Homologs of vascular endothelial growth factor are encoded by the poxvirus orf virus

Constitutive and inducible expression and regulation of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF), which was originally discovered as vascular permeability factor, is critical to human cancer angiogenesis through its potent functions as a stimulator of endothelial cell survival, mitogenesis, migration, differentiation and self-assembly, as well as vascular permeability, immunosuppression and mobilization of endothelial progenitor cells from the bone marrow into the peripheral circulation. Genetic alterations and a chaotic tumor microenvironment, such as hypoxia, acidosis, free radicals, and cytokines, are clearly attributed to numerous abnormalities in the expression and signaling of VEGF and its receptors. These perturbations confer a tremendous survival and growth advantage to vascular endothelial cells as manifested by exuberant tumor angiogenesis and a consequent malignant phenotype. Understanding the regulatory mechanisms of both inducible and constitutive VEGF expression will be crucial in designing effective therapeutic strategies targeting VEGF to control tumor growth and metastasis. In this review, molecular regulation of VEGF expression in tumor cells is discussed.

Vascular endothelial growth factor: basic science and clinical progress

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. Hypoxia has been shown to be a major inducer of VEGF gene transcription. The tyrosine kinases Flt-1 (VEGFR-1) and Flk-1/KDR (VEGFR-2) are high-affinity VEGF receptors. The role of VEGF in developmental angiogenesis is emphasized by the finding that loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF is critical also for reproductive and bone angiogenesis. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Anti-VEGF monoclonal antibodies and other VEGF inhibitors block the growth of several tumor cell lines in nude mice. Clinical trials with various VEGF inhibitors in a variety of malignancies are ongoing. Very recently, an anti-VEGF monoclonal antibody (bevacizumab; Avastin) has been approved by the Food and Drug Administration as a first-line treatment for metastatic colorectal cancer in combination with chemotherapy. Furthermore, VEGF is implicated in intraocular neovascularization associated with diabetic retinopathy and age-related macular degeneration.

Zoonotic poxvirus infections in humans

PURPOSE OF REVIEW

The 2003 USA monkeypox epidemic caused by imported African rodents, newly emergent poxvirus zoonoses in Brazil and the possible use of variola virus for biological warfare has led to renewed interest in poxviruses and anti-poxviral therapies. Increasing foreign travel and importation of exotic animal species increases the likelihood of poxvirus infections occurring outside their usual geographical range and diagnostic delay has important implications. The present review provides an overview of these rare zoonoses.

RECENT FINDINGS

Three genera of Poxviridae are known to cause human zoonoses: orthopoxviruses, parapoxviruses and yatapoxvirus. Most cases are occupational, sporadic and have few cutaneous lesions with low morbidity. The exception is monkeypox, similar to smallpox, with significant morbidity and childhood mortality. Molecular characterization using polymerase chain reaction (PCR) amplification and other methods provides accurate phylogenetic identification and suggests that a cowpox-like virus is the probable ancestor of variola and other zoonotic poxviruses. DNA genomic sequencing of the Brazilian Cantagalo and Araçatuba viruses shows a close relationship to vaccinia virus. Poxviruses have potential in cancer immunotherapy and their ability to evade host-cell immune responses may provide a basis for new antipoxvirus therapies. Other agents, particularly nucleoside phosphonates such as cidofovir, show therapeutic action against poxviruses.

SUMMARY

Human zoonotic poxvirus infections are rare but increasingly encountered outside their usual geographical range. The 2003 USA monkeypox outbreak emphasizes the importance of early accurate diagnosis, particularly because increasing numbers of immunosuppressed individuals increases the potential for severe or fatal infections. PCR methodology enables accurate phylogenetic typing and has identified new diseases, but rapid, reliable methods must be made available for clinicians. More research into therapeutic agents for the prevention and treatment of poxvirus infections is required.

Genomes of the parapoxviruses ORF virus and bovine papular stomatitis virus

Bovine papular stomatitis virus (BPSV) and orf virus (ORFV), members of the genus Parapoxvirus of the Poxviridae, are etiologic agents of worldwide diseases affecting cattle and small ruminants, respectively. Here we report the genomic sequences and comparative analysis of BPSV strain BV-AR02 and ORFV strains OV-SA00, isolated from a goat, and OV-IA82, isolated from a sheep. Parapoxvirus (PPV) BV-AR02, OV-SA00, and OV-IA82 genomes range in size from 134 to 139 kbp, with an average nucleotide composition of 64% G+C. BPSV and ORFV genomes contain 131 and 130 putative genes, respectively, and share colinearity over 127 genes, 88 of which are conserved in all characterized chordopoxviruses. BPSV and ORFV contain 15 and 16 open reading frames (ORFs), respectively, which lack similarity to other poxvirus or cellular proteins. All genes with putative roles in pathogenesis, including a vascular endothelial growth factor (VEGF)-like gene, are present in both viruses; however, BPSV contains two extra ankyrin repeat genes absent in ORFV. Interspecies sequence variability is observed in all functional classes of genes but is highest in putative virulence/host range genes, including genes unique to PPV. At the amino acid level, OV-SA00 is 94% identical to OV-IA82 and 71% identical to BV-AR02. Notably, ORFV 006/132, 103, 109, 110, and 116 genes (VEGF, homologues of vaccinia virus A26L, A33R, and A34R, and a novel PPV ORF) show an unusual degree of intraspecies variability. These genomic differences are consistent with the classification of BPSV and ORFV as two PPV species. Compared to other mammalian chordopoxviruses, PPV shares unique genomic features with molluscum contagiosum virus, including a G+C-rich nucleotide composition, three orthologous genes, and a paucity of nucleotide metabolism genes. Together, these data provide a comparative view of PPV genomics.

Gene Transfer for Therapeutic Vascular Growth in Myocardial and Peripheral Ischemia

Therapeutic vascular growth in the treatment of peripheral and myocardial ischemia has not yet fulfilled its expectations in clinical trials. Randomized, double-blinded placebo-controlled trials have predominantly shown the safety and feasibility but not the clear-cut clinically relevant efficacy of angiogenic gene or recombinant growth factor therapy. It is likely that growth factor levels achieved with single injections of recombinant protein or naked plasmid DNA are too low to induce any relevant angiogenic effects. Also, the route of administration of gene transfer vectors has not been optimal in many cases leading to low gene-transfer efficacy. Animal experiments using intramuscular or intramyocardial injections of adenovirus encoding vascular endothelial growth factor (VEGF, VEGF-A), the mature form of VEGF-D, and fibroblast growth factors (FGF-1, -2, and -4) have shown high angiogenic efficacy. Adenoviral overexpression of VEGF receptor-2 ligands, VEGF-A and the mature form of VEGF-D, enlarge the preexisting capillaries in skeletal muscle and myocardium via nitric oxide(NO)-mediated mechanisms and via proliferation of both endothelial cells and pericytes, resulting in markedly increased tissue perfusion. VEGF also enhances collateral growth, which is probably secondary to increased peripheral capillary blood flow and shear stress. As a side effect of VEGF overexpression and rapid microvessel enlargement, vascular permeability increases and may result in substantial tissue edema and pericardial effusion in the heart. Because of the transient adenoviral gene expression, the majority of angiogenic effects and side effects return to baseline by 2 weeks after the gene transfer. In contrast, VEGF overexpression lasting over 4 weeks has been shown to induce the growth of a persistent vascular network in preclinical models. To improve efficacy, the choice of the vascular growth factor, gene transfer vector, and route of administration should be optimized in future clinical trials. This review is focused on these issues.

