VEGF receptor signalling - in control of vascular function

VEGFR-2 inhibition augments cigarette smoke-induced oxidative stress and inflammatory responses leading to endothelial dysfunction

Vascular endothelial growth factor (VEGF) induces phosphorylation of VEGF receptor-2 (VEGFR-2) and activates the downstream signaling pathway resulting in endothelial cell migration, proliferation, and survival. Cigarette smoking is associated with abnormal vascular and endothelial function, leading to airspace enlargement. Herein, we investigated the mechanism of cigarette smoke (CS) -induced endothelial dysfunction by studying the VEGF-VEGFR-2 signaling in mouse lung and human endothelial cells. CS exposure caused oxidative stress, as shown by increased levels of 4-hydroxy-2-nonenal-adducts in mouse lung and reactive oxygen species generation in human lung microvascular endothelial cells (HMVEC-Ls). Inhibition of VEGFR-2 by a specific kinase inhibitor (NVP-AAD777) enhanced the CS-induced oxidative stress, causing augmented inflammatory cell influx and proinflammatory mediators release in mouse lung. The levels of endothelial nitric oxide synthase (eNOS) and phosphorylated (p) -eNOS in the lungs of mice exposed to CS and/or treated with VEGFR-2 inhibitor were decreased. CS down-regulated VEGFR-2 expression, eNOS levels, and VEGF-induced VEGFR-2 phosphorylation in HMVEC-Ls, resulting in impaired VEGF-induced endothelial cell migration and angiogenesis. Overall, these data show that inhibition of VEGFR-2 augmented CS-induced oxidative stress and inflammatory responses leading to endothelial dysfunction. This explains the mechanism of endothelial dysfunction in smokers and has implications in understanding the pathogenesis of pulmonary and cardiovascular diseases.

Age-related differences in expression of vascular endothelial growth factor by periodontal ligament cells in vitro

The purpose of this study was to evaluate age-related differences in expression of vascular endothelial growth factor (VEGF) by periodontal ligament (PDL) cells. PDL cells were obtained from Wistar male rats weighing approximately 150 g each in the young group and 350 g each in the old group. PDL cells derived from upper and lower incisors were seeded in 35-mm culture dishes after primary culture. For cell proliferation assays, cells were detached and counted at 1, 3, 5, 7, 11 and 14 days after culture. VEGF mRNA expression was analyzed with TaqMan. The number of cells in both groups increased day by day, but the rate of increase in the young group was higher than that in the old group. VEGF mRNA expression in the young group increased from 3 to 14 days, but in the old group increased only slightly over the same time period. Expression ratios in the young group were higher than those in the old group, and there were significant differences between the young and old groups at 7 and 14 days of culture. In conclusion, the data revealed that PDL cells varied with age, and suggest that in view of such changes in cell proliferation and VEGF mRNA expression, age should be taken into consideration in periodontal treatment.

Phosphatidylinositol inhibits vascular endothelial growth factor-A--induced migration of human umbilical vein endothelial cells

Phosphatidylinositol (PI), a phospholipid in component of cell membranes, is widely distributed in animals, plants, and microorganisms. Here, we examined in vitro whether PI inhibits the angiogenesis induced by vascular endothelial growth factor-A (VEGF-A). PI concentration-relatedly and significantly (at 10 and 30 microg/ml) inhibited VEGF-A-induced tube formation in a co-culture of human umbilical vein endothelial cells (HUVECs) and fibroblasts. PI also inhibited the migration, but not proliferation, induced in HUVECs by VEGF-A. Furthermore, PI at 30 microg/ml inhibited the VEGF-A-induced phosphorylation of serine/threonine protein kinase family protein kinase B (Akt) and p38 mitogen activate kinase (p38MAPK), key molecules in cell migration, but not phosphorylation of extracellular signal-regulated kinase 1/2 (ERK1/2), a key molecule in cell proliferation. These findings indicate that PI inhibits VEGF-induced angiogenesis by inhibiting HUVECs migration and that inhibition of phosphorylated-Akt and -p38MAPK may be involved in the mechanism. Therefore, PI may be expected to prevent some diseases caused by angiogenesis.

A novel role of vascular endothelial cadherin in modulating c-Src activation and downstream signaling of vascular endothelial growth factor

Vascular endothelial growth factor (VEGF) is a potent mediator of angiogenesis and vascular permeability, in which c-Src tyrosine kinase plays an essential role. However, the mechanisms by which VEGF stimulates c-Src activation have remained unclear. Here, we demonstrate that vascular endothelial cadherin (VE-cadherin) plays a critical role in regulating c-Src activation in response to VEGF. In vascular endothelial cells, VE-cadherin was basally associated with c-Src and Csk (C-terminal Src kinase), a negative regulator of Src activation. VEGF stimulated Csk release from VE-cadherin by recruiting the protein tyrosine phosphatase SHP2 to VE-cadherin signaling complex, leading to an increase in c-Src activation. Silencing VE-cadherin with small interference RNA significantly reduced VEGF-stimulated c-Src activation. Disrupting the association of VE-cadherin and Csk through the reconstitution of Csk binding-defective mutant of VE-cadherin also diminished Src activation. Moreover, inhibiting SHP2 by small interference RNA and adenovirus-mediated expression of a catalytically inactive mutant of SHP2 attenuated c-Src activation by blocking the disassociation of Csk from VE-cadherin. Furthermore, VE-cadherin and SHP2 differentially regulates VEGF downstream signaling. The inhibition of c-Src, VE-cadherin, and SHP2 diminished VEGF-mediated activation of Akt and endothelial nitric-oxide synthase. In contrast, inhibiting VE-cadherin and SHP2 enhanced ERK1/2 activation in response to VEGF. These findings reveal a novel role for VE-cadherin in modulating c-Src activation in VEGF signaling, thus providing new insights into the importance of VE-cadherin in VEGF signaling and vascular function.

A conserved role of the VEGF pathway in angiogenesis of an ectodermally-derived vasculature

Angiogenesis, the growth and remodeling of a vascular network, is an essential process during development, growth and disease. Here we studied the role of the vascular endothelial growth factor receptor (VEGFR) in experimentally-induced angiogenesis in the colonial ascidian Botryllus schlosseri (Tunicata, Ascidiacea). The circulatory system of B. schlosseri is composed of two distinct, but interconnected regions: a plot of sinuses and lacunae which line the body, and a transparent, macroscopic extracorporeal vascular network. The vessels of the extracorporeal vasculature are morphologically inverted in comparison to the vasculature in vertebrates: they consist of a single layer of ectodermally-derived cells with the basal lamina lining the lumen of the vessel. We found that when the peripheral circulatory system of a colony is surgically removed, it can completely regenerate within 24 to 48 h and this regeneration is dependent on proper function of the VEGF pathway: siRNA-mediated knockdown of the VEGFR blocked vascular regeneration, and interfered with vascular homeostasis. In addition, a small molecule, the VEGFR kinase inhibitor PTK787/ZK222584, phenocopied the siRNA knockdown in a reversible manner. Despite the disparate germ layer origins and morphology of the vasculature, the developmental program of branching morphogenesis during angiogenesis is controlled by similar molecular mechanisms, suggesting that the function of the VEGF pathway may be co-opted during the regeneration of an ectoderm-derived tubular structure.

