Exosome-transmitted ANGPTL1 suppresses angiogenesis in glioblastoma by inhibiting the VEGFA/VEGFR2/Akt/eNOS pathway

OBJECTIVE

Glioblastoma (GBM) is a highly vascularized malignancy that relies on new vessel generation, and thus targeting angiogenesis has been a promising anti-GBM approach. ANGPTL1 is well-known for its anti-angiogenic property; nevertheless, its role in GBM is yet to be explored. Recently, the crucial role of exosomes (Exos) as intercellular communication mediators has gained prominence in GBM therapy. This work aimed to explore the role of exosomal ANGPTL1 in GBM angiogenesis and its mechanisms.

METHODS

Bioinformatic analysis was performed to evaluate ANGPTL expression in GBM. Human GBM cell lines (U87 and U251) and a xenograft mouse model were employed. Exos were isolated from oe-NC- and oe-ANGPTL-transfected bone mesenchymal stem cells and identified. Cell proliferation, migration, and apoptosis were detected. Immunofluorescence, qRT-PCR, western blotting, co-immunoprecipitation, and immunohistochemistry were used to determine the molecular mechanisms underlying exosomal ANGPTL1 against GBM angiogenesis. Besides, tube generation and transmission electron microscope assays were conducted to assess GBM angiogenesis.

RESULTS

Low ANGPTL1 expression was observed in GBM tumor tissues and cells. Functionally, e-ANGPTL-Exos inhibited GBM malignant progression and angiogenesis in vitro and in vivo. Mechanically, e-ANGPTL-Exos reduced VEGFA expression and blocked the VEGFR2/Akt/eNOS pathway in GBM cells and tumor tissues. Co-immunoprecipitation revealed a link between ANGPTL1 and VEGFA in GBM cells. Notably, oe-VEGFA abolished the suppressive functions of e-ANGPTL-Exos in GBM progression and angiogenesis and the VEGFR2/Akt/eNOS axis. The VEGFR2 inhibitor, vandetanib, eliminated the promotive effects of oe-VEGFA on GBM angiogenesis with suppressed VEGFR2/Akt/eNOS pathway.

CONCLUSIONS

Exosomal ANGPTL1 suppressed GBM angiogenesis by inhibiting the VEGFA/VEGFR2/Akt/eNOS axis.

Mining the role of angiopoietin-like protein family in gastric cancer and seeking potential therapeutic targets by integrative bioinformatics analysis

Background

The indistinctive effects of antiangiogenesis agents in gastric cancer (GC) can be attributed to multifaceted gene dysregulation associated with angiogenesis. Angiopoietin‐like (ANGPTL) proteins are secreted proteins regulating angiogenesis. They are also involved in inflammation and metabolism. Emerging evidences have revealed their various roles in carcinogenesis and metastasis development. However, the mRNA expression profiles, prognostic values, and biological functions of ANGPTL proteins in GC are still elucidated.

Methods

We compared the transcriptional expression levels of ANGPTL proteins between GC and normal gastric tissues using ONCOMINE and TCGA‐STAD. The prognostic values were evaluated by LinkedOmics and Kaplan–Meier Plotter, while the association of expression levels with clinicopathological features was generated through cBioPortal. We conducted the functional enrichment analysis with Metascape.

Results

The expression of ANGPTL1/3/6 was lower in GC tissues than in normal gastric tissues. High expression of ANGPTL1/2/4 was correlated with short overall survival and post‐progression survival in GC patients. Upregulated ANGPTL1/2 was correlated with higher histological grade, non‐intestinal Lauren classification, and advanced T stage, while ANGPTL4 exhibited high expression in early T stage, M1 stage, and non‐intestinal Lauren classification.

Conclusions

Integrative bioinformatics analysis suggests that ANGPTL1/2/4 may be potential therapeutic targets in GC patients. Among them, ANGPTL2 acts as a GC promoter, while ANGPTL1/4’s role in GC is still uncertain.

