Quantitation of angiogenesis and its correlation with vascular endothelial growth factor expression in astrocytic tumors

OBJECTIVE: To quantitate tumor angiogenesis by establishing intratumoral microvessel density (IMD), to study vascular endothelial growth factor (VEGF) expression in different grades of astrocytomas and to correlate VEGF expression with tumor angiogenesis. STUDY DESIGN: Forty cases of astrocytic neoplasms (10 of each grade) were assessed for tumor angiogenesis and VEGF expression. The panendothelial marker CD31 was used to highlight microvessels. Tumor angiogenesis was quantitated as IMD count per square millimeter in areas of high vascularity, or "hot spots," using an image analyzer. VEGF expression was studied in sections of the tumors. IMD counts per square millimeter and VEGF expression were correlated with histologic grade. The angiogenic potential of tumors as reflected by IMD counts per square millimeter was correlated with the intensity of VEGF expression. RESULTS: Vascular proliferation in high grade gliomas was significantly higher as compared to that in low grade gliomas. IMD count per square millimeter revealed a positive correlation with histologic grade in high grade gliomas. Pilocytic astrocytoma and low grade astrocytoma as a group had comparable IMD counts per square millimeter. VEGF expression paralleled IMD counts in rare high grade gliomas only. CONCLUSION: Malignant progression in astrocytoma is heralded and accompanied by increased angiogenesis. VEGF is an important angiogenic factor in high grade gliomas since its expression parallels the increased IMD counts in these tumors. In contrast, in low grade gliomas, angiogenic factors other than VEGF may contribute to vascular proliferation. The results emphasize the role of antiangiogenic therapy as an optimal tool in therapeutic strategies as they become available.     

人骨肉瘤OS-732细胞系诱导血管生成作用及相关因子表达的研究

应用鸡胚绒毛尿囊膜模型（chick embryo chorioallantoic membrane,CAM),观察人骨肉瘤OS-732细胞系诱导血管生成过程及血管生长相关因子的表达。结果显示,本细胞系具有较强的促血管生成能力并表达血管内皮生长因子（vacular endothelial growth factor,VEGF),碱性成纤维细胞生长因子（basic fibroblast growth factor,bFGF0,鸡胚绒毛尿囊膜OS-732细胞系接种瘤细胞中血管内皮生长因子（VEGF）,转化生长因子β1(Transforming growth factor,TGF-β1）均呈阳性表达,而且VEGF呈持续高表达,结果表明VEGF,bFGF、TGF-β1可能共同参与骨肉瘤OS-732细胞系诱导的血管生成,而VEGF可能起着主要作用,提示阻断VEGF的作用可能影响骨肉瘤OS-732细胞系诱导的血管生成,此研究为以VEGF为靶点进行抗血管生成实验提供了依据。     

Potent synergism between vascular endothelial growth factor and basic fibroblast growth factor in the induction of angiogenesis in vitro.

Vascular endothelial growth factor (VEGF), also known as vascular permeability factor or vasculotropin, is a recently characterized endothelial-specific mitogen which is angiogenic in vivo. Here we demonstrate that VEGF is angiogenic in vitro: when added to microvascular endothelial cells grown on the surface of three-dimensional collagen gels, VEGF induces the cells to invade the underlying matrix and to form capillary-like tubules, with an optimal effect at approximately 2.2nM (100ng/ml). When compared to basic fibroblast growth factor (bFGF) at equimolar (0.5nM) concentrations, VEGF was about half as potent. The most striking effect was seen in combination with bFGF: when added simultaneously, VEGF and bFGF induced an in vitro angiogenic response which was far greater than additive, and which occurred with greater rapidity than the response to either cytokine alone. These results demonstrate that like bFGF, VEGF induces an angiogenic response via a direct effect on endothelial cells, and that by acting in concert, these two cytokines have a potent synergistic effect on the induction of angiogenesis in vitro. We suggest that the synergism between VEGF and bFGF plays an important role in the control of angiogenesis in vivo.     

