In vitro rapid organization of endothelial cells into capillary-like networks is promoted by collagen matrices

Collagen production by human smooth muscle cells isolated during intestinal organogenesis

The extracellular matrix influences organogenesis by modulating cell behavior. In humans, collagen is the major matrix constituent of the adult intestinal wall and is synthesized by smooth muscle cells. The objective of the current study was to examine collagen production by fetal human intestinal smooth muscle cells isolated during intestinal morphogenesis. Techniques were developed for the isolation and culture of human fetal intestinal smooth muscle cells. The cultured cells were confirmed as muscle by immunohistochemical stains for cytoskeletal filaments and documentation of contractile behavior. In culture, these cells stained for mesenchymal and muscle cytoskeletal proteins: vimentin, actin, and desmin, and did not stain for neural or epithelial markers. The muscle cells contracted in response to acetylcholine, in contrast to human fetal dermal fibroblasts which did not contract appreciably. Collagen production was assayed by the uptake of [3H]-proline into collagenase-digestible protein. Collagen production was greatest at 11 weeks gestation, the youngest age studied. By 20 weeks gestation, collagen production had decreased to adult levels. However, when compared to another matrix-producing fetal mesenchymal cell, the dermal fibroblast, intestinal smooth muscle cells produced twice as much collagen. Collagen types were determined by polyacrylamide slab gel electrophoresis. Smooth muscle cells predominantly produced types I and III collagen alpha chains. Therefore, collagen production is a significant function of human fetal intestinal smooth muscle cells, and probably plays a major role in the development of intestinal structure. The in vitro model presented here provides a means of studying the regulation of this collagen production throughout intestinal organogenesis.

Histopathological follow-up study of 66 cerebral arteriovenous malformations after therapeutic embolization with polyvinyl alcohol

Embolization with polyvinyl alcohol (PVA) is an accepted method of rendering complex arteriovenous malformations (AVM's) more amenable to surgery, but its effects on human vascular tissues have not been adequately documented. The authors reviewed the histopathology of 66 intracranial AVM's resected 1 to 76 days after embolization with PVA. The mean age of the patients was 36 years, and their AVM's were located in the cerebral hemispheres (92%), the cerebellum (6%), or the corpus callosum (2%). In 79% of cases, at least one vessel contained PVA particles; in most cases, the vessel was filled with sharp, angular PVA particles in a serpiginous pattern. Polyvinyl alcohol particles indented the endothelium in 69% of cases but were rarely found subendothelially. Clotted blood and fibroblasts were present among the particles, and abundant intraluminal mononuclear and polymorphonuclear inflammatory cells were found in all vessels containing PVA particles. Foreign-body giant cells appeared 2 to 14 days after embolization in the majority of cases. Patchy mural angionecrosis and necrotizing vasculitis were found in 39% of the cases. Recanalized lumina were seen in 18% of PVA-embolized vessels. Foreign materials resembling cotton fibers and other particulate substances, which were probably contaminants of the contrast solution or the embolic material, were found in 65% of the cases. These findings suggest a specific chain of events in the interaction between PVA and vessel wall components and may explain some important sequelae of embolization therapy.

Protein kinase C regulates endothelial cell tube formation on basement membrane matrix, Matrigel

Human umbilical vein endothelial cells differentiate within 12 h to form capillary-like networks of tube structures when the cells are plated on Matrigel, a mixture of basement membrane proteins. Nothing is known about the intracellular signaling events involved in this differentiation. As a first step to define the process, we investigated the possible role of protein kinase C activation by beta-phorbol 12-myristate 13-acetate (PMA) in regulating the formation of the tube structures. In this model, PMA increased tube formation several-fold in a dose-dependent manner with half-maximum stimulation of tube formation at approximately 5 nM PMA. In the absence of serum, essentially little or no tubes were formed on Matrigel unless PMA was added to the medium. Only active phorbol analogs increased tube formation, while the protein kinase C inhibitor, H-7, blocked tube formation. The protein kinase C activators and inhibitors were effective only when added at or just after plating of the cells and did not affect already formed tubes. This study suggests that protein kinase C is involved in the early events of in vitro endothelial cell tube formation on Matrigel.

Possible mechanisms of type I collagen-induced vascular tube formation

We investigated the effect of type I collagen on endothelial behaviour following its contact with the apical versus basal surface of cultured human endothelial cells. When endothelial cells were plated onto type I collagen they attached via their basal surface and formed a confluent monolayer. However, when type I collagen (100 micrograms/ml) was added directly to the growth medium, so that it made contact with the apical surface of endothelial cells, it induced rapid capillary-like tube formation. Possible mechanisms were assessed using a) polyclonal (anti-VLA-2) and monoclonal (AK7) antibodies to different epitopes on the alpha 2 beta 1 integrin receptor for collagen and b) drugs (chlorpromazine and trifluoperazine) that inhibit protein kinase C activity. Both anti-VLA-2 and AK7 (1-50 micrograms/ml) showed a dose-dependent inhibition of tube formation and cell attachment. At 50 micrograms/ml, anti-VLA-2 completely inhibited tube formation whereas AK7 caused only partial inhibition (less than 50%). By contrast, AK7 was a more potent inhibitor of cell attachment than anti-VLA-2. Both chlorpromazine and trifluoperazine prevented tube formation.

CONCLUSIONS

1) The alpha 2 beta 1 integrin receptor plays a role in both endothelial cell attachment and the induction of tube formation by type I collagen. 2) Protein kinase C may be involved in collagen-induced tube formation.

Antibodies to beta 1-integrins cause alterations of aortic vasculogenesis, in vivo

Vasculogenesis is the de novo formation of blood vessels from mesoderm. This process occurs very early in development and provides a convenient system for studying morphogenesis in higher vertebrates. The cell-extracellular matrix (ECM) interactions that occur during dorsal aortic vasculogenesis were examined using the monoclonal antibody, CSAT, a reagent known to neutralize the ligand-binding activity of avian beta 1-integrins. We injected CSAT into quail embryos during a period of active vasculogenesis (4-10 somites). The CSAT antibodies, but not controls, had a marked and reproducible effect on aortic vessel formation. Vasculogenesis appeared to be arrested at the stage when slender cord-like assemblies of angioblasts rearrange to form tubules. Indeed, aortic primordia near the site of CSAT injection did not form patent vessels.

