Vascular endothelial growth factor promotes sensitivity to ultraviolet B-induced cutaneous photodamage

Acute ultraviolet B (UVB) irradiation of the skin results in erythema, vasodilation, edema, and angiogenesis, which is associated with the expression of vascular endothelial growth factor (VEGF) by epidermal keratinocytes. It is unclear, however, whether VEGF is required for the damage or repair process that occurs in the skin on UVB exposure. We subjected transgenic mice that overexpress VEGF, and their wild-type littermates, to graded doses of acute UVB irradiation. The skin of VEGF-overexpressing mice was highly photosensitive and became erythematic when exposed to half the UVB dose required to induce erythema in wild-type mice. Erythema was associated with proliferating dermal endothelial cells, cutaneous edema, and inflammatory cell infiltration. When subjected to 10 weeks of low-level UVB irradiation, no major changes were observed in wild-type mice, whereas VEGF transgenic mice developed skin damage associated with degradation of the dermal matrix and enhanced vascularization. Systemic treatment with an anti-VEGF blocking antibody reduced the sensitivity of wild-type mice to acute UVB irradiation without inhibiting post-UVB repair. Our results reveal that VEGF promotes the cutaneous damage that occurs after UVB exposure and that the VEGF signaling pathway might serve as a novel target for the prevention of UVB-induced photodamage.     

Increased and prolonged inflammation and angiogenesis in delayed-type hypersensitivity reactions elicited in the skin of thrombospondin-2--deficient mice.

Angiogenesis and enhanced microvascular permeability are hallmarks of a large number of inflammatory diseases. Although up-regulation of proangiogenic factors such as vascular endothelial growth factor and interleukin-8 have been previously reported in inflamed tissue, the biologic role of endogenous inhibitors of angiogenesis in inflammation has remained unclear. To investigate the biologic role of the potent angiogenesis inhibitor thrombospondin-2 (TSP-2) in the control of cutaneous inflammation, delayed-type hypersensitivity reactions were elicited in the ear skin of wild-type and TSP-2-deficient mice by topical sensitization and challenge with oxazolone. Cutaneous TSP-2 expression was up-regulated in the inflamed skin of wild-type mice, predominantly in dermal fibroblasts and microvessels. Lack of TSP-2 resulted in a significantly enhanced inflammatory response with increased angiogenesis, edema formation, and inflammatory infiltration. Ear swelling and inflammation persisted for more than 2 weeks in TSP-2-deficient mice, as compared with 1 week in wild-type mice. Although baseline vascular permeability was unchanged, significantly enhanced microvascular leakage was found in the inflamed skin of TSP-2-deficient mice. Moreover, the fraction of rolling leukocytes was significantly increased in the untreated skin of TSP-2-deficient mice. These results reveal an important role of TSP-2 in limiting the extent and the duration of edema formation, angiogenesis, and inflammatory cell infiltration during acute and chronic inflammation.     

Induction of cutaneous delayed-type hypersensitivity reactions in VEGF-A transgenic mice results in chronic skin inflammation associated with persistent lymphatic hyperplasia

Vascular endothelial growth factor-A (VEGF-A) expression is up-regulated in several inflammatory diseases including psoriasis, delayed-type hypersensitivity (DTH) reactions, and rheumatoid arthritis. To directly characterize the biologic function of VEGF-A in inflammation, we evaluated experimental DTH reactions induced in the ear skin of transgenic mice that overexpress VEGF-A specifically in the epidermis. VEGF-A transgenic mice underwent a significantly increased inflammatory response that persisted for more than 1 month, whereas inflammation returned to baseline levels within 7 days in wild-type mice. Inflammatory lesions in VEGF-A transgenic mice closely resembled human psoriasis and were characterized by epidermal hyperplasia, impaired epidermal differentiation, and accumulation of dermal CD4+ T-lymphocytes and epidermal CD8+ lymphocytes. Surprisingly, VEGF-A also promoted lymphatic vessel proliferation and enlargement, which might contribute to the increased inflammatory response, as lymphatic vessel enlargement was also detected in human psoriatic skin lesions. Combined systemic treatment with blocking antibodies against VEGF receptor-1 (VEGFR-1) and VEGFR-2 potently inhibited inflammation and also decreased lymphatic vessel size. Together, these findings reveal a central role of VEGF-A in promoting lymphatic enlargement, vascular hyperpermeability, and leukocyte recruitment, thereby leading to persistent chronic inflammation. They also indicate that inhibition of VEGF-A bioactivity might be a new approach to anti-inflammatory therapy.     

