Evaluating integrin function in models of angiogenesis and vascular permeability

Assessment of the Biocompatibility Ability and Differentiation Capacity of Mesenchymal Stem Cells on Biopolymer/Gold Nanocomposites

This study assessed the biocompatibility of two types of nanogold composites: fibronectin-gold (FN-Au) and collagen-gold (Col-Au). It consisted of three main parts: surface characterization, in vitro biocompatibility assessments, and animal models. To determine the structural and functional differences between the materials used in this study, atomic force microscopy, Fourier-transform infrared spectroscopy, and ultraviolet-visible spectrophotometry were used to investigate their surface topography and functional groups. The F-actin staining, proliferation, migration, reactive oxygen species generation, platelet activation, and monocyte activation of mesenchymal stem cells (MSCs) cultured on the FN-Au and Col-Au nanocomposites were investigated to determine their biological and cellular behaviors. Additionally, animal biocompatibility experiments measured capsule formation and collagen deposition in female Sprague-Dawley rats. The results showed that MSCs responded better on the FN-Au and Col-AU nanocomposites than on the control (tissue culture polystyrene) or pure substances, attributed to their incorporation of an optimal Au concentration (12.2 ppm), which induced significant surface morphological changes, nano topography cues, and better biocompatibility. Moreover, neuronal, endothelial, bone, and adipose tissues demonstrated better differentiation ability on the FN-Au and Col-Au nanocomposites. Nanocomposites have a crucial role in tissue engineering and even vascular grafts. Finally, MSCs were demonstrated to effectively enhance the stability of the endothelial structure, indicating that they can be applied as promising alternatives to clinics in the future.

γ-Bungarotoxin impairs the vascular endothelial barrier function by inhibiting integrin α5

γ-bungarotoxin (γ-BGT) is an RGD motif-containing protein, derived from the venom of Bungarus multicinctus, leading to acute death in mice. These RGD motif-containing proteins from snake venom belonging to the disintegrin family can interfere with vascular endothelial homeostasis by directly binding cell surface integrins. Targeting integrins that generate vascular endothelial dysfunction may contribute to γ-BGT poisoning, however, the underlying mechanisms have not been investigated in detail. In this study, the results showed that γ-BGT played a role in -promoting the permeability of the vascular endothelial barrier. Depending on its selective binding to integrin α5 in vascular endothelium (VE), γ-BGT initiated downstream events, including focal adhesion kinase dephosphorylation and cytoskeleton remodeling, resulting in the intercellular junction interruption. Those alternations facilitated paracellular permeability of VE and barrier dysfunction. Proteomics profiling identified that as a downstream effector of the integrin α5 / FAK signaling pathway cyclin D1 partially mediated the cellular structural changes and barrier dysfunction. Furthermore, VE-released plasminogen activator urokinase and platelet-derived growth factor D could serve as potential diagnostic biomarkers for γ-BGT-induced vascular endothelial dysfunction. Our results indicate the mechanisms through which γ-BGT as a novel disintegrin directly interacts with the VE, with consequences for barrier dysfunction.

Targeting P21-Activated Kinase-1 for Metastatic Prostate Cancer

Metastatic prostate cancer (mPCa) has limited therapeutic options and a high mortality rate. The p21-activated kinase (PAK) family of proteins is important in cell survival, proliferation, and motility in physiology, and pathologies such as infectious, inflammatory, vascular, and neurological diseases as well as cancers. Group-I PAKs (PAK1, PAK2, and PAK3) are involved in the regulation of actin dynamics and thus are integral for cell morphology, adhesion to the extracellular matrix, and cell motility. They also play prominent roles in cell survival and proliferation. These properties make group-I PAKs a potentially important target for cancer therapy. In contrast to normal prostate and prostatic epithelial cells, group-I PAKs are highly expressed in mPCA and PCa tissue. Importantly, the expression of group-I PAKs is proportional to the Gleason score of the patients. While several compounds have been identified that target group-I PAKs and these are active in cells and mice, and while some inhibitors have entered human trials, as of yet, none have been FDA-approved. Probable reasons for this lack of translation include issues related to selectivity, specificity, stability, and efficacy resulting in side effects and/or lack of efficacy. In the current review, we describe the pathophysiology and current treatment guidelines of PCa, present group-I PAKs as a potential druggable target to treat mPCa patients, and discuss the various ATP-competitive and allosteric inhibitors of PAKs. We also discuss the development and testing of a nanotechnology-based therapeutic formulation of group-I PAK inhibitors and its significant potential advantages as a novel, selective, stable, and efficacious mPCa therapeutic over other PCa therapeutics in the pipeline.

