Podoplanin in cancer cells is experimentally able to attenuate prolymphangiogenic and lymphogenous metastatic potentials of lung squamoid cancer cells
© Suzuki et al; licensee BioMed Central Ltd. 2010
Received: 18 January 2010
Accepted: 31 October 2010
Published: 31 October 2010
Podoplanin, a mucin-like transmembrane glycoprotein, is reportedly expressed in a variety of malignant cells and is generally regarded as a factor for promoting tumor progression in conventional studies. By contrast, a clinicopathologically conflicting role for podoplanin, namely as a favorable prognostic factor for patients with lung/cervical squamous cell carcinoma (SCC), has recently been reported. Here, we investigated the role of podoplanin expressed in lung squamoid cancer cells (LSCCs) in experimental tumor progression.
Using EBC-1 cells, a lung SCC cell line without podoplanin expression and with lymphogenous metastatic potential, stable transformants with or without an exogenous human podoplanin gene were established and applied to a mouse tumor implantation model. In vivo examinations revealed that exogenous podoplanin had no influence on tumor growth, whereas it significantly restrained axillary lymph node metastasis associated with the suppression of lymphangiogenesis but not angiogenesis and with the downregulation of EBC-1-derived VEGF-C but not other lymphangiogenesis-related factor mRNAs in implanted tumor tissue. In vitro examinations to clarify the mechanisms underlying the in vivo phenomena revealed that exogenous podoplanin significantly suppressed the expression of VEGF-C mRNA and of the protein, and also increased the level of phosphorylated c-jun N terminal kinase (JNK) in EBC-1 cells. The former effect of exogenous podoplanin was impaired by treatment with either JNK inhibitor sp600125 or podoplanin-siRNA, and the latter effect was impaired by treatment with podoplanin-siRNA, suggesting that podoplanin was able to activate JNK, thereby downregulating VEGF-C gene expression in LSCCs (podoplanin-JNK-VEGF-C axis). Furthermore, supporting evidence in regard to the axis present in LSCCs was obtained from similar experiments using H157 cells, another lung SCC cell line expressing endogenous podoplanin.
Our findings suggested that LSCC-associated podoplanin was functional and could attenuate the potential for lymph node metastasis, possibly based on the suppression of tumor lymphangiogenesis; thus, podoplanin in cancer cells may become a useful biomarker to measure the malignancy of lung SCC.
Lymphogenous and hematogenous metastases are major events in malignant tumor progression and important prognostic determinants of patients with cancer. Metastasis of cancer cells is a multi-step process, including malignant cell growth, cell detachment, invasion into adjacent tissue, blood or lymphatic permeation, entry into the blood or lymph flow, arrival at remote organ or lymph node, capillary arrest, extravasation, and proliferation within target organs . Numerous factors expressed in tumor cells are implicated in the process of metastasis.
Lymph node status is one of the critical prognostic indicators in patients with malignant tumors, and tumor-associated lymphangiogenesis is thought to be a key step in promoting lymphogenous metastasis of malignant cells. A number of experimental and clinicopathological studies have supported the significance of lymphangiogenesis in tumor progression, including non-small cell lung carcinoma [2–5]. Tumor lymphangiogenesis is regulated by lymphangiogenesis-related growth factors expressed in malignant cells and cognate receptors expressed in host lymphatic vessels [6–17]. Especially, paracrine interaction between vascular endothelial growth factors (VEGF)-C and -D, and their cognate receptor, VEGF receptor-3, plays a central role in tumor lymphangiogenesis in a variety of malignancies . In many cases, a high expression level of VEGF-C in malignant tumor cells correlates with increased density of peritumoral lymphatic vessels, increased incidence of lymph node metastasis, and poor prognosis .
Podoplanin is a mucin-like transmembrane glycoprotein . Since its expression is completely restricted in lymphatic endothelial cells in the vascular system, it is now available as a useful marker to distinguish lymphatic vessels immunohistochemically from blood vessels [19, 20]. Podoplanin is also expressed in a variety of non-neoplastic cells such as podocytes and alveolar type-I cells [18–23]. According to a recent gene targeting study, podoplanin-/- mice showed systemic edema due to aplastic lymphatic vessels during fetal development, and neonatal death due to respiratory failure [24, 25]. These findings are suggestive of the multi-physiological functioning of podoplanin in a cell-type-specific manner.
