Identification of Placenta Growth Factor Determinants for Binding and Activation of Flt-1 Receptor

Biomarkers and point of care screening approaches for the management of preeclampsia

Preeclampsia is a multi-organ pregnancy complication, that is primarily detected when pregnant people have high blood pressure, and is confirmed by testing for the presence of protein in the urine. While more specific and accurate diagnostic and imaging tests are becoming available, they are still in the process of undergoing widespread regulatory adoption, and so are not yet the standard of care. Since biochemical processes are a precursor to the systemic progression of disease, we review some established, emerging, and promising biomarkers that are proposed to be associated with preeclampsia, and newly developed approaches for screening them at the point of care, to reduce the burden of the disease.

PlGF and VEGF-A/PlGF Heterodimer are Crucial for Recruitment and Activation of Immune Cells During Choroid Neovascularization

Purpose

Recruitment and activation of inflammatory cells, such as retinal microglia/macrophages, in the subretinal space contribute significantly to the pathogenesis of age-related macular degeneration (AMD). This study aims to explore the functional role of vascular endothelial growth factor (VEGF-A), placental growth factor (PlGF) and VEGF-A/PlGF heterodimer in immune homeostasis and activation during pathological laser-induced choroidal neovascularization (CNV).

Methods

To investigate these roles, we utilized the PlGF-DE knockin (KI) mouse model, which is the full functional knockout (KO) of PlGF. In this model, mice express a variant of PlGF, named PlGF-DE, that is unable to bind and activate VEGFR-1 but can still form heterodimer with VEGF-A.

Results

Our findings demonstrate that, although there is no difference in healthy conditions, PlGF-DE-KI mice exhibit decreased microglia reactivity and reduced recruitment of both microglia and monocyte-macrophages, compared to wild-type mice during laser-induced CNV. This impairment is associated with a reduction in VEGF receptor 1 (VEGFR-1) phosphorylation in the retinae of PlGF-DE-KI mice compared to C57Bl6/J mice. Corroborating these data, intravitreal delivery of PlGF or VEGF-A/PlGF heterodimer in PlGF-DE-KI mice rescued the immune cell response at the early phase of CNV compared to VEGF-A delivery.

Conclusions

In summary, our study suggests that targeting PlGF and the VEGF-A/PlGF heterodimer, thereby preventing VEGFR-1 activation, could represent a potential therapeutic approach for the management of inflammatory processes in diseases such as AMD.

Placental growth factor regulates the generation of TH17 cells to link angiogenesis with autoimmunity

Helper T cells actively communicate with adjacent cells by secreting soluble mediators, yet crosstalk between helper T cells and endothelial cells remains poorly understood. Here we found that placental growth factor (PlGF), a homolog of the vascular endothelial growth factor that enhances an angiogenic switch in disease, was selectively secreted by the T_H17 subset of helper T cells and promoted angiogenesis. Interestingly, the 'angio-lymphokine' PlGF, in turn, specifically induced the differentiation of pathogenic T_H17 cells by activating the transcription factor STAT3 via binding to its receptors and replaced the activity of interleukin-6 in the production of interleukin-17, whereas it suppressed the generation of regulatory T cells. Moreover, T cell-derived PlGF was required for the progression of autoimmune diseases associated with T_H17 differentiation, including experimental autoimmune encephalomyelitis and collagen-induced arthritis, in mice. Collectively, our findings provide insights into the PlGF-dictated links among angiogenesis, T_H17 cell development and autoimmunity.

Hypoxia activates placental growth factor expression in lymphatic endothelial cells

Placental growth factor (PlGF), a proangiogenic member of vascular endothelial growth family, is active during pathological conditions like cancer, metastasis formation and hind limb ischemia and in wound healing. Endothelial cells express PlGF and hypoxia positively modulates in vitro its expression. To verify whether hypoxia modulates PlGF expression in different cellular contexts and in vivo, we first analyzed five human and five mouse cancer cell lines showing that in eight of them hypoxia positively modulates PlGF. Next, we analyzed xenograft colorectal cancer tumors showing that human cancer cells were able to express PlGF in hypoxic area of the tumor. Surprisingly, we did not visualize mouse PlGF in CD31 positive tumor vessels, but in low CD31 positive vessels, a characteristic of lymphatic vessels. We found that hypoxia effectively activates PlGF expression in lymphatic endothelial cells as well as in LYVE1 positive tumor vessels. We also investigated two additional mouse angiogenic models, hind limb ischemia and wound healing, and we confirmed that lymphatic vessels of both ischemic muscles and skin express PlGF. These results show for the first time that hypoxia activates PlGF expression in lymphatic endothelial cells, which have to be considered an additional source for PlGF production in pathological contexts.

Placental growth factor deficiency is associated with impaired cerebral vascular development in mice

STUDY HYPOTHESIS

Placental growth factor (PGF) is expressed in the developing mouse brain and contributes to vascularization and vessel patterning.

STUDY FINDING

PGF is dynamically expressed in fetal mouse brain, particularly forebrain, and is essential for normal cerebrovascular development.

WHAT IS KNOWN ALREADY

PGF rises in maternal plasma over normal human and mouse pregnancy but is low in many women with the acute onset hypertensive syndrome, pre-eclampsia (PE). Little is known about the expression of PGF in the fetus during PE. Pgf  (-/-) mice appear normal but recently cerebral vascular defects were documented in adult Pgf  (-/-) mice.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Here, temporal-spatial expression of PGF is mapped in normal fetal mouse brains and cerebral vasculature development is compared between normal and congenic Pgf  (-/-) fetuses to assess the actions of PGF during cerebrovascular development. Pgf/PGF, Vegfa/VEGF, Vegf receptor (Vegfr)1 and Vegfr2 expression were examined in the brains of embryonic day (E)12.5, 14.5, 16.5 and 18.5 C57BL/6 (B6) mice using quantitative PCR and immunohistochemistry. The cerebral vasculature was compared between Pgf  (-/-) and B6 embryonic and adult brains using whole mount techniques. Vulnerability to cerebral ischemia was investigated using a left common carotid ligation assay.