Vascular smooth muscle stiffness and its role in aging

Vascular smooth muscle cells (VSMC) are now considered important contributors to the pathophysiological and biophysical mechanisms underlying arterial stiffening in aging. Here, we review mechanisms whereby VSMC stiffening alters vascular function and contributes to the changes in vascular stiffening observed in aging and cardiovascular disease. Vascular stiffening in arterial aging was historically associated with changes in the extracellular matrix; however, new evidence suggests that endothelial and vascular smooth muscle cell stiffness also contribute to overall blood vessel stiffness. Furthermore, VSMC play an integral role in regulating matrix deposition and vessel wall contractility via interaction between the actomyosin contractile unit and adhesion structures that anchor the cell within the extracellular matrix. Aged-induce phenotypic modulation of VSMC from a contractile to a synthetic phenotype is associated with decreased cellular contractility and increased cell stiffness. Aged VSMC also display reduced mechanosensitivity and adaptation to mechanical signals from their microenvironment due to impaired intracellular signaling. Finally, evidence for decreased contractility in arteries from aged animals demonstrate that changes at the cellular level result in decreased functional properties at the tissue level.

Extracellular vesicles in acute respiratory distress syndrome: Recent developments from bench to bedside

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS), characterized by a large number of inflammatory cell aggregation and alveolar cell damage in pathophysiology, have extremely high morbidity and mortality in critically ill patients. In recent years, more and more studies have found that there are abundant extracellular vesicles (EVs) in animal models and patients with ALI/ARDS, and they play a critical role in the pathogenesis of lung injury. Clarifying the mechanisms of EVs in lung injury is of great significance in the diagnosis and treatment of ALI/ARDS. In this review, we will summarize the recent findings on the roles of EVs derived from different cells in ALI/ARDS, along with the formation, function, and related effects of EVs, and explore their potential clinical application for the diagnosis and treatment of ALI/ARDS.

Soluble CD146, a cerebrospinal fluid marker for neuroinflammation, promotes blood-brain barrier dysfunction

The blood-brain barrier (BBB) dysfunction is an initial event of various neuroinflammatory diseases. However, the absence of reliable markers and mechanisms for BBB damage greatly limits the diagnosis and treatment of neuroinflammatory diseases. Soluble CD146 (sCD146) is mainly derived from vascular endothelial cells (ECs) and highly elevated in inflammatory settings. Based on a small cohort, our previous study showed that sCD146 is elevated in the cerebrospinal fluid (CSF) of multiple sclerosis (MS), which is accompanied with BBB damage. Nevertheless, whether sCD146 monitors and regulates the BBB dysfunction remains unknown.

Methods: Coupled serum and CSF samples from patients with or without neuroinflammatory diseases were collected via multicenter collaborations. sCD146 was measured by sandwich ELISA using anti-CD146 antibodies AA1 and AA98, both of which were generated in our laboratory. The correlations between sCD146 and other clinical parameters or inflammatory factors were analyzed by Spearman's correlation coefficient analysis. The role of sCD146 on BBB function was examined in an in vitro BBB model.

Results: Between July 20, 2011, and February 31, 2017, we collected coupled serum and CSF samples from 823 patients, of which 562 (68.3%) had neuroinflammatory diseases, 44 (5.3%) had remitting MS, and 217 (26.4%) had non-inflammatory neurological diseases (NIND). We found that sCD146 in CSF, but not in serum, is abnormally elevated in neuroinflammatory diseases (37.3 ± 13.3 ng/mL) compared with NIND (4.7 ± 2.9 ng/mL) and remitting MS (4.6 ± 3.5 ng/mL). Abnormally elevated CSF sCD146 is significantly correlated with the hyperpermeability-related clinical parameters of BBB and neuroinflammation-related factors. Moreover, CSF sCD146 shows higher sensitivity and specificity for evaluating BBB damage. Using an in vitro BBB model, we found that sCD146 impairs BBB function by promoting BBB permeability via an association with integrin αvβ1. Blocking integrin αvβ1 significantly attenuates sCD146-induced hyperpermeability of the BBB.

