Hepatocyte growth factor enhances endothelial cell barrier function and cortical cytoskeletal rearrangement: potential role of glycogen synthase kinase-3beta

A Razor's Edge: Vascular Responses to Acute Inflammatory Lung Injury/Acute Respiratory Distress Syndrome

Historically considered a metabolically inert cellular layer separating the blood from the underlying tissue, the endothelium is now recognized as a highly dynamic, metabolically active tissue that is critical to organ homeostasis. Under homeostatic conditions, lung endothelial cells (ECs) in healthy subjects are quiescent, promoting vasodilation, platelet disaggregation, and anti-inflammatory mechanisms. In contrast, lung ECs are essential contributors to the pathobiology of acute respiratory distress syndrome (ARDS), as the quiescent endothelium is rapidly and radically altered upon exposure to environmental stressors, infectious pathogens, or endogenous danger signals into an effective and formidable regulator of innate and adaptive immunity. These dramatic perturbations, produced in a tsunami of inflammatory cascade activation, result in paracellular gap formation between lung ECs, sustained lung edema, and multi-organ dysfunction that drives ARDS mortality. The astonishing plasticity of the lung endothelium in negotiating this inflammatory environment and efforts to therapeutically target the aberrant ARDS endothelium are examined in further detail in this review.

Pseudomonas aeruginosa LecB suppresses immune responses by inhibiting transendothelial migration

Pseudomonas aeruginosa is a Gram-negative bacterium causing morbidity and mortality in immuno-compromised humans. It produces a lectin, LecB, that is considered a major virulence factor, however, its impact on the immune system remains incompletely understood. Here we show that LecB binds to endothelial cells in human skin and mice and disrupts the transendothelial passage of leukocytes in vitro. It impairs the migration of dendritic cells into the paracortex of lymph nodes leading to a reduced antigen-specific T cell response. Under the effect of the lectin, endothelial cells undergo profound cellular changes resulting in endocytosis and degradation of the junctional protein VE-cadherin, formation of an actin rim, and arrested cell motility. This likely negatively impacts the capacity of endothelial cells to respond to extracellular stimuli and to generate the intercellular gaps for allowing leukocyte diapedesis. A LecB inhibitor can restore dendritic cell migration and T cell activation, underlining the importance of LecB antagonism to reactivate the immune response against P. aeruginosa infection.

Immuno-engineered mRNA combined with cell adhesive niche for synergistic modulation of the MSC secretome

In vitro transcribed (IVT-)mRNA has entered center stage for vaccine development due to its immune co-stimulating properties. Given the widely demonstrated safety of IVT-mRNA-based vaccines, we aimed to adopt IVT-mRNA encoding VEGF for secretory phenotype modulation of therapeutic cells. However, we observed that the immunogenicity of IVT-mRNA impairs the endogenous secretion of pro-angiogenic mediators from transfected mesenchymal stromal cells, instead inducing anti-angiogenic chemokines. This inflammatory secretome modulation limits the application potential of unmodified IVT-mRNA for cell therapy manufacturing, pro-angiogenic therapy and regenerative medicine. To uncouple immunogenicity from the protein expression functionality, we immuno-engineered IVT-mRNA with different chemically modified ribonucleotides. 5-Methoxy-uridine-modification of IVT-mRNA rescued the endogenous secretome pattern of transfected cells and prolonged secretion of IVT-mRNA-encoded VEGF. We found that high secretion of IVT-mRNA-encoded protein further depends on optimized cell adhesion. Cell encapsulation in a collagen-hyaluronic acid hydrogel increased secretion of IVT-mRNA-encoded VEGF and augmented the endogenous secretion of supporting pro-angiogenic mediators, such as HGF. Integrating minimally immunogenic mRNA technology with predesigned matrix-derived cues allows for the synergistic combination of multiple dimensions of cell manipulation and opens routes for biomaterial-based delivery of mRNA-engineered cell products. Such multimodal systems could present a more biologically relevant way to therapeutically address complex multifactorial processes such as tissue ischemia, angiogenesis, and regeneration.

Microvascular and proteomic signatures overlap in COVID-19 and bacterial sepsis: the MICROCODE study

AIMS

Although coronavirus disease 2019 (COVID-19) and bacterial sepsis are distinct conditions, both are known to trigger endothelial dysfunction with corresponding microcirculatory impairment. The purpose of this study was to compare microvascular injury patterns and proteomic signatures in COVID-19 and bacterial sepsis patients.

METHODS AND RESULTS

This multi-center, observational study included 22 hospitalized adult COVID-19 patients, 43 hospitalized bacterial sepsis patients, and 10 healthy controls from 4 hospitals. Microcirculation and glycocalyx dimensions were quantified via intravital sublingual microscopy. Plasma proteins were measured using targeted proteomics (Olink). Coregulation and cluster analysis of plasma proteins was performed using a training-set and confirmed in a test-set. An independent external cohort of 219 COVID-19 patients was used for validation and outcome analysis. Microcirculation and plasma proteome analysis found substantial overlap between COVID-19 and bacterial sepsis. Severity, but not disease entity explained most data variation. Unsupervised correlation analysis identified two main coregulated plasma protein signatures in both diseases that strictly counteract each other. They were associated with microvascular dysfunction and several established markers of clinical severity. The signatures were used to derive new composite biomarkers of microvascular injury that allow to predict 28-day mortality or/and intubation (area under the curve 0.90, p < 0.0001) in COVID-19.

CONCLUSION

Our data imply a common biological host response of microvascular injury in both bacterial sepsis and COVID-19. A distinct plasma signature correlates with endothelial health and improved outcomes, while a counteracting response is associated with glycocalyx breakdown and high mortality. Microvascular health biomarkers are powerful predictors of clinical outcomes.

The role of brain barriers in the neurokinetics and pharmacodynamics of lithium

Lithium (Li) is the most widely used mood stabilizer in treating patients with bipolar disorder. However, more than half of the patients do not or partially respond to Li therapy, despite serum Li concentrations in the serum therapeutic range. The exact mechanisms underlying the pharmacokinetic-pharmacodynamic (PK-PD) relationships of lithium are still poorly understood and alteration in the brain pharmacokinetics of lithium may be one of the mechanisms explaining the variability in the clinical response to Li. Brain barriers such as the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB) play a crucial role in controlling blood-to-brain and brain-to-blood exchanges of various molecules including central nervous system (CNS) drugs. Recent in vivo studies by nuclear resonance spectroscopy revealed heterogenous brain distribution of Li in human that were not always correlated with serum concentrations, suggesting regional and variable transport mechanisms of Li through the brain barriers. Moreover, alteration in the functionality and integrity of brain barriers is reported in various CNS diseases, as a cause or a consequence and in this regard, Li by itself is known to modulate BBB properties such as the expression and activity of various transporters, metabolizing enzymes, and the specialized tight junction proteins on BBB. In this review, we will focus on recent knowledge into the role of the brain barriers as key-element in the Li neuropharmacokinetics which might improve the understanding of PK-PD of Li and its interindividual variability in drug response.

Ursodesoxycholic acid alleviates liver fibrosis via proregeneration by activation of the ID1-WNT2/HGF signaling pathway

BACKGROUND

The human liver possesses a remarkable capacity for self-repair. However, liver fibrosis remains a serious medical concern, potentially progressing to end-stage liver cirrhosis and even death. Liver fibrosis is characterized by excess accumulation of extracellular matrix in response to chronic injury. Liver regenerative ability, a strong indicator of liver health, is important in resisting fibrosis. In this study, we provide evidence that ursodesoxycholic acid (UDCA) can alleviate liver fibrosis by promoting liver regeneration via activation of the ID1-WNT2/hepatocyte growth factor (HGF) pathway.

METHODS

Bile duct ligation (BDL) and partial hepatectomy (PH) mouse models were used to verify the effects of UDCA on liver fibrosis, regeneration, and the ID1-WNT2/HGF pathway. An Id1 knockdown mouse model was also used to assess the role of Id1 in UDCA alleviation of liver fibrosis.

RESULTS

Our results demonstrate that UDCA can alleviate liver fibrosis in the BDL mice and promote liver regeneration via the ID1-WNT2/HGF pathway in PH mice. In addition, Id1 knockdown abolished the protection afforded by UDCA in BDL mice.

