Synthesis and physiological activity of heterodimers comprising different splice forms of vascular endothelial growth factor and placenta growth factor

Physical activity differentially regulates VEGF, PlGF, and their receptors in the human placenta

Physical activity (PA) has beneficial effects on the function of many organs by modulating their vascular development. Regular PA during pregnancy is associated with favorable short‐ and long‐term outcomes for both mother and fetus. During pregnancy, appropriate vascularization of the placenta is crucial for adequate maternal–fetal nutrient and gas exchange. How PA modulates angiogenic factors, VEGF, and its receptors in the human placenta, is as of yet, unknown. We objectively measured the PA of women at 24–28 and 34–38 weeks of gestation. Participants were considered “active” if they had met or exceeded 150 min of moderate‐intensity PA per week during their 2nd trimester. Term placenta tissues were collected from active (n = 23) or inactive (n = 22) women immediately after delivery. We examined the expression of the angiogenic factors VEGF, PlGF, VEGFR‐1, and VEGFR‐2 in the placenta. Western blot analysis showed VEGF and its receptor, VEGFR‐1 was significantly (p < 0.05) higher both at the protein and mRNA levels in placenta from physically active compared to inactive women. No difference in VEGFR‐2 was observed. Furthermore, immunohistochemistry showed differential staining patterns of VEGF and its receptors in placental endothelial, stromal, and trophoblast cells and in the syncytial brush border. In comparison, PlGF expression did not differ either at the protein or mRNA level in the placenta from physically active or inactive women. The expression and localization pattern of VEGF and its receptors suggest that PA during pregnancy may support a pro‐angiogenic milieu to the placental vascular network.

VEGF₁₂₁b and VEGF₁₆₅b are weakly angiogenic isoforms of VEGF-A

BACKGROUND

Different isoforms of VEGF-A (mainly VEGF₁₂₁, VEGF₁₆₅ and VEGF189) have been shown to display particular angiogenic properties in the generation of a functional tumor vasculature. Recently, a novel class of VEGF-A isoforms, designated as VEGF(xxx)b, generated through alternative splicing, have been described. Previous studies have suggested that these isoforms may inhibit angiogenesis. In the present work we have produced recombinant VEGF₁₂₁/₁₆₅b proteins in the yeast Pichia pastoris and constructed vectors to overexpress these isoforms and assess their angiogenic potential.

RESULTS

Recombinant VEGF₁₂₁/₁₆₅b proteins generated either in yeasts or mammalian cells activated VEGFR2 and its downstream effector ERK1/2, although to a lesser extent than VEGF₁₆₅. Furthermore, treatment of endothelial cells with VEGF₁₂₁/₁₆₅b increased cell proliferation compared to untreated cells, although such stimulation was lower than that induced by VEGF₁₆₅. Moreover, in vivo angiogenesis assays confirmed angiogenesis stimulation by VEGF₁₂₁/₁₆₅b isoforms. A549 and PC-3 cells overexpressing VEGF₁₂₁b or VEGF₁₆₅b (or carrying the PCDNA3.1 empty vector, as control) and xenotransplanted into nude mice showed increased tumor volume and angiogenesis compared to controls. To assess whether the VEGF(xxx)b isoforms are differentially expressed in tumors compared to healthy tissues, immunohistochemical analysis was conducted on a breast cancer tissue microarray. A significant increase (p < 0.05) in both VEGF(xxx)b and total VEGF-A protein expression in infiltrating ductal carcinomas compared to normal breasts was observed. A positive significant correlation (r = 0.404, p = 0.033) between VEGF(xxx)b and total VEGF-A was found.

CONCLUSIONS

Our results demonstrate that VEGF₁₂₁/₁₆₅b are not anti-angiogenic, but weakly angiogenic isoforms of VEGF-A. In addition, VEGF(xxx)b isoforms are up-regulated in breast cancer in comparison with non malignant breast tissues. These results are to be taken into account when considering a possible use of VEGF₁₂₁/₁₆₅b-based therapies in patients.

Is scleroderma a vasculopathy?

Described as an autoimmune collagen vascular disease, the most striking feature of scleroderma may be a systemic vasculopathy. This vasculopathy includes characteristic noninflammatory macrovascular and microvascular changes with dramatic and possibly occlusive formation of a thickened neointima. Scleroderma vessels also have an unusual endothelial phenotype, with loss of normal markers including vascular endothelial (VE)-cadherin. These endothelial cells express type 1 interferon and regulator of G protein signaling 5 (RGS5), two molecules associated with vascular rarefaction. These genes may be important because tissue is hypoxic with high levels of vascular endothelial growth factor (VEGF), especially early in the disease. The combination of VEGF and rarefaction is not necessarily paradoxical. VEGF-mediated angiogenesis creates labile vessels that may not survive unless the vessel acquires a smooth muscle coat. The combination of interferon and RGS5 is consistent with an antiangiogenic phenotype. We offer a hypothesis that places vascular injury at the center of this disease and also suggest possible clinical approaches for arresting and/or reversing the disease.

Is scleroderma a vasculopathy?