MAIN RESULTS AND THE ROLE OF CHANCE

Pgf/PGF and Vegfr1 are highly expressed in E12.5-14.5 forebrain relative to VEGF and Vegfr2. Vegfa/VEGF is relatively more abundant in hindbrain (HB). PGF and VEGF expression were similar in midbrain. Delayed HB vascularization was seen at E10.5 and 11.5 in Pgf  (-/-) brains. At E14.5, Pgf  (-/-) circle of Willis showed unilateral hypoplasia and fewer collateral vessels, defects that persisted post-natally. Functionally, adult Pgf  (-/-) mice experienced cerebral ischemia after left common carotid arterial occlusion while B6 mice did not.

LIMITATIONS, REASONS FOR CAUTION

Since Pgf  (-/-) mice were used, consequences of complete absence of maternal and fetal PGF were defined. Therefore, the effects of maternal versus fetal PGF deficiency on cerebrovascular development cannot be separated. However, as PGF was strongly expressed in the developing brain at all timepoints, we suggest that local PGF has a more important role than distant maternal or placental sources. Full PGF loss is not expected in PE pregnancies, predicting that the effects of PGF deficiency identified in this model will be more severe than any effects in PE-offspring.

WIDER IMPLICATIONS OF THE FINDINGS

These studies provoke the question of whether PGF expression is decreased and cerebral vascular maldevelopment occurs in fetuses who experience a preeclamptic gestation. These individuals have already been reported to have elevated risk for stroke and cognitive impairments.

LARGE SCALE DATA

N/A.

STUDY FUNDING AND COMPETING INTERESTS

This work was supported by awards from the Natural Sciences and Engineering Research Council, the Canada Research Chairs Program and the Canadian Foundation for Innovation to B.A.C. and by training awards from the Universidade Federal de Pernambuco and Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), Brazil to R.L.L.; Queen's University to V.R.K. and the Canadian Institutes of Health Research to M.T.R. The work of P.C. is supported by the Belgian Science Policy BELSPO-IUAP7/03, Structural funding by the Flemish Government-Methusalem funding, and the Flemish Science Fund-FWO grants. There were no competing interests.

Placental growth factor inhibition modulates the interplay between hypoxia and unfolded protein response in hepatocellular carcinoma

Background

Hepatocellular carcinoma (HCC) is a leading cause of cancer-related mortality. We previously showed that the inhibition of placental growth factor (PlGF) exerts antitumour effects and induces vessel normalisation, possibly reducing hypoxia. However, the exact mechanism underlying these effects remains unclear. Because hypoxia and endoplasmic reticulum stress, which activates the unfolded protein response (UPR), have been implicated in HCC progression, we assessed the interactions between PlGF and these microenvironmental stresses.

Methods

PlGF knockout mice and validated monoclonal anti-PlGF antibodies were used in a diethylnitrosamine-induced mouse model for HCC. We examined the interactions among hypoxia, UPR activation and PlGF induction in HCC cells.

Results

Both the genetic and pharmacological inhibitions of PlGF reduced the chaperone levels and the activation of the PKR-like endoplasmic reticulum kinase (PERK) pathway of the UPR in diethylnitrosamine-induced HCC. Furthermore, we identified that tumour hypoxia was attenuated, as shown by reduced pimonidazole binding. Interestingly, hypoxic exposure markedly activated the PERK pathway in HCC cells in vitro, suggesting that PlGF inhibition may diminish PERK activation by improving oxygen delivery. We also found that PlGF expression is upregulated by different chemical UPR inducers via activation of the inositol-requiring enzyme 1 pathway in HCC cells.

Conclusions

PlGF inhibition attenuates PERK activation, likely by tempering hypoxia in HCC via vessel normalisation. The UPR, in turn, is able to regulate PlGF expression, suggesting the existence of a feedback mechanism for hypoxia-mediated UPR that promotes the expression of the angiogenic factor PlGF. These findings have important implications for our understanding of the effect of therapies normalising tumour vasculature.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-015-1990-6) contains supplementary material, which is available to authorized users.

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

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

Vascular endothelial growth factors: multitasking functionality in metabolism, health and disease

Vascular endothelial growth factors (VEGFs) bind to VEGF receptor tyrosine kinases (VEGFRs). The VEGF and VEGFR gene products regulate diverse regulatory pathways in mammalian development, health and disease. The interaction between a particular VEGF and its cognate VEGFR activates multiple signal transduction pathways which regulate different cellular responses including metabolism, gene expression, proliferation, migration, and survival. The family of VEGF isoforms regulate vascular physiology and promote tissue homeostasis. VEGF dysfunction is implicated in major chronic disease states including atherosclerosis, diabetes, and cancer. More recent studies implicate a strong link between response to VEGF and regulation of vascular metabolism. Understanding how this family of multitasking cytokines regulates cell and animal function has implications for treating many different diseases.

Inhibition of Hedgehog signalling by NVP-LDE225 (Erismodegib) interferes with growth and invasion of human renal cell carcinoma cells

Background:

Multiple lines of evidence support that the Hedgehog (Hh) signalling has a role in the maintenance and progression of different human cancers. Therefore, inhibition of the Hh pathway represents a valid anticancer therapeutic approach for renal cell carcinoma (RCC) patients. NVP-LDE225 is a Smoothened (Smo) antagonist that induces dose-related inhibition of Hh and Smo-dependent tumour growth.

Methods:

We assayed the effects of NVP-LDE225 alone or in combination with everolimus or sunitinib on the growth and invasion of human RCC models both in vitro and in vivo. To this aim, we used a panel of human RCC models, comprising cells with acquired resistance to sunitinib – a multiple tyrosine kinase inhibitor approved as a first-line treatment for RCC.

Results:

NVP-LDE225 cooperated with either everolimus or sunitinib to inhibit proliferation, migration, and invasion of RCC cells even in sunitinib-resistant (SuR) cells. Some major transducers involved in tumour cell motility, including paxillin, were also efficiently inhibited by the combination therapy, as demonstrated by western blot and confocal microscopy assays. Moreover, these combined treatments inhibited tumour growth and increased animal survival in nude mice xenografted with SuR RCC cells. Finally, lung micrometastasis formation was reduced when mice were treated with NVP-LDE225 plus everolimus or sunitinib, as evidenced by artificial metastatic assays.