CONCLUSIONS

We conclude that UDCA protects against liver fibrosis by proregeneration via activation of the ID1-WNT2/HGF pathway.

Tyrosine phosphorylation of the scaffold protein IQGAP1 in the MET pathway alters function

IQGAP1 is a key scaffold protein that regulates numerous cellular processes and signaling pathways. Analogous to many other cellular proteins, IQGAP1 undergoes post-translational modifications, including phosphorylation. Nevertheless, very little is known about the specific sites of phosphorylation or the effects on IQGAP1 function. Here, using several approaches, including MS, site-directed mutagenesis, siRNA-mediated gene silencing, and chemical inhibitors, we identified the specific tyrosine residues that are phosphorylated on IQGAP1 and evaluated the effect on function. Tyr-172, Tyr-654, Tyr-855, and Tyr-1510 were phosphorylated on IQGAP1 when phosphotyrosine phosphatase activity was inhibited in cells. IQGAP1 was phosphorylated exclusively on Tyr-1510 under conditions with enhanced MET or c-Src signaling, including in human lung cancer cell lines. This phosphorylation was significantly reduced by chemical inhibitors of MET or c-Src or by siRNA-mediated knockdown of MET. To investigate the biological sequelae of phosphorylation, we generated a nonphosphorylatable IQGAP1 construct by replacing Tyr-1510 with alanine. The ability of hepatocyte growth factor, the ligand for MET, to promote AKT activation and cell migration was significantly greater when IQGAP1-null cells were reconstituted with IQGAP1 Y1510A than when cells were reconstituted with WT IQGAP1. Collectively, our data suggest that phosphorylation of Tyr-1510 of IQGAP1 alters cell function. Because increased MET signaling is implicated in the development and progression of several types of carcinoma, IQGAP1 may be a potential therapeutic target in selected malignancies.

Distinct Factors Secreted by Adipose Stromal Cells Protect the Endothelium From Barrier Dysfunction and Apoptosis

We have shown previously that adipose stromal cell (ASC)-derived conditioned media (CM) limited lung injury, endothelial barrier dysfunction, and apoptosis. Here, we used endothelial hyperpermeability and apoptosis assays to investigate how concentration processes affect endothelium-directed bioactivity of ASC-CM and to gain information on the nature of bioactive factors. Comparison of ASC-CM concentrated with differential molecular weight (MW) cutoff filters showed that endothelial barrier protection depended on the species-specific factors in ASC-CM fractionated with MW > 50 kDa. Known barrier regulators-keratin growth factor (KGF), vascular endothelial growth factor (VEGF), and hepatocyte growth factor (HGF)-were detected in ASC-CM fraction of > 100 kDa. Pretreatment of endothelial monolayers with concentrations of KGF, VEGF, and HGF detected in ASC-CM showed that only KGF and HGF protect the endothelium from barrier dysfunction. Depletion of KGF and HGF from ASC-CM attenuated ASC-CM's ability to protect the endothelial barrier. In contrast to barrier-protective factors, apoptosis-protective factors fractionated with MW < 3 kDa and were not species-specific. Application of donors of apoptosis-mitigating gases showed that the CO donor carbon monoxide-releasing molecule 2 (CORM2) protected the endothelium from apoptosis, while the H₂S donor NaSH did not. Knockdown of CO-generating heme oxygenase 1 in ASC attenuated ASC-CM's ability to protect the endothelium from apoptosis. We have shown that tumor necrosis factor alpha (TNFα)-induced apoptosis in endothelium is c-Jun N-terminal kinase (JNK)-dependent, and JNK activation is inhibited by ASC-CM pretreatment of endothelial cells. ASC-CM from heme oxygenase 1-depleted ASC displayed attenuated ability to suppress endothelial JNK activation, suggesting that CO-mediated protection of the endothelium from apoptosis is achieved by the downregulation of the JNK pathway. Altogether, our results demonstrate that the concentration of ASC-CM with low MW cutoff filters significantly reduces its anti-apoptotic activity while preserving its barrier-protective activity.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Mechanosensing and Mechanoregulation of Endothelial Cell Functions

Vascular endothelial cells (ECs) form a semiselective barrier for macromolecules and cell elements regulated by dynamic interactions between cytoskeletal elements and cell adhesion complexes. ECs also participate in many other vital processes including innate immune reactions, vascular repair, secretion, and metabolism of bioactive molecules. Moreover, vascular ECs represent a unique cell type exposed to continuous, time-dependent mechanical forces: different patterns of shear stress imposed by blood flow in macrovasculature and by rolling blood cells in the microvasculature; circumferential cyclic stretch experienced by the arterial vascular bed caused by heart propulsions; mechanical stretch of lung microvascular endothelium at different magnitudes due to spontaneous respiration or mechanical ventilation in critically ill patients. Accumulating evidence suggests that vascular ECs contain mechanosensory complexes, which rapidly react to changes in mechanical loading, process the signal, and develop context-specific adaptive responses to rebalance the cell homeostatic state. The significance of the interactions between specific mechanical forces in the EC microenvironment together with circulating bioactive molecules in the progression and resolution of vascular pathologies including vascular injury, atherosclerosis, pulmonary edema, and acute respiratory distress syndrome has been only recently recognized. This review will summarize the current understanding of EC mechanosensory mechanisms, modulation of EC responses to humoral factors by surrounding mechanical forces (particularly the cyclic stretch), and discuss recent findings of magnitude-specific regulation of EC functions by transcriptional, posttranscriptional and epigenetic mechanisms using -omics approaches. We also discuss ongoing challenges and future opportunities in developing new therapies targeting dysregulated mechanosensing mechanisms to treat vascular diseases. © 2019 American Physiological Society. Compr Physiol 9:873-904, 2019.

Pro-angiogenic Ginsenosides F1 and Rh1 Inhibit Vascular Leakage by Modulating NR4A1

Vascular endothelial growth factor (VEGF) plays a key role in angiogenesis, but VEGF-induced angiogenesis is often accompanied by a vascular permeability response. Ginsenosides are triterpenoid saponins from the well-known medicinal plant, ginseng, and have been considered a candidate for modulating angiogenesis. Here, we systemically investigated the effects of 10 different ginsenosides on human umbilical vein endothelial cells and newly identified that two PPT-type ginsenosides, F1 and Rh1 induce the migration and proliferation of endothelial cells. Interestingly, RNA transcriptome analysis showed that gene regulation induced by VEGF in endothelial cells is distinct from that of ginsenoside F1 and Rh1. In addition, F1 and Rh1 significantly inhibited vascular leakage both in vitro and in vivo, which are induced by vascular endothelial growth factor. Furthermore, comparative transcriptome analysis revealed that these effects of F1 and Rh1 on vascular leakage restoration are mainly caused by changes in VEGF-mediated TNFα signaling via NFκB, particularly by the suppression of expression and transcriptional activity of NR4A1 by F1 and Rh1, even in the presence of VEGF. These findings demonstrate that ginsenosides F1 and Rh1 can be a promising herbal remedy for vessel normalization in ischemic disease and cancer and that NR4A1 is the key target.

Myosin light chain kinase ( MYLK) coding polymorphisms modulate human lung endothelial cell barrier responses via altered tyrosine phosphorylation, spatial localization, and lamellipodial protrusions

Sphingosine 1-phosphate (S1P) is a potent bioactive endogenous lipid that signals a rearrangement of the actin cytoskeleton via the regulation of non-muscle myosin light chain kinase isoform (nmMLCK). S1P induces critical nmMLCK Y⁴⁶⁴ and Y⁴⁷¹ phosphorylation resulting in translocation of nmMLCK to the periphery where spatially-directed increases in myosin light chain (MLC) phosphorylation and tension result in lamellipodia protrusion, increased cell-cell adhesion, and enhanced vascular barrier integrity. MYLK, the gene encoding nmMLCK, is a known candidate gene in lung inflammatory diseases, with coding genetic variants (Pro21His, Ser147Pro, Val261Ala) that confer risk for inflammatory lung injury and influence disease severity. The functional mechanisms by which these MYLK coding single nucleotide polymorphisms (SNPs) affect biologic processes to increase disease risk and severity remain elusive. In the current study, we utilized quantifiable cell immunofluorescence assays to determine the influence of MYLK coding SNPs on S1P-mediated nmMLCK phosphorylation and translocation to the human lung endothelial cell (EC) periphery . These disease-associated MYLK variants result in reduced levels of S1P-induced Y⁴⁶⁴ phosphorylation, a key site for nmMLCK enzymatic regulation and activation. Reduced Y⁴⁶⁴ phosphorylation resulted in attenuated nmMLCK protein translocation to the cell periphery. We further conducted EC kymographic assays which confirmed that lamellipodial protrusion in response to S1P challenge was retarded by expression of a MYLK transgene harboring the three MYLK coding SNPs. These data suggest that ARDS/severe asthma-associated MYLK SNPs functionally influence vascular barrier-regulatory cytoskeletal responses via direct alterations in the levels of nmMLCK tyrosine phosphorylation, spatial localization, and lamellipodial protrusions.

