Effect of VEGF and VEGF Trap on vascular endothelial cell signaling in tumors

Vascular endothelial growth factor (VEGF) A is a major promoter of tumor angiogenesis and a prime target of antiangiogenic cancer therapy. To examine whether endothelial cell signaling might provide histological biomarkers of angiogenesis and VEGF activity in vivo, normal mouse organs and multiple tumor models were studied immunohistochemically for endothelial expression of activated ERK, STAT3, and AKT. Phospho(p)-ERK and p-STAT3 expression was negligible in the endothelia of normal organs but was significantly elevated in tumor endothelium. p-AKT was present at significant and comparable levels in both tumor and normal endothelia. In K1735 tumors induced to express more VEGF, endothelial p-ERK, p-STAT3 and p-AKT increased accompanied by signs of accelerated angiogenesis. Treatment of K1735 and Colo-205 tumors with the VEGF inhibitor, VEGF Trap (aflibercept), decreased tumor endothelial p-ERK, p-STAT3 and p-AKT expression accompanied by signs of antiangiogenic effect. These results show that endothelial p-ERK and p-STAT3 (but not p-AKT) distinguish tumor from normal vessels and that the presence of these two signaling intermediates may be useful indicators of tumor angiogenic activity and angiogenesis inhibition by VEGF antagonist.     

Activated STAT3 is a mediator and biomarker of VEGF endothelial activation

STAT3 plays important roles in cell proliferation and survival signaling and is often constitutively activated in transformed cells. In this study, we examined STAT3 activation in endothelial cells (EC) during angiogenic activation and therapeutic angiogenesis inhibition. VEGF stimulation of cultured EC induced STAT3 phosphorylation by a VEGFR2- and Src-dependent mechanism. FGF2 but not PlGF also induced EC STAT3 activation in vitro. Activated STAT3 mediated VEGF induction of EC Bcl-2 and contributed to VEGF protection of EC from apoptosis. In vivo, p-STAT3 was absent by immunohistological staining in the vascular EC of most normal mouse organs but was present in the vessels of mouse and human tumors. Tumor vascular p-STAT3 increased as tumors were induced to overexpress VEGF, indicating that VEGF is an activator of EC p-STAT3 in vivo. Tumor vascular p-STAT3 decreased during angiogenesis inhibition by antagonists of VEGF-VEGFR signaling, VEGF Trap and SU5416, indicating that VEGF contributed to the EC STAT3 activation seen in the tumors prior to treatment and that p-STAT3 may be used to monitor therapy. These studies show that p-STAT3 is a mediator and biomarker of endothelial activation that reports VEGF-VEGFR2 activity and may be useful for studying the pharmacodynamics of targeted angiogenesis inhibitors.     

Transfection of the genes for interleukin-12 into the K1735 melanoma and the EMT6 mammary sarcoma murine cell lines reveals distinct mechanisms of antitumor activity

Interleukin 12 (IL-12) is a pleiotropic cytokine with multiple effects on the immune system. The antitumor effects of locally produced IL-12 were examined in 2 tumor model systems. IL-12 expressing EMT6 mammary sarcomas (EMT6/IL-12) grew temporarily and then regressed resulting in mice that were immune to a further challenge of EMT6 cells. Interestingly, the IL-12 expressing K1735 melanomas (K1735/IL-12) maintained a lag phase of nonmeasurable growth for several weeks, followed by tumor outgrowth that was associated with a loss of IL-12 production. Tumor-infiltrating lymphocytes (TILs) isolated from EMT6/IL-12 tumors effectively lysed EMT6 target cells, whereas K1735/IL-12 TILs lacked lytic activity. Both IL-12 expressing tumors, however, grew progressively in nude mice indicating an important role for T cells in each case. Recombinant murine interferon gamma (rmIFN-gamma) inhibited the growth of EMT6 cells, but not K1735 cells in vitro, and strongly induced the expression of the antiangiogenic chemokine interferon-inducible protein 10 (IP-10) by both cell lines. Of interest, only the EMT6 cell line was able to secrete the proangiogenic molecule, vascular endothelial growth factor (VEGF), in response to low oxygen conditions. Fluorescent staining of the vascular endothelium at the tumor injection site provided images depicting early stages of angiogenesis prior to K1735/IL-12 tumor outgrowth. These results indicate that locally produced IL-12 likely mediates the rejection of EMT6 tumors through tumor cell lysis by host immune cells, whereas its antiangiogenic potential may be counterbalanced by the strong induction of VEGF by hypoxic tumor cells. In contrast, IL-12 does not induce protective immunity to K1735 tumors. However, an antiangiogenic mechanism may be responsible for controlling tumor growth.     

