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

Tumor cell invasion of collagen matrices requires coordinate lipid agonist-induced G-protein and membrane-type matrix metalloproteinase-1-dependent signaling

Background

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are bioactive lipid signaling molecules implicated in tumor dissemination. Membrane-type matrix metalloproteinase 1 (MT1-MMP) is a membrane-tethered collagenase thought to be involved in tumor invasion via extracellular matrix degradation. In this study, we investigated the molecular requirements for LPA- and S1P-regulated tumor cell migration in two dimensions (2D) and invasion of three-dimensional (3D) collagen matrices and, in particular, evaluated the role of MT1-MMP in this process.

Results

LPA stimulated while S1P inhibited migration of most tumor lines in Boyden chamber assays. Conversely, HT1080 fibrosarcoma cells migrated in response to both lipids. HT1080 cells also markedly invaded 3D collagen matrices (~700 μm over 48 hours) in response to either lipid. siRNA targeting of LPA₁ and Rac1, or S1P₁, Rac1, and Cdc42 specifically inhibited LPA- or S1P-induced HT1080 invasion, respectively. Analysis of LPA-induced HT1080 motility on 2D substrates vs. 3D matrices revealed that synthetic MMP inhibitors markedly reduced the distance (~125 μm vs. ~45 μm) and velocity of invasion (~0.09 μm/min vs. ~0.03 μm/min) only when cells navigated 3D matrices signifying a role for MMPs exclusively in invasion. Additionally, tissue inhibitors of metalloproteinases (TIMPs)-2, -3, and -4, but not TIMP-1, blocked lipid agonist-induced invasion indicating a role for membrane-type (MT)-MMPs. Furthermore, MT1-MMP expression in several tumor lines directly correlated with LPA-induced invasion. HEK293s, which neither express MT1-MMP nor invade in the presence of LPA, were transfected with MT1-MMP cDNA, and subsequently invaded in response to LPA. When HT1080 cells were seeded on top of or within collagen matrices, siRNA targeting of MT1-MMP, but not other MMPs, inhibited lipid agonist-induced invasion establishing a requisite role for MT1-MMP in this process.

Conclusion

LPA is a fundamental regulator of MT1-MMP-dependent tumor cell invasion of 3D collagen matrices. In contrast, S1P appears to act as an inhibitory stimulus in most cases, while stimulating only select tumor lines. MT1-MMP is required only when tumor cells navigate 3D barriers and not when cells migrate on 2D substrata. We demonstrate that tumor cells require coordinate regulation of LPA/S1P receptors and Rho GTPases to migrate, and additionally, require MT1-MMP in order to invade collagen matrices during neoplastic progression.

Coregulation of vascular tube stabilization by endothelial cell TIMP-2 and pericyte TIMP-3

The endothelial cell (EC)–derived tissue inhibitor of metalloproteinase-2 (TIMP-2) and pericyte-derived TIMP-3 are shown to coregulate human capillary tube stabilization following EC–pericyte interactions through a combined ability to block EC tube morphogenesis and regression in three-dimensional collagen matrices. EC–pericyte interactions strongly induce TIMP-3 expression by pericytes, whereas ECs produce TIMP-2 in EC–pericyte cocultures. Using small interfering RNA technology, the suppression of EC TIMP-2 and pericyte TIMP-3 expression leads to capillary tube regression in these cocultures in a matrix metalloproteinase-1 (MMP-1)–, MMP-10–, and ADAM-15 (a disintegrin and metalloproteinase-15)–dependent manner. Furthermore, we show that EC tube morphogenesis (lumen formation and invasion) is primarily controlled by the TIMP-2 and -3 target membrane type (MT) 1 MMP. Additional targets of these inhibitors include MT2-MMP and ADAM-15, which also regulate EC invasion. Mutagenesis experiments reveal that TIMP-3 requires its proteinase inhibitory function to induce tube stabilization. Overall, these data reveal a novel role for both TIMP-2 and -3 in the pericyte-induced stabilization of newly formed vascular networks that are predisposed to undergo regression and reveal specific molecular targets of the inhibitors regulating these events.

