The specificity of receptor binding by vascular endothelial growth factor-d is different in mouse and man

Modeling the effects of vasculature evolution on early brain tumor growth

Mathematical modeling of both tumor growth and angiogenesis have been active areas of research for the past several decades. Such models can be classified into one of two categories: those that analyze the remodeling of the vasculature while ignoring changes in the tumor mass, and those that predict tumor expansion in the presence of a non-evolving vasculature. However, it is well accepted that vasculature remodeling and tumor growth strongly depend on one another. For this reason, we have developed a two-dimensional hybrid cellular automaton model of early brain tumor growth that couples the remodeling of the microvasculature with the evolution of the tumor mass. A system of reaction-diffusion equations has been developed to track the concentration of vascular endothelial growth factor (VEGF), Ang-1, Ang-2, their receptors and their complexes in space and time. The properties of the vasculature and hence of each cell are determined by the relative concentrations of these key angiogenic factors. The model exhibits an angiogenic switch consistent with experimental observations on the upregulation of angiogenesis. Particularly, we show that if the pathways that produce and respond to VEGF and the angiopoietins are properly functioning, angiogenesis is initiated and a tumor can grow to a macroscopic size. However, if the VEGF pathway is inhibited, angiogenesis does not occur and tumor growth is thwarted beyond 1-2mm in size. Furthermore, we show that tumor expansion can occur in well-vascularized environments even when angiogenesis is inhibited, suggesting that anti-angiogenic therapies may not be sufficient to eliminate a population of actively dividing malignant cells.

Molecular and functional diversity of vascular endothelial growth factors

Members of the vascular endothelial growth factor (VEGF) family are crucial regulators of neovascularization and are classified as cystine knot growth factors that specifically bind cellular receptor tyrosine kinases VEGFR-1, VEGFR-2, and VEGFR-3 with high but variable affinity and selectivity. The VEGF family has recently been expanded and currently comprises seven members: VEGF-A, VEGF-B, placenta growth factor (PlGF), VEGF-C, VEGF-D, viral VEGF (also known as VEGF-E), and snake venom VEGF (also known as VEGF-F). Although all members are structurally homologous, there is molecular diversity among the subtypes, and several isoforms, such as VEGF-A, VEGF-B, and PlGF, are generated by alternative exon splicing. These splicing isoforms exhibit differing properties, particularly in binding to co-receptor neuropilins and heparin. VEGF family proteins play multiple physiological roles, such as angiogenesis and lymphangiogenesis, while exogenous members (viral and snake venom VEGFs) display activities that are unique in physiology and function. This review will highlight the molecular and functional diversity of VEGF family proteins.

Functional interaction of VEGF-C and VEGF-D with neuropilin receptors

Lymphatic vascular development is regulated by vascular endothelial growth factor receptor-3 (VEGFR-3), which is activated by its ligands VEGF-C and VEGF-D. Neuropilin-2 (NP2), known to be involved in neuronal development, has also been implicated to play a role in lymphangiogenesis. We aimed to elucidate the mechanism by which NP2 is involved in lymphatic endothelial cell signaling. By in vitro binding studies we found that both VEGF-C and VEGF-D interact with NP2, VEGF-C in a heparin-independent and VEGF-D in a heparin-dependent manner. We also mapped the domains of VEGF-C and NP2 required for their binding. The functional importance of the interaction of NP2 with the lymphangiogenic growth factors was demonstrated by cointernalization of NP2 along with VEGFR-3 in endocytic vesicles of lymphatic endothelial cells upon stimulation with VEGF-C or VEGF-D. NP2 also interacted with VEGFR-3 in coprecipitation studies. Our results show that NP2 is directly involved in an active signaling complex with the key regulators of lymphangiogenesis and thus suggest a mechanism by which NP2 functions in the development of the lymphatic vasculature.

