High affinity VEGF binding and developmental expression suggest Flk-1 as a major regulator of vasculogenesis and angiogenesis

The in vivo activity of vascular endothelial growth factor isoforms in the avian embryo

The activity of the different isoforms of quail vascular endothelial growth factor (VEGF; 122, 146, 166 and 190 amino acids) were quantitatively examined in vivo by overexpression in the chicken embryo using the retroviral expression vector RCAS. All isoforms were potent inducers of vascularization and permeability. A linear relationship between expression of their mRNA and induction of vascularization and permeability was observed for all isoforms. Pattern formation and morphogenesis was otherwise not altered. Overexpression of qVEGF122 and 146 in the eye of chicken embryos did not induce vascularization of either the cornea or retina-which are avascular tissues in birds. We conclude that all isoforms of VEGF are potent inducers of angiogenesis dependent on a permissive environment.

Automated evaluation of angiogenic effects mediated by VEGF and PlGF homo- and heterodimers

The effects of growth factors on the blood vessel pattern of chick chorioallantoic membrane (CAM) were assessed with a fast and automated method (extended counting method, XCM; Sandau, 1996) that measures complexity, without assumptions about a fractal structure. XCM is a reliable measure of complexity not only in theory but also in practice: (1) it is robust with respect to thresholding; (2) it shows reduced variance due to pattern translation and rotation; (3) its properties come close to requirements of fractal geometry. It hence is superior to established fractal methods for distinguishing effects induced by various isoforms of vascular endothelial growth factor (VEGF121 and VEGF165), placenta growth factor (PlGF) isoforms, and control treatment. We here show that VEGF homo- and heterodimers and VEGF121/PlGF1 heterodimers increase vascular complexity, whereas PlGF1 and PlGF2 are not effective. PlGF1 and VEGF121 did not mutually influence each other when applied in adjacent fields on the same CAM. Since blood vessels in the CAM originate via nonfractal growth processes, their growth should be analyzed accordingly.

Inhibition of vascular endothelial growth factor (VEGF)-induced endothelial cell proliferation by a peptide corresponding to the exon 7-encoded domain of VEGF165

Vascular endothelial growth factor (VEGF) is a potent mitogen for endothelial cells (EC) in vitro and a major regulator of angiogenesis in vivo. VEGF121 and VEGF165 are the most abundant of the five known VEGF isoforms. The structural difference between these two is the presence in VEGF165 of 44 amino acids encoded by exon 7 lacking in VEGF121. It was previously shown that VEGF165 and VEGF121 both bind to KDR/Flk-1 and Flt-1 but that VEGF165 binds in addition to a novel receptor (Soker, S., Fidder, H., Neufeld, G., and Klagsbrun, M. (1996) J. Biol. Chem. 271, 5761-5767). The binding of VEGF165 to this VEGF165-specific receptor (VEGF165R) is mediated by the exon 7-encoded domain. To investigate the biological role of this domain further, a glutathione S-transferase fusion protein corresponding to the VEGF165 exon 7-encoded domain was prepared. The fusion protein inhibited binding of 125I-VEGF165 to VEGF165R on human umbilical vein-derived EC (HUVEC) and MDA-MB-231 tumor cells. The fusion protein also inhibited significantly 125I-VEGF165 binding to KDR/Flk-1 on HUVEC but not on porcine EC which express KDR/Flk-1 alone. VEGF165 had a 2-fold higher mitogenic activity for HUVEC than did VEGF121. The exon 7 fusion protein inhibited VEGF165-induced HUVEC proliferation by 60% to about the level stimulated by VEGF121. Unexpectedly, the fusion protein also inhibited HUVEC proliferation in response to VEGF121. Deletion analysis revealed that a core inhibitory domain exists within the C-terminal 23-amino acid portion of the exon 7-encoded domain and that a cysteine residue at position 22 in exon 7 is critical for inhibition. It was concluded that the exon 7-encoded domain of VEGF165 enhances its mitogenic activity for HUVEC by interacting with VEGF165R and modulating KDR/Flk-1-mediated mitogenicity indirectly and that exon 7-derived peptides may be useful VEGF antagonists in angiogenesis-associated diseases.

Targeting the tumor vasculature: inhibition of tumor growth by a vascular endothelial growth factor-toxin conjugate

Tumor-derived vascular endothelial growth factor (VEGF)/ vascular permeability factor (VPF) plays an important role in neovascularization and the development of tumor stroma. Furthermore, VEGF receptors are over-expressed in the endothelial cells of tumor vasculature and almost non-detectable in the vascular endothelium of adjoining normal tissues. The differential expression of receptor offers a selective advantage for targeting cytotoxic toxin polypeptides. We have prepared a vascular targeting reagent by chemically linking recombinant VEGF to a truncated form of diphtheria toxin. The VEGF-toxin conjugate was selectively toxic to endothelial cell lines and inhibited experimental neovascularization of the chick chorioallantoic membrane. In the present study, we examined the effects of VEGF-toxin conjugate on solid tumor growth. Athymic nude mice with established subcutaneous tumors were treated with daily intraperitoneal injections of the VEGF-toxin conjugate or free toxin. When compared with control animals treated with the toxin polypeptide alone, the conjugate-treated animals displayed a significant inhibition of tumor growth. Histological analysis of tumors from conjugate-treated animals revealed hemorrhagic necrosis consistent with a vascular-mediated injury. In contrast, highly vascularized normal tissues from conjugate-treated animals demonstrated no evidence of hemorrhage or tissue injury. The conjugate was well tolerated without apparent toxicities. Our results illustrate the anti-tumor activity of a VEGF-toxin conjugate selectively targeting the tumor neovasculature.

Angiogenesis promoted by vascular endothelial growth factor: regulation through alpha1beta1 and alpha2beta1 integrins

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor, is a cytokine of central importance for the angiogenesis associated with cancers and other pathologies. Because angiogenesis often involves endothelial cell (EC) migration and proliferation within a collagen-rich extracellular matrix, we investigated the possibility that VEGF promotes neovascularization through regulation of collagen receptor expression. VEGF induced a 5- to 7-fold increase in dermal microvascular EC surface protein expression of two collagen receptors-the alpha1beta1 and alpha2beta1 integrins-through induction of mRNAs encoding the alpha1 and alpha2 subunits. In contrast, VEGF did not induce increased expression of the alpha3beta1 integrin, which also has been implicated in collagen binding. Integrin alpha1-blocking and alpha2-blocking antibodies (Ab) each partially inhibited attachment of microvascular EC to collagen I, and alpha1-blocking Ab also inhibited attachment to collagen IV and laminin-1. Induction of alpha1beta1 and alpha2beta1 expression by VEGF promoted cell spreading on collagen I gels which was abolished by a combination of alpha1-blocking and alpha2-blocking Abs. In vivo, a combination of alpha1-blocking and alpha2-blocking Abs markedly inhibited VEGF-driven angiogenesis; average cross-sectional area of individual new blood vessels was reduced 90% and average total new vascular area was reduced 82% without detectable effects on the pre-existing vasculature. These data indicate that induction of alpha1beta1 and alpha2beta1 expression by EC is an important mechanism by which VEGF promotes angiogenesis and that alpha1beta1 and alpha2beta1 antagonists may prove effective in inhibiting VEGF-driven angiogenesis in cancers and other important pathologies.

VEGF mRNA induction correlates with changes in the vascular architecture upon spinal cord damage in the rat

The multiple cellular and molecular processes induced by injury to the central nervous system (CNS) are still poorly understood. In the present study, we investigated the response of the vasculature and the expression of mRNA for the angiogenic vascular endothelial growth factor (VEGF) following X-irradiation of the spinal cord in the newborn and following traumatic spinal cord injury in the adult rat. Both lesion models induced changes in the density and the distribution pattern of blood vessels: while X-irradiation led to a permanent local increase in vascular density in the fibre tracts of the exposed segments, a transient local sprouting of vessels was induced upon traumatic spinal cord injury. In situ hybridization showed that an increase of VEGF mRNA anticipated and overlapped with the vascular responses in both lesion models. In addition to the temporal correlation of VEGF expression and vascular sprouting, there was a clear correlation in the spatial distribution patterns. Following X-irradiation, the expression of VEGF mRNA was restricted to the fibre tracts, precisely the areas where the changes in the vasculature were observed later on. Upon transection in the adult animal, VEGF was mainly detectable at the border of the lesion area, where the transient increase in vascular density could be observed. Interestingly, according to the type of lesion applied, astrocytes (X-irradiation) or inflammatory cells (presumably microglial cells or macrophages; traumatic lesion) are the cellular sources of VEGF mRNA. Our results strongly indicate that VEGF is crucially involved in mediating vascular changes following different types of injury in the CNS.

