Matrix-specific activation of Src and Rho initiates capillary morphogenesis of endothelial cells

Molecular mechanisms controlling vascular lumen formation in three-dimensional extracellular matrices

Considerable progress has been made toward a molecular understanding of how cells form lumen and tube structures in three-dimensional (3D) extracellular matrices (ECM). This progress has occurred through work performed with endothelial and epithelial cell models using both in vitro and in vivo approaches. Despite the apparent similarities between endothelial and epithelial cell lumen and tube formation mechanisms, there are clear distinctions that directly relate to their functional differences. This review will focus on endothelial cell (EC) lumen formation mechanisms which control blood vessel formation during development and postnatal life. Of great interest is that an EC lumen signaling complex has been identified which controls human EC lumen and tube formation in 3D matrices and which coordinates integrin-ECM contacts, cell surface proteolysis, cytoskeletal rearrangements, and cell polarity. This complex consists of the collagen-binding integrin α2β1, the collagen-degrading membrane-type 1 matrix metalloproteinase (MT1-MMP), junction adhesion molecule (Jam)C, JamB, polarity proteins Par3 and Par6b, and the Rho GTPase Cdc42-GTP. These interacting proteins are necessary to stimulate 3D matrix-specific signaling events (including activation of protein kinase cascades that regulate the actin and microtubule cytoskeletons) to control the formation of EC lumens and tube networks. Also, EC lumen formation is directly coupled to the generation of vascular guidance tunnels, enzymatically generated ECM conduits that facilitate EC tube remodeling and maturation. Mural cells such as pericytes are recruited along EC tubes within these tunnel spaces to control ECM remodeling events resulting in vascular basement membrane matrix assembly, a key step in tube maturation and stabilization.

The requirement for fibroblasts in angiogenesis: fibroblast-derived matrix proteins are essential for endothelial cell lumen formation

A role for fibroblasts in physiological and pathological angiogenesis is now well recognized; however, the precise mechanisms underlying their action have not been determined. Using an in vitro angiogenesis model in combination with a candidate gene approach, column chromatography, and mass spectrometry, we identify two classes of fibroblast-derived factors--one that supports vessel sprouting but not lumen formation, and one that promotes lumen formation. In the absence of fibroblasts a combination of angiopoietin-1, angiogenin, hepatocyte growth factor, transforming growth factor-α, and tumor necrosis factor drives robust endothelial cell (EC) sprouting; however, lumens fail to form. Subsequent addition of fibroblast-conditioned medium restores lumenogenesis. Using small interfering RNA-mediated knockdown, we show that five genes expressed in fibroblasts--collagen I, procollagen C endopeptidase enhancer 1, secreted protein acidic and rich in cysteine, transforming growth factor-β-induced protein ig-h3, and insulin growth factor-binding protein 7--are necessary for lumen formation. Moreover, lumen formation can be rescued by addition of purified protein to knockdown cultures. Finally, using rheology, we demonstrate that the presence of these matricellular proteins results in significantly stiffer gels, which correlates with enhanced lumen formation. These findings highlight the critical role that fibroblast-derived extracellular matrix components play in EC lumen formation and provide potential insight into the role of fibroblasts in the tumor microenvironment.

Fabrication of Multifaceted Micropatterned Surfaces with Laser Scanning Lithography

The implementation of engineered surfaces presenting micrometer-sized patterns of cell adhesive ligands against a biologically inert background has led to numerous discoveries in fundamental cell biology. While existing surface patterning strategies allow for pattering of a single ligand it is still challenging to fabricate surfaces displaying multiple patterned ligands. To address this issue we implemented Laser Scanning Lithography (LSL), a laser-based thermal desorption technique, to fabricate multifaceted, micropatterned surfaces that display independent arrays of subcellular-sized patterns of multiple adhesive ligands with each ligand confined to its own array. We demonstrate that LSL is a highly versatile "maskless" surface patterning strategy that provides the ability to create patterns with features ranging from 450 nm to 100 μm, topography ranging from -1 to 17 nm, and to fabricate both stepwise and smooth ligand surface density gradients. As validation for their use in cell studies, surfaces presenting orthogonally interwoven arrays of 1×8 μm elliptical patterns of Gly-Arg-Gly-Asp-terminated alkanethiol self-assembled monolayers and human plasma fibronectin are produced. Human umbilical vein endothelial cells cultured on these multifaceted surfaces form adhesion sites to both ligands simultaneously and utilize both ligands for lamella formation during migration. The ability to create multifaceted, patterned surfaces with tight control over pattern size, spacing, and topography provides a platform to simultaneously investigate the complex interactions of extracellular matrix geometry, biochemistry, and topography on cell adhesion and downstream cell behavior.

Angiogenesis

Extracellular matrix (ECM) is essential for all stages of angiogenesis. In the adult, angiogenesis begins with endothelial cell (EC) activation, degradation of vascular basement membrane, and vascular sprouting within interstitial matrix. During this sprouting phase, ECM binding to integrins provides critical signaling support for EC proliferation, survival, and migration. ECM also signals the EC cytoskeleton to initiate blood vessel morphogenesis. Dynamic remodeling of ECM, particularly by membrane-type matrix metalloproteases (MT-MMPs), coordinates formation of vascular tubes with lumens and provides guidance tunnels for pericytes that assist ECs in the assembly of vascular basement membrane. ECM also provides a binding scaffold for a variety of cytokines that exert essential signaling functions during angiogenesis. In the embryo, ECM is equally critical for angiogenesis and vessel stabilization, although there are likely important distinctions from the adult because of differences in composition and abundance of specific ECM components.

Role of the cytoskeleton in formation and maintenance of angiogenic sprouts

Angiogenesis is the formation of new blood vessels from pre-existing structures, and is a key step in tissue and organ development, wound healing and pathological events. Changes in cell shape orchestrated by the cytoskeleton are integral to accomplishing the various steps of angiogenesis, and an intact cytoskeleton is also critical for maintaining newly formed structures. This review focuses on how the 3 main cytoskeletal elements--microfilaments, microtubules, and intermediate filaments--regulate the formation and maintenance of angiogenic sprouts. Multiple classes of compounds target microtubules and microfilaments, revealing much about the role of actin and tubulin and their associated molecules in angiogenic sprout formation and maintenance. In contrast, intermediate filaments are much less studied, yet intriguing evidence suggests a vital, but unresolved, role in angiogenic sprouting. This review discusses evidence for regulatory molecules and pharmacological compounds that affect actin, microtubule and intermediate filament dynamics to alter various steps of angiogenesis, including endothelial sprout formation and maintenance.