Analysis of an orf virus chemokine-binding protein: Shifting ligand specificities among a family of poxvirus viroceptors

We identify a secreted chemokine inhibitor encoded by orf virus (ORFV), the prototypic poxvirus of the Parapoxvirus genus, and show that it is related to the poxvirus type II CC-chemokine-binding proteins (CBP-II) produced by members of the Orthopoxvirus and Leporipoxvirus genera. The ORFV chemokine-binding protein (CBP) is functionally similar to the CBP-II proteins in its ability to bind and inhibit many CC-chemokines with high affinity. However, unlike CBP-II, the ORFV CBP also binds with high affinity to lymphotactin, a member of the C-chemokine family, demonstrating that the ORFV CBP possesses an altered binding specificity. Interestingly, the amino acid sequence of ORFV CBP more closely resembles the granulocyte-macrophage colony-stimulating factor/IL-2 inhibitory factor also produced by ORFV, implicating the granulocyte-macrophage colony-stimulating factor/IL-2 inhibitory factor protein as a highly diverged, but related, member of the CBP-II protein family. Notably, these findings suggest that the genes that encode these proteins derive from a common poxvirus ancestral gene that has since been modified in binding specificity during speciation of the poxvirus genera. Overall, these findings illustrate the concept of evolution of viral proteins at the biophysical and molecular interface.

VEGF-receptor signal transduction

The vascular endothelial growth factor (VEGF) family of ligands and receptors has been the focus of attention in vascular biology for more than a decade. There is now a consensus that the VEGFs are crucial for vascular development and neovascularization in physiological and pathological processes in both embryo and adult. This has facilitated a rapid transition to their use in clinical applications, for example, administration of VEGF ligands to enhance vascularization of ischaemic tissues and, conversely, inhibitors of VEGF-receptor function in anti-angiogenic therapy. More recent data indicate essential roles for the VEGFs in haematopoietic cell function and in lymphangiogenesis.

A novel poxvirus lethal to red squirrels (Sciurus vulgaris)

A parapoxvirus has been implicated in the decline of the red squirrel in the United Kingdom. Virus was isolated from an outbreak of lethal disease in red squirrels in the north-east of England. Experimental infection of captive-bred red squirrels confirmed that this virus was the cause of the severe skin lesions observed. Electron microscopic examination of the virus showed that it had a morphology typical of parapoxviruses whilst preliminary sequence data suggested a genomic G+C composition of approximately 66 %, again similar to that found in other parapoxviruses. However Southern hybridization analysis failed to detect three known parapoxvirus genes, two of which have been found so far only in the genus Parapoxvirus. Comparative sequence analysis of two other genes, conserved across the eight recognized chordopoxvirus genera, suggests that the squirrel virus represents a previously unrecognized genus of the Chordopoxviridae.

Transcript mapping of the ‘early’ genes of Orf virus

The full complement of genes encoded by Orf virus (ORFV) is not yet known. A cDNA library was constructed using mRNA isolated 5 h post-infection from cells infected with ORFV in vitro and grown in the presence of cytosine arabinoside. Using 12 non-overlapping probes representing the entire genome of the Orf-11 strain of the virus, cDNA clones representing individual genes expressed early in infection were isolated. Thirty-eight early genes were identified, either via isolation of their cDNA from the library or via Northern blotting. Twenty-nine of the isolated cDNAs represented orthologues of other poxvirus genes or had been identified previously as genes of ORFV, whilst seven appeared unrelated to any known poxvirus gene or indeed to any known gene in the DNA databases. The sequences described in this paper constitute approximately 30 kb of the ORFV genome and contain the complete or partial sequence of 47 genes.

Viral vascular endothelial growth factors vary extensively in amino acid sequence, receptor-binding specificities, and the ability to induce vascular permeability yet are uniformly active mitogens

Infections of humans and ungulates by parapoxviruses result in skin lesions characterized by extensive vascular changes that have been linked to viral-encoded homologues of vascular endothelial growth factor (VEGF). VEGF acts via a family of receptors (VEGFRs) to mediate endothelial cell proliferation, vascular permeability, and angiogenesis. The VEGF genes from independent parapoxvirus isolates show an extraordinary degree of inter-strain sequence variation. We conducted functional comparisons of five representatives of the divergent viral VEGFs. These revealed that despite the sequence divergence, all were equally active mitogens, stimulating proliferation of human endothelial cells in vitro and vascularization of sheep skin in vivo with potencies equivalent to VEGF. This was achieved even though the viral VEGFs bound VEGFR-2 less avidly than did VEGF. Surprisingly the viral VEGFs varied in their ability to cross-link VEGFR-2, induce vascular permeability and bind neuropilin-1. Correlations between these three activities were detected. In addition it was possible to correlate these functional variations with certain sequence and structural motifs specific to the viral VEGFs. In contrast to the conserved ability to bind human VEGFR-2, the viral growth factors did not bind either VEGFR-1 or VEGFR-3. We propose that the extensive sequence divergence seen in the viral VEGFs was generated primarily by selection against VEGFR-1 binding.

Vascular endothelial growth factor receptor-2: its unique signaling and specific ligand, VEGF-E

Vascular endothelial growth factor receptor-2 (VEGFR-2/KDR/Flk-1) is a high-affinity receptor for vascular endothelial growth factor-A (VEGF-A), and mediates most of the endothelial growth and survival signals from VEGF-A. VEGFR-2 has a typical tyrosine kinase receptor structure with seven immunoglobulin (Ig)-like domains in the extracellular region, as well as a long kinase insert in the tyrosine kinase domain. It utilizes a unique signaling system for DNA synthesis in vascular endothelial cells, i.e. a phospholipase C gamma-protein kinase C-Raf-MAP kinase pathway. Although VEGF-A binds two receptors, VEGFR-1 and -2, a newly isolated ligand VEGF-E (Orf-virus-derived VEGF) binds and activates only VEGFR-2. Transgenic mice expressing VEGF-E(NZ-7) showed a dramatic increase in angiogenesis with very few side effects (such as edema and hemorrhagic spots), suggesting strong angiogenic signaling and a potential clinical utility of VEGF-E. VEGF family members bear three loops produced via three intramolecular disulfide bonds, and cooperation between loop-1 and loop-3 is necessary for the specific binding and activation of VEGFR-2 for angiogenesis. As it directly upregulates tumor angiogenesis, VEGFR-2 is an appropriate target for suppression of solid tumor growth using exogenous antibodies, small inhibitory molecules and in vivo stimulation of the immune system.