Conclusions:

Hedgehog inhibition by NVP-LDE225 plus sunitinib or everolimus bolsters antitumour activity by interfering with tumour growth and metastatic spread, even in SuR cells. Thus, this new evidence puts forward a new promising therapeutic approach for RCC patients.

Epigenetic control of hypoxia inducible factor-1α-dependent expression of placental growth factor in hypoxic conditions

Hypoxia plays a crucial role in the angiogenic switch, modulating a large set of genes mainly through the activation of hypoxia-inducible factor (HIF) transcriptional complex. Endothelial cells play a central role in new vessels formation and express placental growth factor (PlGF), a member of vascular endothelial growth factor (VEGF) family, mainly involved in pathological angiogenesis. Despite several observations suggest a hypoxia-mediated positive modulation of PlGF, the molecular mechanism governing this regulation has not been fully elucidated. We decided to investigate if epigenetic modifications are involved in hypoxia-induced PlGF expression. We report that PlGF expression was induced in cultured human and mouse endothelial cells exposed to hypoxia (1% O 2), although DNA methylation at the Plgf CpG-island remains unchanged. Remarkably, robust hyperacetylation of histones H3 and H4 was observed in the second intron of Plgf, where hypoxia responsive elements (HREs), never described before, are located. HIF-1α, but not HIF-2α, binds to identified HREs. Noteworthy, only HIF-1α silencing fully inhibited PlGF upregulation. These results formally demonstrate a direct involvement of HIF-1α in the upregulation of PlGF expression in hypoxia through chromatin remodeling of HREs sites. Therefore, PlGF may be considered one of the putative targets of anti-HIF therapeutic applications.

Inhibition of choroidal and corneal pathologic neovascularization by Plgf1-de gene transfer

PURPOSE

Ocular neovascularization (NV), the primary cause of blindness, typically is treated via inhibition of VEGF-A activity. However, besides VEGF-A, other proteins of the same family, including VEGF-B and placental growth factor (PlGF, all together VEGFs), have a crucial role in the angiogenesis process. PlGF and VEGF, which form heterodimers if co-expressed, both are required for pathologic angiogenesis. We generated a PlGF1 variant, named PlGF1-DE, which is unable to bind and activate VEGFR-1, but retains the ability to form heterodimer. PlGF1-DE acts as dominant negative of VEGF-A and PlGF1wt through heterodimerization mechanism. The purpose of our study was to explore the therapeutic potential of Plgf1-de gene in choroid and cornea NV context.

METHODS

In the model of laser-induced choroidal neovascularization (CNV), Plgf1-de gene, and as control Plgf1wt, LacZ, or gfp genes, were delivered using adeno-associated virus (AAV) vector by subretinal injection 14 days before the injury. After 7 days CNV volume was assessed. Corneal NV was induced by scrape or suture procedures. Expression vectors for PlGF1wt or PlGF1-DE, and as control the empty vector pCDNA3, were injected in the mouse cornea after the vascularization insults. NV was evaluated with CD31 and LYVE-1 immunostaining.

RESULTS

The expression of Plgf1-de induced significant inhibition of choroidal and corneal NV by reducing VEGF-A homodimer production. Conversely, the delivery of Plgf1wt, despite induced similar reduction of VEGF-A production, did not affect NV.

CONCLUSIONS

Plgf1-de gene is a new therapeutic tool for the inhibition of VEGFs driven ocular NV.

Inhibition of ocular neovascularization by a novel peptide derived from human placenta growth factor-1

PURPOSE

To evaluate the effect of ZY1, a novel 21-amino acid peptide from human placenta growth factor-1 (PlGF-1), against ocular neovascularization, and to study its possible toxicity to the retina and the underlying mechanism of antiangiogenic effect.

METHODS

MTS assays, a modified Boyden chamber and Matrigel(™) were used to evaluate the effect of ZY1 on the proliferation, migration and tube formation of RF/6A rhesus macaque choroid-retina endothelial cells induced by vascular endothelial growth factor (VEGF) in vitro. The antiangiogenic effect of ZY1 was also studied with corneal micropocket angiogenesis assays and oxygen-induced retinopathy (OIR) assays in mice. Electrophysiological tests and histological examinations were used to study the possible toxicity of ZY1 against mouse neuroretina. Competitive ELISA and Western blotting were performed to elucidate the underlying mechanism of ZY1.

RESULTS

ZY1 inhibited VEGF-induced RF/6A proliferation, migration and tube formation. It also inhibited ocular neovascularization when applied to the corneal micropocket angiogenesis assays and OIR assays in mice. Electrophysiological tests and histological examinations revealed no evident functional or morphologic abnormalities in mouse neuroretina after ZY1 injection. ZY1 competed for binding to VEGFR-1 against PlGF and VEGF and inhibited VEGFR-1/ERK/AKT activation.

CONCLUSION

 It is concluded that the novel peptide ZY1 is an effective inhibitor of ocular pathologic angiogenesis and may provide a promising alternative for ocular antiangiogenic therapy.

Elevated vascular endothelial growth factor receptor-2 abundance contributes to increased angiogenesis in vascular endothelial growth factor receptor-1-deficient mice

BACKGROUND

Vascular endothelial growth factor receptor-1 (VEGFR-1/Flt-1) is a potential therapeutic target for cardiovascular diseases, but its role in angiogenesis remains controversial. Whereas germline Vegfr-1(-/-) embryos die of abnormal vascular development in association with excessive endothelial differentiation, mice lacking only the kinase domain appear healthy.

METHODS AND RESULTS

We performed Cre-loxP-mediated knockout to abrogate the expression of all known VEGFR-1 functional domains in neonatal and adult mice and analyzed developmental, pathophysiological, and molecular consequences. VEGFR-1 deficiency promoted tip cell formation and endothelial cell proliferation and facilitated angiogenesis of blood vessels that matured and perfused properly. Vascular permeability was normal at the basal level but elevated in response to high doses of exogenous VEGF-A. In the postinfarct ischemic cardiomyopathy model, VEGFR-1 deficiency supported robust angiogenesis and protected against myocardial infarction. VEGFR-1 knockout led to abundant accumulation of VEGFR-2 at the protein level, increased VEGFR-2 tyrosine phosphorylation transiently, and enhanced serine phosphorylation of Akt and ERK. Interestingly, increased angiogenesis, tip cell formation, vascular permeability, VEGFR-2 accumulation, and Akt phosphorylation could be partially rescued or suppressed by one or more of the following manipulations, including injection of the VEGFR-2 selective inhibitor SU1498, anti-VEGF-A, or introduction of Vegfr-2(+/-) heterozygosity into Vegfr-1 somatic knockout mice.

