Encyclopedia of Cancer, 2015
Lymphangiogenesis is the process whereby new lymphatic vessels develop within a tissue. Most commonly, lymphangiogenesis refers to the growth of lymphatic vessels by sprouting of new vessels from preexisting lymphatic vessels. Additionally, lymphangiogenesis refers to the initial formation of the lymphatic system during embryonic development.
Lymphangiogenesis, the growth of new lymphatic vessels, occurs during embryonic development and in tumors and lymph nodes of tumor-bearing animals. This key process also occurs in wounds and in inflamed tissues. Lymphangiogenesis in tumors has been linked to the formation of lymph node metastases. Importantly, lymph nodes are the initial or frequent sites of metastasis for many tumors, including human pancreatic, gastric, breast, and prostate carcinomas, melanomas, and other tumors, and recent studies indicate that lymphangiogenesis likely plays an important role in driving tumor metastasis.
The lymphatic system is comprised of a network of blind-ended, thin-walled lymphatic capillaries, collecting vessels, and specialized secondary immune organs, including lymph nodes, tonsils, Peyer’s patches, and spleen. This system is connected to the vascular system through the thoracic duct. Lymphatic vessels drain protein-rich interstitial fluid and immune cells from tissues through lymph nodes. A thin network of lymphatic capillaries is found in the outer rim or capsule of the normal lymph node. Lymphatic capillaries are comprised of a single layer of lymphatic endothelial cells, which share many molecular characteristics with vascular endothelial cells. Collecting vessels in the lymphatic system are comprised of endothelia surrounded by a layer of pericytes. During early embryonic development, the lymphatic system forms by branching off of the cardinal vein and expanding into an alternate network of thin- walled vessels.
Lymphatic vessels differ from blood vessels in several ways. Large collecting lymphatic vessels contain vascular smooth muscle cells in their walls, as well as valves, which prevent the backflow of lymph. However, lymphatic capillaries, unlike typical blood capillaries, lack pericytes and a continuous basal lamina and contain large inter-endothelial valve-like openings. Lymphatic capillaries consist of loosely overlapping cells. Due to their greater permeability, lymphatic capillaries may be more effective than blood capillaries in allowing tumor cells to pass into the vessel lumen.
Fig. 1. Model of tumor lymphangiogenesis
The recent identification of selective markers of lymphatic versus vascular endothelial cells has allowed identification of the mechanisms that regulate lymphangiogenesis. Lymphatic endothelia selectively express LYVE-1, a member of the CD44 hyaluronic acid receptor family; Prox-1, a lymphatic vessel-specific homeobox transcription factor; podoplanin, a mucin-type glycoprotein; and VEGFR3, a receptor for VEGF-C and VEGF-D. Lymphatic capillaries, unlike typical blood capillaries, lack pericytes and a continuous basal lamina.
Tumor-secreted factors such as VEGF-C and VEGF-A have been shown to promote lymphangiogenesis within tumors. These factors activate VEGFR3, a tyrosine kinase VEGF family receptor that is expressed primarily on lymphatic endothelium. Expression of VEGF-C is correlated with increased lymph node metastasis and poor clinical outcome in a variety of tumors. Indeed, in animal models of metastasis, inhibitors of VEGF-C (soluble VEGFR3) inhibited tumor lymphangiogenesis and tumor metastasis to lymph nodes.
Tumors spread by lymphatic routes to lymph nodes but may also spread by hematogenous (vascular) or lymphatic routes to distant organs. Tumors secrete a number of factors including VEGF-C and others that induce both lymphangiogenesis and angiogenesis (Fig. 1).