Endorepellin In Vivo: Targeting the Tumor Vasculature and Retarding Cancer Growth and Metabolism

BACKGROUND

The antiangiogenic approach to controlling cancer requires a better understanding of angiogenesis and the discovery of new compounds that modulate this key biological process. Here we investigated the role of endorepellin, an angiostatic protein fragment that is derived from the C-terminus of perlecan, a heparan sulfate proteoglycan, in controlling tumor angiogenesis in vivo.

METHODS

We administered human recombinant endorepellin systemically to mice bearing orthotopic squamous carcinoma xenografts or syngeneic Lewis lung carcinoma tumors. We monitored tumor growth, angiogenesis, metabolism, hypoxia, and mitotic index by using quantitative immunohistochemistry and positron emission tomography scan imaging. In addition, we determined the localization of injected endorepellin using near-infrared labeling and immunohistochemistry of frozen tumor sections. Finally, we isolated tumor-derived endothelial cells and tested whether endorepellin could interact with these cells and disrupt in vitro capillary morphogenesis. All statistical tests were two-sided.

RESULTS

Endorepellin specifically targeted the tumor vasculature as determined by immunohistochemical analysis and accumulated in the tumor perivascular zones where it persisted for several days as discrete deposits. This led to inhibition of tumor angiogenesis (as measured by decreased CD31-positive cells, mean control = 1902 CD31-positive pixels, mean endorepellin treated = 343.9, difference between means = 1558, 95% confidence interval [CI] = 1296 to 1820, P<.001), enhanced tumor hypoxia, and a statistically significant decrease in tumor metabolism and mitotic index (as measured by decreased Ki67-positive cells, mean control Ki67 pixels = 5970, mean endorepellin-treated Ki67 pixels = 3644, difference between means = 2326, 95% CI = 1904 to 2749, P<.001) compared to untreated controls. Endorepellin was actively internalized by tumor-derived endothelial cells causing a redistribution of alpha2beta1 integrin such that both proteins colocalized to punctate deposits in the perivascular region. Endorepellin treatment inhibited in vitro capillary morphogenesis of both normal and tumor-derived endothelia.

CONCLUSIONS

Our results provide support for the hypothesis that endorepellin is an effective antitumor vasculature agent that could be used as a therapeutic modality to combat cancer.

Humoral and cellular factors responsible for coronary collateral formation

Clinical observations suggest that patients with coronary artery disease (CAD) display a marked heterogenerty in collateral formation despite similar degrees of coronary obstruction. The development of coronary collaterals helps protect the myocardium from ischemic damage, yet the factors responsible for collateral formation are poorly understood. To better understand the biochemical and cellular mechanisms of collateral artery formation, monocyte function and circulating levels of pro- and antiangiogenic factors were measured in 101 patients with angiographically assessed CAD and extensively developed (score 2, n = 33) or absent (score 0, n = 68) collateral circulations. Compared with patients with score 0, those with score 2 were slightly older and had more advanced CAD. The score 2 group was also more likely to have had a previous myocardial infarction or coronary artery bypass grafting and a family history of CAD. At the same time, there were no significant differences between groups with regard to circulating levels of vascular endothelial growth factor-A(165), platelet-derived growth factor-betabeta, fibroblast growth factor-2, fibroblast growth factor-4, hepatocyte growth factor, tumor necrosis factor-alpha, interleukin-1beta, endostatin, matrix metalloproteinase-9, promatrix metalloproteinase-1, and CD40 ligand. Monocytes isolated from patients with score 2 and 0 collateral circulations demonstrated no differences in migration assays. However, adhesion to fibrinogen and collagen was significantly higher for monocytes from patients with score 0 (p = 0.05 and 0.04, respectively). In conclusion, these data suggest that the degree of coronary collateral formation is not determined by differences in systemically measurable levels of pro- or antiangiogenic factors assessed in this study. Rather, cellular properties, such as cell adhesion, or genetic differences between patients may be the driving force for collateral development.

