Specific binding of vascular permeability factor to endothelial cells

Novel non-angiogenic role for mesenchymal stem cell-derived vascular endothelial growth factor on keratinocytes during wound healing

With chronic wounds remaining a substantial healthcare issue, new therapies are sought to improve patient outcomes. Various studies have explored the benefits of promoting angiogenesis in wounds by targeting proangiogenic factors such as Vascular Endothelial Growth Factor (VEGF) family members to improve wound healing. Along similar lines, Mesenchymal Stem Cell (MSC) secretions, usually containing VEGF, have been used to improve angiogenesis in wound healing via a paracrine mechanism. Recent evidence for keratinocyte VEGF receptor expression, as well as proliferative and chemotactic responses by keratinocytes to exogenous VEGFA in vitro implies distinct non-angiogenic actions for VEGF during wound healing. In this review, we discuss the expression of VEGF family members and their receptors in keratinocytes in relation to the potential for wound healing treatments. We also explore recent findings of MSC secreted paracrine wound healing activity on keratinocytes. We report here the concept of keratinocyte wound healing responses driven by MSC-derived VEGF that is supported in the literature, providing a new mechanism for cell-free therapy of chronic wounds.

Evaluation of the Anti-angiogenic Activity of Saponins from Maesa lanceolata by Different Assays

Angiogenesis, in which a vascular network is established from pre-existing vessels, is a complex multistep process. Mechanisms underlying angiogenesis can be investigated using a variety of in vitro, ex vivo and in vivo approaches. Evaluation of several promising plants and plant metabolites, including terpenoids, revealed promising anti-angiogenic activity. Since the maesasaponins displayed anti-angiogenic activity in the chick chorioallantoic membrane (CAM) assay, their activity was further investigated in several test systems. The rat aorta ring assay was compared with the placental vein assay and then selected for the ex vivo investigation of the saponins. Besides their effect on the viability of HUVEC, the anti-angiogenic capacity of the compounds was also investigated in an in vivo zebrafish assay. The activity of the saponins in the viability assay was more pronounced than in the rat aorta ring assay and similar to the effect observed in the CAM assay. The use of different test systems, however, implies different results in the case of saponins.

HSPG-binding peptide corresponding to the exon 6a-encoded domain of VEGF inhibits tumor growth by blocking angiogenesis in murine model

Vascular endothelial growth factor VEGF₁₆₅ is a critical element for development of the vascular system in physiological and pathological angiogenesis. VEGF isoforms have different affinities for heparan sulphate proteoglycan (HSPG) as well as for VEGF receptors; HSPGs are important regulators in vascular development. Therefore, inhibition of interactions between VEGF and HSPGs may prevent angiogenesis. Here, we demonstrate that an HSPG-binding synthetic peptide, corresponding to exon 6a-encoded domain of VEGF gene, has anti-angiogenic property. This 20 amino acids synthetic peptide prevents VEGF₁₆₅ binding to several different cell types, mouse embryonic sections and inhibits endothelial cell migration, despite its absence in VEGF₁₆₅ sequence. Our in vivo anti-tumor studies show that the peptide inhibits tumor growth in both mouse Lewis-Lung Carcinoma and human Liposarcoma tumor-bearing animal models. This is the first evidence that a synthetic VEGF fragment corresponding to exon 6a has functional antagonism both in vitro and in vivo. We conclude that the above HPSG binding peptide (6a-P) is a potent inhibitor of angiogenesis-dependent diseases.

Bone and wound healing augmentation with platelet-rich plasma

Over the past two decades, autologous platelets that have been sequestered, concentrated, and mixed with thrombin to generate growth factor-concentrated platelet-rich plasma for application to bone and wounds to aide healing have been a subject of great interest. This article reviews the literature related to the use of autologous platelet-rich plasma in bone and wound healing, and reviews the processes necessary to secure a high concentration of viable platelets. Although not yet definitive, autologous platelet-rich plasma has been shown to be safe, reproducible, and effective in mimicking the natural process of bone and wound healing.

Reduction of circulating soluble Flt-1 alleviates preeclampsia-like symptoms in a mouse model

Preeclampsia (PE) is characterized by widespread endothelial damage with hypertension, proteinuria, glomeruloendotheliosis and elevated soluble Flt-1 (sFlt-1), a natural occurring antagonist of vascular endothelial growth factor (VEGF). Cancer patients receiving anti-VEGF therapy exhibit similar symptoms. We suggested that a decrease in circulating sFlt-1 would alleviate the symptoms associated with PE. Adenoviral (Adv) overexpression of sFlt-1 induced proteinuria, caused glomerular damage and increase in blood pressure in female Balb/c mice. Circulating level of sFlt-1 above 50 ng/ml plasma induced severe vascular damage and glomerular endotheliosis. Albumin concentration in urine was elevated up to 30-fold, compared to control AdvGFP-treated animals. The threshold of kidney damage was in the range of 20–30 ng/ml sFlt-1 in plasma (8–15 ng/ml in urine). Co-administration of AdvsFlt-1 with AdvVEGF to neutralize circulating sFlt-1 resulted in more than a 70% reduction in free sFlt-1 in plasma, more than 80% reduction in urine and rescued the damaging effect of sFlt-1 on the kidneys. This demonstrates that below a critical threshold sFlt-1 fails to elicit damage to the fenestrated endothelium and that co-expression of VEGF is able to rescue effects mediated by sFlt-1 overexpression.

The role of vascular endothelial growth factor in wound healing

BACKGROUND

A chronic wound is tissue with an impaired ability to heal. This is often a consequence of one of the following etiologies: diabetes, venous reflux, arterial insufficiency sickle cell disease, steroids, and/or pressure. Healing requires granulation tissue depending on epithelialization and angiogenesis. Currently no growth factor is available to treat patients with impaired healing that stimulates both epithelialization and angiogenesis. The objective is to review is the multiple mechanisms of vascular endothelial growth factor (VEGF) in wound healing.

MATERIALS AND METHODS

The authors reviewed the literature on the structure and function of VEGF, including its use for therapeutic angiogenesis. Particular attention is given to the specific role of VEGF in the angiogenesis cascade, its relationship to other growth factors and cells in a healing wound.

RESULTS

VEGF is released by a variety of cells and stimulates multiple components of the angiogenic cascade. It is up-regulated during the early days of healing, when capillary growth is maximal. Studies have shown the efficacy of VEGF in peripheral and cardiac ischemic vascular disease with minimal adverse effects. Experimental data supports the hypothesis that VEGF stimulates epithelialization and collagen deposition in a wound.

CONCLUSION

VEGF stimulates wound healing through angiogenesis, but likely promotes collagen deposition and epithelialization as well. Further study of the molecule by utilizing the protein itself, or novel forms of delivery such as gene therapy, will increase its therapeutic possibilities to accelerate closure of a chronic wound.

