Getting Tie(2)d up in angiogenesis

Inflammation-Mediated Angiogenesis in Ischemic Stroke

Stroke is the leading cause of disability and mortality in the world, but the pathogenesis of ischemic stroke (IS) is not completely clear and treatments are limited. Mounting evidence indicate that neovascularization is a critical defensive reaction to hypoxia that modulates the process of long-term neurologic recovery after IS. Angiogenesis is a complex process in which the original endothelial cells in blood vessels are differentiated, proliferated, migrated, and finally remolded into new blood vessels. Many immune cells and cytokines, as well as growth factors, are directly or indirectly involved in the regulation of angiogenesis. Inflammatory cells can affect endothelial cell proliferation, migration, and activation by secreting a variety of cytokines via various inflammation-relative signaling pathways and thus participate in the process of angiogenesis. However, the mechanism of inflammation-mediated angiogenesis has not been fully elucidated. Hence, this review aimed to discuss the mechanism of inflammation-mediated angiogenesis in IS and to provide new ideas for clinical treatment of IS.

Development and validation of the prognostic value of the immune-related genes in clear cell renal cell carcinoma

BACKGROUND

Clear cell renal cell carcinoma (ccRCC) is a highly heterogeneous tumor, resulting a challenge of developing target therapeutics. Not long ago, immune checkpoint blockade regimens combine with tyrosin kinase inhibitors have evolved frontline options in metastatic RCC, which implies arrival of the era of tumor immunotherapy. Studies have demonstrated immune-related genes (IRGs) could characterize tumor milieu and related to patient survival. Nevertheless, the clinical significance of classifier depending on IRGs in ccRCC has not been well established.

METHODS

The R package limma, univariate and LASSO cox regression analysis were used to screen the prognostic related IRGs from TCGA database. Multivariate cox regression was utilized to establish a risk prediction model for candidate genes. Quantitative real-time PCR was used to confirm the expression of candidates in clinical samples from our institution. CIBERSORT algorithm and correlation analysis were applied to explore tumor-infiltrating immune cells signature between different risk groups. A clinical nomogram was also developed to predict OS by using the rms R package based on the risk prediction model and other independent risk factors. The ICGC data was used for external validation of either gene risk model or nomogram.

RESULTS

We identified 382 differentially expressed immune related genes. Four unique prognostic IRGs (CRABP2, LTB4R, PTGER1 and TEK) were finally affirmed to associate with tumor survival independently and utilized to establish the risk score model. All candidates' expression was successfully laboratory confirmed by q-PCR. CIBERSORT analysis implied patients in unfavorable-risk group with high CD8 T cell, regulatory T cell and NK cell infiltration, as well as high expression of PD-1, CTLA4, TNFRSF9, TIGIT and LAG3. A nomogram combined IRGs risk score with age, gender, TNM stage, Fuhrman grade, necrosis was further generated to predict of 3- and 5-year OS, which exhibited superior discriminative power (AUCs were 0.811 and 0.795).

CONCLUSIONS

Our study established and validated a survival prognostic model system based on 4 unique immune related genes in ccRCC, which expands knowledge in tumor immune status and provide a potent prediction tool in future.

Pathologic angiogenesis in the bone marrow of humanized sickle cell mice is reversed by blood transfusion

Sickle cell disease (SCD) is a monogenic red blood cell (RBC) disorder with high morbidity and mortality. Here, we report, for the first time, the impact of SCD on the bone marrow (BM) vascular niche, which is critical for hematopoiesis. In SCD mice, we find a disorganized and structurally abnormal BM vascular network of increased numbers of highly tortuous arterioles occupying the majority of the BM cavity, as well as fragmented sinusoidal vessels filled with aggregates of erythroid and myeloid cells. By in vivo imaging, sickle and control RBCs have significantly slow intravascular flow speeds in sickle cell BM but not in control BM. In sickle cell BM, we find increased reactive oxygen species production in expanded erythroblast populations and elevated levels of HIF-1α. The SCD BM exudate exhibits increased levels of proangiogenic growth factors and soluble vascular cell adhesion molecule-1. Transplantation of SCD mouse BM cells into wild-type mice recapitulates the SCD vascular phenotype. Our data provide a model of SCD BM, in which slow RBC flow and vaso-occlusions further diminish local oxygen availability in the physiologic hypoxic BM cavity. These events trigger a milieu that is conducive to aberrant vessel growth. The distorted neovascular network is completely reversed by a 6-week blood transfusion regimen targeting hemoglobin S to <30%, highlighting the plasticity of the vascular niche. A better insight into the BM microenvironments in SCD might provide opportunities to optimize approaches toward efficient and long-term hematopoietic engraftment in the context of curative therapies.

