A naturally occurring soluble form of vascular endothelial growth factor receptor 2 detected in mouse and human plasma

Angiogenesis and vasculogenesis are regulated in large part by several different growth factors and their associated receptor tyrosine kinases (RTKs). Foremost among these is the vascular endothelial growth factor (VEGF) family including VEGF receptor (VEGFR)-2 and -1. VEGFR ligand binding and biological activity are regulated at many levels, one of which is by a soluble, circulating form of VEGFR-1 (sVEGFR-1). This sVEGFR-1 can act as a competitive inhibitor of its ligand, serve as a possible biomarker, and play important roles in cancer and other diseases such as preeclampsia. Recombinant forms of sVEGFR-2 have been shown to have antiangiogenic activity, but a naturally occurring sVEGFR-2 has not been described previously. Here, we report such an entity. Having a molecular weight of approximately 160 kDa, sVEGFR-2 can be detected in mouse and human plasma with several different monoclonal and polyclonal anti-VEGFR-2 antibodies using both ELISA and immunoprecipitation techniques. In vitro studies have determined that the sVEGFR-2 fragment can be found in the conditioned media of mouse and human endothelial cells, thus suggesting that it may be secreted, similar to sVEGFR-1, or proteolytically cleaved from the cell. Potential biological activity of this protein was inferred from experiments in which mouse sVEGFR-2 could bind to VEGF-coated plates. Similar to sVEGFR-1 and other soluble circulating RTKs, sVEGFR-2 may have regulatory consequences with respect to VEGF-mediated angiogenesis as well as potential to serve as a quantitative biomarker of angiogenesis and antiangiogenic drug activity, particularly for drugs that target VEGF or VEGFR-2.

Evaluation of novel antimouse VEGFR2 antibodies as potential antiangiogenic or vascular targeting agents for tumor therapy

We generated a panel of eight rat IgG(2a) monoclonal antibodies with high affinity for mouse VEGFR2 (KDR/Flk-1), the main receptor that mediates the angiogenic effect of VEGF-A. The antibodies (termed RAFL, R at Anti Flk) bound to dividing endothelial cells more strongly than they did to nondividing cells. Most of the RAFL antibodies blocked [(125)I]VEGF(165) binding to VEGFR2. Three of eight antibodies localized to VEGFR2-positive tumor endothelium after intravenous injection into mice bearing orthotopic MDA-MB-231 breast carcinomas, as judged by indirect immunohistochemistry. An average of 60% of vessels in the tumors was stained. The majority (50-80%) of vessels were also stained in a variety of other human and murine tumors growing in mice. The antibodies did not bind detectably to the vascular endothelium in normal heart, lung, liver, and brain cortex, whereas the vascular endothelium in kidney glomerulus and pancreatic islets was stained. Treatment of mice bearing orthotopic MDA-MB-231 tumors with RAFL-1 antibody inhibited tumor growth by an average of 48% and reduced vascular density by 65%, compared to tumors in mice treated with control IgG. Vascular damage was not observed in normal organs, including kidneys and pancreas. These studies demonstrate that anti-VEGFR2 antibodies have potential for vascular targeting and imaging of tumors in vivo.

Exogenous BH4/Bcl-2 Peptide Reverts Coronary Endothelial Cell Apoptosis Induced by Oxidative Stress

BACKGROUND

Vascular endothelium undergoes apoptosis when exposed to reactive oxygen species (ROS), including hydrogen peroxide and superoxide radicals. ROS are believed to be the cause of damage to small vessels during ischemia-reperfusion injury and of arterial damage during atherosclerosis. Hydrogen peroxide-induced apoptosis is mediated through the inhibition of Bcl-xl activity and caspase-3 and caspase-9 activation. The BH4 domain of the Bcl-2 family members is responsible for their antiapoptotic activity. The BH4 domains of Bcl-2 and Bcl-xl inhibit cytochrome c release and the loss of mitochondrial membrane potential.

