The human VE-cadherin promoter is subjected to organ-specific regulation and is activated in tumour angiogenesis

Transcription factors regulating vasculogenesis and angiogenesis

Transcription factors (TFs) play a crucial role in regulating the dynamic and precise patterns of gene expression required for the initial specification of endothelial cells (ECs), and during endothelial growth and differentiation. While sharing many core features, ECs can be highly heterogeneous. Differential gene expression between ECs is essential to pattern the hierarchical vascular network into arteries, veins and capillaries, to drive angiogenic growth of new vessels, and to direct specialization in response to local signals. Unlike many other cell types, ECs have no single master regulator, instead relying on differing combinations of a necessarily limited repertoire of TFs to achieve tight spatial and temporal activation and repression of gene expression. Here, we will discuss the cohort of TFs known to be involved in directing gene expression during different stages of mammalian vasculogenesis and angiogenesis, with a primary focus on development.

Robust genome editing in adult vascular endothelium by nanoparticle delivery of CRISPR-Cas9 plasmid DNA

Vascular endothelium plays a crucial role in vascular homeostasis and tissue fluid balance. To target endothelium for robust genome editing, we developed poly(ethylene glycol) methyl ether-block-poly(lactide-co-glycolide) (PEG-b-PLGA) copolymer-based nanoparticle formulated with polyethyleneimine. A single i.v. administration of mixture of nanoparticles and plasmid DNA expressing Cas9 controlled by CDH5 promoter and guide RNA (U6 promoter) induced highly efficient genome editing in endothelial cells (ECs) of the vasculatures, including lung, heart, aorta, and peripheral vessels in adult mice. Western blotting and immunofluorescent staining demonstrated an ∼80% decrease of protein expression selectively in ECs, resulting in a phenotype similar to that of genetic knockout mice. Nanoparticle delivery of plasmid DNA could induce genome editing of two genes or genome editing and transgene expression in ECs simultaneously. Thus, nanoparticle delivery of plasmid DNA is a powerful tool to rapidly and efficiently alter expression of gene(s) in ECs for cardiovascular research and potential gene therapy.

Biomaterial-induced conversion of quiescent cardiomyocytes into pacemaker cells in rats

Pacemaker cells can be differentiated from stem cells or transdifferentiated from quiescent mature cardiac cells via genetic manipulation. Here we show that the exposure of rat quiescent ventricular cardiomyocytes to a silk-fibroin hydrogel activates the direct conversion of the quiescent cardiomyocytes to pacemaker cardiomyocytes by inducing the ectopic expression of the vascular endothelial cell-adhesion glycoprotein cadherin. The silk-fibroin-induced pacemaker cells exhibited functional and morphological features of genuine sinoatrial-node cardiomyocytes in vitro, and pacemaker cells generated via the injection of silk fibroin in the left ventricles of rats functioned as a surrogate in situ sinoatrial node. Biomaterials with suitable surface structure, mechanics and biochemistry could facilitate the scalable production of biological pacemakers for human use.

Cardioids reveal self-organizing principles of human cardiogenesis

Organoids capable of forming tissue-like structures have transformed our ability to model human development and disease. With the notable exception of the human heart, lineage-specific self-organizing organoids have been reported for all major organs. Here, we established self-organizing cardioids from human pluripotent stem cells that intrinsically specify, pattern, and morph into chamber-like structures containing a cavity. Cardioid complexity can be controlled by signaling that instructs the separation of cardiomyocyte and endothelial layers and by directing epicardial spreading, inward migration, and differentiation. We find that cavity morphogenesis is governed by a mesodermal WNT-BMP signaling axis and requires its target HAND1, a transcription factor linked to developmental heart chamber defects. Upon cryoinjury, cardioids initiated a cell-type-dependent accumulation of extracellular matrix, an early hallmark of both regeneration and heart disease. Thus, human cardioids represent a powerful platform to mechanistically dissect self-organization, congenital heart defects and serve as a foundation for future translational research.

A 3D iPSC-differentiation model identifies interleukin-3 as a regulator of early human hematopoietic specification

Hematopoietic development is spatiotemporally tightly regulated by defined cell-intrinsic and extrinsic modifiers. The role of cytokines has been intensively studied in adult hematopoiesis; however, their role in embryonic hematopoietic specification remains largely unexplored. Here, we used induced pluripotent stem cell (iPSC) technology and established a 3-dimensional (3D), organoid-like differentiation system (“hemanoid”) maintaining the structural cellular integrity to evaluate the effect of cytokines on embryonic hematopoietic development. We show that defined stages of early human hematopoietic development were recapitulated within the generated hemanoids. We identified KDR+/CD34^high/CD144+/CD43^–/CD45^– hemato-endothelial progenitors (HEP) forming organized, vasculature-like structures and giving rise to CD34^low/CD144^–/CD43⁺/CD45⁺ hematopoietic progenitor cells. We demonstrate that the endothelial to hematopoietic transition of HEP is dependent on the presence of interleukin 3 (IL-3). Inhibition of IL-3 signaling blocked hematopoietic differentiation and arrested the cells in the HEP stage. Thus, our data suggest an important role for IL-3 in early human hematopoiesis by supporting the endothelial to hematopoietic transition of HEP and highlight the potential of a hemanoid-based model to study human hematopoietic development.

VE-cadherin fusion protein substrate enhanced the vasculogenic mimicry capability of hepatocellular carcinoma cells

To investigate the VE-cadherin-based intercellular crosstalk in tumor cells, a fusion protein consisting of a human VE-cadherin extracellular domain and an immunoglobulin G Fc region (hVE-cad-Fc) was prepared as an artificial extracellular matrix (ECM) for the culture of hepatocellular carcinoma cells (Bel7402 cells). Compared with cells cultured on TCPS and collagen coated plates, the Bel7402 cells cultured on a series concentration of hVE-cad-Fc coated plates showed elongated morphology, inhibited proliferation, and significantly enhanced migration and secretion of ECM compounds and cytokines in a concentration-dependent manner. When the concentration of hVE-cad-Fc reached 15 μg mL-1, the Bel7402 cells formed a patterned network with positive-staining of PAS. The high-density hVE-cad-Fc substrate markedly elevated the expression of endogenous VE-cadherin and EphA2, and subsequently activated the intracellular signal transduction pathways including VE-cadherin/PI3K/MMPs and VE-cadherin/EphA2/FAK/p-VE-cadherin axes as well as epithelial-mesenchymal transition (EMT). Therefore, the high-density hVE-cad-Fc substrate was able to induce the vasculogenic mimicry (VM) formation of Bel7402 cells, and exhibit the potential for the construction of an in vitro tumor model of VM. Moreover, hVE-cad-Fc appeared to be a promising candidate for the investigation of VE-cadherin mediated cell-cell interactions in tumor development.