Conclusion: Our study provides convincing evidence that CSF sCD146 is a sensitive marker of BBB damage and neuroinflammation. Furthermore, sCD146 is actively involved in BBB dysfunction.

PKN3 is the major regulator of angiogenesis and tumor metastasis in mice

PKN, a conserved family member related to PKC, was the first protein kinase identified as a target of the small GTPase Rho. PKN is involved in various functions including cytoskeletal arrangement and cell adhesion. Furthermore, the enrichment of PKN3 mRNA in some cancer cell lines as well as its requirement in malignant prostate cell growth suggested its involvement in oncogenesis. Despite intensive research efforts, physiological as well as pathological roles of PKN3 in vivo remain elusive. Here, we generated mice with a targeted deletion of PKN3. The PKN3 knockout (KO) mice are viable and develop normally. However, the absence of PKN3 had an impact on angiogenesis as evidenced by marked suppressions of micro-vessel sprouting in ex vivo aortic ring assay and in vivo corneal pocket assay. Furthermore, the PKN3 KO mice exhibited an impaired lung metastasis of melanoma cells when administered from the tail vein. Importantly, PKN3 knock-down by small interfering RNA (siRNA) induced a glycosylation defect of cell-surface glycoproteins, including ICAM-1, integrin β1 and integrin α5 in HUVECs. Our data provide the first in vivo genetic demonstration that PKN3 plays critical roles in angiogenesis and tumor metastasis, and that defective maturation of cell surface glycoproteins might underlie these phenotypes.

Effect of mixed transplantation of autologous and allogeneic microskin grafts on wound healing in a rat model of acute skin defect

The treatment of extensive thermal injuries with insufficient autologous skin remains a great challenge to burn surgeons. In this study, we investigated the influence of the ratio of autologous and allogeneic tissue in mixed microskin grafts on wound healing in order to develop an effective method for using limited donor skin to cover a large open wound. Four different mixtures were tested: autologous microskin at an area expansion ratio of 10∶1 with allogeneic microskin at an area expansion ratio of 10∶1 or 10∶3 and autologous microskin at an expansion ratio of 20∶1 with allogeneic microskin at an expansion ratio of 20∶3 or 20∶6. Wound healing, wound contraction, and integrin β1 expression were measured. Mixed microskin grafting facilitated wound healing substantially. The mixture of autologous microskin at an expansion ratio of 10∶1 with the same amount of allogeneic microskin achieved the most satisfactory wound healing among the 4 tested mixtures. Histological examination revealed the presence of obviously thickened epidermis and ectopic integrin β1 expression. Keratinocytes expressing integrin β1 were scattered in the suprabasal layer. Higher levels of integrin β1 expression were associated with faster wound healing, implying that ectopic expression of integrin β1 in keratinocytes may play a pivotal role in wound healing. In conclusion, this study proves that this new skin grafting technique may improve wound healing.

Drug testing with angiogenesis models

BACKGROUND

The possibility to treat cancers and several angiogenesis- dependent diseases with non-toxic, antiangiogenic agents has revolutionized the therapeutic capabilities in the fields of oncology and ophthalmology, whereas therapeutic angiogenesis, governed by angiogenesis stimulators, is about to enter clinical medicine.

OBJECTIVE

To describe and critically evaluate the advantages and limitations of the most important and most frequently used preclinical in vivo angiogenesis assays as well as to appraise the preclinical models that are most widely used for studying antiangiogenic effects in tumors.

METHODS

Up-to-date literature survey.

RESULTS/CONCLUSION

Only few angiogenesis and tumor models appear to meet realistic standards fully in terms of biological relevance. Improvement of the biological pertinence and sensitivity of such models would apparently facilitate the translatability of preclinical data into clinical practice.

