Phorbol esters induce angiogenesis in vitro from large-vessel endothelial cells

Effect of cellular communication network factor 2/connective tissue growth factor on tube formation by endothelial cells derived from human periodontal ligaments

OBJECTIVES

To clarify the role of cellular communication network factor 2/connective tissue growth factor (CCN2/CTGF) in periodontal tissue regeneration by investigating, the proliferative and tubulogenic responses of human endothelial cells obtained from the periodontal ligament to CCN2/CTGF.

DESIGN

Endothelial cells were seeded on agar gel medium with or without 50 ng/mL recombinant CCN2/CTGF (rCCN2/CTGF) and cultured for 6 h. Cells were morphologically and phenotypically analyzed by immunofluorescent microscopy. A colorimetric assay was used to evaluate cell proliferation, and transmission electron microscopy (TEM) was used for ultrastructural analysis.

RESULTS

The proliferation of endothelial cells was best promoted by rCCN2/CTGF at 50 ng/mL. In the control group, tube formation was not observed within 6 h. In contrast, endothelial cells seeded on the agar with 50 ng/mL rCCN2/CTGF clearly showed proliferation with network formation. Under a two-dimensional culture condition, a dense network of endothelial cells was not constructed on the plastic bottom. However, drastic morphological change was observed in the endothelial cells on the agar containing rCCN2/CTGF. The endothelial cells in the dense network were interconnected with each other and showed a tube-like structure. Tight junctions or adherens junctions were observed between the adjoining endothelial cells in the dense network.

CONCLUSIONS

CCN2/CTGF was found to promote the proliferation and tubulogenesis of endothelial cells from the periodontal ligament. These results suggest that CCN2/CTGF may contribute to the regeneration of damaged periodontal tissue by activating the remaining endothelial cells.

Local pharmacological induction of angiogenesis: Drugs for cells and cells as drugs

The past decades have seen significant advances in pro-angiogenic strategies based on delivery of molecules and cells for conditions such as coronary artery disease, critical limb ischemia and stroke. Currently, three major strategies are evolving. Firstly, various pharmacological agents (growth factors, interleukins, small molecules, DNA/RNA) are locally applied at the ischemic region. Secondly, preparations of living cells with considerable bandwidth of tissue origin, differentiation state and preconditioning are delivered locally, rarely systemically. Thirdly, based on the notion, that cellular effects can be attributed mostly to factors secreted in situ, the cellular secretome (conditioned media, exosomes) has come into the spotlight. We review these three strategies to achieve (neo)angiogenesis in ischemic tissue with focus on the angiogenic mechanisms they tackle, such as transcription cascades, specific signalling steps and cellular gases. We also include cancer-therapy relevant lymphangiogenesis, and shall seek to explain why there are often conflicting data between in vitro and in vivo. The lion's share of data encompassing all three approaches comes from experimental animal work and we shall highlight common technical obstacles in the delivery of therapeutic molecules, cells, and secretome. This plethora of preclinical data contrasts with a dearth of clinical studies. A lack of adequate delivery vehicles and standardised assessment of clinical outcomes might play a role here, as well as regulatory, IP, and manufacturing constraints of candidate compounds; in addition, completed clinical trials have yet to reveal a successful and efficacious strategy. As the biology of angiogenesis is understood well enough for clinical purposes, it will be a matter of time to achieve success for well-stratified patients, and most probably with a combination of compounds.

Phorbol ester-induced angiogenesis of endothelial progenitor cells: The role of NADPH oxidase-mediated, redox-related matrix metalloproteinase pathways

Endothelial progenitor cells (EPCs) may contribute to ischemia-induced angiogenesis in atherosclerotic diseases. The protein kinase C (PKC) family is involved in the regulation of angiogenesis, however the role of PKCα in EPCs during angiogenesis is unclear. The aim of this study was to evaluate the role of PKCα in EPCs during angiogenesis. Phorbol-12-myristate-13-acetate (PMA), a PKCα activator, significantly increased the activity and expression of matrix metalloproteinases (MMP) -2 and -9 in human (late outgrowth) EPCs in vitro. The MMPs promoted the migratory function and vascular formation of EPCs, which then contributed to neovascularization in a mouse hindlimb-ischemia model. Reactive oxygen species derived from nicotinamide adenine dinucleotide phosphate (NADPH) oxidase enhanced the expression of MMPs to increase the bioactivity of EPCs during angiogenesis. The mitogen-activated protein kinase (MAPK) signal pathway was associated with the activation of NADPH oxidase. PMA extensively activated the extracellular signal–regulated kinase (Erk) signal pathway to increase the expression of MMP-9. PMA also activated the p38, Erk, and c-Jun N-terminal kinase signal pathways to increase the expression of MMP-2. PMA-stimulated EPCs enhanced neovascularization in a mouse model of hindlimb ischemia via nuclear factor-κB translocation to up-regulation of the expression of MMP-2 and MMP-9. PMA could activate PKCα and promote the angiogenesis capacity of human EPCs via NADPH oxidase-mediated, redox-related, MMP-2 and MMP-9 pathways. The PKCα-activated, NADPH oxidase-mediated, redox-related MMP pathways could contribute to the function of human EPCs for ischemia-induced neovascularization, which may provide novel insights into the potential modification of EPCs for therapeutic angiogenesis.

Cooperative Effects of Vascular Angiogenesis and Lymphangiogenesis

In this study, we modeled lymphangiogenesis and vascular angiogenesis in a microdevice using a tissue engineering approach. Lymphatic vessels (LV) and blood vessels (BV) were fabricated by sacrificial molding with seeding human lymphatic endothelial cells and human umbilical vein endothelial cells into molded microchannels (600 μm diameter). During subsequent perfusion culture, lymphangiogenesis and vascular angiogenesis were induced by addition of phorbol 12-myristate 13-acetate (PMA) and VEGF-C or VEGF-A characterized by podoplanin and Prox-1 expression. The lymphatic capillaries formed button-like junctions treated with dexamethasone. To test the potential for screening anti-angiogenic (vascular and lymphatic) factors, antagonists of VEGF were introduced. We found that an inhibitor of VEGF-R3 did not completely suppress lymphatic angiogenesis with BVs present, although lymphatic angiogenesis was selectively prevented by addition of a VEGF-R3 inhibitor without BVs. To probe the mechanism of action, we focus on matrix metalloproteinase (MMP) secretion by vascular endothelial cells and lymphatic endothelial cells under monoculture or co-culture conditions. We found that vascular angiogenesis facilitated lymphangiogenesis via remodeling of the local microenvironment by the increased secretion of MMP, mainly by endothelial cells. Applications of this model include a drug screening assay for corneal disease and models for tumorigenesis including lymphatic angiogenesis and vascular angiogenesis.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Investigating Glioblastoma Angiogenesis Using A 3D in Vitro GelMA Microwell Platform

Angiogenesis is an indispensable mechanism in physiological and pathological development of tumors that requires an adequate blood supply. Therefore, understanding the angiogenesis mechanism of tumors has become an important research area to develop reliable and effective therapies for the treatment of tumors. Although several in vivo and in vitro models were developed and used to study the underlying mechanism of angiogenesis, they showed limited success. Therefore, there is an urgent need to build a stable and cost-effective three-dimensional (3D) in vitro angiogenesis model to investigate the tumor formation. In this study, we designed a 3D in vitro angiogenesis model based on gelatin methacrylate (GelMA) hydrogel microwells to mimic an in vivo-like microenvironment for co-cultured glioblastoma and endothelial cells. Our results confirmed the in vitro formation of microtubules during the angiogenic process. We believe that our cost-effective platform can be used for the high-throughput screening of anti-angiogenesis drugs and even for the development of better treatment strategies.