Lymphangiogenesis in the developing lung promoted by VEGF-A

Understanding the basic processes of late-stage pulmonary vascular development is essential as this period corresponds to the stage when preterm infants have increased chance of survival. During this period, refinement of the gas exchange unit leads to close apposition of the capillary vasculature and airway epithelium through thinning of the mesenchyme, formation of alveolar septae and functional adaptation of endothelial cells into vessels including pulmonary lymphatics. The pulmonary lymphatic network promotes efficient gas exchange through maintaining interstitial fluid balance. Through conditional transgene regulation, we found that a modest, pathologically relevant increase in vascular endothelial growth factor A (VEGF-A) in distal lung during only the perinatal period adversely affected final refinement of the gas exchange unit. VEGF-A induction disrupted the established vascular network, increased endothelial cell number, altered endothelial ultrastructure and reduced mesenchymal thinning. In addition, VEGF-A induction caused a 3-fold increase in small vessels identified as lymphatics in distal lung. mRNA levels of lymphangiogenic factors VEGF-D/-C were unchanged, while levels of the cognate receptor VEGFR-3 increased. The responses to VEGF-A induction in the perinatal period differ from those during early lung development when endothelial migration, but not proliferation altered initial vascular patterning (Akeson, A.L., Greenberg, J.M., Cameron, J.E., Thompson, F.Y., Brooks, S.K., Wiginton, D., Whitsett, J.A., 2003. Temporal and spatial regulation of VEGF-A controls vascular patterning in the embryonic lung. Dev. Biol. 264, 443-455). The late-stage response resembles that of adult lung to VEGF-inducing stimuli including injury and disease. These data suggest that VEGF-A influences the balance between development of blood and lymphatic vasculature during lung organogenesis.

Vascular Endothelial Growth Factor-D Activates VEGFR-3 Expressed in Osteoblasts Inducing Their Differentiation

Vascular endothelial growth factor (VEGF)-D is a member of the VEGF family of angiogenic growth factors that recognizes and activates the vascular endothelial growth factor receptor (VEGFR)-2 and VEGFR-3 on blood and/or lymphatic vessels. We show that in the long bones of newborn mice, VEGF-D and VEGFR-3 are expressed in the osteoblasts of the growing plate. The treatment of primary human osteoblasts with recombinant VEGF-D induces the expression of osteocalcin and the formation of mineralized nodules in a dose-dependent manner. A monoclonal neutralizing antibody, anti-VEGF-D, or silencing of VEGFR-3 by lentiviral-mediated expression of VEGFR-3 small hairpin RNA affects VEGF-D-dependent osteocalcin expression and nodule formation. Moreover, in primary human osteoblasts, VEGF-D expression is under the control of VEGF, and inhibition of VEGF-D/VEGFR-3 signaling, by monoclonal antibodies or VEGFR-3 silencing, affects VEGF-dependent osteoblast differentiation. These experiments establish that VEGF-D/VEGFR-3 signaling plays a critical role in osteoblast maturation and suggest that VEGF-D is a downstream effector of VEGF in osteogenesis.

The lymphatic vasculature: recent progress and paradigms

The field of lymphatic research has been recently invigorated by the identification of genes and mechanisms that control various aspects of lymphatic development. We are beginning to understand how, starting from a subgroup of embryonic venous endothelial cells, the whole lymphatic system forms in a stepwise manner. The generation of genetically engineered mice with defects in different steps of the lymphangiogenic program has provided models that are increasing our understanding of the lymphatic system in health and disease. This knowledge, in turn, should lead to the development of better diagnostic methods and treatments of lymphatic disorders and tumor metastasis.

Vascular endothelial growth factor D is dispensable for development of the lymphatic system

Vascular endothelial growth factor receptor 3 (Vegfr-3) is a tyrosine kinase that is expressed on the lymphatic endothelium and that signals for the growth of the lymphatic vessels (lymphangiogenesis). Vegf-d, a secreted glycoprotein, is one of two known activating ligands for Vegfr-3, the other being Vegf-c. Vegf-d stimulates lymphangiogenesis in tissues and tumors; however, its role in embryonic development was previously unknown. Here we report the generation and analysis of mutant mice deficient for Vegf-d. Vegf-d-deficient mice were healthy and fertile, had normal body mass, and displayed no pathologic changes consistent with a defect in lymphatic function. The lungs, sites of strong Vegf-d gene expression during embryogenesis in wild-type mice, were normal in Vegf-d-deficient mice with respect to tissue mass and morphology, except that the abundance of the lymphatics adjacent to bronchioles was slightly reduced. Dye uptake experiments indicated that large lymphatics under the skin were present in normal locations and were functional. Smaller dermal lymphatics were similar in number, location, and function to those in wild-type controls. The lack of a profound lymphatic phenotype in Vegf-d-deficient mice suggests that Vegf-d does not play a major role in lymphatic development or that Vegf-c or another, as-yet-unknown activating Vegfr-3 ligand can compensate for Vegf-d during development.