An important role of lymphatic vessel activation in limiting acute inflammation

In contrast to the established role of blood vessel remodeling in inflammation, the biologic function of the lymphatic vasculature in acute inflammation has remained less explored. We studied 2 established models of acute cutaneous inflammation, namely, oxazolone-induced delayed-type hypersensitivity reactions and ultraviolet B irradiation, in keratin 14-vascular endothelial growth factor (VEGF)-C and keratin 14-VEGF-D transgenic mice. These mice have an expanded network of cutaneous lymphatic vessels. Transgenic delivery of the lymphangiogenic factors VEGF-C and the VEGFR-3 specific ligand mouse VEGF-D significantly limited acute skin inflammation in both experimental models, with a strong reduction of dermal edema. Expression of VEGFR-3 by lymphatic endothelium was strongly down-regulated at the mRNA and protein level in acutely inflamed skin, and no VEGFR-3 expression was detectable on inflamed blood vessels and dermal macrophages. There was no major change of the inflammatory cell infiltrate or the composition of the inflammatory cytokine milieu in the inflamed skin of VEGF-C or VEGF-D transgenic mice. However, the increased network of lymphatic vessels in these mice significantly enhanced lymphatic drainage from the ear skin. These results provide evidence that specific lymphatic vessel activation limits acute skin inflammation via promotion of lymph flow from the skin and reduction of edema formation.     

Soluble neuropilin targeted to the skin inhibits vascular permeability

Neuropilin 1 (NRP1) is a co-receptor for vascular endothelial growth factor (VEGF₁₆₅), an inducer of vascular permeability and angiogenesis. Numerous physiological factors enhance VEGF expression and function but only a few have been shown to be negative regulators. Previously, we have shown that the naturally occurring soluble form of NRP1 (sNRP1) inhibits binding of VEGF₁₆₅ to endothelial cells in vitro and impairs tumor growth in vivo. To investigate the role of sNRP1 in the regulation of vascular development and function, sNRP1 expression was targeted to the skin, where it is not normally expressed, using a keratin 14 (K14) promoter expression construct. K14-sNRP1 transgenic mice displayed normal skin architecture with a subtle abnormal vascular phenotype. While the overall number of skin blood vessels remained unchanged, the lumen size of smooth muscle-associated dermal vessels was reduced. K14-sNRP1 mice had reduced vascular permeability in response to VEGF₁₆₅, but also to VEGF₁₂₁ and platelet activating factor, suggesting that the lack of permeability was not solely due to the sequestration of VEGF. sNRP1 also reversed the increase in inflammation and edema induced by transgenic VEGF overexpression in cutaneous delayed-type hypersensitivity reactions. In summary, sNRP1 appears to primarily regulate vessel permeability while its effect on physiological angiogenesis is less evident in this model.     

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.     