Molecular basis for endothelial lumen formation and tubulogenesis during vasculogenesis and angiogenic sprouting

Many studies reveal a fundamental role for extracellular matrix-mediated signaling through integrins and Rho GTPases as well as matrix metalloproteinases (MMPs) in the molecular control of vascular tube morphogenesis in three-dimensional (3D) tissue environments. Recent work has defined an endothelial cell (EC) lumen signaling complex of proteins that controls these vascular morphogenic events. These findings reveal a signaling interdependence between Cdc42 and MT1-MMP to control the 3D matrix-specific process of EC tubulogenesis. The EC tube formation process results in the creation of a network of proteolytically generated vascular guidance tunnels in 3D matrices that are utilized to remodel EC-lined tubes through EC motility and could facilitate processes such as flow-induced remodeling and arteriovenous EC sorting and differentiation. Within vascular guidance tunnels, key dynamic interactions occur between ECs and pericytes to affect vessel remodeling, diameter, and vascular basement membrane matrix assembly, a fundamental process necessary for endothelial tube maturation and stabilization. Thus, the EC lumen and tube formation mechanism coordinates the concomitant establishment of a network of vascular tubes within tunnel spaces to allow for flow responsiveness, EC-mural cell interactions, and vascular extracellular matrix assembly to control the development of the functional microcirculation.

Laminin-3B11, a novel vascular-type laminin capable of inducing prominent lamellipodial protrusions in microvascular endothelial cells

The basement membrane (BM) proteins laminins, which consist of α, β, and γ chains, support tissue structures and cellular functions. To date only α4 and α5 types of laminins have been identified in the BMs of blood vessels. Our recent study suggested the presence of novel α3B-containing laminins in vascular BMs. Here we identified and characterized the third member of vascular laminins, laminin-3B11 (Lm3B11). RT-PCR analysis showed that microvascular endothelial (MVE) cells and umbilical vein endothelial cells expressed the messages for the α3B, β1, β2, and γ1 chains. In the culture of MVE cells, α3B was associated with β1 and γ1, producing Lm3B11. Recombinant Lm3B11 was overexpressed by introducing the cDNAs of the three chains into HEK-293 cells and purified to homogeneity. Purified Lm3B11 exhibited relatively weak cell adhesion activity through both α3β1 and α6β1 integrins. Most characteristically, Lm3B11 strongly stimulated MVE cells to extend many lamellipodial protrusions. This pseudopodial branching was blocked by an inhibitor for Src or phosphatidylinositol 3-kinase. Consistently, Lm3B11 stimulated the phosphorylation of Src and Akt more strongly than other laminins, suggesting that the integrin-derived signaling is mediated by these factors. The unique activity of Lm3B11 appears to be favorable to the branching of capillaries and venules.

EGFL7 meets miRNA-126: an angiogenesis alliance

Blood vessels form de novo through the tightly regulated programs of vasculogenesis and angiogenesis. Both processes are distinct but one of the steps they share is the formation of a central lumen, when groups of cells organized as vascular cords undergo complex changes to achieve a tube-like morphology. Recently, a protein termed epidermal growth factor-like domain 7 (EGFL7) was described as a novel endothelial cell-derived factor involved in the regulation of the spatial arrangement of cells during vascular tube assembly. With its impact on tubulogenesis and vessel shape EGFL7 joined the large family of molecules governing blood vessel formation. Only recently, the molecular mechanisms underlying EGFL7's effects have been started to be elucidated and shaping of the extracellular matrix (ECM) as well as Notch signaling might very well play a role in mediating its biological effects. Further, findings in knock-out animal models suggest miR-126, a miRNA located within the egfl7 gene, has a major role in vessel development by promoting VEGF signaling, angiogenesis and vascular integrity. This review summarizes our current knowledge on EGFL7 and miR-126 and we will discuss the implications of both bioactive molecules for the formation of blood vessels.

Targeted in vivo extracellular matrix formation promotes neovascularization in a rodent model of myocardial infarction

Background

The extracellular matrix plays an important role in tissue regeneration. We investigated whether extracellular matrix protein fragments could be targeted with antibodies to ischemically injured myocardium to promote angiogenesis and myocardial repair.

Methodology/Principal Findings

Four peptides, 2 derived from fibronectin and 2 derived from Type IV Collagen, were assessed for in vitro and in vivo tendencies for angiogenesis. Three of the four peptides—Hep I, Hep III, RGD—were identified and shown to increase endothelial cell attachment, proliferation, migration and cell activation in vitro. By chemically conjugating these peptides to an anti-myosin heavy chain antibody, the peptides could be administered intravenously and specifically targeted to the site of the myocardial infarction. When administered into Sprague-Dawley rats that underwent ischemia-reperfusion myocardial infarction, these peptides produced statistically significantly higher levels of angiogenesis and arteriogenesis 6 weeks post treatment.

Conclusions/Significance

We demonstrated that antibody-targeted ECM-derived peptides alone can be used to sufficiently alter the extracellular matrix microenvironment to induce a dramatic angiogenic response in the myocardial infarct area. Our results indicate a potentially new non-invasive strategy for repairing damaged tissue, as well as a novel tool for investigating in vivo cell biology.

Modulation of human vascular endothelial cell behaviors by nanotopographic cues

Basement membranes possess a complex three-dimensional topography in the nanoscale and submicron range which have been shown to profoundly modulate a large menu of fundamental cell behaviors. Using the topographic features found in native vascular endothelial basement membranes as a guide, polyurethane substrates were fabricated containing anisotropically ordered ridge and groove structures and isotropically ordered pores from 200 nm to 2000 nm in size. We investigated the impact of biomimetic length-scale topographic cues on orientation/elongation, proliferation and migration on four human vascular endothelial cell-types from large and small diameter vessels. We found that all cell-types exhibited orientation and alignment with the most pronounced response on anisotropically ordered ridges > or =800 nm. HUVEC cells were the only cell-type examined to demonstrate a decrease in proliferation in response to the smallest topographic features regardless of surface order. On anisotropically ordered surfaces all cell-types migrated preferentially parallel to the long axis of the ridges, with the greatest increase in cell migration being observed on the 1200 nm pitch. In contrast, cells did not exhibit any preference in direction or increase in migration speed on isotropically ordered surfaces. Overall, our data demonstrate that surface topographic features impact vascular endothelial cell behavior and that the impact of features varies with the cell behavior being considered, topographic feature scale, surface order, and the anatomic origin of the cell being investigated.

