VLA-2 mediates the interaction of collagen with endothelium during in vitro vascular tube formation

The Role of Methionine Aminopeptidase 2 in Lymphangiogenesis

During the metastasis process, tumor cells invade the blood circulatory system directly from venous capillaries or indirectly via lymphatic vessels. Understanding the relative contribution of each pathway and identifying the molecular targets that affect both processes is critical for reducing cancer spread. Methionine aminopeptidase 2 (MetAp2) is an intracellular enzyme known to modulate angiogenesis. In this study, we investigated the additional role of MetAp2 in lymphangiogenesis. A histological staining of tumors from human breast-cancer donors was performed in order to detect the level and the localization of MetAp2 and lymphatic capillaries. The basal enzymatic level and activity in vascular and lymphatic endothelial cells were compared, followed by loss of function studies determining the role of MetAp2 in lymphangiogenesis in vitro and in vivo. The results from the histological analyses of the tumor tissues revealed a high MetAp2 expression, with detectable sites of co-localization with lymphatic capillaries. We showed slightly reduced levels of the MetAp2 enzyme and MetAp2 mRNA expression and activity in primary lymphatic cells when compared to the vascular endothelial cells. The genetic and biochemical manipulation of MetAp2 confirmed the dual activity of the enzyme in both vascular and lymphatic remodulation in cell function assays and in a zebrafish model. We found that cancer-related lymphangiogenesis is inhibited in murine models following MetAp2 inhibition treatment. Taken together, our study provides an indication that MetAp2 is a significant contributor to lymphangiogenesis and carries a dual role in both vascular and lymphatic capillary formation. Our data suggests that MetAp2 inhibitors can be effectively used as anti-metastatic broad-spectrum drugs.

Integrins in Vascular Development and Pathology

During vascular development, endothelial cells (ECs) and neighboring stromal cells interact and communicate through autocrine and paracrine signaling mechanisms involving extracellular matrix (ECM) proteins and their cell surface integrin adhesion receptors. Integrin-mediated adhesion and signaling pathways are crucial for normal vascular development and physiology, and alterations in integrin expression and/or function drive several vascular-related pathologies including thrombosis, autoimmune disorders, and cancer. The purpose of this chapter is to discuss integrin adhesion and signaling pathways important for EC growth, survival, and migration. Integrin-mediated paracrine links between ECs and surrounding stromal cells in the organ microenvironment will also be discussed. Lastly, we will review roles for integrins in vascular pathologies and discuss possible targets for therapeutic intervention.

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.

Distinct Patterns of Microvascular Endothelial Cell Morphology Are Determined by Extracellular Matrix Composition

Endothelial interactions with the extracellular matrix (ECM) play important roles in angiogenesis but whether specific ECM signals can determine specific cellular morphologies is unclear. The authors compared in vitro ECM-induced morphological responses of the phenotypically distinct human placental microvascular endothelial cells (HPMECs) with large vessel endothelial cells (HUVECs). HPMECs showed distinct patterns of reorganization in response to collagen-I or collagen-IV (monolayer disruption, sprouting, migration) and Matrigel or laminin-A (intussusception, cord formation, tubulogenesis), and an intermediate response to fibrin; whereas HUVECs responded similarly to collagen-1 and Matrigel (elongation, lattice formation, vacuolation) and showed little response to fibrin. Although the extent of collagen and Matrigel responses of HPMECs were increased by serum, acidic or basic fibroblast growth factor (aFGF, bFGF), or vascular endothelial growth factor (VEGF), and varied with matrix protein concentration, the basic patterns were matrix specific, and were independent of fibronectin. The collagen responses correlated with disruption of adherens and tight junctions and the formation of filopodial protrusions. Matrigel responses were associated with up-regulated junctional localization of VE-cadherin, and tubulogenesis developed mainly through paracellular remodeling rather than intracellular vacuolation. Overall, these findings suggest that distinct ECM interactions stimulate specific morphological responses. These signals may regulate morphological behaviour in the angiogenesis cycle, switching endothelial cells between migratory and vasculogenic phenotypes.

