Depletion of Arg/Abl2 improves endothelial cell adhesion and prevents vascular leak during inflammation

Endothelial barrier disruption and vascular leak importantly contribute to organ dysfunction and mortality during inflammatory conditions like sepsis and acute respiratory distress syndrome. We identified the kinase Arg/Abl2 as a mediator of endothelial barrier disruption, but the role of Arg in endothelial monolayer regulation and its relevance in vivo remain poorly understood. Here we show that depletion of Arg in endothelial cells results in the activation of both RhoA and Rac1, increased cell spreading and elongation, redistribution of integrin-dependent cell-matrix adhesions to the cell periphery, and improved adhesion to the extracellular matrix. We further show that Arg is activated in the endothelium during inflammation, both in murine lungs exposed to barrier-disruptive agents, and in pulmonary microvessels of septic patients. Importantly, Arg-depleted endothelial cells were less sensitive to barrier-disruptive agents. Despite the formation of F-actin stress fibers and myosin light chain phosphorylation, Arg depletion diminished adherens junction disruption and intercellular gap formation, by reducing the disassembly of cell-matrix adhesions and cell retraction. In vivo, genetic deletion of Arg diminished vascular leak in the skin and lungs, in the presence of a normal immune response. Together, our data indicate that Arg is a central and non-redundant regulator of endothelial barrier integrity, which contributes to cell retraction and gap formation by increasing the dynamics of adherens junctions and cell-matrix adhesions in a Rho GTPase-dependent fashion. Therapeutic inhibition of Arg may provide a suitable strategy for the treatment of a variety of clinical conditions characterized by vascular leak.

Nintedanib improves cardiac fibrosis but leaves pulmonary vascular remodelling unaltered in experimental pulmonary hypertension

Aims

Pulmonary arterial hypertension (PAH) is associated with increased levels of circulating growth factors and corresponding receptors such as platelet derived growth factor, fibroblast growth factor and vascular endothelial growth factor. Nintedanib, a tyrosine kinase inhibitor targeting primarily these receptors, is approved for the treatment of patients with idiopathic pulmonary fibrosis. Our objective was to examine the effect of nintedanib on proliferation of human pulmonary microvascular endothelial cells (MVEC) and assess its effects in rats with advanced experimental pulmonary hypertension (PH).

Methods and results

Proliferation was assessed in control and PAH MVEC exposed to nintedanib. PH was induced in rats by subcutaneous injection of Sugen (SU5416) and subsequent exposure to 10% hypoxia for 4 weeks (SuHx model). Four weeks after re-exposure to normoxia, nintedanib was administered once daily for 3 weeks. Effects of the treatment were assessed with echocardiography, right heart catheterization, and histological analysis of the heart and lungs. Changes in extracellular matrix production was assessed in human cardiac fibroblasts stimulated with nintedanib. Decreased proliferation with nintedanib was observed in control MVEC, but not in PAH patient derived MVEC. Nintedanib treatment did not affect right ventricular (RV) systolic pressure or total pulmonary resistance index in SuHx rats and had no effects on pulmonary vascular remodelling. However, despite unaltered pressure overload, the right ventricle showed less dilatation and decreased fibrosis, hypertrophy, and collagen type III with nintedanib treatment. This could be explained by less fibronectin production by cardiac fibroblasts exposed to nintedanib.

Conclusion

Nintedanib inhibits proliferation of pulmonary MVECs from controls, but not from PAH patients. While in rats with experimental PH nintedanib has no effects on the pulmonary vascular pathology, it has favourable effects on RV remodelling.

DLC1 is a direct target of activated YAP/TAZ that drives collective migration and sprouting angiogenesis

Endothelial YAP/TAZ (YAP is also known as YAP1, and TAZ as WWTR1) signaling is crucial for sprouting angiogenesis and vascular homeostasis. However, the underlying molecular mechanisms that explain how YAP/TAZ control the vasculature remain unclear. This study reveals that the focal adhesion protein deleted-in-liver-cancer 1 (DLC1) is a direct transcriptional target of the activated YAP/TAZ-TEAD complex. We find that substrate stiffening and VEGF stimuli promote expression of DLC1 in endothelial cells. In turn, DLC1 expression levels are YAP and TAZ dependent, and constitutive activation of YAP is sufficient to drive DLC1 expression. DLC1 is needed to limit F-actin fiber formation, integrin-based focal adhesion lifetime and integrin-mediated traction forces. Depletion of endothelial DLC1 strongly perturbs cell polarization in directed collective migration and inhibits the formation of angiogenic sprouts. Importantly, ectopic expression of DLC1 is sufficient to restore migration and angiogenic sprouting in YAP-depleted cells. Together, these findings point towards a crucial and prominent role for DLC1 in YAP/TAZ-driven endothelial adhesion remodeling and collective migration during angiogenesis.This article has an associated First Person interview with the first author of the paper.

A functional siRNA screen identifies RhoGTPase-associated genes involved in thrombin-induced endothelial permeability

Thrombin and other inflammatory mediators may induce vascular permeability through the disruption of adherens junctions between adjacent endothelial cells. If uncontrolled, hyperpermeability leads to an impaired barrier, fluid leakage and edema, which can contribute to multi-organ failure and death. RhoGTPases control cytoskeletal dynamics, adhesion and migration and are known regulators of endothelial integrity. Knowledge of the precise role of each RhoGTPase, and their associated regulatory and effector genes, in endothelial integrity is incomplete. Using a combination of a RNAi screen with electrical impedance measurements, we quantified the effect of individually silencing 270 Rho-associated genes on the barrier function of thrombin-activated, primary endothelial cells. Known and novel RhoGTPase-associated regulators that modulate the response to thrombin were identified (RTKN, TIAM2, MLC1, ARPC1B, SEPT2, SLC9A3R1, RACGAP1, RAPGEF2, RHOD, PREX1, ARHGEF7, PLXNB2, ARHGAP45, SRGAP2, ARHGEF5). In conclusion, with this siRNA screen, we confirmed the roles of known regulators of endothelial integrity but also identified new, potential key players in thrombin-induced endothelial signaling.

RhoA, RhoB and RhoC differentially regulate endothelial barrier function

RhoGTPases are known regulators of intracellular actin dynamics that are important for maintaining endothelial barrier function. RhoA is most extensively studied as a key regulator of endothelial barrier function, however the function of the 2 highly homologous family-members (> 88%) RhoB and RhoC in endothelial barrier function is still poorly understood.

This study aimed to determine whether RhoA, RhoB and RhoC have overlapping or distinct roles in barrier function and permeability in resting and activated endothelium. By using primary endothelial cells in combination with siRNA transfection to establish individual, double or triple knockdown of the RhoA/B/C RhoGTPases, we found that RhoB, but not RhoA or RhoC, is in resting endothelium a negative regulator of permeability. Loss of RhoB accounted for an accumulation of VE-cadherin at cell-cell contacts. Thrombin-induced loss of endothelial integrity is mediated primarily by RhoA and RhoB. Combined loss of RhoA/B showed decreased phosphorylation of Myosin Light Chain and increased expression of VE-cadherin at cell-cell contacts after thrombin stimulation. RhoC contributes to the Rac1-dependent restoration of endothelial barrier function.

In summary, this study shows that these highly homologous RhoGTPases differentially control the dynamics of endothelial barrier function.

Delayed Microvascular Shear Adaptation in Pulmonary Arterial Hypertension. Role of Platelet Endothelial Cell Adhesion Molecule-1 Cleavage

RATIONALE

Altered pulmonary hemodynamics and fluid flow-induced high shear stress (HSS) are characteristic hallmarks in the pathogenesis of pulmonary arterial hypertension (PAH). However, the contribution of HSS to cellular and vascular alterations in PAH is unclear.