Delivering the goods: viral and non-viral gene therapy systems and the inherent limits on cargo DNA and internal sequences

Viruses have long been considered to be the most promising tools for human gene therapy. However, the initial enthusiasm for the use of viruses has been tarnished in the light of potentially fatal side effects. Transposons have a long history of use with bacteria in the laboratory and are now routinely applied to eukaryotic model organisms. Transposons show promise for applications in human genetic modification and should prove a useful addition to the gene therapy tool kit. Here we review the use of viruses and the limitations of current approaches to gene therapy, followed by a more detailed analysis of transposon length and the physical properties of internal sequences, which both affect transposition efficiency. As transposon length increases, transposition decreases: this phenomenon is known as length-dependence, and has implications for vector cargo capacity. Disruption of internal sequences, either via deletion of native DNA or insertion of exogenous DNA, may reduce or enhance genetic mobility. These effects may be related to host factor binding, essential spacer requirements or other influences yet to be elucidated. Length-dependence is a complex phenomenon driven not simply by the distance between the transposon ends, but by host proteins, the transposase and the properties of the DNA sequences encoded within the transposon.

Functional and transcriptional characterization of human embryonic stem cell-derived endothelial cells for treatment of myocardial infarction

BACKGROUND

Differentiation of human embryonic stem cells into endothelial cells (hESC-ECs) has the potential to provide an unlimited source of cells for novel transplantation therapies of ischemic diseases by supporting angiogenesis and vasculogenesis. However, the endothelial differentiation efficiency of the conventional embryoid body (EB) method is low while the 2-dimensional method of co-culturing with mouse embryonic fibroblasts (MEFs) require animal product, both of which can limit the future clinical application of hESC-ECs. Moreover, to fully understand the beneficial effects of stem cell therapy, investigators must be able to track the functional biology and physiology of transplanted cells in living subjects over time.

METHODOLOGY

In this study, we developed an extracellular matrix (ECM) culture system for increasing endothelial differentiation and free from contaminating animal cells. We investigated the transcriptional changes that occur during endothelial differentiation of hESCs using whole genome microarray, and compared to human umbilical vein endothelial cells (HUVECs). We also showed functional vascular formation by hESC-ECs in a mouse dorsal window model. Moreover, our study is the first so far to transplant hESC-ECs in a myocardial infarction model and monitor cell fate using molecular imaging methods.

CONCLUSION

Taken together, we report a more efficient method for derivation of hESC-ECs that express appropriate patterns of endothelial genes, form functional vessels in vivo, and improve cardiac function. These studies suggest that hESC-ECs may provide a novel therapy for ischemic heart disease in the future.

An approach for formation of vascularized liver tissue by endothelial cell-covered hepatocyte spheroid integration

Tissue vascularization in vitro is necessary for cell transplantation and is a major challenge in tissue engineering. To construct large and regularly vascularized tissue, we focused on the integration of endothelial cell-covered spheroids. Primary rat hepatocytes were cultured on a rotary shaker, and 100-150 mum spheroids were obtained by filtration. The hepatocyte spheroids were coated with collagen by conjugation with a type 1 collagen solution. Collagen-coated hepatocyte spheroids were cocultured with human umbilical vein endothelial cells (HUVECs), and monolayered HUVEC-covered hepatocyte spheroids were constructed. Without a collagen coat, many HUVECs invaded hepatocyte spheroids but did not cover the spheroid surface. To construct regularly vascularized tissue, we packed HUVEC-covered hepatocyte spheroids in hollow fibers used for plasma separation. Packed spheroids attached to each other forming a large cellular tissue with regular distribution of HUVECs. At day 9 after packing, HUVECs invaded the hepatocyte spheroids and a dense vascular network was constructed. Collagen coating of spheroids is useful for the formation of endothelial cell-covered spheroids and subsequent regular vascularized tissue construction.

Dependence of proliferative vascular smooth muscle cells on CD98hc (4F2hc, SLC3A2)

Activation of vascular smooth muscle cells (VSMCs) to migrate and proliferate is essential for the formation of intimal hyperplasia. Hence, selectively targeting activated VSMCs is a potential strategy against vaso-occlusive disorders such as in-stent restenosis, vein-graft stenosis, and transplant vasculopathy. We show that CD98 heavy chain (CD98hc) is markedly up-regulated in neointimal and cultured VSMCs, and that activated but not quiescent VSMCs require CD98hc for survival. CD98hc mediates integrin signaling and localizes amino acid transporters to the plasma membrane. SMC-specific deletion of CD98hc did not affect normal vessel morphology, indicating that CD98hc was not required for the maintenance of resident quiescent VSMCs; however, CD98hc deletion reduced intimal hyperplasia after arterial injury. Ex vivo and in vitro, loss of CD98hc suppressed proliferation and induced apoptosis in VSMCs. Furthermore, reconstitution with CD98hc mutants showed that CD98hc interaction with integrins was necessary for the survival of VSMCs. These studies establish the importance of CD98hc in VSMC proliferation and survival. Furthermore, loss of CD98hc was selectively deleterious to activated VSMCs while sparing resident quiescent VSMCs, suggesting that activated VSMCs are physiologically dependent on CD98hc, and hence, CD98hc is a potential therapeutic target in vaso-occlusive disorders.

Laminin receptor activation inhibits endothelial tissue factor expression

Tissue factor (TF) is an important trigger of arterial thrombosis. The green tea catechin epigallocatechin-3-gallate (EGCG) is a ligand of the 67-kDa laminin receptor (67LR) and exhibits cardioprotective effects. This study investigates whether 67LR regulates TF expression in human endothelial cells. Immunofluorescence demonstrated that human aortic endothelial cells expressed 67LR. Cells grown on laminin expressed 35% less TF in response to TNF-alpha (TNF-alpha) than those grown on fibronectin (n=6; p<0.001). EGCG (1-30 microM) inhibited TNF-alpha and histamine induced endothelial TF expression and activity in a concentration dependent manner resulting in 87% reduction of TF expression (n=5; p<0.001); in contrast, expression of tissue factor pathway inhibitor was not affected (n=4; p=NS). In vivo administration of EGCG (30 mg/kg/day) inhibited TF activity in carotid arteries of C57BL6 mice. Real-time PCR and promoter studies revealed that EGCG decreased TF expression at the transcriptional level and impaired activation of the mitogen activated protein (MAP) kinase JNK 1/2, but not ERK or p38. Similarly, the JNK 1/2 inhibitor SP600125 (1 microM) impaired TF promoter activity (n=4; p<0.001) and protein expression (n=4; p<0.001). 67LR blocking antibodies blunted the inhibitory effect of EGCG on both TF protein expression and JNK activation. In contrast, vascular cell adhesion molecule 1 (VCAM-1) was not affected by laminin nor EGCG, and its expression was not regulated by JNK. EGCG did not affect TNF-alpha stimulated NFkB activation. Laminin receptor activation inhibits endothelial TF expression by impairing JNK phosphorylation. Thus, 67LR may be a potential target for the development of novel anti-thrombotic therapies.