METHODS AND RESULTS

The purpose of this project was to study the antiapoptotic effect of cell-permeant derivative of Bcl-2 (BH4 peptide) on endothelial cells exposed to stress conditions. BH4 peptide was conjugated to the cell-permeable peptide TAT and was applied to endothelial cells under conditions of serum starvation and hydrogen peroxide treatment. TAT-BH4 reduced caspase-3 activity and prevented apoptotic cell death.

CONCLUSION

Our results indicate that TAT-BH4 peptide can protect endothelial cells from ROS-induced apoptosis.

Characterization of the cell-surface procoagulant activity of T-lymphoblastoid cell lines

The procoagulant activity (PCA) of four T-lymphoblastoid cell lines (CEM-CCRF, Jurkat, Molt-4 and A3.01) at different stages of differentiation has been characterized and compared with that of a monocytoid cell line (THP-1). Four assay systems were employed; the activated partial thromboplastin time (APTT); prothrombin time/tissue factor (TF) activity; a purified factor (F)Xa generation system and cancer procoagulant. High levels of TF activity were seen only with the monocytic cells. However the more differentiated of the T-lymphoblastoid cells (Molt-4 and A3.01) were more active than monocytic cells in supporting FXa generation. This pattern was not repeated for the APTT assay, which was related to cell-surface TF activity, since it was partially inhibited by antiTF antibody. Annexin V totally inhibited the activity observed in all three assay systems, indicating that the PCA of T-lymphoblastoid cells is primarily due to expression of negatively charged phospholipids. However, antiphosphatidylserine antibody even at a high concentration gave only partial inhibition of the activity observed in the APTT and FXa generation systems for the cells compared with almost total inhibition for the phospholipid standard, suggesting either that cellular phosphatidylserine (PS) is less accessible to the antibody, or that PS is not the sole negatively charged phospholipid responsible for this activity. Flow cytometry studies using propidium iodide and annexin V showed that the PCA, although linked to PS exposure, was not the result of apoptosis.

Oxygenation in a human tumor xenograft: manipulation through respiratory challenge and antibody-directed infarction

We recently demonstrated the use of 19F NMR relaxometry of hexafluorobenzene to monitor regional tumor oxygen tension dynamics in rats. We have now extended the application to human tumors implanted in immunocompromised (SCID) mice. This has allowed us both to investigate dynamic response to respiratory challenge (carbogen) and to probe the mechanisms of a new anti-vascular therapy designed to produce tumor-specific infarction.

Quantitative assessment of tumor oxygen dynamics: molecular imaging for prognostic radiology

One of the fundamental molecules governing the survival of mammalian cells is oxygen. Oxygen has gained particular significance in tumor developmental biology and oncology. An increasingly diverse array of methods is now available to characterize tumor oxygenation. This Prospect will consider a new method, Fluorocarbon Relaxometry using Echo planar imaging for Dynamic Oxygen Mapping (FREDOM), which we have recently developed for oximetry, examine application to a specific therapeutic example and place this technique in the context of other approaches.

A monoclonal antibody that blocks VEGF binding to VEGFR2 (KDR/Flk-1) inhibits vascular expression of Flk-1 and tumor growth in an orthotopic human breast cancer model

Vascular endothelial growth factor (VEGF) is a primary stimulant of tumor angiogenesis. We previously raised a neutralizing anti-VEGF monoclonal antibody 2C3 that blocks the interaction of VEGF with VEGFR2 (KDR/Flk-1) but not with VEGFRI (FLT-1/flt-1). Here, we describe the therapeutic effects of 2C3 on tumor growth in an orthotopic model of MDA-MB-231 human breast carcinoma implanted in the mammary fat pads (MFP) of nude mice. Administration of 2C3 to mice with 100-150 mm3 tumors inhibited tumor growth by 75%, as compared to recipients of the isotype-matched irrelevant control IgG, C44. Treatment with 2C3 also inhibited the establishment of tumor colonies and reduced tumor burden in the lungs of mice injected intravenously with MDA-MB-231 cells. No toxicity was observed in these studies. The mean microvascular density (MVD) of tumors in 2C3-treated mice was 55 +/- 5 per mm2, as compared to 188 +/- 5 per mm2 in the C44-treated control group. The decrease in MVD closely correlated with the degree of inhibition of tumor growth. Treated tumors mostly contained mid-size and large vessels. Microvessels were mainly confined to the peripheral layer of tumor that bordered on the normal MFP epithelium. Tumor vessels had decreased expression of VEGFR2, indicating that neutralization of tumor-derived VEGF by 2C3 down-regulates the expression of VEGFR2 on tumor vasculature. This, in turn, may limit reinitiation of angiogenesis by either tumor-derived or stromal VEGF. These findings suggest that 2C3 is a candidate for treating primary cancer and for preventing the outgrowth of tumor metastases in cancer patients.