Differential expression of integrin cell-substratum adhesion receptors on endothelium

The interactions of the endothelial cells with their underlying extracellular matrix is of fundamental importance to angiogenesis. Therefore, the distribution of integrin cell-substratum adhesion receptors on cultured and in situ endothelial cells was determined. Cultured endothelium from large and small vessels and large vessel endothelial cells in situ expressed integrins that bound to both basement membrane and inflammatory matrix components. In contrast, in situ microvessels primarily expressed basement membrane binding integrins. Alterations in the expression of the integrins on microvessel endothelial cells may be important for microvascular remodeling during wound healing and/or angiogenesis.

Adhesive interactions in angiogenesis and metastasis

A variety of adhesive interactions must take place between the tumor cell and the host vasculature in order to potentiate both tumor expansion and metastatic tumor spread. The study of tumor cell and blood vessel adhesive interactions becomes essential for our understanding of the malignant process, especially with regard to organ-specific tumor metastasis. In this article we will review recent progress made in our understanding of the nature of (i) receptor mediated adhesion of endothelial cells to extracellular matrix components and (ii) adhesion of tumor cells to endothelial adhesion molecules and to components of the subendothelial basement membrane.

Experimental modulation of the differentiated phenotype of keratinocytes from epidermis and hair follicle outer root sheath and matrix cells

Follicles of human anagen hair were separated into morphologically distinct compartments (by sequential trypsinization and microdissection) for the biochemical and immunological analysis of keratins as differentiation markers to diagnose the type of epithelial differentiation. While outer root sheath contained throughout the "soft" (cyto)keratins K5, 6, 14, 16, and 17, and hair cortex contained exclusively a set of acidic and basic "hard" alpha-keratins (consistent up to the hair tip), in inner root sheath and hair cuticle peptides related or derived from suprabasal epidermal keratins K1 and 10 were detected. These keratin profiles served as in vivo correlates for the evaluation of type and degree of differentiation achieved by the respective isolated epithelial cells, comparing different growth or culture conditions. Cultures of ORS cells and hair matrix cells (PHS cells) as well as normal keratinocytes were initiated using postmitotic human dermal fibroblasts as efficient feeder cells. On lifted collagen gels populated with HDF ("surface" cultures), ORS and PHS cells formed stratified epithelial expressing epidermal differentiation markers such as keratins K1 and 10, involucrin, and filaggrin. Compared with NEK "surface" cultures, balance between growth and differentiation was better maintained by both follicular cell types. In contrast, epidermal tissue homeostasis was largely normalized in transplants on nude mice regardless of the epithelial cell type, apparent from orderly tissue structure, regular distribution of keratin K10, filaggrin, and involucrin, and distinct continuous deposition of basement membrane components at the epithelium-collagen interface. Embedded in Matrigel (on top of HDF collagen gels) ORS cells and NEK formed spheroids exhibiting inward-directed epidermoid differentiation, increasing with time. All epidermal maturation products found in "surface" cultures were likewise expressed, and again differentiation greatly outbalanced proliferation in spheroids of NEK but not of ORS cells. PHS cells embedded together with HDF in Matrigel produced similar spheroids as ORS cells. Size of spheroids and degree of epidermoid differentiation were dramatically reduced when HDF were replaced by follicular DP cells, demonstrating the crucial role of the mesenchymal "companion" cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of angiogenesis in vitro by collagen metabolism

The role of collagen in microvascular growth was investigated using the aortic ring model of angiogenesis. Collagen production by vasoformative outgrowths in plasma clot culture of rat aorta was either stimulated with ascorbic acid or inhibited with the proline analogue cis-hydroxyproline. Microvessels proliferating in the absence of ascorbic acid supplements became ecstatic and developed large lumina. In contrast, newly formed microvessels in the presence of ascorbic acid remained small and maintained thin lumina throughout the angiogenic process. Biochemical studies demonstrated enhanced collagen production and deposition in cultures treated with ascorbic acid. Ultrastructural studies of these cultures showed a marked increase in newly formed interstitial collagen in the perivascular matrix and in regions of the plasma clot containing nonendothelial mesenchymal cells. Small microvessels with thin lumina similar to the ones observed in ascorbic acid-treated plasma clot cultures were obtained by growing aortic explants in gels of interstitial collagen in the absence of ascorbic acid. Inhibition of collagen production with the proline analogue cis-hydroxyproline had a marked anti-angiogenic effect in both plasma clot and collagen gel cultures. The anti-angiogenic effect of cis-hydroxyproline was abolished by adding L-proline to the culture medium, thereby restoring normal metabolism. These results support the hypothesis that angiogenesis is regulated by collagen production and suggest that the size of newly formed microvessels is influenced by the degree of collagenization of the extracellular matrix.

Effects of glucose on migration, proliferation and tube formation by vascular endothelial cells

In order to elucidate the association between hyperglycemia and the vascular complications of diabetes, the effects of high glucose concentrations on the migration, proliferation and tube formation of bovine carotid artery endothelial cells were investigated. Cells treated with 16.7 and 33.3 mM glucose for 6 days showed 1.69- and 1.75-fold increase in serum-induced migration compared with cells treated with 5.6 mM glucose (p less than 0.05). The effect of glucose on cell proliferation was affected by serum concentration. When this was below 0.5%, a high glucose concentration stimulated cell growth to a maximum of 1.73 times that at a serum concentration of 0.05% (p less than 0.01) whereas at a serum concentration of 10%, growth was inhibited (p less than 0.05). Tube formation was studied by culturing the cells between two layers of collagen gel. Ultrastructurally, tubular structures were composed of one to several endothelial cells containing pinocytotic vesicles and cytoplasmic projections, and linked by junctional complexes. A basal lamina-like structure surrounded the abluminal surface. Treatment of the cells with 16.7 and 27.8 mM glucose for 4 days stimulated tubular elongation 1.85 and 1.71 times, respectively (p less than 0.01). Other osmogenic molecules such as mannitol and sucrose did not affect tube formation. These data imply that high glucose concentrations mimicking diabetic hyperglycemia may not inhibit the repair of endothelial injury and could act as a stimulator of neovascularization.