Vegfb gene knockout mice display reduced pathology and synovial angiogenesis in both antigen-induced and collagen-induced models of arthritis

OBJECTIVE

To determine the role of vascular endothelial growth factor B (VEGF-B) in 2 mouse models of arthritis, antigen-induced arthritis (AIA) and collagen-induced arthritis (CIA).

METHODS

For AIA studies, monarticular AIA was induced by methylated bovine serum albumin (mBSA) priming of Vegfb gene knockout (Vegfb(-/-)) and wild-type (Vegfb(+/+)) mice, followed by intraarticular injection of mBSA or saline control 8 days later. CIA was induced in Vegfb(-/-) and Vegfb(+/+) mice by intradermal injection of chick type II collagen in adjuvant. Arthritis was monitored in both models using defined criteria (clinical and histologic). Angiogenesis was measured by synovial vessel density in diseased and control joints.

RESULTS

In AIA studies, Vegfb(+/+) mice displayed significant knee joint swelling and synovial inflammation 7 days after intraarticular injection of antigen. Synovial inflammation was associated with angiogenesis, since vessel density in AIA synovium was significantly higher in arthritic than in control joints from the same animal. Knee joint swelling, synovial inflammation, and inflammation-associated vessel density in arthritic joints were reduced in Vegfb(-/-) mice compared with arthritic joints from Vegfb(+/+) mice. Similarly, in CIA, both disease incidence and mean clinical severity scores were significantly reduced in Vegfb(-/-) mice compared with Vegfb(+/+) mice. Mean histologic severity scores and mean synovial vessel density were reduced in diseased joints from Vegfb(-/-) mice when compared with diseased joints from Vegfb(+/+) mice.

CONCLUSION

The reduction in inflammation-associated synovial angiogenesis in Vegfb(-/-) mice implicates VEGF-B in pathologic vascular remodeling in inflammatory arthritis. VEGF-B may be an attractive target in the design of anti-angiogenic therapies for rheumatoid arthritis.

Relatedness and heterogeneity at the near-terminal end of the genome of a parapoxvirus bovis 1 strain (B177) compared with parapoxvirus ovis (Orf virus)

The present study provides for the first time an extended investigation of individual genes located at the near-terminal right end of the genome of parapoxvirus bovis 1, Bovine papular stomatitis virus (BPSV) strain B177 and Orf virus (ORFV). Comparison of the respective DNA sequences of ORFV strain D1701 (9.9 kbp) and BPSV B177 (7.7 kbp) revealed a very similar organization of closely related genes transcribed in a rightward orientation. The most salient findings of this study were: (i) the absence of the ORFV-specific vascular endothelial growth factor (VEGF-E) gene in the BPSV isolate; (ii) the presence of an interleukin-10 (IL-10) orthologue; and (iii) the detection of three new genes encoding ankyrin-repeat-containing polypeptides. These results not only contribute to potential improvements of future molecular differentiation between the parapoxvirus species, but also shed new light on different pathobiologies among parapoxviruses.

Vascular endothelial growth factor is a chemoattractant for trophoblast cells

A role for angiogenic growth factors in trophoblast invasion has been postulated. Directional motility (chemotaxis) is an important function of trophoblast cells. We have previously shown that vascular endothelial growth factor (VEGF) increases the random movement of trophoblast cells although placental growth factor (PlGF) has no effect. Heparin inhibited this effect of VEGF. Motility of trophoblast cells has been proposed to be mediated by a nitric oxide (NO) pathway. We hypothesized that VEGF but not PlGF would be chemotactic for trophoblast cells. Chemotaxis of a first trimester extravillous trophoblast cell line, SGHPL-4, and primary isolates of first trimester and term trophoblast cells was measured using a Boyden chamber. Initial experiments to optimize the time of the experiment and identify a positive control were performed. Subsequent experiments ran for 20 h, used 0.5 per cent FBS or 10 ng/ml PDGF as negative and positive controls and were performed in triplicate. VEGF (1, 10 and 100 ng/ml+/-1 microg/ml heparin or +/-100 microM L-NAME) and PlGF (1, 10, 100 ng/ml) were tested. The chamber was placed in a 5 per cent CO(2) in air, 37 degrees C incubator. The number of cells in the lower chamber were counted. There was a dose dependent increase in chemotactic motility of the SGHPL-4 cell line and term trophoblast cells in response to VEGF. PlGF had no effect on the movement of the first trimester trophoblast cell line but did increase the motility of the term trophoblast cells in a dose dependent manner. Heparin increased the cellular motility of both cell types alone. It also further enhanced the chemoactivity of VEGF on the term trophoblast cells but not the cell line. L-NAME did not affect the VEGF-stimulated motility of the first trimester cell line. However, in the term trophoblast cells L-NAME increased the directional cellular motility in the absence of, or in the presence of low concentrations of VEGF. In conclusion, the first trimester and term trophoblast cells appeared to respond differently to the various factors tested in the present study that may reflect differential cellular function as gestation progresses.

A set of loop-1 and -3 structures in the novel vascular endothelial growth factor (VEGF) family member, VEGF-ENZ-7, is essential for the activation of VEGFR-2 signaling

The vascular endothelial growth factor (VEGF) family plays important roles in angiogenesis and vascular permeability. Novel members of the VEGF family encoded in the Orf virus genome, VEGF-E, function as potent angiogenic factors by specifically binding and activating VEGFR-2 (KDR). VEGF-E is about 45% homologous to VEGF-A at amino acid levels, however, the amino acid residues in VEGF-A crucial for the VEGFR-2-binding are not conserved in VEGF-E. To understand the molecular basis of the biological activity of VEGF-E, we have functionally mapped residues important for interaction of VEGF-E with VEGFR-2 by exchanging the domains between VEGF-E(NZ-7) and PlGF, which binds only to VEGFR-1 (Flt-1). Exchange on the amino- and carboxyl-terminal regions had no suppressive effect on biological activity. However, exchange on either the loop-1 or -3 region of VEGF-E(NZ-7) significantly reduced activities. On the other hand, introduction of the loop-1 and -3 of VEGF-E(NZ-7) to placenta growth factor rescued the biological activities. The chimera between VEGF-A and VEGF-E(NZ-7) gave essentially the same results. These findings strongly suggest that a common rule exists for VEGFR-2 ligands (VEGF-E(NZ-7) and VEGF-A) that they build up the binding structure for VEGFR-2 through the appropriate interaction between loop-1 and -3 regions.