Phosphoinositide 3-kinase gamma/delta inhibition limits infarct size after myocardial ischemia/reperfusion injury

Although phosphoinositide 3-kinases (PI3Ks) play beneficial pro-cell survival roles during tissue ischemia, some isoforms (gamma and delta) paradoxically contribute to the inflammation that damages these same tissues upon reperfusion. We therefore considered the possibility that selectively inhibiting proinflammatory PI3K isoforms during the reperfusion phase could ultimately limit overall tissue damage seen in ischemia/reperfusion injuries such as myocardial infarction. Panreactive and isoform-restricted PI3K inhibitors were identified by screening a novel chemical family; molecular modeling studies attributed isoform specificity based on rotational freedom of substituent groups. One compound (TG100-115) identified as a selective PI3K gamma/delta inhibitor potently inhibited edema and inflammation in response to multiple mediators known to participate in myocardial infarction, including vascular endothelial growth factor and platelet-activating factor; by contrast, endothelial cell mitogenesis, a repair process important to tissue survival after ischemic damage, was not disrupted. In rigorous animal MI models, TG100-115 provided potent cardioprotection, reducing infarct development and preserving myocardial function. Importantly, this was achieved when dosing well after myocardial reperfusion (up to 3 h after), the same time period when patients are most accessible for therapeutic intervention. In conclusion, by targeting pathologic events occurring relatively late in myocardial damage, we have identified a potential means of addressing an elusive clinical goal: meaningful cardioprotection in the postreperfusion time period.

Vascular endothelial growth factor in thyroid cancers

Vascular endothelial growth factor (VEGF) is an essential peptide in new vessel growth in physiology (endometrial growth, embryonic development); pathological conditions (diabetic retinopathy, rheumatoid arthritis); as well as in tumor cell growth, particularly distant metastases. This study focused on VEGF structure, receptors, and angiogensis in tumors, especially their roles in thyroid cancer. The VEGF mRNA undergoes alternative splicing events that generate four homodimeric isoforms, including VEGF121, VEGF165, VEGF189, or VEGF206. Using VEGF purified from a culture medium conditioned by A-431 human epidermoid carcinoma cells, VEGF-binding site complexes of 230, 170, and 125 kDa were detected on human umbilical vein endothelial cells. The VEGF specifically induced the tyrosine phosphorylation of a 190-kDa polypeptide, which had similar mass to the largest binding site detected through affinity cross-linking. A transmembrane receptor belongs to the tyrosine kinase family, fms-like tyrosine kinase (FLT). These receptor tyrosine kinases encoded by the FLT gene family have distinct functions in regulating blood vessel growth and differentiation. Regulation of VEGF is a complex, multistep mechanism in various kinds of cells and tissues. Hypoxia-dependent and -independent mechanisms are illustrated in different cancer tissues. Hypoxic tumor cells may switch to a proangiogenic phenotype, which increases VEGF transcription. Clinical applications of VEGF in cancer have included diagnosis, prediction of prognosis, and treatment in different solid tumors, including thyroid tumors. Studies involving thyroid cancer cell lines, serum level determination, immunohistocytochemical staining, molecular biological studies, and gene therapy to the in vivo clinical trials, have shown that antiangiogensis therapy can provide another treatment modality for thyroid cancer. Future studies focused on recombinant human anti-VEGF research involving patients with advanced thyroid cancer, and investigation of the protection of high-risk patients by using novel antiangiogenic vaccines, are warranted.

Promising though not yet proven: Emerging strategies to promote myocardial salvage

Remarkable advances in our ability to achieve early and sustained culprit vessel patency in acute myocardial infarction have been satisfying, but our enthusiasm must be tempered by the knowledge that the overall treatment strategy often leaves an inadequate long term clinical result. Early success of percutaneous therapy as judged at angiography does not ensure recovery of normal left ventricular function, the most important determinant of survival in acute myocardial infarction. That congestive heart failure and death still complicate apparently successful percutaneous procedures underscores the need to develop novel therapies which salvage jeopardized myocardium, limit infarct size and preserve left ventricular function. An ever-increasing body of data demonstrates a multifactorial mechanism of myocyte injury and microvascular collapse and also demonstrates that these injuries seem to have a profound impact on long-term outcomes. Given these findings, microvascular protection during the acute event has become the focus of a variety of emerging technologies. The goal of these mechanical and pharmacologic therapies is the restoration of normal metabolic function at the myocyte level. The acute pathologic mechanisms which contribute to sustained left ventricular dysfunction despite angiographically successful revascularization will be reviewed as will be several strategies being developed to counter these pathologic mechanisms.

Novel Function for Vascular Endothelial Growth Factor Receptor-1 on Epidermal Keratinocytes

Vascular endothelial growth factor (VEGF-A), a potent stimulus for angiogenesis, is up-regulated in the skin after wounding. Although studies have shown that VEGF is important for wound repair, it is unclear whether this is based solely on its ability to promote angiogenesis or if VEGF can also promote healing by acting directly on non-endothelial cell types. By immunohistochemistry and reverse transcriptase-polymerase chain reaction, expression of VEGF receptor-1 (VEGFR-1), but not VEGFR-2, was detected in murine keratinocytes during wound repair and in normal human epidermal keratinocytes (NHEKs). The presence of VEGF receptors on NHEKs was verified by binding studies with 125I-VEGF. In vitro, VEGF stimulated the proliferation of NHEKs, an effect that could be blocked by treatment with neutralizing VEGFR-1 antibodies. A role for VEGFR-1 in keratinocytes was also shown in vivo because treatment of excisional wounds with neutralizing VEGFR-1 antibodies delayed re-epithelialization. Treatment with anti-VEGFR-1 antibodies also reduced the number of proliferating keratinocytes at the leading edge of the wound, suggesting that VEGF sends a proliferative signal to these cells. Together, these data describe a novel role for VEGFR-1 in keratinocytes and suggest that VEGF may play several roles in cutaneous wound repair.

Characterization of the vascular endothelial growth factor-receptor interaction and determination of the recombinant protein by an optical receptor sensor

Vascular endothelial growth factor (VEGF) is one of the most important factors controlling angiogenesis. It is a homodimeric glycoprotein belonging to the family of cysteine-knot proteins. The biological activity is transduced via membrane-spanning receptors of the tyrosine kinase receptor family. Each biologically active VEGF has two receptor binding sites leading to receptor dimerization as first step following ligand binding. The ligand-binding site of the receptor is localized on extracellular Ig-like domains. The extracellular part of the receptor Flt-1 (VEGFR-1) was expressed as soluble protein and was used as receptor in an optical affinity sensor system (BIAcore). Suitable conditions allowed the determination of the association and dissociation rate constants as k(a)=4+/-1.2 x 10(6) M(-1) s(-1) and k(d)=3+/-0.8 x 10(-5) s(-1), respectively, leading to an affinity constant of K(D)=7.5+/-3 pM, which is within the range published already from other investigations and methods. Increasing receptor loadings of the sensor surface decreased the binding efficiency, as the ratio of bound VEGF-molecules to theoretically available binding sites increased from 1:1.5 to 1:2.6. Increasing the surface loading further, allowed the establishment of a quantitative assay with the analytical performance being influenced by the receptor loading and the contact time between sample and immobilized receptor, i.e. sample volume. This assay was used for VEGF determination during the cultivation of a recombinant Pichia pastoris strain.