CONCLUSIONS

Upregulation of VEGFR-2 abundance at the protein level contributes in part to increased angiogenesis in VEGFR-1-deficient mice.

The discovery of placenta growth factor and its biological activity

Angiogenesis is a complex biological phenomenon crucial for a correct embryonic development and for post-natal growth. In adult life, it is a tightly regulated process confined to the uterus and ovary during the different phases of the menstrual cycle and to the heart and skeletal muscles after prolonged and sustained physical exercise. Conversly, angiogenesis is one of the major pathological changes associated with several complex diseases like cancer, atherosclerosis, arthritis, diabetic retinopathy and age-related macular degeneration. Among the several molecular players involved in angiogenesis, some members of VEGF family, VEGF-A, VEGF-B and placenta growth factor (PlGF), and the related receptors VEGF receptor 1 (VEGFR-1, also known as Flt-1) and VEGF receptor 2 (VEGFR-2, also known as Flk-1 in mice and KDR in human) have a decisive role. In this review, we describe the discovery and molecular characteristics of PlGF, and discuss the biological role of this growth factor in physiological and pathological conditions.

The biflavonoid amentoflavone inhibits neovascularization preventing the activity of proangiogenic vascular endothelial growth factors

The proangiogenic members of VEGF family and related receptors play a central role in the modulation of pathological angiogenesis. Recent insights indicate that, due to the strict biochemical and functional relationship between VEGFs and related receptors, the development of a new generation of agents able to target contemporarily more than one member of VEGFs might amplify the antiangiogenic response representing an advantage in term of therapeutic outcome. To identify molecules that are able to prevent the interaction of VEGFs with related receptors, we have screened small molecule collections consisting of >100 plant extracts. Here, we report the isolation and identification from an extract of the Malian plant Chrozophora senegalensis of the biflavonoid amentoflavone as an antiangiogenic bioactive molecule. Amentoflavone can to bind VEGFs preventing the interaction and phosphorylation of VEGF receptor 1 and 2 (VEGFR-1,VEGFR-2) and to inhibit endothelial cell migration and capillary-like tube formation induced by VEGF-A or placental growth factor 1 (PlGF-1) at low μm concentration. In vivo, amentoflavone is able to inhibit VEGF-A-induced chorioallantoic membrane neovascularization as well as tumor growth and associated neovascularization, as assessed in orthotropic melanoma and xenograft colon carcinoma models. In addition structural studies performed on the amentoflavone·PlGF-1 complex have provided evidence that this biflavonoid effectively interacts with the growth factor area crucial for VEGFR-1 receptor recognition. In conclusion, our results demonstrate that amentoflavone represents an interesting new antiangiogenic molecule that is able to prevent the activity of proangiogenic VEGF family members and that the biflavonoid structure is a new chemical scaffold to develop powerful new antiangiogenic molecules.

Biochemical and structural analysis of the binding determinants of a vascular endothelial growth factor receptor peptidic antagonist

Cyclic peptide antagonist c[YYDEGLEE]-NH(2), which disrupts the interaction between vascular endothelial growth factor (VEGF) and its receptors (VEGFRs), represents a promising tool in the fight against cancer and age-related macular degeneration. Furthermore, coupled to a cyclen derivative, this ligand could be used as a medicinal imaging agent. Nevertheless, before generating such molecular probes, some preliminary studies need to be undertaken in order to define the more suitable positions for introduction of the cyclen macrocycle. Through an Ala-scan study on this peptide, we identified its binding motif, and an NMR study highlights its binding sites on the VEGFR-1D2 Ig-like domain. Guided by the structural relationship results deduced from the effect of the peptides on endothelial cells, new peptides were synthesized and grafted on beads. Used in a pull-down assay, these new peptides trap the VEGFRs, thus confirming that the identified amino acid positions are suitable for further derivatization.

Angiotensin II upregulates the expression of placental growth factor in human vascular endothelial cells and smooth muscle cells

Background

Atherosclerosis is now recognized as a chronic inflammatory disease. Angiotensin II (Ang II) is a critical factor in inflammatory responses, which promotes the pathogenesis of atherosclerosis. Placental growth factor (PlGF) is a member of the vascular endothelial growth factor (VEGF) family cytokines and is associated with inflammatory progress of atherosclerosis. However, the potential link between PlGF and Ang II has not been investigated. In the current study, whether Ang II could regulate PlGF expression, and the effect of PlGF on cell proliferation, was investigated in human vascular endothelial cells (VECs) and smooth muscle cells (VSMCs).

Results

In growth-arrested human VECs and VSMCs, Ang II induced PlGF mRNA expression after 4 hour treatment, and peaked at 24 hours. 10^-6 mol/L Ang II increased PlGF protein production after 8 hour treatment, and peaked at 24 hours. Stimulation with Ang II also induced mRNA expression of VEGF receptor-1 and -2(VEGFR-1 and -2) in these cells. The Ang II type I receptor (AT₁R) antagonist blocked Ang II-induced PlGF gene expression and protein production. Several intracellular signals elicited by Ang II were involved in PlGF synthesis, including activation of protein kinase C, extracellular signal-regulated kinase 1/2 (ERK1/2) and PI3-kinase. A neutralizing antibody against PlGF partially inhibited the Ang II-induced proliferation of VECs and VSMCs. However, this antibody showed little effect on the basal proliferation in these cells, whereas blocking antibody of VEGF could suppress both basal and Ang II-induced proliferation in VECs and VSMCs.

Conclusion

Our results showed for the first time that Ang II could induce the gene expression and protein production of PlGF in VECs and VSMCs, which might play an important role in the pathogenesis of vascular inflammation and atherosclerosis.

