The Cdc42 and Rac1 GTPases are required for capillary lumen formation in three-dimensional extracellular matrices

Rho, Rac, Pak and angiogenesis: old roles and newly identified responsibilities in endothelial cells

Angiogenesis-the develoment of microvasculature-requires, in part, directed endothelial cell motility and responsiveness to external signals. Several of the proteins, which modulate and/or direct endothelial cell motility and morphology in angiogenesis are the Rho GTPases (Rho, Rac, and Cdc42) and Pak (a downstream effector of Rac and Cdc42). Previously, overexpression and activation of Rho GTPases and Pak had been implicated in the development of cancer, through their roles in cancer cell transformation, stimulation of proliferation, inhibition of apoptosis, and migration. Yet regardless of the transformed status of cells within a tumor, without a blood supply most tumors cannot grow larger than 1-2 mm. The blood supply in tumors is provided by capillaries formed of endothelial cells in a process called angiogenesis. Consequently, there is enormous interest in the role of the wild type endothelial cells-and the signaling mechanisms required to support angiogenesis and subsequent growth of metastatic and aggressive cancers. Recent work has begun to uncover the roles of the Rho GTPases and Pak in the regulation of normal endothelial cell function. This review will discuss the current literature regarding the roles of Rho and Rac, and the Rac effector-Pak, in endothelial cells, and we will propose new avenues of research for interaction of the AGC kinase-PKG, with the Rho GTPases and Pak in the cell motility and cell morphology of endothelial cells.

Multifaceted role of Rho proteins in angiogenesis

The Rho family of GTPases is part of the Ras superfamily. The Rho, Rac, and Cdc42 members of the family are present in mammalian cells and have been the subject of attention of researchers due to their vast spectrum of functions. Rac 1, Cdc42, and RhoA are well-known for their role in the regulation of the actin cytoskeleton in promoting the formation of lamellipodia, filopodia, and stress fibers, respectively. The Rho proteins also participate in the control of cell growth, motility, cell-cell adhesions, morphogenesis, cytoskeletal dynamics, and cellular trafficking. The mechanisms for eliciting these functions have become clearer during the last decade. Concordant with their roles in multiple processes of cellular control, the Rho proteins have been shown to be involved in tumor growth, progression, metastasis, and now angiogenesis.

Antibody against junctional adhesion molecule-C inhibits angiogenesis and tumor growth

The junctional adhesion molecule-C (JAM-C) was recently described as an adhesion molecule localized at interendothelial contacts and involved in leukocyte transendothelial migration. The protein JAM-C interacts with polarity complex molecules and regulates the activity of the small GTPase Cdc42. The angiogenesis process involves rearrangement of endothelial junctions and implicates modulation of cell polarity. We tested whether JAM-C plays a role in angiogenesis using tumor grafts and hypoxia-induced retinal neovascularization. Treatment with a monoclonal antibody directed against JAM-C reduces tumor growth and infiltration of macrophages into tumors. The antibody decreases angiogenesis in the model of hypoxia-induced retinal neovascularization in vivo and vessel outgrowth from aortic rings in vitro. Importantly, the antibody does not induce pathologic side effects in vivo. These findings show for the first time a role for JAM-C in angiogenesis and define JAM-C as a valuable target for antitumor therapies.

Critical roles of CD146 in zebrafish vascular development

In this report, we use zebrafish as a model system to understand the importance of CD146 in vascular development. Endothelial-specific expression of CD146 was verified by whole-mount in situ hybridization. Suppression of CD146 protein expression by antisense morpholino oligonucleotides (MO) resulted in poorly developed intersomitic vessels (ISVs). In CD146 morphants, we observed a lack of blood flow through the ISV region, despite that fluorescence microangiography showed that the ISVs were present. This finding suggests that the lumens of the developing ISVs may be too narrow for proper circulation. Additionally, remodeling of the caudal vein plexus into functional vascular tubes appeared to be affected. Suppression of CD146 protein expression resulted in a circulation shunt that caused the circulation to by-pass part of the caudal artery/vein system. The same vascular defects were recapitulated by using a second morpholino oligonucleotide. This morphant expressed a truncated CD146 protein with amino acids V32 to T57 at the N terminus deleted in an in-frame manner. This region, therefore, is likely to contain elements critical for CD146 function. This study provides the first in vivo functional assessment of CD146 in embryonic development by showing that knockdown of CD146 protein expression severely hinders vascular development in zebrafish.