Role of the Ang2-Tie2 Axis in Vascular Damage Driven by High Glucose or Nucleoside Diphosphate Kinase B Deficiency

Ablation of nucleoside diphosphate kinase B (NDPK-B) in mice causes a breakdown of the neurovascular unit in the retina, mimicking diabetic retinopathy. The NDPK-B deficiency-induced vascular damage is mediated by excessive angiopoietin 2 (Ang2). Herein, the potential involvement of its receptor, Tie2, was investigated. NDPK-B-deficient mouse retinas showed an upregulation of Tie2, specifically in the deep capillary layer. A similar upregulation of Tie2 was observed in cultured endothelial cells (ECs) from different origins upon NDPK-B depletion, whereas high glucose (HG) treatment did not alter Tie2 expression. Immunofluorescence staining and subcellular fractionation showed that the majority of Tie2 upregulation occurred at the plasma membrane. Similar to HG, however, NDPK-B depletion reduced Tie2 tyrosine phosphorylation. Compared to HG, a stronger increase of Ang2 was observed in NDPK-B depleted ECs. Treatment of ECs with soluble Tie2 or siRNA-mediated Tie2 knockdown attenuated NDPK-B depletion- but not HG-induced Ang2 upregulation. Like NDPK-B depletion, overexpression of recombinant Ang2 in ECs enhanced Ang2 secretion and concomitantly promoted the upregulation of Tie2. Thus, we identified a new mechanism showing that after reaching a threshold level of secretion, Ang2 sustains its own expression and secretion by a Tie2-dependent positive feedback loop.

Evidence for critical role of Tie2/Ang1 interaction in metastatic oral cancer

Angiopoietin-1 (Ang1) is a binding partner of endothelial cell-specific tyrosine-protein kinase receptor (Tie2), which serves important roles in vascular development and angiogenesis. Tie2 is closely associated with the metastasis of oral squamous cell carcinomas (OSCCs) however, little is known about the correlation between Tie2 and Ang1. In the present study, the functional mechanisms of the Tie2/Ang1 interaction were investigated using Tie2 overexpressed (oeTie2) OSCC cells and recombinant Ang1 protein. oeTie2 cells had increased cell-cell and cell-extracellular matrix adhesions compared with the control cells. Additionally, the adhesive activities increased following treatment with exogenous Ang1, indicating that Ang1 directly enhances Tie2 functions. In the clinical OSCC data from 10 cases positive for regional lymph node metastasis, all cases were negative for Tie2 expression and eight cases (80%) were negative for Ang1 expression. These results suggest that Tie2 and Ang1 serve important roles in cancer metastasis and may be potential biomarkers and therapeutic targets for OSCC metastasis.

Regulation of retinal angiogenesis by phospholipase C-β3 signaling pathway

Angiogenesis has an essential role in many pathophysiologies. Here, we show that phospholipase C-β3 (PLC-β3) isoform regulates endothelial cell function and retinal angiogenesis. Silencing of PLC-β3 in human umbilical vein endothelial cells (HUVECs) significantly delayed proliferation, migration and capillary-like tube formation. In addition, mice lacking PLC-β3 showed impaired retinal angiogenesis with delayed endothelial proliferation, reduced endothelial cell activation, abnormal vessel formation and hemorrhage. Finally, tumor formation was significantly reduced in mice lacking PLC-β3 and showed irregular size and shape of blood vessels. These results suggest that regulation of endothelial function by PLC-β3 may contribute to angiogenesis.