Laminin-1 induces E-cadherin expression in 3-dimensional cultured breast cancer cells by inhibiting DNA methyltransferase 1 and reversing promoter methylation status

This study determines the role of laminin-1 in promoting metastatic colonization during breast cancer. For this purpose, human mammary epithelial cell lines representing normal (MCF-10A), adenocarcinoma (MCF-7), and malignant carcinoma (MDA-MB-231) were propagated in 3-dimensional cultures composed of laminin-1, collagen I, or mixtures of the two, and analyzed by Western blot, immunocytochemistry, semiquantitative reverse transcription polymerase chain reaction, and methylation-specific PCR. Here we demonstrate that laminin-1 decreases methylation of the E-cadherin promoter, resulting in increased mRNA and protein expression for malignant mammary epithelial cells. This decreased methylation is associated with dramatic changes in the cellular and structural morphology as well as a 70-fold decrease in DNA methyltransferase 1 (DNMT1) and a 6-fold decrease in cadherin 11 protein expression. To control for specificity of laminin-1 interactions, cells were also cultured on 2-dimensional plastic substrata and collagen I hydrogels for analysis, and the MCF-10A and MCF-7 were used as nonmalignant controls. Using a 3-dimensional model, we present evidence that laminin-1 is capable of inducing epigenetic change by inhibiting expression of DNMT1 and preventing methylation of the E-cadherin promoter, resulting in E-cadherin expression and the formation of cell-cell bonds in malignant carcinoma.

Adhesion signaling - crosstalk between integrins, Src and Rho

Interactions between cells and the extracellular matrix coordinate signaling pathways that control various aspects of cellular behavior. Integrins sense the physical properties of the extracellular matrix and organize the cytoskeleton accordingly. In turn, this modulates signaling pathways that are triggered by various other transmembrane receptors and augments the cellular response to growth factors. Over the past years, it has become clear that there is extensive crosstalk between integrins, Src-family kinases and Rho-family GTPases at the heart of such adhesion signaling. In this Commentary, we discuss recent advances in our understanding of the dynamic regulation of the molecular connections between these three protein families. We also discuss how this signaling network can regulate a range of cellular processes that are important for normal tissue function and disease, including cell adhesion, spreading, migration and mechanotransduction.

Formation of endothelial lumens requires a coordinated PKCepsilon-, Src-, Pak- and Raf-kinase-dependent signaling cascade downstream of Cdc42 activation

In this study, we present data showing that Cdc42-dependent lumen formation by endothelial cells (ECs) in three-dimensional (3D) collagen matrices involves coordinated signaling by PKCepsilon in conjunction with the Src-family kinases (SFKs) Src and Yes. Activated SFKs interact with Cdc42 in multiprotein signaling complexes that require PKCepsilon during this process. Src and Yes are differentially expressed during EC lumen formation and siRNA suppression of either kinase, but not Fyn or Lyn, results in significant inhibition of EC lumen formation. Concurrent with Cdc42 activation, PKCepsilon- and SFK-dependent signaling converge to activate p21-activated kinase (Pak)2 and Pak4 in steps that are also required for EC lumen formation. Pak2 and Pak4 further activate two Raf kinases, B-Raf and C-Raf, leading to ERK1 and ERK2 (ERK1/2) activation, which all seem to be necessary for EC lumen formation. This work reveals a multicomponent kinase signaling pathway downstream of integrin-matrix interactions and Cdc42 activation involving PKCepsilon, Src, Yes, Pak2, Pak4, B-Raf, C-Raf and ERK1/2 to control EC lumen formation in 3D collagen matrices.

Interstitial fluid flow intensity modulates endothelial sprouting in restricted Src-activated cell clusters during capillary morphogenesis

Development of tissues in vitro with dimensions larger than 150 to 200 microm requires the presence of a functional vascular network. Therefore, we have studied capillary morphogenesis under controlled biological and biophysical conditions with the aim of promoting vascular structures in tissue constructs. We and others have previously demonstrated that physiological values of interstitial fluid flow normal to an endothelial monolayer in combination with vascular endothelial growth factor play a critical role during capillary morphogenesis by promoting cell sprouting. In the present work, we studied the effect that a range of interstitial flow velocities (0-50 microm/min) has in promoting the amount, length, and branching of developing sprouts during capillary morphogenesis. The number of capillary-like structures developed from human umbilical vein endothelial cell monolayers across the interstitial flow values tested was not significantly affected. Instead, the length and branching degree of the sprouts presented a significant maximum at flow velocities of 10 to 20 microm/min. More-over, at these same flow values, the phosphorylation level of Src also showed its peak. We discovered that capillary morphogenesis is restricted to patches of Src-activated cells (phosphorylated Src (pSrc)) at the monolayer, suggesting that the transduction pathway in charge of sensing the mechanical stimulus induced by flow is promoting predetermined mechanically sensitive areas (pSrc) to undergo capillary morphogenesis

G3139, an anti-Bcl-2 antisense oligomer that binds heparin-binding growth factors and collagen I, alters in vitro endothelial cell growth and tubular morphogenesis

PURPOSE

We examined the effects of G3139 on the interaction of heparin-binding proteins [e.g., fibroblast growth factor 2 (FGF2) and collagen I] with endothelial cells. G3139 is an 18-mer phosphorothioate oligonucleotide targeted to the initiation codon region of the Bcl-2 mRNA. A randomized, prospective global phase III trial in advanced melanoma (GM301) has evaluated G3139 in combination with dacarbazine. However, the mechanism of action of G3139 is incompletely understood because it is unlikely that Bcl-2 silencing is the sole mechanism for chemosensitization in melanoma cells.

EXPERIMENTAL DESIGN

The ability of G3139 to interact with and protect heparin-binding proteins was quantitated. The effects of G3139 on the binding of FGF2 to high-affinity cell surface receptors and the induction of cellular mitogenesis and tubular morphogenesis in HMEC-1 and human umbilical vascular endothelial cells were determined.

RESULTS

G3139 binds with picomolar affinity to collagen I. By replacing heparin, the drug can potentiate the binding of FGF2 to FGFR1 IIIc, and it protects FGF from oxidation and proteolysis. G3139 can increase endothelial cell mitogenesis and tubular morphogenesis of HMEC-1 cells in three-dimensional collagen gels, increases the mitogenesis of human umbilical vascular endothelial cells similarly, and induces vessel sprouts in the rat aortic ring model.

CONCLUSIONS

G3139 dramatically affects the behavior of endothelial cells. There may be a correlation between this observation and the treatment interaction with lactate dehydrogenase observed clinically.

