Large-vessel endothelium switches to a microvascular phenotype during angiogenesis in collagen gel culture of rat aorta

Soluble Mediators Produced by Pro-Resolving Macrophages Inhibit Angiogenesis

Different subtypes of macrophages have been shown to participate in different stages of inflammation and tissue repair. In the late stage of tissue repair, the macrophages, following their engulfment of apoptotic neutrophils, acquire a new phenotype termed alternatively activated macrophages. These macrophages produce growth factors, such as vascular endothelial growth factor (VEGF), that facilitate the angiogenic response as part of tissue restoration. Then, in the later stages of tissue healing, capillary regression takes place. It is presently unknown whether macrophages play an antiangiogenic role in the final stages of tissue repair. Here, we examined whether soluble mediators secreted by pro-resolving CD11b^low macrophages (Mres) inhibit angiogenesis in the context of the resolution of tissue repair. Our findings indicate that soluble mediators produced by ex vivo generated Mres (CM-Mres) attenuate angiogenesis in vitro by inhibiting human umbilical vein endothelial cell (HUVEC) proliferation by lowering their cyclin D1 expression. In addition, CM-Mres lowered HUVEC survival by inducing caspase 3/7 activation, and also inhibited VEGFR2 activation via VEGF. HUVEC migration and differentiation to tubular-like structure was also inhibited by CM-Mres. Similarly, CM-Mres significantly inhibited neovascularization as depicted ex vivo by utilizing the rat aorta ring assay and in vivo by utilizing the chick chorioallantoic membrane assay. Notably endostatin, which was shown previously to exert its antiangiogenic effect by inhibiting proliferation, survival, motility, and morphogenesis of endothelial cells via inhibition of VEGFR2 activation, is produced by Mres. Taken together, our results suggest that a specialized subset of macrophages that appear during the resolution of inflammation can produce antiangiogenic mediators, such as endostatin. These mediators can halt angiogenesis, thereby restoring tissue structure.

Zerumbone Suppresses Angiogenesis in HepG2 Cells through Inhibition of Matrix Metalloproteinase-9, Vascular Endothelial Growth Factor, and Vascular Endothelial Growth Factor Receptor Expressions

Context

Due to increase in the number of patients with impaired immunity, the incidence of liver cancer has increased considerably.

Aims

The aim of this study is the investigation the in vitro anticancer effect of zerumbone (ZER) on hepatocellular carcinoma (HCC).

Materials and Methods

The anticancer mechanism of ZER was determined by the rat aortic ring, human umbilical vein endothelial cells (HUVECs) proliferation, chorioallantoic membrane, cell migration, and proliferation inhibition assays.

Results

Our results showed that ZER reduced tube formation by HUVECs effectively inhibits new blood vessel and tissue matrix formation. Western blot analysis revealed that ZER significantly (P < 0.05) decreased expression of molecular effectors of angiogenesis, the matrix metalloproteinase-9, vascular endothelial growth factor (VEGF), and VEGF receptor proteins. We found that ZER inhibited the proliferation and suppressed migration of HepG2 cell in dose-dependent manner.

Statistical Analysis Used

Statistical analyses were performed according to the Statistical Package for Social Science (SPSS) version 17.0. The data were expressed as the mean ± standard deviation and analyzed using a one-way analysis of variance. A P < 0.05 was considered statistically significant.

Conclusion

The study for the first time showed that ZER is an inhibitor angiogenesis, tumor growth, and spread, which is suggested to be the mechanisms for its anti-HCC effect.

SUMMARY

Tumor angiogenesis has currently become an important research area for the control of cancer growth and metastasis. The current study determined the effect of zerumbone on factors associated with angiogenesis that occurs in tumor formation. Abbreviations used: ZER: Zerumbone, MMP-9: Matrix metalloproteinase-9, VEGF: Vascular endothelial growth factor, VEGFR: Vascular endothelial growth factor receptor, HUVECs: Human umbilical vein endothelial cells, HCC: Hepatocellular carcinoma, HIFCS: Heat inactivated fetal calf serum, DMSO: Dimethyl sulfoxide, EDTA: Ethyldiaminetetraacetic acid, Ig: Immunoglobulin, CAM: Chorioallantoic membrane, HRP: Horseradish peroxidase, NIH: National Institutes of Health, MTT: Microtetrazolium, SPSS: Statistical Package for Social Science.