OBJECTIVES

We hypothesize that failing shear adaptation is an essential part of the endothelial dysfunction in all forms of PAH and tested whether microvascular endothelial cells (MVECs) or pulmonary arterial endothelial cells (PAECs) from lungs of patients with PAH adapt to HSS and if the shear defect partakes in vascular remodeling in vivo.

METHODS

PAH MVEC (n = 7) and PAH PAEC (n = 3) morphology, function, protein, and gene expressions were compared with control MVEC (n = 8) under static culture conditions and after 24, 72, and 120 hours of HSS.

MEASUREMENTS AND MAIN RESULTS

PAH MVEC showed a significantly delayed morphological shear adaptation (P = 0.03) and evidence of cell injury at sites of nonuniform shear profiles that are critical loci for vascular remodeling in PAH. In clear contrast, PAEC isolated from the same PAH lungs showed no impairments. PAH MVEC gene expression and transcriptional shear activation were not altered but showed significant decreased protein levels (P = 0.02) and disturbed interendothelial localization of the shear sensor platelet endothelial cell adhesion molecule-1 (PECAM-1). The decreased PECAM-1 levels were caused by caspase-mediated cytoplasmic cleavage but not increased cell apoptosis. Caspase blockade stabilized PECAM-1 levels, restored endothelial shear responsiveness in vitro, and attenuated occlusive vascular remodeling in chronically hypoxic Sugen5416-treated rats modeling severe PAH.

CONCLUSIONS

Delayed shear adaptation, which promotes shear-induced endothelial injury, is a newly identified dysfunction specific to the microvascular endothelium in PAH. The shear response is normalized on stabilization of PECAM-1, which reverses intimal remodeling in vivo.

Traction force dynamics predict gap formation in activated endothelium

In many pathological conditions the endothelium becomes activated and dysfunctional, resulting in hyperpermeability and plasma leakage. No specific therapies are available yet to control endothelial barrier function, which is regulated by inter-endothelial junctions and the generation of acto-myosin-based contractile forces in the context of cell-cell and cell-matrix interactions. However, the spatiotemporal distribution and stimulus-induced reorganization of these integral forces remain largely unknown. Traction force microscopy of human endothelial monolayers was used to visualize contractile forces in resting cells and during thrombin-induced hyperpermeability. Simultaneously, information about endothelial monolayer integrity, adherens junctions and cytoskeletal proteins (F-actin) were captured. This revealed a heterogeneous distribution of traction forces, with nuclear areas showing lower and cell-cell junctions higher traction forces than the whole-monolayer average. Moreover, junctional forces were asymmetrically distributed among neighboring cells. Force vector orientation analysis showed a good correlation with the alignment of F-actin and revealed contractile forces in newly formed filopodia and lamellipodia-like protrusions within the monolayer. Finally, unstable areas, showing high force fluctuations within the monolayer were prone to form inter-endothelial gaps upon stimulation with thrombin. To conclude, contractile traction forces are heterogeneously distributed within endothelial monolayers and force instability, rather than force magnitude, predicts the stimulus-induced formation of intercellular gaps.

Using cultured endothelial cells to study endothelial barrier dysfunction: Challenges and opportunities

Despite considerable progress in the understanding of endothelial barrier regulation and the identification of approaches that have the potential to improve endothelial barrier function, no drug- or stem cell-based therapy is presently available to reverse the widespread vascular leak that is observed in acute respiratory distress syndrome (ARDS) and sepsis. The translational gap suggests a need to develop experimental approaches and tools that better mimic the complex environment of the microcirculation in which the vascular leak develops. Recent studies have identified several elements of this microenvironment. Among these are composition and stiffness of the extracellular matrix, fluid shear stress, interaction of endothelial cells (ECs) with pericytes, oxygen tension, and the combination of toxic and mechanic injurious stimuli. Development of novel cell culture techniques that integrate these elements would allow in-depth analysis of EC biology that closely approaches the (patho)physiological conditions in situ. In parallel, techniques to isolate organ-specific ECs, to define EC heterogeneity in its full complexity, and to culture patient-derived ECs from inducible pluripotent stem cells or endothelial progenitor cells are likely to advance the understanding of ARDS and lead to development of therapeutics. This review 1) summarizes the advantages and pitfalls of EC cultures to study vascular leak in ARDS, 2) provides an overview of elements of the microvascular environment that can directly affect endothelial barrier function, and 3) discusses alternative methods to bridge the gap between basic research and clinical application with the intent of improving the translational value of present EC culture approaches.

Intracellular RIG-I Signaling Regulates TLR4-Independent Endothelial Inflammatory Responses to Endotoxin

Sepsis is a systemic inflammatory response to infections associated with organ failure that is the most frequent cause of death in hospitalized patients. Exaggerated endothelial activation, altered blood flow, vascular leakage, and other disturbances synergistically contribute to sepsis-induced organ failure. The underlying signaling events associated with endothelial proinflammatory activation are not well understood, yet they likely consist of molecular pathways that act in an endothelium-specific manner. We found that LPS, a critical factor in the pathogenesis of sepsis, is internalized by endothelial cells, leading to intracellular signaling without the need for priming as found recently in immune cells. By identifying a novel role for retinoic acid-inducible gene-I (RIG-I) as a central regulator of endothelial activation functioning independent of TLR4, we provide evidence that the current paradigm of TLR4 solely being responsible for LPS-mediated endothelial responses is incomplete. RIG-I, as well as the adaptor protein mitochondrial antiviral signaling protein, regulates NF-κB-mediated induction of adhesion molecules and proinflammatory cytokine expression in response to LPS. Our findings provide essential new insights into the proinflammatory signaling pathways in endothelial cells and suggest that combined endothelial-specific inhibition of RIG-I and TLR4 will provide protection from aberrant endothelial responses associated with sepsis.

Transient Intervals of Hyper-Gravity Enhance Endothelial Barrier Integrity: Impact of Mechanical and Gravitational Forces Measured Electrically

Background

Endothelial cells (EC) guard vascular functions by forming a dynamic barrier throughout the vascular system that sensitively adapts to ‘classical’ biomechanical forces, such as fluid shear stress and hydrostatic pressure. Alterations in gravitational forces might similarly affect EC integrity, but remain insufficiently studied.

Methods

In an unique approach, we utilized Electric Cell-substrate Impedance Sensing (ECIS) in the gravity-simulators at the European Space Agency (ESA) to study dynamic responses of human EC to simulated micro- and hyper-gravity as well as to classical forces.

Results

Short intervals of micro- or hyper-gravity evoked distinct endothelial responses. Stimulated micro-gravity led to decreased endothelial barrier integrity, whereas hyper-gravity caused sustained barrier enhancement by rapid improvement of cell-cell integrity, evidenced by a significant junctional accumulation of VE-cadherin (p = 0.011), significant enforcement of peripheral F-actin (p = 0.008) and accompanied by a slower enhancement of cell-matrix interactions. The hyper-gravity triggered EC responses were force dependent and nitric-oxide (NO) mediated showing a maximal resistance increase of 29.2±4.8 ohms at 2g and 60.9±6.2 ohms at 4g vs. baseline values that was significantly suppressed by NO blockage (p = 0.011).

