The Cell Surface GRP78 Facilitates the Invasion of Hepatocellular Carcinoma Cells

Zhang, Xiu-Xiu; Li, Hong-Dan; Zhao, Song; Zhao, Liang; Song, Hui-Juan; Wang, Guan; Guo, Qing-Jun; Luan, Zhi-Dong; Su, Rong-Jian

doi:https://doi.org/10.1155/2013/917296

BioMed Research International

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Research Article | Open Access

Volume 2013 | Article ID 917296 | https://doi.org/10.1155/2013/917296

The Cell Surface GRP78 Facilitates the Invasion of Hepatocellular Carcinoma Cells

Xiu-Xiu Zhang,^1,2Hong-Dan Li,¹Song Zhao,¹Liang Zhao,³Hui-Juan Song,¹Guan Wang,¹Qing-Jun Guo,⁴Zhi-Dong Luan,^1,5and Rong-Jian Su^1,6

Academic Editor: Paul Higgins

Received06 Oct 2013

Accepted18 Nov 2013

Published08 Dec 2013

Abstract

Invasion is a major characteristic of hepatocellular carcinoma and one of the main causes of refractory to treatment. We have previously reported that GRP78 promotes the invasion of hepatocellular carcinoma although the mechanism underlying this change remains uncertain. In this paper, we explored the role of the cell surface GRP78 in the regulation of cancer cell invasion in hepatocellular carcinoma cells. We found that neutralization of the endogenous cell surface GRP78 with the anti-GRP78 antibody inhibited the adhesion and invasion in hepatocellular carcinoma cell lines Mahlavu and SMMC7721. However, forced expression of the cell surface GRP78 facilitated the adhesion and invasion in SMMC7721. We further demonstrated that inhibition of the endogenous cell surface GRP78 specifically inhibited the secretion and activity of MMP-2 but did not affect the secretion and activity of MMP-9. We also found that inhibition of the cell surface GRP78 increased E-Cadherin expression and decreased N-Cadherin level. On the contrary, forced expression of the cell surface GRP78 increased N-Cadherin expression and decreased E-Cadherin level, suggesting that the cell surface GRP78 plays critical role in the regulation of EMT process. These findings suggest that the cell surface GRP78 plays a stimulatory role in the invasion process and may be a potential anti-invasion target for the treatment of hepatocellular carcinoma.

1. Introduction

Hepatocellular carcinoma (HCC) is the third leading cause of cancer-related death worldwide [1]. Although new therapeutic strategies have been continuously developed and applied to clinical treatment of HCC, the prognosis is still very poor [2]. The invasion and metastasis are one of the most important reasons for the mortality of HCC [3]. Therefore, understanding the mechanisms that facilitate the invasion and metastasis is critical for exploring new strategies for the treatment of HCC.

The glucose regulated protein 78 (GRP78) is traditionally regarded as a resident protein of the endoplasmic reticulum (ER) and functions as a molecular chaperone [4]. In addition to its chaperoning function, many data suggest that GRP78 is a multifunctional protein and plays critical roles in the resistance to chemotherapy agents, proliferation, invasion, and metastasis of many human cancers [5–9].

GRP78 is expressed in the endoplasmic reticulum in normal conditions but also is expressed at an elevated level on the surface of many tumors and disseminated tumor cells [10, 11]. The cell surface GRP78 functions as a signaling receptor and plays important roles in the regulation of the proproliferative/antiapoptotic and promigratory signaling pathways [12, 13].

Most information about its functions is derived from treatment of cancer cells with antibody directed against the C-terminal domain or N-terminal domain of GRP78. Treatment of prostate cancer (1-LN, DU145) and melanoma cells (A375), which express GRP78 on the cell surface, with antibody directed against the C-terminal domain of GRP78, inhibited cell proliferation and induced apoptosis by activating p53 and suppressing Ras/MAPK, PI3K/AKT signaling pathways [14, 15]. Ligation of the cell surface GRP78 in teratoma cell line (NCCIT) and breast cancer cell line (MCF-7) with antibody directed against the N-terminal domain of GRP78 decreased cell proliferation and cell adhesion by inhibiting MAPK/PI3K signaling pathway [16, 17].