Control of receptor internalization, signaling level, and precise arrival at the target in guided cell migration

Activation of the chemokine receptor CXCR4 by SDF1 controls a variety of biological processes in development, immune response, and disease [1-5]. The carboxyl-terminal region of CXCR4 is subject to phosphorylation that allows binding of regulatory proteins [5]; this results in downregulation of CXCR4 signaling and receptor internalization [6]. Notably, truncations of this part of CXCR4 have been implicated in WHIM syndrome, a dominantly inherited immunodeficiency disorder [7, 8]. Despite its importance in receptor signaling and the clinical relevance of its regulation, the precise function of regulating signaling level and internalization in controlling cell behavior is not known. Whereas a number of in vitro studies suggested that the carboxyl terminus of CXCR4 positively regulates chemotaxis (e.g., [9]), others reached the opposite conclusion [8, 10, 11]. These conflicting results highlight the importance of investigating this process under physiological conditions in the live animal. In this study, we demonstrate the significance of internalization and of controlling receptor signaling level for SDF-1-guided migration. We found that whereas internalization and the control over signaling intensity are dispensable for cell motility and directional sensing, they are essential for fine-tuning of migration in vivo, allowing precise arrival of zebrafish PGCs at their target, the region where the gonad develops.

Prognostic significance of VEGF, VEGF receptors, and microvessel density in diffuse large B cell lymphoma treated with anthracycline-based chemotherapy

Vascular endothelial growth factor-mediated signaling has at least two potential roles in diffuse large B cell lymphoma: potentiation of angiogenesis, and potentiation of lymphoma cell proliferation and/or survival induced by autocrine vascular endothelial growth factor receptor-mediated signaling. We have recently shown that diffuse large B cell lymphomas expressing high levels of vascular endothelial growth factor protein also express high levels of vascular endothelial growth factor receptor-1 and vascular endothelial growth factor receptor-2. We have now assessed a larger multi-institutional cohort of patients with de novo diffuse large B cell lymphoma treated with anthracycline-based therapy to address whether tumor vascularity, or expression of vascular endothelial growth factor protein and its receptors, contribute to patient outcomes. Our results show that increased tumor vascularity is associated with poor overall survival (P=0.047), and is independent of the international prognostic index. High expression of vascular endothelial growth factor receptor-1 by lymphoma cells by contrast is associated with improved overall survival (P=0.044). The combination of high vascular endothelial growth factor and vascular endothelial growth factor receptor-1 protein expression by lymphoma cells identifies a subgroup of patients with improved overall (P=0.003) and progression-free (P=0.026) survival; these findings are also independent of the international prognostic index. The prognostic significance of overexpression of this ligand-receptor pair suggests that autocrine signaling via vascular endothelial growth factor receptor-1 may represent a survival or proliferation pathway in diffuse large B cell lymphoma. Dependence on autocrine vascular endothelial growth factor receptor-1-mediated signaling may render a subset of diffuse large B-cell lymphomas susceptible to anthracycline-based therapy.

Modulation of vascular endothelial growth factor (VEGF) expression in motor neurons and its electrophysiological effects

Previous studies have shown that VEGF expression in forebrain increases after experimental manipulations that increase neuronal activity. One question is whether this also occurs in motor neurons. If so, it could be potentially advantageous from a therapeutic perspective, because VEGF prevents motor neuron degeneration. Therefore, we asked whether endogenous VEGF expression in motor neurons could be modulated. We also asked how VEGF exposure would influence motor neurons using electrophysiology. Immunocytochemistry showed that motor neuron VEGF expression increased after a stimulus that increases neuronal and motor activity, i.e., convulsive seizures. The increase in VEGF immunoreactivity occurred in all motor neuron populations that were examined 24h later. This effect was unlikely to be due to seizure-induced toxicity, because silver degeneration stain did not show the typical appearance of a dying or dead neuron. To address the effects of VEGF on motor neuron function, VEGF was applied directly to motor neurons while recording intracellularly, using a brainstem slice preparation. Exposure to exogenous VEGF (200 ng/ml) in normal conditions depressed stimulus-evoked depolarization of hypoglossal motor neurons. There was no detectable effect of VEGF on membrane properties or firing behavior. We suggest that VEGF is upregulated in neurons when they are strongly activated, and VEGF depresses neuronal excitation as a compensatory mechanism. Failure of this mechanism may contribute to diseases that involve a dysregulation of VEGF, excessive excitation of motor neurons, and motor neuron loss, such as amyotrophic lateral sclerosis (ALS).

A 10,000 member PNA-encoded peptide library for profiling tyrosine kinases

A 10,000 member peptide nucleic acid (PNA) encoded peptide library was prepared, treated with the Abelson tyrosine kinase (Abl), and decoded using a DNA microarray and a fluorescently labeled secondary antiphosphotyrosine antibody. A dual-color approach ensured internal referencing for each and every member of the library and the generation of robust data sets. Analysis identified 155 peptides (out of 10,000) that were strongly phosphorylated by Abl in full agreement with known Abl specificities. BLAST analysis identified known cellular Abl substrates such as c-Jun amino-terminal kinase as well as new potential target proteins such as the G-protein coupled receptor kinase 6 and diacylglycerol kinase gamma. To illustrate the generalization of this approach, two other tyrosine kinases, human epidermal growth factor 2 (Her2) and vascular endothelial growth factor receptor 2/kinase insert domain protein receptor (VEGFR2/KDR), were profiled allowing characterization of specific peptide sequences known to interact with these kinases; under these conditions Her2 was demonstrated to have a marked preference for D-proline perhaps offering a unique means of targeting and inhibiting this kinase.

FGFR-1 regulates angiogenesis through cytokines interleukin-4 and pleiotrophin

The role of fibroblast growth factors (FGFs) in blood vessel formation has remained unclear. We used differentiating stem-cell cultures (embryoid bodies) and teratomas to show that FGF receptor-1 (FGFR-1) exerts a negative regulatory effect on endothelial cell function in these models. Embryoid bodies lacking expression of FGFR-1 as a result of gene targeting (Fgfr-1−/−) displayed increased vascularization and a distinct, elongated vessel morphology. Teratomas derived from FGFR-1–deficient stem cells were characterized by an increased growth rate and abundant, morphologically distinct vessels. Transmission electron microscopy of the Fgfr-1−/− teratomas showed a compact and voluminous but functional endothelium, which anastomosed with the host circulation. The increased vascularization and altered endothelial cell morphology was dependent on secreted factor(s), based on the transfer of the Fgfr-1−/− vascular phenotype by conditioned medium to Fgfr-1+/− embryoid bodies. Antibody and transcript arrays showed down-regulation of interleukin-4 (IL-4) and up-regulation of pleiotrophin in Fgfr-1−/− embryoid bodies, compared with the heterozygous cultures. We used neutralizing antibodies to show that IL-4 and pleiotrophin act as negative and positive angiogenic regulators, respectively. We conclude that FGFR-1 negatively regulates endothelial cell function by altering the balance of modulatory cytokines.

