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

Lipopolysaccharide-induced phosphorylation of c-Met tyrosine residue 1003 regulates c-Met intracellular trafficking and lung epithelial barrier function

c-Met, the receptor tyrosine kinase whose natural ligand is hepatocyte growth factor, is known to have a key role in cell motility. We have previously shown that lysophosphatidic acid (LPA) induced a decrease in c-Met activation via serine phosphorylation of c-Met at cell-cell contacts. Here, we demonstrate that lipopolysaccharide (LPS) treatment of human bronchial epithelial cells induced internalization of c-Met via phosphorylation at its tyrosine residue 1003. In addition, it induced epithelial barrier dysfunction as evidenced by a decrease in transepithelial resistance (TER) in a time-dependent manner. Pretreatment with a c-Met inhibitor (PHA-665752) or inhibition of protein kinase C (PKC)-α attenuated the LPS-mediated phosphorylation of c-Met and its internalization. LPS-induced c-Met tyrosine 1003 phosphorylation, activation of PKCα, and c-Met internalization were, however, reversed by pretreatment of cells with LPA, which increased c-Met accumulation at cell-cell contacts. Inhibition of LPS-mediated c-Met tyrosine (Y1003) phosphorylation and internalization by prior treatment with PHA-665752, inhibition of PKCα, or overexpression of c-MetY1003A mutant attenuated LPS-induced reduction of TER. Furthermore, we found that c-Met accumulation at cell-cell contacts contributed to LPA-enhanced epithelial barrier integrity, since downregulation of c-Met by specific small-interfering RNA attenuated LPA-increased TER. The data reveal a novel biological function of c-Met in the regulation of lung epithelial barrier integrity.

Adenosine A1 receptors promote vasa vasorum endothelial cell barrier integrity via Gi and Akt-dependent actin cytoskeleton remodeling

BACKGROUND

In a neonatal model of hypoxic pulmonary hypertension, a dramatic pulmonary artery adventitial thickening, accumulation of inflammatory cells in the adventitial compartment, and angiogenic expansion of the vasa vasorum microcirculatory network are observed. These pathophysiological responses suggest that rapidly proliferating vasa vasorum endothelial cells (VVEC) may exhibit increased permeability for circulating blood cells and macromolecules. However, the molecular mechanisms underlying these observations remain unexplored. Some reports implicated extracellular adenosine in the regulation of vascular permeability under hypoxic and inflammatory conditions. Thus, we aimed to determine the role of adenosine in barrier regulation of VVEC isolated from the pulmonary arteries of normoxic (VVEC-Co) or chronically hypoxic (VVEC-Hyp) neonatal calves.

PRINCIPAL FINDINGS

We demonstrate via a transendothelial electrical resistance measurement that exogenous adenosine significantly enhanced the barrier function in VVEC-Co and, to a lesser extent, in VVEC-Hyp. Our data from a quantitative reverse transcription polymerase chain reaction show that both VVEC-Co and VVEC-Hyp express all four adenosine receptors (A1, A2A, A2B, and A3), with the highest expression level of A1 receptors (A1Rs). However, A1R expression was significantly lower in VVEC-Hyp compared to VVEC-Co. By using an A1R-specific agonist/antagonist and siRNA, we demonstrate that A1Rs are mostly responsible for adenosine-induced enhancement in barrier function. Adenosine-induced barrier integrity enhancement was attenuated by pretreatment of VVEC with pertussis toxin and GSK690693 or LY294002, suggesting the involvement of Gi proteins and the PI3K-Akt pathway. Moreover, we reveal a critical role of actin cytoskeleton in VVEC barrier regulation by using specific inhibitors of actin and microtubule polymerization. Further, we show that adenosine pretreatment blocked the tumor necrosis factor alpha (TNF-α)-induced permeability in VVEC-Co, validating its anti-inflammatory effects.

CONCLUSIONS

We demonstrate for the first time that stimulation of A1Rs enhances the barrier function in VVEC by activation of the Gi/PI3K/Akt pathway and remodeling of actin microfilament.

Inhibition of glycogen synthase kinase 3β promotes tight junction stability in brain endothelial cells by half-life extension of occludin and claudin-5

Neuroinflammatory conditions often involve dysfunction of the Blood-Brain Barrier (BBB). Therefore, identifying molecular targets that can maintain barrier fidelity is of clinical importance. We have previously reported on the anti-inflammatory effects that glycogen synthase kinase 3β (GSK3β) inhibition has on primary human brain endothelial cells. Here we show that GSK3β inhibitors also promote barrier tightness by affecting tight junction (TJ) protein stability. Transendothelial electrical resistance (TEER) was used to evaluate barrier integrity with both pharmacological inhibitors and mutants of GSK3β. Inhibition of GSK3β produced a gradual and sustained increase in TEER (as much as 22% over baseline). Analysis of subcellular membrane fractions revealed an increase in the amount of essential tight junction proteins, occludin and claudin-5, but not claudin-3. This phenomenon was attributed to a decrease in TJ protein turnover and not transcriptional regulation. Using a novel cell-based assay, inactivation of GSK3β significantly increased the half-life of occludin and claudin-5 by 32% and 43%, respectively. A correlation was also established between the enhanced association of β-catenin with ZO-1 as a function of GSK3β inhibition. Collectively, our findings suggest the possibility of using GSK3β inhibitors as a means to extend the half-life of key tight junction proteins to promote re-sealing of the BBB during neuroinflammation.

Inhibitory phosphorylation of GSK-3β by AKT, PKA, and PI3K contributes to high NaCl-induced activation of the transcription factor NFAT5 (TonEBP/OREBP)

High NaCl activates the transcription factor nuclear factor of activated T cells 5 (NFAT5), leading to increased transcription of osmoprotective target genes. Kinases PKA, PI3K, AKT1, and p38α were known to contribute to the high NaCl-induced increase of NFAT5 activity. We now identify another kinase, GSK-3β. siRNA-mediated knock-down of GSK-3β increases NFAT5 transcriptional and transactivating activities without affecting high NaCl-induced nuclear localization of NFAT5 or NFAT5 protein expression. High NaCl increases phosphorylation of GSK-3β-S9, which inhibits GSK-3β. In GSK-3β-null mouse embryonic fibroblasts transfection of GSK-3β, in which serine 9 is mutated to alanine, so that it cannot be inhibited by phosphorylation at that site, inhibits high NaCl-induced NFAT5 transcriptional activity more than transfection of wild-type GSK-3β. High NaCl-induced phosphorylation of GSK-3β-S9 depends on PKA, PI3K, and AKT, but not p38α. Overexpression of PKA catalytic subunit α or of catalytically active AKT1 reduces inhibition of NFAT5 by GSK-3β, but overexpression of p38α together with its catalytically active upstream kinase, MKK6, does not. Thus, GSK-3β normally inhibits NFAT5 by suppressing its transactivating activity. When activated by high NaCl, PKA, PI3K, and AKT1, but not p38α, increase phosphorylation of GSK-3β-S9, which reduces the inhibitory effect of GSK-3β on NFAT5, and thus contributes to activation of NFAT5.

Tetraspanin CD82 inhibits protrusion and retraction in cell movement by attenuating the plasma membrane-dependent actin organization

To determine how tetraspanin KAI1/CD82, a tumor metastasis suppressor, inhibits cell migration, we assessed which cellular events critical for motility are altered by KAI1/CD82 and how KAI1/CD82 regulates these events. We found that KAI1/CD82-expressing cells typically exhibited elongated cellular tails and diminished lamellipodia. Live imaging demonstrated that the polarized protrusion and retraction of the plasma membrane became deficient upon KAI1/CD82 expression. The deficiency in developing these motility-related cellular events was caused by poor formations of actin cortical network and stress fiber and by aberrant dynamics in actin organization. Rac1 activity was reduced by KAI1/CD82, consistent with the diminution of lamellipodia and actin cortical network; while the growth factor-stimulated RhoA activity was blocked by KAI1/CD82, consistent with the loss of stress fiber and attenuation in cellular retraction. Upon KAI1/CD82 expression, Rac effector cofilin was not enriched at the cell periphery to facilitate lamellipodia formation while Rho kinase exhibited a significantly lower activity leading to less retraction. Phosphatidylinositol 4, 5-biphosphate, which initiates actin polymerization from the plasma membrane, became less detectable at the cell periphery in KAI1/CD82-expressing cells. Moreover, KAI1/CD82-induced phenotypes likely resulted from the suppression of multiple signaling pathways such as integrin and growth factor signaling. In summary, at the cellular level KAI1/CD82 inhibited polarized protrusion and retraction events by disrupting actin reorganization; at the molecular level, KAI1/CD82 deregulated Rac1, RhoA, and their effectors cofilin and Rho kinase by perturbing the plasma membrane lipids.

Critical role of S1PR1 and integrin β4 in HGF/c-Met-mediated increases in vascular integrity

Vascular endothelial cell (EC) barrier integrity is critical to vessel homeostasis whereas barrier dysfunction is a key feature of inflammatory disorders and tumor angiogenesis. We previously reported that hepatocyte growth factor (HGF)-mediated increases in EC barrier integrity are signaled through a dynamic complex present in lipid rafts involving its receptor, c-Met. We extended these observations to confirm that S1PR1 (sphingosine 1-phosphate receptor 1) and integrin β4 (ITGB4) are essential participants in HGF-induced EC barrier enhancement. Immunoprecipitation experiments demonstrated HGF-mediated recruitment of c-Met, ITGB4 and S1PR1 to caveolin-enriched lipid rafts in human lung EC with direct interactions of c-Met with both S1PR1 and ITGB4 accompanied by c-Met-dependent S1PR1 and ITGB4 transactivation. Reduced S1PR1 expression (siRNA) attenuated both ITGB4 and Rac1 activation as well as c-Met/ITGB4 interaction and resulted in decreased transendothelial electrical resistance. Furthermore, reduced ITGB4 expression attenuated HGF-induced c-Met activation, c-Met/S1PR1 interaction, and effected decreases in S1P- and HGF-induced EC barrier enhancement. Finally, the c-Met inhibitor, XL880, suppressed HGF-induced c-Met activation as well as S1PR1 and ITGB4 transactivation. These results support a critical role for S1PR1 and ITGB4 transactivation as rate-limiting events in the transduction of HGF signals via a dynamic c-Met complex resulting in enhanced EC barrier integrity.

