Endothelial adhesion molecules in the development of the vascular tree: the garden of forking paths

The Role of Actin Dynamics and Actin-Binding Proteins Expression in Epithelial-to-Mesenchymal Transition and Its Association with Cancer Progression and Evaluation of Possible Therapeutic Targets

Metastasis causes death of 90% of cancer patients, so it is the most significant issue associated with cancer disease. Thus, it is no surprise that many researchers are trying to develop drugs targeting or preventing them. The secondary tumour site formation is closely related to phenomena like epithelial-to-mesenchymal and its reverse, mesenchymal-to-epithelial transition. The change of the cells' phenotype to mesenchymal involves the acquisition of migratory potential. Cancer cells movement is possible due to the development of invasive structures like invadopodia, lamellipodia, and filopodia. These changes are dependent on the reorganization of the actin cytoskeleton. In turn, the polymerization and depolymerization of actin are controlled by actin-binding proteins. In many tumour cells, the actin and actin-associated proteins are accumulated in the cell nucleus, suggesting that it may also affect the progression of cancer by regulating gene expression. Once the cancer cell reaches a new habitat it again acquires epithelial features and thus proliferative activity. Targeting of epithelial-to-mesenchymal or/and mesenchymal-to-epithelial transitions through regulation of their main components expression may be a potential solution to the problem of metastasis. This work focuses on the role of these processes in tumour progression and the assessment of therapeutic potential of agents targeting them.

In vivo analysis of Arg-Gly-Asp sequence/integrin α5β1-mediated signal involvement in embryonic enchondral ossification by exo utero development system

Enchondral ossification is a fundamental mechanism for longitudinal bone growth during vertebrate development. In vitro studies suggested that functional blockade with RGD peptides or with an antibody that interferes with integrin α5β1-ligand interactions inhibited pre-hypertrophic chondrocyte differentiation. The purpose of this study is to elucidate in vivo the roles of the integrin α5β1-mediated signal through the Arg-Gly-Asp (RGD) sequence in the cell-extracellular matrix (ECM) interaction in embryonic enchondral ossification by an exo utero development system. We injected Arg-Gly-Asp-Ser (RGDS) peptides and anti-integrin α5β1 antibody (α5β1 ab) in the upper limbs of mouse embryos at embryonic day (E) 15.5 (RGDS-injected limbs, α5β1 ab-injected limbs), and compared the effects on enchondral ossification with those found in the control limbs (Arg-Gly-Glu-Ser peptide-, mouse IgG-, or vehicle-injected, and no surgery) at E16.5. In the RGDS-injected limbs, the humeri were shorter and there were fewer BrdU-positive cells than in the control limbs. The ratios of cartilage length and area to those of the humerus were higher in the RGDS-injected limbs. The ratios of type X collagen to type 2 collagen mRNA and protein (Coll X/Coll 2) were significantly lower in the RGDS-injected limbs. In those limbs, TUNEL-positive cells were hardly observed, and the ratios of fractin to the Coll X/Coll 2 ratio were lower than in the control limbs. Furthermore, the α5β1 ab-injected limbs showed results similar to those of RGDS-injected limbs. The present in vivo study by exo utero development system showed that RGDS and α5β1 ab injection decreased chondrocyte proliferation, differentiation, and apoptosis in enchondral ossification, and suggested that the integrin α5β1-mediated ECM signal through the RGD sequence is involved in embryonic enchondral ossification.

The apical junctional complex in respiratory diseases

The epithelium, including the respiratory system, acts as a selective gate between the outside environment and underlying tissue. Epithelial cells are polarized due to the formation of the apical junctional complex, which includes adherent junctions and tight junctions. Endothelial cells are one of the most important cellular constituents of blood vessels. Endothelial junctional proteins play important roles in tissue integrity as well as in vascular permeability, leukocyte extravasation, and angiogenesis. This review focuses on the apical junctional complex in respiratory diseases.

Molecular basis of the inner blood-retinal barrier and its breakdown in diabetic macular edema and other pathological conditions

Breakdown of the inner endothelial blood-retinal barrier (BRB), as occurs in diabetic retinopathy, age-related macular degeneration, retinal vein occlusions, uveitis and other chronic retinal diseases, results in vasogenic edema and neural tissue damage, causing loss of vision. The central mechanism of altered BRB function is a change in the permeability characteristics of retinal endothelial cells caused by elevated levels of growth factors, cytokines, advanced glycation end products, inflammation, hyperglycemia and loss of pericytes. Subsequently, paracellular but also transcellular transport across the retinal vascular wall increases via opening of endothelial intercellular junctions and qualitative and quantitative changes in endothelial caveolar transcellular transport, respectively. Functional changes in pericytes and astrocytes, as well as structural changes in the composition of the endothelial glycocalyx and the basal lamina around BRB endothelium further facilitate BRB leakage. As Starling's rules apply, active transcellular transport of plasma proteins by the BRB endothelial cells causing increased interstitial osmotic pressure is probably the main factor in the formation of macular edema. The understanding of the complex cellular and molecular processes involved in BRB leakage has grown rapidly in recent years. Although appropriate animal models for human conditions like diabetic macular edema are lacking, these insights have provided tools for rational design of drugs aimed at restoring the BRB as well as for design of effective transport of drugs across the BRB, to treat the chronic retinal diseases such as diabetic macular edema that affect the quality-of-life of millions of patients.

Mechanistic study of the proangiogenic effect of osteoprotegerin

Osteoprotegerin (OPG), a soluble tumour necrosis factor receptor superfamily member, inhibits RANKL-mediated osteoclastogenesis. We have previously reported that OPG enhances the proangiogenic properties of endothelial colony-forming cells (ECFCs) in vitro, and promotes vasculogenesis in vivo. Here we investigated how OPG promotes neovascularisation. Proteomic experiments showed that OPG pretreatment affected ECFCs protein expression in two ways, 23 spots being down-regulated and 6 upregulated. These spots corresponded to proteins involved in cell motility, adhesion, signal transduction and apoptosis. In keeping with these proteomic results, we found that OPG induced ECFCs adhesion to activated endothelium in shear stress conditions, promoting intermediate but not focal adhesion to fibronectin and collagen. Treatment with OPG induced a reorganization of the ECFCs cytoskeleton, with the emergence of cell protrusions characteristic of a migratory phenotype. These effects correlated with decreased FAK phosphorylation and enhanced integrin αVβ3 expression. OPG drastically reduced caspase-3/7 activities and maintained ECFCs viability after 48 h of treatment. All these effects were significantly attenuated by ECFCs incubation with the CXCR4 antagonist AMD-3100, and by prior heparan sulphate proteoglycan disruption. The proangiogenic properties of OPG appeared to be mediated by the proteoglycan syndecan-1, although OPG 1-194 lacking its heparin-binding domain still had pro-vasculogenic effects in vitro and in vivo. These results suggest that OPG may interact with ECFCs by binding to HSPGs/syndecan-1, thereby induce an anti-adhesive effect and promoting ECFCs migration through a SDF-1/CXCR4 dependent pathway.