VEGF Trap induces antiglioma effect at different stages of disease

Pathological angiogenesis is a hallmark of cancer, specifically of glioblastomas, the most malignant and common primary brain tumor. Vascular endothelial growth factor (VEGF) is the key protein in the regulation of the hypervascular phenotype of primary malignant brain tumors. In this study, we tested VEGF Trap, a soluble decoy receptor for VEGF, in an intracranial glioma model. VEGF Trap was administered in short or prolonged schedules to animals bearing human gliomas at different stages of disease. Of importance, VEGF Trap treatment was efficacious in both initial and advanced phases of tumor development by significantly increasing overall survival. Furthermore, this effect was enhanced in animals treated with more prolonged regimens. In addition, we observed the emergence of a VEGF Trap-resistant phenotype characterized by tumor growth and increased invasiveness. Our results suggest that VEGF Trap will be effective in treating both patients with recurrent or progressive resectable glioblastoma and patients that have undergone extensive initial surgery. Finally, our results indicate that the clinical success of VEGF Trap may depend on a prolonged treatment in combined therapy aiming to simultaneously inhibit angiogenesis and tumor invasion.     

Expression of vascular endothelial growth factor (VEGF) and VEGF receptors in tumor angiogenesis and malignancies

Angiogenesis is a process by which new blood vessels are formed from preexisting vessels. New blood vessel formation by angiogenesis involves the degradation of extra-cellular matrix combined with sprouting and migration of endothelial cells from preexisting capillaries. Solid tumors consist of several components, including normal and stromal cells, extracellular matrix, and vasculature. To grow and metastasize, tumors must stimulate the development of new vasculature through angiogenesis. Vascular endothelial growth factor (VEGF) is a potent angiogenic peptide with biologic effects that include regulation of hematopoietic stem cell development, extracellular matrix remodeling, and inflammatory cytokine regeneration. VEGF is both a vascular growth factor and a vascular permeability factor. Its expression can upregulate several proangiogenic and prometa-static molecules. As a central mediator of angiogenesis, VEGF has emerged as an important target for antiangiogenic therapy. In this review, the authors describe the essential characteristics of VEGF and the VEGF family of ligands and their receptors. They also provide an overview of the central role of VEGF in physiologic and pathologic angiogenesis, directly or indirectly. This review sheds light on the importance of VEGF-targeted antiangiogenic therapy based on the monoclonal antibodies against VEGF, small interfering RNA, and therapy directed against VEGF-VEGFR kinase. It also gives a brief overview of the natural products or dietary compounds that could be used as antiangiogenic agents. Therapeutic inhibition of vessel formation could be best suited to preventive strategies aimed at the suppression of angiogenesis in primary tumors in subjects at risk or of micrometastases after surgical removal of primary tumor.     

VEGF165b, an inhibitory vascular endothelial growth factor splice variant: mechanism of action, in vivo effect on angiogenesis and endogenous protein expression

Growth of new blood vessels (angiogenesis), required for all tumor growth, is stimulated by the expression of vascular endothelial growth factor (VEGF). VEGF is up-regulated in all known solid tumors but also in atherosclerosis, diabetic retinopathy, arthritis, and many other conditions. Conventional VEGF isoforms have been universally described as proangiogenic cytokines. Here, we show that an endogenous splice variant, VEGF(165)b, is expressed as protein in normal cells and tissues and is circulating in human plasma. We also present evidence for a sister family of presumably inhibitory splice variants. Moreover, these isoforms are down-regulated in prostate cancer. We also show that VEGF(165)b binds VEGF receptor 2 with the same affinity as VEGF(165) but does not activate it or stimulate downstream signaling pathways. Moreover, it prevents VEGF(165)-mediated VEGF receptor 2 phosphorylation and signaling in cultured cells. Furthermore, we show, with two different in vivo angiogenesis models, that VEGF(165)b is not angiogenic and that it inhibits VEGF(165)-mediated angiogenesis in rabbit cornea and rat mesentery. Finally, we show that VEGF(165)b expressing tumors grow significantly more slowly than VEGF(165)-expressing tumors, indicating that a switch in splicing from VEGF(165) to VEGF(165)b can inhibit tumor growth. These results suggest that regulation of VEGF splicing may be a critical switch from an antiangiogenic to a proangiogenic phenotype.     

