Integrins and angiogenesis

Molecular basis for endothelial lumen formation and tubulogenesis during vasculogenesis and angiogenic sprouting

Many studies reveal a fundamental role for extracellular matrix-mediated signaling through integrins and Rho GTPases as well as matrix metalloproteinases (MMPs) in the molecular control of vascular tube morphogenesis in three-dimensional (3D) tissue environments. Recent work has defined an endothelial cell (EC) lumen signaling complex of proteins that controls these vascular morphogenic events. These findings reveal a signaling interdependence between Cdc42 and MT1-MMP to control the 3D matrix-specific process of EC tubulogenesis. The EC tube formation process results in the creation of a network of proteolytically generated vascular guidance tunnels in 3D matrices that are utilized to remodel EC-lined tubes through EC motility and could facilitate processes such as flow-induced remodeling and arteriovenous EC sorting and differentiation. Within vascular guidance tunnels, key dynamic interactions occur between ECs and pericytes to affect vessel remodeling, diameter, and vascular basement membrane matrix assembly, a fundamental process necessary for endothelial tube maturation and stabilization. Thus, the EC lumen and tube formation mechanism coordinates the concomitant establishment of a network of vascular tubes within tunnel spaces to allow for flow responsiveness, EC-mural cell interactions, and vascular extracellular matrix assembly to control the development of the functional microcirculation.

Dynamic magnetic resonance imaging assessment of vascular targeting agent effects in rat intracerebral tumor models

We used dynamic MRI to evaluate the effects of monoclonal antibodies targeting brain tumor vasculature. Female athymic rats with intracerebral human tumor xenografts were untreated or treated with intetumumab, targeting α(V)-integrins, or bevacizumab, targeting vascular endothelial growth factor (n = 4-6 per group). Prior to treatment and at 1, 3, and 7 days after treatment, we performed standard MRI to assess tumor volume, dynamic susceptibility-contrast MRI with the blood-pool iron oxide nanoparticle ferumoxytol to evaluate relative cerebral blood volume (rCBV), and dynamic contrast-enhanced MRI to assess tumor vascular permeability. Tumor rCBV increased by 27 ± 13% over 7 days in untreated rats; intetumumab increased tumor rCBV by 65 ± 10%, whereas bevacizumab reduced tumor rCBV by 31 ± 10% at 7 days (P < .001 for group and day). Similarly, intetumumab increased brain tumor vascular permeability compared with controls at 3 and 7 days after treatment, whereas bevacizumab decreased tumor permeability within 24 hours (P = .0004 for group, P = .0081 for day). All tumors grew over the 7-day assessment period, but bevacizumab slowed the increase in tumor volume on MRI. We conclude that the vascular targeting agents intetumumab and bevacizumab had diametrically opposite effects on dynamic MRI of tumor vasculature in rat brain tumor models. Targeting α(V)-integrins increased tumor vascular permeability and blood volume, whereas bevacizumab decreased both measures. These findings have implications for chemotherapy delivery and antitumor efficacy.

Multifunctional agents for concurrent imaging and therapy in cardiovascular disease

The development of agents for the simultaneous detection and treatment of disease has recently gained significant attention. These multifunctional theranostic agents posses a number of advantages over their monofunctional counterparts, as they potentially allow for the concomitant determination of agent localization, release, and efficacy. Whereas the development of these agents for use in cancers has received the majority of the attention, their use in cardiovascular disease is steadily increasing. As such, this review summarized some of the most poignant recent advances in the development of theranostic agents for the treatment of this class of diseases.

Galectin-3 is an important mediator of VEGF- and bFGF-mediated angiogenic response

Recent studies have shown that a carbohydrate-binding protein, galectin-3, is a novel pro-angiogenic molecule. The mechanism by which galectin-3 promotes angiogenesis remains unknown. We demonstrate here that galectin-3 is a mediator of vascular endothelial growth factor (VEGF)- and basic fibroblast growth factor (bFGF)-mediated angiogenic response. Angiogenesis assays revealed that galectin-3 inhibitors, β-lactose and dominant-negative galectin-3, reduce VEGF- and bFGF-mediated angiogenesis in vitro and that VEGF- and bFGF-mediated angiogenic response is reduced in galectin-3 knockdown cells and Gal3^−/− animals. Integrin αvβ3 was identified as the major galectin-3–binding protein and anti-αv, -β3, and -αvβ3 integrin function-blocking antibodies significantly inhibited the galectin-3–induced angiogenesis. Furthermore, galectin-3 promoted the clustering of integrin αvβ3 and activated focal adhesion kinase. Knockdown of GnTV, an enzyme that synthesizes high-affinity glycan ligands for galectin-3, substantially reduced: (a) complex N-glycans on αvβ3 integrins and (b) VEGF- and bFGF-mediated angiogenesis. Collectively, these data suggest that galectin-3 modulates VEGF- and bFGF-mediated angiogenesis by binding via its carbohydrate recognition domain, to the GnTV synthesized N-glycans of integrin αvβ3, and subsequently activating the signaling pathways that promote the growth of new blood vessels. These findings have broad implications for developing novel, carbohydrate-based therapeutic agents for inhibition of angiogenesis.

