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

Vascular Endothelial Growth Factor-D (VEGF-D): An Angiogenesis Bypass in Malignant Tumors

Vascular endothelial growth factors (VEGFs) are the key regulators of vasculogenesis in normal and oncological development. VEGF-A is the most studied angiogenic factor secreted by malignant tumor cells under hypoxic and inflammatory stress, which made VEGF-A a rational target for anticancer therapy. However, inhibition of VEGF-A by monoclonal antibody drugs led to the upregulation of VEGF-D. VEGF-D was primarily described as a lymphangiogenic factor; however, VEGF-D's blood angiogenic potential comparable to VEGF-A has already been demonstrated in glioblastoma and colorectal carcinoma. These findings suggested a role for VEGF-D in facilitating malignant tumor growth by bypassing the anti-VEGF-A antiangiogenic therapy. Owing to its high mitogenic ability, higher affinity for VEGFR-2, and higher expression in cancer, VEGF-D might even be a stronger angiogenic driver and, hence, a better therapeutic target than VEGF-A. In this review, we summarized the angiogenic role of VEGF-D in blood vasculogenesis and its targetability as an antiangiogenic therapy in cancer.

VEGF-targeting drugs for the treatment of retinal neovascularization in diabetic retinopathy

Diabetic retinopathy (DR) is the most common microangiopathic complication of diabetes mellitus, representing a major cause of visual impairment in developed countries. Proliferative DR (PDR) represents the last stage of this extremely complex retinal disease, characterized by the development of neovascularization induced by the abnormal production and release of vascular endothelial growth factor (VEGF). The term VEGF includes different isoforms; VEGF-A represents one of the most important pathogenic factors of DR. Anti-VEGF intravitreal therapies radically changed the outcome of DR, due to combined anti-angiogenic and anti-edematous activities. Nowadays, several anti-VEGF molecules exist, characterized by different pharmacological features and duration. With respect to PDR, although anti-VEGF treatments represented a fundamental step forward in the management of this dramatic complication, a big debate is present in the literature regarding the role of anti-VEGF as substitute of panretinal photocoagulation or if these two approaches may be used in combination. In the present review, we provided an update on VEGF isoforms and their role in DR pathogenesis, on current anti-VEGF molecules and emerging new drugs, and on the current management strategies of PDR. There is an overall agreement regarding the relative advantage provided by anti-VEGF, especially looking at the management of PDR patients requiring vitrectomy, with respect to laser. Based on the current data, laser approaches might be avoided when a perfectly planned anti-VEGF therapeutic strategy can be adopted. Conversely, laser treatment may have a role for those patients unable to guarantee enough compliance to anti-VEGF injections.Key messagesVEGF increased production, stimulated by retinal hypoperfusion and ischaemia, is a major pathogenic factor of neovascular complication onset in diabetic retinopathy and of DR stages progression.Nowadays, several anti-VEGF molecules are available in clinical practice and other molecules are currently under investigation. Each anti-VEGF molecule is characterized by different targets and may interact with multiple biochemical pathways within the eye.All the data agreed in considering anti-VEGF molecules as a first line choice for the management of diabetic retinopathy. Laser treatments may have a role in selected advanced cases and for those patients unable to guarantee enough compliance to intravitreal treatments schemes.

Snake venom vascular endothelial growth factors (svVEGFs): Unravelling their molecular structure, functions, and research potential

Vascular endothelial growth factor (VEGF) is a key regulator of angiogenesis, a physiological process characterized by the formation of new vessels from a preexisting endothelium. VEGF has also been implicated in pathologic states, such as neoplasias, intraocular neovascular disorders, among other conditions. VEGFs are distributed in seven different families: VEGF-A, B, C, D, and PIGF (placental growth factor), which are identified in mammals; VEGF-E, which are encountered in viruses; and VEGF-F or svVEGF (snake venom VEGF) described in snake venoms. This is the pioneer review of svVEGF family, exploring its distribution among the snake venoms, molecular structure, main functions, and potential applications.

Emerging Roles for VEGF-D in Human Disease

Blood vessels and lymphatic vessels are located in many tissues and organs throughout the body, and play important roles in a wide variety of prevalent diseases in humans. Vascular endothelial growth factor-D (VEGF-D) is a secreted protein that can promote the remodeling of blood vessels and lymphatics in development and disease. Recent fundamental and translational studies have provided insight into the molecular mechanisms by which VEGF-D exerts its effects in human disease. Hence this protein is now of interest as a therapeutic and/or diagnostic target, or as a potential therapeutic agent, in a diversity of indications in cardiovascular medicine, cancer and the devastating pulmonary condition lymphangioleiomyomatosis. This has led to clinical trial programs to assess the effect of targeting VEGF-D signaling pathways, or delivering VEGF-D, in angina, cancer and ocular indications. This review summarizes our understanding of VEGF-D signaling in human disease, which is largely based on animal disease models and clinicopathological studies, and provides information about the outcomes of recent clinical trials testing agonists or antagonists of VEGF-D signaling.

The fibrinolysis inhibitor α2-antiplasmin restricts lymphatic remodelling and metastasis in a mouse model of cancer

Remodelling of lymphatic vessels in tumours facilitates metastasis to lymph nodes. The growth factors VEGF-C and VEGF-D are well known inducers of lymphatic remodelling and metastasis in cancer. They are initially produced as full-length proteins requiring proteolytic processing in order to bind VEGF receptors with high affinity and thereby promote lymphatic remodelling. The fibrinolytic protease plasmin promotes processing of VEGF-C and VEGF-D in vitro, but its role in processing them in cancer was unknown. Here we explore plasmin's role in proteolytically activating VEGF-D in vivo, and promoting lymphatic remodelling and metastasis in cancer, by co-expressing the plasmin inhibitor α₂-antiplasmin with VEGF-D in a mouse tumour model. We show that α₂-antiplasmin restricts activation of VEGF-D, enlargement of intra-tumoural lymphatics and occurrence of lymph node metastasis. Our findings indicate that the fibrinolytic system influences lymphatic remodelling in tumours which is consistent with previous clinicopathological observations correlating fibrinolytic components with cancer metastasis.

Neoadjuvant Locoregional Therapy and Recurrent Hepatocellular Carcinoma after Liver Transplantation

BACKGROUND

Neoadjuvant locoregional therapies (LRTs) have been widely used to reduce tumor burden or to downstage hepatocellular carcinoma (HCC) before orthotopic liver transplantation (OLT). We examined the impact of LRT response on HCC recurrence after OLT.