Matrix density alters zyxin phosphorylation, which limits peripheral process formation and extension in endothelial cells invading 3D collagen matrices

This study was designed to determine the optimal conditions required for known pro-angiogenic stimuli to elicit successful endothelial sprouting responses. We used an established, quantifiable model of endothelial cell (EC) sprout initiation where ECs were tested for invasion in low (1 mg/mL) and high density (5 mg/mL) 3D collagen matrices. Sphingosine 1-phosphate (S1P) alone, or S1P combined with stromal derived factor-1α (SDF) and phorbol ester (TPA), elicited robust sprouting responses. The ability of these factors to stimulate sprouting was more effective in higher density collagen matrices. S1P stimulation resulted in a significant increase in invasion distance, and with the exception of treatment groups containing phorbol ester, invasion distance was longer in 1mg/mL compared to 5mg/mL collagen matrices. Closer examination of cell morphology revealed that increasing matrix density and supplementing with SDF and TPA enhanced the formation of multicellular structures more closely resembling capillaries. TPA enhanced the frequency and size of lumen formation and correlated with a robust increase in phosphorylation of p42/p44 Erk kinase, while S1P and SDF did not. Also, a higher number of significantly longer extended processes formed in 5mg/mL compared to 1mg/mL collagen matrices. Because collagen matrices at higher density have been reported to be stiffer, we tested for changes in the mechanosensitive protein, zyxin. Interestingly, zyxin phosphorylation levels inversely correlated with matrix density, while levels of total zyxin did not change significantly. Immunofluorescence and localization studies revealed that total zyxin was distributed evenly throughout invading structures, while phosphorylated zyxin was slightly more intense in extended peripheral processes. Silencing zyxin expression increased extended process length and number of processes, while increasing zyxin levels decreased extended process length. Altogether these data indicate that ECs integrate signals from multiple exogenous factors, including changes in matrix density, to accomplish successful sprouting responses. We show here for the first time that zyxin limited the formation and extension of fine peripheral processes used by ECs for matrix interrogation, providing a molecular explanation for altered EC responses to high and low density collagen matrices.

Harvesting the potential of the human umbilical cord: isolation and characterisation of four cell types for tissue engineering applications

The human umbilical cord (UC) has attracted interest as a source of cells for many research applications. UC solid tissues contain four cell types: epithelial, stromal, smooth muscle and endothelial cells. We have developed a unique protocol for the sequential extraction of all four cell types from a single UC, allowing tissue reconstruction using multiple cell types from the same source. By combining perfusion, immersion and explant techniques, all four cell types have been successfully expanded in monolayer cultures. We have also characterised epithelial and Wharton's jelly cells (WJC) by immunolabelling of specific proteins. Epithelial cell yields averaged at 2.3 × 10(5) cells per centimetre UC, and the cells expressed an unusual combination of keratins typical of simple, mucous and stratified epithelia. Stromal cells in the Wharton's jelly expressed desmin, α-smooth muscle actin, elastin, keratins (K12, K16, K18 and K19), vimentin and collagens. Expression patterns in cultured cells resembled those found in situ except for basement membrane components and type III collagen. These stromal cells featured a sustained proliferation rate up to passage 12 after thawing. The mesenchymal stem cell (MSC) character of the WJC was confirmed by their expression of typical MSC surface markers and by adipogenic and osteogenic differentiation assays. To emphasise and demonstrate their potential for regenerative medicine, UC cell types were successfully used to produce human tissue-engineered constructs. Both bilayered stromal/epithelial and vascular substitutes were produced, establishing the versatility and importance of these cells for research and therapeutic applications.

Notch modulates VEGF action in endothelial cells by inducing Matrix Metalloprotease activity

BACKGROUND

In the vasculature, Notch signaling functions as a downstream effecter of Vascular Endothelial Growth Factor (VEGF) signaling. VEGF regulates sprouting angiogenesis in part by inducing and activating matrix metalloproteases (MMPs). This study sought to determine if VEGF regulation of MMPs was mediated via Notch signaling and to determine how Notch regulation of MMPs influenced endothelial cell morphogenesis.

METHODS AND RESULTS

We assessed the relationship between VEGF and Notch signaling in cultured human umbilical vein endothelial cells. Overexpression of VEGF-induced Notch4 and the Notch ligand, Dll4, activated Notch signaling, and altered endothelial cell morphology in a fashion similar to that induced by Notch activation. Expression of a secreted Notch antagonist (Notch1 decoy) suppressed VEGF-mediated activation of endothelial Notch signaling and endothelial morphogenesis. We demonstrate that Notch mediates VEGF-induced matrix metalloprotease activity via induction of MMP9 and MT1-MMP expression and activation of MMP2. Introduction of a MMP inhibitor blocked Notch-mediated endothelial morphogenesis. In mice, analysis of VEGF-induced dermal angiogenesis demonstrated that the Notch1 decoy reduced perivascular MMP9 expression.

CONCLUSIONS

Taken together, our data demonstrate that Notch signaling can act downstream of VEGF signaling to regulate endothelial cell morphogenesis via induction and activation of specific MMPs. In a murine model of VEGF-induced dermal angiogenesis, Notch inhibition led to reduced MMP9 expression.

The aortic ring model of angiogenesis: a quarter century of search and discovery

The aortic ring model has become one of the most widely used methods to study angiogenesis and its mechanisms. Many factors have contributed to its popularity including reproducibility, cost effectiveness, ease of use and good correlation with in vivo studies. In this system aortic rings embedded in biomatrix gels and cultured under chemically defined conditions generate arborizing vascular outgrowths which can be stimulated or inhibited with angiogenic regulators. Originally based on the rat aorta, the aortic ring model was later adapted to the mouse for the evaluation of specific molecular alterations in genetically modified animals. Viral transduction of the aortic rings has enabled investigators to overexpress genes of interest in the aortic cultures. Experiments on angiogenic mechanisms have demonstrated that formation of neovessels in aortic cultures is regulated by macrophages, pericytes and fibroblasts through a complex molecular cascade involving growth factors, inflammatory cytokines, axonal guidance cues, extracellular matrix (ECM) molecules and matrix-degrading proteolytic enzymes. These studies have shown that endothelial sprouting can be effectively blocked by depleting the aortic explants of macrophages or by interfering with the angiogenic cascade at multiple levels including growth factor signalling, cell adhesion and proteolytic degradation of the ECM. In this paper, we review the literature in this field and retrace the journey from our first morphological descriptions of the aortic outgrowths to the latest breakthroughs in the cellular and molecular regulation of aortic vessel growth and regression.

A notch1 ectodomain construct inhibits endothelial notch signaling, tumor growth, and angiogenesis

Notch signaling is required for vascular development and tumor angiogenesis. Although inhibition of the Notch ligand Delta-like 4 can restrict tumor growth and disrupt neovasculature, the effect of inhibiting Notch receptor function on angiogenesis has yet to be defined. In this study, we generated a soluble form of the Notch1 receptor (Notch1 decoy) and assessed its effect on angiogenesis in vitro and in vivo. Notch1 decoy expression reduced signaling stimulated by the binding of three distinct Notch ligands to Notch1 and inhibited morphogenesis of endothelial cells overexpressing Notch4. Thus, Notch1 decoy functioned as an antagonist of ligand-dependent Notch signaling. In mice, Notch1 decoy also inhibited vascular endothelial growth factor-induced angiogenesis in skin, establishing a role for Notch receptor function in this process. We tested the effects of Notch1 decoy on tumor angiogenesis using two models: mouse mammary Mm5MT cells overexpressing fibroblast growth factor 4 (Mm5MT-FGF4) and NGP human neuroblastoma cells. Exogenously expressed FGF4 induced Notch ligand expression in Mm5MT cells and xenografts. Notch1 decoy expression did not affect tumorigenicity of Mm5MT-FGF4 cells in vitro but restricted Mm5MT-FGF4 xenograft growth in mice while markedly impairing neoangiogenesis. Similarly, Notch1 decoy expression did not affect NGP cells in vitro but disrupted vessels and decreased tumor viability in vivo. These results strongly suggest that Notch receptor signaling is required for tumor neoangiogenesis and provides a new target for tumor therapy.

In vitro assays of angiogenesis for assessment of angiogenic and anti-angiogenic agents

Blood vessels, either in insufficient numbers or in excess, contribute to the pathogenesis of many diseases. Agents that stimulate angiogenesis can improve blood flow in patients with ischemic diseases, whereas anti-angiogenic agents are used to treat disorders ranging from macular degeneration to cancer. In this review I describe in vitro assays that can be used to assess the activity of agents that affect angiogenesis. Means of quantifying endothelial cell matrix degradation, migration, proliferation, apoptosis and morphogenesis are discussed, as are embryoid body, aortic ring and metatarsal assays of vessel outgrowth. Strengths and limitations of these techniques are also addressed.