The first international conference on vascular targeting: meeting overview

The First International Conference on Vascular Targeting focused on vascular targeting agents (VTAs) that occlude or destroy the pre-existing blood vessels of solid tumors. The VTAs cause a rapid shutdown in the blood supply to the tumor that kills tumor cells by depriving them of oxygen and nutrients. The VTAs are distinct from antiangiogenic agents, which prevent new blood vessel formation. Two major types of VTAs are being developed for cancer: the ligand-directed VTAs that use antibodies, peptides, and growth factors to deliver toxins, procoagulants, and proapoptotic effectors to tumor endothelium, and the small molecule VTAs that do not specifically localize to tumor endothelium but exploit pathophysiological differences between tumor and normal tissue endothelia to induce acute vascular shutdown in tumors. Both approaches were described at the meeting and highlighted the variety of VTAs in preclinical development, their selectivity for tumor endothelium, their rapid antitumor effects, and the improved activity seen when combined with other anticancer approaches (antiproliferative chemotherapeutic drugs, radiation, radiolabeled antibodies, nitric oxide synthetase inhibitors, and antiangiogenic agents). Early clinical studies were summarized for the small molecule VTAs: the antitubulin drugs, combretastatin A4 phosphate (CA4P) and ZD6126, and the flavonoid, 5,6-dimethylxanthenone-4-acetic acid (DMXAA). The agents lacked the bone marrow and gastrointestinal toxicities associated with antiproliferative chemotherapy. As a marker of biological effect, blood flow reductions in tumors were measured using magnetic resonance imaging or positron emission tomography for all of the agents tested, and single-agent clinical activity was seen. These agents are now being evaluated in combined modality studies to see whether the impressive results obtained in experimental models can be translated into humans.

Phosphatidylserine is a marker of tumor vasculature and a potential target for cancer imaging and therapy

PURPOSE

(1) To determine whether exposure of phosphatidylserine (PS) occurs on vascular endothelium in solid tumors in mice. (2) To determine whether PS exposure can be induced on viable endothelial cells in tissue culture by conditions present in the tumor microenvironment.

METHODS AND MATERIALS

Externalized PS in vivo was detected by injecting mice with a monoclonal anti-PS antibody and examining frozen sections of tumors and normal tissues for anti-PS antibody bound to vascular endothelium. Apoptotic cells were identified by anti-active caspase-3 antibody or by TUNEL assay. PS exposure on cultured endothelial cells was determined by 125I-annexin V binding.

RESULTS

Anti-PS antibody bound specifically to vascular endothelium in six tumor models. The percentage of PS-positive vessels ranged from 4% to 40% in different tumor types. Vascular endothelium in normal organs was unstained. Very few tumor vessels expressed apoptotic markers. Hypoxia/reoxygenation, acidity, inflammatory cytokines, thrombin, or hydrogen peroxide induced PS exposure on cultured endothelial cells without causing loss of viability.

CONCLUSIONS

Vascular endothelial cells in tumors, but not in normal tissues, externalize PS. PS exposure might be induced by tumor-associated oxidative stress and activating cytokines. PS is an abundant and accessible marker of tumor vasculature and could be used for tumor imaging and therapy.