In Development-A New Paradigm for Understanding Vascular Disease

Under physiological conditions, the arterial endothelium exerts a powerful protective influence to maintain vascular homeostasis. However, during the development of vascular disease, these protective activities are lost, and dysfunctional endothelial cells actually promote disease pathogenesis. Numerous investigations have analyzed the characteristics of dysfunctional endothelium with a view to understanding the processes responsible for the dysfunction and to determining their role in vascular pathology. This review adopts an alternate approach: reviewing the mechanisms that contribute to the initial formation of a healthy protective endothelium and on how those mechanisms may be disrupted, precipitating the appearance of dysfunctional endothelial cells and the progression of vascular disease. This approach, which highlights the role of endothelial adherens junctions and vascular endothelial-cadherin in endothelial maturation and endothelial dysfunction, provides new insight into the remarkable biology of this important cell layer and its role in vascular protection and vascular disease.

An Accessible Organotypic Microvessel Model Using iPSC-Derived Endothelium

While organotypic approaches promise increased relevance through the inclusion of increased complexity (e.g., 3D extracellular microenvironment, structure/function relationships, presence of multiple cell types), cell source is often overlooked. Induced pluripotent stem cell (iPSC)-derived cells are potentially more physiologically relevant than cell lines, while also being less variable than primary cells, and recent advances have made them commercially available at costs similar to cell lines. Here, the use of induced pluripotent stem cell-derived endothelium for the generation of a functional microvessel model is demonstrated. High precision structural and microenvironmental control afforded by the design approach synergizes with the advantages of iPSC to produce microvessels for modeling endothelial biology in vitro. iPSC microvessels show endothelial characteristics, exhibit barrier function, secrete angiogenic and inflammatory mediators, and respond to changes in the extracellular microenvironment by altering vessel phenotype. Importantly, when deployed in the investigation of neutrophils during innate immune recruitment, the presence of the iPSC endothelial vessel facilitates neutrophil extravasation and migration toward a chemotactic source. Relevant cell sources, such as iPSC, combine with organotypic models to open the way for improved and increasingly accessible in vitro tissue, disease, and patient-specific models.

Injured lung endothelium: mechanisms of self-repair and agonist-assisted recovery (2017 Grover Conference Series)

The lung endothelium is vulnerable to both exogenous and endogenous insults, so a properly coordinated efficient repair system is essential for the timely recovery of the lung after injury. The agents that cause endothelial injury and dysfunction fall into a broad range from mechanical forces such as pathological cyclic stretch and shear stress to bacterial pathogens and their virulent components, vasoactive agonists including thrombin and histamine, metabolic causes including high glucose and oxidized low-density lipoprotein (OxLDL), circulating microparticles, and inflammatory cytokines. The repair mechanisms employed by endothelial cells (EC) can be broadly categorized into three groups: (1) intrinsic mechanism of recovery regulated by the cross-talk between small GTPases as exemplified by Rap1-mediated EC barrier recovery from Rho-mediated thrombin-induced EC hyperpermeability; (2) agonist-assisted recovery facilitated by the activation of Rac and Rap1 with subsequent inhibition of Rho signaling as observed with many barrier protective agonists including oxidized phospholipids, sphingosine 1-phosphate, prostacyclins, and hepatocyte growth factor; and (3) self-recovery of EC by the secretion of growth factors and other pro-survival bioactive compounds including anti-inflammatory molecules such as lipoxins during the resolution of inflammation. In this review, we will discuss the molecular and cellular mechanisms of pulmonary endothelium repair that is critical for the recovery from various forms of lung injuries.

Molecular Regulation of Sprouting Angiogenesis

The term angiogenesis arose in the 18th century. Several studies over the next 100 years laid the groundwork for initial studies performed by the Folkman laboratory, which were at first met with some opposition. Once overcome, the angiogenesis field has flourished due to studies on tumor angiogenesis and various developmental models that can be genetically manipulated, including mice and zebrafish. In addition, new discoveries have been aided by the ability to isolate primary endothelial cells, which has allowed dissection of various steps within angiogenesis. This review will summarize the molecular events that control angiogenesis downstream of biochemical factors such as growth factors, cytokines, chemokines, hypoxia-inducible factors (HIFs), and lipids. These and other stimuli have been linked to regulation of junctional molecules and cell surface receptors. In addition, the contribution of cytoskeletal elements and regulatory proteins has revealed an intricate role for mobilization of actin, microtubules, and intermediate filaments in response to cues that activate the endothelium. Activating stimuli also affect various focal adhesion proteins, scaffold proteins, intracellular kinases, and second messengers. Finally, metalloproteinases, which facilitate matrix degradation and the formation of new blood vessels, are discussed, along with our knowledge of crosstalk between the various subclasses of these molecules throughout the text. Compr Physiol 8:153-235, 2018.

Mechanical Stress and Single Nucleotide Variants Regulate Alternative Splicing of the MYLK Gene

The nonmuscle (nm) myosin light-chain kinase isoform (MLCK), encoded by the MYLK gene, is a vital participant in regulating vascular barrier responses to mechanical and inflammatory stimuli. We determined that MYLK is alternatively spliced, yielding functionally distinct nmMLCK splice variants including nmMLCK2, a splice variant highly expressed in vascular endothelial cells (EC) and associated with reduced EC barrier integrity. We demonstrated previously that the nmMLCK2 variant lacks exon 11, which encodes a key regulatory region containing two differentially phosphorylated tyrosine residues (Y⁴⁶⁴ and Y⁴⁷¹) that influence vascular barrier function during inflammation. In this study, we used minigene constructs and RT-PCR to interrogate biophysical factors (mechanical stress) and genetic variants (MYLK single-nucleotide polymorphisms [SNPs]) that are potentially involved in regulating MYLK alternative splicing and nmMLCK2 generation. Human lung EC exposed to pathologic mechanical stress (18% cyclic stretch) produced increased nmMLCK2 expression relative to levels of nmMLCK1 with alternative splicing significantly influenced by MYLK SNPs rs77323602 and rs147245669. In silico analyses predicted that these variants would alter exon 11 donor and acceptor sites for alternative splicing, computational predictions that were confirmed by minigene studies. The introduction of rs77323602 favored wild-type nmMLCK expression, whereas rs147245669 favored alternative splicing and deletion of exon 11, yielding increased nmMLCK2 expression. Finally, lymphoblastoid cell lines selectively harboring these MYLK SNPs (rs77323602 and rs147245669) directly validated SNP-specific effects on MYLK alternative splicing and nmMLCK2 generation. Together, these studies demonstrate that mechanical stress and MYLK SNPs regulate MYLK alternative splicing and generation of a splice variant, nmMLCK2, that contributes to the severity of inflammatory injury.

