Tumor-induced lymphangiogenesis in cervical lymph nodes in oral melanoma-bearing mice
© Ozasa et al.; licensee BioMed Central Ltd. 2012
Received: 12 September 2012
Accepted: 29 September 2012
Published: 2 October 2012
Metastasis via the lymphatic system is promoted by lymphangiogenesis. Alterations of the lymphatic channels during the progression of metastasis to regional lymph nodes (LNs) remain unexplored. To examine whether tumor-induced LN lymphangiogenesis controls metastasis to regional LNs, we investigated cervical LN metastasis in a mouse model of oral melanoma.
Injection of B16F10 melanoma cells into mouse tongues replicated spontaneous cervical LN metastasis. We performed histological, immunofluorescent, and histomorphometric analyses of tumor-reactive lymphadenopathy and lymphangiogenesis in tumor-associated LNs. We investigated the expression of vascular endothelial growth factor (VEGF)-C and its receptor, VEGF receptor-3 (VEGFR-3), in tumor cells and tissues, and LNs by reverse transcription polymerase chain reaction and immunofluorescence.
Tumor-associated LNs comprised sentinel LNs (SLNs) before and after tumor cell invasion (tumor-bearing SLNs), and LNs adjacent or contralateral to tumor-bearing SLNs. Extensive lymphangiogenesis appeared in SLNs before evidence of metastasis. After metastasis was established in SLNs, both LNs adjacent and contralateral to tumor-bearing SLNs demonstrated lymphangiogenesis. Interaction between VEGF-C-positive melanoma cells and VEGFR-3-positive lymphatic vessels was evident in tumor-associated LNs.
LN lymphangiogenesis contributes a progression of tumor metastasis from SLNs to other regional LNs.
KeywordsSentinel lymph node Tumor-bearing lymph node Oral melanoma Lymphangioegnesis Lymphatic metastasis
Sentinel lymph node
Vascular endothelial growth factor
Lymphatic Vessel Endothelial hyaluronan receptor 1
Dulbecoo’s Modified Eagle’s medium
Hematoxylin and Eosin
Tyrosinase-related Protein 1
Immunoglobulin G Flt-4: Fms-related tyrosine kinase 4
Reverse Transcription-polymerase Chain Reaction.
The lymphatic system functions in regulating tissue fluid balance and immune cell trafficking, and it is involved in the pathogenesis of edema and metastasis. Tumor cell dissemination to lymph nodes (LNs) through the lymphatic system is common and early event in human malignant tumors. LN metastasis is the first sign of tumor progression in most malignant tumors, and is a crucial determinant in their staging, prognosis, and treatment . Lymphatic metastasis was considered a passive process, where detached tumor cells entered LNs via pre-existing lymphatic vessels proximate to the primary tumors . Sentinel LNs (SLNs) are defined as the first LNs to receive cells and fluid from primary tumors through lymphatic vessels . Malignant cells at SLNs were believed to then enter the blood stream via high endothelial venules or continue through the lymphatic drainage system, exiting into the blood stream via anastomoses such as the thoracic duct .
Changes in LNs begin before metastasis, a process termed tumor-reactive lymphadenopathy . Regional LNs proximate to the primary tumors are commonly enlarged because of reactive lymphadenopathy, tumor metastasis, or both, suggesting that LN alteration results from interactions between tumors and the lymphatic system. Experimental tumor models and human clinicopathological data indicate that lymphatic vessel growth near solid tumors is often associated with LN metastasis [6, 7]. In melanoma, the level of tumor-related lymphangiogenesis correlates with the rate of SLN metastases . Moreover, recent studies demonstrated that tumor cells in several malignancies can induce lymphangiogenesis in SLNs before metastasis [6, 9–12]. Although it is known that structural changes to SLNs are required for premetastatic conditions, changes to regional LNs remain unexplored.
