Distinct roles of vascular endothelial growth factor-D in lymphangiogenesis and metastasis.

In many human carcinomas, expression of the lymphangiogenic factor vascular endothelial growth factor-D (VEGF-D) correlates with up-regulated lymphangiogenesis and regional lymph node metastasis. Here, we have used the Rip1Tag2 transgenic mouse model of pancreatic beta-cell carcinogenesis to investigate the functional role of VEGF-D in the induction of lymphangiogenesis and tumor progression. Expression of VEGF-D in beta cells of single-transgenic Rip1VEGF-D mice resulted in the formation of peri-insular lymphatic lacunae, often containing leukocyte accumulations and blood hemorrhages. When these mice were crossed to Rip1Tag2 mice, VEGF-D-expressing tumors also exhibited peritumoral lymphangiogenesis with lymphocyte accumulations and hemorrhages, and they frequently developed lymph node and lung metastases. Notably, tumor outgrowth and blood microvessel density were significantly reduced in VEGF-D-expressing tumors. Our results demonstrate that VEGF-D induces lymphangiogenesis, promotes metastasis to lymph nodes and lungs, and yet represses hemangiogenesis and tumor outgrowth. Because a comparable transgenic expression of vascular endothelial growth factor-C (VEGF-C) in Rip1Tag2 has been shown previously to provoke lymphangiogenesis and lymph node metastasis in the absence of any distant metastasis, leukocyte infiltration, or angiogenesis-suppressing effects, these results reveal further functional differences between VEGF-D and VEGF-C.

Lymphangiogenic growth factor responsiveness is modulated by postnatal lymphatic vessel maturation.

Lymphatic vessel plasticity and stability are of considerable importance when attempting to treat diseases associated with the lymphatic vasculature. Development of lymphatic vessels during embryogenesis is dependent on vascular endothelial growth factor (VEGF)-C but not VEGF-D. Using a recombinant adenovirus encoding a soluble form of their receptor VEGFR-3 (AdVEGFR-3-Ig), we studied lymphatic vessel dependency on VEGF-C and VEGF-D induced VEGFR-3 signaling in postnatal and adult mice. Transduction with AdVEGFR-3-Ig led to regression of lymphatic capillaries and medium-sized lymphatic vessels in mice under 2 weeks of age without affecting collecting lymphatic vessels or the blood vasculature. No effect was observed after this period. The lymphatic capillaries of neonatal mice also regressed partially in response to recombinant VEGFR-3-Ig or blocking antibodies against VEGFR-3, but not to adenovirus-encoded VEGFR-2-Ig. Despite sustained inhibitory VEGFR-3-Ig levels, lymphatic vessel regrowth was observed at 4 weeks of age. Interestingly, whereas transgenic expression of VEGF-C in the skin induced lymphatic hyperplasia even during embryogenesis, similar expression of VEGF-D resulted in lymphangiogenesis predominantly after birth. These results indicate considerable plasticity of lymphatic vessels during the early postnatal period but not thereafter, suggesting that anti-lymphangiogenic therapy can be safely applied in adults.

Moderate antiangiogenic activity by local, transgenic expression of endostatin in Rip1Tag2 transgenic mice.

Many previous reports have demonstrated that systemic administration of endostatin (ES), a proteolytic cleavage product of collagen type XVIII and an endogenous angiogenesis inhibitor, represses tumor angiogenesis in different preclinical tumor models with varying efficacy. For example, systemic delivery of recombinant ES to rat insulin promoter 1 (Rip1)T-antigen 2 (Tag2)-transgenic mice, a mouse model of pancreatic beta-cell carcinogenesis, has repressed tumor angiogenesis efficiently and with it, tumor growth. Here, we report that the transgenic expression of ES in Rip1ES-transgenic mice only interferes moderately with tumor growth in Rip1Tag2;Rip1ES double-transgenic mice. Tumor incidence is not reduced by the local expression of ES, and tumor outgrowth and progression to tumor malignancy are only retarded slightly. A significant effect of local ES expression on tumor angiogenesis is only apparent during the early stages of tumor development, where less angiogenic hyperplastic lesions are observed. Although efficiently produced and secreted by transgenic beta cells, locally expressed ES appears to be sequestered in the microenvironment, and its systemic levels are not increased. The results indicate that the antiangiogenic functions of ES critically depend on the mode of delivery and the site of expression: although its systemic application represses tumor angiogenesis and tumor growth efficiently, locally expressed ES appears to be less effective, and hence, additional mechanisms of solubilization or activation of latent ES seem to be required. These results have important implications about the modes of delivery used in antiangiogenic, therapeutic strategies, which are based on the antiangiogenic activities of ES.