Role of Sphingosine Kinase 1 and S1P Transporter Spns2 in HGF-mediated Lamellipodia Formation in Lung Endothelium

Hepatocyte growth factor (HGF) signaling via c-Met is known to promote endothelial cell motility and angiogenesis. We have previously reported that HGF stimulates lamellipodia formation and motility of human lung microvascular endothelial cells (HLMVECs) via PI3K/Akt signal transduction and reactive oxygen species generation. Here, we report a role for HGF-induced intracellular sphingosine-1-phosphate (S1P) generation catalyzed by sphingosine kinase 1 (SphK1), S1P transporter, spinster homolog 2 (Spns2), and S1P receptor, S1P₁, in lamellipodia formation and perhaps motility of HLMVECs. HGF stimulated SphK1 phosphorylation and enhanced intracellular S1P levels in HLMVECs, which was blocked by inhibition of SphK1. HGF enhanced co-localization of SphK1/p-SphK1 with actin/cortactin in lamellipodia and down-regulation or inhibition of SphK1 attenuated HGF-induced lamellipodia formation in HLMVECs. In addition, down-regulation of Spns2 also suppressed HGF-induced lamellipodia formation, suggesting a key role for inside-out S1P signaling. The HGF-mediated phosphorylation of SphK1 and its localization in lamellipodia was dependent on c-Met and ERK1/2 signaling, but not the PI3K/Akt pathway; however, blocking PI3K/Akt signaling attenuated HGF-mediated phosphorylation of Spns2. Down-regulation of S1P₁, but not S1P₂ or S1P₃, with specific siRNA attenuated HGF-induced lamellipodia formation. Further, HGF enhanced association of Spns2 with S1P₁ that was blocked by inhibiting SphK1 activity with PF-543. Moreover, HGF-induced migration of HLMVECs was attenuated by down-regulation of Spns2_. Taken together, these results suggest that HGF/c-Met-mediated lamellipodia formation, and perhaps motility is dependent on intracellular generation of S1P via activation and localization of SphK1 to cell periphery and Spns2-mediated extracellular transportation of S1P and its inside-out signaling via S1P₁.

Differential elastic responses to barrier-altering agonists in two types of human lung endothelium

Vascular integrity is primarily determined by endothelial cell (EC) cytoskeletal structure that is differentially regulated by various stimuli. In this study, atomic force microscopy (AFM) was used to characterize structural and mechanical properties in the cytoskeleton of cultured human pulmonary artery EC (HPAEC) and human lung microvascular EC (HLMVEC) by determining elastic properties (Young's modulus) in response to endogenous barrier protective agents sphingosine 1-phosphate (S1P) and hepatocyte growth factor (HGF), or the barrier disruptive molecule thrombin. Initial studies in unstimulated cells indicate higher baseline peripheral elastic modulus values in HPAEC (mean 2.9 KPa) than in HLMVEC (1.8 KPa). After 30 min of stimulation, S1P induced the highest Young's modulus increase (6.1 KPa) compared to the other barrier enhancing stimuli, HGF (5.8 KPa) and the pharmaceutical agent and S1P analog FTY720 (4.1 KPa). In contrast, the barrier disruptive agent thrombin decreased values from 2.5 KPa to 0.7 KPa depending on the cell type and treatment time. AFM topographical imaging supports these quantitative biophysical data regarding differential peripheral elastic properties in EC. Overall, these AFM studies provide novel insights into the biomechanical properties of human lung EC that regulate vascular barrier function and have potential applicability to pathophysiologic vascular leak syndromes such as acute lung injury.

Cellular and molecular mechanisms of HGF/Met in the cardiovascular system

Met tyrosine kinase receptor, also known as c-Met, is the HGF (hepatocyte growth factor) receptor. The HGF/Met pathway has a prominent role in cardiovascular remodelling after tissue injury. The present review provides a synopsis of the cellular and molecular mechanisms underlying the effects of HGF/Met in the heart and blood vessels. In vivo, HGF/Met function is particularly important for the protection of the heart in response to both acute and chronic insults, including ischaemic injury and doxorubicin-induced cardiotoxicity. Accordingly, conditional deletion of Met in cardiomyocytes results in impaired organ defence against oxidative stress. After ischaemic injury, activation of Met provides strong anti-apoptotic stimuli for cardiomyocytes through PI3K (phosphoinositide 3-kinase)/Akt and MAPK (mitogen-activated protein kinase) cascades. Recently, we found that HGF/Met is also important for autophagy regulation in cardiomyocytes via the mTOR (mammalian target of rapamycin) pathway. HGF/Met induces proliferation and migration of endothelial cells through Rac1 (Ras-related C3 botulinum toxin substrate 1) activation. In fibroblasts, HGF/Met antagonizes the actions of TGFβ1 (transforming growth factor β1) and AngII (angiotensin II), thus preventing fibrosis. Moreover, HGF/Met influences the inflammatory response of macrophages and the immune response of dendritic cells, indicating its protective function against atherosclerotic and autoimmune diseases. The HGF/Met axis also plays an important role in regulating self-renewal and myocardial regeneration through the enhancement of cardiac progenitor cells. HGF/Met has beneficial effects against myocardial infarction and endothelial dysfunction: the cellular and molecular mechanisms underlying repair function in the heart and blood vessels are common and include pro-angiogenic, anti-inflammatory and anti-fibrotic actions. Thus administration of HGF or HGF mimetics may represent a promising therapeutic agent for the treatment of both coronary and peripheral artery disease.

Low-Dose Endothelial Monocyte-Activating Polypeptide-II Induces Blood-Tumor Barrier Opening Via the cAMP/PKA/Rac1 Pathway

Previous studies have demonstrated that low-dose endothelial monocyte-activating polypeptide-II (EMAP-II) induces blood-tumor barrier (BTB) hyperpermeability via both paracellular and transcellular pathways. In a recent study, we revealed that cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)-dependent signaling pathway is involved in EMAP-II-induced BTB hyperpermeability. This study further investigated the exact mechanisms through which the cAMP/PKA-dependent signaling pathway affects EMAP-II-induced BTB hyperpermeability. In an in vitro BTB model, low-dose EMAP-II (0.05 nM) induced a significant decrease in Rac1 activity in rat brain microvascular endothelial cells (RBMECs). Pretreatment with forskolin to elevate intracellular cAMP concentration completely blocked EMAP-II-induced inactivation of Rac1. Besides, pretreatment with 6Bnz-cAMP to activate PKA partially attenuated EMAP-II-induced Rac1 inactivation. Moreover, 6Bnz-cAMP pretreatment significantly diminished EMAP-II-induced changes in BTB permeability, myosin light chain (MLC) phosphorylation, expression and distribution of ZO-1, and actin cytoskeleton arrangement in RBMECs. These effects of 6Bnz-cAMP were completely blocked in the presence of NSC-23766 (the specific inhibitor of Rac1). In conclusion, this study demonstrates that low-dose EMAP-II induces BTB hyperpermeability via the cAMP/PKA/Rac1 signaling pathway.

Structure-Function Analysis of the Non-Muscle Myosin Light Chain Kinase (nmMLCK) Isoform by NMR Spectroscopy and Molecular Modeling: Influence of MYLK Variants

The MYLK gene encodes the multifunctional enzyme, myosin light chain kinase (MLCK), involved in isoform-specific non-muscle and smooth muscle contraction and regulation of vascular permeability during inflammation. Three MYLK SNPs (P21H, S147P, V261A) alter the N-terminal amino acid sequence of the non-muscle isoform of MLCK (nmMLCK) and are highly associated with susceptibility to acute lung injury (ALI) and asthma, especially in individuals of African descent. To understand the functional effects of SNP associations, we examined the N-terminal segments of nmMLCK by ¹H-¹⁵N heteronuclear single quantum correlation (HSQC) spectroscopy, a 2-D NMR technique, and by in silico molecular modeling. Both NMR analysis and molecular modeling indicated SNP localization to loops that connect the immunoglobulin-like domains of nmMLCK, consistent with minimal structural changes evoked by these SNPs. Molecular modeling analysis identified protein-protein interaction motifs adversely affected by these MYLK SNPs including binding by the scaffold protein 14-3-3, results confirmed by immunoprecipitation and western blot studies. These structure-function studies suggest novel mechanisms for nmMLCK regulation, which may confirm MYLK as a candidate gene in inflammatory lung disease and advance knowledge of the genetic underpinning of lung-related health disparities.

Acute alcohol intoxication-induced microvascular leakage

BACKGROUND

Alcohol intoxication can increase inflammation and worsen injury, yet the mechanisms involved are not clear. We investigated whether acute alcohol intoxication increases microvascular permeability and investigated potential signaling mechanisms in endothelial cells that may be involved.

METHODS

Conscious rats received a 2.5 g/kg alcohol bolus via gastric catheters to produce acute intoxication. Microvascular leakage of intravenously administered fluorescein isothiocyanate (FITC)-conjugated albumin (FITC-albumin) from the mesenteric microcirculation was assessed by intravital microscopy. Endothelial-specific mechanisms were studied using cultured endothelial cell monolayers. Transendothelial electrical resistance (TER) served as an index of barrier function, before and after treatment with alcohol or its metabolite acetaldehyde. Pharmacologic agents were used to test the roles of alcohol metabolism, oxidative stress, p38 mitogen-activated protein kinase (MAPK), myosin light-chain kinase (MLCK), rho kinase (ROCK), and exchange protein activated by cAMP (Epac). VE-cadherin localization was investigated to assess junctional integrity. Rac1 and RhoA activation was assessed by ELISA assays.