VEGF receptor-2-specific signaling mediated by VEGF-E induces hemangioma-like lesions in normal and in malignant tissue

Viral VEGF-E (ovVEGF-E), a homolog of VEGF-A, was discovered in the genome of Orf virus. Together with VEGF-A, B, C, D, placental growth factor (PlGF) and snake venom VEGF (svVEGF), ovVEGF-E is a member of the VEGF family of potent angiogenesis factors with a bioactivity similar to VEGF-A: it induces proliferation, migration and sprouting of cultured vascular endothelial cells and proliferative lesions in the skin of sheep, goat and man that are characterized by massive capillary proliferation and dilation. These biological functions are mediated exclusively via its interaction with VEGF receptor-2 (VEGFR-2). Here, we have generated transgenic mice specifically expressing ovVEGF-E in β-cells of the endocrine pancreas (Rip1VEGF-E; RVE). RVE mice show an increase in number and size of the islets of Langerhans and a distorted organization of insulin and glucagon-expressing cells. Islet endothelial cells of RVE mice hyper-proliferate and form increased numbers of functional blood vessels. In addition, the formation of disorganized lymphatic vessels and increased immune cell infiltration is observed. Upon crossing RVE single-transgenic mice with Rip1Tag2 (RT2) transgenic mice, a well-studied model of pancreatic β-cell carcinogenesis, double-transgenic mice (RT2;RVE) display hyper-proliferation of endothelial cells resulting in the formation of hemangioma-like lesions. In addition, RT2;RVE mice exhibit activated lymphangiogenesis at the tumor periphery and increased neutrophil and macrophage tumor infiltration and micro-metastasis to lymph nodes and lungs. These phenotypes markedly differ from the phenotypes observed with the transgenic expression of the other VEGF family members in β-cells of normal mice and of RT2 mice.     

The biology of vascular endothelial growth factors

The discovery of the vascular endothelial growth factor (VEGF) family members VEGF, VEGF-B, placental growth factor (PlGF), VEGF-C and VEGF-D and their receptors VEGFR-1, -2 and -3 has provided tools for studying the vascular system in development as well as in diseases ranging from ischemic heart disease to cancer. VEGF has been established as the prime angiogenic molecule during development, adult physiology and pathology. PlGF may primarily mediate arteriogenesis, the formation of collateral arteries from preexisting arterioles, with potential future therapeutic use in for example occlusive atherosclerotic disease. VEGF-C and VEGF-D are primarily lymphangiogenic factors, but they can also induce angiogenesis in some conditions. While many studies have addressed the role of angiogenesis and the blood vasculature in human physiology, the lymphatic vascular system has until recently attracted very little attention. In this review, we will discuss recent advances in angiogenesis research and provide an overview of the molecular players involved in lymphangiogenesis.     

Vascular endothelial growth factor-dependent spatiotemporal dual roles of placental growth factor in modulation of angiogenesis and tumor growth

Placental growth factor (PlGF) remodels tumor vasculatures toward a normalized phenotype, which affects tumor growth, invasion and drug responses. However, the coordinative and spatiotemporal relation between PlGF and VEGF in modulation of tumor angiogenesis and vascular remodeling is less understood. Here we report that PlGF positively and negatively modulate tumor growth, angiogenesis, and vascular remodeling through a VEGF-dependent mechanism. In two independent tumor models, we show that PlGF inhibited tumor growth and angiogenesis and displayed a marked vascular remodeling effect, leading to normalized microvessels with infrequent vascular branches and increased perivascular cell coverage. Surprisingly, elimination of VEGF gene (i.e., VEGF-null) in PlGF-expressing tumors resulted in (i) accelerated tumor growth rates and angiogenesis and (ii) complete attenuation of PlGF-induced vascular normalization. Thus, PlGF positively and negatively modulates tumor growth, angiogenesis, and vascular remodeling through VEGF-dependent spatiotemporal mechanisms. Our data uncover molecular mechanisms underlying the complex interplay between PlGF and VEGF in modulation of tumor growth and angiogenesis, and have conceptual implication for antiangiogenic cancer therapy.     