Aberrant expression of angiopoietin-like proteins 1 and 2 in cumulus cells is potentially associated with impaired oocyte developmental competence in polycystic ovary syndrome

Polycystic ovary syndrome (PCOS) is a heterogeneous endocrine disorder associated with obesity, insulin resistance, hyperandrogenism, alterations in ovarian angiogenesis and impaired oocyte competence. Emerging evidence demonstrates that angiopoietin-like protein 1 (ANGPTL1) and angiopoietin-like protein 2 (ANGPTL2) have an important influence on angiogenesis, androgen biosynthesis, insulin resistance and adipocytes function. In this study, we set out to determine the potential relationship between ANGPTL1, ANGPTL2 and oocyte competence in PCOS through analyzing the expression levels and dynamic pattern of the two genes in cumulus cells (CCs) during different phases of nuclear maturation of PCOS patients and control groups undergoing controlled ovarian hyperstimulation (COH) for in vitro fertilization and embryo transfer. We found that the relative abundance of ANGPTL1 and ANGPTL2 transcripts in CCs from patients with PCOS showed dynamic changes during oocyte maturation. Specifically, their expressions were increased significantly at the Metaphase II stage. In summary, the present novel evidence indicates that the expression patterns of ANGPTL1 and ANGPTL2 mRNAs are disordered during oocyte maturation in PCOS, which were potentially related to aberrant oocyte quality and developmental potency, at least in part, via pathological angiogenesis and metabolism.

Expression change in Angiopoietin-1 underlies change in relative brain size in fish

Brain size varies substantially across the animal kingdom and is often associated with cognitive ability; however, the genetic architecture underpinning natural variation in these key traits is virtually unknown. In order to identify the genetic architecture and loci underlying variation in brain size, we analysed both coding sequence and expression for all the loci expressed in the telencephalon in replicate populations of guppies (Poecilia reticulata) artificially selected for large and small relative brain size. A single gene, Angiopoietin-1 (Ang-1), a regulator of angiogenesis and suspected driver of neural development, was differentially expressed between large- and small-brain populations. Zebra fish (Danio rerio) morphants showed that mild knock down of Ang-1 produces a small-brained phenotype that could be rescued with Ang-1 mRNA. Translation inhibition of Ang-1 resulted in smaller brains in larvae and increased expression of Notch-1, which regulates differentiation of neural stem cells. In situ analysis of newborn large- and small-brained guppies revealed matching expression patterns of Ang-1 and Notch-1 to those observed in zebrafish larvae. Taken together, our results suggest that the genetic architecture affecting brain size in our population may be surprisingly simple, and Ang-1 may be a potentially important locus in the evolution of vertebrate brain size and cognitive ability.

The role of tumor necrosis factor-α and interferon-γ in regulating angiomotin-like protein 1 expression in lung microvascular endothelial cells

BACKGROUND

Angiogenesis in the alveolar septa is thought be a critical factor in pulmonary emphysema. Angiomotin-like protein 1 (AmotL1) is involved in angiogenesis via regulating endothelial cell function. However, the role of AmotL1 in the pathogenesis of pulmonary emphysema has not been elucidated. The objective of this study is to evaluate the expression of AmotL1 in lung tissues from a murine model with emphysema, as well as from patients with chronic obstructive pulmonary disease (COPD). Furthermore, we analyzed the regulation of AmotL1 expression by TNF-α and IFN-γ in endothelial cells in vitro.

METHODS

Nrf2 knockout mice were exposed to cigarette smoke (CS) for 4 weeks, and the down-regulated genes affecting vascularity in the whole lung were identified by microarray analysis. This analysis revealed that the mRNA expression of AmotL1 decreased in response to CS when compared with air exposure. To confirm the protein levels that were indicated in the microarray data, we determined the expression of AmotL1 in lung tissues obtained from patients with COPD and also determined the expression of AmotL1, NFκB and IκBα in cultured normal human lung microvascular endothelial cells (HLMVECs) that were stimulated by TNF-α and IFN-γ.

RESULTS

We found that the number of AmotL1-positive vessels decreased in the emphysema lungs compared with the normal and bronchial asthmatic lungs. IFN-γ pretreatment diminished the TNF-α-induced AmotL1 in the cultured HLMVECs by blocking the degradation of IκBα.

CONCLUSIONS

These results suggested that IFN-γ exhibits anti-angiogenesis effects by regulating the expression of TNF-α-induced AmotL1 via NFκB in emphysema lungs.