Discovery of Novel Antiangiogenic Marine Natural Product Scaffolds

Marine natural products (MNPs) are recognized for their structural complexity, diversity, and novelty. The vast majority of MNPs are pharmacologically relevant through their ability to modulate macromolecular targets underlying human diseases. Angiogenesis is a fundamental process in cancer progression and metastasis. Targeting angiogenesis through selective modulation of linked protein kinases is a valid strategy to discover novel effective tumor growth and metastasis inhibitors. An in-house marine natural products mini-library, which comprises diverse MNP entities, was submitted to the Lilly’s Open Innovation Drug Discovery platform. Accepted structures were subjected to in vitro screening to discover mechanistically novel angiogenesis inhibitors. Active hits were subjected to additional angiogenesis-targeted kinase profiling. Some natural and semisynthetic MNPs, including multiple members of the macrolide latrunculins, the macrocyclic oxaquinolizidine alkaloid araguspongine C, and the sesquiterpene quinone puupehenone, showed promising results in primary and secondary angiogenesis screening modules. These hits inhibited vascular endothelial growth factor (VEGF)-mediated endothelial tube-like formation, with minimal cytotoxicity at relevant doses. Secondary kinase profiling identified six target protein kinases, all involved in angiogenesis signaling pathways. Molecular modeling and docking experiments aided the understanding of molecular binding interactions, identification of pharmacophoric epitopes, and deriving structure-activity relationships of active hits. Marine natural products are prolific resources for the discovery of chemically and mechanistically unique selective antiangiogenic scaffolds.

Granulocyte colony-stimulating factor attenuates delayed tPA-induced hemorrhagic transformation in ischemic stroke rats by enhancing angiogenesis and vasculogenesis

Treatment with tissue plasminogen activator (tPA) beyond the therapeutic time window (>4.5 hours post stroke) may produce hemorrhagic transformation (HT). Strategies that could extend the narrow time window of tPA will benefit a significant number of stroke patients. Male Sprague-Dawley rats underwent middle cerebral artery occlusion (MCAo) and given vehicle, tPA (10 mg/kg), or tPA and granulocyte colony-stimulating factor (G-CSF, 300 μg/kg), at 6 hours after MCAo. Twenty-four hours post treatment, G-CSF+tPA-treated stroke rats displayed 25% improvement in neurological functions and 38.9% reduction of hemorrhage, with Western blots showing 1.9- and 1.2-fold increments in Ang-2 expression in the ischemic cortex and striatum, respectively, and 3-fold increase in phosphorylated endothelial nitric oxide synthase expression in the ipsilateral cortex relative to tPA-treated rats. Immunohistochemistry also showed 2- and 2.8-fold increase in von-Willebrand expression, 3.2- and 2.2-fold increased CD34+ expression, and 4- and 13-fold upregulation of VEGFR-2 expression in the ischemic cortex and striatum, respectively, in G-CSF+tPA-treated stroke rats relative to tPA-treated subjects. Altogether, these findings indicate that G-CSF attenuated delayed tPA-induced HT likely via the enhancement of angiogenesis and vasculogenesis. The use of G-CSF to protect the vasculature may improve the clinical outcome of tPA even outside the currently indicated therapeutic window for ischemic stroke.

Activation of the endothelin system mediates pathological angiogenesis during ischemic retinopathy

Retinopathy of prematurity adversely affects premature infants because of oxygen-induced damage of the immature retinal vasculature, resulting in pathological neovascularization (NV). Our pilot studies using the mouse model of oxygen-induced retinopathy (OIR) showed marked increases in angiogenic mediators, including endothelins and endothelin receptor (EDNR) A. We hypothesized that activation of the endothelin system via EDNRA plays a causal role in pathological angiogenesis and up-regulation of angiogenic mediators, including vascular endothelial growth factor A (VEGFA) in OIR. Mice were exposed to 75% oxygen from post-natal day P7 to P12, treated with either vehicle or EDNRA antagonist BQ-123 or EDNRB antagonist BQ-788 on P12, and kept at room air from P12 to P17 (ischemic phase). RT-PCR analysis revealed increased levels of EDN2 and EDNRA mRNA, and Western blot analysis revealed increased EDN2 expression during the ischemic phase. EDNRA inhibition significantly increased vessel sprouting, resulting in enhanced physiological angiogenesis and decreased pathological NV, whereas EDNRB inhibition modestly improved vascular repair. OIR triggered significant increases in VEGFA protein and mRNA for delta-like ligand 4, apelin, angiopoietin-2, and monocyte chemoattractant protein-1. BQ-123 treatment significantly reduced these alterations. EDN2 expression was localized to retinal glia and pathological NV tufts of the OIR retinas. EDN2 also induced VEGFA protein expression in cultured astrocytes. In conclusion, inhibition of the EDNRA during OIR suppresses pathological NV and promotes physiological angiogenesis.

