Isolated lymphatic endothelial cells transduce growth, survival and migratory signals via the VEGF-C/D receptor VEGFR-3

Lymphatic endothelial reprogramming of vascular endothelial cells by the Prox-1 homeobox transcription factor

Lymphatic vessels are essential for fluid homeostasis, immune surveillance and fat adsorption, and also serve as a major route for tumor metastasis in many types of cancer. We found that isolated human primary lymphatic and blood vascular endothelial cells (LECs and BECs, respectively) show interesting differences in gene expression relevant for their distinct functions in vivo. Although these phenotypes are stable in vitro and in vivo, overexpression of the homeobox transcription factor Prox-1 in the BECs was capable of inducing LEC-specific gene transcription in the BECs, and, surprisingly, Prox-1 suppressed the expression of approximately 40% of the BEC-specific genes. Prox-1 did not have global effects on the expression of LEC-specific genes in other cell types, except that it up-regulated cyclin E1 and E2 mRNAs and activated the cyclin e promoter in various cell types. These data suggest that Prox-1 acts as a cell proliferation inducer and a fate determination factor for the LECs. Furthermore, the data provide insights into the phenotypic diversity of endothelial cells and into the possibility of transcriptional reprogramming of differentiated endothelial cells.

Tumor-associated macrophages express lymphatic endothelial growth factors and are related to peritumoral lymphangiogenesis

Formation of lymphatic metastasis is the initial step of generalized spreading of tumor cells and predicts poor clinical prognosis. Lymphatic vessels generally arise within the peritumoral stroma, although the lymphangiopoietic vascular endothelial growth factors (VEGF)-C and -D are produced by tumor cells. In a carefully selected collection of human cervical cancers (stage pT1b1) we demonstrate by quantitative immunohistochemistry and in situ hybridization that density of lymphatic microvessels is significantly increased in peritumoral stroma, and that a subset of stromal cells express large amounts of VEGF-C and VEGF-D. The density of cells producing these vascular growth factors correlates with peritumoral inflammatory stroma reaction, lymphatic microvessel density, and indirectly with peritumoral carcinomatous lymphangiosis and frequency of lymph node metastasis. The VEGF-C- and VEGF-D-producing stroma cells were identified in situ as a subset of activated tumor-associated macrophages (TAMs) by expression of a panel of macrophage-specific markers, including CD68, CD23, and CD14. These TAMs also expressed the VEGF-C- and VEGF-D-specific tyrosine kinase receptor VEGFR-3. As TAMs are derived from monocytes in the circulation, a search in peripheral blood for candidate precursors of VEGFR-3-expressing TAMs revealed a subfraction of CD14-positive, VEGFR-3-expressing monocytes, that, however, failed to express VEGF-C and VEGF-D. Only after in vitro incubation with tumor necrosis factor-alpha, lipopolysaccharide, or VEGF-D did these monocytes start to synthesize VEGF-C de novo. In conclusion VEGF-C-expressing TAMs play a novel role in peritumoral lymphangiogenesis and subsequent dissemination in human cancer.

Lymphangiogenesis and cancer metastasis

Lymphatic vessels are important for the spread of solid tumours, but the mechanisms that underlie lymphatic spread and the role of lymphangiogenesis (the growth of lymphatics) in tumour metastasis has been less clear. This article reviews recent experimental and clinico-pathological data indicating that growth factors that stimulate lymphangiogenesis in tumours are associated with an enhanced metastatic process.

A tumor-homing peptide with a targeting specificity related to lymphatic vessels

Blood vessels of tumors carry specific markers that are usually angiogenesis-related. We previously used phage-displayed peptide libraries in vivo to identify peptides that home to tumors through the circulation and that specifically bind to the endothelia of tumor blood vessels. Here we devised a phage screening procedure that would favor tumor-homing to targets that are accessible to circulating phage, but are not blood vessels. Screening on MDA-MB-435 breast carcinoma xenografts yielded multiple copies of a phage that displays a cyclic 9-amino-acid peptide, LyP-1. Homing and binding to tumor-derived cell suspensions indicated that LyP-1 also recognizes an osteosarcoma xenograft, and spontaneous prostate and breast cancers in transgenic mice, but not two other tumor xenografts. Fluorescein-labeled LyP-1 peptide was detected in tumor structures that were positive for three lymphatic endothelial markers and negative for three blood vessel markers. LyP-1 accumulated in the nuclei of the putative lymphatic cells, and in the nuclei of tumor cells. These results suggest that tumor lymphatics carry specific markers and that it may be possible to specifically target therapies into tumor lymphatics.