Described as an autoimmune collagen vascular disease, the most striking feature of scleroderma may be a systemic vasculopathy. This vasculopathy includes characteristic noninflammatory macrovascular and microvascular changes with dramatic and possibly occlusive formation of a thickened neointima. Scleroderma vessels also have an unusual endothelial phenotype, with loss of normal markers including vascular endothelial (VE)-cadherin. These endothelial cells express type 1 interferon and regulator of G protein signaling 5 (RGS5), two molecules associated with vascular rarefaction. These genes may be important because tissue is hypoxic with high levels of vascular endothelial growth factor (VEGF), especially early in the disease. The combination of VEGF and rarefaction is not necessarily paradoxical. VEGF-mediated angiogenesis creates labile vessels that may not survive unless the vessel acquires a smooth muscle coat. The combination of interferon and RGS5 is consistent with an antiangiogenic phenotype. We offer a hypothesis that places vascular injury at the center of this disease and also suggest possible clinical approaches for arresting and/or reversing the disease.

Positive and negative modulation of angiogenesis by VEGFR1 ligands

Vascular endothelial growth factor-A (VEGF-A) is a key target for new antiangiogenic drugs for the treatment of both malignant and nonmalignant human diseases. Vascular effects of VEGF family members are mainly mediated by VEGF receptor 2 (VEGFR2). Conversely, the function and signaling of VEGFR1, which is present on endothelial and nonendothelial cells, are poorly understood. Intriguingly, two of five members in the VEGF family--VEGF-B and placental growth factor (PlGF)--are exclusive ligands for VEGFR1 and do not interact with the other VEGFRs, VEGFR2 and VEGFR3. These VEGFR1-specific ligands may be important therapeutic targets for the treatment of cancer. This Review discusses the distinctive roles of VEGFR1 and its ligands PlGF and VEGF-B in the mediation of angiogenic signaling and considers the therapeutic potential of targeting these particular vascular factors.

Melanoma cell-derived vascular endothelial growth factor induces endothelial tubulogenesis within fibrin gels by a metalloproteinase-mediated mechanism

Angiogenesis is a process required not only for embryonal development but is encountered in wound healing and in pathological situations such as tumour growth. In vitro, formation of capillary-like structures can be induced by seeding human microvascular endothelial cells (HDMECs) on top of a fibrin matrix in the presence of phorbol 12-myristate 13-acetate (PMA) as a stimulating agent. In this study, we show that supernatants collected from high-invasive melanoma cells (BLM) induce the formation of tubular structures similar to PMA treatment whereas supernatants from low-invasive cells (WM164) did not. Analysis of proteins secreted into the supernatant of both melanoma cell lines identified differential expression of several pro-angiogenic proteins in high- and low-invasive melanoma cells. Vascular endothelial growth factor (VEGF) was strongly expressed by high- but not by low-invasive melanoma cells. Neutralisation of VEGF as well as inhibition of matrix metalloproteases (MMPs) using the broad spectrum MMP inhibitor 1,10-phenanthroline, both strongly reduced the melanoma-induced tube formation. PMA treatment of HDMECs on a fibrin matrix stimulated MT1-MMP synthesis, indicating that this protease is involved in PMA-induced angiogenesis. In addition, stimulation of HDMECs by supernatants of BLM melanoma cells resulted in a strong induction of ADAM-15, which is known to act as a metalloproteinase. In conclusion, these results show that VEGF released by melanoma cells is an important mediator of neo-vascularisation and that this process depends on the presence of metalloproteinases.

Vascular endothelial growth factor and placenta growth factor in intrauterine growth-restricted fetuses and neonates

The angiogenic factors vascular endothelial growth factor (VEGF) and placenta growth factor (PlGF) are respectively up- and downregulated by hypoxia. We aimed to study circulating levels of the above factors in intrauterine growth restriction (IUGR) and to correlate their levels with the customized centiles of the infants. The study included 25 IUGR and 25 appropriate for gestational age (AGA) full-term, singleton infants and their mothers. Maternal (MS), fetal (UC), and neonatal day 1 (N1) and 4 (N4) blood was examined. MS and N1 PlGF, as well as UC VEGF levels correlated with the customized centiles of the infants (r = 0.39, P = .007, r = 0.34, P = .01, and r = −0.41, P = .004, resp). Furthermore, UC, N1, and N4 VEGF levels were higher in girls (r = 0.36, P = .01, r = 0.33, P = .02, and r = 0.41, P = .005 resp). In conclusion, positive and negative correlations of examined factors with the customized centiles of the infant could rely on placental function and intrauterine oxygen concentrations—both being usually lower in IUGR cases—while higher VEGF levels in girls should possibly be attributed to the stimulating action of estrogens.

Engineered fibrin matrices for functional display of cell membrane-bound growth factor-like activities: Study of angiogenic signaling by ephrin-B2

With the rapid increase in approaches to pro- or anti-angiogenic therapy, new and effective methodologies for administration of cell-bound growth factors will be required. We sought to develop the natural hydrogel matrix fibrin as platform for extensive interactions and continuous signaling by the vascular morphogen ephrin-B2 that normally resides in the plasma membrane and requires multivalent presentation for ligation and activation of Eph receptors on apposing endothelial cell surfaces. Using fibrin and protein engineering technology to induce multivalent ligand presentation, a recombinant mutant ephrin-B2 receptor binding domain was covalently coupled to fibrin networks at variably high densities. The ability of fibrin-bound ephrin-B2 to act as ligand for endothelial cells was preserved, as demonstrated by a concomitant, dose-dependent increase of endothelial cell binding to engineered ephrin-B2-fibrin substrates in vitro. The therapeutic relevance of ephrin-B2-fibrin implant matrices was demonstrated by a local angiogenic response in the chick embryo chorioallontoic membrane evoked by the local and prolonged presentation of matrix-bound ephrin-B2 to tissue adjacing the implant. This new knowledge on biomimetic fibrin vehicles for precise local delivery of membrane-bound growth factor signals may help to elucidate specific biological growth factor function, and serve as starting point for development of new treatment strategies.

