Mechanobiology and developmental control.

Morphogenesis is the remarkable process by which cells self-assemble into complex tissues and organs that exhibit specialized form and function during embryological development. Many of the genes and chemical cues that mediate tissue and organ formation have been identified; however, these signals alone are not sufficient to explain how tissues and organs are constructed that exhibit their unique material properties and three-dimensional forms. Here, we review work that has revealed the central role that physical forces and extracellular matrix mechanics play in the control of cell fate switching, pattern formation, and tissue development in the embryo and how these same mechanical signals contribute to tissue homeostasis and developmental control throughout adult life.

EPAS1 Attenuates Atherosclerosis Initiation at Disturbed Flow Sites Through Endothelial Fatty Acid Uptake.

BACKGROUND: Atherosclerotic plaques form unevenly due to disturbed blood flow, causing localized endothelial cell (EC) dysfunction. Obesity exacerbates this process, but the underlying molecular mechanisms are unclear. The transcription factor EPAS1 (HIF2A) has regulatory roles in endothelium, but its involvement in atherosclerosis remains unexplored. This study investigates the potential interplay between EPAS1, obesity, and atherosclerosis. METHODS: Responses to shear stress were analyzed using cultured porcine aortic EC exposed to flow in vitro coupled with metabolic and molecular analyses and by en face immunostaining of murine aortic EC exposed to disturbed flow in vivo. Obesity and dyslipidemia were induced in mice via exposure to a high-fat diet or through Leptin gene deletion. The role of Epas1 in atherosclerosis was evaluated by inducible endothelial Epas1 deletion, followed by hypercholesterolemia induction (adeno-associated virus-PCSK9 [proprotein convertase subtilisin/kexin type 9]; high-fat diet). RESULTS: En face staining revealed EPAS1 enrichment at sites of disturbed blood flow that are prone to atherosclerosis initiation. Obese mice exhibited substantial reduction in endothelial EPAS1 expression. Sulforaphane, a compound with known atheroprotective effects, restored EPAS1 expression and concurrently reduced plasma triglyceride levels in obese mice. Consistently, triglyceride derivatives (free fatty acids) suppressed EPAS1 in cultured EC by upregulating the negative regulator PHD2. Clinical observations revealed that reduced serum EPAS1 correlated with increased endothelial PHD2 and PHD3 in obese individuals. Functionally, endothelial EPAS1 deletion increased lesion formation in hypercholesterolemic mice, indicating an atheroprotective function. Mechanistic insights revealed that EPAS1 protects arteries by maintaining endothelial proliferation by positively regulating the expression of the fatty acid-handling molecules CD36 (cluster of differentiation 36) and LIPG (endothelial type lipase G) to increase fatty acid beta-oxidation. CONCLUSIONS: Endothelial EPAS1 attenuates atherosclerosis at sites of disturbed flow by maintaining EC proliferation via fatty acid uptake and metabolism. This endothelial repair pathway is inhibited in obesity, suggesting a novel triglyceride-PHD2 modulation pathway suppressing EPAS1 expression. These findings have implications for therapeutic strategies addressing vascular dysfunction in obesity.

Rap1A Modulates Store-Operated Calcium Entry in the Lung Endothelium: A Novel Mechanism Controlling NFAT-Mediated Vascular Inflammation and Permeability.