Vascular endothelial growth factor increases the mitogenic response to fibroblast growth factor-2 in vascular smooth muscle cells in vivo via expression of fms-like tyrosine kinase-1

Vascular endothelial growth factor (VEGF) has traditionally been considered an endothelial cell-specific factor inducing angiogenesis and vascular permeability in vivo. In the present study, expression of VEGF and its receptors, fetal liver kinase-1 (flk-1) and fms-like tyrosine kinase-1 (flt-1), was examined in rat carotid arteries after balloon injury. Although VEGF and flk-1 were not detectable, high levels of flt-1 mRNA and protein were expressed by smooth muscle cells (SMCs) in the neointima, as demonstrated by en face in situ hybridization and Western blotting. Intimal SMC proliferation in chronically denuded rat carotid arteries was unaffected by intraluminal infusion of VEGF, whereas fibroblast growth factor (FGF)-2 increased the number of replicating SMCs 4-fold. Pretreatment with VEGF doubled the mitogenic response to infused FGF-2 by increasing SMC replication in deeper layers of the intima. VEGF increased the permeability of chronically denuded vessels to plasma proteins but had no effect on the uptake of locally infused biotinylated FGF-2. These findings demonstrate that vascular SMCs express functional flt-1 receptors after arterial injury and that VEGF has synergistic effects with FGF-2 on SMC proliferation. These effects are likely to be mediated by a VEGF-mediated increase in permeability as well as a direct interaction between the VEGF and FGF signaling pathways.

Expressions of PDGF receptor alpha, c-Kit and Flk1 genes clustering in mouse chromosome 5 define distinct subsets of nascent mesodermal cells

In gastrulating embryos, various types of cells are generated before differentiation into specific lineages. The mesoderm of the gastrulating mouse embryo represents a group of such intermediate cells. PDGF receptor alpha (PDGFRalpha), c-Kit and fetal liver kinase 1 (Flk1) are expressed in distinctive mesodermal derivatives of post-gastrulation embryos. Their expressions during gastrulation were examined by whole mount immunostaining with monoclonal antibodies against these three receptors. The antibodies stained different mesodermal subsets in gastrulating embryos. Flow cytometry of head fold stage embryos revealed that Flk1+ mesodermal cells could be further classified by the level of c-Kit expression. To examine the possibility that hematopoietic cell differentiation is initiated from the Flk1+ mesoderm, embryonic stem (ES) cells were cultured on the OP9 or PA6 stromal cell layer; the former but not the latter supported in vitro hematopoiesis from ES cells. Flk1+ cells were detected only on the OP9 cell layer from day 3 of differentiation before the appearance of hematopoietic cells. Thus, Flk1+ cells will be required for in vitro ES cell differentiation into hematopoietic cells. The results suggest that these three receptor tyrosine kinases will be useful for defining and sorting subsets of mesodermal cells from embryos or in vitro cultured ES cells.

Vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is a hypoxia-inducible angiogenesis and vascular permeability factor which is expressed in high amounts in perinecrotic palisading cells in human glioblastomas. In vitro VEGF gene expression is enhanced approximately ten times by hypoxia. Current evidence suggests, that hypoxia is also the driving force for VEGF gene expression in glioblastoma cells in vivo and represents the most important trigger for tumor angiogenesis and edema. Our approaches to inhibit tumor angiogenesis and edema formation in glioblastoma patients will concentrate on the disruption of VEGF/VEGF receptor signal transduction pathway in vivo.

Crystal structure at 1.7 A resolution of VEGF in complex with domain 2 of the Flt-1 receptor

Vascular endothelial growth factor (VEGF) is a homodimeric hormone that induces proliferation of endothelial cells through binding to the kinase domain receptor and the Fms-like tyrosine kinase receptor (Flt-1), the extracellular portions of which consist of seven immunoglobulin domains. We show that the second and third domains of Flt-1 are necessary and sufficient for binding VEGF with near-native affinity, and that domain 2 alone binds only 60-fold less tightly than wild-type. The crystal structure of the complex between VEGF and the second domain of Flt-1 shows domain 2 in a predominantly hydrophobic interaction with the "poles" of the VEGF dimer. Based on this structure and on mutational data, we present a model of VEGF bound to the first four domains of Flt-1.

Mixed cultures of avian blastoderm cells and the quail mesoderm cell line QCE-6 provide evidence for the pluripotentiality of early mesoderm

During the early stages of embryogenesis, the mesoderm gives rise to cells of the cardiovascular system which include cardiac myocytes and vascular endothelial and red blood cells. We have investigated the development of these cell phenotypes using aggregate cultures of avian blastoderm cells, which replicated mesodermal cell diversification. The cell phenotypes expressed by the blastoderm cells were dependent upon the age of the blastoderm cells, with Hamburger-Hamilton stage 3 or 4 cells giving rise to endothelial and red blood cells and stage 5 cells producing endothelial and myocardial cells. To begin to understand the stage dependency of the cellular diversification of these aggregate cultures, we treated the cultures with various signaling factors that have been shown to be present in the early avian embryo. These experiments showed that stem cell factor and TGF alpha altered cell phenotypes by stimulating red blood cell and myocardial differentiation, respectively. The ability of these growth factors to shift the differentiation profile of aggregate cultures demonstrated the plasticity of early embryonic cells. To explore the diversification of individual mesodermal cells, labeled QCE-6 cells were incorporated within these blastoderm aggregate cultures. Previous studies have shown that this quail mesodermal cell line possesses characteristics of early nondifferentiated mesodermal cells and can be induced to express either myocardial or endothelial cell phenotypes (C. A. Eisenberg and D. M. Bader, 1996, Circ. Res. 78, 205-216). In the present study, we show that when these cells were cultured as a component of blastoderm cell aggregates, they differentiated into fully contractile cardiomyocytes or endothelial or red blood cells. Moreover, QCE-6 cell differentiation was in accordance with that displayed by the blastoderm cells. Specifically, QCE-6 cells differentiated into red blood cells when cultured within stage 3 or stage 4, but not stage 5, blastoderm cell aggregates. Accordingly, the differentiation of QCE-6 cells into beating cardiomyocytes only occurred when these cells were incorporated into stage 5 blastoderm cell aggregates. The identical sorting and differentiation patterns that were exhibited by QCE-6 and blastoderm cells suggest that expression of differentiated cell types within the early mesoderm is directed by the surrounding environment without immediate cellular commitment. In addition, these results provide further evidence that QCE-6 cells are representative of a multipotential mesodermal stem cell and that they possess the potential to exhibit fully differentiated cell phenotypes.

Adrenergic activation of vascular endothelial growth factor mRNA expression in rat brown adipose tissue: implication in cold-induced angiogenesis

Cold exposure produces adaptive hyperplasia and growth of brown adipose tissue (BAT), the major site of non-shivering thermogenesis in rodents, associated with increased angiogenesis in this tissue. Vascular endothelial growth factor (VEGF), one of the most potent angiogenic factors, was found to be expressed abundantly in BAT of the rat. When rats were exposed to cold at 4 degrees C, the VEGF mRNA level in BAT was increased by 2-3-fold in 1-4 h, but returned to the basal level within 24 h. VEGF expression in other tissues such as heart, kidney and lung did not change after cold exposure. The cold-induced increase in VEGF mRNA was abolished by surgical sympathetic denervation, but mimicked by administration of noradrenaline or a beta3-adrenoceptor agonist CL316,243, indicating the critical role of the beta-adrenergic pathway in VEGF expression in BAT. Among three isoforms of VEGF, the mRNA of a short form (VEGF120) lacking heparin-binding activity was preferentially increased after cold exposure and treatment with the adrenergic agonists. These results suggest that cold exposure activates the sympathetic nerves and leads to a rapid increase in synthesis of VEGF in BAT, which in turn stimulates the proliferation of surrounding vascular endothelial cells.