ETS factors are required but not sufficient for specific patterns of enhancer activity in different endothelial subtypes

Correct vascular differentiation requires distinct patterns of gene expression in different subtypes of endothelial cells. Members of the ETS transcription factor family are essential for the transcriptional activation of arterial and angiogenesis-specific gene regulatory elements, leading to the hypothesis that they play lineage-defining roles in arterial and angiogenic differentiation directly downstream of VEGFA signalling. However, an alternative explanation is that ETS binding at enhancers and promoters is a general requirement for activation of many endothelial genes regardless of expression pattern, with subtype-specificity provided by additional factors. Here we use analysis of Ephb4 and Coup-TFII (Nr2f2) vein-specific enhancers to demonstrate that ETS factors are equally essential for vein, arterial and angiogenic-specific enhancer activity patterns. Further, we show that ETS factor binding at these vein-specific enhancers is enriched by VEGFA signalling, similar to that seen at arterial and angiogenic enhancers. However, while arterial and angiogenic enhancers can be activated by VEGFA in vivo, the Ephb4 and Coup-TFII venous enhancers are not, suggesting that the specificity of VEGFA-induced arterial and angiogenic enhancer activity occurs via non-ETS transcription factors. These results support a model in which ETS factors are not the primary regulators of specific patterns of gene expression in different endothelial subtypes.

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Vein-specific enhancers can contain essential ETS motifs.

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VEGFA induced an increase in ETS binding at vein, arterial and angiogenic enhancers.

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VEGFA stimulation cannot induce vein-specific enhancer activity.

Sonoselective transfection of cerebral vasculature without blood-brain barrier disruption

Treatment of many pathologies of the brain could be improved markedly by the development of noninvasive therapeutic approaches that elicit robust, endothelial cell-selective gene expression in specific brain regions that are targeted under MR image guidance. While focused ultrasound (FUS) in conjunction with gas-filled microbubbles (MBs) has emerged as a noninvasive modality for MR image-guided gene delivery to the brain, it has been used exclusively to transiently disrupt the blood-brain barrier (BBB), which may induce a sterile inflammation response. Here, we introduce an MR image-guided FUS method that elicits endothelial-selective transfection of the cerebral vasculature (i.e., "sonoselective" transfection), without opening the BBB. We first determined that activating circulating, cationic plasmid-bearing MBs with pulsed low-pressure (0.1 MPa) 1.1-MHz FUS facilitates sonoselective gene delivery to the endothelium without MRI-detectable disruption of the BBB. The degree of endothelial selectivity varied inversely with the FUS pressure, with higher pressures (i.e., 0.3-MPa and 0.4-MPa FUS) consistently inducing BBB opening and extravascular transfection. Bulk RNA sequencing analyses revealed that the sonoselective low-pressure regimen does not up-regulate inflammatory or immune responses. Single-cell RNA sequencing indicated that the transcriptome of sonoselectively transfected brain endothelium was unaffected by the treatment. The approach developed here permits targeted gene delivery to blood vessels and could be used to promote angiogenesis, release endothelial cell-secreted factors to stimulate nerve regrowth, or recruit neural stem cells.

Loss of Renal Peritubular Capillaries in Hypertensive Patients Is Detectable by Urinary Endothelial Microparticle Levels

Hypertension, an important cause of chronic kidney disease, is characterized by peritubular capillary (PTC) loss. Circulating levels of endothelial microparticles (EMPs) reflect systemic endothelial injury. We hypothesized that systemic and urinary PTC-EMPs levels would reflect renal microvascular injury in hypertensive patients. We prospectively measured by flow cytometry renal vein, inferior vena cava, and urinary levels of EMPs in essential (n=14) and renovascular (RVH; n=24) hypertensive patients and compared them with peripheral blood and urinary levels in healthy volunteers (n=14). PTC-EMPs were identified as urinary exosomes positive for the PTC marker plasmalemmal-vesicle-associated protein. In 7 RVH patients, PTC and fibrosis were also quantified in renal biopsy, and in 18 RVH patients, PTC-EMPs were measured again 3 months after continued medical therapy with or without stenting (n=9 each). Renal vein and systemic PTC-EMPs levels were not different among the groups, whereas their urinary levels were elevated in both RVH and essential hypertension versus healthy volunteers (56.8%±12.7% and 62.8%±10.7% versus 34.0%±17.8%; both P≤0.001). Urinary PTC-EMPs levels correlated directly with blood pressure and inversely with estimated glomerular filtration rate. Furthermore, in RVH, urinary PTC-EMPs levels correlated directly with stenotic kidney hypoxia, histological PTC count, and fibrosis and inversely with cortical perfusion. Three months after treatment, the change in urinary PTC-EMPs levels correlated inversely with a change in renal function ( r=-0.582; P=0.011). Therefore, urinary PTC-EMPs levels are increased in hypertensive patients and may reflect renal microcirculation injury, whereas systemic PTC-EMPs levels are unchanged. Urinary PTC-EMPs may be useful as novel biomarkers of intrarenal capillary loss.

Generation of an immortalized human choroid endothelial cell line (iChEC-1) using an endothelial cell specific promoter

Age-related macular degeneration (AMD) is a common cause of blindness worldwide. While recent studies have revealed that the loss of choroidal endothelial cells (ChECs) is critical to the disease pathogenesis of dry AMD, in vitro studies are needed to fully elucidate the disease mechanism. However, these studies remain hindered due to the lack of publically available human ChEC lines. To address this need, ChECs were harvested form donor tissue and enriched for by using magnetic cell separation using anti-CD31 conjugated microbeads. Next, lenti-viral vectors with endothelial-specific promoters driving genes necessary for immortalization, CDH5p-hTERT and CDH5p TAg, were generated. Stable integration of both gene cassettes allowed cells to maintain their proliferative state and yielded an immortalized cell line (iChEC-1). Immunocytochemical analysis of iChEC-1 confirmed the expression of important ChEC markers such as CA4, a marker of choriocapillaris endothelial cells, CDH5, and CD34, pan-endothelial cell markers. qRT-PCR analysis of expanded clones from iChEC-1 further showed that the line maintained expression of other important endothelial markers, vWF, PECAM1, and PLVAP, similar to primary cells. Functional responses were characterized by tube-forming assays and repopulation of decellularized choroid with the immortalized cell line. In conclusion, the iChEC-1 line presents a suitable immortalized human ChEC line for future in vitro studies of AMD.

Anti-angiogenic agents for the treatment of solid tumors: Potential pathways, therapy and current strategies - A review

Recent strategies for the treatment of cancer, other than just tumor cell killing have been under intensive development, such as anti-angiogenic therapeutic approach. Angiogenesis inhibition is an important strategy for the treatment of solid tumors, which basically depends on cutting off the blood supply to tumor micro-regions, resulting in pan-hypoxia and pan-necrosis within solid tumor tissues. The differential activation of angiogenesis between normal and tumor tissues makes this process an attractive strategic target for anti-tumor drug discovery. The principles of anti-angiogenic treatment for solid tumors were originally proposed in 1972, and ever since, it has become a putative target for therapies directed against solid tumors. In the early twenty first century, the FDA approved anti-angiogenic drugs, such as bevacizumab and sorafenib for the treatment of several solid tumors. Over the past two decades, researches have continued to improve the performance of anti-angiogenic drugs, describe their drug interaction potential, and uncover possible reasons for potential treatment resistance. Herein, we present an update to the pre-clinical and clinical situations of anti-angiogenic agents and discuss the most recent trends in this field.