Integrin-assisted drug delivery of nano-scaled polymer therapeutics bearing paclitaxel

Angiogenesis plays a prominent role in cancer progression. Anti-angiogenic therapy therefore, either alone or in combination with conventional cytotoxic therapy, offers a promising therapeutic approach. Paclitaxel (PTX) is a widely-used potent cytotoxic drug that also exhibits anti-angiogenic effects at low doses. However, its use, at its full potential, is limited by severe side effects. Here we designed and synthesized a targeted conjugate of PTX, a polymer and an integrin-targeted moiety resulting in a polyglutamic acid (PGA)-PTX-E-[c(RGDfK)(2)] nano-scaled conjugate. Polymer conjugation converted PTX to a macromolecule, which passively targets the tumor tissue exploiting the enhanced permeability and retention effect, while extravasating via the leaky tumor neovasculature. The cyclic RGD peptidomimetic enhanced the effects previously seen for PGA-PTX alone, utilizing the additional active targeting to the α(v)β(3) integrin overexpressed on tumor endothelial and epithelial cells. This strategy is particularly valuable when tumors are well-vascularized, but they present poor vascular permeability. We show that PGA is enzymatically-degradable leading to PTX release under lysosomal acidic pH. PGA-PTX-E-[c(RGDfK)(2)] inhibited the growth of proliferating α(v)β(3)-expressing endothelial cells and several cancer cells. We also showed that PGA-PTX-E-[c(RGDfK)(2)] blocked endothelial cells migration towards vascular endothelial growth factor; blocked capillary-like tube formation; and inhibited endothelial cells attachment to fibrinogen. Orthotopic studies in mice demonstrated preferential tumor accumulation of the RGD-bearing conjugate, leading to enhanced anti-tumor efficacy and a marked decrease in toxicity as compared with free PTX-treated mice.

Albumin causes increased myosin light chain kinase expression in astrocytes via p38 mitogen-activated protein kinase

Myosin light chain kinase (MLCK) plays an important role in the reorganization of the cytoskeleton, leading to disruption of vascular barrier integrity in multiple organs, including the blood-brain barrier (BBB), after traumatic brain injury (TBI). MLCK has been linked to transforming growth factor (TGF) and rho kinase signaling pathways, but the mechanisms regulating MLCK expression following TBI are not well understood. Albumin leaks into the brain parenchyma following TBI, activates glia, and has been linked to TGF-β receptor signaling. We investigated the role of albumin in the increase of MLCK in astrocytes and the signaling pathways involved in this increase. After midline closed-skull TBI in mice, there was a significant increase in MLCK-immunoreactive (IR) cells and albumin extravasation, which was prevented by treatment with the MLCK inhibitor ML-7. Using immunohistochemical methods, we identified the MLCK-IR cells as astrocytes. In primary astrocytes, exposure to albumin increased both isoforms of MLCK, 130 and 210. Inhibition of the TGF-β receptor partially prevented the albumin-induced increase in both isoforms, which was not prevented by inhibition of smad3. Inhibition of p38 MAPK, but not ERK, JNK, or rho kinase, also prevented this increase. These results are further evidence of a role of MLCK in the mechanisms of BBB compromise following TBI and identify astrocytes as a cell type, in addition to endothelium in the BBB, that expresses MLCK. These findings implicate albumin, acting through p38 MAPK, in a novel mechanism by which activation of MLCK following TBI may lead to compromise of the BBB.

Angiogenesis in Balb/c mice under beta-carotene supplementation in diet

Angiogenesis is a process of new blood vessel formation from pre-existing ones. The most important steps in angiogenesis include detachment, proliferation, migration, homing and differentiation of vascular wall cells, which are mainly endothelial cells and their progenitors. The study focused on the effect of beta-carotene (BC) supplementation (12,000 mg/kg) in the diet on angiogenesis in Balb/c mice. Female Balb/c mice were fed for 5 weeks with two different diets: with BC or without BC supplementation. After 4 weeks of feeding, Balb/c mice were injected subcutaneously with two matrigel plugs with or without basic fibroblast growth factor (bFGF). Six days later, the animals were killed, and the matrigel plugs were used for immunohistochemical staining with CD31 antibody and for gene expression analysis. Microarray and Real-Time PCR data showed down-regulation of genes involved in proliferation and up-regulation of genes encoding inhibitors of apoptosis, proteins regulating cell adhesion, matrix-degrading enzymes and proteins involved in the VEGF pathway. The results of this study demonstrated that BC proangiogenic activity (with or without bFGF) in vivo seemed to be more significantly associated with cells' protection from apoptosis and their stimulation of chemotaxis/homing than cell proliferation.

The role of apoptosis in LDL transport through cultured endothelial cell monolayers

We have previously shown that leaky junctions associated with dying or dividing cells are the dominant pathway for low density lipoprotein (LDL) transport under convective conditions, accounting for more than 90% of the transport [Cancel LM, Fitting A, Tarbell JM. In vitro study of LDL transport under pressurized (convective) conditions. Am J Physiol Heart Circ Physiol 2007;293:H126-32]. To explore the role of apoptosis in the leaky junction pathway, TNFalpha and cycloheximide (TNFalpha/CHX) were used to induce an elevated rate of apoptosis in cultured bovine aortic endothelial cell (BAEC) monolayers and the convective fluxes of LDL and water were measured. Treatment with TNFalpha/CHX induced a 18.3-fold increase in apoptosis and a 4.4-fold increase in LDL permeability. Increases in apoptosis and permeability were attenuated by treatment with the caspase inhibitor Z-VAD-FMK. Water flux increased by 2.7-fold after treatment with TNFalpha/CHX, and this increase was not attenuated by treatment with Z-VAD-FMK. Immunostaining of the tight junction protein ZO-1 showed that TNFalpha/CHX treatment disrupts the tight junction in addition to inducing apoptosis. This disruption is present even when Z-VAD-FMK is used to inhibit apoptosis, and likely accounts for the increase in water flux. We found a strong correlation between the rate of apoptosis and the permeability of BAEC monolayers to LDL. These results demonstrate the potential of manipulating endothelial monolayer permeability by altering the rate of apoptosis pharmacollogicaly. This has implications for the treatment of atherosclerosis.

Suppression of retinal neovascularization with an antagonist to vascular endothelial cadherin

OBJECTIVES

To examine the role of vascular endothelial cadherin (VE-cadherin) in cellular processes underlying angiogenesis and the effects of VE-cadherin inhibition on retinal angiogenesis.

METHODS

Retinal neovascularization was induced in newborn mice by exposure to 75% oxygen (postnatal days 7-12) followed by room air and quantitated from histological sections. Mice received daily intraperitoneal injections of either a VE-cadherin antagonist or a control peptide from postnatal days 12 to 17. In vitro cell migration, proliferation, and tubule formation were examined in the presence of the VE-cadherin antagonist. The effect of antagonist treatment on the integrity of established cell junctions was examined by fluorescein isothiocyanate-dextran monolayer permeability and VE-cadherin immunocytochemistry.

RESULTS

Treatment with the VE-cadherin antagonist significantly reduced retinal angiogenesis. Inhibition of VE-cadherin function suppressed tubule formation in endothelial cells. The antagonist treatment also decreased cell migration and proliferation. The antagonist treatment did not affect the integrity of existing cell junctions. Immunostaining for VE-cadherin and rates of monolayer permeability were comparable to those in untreated controls.

CONCLUSION

Our study points to a pivotal role played by VE-cadherin in the angiogenic process.

CLINICAL RELEVANCE

Inhibition of VE-cadherin might be an effective strategy for pharmacological inhibition in proliferative retinopathies.

Contribution of annexin 2 to the architecture of mature endothelial adherens junctions

The vascular endothelial cadherin (VE-cad)-based complex is involved in the maintenance of vascular endothelium integrity. Using immunoprecipitation experiments, we have demonstrated that, in confluent human umbilical vein endothelial cells, the VE-cad-based complex interacts with annexin 2 and that annexin 2 translocates from the cytoplasm to the cell-cell contact sites as cell confluence is established. Annexin 2, located in cholesterol rafts, binds to both the actin cytoskeleton and the VE-cad-based complex so the complex is docked to cholesterol rafts. These multiple connections prevent the lateral diffusion of the VE-cad-based complex, thus strengthening adherens junctions in the ultimate steps of maturation. Moreover, we observed that the down-regulation of annexin 2 by small interfering RNA induces a delocalization of VE-cad from adherens junctions and consequently a destabilization of these junctions. Furthermore, our data indicate that the decoupling of the annexin 2/p11 complex from the VE-cad-based junction, triggered by vascular endothelial growth factor treatment, facilitates the switch from a quiescent to an immature state.