MMP-1 activation by serine proteases and MMP-10 induces human capillary tubular network collapse and regression in 3D collagen matrices

Previous work has shown that endothelial cell (EC)-derived matrix metalloproteinases (MMPs) regulate regression of capillary tubes in vitro in a plasmin- and MMP-1 dependent manner. Here we report that a number of serine proteases can activate MMP-1 and cause capillary tube regression; namely plasma kallikrein, trypsin, neutrophil elastase, cathepsin G, tryptase and chymase. Plasma prekallikrein failed to induce regression without coactivators such as high molecular weight kininogen (HMWK) or coagulation Factor XII. The addition of trypsin, the neutrophil serine proteases (neutrophil elastase and cathepsin G) and the mast cell serine proteases (tryptase and chymase) each caused MMP-1 activation and collagen type I proteolysis, capillary tubular network collapse, regression and EC apoptosis. Capillary tube collapse is accompanied by collagen gel contraction, which is strongly related to the wound contraction that occurs during regression of granulation tissue in vivo. We also report that proMMP-10 protein expression is markedly induced in ECs undergoing capillary tube morphogenesis. Addition of each of the serine proteases described above led to activation of proMMP-10, which also correlated with MMP-1 activation and capillary tube regression. Treatment of ECs with MMP-1 or MMP-10 siRNA markedly delayed capillary tube regression, whereas gelatinase A (MMP-2), gelatinase B (MMP-9) and stromelysin-1 (MMP-3) siRNA-treated cells behaved in a similar manner to controls and regressed normally. Increased expression of MMP-1 or MMP-10 in ECs using recombinant adenoviral delivery markedly accelerated serine protease-induced capillary tube regression. ECs expressing increased levels of MMP-10 activated MMP-1 to a greater degree than control ECs. Thus, MMP-10-induced activation of MMP-1 correlated with tube regression and gel contraction. In summary, our work demonstrates that MMP-1 zymogen activation is mediated by multiple serine proteases and MMP-10, and that these events are central to EC-mediated collagen degradation and capillary tube regression in 3D collagen matrices.

Cytoskeletal rearrangement and caspase activation in sphingosine 1-phosphate-induced lung capillary tube formation

Angiogenesis is a multistep process involving the endothelial cell (EC) cytoskeleton in migration, proliferation, and barrier stabilization. Although precise intracellular pathways by which angiogenic tube formation occurs remain poorly understood, we speculated that interactions between the cytoskeleton and apoptosis are involved and explored cytoskeletal rearrangement and caspase activation in human lung microvascular EC capillary-like tube formation induced by sphingosine 1-phosphate (Sph 1-P) and vascular endothelial growth factor (VEGF). Sph 1-P and VEGF enhance tube formation quantified by a Tube Immaturity Index (TII) generated from the ratio of cell number to tube length, with concomitant morphologic and actomyosin network changes. Angiogenesis was temporally grouped into three stages with early changes characterized by cortical actin localization, whereas midstage tube development demonstrated elongated EC with peripheral actin labeling with transcellular stress fibers. Late tube formation was characterized by broad actin distribution and presence of caspase-positive EC. Phosphorylated MLC immunoreactivity was present at all stages, suggesting that coordinate Rho kinase and MLCK involvement is important to Sph 1-P-induced cell motility; however, chemical inhibition of either MLCK or Rho kinase failed to alter early tube formation. To address whether gaps created by apoptosis expand the lumen, Sph 1-P-induced tubes were differentiated in the presence of caspase inhibitor z-Val-Ala-Asp-fluoromethylketone (zVAD-FMK). Capillary-like tube maturation, but not length, was decreased by zVAD-FMK treatment. These studies suggest that Sph 1-P may induce EC tube formation by regulating early cytoskeletal rearrangement, whereas EC apoptosis within capillary-like tubes is necessary for late stage Sph 1-P-induced tube maturation and lumen formation.