Angiopoietin-like protein 3: development, analytical characterization, and clinical testing of a new ELISA

The aim of our work was to develop an assay for the determination of angiopoietin-like protein 3 (Angptl3) in human blood, and investigate its levels in healthy volunteers and donors suffer from metabolic syndrome and familiar hypercholesterolemia. We developed and evaluated the sandwich ELISA method for the quantitative determination of human Angptl3 in serum samples. We conducted also the pilot study on individuals with metabolic syndrome or familiar hypercholesterolemia and healthy probands. The following parameters were measured: blood pressure, waist circumference, Angptl3 serum levels, serum cholesterol, triglycerides, HDL-cholesterol, LDL-cholesterol, insulin, glucose, A-FABP, and BMI and Quicki insulin sensitivity index was calculated. In the study on 93 healthy volunteers we demonstrated that sex or age is not the determinant for Angptl3 serum values. Futhermore, 118 individuals with metabolic syndrome and 200 patients with familiar hypercholesterolemia were tested and it was found that probands with metabolic syndrome or familiar hypercholesterolemia had higher Angptl3 values than healthy individuals from the first study (medians 289.5 vs. 277.1 vs. 224.8 ng/ml, p < 0.01). All of groups did not differ in sex or age. Angptl3 values correlated with the systolic blood pressure, LDL and A-FABP (p < 0.05). No connection of Angptl3 with triglycerides was found (presumably influences of statins, fibrates via PPARs, etc). However, we performed stepwise regression and found A-FABP and Angptl3 serum values as the independent markers for metabolic syndrome presence only (F ratio 29, p < 0.01). Then we adjusted Angptl3 to A-FABP (reputable metabolic syndrome marker) and recognised that Angptl3 is the A-FABP-independent marker. The pilot study supports the hypothesis about the role of Angptl3 as a new class of lipid metabolism modulator. Their values could be a new key predictors of metabolic syndrome. Further research is necessary to confirm our findings in individuals with dyslipidemia, obesity, CAD and different medication in order to assess Angptl3 value as a risk predictor of accelerated atherosclerosis.

Molecular characterization of angiomotin/JEAP family proteins: interaction with MUPP1/Patj and their endogenous properties

We have previously shown that MUPP1, which has an MRE domain and 13 PDZ domains, is expressed in epithelial cells and localize at tight junctions (TJs) and apical membranes. Using yeast two-hybrid screening, we found here that MUPP1 interacts with angiomotin (Amot), JEAP/Amot-like 1 and MASCOT/Amot-like 2, which we refer to as Amot/JEAP family proteins. PDZ2 and -3 were responsible for MUPP1's interaction with Amot and MASCOT, whereas only PDZ3 was responsible for its interaction with JEAP. All the Amot/JEAP family proteins also interacted with Patj, a close relative of MUPP1. The C-terminal PDZ-binding motives of the Amot/JEAP family were required for these interactions. We successfully generated specific antibodies for these proteins and analyzed the endogenous molecular properties of the family in parallel. Immunofluorescence microscopy of cultured epithelial cells showed that in subcellular distribution, the Amot/JEAP family proteins were indistinguishable; they were apparent at TJs as well as apical membranes, and mostly co-localized with MUPP1. They were also located at TJs in several mouse tissues, but each protein showed a distinct tissue distribution. In biochemical fractionation assays, the Amot/JEAP family behaved not as transmembrane but as peripheral membrane proteins. Unexpectedly, the PDZ-binding motives were not necessarily required for their localization to TJs, and dominant negative MUPP1 or Patj did not affect the localization of Amot/JEAP family proteins, suggesting that the interaction with MUPP1/Patj is not necessarily responsible for their proper subcellular distribution.

Angiomotin Regulates Endothelial Cell-Cell Junctions and Cell Motility

We have previously identified angiomotin by its ability to bind to and mediate the anti-angiogenic properties of angiostatin. In vivo and in vitro data indicate an essential role of angiomotin in endothelial cell motility. Here we show that angiostatin binds angiomotin on the cell surface and provide evidence for a transmembrane model for the topology of both p80 and p130 angiomotin isoforms. Immunofluorescence analysis shows that angiomotin co-localized with ZO-1 in cell-cell contacts in endothelial cells in vitro and in angiogenic blood vessels of the postnatal mouse retina in vivo. Transfection of p80 as well as p130 angiomotin in Chinese hamster ovary cells resulted in junctional localization of both isoforms. Furthermore, p130 angiomotin could recruit ZO-1 to actin stress fibers. The p130 but not p80 isoform could be coprecipitated with MAGI-1b, a component of endothelial tight junctions. Paracellular permeability, as measured by diffusion of fluorescein isothiocyanate-dextran, was reduced by p80 and p130 angiomotin expression with 70 and 88%, respectively, compared with control. Angiostatin did not have any effect on cell permeability but inhibited the migration of angiomotin-expressing cells in the Boyden chamber assay. We conclude that angiomotin, in addition to controlling cell motility, may play a role in the assembly of endothelial cell-cell junctions.