Activation of vascular endothelial growth factor receptor-1 sustains angiogenesis and Bcl-2 expression via the phosphatidylinositol 3-kinase pathway in endothelial cells

Vascular insufficiency and retinal ischemia precede many proliferative retinopathies and stimulate secretion of various vasoactive growth factors, including vascular endothelial growth factor (VEGF) and placenta growth factor (PlGF). It is unclear, however, how PlGF, which is elevated in proliferative diabetic retinopathy and is a VEGF homolog that binds only to VEGF receptor (VEGFR)-1, promotes pathological angiogenesis. When primary microvascular endothelial cells were grown on collagen gels, PlGF-containing ligands upregulated Bcl-2 expression and stimulated the formation of capillary-like tube networks that were retained for up to 14 days in culture. The inhibition of VEGFR-1 results in a dramatic decrease in the number of capillary connections, indicating that VEGFR-1 ligands promote branching angiogenesis. In contrast, VEGF-induced tube formations and Bcl-2 expression were significantly decreased at the end of this period. Flow cytometry analysis of annexin-V/propidium iodide-stained cells revealed that PlGF and PlGF/VEGF heterodimer inhibited apoptosis in serum-deprived endothelial cells. These two growth factors stimulated a survival signaling pathway phosphatidylinositol 3-kinase (PI3K), as identified by increased Akt phosphorylation and because blocking PI3K signalling by adenovirus-mediated overexpression of wild-type phosphatase and tensin homolog on chromosome 10 (PTEN) disrupted angiogenesis and decreased Bcl-2 expression by PlGF and PlGF/VEGF heterodimer, whereas a dominant-negative PTEN mutant enhanced endothelial sprout formation and Bcl-2 expression. Together, these findings indicate that PlGF-containing ligands contribute to pathological angiogenesis by prolonging cell survival signals and maintaining vascular networks.

Role of vascular endothelial growth factor and placental growth factor in basal adhesion formation and in carbon dioxide pneumoperitoneum-enhanced adhesion formation after laparoscopic surgery in transgenic mice

OBJECTIVE

To evaluate the role of vascular endothelial growth factor (VEGF) and placental growth factor (PlGF) in adhesion formation after laparoscopic surgery.

DESIGN

Prospective, randomized study.

SETTING

Academic research center.

ANIMAL(S)

Female wild-type mice and transgenic mice (n = 110), expressing exclusively VEGF-A(164) (VEGF-A(164/164)) or deficient for VEGF-B (VEGF-B(-/-)) or for PlGF (PlGF(-/-)).

INTERVENTION(S)

Adhesions were induced during laparoscopy. To evaluate "basal adhesions" and "CO(2) pneumoperitoneum-enhanced adhesions," the pneumoperitoneum was maintained for a minimum (10 minutes) or prolonged (60 minutes) period. The role of PlGF was also evaluated by administration of antibodies.

MAIN OUTCOME MEASURE(S)

Adhesions were blindly scored after 7 days.

RESULT(S)

In all wild-type mice, CO(2) pneumoperitoneum enhanced adhesion formation. In comparison with wild-type mice, basal adhesions were higher in VEGF-A(164/164) mice and similar in VEGF-B(-/-) and PlGF(-/-) mice. Pneumoperitoneum did not enhance adhesions in any of these transgenic mice. The effects observed in PlGF(-/-) mice were confirmed in PlGF antibody-treated mice.

CONCLUSION(S)

The data demonstrate that the VEGF family plays a role in adhesion formation and confirm that CO(2) pneumoperitoneum enhances adhesions. VEGF-A(164) has a direct role in basal adhesions. Absence of pneumoperitoneum-enhanced adhesions in VEGF-A(164/164), VEGF-B(-/-), and PlGF(-/-) mice indicates up-regulation of VEGF-A(164), VEGF-B, and PlGF by CO(2) pneumoperitoneum as a mechanism for pneumoperitoneum-enhanced adhesion formation.