Microfibrils and fibrillin-1 induce integrin-mediated signaling, proliferation and migration in human endothelial cells

Microfibrils are macromolecular complexes associated with elastin to form elastic fibers that endow extensible tissues, such as arteries, lungs, and skin, with elasticity property. Fibrillin-1, the main component of microfibrils, is a 350-kDa glycoprotein for which genetic haploinsufficiency in humans can lead to Marfan syndrome, a severe polyfeatured pathology including aortic aneurysms and dissections. Microfibrils and fibrillin-1 fragments mediate adhesion of several cell types, including endothelial cells, while fibrillin-1 additionally triggers lung and mesangial cell migration. However, fibrillin-1-induced intracellular signaling is unknown. We have studied the signaling events induced in human umbilical venous endothelial cells (HUVECs) by aortic microfibrils as well as recombinant fibrillin-1 Arg-Gly-Asp (RGD)-containing fragments PF9 and PF14. Aortic microfibrils and PF14, not PF9, substantially and dose dependently increased HUVEC cytoplasmic and nuclear calcium levels measured using the fluorescent dye Fluo-3. This effect of PF14 was confirmed in bovine aortic endothelial cells. PF14 action in HUVECs was mediated by αvβ3 and α5β1 integrins, phospholipase-C, inosital 1,4,5-trisphosphate, and mobilization of intracellular calcium stores, whereas membrane calcium channels were not or only slightly implicated, as shown in patch-clamp experiments. Finally, PF14 enhanced endothelial cell proliferation and migration. Hence, fibrillin-1 sequences may physiologically activate endothelial cells. Genetic fibrillin-1 deficiency could alter normal endothelial signaling and, since endothelium dysfunction is an important contributor to Marfan syndrome, participate in the arterial anomalies associated with this developmental disease.

Strategies in the design of nanoparticles for therapeutic applications

Engineered nanoparticles have the potential to revolutionize the diagnosis and treatment of many diseases; for example, by allowing the targeted delivery of a drug to particular subsets of cells. However, so far, such nanoparticles have not proved capable of surmounting all of the biological barriers required to achieve this goal. Nevertheless, advances in nanoparticle engineering, as well as advances in understanding the importance of nanoparticle characteristics such as size, shape and surface properties for biological interactions, are creating new opportunities for the development of nanoparticles for therapeutic applications. This Review focuses on recent progress important for the rational design of such nanoparticles and discusses the challenges to realizing the potential of nanoparticles.

Molecular mediators of angiogenesis

Angiogenesis, or the formation of new blood vessels from the preexisting vasculature, is a key component in numerous physiologic and pathologic responses and has broad impact in many medical and surgical specialties. In this review, we discuss the key cellular steps that lead to the neovascularization of tissues and highlight the main molecular mechanisms and mediators in this process. We include discussions on proteolytic enzymes, cell-matrix interactions, and pertinent cell signaling pathways and end with a survey of the mechanisms that lead to the stabilization and maturation of neovasculatures.

MVL-PLA2, a snake venom phospholipase A2, inhibits angiogenesis through an increase in microtubule dynamics and disorganization of focal adhesions

Integrins are essential protagonists of the complex multi-step process of angiogenesis that has now become a major target for the development of anticancer therapies. We recently reported and characterized that MVL-PLA2, a novel phospholipase A2 from Macrovipera lebetina venom, exhibited anti-integrin activity. In this study, we show that MVL-PLA2 also displays potent anti-angiogenic properties. This phospholipase A2 inhibited adhesion and migration of human microvascular-endothelial cells (HMEC-1) in a dose-dependent manner without being cytotoxic. Using Matrigel and chick chorioallantoic membrane assays, we demonstrated that MVL-PLA2, as well as its catalytically inactivated form, significantly inhibited angiogenesis both in vitro and in vivo. We have also found that the actin cytoskeleton and the distribution of alphav beta3 integrin, a critical regulator of angiogenesis and a major component of focal adhesions, were disturbed after MVL-PLA2 treatment. In order to further investigate the mechanism of action of this protein on endothelial cells, we analyzed the dynamic instability behavior of microtubules in living endothelial cells. Interestingly, we showed that MVL-PLA2 significantly increased microtubule dynamicity in HMEC-1 cells by 40%. We propose that the enhancement of microtubule dynamics may explain the alterations in the formation of focal adhesions, leading to inhibition of cell adhesion and migration.

CC-PLA2-1 and CC-PLA2-2, two Cerastes cerastes venom-derived phospholipases A2, inhibit angiogenesis both in vitro and in vivo

Integrins are essential in the complex multistep process of angiogenesis and are thus attractive targets for the development of antiangiogenic therapies. Integrins are antagonized by disintegrins and C-type lectin-like proteins, two protein families from snake venom. Here, we report that CC-PLA2-1 and CC-PLA2-2, two novel secreted phospholipases A(2) (PLA(2)) isolated from Cerastes cerastes venom, also showed anti-integrin activity. Indeed, both PLA(2)s efficiently inhibited human brain microvascular endothelial cell adhesion and migration to fibrinogen and fibronectin in a dose-dependent manner. Interestingly, we show that this anti-adhesive effect was mediated by alpha5beta1 and alphav-containing integrins. CC-PLA2s also impaired in vitro human brain microvascular endothelial cell tubulogenesis on Matrigel and showed antiangiogenic activity in vivo in chicken chorioallantoic membrane assay. The complete PLA(2) cDNAs were cloned from a venom gland cDNA library. Mature CC-PLA2-1 and CC-PLA2-2 contain 121 and 120 amino acids, respectively, including 14 cysteines each and showed 83% identity. Tertiary model structures of CC-PLA2-1 and CC-PLA2-2 were generated by homology modeling. This is thus the first study describing an antiangiogenic effect for snake venom PLA(2)s and reporting first clues to their mechanism of action on endothelial cells.