Tie2 and Eph receptor tyrosine kinase activation and signaling

The Eph and Tie cell surface receptors mediate a variety of signaling events during development and in the adult organism. As other receptor tyrosine kinases, they are activated on binding of extracellular ligands and their catalytic activity is tightly regulated on multiple levels. The Eph and Tie receptors display some unique characteristics, including the requirement of ligand-induced receptor clustering for efficient signaling. Interestingly, both Ephs and Ties can mediate different, even opposite, biological effects depending on the specific ligand eliciting the response and on the cellular context. Here we discuss the structural features of these receptors, their interactions with various ligands, as well as functional implications for downstream signaling initiation. The Eph/ephrin structures are already well reviewed and we only provide a brief overview on the initial binding events. We go into more detail discussing the Tie-angiopoietin structures and recognition.

Enhanced expression of VEGF-A in β cells increases endothelial cell number but impairs islet morphogenesis and β cell proliferation

There is a reciprocal interaction between pancreatic islet cells and vascular endothelial cells (EC) in which EC-derived signals promote islet cell differentiation and islet development while islet cell-derived angiogenic factors promote EC recruitment and extensive islet vascularization. To examine the role of angiogenic factors in the coordinated development of islets and their associated vessels, we used a "tet-on" inducible system (mice expressing rat insulin promoter-reverse tetracycline activator transgene and a tet-operon-angiogenic factor transgene) to increase the β cell production of vascular endothelial growth factor-A (VEGF-A), angiopoietin-1 (Ang1), or angiopoietin-2 (Ang2) during islet cell differentiation and islet development. In VEGF-A overexpressing embryos, ECs began to accumulate around epithelial tubes residing in the central region of the developing pancreas (associated with endocrine cells) as early as embryonic day 12.5 (E12.5) and increased dramatically by E16.5. While α and β cells formed islet cell clusters in control embryos at E16.5, the increased EC population perturbed endocrine cell differentiation and islet cell clustering in VEGF-A overexpressing embryos. With continued overexpression of VEGF-A, α and β cells became scattered, remained adjacent to ductal structures, and never coalesced into islets, resulting in a reduction in β cell proliferation and β cell mass at postnatal day 1. A similar impact on islet morphology was observed when VEGF-A was overexpressed in β cells during the postnatal period. In contrast, increased expression of Ang1 or Ang2 in β cells in developing or adult islets did not alter islet differentiation, development, or morphology, but altered islet EC ultrastructure. These data indicate that (1) increased EC number does not promote, but actually impairs β cell proliferation and islet formation; (2) the level of VEGF-A production by islet endocrine cells is critical for islet vascularization during development and postnatally; (3) angiopoietin-Tie2 signaling in endothelial cells does not have a crucial role in the development or maintenance of islet vascularization.

NKX2-3 transcriptional regulation of endothelin-1 and VEGF signaling in human intestinal microvascular endothelial cells

Background

NKX2-3 is associated with inflammatory bowel disease (IBD). NKX2-3 is expressed in microvascular endothelial cells and the muscularis mucosa of the gastrointestinal tract. Human intestinal microvascular endothelial cells (HIMECs) are actively involved in the pathogenesis of IBD and IBD-associated microvascular dysfunction. To understand the cellular function of NKX2-3 and its potential role underlying IBD pathogenesis, we investigated the genes regulated by NKX2-3 in HIMEC using cDNA microarray.

Methodology/Principal Findings

NKX2-3 expression was suppressed by shRNA in two HIMEC lines and gene expression was profiled by cDNA microarray. Pathway Analysis was used to identify gene networks according to biological functions and associated pathways. Validation of microarray and genes expression in intestinal tissues was assessed by RT-PCR. NKX2-3 regulated genes are involved in immune and inflammatory response, cell proliferation and growth, metabolic process, and angiogenesis. Several inflammation and angiogenesis related signaling pathways that play important roles in IBD were regulated by NKX2-3, including endothelin-1 and VEGF-PI3K/AKT-eNOS. Expression levels of NKX2-3, VEGFA, PI3K, AKT, and eNOS are increased in intestinal tissues from IBD patients and expression levels of EDN1 are decreased in intestinal tissues from IBD patients. These results demonstrated the important roles of NKX2-3, VEGF, PI3K, AKT, eNOS, and EDN1 in IBD pathogenesis. Correlation analysis showed a positive correlation between mRNA expression of NKX2-3 and VEGFA and a negative correlation between mRNA expression of NKX2-3 and EDN1 in intestinal tissues from IBD patients.