Induction of KDR expression in bovine arterial endothelial cells by thrombin: involvement of nitric oxide

Thrombin, a multifunctional serine protease, is generated at the site with vascular injuries. It not only participates in the coagulation cascade, but also can induce a lot of events related to cell mitogenesis and migration. In this study, we investigated the effect of thrombin on endothelial cell proliferation induced by vascular endothelial growth factor (VEGF). Thrombin promoted proliferation of cultured bovine carotid endothelial cells in a time- and dose-dependent manner. Moreover, it drastically enhanced the cell growth stimulated by VEGF. This stimulatory effect was reduced by inhibitors of either protein kinase C (PKC) or mitogen-activated protein kinase kinase (MAPKK). Thrombin induced a significant increase in the level of mRNA of the kinase domain-containing receptor (KDR), but not tms-like tyrosine kinase (Flt-1), in a time-dependent manner, which reached the maximum after 24 h of stimulation. This increase coincides well with the KDR protein expression. The luciferase assay showed that thrombin induced an about 7.5-fold increase in the KDR promoter activity compared with the control. This enhanced KDR promoter activity was also abolished by inhibitors of either PKC or MAPKK. The deletion analyses indicated that the region between -115 and -97 (containing Sp1 binding region) within the KDR promoter gene was required for the enhanced KDR expression induced by thrombin and VEGF. Moreover, the nitric oxide synthase (NOS) inhibitor abolished both the accelerated cell proliferation and the increased KDR expression induced by thrombin and VEGF. This inhibition was abrogated by DETA NONOate, a NO donor with long half-life. These findings suggest that thrombin might potentiate the VEGF-induced angiogenic activity through increasing the level of the VEGF receptor KDR, in which production of NO is involved.

Co-localization of vascular endothelial growth factor and its two receptors flt-1 and kdr in the mink placenta

Placental angiogenesis plays an important role in placental development and morphogenesis. Vascular endothelial growth factor (VEGF) is a well-known angiogenic growth factor, which has previously been localized in different epitheliochorial and haemochorial placenta types. In the present study VEGF and its Flt-1(VEGFR-1) and KDR (VEGFR-2) receptors were immunolocalized in the endotheliochorial mink placenta throughout gestation. VEGF, Flt-1 and KDR co-localized to fetal and maternal microvascular endothelial cells, but with a temporal difference, displaying KDR in endothelial cells throughout gestation, whereas the VEGF and Flt-1 maternal endothelial cell staining was most intense during late gestation. Additionally, KDR was found in vascular related mesenchymal cells. The VEGF-receptors were also localized in non-endothelial cells, e.g. the uterine luminal and glandular epithelium as well as the trophoblast. Our results are in agreement with former studies, showing the different effects of the Flt-1-and KDR receptors in respect of angiogenesis. More importantly, the present study of the endotheliochorial placenta localizes the VEGF-ligand-receptor system in non-endothelial cells, and thereby strengthen the hypothesis that VEGF, apart from its well-established angiogenic properties, must also have additional functional roles in the establishment and development of the placenta.

The human internal thoracic artery releases more nitric oxide in response to vascular endothelial growth factor than the human saphenous vein

OBJECTIVE

Endothelial nitric oxide inhibits smooth muscle cell proliferation, reducing the chance of vascular intimal thickening. In this study we investigated whether the superior long-term patency of the internal thoracic artery in human coronary bypass grafting compared with that of the saphenous vein could be explained by different levels of nitric oxide production.

METHODS

The baseline endogenous nitric oxide production appeared to be 50% higher in the internal thoracic artery than in the saphenous vein. Previously, it was shown that vascular endothelial growth factor and the vascular endothelial growth factor receptors KDR (Flk-1) and Flt-1 are expressed in both internal thoracic arteries and saphenous veins and that vascular endothelial growth factor receptor density was higher in internal thoracic arteries than in saphenous veins. Therefore, we also investigated the influence of vascular endothelial growth factor on nitric oxide release in both the internal thoracic artery and the saphenous vein.

RESULTS

Vascular endothelial growth factor augmented nitric oxide production by approximately 50% in the saphenous vein and 100% in the internal thoracic artery. As shown by means of immunohistochemistry, expression of endothelial constitutive nitric oxide synthase was similar in the internal thoracic artery and the saphenous vein, and no inducible nitric oxide synthase was expressed in any of the vascular segments.

CONCLUSION

Vascular endothelial growth factor augments endothelial constitutive nitric oxide synthase-dependent nitric oxide release to a greater extent in the internal thoracic artery than in the saphenous vein. These findings may help to explain the long-term superiority of the internal thoracic artery versus the saphenous vein as a conduit for coronary artery bypass.

Effect of ionizing radiation on thymidine uptake, differentiation, and VEGFR2 receptor expression in endothelial cells: the role of VEGF(165)

PURPOSE

Late thrombosis of irradiated vascular segments may be the consequence of endothelial cell (EC) dysfunction after radiation therapy. We investigated the effects of beta ionizing radiation on human EC viability, thymidine uptake, and differentiation.

METHODS AND MATERIALS

Endothelial cells were exposed to (32)P-labeled DNA oligonucleotides in incremental doses of 2, 6, and 10 Gy. The modulation of the VEGFR2 receptor expression after irradiation and the overall potential radioprotective effect of VEGF(165) on these functions were assayed.

RESULTS

A dose-dependent inhibitory effect of beta irradiation on ECs' thymidine uptake and differentiation was observed. EC viability, however, was not affected at levels of radiation up to 10 Gy. VEGF(165) proved to have a radioprotective effect as ECs' thymidine uptake, after radiation doses of 2, 6, and 10 Gy, was increased by 1.5-, 2-, and 4-fold, respectively, in the presence of 10 ng/ml of VEGF(165) (p < 0.05 vs. LacZ). This concentration of VEGF(165) also proved beneficial in maintaining cell differentiation at 16 h postirradiation when compared to controls. These biologic effects were in direct correlation with the upregulation of VEGFR2 receptor expression in irradiated ECs.

CONCLUSIONS

beta irradiation interacts directly with EC functions by significantly reducing their ability to differentiate and proliferate, associated with upregulation of VEGFR2. These effects can be prevented in part by pretreating cells with VEGF(165), an effect potentially favored by the upregulation of VEGFR2 receptor expression after irradiation.

Kinetics of placenta growth factor/vascular endothelial growth factor synergy in endothelial hydraulic conductivity and proliferation

Vascular endothelial growth factor (VEGF) was originally discovered as vascular permeability factor because of its ability to increase microvascular permeability to plasma proteins. Since then, it has been shown to induce proliferation and migration in endothelial cells. Placenta growth factor (PlGF) is a member of the VEGF family of growth factors, but has little or undetectable mitogenic activity on endothelial cells. Intriguingly, however, PlGF was able to potentiate the action of low concentrations of VEGF on endothelial cell growth and macromolecule permeability in vitro. Here we show that PlGF can potentiate the effects of VEGF on the hydraulic conductivity of certain endothelial cells and that the duration of pretreatment with PlGF determines the resulting response. Hydraulic conductivity (Lp) was calculated from the water flux across the monolayer of human umbilical vein endothelial cells (HUVECs) or bovine aortic endothelial cells (BAECs). After 2 h of exposure to VEGF(165), the Lp of BAEC monolayers increased threefold, but the Lp of HUVEC monolayers did not increase. PlGF alone induced a small (63%) increase in Lp in BAECs, but not in HUVECs. BAEC, but not HUVEC, monolayers exposed first to PlGF and then to VEGF exhibited a seven- to eightfold increase in Lp. This enhancement in BAEC Lp could be observed for 4 h after the administration of PlGF. PlGF also potentiated the effect of VEGF on BAEC proliferation. Thus, augmentation of VEGF action by PlGF depends on the duration of PlGF exposure and on the origin of endothelial cells.