VEGFR-3 expression in breast cancer tissue is not restricted to lymphatic vessels

We examined the immunohistochemical reactivity for vascular endothelial growth factor receptor 3 (VEGFR-3), a protein playing an important role in lymphangiogenesis, in breast cancer. A retrospective series of 77 invasive ductal breast carcinomas was investigated. The relationship between VEGFR-3 expression and clinicopathologic parameters was examined for statistical significance using Pearson's chi-square (chi2) test and Fisher's exact test (when n<5). Threshold for significance was p<0.05. Patient age ranged from 31 to 77 years (mean: 55 years). The VEGFR-3 immunoreactivity was as follows: 5 cases were negative (6.5%), 35 + (45.4%), 27+ + (35.1%), and 10+ + + (13.0%). Reactions were positive for both lymphatic and blood vessels in several cases. VEGFR-3-positive reactions were more frequent in the tumor periphery than within the tumor. Immunoreactivity was also observed in myoepithelial cells surrounding both normal ducts and ducts with ductal carcinoma in situ. Statistical analysis of VEGFR-3 reactions was not significantly related to node status, microvessel density, and tumor grade. Ploidy showed a tendency towards significance (p=0.063); however, owing to the limited number of cases, statistical significance was not reached. VEGFR-3 lacks lymphatic vessel specificity and is also expressed in blood vessels, myoepithelial cells, and neoplastic cells.

Vascular endothelial growth factor and related molecules in acute lung injury

VEGFs and their receptors have been implicated in the regulation of vascular permeability in many organ systems, including the lung. Increased permeability and interstitial and pulmonary edema are prominent features of acute lung injury (ALI)/acute respiratory distress syndrome (ARDS). Extrapolating data from other organ systems and animal experiments have suggested that overexpression of VEGF functions primarily as proinjurious molecules in the lung. Recent data, from animal models as well as from patients with ARDS, have shown decreased levels of VEGF in the lung. The role of VEGF and related molecules in ALI/ARDS is, therefore, controversial: what has become clear is that there are many unique features in the regulation of pulmonary vascular permeability and in VEGF expression in the lung. In this review, we explore a growing body of literature looking at the expression and function of VEGF and related molecules in different models of ALI and in patients with ALI/ARDS. Novel evidence points to a potential role of VEGF in promoting repair of the alveolar-capillary membrane during recovery from ALI/ARDS. Understanding the role of VEGF in this disease process is crucial for developing new therapeutic strategies for ALI/ARDS.

Lymphangiogenic growth factors as markers of tumor metastasis

Understanding the complex process of tumor metastasis is a problem which has challenged both clinician and scientist for well over 100 years. Defining molecular markers which reflect the metastatic potential of a tumor has also proved elusive. Recently, members of the vascular endothelial growth factor (VEGF) family of glycoproteins have been demonstrated to be potent mediators of both blood vessel and lymphatic vessel formation in the context of tumor biology. Experimental studies in animal models combined with extensive clinicopathological data provide a compelling case indicating that members of the VEGF family play a key role in the formation of metastases in a broad range of solid tumors. The question of whether VEGF signaling pathways can now serve as therapeutic targets alone, or in combination with other forms of anti-cancer agents, needs to be addressed.

Vascular endothelial growth factor receptor 3 in hypoxia‐induced vascular development

Reduced tissue oxygen tension (hypoxia) is appreciated as an efficient stimulus for neovascularization. The effect of hypoxia on the very first stages of vascular development is, however, less well characterized. Here we show that hypoxic conditions (1% O2) potently stimulated formation of an extensive vascular network during a discrete stage of mouse embryonal stem cell differentiation. The morphological changes correlated with an expanding pool of endothelial cells and with activation of the vascular endothelial growth factor-d (Vegf-d) and Vegf receptor-3 genes. VEGF receptor-3 expression was confined to vascular endothelial cells and analysis of the lymphatic marker Prox-1 revealed no expansion of lymphatic endothelial cells. Administration of neutralizing antibodies against either VEGF receptor-3 or VEGF receptor-2 impaired vascular network formation, whereas neutralizing antibodies against VEGF receptor-1 potentiated development of immature vascular structures. In addition, sequestering of VEGF receptor-3 ligands reduced vascularization in a manner similar to neutralization of VEGF receptor-3. We conclude that hypoxia-driven vascular development requires the activity of VEGF receptor-3.