Cell‐demanded release of VEGF from synthetic, biointeractive cell‐ingrowth matrices for vascularized tissue growth

Local, controlled induction of angiogenesis remains a challenge that limits tissue engineering approaches to replace or restore diseased tissues. We present a new class of bioactive synthetic hydrogel matrices based on poly(ethylene glycol) (PEG) and synthetic peptides that exploits the activity of vascular endothelial growth factor (VEGF) alongside the base matrix functionality for cellular ingrowth, that is, induction of cell adhesion by pendant RGD-containing peptides and provision of cell-mediated remodeling by cross-linking matrix metalloproteinase substrate peptides. By using a Michael-type addition reaction, we incorporated variants of VEGF121 and VEGF165 covalently within the matrix, available for cells as they invade and locally remodel the material. The functionality of the matrix-conjugated VEGF was preserved and was critical for in vitro endothelial cell survival and migration within the matrix environment. Consistent with a scheme of locally restricted availability of VEGF, grafting of these VEGF-modified hydrogel matrices atop the chick chorioallontoic membrane evoked strong new blood vessel formation precisely at the area of graft-membrane contact. When implanted subcutaneously in rats, these VEGF-containing matrices were completely remodeled into native, vascularized tissue. This type of synthetic, biointeractive matrix with integrated angiogenic growth factor activity, presented and released only upon local cellular demand, could become highly useful in a number of clinical healing applications of local therapeutic angiogenesis.

Effects of angiogenic growth factor combinations on retinal endothelial cells

The aim of this paper was to determine if growth factors, known to be upregulated in proliferative diabetic retinopathy, exerted combined effects on retinal endothelial cells. The authors explored the individual and collective actions of insulin-like growth factor I (IGF-I), vascular endothelial growth factor (VEGF), platelet-derived growth factor-BB (PDGF-BB), fibroblast growth factor-2 (FGF-2) and placenta growth factor (PlGF) on several parameters that reflect the angiogenic potential of endothelial cells. The effect of growth factors on cell migration and survival/proliferation was examined using primary cultures of bovine retinal endothelial cells (BREC). The authors also determined the growth factor action on capillary-like tube formation on a reconstituted basement membrane matrix and on the newly described phenomenon of secondary sprouting, in which endothelial cell colonies spontaneously survive, proliferate, migrate and invade the matrix after the original capillary-like tubes have collapsed. Sprouting cells were positive for von Willebrand factor and could aggregate into larger tubes with lumens. Incubation with VEGF+IGF-I or PlGF+FGF-2 enhanced tube stability by 40-50%, more than each growth factor alone or other combinations (5-20%). The concurrent addition of four growth factors did not improve the response seen with growth factor pairs. Surprisingly, PDGF-BB induced tube collapse. IGF-I and FGF-2 mildly enhanced BREC proliferation/survival (5-15%). However, VEGF+IGF-I or PlGF+FGF-2 increased BREC proliferation/survival by 25% under low serum conditions, whereas combinations of all four growth factors exerted a clearly synergistic effect (250% increase). PDGF-BB or FGF-2 stimulated secondary sprouting and were the only factors capable of exerting this effect alone. Even though VEGF, IGF-I or PlGF were not effective, if administered in pairs, they demonstrated increased responses. PDGF-BB was also able to enhance the effect of FGF-2+IGF-I+VEGF on BREC secondary sprouting, but not of any of them individually. No other growth factor tested was able to significantly improve the action of combinations of three other growth factors. VEGF increased cell migration in a wounded monolayer assay two-fold and PDGF-BB, 2.5 times, but other individual growth factors were ineffective. PlGF+FGF-2 enhanced cell migration more than each factor alone. VEGF+IGF-I+PlGF+FGF-2, however, increased cell migration four-fold. In summary, this study indicates that growth factors, overexpressed in diabetic retinopathy eyes, enhance the angiogenic characteristics of cultured cells (tube formation, proliferation, secondary sprouting and migration). Their effects, however, can be greatly augmented by other growth factors that alone exert little or no action. Therefore, diabetic retinal neovascularization may result from the additive or synergistic action of several growth factors.

Placental expression of VEGF, PlGF and their receptors in a model of placental insufficiency-intrauterine growth restriction (PI-IUGR)

Placental development requires adequate and organized interaction of vascular growth factors and their receptors, including vascular endothelial growth factor (VEGF) and placental growth factor (PlGF). Both VEGF and PlGF, acting through the tyrosine kinase receptors VEGFR-1 and VEGFR-2, have been implicated in playing a role in ovine placental vascular development. The present studies describe the placental expression of components of the VEGF family at two maturational time points (55 and 90 days post coitus, dpc) in a hyperthermic-induced ovine model of placental insufficiency-intrauterine growth restriction (PI-IUGR). Both caruncular and cotyledonary VEGF and PlGF mRNA concentration increased with gestational age (P< 0.05), whereas only cotyledonary VEGF and PlGF protein concentration increased over gestation (P< 0.002). At 55 dpc, VEGF mRNA concentration was elevated in hyperthermic (HT) ewes, compared to control thermoneutral (TN) animals (TN; 0.52+/-0.08 vs HT; 1.27+/-0.17 VEGF/GAPDH, P< 0.001). At 90 dpc, expression of PlGF and VEGF mRNA was not altered by the HT treatment. Both TN cotyledonary VEGFR-1 and VEGFR-2 mRNA expression levels rose significantly over the period studied (P< 0.05 and P< 0.01 respectively). Receptor mRNA concentration in HT cotyledonary tissue was significantly reduced at 90 dpc (VEGFR-1; TN 0.21+/-0.02 vs HT 0.11+/-0.01 VEGFR-1/actin, P< 0.05, VEGFR-2; TN 0.18+/-0.05 vs HT 0.07+/-0.01 VEGFR-2/actin, P< 0.01). Soluble VEGFR-1 (sVEGFR-1) mRNA was not detected in these tissues. These alterations in growth factor and growth factor receptor mRNA expression, as a result of environmental heat stress early in placental development, could impair normal placental vascular development. Furthermore, alterations in VEGF, VEGFR-1 and VEGFR-2 mRNA expression, during the period of maximal placental growth, may contribute to the development of placental insufficiency, and ultimately intrauterine growth restriction.