STUDY DESIGN

We performed a retrospective study of 384 patients with HCC treated by OLT. Tumor necrosis was determined by pathologic evaluation. The vascular and lymphatic vessels were localized by immunofluorescence staining in formalin-fixed, paraffin-embedded tissue; expressions of vascular endothelial growth factor receptor (VEGFR)-2 and VEGFR-3 were analyzed by Western blot. Plasma vascular endothelial growth factor (VEGF)-A and VEGF-C levels of a consecutive cohort of 171 HCC patients were detected by ELISA.

RESULTS

Of the 384 patients with HCC, 268 had undergone pretransplantation neoadjuvant LRTs. Patients with no tumor necrosis (n = 58; 5.2% recurrence) or complete tumor necrosis (n = 70; 6.1% recurrence) had significantly lower 5-year recurrence rates than those with partial tumor necrosis (n = 140; 22.6% recurrence; p < 0.001). Lymphatic metastases were significantly more numerous in patients with partial tumor necrosis than in those without tumor necrosis after OLT (p < 0.001). With immunofluorescence staining of peritumor zone, lymphatics were visualized around partially necrotic tumors, but not around tumors without necrosis. Plasma levels of VEGF-A and VEGF-C were elevated significantly in patients with evidence of tumor necrosis (n = 102) compared with those without necrosis (n = 69; p < 0.001). By Western blot, VEGFR-2 and VEGFR-3 expression in the peritumoral tissue associated with partially necrotic tumors was significantly higher than in peritumoral tissue of non-necrosis tumors (n = 3/group, p < 0.020 and p < 0.006, respectively).

CONCLUSIONS

Locoregional therapy-induced or spontaneous partially necrotic HCC was associated with increased risk of lymphatic metastases compared with tumors with no or complete tumor necrosis. Anti-lymphangiogenic agents with neoadjuvant LRTs can decrease the pattern of lymphatic metastasis after OLT.

Vegfd modulates both angiogenesis and lymphangiogenesis during zebrafish embryonic development

Vascular endothelial growth factors (VEGFs) control angiogenesis and lymphangiogenesis during development and in pathological conditions. In the zebrafish trunk, Vegfa controls the formation of intersegmental arteries by primary angiogenesis and Vegfc is essential for secondary angiogenesis, giving rise to veins and lymphatics. Vegfd has been largely thought of as dispensable for vascular development in vertebrates. Here, we generated a zebrafish vegfd mutant by genome editing. vegfd mutants display significant defects in facial lymphangiogenesis independent of vegfc function. Strikingly, we find that vegfc and vegfd cooperatively control lymphangiogenesis throughout the embryo, including during the formation of the trunk lymphatic vasculature. Interestingly, we find that vegfd and vegfc also redundantly drive artery hyperbranching phenotypes observed upon depletion of Flt1 or Dll4. Epistasis and biochemical binding assays suggest that, during primary angiogenesis, Vegfd influences these phenotypes through Kdr (Vegfr2) rather than Flt4 (Vegfr3). These data demonstrate that, rather than being dispensable during development, Vegfd plays context-specific indispensable and also compensatory roles during both blood vessel angiogenesis and lymphangiogenesis.

Differential Receptor Binding and Regulatory Mechanisms for the Lymphangiogenic Growth Factors Vascular Endothelial Growth Factor (VEGF)-C and -D

VEGF-C and VEGF-D are secreted glycoproteins that induce angiogenesis and lymphangiogenesis in cancer, thereby promoting tumor growth and spread. They exhibit structural homology and activate VEGFR-2 and VEGFR-3, receptors on endothelial cells that signal for growth of blood vessels and lymphatics. VEGF-C and VEGF-D were thought to exhibit similar bioactivities, yet recent studies indicated distinct signaling mechanisms (e.g. tumor-derived VEGF-C promoted expression of the prostaglandin biosynthetic enzyme COX-2 in lymphatics, a response thought to facilitate metastasis via the lymphatic vasculature, whereas VEGF-D did not). Here we explore the basis of the distinct bioactivities of VEGF-D using a neutralizing antibody, peptide mapping, and mutagenesis to demonstrate that the N-terminal α-helix of mature VEGF-D (Phe⁹³–Arg¹⁰⁸) is critical for binding VEGFR-2 and VEGFR-3. Importantly, the N-terminal part of this α-helix, from Phe⁹³ to Thr⁹⁸, is required for binding VEGFR-3 but not VEGFR-2. Surprisingly, the corresponding part of the α-helix in mature VEGF-C did not influence binding to either VEGFR-2 or VEGFR-3, indicating distinct determinants of receptor binding by these growth factors. A variant of mature VEGF-D harboring a mutation in the N-terminal α-helix, D103A, exhibited enhanced potency for activating VEGFR-3, was able to promote increased COX-2 mRNA levels in lymphatic endothelial cells, and had enhanced capacity to induce lymphatic sprouting in vivo. This mutant may be useful for developing protein-based therapeutics to drive lymphangiogenesis in clinical settings, such as lymphedema. Our studies shed light on the VEGF-D structure/function relationship and provide a basis for understanding functional differences compared with VEGF-C.

Hyperplasia, de novo lymphangiogenesis, and lymphatic regression in mice with tissue-specific, inducible overexpression of murine VEGF-D

Lymphatic vessels modulate tissue fluid balance and inflammation and provide a conduit for endocrine and lipid transport. The growth of new lymphatic vessels in the adult, lymphangiogenesis, is predominantly mediated through vascular endothelial growth factor receptor-3 (VEGFR-3) signaling. We took advantage of the unique binding of murine VEGF-D specifically to VEGFR-3 and generated mice capable of inducible, tissue-specific expression of murine VEGF-D under a tightly-controlled tetracycline response element (TRE) promoter to stimulate adult tissue lymphangiogenesis. With doxycycline-activated expression, TRE-VEGF-D mouse crossed to mice with tissue-specific promoters for the lung [Clara cell secretory protein-reverse tetracycline transactivator (rtTA)] developed pulmonary lymphangiectasia. In the kidney, (kidney-specific protein-rtTA × TRE-VEGF-D) mice exhibited rapid lymphatic hyperplasia on induction of VEGF-D expression. Crossed with adipocyte-specific adiponectin-rtTA mice [Adipo-VEGF-D (VD)], chronic VEGF-D overexpression was capable of inducing de novo lymphangiogenesis in white adipose tissue and a massive expansion of brown adipose tissue lymphatics. VEGF-D expression in white adipose tissue also increased macrophage infiltration and tissue fibrosis in the tissue. Expression did not, however, measurably affect peripheral fluid transport, the blood vasculature, or basal metabolic parameters. On removal of the doxycycline stimulus, VEGF-D expression returned to normal, and the expanded adipose tissue lymphatics regressed in Adipo-VD mice. The inducible TRE-VEGF-D mouse thus provides a novel murine platform to study the adult mechanisms and therapies of an array of disease- and tissue-specific models of lymphangiogenesis.

