Pathways targeting tumor lymphangiogenesis.

Tumor metastasis to sentinel lymph nodes represents the first step of tumor dissemination in most human cancers and serves as a major prognostic indicator for disease progression. Recent studies have revealed that tumors can actively induce the formation of lymphatic vessels, and that tumor lymphangiogenesis is correlated with lymph node metastasis in experimental cancer models and in several types of human cancers. Metastatic tumor cells may continue to promote lymphatic vessel growth even after their metastasis to sentinel lymph nodes, likely promoting further cancer spread. Vascular endothelial growth factor-C (VEGF-C) and VEGF-D were the first specific lymphangiogenesis factors identified, acting predominantly via VEGF receptor-3 (VEGFR-3) that is expressed by lymphatic endothelial cells, and a large number of clinical studies have shown a correlation between tumor expression of VEGF-C or VEGF-D and lymph node metastasis. VEGFR-3 activation promotes lymphatic endothelial cell proliferation, migration, and survival via the extracellular signal-regulated kinase 1/2, the phosphatidylinositol 3-kinase/AKT, and the c-Jun NH(2)-terminal kinase 1/2 pathways. Additional tumor lymphangiogenesis factors have been recently identified, including VEGF-A. Importantly, blockade of the VEGFR-3 pathway by specific antibodies, by soluble receptor constructs, and by small molecule kinase inhibitors efficiently inhibits experimental tumor lymphangiogenesis and metastasis and might also represent a novel therapeutic avenue for the treatment of human cancers.     

Vascular endothelial cell growth factor receptor 3-mediated activation of lymphatic endothelium is crucial for tumor cell entry and spread via lymphatic vessels

Lymphangiogenic growth factors vascular endothelial growth factor (VEGF)-C and VEGF-D have been shown to promote lymphatic metastasis by inducing tumor-associated lymphangiogenesis. In this study, we have investigated how tumor cells gain access into lymphatic vessels and at what stage tumor cells initiate metastasis. We show that VEGF-C produced by tumor cells induced extensive lymphatic sprouting towards the tumor cells as well as dilation of the draining lymphatic vessels, suggesting an active role of lymphatic endothelial cells in lymphatic metastasis. A significant increase in lymphatic vessel growth occurred between 2 and 3 weeks after tumor xenotransplantation, and lymph node metastasis occurred at the same stage. These processes were blocked dose-dependently by inhibition of VEGF receptor 3 (VEGFR-3) signaling by systemic delivery of a soluble VEGFR-3-immunoglobulin (Ig) fusion protein via adenoviral or adeno-associated viral vectors. However, VEGFR-3-Ig did not suppress lymph node metastasis when the treatment was started at a later stage after the tumor cells had already spread out, suggesting that tumor cell entry into lymphatic vessels is a key step during tumor dissemination via the lymphatics. Whereas lymphangiogenesis and lymph node metastasis were significantly inhibited by VEGFR-3-Ig, some tumor cells were still detected in the lymph nodes in some of the treated mice. This indicates that complete blockade of lymphatic metastasis may require the targeting of both tumor lymphangiogenesis and tumor cell invasion.     

Suppression of tumor lymphangiogenesis and lymph node metastasis by blocking vascular endothelial growth factor receptor 3 signaling