The molecular basis of VEGFR-1 signal transduction pathways in primary human monocytes

OBJECTIVE

Arteriogenesis, the growth of preexisting arterioles into functional arteries, is dependent on the proper function of monocytes. Likewise, wound healing is monocyte-dependent. The activation of vascular endothelial growth factor receptor-1 (VEGFR-1) in monocytes induces a chemotactic response, triggers the expression of tissue factor, and gene expression of cytokines and chemokines. Little is known about intracellular signaling pathways mediating the biological functions triggered by VEGFR-1 in primary monocytes.

METHODS AND RESULTS

Monocytes were isolated from peripheral venous blood of young healthy individuals using indirect magnetic labeling. Stimulation of monocytes with either vascular endothelial growth factor-A (VEGF-A) or placenta growth factor (PlGF-1) triggered VEGFR-1 autophosphorylation and phosphorylation of distinct downstream proteins: phosphatidylinositol-3 kinase (PI-3K), Akt, p38, and extracellular signal-regulated kinase-1/2 (ERK1/2). PI-3K appears to be a central regulator in VEGFR-1 signaling in monocytes as the activation of Akt, p38, and ERK1/2 are PI-3-K-dependent. In addition, Akt activation functions downstream of p38 kinase. VEGFR-1-mediated chemotaxis of monocytes is dependent on the activation of PI-3K, p38 kinase, Akt, and ERK1/2, when assessed in a modified Boyden chamber.

CONCLUSIONS

Both PlGF-1 and VEGF-A can activate VEGFR-1-dependent signaling pathways in primary human monocytes, leading to the activation of several intracellular signaling pathways. These pathways are critically involved in primary monocyte chemotaxis.

The C-terminus of viral vascular endothelial growth factor-E partially blocks binding to VEGF receptor-1

Vascular endothelial growth factor (VEGF) family members play important roles in embryonic development and angiogenesis during wound healing and in pathological conditions such as tumor formation. Parapoxviruses express a new member of the VEGF family which is a functional mitogen that specifically activates VEGF receptor (VEGFR)-2 but not VEGFR-1. In this study, we show that deletion from the viral VEGF of a unique C-terminal region increases both VEGFR-1 binding and VEGFR-1-mediated monocyte migration. Enzymatic removal of O-linked glycosylation from the C-terminus also increased VEGFR-1 binding and migration of THP-1 monocytes indicating that both the C-terminal residues and O-linked sugars contribute to blocking viral VEGF binding to VEGFR-1. The data suggest that conservation of the C-terminal residues throughout the viral VEGF subfamily may represent a means of reducing the immunostimulatory activities associated with VEGFR-1 activation while maintaining the ability to induce angiogenesis via VEGFR-2.

Laminin deposition is dispensable for vasculogenesis but regulates blood vessel diameter independent of flow

Basement membranes (BMs) consisting of laminins, collagens, and heparan sulfate proteoglycans (HSPGs) are vital for proper endothelial cell function, but many aspects of their role in vascular development remain unknown. Here, we demonstrate that vascular structures within differentiating embryoid bodies are wrapped in a BM composed of alpha4- and alpha5-chain laminins, fibronectin, collagen IV, and HSPGs. In sprouting angiogenesis, laminins were produced by stalk cells, as well as the leading tip cell, and deposited along the sprout length, including tip cell filopodia. In embryonic stem cells deficient in laminins, due to lamc1 (laminin gamma1) deletion, vascular development and organization were largely unaffected. However, the frequency of vessels with wide lumens was increased 4-fold. Laminin-deficient vessels were moreover characterized by increased fibronectin levels and enhanced endothelial cell proliferation. We conclude that laminins are dispensable for vascular development but that they regulate lumen formation in the absence of flow and vascular tone.

Comparison of molecular determinants of angiogenesis and lymphangiogenesis in lymph node metastases and in primary tumours of patients with breast cancer

Angiogenesis and lymphangiogenesis are complex processes, driven by multiple factors. In primary breast tumours (PTs), VEGFA, -C and -D are the most important (lymph)angiogenic factors. The induction of lymphangiogenesis in axillary lymph node (LN) metastases of patients with breast cancer was described recently. To compare the molecular determinants of (lymph)angiogenesis in LN metastases and PTs of breast cancer patients, RNA was isolated from formalin-fixed, paraffin-embedded tissue sections of a metastatically involved and uninvolved LN and the PT from 26 lymph node-positive patients. The expression of 12 (lymph)angiogenic markers was measured by qRT-PCR. Expression was correlated with tumour cell proliferation, angiogenesis and lymphangiogenesis, quantified by tumour cell proliferation fraction (TCP%) and (lymphatic) endothelial cell proliferation fraction [(L)ECP%]. TCP%, ECP% and LECP% were assessed on immunohistochemical double stains for CD34/Ki-67 and D2-40/Ki-67, respectively. In involved LNs, the relative gene expression levels of PROX1 (p < 0.001) and FGF2 (p = 0.008) were decreased and the expression levels of VEGFA (p = 0.01) and PDGFB (p = 0.002) were increased compared to uninvolved LNs. The expression of most markers was increased in PTs compared to involved LNs. In metastatically involved LNs, the expression of VEGFA correlated with ECP% (r = 0.54, p = 0.009) and LECP% (r = 0.76, p < 0.001). In PTs, VEGFA correlated only with ECP% (r = 0.74, p < 0.001). VEGFD correlated with peritumoural LECP% (r = 0.61, p = 0.001) and with VEGFC (r = 0.78, p < 0.001). Linear regression analysis confirmed the expression of VEGFA as an independent predictor of ECP% in both PTs and LN metastases and of LECP% in LN metastases. The expression of VEGFD, but not of VEGFA, independently predicted peritumoural LECP% in PTs. Our results confirm existing data that, in PTs, angiogenesis and lymphangiogenesis are respectively driven by VEGFA and VEGFD. In contrast, in LN metastases, both processes seem to be driven by VEGFA. Lymphangiogenesis in PTs and in LN metastases might thus be driven by different factors.