VEGF-D promotes pulmonary oedema in hyperoxic acute lung injury

Leakage of fluid from blood vessels, leading to oedema, is a key feature of many diseases including hyperoxic acute lung injury (HALI), which can occur when patients are ventilated with high concentrations of oxygen (hyperoxia). The molecular mechanisms driving vascular leak and oedema in HALI are poorly understood. VEGF‐D is a protein that promotes blood vessel leak and oedema when overexpressed in tissues, but the role of endogenous VEGF‐D in pathological oedema was unknown. To address these issues, we exposed Vegfd‐deficient mice to hyperoxia. The resulting pulmonary oedema in Vegfd‐deficient mice was substantially reduced compared to wild‐type, as was the protein content of bronchoalveolar lavage fluid, consistent with reduced vascular leak. Vegf‐d and its receptor Vegfr‐3 were more highly expressed in lungs of hyperoxic, versus normoxic, wild‐type mice, indicating that components of the Vegf‐d signalling pathway are up‐regulated in hyperoxia. Importantly, VEGF‐D and its receptors were co‐localized on blood vessels in clinical samples of human lungs exposed to hyperoxia; hence, VEGF‐D may act directly on blood vessels to promote fluid leak. Our studies show that Vegf‐d promotes oedema in response to hyperoxia in mice and support the hypothesis that VEGF‐D signalling promotes vascular leak in human HALI. © 2016 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

A Simple Bioassay for the Evaluation of Vascular Endothelial Growth Factors

The analysis of receptor tyrosine kinases and their interacting ligands involved in vascular biology is often challenging due to the constitutive expression of families of related receptors, a broad range of related ligands and the difficulty of dealing with primary cultures of specialized endothelial cells. Here we describe a bioassay for the detection of ligands to the vascular endothelial growth factor receptor-2 (VEGFR-2), a key transducer of signals that promote angiogenesis and lymphangiogenesis. A cDNA encoding a fusion of the extracellular (ligand-binding) region of VEGFR-2 with the transmembrane and cytoplasmic regions of the erythropoietin receptor (EpoR) is expressed in the factor-dependent cell line Ba/F3. This cell line grows in the presence of interleukin-3 (IL-3) and withdrawal of this factor results in death of the cells within 24 hr. Expression of the VEGFR-2/EpoR receptor fusion provides an alternative mechanism to promote survival and potentially proliferation of stably transfected Ba/F3 cells in the presence of a ligand capable of binding and cross-linking the extracellular portion of the fusion protein (i.e., one that can cross-link the VEGFR-2 extracellular region). The assay can be performed in two ways: a semi-quantitative approach in which small volumes of ligand and cells permit a rapid result in 24 hr, and a quantitative approach involving surrogate markers of a viable cell number. The assay is relatively easy to perform, is highly responsive to known VEGFR-2 ligands and can accommodate extracellular inhibitors of VEGFR-2 signaling such as monoclonal antibodies to the receptor or ligands, and soluble ligand traps.

Pharmacological targeting of VEGFR signaling with axitinib inhibits Tsc2-null lesion growth in the mouse model of lymphangioleiomyomatosis

Pulmonary lymphangioleiomyomatosis (LAM), a rare progressive lung disease associated with mutations of the tuberous sclerosis complex 2 (Tsc2) tumor suppressor gene, manifests by neoplastic growth of LAM cells, induction of cystic lung destruction, and respiratory failure. LAM severity correlates with upregulation in serum of the prolymphangiogenic vascular endothelial growth factor D (VEGF-D) that distinguishes LAM from other cystic diseases. The goals of our study was to determine whether Tsc2 deficiency upregulates VEGF-D, and whether axitinib, the Food and Drug Administration-approved small-molecule inhibitor of VEGF receptor (VEGFR) signaling, will reduce Tsc2-null lung lesion growth in a mouse model of LAM. Our data demonstrate upregulation of VEGF-D in the serum and lung lining in mice with Tsc2-null lesions. Progressive growth of Tsc2-null lesions induces recruitment and activation of inflammatory cells and increased nitric oxide production. Recruited cells isolated from the lung lining of mice with Tsc2-null lesions demonstrate upregulated expression of provasculogenic Vegfa, prolymphangiogenic Figf, and proinflammatory Nos2, Il6, and Ccl2 genes. Importantly, axitinib is an effective inhibitor of Tsc2-null lesion growth and inflammatory cell recruitment, which correlates with reduced VEGF-D levels in serum and lung lining. Our data demonstrate that pharmacological inhibition of VEGFR signaling with axitinib inhibits Tsc2-null lesion growth, attenuates recruitment and activation of inflammatory cells, and reduces VEGF-D levels systemically and in the lung lining. Our study suggests a potential therapeutic benefit of inhibition of VEGFR signaling for treatment of LAM.

The cellular response to vascular endothelial growth factors requires co-ordinated signal transduction, trafficking and proteolysis

VEGFs (vascular endothelial growth factors) are a family of conserved disulfide-linked soluble secretory glycoproteins found in higher eukaryotes. VEGFs mediate a wide range of responses in different tissues including metabolic homoeostasis, cell proliferation, migration and tubulogenesis. Such responses are initiated by VEGF binding to soluble and membrane-bound VEGFRs (VEGF receptor tyrosine kinases) and co-receptors. VEGF and receptor splice isoform diversity further enhances complexity of membrane protein assembly and function in signal transduction pathways that control multiple cellular responses. Different signal transduction pathways are simultaneously activated by VEGFR-VEGF complexes with membrane trafficking along the endosome-lysosome network further modulating signal output from multiple enzymatic events associated with such pathways. Balancing VEGFR-VEGF signal transduction with trafficking and proteolysis is essential in controlling the intensity and duration of different intracellular signalling events. Dysfunction in VEGF-regulated signal transduction is important in chronic disease states including cancer, atherosclerosis and blindness. This family of growth factors and receptors is an important model system for understanding human disease pathology and developing new therapeutics for treating such ailments.

Nuclear localization of vascular endothelial growth factor-D and regulation of c-Myc-dependent transcripts in human lung fibroblasts

Lymphangiogenesis and angiogenesis are processes that are, in part, regulated by vascular endothelial growth factor (VEGF)-D. The formation of lymphatic structures has been implicated in multiple lung diseases, including pulmonary fibrosis. VEGF-D is a secreted protein produced by fibroblasts and macrophages, which induces lymphangiogenesis by signaling via VEGF receptor-3, and angiogenesis through VEGF receptor-2. VEGF-D contains a central VEGF homology domain, which is the biologically active domain, with flanking N- and C-terminal propeptides. Full-length VEGF-D (∼ 50 kD) is proteolytically processed in the extracellular space, to generate VEGF homology domain that contains the VEGF-D receptor-binding sites. Here, we report that, independent of its cell surface receptors, full-length VEGF-D accumulated in nuclei of fibroblasts, and that this process appears to increase with cell density. In nuclei, full-length VEGF-D associated with RNA polymerase II and c-Myc. In cells depleted of VEGF-D, the transcriptionally regulated genes appear to be modulated by c-Myc. These findings have potential clinical implications, as VEGF-D was found in fibroblast nuclei in idiopathic pulmonary fibrosis, a disease characterized by fibroblast proliferation. These findings are consistent with actions of full-length VEGF-D in cellular homeostasis in health and disease, independent of its receptors.