Targeting activated integrin alphavbeta3 with patient-derived antibodies impacts late-stage multiorgan metastasis

Advanced metastatic disease is difficult to manage and specific therapeutic targets are rare. We showed earlier that metastatic breast cancer cells use the activated conformer of adhesion receptor integrin alphavbeta3 for dissemination. We now investigated if targeting this form of the receptor can impact advanced metastatic disease, and we analyzed the mechanisms involved. Treatment of advanced multi-organ metastasis in SCID mice with patient-derived scFv antibodies specific for activated integrin alphavbeta3 caused stagnation and regression of metastatic growth. The antibodies specifically localized to tumor lesions in vivo and inhibited alphavbeta3 ligand binding at nanomolar levels in vitro. At the cellular level, the scFs associated rapidly with high affinity alphavbeta3 and dissociated extremely slowly. Thus, the scFvs occupy the receptor on metastatic tumor cells for prolonged periods of time, allowing for inhibition of established cell interaction with natural alphavbeta3 ligands. Potential apoptosis inducing effects of the antibodies through interaction with caspase-3 were studied as potential additional mechanism of treatment response. However, in contrast to a previous concept, neither the RGD-containing ligand mimetic scFvs nor RGD peptides bound or activated caspase-3 at the cellular or molecular level. This indicates that the treatment effects seen in the animal model are primarily due to antibody interference with alphavbeta3 ligation. Inhibition of advanced metastatic disease by treatment with cancer patient derived single chain antibodies against the activated conformer of integrin alphavbeta3 identifies this form of the receptor as a suitable target for therapy.

Cell-targeting and cell-penetrating peptides for delivery of therapeutic and imaging agents

This review will discuss the basic concepts concerning the use of cell-targeting peptides (CTPs) and cell-penetrating peptides (CPPs) in the context of nanocarrier technology. It deals with the discovery and subsequent evolution of CTPs and CPPs, issues concerning their interactions with cells and their biodistribution in vivo, and their potential advantages and disadvantages as delivery agents. The article also briefly discusses several specific examples of the use of CTPs or CPPs to assist in the delivery of nanoparticles, liposomes, and other nanocarriers.

Targeted contrast-enhanced ultrasound imaging of tumor angiogenesis with contrast microbubbles conjugated to integrin-binding knottin peptides

Targeted contrast-enhanced ultrasound imaging is increasingly being recognized as a powerful imaging tool for the detection and quantification of tumor angiogenesis at the molecular level. The purpose of this study was to develop and test a new class of targeting ligands for targeted contrast-enhanced ultrasound imaging of tumor angiogenesis with small, conformationally constrained peptides that can be coupled to the surface of ultrasound contrast agents.

METHODS

Directed evolution was used to engineer a small, disulfide-constrained cystine knot (knottin) peptide that bound to alpha(v)beta(3) integrins with a low nanomolar affinity (Knottin(Integrin)). A targeted contrast-enhanced ultrasound imaging contrast agent was created by attaching Knottin(Integrin) to the shell of perfluorocarbon-filled microbubbles (MB-Knottin(Integrin)). A knottin peptide with a scrambled sequence was used to create control microbubbles (MB-Knottin(Scrambled)). The binding of MB-Knottin(Integrin) and MB-Knottin(Scrambled) to alpha(v)beta(3) integrin-positive cells and control cells was assessed in cell culture binding experiments and compared with that of microbubbles coupled to an anti-alpha(v)beta(3) integrin monoclonal antibody (MB(alphavbeta3)) and microbubbles coupled to the peptidomimetic agent c(RGDfK) (MB(cRGD)). The in vivo imaging signals of contrast-enhanced ultrasound with the different types of microbubbles were quantified in 42 mice bearing human ovarian adenocarcinoma xenograft tumors by use of a high-resolution 40-MHz ultrasound system.

RESULTS

MB-Knottin(Integrin) attached significantly more to alpha(v)beta(3) integrin-positive cells (1.76 +/- 0.49 [mean +/- SD] microbubbles per cell) than to control cells (0.07 +/- 0.006). Control MB-Knottin(Scrambled) adhered less to alpha(v)beta(3) integrin-positive cells (0.15 +/- 0.12) than MB-Knottin(Integrin). After blocking of integrins, the attachment of MB-Knottin(Integrin) to alpha(v)beta(3) integrin-positive cells decreased significantly. The in vivo ultrasound imaging signal was significantly higher after the administration of MB-Knottin(Integrin) than after the administration of MB(alphavbeta3) or control MB-Knottin(Scrambled). After in vivo blocking of integrin receptors, the imaging signal after the administration of MB-Knottin(Integrin) decreased significantly (by 64%). The imaging signals after the administration of MB-Knottin(Integrin) were not significantly different in the groups of tumor-bearing mice imaged with MB-Knottin(Integrin) and with MB(cRGD). Ex vivo immunofluorescence confirmed integrin expression on endothelial cells of human ovarian adenocarcinoma xenograft tumors.

CONCLUSION

Integrin-binding knottin peptides can be conjugated to the surface of microbubbles and used for in vivo targeted contrast-enhanced ultrasound imaging of tumor angiogenesis. Our results demonstrate that microbubbles conjugated to small peptide-targeting ligands provide imaging signals higher than those provided by a large antibody molecule.