BACKGROUND: Vascular endothelial growth factor C (VEGF-C) stimulates tumor lymphangiogenesis (i.e., formation of lymphatic vessels) and metastasis to regional lymph nodes by interacting with VEGF receptor 3 (VEGFR-3). We sought to determine whether inhibiting VEGFR-3 signaling, and thus tumor lymphangiogenesis, would inhibit tumor metastasis. METHODS: We used the highly metastatic human lung cancer cell line NCI-H460-LNM35 (LNM35) and its parental line NCI-H460-N15 (N15) with low metastatic capacity. We inserted genes by transfection and established a stable N15 cell line secreting VEGF-C and a LNM35 cell line secreting the soluble fusion protein VEGF receptor 3-immunoglobulin (VEGFR-3-Ig, which binds VEGF-C and inhibits VEGFR-3 signaling). Control lines were transfected with mock vectors. Tumor cells were implanted subcutaneously into severe combined immunodeficient mice (n = 6 in each group), and tumors and metastases were examined 6 weeks later. In another approach, recombinant adenoviruses expressing VEGFR-3-Ig (AdR3-Ig) or beta-galactosidase (AdLacZ) were injected intravenously into LNM35 tumor-bearing mice (n = 14 and 7, respectively). RESULTS: LNM35 cells expressed higher levels of VEGF-C RNA and protein than did N15 cells. Xenograft mock vector-transfected LNM35 tumors showed more intratumoral lymphatic vessels (15.3 vessels per grid; 95% confidence interval [CI] = 13.3 to 17.4) and more metastases in draining lymph nodes (12 of 12) than VEGFR-3-Ig-transfected LNM35 tumors (4.1 vessels per grid; 95% CI = 3.4 to 4.7; P<.001, two-sided t test; and four lymph nodes with metastases of 12 lymph nodes examined). Lymph node metastasis was also inhibited in AdR3-Ig-treated mice (AdR3-Ig = 0 of 28 lymph nodes; AdLacZ = 11 of 14 lymph nodes). However, metastasis to the lungs occurred in all mice, suggesting that LNM35 cells can also spread via other mechanisms. N15 tumors overexpressing VEGF-C contained more lymphatic vessels than vector-transfected tumors but did not have increased metastatic ability. CONCLUSIONS: Lymph node metastasis appears to be regulated by additional factors besides VEGF-C. Inhibition of VEGFR-3 signaling can suppress tumor lymphangiogenesis and metastasis to regional lymph nodes but not to lungs.     

血管内皮生长因子D及其受体3在乳腺癌淋巴转移中作用的研究进展

乳腺癌是女性最常见的恶性肿瘤。乳腺癌淋巴转移与患者的预后密切相关。最近研究表明淋巴管生成可能会主动促进淋巴转移的发生,而血管内皮生长因子家族的部分成员在这一过程中发挥了重要作用,如血管内皮生长因子C、血管内皮生长因子D及其受体3等。但是,对乳腺癌中血管内皮生长因子D的作用及其预后价值、血管内皮生长因子受体3与乳腺癌淋巴管生成的关系等问题仍有争议。本文对近年国内外有关血管内皮生长因子D及其受体3在乳腺癌淋巴转移中作用的研究进展作一综述。     

Tumor-induced activation of lymphatic endothelial cells via vascular endothelial growth factor receptor-2 is critical for prostate cancer lymphatic metastasis

Prostate cancer disseminates initially and primarily to regional lymph nodes. However, the nature of interactions between tumor cells and lymphatic endothelial cells (LEC) is poorly understood. In the current study, we have isolated prostate LECs and developed a series of two-dimensional and three-dimensional in vitro coculture systems and in vivo orthotopic prostate cancer models to investigate the interactions of prostate cancer cells with prostate LECs. In vitro, highly lymph node metastatic prostate cancer cell lines (PC-3 and LNCaP) and their conditioned medium enhanced prostate LEC tube formation and migration, whereas poorly lymph node metastatic prostate cancer cells (DU145) or normal prostate epithelial cells (RWPE-1) or their conditioned medium had no effect. In vivo, the occurrence of lymphatic invasion and lymph node metastasis was observed in PC-3 and LNCaP xenografts but not in DU145 xenografts. Furthermore, vascular endothelial growth factor (VEGF) receptor (VEGFR)-2 is expressed by prostate LECs, and its ligands VEGF-A, VEGF-C, and VEGF-D are up-regulated in highly lymph node metastatic prostate cancer cells. Recombinant VEGF-A and VEGF-C, but not VEGF-C156S, potently promoted prostate LEC tube formation, migration, and proliferation in vitro, indicating that signaling via VEGFR-2 rather than VEGFR-3 is involved in these responses. Consistent with this, blockade of VEGFR-2 significantly reduced tumor-induced activation of LECs. These results show that the interaction of prostate tumor cells with LECs via VEGFR-2 modulates LEC behavior and is related to the ability of tumor cells to form lymph node metastases.     