Methodological Approach to Use Fresh and Cryopreserved Vessels as Tools to Analyze Pharmacological Modulation of the Angiogenic Growth

The sprouting of new vessels is greatly influenced by the procedure chosen. We sought to optimize the experimental conditions of the angiogenic growth of fresh and cryopreserved vessels cultured in Matrigel with the aim to use this system to analyze the pharmacological modulation of the process. Segments of second-order branches of rat mesenteric resistance arteries, thoracic aorta of rat or mouse, and cryopreserved rat aorta and human femoral arteries were cultured in Matrigel for 7-21 days in different mediums, as well as in the absence of endothelial or adventitia layer. Quantification of the angiogenic growth was performed by either direct measurement of the mean length of the neovessels or by calcein AM staining and determination of fluorescence intensity and area. Fresh and cryopreserved arterial rings incubated in Matrigel exhibited a spontaneous angiogenic response that was strongly accelerated by fetal calf serum. Addition of vascular endothelial growth factor, fibroblast growth factor, endothelial growth factor, or recombinant insulin-like growth factor failed to increase aortic sprouting, unless all were added together. Removal of adventitia, but not the endothelial layer, abrogated the angiogenic response of aortic rings. Determination of the mean neovessel length is an easy and accurate method to quantify the angiogenic growth devoid of confounding factors, such as inclusion of other cellular types surrounding the neovessels. Activity of a α1-adrenoceptor agonist (phenylephrine) and its inhibition by a selective antagonist (prazosin) were analyzed to prove the usefulness of the Matrigel system to evaluate the pharmacological modulation of the angiogenic growth.

A Modified Aortic Ring Assay to Assess Angiogenic Potential In Vitro

Angiogenesis, an integral part of many physiological and pathological processes, is a tightly regulated multistep process. Angiogenesis assays are used to clarify the molecular mechanisms and screen for pharmacological inhibitors. However, most in vitro angiogenesis models measure only one aspect of this process, whereas in vivo assays are complex and difficult to interpret. The ex vivo aortic ring model allows the study of many key features of angiogenesis, such as endothelial activation, branching, and remodeling as well as later steps such as pericyte acquisition. This model can be modified to include genetic manipulation and can be used to assess the pro- or anti-angiogenic effects of compounds in a relatively controlled system.

Microvasculature alters the dispersion properties of shear waves--a multi-frequency MR elastography study

Magnetic Resonance Elastography (MRE) uses macroscopic shear wave propagation to quantify mechanical properties of soft tissues. Micro-obstacles are capable of affecting the macroscopic dispersion properties of shear waves. Since disease or therapy can change the mechanical integrity and organization of vascular structures, MRE should be able to sense these changes if blood vessels represent a source for wave scattering. To verify this, MRE was performed to quantify alteration of the shear wave speed cs due to the presence of vascular outgrowths using an aortic ring model. Eighteen fragments of rat aorta included in a Matrigel matrix (n=6 without outgrowths, n=6 with a radial outgrowth extent of ~600 µm and n=6 with ~850 µm) were imaged using a 7 Tesla MR scanner (Bruker, PharmaScan). High resolution anatomical images were acquired in addition to multi-frequency MRE (ν = 100, 115, 125, 135 and 150 Hz). Average cs was measured within a ring of ~900 µm thickness encompassing the aorta and were normalized to cs0 of the corresponding Matrigel. The frequency dependence was fit to the power law model cs ~ν(y). After scanning, optical microscopy was performed to visualize outgrowths. Results demonstrated that in presence of vascular outgrowths (1) normalized cs significantly increased for the three highest frequencies (Kruskal-Wallis test, P = 0.0002 at 125 Hz and P = 0.002 at 135 Hz and P = 0.003 at 150 Hz) but not for the two lowest (Kruskal-Wallis test, P = 0.63 at 100 Hz and P = 0.87 at 115 Hz), and (2) normalized cs followed a power law behavior not seen in absence of vascular outgrowths (ANOVA test, P < 0.0001). These results showed that vascular outgrowths acted as micro-obstacles altering the dispersion relationships of propagating shear waves and that MRE could provide valuable information about microvascular changes.