Clinical relevance of serum angiogenic activity in patients with transitional cell carcinoma of the bladder

Angiogenesis is essential for tumor growth and progression and is mediated by positive and negative regulators of vessel growth. Since angiogenic mediators found in patient serum have been postulated to reflect the angiogenic potential of a malignant tumor, we investigated the angiogenic activity in the serum of patients with transitional cell carcinoma (TCC). The data were correlated to tumor characteristics and the clinical course of the patients. Eighty-one patients with transitional cell carcinoma and 53 control persons were included in the study. Preoperative serum samples were collected and both vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) were quantified by ELISA. Additionally, the serum evoked proliferative activity on human umbilical vein endothelial cells (HUVEC) was evaluated. Data were compared to the clinical course of the patients. Serum of tumor patients significantly enhanced the proliferative capacity of HUVEC, compared to cells grown in standard culture medium (p = 0.0032), but not when compared to serum from control persons. Serum from patients with superficial TCC and well differentiated tumors induced a significantly higher angiogenic response (ANG(hi)) than serum from patients with poorly differentiated and invasive carcinomas (ANG(lo); p = 0.037). VEGF level of ANG(hi) serum was 384.22 +/- 247.76 pg/ml (n = 37) which significantly differed from mean VEGF level detected in ANG(lo) serum (247.72 +/- 211.93 pg/ml, n = 42; p = 0.019). Similarly, mean bFGF levels were 9.58 +/- 5.91 pg/ml in ANG(hi) serum versus 5.74 +/- 3.52 pg/ml) in ANG(lo) serum (p = 0.0043). A negative correlation was established between VEGF/bFGF serum concentration and patient prognosis. The experiments demonstrate a positive correlation between VEGF and bFGF serum level and endothelial proliferation in vitro. The inverse relationship between angiogenic activity and tumor stage might disclose information about angiogenesis and tumor progression in TCC.     

Distinct angiogenic mediators are required for basic fibroblast growth factor- and vascular endothelial growth factor-induced angiogenesis: the role of cytoplasmic tyrosine kinase c-Abl in tumor angiogenesis

Signaling pathways engaged by angiogenic factors bFGF and VEGF in tumor angiogenesis are not fully understood. The current study identifies cytoplasmic tyrosine kinase c-Abl as a key factor differentially mediating bFGF- and VEGF-induced angiogenesis in microvascular endothelial cells. STI571, a c-Abl kinase inhibitor, only inhibited bFGF- but not VEGF-induced angiogenesis. bFGF induced membrane receptor cooperation between integrin beta(3) and FGF receptor, and triggered a downstream cascade including FAK, c-Abl, and MAPK. This signaling pathway is different from one utilized by VEGF that includes integrin beta(5), VEGF receptor-2, Src, FAK, and MAPK. Ectopic expression of wild-type c-Abl sensitized angiogenic response to bFGF, but kinase dead mutant c-Abl abolished this activity. Furthermore, the wild-type c-Abl enhanced angiogenesis in both Matrigel implantation and tumor xenograft models. These data provide novel insights into c-Abl's differential functions in mediating bFGF- and VEGF-induced angiogenesis.     

Hypoxic induction of endothelial cell growth factors in retinal cells: identification and characterization of vascular endothelial growth factor (VEGF) as the mitogen.