Protein kinase C and downstream signaling pathways in a three-dimensional model of phorbol ester-induced angiogenesis

Angiogenesis, a critical process in both health and disease, is mediated by a number of signaling pathways. Although proangiogenic stimuli, including vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and the phorbol ester phorbol-12 myristate-13 acetate (PMA) are known to promote blood vessel formation, their downstream targets are ill defined. We sought to investigate the signaling pathways required for vessel assembly by utilizing a three-dimensional collagen matrix in which human umbilical vein endothelial cells (HUVECs) form tubular structures. Our data show that PMA is sufficient for the induction of angiogenesis, and that protein kinase C (PKC) is necessary for this process. Evaluation of PKC isoforms alpha and sigma revealed that these proteins are uniquely regulated. Characterization of an additional PMA target, protein kinase D (PKD) demonstrated that this enzyme becomes phosphorylated in HUVECs, and may therefore be involved in proangiogenic signaling. Further examination of downstream effectors of PKC showed that extracellular signal-regulated kinase (ERK) is critical for angiogenesis, and is accordingly phosphorylated in response to PMA. Surprisingly however, phosphorylation of ERK is independent of PKC activity. In addition, we show that the PKC target sphingosine kinase (SPK) is required for vessel formation. These findings illustrate the complexities of blood vessel formation, and suggest that activators utilize multiple independent pathways to invoke a complete angiogenic response.

Endothelial cell regulation of matrix metalloproteinases

The process of sprouting angiogenesis requires that the endothelial cells degrade the basement membrane matrix and migrate into the interstitial matrix. Matrix metalloproteinases are enzymes capable of cleaving numerous extracellular matrix proteins. Increased production and activity of matrix metalloproteinases in any cell type is associated with a more migratory and invasive phenotype. This paper describes results of recent in-vitro studies of the regulation of transcription and activation of MMP-2 and MT1-MMP in endothelial cells, as well as studies that examined roles of matrix metalloproteinases in activity-induced angiogenesis.

The roles of tissue engineering and vascularisation in the development of micro-vascular networks: a review

The construction of tissue-engineered devices for medical applications is now possible in vitro using cell culture and bioreactors. Although methods of incorporating them back into the host are available, current constructs depend purely on diffusion which limits their potential. The absence of a vascular network capable of distributing oxygen and other nutrients within the tissue-engineered device is a major limiting factor in creating vascularised artificial tissues. Though bio-hybrid prostheses such as vascular bypass grafts and skin substitutes have already been developed and are being used clinically, the absence of a capillary bed linking the two systems remains the missing link. In this review, the different approaches currently being or that have been applied to vascularise tissues are identified and discussed.

Alternagin-C, a Disintegrin-like Protein, Induces Vascular Endothelial Cell Growth Factor (VEGF) Expression and Endothelial Cell Proliferation in Vitro

Alternagin-C (ALT-C), a disintegrin-like protein purified from the venom of the Brazilian snake Bothrops alternatus, interacts with the major collagen I receptor, the alpha(2)beta(1) integrin, inhibiting collagen binding. Here we show that ALT-C also inhibits the adhesion of a mouse fibroblast cell line (NIH-3T3) to collagen I (IC(50) 2.2 microm). In addition, when immobilized on plate wells, ALT-C supports the adhesion of this cell line as well as of human vein endothelial cell (HUVEC). ALT-C (3 microm) does not detach cells that were previously bound to collagen I. ALT-C (5 nm) induces HUVEC proliferation in vitro, and it inhibits the positive effect of vascular endothelial growth factor (VEGF) or FGF-2 on the proliferation of these cells, thus suggesting a common mechanism for these proteins. Gene expression analysis of human fibroblasts growing on ALT-C- or collagen-coated plates showed that ALT-C and collagen I induce a very similar pattern of gene expression. When compared with cells growing on plastic only, ALT-C up-regulates the expression of 45 genes including the VEGF gene and down-regulates the expression of 30 genes. Fibroblast VEGF expression was confirmed by RT-PCR and ELISA assay. Up-regulation of the VEGF gene and other growth factors could explain the positive effect on HUVEC proliferation. ALT-C also strongly activates Akt/PKB phosphorylation, a signaling event involved in endothelial survival and angiogenesis. In conclusion, ALT-C acts as a survival factor, promoting adhesion and endothelial cell proliferation.

Sphingosine-1-phosphate signaling in endothelial activation

The signaling and functions of the Endothelial Differentiation Gene (EDG) family of G protein-coupled receptors have been extensively elucidated. All the members of EDG family were shown to be receptors for lysosphingolipids or lysophospholipids. EDG-1, the prototype of EDG family receptors, is a high affinity receptor for serum-borne bioactive lipid, Sphingosine-1-phosphate (S1P). S1P, secreted by thrombotic platelets, has been shown to regulate a variety of cellular responses, including survival, cytoskeletal remodeling, chemotaxis etc, via the activation of cell surface EDG receptors. Recently, a novel function of S1P in modulating angiogenic response has been demonstrated. This review will focus on S1P/ EDG-1 signaling in endothelial activation, in particular in the S1P-mediated adherens junctions assembly and chemotaxis in endothelial cells.

The role of COX-2 in angiogenesis and rheumatoid arthritis

Recent evidence suggests that cyclooxygenase (COX)-2 is a mediator of angiogenesis, and COX-2 activity is known to be upregulated in the rheumatoid arthritis (RA) synovium. We examined whether mediation of angiogenesis by COX-2 was occuring in cells of the RA synovium and in microvascular endothelial cells (ECs) that are similar to those found in the RA synovium. We demonstrate that rofecoxib, a selective COX-2 inhibitor, acts directly on human dermal microvascular ECs (HMVECs) to inhibit their chemotactic and tube forming ability. Likewise, pretreatment of HMVECs with rofecoxib significantly inhibited their ability to form tubes induced by conditioned media (CM) of activated RA synovial fibroblasts. When RA synovial fibroblasts were pretreated with rofecoxib for 16 h and then stimulated with interleukin (IL)-1beta, their CM induced significantly less HMVEC tube formation when compared with CM from vehicle-treated RA synovial fibroblasts. ELISAs performed on activated RA fibroblast CM for known proangiogenic factors demonstrated a significant reduction in bFGF, in addition to the expected decrease in PGE(2). Our studies suggest that COX-2-induced angiogenic activity is an active mechanism within diseased synovium and may provide an additional rationale for the use of COX-2 inhibitors in RA.

Synthesis and angiogenetic activity in the chick chorioallantoic membrane model of thymosin beta-15

Thymosin beta-15 (Tbeta15), a 44 amino acid peptide (MW = 5173) localized in human prostate and breast cancer tissues was successfully synthesized in multigram quantities using Fmoc solid-phase peptide synthesis. The synthesized product was shown to have the right structure by ESI and MALDI mass spectral techniques and amino acid analysis. Relatively high yield was achieved, which might be due to enhanced acid stability of the p-cyanotrityl resin used. The effect of the synthesized Tbeta15 on the angiogenesis process was investigated using the chick chorioallantoic membrane (CAM) in vivo model. At concentrations above 1 microg/10 microl per disc, Tbeta15 exhibited a positive effect on angiogenesis, comparable to the effect of the intense angiogenetic factor phorbol 12-myristate 13-acetate at a standard concentration of 0.1 microg/10 microl per disc. The results of this study contribute to the further elucidation of the biological regulatory role of thymosin peptides and provide helpful information in the investigation of their possible therapeutic potential.

VEGF receptor signal transduction

The family of vascular endothelial growth factors (VEGFs) currently includes VEGF-A, -B, -C, -D, -E, and placenta growth factor (PlGF). Several of these factors, notably VEGF-A, exist as different isoforms, which appear to have unique biological functions. The VEGF family proteins bind in a distinct pattern to three structurally related receptor tyrosine kinases, denoted VEGF receptor-1, -2, and -3. Neuropilins, heparan-sulfated proteoglycans, cadherins, and integrin alphavbeta3 serve as coreceptors for certain but not all VEGF proteins. Moreover, the angiogenic response to VEGF varies between different organs and is dependent on the genetic background of the animal. Inactivation of the genes for VEGF-A and VEGF receptor-2 leads to embryonal death due to the lack of endothelial cells. Inactivation of the gene encoding VEGF receptor-1 leads to an increased number of endothelial cells, which obstruct the vessel lumen. Inactivation of VEGF receptor-3 leads to abnormally organized vessels and cardiac failure. Although VEGF receptor-3 normally is expressed only on lymphatic endothelial cells, it is up-regulated on vascular as well as nonvascular tumors and appears to be involved in the regulation of angiogenesis. A large body of data, such as those on gene inactivation, indicate that VEGF receptor-1 exerts a negative regulatory effect on VEGF receptor-2, at least during embryogenesis. Recent data imply a positive regulatory role for VEGF receptor-1 in pathological angiogenesis. The VEGF proteins are in general poor mitogens, but binding of VEGF-A to VEGF receptor-2 leads to survival, migration, and differentiation of endothelial cells and mediation of vascular permeability. This review outlines the current knowledge about the signal transduction properties of VEGF receptors, with focus on VEGF receptor-2.