Increased exposure of anionic phospholipids on the surface of tumor blood vessels

Anionic phospholipids are largely absent from the external leaflet of the plasma membrane of mammalian cells under normal conditions. Exposure of phosphatidylserine on the cell surface occurs during apoptosis, necrosis, cell injury, cell activation, and malignant transformation. In the present study, we determined whether anionic phospholipids become exposed on tumor vasculature. A monoclonal antibody, 9D2, which specifically recognizes anionic phospholipids, was injected into mice bearing a variety of orthotopic or ectopic tumors. Other mice received annexin V, a natural ligand that binds to anionic phospholipids. Both 9D2 and annexin V specifically localized to vascular endothelium in all of the tumors, and also to tumor cells in and around regions of necrosis. Between 15 and 40% of endothelial cells in tumor vessels were stained. No localization was detected on normal endothelium. Various factors and tumor-associated conditions known to be present in the tumor microenvironment were examined for their ability to cause exposure of anionic phospholipids in cultured endothelial cells, as judged by 9D2 and annexin V binding. Hypoxia/reoxygenation, acidity, thrombin, and inflammatory cytokines all induced exposure of anionic phospholipids. Hydrogen peroxide was also a strong inducer. Combined treatment with inflammatory cytokines and hypoxia/reoxygenation had greater than additive effects. Possibly, injury and activation of tumor endothelium by cytokines and reactive oxygen species induce exposure of anionic phospholipids, most likely phosphatidylserine. Anionic phospholipids on tumor vessels could potentially provide markers for tumor vessel targeting and imaging.

Soluble Eph A receptors inhibit tumor angiogenesis and progression in vivo

The Eph family of receptor tyrosine kinases and their ligands, known as ephrins, play a crucial role in vascular development during embryogenesis. The function of these molecules in adult angiogenesis has not been well characterized. Here, we report that blocking Eph A class receptor activation inhibits angiogenesis in two independent tumor types, the RIP-Tag transgenic model of angiogenesis-dependent pancreatic islet cell carcinoma and the 4T1 model of metastatic mammary adenocarcinoma. Ephrin-A1 ligand was expressed in both tumor and endothelial cells, and EphA2 receptor was localized primarily in tumor-associated vascular endothelial cells. Soluble EphA2-Fc or EphA3-Fc receptors inhibited tumor angiogenesis in cutaneous window assays, and tumor growth in vivo. EphA2-Fc or EphA3-Fc treatment resulted in decreased tumor vascular density, tumor volume, and cell proliferation, but increased cell apoptosis. However, EphA2-Fc had no direct effect on tumor cell growth or apoptosis in culture, yet inhibited migration of endothelial cells in response to tumor cells, suggesting that the soluble receptor inhibited blood vessel recruitment by the tumor. These data provide the first functional evidence for Eph A class receptor regulation of pathogenic angiogenesis induced by tumors and support the function of A class Eph receptors in tumor progression.

Vascular endothelial growth factor isoforms display distinct activities in promoting tumor angiogenesis at different anatomic sites

The gene for the major angiogenic factor, vascular endothelial growth factor (VEGF), encodes several spliced isoforms. We reported previously that overexpression of two VEGF isoforms, VEGF(121) and VEGF(165), by human glioma U87 MG cells induced tumor-associated intracerebral hemorrhage, whereas expression of a third form, VEGF(189), did not cause vessel rupture. Here, we test whether these VEGF isoforms have distinct activities for enhancing vascularization and growth of gliomas in mice. U87 MG cells that overexpressed VEGF(165) or VEGF(189) grew more rapidly than the parental cells in both s.c. and intracranial (i.c.) locations. However, cells that overexpressed VEGF(121) only showed enhancement of i.c. tumor growth but had a minimal effect on s.c. glioma progression. At both anatomical sties, VEGF(165) and VEGF(189) strongly augmented neovascularization, whereas VEGF(121) only increased vessel density in brain tumors. In each type of glioma, expression of VEGF receptors -1 and -2 largely phenocopied the tumor vasculature, because increased VEGF/VEGF receptor-activated microvessel densities were strongly correlated with the angiogenicity and tumorigenicity elicited by the VEGF isoforms at both anatomical sites. One notable difference between the sites was the expression of vitronectin, a prototypic ligand of alpha(v)beta(3) and alpha(v)beta(5) integrins, detected in i.c. but not in s.c., gliomas. Endothelial cell migration stimulated by VEGF(121) was potentiated by vitronectin to a greater extent than that stimulated by VEGF(165). This data demonstrates that VEGF isoforms have distinct activities at different anatomical sites and suggest that the microenvironment of different tissues affects the function of VEGF isoforms.