Ischemia-induced cleavage of cadherins in NRK cells: evidence for a role of metalloproteinases

Although ischemia has been shown to disrupt cell adhesion, the underlying molecular mechanism is unknown. In these studies, we adapted a model of ischemia-reperfusion to normal rat kidney (NRK) cells, examined disruption of the cadherin/catenin complex, and identified a role for matrix metalloproteinases (MMPs) in ischemia-induced cleavage of cadherins. In NRK cells, ischemia was induced by applying a thin layer of PBS solution supplemented with calcium and magnesium and a layer of mineral oil, which restricts exposure to oxygen. NRK cells exhibited extracellular 80-kDa and intracellular 40-kDa E-cadherin fragments after 4 h of ischemia, and at 6 h the expression of full-length E-cadherin decreased. While no fragments of N-cadherin, alpha-catenin, and gamma-catenin were observed at any time point, the detectable levels of these proteins decreased during ischemia. Ischemia was detected by an increase in pimonidazole adducts, as well as an increase in glucose transporter-1 protein expression. Ischemia did not decrease cell number, but there was a decrease in ATP levels. In addition, there was no evidence of cleaved caspase 3 or 9 during 6 h of ischemia. The MMP inhibitors GM-6001 and TAPI-O inhibited cleavage and/or loss of E- and N-cadherin protein expression. Tissue inhibitors of metalloproteinases (TIMP)-3 and to a lesser extent TIMP-2, but not TIMP-1, inhibit ischemic cleavage and/or loss of E- and N-cadherin. These results demonstrate that ischemia induces a selective metalloproteinase-dependent cleavage of E-cadherin and decrease in N-cadherin that are associated with a disruption of junctional contacts.

Nitric oxide modulation of early angiogenesis

Angiogenesis is the process of new vessel formation from an existing vasculature network. In all but a few circumstances it is tightly controlled and suppressed. Precise understanding of the factors involved in modulation of angiogenesis has significant potential clinical value. One agent believed to play a role in angiogenesis is nitric oxide. However, there remain substantial uncertainties concerning the specifics of this role. The present study was undertaken to better define the role nitric oxide plays in angiogenesis associated with acute wound healing. Muscle biopsies from the pectoralis major of C57B6 mice were embedded in 500 microl of type I collagen matrix, and incubated in the presence of growth medium for 14 days. Treatment wells received L-Arginine (2 mM), L-NAME (300 microM), or SNAP (10-20 microM). Angiogenic response was quantified as the measure of cell migration through the matrix and as the total cells recovered from the matrix. Whole lung specimens and aortic segments served as sources of endothelial and vascular smooth muscle cells respectively for proliferation studies under similar treatment conditions. Nitric oxide was found to exert either a stimulatory or inhibitory effect on angiogenesis and cell proliferation that was subject to the assay system and specific vascular cell types present. These results suggest that the role of nitric oxide in angiogenesis is context dependent.

A vascular cell-restricted RhoGAP, p73RhoGAP, is a key regulator of angiogenesis

Angiogenesis is a major therapeutic target. Ideal drug targets are genes expressed only in endothelial cells (ECs) or only during the angiogenic process. Here, we describe a gene, p73RhoGAP (p73), that has both of these properties. By using a PCR-based subtraction-hybridization approach to clone cDNAs from ECs undergoing capillary-tube formation, we identified a RhoGAP member, p73. p73 displays GTPase activity to Rho but not to Rac or Cdc42. Knockdown of p73 protein, achieved by adenovirus delivery of p73 antisense and by small interfering RNA into ECs, demonstrated the importance of this protein in EC function. Under such conditions, EC migration, proliferation, and capillary-tube formation were inhibited. Furthermore, angiogenesis in vivo was also inhibited by antisense p73. A mutant R82A alteration achieved a similar phenotype in vitro to the antisense, demonstrating the importance of the GTPase-activating protein activity to p73 function. Expression profiling of p73 shows that it is vascular cell-selective, being highly expressed in ECs and smooth-muscle cells but not in other cell types. Finally, we show that the mRNA of p73 is up-regulated in an angiogenic milieu with little or no regulation seen under nonangiogenic conditions. p73, a vascular cell-specific GTPase-activating protein, is an important modulator of angiogenesis and displays many of features that make it worthy of being a drug target.