Differential mitogenic responses of human macrovascular and microvascular endothelial cells to cytokines underline their phenotypic heterogeneity

A variety of growth factors promote the complex multistep process of angiogenesis. The mitogenic activity of vascular endothelial growth factors (VEGFs) and placental growth factors (PlGFs), known as cytokines acting predominantly on endothelial cells, was tested on human umbilical vein endothelial cells (HUVEC) and microvascular endothelial cells (MIEC) and compared with the potency of the universally acting basic fibroblast growth factor (FGF-2). The cells were seeded at different cell numbers and incubated with various doses of growth factors for a period of 24-72 h in culture medium +/- serum. Proliferation was determined by measuring the optical density after staining the cells with the tetrazolium salt WST-1. VEGF121 and VEGF165 increased the number of HUVEC and MIEC at low and high seeding densities various doses and incubation times. The efficiency of FGF-2 was less pronounced at high seeding densities of the cells under serum-free conditions. PlGF-1 and PlGF-2 stimulated mitogenesis on HUVEC only at low cell numbers and after a short incubation time by 125 +/- 3% and 102 +/- 5% (P < 0.001), respectively. Longer incubation times with the lower seeding density in the absence of FCS did not induce a significant stimulatory effect of the PlGFs. MIEC responded stronger to all growth factors. In particular under serum free conditions, PlGF-1 and PlGF-2 effectively stimulated cell proliferation by 247 +/- 54% (P < 0.01) and 288 +/- 40% (P < 0.05) at low cell numbers, and by 81 +/- 13% (P < 0.05) and 49 +/- 13% (P < 0.01), respectively, at high cell numbers. The addition of fetal calf serum caused a reduced proliferative response of all growth factors on both cell types related to the controls. In conclusion, MIEC and HUVEC differ in their proliferative response to VEGFs, PlGFs and FGF-2.

Covalently conjugated VEGF--fibrin matrices for endothelialization

Vascular endothelial growth factor (VEGF) is a key factor in endothelial cell biology and blood vessel formation and a candidate therapeutic for the stimulation of angiogenesis-dependent tissue regeneration. The objective of this study was to confer the angiogenic activity of VEGF(121) upon the biomaterial fibrin, a natural substrate for endothelial cell growth and clinically accepted as 'fibrin glue'. To achieve this, we engineered fibrin-based hydrogels that were covalently modified with VEGF(121). Our laboratory has recently developed novel methodology that allows the covalent incorporation of exogenous bioactive peptides by the transglutaminase activity of factor XIIIa into fibrin during coagulation. Here, this ability of factor XIIIa to crosslink additional proteins within fibrin was employed to covalently incorporate VEGF(121). By recombinant DNA methodology, a mutant VEGF(121) variant, alpha(2)-PI(1--8)-VEGF(121), which contains an additional factor XIIIa substrate sequence NQEQVSPL at the aminoterminus, was expressed in E. coli. In soluble form, the mutant protein fully retained its mitogenic activity for endothelial cells. Using (125)I-labeled alpha(2)-PI(1--8)-VEGF(121), its covalent incorporation and the efficiency of incorporation into fibrin was demonstrated and characterized. The immobilized, fibrin-conjugated VEGF(121) protein remained an active and very efficient mitogen for human endothelial cells grown on two-dimensional VEGF(121)-modified fibrin surfaces, and the incorporation of increasing amounts of alpha(2)-PI(1--8)-VEGF(121) resulted in dose-dependent enhancement of endothelial cell growth. The VEGF-modified fibrin matrices can be formed as injectable gels in a single-step reaction under physiological conditions in vivo. When used as a ingrowth matrix, such VEGF incorporating materials could be useful in a variety of clinical situations that require an angiogenic response into an ischemic region or inplant.

Angiogenesis and intrauterine growth restriction

Human placental development involves co-ordinated angiogenesis and trophoblast outgrowth that are compromised in intrauterine growth restriction. Adaptive angiogenesis in IUGR placental villi is a result of an imbalance in the orderly progression of the expression profile of vascular endothelial growth factor, placenta growth factor and angiopoietin during placental development. VEGF receptors and the angiopoietin receptor Tie-2 are expressed on trophoblast, and their activation leads to trophoblast proliferation, migration and production of nitric oxide. Thus, these vascular factors act as autocrine regulators of trophoblast behaviour in the development of the utero-/feto-placental circulation, an action independent of their well-established roles in vascular endothelium.

Molecular basis of angiogenesis. Role of VEGF and VE-cadherin

The formation of new blood vessels (angiogenesis) is essential for embryonic development and contributes to the pathogenesis of numerous disorders. In contrast, insufficient angiogenesis may lead to tissue ischemia and failure. The recent discovery of novel angiogenic molecules has initiated efforts to improve tissue perfusion via therapeutic angiogenesis. However, rational design of such treatment strategies mandates a better understanding of the molecular mechanisms of angiogenesis. In this brief review, the role of a prime angiogenic candidate, namely vascular endothelial growth factor (VEGF) and its homologues, in physiological and pathological angiogenesis will be discussed with particular attention to myocardial ischemia and heart failure. In addition, a novel interaction between the junctional protein vascular endothelial-cadherin (VE-cadherin) and VEGF, essential for the endothelial survival function of VEGF, will be reviewed.