Vascular growth factors and lymphangiogenesis

Blood and lymphatic vessels develop in a parallel, but independent manner, and together form the circulatory system allowing the passage of fluid and delivering molecules within the body. Although the lymphatic vessels were discovered already 300 years ago, at the same time as the blood circulation was described, the lymphatic system has remained relatively neglected until recently. This is in part due to the difficulties in recognizing these vessels in tissues because of a lack of specific markers. Over the past few years, several molecules expressed specifically in the lymphatic endothelial cells have been characterized, and knowledge about the lymphatic system has started to accumulate again. The vascular endothelial growth factor (VEGF) family of growth factors and receptors is involved in the development and growth of the vascular endothelial system. Two of its family members, VEGF-C and VEGF-D, regulate the lymphatic endothelial cells via their receptor VEGFR-3. With the aid of these molecules, lymphatic endothelial cells can be isolated and cultured, allowing detailed studies of the molecular properties of these cells. Also the role of the lymphatic endothelium in immune responses and certain pathological conditions can be studied in more detail, as the blood and lymphatic vessels seem to be involved in many diseases in a coordinated manner. Discoveries made so far will be helpful in the diagnosis of certain vascular tumors, in the design of specific treatments for lymphedema, and in the prevention of metastatic tumor spread via the lymphatic system.

The role of tumor lymphangiogenesis in metastatic spread

The high mortality rates associated with cancer can be attributed to the metastatic spread of tumor cells from the site of their origin. Tumor cells invade either the blood or lymphatic vessels to access the general circulation and then establish themselves in other tissues. Clinicopathological data suggest that the lymphatics are an initial route for the spread of solid tumors. Detection of sentinel lymph nodes by biopsy provides significant information for staging and designing therapeutic regimens. The role of angiogenesis in facilitating the growth of solid tumors has been well established, but the presence of lymphatic vessels and the relevance of lymphangiogenesis to tumor spread are less clear. Recently, the molecular pathway that signals for lymphangiogenesis and relatively specific markers for lymphatic endothelium have been described allowing analyses of tumor lymphangiogenesis to be performed in animal models. These studies demonstrate that tumor lymphangiogenesis is a major component of the metastatic process and implicate members of the VEGF family of growth factors as key mediators of lymphangiogenesis in both normal biology and tumors.

Blockade of vascular endothelial growth factor receptor-3 signaling inhibits fibroblast growth factor-2-induced lymphangiogenesis in mouse cornea

Vascular endothelial growth factor receptor-3 (VEGFR-3) is a major mediator of lymphangiogenesis. Recently, VEGFR-3 ligands, VEGF-C, and VEGF-D were reported to promote tumor lymphangiogenesis and lymphatic metastasis, and these processes were inhibited by blocking of the VEGFR-3-signaling pathway. Here, we have adapted the mouse corneal angiogenesis assay to study potential lymphangiogenic factors and inhibitors. Immunohistochemical analysis with lymphatic endothelial markers showed that VEGF-C induces lymphatic as well as blood vessel growth in the cornea. By contrast, VEGF induced angiogenesis but not lymphangiogenesis. Fibroblast growth factor-2 (FGF-2) stimulated both lymphangiogenesis and angiogenesis. FGF-2 up-regulated VEGF-C expression in vascular endothelial and perivascular cells. Furthermore, administration of blocking anti-VEGFR-3 antibodies inhibited the FGF-2-induced lymphangiogenesis. These findings show that VEGFR-3 can mediate lymphangiogenesis induced by other growth factors. Because increased expression of FGF-2 and VEGF-C has been associated with lymphatic metastasis, our results provide a potential strategy for the inhibition of lymphatic metastasis in cancer therapy.