Activation of cannabinoid receptor 2 attenuates leukocyte-endothelial cell interactions and blood-brain barrier dysfunction under inflammatory conditions

Previous studies have shown that modulation of the receptor-mediated cannabinoid system during neuroinflammation can produce potent neuroprotective and anti-inflammatory effects. However, in this context, little is known about how selective activation of the cannabinoid type-2 receptor (CB2R) affects the activated state of the brain endothelium and blood-brain barrier (BBB) function. Using human brain tissues and primary human brain microvascular endothelial cells (BMVECs), we demonstrate that the CB2R is highly upregulated during inflammatory insult. We then examined whether the CB2R agonists could attenuate inflammatory responses at the BBB using a mouse model of LPS-induced encephalitis and highly selective CB2R agonists. Visualization by intravital microscopy revealed that administration of JWH133 [(6aR,10aR)-3-(1,1-dimethylbutyl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6H-dibenzo[b,d]pyran] or a novel resorcinol-based compound, O-1966 (1-[4-(1,1-dimethyl-heptyl)-2,6-dimethoxy-phenyl]-3-methyl-cyclohexanol), greatly attenuated leukocyte adhesion in surface pial vessels and in deep ascending cortical postcapillary venules. BBB permeability assessments with small and large fluorescent tracers showed that CB2R agonists were effective at preventing barrier leakiness after LPS administration. To determine whether the effects by CB2R agonists on barrier protection are not only due to the CB2R modulation of immune cell function, we tested the agonists in vitro with barrier-forming primary BMVECs. Remarkably, the addition of CB2R agonist increased transendothelial electrical resistance and increased the amount of tight junction protein present in membrane fractions. Furthermore, CB2R agonists decreased the induction of intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 surface expression in BMVECs exposed to various proinflammatory mediators. Together, these results suggest that pharmacological CB2R ligands offer a new strategy for BBB protection during neuroinflammation.

Insights into gene therapy for critical limb ischemia: the devil is in the details

Patients with critical limb ischemia (CLI) without potential for revascularization are currently without alternate therapies. Several gene therapy trials have tested angiogenesis factors, hepatic growth factor, vascular endothelial growth factor, and basic fibroblast growth factor, in rescuing CLI patients from amputation and mortality, and for improved quality of life including decreased pain, improved healing, and blood flow. Trial results have been variable, with HGF gene therapy being the most successful. New studies examining each of these angiogenic factors provide insights that will be useful for the design of effective therapeutic strategies.

Role of sphingomyelin synthesis in pulmonary endothelial cell cytoskeletal activation and endotoxin-induced lung injury

Sphingomyelin (SM), a major sphingolipid in the lipid raft microdomains of the cell membrane, is synthesized by plasma membrane-bound sphingomyelin synthase 2 (SMS2). SMS2 is required for the maintenance of plasma membrane microdomain fluidity and receptor-mediated responses to inflammation in macrophages. However, the exact mechanism of SMS2 activation in endothelial barrier disruption and lung injury is not fully understood. To define the role of SMS activation in lung injury, we hypothesized that the inhibition of SM synthesis may provide protection against acute lung injury (ALI) by preserving endothelial barrier function. Using SMS2-silencing RNA (siRNA) treatment in human pulmonary endothelial cells (HPAECs) and tricyclodecan-9-yl-xanthogenate (D609), a competitive inhibitor of SMS, and phosphatidylcholine-specific phospholipase C in a murine model of bacterial LPS injury, we studied the role of sphingomyelin synthesis in ALI. Results show that pretreating mice with D609 significantly attenuated LPS-induced lung injury, as measured by a significant decrease in wet to dry ratio, bronchoalveolar lavage fluid cell and protein counts, and myeloperoxidase activity in lung tissue. Similarly, LPS-induced endothelial barrier disruption was significantly reduced in HPAECs pretreated with D609 or SMS2 siRNA, as demonstrated by an increase in paracellular integrity on an FITC-dextran assay, by the inhibition of LPS-induced stress fibers, and by the formation of cortical actin rings and lamellipodia at the periphery. These results indicate that D609 attenuates LPS-mediated endothelial barrier dysfunction and lung injury in mice through inhibition of SMS, suggesting a novel and essential role of SMS inhibition in modulating endothelial barrier integrity via actin cytoskeletal activation, with a potential therapeutic role in ALI.

Role of growth arrest and DNA damage-inducible α in Akt phosphorylation and ubiquitination after mechanical stress-induced vascular injury

RATIONALE

The stress-induced growth arrest and DNA damage-inducible a (GADD45a) gene is up-regulated by mechanical stress with GADD45a knockout (GADD45a(-/-)) mice demonstrating both increased susceptibility to ventilator-induced lung injury (VILI) and reduced levels of the cell survival and vascular permeability signaling effector (Akt). However, the functional role of GADD45a in the pathogenesis of VILI is unknown.

OBJECTIVES

We sought to define the role of GADD45a in the regulation of Akt activation induced by mechanical stress.

METHODS

VILI-challenged GADD45a(-/-) mice were administered a constitutively active Akt1 vector and injury was assessed by bronchoalveolar lavage cell counts and protein levels. Human pulmonary artery endothelial cells (EC) were exposed to 18% cyclic stretch (CS) under conditions of GADD45a silencing and used for immunoprecipitation, Western blotting or immunofluoresence. EC were also transfected with mutant ubiquitin vectors to characterize site-specific Akt ubiquitination. DNA methylation was measured using methylspecific polymerase chain reaction assay.

MEASUREMENTS AND MAIN RESULTS

Studies exploring the linkage of GADD45a with mechanical stress and Akt regulation revealed VILI challenged GADD45a(-/-) mice to have significantly reduced lung injury on overexpression of Akt1 transgene. Increased mechanical stress with 18% CS in EC induced Akt phosphorylation via E3 ligase tumor necrosis factor receptor–associated factor 6 (TRAF6)–mediated Akt K63 ubiquitination resulting in Akt trafficking and activation at the membrane. GADD45a is essential to this process because GADD45a silenced endothelial cells and GADD45a(-/-) mice exhibited increased Akt K48 ubiquitination leading to proteasomal degradation. These events involve loss of ubiquitin carboxyl terminal hydrolase 1(UCHL1), a deubiquitinating enzyme that normally removes K48 polyubiquitin chains bound to Akt thus promoting Akt K63 ubiquitination. Loss of GADD45a significantly reduces UCHL1 expression via UCHL1 promoter methylation resulting in increased Akt K48 ubiquitination and reduced Akt levels.

CONCLUSIONS

These studies highlight a novel role for GADD45a in the regulation of site-specific Akt ubiquitination and activation and implicate a significant functional role for GADD45a in the clinical predisposition to VILI.

Differential involvement of ezrin/radixin/moesin proteins in sphingosine 1-phosphate-induced human pulmonary endothelial cell barrier enhancement

Endothelial cell (EC) barrier dysfunction induced by inflammatory agonists is a frequent pathophysiologic event in multiple diseases. The platelet-derived phospholipid sphingosine-1 phosphate (S1P) reverses this dysfunction by potently enhancing the EC barrier through a process involving Rac GTPase-dependent cortical actin rearrangement as an integral step. In this study we explored the role of the ezrin, radixin, and moesin (ERM) family of actin-binding linker protein in modulating S1P-induced human pulmonary EC barrier enhancement. S1P induces ERM translocation to the EC periphery and promotes ERM phosphorylation on a critical threonine residue (Ezrin-567, Radixin-564, Moesin-558). This phosphorylation is dependent on activation of PKC isoforms and Rac1. The majority of ERM phosphorylation on these critical threonine residues after S1P occurs in moesin and ezrin. Baseline radixin phosphorylation is higher than in the other two ERM proteins but does not increase after S1P. S1P-induced moesin and ezrin threonine phosphorylation is not mediated by the barrier enhancing receptor S1PR1 because siRNA downregulation of S1PR1 fails to inhibit these phosphorylation events, while stimulation of EC with the S1PR1-specific agonist SEW2871 fails to induce these phosphorylation events. Silencing of either all ERM proteins or radixin alone (but not moesin alone) reduced S1P-induced Rac1 activation and phosphorylation of the downstream Rac1 effector PAK1. Radixin siRNA alone, or combined siRNA for all three ERM proteins, dramatically attenuates S1P-induced EC barrier enhancement (measured by transendothelial electrical resistance (TER), peripheral accumulation of di-phospho-MLC, and cortical cytoskeletal rearrangement. In contrast, moesin depletion has the opposite effects on these parameters. Ezrin silencing partially attenuates S1P-induced EC barrier enhancement and cytoskeletal changes. Thus, despite structural similarities and reported functional redundancy, the ERM proteins differentially modulate S1P-induced alterations in lung EC cytoskeleton and permeability. These results suggest that ERM activation is an important regulatory event in EC barrier responses to S1P.