Biphasic function of focal adhesion kinase in endothelial tube formation induced by fibril-forming collagens

Migration and tube formation of endothelial cells are important in angiogenesis and require a coordinated response to the extra-cellular matrix (ECM) and growth factor. Since focal adhesion kinase (FAK) integrates signals from both ECM and growth factor, we investigated its role in angiogenesis. Type I and II collagens are fibril-forming collagens and stimulate human umbilical vein endothelial cells (HUVECs) to form tube structure. Although knockdown of FAK restrained cell motility and resulted in inhibition of tube formation, FAK degradation and tube formation occurred simultaneously after incubation with fibril-forming collagens. The compensation for the FAK degradation by a calpain inhibitor or transient over-expression of FAK resulted in disturbance of tube formation. These phenomena are specific to fibril-forming collagens and mediated via alpha2beta1 integrin. In conclusion, our data indicate that FAK is functioning in cell migration, but fibril-forming collagen-induced FAK degradation is necessary for endothelial tube formation.

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.

Histological quantitation of tumour angiogenesis

Tumours establish their blood supply via a number of processes in addition to angiogenesis. These include vasculogenesis, vascular remodelling, intussusception and possibly vascular mimicry in certain tumours. The mainstay of the assessment of tumour vascularity has been counting the number of immunohistochemically identified microvessels in vascular hot spots. Nevertheless, several other techniques are available, including Chalkley counting, vascular grade and the use of image analysis systems. Angiogenic activity can furthermore be assessed in histological samples by measuring the molecules involved in the establishment of the tumour vasculature, including angiogenic growth factors and their receptors, cell adhesion molecules, proteases and markers of activated, proliferating, cytokine stimulated or angiogenic vessels, such as CD105. Measuring the maturity of vessels may give an indication of the proportion of the tumour vasculature that is functional. Other reagents that can identify hypoxia-activated pathways are also being developed. The histological assessment of tumour vascularity is mainly used in the research setting but may also have applications in the clinic if appropriate methodology and trained observers perform the studies. Gene arrays may be able to provide an angiogenesis profile. Continued study into the processes involved in generating a tumour blood supply is likely to identify new markers that may be more accurate measures.

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

Interstitial collagen I stimulates microvascular endothelial cells to form solid cords that imitate precapillary structures found during angiogenesis. Time-lapse microscopy identified cell retraction and disruption of cell-cell contacts as early critical steps in collagen I-induced capillary morphogenesis. These early stages paralleled collagen I activation of Src kinase and GTPase Rho through beta1 integrins. The Src inhibitor PP2, dominant-negative Src, and Rho inhibitor exoenzyme C3 transferase each inhibited collagen I induction of actin stress fibers that mediate cell retraction and each inhibited capillary morphogenesis. Collagen I also disrupted VE-cadherin from intercellular junctions through a Src-dependent mechanism; both the Src inhibitor PP2 and dominant-negative Src preserved VE-cadherin localization to regions of cell-cell contact. An active Src mutant disrupted VE-cadherin and cell-cell contacts similarly to collagen I. In sharp contrast, laminin-1 did not induce capillary morphogenesis, and laminin-1 did not induce activation of Src or Rho. Rather, laminin-1 induced persistent activation of the GTPase Rac. Thus, these studies identify activation of Src and Rho as key mechanisms by which collagen I provokes capillary morphogenesis of microvascular endothelial cells, and they define marked differences between the functions of collagen I and laminin-1 in regulating endothelial cell morphogenesis.

