Blocking neuropilin-1 function has an additive effect with anti-VEGF to inhibit tumor growth.

Neuropilin-1 (NRP1) guides the development of the nervous and vascular systems. Binding to either semaphorins or VEGF, NRP1 acts with plexins to regulate neuronal guidance, or with VEGFR2 to mediate vascular development. We have generated two monoclonal antibodies that bind to the Sema- and VEGF-binding domains of NRP1, respectively. Both antibodies reduce angiogenesis and vascular remodeling, while having little effect on other VEGFR2-mediated events. Importantly, anti-NRP1 antibodies have an additive effect with anti-VEGF therapy in reducing tumor growth. Vessels from tumors treated with anti-VEGF show a close association with pericytes, while tumors treated with both anti-NRP1 and anti-VEGF lack this organization. We propose that blocking NRP1 function inhibits vascular remodeling, rendering vessels more susceptible to anti-VEGF therapy.

CCBE1 is essential for mammalian lymphatic vascular development and enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo.

RATIONALE: Collagen- and calcium-binding EGF domains 1 (CCBE1) has been associated with Hennekam syndrome, in which patients have lymphedema, lymphangiectasias, and other cardiovascular anomalies. Insight into the molecular role of CCBE1 is completely lacking, and mouse models for the disease do not exist. OBJECTIVE: CCBE1 deficient mice were generated to understand the function of CCBE1 in cardiovascular development, and CCBE1 recombinant protein was used in both in vivo and in vitro settings to gain insight into the molecular function of CCBE1. METHODS AND RESULTS: Phenotypic analysis of murine Ccbe1 mutant embryos showed a complete lack of definitive lymphatic structures, even though Prox1(+) lymphatic endothelial cells get specified within the cardinal vein. Mutant mice die prenatally. Proximity ligation assays indicate that vascular endothelial growth factor receptor 3 activation appears unaltered in mutants. Human CCBE1 protein binds to components of the extracellular matrix in vitro, and CCBE1 protein strongly enhances vascular endothelial growth factor-C-mediated lymphangiogenesis in a corneal micropocket assay. CONCLUSIONS: Our data identify CCBE1 as a factor critically required for budding and migration of Prox-1(+) lymphatic endothelial cells from the cardinal vein. CCBE1 probably exerts these effects through binding to components of the extracellular matrix. CCBE1 has little lymphangiogenic effect on its own but dramatically enhances the lymphangiogenic effect of vascular endothelial growth factor-C in vivo. Thus, our data suggest CCBE1 to be essential but not sufficient for lymphangiogenesis.

Inhibition of Dll4 signalling inhibits tumour growth by deregulating angiogenesis.

Haploinsufficiency of Dll4, a vascular-specific Notch ligand, has shown that it is essential for embryonic vascular development and arteriogenesis. Mechanistically, it is unclear how the Dll4-mediated Notch pathway contributes to complex vascular processes that demand meticulous coordination of multiple signalling pathways. Here we show that Dll4-mediated Notch signalling has a unique role in regulating endothelial cell proliferation and differentiation. Neutralizing Dll4 with a Dll4-selective antibody rendered endothelial cells hyperproliferative, and caused defective cell fate specification or differentiation both in vitro and in vivo. In addition, blocking Dll4 inhibited tumour growth in several tumour models. Remarkably, antibodies against Dll4 and antibodies against vascular endothelial growth factor (VEGF) had paradoxically distinct effects on tumour vasculature. Our data also indicate that Dll4-mediated Notch signalling is crucial during active vascularization, but less important for normal vessel maintenance. Furthermore, unlike blocking Notch signalling globally, neutralizing Dll4 had no discernable impact on intestinal goblet cell differentiation, supporting the idea that Dll4-mediated Notch signalling is largely restricted to the vascular compartment. Therefore, targeting Dll4 might represent a broadly efficacious and well-tolerated approach for the treatment of solid tumours.

Cardiovascular and systemic microvascular effects of anti-vascular endothelial growth factor therapy for cancer.