Blood pressure rise following angiogenesis inhibition by bevacizumab. A crucial role for microcirculation

Arterial hypertension (HT) has been reported in all studies involving bevacizumab, an antiangiogenic agent designed to target vascular endothelial growth factor (VEGF). The mechanism underlying bevacizumab-related HT is not yet clearly understood. As far as endothelial dysfunction and microvascular rarefaction are hallmarks in all forms of HT, we tested the hypothesis that anti-VEGF therapy could alter the microcirculation in nontumor tissues and, thus, result in an increase in blood pressure (BP). We used intravital video microscopy to measure dermal capillary densities in the dorsum of the fingers. Microvascular endothelial function was assessed by laser Doppler flowmetry combined with iontophoresis of pilocarpine (acetylcholine analogue). All measurements were carried out in 18 patients before and after a 6-month treatment with bevacizumab (mean cumulative dose: 3.16 +/- 0.90 g). Mean BP was increased after 6 months of therapy compared with baseline, from 129 +/- 13/75 +/- 7 mmHg to 145 +/- 17/82 +/- 7 mmHg for systolic BP and diastolic BP, respectively (P < 0.0001). Compared with the baseline, mean dermal capillary density at 6 months was significantly lower (75 +/- 12 versus 83 +/- 13/mm(2); P < 0.0001), as well as pilocarpine-induced vasodilation (P < 0.05). Thus, bevacizumab treatment resulted in endothelial dysfunction and capillary rarefaction; both changes are closely associated and could be responsible for the rise in BP observed in most patients.

Placental growth factor-1 attenuates vascular endothelial growth factor-A-dependent tumor angiogenesis during beta cell carcinogenesis

Members of the vascular endothelial growth factor (VEGF) family are critical players in angiogenesis and lymphangiogenesis. Although VEGF-A has been shown to exert fundamental functions in physiologic and pathologic angiogenesis, the exact role of the VEGF family member placental growth factor (PlGF) in tumor angiogenesis has remained controversial. To gain insight into PlGF function during tumor angiogenesis, we have generated transgenic mouse lines expressing human PlGF-1 in the beta cells of the pancreatic islets of Langerhans (Rip1PlGF-1). In single-transgenic Rip1PlGF-1 mice, intra-insular blood vessels are found highly dilated, whereas islet physiology is unaffected. Upon crossing of these mice with the Rip1Tag2 transgenic mouse model of pancreatic beta cell carcinogenesis, tumors of double-transgenic Rip1Tag2;Rip1PlGF-1 mice display reduced growth due to attenuated tumor angiogenesis. The coexpression of transgenic PlGF-1 and endogenous VEGF-A in the beta tumor cells of double-transgenic animals causes the formation of low-angiogenic hPlGF-1/mVEGF-A heterodimers at the expense of highly angiogenic mVEGF-A homodimers resulting in diminished tumor angiogenesis and reduced tumor infiltration by neutrophils, known to contribute to the angiogenic switch in Rip1Tag2 mice. The results indicate that the ratio between the expression levels of two members of the VEGF family of angiogenic factors, PlGF-1 and VEGF-A, determines the overall angiogenic activity and, thus, the extent of tumor angiogenesis and tumor growth.

The lysine 831 of vascular endothelial growth factor receptor 1 is a novel target of methylation by SMYD3

We previously identified SMYD3 as a histone methyltransferase and showed that its expression was elevated in colorectal, hepatocellular, and breast carcinomas. In the investigation of methyltransferase activity of SMYD3, we have found that vascular endothelial growth factor receptor 1 (VEGFR1) was also methylated by SMYD3. We further identified the methylated residue at VEGFR1 lysine 831, which is located in the kinase domain and is conserved among VEGFR1 orthologues. We also found that the lysine is followed by serine, which is conserved among some of the methylation targets of histone methyltransferases. Furthermore, methylation of VEGFR1 enhanced its kinase activity in cells. These data should be helpful for the profound understanding of the biological role of SMYD3 and regulatory mechanisms of VEGFR1. Additionally our finding may facilitate the development of strategies that may inhibit the progression of cancer cells.

Vascular endothelial growth factor signaling pathways: therapeutic perspective

The establishment of a vascular supply is one of the earliest and most important events occurring during embryonic development. Growth and maturation of a functional vascular network are complex and still incompletely understood processes involving orchestrated activation of vascular progenitors in the early stages of embryonic development followed by vasculogenesis and angiogenesis. These processes require a tightly regulated activation of several growth factors and their receptors. The role of vascular endothelial growth factors (VEGF) and their receptors has been studied extensively due to their prominent role during blood vessel formation. Mice deficient in various VEGF ligands or receptors show serious defects in vascular formation and maturation. Moreover, members of the VEGF family are involved in other significant biological processes, including lymphangiogenesis, vascular permeability, and hematopoiesis. Importantly, VEGF is released by tumor cells and induces tumor neovascularization. It is now well established that the VEGF axis represents an important target for antitumor therapy. Aberrant VEGF signaling is also a feature of several other pathologic conditions, such as age-related macular degeneration and rheumatoid arthritis.

Activation of the UNC5B receptor by Netrin-1 inhibits sprouting angiogenesis

Netrins are secreted molecules with roles in axonal growth and angiogenesis. The Netrin receptor UNC5B is required during embryonic development for vascular patterning, suggesting that it may also contribute to postnatal and pathological angiogenesis. Here we show that unc5b is down-regulated in quiescent adult vasculature, but re-expressed during sprouting angiogenesis in matrigel and tumor implants. Stimulation of UNC5B-expressing neovessels with an agonist (Netrin-1) inhibits sprouting angiogenesis. Genetic loss of function of unc5b reduces Netrin-1-mediated angiogenesis inhibition. Expression of UNC5B full-length receptor also triggers endothelial cell repulsion in response to Netrin-1 in vitro, whereas a truncated UNC5B lacking the intracellular signaling domain fails to induce repulsion. These data show that UNC5B activation inhibits sprouting angiogenesis, thus identifying UNC5B as a potential anti-angiogenic target.

Thrombospondins use the VLDL receptor and a nonapoptotic pathway to inhibit cell division in microvascular endothelial cells

TSPs 1 and 2 function as endogenous inhibitors of angiogenesis. Although thrombospondins (TSPs) have been shown to induce apoptosis in HMVECs, we reasoned that a homeostatic mechanism would also be needed to inhibit EC growth without causing cell death, e.g., in the maintenance of a normal vascular endothelium. HMVECs, cultured in low serum, responded to VEGF with an increase in [(3)H]thymidine incorporation that was inhibited by TSPs and was accompanied by decreases in the phosphorylation of Akt and MAPK, without an increase in apoptosis. RAP, an inhibitor of the low-density lipoprotein (LDL) family of endocytic receptors, and blocking antibodies to VLDLR were as effective as TSPs in the inhibition of thymidine uptake in response to VEGF, and the effects of these agents were not additive. Supportive evidence for the role of the VLDLR in mediating this inhibition was provided by the demonstration of a high-affinity interaction between TSPs and the VLDLR. We propose that TSP1 and TSP2, together with the VLDLR, initiate a nonapoptotic pathway for maintenance of the normal adult vascular endothelium in a quiescent state, similar to that invoked for the regulation of mitogenesis by PDGF, but involving signaling via the VLDLR rather than LRP1.