Endothelial p110γPI3K Mediates Endothelial Regeneration and Vascular Repair After Inflammatory Vascular Injury

BACKGROUND

The integrity of endothelial monolayer is a sine qua non for vascular homeostasis and maintenance of tissue-fluid balance. However, little is known about the signaling pathways regulating regeneration of the endothelial barrier after inflammatory vascular injury.

METHODS AND RESULTS

Using genetic and pharmacological approaches, we demonstrated that endothelial regeneration selectively requires activation of p110γPI3K signaling, which thereby mediates the expression of the endothelial reparative transcription factor Forkhead box M1 (FoxM1). We observed that FoxM1 induction in the pulmonary vasculature was inhibited in mice treated with a p110γ-selective inhibitor and in Pik3cg(-/-) mice after lipopolysaccharide challenge. Pik3cg(-/-) mice exhibited persistent lung inflammation induced by sepsis and sustained increase in vascular permeability. Restoration of expression of either p110γ or FoxM1 in pulmonary endothelial cells of Pik3cg(-/-) mice restored endothelial regeneration and normalized the defective vascular repair program. We also observed diminished expression of p110γ in pulmonary vascular endothelial cells of patients with acute respiratory distress syndrome, suggesting that impaired p110γ-FoxM1 vascular repair signaling pathway is a critical factor in persistent leaky lung microvessels and edema formation in the disease.

CONCLUSIONS

We identify p110γ as the critical mediator of endothelial regeneration and vascular repair after sepsis-induced inflammatory injury. Thus, activation of p110γ-FoxM1 endothelial regeneration may represent a novel strategy for the treatment of inflammatory vascular diseases.

Anti-angiogenic alternatives to VEGF blockade

Angiogenesis is a major requirement for tumour formation and development. Anti-angiogenic treatments aim to starve the tumour of nutrients and oxygen and also guard against metastasis. The main anti-angiogenic agents to date have focused on blocking the pro-angiogenic vascular endothelial growth factors (VEGFs). While this approach has seen some success and has provided a proof of principle that such anti-angiogenic agents can be used as treatment, the overall outcome of VEGF blockade has been somewhat disappointing. There is a current need for new strategies in inhibiting tumour angiogenesis; this article will review current and historical examples in blocking various membrane receptors and components of the extracellular matrix important in angiogenesis. Targeting these newly discovered pro-angiogenic proteins could provide novel strategies for cancer therapy.

Development of Endothelial-Specific Single Inducible Lentiviral Vectors for Genetic Engineering of Endothelial Progenitor Cells

Endothelial progenitor cells (EPC) are able to migrate to tumor vasculature. These cells, if genetically modified, can be used as vehicles to deliver toxic material to, or express anticancer proteins in tumor. To test this hypothesis, we developed several single, endothelial-specific, and doxycycline-inducible self-inactivating (SIN) lentiviral vectors. Two distinct expression cassettes were inserted into a SIN-vector: one controlled by an endothelial lineage-specific, murine vascular endothelial cadherin (mVEcad) promoter for the expression of a transactivator, rtTA2S-M2; and the other driven by an inducible promoter, TREalb, for a firefly luciferase reporter gene. We compared the expression levels of luciferase in different vector constructs, containing either the same or opposite orientation with respect to the vector sequence. The results showed that the vector with these two expression cassettes placed in opposite directions was optimal, characterized by a robust induction of the transgene expression (17.7- to 73-fold) in the presence of doxycycline in several endothelial cell lines, but without leakiness when uninduced. In conclusion, an endothelial lineage-specific single inducible SIN lentiviral vector has been developed. Such a lentiviral vector can be used to endow endothelial progenitor cells with anti-tumor properties.

Catalpol downregulates vascular endothelial‑cadherin expression and induces vascular hyperpermeability

Catalpol, an iridiod glucoside isolated from Rehmannia glutinosa, has been reported to possess anti‑inflammatory properties. However, the molecular mechanisms underlying this effect have not been fully elucidated. This study aimed to investigate the effects of catalpol on vascular permeability. Using Transwell permeability assays and measurements of trans‑endothelial electrical resistance (TEER), it was demonstrated that 1 mM catalpol induces a significant increase in the permeability of the monolayers of human umbilical vein endothelial cells (HUVECs). Western blotting and immunofluorescence demonstrated that catalpol inhibits the expression of vascular endothelial (VE)‑cadherin, the key component of adherens junctions, but not occludin, the major constituent of tight junctions. In addition, catalpol inhibits the ETS transcription factor ERG, a positive regulator of VE‑cadherin. Knockdown of ERG expression compromised the catalpol‑induced reduction of TEER in HUVECs. The present study revealed a novel effect of catalpol on vascular permeability and gave insight into the multifaceted roles of catalpol in inflammation.

Endothelial permeability and VE-cadherin: a wacky comradeship

The endothelium forms a selective semi-permeable barrier controlling bidirectional transfer between blood vessel and irrigated tissues. This crucial function relies on the dynamic architecture of endothelial cell–cell junctions, and in particular, VE -cadherin-mediated contacts. VE -cadherin indeed chiefly organizes the opening and closing of the endothelial barrier, and is central in permeability changes. In this review, the way VE -cadherin-based contacts are formed and maintained is first presented, including molecular traits of its expression, partners, and signaling. In a second part, the mechanisms by which VE -cadherin adhesion can be disrupted, leading to cell–cell junction weakening and endothelial permeability increase, are described. Overall, the molecular basis for VE -cadherin control of the endothelial barrier function is of high interest for biomedical research, as vascular leakage is observed in many pathological conditions and human diseases.

Brain vascular pericytes following ischemia have multipotential stem cell activity to differentiate into neural and vascular lineage cells

Brain vascular pericytes (PCs) are a key component of the blood-brain barrier (BBB)/neurovascular unit, along with neural and endothelial cells. Besides their crucial role in maintaining the BBB, increasing evidence shows that PCs have multipotential stem cell activity. However, their multipotency has not been considered in the pathological brain, such as after an ischemic stroke. Here, we examined whether brain vascular PCs following ischemia (iPCs) have multipotential stem cell activity and differentiate into neural and vascular lineage cells to reconstruct the BBB/neurovascular unit. Using PCs extracted from ischemic regions (iPCs) from mouse brains and human brain PCs cultured under oxygen/glucose deprivation, we show that PCs developed stemness presumably through reprogramming. The iPCs revealed a complex phenotype of angioblasts, in addition to their original mesenchymal properties, and multidifferentiated into cells from both a neural and vascular lineage. These data indicate that under ischemic/hypoxic conditions, PCs can acquire multipotential stem cell activity and can differentiate into major components of the BBB/neurovascular unit. Thus, these findings support the novel concept that iPCs can contribute to both neurogenesis and vasculogenesis at the site of brain injuries.