Recently, podoplanin has been reported to be expressed in a variety of malignant tumor cells, such as squamous cell carcinoma, methothelioma, and germ cell tumors [22, 26], and evidence suggesting the involvement of podoplanin in malignant potential from various studies has accumulated: 1) Podoplanin can alter cell morphology and motility, by which tumor invasive/migratory activity is promoted [27, 28]; 2) Podoplanin can induce the epithelial-mesenchymal transition ; and 3) Podoplanin can induce platelet activation/aggregation mediated by its platelet aggregation-stimulating (PLAG) domain, resulting in a greater ability to achieve hematogenous metastasis of circulating tumor cells [30–32]. Together, previous in vitro and in vivo experimental studies have suggested that podoplanin is an enhancer that promotes tumor progression
The role of podoplanin in tumor cells, however, seems to be controversial in recent clinicopathological studies of human cancers. For example, Yuan et al. and Chuang et al. demonstrated that a higher expression level of podoplanin in cancer cells significantly correlated with poor prognosis and a higher incidence of lymph node metastasis in head and neck SCC [33, 34]. Conversely, current several reports demonstrate that a lower expression level of podoplanin in cancer cells significantly correlated with a poor prognosis and a higher incidence of lymph node metastasis in both lung and cervical SCCs [35–37]. Hitherto-existing evidence relating to podoplanin functions can not explain the mechanism underlying the tumor suppression. The inconsistent and elusive results of these traditional experimental and clinicopathological studies may suggest that podoplanin exerts context-dependent multi-functions in different organ environments and/or different malignant cells, and that it may act as an enhancer in some cases and a suppressor in others in tumor progression.
Here, we investigated the role of cancer cell-associated podoplanin in the progression of lung SCC using an animal model, and found novel functions of podoplanin as a suppressor for cancer progression.
Podoplanin does not affect activities of cell growth and migration in vitro
Podoplanin attenuates lymphogenous metastatic potential in vivo
Podoplanin inhibits tumor-associated lymphangiogenesis but not angiogenesis
Podoplanin suppresses endogenous VEGF-C but not other lymphangiogenesis-related growth factor gene expressions in vivo and in vitro
Podoplanin-induced VEGF-C downregulation is mediated by an increase in the level of activated JNK in LSCCs
Using stable lung SCC cell line-derived transformants exogenously expressing podoplanin, we herein found direct evidence suggesting the role of podoplanin in experimental tumor progression. The body of our findings is that podoplanin in LSCCs can induce VEGF-C downregulation via the JNK signaling pathway(s), and can impair tumor-associated lymphangiogenesis and lymphogenous metastasis
Current immunohistochemical studies have revealed the expression of podoplanin in a variety of malignant cells. In the case of SCCs, podoplanin is mainly expressed in peripheral cancer cells in solid nests [22, 34, 36]. A series of these past studies relevant to tumor cell-associated podoplanin suggest its role in promoting cancer progression, especially in enhancing the potential of cancer cell invasion, and this hypothesis has been supported by several past studies . Indeed, previous reports have shown experimental evidence of interesting and important cytophysiological and cytochemical phenomena mediated by podoplanin. Notably, as evidenced by previous studies, podoplanin does not exert the same function in all cell types. For example, although podoplanin can induce RhoA activation and an epithelial-mesenchymal transition (EMT) in MDCK cells , it attenuated RhoA activity and could not induce EMT in a breast carcinoma cell line , suggesting that podoplanin exerts cell type-specific functions. Therefore, we are concerned about the interpretation of podoplanin's functions in several past studies. In fact, the function-related data were often from experiments using malignant cell lines that lacked podoplanin expression in actual human lesions. For example, since it has been immunohistochemically reported that almost all adenocarcinomas cells, including lung cancer, hardly express podoplanin [22, 23], the evidence from experimental studies on podoplanin functions using such cell lines/animal models may be rather different from the pathophysiological roles it plays in human malignancies. Consequently, experimental studies should be designed with human cell lines that actually express podoplanin in human malignancies in order to understand its true role in human pathology. In the present study with lung SCC cell line EBC-1, several different results relating to podoplanin functions were observed compared to those reported in past studies: 1) podoplanin did not promote cell migration, and 2) podoplanin-mediated morphological change, such as filopodia-like formation, was not observed. These findings should be interpreted as relating only to LSCC-associated podoplanin function.