Expression of vascular endothelial growth factor D is associated with hypoxia inducible factor (HIF-1alpha) and the HIF-1alpha target gene DEC1, but not lymph node metastasis in primary human breast carcinomas

BACKGROUND

Vascular endothelial growth factor D (VEGF-D) induces angiogenesis and lymphangiogenesis. Nodal metastasis is recognised as a powerful prognostic marker in breast carcinoma, but the molecular mechanisms underlying this process are unknown. Although it has been suggested that VEGF-D may regulate nodal metastasis, this is based largely on animal models, its role in human disease being unclear.

AIMS

To measure the pattern and degree of VEGF-D protein expression in normal and neoplastic human breast tissues.

METHODS

The pattern and degree of VEGF-D expression was measured in normal tissue and invasive carcinomas, and expression was correlated with clinicopathological parameters, hypoxia markers, and survival. Because other VEGF family members are affected by oestrogen, whether VEGF-D is regulated by oestrogen in breast cancer cell lines was also assessed.

RESULTS

VEGF-D was significantly positively associated with hypoxia inducible factor (HIF-1alpha) (p = 0.03) and the HIF-1alpha regulated gene DEC1 (p = 0.001), but not lymph node status, the number of involved lymph nodes, patient age, tumour size, tumour grade, lymphovascular invasion, oestrogen receptor, progesterone receptor, c-erb-B2, or tumour histology (all p>0.05). There was no significant relation between tumour VEGF-D expression and relapse free (p = 0.78) or overall (p = 0.94) survival. VEGF-D expression was enhanced by oestrogen in MCF-7 and T47D breast cancer cells, and was blocked by hydroxytamoxifen.

CONCLUSION

These findings support a role for hypoxia and oestrogen induced VEGF-D in human breast cancer and also suggest that tamoxifen and related oestrogen antagonists may exert some of their antitumour effects through the abrogation of VEGF-D induced function.

Dose-dependent response of FGF-2 for lymphangiogenesis

Spatio-temporal studies on the growth of capillary blood vessels and capillary lymphatic vessels in tissue remodeling have suggested that lymphangiogenesis is angiogenesis-dependent. We revisited this concept by using fibroblast growth factor 2 (FGF-2) (80 ng) to stimulate the growth of both vessel types in the mouse cornea. When we lowered the dose of FGF-2 in the cornea 6.4-fold (12.5 ng), the primary response was lymphangiogenic. Further investigation revealed that vascular endothelial growth factor-C and -D are required for this apparent lymphangiogenic property of FGF-2, and when the small amount of accompanying angiogenesis was completely suppressed, lymphangiogenesis remained unaffected. Our findings demonstrate that there is a dose-dependent response of FGF-2 for lymphangiogenesis, and lymphangiogenesis can occur in the absence of a preexisting or developing vascular bed, i.e., in the absence of angiogenesis, in the mouse cornea.

Molecular regulation of the VEGF family – inducers of angiogenesis and lymphangiogenesis

The vascular endothelial growth factor (VEGF) family of secreted glycoproteins are critical inducers of angiogenesis (growth of blood vessels) and lymphangiogenesis (growth of lymphatic vessels). These proteins are attractive therapeutic targets for blocking growth of blood vessels and lymphatics in tumors and thereby inhibiting the growth and spread of cancer -- in fact, the first VEGF inhibitor has recently entered the clinic for treatment of cancer. In addition, the VEGFs are being considered for stimulation of angiogenesis in the context of ischemic disease and lymphangiogenesis for treatment of lymphedema. These therapeutic possibilities have focused great interest on the molecular regulation of VEGF family members. Much has been learned in the past five years about the mechanisms controlling the action of the VEGFs, including the importance of hypoxia, proteolysis, transcription factors and RNA splicing. An understanding of these mechanisms offers broader opportunities to manipulate expression and activity of the VEGFs for treatment of disease.