The angiogenic "vascular endothelial growth factor/flk-1(KDR) receptor" pathway in patients with endometrial carcinoma: prognostic and therapeutic implications

BACKGROUND

Vascular endothelial growth factor (VEGF) is an important endothelial cell mitogen associated with increased angiogenesis and aggressive tumor behavior. Its stimulating effect on endothelial cells basically is dependent on the presence of specific VEGF receptors, such as the flk-1(KDR) receptor. This study investigates the roles of VEGF and of a functionally intact angiogenic pathway, "VEGF/flk-1(KDR)," in patients with endometrial carcinoma and their significance in prognosis and therapy.

METHODS

A series of 121 endometrial carcinomas were studied. The expression of VEGF by endometrial tumor cells was assessed using the monoclonal antibody (MoAb) VG1. VEGF/KDR complexes on tumor endothelium or activated microvessel density (aMVD) were identified using the MoAb 11B5. In addition, the standard microvessel density (sMVD) was assessed with anti-CD31. In all tumors, the alkaline phosphatase/antialkaline phosphatase technique was employed. A Fisher exact test or an unpaired, two-tailed t test was used for testing correlations between categoric tumor variables, whereas a log-rank test was used to determine statistical differences between life tables. A Cox proportional hazards model was used to assess the effect of tumor variables on overall survival.

RESULTS

Cytoplasmic VEGF expression in > 50% of tumor cells was associated significantly with aMVD (P < 0.0001) and with sMVD (P < 0.003). In univariate survival analysis, VEGF (P = 0.0002), aMVD (P = 0.001), and sMVD (P = 0.0009) were significant prognostic variables. Equally important were the histologic parameters tumor type (P = 0.03), tumor grade (P = 0.003), and disease stage (P < 0.0001). In multivariate analysis, disease stage was the most important independent prognostic factor (P < 0.0001), followed by VEGF/KDR (P < 0.01), and VEGF (P < 0.04). Furthermore, VEGF and VEGF/KDR were the only independent prognostic variables for patients with Stage I endometrioid adenocarcinoma.

CONCLUSIONS

sMVD and the angiogenic factor VEGF are important indicators of a poor prognosis in patients with endometrial carcinoma. VEGF/KDR complexes define a subgroup of patients with endometrial carcinoma with an even worse prognosis.

Vascular endothelial growth factor and vascular targeting of solid tumors

Vascular targeting agents, which selectively destroy tumor blood vessels, are attractive agents for the treatment of solid tumors. They differ from anti-angiogenic agents in that they target the mature, blood-conducting vessels of the tumors. They are better suited for larger tumors where angiogenesis can occur less frequently. For application in man, target molecules are needed that are selectively expressed on the vascular endothelium of tumors. Such markers include the complexes that are formed when vascular endothelial growth factor (VEGF) binds to its receptors (VEGFR). VEGF production by tumor cells is induced by oncogenic gene mutations and by the hypoxic conditions within the tumor mass. The receptors, VEGFR1 (FLT-1) and VEGFR2 (KDR/Flk-1), are upregulated on vascular endothelial cells in tumors by hypoxia and by the increased local concentration of VEGF. Consequently, there is a high concentration of occupied receptors on tumor vascular endothelium. Here, we review the concept of vascular targeting and the development of monoclonal antibodies that bind to VEGF: VEGFR complexes and their use as tumor vascular targeting agents. A promising monoclonal antibody is 2C3, which blocks VEGF from binding to VEGFR2 but not VEGFR1. We conclude that 2C3 might have dual activity as an anti-angiogenic agent by inhibiting VEGFR2 activity and as a vascular targeting agent for selective drug delivery to tumor vessels.