Signal Transduction in Human Hematopoietic Cells by Vascular Endothelial Growth Factor Related Protein, a Novel Ligand for the FLT4 Receptor

We have recently identified a novel ligand of the vascular endothelial growth factor (VEGF) family termed VEGF-related protein (VRP), which specifically binds to the FLT4 receptor. To characterize the signaling events after VRP engagement of its cognate receptor in hematopoietic cells, a population of human erythroleukemia (HEL) cells, termed HEL-JW, expressing high levels of FLT4 receptor was isolated. Stimulation of HEL-JW cells with VRP alone and in combination with the c-kit ligand/stem cell factor increased cell growth. VRP induced tyrosine phosphorylation of various proteins, including the FLT4 receptor. Further characterization of these tyrosine phosphorylated molecules revealed that Shc, Grb2, and SOS form a complex with the activated FLT4 receptor. HEL-JW cells also expressed RAFTK, a recently identified member of the focal adhesion kinase family. RAFTK was phosphorylated and activated upon VRP treatment, and there was an enhanced association of this kinase with the adaptor protein Grb2. Furthermore, the c-Jun NH2-terminal kinase (JNK), involved in growth activation and shown to mediate RAFTK signaling in other cell types, was activated by VRP stimulation. We also observed that VRP treatment of HEL-JW cells resulted in the phosphorylation of the cytoskeletal protein paxillin. This treatment resulted in an increased association of paxillin with RAFTK, which was mediated by the C-terminal region of RAFTK. These studies indicate that VRP stimulation induced the formation of a signaling complex at its activated receptor as well as activation of RAFTK. VRP-mediated activation of RAFTK may facilitate signal transduction to the cytoskeleton and downstream to the JNK pathway in FLT4-expressing blood cells.

Differential hormonal regulation of vascular endothelial growth factors VEGF, VEGF-B, and VEGF-C messenger ribonucleic acid levels in cultured human granulosa-luteal cells

The development of ovarian follicles and subsequent corpus luteum formation is accompanied by very active angiogenesis. Ovarian granulosa cells produce vascular endothelial growth factor (VEGF), which is a potent endothelial cell mitogen and an angiogenic agent. The complementary DNAs of two other factors structurally related to VEGF, namely VEGF-B and VEGF-C, were recently cloned, but little is known of their regulation in the ovary. We first studied the expression of the messenger RNAs (mRNAs) of the three VEGF isotypes in freshly isolated human granulosa-luteal (GL) cells obtained at oocyte retrieval for in vitro fertilization. The hormonal regulation of these mRNAs was subsequently studied in primary cultures of human GL cells. Analysis of cultured GL cell RNA by reverse transcription-PCR revealed that these cells express the alternatively spliced transcripts representing 121-, 145-, and 165-amino acid VEGF isoforms. Northern blot hybridization analyses indicated that transcripts of 4.5 and 3.7 kilobases for VEGF, and 1.4 and 2.4 kilobases for VEGF-B and VEGF-C, respectively, are expressed in human GL cells. The basal VEGF mRNA levels declined steadily, whereas VEGF-B mRNA levels were rather invariant over a 10-day culture period of GL cells. In contrast, VEGF-C mRNA levels increased toward the end of culture. For studying the hormonal regulation of VEGF isotype mRNAs, GL cells were treated with hCG, recombinant human FSH, PGE2, as well as 8-bromo-cAMP and 12-O-tetradecanoylphorbol 13-acetate, which activate protein kinase A- and protein kinase C-dependent signaling pathways, respectively. All test agents stimulated the expression of VEGF mRNA levels in a concentration-dependent manner. Time-course studies indicated that all treatments induced VEGF mRNA levels as early as incubation for 2 h, and the effect was sustained up to 48 h. VEGF-B mRNA levels were not regulated by any of the test agents. However, we found that hCG and 8-bromo-cAMP decreased VEGF-C mRNA levels with a maximal response observed at 24 and 48 h after cellular treatment. We conclude that the mRNAs of VEGF, VEGF-B, and VEGF-C are expressed in human GL cells and that their mRNA steady state levels are regulated in cultured human GL cells in an isotype-specific manner. The differential regulation of VEGF, VEGF-B, and VEGF-C in human GL cells suggests that distinct VEGF isotypes may play different roles during the vascularization of the human ovarian follicle and corpus luteum.

Vascular endothelial growth factor, placenta growth factor and their receptors in isolated human trophoblast

The expression of the angiogenic growth factors, vascular endothelial cell growth factor (VEGF) and placenta growth factor (PIGF) was demonstrated in isolated human term cytotrophoblast and in vitro differentiated syncytiotrophoblast. RNase protection assays demonstrated VEGF expression in both cytotrophoblast and syncytiotrophoblast while prominent PIGF expression was detected in both types of trophoblast by Northern blot analyses. VEGF expression increased approximately eightfold in trophoblast cultured under hypoxic conditions (1 per cent O2) yet PIGF expression decreased 73 +/- 5.5 per cent in the same trophoblast. These results suggest distinct regulatory mechanisms govern expression of VEGF and PIGF in trophoblast. Characterization of the VEGF/PIGF receptors, KDR and flt-1, revealed the presence of flt-1 mRNA in isolated cytotrophoblast and in vitro differentiated syncytiotrophoblast. KDR was not detected in the isolated trophoblast. Exogenous rhVEGF induced c-Jun N-terminal kinase (JNK) activity in the normal trophoblast indicating that the flt-1 receptors on trophoblast are functional. Trophoblast-derived VEGF/PIGF could act in a paracrine fashion to promote uterine angiogenesis and vascular permeability within the placental bed. In addition, presence of function flt-1 on normal trophoblast suggests that VEGF/PIGF functions in an autocrine manner to perform an as yet undefined role in trophoblast invasion, differentiation, and/or metabolic activity during placentation.

Halting angiogenesis suppresses carcinoma cell invasion

The importance of angiogenesis in malignant tumor growth has been interpreted mainly in terms of oxygen and nutrient supply. Here we demonstrate its fundamental role for tumor invasion of malignant human keratinocytes in surface transplants on nude mice. Distinct patterns of angiogenesis and vascular endothelial growth factor receptor-2 (VEGFR-2) expression allowed us to distinguish between benign and malignant cells. Functional inactivation of VEGF-R2 by a blocking antibody disrupted ongoing angiogenesis and prevented invasion of malignant cells, without reducing tumor cell proliferation. The reversion of a malignant into a benign phenotype by halting angiogenesis demonstrates a significant function of vascular endothelium for tumor invasion.

Arterial wall neovascularization--potential role in atherosclerosis and restenosis

Neointimal formation and arterial wall remodeling are pivotal causes of luminal narrowing in atherogenesis and restenosis. Arterial remodeling refers to a series of dynamic structural changes that arteries may undergo in response to various stimuli, including changes in blood flow and pressure, and acute injury. The biological mechanisms involved in arterial remodeling are poorly understood and are currently a main target for research. We have recently focused on the role of the arterial wall microcirculation (ie, vasa vasorum) in arterial remodeling after injury. In the past, a correlation between arterial wall neovascularization and the accumulation of arterial plaque has been documented; however, the dynamic role of these microvessels in arterial repair and luminal narrowing has not been examined. The type of arterial injury, the nature of the lesion that develops, and the arterial compartment in which angiogenesis occurs may determine the role of the vasa vasorum in arterial narrowing. In this review, we highlight the data that link arterial wall neovascularization with lesion formation and the process of arterial remodeling.