Effect of thymosin peptides on the chick chorioallantoic membrane angiogenesis model

The effect of alpha- and beta-thymosin peptides, namely prothymosin alpha (ProT(alpha)), thymosin alpha(1) (T(alpha)1), parathymosin alpha (ParaT(alpha)), thymosin beta(4) (Tbeta4), thymosin beta(10) (Tbeta10), and thymosin beta(9) (Tbeta9), on the angiogenesis process was investigated using the chick chorioallantoic membrane as an in vivo angiogenesis model. The thymosin peptides tested were applied in 10 microl aliquots containing 0.01-4 nmoles of Tbeta4, Tbeta10 or Tbeta9, 0.016-6.66 nmoles of T(alpha)1, 4.1 pmoles-1.66 nmoles of ProT(alpha), and 4.4 pmoles-1.76 nmoles of ParaT(alpha). Phorbol 12-myristate 13-acetate and hydrocortisone were also used as positive and negative control, respectively. Tbeta4, ProT(alpha) and T(alpha)1 were found to enhance angiogenesis, while Tbeta10, Tbeta9 and ParaT(alpha) exhibited an inhibitory effect on the angiogenesis process. When mixtures of Tbeta4 and Tbeta10 containing active amounts of the two peptides at different proportions were applied, the promoting effect of Tbeta4 on angiogenesis was reversed in the presence of increasing concentrations of Tbeta10 and vice versa. The effect of Tbeta10, Tbeta9, ProT(alpha) and ParaT(alpha), in parallel with Tbeta4 and T(alpha)1, on the angiogenesis process was investigated for the first time as far as we know and the results of this study offer more insight into the biological regulatory roles of thymosin peptides, and provide helpful information about their therapeutic potential. Whether these agents could be used either as inhibitors of angiogenesis in disease states where uncontrolled angiogenesis is involved, e.g. in carcinogenesis, or as angiogenesis promoters that could be useful in wound healing, fracture repair, peptic ulcers etc., remains to be further studied.

Response of bovine endothelial cells to FGF-2 and VEGF is dependent on their site of origin: Relevance to the regulation of angiogenesis

Angiogenesis, the formation of new capillary blood vessels, occurs almost exclusively in the microcirculation. This process is controlled by the interaction between factors with positive and negative regulatory activity. In this study, we have compared the effect of two well described positive regulators, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (FGF-2) on bovine adrenal cortex-derived microvascular endothelial (BME) and bovine aortic endothelial (BAE) cells. The parameters we assessed included (a) cellular reorganization and lumen formation following exposure of the apical cell surface to a three-dimensional collagen gel; (b) organization of the actin cytoskeleton; (c) expression of thrombospondin-1 (TSP-1), an endogenous negative regulator of angiogenesis; and (d) extracellular proteolytic activity mediated by the plasminogen activator (PA)/plasmin system. We found that (a) collagen gel overlay induces rapid reorganization and lumen formation in BME but not BAE cells; (b) FGF-2 but not VEGF induced dramatic reorganization of actin microfilaments in BME cells, with neither cytokine affecting BAE cells; (c) FGF-2 decreased TSP-1 protein and mRNA expression in BME cells, an effect which was specific for FGF-2 and BME cells, since TSP-1 protein levels were unaffected by VEGF in BME cells, or by FGF-2 or VEGF in BAE cells; (d) FGF-2 induced urokinase-type PA (uPA) in BME and BAE cells, while VEGF induced uPA and tissue-type PA in BME cells with no effect on BAE cells. Taken together, these findings reveal endothelial cell-type specific responses to FGF-2 and VEGF, and point to the greater specificity of these cytokines for endothelial cells of the microvasculature than for large vessel (aortic) endothelial cells. Furthermore, when viewed in the context of our previous observation on the synergistic interaction between VEGF and FGF-2, our present findings provide evidence for complementary mechanisms which, when acting in concert, might account for the synergistic effect.

Sustained ERK phosphorylation is necessary but not sufficient for MMP-9 regulation in endothelial cells: involvement of Ras-dependent and -independent pathways

Endothelial expression of matrix metalloproteinase-9 (MMP-9), which degrades native type IV collagen, was implicated as a prerequisite for angiogenesis. Therefore, the aim of this study was to determine signaling requirements that regulate MMP-9 expression in endothelial cells. Both, primary and permanent human umbilical vein endothelial cells (HUVEC and ECV304, respectively) were stimulated with phorbol 12-myristate 13-acetate (PMA) and the cytokine tumor necrosis factor-(alpha) (TNF(alpha)) to induce MMP-9 expression. While both cell types responded to PMA at the protein, mRNA and promoter level by induction of MMP-9, TNF(alpha) caused this response only in ECV304. Inhibitors specific for mitogen-activated protein/ERK kinase 1/2 (MEK1/2), protein kinase C (PKC), and Ras and co-transfections of wild-type and mutant Raf were used to elucidate the signaling cascades involved. Thus, we could show that the Raf/MEK/ERK cascade is mainly responsible for MMP-9 induction in endothelial cells and that this cascade is regulated independently of PKC and Ras subsequent to TNF(alpha) stimulation and in a PKC-dependent manner as a result of PMA treatment. In addition, PMA triggers a Ras-dependent signal transduction pathway bypassing the phosphorylation of ERK. Finally, we provide evidence that sustained phosphorylation of ERK1/2 is necessary but not sufficient for expression of MMP-9.

An in vitro model for the study of human bone marrow angiogenesis: role of hematopoietic cytokines

This study describes a human bone marrow endothelial cell culture in which endothelial cells are organized into capillary tubes. These endothelial cells were positive for von Willebrand Factor, expressed CD34, CD31, and L-fucose residues, took up acetylated low-density lipoproteins, contained Weibel-Palade bodies, and were ensheathed in a basal lamina (which included laminin beta1, EDa+ and EDb+ fibronectin, and collagen type iv). Pericytes expressing alpha-smooth muscle (alpha-SM) actin were spatially associated with the capillary tubes and there was a highly significant correlation between the number of capillary tubes and pericytes. In this model, basal angiogenesis was found to be vascular endothelial growth factor (VEGF)-dependent, because neutralization of endogenous VEGF induced a dramatic regression in the number of tubes. However, the presence of alpha-SM actin-expressing pericytes in the linings of endothelial tubes partially prevented the VEGF-neutralized tube regression. We also observed that nitric oxide production contributed to basal angiogenesis and that upregulation of nitric oxide increased the number of tubes. Tube numbers also decreased when antibodies neutralizing the integrin alphavbeta5 were applied to the cultures. Moreover, addition of any of the hematopoietic cytokines, erythropoietin, stem cell factor, granulocytic colony stimulating factor, or granulomonocytic colony stimulating factor induced a highly significant increase in tube formation. When erythropoietin and granulocytic colony stimulating factor were added, this increase was larger than the maximum increase observed with VEGF. Thus, we have described an in vitro model for human bone marrow angiogenesis in which pericytes and basal lamina matrix were associated with endothelial cells and formed fully organized capillary tubes. In this model, cytokines known to regulate hematopoiesis also seemed to be mediators of angiogenesis. This culture system may therefore prove to be a valuable tool for the study of hematopoietic cytokines on angiogenesis.