Adenovirus encoding vascular endothelial growth factor-D induces tissue-specific vascular patterns in vivo

The capacity of an adenovirus encoding the mature form of vascular endothelial growth factor (VEGF)-D, VEGF-D Delta N Delta C, to induce angiogenesis, lymphangiogenesis, or both was analyzed in 2 distinct in vivo models. We first demonstrated in vitro that VEGF-D Delta N Delta C encoded by the adenovirus (Ad-VEGF-D Delta N Delta C) is capable of inducing endothelial cell proliferation and migration and that the latter response is primarily mediated by VEGF receptor-2 (VEGFR-2). Second, we characterized a new in vivo model for assessing experimental angiogenesis, the rat cremaster muscle, which permits live videomicroscopy and quantitation of functional blood vessels. In this model, a proangiogenic effect of Ad-VEGF-D Delta N Delta C was evident as early as 5 days after injection. Immunohistochemical analysis of the cremaster muscle demonstrated that neovascularization induced by Ad-VEGF-D Delta N Delta C and by Ad-VEGF-A(165) (an adenovirus encoding the 165 isoform of VEGF-A) was composed primarily of laminin and VEGFR-2-positive vessels containing red blood cells, thus indicating a predominantly angiogenic response. In a skin model, Ad-VEGF-D Delta N Delta C induced angiogenesis and lymphangiogenesis, as indicated by staining with laminin, VEGFR-2, and VEGFR-3, whereas Ad-VEGF-A(165) stimulated the selective growth of blood vessels. These data suggest that the biologic effects of VEGF-D are tissue-specific and dependent on the abundance of blood vessels and lymphatics expressing the receptors for VEGF-D in a given tissue. The capacity of Ad-VEGF-D Delta N Delta C to induce endothelial cell proliferation, angiogenesis, and lymphangiogenesis demonstrates that its potential usefulness for the treatment of coronary artery disease, cerebral ischemia, peripheral vascular disease, restenosis, and tissue edema should be tested in preclinical models.

Lymphatic metastasis in the absence of functional intratumor lymphatics

Lymphatic metastasis contributes to mortality from solid tumors. Whether metastasizing cancer cells reach lymph nodes via intratumor lymphatic vessels is unknown. Here, we examine functional lymphatics associated with mouse tumors expressing normal or elevated levels of vascular endothelial growth factor-C (VEGF-C), a molecule that stimulates lymphangiogenesis. Although VEGF-C overexpression increased lymphatic surface area in the tumor margin and lymphatic metastasis, these tumors contained no functional lymphatics, as assessed by four independent functional assays and immunohistochemical staining. These findings suggest that the functional lymphatics in the tumor margin alone are sufficient for lymphatic metastasis and should be targeted therapeutically.

AIDS-related malignancies

Cancer remains a significant burden for human immunodeficiency virus (HIV)-infected individuals. Most cancers that are associated with HIV infection are driven by oncogenic viruses, such as Epstein-Barr virus, Kaposi's sarcoma-associated herpesvirus and human papillomavirus. Gaining insight into the epidemiology and mechanisms that underlie AIDS-related cancers has provided us with a better understanding of cancer immunity and viral oncogenesis.

Molecular mechanisms of lymphangiogenesis in health and disease

Studies of the last decades have revealed the importance of angiogenesis for normal growth and for the pathogenesis of numerous diseases. Much less studied is lymphangiogenesis, the growth of lymphatic vessels, which drain extravasated fluid, proteins, and cells and transport them back to the venous circulation. Nonetheless, insufficient lymphangiogenesis causes incapacitating lymphedema, while lymphatic growth around tumors may facilitate metastatic spread of malignant cells that ultimately kill the patient. The recent discovery of the key lymphangiogenic factors VEGF-C and VEGF-D and their receptor VEGFR-3 has allowed novel insights into how the lymphatic vessels and blood vessels coordinately grow and affect human disease. In addition, these studies have opened novel diagnostic and therapeutic avenues for the treatment of lymphedema and metastasis. This overview highlights the recent insights and developments in the field of lymphatic vascular research.