Glycogen synthase kinase 3 beta positively regulates Notch signaling in vascular smooth muscle cells: role in cell proliferation and survival

The role of glycogen synthase kinase 3 beta (GSK-3β) in modulating Notch control of vascular smooth muscle cell (vSMC) growth (proliferation and apoptosis) was examined in vitro under varying conditions of cyclic strain and validated in vivo following changes in medial tension and stress. Modulation of GSK-3β in vSMC following ectopic expression of constitutively active GSK-3β, siRNA knockdown and pharmacological inhibition with SB-216763 demonstrated that GSK-3β positively regulates Notch intracellular domain expression, CBF-1/RBP-Jκ transactivation and downstream target gene mRNA levels, while concomitantly promoting vSMC proliferation and inhibiting apoptosis. In contrast, inhibition of GSK-3β attenuated Notch signaling and decreased vSMC proliferation and survival. Exposure of vSMC to cyclic strain environments in vitro using both a Flexercell™ Tension system and a novel Sylgard™ phantom vessel following bare metal stent implantation revealed that cyclic strain inhibits GSK-3β activity independent of p42/p44 MAPK and p38 activation concomitant with reduced Notch signaling and decreased vSMC proliferation and survival. Exposure of vSMC to changes in medial strain microenvironments in vivo following carotid artery ligation revealed that enhanced GSK-3β activity was predominantly localized to medial and neointimal vSMC concomitant with increased Notch signaling, proliferating nuclear antigen and decreased Bax expression, respectively, as vascular remodeling progressed. GSK-3β is an important modulator of Notch signaling leading to altered vSMC cell growth where low strain/tension microenvironments prevail.

Factor VIIa bound to endothelial cell protein C receptor activates protease activated receptor-1 and mediates cell signaling and barrier protection

Recent studies have shown that factor VIIa (FVIIa) binds to the endothelial cell protein C receptor (EPCR), a cellular receptor for protein C and activated protein C, but the physiologic significance of this interaction is unclear. In the present study, we show that FVIIa, upon binding to EPCR on endothelial cells, activates endogenous protease activated receptor-1 (PAR1) and induces PAR1-mediated p44/42 mitogen-activated protein kinase (MAPK) activation. Pretreatment of endothelial cells with FVIIa protected against thrombin-induced barrier disruption. This FVIIa-induced, barrier-protective effect was EPCR dependent and did not involve PAR2. Pretreatment of confluent endothelial monolayers with FVIIa before thrombin reduced the development of thrombin-induced transcellular actin stress fibers, cellular contractions, and paracellular gap formation. FVIIa-induced p44/42 MAPK activation and the barrier-protective effect are mediated via Rac1 activation. Consistent with in vitro findings, in vivo studies using mice showed that administration of FVIIa before lipopolysaccharide (LPS) treatment attenuated LPS-induced vascular leakage in the lung and kidney. Overall, our present data provide evidence that FVIIa bound to EPCR on endothelial cells activates PAR1-mediated cell signaling and provides a barrier-protective effect. These findings are novel and of great clinical significance, because FVIIa is used clinically for the prevention of bleeding in hemophilia and other bleeding disorders.

p190RhoGAP mediates protective effects of oxidized phospholipids in the models of ventilator-induced lung injury

Products resulting from oxidation of cell membrane phospholipid 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC) exhibit potent protective effects against lung endothelial cell (EC) barrier dysfunction caused by pathologically relevant mechanical forces and inflammatory agents. These effects were linked to enhancement of peripheral cytoskeleton and cell adhesion interactions mediated by small GTPase Rac and inhibition of Rho-mediated barrier-disruptive signaling. However, the mechanism of OxPAPC-induced, Rac-dependent Rho downregulation critical for vascular barrier protection remains unclear. This study tested the hypothesis that Rho negative regulator p190RhoGAP is essential for OxPAPC-induced lung barrier protection against ventilator-induced lung injury (VILI), and investigated potential mechanism of p190RhoGAP targeting to adherens junctions (AJ) via p120-catenin. OxPAPC induced peripheral translocation of p190RhoGAP, which was abolished by knockdown of Rac-specific guanine nucleotide exchange factors Tiam1 and Vav2. OxPAPC also induced Rac-dependent tyrosine phosphorylation and association of p190RhoGAP with AJ protein p120-catenin. siRNA-induced knockdown of p190RhoGAP attenuated protective effects of OxPAPC against EC barrier compromise induced by thrombin and pathologically relevant cyclic stretch (18% CS). In vivo, p190RhoGAP knockdown significantly attenuated protective effects of OxPAPC against ventilator-induced lung vascular leak, as detected by increased cell count and protein content in the bronchoalveolar lavage fluid, and tissue neutrophil accumulation in the lung. These results demonstrate for the first time a key role of p190RhoGAP for the vascular endothelial barrier protection in VILI.

Active Rac1 improves pathologic VEGF neovessel architecture and reduces vascular leak: mechanistic similarities with angiopoietin-1

Architecturally defective, leaky blood vessels typify pathologic angiogenesis induced by vascular endothelial growth factor-A (VEGF-A). Such neovascular defects aggravate disease pathology and seriously compromise the therapeutic utility of VEGF. Endothelial cell (EC) transduction with active L61Rac1 strongly improved VEGF-driven angiogenesis in vivo as measured by increased neovascular density, enhanced lumen formation, and reduced vessel leakiness. Conversely, transduction with dominant-negative N17Rac1 strongly inhibited neovascularization. In vitro, active L61Rac1 promoted organization of cortical actin filaments and vascular cords and improved EC-EC junctions, indicating that improved cytoskeletal dynamics are important to the mechanism by which active L61Rac1 rectifies VEGF-driven angiogenesis. SEW2871, a sphingosine 1-phosphate receptor-1 agonist that activates Rac1 in ECs, improved cord formation and EC-EC junctions in vitro similarly to active L61Rac. Moreover, SEW2871 administration in vivo markedly improved VEGF neovessel architecture and reduced neovascular leak. Angiopoietin-1, a cytokine that "normalizes" VEGF neovessels in vivo, activated Rac1 and improved cord formation and EC-EC junctions in vitro comparably to active L61Rac1, and a specific Rac1 inhibitor blocked these effects. These studies distinguish augmentation of Rac1 activity as a means to rectify the pathologic angioarchitecture and dysfunctionality of VEGF neovessels, and they identify a rational pharmacologic strategy for improving VEGF angiogenesis.

Septin-2 mediates airway epithelial barrier function in physiologic and pathologic conditions

Epithelial cells have the ability to regulate paracellular permeability dynamically in response to extracellular stimuli. With every respiratory effort, airway epithelial cells are exposed to both physiologic as well as pathologic stimuli, and regulation of the epithelial barrier in response to these stimuli is crucial to respiratory function. We report that increased membrane septin-2 localization mediates decreases in paracellular permeability by altering cortical actin arrangement in human airway epithelial cells. This phenomenon occurs in response to both physiologic levels of shear stress and a pathologic stimulus, particular matter exposure. The resulting changes in barrier function in response to septin-2 redistribution have a significant impact on the ability of the apical ligand, epidermal growth factor, to interact with its receptor, epidermal growth factor receptor, which is segregated to the basolateral side in airway epithelial cells. This suggests that the dynamic regulation of the epithelial barrier function is essential in regulating signaling responses to extracellular stimuli. These findings indicate that septin-2 plays a fundamental role in regulating barrier function by altering cortical actin expression.

Abl tyrosine kinase phosphorylates nonmuscle Myosin light chain kinase to regulate endothelial barrier function

This study identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy and examined their influence on nmMLCK function and human lung endothelial barrier regulation. The data indicate an essential role for Abl kinase in vascular barrier regulation via phosphorylation of nmMLCK and the actin-binding protein cortactin.

Nonmuscle myosin light chain kinase (nmMLCK), a multi-functional cytoskeletal protein critical to vascular homeostasis, is highly regulated by tyrosine phosphorylation. We identified multiple novel c-Abl–mediated nmMLCK phosphorylation sites by mass spectroscopy analysis (including Y²³¹, Y⁴⁶⁴, Y⁵⁵⁶, Y⁸⁴⁶) and examined their influence on nmMLCK function and human lung endothelial cell (EC) barrier regulation. Tyrosine phosphorylation of nmMLCK increased kinase activity, reversed nmMLCK-mediated inhibition of Arp2/3-mediated actin polymerization, and enhanced binding to the critical actin-binding phosphotyrosine protein, cortactin. EC challenge with sphingosine 1-phosphate (S1P), a potent barrier-enhancing agonist, resulted in c-Abl and phosphorylated nmMLCK recruitment into caveolin-enriched microdomains, rapid increases in Abl kinase activity, and spatial targeting of c-Abl to barrier-promoting cortical actin structures. Conversely, reduced c-Abl expression in EC (siRNA) markedly attenuated S1P-mediated cortical actin formation, reduced the EC modulus of elasticity (assessed by atomic force microscopy), reduced nmMLCK and cortactin tyrosine phosphorylation, and attenuated S1P-mediated barrier enhancement. These studies indicate an essential role for Abl kinase in vascular barrier regulation via posttranslational modification of nmMLCK and strongly support c-Abl-cortactin-nmMLCK interaction as a novel determinant of cortical actin-based cytoskeletal rearrangement critical to S1P-mediated EC barrier enhancement.