Use of soluble recombinant decoy receptor vascular endothelial growth factor trap (VEGF Trap) to inhibit vascular endothelial growth factor activity

Primary tumors and metastases require blood vessel formation to support their continued growth and eventual metastasis. They use existing vasculature during initial growth but eventually must orchestrate the development and maintenance of new vessels--a process termed angiogenesis--to grow beyond a small size and spread. Angiogenesis is regulated by a number of soluble factors, the relative proportions of which can exacerbate or inhibit the process. Vascular endothelial growth factor (VEGF) is a potent stimulator of angiogenesis, produced by the majority of human solid tumors. Inhibitors of VEGF might have an impact on the growth and metastasis of these cancers. The relevance of this strategy to the treatment of colorectal cancer was first successfully demonstrated in human clinical trials using a monoclonal antibody against VEGF. A potent antiangiogenic soluble recombinant decoy, VEGF Trap is a protein constructed from VEGF receptor-binding domains linked to an immunoglobulin G(1) constant region. It possesses an affinity for VEGF that is significantly higher than that of the monoclonal antibody. VEGF Trap has demonstrated marked efficacy in halting angiogenesis and shrinking tumors in preclinical animal models and is currently being studied in phase I clinical trials in humans with advanced solid malignancies.     

ErbB2 increases vascular endothelial growth factor protein synthesis via activation of mammalian target of rapamycin/p70S6K leading to increased angiogenesis and spontaneous metastasis of human breast cancer cells

ErbB2 overexpression in breast tumors results in increased metastasis and angiogenesis and reduced survival. To study ErbB2 signaling mechanisms in metastasis and angiogenesis, we did a spontaneous metastasis assay using MDA-MB-435 human breast cancer cells stably transfected with constitutively active ErbB2 kinase (V659E), a kinase-dead mutant of ErbB2 (K753M), or vector control (neo). Mice injected with V659E had increased metastasis incidence and tumor microvessel density than mice injected with K753M or control. Increased angiogenesis in vivo from the V659E transfectants paralleled increased angiogenic potential in vitro. V659E produced increased vascular endothelial growth factor (VEGF) through increased VEGF protein synthesis. This was mediated through signaling events involving extracellular signal-regulated kinase, phosphatidylinositol 3-kinase/Akt, mammalian target of rapamycin (mTOR), and p70S6K. The V659E xenografts also had significantly increased phosphorylated Akt, phosphorylated p70S6K, and VEGF compared with controls. To validate the clinical relevance of these findings, we examined 155 human breast tumor samples. Human tumors that overexpressed ErbB2, which have been previously shown to have higher VEGF expression, showed significantly higher p70S6K phosphorylation as well. Increased VEGF expression also significantly correlated with higher levels of Akt and mTOR phosphorylation. Additionally, patients with tumors having increased p70S6K phosphorylation showed a trend for worse disease-free survival and increased metastasis. Our findings show that ErbB2 increases VEGF protein production by activating p70S6K in cell lines, xenografts, and in human cancers and suggest that these signaling molecules may serve as targets for antiangiogenic and antimetastatic therapies.     

Notch signaling from tumor cells: a new mechanism of angiogenesis

Notch signaling is an evolutionarily conserved pathway and plays key roles in embryonic vascular development and angiogenesis. Multiple components of the Notch pathway are expressed in vasculature, and mice deficient for a variety of these components display embryonic lethality with vascular remodeling defects. Alteration of Notch signaling in various endothelial cells generates profound effects on angiogenesis in vitro. New evidence shows that Notch signaling from tumor cells is able to activate endothelial cells and trigger tumor angiogenesis in vitro and in a xenograft mouse tumor model. Selective interruption of Notch signaling within tumors may provide an antiangiogenic strategy.     