Sorting of murine vascular smooth muscle cells during wound healing in the chicken chorioallantoic membrane

The vascular wall is built up of a heterogeneous population of smooth muscle cells, which exhibit not only morphological distinctions but also important differences in the composition of their structural and contractile proteins. "Epithelioid" smooth muscle cells correspond to an intimal-like type and display features associated with immaturity, whereas "spindle-shaped" cells closely resemble the more typical medial smooth muscle population. We have investigated the integration of these two cell types into the vascular architecture of an in vivo wound-healing model. Stably transfected with the beta-galactosidase gene, intima- and media-like cells were injected intravenously into the chicken chorioallantoic membrane, within which superficial foci of granulation tissue had been created by thermal or chemical injury. At 24 to 72 h after injection, cells had honed in on the lesion sites and were observed in juxtaposition to the endothelial lining of the capillaries. They began to deposit laminin, thereby indicating an impending role in the formation of the vascular wall. Intima- and media-like smooth muscle cells did not differ in their capacity to associate with capillaries, and, in so doing, their biochemical lineage characteristics became indistinguishable from one another. However, intima-like cells also penetrated the adventitial and medial layers of arteries. These findings reveal vascular smooth muscle cells to possess an extraordinary degree of plasticity, being able to adapt flexibly to changes in functional demands.

Angiopoietin-1 and its receptor Tie-2 participate in the regulation of capillary-like tubule formation and survival of endothelial cells

Angiopoietin-1 (Ang-1) and its receptor Tie-2, a trans-membrane tyrosine kinase uniquely expressed by endothelial cells, are shown by null mutation studies to be essential to developmental angiogenesis. The phenotypic abnormalities in these knockout animals suggest that Tie-2 signaling is necessary for the maintenance and expansion of the primitive capillary network. We present in vitro evidence indicating that the Ang-1/Tie-2 system participates in the regulation of capillary tubule formation and is necessary for the survival of confluent endothelial cells. Although recombinant Ang-1, which induces Tie-2 phosphorylation, has no effect on the proliferation of endothelial cells, treatment of confluent adult bovine aortic endothelial cells (ABAE) cells grown on collagen gels with Ang-1 (100 ng/ml) causes the cells to migrate into the collagen gel and form capillary-like tubules. The tubule-forming effect of Ang-1 is similar to the effect caused by FGF-2. A soluble form of the Tie-2 extracellular domain, in fivefold molar excess, blocks Ang-1-induced tubule formation. Specific elimination of Tie-2 protein expression in cultured ABAE cells as a result of transfection with an antisense oligonucleotide causes cell death in a dose-dependent manner (IC(50) = 50 nM). The antisense treatment has no effect on cells that do not express Tie-2. Cells treated with antisense oligonucleotide show a sixfold increase in the rate of apoptosis as assessed by in situ end labeling of fragmented DNA. These findings are consistent with the view that Ang-1/Tie-2 signaling is essential for both angiogenesis and endothelial cell survival.

Molecular cloning of EDG-3 and N-Shc genes from the puffer fish, Fugu rubripes, and conservation of synteny with the human genome

EDG-3 is a receptor for sphingosine-1-phosphate mapped on human chromosome 9q22.1-q22.2. We used the compact Fugu genome for its linkage analysis. The Fugu EDG-3 was composed of one intron and two exons, encoding a 384 amino acid protein that has 56.9% homology with the human EDG-3. Approximately 3 kb apart, a neuronal Shc (N-Shc) gene was identified. It spans 7 kb containing 12 coding exons, and has an overall 53.4% similarity with the human protein. We mapped the human N-Shc gene to chromosome 9q21.3-q22.2. This is the first report of the genomic structure and the linkage of these two genes conserved between Fugu and human.

Examining the relationship between the gelatinolytic balance and the invasive capacity of endothelial cells

Angiogenesis and the formation of new blood vessels requires coordinated regulation of matrix proteolysis and endothelial cell migration. Cellular proteolytic capacity is the balance between secreted matrix metalloproteinases (MMP) and their inhibitors (TIMPs). We have examined the regulation of the gelatinase/TIMP balance by transforming growth factor-beta1 (TGF-beta1) and phorbol myristate acetate (PMA) in bovine endothelial cells. The low constitutive expression of gelatinase A/MMP-2 was upregulated by TGF-beta1 in a dose-dependent manner. Gelatinase B/MMP-9 was only detected upon treatment with either PMA or TGF-beta1. However, addition of both factors together revealed a striking synergistic effect causing upregulation of MMP-9 and downregulation of TIMPs, thereby increasing the net MMP-9/TIMP balance and the gelatinolytic capacity. These effects were observed at both the protein and mRNA levels. We demonstrate that changes in different members of the Jun oncogene family with distinct transactivation properties may account for this synergistic effect. We investigated the contribution of these changes in gelatinolytic balance to endothelial cell migration and invasion. The endothelial cells showed increased cell motility in response to PMA, but the addition of TGF-beta1 had an inhibitory effect. Hence, regulation of the MMP-9/TIMP balance failed to correlate with the migratory or invasive capacity. These results question a direct role for MMP-9 in endothelial cell motility and suggest that gelatinases may contribute in alternative ways to the angiogenic process.

A novel, quantitative model for study of endothelial cell migration and sprout formation within three-dimensional collagen matrices

Interactions between migratory endothelial cells (ECs) and surrounding extracellular matrix (ECM) are of central importance to vascular growth. Here, we present a new model of EC migration and morphogenesis within three-dimensional ECM termed "radial invasion of matrix by aggregated cells" (RIMAC). In the RIMAC model, single aggregates of defined numbers of bovine aortic ECs were embedded within small, lenticular gels of type I collagen supported by annuli of nylon mesh. Culture of the gels in nutrient media resulted in quantifiable, reproducible, radial migration of ECs into the collagen. The angiogenic proteins basic fibroblast growth factor (bFGF) and vascular endothelial growth factor (VEGF) each stimulated migration of ECs in a concentration-dependent manner. In combination, bFGF and VEGF stimulated migration synergistically. In contrast, transforming growth factor-beta1 inhibited migration of ECs. Low concentrations (0.1-0.5 ng/ml) of VEGF induced ECs to form multicellular sprouts, some of which possessed lumen-like spaces. Mitomycin C, an inhibitor of cell proliferation, did not affect the migration of ECs into collagen induced by 0.5 ng/ml VEGF but moderately inhibited migration induced by 5 ng/ml VEGF. Increasing the density (concentration) of the collagen gel inhibited the migration of single ECs and increased the branching and anastomosis of multicellular sprouts. We conclude that the RIMAC model is a highly efficacious assay for the screening of potentially angiogenic and angiostatic compounds and, moreover, is advantageous for mechanistic studies of vascular morphogenesis.

Distinct signal transduction pathways are utilized during the tube formation and survival phases of in vitro angiogenesis

Angiogenesis, the formation of new blood vessels from pre-existing ones, occurs during development, wound healing and cancer and involves stages that orchestrate a network of cooperative interactions. Peptide growth factors and extracellular matrix (ECM) components are two major groups of angiogenesis mediators. Among the different ECM proteins, collagens have been well-associated with in vivo angiogenesis. Using human umbilical vein endothelial cells (HUVEC) grown in 3-D collagen gels we show that: (1) HUVEC do not survive well in 3-D collagen gels due to rapid induction of apoptosis. (2) VEGF, a potent in vivo angiogenic factor, fails to induce tube formation. (3) PMA was effective in inducing tube formation and survival in HUVEC dispersed in 3-D collagen gels, activating MAP kinase, phosphoinositide 3-OH kinase (PI-3-kinase) and Akt/PKB (protein kinase B) pathways. (4) VEGF was effective in preventing PMA-induced tube-like structure regression after PMA-withdrawal by (5) activating the mitogen activated protein kinase (MAPK), rather than the Akt/PKB, signaling pathway.

Regulation of angiopoietin-2 mRNA levels in bovine microvascular endothelial cells by cytokines and hypoxia

Angiopoietin-2 (Ang2) is a ligand for the endothelial cell tyrosine kinase receptor Tie2 and counteracts blood vessel maturation/stability mediated by angiopoietin-1 (Ang1), the other known ligand of Tie2. Using degenerate oligonucleotides and reverse transcriptase-polymerase chain reaction, we have screened bovine microvascular endothelial (BME), aortic, lymphatic, pulmonary artery, and transformed fetal aortic endothelial cells, as well as rat smooth muscle cells for Ang1 and Ang2 expression. Except for high Ang2 mRNA levels found in BME cells, none of the endothelial cell types studied expressed appreciable levels of Ang1 or Ang2 mRNAs, whereas smooth muscle cells expressed both Ang1 and Ang2. BME cell Ang2 mRNA levels were increased by vascular endothelial growth factor (1.9- to 2.9-fold), basic fibroblast growth factor (1.6- to 2-fold), both cytokines in combination (2.9- to 4-fold), and hypoxia (3.1- to 5.6-fold) and were decreased by Ang1 (31% to 70%) or transforming growth factor-ss1 (64% to 81%). Ang2 also decreased (60% to 82%) BME cell Ang2 mRNA. mRNA levels for the Tie1 or Tie2 receptors were only slightly modulated under the conditions described above. These findings suggest that the angiogenic effect of a number of regulators may be achieved in part through the regulation of an autocrine loop of Ang2 activity in microvascular endothelial cells.