Underexpression of mineralocorticoid receptor in colorectal carcinomas and association with VEGFR-2 overexpression

BACKGROUND

The human mineralocorticoid receptor (MR) is a steroid receptor widely expressed in colorectal mucosa. A significant role for the MR in the reduction of vascular endothelial growth factor receptor-2 (VEGFR-2) mRNA levels has been demonstrated in vitro. To evaluate a potential contribution of MR to colorectal carcinoma progression, we analyzed the expression of MR in relation to VEGFR-2.

METHODS

Fresh human colorectal cancer tissue and adjacent normal mucosa were harvested from 48 consecutive patients. MR and VEGFR-2 mRNA expression levels were determined by real-time reverse transcriptase-polymerase chain reaction and correlated with clinicopathological parameters.

RESULTS

A decline of MR expression was observed in all carcinomas compared to normal mucosa. Expression of MR was a median of 11-fold lower in carcinoma compared to the normal mucosa, irrespective of the location, size, stage, and differentiation. MR was a median of 20-fold underexpressed in carcinomas with VEGFR-2 overexpression vs only 9-fold in carcinomas with VEGFR-2 underexpression (p = 0.035, Mann-Whitney test).

CONCLUSIONS

These findings support the hypothesis that reduction of MR expression may be one of the early events involved in colorectal carcinoma progression. The inverse association between MR and VEGFR-2 expression in carcinoma suggests a potential tumor-suppressive function for MR.

Increased expression of tissue vascular endothelial growth factor and foetal liver kinase-1 receptor in seasonal allergic rhinitis and relevance to asthma component

BACKGROUND

There is a difference in the extent of remodelling in allergic rhinitis (AR) and asthma. This may be attributed to the difference in local tissue response to these mediators.

OBJECTIVES

The aim of this study was to compare vascular endothelial growth factor (VEGF) and its receptor foetal liver kinase (Flk)-1 expression between seasonal AR patients with or without asthma and non-allergic controls as well as that between AR patients with and without asthma.

METHODS

Thirteen subjects with seasonal AR and six non-allergic controls were included in the study. Allergic sensitization was demonstrated by a skin prick test. Inferior turbinate thiny biopsies were obtained from both groups. Monoclonal mouse antibodies were used to demonstrate VEGF and Flk-1. Nasal mucosal endothelial cells' staining intensity was graded semi-quantitatively and the histochemical score (HSCORE) was calculated. In all samples, VEGF- and Flk-1-labelled vessels were counted for the assessment of vascular surface density (VSD).

RESULTS

The mean HSCORE for VEGF and anti-VEGF-based VSD were significantly higher in the patient group (P=0.001 and 0.002, respectively). The mean HSCORE for Flk-1 and anti-Flk-1-based VSD in the patient group were significantly higher than those in the control group (P=0.016 and 0.028, respectively). Differences between the mean HSCORE for VEGF and anti-VEGF-based VSD in patients with pure AR and AR and asthma were insignificant (P=0.16 and 0.39, respectively). The mean HSCORE for Flk-1 and anti-Flk-1-based VSD in patients with pure AR were significantly lower than those in patients with AR and asthma (P=0.004 and 0.018, respectively).

CONCLUSION

Angiogenic factor VEGF and its receptor Flk-1 is increased in AR. A similar increase in VEGF in AR with and without asthma despite a higher Flk-1 in AR patients with asthma may be a possible explanation for the presence of angiogenesis in the airway wall in patients with asthma but not in those with pure AR.

Impaired Angiogenesis After Hindlimb Ischemia in Type 2 Diabetes Mellitus

Deficient angiogenesis after ischemia may contribute to worse outcomes of peripheral arterial disease in patients with diabetes mellitus (DM). Vascular endothelial growth factor (VEGF) and its receptors promote angiogenesis. We hypothesized that in peripheral arterial disease, maladaptive changes in VEGF ligand/receptor expression could account for impaired angiogenesis in DM. Skeletal muscle from diet-induced, type 2 diabetic (DM) and age-matched normal chow (NC)-fed mice was collected at baseline and 3 and 10 days after hindlimb ischemia and analyzed for expression of VEGF (n=10 per group), full-length VEGF receptor (VEGFR)-1, soluble VEGFR-1, and markers of downstream VEGF signaling (n=20 per group) using ELISA, reverse transcriptase-polymerase chain reaction, and Western blots. In the absence of ischemia, DM mice had increased VEGF (NC versus DM: 26.6+/-2.6 versus 53.5+/-8.8 pg/mg protein; P<0.05), decreased soluble and membrane-bound VEGFR-1 (NC versus DM: 1.44+/-0.30 versus 0.85+/-0.08 and 1.03+/-0.10 versus 0.72+/-0.10, respectively; P<0.05), decreased phospho-AKT/AKT and phospho-endothelial NO synthase/endothelial NO synthase (NC versus DM: 0.76+/-0.2 versus 0.38+/-0.1 and 0.36+/-0.06 versus 0.25+/-0.04, respectively; P<0.05), and no change in VEGFR-2. After ischemia, both DM and NC had comparable increases in VEGF-A. VEGFR-1 and soluble VEGFR-1 expression increased in both groups, but the fold increase was significantly greater in DM. These data demonstrate that soluble VEGFR-1, an angiogenesis inhibitor, is regulated in skeletal muscle by type 2 DM and ischemia. In the absence of ischemia, despite reductions in both soluble VEGFR-1 and VEGFR-1, VEGF ligand signaling is lower in DM compared with controls. After ischemia, maladaptive upregulation of these receptors further reduces the capacity of VEGF to induce an angiogenic response, which may provide a novel target for therapy.

Angiomotin regulates endothelial cell migration during embryonic angiogenesis

The development of the embryonic vascular system into a highly ordered network requires precise control over the migration and branching of endothelial cells (ECs). We have previously identified angiomotin (Amot) as a receptor for the angiogenesis inhibitor angiostatin. Furthermore, DNA vaccination targeting Amot inhibits angiogenesis and tumor growth. However, little is known regarding the role of Amot in physiological angiogenesis. We therefore investigated the role of Amot in embryonic neovascularization during zebrafish and mouse embryogenesis. Here we report that knockdown of Amot in zebrafish reduced the number of filopodia of endothelial tip cells and severely impaired the migration of intersegmental vessels. We further show that 75% of Amot knockout mice die between embryonic day 11 (E11) and E11.5 and exhibit severe vascular insufficiency in the intersomitic region as well as dilated vessels in the brain. Furthermore, using ECs differentiated from embryonic stem (ES) cells, we demonstrate that Amot-deficient cells have intact response to vascular endothelial growth factor (VEGF) in regard to differentiation and proliferation. However, the chemotactic response to VEGF was abolished in Amot-deficient cells. We provide evidence that Amot is important for endothelial polarization during migration and that Amot controls Rac1 activity in endothelial and epithelial cells. Our data demonstrate a critical role for Amot during vascular patterning and endothelial polarization.

The VEGF family, the inside story

The role of paracrine signaling by vascular endothelial growth factor (VEGF) in the formation and maintenance of blood vessels has been studied extensively. In this issue, Lee et al. (2007) report unexpected results showing that endogenous VEGF produced by endothelial cells is also crucial for vascular homeostasis.