Anti-neuropilin-1 peptide inhibition of synoviocyte survival, angiogenesis, and experimental arthritis

OBJECTIVE

To delineate the role of neuropilin-1 (NP-1), a vascular endothelial growth factor receptor (VEGFR), in rheumatoid inflammation and to determine whether blockade of NP-1 could suppress synoviocyte survival and angiogenesis.

METHODS

VEGF(111-165) peptide, which encompasses the NP-1 binding domain of VEGF(165), was generated by cleaving VEGF(165) with plasmin. The effect of this peptide on the interaction between VEGF(165) and its receptor was determined by (125)I-VEGFR binding assay. Assays to determine synoviocyte apoptosis, adhesion, and migration were performed in the presence of VEGF(165) and/or the peptide. VEGF(165)-induced angiogenesis was assessed by measuring the proliferation, tube formation, and wounding migration of endothelial cells (ECs). Mice were immunized with type II collagen to induce experimental arthritis.

RESULTS

VEGF(111-165) peptide specifically inhibited the binding of (125)I-VEGF(165) to NP-1 on rheumatoid synoviocytes and ECs. The peptide eliminated the VEGF(165)-mediated increase in synoviocyte survival and activation of p-ERK and Bcl-2. The peptide also completely inhibited a VEGF(165)-induced increase in synoviocyte adhesion and migration. In addition, the anti-NP-1 peptide blocked VEGF(165)-stimulated proliferation, capillary tube formation, and wounding migration of ECs in vitro. VEGF(165)-induced neovascularization in a Matrigel plug in mice was also blocked by treatment with the peptide. Finally, subcutaneous injection of anti-NP-1 peptide suppressed arthritis severity and autoantibody formation in mice with experimental arthritis and inhibited synoviocyte hyperplasia and angiogenesis in arthritic joints.

CONCLUSION

Anti-NP-1 peptide suppressed VEGF(165)-induced increases in synoviocyte survival and angiogenesis, and thereby blocked experimental arthritis. Our findings suggest that anti-NP-1 peptide could be useful in alleviating chronic arthritis.

A placental growth factor variant unable to recognize vascular endothelial growth factor (VEGF) receptor-1 inhibits VEGF-dependent tumor angiogenesis via heterodimerization

Angiogenesis is one of the crucial events for cancer development and growth. Two members of the vascular endothelial growth factor (VEGF) family, VEGF-A and placental growth factor (PlGF), which are able to heterodimerize if coexpressed in the same cell, are both required for pathologic angiogenesis. We have generated a PlGF1 variant, named PlGF1-DE in which the residues Asp72 and Glu73 were substituted with Ala, which is unable to bind and activate VEGF receptor-1 but is still able to heterodimerize with VEGF. Here, we show that overexpression in tumor cells by adenoviral delivery or stable transfection of PlGF1-DE variant significantly reduces the production of VEGF homodimer via heterodimerization, determining a strong inhibition of xenograft tumor growth and neoangiogenesis, as well as significant reduction of vessel lumen and stabilization, and monocyte-macrophage infiltration. Conversely, the overexpression of PlGF1wt, also reducing the VEGF homodimer production comparably with PlGF1-DE variant through the generation of VEGF/PlGF heterodimer, does not inhibit tumor growth and vessel density compared with controls but induces increase of vessel lumen, vessel stabilization, and monocyte-macrophage infiltration. The property of PlGF and VEGF-A to generate heterodimer represents a successful strategy to inhibit VEGF-dependent angiogenesis. The PlGF1-DE variant, and not PlGF1wt as previously reported, acts as a "dominant negative" of VEGF and is a new candidate for antiangiogenic gene therapy in cancer treatment.

Structure-function analysis of VEGF receptor activation and the role of coreceptors in angiogenic signaling

Vascular endothelial growth factors (VEGFs) constitute a family of six polypeptides, VEGF-A, -B, -C, -D, -E and PlGF, that regulate blood and lymphatic vessel development. VEGFs specifically bind to three type V receptor tyrosine kinases (RTKs), VEGFR-1, -2 and -3, and to coreceptors such as neuropilins and heparan sulfate proteoglycans (HSPG). VEGFRs are activated upon ligand-induced dimerization mediated by the extracellular domain (ECD). A study using receptor constructs carrying artificial dimerization-promoting transmembrane domains (TMDs) showed that receptor dimerization is necessary, but not sufficient, for receptor activation and demonstrates that distinct orientation of receptor monomers is required to instigate transmembrane signaling. Angiogenic signaling by VEGF receptors also depends on cooperation with specific coreceptors such as neuropilins and HSPG. A number of VEGF isoforms differ in binding to coreceptors, and ligand-specific signal output is apparently the result of the specific coreceptor complex assembled by a particular VEGF isoform. Here we discuss the structural features of VEGF family ligands and their receptors in relation to their distinct signal output and angiogenic potential.

Modulation of angiogenesis by a tetrameric tripeptide that antagonizes vascular endothelial growth factor receptor 1

Vascular endothelial growth factor receptor-1 (VEGFR-1, also known as Flt-1) is involved in complex biological processes often associated to severe pathological conditions like cancer, inflammation, and metastasis formation. Consequently, the search for antagonists of Flt-1 has recently gained a growing interest. Here we report the identification of a tetrameric tripeptide from a combinatorial peptide library built using non-natural amino acids, which binds Flt-1 and inhibits in vitro its interaction with placental growth factor (PlGF) and vascular endothelial growth factor (VEGF) A and B (IC(50) approximately 10 microm). The peptide is stable in serum for 7 days and prevents both Flt-1 phosphorylation and the capillary-like tube formation of human primary endothelial cells stimulated by PlGF or VEGF-A. Conversely, the identified peptide does not interfere in VEGF-induced VEGFR-2 activation. In vivo, this peptide inhibits VEGF-A- and PlGF-induced neoangiogenesis in the chicken embryo chorioallantoic membrane assay. In contrast, in the cornea, where avascularity is maintained by high levels of expression of the soluble form of Flt-1 receptor (sFlt-1) that prevents the VEGF-A activity, the peptide is able to stimulate corneal mouse neovascularization in physiological condition, as reported previously for others neutralizing anti-Flt-1 molecules. This tetrameric tripeptide represents a new, promising compound for therapeutic approaches in pathologies where Flt-1 activation plays a crucial role.