BACKGROUND: Store-operated calcium entry mediated by STIM (stromal interaction molecule)-1-Orai1 is essential in endothelial cell (EC) functions, affecting signaling, NFAT (nuclear factor for activated T cells)-induced transcription, and metabolic programs. While the small GTPase Rap1 isoforms, including the predominant Rap1B, are known for their role in cadherin-mediated adhesion, EC deletion of Rap1A after birth uniquely disrupts lung endothelial barrier function. Here, we elucidate the specific mechanisms by which Rap1A modulates lung vascular integrity and inflammation. METHODS: The role of EC Rap1A in lung inflammation and permeability was examined using in vitro and in vivo approaches. RESULTS: We explored Ca2+ signaling in human ECs following siRNA-mediated knockdown of Rap1A or Rap1B. Rap1A knockdown, unlike Rap1B, significantly increased store-operated calcium entry in response to a GPCR (G-protein-coupled receptor) agonist, ATP (500 µmol/L), or thapsigargin (250 nmol/L). This enhancement was attenuated by Orai1 channel blockers 10 µmol/L BTP2, 10 µmol/L GSK-7975A, and 5 µmol/L Gd3+. Whole-cell patch clamp measurements revealed enhanced Ca2+ release-activated Ca2+ current density in siRap1A ECs. Rap1A depletion in ECs led to increased NFAT1 nuclear translocation and activity and elevated levels of proinflammatory cytokines (CXCL1, CXCL11, CCL5 [chemokine (C-C motif) ligand 5], and IL-6 [interleukin-6]). Notably, reducing Orai1 expression in siRap1A ECs normalized store-operated calcium entry, NFAT activity, and endothelial hyperpermeability in vitro. EC-specific Rap1A knockout (Rap1AiΔEC) mice displayed an inflammatory lung phenotype with increased lung permeability and inflammation markers, along with higher Orai1 expression. Delivery of siRNA against Orai1 to lung endothelium using lipid nanoparticles effectively normalized Orai1 levels in lung ECs, consequently reducing hyperpermeability and inflammation in Rap1AiΔEC mice. CONCLUSIONS: Our findings uncover a novel role of Rap1A in regulating Orai1-mediated Ca2+ entry and expression, crucial for NFAT-mediated transcription and endothelial inflammation. This study distinguishes the unique function of Rap1A from that of the predominant Rap1B isoform and highlights the importance of normalizing Orai1 expression in maintaining lung vascular integrity and modulating endothelial functions.

Studying the Pulmonary Endothelium in Health and Disease: An Official American Thoracic Society Workshop Report.

Lung endothelium resides at the interface between the circulation and the underlying tissue, where it senses biochemical and mechanical properties of both the blood as it flows through the vascular circuit and the vessel wall. The endothelium performs the bidirectional signaling between the blood and tissue compartments that is necessary to maintain homeostasis while physically separating both, facilitating a tightly regulated exchange of water, solutes, cells, and signals. Disruption in endothelial function contributes to vascular disease, which can manifest in discrete vascular locations along the artery-to-capillary-to-vein axis. Although our understanding of mechanisms that contribute to endothelial cell injury and repair in acute and chronic vascular disease have advanced, pathophysiological mechanisms that underlie site-specific vascular disease remain incompletely understood. In an effort to improve the translatability of mechanistic studies of the endothelium, the American Thoracic Society convened a workshop to optimize rigor, reproducibility, and translation of discovery to advance our understanding of endothelial cell function in health and disease.

Molecular mapping of transmembrane mechanotransduction through the ß1 integrin-CD98hc-TRPV4 axis.

One of the most rapid (less than 4 ms) transmembrane cellular mechanotransduction events involves activation of transient receptor potential vanilloid 4 (TRPV4) ion channels by mechanical forces transmitted across cell surface ß1 integrin receptors on endothelial cells, and the transmembrane solute carrier family 3 member 2 (herein denoted CD98hc, also known as SLC3A2) protein has been implicated in this response. Here, we show that ß1 integrin, CD98hc and TRPV4 all tightly associate and colocalize in focal adhesions where mechanochemical conversion takes place. CD98hc knockdown inhibits TRPV4-mediated calcium influx induced by mechanical forces, but not by chemical activators, thus confirming the mechanospecificity of this signaling response. Molecular analysis reveals that forces applied to ß1 integrin must be transmitted from its cytoplasmic C terminus via the CD98hc cytoplasmic tail to the ankyrin repeat domain of TRPV4 in order to produce ultrarapid, force-induced channel activation within the focal adhesion.

Endothelial YAP1 in Regenerative Lung Growth through the Angiopoietin-Tie2 Pathway.