Inhibition of breast carcinoma and trophoblast cell invasiveness by vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is a potent endothelial cell mitogen and angiogenic growth factor that enhances endothelial cell invasion through the extracellular matrix (ECM). While various cell types express VEGF receptors, little is known about the biological actions of VEGF on nonendothelial cells. Therefore, the main objective of the present study was to determine the effect of VEGF on the in vitro invasiveness and proliferation of human MDA-MB-231 breast carcinoma cells and human HTR-8/SVneo trophoblast cells. Reverse-transcriptase polymerase chain reaction analysis demonstrated the presence of transcripts encoding VEGF receptors (VEGFR) -1, -2, and -3 as well as neuropilins-1 and -2 in the trophoblast cells, and the presence of transcripts encoding VEGFR-2 and neuropilins-1 and -2 in the breast carcinoma cells. Both cell lines also expressed transcripts for VEGF-A, -B, -C and -D, as well as for placenta growth factor (PlGF). Although incubation with exogenous VEGF-A(165) or VEGF-A(121) did not affect the rate of proliferation of either the trophoblast or the breast carcinoma cells, incubation with these molecules reduced their ability to invade through reconstituted ECM (Matrigel). The effect of VEGF-A(165) on the invasiveness of both cell lines was inhibited by the inclusion of a neutralizing antibody to VEGF. Exogenous VEGF-A(165) also decreased the cell surface expression of the urokinase-type plasminogen activator (a molecule required for invasion) by the breast carcinoma and trophoblast cells. These results indicate that the biological actions of VEGF on certain cell types may differ from the effects of this molecule on vascular endothelial cells, and therefore are relevant to angiogenesis-based therapies.

VEGFR-2-specific ligand VEGF-E induces non-edematous hyper-vascularization in mice

VEGF family members play important roles in angiogenesis and vascular permeability. VEGF-A-transgenic mice showed an increased vascularization with edema due to hyper-vascular permeability and subcutaneous hemorrhage as side effects. VEGF-A binds and activates two receptors, VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1). To dissect the signals of these two receptors, we generated transgenic mice overexpressing either the VEGFR-2-specific ligand VEGF-E(NZ-7) or VEGFR-1-specific ligand PlGF-II under the control of the Keratin-14 promoter. VEGF-E-mice showed a significant increase in vascularization (about 10-fold compared to control mice) in subcutaneous tissues, whereas PlGF-mice showed only a 2-3-fold increase. Interestingly, VEGF-E-mice did not show any clear edematous lesions or hemorrhagic spots on the skin. Microscopically, VEGF-E-induced capillary networks have a well organized structure with the recruitment of pericytes. These results indicate that VEGF-E is a new angiogenic agent with less side effects for clinical usage.

Pseudocowpox virus encodes a homolog of vascular endothelial growth factor

We have identified a gene encoding a homolog of vascular endothelial growth factor (VEGF) in the Pseudocowpox virus (PCPV) genome. The predicted protein shows 27% amino acid identity to human VEGF-A. It also shows 41 and 61% amino acid identity to VEGFs encoded by orf virus (ORFV) strains NZ2 and NZ7, respectively. Assays of the expressed VEGF-like protein of PCPV (PCPV(VR634)VEGF) demonstrated that PCPV(VR634)VEGF is mitogenic for endothelial cells and is capable of inducing vascular permeability. PCPV(VR634)VEGF bound VEGF receptor-2 (VEGFR-2) but did not bind VEGFR-1 or VEGFR-3. These results indicate that PCPV(VR634)VEGF is a biologically active member of the VEGF family which shares with the ORFV-encoded VEGFs a receptor binding profile that differs from those of all cellular members of the VEGF family. It seems likely that the biological activities of PCPV(VR634)VEGF contribute to the proliferative and highly vascularized nature of PCPV lesions.

A comparison of the anti-inflammatory and immuno-stimulatory activities of orf virus and ovine interleukin-10

Orf virus causes pustular skin lesions (orf) in sheep, goats and humans. The virus encodes an interleukin-10 (orfvIL-10) that is identical in amino acid composition to ovine IL-10 (ovIL-10) over the C terminal two-thirds of the polypeptide, but not in the N terminal third. The immuno-suppressive and immuno-stimulatory activities of orfvIL-10 and ovIL-10 were compared. Both orfvIL-10 and ovIL-10 inhibited TNF-alpha and IL-8 cytokine production from stimulated ovine macrophages and keratinocytes and IFN-gamma and GM-CSF production from peripheral blood lymphocytes. OrfvIL-10 and ovIL-10 co-stimulated both ovine and murine mast cell proliferation in conjunction with IL-3 (ovine) or IL-4 (murine). Isoleucine at position 87 (Ile(87)) of the mature human IL-10 (huIL-10) has been reported as essential for the immuno-stimulatory activity of huIL-10. In spite of the differences in amino acids within the N-terminal third of orfvIL-10 compared with ovIL-10 and substitution of Ile(87) with Ala(87) in ovIL-10, these variants of ovIL-10 and orfvIL-10 all co-stimulated mast cell proliferation and inhibited macrophage IL-8 production. As ovIL-10 and orfvIL-10 have a similar structure to huIL-10 and conserved receptor-binding residues, it was concluded that Ile(87) is not essential for IL-10 immuno-stimulatory activity. Finally, ovine keratinocytes do not express ovIL-10. This might explain why orf virus has evolved a viral IL-10.

Vascular endothelial growth factors encoded by Orf virus show surprising sequence variation but have a conserved, functionally relevant structure

The first report of a vascular endothelial growth factor (VEGF)-like gene in Orf virus included the surprising observation that the genes from two isolates (NZ2 and NZ7) shared only 41.1% amino acid sequence identity. We have examined this sequence disparity by determining the VEGF gene sequence of 21 isolates of Orf virus derived from diverse sources. Most isolates carried NZ2-like VEGF genes but their predicted amino acid sequences varied by up to 30.8% with an average amino acid identity between pairs of NZ2-like sequences of 86.1%. This high rate of sequence variation is more similar to interspecies than intraspecies variability. In contrast, only three isolates carried an NZ7-like VEGF gene and these varied from the NZ7 sequence by no more than a single nucleotide. The VEGF family are ligands for a set of tyrosine kinase receptors. The viral VEGFs are unique among the family in that they recognize VEGF receptor 2 (VEGFR-2) but not VEGFR-1 or VEGFR-3. Comparisons of the viral VEGFs with other family members revealed some correlations between conserved residues and the ability to recognize specific VEGF receptors. Despite the sequence variations, structural predictions for the viral VEGFs were very similar to each other and to the structure determined by X-ray crystallography for human VEGF-A. Structural modelling also revealed that a groove seen in the VEGF-A homodimer and believed to play a role in its binding to VEGFR-1 is blocked in the viral VEGFs. This may contribute to the inability of the viral VEGFs to bind VEGFR-1.

Orf virus immuno-modulation and the host immune response

Orf virus encodes a range of immuno-modulatory genes that interfere with host anti-virus immune and inflammatory effector mechanisms. The function of these reflects the pathogenesis of orf. The orf virus interferon resistance protein (OVIFNR) and virus IL-10 (vIL-10) inhibit interferon production and activity. In addition the vIL-10 suppresses inflammatory cytokine production by activated macrophages and keratinocytes. The virus GM-CSF inhibitory factor (GIF) is a novel virus protein that binds to and inhibits the biological activity of GM-CSF and IL-2. Together, these immuno-modulators target key effector mechanisms of host anti-virus immunity to allow time for virus replication in epidermal cells.