Enzastaurin inhibits tumours sensitive and resistant to anti-EGFR drugs

We investigated the antitumour effect and ability to overcome the resistance to anti-EGFR drugs of enzastaurin, an inhibitor of VEGFR-dependent PKCβ signalling. Enzastaurin was evaluated alone and in combination with the EGFR inhibitor gefitinib, on growth and signalling protein expression in human cancer cells sensitive and resistant to anti-EGFR drugs, both in vitro and in nude mice. We demonstrated the marked inhibitory activity of enzastaurin against GEO colon and PC3 prostate cancer cells and their gefitinib-resistant counterparts GEO-GR and PC3-GR, accompanied by inhibition of pAkt and its effector pp70S6K, pGSK3β and VEGF expression and secretion. Moreover, enzastaurin showed a cooperative effect with gefitinib in parental and in gefitinib-resistant cells. Remarkably, these results were confirmed in vivo, where enzastaurin showed antitumour activity and cooperativity with gefitinib in mice grafted with GEO and GEO-GR tumours, incrementing their median survival and inhibiting the aforesaid protein expression and secretion in tumour specimens. In conclusion, enzastaurin by interfering with signalling proteins implicated in EGFR drug resistance markedly cooperates with gefitinib in sensitive and gefitinib-resistant tumours, thus overcoming and reverting such resistance and providing a rational basis for its development in patients resistant to anti-EGFR drugs.

Vascular endothelial growth factor receptor-1 contributes to resistance to anti-epidermal growth factor receptor drugs in human cancer cells

PURPOSE

The resistance to selective EGFR inhibitors involves the activation of alternative signaling pathways, and Akt activation and VEGF induction have been described in EGFR inhibitor-resistant tumors. Combined inhibition of EGFR and other signaling proteins has become a successful therapeutic approach, stimulating the search for further determinants of resistance as basis for novel therapeutic strategies.

EXPERIMENTAL DESIGN

We established human cancer cell lines with various degrees of EGFR expression and sensitivity to EGFR inhibitors and analyzed signal transducers under the control of EGFR-dependent and EGFR-independent pathways.

RESULTS

Multitargeted inhibitor vandetanib (ZD6474) inhibited the growth and the phosphorylation of Akt and its effector p70S6 kinase in both wild-type and EGFR inhibitor-resistant human colon, prostate, and breast cancer cells. We found that the resistant cell lines exhibit, as common feature, VEGFR-1/Flt-1 overexpression, increased secretion of VEGF and placental growth factor, and augmented migration capabilities and that vandetanib is able to antagonize them. Accordingly, a new kinase assay revealed that in addition to VEGF receptor (VEGFR)-2, RET, and EGFR, vandetanib efficiently inhibits also VEGFR-1. The contribution of VEGFR-1 to the resistant phenotype was further supported by the demonstration that VEGFR-1 silencing in resistant cells restored sensitivity to anti-EGFR drugs and impaired migration capabilities, whereas exogenous VEGFR-1 overexpression in wild-type cells conferred resistance to these agents.

CONCLUSIONS

This study shows that VEGFR-1 contributes to anti-EGFR drug resistance in different human cancer cells. Moreover, vandetanib inhibits VEGFR-1 activation, cell proliferation, and migration, suggesting its potential utility in patients resistant to EGFR inhibitors.

Vascular Endothelial Growth Factor Receptor-1 Regulates Postnatal Angiogenesis Through Inhibition of the Excessive Activation of Akt

Vascular endothelial growth factor (VEGF) binds both VEGF receptor-1 (VEGFR-1) and VEGF receptor-2 (VEGFR-2). Activation of VEGFR-2 is thought to play a major role in the regulation of endothelial function by VEGF. Recently, specific ligands for VEGFR-1 have been reported to have beneficial effects when used to treat ischemic diseases. However, the role of VEGFR-1 in angiogenesis is not fully understood. In this study, we showed that VEGFR-1 performs “fine tuning” of VEGF signaling to induce neovascularization. We examined the effects of retroviral vectors expressing a small interference RNA that targeted either the VEGFR-1 gene or the VEGFR-2 gene. Deletion of either VEGFR-1 or VEGFR-2 reduced the ability of endothelial cells to form capillaries. Deletion of VEGFR-1 markedly reduced endothelial cell proliferation and induced premature senescence of endothelial cells. In contrast, deletion of VEGFR-2 significantly impaired endothelial cell survival. When VEGFR-1 expression was blocked, VEGF constitutively activated Akt signals and thus induced endothelial cell senescence via a p53-dependent pathway. VEGFR-1 +/− mice exhibited an increase of endothelial Akt activity and showed an impaired neovascularization in response to ischemia, and this impairment was ameliorated in VEGFR-1 +/− Akt1 +/− mice. These results suggest that VEGFR-1 plays a critical role in the maintenance of endothelial integrity by modulating the VEGF/Akt signaling pathway.

Orf virus VEGF-E NZ2 promotes paracellular NRP-1/VEGFR-2 coreceptor assembly via the peptide RPPR

Vascular endothelial growth factors (VEGFs) interact with the receptor tyrosine kinases (RTKs) VEGFR-1, -2, and -3; neuropilins (NRPs); and heparan sulfate (HS) proteoglycans. VEGF RTKs signal to downstream targets upon ligand-induced tyrosine phosphorylation, while NRPs and HS act as coreceptors that lack enzymatic activity yet modulate signal output by VEGF RTKs. VEGFs exist in various isoforms with distinct receptor specificity and biological activity. Here, a series of mammalian VEGF-A splice variants and orf virus VEGF-Es, as well as chimeric and mutant VEGF variants, were characterized to determine the motifs required for binding to NRP-1 in the absence (VEGF-E) or presence (VEGF-A(165)) of an HS-binding sequence. We identified the carboxyterminal peptides RPPR and DKPRR as the NRP-1 binding motifs of VEGF-E and VEGF-A, respectively. RPPR had significantly higher affinity for NRP-1 than DKPRR. VEGFs containing an RPPR motif promoted HS-independent coreceptor complex assembly between VEGFR-2 and NRP-1, independent of whether these receptors were expressed on the same or separate cells grown in cocultures. Functional studies showed that stable coreceptor assembly by VEGF correlated with its ability to promote vessel formation in an embryoid body angiogenesis assay.