The cell surface GRP78 is also involved in the regulation of the invasion and metastasis of many human cancers including prostate and colorectal cancers [18, 19]. In prostate cancer, the cell surface GRP78 activates the p21-activated kinase-2 (PAK2) signaling pathway and therefore facilitates the invasion and metastasis by binding with α2-macroglobulin [18]. In colorectal cancer, cell surface GRP78 promotes the invasion and metastasis of cancer cells by activating the uPA/uPAR protease system [19].

The invasion and metastasis is a complex process that is regulated by many signaling molecules [20]. Among them, matrix metalloproteinases (MMPs), especially MMP-2 and MMP-9, have been demonstrated to play important roles in this process by degrading the extracellular matrix [21, 22]. We previously reported that overexpression of GRP78 promoted the invasion and metastasis of HCC and knockdown of GRP78 reduced the activity of MMPs [23, 24]. However, the role of cell surface GRP78 in the regulation of MMPs is still unknown.

The epithelial-to-mesenchymal transition (EMT), a phenomenon reported in embryonic development, has been gradually demonstrated as a potential mechanism underlying cancer progression and metastasis. Many cell signaling pathways are involved in the regulation of EMT and contribute to cancer cell invasion and metastasis. However, the role of the cell surface GRP78 in the regulation of EMT has not been elucidated.

In this paper, we inhibited the cell surface GRP78 using antibody against GRP78 and overexpressed GRP78 at the cell surface in hepatocellular carcinoma cells Mahlavu and SMMC7721 and explored whether cell surface GRP78 affected the invasion of hepatocellular carcinoma cells.

2. Materials and Methods

2.1. Cell Culture

Human hepatocellular carcinoma cell lines Mahlavu, PLC, and SMMC7721 were kindly given by Dr Li-Jian Hui of the Institute of Biochemistry and Cell Biology of Chinese Academy of Sciences (Shanghai, China) and cultured in Dulbecco’s modified Eagle’s medium (DMEM, Hyclone) containing 10% fetal bovine serum (Hyclone) and penicillin/streptomycin (100 units/mL and 100 μg/mL, resp., Gibco). All cells were maintained at 37°C in a humidified atmosphere of 5% CO₂, 95% air.

2.2. Reagents and Antibodies

Polyclonal antibodies against the N-terminal region of GRP78 (N-20) and β-actin were from Santa Cruz (Santa Cruz, CA). Monoclonal antibodies raised against GRP78, MMP-2, MMP-9, E-cad, and N-cad were from CST (Danvers, USA). Fluorescein isothiocyanate- (FITC-) conjugated anti-rabbit IgG was from Abcam. Fibronectin (FN), gelatin, and bovine serum albumin (BSA) were purchased from Aldrich-Sigma (USA). Goat and rabbit isotype IgG were from Abcam. Protease inhibitor tablet was from Roche (USA).

2.3. Flow Cytometric Analysis

The cells were incubated with the anti-GRP78 antibody (Abcam) or rabbit isotype IgG for 1 h on ice, washed with cold PBS for 3 times, and then incubated with saturating concentrations of FITC-conjugated goat anti-rabbit IgG for 30 min at 4°C. After washing with PBS, the cells were fixed with 2% formaldehyde in PBS. The cell surface GRP78 was analyzed by flow cytometry performed on a FACScan (BD Biosciences).

2.4. Invasion Assay

24-well Transwell chamber inserts (Corning Costar, USA) with 8 μM porous polycarbonate filters were coated with 80 μg of basement membrane Matrigel (BD Biosciences) onto the upper surface and with FN (10 μg/mL) onto the lower surface. The cells were counted and dispensed into the upper chambers (2 × 10⁴ cells/well) and incubated at 37°C for 48 h. Cells were fixed with 3.7% formaldehyde and stained with 1% crystal violet. Noninvading cells on the upper surface of the filter were removed with a cotton swab, and the filter was excised and mounted on a microscope slide. Invasiveness was quantified by counting cells on the lower surface of the filter.

2.5. Attachment Assay

96-well culture plates were coated with FN (10 μg/mL) overnight at 4°C, washed with PBS, and then incubated with 1% heat-inactivated bovine serum albumin (BSA) for 1 h at 37°C to block nonspecific binding sites. Cells (5 × 10³ each well) suspended in serum free DMEM were dispensed into each well, incubated at 37°C for 1 h, and washed 3 times with PBS. Cells attached to the bottom of plate were stained with 1% crystal violet, dissolved with DMSO overnight at room temperature, and the absorbance at 595 nm was examined by a microplate reader (Synergy 2, Biotek, USA).

