Cooperation between VEGF and beta3 integrin during cardiac vascular development

Gastrointestinal bleeding in von Willebrand patients: special diagnostic and management considerations

INTRODUCTION

Severe and recurrent gastrointestinal (GI) bleeding caused by angiodysplasia is a significant problem in patients with von Willebrand disease (VWD) and in those with acquired von Willebrand syndrome (AVWS). At present, angiodysplasia-related GI bleeding is often refractory to standard treatment including replacement therapy with von Willebrand factor (VWF) concentrates and continues to remain a major challenge and cause of significant morbidity in patients despite advances in diagnostics and therapeutics.

AREAS COVERED

This paper reviews the available literature on GI bleeding in VWD patients, examines the molecular mechanisms implicated in angiodysplasia-related GI bleeding, and summarizes existing strategies in the management of bleeding GI angiodysplasia in patients with VWF abnormalities. Suggestions are made for further research directions.

EXPERT OPINION

Bleeding from angiodysplasia poses a significant challenge for individuals with abnormal VWF. Diagnosis remains a challenge and may require multiple radiologic and endoscopic investigations. Additionally, there is a need for enhanced understanding at a molecular level to identify effective therapies. Future studies of VWF replacement therapies using newer formulations as well as other adjunctive treatments to prevent and treat bleeding will hopefully improve care.

Advances in SEMA3F regulation of clinically high-incidence cancers

Cancer has become a leading cause of morbidity and mortality in recent years. Its high prevalence has had a severe impact on society. Researchers have achieved fruitful results in the causative factors, pathogenesis, treatment strategies, and cancer prevention. Semaphorin 3F (SEMA3F), a member of the signaling family, was initially reported in the literature to inhibit the growth, invasion, and metastasis of cancer cells in lung cancer. Later studies showed it has cancer-inhibiting effects in malignant tumors such as breast, colorectal, ovarian, oral squamous cell carcinoma, melanoma, and head and neck squamous carcinoma. In contrast, recent studies have reported that SEMA3F is expressed more in hepatocellular carcinoma than in normal tissue and promotes metastasis of hepatocellular carcinoma. We chose lung, breast, colorectal, and hepatocellular carcinomas with high clinical prevalence to review the roles and molecular mechanisms of SEMA3F in these four carcinomas. We concluded with an outlook on clinical interventions for patients targeting SEMA3F.

Revascularization of chronic total occlusion coronary artery and cardiac regeneration

Coronary chronic total occlusion (CTO) contributes to the progression of heart failure in patients with ischemic cardiomyopathy. Randomized controlled trials demonstrated that percutaneous coronary intervention (PCI) for CTO significantly improves angina symptoms and quality of life but fails to reduce clinical events compared with optimal medical therapy. Even so, intervening physicians strongly support CTO-PCI. Cardiac regeneration therapy after CTO-PCI should be a promising approach to improving the prognosis of ischemic cardiomyopathy. However, the relationship between CTO revascularization and cardiac regeneration has rarely been studied, and experimental studies on cardiac regeneration usually employ rodent models with permanent ligation of the coronary artery rather than reopening of the occlusive artery. Limited early-stage clinical trials demonstrated that cell therapy for cardiac regeneration in ischemic cardiomyopathy reduces scar size, reverses cardiac remodeling, and promotes angiogenesis. This review focuses on the status quo of CTO-PCI in ischemic cardiomyopathy and the clinical prospect of cardiac regeneration in this setting.

Microvascular remodeling and altered angiogenic signaling in human kidneys distal to occlusive atherosclerotic renal artery stenosis

BACKGROUND

Renal artery stenosis (RAS) is an important cause of chronic kidney disease and secondary hypertension. In animal models, renal ischemia leads to downregulation of growth-factor expression and loss of intrarenal microcirculation. However, little is known about the sequelae of large vessel occlusive disease on the microcirculation within human kidneys.

METHOD

This study included 5 patients who underwent nephrectomy due to renovascular occlusion, and 7 non-stenotic discarded donor kidneys (4 deceased donors). Micro-computed tomography was performed to assess microvascular spatial densities and tortuosity, an index of microvascular immaturity. Renal protein expression, gene expression, and histology were studied in-vitro using immunoblotting, polymerase-chain-reaction, and staining.

RESULTS

RAS demonstrated loss of medium-sized vessels (0.2-0.3mm) compared to donor kidneys (p = 0.037) and increased microvascular tortuosity. RAS kidneys had greater protein expression of angiopoietin-1, hypoxia-inducible factor (HIF)-1α, and thrombospondin (TSP)-1, but lower protein expression of vascular endothelial growth factor (VEGF) than donor kidneys. Renal fibrosis, loss of peritubular capillaries (PTC) and pericyte detachment were greater in RAS, yet they had more newly-formed PTC than donor kidneys. Therefore, our study quantified significant microvascular remodeling in the post-stenotic human kidney. RAS induced renal microvascular loss, vascular remodeling, and fibrosis. Despite downregulated VEGF, stenotic kidneys upregulated compensatory angiogenic pathways related to angiopoietin-1.

CONCLUSIONS

These observations underscore the nature of human RAS as a microvascular disease distal to main vessel stenosis, and support therapeutic strategies directly targeting the post-stenotic kidney microcirculation in patients with RAS.

Integrins Within the Tumor Microenvironment: Biological Functions, Importance for Molecular Targeting, and Cancer Therapeutics Innovation

Many cellular functions important for solid tumor initiation and progression are mediated by members of the integrin family, a diverse family of cell attachment receptors. With recent studies emphasizing the role of the tumor microenvironment (TME) in tumor initiation and progression, it is not surprising that considerable attention is being paid to integrins. Several integrin antagonists are under clinical trials, with many demonstrating promising activity in patients with different cancers. A deeper knowledge of the functions of integrins within the TME is still required and might lead to better inhibitors being discovered. Integrin expression is commonly dysregulated in many tumors with integrins playing key roles in signaling as well as promotion of tumor cell invasion and migration. Integrins also play a major role in adhesion of circulating tumor cells to new sites and the resulting formation of secondary tumors. Furthermore, integrins have demonstrated the ability to promoting stem cell-like properties in tumor cells as well as drug resistance. Anti-integrin therapies rely heavily on the doses or concentrations used as these determine whether the drugs act as antagonists or as integrin agonists. This expert review offers the latest synthesis in terms of the current knowledge of integrins functions within the TME and as potential molecular targets for cancer therapeutics innovation.