Detection of the Hematopoietic Stem and Progenitor Cell Marker CD133 during Angiogenesis in Three-Dimensional Collagen Gel Culture

We detected the hematopoietic stem and progenitor cell marker CD133 using immunogold labeling during angiogenesis in a three-dimensional collagen gel culture. CD133-positive cells were present in capillary tubes newly formed from aortic explants in vitro. The CD133-positive cell population had the capacity to form capillary tubes. Lovastatin strongly inhibited cell migration from aortic explants and caused the degradation of the capillary tubes. The present study provides insight into the function of CD133 during angiogenesis as well as an explanation for the antiangiogenic effect of statins.

Miniaturized assays of angiogenesis in vitro

Assays of angiogenesis in vitro provide insights into vascular development and are useful in studies of agents that modulate blood vessel formation. This chapter describes techniques to induce angiogenesis-like sprouting from aortic and microvascular explants cultured in 3-dimensional native collagen gels. The chapter focuses on explants derived from mice and use of a miniaturized format that permits efficient utilization of reagents, ease of processing, rapid analysis, and conventional imaging.

Angiogenesis In Vitro Utilizing Murine Vascular Explants in Miniaturized 3-Dimensional Collagen Gels

Models of angiogenesis in vitro are used to study blood vessel morphogenesis and the effects of compounds that influence vascular growth. Herein, we describe techniques to induce angiogenesis-like sprouting from explants of mouse aortae and microvessels cultured in 3-dimensional gels of native type I collagen. The gels are supported by rings of nylon mesh that are sized to fit in 96-well culture plates. This mechanically-supported, miniaturized, 3-dimensional culture system requires only small quantities of cells and reagents and facilitates handling, staining, and imaging by conventional and confocal microscopy.

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.

Sertoli cells enhance formation of capillary-like structures in vitro

Sertoli cells isolated from the testis (referred to as extratesticular Sertoli cells) have been shown to facilitate allo- and xenogeneic cell transplantations. It appears likely that the ability of these cells to enhance the success of cell engraftment is due, in part, to the retention of their intratesticular functions of trophic support and immunoprotection. Sertoli cells also are involved in the regulation of angiogenesis in the testis, which may also contribute to enhanced cell engraftment success facilitated by extratesticular Sertoli cells. Because the maintenance of the cell's intratesticular angiogenic function has not yet been evaluated for extratesticular Sertoli cells, this study examined the cell's ability to enhance angiogenesis in vitro. Sertoli cell conditioned media were derived from isolated rat Sertoli cell cultures and used in a rat aortic model of induced angiogenesis, in endothelial and smooth muscle cell monocultures, and in endothelial smooth muscle cocultures. An angiogenic rat cytokine array identified angiogenic factors in the control and conditioned media. Aorta sections incubated with Sertoli cell conditioned media showed a marked increase in the formation of capillary-like structures when compared to controls. Likewise, endothelial cells incubated in conditioned media organized into capillary-like structures not observed when incubated in control media. In coculture, smooth muscle cells were associated with endothelial cell-derived capillary-like structures only when incubated in conditioned media. Cytokine arrays indicated the presence and a qualitative increase of specific angiogenic growth factors in Sertoli cell conditioned media not observed in control media. Results indicate that extratesticular Sertoli cells retain their intratesticular angiogenic function in vitro.

Three-dimensional in vitro anglogenesis in the rat aortic ring model

Angiogenesis is a complex sequential process involving endothelial activation, basement membrane degradation, endothelial sprouting from the parent vessel, invasion of the extracellular matrix, endothelial proliferation, vessel elongation, branching, anastomosis, increases in vessel diameter, basement membrane formation, pericyte acquisition, and remodelling. Most in vitro angiogenesis assays are two-dimensional and measure only one facet of this process, generally endothelial proliferation, migration, or tube formation. The two-dimensional nature of the assays also ignores the differences in endothelial phenotype seen in three-dimensional models and in vivo. The in vitro serum-free three-dimensional rat aortic model closely approximates the complexities of angiogenesis in vivo, from endothelial activation to pericyte acquisition and remodelling, and most of these can be quantified by image analysis, immunohistochemistry, and biochemical analysis. It is easily manipulated using molecular biological intervention or exogenous inhibitors and activators in a relatively controlled system.

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.