Conclusion

In conclusion, short-term application of hyper-gravity caused a sustained improvement of endothelial barrier integrity, whereas simulated micro-gravity weakened the endothelium. In clear contrast, classical forces of shear stress and hydrostatic pressure induced either short-lived or no changes to the EC barrier. Here, ECIS has proven a powerful tool to characterize subtle and distinct EC gravity-responses due to its high temporal resolution, wherefore ECIS has a great potential for the study of gravity-responses such as in real space flights providing quantitative assessment of a variety of cell biological characteristics of any adherent growing cell type in an automated and continuous fashion.

Globular adiponectin controls insulin-mediated vasoreactivity in muscle through AMPKα2

Decreased tissue perfusion increases the risk of developing insulin resistance and cardiovascular disease in obesity, and decreased levels of globular adiponectin (gAdn) have been proposed to contribute to this risk. We hypothesized that gAdn controls insulin's vasoactive effects through AMP-activated protein kinase (AMPK), specifically its α2 subunit, and studied the mechanisms involved. In healthy volunteers, we found that decreased plasma gAdn levels in obese subjects associate with insulin resistance and reduced capillary perfusion during hyperinsulinemia. In cultured human microvascular endothelial cells (HMEC), gAdn increased AMPK activity. In isolated muscle resistance arteries gAdn uncovered insulin-induced vasodilation by selectively inhibiting insulin-induced activation of ERK1/2, and the AMPK inhibitor compound C as well as genetic deletion of AMPKα2 blunted insulin-induced vasodilation. In HMEC deletion of AMPKα2 abolished insulin-induced Ser(1177) phosphorylation of eNOS. In mice we confirmed that AMPKα2 deficiency decreases insulin sensitivity, and this was accompanied by decreased muscle microvascular blood volume during hyperinsulinemia in vivo. This impairment was accompanied by a decrease in arterial Ser(1177) phosphorylation of eNOS, which closely related to AMPK activity. In conclusion, globular adiponectin controls muscle perfusion during hyperinsulinemia through AMPKα2, which determines the balance between NO and ET-1 activity in muscle resistance arteries. Our findings provide a novel mechanism linking reduced gAdn-AMPK signaling to insulin resistance and impaired organ perfusion.

ROCK2 primes the endothelium for vascular hyperpermeability responses by raising baseline junctional tension

Rho kinase mediates the effects of inflammatory permeability factors by increasing actomyosin-generated traction forces on endothelial adherens junctions, resulting in disassembly of intercellular junctions and increased vascular leakage. In vitro, this is accompanied by the Rho kinase-driven formation of prominent radial F-actin fibers, but the in vivo relevance of those F-actin fibers has been debated, suggesting other Rho kinase-mediated events to occur in vascular leak. Here, we delineated the contributions of the highly homologous isoforms of Rho kinase (ROCK1 and ROCK2) to vascular hyperpermeability responses. We show that ROCK2, rather than ROCK1 is the critical Rho kinase for regulation of thrombin receptor-mediated vascular permeability. Novel traction force mapping in endothelial monolayers, however, shows that ROCK2 is not required for the thrombin-induced force enhancements. Rather, ROCK2 is pivotal to baseline junctional tension as a novel mechanism by which Rho kinase primes the endothelium for hyperpermeability responses, independent from subsequent ROCK1-mediated contractile stress-fiber formation during the late phase of the permeability response.

Abstract 240: RhoGTPases and RNAi: A Large-scale Screen Using Ecis® to Measure Endothelial Integrity in Real-time

Introduction: Vascular leakage is a hallmark of many cardiovascular diseases and albeit its importance no specialized therapies are available to prevent or reduce it. RhoGTPases, their on/off switchers GEFs and GAPs and downstream effectors are known regulators of different cellular processes and studies have shown their positive and negative effects on the integrity of the endothelial barrier. Still, with data available on only a few family members, precise knowledge about this mechanism remains fragmentary. We hypothesize that there is a tight spatiotemporal regulation of a yet unknown combination of these regulatory proteins and that a systematic study of all members will provide more information on this mechanism. Using state-of-art RNAi and Electric Cell-substrate Impedance Sensing we measured the silencing effect of given genes on the endothelial barrier.

Methods: A selective RNAi library of 270 siRNAs targeting RhoGTPases, GAPs, GEFs and Effectors was ordered. HUVECs of 12 donors were isolated, pooled and plated on 96 wells ECIS arrays. Before reaching confluence, cells were transfected and 72h later stimulated with 1U/ml of thrombin. Positive and negative controls were carefully chosen and three independent screens were carried out on different pools of cells. Trans endothelial electrical resistance was measured in real-time and recorded throughout the whole procedure.

Results: Analysis of 3 critical time-points (72h post-transfection, direct thrombin response and recovery phase) confirmed already known key players such as Gef-H1, Larg or RhoA, which supports the reliability of this technique and its accuracy. However, it also revealed more than 10 new candidates involved in the regulation of the endothelial barrier. A few promoted a tighter endothelial barrier, some showed a protective effect upon thrombin challenge and others an intense disruptive response to the stimulus. Further characterization studies will be carried out and validation is ongoing.

Conclusions: Using a novel screening approach we identified previously unknown candidate regulators involved with endothelial barrier formation, dysruption and restoration. These new molecular targets provide opportunities for future pharmacological intervention.

Targeting Abl kinases to regulate vascular leak during sepsis and acute respiratory distress syndrome

The vascular endothelium separates circulating fluid and inflammatory cells from the surrounding tissues. Vascular leak occurs in response to wide-spread inflammatory processes, such as sepsis and acute respiratory distress syndrome, because of the formation of gaps between endothelial cells. Although these disorders are leading causes of mortality in the intensive care unit, no medical therapies exist to restore endothelial cell barrier function. Recent evidence highlights a key role for the Abl family of nonreceptor tyrosine kinases in regulating vascular barrier integrity. These kinases have well-described roles in cancer progression and neuronal morphogenesis, but their functions in the vasculature have remained enigmatic until recently. The Abl family kinases, c-Abl (Abl1) and Abl related gene (Arg, Abl2), phosphorylate several cytoskeletal effectors that mediate vascular permeability, including nonmuscle myosin light chain kinase, cortactin, vinculin, and β-catenin. They also regulate cell-cell and cell-matrix junction dynamics, and the formation of actin-based cellular protrusions in multiple cell types. In addition, both c-Abl and Arg are activated by hyperoxia and contribute to oxidant-induced endothelial cell injury. These numerous roles of Abl kinases in endothelial cells and the current clinical usage of imatinib and other Abl kinase inhibitors have spurred recent interest in repurposing these drugs for the treatment of vascular barrier dysfunction. This review will describe the structure and function of Abl kinases with an emphasis on their roles in mediating vascular barrier integrity. We will also provide a critical evaluation of the potential for exploiting Abl kinase inhibition as a novel therapy for inflammatory vascular leak syndromes.

Rac1 functions as a reversible tension modulator to stabilize VE-cadherin trans-interaction

The role of the RhoGTPase Rac1 in stabilizing mature endothelial adherens junctions (AJs) is not well understood. In this paper, using a photoactivatable probe to control Rac1 activity at AJs, we addressed the relationship between Rac1 and the dynamics of vascular endothelial cadherin (VE-cadherin). We demonstrated that Rac1 activation reduced the rate of VE-cadherin dissociation, leading to increased density of VE-cadherin at AJs. This response was coupled to a reduction in actomyosin-dependent tension across VE-cadherin adhesion sites. We observed that inhibiting myosin II directly or through photo-release of the caged Rho kinase inhibitor also reduced the rate of VE-cadherin dissociation. Thus, Rac1 functions by stabilizing VE-cadherin trans-dimers in mature AJs by counteracting the actomyosin tension. The results suggest a new model of VE-cadherin adhesive interaction mediated by Rac1-induced reduction of mechanical tension at AJs, resulting in the stabilization of VE-cadherin adhesions.