Cooperative interaction of Angiopoietin-like proteins 1 and 2 in zebrafish vascular development

Angiopoietin-like protein (Angptl) 1 and Angptl2, which are considered orphan ligands, are highly homologous, particularly in the fibrinogen-like domain containing the putative receptor binding site. This similarity suggests potentially cooperative functions between the two proteins. In this report, the function of Angptl1 and Angptl2 is analyzed by using morpholino antisense technology in zebrafish. Knockdown of both Angptl1 and Angptl2 produced severe vascular defects due to increased apoptosis of endothelial cells at the sprouting stage. In vitro studies showed that Angptl1 and Angptl2 have antiapoptotic activities through the phosphatidylinositol 3-kinase/Akt pathway, and coinjection of constitutively active Akt/protein kinase B mRNA rescued impaired vascular development seen in double knockdown embryos. These results provide a physiological demonstration of the cooperative interaction of Angptl1 and Angptl2 in endothelial cells through phosphatidylinositol 3-kinase/Akt mediated antiapoptotic activities.

Angioarrestin: A unique angiopoietin-related protein with anti-angiogenic properties

The process of angiogenesis plays a pivotal role in embryogenesis, wound healing, and tumorigenesis through the growth of new blood vessels from pre-existing vasculature. Among the angiogenic factors recently identified as specific for vascular endothelium are the angiopoietins. In depth characterization of the angiopoietins has allowed investigators to better understand the molecular basis of blood vessel formation and vascular endothelial cell function. In this review, we describe angiopoietins and related family members, with particular emphasis on a recently identified protein known as angioarrestin. Our investigations clearly demonstrate that angioarrestin is an anti-angiogenic molecule. The effects of angioarrestin on tumor cell progression and specific aspects of the angiogenic cascade in in vitro models are further discussed.

Isolation and expression patterns of genes for three angiopoietin-like proteins, Angptl1, 2 and 6 in zebrafish

Angiopoietin-like proteins (Angptls) are known to possess biological activities not only in the vascular system, but in the other mammalian tissues; however, their expression patterns and function in embryogenesis have not been extensively characterized. Here, we identify three zebrafish genes (Zangptl1, Zangptl2 and Zangptl6) highly homologous to mammalian Angptl1/ARP1, Angptl2/ARP2 and Angptl6/AGF, and describe their adult and embryonic temporal and spatial expression patterns. Zangptl1 is expressed faintly in the somites, while Zangptl2 is first detected in the yolk sac extension, spinal cord and branchial arches and is later expressed in the liver primordium and pectoral fin buds. Zangptl6 is expressed in the notochord. In addition to its embryonic expression, Zangptl2 is induced in adult fish during fin regeneration.

[Screening for new binding proteins which interact with BM2 of influenza B virus with yeast two-hybrid system]

OBJECTIVE

To explore the role of BM2 protein in the life cycle of influenza B virus.

METHODS

The authors screened human kidney MATCHMAKER cDNA library for new binding partners of BM2 of influenza B virus by using the yeast two hybrid system with truncated BM2 (26-109 aa) as the bait.

RESULTS

Six positive plasmids encoding N-acetylneuraminate pyruvate lyase, angiopoietin 3, zinc finger protein 251, ribosomal protein S20, protein arginine N-methyltransferase 1 variant 1 (PRMT) and transcription factor-like 1 (TCFL1) were obtained.

CONCLUSION

The results suggest that BM2 may play an important role in the life cycle of influenza B virus.

Biological characterization of angiopoietin-3 and angiopoietin-4

The angiopoietin (Ang) family of growth factors includes Ang1, Ang2, Ang3, and Ang4, all of which bind to the endothelial receptor tyrosine kinase Tie2. Ang3 (mouse) and Ang4 (human) are interspecies orthologs. In experiments with human endothelial cell lines, Ang3 was identified as an antagonist of Tie2 and Ang4 was identified as an agonist of Tie2. However, the biological roles of Ang3 and Ang4 are unknown. We examined the biological effect of recombinant Ang3 and Ang4 proteins in primary cultured endothelial cells and in vivo in mice. Recombinant Ang3 and Ang4 formed disulfide-linked dimers. Ang4 (400 ng/mL) markedly increased Tie2 and Akt phosphorylation in primary cultured HUVECs whereas Ang3 (400 ng/mL) did not produce significant changes. Accordingly, Ang4, but not Ang3, induced survival and migration in primary cultured HUVECs. Unexpectedly, intravenously administered Ang3 (30 microg) was more potent than Ang4 (30 microg) in phosphorylating the Tie2 receptor in lung tissue from mice in vivo. Accordingly, Ang3 was more potent than Ang4 in phosphorylating Akt in primary cultured mouse lung microvascular endothelial cells. Ang3 and Ang4 both produced potent corneal angiogenesis extending from the limbus across the mouse cornea in vivo. Thus, Ang3 and Ang4 are agonists of Tie2, but mouse Ang3 has strong activity only on endothelial cells of its own species.