In vitro reconstruction of a human capillary-like network in a tissue-engineered skin equivalent

For patients with extensive burns, wound coverage with an autologous in vitro reconstructed skin made of both dermis and epidermis should be the best alternative to split-thickness graft. Unfortunately, various obstacles have delayed the widespread use of composite skin substitutes. Insufficient vascularization has been proposed as the most likely reason for their unreliable survival. Our purpose was to develop a vascular-like network inside tissue-engineered skin in order to improve graft vascularization. To reach this aim, we fabricated a collagen biopolymer in which three human cell types keratinocytes, dermal fibroblasts, and umbilical vein endothelial cells were cocultured. We demonstrated that the endothelialized skin equivalent (ESE) promoted spontaneous formation of capillary-like structures in a highly differentiated extracellular matrix. Immunohistochemical analysis and transmission electron microscopy of the ESE showed characteristics associated with the microvasculature in vivo (von Willebrand factor, Weibel-Palade bodies, basement membrane material, and intercellular junctions). We have developed the first endothelialized human tissue-engineered skin in which a network of capillary-like tubes is formed. The transplantation of this ESE on human should accelerate graft revascularization by inosculation of its preexisting capillary-like network with the patient's own blood vessels, as it is observed with autografts. In addition, the ESE turns out to be a promising in vitro angiogenesis model.

Regulation of endothelial cell motility by complexes of tetraspan molecules CD81/TAPA-1 and CD151/PETA-3 with alpha3 beta1 integrin localized at endothelial lateral junctions

Cell-to-cell junction structures play a key role in cell growth rate control and cell polarization. In endothelial cells (EC), these structures are also involved in regulation of vascular permeability and leukocyte extravasation. To identify novel components in EC intercellular junctions, mAbs against these cells were produced and selected using a morphological screening by immunofluorescence microscopy. Two novel mAbs, LIA1/1 and VJ1/16, specifically recognized a 25-kD protein that was selectively localized at cell-cell junctions of EC, both in the primary formation of cell monolayers and when EC reorganized in the process of wound healing. This antigen corresponded to the recently cloned platelet-endothelial tetraspan antigen CD151/PETA-3 (platelet-endothelial tetraspan antigen-3), and was consistently detected at EC cell-cell contact sites. In addition to CD151/PETA-3, two other members of the tetraspan superfamily, CD9 and CD81/ TAPA-1 (target of antiproliferative antibody-1), localized at endothelial cell-to-cell junctions. Biochemical analysis demonstrated molecular associations among tetraspan molecules themselves and those of CD151/ PETA-3 and CD9 with alpha3 beta1 integrin. Interestingly, mAbs directed to both CD151/PETA-3 and CD81/ TAPA-1 as well as mAb specific for alpha3 integrin, were able to inhibit the migration of ECs in the process of wound healing. The engagement of CD151/PETA-3 and CD81/TAPA-1 inhibited the movement of individual ECs, as determined by quantitative time-lapse video microscopy studies. Furthermore, mAbs against the CD151/PETA-3 molecule diminished the rate of EC invasion into collagen gels. In addition, these mAbs were able to increase the adhesion of EC to extracellular matrix proteins. Together these results indicate that CD81/TAPA-1 and CD151/PETA-3 tetraspan molecules are components of the endothelial lateral junctions implicated in the regulation of cell motility, either directly or by modulation of the function of the associated integrin heterodimers.

Endothelial cell monolayers as a tool for studying microvascular pathophysiology

Endothelial cells contribute to a variety of biological responses that facilitate organ function. This critical role of the endothelial cell has resulted in the development of different in vitro models that utilize monolayers of cultured cells to simulate conditions that exist in the intact animal. This review focuses on endothelial cell monolayers as a model system for research on certain pathophysiological conditions affecting the gastrointestinal tract. The advantages and limitations of endothelial cell monolayers are addressed, along with evolving technologies and strategies that hold promise for extending the utility of this in vitro model for studies of gastrointestinal function and disease.

The immediate-early gene product MAD-3/EDG-3/IkappaB alpha is an endogenous modulator of fibroblast growth factor-1 (FGF-1) dependent human endothelial cell growth

The tumor promoter phorbol 12-myristic 13-acetate inhibits the growth of human endothelial cells and induces the formation of capillary-like, tubular structures. We report the novel growth regulatory function of the immediate-early gene, edg-3, which is identical to the IkappaB alpha/MAD-3 gene. We employed phosphothioate oligonucleotides (PTO) directed against the translation initiation site of IkappaB alpha to inhibit its expression. The antisense IkappaB alpha PTO-treated cells exhibited an exaggerated growth response to fibroblast growth factor-1 (FGF-1). In contrast, IL-1-induced growth arrest response was not modulated. These data suggest that the early response gene IkappaB alpha is an endogenous regulator of endothelial cell growth in vitro.

The mouse gene for the inducible G-protein-coupled receptor edg-1

Edg-1, an immediate-early gene induced during the in vitro differentiation of human endothelial cells, encodes a G-protein-coupled receptor (GPR) that signals via the G(i)/mitogen-activated protein kinase (MAP kinase) pathway (Lee, M.-J., Evans, M., and Hla, T. (1996) J. Biol. Chem. 271, 11272-11279). It is a prototypical member of the subfamily of "orphan" receptors that are expressed in the cardiovascular and nervous systems. In this report, the mouse edg-1 gene was cloned and sequenced, and its expression patterns were defined. The edg-1 transcript was expressed in a wide variety of adult tissues including the brain, lung, liver, heart, and spleen. However, during embryogenesis, the edg-1 mRNA was induced late in development (after Embryonic Day 15.5) at centers of ossification. As a first step toward understanding the molecular basis of tissue-specific and inducible expression, the mouse edg-1 gene and its promoter were characterized. The mouse edg-1 gene is composed of two exons and is 4.9 kb in length. The second exon is large and contains the entire coding region and the 3'-untranslated region. The edg-1 promoter is TATA-less and contains GC-rich elements, and transcription initiation occurs from a single start site. The 5'-flanking region of the promoter contains several enhancer elements. However, the activity of the 5'-flanking region was suppressed by the repressor activity within the first intron. These data provide the basis for the further characterization of the regulation of the orphan GPR edg-1.

pp60c-src is required for the induction of a quiescent mesangial cell phenotype

The tyrosine kinase c-src associates with growth factor receptors, focal contacts and cytoskeletal proteins and is involved in signaling events. The aim of this study was to investigate the role of src in the regulation of mesangial cell (MC) proliferation and differentiation in three-dimensional (3D) culture in collagen gels. Using retroviral gene transfer we have overexpressed wild-type c-src, a kinase-negative c-src mutant (c-src295) and transforming v-src in MC. The MC differentiation in 3D culture was characterized by the formation of a nonproliferating multicellular network in control cells and in cells expressing wild-type c-src. Immunoblotting demonstrated a rapid down-regulation of the alpha-smooth muscle actin expression. The kinase-negative MC (c-src295) failed to differentiate, maintained a significant proliferative rate, and the alpha-smooth muscle actin expression remained stable during 3D culture. MC transformed with v-src showed a high level of tyrosine phosphorylation and proliferation in 3D culture. Analyses of proteins involved in cell cycle regulation demonstrated dephosphorylation of the retinoblastoma protein (Rb) during 3D culture in control and c-src transfected cells. Expression of v-src resulted in sustained Rb phosphorylation. Zymographic analysis of plasminogen activator (u-PA) revealed an inhibition of u-PA secretion in MC transfected with c-src295. These results indicate that c-src exerts regulatory effects on MC proliferation, cytoskeletal organization, matrix proteases and differentiation. Targeted manipulation of the c-src kinase may be useful in modulating MC behavior in vivo.