Immunity and counter-immunity during infection with the parapoxvirus orf virus

Orf virus is a DNA parapoxvirus that causes orf, an acute debilitating skin disease of sheep, goats and humans. In sheep, a vigorous immune response involving neutrophils, dermal dendritic cells, T cells, B cells and antibody is generated after infection. CD4(+) T cells, IFN-gamma and to a lesser extent CD8(+) T cells are involved in partial protection against infection. In spite of this, orf virus can repeatedly infect sheep albeit with reduced lesion size and time to resolution compared to primary infection. This is due at least in part to the action of virus immuno-modulator proteins that interfere with host immune and inflammatory responses. These include: an interferon resistance protein; a viral orthologue of mammalian IL-10 (vIL-10) that is an anti-inflammatory cytokine; and a novel inhibitor of the cytokines GM-CSF and IL-2 (GIF). The virus also encodes a virulence protein that is an orthologue of mammalian vascular endothelial growth factor. The study of the immuno-modulator proteins provides an insight into disease pathogenesis and important elements of a host protective response. This information will be used to devise a rational disease control strategy.

Vascular growth factors and lymphangiogenesis

Blood and lymphatic vessels develop in a parallel, but independent manner, and together form the circulatory system allowing the passage of fluid and delivering molecules within the body. Although the lymphatic vessels were discovered already 300 years ago, at the same time as the blood circulation was described, the lymphatic system has remained relatively neglected until recently. This is in part due to the difficulties in recognizing these vessels in tissues because of a lack of specific markers. Over the past few years, several molecules expressed specifically in the lymphatic endothelial cells have been characterized, and knowledge about the lymphatic system has started to accumulate again. The vascular endothelial growth factor (VEGF) family of growth factors and receptors is involved in the development and growth of the vascular endothelial system. Two of its family members, VEGF-C and VEGF-D, regulate the lymphatic endothelial cells via their receptor VEGFR-3. With the aid of these molecules, lymphatic endothelial cells can be isolated and cultured, allowing detailed studies of the molecular properties of these cells. Also the role of the lymphatic endothelium in immune responses and certain pathological conditions can be studied in more detail, as the blood and lymphatic vessels seem to be involved in many diseases in a coordinated manner. Discoveries made so far will be helpful in the diagnosis of certain vascular tumors, in the design of specific treatments for lymphedema, and in the prevention of metastatic tumor spread via the lymphatic system.

Orf virus encodes a functional dUTPase gene

The present study is the first report on the functional activity of a parapoxvirus-encoded dUTPase. The dUTPase gene of the attenuated orf virus (ORFV), strain D1701, was expressed as a bacterial thioredoxin fusion protein. In vitro assays showed that ORFV dUTPase was highly specific for dUTP as substrate. The enzyme was active over a broad pH range (pH 6.0-9.0), with maximal enzymatic activity at pH 7.0 in the presence of Mg(2+) cations. Kinetic studies of the recombinant ORFV dUTPase revealed an apparent K(m) of 4.0 microM, which is more similar to that of the mammalian or African swine fever virus enzyme than to the K(m) of vaccinia virus dUTPase. Enzyme activity was also found with purified ORFV particles, indicating its virion association.

Mechanisms of normal and tumor-derived angiogenesis

Often those diseases most evasive to therapeutic intervention usurp the human body's own cellular machinery or deregulate normal physiological processes for propagation. Tumor-induced angiogenesis is a pathological condition that results from aberrant deployment of normal angiogenesis, an essential process in which the vascular tree is remodeled by the growth of new capillaries from preexisting vessels. Normal angiogenesis ensures that developing or healing tissues receive an adequate supply of nutrients. Within the confines of a tumor, the availability of nutrients is limited by competition among actively proliferating cells, and diffusion of metabolites is impeded by high interstitial pressure (Jain RK. Cancer Res 47: 3039-3051, 1987). As a result, tumor cells induce the formation of a new blood supply from the preexisting vasculature, and this affords tumor cells the ability to survive and propagate in a hostile environment. Because both normal and tumor-induced neovascularization fulfill the essential role of satisfying the metabolic demands of a tissue, the mechanisms by which cancer cells stimulate pathological neovascularization mimic those utilized by normal cells to foster physiological angiogenesis. This review investigates mechanisms of tumor-induced angiogenesis. The strategies used by cancer cells to develop their own blood supply are discussed in relation to those employed by normal cells during physiological angiogenesis. With an understanding of blood vessel growth in both normal and abnormal settings, we are better suited to design effective therapeutics for cancer.

Placental development in normal and compromised pregnancies-- a review

Intrauterine growth restriction (IUGR) is a significant cause of infant mortality and morbidity. It is now clear that IUGR infants exhibit higher rates of coronary heart disease, type 2-diabetes, hypertension and stroke as adults. Therefore, fetal growth not only impacts the outcome of the perinatal period, but also impacts adult well-being. The etiologies of IUGR are numerous, but are often associated with abnormalities in placental structure and function. The process of implantation and placentation requires the production of a plethora of growth factors, cell-adhesion molecules, extracellular matrix proteins, hormones and transcription factors. Many of these exhibit altered expression within the placenta of IUGR pregnancies. However, it has been difficult to fully assess their role during the development of placental insufficiency (PI) in the human, underscoring the need for animal models. Using an ovine model of PI-IUGR we have observed changes in the expression of vascular endothelial growth factor, placental growth factor, their common receptors, as well as angiopoietin 2 and its receptor, Tie 2. We found that changes in these growth factors can be associated with both acute and chronic changes in placental vascular structure and function. These studies and others are providing needed insight into the developmental chronology of placental insufficiency.

Molecular cloning and expression of a functional snake venom vascular endothelium growth factor (VEGF) from the Bothrops insularis pit viper. A new member of the VEGF family of proteins

During the generation of abundant expressed sequence tags from the Viperidae snake Bothrops insularis venom glands, we identified for the first time a cDNA coding for a putative vascular endothelial growth factor-like (VEGF-like) protein. The deduced primary sequence, after complete sequencing of the longest snake venom VEGF (svVEGF) cDNA, displayed similarity with vertebrate VEGFs and with the hypotensive factor from Vipera aspis venom. Its cDNA was subcloned, expressed in Escherichia coli with a His(6) tag as an insoluble monomer, and purified by Ni(2+)-affinity chromatography after 8 m urea extraction. Antiserum against svVEGF was generated and tested in Western blot against proteins from snake venoms and cellular extracts. The mature svVEGF appears to be ubiquitously distributed throughout snake venoms and was also confirmed by Northern blot studies of other related Viperidae species and by cDNA cloning of svVEGF from Bothrops jararaca pit viper. The produced recombinant protein dimerizes after refolding processes and was biologically characterized, showing ability to increase vascular permeability. These results established that svVEGF is a novel and important active toxin during the early stages of bothropic snake bite envenoming and represents a new member of the VEGF family of proteins.

Poxvirus homologues of cellular genes

Over the course of time poxviruses have acquired or "captured" numerous homologues of cellular genes and incorporated them into their large DNA genomes. With more poxvirus genome sequencing data becoming available, the number of newly discovered poxviral cellular homologues is constantly increasing. A common feature of these genes is that they are nonessential for virus replication in vitro and they confer selective advantages in dealing with host cell differentiation and immune defense mechanisms in vivo. Poxviral cellular homologues are reviewed in this synopsis considering the specific viral habitats of different poxviruses and the immune defence capabilities of their respective hosts. Possible mechanisms of cellular gene acquisition by poxviruses as suggested by the analysis of mobile genetic elements in large DNA viruses are discussed. The investigation of poxvirus homologues of cellular genes is essential for our understanding of the mechanisms that regulate virus/host interactions on the cellular level and the host response against infection.