Culture of murine aortic explants in 3-dimensional extracellular matrix: a novel, miniaturized assay of angiogenesis in vitro

Assays of angiogenesis in vitro are critical to the study of vascular morphogenesis and to the evaluation of therapeutic compounds that promote or inhibit vascular growth. Culture of explanted aortic segments from rats or mice in a 3-dimensional extracellular matrix (ECM) is one of the most effective ways to generate capillary-like endothelial sprouts in vitro. We have modified the classic aortic explant model by placing the aortic segments from mice within small (5.6 mm diameter, 30 microl volume) lenticular hydrogels of type I collagen supported at the edge by nylon mesh rings. This method of culture, referred to as the "miniature ring-supported gel" (MRSG) assay, optimizes handling, cytological staining, and conventional imaging of the specimen and permits use of minimal volumes of reagents in a 96-well tissue culture format. We have used the MRSG assay to quantify the impaired angiogenic response of aged mice relative to young mice and to show that aged mice have significantly decreased sprout formation, but have similar levels of invasion of vascular smooth muscle cells into the supportive ECM. The MRSG assay, which combines low volume, physically robust gels in conjunction with mouse aortic segments, may prove to be a highly useful tool in studies of the process and control of vascular growth.

Inhibition of angiogenesis and tumor growth by beta and gamma-secretase inhibitors

The involvement of beta-secretase and gamma-secretase in producing the beta-amyloid component of senile plaques found in the brain of Alzheimer's patients has fueled a major research effort to design selective inhibitors of these proteases. Interestingly, gamma-secretase cleaves several proteins including Notch, E-cadherin, CD44 and ErbB-4 (erythroblastic leukemia viral oncogene homolog 4), which are important modulators of angiogenesis. The beta-amyloid precursor protein, which is cleaved by beta-secretase and gamma-secretase to produce beta-amyloid, is highly expressed in the endothelium of neoforming vessels suggesting that it might play a role during angiogenesis. These data prompted us to explore the effects of beta and gamma-secretase inhibitors of different structures on angiogenesis and tumor growth. Both the gamma and beta-secretase inhibitors tested reduce endothelial cell proliferation without inducing cellular toxicity, suppress the formation of capillary structures in vitro and oppose the sprouting of microvessel outgrowths in the rat aortic ring model of angiogenesis. Moreover, they potently inhibit the growth and vascularization of human glioblastoma and human lung adenocarcinoma tumors xenotransplanted into nude mice. Altogether these data suggest that the gamma and beta-secretases play an essential role during angiogenesis and that inhibitors of the beta and gamma-secretases may constitute new classes of anti-angiogenic and anti-tumoral compounds.

Use of embryonic stem cell-derived endothelial cells as a cell source to generate vessel structures in vitro

Embryonic stem (ES) cells could potentially serve as an excellent cell source for various applications in regenerative medicine and tissue engineering. Our laboratory is particularly interested in generating a reproducible endothelial cell source for the development of prevascularized materials for tissue/organ reconstruction. After developing methods to isolate highly purified (>96%) proliferating populations of endothelial cells from mouse embryonic stem cells, we tested their ability to form three-dimensional (3-D) vascular structures in vitro. The ES cell-derived endothelial cells were embedded in 3-D collagen gel constructs with rat tail collagen type I (2 mg/mL) at a concentration of 10(6) cells/mL of gel. The gels were observed daily with a phase-contrast microscope to analyze the time course for endothelial cell assembly. The first vessels were observed between days 3 and 5 after gel construct formation. The number and complexity of structures steadily increased, reaching a maximum before beginning to regress. By 2 weeks, all vessel-like structures had regressed back to single cells. Histology and fluorescent images of the vessel-like structures verified that tube structures were multicellular and could develop patent lumens. We have shown that endothelial cells derived, purified and expanded in vitro from ES cells sustain an important endothelial cell function, the ability to undergo vasculogenesis in collagen gels, indicating that endothelial products derived in vitro from stem cells could be useful in regenerative medicine applications.

STI-571 inhibits in vitro angiogenesis

Compounds that block angiogenesis are effective in the treatment of certain cancers and other angiogenesis-related diseases. Many of these compounds specifically target the rapidly proliferating and migrating endothelial cell. However, angiogenesis is a multi-faceted process involving heterotypic interactions between various cell types. For example, PDGFBB is an important cytokine secreted by endothelial cells that attracts smooth muscle cells to surround and stabilize a nascent vessel. Therefore, we hypothesized that STI-571, a tyrosine kinase inhibitor with PDGFbeta receptor activity, would inhibit angiogenesis through an anti-migratory effect on smooth muscle cells. We demonstrate that STI-571 completely inhibits in vitro angiogenesis in fibrinogen-embedded mouse aorta. Furthermore, this angiostatic property was due mainly to an anti-migratory and anti-proliferative effect upon smooth muscle cells. These data suggest that STI-571, in addition to its efficacy in the treatment of certain cancers, may also prove to be clinically useful in diseases characterized by unregulated angiogenesis.