Conclusion/Relevance

NKX2-3 may play an important role in IBD pathogenesis by regulating endothelin-1 and VEGF signaling in HIMECs.

Retinopathy of prematurity: understanding ischemic retinal vasculopathies at an extreme of life

Retinopathy of prematurity (ROP) is a major complication of preterm birth. It encompasses a spectrum of pathologies that affect vision, from mild disease that resolves spontaneously to severe disease that causes retinal detachment and subsequent blindness. The pathologies are characterized by an arrest in normal retinal vascular development associated with microvascular degeneration. The resulting ischemia and retinal hypoxia lead to excessive abnormal compensatory blood vessel growth. However, this neovascularization can lead to fibrous scar formation and culminate in retinal detachment. Present therapeutic modalities to limit the adverse consequences of aberrant neovascularization are invasive and/or tissue-destructive. In this Review, we discuss current concepts on retinal microvascular degeneration, neovascularization, and available treatments, as well as present future perspectives toward more profound elucidation of the pathogenesis of ROP.

Tie1-Tie2 interactions mediate functional differences between angiopoietin ligands

The Tie family of endothelial-specific receptor tyrosine kinases is essential for cell proliferation, migration, and survival during angiogenesis. Despite considerable similarity, experiments with Tie1- or Tie2-deficient mice highlight distinct functions for these receptors in vivo. The Tie2 receptor is further unique with respect to its structurally homologous ligands. Angiopoietin-2 and -3 can function as agonists or antagonists; angiopoietin-1 and -4 are constitutive agonists. To address the role of Tie1 in angiopoietin-mediated Tie2 signaling and determine the basis for the behavior of the individual angiopoietins, we used an in vivo FRET-based proximity assay to monitor Tie1 and -2 localization and association. We provide evidence for Tie1-Tie2 complex formation on the cell surface and identify molecular surface areas essential for receptor-receptor recognition. We further demonstrate that the Tie1-Tie2 interactions are dynamic, inhibitory, and differentially modulated by angiopoietin-1 and -2. Based on the available data, we propose a unified model for angiopoietin-induced Tie2 signaling.

Cell-responsive hydrogel for encapsulation of vascular cells

The in vitro potential of a synthetic matrix metalloproteinase (MMP)-responsive poly(ethylene glycol) (PEG)-based hydrogel as a bioactive co-encapsulation system for vascular cells and a small bioactive peptide, thymosin beta4 (Tbeta4), was examined. We show that the physical incorporation of Tbeta4 in this bioactive matrix creates a three-dimensional (3D) environment conducive for human umbilical vein endothelial cell (HUVEC) adhesion, survival, migration and organization. Gels with entrapped Tbeta4 increased the survival of HUVEC compared to gels without Tbeta4, and significantly up-regulated the endothelial genes vascular endothelial-cadherin and angiopoietin-2, whereas von Willebrand factor was significantly down-regulated. Incorporation of Tbeta4 significantly increased MMP-2 and MMP-9 secretion of encapsulated HUVEC. The gel acts as a controlled Tbeta4-release system, as MMP-2 and MMP-9 enzymes trigger the release. In addition, Tbeta4 facilitated HUVEC attachment and induced vascular-like network formation upon the PEG-hydrogels. These MMP-responsive PEG-hydrogels may thus serve as controlled co-encapsulation system of vascular cells and bioactive factors for in situ regeneration of ischemic tissues.

Arsenic requires sphingosine-1-phosphate type 1 receptors to induce angiogenic genes and endothelial cell remodeling

