Search

BACKGROUND: New lymphatic growth may contribute to tumor metastasis. Activation of vascular endothelial growth factor receptor 3 (VEGFR-3) by its ligands VEGF-C and -D is necessary for embryonic and tumor lymphangiogenesis. However, the exact role of VEGFR-3 signaling in adult lymphangiogenesis and in lymphatic vessel survival and regeneration is unclear. METHODS: A novel rat monoclonal antibody to murine VEGFR-3, mF4-31C1, which potently antagonizes the binding of VEGF-C to VEGFR-3, was developed. We tested the effects of systemic mF4-31C1 administration in a mouse tail skin model of lymphatic regeneration, either with or without local overexpression of VEGF-C, and we observed lymphatic and blood vessel regeneration over time using microlymphangiography and immunostaining. RESULTS: Normal mice regenerated complete and functional lymphatic vessels within 60 days of surgery. In athymic mice implanted with VEGF-C-overexpressing human breast carcinoma cells, lymphatic regeneration took place over 25 days and resulted in hyperplastic vessels. Under either condition, no lymphatic regeneration occurred in mice receiving mF4-31C1 during the regeneration period. Blood angiogenesis and preexisting lymphatic vessels were unaffected, both in morphology and in function. CONCLUSIONS: Blocking VEGFR-3 completely and specifically prevented both physiologically normal and tumor VEGF-C-enhanced lymphangiogenesis in the adult mouse but had no effect on either blood angiogenesis or the survival or function of existing lymphatic vessels. Thus, targeting VEGFR-3 with specific inhibitors may block new lymphatic growth exclusively.

The lymphatic microvasculature is uniquely adapted for the continuous removal of interstitial fluid and proteins, and is an important point of entry for leukocytes and tumor cells. The traditional view that lymphatic capillaries are passive participants in these tasks is currently being challenged. This overview highlights recent advances in our understanding of the molecular mechanisms underlying the formation and function of lymphatic vessels.

The lymphatic system transports interstitial fluid and macromolecules from tissues back to the blood circulation, and plays an important role in the immune response by directing the traffic of lymphocytes and antigen-presenting cells. The lymphatic system also constitutes one of the most important pathways of tumor dissemination. In many human cancers, increased expression of vascular endothelial growth factor-C (VEGF-C) is correlated with regional lymph node metastases. Experimental studies using transgenic mice overexpressing VEGF-C or xenotransplantation of VEGF-C-expressing tumor cells into immunodeficient mice have demonstrated a role for VEGF-C in tumor lymphangiogenesis and the subsequent formation of lymph node metastases. However, there is at present little evidence for lymphangiogenesis in human tumors and the relative importance of preexisting vs. newly formed lymphatics for metastasis in humans remains to be determined. Nonetheless, the striking correlation between the levels of VEGF-C in primary human tumors and lymph node metastases predicts its importance in cancer spread. Aside from promoting lymphangiogenesis, VEGF-C may also activate lymphatics to promote tumor cell chemotaxis, lymphatic intravasation and hence tumor cell dissemination.

Chronic regional impairments of the lymphatic circulation often lead to striking architectural abnormalities in the lymphedematous tissues. Lymphedema is a common, disabling disease that currently lacks a cure. Vascular endothelial growth factors C and D mediate lymphangiogenesis through the VEGFR-3 receptor on lymphatic endothelia. The purpose of this study was to investigate the therapeutic potential for lymphangiogenesis with VEGF-C. We developed a rabbit ear model to simulate human chronic postsurgical lymphatic insufficiency. Successful, sustained surgical ablation of the ear lymphatics was confirmed by water displacement volumetry. After complete healing, the experimental animals (n=8) received a single, s.c. 100 microg dose of VEGF-C in the operated ear; controls (n=8) received normal saline. Radionuclide lymphoscintigraphy was performed to quantitate lymphatic function. Immunohistochemistry (IHC) was performed 7-8 days following treatment. After VEGF-C, there was a quantifiable amelioration of lymphatic function. IHC confirmed a significant increase in lymphatic vascularity, along with reversal of the intense tissue hypercellularity of untreated lymphedema. This study confirms the capacity of a single dose of VEGF-C to induce therapeutic lymphangiogenesis in acquired lymphedema. In addition to improving lymphatic function and vascularity, VEGF-C can apparently reverse the abnormalities in tissue architecture that accompany chronic lymphatic insufficiency.