A dual-labeled knottin peptide for PET and near-infrared fluorescence imaging of integrin expression in living subjects

Previously, we used directed evolution to engineer mutants of the Ecballium elaterium trypsin inhibitor (EETI-II) knottin that bind to αvβ3 and αvβ5 integrin receptors with low nanomolar affinity, and showed that Cy5.5- or (64)Cu-DOTA-labeled knottin peptides could be used to image integrin expression in mouse tumor models using near-infrared fluorescence (NIRF) imaging or positron emission tomography (PET). Here, we report the development of a dual-labeled knottin peptide conjugated to both NIRF and PET imaging agents for multimodality imaging in living subjects. We created an orthogonally protected peptide-based linker for stoichiometric coupling of (64)Cu-DOTA and Cy5.5 onto the knottin N-terminus and confirmed that conjugation did not affect binding to αvβ3 and αvβ5 integrins. NIRF and PET imaging studies in tumor xenograft models showed that Cy5.5 conjugation significantly increased kidney uptake and retention compared to the knottin peptide labeled with (64)Cu-DOTA alone. In the tumor, the dual-labeled (64)Cu-DOTA/Cy5.5 knottin peptide showed decreased wash-out leading to significantly better retention (p < 0.05) compared to the (64)Cu-DOTA-labeled knottin peptide. Tumor uptake was significantly reduced (p < 0.05) when the dual-labeled knottin peptide was coinjected with an excess of unlabeled competitor and when tested in a tumor model with lower levels of integrin expression. Finally, plots of tumor-to-background tissue ratios for Cy5.5 versus (64)Cu uptake were well-correlated over several time points post injection, demonstrating pharmacokinetic cross validation of imaging labels. This dual-modality NIRF/PET imaging agent is promising for further development in clinical applications where high sensitivity and high resolution are desired, such as detection of tumors located deep within the body and image-guided surgical resection.

Angiogenesis inhibition in cancer therapy: platelet-derived growth factor (PDGF) and vascular endothelial growth factor (VEGF) and their receptors: biological functions and role in malignancy

Vascular endothelial growth factor (VEGF) is an endothelial cell-specific mitogen in vitro and an angiogenic inducer in a variety of in vivo models. VEGF gene transcription is induced in particular in hypoxic cells. In developmental angiogenesis, the role of VEGF is demonstrated by the finding that the loss of a single VEGF allele results in defective vascularization and early embryonic lethality. Substantial evidence also implicates VEGF as a mediator of pathological angiogenesis. In situ hybridization studies demonstrate expression of VEGF mRNA in the majority of human tumors. Platelet-derived growth factor (PDGF) is mainly believed to be an important mitogen for connective tissue, and also has important roles during embryonal development. Its overexpression has been linked to different types of malignancies. Thus, it is important to understand the physiology of VEGF and PDGF and their receptors as well as their roles in malignancies in order to develop antiangiogenic strategies for the treatment of malignant disease.

Engineered cystine knot peptides that bind alphavbeta3, alphavbeta5, and alpha5beta1 integrins with low-nanomolar affinity

There is a critical need for compounds that target cell surface integrin receptors for applications in cancer therapy and diagnosis. We used directed evolution to engineer the Ecballium elaterium trypsin inhibitor (EETI-II), a knottin peptide from the squash family of protease inhibitors, as a new class of integrin-binding agents. We generated yeast-displayed libraries of EETI-II by substituting its 6-amino acid trypsin binding loop with 11-amino acid loops containing the Arg-Gly-Asp integrin binding motif and randomized flanking residues. These libraries were screened in a high-throughput manner by fluorescence-activated cell sorting to identify mutants that bound to alpha(v)beta(3) integrin. Select peptides were synthesized and were shown to compete for natural ligand binding to integrin receptors expressed on the surface of U87MG glioblastoma cells with half-maximal inhibitory concentration values of 10-30 nM. Receptor specificity assays demonstrated that engineered knottin peptides bind to both alpha(v)beta(3) and alpha(v)beta(5) integrins with high affinity. Interestingly, we also discovered a peptide that binds with high affinity to alpha(v)beta(3), alpha(v)beta(5), and alpha(5)beta(1) integrins. This finding has important clinical implications because all three of these receptors can be coexpressed on tumors. In addition, we showed that engineered knottin peptides inhibit tumor cell adhesion to the extracellular matrix protein vitronectin, and in some cases fibronectin, depending on their integrin binding specificity. Collectively, these data validate EETI-II as a scaffold for protein engineering, and highlight the development of unique integrin-binding peptides with potential for translational applications in cancer.

Vascular regeneration: engineering the stem cell microenvironment

Vascular diseases are a major threat to human health nowadays. While current treatments can cure some vascular diseases, their beneficial effects are only temporary; vascular regeneration holds the promise of permanent, effective treatments for many vascular diseases. Stem cells and endothelial progenitor cells can differentiate into vascular lineages and therefore have the potential to repair vascular systems. However, engineering appropriate microenvironments that will allow cell maturation and delivery remains the major challenge to the successful implementation of this treatment. This review introduces the cells that are being studied for vascular differentiation and regeneration; we then consider recent approaches to engineering microenvironments, including proper signaling cues and biodegradable scaffolds that will guide the development of these cells into vessels suitable for cell-based vascular therapy.

Cationic amphiphiles: promising carriers of genetic materials in gene therapy

The clinical success of gene therapy critically depends on the use of efficient and safe gene delivery reagents. The present tutorial review is aimed at inspiring young researchers and students to take up the unsolved challenges in using cationic amphiphiles as safe gene transfer reagents. The review highlights important structure-activity studies in the field to date including the use of cationic amphiphiles for receptor specific targeted gene therapy and for delivery of siRNAs in the emerging field of RNA interference.