γ-Actin plays a key role in endothelial cell motility and neovessel maintenance

Background

Angiogenesis plays a crucial role in development, wound healing as well as tumour growth and metastasis. Although the general implication of the cytoskeleton in angiogenesis has been partially unravelled, little is known about the specific role of actin isoforms in this process. Herein, we aimed at deciphering the function of γ-actin in angiogenesis.

Methods

Localization of β- and γ-actin in vascular endothelial cells was investigated by co-immunofluorescence staining using monoclonal antibodies, followed by the functional analysis of γ-actin using siRNA. The impact of γ-actin knockdown on the random motility and morphological differentiation of endothelial cells into vascular networks was investigated by timelapse videomicroscopy while the effect on chemotaxis was assessed using modified Boyden chambers. The implication of VE-cadherin, VEGFR-2 and ROCK signalling was then examined by Western blotting and using pharmacological inhibitors.

Results

The two main cytoplasmic isoforms of actin strongly co-localized in vascular endothelial cells, albeit with some degree of spatial preference. While β-actin knockdown was not achievable without major cytotoxicity, γ-actin knockdown did not alter the viability of endothelial cells. Timelapse videomicroscopy experiments revealed that γ-actin knockdown cells were able to initiate morphological differentiation into capillary-like tubes but were unable to maintain these structures, which rapidly regressed. This vascular regression was associated with altered regulation of VE-cadherin expression. Interestingly, knocking down γ-actin expression had no effect on endothelial cell adhesion to various substrates but significantly decreased their motility and migration. This anti-migratory effect was associated with an accumulation of thick actin stress fibres, large focal adhesions and increased phosphorylation of myosin regulatory light chain, suggesting activation of the ROCK signalling pathway. Incubation with ROCK inhibitors, H-1152 and Y-27632, completely rescued the motility phenotype induced by γ-actin knockdown but only partially restored the angiogenic potential of endothelial cells.

Conclusions

Our study thus demonstrates for the first time that β-actin is essential for endothelial cell survival and γ-actin plays a crucial role in angiogenesis, through both ROCK-dependent and -independent mechanisms. This provides new insights into the role of the actin cytoskeleton in angiogenesis and may open new therapeutic avenues for the treatment of angiogenesis-related disorders.

Electronic supplementary material

The online version of this article (doi:10.1186/s13221-014-0027-2) contains supplementary material, which is available to authorized users.

Endothelial permeability and VE-cadherin: a wacky comradeship

The endothelium forms a selective semi-permeable barrier controlling bidirectional transfer between blood vessel and irrigated tissues. This crucial function relies on the dynamic architecture of endothelial cell-cell junctions, and in particular, VE-cadherin-mediated contacts. VE-cadherin indeed chiefly organizes the opening and closing of the endothelial barrier, and is central in permeability changes. In this review, the way VE-cadherin-based contacts are formed and maintained is first presented, including molecular traits of its expression, partners, and signaling. In a second part, the mechanisms by which VE-cadherin adhesion can be disrupted, leading to cell-cell junction weakening and endothelial permeability increase, are described. Overall, the molecular basis for VE-cadherin control of the endothelial barrier function is of high interest for biomedical research, as vascular leakage is observed in many pathological conditions and human diseases.

The C-terminus region of β-arrestin1 modulates VE-cadherin expression and endothelial cell permeability

Background

The endothelial specific cell-cell adhesion molecule, VE-cadherin, modulates barrier function and vascular homeostasis. In this context, we have previously characterized that VEGF (vascular endothelial growth factor) leads to VE-cadherin phosphorylation, β-arrestin2 recruitment and VE-cadherin internalization in mouse endothelial cells. However, exactly how this VE-cadherin/β-arrestin complex contributes to VEGF-mediated permeability in human endothelial cells remains unclear. In this study, we investigated in-depth the VE-cadherin/β-arrestin interactions in human endothelial cells exposed to VEGF.

Findings

First, we demonstrated that VEGF induces VE-cadherin internalization in a clathrin-dependent manner in human umbilical vein endothelial cells (HUVEC). In addition to the classical components of endocytic vesicles, β-arrestin1 was recruited and bound to phosphorylated VE-cadherin. Molecular mapping of this interaction uncovered that the C-terminus tail of β-arrestin1, that comprises amino acids 375 to 418, was sufficient to directly interact with the phosphorylated form of VE-cadherin. Interestingly, the expression of the C-terminus tail of β-arrestin1 induced loss of surface exposed-VE-cadherin, promoted monolayer disorganization and enhanced permeability. Finally, this effect relied on decreased VE-cadherin expression at the transcriptional level, through inhibition of its promoter activity.

Conclusions

Altogether, our results demonstrate that β-arrestin1 might play multiple functions collectively contributing to endothelial barrier properties. Indeed, in addition to a direct implication in VE-cadherin endocytosis, β-arrestin1 could also control VE-cadherin transcription and expression. Ultimately, understanding the molecular mechanisms involved in VE-cadherin function might provide therapeutic tools for many human diseases where the vascular barrier is compromised.

A mechanistic role for DNA methylation in endothelial cell (EC)-enriched gene expression: relationship with DNA replication timing

Proximal promoter DNA methylation has been shown to be important for regulating gene expression. However, its relative contribution to the cell-specific expression of endothelial cell (EC)-enriched genes has not been defined. We used methyl-DNA immunoprecipitation and bisulfite conversion to analyze the DNA methylation profile of EC-enriched genes in ECs vs nonexpressing cell types, both in vitro and in vivo. We show that prototypic EC-enriched genes exhibit functional differential patterns of DNA methylation in proximal promoter regions of most (eg, CD31, von Willebrand factor [vWF], VE-cadherin, and intercellular adhesion molecule-2), but not all (eg, VEGFR-1 and VEGFR-2), EC-enriched genes. Comparable findings were evident in cultured ECs, human blood origin ECs, and murine aortic ECs. Promoter-reporter episomal transfection assays for endothelial nitric oxide synthase, VE-cadherin, and vWF indicated functional promoter activity in cell types where the native gene was not active. Inhibition of DNA methyltransferase activity indicated important functional relevance. Importantly, profiling DNA replication timing patterns indicated that EC-enriched gene promoters with differentially methylated regions replicate early in S-phase in both expressing and nonexpressing cell types. Collectively, these studies highlight the functional importance of promoter DNA methylation in controlling vascular EC gene expression.