Extracellular matrix-bound angiopoietin-like 4 inhibits endothelial cell adhesion, migration, and sprouting and alters actin cytoskeleton

Angiopoietin-like 4 (ANGPTL4) is a secreted protein that belongs to the angiopoietin family and is involved in angiogenesis and metabolism regulation. We previously reported the induction of angptl4 by hypoxia in endothelial cells and in human ischemic tissues from peripheral artery disease. We here observed in a mouse model of hindlimb ischemia that the mRNA upregulation in the vessels correlates with the accumulation of the full-length protein in ischemic tissues. We then investigated its functions in endothelial cells. In response to hypoxia, endogenous ANGPTL4 accumulates in the subendothelial extracellular matrix (ECM). Although the secreted protein undergoes proteolysis leading to truncated fragments present in the medium, only full-length ANGPTL4 interacts with the ECM. Competition and direct binding assays indicate that the strong interaction of ANGPTL4 with the ECM is heparin/heparan sulfate proteoglycan dependent. The balance between matrix-associated and soluble forms of ANGPTL4 points to the role of the ECM in the regulation of its bioavailability. The angiogenic function of the ECM-bound full-length protein was investigated using either the form associated with the conditioned ECM from ANGPTL4-transfected HEK293 cells or the purified immobilized protein. We show that matrix-associated and immobilized ANGPTL4 limit the formation of actin stress fibers and focal contacts in the adhering endothelial cells and inhibit their adhesion. Immobilized ANGPTL4 also decreases motility of endothelial cells and inhibits the sprouting and tube formation. Altogether, these findings show that hypoxic endothelial cells accumulate ANGPTL4 in the ECM, which in turn negatively regulates their angiogenic capacities through an autocrine pathway.

Low level arsenic promotes progressive inflammatory angiogenesis and liver blood vessel remodeling in mice

The vascular effects of arsenic in drinking water are global health concerns contributing to human disease worldwide. Arsenic targets the endothelial cells lining blood vessels, and endothelial cell activation or dysfunction may underlie the pathogenesis of both arsenic-induced vascular diseases and arsenic-enhanced tumorigenesis. The purpose of the current studies was to demonstrate that exposing mice to drinking water containing environmentally relevant levels of arsenic promoted endothelial cell dysfunction and pathologic vascular remodeling. Increased angiogenesis, neovascularization, and inflammatory cell infiltration were observed in Matrigel plugs implanted in C57BL/6 mice following 5-week exposures to 5-500 ppb arsenic [Soucy, N.V., Mayka, D., Klei, L.R., Nemec, A.A., Bauer, J.A., Barchowsky, A., 2005. Neovascularization and angiogenic gene expression following chronic arsenic exposure in mice. Cardiovasc.Toxicol 5, 29-42]. Therefore, functional in vivo effects of arsenic on endothelial cell function and vessel remodeling in an endogenous vascular bed were investigated in the liver. Liver sinusoidal endothelial cells (LSEC) became progressively defenestrated and underwent capillarization to decrease vessel porosity following exposure to 250 ppb arsenic for 2 weeks. Sinusoidal expression of PECAM-1 and laminin-1 proteins, a hallmark of capillarization, was also increased by 2 weeks of exposure. LSEC caveolin-1 protein and caveolae expression were induced after 2 weeks of exposure indicating a compensatory change. Likewise, CD45/CD68-positive inflammatory cells did not accumulate in the livers until after LSEC porosity was decreased, indicating that inflammation is a consequence and not a cause of the arsenic-induced LSEC phenotype. The data demonstrate that the liver vasculature is an early target of pathogenic arsenic effects and that the mouse liver vasculature is a sensitive model for investigating vascular health effects of arsenic.

Fine-tuning of a three-dimensional microcarrier-based angiogenesis assay for the analysis of endothelial-mesenchymal cell co-cultures in fibrin and collagen gels