Roles of integrins in tumor angiogenesis and lymphangiogenesis

The lifelong dedication of Dr. Judah Folkman to understand how tumors co-opt vasculature to promote tumor growth and spread resulted in the development of an astounding body of knowledge and development of new clinical therapeutics for cancer. Angiogenesis is a critical point in the development and dissemination of most human tumors. Tumor-associated lymphangiogenesis also plays an important role in mediating tumor spread to lymph nodes. The molecular regulations of these processes are complex, and many key molecular families have been implicated in the regulation of angiogenesis and lymphangiogenesis. By regulating cell-cell and cell-matrix contacts, integrins participate in blood and lymphatic vessel growth by promoting endothelial cell migration and survival. Understanding the underlying mechanisms by which integrins promote tumor-associated blood and lymphatic vessel development might provide important modalities for the therapeutic intervention of metastatic spread. This review focuses on the role of integrins in angiogenesis and lymphangiogenesis. Integrins represent potential targets for pharmacological agents and open new avenues for the control of metastatic spread in the treatment of malignancies. This article is dedicated to the memory of Dr. Judah Folkman, an amazing and caring teacher, scientist, physician, and friend.

Mechanotransduction - a field pulling together?

Mechanical stresses are ever present in the cellular environment, whether through external forces that are applied to tissues or endogenous forces that are generated within the active cytoskeleton. Despite the wide array of studies demonstrating that such forces affect cellular signaling and function, it remains unclear whether mechanotransduction in different contexts shares common mechanisms. Here, I discuss possible mechanisms by which applied forces, cell-generated forces and changes in substrate mechanics could exert changes in cell function through common mechanotransduction machinery. I draw from examples that are primarily focused on the role of adhesions in transducing mechanical forces. Based on this discussion, emerging themes arise that connect these different areas of inquiry and suggest multiple avenues for future studies.

Angiogenesis alteration by defibrotide: implications for its mechanism of action in severe hepatic veno-occlusive disease

Defibrotide (DF) is a mixture of porcine-derived single-stranded phosphodiester oligonucleotides (9-80-mer; average, 50-mer) that has been successfully used to treat severe hepatic veno-occlusive disease (sVOD) with multiorgan failure (MOF) in patients who have received cytotoxic chemotherapy in preparation for bone marrow transplantation. However, its mechanism of action is unknown. Herein, we show that DF and phosphodiester oligonucleotides can bind to heparin-binding proteins (eg, basic fibroblast growth factor [bFGF] but not vascular endothelial growth factor [VEGF] 165) with low nanomolar affinity. This binding occurred in a length- and concentration-dependent manner. DF can mobilize proangiogenic factors such as bFGF from their depot or storage sites on bovine corneal endothelial matrix. However, these molecules do not interfere with high-affinity binding of bFGF to FGFR1 IIIc but can replace heparin as a required cofactor for binding and hence cellular mitogenesis. DF also protects bFGF against digestion by trypsin and chymotrypsin and from air oxidation. In addition, DF binds to collagen I with low nanomolar affinity and can promote human microvascular endothelial cell-1 (HMEC-1) cell mitogenesis and tubular morphogenesis in three-dimensional collagen I gels. Thus, our data suggest that DF may provide a stimulus to the sinusoidal endothelium of a liver that has suffered a severe angiotoxic event, thus helping to ameliorate the clinical sVOD/MOF syndrome.

Extracellular matrix mediates a molecular balance between vascular morphogenesis and regression

PURPOSE OF REVIEW

We discuss very recent studies that address the critical role of extracellular matrix in controlling the balance between vascular morphogenesis and regression. Much of this work suggests that a balance mechanism exists for controlling the extent of tissue vascularization involving downstream signaling events regulating endothelial cell behaviors in relation to their interactions with extracellular matrix molecules.

RECENT FINDINGS

Endothelial gene expression changes and signaling lead to events that not only stimulate vascular morphogenesis but also suppress mechanisms mediated through pro-regression factors such as Rho kinase. At the same time, vascular networks are susceptible to regression mediated by factors such as matrix metalloproteinase-1, matrix metalloproteinase-10, thrombospondin-1, extracellular matrix matricryptic fragments and angiopoietin-2. Pericyte recruitment to such vascular tubes can prevent regression events by delivering molecules such as tissue inhibitor of metalloproteinase-3 and angiopoietin-1 that promote vascular stabilization by decreasing tube susceptibility to these regression stimuli.

SUMMARY

Extracellular matrix-derived signals lead to critical morphologic changes mediated through cytoskeletal rearrangements that control the shape, function and signaling events in endothelial cell-lined vessels regulating tube formation, remodeling, stabilization and regression. These signals control both vascular morphogenic and regression events, and thus a molecular balance exists to control the extent and function of vascular tube networks within tissues.

Liposome-nucleic acid immunotherapeutics

Cationic liposome-nucleic acid complexes, which were originally developed for use as non-viral gene delivery vectors, may now have an equally important application as immunotherapeutic drugs. Recent studies have highlighted the ability of cationic liposomes to potently activate the innate immune system when used to deliver certain Toll-like receptor (TLR) agonists. The immune-enhancing properties of cationic liposomes have been most clearly demonstrated when combined with nucleic acid agonists for endosomally located TLRs, including TLR3, TLR7/8 and TLR9. Immune potentiation by cationic liposomes likely results from the combined effects of endosomal targeting, protection of nucleic acids from extracellular degradation, and from signaling via newly identified cytoplasmic receptors for nucleic acids. The potent innate immune stimulatory properties of liposome-nucleic acid complexes make them particularly attractive as non-specific immunotherapeutics and as vaccine adjuvants. Liposome-nucleic acid complexes have demonstrated impressive anticancer activity in a number of different animal tumor models. Moreover, liposome-nucleic acid complexes have also been shown to be effective for immunotherapy of acute viral and bacterial infections, as well as chronic fungal infections. When used as vaccine adjuvants, liposome-nucleic acid complexes target antigens for efficient uptake by dendritic cells and are particularly effective in eliciting CD8(+) T-cell responses to protein antigens. Thus, liposome-nucleic acid complexes form a potent and versatile immunotherapeutic platform.