Differences in laminin fragment interactions of normal and transformed endothelial cells

Bovine aortic and microvascular endothelial cells showed good adhesion with spreading on fibronectin or collagen IV and to a lower extent on laminin. Recognition of native laminin was due to its long arm fragment E8 and was mediated by alpha 6 integrins as demonstrated by antibody inhibition. A considerably stronger, RGD-dependent interaction was observed with the isolated laminin short arm fragment P1 previously shown to represent a cryptic cell-binding site. No adhesion was observed with the heparin-binding fragment E3. In contrast, murine microvascular endothelial cells transformed by the polyoma middle T oncogene showed preferential adherence and spreading on laminin via its E8 cell-binding site and also showed adhesion to fragment E3. Attachment to laminin fragment P1 and to collagen IV was low or negative and was never followed by spreading. These data show that the transformation of microvascular endothelial cells, which give them the property to form hemangiomas, also leads to changes in cell adhesion to extracellular matrix proteins, particularly to laminin fragments.

Extracellular matrix permits the expression of von Willebrand's factor, uptake of di-I-acetylated low density lipoprotein and secretion of prostacyclin in cultures of endothelial cells from rat brain microvessels

Microvascular endothelial cells from the adult rat brain were cultured on Matrigel and found to express many differentiated properties including secretion of prostacyclin (PGI2) and von Willebrand's factor (vWF). Brain microvascular endothelial cells (BMECs) were purified by dextran and percoll gradients after enzymatic treatment and cultured under various conditions. BMECs that were plated on Matrigel stained positively for factor VIII-related antigen and incorporated Di-I-acetylated low density lipoprotein, whereas BMEC plated on fibronectin, gelatin, or uncoated dishes did not express any of the above properties which are characteristic of endothelial cells. vWF was measured by a sensitive ELISA in the culture media of BMECs plated on different types of matrices. Specificity of the anti-human vWF antibodies for the rat vWF was verified by immunoabsorption on a solid phase, sodium dodecyl sulfate, and Western blot analysis. BMECs also secreted vWF into the culture media only when the cells were plated on Matrigel, and this secretion was augmented after a 6 h incubation with an interleukin-1 tumor necrosis factor-alpha mixture, but not by lipopolysaccharide. From different matrices tested, only Matrigel permitted the secretion of PGI2 by BMECs. Cells also proved to be sensitive to mechanical stimulation and became refractory to secretagogue if the mechanical stimulation was serially repeated. Under the best conditions, stimulation of the cells with bradykinin (1 microM) substantially increased PGI2 secretion. These data indicate that growth of BMECs on Matrigel in vitro permits the expression of classical endothelial cell markers in a manner similar to the behavior of these cells in situ.

Progressive modulation of endothelial phenotype during in vitro blood vessel formation

"Sprouting" vascular endothelial cells were used as an in vitro model system to study the progressive morphologic and biosynthetic changes associated with the formation of tubular structures. In vitro, sprouting endothelial cells formed spontaneously without the addition of any exogenous factors from cultures of cloned endothelium exhibiting a polygonal/cobblestone phenotype. These phenotypically variant endothelial cells differentiated to form associated cell networks or nodules which gradually reorganized into tubular structures. Concomitant with these morphologic changes, the biosynthesis of extracellular matrix proteins was modulated, as determined by Northern blot analysis, metabolic labeling, and immunocytochemistry. The initial sprouting phase was characterized by the induction of type I collagen synthesis and the appearance of fibronectin containing the ED-A domain, in comparison to their absence in cloned cultures displaying a stable polygonal/cobblestone phenotype. The organizational stage, where the sprouting endothelial cells assembled into tubular structures, was additionally characterized by the expression of type IV collagen. These studies demonstrate that the progression from polygonal/cobblestone to sprouting cultures, and subsequent tubular organization, involves major alterations in extracellular matrix protein expression. This developmental phenomenon, although not completely analogous to blood vessel formation in vivo, nevertheless may be helpful in understanding the role of matrix macromolecules in the angiogenic process.

Glutamine stimulates growth in rat cerebral endothelial cell culture

Endothelial cells were isolated from rat cerebral cortices using combined enzymatic digestions and Percoll gradient centrifugation. Primary cultures were subsequently grown on collagen-covered dishes in a medium containing 20% fetal calf serum and 0.6 mmol glutamine. The majority of cultures became confluent by day 7 or 8, but some could not reach confluence. The cells were fusiform in shape and exhibited immunoreactivity to factor VIII-related antigen and binding to the lectin Griffonia simplicifolia. Exposure of cultures to media containing 2.6 mmol glutamine resulted in accelerated growth (in cultures were confluent at days 3-4) and change in culture morphology, namely the formation of circular, cell-free areas. However, this treatment did not restore gamma-glutamyl transpeptidase activity that was lost during cultivation. As for other amino acids, asparagine was less potent, glycine and phenylalanine failed to mimic the glutamine effect. In summary, glutamine stimulates growth of cerebral endothelial cells in vitro and so it may supplement for other growth factors in the culture media.

Inhibition of angiogenesis in vitro by Arg-Gly-Asp-containing synthetic peptide

This study was designed to evaluate the effect of the synthetic peptide Gly-Arg-Gly-Asp-Ser (GRGDS) on angiogenesis in serum-free collagen gel culture of rat aorta. The GRGDS peptide contains the amino acid sequence Arg-Gly-Asp (RGD), which has been implicated as a recognition site in interactions between extracellular matrix (ECM) molecules and cell membrane receptors. RGD-containing synthetic peptides are known to inhibit attachment of endothelial cells to substrates, but their effect on angiogenesis has not been fully characterized. Aortic explants embedded in collagen gel in the absence of GRGDS generated branching microvessels through a process of endothelial migration and proliferation. Addition of GRGDS to the culture medium caused a marked inhibition of angiogenesis. In contrast, GRGES, a control peptide lacking the RGD sequence, failed to inhibit angiogenesis. The inhibitory effect of GRGDS was nontoxic and reversible. The angiogenic activity of aortic explants previously inhibited with GRGDS could be restored by incubating the cultures in GRGDS-free medium. These findings suggest that angiogenesis is an anchorage-dependent process that can be inhibited by interfering with the attachment of endothelial cells to the ECM. It also indicates that synthetic peptides can be used as probes to study the mechanisms by which the ECM regulates angiogenesis.