Mast Cell Protease 7 Promotes Angiogenesis by Degradation of Integrin Subunits

Previous studies from our laboratory have shown that during angiogenesis in vitro, rmMCP-7 (recombinant mouse mast cell protease-7) stimulates endothelial cell spreading and induces their penetration into the matrix. The ability of rmMCP-7 to induce angiogenesis in vivo was assessed in the present study using a directed in vivo angiogenesis assay (DIVAA™). Vessel invasion of the angioreactor was observed in the presence of rmMCP-7 but was not seen in the control. Since integrins are involved in endothelial cell migration, the relationship between rmMCP-7 and integrins during angiogenesis was investigated. Incubation with rmMCP-7 resulted in a reduction in the levels of integrin subunits αv and β1 on SVEC4-10 endothelial cells during angiogenesis in vitro. Furthermore, the degradation of integrin subunits occurs both through the direct action of rmMCP-7 and indirectly via the ubiquitin/proteasome system. Even in the presence of a proteasome inhibitor, incubation of endothelial cells with rmMCP-7 induced cell migration and tube formation as well as the beginning of loop formation. These data indicate that the direct degradation of the integrin subunits by rmMCP-7 is sufficient to initiate angiogenesis. The results demonstrate, for the first time, that mMCP-7 acts in angiogenesis through integrin degradation.

MFG-E8/integrin β3 signaling contributes to airway inflammation response and airway remodeling in an ovalbumin-induced murine model of asthma

Asthma is the most common chronic childhood disease worldwide, characterized by airway remodeling and chronic inflammation, orchestrated primarily by Th2 cytokines. The aim of the current study was to explore the influences of milk fat globule epidermal growth factor 8 (MFG-E8)/integrin β3 signaling involved in airway inflammation and remodeling in asthma. BALB/c mice were sensitized by intraperitoneal injection of ovalbumin (OVA), followed by OVA nebulization. The levels of MFG-E8 expression were declined markedly in the OVA-induced allergy murine model. In addition, administration of MFG-E8 strongly reduced the accumulation of T-helper type 2 (Th2)-associated cytokines (such as interleukin-4, -5, and -13) as well as chemokine CCL11 (eotaxin) in bronchoalveolar lavage fluid and tissues in the OVA-sensitized mice. Moreover, MFG-E8 remarkably repressed the total immunoglobulin E and OVA-specific immunoglobulin E in serum in OVA-challenged mice. Meanwhile, treatment with recombinant murine MFG-E8 noticeably prevented inflammatory cell infiltration into the airways, as showed by a marked decrease in the numbers of total immune cells, eosinophils, neutrophils, macrophages, and lymphocytes in the bronchoalveolar lavage fluid in response to OVA challenge. Importantly, MFG-E8 apparently alleviated OVA-driven airway remodeling, which were evidenced by declined secretion of important mediators of airway remodeling, including transforming growth factor-β1, matrix metalloproteinase 9, ADAM8, and vascular endothelial growth factor, and reduced airway collagen deposition and inhibited goblet cell hyperplasia in OVA-induced asthma in mice. Mechanistically, integrin 3 contributes to the protective effect of MFG-E8 in inhibiting airway inflammation and remodeling in OVA-driven features of allergic asthma. Overall, MFG-E8, as a candidate molecule to evaluate airway inflammation and remodeling, could be a potential target for the management and prevention of asthma exacerbations, suggesting that MFG-E8/integrin β3 signaling may serve as a promising therapeutic agent for childhood asthma.

Coronary Artery Development: Origin, Malformations, and Translational Vascular Reparative Therapy

After thickening of the cardiac chamber walls during embryogenesis, oxygen and nutrients can no longer be adequately supplied to cardiac cells via passive diffusion; therefore, a primitive vascular network develops to supply these vital structures. This plexus further matures into coronary arteries and veins, which ensures continued development of the heart. Various models have been proposed to account for the growth of the coronary arteries. However, lineage-tracing studies in the last decade have identified 3 major sources, namely, the proepicardium, the sinus venosus, and endocardium. Although the exact contribution of each source remains unknown, the emerging model depicts alternative pathways and progenitor cells, which ensure successful coronary angiogenesis. We aim to explore the current trends in coronary artery development, the cellular and molecular signals regulating heart vascularization, and its implications for heart disease and vascular regeneration.

Integrins in wound healing, fibrosis and tumor stroma: High potential targets for therapeutics and drug delivery

Wound healing is a complex process, which ultimately leads to fibrosis if not repaired well. Pathologically very similar to fibrosis is the tumor stroma, found in several solid tumors which are regarded as wounds that do not heal. Integrins are heterodimeric surface receptors which control various physiological cellular functions. Additionally, integrins also sense ECM-induced extracellular changes during pathological events, leading to cellular responses, which influence ECM remodeling. The purpose and scope of this review is to introduce integrins as key targets for therapeutics and drug delivery within the scope of wound healing, fibrosis and the tumor stroma. This review provides a general introduction to the biology of integrins including their types, ligands, means of signaling and interaction with growth factor receptors. Furthermore, we highlight integrins as key targets for therapeutics and drug delivery, based on their biological role, expression pattern within human tissues and at cellular level. Next, therapeutic approaches targeting integrins, with a focus on clinical studies, and targeted drug delivery strategies based on ligands are described.

Hydrogels with precisely controlled integrin activation dictate vascular patterning and permeability

Integrin binding to bioengineered hydrogel scaffolds is essential for tissue regrowth and regeneration, yet not all integrin binding can lead to tissue repair. Here, we show that through engineering hydrogel materials to promote α3/α5β1 integrin binding, we can promote the formation of a space-filling and mature vasculature compared with hydrogel materials that promote αvβ3 integrin binding. In vitro, α3/α5β1 scaffolds promoted endothelial cells to sprout and branch, forming organized extensive networks that eventually reached and anastomosed with neighbouring branches. In vivo, α3/α5β1 scaffolds delivering vascular endothelial growth factor (VEGF) promoted non-tortuous blood vessel formation and non-leaky blood vessels by 10 days post-stroke. In contrast, materials that promote αvβ3 integrin binding promoted endothelial sprout clumping in vitro and leaky vessels in vivo. This work shows that precisely controlled integrin activation from a biomaterial can be harnessed to direct therapeutic vessel regeneration and reduce VEGF-induced vascular permeability in vivo.