The relative magnitudes of endothelial force generation and matrix stiffness modulate capillary morphogenesis in vitro

When suspended in collagen gels, endothelial cells elongate and form capillary-like networks containing lumens. Human blood outgrowth endothelial cells (HBOEC) suspended in relatively rigid 3 mg/ml floating collagen gels, formed in vivo-like, thin, branched multi-cellular structures with small, thick-walled lumens, while human umbilical vein endothelial cells (HUVEC) formed fewer multi-cellular structures, had a spread appearance, and had larger lumens. HBOEC exert more traction on collagen gels than HUVEC as evidenced by greater contraction of floating gels. When the stiffness of floating gels was decreased by decreasing the collagen concentration from 3 to 1.5 mg/ml, HUVEC contracted gels more and formed thin, multi-cellular structures with small lumens, similar in appearance to HBOEC in floating 3 mg/ml gels. In contrast to floating gels, traction forces exerted by cells in mechanically constrained gels encounter considerable resistance. In constrained collagen gels (3 mg/ml), both cell types appeared spread, formed structures with fewer cells, had larger, thinner-walled lumens than in floating gels, and showed prominent actin stress fibers, not seen in floating gels. These results suggest that the relative magnitudes of cellular force generation and apparent matrix stiffness modulate capillary morphogenesis in vitro and that this balance may play a role in regulating angiogenesis in vivo.

Rho activity critically and selectively regulates endothelial cell organization during angiogenesis

The mechanisms that control organization of endothelial cells (ECs) into new blood vessels are poorly understood. We hypothesized that the GTPase Rho, which regulates cytoskeletal architecture, is important for EC organization during neovascularization. To test this hypothesis, we designed a highly versatile mouse skin model that used vascular endothelial growth factor-expressing cells together with packaging cells producing retroviruses encoding RhoA GTPase mutants. In this animal model, dominant negative N19RhoA selectively impaired assembly of ECs into new blood vessels; and, in contrast, active V14RhoA stimulated ECs to form blood vessels with functional lumens. In vitro, dominant negative N19RhoA reduced EC actin stress fibers and prevented ECs from contracting and reorganizing into precapillary cords within collagen gels. In contrast, active V14RhoA promoted EC stress fiber formation, contractility, and organization into cords. Neither N19RhoA nor V14RhoA significantly affected EC proliferation or migration in vitro; and, similarly, neither mutant significantly affected EC density during angiogenesis in vivo. Thus, these studies identify a critical and selective role for Rho activity in regulating EC assembly into new blood vessels, and they identify both negative and positive manipulation of Rho activity, respectively, as strategies for suppressing or promoting the organizational stages of neovascularization.

Rac regulates integrin-mediated endothelial cell adhesion and migration on laminin-8

Blood vessel formation requires endothelial cell interactions with the extracellular matrix through cell surface receptors, and signaling events that control endothelial cell adhesion, migration, and lumen formation. Laminin-8 (alpha4beta1gamma1) is present in all basement membranes of blood vessels in fetal and adult tissues, but despite its importance in vessel formation, its role in endothelial cell adhesion and migration remains undefined. We examined adhesion and migration of HMEC-1 human microvascular endothelial cells on laminin-8 with an emphasis on the integrin-mediated signaling events, as compared with those on laminin-10/11 and fibronectin. We found that laminin-8 was less potent in HMEC-1 cell adhesion than laminin-1, laminin-10/11, and fibronectin, and mediated cell adhesion through alpha6beta1 integrin. Despite its weak cell-adhesive activity, laminin-8 was as potent as laminin-10/11 in promoting cell migration. Cells adhering to laminin-8 displayed streaks of thin actin filaments and formed lamellipodia at the leading edge of the cells, as observed with cells adhering to laminin-10/11, while cells on fibronectin showed thick actin stress fibers and large focal adhesions. Pull-down assays of GTP-loaded Rho, Rac, and Cdc42 demonstrated that Rac, but not Rho or Cdc42, was preferentially activated on laminin-8 and laminin-10/11, when compared with fibronectin. Furthermore, a dominant-negative mutant of Rac suppressed cell spreading, lamellipodial formation, and migration on laminin-8, but not on fibronectin. These results, taken together, indicate that Rac is activated during endothelial cell adhesion to laminin-8, and is pivotal for alpha6beta1 integrin-mediated cell spreading and migration on laminin-8.