Role of vascular endothelial growth factor in regulation of physiological angiogenesis

Evidence accumulating over the last decade has established the fundamental role of vascular endothelial growth factor (VEGF) as a key regulator of normal and abnormal angiogenesis. The biological effects of VEGF are mediated by two tyrosine kinase receptors, Flt-1 (VEGFR-1) and KDR (VEGFR-2). The signaling and biological properties of these two receptors are strikingly different. VEGF is essential for early development of the vasculature to the extent that inactivation of even a single allele of the VEGF gene results in embryonic lethality. VEGF is also required for female reproductive functions and endochondral bone formation. Substantial evidence also implicates VEGF as an angiogenic mediator in tumors and intraocular neovascular syndromes, and numerous clinical trials are presently testing the hypothesis that inhibition of VEGF may have therapeutic value.

Subversion and piracy: DNA viruses and immune evasion

During the co-evolution of viruses with their vertebrate hosts, the DNA viruses have acquired an impressive array of immunomodulatory genes to combat host immune responses and their hosts have developed a sophisticated immune system to contain virus infections. In order to replicate, the viruses have evolved mechanisms to inhibit key host anti-virus responses that include apoptosis, interferon production, chemokine production, inflammatory cytokine production, and the activity of cytotoxic T-cells, natural killer cells and antibody. In addition, some of the viruses encode cytokine or chemokine homologues that recruit or expand cell numbers for infection or that subvert the host cellular response from a protective response to a benign one. The specificity of the viral immunomodulatory molecules reflects the life cycle and the pathogenesis of the viruses. Herpesviruses achieve latency in host cells by inducing cell survival and protecting infected cells from immune recognition. This involves interference with cell signal transduction pathways. Many of the viral immunomodulatory proteins are homologues of host proteins that appear to have been pirated from the host and reassorted in the virus genomes. Some of these have unique functions and indicate novel or important aspects of both viral pathogenesis and host immunity to viruses. The specific example of orf virus infection of sheep is described.

Cellular interactions in vascular growth and differentiation

In nature, mammalian cells do not exist in isolation, but rather are involved in interactions with other cells and matrix. In this review, several aspects of cellular interactions that are important in vascular growth and development will be highlighted. The cardiovascular system is the earliest to develop in the embryo. A number of growth factors and their receptors mediate the complex stages of migration, assembly, organization, and stabilization of developing vessels. In the adult organism, normal angiogenesis is restricted primarily to tissue growth (such as muscle and fat), the wound healing process and the female reproductive system. However, pathological angiogenesis, such as with tumor growth, diabetic retinopathy, and arthritis, is of great concern. The identification and/or development of exogenous and endogenous angiogenesis inhibitors has added to the understanding of these pathological processes. In addition to cellular interactions via ligands and receptors, cells also interact directly through physical contacts. These interactions facilitate anchorage, communication, and permeability. Since vessels serve as non-leaky conduits for blood flow as well as interfaces for molecular diffusion, the physical interactions between the cells that make up vessels must be specific for the function at hand. Permeability is a specialized function of vessels and is mediated by intracellular mechanisms and intercellular interactions. Cells also interact with the surrounding extracellular matrix. Integrin-matrix interaction is a two-way exchange critical for angiogenesis. Matrix metalloproteinases and tissue inhibitors of matrix metalloproteinases play major roles in embryonic remodeling, adult injury, and pathological conditions. Several experimental model systems have been useful in our understanding of cellular interactions. These in vitro models incorporate heterotypic cell-cell interactions and/or allow cell-matrix interactions to occur.

Virokines: novel immunomodulatory agents

Since the discovery of virokines in the 1980s, much time and research has been dedicated to exploring their potential use as therapeutic agents. Simply put, virokines are virally encoded proteins that are secreted from the infected host cell. Most of these proteins possess the ability to modulate different aspects of the host immune system, to better maintain a suitable habitat for viral replication. These proteins are often highly homologous to host immune proteins but are often smaller and more powerful. Examples of virokines include viral secreted proteins that: block components of the complement system, act as serine protease inhibitors, function as chemokine and cytokine agonists or antagonists and contribute to cell proliferation. Many of these proteins are currently being investigated for use as novel therapeutic immunomodulators to manage immune disorders, inflammation after trauma, graft rejection and autoimmune diseases.

The splice variants of vascular endothelial growth factor (VEGF) and their receptors

Vascular endothelial growth factor (VEGF) is a secreted mitogen highly specific for cultured endothelial cells. In vivo VEGF induces microvascular permeability and plays a central role in both angiogenesis and vasculogenesis. VEGF is a promising target for therapeutic intervention in certain pathological conditions that are angiogenesis dependent, most notably the neovascularisation of growing tumours. Through alternative mRNA splicing, a single gene gives rise to several distinct isoforms of VEGF, which differ in their expression patterns as well as their biochemical and biological properties. Two VEGF receptor tyrosine kinases (VEGFRs) have been identified, VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1). VEGFR-2 seems to mediate almost all observed endothelial cell responses to VEGF, whereas roles for VEGFR-1 are more elusive. VEGFR-1 might act predominantly as a ligand-binding molecule, sequestering VEGF from VEGFR-2 signalling. Several isoform-specific VEGF receptors exist that modulate VEGF activity. Neuropilin-1 acts as a co-receptor for VEGF(165), enhancing its binding to VEGFR-2 and its bioactivity. Heparan sulphate proteoglycans (HSPGs), as well as binding certain VEGF isoforms, interact with both VEGFR-1 and VEGFR-2. HSPGs have a wide variety of functions, such as the ability to partially restore lost function to damaged VEGF(165) and thereby prolonging its biological activity.

A review of therapeutic angiogenesis and consideration of its potential applications to plastic and reconstructive surgery

The use of exogenous agents to stimulate the growth of new blood vessels into ischaemic tissue is a potentially revolutionary therapy in a wide variety of clinical specialties. Therapeutic angiogenesis research has been mostly confined to ischaemia of the heart and the lower limb. There has been relatively little research into the potential applications of the technique to plastic, reconstructive and burns surgery. In this paper, relevant published work is reviewed and potential applications of therapeutic angiogenesis to our specialty are considered.

Viral vascular endothelial growth factor plays a critical role in orf virus infection

Infection by the parapoxvirus orf virus causes proliferative skin lesions in which extensive capillary proliferation and dilation are prominent histological features. This infective phenotype may be linked to a unique virus-encoded factor, a distinctive new member of the vascular endothelial growth factor (VEGF) family of molecules. We constructed a recombinant orf virus in which the VEGF-like gene was disrupted and show that inactivation of this gene resulted in the loss of three VEGF activities expressed by the parent virus: mitogenesis of vascular endothelial cells, induction of vascular permeability, and activation of VEGF receptor 2. We used the recombinant orf virus to assess the contribution of the viral VEGF to the vascular response seen during orf virus infection of skin. Our results demonstrate that the viral VEGF, while recognizing a unique profile of the known VEGF receptors (receptor 2 and neuropilin 1), is able to stimulate a striking proliferation of blood vessels in the dermis underlying the site of infection. Furthermore, the data demonstrate that the viral VEGF participates in promoting a distinctive pattern of epidermal proliferation. Loss of a functional viral VEGF resulted in lesions with markedly reduced clinical indications of infection. However, viral replication in the early stages of infection was not impaired, and only at later times did it appear that replication of the recombinant virus might be reduced.