Tumor-secreted vascular endothelial growth factor-C is necessary for prostate cancer lymphangiogenesis, but lymphangiogenesis is unnecessary for lymph node metastasis

Dissemination to draining lymph nodes is a frequent first step in prostate cancer metastasis. Although tumors metastasize to lymph nodes via the lymphatics, the importance of lymphangiogenesis in mediating the process remains controversial. Here, we inhibit intratumoral lymphangiogenesis in s.c. and surgical orthotopic implantation mouse models of human prostate cancer using several strategies. Stable expression of small interfering RNAs (siRNA) targeted against human vascular endothelial growth factor-C (VEGF-C) in PC-3 cells reduced intratumoral lymphatics by 99% in s.c. tumors, indicating that tumor-secreted VEGF-C is necessary for lymphangiogenesis. Expression of siRNAs against human VEGF-A somewhat reduced tumor lymphangiogenesis. Secretion of a soluble VEGF receptor-3/Flt4 fusion protein by PC-3 cells reduced intratumoral lymphatics by 100% in s.c. tumors. Combination of soluble Flt4 and VEGF-C siRNA yielded >92% reduction of intratumoral lymphatics in orthotopic prostate tumors. However, metastasis to lymph nodes was not significantly affected regardless of intratumoral lymphatic vessel density. The abundance of marginal lymphatics at the tumor-stromal interface was unchanged in orthotopic tumors whose intratumoral lymphatics were inhibited, suggesting that these marginal vessels could be sufficient for lymph node metastasis. Hematogenous metastasis (blood tumor burden, lung metastasis) correlated with degree of lymph node invasion. We also analyzed the lymphatics in spontaneous transgenic adenocarcinomas of the mouse prostate which metastasize to lymph nodes. Progression from well-differentiated prostate intraepithelial neoplasia to metastatic, undifferentiated adenocarcinoma was accompanied by loss of lymphatics. These results suggest that tumor-secreted VEGF-C and, to a lesser extent, VEGF-A, are important for inducing prostate cancer intratumoral lymphangiogenesis but are unnecessary for lymph node metastasis.     

VEGF-D promotes tumor growth and lymphatic spread in a mouse model of hepatocellular carcinoma

Lymphatic spread is an important clinical determinant for the prognosis of hepatocellular carcinoma (HCC), but little is known about the control of lymphangiogenesis in HCC. We addressed expression and biological role of the pro-(lymph), angiogenic protein VEGF-D in this tumor entity. Using immunohistochemistry and in situ hybridization on specimens of HCC, cirrhotic and normal liver we found abundant expression of VEGF-D exclusively in the tumor cells. The cognate receptor VEGFR-3 was detected on blood and lymphatic vessels. By clinicopathological analysis VEGF-D expression was correlated with pT-stage of the primary, lymph node metastasis and lymphangiosis carcinomatosa. Three out of 4 human HCC cell lines expressed and secreted VEGF-D. To approach its biological function, VEGF-D deficient SKHep-1 cells were stably transfected with VEGF-D cDNA and effects on tumor progression were determined in vivo. Compared to mock-transfected controls, subcutaneous tumors derived from VEGF-D expressing cells were larger and more frequently metastasized to regional lymph nodes. VEGF-D expressing tumors exhibited increased microvessel density and increased abundance of peri- and intratumoral lymphatics, as assessed by immunostaining for CD31 and for LYVE-1 and/or podoplanin, respectively. Furthermore, coexpression of the soluble extracellular VEGFR-3 domain blocked VEGF-D-induced tumor growth and lymphatic spread via reduction of angiogenesis and lymphangiogenesis. In the orthotopic approach, VEGF-D expression resulted in an increased rate of intra- and extrahepatic as well as lymph node metastasis. In conclusion, our study suggests that expression of VEGF-D is involved in growth and lymphatic spread of HCC. Therefore, VEGF-D might represent a therapeutic target in HCC.     

MIA-dependent angiogenesis and lymphangiogenesis are closely associated with progression,nodal metastasis and poor prognosis in tongue squamous cell carcinoma