Microtubule depolymerization rapidly collapses capillary tube networks in vitro and angiogenic vessels in vivo through the small GTPase Rho

Maintenance of endothelial cell tube integrity is dependent on an intact cytoskeleton. We present data indicating that rapid collapse of endothelial tubular networks in vitro occurs in a dose-dependent manner after administration of microtubule-depolymerizing reagents but not after actin depolymerization. Pretreatment of endothelial cell networks with C3 exoenzyme or recombinant adenoviruses expressing dominant negative RhoA resulted in complete blockade of tube collapse, indicating a role for RhoA in these events. Microtubule depolymerization also resulted in activation of RhoA, whereas increased expression of constitutively active RhoA induced cell rounding and apoptosis of endothelial cells. Furthermore, following treatment with the chemotherapeutic agent vinblastine, rapid capillary tube network collapse occurred followed by endothelial cell apoptosis. Vinblastine, but not control agents, induced cleavage of procaspase-3, procaspase-9, and procaspase-8, along with the known caspase targets p21-activated kinase-2 and gelsolin, indicating that tube collapse caused a defined apoptotic response. Using a model of vascular endothelial growth factor-stimulated angiogenesis in vivo, vinblastine treatment also resulted in collapse and apoptosis of angiogenic blood vessels. Apoptotic endothelial cells stained strongly for cleaved caspase-3, and terminal dUTP nick-end labeling staining revealed fragmented nuclei in vinblastine-treated but not control angiogenic areas. Together, these findings indicate that microtubule-depolymerizing agents directly induce endothelial network collapse in vitro and in vivo leading to endothelial cell apoptosis in a manner dependent on the small GTPase, RhoA. In addition, these findings reveal a novel function for microtubule disrupting chemotherapeutic agents, namely their ability to rapidly collapse newly formed angiogenic vessels, which may contribute to their effectiveness in limiting angiogenesis and tumor growth.

Temporal and spatial modulation of Rho GTPases during in vitro formation of capillary vascular network. Adherens junctions and myosin light chain as targets of Rac1 and RhoA

Endothelial cells (ECs) self-organize into capillary networks when plated on extracellular matrix. In this process, Rho GTPases-mediated cytoskeletal dynamics control cell movement and organization of cell-to-matrix and cell-to-cell contacts. Time course analysis of RhoA and Rac1 activation matches specific morphological aspects of nascent pattern. RhoA-GTP increases early during EC adhesion and accumulates at sites of membrane ruffling. Rac1 is activated later and localizes in lamellipodia and at cell-to-cell contacts of organized cell chains. When ECs stretch and remodel to form capillary structures, RhoA-GTP increases again and associates with stress fibers running along the major cell axis. N17Rac1 and N19RhoA mutants impair pattern formation. Cell-to-cell contacts and myosin light chains (MLC) are targets of Rac1 and RhoA, respectively. N17Rac1 reduces the shift of beta-catenin and vascular endothelial cadherin to Triton X-100-insoluble fraction and impairs beta-catenin distribution at adherens junctions, suggesting that Rac1 controls the dynamics of cadherin-catenin complex with F-actin. During the remodeling phase of network formation, ECs show an intense staining for phosphorylated MLC along the plasma membrane; in contrast, MLC is less phosphorylated and widely diffused in N19RhoA ECs. Both N17Rac1 and N19RhoA have been used to investigate the role of wild type molecules in the main steps characterizing in vitro angiogenesis: (i) cell adhesion to the substrate, (ii) cell movement, and (iii) mechanical remodeling of matrix. N17Rac1 has a striking inhibitory effect on haptotaxis, whereas N19RhoA slightly inhibits EC adhesion and motility but more markedly Matrigel contraction. We conclude that different Rho GTPases control distinct morphogenetic aspects of vascular morphogenesis.

Sphingosine-1-phosphate markedly induces matrix metalloproteinase and integrin-dependent human endothelial cell invasion and lumen formation in three-dimensional collagen and fibrin matrices