Acute pharmacodynamic and antivascular effects of the vascular endothelial growth factor signaling inhibitor AZD2171 in Calu-6 human lung tumor xenografts

The vascular endothelial growth factor-A (VEGF-A) signaling pathway, a key stimulant of solid tumor vascularization, is primarily dependent on the activation of the endothelial cell surface receptor VEGF receptor-2 (VEGFR-2). AZD2171 is an oral, highly potent small-molecule inhibitor of VEGFR tyrosine kinase activity that inhibits angiogenesis and the growth of human tumor xenografts in vivo. Here, we show pharmacodynamic changes in VEGFR-2 phosphorylation induced by AZD2171. In mouse lung tissue, a single dose of AZD2171 at 6 mg/kg inhibited VEGF-A-stimulated VEGFR-2 phosphorylation by 87% at 2 h with significant inhibition (>or=60%) maintained to 24 h. To examine inhibition of VEGFR-2 phosphorylation in tumor vasculature by immunohistochemistry, a comprehensive assessment of antibodies to various phosphorylation sites on the receptor was undertaken. Antibodies to the phosphotyrosine epitopes pY1175/1173 and pY1214/1212 were found suitable for this application. Calu-6 human lung tumor xenografts, from mice receiving AZD2171 or vehicle treatment (p.o., once daily), were examined by immunohistochemistry. A significant reduction in tumor vessel staining of phosphorylated VEGFR-2 (pVEGFR-2) was evident within 28 h of AZD2171 treatment (6 mg/kg). This effect preceded a significant reduction in tumor microvessel density, which was detectable following 52 h of AZD2171 treatment. These data show that AZD2171 is a potent inhibitor of VEGFR-2 activation in vivo and suggest that AZD2171 delivers therapeutic benefit in Calu-6 tumors by targeting vessels dependent on VEGFR-2 signaling for survival. In addition, this work highlights the utility of measuring either pY1175/1173 or pY1214/1212 on VEGFR-2 as a pharmacodynamic marker of VEGFR-2 activation.

Rational Design, Structure, and Biological Evaluation of Cyclic Peptides Mimicking the Vascular Endothelial Growth Factor

Angiogenesis is the development of a novel vascular network from a pre-existing structure. Blocking angiogenesis is an attractive strategy to inhibit tumor growth and metastasis formation. Based on structural and mutagenesis data, we have developed novel cyclic peptides that mimic, simultaneously, two regions of the VEGF crucial for the interaction with the VEGF receptors. The peptides, displaying the best affinity for VEGF receptor 1 on a competition assay, inhibited endothelial cell transduction pathway, migration, and capillary-like tubes formation. The specificity of these peptides for VEGF receptors was demonstrated by microscopy using a fluorescent peptide derivative. The resolution of the structure of some cyclic peptides by NMR and molecular modeling has allowed the identification of various factors accounting for their inhibitory activity. Taken together, these results validate the selection of these two regions as targets to develop molecules able to disturb the development of cancer and angiogenesis-associated diseases.

Platelet-Derived Growth Factor Receptor-β Constitutive Activity Promotes Angiogenesis In Vivo and In Vitro

OBJECTIVE

Knockout studies have demonstrated crucial roles for the platelet-derived growth factor-B and its cognate receptor, platelet-derived growth factor receptor-beta (PDGFR-beta), in blood vessel maturation, that is, the coverage of newly formed vessels with mural cells/pericytes. This study describes the consequences of a constitutively activating mutation of the PDGFR-beta (Pdgfrb(D849V)) introduced into embryonic stem cells with respect to vasculogenesis/angiogenesis in vitro and in vivo.

METHODS AND RESULTS

Embryonic stem cells were induced to either form teratomas in vivo or embryoid bodies, an in vitro model for mouse embryogenesis. Western blotting studies on embryoid bodies showed that expression of a single allele of the mutant Pdgfrb led to increased levels of PDGFR-beta tyrosine phosphorylation and augmented downstream signal transduction. This was accompanied by enhanced vascular development, followed by exaggerated angiogenic sprouting with abundant pericyte coating as shown by immunohistochemistry/immunofluorescence. Pdgfrb(D849V/+) embryoid bodies were characterized by increased expression of vascular endothelial growth factor (VEGF)-A and VEGF receptor-2; neutralizing antibodies against VEGF-A/VEGF receptor-2 blocked vasculogenesis and angiogenesis in mutant embryoid bodies. Moreover, Pdgfrb(D849V/+) embryonic stem cell-derived teratomas in nude mice were more densely vascularized than wild-type teratomas.

CONCLUSIONS

Increased PDGFR-beta kinase activity is associated with elevated expression of VEGF-A and VEGF receptor-2, acting directly on endothelial cells and resulting in increased vessel formation.

NADPH oxidase activity selectively modulates vascular endothelial growth factor signaling pathways

Vascular endothelial growth factor (VEGF) and reactive oxygen species (ROS) play critical roles in vascular physiology and pathophysiology. We have demonstrated previously that NADPH oxidase-derived ROS are required for VEGF-mediated migration and proliferation of endothelial cells. The goal of this study was to determine the extent to which VEGF signaling is coupled to NADPH oxidase activity. Human umbilical vein endothelial cells and/or human coronary artery endothelial cells were transfected with short interfering RNA against the p47(phox) subunit of NADPH oxidase, treated in the absence or presence of VEGF, and assayed for signaling, gene expression, and function. We show that NADPH oxidase activity is required for VEGF activation of phosphoinositide 3-kinase-Akt-forkhead, and p38 MAPK, but not ERK1/2 or JNK. The permissive role of NADPH oxidase on phosphoinositide 3-kinase-Akt-forkhead signaling is mediated at post-VEGF receptor levels and involves the nonreceptor tyrosine kinase Src. DNA microarrays revealed the existence of two distinct classes of VEGF-responsive genes, one that is ROS-dependent and another that is independent of ROS levels. VEGF-induced, thrombomodulin-dependent activation of protein C was dependent on NADPH oxidase activity, whereas VEGF-induced decay-accelerating factor-mediated protection of endothelial cells against complement-mediated lysis was not. Taken together, these findings suggest that NADPH oxidase-derived ROS selectively modulate some but not all the effects of VEGF on endothelial cell phenotypes.

Regulation of angiogenesis and vascular permeability by Src family kinases: opportunities for therapeutic treatment of solid tumors

Aberrant expression or activation of protein tyrosine kinases, including Src and related Src family kinases, is a common occurrence in many human cancers, resulting in deregulation of expression of numerous mediators of cellular functions, including pro-angiogenic molecules. In addition, Src activation regulates vascular permeability of endothelial cells. How these processes contribute to tumor progression and metastasis are the subjects of this review. As Src-selective inhibitors have entered clinical trials for a number of solid tumors, further understanding of the roles of Src kinases in mediating tumor angiogenesis as well as modulating tumor/microenvironment interactions will provide insights into the best use of these inhibitors in treating patients afflicted with tumors in which Src is activated.