Lycorine hydrochloride inhibits metastatic melanoma cell-dominant vasculogenic mimicry

Melanoma cells actively participate in tumor angiogenesis and vasculogenic mimicry. However, anti-angiogenic therapy in patients with melanoma has not shown a significant survival gain. Thus, new anti-melanoma angiogenic and vasculogenic drugs are highly desired. Using the metastatic melanoma cell line C8161 as a model, we explored melanoma vasculogenic inhibitors and found that lycorine hydrochloride (LH) effectively suppressed C8161 cell-dominant formation of capillary-like tubes in vitro and generation of tumor blood vessels in vivo with low toxicity. Mechanistic studies revealed that LH markedly hindered expression of VE-cadherin in C8161 cells, but did not affect expression of six other important angiogenic and vasculogenic genes. Luciferase assays showed that LH significantly impeded promoter activity of the VE-cadherin gene in a dose-dependent manner. Together, these data suggest that LH inhibits melanoma C8161 cell-dominant vasculogenic mimicry by reducing VE-cadherin gene expression and diminishing cell surface exposure of the protein.

iNOS activation regulates β-catenin association with its partners in endothelial cells

BACKGROUND

Signals that disrupt β-catenin association to cadherins may influence the translocation of β-catenin to the nucleus to regulate transcription. Post-translational modification of proteins is a signalling event that may lead to changes in structural conformation, association or function of the target proteins. NO and its derivatives induce nitration of proteins during inflammation. It has been described that animals treated with NO donors showed increased permeability due to modulation of VE-cadherin/catenin complex. We, therefore, aim to evaluate the effect of iNOS activation on the expression, nuclear localisation and function of β-catenin in endothelial cells.

METHODOLOGY/PRINCIPAL FINDINGS

Expression, nuclear localisation, post-translational modifications and function of β-catenin was analysed by cell fractionation, immunoprecipitation, immunoblots, QRT-PCR and permeability assays in murine endothelial cells (H5V). Influence of macrophage activation on expression of VE-cadherin/p120-catenin/β-catenin complex in co-cultured H5V cells was also assessed. Activation of macrophages to produce NO provoked a decrease in VE-cadherin/p120-catenin/β-catenin expression in H5V cells. Phosphorylation of β-catenin, p120-catenin and VE-cadherin, and reduction in the barrier properties of the cell monolayer was associated with iNOS induction. Moreover, high NO levels provoked nitration of β-catenin, and induced its translocation to the nucleus. In the nucleus of NOS activated cells, nitration levels of β-catenin influenced its association with TCF4 and p65 proteins. High levels of NO altered β-catenin mediated gene expression of NFκB and Wnt target genes without affecting cell viability.

CONCLUSIONS

NOS activity modulates β-catenin post-translational modifications, function and its association with different partners to promote endothelial cell survival. Therapeutic manipulation of iNOS levels may remove a critical cytoprotective mechanism of importance in tumour angiogenesis.

Placenta growth factor-1 exerts time-dependent stabilization of adherens junctions following VEGF-induced vascular permeability

Increased vascular permeability is an early event characteristic of tissue ischemia and angiogenesis. Although VEGF family members are potent promoters of endothelial permeability the role of placental growth factor (PlGF) is hotly debated. Here we investigated PlGF isoforms 1 and 2 and present in vitro and in vivo evidence that PlGF-1, but not PlGF-2, can inhibit VEGF-induced permeability but only during a critical window post-VEGF exposure. PlGF-1 promotes VE-cadherin expression via the trans-activating Sp1 and Sp3 interaction with the VE-cadherin promoter and subsequently stabilizes transendothelial junctions, but only after activation of endothelial cells by VEGF. PlGF-1 regulates vascular permeability associated with the rapid localization of VE-cadherin to the plasma membrane and dephosphorylation of tyrosine residues that precedes changes observed in claudin 5 tyrosine phosphorylation and membrane localization. The critical window during which PlGF-1 exerts its effect on VEGF-induced permeability highlights the importance of the translational significance of this work in that PLGF-1 likely serves as an endogenous anti-permeability factor whose effectiveness is limited to a precise time point following vascular injury. Clinical approaches that would pattern nature's approach would thus limit treatments to precise intervals following injury and bring attention to use of agents only during therapeutic windows.

Wilms' tumour protein Wt1 stimulates transcription of the gene encoding vascular endothelial cadherin

The Wilms' tumour gene, Wt1, encodes a zinc finger protein, which is mutated in a subset of paediatric renal carcinomas known as Wilms' tumours (nephroblastomas). Recent findings indicate that Wt1, beside its role in genitourinary development, is also necessary for normal vascularisation of the embryonic heart, and may even be involved in tumour angiogenesis. The original purpose of this study was to decipher potential downstream signalling pathways of Wt1 for blood vessel formation. We found that the Wt1(-KTS) protein, which functions as a transcription factor, stimulated the expression of cadherin 5 (CDH5, vascular endothelial (VE) cadherin) and other vascular genes, i.e. those encoding vascular endothelial growth factor receptors 1 and 2, and angiopoietin-2. Furthermore, an enhancer element was identified in the first intron of the CDH5 gene, which bound to the Wt1(-KTS) protein and was necessary for reporter gene activation by Wt1(-KTS) in transiently transfected cell lines. Wt1 and VE-cadherin proteins could be co-localised by double immunofluorescence staining in maturating glomeruli of embryonic murine kidneys. VE-cadherin transcripts were reduced in some but not all tissues of Wt1-deficient mouse embryos. These results indicate that Wt1 can stimulate vascular gene transcription. By demonstrating that Wt1(-KTS) protein trans-activates an enhancer element in the first intron we identified CDH5 as a novel target gene of Wt1. It is suggested that transcriptional activation of CDH5 by Wt1 fulfils regulatory functions during vascular development and kidney formation.

Bioinformatic identification and characterization of human endothelial cell-restricted genes

Background

In this study, we used a systematic bioinformatics analysis approach to elucidate genes that exhibit an endothelial cell (EC) restricted expression pattern, and began to define their regulation, tissue distribution, and potential biological role.

Results

Using a high throughput microarray platform, a primary set of 1,191 transcripts that are enriched in different primary ECs compared to non-ECs was identified (LCB >3, FDR <2%). Further refinement of this initial subset of transcripts, using published data, yielded 152 transcripts (representing 109 genes) with different degrees of EC-specificity. Several interesting patterns emerged among these genes: some were expressed in all ECs and several were restricted to microvascular ECs. Pathway analysis and gene ontology demonstrated that several of the identified genes are known to be involved in vasculature development, angiogenesis, and endothelial function (P < 0.01). These genes are enriched in cardiovascular diseases, hemorrhage and ischemia gene sets (P < 0.001). Most of the identified genes are ubiquitously expressed in many different tissues. Analysis of the proximal promoter revealed the enrichment of conserved binding sites for 26 different transcription factors and analysis of the untranslated regions suggests that a subset of the EC-restricted genes are targets of 15 microRNAs. While many of the identified genes are known for their regulatory role in ECs, we have also identified several novel EC-restricted genes, the function of which have yet to be fully defined.

Conclusion

The study provides an initial catalogue of EC-restricted genes most of which are ubiquitously expressed in different endothelial cells.