Intracellular mechanisms involved in basement membrane induced blood vessel differentiation in vitro

The extracellular matrix, particularly basement membranes, plays an important role in angiogenesis (blood vessel formation). Previous work has demonstrated that a basement membranelike substrate (Matrigel) induces human umbilical vein endothelial cells to rapidly form vessel-like tubes (Kubota, et al., 1988; Grant et al., 1989b); however, the precise mechanism of tube formation is unclear. Using this in vitro model, we have investigated morphologic changes occurring during tube formation and the cytoskeletal and protein synthesis requirements of this process. Electron microscopy showed that endothelial cells attach to the Matrigel surface, align, and form cylindrical structures that contain a lumen and polarized cytoplasmic organelles. The cytoskeleton is reorganized into bundles of actin filaments oriented along the axis of the tubes and is located at the periphery of the cells. The addition of colchicine or cytochalasin D blocked tube formation, indicating that both microfilaments and microtubules are involved in this process. Cycloheximide blocked tube formation by 100%, indicating that the process also required protein synthesis. In particular, collagen synthesis seems to be required for tube formation because cis-hydroxyproline inhibited tube formation, whereas either the presence of ascorbic acid or the addition of exogenous collagen IV to the Matrigel increased tube formation. Our results indicate that endothelial cell attachment to Matrigel induces the reorganization of the cytoskeleton and elicits the synthesis of specific proteins required for the differentiated phenotype of the cells.

Time course of wound healing

Wound healing is a special kind of inflammation. Undisturbed wound healing is subject to a fixed time schedule of biochemical and cellular events. It is virtually impossible to deal with the time course of wound healing without describing the cellular and non-cellular events involved. The activity and mode of cell action after injury are coordinated by spatial and chronological factors, as well as by different mediators and cell-cell interacting signals. During wound healing the sequence of different signals and message substances, such as mediators of inflammation, fulfill a key function in wound repair. The report describes the time course of healing and the control of cellular events by different mediators and cell interactions. Emphasis is placed on temporal aspects, including the various signals leading to typical cellular events in wound healing.

Growth characteristics of retinal capillary endothelial cells compared with pulmonary vein endothelial cells in culture. The effect of pericytes on differentiation of endothelial cells

Bovine retinal capillary endothelial cells (RCECs) and pulmonary vein endothelial cells (PVECs) were isolated and investigated in plate culture, three-dimensional culture and in co-culture with pericytes. In plate culture, RCECs required growth factor in the medium for growth whereas PVECs did not. Phenotypic modulation (a tendency to become similar morphologically to smooth muscle cells, and to accumulate into thread-like structures) was observed in PVECs but not in RCECs. In three-dimensional culture, RCECs contracted, aggregated and were unable to proliferate. Proliferation was elicited when the gel matrix was adsorbed by fibronectin or upon co-culture with pericytes. In contrast, PVECs not only proliferated but also formed tubular structures. In co-culture with pericytes, PVECs in close contact with, or in near apposition to pericytes formed tubular structures earlier than those without contact in the same dish. These results provide new findings about differences in the growth characteristics of endothelial cells between microvessels and large vessels. In addition, it is considered that pericytes may promote tube formation by endothelial cells in three-dimensional culture.

Modulation of endothelial cell surface glycoconjugate expression by organ-derived biomatrices

Cell surface molecules play an important role in cellular communication, migration, and adherence. Here, we show the effect of organ-derived biomatrices on endothelial cell surface glycosylation. Five different lectins (with and without neuraminidase treatment) have been used as probes in an enzyme-linked lectin assay to quantitatively detect glycoconjugates on endothelial cells (BAEC) grown on tissue culture plastic or biomatrices isolated from bovine lung, liver, and kidney. BAEC generally exhibit strong binding of concanavalin A (Con A), Ricinus communis agglutinin I (RCA-I), wheat germ agglutinin (WGA), and soybean agglutinin, and peanut agglutinin after neuraminidase pretreatment of cells (Neu-SBA and Neu-PNA), while SBA and PNA consistently bind weakly to BAEC. BAEC grown on organ-derived biomatrices exhibit significantly altered binding intensities of Con A, RCA-I, WGA, and Neu-PNA: BAEC cultured on lung- or kidney-derived biomatrices express significantly stronger binding affinities for Con A and RCA-I than BAEC grown on liver-derived biomatrix or tissue culture plastic. In contrast, BAEC binding of WGA and PNA (after treatment of cells with neuraminidase) is significantly reduced when BAEC are grown on liver- or kidney-derived biomatrix. Quantitative lectin immunogold electron microscopy reveals consistently stronger lectin binding over nuclear regions compared to junctional regions between neighboring cells. These results indicate that extracellular matrix components regulate endothelial cell surface glycoconjugate expression, which determines cellular functions, e.g., preferential adhesion of lymphocytes or metastatic tumor cells.

Type I collagen fibrils promote rapid vascular tube formation upon contact with the apical side of cultured endothelium

Extracellular matrix (ECM) components phenotypically modulate cultured endothelium. This paper examined the ability of ECM components to promote tube formation in vitro. When collagen type I was added to the culture medium of confluent neonatal foreskin or human umbilical vein endothelium at 10-100 micrograms/ml tube-like structures formed rapidly. Tube formation did not occur with the addition of other ECM components at similar concentrations. Electron microscopy revealed that the lumen of the tubes consisted of collagen fibrils, with the surrounding cells having typical endothelial junctional complexes. These findings demonstrate that when collagen fibrils make contact with the apical side of endothelium they act as a stimulus and provide a template for vascular tube formation.