Angiogenesis, the formation of new blood capillaries, plays a key role in organ development and regeneration. Inhibition of lung angiogenesis through the blockade of angiogenic signaling pathways impairs compensatory and regenerative lung growth after unilateral pneumonectomy (PNX). The Hippo signaling transducer, Yes-associated protein (YAP) 1 binds to TEA domain transcription factor (TEAD) and controls organ size and regeneration. However, the role of endothelial YAP1 in lung vascular and alveolar morphogenesis remains unclear. In this report, we demonstrate that knockdown of YAP1 in endothelial cells (ECs) decreases angiogenic factor receptor Tie2 expression, and inhibits EC sprouting and epithelial cell budding in vitro and vascular and alveolar morphogenesis in the gel implanted on the mouse lung. The expression levels of YAP1, TEAD1, and Tie2 increase in ECs isolated from the remaining mouse lungs after unilateral PNX and vascular formation is stimulated in the post-PNX mouse lungs. Knockdown of endothelial YAP1 inhibits compensatory lung growth and vascular and alveolar morphogenesis after unilateral PNX. These findings suggest that endothelial YAP1 is required for lung vascular and alveolar regeneration and modulation of YAP1 in ECs may be novel interventions for the improvement of lung regeneration.

Twist1 in Hypoxia-induced Pulmonary Hypertension through Transforming Growth Factor-ß-Smad Signaling.

Pulmonary hypertension (PH) is a devastating pulmonary vascular disease characterized by aberrant muscularization of the normally nonmuscularized distal pulmonary arterioles. The expression of the transcription factor, Twist1, increases in the lungs of patients with pulmonary arterial hypertension. However, the mechanisms by which Twist1 controls the pathogenesis of PH remain unclear. It is becoming clear that endothelial-to-mesenchymal transition (EndMT) contributes to various vascular pathologies, including PH; Twist1 is known to mediate EndMT. In this report, we demonstrate that Twist1 overexpression increases transforming growth factor (TGF) ß receptor2 (TGF-ßR2) expression and Smad2 phosphorylation, and induces EndMT in cultured human pulmonary arterial endothelial (HPAE) cells, whereas a mutant construct of Twist1 at the serine 42 residue (Twist1S42A) fails to induce EndMT. We also implanted fibrin gel supplemented with HPAE cells on the mouse lung, and found that these HPAE cells form vascular structures and that Twist1-overexpressing HPAE cells undergo EndMT in the gel, whereas Twist1S42A-overexpressing cells do not. Furthermore, hypoxia-induced EndMT is inhibited in endothelial cells overexpressing Twist1S42A mutant construct in vitro. Hypoxia-induced accumulation of α-smooth muscle actin-positive cells in the pulmonary arterioles is attenuated in Tie2-specific Twist1 conditional knockout mice in vivo. These findings suggest that Twist1 serine 42 phosphorylation plays a key role in EndMT through TGF-ß signaling and that modulation of Twist1 phosphorylation could be an effective strategy for managing PH.

YAP1-TEAD1 signaling controls angiogenesis and mitochondrial biogenesis through PGC1α.

Mitochondria contribute to key processes of cellular function, while mitochondrial dysfunction is implicated in metabolic disorders, neurodegenerative diseases, and cardiovascular diseases, in which angiogenesis - the formation of new blood capillaries - is dysregulated. The Hippo signaling transducer, Yes-associated protein (YAP1) binds to the TEA domain (TEAD1) transcription factor and controls angiogenesis. YAP1 also regulates glucose metabolism through peroxisome proliferator-activated receptor gamma co-activator 1-alpha (PGC1α), a major player controlling mitochondrial biogenesis. However, the role of YAP1-TEAD1-PGC1α signaling in mitochondrial structure, cellular metabolism, and angiogenesis in endothelial cells (ECs) remains unclear. We now find that knockdown of TEAD1 decreases the expression of PGC1α and suppresses mitochondrial biogenesis, glycolysis, and oxygen consumption in ECs. A YAP1 mutant construct, YAP1S127A, which stimulates binding of YAP1 to TEAD1, upregulates the expression of PGC1α, induces mitochondrial biogenesis, and increases oxygen consumption and glycolytic flux in ECs; in contrast, YAP1S94A, which fails to bind to TEAD1, attenuates these effects. PGC1α knockdown inhibits YAP1S127A-induced EC sprouting in vitro and vascular morphogenesis in the fibrin gel subcutaneously implanted on mice, while overexpression of PGC1α reverses vascular morphogenesis suppressed by YAP1S94A. These results suggest that YAP1-TEAD1 signaling induces mitochondrial biogenesis in ECs and stimulates angiogenesis through PGC1α. Modulation of YAP1-TEAD1-PGC1α signaling in ECs may provide a novel intervention for angiogenesis-related diseases.