Vascular endothelial growth factor/vascular permeability factor produces nitric oxide-dependent hypotension. Evidence for a maintenance role in quiescent adult endothelium

In vitro studies suggest that vascular endothelial growth factor/vascular permeability factor (VEGF/VPF) may stimulate release of nitric oxide (NO) from endothelial cells. To investigate the hemodynamic consequences of recombinant VEGF/VPF administered in vivo, recombinant human VEGF/VPF was administered as a bolus dose of 500 micrograms to anesthetized (n = 6) or conscious (n = 5) New Zealand White rabbits, as well as anesthetized rabbits with diet-induced hypercholesterolemia (HC; n = 7). Anesthetized Yorkshire farm pigs (no specific dietary pretreatment) were studied before and after receiving 500 micrograms intravenous (IV; n = 5) or intracoronary (IC; n = 5) VEGF/VPF. In anesthetized, normal rabbits, mean arterial pressure (MAP) fell by 20.5 +/- 1.4% (P < .05 versus baseline) within 3 minutes after IV VEGF/VPF. Pretreatment with N omega-nitro-L-arginine caused a significant inhibition of VEGF/VPF-induced hypotension. In conscious, normal rabbits, VEGF/VPF produced a consistent though lesser reduction in MAP. The fall in MAP induced by VEGF/VPF in anesthetized, HC rabbits (21.5 +/- 2.5% from baseline) was no different from that observed in normal anesthetized rabbits. In pigs, both IV and IC administration of VEGF/VPF produced a prompt reduction in MAP. Heart rate increased, while cardiac output, stroke volume, left atrial pressure, and total peripheral resistance all declined to a similar, statistically significant degree in both IV and IC groups. Epicardial echocardiography disclosed neither global nor segmental wall motion abnormalities in response to VEGF/VPF. We conclude that (1) VEGF/VPF-stimulated release of NO, previously suggested in vitro, occurs in vivo; (2) this finding suggests that functional VEGF/VPF receptors are present on quiescent adult endothelium, consistent with a maintenance function for VEGF/VPF, which may include regulation of NO; and (3) the preserved response of HC rabbits suggests that endothelial cell receptors for VEGF/VPF are spared in the setting of hypercholesterolemia.

Fibronectins Are Essential for Heart and Blood Vessel Morphogenesis But Are Dispensable for Initial Specification of Precursor Cells

The underlying mechanisms of lethal cardiovascular defects associated with the fibronectin-null (FN.null) mutation in mouse embryos were investigated by lineage analysis of myocardial, endocardial, and endothelial cells. A wide variation in phenotype was observed on two genetic backgrounds. In the less severe class (C57/BL6 background), FN.null embryos display a defective heart. Myocardial cells express the specific marker MF-20 and are correctly localized in the anterior trunk region, but myocardial organization is disrupted, resulting in a bulbous heart tube. Endocardial cells express the specific marker platelet-endothelial cell adhesion molecule-1 (PECAM-1) and are localized within the myocardium, but the endocardium appears collapsed. Endothelial cells of two vascular beds are specified, but the aortae are distended and lack contact with the surrounding mesenchyme, while no vessels form in the yolk sac. Defects in the more severe class suggest that FNs are essential earlier in development on the 129/Sv background. Myocardial and endocardial cells are specified, but morphogenesis of the myocardium and endocardium does not occur. Aortic endothelial cells are specified and localized normally, but remain scattered. Yolk sac endothelial cells resemble those of the less severe class. We conclude that FNs are essential for organization of heart and blood vessels, but are dispensable for cellular specification in the appropriate regions within the embryo.

VEGF gene transfer reduces intimal thickening via increased production of nitric oxide in carotid arteries

Thickening of the arterial intima and smooth muscle cell (SMC) proliferation remain major problems after vascular surgery and other types of vascular manipulations. We studied the effect of endothelial cell (EC)-specific vascular endothelial growth factor (VEGF) gene transfer on the thickening of the intima using a silicone collar inserted around carotid arteries that acted both as the agent that caused intimal SMC growth and as a reservoir for the transfected gene. The model preserved EC integrity and permitted direct extravascular gene transfer without any intravascular manipulation. Compared to beta-galactosidase (lacZ)-transfected control arteries, plasmid/liposome-mediated VEGF gene transfer significantly reduced intimal thickening 1 week after the gene transfer. Administration to the experimental animals of the nitric oxide (NO) synthase inhibitor L-NAME abolished the difference in intimal thickening between VEGF and lacZ-transfected arteries. Furthermore, VEGF caused NO release from cultured human umbilical vein EC. It is concluded that extravascular VEGF gene transfer attenuates intimal growth and could be useful for the prevention of intimal thickening during vascular surgery. Our results further suggest that VEGF may reduce SMC proliferation via a mechanism that involves VEGF-induced NO production from the endothelium.

Novel antibodies directed against the extracellular domain of the human VEGF-receptor type II

Vascular endothelial cell growth factor (VEGF) plays a pivotal role in the regulation of angiogenesis by binding to its cognate receptor molecule type II (VEGFr-II, KDR). VEGFr-II is an endothelial cell-specific transmembrane tyrosine kinase important for vascular endothelial cell development and differentiation during embryogenesis, angiogenic processes under physiological conditions, and various diseases. An increasing number of reports indicate that VEGF/VEGFr-II also play a fundamental role for tumor angiogenesis. We present the generation and in vitro characterization of the monoclonal antibodies 2-7-9 and 2-10-1. Both antibodies are highly specific for VEGFr-II as demonstrated by Western blotting and immunoprecipitation. MAbs 2-10-1 and 2-7-9 bind to a disulphide bridge-stabilized epitope within domains 6 and 7 of the human VEGFr-II with an affinity of 8 and 80 nM, respectively. Furthermore, the antibodies are suitable for immunohistochemistry and ELISA techniques. Because both antibodies recognize their epitope on living cells, they also have the potential for drug targeting and diagnostic purposes.

1H, 13C, and 15N backbone assignment and secondary structure of the receptor-binding domain of vascular endothelial growth factor

Nearly complete sequence-specific 1H, 13C, and 15N resonance assignments are reported for the backbone atoms of the receptor-binding domain of vascular endothelial growth factor (VEGF), a 23-kDa homodimeric protein that is a major regulator of both normal and pathological angiogenesis. The assignment strategy relied on the use of seven 3D triple-resonance experiments [HN(CO)CA, HNCA, HNCO, (HCA)CONH, HN(COCA)HA, HN(CA)HA, and CBCA-(CO)NH] and a 3D 15N-TOCSY-HSQC experiment recorded on a 0.5 mM (12 mg/mL) sample at 500 MHz, pH 7.0, 45 degrees C. Under these conditions, 15N relaxation data show that the protein has a rotational correlation time of 15.0 ns. Despite this unusually long correlation time, assignments were obtained for 94 of the 99 residues; 8 residues lack amide 1H and 15N assignments, presumably due to rapid exchange of the amide 1H with solvent under the experimental conditions used. The secondary structure of the protein was deduced from the chemical shift indices of the 1H alpha, 13C alpha, 13C beta, and 13CO nuclei, and from analysis of backbone NOEs observed in a 3D 15N-NOESY-HSQC spectrum. Two helices and a significant amount of beta-sheet structure were identified, in general agreement with the secondary structure found in a recently determined crystal structure of a similar VEGF construct [Muller YA et al., 1997, Proc Natl Acad Sci USA 94:7192-7197].

Characterization of Signal Transduction Pathways in Human Bone Marrow Endothelial Cells

Human bone marrow endothelial cells immortalized with the T antigen of SV40 (TrHBMEC) have previously been characterized by us with regard to their properties that are similar to primary marrow endothelial cells and their utility as a model system. We now report that TrHBMEC express a recently discovered signal transduction molecule termed RAFTK (related adhesion focal tyrosine kinase), also called Pyk2 or CAK-β. RAFTK, the second member of the focal adhesion kinase (FAK) family, is known to be activated in response to calcium flux in neuronal cells and integrin stimulation in megakaryocytes and B cells. We have studied the effects of cytokines on RAFTK activation in TrHBMEC. Treatment of TrHBMEC with the vascular endothelial growth factor (VEGF ), as well as the VEGF-related protein (VRP), the recently identified ligand for the FLT-4 receptor, resulted in enhanced tyrosine phosphorylation of RAFTK. Similar changes in RAFTK phosphorylation were observed upon stimulation of TrHBMEC with basic fibroblast growth factor (bFGF ) or oncostatin M (OSM). Stimulation of these cells with growth factors also resulted in an increase in RAFTK activity and the c-Jun NH2 -terminal kinase (JNK). RAFTK coimmunoprecipitated with the cytoskeletal protein paxillin through its C-terminal proline-rich domain in TrHBMEC. These results suggest that, in marrow endothelium, activation of RAFTK by VEGF, VRP, OSM, and bFGF represents a new element in the signal transduction pathways used by these growth factors and likely acts to coordinate signaling from their surface receptors to the cytoskeleton, thereby modulating cell growth and function.