Should studies on Glanzmann thrombasthenia not be telling us more about cardiovascular disease and other major illnesses?

Glanzmann thrombasthenia (GT) is a rare inherited bleeding disorder caused by loss of αIIbβ3 integrin function in platelets. Most genetic variants of β3 also affect the widely expressed αvβ3 integrin. With brief mention of mouse models, I now look at the consequences of disease-causing ITGA2B and ITGB3 mutations on the non-hemostatic functions of platelets and other cells. Reports of arterial thrombosis in GT patients are rare, but other aspects of cardiovascular disease do occur including deep vein thrombosis and congenital heart defects. Thrombophilic and other risk factors for thrombosis and lessons from heterozygotes and variant forms of GT are discussed. Assessed for GT patients are reports of leukemia and cancer, loss of fertility, bone pathology, inflammation and wound repair, infections, kidney disease, autism and respiratory disease. This survey shows an urgent need for a concerted international effort to better determine how loss of αIIbβ3 and αvβ3 influences health and disease.

Von Willebrand factor and angiogenesis: basic and applied issues

The recent discovery that von Willebrand factor (VWF) regulates blood vessel formation has opened a novel perspective on the function of this complex protein. VWF was discovered as a key component of hemostasis, capturing platelets at sites of endothelial damage and synthesized in megakaryocytes and endothelial cells (EC). In recent years, novel functions and binding partners have been identified for VWF. The finding that loss of VWF in EC results in enhanced, possibly dysfunctional, angiogenesis is consistent with the clinical observations that in some patients with von Willebrand disease (VWD), vascular malformations can cause severe gastrointestinal (GI) bleeding. In vitro and in vivo studies indicate that VWF can regulate angiogenesis through multiple pathways, both intracellular and extracellular, although their relative importance is still unclear. Investigation of these pathways has been greatly facilitated by the ability to isolate EC from progenitors circulating in the peripheral blood of normal controls and patients with VWD. In the next few years, these will yield further evidence on the molecular pathways controlled by VWF and shed light on this novel and fascinating area of vascular biology. In this article, we will review the evidence supporting a role for VWF in blood vessel formation, the link between VWF dysfunction and vascular malformations causing GI bleeding and how they may be causally related. Finally, we will discuss how these findings point to novel therapeutic approaches to bleeding refractory to VWF replacement therapy in VWD.

Myosin VI and cardiomyopathy: Left ventricular hypertrophy, fibrosis, and both cardiac and pulmonary vascular endothelial cell defects in the Snell's waltzer mouse

In mice and humans, loss of myosin VI (Myo6) function results in deafness, and certain Myo6 mutations also result in cardiomyopathies in humans. The current studies have utilized the Snell's waltzer (sv) mouse (a functional null mutation for Myo6) to determine if this mouse also exhibits cardiac defects and thus used to determine the cellular and molecular basis for Myo6-associated heart disease. Myo6 is expressed in mouse heart where it is predominantly expressed in vascular endothelial cells (VECs) based on co-localization with the VEC cell marker CD31. Sv/sv heart mass is significantly greater than that of sv/+ littermates, a result of left ventricle hypertrophy. The left ventricle of the sv/sv exhibits extensive fibrosis, both interstitial and perivascular, based on histologic staining, and immunolocalization of several markers for fibrosis including fibronectin, collagen IV, and the fibroblast marker vimentin. Myo6 is also expressed in lung VECs but not in VECs of intestine, kidney, or liver. Sv/sv lungs exhibit increased periaveolar fibrosis and enlarged air sacs. Electron microscopy of sv/sv cardiac and lung VECs revealed abnormal ultrastructure, including luminal protrusions and increased numbers of cytoplasmic vesicles. Previous studies have shown that loss of function of either Myo6 or its adaptor binding partner synectin/GIPC results in impaired arterial development due to defects in VEGF signaling. However, examination of synectin/GIPC-/- heart revealed no fibrosis or significantly altered VEC ultrastructure, suggesting that the cardiac and lung defects observed in the sv/sv mouse are not due to Myo6 function in arterial development.

Matrix metalloproteinase 14 modulates signal transduction and angiogenesis in the cornea

The cornea is transparent and avascular, and retention of these characteristics is critical to maintaining vision clarity. Under normal conditions, wound healing in response to corneal injury occurs without the formation of new blood vessels; however, neovascularization may be induced during corneal wound healing when the balance between proangiogenic and antiangiogenic mediators is disrupted to favor angiogenesis. Matrix metalloproteinases (MMPs), which are key factors in extracellular matrix remodeling and angiogenesis, contribute to the maintenance of this balance, and in pathologic instances, can contribute to its disruption. Here, we elaborate on the facilitative role of MMPs, specifically MMP-14, in corneal neovascularization. MMP-14 is a transmembrane MMP that is critically involved in extracellular matrix proteolysis, exosome transport, and cellular migration and invasion, processes that are critical for angiogenesis. To aid in developing efficacious therapies that promote healing without neovascularization, it is important to understand and further investigate the complex pathways related to MMP-14 signaling, which can also involve vascular endothelial growth factor, basic fibroblast growth factor, Wnt/β-catenin, transforming growth factor, platelet-derived growth factor, hepatocyte growth factor or chemokines, epidermal growth factor, prostaglandin E2, thrombin, integrins, Notch, Toll-like receptors, PI3k/Akt, Src, RhoA/RhoA kinase, and extracellular signal-related kinase. The involvement and potential contribution of these signaling molecules or proteins in neovascularization are the focus of the present review.

Integrin function in vascular biology: a view from 2013

PURPOSE OF REVIEW

This review considers recent developments concerning the role of integrins in vascular biology with a specific emphasis on integrin activation, and the crosstalk between integrins and growth factor receptors.

RECENT FINDINGS

Recent studies have shown leukocytes can mediate direct transfer of molecules into endothelial cells, how specific integrins can be used to transduce signaling events, in particular in vascular beds, and how endothelial cell integrins can be targeted with specific ligands for the delivery of therapeutics. Kindlin and talin are both essential for integrin activation based on in-vivo studies of mice and humans in which the genes encoding for these proteins have been inactivated. Recent studies have attempted to translate these in-vivo realities into in-vitro models with mixed results.