Repressors NFI and NFY participate in organ-specific regulation of von Willebrand factor promoter activity in transgenic mice

OBJECTIVE

To determine the role of repressors in cell type and organ-specific activation of von Willebrand factor (VWF) promoter sequences -487 to 247 in vivo.

METHODS AND RESULTS

Activation patterns of wild-type and mutant VWF promoters (sequences -487 to 247) containing mutations in repressors nuclear factor-I (NFI)- and nuclear factor Y (NFY)-binding sites were analyzed in transgenic mice. Mutation of the NFI-binding site activated the promoter in heart and lung endothelial cells, whereas mutation of the NFY-binding site activated the promoter in kidney vasculature. Immunofluorescence analyses showed that NFIB was predominant in heart and lung endothelial cells, whereas NFIX was predominantly detected in kidney endothelial cell nuclei. By using chromatin immunoprecipitation, we demonstrated that the distal lung-specific enhancer (containing a YY1 site) of the VWF gene is brought in proximity to the NFI binding site.

CONCLUSIONS

The NFI and NFY repressors contribute differentially to organ-specific regulation of the VWF promoter, and the organ-specific action of NFI may reflect its organ-specific isoform distribution. In addition, the lung-specific enhancer region of the endogenous VWF gene may inhibit NFI repressor function through chromatin looping, which can approximate the 2 regions.

[Genes that make the endothelial identity]

The endothelium is a tissue with a distinct identity due to the specific expression of molecular markers by endothelial cells. Further, the endothelium displays a structural heterogeneity illustrated by the expression of specific markers in arteries and in veins. Here, we present a review of the transcriptional and epigenetic mechanisms regulating the expression of the main markers of endothelial cells in man and mouse, demonstrating that there is no common and unique mechanism of specific expression of genes in these cells.

[The bHLH TAL1 protein: a key molecule in the hematopoietic and endothelial systems]

The formation of blood cells and vascular networks occurs simultaneously during development, and both lineages remain in close association in all adult tissues. The functional setting of both systems within the embryo and their renewal during adult life are highly complex processes, and require the involvement of numerous molecular actors, the activities of which are often overlapping. Here, I review the activity of TAL-1, a basic-helix-loop-helix transcription factor, which plays a key role in the formation and functioning of both blood and endothelial systems, with a particular emphasis on recent data that associate TAL-1 with angiogenesis.

Transcriptional control of endothelial cell development

The transcription factors that regulate endothelial cell development have been a focus of active research for several years, and many players in the endothelial transcriptional program have been identified. This review discusses the function of several major regulators of endothelial transcription, including members of the Sox, Ets, Forkhead, GATA, and Kruppel-like families. This review also highlights recent developments aimed at unraveling the combinatorial mechanisms and transcription factor interactions that regulate endothelial cell specification and differentiation during vasculogenesis and angiogenesis.

Combinatorial regulation of endothelial gene expression by ets and forkhead transcription factors

Vascular development begins when mesodermal cells differentiate into endothelial cells, which then form primitive vessels. It has been hypothesized that endothelial-specific gene expression may be regulated combinatorially, but the transcriptional mechanisms governing specificity in vascular gene expression remain incompletely understood. Here, we identify a 44 bp transcriptional enhancer that is sufficient to direct expression specifically and exclusively to the developing vascular endothelium. This enhancer is regulated by a composite cis-acting element, the FOX:ETS motif, which is bound and synergistically activated by Forkhead and Ets transcription factors. We demonstrate that coexpression of the Forkhead protein FoxC2 and the Ets protein Etv2 induces ectopic expression of vascular genes in Xenopus embryos, and that combinatorial knockdown of the orthologous genes in zebrafish embryos disrupts vascular development. Finally, we show that FOX:ETS motifs are present in many known endothelial-specific enhancers and that this motif is an efficient predictor of endothelial enhancers in the human genome.

Tumor-induced upregulation of Twist, Snail, and Slug represses the activity of the human VE-cadherin promoter

Endothelial integrity is dependent on intracellular adherens junctions formed by complexes of vascular endothelial (VE)-cadherin and catenins. We have previously demonstrated that exposing endothelial cells (EC) to breast cancer cell-conditioned media (CM) for 24h results in a reduction in VE-cadherin protein and mRNA levels. Herein, we examined the mechanism(s) involved in the downregulation of VE-cadherin by CM. Human dermal microvascular EC exposed to CM showed a downregulation in VE-cadherin promoter activity and upregulation of Twist, Slug, and Snail expression. Reporter gene analysis demonstrated a direct repression of the VE-cadherin promoter by Slug, Snail, and Twist expression plasmids. At least two E-box motifs appear to be involved in this regulatory process as shown by electrophoretic mobility shift assays. These results suggest that factors released by breast cancer cells are able to upregulate Twist, Slug, and Snail expression in EC, which in turn downregulate the activity of the VE-cadherin promoter.

Slug is a direct Notch target required for initiation of cardiac cushion cellularization

Snail family proteins are key regulators of epithelial-mesenchymal transition, but their role in endothelial-to-mesenchymal transition (EMT) is less well studied. We show that Slug, a Snail family member, is expressed by a subset of endothelial cells as well as mesenchymal cells of the atrioventricular canal and outflow tract during cardiac cushion morphogenesis. Slug deficiency results in impaired cellularization of the cardiac cushion at embryonic day (E)–9.5 but is compensated by increased Snail expression at E10.5, which restores cardiac cushion EMT. We further demonstrate that Slug, but not Snail, is directly up-regulated by Notch in endothelial cells and that Slug expression is required for Notch-mediated repression of the vascular endothelial cadherin promoter and for promoting migration of transformed endothelial cells. In contrast, transforming growth factor β (TGF-β) induces Snail but not Slug. Interestingly, activation of Notch in the context of TGF-β stimulation results in synergistic up-regulation of Snail in endothelial cells. Collectively, our data suggest that combined expression of Slug and Snail is required for EMT in cardiac cushion morphogenesis.

