Identification of ligand-induced proteolytic cleavage and ectodomain shedding of VEGFR-1/FLT1 in leukemic cancer cells

γ-Secretase and presenilin mediate cleavage and phosphorylation of vascular endothelial growth factor receptor-1

We have reported previously that pigment epithelium-derived factor (PEDF) can, via γ-secretase-mediated events, inhibit VEGF-induced angiogenesis in microvascular endothelial cells by both (a) cleavage and intracellular translocation of a C-terminal fragment of VEGF receptor-1 (VEGFR1) and (b) inhibition of VEGF-induced phosphorylation of VEGFR1. Using site-direct mutagenesis and transfection of wild type and mutated receptors into endothelial cells, we showed that transmembrane cleavage of VEGFR1 occurs at valine 767 and that a switch from valine to alanine at this position prevented cleavage and formation of a VEGFR1 intracellular fragment. Using siRNA to selectively knock down protein-tyrosine phosphatases (PTPs) in endothelial cells, we demonstrated that vascular endothelial PTP is responsible for dephosphorylation of activated VEGFR1. PEDF up-regulation of full-length presenilin 1 (Fl.PS1) facilitated the association of vascular endothelial PTP and VEGFR1. Knockdown of Fl.PS1 prevented dephosphorylation of VEGFR1, whereas up-regulation of Fl.PS1 stimulated VEGFR1 dephosphorylation. Fl.PS1 associated with VEGFR1 within 15 min after PEDF treatment. In conclusion, we determined the PEDF-mediated events responsible for VEGFR1 signaling and identified full-length presenilin as a critical adaptor molecule in the dephosphorylation of VEGFR1. This greater understanding of the regulation of VEGFR1 signaling will help identify novel anti-VEGF therapeutic strategies.

Placental growth factor regulates cardiac inflammation through the tissue inhibitor of metalloproteinases-3/tumor necrosis factor-α-converting enzyme axis: crucial role for adaptive cardiac remodeling during cardiac pressure overload

BACKGROUND

Heart failure is one of the leading causes of mortality and is primarily the final stage of several overload cardiomyopathies, preceded by an early adaptive hypertrophic response and characterized by coordinated cardiomyocyte growth, angiogenesis, and inflammation. Therefore, growth factors and cytokines have to be critically regulated during cardiac response to transverse aortic constriction. Interestingly, the dual properties of placental growth factor as an angiogenic factor and cytokine make it a candidate to participate in cardiac remodeling in response to hemodynamic overload.

METHODS AND RESULTS

After transverse aortic constriction, placental growth factor knockout mice displayed a dysregulation of cardiac remodeling, negatively affecting muscle growth. Molecular insights underscored that this effect was ascribable mainly to a failure in the establishment of adequate inflammatory response owing to an impaired activity of tumor necrosis factor-α-converting enzyme. Interestingly, after transverse aortic constriction, placental growth factor knockout mice had strongly increased levels of tissue inhibitor of metalloproteinases-3, the main natural TACE inhibitor, thus indicating an unbalance of the tissue inhibitor of metalloproteinases-3/tumor necrosis factor-α-converting enzyme axis. Strikingly, when we used an in vivo RNA interference approach to reduce tissue inhibitor of metalloproteinases-3 levels in placental growth factor knockout mice during transverse aortic constriction, we obtained a complete phenotype rescue of early dilated cardiomyopathy.

CONCLUSIONS

Our results demonstrate that placental growth factor finely tunes a balanced regulation of the tissue inhibitor of metalloproteinases-3/tumor necrosis factor-α-converting enzyme axis and the consequent TNF-α activation in response to transverse aortic constriction, thus allowing the establishment of an inflammatory response necessary for adaptive cardiac remodeling.

PEST motif serine and tyrosine phosphorylation controls vascular endothelial growth factor receptor 2 stability and downregulation