In vitro three dimensional collagen matrix models of endothelial lumen formation during vasculogenesis and angiogenesis

Discovery and comprehension of detailed molecular signaling pathways underlying endothelial vascular morphogenic events including endothelial lumen formation are key steps in understanding their roles during embryonic development, as well as during various disease states. Studies that used in vitro three-dimensional (3D) matrix endothelial cell morphogenic assay models, in conjunction with in vivo studies, have been essential to identifying molecules and explaining their related signaling pathways that regulate endothelial cell morphogenesis. We present methods to study molecular mechanisms controlling EC lumen formation in 3D collagen matrices. In vitro models representing vasculogenesis and angiogenesis, whereby EC lumen formation and tube morphogenesis readily occur, are described. We also detail different methods of gene manipulation in ECs and their application to analyze critical signaling events regulating EC lumen formation.

alpha2beta1 integrin expression in the tumor microenvironment enhances tumor angiogenesis in a tumor cell-specific manner

To define the role of the alpha2beta1 integrin in pathologic angiogenesis, we investigated tumor-associated growth and angiogenesis in wild-type and alpha2-null mice. Our findings reveal that the alpha2beta1 integrin plays an important role in angiogenesis via regulation of VEGFR1 expression. When challenged with B16F10 melanoma cells, mice lacking alpha2beta1 integrin ex-pression exhibit increased tumor angiogenesis associated with up-regulated VEGFR1 expression. In contrast, there was no alpha2beta1 integrin-dependent difference in the angiogenic response to Lewis lung carcinoma (LLC) cells. Interestingly, whereas B16F10 cells secrete high levels of placental growth factor (PLGF), LLC cells produce high levels of VEGF, but low levels of PLGF. The alpha2beta1 integrin-dependent difference in angiogenesis was restored to LLC cells by expression of PLGF, strongly suggesting that the angiogenic phenotype and tumor growth in the alpha2-null host is dependent on specific interactions between the tumor cell and the genetically defined integrin repertoire of the host microenvironment. Thus integrin alpha2-null mice represent an example of genetic alterations of "the soil" determining response to the "seed."

The inhibition of tube formation in a collagen-fibrinogen, three-dimensional gel by cleaved kininogen (HKa) and HK domain 5 (D5) is dependent on Src family kinases

Cleaved high molecular weight kininogen (HKa), as well as its domain 5 (D5), inhibits migration and proliferation induced by angiogenic factors and induces apoptosis in vitro. To study its effect on tube formation we utilized a collagen-fibrinogen, three-dimensional gel, an in vitro model of angiogenesis. HKa, GST-D5 and D5 had a similar inhibitory effect of tube length by 90+/-4.5%, 86+/-5.5% and 77+/-12.9%, respectively. D5-derived synthetic peptides: G440-H455 H475-H485 and G486-K502 inhibited tube length by 51+/-3.7%, 54+/-3.8% and 77+/-1.7%, respectively. By a comparison of its inhibitory potency and its sequences, a functional sequence of HKa was defined to G486-G496. PP2, a Src family kinase inhibitor, prevented tube formation in a dose-dependent manner (100-400 nM), but PP3 at 5 microM, an inactive analogue of PP2, did not. HKa and D5 inhibited Src 416 phosphorylation by 62+/-12.3% and 83+/-6.1%, respectively. The C-terminal Src kinase (Csk) inhibits Src kinase activity. Using a siRNA to Csk, expression of Csk was down-regulated by 86+/-7.0%, which significantly increased tube length by 27+/-5.8%. The addition of HKa and D5 completely blocked this effect. We further showed that HKa inhibited Src family kinase activity by disrupting the complex of uPAR, alphavbeta3 integrin and Src. Our results indicate that the anti-angiogenic effect of HKa and D5 is mediated at least in part through Src family kinases and identify a potential novel target for therapeutic inhibition of neovascularization in cancer and inflammatory arthritis.

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.

AMELIORATION OF CARDIAC FUNCTION IN CHRONIC MYOCARDIAL INFARCTED RATS FOLLOWING ADMINISTRATION OF VECTOR pcDNA3.1AM

1. The present study was designed to examine the cardiovascular effects of intravenously administered pcDNA3.1AM, a recombinant non-virus vector carrying a rat adrenomedullin (AM) gene translation fragment, in rats with chronic cardiac dysfunction induced by ligation of the left descending coronary artery. 2. Haemodynamic parameters were recorded by intraventricular catheterization. In situ hybridization and polymerase chain reaction (PCR) were performed to identify the distribution of the introduced vector. The concentration of AM was determined by radioimmunoassay. 3. Progressive cardiac dysfunction was observed following coronary artery ligation, as indicated by a significant reduction in mean arterial pressure (MAP) and increases in both central venous pressure (CVP) and end-diastolic pressure of the left ventricle (LVEDP; P < 0.01). Administration of pcDNA3.1AM significantly attenuated the progressive cardiac dysfunction and lowered the elevated CVP and LVEDP. The introduced vector was widely distributed in different organs, including the lungs, kidney, heart, liver, spleen and brain. However, intense staining of pcDNA3.1 AM was observed in the lungs and kidneys. The introduced vector was localized mainly in the endothelial cells of blood vessels. Radioimmunoassay showed elevated levels of AM in the plasma and lung and heart after surgery, but there was no significant further increase in the concentration of AM after pcDNA3.1AM delivery. 4. The present study has provided some novel findings on the potential beneficial effects of AM gene delivery on chronic cardiac function in rats. Expression of AM by a non-virus vector may also have therapeutic value against cardiac dysfunction in vivo.

The extracellular matrix and blood vessel formation: not just a scaffold

The extracellular matrix plays a number of important roles, among them providing structural support and information to cellular structures such as blood vessels imbedded within it. As more complex organisms have evolved, the matrix ability to direct signalling towards the vasculature and remodel in response to signalling from the vasculature has assumed progressively greater importance. This review will focus on the molecules of the extracellular matrix, specifically relating to vessel formation and their ability to signal to the surrounding cells to initiate or terminate processes involved in blood vessel formation.

Mechanisms of HER2-induced endothelial cell retraction

BACKGROUND

HER2 overexpression imparts a metastatic advantage in breast cancer. We have shown that HER2 signaling in breast cancer cells induces adjacent endothelial cell (EC) retraction, disrupting endothelial integrity. Because endothelial integrity is dependent on the adherens junctions, we postulated that the mechanism of tumor cell-induced EC retraction involves dissociation of catenin proteins from vascular endothelial (VE) cadherin. In this study, we report a loss of VE-cadherin in tumor-associated EC. We also tested for a change of catenin dissociation from VE-cadherin by manipulating HER2 signaling in tumor cells.

METHODS

We tested confluent monolayers of human EC for downregulation of VE cadherin and dissociation of catenins from VE cadherin after exposure to breast cancer cells or conditioned media. Using immunoprecipitation, we quantitated the remaining complexed catenins to VE-cadherin in tumor-associated EC after different treatments to manipulate HER2 signaling.

RESULTS

Treatment of EC with conditioned media from MCF-7 cells expressing HER2 induced a loss of VE-cadherin expression, and time-dependent dissociation of catenins from VE cadherin. Catenin dissociation from VE-cadherin was enhanced by Heregulin beta1 (P < .05) stimulation and decreased by trastuzumab (P < .05) blockade of HER2 signaling in cancer cells. An increase in EC phosphoSrc (Tyr 416) was seen by 8 hours.