Lymphangiogenesis and lymphatic vessel remodelling in cancer

The generation of new lymphatic vessels through lymphangiogenesis and the remodelling of existing lymphatics are thought to be important steps in cancer metastasis. The past decade has been exciting in terms of research into the molecular and cellular biology of lymphatic vessels in cancer, and it has been shown that the molecular control of tumour lymphangiogenesis has similarities to that of tumour angiogenesis. Nevertheless, there are significant mechanistic differences between these biological processes. We are now developing a greater understanding of the specific roles of distinct lymphatic vessel subtypes in cancer, and this provides opportunities to improve diagnostic and therapeutic approaches that aim to restrict the progression of cancer.

Lymphovascular and neural regulation of metastasis: shared tumour signalling pathways and novel therapeutic approaches

The progression of cancer is supported by a wide variety of non-neoplastic cell types which make up the tumour stroma, including immune cells, endothelial cells, cancer-associated fibroblasts and nerve fibres. These host cells contribute molecular signals that enhance primary tumour growth and provide physical avenues for metastatic dissemination. This article provides an overview of the role of blood vessels, lymphatic vessels and nerve fibres in the tumour microenvironment and highlights the interconnected molecular signalling pathways that control their development and activation in cancer. Further, this article highlights the known pharmacological agents which target these pathways and discusses the potential therapeutic uses of drugs that target angiogenesis, lymphangiogenesis and stress-response pathways in the different stages of cancer care.

Gene and protein therapies utilizing VEGF for ALS

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease that is usually fatal within 2-5years. Unfortunately, the only treatment currently available is riluzole, which has a limited efficacy. As a redress, there is an expanding literature focusing on other potential treatments. One such potential treatment option utilizes the vascular endothelial growth factor (VEGF) family, which includes factors that are primarily associated with angiogenesis but are now increasingly recognized to have neurotrophic effects. Reduced expression of a member of this family, VEGF-A, in mice results in neurodegeneration similar to that of ALS, while treatment of animal models of ALS with either VEGF-A gene therapy or VEGF-A protein has yielded positive therapeutic outcomes. These basic research findings raise the potential for a VEGF therapy to be translated to the clinic for the treatment of ALS. This review covers the VEGF family, its receptors and neurotrophic effects as well as VEGF therapy in animal models of ALS and advances towards clinical trials.

The propeptides of VEGF-D determine heparin binding, receptor heterodimerization, and effects on tumor biology

VEGF-D is an angiogenic and lymphangiogenic glycoprotein that can be proteolytically processed generating various forms differing in subunit composition due to the presence or absence of N- and C-terminal propeptides. These propeptides flank the central VEGF homology domain, that contains the binding sites for VEGF receptors (VEGFRs), but their biological functions were unclear. Characterization of propeptide function will be important to clarify which forms of VEGF-D are biologically active and therefore clinically relevant. Here we use VEGF-D mutants deficient in either propeptide, and in the capacity to process the remaining propeptide, to monitor the functions of these domains. We report for the first time that VEGF-D binds heparin, and that the C-terminal propeptide significantly enhances this interaction (removal of this propeptide from full-length VEGF-D completely prevents heparin binding). We also show that removal of either the N- or C-terminal propeptide is required for VEGF-D to drive formation of VEGFR-2/VEGFR-3 heterodimers which have recently been shown to positively regulate angiogenic sprouting. The mature form of VEGF-D, lacking both propeptides, can also promote formation of these receptor heterodimers. In a mouse tumor model, removal of only the C-terminal propeptide from full-length VEGF-D was sufficient to enhance angiogenesis and tumor growth. In contrast, removal of both propeptides is required for high rates of lymph node metastasis. The findings reported here show that the propeptides profoundly influence molecular interactions of VEGF-D with VEGF receptors, co-receptors, and heparin, and its effects on tumor biology.

Vascular endothelial growth factor-D: signaling mechanisms, biology, and clinical relevance

Vascular endothelial growth factor-D (VEGF-D) is a secreted glycoprotein that promotes growth of blood vessels (angiogenesis) and lymphatic vessels (lymphangiogenesis), and can induce remodeling of large lymphatics. VEGF-D enhances solid tumor growth and metastatic spread in animal models of cancer, and in some human cancers VEGF-D correlates with metastatic spread, poor patient outcome, and, potentially, with resistance to anti-angiogenic drugs. Hence, VEGF-D signaling is a potential target for novel anti-cancer therapeutics designed to enhance anti-angiogenic approaches and to restrict metastasis. In the cardiovascular system, delivery of VEGF-D in animal models enhanced angiogenesis and tissue perfusion, findings which have led to a range of clinical trials testing this protein for therapeutic angiogenesis in cardiovascular diseases. Despite these experimental and clinical developments, our knowledge of the signaling mechanisms driven by VEGF-D is still evolving--here we explore the biology of VEGF-D, its signaling mechanisms, and the clinical relevance of this growth factor.

Tumor lymphangiogenesis as a potential therapeutic target

Metastasis the spread of cancer cells to distant organs, is the main cause of death for cancer patients. Metastasis is often mediated by lymphatic vessels that invade the primary tumor, and an early sign of metastasis is the presence of cancer cells in the regional lymph node (the first lymph node colonized by metastasizing cancer cells from a primary tumor). Understanding the interplay between tumorigenesis and lymphangiogenesis (the formation of lymphatic vessels associated with tumor growth) will provide us with new insights into mechanisms that modulate metastatic spread. In the long term, these insights will help to define new molecular targets that could be used to block lymphatic vessel-mediated metastasis and increase patient survival. Here, we review the molecular mechanisms of embryonic lymphangiogenesis and those that are recapitulated in tumor lymphangiogenesis, with a view to identifying potential targets for therapies designed to suppress tumor lymphangiogenesis and hence metastasis.