Actions of TNF and IFN-gamma on angiogenesis in vitro

We have developed a model system for studying angiogenesis in which microvascular fragments and myofibroblasts (Mf) isolated from lipid tissues are grown in co-culture. We have found that Mf induce capillary formation by producing an endothelial cell growth factor and by secreting an extracellular matrix that causes endothelial cells to form a cordlike structure. This system appeared to be well suited for examining the effects of vasoactive substances such as the TNF and INF-gamma on capillary growth. TNF-alpha,beta, and IFN-gamma not only significantly inhibited capillary growth induced by Mf, but also blocked capillary development induced by fibroblast growth factors (FGF), well-known potent angiogenic factors. Recently, we have found that platelet-derived growth factor (PDGF) enhances in vitro capillary formation, probably at least in part by acting on Mf. Just as with FGF, capillary growth in the presence of PDGF was almost completely blocked by IFN-gamma. We examined the mode by which IFN-gamma inhibits angiogenesis and found that IFN-gamma inhibits both the proliferation of endothelial cells and collagen(s) synthesis by Mf. These actions of TNF or IFN-gamma could limit vascular formation in solid tumors.

Behaviour of chick embryo aortic cells obtained through nonenzymatic means cultured onto collagen gels

In the present paper we used a method whereby some of the cellular events that take place in the aortic wall during chick embryo development can be studied in vitro. Collagen gels were utilized to culture endothelial cells obtained through nonenzymatic means from aortic explants isolated from 12- to 14-day-old chick embryos. These cells were characterized by morphological and immunocytochemical criteria. After 72 h, explanted endothelial cells from embryonic aorta formed a monolayer of polygonal cells, which gave rise to elongated cells that migrate into the collagen gel. These cells are similar to those of mesenchyme-like cells observed in vivo at the subendothelial region of 14-day-old chick embryonic aorta. In long-term cultures, some of these cells acquired features either of synthetic smooth muscle cell phenotype, or of fibroblast-like cells very similar to those found in developing aorta. These results indicate that the culture of explants of chick embryo aorta on three-dimensional collagen gel is a valuable system for studying some of the complex morphogenetic events that occur during the development of the aorta.

Avian vasculogenesis and the distribution of collagens I, IV, laminin, and fibronectin in the heart primordia

The heart-forming regions of the early embryo are composed of splanchnic mesoderm, endoderm, and the associated ECM. The ECM of the heart-forming regions in stage 7-9 chicken embryos was examined using immunofluorescence. Affinity purified antibodies to chicken collagens type I and IV, chicken fibronectin, and mouse laminin were used as probes. We report that (1) the basement membrane of the endoderm contains immunoreactive laminin and collagen IV; (2) the nascent basement membrane of the heart splanchnic mesoderm contains immunoreactive laminin, but not type IV collagen, and (3) the prominent ECM between the splanchnic mesoderm and the endoderm (the primitive-heart ECM) contains collagen IV, collagen I, fibronectin, but not laminin. In addition, we describe microscopic observations on the spatial relationship of cardiogenic cells to the primitive-heart ECM and the endodermal basement membrane.

Increased proteolytic activity is responsible for the aberrant morphogenetic behavior of endothelial cells expressing the middle T oncogene

Expression of the polyoma virus middle T (mT) oncogene in vivo is associated with a profound subversion of normal vascular development, which results in the formation of endothelial tumors (hemangiomas). In an attempt to understand the molecular mechanisms responsible for this phenomenon, we have investigated, in an in vitro system, the morphogenetic properties of endothelial cells expressing this oncogene. mT-expressing endothelioma (End) cells grown within fibrin gels formed large hemangioma-like cystic structures. All End cell lines examined expressed high levels of fibrinolytic activity resulting from increased production of urokinase-type plasminogen activator and decreased production of plasminogen activator inhibitors. Neutralization of excess proteolytic activity by exogenously added serine protease inhibitors corrected the aberrant in vitro behavior of End cells and allowed the formation of capillary-like tubules. These results suggest that tightly controlled proteolytic activity is essential for vascular morphogenesis and that physiological protease inhibitors play an important regulatory role in angiogenesis.

Proliferative response and macromolecular synthesis by ocular cells cultured on extracellular matrix materials

To investigate the effects of extracellular matrix components on cellular function, we cultured several types of ocular cells on substrates composed of extracellular matrix materials that were layered on culture dishes either as dried films or as gels. We measured cellular proliferation on these substrates and on a series of gels composed of varying proportions of rat tail tendon type I collagen and Matrigel, a commercially available extract of a basement membrane-producing murine tumor. In addition, we studied the biosynthesis of collagens and of proteoglycans by these cultured cells using [3H]-L-proline and [35S]-sulfate. The proliferative abilities of the various types of ocular cells on the dried film substrates, on uncoated plastic culture vessels, and on pure type I collagen gel, were similar. However, proliferation of ocular cells cultured on gels composed of greater than or equal to 90% Matrigel was markedly reduced. There was little or no inhibition of growth of two types of non-ocular cells: rat C6 astrocytoma cells, and human dermal fibroblasts. Histologic studies showed that the ocular cells tested often formed long strands and capillary-like tubes, and tended to "burrow" beneath the surface of substrates containing high percentages of Matrigel. Fibroblasts infrequently formed tubes, and exhibited the burrowing property also on gels containing primarily type I collagen, while C6 cells showed neither of these behaviors on any of the matrices tested. The elution pattern of newly synthesized [3H]-labeled and [35S]-labeled macromolecules produced by all of the cultured cell types, and detected by Sepharose CL-4B chromatography in the medium and in the cell layer plus matrix fractions did not vary following culture on the different substrates. Approximately twofold more of the newly synthesized collagens and proteoglycans were deposited in the cell layer plus matrix, and proportionately less appeared in the medium, when cells were cultured on type I collagen gels and on Matrigel than on the dried film substrates. These experiments demonstrate the influence of the extracellular matrix on several aspects of cell behavior, and provide further evidence that modification of the composition of the extracellular matrix may be an important determinant of normal or pathological cell function.

The role of basement membranes in vascular development

Endothelial cells produce and bind to multiple basement membrane components. Fibronectin and interstitial collagens seem to promote migration and proliferation, whereas basement membrane collagen and laminin stimulate attachment and differentiation. Human umbilical vein endothelial cells will rapidly form capillary-like structures when plated on a reconstituted basement membrane gel. This morphological differentiation involves the alignment of the cells followed by their close association with one another and the formation of a central lumen. Using antibodies to basement membrane components, we find that the formation of these vessels is a complex process involving multiple interactions with several matrix components. Synthetic peptides to active sequences in laminin have demonstrated that at least two sites in laminin participate in tube formation. An RGD-containing site on the A chain appears to mediate cell to matrix adhesion, and synthetic RGD-containing peptides block cell to matrix adhesion during tube formation. A YIGSR-containing site on the B1 chain appears to mediate cell to cell adhesion and promote tube formation because synthetic peptides block the strong cell interactions involved in tube formation. Our data with laminin peptides show that for at least one protein, multiple sites are recognized. Such data would also suggest that several cellular receptors are involved in a concerted process in laminin-induced differentiation of endothelial cells. We conclude that vessel formation is a complex, multistep process. Identification of active sites that block this process may have potential use in blocking angiogenesis in diseases such as diabetic retinopathy and Kaposi's sarcoma.