Cdc42-mediated inhibition of GSK-3β improves angio-architecture and lumen formation during VEGF-driven pathological angiogenesis

Vascular endothelial growth factor-A (VEGF) typically induces abnormal angiogenesis in the adult, thereby aggravating disease pathology and limiting utility of VEGF for therapeutic angiogenesis. To identify strategies for rectifying defects in pathological VEGF neovessels, we investigated consequences of modulating the Rho GTPase Cdc42. In a mouse skin model of VEGF-driven pathological angiogenesis, transduction with active Cdc42 (L28Cdc42) markedly improved VEGF neovessels, as measured by increased lumen formation, enlarged vessel diameter, and enhanced perfusion of macromolecular tracers. Conversely, transduction with dominant negative Cdc42 (N17Cdc42) impaired endothelial cell (EC) assembly into lumenized blood vessels and reduced neovessel diameter and tracer perfusion. In vitro, active Cdc42 improved coordination between actin filaments and microtubules and enhanced formation of vascular cords, suggesting that active Cdc42 rectifies defects in angiogenesis by improving cytoskeletal dynamics and capillary morphogenesis. Analyses of Cdc42 signaling in microvascular ECs indicated that active Cdc42 also inhibits glycogen synthase kinase-3β (GSK-3β), a multi-functional serine/threonine protein kinase. Pharmacological inhibition of GSK-3β improved vascular cord formation in vitro and promoted proper neovessel formation in vivo comparably to active Cdc42, thus linking GSK-3β inhibition to the mechanism by which active Cdc42 rectifies pathological neovascularization. These studies identify activation of Cdc42 and inhibition of GSK-3β as novel strategies for correcting abnormalities associated with VEGF-driven angiogenesis, and they suggest new approaches for achieving improved therapeutic neovascularization with VEGF.

Atrial natriuretic peptide attenuates LPS-induced lung vascular leak: role of PAK1

Increased levels of atrial natriuretic peptide (ANP) in the models of sepsis, pulmonary edema, and acute respiratory distress syndrome (ARDS) suggest its potential role in the modulation of acute lung injury. We have recently described ANP-protective effects against thrombin-induced barrier dysfunction in pulmonary endothelial cells (EC). The current study examined involvement of the Rac effector p21-activated kinase (PAK1) in ANP-protective effects in the model of lung vascular permeability induced by bacterial wall LPS. C57BL/6J mice or ANP knockout mice (Nppa(-/-)) were treated with LPS (0.63 mg/kg intratracheal) with or without ANP (2 μg/kg iv). Lung injury was monitored by measurements of bronchoalveolar lavage protein content, cell count, Evans blue extravasation, and lung histology. Endothelial barrier properties were assessed by morphological analysis and measurements of transendothelial electrical resistance. ANP treatment stimulated Rac-dependent PAK1 phosphorylation, attenuated endothelial permeability caused by LPS, TNF-α, and IL-6, decreased LPS-induced cell and protein accumulation in bronchoalveolar lavage fluid, and suppressed Evans blue extravasation in the murine model of acute lung injury. More severe LPS-induced lung injury and vascular leak were observed in ANP knockout mice. In rescue experiments, ANP injection significantly reduced lung injury in Nppa(-/-) mice caused by LPS. Molecular inhibition of PAK1 suppressed the protective effects of ANP treatment against LPS-induced lung injury and endothelial barrier dysfunction. This study shows that the protective effects of ANP against LPS-induced vascular leak are mediated at least in part by PAK1-dependent signaling leading to EC barrier enhancement. Our data suggest a direct role for ANP in endothelial barrier regulation via modulation of small GTPase signaling.

Glycogen Synthase Kinase 3 (GSK-3) influences epithelial barrier function by regulating occludin, claudin-1 and E-cadherin expression

The Apical Junctional Complex (AJC) encompassing the tight junction (TJ) and adherens junction (AJ) plays a pivotal role in regulating epithelial barrier function and epithelial cell proliferative processes through signaling events that remain poorly characterized. A potential regulator of AJC protein expression is Glycogen Synthase Kinase-3 (GSK-3). GSK-3 is a constitutively active kinase that is repressed during epithelial-mesenchymal transition (EMT). In the present study, we report that GSK-3 activity regulates the structure and function of the AJC in polarized model intestinal (SK-CO15) and kidney (Madin-Darby Canine Kidney (MDCK)) epithelial cells. Reduction of GSK-3 activity, either by small molecule inhibitors or siRNA targeting GSK-3 alpha and beta mRNA, resulted in increased permeability to both ions and bulk solutes. Immunofluorescence labeling and immunoblot analyses revealed that the barrier defects correlated with decreased protein expression of AJC transmembrane proteins Occludin, Claudin-1 and E-cadherin without influencing other TJ proteins, Zonula Occludens-1 (ZO-1) and Junctional Adhesion Molecule A (JAM-A). The decrease in Occludin and E-cadherin protein expression correlated with downregulation of the corresponding mRNA levels for these respective proteins following GSK-3 inhibition. These observations implicate an important role of GSK-3 in the regulation of the structure and function of the AJC that is mediated by differential modulation of mRNA transcription of key AJC proteins, Occludin, Claudin-1 and E-cadherin.

T-cadherin modulates endothelial barrier function

T-cadherin is an atypical member of the cadherin family, which lacks the transmembrane and intracellular domains and is attached to the plasma membrane via a glycosylphosphatidylinositol anchor. Unlike canonical cadherins, it is believed to function primarily as a signaling molecule. T-cadherin is highly expressed in endothelium. Using transendothelial electrical resistance measurements and siRNA-mediated depletion of T-cadherin in human umbilical vein endothelial cells, we examined its involvement in regulation of endothelial barrier. We found that in resting confluent monolayers adjusted either to 1% or 10% serum, T-cadherin depletion modestly, but consistently reduced transendothelial resistance. This was accompanied by increased phosphorylation of Akt and LIM kinase, reduced phosphorylation of p38 MAP kinase, but no difference in tubulin acetylation and in phosphorylation of an actin filament severing protein cofilin and myosin light chain kinase. Serum stimulation elicited a biphasic increase in resistance with peaks at 0.5 and 4-5 h, which was suppressed by a PI3 kinase/Akt inhibitor wortmannin and a p38 inhibitor SB 239063. T-cadherin depletion increased transendothelial resistance between the two peaks and reduced the amplitude of the second peak. T-cadherin depletion abrogated serum-induced Akt phosphorylation at Thr308 and reduced phosphorylation at Ser473, reduced phosphorylation of cofilin, and accelerated tubulin deacetylation. Adiponectin slightly improved transendothelial resistance irrespectively of T-cadherin depletion. T-cadherin depletion also resulted in a reduced sensitivity and delayed responses to thrombin. These data implicate T-cadherin in regulation of endothelial barrier function, and suggest a complex signaling network that links T-cadherin and regulation of barrier function.

Inhibition of glycogen synthase kinase 3beta (GSK3beta) decreases inflammatory responses in brain endothelial cells

Immune mediators and leukocyte engagement of brain microvascular endothelial cells (BMVECs) contribute to blood-brain barrier impairment during neuroinflammation. Glycogen synthase kinase 3beta (GSK3beta) was recently identified as a potent regulator of immune responses in in vitro systems and animal models. However, the role of GSK3beta in regulation of immune endothelial functions remains undetermined. Here we evaluated the effect of GSK3beta inhibition on the regulation of inflammatory responses in BMVECs. A focused PCR gene array of 84 genes was performed to identify the cytokine and chemokine gene expression profile in tumor necrosis factor (TNF) alpha-stimulated BMVECs after GSK3beta inactivation by specific inhibitors. Fifteen of 39 genes induced by TNFalpha stimulation were down-regulated after GSK3beta inhibition. Genes known to contribute to neuroinflammation that were most negatively affected by GSK3beta inactivation included IP-10/CXCL10, MCP-1/CCL2, IL-8/CXCL8, RANTES/CCL5, and Groalpha/CXCL1. GSK3beta suppression resulted in diminished secretion of these proinflammatory mediators by inflamed BMVECs detected by ELISA. GSK3beta inhibition in BMVECs reduced adhesion molecule expression as well as monocyte adhesion to and migration across cytokine stimulated BMVEC monolayers. Interactions of monocytes with TNFalpha-activated BMVECs led to barrier disruption, and GSK3beta suppression in the endothelium restored barrier integrity. GSK3beta inhibition in vivo substantially decreased leukocyte adhesion to brain endothelium under inflammatory conditions. In summary, inhibition of GSK3beta emerges as an important target for stabilization of the blood-brain barrier in neuroinflammation.

Quantitative distribution and colocalization of non-muscle myosin light chain kinase isoforms and cortactin in human lung endothelium

Vascular barrier regulation is intimately linked to alterations in the distribution and configuration of the endothelial cell (EC) cytoskeleton in response to angiogenic and edemagenic agonists. Critical actin cytoskeletal rearrangement includes spatially directed increases in myosin light chain (MLC) phosphorylation, catalyzed by Ca(2+)/calmodulin-dependent non-muscle myosin light chain kinase variants (nmMLCK1- and -2), as well as association of nmMLCK with the actin-binding protein, cortactin. As these associations have proven difficult to quantify in a spatially specific manner, we now describe the utility of intensity correlation image analysis and the intensity correlation quotient (ICQ) to quantify colocalization in fixed and live cell imaging assays in human pulmonary artery EC. From baseline ICQ values averaging 0.216 reflecting colocalization of cortactin-DsRed with EGFP-nmMLCK fusion proteins in resting EC, thrombin-induced EC contraction significantly reduced cortactin-DsRed-EGFP-nmMLCK colocalization (nmMLCK1: ICQ=0.118; nmMLCK2: ICQ=0.091) whereas the potent EC barrier-protective agonist, sphingosine 1-phosphate (S1P), significantly increased nmMLCK-cortactin colocalization within lamellipodia (nmMLCK1: ICQ=0.275; nmMLCK2: ICQ=0.334). Over-expression of a cortactin-DsRed mutant fusion protein lacking the SH3 domain, known to be essential for cortactin-nmMLCK association, reduced baseline and S1P-mediated live cell colocalization with each nmMLCK variant (nmMLCK1: ICQ=0.160; nmMLCK2: ICQ=0.157). Similarly, expression of a truncated EGFP-nmMLCK2 mutant lacking cortactin- and actin-binding domains, markedly reduced basal localization in lamellipodia and abolished colocalization with cortactin-DsRed in lamellipodia after S1P (ICQ=-0.148). These data provide insights into the molecular basis for vascular barrier-regulatory cytoskeletal responses and support the utility of sophisticated imaging analyses and methodological assessment to quantify the critical nmMLCK and cortactin interaction during vascular barrier regulation.