The internalization and degradation of vascular endothelial growth factor receptor 2 (VEGFR-2), a potent angiogenic receptor tyrosine kinase, is a central mechanism for the regulation of the coordinated action of VEGF in angiogenesis. Here, we show that VEGFR-2 is ubiquitinated in response to VEGF, and Lys 48-linked polyubiquitination controls its degradation via the 26S proteosome. The degradation and ubiquitination of VEGFR-2 is controlled by its PEST domain, and the phosphorylation of Ser1188/Ser1191 is required for the ubiquitination of VEGFR-2. F-box-containing β-Trcp1 ubiquitin E3 ligase is recruited to S1188/S1191 VEGFR-2 and mediates the ubiquitination and degradation of VEGFR-2. The PEST domain also controls the activation of p38 mitogen-activated protein kinase (MAPK) through phospho-Y1173. The activation of p38 stabilizes VEGFR-2, and its inactivation accelerates VEGFR-2 downregulation. The VEGFR-2-mediated activation of p38 is established through the protein kinase A (PKA)/MKK6 pathway. PKA is recruited to VEGFR-2 through AKAP1/AKAP149, and its phosphorylation requires Y1173 of VEGFR-2. The study has identified a unique mechanism in which VEGFR-2 stability and degradation is modulated. The PEST domain acts as a dual modulator of VEGFR-2; the phosphorylation of S1188/S1191 controls ubiquitination and degradation via β-Trcp1, where the phosphorylation of Y1173 through PKA/p38 MAPK controls the stability of VEGFR-2.

Elevated Soluble fms-Like Tyrosine Kinase-1 Levels in Acute Coronary Occlusion

Objective—

Early recognition of an acute coronary occlusion (ACO) improves clinical outcomes. Soluble fms-like tyrosine kinase-1 (sFLT1) is an endothelium-derived protein induced by hypoxia. We tested whether sFLT1 levels are elevated in ACO.

Methods and Results—

Serum sFLT1 levels were measured by enzyme-linked immunosorbent assay in patients with ST-segment elevations and angiographically confirmed ACO, unstable angina/non ST-segment elevation myocardial infarction, and 2 control groups. To further explore sFLT1 release, a mouse model of ACO and in vitro human coronary artery endothelial cell injury were used. sFLT1 levels were increased in ACO compared with unstable angina/non-ST-elevation myocardial infarction, catheterized controls, or healthy volunteers (200.7±15.5 versus 70.7±44.0 versus 10.2±4.0 versus 11.7±1.7 pg/mL respectively, P <0.001 versus ACO). At presentation, all ACO patients had elevated sFLT1 levels (>15 pg/mL, 99th percentile in controls), whereas 57% had levels of the MB isoform of creatine kinase levels >10 ng/mL ( P <0.01) and 85% had ultrasensitive troponin I levels >0.05 ng/mL ( P <0.05). Within 60 minutes after symptom onset, sFLT1 was more sensitive than the MB isoform of creatine kinase or ultrasensitive troponin I for ACO (100% versus 20% versus 20% respectively; P ≤0.01 for each). Within 60 minutes of ACO in mice, sFLT1 levels were elevated. Hypoxia and thrombin increased sFLT1 levels within 15 minutes in human coronary artery endothelial cells.

Conclusion—

sFLT1 levels may be an early indicator of endothelial hypoxia in ACO.

c-Cbl inhibits angiogenesis and tumor growth by suppressing activation of PLCγ1

Angiogenesis is regulated by highly coordinated function of various proteins with pro- and anti-angiogenic functions. Among the many cytoplasmic signaling proteins that are activated by VEGFR-2, activation of PLCγ1 is considered to have a pivotal role in angiogenic signaling. In previous study we have identified c-Cbl as a negative regulator of PLCγ1 in endothelial cells, the biochemical and biological significance of c-Cbl, however, in angiogenesis in vivo and molecular mechanisms involved were remained elusive. In this study, we report that genetic inactivation of c-Cbl in mice results in enhanced tumor angiogenesis and retinal neovascularization. Endothelial cells derived from c-Cbl null mice displayed elevated cell proliferation and tube formation in response to VEGF stimulation. Loss of c-Cbl also resulted in robust activation of PLCγ1 and increased intracellular calcium release. c-Cbl-dependent ubiquitination selectively inhibited tyrosine phosphorylation of PLCγ1 and mostly refrained from ubiquitin-mediated degradation. Hence, we propose c-Cbl as an angiogenic suppressor protein where upon activation it uniquely modulates PLCγ1 activation by ubiquitination and subsequently inhibits VEGF-driven angiogenesis.