The lymphatic system serves as the primary route for the metastasis of breast cancer, and the extent of lymph node involvement is a key prognostic factor for the outcome of the disease. Whereas the significance of angiogenesis for tumor progression has been well documented, the ability of tumor cells to induce the growth of lymphatic vessels, (lymphangiogenesis) and the presence of intratumoral lymphatic vessels have been questioned. We demonstrate occurrence of intratumoral lymphangiogenesis within human breast cancers alter orthotopic transplantation onto nude mice, using a novel marker for lymphatic endothelium, LYVE-1-I. We show using green fluorescent protein-tagging that breast tumor cells which overexpress vascular endothelial growth factor-C (VEGF-C), display a potent increase in intratumoral lymphangiogenesis, and significantly enhanced metastasis to both regional lymph nodes and lungs. Furthermore we have found that the degree of tumor lymphangiogenesis is highly correlated with the extent of both lymph node and lung metastases. These results establish the occurrence and biological significance of intratumoral lymphangiogenesis in breast cancer and identify VEGF-C as a molecular link between tumor lymphangiogenesis and metastasis.

The distinct roles of angiopoietin (Ang)-1 and Ang2, counteracting ligands for the endothelium-specific Tie2 receptor, in tumor development and progression have remained poorly understood. We investigated the expression of Ang1 and Ang2 during multistep mouse skin carcinogenesis and in human squamous cell carcinoma (SCC) xenografts. Expression of Ang2, but not of Ang1, was up-regulated in angiogenic tumor vessels already in early stages of skin carcinogenesis and was also strongly increased in SCCs. Stable overexpression of Ang1 in human A431 SCCs resulted in a more than 70% inhibition of tumor growth, associated with enhanced Tie2 phosphorylation levels, as compared with low levels in control transfected tumors. No major changes in the vascular density, vascular endothelial growth factor mRNA and protein expression, and vascular endothelial growth factor receptor-2 phosphorylation levels were observed in Ang1-expressing tumors. However, the fraction of tumor blood vessels with coverage by alpha-smooth muscle actin-positive periendothelial cells was significantly increased, indicative of an increased vascular maturation status. These findings identify an inhibitory role of Ang1/Tie2 receptor-mediated vessel maturation in SCC growth and suggest that up-regulation of its antagonist, Ang2, during early-stage epithelial tumorigenesis contributes to the angiogenic switch by counteracting specific vessel-stabilizing effects of Ang1.

The above summarized findings of this laboratory on tumor-stroma interactions, together with a growing number of other reports contribute to the accumulating evidence that the stromal part of epithelial tumors plays a more active role in epithelial tumor development and progression than until recently recognized. The transplantation assay developed by us represents in in vivo system particularly suited to highlight early tumor-stroma interactions and to discriminate tumor stage-dependent characteristics in this interplay. Furthermore, by the unique assembly and geometry of this model the essential role of tumor-stroma interactions for tumor invasion has been documented unimbigously for the first time. Thus, the significance of extracellular matrix composition, stromal protease activity and growth factor interactions for modulating the tumor phenotype has been evidenced in different experimental settings. In addition to the transplantation assay comparable culture systems in vitro provide relevant models to study the mechanistic details of the complex interplay between tumor and stroma cells. These investigations will not only increase our knowledge on the important role of stromal cells in tumor biology, but may yield additional targets for novel therapeutic strategies aiming at genetically normal cells according to the principle of antiangiogenic therapy.

The lymphatic system serves as the primary route for the metastasis of many cancers and the extent of lymph node involvement is the most important indicator of tumor aggressiveness. Despite the apparent importance of the lymphatic vessels for tumor dissemination, it has remained unclear whether activation of lymphatic endothelial cells may affect tumor progression and metastasis and the molecular mechanisms of lymphangiogenesis are just beginning to be elucidated. This overview describes the unique structural and functional characteristics of the lymphatic vessels that render them particularly suitable for invasion by tumor cells and for their efficient transport to lymph nodes. Recent evidence indicates occurrence of tumor lymphangiogenesis and its correlation with metastasis. Molecular regulation of tumor lymphangiogenesis, its significance for tumor metastasis, and implications for cancer therapy are discussed.