Rat aorta-derived mural precursor cells express the Tie2 receptor and respond directly to stimulation by angiopoietins

Recent studies have implicated the Tie2 tyrosine-kinase receptor and its main ligands--angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2)--as crucial regulators of mural cell recruitment during angiogenesis. Angiopoietin-mediated activation of Tie2 promotes perivascular mural cell assembly, but the mechanisms regulating this process are poorly understood because differentiated mural cells do not have the Tie2 receptor, which is reportedly expressed only in endothelial cells. There is also no direct evidence that Tie2 activation results in production of mural cell chemoattractants by the endothelium. In the rat aorta model of angiogenesis, developing microvessels recruit mural cells from the intimal/subintimal layers of the aortic wall. Ang-1 and Ang-2 promote angiogenesis in this system, stimulating branching morphogenesis and mural cell assembly. Mural precursor cells (MPCs) isolated with a nonenzymatic method from the intimal aspect of the rat aorta were positive for smooth muscle cell markers (alpha-smooth muscle actin and calponin) and negative for endothelial markers (factor-VIII-related antigen and CD31). These cells responded chemotactically to Ang-1 and Ang-2, and secreted MMP-2 when treated with these factors. Western-blot analysis, immunocytochemistry and RT-PCR demonstrated that MPCs express the Tie2 receptor. Immunoprecipitation showed phosphorylation of MPC Tie2 on tyrosine residues upon stimulation with Ang-1 or Ang-2. Surface expression of Tie2 was further demonstrated by isolating Tie2+/alpha-smooth muscle actin+ MPCs from primary aortic outgrowths with anti-Tie2-IgG-coated magnetic beads. Immunostaining of the rat aorta confirmed expression of Tie2 not only in endothelial cells but also in nonendothelial mesenchymal cells located in the aortic intimal/subintimal layers, which are the source of MPCs. These data indicate that the aortic wall contains Tie2+ nonendothelial mesenchymal cells and suggest that Tie2-related recruitment of mural cells during angiogenesis may occur through angiopoietin-mediated direct stimulation of these cells.

Isolation and characterization of large numbers of endothelial cells for studies of cell signaling

Studies of endothelial cell signaling involving cholesterol-rich domains require large numbers of cells of bona fide origin. The growth of any cell in culture, particularly for extended periods, results in an altered phenotype that could include changes in the properties of caveolae and lipid raft structures. While continuously propagated cells are used to study specific questions because their origin is known and because proteins of interest are still expressed, such reasoning is an oversimplification and can lead to findings that are descriptive of the cell's adaptation to culture rather than its original phenotype. We are particularly cognizant of this concern as we examine caveolar signaling domains in endothelial cells. Here we present a reproducible method for the isolation and characterization of large numbers of bona fide endothelial cells suitable for studies of the regulation of receptor signal transduction. Digestion of guinea pig hearts with collagenase results in the liberation of cells that adhere to collagen-coated plastic and express platelet-endothelial cell adhesion molecule 1 (PECAM-1) and binding sites for Ulex europaeus agglutinin 1 (UEA-1) that permit segregation of cells using fluorescence-activated cell sorting. Growth of cells over 7 doublings results in enrichment in the expression of both PECAM-1 and UEA-1 and retention of functional low-density lipoprotein receptor. The ability of cells to differentiate into endothelial tubes at any stage during their characterization up to 20 doublings in culture suggests that this method can be employed to generate endothelial cells that are minimally altered from their site of origin.