Hypoxia: a key regulator of angiogenesis in cancer

Angiogenesis is an important mediator of tumor progression. As tumors expand, diffusion distances from the existing vascular supply increases resulting in hypoxia. Sustained expansion of a tumor mass requires new blood vessel formation to provide rapidly proliferating tumor cells with an adequate supply of oxygen and metabolites. The key regulator of hypoxia-induced angiogenesis is the transcription factor hypoxia inducible factor (HIF)-1. Multiple HIF-1 target genes have been shown to modulate angiogenesis by promoting the mitogenic and migratory activities of endothelial cells. Because of this, hypoxia-induced angiogenesis has become an attractive target for cancer therapy, however the mechanisms involved during this process and how best to target it for cancer therapy are still under investigation. This review will cover the current understanding of hypoxia-induced tumor angiogenesis and discuss the caveats of hypoxia-targeted antiangiogenic therapy for the treatment of cancer.

The influence of physical training on the angiopoietin and VEGF-A systems in human skeletal muscle

Eleven subjects performed one-legged exercise four times per week for 5 wk. The subjects exercised one leg for 45 min with restricted blood flow (R leg), followed by exercise with the other leg at the same absolute workload with unrestricted blood flow (UR leg). mRNA and protein expression were measured in biopsies from the vastus lateralis muscle obtained at rest before the training period, after 10 days, and after 5 wk of training, as well as 120 min after the first and last exercise bouts. Basal Ang-2 and Tie-1 mRNA levels increased in both legs with training. The Ang-2-to-Ang-1 ratio increased to a greater extent in the R leg. The changes in Ang-2 mRNA were followed by similar changes at the protein level. In the R leg, VEGF-A mRNA expression responded transiently after acute exercise both before and after the 5-wk training program. Over the course of the exercise program, there was a concurrent increase in basal VEGF-A protein and VEGFR-2 mRNA in the R leg. Ki-67 mRNA showed a greater increase in the R leg and the protein was localized to the endothelial cells. In summary, the increased translation of VEGF-A is suggested to be caused by the short mRNA burst induced by each exercise bout. The concurrent increase in the Ang-2-to-Ang-1 ratio and the VEGF-expression combined with the higher level of Ki-67 mRNA in the R leg indicate that changes in these systems are of importance also in nonpathological angiogenic condition such as voluntary exercise in humans. It further establish that hypoxia/ischemia-related metabolic perturbation is likely to be involved as stimuli in this process in human skeletal muscle.

Targeting endothelial and tumor cells with semaphorins

Neuropilins (NRP) are receptors for the class 3 semaphorin (SEMA3) family of axon guidance molecules and the vascular endothelial growth factor (VEGF) family of angiogenesis factors. Although the seminal studies on SEMA3s and NRPs first showed them to be mediators of axon guidance, it has become very apparent that these proteins play an important role in vascular and tumor biology as well. Neuronal guidance and angiogenesis are regulated similarly at the molecular level. For example, SEMA3s not only repel neurons and collapse axon growth cones, but have similar effects on endothelial cells and tumor cells. Preclinical studies indicate that SEMA3F is a potent inhibitor of tumor angiogenesis and metastasis. In addition, neutralizing antibodies to NRP1 enhance the effects of anti-VEGF antibodies in suppressing tumor growth in xenograft models. This article reviews NRP and SEMA3 structural interactions and their role in developmental angiogenesis, tumor angiogenesis and metastasis based on cell culture, zebrafish and murine studies.

Regulated proteolytic processing of Tie1 modulates ligand responsiveness of the receptor-tyrosine kinase Tie2

Regulated ectodomain shedding followed by intramembrane proteolysis has recently been recognized as important in cell signaling and for degradation of several type I transmembrane proteins. The receptor-tyrosine kinase Tie1 is known to undergo ectodomain cleavage generating a membrane-tethered endodomain. Here we show Tie1 is a substrate for regulated intramembrane proteolysis. After Tie1 ectodomain cleavage the newly formed 45-kDa endodomain undergoes additional proteolytic processing mediated by gamma-secretase to generate an amino-terminal-truncated 42-kDa fragment that is subsequently degraded by proteasomal activity. This sequential processing occurs constitutively and is stimulated by phorbol ester and vascular endothelial growth factor. To assess the biological significance of regulated Tie1 processing, we analyzed its effects on angiopoietin signaling. Activation of ectodomain cleavage causes loss of phosphorylated Tie1 holoreceptor and generation of phosphorylated receptor fragments in the presence of cartilage oligomeric protein angiopoietin 1. A key function of gamma-secretase is in preventing accumulation of these phosphorylated fragments. We also find that regulated Tie1 processing modulates ligand responsiveness of the Tie-1-associated receptor Tie2. Activation of Tie1 ectodomain cleavage increases cartilage oligomeric protein angiopoietin 1 activation of Tie2. This correlates with increased ability of Tie2 to bind ligand after shedding of the Tie1 extracellular domain. A similar enhancement of ligand activation of Tie2 is seen when Tie1 expression is suppressed by RNA interference. Together these data indicate that Tie1, via its extracellular domain, limits the ability of ligand to bind and activate Tie2. Furthermore the data suggest that regulated processing of Tie1 may be an important mechanism for controlling signaling by Tie2.

Antibody GD3G7 selected against embryonic glycosaminoglycans defines chondroitin sulfate-E domains highly up-regulated in ovarian cancer and involved in vascular endothelial growth factor binding

Chondroitin sulfate (CS) is abundantly present in the tumor stroma, and tumor-specific CS modifications might be potential targets to influence tumor development. We applied the phage display technology to select antibodies that identify these tumor-specific CS modifications. Antibody GD3G7 was selected against embryonic glycosaminoglycans, and it reacted strongly with CS-E (rich in GlcA-GalNAc4S6S units). In ovarian adenocarcinomas, strong expression of this CS-E epitope was found in the extracellular matrix, and occasionally on tumor cells. No expression was found in normal ovary and cystadenomas. Differential expression was found in ovarian carcinoma cell lines, which correlated with the gene expression of the GalNAc4S-6st enzyme, involved in biosynthesis of CS-E. Vascular endothelial growth factor (VEGF)-sensitive fenestrated (in normal tissues) and tumor blood vessels were both identified by antibody GD3G7, which might implicate a role for CS-E in VEGF biology. VEGF bound to CS-E and antibody GD3G7 could compete for binding of VEGF to CS-E. In conclusion, antibody GD3G7 identified rare CS-E-like structures that were strongly expressed in ovarian adenocarcinomas. This antibody might therefore be instrumental for identifying tumor-related CS alterations.