Role of lysophosphatidylcholine in the inhibition of endothelial cell motility by oxidized low density lipoprotein

Endothelial cell (EC) movement is required for the development and repair of blood vessels. We have previously shown that LDL oxidized by transition metals almost completely suppressed the wound-healing migratory response of vascular EC in vitro. We now report that lysophosphatidylcholine (lysoPC), a lipid component of oxidized LDL, has an important role in the antimigratory activity of the lipoprotein. Purified 1-palmitoyl lysoPC inhibited movement with a half-maximal activity at 12-15 micrometers, and near complete inhibition at 20 micrometers; the inhibitory concentration of lysoPC was consistent with its abundance in oxidized LDL. The inhibition was not due to cytotoxicity since protein synthesis was unaffected and since EC movement was restored after removal of lysoPC. Lysophospholipid activity was dependent on lipid structure. LysoPC's containing 1-position C16 or C18 saturated fatty acids were antimigratory, but those containing C < or = 14 saturated fatty acids or polyunsaturated fatty acids were not. The activity of 1-palmitoyl lysolipids with various head groups was examined. Lysophosphatidylinositol was more antimigratory than lysophosphatidylglycerol and lysophosphatidylcholine, which were more potent than lysophosphatidylserine and lysophosphatidylethanolamine. Monoglyceride was inactive while lysophosphatidate had promigratory activity. These results are consistent with head group size rather than charge as a critical determinant of activity. To show that lysophospholipids within an intact lipoprotein were active, LDL was treated with bee venom phospholipase A2 (PLA2). The modified lipoprotein inhibited EC movement to the same extent as iron-oxidized LDL and antimigratory activity correlated with the amount of lysoPC formed. To determine antimigratory activity of lysoPC present in oxidized LDL, lipid extracts from oxidized LDL were fractionated by normal phase HPLC. The fraction comigrating with lysoPC had nearly the same activity as the total extract confirming that lysoPC (or a co-eluting lipid) was a major antimigratory molecule in oxidized LDL. These studies demonstrate that lysoPC in oxidized LDL limit EC wound healing responses in vitro, and suggest a possible role for lysolipids in limiting endothelial regeneration after a denuding injury in vivo.

CD44-related chondroitin sulfate proteoglycan, a cell surface receptor implicated with tumor cell invasion, mediates endothelial cell migration on fibrinogen and invasion into a fibrin matrix

Microvascular endothelial cell invasion into the fibrin provisional matrix is an integral component of angiogenesis during wound repair. Cell surface receptors which interact with extracellular matrix proteins participate in cell migration and invasion. Malignant cells use CD44-related chondroitin sulfate proteoglycan (CSPG) as a matrix receptor to mediate migration and invasion. In this study, we examine whether cell surface CSPG can mediate similar events in nonmalignant wound microvascular endothelial cells or whether use of CSPG for migration and invasion is a property largely restricted to malignant cells. After inhibiting CSPG synthesis with p-nitrophenyl beta-d xylopyranoside (beta-d xyloside), wound microvascular endothelial cells were capable of attaching and spreading on the surface of a fibrin gel; however, their ability to invade the fibrin matrix was virtually eliminated. To begin to examine the mechanism by which endothelial cells use CSPG to invade fibrin matrices, cell adhesion and migration on fibrinogen was examined. Endothelial cell adhesion and migration on fibrinogen were inhibited by both beta-d xyloside and after cleavage of chondroitin sulfate from the core protein by chondroitinase ABC. We have determined that wound microvascular endothelial cells express the majority of their proteoglycan as CSPG and that the CSPG core protein is immunologically related to CD44. PCR studies show that these cells express both the "standard" (CD44H) isoform and an isoform containing the variably spliced exon V3. In addition, anti-CD44 antibody blocks endothelial cell migration on fibrinogen. Affinity chromatography studies reveal that partially purified microvascular endothelial cell CSPG binds fibrinogen. These findings suggest that CD44-related CSPG, a molecule implicated in the invasive behavior of tumor cells, is capable of binding fibrinogen/fibrin, thereby mediating endothelial cell migration and invasion into the fibrin provisional matrix during wound repair.

The inducible G protein-coupled receptor edg-1 signals via the G(i)/mitogen-activated protein kinase pathway

The edg-1 gene encodes an inducible G protein-coupled receptor (GPR) homologue that is induced during the in vitro differentiation of human endothelial cells. The aim of this study was to investigate the G protein-coupling and -signaling properties of the edg-1 polypeptide. The third cytosolic loop (i3) of edg-1 associates with G(i) alpha and G(o) alpha polypeptides in a guanosine 5'-O-(thiotriphosphate)-sensitive manner. Immunoprecipitation of the edg-1 polypeptide in transfected cells results in the co-precipitation of G(i) alpha 1 and G(i) alpha 3 polypeptides. These data strongly suggest that edg-1 is capable of coupling to the Gi pathway. Overexpression of the edg-1 GPR in human embryonic kidney 293 cells results in the sustained activation of the MAP kinase activity that is blocked by pertussis toxin treatment. Moreover, NIH3T3 cells permanently transfected with edg-1 exhibit enhanced MAP kinase and phospholipase A2 activities. These data suggest that the G(i)/mitogen-activated protein kinase pathway is a major signaling pathway regulated by the orphan receptor edg-1.

Contraction of fibrillar type I collagen by endothelial cells: a study in vitro

The formation of microvascular sprouts during angiogenesis requires that endothelial cells move through an extracellular matrix. Endothelial cells that migrate in vitro generate forces of traction that compress (i.e., contract) and reorganize vicinial extracellular matrix, a process that might be important for angiogenic invasion and morphogenesis in vivo. To study potential relationships between traction and angiogenesis, we have measured the contraction of fibrillar type I collagen gels by endothelial cells in vitro. We found that the capacity of bovine aortic endothelial (BAE) cells to remodel type I collagen was similar to that of human dermal fibroblasts--a cell type that generates high levels of traction. Contraction of collagen by BAE cells was stimulated by fetal bovine serum, human plasma-derived serum, bovine serum albumin, and the angiogenic factors phorbol myristate acetate and basic fibroblast growth factor (bFGF). In contrast, fibronectin and immunoglobulin from bovine serum, several nonserum proteins, and polyvinyl pyrrolidone (a nonproteinaceous substitute for albumin in artificial plasma) were not stimulatory. Contraction of collagen by BAE cells was diminished by an inhibitor of metalloproteinases (1,10-phenanthroline) at concentrations that were not obviously cytotoxic. Zymography of proteins secreted by BAE cells that had contracted collagen gels revealed matrix metalloproteinase 2. Subconfluent BAE cells that were migratory and proliferating were more effective contractors of collagen than were quiescent, confluent cells of the same strain. Moreover, bovine capillary endothelial cells contracted collagen gels to a greater degree than was seen with BAE cells. Collectively, our observations indicate that traction-driven reorganization of fibrillar type I collagen by endothelial cells is sensitive to different mediators, some of which, e.g., bFGF, are known regulators of angiogenesis in vivo.

Requirement for protein kinase C activation in basic fibroblast growth factor-induced human endothelial cell proliferation

The intracellular signaling mechanisms that mediate basic fibroblast growth factor (bFGF)-induced angiogenesis have not been fully identified. In particular, whether activation of the intracellular enzyme protein kinase C (PKC) is necessary or sufficient for bFGF-induced mitogenesis of human endothelial cells is not clear. Accordingly, the effect of bFGF stimulation on the Ca2+ increase and PKC activity of normal human endothelial cells (HEC) was studied, as was the effect of inhibition of PKC and the distribution of PKC isoenzymes in these cells. The addition of bFGF to cultured HEC increased overall PKC activity in the absence of an increase in intracellular Ca2+ and markedly stimulated their proliferation, as did the addition of PKC-activating phorbol esters. bFGF-induced proliferation was prevented by the PKC inhibitors chelerythrine and H-7 and by downregulation of PKC after prolonged incubation with phorbol esters. In contrast, these inhibitors did not prevent HEC proliferation induced by epidermal growth factor. Because of the failure of bFGF to increase Ca2+, we determined whether bFGF-induced proliferation could be mediated by novel or atypical PKC isoenzymes (which are not regulated by Ca2+). Investigation of the isoenzyme distribution of confluent and subconfluent HEC by immunoblotting, Northern transfer analysis, and polymerase chain reaction of reverse-transcribed RNA revealed the presence of several novel and atypical isoenzymes (PKC-delta, -eta, -theta, and -zeta) as well as small amounts of the conventional (Ca(2+)-regulated) isoenzymes PKC-alpha and -beta. Activation of PKC by bFGF, in the absence of an increase in intracellular Ca2+, suggests that one or more of these Ca(2+)-independent PKC isoenzymes are both necessary and sufficient for HEC proliferation after bFGF.