A prerequisite for successful tissue engineering is the existence of a functional microvascular network. We hypothesized that such networks can be created and quantified in an in vitro setting by co-culturing endothelial cells (ECs) with tissue-specific 'bystander cells' in 3-D gel matrices. To test this hypothesis we adapted a previously described in vitro microcarrier-based angiogenesis assay (V. Nehls and D. Drenckhahn, 1995, Microvasc Res 50: 311-322). On optimizing this assay, we noted that the initial EC-microcarrier coverage depended on EC type and seeding technique employed to coat the microcarrier beads with the ECs. A confluent EC monolayer on the microcarrier surfaces formed only when bovine aortic endothelial cells (BAECs) were admixed to the beads under gentle agitation on an orbital shaker. After embedding BAEC-covered microcarrier beads into a sandwich-like arrangement of collagen or fibrin gels, we assessed cellular outgrowth at different serum concentrations in terms of migration distance and sprout formation. Quantifiable sprout formation was highest at 1% fetal bovine serum (FBS) in collagen matrices and at 0.1% FBS in fibrin matrices. At higher serum concentration, excess cell migration and formation of clusters prevented quantitative analysis of sprouting. Following the fine-tuning of this angiogenesis assay, we co-cultured BAECs with adipose tissue-derived fibroblasts (FBs) and vascular smooth muscle cells (SMCs). While FBs were able to increase the average migration distance of BAECs in both matrices, SMCs enhanced BAEC migration in fibrin, but not in collagen gels. By contrast, the number of newly formed sprouts in fibrin gels was increased by both cell types. We conclude that in this model bystander cells enhance EC network formation in a matrix-dependent manner. Additionally, these results stress the importance of carefully selecting the experimental parameters of a given in vitro angiogenesis model.

Vascular patterning of the quail coronary system during development

Recent studies have provided insights into specific events that contribute to vasculogenesis and angiogenesis in the developing coronary vasculature. This study focused on the developmental progression of coronary vascularization beginning with tube formation and ending with the establishment of a coronary arterial tree. We used electron microscopy, histology of serial sections, and immunohistochemistry in order to provide a comprehensive view of coronary vessel formation during the embryonic and fetal periods of the quail heart, a species that has been used in a number of studies addressing myocardial vascularization. Our data reveal features of progenitor cells and blood islands, tubular formation, and the anatomical relationship of a transformed periarterial tubular network and sympathetic ganglia to the emergence and branching of the right and left coronary arteries. We have traced the pattern of coronary artery branching and documented its innervation. Finally, our data include the relationship of fibronectin, laminin, and apoptosis to coronary artery growth. Our findings bring together morphological events that occur over the embryonic and fetal periods and provide a baseline for studies into the mechanisms that regulate the various events that occur during these time periods.

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.

Heparin-II domain of fibronectin is a vascular endothelial growth factor-binding domain: enhancement of VEGF biological activity by a singular growth factor/matrix protein synergism

We describe extracellular interactions between fibronectin (Fn) and vascular endothelial growth factor (VEGF) that influence integrin-growth factor receptor crosstalk and cellular responses. In previous work, we found that VEGF bound specifically to fibronectin (Fn) but not vitronectin or collagens. Herein we report that VEGF binds to the heparin-II domain of Fn and that the cell-binding and VEGF-binding domains of Fn, when physically linked, are necessary and sufficient to promote VEGF-induced endothelial cell proliferation, migration, and Erk activation. Using recombinant Fn domains, the C-terminal heparin-II domain of Fn (type III repeats 13 to 14) was identified as a key VEGF-binding site. Mutation of the heparin-binding residues on FnIII(13-14) abolished VEGF binding, and peptides corresponding to the heparin-binding sequences in FnIII(13-14) inhibited VEGF binding to Fn. Fn fragments containing both the alpha5beta1 integrin-binding domain (III 9 to 10) and the VEGF-binding domain (III 13 to 14) significantly enhanced VEGF-induced EC migration and proliferation and induced strong phosphorylation of the VEGF receptor and Erk. Neither the cell-binding or VEGF-binding fragment of Fn alone had comparable VEGF-promoting effects. These results suggest that the mechanism of VEGF/Fn synergism is mediated extracellularly by the formation of a novel VEGF/Fn complex requiring both the cell-binding and VEGF-binding domains linked in a single molecular unit. These data also highlight a new function for the Fn C-terminal heparin-binding domain that may have important implications for angiogenesis and tumor growth.