CONCLUSIONS

Our data suggest that HER2 induction of EC retraction involves both down-regulation of VE-cadherin and dissociation of catenins. HER2 signaling appears to regulate this potential metastatic mechanism. Further, Src phosphorylation suggests that this pathway may be involved in this mechanism.

JunB is required for endothelial cell morphogenesis by regulating core-binding factor beta

The molecular mechanism triggering the organization of endothelial cells (ECs) in multicellular tubules is mechanistically still poorly understood. We demonstrate that cell-autonomous endothelial functions of the AP-1 subunit JunB are required for proper endothelial morphogenesis both in vivo in mouse embryos with endothelial-specific ablation of JunB and in in vitro angiogenesis models. By cDNA microarray analysis, we identified core-binding factor beta (CBFbeta), which together with the Runx proteins forms the heterodimeric core-binding transcription complex CBF, as a novel JunB target gene. In line with our findings, expression of the CBF target MMP-13 was impaired in JunB-deficient ECs. Reintroduction of CBFbeta into JunB-deficient ECs rescued the tube formation defect and MMP-13 expression, indicating an important role for CBFbeta in EC morphogenesis.

Deletion of tetraspanin Cd151 results in decreased pathologic angiogenesis in vivo and in vitro

Tetraspanin protein CD151 is abundant on endothelial cells. To determine whether CD151 affects angiogenesis, Cd151-null mice were prepared. Cd151-null mice showed no vascular defects during normal development or during neonatal oxygen-induced retinopathy. However, Cd151-null mice showed impaired pathologic angiogenesis in other in vivo assays (Matrigel plug, corneal micropocket, tumor implantation) and in the ex vivo aortic ring assay. Cd151-null mouse lung endothelial cells (MLECs) showed normal adhesion and proliferation, but marked alterations in vitro, in assays relevant to angiogenesis (migration, spreading, invasion, Matrigel contraction, tube and cable formation, spheroid sprouting). Consistent with these functional impairments, and with the close, preferential association of CD151 with laminin-binding integrins, Cd151-null MLECs also showed selective signaling defects, particularly on laminin substrate. Adhesion-dependent activation of PKB/c-Akt, e-NOS, Rac, and Cdc42 was diminished, but Raf, ERK, p38 MAP kinase, FAK, and Src were unaltered. In Cd151-null MLECs, connections were disrupted between laminin-binding integrins and at least 5 other proteins. In conclusion, CD151 modulates molecular organization of laminin-binding integrins, thereby supporting secondary (ie, after cell adhesion) functions of endothelial cells, which are needed for some types of pathologic angiogenesis in vivo. Selective effects of CD151 on pathologic angiogenesis make it a potentially useful target for anticancer therapy.

Intravenous Delivery of Anti-RhoA Small Interfering RNA Loaded in Nanoparticles of Chitosan in Mice: Safety and Efficacy in Xenografted Aggressive Breast Cancer

Overexpression of RhoA in cancer indicates a poor prognosis, because of increased tumor cell proliferation and invasion and tumor angiogenesis. We showed previously that anti-RhoA small interfering RNA (siRNA) inhibited aggressive breast cancer more effectively than conventional blockers of Rho-mediated signaling pathways. This study reports the efficacy and lack of toxicity of intravenously administered encapsulated anti-RhoA siRNA in chitosan-coated polyisohexylcyanoacrylate (PIHCA) nanoparticles in xenografted aggressive breast cancers (MDA-MB-231). The siRNA was administered every 3 days at a dose of 150 or 1500 microg/kg body weight in nude mice. This treatment inhibited the growth of tumors by 90% in the 150-microg group and by even more in the 1500-microg group. Necrotic areas were observed in tumors from animals treated with anti-RhoA siRNA at 1500 microg/kg, resulting from angiogenesis inhibition. In addition, this therapy was found to be devoid of toxic effects, as evidenced by similarities between control and treated animals for the following parameters: body weight gain; biochemical markers of hepatic, renal, and pancreatic function; and macroscopic appearance of organs after 30 days of treatment. Because of its efficacy and the absence of toxicity, it is suggested that this strategy of anti-RhoA siRNA holds significant promise for the treatment of aggressive cancers.

Silencing profilin-1 inhibits endothelial cell proliferation, migration and cord morphogenesis

Expression of several actin-binding proteins including profilin-1 is up-regulated during capillary morphogenesis of endothelial cells, the biological significance of which remains unknown. Specifically, we hypothesized that profilin-1 is important for endothelial migration and proliferation. In this study, we suppressed profilin-1 expression in human umbilical vein endothelial cells by RNA-interference. Gene silencing of profilin-1 led to significant reduction in the formation of actin filaments and focal adhesions. Loss of profilin-1 expression was also associated with reduced dynamics of cell-cell adhesion. Data from both wound-healing experiments and time-lapse imaging of individual cells showed inhibition of cell migration when profilin-1 expression was suppressed. Cells lacking profilin-1 exhibited defects in membrane protrusion, both in terms of its magnitude and directional persistence. Furthermore, loss of profilin-1 expression inhibited cell growth without compromising cell survival, at least in the short-term, thus suggesting that profilin-1 also plays an important role in endothelial proliferation as hypothesized. Finally, silencing profilin-1 expression suppressed matrigel-induced early cord morphogenesis of endothelial cells. Taken together, our data suggest that profilin-1 may play important role in biological events that involve endothelial proliferation, migration and morphogenesis.

Methylnaltrexone inhibits opiate and VEGF-induced angiogenesis: role of receptor transactivation

Angiogenesis or the formation of new blood vessels is important in the growth and metastatic potential of various cancers. Therefore, agents that inhibit angiogenesis have important therapeutic implications in numerous malignancies. We examined the effects of methylnaltrexone (MNTX), a peripheral mu opioid receptor antagonist, on agonist-induced human EC proliferation and migration, two key components in angiogenesis. Using human dermal microvascular EC, we observed that morphine sulfate (MS), the active metabolite, morphine-6-glucuronide (M6G), DAMGO ([d-Ala(2), N-Me-Phe(4), Gly(5)-ol]enkaphalin) and VEGF induced migration which were inhibited by pretreatment with MNTX at therapeutically relevant concentration (0.1 microM). The biologically inactive metabolite morphine-3-glucuronide (M3G) did not affect EC migration. We next examined the mechanism(s) by which MNTX inhibits opioid and VEGF-induced angiogenesis using human pulmonary microvascular EC. MS and DAMGO induced Src activation which was required for VEGF receptor transactivation and opioid-induced EC proliferation and migration. MNTX inhibited MS, DAMGO and VEGF induced tyrosine phosphorylation (transactivation) of VEGF receptors 1 and 2. Furthermore, MS, DAMGO and VEGF induced RhoA activation which was inhibited by MNTX or VEGF receptor tyrosine kinase inhibition. Finally, MNTX or silencing RhoA expression (siRNA) blocked MS, DAMGO and VEGF-induced EC proliferation and migration. Taken together, these results indicate that MNTX inhibits opioid-induced EC proliferation and migration via inhibition of VEGF receptor phosphorylation/transactivation with subsequent inhibition of RhoA activation. These results suggest that MNTX inhibition of angiogenesis can be a useful therapeutic intervention for cancer treatment.