Intrinsic versus microenvironmental regulation of lymphatic endothelial cell phenotype and function.

Vascular endothelial cells are characterized by a high degree of functional and phenotypic plasticity, which is controlled both by their pericellular microenvironment and their intracellular gene expression programs. To gain further insight into the mechanisms regulating the endothelial cell phenotype, we have compared the responses of lymphatic endothelial cells (LECs) and blood vascular endothelial cells (BECs) to vascular endothelial growth factors (VEGFs). VEGFR-3-specific signals are sufficient for LEC but not BEC proliferation, as shown by the ability of the specific ligand VEGF-C156S to stimulate cell cycle entry only in LECs. On the other hand, we found that VEGFR-3 stimulation did not induce LEC cell shape changes typical of VEGFR-2-stimulated LECs, indicating receptor-specific differences in the cytoskeletal responses. Genes induced via VEGFR-2 also differed between BECs and LECs: angiopoietin-2 (Ang-2) was induced via VEGFR-2 in BECs and LECs, but the smooth muscle cell (SMC) chemoattractant BMP-2 was induced only in BECs. Both BECs and LECs were able to promote SMC chemotaxis, but contact with SMCs led to down-regulation of VEGFR-3 expression in BECs in a 3-dimensional coculture system. This was consistent with the finding that VEGFR-3 is down-regulated in vivo at sites of endothelial cell-pericyte/smooth muscle cell contacts. Collectively, these data show intrinsic cell-specific differences of BEC and LEC responses to VEGFs and identify a pericellular regulatory mechanism for VEGFR-3 down-regulation in endothelial cells.

Lymphangiogenic growth factors, receptors and therapies.

The lymphatic vasculature is essential for the maintenance of normal fluid balance and for the immune responses, but it is also involved in a variety of diseases. Hypoplasia or dysfunction of the lymphatic vessels can lead to lymphedema, whereas hyperplasia or abnormal growth of these vessels are associated with lymphangiomas and lymphangiosarcomas. Lymphatic vessels are also involved in lymph node and systemic metastasis of cancer cells. Recent novel findings on the molecular mechanisms involved in lymphatic vessel development and regulation allow the modulation of the lymphangiogenic process and specific targeting of the lymphatic endothelium. Recent results show that the homeodomain transcription factor Prox-1 is an important lymphatic endothelial cell (LEC) fate-determining factor which can induce LEC-specific gene transcription even in blood vascular endothelial cells (BECs). This suggests that the distinct phenotypes of cells in the adult vascular endothelium are plastic and sensitive to transcriptional reprogramming, which might be useful for future therapeutic applications involving endothelial cells. Vascular endothelial growth factor-C (VEGF-C) and VEGF-D are peptide growth factors capable of inducing the growth of new lymphatic vessels in vivo in a process called lymphangiogenesis. They belong to the larger family which also includes VEGF, placenta growth factor (PlGF) and VEGF-B, VEGF-C and VEGF-D are ligands for the endothelial cell specific tyrosine kinase receptors VEGFR-2 and VEGFR-3. In adult human as well as mouse tissues VEGFR-3 is expressed predominantly in lymphatic endothelial cells which line the inner surface of lymphatic vessels. While VEGFR-2 is thought to be the main mediator of angiogenesis, VEGFR-3 signaling is crucial for the development of the lymphatic vessels. Heterozygous inactivation of the VEGFR-3 tyrosine kinase leads to primary lymphedema due to defective lymphatic drainage in the limbs. Other factors that seem to be involved in lymphangiogenesis include the Tie/angiopoietin system, neuropilin-2 and integrin alpha 9. VEGF-C induces lymphatic vessel growth, but high levels of VEGF-C also resulted in blood vessel leakiness and growth. The VEGFR-3-specific mutant form of VEGF-C called VEGF-C156S lacks blood vascular side effects but is sufficient for therapeutic lymphangiogenesis in a mouse model of lymphedema. As VEGF-C156S is a specific lymphatic endothelial growth factor in the skin, it provides an attractive molecule for pro-lymphangiogenic therapy.

A fluorescent Tie1 reporter allows monitoring of vascular development and endothelial cell isolation from transgenic mouse embryos.