Lipopeptide with a RGDK tetrapeptide sequence can selectively target genes to proangiogenic alpha5beta1 integrin receptor and mouse tumor vasculature

Integrins, the major class of alphabeta heterodimeric transmembrane glycoprotein receptors, play crucial roles in mediating tumor angiogenesis. Genetic ablation experiments combined with use of antibodies/peptide ligands for blocking either alpha(5) or beta(1) integrins have convincingly demonstrated alpha(5)beta(1) integrin to be unquestionably proangiogenic among the 24 known integrin receptors. Herein, we report on a novel RGDK-lipopeptide 1 that targets selectively alpha(5)beta(1) integrin and is capable of targeting genes to mouse tumor vasculatures.

Endothelial cell lumen and vascular guidance tunnel formation requires MT1-MMP-dependent proteolysis in 3-dimensional collagen matrices

Here we show that endothelial cells (EC) require matrix type 1-metalloproteinase (MT1-MMP) for the formation of lumens and tube networks in 3-dimensional (3D) collagen matrices. A fundamental consequence of EC lumen formation is the generation of vascular guidance tunnels within collagen matrices through an MT1-MMP-dependent proteolytic process. Vascular guidance tunnels represent a conduit for EC motility within these spaces (a newly remodeled 2D matrix surface) to both assemble and remodel tube structures. Interestingly, it appears that twice as many tunnel spaces are created than are occupied by tube networks after several days of culture. After tunnel formation, these spaces represent a 2D migratory surface within 3D collagen matrices allowing for EC migration in an MMP-independent fashion. Blockade of EC lumenogenesis using inhibitors that interfere with the process (eg, integrin, MMP, PKC, Src) completely abrogates the formation of vascular guidance tunnels. Thus, the MT1-MMP-dependent proteolytic process that creates tunnel spaces is directly and functionally coupled to the signaling mechanisms required for EC lumen and tube network formation. In summary, a fundamental and previously unrecognized purpose of EC tube morphogenesis is to create networks of matrix conduits that are necessary for EC migration and tube remodeling events critical to blood vessel assembly.

Cellular delivery and biological activity of antisense oligonucleotides conjugated to a targeted protein carrier

Targeted delivery can potentially improve the pharmacological effects of antisense and siRNA oligonucleotides. Here, we describe a novel bioconjugation approach to the delivery of splice-shifting antisense oligonucleotides (SSOs). The SSOs are linked to albumin via reversible S-S bonds. The albumin is also conjugated with poly(ethylene glycol) (PEG) chains that terminate in an RGD ligand that selectively binds the alphavbeta3 integrin. As a test system, we utilized human melanoma cells that express the alphavbeta3 integrin and that also contain a luciferase reporter gene that can be induced by delivery of SSOs to the cell nucleus. The RGD-PEG-SSO-albumin conjugates were endocytosed by the cells in an RGD-dependent manner; using confocal fluorescence microscopy, evidence was obtained that the SSOs accumulate in the nucleus. The conjugates were able to robustly induce luciferase expression at concentrations in the 25-200 nM range. At these levels, little short-term or long-term toxicity was observed. Thus, the RGD-PEG-albumin conjugates may provide an effective tool for targeted delivery of oligonucleotides to certain cells and tissues.

Engineered knottin peptides: a new class of agents for imaging integrin expression in living subjects

There is a critical need for molecular imaging agents to detect cell surface integrin receptors that are present in human cancers. Previously, we used directed evolution to engineer knottin peptides that bind with high affinity ( approximately 10 to 30 nmol/L) to integrin receptors that are overexpressed on the surface of tumor cells and the tumor neovasculature. To evaluate these peptides as molecular imaging agents, we site-specifically conjugated Cy5.5 or (64)Cu-1,4,7,10-tetra-azacyclododecane-N,N',N'',N'''-tetraacetic acid (DOTA) to their N termini, and used optical and positron emission tomography (PET) imaging to measure their uptake and biodistribution in U87MG glioblastoma murine xenograft models. NIR fluorescence and microPET imaging both showed that integrin binding affinity plays a strong role in the tumor uptake of knottin peptides. Tumor uptake at 1 hour postinjection for two high-affinity (IC(50), approximately 20 nmol/L) (64)Cu-DOTA-conjugated knottin peptides was 4.47% +/- 1.21% and 4.56% +/- 0.64% injected dose/gram (%ID/g), compared with a low-affinity knottin peptide (IC(50), approximately 0.4 mumol/L; 1.48 +/- 0.53%ID/g) and c(RGDyK) (IC(50), approximately 1 mumol/L; 2.32 +/- 0.55%ID/g), a low-affinity cyclic pentapeptide under clinical development. Furthermore, (64)Cu-DOTA-conjugated knottin peptides generated lower levels of nonspecific liver uptake ( approximately 2%ID/g) compared with c(RGDyK) ( approximately 4%ID/g) 1 hour postinjection. MicroPET imaging results were confirmed by in vivo biodistribution studies. (64)Cu-DOTA-conjugated knottin peptides were stable in mouse serum, and in vivo metabolite analysis showed minimal degradation in the blood or tumor upon injection. Thus, engineered integrin-binding knottin peptides show great potential as clinical diagnostics for a variety of cancers.

Cooperation between integrin alphavbeta3 and VEGFR2 in angiogenesis

The cross-talk between receptor tyrosine kinases and integrin receptors are known to be crucial for a number of cellular functions. On endothelial cells, an interaction between integrin alphavbeta3 and VEGFR2 seems to be particularly important process during vascularization. Importantly, the functional association between VEGFR2 and integrin alphavbeta3 is of reciprocal nature since each receptor is able to promote activation of its counterpart. This mutually beneficial relationship regulates a number of cellular activities involved in angiogenesis, including endothelial cell migration, survival and tube formation, and hematopoietic cell functions within vasculature. This article discusses several possible mechanisms reported by different labs which mediate formation of the complex between VEGFR-2 and alphavbeta3 on endothelial cells. The pathological consequences and regulatory events involved in this receptor cross-talk are also presented.