Cross-talk between the VEGF-A and HGF signalling pathways in endothelial cells

BACKGROUND INFORMATION

Endothelial cells play a major role in angiogenesis, the process by which new blood vessels arise from a pre-existing vascular bed. VEGF-A (vascular endothelial growth factor-A) is a key regulator of angiogenesis during both development and in adults. HGF (hepatocyte growth factor) is a pleiotropic cytokine that may promote VEGF-A-driven angiogenesis, although the signalling mechanisms underlying this co-operation are not completely understood.

RESULTS

We analysed the effects of the combination of VEGF-A and HGF on the activation of VEGFR-2 (VEGF receptor-2) and c-met receptors, and on the stimulation of downstream signalling pathways in endothelial cells. We found that VEGFR-2 and c-met do not physically associate and do not transphosphorylate each other, suggesting that co-operation involves signalling events more distal from receptor activation. We demonstrate that the VEGF isoform VEGF-A(165) and HGF stimulate a similar set of MAPKs (mitogen-activated protein kinases), although the kinetics and strengths of the activation differ depending on the growth factor and pathway. An enhanced activation of the signalling was observed when endothelial cells were stimulated by the combination of VEGF-A(165) and HGF. Moreover, the combination of VEGF-A and HGF results in a statistically significant synergistic activation of ERK1/2 (extracellular-signal-regulated kinase 1/2) and p38 kinases. We demonstrated that VEGF-A(165) and HGF activate FAK (focal adhesion kinase) with different kinetics and stimulate the recruitment of phosphorylated FAK to different subsets of focal adhesions. VEGF-A(165) and HGF regulate distinct morphogenic aspects of the cytoskeletal remodelling that are associated with the preferential activation of Rho or Rac respectively, and induce structurally distinct vascular-like patterns in vitro in a Rho- or Rac-dependent manner.

CONCLUSIONS

Under angiogenic conditions, combining VEGF-A with HGF can promote neovascularization by enhancing intracellular signalling and allowing more finely regulated control of the signalling molecules involved in the regulation of the cytoskeleton and cellular migration and morphogenesis.

Molecular mechanisms of endothelial hyperpermeability: implications in inflammation

Endothelial hyperpermeability is a significant problem in vascular inflammation associated with trauma, ischaemia-reperfusion injury, sepsis, adult respiratory distress syndrome, diabetes, thrombosis and cancer. An important mechanism underlying this process is increased paracellular leakage of plasma fluid and protein. Inflammatory stimuli such as histamine, thrombin, vascular endothelial growth factor and activated neutrophils can cause dissociation of cell-cell junctions between endothelial cells as well as cytoskeleton contraction, leading to a widened intercellular space that facilitates transendothelial flux. Such structural changes initiate with agonist-receptor binding, followed by activation of intracellular signalling molecules including calcium, protein kinase C, tyrosine kinases, myosin light chain kinase, and small Rho-GTPases; these kinases and GTPases then phosphorylate or alter the conformation of different subcellular components that control cell-cell adhesion, resulting in paracellular hypermeability. Targeting key signalling molecules that mediate endothelial-junction-cytoskeleton dissociation demonstrates a therapeutic potential to improve vascular barrier function during inflammatory injury.

Multifaceted role of Rho, Rac, Cdc42 and Ras in intercellular junctions, lessons from toxins

Tight junctions (TJs) and adherens junctions (AJs) are dynamic structures linked to the actin cytoskeleton, which control the paracellular permeability of epithelial and endothelial barriers. TJs and AJs are strictly regulated in a spatio-temporal manner by a complex signaling network, including Rho/Ras-GTPases, which have a pivotal role. Rho preferentially regulates TJs by controlling the contraction of apical acto-myosin filaments, whereas Rac/Cdc42 mainly coordinate the assembly-disassembly of AJ components. However, a subtle balance of Rho/Ras-GTPase activity and interplay between these molecules is required to maintain an optimal organization and function of TJs and AJs. Conversely, integrity of intercellular junctions generates signals through Rho-GTPases, which are involved in the regulation of multiple cellular processes. Rho/Ras-GTPases and the control of intercellular junctions are the target of various bacterial toxins responsible for severe diseases in man and animals, and are part of their mechanism of action. This review focuses on the regulation of TJs and AJs by Rho/Ras-GTPases through molecular approaches and bacterial toxins.

Statins in endothelial signaling and activation

The beneficial effects of statins, the most widely prescribed class of drugs in the world, are now recognized to extend well beyond their lipid-lowering properties. Through a combination of both distinct and interdependent effects on endothelial cell (EC) Rho GTPase regulation, NAPDH oxidase activity, NO bioavailability, and differential gene expression, statins confer significant protection of the vasculature. Abundant in vitro data, in addition to myriad reports relying on a range of animal models, now firmly support the idea that these drugs may serve as novel and effective therapeutic agents in a variety of disease states characterized by vascular dysfunction.

Protective effects of high-molecular weight polyethylene glycol (PEG) in human lung endothelial cell barrier regulation: role of actin cytoskeletal rearrangement

Acute lung injury represents the result of multiple pathways initiated by local or systemic insults and is characterized by profound vascular permeability, pulmonary edema, and life-threatening respiratory failure. Permeability-reducing therapies are of potential clinical utility but are currently unavailable. We hypothesized that polyethylene glycol (PEG) compounds, inert and non-toxic polymers that serve as a surrogate mucin lining in intestinal epithelium, may attenuate agonist-mediated lung endothelial cell (EC) barrier dysfunction. High molecular weight PEG (PEG15-20) produced rapid, dose-dependent increases in transendothelial electrical resistance (TER) in human lung endothelium cultured on gold microelectrodes, reflecting increased paracellular integrity. The maximal effective concentration of 8% PEG induced a sustained 125% increase in TER (40 h), results similar to barrier-enhancing agonists such as sphingosine 1-phosphate (40% increase in TER). Maximal PEG barrier enhancement was achieved at 45-60 min and PEG effectively reversed both thrombin- and LPS-induced EC barrier dysfunction. Consistent with the increase in TER, immunofluorescent studies demonstrated that PEG produced significant cytoskeletal rearrangement with formation of well-defined cortical actin rings and lamellipodia containing the actin-binding proteins, cortactin and MLCK, known participants in cell-matrix and cell-cell junctional adhesion. Finally, PEG challenge induced rapid alterations in levels of MAP kinase and MLC phosphorylation. In summary, PEG joins a number of EC barrier-regulatory agents which rapidly activate barrier-enhancing signal transduction pathways which target the cytoskeleton and provides a potential therapeutic strategy in inflammatory lung injury.

TNF-induced activation of pulmonary microvessel endothelial cells: a role for GSK3beta

The hypothesis tested was PKCalpha mediates the phosphorylation of glycogen synthetase kinase 3beta (GSK3beta) and that the GSK3beta inhibition modulates the response to tumor necrosis factor-alpha (TNF) in rat pulmonary microvessel endothelial cells (PMEC). PMEC were treated with TNF for 4.0 h (100 ng/ml) or vehicle. First, to assess the role of PKCalpha in the phosphorylation of GSK3beta (i.e., an indicator of GSK3beta inhibition), PMEC were pretreated with 1) nonsense-RNA-PKCalpha, 2) siRNA-PKCalpha, and 3) the PKC inhibitor Gö6983. In the nonsense RNA-PKCalpha+TNF and TNF groups, there was increased phosphorylated GSK3beta-Ser9 that did not occur in the Gö6983+TNF group. In the TNF groups, there was a significant correlation between PKCalpha protein and phosphorylated GSK3beta-Ser9 that did not occur in the groups without TNF. Second, to assess the role of GSK3beta in beta-catenin activity, PMEC were pretreated with 1) wild-type (w) GSK3beta plasmid to enhance GSK3beta activity, 2) kinase dead (kd)-GSK3beta plasmid, and 3) the GSK3beta inhibitor SB-216763. In the TNF group, there was increased unphosphorylated beta-catenin-Ser37/33 compared with the control group. In the GSK3beta-inhibited groups (i.e., SB-216763 and kdGSK3beta) +/- TNF, the unphosphorylated beta-catenin-Ser37/33 was similar to the TNF group. In the GSK3beta-enhanced group +/- TNF, the unphosphorylated beta-catenin-Ser37/33 was similar to the control. Finally, PMEC were also treated with TOPflash, a beta-catenin-dependent promoter luciferase reporter, or the mutant construct FOPflash, 2 days before treatment with TNF. In the TNF group, there was an increased TOPflash/FOPflash activity ratio compared with the control group. In the GSK3beta-inhibited groups (i.e., SB-216763 and kdGSK3beta) +/- TNF, the TOPflash/FOPflash activity ratio was similar to the TNF group. In the GSK3beta-enhanced group +/- TNF, the TOPflash/FOPflash activity ratio was similar to the control. The data indicate that TNF induces endothelial activation that is modulated by a PKCalpha-dependent inhibition of GSK3beta.