Characterization of Signal Transduction Pathways in Human Bone Marrow Endothelial Cells

Human bone marrow endothelial cells immortalized with the T antigen of SV40 (TrHBMEC) have previously been characterized by us with regard to their properties that are similar to primary marrow endothelial cells and their utility as a model system. We now report that TrHBMEC express a recently discovered signal transduction molecule termed RAFTK (related adhesion focal tyrosine kinase), also called Pyk2 or CAK-β. RAFTK, the second member of the focal adhesion kinase (FAK) family, is known to be activated in response to calcium flux in neuronal cells and integrin stimulation in megakaryocytes and B cells. We have studied the effects of cytokines on RAFTK activation in TrHBMEC. Treatment of TrHBMEC with the vascular endothelial growth factor (VEGF ), as well as the VEGF-related protein (VRP), the recently identified ligand for the FLT-4 receptor, resulted in enhanced tyrosine phosphorylation of RAFTK. Similar changes in RAFTK phosphorylation were observed upon stimulation of TrHBMEC with basic fibroblast growth factor (bFGF ) or oncostatin M (OSM). Stimulation of these cells with growth factors also resulted in an increase in RAFTK activity and the c-Jun NH2 -terminal kinase (JNK). RAFTK coimmunoprecipitated with the cytoskeletal protein paxillin through its C-terminal proline-rich domain in TrHBMEC. These results suggest that, in marrow endothelium, activation of RAFTK by VEGF, VRP, OSM, and bFGF represents a new element in the signal transduction pathways used by these growth factors and likely acts to coordinate signaling from their surface receptors to the cytoskeleton, thereby modulating cell growth and function.

Identification of a 190-kDa vascular endothelial growth factor 165 cell surface binding protein on a human glioma cell line

Vascular endothelial growth factor (VEGF) is an angiogenesis factor for which two signaling protein tyrosine kinase receptors, Flt1 and KDR, have been identified. We describe here a 190-kDa component present on a human glioma cell line that binds VEGF165 with high affinity. In contrast, VEGF121 is bound only with low affinity, suggesting that the C-terminal part of VEGF165 is important for interaction with the 190-kDa component. No internalization or stimulation of tyrosine phosphorylation was recorded after ligand binding to the 190-kDa component, suggesting that it may not be directly involved in signaling; its function may be to present ligand or stabilize ligand binding to signaling receptors.

Characterization of the extracellular domain in vascular endothelial growth factor receptor-1 (Flt-1 tyrosine kinase)

Flt-1 tyrosine kinase, vascular endothelial growth factor (VEGF) receptor-1, binds VEGF and a new VEGF-related ligand, placenta growth factor, but KDR/Flk-1 (VEGF receptor-2) binds only VEGF. To characterize the functional regions in the Flt-1 extracellular domain such as the ligand binding region and the dimer formation of the receptor, we constructed a series of mutants of the Flt-1 extracellular domain as soluble forms in a baculovirus system. We found that a region carrying the N-terminal 1st to 3rd immunoglobulin (Ig)-like domains of Flt-1 binds both ligands with high affinity. However, for dimer formation of soluble Flt-1, a region further downstream in the Flt-1 extracellular domain was required. Mutant Flt-1 receptors expressed in COS cells confirmed the requirement of the 4th to 7th Ig region for the activation of Flt-1 tyrosine kinase. Soluble Flt-1 carrying the N-terminal 1st to 3rd Ig region suppressed VEGF-dependent endothelial proliferation in vitro to the same level as the larger forms of soluble Flt-1, suggesting that the binding of one soluble Flt-1 molecule to one subunit of the VEGF homodimer may be sufficient to block the VEGF activity.

Neovascularization of the Xenopus embryo

The receptor tyrosine kinase, Flk-1 or VEGFR-2, and its ligand, vascular endothelial growth factor (VEGF) are required for the development of the embryonic vasculature. Targeted disruption of either gene in mice results in the failure of vascular system formation. The Xenopus homologues of flk-1 and VEGF have been cloned and their expression has been examined throughout early embryonic development. These studies indicate that flk-1 is expressed in groups of endothelial precursor cells which will form the major blood vessels of the embryo, including the posterior cardinal veins, the dorsal aorta, the vitelline veins, and the endocardium. VEGF expression is found in tissues adjacent to the mesenchyme containing the flk-1-expressing endothelial precursors. Expression of both flk-1 and VEGF is transient, appearing as the primary vascular plexus is forming and declining steadily after the onset of functional embryonic circulation. After establishment of the primary vascular structures, flk-1 expression is also observed in the intersegmental veins which form by an angiogenic mechanism. Overall, these results support a role for VEGF/flk-1 signaling in both vasculogenesis and angiogenesis in the Xenopus embryo. When VEGF is expressed ectopically in Xenopus embryos by microinjection of either plasmid DNA or synthetic mRNA, large, disorganized vascular structures are produced. This result indicates that ectopic VEGF is capable of altering the architecture of the developing vascular network.

Tat–Human Immunodeficiency Virus-1 Induces Human Monocyte Chemotaxis by Activation of Vascular Endothelial Growth Factor Receptor-1

Human immunodeficiency virus-1 (HIV-1) Tat protein can be released by infected cells and activates mesenchymal cells. Among these, monocytes respond to Tat by migrating into tissues and releasing inflammatory mediators. In the present study, we have examined the molecular mechanism of monocyte activation by Tat, showing that this viral protein signals inside the cells through the tyrosine kinase receptor for vascular endothelial growth factor encoded by fms-like tyrosine kinase gene (VEGFR-1/Flt-1). Subnanomolar concentrations of Tat induced monocyte chemotaxis, which was inhibited by cell preincubation with vascular-endothelial growth factor-A (VEGF-A). This desensitisation was specific for VEGF-A, because it not was observed with FMLP. In addition, the soluble form of VEGFR-1 specifically inhibited polarization and migration induced by Tat and VEGF-A, thus confirming the common use of this receptor. Binding studies performed at equilibrium by using radiolabeled Tat showed that monocytes expressed a unique class of binding site, with a kd of approximately 0.2 nmol/L. The binding of radiolabeled Tat to monocyte surface and the cross-linking to a protein of 150 kD was inhibited specifically by an excess of cold Tat or VEGF-A. Western blot analysis with an antibody anti–VEGFR-1/Flt-1 performed on monocyte phosphoproteins immunoprecipitated by an monoclonal antibody antiphosphotyrosine showed that Tat induced a rapid phosphorylation in tyrosine residue of the 150-kD VEGFR-1/Flt-1. Taken together, these results suggest that biologic activities of HIV-1 Tat in human monocytes may, at least in part, be elicited by activation of VEGFR-1/Flt-1.

Tat–Human Immunodeficiency Virus-1 Induces Human Monocyte Chemotaxis by Activation of Vascular Endothelial Growth Factor Receptor-1

Human immunodeficiency virus-1 (HIV-1) Tat protein can be released by infected cells and activates mesenchymal cells. Among these, monocytes respond to Tat by migrating into tissues and releasing inflammatory mediators. In the present study, we have examined the molecular mechanism of monocyte activation by Tat, showing that this viral protein signals inside the cells through the tyrosine kinase receptor for vascular endothelial growth factor encoded by fms-like tyrosine kinase gene (VEGFR-1/Flt-1). Subnanomolar concentrations of Tat induced monocyte chemotaxis, which was inhibited by cell preincubation with vascular-endothelial growth factor-A (VEGF-A). This desensitisation was specific for VEGF-A, because it not was observed with FMLP. In addition, the soluble form of VEGFR-1 specifically inhibited polarization and migration induced by Tat and VEGF-A, thus confirming the common use of this receptor. Binding studies performed at equilibrium by using radiolabeled Tat showed that monocytes expressed a unique class of binding site, with a kd of approximately 0.2 nmol/L. The binding of radiolabeled Tat to monocyte surface and the cross-linking to a protein of 150 kD was inhibited specifically by an excess of cold Tat or VEGF-A. Western blot analysis with an antibody anti–VEGFR-1/Flt-1 performed on monocyte phosphoproteins immunoprecipitated by an monoclonal antibody antiphosphotyrosine showed that Tat induced a rapid phosphorylation in tyrosine residue of the 150-kD VEGFR-1/Flt-1. Taken together, these results suggest that biologic activities of HIV-1 Tat in human monocytes may, at least in part, be elicited by activation of VEGFR-1/Flt-1.