Recombinant angioarrestin secreted from mouse melanoma cells inhibits growth of primary tumours

Angioarrestin is a recently described anti-angiogenic protein whose expression is down-regulated in solid tumours of various origins. It has a sequence identical to angiopoietin related protein-1. In this study we investigated anti-tumour properties of angioarrestin in B16 (F10) melanoma tumour model. We constructed an expression vector encoding human angioarrestin under the control of EF-1alpha promoter. This vector was transferred to B16 (F10) cells and recombinant angioarrestin secreted from the transfected cells was tested for anti-angiogenic activity using endothelial cell proliferation assay. Finally, mice were injected subcutaneously with cells that had been transfected with either angioarrestin-encoding vector or empty vector and tumor growth was compared. The obtained recombinant angioarrestin inhibited proliferation of bovine aortic endothelial cells. Tumours derived from an angioarrestin-secreting B16 (F10) cell clone grew in vivo more slowly than tumours derived from a cell clone transfected with empty vector. These data show, to our knowledge for the first time, that angioarrestin can inhibit primary melanoma tumour growth.

Angiopoietin-3 inhibits pulmonary metastasis by inhibiting tumor angiogenesis

Angiopoietins (Ang-1, Ang-2, and Ang-3) are the ligands of Tie-2 receptor tyrosine kinase. The essential roles of Ang-1 and Tie-2 in embryonic angiogenesis have been established, and studies have demonstrated the involvement of Ang-1 and Ang-2 in tumor angiogenesis. However, the role of Ang-3 in tumor angiogenesis and metastasis and the mechanism underlying its function are totally unknown. We have shown recently that Ang-3 is tethered on cell surface via heparan sulfate proteoglycans. In our current study, we have demonstrated that overexpression of Ang-3 inhibits pulmonary metastasis of Lewis lung carcinoma and TA3 mammary carcinoma (TA3) cells by inhibiting tumor angiogenesis and promoting apoptosis of the tumor cells. In addition, we have demonstrated that the binding of Ang-3 to the cell surface is required for the effective inhibition of Ang-3 on tumor metastasis and that Ang-3 inhibits endothelial cell proliferation and survival and blocks Ang-1- and vascular endothelial growth factor-induced activation of extracellular signal-regulated kinase 1/2 and Akt kinases, which likely underlie the Ang-3-mediated inhibition on tumor angiogenesis and metastasis.

Angiopoietin-3 is tethered on the cell surface via heparan sulfate proteoglycans

Angiopoietins are a family of factors that play important roles in angiogenesis, and their receptor, Tie-2 receptor tyrosine kinase, is expressed primarily by endothelial cells. Three angiopoietins have been identified so far, angiopoietin-1 (Ang-1), angiopietin-2 (Ang-2), and angiopoietin-3 (Ang-3). It has been established that Ang-1 and Tie-2 play essential roles in embryonic angiogenesis. We have demonstrated recently that, unlike Ang-2, Ang-1 binds to the extracellular matrix, which regulates the availability and activity of Ang-1 (Xu, Y., and Yu, Q. (2001) J. Biol. Chem. 276, 34990-34998). However, the role and biochemical characteristics of Ang-3 are unknown. In our current study, we demonstrated that, unlike Ang-1 and Ang-2, Ang-3 is tethered on cell surface via heparan sulfate proteoglycans (HSPGs), especially perlecan. The cell surface-bound Ang-3 is capable of binding to its receptor, Tie-2; suggesting HSPGs concentrate Ang-3 on the cell surface and present Ang-3 to its receptor to elicit specific local reaction. Mutagenesis experiment revealed that the coiled-coil domain of Ang-3 is responsible for its binding to the cell surface. In addition, we demonstrated that the cell surface-bound Ang-3 but not soluble Ang-3 induces retraction and loss of integrity of endothelial monolayer, indicating the binding of Ang-3 to the cell surface via HSPGs is required for this bioactivity of Ang-3.