RESULTS

Alcohol significantly increased FITC-albumin extravasation from the mesenteric microcirculation. Alcohol also significantly decreased TER and disrupted VE-cadherin organization at junctions. Acetaldehyde significantly decreased TER, but inhibition of alcohol dehydrogenase or application of a superoxide dismutase mimetic failed to prevent alcohol-induced decreases in TER. Inhibition of p38 MAPK, but not MLCK or ROCK, significantly attenuated the alcohol-induced barrier dysfunction. Alcohol rapidly decreased GTP-bound Rac1 but not RhoA during the drop in TER. Activation of Epac increased TER, but did not prevent alcohol from decreasing TER. However, activation of Epac after initiation of alcohol-induced barrier dysfunction quickly resolved TER to baseline levels.

CONCLUSIONS

Our results suggest that alcohol intoxication increases microvascular permeability to plasma proteins. The data also suggest the endothelial-specific mechanism involves the p38 MAPK, Rac1, and reorganization of VE-cadherin at junctions. Last, activation of Epac can quickly resolve alcohol-induced endothelial barrier dysfunction.

Human epicardial cell-conditioned medium contains HGF/IgG complexes that phosphorylate RYK and protect against vascular injury

AIMS

The aim of this study was to evaluate the paracrine activity of human epicardial-derived cells (hEPDCs) to screen for secreted vasoprotective factors and develop therapeutics to treat vascular reperfusion injury.

METHODS AND RESULTS

Epicardial cells support cardiac development, repair, and remodelling after injury in part, through paracrine activity. We hypothesized that secreted ligands from hEPDCs would protect vascular integrity after myocardial infarction (MI) with reperfusion. During simulated ischaemia in culture (24-48 h), concentrated hEPDC-conditioned medium (EPI CdM) increased survival of primary cardiac endothelial cells. In a rat MI model, EPI CdM treatment reduced vascular injury in vivo after reperfusion. By phospho-receptor tyrosine kinase (RTK) arrays, ELISA, and neutralizing antibody screens, we identified hepatocyte growth factor (HGF) as a key vasoprotective factor in EPI CdM. Unexpectedly, we observed that some of the HGF in EPI CdM formed complexes with polyclonal IgG. Following reperfusion, preparations of HGF/IgG complexes provided greater vascular protection than free HGF with IgG. HGF/IgG complexes localized to blood vessels in vivo and increased HGF retention time after administration. In subsequent screens, we found that 'related to tyrosine kinase' (RYK) receptor was phosphorylated after exposure of cardiac endothelial cells to HGF/IgG complexes, but not to free HGF with IgG. The enhanced protection conferred by HGF/IgG complexes was lost after antibody blockade of RYK. Notably, the HGF/IgG complex is the first 'ligand' shown to promote phosphorylation of RYK.

CONCLUSION

Early treatment with HGF/IgG complexes after myocardial ischaemia with reperfusion may rescue tissue through vasoprotection conferred by c-Met and RYK signalling.

Junctional complex and focal adhesion rearrangement mediates pulmonary endothelial barrier enhancement by FTY720 S-phosphonate

RATIONALE

Modulation of pulmonary vascular barrier function is an important clinical goal given the devastating effects of vascular leak in acute lung injury (ALI). We previously demonstrated that FTY720 S-phosphonate (Tys), an analog of sphingosine 1-phosphate (S1P) and FTY720, has more potent pulmonary barrier protective effects than these agents in vitro and in mouse models of ALI. Tys preserves expression of the barrier-promoting S1P1 receptor (S1PR1), whereas S1P and FTY720 induce its ubiquitination and degradation. Here we further characterize the novel barrier promoting effects of Tys in cultured human pulmonary endothelial cells (EC).

METHODS/RESULTS

In human lung EC, Tys significantly increased peripheral redistribution of adherens junction proteins VE-cadherin and β-catenin and tight junction protein ZO-1. Inhibition of VE-cadherin with blocking antibody significantly attenuated Tys-induced transendothelial resistance (TER) elevation, while ZO-1 siRNA partially inhibited this elevation. Tys significantly increased focal adhesion formation and phosphorylation of focal adhesion kinase (FAK). Pharmacologic inhibition of FAK significantly attenuated Tys-induced TER elevation. Tys significantly increased phosphorylation and peripheral redistribution of the actin-binding protein, cortactin, while cortactin siRNA partially attenuated Tys-induced TER elevation. Although Tys significantly increased phosphorylation of Akt and GSK3β, neither PI3 kinase nor GSK3β inhibition altered Tys-induced TER elevation. Tys significantly increased Rac1 activity, while inhibition of Rac1 activity significantly attenuated Tys-induced VE-cadherin redistribution and TER elevation.

CONCLUSION

Junctional complex, focal adhesion rearrangement and Rac1 activation play critical roles in Tys-mediated barrier protection in pulmonary EC. These results provide mechanistic insights into the effects of this potential ALI therapy.

Fluid shear, intercellular stress, and endothelial cell alignment

Endothelial cell alignment along the direction of laminar fluid flow is widely understood to be a defining morphological feature of vascular homeostasis. While the role of associated signaling and structural events have been well studied, associated intercellular stresses under laminar fluid shear have remained ill-defined and the role of these stresses in the alignment process has remained obscure. To fill this gap, we report here the tractions as well as the complete in-plane intercellular stress fields measured within the human umbilical vein endothelial cell (HUVEC) monolayer subjected to a steady laminar fluid shear of 1 Pa. Tractions, intercellular stresses, as well as their time course, heterogeneity, and anisotropy, were measured using monolayer traction microscopy and monolayer stress microscopy. Prior to application of laminar fluid flow, intercellular stresses were largely tensile but fluctuated dramatically in space and in time (317 ± 122 Pa). Within 12 h of the onset of laminar fluid flow, the intercellular stresses decreased substantially but continued to fluctuate dramatically (142 ± 84 Pa). Moreover, tractions and intercellular stresses aligned strongly and promptly (within 1 h) along the direction of fluid flow, whereas the endothelial cell body aligned less strongly and substantially more slowly (12 h). Taken together, these results reveal that steady laminar fluid flow induces prompt reduction in magnitude and alignment of tractions and intercellular stress tensor components followed by the retarded elongation and alignment of the endothelial cell body. Appreciably smaller intercellular stresses supported by cell-cell junctions logically favor smaller incidence of gap formation and thus improved barrier integrity.

Drug target optimization in chronic myeloid leukemia using innovative computational platform

Chronic Myeloid Leukemia (CML) represents a paradigm for the wider cancer field. Despite the fact that tyrosine kinase inhibitors have established targeted molecular therapy in CML, patients often face the risk of developing drug resistance, caused by mutations and/or activation of alternative cellular pathways. To optimize drug development, one needs to systematically test all possible combinations of drug targets within the genetic network that regulates the disease. The BioModelAnalyzer (BMA) is a user-friendly computational tool that allows us to do exactly that. We used BMA to build a CML network-model composed of 54 nodes linked by 104 interactions that encapsulates experimental data collected from 160 publications. While previous studies were limited by their focus on a single pathway or cellular process, our executable model allowed us to probe dynamic interactions between multiple pathways and cellular outcomes, suggest new combinatorial therapeutic targets, and highlight previously unexplored sensitivities to Interleukin-3.

IQGAPs choreograph cellular signaling from the membrane to the nucleus

Since its discovery in 1994, recognized cellular functions for the scaffold protein IQGAP1 have expanded immensely. Over 100 unique IQGAP1-interacting proteins have been identified, implicating IQGAP1 as a critical integrator of cellular signaling pathways. Initial research established functions for IQGAP1 in cell-cell adhesion, cell migration, and cell signaling. Recent studies have revealed additional IQGAP1 binding partners, expanding the biological roles of IQGAP1. These include crosstalk between signaling cascades, regulation of nuclear function, and Wnt pathway potentiation. Investigation of the IQGAP2 and IQGAP3 homologs demonstrates unique functions, some of which differ from those of IQGAP1. Summarized here are recent observations that enhance our understanding of IQGAP proteins in the integration of diverse signaling pathways.