TWIST1 Integrates Endothelial Responses to Flow in Vascular Dysfunction and Atherosclerosis.

RATIONALE: Blood flow-induced shear stress controls endothelial cell (EC) physiology during atherosclerosis via transcriptional mechanisms that are incompletely understood. The mechanosensitive transcription factor TWIST is expressed during embryogenesis, but its role in EC responses to shear stress and focal atherosclerosis is unknown. OBJECTIVE: To investigate whether TWIST regulates endothelial responses to shear stress during vascular dysfunction and atherosclerosis and compare TWIST function in vascular development and disease. METHODS AND RESULTS: The expression and function of TWIST1 was studied in EC in both developing vasculature and during the initiation of atherosclerosis. In zebrafish, twist was expressed in early embryonic vasculature where it promoted angiogenesis by inducing EC proliferation and migration. In adult porcine and murine arteries, TWIST1 was expressed preferentially at low shear stress regions as evidenced by quantitative polymerase chain reaction and en face staining. Moreover, studies of experimental murine carotid arteries and cultured EC revealed that TWIST1 was induced by low shear stress via a GATA4-dependent transcriptional mechanism. Gene silencing in cultured EC and EC-specific genetic deletion in mice demonstrated that TWIST1 promoted atherosclerosis by inducing inflammation and enhancing EC proliferation associated with vascular leakiness. CONCLUSIONS: TWIST expression promotes developmental angiogenesis by inducing EC proliferation and migration. In addition to its role in development, TWIST is expressed preferentially at low shear stress regions of adult arteries where it promotes atherosclerosis by inducing EC proliferation and inflammation. Thus, pleiotropic functions of TWIST control vascular disease and development.

Bone marrow-on-a-chip replicates hematopoietic niche physiology in vitro.

Current in vitro hematopoiesis models fail to demonstrate the cellular diversity and complex functions of living bone marrow; hence, most translational studies relevant to the hematologic system are conducted in live animals. Here we describe a method for fabricating 'bone marrow-on-a-chip' that permits culture of living marrow with a functional hematopoietic niche in vitro by first engineering new bone in vivo, removing it whole and perfusing it with culture medium in a microfluidic device. The engineered bone marrow (eBM) retains hematopoietic stem and progenitor cells in normal in vivo-like proportions for at least 1 week in culture. eBM models organ-level marrow toxicity responses and protective effects of radiation countermeasure drugs, whereas conventional bone marrow culture methods do not. This biomimetic microdevice offers a new approach for analysis of drug responses and toxicities in bone marrow as well as for study of hematopoiesis and hematologic diseases in vitro.

Mechanotransduction of fluid stresses governs 3D cell migration.

Solid tumors are characterized by high interstitial fluid pressure, which drives fluid efflux from the tumor core. Tumor-associated interstitial flow (IF) at a rate of ∼3 µm/s has been shown to induce cell migration in the upstream direction (rheotaxis). However, the molecular biophysical mechanism that underlies upstream cell polarization and rheotaxis remains unclear. We developed a microfluidic platform to investigate the effects of IF fluid stresses imparted on cells embedded within a collagen type I hydrogel, and we demonstrate that IF stresses result in a transcellular gradient in ß1-integrin activation with vinculin, focal adhesion kinase (FAK), FAK(PY397), F actin, and paxillin-dependent protrusion formation localizing to the upstream side of the cell, where matrix adhesions are under maximum tension. This previously unknown mechanism is the result of a force balance between fluid drag on the cell and matrix adhesion tension and is therefore a fundamental, but previously unknown, stimulus for directing cell movement within porous extracellular matrix.