Tumor specific activation of the VEGF/KDR angiogenic pathway in a subset of locally advanced squamous cell head and neck carcinomas

Vascular endothelial growth factor (VEGF) and its receptors, Flt-1 and flk-1(KDR), constitute an important angiogenic pathway which, under hypoxic conditions, is up-regulated in many solid tumours. We used the monoclonal antibody 11B5, specific for recognizing VEGF expression and the 'VEGF/flk-1(KDR) complex' on tumour endothelium, to assess free VEGF protein expression and VEGF/receptor activated microvessel density (aMVD) in a series of 104 inoperable locally advanced squamous cell carcinomas of the head and neck, treated with chemo-radiotherapy. High VEGF expression in cancer cells was strongly associated with high VEGF/receptor expression in the vasculature. The high VEGF expression and the aMVD were not associated with the standard microvessel density (sMVD), as assessed with the monoclonal antibody anti-CD31 and, were not detected in normal tissue. An increased sMVD, however, was significantly related with the expression thymidine phosphorylase (TP), and also with the nuclear accumulation of the oncoprotein p53, but neither p53 nor TP was associated with VEGF expression by cancer cells or VEGF/receptor complex aMVD. In 35% of cancer cases examined, more than 20% of the microvessels assessed with anti-CD31 also expressed the VEGF/KDR complex. The vasculature of the normal head and neck mucosa did not express the VEGF/KDR complex. There was no association between VEGF expression or VEGF/receptor complex aMVD and response to chemo-radiotherapy or patient's survival. It is concluded that activation of the angiogenic pathway VEGF/flk-1(KDR) is tumor specific in a subgroup of locally advanced squamous cell carcinomas of the head and neck. Selective destruction of this type of vasculature, using immunoconjugates directed against the VEGF/receptor complex, may prove therapeutically useful for patients with a high tumoral VEGF/flk-1(KDR) activated microvessel fraction.

VEGF-VEGF receptor complexes as markers of tumor vascular endothelium

Vascular endothelial growth factor (VEGF) is a primary stimulant of the vascularization of solid tumors and has therefore been the focus of intense research aimed at blocking its activity in solid tumors. VEGF production by tumor cells is induced by oncogenic gene mutations and hypoxic conditions inside the tumor mass. VEGF receptor expression on endothelial cells lining blood vessels in the tumor is also induced by hypoxia and the increased local concentration of VEGF. Therefore in the tumor microenvironment there is an upregulation of both VEGF and its receptor leading to a high concentration of occupied receptor on tumor vascular endothelium. The VEGF-VEGF receptor complex (VEGF-VEGFR) presents an attractive target for the specific delivery of drugs or other effectors to tumor endothelium. Herein we review the development of monoclonal antibodies that selectively bind to the VEGF-VEGFR and their use as targeting agents that selectively bind to VEGF activated blood vessels. Additionally, we summarize the properties of 2C3, a novel monoclonal anti-VEGF antibody that blocks VEGF from binding to VEGFR2 but not VEGFR1. 2C3 may be utilized as both an anti-angiogenic agent by inhibiting VEGFR2 activity and potentially as a vascular targeting agent by binding to blood vessels that express the VEGF-VEGFR1 complex.

The angiogenic pathway "vascular endothelial growth factor/flk-1(KDR)-receptor" in rheumatoid arthritis and osteoarthritis