2.6. Treatment of the Cells by the Anti-GRP78 Antibody

Cells were grown to confluence in 6-well culture plates coated with FN (10 μg/mL), rinsed with serum-free media, and serum-starved for 3 hours. To block the cell surface GRP78, the anti-GRP78 antibody (N20, Santa Cruz) was added and incubated for 12 hours in normal culture condition [15, 23].

2.7. Gelatin Zymography Assay

The conditioned medium was collected and the protein concentrations were determined by BCA assay. Equal protein samples were separated by 10% SDS-PAGE containing 0.1% (w/v) gelatin in the absence of reducing reagent. The gels were washed for 3 times in 2.5% (w/v) Triton X-100 for 30 min at room temperature and incubated in 50 mM Tris-HCl (pH 7.6), 200 mM NaCl, 5 mM CaCl₂, and 0.02% (w/v) Brij35 for 24 h at 37°C under gentle agitation. The gels were finally stained with 0.5% Coomassie Brilliant Blue R-250 in 50% trichloroacetic acid, destained in 10% acetic acid and 20% methanol, and photographed by a bioimaging system (I-BOX, UVP, USA).

2.8. In-Cell Western Assay

The cells were harvested and diluted with DMEM containing 10% FBS to 75,000 cells/mL. Under sterile conditions, dispense 200 μL of the cell suspension per well in a Nunc 96 Microwell Plate. After 24 h, we removed supernatant and dispensed 100 μL of serum-free media (DMEM) per well of the 96-well microplate in 4–6 h. Then, remove media manually. Immediately fix cells with 4% formaldehyde in 1× PBS for 20 minutes at room temperature. Wash five times with 1× PBS containing 0.1% Triton X-100 (cell permeabilization) for 5 minutes per wash (the expression of Grp78 in cell surface without permeabilization). The wells were blocked by adding 150 μL of LI-COR Odyssey Blocking Buffer to each well in 1.5 h, removed blocking buffer from the blocking step, and added 50 μL of primary antibody (GRP78, 1 : 100, GAPDH, 1 : 100), incubated overnight at 4°C, and washed the plate five times with 1× PBST (0.1% Tween-20) for 5 minutes at RT with gentle shaking, using a generous amount of buffer, and added 50 μL of the secondary antibody (IRDye 800CW-labeled secondary antibody, 1 : 100,000) solution to each well, incubated for 60 minutes with gentle shaking at RT and protected plate from light during incubation, and washed the plate five times with 1× PBST for 5 minutes at RT with gentle shaking, using a generous amount of buffer. The plates were scanned at 800 nm by Odyssey imaging system.

2.9. Western Blotting

The cell lysates were prepared as previously described. The proteins (50 μg) were separated on 10% SDS-PAGE and transferred onto PVDF membranes (Millipore, USA). The membranes were incubated at room temperature for 2 h in 50 mM Tris-HCl (pH 7.4), 150 mM NaCl, 0.1% (v/v) Tween-20, 0.5% (w/v) BSA, and then overnight at 4°C on a rotator with specific primary antibodies (1 : 1000 in TBST/BSA). The membranes were washed for 3 times with TBST and incubated for 1 h with appropriate horseradish peroxidase- (HRP-) conjugated secondary antibody (1 : 5000 in TBST/BSA). The signals were visualized with enhanced chemiluminescence western blotting detection reagent (ECL, Pierce, USA) using a bioimaging system (I-BOX, UVP, USA). Densitometric analysis was performed using Quantity one software (Bio-Rad, USA).

2.10. ΔKDEL Recombinant Transfection and Selection of Stable Transfectants

The GRP78 ΔKDEL mutant in which the ER resident motif KDEL is deleted was constructed as previously reported [13]. For establishing the cell lines stably overexpressing GRP78 on the cell surface, the GRP78 ΔKDEL mutant was transfected into SMMC7721 cells using Lipofectamine 2000 (Invitrogen, USA) according to the manufacturer’s instructions. pcDNA3.1 vector was also transfected as control. After 24 h, the cells were subcultured at 1 : 3 ratio and the ΔKDEL recombinant transfectants were selected by adding G418 (400 μg/mL) in DMEM containing 10% FBS and identified by western blotting and flow cytometry, respectively.