Lymphangiogenic factors promoting formation of tumor lymphatics and metastasis of tumor cells to LNs have been identified [13, 14]. These factors include the secreted glycoproteins vascular endothelial growth factor (VEGF)-C and VEGF-D, which activate VEGF receptor-3 (VEGFR-3), a cell surface receptor tyrosine kinase expressed on lymphatic endothelium [15, 16]. VEGF-C or VEGF-D overexpression is known to promote tumor lymphangiogenesis and tumor dissemination in animal models [17–19], whereas inhibition of VEGFR-3 signaling blocks these phenomena . Similarly, in human cancers, increased VEGF-C or VEGF-D expression is related to metastasis and poor prognosis [13, 14], whereas VEGF-A and VEGF-C-induced lymphangiogenesis in LNs contributes to metastasis [10, 12]. These observations support that VEGF-C or VEGF-D and VEGFR-3 signaling pathway is required for tumor lymphangiogenesis induction. However, much remains undiscovered about contribution of this pathway to lymphangiogenesis in the regional LNs proximal to tumors.
To histologically characterize regional LNs proximal to tumors.
To investigate increased lymphangiogenesis in LNs by histomorphometric analysis of lymphatic vessel endothelial hyaluronan receptor 1 (LYVE-1) -positive areas.
To examine an interaction of VEGF-C with VEGFR-3 in LN lymphangiogenesis using dual immunofluorescence.
Our results indicate that tumor-associated LNs show extensive lymphangiogenesis, which may facilitate further metastasis.
The mouse melanoma cell line, B16/F10 (RCB2630), was provided by the RIKEN BRC through the National BioResource Center through the National Bio-Resource Project of the Ministry of Education, Culture, Sports and Technology (Ibaraki, Japan). Cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM; Invitrogen, Carlsbad, CA, USA) supplemented with 10% fetal calf serum and penicillin/streptomycin. Cells were cultured in vitro until confluent and were detached with 0.25% trypsin/0.02% ethylenediaminetetraacetic acid (EDTA) solution. These cells were then used for the metastatic model, cell immunostaining, and total RNA extraction.
Animals and the spontaneous LN metastasis model
Female C57BL/6 mice (6–8 weeks old) were purchased from Kyudo Co., Ltd. (Saga, Japan). All animal studies were conducted using protocols approved by the Animal Care and Use Committee, Fukuoka Dental College. For the spontaneous LN metastasis model, tumor cells (1 x 105 in 50 μl DMEM) were injected submucosally into the left border of the tongue . Control mice were untreated.
To trace lymphatic drainage, 10 μl Evan’s blue dye (0.4%) in phosphate-buffered saline (PBS) was injected into sites of melanoma cell inoculation 15 min before sacrifice.
Cervical LNs were excised 1–21 days after injection from three animals in each treatment group. On the terminal day, the weight of each LN was measured, and the specimens immediately frozen in liquid nitrogen. Frozen specimens were cut into sections of 6-μm thickness and stained with hematoxylin and eosin (HE) to visualize histopathological changes. Frozen sections were also used for immunofluorescence and extraction of total RNA.