Suramin analogs inhibit human angiogenesis in vitro

BACKGROUND

Suramin is a polysulfonated naphthylurea that inhibits tumor cell proliferation and angiogenesis, but the widespread use of this drug has been limited by significant neurologic toxicity. A series of suramin analogs that may exhibit less toxicity in vivo have been synthesized. We hypothesized that these novel analogs would have antiangiogenic properties equal to or greater than those of suramin when evaluated in an in vitro human placental vein angiogenesis model.

METHODS

Human placental veins (n = 72 per group) were cultured in a 0.3% fibrin clot for a period of 14 days. Three suramin analogs (NF 145, NF 248, NF 293) and suramin were tested at 56 and 560 microM concentrations to determine their effect on the development of an angiogenic response. Experiments were repeated for each analog on veins from three different placentas. The percentage of wells that initiated an angiogenic response was calculated and compared with initiation in a control group (n = 141).

RESULTS

The three suramin analogs inhibited angiogenesis in a dose-dependent fashion, with all compounds exhibiting near-complete inhibition of angiogenesis at 560 microM. The effects of these analogs were equal to or greater than those of suramin.

CONCLUSION

Suramin analogs with structural alterations inhibit human angiogenesis at concentrations equivalent to those seen in vivo. These analogs may be more effective antiangiogenic agents than suramin and may have less potential for toxicity.

Expression of vascular endothelial growth factor (VEGF) and VEGF receptors in human neuroblastomas

BACKGROUND

Vascular endothelial growth factor (VEGF) is a specific endothelial cell mitogen that stimulates angiogenesis and plays a crucial role in tumor growth. The aim of the present study was to evaluate the expression of VEGF and of its two high-affinity tyrosine kinase receptors (KDR and Flt-1) in neuroblastoma surgical samples and cell lines.

PROCEDURE

The VEGF, KDR, and Flt-1 mRNA expression in neuroblastoma surgical samples and cell lines was studied by RT-PCR. The receptors were identified in [(125)I]VEGF binding and in functional studies (effect on cell growth). VEGF production by neuroblastomas was investigated by the ELISA method.

RESULTS

It was possible to observe the mRNAs encoding for VEGF and its two receptors in some of the surgical specimens examined, including most of the high-grade tumors. It was also possible to demonstrate that the SK-N-BE cell line expressed VEGF, KDR, and Flt-1 mRNAs as well as biologically active receptors: The cells bound [(125)I]-VEGF, and their growth was stimulated by exogenous VEGF. Moreover, VEGF protein could be detected in their culture conditioned medium.

CONCLUSIONS

These results suggest that, in addition to its effect on angiogenesis, VEGF may affect neuroblastoma cell growth directly and could be an autocrine growth factor.

Effect of vascular endothelial growth factor on cultured endothelial cell monolayer transport properties

Vascular endothelial growth factor (VEGF) is a potent enhancer of microvascular permeability in vivo. To date, its effects on hydraulic conductivity (L(p)) and diffusive albumin permeability (P(e)) of endothelial monolayers have not been thoroughly assessed in vitro. We hypothesized that VEGF affects endothelial transport properties differently depending on vessel location and endothelial phenotype. Using three well-established endothelial cell culture models-human umbilical vein endothelial cells (HUVECs), bovine aortic endothelial cells (BAECs), and bovine retinal microvascular cells (BRECs)-grown on porous, polycarbonate filters we were able to produce baseline transport properties characteristic of restrictive barriers. Our results show 3.1-fold and 5.7-fold increases in endothelial L(p) for BAEC and BREC monolayers, respectively, at the end of 3 h of VEGF (100 ng/ml) exposure. HUVECs, however, showed no significant alteration in L(p) after 3 h (100 ng/ml) or 24 h (25 ng/ml) of incubation with VEGF even though they were responsive to the inflammatory mediators, thrombin (1 U/ml; 27-fold increase in L(p) in 25 min) and bradykinin (10 microM; 4-fold increase in L(p) in 20 min). Protein kinase C (PKC) and nitric oxide (NO) are downstream effectors of VEGF signaling. BAEC L(p) was responsive to activation of NO (SNAP) and PKC (PMA), whereas these agents had no effect in altering HUVEC L(p). Moreover, BAECs exposed to the PKC inhibitor, staurosporine (50 ng/ml), exhibited significant attenuation of VEGF-induced increase in L(p), but inhibition of nitric oxide synthase (NOS) with L-NMMA (100 microM) had no effect in altering the VEGF-induced increase in L(p). These data provide strong evidence that in BAECs, the VEGF-induced increase in L(p) is mediated by a PKC-dependent mechanism. Regarding diffusive albumin P(e), at the end of 3 h, BAECs and BRECs showed 6.0-fold and 9. 9-fold increases in P(e) in response to VEGF (100 ng/ml), whereas VEGF had no significant effect after 3 h (100 ng/ml) or 24 h (25 ng/ml) in changing HUVEC P(e). In summary, these data indicate that VEGF affects endothelial transport properties differently depending on the vessel type and that differences in cell signaling pathways underlie the differences in VEGF responsiveness.

Angiostatic effects of suramin analogs in vitro

Suramin analogs are polyanionic naphthylureas structurally related to suramin, an antitumor agent with a narrow therapeutic window. The angiostatic activities of suramin and 16 suramin analogs were investigated using an easily quantifiable in vitro angiogenesis system. In addition, the antiproliferative activities of the analogs were studied in four different human tumor cell lines and in porcine aortic endothelial cells. The suramin analogs encompassed two main structural variations, i.e. their molecular size, and the number and substitution pattern of the sulfonate groups. Some suramin analogs with a reduced number of sulfonate groups (NF062, NF289 and NF326) showed significant dose-dependent angiostatic and also antiproliferative activities. The disulfonate NF062 was superior to suramin in inhibiting HT29 and T47D tumor cells while demonstrating a similar angiostatic potential as suramin. Therefore, the sulfonate groups in the para position of the amino groups of the naphthyl residues of suramin seem to be of special importance. The very small disulfonates (NF108, NF109, NF499, NF500 and NF241) and the asymmetric compound NF520, one half of the suramin molecule, are inactive. Therefore, a minimal molecule size seems to be essential for the biological activity. Suramin is a rather rigid molecule. The highly flexible analogs (NF527, NF528 and NF529) are inactive. This indicates that the molecular rigidity is important for the biological activity.