BACKGROUND: New vessel growth is often associated with ischemia, and hypoxic tissue has been identified as a potential source of angiogenic factors. In particular, ischemia is associated with the development of neovascularization in a number of ocular pathologies. For this reason, we have studied the induction of endothelial cell mitogens by hypoxia in retinal cells. MATERIALS AND METHODS: Human retinal pigment epithelium (hRPE) were grown under normoxic and hypoxic conditions and examined for the production of endothelial mitogens. Northern analysis, biosynthetic labeling and immunoprecipitation, and ELISA were used to assess the levels of vascular endothelial growth factor/vascular permeability factor (VEGF) and basic fibroblast growth factor (bFGF), two endothelial cell mitogens and potent angiogenic factors. Soluble receptors for VEGF were employed as competitive inhibitors to determine the contribution of the growth factor to the hypoxia-stimulated mitogen production. RESULTS: Following 6-24 hr of hypoxia, confluent and growing cultures of hRPE increase their levels of VEGF mRNA and protein synthesis. Biosynthetic labeling studies and RT-PCR analysis indicate that the cells secrete VEGF121 and VEGF165, the soluble forms of the angiogenic factor. In contrast, hRPE cultured under hypoxic conditions show reduced steady-state levels of basic fibroblast growth factor (bFGF) mRNA and decreased bFGF protein synthesis. Unlike VEGF, bFGF is not found in conditioned media of hRPE following 24 hr of hypoxia. Using a soluble high-affinity VEGF receptor as a competitive inhibitor of VEGF, we demonstrate that a VEGF-like activity is the sole hypoxia-inducible endothelial mitogen produced by cultured hRPE. CONCLUSIONS: From this comparison we conclude that hRPE do not respond to hypoxia with a general, nonspecific increase in the overall levels of growth factors, as is seen during cell wounding responses or serum stimulation. The physiological relevance of data from this in vitro model are affirmed by separate studies in an animal model of retinal ischemia-induced ocular neovascularization (1) in which retina-derived VEGF levels have been shown to correlate spatio-temporally with the onset of angiogenesis. Taken together, these data support the hypothesis that the induction of VEGF by hypoxia mediates the rapid, initial angiogenic response to retinal ischemia.     

Prognostic significance of IL-8-STAT-3 pathway in astrocytomas: correlation with IL-6, VEGF and microvessel morphometry

Malignant astrocytomas are highly vascular neoplasms characterized by a potent angiogenic and immunosuppressive phenotype. Th2-cytokines (IL-6/IL-8) are implicated as major regulators of glioma cell growth and invasiveness. STAT-3, a downstream transducer of cytokine signaling is positively associated with tumor angiogenesis. The present study aimed to investigate the expression of IL-8 and p-STAT-3 in 97 diffusely infiltrating astrocytomas of various grades, in relation to IL-6, VEGF, clinicopathological features, microvascular characteristics and patients' survival. IL-8 expression was localized in neoplastic cells, being associated with p-STAT-3 (p = 0.0013), IL-6 (p = 0.0004) and VEGF (p < 0.0001) around areas of necrosis as well as in perivascular inflammatory and endothelial cells. All the molecules under study correlated with tumor grade and degree of necrosis (p < 0.05, respectively). p-STAT-3, IL-8 and VEGF expression was positively associated with microvessel density (p = 0.0491, p < 0.0001 and p = 0.0118, respectively). Univariate analysis indicated that overexpression of IL-8 and IL-6 adversely affected survival in the entire cohort whereas increased p-STAT-3 expression was predictive of improved survival in high grade (III/IV) astrocytomas (p = 0.0032). In multivariate analysis only IL-8 expression (p = 0.043) retained its significance. The prognostic significance of IL-8 expression and its correlation with p-STAT-3 and VEGF implicates this novel signaling pathway in astroglial tumors progression providing new targets for effective immunotherapy.     

Hepatocyte growth factor is a major mediator in heparin-induced angiogenesis

Heparin has a potent angiogenic effect in experimental animals and patients with ischemic diseases; however, the precise mechanism behind this angiogenesis remains to be clarified. The aim of this study was to determine whether the administration of heparin affects the levels of heparin-binding angiogenic factors in human plasma, and to identify the molecule responsible for heparin-induced angiogenesis. Plasma levels of hepatocyte growth factor (HGF), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) were measured before and after administration of 100 U, 3,000 U or 10,000 U of heparin in patients with coronary artery disease. Administration of 3,000 U or 10,000 U of heparin caused significant increases in plasma HGF (40- and 54-fold, respectively), in absence of obvious increases in bFGF and VEGF levels. Furthermore, compared with the serum collected before heparin administration, the serum collected after heparin administration had more prominent growth-promoting and vascular tube-inducing properties on endothelial cells, and these increased activities were completely inhibited by neutralization of HGF, whereas neutralization of bFGF and VEGF had no effect. These findings suggest that HGF plays a significant role in heparin-induced angiogenesis.     