Integrin affinity modulation in angiogenesis

Integrins, transmembrane glycoprotein receptors, play vital roles in pathological angiogenesis, but their precise regulatory functions are not completely understood and remain controversial. This study aims to assess the regulatory functions of individual beta subunits of endothelial integrins in angiogenic responses induced by vascular endothelial growth factor (VEGF). Inhibition of expression of beta(1), beta(3), or beta(5) integrins in endothelial cells resulted in down regulation of EC adhesion and migration on the primary ligand for the corresponding integrin receptor, while no effects on the recognition of other ligands were detected. Although inhibition of expression of each subunit substantially affected capillary growth stimulated by VEGF, the loss of beta(3) integrin was the most inhibitory.

Vascular endothelial cadherin regulates vascular permeability: Implications for ovarian hyperstimulation syndrome

CONTEXT

Ovarian hyperstimulation syndrome (OHSS) is an iatrogenic complication of treatment with fertility drugs. It is characterized by increased vascular permeability and simultaneous overexpression of vascular endothelial growth factor (VEGF) in ovarian cells.

OBJECTIVE

We tested the hypothesis that the endothelium and endothelial cell-to-cell junctions are downstream targets of VEGF during OHSS pathogenesis. We investigated the potential involvement of vascular endothelial (VE)-cadherin, an interendothelial adhesion molecule, in the capillary hyperpermeability in OHSS.

DESIGN

Human endothelial cells from umbilical veins (HUVEC) were used as an in vitro model of OHSS.

INTERVENTION

Cell cultures were treated with varying doses of estradiol (E2), human chorionic gonadotropin (hCG), VEGF, and antihuman VEGF antibodies, either alone or in combination, and the effect on VE-cadherin release was evaluated at different time points. Permeability assays were performed using fluoresceinisothiocyanate-labeled albumin, and actin filaments rearrangement was evaluated by fluorescent microscopy.

RESULTS

Culturing of HUVEC with high doses of E2 produced no significant changes in VE-cadherin concentration, but hCG and VEGF produced a significant increase in VE-cadherin release. Time-course experiments showed that VE-cadherin was secreted 12 h after VEGF addition. Antihuman VEGF antibodies prevented these changes. Permeability assays demonstrated that, although E2 did not alter the arrangement of HUVEC in vitro, hCG and VEGF caused changes in the actin fibers indicative of increased capillary permeability. VEGF also induced an increase in paracellular permeability of HUVEC at the same doses used in the previous experiments.

CONCLUSIONS

Adhesion molecules like VE-cadherin may play a role in the development and progression of increased capillary permeability in severe OHSS.

Cadherin-2 participates in the morphogenesis of the zebrafish inner ear

Molecular mechanisms that control inner ear morphogenesis from the placode to the three-dimensional functional organ are not well understood. We hypothesize that cell-cell adhesion, mediated by cadherin molecules, contributes significantly to various stages of inner ear formation. Cadherin-2 (Cdh2) function during otic vesicle morphogenesis was investigated by examining morpholino antisense oligonucleotide knockdown and glass onion (glo) (Cdh2 mutant) zebrafish embryos. Placode formation, vesicle cavitation and specification occurred normally, but morphogenesis of the otic vesicle was affected by Cdh2 deficiency: semicircular canals were reduced or absent. Phalloidin staining of the hair cell stereocillia demonstrated that cadherin-2 (cdh2) loss-of-function did not affect hair cell number, but acetylated tubulin labeling showed that hair cell kinocilia were shorter and irregularly shaped. Statoacoustic ganglion size was significantly reduced, which suggested that neuron differentiation or maturation was affected. Furthermore, cdh2 loss-of-function did not cause a general developmental delay, since differentiation of other tissues, including eye, proceeded normally. These findings demonstrate that Cdh2 selectively affects epithelial morphogenetic cell movements, particularly semicircular canal formation, during normal ear mophogenesis.

Targeting of angiogenic endothelial cells at sites of inflammation by dexamethasone phosphate-containing RGD peptide liposomes inhibits experimental arthritis

OBJECTIVE

To investigate whether RGD peptide-exposing long circulating polyethylene glycol (PEG) liposomes (RGD-PEG-L) targeted to alphavbeta3 integrins expressed on angiogenic vascular endothelial cells (VECs) are able to bind VECs at sites of inflammation and whether such liposomes containing dexamethasone phosphate (DEXP) can be used as carriers to interfere with the development of experimental arthritis.

METHODS

Binding and internalization of RGD-PEG-L were studied by fluorescence-activated cell sorting and confocal microscopy using fluorescently labeled liposomes. Radiolabeled liposomes were used to test in vivo pharmacokinetics and inflammation site targeting in lipopolysaccharide (LPS)-induced inflammation and adjuvant-induced arthritis (AIA) in rats. In vivo inflammation targeting was visualized by intravital microscopy using fluorescently labeled RGD-PEG-L. Therapeutic efficacy of DEXP-encapsulating RGD-PEG-L compared with nontargeted liposomes was evaluated in rats with AIA.

RESULTS

RGD-PEG-L bound to and were taken up by proliferating human VECs in vitro. In vivo, increased targeting of radiolabeled RGD-PEG-L to areas of LPS-induced inflammation in rats was observed. Specific association with the blood vessel wall at the site of inflammation was confirmed by intravital microscopy. One single intravenous injection of DEXP encapsulated in RGD-PEG-L resulted in a strong and long-lasting antiarthritic effect in rat AIA.

CONCLUSION

RGD-targeted PEG liposomes represent an endothelial cell-specific drug delivery system that targets VECs at sites of inflammation. Use of these liposomes to deliver DEXP to VECs at arthritis-affected sites proved efficacious in rat adjuvant arthritis. These data indicate that VECs have an essential role in the inflammation process and suggest the possibility of using VEC targeting for therapeutic intervention in inflammatory processes such as arthritis.

Modulation of response to tumor therapies by the extracellular matrix

The composition of the extracellular matrix in tumors is vastly different from that found in the normal tissue counterparts. As the extracellular matrix can signal to cells via integrin binding and activation, which is known to modulate cell proliferation, survival and migration, it may influence the response of both tumor and endothelial cells to anticancer therapies. Certain tumor-associated extracellular matrix proteins have been shown to confer resistance to chemotherapeutic drugs, radiation and anti-angiogenic factors. The current literature regarding this phenomenon and the potential therapeutic modalities to overcome extracellular matrix-induced resistance will be discussed.

Inhibition of tumor angiogenesis by a natural fragment of a tRNA synthetase

Human tyrosyl- and tryptophanyl-tRNA synthetases (TyrRS and TrpRS, respectively) link protein synthesis to signal-transduction pathways, including angiogenesis. Fragments of TyrRS stimulate angiogenesis, whereas those of TrpRS (T2-TrpRS) inhibit angiogenesis. Thus, these two synthetases acquired opposing activities during evolution, possibly as a coordinated mechanism for regulating angiogenesis. The recent identification of the cellular target of T2-TrpRS sheds light into the mechanism of angiogenesis inhibition. This mechanism provides a molecular basis for the lack of effect of T2-TrpRS on the normal vasculature. With these features, we suggest that this fragment of a tRNA synthetase might safely be used to arrest neovascularization of tumors. In particular, an anti-angiogenesis agent that stops the growth of tumor vessels without affecting normal vessels might serve as an adjunct to cytotoxic therapy.