Angiopoietin-related/angiopoietin-like proteins regulate angiogenesis

A general understanding of the molecular mechanisms underlying angiogenesis is emerging from the analysis of targeted mutations in vasculature-related genes. These analyses reveal that angiopoietin signaling through the TIE2 receptor is involved in regulating angiogenesis. Recently, we and several other groups have independently identified several molecules containing a coiled-coil domain and a fibrinogen-like domain, both of which are structurally conserved in angiopoietins. Because these molecules do not bind to the angiopoietin-specific receptor,TIE2, they have been named angiopoietin-related proteins (ARPs) or angiopoietin-like proteins (Angptls). ARPs/Angptls, which are all currently orphan ligands, also have potent activity for regulating angiogenesis as proangiogenic or antiangiogenic factors, suggesting that their receptors may be expressed on endothelial cells. In addition, ARPs/Angptls show pleiotropic effects not only on vascular cells but also on cells of other lineages, such as skin and chondrocyte cells. More recent studies have proposed that ARPs/Angptls are involved in various pathologies, such as tumor angiogenesis and metabolic diseases. To summarize the current findings relating to these proteins, we focus in this review on the functions of ARPs/Angptls as new angiogenic modulating factors in the vascular system and discuss the pleiotropic functions of ARPs/Angptls in nonvascular cell lineages.

Angioarrestin mRNA expression in early-stage lung cancers

AIMS

Angioarrestin is a recently isolated gene, which has a novel function as an angiogenesis inhibitor. Angiogenesis plays an important role in tumorigenesis. It has been reported that the angioarrestin expression was decreased in lung cancer. We attempted to determine the influence of angioarrestin expression on clinicopathological features in patients with lung cancer who had undergone surgery.

METHODS

Expression of angioarrestin messenger RNA was evaluated by a quantitative reverse transcription-polymerase chain reaction (RT-PCR) in 93 lung carcinomas and adjacent histological normal lung samples using LightCycler.

RESULTS

Angioarrestin/GAPDH mRNA expression was significantly decreased in the tumor of lung cancer tissue (86.676+/-123.505) than in the normal lung tissue (1154.218+/-2003.508, p<0.0001), although only four lung cancer tissues had more than one tumor/normal ratio of angioarrestin/GAPDH mRNA expression. There was no relationship between angioarrestin gene expression and age, gender or T-status. However, decreased angioarrestin/GAPDH expression was especially seen at stage I lung cancer (54.156+/-62.783) when compared to stage II-IV lung cancer (110.315+/-151.359, p=0.0316). Decreased angioarrestin/GAPDH expression was especially seen at N0 lung cancer (56.396+/-69.941) when compared to N2 lung cancer (137.522+/-180.489, p=0.0362).

CONCLUSIONS

The decreased expression of angioarrestin mRNA was the early phase phenomena for tumor progression from lung cancer. Alternatively, loss of antianiogenesis might play a role in oncogenesis for lung cancer.

Loss of responsiveness to chemotactic factors by deletion of the C-terminal protein interaction site of angiomotin

We have recently identified a novel protein, named angiomotin, by its ability to bind the angiogenesis inhibitor angiostatin in the yeast two-hybrid system. Angiomotin belongs to a family with two other members, AmotL-1 and -2 characterized by coiled-coil and C-terminal PDZ binding domains. Here we show that the putative PDZ binding motif of angiomotin serves as a protein recognition site and that deletion of three amino acids in this site results in inhibition of chemotaxis. Furthermore, endothelial cells expressing mutant angiomotin failed to migrate and form tubes in an in vitro tube formation assay. To study the effect of angiomotin on embryonic angiogenesis, we generated transgenic mice expressing wild-type angiomotin and the C-terminal deletion mutant driven by the endothelial cell-specific receptor tyrosine kinase (TIE) promoter. Expression of mutant angiomotin in endothelial cells inhibited migration into the neuroectoderm and intersomitic regions resulting in death at embryonic day 9.5. In contrast, mice expressing wild-type angiomotin developed normally and were fertile. These results suggest that the putative PDZ binding motif of angiomotin plays a critical role in regulating the responsiveness of endothelial cells to chemotactic cues.