SUMMARY

Mechanisms and consequences of integrin-ligand interactions on blood and vascular cells remain a major topic of hematological research. Crucial to the ligand binding function of integrins are two intracellular binding partners, talin and kindlin. In seeking to define the molecular basis for 'integrin activation', a mechanism must be envisioned in which both proteins talin and kindlin are required to produce a productive functional response, be it platelet aggregation or leukocyte extravasation. On endothelial cells, integrins and vascular endothelial growth factor receptor 2 influence the activation of one another by virtue of their direct physical interaction. It has been shown that this bidirectional communication is subject to regulation during angiogenesis.

The integrin adhesome: from genes and proteins to human disease

The adhesive interactions of cells with their environment through the integrin family of transmembrane receptors have key roles in regulating multiple aspects of cellular physiology, including cell proliferation, viability, differentiation and migration. Consequently, failure to establish functional cell adhesions, and thus the assembly of associated cytoplasmic scaffolding and signalling networks, can have severe pathological effects. The roles of specific constituents of integrin-mediated adhesions, which are collectively known as the 'integrin adhesome', in diverse pathological states are becoming clear. Indeed, the prominence of mutations in specific adhesome molecules in various human diseases is now appreciated, and experimental as well as in silico approaches provide insights into the molecular mechanisms underlying these pathological conditions.

Sprouty4 regulates endothelial cell migration via modulating integrin β3 stability through c-Src

Angiogenesis is mediated by signaling through receptor tyrosine kinases (RTKs), Src family kinases and adhesion receptors such as integrins, yet the mechanism how these signaling pathways regulate one another remains incompletely understood. The RTK modulator, Sprouty4 (Spry4) inhibits endothelial cell functions and angiogenesis, but the mechanisms remain to be fully elucidated. In this study, we demonstrate that Spry4 regulates angiogenesis in part by regulating endothelial cell migration. Overexpression of Spry4 in human endothelial cells inhibited migration and adhesion on vitronectin (VTN), whereas knockdown of Spry4 enhanced these behaviors. These activities were shown to be c-Src-dependent and Ras-independent. Spry4 disrupted the crosstalk between vascular endothelial growth factor-2 and integrin αVβ3, the receptor for VTN. Spry4 overexpression resulted in decreased integrin β3 protein levels in a post-transcriptional manner in part by modulating its tyrosine phosphorylation by c-Src. Conversely, knockdown of Spry4 resulted in increased integrin β3 protein levels and tyrosine phosphorylation. Moreover, in vivo analysis revealed that Spry4 regulated integrin β3 levels in murine embryos and yolk sacs. Our findings identify an unanticipated role for Spry4 in regulating c-Src activity and integrin β3 protein levels, which contributes to the regulation of migration and adhesion of endothelial cells. Thus, targeting Spry4 may be exploited as a target in anti-angiogenesis therapies.

Tips, stalks, tubes: notch-mediated cell fate determination and mechanisms of tubulogenesis during angiogenesis

Angiogenesis is the process of developing vascular sprouts from existing blood vessels. Luminal endothelial cells convert into "tip" cells that contribute to the development of a multicellular stalk, which then undergoes lumen formation. In this review, we consider a variety of cellular and molecular pathways that mediate these transitions. We focus first on Notch signaling in cell fate determination as a mechanism to define tip and stalk cells. We next discuss the current models of lumen formation and describe new players in this process, such as chloride intracellular channel proteins. Finally, we consider the possible medical therapeutic benefits of understanding these processes and acknowledge potential obstacles in drug development.

αV integrins in angiogenesis and cancer

During angiogenesis, αv integrins are overexpressed on the endothelial cell surface to facilitate the growth and survival of newly forming vessels. Accordingly, blocking αv integrin function by disrupting ligand binding can produce an antiangiogenic effect. Although the integrin ectodomain regulates ligand binding specificity, the short cytoplasmic tail facilitates intracellular signaling pathways through the recruitment and activation of specific kinases and signaling intermediates. This in turn controls endothelial cell adhesion, morphology, migration, invasion, proliferation, and survival. These same integrin-mediated signaling pathways are exploited in cancer to promote the invasiveness and survival of tumor cells and to manipulate the host microenvironment to provide ample blood vessel and stromal resources to support tumor growth and metastatic spread. Because expression of αv integrins on distinct cell types contributes to cancer growth, αv integrin antagonists have the potential to disrupt multiple aspects of disease progression.

The platelet fibrinogen receptor: from megakaryocyte to the mortuary

Platelets are integral to normal haemostatic function and act to control vascular haemorrhage with the formation of a stable clot. The fibrinogen receptor (glycoprotein IIb/IIIa [GPIIb/IIIa]) is the most abundant platelet integrin and, by binding fibrinogen, facilitates irreversible binding of platelets to the exposed extracellular matrix and enables the cross-linking of adjacent platelets. The vital role of GPIIb/IIIa requires tight control of both its synthesis and function. After transcription from distinct domains on chromosome 17, the two subunits of the heterodimer are carefully directed through organelles with intricate regulatory steps designed to prevent the cellular expression of a dysfunctional receptor. Similarly, exquisite control of platelet activation via bidirectional signalling acts to limit the inappropriate and excessive formation of platelet-mediated thrombus. However, the enormous diversity of genetic mutations in the fibrinogen receptor has resulted in a number of allelic variants becoming established. The Pro³³ polymorphism in GPIIIa is associated with increased cardiovascular risk due to a pathological persistence of outside-in signalling once fibrinogen has dissociated from the receptor. The polymorphism has also been associated with the phenomenon of aspirin resistance, although larger epidemiological studies are required to establish this conclusively. A failure of appropriate receptor function due to a diverse range of mutations in both structural and signalling domains, results in the bleeding diathesis Glanzmann's thrombasthaenia. GPIIb/IIIa inhibitors were the first rationally designed anti-platelet drugs and have proven to be a successful therapeutic option in high-risk primary coronary intervention. As our understanding of bidirectional signalling improves, more subtle and directed therapeutic strategies may be developed.

Absence of integrin αvβ3 enhances vascular leak in mice by inhibiting endothelial cortical actin formation

RATIONALE

Sepsis and acute lung injury (ALI) have devastatingly high mortality rates. Both are associated with increased vascular leak, a process regulated by complex molecular mechanisms.

OBJECTIVES

We hypothesized that integrin αvβ3 could be an important determinant of vascular leak and endothelial permeability in sepsis and ALI.