Localization of C-Fos-Induced Growth Factor (Figf) mRNA Expression in the Mouse Uterus during Implantation

The purpose of this study was to characterize the localization of Figf mRNA in the mouse uterus during embryo implantation. Strong Figf mRNA hybridization signals were seen in the primary decidual zone just after the onset of implantation from Days 4.5–6.5. On Day 7.5, this expression continued around the concept us, but in addition we observed high expression of Figf mRNA in the endothelial cells that line the forming vascular sinusoids in the lateral me some trial decidua. Interestingly, on Days 8.5 this high expression continued in the endothelial cells of sinusoids in the lateral me some trial decidual tissue but not in the decidual cells surrounding the concept us. As implantation and placental development finished, Figf mRNA expression remained in the endothelial cells of the sinusoids and spiral arterioles of the decidua basalis. Interestingly, Flt4 mRNA was localized to the endothelial cells lining the sinusoids that form during implantation. Since the endothelial cells of the me some trial sinusoids exhibit a high level of proliferation, we speculate that FIGF-FLT4 signaling may play a role in their formation and function during implantation. This work will provide a basis for further research on the potential role of FIGF-FLT4 signaling in endometrial angiogenesis during implantation in mice.

Conserved signaling through vascular endothelial growth (VEGF) receptor family members in murine lymphatic endothelial cells

Lymphatic vessels guide interstitial fluid, modulate immune responses by regulating leukocyte and antigen trafficking to lymph nodes, and in a cancer setting enable tumor cells to track to regional lymph nodes. The aim of the study was to determine whether primary murine lymphatic endothelial cells (mLECs) show conserved vascular endothelial growth factor (VEGF) signaling pathways with human LECs (hLECs). LECs were successfully isolated from murine dermis and prostate. Similar to hLECs, vascular endothelial growth factor (VEGF) family ligands activated MAPK and pAkt intracellular signaling pathways in mLECs. We describe a robust protocol for isolation of mLECs which, by harnessing the power of transgenic and knockout mouse models, will be a useful tool to study how LEC phenotype contributes to alterations in lymphatic vessel formation and function.

Proteolytic processing of vascular endothelial growth factor-D is essential for its capacity to promote the growth and spread of cancer

VEGF-D is a mitogen for endothelial cells that promotes tumor growth and metastatic spread in animal models, and expression of which correlates with lymph node metastasis in some human cancers. It is secreted from the cell as a full-length form with propeptides flanking a central region containing binding sites for VEGFR-2 and VEGFR-3, receptors that signal for angiogenesis and lymphangiogenesis. The propeptides can be cleaved from VEGF-D, enhancing affinity for VEGFR-2 and VEGFR-3 in vitro; however, the importance of this processing in cancer is unclear. To explore the necessity of processing for the effects of VEGF-D in cancer, we use a mutant full-length form that cannot be processed, and show that, in contrast to full-length VEGF-D that is processed, this mutant does not promote tumor growth and lymph node metastasis in a mouse tumor model. Processing of VEGF-D is required for tumor angiogenesis, lymphangiogenesis, and recruitment of tumor-associated macrophages. These observations may be explained by the requirement of processing for VEGF-D to bind neuropilin receptors and activate VEGFR-2. Our results indicate that proteolytic processing is necessary for VEGF-D to promote the growth and spread of cancer, and suggest that enzymes catalyzing this processing could be targets for antimetastatic therapeutics.

Lovastatin inhibits VEGFR and AKT activation: synergistic cytotoxicity in combination with VEGFR inhibitors

Background

In a recent study, we demonstrated the ability of lovastatin, a potent inhibitor of mevalonate synthesis, to inhibit the function of the epidermal growth factor receptor (EGFR). Lovastatin attenuated ligand-induced receptor activation and downstream signaling through the PI3K/AKT pathway. Combining lovastatin with gefitinib, a potent EGFR inhibitor, induced synergistic cytotoxicity in a variety of tumor derived cell lines. The vascular endothelial growth factor receptor (VEGFR) and EGFR share similar activation, internalization and downstream signaling characteristics.

Methodology/Principal Findings

The VEGFRs, particularly VEGFR-2 (KDR, Flt-1), play important roles in regulating tumor angiogenesis by promoting endothelial cell proliferation, survival and migration. Certain tumors, such as malignant mesothelioma (MM), also express both the VEGF ligand and VEGFRs that act in an autocrine loop to directly stimulate tumor cell growth and survival. In this study, we have shown that lovastatin inhibits ligand-induced VEGFR-2 activation through inhibition of receptor internalization and also inhibits VEGF activation of AKT in human umbilical vein endothelial cells (HUVEC) and H28 MM cells employing immunofluorescence and Western blotting. Combinations of lovastatin and a VEGFR-2 inhibitor showed more robust AKT inhibition than either agent alone in the H28 MM cell line. Furthermore, combining 5 µM lovastatin treatment, a therapeutically relevant dose, with two different VEGFR-2 inhibitors in HUVEC and the H28 and H2052 mesothelioma derived cell lines demonstrated synergistic cytotoxicity as demonstrated by MTT cell viability and flow cytometric analyses.

Conclusions/Significance

These results highlight a novel mechanism by which lovastatin can regulate VEGFR-2 function and a potential therapeutic approach for MM through combining statins with VEGFR-2 inhibitors.

The yin and yang of VEGF and PEDF: multifaceted neurotrophic factors and their potential in the treatment of Parkinson's Disease

Over the last few decades, vascular endothelial growth factor (VEGF) and pigment epithelium-derived factor (PEDF) have emerged as multifaceted players in not only the pathogenesis, but potential treatment, of numerous diseases. They activate diverse intracellular signaling cascades known to have extensive crosstalk, and have been best studied for their effects in cardiology and cancer biology. Recent work with the two factors indicates that the activity of one growth factor is often directly related to the action of the other. Their respective neuroprotective effects, in particular, raise important questions regarding the treatment of neurodegenerative disorders, including Parkinson’s disease.

Characterization of novel VEGF (vascular endothelial growth factor)-C splicing isoforms from mouse

VEGF (vascular endothelial growth factor)-C is a major growth factor implicated in various physiological processes, such as angiogenesis and lymphangiogenesis. In the present paper, we report the identification of three short VEGF-C splicing isoforms (VEGF-C62, VEGF-C129 and VEGF-C184) from immortalized mouse kidney PTECs (proximal tubular epithelial cells). Semi-quantitative RT (reverse transcription)-PCR analysis showed these isoforms were universally expressed to varying degrees in different tissues with high expression levels in the kidney. In immortalized PTECs and podocytes, VEGF-C62 can activate phosphorylation of FAK (focal adhesion kinase) and promote cell adhesion to substratum. Cell survival was also increased by VEGF-C62 treatment in the absence of serum. VEGF-C62 can also reduce cell proliferation in PTECs and podocytes. Nucleolin was one of the proteins that associated with VEGF-C62 in pull-down assays using GST (glutathione transferase) fusion proteins as bait, indicating different protein binding requirements for VEGF-C62 compared with VEGF-C. In conclusion, these newly identified VEGF-C isoforms represent a new class of proteins, which are potentially involved in epithelial cell adhesion and proliferation through novel receptor pathways.