Slit2/Robo4 signaling modulates HIV-1 gp120-induced lymphatic hyperpermeability

Dissemination of HIV in the host involves transit of the virus and virus-infected cells across the lymphatic endothelium. HIV may alter lymphatic endothelial permeability to foster dissemination, but the mechanism is largely unexplored. Using a primary human lymphatic endothelial cell model, we found that HIV-1 envelope protein gp120 induced lymphatic hyperpermeability by disturbing the normal function of Robo4, a novel regulator of endothelial permeability. HIV-1 gp120 induced fibronectin expression and integrin α₅β₁ phosphorylation, which led to the complexing of these three proteins, and their subsequent interaction with Robo4 through its fibronectin type III repeats. Moreover, pretreatment with an active N-terminus fragment of Slit2, a Robo4 agonist, protected lymphatic endothelial cells from HIV-1 gp120-induced hyperpermeability by inhibiting c-Src kinase activation. Our results indicate that targeting Slit2/Robo4 signaling may protect the integrity of the lymphatic barrier and limit the dissemination of HIV in the host.

New Methods for Labeling RGD Peptides with Bromine-76

Direct bromination of the tyrosine residues of peptides and antibodies with bromine-76, to create probes for PET imaging, has been reported. For peptides that do not contain tyrosine residues, however, a prosthetic group is required to achieve labeling via conjugation to other functional groups such as terminal α-amines or lysine ε-amines. The goal of this study was to develop new strategies for labeling small peptides with Br-76 using either a direct labeling method or a prosthetic group, depending on the available functional group on the peptides. A new labeling agent, N-succinimidyl-3-[(76)Br]bromo-2,6-dimethoxybenzoate ([(76)Br]SBDMB) was prepared for cyclic RGD peptide labeling. N-succinimidyl-2, 6-dimethoxybenzoate was also used to pre-attach a 2, 6-dimethoxybenzoyl (DMB) moiety to the peptide, which could then be labeled with Br-76. A competitive cell binding assay was performed to determine the binding affinity of the brominated peptides. PET imaging of U87MG human glioblastoma xenografted mice was performed using [(76)Br]-BrE[c(RGDyK)](2) and [(76)Br]-BrDMB-E[c(RGDyK)](2). An ex vivo biodistribution assay was performed to confirm PET quantification. The mechanisms of bromination reaction between DMB-c(RGDyK) and the brominating agent CH(3)COOBr were investigated with the SCRF-B3LYP/6-31G* method with the Gaussian 09 program package. The yield for direct labeling of c(RGDyK) and E[c(RGDyK)](2) using chloramine-T and peracetic acid at ambient temperature was greater than 50%. The yield for [(76)Br]SBDMB was over 60% using peracetic acid. The conjugation yields for labeling c(RGDfK) and c(RGDyK) were over 70% using the prosthetic group at room temperature. Labeling yield for pre-conjugated peptides was over 60%. SDMB conjugation and bromination did not affect the binding affinity of the peptides with integrin receptors. Both [(76)Br]Br-E[c(RGDyK)](2) and [(76)Br]BrDMB-E[c(RGDyK)](2) showed high tumor uptake in U87MG tumor bearing mice. The specificity of the imaging tracers was confirmed by decreased tumor uptake after co-administration of unlabeled dimeric RGD peptides. The energy barrier of the transition state of bromination for the dimethoxybenzoyl group was about 9 kcal/mol lower than that for the tyrosine residue. In conclusion, the newly developed N-succinimidyl-2, 6-dimethoxybenzoate molecule can be used either for one step labeling through pre-conjugation or as the precursor for a Br-76 labeled prosthetic group for indirect labeling. Radiobromination on a dimethoxybenzoyl group has selectivity over radiobromination on tyrosine. The energy barrier difference of the transition states of bromination between the dimethoxybenzoyl group and the tyrosine residue may account for the reaction selectivity when both groups are present in the same molecule.