Asef mediates HGF protective effects against LPS-induced lung injury and endothelial barrier dysfunction

Increased vascular endothelial permeability and inflammation are major pathological mechanisms of pulmonary edema and its life-threatening complication, the acute respiratory distress syndrome (ARDS). We have previously described potent protective effects of hepatocyte growth factor (HGF) against thrombin-induced hyperpermeability and identified the Rac pathway as a key mechanism of HGF-mediated endothelial barrier protection. However, anti-inflammatory effects of HGF are less understood. This study examined effects of HGF on the pulmonary endothelial cell (EC) inflammatory activation and barrier dysfunction caused by the gram-negative bacterial pathogen lipopolysaccharide (LPS). We tested involvement of the novel Rac-specific guanine nucleotide exchange factor Asef in the HGF anti-inflammatory effects. HGF protected the pulmonary EC monolayer against LPS-induced hyperpermeability, disruption of monolayer integrity, activation of NF-kB signaling, expression of adhesion molecules intercellular adhesion molecule-1 and vascular cell adhesion molecule-1, and production of IL-8. These effects were critically dependent on Asef. Small-interfering RNA-induced downregulation of Asef attenuated HGF protective effects against LPS-induced EC barrier failure. Protective effects of HGF against LPS-induced lung inflammation and vascular leak were also diminished in Asef knockout mice. Taken together, these results demonstrate potent anti-inflammatory effects by HGF and delineate a key role of Asef in the mediation of the HGF barrier protective and anti-inflammatory effects. Modulation of Asef activity may have important implications in therapeutic strategies aimed at the treatment of sepsis and acute lung injury/ARDS-induced gram-negative bacterial pathogens.

Asef controls vascular endothelial permeability and barrier recovery in the lung

This is the first report of Asef involvement in the regulation of endothelial vascular permeability in vitro and in vivo. Asef activation in endothelial cells by hepatocyte growth factor suppressed the Rho-dependent pathway of agonist-induced endothelial permeability and promoted Rac1-dependent endothelial barrier recovery.

Increased levels of hepatocyte growth factor (HGF) in injured lungs may reflect a compensatory response to diminish acute lung injury (ALI). HGF-induced activation of Rac1 GTPase stimulates endothelial barrier protective mechanisms. This study tested the involvement of Rac-specific guanine nucleotide exchange factor Asef in HGF-induced endothelial cell (EC) cytoskeletal dynamics and barrier protection in vitro and in a two-hit model of ALI. HGF induced membrane translocation of Asef and stimulated Asef Rac1-specific nucleotide exchange activity. Expression of constitutively activated Asef mutant mimicked HGF-induced peripheral actin cytoskeleton enhancement. In contrast, siRNA-induced Asef knockdown or expression of dominant-negative Asef attenuated HGF-induced Rac1 activation evaluated by Rac-GTP pull down and FRET assay with Rac1 biosensor. Molecular inhibition of Asef attenuated HGF-induced peripheral accumulation of cortactin, formation of lamellipodia-like structures, and enhancement of VE-cadherin adherens junctions and compromised HGF-protective effect against thrombin-induced RhoA GTPase activation, Rho-dependent cytoskeleton remodeling, and EC permeability. Intravenous HGF injection attenuated lung inflammation and vascular leak in the two-hit model of ALI induced by excessive mechanical ventilation and thrombin signaling peptide TRAP6. This effect was lost in Asef^−/− mice. This study shows for the first time the role of Asef in HGF-mediated protection against endothelial hyperpermeability and lung injury.

Hepatocyte growth factor-induced Asef-IQGAP1 complex controls cytoskeletal remodeling and endothelial barrier

Hepatocyte growth factor (HGF) attenuates agonist-induced endothelial cell (EC) permeability and increases pulmonary endothelial barrier function via Rac-dependent enhancement of the peripheral actin cytoskeleton. However, the precise mechanisms of HGF effects on the peripheral cytoskeleton are not well understood. This study evaluated a role for Rac/Cdc42-specific guanine nucleotide exchange factor Asef and the multifunctional Rac effector, IQGAP1, in the mechanism of HGF-induced EC barrier enhancement. HGF induced Asef and IQGAP1 co-localization at the cell cortical area and stimulated formation of an Asef-IQGAP1 functional protein complex. siRNA-induced knockdown of Asef or IQGAP1 attenuated HGF-induced EC barrier enhancement. Asef knockdown attenuated HGF-induced Rac activation and Rac association with IQGAP1, and it abolished both IQGAP1 accumulation at the cell cortical layer and IQGAP1 interaction with actin cytoskeletal regulators cortactin and Arp3. Asef activation state was essential for Asef interaction with IQGAP1 and protein complex accumulation at the cell periphery. In addition to the previously reported role of the IQGAP1 RasGAP-related domain in the Rac-dependent IQGAP1 activation and interaction with its targets, we show that the IQGAP1 C-terminal domain is essential for HGF-induced IQGAP1/Asef interaction and Asef-Rac-dependent activation leading to IQGAP1 interaction with Arp3 and cortactin as a positive feedback mechanism of IQGAP1 activation. These results demonstrate a novel feedback mechanism of HGF-induced endothelial barrier enhancement via Asef/IQGAP1 interactions, which regulate the level of HGF-induced Rac activation and promote cortical cytoskeletal remodeling via IQGAP1-Arp3/cortactin interactions.

Hepatocyte growth factor triggers distinct mechanisms of Asef and Tiam1 activation to induce endothelial barrier enhancement

Previous reports described an important role of hepatocyte growth factor (HGF) in mitigation of pulmonary endothelial barrier dysfunction and cell injury induced by pathologic agonists and mechanical forces. HGF protective effects have been associated with Rac-GTPase signaling pathway activated by Rac-specific guanine nucleotide exchange factor Tiam1 and leading to enhancement of intercellular adherens junctions. This study tested involvement of a novel Rac-specific activator, Asef, in endothelial barrier enhancement by HGF and investigated a mechanism of HGF-induced Asef activation. Si-RNA-based knockdown of Tiam1 and Asef had an additive effect on attenuation of HGF-induced Rac activation and endothelial cell (EC) barrier enhancement. Tiam1 and Asef activation was abolished by pharmacologic inhibitors of HGF receptor and PI3-kinase. In contrast to Tiam1, Asef interacted with APC and associated with microtubule fraction upon HGF stimulation. EC treatment by low dose nocodazole to inhibit peripheral microtubule dynamics partially attenuated HGF-induced Asef peripheral translocation, but had negligible effect on Tiam1 translocation. These effects were associated with attenuation of HGF-induced barrier enhancement in EC pretreated with low ND dose and activation of Rac and its cytoskeletal effectors PAK1 and cortactin. These data demonstrate, that in addition to microtubule-independent Tiam1 activation, HGF engages additional microtubule- and APC-dependent pathway of Asef activation. These mechanisms may complement each other to provide the fine tuning of Rac signaling and endothelial barrier enhancement in response to various agonists.

SC5b-9-induced pulmonary microvascular endothelial hyperpermeability participates in ventilator-induced lung injury

Mechanical ventilation with large tidal volumes can increase lung alveolar permeability and initiate inflammatory responses, termed ventilator-induced lung injury (VILI). VILI is characterized by an influx of inflammatory cells, increased pulmonary permeability, and endothelial and epithelial cell death. But the underlying molecular mechanisms that regulate VILI remain unclear. The purpose of this study was to investigate the mechanisms that regulate pulmonary endothelial barrier in an animal model of VILI. These data suggest that SC5b-9, as the production of the complement activation, causes increase in rat pulmonary microvascular permeability by inducing activation of RhoA and subsequent phosphorylation of myosin light chain and contraction of endothelial cells, resulting in gap formation. In general, the complement-mediated increase in pulmonary microvascular permeability may participate in VILI.