Antagonism of VEGF by genetically engineered dendritic cells is essential to induce antitumor immunity against malignant ascites

Malignant ascitis (MA) is a highly intractable and immunotherapy-resistant state of advanced gastrointestinal and ovarian cancers. Using a murine model of MA with CT26 colon cancer cells, we here determined that the imbalance between the VEGF-A/vascular permeability factor and its decoy receptor, soluble fms-like tryrosine kinase receptor-1 (sFLT-1), was a major cause of MA resistance to dendritic cell (DC)-based immunotherapy. We found that the ratio of VEGF-A/sFLT-1 was increased not only in murine but also in human MA, and F-gene-deleted recombinant Sendai virus (rSeV/dF)-mediated secretion of human sFLT-1 by DCs augmented not only the activity of DCs themselves, but also dramatically improved the survival of tumor-bearing animals associated with enhanced CTL activity and its infiltration to peritoneal tumors. These findings were not seen in immunodeficient mice, indicating that a VEGF-A/sFLT-1 imbalance is critical for determining the antitumor immune response by DC-vaccination therapy against MA.

Soluble VEGFR-2: an antilymphangiogenic variant of VEGF receptors

The vascular endothelial growth factor (VEGF) family of secreted proteins and their receptors are major regulators of blood vessel development (hemangiogenesis) and lymphatic vessel development (lymphangiogenesis). VEGF acts through a complex system of receptor tyrosine kinases, which can be membrane bound or soluble. New data concerning the receptor system are still emerging, thus contributing to the complexity of the system. Very recently a soluble form of VEGFR-2, termed sVEGFR-2, which is a result of alternative splicing, has been discovered. Earlier, it has been shown that a secreted/soluble form of VEGFR-1, termed sVEGFR-1, is produced by alternative splicing and exerts an antihemangiogenic effect by binding VEGF-A. The newly discovered spliced variant of sVEGFR-2 binds the lymphangiogenic growth factor VEGF-C and thus inhibits VEGF-C-induced activation of VEGFR-3, consequently inhibiting lymphatic endothelial cell proliferation. Its inactivation in murine embryos permits hyperplasia of dermal lymphatics and invasion of lymphatics into the cornea. Tumor lymphangiogenesis seems to influence the metastatic behavior of malignant cells. A correlation has been found between the downregulation of  sVEGFR-2 and the malignant progression of neuroblastoma, which is characterized by lymphogenic metastases in progressed stages. Data show that lymphangiogenesis is regulated by both activators and inhibitors, and its balance is crucial in health and disease.

A systems biology perspective on sVEGFR1: its biological function, pathogenic role and therapeutic use

Angiogenesis is the growth of new capillaries from pre-existent microvasculature. A wide range of pathological conditions, from atherosclerosis to cancer, can be attributed to either excessive or deficient angiogenesis. Central to the physiological regulation of angiogenesis is the vascular endothelial growth factor (VEGF) system – its ligands and receptors (VEGFRs) are thus prime molecular targets of pro-angiogenic and anti-angiogenic therapies. Of growing interest as a prognostic marker and therapeutic target in angiogenesis-dependent diseases is soluble VEGF receptor-1 (sVEGFR1, also known as sFlt-1) – a truncated version of the cell membrane-spanning VEGFR1. For instance, it is known that sVEGFR1 is involved in the endothelial dysfunction characterizing the pregnancy disorder of pre-eclampsia, and sVEGFR1’s therapeutic potential as an anti-angiogenic agent is being evaluated in pre-clinical models of cancer. This mini review begins with an examination of the protein domain structure and biomolecular interactions of sVEGFR1 in relation to the full-length VEGFR1. A synopsis of known and inferred physiological and pathological roles of sVEGFR1 is then given, with emphasis on the utility of computational systems biology models in deciphering the molecular mechanisms by which sVEGFR1’s purported biological functions occur. Finally, we present the need for a systems biology perspective in interpreting circulating VEGF and sVEGFR1 concentrations as surrogate markers of angiogenic status in angiogenesis-dependent diseases.

Lymphoma cell VEGFR2 expression detected by immunohistochemistry predicts poor overall survival in diffuse large B cell lymphoma treated with immunochemotherapy (R-CHOP)