Enhancement of pituitary adenoma cell invasion and adhesion is mediated by discoidin domain receptor-1

The discoidin domain receptor-1 (DDR1) tyrosine kinases are a family of cell surface receptors that bind to several types of collagen and facilitate cell adhesion that is known association with several cancers. However, no previous study has examined the expression and function of DDR1 in pituitary adenoma. Tissue microarray analysis of DDR1 expression levels in 52 pituitary adenoma tissues revealed that DDR1 expression was significantly related to hormonal background (Kruskal-Wallis test; P < 0.0001). To further elucidate the function of DDR1 in pituitary adenoma, we developed DDR1 over- and under-expressing cell lines using DDR1 clone transfection and short interfering ribonucleic acids (siRNA)-based DDR1 gene silencing, respectively, in a human pituitary adenoma cell line (HP-75). Real-time reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting confirmed that expression of both DDR1 isoforms (DDR1a and DDR1b) was elevated by clone transfection and diminished by siRNA. Matrigel invasion assays revealed that cell invasion was increased in HP-75 cells over-expressing DDR1 and decreased in cells under-expressing DDR1. Consistent with this, zymography revealed that the activation levels of matrix metalloproteinase (MMP)-2 and -9 were increased and decreased in cells over- and under-expressing DDR1, respectively. Examination of in vitro cell adhesion to collagen types I, II, III, and IV with respect to MMP-2 and -9 expression revealed that DDR1 regulated cell adhesion to collagen type I, which was responsible for accelerating secretion of MMP-2 and -9 in DDR1 over-expressing cells. Taken together, these results strongly suggest that DDR1 mediates cell invasion-related signaling between collagen type I and MMP-2 and -9 in pituitary adenoma cells.

Redox regulation of 4-hydroxy-2-nonenal-mediated endothelial barrier dysfunction by focal adhesion, adherens, and tight junction proteins

4-Hydroxy-2-nonenal (4-HNE), one of the major biologically active aldehydes formed during inflammation and oxidative stress, has been implicated in a number of cardiovascular and pulmonary disorders. 4-HNE has been shown to increase vascular endothelial permeability; however, the underlying mechanisms are unclear. Hence, in the current study, we tested our hypothesis that 4-HNE-induced changes in cellular thiol redox status may contribute to modulation of cell signaling pathways that lead to endothelial barrier dysfunction. Exposure of bovine lung microvascular endothelial cells (BLMVECs) to 4-HNE induced reactive oxygen species generation, depleted intracellular glutathione, and altered cell-cell adhesion as measured by transendothelial electrical resistance. Pretreatment of BLM-VECs with thiol protectants, N-acetylcysteine and mercaptopropionyl glycine, attenuated 4-HNE-induced decrease in transendothelial electrical resistance, reactive oxygen species generation, Michael protein adduct formation, protein tyrosine phosphorylation, activation of ERK, JNK, and p38 MAPK, and actin cytoskeletal rearrangement. Treatment of BLMVECs with 4-HNE resulted in the redistribution of FAK, paxillin, VE-cadherin, beta-catenin, and ZO-1, and intercellular gap formation. Western blot analyses confirmed the formation of 4-HNE-derived Michael adducts with the focal adhesion and adherens junction proteins. Also, 4-HNE decreased tyrosine phosphorylation of FAK without affecting total cellular FAK contents, suggesting the modification of integrins, which are natural FAK receptors. 4-HNE caused a decrease in the surface integrin in a time-dependent manner without altering total alpha5 and beta3 integrins. These results, for the first time, revealed that 4-HNE in redox-dependent fashion affected endothelial cell permeability by modulating cell-cell adhesion through focal adhesion, adherens, and tight junction proteins as well as integrin signal transduction that may lead dramatic alteration in endothelial cell barrier dysfunction during heart infarction, brain stroke, and lung diseases.

Surfactant protein A promoter directs the expression of Cre recombinase in brain microvascular endothelial cells of transgenic mice

Brain microvascular endothelial cells (ECs) have unique characteristics distinguished from peripheral ECs and play important roles in blood-brain barrier (BBB). To investigate the physiological control of the brain ECs, we generated a transgenic mouse line in which the expression of Cre recombinase was driven by the promoter of the mouse surfactant protein A (SP-A) gene. The Cre activity was detected in blood vessels of brain, alveolar type II cells of lung and epithelium of gland stomach. In brain ECs, the Cre activity started at embryonic day 11.5, indicating that the subpopulation of ECs in brain could be molecularly defined at early embryonic stages. The use of SP-A-Cre mice should facilitate analysis of gene function in the brain ECs.