Collagen I initiates endothelial cell morphogenesis by inducing actin polymerization through suppression of cyclic AMP and protein kinase A

Collagen I provokes endothelial cells to assume a spindle-shaped morphology and to align into solid cord-like assemblies. These cords closely imitate the solid pre-capillary cords of embryonic angiogenesis, raising interesting questions about underlying mechanisms. Studies described here identify a critical mechanism beginning with collagen I ligation of integrins alpha(1)beta(1) and alpha(2)beta(1), followed by suppression of cyclic AMP and cyclic AMP (cAMP)-dependent protein kinase A, and marked induction of actin polymerization to form prominent stress fibers. In contrast to collagen I, laminin-1 neither suppressed cAMP nor protein kinase A activity nor induced actin polymerization or changes in cell shape. Moreover, fibroblasts did not respond to collagen I with changes in cAMP, actin polymerization, or cell shape, thus indicating that collagen signaling, as observed in endothelial cells, does not extend to all cell types. Pharmacological elevation of cAMP blocked collagen-induced actin polymerization and formation of cords by endothelial cells; conversely, pharmacological suppression of either cAMP or protein kinase A induced actin polymerization. Collectively, these studies identify a previously unrecognized and critical mechanism, involving suppression of cAMP-dependent protein kinase A and induction of actin polymerization, through which collagen I drives endothelial cell organization into multicellular pre-capillary cords.

The alpha(1)beta(1) and alpha(2)beta(1) integrins provide critical support for vascular endothelial growth factor signaling, endothelial cell migration, and tumor angiogenesis

Angiogenesis is a complex process, involving functional cooperativity between cytokines and endothelial cell (EC) surface integrins. In this study, we investigated the mechanisms through which the alpha(1)beta(1) and alpha(2)beta(1) integrins support angiogenesis driven by vascular endothelial growth factor (VEGF). Dermal microvascular EC attachment through either alpha(1)beta(1) or alpha(2)beta(1) supported robust VEGF activation of the Erk1/Erk2 (p44/42) mitogen-activated protein kinase signal transduction pathway that drives EC proliferation. Haptotactic EC migration toward collagen I was dependent on alpha(1)beta(1) and alpha(2)beta(1) as was VEGF-stimulated chemotaxis of ECs in a uniform collagen matrix. Consistent with the functions of alpha(1)beta(1) and alpha(2)beta(1) in supporting signal transduction and EC migration, antibody antagonism of either integrin resulted in potent inhibition of VEGF-driven angiogenesis in mouse skin. Moreover, combined antagonism of alpha(1)beta(1) and alpha(2)beta(1) substantially reduced tumor growth and angiogenesis of human squamous cell carcinoma xenografts. Collectively, these studies identify critical collaborative functions for the alpha(1)beta(1) and alpha(2)beta(1) integrins in supporting VEGF signal transduction, EC migration, and tumor angiogenesis.

Protein phosphatases 1 and 2A maintain endothelial cells in a resting state, limiting the motility that is needed for the morphogenic process of angiogenesis

Angiogenesis that is induced by cancers, including those of the head and neck, requires endothelial cells to shift from a nonmotile resting state to an increased level of motility. Using a human microvascular endothelial cell line, this study shows the importance of the serine/threonine protein phosphatases 1 (PP1) and 2A (PP2A) in restricting endothelial cell motility. Treatment of endothelial cells with increasing concentrations of the PP1 and PP2A inhibitor okadaic acid resulted in cell rounding and increased motility, which was accompanied by cytoskeletal disorganization involving a loss of filamentous beta-tubulin and F-actin. These effects occurred at okadaic acid levels that selectively inhibit PP2A and became more prominent with higher levels that inhibit both PP2A and PP1. This study shows the importance of PP1 and PP2A in maintaining cytoskeletal organization, thereby limiting endothelial cell motility, and suggests that pharmacologic approaches to enhance PP1 and PP2A activities may be useful in preventing key events of the angiogenic process.

Extracellular matrix-driven matrix metalloproteinase production in endothelial cells: implications for angiogenesis

The process of new blood vessel growth, angiogenesis, involves orchestrated alterations in endothelial cell interactions with adjacent cells and with components of the underlying basement membrane matrix. The activity of matrix metalloproteinases (MMPs), proteases that can cleave basement membrane and interstitial matrix molecules, has been shown to be necessary for angiogenesis as it occurs in several different in vivo and in vitro models. This review discusses the potential roles of two particular MMPs, MMP-2 and MT1-MMP, in angiogenesis, with emphasis on current understanding of how endothelial cell-extracellular matrix interactions may regulate the production of these MMPs via matrix-induced signaling leading to transcriptional activation and subsequent formation of active multiprotease complexes on the cell surface.