Spectrum of activity and mechanism of action of VEGF/PDGF inhibitors

BACKGROUND

Angiogenesis plays an important role in tumor growth and metastasis.

METHODS

We review the function of the vascular endothelial growth factor (VEGF) in vessel formation that is complemented by platelet-derived growth factor (PDGF). We also review the agents designed to target VEGF, PDGF, and/or their receptors.

RESULTS

VEGF plays a central role in tumor angiogenesis. It is expressed at increased levels in colorectal, liver, lung, thyroid, breast, as well as in bladder, ovary, uterine cancers, and in angiosarcomas, germ cell tumors, intracranial tumors, and others. VEGF blockade has been shown to have a direct and rapid antivascular effect in both animal and human tumors, through deprivation of tumor vascular supply and inhibition of endothelial proliferation. Overexpression of PDGFs and their receptors has also been reported in many types of cancers such as prostate, ovarian, and non-small-cell lung cancer. Many VEGF and PDGF inhibitors are available. The use of some of these inhibitors has significantly improved the survival of cancer patients. Several agents are in development and currently are being tested in clinical trials.

CONCLUSIONS

Angiogenic agents inhibiting VEGF and PDGF have shown promising clinical results. Targeting more than one pathway by combining different agents may increase the antitumor activity of these drugs. The implementation of reliable radiologic and pathologic angiogenesis monitoring techniques is necessary to implement antiangiogenic therapies in cancer.

TLR9 agonist acts by different mechanisms synergizing with bevacizumab in sensitive and cetuximab-resistant colon cancer xenografts

Synthetic agonists of Toll-like receptor 9 (TLR9), a class of agents that induce specific immune response, exhibit antitumor activity and are currently being investigated in cancer patients. Intriguingly, their mechanisms of action on tumor growth and angiogenesis are still incompletely understood. We recently discovered that a synthetic agonist of TLR9, immune modulatory oligonucleotide (IMO), acts by impairing epidermal growth factor receptor (EGFR) signaling and potently synergizes with anti-EGFR antibody cetuximab in GEO human colon cancer xenografts, whereas it is ineffective in VEGF-overexpressing cetuximab-resistant GEO cetuximab-resistant (GEO-CR) tumors. VEGF is activated by EGFR, and its overexpression causes resistance to EGFR inhibitors. Therefore, we used IMO and the anti-VEGF antibody bevacizumab as tools to study IMO's role on EGFR and angiogenesis and to explore its therapeutic potential in GEO, LS174T, and GEO-CR cancer xenografts. We found that IMO enhances the antibody-dependent cell-mediated cytotoxicity (ADCC) activity of cetuximab, that bevacizumab has no ADCC, and IMO is unable to enhance it. Nevertheless, the IMO-plus-bevacizumab combination synergistically inhibits the growth of GEO and LS174T as well as of GEO-CR tumors, preceded by inhibition of signaling protein expression, microvessel formation, and human, but not murine, VEGF secretion. Moreover, IMO inhibited the growth, adhesion, migration, and capillary formation of VEGF-stimulated endothelial cells. The antitumor activity was irrespective of the TLR9 expression on tumor cells. These studies demonstrate that synthetic agonists of TLR9 interfere with growth and angiogenesis also by EGFR- and ADCC-independent mechanisms affecting endothelial cell functions and provide a strong rationale to combine IMO with bevacizumab and EGFR inhibitory drugs in colon cancer patients.

Role of placenta growth factor in malignancy and evidence that an antagonistic PlGF/Flt-1 peptide inhibits the growth and metastasis of human breast cancer xenografts

The angiogenic growth factor placenta growth factor (PlGF) is implicated in several pathologic processes, including the growth and spread of cancer. We found by immunohistochemistry that 36% to 60% and 65% of primary breast cancers express PlGF and its receptor Flt-1, respectively. These findings suggest that PlGF may be active in tumor growth and metastasis beyond its role in angiogenesis. It was found that exogenously added PlGF (2 nmol/L), in contrast to vascular endothelial growth factor (2 nmol/L), significantly stimulated in vitro motility and invasion of the human breast tumor lines MCF-7 and MDA-MB-231. A PlGF-2/Flt-1-inhibiting peptide, binding peptide 1 (BP1), that binds Flt-1 at or near the heparin-binding site was identified and synthesized. Both PlGF-stimulated motility and invasion were prevented by treatment with BP1 (P < 0.05), as well as by anti-PlGF antibody. Treatment of mice bearing s.c. MDA-MB-231 with BP1 (200 mug i.p., twice per week) decreased the number of spontaneous metastatic lung nodules by 94% (P < 0.02), whereas therapy of animals with orthotopic mammary fat pad tumors decreased pulmonary metastases by 82% (P < 0.02). These results indicate, for the first time, that PlGF stimulates the metastatic phenotype in these breast cancer cells, whereas therapy with a PlGF-2/Flt-1 heparin-blocking peptide reduces the growth and metastasis of human breast cancer xenografts.

Molecular and functional diversity of vascular endothelial growth factors

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

Crystal Structure of the Orf Virus NZ2 Variant of Vascular Endothelial Growth Factor-E

Mammalian vascular endothelial growth factors constitute a family of polypeptides, vascular endothelial growth factor (VEGF)-A, -B, -C, -D and placenta growth factor (PlGF), that regulate blood and lymphatic vessel development. VEGFs bind to three types of receptor tyrosine kinases, VEGF receptors 1, 2, and 3, that are predominantly expressed on endothelial and some hematopoietic cells. Pox viruses of the Orf family encode highly related proteins called VEGF-E that show only 25-35% amino acid identity with VEGF-A but bind with comparable affinity to VEGFR-2. The crystal structure of VEGF-E NZ2 described here reveals high similarity to the known structural homologs VEGF-A, PlGF, and the snake venoms Vammin and VR-1, which are all homodimers and contain the characteristic cysteine knot motif. Distinct conformational differences are observed in loop L1 and particularly in L3, which contains a highly flexible GS-rich motif that differs from all other structural homologs. Based on our structure, we created chimeric proteins by exchanging selected segments in L1 and L3 with the corresponding sequences from PlGF. Single loop mutants did not bind to either receptor, whereas a VEGF-E mutant in which both L1 and L3 were replaced gained affinity for VEGFR-1, illustrating the possibility to engineer receptor-specific chimeric VEGF molecules. In addition, changing arginine 46 to isoleucine in L1 significantly increased the affinity of VEGF-E for both VEGF receptors.