Abstract 179: Rho Kinase Isoform Contribution to Endothelial Contractility

Background: Rho kinase (ROCK) is the major effector protein of RhoA and is known to mediate F-actin stress fibers. ROCK activates myosin motors. Actomyosin-generated contractile forces can be transmitted to cell-cell junctions, resulting in increased junctional tension, junctional disassembly and endothelial hyperpermeability. In apparent contrast, basal ROCK activity is required for maintenance of barrier integrity . We hypothesize that these dual roles of ROCK could be contributed to the two highly homologous expressed isoforms.

Methods: Traction force microscopy was utilized to study the endothelium-generated contractile forces after depletion of ROCK1 and/or ROCK2 by SiRNA approach in the absence and presence of the vaso-active agent thrombin.

Results: The traction force landscape showed that ROCK is a major contributor to endothelial contraction and permeability. Depletion of ROCK1 and ROCK2 resulted in a blockade of contractile response to thrombin and a marked reduction in endothelial hyperpermeability. Detailed force distribution maps revealed that the absence of ROCK1, but not of ROCK2, resulted in an increase in baseline contractile forces (siROCK1: 88.1 ± 4.1 Pa, siROCK2: 53.6 ± 3.4 Pa, Control: 56.2 ± 6.1 Pa), which was associated with the formation of numerous inter-endothelial gaps upon stimulation with thrombin. Knockdown of ROCK2 stabilized the endothelial barrier and largely prevented traction force enhancements. Exposure to thrombin resulted in all conditions in an increase of traction forces and hyperpermeability.

Conclusion: Both ROCK isoforms have a distinct function in endothelial contractility and permeability. ROCK1 is predominantly contributing to barrier maintenance under baseline conditions, whereas ROCK2 mediates the thrombin-induced contractile force enhancements and subsequent barrier dysfunction.

Funding: Dutch Heart Foundation 2011T072

Abstract 530: Impaired Ability of Pulmonary Microvascular, but Not Macrovascular, Endothelial Cells From Patients With Pulmonary Arterial Hypertension to Respond to High Shear Stress

Rationale: We hypothesize that severe pulmonary vascular remodelling in pulmonary arterial hypertension (PAH) results from sustained endothelial injury caused by increased shear stress (SS) in pre-capillary endothelial cells. Therefore we tested whether human pulmonary arterial and microvascular endothelial cells (HPAEC and MVEC resp.) from PAH patients are capable to adapt to high levels of SS.

Methods & Results: HPMVEC were isolated from PAH patients and controls (lung tumor patients). Static controls were compared to cells under physiological low (LSS, 2.3 dyn/cm 2 ) and pathological high SS (HSS, up to 25 dyn/cm 2 ). Different flow profiles (laminar, bifurcation-like) were applied and electrical resistance of the monolayers was recorded as a measure of cell proliferation and barrier function. Under static culture conditions, PAH cells showed a decreased and irregular distribution and organization of VE-cadherin and CD31 at the cell periphery, indicating weakened adherens junctions. PAH and control HPMVEC formed similar endothelial barriers with a baseline resistance of 7767 vs. 8102 ohms, and responded to challenge with the hyper-permeability inducer thrombin (1 U/mL) with a specific maximal drop in resistance (to 2109 vs. 2083 ohms). Control HPMVEC re-aligned in flow direction, whereas HPMVEC, but not HPAEC from PAH-patients showed a delayed alignment response. Furthermore, application of HSS to control and PAH HPMVEC caused severe injury when bifurcation-like flow patterns were used. In contrast HPMVEC in laminar flow regions were able to withstand HSS challenge. Known shear-response genes were not affected (normal induction of KLF2, thrombomodulin, eNOS, SMAD6/7 and downregulation of MCP1). Protein analysis showed reduced expression of PECAM1 as part of a shear sensing complex (PECAM1/VEGFR2/VEcadherin) and its downstream signaling molecule Tiam1. Partial depletion of PECAM1 in control HPMEC mimicked the impaired realignment response.

Conclusion: HPMVEC, but HPAEC from PAH-patients have defects in the shear sensing complex accompanied by a hampered ability to adapt to pathological HSS, with severe endothelial damage at bifurcations.

Electric cell-substrate impedance sensing for the quantification of endothelial proliferation, barrier function, and motility

Electric Cell-substrate Impedance Sensing (ECIS) is an in vitro impedance measuring system to quantify the behavior of cells within adherent cell layers. To this end, cells are grown in special culture chambers on top of opposing, circular gold electrodes. A constant small alternating current is applied between the electrodes and the potential across is measured. The insulating properties of the cell membrane create a resistance towards the electrical current flow resulting in an increased electrical potential between the electrodes. Measuring cellular impedance in this manner allows the automated study of cell attachment, growth, morphology, function, and motility. Although the ECIS measurement itself is straightforward and easy to learn, the underlying theory is complex and selection of the right settings and correct analysis and interpretation of the data is not self-evident. Yet, a clear protocol describing the individual steps from the experimental design to preparation, realization, and analysis of the experiment is not available. In this article the basic measurement principle as well as possible applications, experimental considerations, advantages and limitations of the ECIS system are discussed. A guide is provided for the study of cell attachment, spreading and proliferation; quantification of cell behavior in a confluent layer, with regard to barrier function, cell motility, quality of cell-cell and cell-substrate adhesions; and quantification of wound healing and cellular responses to vasoactive stimuli. Representative results are discussed based on human microvascular (MVEC) and human umbilical vein endothelial cells (HUVEC), but are applicable to all adherent growing cells.

Nitrosation-dependent caveolin 1 phosphorylation, ubiquitination, and degradation and its association with idiopathic pulmonary arterial hypertension

In the present study, we tested the hypothesis that chronic inflammation and oxidative/nitrosative stress induce caveolin 1 (Cav-1) degradation, providing an underlying mechanism of endothelial cell activation/dysfunction and pulmonary vascular remodeling in patients with idiopathic pulmonary arterial hypertension (IPAH). We observed reduced Cav-1 protein despite increased Cav-1 messenger RNA expression and also endothelial nitric oxide synthase (eNOS) hyperphosphorylation in human pulmonary artery endothelial cells (PAECs) from patients with IPAH. In control human lung endothelial cell cultures, tumor necrosis factor α-induced nitric oxide (NO) production and S-nitrosation (SNO) of Cav-1 Cys-156 were associated with Src displacement and activation, Cav-1 Tyr-14 phosphorylation, and destabilization of Cav-1 oligomers within 5 minutes that could be blocked by eNOS or Src inhibition. Prolonged stimulation (72 hours) with NO donor DETANONOate reduced oligomerized and total Cav-1 levels by 40%-80%, similar to that observed in IPAH patient-derived PAECs. NO donor stimulation of endothelial cells for >72 hours, which was associated with sustained Src activation and Cav-1 phosphorylation, ubiquitination, and degradation, was blocked by NOS inhibitor L-NAME, Src inhibitor PP2, and proteosomal inhibitor MG132. Thus, chronic inflammation, sustained eNOS and Src signaling, and Cav-1 degradation may be important causal factors in the development of IPAH by promoting PAEC dysfunction/activation via sustained oxidative/nitrosative stress.