GADD45a is a novel candidate gene in inflammatory lung injury via influences on Akt signaling

We explored the mechanistic involvement of the growth arrest and DNA damage-inducible gene GADD45a in lipopolysaccharide (LPS)- and ventilator-induced inflammatory lung injury (VILI). Multiple biochemical and genomic parameters of inflammatory lung injury indicated that GADD45a(-/-) mice are modestly susceptible to intratracheal LPS-induced lung injury and profoundly susceptible to high tidal volume VILI, with increases in microvascular permeability and bronchoalveolar lavage levels of inflammatory cytokines. Expression profiling of lung tissues from VILI-challenged GADD45a(-/-) mice revealed strong dysregulation in the B-cell receptor signaling pathway compared with wild-type mice and suggested the involvement of PI3 kinase/Akt signaling components. Western blot analyses of lung homogenates confirmed approximately 50% reduction in Akt protein levels in GADD45a(-/-) mice accompanied by marked increases in Akt ubiquitination. Electrical resistance measurements across human lung endothelial cell monolayers with either reduced GADD45a or Akt expression (siRNAs) revealed significant potentiation of LPS-induced human lung endothelial barrier dysfunction, which was attenuated by overexpression of a constitutively active Akt1 transgene. These studies validate GADD45a as a novel candidate gene in inflammatory lung injury and a significant participant in vascular barrier regulation via effects on Akt-mediated endothelial signaling.

Endothelial permeability is controlled by spatially defined cytoskeletal mechanics: atomic force microscopy force mapping of pulmonary endothelial monolayer

Actomyosin contraction directly regulates endothelial cell (EC) permeability, but intracellular redistribution of cytoskeletal tension associated with EC permeability is poorly understood. We used atomic force microscopy (AFM), EC permeability assays, and fluorescence microscopy to link barrier regulation, cell remodeling, and cytoskeletal mechanical properties in EC treated with barrier-protective as well as barrier-disruptive agonists. Thrombin, vascular endothelial growth factor, and hydrogen peroxide increased EC permeability, disrupted cell junctions, and induced stress fiber formation. Oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine, hepatocyte growth factor, and iloprost tightened EC barriers, enhanced peripheral actin cytoskeleton and adherens junctions, and abolished thrombin-induced permeability and EC remodeling. AFM force mapping and imaging showed differential distribution of cell stiffness: barrier-disruptive agonists increased stiffness in the central region, and barrier-protective agents decreased stiffness in the center and increased it at the periphery. Attenuation of thrombin-induced permeability correlates well with stiffness changes from the cell center to periphery. These results directly link for the first time the patterns of cell stiffness with specific EC permeability responses.

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

OBJECTIVE

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

METHODS AND RESULTS

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

CONCLUSIONS

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

Cell-cell junctions and vascular endothelial growth factor in rat lung as affected by ischemia/reperfusion and preconditioning with inhaled nitric oxide

BACKGROUND

Previous investigations have shown that short term inhalation of nitric oxide (NO) before ischemia and reperfusion (I/R) prevents I/R-related consequences on lung function. Here we correlate effects of NO-induced preconditioning, especially on the lung permeability barrier, with analysis of cell junction proteins and the level of vascular endothelial growth factor (VEGF).

METHODS

A rat model of left lung in situ I/R was used. After left lateral thoracotomy, left lung ischemia was maintained for 60 min, followed by 30 min or 4 h (h) reperfusion (I/R groups). In the NO groups, inhalation of NO (10 min, 15 ppm) preceded I/R. Animals in control groups underwent sham surgery without NO inhalation and ischemia. The extent of I/R injury was assessed in terms of oxygenation (arterial PO(2)) and lung permeability (Evans blue extravasation). Expression of junctional proteins and phosphorylation was determined in complete protein extracts from lung tissue, whereas the adherens junction (AJ) core complex was analyzed in Triton extracts by co-immunoprecipitation using antibodies against E-cadherin and VE-cadherin.

RESULTS

The inhalation of NO prevented the I/R-induced increase of permeability at 30 min reperfusion, and the PO(2) increased from 27% of controls in the I/R group to 77% in the NO group. Left lung I/R correlated with a progressive loss of cadherins (VE-cadherin, E-cadherin, desmoglein 1) during reperfusion, whereas AJ catenins were largely preserved. Preconditioning with NO resulted in an increased ratio of catenins (alpha- and beta-catenin) to E-cadherin in immunoprecipitates and in reduced phosphorylation of beta-catenin. A reduction of VEGF in left lung lavage fluid was observed at 4 h but not at 30 min reperfusion.

CONCLUSIONS

The NO-induced changes of the AJ complex may have contributed to the stabilization of the lung permeability barrier during reperfusion.

Magnitude-dependent effects of cyclic stretch on HGF- and VEGF-induced pulmonary endothelial remodeling and barrier regulation

Mechanical ventilation at high tidal volumes compromises the blood-gas barrier and increases lung vascular permeability, which may lead to ventilator-induced lung injury and pulmonary edema. Using pulmonary endothelial cell (ECs) exposed to physiologically [5% cyclic stretch (CS)] and pathologically (18% CS) relevant magnitudes of CS, we evaluated the potential protective effects of hepatocyte growth factor (HGF) on EC barrier dysfunction induced by CS and vascular endothelial growth factor (VEGF). In static culture, HGF enhanced EC barrier function in a Rac-dependent manner and attenuated VEGF-induced EC permeability and paracellular gap formation. The protective effects of HGF were associated with the suppression of Rho-dependent signaling triggered by VEGF. Five percent CS promoted HGF-induced enhancement of the cortical F-actin rim and activation of Rac-dependent signaling, suggesting synergistic barrier-protective effects of physiological CS and HGF. In contrast, 18% CS further enhanced VEGF-induced EC permeability, activation of Rho signaling, and formation of actin stress fibers and paracellular gaps. These effects were attenuated by HGF pretreatment. EC preconditioning at 5% CS before HGF and VEGF further promoted EC barrier maintenance. Our data suggest synergistic effects of HGF and physiological CS in the Rac-mediated mechanisms of EC barrier protection. In turn, HGF reduced the barrier-disruptive effects of VEGF and pathological CS via downregulation of the Rho pathway. These results support the importance of HGF-VEGF balance in control of acute lung injury/acute respiratory distress syndrome severity via small GTPase-dependent regulation of lung endothelial permeability.

Paxillin is involved in the differential regulation of endothelial barrier by HGF and VEGF

Circulating levels of hepatocyte growth factor (HGF) and vascular endothelial growth factor (VEGF) are increased during acute lung injury; however, combined effects of HGF and VEGF on pulmonary endothelial cell (EC) permeability remain to be elucidated. We have previously shown differential remodeling of focal adhesions (FA) caused by barrier-protective and barrier-disruptive mechanical and chemical stimuli. This study examined a role of FA protein paxillin in the pulmonary EC barrier responses induced by HGF and VEGF. VEGF increased, but HGF decreased, pulmonary EC permeability. These effects were accompanied by differential patterns of site-specific phosphorylation of focal adhesion kinase (FAK) and paxillin and FA redistribution. HGF antagonized random FA formation caused by VEGF challenge and promoted FA accumulation at the cell periphery. HGF attenuated VEGF-induced paxillin redistribution, FA remodeling, and endothelial permeability. SiRNA-based paxillin knockdown attenuated VEGF-induced EC permeability, myosin light chain phosphorylation, and stress fiber and paracellular gap formation. Paxillin knockdown also decreased HGF-induced EC barrier enhancement and suppressed activation of Rac and its effector PAK1. Expression of paxillin-S(273) deficient on PAK1 phosphorylation site prevented HGF-induced cytoskeletal remodeling. These data show a dual role of paxillin in the HGF- and VEGF-mediated endothelial barrier regulation and suggest essential paxillin role in the modulation of Rac-Rho crosstalk. Our results also support a model of pulmonary EC barrier recovery during resolution of ALI via switch from VEGF to HGF signaling.

Endothelial cell barrier protection by simvastatin: GTPase regulation and NADPH oxidase inhibition

The statins, hydroxy-3-methylglutaryl-CoA reductase inhibitors that lower serum cholesterol, exhibit myriad clinical benefits, including enhanced vascular integrity. One potential mechanism underlying increased endothelial cell (EC) barrier function is inhibition of geranylgeranylation, a covalent modification enabling translocation of the small GTPases Rho and Rac to the cell membrane. While RhoA inhibition attenuates actin stress fiber formation and promotes EC barrier function, Rac1 inhibition at the cell membrane potentially prevents activation of NADPH oxidase and subsequent generation of superoxides known to induce barrier disruption. We examined the relative regulatory effects of simvastatin on RhoA, Rac1, and NADPH oxidase activities in the context of human pulmonary artery EC barrier protection. Confluent EC treated with simvastatin demonstrated significantly decreased thrombin-induced FITC-dextran permeability, a reflection of vascular integrity, which was linked temporally to simvastatin-mediated actin cytoskeletal rearrangement. Compared with Rho inhibition alone (Y-27632), simvastatin afforded additional protection against thrombin-mediated barrier dysfunction and attenuated LPS-induced EC permeability and superoxide generation. Statin-mediated inhibition of both Rac translocation to the cell membrane and superoxide production were attenuated by geranylgeranyl pyrophosphate (GGPP), indicating that these effects are due to geranylgeranylation inhibition. Finally, thrombin-induced EC permeability was modestly attenuated by reduced Rac1 expression (small interfering RNA), whereas these effects were made more pronounced by simvastatin pretreatment. Together, these data suggest EC barrier protection by simvastatin is due to dual inhibitory effects on RhoA and Rac1 as well as the attenuation of superoxide generation by EC NADPH oxidase and contribute to the molecular mechanistic understanding of the modulation of EC barrier properties by simvastatin.