We examined the role of angiogenesis/lymphangiogenesis and the relationship between melanoma inhibitory activity (MIA) and angiogenesis or lymphangiogenesis in oral squamous cell carcinoma (OSCC). One hundred and one formalin-fixed, paraffin-embedded specimens of primary OSCC were evaluated for microvessel density (MVD), lymphovessel density (LVD), expression of vascular endothelial growth factor (VEGF), VEGF-C, VEGF-D and MIA. Fresh frozen 18 samples of primary OSCC were further examined for the expression of VEGF, VEGF-C, VEGF-D and MIA protein by enzyme-linked immunosorbent assay (ELISA). In in vitro analysis, we studied the change of VEGF, VEGF-C and VEGF-D expression after MIA siRNA treatment. Higher MVD, LVD and VEGF expression levels were closely associated with tumour progression, nodal metastasis and poor prognosis. Expression levels of VEGF-C and VEGF-D were only related with nodal metastasis. MIA expression was significantly associated with MVD, LVD, VEGF, VEGF-C and VEGF-D expression by immunohistochemistry and ELISA assay. VEGF, VEGF-C, VEGF-D and MIA expression levels of metastatic tongue cancer HSC-3 cells were higher than those with no metastatic HSC-4 cells, and VEGF, VEGF-C and VEGF-D expression levels were decreased by MIA siRNA treatment in both cells. MIA-dependent angiogenesis/lymphangiogenesis might be a useful therapeutic target in progressive and metastatic OSCC.     

Expression of vascular endothelial growth factor receptor-3 by lymphatic endothelial cells is associated with lymph node metastasis in prostate cancer.

PURPOSE: The molecular mechanisms underlying lymph node metastasis are poorly understood, despite the well-established clinical importance of lymph node status in many human cancers. Recently, vascular endothelial growth factor (VEGF)-C and VEGF-D have been implicated in the regulation of tumor lymphangiogenesis and enhancement of lymphatic invasion via activation of VEGF receptor-3. The purpose of this study was to determine the expression pattern of the VEGF-C/VEGF-D/VEGF receptor-3 axis in prostate cancer and its relationship with lymph node metastasis. EXPERIMENTAL DESIGN: The expression pattern of VEGF-C, VEGF-D, and VEGF receptor-3 in localized prostate cancer specimens (n = 37) was determined using immunohistochemistry. RESULTS: Widespread, heterogeneous staining for VEGF-C and VEGF-D was observed in all cancer specimens. Intensity of VEGF-C staining was lower in benign prostate epithelium than in adjacent carcinoma, whereas no difference between benign epithelium and carcinoma was observed for VEGF-D staining. VEGF receptor-3 immunostaining was detected in endothelial cells of lymphatic vessels in 18 of 37 tissue samples. The presence of VEGF receptor-3-positive vessels was associated with lymph node metastasis (P = 0.0002), Gleason grade (P < 0.0001), extracapsular extension (P = 0.0382), and surgical margin status (P = 0.0069). In addition, VEGF receptor-3 staining highlighted lymphatic invasion by VEGF-C-positive/VEGF-D-positive carcinoma cells. CONCLUSIONS: Together, these results suggest that paracrine activation of lymphatic endothelial cell VEGF receptor-3 by VEGF-C and/or VEGF-D may be involved in lymphatic metastasis. Thus the VEGF-C/VEGF-D/VEGF receptor-3 signaling pathway may provide a target for antilymphangiogenic therapy in prostate cancer.     

Mechanisms and role of lymphangiogenesis in cancer metastasis

An important critical point in tumor progression is the acquisition of metastatic potential. The presence of metastases in regional lymph nodes is an indicator of poor survival. The vascular endothelial growth factor (VEGF) family of growth factors and receptors is involved in vasculogenesis and angiogenesis. Among them, VEGF-C and VEGF-D regulate the lymphatic vessels development and growth via their binding to their receptor VEGFR3. The expression of VEGF-C or VEGF-D is demonstrated in various human tumors and can be used as pronostic factors in some of them. With the aid of these molecules and the discovery of specific lymphatic markers, lymphatic endothelial cells can be isolated and lymphatic vessels can be identified within tumors. The role of lymphangiogenesis in promoting the metastatic spread of tumor cells has been studied in animal models.     

Quantitative analysis of lymphangiogenic markers for predicting metastasis of human gastric carcinoma to lymph nodes

The spread of tumor cells to regional lymph nodes is an early event of gastric cancer metastasis. In our study, we assessed the expression of lymphangiogenic factors and lymphatic endothelial markers in gastric carcinoma tissues and compared expression levels with the status of lymph node metastasis. We also examined the correlation between lymphatic vessel density (LVD) in primary tumors and lymph node metastasis. Paired biopsy samples (tumor and corresponding normal mucosa) of gastric tissue were obtained from 39 patients with gastric carcinoma. The expression of VEGF-C, VEGF-D, VEGFR-3 and podoplanin mRNAs was assessed by real-time quantitative PCR. The expression of VEGF-C (but not of VEGF-D) was significantly greater in patients with lymph node metastasis than in those without metastasis. The expression of lymphatic endothelial markers VEGFR-3 and podoplanin was also significantly greater in the node-positive group. LVD, as assessed by immunohistochemistry for podoplanin, was correlated with lymph node metastasis. These results indicate that quantitative analysis of lymphangiogenic markers in gastric biopsy specimens may be useful in predicting metastasis of gastric cancer to regional lymph nodes.     