Lysophospholipids enhance matrix metalloproteinase-2 expression in human endothelial cells

Lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) are both low-molecular-weight lysophospholipids, which promote cell proliferation, migration, and invasion via interaction with a family of specific G protein-coupled receptors. Matrix metalloproteinases (MMPs) are zinc-dependent proteolytic enzymes, which are involved in degradation of the extracellular matrix and play critical roles in endothelial cell migration and matrix remodeling during angiogenesis. Among these MMPs, MMP-2 is known to trigger cell migration. In our present study, we examined the effects of LPA and S1P on MMP-2 expression in human endothelial cells. We showed that LPA and S1P enhanced MMP-2 expression in mRNA, protein levels, and also enzymatic activity of cells of the EAhy926 human endothelial cell line. The enhancement effects occurred in concentration- and time-dependent manners. Results from real-time PCR, Western blots, and substrate gels indicated that these enhancement effects were mediated through MAPK kinase/ERK-, nuclear factor-kappaB-, and calcium influx-dependent pathways. Furthermore, we show that endothelial cell invasion of the gel was enhanced by lysophospholipids, and the induction could be prevented by an MMP inhibitor, GM6001. These observations suggest that LPA and S1P may play important roles in endothelial cell invasion by regulating the expression of MMP-2.

Proangiogenic activity of beta-carotene is coupled with the activation of endothelial cell chemotaxis

Endothelial cells play an important role in angiogenesis (formation of new vessels from preexisting ones), which is essential for organogenesis, tissue remodeling but also inflammatory response, carcinogenesis in all periods of our life. Beta-carotene (BC) in non-toxic concentrations (up to 3 microM) had no detectable effect on HUVECs (human umbilical vein endothelial cells) proliferation or apoptosis, despite significant changes of the expression patterns of pro- and anti-apoptotic genes. However beta-carotene did not change the tubulogenic activity of HUVEC in the in vitro angiogenesis model, it potently accelerated the bFGF-induced development of microcapillaries, as well as the migration of endothelial cells, in matrigel plug injected subcutaneously to mice. Potent activation of endothelial cell migration in the in vitro model of chemotaxis was also observed. According to the microarray data, genes involved in cell/cell and cell/matrix adhesion, matrix reorganization, activation of chemotaxis, the G-protein regulated intracellular signaling as well as genes involved in the rapid remodeling of protein cytoskeleton were the most affected by BC in HUVEC. We conclude that beta-carotene in the physiological concentration range stimulates early steps of angiogenesis by the activation of cellular migration as well as matrix reorganization and decrease of cell adhesion.

Identification of a transiently exposed VE-cadherin epitope that allows for specific targeting of an antibody to the tumor neovasculature

VE-cadherin is an adhesion molecule localized at the adherens junctions of endothelial cells. It is crucial for the proper assembly of vascular structures during angiogenesis and maintaining vascular integrity. We have studied 3 monoclonal antibodies (mAbs) against murine VE-cadherin that inhibit angiogenesis and tumor growth. Two of these, BV13 and 10G4, also disrupted normal vessels, resulting in severe vascular leakage, whereas the third, E4G10, did not. The goal of the current report was to identify the epitope of E4G10 and distinguish it from those of the disruptive mAbs. We mapped the epitope of E4G10 to within the first 10 amino acids of mature VE-cadherin and demonstrated that conserved tryptophan residues in this sequence are required for VE-cadherin-mediated trans-adhesion. The disruptive mAbs target a different epitope within amino acids 45 to 56, which structural homology modeling suggests is not involved in trans-adhesion. From our studies, we hypothesize that E4G10 can only bind the neovasculature, where VE-cadherin has not yet engaged in trans-adhesion and its epitope is fully exposed. Thus, E4G10 can inhibit junction formation and angiogenesis but is unable to target normal vasculature because its epitope is masked. In contrast, BV13 and 10G4 bind an epitope that is accessible regardless of VE-cadherin interactions, leading to the disruption of adherens junctions. Our findings establish the immediate N-terminal region of VE-cadherin as a novel target for inhibiting angiogenesis.

Transmembrane CEACAM1 affects integrin-dependent signaling and regulates extracellular matrix protein-specific morphology and migration of endothelial cells

Carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1/CD66a), expressed on leukocytes, epithelia, and endothelia mediates homophilic cell adhesion. It plays an important role in cell morphogenesis and, recently, soluble CEACAM1 isoforms have been implicated in angiogenesis. In the present study, we investigated the function of long transmembrane isoform of CEACAM1 (CEACAM1-L) in cultured rat brain endothelial cells. We observed that expression of CEACAM1-L promotes network formation on basement membrane Matrigel and increased cell motility after monolayer injury. During cell-matrix adhesion, CEACAM1-L translocated into the Triton X-100-insoluble cytoskeletal fraction and affected cell spreading and cell morphology on Matrigel and laminin-1 but not on fibronectin. On laminin-1, CEACAM1-L-expressing cells developed protrusions with lamellipodia, showed less stress fiber formation, reduced focal adhesion kinase (FAK) tyrosine phosphorylation, and decreased focal adhesion formation leading to high motility. CEACAM1-L-mediated morphologic alterations were sensitive to RhoA activation via lysophosphatidic acid (LPA) treatment and dependent on Rac1 activation. Furthermore, we demonstrate a matrix protein-dependent association of CEACAM1-L with talin, an important regulator of integrin function. Taken together, our results suggest that transmembrane CEACAM1-L expressed on endothelial cells is implicated in the activation phase of angiogenesis by affecting the cytoskeleton architecture and integrin-mediated signaling.

Endothelial cell-to-cell junctions: molecular organization and role in vascular homeostasis

Intercellular junctions mediate adhesion and communication between adjoining endothelial and epithelial cells. In the endothelium, junctional complexes comprise tight junctions, adherens junctions, and gap junctions. The expression and organization of these complexes depend on the type of vessels and the permeability requirements of perfused organs. Gap junctions are communication structures, which allow the passage of small molecular weight solutes between neighboring cells. Tight junctions serve the major functional purpose of providing a "barrier" and a "fence" within the membrane, by regulating paracellular permeability and maintaining cell polarity. Adherens junctions play an important role in contact inhibition of endothelial cell growth, paracellular permeability to circulating leukocytes and solutes. In addition, they are required for a correct organization of new vessels in angiogenesis. Extensive research in the past decade has identified several molecular components of the tight and adherens junctions, including integral membrane and intracellular proteins. These proteins interact both among themselves and with other molecules. Here, we review the individual molecules of junctions and their complex network of interactions. We also emphasize how the molecular architectures and interactions may represent a mechanistic basis for the function and regulation of junctions, focusing on junction assembly and permeability regulation. Finally, we analyze in vivo studies and highlight information that specifically relates to the role of junctions in vascular endothelial cells.