2.11. Statistical Analysis

The data are presented as means ± standard deviations (SD). The data were analyzed with student’s -test, chi-squared test, and one-way ANOVA.

3. Results

3.1. GRP78 Is Localized on the Cell Surface of Hepatocellular Carcinoma Cells

We first determined whether GRP78 is expressed on the cell surface of hepatocellular carcinoma cells. In-cell western analysis revealed that GRP78 was identified on the cell surface of Mahlavu, SMMC7721, and PLC cells with different levels. As shown in Figure 1(a), Mahlavu cells showed the highest cell surface GRP78 level, while PLC cells showed the lowest cell surface GRP78. We next compared the invasion potential of hepatocellular carcinoma cells. Transwell analysis revealed that the invasion potential of Mahlavu cells was the highest among these cell lines, whereas the invasion potential of PLC cells was the lowest (Figure 1(b)).

(a)

(b)

3.2. Blockade of Endogenous Cell Surface GRP78 Inhibits the Invasion and Adhesion of Hepatocellular Carcinoma Cells

To explore whether the cell surface GRP78 plays a role in the invasion of hepatocellular carcinoma cells, we neutralized the cell surface GRP78 using the N-20 antibody, which has been demonstrated to block the signaling transduction downstream of the cell surface GRP78 effectively in many human cancers, in SMMC7721 and Mahlavu cells and examined the adhesion and invasion status. Cells treated with the goat isotype IgG were used as control. Transwell assay showed that the N-20 antibody markedly inhibited the invasive potential of Mahlavu cells and SMMC7721 cells. The N-20 antibody decreased the invasive potential of cancer cells to ~50% in Mahlavu cells and to ~58.5% in SMMC7721 cells as compared with isotype goat IgG treated cells, respectively (, chi-squared test) (Figures 2(a) and 2(b)). Cell adhesion assay revealed that the N-20 antibody significantly decreased the binding abilities of cancer cells to FN-coated culture dishes. The N-20 antibody reduced the adhesion of cancer cells to FN to ~51% in Mahlavu cells, ~60% in SMMC7721 cells as compared with control cells (, chi-squared test) (Figures 2(c) and 2(d)). These data suggested that the endogenous cell surface GRP78 facilitates the adhesion and invasion of hepatocellular carcinoma cells.

(a)

(b)

(c)

(d)

Figure 2

Immunoneutralization of the endogenous cell surface GRP78 inhibited the FN induced adhesion and invasion. ((a) and (b)) Transwell analysis of the invasive potential of Mahlavu and SMMC7721 cells treated with the N20 antibody. (Original magnification: 100x.) ((c) and (d)) Cell adhesion analysis of the binding ability of Mahlavu and SMMC7721 cells with the FN-coated substrate when treated with the N20 antibody. Data represent the means ± SD of triplicate determinations in three independent experiments. Asterisks indicate that the differences are statistically significant ( versus iostype IgG treated cells; one-way ANOVA); UT, untreated; IgG goat isotype IgG; N20, the N20 antibody.

3.3. Overexpression of the Cell Surface GRP78 Promotes the Invasion and Adhesion of Hepatocellular Carcinoma Cells

To further investigate the effect of exogenous cell surface GRP78 on the invasive potential of hepatocellular carcinoma cells, we constructed GRP78 KDEL motif deleted mutant (ΔKDEL) and transfected SMMC7721 cells with the ΔKDEL mutant and the cells stably overexpressing GRP78 on the cell surface were selected by G418 (400 μg/mL). The cells transfected with an empty pcDNA3.1 (+) vector served as the mock transfectants. The cell surface GRP78 in the ΔKDEL transfectants was determined by in-cell western and flow cytometry, respectively. In-cell western showed that the level of the cell surface GRP78 in the ΔKDEL transfectants appeared to be ~2-fold higher than that on the cell surface of mock transfectants (Figure 3(a)). Flow cytometry showed that the percentage of the cell surface GRP78 positive cells increased significantly in the ΔKDEL transfectants as compared with the mock transfectants (56.3% versus 95.4%, , chi-squared test) (Figure 3(b)).