Dexamethasone treatment and ICAM-1 deficiency impair VEGF-induced angiogenesis in adult brain

BACKGROUND: Infusion of exogenous vascular endothelial growth factor (VEGF) into adult brain at doses above 60 ng/day induces dramatic angiogenesis accompanied by vascular leak and inflammation. Blood vessels formed by this treatment are dilated and tortuous, exhibiting a pathological morphology. Pathological VEGF-induced angiogenesis is preceded by vascular leak and inflammation, which have been proposed to mediate subsequent angiogenesis. METHODS: To test this hypothesis, we infused VEGF into the brains of adult rats to induce pathological angiogenesis. Some of these rats were treated with dexamethasone, a potent anti-inflammatory glucocorticoid, to inhibit inflammation and edema. RESULTS: We demonstrate that inhibition of inflammation by treatment with dexamethasone significantly attenuated VEGF-induced pathological angiogenesis. To present converging evidence that inflammation may be important in this angiogenic process, we also demonstrate that mice genetically deficient in the inflammatory mediator intercellular adhesion molecule-1 have attenuated VEGF-induced angiogenesis. These same mice showed normal amounts of physiological angiogenesis in response to enriched environments, however, suggesting that a generalized reduction in vascular plasticity could not account for their poor angiogenic response to VEGF. CONCLUSIONS: Taken together, the data from these experiments suggest that the inflammation which occurs before or during VEGF-induced pathological brain angiogenesis plays a contributory role in the pathological angiogenic process.     

VEGF function for upregulation of endogenous PlGF expression during FGF-2-mediated therapeutic angiogenesis

Vascular endothelial growth factor (VEGF) is a major positive angiogenic factor. Using a murine hindlimb ischemia model, we previously showed that fibroblast growth factor-2 (FGF-2) enhances the expression of endogenous VEGF which highly contribute to the therapeutic effect of FGF-2 gene transfer. Recently, placental growth factor (PlGF) has been shown to play an important role in angiogenesis. However, the molecular mechanism of endogenous PlGF during FGF-2-mediated angiogenesis has not been elucidated. Severe hindlimb ischemia stimulated PlGF expression that was more strongly enhanced by FGF-2 gene transfer, and a blockade of PlGF activity diminished the recovery of blood flow by FGF-2-mediated angiogenesis. The PlGF expression in cultured endothelial cells was significantly enhanced by VEGF stimulation, but not by FGF-2. The upregulation of endogenous PlGF expression was significantly decreased by the inhibition of endogenous VEGF activity in vivo. Subsequent signal inhibition experiments revealed that the PKC, MEK, and possibly NF-κB-related pathways were essential in stimulating PlGF expression with VEGF, while p70S6K is the regulator for VEGF expression. These results indicate that the FGF-2-mediated enhancement of PlGF expression was dependent on VEGF function, and the FGF-2/VEGF axis participates in FGF-2-mediated angiogenesis indirectly via PlGF as well as directly.     

Structure and function of placental growth factor

Placental growth factor (PlGF) belongs to the same family as the vascular endothelial growth factor A (VEGF-A). Recent gene inactivation studies in mice have demonstrated that loss of PlGF does not affect development, reproduction, or normal postnatal life. However, the mice show significantly impaired angiogenesis and arteriogenesis during pathological conditions such as ischemia and tumor formation, conditions in which the expression of VEGF-A is normally increased. Mice expressing a truncated form of the specific receptor for PlGF, the VEGF receptor 1 (VEGFR-1), show impaired angiogenesis similar to that observed in Plgf(-/-)mice. These data suggest a pivotal role for PlGF and VEGFR-1 in regulating VEGF-A-dependent angiogenesis under pathological conditions. VEGF-A has been utilized for the therapeutic stimulation of new blood vessels in ischemic hearts and limbs, with controversial results from the initial clinical experience. This review discusses the possibility of using the PlGF/VEGFR-1 pathway as an alternative target for angiogenic therapy.     