In Vitro Angiogenic and Proteolytic Properties of Bovine Lymphatic Endothelial Cells

The aim of the present study was to determine whether angiogenic cytokines, which induce neovascularization in the blood vascular system, might also be operative in the lymphatic system. In an assay of spontaneous in vitro angiogenesis, endothelial cells isolated from bovine lymphatic vessels retained their histotypic morphogenetic properties by forming capillary-like tubes. In a second assay, in which endothelial cells could be induced to invade a three-dimensional collagen gel within which they formed tube-like structures, lymphatic endothelial cells responded to basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) in a manner similar to what has previously been observed with endothelial cells derived from the blood vascular system. Finally, since angiogenesis is believed to require extracellular proteolytic activity, we investigated the effects of bFGF and VEGF on lymphatic endothelial cell proteolytic properties by focussing on the plasminogen activator (PA) system. bFGF and VEGF increased urokinase, urokinase receptor, and tissue-type PA expression. This was accompanied by an increase in PA inhibitor-l, which is thought to play an important permissive role in angiogenesis by protecting the extracellular matrix against excessive proteolytic degradation. Taken together, these results demonstrate that with respect to in vitro morphogenetic and proteolytic properties, lymphatic endothelial cells respond to the previously described angiogenic factors, bFGF and VEGF, in a manner very similar to what has been described for endothelial cells derived from the blood vascular system.     

Marked induction of the IAP family antiapoptotic proteins survivin and XIAP by VEGF in vascular endothelial cells.

Vascular endothelial growth factor (VEGF) is a potent angiogenic factor that has been shown to act as an endothelial cell mitogen as well as a vascular permeability factor. Several recent reports have also implicated VEGF as a major survival factor for endothelial cells during angiogenesis and vasculogenesis along with other growth factors such as bFGF and angiopoietin-1. VEGF has been shown to mediate this additional function, at least in part through the induction of bcl-2 and the activation of the PI3 kinase-Akt/PKB signaling pathway. We report here that VEGF can also mediate the induction/upregulation of members of a newly discovered family of antiapoptotic proteins, namely the Inhibitors of Apoptosis (IAP), in vascular endothelial cells. We show that VEGF(165) leads to the induction of XIAP (2.9-fold) and survivin (19.1-fold) protein in human umbilical vein endothelial cells (HUVECs). In contrast, bFGF had little effect on XIAP expression, but produced approximately a 10-fold induction on survivin. VEGF-dependent upregulation of survivin could be prevented by cell cycle arrest in the G1 and S phases. These findings implicate that the survival and mitotic functions of VEGF in an angiogenic context may be more intrinsically related than previously anticipated. Moreover, they also raise the possibility of therapeutically targeting XIAP or survivin in antiangiogenic therapy as a means of suppressing tumor growth, in addition to directly targeting tumor cells which express these survival proteins.     

Site-dependent angiogenic cytokine production in human tumor xenografts

Tumor microenvironment plays a critical role in tumor growth, angiogenesis, and metastasis. Differences in site of tumor implantation result in differences in tumor growth, metastasis, as well as response to chemotherapy. We hypothesized that tumor-induced angiogenic growth factor production into the plasma will also be influenced by site of tumor implantation. We evaluated the site-dependent production of angiogenic growth factors in the plasma of tumor bearing animals at two different sites of implantation. Plasma levels of tumor necrosis factor-alpha (TNF-alpha), basic fibroblast growth factor (bFGF), and vascular endothelial growth factor (VEGF) were evaluated in nude mice bearing A2780, SKOV-3, or OVCAR-3 human ovarian tumors, as well as Panc-1, AsPC-1, or BxPC-3 human pancreatic tumors grown as subcutaneous (SC) xenografts or in the intraperitoneal (IP) cavity. Plasma VEGF and bFGF levels produced by two ovarian tumor lines and two pancreatic tumor lines were substantially higher when the tumors were implanted in the IP cavity than in the SC space. These studies indicated that the site of tumor implantation was an important determinant in the production of plasma VEGF and bFGF levels. As more and more anti-angiogenic agents are developed, the need for appropriate animal models becomes apparent. These results suggest the demand for an appropriate model for the in vivo evaluation of anti-angiogenesis.     

Angiogenesis activators and inhibitors differentially regulate caveolin-1 expression and caveolae formation in vascular endothelial cells. Angiogenesis inhibitors block vascular endothelial growth factor-induced down-regulation of caveolin-1.