Epac/Rap and PKA are novel mechanisms of ANP-induced Rac-mediated pulmonary endothelial barrier protection

Acute lung injury, sepsis, lung inflammation, and ventilator-induced lung injury are life-threatening conditions associated with lung vascular barrier dysfunction, which may lead to pulmonary edema. Increased levels of atrial natriuretic peptide (ANP) in lung circulation reported in these pathologies suggest its potential role in the modulation of lung injury. Besides well recognized physiological effects on vascular tone, plasma volume, and renal function, ANP may exhibit protective effects in models of lung vascular endothelial cell (EC) barrier dysfunction. However, the molecular mechanisms of ANP protective effects are not well understood. The recently described cAMP-dependent guanine nucleotide exchange factor (GEF) Epac activates small GTPase Rap1, which results in activation of small GTPase Rac-specific GEFs Tiam1 and Vav2 and Rac-mediated EC barrier protective responses. Our results show that ANP stimulated protein kinase A and the Epac/Rap1/Tiam/Vav/Rac cascade dramatically attenuated thrombin-induced pulmonary EC permeability and the disruption of EC monolayer integrity. Using pharmacological and molecular activation and inhibition of cAMP-and cGMP-dependent protein kinases (PKA and PKG), Epac, Rap1, Tiam1, Vav2, and Rac we linked ANP-mediated protective effects to the activation of Epac/Rap and PKA signaling cascades, which dramatically inhibited the Rho pathway of thrombin-induced EC hyper-permeability. These results suggest a novel mechanism of ANP protective effects against agonist-induced pulmonary EC barrier dysfunction via inhibition of Rho signaling by Epac/Rap1-Rac and PKA signaling cascades.

Regulation of the micromechanical properties of pulmonary endothelium by S1P and thrombin: role of cortactin

Disruption of pulmonary endothelial cell (EC) barrier function is a critical pathophysiologic event in highly morbid inflammatory conditions such as sepsis and acute respiratory disease stress syndrome. Actin cytoskeleton, an essential regulator of endothelial permeability, is a dynamic structure whose stimuli-induced rearrangement is linked to barrier modulation. Here, we used atomic force microscopy to characterize structural and mechanical changes in the F-actin cytoskeleton of cultured human pulmonary artery EC in response to both barrier-enhancing (induced by sphingosine 1-phosphate (S1P)) and barrier-disrupting (induced by thrombin) conditions. Atomic force microscopy elasticity measurements show differential effects: for the barrier protecting molecule S1P, the elastic modulus was elevated significantly on the periphery; for the barrier-disrupting molecule thrombin, on the other hand, it was elevated significantly in the central region of the cell. The force and elasticity maps correlate with F-actin rearrangements as identified by immunofluorescence analysis. Significantly, reduced expression (via siRNA) of cortactin, an actin-binding protein essential to EC barrier regulation, resulted in a shift in the S1P-mediated elasticity pattern to more closely resemble control, unstimulated endothelium.

Thrombin enhances the barrier function of rat microvascular endothelium in a PAR-1-dependent manner

Thrombin is a multifunctional coagulation protease with pro- and anti-inflammatory vascular effects. We questioned whether thrombin may have segmentally differentiated effects on pulmonary endothelium. In cultured rat endothelial cells, rat thrombin (10 U/ml) recapitulated the previously reported decrease in transmonolayer electrical resistance (TER), F-actin stress fiber formation, paracellular gap formation, and increased permeability. In contrast, in rat pulmonary microvascular endothelial cells (PMVEC), isolated on the basis of Griffonia simplicifolia lectin recognition, thrombin increased TER, induced fewer stress fibers, and decreased permeability. To assess for differential proteinase-activated receptor (PAR) expression as a basis for the different responses, PAR family expression was analyzed. Both pulmonary artery endothelial cells and PMVEC expressed PAR-1 and PAR-2; however, only PMVEC expressed PAR-3, as shown by both RT-PCR and Western analysis. PAR-1 activating peptides (PAR-APs: SFLLRN-NH(2) and TFLLRN-NH(2)) were used to confirm a role for the PAR-1 receptor. PAR-APs (25-250 muM) also increased TER, formed fewer stress fibers, and did not induce paracellular gaps in PMVEC in contrast to that shown in pulmonary artery endothelial cells. These results were confirmed in isolated perfused rat lung preparations. PAR-APs (100 mug/ml) induced a 60% increase in the filtration coefficient over baseline. However, by transmission electron microscopy, perivascular fluid cuffs were seen only along conduit veins and arteries without evidence of intra-alveolar edema. We conclude that thrombin exerts a segmentally differentiated effect on endothelial barrier function in vitro, which corresponds to a pattern of predominant perivascular fluid cuff formation in situ. This may indicate a distinct role for thrombin in the microcirculation.

Heterogeneity of barrier function in the lung reflects diversity in endothelial cell junctions

Endothelial cells assemble unique barriers that confer specific permeability requirements at different vascular segments. We examined lung microvascular and artery endothelial cells to gain insight into mechanisms for segment-specific barrier functions. Transendothelial electrical resistance was significantly higher in microvascular barriers, and a 50% reduction in barrier function required 5-fold higher concentration of cytochalasin D in the microvascular compared to the arterial barrier. Transcriptional profiling studies identified N-cadherin and activated leukocyte cell adhesion molecule (ALCAM) to be most highly expressed in microvascular than in pulmonary artery endothelial cells. ALCAM was detected in microvascular endothelial cells in the alveolar septum but not in endothelial cells in larger pulmonary vessels in situ. This pattern was retained in culture as ALCAM was recruited to cell junctions in pulmonary microvascular endothelial cells but remained predominantly cytosolic in pulmonary artery endothelial cells. Confocal analysis revealed ALCAM in the lateral plasma membrane domain where it co-localized with N- and VE-cadherin. This finding was supported by co-immunoprecipitation studies demonstrating the presence of ALCAM in multiple adherens junction protein complexes. These functional, biophysical and molecular findings suggest specialization of the adherens junction as a basis for a highly restrictive endothelial barrier to control fluid flux into the alveolar airspace.

CD44 regulates hepatocyte growth factor-mediated vascular integrity. Role of c-Met, Tiam1/Rac1, dynamin 2, and cortactin

The preservation of vascular endothelial cell (EC) barrier integrity is critical to normal vessel homeostasis, with barrier dysfunction being a feature of inflammation, tumor angiogenesis, atherosclerosis, and acute lung injury. Therefore, agents that preserve or restore vascular integrity have important therapeutic implications. In this study, we explored the regulation of hepatocyte growth factor (HGF)-mediated enhancement of EC barrier function via CD44 isoforms. We observed that HGF promoted c-Met association with CD44v10 and recruitment of c-Met into caveolin-enriched microdomains (CEM) containing CD44s (standard form). Treatment of EC with CD44v10-blocking antibodies inhibited HGF-mediated c-Met phosphorylation and c-Met recruitment to CEM. Silencing CD44 expression (small interfering RNA) attenuated HGF-induced recruitment of c-Met, Tiam1 (a Rac1 exchange factor), cortactin (an actin cytoskeletal regulator), and dynamin 2 (a vesicular regulator) to CEM as well as HGF-induced trans-EC electrical resistance. In addition, silencing Tiam1 or dynamin 2 reduced HGF-induced Rac1 activation, cortactin recruitment to CEM, and EC barrier regulation. We observed that both HGF- and high molecular weight hyaluronan (CD44 ligand)-mediated protection from lipopolysaccharide-induced pulmonary vascular hyperpermeability was significantly reduced in CD44 knock-out mice, thus validating these in vitro findings in an in vivo murine model of inflammatory lung injury. Taken together, these results suggest that CD44 is an important regulator of HGF/c-Met-mediated in vitro and in vivo barrier enhancement, a process with essential involvement of Tiam1, Rac1, dynamin 2, and cortactin.

Pulmonary endothelial cell barrier enhancement by FTY720 does not require the S1P1 receptor

Novel therapeutic strategies are needed to reverse the loss of endothelial cell (EC) barrier integrity that occurs during inflammatory disease states such as acute lung injury. We previously demonstrated potent EC barrier augmentation in vivo and in vitro by the platelet-derived phospholipid, sphingosine 1-phosphate (S1P) via ligation of the S1P1 receptor. The S1P analogue, FTY720, similarly exerts barrier-protective vascular effects via presumed S1P1 receptor ligation. We examined the role of the S1P1 receptor in sphingolipid-mediated human lung EC barrier enhancement. Both S1P and FTY-induced sustained, dose-dependent barrier enhancement, reflected by increases in transendothelial electrical resistance (TER), which was abolished by pertussis toxin indicating Gi-coupled receptor activation. FTY-mediated increases in TER exhibited significantly delayed onset and intensity relative to the S1P response. Reduction of S1P1R expression (via siRNA) attenuated S1P-induced TER elevations whereas the TER response to FTY was unaffected. Both S1P and FTY rapidly (within 5 min) induced S1P1R accumulation in membrane lipid rafts, but only S1P stimulated S1P1R phosphorylation on threonine residues. Inhibition of PI3 kinase activity attenuated S1P-mediated TER increases but failed to alter FTY-induced TER elevation. Finally, S1P, but not FTY, induced significant myosin light chain phosphorylation and dramatic actin cytoskeletal rearrangement whereas reduced expression of the cytoskeletal effectors, Rac1 and cortactin (via siRNA), attenuated S1P-, but not FTY-induced TER elevations. These results mechanistically characterize pulmonary vascular barrier regulation by FTY720, suggesting a novel barrier-enhancing pathway for modulating vascular permeability.