Endothelial cell invasion is a key step in angiogenic blood vessel formation. Sphingosine-1-phosphate (S1P) has been previously reported to play a role in endothelial cell proliferation, survival, migration, and angiogenesis. Here, we examine the ability of S1P to regulate human endothelial cell invasion into three-dimensional collagen or fibrin matrices. We show that S1P potently stimulated human endothelial cell invasion, lumen formation, and branching morphogenesis in collagen, and fibrin matrices, (5- and 15-fold increases in invasion were observed, respectively). The S1P-induced invasion response was pertussis-toxin sensitive and completely dependent on integrins. Addition of integrin blocking reagents revealed that the alpha2beta1 integrin regulated invasion in collagen matrices, while a combination of alphavbeta3 and alpha5beta1 integrins regulated invasion in fibrin. Additionally, the S1P-induced invasion response was dependent on matrix metalloproteinases (MMPs). Tissue inhibitor of metalloproteinase-3 (TIMP-3) was the only physiologic inhibitor of metalloproteinases that completely inhibited the potent stimulation of invasion induced by S1P. In contrast, TIMP-1 had no blocking effect on invasion or morphogenesis, while TIMP-2 and TIMP-4 partially reduced invasion but completely blocked lumen formation events. Collectively, these data reveal a marked ability of S1P to induce metalloproteinase- and integrin-dependent human endothelial cell invasion and morphogenesis in both collagen and fibrin three-dimensional matrices, the two most physiologically relevant matrices for angiogenesis.

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II: modulated by G proteins, kinases, and myosin phosphatase

Ca2+ sensitivity of smooth muscle and nonmuscle myosin II reflects the ratio of activities of myosin light-chain kinase (MLCK) to myosin light-chain phosphatase (MLCP) and is a major, regulated determinant of numerous cellular processes. We conclude that the majority of phenotypes attributed to the monomeric G protein RhoA and mediated by its effector, Rho-kinase (ROK), reflect Ca2+ sensitization: inhibition of myosin II dephosphorylation in the presence of basal (Ca2+ dependent or independent) or increased MLCK activity. We outline the pathway from receptors through trimeric G proteins (Galphaq, Galpha12, Galpha13) to activation, by guanine nucleotide exchange factors (GEFs), from GDP. RhoA. GDI to GTP. RhoA and hence to ROK through a mechanism involving association of GEF, RhoA, and ROK in multimolecular complexes at the lipid cell membrane. Specific domains of GEFs interact with trimeric G proteins, and some GEFs are activated by Tyr kinases whose inhibition can inhibit Rho signaling. Inhibition of MLCP, directly by ROK or by phosphorylation of the phosphatase inhibitor CPI-17, increases phosphorylation of the myosin II regulatory light chain and thus the activity of smooth muscle and nonmuscle actomyosin ATPase and motility. We summarize relevant effects of p21-activated kinase, LIM-kinase, and focal adhesion kinase. Mechanisms of Ca2+ desensitization are outlined with emphasis on the antagonism between cGMP-activated kinase and the RhoA/ROK pathway. We suggest that the RhoA/ROK pathway is constitutively active in a number of organs under physiological conditions; its aberrations play major roles in several disease states, particularly impacting on Ca2+ sensitization of smooth muscle in hypertension and possibly asthma and on cancer neoangiogenesis and cancer progression. It is a potentially important therapeutic target and a subject for translational research.

Tie-2-dependent activation of RhoA and Rac1 participates in endothelial cell motility triggered by angiopoietin-1

Angiopoietin-1 is implicated in the maturation and remodeling of the vascular network during embryo development and in adult life. Through its tyrosine kinase receptor Tie-2 it stimulates endothelial cells to migrate and change shape. Here we show that angiopoietin-1 elicits chemokinesis of endothelial cells by a phosphoinositide 3-OH kinase/son of sevenless-dependent modulation of Rac1 and RhoA. The resulting temporal events are associated with cytoskeletal rearrangements and occur in discrete zones of the cell. Endothelial cells carrying dominant-negative mutants of RhoA and Rac1 or treated with LY294002, an inhibitor of phosphoinositide 3-OH kinase, dramatically decrease their chemokinetic velocity. Taken together, these results further expand our understanding of angiopoietin-1-mediated endothelial cell motility during vascular network assembly and angiogenesis.

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.