Multiple expressions of lymphatic markers and morphological evolution of newly formed lymphatics in lymphangioma and lymph node lymphangiogenesis

The rapid evolution of reliable technology combined with increasing number of specific markers for lymphatic endothelial cells (LECs) facilitates the investigation of lymphangiogenesis in developing and diseased tissues. Here, we injected incomplete Freund's adjuvant (IFA) peritoneally into BALB/c and nonobese diabetic (NOD) mice to induce lymphangioma and found atypical lymphatic accumulations with intervening fibrous tissue and lymphoid aggregates. Lymphatic markers, LYVE-1 and podoplanin, were used to specifically define the morphological features of the neoplastic lymphatics. The NOD mice (affected by an autoimmune disorder) had fewer and smaller lymphangiomas than the BALB/c mice. Injection of IFA in the footpad skin of the mice also disturbed draining regional lymph node lymphangiogenesis and caused enlargement of popliteal lymph nodes. Molecular analyses of the LECs indicated potential interventions for lymphangioma through vascular endothelial growth factor (VEGF)-A/-C/-D and their receptors, VEGF receptors-2/-3, and Prox-1 signaling pathways. These findings represent an important link between multiple factors and lymphatic disorders.

Anti-angiogenesis effect of the novel anti-inflammatory and pro-resolving lipid mediators

PURPOSE

Resolvins and lipoxins are lipid mediators generated from essential polyunsaturated fatty acids that are the first dual anti-inflammatory and pro-resolving signals identified in the resolution phase of inflammation. Here the authors investigated the potential of aspirin-triggered lipoxin (LX) A4 analog (ATLa), resolving (Rv) D1, and RvE1, in regulating angiogenesis in a murine model.

METHODS

ATLa and RvE1 receptor expression was tested in different corneal cell populations by RT-PCR. Corneal neovascularization (CNV) was induced by suture or micropellet (IL-1 beta, VEGF-A) placement. Mice were then treated with ATLa, RvD1, RvE1, or vehicle, subconjunctivally at 48-hour intervals. Infiltration of neutrophils and macrophages was quantified after immunofluorescence staining. The mRNA expression levels of inflammatory cytokines, VEGFs, and VEGFRs were analyzed by real-time PCR. CNV was evaluated intravitally and morphometrically.

RESULTS

The receptors for LXA4, ALX/Fpr-rs-2 and for RvE1, ChemR23 were each expressed by epithelium, stromal keratocytes, and infiltrated CD11b(+) cells in corneas. Compared to the vehicle-treated eye, ATLa-, RvD1-, and RvE1-treated eyes had reduced numbers of infiltrating neutrophils and macrophages and reduced mRNA expression levels of TNF-alpha, IL-1 alpha, IL-1 beta, VEGF-A, VEGF-C, and VEGFR2. Animals treated with these mediators had significantly suppressed suture-induced or IL-1 beta-induced hemangiogenesis (HA) but not lymphangiogenesis. Interestingly, only the application of ATLa significantly suppressed VEGF-A-induced HA.

CONCLUSIONS

ATLa, RvE1, and RvD1 all reduce inflammatory corneal HA by early regulation of resolution mechanisms in innate immune responses. In addition, ATLa directly inhibits VEGF-A-mediated angiogenesis and is the most potent inhibitor of NV among this new genus of dual anti-inflammatory and pro-resolving lipid mediators.

Molecular control of lymphatic metastasis

The metastatic spread of tumor cells is the most lethal aspect of cancer and often occurs via the lymphatic vasculature. Both experimental tumor models and human clinicopathologic data indicate that growth of lymphatic vessels (lymphangiogenesis) near solid tumors is often associated with lymph node metastasis. Changes in the adhesive properties of lymphatic endothelium near tumors may also facilitate metastatic spread via the lymphatics. Lymphangiogenic growth factors have been identified that promote formation of tumor lymphatics and metastatic spread of tumor cells to lymph nodes. These include the secreted glycoproteins vascular endothelial growth factor-C (VEGF-C) and VEGF-D, which act via their cognate receptor tyrosine kinase VEGF receptor-3 (VEGFR-3) located on lymphatic endothelial cells. Other signaling molecules that have been reported to promote lymphangiogenesis and/or lymphatic metastasis in cancer include VEGF-A, platelet-derived growth factor-BB, and hepatocyte growth factor. However, the quantitative contribution of these proteins to tumor lymphangiogenesis and lymphatic metastasis in different tumor types requires further investigation. In addition, chemokines are thought to play a role in attracting tumor cells and lymphatic vessels to each other. Moreover, it has recently been shown that lymphangiogenic growth factors secreted from a primary tumor can induce lymphangiogenesis in nearby lymph nodes, even before arrival of tumor cells, which may facilitate further metastasis. This article provides an overview of the molecular mechanisms that control lymphatic metastasis and discusses potential therapeutic approaches for inhibiting this process in human cancer.

Identification and expression analysis of an N-terminally truncated isoform of human PDGF-C

Platelet-derived growth factor C (PDGF-C) is a member of the PDGF family that plays an important role in developmental and physiological processes, and in human diseases. Here, we report a novel splice variant of human PDGF-C (PDGF-Cb), which encodes an N-terminally truncated protein, lacking the signal peptide and CUB domain. This variant is coexpressed with PDGF-C in all normal tissues analyzed. PDGF-Cb is produced as a cytoplasmic protein, and has a similar intracellular localization to PDGF-C, but is not secreted from transfected cells. Further, we show that PDGF-Cb can form heterodimers (PDGF-CCb) with PDGF-C, which is thereby retained and degraded within cells. In primary renal cell carcinoma (RCC), expression of the two alternatively spliced transcripts was different. Generally, expression of the full-length PDGF-C transcript was increased in RCC tumors, whereas expression of PDGF-Cb was not in the 30 analyzed cases with paired RCC tumor tissues and normal renal tissues. Based on these findings, we suggest that PDGF-Cb might act as a dominant negative molecule regulating the secretion of PDGF-C, and that deregulation of full-length PDGF-C is involved in RCC tumorigenesis.