Cyclic strain-mediated matrix metalloproteinase regulation within the vascular endothelium: a force to be reckoned with

The vascular endothelium is a dynamic cellular interface between the vessel wall and the bloodstream, where it regulates the physiological effects of humoral and biomechanical stimuli on vessel tone and remodeling. With respect to the latter hemodynamic stimulus, the endothelium is chronically exposed to mechanical forces in the form of cyclic circumferential strain, resulting from the pulsatile nature of blood flow, and shear stress. Both forces can profoundly modulate endothelial cell (EC) metabolism and function and, under normal physiological conditions, impart an atheroprotective effect that disfavors pathological remodeling of the vessel wall. Moreover, disruption of normal hemodynamic loading can be either causative of or contributory to vascular diseases such as atherosclerosis. EC-matrix interactions are a critical determinant of how the vascular endothelium responds to these forces and unquestionably utilizes matrix metalloproteinases (MMPs), enzymes capable of degrading basement membrane and interstitial matrix molecules, to facilitate force-mediated changes in vascular cell fate. In view of the growing importance of blood flow patterns and mechanotransduction to vascular health and pathophysiology, and considering the potential value of MMPs as therapeutic targets, a timely review of our collective understanding of MMP mechanoregulation and its impact on the vascular endothelium is warranted. More specifically, this review primarily summarizes our current knowledge of how cyclic strain regulates MMP expression and activation within the vascular endothelium and subsequently endeavors to address the direct and indirect consequences of this on vascular EC fate. Possible relevance of these phenomena to vascular endothelial dysfunction and pathological remodeling are also addressed.

Gene expression changes in human cells after exposure to mobile phone microwaves

Possible biological effects of mobile phone microwaves were investigated in vitro. In this study, which was part of the 5FP EU project REFLEX (Risk Evaluation of Potential Environmental Hazards From Low-Energy Electromagnetic Field Exposure Using Sensitive in vitro Methods), six human cell types, immortalized cell lines and primary cells, were exposed to 900 and 1800 MHz. RNA was isolated from exposed and sham-exposed cells and labeled for transcriptome analysis on whole-genome cDNA arrays. The results were evaluated statistically using bioinformatics techniques and examined for biological relevance with the help of different databases. NB69 neuroblastoma cells, T lymphocytes, and CHME5 microglial cells did not show significant changes in gene expression. In EA.hy926 endothelial cells, U937 lymphoblastoma cells, and HL-60 leukemia cells we found between 12 and 34 up- or down-regulated genes. Analysis of the affected gene families does not point towards a stress response. However, following microwave exposure, some but not all human cells might react with an increase in expression of genes encoding ribosomal proteins and therefore up-regulating the cellular metabolism.

Molecular balance of capillary tube formation versus regression in wound repair: role of matrix metalloproteinases and their inhibitors

In this review, we discuss the identification of distinct matrix metalloproteinases (MMPs) and their inhibitors that differentially control the processes of capillary tube formation (morphogenesis) versus capillary tube regression in three-dimensional (3D) collagen matrices. This work directly relates to both granulation tissue formation and regression during wound repair. The membrane metalloproteinase, MT1-MMP (MMP-14), is required for endothelial cell (EC) tube formation using in vitro assays that mimic vasculogenesis or angiogenic sprouting in 3D collagen matrices. These events are markedly blocked by small interfering RNA (siRNA) suppression of MT1-MMP in ECs or by addition of tissue inhibitor of metalloproteinases (TIMPs)-2,-3, and -4 but not TIMP-1. In contrast, MMP-1 and MMP-10 are strongly induced during EC tube formation to regulate the process of tube regression (following activation by serine proteases) rather than formation. TIMP-1, which selectively inhibits soluble MMPs, blocks tube regression by inhibiting MMP-1 and MMP-10 while having no influence on EC tube formation. siRNA suppression of MMP-1 and MMP-10 markedly blocks tube regression without affecting tube formation. Furthermore, we discuss that pericyte-induced stabilization of EC tube networks in our model system appears to occur through EC-derived TIMP-2 and pericyte-derived TIMP-3 to block both the capillary tube formation and regression pathways.