Rho kinase and matrix metalloproteinase inhibitors cooperate to inhibit angiogenesis and growth of human prostate cancer xenotransplants

The purpose of this study was to determine the effects of inhibitors of Rho kinase (ROK) and matrix metalloproteinases (MMPs) on angiogenesis and tumor growth and to evaluate ROK activity in human prostate cancer PC3 cells and endothelial cells (HUVECs). Vacuolation by endothelial cells and lumen formation, the earliest detectable stages of angiogenesis, were inhibited by the ROK inhibitor Wf-536. Combining Wf-536 with the MMP inhibitor Marimastat greatly enhanced in vitro inhibition of endothelial vacuolation, lumen and cord formation, and VEGF- and HGF-stimulated endothelial sprout formation from aorta. Inhibition of sprout formation by the two inhibitors was synergistic. Both agents inhibited migration of HUVECs. The regulatory subunit (MYPT1) of the myosin phosphatase was phosphorylated in PC3 cells and HUVECs, and phosphorylation of MYPT1 and the myosin regulatory light chain was reduced by Wf-536, providing direct evidence of ROK activity. Early treatment of immuno-incompetent mice bearing xenotransplants of PC3 cells with a combination of Wf-536 plus Marimastat with or without Paclitaxel, significantly inhibited tumor growth, prevented tumor growth escape after discontinuation of Paclitaxel, and increased survival.

Tube morphogenesis: making and shaping biological tubes

Many organs are composed of epithelial tubes that transport vital fluids. Such tubular organs develop in many different ways and generate tubes of widely varying sizes and structures, but always with the apical epithelial surface lining the lumen. We describe recent progress in several diverse cell culture and genetic models of tube morphogenesis, which suggest apical membrane biogenesis, vesicle fusion, and secretion play central roles in tube formation and growth. We propose a unifying mechanism of tube morphogenesis that has been modified to create tube diversity and describe how defects in the tube size-sensing step can lead to polycystic kidney disease.

Lumen formation: in vivo versus in vitro observations

Lumen formation must accompany the de novo growth of blood vessels during embryological development, the production of new vessels (vasculogenesis), and the expansion or remodeling of the microcirculation in differentiated tissue (angiogenesis). The debate over lumen origin centers on whether this is an intracellular or intercellular phenomenon, entailing vesicle accretion or loss of endothelial cell (EC) contact, and whether this represents an intrinsic property of ECs or relies on extrinsic signals. In addition, recent in vivo data suggest that a third mechanism, that of longitudinal division, may be used to expand existing capillary networks. Importantly, more than one mechanism of lumen formation may be found in response to a given angiogenic signal. Tubule formation by ECs in a matrix is an increasingly popular form of in vitro angiogenesis assay, and it may offer insights into the mechanisms involved during growth in embryos or under pathological conditions in adults. Crucial to the validity of in vitro preparations is the extent to which tubule assembly and lumen formation mirrors that observed in vivo, although these data cannot elucidate the controls operative during adaptive remodeling of the vascular bed. Similar structures may be observed in vivo and in vitro, and may represent the situation found during angiogenesis and vasculogenesis, respectively. Lumen formation during angiogenesis, and tubule formation during EC culture, require the existence of cell polarity. As tubule formation is not a unique property of ECs, how this is developed is a key area where in vitro studies may extend our understanding of EC biology.

Molecular basis of endothelial cell morphogenesis in three-dimensional extracellular matrices

Although many studies have focused on blood vessel development and new blood vessel formation associated with disease processes, the question of how endothelial cells (ECs) assemble into tubes in three dimensions (i.e., EC morphogenesis) remains unanswered. EC morphogenesis is particularly dependent on a signaling axis involving the extracellular matrix (ECM), integrins, and the cytoskeleton, which regulates EC shape changes and signals the pathways necessary for tube formation. Recent studies reveal that genes regulating this matrix-integrin-cytoskeletal (MIC) signaling axis are differentially expressed during EC morphogenesis. The Rho GTPases represent an important class of molecules involved in these events. Cdc42 and Rac1 are required for the process of EC intracellular vacuole formation and coalescence that regulates EC lumen formation in three-dimensional (3D) extracellular matrices, while RhoA appears to stabilize capillary tube networks. Once EC tube networks are established, supporting cells, such as pericytes, are recruited to further stabilize these networks, perhaps by regulating EC basement membrane matrix assembly. Furthermore, we consider recent work showing that EC morphogenesis is balanced by a tendency for newly formed tubes to regress. This morphogenesis-regression balance is controlled by differential gene expression of such molecules as VEGF, angiopoietin-2, and PAI-1, as well as a plasmin- and matrix metalloproteinase-dependent mechanism that induces tube regression through degradation of ECM scaffolds that support EC-lined tubes. It is our hope that this review will stimulate increased interest and effort focused on the basic mechanisms regulating capillary tube formation and regression in 3D extracellular matrices.