We have previously reported that VEGF-C expression is positively regulated by p42/44 MAPK, protein kinase C (PKC) or p38 MAPK in oral squamoid cancer cells . In contrast, a negative regulatory mechanism has not been reported in any past studies. Here, we found the critical intracellular signaling pathway, JNK, for the negative regulation of VEGF-C gene. A decreased level of phosphorylated JNK but not of total JNK was induced by siRNA-mediated podoplanin knockdown (Figures 6D and 7C), suggesting that podoplanin could induce JNK phosphorylation. By contrast, as shown in Figure 6B, not only the phosphorylated JNK but also the total JNK levels were apparently increased in stable transformants exogenously expressing podoplanin. These phenomena may be due to the podoplanin-dependent phenotypic change of EBC-1 cells. As described above, podoplanin has the potential to induce EMT , so forced and long-term expression of podoplanin in our experimental conditions may cause several phenotypic changes of EBC-1 cells, resulting in the increased level of total JNK. Taken together, podoplanin-dependent downregulation of VEGF-C is mediated by the direct and/or indirect increase of active JNK level in LSCCs.
EBC1-induced lymphatic vessels in the implanted tumor were markedly dilated, frequently resulting in lymph node metastasis. In this model, podoplanin narrowed lymphatic vessels in luminal size. Since VEGF-C is reportedly an inducer not only in proliferation/migration of lymphatic endothelial cells but also in the enlargement of lymphatic vessels , podoplanin-mediated dowregulation of VEGF-C is a consistent mechanism for the decreased area and perimeter of lymphatic vessels. By contrast, the reason why podoplanin-mediated dowregulation of VEGF-C has no influence on the lymphatic vessel density remains unclear. Inoculated EBC-1 cells might be able to permeate the existing lymphatic vessels in an early phase of tumor growth and gradually proliferate in solid nests in lymphatic vessels. As a consequence, intralymphatic EBC-1 cell-derived VEGF-C acts on the free surface of lymphatic endothelial cells, thereby impairing concentration gradient-dependent vessel migration/sprouting but not affecting vessel proliferation/enlargement in our animal model.
Considering that a positive correlation between tumor VEGF-C and tumor lymphangiogenesis/lymphogenous metastasis has been reported in numerous malignancies including non-small cell lung carcinoma , we herein found a novel molecular mechanism, the podoplanin-JNK-VEGF-C axis, by which podoplanin impaired lymphogenous metastasis in our animal model. We could not, however, demonstrate any direct evidence suggesting a linkage between the podoplanin-JNK-VEGF-C axis and the podoplanin-dependent impairment of lymphangiogenesis/lymphogenous metastasis. Therefore, further examination is necessary to clarify the mechanism by which podoplanin suppresses lymph node metastasis, including the possibility that podoplanin is critical in other lymphangiogenesis-independent steps to achieve lymph node metastasis.
All together, the findings obtained from our animal model in the present study were experimentally able to support recent clinicopathological evidence suggesting that the expression of podoplanin in cancer cells is a favorable prognostic marker of patients with lung SCC. Moreover, the podoplanin-mediated downregulation of the VEGF-C gene via JNK (the podoplanin-JNK-VEGF-C axis) was newly found as one of the possible underlying mechanisms. Therefore, podoplanin may be a useful prognostic biomarker to determine the malignancy of lung SCC. Further advanced study to understand the pathophysiological functions of podoplanin, including the podoplanin-mediated decreased potential for lymph node metastasis in cancer cells, would provide beneficial information to explore a novel therapeutic strategy for patients with lung SCC.
Cells and Reagents
The lung squamoid cancer cell lines EBC-1 and H157 were maintained with RPMI 1640 (Sigma-Aldrich Japan, Tokyo, Japan) supplemented with 100 units/mL penicillin/streptomycin and 10% fetal bovine serum (FBS). Human fibroblast (MRC5) and mouse fibroblast (NIH3T3) were purchased from the American Type Culture Collection and maintained with Dulbecco's Modified Eagle's Medium supplemented with 10% FBS. Human microvascular endothelial cells (HMVECs) were purchased from Kurabo Co. Ltd., Tokyo, Japan, and maintained with EGM2 medium (Kurabo). c-Jun N-terminal kinase (JNK) inhibitor sp600125 and ROCK/Rho kinase inhibitor Y-27632 were purchased from Promega K.K., Tokyo, Japan. Anti-human podoplanin mouse monoclonal antibody was purchased from AngioBio Co., Del Mar, CA. Anti-total JNK, anti-phospho-JNK, anti-phospho-ERM and anti-total ERM were purchased from Cell Signaling Technology, Beverly, MA. Anti-β-actin rabbit polyclonal antibody was purchased from Sigma-Aldrich Japan, Tokyo, Japan. Anti-murine lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1) rabbit polyclonal antibody was produced in our laboratory as described previously , and anti-mouse CD31 rabbit polyclonal antibody was purchased from Abcam Inc., Cambridge, MA.