Angiopoietins in angiogenesis and beyond

The angiopoietin (Ang) family of growth factors includes four members, all of which bind to the endothelial receptor tyrosine kinase Tie2. Two of the Angs, Ang-1 and Ang-4, activate the Tie2 receptor, whereas Ang-2 and Ang-3 inhibit Ang-1-induced Tie2 phosphorylation. While genetic models have underscored the importance of Angs in the developing cardiovascular system, other studies have demonstrated that Ang-1 promotes endothelial cell survival, sprouting and tube formation. More recently, a new aspect of the biology of this class of growth factors has emerged, namely the ability of Ang-1 to reduce inflammation. This review presents an outline of Angs and their receptors, examining their structure, expression, signalling, regulation and biological significance and comments on the role and potential usefulness of Angs in medicine.

Angioarrestin: an antiangiogenic protein with tumor-inhibiting properties

The angiopoietins comprise a family of proteins that have pro or antiangiogenic activities. Through a proprietary technology designed to identify transcripts of all expressed genes, we isolated a cDNA encoding an angiopoietin-related protein that we designate angioarrestin. The mRNA expression profile of angioarrestin was striking in that it was down-regulated in many tumor tissues when compared with adjacent nontumor tissue, suggesting a role for this protein in tumor inhibition. To test this hypothesis, we ectopically expressed angioarrestin in HT1080 tumor cells and measured pulmonary tumor nodule formation in nude mice. HT1080 cells expressing angioarrestin showed a marked reduction in the number and size of tumor nodules. In vitro, the recombinant protein was systematically tested in a number of endothelial cell assays and found to block critical processes involved in the angiogenic cascade, such as vascular endothelial growth factor/basic fibroblast growth factor-mediated endothelial cell proliferation, migration, tubular network formation, and adhesion to extracellular matrix proteins. These findings reveal a novel function for angioarrestin as an angiogenesis inhibitor and indicate that the molecule may be a potential cancer therapeutic.

The angiopoietins and Tie2/Tek: adding to the complexity of cardiovascular development

The expansion or remodelling of pre-existing blood vessels, known as angiogenesis, by either nascent sprouting, intercalated or intussusceptive growth is a highly regulated process. Angiogenesis is critical not only during normal embryonic vascular development, but also in the progression of several diseases, including cancer, psoriasis, and diabetes. Mouse molecular genetic experiments have shown that the angiopoietins and their receptor Tie2/Tek are indispensable for embryonic vessel development. The importance of the angiopoietin-signalling pathway has also been shown to extend beyond development, into in vitro and in vivo experimental models of angiogenic growth. Currently the precise role of the angiopoietins remains unclear. However, what is emerging from genetic, xenograft transplant, histochemical and cell culture experiments are that the response of endothelial cells to angiopoietins appears to be context and endothelial cell type specific.

Differential expression of Tie-2 receptors and angiopoietins in response to in vivo hypoxia in rats

In this study, we assessed the effects of in vivo hypoxia on the expression of Tie-2 receptors and angiopoietins in various organs of conscious rats and correlated these effects with the expression of hypoxia-inducible factor-1 (HIF-1). RT-PCR and Southern blotting were used to amplify mRNA expression of angiopoietin-1, -2, and -3, Tie-2, and HIF-1 alpha in tissues of normoxic and hypoxic (fraction of inspired oxygen of 9--10% for either 12 or 48 h) rats. Hypoxia provoked a decline in angiopoietin-1 mRNA and Tie-2 mRNA, protein, and phosphorylation levels in the lung, liver, cerebellum, and heart but not in the kidney and diaphragm. In comparison, hypoxia raised the levels of angiopoietin-2 mRNA in the cerebellum and angiopoietin-3 mRNA in the lung, kidney, and diaphragm. HIF-1 alpha mRNA was abundant in most organs of normoxic rats but was significantly induced in the kidney and diaphragm of hypoxic rats. We conclude that in vivo hypoxia exerts inhibitory effects on the activity of the angiopoietin-1/Tie-2 receptor pathway through reduction of angiopoietin-1 and upregulation of angiopoietin-2 and -3. Induction of angiopoietin-3 in the kidney and diaphragm of hypoxic rats could be mediated through the HIF-1 transcription factor.