Angiogenesis is the process by which new blood vessels are formed via proliferation of vascular endothelial cells. A variety of angiogenesis inhibitors that antagonize the effects of vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF) have recently been identified. However, the mechanism by which these diverse angiogenesis inhibitors exert their common effects remains largely unknown. Caveolin-1 and -2 are known to be highly expressed in vascular endothelial cells both in vitro and in vivo. Here, we examine the potential role of caveolins in the angiogenic response. For this purpose, we used the well established human umbilical vein endothelial cell line, ECV 304. Treatment of ECV 304 cells with known angiogenic growth factors (VEGF, bFGF, or hepatocyte growth factor/scatter factor), resulted in a dramatic reduction in the expression of caveolin-1. This down-regulation event was selective for caveolin-1, as caveolin-2 levels remained constant under these conditions of growth factor stimulation. VEGF-induced down-regulation of caveolin-1 expression also resulted in the morphological loss of cell surface caveolae organelles as seen by transmission electron microscopy. A variety of well characterized angiogenesis inhibitors (including angiostatin, fumagillin, 2-methoxy estradiol, transforming growth factor-beta, and thalidomide) effectively blocked VEGF-induced down-regulation of caveolin-1 as seen by immunoblotting and immunofluorescence microscopy. However, treatment with angiogenesis inhibitors alone did not significantly affect the expression of caveolin-1. PD98059, a specific inhibitor of mitogen-activated protein kinase and a known angiogenesis inhibitor, also blocked the observed VEGF-induced down-regulation of caveolin-1. Furthermore, we show that caveolin-1 can function as a negative regulator of VEGF-R (KDR) signal transduction in vivo. Thus, down-regulation of caveolin-1 may be an important step along the pathway toward endothelial cell proliferation.     

Epoxyeicosatrienoic acids are part of the VEGF-activated signaling cascade leading to angiogenesis

Cytochrome P-450 (CYP) epoxygenases metabolize arachidonic acid to epoxyeicosatrienoic acid (EET) regioisomers, which activate several signaling pathways to promote endothelial cell proliferation, migration, and angiogenesis. Since vascular endothelial growth factor (VEGF) plays a key role in angiogenesis, we assessed a possible role of EETs in the VEGF-activated signal transduction cascade. Stimulation with VEGF increased CYP2C promoter activity in endothelial cells and enhanced CYP2C8 mRNA and protein expression resulting in increased intracellular EET levels. VEGF-induced endothelial cell tube formation was inhibited by the EET antagonist 14,15-epoxyeicosa-5(Z)-enoicacid (14,15-EEZE), which did not affect the VEGF-induced phosphorylation of its receptor or basic fibroblast growth factor (bFGF)-stimulated tube formation. Moreover, VEGF-stimulated endothelial cell sprouting in a modified spheroid assay was reduced by CYP2C antisense oligonucleotides. Mechanistically, VEGF stimulated the phosphorylation of the AMP-activated protein kinase (AMPK), which has also been linked to CYP induction, and the overexpression of a constitutively active AMPK mutant increased CYP2C expression. On the other hand, a dominant-negative AMPK mutant prevented the VEGF-induced increase in CYP2C RNA and protein expression in human endothelial cells. In vivo (Matrigel plug assay) in mice, endothelial cells were recruited into VEGF-impregnated plugs; an effect that was sensitive to 14,15-EEZE and the inclusion of small interfering RNA directed against the AMPK. The EET antagonist did not affect responses observed in plugs containing bFGF. Taken together, our data indicate that CYP2C-derived EETs participate as second messengers in the angiogenic response initiated by VEGF and that preventing the increase in CYP expression curtails the angiogenic response to VEGF.     