Arsenic in drinking water is a major public health concern as it increases risk and incidence of cardiovascular disease and cancer. Arsenic exposure affects multiple vascular beds, promoting liver sinusoidal capillarization and portal hypertension, ischemic heart disease, peripheral vascular disease, and tumor angiogenesis. While Rac1-GTPase and NADPH oxidase activities are essential for arsenic-stimulated endothelial cell signaling for angiogenesis or liver sinusoid capillarization, the mechanism for initiating these effects is unknown. We found that arsenic-stimulated cell signaling and angiogenic gene expression in human microvascular endothelial cells were Pertussis toxin sensitive, indicating a G-protein coupled signaling pathway. Incubating human microvascular endothelial cells with the sphingosine-1-phosphate type 1 receptor (S1P(1)) inhibitor VPC23019 or performing small interfering RNA knockdown of S1P(1) blocked arsenic-stimulated HMVEC angiogenic gene expression and tube formation, but did not affect induction of either HMOX1 or IL8. Liver sinusoidal endothelial cells (LSECs) defenestrate and capillarize in response to aging and environmental oxidant stresses. We found that S1P(1) was enriched on LSECs in vivo and in primary cell culture and that VPC23019 inhibited both sphingosine-1-phosphate-stimulated and arsenic-stimulated LSEC oxidant generation and defenestration. These studies identified novel roles for S1P(1) in mediating arsenic stimulation of both angiogenesis and pathogenic LSEC capillarization, as well as demonstrating a role for S1P(1) in mediating environmental responses in the liver vasculature, providing possible mechanistic insight into arsenic-induced vascular pathogenesis and disease.

The periphery of the human fetal adrenal gland is a site of angiogenesis: zonal differential expression and regulation of angiogenic factors

CONTEXT

Although the inner fetal zone (FZ) of the mid-gestation human fetal adrenal (HFA) produces dehydroepiandrosterone sulfate, the function of the outer definitive zone (DZ) remains less clear. We have proposed that the DZ phenotype is that of a pool of progenitor cells, many of which are mitotically active. Recently, we studied HFA expression of a family of vascular endothelial cell-specific angiogenic factors, the angiopoietins (Angs), and demonstrated that Ang2 was localized predominantly in the periphery of the gland. Ang1 stabilizes, whereas Ang2 destabilizes, vessels, increasing responsiveness to angiogenic stimuli such as vascular endothelial growth factor (VEGF)-A and fibroblast growth factor (FGF)-2.

OBJECTIVE

Our objective was to test the hypothesis that the periphery of the HFA is a site of angiogenesis.

DESIGN

Studies were conducted involving RNA, frozen sections, and primary cell cultures from midgestation HFAs.

MAIN OUTCOME MEASURES

Immunofluorescence, laser capture microdissection, and real-time quantitative RT-PCR were used.

RESULTS

Double immunostaining demonstrated that proliferating endothelial cells were limited to the DZ and DZ/FZ border. Ang2 mRNA was primarily expressed in the DZ, whereas Ang1 mRNA was primarily in the FZ. VEGF-A and FGF-2 mRNA levels were higher in the DZ. FGF-2 (10 ng/ml) induced Ang2 mRNA by 4-fold in both zones of cells (P < 0.01, at 24 h), but not Ang1 or VEGF-A mRNA.

CONCLUSION

Data suggest that angiogenesis occurs at the periphery of the HFA. The DZ-predominant expression of Ang2 may be explained, in part, by the parallel pattern of FGF-2 expression.

Maturation of capillary-like structures in a tube-like construct in perfusion and rotation culture

In an effort to engineer capillary-like networks in vitro, different cultivation methods were compared. Five small-diameter tube-like constructs ('vessel equivalents') were fabricated from porcine gelatin scaffolds and seeded with human adipose tissue stromal cells and umbilical vein endothelial cells. After initial growth, the vessel equivalents were divided. One segment was exposed to pulsatile perfusion and the other was kept in rotating culture. Specimens harvested at the start of the experiments and after 16 days of rotation or perfusion were compared histomorphometrically with respect to capillary-like network formation in the vessel wall. Most capillary-like structures were found in the luminal portion of perfused vessel equivalents. Maturation of these capillary-like structures ascertained by recruitment of alpha-actin-positive cells also reached the highest degree in the luminal portion of perfused specimens. Perfused specimens showed significantly less apoptosis. Pulsatile perfusion promotes the development and maturation of a capillary-like network in this in-vitro approach.

Angiopoietin/Tie2 Pathway Influences Smooth Muscle Hyperplasia in Idiopathic Pulmonary Hypertension

RATIONALE

Angiopoietins are involved in blood vessel maturation and remodeling.

OBJECTIVES

One consequence of endothelium-specific tyrosine kinase-2 (Tie2) receptor activation by angiopoietin-1 (Ang1) is the release of endothelium-derived growth factors that recruit vascular wall cells. We investigated this process in idiopathic pulmonary arterial hypertension (iPAH).