IQGAP1 regulates endothelial barrier function via EB1-cortactin cross talk

Cross talk between the actin cytoskeleton and microtubules (MT) has been implicated in the amplification of agonist-induced Rho signaling, leading to increased vascular endothelial permeability. This study tested the involvement of actin-MT cross talk in the mechanisms of barrier enhancement induced by hepatocyte growth factor (HGF) and evaluated the role of the adaptor protein IQGAP1 in integrating the MT- and actin-dependent pathways of barrier enhancement. IQGAP1 knockdown by small interfering RNA attenuated the HGF-induced increase in endothelial barrier properties and abolished HGF-activated cortical actin dynamics. IQGAP1 reduction abolished HGF-induced peripheral accumulation of Rac cytoskeletal effector cortactin and cortical actin remodeling. In addition, HGF stimulated peripheral MT growth in an IQGAP1-dependent fashion. HGF also induced Rac1-dependent IQGAP1 association with the MT fraction and the formation of a protein complex containing end-binding protein 1 (EB1), IQGAP1, and cortactin. Decreasing endogenous IQGAP1 abolished HGF-induced EB1-cortactin colocalization at the cell periphery. In turn, expression of IQGAP1ΔC (IQGAP1 lacking the C-terminal domain) attenuated the cortactin association with EB1 and suppressed HGF-induced endothelial cell peripheral actin cytoskeleton enhancement. These results demonstrate for the first time the MT-actin cross talk mechanism of HGF-induced endothelial barrier enhancement and suggest that IQGAP1 functions as a hub linking HGF-induced signaling to MT and actin remodeling via EB1-IQGAP1-cortactin interactions.

Stromal cell-derived factor-1α/C-X-C chemokine receptor type 4 axis promotes endothelial cell barrier integrity via phosphoinositide 3-kinase and Rac1 activation

OBJECTIVE

Although stromal cell-derived factor (SDF)-1αis well known to modulate the mobilization of hematopoietic stem cells and endothelial progenitor cells, its effects on some pre-existing vascular functions remain unknown. We have investigated here the role of SDF-1αsignaling in endothelial barrier function.

APPROACH AND RESULTS

Treatment with SDF-1α elevated transendothelial electrical resistance and inhibited the dextran hyperpermeability elicited by thrombin in bovine aortic endothelial cells, both indicating an increase in endothelial barrier function. SDF-1α binds to 2 receptors, C-X-C chemokine receptor types 4 and 7 (CXCR4 and CXCR7). Pretreatment with a CXCR4 antagonist or CXCR4 gene depletion by small interfering RNA (siRNA) eliminated SDF-1α-induced endothelial barrier enhancement. In contrast, CXCR7 antagonist or CXCR7 gene depletion by siRNA did not influence SDF-1α-induced barrier enhancement. Pretreatment with a Gi-protein inhibitor, a phosphoinositide 3-kinase (PI3K) inhibitor, or PI3K p110γsubunit gene depletion by siRNA also inhibited SDF-1α-induced barrier enhancement significantly. Western blot analysis revealed that SDF-1α phosphorylated Akt(Ser473) in endothelial cells, suggesting PI3K activation. Immunostaining showed that treatment with SDF-1αformed a cortical actin rim, which was accompanied by Rac1 activation. In vivo, SDF-1αinhibited croton oil-induced vascular leakage indexed by dye extravasation, which is attenuated by a pretreatment with a CXCR4 antagonist.

CONCLUSIONS

We have identified SDF-1α as a novel suppressor of endothelial permeability. Specifically, SDF-1α stimulates the CXCR4/PI3K/Rac1 signaling pathway and the subsequent cytoskeletal rearrangement.

Interaction of integrin β4 with S1P receptors in S1P- and HGF-induced endothelial barrier enhancement

We previously reported sphingosine 1-phosphate (S1P) and hepatocyte growth factor (HGF) augment endothelial cell (EC) barrier function and attenuate murine acute lung inury (ALI). While the mechanisms underlying these effects are not fully understood, S1P and HGF both transactivate the S1P receptor, S1PR1 and integrin β4 (ITGB4) at membrane caveolin-enriched microdomains (CEMs). In the current study, we investigated the roles of S1PR2 and S1PR3 in S1P/HGF-mediated EC signaling and their associations with ITGB4. Our studies confirmed ITGB4 and S1PR2/3 are recruited to CEMs in human lung EC in response to either S1P (1 µM, 5 min) or HGF (25 ng/ml, 5 min). Co-immunoprecipitation experiments identified an S1P/HGF-mediated interaction of ITGB4 with both S1PR2 and S1PR3. We then employed an in situ proximity ligation assay (PLA) to confirm a direct ITGB4-S1PR3 association induced by S1P/HGF although a direct association was not detectable between S1PR2 and ITGB4. S1PR1 knockdown (siRNA), however, abrogated S1P/HGF-induced ITGB4-S1PR2 associations while there was no effect on ITGB4-S1PR3 associations. Moreover, PLA confirmed a direct association between S1PR1 and S1PR2 induced by S1P and HGF. Finally, silencing of S1PR2 significantly attenuated S1P/HGF-induced EC barrier enhancement as measured by transendothelial resistance while silencing of S1PR3 significantly augmented S1P/HGF-induced barrier enhancement. These results confirm an important role for S1PR2 and S1PR3 in S1P/HGF-mediated EC barrier responses that are associated with their complex formation with ITGB4. Our findings elucidate novel mechanisms of EC barrier regulation that may ultimately lead to new therapeutic targets for disorders characterized by increased vascular permeability including ALI.

HGF and the regulation of tight junctions in human prostate cancer cells

Hepatocyte growth factor (HGF) may impact the metastasis of prostate cancer via its action on prostate stem cells or their progeny. Tight junctions (TJs) are crucial to the process of metastasis and have been previously shown to be regulated by HGF. The present study aimed to evaluate the effect of HGF on the function of TJs in human prostate epithelial, prostate stem cell-like and prostate cancer cell lines. Four human prostate cancer cell lines (PC-3, DU-145, PZHPV-7, CaHPV-10), normal adult prostate parental epithelial cells (RWPE-1) and a stem cell-like derivative of RWPE-1 (WPE-STEM) were used to assess HGF-induced changes in TJs. A significant difference was noted in the behaviour between the WPE-STEM, RWPE-1 and the cancer cell lines which was HGF concentration-dependent. However, in the WPE-STEM cells, the effect was biphasic, with the cells seemingly resistant to HGF-modulated TJ disruption. Closer examination revealed that HGF affected the redistribution of ZO-1, ZO-2 and ZO-3 away from the TJs of confluent cells with concurrent loss of claudin-1 and claudin-5, and western blot analysis revealed a loss in TJ protein expression of ZO-1 and ZO-2. We demonstrated for the first time that HGF regulates TJ function in human prostate cells. Moreover, this regulation was dependent on the tumourigenicity of the cells, with the most aggressive cells most susceptible and the stem cell-like cells least susceptible. These data offer an intriguing glimpse of how TJs affect the behaviour of prostate cancer cells and how HGF modulates the expression and function of the molecules maintaining TJ structure and function.

Hepatocyte growth factor enhances the barrier function in primary cultures of rat brain microvascular endothelial cells

The effects of hepatocyte growth factor (HGF) on barrier functions were investigated by a blood-brain barrier (BBB) in vitro model comprising a primary culture of rat brain capillary endothelial cells (RBEC). In order to examine the response of the peripheral endothelial cells to HGF, human umbilical vascular endothelial cells (HUVEC) and human dermal microvascular endothelial cells (HMVEC) were also treated with HGF. HGF decreased the permeability of RBEC to sodium fluorescein and Evans blue albumin, and dose-dependently increased transendothelial electrical resistance (TEER) in RBEC. HGF altered the immunochemical staining pattern of F-actin bands and made ZO-1 staining more distinct on the linear cell borders in RBEC. In contrast, HGF increased sodium fluorescein and Evans blue albumin permeability in HMVEC and HUVEC, and decreased TEER in HMVEC. In HMVEC, HGF reduced cortical actin bands and increased stress fiber density, and increased the zipper-like appearance of ZO-1 staining. Western blot analysis showed that HGF significantly increased the amount of ZO-1 and VE-cadherin. HGF seems to act on the BBB to strengthen BBB integrity. These findings indicated that cytoskeletal rearrangement and cell-cell adhesion, such as through VE-cadherin and ZO-1, are candidate mechanisms for the influence of HGF on the BBB. The possibility that HGF has therapeutic significance in protecting the BBB from damage needs to be considered.