Localized RhoA GTPase activity regulates dynamics of endothelial monolayer integrity

AIMS

Endothelial cells (ECs) control vascular permeability by forming a monolayer that is sealed by extracellular junctions. Various mediators modulate the endothelial barrier by acting on junctional protein complexes and the therewith connected F-actin cytoskeleton. Different Rho GTPases participate in this modulation, but their mechanisms are still partly resolved. Here, we aimed to elucidate whether the opening and closure of the endothelial barrier are associated with distinct localized RhoA activities at the subcellular level.

METHODS AND RESULTS

Live fluorescence resonance energy transfer (FRET) microscopy revealed spatially distinct RhoA activities associated with different aspects of the regulation of endothelial monolayer integrity. Unstimulated ECs were characterized by hotspots of RhoA activity at their periphery. Thrombin receptor activation in the femoral vein of male wistar rats and in cultured ECs enhanced RhoA activity at membrane protrusions, followed by a more sustained RhoA activity associated with cytoplasmic F-actin filaments, where prolonged RhoA activity coincided with cellular contractility. Unexpectedly, thrombin-induced peripheral RhoA hotspots were not spatially correlated to the formation of large inter-endothelial gaps. Rather, spontaneous RhoA activity at membrane protrusions coincided with the closure of inter-endothelial gaps. Electrical impedance measurements showed that RhoA signalling is essential for this protrusive activity and maintenance of barrier restoration.

CONCLUSION

Spontaneous RhoA activity at membrane protrusions is spatially associated with closure, but not formation of inter-endothelial gaps, whereas RhoA activity at distant contractile filaments contributes to thrombin-induced disruption of junctional integrity. Thus, these data indicate that distinct RhoA activities are associated with disruption and re-annealing of endothelial junctions.

Activated protein C in the treatment of acute lung injury and acute respiratory distress syndrome

BACKGROUND

Acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) frequently necessitate mechanical ventilation in the intensive care unit. The syndromes have a high mortality rate and there is at present no treatment specifically directed at the underlying pathogenesis. Central in the pathophysiology of ALI/ARDS is alveolocapillary inflammation leading to permeability edema. As a result of the crosstalk between inflammation and coagulation, activation of proinflammatory and procoagulant/antifibrinolytic pathways contributes to disruption of the endothelial barrier. Protein C (PC) plays a central role in maintaining the equilibrium between coagulation and inflammation. Additionally, natural anticoagulants, such as PC, are depleted, both in blood as well as in the lung. Therefore, the PC system is of interest as a therapeutic target in patients with ALI/ARDS.

METHOD

This review is based on a Medline search of relevant basic and clinical studies.

OBJECTIVE

It discusses the potential role of activated PC in modulating the proinflammatory/procoagulant state for enhancing endothelial barrier function in animal models and human ALI/ARDS.

Effective treatment of edema and endothelial barrier dysfunction with imatinib

BACKGROUND

Tissue edema and endothelial barrier dysfunction as observed in sepsis and acute lung injury carry high morbidity and mortality, but currently lack specific therapy. In a recent case report, we described fast resolution of pulmonary edema on treatment with the tyrosine kinase inhibitor imatinib through an unknown mechanism. Here, we explored the effect of imatinib on endothelial barrier dysfunction and edema formation.

METHODS AND RESULTS

We evaluated the effect of imatinib on endothelial barrier function in vitro and in vivo. In human macro- and microvascular endothelial monolayers, imatinib attenuated endothelial barrier dysfunction induced by thrombin and histamine. Small interfering RNA knock-downs of the imatinib-sensitive kinases revealed that imatinib attenuates endothelial barrier dysfunction via inhibition of Abl-related gene kinase (Arg/Abl2), a previously unknown mediator of endothelial barrier dysfunction. Indeed, Arg was activated by endothelial stimulation with thrombin, histamine, and vascular endothelial growth factor. Imatinib limited Arg-mediated endothelial barrier dysfunction by enhancing Rac1 activity and enforcing adhesion of endothelial cells to the extracellular matrix. Using mouse models of vascular leakage as proof-of-concept, we found that pretreatment with imatinib protected against vascular endothelial growth factor-induced vascular leakage in the skin, and effectively prevented edema formation in the lungs. In a murine model of sepsis, imatinib treatment (6 hours and 18 hours after induction of sepsis) attenuated vascular leakage in the kidneys and the lungs (24 hours after induction of sepsis).

CONCLUSIONS

Thus, imatinib prevents endothelial barrier dysfunction and edema formation via inhibition of Arg. These findings identify imatinib as a promising approach to permeability edema and indicate Arg as novel target for edema treatment.

Direct grafting of RGD-motif-containing peptide on the surface of polycaprolactone films

Direct surface modification of biodegradable polycaprolactone (PCL) was performed without the necessity of synthesis of functionisable co-polymers. An easy-to-perform three-step procedure consisting of amination, reaction with hetero-bifunctional cross-linkers and conjugation of an RGD-motif-containing peptide was used to modify polymer films and improve the attachment of endothelial cells. The biological activity of modified surfaces was assessed by estimating microvascular endothelial cell attachment. Covalent coating with RGD resulted in an approximately 11-fold increase of endothelial cell attachment on modified PCL surfaces compared with untreated polymer. The specificity of the attachment enhancement was confirmed by using a control peptide. It is concluded that chemical surface modification is an appropriate method of rendering degradable polymers, such as PCL, cell-adhesive.

Nitric oxide-dependent Src activation and resultant caveolin-1 phosphorylation promote eNOS/caveolin-1 binding and eNOS inhibition

The mechanism of caveolin-1–dependent eNOS inactivation is not clear. These studies reveal that NO-mediated Src kinase activation and caveolin-1 phosphorylation promote eNOS binding and inactivation, that is, eNOS negative feedback regulation.

Endothelial nitric oxide synthase (eNOS)–mediated NO production plays a critical role in the regulation of vascular function and pathophysiology. Caveolin-1 (Cav-1) binding to eNOS holds eNOS in an inactive conformation; however, the mechanism of Cav-1–mediated inhibition of activated eNOS is unclear. Here the role of Src-dependent Cav-1 phosphorylation in eNOS negative feedback regulation is investigated. Using fluorescence resonance energy transfer (FRET) and coimmunoprecipitation analyses, we observed increased interaction between eNOS and Cav-1 following stimulation of endothelial cells with thrombin, vascular endothelial growth factor, and Ca²⁺ ionophore A23187, which is corroborated in isolated perfused mouse lung. The eNOS/Cav-1 interaction is blocked by eNOS inhibitor l-N^G-nitroarginine methyl ester (hydrochloride) and Src kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo [3, 4-d] pyrimidine. We also observe increased binding of phosphomimicking Y14D-Cav-1 mutant transduced in human embryonic kidney cells overexpressing eNOS and reduced Ca²⁺-induced NO production compared to cells expressing the phosphodefective Y14F-Cav-1 mutant. Finally, Src FRET biosensor, eNOS small interfering RNA, and NO donor studies demonstrate NO-induced Src activation and Cav-1 phosphorylation at Tyr-14, resulting in increased eNOS/Cav-1 interaction and inhibition of eNOS activity. Taken together, these data suggest that activation of eNOS promotes Src-dependent Cav-1–Tyr-14 phosphorylation and eNOS/Cav-1 binding, that is, eNOS feedback inhibition.

Opposing effects of the angiopoietins on the thrombin-induced permeability of human pulmonary microvascular endothelial cells

Background

Angiopoietin-2 (Ang-2) is associated with lung injury in ALI/ARDS. As endothelial activation by thrombin plays a role in the permeability of acute lung injury and Ang-2 may modulate the kinetics of thrombin-induced permeability by impairing the organization of vascular endothelial (VE-)cadherin, and affecting small Rho GTPases in human pulmonary microvascular endothelial cells (HPMVECs), we hypothesized that Ang-2 acts as a sensitizer of thrombin-induced hyperpermeability of HPMVECs, opposed by Ang-1.