Isolation and expression analysis of three zebrafish angiopoietin genes

The Tie1 and Tie2 receptor tyrosine kinases and the Tie2 ligands, the angiopoietins, play critical roles in vertebrate vascular embryogenesis, helping to mediate the interaction between endothelial cells and the pericytes or vascular smooth muscle cells that envelop and support them. We have obtained full-length cDNA sequences for zebrafish orthologs of angiopoietin-1 (ang1), angiopoietin-2 (ang2), and angiopoietin-like-3 (angptl3). Ang1 is expressed in head ventral mesenchyme, in the ventromedial region of somites, in mesenchyme surrounding trunk axial vessels, and in the hypochord, a transient embryonic structure of endodermal origin that has been implicated in dorsal aorta assembly in both zebrafish and Xenopus. Ang2 is expressed in head and anterior trunk ventral mesenchyme and the developing pronephric glomeruli. Angptl3 is expressed in the yolk syncytial layer.

Angiotensin II induces expression of the Tie2 receptor ligand, angiopoietin-2, in bovine retinal endothelial cells

Recent studies have shown that angiopoietins (Angs) and their receptor, Tie2, play a role in vascular integrity and neovascularization. The renin-angiotensin system has been hypothesized to contribute to the development of diabetic retinopathy. In this study, we investigated the effect of angiotensin II (AII) on Ang1 and Ang2 expression in cultured bovine retinal endothelial cells (BRECs). AII stimulated Ang2 but not Ang1 mRNA expression in a dose- and time-dependent manner. This response was inhibited completely by angiotensin type 1 receptor (AT1) antagonist. AII increased the transcription of Ang2 mRNA, but did not change the half-life. Protein kinase C (PKC) inhibitor completely inhibited AII-induced Ang2 expression, and the mitogen-activated protein kinase (MAPK) inhibitor also inhibited it by 69.4+/-15.6%. In addition, we confirmed the upregulation of Ang2 in an AII-induced in vivo rat corneal neovascularization model. These data suggest that AII stimulates Ang2 expression through AT1 receptor-mediated PKC and MAPK pathways in BREC, and AII may play a novel role in retinal neovascularization.

Angiopoietin-3, a novel member of the angiopoietin family

A cDNA clone encoding angiopoietin-3 protein (Ang3), a novel member of the angiopoietin family, was identified. Ang3 cDNA was cloned from a human aorta cDNA library. Ang3 is a 503 amino acid protein having 45.1% and 44.7% identity with human angiopoietin-1 and human angiopoietin-2, respectively. Ang3 mRNA is expressed in lung and cultured human umbilical vein endothelial cells (HUVECs). Ang3 mRNA expression in HUVECs was slightly decreased by vascular endothelial cell growth factor treatment, suggesting that the regulation of Ang3 mRNA expression is different from that of Ang2.

Angiopoietins 3 and 4: diverging gene counterparts in mice and humans

The angiopoietins have recently joined the members of the vascular endothelial growth factor family as the only known growth factors largely specific for vascular endothelium. The angiopoietins include a naturally occurring agonist, angiopoietin-1, as well as a naturally occurring antagonist, angiopoietin-2, both of which act by means of the Tie2 receptor. We now report our attempts to use homology-based cloning approaches to identify new members of the angiopoietin family. These efforts have led to the identification of two new angiopoietins, angiopoietin-3 in mouse and angiopoietin-4 in human; we have also identified several more distantly related sequences that do not seem to be true angiopoietins, in that they do not bind to the Tie receptors. Although angiopoietin-3 and angiopoietin-4 are strikingly more structurally diverged from each other than are the mouse and human versions of angiopoietin-1 and angiopoietin-2, they appear to represent the mouse and human counterparts of the same gene locus, as revealed in our chromosomal localization studies of all of the angiopoietins in mouse and human. The structural divergence of angiopoietin-3 and angiopoietin-4 appears to underlie diverging functions of these counterparts. Angiopoietin-3 and angiopoietin-4 have very different distributions in their respective species, and angiopoietin-3 appears to act as an antagonist, whereas angiopoietin-4 appears to function as an agonist.

Molecular cloning and characterization of a novel angiopoietin family protein, angiopoietin-3

Using homology-based PCR, we have isolated cDNA encoding a novel member (491 amino acids) of the angiopoietin (Ang) family from human adult heart cDNA and have designated it angiopoietin-3 (Ang3). The NH2-terminal and COOH-terminal portions of Ang-3 contain the characteristic coiled-coil domain and fibrinogen-like domain that are conserved in other known Angs. Ang3 has a highly hydrophobic region at the N-terminus (approximately 21 amino acids) that is typical of a signal sequence for protein secretion. Ang3 mRNA is most abundant in adrenal gland, placenta, thyroid gland, heart and small intestine in human adult tissues. Additionally, Ang3 is a secretory protein, but is not a mitogen in endothelial cells.