Desmoplakin is required for microvascular tube formation in culture

Desmoplakin (DP) is a key component of cellular adhesion junctions known as desmosomes; however, recent investigations have revealed a novel location for DP in junctions separate from desmosomes termed complexus adherens junctions. These junctions are found at contact sites between endothelial cells that line capillaries. Few studies have focused on the function of DP in de novo capillary formation (vasculogenesis) and branching (angiogenesis) during tumorigenesis, embryonic development, cardiovascular development or wound healing. Only recently have investigations begun to determine the effect the loss of DP has on capillaries during embryogenesis (i.e. in DP-/- mice). Evidence shows that the loss of desmoplakin in vivo results in leaky capillaries and/or capillary malformation. Consequently, the goal of this study was to determine the function of DP in complexus adherens junctions during capillary formation. To accomplish this goal, we used siRNA technology to knock down desmoplakin expression in endothelial cells before they were induced to form microvascular tubes on matrigel. DP siRNA treated cells sent out filopodia and came in close contact with each other when plated onto matrigel; however, in most cases they failed to form tubes as compared with control endothelial cells. Interestingly, after siRNA degradation, endothelial cells were then capable of forming microvascular tubes. In depth analyses into the function of DP in capillary formation were not previously possible because the tools and experimental approaches only recently have become available (i.e. siRNA). Consequently, fully understanding the role of desmoplakin in capillary formation may lead to a novel approach for inhibiting vasculo- and angiogenesis in tumor formation.

Endothelial expression of the alpha6beta4 integrin is negatively regulated during angiogenesis

Development and homeostasis of the vascular system requires integrin-facilitated cellular adhesion, migration, proliferation and survival. A specific role for the alpha6beta4 integrin in the vasculature, however, has not been identified. Using immunohistochemistry, we observed alpha6beta4 expression on the dermal microvasculature of human foreskin. Analysis of individual cells isolated from trypsin-disrupted foreskin tissue indicated that alpha6beta4 was expressed by a subset of epithelial and endothelial cells, and not by smooth muscle cells. Expression of alpha6beta4 was also analyzed during new vessel growth using explants of human saphenous vein cultured in fibrinogen gels. The results indicate that alpha6beta4 is not expressed by outgrowing endothelial cells, and is downregulated by the original alpha6beta4-positive endothelial cells of the explant. To determine whether alpha6beta4 is expressed during angiogenesis in vivo, the expression of the beta4 subunit was analyzed during the development of the mouse mystacial (whisker) pad. Immunohistochemical staining of the whisker pad indicates that beta4 is expressed by the adult vasculature. To identify when and where beta4 is turned on in the vasculature, we examined the whisker pads from the developing embryo (E19.5 pc), and from postnatal days zero (P0), three (P3) and seven (P7) pups. The expression of alpha6beta4 was found to be turned on spatially and temporally from caudal to rostral regions and from the deep to superficial vasculature, correlating with the maturation of the whisker pad and its corresponding vasculature. Together, these findings suggest a potential role for alpha6beta4 as a negative component of the angiogenic switch, whereas expression of alpha6beta4 on the adult vasculature may indicate regions requiring additional adhesive mechanisms.

Molecular plasticity of human melanoma cells

The molecular analysis of tumors, such as melanoma, has benefited significantly from microarray technology that can facilitate the classification of tumors based on the differential expression of genes. The data summarized in this review describe the molecular profile of aggressive cutaneous and uveal melanoma cells as that of multiple phenotypes similar to a pluripotent, embryonic-like stem cell. A noteworthy example of the plasticity of the aggressive melanoma cell phenotype is demonstrated by the ability of these tumor cells to engage in vasculogenic mimicry and neovascularization. A review of the current evidence demonstrating important cellular and molecular determinants of melanoma vasculogenic mimicry is presented. In addition, novel signaling pathways are discussed, involving VE-cadherin, EphA2, FAK, and PI 3-kinase, which promote cell migration, invasion, and matrix remodeling. The observations summarized in this review describe some of the key molecular events that regulate the process of melanoma vasculogenic mimicry and identify new signal transduction pathways that can serve as putative targets for therapeutic intervention.

ECM regulates MT1-MMP localization with beta1 or alphavbeta3 integrins at distinct cell compartments modulating its internalization and activity on human endothelial cells

Regulation of membrane-type 1 matrix metalloproteinase (MT1-MMP) by different extracellular matrices (ECMs) on human endothelial cells (ECs) has been investigated. First, MT1-MMP is found at the intercellular contacts of confluent ECs grown on beta1 integrin-dependent matrix such as type 1 collagen (COL I), fibronectin (FN), or fibrinogen (FG), but not on gelatin (GEL) or vitronectin (VN). The novel localization of MT1-MMP at cell-cell contacts is assessed by confocal videomicroscopy of MT1-MMP-GFP-transfected ECs. Moreover, MT1-MMP colocalizes with beta1 integrins at the intercellular contacts, whereas it is preferentially found with alphavbeta3 integrin at motility-associated structures on migrating ECs. In addition, clustered integrins recruit MT1-MMP and neutralizing anti-beta1 or anti-alphav integrin mAb displace MT1-MMP from its specific sites, pointing to a biochemical association that is finally demonstrated by coimmunoprecipitation assays. On the other hand, COL I, FN, or FG up-regulate cell surface MT1-MMP on confluent ECs by an impairment of its internalization, whereas expression and internalization are not modified on GEL or VN. In addition, MT1-MMP activity is diminished in confluent ECs on COL I, FN, or FG. Finally, MT1-MMP participates and cooperates with beta1 and alphavbeta3 integrins in the migration of ECs on different ECM. These data show a novel mechanism by which ECM regulates MT1-MMP association with beta1 or alphavbeta3 integrins at distinct cellular compartments, thus modulating its internalization, activity, and function on human ECs.

Identification of an occludin cell adhesion recognition sequence

The molecular mechanisms by which the tight junction integral membrane protein, occludin promotes cell adhesion and establishes an endothelial monolayer permeability barrier have not been elucidated. In particular, the amino acid sequences of the occludin cell adhesion recognition (CAR) sites have not been determined. Here we demonstrate that a cyclic peptide containing the sequence LYHY, which is found in the second extracellular domain of occludins in all mammalian species, inhibits the establishment of endothelial cell barriers in vitro and in vivo. This cyclic peptide also prevents the aggregation of fibroblasts stably transfected with cDNA encoding occludin. The data suggest that the LYHY motif is an occludin CAR sequence.

A monoclonal antibody to vascular endothelial-cadherin inhibits tumor angiogenesis without side effects on endothelial permeability

Vascular endothelial cadherin (VE-cadherin) is an endothelial-specific, trans-membrane protein that promotes homophilic cell adhesion. Inhibition of VE-cadherin by the blocking monoclonal antibody (mAb) BV13 inhibited angiogenesis and tumor growth in vivo. However, this effect was accompanied by a marked increase in lung and heart permeability. In the present paper, we characterize a different VE-cadherin mAb (BV14) that is able to inhibit angiogenesis without affecting vascular permeability. In vitro studies show that BV14, in contrast to BV13, did not increase paracellular permeability of endothelial monolayers and did not disrupt VE-cadherin clusters at junctions. However, both antibodies could inhibit formation of vascularlike structures in collagen gels and increase migration of endothelial cells into wounded areas. In vivo, BV14 and BV13 were equally active in inhibiting angiogenesis in the mouse cornea and in reducing the growth of hemangioma and C6 glioma. In contrast to BV13, BV14 did not change vascular permeability in all the organs tested and at any dose used. BV14 and BV13 bind to VE-cadherin extracellular repeats EC4 and EC1, respectively. We propose that, in resting vessels, where junctions are stable and well-structured, antibody binding to EC1 but not EC4 disrupts their organization and increases permeability. In contrast, in growing vessels, where endothelial cells are migrating and junctions are weaker, antibody binding to EC4 may be sufficient to disrupt cell-to-cell adhesion and inhibit assembly of new vascular structures.