TGFB1 disrupts the angiogenic potential of microvascular endothelial cells of the corpus luteum

Cyclical formation and regression of the ovarian corpus luteum is required for reproduction. During luteal regression, the microvasculature of the corpus luteum is extensively disrupted. Prostaglandin F2α, a primary signal for luteal regression, induces the expression of transforming growth factor β1 (TGFB1) in the corpus luteum. This study determined the actions of TGFB1 on microvascular endothelial cells isolated from the bovine corpus luteum (CLENDO cells). We hypothesized that TGFB1 participates in the disruption of the microvasculature during luteal regression. TGFB1 activated the canonical SMAD signaling pathway in CLENDO cells. TGFB1 (1 ng/ml) significantly reduced both basal and fetal-calf-serum-stimulated DNA synthesis, without reducing cell viability. TGFB1 also significantly reduced CLENDO cell transwell migration and disrupted the formation of capillary-like structures when CLENDO cells were plated on Matrigel. By contrast, CLENDO cells plated on fibrillar collagen I gels did not form capillary-like structures and TGFB1 induced cell death. Additionally, TGFB1 caused loss of VE-cadherin from cellular junctions and loss of cell-cell contacts, and increased the permeability of confluent CLENDO cell monolayers. These studies demonstrate that TGFB1 acts directly on CLENDO cells to limit endothelial cell function and suggest that TGFB1 might act in the disassembly of capillaries observed during luteal regression.

Overview: studying integrins in vivo

Integrins are adhesive proteins that have evolved to mediate cell-cell and cell-matrix communication that is indispensable for development and postnatal physiology. Despite their widespread expression, the genetic deletion of specific integrin family members in lower organisms as well as mammals leads to relatively distinct abnormalities. Many of the processes in which integrins participate have a requirement for strong adhesion coincident with times of mechanical stress. In Drosophila, the absence of specific integrins leads to detachment of muscle from the gut and body wall and separation of the two epithelial layers in the wing. In mice and humans, a deletion of either subunit of the laminin-binding integrin, α6β4 leads to severe skin blistering and defects in other epithelial layers. In addition, integrins have also evolved to serve more subspecialized roles ranging from the establishment of a stem cell niche in Drosophila and mammals, to the regulation of pathogenic tumor vascularization, platelet adhesion, and leukocyte transmigration in mammalian systems. However, some cells seem to function normally in the absence of all integrins, as revealed by the very surprising finding that deletion of all the major integrin types on dendritic cells of mice has no effect on the ability of these cells to migrate within the interstitium of the skin and enter into lymphatics. In addition to serving as transmembrane mechanical links, integrins in vertebrates synergize with a number of receptors including growth factor receptors, to enhance responses. This leads to the activation of a large signaling network that affects cell proliferation and differentiation, as well as cell shape and migration. In vivo studies, in lower organisms, knockout mouse models as well as in inherited human diseases together have provided important insights into how this major, primordial family of adhesion receptors have remained true to their name "integrins" as their diverse functions have in common the ability to integrate extracellular stimuli into intracellular signals that affect cell behavior.

18F-FPPRGD2 and 18F-FDG PET of response to Abraxane therapy

Abraxane (nanoparticle albumin-bound paclitaxel) is an anticancer drug approved by the Food and Drug Administration. However, the mechanism of action of Abraxane is complex, and no established biomarker is available to accurately monitor its treatment outcomes. The aim of this study was to investigate whether the integrin-specific PET tracer 18F-FPPRGD2 (investigational new drug 104150) can be used to monitor early response of tumors to Abraxane therapy.

METHODS

Orthotopic MDA-MB-435 breast cancer mice were treated with Abraxane (25 mg/kg every other day, 3 doses) or phosphate-buffered saline. Tumor volume was monitored by caliper measurement. PET scans were obtained before and at different times after the start of treatment (days 0, 3, 7, 14, and 21) using 18F-FPPRGD2 and 18F-FDG. The tumoricidal effect was also assessed ex vivo by immunohistochemistry.

RESULTS

Abraxane treatment inhibited the tumor growth, and a significant difference in tumor volume could be seen at day 5 after the initiation of treatment. The tumor uptake of 18F-FPPRGD2 in the Abraxane-treated group was significantly lower on days 3 and 7 than at baseline but returned to the baseline level at days 14 and 21, indicative of relapse of the tumors after the treatment was halted. Immunohistologic staining confirmed that the change of 18F-FPPRGD2 uptake correlated with the variation of integrin level in the tumor vasculature induced by Abraxane treatment. No significant change of tumor (rather than vascular) integrin expression was observed throughout the study. No significant decrease of 18F-FDG uptake was found between the treated and the control tumors on days 3, 14, and 21, although an increase in 18F-FDG tumor uptake of treated mice, as compared with the control mice, was found on day 7. The increase of 18F-FDG on day 7 was related to the inflammatory response during therapy.