The paracrine role of tumour-derived mIL-4 on tumour-associated endothelium

Interleukin-4 (IL-4) has been demonstrated to possess anti-tumourigenic properties in vivo which is initially attributed to the infiltration of eosinophils proposed to occur by IL-4 binding to its receptors on endothelial cells, thereby mediating eosinophil adhesion. We have investigated whether the binding of IL-4 to receptors on endothelial cells could elicit other biological responses which may also play a role in tumour inhibition, such as angiogenesis. We have demonstrated that mouse IL-4 (mIL-4) down-regulates the expression of one of the receptors for VEGF, VEGF-R2, on endothelial cells in vitro. By generating stable transfectants of C6 glioma cells that express mIL-4 under a tetracycline-responsive promoter system, we were able to apply tight regulatory control of mIL-4 expression in vivo. Subcutaneous implantation of mIL-4/C6 cell lines in nu/nu mice revealed that tumour growth is inhibited by mIL-4 expression. mIL-4-expressing tumours were demonstrated to have a reduced level of vascularisation compared with controls, in addition to a high degree of eosinophil infiltration. Our results suggest that mIL-4 has bimodal biological roles in potentiating tumour inhibition in athymic mice: the suppression of angiogenesis and the augmentation of the host local immune response.

VEGF and VEGF-C: specific induction of angiogenesis and lymphangiogenesis in the differentiated avian chorioallantoic membrane

The lymphangiogenic potency of endothelial growth factors has not been studied to date. This is partially due to the lack of in vivo lymphangiogenesis assays. We have studied the lymphatics of differentiated avian chorioallantoic membrane (CAM) using microinjection of Mercox resin, semi- and ultrathin sectioning, immunohistochemical detection of fibronectin and alpha-smooth muscle actin, and in situ hybridization with VEGFR-2 and VEGFR-3 probes. CAM is drained by lymphatic vessels which are arranged in a regular pattern. Arterioles and arteries are accompanied by a pair of interconnected lymphatics and form a plexus around bigger arteries. Veins are also associated with lymphatics, particularly larger veins, which are surrounded by a lymphatic plexus. The lymphatics are characterized by an extremely thin endothelial lining, pores, and the absence of a basal lamina. Patches of the extracellular matrix can be stained with an antibody against fibronectin. Lymphatic endothelial cells of differentiated CAM show ultrastructural features of this cell type. CAM lymphatics do not possess mediae. In contrast, the lymphatic trunks of the umbilical stalk are invested by a single but discontinuous layer of smooth muscle cells. CAM lymphatics express VEGFR-2 and VEGFR-3. Both the regular pattern and the typical structure of these lymphatics suggest that CAM is a suitable site to study the in vivo effects of potential lymphangiogenic factors. We have studied the effects of VEGF homo- and heterodimers, VEGF/PlGF heterodimers, and PlGF and VEGF-C homodimers on Day 13 CAM. All the growth factors containing at least one VEGF chain are angiogenic but do not induce lymphangiogenesis. PlGF-1 and PlGF-2 are neither angiogenic nor lymphangiogenic. VEGF-C is the first lymphangiogenic factor and seems to be highly chemoattractive for lymphatic endothelial cells. It induces proliferation of lymphatic endothelial cells and development of new lymphatic sinuses which are directed immediately beneath the chorionic epithelium. Our studies show that VEGF and VEGF-C are specific angiogenic and lymphangiogenic growth factors, respectively.

Reciprocal relation between VEGF and NO in the regulation of endothelial integrity

Balloon angioplasty disrupts the protective endothelial lining of the arterial wall, rendering arteries susceptible to thrombosis and intimal thickening. We show here that vascular endothelial growth factor (VEGF), an endothelial cell mitogen, is upregulated in medial smooth muscle cells of the arterial wall in response to balloon injury. Both protein kinase C (PKC) and tyrosine kinase pp60src mediate augmented VEGF expression. In contrast, nitric oxide (NO) donors inhibit PKC-induced VEGF upregulation by interfering with binding of the transcription factor activator protein-1 (AP-1) to the VEGF promoter. Inhibition of VEGF promoter activation suggests that NO secreted by a restored endothelium functions as the negative feedback mechanism that downregulates VEGF expression to basal levels. Administration of a neutralizing VEGF antibody impaired reendothelialization following balloon injury performed in vivo. These findings establish a reciprocal relation between VEGF and NO in the endogenous regulation of endothelial integrity following arterial injury.

The XHex homeobox gene is expressed during development of the vascular endothelium: overexpression leads to an increase in vascular endothelial cell number

The Hex/Prh homeobox gene is expressed in a subset of adult blood cell types and may play a role in the differentiation of the myeloid and B-cell lineages. In a search for homeobox genes involved in cardiovascular development, we have independently isolated a Xenopus laevis cDNA which appears to be the amphibian orthologue of Hex/Prh. Based on high sequence similarity in a number of regions, particularly the critical homeobox, we have named this gene XHex. This developmentally regulated gene is first expressed in the dorsal endomesoderm of the gastrula stage embryo. This tissue goes on to contribute to the structures of the embryonic liver and XHex continues to be expressed in the liver throughout development. From the tailbud stage, XHex is expressed in vascular endothelial cells throughout the developing vascular network. Vascular expression of XHex is transient and commences slightly after expression of the receptor tyrosine kinase gene, flk-1, which is known to be essential for vascular development. This observation raises the possibility that XHex is one of the transcription factors that responds to the VEGF/Flk-1 signal transduction pathway leading to differentiation of vascular endothelial cells. XHex is unique amongst homeobox genes in displaying expression in the endothelial layer throughout the developing vasculature. Overexpression of XHex sequences in the frog embryo causes disruption to developing vascular structures and an increase in the number of vascular endothelial cells, suggesting a possible role in regulation of cell proliferation.

Vascular endothelial growth factor and its receptors in normal human testicular tissue

Expression of vascular endothelial growth factor (VEGF), also known as vascular permeability factor (VPF), and its receptors Flt-1 and KDR (Flk-1 in mouse) and their localization in the human testis were analyzed by means of reverse transcriptase-polymerase chain reaction (RT-PCR), Western blotting and immunohistochemistry. VEGF mRNA was detected in the human testicular tissue and in fragments of seminiferous tubules by means of RT-PCR, while fragments of blood vessels isolated from testes were negative. Western blotting procedure using a specific VEGF antibody, revealed two protein bands corresponding to 24 and 49 kDa in the extracts prepared from the whole testis and in the seminiferous tubules while no such bands were found in isolated fragments of human testicular blood vessels. Also immunohistochemically, human testicular blood vessels show no VEGF immunoreactivity, while Leydig cells and Sertoli cells were positive. The mRNA of the VEGF receptor Flt-1 was found to be expressed in human testicular tissue, in isolated fragments of testicular blood vessels and in seminiferous tubules as determined by RT-PCR procedure. In accordance with these results, the Flt-1 protein was immunohistochemically localized in Leydig, Sertoli and perivascular cells. Endothelial cells of certain segments of human testicular microvasculature also stained positive for Flt-1. Expression of VEGF receptor, KDR, could be demonstrated in human testicular tissue, in isolated seminiferous tubules and in isolated fragments of human testicular blood vessels by means of RT-PCR. Immunohistochemically, the KDR protein was localized in endothelial cells and perivascular cells of capillaries within the lamina propria of seminiferous tubules. Leydig cells and Sertoli cells show KDR immunoreactivity, too. Thus we demonstrate the presence of both types of VEGF receptors Flt-1 and KDR on Leydig as well as on Sertoli cells which are normal non-endothelial cells, suggesting hitherto unrecognized and novel functions for such receptors. The results obtained permit us to suggest VEGF as a paracrine mitogenic and angiogenic factor, responsible for modulating the capillarization of the human testicular tissue and maintaining the functions of testicular microvasculature. VEGF may also influence the permeability of capillaries passing through the groups of Leydig cells and those localized within the lamina propria of human seminiferous tubules. The differences in the expression pattern of the VEGF receptors in the human testicular tissue probably reflect different VEGF effects in different compartments of human testis.