Regulation of placental vascular endothelial growth factor (VEGF) and placenta growth factor (PIGF) and soluble Flt-1 by oxygen--a review

Morphological studies show poor placental vascular development and an increase in the mitotic index of cytotrophoblast cells in intrauterine growth restriction (IUGR). We hypothesized that the reported relatively high oxygen level in the intervillous space in contact with IUGR placental villi will limit angiogenesis by changes in vascular endothelial growth factor (VEGF) and placenta growth factor (PIGF) expression and function. Western immunoblot analysis demonstrates a diametric expression of PIGF and VEGF proteins throughout pregnancy, with P1GF levels increasing and VEGF levels decreasing, consistent with placental oxygenation. PIGF mRNA and protein is increased in IUGR as compared to gestationally matched normal placentae. Increasing oxygen tension upregulates P1GF protein in term placental villous explants, whereas hypoxia downregulates P1GF and VEGFR-1 (Flt-1) autophosphorylation in term trophoblast choriocarcinoma cell line (BeWo). Levels of soluble Flt-1 (sFlt-1) protein in supernatant of term villous explants were upregulated by 1 per cent hypoxia, whereas hyperoxia (40 per cent) decreased sFlt-1 levels, indicating that under conditions of increasing oxygen tension, PlGF function may remain unopposed. The addition of PlGF-1 to a spontaneously transformed first trimester cytotrophoblast cell line (ED27) stimulated cell proliferation while PlGF-2 had little effect. In contrast, the addition of PlGF-1 had little effect on endothelial cell proliferation while this was inhibited by PIGF-2. Taken together these changes provide a molecular explanation for the observed poor angiogenesis in the pathogenesis of IUGR.

Transgenic mouse models in angiogenesis and cardiovascular disease

Novel gene technologies have allowed us to manipulate the genetic balance of candidate molecules in mice in a controllable manner. Homologous or site-specific recombination in embryonic stem cells allows us to study the consequences of deficiencies, mutations, and conditional or tissue-specific expression of gene products in transgenic mice. These technological breakthroughs have significantly advanced biomedical research and broadened our understanding of the pathophysiological role of candidate disease genes. In addition, gene transfer allows us to test the possible therapeutic use of gene products for gene therapy. A variety of assays have been miniaturized, allowing analysis of cardiovascular physiology in the mouse. With the advent of genome sequencing programmes, these gene technologies provide means of studying gene function in a conclusive manner. Furthermore, disease models can be generated which can be used as test models for (gene) therapy or for the discovery of novel genes using differential gene profiling techniques. The present review will focus on the molecular basis of how blood vessels form (angiogenesis and arteriogenesis) and how they become diseased. A selected number of molecules that have been studied in the authors' laboratory will be reviewed in more detail.

Expression and localization of placenta growth factor and PlGF receptors in human meningiomas

It has previously been suggested that in human brain tumours, endothelial cell proliferation during angiogenesis is regulated by a paracrine mechanism involving vascular endothelial growth factor (VEGF) and its receptors (VEGF receptor 1 and VEGF receptor 2). The mechanism of growth factor up-regulation is based on hypoxic activation of mRNA expression and mRNA stabilization and genetic events, leading to an increase of growth factor gene expression. The role of the other newly discovered VEGF family members with a high specificity for endothelial cells in the pathogenesis of glial neoplasms is unknown. To investigate which other members of the VEGF family are overexpressed in human brain tumours, the mRNA levels of placenta growth factor (PlGF), VEGF-A, and VEGF-B genes were determined by northern blot analysis in surgically obtained human meningiomas. In the 16 meningiomas examined, the mRNA for PlGF was highly expressed in four tumours and VEGF-A mRNA was highly abundant in three tumour samples. There was no close correlation between PlGF mRNA levels and VEGF-A expression levels. VEGF-B mRNA was abundantly expressed in all tumour samples at uniform levels. In a PlGF-positive tumour sample, immunoreactive VEGFR-1 and VEGFR-2 were detected in endothelial cells of the blood vessels. PlGF protein was detectable in most but not all capillaries of the tumour. PlGF is thus highly up-regulated in a subset of human meningiomas and may therefore have functions, in some tumour vessels, connected to endothelial cell maturation and tube formation. These findings suggest that PlGF, in addition to VEGF-A, may be another positive factor in tumour angiogenesis in human meningiomas.