Defining the domains of type I collagen involved in heparin- binding and endothelial tube formation

Cell surface heparan sulfate proteoglycan (HSPG) interactions with type I collagen may be a ubiquitous cell adhesion mechanism. However, the HSPG binding sites on type I collagen are unknown. Previously we mapped heparin binding to the vicinity of the type I collagen N terminus by electron microscopy. The present study has identified type I collagen sequences used for heparin binding and endothelial cell-collagen interactions. Using affinity coelectrophoresis, we found heparin to bind as follows: to type I collagen with high affinity (Kd approximately 150 nM); triple-helical peptides (THPs) including the basic N-terminal sequence alpha1(I)87-92, KGHRGF, with intermediate affinities (Kd approximately 2 microM); and THPs including other collagenous sequences, or single-stranded sequences, negligibly (Kd >> 10 microM). Thus, heparin-type I collagen binding likely relies on an N-terminal basic triple-helical domain represented once within each monomer, and at multiple sites within fibrils. We next defined the features of type I collagen necessary for angiogenesis in a system in which type I collagen and heparin rapidly induce endothelial tube formation in vitro. When peptides, denatured or monomeric type I collagen, or type V collagen was substituted for type I collagen, no tubes formed. However, when peptides and type I collagen were tested together, only the most heparin-avid THPs inhibited tube formation, likely by influencing cell interactions with collagen-heparin complexes. Thus, induction of endothelial tube morphogenesis by type I collagen may depend upon its triple-helical and fibrillar conformations and on the N-terminal heparin-binding site identified here.

Three-dimensional type I collagen lattices induce coordinate expression of matrix metalloproteinases MT1-MMP and MMP-2 in microvascular endothelial cells

Matrix metalloproteinases (MMPs) are hypothesized to play a key role in the processes of endothelial cell migration and matrix remodeling during angiogenesis. We utilized an in vitro model of microvascular endothelial cell angiogenesis, cells cultured within a collagen matrix, to investigate the MMP profile of endothelial cells undergoing angiogenesis. We demonstrated by gelatin zymography that monolayer cultures (two-dimensional) of endothelial cells constitutively expressed low levels of latent MMP-2, but that culture in a three-dimensional collagen matrix increased the total amount of MMP-2 mRNA and protein. Furthermore, 51% of total MMP-2 protein was activated in the three-dimensional culture lysates, compared with 3.5% in two-dimensional culture. The mRNA and protein of MT1-MMP, the putative activator of MMP-2, were up-regulated in endothelial cells cultured in three-dimensional as compared with two-dimensional culture. Treatment of cultures with MMP inhibitors blocked activation of MMP-2 and inhibited formation of endothelial cell networks within the collagen gel. Induction of MT1-MMP and MMP-2 appeared to be specific to collagen, inasmuch as culture of the endothelial cells on top of, or within, a Matrigel(R) matrix neither increased total MMP-2 nor increased activation of MMP-2. These results suggest that MT1-MMP activation of MMP-2 occurs in endothelial cells undergoing angiogenesis, that this activation has a functional role in endothelial cell organization, and that specific matrix interactions may be critical for the increased expression of MT1-MMP and MMP-2.