METHODS

Ang1, Ang2, and total and phosphorylated Tie2 expression (mRNA and protein) was evaluated in human lung specimens and in cultured pulmonary artery smooth muscle cells (PA-SMCs) and pulmonary endothelial cells (P-ECs) isolated from patients with iPAH and control subjects. Media collected from Ang1-treated P-ECs were assessed for their PA-SMC growth-promoting effect.

MEASUREMENTS AND MAIN RESULTS

Tie2 receptor was fourfold higher in lungs and P-ECs from patients with iPAH than in those from control subjects, with a parallel increase in phosphorylated lung Tie2 receptor. In contrast, Ang1 and Ang2 expression in lungs, P-ECs, and PA-SMCs did not differ. Incubation of PA-SMCs with medium collected from P-EC cultures induced marked proliferation, and this effect was stronger when using P-ECs from patients with iPAH than from control subjects. Ang1 pretreatment of P-ECs from either patients or control subjects induced a further increase in PA-SMC proliferation. Fluoxetine, an inhibitor of the mitogenic action of serotonin, reduced the growth-promoting effect of P-EC media. Ang1 added to P-ECs from patients with iPAH increased the production of endothelin-1 (ET-1) and serotonin, but not of platelet-derived growth factor-BB or epidermal growth factor, and increased the amount of mRNA encoding tryptophan hydroxylase-1 (the rate-limiting enzyme of serotonin synthesis), preproET-1, and ET-1-converting enzyme.

CONCLUSIONS

The Ang1/Tie2 pathway is potentiated in iPAH, contributing to PA-SMC hyperplasia via increased stimulation of endothelium-derived growth factors synthesis by P-ECs.

Rapid vessel regression, protease inhibition, and stromal normalization upon short-term vascular endothelial growth factor receptor 2 inhibition in skin carcinoma heterotransplants

Vascular endothelial growth factor (VEGF) plays a key role in tumor angiogenesis, and blockade of VEGF receptor 2 (VEGFR-2), with the monoclonal antibody DC101, inhibits angiogenesis and tumor growth. To examine the short-term effects of DC101, we surface transplanted the squamous cell carcinoma cell line A5-RT3 onto nude mice. After short-term treatment with DC101, we observed rapid reduction in vascularization and reversion of the tumor phenotype. Beginning 24 hours after treatment, VEGFR-2 inhibition resulted in decreased vessel density within the tenascin-c-staining tumor-associated stroma and reduced endothelial cell proliferation. Stromal expression of matrix metalloproteinase-9 and -13 was drastically reduced 96 hours after VEGFR-2 inhibition as detected by in situ hybridization and in situ zymography. Moreover, the morphology of the tumor-stroma border changed from a highly invasive carcinoma to a well-demarcated, premalignant phenotype. The latter was characterized by the appearance of a regular basement membrane in immunostaining and ultrastructural analyses. These findings suggest that VEGFR-2 inhibition by DC101 evokes very rapid reduction of preformed vessels and decreases both stromal protease expression and gelatinolytic activity, resulting in the modulation of the tumor-stroma border zone and reversion of the tumor phenotype. Thus, short-term inhibition of VEGF signaling results in complex stromal alterations with crucial consequences for the tumor phenotype.

The role of pericytes in blood-vessel formation and maintenance

Blood vessels are composed of two interacting cell types. Endothelial cells form the inner lining of the vessel wall, and perivascular cells--referred to as pericytes, vascular smooth muscle cells or mural cells--envelop the surface of the vascular tube. Over the last decades, studies of blood vessels have concentrated mainly on the endothelial cell component, especially when the first angiogenic factors were discovered, while the interest in pericytes has lagged behind. Pericytes are, however, functionally significant; when vessels lose pericytes, they become hemorrhagic and hyperdilated, which leads to conditions such as edema, diabetic retinopathy, and even embryonic lethality. Recently, pericytes have gained new attention as functional and critical contributors to tumor angiogenesis and therefore as potential new targets for antiangiogenic therapies. Pericytes are complex. Their ontogeny is not completely understood, and they perform various functions throughout the body. This review article describes the current knowledge about the nature of pericytes and their functions during vessel growth, vessel maintenance, and pathological angiogenesis.