[Vascular endothelial factor (VEGF): therapeutic angiogenesis and vasculogenesis in the treatment of cardiovascular disease]

The formation of new blood vessel is essential for a variety of physiological processes like embryogenesis and the female reproduction as well as pathological processes like tumor growth, wound healing and neovascularization of ischemic tissue. Vasculogenesis and angiogenesis are the mechanisms responsible for the development of the blood vessels. While angiogenesis refers to the formation of capillaries from pre-existing vessels in the embryo and adult organism, vasculogenesis, the development of new blood vessels from in situ differentiating endothelial cells, has been previously considered restricted to embryogenesis. Recent investigations, however, show the existence of endothelial progenitor cells (EPCs) in the peripheral blood of the adult and their participation in ongoing neovascularization. Molecular and cell-biological experiments suggest that different cytokines and growth factors have a stimulatory effect on these bone-marrow derived EPCs. Results with GM-CSF (granulocyte macrophage-colony stimulating factor) and VEGF (vascular endothelial growth factor) open a new insight into the clinical use of cytokines and in particular the use of growth factors in gene therapy. The administration via protein or plasmid-DNA for neovascularization seems to enhance both pathways, angiogenesis and vasculogenesis.

A vascular endothelial growth factor antagonist is produced by the human placenta and released into the maternal circulation

Vascular endothelial growth factor (VEGF) is a potent secreted factor that promotes angiogenesis and maintains the integrity of the endothelium. Levels of VEGF are increased in many tumors and are elevated in women with pre-eclampsia, a serious disease of pregnancy. Here we show by in situ hybridization that the trophoblast contains the mRNA encoding a soluble version of the VEGF receptor known as Flt-1 (sFlt-1: initially described by Kendall and Thomas, PNAS 90:10705-10709). Binding assays and Western blotting of villus-conditioned media confirmed the production of sFlt-1. Serum from pregnant women was found to contain a VEGF-binding protein that was not present in serum from men or nonpregnant women. As determined by heparin affinity, column fractionation, and cross-linking, this protein was identical to sFlt-1. Taken together, these results show that the placenta secretes sFlt-1, which would be expected to be a VEGF antagonist. This is the first report of production of the sFlt-1 receptor in vivo, and it reveals a new mechanism for naturally regulating this potent angiogenic agent. The presence of such an antagonist suggests that regulation of VEGF action is essential to successful pregnancy. This has important implications for the activity of VEGF locally and systemically in other conditions.

Functions of the VEGF Receptor-1 (FLT-1) in the Vasculature

Vascular endothelial growth factor (VEGF) is a major inducer of angiogenesis and vasculogenesis. Two distinct receptors for VEGF, the tyrosine kinase receptors VEGFR-1 (Flt-1) and VEGFR-2 (Flk-1/KDR), have been identified. Transfection studies could demonstrate biological activities for the Flk-1/KDR-, but not for the Flt-1-receptor, which led to the hypothesis that Flt-1 is a decoy receptor for VEGF. However, Flt-1 is biologically active in non-endothelial cells, namely monocytes, which exclusively express this receptor. In addition, the Flt-1 ligand placenta growth factor (PlGF), which is unable to bind and activate Flk-1/KDR, elicits activities in both monocytes and endothelial cells. The implications of Flt-1 mediated monocyte transmigration through endothelial monolayers and induction of the procoagulant tissue factor on monocytes for the field of vascular medicine are discussed.

Vascular endothelial growth factor mediates angiogenic activity during the proliferative phase of wound healing

Angiogenesis is an essential component of normal wound repair, yet the primary mediators of wound angiogenesis have not been well described. The current study characterizes the contribution of vascular endothelial cell growth factor (VEGF) to the angiogenic environment of human surgical wounds. Surgical wound fluid samples (n = 70) were collected daily for up to 7 postoperative days (POD) from 14 patients undergoing mastectomy or neck dissection. VEGF levels in surgical wound fluid were lowest on POD 0, approximating values of serum, but increased steadily through POD 7. An opposite pattern was noted for basic fibroblast growth factor-2. Fibroblast growth factor-2, which has been previously described as a wound angiogenic factor, exhibited highest levels at POD 0, declining to near serum levels by POD 3. Surgical wound fluid form all time points stimulated marked endothelial cell chemotaxis and induced a brisk neovascular response in the rat corneal micropocket angiogenesis assay. Antibody neutralization of VEGF did not affect the in vitro chemotactic or the in vivo angiogenic activity early wound samples (POD 0). In contrast, VEGF neutralization significantly attenuated both chemotactic activity (mean decrease 76 +/- 13%, P < 0.01) and angiogenic activity (5 of 5 samples affected) of later wound samples (POD 3 and 6). The results suggest a model of wound angiogenesis in which an initial angiogenic stimulus is supplied by fibroblast growth factor-2, followed by a subsequent and more prolonged angiogenic stimulus mediated by VEGF.

Estrogen restores endothelial cell function in an experimental model of vascular injury

BACKGROUND

It has been suggested that reendothelialization of damaged blood vessels protects against the vascular injury response. The goal of the present study was to determine whether estrogen restores endothelial cell function in balloon-injured rat carotid arteries.

METHODS AND RESULTS

Ten-week-old male and female Sprague-Dawley rats with intact gonads underwent balloon injury to the right common carotid artery. Female rats were randomized to receive either daily subcutaneous injections of 17beta-estradiol (17betaE[2]; 20 microg x kg[-1] x d[-1]) or vehicle over the course of the study. Vessel morphology was assessed 2 weeks after injury. Significant neointima formation was observed in vehicle-treated males. This response was blunted in vehicle-treated and 17beta-E(2)-supplemented females. Intima-to-media ratios were 1.28+/-0.23 (males), 0.72+/-0.07 (vehicle-treated females), and 0.49+/-0.07 (17beta-E[2]supplemented females). To test whether reductions in neointimal lesion formation were related to the functional reendothelialization of the damaged vessel, endothelium-dependent relaxation was tested in isolated ring segments from the three experimental groups. Vessels were precontracted with phenylephrine followed by cumulative administration of acetylcholine, an endothelium-dependent vasodilator. Maximum relaxation to acetylcholine was 8.13+/-1.70% (males), 22.06+/-4.36% (vehicle-treated females), and 46.47+/-3.48% (17beta-E[2]-supplemented females). The enhanced endothelium-dependent relaxation of rings from 17betaE(2)-supplemented females correlated with reduced neointimal proliferation in this group. The concentration of nitric oxide metabolites in plasma correlated positively with plasma 17beta-E(2) concentration.

CONCLUSIONS

These results suggest that estrogen protects against neointimal injury in the balloon-injured rat, at least in part, by facilitating the reendothelialization of the damaged vessel.