OBJECTIVES: This study sought to evaluate the contribution of microvascular functional rarefaction and changes in vascular mechanical properties to the development of hypertension and secondary ventricular remodeling that occurs with anti-vascular endothelial growth factor (VEGF) therapy. BACKGROUND: Hypertension is a common side effect of VEGF inhibitors used in cancer medicine. METHODS: Mice were treated for 5 weeks with an anti-murine VEGF-A monoclonal antibody, antibody plus ramipril, or sham treatment. Microvascular blood flow (MBF) and blood volume (MBV) were quantified by contrast-enhanced ultrasound in skeletal muscle, left ventricle (LV), and kidney. Echocardiography and invasive hemodynamics were used to assess ventricular function, dimensions and vascular mechanical properties. RESULTS: Ambulatory blood pressure increased gradually over the first 3 weeks of anti-VEGF therapy. Compared with controls, anti-VEGF-treated mice had similar aortic elastic modulus and histological appearance, but a marked increase in arterial elastance, indicating increased afterload, and elevated plasma angiotensin II. Increased afterload in treated mice led to concentric LV remodeling and reduced stroke volume without impaired LV contractility determined by LV peak change in pressure over time (dp/dt) and the end-systolic dimension-pressure relation. Anti-VEGF therapy did not alter MBF or MBV in skeletal muscle, myocardium, or kidney; but did produce cortical mesangial glomerulosclerosis. Ramipril therapy almost entirely prevented the adverse hemodynamic effects, increased afterload, and LV remodeling in anti-VEGF-treated mice. CONCLUSIONS: Neither reduced functional microvascular density nor major alterations in arterial mechanical properties are primary causes of hypertension during anti-VEGF therapy. Inhibition of VEGF leads to an afterload mismatch state, increased angiotensin II, and LV remodeling, which are all ameliorated by angiotensin-converting enzyme inhibition.

Robo4 maintains vessel integrity and inhibits angiogenesis by interacting with UNC5B.

Robo4 is an endothelial cell-specific member of the Roundabout axon guidance receptor family. To identify Robo4 binding partners, we performed a protein-protein interaction screen with the Robo4 extracellular domain. We find that Robo4 specifically binds to UNC5B, a vascular Netrin receptor, revealing unexpected interactions between two endothelial guidance receptors. We show that Robo4 maintains vessel integrity by activating UNC5B, which inhibits signaling downstream of vascular endothelial growth factor (VEGF). Function-blocking monoclonal antibodies against Robo4 and UNC5B increase angiogenesis and disrupt vessel integrity. Soluble Robo4 protein inhibits VEGF-induced vessel permeability and rescues barrier defects in Robo4(-/-) mice, but not in mice treated with anti-UNC5B. Thus, Robo4-UNC5B signaling maintains vascular integrity by counteracting VEGF signaling in endothelial cells, identifying a novel function of guidance receptor interactions in the vasculature.

Blocking neuropilin-2 function inhibits tumor cell metastasis.

Metastasis, which commonly uses lymphatics, accounts for much of the mortality associated with cancer. The vascular endothelial growth factor (VEGF)-C coreceptor, neuropilin-2 (Nrp2), modulates but is not necessary for developmental lymphangiogenesis, and its significance for metastasis is unknown. An antibody to Nrp2 that blocks VEGFC binding disrupts VEGFC-induced lymphatic endothelial cell migration, but not proliferation, in part independently of VEGF receptor activation. It does not affect established lymphatics in normal adult mice but reduces tumoral lymphangiogenesis and, importantly, functional lymphatics associated with tumors. It also reduces metastasis to sentinel lymph nodes and distant organs, apparently by delaying the departure of tumor cells from the primary tumor. Our results demonstrate that Nrp2, which was originally identified as an axon-guidance receptor, is an attractive target for modulating metastasis.

Characterization of endocrine gland-derived vascular endothelial growth factor signaling in adrenal cortex capillary endothelial cells.