Role of Twist1 Phosphorylation in Angiogenesis and Pulmonary Fibrosis.

Idiopathic pulmonary fibrosis is a chronic and progressive lung disease in which microvessel remodeling is deregulated. However, the mechanism by which deregulated angiogenesis contributes to the pathogenesis of pulmonary fibrosis remains unclear. Here we show that a transcription factor, Twist1, controls angiogenesis through the angiopoietin-Tie2 pathway, and that deregulation of this mechanism mediates pathological angiogenesis and collagen deposition in a bleomycin-induced mouse pulmonary fibrosis model. Twist1 knockdown decreases Tie2 expression and attenuates endothelial cell sprouting in vitro. Angiogenesis is also inhibited in fibrin gel implanted on Tie2-specific Twist1 conditional knockout (Twist1fl/fl/Tie2-cre) mouse lung in vivo. Inhibition of Twist1 phosphorylation at the serine 42 (Ser42) residue by treating endothelial cells with a mutant construct (Twist1S42A) decreases Tie2 expression and attenuates angiogenesis compared with full-length Twist1 in vitro and in vivo. Bleomycin challenge up-regulates Twist1 Ser42 phosphorylation and Tie2 expression, increases blood vessel density, and induces collagen deposition in the mouse lung, whereas these effects are attenuated in Twist1fl/fl/Tie2-cre mice or in mice treated with Twist1S42A mutant construct. These results indicate that Twist1 Ser42 phosphorylation contributes to the pathogenesis of bleomycin-induced pulmonary fibrosis through angiopoietin-Tie2 signaling.

Acceleration of Lung Regeneration by Platelet-Rich Plasma Extract through the Low-Density Lipoprotein Receptor-Related Protein 5-Tie2 Pathway.

Angiogenesis, the growth of new blood vessels, plays a key role in organ development, homeostasis, and regeneration. The cooperation of multiple angiogenic factors, rather than a single factor, is required for physiological angiogenesis. Recently, we have reported that soluble platelet-rich plasma (PRP) extract, which contains abundant angiopoietin-1 and multiple other angiogenic factors, stimulates angiogenesis and maintains vascular integrity in vitro and in vivo. In this report, we have demonstrated that mouse PRP extract increases phosphorylation levels of the Wnt coreceptor low-density lipoprotein receptor-related protein 5 (LRP5) and thereby activates angiogenic factor receptor Tie2 in endothelial cells (ECs) and accelerates EC sprouting and lung epithelial cell budding in vitro. PRP extract also increases phosphorylation levels of Tie2 in the mouse lungs and accelerates compensatory lung growth and recovery of exercise capacity after unilateral pneumonectomy in mice, whereas soluble Tie2 receptor or Lrp5 knockdown attenuates the effects of PRP extract. Because human PRP extract is generated from autologous peripheral blood and can be stored at -80°C, our findings may lead to the development of novel therapeutic interventions for various angiogenesis-related lung diseases and to the improvement of strategies for lung regeneration.

Paxillin controls endothelial cell migration and tumor angiogenesis by altering neuropilin 2 expression.

Although a number of growth factors and receptors are known to control tumor angiogenesis, relatively little is known about the mechanism by which these factors influence the directional endothelial cell migration required for cancer microvessel formation. Recently, it has been shown that the focal adhesion protein paxillin is required for directional migration of fibroblasts in vitro. Here, we show that paxillin knockdown enhances endothelial cell migration in vitro and stimulates angiogenesis during normal development and in response to tumor angiogenic factors in vivo. Paxillin produces these effects by decreasing expression of neuropilin 2 (NRP2). Moreover, soluble factors secreted by tumors that stimulate vascular ingrowth, including vascular endothelial growth factor (VEGF), also decrease endothelial cell expression of paxillin and NRP2, and overexpression of NRP2 reverses these effects. These results suggest that the VEGF-paxillin-NRP2 pathway could represent a new therapeutic target for cancer and other angiogenesis-related diseases.