Interactions of tumor cells with lymphatic vessels are of paramount importance for tumor progression, however, the underlying molecular mechanisms are poorly understood. Whereas enlarged lymphatic vessels are frequently observed at the periphery of malignant melanomas, it has remained unclear whether intratumoral lymphangiogenesis occurs within these tumors. Here, we demonstrate the presence of intratumoral lymphatics and enlargement of lymphatic vessels at the tumor periphery in vascular endothelial growth factor (VEGF)-C-overexpressing human melanomas transplanted onto nude mice. VEGF-C expression also resulted in enhanced tumor angiogenesis, indicating a coordinated regulation of lymphangiogenesis and angiogenesis in melanoma progression. The specific biological effects of VEGF-C were critically dependent on its proteolytic processing in vivo. Furthermore, VEGF-C induced chemotaxis of macrophages in vitro and in vivo, revealing a potential function of VEGF-C as an immunomodulator. Taken together, our results identify VEGF-C as multifunctional factor involved in regulating tumor lymphangiogenesis, angiogenesis, and immune response.

Metastasis of breast cancer occurs primarily through the lymphatic system, and the extent of lymph node involvement is a key prognostic factor for the disease. Whereas the significance of angiogenesis for tumor progression has been well documented, the ability of tumor cells to induce the growth of lymphatic vessels (lymphangiogenesis) and the presence of intratumoral lymphatic vessels have been controversial. Using a novel marker for lymphatic endothelium, LYVE-1, we demonstrate here the occurrence of intratumoral lymphangiogenesis within human breast cancers after orthotopic transplantation onto nude mice. Vascular endothelial growth factor (VEGF)-C overexpression in breast cancer cells potently increased intratumoral lymphangiogenesis, resulting in significantly enhanced metastasis to regional lymph nodes and to lungs. The degree of tumor lymphangiogenesis was highly correlated with the extent of lymph node and lung metastases. These results establish the occurrence and biological significance of intratumoral lymphangiogenesis in breast cancer and identify VEGF-C as a molecular link between tumor lymphangiogenesis and metastasis.

Disruption of the precise balance of positive and negative molecular regulators of blood and lymphatic vessel growth can lead to myriad diseases. Although dozens of natural inhibitors of hemangiogenesis have been identified, an endogenous selective inhibitor of lymphatic vessel growth has not to our knowledge been previously described. We report the existence of a splice variant of the gene encoding vascular endothelial growth factor receptor-2 (Vegfr-2) that encodes a secreted form of the protein, designated soluble Vegfr-2 (sVegfr-2), that inhibits developmental and reparative lymphangiogenesis by blocking Vegf-c function. Tissue-specific loss of sVegfr-2 in mice induced, at birth, spontaneous lymphatic invasion of the normally alymphatic cornea and hyperplasia of skin lymphatics without affecting blood vasculature. Administration of sVegfr-2 inhibited lymphangiogenesis but not hemangiogenesis induced by corneal suture injury or transplantation, enhanced corneal allograft survival and suppressed lymphangioma cellular proliferation. Naturally occurring sVegfr-2 thus acts as a molecular uncoupler of blood and lymphatic vessels; modulation of sVegfr-2 might have therapeutic effects in treating lymphatic vascular malformations, transplantation rejection and, potentially, tumor lymphangiogenesis and lymphedema (pages 993-994).