Rac regulates endothelial morphogenesis and capillary assembly

Endothelial cells undergo branching morphogenesis to form capillary tubes. We have utilized an in vitro Matrigel overlay assay to analyze the role of the cytoskeleton and Rho GTPases during this process. The addition of matrix first induces changes in cell morphology characterized by the formation of dynamic cellular protrusions and the assembly of discrete aggregates or cords of aligned cells resembling primitive capillary-like structures, but without a recognizable lumen. This is followed by cell migration leading to the formation of a complex interconnecting network of capillary tubes with readily identifiable lumens. Inhibition of actin polymerization or actin-myosin contraction inhibits cell migration but has no effect on the initial changes in endothelial cell morphology. However, inhibition of microtubule dynamics prevents both the initial cell shape changes as well as cell migration. We find that the small GTPase Rac is essential for the matrix-induced changes in endothelial cell morphology, whereas p21-activated kinase, an effector of Rac, is required for cell motility. We conclude that Rac integrates signaling through both the actin and microtubule cytoskeletons to promote capillary tube assembly.

Functional microarray analysis of mammary organogenesis reveals a developmental role in adaptive thermogenesis

The use of DNA microarrays to study vertebrate organogenesis presents unique analytical challenges compared with expression profiling of homogeneous cell populations. We have used a general approach that permits the automated, unbiased identification of biologically relevant patterns of gene expression to study murine mammary gland development. Our studies confirm the utility of this approach by demonstrating the ready identification of cellular processes and pathways of known functional importance in mammary development. Additionally, this approach permitted the identification of genetic pathways with unpredicted patterns of developmental regulation, including those involved in angiogenesis, extracellular matrix synthesis, and the beta-oxidation of fatty acids. Surprisingly, our findings demonstrate that the coordinate regulation of genes involved in the beta-oxidation of fatty acids reflects the presence of an abundant, yet previously unrecognized stromal compartment within the mammary gland that is composed of brown adipose tissue. Our data demonstrate that the amount of brown adipose tissue present in the mammary gland is developmentally regulated; that PPARalpha, Ucp1, and genes involved in fatty acid oxidation are spatially and temporally coregulated during development; that the mammary gland plays a functional role in adaptive thermogenesis; and that the transcriptional control of this adaptive response to cold is itself developmentally regulated.

Transcytosis of plasma macromolecules in endothelial cells: a cell biological survey

The modern exploration of endothelial cell biology is a largely interdisciplinary exercise. Cell biological, physiological, and more recently molecular biology approaches were used to study the pathways and the organelles involved in transcytosis of macromolecules in endothelial cell (EC). Here we discuss mainly the cell biological findings that revealed that EC have the attributes to fulfill the transport function. They are polarized cells, heterogeneous, and, thus, structurally and functionally adapted to the vascular bed in which they reside. The structural heterogeneity involves the number and distribution of plasmalemmal vesicles (caveolae), their generated channels, and the organization of intercellular junctions. The closely related functional heterogeneity comprises the degree of permeability for plasma molecules that vary as a function of organ. The EC are endowed with the cellular machinery to perform (1) endocytosis, that is to take up plasma proteins and the molecules they carry to be used for themselves (cholesterol-carrying low density lipoproteins, fatty acid carrying albumin, iron carrying transferrin, etc.), and (2) transcytosis, which implies to transport plasma proteins to the subjacent cells and tissues. The possible pathways for transport of molecules are transcellular, via caveolae and channels, and paracellular via intercellular junctions. Most of the results obtained, so far, indicate that transcytosis of albumin, low-density lipoproteins, metaloproteases, and insulin, is performed by cargo-vesicles and their generated channels. The paracellular pathway can be used for water and ions; in postcapillary venules, at the level of which approximately 30% of junctions are open to a space of 6 nm, small molecules may take this route. Recent data obtained by molecular biology techniques revealed that caveolae are endowed with the molecular machinery for fusion/fission, docking, and movement across cells. Moreover, the various and numerous molecules that have been detected in the caveolae membrane and the different functions assumed by this differentiated microdomain strengthen the postulate that there are at least two or more types of vesicles molecularly tailored for the local physiological requirements.

Activated Notch4 inhibits angiogenesis: role of beta 1-integrin activation

Notch4 is a member of the Notch family of transmembrane receptors that is expressed primarily on endothelial cells. Activation of Notch in various cell systems has been shown to regulate cell fate decisions. The sprouting of endothelial cells from microvessels, or angiogenesis, involves the modulation of the endothelial cell phenotype. Based on the function of other Notch family members and the expression pattern of Notch4, we postulated that Notch4 activation would modulate angiogenesis. Using an in vitro endothelial-sprouting assay, we show that expression of constitutively active Notch4 in human dermal microvascular endothelial cells (HMEC-1) inhibits endothelial sprouting. We also show that activated Notch4 inhibits vascular endothelial growth factor (VEGF)-induced angiogenesis in the chick chorioallantoic membrane in vivo. Activated Notch4 does not inhibit HMEC-1 proliferation or migration through fibrinogen. However, migration through collagen is inhibited. Our data show that Notch4 cells exhibit increased beta1-integrin-mediated adhesion to collagen. HMEC-1 expressing activated Notch4 do not have increased surface expression of beta 1-integrins. Rather, we demonstrate that Notch4-expressing cells display beta1-integrin in an active, high-affinity conformation. Furthermore, using function-activating beta 1-integrin antibodies, we demonstrate that activation of beta1-integrins is sufficient to inhibit VEGF-induced endothelial sprouting in vitro and angiogenesis in vivo. Our findings suggest that constitutive Notch4 activation in endothelial cells inhibits angiogenesis in part by promoting beta 1-integrin-mediated adhesion to the underlying matrix.

Characterization of rat aortic fragment within collagen gel as an angiogenesis model; capillary morphology may reflect the action mechanisms of angiogenesis inhibitors

A fragment of rat thoracic aorta within type I collagen gel was employed as a model of angiogenesis, including the processes of cell migration, proliferation and capillary tube formation. Endogenous angiogenic factors in this model were studied. Expressions of vascular endothelial growth factor (VEGF) and its receptor, and proteolytic enzyme activities (matrix metalloprotease-2; MMP-2 and plasminogen activator; PA) increased during angiogenesis. The angiogenesis was inhibited by VEGF receptor kinase inhibitor and MMP inhibitor, confirming that these endogenous factors played an important role in angiogenesis. Interestingly, these inhibitors induced different capillary morphologies, including differences of cell migration and sprouting. Furthermore, dexamethasone (a down-regulator of MMP and PA) and TNP-470 (an endothelial cell growth inhibitor) induced another capillary morphology. The results suggest that the capillary structure in this model is dramatically influenced by the inhibition of angiogenic signalling and extracellular matrix (ECM) degradation. We also found that a novel angiogenesis inhibitor, the microbial metabolite luminacin, which was recently identified by us (Wakabayashi et al., J. Antiobiot., 53, 591-596 (2000)), induced a different morphology compared with other inhibitors examined, suggesting that it has a unique mechanism of action. Our results indicate that this rat aorta model should be useful for screening novel angiogenesis inhibitors.

Keratinocyte junctions and the epidermal barrier: how to make a skin-tight dress

Although intercellular junctions are known to be the major regulators of permeability of simple epithelia, they had not been thought to be important in regulating the permeability of stratified mammalian epithelia. Furuse et al. (2002)(this issue) demonstrate that functional tight junctions may indeed be a necessary part of the permeability barrier of the skin.