Vascular endothelial growth factor (VEGF)-D and VEGF-A differentially regulate KDR-mediated signaling and biological function in vascular endothelial cells

Vascular endothelial growth factor (VEGF)-D binds to VEGF receptors (VEGFR) VEGFR2/KDR and VEGFR3/Flt4, but the signaling mechanisms mediating its biological activities in endothelial cells are poorly understood. Here we investigated the mechanism of action of VEGF-D, and we compared the signaling pathways and biological responses induced by VEGF-D and VEGF-A in endothelial cells. VEGF-D induced KDR and phospholipase C-gamma tyrosine phosphorylation more slowly and less effectively than VEGF-A at early times but had a more sustained effect and was as effective as VEGF-A after 60 min. VEGF-D activated extracellular signal-regulated protein kinases 1 and 2 with similar efficacy but slower kinetics compared with VEGF-A, and this effect was blocked by inhibitors of protein kinase C and mitogen-activated protein kinase kinase. In contrast to VEGF-A, VEGF-D weakly stimulated prostacyclin production and gene expression, had little effect on cell proliferation, and stimulated a smaller and more transient increase in intracellular [Ca(2+)]. VEGF-D induced strong but more transient phosphatidylinositol 3-kinase (PI3K)-mediated Akt activation and increased PI3K-dependent endothelial nitric-oxide synthase phosphorylation and cell survival more weakly. VEGF-D stimulated chemotaxis via a PI3K/Akt- and endothelial nitric-oxide synthase-dependent pathway, enhanced protein kinase C- and PI3K-dependent endothelial tubulogenesis, and stimulated angiogenesis in a mouse sponge implant model less effectively than VEGF-A. VEGF-D-induced signaling and biological effects were blocked by the KDR inhibitor SU5614. The finding that differential KDR activation by VEGF-A and VEGF-D has distinct consequences for endothelial signaling and function has important implications for understanding how multiple ligands for the same VEGF receptors can generate ligand-specific biological responses.

Gene Transfer for Therapeutic Vascular Growth in Myocardial and Peripheral Ischemia

Therapeutic vascular growth in the treatment of peripheral and myocardial ischemia has not yet fulfilled its expectations in clinical trials. Randomized, double-blinded placebo-controlled trials have predominantly shown the safety and feasibility but not the clear-cut clinically relevant efficacy of angiogenic gene or recombinant growth factor therapy. It is likely that growth factor levels achieved with single injections of recombinant protein or naked plasmid DNA are too low to induce any relevant angiogenic effects. Also, the route of administration of gene transfer vectors has not been optimal in many cases leading to low gene-transfer efficacy. Animal experiments using intramuscular or intramyocardial injections of adenovirus encoding vascular endothelial growth factor (VEGF, VEGF-A), the mature form of VEGF-D, and fibroblast growth factors (FGF-1, -2, and -4) have shown high angiogenic efficacy. Adenoviral overexpression of VEGF receptor-2 ligands, VEGF-A and the mature form of VEGF-D, enlarge the preexisting capillaries in skeletal muscle and myocardium via nitric oxide(NO)-mediated mechanisms and via proliferation of both endothelial cells and pericytes, resulting in markedly increased tissue perfusion. VEGF also enhances collateral growth, which is probably secondary to increased peripheral capillary blood flow and shear stress. As a side effect of VEGF overexpression and rapid microvessel enlargement, vascular permeability increases and may result in substantial tissue edema and pericardial effusion in the heart. Because of the transient adenoviral gene expression, the majority of angiogenic effects and side effects return to baseline by 2 weeks after the gene transfer. In contrast, VEGF overexpression lasting over 4 weeks has been shown to induce the growth of a persistent vascular network in preclinical models. To improve efficacy, the choice of the vascular growth factor, gene transfer vector, and route of administration should be optimized in future clinical trials. This review is focused on these issues.

Vascular endothelial growth factor C is required for sprouting of the first lymphatic vessels from embryonic veins

Lymphatic vessels are essential for immune surveillance, tissue fluid homeostasis and fat absorption. Defects in lymphatic vessel formation or function cause lymphedema. Here we show that the vascular endothelial growth factor C (VEGF-C) is required for the initial steps in lymphatic development. In Vegfc-/- mice, endothelial cells commit to the lymphatic lineage but do not sprout to form lymph vessels. Sprouting was rescued by VEGF-C and VEGF-D but not by VEGF, indicating VEGF receptor 3 specificity. The lack of lymphatic vessels resulted in prenatal death due to fluid accumulation in tissues, and Vegfc+/- mice developed cutaneous lymphatic hypoplasia and lymphedema. Our results indicate that VEGF-C is the paracrine factor essential for lymphangiogenesis, and show that both Vegfc alleles are required for normal lymphatic development.