Plgf−/−eNos−/−mice show defective angiogenesis associated with increased oxidative stress in response to tissue ischemia

Neo-angiogenesis is a complex phenomenon modulated by the concerted action of several molecular factors. We have generated a congenic line of knockout mice carrying null mutations of both placental growth factor (PlGF) and endothelial nitric oxide synthase (eNOS), two genes that play a pivotal role in the regulation of pathological angiogenesis. In the present study, we describe the phenotype of this new experimental animal model after surgically induced hind-limb ischemia. Plgf-/-, eNos-/-, Plgf-/- eNos-/-, and wild-type C57BL/6J mice were studied. Plgf-/- eNos-/- mice showed the most severe phenotype: self-amputation, and death occurred in up to 47% of the animals studied; in ischemic legs, capillary density was severely reduced; macrophage infiltration and oxidative stress increased as compared to the other groups of animals. These changes were associated with an up-regulation of both inducible NOS (iNOS) expression and vascular endothelial growth factor (VEGF) protein levels in ischemic limbs, and to an increased extent of protein nitration. Our results demonstrate that the deletion of these two genes, Plgf, which acts in synergism with VEGF, and eNos, a downstream mediator of VEGF, determines a significant change in the vascular response to an ischemic stimulus and that oxidative stress within the ischemic tissue represents a crucial factor to maintain tissue homeostasis.

The C. elegans pvf-1 gene encodes a PDGF/VEGF-like factor able to bind mammalian VEGF receptors and to induce angiogenesis

Members of the platelet-derived growth factor/vascular endothelial growth factor (PDGF/VEGF) family have been implicated in a variety of functions in vertebrates, especially angiogenesis. Here we identify and characterize a PDGF/VEGF-like factor (named PVF-1) from the nematode C. elegans. We show that PVF-1 has biochemical properties similar to vertebrate PDGF/VEGF growth factors. More important, PVF-1 binds to the human receptors VEGFR-1 (Flt-1) and VEGFR-2 (KDR) and is able to induce angiogenesis in two model systems derived from vertebrates. Our results highlight the widespread evolutionary conservation of this important class of growth factors and raise the possibility that C. elegans can provide a simple experimental system in which to investigate how these factors function.

The role of VEGF receptors in angiogenesis; complex partnerships

Vascular endothelial growth factors (VEGFs) regulate blood and lymphatic vessel development and homeostasis but also have profound effects on neural cells. VEGFs are predominantly produced by endothelial, hematopoietic and stromal cells in response to hypoxia and upon stimulation with growth factors such as transforming growth factors, interleukins or platelet-derived growth factor. VEGFs bind to three variants of type III receptor tyrosine kinases, VEGF receptor 1, 2 and 3. Each VEGF isoform binds to a particular subset of these receptors giving rise to the formation of receptor homo- and heterodimers that activate discrete signaling pathways. Signal specificity of VEGF receptors is further modulated upon recruitment of coreceptors, such as neuropilins, heparan sulfate, integrins or cadherins. Here we summarize the knowledge accumulated since the discovery of these proteins more than 20 years ago with the emphasis on the signaling pathways activated by VEGF receptors in endothelial cells during cell migration, growth and differentiation.

The vascular endothelial growth factor (VEGF)/VEGF receptor system and its role under physiological and pathological conditions

The VEGF (vascular endothelial growth factor) family and its receptors are essential regulators of angiogenesis and vascular permeability. Currently, the VEGF family consists of VEGF-A, PlGF (placenta growth factor), VEGF-B, VEGF-C, VEGF-D, VEGF-E and snake venom VEGF. VEGF-A has at least nine subtypes due to the alternative splicing of a single gene. Although the VEGF165 isoform plays a central role in vascular development, recent studies have demonstrated that each VEGF isoform plays distinct roles in vascular patterning and arterial development. VEGF-A binds to and activates two tyrosine kinase receptors, VEGFR (VEGF receptor)-1 and VEGFR-2. VEGFR-2 mediates most of the endothelial growth and survival signals, but VEGFR-1-mediated signalling plays important roles in pathological conditions such as cancer, ischaemia and inflammation. In solid tumours, VEGF-A and its receptor are involved in carcinogenesis, invasion and distant metastasis as well as tumour angiogenesis. VEGF-A also has a neuroprotective effect on hypoxic motor neurons, and is a modifier of ALS (amyotrophic lateral sclerosis). Recent progress in the molecular and biological understanding of the VEGF/VEGFR system provides us with novel and promising therapeutic strategies and target proteins for overcoming a variety of diseases.

Placenta growth factor is not required for exercise-induced angiogenesis

Angiogenesis is a tightly regulated process, both during development and adult life. Animal models with mutations in the genes coding for placental growth factor (PlGF), a member of vascular endothelial growth factor (VEGF) family, or the tyrosine kinase domain of the PlGF receptor (Flt-1) have revealed differences between normal physiological angiogenesis and pathological angiogenesis associated with conditions such as tumor growth, arthritis and atherosclerosis. In the present paper, we investigated the potential role of PlGF in regulating physiological angiogenesis by analyzing vascular changes in heart and skeletal muscles of wild-type and Plgf-/- mice following prolonged and sustained physical training. Sedentary Plgf-/- mice showed a reduced capillary density in both heart and skeletal muscles as compared to wild-type mice (P < 0.05). However, after a 6-week training period, heart/body weight ratio, citrate synthase activity, vessel density and capillary/myocyte ratio were significantly increased in both wild-type and Plgf-/- mice (all P < 0.05). At the same time intercapillary distance was significantly reduced. Finally, acute exercise was not associated with any change in PlGF protein level in the skeletal muscle. Our results demonstrate that PlGF is not necessary for exercise-training-induced angiogenesis. We thus suggest that the role of PlGF is confined to the selective regulation of angiogenesis only under pathological conditions.