Modulation of microvascular growth and morphogenesis by reconstituted basement membrane gel in three-dimensional cultures of rat aorta: a comparative study of angiogenesis in matrigel, collagen, fibrin, and plasma clot

Rings of rat aorta cultured in Matrigel, a reconstituted gel composed of basement membrane molecules, gave rise to three-dimensional networks composed of solid cellular cords and occasional microvessels with slitlike lumina. Immunohistochemical and ultrastructural studies showed that the solid cords were composed of endothelial sprouts surrounded by nonendothelial mesenchymal cells. The angiogenic response of the aortic rings in Matrigel was compared to that obtained in interstitial collagen, fibrin, or plasma clot. Morphometric analysis demonstrated that the mean luminal area of the microvascular sprouts and channels was significantly smaller in Matrigel than in collagen, fibrin, or plasma clot. The percentage of patent microvessels in Matrigel was also markedly reduced. Autoradiographic studies of 3H-thymidine-labeled cultures showed reduced DNA synthesis by developing microvessels in Matrigel. The overall number of solid endothelial cords and microvessels was lower in Matrigel than in fibrin or plasma clot. A mixed cell population isolated from Matrigel cultures formed a monolayer in collagen or fibrin-coated dishes but rapidly reorganized into a polygonal network when plated on Matrigel. The observation that gels composed of basement membrane molecules modulate the canalization, proliferation, and organization into networks of vasoformative endothelial cells in three-dimensional cultures supports the hypothesis that the basement membrane is a potent regulator of microvascular growth and morphogenesis.

Factors affecting the migration and growth of endothelial cells from microvessels of bovine retina

We have developed a retinal microvessel culture system which permits study of the initial events of endothelial cell activation and migration during the angiogenic response. Enzyme digest experiments indicate that Type IV collagen is the major basement membrane component regulating migration and growth of endothelial cells. Following removal of basement membrane collagen, further cell migration and proliferation require a suitable substrate. Laminin, fibronectin and fibrin(ogen) provide excellent substrates for endothelial cell outgrowths while Type I collagen, even if prepared as a three-dimensional gel, or Type IV collagen fails to promote typical cell growth. In contrast to fibrin and fibronectin, plasmin was a poor substrate for cell outgrowth and it is suggested that cell-associated protease activity may exert a regulatory role over endothelial cell-matrix interactions during cell migration.

Morphologic plasticity and periodicity: porcine cerebral microvascular cells in culture

Porcine cerebral microvascular (PCMV) endothelial cell cultures and pericyte-endothelial cell cocultures were established and the self-organizational properties of the cells were examined in various culture conditions. Cultured PCMV endothelial cells were characterized by the capacity to produce prostacyclin in response to bradykinin. Cultured PCMV pericytes were identified with a smooth muscle actin-specific stain. PCMV endothelial cells organized into cord structures when left in culture for several weeks without passage. Lumina were observed in cross sections of these cords and appeared to form through a process of cell-selective autolysis. PCMV endothelial cells required three dimensions for self-organization, forming suspended cords in planes that either intersected or paralleled the culture vessel floor. After formation, suspended cords continued to exhibit a morphologic plasticity punctuated by the coordinated migrations of PCMV endothelial cells en masse. Sequential propagation of PCMV endothelial cell monolayers and development of suspended capillarylike cords recurred cyclically when cells were left in culture without passage for several weeks. Cord development was also observed in PCMV pericyte-endothelial cell cocultures with large proportions of pericytes. However, pericytes were not located in cross sections of suspended cords formed in coculture. Apparently, in some conditions of PCMV coculture, populations of PCMV endothelial cells and pericytes segregate. Retina-derived growth factor (RDGF) promoted this cell-type segregation and the subsequent formation of suspended cords in PCMV cocultures, although its exact mode of action is unclear. These results indicate that cultured cerebral microvascular endothelial cells and pericytes have capacities for complex, temporal self-organization that varies according to culture conditions.

Morphological behavior of cultured bovine adrenal medulla capillary endothelial cells

Bovine adrenal medulla capillary endothelial cells were isolated and cloned, and their morphological behaviors in vitro were examined. In the culture of primary or early passage, one type of colony formed intracellular lumina both on the dish and in the three dimensional collagen gel. Another type proliferated well and showed morphology ranging from slender-shape to cobblestone shape, and were easily cloned. Cloned cells which showed slender-shapes formed tubular network on plastic dish after addition of PMA, OAG or vanadate, and these cells also formed multicellular tubules in the three dimensional collagen gel. However, the formation of diaphragmed fenestrae by these slender-shape clones was rare. One clone which showed cobblestone shape formed diaphragmed fenestrae, when cultured on collagen gel for more than one month. Isolated colonies or clones showed heterogeneity of cell shape, angiogenic behaviors and fenestrae formation.

Chitosan--as a biomaterial

Chitosan [a (1----4) 2-amino-2-deoxy-beta-D-Glucan] is a unique polysaccharide derived from chitin. Several attempts have been made to use this biopolymer in biomedical field. The use of this material in the development of hemodialysis membranes, artificial skin, drug targetting and other applications are discussed. It appears, this novel biomolecule, biodegradable, and biocompatible, find applications in substituting or regenerating the blood/tissue interfaces. This polysaccharide having structural characteristics similar to glycosaminoglycans, seems to mimic their functional behaviour.