Vascular permeability factor/vascular endothelial growth factor: a multifunctional angiogenic cytokine

VPF/VEGF is a multifunctional cytokine that contributes to angiogenesis by both direct and indirect mechanisms. On the one hand, VPF/VEGF stimulates the endothelial cells lining nearby microvessels to proliferate, to migrate and to alter their pattern of gene expression. On the other hand, VPF/VEGF renders these same microvascular endothelial cells hyperpermeable so that they spill plasma proteins into the extravascular space, leading to profound alterations in the extracellular matrix that favor angiogenesis. These same principles apply in tumors, in several examples of non-neoplastic pathology, and in physiological processes that involve angiogenesis and new stroma generation. In all of these examples, microvascular hyperpermeability and the introduction of a provisional, plasma-derived matrix precede and accompany the onset of endothelial cell division and new blood vessel formation. It would seem, therefore, that tumors have made use of fundamental pathways that developed in multicellular organisms for purposes of tissue defense, renewal and repair. VPF/VEGF, therefore, has taught us something new about angiogenesis; namely, that vascular hyperpermeability and consequent plasma protein extravasation are important--perhaps essential--elements in its generation. However, this finding raises a paradox. While VPF/VEGF induces vascular hyperpermeability, other potent angiogenic factors apparently do not, at least in sub-toxic concentrations that are more than sufficient to induce angiogenesis (Connolly et al., 1989a). Nonetheless, wherever angiogenesis has been studied, the newly generated vessels have been found to be hyperpermeable. How, therefore, do angiogenic factors other than VPF/VEGF lead to the formation of new and leaky blood vessels? We do not as yet have a complete answer to this question. One possibility is that at least some angiogenic factors mediate their effect by inducing or stimulating VPF/VEGF expression. In fact, there are already clear example of this. A number of putative angiogenic factors including small molecules (e.g. prostaglandins, adenosine) as well as many cytokines (e.g. TGF-alpha, bFGF, TGF-beta, TNF-alpha, KGF, PDGF) have all been shown to upregulate VPF/VEGF expression. Further studies that elucidate the crosstalk among various angiogenic factors are likely to contribute significantly to a better understanding of the mechanisms by which new blood vessels are formed in health and in disease.

VEGF and flt. Expression time kinetics in rat brain infarct

BACKGROUND AND PURPOSE

Vascular endothelial growth/vascular permeability factor (VEGF) is a candidate for an angiogenic and hyperpermeability inducing factor in an infarct because it is a secretable mitogen specific for endothelial cells and is upregulated by hypoxia. Our study attempts to clarify the chronological expression of VEGF and its receptor (flt) system in experimental cerebral infarction.

METHODS

With the use of a reproducible middle cerebral artery occlusion model in rats, VEGF expression was identified by Western blotting with anti-VEGF antibody. The chronological expression of the VEGF/flt system was analyzed semiquantitatively by immunohistochemical means in infarcts with different time courses from 3 hours to 3 weeks.

RESULTS

VEGF and flt were expressed exclusively in the ischemic brain. The bands obtained on the immunoblot at 38 and 45 kD are related to those of VEGF121 and VEGF165 isoforms. Macrophages, neurons, and glial cells chronologically expressed VEGF immunoreactivity in a different fashion. Both VEGF (bound) and flt were detected in endothelial cells along with the development of angiogenesis.

CONCLUSIONS

In the ischemic brain the macrophages, neurons, and glial cells appear to contain VEGF. The VEGF receptor flt was induced in endothelial cells along with the progression of angiogenesis in infarct. The VEGF/flt system is thus considered to be involved in the healing process of brain infarct.

The vascular endothelial growth factor receptor Flt-1 mediates biological activities. Implications for a functional role of placenta growth factor in monocyte activation and chemotaxis

Two distinct receptors for vascular endothelial growth factor (VEGF), the tyrosine kinase receptors Flt-1 and Flk-1/KDR, have been described. In this study we show that monocytes, in contrast to endothelium, express only the VEGF receptor Flt-1, and that this receptor specifically binds also the VEGF homolog placenta growth factor (PlGF). Both VEGF and PlGF stimulate tissue factor production and chemotaxis in monocytes at equivalent doses. In contrast, endothelial cells expressing both the Flt-1 and the Flk-1/KDR receptors produce more tissue factor upon stimulation with VEGF than after stimulation with PlGF. Neutralizing antibodies to the KDR receptor reduce the VEGF-stimulated tissue factor induction in endothelial cells to levels obtained by stimulation with PlGF alone, but do not affect PlGF-induced tissue factor induction in endothelial cells nor the VEGF-dependent tissue factor production in monocytes. These findings strongly suggest Flt-1 as a functional receptor for VEGF and PlGF in monocytes and endothelial cells and identify this receptor as a mediator of monocyte recruitment and procoagulant activity.

Vascular endothelial growth factor is an autocrine growth factor for hair dermal papilla cells

The transition of the late anagen to the catagen phase is concomitant with the disappearance of perifollicular capillaries, and therefore cyclical hair growth might depend on the ability of the dermal papilla to synthesize and release soluble growth and differentiation factors toward pre-existing capillaries. We characterized an angiogenic growth factor in the conditioned medium of dermal papilla cells indistinguishable from vascular endothelial growth factor as judged by biochemical and immunologic criteria. In addition, these cells bind vascular endothelial, growth factor on two binding sites and proliferate or migrate in the presence of this growth factor. Moreover, neutralizing antibodies inhibit these biologic effects, confirming that vascular endothelial growth factor might contribute to hair growth either by acting directly on papilla cells or by stimulating the local vascularization.

Vasculotropin/vascular endothelial growth factor is an autocrine growth factor for human retinal pigment epithelial cells cultured in vitro

Vasculotropin (VAS), also called vascular endothelial growth factor (VEGF) or vascular permeability factor, is a secreted growth factor whose target cell specificity has been reported as restricted to vascular endothelium. Its effects are mediated by at least two distinct membrane-spanning tyrosine kinase receptors, KDR and flt-1; the expression of which also seems restricted to vascular endothelium. We describe here that cultured human retinal pigment epithelial (HRPE) cells express both KDR and flt-1 receptors, bind VAS/VEGF on two high affinity sites (apparent Kd of 9 and 210 pM corresponding to 940 and 18,800 sites per cell) and proliferate or migrate upon recombinant VAS/VEGF addition. HRPE cells also express the mRNA corresponding to the 121 and 165 amino acid forms of VAS/VEGF. HRPE cells release in their own culture medium and store in their extracellular matrix self-mitogenic and chemoattractant factors indistinguishable from 121 and 165 VAS/VEGF isoforms. The autocrine role of VAS/VEGF was confirmed by the inhibition of these bioactivities by neutralizing specific anti-VAS/VEGF antibodies.

Activation of mitogen-activated protein kinases by vascular endothelial growth factor and basic fibroblast growth factor in capillary endothelial cells is inhibited by the antiangiogenic factor 16-kDa N-terminal fragment of prolactin

A number of factors both stimulating and inhibiting angiogenesis have been described. In the current work, we demonstrate that the angiogenic factor vascular endothelial growth factor (VEGF) activates mitogen-activated protein kinase (MAPK) as has been previously shown for basic fibroblast growth factor. The antiagiogenic factor 16-kDa N-terminal fragment of human prolactin inhibits activation of MAPK distal to autophosphorylation of the putative VEGF receptor, Flk-1, and phospholipase C-gamma. These data show that activation and inhibition of MAPK may play a central role in the control of angiogenesis.