A critical role of vascular endothelial growth factor D in zebrafish embryonic vasculogenesis and angiogenesis

The VEGF family comprises seven members that are designated VEGF-A, VEGF-B, VEGF-C, VEGF-D, VEGF-E, placental growth factor (PlGF), and VEGF-F. Of these factors, VEGF-D plays important roles for angiogenesis and lymphangiogenesis, and could promote tumor growth and lymphatic metastasis. In this study, we identified a zebrafish VEGF-D homolog that encodes a 272 amino acid protein including a PDGF (platelet-derived growth factor) domain characteristic to VEGF family. Expression profile demonstrated that the VEGF-D began expressed from 13 somite stage. Microinjecting zVEGF-D mRNA into zebrafish 1-cell stage embryos resulted in severe misguidance of intersegmental vessels (ISV) and abnormal connection between dorsal aorta and caudal vein. Microangiography indicated that these abnormal ISVs were not functional. Our studies therefore identified the first non-mammalian VEGF-D and established its in vivo role for vascular system development during vertebrate embryogenesis and provided an alternative animal model to further reveal functions of VEGF-D.

Vascular endothelial growth factor: biology and therapeutic applications

While the development of anti-angiogenic therapy, as it pertains to cancer treatment, may still be in its infancy relative to well-established modalities such as chemotherapy, radiation, and surgery, major strides made in the past several decades have allowed translation of basic science discoveries in this field into clinical reality. The discovery of key molecular modulators of angiogenesis, notably vascular endothelial growth factor (VEGF), has catalyzed the development of numerous neutralizing therapeutic agents. The validity of VEGF inhibition as a therapeutic strategy has been well supported in randomized clinical trials, as well as U.S. Food and Drug Administration approval of the VEGF antagonists bevacizumab, sunitinib malate, sorafenib, pegaptinib and ranibizumab. Accordingly, this review will (1) briefly review the basic molecular biology of VEGF and (2) summarize recent progress in targeting the VEGF molecular pathway as therapy for angiogenic diseases such as cancer and age-related macular degeneration.

VEGF and VEGF receptors are differentially expressed in chondrocytes

During long bone development, cartilage replacement by bone is governed in part by angiogenesis. Although it has been demonstrated that vascular endothelial growth factor (VEGF-A) is crucial during endochondral ossification, little is known about the involvement of the other VEGF family members. Thus, we examined the expression and production of these members on primary chondrocytes and ATDC5 chondrogenic cells. VEGF-A, VEGF-B, VEGF-C and VEGF-D were shown to be expressed and synthesized demonstrating that numerous angiogenic factors can be produced by chondrocytes. In ATDC5 VEGF-A, VEGF-B and VEGF-C were over-expressed in the presence of chondrogenic and bone morphogenetic protein (BMP)-2 treatment suggesting that these factors play an important role during chondrogenesis. In addition, neuropilin-1, VEGF receptor-2 and VEGF receptor-3 gene expression were observed with an increase in VEGF-R2 expression under chondrogenic and BMP-2 treatment, suggesting that VEGF proteins could act in an autocrine/paracrine manner in addition to their angiogenic function. In conclusion, we demonstrated for the first time that chondrocytes secreted the four members of the VEGF family. We also showed that VEGF-B, VEGF-C and VEGF-D were secreted as processed proteins. The up-regulation of VEGF-B and VEGF-C at the mRNA and protein levels under chondrogenic stimulation strongly suggests a major role for these proteins in growth plate physiology.

Proprotein convertases promote processing of VEGF-D, a critical step for binding the angiogenic receptor VEGFR-2

Vascular endothelial growth factor (VEGF)-D is a secreted glycoprotein that induces angiogenesis and lymphangiogenesis. It consists of a central domain, containing binding sites for VEGF receptor-2 (VEGFR-2) and VEGFR-3, and N- and C-terminal propeptides. It is secreted from the cell as homodimers of the full-length form that can be proteolytically processed to remove the propeptides. It was recently shown, using adenoviral gene delivery, that fully processed VEGF-D induces angiogenesis in vivo, whereas full-length VEGF-D does not. To better understand these observations, we monitored the effect of VEGF-D processing on receptor binding using a full-length VEGF-D mutant that cannot be processed. This mutant binds VEGFR-2, the receptor signaling for angiogenesis, with approximately 17,000-fold lower affinity than mature VEGF-D, indicating the importance of processing for interaction with this receptor. Further, we show that members of the proprotein convertase (PC) family of proteases promote VEGF-D processing, which facilitates the VEGF-D/VEGFR-2 interaction. The PCs furin and PC5 promote cleavage of both propeptides, whereas PC7 promotes cleavage of the C-terminal propeptide only. The finding that PCs promote activation of VEGF-D and other proteins with roles in cancer such as matrix metalloproteinases, emphasizes the importance of these enzymes as potential regulators of tumor progression and metastasis.

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.

Signal transduction by VEGF receptors in regulation of angiogenesis and lymphangiogenesis

The VEGF/VPF (vascular endothelial growth factor/vascular permeability factor) ligands and receptors are crucial regulators of vasculogenesis, angiogenesis, lymphangiogenesis and vascular permeability in vertebrates. VEGF-A, the prototype VEGF ligand, binds and activates two tyrosine kinase receptors: VEGFR1 (Flt-1) and VEGFR2 (KDR/Flk-1). VEGFR1, which occurs in transmembrane and soluble forms, negatively regulates vasculogenesis and angiogenesis during early embryogenesis, but it also acts as a positive regulator of angiogenesis and inflammatory responses, playing a role in several human diseases such as rheumatoid arthritis and cancer. The soluble VEGFR1 is overexpressed in placenta in preeclampsia patients. VEGFR2 has critical functions in physiological and pathological angiogenesis through distinct signal transduction pathways regulating proliferation and migration of endothelial cells. VEGFR3, a receptor for the lymphatic growth factors VEGF-C and VEGF-D, but not for VEGF-A, regulates vascular and lymphatic endothelial cell function during embryogenesis. Loss-of-function variants of VEGFR3 have been identified in lymphedema. Formation of tumor lymphatics may be stimulated by tumor-produced VEGF-C, allowing increased spread of tumor metastases through the lymphatics. Mapping the signaling system of these important receptors may provide the knowledge necessary to suppress specific signaling pathways in major human diseases.

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.

The vascular endothelial growth factor (VEGF) family: angiogenic factors in health and disease

Vascular endothelial growth factors (VEGFs) are a family of secreted polypeptides with a highly conserved receptor-binding cystine-knot structure similar to that of the platelet-derived growth factors. VEGF-A, the founding member of the family, is highly conserved between animals as evolutionarily distant as fish and mammals. In vertebrates, VEGFs act through a family of cognate receptor kinases in endothelial cells to stimulate blood-vessel formation. VEGF-A has important roles in mammalian vascular development and in diseases involving abnormal growth of blood vessels; other VEGFs are also involved in the development of lymphatic vessels and disease-related angiogenesis. Invertebrate homologs of VEGFs and VEGF receptors have been identified in fly, nematode and jellyfish, where they function in developmental cell migration and neurogenesis. The existence of VEGF-like molecules and their receptors in simple invertebrates without a vascular system indicates that this family of growth factors emerged at a very early stage in the evolution of multicellular organisms to mediate primordial developmental functions.