Methodology/Principal Findings

Permeability was assessed by measuring macromolecule passage and transendothelial electrical resistance (TEER). Angiopoietins did not affect basal permeability. Nevertheless, they had opposing effects on the thrombin-induced permeability, in particular in the initial phase. Ang-2 enhanced the initial permeability increase (passage, P = 0.010; TEER, P = 0.021) in parallel with impairment of VE-cadherin organization without affecting VE-cadherin Tyr685 phosphorylation or increasing RhoA activity. Ang-2 also increased intercellular gap formation. Ang-1 preincubation increased Rac1 activity, enforced the VE-cadherin organization, reduced the initial thrombin-induced permeability (TEER, P = 0.027), while Rac1 activity simultaneously normalized, and reduced RhoA activity at 15 min thrombin exposure (P = 0.039), but not at earlier time points. The simultaneous presence of Ang-2 largely prevented the effect of Ang-1 on TEER and macromolecule passage.

Conclusions/Significance

Ang-1 attenuated thrombin-induced permeability, which involved initial Rac1 activation-enforced cell-cell junctions, and later RhoA inhibition. In addition to antagonizing Ang-1, Ang-2 had also a direct effect itself. Ang-2 sensitized the initial thrombin-induced permeability accompanied by destabilization of VE-cadherin junctions and increased gap formation, in the absence of increased RhoA activity.

Substrate stiffening promotes endothelial monolayer disruption through enhanced physical forces

A hallmark of many, sometimes life-threatening, inflammatory diseases and disorders is vascular leakage. The extent and severity of vascular leakage is broadly mediated by the integrity of the endothelial cell (EC) monolayer, which is in turn governed by three major interactions: cell-cell and cell-substrate contacts, soluble mediators, and biomechanical forces. A potentially critical but essentially uninvestigated component mediating these interactions is the stiffness of the substrate to which the endothelial monolayer is adherent. Accordingly, we investigated the extent to which substrate stiffening influences endothelial monolayer disruption and the role of cell-cell and cell-substrate contacts, soluble mediators, and physical forces in that process. Traction force microscopy showed that forces between cell and cell and between cell and substrate were greater on stiffer substrates. On stiffer substrates, these forces were substantially enhanced by a hyperpermeability stimulus (thrombin, 1 U/ml), and gaps formed between cells. On softer substrates, by contrast, these forces were increased far less by thrombin, and gaps did not form between cells. This stiffness-dependent force enhancement was associated with increased Rho kinase activity, whereas inhibition of Rho kinase attenuated baseline forces and lessened thrombin-induced inter-EC gap formation. Our findings demonstrate a central role of physical forces in EC gap formation and highlight a novel physiological mechanism. Integrity of the endothelial monolayer is governed by its physical microenvironment, which in normal circumstances is compliant but during pathology becomes stiffer.

A plethora of angiopoietin-2 effects during clinical sepsis

The interesting study by Davis and colleagues in the current issue of Critical Care expands on the increasingly recognized role of angiopoietins in human sepsis but raises a number of questions, which are discussed in this commentary. The authors describe an association between elevated angiopoietin (ang)-2 levels and impaired vascular reactivity, measured by the partly nitric oxide-dependent finger hyperemic response to forearm vascular occlusion, in patients with sepsis. This suggests that the ang-1/2-Tie2 system is involved in a number of pathophysiologic, phenotypic and perhaps prognostic alterations in human sepsis, on top of the effect on pulmonary endothelial barrier function. The novel inflammatory route may be a target for future therapeutic studies in human sepsis and acute lung injury, including those with activated protein C.

The interaction of soluble Tie2 with angiopoietins and pulmonary vascular permeability in septic and nonseptic critically ill patients

Circulating angiopoietin (Ang) 1 may inhibit and Ang-2 may enhance pulmonary vascular permeability in septic and nonseptic patients with or at risk for acute lung injury or acute respiratory distress syndrome. We hypothesized that the soluble form of the Ang-binding Tie2 receptor (sTie2), whose shedding may be induced by vascular endothelial growth factor (VEGF) levels, may bind circulating Angs and thereby inhibit their effects on pulmonary vascular permeability. In 24 septic and 40 nonseptic mechanically ventilated patients, sTie2, Ang-1, Ang-2, and VEGF plasma levels were measured together with the pulmonary leak index (PLI) for (67)Gallium-labeled transferrin as a measure of pulmonary vascular permeability. Soluble Tie2 and VEGF levels correlated (r = 0.53, P = 0.001). Soluble Tie2 was higher in septic than in nonseptic patients (7.43 [6.57 - 8.40] vs. 5.03 [4.57 - 5.54] ng/mL; P < 0.001). Soluble Tie2 was associated with the PLI (standardized regression coefficient [beta] = 0.26; P = 0.006) but lost its association with the PLI when the Angs were included in a multivariate model. Soluble Tie2 did not affect the association between Ang-1 or Ang-2 and the PLI (beta = -0.39, P < 0.001; beta = 0.52, P < 0.001, respectively), independently of underlying disease. Although limited to correlations and associations, the clinical data support in vivo shedding of sTie2 through VEGF signaling upon pulmonary vascular injury. However, this shedding may not prevent a direct role of Angs in pulmonary vascular permeability.

Role of protein kinase Czeta in thrombin-induced RhoA activation and inter-endothelial gap formation of human dermal microvessel endothelial cell monolayers

We studied the potential involvement of the Ca(2+)-independent atypical protein kinase C isoform PKCzeta in mediating the thrombin-induced increase in endothelial permeability. Studies were done using human dermal microvessel endothelial cells (HMEC), which we showed constitutively expressed PKCzeta. We quantified the patency of inter-endothelial junctions (IEJs) and endothelial barrier function by measuring transendothelial electrical resistance (TER) in confluent HMEC monolayers. In control monolayers, thrombin decreased TER by approximately 50%, indicating thrombin-dependent opening of IEJs. Thrombin also elicited increases in cytosolic Ca(2+) concentration [Ca(2+)](i), actin stress fiber formation, and myosin light chain (MLC) phosphorylation. Pan-PKC inhibitors, calphostin C and chelerythrine, abrogated these responses. Thrombin also decreased TER after depletion of conventional and novel Ca(2+)-dependent PKC isoforms using phorbol 12-myristate 13-acetate (PMA). In these PMA-treated cells, thrombin induced inter-endothelial gap formation, MLC phosphorylation, and actin stress fiber formation, but failed to increase [Ca(2+)](i). Inhibition of PKCzeta activation using the PKCzeta pseudosubstrate peptide (PSI), depletion of PKCzeta protein with siRNA, and competitive inhibition of PKCzeta activity using dominant-negative (dn) PKCzeta mutant all prevented the thrombin-induced decrease in TER and MLC phosphorylation. Expression of dn-PKCzeta also inhibited thrombin-induced RhoA activation. These findings reveal a novel Ca(2+)-independent, PKCzeta-dependent mechanism of thrombin-induced increase in endothelial permeability. The results raise the possibility that inhibition of PKCzeta may be a novel drug target for thrombin-induced inflammatory hyperpermeability.

Activation of AMP-activated protein kinase by 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside in the muscle microcirculation increases nitric oxide synthesis and microvascular perfusion

OBJECTIVE

To investigate the effects of activation of the AMP-activated protein kinase (AMPK) on muscle perfusion and to elucidate the mechanisms involved.