Male BALB/c nu/nu mice (5 weeks old) were from Kyudo Co., Ltd. (Tosu, Saga, Japan). All animal experiments were done under approved protocols and in accordance with recommendations for the proper care and use of laboratory animals by the Committee for Animal, Recombinant DNA, and Infectious Pathogen Experiments at Kyushu University and according to the Law (No. 105) and Notification (No. 6) of the Japanese Government.
Reverse transcription polymerase chain reaction (RT-PCR) and real-time RT-PCR
Total cellular RNA was extracted from culture cells or implanted tumor tissues with the ISOGEN system (Wako Pure Chemical Industries, Osaka, Japan) according to the manufacturer's instructions, and treated with RNase-free DNase I (Behringer Roche Applied Science Japan, Tokyo, Japan). Subsequently, aliquots (25 ng) of total RNA were reverse-transcribed and used for PCR templates. Real-time RT-PCR was performed for quantification of gene expression levels. Amplification of target genes was monitored in real-time, and gene expression levels were quantified using the Sequence Detection System, model 7000 (Applied Biosystems Ltd., Tokyo, Japan), according to the manufacturer's instructions for TaqMan methods. The oligonucleotide sequences of PCR primer sets and TaqMan probes were listed in additional file 4. The target gene expression level was normalized with GAPDH level and expressed as relative fold increases compared to mean level in control group.
Construction of plasmid vector and plasmid templates
The PCR primers incorporating Hind-III and Xho-I sites for amplification of human podoplanin are as follows: 5'-GATGTGGAAGGTGTCAGCTC-3'(forward) and 5'-GATCCTCGATGCGAATGCCT-3' (reverse). A PCR amplicon using cDNA from HMVECs was inserted into a plasmid vector, pCEP4, for mammalian cell expression according to general subcloning methods. The primer sequences for construction of plasmid templates are as follows: human and mouse vegf-c, 5'-GAAATTACAGTGCCTCTCTC-3' (forward) and 5'-CTAGTTCTTTGTGGGTCCAC-3' (reverse). Each PCR amplicon using cDNA from MRC5 or NIH3T3 was inserted into a plasmid with a pCRII TA cloning kit (Invitrogen, Carlsbad, CA), according to the manufacturer's instructions. The sequence of the insert was examined using the CEQ 2000 Sequence Detection System (Beckman Coulter, Inc., Fullerton, CA), and complete matching was confirmed through comparison to those reported in GenBank (accession nos. AF 390106, NM 005429, and NM 009506 for human podoplanin, human vegf-c, and mouse vegf-c, respectively) was confirmed.
Establishment of stable transformant
Constructed pCEP-4 inserted human podoplanin cDNA and empty pCEP-4 were transfected into EBC-1 cells using LipofectAMINE 2000 (LF2000) reagent (Invitrogen) according to the manufacturer's instructions. Forty-eight hours after transfection, the culture medium was replaced with medium containing 400 μg/mL hygromysin (RPMI-hygro). At that concentration, wild EBC-1 cells were completely killed. The cells were then maintained with RPMI-hygro until the selected cells had grown appropriately. Next, the selected cells were spread onto 96-multiwell plates for single-cell culture and were maintained with RPMI-hygro until they reached confluence. Single-cell-derived confluent cells were continuously maintained in RPMI-hygro in larger shares.
Tumor implantation model
Under sufficient anesthesia by an i.p. injection of sodium pentobarbital, 1 × 106 of EBC1-V1, EBC1-V2, EBC1-P4, and EBC1-P15 cells were subcutaneously injected into the dorsal region. After inoculation, tumor length and width were measured every 3 days for 3 weeks, and tumor volume was estimated by the formula V = π/6 × a 2 × b, where a was the short axis and b the long . 21 days after implantation, mice were sacrificed and the primary tumors and axillary lymph nodes (two lymph nodes per mouse) were harvested. Each harvested primary tumor tissue was sliced in two, and the slices were used as samples for histological and molecular biological experiments. Harvested axillary lymph nodes were subjected to HE staining. Through microscopic findings, metastatic status was divided into two cases--positive and negative--irrespective of the area of metastatic foci. The positive lymph nodes were counted, and the incidence of metastasis was expressed as the ratio of positive lymph nodes to the total number of lymph nodes in each group.