Regulation by vascular endothelial growth factor of human colon cancer tumorigenesis in a mouse model of experimental liver metastasis

To investigate the relationship between angiogenesis and hepatic tumorigenesis, we examined the expression of vascular endothelial growth factor (VEGF) in 8 human colon carcinoma cell lines and in 30 human colorectal cancer liver metastases. Abundant message for VEGF was found in all tumors, localized to the malignant cells within each neoplasm. Two receptors for VEGF, KDR and flt1, were also demonstrated in most of the tumors examined. KDR and flt1 mRNA were limited to tumor endothelial cells and were more strongly expressed in the hepatic metastases than in the sinusoidal endothelium of the surrounding liver parenchyma. VEGF monoclonal antibody administration in tumor-bearing athymic mice led to a dose- and time-dependent inhibition of growth of subcutaneous xenografts and to a marked reduction in the number and size of experimental liver metastases. In hepatic metastases of VEGF antibody-treated mice, neither blood vessels nor expression of the mouse KDR homologue flk-1 could be demonstrated. These data indicate that VEGF is a commonly expressed angiogenic factor in human colorectal cancer metastases, that VEGF receptors are up-regulated as a concomitant of hepatic tumorigenesis, and that modulation of VEGF gene expression or activity may represent a potentially effective antineoplastic therapy in colorectal cancer.

The role of vascular permeability factor and basic fibroblast growth factor in tumor angiogenesis

In the last decade a considerable amount of research has been dedicated to studying the process of angiogenesis. In the field of tumor biology angiogenesis is a relevant subject of investigation as well, since newly formed blood vessels are required for the growth of tumors and provide an exit route for metastasizing tumor cells. In this review we discuss some aspects of tumor angiogenesis with emphasis on the role that growth factors bFGF and VPF play in this process. A number of biochemical characteristics and biological properties of the two factors and their receptors are reviewed, and the expression of bFGF and VPF in both normal tissues and in tumors is discussed. Finally, we speculate on the use of bFGF and VPF expression as a diagnostic parameter and on possible clinical applications.

Regulation of the expression of the VEGF/VPS and its receptors: role in tumor angiogenesis

Vascular endothelial growth factor (VEGF)/vascular permeability factor (VPS) plays a crucial role for the vascularization of tumors including breast cancers. Tumors produce ample amounts of VEGF, which stimulates the proliferation and migration of endothelial cells (ECs), thereby inducing tumor vascularization by a paracrine mechanism. VEGF receptors (VEGF-Rs) are highly expressed by the ECs in tumor blood vessels. VEGF expression can be induced in various cell types by a number of stimuli including hypoxia, differentiation, growth factors and tumor promoters of the phorbol ester class, such as TPA. The VEGF inductive pathways comprise kinases, oncogenes, tumor suppressor genes, and steroid hormone transcription factors, many of which seem to converge on the activator protein (AP-1) transcription factor. Much less is known about the regulation of VEGF-R expression, which is restricted to ECs. This expression is greatly enhanced in diseased tissue such as solid tumors. So far, it appears that growth factors, cytokines, and tumor promoters are involved in the control of VEGF-R expression. Here we review current knowledge about the regulation of the expression of VEGF and its receptors.

Suppression of VEGF-induced angiogenesis by the protein tyrosine kinase inhibitor, lavendustin A

1. Vascular endothelial growth factor (VEGF) is a heparin-binding angiogenic factor which specifically acts on endothelial cells via distinct membrane-spanning tyrosine kinase receptors. Here we used the rat sponge implant model to test the hypothesis that the angiogenic activity of VEGF can be suppressed by protein tyrosine kinase (PTK) inhibitors. 2. Neovascular responses in subcutaneous sponge implants were determined by measurements of relative sponge blood flow by use of a 133Xe clearance technique, and confirmed by histological studies and morphometric analysis. 3. Daily local administration of 250 ng VEGF165 accelerated the rate of 133Xe clearance from the sponges and induced an intense neovascularisation. This VEGF165-induced angiogenesis was inhibited by daily co-administration of the selective PTK inhibitor, lavendustin A (10 micrograms), but not its negative control, lavendustin B (10 micrograms). Blood flow measurements and morphometric analysis of 8-day-old sponges showed that lavendustin A reduced the 133Xe clearance of VEGF165-treated sponges from 32.9 +/- 1.5% to 20.9 +/- 1.6% and the total fibrovascular growth area from 62.4 +/- 6.1% to 21.6 +/- 6.8% (n = 12, P < 0.05). 4. Co-injection of suramin (3 mg), an inhibitor of heparin-binding growth factors, also suppressed the VEGF165-elicited neovascular response. In contrast, neither lavendustin A nor suramin produced any effect on the basal sponge-induced angiogenesis. 5. When given alone, low doses of VEGF165 (25 ng) or basic fibroblast growth factor (bFGF; 10 ng) did not modify the basal sponge-induced neovascularisation. However, co-administration of these two peptides to a single sponge together caused a significant increase in the rate of 133Xe clearance and angiogenesis similar to that seen with the high dose of VEGF165 (250 ng) acting alone. This VEGF/bFGF neovascular response was also blocked by daily co-administration of lavendustin A (10 jig),suramin (3 mg) or a monoclonal anti-bFGF antibody (DG2, I jig), but not lavendustin B (10 g).6 These results suggest that selective inhibition of PTK could have therapeutic potential in angiogenic diseases where VEGF plays a dominant role. Furthermore, blockade of the angiogenic activity of VEGF and VEGF,/bFGF by suramin reveals an alternative strategy in angio suppression.

Detection and quantification of vascular endothelial growth factor/vascular permeability factor in brain tumor tissue and cyst fluid: the key to angiogenesis?

In primary malignant brain tumors increased vascularity and marked edema strongly suggest a possible role of the vascular endothelial growth factor/vascular permeability factor (VEGF/VPF). This was confirmed by earlier in situ hybridization studies, by analysis of the expression of the mitogen in different subsets of glioblastoma cells, and by the fact that the VEGF/VPF receptor flt-1 (fms-like tyrosine kinase) is up-regulated in tumor cells in vivo. To assess and quantify the expression of the VEGF/VPF gene and of the receptor gene, 26 surgical specimens of brain tumor tissue from 24 patients were analyzed. In most malignant gliomas, the expression level of the VEGF/VPF gene is elevated and can be increased up to 20- to 50-fold in comparison with low-grade tumors. Using polymerase chain reaction-based amplification, it could be shown that the messenger RNAs of three different VEGF/VPF forms are synthesized in tumor tissue samples. Northern blot studies revealed that in some samples a significant expression of the gene coding for placenta growth factor, a growth factor closely related to VEGF/VPF, was observed. In addition, using a radioreceptor assay it was possible to detect high VEGF/VPF-like activity in the cyst fluids of brain tumors, indicating the accumulation of the mitogen and permeability factor in brain tumor cysts. Further investigations revealed that astrocytoma and glioblastoma cells in culture express the VEGF/VPF gene and secrete the VEGF/VPF protein, whereas gene expression of the two known VEGF/VPF receptors, kinase insert domain-containing receptor and flt-1, could not be detected. These data support previous reports, which stated that VEGF/VPF acts as a paracrine growth and permeability factor in brain tumors and may contribute to tumor growth by initiating tumor angiogenesis.