EGCG对胃癌血管新生中VEGF信号通路的多靶点作用

目的：探讨EGCG对胃癌血管生成抑制作用及其信号通路。方法：建立裸鼠异位胃癌模型,经腹腔注射EGCG,检测肿瘤生长及肿瘤组织微血管密度;不同浓度EGCG处理胃癌细胞24 h,检测胃癌VEGF蛋白和mRNA表达及VEGF分泌;同时不同浓度EGCG处理脐静脉内皮细胞,检测内皮细胞生长、迁移和体外小管形成。结果：EGCG显著抑制胃癌生长和肿瘤血管生成,平均肿瘤抑制率60.4%;EGCG显著抑制胃癌VEGF蛋白、mRNA表达和VEGF分泌;EGCG时间和剂量依赖性地抑制VEGF诱导的内皮细胞增殖,同时也剂量依赖性地抑制VEGF诱导的内皮细胞的迁移和小管生成。结论：EGCG多靶点作用于VEGF信号通路,抑制胃癌生长和血管生成。     

VEGF receptor signaling in tumor angiogenesis

The growth of human tumors and development of metastases depend on the de novo formation of blood vessels. The formation of new blood vessels is tightly regulated by specific growth factors that target receptor tyrosine kinases (RTKs). Vascular endothelial growth factor (VEGF) and the Flk-1/KDR RTK have been implicated as the key endothelial cell-specific factor signaling pathway required for pathological angiogenesis, including tumor neovascularization. Inhibition of the VEGF tyrosine kinase signaling pathway blocks new blood vessel formation in growing tumors, leading to stasis or regression of tumor growth. Advances in understanding the biology of angiogenesis have led to the development of several therapeutic modalities for the inhibition of the VEGF tyrosine kinase signaling pathway. A number of these modalities are under investigation in clinical studies to evaluate their potential to treat human cancers.     

Importance of fibroblast growth factor receptor in neovascularization and tumor escape from antiangiogenic therapy

Therapeutic inhibition of pathways involved in angiogenesis has become the standard of care in renal cell carcinoma (RCC). Most currently available antiangiogenic agents inhibit the vascular endothelial growth factor (VEGF) pathway. Although these drugs have produced exciting benefits, some tumors do not respond to these agents. In addition most if not all tumors that initially respond will eventually develop resistance. Tumor escape from antiangiogenic therapy may include various signaling pathways that are involved in angiogenesis, including the fibroblast growth factor (FGF) signaling pathway. Emerging preclinical data suggest that FGF and VEGF act distinctly and synergistically to promote tumor vascularization. The current review discusses the role of FGF signaling in resistance to anti-VEGF therapies and outlines potential therapeutic implications.     

Inhibition of infiltration and angiogenesis by thrombospondin-1 in papillary thyroid carcinoma.

PURPOSE: Angiogenesis is essential for tumor growth and is controlled by the balance between angiogenic and antiangiogenic factors. We studied the expression of angiogenic factors and antiangiogenic factors in papillary thyroid carcinoma. EXPERIMENTAL DESIGN: We investigated immunohistochemically the expression patterns and levels of antiangiogenic factor and its receptor, thrombospondin-1 (TSP-1) and CD36, and four angiogenic factors, vascular endothelial growth factor (VEGF), VEGF-C, angiopoietin-2 (Ang-2), and Tie-2, in the primary tumors of 75 papillary thyroid carcinoma patients. We also examined the microvessel count (MVC), using CD31 staining. RESULTS: VEGF expression strongly correlated with other angiogenic factors. The cytoplasm of cancer cells stained positive for all factors. Tie-2 and TSP-1 receptor also appeared in endothelia of microvessels. TSP-1 inversely correlated with the degree of invasion of the primary tumor to other adjacent organs and with MVC. A higher MVC correlated with poorer survival. To clarify the balance between angiogenic and antiangiogenic factors in the same tumor, we calculated the ratio of each angiogenic factor against TSP-1 as the antiangiogenic factor. The ratios VEGF/TSP-1, VEGF-C/TSP-1, and Ang-2/TSP-1 significantly correlated with a higher MVC. Furthermore, the ratios VEGF/TSP-1 and Ang-2/TSP-1 significantly correlated with the degree of infiltration. CONCLUSIONS: To the best of our knowledge, this is the first report demonstrating that the balance between angiogenic and antiangiogenic factors correlates with distinct invasion to other organs and neovascularization of papillary thyroid carcinoma.     