Diffuse large B cell lymphoma (DLBCL) is clinically and biologically heterogeneous. In most cases of DLBCL, lymphoma cells co-express vascular endothelial growth factor (VEGF) and its receptors VEGFR1 and VEGFR2, suggesting autocrine in addition to angiogenic effects. We enumerated microvessel density and scored lymphoma cell expression of VEGF, VEGFR1, VEGFR2 and phosphorylated VEGFR2 in 162 de novo DLBCL patients treated with R-CHOP (rituximab, cyclophosphamide, vincristine, doxorubicin and prednisone)-like regimens. VEGFR2 expression correlated with shorter overall survival (OS) independent of International Prognostic Index (IPI) (P = 0.0028). Phosphorylated VEGFR2 (detected in 13% of cases) correlated with shorter progression-free survival (PFS, P = 0.044) and trended toward shorter OS on univariate analysis. VEGFR1 was not predictive of survival on univariate analysis, but it did correlate with better OS on multivariate analysis with VEGF, VEGFR2 and IPI (P = 0.036); in patients with weak VEGFR2, lack of VEGFR1 coexpression was significantly correlated with poor OS independent of IPI (P = 0.01). These results are concordant with our prior finding of an association of VEGFR1 with longer OS in DLBCL treated with chemotherapy alone. We postulate that VEGFR1 may oppose autocrine VEGFR2 signalling in DLBCL by competing for VEGF binding. In contrast to our prior results with chemotherapy alone, microvessel density was not prognostic of PFS or OS with R-CHOP-like therapy.

Regulation of soluble vascular endothelial growth factor receptor 1 secretion from human endothelial cells by tissue inhibitor of metalloproteinase 1

Vascular endothelial growth factor (VEGF) and its soluble receptor (sVEGFR-1) are key regulators in human ovarian angiogenesis. Produced by granulosa and ovarian theca interna cells, VEGF promotes blood vessel growth during follicular development and corpus luteum formation, whereas sVEGFR-1, which is secreted by endothelial cells, functions as an antagonist to VEGF activity by binding it. In order to gain further insights into the regulatory mechanisms of ovarian angiogenesis, the aim of the present study was to analyze the influence of tissue inhibitor of metalloproteinase 1 (TIMP-1), which is actively involved in the degradation and remodeling of the extracellular matrix, on sVEGFR-1 secretion of cultured human umbilical vein endothelial cells. sVEGFR-1 production was determined in the culture supernatant by Sandwich-ELISA. We showed that TIMP-1 produced by human granulosa cells and recombinant human TIMP-1 both significantly increased the production of sVEGFR-1 in endothelial cells. Also, the down-regulation of TIMP-1 expression by RNA interference resulted in a significant reduction of endothelial sVEGFR-1 secretion into the culture medium. Furthermore, TIMP-1 weakly inhibited proliferation of VEGF-stimulated endothelial cells. In conclusion, our results provide evidence that TIMP-1 increases the production of sVEGFR-1 in endothelial cells and thus may reduce VEGF bioavailability, leading to reduced blood vessel growth in the ovary.

Computational kinetic model of VEGF trapping by soluble VEGF receptor-1: effects of transendothelial and lymphatic macromolecular transport

Vascular endothelial growth factor (VEGF) signal transduction through the cell surface receptors VEGFR1 and VEGFR2 regulates angiogenesis-the growth of new capillaries from preexistent microvasculature. Soluble VEGF receptor-1 (sVEGFR1), a nonsignaling truncated variant of VEGFR1, has been postulated to inhibit angiogenic signaling via direct sequestration of VEGF ligands or dominant-negative heterodimerization with surface VEGFRs. The relative contributions of these two mechanisms to sVEGFR1's purported antiangiogenic effects in vivo are currently unknown. We previously developed a computational model for predicting the compartmental distributions of VEGF and sVEGFR1 throughout the healthy human body by simulating the molecular interaction networks of the VEGF ligand-receptor system as well as intercompartmental macromolecular biotransport processes. In this study, we decipher the dynamic processes that led to our prior prediction that sVEGFR1, through its ligand trapping mechanism alone, does not demonstrate significant steady-state antiangiogenic effects. We show that sVEGFR1-facilitated tissue-to-blood shuttling of VEGF accounts for a counterintuitive and drastic elevation in plasma free VEGF concentrations after both intramuscular and intravascular sVEGFR1 infusion. While increasing intramuscular VEGF production reduces free sVEGFR1 levels through increased VEGF-sVEGFR1 complex formation, we demonstrate a competing and opposite effect in which increased VEGF occupancy of neuropilin-1 (NRP1) and the corresponding reduction in NRP1 availability for internalization of sVEGFR1 unexpectedly increases free sVEGFR1 levels. In conclusion, dynamic intercompartmental transport processes give rise to our surprising prediction that VEGF trapping alone does not account for sVEGFR1's antiangiogenic potential. sVEGFR1's interactions with cell surface receptors such as NRP1 are also expected to affect its molecular interplay with VEGF.