Update on therapeutic vascularization strategies

The ability to exploit angiogenesis and vascularization as a therapeutic strategy will be of enormous benefit to a wide range of medical and tissue-engineering applications. Angiogenic growth factor and cell-based therapies have thus far failed to produce a robust healing response in clinical trials for a variety of ischemic diseases, while engineered tissue substitutes are still size-limited by a lack of vascularization. The purpose of this review is to investigate current research advances in therapeutic vascularization strategies applied to ischemic disease states, tissue engineering and regenerative medicine. Recent advances are discussed that focus on better regulation of growth factor delivery and attempts to better mimic natural processes by delivering combinations of multiple growth factors, cells and bioactive materials in the right spatial and temporal setting. Some unconventional approaches and novel therapeutic targets that hold significant potential are also discussed.

Integrin alpha(v)beta(3) is a pleiotrophin receptor required for pleiotrophin-induced endothelial cell migration through receptor protein tyrosine phosphatase beta/zeta

We have previously shown that the angiogenic growth factor pleiotrophin (PTN) induces migration of endothelial cells through binding to its receptor protein tyrosine phosphatase beta/zeta (RPTPbeta/zeta). In this study, we show that a monoclonal antibody against alpha(nu)beta(3) but not alpha(5)beta(1) integrin abolished PTN-induced human endothelial cell migration in a concentration-dependent manner. Integrin alpha(nu)beta(3) was found to directly interact with PTN in an RGD-independent manner, whereas a synthetic peptide corresponding to the specificity loop of the beta(3) integrin extracellular domain ((177)CYDMKTTC(184)) inhibited PTN-alpha(nu)beta(3) interaction and totally abolished PTN-induced endothelial cell migration. Interestingly, alpha(nu)beta(3) was also found to directly interact with RPTPbeta/zeta, and PTN-induced Y773 phosphorylation of beta(3) integrin was dependent on both RPTPbeta/zeta and the downstream c-src kinase activation. Midkine was found to interact with RPTPbeta/zeta, but not with alpha(nu)beta(3), and caused a small but statistically significant decrease in cell migration. In the same line, PTN decreased migration of different glioma cell lines that express RPTPbeta/zeta but do not express alpha(nu)beta(3), while it stimulated migration of U87MG cells that express alpha(nu)beta(3) on their cell membrane. Overexpression or down-regulation of beta(3) stimulated or abolished, respectively, the effect of PTN on cell migration. Collectively, these data suggest that alpha(nu)beta(3) is a key molecule that determines the stimulatory or inhibitory effect of PTN on cell migration.

Integrin and growth factor receptor alliance in angiogenesis

A sequence of events in vascular and stromal cells maintained in a highly coordinated manner regulates angiogenesis and tissue remodeling. These processes are mediated by the ability of cells to respond to environmental cues and activate surface integrins. Physiological and pathological processes in vascular biology are dependent on the specificity of important signaling mechanisms that are activated through the association between growth factors, their receptors, integrins, and their specific extracellular matrix ligands. A large body of evidence from in vitro and in vivo models demonstrates the importance of coordination of signals from the extracellular environment that activates specific tyrosine kinase receptors and integrins in order to regulate angiogenic processes in vivo. In addition to complex formation between growth factor receptors and integrins, growth factors and cytokines also directly interact with integrins, depending upon their concentration levels in the environment, and differentially regulate integrin-related processes. Recent studies from a number of laboratories including ours have provided important novel insights into the involvement of many signaling events that improve our existing knowledge on the cross-talk between growth factor receptors and integrins in the regulation of angiogenesis. In this review, our focus will be on updating the recent developments in the field of integrin-growth factor receptor associations and their implications in the vascular processes.

Cooperation between integrin alpha5 and tetraspan TM4SF5 regulates VEGF-mediated angiogenic activity

Tetraspan TM4SF5 is highly expressed in a diverse number of tumor types. Here we explore the mechanistic roles of TM4SF5 in angiogenesis. We found that TM4SF5 overexpression correlates with vascular endothelial growth factor (VEGF) expression in SNU449 hepatocytes and with vessel formation in clinical hepatocarcinoma samples. Conditioned media from TM4SF5-expressing cells enhanced viability and tube formation of primary human umbilical vein endothelial cells, and outgrowth of endothelial cells from aorta ring segments, which was abolished by treatment with an anti-VEGF antibody. TM4SF5 retained integrin alpha(5) on the cell surface for VEGF induction, and preincubation with anti-integrin alpha(5) antibody abolished TM4SF5-mediated VEGF expression and secretion. TM4SF5-mediated effects required integrin alpha(5), c-Src, and signal transducer and activator of transcription 3 (STAT3). In addition, tumors from nude mice injected with TM4SF5-expressing cells and from clinical human hepatocarcinoma tissues showed enhanced integrin alpha(5) expression, vessel formation, and signaling activity, which were inhibited by administration of anti-integrin alpha(5) or -VEGF antibody. This study suggests that TM4SF5 facilitates angiogenesis of neighboring endothelial cells through VEGF induction, mediated by cooperation between TM4SF5 and integrin alpha(5) of epithelial cells.