Effects of collagen gel configuration on behavior of vascular smooth muscle cells in vitro: association with vascular morphogenesis

The growth, behavior, and contractile protein expression of rabbit aortic smooth muscle cells (SMC) grown on, between layers, or within a collagen gel was investigated by confocal laser scanning fluorescence microscopy and Western analysis. SMC grown on collagen gel behaved similarly to those on conventional culture dishes. However, when a second layer of collagen was overlaid, cells underwent an elongated quiescent phase before onset of proliferation and a more than threefold lower logarithmic growth rate was observed. These cells self-organized into a network with ring-like structures. With increasing culture time, some of the rings developed into funnel-like, incomplete or complete tubular structures. If a tubular template preexisted within the gel, the SMC established a cylinder-shaped tube with several circularly arranged muscular layers (similar to an artery wall). This behavior mimicked endothelial cells during angiogenesis in vitro. A similar phenomenon occurred in cultures in which SMC were randomly mixed in a collagen gel, but here their behavior and morphology varied with their position within the gel. Western blot analysis showed that the SMC differentiation marker, smooth muscle myosin heavy chain-2 (SM-2), rapidly decreased, disappearing by day 10 in SMC grown on collagen, but was still detectable until day 25 in cells cultured between or within the same gel. These findings indicate that like endothelial cells, vascular SMC can display blood vessel formation behavior in vitro when an appropriate three-dimensional matrix environment is provided to keep them in a relatively higher-differentiated and low-proliferative state.

HCPTPA, a protein tyrosine phosphatase that regulates vascular endothelial growth factor receptor-mediated signal transduction and biological activity

Angiogenesis is a tightly controlled process in which signaling by the receptors for vascular endothelial growth factor (VEGF) plays a key role. In order to define signaling pathways downstream of VEGF receptors (VEGFR), the kinase domain of VEGFR2 (Flk-1) was used as a bait to screen a human fetal heart library in the yeast two-hybrid system. One of the signaling molecules identified in this effort was HCPTPA, a low molecular weight, cytoplasmic protein tyrosine phosphatase. Although HCPTPA possesses no identifiable phosphotyrosine binding domains (i.e. SH2 or phosphotyrosine binding domains), it bound specifically to active, autophosphorylated VEGFR2 but not to a mutated, kinase-inactive VEGFR2. Recombinant VEGFR2 and endogenous VEGFR2 were substrates for recombinant HCPTPA, and HCPTPA was co-expressed with VEGFR2 in endothelial cell lines, suggesting that HCPTPA may be a negative regulator of VEGFR2 signal transduction. To pursue this possibility, an adenovirus directing the expression of HCPTPA was constructed. When used to infect cultured endothelial cells, this adenovirus directed high level expression of HCPTPA that resulted in impairment of VEGF-mediated VEGFR2 autophosphorylation and mitogen-activated protein kinase activation. Adenovirus-mediated overexpression of HCPTPA also inhibited VEGF-induced cellular responses (endothelial cell migration and proliferation) and inhibited angiogenesis in the rat aortic ring assay. Taken together, these findings indicate that HCPTPA may be an important regulator of VEGF-mediated signaling and biological activity. Potential interactions with other signaling pathways and possible therapeutic implications are discussed.

The role of matrix metalloproteinase activity in the maturation of human capillary endothelial cells in vitro

Vessel maturation during angiogenesis (the formation of new blood vessels) is characterized by the deposition of new basement membrane and the downregulation of endothelial cell proliferation in the new vessels. Matrix remodeling plays a crucial, but still poorly understood role, in angiogenesis regulation. We present here a novel assay system with which to study the maturation of human capillary endothelial cells in vitro. When human dermal microvascular endothelial cells (HDMEC) were cultured in the presence of dibutyryl cAMP (Bt2) and hydrocortisone (HC), the deposition of a fibrous lattice of matrix molecules consisting of collagens type IV, type XVIII, laminin and thrombospondin was induced. In basal medium (without Bt2 and HC), HDMEC released active matrix metalloproteinases (MMPs) into the culture medium. However, MMP protein levels were significantly reduced by treatment with Bt2 and HC, while protein levels and activity of endogenous tissue inhibitor of MMPs (TIMP) increased. This shift in the proteolytic balance and matrix deposition was inhibited by the specific protein kinase A inhibitors RpcAMP and KT5720 or by substituting analogues without reported glucocorticoid activity for HC. The addition of MMP inhibitors human recombinant TIMP-1 or 1,10-phenanthroline to cultures under basal conditions induced matrix deposition in a dose-dependent manner, which was not observed with the serine protease inhibitor epsilon-amino-n-caproic acid (ACA). The deposited basement membrane-type of matrix reproducibly suppressed HDMEC proliferation and increased HDMEC adhesion to the substratum. These processes of matrix deposition and downregulation of endothelial cell proliferation, hallmarks of differentiating new capillaries in the end of angiogenesis, were recapitulated in our cell culture system by decreasing the matrix-degrading activity. These data suggest that our cell culture assay provides a simple and feasible model system for the study of capillary endothelial cell differentiation and vessel maturation in vitro.