Role of Sox-9, ER81 and VE-cadherin in retinoic acid-mediated trans-differentiation of breast cancer cells

Many aspects of development, tumor growth and metastasis depend upon the provision of an adequate vasculature. This can be a result of regulated angiogenesis, recruitment of circulating endothelial progenitors and/or vascular trans-differentiation. The present study demonstrates that treatment of SKBR-3 breast cancer cells with retinoic acid (RA), an important regulator of embryogenesis, cancer and other diseases, stimulates the formation of networks in Matrigel. RA-treatment of SKBR-3 cells co-cultured with human umbilical vein endothelial cells resulted in the formation of mixed structures. RA induces expression of many endothelial genes including vascular endothelial (VE) cadherin. VE-cadherin was also induced by RA in a number of other breast cancer cells. We show that RA-induced VE-cadherin is responsible for the RA-induced morphological changes. RA rapidly induced the expression of Sox-9 and ER81, which in turn form a complex on the VE-cadherin promoter and are required to mediate the transcriptional regulation of VE-cadherin by RA. These data indicate that RA may promote the expression of endothelial genes resulting in endothelial-like differentiation, or provide a mechanism whereby circulating endothelial progenitor cells could be incorporated into a growing organ or tumor.

gamma-Secretase: a multifaceted regulator of angiogenesis

Physiological angiogenesis is essential for development, homeostasis and tissue repair but pathological neovascularization is a major feature of tumours, rheumatoid arthritis and ocular complications. Studies over the last decade have identified γ-secretase, a presenilin-dependent protease, as a key regulator of angiogenesis through: (i) regulated intramembrane proteolysis and transmembrane cleavage of receptors (e.g. VEGFR-1, Notch, ErbB-4, IGFI-R) followed by translocation of the intracellular domain to the nucleus, (ii) translocation of full length membrane-bound receptors to the nucleus (VEGFR-1), (iii) phosphorylation of membrane bound proteins (VEGFR-1 and ErbB-4), (iv) modulation of adherens junctions (cadherin) and regulation of permeability and (v) cleavage of amyloid precursor protein to amyloid-β which is able to regulate the angiogenic process. The γ-secretase-induced translocation of receptors to the nucleus provides an alternative intracellular signalling pathway, which acts as a potent regulator of transcription. γ-secretase is a complex composed of four different integral proteins (presenilin, nicastrin, Aph-1 and Pen-2), which determine the stability, substrate binding, substrate specificity and proteolytic activity of γ-secretase. This seeming complexity allows numerous possibilities for the development of targeted γ-secretase agonists/antagonists, which can specifically regulate the angiogenic process. This review will consider the structure and function of γ-secretase, the growing evidence for its role in angiogenesis and the substrates involved, γ-secretase as a therapeutic target and future challenges in this area.

Transcription factor Erg regulates angiogenesis and endothelial apoptosis through VE-cadherin

Tight regulation of the balance between apoptosis and survival is essential in angiogenesis. The ETS transcription factor Erg is required for endothelial tube formation in vitro. Inhibition of Erg expression in human umbilical vein endothelial cells (HUVECs), using antisense oligonucleotides, resulted in detachment of cell-cell contacts and increased cell death. Inhibition of Erg expression by antisense in HUVECs also lowered expression of the adhesion molecule vascular endothelial (VE)-cadherin, a key regulator of endothelial intercellular junctions and survival. Using chromatin immunoprecipitation, we showed that Erg binds to the VE-cadherin promoter. Furthermore, Erg was found to enhance VE-cadherin promoter activity in a transactivation assay. Apoptosis induced by inhibition of Erg was partly rescued by overexpression of VE-cadherin-GFP, suggesting that VE-cadherin is involved in the Erg-dependent survival signals. To show the role of Erg in angiogenesis in vivo, we used siRNA against Erg in a Matrigel plug model. Erg inhibition resulted in a significant decrease in vascularization, with increase in caspase-positive endothelial cells (ECs). These results identify a new pathway regulating angiogenesis and endothelial survival, via the transcription factor Erg and the adhesion molecule VE-cadherin.

Sequences in intron 51 of the von Willebrand factor gene target promoter activation to a subset of lung endothelial cells in transgenic mice

In vivo analyses of the VWF promoter previously demonstrated that a fragment spanning sequences -487 to +247 targets promoter activation to brain vascular endothelial cells, whereas a longer fragment including 2182 bp of the 5'-flanking sequences, the first exon, and the first intron activated expression in endothelial cells of the heart and muscles as well as the brain of transgenic mice. These results suggested that additional VWF gene sequences were required for expression in other vascular endothelial cells in vivo. We have now identified a region within intron 51 of the VWF gene that is DNase I-hypersensitive (HSS) specifically in non-endothelial cells and interacts with endothelial and non-endothelial specific complexes that contain YY1. We demonstrate that beta-actin is associated with YY1 specifically in the nucleus of non-endothelial cells and is a component of the nuclear protein complexes that interact with the DNase I-hypersensitive region. In vitro transfection analyses demonstrated that HSS sequences containing this YY1-binding site do not significantly affect VWF promoter activity. However, in vivo analyses demonstrated that addition of these sequences to the VWF promoter (-487 to +247) results in promoter activation in lung and brain vascular endothelial cells. These results demonstrate that the HSS sequences in intron 51 of the VWF gene contain cis-acting elements that are necessary for the VWF gene transcription in a subset of lung endothelial cells in vivo.

JNK signaling pathway is required for bFGF-mediated surface cadherin downregulation on HUVEC

Angiogenesis, the process of new blood vessel formation, is important in wound healing, inflammation, tumorigenesis and metastases. During this process, it is a critical step of the loosening of cellular interactions between endothelial cells, which are dependent on the architecture of adherens junction constructed by homophilic interactions of cell surface cadherins. Several studies suggested that the dynamic changes of cadherins are necessary during angiogenesis. However, the mechanism of cadherins regulation on endothelial cells requires further delineation. Here, we showed that basic fibroblast growth factor (bFGF), a pivotal pro-angiogenic factor, can downregulate typical cadherins (E-, N-, P- and VE-cadherin) expression on the surface of human umbilical vein endothelial cells (HUVECs) via FGF receptor 1 (FGFR1) signaling. The bFGF-mediated surface cadherin downregulation was significantly reversed only when the HUVECs were treated with JNK inhibitor (SP600125), but not ERK (PD98059) or p38 inhibitor (SB203580). Infecting HUVECs with a dominant negative H-Ras mutant (Ras(S17N)) interferes bFGF-mediated cadherin downregulation, and the result suggests that bFGF attenuates surface cadherin expression on HUVECs via FGFR1 and intracellular Ras-JNK signaling. However, after growth factors withdrawal, FGFR1 blockade or JNK inhibition for 16 h, cadherins were re-expressed on cell surface of HUVECs. But the mRNA or total protein of cadherins had no significant change, suggesting that the effect of bFGF on cadherin expression may work through a post-translational control. Our data first suggest that JNK participates in bFGF-mediated surface cadherin downregulation. Loss of surface cadherins may affect the cell-cell interaction between endothelial cells and facilitate angiogenesis.

Non-invasive in vivo optical imaging of the lacZ and luc gene expression in mice

The bacterial lacZ gene encoding for beta-galactosidase (beta-gal) is a common reporter gene used in transgenic mice. Nonetheless, the absence of fluorigenic substrates usable in live animals greatly hampered the non-invasive follow-up of this reporter gene expression. We used far-red fluorescence for imaging beta-Gal expression in live cells in vitro or in vivo. The 9H-(1,3-dichloro-9,9-dimethylacridin- 2-one-7-yl) beta-D-galactopyranoside substrate was used to monitor beta-Gal expression as a reporter of tumor growth, or of the physiological levels of an endogenous gene or of gene transfer in lung. A quantitative evaluation of this method as well as a comparison of its sensitivity with Firefly Luciferase-based bioluminescence was also performed. In vivo measurements showed that 10(3) beta-Gal tumor cells located under the skin were detectable. In deeper organs like lung, as little as 5 ng of beta-Gal or Luciferase enzymes per mg of proteins were measured, confirming that both techniques reached similar sensibilities. Nonetheless, quantitative comparison of beta-Gal levels measured with far-red imaging or with a standardized enzymatic evaluation after killing revealed that the 2D-fluorescent reflectance imaging method is submitted to a color-dependent disparity of the organs and cannot supply quantitative measurements but that a simple correction can be applied.