Lysophosphatidylcholine impairs endothelial barrier function through the G protein-coupled receptor GPR4

Abundant evidence indicates that lysophosphatidylcholine (LPC) is proinflammatory and atherogenic. In the vascular endothelium, LPC increases permeability and expression of proinflammatory molecules such as adhesion molecules and cytokines. Yet, mechanisms by which LPC mediates these activities remain unclear and controversial. Recent evidence implicates involvement of a novel subfamily of G protein-coupled receptors (GPR4, G2A, OGR1, and TDAG8) that are sensitive to lysolipids and protons. We previously reported that one of these receptors, GPR4, is selectively expressed by a variety of endothelial cells and therefore hypothesize that the LPC-stimulated endothelial barrier dysfunction is mediated through GPR4. We developed a peptide Ab against GPR4 that detected GPR4 expression in transfected COS 7 cells and endogenous GPR4 expression in endothelial cells by Western blot. Endothelial cells infected with a retrovirus containing small interference RNA (siRNA) to GPR4 resulted in 40–50% decreased GPR4 expression, which corresponded with partial prevention of the LPC-induced 1) decrease in transendothelial resistance, 2) stress fiber formation, and 3) activation of RhoA. Furthermore, coexpression of the siRNA-GPR4 with a siRNA-resistant mutant GPR4 fully restored the LPC-induced resistance decrease. However, extracellular pH of <7.4 did not alter baseline or LPC-stimulated resistances. The results provide strong evidence that the LPC-mediated endothelial barrier dysfunction is regulated by endogenous GPR4 in endothelial cells and suggest that GPR4 may play a critical role in the inflammatory responses activated by LPC.

Endothelial extracellular matrix: biosynthesis, remodeling, and functions during vascular morphogenesis and neovessel stabilization

The extracellular matrix (ECM) is critical for all aspects of vascular biology. In concert with supporting cells, endothelial cells (ECs) assemble a laminin-rich basement membrane matrix that provides structural and organizational stability. During the onset of angiogenesis, this basement membrane matrix is degraded by proteinases, among which membrane-type matrix metalloproteinases (MT-MMPs) are particularly significant. As angiogenesis proceeds, ECM serves essential functions in supporting key signaling events involved in regulating EC migration, invasion, proliferation, and survival. Moreover, the provisional ECM serves as a pliable scaffold wherein mechanical guidance forces are established among distal ECs, thereby providing organizational cues in the absence of cell-cell contact. Finally, through specific integrin-dependent signal transduction pathways, ECM controls the EC cytoskeleton to orchestrate the complex process of vascular morphogenesis by which proliferating ECs organize into multicellular tubes with functional lumens. Thus, the composition of ECM and therefore the regulation of ECM degradation and remodeling serves pivotally in the control of lumen and tube formation and, finally, neovessel stability and maturation.

RhoA and Rac mediate endothelial cell polarization and detachment induced by T-cadherin

T-cadherin (T-cad) is an atypical GPI-anchored member of the cadherin superfamily. Ligation of T-cad receptors on endothelial cells prevents cell spreading, promotes elongation and polarization, decreases adhesion to the matrix, and facilitates migration. This study investigates involvement of Rho GTPases in T-cad signaling. Human umbilical vein endothelial cells were infected with adenoviral vectors expressing dominant-negative and/or constitutively active mutants of RhoA (N19RhoA/RhoA63), ROCK (RB/PH(TT)/CAT), and Rac1 (N17RAC). Mutant-infected and empty vector-infected cells were compared with respect to their ability to detach and polarize when plated on substratum containing recombinant T-cad protein used as a ligand mimicking homophilic T-cad interactions. ROCK involvement was also studied using specific inhibitor Y-27632. Adhesion assays, analysis of cell phenotype, and actin cytoskeleton organization using TRITC-labeled phalloidin demonstrated that T-cad-induced cell polarization includes two complementary components: RhoA/ROCK pathway is necessary for cell contraction, stress fiber assembly, and inhibition of spreading, whereas Rac is required for formation of actin-rich lamellipodia at the leading edges of polarized cells. Individual repression of either pathway only partially prevented cell polarization and detachment, while simultaneous repression of RhoA and Rac pathways fully eliminated responses to homophilic T-cad ligation. In conclusion, these data suggest that T-cad induces cell deadhesion and polarization via RhoA-ROCK- and Rac-dependent mechanisms.

Vascular endothelial-cadherin tyrosine phosphorylation in angiogenic and quiescent adult tissues

Vascular endothelial-cadherin (VE-cadherin) plays a key role in angiogenesis and in vascular permeability. The regulation of its biological activity may be a central mechanism in normal or pathological angiogenesis. VE-cadherin has been shown to be phosphorylated on tyrosine in vitro under various conditions, including stimulation by VEGF. In the present study, we addressed the question of the existence of a tyrosine phosphorylated form of VE-cadherin in vivo, in correlation with the quiescent versus angiogenic state of adult tissues. Phosphorylated VE-cadherin was detected in mouse lung, uterus, and ovary but not in other tissues unless mice were injected with peroxovanadate to block protein phosphatases. Remarkably, VE-cadherin tyrosine phosphorylation was dramatically increased in uterus and ovary, and not in other organs, during PMSG/hCG-induced angiogenesis. In parallel, we observed an increased association of VE-cadherin with Flk1 (VEGF receptor 2) during hormonal angiogenesis. Additionally, Src kinase was constitutively associated with VE-cadherin in both quiescent and angiogenic tissues and increased phosphorylation of VE-cadherin-associated Src was detected in uterus and ovary after hormonal treatment. Src-VE-cadherin association was detected in cultured endothelial cells, independent of VE-cadherin phosphorylation state and Src activation level. In this model, Src inhibition impaired VEGF-induced VE-cadherin phosphorylation, indicating that VE-cadherin phosphorylation was dependent on Src activation. We conclude that VE-cadherin is a substrate for tyrosine kinases in vivo and that its phosphorylation, together with that of associated Src, is increased by angiogenic stimulation. Physical association between Flk1, Src, and VE-cadherin may thus provide an efficient mechanism for amplification and perpetuation of VEGF-stimulated angiogenic processes.