Figure 3

Forced expression of the ΔKDEL recombinant promotes the adhesion and invasion of hepatocellular carcinoma cells. (a) In-cell western analysis of the cell surface GRP78 in the ΔKDEL transfectants. β-actin served as loading control. sGRP78, cell surface GRP78. (b) Flow cytometry analysis of the cell surface GRP78 in the ΔKDEL transfectants. Red, rabbit isotype IgG. Green, anti-GRP78 antibody. (c) The morphology observation of the ΔKDEL transfectants. The ΔKDEL transfectants were seeded on FN-coated culture dishes, allowed to spread for 2 h. Scale bar, 50 μM. (d) Cell adhesion analysis of the binding ability of the ΔKDEL transfectants with FN-coated substrate. (e) Transwell analysis of the invasive potential of the ΔKDEL transfectants. (Original magnification: 100x.) Data represent the means ± SD of triplicate determinations in three independent experiments. Asterisks indicate that the differences are statistically significant ( versus Mock transfectant; student’s -test).

We then observed whether forced expression of the cell surface GRP78 in SMMC7721 cells caused the morphology change. To do this, the ΔKDEL transfectants were seeded on the FN-coated culture dishes, allowed to spread for 2 h. We found that the ΔKDEL transfectants appeared to be an irregular, elongated morphology. In contrast, the mock transfectant appeared to be a regular, polygon shape (Figure 3(c)). We also examined whether forced expression of the cell surface GRP78 affected the adhesion of hepatocellular carcinoma cells to FN. Cell adhesion assay revealed that the ΔKDEL transfectants exhibited increased cell adhesion to FN-coated culture dishes as compared with the mock transfectant (Figure 3(d)). We next compared the invasive potential of the ΔKDEL transfectants with the mock transfectants. Transwell assay revealed that the invasive potential of ΔKDEL transfectants increased significantly as compared with the mock transfectants (, student’s -test) (Figure 3(e)). These data suggested that exogenous cell surface GRP78 promotes the adhesion and invasion of hepatocellular carcinoma cells, suggesting that both exogenous and endogenous cell surface GRP78 promotes the invasion of hepatocellular carcinoma cells.

3.4. Overexpression of the Cell Surface GRP78 Enhances MMP-2 Expression and EMT

To determine whether cell surface GRP78 affected the expression of MMP-2 and MMP-9 in hepatocellular carcinoma cells, we examined the activity of MMP-2 and MMP-9 in serum-free medium conditioned by SMMC7721 and Mahlavu cells, which were treated by the N-20 antibody. Gelatin zymography analysis revealed that the N-20 antibody significantly inhibited MMP-2 activity in both Mahlavu and SMMC7721 cells, while it did not affect MMP-9 activity (Figure 4(a)). We further analyzed whether blockade of cell surface GRP78 influenced the secretion of MMP-2 and MMP-9 by examining the level of MMP-2 and MMP-9 in the conditioned medium. We found that treatment with the N-20 antibody significantly decreased MMP-2 content in the condition medium, indicating that blockade of cell surface GRP78 inhibited the secretion of MMP-2 in hepatocellular carcinoma cells (Figure 4(b)). On the contrary, the ΔKDEL transfectants exhibited markedly increased activity and secretion of MMP-2 (Figure 4(c)).

Figure 4

The cell surface GRP78 regulates the activity and secretion of MMP-2 and EMT. (a) Gelatin zymography analysis of the activity of MMP-2 and MMP-9 in Mahlavu and SMMC7721 cells treated with the N-20 antibody. (b) Western blot analysis of the secretion of MMP-2 and MMP-9 in Mahlavu and SMMC7721 cells treated with the N-20 antibody. (c) Gelatin zymography analysis of the activity and secretion of MMP-2 in the ΔKDEL transfectants. (d) Western blot analysis of the expression of E-Cadherin and N-Cadherin in Mahlavu cells treated with the N-20 antibody. (e) Western blot analysis of the expression of E-Cadherin and N-Cadherin in SMMC7721 cells treated with the N-20 antibody. (f) Western blot analysis of the expression of E-Cadherin and N-Cadherin in the ΔKDEL transfectants. UT, untreated. IgG goat isotype IgG. N20, the N20 antibody.