Extracellular angiogenic growth factor interactions: an angiogenesis interactome survey

Angiogenesis plays a key role in various physiological and pathological processes, including inflammation and tumor growth. Numerous angiogenic growth factors (AGFs) have been identified. Usually, the angiogenic process is assumed to represent the outcome of a straightforward interaction of AGFs with specific signalling receptors of the endothelial cell (EC) surface. Actually, the mechanisms by which AGFs induce neovascularization are much more complex. Indeed, angiogenesis is the result of the simultaneous actions of various AGFs and angiogenesis modulators; multiple EC surface receptors with different structure and biological properties are engaged by AGFs to exert a full angiogenic response; AGFs bind a variety of free and immobilized proteins, polysaccharides, and complex lipids of the extracellular milieu that affect AGF integrity, stability, and bioavailability; some of the AGF-binding molecules interact also with AGF receptors. In this review the authors summarize literature data and discuss the current knowledge about the extracellular molecules able to interact with AGFs, thus representing possible key regulators of the angiogenesis process and targets/templates for the development of novel antiangiogenic drugs. This work represents an attempt to highlight common theme in the AGF interactome that occurs at the extracellular level during neovascularization.

Preclinical testing of tissue-engineered heart valves re-endothelialized under simulated physiological conditions

BACKGROUND

The in vivo regeneration capacity of decellularized heart valve grafts is still controversial. The aim of this study was to evaluate function, morphological changes, and cellular composition of decellularized versus re-endothelialized ovine pulmonary valves (PV) after implantation into lambs for 1 or 3 months.

METHODS AND RESULTS

PV (n=21) were decellularized using detergents. Twelve PV were repopulated with autologous jugular veins endothelial cells (ECs) in a dynamic pulsatile bioreactor under simulated physiological conditions. Morphological evaluation before implantation included histological stainings (H&E, Movat-pentachrome, von-Kossa, DAPI), immunostainings (anti-perlecan, anti-eNOS, anti-procollagen-I, anti-SM-alpha-actin), electron microscopy (EM), and DNA extraction. Decellularization led to cell-free scaffolds with preserved extracellular matrix (ECM) including basement membrane. Reseeded PV (n=5) were completely covered with ECs expressing endothelial nitric oxide synthase (eNOS) and von Willebrand factor (vWF). The function of orthotopically implanted decellularized and re-endothelialized PV (n=7, each) was analyzed after 1 and 3 months by echocardiography and revealed no differences in competence between both groups. A confluent EC monolayer expressing eNOS/vWF was only found in re-endothelialized PV but not in decellularized PV, whereas the valve matrices were comparable repopulated with interstitial cells expressing SM-alpha-actin and procollagen-I. More thrombotic and neointima formations were observed in decellularized PV. No signs of calcification were detected in both PV types.

CONCLUSIONS

In vitro re-endothelialization of detergent-decellularized valves with autologous ECs under simulated physiological conditions significantly improves total EC valve coverage 3 months after implantation, whereas the valve repopulation with interstitial cells in vivo occurs most likely by cell migration inside the scaffold.

Thrombospondin-1 C-terminal-derived peptide protects thyroid cells from ceramide-induced apoptosis through the adenylyl cyclase pathway

Thrombospondin-1, a multi-modular matrix protein is able to interact with a variety of matrix proteins and cell-surface receptors. Thus it is multifunctional. In this work, we examined the role of thrombospondin-1 in ceramide-induced thyroid apoptosis. We focused on the VVM containing sequence localized in the C-terminal domain of the molecule. Primary cultured thyroid cells synthesize thrombospondin-1 depending on their morphological organization. As it leads thyrocytes to organize into monolayers before inducing apoptosis ceramide can modulate this organization. Here, we established that C(2)-ceramide treatment decreased thrombospondin-1 expression by interfering with the adenylyl cyclase pathway, thus leading to apoptosis. Furthermore, we demonstrated that the thrombospondin-1-derived peptide 4N1 (RFYVVMWK) abolished ceramide-induced thyroid cell death by preventing intracellular cAMP levels from dropping. Finally, we reported that 4N1-mediated inhibition of ceramide-induced apoptosis was consistently associated with a down-regulation of the caspase-3 processing. Integrin-associated protein receptor (IAP or CD47) was identified as a molecular relay mediating the observed 4N1 effects. Taken together, our results shed light for the first time on anti-apoptotic activities of the thrombospondin-1-derived peptide 4N1 and provide new information on how thrombospondin-1 may control apoptosis of non-tumoral cells.