Transgenic delivery of VEGF to mouse skin leads to an inflammatory condition resembling human psoriasis

Gene therapy approaches involving vascular endothelial growth factor (VEGF) to promote therapeutic angiogenesis are under consideration for conditions ranging from ischemic heart disease to nonhealing skin ulcers. Here we make the surprising observation that the transgenic delivery of VEGF to the skin results in a profound inflammatory skin condition with many of the cellular and molecular features of psoriasis, including the characteristic vascular changes, epidermal alterations, and inflammatory infiltrates. Even longstanding psoriatic disease remains dependent on the transgenic VEGF in this model because it can be effectively reversed by the addition of VEGF Trap, a potent VEGF antagonist. Previous attempts to faithfully replicate the psoriatic phenotype through the transgenic delivery of epidermal keratinocyte growth factors or inflammatory mediators generated phenotypes with only partial resemblance to human psoriasis, leaving unanswered questions about the etiology of this disease. The ability of transgenic VEGF to induce a psoriasiform phenotype suggests a new etiology and treatment approach for this disease and further substantiates emerging concerns about possible proinflammatory adverse effects that might be associated with therapeutic attempts to deliver VEGF.     

Elevated placental soluble vascular endothelial growth factor receptor-1 inhibits angiogenesis in preeclampsia

Preeclampsia is an inflammatory disorder in which serum levels of vascular endothelial growth factor (VEGF) and its soluble receptor-1 (sVEGFR-1, also known as sFlt-1) are elevated. We hypothesize that VEGF and placenta growth factor (PlGF) are dysregulated in preeclampsia due to high levels of sVEGFR-1, which leads to impaired placental angiogenesis. Analysis of supernatants taken from preeclamptic placental villous explants showed a four-fold increase in sVEGFR-1 than normal pregnancies, suggesting that villous explants in vitro retain a hypoxia memory reflecting long-term fetal programming. The relative ratios of VEGF to sVEGFR-1 and PlGF to sVEGFR-1 released from explants decreased by 53% and 70%, respectively, in preeclampsia compared with normal pregnancies. Exposure of normal villous explants to hypoxia increased sVEGFR-1 release compared with tissue normoxia (P<0.001), as did stimulation with tumor necrosis factor-alpha (P<0.01). Conditioned medium (CM) from normal villous explants induced endothelial cell migration and in vitro tube formation, which were both attenuated by pre-incubation with exogenous sVEGFR-1 (P<0.001). In contrast, endothelial cells treated with preeclamptic CM showed substantially reduced angiogenesis compared with normal CM (P<0.001), which was not further decreased by the addition of exogenous sVEGFR-1, indicating a saturation of the soluble receptor. Removal of sVEGFR-1 by immunoprecipitation from preeclamptic CM significantly restored migration (P<0.001) and tube formation (P<0.001) to levels comparable to that induced by normal CM, demonstrating that elevated levels of sVEGFR-1 in preeclampsia are responsible for inhibiting angiogenesis. Our finding demonstrates the dysregulation of the VEGF/PlGF axis in preeclampsia and offers an entirely new therapeutic approach to its treatment.     

The discovery of the placental growth factor and its role in angiogenesis: a historical review

The placental growth factor (PlGF) is an angiogenic protein belonging to the vascular endothelial growth factor (VEGF) family, which was discovered in 1991 by an Italian scientist, Maria Graziella Persico. Dr Persico cloned and purified PlGF and determined its structure by crystallography resolution. Furthermore, she identified VEGF receptor-1 (VEGFR-1) as the receptor for PlGF, and in collaboration with Dr Peter Carmeliet in Leuven, she generated evidence that loss of PlGF does not affect development, reproduction, or postnatal life. PlGF is expressed primarily in the placenta and is up-regulated in several pathological conditions, although its role is still controversial. Some data in literature reported that PlGF enhances pathological angiogenesis by initiating a cross-talk between VEGFR-1 and VEGFR-2, whereas other studies did not confirm these findings. Regarding the potential therapeutic employment of PlGF, recent evidence has shown that an anti-PlGF antibody may act as a potent antiangiogenic agent, and that it has the advantage of minor toxicity when combined with anti-VEGF strategies.     