cAMP with other signaling cues converges on Rac1 to stabilize the endothelial barrier- a signaling pathway compromised in inflammation

cAMP is one of the most potent signaling molecules to stabilize the endothelial barrier, both under resting conditions as well as under challenge of barrier-destabilizing mediators. The two main signaling axes downstream of cAMP are activation of protein kinase A (PKA) as well as engagement of exchange protein directly activated by cAMP (Epac) and its effector GTPase Rap1. Interestingly, both pathways activate GTP exchange factors for Rac1, such as Tiam1 and Vav2 and stabilize the endothelial barrier via Rac1-mediated enforcement of adherens junctions and strengthening of the cortical actin cytoskeleton. On the level of Rac1, cAMP signaling converges with other barrier-enhancing signaling cues induced by sphingosine-1-phosphate (S1P) and angiopoietin-1 (Ang1) rendering Rac1 as an important signaling hub. Moreover, activation of Rap1 and inhibition of RhoA also contribute to barrier stabilization, emphasizing that regulation of small GTPases is a central mechanism in this context. The relevance of cAMP/Rac1-mediated barrier protection under pathophysiologic conditions can be concluded from data showing that inflammatory mediators causing multi-organ failure in systemic inflammation or sepsis interfere with this signaling axis on the level of cAMP or Rac1. This is in line with the well-known efficacy of cAMP to abrogate the barrier breakdown in response to most barrier-compromising stimuli. New is the notion that the tight endothelial barrier under resting conditions is maintained by (1) continuous cAMP formation induced by hormones such as epinephrine or (2) by activation of Rac1 downstream of S1P that is secreted by erythrocytes and activated platelets.

Barrier enhancing signals in pulmonary edema

Increased endothelial permeability and reduction of alveolar liquid clearance capacity are two leading pathogenic mechanisms of pulmonary edema, which is a major complication of acute lung injury, severe pneumonia, and acute respiratory distress syndrome, the pathologies characterized by unacceptably high rates of morbidity and mortality. Besides the success in protective ventilation strategies, no efficient pharmacological approaches exist to treat this devastating condition. Understanding of fundamental mechanisms involved in regulation of endothelial permeability is essential for development of barrier protective therapeutic strategies. Ongoing studies characterized specific barrier protective mechanisms and identified intracellular targets directly involved in regulation of endothelial permeability. Growing evidence suggests that, although each protective agonist triggers a unique pattern of signaling pathways, selected common mechanisms contributing to endothelial barrier protection may be shared by different barrier protective agents. Therefore, understanding of basic barrier protective mechanisms in pulmonary endothelium is essential for selection of optimal treatment of pulmonary edema of different etiology. This article focuses on mechanisms of lung vascular permeability, reviews major intracellular signaling cascades involved in endothelial monolayer barrier preservation and summarizes a current knowledge regarding recently identified compounds which either reduce pulmonary endothelial barrier disruption and hyperpermeability, or reverse preexisting lung vascular barrier compromise induced by pathologic insults.

Small G Proteins in the Cardiovascular System: Physiological and Pathological Aspects

Small G proteins exist in eukaryotes from yeast to human and constitute the Ras superfamily comprising more than 100 members. This superfamily is structurally classified into five families: the Ras, Rho, Rab, Arf, and Ran families that control a wide variety of cell and biological functions through highly coordinated regulation processes. Increasing evidence has accumulated to identify small G proteins and their regulators as key players of the cardiovascular physiology that control a large panel of cardiac (heart rhythm, contraction, hypertrophy) and vascular functions (angiogenesis, vascular permeability, vasoconstriction). Indeed, basal Ras protein activity is required for homeostatic functions in physiological conditions, but sustained overactivation of Ras proteins or spatiotemporal dysregulation of Ras signaling pathways has pathological consequences in the cardiovascular system. The primary object of this review is to provide a comprehensive overview of the current progress in our understanding of the role of small G proteins and their regulators in cardiovascular physiology and pathologies.

Linking metabolism and epigenetic regulation in development of hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the fifth most common form of cancer globally and is rarely curable once detected. The 5-year survival rate of patients diagnosed with late-stage HCC may be as low as 27%. HCC is a cancer largely driven by epigenetic changes that arise from exposure to exogenous environmental factors rather than coding sequence mutations. The liver is susceptible to effects from Hepatitis C and Hepatitis B viruses, exposure to aflatoxin and continuous excessive consumption of alcohol. The liver is a highly metabolic organ balancing many vital biochemical processes; exposure to any of the above environmental factors is associated with loss of liver function and is a major risk factor for the development of HCC. Emerging studies aim to examine the underlying metabolic processes that are abrogated in cancer and lead to the altered flux and availability of key metabolites important for epigenetic processes. Metabolites have been shown to act as substrates for many canonical epigenetic regulators. These enzymes are responsible for regulating histone modification, DNA methylation and micro RNA expression. By studying the impact of altered liver metabolism, we may better understand the long-term epigenetic processes, which lead to the development and progression of HCC.

Rk1, a ginsenoside, is a new blocker of vascular leakage acting through actin structure remodeling

Endothelial barrier integrity is essential for vascular homeostasis and increased vascular permeability and has been implicated in many pathological processes, including diabetic retinopathy. Here, we investigated the effect of Rk1, a ginsenoside extracted from sun ginseng, on regulation of endothelial barrier function. In human retinal endothelial cells, Rk1 strongly inhibited permeability induced by VEGF, advanced glycation end-product, thrombin, or histamine. Furthermore, Rk1 significantly reduced the vessel leakiness of retina in a diabetic mouse model. This anti-permeability activity of Rk1 is correlated with enhanced stability and positioning of tight junction proteins at the boundary between cells. Signaling experiments revealed that Rk1 induces phosphorylation of myosin light chain and cortactin, which are critical regulators for the formation of the cortical actin ring structure and endothelial barrier. These findings raise the possibility that ginsenoside Rk1 could be exploited as a novel prototype compound for the prevention of human diseases that are characterized by vascular leakage.

Inhibition of GSK3α/β promotes increased pulmonary endothelial permeability to albumin by reactive oxygen/nitrogen species

Glycogen synthase kinase 3α/β (GSK3α/β) is a serine/threonine kinase that participates in numerous processes in many cell types. Importantly, the role of GSK3α/β in homeostatic maintenance of the pulmonary endothelial cell barrier to protein is not known. We tested the hypothesis that GSK3α/β regulates endothelial barrier function by measuring the permeability to albumin of a rat pulmonary microvessel endothelial cell monolayer (PMECM) treated with and without the selective GSK3α/β inhibitor SB 216763 (1.0, 5.0 and 10 uM) for 1 h. The treatment with the inhibitor SB 216763 caused a dose dependent decrease in phospho-β-catenin-Ser(33/37) levels indicating effective suppression of GSK3α/β. SB216763 caused an increase in both permeability to albumin and DCFDA (6-Carboxy-2',7'-Dichlorodihydrofluorescein Diacetate, Di(Acetoxymethyl Ester)) oxidation that were prevented by co-treatment with the anti-oxidant tiron or the nitric oxide synthase inhibitor L-NAME (Nω-nitro-l-arginine-methyl ester). In separate studies PMECMs were treated with the Akt inhibitor triciribine (12.5 uM) for 1 h to unmask Akt dependent constitutive suppression of GSK3α/β. Triciribine decreased phospho-GSK3α/β-Ser(21)/9 (i.e., the product of Akt) which was associated with an increase in phospho-β-catenin-Ser(33/37) (i.e., the product of GSK3α/β) indicating constitutive activity of Akt for GSK3α/β-Ser(21/9). The data indicates GSK3α/β inhibition causes increased endothelial monolayer protein permeability which is mediated by reactive oxygen/nitrogen species.

Glassy dynamics, cell mechanics, and endothelial permeability

A key feature of all inflammatory processes is disruption of the vascular endothelial barrier. Such disruption is initiated in part through active contraction of the cytoskeleton of the endothelial cell (EC). Because contractile forces are propagated from cell to cell across a great many cell-cell junctions, this contractile process is strongly cooperative and highly nonlocal. We show here that the characteristic length scale of propagation is modulated by agonists and antagonists that impact permeability of the endothelial barrier. In the presence of agonists including thrombin, histamine, and H2O2, force correlation length increases, whereas in the presence of antagonists including sphingosine-1-phosphate, hepatocyte growth factor, and the rho kinase inhibitor, Y27632, force correlation length decreases. Intercellular force chains and force clusters are also evident, both of which are reminiscent of soft glassy materials approaching a glass transition.