Tissue sections and B16F10 cells were fixed with 4% paraformaldehyde in PBS for 15 min at 4°C, then washed in PBS. To evaluate lymphangiogenesis in tumor-associated LNs, we simultaneously performed three types of double immunofluorescent staining on frozen sections comprising two mixtures of two primary antibodies, goat anti-mouse/rat tyrosinase-related protein 1 (TRP-1, 1:100; Santa Cruz Biotechnology, Inc., Sata Cruz, CA, USA) and biotinylated anti-mouse LYVE-1 (1:200; R&D Systems, Minneapolis, MN, USA) and rat anti-mouse CD45RB (1:100; Acris Antibodies, Herford, Germany) and biotinylated anti-mouse LYVE-1 and a mixture of rat anti-mouse CD31 (1:100; Becton Dickinson and Co., Franklin Lakes, NJ, USA) and biotinylated anti-mouse LYVE-1 for 2 h at room temperature. After washing with PBS, sections were incubated in a mixture of anti-goat immunoglobulin G (IgG) antibody conjugated with Alexa Fluor 488 or anti-rat IgG antibody conjugated with Alexa Flour 488 (1:200; Molecular Probes, Eugene, OR, USA), and streptavidin conjugated with Alexa Fluor 568 (1:400; Molecular Probes) for 30 min at room temperature. These two simultaneously incubated double immunofluorescence stainings were applied to examine the codistribution of VEGF-C and Fms-related tyrosine kinase 4 (Flt-4, or VEGFR-3) in tumor-associated LNs. A mixture of anti-rabbit IgG conjugated with Alexa Flour 488 (1:200; Molecular Probes) and anti-rat IgG conjugated with Alexa Flour 568 (1:200; Molecular Probe) was overlaid on tissue sections for 45 min at room temperature, followed by preincubation with mixture of rabbit anti-mouse VEGF-C (1:00; Angio-Proteomie, Boston, MA, USA) and rat anti-mouse VEGFR-3 (Flt-4, 1:100; BioLegend, San Diego, CA, USA) for 2 h. For immunofluorescent staining of B16F10 cells, paraformaldehyde-fixed cells were incubated with VEGF-C antibody (1:200; Angio-Proteomie) for 1 h and were then visualized with anti-rabbit IgG conjugated with Alexa Flour 488 (1:200; Molecular Probes) for 30 min at room temperature. Immunostained sections and cells were then counterstained with 4, 6-diamidino- 2-phenylindole (DAPI; Vector Laboratories, Inc., Burlingame, CA, USA).
Lymphatic vessel area
Lymphatic vessel area was measured in 616 x 484-mm LYVE-1-stained LN section images at 100x magnification using ImageJ (National Institutes of Health, Bethesda, MD, USA). Statistical analysis was performed with the two-tailed Student’s t-test. Data were presented as the mean ±standard error and P values of < 0.05 were considered statistically significant.
Total RNA was isolated from B16F10 cells and serial frozen sections of tumor-bearing LNs by acid guanidiniumthiocyanate-phenol-chloroform extraction using an ISOGEN kit (Nippon Gene Co., Ltd., Tokyo, Japan). Isolates were quantified, and their purity evaluated spectrophotometrically. Reverse transcription PCR (RT-PCR) was performed using the Access RT-PCR System (Promega Corp., Fitchburg, WI, USA) according to the manufacturer’s instructions. We used the following primers: human VEGF-C, 5’-TTACAGACGGCCATGTACGA-3’ (forward) and 5’-TTTGTTAGCATGGACCCACA-3’ (reverse: product size 288 bp), and human glyceraldehyde-3-phosphate dehydrogenase (G3PDH), 5’-TCCACCACCCTGTTGCTGTA-3’ (forward) and 5’-ACCACAGTCCATGCCAT-3’ (reverse: product size 450 bp). Amplification was performed by a thermal cycler for 35 cycles as follows: 30s of denaturation at 94°C, 30 s of annealing at 60°C, and 1 min of extension at 72°C for all primers. Amplified products were resolved on 1.2% agarose/Tris-acetate EDTA gels (NacalaiTesque, Inc., Kyoto, Japan) electrophoresed at 100 mV, and then visualized with ethidium bromide.
Tumor-associated LN enlargement
LNs adjacent or contralateral to tumor-bearing SLNs.
LNs proximal to tumor-bearing SLNs
Lymphangiogenesis occurs in cervical LNs showing tumor-reactive lymphadenopathy
Immunohistochemical interactions between VEGF-C and VEGFR-3 in tumor-associated LNs
Despite increasing evidence supporting involvement of the lymphatic system in the metastasis of various malignant tumors, little is known about the mechanism of continuous spreading of tumors via regional LNs. In this study, we established an experimental model of cervical LN metastasis to investigate changes in tumor-associated LNs such as SLNs before metastasis, tumor-bearing SLNs, and LNs adjacent or contralateral to tumor-bearing SLNs. We present three lines of evidence to support the conclusion that lymphangiogenesis is evident in tumor-associated regional LNs. First, all tumor-associated LNs exhibited tumor-reactive lymphadenopathy. Second, measurement of the LYVE-1-positive areas in tumor-associated LNs indicated extensive lymphangiogenesis. Third, immunohistochemical interaction of VEGF-C with VEGFR-3 was examined in LN lymphangiogenesis.