Capillary growth of rat aortic segments cultured in collagen gel without serum

The process of angiogenesis was studied under serum-free conditions using rat aortic segments in three-dimensional collagen gel. In serum-free and growth-factor free conditions, the capillaries formed networks and tube-like structures, and the endothelial cells produced von Willebrand factor, laminin and type IV collagen, but the number of capillaries was lower and their growth was slower than in medium containing 20% fetal calf serum (FCS). Incorporation of bromodeoxyuridine (BrdU) and inhibition of growth by hydroxyurea suggested that capillary growth depended mainly on cell proliferation and not on migration. Capillary growth in PRMI 1640 or DMEM was similar and more efficient than in MEM. Only slight growth was seen in Medium 199 and HAM-F12. The addition of serum to the medium accelerated capillary growth in proportion to the amount added. In serum-free conditions, ITS(+) and fibroblast growth factor (FGF) promoted capillary growth, but not to a significant extent. There ware no differences in capillary growth among the gel matrices used (type I collagen, type I + II collagen, type I + IV collagen, fibrin and plasma clot).

Phenotypic comparison between mesothelial and microvascular endothelial cell lineages using conventional endothelial cell markers, cytoskeletal protein markers and in vitro assays of angiogenic potential

Endothelial and mesothelial cells are mesodermally derived simple squamous epithelial cells. A controversy concerning the ontogenetic origin of neoplasms derived from these cell types, commonly cited in the literature, is whether Kaposi's sarcoma is a mesothelioma or an angioma. To assess the similarities and differences between these cell types, pulmonary microvascular endothelial cells (PMVEC) and pericardial mesothelial cells (PMC) were cultured in vitro. PMVEC and PMC were found to be difficult to distinguish from one another by histological criteria alone. Both cell types formed contact-inhibited, and 'cobblestone', monolayers typical of simple epithelial cells. PMVEC and PMC demonstrated positive immunoreactivity to Factor VIII-related antigen and angiotensin-converting enzyme (ACE) antigen. They also showed uptake of 1,1'-dioctacecyl-1,3,3,3',3-tetramethyl-indocarbocyanine perchlorate acetylated low density lipoprotein (DiI-Ac-LDL) in 4 h. Both PMVEC and PMC expressed low ACE activities when compared to macrovessel endothelial cells. PMVEC and PMC shared similar isoform profiles for vimentin and actin. Both cell types expressed the simple epithelial keratins, cytokeratins 8 and 19, though PMC contained 50% more cytokeratins than PMVEC. Additionally, PMC contained cytokeratin 18, an intermediate filament protein not detectable in PMVEC. PMC formed 15 times as many epithelial ringlets or "stomata" as PMVEC. PMVEC but not PMC could be induced in vitro to differentiate into branching tube-like structures in response to their culture environment. Reorganization of PMVEC into vessel-like structures was more rapid and complete than PMC when embedded in three-dimensional collagen I lattices, cultured on Matrigel or exposed to a shaped-pulsed electromagnetic field. The angiogenic response of PMVEC to specialized culture conditions in vitro may reflect their phenotypic differentiation state characterized by anastomosing vascular structures in vivo, whereas PMC remain differentiated into monolayer sheet-like structures.

Two different laminin domains mediate the differentiation of human endothelial cells into capillary-like structures in vitro

Endothelial cells, both microvascular as well as large vessel, undergo differentiation slowly in culture under most conditions. When endothelial cells are cultured on Matrigel, a solid gel of basement membrane proteins, they rapidly align and form hollow tube-like structures. We show here that tube formation is a multi-step process induced by laminin. An RGD-containing sequence in the A chain of laminin through an integrin receptor on the endothelial cell induces their attachment to the protein while a YIGSR site in the B1 chain induces cell-cell interactions and the resulting tube formation. We also show that the laminin-derived synthetic peptide YIGSR contains sufficient information to induce single endothelial cells to form ring-like structures surrounding a hollow lumen, the basic putative unit in the formation of capillaries.

Isolation and propagation of endothelial cells derived from rheumatoid synovial microvasculature

Synovial angiogenesis may play an important part in the destruction of articular cartilage in patients with rheumatoid arthritis (RA). As an important first step towards developing in vitro models of synovial angiogenesis, microvascular endothelial cells have been isolated, purified, and cultured from operative synovial specimens obtained from adult patients with RA.

Induction of morphologic differentiation of endothelial cells in culture

Human endothelial cells when grown in cell culture assume a "cobblestone" morphology and do not form tubes or capillarylike structures. We have recently identified a culture substrate containing basement membrane-derived proteins that promotes morphologic differentiation of human umbilical vein and human dermal microvascular endothelial cells into capillarylike tubes. This differentiation is rapid, beginning within 1 h and is complete by 8-12 h. On electron microscopy these cells form a lumen, derived from remodeling of multiple cells and also by forming holes in the cytoplasm of individual cells. The endothelial cells no longer proliferate when cultured on this substrate known as matrigel, but can be induced to do so when cultured on fibronectin. We have also identified a critical molecular signal for endothelial cell differentiation induced by matrigel. Laminin, a prime constituent of matrigel, and to lesser extent collagen IV appear to be key elements in the differentiation of endothelial cells induced by matrigel.

Long-term culture of microvascular endothelial cells derived from Mongolian gerbil brain

A method for long-term culture of microvascular endothelial cells from Mongolian gerbil brain and their biologic properties in vitro are described. Microvessels were isolated from Mongolian gerbil brain by a combination of enzymatic treatment, filtration, and centrifugation and were seeded onto a gelatin-coated dish. A morphologically homogeneous cell plaque showing a cobblestone appearance was removed 2 to 3 weeks after the seeding, and the cells were subcultured. The cultured cells grew as monolayers of flat polygonal cells and were carried for more than 20 passages without morphologic change. These cells synthesized prostacyclin and retained an endothelial specific marker, factor VIII-related antigen. When the cells were cultured in a collagen gel, they rapidly formed capillarylike tubular structures without endothelial cell growth factor or special substrata. Long-term culture of purified microvascular endothelial cells derived from Mongolian gerbil brain will facilitate the study of the function of microvascular endothelial cells in human brain under normal and pathologic conditions.