Relationship between Lymphatic Vessel Density and Lymph Node Metastasis of Invasive Micropapillary Carcinoma of the Breast

OBJECTIVE To investigate the relationship between lymphatic vessel density and lymph node metastasis of invasive micropapillary carcinoma (IMPC) of the breast. METHODS The immunohistochemical study for vascular endothelial growth factor-c (VEGF-C), VEGF Receptor-3 (VEGFR-3) and lymphatic vessel density of 51 cases of IMPC were performed, and lymph node metastases were examined by microscopic analysis of these cases. RESULTS In IMPC, VEGF-C was expressed in the cytoplasm and/or on the membrane of the tumor cells, and the expression of VEGF-C showed a positive correlation with lymph node metastasis (P<0.01). Lymphatic vessel density was determined by the number of micro-lymphatic vessels with VEGFR-3 positive staining. Lymphatic vessel density was positively correlated with VEGF-C expression (P<0.01) and lymph node metastasis (P<0.01). The percentage of IMPC in the tumor was not associated with the incidence of lymph node metastasis. The metastatic foci in lymph nodes were either pure or predominant micropapillary carcinoma. CONCLUSION The results suggested that VEGF-C overexpression stimulated tumor lymphangiogenesis, and the increased lymphatic vessel density may be the key factor that influenced lymph node metastasis of IMPC.     

VEGF-C induced lymphangiogenesis is associated with lymph node metastasis in orthotopic MCF-7 tumors

The spread of cancer cells to regional lymph nodes through the lymphatic system is the first step in the dissemination of breast cancer. In several human cancers including those of the breast and prostate, the expression of vascular endothelial growth factor C (VEGF-C) is associated with lymph node metastasis. Our study was undertaken to evaluate the effect of VEGF-C on metastasis of poorly invasive, estrogen dependent human MCF-7 breast cancer cells. MCF-7 breast cancer cells transfected with VEGF-C (MCF-7-VEGF-C) were grown as tumors in the mammary fat pads of nude mice implanted with subcutaneous estrogen pellets. Tumor lymphangiogenesis and lymph node metastasis were studied immunohistochemically using antibodies against lymphatic vessel hyaluronan receptor -1 (LYVE-1), VEGF receptor-3 (VEGFR-3), PECAM-1, pan-cytokeratin and estrogen dependent pS2 protein. Overexpression of VEGF-C in transfected MCF-7 cells stimulated in vivo tumor growth in xenotransplanted mice without affecting estrogen responsiveness. The resulting tumors metastasized to the regional lymph nodes in 75% (in 6 mice out of 8, Experiment I) and in 62% (in 5 mice out of 8, Experiment II) of mice bearing orthotopic tumors formed by MCF-7-VEGF-C cells whereas no metastases were observed in mice bearing tumors of control vector-transfected MCF-7 cells (MCF-7-Mock). The density of intratumoral and peritumoral lymphatic vessels was increased in tumors derived from MCF-7-VEGF-C cells but not MCF-7-Mock cells. Taken together, our results show that VEGF-C overexpression stimulates tumor lymphangiogenesis and induces normally poorly metastatic estrogen-dependent MCF-7 tumors to disseminate to local lymph nodes. These data suggest that VEGF-C has an important role in lymph node metastasis of breast cancer even at its hormone-dependent early stage.     