A structure-activity analysis of antagonism of the growth factor and angiogenic activity of basic fibroblast growth factor by suramin and related polyanions

The ability of a series of polysulphonated naphthylureas structurally related to suramin to inhibit basic fibroblast growth factor (bFGF) or serum-stimulated growth of endothelial cells [either large vessel, human umbilical vein endothelial cells (HUVEC) or microvascular, bovine adrenal capillary endothelial (BACE) cells] and angiogenesis in vivo has been examined. The polyanions encompassed two main structural variations, namely the number of aromatic amide groups intervening between two terminal naphthyl rings and/or variation in the substitution pattern of the naphthyl rings. The polyanions were either inactive (group I) or inhibited (group II) bFGF-stimulated uptake of [3H]methylthymidine by BACE cells. Group I compounds shared a common structural feature in that they were simple binaphthyl-substituted ureas. In contrast, group II compounds all had an extended multiple ring structure with at least two aromatic groups intervening between the two terminal naphthyl rings. Compounds with either two or four intervening groups were equipotent in blocking bFGF in vitro. However, compounds with two bridging aromatic groups were 5- to 10-fold less toxic than suramin in mice, suggesting a potential for an improved therapeutic ratio. The ability of the polyanions to block bFGF-driven endothelial cell proliferation in vitro correlated with antiangiogenic activity in vivo as shown by use of the rat sponge angiogenesis model. These observations could substantially widen the anti-tumour therapeutic opportunities for this class of compound.

Transfected MCF-7 cells as a model for breast-cancer progression

The MCF-7 human breast carcinoma cell line has been used as a recipient for eukaryotic plasmid expression vectors to determine the effects of growth factor and growth factor receptor overexpression on the estrogen-dependent, antiestrogen sensitive and poorly metastatic phenotypes exhibited by this line. Overexpression of some members of the erbB family of ligands and receptors were found to have some effects on these phenotypes. However, only when two members of the fibroblast growth factor family, FGF-1 and FGF-4, were overexpressed was progressive in vivo growth observed is either ovariectomized nude mice without estrogen supplementation or in mice that received tamoxifen treatment. FGF transfected cells also exhibited an increased ability to form micrometastases. The implications of these results with regard to the possible role of the paracrine and autocrine effects of angiogenic growth factor production in breast cancer progression are discussed.

Vascular endothelial growth factor and its receptors

Vascular endothelial growth factor (VEGF) is a highly specific mitogen for vascular endothelial cells and an angiogenic factor that is structurally related to platelet derived growth factor (PDGF). It is also known as the vascular permeability factor (VPF) because it efficiently potentiates the permeabilization of blood vessels. Five types of VEGF mRNA encoding VEGF species which differ in their molecular mass and in their biological properties are transcribed from a single gene as a result of alternative splicing. VEGFs are produced and secreted by several normal cell types including smooth muscle, luteal and adrenal cortex cells. VEGFs are also produced by different tumorigenic cells, and appear to play a major role in tumour angiogenesis. Antibodies directed against VEGF can inhibit the growth of a variety of VEGF producing tumours. Of the various VEGF species, the best characterized is the 165 amino acid long form (VEGF165). VEGF165 is a heparin binding growth factor, and its interaction with VEGF receptors on the cell surface of vascular endothelial cells depends on the presence of heparin-like molecules. Several cell types which do not proliferate in response to VEGF such as bovine corneal endothelial cells, HeLa cells and human melanoma cells also express cell surface VEGF receptors, but the function of the VEGF receptors in these cells is unclear. Recently, the tyrosine-kinase receptors encoded by the flt and KDR/flk-1 genes were found to function as VEGF165 receptors.

Differential expression of the two VEGF receptors flt and KDR in placenta and vascular endothelial cells

Vascular endothelial growth factor (VEGF) is a newly identified growth and permeability factor with a unique specificity for endothelial cells. Recently the flt-encoded tyrosine kinase was characterized as a receptor for VEGF. A novel tyrosine kinase receptor encoded by the KDR gene was also found to bind VEGF with high affinity when expressed in CMT-3 cells. Screening for flt and KDR expression in a variety of species and tissue-derived endothelial cells demonstrates that flt is predominantly expressed in human placenta and human vascular endothelial cells. Placenta growth factor (PIGF), a growth factor significantly related to VEGF, is coexpressed with flt in placenta and human vascular endothelial cells. KDR is more widely distributed and expressed in all vessel-derived endothelial cells. These data demonstrate that cultured human endothelial cells isolated from different tissues express both VEGF receptors in relative high levels and, additionally, that all investigated nonhuman endothelial cells in culture are also positive for KDR gene expression.

Fetal liver kinase 1 is a receptor for vascular endothelial growth factor and is selectively expressed in vascular endothelium

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor, induces endothelial proliferation in vitro and vascular permeability in vivo. The human transmembrane c-fms-like tyrosine kinase Flt-1 has recently been identified as a VEGF receptor. Flt-1 kinase has seven immunoglobulin-like extracellular domains and a kinase insert sequence, features shared by two other human gene-encoded proteins, kinase insert domain-containing receptor (KDR) and FLT-4. In this study we show that the mouse homologue of KDR, Flk-1, is a second functional VEGF receptor. Flk-1 binds VEGF with high affinity, undergoes autophosphorylation, and mediates VEGF-dependent Ca2+ efflux in Xenopus oocytes injected with Flk-1 mRNA. We also demonstrate by in situ hybridization that Flk-1 protein expression in the mouse embryo is restricted to the vascular endothelium and the umbilical cord stroma. VEGF and its receptors Flk-1/KDR and Flt-1 may play a role in vascular development and regulation of vascular permeability.

Characterization of the increase in vascular permeability induced by vascular permeability factor in vivo

1. Vascular permeability factor (VPF) is a protein secreted from a variety of human and rodent tumour and normal tissue cells. In addition to mediating angiogenesis and endothelial cell growth, VPF has been reported to be a potent mediator of increased microvascular permeability in vivo. In this study we have investigated these permeability changes in vivo using a quantitative model of local plasma leakage in rabbit skin. 2. Our results reveal that VPF is a potent mediator of plasma leakage which, in the rabbit, depends on a synergistic interaction with arteriolar vasodilators such as prostaglandin E2. The requirement for an exogenous vasodilator further suggest that VPF does not act to increase blood flow in this model. 3. We show that this response does not require the presence of circulating neutrophils and in this respect is similar to direct-action permeability increasing mediators such as histamine and bradykinin. Similarly, the time course of plasma leakage induced by VPF resembles that of direct-action mediators, where the greatest response occurs over the first 30 min. In contrast, the neutrophil-dependent plasma leakage induced by the active component of zymosan-activated plasma, C5ades arg, was maintained at a similar level over 2.5 h. 4. Further, using mediator antagonists and enzyme inhibitors we demonstrate that the mechanism of action of VPF is not via activation of histamine, kinin, or platelet-activating factor pathways.