Tie1 is an endothelial receptor tyrosine kinase essential for development and maintenance of the vascular system. Here we report generation of transgenic mice expressing enhanced green fluorescent protein (EGFP) or a chimeric protein consisting of a Zeosin resistance marker and EGFP under the control of mouse Tie1 promoter. Intravital monitoring of fluorescence showed that the EGFP reporter recapitulates the Tie1 expression pattern in the developing vasculature, and flow cytometry using EGFP allowed the isolation of essentially pure Tie1-expressing endothelial cells from transgenic mouse embryos. However, EGFP and LacZ transgenic markers were strongly down-regulated in the adult vasculature; unlike the Tie1-LacZ knock-in locus, the promoter was not reactivated during tumor neovascularization, indicating the presence of additional regulatory elements in the Tie1 locus. Starting at midgestation, Tie1 promoter activity became stronger in the arterial than in the venous endothelium; in adult mice, promoter activity was observed in arterioles, capillaries, and lymphatic vessels, indicating a significant degree of specificity in different types of endothelial cells. Our results establish Tie1-Z/EGFP transgenic mice as a useful model to study embryonic vascular development and a convenient source for the isolation of primary endothelial cells.

Dual role of Ang2 in postnatal angiogenesis and lymphangiogenesis.

The maturation of the vascular system and the adjustment of blood vessel density in tissues require the opposing processes of vessel growth and regression. A new study in this issue of Developmental Cell shows that Angiopoietin-2 (Ang2), a ligand for the endothelial Tie2 receptor tyrosine kinase, has a dual function in the processes of postnatal angiogenesis and vascular remodeling. Also, Ang2 signals are required for the proper development and function of the lymphatic vessels.

Lymphangiogenic gene therapy with minimal blood vascular side effects.

Recent work from many laboratories has demonstrated that the vascular endothelial growth factor-C/VEGF-D/VEGFR-3 signaling pathway is crucial for lymphangiogenesis, and that mutations of the Vegfr3 gene are associated with hereditary lymphedema. Furthermore, VEGF-C gene transfer to the skin of mice with lymphedema induced a regeneration of the cutaneous lymphatic vessel network. However, as is the case with VEGF, high levels of VEGF-C cause blood vessel growth and leakiness, resulting in tissue edema. To avoid these blood vascular side effects of VEGF-C, we constructed a viral vector for a VEGFR-3-specific mutant form of VEGF-C (VEGF-C156S) for lymphedema gene therapy. We demonstrate that VEGF-C156S potently induces lymphangiogenesis in transgenic mouse embryos, and when applied via viral gene transfer, in normal and lymphedema mice. Importantly, adenoviral VEGF-C156S lacked the blood vascular side effects of VEGF and VEGF-C adenoviruses. In particular, in the lymphedema mice functional cutaneous lymphatic vessels of normal caliber and morphology were detected after long-term expression of VEGF-C156S via an adeno associated virus. These results have important implications for the development of gene therapy for human lymphedema.

Adenoviral VEGF-C overexpression induces blood vessel enlargement, tortuosity, and leakiness but no sprouting angiogenesis in the skin or mucous membranes.

Vascular endothelial growth factors (VEGFs) and their receptors (VEGFRs) are important regulators of blood and lymphatic vessel growth and vascular permeability. The VEGF-C/VEGFR-3 signaling pathway is crucial for lymphangiogenesis, and heterozygous inactivating missense mutations of the VEGFR-3 gene are associated with hereditary lymphedema. However, VEGF-C can have potent effects on blood vessels because its receptor VEGFR-3 is expressed in certain blood vessels and because the fully processed form of VEGF-C also binds to the VEGFR-2 of blood vessels. To characterize the in vivo effects of VEGF-C on blood and lymphatic vessels, we have overexpressed VEGF-C via adenovirus- and adeno-associated virus-mediated transfection in the skin and respiratory tract of athymic nude mice. This resulted in dose-dependent enlargement and tortuosity of veins, which, along with the collecting lymphatic vessels were found to express VEGFR-2. Expression of angiopoietin 1 blocked the increased leakiness of the blood vessels induced by VEGF-C whereas vessel enlargement and lymphangiogenesis were not affected. However, angiogenic sprouting of new blood vessels was not observed in response to AdVEGF-C or AAV-VEGF-C. These results show that virally produced VEGF-C induces blood vessel changes, including vascular leak, but its angiogenic potency is much reduced compared with VEGF in normal skin.