METHODS

β3 subunit knockout mice were tested for lung vascular leak after endotracheal LPS, and systemic vascular leak and mortality after intraperitoneal LPS and cecal ligation and puncture. Possible contributory effects of β3 deficiency in platelets and other hematopoietic cells were excluded by bone marrow reconstitution experiments. Endothelial cells treated with αvβ3 antibodies were evaluated for sphingosine-1 phosphate (S1P)–mediated alterations in barrier function, cytoskeletal arrangement, and integrin localization.

MEASUREMENTS AND MAIN RESULTS

β3 knockout mice had increased vascular leak and pulmonary edema formation after endotracheal LPS, and increased vascular leak and mortality after intraperitoneal LPS and cecal ligation and puncture. In endothelial cells, αvβ3 antibodies inhibited barrier-enhancing and cortical actin responses to S1P. Furthermore, S1P induced translocation of αvβ3 from discrete focal adhesions to cortically distributed sites through Gi- and Rac1-mediated pathways. Cortical αvβ3 localization after S1P was decreased by αvβ3 antibodies, suggesting that ligation of the αvβ3 with its extracellular matrix ligands is required to stabilize cortical αvβ3 focal adhesions.

CONCLUSIONS

Our studies identify a novel mechanism by which αvβ3 mitigates increased vascular leak, a pathophysiologic function central to sepsis and ALI. These studies suggest that drugs designed to block αvβ3 may have the unexpected side effect of intensifying sepsis- and ALI-associated vascular endothelial leak.

Bone morphogenetic protein-2-induced signaling and osteogenesis is regulated by cell shape, RhoA/ROCK, and cytoskeletal tension

Osteogenic differentiation of human mesenchymal stem cells (hMSCs) is classically thought to be mediated by different cytokines such as the bone morphogenetic proteins (BMPs). Here, we report that cell adhesion to extracellular matrix (ECM), and its effects on cell shape and cytoskeletal mechanics, regulates BMP-induced signaling and osteogenic differentiation of hMSCs. Using micropatterned substrates to progressively restrict cell spreading and flattening against ECM, we demonstrated that BMP-induced osteogenesis is progressively antagonized with decreased cell spreading. BMP triggered rapid and sustained RhoA/Rho-associated protein kinase (ROCK) activity and contractile tension only in spread cells, and this signaling was required for BMP-induced osteogenesis. Exploring the molecular basis for this effect, we found that restricting cell spreading, reducing ROCK signaling, or inhibiting cytoskeletal tension prevented BMP-induced SMA/mothers against decapentaplegic (SMAD)1 c-terminal phosphorylation, SMAD1 dimerization with SMAD4, and SMAD1 translocation into the nucleus. Together, these findings demonstrate the direct involvement of cell spreading and RhoA/ROCK-mediated cytoskeletal tension generation in BMP-induced signaling and early stages of in vitro osteogenesis, and highlight the essential interplay between biochemical and mechanical cues in stem cell differentiation.

Glanzmann thrombasthenia: a review of ITGA2B and ITGB3 defects with emphasis on variants, phenotypic variability, and mouse models

Characterized by mucocutaneous bleeding arising from a lack of platelet aggregation to physiologic stimuli, Glanzmann thrombasthenia (GT) is the archetype-inherited disorder of platelets. Transmitted by autosomal recessive inheritance, platelets in GT have quantitative or qualitative deficiencies of the fibrinogen receptor, αIIbβ3, an integrin coded by the ITGA2B and ITGB3 genes. Despite advances in our understanding of the disease, extensive phenotypic variability with respect to severity and intensity of bleeding remains poorly understood. Importantly, genetic defects of ITGB3 also potentially affect other tissues, for β3 has a wide tissue distribution when present as αvβ3 (the vitronectin receptor). We now look at the repertoire of ITGA2B and ITGB3 gene defects, reexamine the relationship between phenotype and genotype, and review integrin structure in the many variant forms. Evidence for modifications in platelet production is assessed, as is the multifactorial etiology of the clinical expression of the disease. Reports of cardiovascular disease and deep vein thrombosis, cancer, brain disease, bone disorders, and pregnancy defects in GT are discussed in the context of the results obtained for mouse models where nonhemostatic defects of β3-deficiency or nonfunction are being increasingly described.

Why integrin as a primary target for imaging and therapy

Integrin-mediated cell adhesion is involved in many essential normal cellular and pathological functions including cell survival, growth, differentiation, migration, inflammatory responses, platelet aggregation, tissue repair and tumor invasion. 24 different heterodimerized transmembrane integrin receptors are combined from 18 different α and 8 different β subunits. Each integrin subunit contains a large extracellular domain, a single transmembrane domain and a usually short cytoplasmic domain. Integrins bind extracellular matrix (ECM) proteins through their large extracellular domain, and engage the cytoskeleton via their short cytoplasmic tails. These integrin-mediated linkages on either side of the plasma membrane are dynamically linked. Thus, integrins communicate over the plasma membrane in both directions, i.e., outside-in and inside-out signaling. In outside-in signaling through integrins, conformational changes of integrin induced by ligand binding on the extracellular domain altered the cytoplasmic domain structures to elicit various intracellular signaling pathways. Inside-out signaling originates from non-integrin cell surface receptors or cytoplasmic molecules and it activates signaling pathways inside the cells, ultimately resulting in the activation/deactivation of integrins. Integrins are one of key family proteins for cell adhesion regulation through binding to a large number of ECM molecules and cell membrane proteins. Lack of expression of integrins may result in a wide variety of effects ranging from blockage in pre-implantation to embryonic or perinatal lethality and developmental defects. Based on both the key role they played in angiogenesis, leukocytes function and tumor development and easy accessibility as cell surface receptors interacting with extracellular ligands, the integrin superfamily represents the best opportunity of targeting both antibodies and small-molecule antagonists for both therapeutic and diagnostic utility in various key diseases so far.

The role of β3-integrins in tumor angiogenesis: context is everything

Integrins are a family of cell-extracellular matrix adhesion molecules that play important roles in tumor angiogenesis. αvβ3-Integrin has received much attention as a potential anti-angiogenic target because it is upregulated in tumor-associated blood vessels. Agents targeting αvβ3-integrin are now showing some success in phase III clinical trails for the treatment of glioblastoma, but the exact function of this integrin in tumor angiogenesis is still relatively unknown. This review highlights some of the recent data illustrating that β3-integrins play both pro-angiogenic and anti-angiogenic roles in tumor angiogenesis depending on the context. Specifically we will discuss how the following differentially influence β3-integrin's role in tumor angiogenesis: first, cell-matrix interactions, second, β3-integrin inhibitor doses, third, cell type, and fourth, other interacting molecules.