Active angiogenesis, together with an up-regulation of angiogenic factors, is evident in the synovium of both rheumatoid arthritis (RA) and osteoarthritis (OA). The present study assessed, by immunohistochemistry, the microvessel density in the synovium of these arthritides and in normal controls, in relation to the expression of the angiogenic factors vascular endothelial growth factor (VEGF) and platelet-derived endothelial cell growth factor (PD-ECGF) and the apoptosis-related proteins bcl-2 and p53. More importantly, using the novel 11B5 MAb, the activated "VEGF/flk-1(KDR)-receptor" microvessel density was assessed. VEGF expression in fibroblasts was diffuse in both RA and OA. Diffuse PD-ECGF expression of fibroblasts was noted in all cases of RA, while fibroblast reactivity was focal in the OA material. The standard microvessel density (sMVD), as assessed with the anti-CD31 monoclonal antibody (MAb), was higher in RA (64+/-12) and in OA (65+/-16) than in normal tissues (52+/-8; p=0.008 and 0.0004, respectively). The activated microvessel density (aMVD), assessed with the 11B5 MAb, was significantly higher in RA (29+/-10) than in OA (17+/-4; p<0.0001) and than in normal tissues (14+/-2; p<0.0001). The "activation ratio" (aMVD/sMVD) was statistically higher in RA (0.46+/-0.17) than in OA and normal synovial tissues, the latter two having a similar ratio (0.28+/-0.08 and 0.26+/-0.03, respectively). Cytoplasmic bcl-2 expression was frequent in the synovial cells of OA, but rare in RA. Nuclear p53 protein accumulation was never observed. It is suggested that the angiogenic pathway VEGF/flk-1(KDR) may play an important role in the pathogenesis of RA and OA. Thus, failure of VEGF/flk-1(KDR) activation, in the presence of increased VEGF expression, may indicate a synovium with an impaired capacity to establish a viable vasculature, consistent with the degenerative nature of OA. On the other hand, the activated angiogenesis in RA shows a functional, still pathologically up-regulated VEGF/flk-1(KDR) pathway. Whether restoration of an impaired VEGF/flk-1(KDR) pathway in OA, or inhibition of this in RA, would prove of therapeutic importance requires further investigation.

Generation and characterization of recombinant vascular targeting agents from hybridoma cell lines

Vascular targeting agents (VTAs) can be produced by linking antibodies or antibody fragments directed against endothelial cell markers to effector moieties. So far, it has been necessary to produce the components of VTAs (antibody, antibody fragment, linker, and effector) separately and, subsequently, to conjugate them by biochemical reactions. We devised a cloning and expression system to allow rapid generation of recombinant VTAs from hybridoma cell lines. The VTAs consist of a single chain Fv antibody fragment as a targeting moiety and either truncated Pseudomonas exotoxin (resulting in immunotoxins) or truncated human tissue factor (resulting in coaguligands) as effectors. The system was applied to generate recombinant immunotoxins and coaguligands directed against endoglin, vascular endothelial growth factor (VEGF):VEGF receptor (VEGFR) complex and vascular cell adhesion molecule 1 (VCAM-1). The fusion proteins exhibited similar functional activity to analogous biochemical constructs. This is the first report to describe the generation and characterization of recombinant coaguligands.

Selective inhibition of vascular endothelial growth factor (VEGF) receptor 2 (KDR/Flk-1) activity by a monoclonal anti-VEGF antibody blocks tumor growth in mice

Vascular endothelial growth factor (VEGF) is a multifunctional angiogenic growth factor that is a primary stimulant of the development and maintenance of a vascular network in embryogenesis and the vascularization of solid tumors. At the present time there are two well-characterized receptors for VEGF that are selectively expressed on endothelium. VEGF receptor 2 [VEGFR2 (KDR/Flk-1)] mediates endothelial cell mitogenesis and permeability increases, whereas the role of VEGF receptor 1 [VEGFR1 (Flt-1)] has not been clearly defined. In the present study, a monoclonal antibody, 2C3, is shown to block the interaction of VEGF with VEGFR2 but not with VEGFR1 through ELISA, receptor binding assays, and receptor activation assays. 2C3 blocks the VEGF-induced vascular permeability increase in guinea pig skin. 2C3 has potent antitumor activity, inhibiting the growth of newly injected and established human tumor xenografts in mice. These findings demonstrate the usefulness of 2C3 in dissecting the pathways that are activated by VEGF in cells that express both VEGFR1 and VEGFR2, as well as highlighting the dominant role of VEGFR2 in mediating VEGF-induced vascular permeability increase and tumor angiogenesis.