Cell response to matrix mechanics: focus on collagen

Many model systems and measurement tools have been engineered for observing and quantifying the effect of mechanics on cellular response. These have contributed greatly to our current knowledge of the molecular events by which mechanical cues affect cell biology. Cell responses to the mechanical properties of type 1 collagen gels are discussed, followed by a description of a model system of very thin, mechanically tunable collagen films that evoke similar responses from cells as do gel systems, but have additional advantages. Cell responses to thin films of collagen suggest that at least some of the mechanical cues that cells can respond to in their environment occur at the sub-micron scale. Mechanical properties of thin films of collagen can be tuned without altering integrin engagement, and in some cases without altering topology, making them useful in addressing questions regarding the roles of specific integrins in transducing or mitigating responses to mechanical cues. The temporal response of cells to differences in ECM may provide insight into mechanisms of signal transduction.

Hepatocyte growth factor inhibits VEGF-forkhead-dependent gene expression in endothelial cells

OBJECTIVE

Recently, we reported that the forkhead transcription factor, FKHR/FOXO1, is required for vascular endothelial growth factor (VEGF)-mediated upregulation of a number of genes in endothelial cells. Here, we tested the hypothesis that hepatocyte growth factor (HGF), a potent activator of PI3K-Akt in endothelial cells, is capable of depleting the nucleus of FKHR/FOXO1 and thus inhibiting VEGF induction of this class of genes.

METHODS AND RESULTS

Incubation of human coronary artery endothelial cells with HGF induced prolonged PI3K/Akt-dependent phosphorylation and nuclear exclusion of FKHR/FOXO1. HGF-mediated inhibition of FKHR/FOXO1 activity resulted in secondary attenuation of VEGF-induced expression of FKHR/FOXO1-dependent genes including vascular cell adhesion molecule-1, manganese superoxide dismutase, endothelial specific molecule-1, CBP/p300 interacting transactivator with ED-rich tail-2, bone morphogenetic protein-2, matrix metalloproteinase (MMP)-10, and MGC5618. At a functional level, preincubation of HGF resulted in inhibition of VEGF-induced vascular cell adhesion molecule (VCAM)-1-mediated monocyte adhesion to endothelial cells. HGF-mediated inhibition of VEGF-inducible VCAM-1 expression and monocyte adhesion was reversed by overexpression of constitutively active phosphorylation-resistant triple mutant (TM)-FKHR.

CONCLUSIONS

These findings suggest that physiological agonists of PI3K-Akt signaling pathway may modulate VEGF-FKHR/FOXO1-dependent gene expression in endothelial cells. The data underscore the importance of the "set point" of the endothelial cell when considering mechanisms of signal transduction.

Antimigratory effect of TK1-2 is mediated in part by interfering with integrin alpha2beta1

The recombinant two kringle domain of human tissue-type plasminogen activator (TK1-2) has been shown to inhibit endothelial cell proliferation, angiogenesis, and tumor cell growth despite of sharing a low amino acid sequence homology with angiostatin. Here, we explored a possible inhibitory mechanism of action of TK1-2 by focusing on antimigratory effect. TK1-2 effectively inhibited endothelial cell migration induced by basic fibroblast growth factor or vascular endothelial growth factor in a dose-dependent manner and tube formation on Matrigel. It blocked basic fibroblast growth factor-induced or vascular endothelial growth factor-induced phosphorylation of extracellular signal-regulated kinase 1/2 and formation of actin stress fibers and focal adhesions. Interestingly, TK1-2 alone induced the weak phosphorylation of focal adhesion kinase, whereas it inhibited focal adhesion kinase phosphorylation induced by growth factors. When immobilized, TK1-2 promoted adhesion and spreading of endothelial cells compared with bovine serum albumin. However, treatment with anti-alpha(2)beta(1) blocking antibody markedly diminished endothelial cell adhesion to immobilized TK1-2 compared with anti-alpha(v)beta(3) or anti-alpha(5)beta(1) antibody. Pretreatment of soluble TK1-2 also altered the binding level of anti-alpha(2)beta(1) antibody to endothelial cells in fluorescence-activated cell sorting analysis. Indeed, a blocking antibody against integrin alpha(2)beta(1) or knocking down of integrin alpha(2) expression prevented the inhibitory effect of TK1-2 in cell migration. Therefore, these results suggest that TK1-2 inhibits endothelial cell migration through inhibition of signaling and cytoskeleton rearrangement in part by interfering with integrin alpha(2)beta(1).

The role of cell adhesion pathways in angiogenesis

Angiogenesis, the formation of new blood vessels from pre-existing vasculature, is prevalent both during normal mammalian development and in certain pathological conditions such as tumor growth. It is stimulated and controlled by a complex network of intracellular signaling mechanisms, many of which are initiated by trans-membrane receptors transducing signals received from other cells and from the extracellular environment. Of these, cytokine signaling is recognized as one of the primary drivers of angiogenesis, but it has become increasingly evident that signaling mechanisms generated as a result of cell adhesion interactions are also crucially important. In addition, cell adhesion pathways are also intimately tied to cytokine signaling often making it difficult to dissect out the relative contribution of each to a particular angiogenic step. Many of these same signaling mechanisms are often manipulated by tumors to stimulate aberrant angiogenesis and enhance their blood supply. As a consequence, there is a great deal of interest in trying to understand the full complement of intracellular signaling pathways in angiogenesis as well as their interplay and timing during the process. Ultimately, understanding the complex network of signaling pathways that function during angiogenesis will provide important avenues for future therapeutic development.