Mechanochemical switching between growth and differentiation during fibroblast growth factor-stimulated angiogenesis in vitro: role of extracellular matrix

The angiogenic factor, basic fibroblast growth factor (FGF), either stimulates endothelial cell growth or promotes capillary differentiation depending upon the microenvironment in which it acts. Analysis of various in vitro models of spontaneous angiogenesis, in combination with time-lapse cinematography, demonstrated that capillary tube formation was greatly facilitated by promoting multicellular retraction and cell elevation above the surface of the rigid culture dish or by culturing endothelial cells on malleable extracellular matrix (ECM) substrata. These observations suggested to us that mechanical (i.e., tension-dependent) interactions between endothelial cells and ECM may serve to regulate capillary development. To test this hypothesis, FGF-stimulated endothelial cells were grown in chemically defined medium on bacteriological (nonadhesive) dishes that were precoated with different densities of fibronectin. Extensive cell spreading and growth were promoted by fibronectin coating densities that were highly adhesive (greater than 500 ng/cm2), whereas cell rounding, detachment, and loss of viability were observed on dishes coated with low fibronectin concentrations (less than 100 ng/cm2). Intermediate fibronectin coating densities (100-500 ng/cm2) promoted cell extension, but they could not completely resist cell tractional forces. Partial retraction of multicellular aggregates resulted in cell shortening, cessation of growth, and formation of branching tubular networks within 24-48 h. Multicellular retraction and subsequent tube formation also could be elicited on highly adhesive dishes by overcoming the mechanical resistance of the substratum using higher cell plating numbers. Dishes coated with varying concentrations of type IV collagen or gelatin produced similar results. These results suggest that ECM components may act locally to regulate the growth and pattern-regulating actions of soluble FGF based upon their ability to resist cell-generated mechanical loads. Thus, we propose that FGF-stimulated endothelial cells may be "switched" between growth, differentiation, and involution modes during angiogenesis by altering the adhesivity or mechanical integrity of their ECM.

Induction of angiogenesis during the transition from hyperplasia to neoplasia

It is now well established that unrestricted growth of tumours is dependent upon angiogenesis. Previous studies on tumour growth, however, have not revealed when or how the transition to an angiogenic state occurs during early tumour development. The advent of transgenic mice carrying oncogenes that reproducibly elicit tumours of specific cell types is providing a new format for studying multi-step tumorigenesis. In one of these models, transgenic mice expressing an oncogene in the beta-cells of the pancreatic islets heritably recapitulate a progression from normality to hyperplasia to neoplasia. We report here that angiogenic activity first appears in a subset of hyperplastic islets before the onset of tumour formation. A novel in vitro assay confirms that hyperplasia per se does not obligate angiogenesis. Rather, a few hyperplastic islets become angiogenic in vitro at a time when such islets are neovascularized in vivo and at a frequency that correlates closely with subsequent tumour incidence. These findings suggest that induction of angiogenesis is an important step in carcinogenesis.

Modulation of actin mRNAs in cultured vascular cells by matrix components and TGF-beta 1

Alpha-smooth muscle actin is currently considered a marker of smooth muscle cell differentiation. However, during various physiologic and pathologic conditions, it can be expressed, sometimes only transiently, in a variety of other cell types, such as cardiac and skeletal muscle cells, as well as in nonmuscle cells. In this report, the expression of actin mRNAs in cultured rat capillary endothelial cells (RFCs) and aortic smooth muscle cells (SMCs) has been studied by Northern hybridization in two-dimensional cultures seeded on individual extracellular matrix proteins and in three-dimensional type I collagen gels. In two-dimensional cultures, in addition to cytoplasmic actin mRNAs which are normally found in endothelial cell populations, RFCs expressed alpha-smooth muscle (SM) actin mRNA at low levels. alpha-SM actin mRNA expression is dramatically enhanced by TGF-beta 1. In addition, double immunofluorescence staining with anti-vWF and anti-alpha-SM-1 (a monoclonal antibody to alpha-SM actin) shows that RFCs co-express the two proteins. In three dimensional cultures, RFCs still expressed vWF, but lost staining for alpha-SM actin, whereas alpha-SM actin mRNA became barely detectable. In contrast to two-dimensional cultures, the addition of TGF-beta 1 to the culture media did not enhance alpha-SM actin mRNA in three-dimensional cultures, whereas it induced rapid capillary tube formation. Actin mRNA expression was modulated in SMCs by extracellular matrix components and TGF-beta 1 with a pattern very different from that of RFCs. Namely, the comparison of RFCs with other cell types such as bovine aortic endothelial cells shows that co-expression of endothelial and smooth muscle cell markers is very unique to RFCs and occurs only in particular culture conditions. This could be related to the capacity of these microvascular endothelial cells to modulate their phenotype in physiologic and pathologic conditions, particularly during angiogenesis, and could reflect different embryologic origins for endothelial cell populations.

Leukotriene C4 stimulates angiogenesis in bovine carotid artery endothelial cells in vitro

In order to investigate the mechanism of angiogenesis involved in inflammatory processes, the effects of leukotrienes and prostaglandin E2 on in vitro tube formation of cultured vascular endothelial cells were examined. Endothelial cells from bovine carotid artery were cultured for 4 days between two layers of collagen gel and the lengths of organized tubes were quantitatively estimated with an image analyzer. Treatment with 10(-8)-10(-6)M of prostaglandin E2 increased the tubular lengths, and leukotriene C4 stimulated tube formation at far lower concentrations (10(-15)-10(-9)M) but leukotriene B4 and D4 were not effective on the tube formation. It was also found that endothelial cell migration was stimulated by almost the same concentrations of leukotriene C4 as those stimulating tube formation. These data suggest that leukotriene C4 is, at least, one of the important factors involved in angiogenesis during inflammatory processes.

Regulation of the activity of a new inhibitor of angiogenesis by a cancer suppressor gene

An inhibitor has been identified in the conditioned medium of hamster cells and hamster-human hybrids that suppresses neovascularization in vivo in the rat cornea. Inhibitory activity was tightly linked to the presence of an active cancer suppressor gene in transformants and revertants, in segregating hybrids, and in temperature-limited transformants. It copurified with a approximately 140 kd glycoprotein. Polyclonal antiserum raised against the purified preparation recognized a 140 kd protein in conditioned medium and was able to adsorb out all antiangiogenic activity. These results define the control of the activity of an inhibitor of neovascularization as one function of the cancer suppressor gene active in BHK21/cl13 cells and simultaneously identify a new inhibitor of angiogenesis, a process vital to the growth of solid tumors.