Endothelial adherens and tight junctions in vascular homeostasis, inflammation and angiogenesis

Endothelial cells lining the vessel wall are connected by adherens, tight and gap junctions. These junctional complexes are related to those found at epithelial junctions but with notable changes in terms of specific molecules and organization. Endothelial junctional proteins play important roles in tissue integrity but also in vascular permeability, leukocyte extravasation and angiogenesis. In this review, we will focus on specific mechanisms of endothelial tight and adherens junctions.

Expression of vascular endothelial-cadherin in human hepatocellular carcinoma tissues

AIM

Angiogenesis is important in tumor growth and progression to metastasis. Vascular endothelial (VE)-cadherin is an endothelial cell-specific cadherin required for angiogenesis, but its expression in hepatocellular carcinoma (HCC) tissues has not been examined.

METHODS

Expression of VE-cadherin was analyzed in 31 HCC frozen tissue specimens by immunohistochemical and immunoelectron microscopic procedures. In addition, the association of its expression with clinicopathological parameters was investigated to determine the possible diagnostic or predictive value of VE-cadherin expression in neoplastic and non-neoplastic liver lesions.

RESULTS

Immunoreactive VE-cadherin expression was faint or barely detectable on sinusoidal endothelial cells of normal liver but was evident on sinusoidal or capillary endothelium of chronic hepatitis, cirrhosis, and HCC tissues. VE-cadherin expression was more intense on capillary endothelium of HCC tissues in 26 (84%) of 31 patients than on sinusoidal endothelium of surrounding non-tumorous liver tissues with chronic liver diseases. The intensity or intracapillary extent of positive stain for VE-cadherin on capillary endothelium of HCC tissues was significantly associated with tumor size, capsular invasion and tumor cell differentiation in HCC.

CONCLUSIONS

Intense VE-cadherin expression was evident in capillary endothelium of HCC tissues, giving the first indication of association with clinicopathological features of HCC patients.

TAL-1/SCL and its partners E47 and LMO2 up-regulate VE-cadherin expression in endothelial cells

The basic helix-loop-helix TAL-1/SCL essential for hematopoietic development is also required during vascular development for embryonic angiogenesis. We reported that TAL-1 acts positively on postnatal angiogenesis by stimulating endothelial morphogenesis. Here, we investigated the functional consequences of TAL-1 silencing in human primary endothelial cells. We found that TAL-1 knockdown caused the inhibition of in vitro tubulomorphogenesis, which was associated with a dramatic reduction in vascular endothelial cadherin (VE-cadherin) at intercellular junctions. Consistently, silencing of TAL-1 as well as of its cofactors E47 and LMO2 down-regulated VE-cadherin at both the mRNA and the protein level. Endogenous VE-cadherin transcription could be activated in nonendothelial HEK-293 cells by the sole concomitant ectopic expression of TAL-1, E47, and LMO2. Transient transfections in human primary endothelial cells derived from umbilical vein (HUVECs) demonstrated that VE-cadherin promoter activity was dependent on the integrity of a specialized E-box associated with a GATA motif and was maximal with the coexpression of the different components of the TAL-1 complex. Finally, chromatin immunoprecipitation assays showed that TAL-1 and its cofactors occupied the VE-cadherin promoter in HUVECs. Together, these data identify VE-cadherin as a bona fide target gene of the TAL-1 complex in the endothelial lineage, providing a first clue to TAL-1 function in angiogenesis.

CpG methylation profiles of endothelial cell-specific gene promoter regions in adipose tissue stem cells suggest limited differentiation potential toward the endothelial cell lineage

In vivo endothelial commitment of adipose stem cells (ASCs) has scarcely been reported, and controversy remains on the contribution of ASCs to vascularization. We address the epigenetic commitment of ASCs to the endothelial lineage. We report a bisulfite sequencing analysis of CpG methylation in the promoters of two endothelial-cell-specific genes, CD31 and CD144, in freshly isolated and in cultures of ASCs before and after induction of endothelial differentiation. In contrast to adipose tissue-derived endothelial (CD31(+)) cells, freshly isolated ASCs display a heavily methylated CD31 promoter and a mosaically methylated CD144 promoter despite basal transcription of both genes. Methylation state of both promoters remains globally stable upon culture. Endothelial stimulation of ASCs in methylcellulose elicits phenotypic changes, marginal upregulation of CD31, and CD144 expression and restrictive induction of a CD31(+)CD144(+) immunophenotype. These events are accompanied by discrete changes in CpG methylation in CD31 and CD144 promoters; however, no global demethylation that marks CD31(+) cells and human umbilical vein endothelial cells occurs. Immunoselection of CD31(+) cells after endothelial stimulation reveals consistent demethylation of one CpG immediately 3' of the transcription start site of the CD31 promoter. Adipogenic or osteogenic differentiation maintains CD31 and CD144 methylation patterns of undifferentiated cells. Methylation profiles of CD31 and CD144 promoters suggest a limited commitment of ASCs to the endothelial lineage. This contrasts with the reported hypomethylation of adipogenic promoters, which reflects a propensity of ASCs toward adipogenic differentiation. Analysis of CpG methylation at lineage-specific promoters provides a robust assessment of epigenetic commitment of stem cells to a specific lineage.

VE-Cadherin-Cre-recombinase transgenic mouse: a tool for lineage analysis and gene deletion in endothelial cells

The ability to target gene deletion to a specific cellular compartment via the Cre/loxP system has been a powerful tool in the analysis of broadly expressed genes. Here, we report the generation of a transgenic mouse line in which expression of Cre-recombinase is under the regulatory control of the VE-Cadherin promoter. Temporal distribution and activity of the enzyme was evaluated with two independent Cre reporter lines. Histological analysis was performed throughout development and in the adult. Recombination of lox P sites with subsequent expression of beta-galactosidase or GFP was detected as early as E7.5 in endothelial cells of the yolk sac. Progressive staining of the embryonic vasculature was noted from E8.5-13.5; however, more contiguous reporter expression was only seen by E14.5 onward in all endothelial compartments including arteries, veins, and capillaries. In addition, we found Cre activity in lymphatic endothelial cells. Unlike other endothelial-specific Cre mice, this model showed expression in the adult quiescent vasculature. Furthermore, the constitutive nature of the VE-Cadherin promoter in the adult can be advantageous for analysis of gene deletion in pathological settings.