Matrix elasticity of void-forming hydrogels controls transplanted-stem-cell-mediated bone formation.

The effectiveness of stem cell therapies has been hampered by cell death and limited control over fate. These problems can be partially circumvented by using macroporous biomaterials that improve the survival of transplanted stem cells and provide molecular cues to direct cell phenotype. Stem cell behaviour can also be controlled in vitro by manipulating the elasticity of both porous and non-porous materials, yet translation to therapeutic processes in vivo remains elusive. Here, by developing injectable, void-forming hydrogels that decouple pore formation from elasticity, we show that mesenchymal stem cell (MSC) osteogenesis in vitro, and cell deployment in vitro and in vivo, can be controlled by modifying, respectively, the hydrogel's elastic modulus or its chemistry. When the hydrogels were used to transplant MSCs, the hydrogel's elasticity regulated bone regeneration, with optimal bone formation at 60 kPa. Our findings show that biophysical cues can be harnessed to direct therapeutic stem cell behaviours in situ.

Epoxyeicosanoids promote organ and tissue regeneration.

Epoxyeicosatrienoic acids (EETs), lipid mediators produced by cytochrome P450 epoxygenases, regulate inflammation, angiogenesis, and vascular tone. Despite pleiotropic effects on cells, the role of these epoxyeicosanoids in normal organ and tissue regeneration remains unknown. EETs are produced predominantly in the endothelium. Normal organ and tissue regeneration require an active paracrine role of the microvascular endothelium, which in turn depends on angiogenic growth factors. Thus, we hypothesize that endothelial cells stimulate organ and tissue regeneration via production of bioactive EETs. To determine whether endothelial-derived EETs affect physiologic tissue growth in vivo, we used genetic and pharmacological tools to manipulate endogenous EET levels. We show that endothelial-derived EETs play a critical role in accelerating tissue growth in vivo, including liver regeneration, kidney compensatory growth, lung compensatory growth, wound healing, corneal neovascularization, and retinal vascularization. Administration of synthetic EETs recapitulated these results, whereas lowering EET levels, either genetically or pharmacologically, delayed tissue regeneration, demonstrating that pharmacological modulation of EETs can affect normal organ and tissue growth. We also show that soluble epoxide hydrolase inhibitors, which elevate endogenous EET levels, promote liver and lung regeneration. Thus, our observations indicate a central role for EETs in organ and tissue regeneration and their contribution to tissue homeostasis.

Mesenchymal condensation-dependent accumulation of collagen VI stabilizes organ-specific cell fates during embryonic tooth formation.

BACKGROUND: Mechanical compression of cells during mesenchymal condensation triggers cells to undergo odontogenic differentiation during tooth organ formation in the embryo. However, the mechanism by which cell compaction is stabilized over time to ensure correct organ-specific cell fate switching remains unknown. RESULTS: Here, we show that mesenchymal cell compaction induces accumulation of collagen VI in the extracellular matrix (ECM), which physically stabilizes compressed mesenchymal cell shapes and ensures efficient organ-specific cell fate switching during tooth organ development. Mechanical induction of collagen VI deposition is mediated by signaling through the actin-p38MAPK-SP1 pathway, and the ECM scaffold is stabilized by lysyl oxidase in the condensing mesenchyme. Moreover, perturbation of synthesis or cross-linking of collagen VI alters the size of the condensation in vivo. CONCLUSIONS: These findings suggest that the odontogenic differentiation process that is induced by cell compaction during mesenchymal condensation is stabilized and sustained through mechanically regulated production of collagen VI within the mesenchymal ECM.

Platelet-rich plasma extract prevents pulmonary edema through angiopoietin-Tie2 signaling.