Vasoactive peptides modulate vascular endothelial cell growth factor production and endothelial cell proliferation and invasion

The proliferation of vascular endothelial cells (EC) is an important event in angiogenesis. The synthesis of the EC growth factor, vascular endothelial cell growth factor (VEGF), is stimulated by a variety of activators; but the effects of important vasoactive peptides are not well understood, and there are no known natural inhibitors of VEGF production. We found that the vasoactive peptides endothelin (ET)-1 and ET-3 stimulated the synthesis of VEGF protein 3-4-fold in cultured human vascular smooth muscle cells, comparable in magnitude to hypoxia. ET-1 and ET-3 acted through the ETA and ETB receptors, respectively, and signaling through protein kinase C was important. Atrial natriuretic peptide (ANP), C-type natriuretic peptide, and C-ANP-(4-23), a ligand for the natriuretic peptide clearance receptor, equipotently inhibited production of VEGF by as much as 88% and inhibited ET- or hypoxia-stimulated VEGF transcription. EC proliferation and invasion of matrix were stimulated by VEGF secreted into the medium by ET-incubated vascular smooth muscle cells. This was inhibited by ANP. Our results identify the natriuretic peptides as the first peptide inhibitors of VEGF synthesis and indicate a novel mechanism by which vasoactive peptides could modulate angiogenesis.

Involvement of interleukin-8, vascular endothelial growth factor, and basic fibroblast growth factor in tumor necrosis factor alpha-dependent angiogenesis

Tumor necrosis factor alpha (TNF-alpha) is a macrophage/monocyte-derived polypeptide which modulates the expression of various genes in vascular endothelial cells and induces angiogenesis. However, the underlying mechanism by which TNF-alpha mediates angiogenesis is not completely understood. In this study, we assessed whether TNF-alpha-induced angiogenesis is mediated through TNF-alpha itself or indirectly through other TNF-alpha-induced angiogenesis-promoting factors. Cellular mRNA levels of interleukin-8 (IL-8), vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and their receptors were increased after the treatment of human microvascular endothelial cells with TNF-alpha (100 U/ml). TNF-alpha-dependent tubular morphogenesis in vascular endothelial cells was inhibited by the administration of anti-IL-8, anti-VEGF, and anti-bFGF antibodies, and coadministration of all three antibodies almost completely abrogated tubular formation. Moreover, treatment with Sp1, NF-kappaB, and c-Jun antisense oligonucleotides inhibited TNF-alpha-dependent tubular morphogenesis by microvascular endothelial cells. Administration of a NF-kappaB antisense oligonucleotide almost completely inhibited TNF-alpha-dependent IL-8 production and partially abrogated TNF-alpha-dependent VEGF production, and an Sp1 antisense sequence partially inhibited TNF-alpha-dependent production of VEGF. A c-Jun antisense oligonucleotide significantly inhibited TNF-alpha-dependent bFGF production but did not affect the production of IL-8 and VEGF. Administration of an anti-IL-8 or anti-VEGF antibody also blocked TNF-alpha-induced neovascularization in the rabbit cornea in vivo. Thus, angiogenesis by TNF-alpha appears to be modulated through various angiogenic factors, both in vitro and in vivo, and this pathway is controlled through paracrine and/or autocrine mechanisms.

Influence of hypoxia on vascular endothelial growth factor and chorionic gonadotrophin production in the trophoblast-derived cell lines: JEG, JAr and BeWo

Growth of trophoblast tissue in early pregnancy is rapid and accomplished in an unusually hypoxic environment. Hypoxia has been reported to upregulate mRNA production of vascular endothelial growth factor (VEGF), and VEGF receptors have been found on trophoblast cells. These observations suggest that VEGF may have an important role in early placentation. This study examines the influence of hypoxia on both the production of the VEGF message and protein and on the production of human chorionic gonadotrophin (hCG) protein by the cell lines JEG, JAr and BeWo. Cells were grown under normoxic and hypoxic conditions for 72 h. The average oxygen tension in the culture media of the hypoxic cultures (6-7 kPa) was significantly less than in the normoxic cultures (19-21 kPa). RNA was extracted and message for VEGF(121), VEGF(165) and VEGF(189) found in all cell lines by reverse transcription and the polymerase chain reaction (RT-PCR). These messages were upregulated by hypoxia; findings confirmed by competitive PCR for VEGF and expression of the house keeping gene GAPDH. hCG and VEGF were measured by immunoassay. Hypoxia resulted in an increase in VEGF production (P<0.05) but had inconsistent effects on hCG production. In some experiments the absolute concentrations of hCG and VEGF in the culture media were noted to be significantly correlated (r>0.5, P<0.05). In addition to its role in angiogenesis, VEGF may have direct effects on trophoblast cells encouraging proliferation and invasion. These effects may be regulated in part through oxygen supply and hCG.

Vascular endothelial growth factor expression in canine peripheral vein bypass grafts

PURPOSE

Autologous veins used as arterial bypass grafts undergo initial loss of the endothelial cell (EC) lining, which is followed by reendothelialization. We characterized the expression of the EC-specific angiogenic mitogen, vascular endothelial growth factor (VEGF), in vascular grafts to help elucidate the molecular and cellular events after bypass procedures.

METHODS

Cephalic vein-femoral artery interposition grafts were placed in mongrel dogs. Vein grafts and arteries were harvested at either 48 hours or 4 weeks after bypass, the total RNA was isolated, and the VEGF mRNA expression was evaluated by Northern blot analysis. Tissue segments from each time period were evaluated by immunohistochemical analysis using anti-VEGF antibodies.

RESULTS

VEGF mRNA expression in vein grafts as compared with control veins was increased 2.5-fold 48 hours after bypass grafting (p = 0.02) but returned to initial control levels in grafts removed at 4 weeks. Distal arterial segments, which included the anastomotic site without attached vein graft, had a 21.4-fold increase in VEGF expression at 48 hours (p = 0.02) and a 6.6-fold increase at 4 weeks (p < 0.01) as compared with control arterial segments. Vessels subjected to arteriotomy or ischemia alone also demonstrated increased VEGF expression. Immunohistochemical analysis revealed VEGF protein within ECs and smooth muscle cells of the venous bypass graft, with maximal levels observed within intimal hyperplasia at the arterial anastomosis.

CONCLUSIONS

After arterial reconstruction procedures using venous conduits, VEGF is significantly increased at 48 hours in the vein graft and arterial anastomosis. VEGF expression in the vein graft normalizes within 4 weeks but remains significantly elevated in the adjacent arterial segment. Increased VEGF production after arterial grafting may facilitate reendothelialization, thus partially accounting for optimal patency rates achieved with autologous vein grafts.

Molecular cloning of a novel vascular endothelial growth factor, VEGF-D

We have identified and characterized a novel vascular endothelial growth factor (VEGF), VEGF-D, which is structurally related to vascular endothelial growth factor C. A full-length cDNA for human VEGF-D was cloned following the identification of an EST obtained through a TFASTA search of public EST databases. The murine VEGF-D was subsequently isolated from a mouse lung cDNA library. The human VEGF-D gene was mapped to human chromosome Xp22.31. Both human and mouse VEGF-D are strongly expressed in lung and encode the eight cysteine residues that are highly conserved among the members of this family. The high level of conservation between mouse and human VEGF-D may emphasize the biological importance of this gene. Recently the murine gene, FIGF, which is identical to mouse VEGF-D, was reported.