Effect of temperature on in vitro proliferative activity of human umbilical vein endothelial cells

Human umbilical vein endothelial cells, skin fibroblasts, and retinal pigment epithelial cells are cultivated in medium supplemented with 15 to 20% serum in our laboratory. The effects of various incubation temperatures on the proliferation of these cells was examined. Our study shows that the mitogenic response of the endothelial cells to a change of temperature differed markedly from that of the fibroblasts and epithelial cells. Cultivation of human umbilical vein endothelial at 37 degrees C required seeding densities as high as 1-2 x 10(4) cells/cm2, and yet resulted in a low growth rate and premature senescence. However, under the same culture conditions, but at 33 degrees C, the proliferative capacity of these endothelial cells was potentiated. The results were striking; at 33 degrees C the cells grew actively and the life span was extended. The number of cumulative population doublings increased fourfold compared with that for the same cells cultivated at 37 degrees C. The inoculum size could be reduced, since at 33 degrees C the endothelial cells were able to replicate at seeding densities as low as 20 cells/cm2. The cells serially subcultured at 33 degrees C retained morphological features and specific immunological markers of endothelial cells.

Organized type I collagen influences endothelial patterns during "spontaneous angiogenesis in vitro": planar cultures as models of vascular development

Selected strains of vascular endothelial cells, grown as confluent monolayers on tissue culture plastic, generate flat networks of cellular cords that resemble beds of capillaries--a phenomenon referred to as "spontaneous angiogenesis in vitro". We have studied spontaneous angiogenic activity by a clonal population (clone A) of bovine aortic endothelial cells to identify processes that mediate the development of cellular networks. Confluent cultures of clone A endothelial cells synthesized type I collagen, a portion of which was incorporated into narrow, extracellular cables that formed a planar network beneath the cellular monolayer. The collagenous cables acted as a template for the development of cellular networks: flattened, polygonal cells of the monolayer that were in direct contact with the cables acquired spindle shapes, associated to form cellular cords, and became elevated above the monolayer. Networks of cables and cellular cords did not form in a strain of bovine aortic endothelial cells that did not synthesize type I collagen, or when traction forces generated by clone A endothelial cells were inhibited with cytochalasin D. In a model of cable development, tension applied by a confluent monolayer of endothelial cells reorganized a sheetlike substrate of malleable type I collagen into a network of cables via the formation and radial enlargement of perforations through the collagen sheet. Our results point to a general involvement of extracellular matrix templates in two-dimensional (planar) models of vascular development in vitro. For several reasons, planar models simulate invasive angiogenesis poorly. In contrast, planar models might offer insights into the growth and development of planar vascular systems in vivo.

Characterization of edg-2, a human homologue of the Xenopus maternal transcript G10 from endothelial cells

We have isolated by polymerase chain reaction-amplified subtractive hybridization technique, several cDNA clones that are induced by phorbol myristic acetate in human umbilical vein endothelial cells (HUVEC). One such clone, termed edg-2, was sequenced and was found to encode a human homologue of a Xenopus maternal transcript G10. The deduced amino acid sequence of edg-2 contains a putative nuclear translocation sequence, an N-terminal acidic domain and a cysteine-rich C-terminal domain containing a putative Zinc-finger structure. The structure of edg-2 polypeptide suggests that it may be a nuclear regulator of transcription. The edg-2 mRNA was expressed ubiquitously in cell lines of epithelial and mesenchymal lineages. In addition, the edg-2 polypeptide sequence is highly conserved in evolution and is expressed by lower organisms such as yeast and C. elegans, suggesting that it may be an important regulator of general nuclear function.

Angiogenesis: models and modulators

Angiogenesis in vivo is distinguished by four stages: subsequent to the transduction of signals to differentiate, stage 1 is defined as an altered proteolytic balance of the cell allowing it to digest through the surrounding matrix. These committed cells then proliferate (stage 2), and migrate (stage 3) to form aligned cords of cells. The final stage is the development of vessel patency (stage 4), generated by a coalescing of intracellular vacuoles. Subsequently, these structures anastamose and the initial flow of blood through the new vessel completes the process. We present and discuss how the available models most closely represent phases of in vivo angiogenesis. The enhancement of angiogenesis by hyaluronic acid fragments, transforming growth factor beta, tumor necrosis factor alpha, angiogenin, okadaic acid, fibroblast growth factor, interleukin 8, vascular endothelial growth factor, haptoglobin, and gangliosides, and the inhibition of the process by hyaluronic acid, estrogen metabolites, genestein, heparin, cyclosporin A, placental RNase inhibitor, steroids, collagen synthesis inhibitors, thrombospondin, fumagellin, and protamine are also discussed.

PDGF-BB modulates endothelial proliferation and angiogenesis in vitro via PDGF beta-receptors

To delineate potential angiogenic roles of platelet-derived growth factor (PDGF), we have investigated PDGF and its receptors on bovine aortic endothelial cells that exhibit spontaneous angiogenesis in vitro (angiogenic endothelial cells). Initiation of cord/tube formation by angiogenic endothelial cells required bovine or human serum. Neutralization of PDGF-BB in human serum with a monoclonal anti-PDGF-BB antibody reduced cord/tube formation by 37 +/- 10%, whereas neutralizing anti-PDGF-AA and an IgG isotype-matched control antibody had no effect. DNA synthesis in response to PDGF-BB increased as the cords and tubes developed; furthermore, PDGF-BB induced the incorporation of BrdU in the nuclei of cells associated with these structures. PDGF beta-receptor (PDGF-beta) mRNA increased concomitantly with cord/tube formation, and PDGFR-beta were specifically localized by immunocytochemistry to developing and mature cords and tubes. However, PDGFR-beta transcripts and protein were undetectable in nonangiogenic endothelial cells, and PDGF alpha-receptor mRNA was not expressed in either endothelial cell strain. In contrast to nonangiogenic endothelial cells, angiogenic endothelial cells did not express the PDGF B-chain, the required ligand for the PDGFR-beta. We conclude that (a) PDGF-BB can contribute to angiogenesis in vitro, (b) PDGFR-beta are specific for cord/tube-forming endothelial cells and mediate endothelial proliferation and cord/tube formation, and (c) in angiogenic and nonangiogenic endothelial cells, the expression of PDGFR-beta and PDGF B-chain is inversely correlated. We therefore suggest that paracrine PDGF might amplify angiogenesis via direct action on endothelially expressed PDGFR-beta.

Intense vascular sprouting from rat femoral vein induced by prostaglandins E1 and E2

The formation of new capillaries from the rat femoral vein was specifically explored to assess whether venous vessels of this caliber may participate in the process of angiogenesis. Prostaglandins of the E series (PGE1 and PGE2) were administered into the soft connective tissue surrounding the rat femoral vessels as angiogenic inducers. In these conditions, between 2 and 7 days, a great number of new capillaries were observed in the media of the femoral vein, arising from the endothelial cells (EC) in the intima. The events of the capillary growth from the femoral vein included EC activation, local degradation of the basal membrane followed by migration and proliferation of EC, solid sprout formation with posterior canalization, development of a new basal membrane, and appearance of pericytes around the new capillary. Although numerous vascular buds were also observed arising from the small venules and capillaries in the periadventitial tissues, they were separated at first from those in the media of the femoral vein by the venous adventitia. Later, connections were observed between both newly formed microcirculations. The present study shows the capacity of PGE1 and PGE2 in the extravascular position of inducing capillary sprouting from veins. Furthermore, the observations provide greater evidence that vessels with characteristics similar to those of the rat femoral vein may contribute to angiogenesis, on occasion with an intense neovascularization. This fact may be of interest for the establishment of a functional circulation after angiogenesis by anastomoses of the new capillaries with those arising from pre-existing vessels of greater caliber than the venules.