Vascular endothelial growth factor/KDR activated microvessel density versus CD31 standard microvessel density in non-small cell lung cancer

Vascular endothelial growth factor (VEGF) is an important angiogenic factor, linked to poor outcome in human malignancies including non-small cell lung carcinoma (NSCLC). We used the 11B5 monoclonal antibody recognizing the VEGF/KDR complex (R. A. Brekken et al., Cancer Res., 58: 1952-1959, 1998) to assess the VEGF expression in cancer cells and the VEGF/KDR activated microvessel density (aMVD) in early operable NSCLC. The JC70 anti-CD31 monoclonal antibody was used to assess the standard MVD (sMVD). The aMVD was significantly higher in the invading front of the tumors and in the normal lung adjacent to the tumors as compared with normal lung distant to the tumor or to inner tumor areas (P < 0.0002). The sMVD was higher in the normal lung and decreased from the invading front to inner tumor areas (P < 0.0001). However, the vascular activation (aMVD:sMVD) was 4-6 times higher in the tumor areas as compared with lung from normal individuals (36-58% versus 9%; P < 0.0001). Fibroblast 11B5 reactivity, noted in 25% of cases, correlated with high aMVD and sMVD in the inner tumor areas. Multivariate analysis showed that aMVD was the most potent and independent prognostic factor (P = 0.001; t-ratio, 3.28). It is concluded that intense VEGF/KDR angiogenic pathway activation is a tumor-specific feature in more than 50% of NSCLC cases and is associated with poor postoperative outcome. Clinical trials involving targeting of the VEGF/KDR-positive vasculature with specific antibodies, such as 11B5, are, therefore, encouraged.

Coexpression of MUC1 glycoprotein with multiple angiogenic factors in non-small cell lung cancer suggests coactivation of angiogenic and migration pathways

We investigated the expression of MUC1 protein and its relationship to the microvessel density and the expression of thymidine phosphorylase, vascular endothelial growth factor (VEGF), VEGF-receptor KDR, basic fibroblast growth factor (bFGF), and bFGF-receptor (FGFR-2) in non-small cell lung cancer. Although MUC1 expression was found equally in poorly and highly vascularized tumors, a significant coexpression with multiple angiogenic factors and their receptors was noted (P = 0.0002, 0.03, 0.19, 0.10, and 0.01 for thymidine phosphorylase, VEGF, KDR, bFGF, and FGFR-2, respectively). In multiple regression analysis, both angiogenesis and MUC1 expression were independent prognostic variables. The present study suggests the existence of an early genetic event leading to the activation of both migration and angiogenesis pathways in non-small cell lung cancer.

Ultrastructural localization of the vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) receptor-2 (FLK-1, KDR) in normal mouse kidney and in the hyperpermeable vessels induced by VPF/VEGF-expressing tumors and adenoviral vectors

Vascular permeability factor/vascular endothelial growth factor (VPF/VEGF) interacts with two high-affinity tyrosine kinase receptors, VEGFR-1 and VEGFR-2, to increase microvascular permeability and induce angiogenesis. Both receptors are selectively expressed by vascular endothelial cells and are strikingly increased in tumor vessels. We used a specific antibody to localize VEGFR-2 (FLK-1, KDR) in microvascular endothelium of normal mouse kidneys and in the microvessels induced by the TA3/St mammary tumor or by infection with an adenoviral vector engineered to express VPF/VEGF. A pre-embedding method was employed at the light and electron microscopic levels using either nanogold or peroxidase as reporters. Equivalent staining was observed on both the luminal and abluminal surfaces of tumor- and adenovirus-induced vascular endothelium, but plasma membranes at interendothelial junctions were spared except at sites connected to vesiculovacuolar organelles (VVOs). VEGFR-2 was also localized to the membranes and stomatal diaphragms of some VVOs. This staining distribution is consistent with a model in which VPF/VEGF increases microvascular permeability by opening VVOs to allow the transendothelial cell passage of plasma and plasma proteins.