Generation of recombinant parapoxviruses: non-essential genes suitable for insertion and expression of foreign genes

Orf virus (OV) is an epitheliotropic poxvirus and belongs to the genus Parapoxvirus (PPV). PPV, especially OV, is regarded as a promising candidate for an expression vector. Among available live vaccines only strain D1701 represents a highly attenuated OV strain with clearly reduced pathogenicity. Therefore, we started to identify potentially non-essential genes or regions of D1701, which might be suitable for insertion and expression of foreign genes. The present contribution reviews some of the progress using the vegf-e (homologue of the mammalian vascular endothelial growth factor) gene locus for the generation of recombinant D1701. The vegf-e gene of D1701 is dispensable for virus growth in vitro and in vivo, and represents a major virulence determinant of OV. It is shown that foreign genes can be inserted and functionally expressed in the vegf-e locus, also leading to the induction of a specific immune response in the non-permissive host. Furthermore, it is reported that adaptation to VERO cells led to the deletion of three further regions of the OV D1701 genome, which seems to be combined with additional virus attenuation in sheep. Molecular analysis of this OV D1701 variant allows the identification of new, potentially non-essential sites in the viral genome.

Inhibition of tumor growth and metastasis by targeting tumor-associated angiogenesis with antagonists to the receptors of vascular endothelial growth factor

Angiogenesis, the formation of new blood vessels, is essential for both tumor growth and metastasis. Recent advances in our understanding of the molecular mechanisms underlying the angiogenesis process and its regulation have led to the discovery of a variety of pharmaceutical agents with anti-angiogenic activity. The potential application of these angiogenesis inhibitors is currently under intense clinical and pre-clinical investigation. Compelling evidence suggests that vascular endothelial growth factor (VEGF) and its receptors play critical roles in tumor-associated angiogenesis, and that they represent good targets for therapeutic intervention. This has been demonstrated in a variety of animal tumor models in which disabling the function of VEGF and its receptors was shown to inhibit both tumor growth and metastasis. We have produced a panel of antibodies directed against the VEGF receptor 2, KDR/F1k-1. These antibodies potently block VEGF/KDR/F1k-1 interaction, and inhibit VEGF-stimulated activation of the receptor and proliferation of human endothelial cells. Further, the antibodies significantly inhibited tumor-associated angiogenesis in several animal models. Antagonists of VEGF and/or its receptors may offer higher specificity towards tumors with reduced side effects, and may be less likely to elicit drug resistance compared to conventional therapy. Anti-angiogenesis therapy represents a novel strategy for the treatment of cancer and other human disorders where pathological angiogenesis is involved.

Orf virus encodes a novel secreted protein inhibitor of granulocyte-macrophage colony-stimulating factor and interleukin-2

The parapoxvirus orf virus encodes a novel soluble protein inhibitor of ovine granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2). The GM-CSF- and IL-2-inhibitory factor (GIF) gene was expressed as an intermediate-late viral gene in orf virus-infected cells. GIF formed homodimers and tetramers in solution, and it bound ovine GM-CSF with a K(d) of 369 pM and ovine IL-2 with a K(d) of 1.04 nM. GIF did not bind human GM-CSF or IL-2 in spite of the fact that orf virus is a human pathogen. GIF was detected in afferent lymph plasma draining the skin site of orf virus reinfection and was associated with reduced levels of lymph GM-CSF. GIF expression by orf virus indicates that GM-CSF and IL-2 are important in host antiviral immunity.

Vascular endothelial growth factor (VEGF) and VEGF-C show overlapping binding sites in embryonic endothelia and distinct sites in differentiated adult endothelia

Vascular endothelial growth factor (VEGF) is a key modulator of angiogenesis during development and in adult tissues, whereas the related VEGF-C has been shown to induce both lymphangiogenesis and angiogenesis. To better understand the specific functions of these growth factors, we have here analyzed their binding to sections of mouse embryonic and adult tissues and compared the distribution of the bound growth factors with the expression patterns of the 3 known members of the VEGF receptor family as well as with neuropilin-1, a coreceptor for VEGF(165). Partially overlapping patterns of VEGF and VEGF-C binding were obtained in embryonic tissues, consistent with the expression of all known VEGF receptors by vascular endothelial cells. However, the most striking differences of binding were observed in the developing and adult heart, in which VEGF decorated all vessels, whereas strong VEGF-C signals were obtained only from epicardial vessels. In the lymph nodes, VEGF and VEGF-C showed distinct binding patterns in agreement with the differential location of their specific receptors. These results show that both VEGF-C and VEGF target embryonic blood vessels, whereas a more selective binding of VEGF-C occurs to its lymphatic vascular receptor in certain adult tissues. Our results suggest that VEGF and VEGF-C have both overlapping and distinct activities via their endothelial receptors.

The effects of angiogenic growth factors on extravillous trophoblast invasion and motility

There is accumulating evidence that deficient trophoblast invasion of the placental bed spiral arteries is crucial to the pathogenesis of pre-eclampsia and intrauterine growth restriction. However, the factors which regulate the process of trophoblast invasion remain unclear. We have investigated whether extravillous trophoblast invasion and motility are mediated by the angiogenic growth factors, vascular endothelial growth factor (VEGF) and placental growth factor (PlGF). The SGHPL-4 extravillous trophoblast cell line was utilized. Expression of mRNA for the receptors of VEGF and PlGF (KDR and flt-1) was determined using the reverse transcriptase polymerase chain reaction. An in vitro model of invasion assessed the number and length of trophoblast processes invading into an extracellular matrix. The motility of cells under standard culture conditions was also quantified. The effect of the addition of VEGF and PlGF (+/-heparin) on trophoblast invasion and motility was determined. The effect of VEGF and PlGF (+/-heparin) on SGHPL-4 cell proliferation was assessed by cell counts at 24, 48 and 72 h post-addition of growth factor. The SGHPL-4 cells expressed mRNA for the flt-1 but not the KDR receptor. The addition of VEGF resulted in a significant decrease in the number of trophoblast processes formed (P< 0.02); this effect was not influenced by the addition of heparin. However, there was no effect on the length of processes formed in response to VEGF (+/-heparin). The addition of PlGF had no effect on either the number or the length of processes formed. The addition of VEGF increased the motility of the SGHPL-4 cells (P< 0.002); the addition of heparin prevented this VEGF-induced increase in motility. The addition of PlGF had no effect on SGHPL-4 motility (+/-heparin). Neither growth factor had any effect on the proliferative ability of SGHPL-4 cells. Contrary to our hypothesis, we did not find that the angiogenic growth factors, VEGF and PlGF, mediated the in vitro invasion of trophoblast cells into an extracellular matrix. However, VEGF did increase trophoblast motility. Our findings of an effect of VEGF on trophoblast motility (and possibly invasion) suggests the presence of functional receptors, which can mediate the actions of VEGF. Caution must be exercised before any extrapolation to the in vivo situation, however, it could be speculated that the increased motility in response to VEGF may be an initial response to attract trophoblast cells to the decidua, and that VEGF might then limit the degree to which trophoblast cells invade.