Effects of thrombospondin-1 on disease course and angiogenesis in rat adjuvant-induced arthritis

Leukocyte extravasation into the synovium is important in rheumatoid arthritis (RA). Thrombospondin (TSP)-1 mediates cell adhesion and migration and inhibits angiogenesis, and it has been implicated in RA. However, little information is available on the role of TSP-1 in arthritis-associated inflammation and neovascularization. Therefore, we analyzed the effects of TSP-1 in adjuvant-induced arthritis (AIA), a rat model for RA. Hydron pellets containing TSP-1 were implanted in one ankle of AIA rats post-adjuvant injection, while the contralateral ankle received sham implants. Body weight loss and joint swelling were determined in comparison to nonimplanted AIA controls. In addition, synovial vessel counts were obtained in TSP-1-versus sham-implanted ankles of the same rat. The implantation of TSP-1 pellets into one ankle resulted in an enhancement of swelling in both ankles. Furthermore, TSP-1 exhibited a biphasic modulatory effect on synovial vessel counts (P < 0.05). In conclusion, TSP-1 implanted into one ankle of AIA rats may augment the severity of the disease. One possible explanation, among others, for the modulating effect of TSP-1 on inflammation may be its effect on arthritis-related angiogenesis.

Establishment and characterization of a novel in vitro angiogenesis model using a microvascular endothelial cell line, F-2C, cultured in chemically defined medium

The behavior of vascular endothelial cells (EC) is an important factor in the processes involved in angiogenesis, but the regulatory mechanisms of angiogenesis, especially underlying the tubulogenesis by EC are not yet clear. Although a number of in vitro experimental models of tubulogenesis have been developed by use of cultured EC, most of those models are too complex to be easily handled and further, the culture media are usually supplemented with serum, creating problems in interpretation of experimental results. To generate a simple in vitro angiogenesis study model under serum-free culture conditions, we adapted a murine microvascular endothelial cell line, F-2, to a chemically defined medium, Cos Medium 001, and successfully established a subline of F-2, designated F-2C, which revealed a unique growth pattern. In Cos Medium 001, F-2C proliferates in a cobblestone pattern at an early growth stage, but, at a late growth stage, spontaneously differentiates to form three-dimensional honeycomblike tubular structures without the supplementation of any specific factors. The cell aggregation activity of F-2C in the presence of Ca2+ was much greater than that of F-2. The amount of subendothelial matrix deposited by F-2C was significantly higher than that by F-2, and increased prominently after the F-2C cells reached the differentiating stage of tubulogenesis. These findings indicate that F-2C is a new EC line in which tubulogenesis is spontaneously induced by the marked deposition of basement membrane analog to the subendothelial matrix and by the enhancement of presumable cadherin activity. We suggest that this cell line, F-2C, represents a simple and useful in vitro angiogenesis model.

Markers of tumor angiogenesis: clinical applications in prognosis and anti-angiogenic therapy

Numerous studies in many tumor types have demonstrated that quantitation by microvessel as a measure of angiogenesis is a powerful prognostic tool. However, the ability to exploit tumor angiogenesis as a prognostic marker is limited by the methods currently used for capillary identification and quantitation. This report critically evaluates all aspects of the techniques and their associated problems used for assessing tumor angiogenesis in tissue sections including the area of tumor assessed, the vascular parameter measured, the method of quantitation, the stratification of patients and the practical utility of computer image analysis systems. The potential of angiogenic factors assays, proteolytic enzymes, and cell adhesion molecules as surrogate endpoints for quantifying tumor angiogenesis are discussed and other methods for quantifying tumor angiogenesis are described. The potential clinical applications of these angiogenic markers in prognosis, stratification for adjuvant treatments (both cytotoxic and anti-angiogenic/vascular targeting) and other aspects of patient management is also discussed, particularly design of phase I and II trials.

Bi-directional stimulation of adherence to extracellular matrix components by human head and neck squamous carcinoma cells and endothelial cells

Head and neck squamous cell carcinoma (HNSCC) cultures were established from two patients to determine if cancer and endothelial cells bi-directionally regulate their adherence to extracellular matrix components (ECM), an important process for tumor vascularization and metastasis. Soluble products from endothelial cells transiently enhanced adherence by HNSCC to ECM and increased surface levels of beta 1 and beta 4 integrins, although not beta 3. HNSCC products enhanced endothelial cell adherence to fibronectin and laminin, and beta 1 and beta 4 expression. These data show bi-directional enhancement of adherence to ECM and integrin expression among endothelial and tumor cells, which may facilitate metastasis and neovascularization.