Le VEGF en physiologie et pathologie thyroïdienne

Angiogenesis is a physiological process involving the growth of new vessels from preexisting vasculature. The vascular endothelial growth factor (VEGF) is an important regulator of both benign and malignant disease process in the thyroid gland. The VEGF family includes seven members called respectively VEGF-A, also known as the VPF (vascular permeability factor), VEGF-B, VEGF-C, VEGF-D, all described in mammalians, VEGF-E (found in parapoxviridae), VEGF-F (also called svVEGF, for snake venom VEGF, found in the venom of viper) and PlGF (for placental growth factor). The thyrocytes are able to synthesize and to secrete the VEGF. VEGF-A is implicated in tumour growth and metastasis via blood vessels while VEGF-C and VEGF-D, implicated in lymphangiogenesis, favour metastasis to the cervical lymph nodes during papillary thyroid carcinomas. The importance of VEGF expression could correlate with a poorer outcome in papillary thyroid carcinomas. Because of its important role in malignant angiogenesis, the VEGF is the privileged target of a new variety of therapeutic agents called angiogenesis inhibitors.     

Overexpression of the soluble vascular endothelial growth factor receptor in preeclamptic patients: pathophysiological consequences

Several growth factors such as vascular endothelial growth factor (VEGF)-A and placental growth factor (PlGF) are involved in the placental vascular development. We investigated whether dysregulation in the VEGF family may explain the defective uteroplacental vascularization characterizing preeclampsia. We compared pregnancies complicated by early onset severe preeclampsia or intrauterine growth retardation to normal pregnancies. Maternal plasma, placentas, and placental bed biopsies were collected. The mRNA levels of VEGF-A, PlGF, and their receptors were quantified in placentas and placental beds. Levels of VEGF-A, PlGF, and soluble VEGF receptor (VEGFR) were assessed in maternal plasma. In compromised pregnancies, elevated levels of VEGF-A and VEGFR-1 mRNAs may reflect the hypoxic status of the placenta. On contrast, the membrane-bound VEGFR-1 was decreased in the placental bed of preeclamptic patients. Preeclampsia was associated with low levels of circulating PlGF and increased levels of total VEGF-A and soluble VEGFR-1. Free VEGF-A was undetectable in maternal blood. Immunohistochemical studies revealed that VEGF-A and PlGF were localized in trophoblastic cells. Altogether, our results suggest two different pathophysiological mechanisms associated with preeclampsia. The first one is related to an overproduction of competitive soluble VEGFR-1 that may lead to suppression of VEGF-A and PlGF effects. The second one is the down-regulation of its membrane bound form (VEGFR-1) in the placental bed, which may result in the defective uteroplacental development.     

Angiogenic growth factors and hypertension

Emerging evidence supports a novel view of hypertension as a disease of inadequate or aberrant responses to angiogenic growth factors (AGF). Patients with hypertension have reduced microvascular density, with some evidence supporting a primary role for rarefaction in causing hypertension. Two clinical models have demonstrated a link between inhibition of AGF activity and hypertension. A major side effect of bevacizumab, a monoclonal antibody to vascular endothelial growth factor (VEGF), is hypertension. Pre-eclampsia is accompanied by high circulating levels of soluble VEGF receptor-1, which forms inactive complexes with VEGF and placental growth factor (PlGF). Paradoxically, early studies have demonstrated high circulating levels of AGF in hypertension. Several mechanisms may account for this finding including increased vascular stretch, tissue ischemia, compensatory responses, decreased clearance or a combination of these mechanisms. High AGF in hypertension could contribute to clinical sequelae such as peripheral and pulmonary edema, microalbuminuria, and progression of atherosclerosis. However, a role for altered angiogenesis in the pathogenesis of hypertension or its sequelae has not been established. Novel studies to understand the roles of AGF in hypertensive patients are warranted.