Potassium Channels Keep Mobile Cells on the Go

Cell motility is a prerequisite for the creation of new life, and it is required for maintaining the integrity of an organism. Under pathological conditions, “too much” motility may cause premature death. Studies over the past few years have revealed that ion channels are essential for cell motility. This review highlights the importance of K+ channels in regulating cell motility.

TIMP-2 disrupts FGF-2-induced downstream signaling pathways

We have previously reported that tissue inhibitor of metalloproteinases-2 (TIMP-2), an endogenous inhibitor of matrix metalloproteinase, modulates angiogenic responses through the MMP inhibition-independent activity. In this study, we investigate the molecular mechanisms of TIMP-2-mediated growth inhibition in response to fibroblast growth factor-2 (FGF-2). Pre-treatment with a protein tyrosine phosphatase inhibitor orthovanadate or expression of a dominant negative Shp-1 mutant fails to induce TIMP-2 inactivation of FGF-2 signaling pathways in human microvascular endothelial cells. We also show that TIMP-2 inhibition of FGF-2-induced p42/44(MAPK) activation and cell proliferation is associated with TIMP-2 binding to integrin alpha3beta1 on endothelial cell surfaces, as demonstrated by use of anti-integrin alpha3 or beta1 blocking antibodies, or disruption of integrin alpha3 expression by siRNA. Collectively, our results indicate that TIMP-2 inhibits FGF-2 signaling pathways through association with integrin alpha3beta1 and Shp-1-dependent inhibition of p42/44(MAPK) signaling, which in turn, results in suppression of FGF-2-stimulated endothelial cell mitogenesis.

Markers for microscopic imaging of lymphangiogenesis and angiogenesis

Imaging of lymphangiogenesis and angiogenesis requires robust and unambiguous markers of lymphatic and blood vessels. Although much progress has been made in recent years in identifying molecules specifically expressed on lymphatic and blood vessels, no perfect marker has been found that works reliably in all species, tissues, vascular beds, and in all physiological and pathologic conditions. The heterogeneity of expression of markers in both blood and lymphatic vessels reflects underlying differences in the phenotype of endothelial cells. Use of only one marker can lead to misleading interpretations, but these pitfalls can usually be avoided by use of multiple markers and three-dimensional whole-mount preparations. LYVE-1, VEGFR-3, Prox1, and podoplanin are among the most useful markers for microscopic imaging of lymphatic vessels, but, depending on histologic location, each marker can be expressed by other cell types, including vascular endothelial cells. Other markers, including CD31, junctional proteins, and receptors, such as VEGF-2, are shared by lymphatic and blood vessels.

Therapeutic potential of novel modulators of neovascularization

Neovacularization is an important biological process whereby new blood vessels develop in both health and disease. During development, blood vessels are formed from mesodermal cells in a process called vasculogenesis. The vascular network then expands by the sprouting of new vessel networks from pre-established vessels in a process known as angiogenesis. However, in adult life, undesirable neovascularization is associated with tumor development and a growing list of 'angiogenesis-dependent' diseases, including cardiovascular complications. Furthermore, diseases characterized by ischemia-induced tissue damage cause a neovascularization response to facilitate tissue repair. Recent research has identified novel molecular and cellular mediators of neovascularization that, in adult life, recapitulate angiogenic processes observed during embryonic development. The discovery of vascular progenitor cells and new molecules that display selective functions in modulating endothelial cell fate, migration and patterning, vessel morphogenesis and the amplification of angiogenic signaling by regulating the master signal VEGF, opens the door to new clinical strategies that target angiogenesis-dependent diseases or that can promote therapeutic neovascularization.

Targeted molecular imaging in oncology: focus on radiation therapy

Anatomically based technologies (computed tomography scans, magnetic resonance imaging, and so on) are in routine use in radiotherapy for planning and assessment purposes. Even with improvements in imaging, however, radiotherapy is still limited in efficacy and toxicity in certain applications. Further advances may be provided by technologies that image the molecular activities of tumors and normal tissues. Possible uses for molecular imaging include better localization of tumor regions and early assay for the radiation response of tumors and normal tissues. Critical to the success of this approach is the identification and validation of molecular probes that are suitable in the radiotherapy context. Recent developments in molecular-imaging probes and integration of functional imaging with radiotherapy are promising. This review focuses on recent advances in molecular imaging strategies and probes that may aid in improving the efficacy of radiotherapy.

Vascular tube formation on matrix metalloproteinase-1-damaged collagen

Connective tissue damage and angiogenesis are both important features of tumour growth and invasion. Here, we show that endothelial cells maintained on a three-dimensional lattice of intact polymerised collagen formed a monolayer of cells with a cobblestone morphology. When the collagen was exposed to organ culture fluid from human basal cell tumours of the skin (containing a high level of active matrix metalloproteinase-1 (MMP-1)), degradation of the collagen matrix occurred. The major degradation products were the $3over 4$- and $1over 4$-sized fragments known to result from the action of MMP-1 on type I collagen. When endothelial cells were maintained on the partially degraded collagen, the cells organised into a network of vascular tubes. Pretreatment of the organ culture fluid with either tissue inhibitor of metalloproteinase-1 (TIMP-1) or neutralising antibody to MMP-1 prevented degradation of the collagen lattice and concomitantly inhibited endothelial cell organisation into the vascular network. Purified (activated) MMP-1 duplicated the effects of skin organ culture fluid, but other enzymes including MMP-9 (gelatinase B), elastase or trypsin failed to produce measurable fragments from intact collagen and also failed to promote vascular tube formation. Together, these studies suggest that damage to the collagenous matrix is itself an important inducer of new vessel formation.