Cell-cell interactions in vascular development

Research into areas as divergent as hemangiopoiesis and cardiogenesis as well as investigations of diseases such as cancer and diabetic retinopathy have converged to form the face of research in vascular development today. This convergence of disparate topics has resulted in rapid advances in many areas of vascular research. The focus of this review has been the role of cell-cell interactions in the development of the vascular system, but we have included discussions of pathology where the mechanism of disease progression may have parallels with developmental processes. A number of intriguing questions remain unanswered. For example, what triggers abnormal angiogenesis in the disease state? Are the mechanisms similar to those that control developmental neovascularization? Perhaps the difference in development in angiogenesis versus in disease is context driven, that is, an adult versus an embryonic organism. If this is the case, can the controls that curtail developmental vessel formation be applied in pathologies? Can cell-cell interactions be targeted as a control point for new vessel formation? For instance, can perivascular cells be stimulated or eliminated to result in increased vessel stability or instability, respectively? If the hypothesis that mural cell association is required for vessel stabilization is accurate, are there mechanisms to promote or inhibit mural cell recruitment and differentiation as needed? These and other questions lie in wait for the next generation of approaches to discern the mechanisms and the nature of the cell-cell interactions and the influence of the microenvironment on vascular development.

Transforming growth factor beta1 (TGF-beta1) promotes endothelial cell survival during in vitro angiogenesis via an autocrine mechanism implicating TGF-alpha signaling

Mouse capillary endothelial cells (1G11 cell line) embedded in type I collagen gels undergo in vitro angiogenesis. Cells rapidly reorganize and form capillary-like structures when stimulated with serum. Transforming growth factor beta1 (TGF-beta1) alone can substitute for serum and induce cell survival and tubular network formation. This TGF-beta1-mediated angiogenic activity depends on phosphatidylinositol 3-kinase (PI3K) and p42/p44 mitogen-activated protein kinase (MAPK) signaling. We showed that specific inhibitors of either pathway (wortmannin, LY-294002, and PD-98059) all suppressed TGF-beta1-induced angiogenesis mainly by compromising cell survival. We established that TGF-beta1 stimulated the expression of TGF-alpha mRNA and protein, the tyrosine phosphorylation of a 170-kDa membrane protein representing the epidermal growth factor (EGF) receptor, and the delayed activation of PI3K/Akt and p42/p44 MAPK. Moreover, we showed that all these TGF-beta1-mediated signaling events, including tubular network formation, were suppressed by incubating TGF-beta1-stimulated endothelial cells with a soluble form of an EGF receptor (ErbB-1) or tyrphostin AG1478, a specific blocker of EGF receptor tyrosine kinase. Finally, addition of TGF-alpha alone poorly stimulated angiogenesis; however, by reducing cell death, it strongly potentiated the action of TGF-beta1. We therefore propose that TGF-beta1 promotes angiogenesis at least in part via the autocrine secretion of TGF-alpha, a cell survival growth factor, activating PI3K/Akt and p42/p44 MAPK.

Membrane type 1-matrix metalloproteinase is activated during migration of human endothelial cells and modulates endothelial motility and matrix remodeling

Matrix metalloproteinases are thought to play an important role in endothelial cell migration and matrix remodeling. We have used an in vitro wound healing migration model and newly generated anti-membrane type 1-matrix metalloproteinase (MT1-MMP) monoclonal antibodies (mAbs) to characterize the role of MT1-MMP during this process. First, the expression and shedding of MT1-MMP are up-regulated upon induction of migration in endothelial cells, as demonstrated by flow cytometry and Western blot analysis. Furthermore, MT1-MMP is concentrated at discrete areas in migrating endothelial cells, in contrast to the diffuse pattern observed in confluent cells. Interestingly, migration of endothelial cells results in the stimulation of MT1-MMP activity, as shown by its ability to process pro-MMP-2 and to degrade fibrinogen assessed by zymography. Moreover, MT1-MMP-mediated gelatin degradation is enriched at migration sites. mAbs generated against the MT1-MMP catalytic domain are shown to inhibit MT1-MMP enzymatic activity and to impair both phorbol 12-myristate 13-acetate-induced endothelial migration and invasion of collagen and fibrin gels. Furthermore, a reduction in the formation of capillary tubes in Matrigel is also observed when endothelial cells are pretreated with the blocking anti-MT1-MMP mAbs. Altogether, these data demonstrate that MT1-MMP plays an important role during endothelial cell migration, and its activity can modulate endothelial migration, invasion, and formation of capillary tubes during the angiogenic response.

Angiopoietin and Tie signaling pathways in vascular development

The angiopoietin ligands and Tie receptors belong to a novel class of ligand/receptor families, which play critical roles in blood vessel formation. They are considered to control numerous signaling pathways that are involved in diverse cellular processes, such as cell migration, proliferation and survival, and reorganization of the actin cytoskeleton. In this review, we summarize the important biochemical and biological properties of this interesting ligand/receptor family. Particular emphasis will be made on potential downstream targets and consequences of the endothelial cell behavior, due to regulation by the angiopoietin/Tie pathway.

Direct cell adhesion to the angiopoietins mediated by integrins

Genetic ablation of angiopoietin-1 (Ang-1) or of its cognate receptor, Tie2, disrupts angiogenesis in mouse embryos. The endothelial cells in growing blood vessels of Ang-1 knockout mice have a rounded appearance and are poorly associated with one another and their underlying basement membranes (Dumont, D. J., Gradwohl, G., Fong, G. H., Puri, M. C., Gertsenstein, M., Auerbach, A., and Breitman, M. L. (1994) Genes Dev. 8, 1897--1909; Sato, T. N., Tozawa, Y., Deutsch, U., Wolburg-Buchholz, K., Fujiwara, Y., Gendron-Maguire, M., Gridley, T., Wolburg, H., Risau, W., and Qin, Y. (1995) Nature 376, 70--74; Suri, C., Jones, P. F., Patan, S., Bartunkova, S., Maisonpierre, P. C., Davis, S., Sato, T. N., and Yancopoulos, G. D. (1996) Cell 87, 1171--1180). It is therefore possible that Ang-1 regulates endothelial cell adhesion. In this study we asked whether Ang-1 might act as a direct substrate for cell adhesion. Human umbilical vein endothelial cells (HUVECs) plated for a brief period on different substrates were found to adhere and spread well on Ang-1. Similar results were seen on angiopoietin-2 (Ang-2)-coated surfaces, although cells did not spread well on Ang-2. Ang-1, but not Ang-2, supported HUVEC migration, and this was independent of growth factor activity. When the same experiments were done with fibroblasts that either lacked, or stably expressed, Tie2, results similar to those with HUVECs were seen, suggesting that adhesion to the angiopoietins was independent of Tie2 and not limited to endothelial cells. Interestingly, when integrin-blocking agents were included in these assays, adhesion to either angiopoietin was significantly reduced. Moreover, Chinese hamster ovary-B2 cells lacking the alpha(5) integrin subunit did not adhere to Ang-1, but they did adhere to Ang-2. Stable expression of the human alpha(5) integrin subunit in these cells rescued adhesion to Ang-1 and promoted an increase in adhesion to Ang-2. We also found that Ang-1 and Ang-2 bind rather selectively to vitronectin. These results suggest that, beyond their role in modulating Tie2 signaling, Ang-1 and Ang-2 can directly support cell adhesion mediated by integrins.