Endocrine gland-derived vascular endothelial growth factor (EG-VEGF) has been recently identified as a mitogen specific for the endothelium of steroidogenic glands. Here we report a characterization of the signal transduction of EG-VEGF in a responsive cell type, bovine adrenal cortex-derived endothelial (ACE) cells. EG-VEGF led to a time- and dose-dependent phosphorylation of p44/42 MAPK. This effect was blocked by pretreatment with pertussis toxin, suggesting that G alpha(i) plays an important role in mediating EG-VEGF-induced activation of MAPK signaling. The inhibitor of p44/42 MAPK phosphorylation PD 98059 resulted in suppression of both proliferation and migration in response to EG-VEGF. EG-VEGF also increased the phosphorylation of Akt in a phosphatidylinositol 3-kinase-dependent manner. Consistent with such an effect, EG-VEGF was a potent survival factor for ACE cells. We also identified endothelial nitric-oxide synthase as one of the downstream targets of Akt activation. Phosphorylation of endothelial nitric-oxide synthase in ACE cells was stimulated by EG-VEGF with a time course correlated to the Akt phosphorylation. Our data demonstrate that EG-VEGF, possibly through binding to a G-protein coupled receptor, results in the activation of MAPK p44/42 and phosphatidylinositol 3-kinase signaling pathways, leading to proliferation, migration, and survival of responsive endothelial cells.

ANGPTL3 stimulates endothelial cell adhesion and migration via integrin alpha vbeta 3 and induces blood vessel formation in vivo.

The angiopoietin family of secreted factors is functionally defined by the C-terminal fibrinogen (FBN)-like domain, which mediates binding to the Tie2 receptor and thereby facilitates a cascade of events ultimately regulating blood vessel formation. By screening expressed sequence tag data bases for homologies to a consensus FBN-like motive, we have identified ANGPTL3, a liver-specific, secreted factor consisting of an N-terminal coiled-coil domain and the C-terminal FBN-like domain. Co-immunoprecipitation experiments, however, failed to detect binding of ANGPTL3 to the Tie2 receptor. A molecular model of the FBN-like domain of ANGPTL3 was generated and predicted potential binding to integrins. This hypothesis was experimentally confirmed by the finding that recombinant ANGPTL3 bound to alpha(v)beta(3) and induced integrin alpha(v)beta(3)-dependent haptotactic endothelial cell adhesion and migration and stimulated signal transduction pathways characteristic for integrin activation, including phosphorylation of Akt, mitogen-activated protein kinase, and focal adhesion kinase. When tested in the rat corneal assay, ANGPTL3 strongly induced angiogenesis with comparable magnitude as observed for vascular endothelial growth factor-A. Moreover, the C-terminal FBN-like domain alone was sufficient to induce endothelial cell adhesion and in vivo angiogenesis. Taken together, our data demonstrate that ANGPTL3 is the first member of the angiopoietin-like family of secreted factors binding to integrin alpha(v)beta(3) and suggest a possible role in the regulation of angiogenesis.

VEGF-null cells require PDGFR alpha signaling-mediated stromal fibroblast recruitment for tumorigenesis.

We generated VEGF-null fibrosarcomas from VEGF-loxP mouse embryonic fibroblasts to investigate the mechanisms of tumor escape after VEGF inactivation. These cells were found to be tumorigenic and angiogenic in vivo in spite of the absence of tumor-derived VEGF. However, VEGF derived from host stroma was readily detected in the tumor mass and treatment with a newly developed anti-VEGF monoclonal antibody substantially inhibited tumor growth. The functional significance of stroma-derived VEGF indicates that the recruitment of stromal cells is critical for the angiogenic and tumorigenic properties of these cells. Here we identified PDGF AA as the major stromal fibroblast chemotactic factor produced by tumor cells, and demonstrated that disrupting the paracrine PDGFR alpha signaling between tumor cells and stromal fibroblasts by soluble PDGFR alpha-IgG significantly reduced tumor growth. Thus, PDGFR alpha signaling is required for the recruitment of VEGF-producing stromal fibroblasts for tumor angiogenesis and growth. Our findings highlight a novel aspect of PDGFR alpha signaling in tumorigenesis.