Viral vascular endothelial growth factors vary extensively in amino acid sequence, receptor-binding specificities, and the ability to induce vascular permeability yet are uniformly active mitogens

Infections of humans and ungulates by parapoxviruses result in skin lesions characterized by extensive vascular changes that have been linked to viral-encoded homologues of vascular endothelial growth factor (VEGF). VEGF acts via a family of receptors (VEGFRs) to mediate endothelial cell proliferation, vascular permeability, and angiogenesis. The VEGF genes from independent parapoxvirus isolates show an extraordinary degree of inter-strain sequence variation. We conducted functional comparisons of five representatives of the divergent viral VEGFs. These revealed that despite the sequence divergence, all were equally active mitogens, stimulating proliferation of human endothelial cells in vitro and vascularization of sheep skin in vivo with potencies equivalent to VEGF. This was achieved even though the viral VEGFs bound VEGFR-2 less avidly than did VEGF. Surprisingly the viral VEGFs varied in their ability to cross-link VEGFR-2, induce vascular permeability and bind neuropilin-1. Correlations between these three activities were detected. In addition it was possible to correlate these functional variations with certain sequence and structural motifs specific to the viral VEGFs. In contrast to the conserved ability to bind human VEGFR-2, the viral growth factors did not bind either VEGFR-1 or VEGFR-3. We propose that the extensive sequence divergence seen in the viral VEGFs was generated primarily by selection against VEGFR-1 binding.

A chemically defined culture of VEGFR2+ cells derived from embryonic stem cells reveals the role of VEGFR1 in tuning the threshold for VEGF in developing endothelial cells

Vascular endothelial growth factor (VEGF) is a major growth factor for developing endothelial cells (ECs). Embryonic lethality due to haploinsufficiency of VEGF in the mouse highlighted the strict dose dependency of VEGF on embryonic vascular development. Here we investigated the dose-dependent effects of VEGF on the differentiation of ES cell-derived fetal liver kinase 1 (Flk-1)/VEGF receptor 2(+) (VEGFR2(+)) mesodermal cells into ECs on type IV collagen under a chemically defined serum-free condition. These cells could grow even in the absence of VEGF, but differentiated mostly into mural cells positive for alpha-smooth muscle actin. VEGF supported in a dose-dependent manner the differentiation into ECs defined by the expression of VE-cadherin, platelet-endothelial cell adhesion molecule 1 (PECAM-1)/ CD31, CD34, and TIE2/TEK. VEGF requirement was greater at late than at early phase of culture during EC development, whereas response of VEGFR2(+) cells to VEGF-E, which is a virus-derived ligand for VEGFR2 but not for Flt-1/VEGFR1, was not dose sensitive even at late phase of culture. Delayed expression of VEGFR1 correlated with increased dose dependency of VEGF. These results suggested that greater requirement of VEGF in the maintenance than induction of ECs was due to the activity of VEGFR1 sequestering VEGF from VEGFR2 signal. The chemically defined serum-free culture system described here provides a new tool for assessing different factors for the proliferation and differentiation of VEGFR2(+) mesodermal cells.

Vascular endothelial growth factor-D is an independent prognostic factor in epithelial ovarian carcinoma

We assessed the presence of vascular endothelial growth factor (VEGF)-C, VEGF-D and their receptor VEGFR-3 by immunohistochemistry in 59 epithelial ovarian carcinomas, 11 borderline tumours and 20 benign cystadenomas. VEGF-C and VEGF-D were generally expressed in tumour cells and also in endothelia adjacent to tumour nests which showed a strong staining for them. VEGFR-3 was expressed in lymphatic and vascular endothelial cells adjacent to tumour nests. Immunoreactivity was significantly more frequent as lesions progressed from a benign tumour to advanced carcinoma. A strong correlation was found between VEGF-C and VEGF-D detected in carcinoma and VEGFR-3 detected in neighbouring endothelial cells. Increased expression of VEGF-C, VEGF-D and VEGFR-3 was significantly associated with lymph node metastasis and peritoneal metastasis outside the pelvis. There was a significant correlation between the high levels of VEGF-C and VEGF-D proteins, and poor survival. The presence of VEGF-D was an independent prognostic indicator by multivariate analysis. We conclude that VEGF-C, VEGF-D and VEGFR-3 play an important role in lymphatic spread and intraperitoneal tumour development in ovarian carcinoma. Since VEGF-D was found to be an independent predictor of poor outcome, its measurement, together with other prognostic markers may improve prospective identification of patients with a poor prognosis.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.