Targeting Angiogenesis in Cancer Therapy: Moving Beyond Vascular Endothelial Growth Factor

Angiogenesis, or the formation of new capillary blood vessels, occurs primarily during human development and reproduction; however, aberrant regulation of angiogenesis is also a fundamental process found in several pathologic conditions, including cancer. As a process required for invasion and metastasis, tumor angiogenesis constitutes an important point of control of cancer progression. Although not yet completely understood, the complex process of tumor angiogenesis involves highly regulated orchestration of multiple signaling pathways. The proangiogenic signaling molecule vascular endothelial growth factor (VEGF) and its cognate receptor (VEGF receptor 2 [VEGFR-2]) play a central role in angiogenesis and often are highly expressed in human cancers, and initial clinical efforts to develop antiangiogenic treatments focused largely on inhibiting VEGF/VEGFR signaling. Such approaches, however, often lead to transient responses and further disease progression because angiogenesis is regulated by multiple pathways that are able to compensate for each other when single pathways are inhibited. The platelet-derived growth factor (PDGF) and PDGF receptor (PDGFR) and fibroblast growth factor (FGF) and FGF receptor (FGFR) pathways, for example, provide potential escape mechanisms from anti-VEGF/VEGFR therapy that could facilitate resumption of tumor growth. Accordingly, more recent treatments have focused on inhibiting multiple signaling pathways simultaneously. This comprehensive review discusses the limitations of inhibiting VEGF signaling alone as an antiangiogenic strategy, the importance of other angiogenic pathways including PDGF/PDGFR and FGF/FGFR, and the novel current and emerging agents that target multiple angiogenic pathways for the treatment of advanced solid tumors.

Implications for Practice:

Significant advances in cancer treatment have been achieved with the development of antiangiogenic agents, the majority of which have focused on inhibition of the vascular endothelial growth factor (VEGF) pathway. VEGF targeting alone, however, has not proven to be as efficacious as originally hoped, and it is increasingly clear that there are many interconnected and compensatory pathways that can overcome VEGF-targeted inhibition of angiogenesis. Maximizing the potential of antiangiogenic therapy is likely to require a broader therapeutic approach using a new generation of multitargeted antiangiogenic agents.     

VEGF in Brain Tumors

Vascular endothelial growth factor (VEGF) is a regulator of angiogenesis, vasculogenesis and vascular permeability. In this contribution, molecular and biological properties of VEGF are described. Furthermore, this article focuses on the evidence that angiogenesis in brain tumors is mediated by VEGF. Among the topics discussed are expression patterns of VEGF and its receptors in different brain tumors, possible regulatory mechanism involved in the VEGF-driven tumor angiogenesis and the involvement of VEGF in the genesis of peritumoral edema. Finally, anti-angiogenesis approaches to target VEGF/VEGF receptors are discussed.     

VEGF trap in combination with radiotherapy improves tumor control in u87 glioblastoma

PURPOSE: To determine the effect of vascular endothelial growth factor VEGF Trap (Regeneron Pharmaceuticals, Tarrytown, NY), a humanized soluble vascular endothelial growth factor (VEGF) receptor protein, and radiation (RT) on tumor growth in U87 glioblastoma xenografts in nude mice. METHODS AND MATERIALS: U87 cell suspensions were implanted subcutaneously into hind limbs of nude mice. VEGF Trap (2.5-25 mg/kg) was administered every 3 days for 3 weeks alone or in combination with a single dose of 10 Gy or fractionated RT (3 x 5 Gy). In addition, three scheduling protocols for VEGF Trap plus fractionated RT were examined. RESULTS: Improved tumor control was seen when RT (either single dose or fractionated doses) was combined with the lowest dose of VEGF Trap (2.5 mg/kg). Scheduling did not significantly affect the efficacy of combined therapy. Although high-dose VEGF Trap (10 mg/kg or 25 mg/kg) significantly reduced tumor growth over that of RT alone, there was no additional benefit to combining high-dose VEGF Trap with RT. CONCLUSIONS: Vascular endothelial growth factor Trap plus radiation is clearly better than radiation alone in a U87 subcutaneous xenograft model. Although high doses of VEGF Trap alone are highly efficacious, it is unclear whether such high doses can be used clinically without incurring normal tissue toxicities. Thus, information on lower doses of VEGF Trap and ionizing radiation is of clinical relevance.     