METHODS AND RESULTS

In a combined approach, we studied the vasoactive actions of AMPK activator by 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) on rat cremaster muscle resistance arteries ( approximately 100 mum) ex vivo and on microvascular perfusion in the rat hindlimb in vivo. In isolated resistance arteries, AICAR increased Thr172 phosphorylation of AMPK in arteriolar endothelium, which was predominantly located in microvascular endothelium. AICAR induced vasodilation (19+/-4% at 2 mmol/L, P<0.01), which was abolished by endothelium removal, inhibition of NO synthase (with N-nitro-L-arginine), or AMPK (with compound C). Smooth muscle sensitivity to NO, determined by studying the effects of the NO donor S-nitroso-N-acetylpenicillamine (SNAP), was not affected by AICAR except at the highest dose. AICAR increased endothelial nitric oxide synthase activity, as indicated by Ser1177 phosphorylation. In vivo, infusion of AICAR markedly increased muscle microvascular blood volume (approximately 60%, P<0.05), as was evidenced by contrast-enhanced ultrasound, without effects on blood pressure, femoral blood flow, or hind leg glucose uptake.

CONCLUSIONS

Activation of AMPK by AICAR activates endothelial nitric oxide synthase in arteriolar endothelium by increasing its Ser1177 phosphorylation, which leads to vasodilation of resistance arteries and recruitment of microvascular perfusion in muscle.

Driving Rho GTPase activity in endothelial cells regulates barrier integrity

In the past decade understanding of the role of the Rho GTPases RhoA, Rac1 and Cdc42 has been developed from regulatory proteins that regulate specific actin cytoskeletal structures - stress fibers, lamellipodia and filopodia - to complex integrators of cytoskeletal structures that can exert multiple functions depending on the cellular context. Fundamental to these functions are three-dimensional complexes between the individual Rho GTPases, their specific activators (GEFs) and inhibitors (GDIs and GAPs), which greatly outnumber the Rho GTPases themselves, and additional regulatory proteins. By this complexity of regulation different vasoactive mediators can induce various cytoskeletal structures that enable the endothelial cell (EC) to respond adequately. In this review we have focused on this complexity and the consequences of Rho GTPase regulation for endothelial barrier function. The permeability inducers thrombin and VEGF are presented as examples of G-protein coupled receptor- and tyrosine kinase receptor-mediated Rho GTPase activation, respectively. These mediators induce complex but markedly different networks of activators, inhibitors and effectors of Rho GTPases, which alter the endothelial barrier function. An interesting feature in this regulation is that Rho GTPases often have both barrier-protecting and barrier-disturbing functions. While Rac1 enforces the endothelial junctions, it becomes part of a barrier-disturbing mechanism as activator of reactive oxygen species generating NADPH oxidase. Similarly RhoA is protective under basal conditions, but becomes involved in barrier dysfunction after activation of ECs by thrombin. The challenge and promise lies in unfolding this complex regulation, as this will provide leads for new therapeutic opportunities.

Circulating angiopoietin-2 levels in the course of septic shock: relation with fluid balance, pulmonary dysfunction and mortality

PURPOSE

To investigate whether angiopoietin-2, von Willebrand factor (VWF) and angiopoietin-1 relate to surrogate indicators of vascular permeability, pulmonary dysfunction and intensive care unit (ICU) mortality throughout the course of septic shock.

METHODS

In 50 consecutive mechanically ventilated septic shock patients, plasma angiopoietin-2, VWF and angiopoietin-1 levels and fluid balance, partial pressure of oxygen/inspiratory oxygen fraction and the oxygenation index as indicators of vascular permeability and pulmonary dysfunction, respectively, were measured until day 28.

RESULTS

Angiopoietin-2 positively related to the fluid balance and pulmonary dysfunction, was higher in non-survivors than in survivors and independently predicted non-survival throughout the course of septic shock. VWF inversely related to the fluid balance and pulmonary dysfunction throughout the course of septic shock, was comparable between survivors and non-survivors and predicted non-survival on day 0 only. Angiopoietin-1 positively related to pulmonary dysfunction throughout the course, but did not differ between survivors and non-survivors.

CONCLUSIONS

In contrast to VWF, plasma angiopoietin-2 positively relates to fluid balance, pulmonary dysfunction and mortality throughout the course of septic shock, in line with a suggested mediator role of the protein.

Endothelin receptor blockade combined with phosphodiesterase-5 inhibition increases right ventricular mitochondrial capacity in pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is often treated with endothelin (ET) receptor blockade or phosphodiesterase-5 (PDE5) inhibition. Little is known about the specific effects on right ventricular (RV) function and metabolism. We determined the effects of single and combination treatment with Bosentan [an ET type A (ET(A))/type B (ET(B)) receptor blocker] and Sildenafil (a PDE5 inhibitor) on RV function and oxidative metabolism in monocrotaline (MCT)-induced PAH. Fourteen days after MCT injection, male Wistar rats were orally treated for 10 days with Bosentan, Sildenafil, or both. RV catheterization and echocardiography showed that MCT clearly induced PAH. This was evidenced by increased RV systolic pressure, reduced cardiac output, increased pulmonary vascular resistance (PVR), and reduced RV fractional shortening. Quantitative histochemistry showed marked RV hypertrophy and fibrosis. Monotreatment with Bosentan or Sildenafil had no effect on RV systolic pressure or cardiac function, but RV fibrosis was reduced and RV capillarization increased. Combination treatment did not reduce RV systolic pressure, but significantly lowered PVR, and normalized cardiac output, RV fractional shortening, and fibrosis. Only combination treatment increased the mitochondrial capacity of the RV, as reflected by increased succinate dehydrogenase and cytochrome c oxidase activities, associated with an activation of PKG, as indicated by increased VASP phosphorylation. Moreover, significant interactions were found between Bosentan and Sildenafil on PVR, cardiac output, RV contractility, PKG activity, and mitochondrial capacity. These data indicate that the combination of Bosentan and Sildenafil may beneficially contribute to RV adaptation in PAH, not only by reducing PVR but also by acting on the mitochondria in the heart.

RhoA-ROCK signaling is involved in contraction-mediated inhibition of SERCA2a expression in cardiomyocytes

In neonatal ventricular cardiomyocytes (NVCM), decreased contractile activity stimulates sarco-endoplasmic reticulum Ca(2+)-ATPase2a (SERCA2a), analogous to reduced myocardial load in vivo. This study investigated in contracting NVCM the role of load-dependent RhoA-ROCK signaling in SERCA2a regulation. Contractile arrest of NVCM resulted in low peri-nuclear localized RhoA levels relative to contracting NVCM. In arrested NVCM, ROCK activity was decreased (59%) and paralleled a loss in F-actin levels. Y-27632-induced ROCK inhibition in contracting NVCM increased SERCA2a messenger RNA expression by 150%. This stimulation was transcriptional, as evident from transfections with the SERCA2a promoter. A reciprocal effect of Y-27632 treatment on the promoter activity of atrial natriuretic factor was observed. SERCA2a transcription was not altered by co-transfection of the RhoA-ROCK-dependent serum response factor (SRF) alone or in combination with myocardin. Furthermore, GATA4, another ROCK-dependent transcription factor, induced rather than repressed SERCA2a transcription. This study shows that contractile activity suppresses SERCA2a gene expression via RhoA-ROCK-dependent transcription modulation. This modulation is likely to be accomplished by a transcription factor other than SRF, myocardin, or GATA4.