Molecular control of lymphangiogenesis

The lymphatic vasculature plays a critical role in the regulation of body fluid volume and immune function. Extensive research into the molecular mechanisms that control blood vessel growth has led to identification of molecules that also regulate development and growth of the lymphatic vessels. This is generating a great deal of interest in the molecular control of the lymphatics in the context of embryogenesis, lymphatic disorders and tumor metastasis. Studies in animal models carried out over the past three years have shown that the soluble protein growth factors, vascular endothelial growth factor (VEGF)-C and VEGF-D, and their cognate receptor tyrosine kinase, VEGF receptor-3 (VEGFR-3), are critical regulators of lymphangiogenesis. Furthermore, disfunction of VEGFR-3 has recently been shown to cause lymphedema. The capacity to induce lymphangiogenesis by manipulation of the VEGF-C/VEGF-D/VEGFR-3 signaling pathway offers new opportunities to understand the function of the lymphatic system and to develop novel treatments for lymphatic disorders.

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.

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.

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.

Mesenchymal expression of vascular endothelial growth factors D and A defines vascular patterning in developing lung

The lung has specific vascular patterning requirements for effective gas exchange at birth, including alignment of airways and blood vessels and lymphatic vessels. Vascular endothelial growth factors (VEGF) are potent effectors of vascular development. We examined the temporal and spatial expression of VEGF-D and specific VEGF-A isoforms at each stage of lung development. VEGF-D, expressed only by cadherin-11-positive cells of the mesenchyme, is first detected at embryonic day (E) 13.5, a period of active vasculogenesis. VEGFR-3, its cognate receptor, is detected earlier on days E11.5 to E14.5, in both blood vessels and lymphatic vessels and later, on day E17.5, in only lymphatic vessels. VEGF-A is expressed in the mesenchyme throughout lung development and also by the epithelium midway through organogenesis. Before E14, the predominant forms of VEGF-A are the soluble isoforms, VEGF-A120 and 164. Not until E14.5 do epithelial cells at the tips of expanding airways express VEGF-A, including VEGF-A188, an isoform with high affinity for extracellular matrix. Our results demonstrate unique temporal and spatial expression of VEGF-D and specific VEGF-A isoforms during lung development and suggest these related factors have distinct functions in vascular and lymphatic patterning of the lung.

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.

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.

A model for gene therapy of human hereditary lymphedema

Primary human lymphedema (Milroy's disease), characterized by a chronic and disfiguring swelling of the extremities, is associated with heterozygous inactivating missense mutations of the gene encoding vascular endothelial growth factor C/D receptor (VEGFR-3). Here, we describe a mouse model and a possible treatment for primary lymphedema. Like the human patients, the lymphedema (Chy) mice have an inactivating Vegfr3 mutation in their germ line, and swelling of the limbs because of hypoplastic cutaneous, but not visceral, lymphatic vessels. Neuropilin (NRP)-2 bound VEGF-C and was expressed in the visceral, but not in the cutaneous, lymphatic endothelia, suggesting that it may participate in the pathogenesis of lymphedema. By using virus-mediated VEGF-C gene therapy, we were able to generate functional lymphatic vessels in the lymphedema mice. Our results suggest that growth factor gene therapy is applicable to human lymphedema and provide a paradigm for other diseases associated with mutant receptors.

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

Vascular endothelial growth factor receptor-3 (VEGFR-3/Flt4) binds two known members of the VEGF ligand family, VEGF-C and VEGF-D, and has a critical function in the remodelling of the primary capillary vasculature of midgestation embryos. Later during development, VEGFR-3 regulates the growth and maintenance of the lymphatic vessels. In the present study, we have isolated and cultured stable lineages of blood vascular and lymphatic endothelial cells from human primary microvascular endothelium by using antibodies against the extracellular domain of VEGFR-3. We show that VEGFR-3 stimulation alone protects the lymphatic endothelial cells from serum deprivation-induced apoptosis and induces their growth and migration. At least some of these signals are transduced via a protein kinase C-dependent activation of the p42/p44 MAPK signalling cascade and via a wortmannin-sensitive induction of Akt phosphorylation. These results define the critical role of VEGF-C/VEGFR-3 signalling in the growth and survival of lymphatic endothelial cells. The culture of isolated lymphatic endothelial cells should now allow further studies of the molecular properties of these cells.