A serum-free in vitro model of renal microvessel development

The differentiation and organization of the embryonic renal vasculature is a crucial event in renal development. To study this process, we developed a serum-free in vitro model of renal microvessel development. Mouse embryonic kidney explants, when embedded specifically in type I collagen, demonstrate outgrowth of microvascular structures when stimulated by the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA, 10-50 ng/ml). Other polypeptide growth factors stimulated little, if any, microvessel outgrowth from the explants. Similar outgrowths were not observed when other embryonic tissue explants were used. The number of microvessels observed depended on the gestational age of the explants. We hypothesize that TPA induces the in situ differentiation of metanephric mesenchymal cells into endothelial cell precursors and that specific matrix proteins and cell-matrix interactions are necessary for the organization of these precursors into microvessels. Our model will allow us to examine in detail the responsiveness of metanephric kidney cells to both growth factors and extracellular matrix molecules and to understand how they influence renal endothelial cell differentiation.

Inhibitory effects of hypercholesterolemia and ox-LDL on angiogenesis-like endothelial growth in rabbit aortic explants. Essential role of basic fibroblast growth factor

Hypercholesterolemic (HC) rabbits exhibit suppressed compensatory vascular growth after restriction of arterial supply. However, neovascularization is commonly found in atheromas containing inflammatory cells. We used an in vitro model to determine the effects of hypercholesterolemia on angiogenesis in the absence or presence of inflammatory cells. HC rabbit aortic explants (1 mm2) with or without (n = 90 each) lesion-forming inflammatory cells were cultured in a collagen matrix with serum-free medium. Explant-derived endothelial cell growth was organized into capillary-like microtubes (CLM) that could be videomicroscopically quantified. CLM growth from lesion-free HC explants was significantly reduced to 13 +/- 4% of the value in explants (n = 90) from normocholesterolemic (NC, n = 15) rabbits (P < .001). In contrast, in lesion-containing HC explants, the matrix was invaded by foam cells, and CLM growth was not inhibited. Immunoassayable basic fibroblast growth factor (bFGF, in pg/mL) in the culture medium was significantly lower in lesion-free HC (< 5) than NC explants (11 +/- 2, P < .01) or HC explants with lesions (14 +/- 3). In addition, CLM growth was reduced in NC explants incubated with oxidized LDL (ox-LDL, 50-100 micrograms/mL). Exogenous bFGF (10 ng/mL) reversed the inhibitory effects of hypercholesterolemia and ox-LDL, whereas bFGF-neutralizing antibody (10 micrograms/mL) abolished CLM growth in all groups. In cultured rabbit aortic endothelial cells, ox-LDL reduced DNA synthesis, but this inhibition was reversed by bFGF. We conclude that hypercholesterolemia and ox-LDL inhibit angiogenesis like endothelial growth because of a suppressed availability of endogenous bFGF. Retained responsiveness to exogenous bFGF suggests that inducing bFGF expression at targeted sites may improve collateral growth in hyperlipidemic arterial disease.

Endothelial cell injury in cardiovascular surgery: the intimal hyperplastic response

Arteries and veins respond to injury by a healing process that includes the development of a neointima. This response to injury is implicated as the primary cause of failure after arterial reconstruction. Because it is an integrator and transmitter of blood flow variations, inflammation, and growth stimuli, the endothelium is a potent regulator of long-term arterial wall mass changes. The contribution of the endothelium to intimal development depends on the type of arterial conduit. In arteries, the growth of the intima stops when the endothelium has regrown. In synthetic grafts, the endothelium stabilizes intimal growth. Hence, the mere presence of endothelial cells can influence intimal changes in arterial conduits. Understanding endothelial biology should help us define methods to prevent cell proliferation, extracellular matrix accumulation, intimal hyperplasia, and vessel narrowing.