The molecular mechanisms that regulate lymphangiogenesis, the growth of lymphatic vessels, have recently begun to be understood. Two members of the VEGF family, VEGF-C and VEGF-D, stimulate lymphangiogenesis by binding to the receptors VEGFR-2 and VEGFR-3 on lymphatic endothelial cells. Indeed, homozygous deletion of VEGF-C genes in mice leads to a complete absence of the lymphatic system. VEGF-A, FGF, and HGF can also stimulate lymphangiogenesis. The discovery of specific markers of lymphatic endothelium has facilitated analysis of the mechanisms regulating lymphangiogenesis. VEGFR3 is expressed only by quiescent lymphatic endothelial cells and not by quiescent vascular endothelial cells; however, both proliferating vascular and lymphatic endothelial cells express this protein. The homeodomain transcription factor Prox-1 is selectively expressed on lymphatic endothelium and not vascular endothelial cells. It is also expressed by developing neuronal cells in flies and mammals. Podoplanin, a mucin-type glycoprotein, is expressed by lymphatic endothelial cells and a few other cell types such as type I lung alveolar cells and kidney podocytes. Mice with genetic loss of podoplanin display paw lymphedema or loss of fluid drainage from the paw. Importantly, the CD44 hyaluronic acid-binding protein family member LYVE-1 is expressed only by lymphatic endothelium as well as liver and spleen sinusoidal endothelial cells. Interestingly, LYVE-1 is downregulated on pericyte- lined collecting vessels of the lymphatic system. Although the function of LYVE-1 is currently unknown, there is a clear association of the absence of LYVE-1 with the presence of pericytes. Expression of LYVE-1 is undetectable on lymphocytes, hematopoietic cells, or vascular endothelial cells. LYVE-1 is thus a useful marker to determine the localization of lymphatic endothelium by immunohistochemistry in vivo and to characterize and purify LEC in vitro.
While growth factors and their receptors play critical roles in angiogenesis and lymphangiogenesis, the integrin family of cell adhesion proteins controls cell attachment to the extracellular matrix and promotes the survival, proliferation, and motility of many cell types. Angiogenesis, the development of new blood vessels, depends not only on soluble growth factors such as VEGF-A but also on survival and migratory signals transduced by the integrins avb3 avb5, a5b1, and/or a4b1. In contrast, only integrins a4b1 and aA sentinel lymph node have been shown to play roles in lymphatic vessel development, as animals lacking integrin a9b1 develop chylothorax (i.e., accumulation of high-fat-containing fluid (chyle) in the abdomen as a result of obstruction of or abnormal development of lymphatic vasculature), and animals lacking a4b1in endothelia do not respond to VEGF-C.
Many tumors express VEGF-C and VEGF-D, growth factors that selectively regulate lymphangiogenesis. While there are three known vascular endothelial growth factor receptors, VEGFR-1, VEGFR-2, and VEGFR-3, only one, VEGFR-3, is expressed predominantly on lymphatic vessel. Importantly, tumor-associated macrophages can release VEGF-C and can stimulate lymphangiogenesis in the absence of added growth factors. In fact, recent studies showed that macrophage secretion of VEGF-C and VEGF-D induces lymphangiogenesis, as well as lymphangiogenesis in tumors. Importantly, a number of studies have shown that antagonists of VEGF-C suppress tumor lymphangiogenesis and lymphatic metastases in animal models of tumor growth.
Congenital or pathologically induced damage to the lymphatic system can result in lymphedema, a condition in which drainage of fluid from tissues is blocked, skin thickens, and adipose tissue accumulates. Mutations in VEGFR3, the forkhead transcription factor FOXC2, and the transcription factor SOX18 each induce distinct forms of congenital or primary lymphedema. Recombinant VEGF-C was able to promote therapeutic lymphangiogenesis in animal models of lymphedema. Additionally, cancer surgery and radiation therapy, especially in breast cancer therapy, can induce secondary lymphedema by damaging the lymphatic system in normal tissues, such as the breast. Diseases of the lymphatic system include lymphedema, lymphangiosarcomas, and lymph node metastasis. Primary lymphedema arises from congenital defects in molecules that regulate development of lymphatic vessels, such as VEGFR3, FOXC2, and SOX18. Secondary lymphedema arises as a consequence of surgery, infection (such as filariasis), or radiation therapy. In these disorders, the normal architecture of the lymphatic vessels and/or lymph nodes is disrupted. Removal of fluids from tissues can be disrupted in these disorders and tissue architecture is altered. An understanding of how lymphatic vessels grow and respond to environmental cues could help to develop therapies for these disorders. In over eighty percent of cancers, malignant tumor cells metastasize to the lymph nodes and travel through the lymphatic system. Tumor cell-derived factors such as VEGF-C stimulate lymphangiogenesis in tumors, and studies have shown that increased lymphatic vessel density is associated with increased metastasis. An understanding of the molecular mechanisms that regulate lymphangiogenesis may lead to new therapies for cancer metastasis and lymphedema.
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