CONCLUSION

Abraxane-mediated downregulation of integrin αvβ3 expression on tumor endothelial cells can be quantitatively visualized by PET. The change of integrin expression precedes that of tumor size. Consequently, 18F-FPPRGD2 PET is superior to 18F-FDG PET in monitoring early response to treatment, favoring its potential clinical translation.

PAK1 as a therapeutic target

IMPORTANCE OF THE FIELD

P21-activated kinases (PAKs) are involved in multiple signal transduction pathways in mammalian cells. PAKs, and PAK1 in particular, play a role in such disorders as cancer, mental retardation and allergy. Cell motility, survival and proliferation, the organization and function of cytoskeleton and extracellular matrix, transcription and translation are among the processes affected by PAK1.

AREAS COVERED IN THIS REVIEW

We discuss the mechanisms that control PAK1 activity, its involvement in physiological and pathophysiological processes, the benefits and the drawbacks of the current tools to regulate PAK1 activity, the evidence that suggests PAK1 as a therapeutic target and the likely directions of future research.

WHAT THE READER WILL GAIN

The reader will gain a better knowledge and understanding of the areas described above.

TAKE HOME MESSAGE

PAK1 is a promising therapeutic target in cancer and allergen-induced disorders. Its suitability as a target in vascular, neurological and infectious diseases remains ambiguous. Further advancement of this field requires progress on such issues as the development of specific and clinically acceptable inhibitors, the choice between targeting one or multiple PAK isoforms, elucidation of the individual roles of PAK1 targets and the mechanisms that may circumvent inhibition of PAK1.

Lowered albumin extravasation rate in heart but not in other organs in beta3-integrin-deficient mice

AIM

The vascular protein permeability is dependent on the integrity of the vascular wall. The heart capillaries in male mice lacking beta3 integrins have an immature phenotype. Previously, we have demonstrated a role for alphavbeta3 integrins in control of interstitial fluid pressure (Pif) and thereby in the fluid flux during inflammation. We wanted to explore a possible role for alphavbeta3 integrins in controlling capillary protein permeability during control situation and inflammation.

METHODS

We performed double-tracer and microdialysis experiments on beta3-integrin-deficient mice and wild type control mice. We also measured blood pressure and heart rate in the two mice strains.

RESULTS

We found reduced albumin extravasation (during 25 min) in the heart capillaries (0.053 +/- 0.003 vs. 0.087 +/- 0.009 mL g(-1) dw, P < 0.05), and an increased cardiac mass/body weight (5.3 x 10(-3) +/- 0.3 x 10(-3) vs. 3.8 x 10(-3) +/- 0.1 x 10(-3), P < 0.01) in the beta3-integrin-deficient mice (n = 6) compared with the controls (n = 6). Heart rate and blood pressure were the same in mice with and without beta3-integrins. No difference in permeability was found in other tissues studied, or under local inflammation.

CONCLUSION

These results show a function for the alphavbeta3 integrin in the regulation of protein permeability, selective for the heart capillaries.

Wnt/beta-catenin signaling is required for CNS, but not non-CNS, angiogenesis

Despite the importance of CNS blood vessels, the molecular mechanisms that regulate CNS angiogenesis and blood-brain barrier (BBB) formation are largely unknown. Here we analyze the role of Wnt/beta-catenin signaling in regulating the formation of CNS blood vessels. First, through the analysis of TOP-Gal Wnt reporter mice, we identify that canonical Wnt/beta-catenin signaling is specifically activated in CNS, but not non-CNS, blood vessels during development. This activation correlates with the expression of different Wnt ligands by neural progenitor cells in distinct locations throughout the CNS, including Wnt7a and Wnt7b in ventral regions and Wnt1, Wnt3, Wnt3a, and Wnt4 in dorsal regions. Blockade of Wnt/beta-catenin signaling in vivo specifically disrupts CNS, but not non-CNS, angiogenesis. These defects include reduction in vessel number, loss of capillary beds, and the formation of hemorrhagic vascular malformations that remain adherent to the meninges. Furthermore, we demonstrate that Wnt/beta-catenin signaling regulates the expression of the BBB-specific glucose transporter glut-1. Taken together these experiments reveal an essential role for Wnt/beta-catenin signaling in driving CNS-specific angiogenesis and provide molecular evidence that angiogenesis and BBB formation are in part linked.