Placental growth throughout the last two thirds of pregnancy in sheep: vascular development and angiogenic factor expression

Morphometric methodologies were developed and applied to investigate the patterns of vascular development in maternal (caruncular; CAR) and fetal (cotyledonary; COT) sheep placentas throughout the last two thirds of gestation. We also examined the expression levels of the major angiogenic factors and their receptors in CAR and COT sheep placentas. Although the vascularity of the CAR tissues increased continuously from Day 50 through Day 140 of pregnancy, those of the COT tissues increased at about twice the instantaneous rate (i.e., the proportionate increase/day) of the CAR. For CAR, vascularity increased 2-fold from Day 50 through Day 140 via relatively small increases in capillary number and 2- to 3-fold increases in capillary diameter. For COT, the increased vascularity resulted from a 12-fold increase in capillary number associated with a concomitant 2-fold decrease in capillary diameter. This large increase in fetal placental capillary number, which was due to increased branching, resulted in 6-fold increases in total capillary cross-sectional area and total capillary surface, per unit of COT tissue. Different patterns of expression of the mRNAs for angiogenic factors and their receptors were observed for CAR and COT. The dilation-like angiogenesis of CAR was correlated with the expression of vascular endothelial growth factor receptor-1 (FLT1), angiopoietin-2 (ANGPT2), and soluble guanylate cyclase (GUCY1B3) mRNAs. The branching-like angiogenesis of COT was correlated with the expression of vascular endothelial growth factor (VEGF), FLT1, angiopoietin-1 (ANGPT1), ANGPT2, and FGF2 mRNAs. Monitoring the expression of angiogenic factors and correlating the levels with quantitative measures of vascularity enable one to model angiogenesis in a spatiotemporal fashion.     

Angiogenic activity of bFGF and VEGF suppressed by proteolytic cleavage by neutrophil elastase

Neutrophil elastase (NE), a serine protease released from the azurophil granules of activated neutrophil, proteolytically cleaves multiple cytokines, and cell surface proteins. In the present study, we examined whether NE affects the biological abilities of angiogenic growth factors such as basic-fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF). NE degraded bFGF and VEGF in a time- and concentration-dependent manner, and these degradations were suppressed by sivelestat, a synthetic inhibitor of NE. The bFGF- or VEGF-mediated proliferative activity of human umbilical vein endothelial cells was inhibited by NE, and the activity was recovered by sivelestat. Furthermore, NE reduced the bFGF- or VEGF-induced tubulogenic response of the mice aortas, ex vivo angiogenesis assay, and these effects were also recovered by sivelestat. Neutrophil-derived NE degraded potent angiogenic factors, resulting in loss of their angiogenic activity. These findings provide additional insight into the role played by neutrophils in the angiogenesis process at sites of inflammation.     

Vascular endothelial growth factor (VEGF)-C synergizes with basic fibroblast growth factor and VEGF in the induction of angiogenesis in vitro and alters endothelial cell extracellular proteolytic activity

Vascular endothelial growth factor-C (VEGF-C) is a recently characterized member of the VEGF family of angiogenic polypeptides. We demonstrate here that VEGF-C is angiogenic in vitro when added to bovine aortic or lymphatic endothelial (BAE and BLE) cells but has little or no effect on bovine microvascular endothelial (BME) cells. As reported previously for VEGF, VEGF-C and basic fibroblast growth factor (bFGF) induced a synergistic in vitro angiogenic response in all three cells lines. Unexpectedly, VEGF and VEGF-C also synergized in the in vitro angiogenic response when assessed on BAE cells. Characterization of VEGF receptor (VEGFR) expression revealed that BME, BAE, and BLE cell lines express VEGFR-1 and -2, whereas of the three cell lines assessed, only BAE cells express VEGFR-3. We also demonstrate that VEGF-C increases plasminogen activator (PA) activity in the three bovine endothelial cell lines and that this is accompanied by a concomitant increase in PA inhibitor-1. Addition of α₂-antiplasmin to BME cells co-treated with bFGF and VEGF-C partially inhibited collagen gel invasion. These results demonstrate, first, that by acting in concert with bFGF or VEGF, VEGF-C has a potent synergistic effect on the induction of angiogenesis in vitro and, second, that like VEGF and bFGF, VEGF-C is capable of altering endothelial cell extracellular proteolytic activity. These observations also highlight the notion of context, i.e., that the activity of an angiogenesis-regulating cytokine depends on the presence and concentration of other cytokines in the pericellular environment of the responding endothelial cell. J. Cell. Physiol. 177:439–452, 1998. © 1998 Wiley-Liss, Inc.