Western blot analysis
Cells were lysed with 200 ml of cell lysis buffer (Promega) containing a cocktail of protease inhibitors (Nacalai Tesque), and the supernatant of the lysed cells was recovered. An aliquot of 20 mg of proteins was subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) under reducing condition, and were then transferred to a PVDF membrane. An hour after being blocked with PBS containing 5% non-fat milk, the membrane was incubated overnight, with each primary antibody diluted with PBS containing 5% BSA and 0.1% Tween 20, at 4°C. The dilution rate was determined according to the manufacturer's instructions. Following several washings with PBS containing 0.1% Tween 20, the membrane was treated for 2 hours with an appropriate HRP-labeled secondary antibody, HistoFine (DAKO, Glostrup, Denmark) at room temperature. The target proteins were visualized by a luminal chemiluminescent reagent, LumiGLO (Cell Signaling Technology). After that, the membrane was subjected to re-probing assay. Briefly, following stripping the binding antibodies using Stripping Solution (Nacalai Tesque, Kyoto Japan) according to the manufacture's instruction, the membrane was washed with PBS and subjected to the WB described above. The band intensity was measured with FMBIO (Hitachi Software Engineering Co. Ltd., Tokyo, Japan).
In vitro proliferation assay
The WST-8 assay kit (Nacalai Tesque) was used to determine the proliferation rates of EBC1-Vs cells and EBC1-Ps cells, which were seeded at 1 × 103 cells/well on 96-well plates (n = 12 each). Cell viability was evaluated every day for 3 days according to the distributor's protocol. The obtained data were expressed as relative fold increases of O.D. values compared to those 1 day after dissemination.
Boyden chamber cell invasion was assayed using a cell culture-chamber-insert system (BD, FALCON) with an 8 μm polyethylene terephthalate (PET) membrane. First, 1 × 105 cells were seeded on the upper chamber in RPMI medium with 1% FBS. The RPMI medium with 1% or 10% FBS was added to the lower chamber. After 18 hours, cells that did not cross the membrane were scraped off the upper side of the membrane with a cotton swab. Cells that had transmigrated to the lower side were fixed with 70% ethanol and subjected to Giemsa staining. The membrane was excised from its support and mounted on a glass slide. Migrating cells in four independent microscopic visual fields (×100) were counted, and expressed as mean number per one field.
Angiogenesis and lymphangiogenesis in implanted tumor tissue were immunohistochemically evaluated. Detail immunohistochemcial procedures were described previously . The lymphatic vessels and blood vessels were identified as LYVE-1-positive and CD31-positive vessels, respectively. Image J Software was used to count the total numbers and measure the areas and perimeters of vessels whose apparent luminal areas were framed by LYVE-1-positive or CD31-positive endothelial cells in viable tumors. The blood and lymphatic vessel number was expressed as that per unit of viable tumor area, and the vessel area was expressed as that per a vessel. All sections were independently evaluated by three persons (K.S., M.O., H.S.).
Human podoplanin siRNA was purchased from Invitrogen. The RNA sequence is as below; 5'-UAUAGCGGUCUUCGCUGGUUCCUGG-3'. Control siRNA High GC Duplex (Invitrogen) was used as a negative control. siRNA transfection was performed with lipofectamine RNAiMAX (Invitrogen) according to the manufacturer's instructions.
Enzyme-linked immunosorbent assay (ELISA)
VEGF-C content in the culture media was measured using Quantikine Immunoassay systems for human VEGF-C (R&D Systems) according to the manufacturer's instructions. Three independent experiments were performed and the obtained data were statistically analyzed. VEGF-C content in the culture media was expressed as the amount of protein secreted from 100,000 cells during 24 hours.
All data were expressed as means ± SD and were analyzed by one-way ANOVA with Fisher's adjustment. Statistical significance was determined using the log-rank test, and p < 0.05 was considered statistically significant. Statistical analysis was done using SPSS software version 9.0 (SPSS, Chicago, IL).
This work was supported by grants-in-aid (to K. S. and Y. Y.) from the Japanese Ministry of Education, Culture, Sports, Science, and Technology. We would like to thank Mr. Hiroshi Fujii for his help with sectioning the tissue samples and histological experiments, Miss Chie Arimatsu for her help with animal experiments, and Miss Haruko Nizato for her help with molecular biological experiments.
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