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.

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.

Comparative expression of vascular endothelial growth factor family members, VEGF-B, -C and -D, by normal human keratinocytes and fibroblasts

The vascular endothelial growth factor (VEGF) family includes the related polypeptides VEGF-B, -C and -D, which contribute to endothelial and lymphatic vessel development. The parental VEGF molecule, VEGF-A, has been widely described in the skin, but the other members of the VEGF family have not yet been reported. The aim of our study was to determine whether the two main skin cells, keratinocytes and fibroblasts, expressed VEGF-B, -C and -D in basal condition and after stimulation by either growth factors or the pro-inflammatory cytokine tumour necrosis factor-alpha (TNF-alpha). Reverse-transcription polymerase chain reaction (RT-PCR) analysis on cultured normal human keratinocytes (NHKs) and normal human fibroblasts (NHFs) allowed the detection of different levels of VEGF-B, -C and -D mRNA, in both cell types with similar RT-PCR products in the skin cells. A semi-quantitative evaluation of the VEGF family proteins by dot blot, using the different human recombinant VEGFs, showed different levels of VEGF-B, -C and -D, in NHKs and NHFs. After cell stimulation by growth factors (epidermal growth factor (EGF) and transforming growth factor-beta1 (TGF-beta1) for NHKs and NHFs, respectively), a significant up-regulation of the VEGF family member proteins was observed in NHFs but not in NHKs. Conversely, TNF-alpha did not exert a significant effect. However, we could not detect any transcriptional modification in stimulated cells, whatever the stimulation duration. The addition of cycloheximide to the cell cultures strongly inhibited the increase of VEGF proteins in TGF-beta1-stimulated NHFs. Taken together, the results underline the major role played by NHFs in the elaboration of the VEGF family proteins known to regulate wound healing, chronic inflammation and tumour angiogenesis and lymphangiogenesis.

Plasmin activates the lymphangiogenic growth factors VEGF-C and VEGF-D

Vascular endothelial growth factor (VEGF) C and VEGF-D stimulate lymphangiogenesis and angiogenesis in tissues and tumors by activating the endothelial cell surface receptor tyrosine kinases VEGF receptor (VEGFR) 2 and VEGFR-3. These growth factors are secreted as full-length inactive forms consisting of NH2- and COOH-terminal propeptides and a central VEGF homology domain (VHD) containing receptor binding sites. Proteolytic cleavage removes the propeptides to generate mature forms, consisting of dimers of the VEGF homology domain, that bind receptors with much greater affinity than the full-length forms. Therefore, proteolytic processing activates VEGF-C and VEGF-D, although the proteases involved were unknown. Here, we report that the serine protease plasmin cleaved both propeptides from the VEGF homology domain of human VEGF-D and thereby generated a mature form exhibiting greatly enhanced binding and cross-linking of VEGFR-2 and VEGFR-3 in comparison to full-length material. Plasmin also activated VEGF-C. As lymphangiogenic growth factors promote the metastatic spread of cancer via the lymphatics, the proteolytic activation of these molecules represents a potential target for antimetastatic agents. Identification of an enzyme that activates the lymphangiogenic growth factors will facilitate development of inhibitors of metastasis.

The FREP gene family in the snail Biomphalaria glabrata: additional members, and evidence consistent with alternative splicing and FREP retrosequences. Fibrinogen-related proteins

Fibrinogen-related proteins (FREPs) found in hemolymph of the snail Biomphalara glabrata are hypothesized to be involved in non-self recognition. Among 150 cloned FREP cDNAs examined, we have identified three additional FREP members, FREPs 3.3, 12.1 and 13.1, bringing the total of FREP subfamilies to 13. The new FREPs each encode two immunoglobulin superfamily domains and a fibrinogen domain. Additionally, five truncated cDNAs with >99% nucleotide identity in coding regions to FREPs 3.2, 12.1 or 13.1 were identified. The truncated forms, the first reported for FREPs, lack a partial exon, one complete exon, or two complete exons plus the 3'UTR. Our preferred hypothesis is that all five truncated cDNAs observed arise from alternative splicing of full-length FREP genes. Genomic sequences lacking at least two introns and corresponding to the 3' ends of the cDNAs of FREP12.1 and its two truncated forms were also recovered. Although these could be the source of the truncated cDNAs, they are believed to be retrosequences.

Modulation of VEGFR-2-mediated endothelial-cell activity by VEGF-C/VEGFR-3

Vascular endothelial growth factor (VEGF) receptor 3 (VEGFR-3), a receptor for VEGF-C, was shown to be essential for angiogenesis as well as for lymphangiogenesis. Targeted disruption of the VEGFR-3 gene in mice and our previous study using an antagonistic monoclonal antibody (MoAb) for VEGFR-3 suggested that VEGF-C/VEGFR-3 signals might be involved in the maintenance of vascular integrity. In this study we used an in vitro embryonic stem (ES) cell culture system to maintain the VEGFR-3(+) endothelial cell (EC) and investigated the role of VEGFR-3 signals at the cellular level. In this system packed clusters of ECs were formed. Whereas addition of exogenous VEGF-A induced EC dispersion, VEGF-C, which can also stimulate VEGFR-2, promoted EC growth without disturbing the EC clusters. Moreover, addition of AFL4, an antagonistic MoAb for VEGFR-3, resulted in EC dispersion. Cytological analysis showed that VEGF-A- and AFL4-treated ECs were indistinguishable in many aspects but were distinct from the cytological profile induced by antagonistic MoAb for VE-cadherin (VECD-1). As AFL4- induced EC dispersion requires VEGF-A stimulation, it is likely that VEGFR-3 signals negatively modulate VEGFR-2. This result provides new insights into the involvement of VEGFR-3 signals in the maintenance of vascular integrity through modulation of VEGFR-2 signals. Moreover, our findings suggest that the mechanisms underlying AFL4-induced EC dispersion are distinct from those underlying VECD-1-induced dispersion for maintenance of EC integrity.

Recent advances in angiogenesis, anti-angiogenesis and vascular targeting

Angiogenesis, the development of new blood vessels, has become a major focus of research. This has been stimulated by the therapeutic opportunities offered by the ability to manipulate the vasculature in pathologies such as cancer. Here, we present an overview of recent advances in angiogenesis. Especially noteworthy is the large volume of information from developmental studies, particularly those that involve transgenic and gene knockout mice. We also discuss the increasing repertoire of drugs with which to manipulate angiogenesis and new endothelial-specific genes with which to target the vasculature.