The angiopoietin-Tie2 system as a therapeutic target in sepsis and acute lung injury

BACKGROUND

Sepsis and acute lung injury (ALI)/acute respiratory distress syndrome (ARDS) are life-threatening syndromes characterised by inflammation and increased vascular permeability. Amongst other factors, the angiopoietin-tyrosine kinase with immunoglobulin-like and EGF-like domains 2 (Tie2) system is involved.

OBJECTIVE

To explore whether the angiopoietin-Tie2 system provides suitable targets for the treatment of sepsis and ALI/ARDS.

METHODS

Original experimental and patient studies on angiopoietins and sepsis/endotoxemia, inflammation, lung injury, hyperpermeability, apoptosis, organ functions and vital outcomes were reviewed.

RESULTS/CONCLUSION

The angiopoietin-Tie2 system controls the responsiveness of the endothelium to inflammatory, hyperpermeability, apoptosis and vasoreactive stimuli. Angiopoietin-2 provokes inflammation and vascular hyperpermeability, while angiopoietin-1 has a protective effect. Targeted angiopoietin-2 inhibition with RNA aptamers or blocking antibodies is a potential anti-inflammatory and anti-vascular hyperpermeability strategy in the treatment of sepsis and ALI/ARDS.

Nuclear targeting of beta-catenin and p120ctn during thrombin-induced endothelial barrier dysfunction

AIMS

Cytosolic and nuclear localization of beta-catenin was observed in leaky vessels and in tumours. Several lines of evidence indicate that nuclear beta-catenin facilitates angiogenesis. We hypothesized that nuclear beta-catenin liberated from endothelial junctional complexes marks the transition from hyperpermeability to angiogenesis. The aim of this study was, therefore, to investigate the fate of beta-catenin and the related catenin p120catenin (p120ctn), during disruption of the endothelial barrier function in human umbilical vein endothelial cells (ECs).

METHODS AND RESULTS

The hyperpermeability-inducer thrombin caused a Rho kinase-dependent redistribution of beta-catenin from the membrane to the cytosol as evidenced by the western blot analysis of membrane and cytosol fractions and by immunohistochemistry. Glycogen synthase kinase 3beta, which phosphorylates cytosolic beta-catenin and thereby facilitates its proteasomal degradation, was inhibited by thrombin. The analysis of nuclear extracts demonstrated a thrombin-induced nuclear accumulation of beta-catenin as well as p120ctn. Thrombin stimulation activated beta-catenin-mediated transcriptional activity as evidenced by reporter assays. Finally, real-time-PCR revealed increased mRNA levels of several beta-catenin target genes.

CONCLUSION

Thrombin induced a cytosolic stabilization of membrane-liberated beta-catenin, which, together with p120ctn, subsequently translocated to the nucleus where it induces several beta-catenin target genes. This supports the suggestion that membrane-liberated beta-catenin and p120ctn contribute to angiogenic responses of ECs following episodes of vascular leakage.

Rho-kinase-dependent F-actin rearrangement is involved in the inhibition of PI3-kinase/Akt during ischemia-reperfusion-induced endothelial cell apoptosis

Activation of cytoskeleton regulator Rho-kinase during ischemia-reperfusion (I/R) plays a major role in I/R injury and apoptosis. Since Rho-kinase is a negative regulator of the pro-survival phosphatidylinositol 3-kinase (PI3-kinase)/Akt pathway, we hypothesized that inhibition of Rho-kinase can prevent I/R-induced endothelial cell apoptosis by maintaining PI3-kinase/Akt activity and that protective effects of Rho-kinase inhibition are facilitated by prevention of F-actin rearrangement. Human umbilical vein endothelial cells were subjected to 1 h of simulated ischemia and 1 or 24 h of simulated reperfusion after treatment with Rho-kinase inhibitor Y-27632, PI3-kinase inhibitor wortmannin, F-actin depolymerizers cytochalasinD and latrunculinA and F-actin stabilizer jasplakinolide. Intracellular ATP levels decreased following I/R. Y-27632 treatment reduced I/R-induced apoptosis by 31% (P < 0.01) and maintained Akt activity. Both effects were blocked by co-treatment with wortmannin. Y-27632 treatment prevented the formation of F-actin bundles during I/R. Similar results were observed with cytochalasinD treatment. In contrast, latrunculinA and jasplakinolide treatment did not prevent the formation of F-actin bundles during I/R and had no effect on I/R-induced apoptosis. Apoptosis and Akt activity were inversely correlated (R (2) = 0.68, P < 0.05). In conclusion, prevention of F-actin rearrangement by Rho-kinase inhibition or by cytochalasinD treatment attenuated I/R-induced endothelial cell apoptosis by maintaining PI3-kinase and Akt activity.

Interference with actin dynamics is superior to disturbance of microtubule function in the inhibition of human ovarian cancer cell motility

Cellular movement is mainly orchestrated by the actin and microtubule cytoskeleton in which Rho GTPases closely collaborate. We studied whether cytoskeleton-interfering agents at subtoxic and 50% growth-inhibiting concentrations affect motility of five unselected human ovarian cancer cell lines. Cisplatin and doxorubicin as control cytotoxic agents were not effective, the microtubule-targeting agents docetaxel, epothilone B and vinblastine only marginally inhibited cell motility, while the actin-targeting agent cytochalasin D was most potent in hampering both cell migration and invasion. Disturbance of microtubule dynamics by docetaxel did not importantly affect the cellular structures of beta-tubulin and F-actin. In contrast, hindrance of actin dynamics by cytochalasin D resulted in loss of lamellipodial extensions, induced thick layers of F-actin and disorder in cellular organization. In OVCAR-3 cells the activity of Rac1 was only slightly diminished by docetaxel, but clearly reduced by cytochalasin D. In conclusion, targeting the actin cytoskeleton might provide a means to prevent metastasis formation.

Microtubule-targeting agents inhibit angiogenesis at subtoxic concentrations, a process associated with inhibition of Rac1 and Cdc42 activity and changes in the endothelial cytoskeleton

Conventional anticancer agents may display antiangiogenic effects, but the underlying mechanism is poorly understood. We determined the antiangiogenic properties of cisplatin, doxorubicin, and the microtubule-targeting agents docetaxel, epothilone B, and vinblastine at concentrations not affecting cell proliferation. We also assessed tubulin and actin morphology and the activity of two key molecules in cell motility, the small Rho GTPases Cdc42 and Rac1. The highest non-toxic concentration (HNTC) of each drug was defined as the concentration inhibiting a maximum of 10% human umbilical vein endothelial cell growth on a 1-hour drug exposure, being for cisplatin 10 micromol/L, doxorubicin 100 nmol/L, docetaxel 10 nmol/L, epothilone B 1 nmol/L, and vinblastine 10 nmol/L. Comparative endothelial cell functional assays using HNTCs for an exposure time of 1 hour indicated that endothelial cell migration in the wound assay, endothelial cell invasion in a transwell invasion system, and endothelial cell formation into tubelike structures on a layer of Matrigel were significantly inhibited by docetaxel, epothilone B, and vinblastine (P < 0.05), but not by cisplatin and doxorubicin. Docetaxel was slightly more efficient in the inhibition of endothelial cell motility than epothilone B and vinblastine. Fluorescence microscopy revealed that only the microtubule-targeting agents affected the integrity of the tubulin and F-actin cytoskeleton, which showed disturbed microtubule structures, less F-actin stress fiber formation, and appearance of nuclear F-actin rings. These observations were associated with early inhibition of Rac1 and Cdc42 activity. In conclusion, HNTCs of microtubule-targeting agents efficiently reduce endothelial cell motility by interference with microtubule dynamics preventing the activation of Rac1/Cdc42 and disorganizing the actin cytoskeleton.