For the critical roles of EMT in tumor invasion, we next examined whether the cell surface GRP78 regulates EMT process. Western blot analysis revealed that the N20 antibody significantly increased E-Cadherin protein expression level while decreased N-Cadherin protein expression level in Mahlavu cells (Figure 4(d)). Similar results were obtained in SMMC7721 cells (Figure 4(e)). These data suggested that blockade of the endogenous cell surface GRP78 inhibited EMT. However, the ΔKDEL transfectants exhibited decreased E-Cadherin protein level and increased N-Cadherin protein level, indicating that the exogenous cell surface GRP78 promoted EMT (Figure 4(f)).

4. Discussion

Hepatocellular carcinoma invasion requires the stimulation and regulation of many signaling pathways. In this study, we demonstrate that the cell surface GRP78 is localized on the cell surface of hepatocellular carcinoma cells and plays important roles in the regulation of the adhesion and invasion.

First, GRP78 is expressed on the cell surface of hepatocellular carcinoma cells. This observation is consistent with previous reports that GRP78 is localized on the cell surface of many other human cancer cells including prostate, melanoma, breast, and lung cancer. Our findings together with others show that the expression of GRP78 on the cell surface may be a general event in human cancers. The significance of this feature has been elucidated, although the mechanism remains unclarified. Accumulating evidence suggests that the cell surface GRP78 functions as a receptor and regulates the processes of cell proliferation, apoptosis, invasion, and metastasis of different cancer cells [15–20].

Second, the cell surface GRP78 promotes the invasion of HCC in hepatocellular carcinoma cells. We previously showed that GRP78 promoted the invasion of hepatocellular carcinoma cells, but the role of cell surface GRP78 in this process remained unclear. We demonstrated that the neutralization of endogenous cell surface GRP78 inhibited both adhesion and invasion of hepatocellular carcinoma cell lines Mahlavu and SMMC7721 cells. In contrast, the forced expression of GRP78ΔKDEL mutant facilitated cancer cell adhesion and invasion in SMMC7721 cells. Thus, the cell surface GRP78 plays a stimulatory role in the adhesion and invasion of hepatocellular carcinoma cells.

Third, MMP-2 but not MMP-9 plays critical role in the cell surface GRP78 induced cancer cell invasion. MMPs, which are a family of zinc-dependent endopeptidases, have long been associated with cancer cell invasion and metastasis. Among them, the roles of MMP-2 and MMP-9 in cancer cell invasion have been wellknown. In this paper, we demonstrated that the cell surface GRP78 specifically regulated the activity and secretion of MMP-2, but not MMP-9, in hepatocellular carcinoma cells. Blockade of the cell surface GRP78 in Mahlavu and SMMC7721 cells inhibited the activity and secretion of MMP-2. In contrast, forced expression of the cell surface GRP78 promoted the secretion and activity of MMP-2. Our results suggested that MMP-2 lies downstream of specific signaling molecule and regulated tumor invasion independently or in combination with MMP-9 by degrading the extracellular matrix. Many publications have obtained similar results. For example, Stat3 activation specifically regulates MMP-2 and promotes the invasion of melanoma. S100A14 protein, a member of EF-hand calcium-binding proteins subfamily, enhances the expression of MMP-2 and, therefore, promotes the motility of colon cancer cells.

Finally, EMT plays essential roles in the initiation of cancer cell invasion. Many data have reported that EMT is perquisite for the invasion of cancer cells. We demonstrated that the cell surface GRP78 regulates EMT process in hepatocellular carcinoma cells. Inhibition of the endogenous cell surface GRP78 decreased the invasive potential of hepatocellular carcinoma cells. However, forced expression of exogenous cell surface GRP78 enhanced the invasive potential of cancer cells.

Taken together, the cell surface GRP78 plays a stimulatory role in the invasion of hepatocellular carcinoma cells. The cell surface GRP78 induced the invasion of cancer cells by enhancing the activity of MMP-2 and regulating EMT process. Our results suggest that GRP78 on the cell surface may be a potential target for inhibiting clinically the invasion of hepatocellular carcinoma cells.

5. Conclusions

Cell surface GRP78 plays critical role in the regulation of EMT process and cell surface GRP78 plays a stimulatory role in the invasion process and may be a potential anti-invasion target for the treatment of hepatocellular carcinoma.

Authors’ Contribution

Xiu-Xiu Zhang and Hong-Dan Li Contributed equally to this paper.

Acknowledgment

This work was financially supported by National Natural Science Foundation of China Grant 81172048.

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Copyright © 2013 Xiu-Xiu Zhang 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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