Lithium inhibits cell cycle progression and induces stabilization of p53 in bovine aortic endothelial cells

Lithium affects development of various organisms and cell fate through the inhibition of glycogen synthase kinase-3 beta and induction of the Wnt/beta-catenin signaling pathway. In this study, we investigated the effects of lithium on primary bovine aortic endothelial cells (BAEC). Lithium treatment of BAEC induced beta-catenin stabilization but failed to activate the transcriptional activity of the beta-catenin/T-cell factor complex. Lithium caused a sustained G(2)/M cell cycle arrest without affecting cell viability. Reversibility of this cell cycle arrest occurred up to 3 days after treatment but was reduced thereafter. Lithium-treated BAEC exhibited a senescent-like morphology with an increase in cells positive for the senescence-associated-beta-galactosidase activity. Lithium also increased the expression of p21(Cip), a cyclin-dependent kinase inhibitor, both at the protein and RNA levels. No change in p21(Cip) mRNA stability was observed, whereas the transcriptional activity of a p21(Cip) promoter-luciferase construct containing p53 binding sites was increased after lithium treatment. Furthermore, lithium caused increased transcription of a reporter gene under the control of a promoter containing the p53 consensus binding sites both in transiently transfected BAEC and in a stably transfected fibroblast cell line. Lithium caused accumulation of p53 protein in BAEC without affecting p53 mRNA levels. Finally, up-regulation of p21(Cip) in response to lithium did not occur in mouse embryonic fibroblasts that were null for p53 alleles, confirming the dependence on a p53 pathway for this lithium effect. These findings demonstrate for the first time that lithium induces also stabilization of the tumor suppressor p53 and reveal a new mechanism that may contribute to the neuroprotective effects of lithium.

Cell adhesion and migration properties of beta 2-integrin negative polymorphonuclear granulocytes on defined extracellular matrix molecules. Relevance for leukocyte extravasation

Regulated adhesion of leukocytes to the extracellular matrix is essential for transmigration of blood vessels and subsequent migration into the stroma of inflamed tissues. Although beta(2)-integrins play an indisputable role in adhesion of polymorphonuclear granulocytes (PMN) to endothelium, we show here that beta(1)- and beta(3)-integrins but not beta(2)-integrin are essential for the adhesion to and migration on extracellular matrix molecules of the endothelial cell basement membrane and subjacent interstitial matrix. Mouse wild type and beta(2)-integrin null PMN and the progranulocytic cell line 32DC13 were employed in in vitro adhesion and migration assays using extracellular matrix molecules expressed at sites of extravasation in vivo, in particular the endothelial cell laminins 8 and 10. Wild type and beta(2)-integrin null PMN showed the same pattern of ECM binding, indicating that beta(2)-integrins do not mediate specific adhesion of PMN to the extracellular matrix molecules tested; binding was observed to the interstitial matrix molecules, fibronectin and vitronectin, via integrins alpha(5)beta(1) and alpha(v)beta(3), respectively; to laminin 10 via alpha(6)beta(1); but not to laminins 1, 2, and 8, collagen type I and IV, perlecan, or tenascin-C. PMN binding to laminins 1, 2, and 8 could not be induced despite surface expression of functionally active integrin alpha(6)beta(1), a major laminin receptor, demonstrating that expression of alpha(6)beta(1) alone is insufficient for ligand binding and suggesting the involvement of accessory factors. Nevertheless, laminins 1, 8, and 10 supported PMN migration, indicating that differential cellular signaling via laminins is independent of the extent of adhesion. The data demonstrate that adhesive and nonadhesive interactions with components of the endothelial cell basement membrane and subjacent interstitium play decisive roles in controlling PMN movement into sites of inflammation and illustrate that beta(2)-integrins are not essential for such interactions.

Expression of angiopoietin-1 in human glioblastomas regulates tumor-induced angiogenesis: in vivo and in vitro studies

To define a role for the angiopoietin/Tie2 system in astrocytoma angiogenesis, we examined the expression of angiopoietin-1 (Ang1) and angiopoietin-2 (Ang2) in these tumors by immunohistochemistry and in situ hybridization. Furthermore, we studied in vitro the effects elicited by glioblastoma cell-secreted Ang1 or by recombinant Ang1 on functions of endothelial cells (ECs). Our observations of astrocytomas show that a stage-specific induction of angiopoietins occurs and is correlated with angiogenic phases of different intensity. Ang1 expression was found in a few astrocytes scattered in the tumor at all stages of astrocytoma progression. In blood vessels, Ang1 mRNA increased progressively in high-grade glioblastomas, in which the number of vessels was higher than in low-grade tumors. Ang2 was detected in tumor cells and in ECs in high-grade astrocytomas, whereas its expression was negligible in low-grade tumors. Coculture of glioblastoma cell lines producing Ang1 with endothelium demonstrated a key role of this ligand in the control of EC network organization. We found that recombinant Ang1 in vitro induces EC spreading and reorganization of the cell monolayer into cordlike structures. These results suggest that Ang1 directly acts on ECs by modulating cell-cell and cell-matrix associations and promoting the differentiation phase of angiogenesis.

Angiogenic properties of the carcinoembryonic antigen-related cell adhesion molecule 1

The carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1), a member of the immunoglobulin superfamily, is expressed in microvessels of proliferating tissues such as endometrium, in tissues after wounding, and in solid human tumors. In microvascular human endothelial cells, purified native and recombinant CEACAM1 stimulates proliferation, chemotaxis, and tube formation. In the chorioallantoic membrane of the chicken, CEACAM1 induces angiogenesis. The angiogenic effects of CEACAM1 are additive to those of the vascular endothelial growth factor (VEGF). The expression of CEACAM1 is up-regulated by VEGF, and VEGF-induced in vitro tube formation is blocked completely by a monoclonal CEACAM1 antibody. These findings indicate that CEACAM1 is an angiogenic factor and an effector of VEGF.

Alagille syndrome. The widening spectrum of arteriohepatic dysplasia

Alagille syndrome was described more than 35 years ago as a genetic entity characterized by five major features: chronic cholestasis resulting from paucity of interlobular bile ducts, peripheral pulmonary stenosis, butterflylike vertebral arch defect, posterior embryotoxon, and peculiar facies. Recently, JAGGED1 has been identified as a responsible gene by demonstration of mutations in AGS patients. Studies of the JAGGED1 expression pattern demonstrate that minor features and almost all the elements in the long list of manifestations described in AGS patients are not coincidental. This finding suggests that the definition of AGS may be reconsidered in the light of JAGGED1 mutations.

JAGGED1 gene expression during human embryogenesis elucidates the wide phenotypic spectrum of Alagille syndrome

Mutations of the JAGGED1 gene, encoding a NOTCH receptor ligand, cause Alagille syndrome (AGS), a complex malformative disorder affecting mainly the liver, heart, vertebrae, eye, and face. Minor and occasional features involving kidney, pharynx, systemic arteries, skeleton, and ear are in some cases associated with the syndrome. To describe the expression of JAGGED1 during human embryogenesis and to study its relationship with all the features of AGS, we performed in situ hybridization studies on human embryos and fetal tissue sections. JAGGED1 was mainly expressed in the cardiovascular system. In the liver, JAGGED1 transcripts were only detected in blood vessels. JAGGED1 was also expressed in other structures of mesenchymal origin (distal mesenchyme of limb buds; mesonephric and metanephric tubules of the kidney) and in epithelial structures including the ciliary margin of the retina and the posterior part of the lens, the ventral epithelium of the otic vesicle, the neurosensory epithelium of the ear vestibule, the epithelium of pharyngeal arches, and the developing central nervous system. The strong JAGGED1 expression during human embryo- and feto-genesis both in the vascular system and in other mesenchymal and epithelial tissues implicates abnormal angiogenesis in the pathogenesis of Alagille syndrome and particularly the paucity of interlobular bile ducts. However, it is probably not the only mechanism of the disease. Except for the central nervous system, there is a strong correlation between JAGGED1 expression and all the features of AGS. This implies that the features occasionally associated with the syndrome are not coincidental.