Specialization of tumour vasculature

Tumour blood vessels express markers that are not present in resting blood vessels of normal tissues, but that can be shared by angiogenic vessels in non-malignant conditions. Many of these proteins are functionally important in the angiogenic process. Some tumours also contain lymphatic vessels, as well as channels that consist of cancer cells and their extracellular matrix. These special features of tumour vessels are good targets for cancer therapies.

Lymphatic endothelial regulation, lymphoedema, and lymph node metastasis

Vascular endothelial growth factor receptor-3 (VEGFR-3) mediates lymphatic endothelial cell (LEC) growth, migration, and survival by binding VEGF-C and VEGF-D. Recent studies have revealed new regulators of the lymphatic endothelium, such as the transcription factor Prox1, and the cell surface proteins podoplanin and lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1). Furthermore, the isolation of LECs now allows detailed molecular studies of the factors regulating the lymphatic vasculature. These studies are aimed at targeting the lymphatic vasculature in the treatment of various diseases, such as tumour metastasis and lymphoedema.

Lymphatic dissemination of tumour cells and the formation of micrometastases

Most human cancers show evidence of metastatic spread to regional lymph nodes, and the extent of lymph-node involvement is directly related to dinical outcome. Increased expression of vascular endothelial growth factor C in primary tumours is associated with increased dissemination of tumour cells to regional lymph nodes in various human carcinomas. Arguments favouring the activation of pre-existing lymphatic endothelium and the de novo formation of lymphatic capillaries (lymphangiogenesis) are discussed. We highlight recent advances in the molecular detection and characterisation of lymph-node micrometastases, as well as potential microenvironmental factors, such as chemokines, which may influence the migration and growth of metastatic tumour cells. Finally, we examine the clinical significance of lymphatic-mediated tumour-cell dissemination and the formation of lymph-node micrometastases.

Lymphatic endothelium: a new frontier of metastasis research

The vascular endothelium is a dynamic tissue with many active functions. Until recently, endothelial cell (EC) biology studies have used cultured ECs from various organs; these cell lines are considered representative of the blood vascular endothelium. Very few lymphatic EC lines have been available, and these were derived from lymphatic tumours or large collecting lymphatic ducts. In the past, lymphatic vessels were defined largely by the lack of erythrocytes in their lumen, a lack of junctional complexes and the lack of a well-defined basement membrane. Now that lymphatic-specific vascular endothelial growth factors (VEGF-C and VEGF-D) and molecular cell surface markers such as the VEGFR-3 receptor have been identified, this definition needs to be updated. Recent developments have highlighted the importance of lymphatic ECs, and they could become the next focus for angiogenesis and metastasis research.

VEGF receptor signal transduction

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin alphavbeta3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.

Multiple forms of mouse vascular endothelial growth factor-D are generated by RNA splicing and proteolysis

The secreted glycoprotein vascular endothelial growth factor-D (VEGF-D) is angiogenic, lymphangiogenic, and promotes metastatic spread of tumor cells via lymphatic vessels. VEGF-D consists of a receptor-binding domain (VEGF homology domain) and N- and C-terminal propeptides. Proteolytic processing produces numerous forms of human VEGF-D, including fully processed derivatives (containing only the VEGF homology domain), partially processed, and unprocessed derivatives. Proteolysis is essential to generate human VEGF-D that binds the angiogenic receptor VEGF receptor-2 (VEGFR-2) and the lymphangiogenic receptor VEGFR-3 with high affinity. Here, we report that alternative use of an RNA splice donor site in exon 6 of the mouse VEGF-D gene produces two different protein isoforms, VEGF-D(358) and VEGF-D(326), with distinct C termini. The two isoforms were both expressed in all adult mouse tissues and embryonic stages of development analyzed. Both isoforms are proteolytically processed in a similar fashion to human VEGF-D to generate a range of secreted derivatives and bind and cross-link VEGFR-3 with similar potency. The isoforms are differently glycosylated when expressed in vitro. This study demonstrates that RNA splicing, protein glycosylation, and proteolysis are mechanisms for generating structural diversity of mouse VEGF-D.