Both macroscopic and microscopic observations indicate that LNs proximate to oral melanoma show tumor-reactive lymphadenopathy regardless of the presence of tumor cells. The dilated lymphatic sinuses evident in tumor-associated LNs differ from those evident in inflammatory lymphadenopathy, which are full of lymphocytes . These differences suggest that alternate mechanisms underlie sinus expansion in tumor-associated LNs. Previous studies demonstrated that expansion of lymphatic sinuses is induced in tumor-draining LNs before metastasis [9, 11]. Our observations in SLNs without metastasis support this hypothesis. Sinus expansion in tumor-bearing LNs was also reported by Harrell et al. . Interestingly, we found that tumor-bearing SLNs could induce changes in both adjacent and contralateral LNs. Both adjacent and contralateral LNs, similarly to SLNs with or without metastases, showed enlargement and sinus expansion. These observations led us to speculate that changes in both adjacent and contralateral LNs constitute premetastatic condition for tumor dissemination via the lymphatic vessels from metastatic SLNs.
Immunohistochemical quantification of the LYVE-1-positive area revealed lymphangiogenesis in all tumor-associated LNs. These results indicate that extensive lymphangiogenesis is significantly correlated with tumor-reactive lymphadenopathy in these LNs. In this study, tumor-induced lymphangiogenesis was evident in tumor-draining SLNs before tumor cell invasion. This supports recent observations that SLN lymphangiogenesis precedes tumor metastasis [9, 11]. SLN lymphangiogenesis occurred mainly in the medullary region, following tumor cell invasion into SLNs. After metastasis was established in SLNs, lymphangiogenesis expanded to LNs adjacent or contralateral to metastatic SLNs. These results suggest that tumors in SLNs act over a distance to induce lymphangiogenesis within regional LNs.
In this study, we considered tumor-reactive lymphadenopathy to result from extensive lymphangiogenesis, suggesting that tumor-derived signals are transported via the lymphatic system to tumor-associated LNs where they induce lymphangiogenesis. Recent studies reported that VEGF-C activates lymphatic vessel growth by stimulating VEGFR-3 expressed on lymphatic endothelium [12, 14]. RT-PCR and immunohistochemical analyses in our study demonstrated expression of VEGF-C mRNA and VEGF-C protein in cultured B16F10 cells and melanoma-bearing tissues. These results suggest that tumor cells are actively responsible for lymphangiogenesis by producing of VEGF-C. Double immunofluorescent staining showed that VEGF-C in tumor cells promotes increased expression of its receptor, Flt-4, on lymphatic endothelia. In both primary tongue tumors and tumor-bearing SLNs, lymphatic vessels close to tumor cells expressed Flt-4. Interestingly, an increase in Flt-4-positive LN sinuses was observed in all tumor-associated LNs. A recent study proposed that VEGF-C-induced lymphangiogenesis in SLNs promotes tumor metastasis to distant sites . In our study, even though only immunohistohcemical results, LN lymphangiogenesisis seems to be partly mediated by VEGF-C/VEGFR-3 signaling and to promote in tumor metastasis from SLNs to adjacent and/or remote LNs. Future work using the knocked-down expression of VEGF-C in tumor cells will address the detailed mechanisms of LN lymphangiogenesis mediated by VEGF-C/VEGFR-3 signaling in this model.
In conclusions, our findings demonstrate that all tumor-associated LNs exhibit tumor-reactive lymphadenopathy, histologically characterized by extensive lymphangiogenesis. These data suggest that LN lymphangiogenesis is premetastatic condition in regional LNs and contributes to metastasis from SLN to remote LNs.
This study was supported in part by a Grant-in-Aid from the Ministry of Education, Culture, Sports, Science and Technology of Japan (#11671876, #13671977 and #1659190 to JO). The authors would like to thank Enago (http://www.enago.jp) for the English language review.
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