Isolation of a morphologically and functionally distinct smooth muscle cell type from the intimal aspect of the normal rat aorta. Evidence for smooth muscle cell heterogeneity

Recent studies indicate that the neointima of injured rat arteries is composed of a subpopulation of smooth muscle cells (SMCs) distinct from medial smooth muscle cells. However, SMC diversity in normal adult aorta has remained elusive. This study characterizes two morphologically and functionally distinct SMC types isolated from different anatomic regions of the normal rat aorta. Rat aortic medial smooth muscle cells (MSMCs) were isolated from the media after removal of the intimal and adventitial cells. Rat aortic intimal smooth muscle cells (ISMCs) were isolated from the intimal aspect of everted rat aortas. The two cell types were characterized morphologically and immunohistochemically and were compared for their capacity to contract collagen gels in response to endothelin-1. MSMCs were spindle-shaped and grew in hills and valleys showing features previously described for vascular SMCs. Conversely, ISMCs displayed a polygonal and epitheloid shape, grew mainly as a monolayer, and had a higher proliferative rate. Both cell types expressed alpha-smooth muscle actin and were negative for Factor VIII-RAg. ISMCs produced large amounts of a laminin and type IV collagen-rich extracellular matrix which had a characteristic pericellular distribution. ISMCs, but not MSMCs, rapidly contracted collagen gels in response to endothelin-1. This study indicates that the normal rat aorta contains two types of SMCs located in anatomically distinct regions of the vessel wall. Because of their functional characteristics, the SMCs isolated from the intimal aspect of the aorta may play an important role in physiologic as well as pathologic conditions.

Isolation and characterization of vasoformative endothelial cells from the rat aorta

We describe here a modified nonenzymatic method for the isolation of rat aortic endothelial cells with vasoformative properties. Aortic rings placed on plastic or gelatin-coated surfaces generated outgrowths primarily composed of endothelial cells. Prompt removal of aortic explants after endothelial migration minimized fibroblast contamination. However, fibroblasts, because of their high proliferative rate tended to overgrow the endothelial cells even when present in small numbers. This potential pitfall was avoided by weeding out fibroblasts with the rounded tip of a bent glass pipette. Primary endothelial colonies free of fibroblasts were segregated in cloning rings, trypsin-treated, and transferred to gelatin-coated dishes. Endothelial cells were cultured in MCDB 131 growth medium containing 10% fetal bovine serum, endothelial cell growth supplement, and heparin. Using this technique, pure endothelial cell strains were obtained from single aortic rings. Confluent endothelial cells formed a contact-inhibited monolayer with typical cobblestone pattern. The endothelial cells were positive for Factor VIII-related antigen, took up DiI-Ac-LDL, and bound the Griffonia Simplicifolia-isolectin-B4. Endothelial cells cultured on collagen gel formed a polarized monolayer, produced basement membrane, displayed Weibel-Palade bodies and caveolae, and were connected by tight junctions. In addition, they reorganized into a network of microvascular cords and tubes when overlaid with a second layer of collagen and formed microvascular sprouts in response to fibroblast-conditioned medium. This isolation procedure yields stable strains of vasoformative endothelial cells, which can be used to study aortic endothelium-related angiogenesis and its mechanisms.

Matrix-bound thrombospondin promotes angiogenesis in vitro

Thrombospondin (TSP) is a multidomain adhesive protein postulated to play an important role in the biological activity of the extracellular matrix. To test this hypothesis, TSP-containing fibrin and collagen matrices were evaluated for their capacity to support angiogenesis and cell growth from explants of rat aorta. This serum-free model allowed us to study the angiogenic effect of TSP without the interference of attachment and growth factors present in serum. TSP promoted dose-dependent growth of microvessels and fibroblast-like cells. The number of microvessels in TSP-containing collagen and fibrin gels increased by 136 and 94%, respectively. The TSP effect was due in part to cell proliferation since a 97% increase in [3H]thymidine incorporation by the aortic culture was observed. The effect was TSP-specific because TSP preparations adsorbed with anti-TSP antibody showed no activity. TSP did not promote angiogenesis directly since no TSP-dependent growth of isolated endothelial cells could be demonstrated. Rather TSP directly stimulated the growth of aortic culture-derived myofibroblasts which in turn promoted microvessel formation when cocultured with the aortic explants. Angiogenesis was also stimulated by myofibroblast-conditioned medium. Partial characterization of the conditioned medium suggests that the angiogenic activity is due to heparin-binding protein(s) with molecular weight > 30 kD. These results indicate that matrix-bound TSP can indirectly promote microvessel formation through growth-promoting effects on myofibroblasts and that TSP may be an important stimulator of angiogenesis and wound healing in vivo.