,” The Journal of Biological Chemistry, vol. 274, no. 19, pp. 13066–13076, 1999. View at: Publisher Site | Google Scholar
  32. S. Y. Shin, J.-S. Nam, Y. Lim, and Y. H. Lee, “TNFα-exposed bone marrow-derived mesenchymal stem cells promote locomotion of MDA-MB-231 breast cancer cells through transcriptional activation of CXCR3 ligand chemokines,” Journal of Biological Chemistry, vol. 285, no. 40, pp. 30731–30740, 2010. View at: Publisher Site | Google Scholar
  33. H. Sato and M. Seiki, “Regulatory mechanism of 92 kDa type IV collagenase gene expression which is associated with invasiveness of tumor cells,” Oncogene, vol. 8, no. 2, pp. 395–405, 1993. View at: Google Scholar

Copyright © 2016 Soon Young Shin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


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