Diminished metastasis in tetraspanin CD151-knockout mice

Tetraspanin protein CD151 on tumor cells supports invasion and metastasis. In the present study, we show that host animal CD151 also plays a critical role. CD151-null mice showed markedly diminished experimental lung metastasis after injection of Lewis lung carcinoma or B16F10 melanoma cells. Diminished tumor cell residence in the lungs was evident 6-24 hours after injection. Consistent with an endothelial cell deficiency, isolated CD151-null mouse lung endothelial cells showed diminished support for B16F10 adhesion and transendothelial migration, diminished B16F10-induced permeability, and diminished B16F10 adhesion to extracellular matrix deposited by CD151-null mouse lung endothelial cells. However, CD151 deletion did not affect the size of metastatic foci or subcutaneous primary B16F10 tumors, tumor aggregation, tumor clearance from the blood, or tumor-induced immune cell activation and recruitment. Therefore, the effects of host CD151 on metastasis do not involve altered local tumor growth or immune surveillance. VEGF-induced endothelial cell signaling through Src and Akt was diminished in CD151-null endothelial cells. However, deficient signaling was not accompanied by reduced endothelial permeability either in vitro (monolayer permeability assay) or in vivo (VEGF-stimulated Miles assay). In summary, diminished metastasis in CD151-null host animals may be due to impaired tumor-endothelial interactions, with underlying defects in mouse lung endothelial cell extracellular matrix production.

Evaluation of VEGF-induced vascular permeability in mice

Vascular endothelial growth factor (VEGF) is a potent inducer of angiogenesis and vascular leak involved in development, wound healing, tumor growth, macular degeneration, and ischemia. Studying the effects of VEGF in vitro is not always sufficient to approximate the complex in vivo response that involves multiple cell types within functioning tissues. Treating mice with an intravenous injection of recombinant VEGF produces a rapid and transient biochemical response that is accompanied by a series of ultrastructural changes. Similar events are induced by hypoxia-induced VEGF in the heart following myocardial infarction or by tumor cell-released VEGF during metastasis. Studying how intact blood vessels respond to VEGF will augment the further development of antipermeability strategies to improve disease progression in a number of pathologies.

Pathogenic hantaviruses direct the adherence of quiescent platelets to infected endothelial cells

Hantavirus infections are noted for their ability to infect endothelial cells, cause acute thrombocytopenia, and trigger 2 vascular-permeability-based diseases. However, hantavirus infections are not lytic, and the mechanisms by which hantaviruses cause capillary permeability and thrombocytopenia are only partially understood. The role of beta(3) integrins in hemostasis and the inactivation of beta(3) integrin receptors by pathogenic hantaviruses suggest the involvement of hantaviruses in altered platelet and endothelial cell functions that regulate permeability. Here, we determined that pathogenic hantaviruses bind to quiescent platelets via a beta(3) integrin-dependent mechanism. This suggests that platelets may contribute to hantavirus dissemination within infected patients and provides a means by which hantavirus binding to beta(3) integrin receptors prevents platelet activation. The ability of hantaviruses to bind platelets further suggested that cell-associated hantaviruses might recruit platelets to the endothelial cell surface. Our findings indicate that Andes virus (ANDV)- or Hantaan virus (HTNV)-infected endothelial cells specifically direct the adherence of calcein-labeled platelets. In contrast, cells comparably infected with nonpathogenic Tula virus (TULV) failed to recruit platelets to the endothelial cell surface. Platelet adherence was dependent on endothelial cell beta(3) integrins and neutralized by the addition of the anti-beta(3) Fab fragment, c7E3, or specific ANDV- or HTNV-neutralizing antibodies. These findings indicate that pathogenic hantaviruses displayed on the surface of infected endothelial cells bind platelets and that a platelet layer covers the surface of infected endothelial cells. This fundamentally changes the appearance of endothelial cells and has the potential to alter cellular immune responses, platelet activation, and endothelial cell functions that affect vascular permeability. Hantavirus-directed platelet quiescence and recruitment to vast endothelial cell beds further suggests mechanisms by which hantaviruses may cause thrombocytopenia and induce hypoxia. These findings are fundamental to our understanding of pathogenic-hantavirus regulation of endothelial cell responses that contribute to vascular permeability.

Novel aspects of corneal angiogenic and lymphangiogenic privilege

In this article, we provide the results of experimental studies demonstrating that corneal avascularity is an active process involving the production of anti-angiogenic factors, which counterbalance the pro-angiogenic/lymphangiogenic factors that are upregulated during wound healing. We also summarize pertinent published reports regarding corneal neovascularization (NV), corneal lymphangiogenesis and corneal angiogenic/lymphangiogenic privilege. We outline the clinical causes of corneal NV, and discuss the angiogenic proteins (VEGF and bFGF) and angiogenesis regulatory proteins. We also describe the role of matrix metalloproteinases MMP-2, -7, and MT1-MMP, anti-angiogenic factors, and lymphangiogenic regulatory proteins during corneal wound healing. Established and potential new therapies for the treatment of corneal neovascularization are also discussed.

Integrins in cancer: biological implications and therapeutic opportunities

The integrin family of cell adhesion receptors regulates a diverse array of cellular functions crucial to the initiation, progression and metastasis of solid tumours. The importance of integrins in several cell types that affect tumour progression has made them an appealing target for cancer therapy. Integrin antagonists, including the alphavbeta3 and alphavbeta5 inhibitor cilengitide, have shown encouraging activity in Phase II clinical trials and cilengitide is currently being tested in a Phase III trial in patients with glioblastoma. These exciting clinical developments emphasize the need to identify how integrin antagonists influence the tumour and its microenvironment.

Lowered albumin extravasation rate in heart but not in other organs in beta3-integrin-deficient mice

AIM

The vascular protein permeability is dependent on the integrity of the vascular wall. The heart capillaries in male mice lacking beta3 integrins have an immature phenotype. Previously, we have demonstrated a role for alphavbeta3 integrins in control of interstitial fluid pressure (Pif) and thereby in the fluid flux during inflammation. We wanted to explore a possible role for alphavbeta3 integrins in controlling capillary protein permeability during control situation and inflammation.