VEGF、VEGF-C和VEGF-D在甲状腺乳头状癌组织中的表达及意义

背景与目的:研究表明血管内皮生长因子C和D(vascularendothelialgrowthfactor-Cand-D,VEGF-C,VEGF-D)与淋巴管的生成有关。局部淋巴结转移是甲状腺乳头状癌的特征,本研究的目是探讨血管内皮生长因子及其C和D(VEGF、VEGF-C和VEGF-D)蛋白在甲状腺乳头状癌的表达及其意义。方法:应用免疫组织化学SP法检测115例甲状腺乳头状癌和20例结节性甲状腺肿中VEGF、VEGF-C和VEGF-D蛋白的表达。结果:甲状腺乳头状癌VEGF、VEGF-C和VEGF-D的阳性率分别为79.1%、87.0%和72.2%。结节性甲状腺肿组织中VEGF、VEGF-C和VEGF-D的阳性率分别为30.0%、15.0%和20.0%。甲状腺乳头状癌VEGF、VEGF-C和VEGF-D的阳性率较结节性甲状腺肿明显增高。VEGF的表达与甲状腺癌的大小有关。VEGF、VEGF-C和VEGF-D在淋巴结转移组和非转移组的阳性率分别为84.7%、93.2%、83.1%和73.2%、80.4%、60.7%。VEGF-C和VEGF-D的阳性表达与甲状腺乳头状癌的淋巴结转移显著相关(P<0.05;P<0.01)。结论:VEGF与甲状腺乳头状癌的生长有关。VEGF-C和VEGF-D与淋巴结转移密切相关,提示甲状腺乳头状癌标本VEGF-C和VEGF-D的检测可作为预测肿瘤转移的指标之一。     

Suppression of VEGFR-3 signaling inhibits lymph node metastasis in gastric cancer

In gastric cancer, lymph node metastasis is one of the major prognostic factors and forms the basis for surgical removal of local lymph nodes. Recently, several studies have demonstrated that overexpression of lymphangiogenic growth factor VEGF-C or VEGF-D induces tumor lymphangiogenesis and promotes lymphatic metastasis in mouse tumor models. We examined whether these processes could be inhibited in naturally metastatic tumors by blocking of their cognate receptor VEGFR-3 signaling pathway. Using a mouse orthotopic gastric cancer model which has a high frequency of lymph node metastasis, we estimated lymphatic vessels in gastric cancers by immunostaining for VEGFR-3 and other specific lymphatic markers, LYVE-1 and prox-1. Then we systemically administered anti-VEGFR-3 blocking antibodies. This treatment resulted in the inhibition of regional lymph node metastasis and reduction of lymphatic vessel density in the primary tumors. In addition, increased density of LYVE-1-positive lymphatic vessels of primary tumors was closely correlated with lymph node metastasis in human samples of gastric cancer. Antilymphangiogenesis by inhibiting VEGFR-3 signaling could provide a potential strategy for the prevention of lymph node metastasis in gastric cancer.     

Vascular endothelial growth factor C promotes human gastric carcinoma lymph node metastasis in mice.

Vascular endothelial growth factor (VEGF)-C, one of several members of the VEGF family, is a relatively specific lymphangiogenic growth factor. VEGF receptor (VEGFR)-3 (or Flt4) is a VEGF-C receptor with expression restricted to lymphatic endothelial cells. Since the mechanisms by which carcinoma cells metastasize to lymph nodes remain unclear, we constructed a VEGF-C transfectant (AZ-VEGF-C) from the AZ521 human gastric carcinoma cell line, which ordinarily shows little nodal metastatic potential and little VEGF-C expression. We orthotopically implanted transfected tumor cells into the stomachs of nude mice. The number of mice developing lymph node metastases and the number of lymph node metastases per mouse with nodal metastases were higher than with implants of mock-transfected control cells. Specifically, percentages of mice with lymph node metastases were 95.5% (21/22) for AZ-VEGF-C and 29.4% (5/17) for controls (P<0.01), while mean numbers of involved lymph nodes were 3.76 for AZ-VEGF-C and 1.00 for controls (P<0.01). No difference was found between AZ-VEGF-C and controls regarding cell growth and chemotactic responses in vitro, or in volumes of tumors arising from implanted cells. When we performed immunohistochemical staining for VEGFR-3 in these tumors to investigate lymphangiogenesis by VEGF-C, the number of vessels stained for VEGFR-3 in tumors and surrounding tissues was higher for AZ-VEGF-C than for controls. VEGFR-3-positive vessels occupied 14.9/1000 of microscopically examined areas for AZ-VEGF-C, but only 1.30/1000 for controls (P<0.001). Our results suggest that VEGF-C is a specific lymphangiogenic growth factor with an important role in lymph node metastasis.