The lymphatic vessels are thought to contribute to metastasis primarily by serving as a transportation system. It is widely believed that tumor cells enter lymph nodes passively, by the flow of lymph. Here, we demonstrate that lymph node lymphatic sinuses control tumor cell entry into the lymph node. In vitro, tumor migration to lymphatic endothelium (LECs) was inhibited by blocking CCR8 or CCL1. Recombinant CCL1 promoted migration of CCR8+ tumor cells. Pro-inflammatory mediators TNF-α, IL-1α and LPS increased CCL1 production by LECs as well as tumor cell migration to LECs. Blocking studies showed that CCL1 is a key molecule mediating tumor cell chemotaxis to inflamed lymphatic endothelium. In mouse and human tissues CCL1 protein was detected in lymph node lymphatic sinuses, but not in the peripheral lymphatics. In addition, CCR8 was strongly expressed by human malignant melanoma. In a mouse model, blocking CCR8 function decreased lymph node metastasis. Notably, inhibition of CCR8 led to the arrest of tumor cells in the collecting lymphatic vessels at the junction with the lymph node subcapsular sinus. These data identify a novel function for CCL1/CCR8 in metastasis and lymph node LECs as a critical check-point for entry of metastases into the lymph nodes. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4368. doi:1538-7445.AM2012-4368

In Response: Our study is the first in which vascular endothelial growth factor receptor-2 (VEGFR-2) and VEGFR-3 signaling have been selectively inhibited and the efficacy of the treatments in inhibiting tumor lymphangiogenesis, angiogenesis, and metastasis have been compared. The key findings

The mechanisms of angiogenesis have been studied extensively over the past years. The focus, however, has been almost exclusively on blood vessels, whereas little effort has been directed toward understanding lymphangiogenesis and the role of lymphatic vessels in physiology and pathology. The lymphatic system, acting in concert with the blood vascular system, is of fundamental importance in maintaining tissue homeostasis, and disorders of the lymphatic system are common, often resulting in chronic, disabling conditions. This overview summarizes the most important aspects of the structure and function of the lymphatic system with emphasis on the skin lymphatic vasculature and the differences between blood and lymphatic vessels. Special attention has been given to the methods employed in research of the lymphatic system. Finally, we describe molecular mechanisms involved in the regulation of lymphangiogenesis. Vascular endothelial growth factor and vascular endothelial growth factor-C, expressed by distinct skin cell populations, play an important role in the molecular control of skin angiogenesis and lymphangiogenesis.

SummaryDendritic cell (DC) migration from the periphery to lymph nodes is regulated by the pattern of genes expressed by DCs themselves and by signals within the surrounding peripheral environment. Here, we report that DC mobilization can also be regulated by signals initiated within the downstream lymph nodes, particularly when lymph nodes enlarge as a consequence of immunization. Lymph node B lymphocytes orchestrate expansion of the lymphatic network within the immunized lymph node. This expanded network in turn supports increased DC migration from the periphery. These results reveal unique relationships between B cells, lymphatic vessels, and migratory DCs. Knowledge that DC migration from the periphery is augmented by B cell-dependent signals reveals new potential strategies to increase DC migration during vaccination.

Kaposi's sarcoma is characterized by clusters of spindle-shaped cells that are considered to be tumor cells and by prominent vasculature. Whereas spindle cells are most likely endothelial in origin, it remains controversial whether they are of lymphatic or blood vascular derivation. To test the hypothesis that the lymphangiogenesis factor vascular endothelial growth factor-C and its receptors, KDR and flt-4, are involved in the pathogenesis of Kaposi's sarcoma, we performed in situ hybridizations and immunofluorescent stainings on human immunodeficiency virus-associated Kaposi's sarcoma. Spindle-shaped tumor cells strongly expressed KDR and flt-4 mRNA. Immunofluorescent staining confirmed expression of the flt-4 receptor in Kaposi's sarcoma cells, and double labeling revealed its colocalization with the endothelial cell marker CD31. Vascular endothelial growth factor-C was strongly expressed in blood vessels associated with Kaposi's sarcoma. In vitro, human dermal microvascular endothelial cells also expressed vascular endothelial growth factor-C mRNA that was further upregulated by vascular permeability factor/vascular endothelial growth factor. Vascular endothelial growth factor-C potently stimulated the proliferation of Kaposi's sarcoma tumor cells in vitro. These results demonstrate important paracrine functions of vascular endothelial growth factor-C, produced by blood vessels, in the pathogenesis of cutaneous Kaposi's sarcoma, and suggest a lymphatic origin and/or differentiation of Kaposi's sarcoma tumor cells.