Signal transduction and biological function of placenta growth factor in primary human trophoblast

Placenta growth factor (PlGF), a member of the vascular endothelial growth factor family of angiogenic factors, is prominently expressed by trophoblast. In addition to its role as a paracrine angiogenic factor within the placenta and endometrium, presence of its receptor, Flt-1, on trophoblast suggests that PlGF also may have an autocrine role(s) in regulating trophoblast function. To elucidate its role in trophoblast, we examined the signal transduction and functional responses of primary human trophoblast to PlGF. Exogenous PlGF induced specific activation of the stress-activated protein kinase (SAPK) pathways, c-Jun-N terminal kinase (JNK) and p38 kinase, in primary term trophoblast with little to no induction of the extracellular signal regulated kinase (ERK-1 and -2) pathways. In contrast, PlGF induced significant ERK-1 and -2 activity in human umbilical vein endothelial cells but did not induce JNK or p38 activity. PlGF-induced activation of the SAPK signaling pathways protected trophoblast from growth factor withdrawal-induced apoptosis, but it did not protect trophoblast from apoptosis induced by the pro-inflammatory cytokines, interferon gamma and tumor necrosis factor alpha. These results provide the first direct evidence of a biochemical and functional role for PlGF/Flt-1 in normal trophoblast and suggest that aberrant PlGF expression during pregnancy may impact upon trophoblast function as well as vascularity within the placental bed.

Vascular endothelial growth factor (VEGF) receptor II-derived peptides inhibit VEGF

Vascular endothelial growth factor (VEGF) directly stimulates endothelial cell proliferation and migration via tyrosine kinase receptors of the split kinase domain family. It mediates vascular growth and angiogenesis in the embryo but also in the adult in a variety of physiological and pathological conditions. The potential binding site of VEGF with its receptor was identified using cellulose-bound overlapping peptides of the extracytosolic part of the human vascular endothelial growth factor receptor II (VEGFR II). Thus, a peptide originating from the third globular domain of the VEGFR II comprising residues 247RTELNVGIDFNWEYP261 was revealed as contiguous sequence stretch, which bound 125I-VEGF165. A systematic replacement with L-amino acids within the peptide representing the putative VEGF-binding site on VEGFR II indicates Asp255 as the hydrophilic key residue for binding. The dimerized peptide (RTELNVGIDFNWEYPAS)2K inhibits VEGF165 binding with an IC50 of 0.5 microM on extracellular VEGFR II fragments and 30 microM on human umbilical vein cells. VEGF165-stimulated autophosphorylation of VEGFR II as well as proliferation and migration of microvascular endothelial cells was inhibited by the monomeric peptide RTELNVGIDFNWEYPASK at a half-maximal concentration of 3-10, 0.1, and 0.1 microM, respectively. We conclude that transduction of the VEGF165 signal can be interrupted with a peptide derived from the third Ig-like domain of VEGFR II by blockade of VEGF165 binding to its receptor.

Placenta growth factor: potential role in pregnancy

PROBLEM

In spite of the known requirement for adequate vascularity during placentation, little is known regarding the regulation of angiogenic growth factor production by trophoblast. Placenta growth factor (PIGF) is a recently discovered angiogenic growth factor whose expression is relatively limited to trophoblast.

METHOD OF STUDY

Current literature of PIGF was reviewed, with emphasis on its expression, regulation, role in angiogenesis, and potential function(s) at the maternal-fetal interface.

RESULTS

PIGF is abundantly expressed by trophoblast, which implies that it could act in a paracrine manner to modulate vascular development, stability, and/or function within the decidua and placental villi. In addition, expression of the PIGF receptor, fms-like tyrosine kinase (flt-1) receptor, on trophoblast raises the potential for an autocrine role of PIGF in regulating trophoblast growth and/or function.

CONCLUSIONS

The potential for PIGF to influence both vascular endothelial cells and trophoblast suggests that aberrant trophoblast production of PIGF could compromise cellular function during gestation and contribute to the vascular and placental pathologies noted in many obstetric complications.

Paracrine and autocrine regulation of vascular endothelial growth factor during tissue differentiation in the quail

The expression of vascular endothelial growth factor (VEGF) has been described to coincide both temporally and spatially with angiogenesis suggesting a role as a paracrine stimulator of endothelial cells. We have used digoxigenin labelled RNA probes to VEGF and the VEGF receptor-2 (Quek1) to investigate the relationship between VEGF expression and vascular events in quail embryos from day 1 to 13 of incubation. Furthermore, the effect of exogenously applied VEGF was studied in day 4 quail embryos using polyclonal anti-VEGF antibodies. Expression of VEGF mRNA was observed in day 1 and 2 embryos in regions of active angiogenesis and hemangiopoiesis. VEGF mRNA expression was found at high levels in the ventral aspect of the neural tube and Quek1 mRNA expression in the accompanying endothelial cells of day 3 embryos, suggesting a function in brain angiogenesis. However, in the neural tube, thyroid gland and cartilaginous skeleton VEGF mRNA was expressed at least 1 day before the ingrowth of vessels, suggesting that additional mechanisms are involved in control of angiogenesis. This is supported by the observation that application of VEGF165 into the midbrain induced dilatation of perineural vessels, while the intraneural vessels remained almost unaffected. Expression of VEGF mRNA was also observed at high levels in podocytes during all stages, indicative of its importance in glomerular development and function. The results of the present study indicate that as angiogenesis occurred in other tissues and organs (day 13 metanephros, dorsal third of the day 7 neural tube, skeletal muscle, and many mesodermal compartments), there was concurrent paracrine expression of VEGF mRNA and Quek1 mRNA. One noteable exception was the hepatocytes of the developing liver which appeared to remain VEGF-negative throughout the study. However, a small number of endothelial cells within liver sinuses, and additionally within the kidney and the elastic arteries, expressed VEGF mRNA. These results suggest that VEGF may also act as an autocrine mediator of angiogenesis, possibly as a result of localised tissue hypoxia.