Cloning of the mouse laminin alpha 4 cDNA. Expression in a subset of endothelium

Endothelial cells express a 400-kDa or 240-kDa laminin alpha chain, depending on their tissue of origin or physiological state [1, 2]. Using differential display and subsequent screening of a mouse endothelial cell cDNA library we here identify the gene coding for the 240-kDa laminin chain as the laminin alpha 4 gene. The complete mouse laminin alpha 4 cDNA sequence is reported and compared with other laminin alpha chains. In situ hybridization of embryonic and new born mouse tissues revealed expression of laminin alpha 4 mRNA in a subset of endothelium, in particular aortic endothelium, endocardium and endothelium of blood vessels in the skin and in the brain. Strong laminin alpha 4 expression by aortic endothelia was confirmed by data obtained from cultured bovine aortic endothelial cells (BAEC). Isolation of laminin from BAEC conditioned medium revealed a Y-shaped molecule in rotary shadowing. Subsequent sequencing of BAEC laminin resulted in laminin alpha 4, beta 1 and gamma 1 amino acid sequences, confirming that laminin alpha 4 is one of the major laminin alpha chains expressed by aortic endothelium not only in the mouse. In addition, strong laminin alpha 4 mRNA expression occurred in peripheral nerves, cardiac muscle, fat, the dermis of the skin and lung stroma of mouse tissues. The data demonstrate a cytokine and progesterone-regulated differential expression of laminin alpha 4 mRNA in mouse endothelium, suggesting a distinct functional role for this laminin chain in endothelium.

A requirement for Flk1 in primitive and definitive hematopoiesis and vasculogenesis

Mouse embryos lacking the receptor tyrosine kinase, Flk1, die without mature endothelial and hematopoietic cells. To investigate the role of Flk1 during vasculogenesis and hematopoiesis, we examined the developmental potential of Flk1-/- embryonic stem cells in chimeras. We show that Flk1 is required cell autonomously for endothelial development. Furthermore, Flk1-/- cells do not contribute to primitive hematopoiesis in chimeric yolk sacs or definitive hematopoiesis in adult chimeras and chimeric fetal livers. We also demonstrate that cells lacking Flk1 are unable to reach the correct location to form blood islands, suggesting that Flk1 is involved in the movement of cells from the posterior primitive streak to the yolk sac and, possibly, to the intraembryonic sites of early hematopoiesis.

Vascular endothelial growth factor stimulates tyrosine phosphorylation and recruitment to new focal adhesions of focal adhesion kinase and paxillin in endothelial cells

Vascular endothelial growth factor (VEGF) stimulated the tyrosine phosphorylation of multiple components in confluent human umbilical vein endothelial cells (HUVECs) including bands of Mr 205,000, corresponding to the VEGF receptors Flt-1 and KDR, and Mr 145,000, 120,000, 97,000, and 65,000-70,000. VEGF caused a striking and transient increase in mitogen-activated protein (MAP) kinase activity and stimulated phospholipase C-gamma tyrosine phosphorylation, but it had no effect on phosphatidylinositol 3'-kinase activity. VEGF caused a marked increase in tyrosine phosphorylation of p125 focal adhesion kinase (p125(FAK)), which was both rapid and concentration-dependent. VEGF produced similar effects on p125(FAK) in the endothelial cell line ECV.304. VEGF stimulated tyrosine phosphorylation of the 68-kDa focal adhesion-associated component, paxillin, with similar kinetics and concentration dependence to that for p125(FAK). Thrombin and the phorbol ester, phorbol 12-myristate 13-acetate, also increased p125(FAK) tyrosine phosphorylation in HUVECs. The effect of VEGF on p125(FAK) tyrosine phosphorylation was completely inhibited by the actin filament-disrupting agent cytochalasin D and was partially inhibited by the protein kinase C inhibitor GF109203X. Inhibition of the MAP kinase pathway using a specific inhibitor of MAP kinase kinase had no effect on p125(FAK) tyrosine phosphorylation. VEGF stimulated migration and actin stress fiber formation in confluent HUVEC, and VEGF-induced p125(FAK)/paxillin tyrosine phosphorylation was accompanied by increased immunofluorescent staining of p125(FAK), paxillin, and phosphotyrosine in focal adhesions in confluent cultures of HUVECs. These findings identify p125(FAK) and paxillin as components in a VEGF-stimulated signaling pathway and suggest a novel mechanism for VEGF regulation of endothelial cell functions.

Monocyte chemotactic protein-1 increases collateral and peripheral conductance after femoral artery occlusion

Monocytes are activated during collateral artery growth in vivo, and monocyte chemotactic protein-1 (MCP-1) has been shown to be upregulated by shear stress in vitro. In order to investigate whether MCP-1 enhances collateral growth after femoral artery occlusion, 12 rabbits were randomly assigned to receive either MCP-1, PBS, or no local infusion via osmotic minipump. Seven days after occlusion, isolated hindlimbs were perfused with autologous blood at different pressures, measuring flows at maximal vasodilation via flow probe and radioactive microspheres, as well as peripheral pressures. This allowed the calculation of collateral (thigh) and peripheral (lower limb) conductances from pressure-flow tracings (slope of the curve). Collateral growth on postmortem angiograms was restricted to the thigh and was markedly enhanced with MCP-1 treatment. Both collateral and peripheral conductances were significantly elevated in animals with MCP-1 treatment compared with the control group, reaching values of nonoccluded hindlimbs after only 1 week of occlusion (collateral conductance, 70.6 +/- 19.23 versus 25.1 +/- 2.59 mL/min per 100 mm Hg; P < .01; peripheral conductance, 119.3 +/- 22.37 versus 45.4 +/- 6.80 mL/min per 100 mm Hg; P < .05). These results suggest that activation of monocytes plays an important role in collateral growth as well as in capillary sprouting.

PDGFR alpha expression during mouse embryogenesis: immunolocalization analyzed by whole-mount immunohistostaining using the monoclonal anti-mouse PDGFR alpha antibody APA5

We investigated the cells that express platelet-derived growth factor receptor alpha (PDGFR alpha) during mouse embryogenesis. PDGFR alpha expression has been identified by in situ hybridization or immunohistochemistry using polyclonal antibodies on tissue sectins. Because no immunostaining study using whole-mount specimens has been published to date, we established a new monoclonal antibody (MAb), APA5, for this purpose. Our results differed in that APA5 stained only the paraxial mesoderm, whereas other investigators concluded that most if not all mesodermal cells expressed PDGFR alpha. Moreover, we did not find PDGFR alpha expression in embryonic erythrocytes, which have been previously suggested to express PDGFR alpha. On the basis of our present results, we wish to revise the proposed PDGFR alpha expression as follows. At the pregastrulation stage, PDGFR alpha is expressed only in primitive endoderm, particularly that in the ectoplacental cone. On gastrulation, it is expressed at high levels in the paraxial mesoderm. This expression continues after its differentiation into the somite. Along with the differentiation and migration of the sclerotome, PDGFR alpha + cells begin to become distributed throughout the embryonal mesenchyme. During organogenesis, particularly intense staining is detected in regions of epithelial and mesenchymal interaction, such as the tooth bud and bronchi. In addition to mesodermal derivatives, the developing lens, apical ectodermal ridge, glial precursor, cardiac valves, and choroid plexus express PDGFR alpha. Our results with whole-mount immunostaining show that PDGFR alpha is abundantly expressed and may play important roles during embryogenesis.

Hyperplasia of lymphatic vessels in VEGF-C transgenic mice

No growth factors specific for the lymphatic vascular system have yet been described. Vascular endothelial growth factor (VEGF) regulates vascular permeability and angiogenesis, but does not promote lymphangiogenesis. Overexpression of VEGF-C, a ligand of the VEGF receptors VEGFR-3 and VEGFR-2, in the skin of transgenic mice resulted in lymphatic, but not vascular, endothelial proliferation and vessel enlargement. Thus, VEGF-C induces selective hyperplasia of the lymphatic vasculature, which is involved in the draining of interstitial fluid and in immune function, inflammation, and tumor metastasis. VEGF-C may play a role in disorders involving the lymphatic system and may be of potential use in therapeutic lymphangiogenesis.