Poly(ADP-ribose) polymerase (PARP) inhibition or PARP-1 gene deletion reduces angiogenesis

Poly(ADP-ribose) polymerase (PARP)-1 has recently been shown to promote tumour progression. Since angiogenesis is an essential requirement for tumour growth, we examined whether PARP inhibition/deletion might affect endothelial cell functions. To this end, the influence of PARP inhibitors on endothelial cell proliferation, migration, tube formation and angiogenesis in PARP-1 knock-out mice, using an in vivo matrigel plug assay, was investigated. The results indicated that the PARP inhibitor GPI 15427 (IC50 on endothelial PARP: 237 +/- 27 nM), at concentrations devoid of cytotoxic effects (0.5-1 microM), abrogated migration in response to vascular endothelial growth factor or placenta growth factor, hampered formation of tubule-like networks and impaired angiogenesis in vivo. The anti-angiogenic effect of the PARP inhibitor was confirmed in PARP-1 knock-out mice that displayed a defect of angiogenesis induced by growth factors. These results provide evidence for targeting PARP for anti-angiogenesis, adding novel therapeutic implications to the use of PARP inhibitors in cancer treatment.

Lentiviral rescue of vascular endothelial growth factor receptor-2 expression in flk1-/- embryonic stem cells shows early priming of endothelial precursors

The vascular endothelial growth factor (VEGF) family and its receptors are important for vascular development and maintenance of blood vessels, as well as for angiogenesis, the formation of new vessels. Loss of VEGF receptor-2 (VEGFR-2; designated Flk-1 in mouse) results in arrest of vascular and hematopoietic development in vivo. We used lentiviral transduction to reconstitute VEGFR-2 expression in flk1-/- embryonic stem (ES) cells. VEGF-induced vasculogenesis and sprouting angiogenesis were rescued in transduced ES cultures differentiating in vitro as EBs. Although the transgene was expressed in the pluripotent stem cells and lacked linage restriction during differentiation, the extent of endothelial recruitment was similar to that in wild-type EBs. Reconstitution of VEGFR-2 in flk1-/- ES cells allowed only precommitted precursors to differentiate into functional endothelial cells able to organize into vascular structures. Chimeric EB cultures composed of wild-type ES cells mixed with flk1-/- ES cells or reconstituted VEGFR-2-expressing ES cells were created. In the chimeric cultures, flk1-/- endothelial precursors were excluded from wild-type vessel structures, whereas reconstituted VEGFR-2-expressing precursors became integrated together with wild-type endothelial cells to form chimeric vessels. We conclude that maturation of endothelial precursors, as well as organization into vascular structures, requires expression of VEGFR-2. Disclosure of potential conflicts of interest is found at the end of this article.

AKT/PKB signaling: navigating downstream

The serine/threonine kinase Akt, also known as protein kinase B (PKB), is a central node in cell signaling downstream of growth factors, cytokines, and other cellular stimuli. Aberrant loss or gain of Akt activation underlies the pathophysiological properties of a variety of complex diseases, including type-2 diabetes and cancer. Here, we review the molecular properties of Akt and the approaches used to characterize its true cellular targets. In addition, we discuss those Akt substrates that are most likely to contribute to the diverse cellular roles of Akt, which include cell survival, growth, proliferation, angiogenesis, metabolism, and migration.

The Tyrosine Kinase Inhibitor, AZD2171, Inhibits Vascular Endothelial Growth Factor Receptor Signaling and Growth of Anaplastic Thyroid Cancer in an Orthotopic Nude Mouse Model

PURPOSE

Anaplastic thyroid cancer (ATC) is a locally aggressive type of thyroid tumor with high rate of distant metastases. With conventional treatment, the median survival ranges from 4 to 12 months; therefore, new treatment options are needed. AZD2171 is a tyrosine kinase inhibitor of the vascular endothelial growth factor receptors (VEGFR) VEGFR-1, VEGFR-2, and VEGFR-3. The objective of the study is to determine whether AZD2171 can inhibit VEGFR-2 signaling and decrease tumor growth and prolong survival of ATC in an orthotopic nude mouse model.

EXPERIMENTAL DESIGN

We examined the effects of AZD2171 on phosphorylation of VEGFR-2, mitogen-activated protein kinase, and AKT in human umbilical vascular endothelial cells. To determine the antiproliferative and antiapoptotic effects of AZD2171, we did 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide and flow cytometry assays, respectively. We assessed the antitumor effects of AZD2171 in a xenograft model of ATC using control, AZD2171, paclitaxel, and combination groups by measuring tumor size and survival.

RESULTS

Treatment with AZD2171 led to dose-dependent inhibition of VEGFR-2 phosphorylation and its downstream signaling in human umbilical vascular endothelial cells (IC(50) for cell proliferation, 500 nmol/L). In the ATC cell lines DRO and ARO, IC(50) was 7.5 micromol/L. AZD2171 induced apoptosis in 50% of endothelial and ATC cells at 3 and 10 micromol/L concentrations, respectively. In vivo, AZD2171 led to a significant reduction in tumor size between control and AZD2171 (P = 0.002) or AZD2171 + paclitaxel group (P = 0.002) but not the paclitaxel alone group (P = 0.11). Survival was significantly higher among AZD2171 (P < 0.001) and combination groups (P < 0.001) compared with control.

CONCLUSIONS

AZD2171 effectively inhibits tumor growth and prolongs survival of ATC-bearing mice. The main effect of AZD2171 is mediated through angiogenesis inhibition.

The VEGFR2 and PKA pathways converge at MEK/ERK1/2 to promote survival in serum deprived neuronal cells

Identifying prosurvival mechanisms in stressed neuronal cells would provide protective strategies to hinder neurodegeneration. Recent evidence shows that vascular endothelial growth factor (VEGF), a well-established mitogen in endothelial cells, can mediate neuroprotection against damaging insults through the activation of its cognate receptor VEGFR2. In addition, growth factor receptor signaling pathways have been shown to crosstalk with cAMP-dependent Protein Kinase A (PKA) to protect neuronal cells from harmful stimuli. Whether a relationship exists between VEGFR2 and PKA in mediating neuroprotection under stressful conditions is unknown. Using SK-N-SH neuronal cells as a model system, we show that serum deprivation induces an upregulation in VEGF and VEGFR2 that concomitantly serves as a prosurvival signaling pathway. Inhibitor studies revealed that PKA functioned concurrently with VEGFR2 pathway to signal the activation of the extracellular signal-regulated protein kinases (ERK1/2) as protection against caspase-3/7 activation and a subsequent cell death. The loss in cell viability induced by VEGFR2 and PKA inhibition was prevented by caspase inhibition or overexpression of ERK1. Overexpression of the antiapoptotic protein Bcl-xL also promoted survival when VEGFR2 function was blocked. However, the protection elicited by all three treatments were prevented by the inclusion of a selective inhibitor of mitogen-activated protein kinase kinase (MEK), the upstream kinase that activates ERK1/2. Taken together, these findings suggested that PKA and VEGFR2 converge at the MEK/ERK1/2 pathway to protect serum starved neuronal cells from a caspase-dependent cell death.