Role of vascular endothelial growth factor in the regulation of angiogenesis

Compelling evidence indicates that vascular endothelial growth factor (VEGF) is a fundamental regulator of normal and abnormal angiogenesis. The loss of a single VEGF allele results in defective vascularization and early embryonic lethality. VEGF plays also a critical role in kidney development, and its inactivation during early postnatal life results in the suppression of glomerular development and kidney failure. Recent evidence indicates that VEGF is also essential for angiogenesis in the female reproductive tract and for morphogenesis of the epiphyseal growth plate and endochondral bone formation. Substantial experimental evidence also implicates VEGF in pathological angiogenesis. Anti-VEGF monoclonal antibodies or other VEGF inhibitors block the growth of several human tumor cell lines in nude mice. Furthermore, the concentrations of VEGF are elevated in the aqueous and vitreous humors of patients with proliferative retinopathies such as the diabetic retinopathy. In addition, VEGF-induced angiogenesis results in a therapeutic benefit in several animal models of myocardial or limb ischemia. Currently, both therapeutic angiogenesis using recombinant VEGF or VEGF gene transfer and inhibition of VEGF-mediated pathological angiogenesis are being pursued clinically.

Parapoxviruses: potential alternative vectors for directing the immune response in permissive and non-permissive hosts

Parapoxvirus (PPV) represents a genus of the poxviridae, and particularly PPV ovis (Orf virus, OV) seems to offer several potential advantages for the use of vector vaccine. Therefore, we started to investigate the genome of the highly attenuated OV strain D1701, which was only poorly characterised until now. Due to recombination of non-homologous sequences, part of the right hand end of the D1701 genome was duplicated and translocated to the opposite end of the genome. As a consequence gene deletion had occurred and the inverted terminal repeat region is increased. Results are described to identify viral genes, which are non-essential for virus replication and potentially influence viral pathogenesis, virulence, and host immunity. In more detail, we analysed the expression and functional activity of the OV-specific vascular endothelial growth factor (VEGF) gene homologue. Finally the construction and production of a D1701 mutant lacking the VEGF gene homologue is reported.

VEGFs, receptors and angiogenesis

Angiogenesis, the formation of new blood vessels from pre--existing ones, is central for both normal development and homeostasis as well as in certain pathological conditions. The vascular endothelial growth factors (VEGFs) and their receptors are prime regulators of both physiological and pathological angiogenesis. The different VEGFs have overlapping but specific roles in controlling the growth of new blood vessels. The VEGF receptors transduce signals mediating endothelial cell proliferation, migration, organization into functional vessels and remodeling of the vessel network. In recent years, rapid progress has been made in understanding the receptor-ligand interactions that orchestrate the neovascularization process.

Vascular endothelial growth factor (VEGF)-like protein from orf virus NZ2 binds to VEGFR2 and neuropilin-1

Orf virus, a member of the poxvirus family, produces a pustular dermatitis in sheep, goats, and humans. The lesions induced after infection with orf virus show extensive proliferation of vascular endothelial cells, dilation of blood vessels and dermal swelling. An explanation for the nature of these lesions may lie in the discovery that orf virus encodes an apparent homolog of the mammalian vascular endothelial growth factor (VEGF) family of molecules. These molecules mediate endothelial cell proliferation, vascular permeability, angiogenesis, and lymphangiogenesis via the endothelial cell receptors VEGFR-1 (Flt1), VEGFR-2 (KDR/Flk1), and VEGFR-3 (Flt4). The VEGF-like protein of orf virus strain NZ2 (ORFV2-VEGF) is most closely related in primary structure to VEGF. In this study we examined the biological activities and receptor specificity of the ORFV2-VEGF protein. ORFV2-VEGF was found to be a disulfide-linked homodimer with a subunit of approximately 25 kDa. ORFV2-VEGF showed mitogenic activity on bovine aortic and human microvascular endothelial cells and induced vascular permeability. ORFV2-VEGF was found to bind and induce autophosphorylation of VEGFR-2 and was unable to bind or activate VEGFR-1 and VEGFR-3, but bound the newly identified VEGF165 receptor neuropilin-1. These results indicate that, from a functional viewpoint, ORFV2-VEGF is indeed a member of the VEGF family of molecules, but is unique, however, in that it utilizes only VEGFR-2 and neuropilin-1.

Subversion of cytokine networks by viruses

Viruses and the immune system have been competitors throughout their co-evolution. It is therefore not surprising that the viruses in circulation today possess a variety of strategies to counteract those aspects of the immune system that are involved in virus clearance. Examination of these virus encoded functions provides an important view of immune function and an appreciation of the complexity of the virus-host interaction. It is clear that viruses, seeking to subvert the immune system, have become adept in blocking the communication channels of the immune system. There are numerous examples of viral proteins that target the cytokine networks, disrupting the processes by which the delicately balanced immune system is regulated. This review focuses on the gene products of poxviruses, adenoviruses and herpesviruses that function primarily as immune-modulators.

A novel vascular endothelial growth factor encoded by Orf virus, VEGF-E, mediates angiogenesis via signalling through VEGFR-2 (KDR) but not VEGFR-1 (Flt-1) receptor tyrosine kinases

The different members of the vascular endothelial growth factor (VEGF) family act as key regulators of endothelial cell function controlling vasculogenesis, angiogenesis, vascular permeability and endothelial cell survival. In this study, we have functionally characterized a novel member of the VEGF family, designated VEGF-E. VEGF-E sequences are encoded by the parapoxvirus Orf virus (OV). They carry the characteristic cysteine knot motif present in all mammalian VEGFs, while forming a microheterogenic group distinct from previously described members of this family. VEGF-E was expressed as the native protein in mammalian cells or as a recombinant protein in Escherichia coli and was shown to act as a heat-stable, secreted dimer. VEGF-E and VEGF-A were found to possess similar bioactivities, i.e. both factors stimulate the release of tissue factor (TF), the proliferation, chemotaxis and sprouting of cultured vascular endothelial cells in vitro and angiogenesis in vivo. Like VEGF-A, VEGF-E was found to bind with high affinity to VEGF receptor-2 (KDR) resulting in receptor autophosphorylation and a biphasic rise in free intracellular Ca2+ concentration, whilst in contrast to VEGF-A, VEGF-E did not bind to VEGF receptor-1 (Flt-1). VEGF-E is thus a potent angiogenic factor selectively binding to VEGF receptor-2. These data strongly indicate that activation of VEGF receptor-2 alone can efficiently stimulate angiogenesis.