METHODS

We performed double-tracer and microdialysis experiments on beta3-integrin-deficient mice and wild type control mice. We also measured blood pressure and heart rate in the two mice strains.

RESULTS

We found reduced albumin extravasation (during 25 min) in the heart capillaries (0.053 +/- 0.003 vs. 0.087 +/- 0.009 mL g(-1) dw, P < 0.05), and an increased cardiac mass/body weight (5.3 x 10(-3) +/- 0.3 x 10(-3) vs. 3.8 x 10(-3) +/- 0.1 x 10(-3), P < 0.01) in the beta3-integrin-deficient mice (n = 6) compared with the controls (n = 6). Heart rate and blood pressure were the same in mice with and without beta3-integrins. No difference in permeability was found in other tissues studied, or under local inflammation.

CONCLUSION

These results show a function for the alphavbeta3 integrin in the regulation of protein permeability, selective for the heart capillaries.

Integrins as "functional hubs" in the regulation of pathological angiogenesis

It is well accepted that complex biological processes such as angiogenesis are not controlled by a single family of molecules or individually isolated signaling pathways. In this regard, new insight into the interconnected mechanisms that regulate angiogenesis might be gained by examining this process from a more global network perspective. The coordination of signaling cues from both outside and inside many different cell types is required for the successful completion of angiogenesis. Evidence is accumulating that the multifunctional integrin family of cell adhesion receptors represent an important group of molecules that play active roles in sensing, integrating, and distributing a diverse set of signals that regulate many cellular events required for angiogenesis. Given the ability of integrins to bind numerous extracellular ligands and transmit signals in a bi-directional fashion, we will discuss the multiple ways by which integrins may serve as a functional hub during pathological angiogenesis. In addition, we will highlight potential imaging and therapeutic strategies based on the expanding new insight into integrin function.

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.

Signal co-operation between integrins and other receptor systems

The multicellular nature of metazoans means that all cellular processes need to be tuned by adhesive interactions between cells and their local microenvironment. The spatial organization of cells within tissues requires sophisticated networks of extracellular signals to control their survival and proliferation, movements and positioning, and differentiated function. These cellular characteristics are mediated by multiple inputs from adhesion systems in combination with soluble and developmental signals. In the present review we explore how one class of adhesion receptor, the integrins, co-operate with other types of receptor to control diverse aspects of cell fate. In particular we discuss: (i) how beta3 and beta1 integrins work together with growth factors to control angiogenesis; (ii) how alpha6beta4 integrin co-operates with receptor tyrosine kinases in normal epithelial function and cancer; (iii) the interplay between beta1 integrins and EGF (epidermal growth factor) receptor; (iv) signal integration connecting integrins and cytokine receptors for interleukins, prolactin and interferons; and (v) how integrins and syndecans co-operate in cell migration.

Cooperation between integrin alphavbeta3 and VEGFR2 in angiogenesis

The cross-talk between receptor tyrosine kinases and integrin receptors are known to be crucial for a number of cellular functions. On endothelial cells, an interaction between integrin alphavbeta3 and VEGFR2 seems to be particularly important process during vascularization. Importantly, the functional association between VEGFR2 and integrin alphavbeta3 is of reciprocal nature since each receptor is able to promote activation of its counterpart. This mutually beneficial relationship regulates a number of cellular activities involved in angiogenesis, including endothelial cell migration, survival and tube formation, and hematopoietic cell functions within vasculature. This article discusses several possible mechanisms reported by different labs which mediate formation of the complex between VEGFR-2 and alphavbeta3 on endothelial cells. The pathological consequences and regulatory events involved in this receptor cross-talk are also presented.

Signal co-operation between integrins and other receptor systems

The multicellular nature of metazoans means that all cellular processes need to be tuned by adhesive interactions between cells and their local microenvironment. The spatial organization of cells within tissues requires sophisticated networks of extracellular signals to control their survival and proliferation, movements and positioning, and differentiated function. These cellular characteristics are mediated by multiple inputs from adhesion systems in combination with soluble and developmental signals. In the present review we explore how one class of adhesion receptor, the integrins, co-operate with other types of receptor to control diverse aspects of cell fate. In particular we discuss: (i) how β3 and β1 integrins work together with growth factors to control angiogenesis; (ii) how α6β4 integrin co-operates with receptor tyrosine kinases in normal epithelial function and cancer; (iii) the interplay between β1 integrins and EGF (epidermal growth factor) receptor; (iv) signal integration connecting integrins and cytokine receptors for interleukins, prolactin and interferons; and (v) how integrins and syndecans co-operate in cell migration.

Dysfunctional microvasculature as a consequence of shb gene inactivation causes impaired tumor growth

Shb (Src homology 2 protein B) is an adapter protein downstream of the vascular endothelial growth factor receptor receptor-2 (VEGFR-2). Previous experiments have suggested a role for Shb in endothelial cell function. Recently, the Shb gene was inactivated and Shb null mice were obtained on a mixed genetic background, but not on C57Bl6 mice. The present study was performed to address endothelial function in the Shb knockout mouse and its relevance for tumor angiogenesis. Tumor growth was retarded in Shb mutant mice, and this correlated with decreased angiogenesis both in tumors and in Matrigel plugs. Shb null mice display an abnormal endothelial ultrastructure in liver sinusoids and heart capillaries with cytoplasmic extensions projecting toward the lumen. Shb null heart VE-cadherin staining was less distinct than that of control heart, exhibiting in the former case a wavy and punctuate pattern. Experiments on isolated endothelial cells suggest that these changes could partly reflect cytoskeletal abnormalities. Vascular permeability was increased in Shb null mice in heart, kidney, and skin, whereas VEGF-stimulated vascular permeability was reduced in Shb null mice. It is concluded that Shb plays an important role in maintaining a functional vasculature in adult mice, and that interference with Shb signaling may provide novel means to regulate tumor angiogenesis.