GPR56 Regulates VEGF production and angiogenesis during melanoma progression

Angiogenesis is a critical step during cancer progression. The VEGF is a major stimulator for angiogenesis and is predominantly contributed by cancer cells in tumors. Inhibition of the VEGF signaling pathway has shown promising therapeutic benefits for cancer patients, but adaptive tumor responses are often observed, indicating the need for further understanding of VEGF regulation. We report that a novel G protein-coupled receptor, GPR56, inhibits VEGF production from the melanoma cell lines and impedes melanoma angiogenesis and growth, through the serine threonine proline-rich segment in its N-terminus and a signaling pathway involving protein kinase Cα. We also present evidence that the two fragments of GPR56, which are generated by autocatalyzed cleavage, played distinct roles in regulating VEGF production and melanoma progression. Finally, consistent with its suppressive roles in melanoma progression, the expression levels of GPR56 are inversely correlated with the malignancy of melanomas in human subjects. We propose that components of the GPR56-mediated signaling pathway may serve as new targets for antiangiogenic treatment of melanoma.     

血管内皮生长因子和Dll4-Notch信号通路在肿瘤血管发生中的作用

抗肿瘤血管发生是一种全新的抗肿瘤治疗策略,目前研究最多的是寻找有效抗血管内皮生长因子(VEGF)信号通路的药物.然而VEGF抑制剂并不是对所有肿瘤有效,因此,有必要进一步探索血管发生的其他信号通路.目前,Dll4-Notch信号通路被认为是抗肿瘤血管发生的新靶点,而且VEGF和Dll4-Notch信号通路在多方面相互作...     

Up-regulation of delta-like 4 ligand in human tumor vasculature and the role of basal expression in endothelial cell function

The Notch signaling pathway and the delta-like 4 ligand (DLL4) play key roles in embryonic vascular development. Many of the pathways involved in embryonic vascular development also play important roles in tumor angiogenesis. In this study, we assessed the expression of DLL4 in primary renal cancer and investigated the biological function of DLL4 in primary endothelial cells. Using real-time quantitative PCR and in situ hybridization, we showed that the expression of DLL4 was up-regulated within the vasculature of clear cell-renal cell carcinoma almost 9-fold more than normal kidney and was correlated with the expression of vascular endothelial growth factor (VEGF). The expression of DLL4 in endothelial cells was up-regulated by VEGF and basic fibroblast growth factor synergistically, and by hypoxia through hypoxia-inducible factor 1alpha. Down-regulation of DLL4 expression with RNA interference led to decreased expression of HEY1 and EphrinB2, and the inhibition of endothelial cell proliferation, migration, and network formation, all of which are important processes in tumor angiogenesis. The inhibition of proliferation occurred via the induction of cell cycle arrest in G0-G1 by increased expression of p21 and decreased phosphorylation of retinoblastoma. We conclude that an optimal window of the DLL4 expression is essential for tumor angiogenesis and that selective modulation of the DLL4 expression within human tumors may represent a potential novel antiangiogenic therapy.     

Inhibition of VEGF-dependent multistage carcinogenesis by soluble EphA receptors

Elevated expression of Eph receptors has long been correlated with the growth of solid tumors. However, the functional role of this family of receptor tyrosine kinases in carcinogenesis and tumor angiogenesis has not been well characterized. Here we report that soluble EphA receptors inhibit tumor angiogenesis and tumor progression in vivo in the RIP-Tag transgenic model of vascular endothelial growth factor (VEGF)-dependent multistage pancreatic islet cell carcinoma. Soluble EphA receptors delivered either by a transgene or an osmotic minipump inhibited the formation of angiogenic islet, a premalignant lesion, and reduced tumor volume of solid islet cell carcinoma. EphA2-Fc or EphA3-Fc treatment resulted in decreased tumor volume but increased tumor and endothelial cell apoptosis in vivo. In addition, soluble EphA receptors inhibited VEGF and betaTC tumor cell-conditioned medium-induced endothelial cell migration in vitro and VEGF-induced cornea angiogenesis in vivo. A dominant negative EphA2 mutant inhibited--whereas a gain-of-function EphA2 mutant enhanced--tumor cell-induced endothelial cell migration, suggesting that EphA2 receptor activation is required for tumor cell-endothelial cell interaction. These data provide functional evidence for EphA class receptor regulation of VEGF-dependent tumor angiogenesis, suggesting that the EphA signaling pathway may represent an attractive novel target for antiangiogenic therapy in cancer.