Effects of fibulin-5 on attachment, adhesion, and proliferation of primary human endothelial cells

BACKGROUND

Fibulin-5 is a novel extracellular protein that is thought to act as a bridging peptide between elastin fibers and cell surface integrins in blood vessel wall. Fibulin-5 binding to endothelial cell (EC) surface integrins may effect cell proliferation and cell attachment to extracellular matrix (ECM) or to artificial surfaces. In this paper, we describe the effects of fibulin-5 on attachment, adhesion, and proliferation of primary human EC. After demonstrating that fibulin-5 over-expression inhibited EC proliferation, we tested the hypothesis that co-expression of fibulin-5 and VEGF165 will lead to unique EC phenotype that will exhibit increased adherence properties and retain its proliferation capacity.

METHODS AND RESULTS

Fibulin-5 and VEGF165 gene transfer to primary human saphenous vein endothelial cells was accomplished using retroviral vectors encoding the two genes. Transgene expression was verified using immunohistochemistry, Western blotting, and ELISA. Fibulin 5 over-expression tended to improve immediate EC attachment (30 min after seeding) and improved significantly adhesion (>40%) under shear stress tested 24h after EC seeding. The effects of fibulin-5 and VEGF165 on EC proliferation in the presence or absence of basic FGF were also tested. EC expressing fibulin-5 had reduced proliferation while VEGF165 co-expression ameliorated this effect.

CONCLUSION

Fibulin-5 improved EC attachment to artificial surfaces. Dual transfer of fibulin-5 and VEGF165 resulted in EC phenotype with increased adhesion and improved proliferation. This unique EC phenotype can be useful for tissue engineering on endovascular prostheses.

Vascular endothelial cadherin-mediated cell-cell adhesion regulated by a small GTPase, Rap1

Vascular endothelial cadherin (VE-cadherin), which belongs to the classical cadherin family, is localized at adherens junctions exclusively in vascular endothelial cells. Biochemical and biomechanical cues regulate the VE-cadherin adhesive potential by triggering the intracellular signals. VE-cadherin-mediated cell adhesion is required for cell survival and endothelial cell deadhesion is required for vascular development. It is therefore crucial to understand how VE-cadherin-based cell adhesion is controlled. This review summarizes the inter-endothelial cell adhesions and introduces our recent advance in Rap1-regulated VE-cadherin adhesion. A further analysis of the VE-cadherin recycling system will aid the understanding of cell adhesion/deadhesion mechanisms mediated by VE-cadherin in response to extracellular stimuli during development and angiogenesis.

Galectin interactions with extracellular matrix and effects on cellular function

In vivo, cells exist within a complex mixture of glycoproteins and proteoglycans termed the extracellular matrix (ECM). The components of the ECM are secreted by the cells in that site, and the ECM provides not only a physical support but also outside-in signals that regulate many cellular functions, including cell proliferation, differentiation, migration, and survival. Altering the composition of the ECM can thus significantly alter cell behavior. Many types of cells, including normal and malignant epithelial and mesenchymal cells, secrete galectin-1, a member of the galectin family of lectins, into the ECM surrounding the cells. Galectin-1 is known to regulate many of these same cellular functions (i.e., proliferation, differentiation, migration, and death), so that the presence of galectin-1 in ECM will modify the effects of ECM on cells. In this chapter, we present three types of assays that allow interrogation of the effects of galectin-1 on cell adhesion to ECM, cell migration through ECM, and cell death in ECM, using T-cell lines as a model cell type.