Increased vascular permeability contributes to life-threatening pathological conditions, such as acute respiratory distress syndrome. Current treatments for sepsis-induced pulmonary edema rely on low-tidal volume mechanical ventilation, fluid management, and pharmacological use of a single angiogenic or chemical factor with antipermeability activity. However, it is becoming clear that a combination of multiple angiogenic/chemical factors rather than a single factor is required for maintaining stable and functional blood vessels. We have demonstrated that mouse platelet-rich plasma (PRP) extract contains abundant angiopoietin (Ang) 1 and multiple other factors (e.g., platelet-derived growth factor), which potentially stabilize vascular integrity. Here, we show that PRP extract increases tyrosine phosphorylation levels of Tunica internal endothelial cell kinase (Tie2) and attenuates disruption of cell-cell junctional integrity induced by inflammatory cytokine in cultured human microvascular endothelial cells. Systemic injection of PRP extract also increases Tie2 phosphorylation in mouse lung and prevents endotoxin-induced pulmonary edema and the consequent decreases in lung compliance and exercise intolerance resulting from endotoxin challenge. Soluble Tie2 receptor, which inhibits Ang-Tie2 signaling, suppresses the ability of PRP extract to inhibit pulmonary edema in mouse lung. These results suggest that PRP extract prevents endotoxin-induced pulmonary edema mainly through Ang-Tie2 signaling, and PRP extract could be a potential therapeutic strategy for sepsis-induced pulmonary edema and various lung diseases caused by abnormal vascular permeability.

A mechanosensitive transcriptional mechanism that controls angiogenesis.

Angiogenesis is controlled by physical interactions between cells and extracellular matrix as well as soluble angiogenic factors, such as VEGF. However, the mechanism by which mechanical signals integrate with other microenvironmental cues to regulate neovascularization remains unknown. Here we show that the Rho inhibitor, p190RhoGAP (also known as GRLF1), controls capillary network formation in vitro in human microvascular endothelial cells and retinal angiogenesis in vivo by modulating the balance of activities between two antagonistic transcription factors, TFII-I (also known as GTF2I) and GATA2, that govern gene expression of the VEGF receptor VEGFR2 (also known as KDR). Moreover, this new angiogenesis signalling pathway is sensitive to extracellular matrix elasticity as well as soluble VEGF. This is, to our knowledge, the first known functional cross-antagonism between transcription factors that controls tissue morphogenesis, and that responds to both mechanical and chemical cues.

Netrin-1 promotes glioblastoma cell invasiveness and angiogenesis by multiple pathways including activation of RhoA, cathepsin B, and cAMP-response element-binding protein.

Glioblastomas are very difficult tumors to treat because they are highly invasive and disseminate within the normal brain, resulting in newly growing tumors. We have identified netrin-1 as a molecule that promotes glioblastoma invasiveness. As evidence, netrin-1 stimulates glioblastoma cell invasion directly through Matrigel-coated transwells, promotes tumor cell sprouting and enhances metastasis to lymph nodes in vivo. Furthermore, netrin-1 regulates angiogenesis as shown in specific angiogenesis assays such as enhanced capillary endothelial cells (EC) sprouting and by increased EC infiltration into Matrigel plugs in vivo, as does VEGF-A. This netrin-1 signaling pathway in glioblastoma cells includes activation of RhoA and cyclic AMP response element-binding protein (CREB). A novel finding is that netrin-1-induced glioblastoma invasiveness and angiogenesis are mediated by activated cathepsin B (CatB), a cysteine protease that translocates to the cell surface as an active enzyme and co-localizes with cell surface annexin A2 (ANXA2). The specific CatB inhibitor CA-074Me inhibits netrin-1-induced cell invasion, sprouting, and Matrigel plug angiogenesis. Silencing of CREB suppresses netrin-1-induced glioblastoma cell invasion, sprouting, and CatB expression. It is concluded that netrin-1 plays an important dual role in glioblastoma progression by promoting both glioblastoma cell invasiveness and angiogenesis in a RhoA-, CREB-, and CatB-dependent manner. Targeting netrin-1 pathways may be a promising strategy for brain cancer therapy.