The alpha-helical domain near the amino terminus is essential for dimerization of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen and a key mediator of aberrant endothelial cell proliferation and vascular permeability in a variety of human pathological situations such as tumor angiogenesis, diabetic retinopathy, or psoriasis. By amino-terminal deletion analysis and by site-directed mutagenesis we have identified a new domain within the amino-terminal alpha-helix that is essential for dimerization of VEGF. VEGF121 variants containing amino acids 8 to 121 or 14 to 121, respectively, either expressed in Escherichia coli and refolded in vitro, or expressed in Chinese hamster ovary cells, were in a dimeric conformation and showed full binding activity to VEGF receptors and stimulation of endothelial cell proliferation as compared with wild-type VEGF. In contrast, a VEGF121 variant covering amino acids 18 to 121, as well as a variant in which the hydrophobic amino acids Val14, Val15, Phe17, and Met18 within the amphipathic alpha-helix near the amino terminus were replaced by serine, failed to form biological active VEGF dimers. From these data we conclude that a domain between amino acids His12 and Asp19 within the amino-terminal alpha-helix is essential for formation of VEGF dimers, and we propose hydrophobic interactions between VEGF monomers to stabilize or favor dimerization.

Vascular endothelial growth factor induces endothelial fenestrations in vitro

Vascular endothelial growth factor (VEGF) is an important regulator of vasculogenesis, angiogenesis, and vascular permeability. In contrast to its transient expression during the formation of new blood vessels, VEGF and its receptors are continuously and highly expressed in some adult tissues, such as the kidney glomerulus and choroid plexus. This suggests that VEGF produced by the epithelial cells of these tissues might be involved in the induction or maintenance of fenestrations in adjacent endothelial cells expressing the VEGF receptors. Here we describe a defined in vitro culture system where fenestrae formation was induced in adrenal cortex capillary endothelial cells by VEGF, but not by fibroblast growth factor. A strong induction of endothelial fenestrations was observed in cocultures of endothelial cells with choroid plexus epithelial cells, or mammary epithelial cells stably transfected with cDNAs for VEGF 120 or 164, but not with untransfected cells. These results demonstrate that, in these cocultures, VEGF is sufficient to induce fenestrations in vitro. Identical results were achieved when the epithelial cells were replaced by an epithelial-derived basal lamina-type extracellular matrix, but not with collagen alone. In this defined system, VEGF-mediated induction of fenestrae was always accompanied by an increase in the number of fused diaphragmed caveolae-like vesicles. Caveolae, but not fenestrae, were labeled with a caveolin-1-specific antibody both in vivo and in vitro. VEGF stimulation led to VEGF receptor tyrosine phosphorylation, but no change in the distribution, phosphorylation, or protein level of caveolin-1 was observed. We conclude that VEGF in the presence of a basal lamina-type extracellular matrix specifically induces fenestrations in endothelial cells. This defined in vitro system will allow further study of the signaling mechanisms involved in fenestrae formation, modification of caveolae, and vascular permeability.

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.

VEGF induces cardiovascular malformation and embryonic lethality

The essential function of vascular endothelial growth factor (VEGF) in embryonic angiogenesis has clearly been documented in murine embryos with targeted deletions of either VEGF or its receptors. The effects of VEGF in the organogenetic phase of development have not been studied to date. Therefore, we applied 0.7 to 0.9 microgram of VEGF via methylcellulose carriers into the midbrain or onto the right forelimb of 4.5-day-old quail embryos. Another group of embryos was treated with 1 microgram of platelet-derived growth factor and controls were carried out using carriers without any growth factor. VEGF-induced cardiovascular malformations resulted in embryonic lethality. The venous area of the vasculature was dilated in almost all organs. The heart was most seriously affected and showed typical characteristics of insufficiency. VEGF strongly increased endocardial cell proliferation and obviously induced impairment of the growth rates of myocardium and endocardium. The myocardium of the ventricles was extremely thin, and septation defects were observed. As a result of the disturbed outflow, the atria were extremely dilated and thin-walled. The morphology of the hearts was reminiscent of that observed in congenital malformations such as Uhl's and Osler's syndromes. Our results show that expression of VEGF has to be tightly controlled during development.

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.

Placenta growth factor and vascular endothelial growth factor are co-expressed during early embryonic development

We have used the polymerase chain reaction to identify mouse proteins similar in primary structure to the endothelial cell mitogen Vascular Endothelial Growth Factor (VEGF). One amplified product encoded mouse Placenta Growth Factor (PIGF). The pattern of PIGF gene expression in mouse embryos was studied by in situ hybridization. Transcripts encoding mouse PIGF were abundant in trophoblastic giant cells associated with the parietal yolk sac at early stages of embryogenesis. VEGF transcripts were also detected in trophoblastic giant cells raising the possibility that these cells may secrete heterodimers consisting of one PIGF subunit and one VEGF subunit. The secretion of PIGF and VEGF by trophoblastic giant cells is likely to be the signal which initiates and co-ordinates vascularization in the deciduum and placenta during early embryogenesis.