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.

Nanoparticle-mediated drug delivery to tumor vasculature suppresses metastasis

Integrin alphanubeta3 is found on a subset of tumor blood vessels where it is associated with angiogenesis and malignant tumor growth. We designed an alphanubeta3-targeted nanoparticle (NP) encapsulating the cytotoxic drug doxorubicin (Dox) for targeted drug delivery to the alphanubeta3-expressing tumor vasculature. We observed real-time targeting of this NP to tumor vessels and noted selective apoptosis in regions of the alphanubeta3-expressing tumor vasculature. In clinically relevant pancreatic and renal cell orthotopic models of spontaneous metastasis, targeted delivery of Dox produced an antimetastatic effect. In fact, alphanubeta3-mediated delivery of this drug to the tumor vasculature resulted in a 15-fold increase in antimetastatic activity without producing drug-associated weight loss as observed with systemic administration of the free drug. These findings reveal that NP-based delivery of cytotoxic drugs to the alphanubeta3-positive tumor vasculature represents an approach for treating metastatic disease.

Endothelial-stromal interactions in angiogenesis

PURPOSE OF REVIEW

Angiogenesis often occurs in the context of a wound or tumor stroma. This review will focus on the recent findings on the interactions between angiogenic endothelial cells and the other components of the stroma - fibroblasts, pericytes and extracellular matrix.

RECENT FINDINGS

Large-scale gene expression arrays have provided a remarkable insight into the diversity of fibroblasts in different tissues and under different conditions. These somewhat neglected cells are now understood to play a critical role in tumor growth, regulating not only the phenotype of the tumor cells but also the angiogenic response that supports them. These advances are leading to an understanding of the soil and seed hypothesis at the molecular level. In addition, there is a new focus on the role of pericytes in regulating angiogenesis and their potential as targets for tumor therapy.

SUMMARY

Initiation of new blood vessel formation requires metalloproteinase induction leading to the degradation of the basement membrane, sprouting of endothelial cells and regulation of pericyte attachment. Fibroblasts and their activated counterpart, the myofibroblast, play a large role in synchronizing these events through the expression of numerous extracellular matrix molecules, growth factors and morphogens, including fibroblast growth factors and transforming growth factor beta.

Compensatory role for Pyk2 during angiogenesis in adult mice lacking endothelial cell FAK

Focal adhesion kinase (FAK) plays a critical role during vascular development because knockout of FAK in endothelial cells (ECs) is embryonic lethal. Surprisingly, tamoxifen-inducible conditional knockout of FAK in adult blood vessels (inducible EC–specific FAK knockout [i-EC-FAK-KO]) produces no vascular phenotype, and these animals are capable of developing a robust growth factor–induced angiogenic response. Although angiogenesis in wild-type mice is suppressed by pharmacological inhibition of FAK, i-EC-FAK-KO mice are refractory to this treatment, which suggests that adult i-EC-FAK-KO mice develop a compensatory mechanism to bypass the requirement for FAK. Indeed, expression of the FAK-related proline-rich tyrosine kinase 2 (Pyk2) is elevated and phosphorylated in i-EC-FAK-KO blood vessels. In cultured ECs, FAK knockdown leads to increased Pyk2 expression and, surprisingly, FAK kinase inhibition leads to increased Pyk2 phosphorylation. Pyk2 can functionally compensate for the loss of FAK because knockdown or pharmacological inhibition of Pyk2 disrupts angiogenesis in i-EC-FAK-KO mice. These studies reveal the adaptive capacity of ECs to switch to Pyk2-dependent signaling after deletion or kinase inhibition of FAK.