Foxc2 transcription factor as a regulator of angiogenesis via induction of integrin beta3 expression

Angiogenesis, the growth of new blood vessels from pre-existing vessels, is a process involving endothelial cell proliferation, migration and vascular tube formation. One of the key molecules that regulate this process is the integrin beta3 subunit, a cell adhesion receptor that forms a heterodimer with the integrin alphav subunit and interacts with extracellular matrix components such as fibronectin and vitronectin. Although the integrin beta3 subunit is not normally expressed in quiescent endothelial cells, its expression increases in pathological and physiological angiogenesis, including the vasculature in the ischemic tissues such as tumors. Therefore, the integrin beta3 subunit is known to be a potential target for cancer therapy to block tumor angiogenesis. However, the molecular mechanisms for the transcriptional regulation of this subunit are not fully understood. Recently, we reported that Forkhead transcription factor Foxc2 directly induces expression of the integrin beta3 subunit thorough Forkhead-binding elements in its promoter, thereby regulating integrin beta3-mediated endothelial cell migration and adhesion. Thus, our work now identifies Foxc2 as a novel regulator of angiogenesis. In this commentary, we summarize our new findings and discuss prospects for future research in Foxc2-mediated angiogenesis.

The angiogenic response is dictated by beta3 integrin on bone marrow-derived cells

Angiogenesis is dependent on the coordinated action of numerous cell types. A key adhesion molecule expressed by these cells is the α_vβ₃ integrin. Here, we show that although this receptor is present on most vascular and blood cells, the key regulatory function in tumor and wound angiogenesis is performed by β₃ integrin on bone marrow–derived cells (BMDCs) recruited to sites of neovascularization. Using knockin mice expressing functionally stunted β₃ integrin, we show that bone marrow transplantation rescues impaired angiogenesis in these mice by normalizing BMDC recruitment. We demonstrate that α_vβ₃ integrin enhances BMDC recruitment and retention at angiogenic sites by mediating cellular adhesion and transmigration of BMDCs through the endothelial monolayer but not their release from the bone niche. Thus, β₃ integrin has the potential to control processes such as tumor growth and wound healing by regulating BMDC recruitment to sites undergoing pathological and adaptive angiogenesis.

Regulation of angiogenesis: apoptotic cues from the ECM

The extracellular matrix (ECM) acts both as a physical scaffold for cells and as a repository for growth factors. Moreover, ECM structure and physical-chemical properties convey precise information to cells that profoundly influences their biology by interactions with cell surface receptors termed integrins. During angiogenesis, the perivascular ECM plays a critical role in determining the proliferative, invasive and survival responses of the local vascular cells to the angiogenic growth factors. Dynamic changes in both the ECM and the local vascular cells act in concert to regulate new blood vessel growth. The digestion of ECM components by proteolysis is critical for the invasive capacity of endothelial cells, but also creates ECM fragments, which antagonize the mechanosensory function of integrins, and can be apoptogenic. Here, we discuss the roles of integrins in modulating cellular responses to a changing ECM, in particular the regulation of survival and invasion among invasive endothelial cells.

Integrins in angiogenesis and lymphangiogenesis

Blood vessels promote tumour growth, and both blood and lymphatic vessels facilitate tumour metastasis by serving as conduits for the transport of tumour cells to new sites. Angiogenesis and lymphangiogenesis are regulated by integrins, which are members of a family of cell surface receptors whose ligands are extracellular matrix proteins and immunoglobulin superfamily molecules. Select integrins promote endothelial cell migration and survival during angiogenesis and lymphangiogenesis, whereas other integrins promote pro-angiogenic macrophage trafficking to tumours. Several integrin-targeted therapeutic agents are currently in clinical trials for cancer therapy. Here, we review the evidence implicating integrins as a family of fundamental regulators of angiogenesis and lymphangiogenesis.

Integrins team up with tyrosine kinase receptors and plexins to control angiogenesis

PURPOSE OF REVIEW

Understanding the role of integrins in the formation of vascular bed is important for designing new therapeutic approaches to ameliorate or inhibit pathological vascularization. Besides regulating cell adhesion and migration, integrins dynamically participate in a network with soluble molecules and their receptors. This study summarizes recent progress in the understanding of the reciprocal interactions between integrins, tyrosine kinase, and semaphorin receptors.

RECENT FINDINGS

During angiogenic remodeling, endothelial cells that line blood vessel walls dynamically modify their integrin-mediated adhesive contacts with the surrounding extracellular matrix. During angiogenesis, opposing autocrine and paracrine loops of growth factors and semaphorins regulate endothelial integrin activation and function through tyrosine kinase receptors and the neuropilin/plexins system. Moreover, proangiogenic and antiangiogenic factors can directly bind integrins and regulate endothelial cell behavior. Studies describing these intense research areas are discussed.

SUMMARY

Alteration in the balance between the angiogenic growth factors and semaphorins results in an impairment of integrin functions and could account for cardiovascular malformation and structural and functional abnormalities of the tumor vasculature.

Modulation of VEGF signalling output by the Notch pathway

The formation of blood vessels within the vascular system entails a variety of cellular processes, including proliferation, migration and differentiation. In many cases, these diverse processes need to be finely coordinated among neighbouring endothelial cells in order to establish a functional vascular network. For instance, during angiogenic sprouting specialized endothelial tip cells follow guidance cues and migrate extensively into avascular tissues while trailing stalk cells must stay connected to the patent blood vessel. The vascular endothelial growth factor (VEGF) and Notch signalling pathways have emerged as the major players in governing these different cellular behaviours. In particular, recent work indicates an important role for Notch signalling in determining how an endothelial cell responds to VEGF. In this review, we provide an overview of these biochemically distinct pathways and discuss how they may interact during endothelial cell differentiation and angiogenesis.

Distinct angiogenic mediators are required for basic fibroblast growth factor- and vascular endothelial growth factor-induced angiogenesis: the role of cytoplasmic tyrosine kinase c-Abl in tumor angiogenesis

Signaling pathways engaged by angiogenic factors bFGF and VEGF in tumor angiogenesis are not fully understood. The current study identifies cytoplasmic tyrosine kinase c-Abl as a key factor differentially mediating bFGF- and VEGF-induced angiogenesis in microvascular endothelial cells. STI571, a c-Abl kinase inhibitor, only inhibited bFGF- but not VEGF-induced angiogenesis. bFGF induced membrane receptor cooperation between integrin beta(3) and FGF receptor, and triggered a downstream cascade including FAK, c-Abl, and MAPK. This signaling pathway is different from one utilized by VEGF that includes integrin beta(5), VEGF receptor-2, Src, FAK, and MAPK. Ectopic expression of wild-type c-Abl sensitized angiogenic response to bFGF, but kinase dead mutant c-Abl abolished this activity. Furthermore, the wild-type c-Abl enhanced angiogenesis in both Matrigel implantation and tumor xenograft models. These data provide novel insights into c-Abl's differential functions in mediating bFGF- and VEGF-induced angiogenesis.