Paxillin-β-catenin interactions are involved in Rac/Cdc42-mediated endothelial barrier-protective response to oxidized phospholipids

Oxidized phospholipids may appear in the pulmonary circulation as a result of acute lung injury or inflammation. We have previously described barrier-protective effects of oxidized 1-palmitoyl-2-arachidonoyl- sn-glycero-3-phosphocholine (OxPAPC) on human pulmonary endothelial cells (EC) mediated by small GTPases Rac and Cdc42. This work examined OxPAPC-induced focal adhesion (FA) and adherens junction (AJ) remodeling and potential interactions between FA and AJ protein complexes involved in OxPAPC-induced EC barrier enhancement. Immunofluorescence analysis, subcellular fractionation, and coimmunoprecipitation assays have shown that OxPAPC induced translocation and peripheral accumulation of FA complexes containing paxillin, focal adhesion kinase, vinculin, GIT1, and GIT2, increased association of AJ proteins vascular endothelial-cadherin, p120-catenin, α-, β-, and γ-catenins, and dramatically enhanced cell junction areas covered by AJ. Coimmunoprecipitation, pulldown assays, and confocal microscopy studies have demonstrated that OxPAPC promoted novel interactions between FA and AJ complexes via paxillin and β-catenin association, which was critically dependent on Rac and Cdc42 activities and was abolished by pharmacological or small interfering RNA (siRNA)-mediated inhibition of Rac and Cdc42. Depletion of β-catenin using the siRNA approach attenuated OxPAPC-induced paxillin translocation to the cell periphery, but also significantly decreased interaction of paxillin with AJ protein complex. In turn, paxillin knockdown by specific siRNA attenuated AJ enhancement in response to OxPAPC. These results show for the first time the novel interactions between FA and AJ protein complexes critical for EC barrier regulation by OxPAPC.

Hepatocyte growth factor promotes the number of PSD-95 clusters in young hippocampal neurons

Hepatocyte growth factor (HGF) and its receptor are expressed in various regions of the brain and have protective effects against excitotoxic injuries. However, their effects on synapse formation remain to be elucidated. To determine whether HGF has the ability to alter synaptic function during development, we investigated changes in the number of synapse detected by double immunostaining for NMDA receptor subunits and a presynaptic marker in cultured young hippocampal neurons. Whereas application of HGF increased the number of cluster of synapsin, a presynaptic protein, the clusters of NMDA receptor subunits NR1 and NR2B were not altered. Interestingly, colocalization of PSD-95, a scaffolding protein of the receptor, with synapsin was increased by HGF treatment without a change in the total amount of it. In addition, we investigated the expression of surface NMDA receptor, neuroligin, and neurexin, which were assessed by use of a cell-surface biotinylation assay. The application of HGF did not change the surface expression of these proteins. Furthermore, we determined the release of glutamate in response to depolarization. Treatment with HGF promoted depolarization-evoked release of glutamate. These results suggest that HGF modulates the expression of the scaffolding protein of the NMDA receptor at the synapse and promotes maturation of excitatory synapses in young hippocampal neurons.

HGF attenuates thrombin-induced endothelial permeability by Tiam1-mediated activation of the Rac pathway and by Tiam1/Rac-dependent inhibition of the Rho pathway

Reorganization of the endothelial cell (EC) cytoskeleton and cell adhesive complexes provides a structural basis for increased vascular permeability implicated in the pathogenesis of many diseases, including asthma, sepsis, and acute respiratory distress syndrome (ARDS). We have recently described the barrier-protective effects of hepatocyte growth factor (HGF) on the human pulmonary EC. In the present study, we explored the involvement of Rac-GTPase and Rac-specific nucleotide exchange factor Tiam1 in the mechanisms of EC barrier protection by HGF. HGF protected EC monolayers from thrombin-induced hyperpermeability, disruption of intercellular junctions, and formation of stress fibers and paracellular gaps by inhibiting thrombin-induced activation of Rho GTPase, Rho association with nucleotide exchange factor p115-RhoGEF, and myosin light chain phosphorylation, which was opposed by stimulation of Rac-dependent signaling. The pharmacological Rac inhibitor or silencing RNA (siRNA) based depletion of either Rac or Tiam1 significantly attenuated HGF-induced peripheral translocation of Rac effector cortactin, cortical actin ring formation, and EC barrier enhancement. Moreover, Tiam1 knockdown using the siRNA approach, attenuated the protective effect of HGF against thrombin-induced activation of Rho signaling, monolayer disruption, and EC hyperpermeability. This study demonstrates the Tiam1/Rac-dependent mechanism of HGF-induced EC barrier protection and provides novel mechanistic insights into regulation of EC permeability via dynamic interactions between Rho- and Tiam1/Rac-mediated pathways.

Prostaglandins PGE(2) and PGI(2) promote endothelial barrier enhancement via PKA- and Epac1/Rap1-dependent Rac activation

Prostaglandin E(2) (PGE(2)) and prostacyclin are lipid mediators produced by cyclooxygenase and implicated in the regulation of vascular function, wound repair, inflammatory processes, and acute lung injury. Although protective effects of these prostaglandins (PGs) are associated with stimulation of intracellular cAMP production, the crosstalk between cAMP-activated signal pathways in the regulation of endothelial cell (EC) permeability is not well understood. We studied involvement of cAMP-dependent kinase (PKA), cAMP-Epac-Rap1 pathway, and small GTPase Rac in the PGs-induced EC barrier protective effects and cytoskeletal remodeling. PGE(2) and PGI(2) synthetic analog beraprost increased transendothelial electrical resistance and decreased dextran permeability, enhanced peripheral F-actin rim and increased intercellular adherens junction areas reflecting EC barrier-protective response. Furthermore, beraprost dramatically attenuated thrombin-induced Rho activation, MLC phosphorylation and EC barrier dysfunction. In vivo, beraprost attenuated lung barrier dysfunction induced by high tidal volume mechanical ventilation. Both PGs caused cAMP-mediated activation of PKA-, Epac/Rap1- and Tiam1/Vav2-dependent pathways of Rac1 activation and EC barrier regulation. Knockdown of Epac, Rap1, Rac-specific exchange factors Tiam1 and Vav2 using siRNA approach, or inhibition of PKA activity decreased Rac1 activation and PG-induced EC barrier enhancement. Thus, our results show that barrier-protective effects of PGE(2) and prostacyclin on pulmonary EC are mediated by PKA and Epac/Rap pathways, which converge on Rac activation and lead to enhancement of peripheral actin cytoskeleton and adherens junctions. These mechanisms may mediate protective effects of PGs against agonist-induced lung vascular barrier dysfunction in vitro and against mechanical stress-induced lung injury in vivo.

Phosphoinositide 3-kinase, Src, and Akt modulate acute ventilation-induced vascular permeability increases in mouse lungs

To determine the role of phosphoinositide 3-OH kinase (PI3K) pathways in the acute vascular permeability increase associated with ventilator-induced lung injury, we ventilated isolated perfused lungs and intact C57BL/6 mice with low and high peak inflation pressures (PIP). In isolated lungs, filtration coefficients (K(f)) increased significantly after ventilation at 30 cmH(2)O (high PIP) for successive periods of 15, 30 (4.1-fold), and 50 (5.4-fold) min. Pretreatment with 50 microM of the PI3K inhibitor, LY-294002, or 20 microM PP2, a Src kinase inhibitor, significantly attenuated the increase in K(f), whereas 10 microM Akt inhibitor IV significantly augmented the increased K(f). There were no significant differences in K(f) or lung wet-to-dry weight (W/D) ratios between groups ventilated with 9 cmH(2)O PIP (low PIP), with or without inhibitor treatment. Total lung beta-catenin was unchanged in any low PIP isolated lung group, but Akt inhibition during high PIP ventilation significantly decreased total beta-catenin by 86%. Ventilation of intact mice with 55 cmH(2)O PIP for up to 60 min also increased lung vascular permeability, indicated by increases in lung lavage albumin concentration and lung W/D ratios. In these lungs, tyrosine phosphorylation of beta-catenin and serine/threonine phosphorylation of Akt, glycogen synthase kinase 3beta (GSK3beta), and ERK1/2 increased significantly with peak effects at 60 min. Thus mechanical stress activation of PI3K and Src may increase lung vascular permeability through tyrosine phosphorylation, but simultaneous activation of the PI3K-Akt-GSK3beta pathway tends to limit this permeability response, possibly by preserving cellular beta-catenin.

Caldesmon is a cytoskeletal target for PKC in endothelium

We have previously shown that treatment of bovine endothelial cell (EC) monolayers with phorbol myristate acetate (PMA) leads to the thinning of cortical actin ring and rearrangement of the cytoskeleton into a grid-like structure, concomitant with the loss of endothelial barrier function. In the current work, we focused on caldesmon, a cytoskeletal protein, regulating actomyosin interaction. We hypothesized that protein kinase C (PKC) activation by PMA leads to the changes in caldesmon properties such as phosphorylation and cellular localization. We demonstrate here that PMA induces both myosin and caldesmon redistribution from cortical ring into the grid-like network. However, the initial step of PMA-induced actin and myosin redistribution is not followed by caldesmon redistribution. Co-immunoprecipitation experiments revealed that short-term PMA (5 min) treatment leads to the weakening of caldesmon ability to bind actin and, to the lesser extent, myosin. Prolonged incubation (15-60 min) with PMA, however, strengthens caldesmon complexes with actin and myosin, which correlates with the grid-like actin network formation. PMA stimulation leads to an immediate increase in caldesmon Ser/Thr phosphorylation. This process occurs at sites distinct from the sites specific for ERK1/2 phosphorylation and correlates with caldesmon dissociation from the actomyosin complex. Inhibition of ERK-kinase MEK fails to abolish grid-like structure formation, although reducing PMA-induced weakening of the cortical actin ring, whereas inhibition of PKC reverses PMA-induced cytoskeletal rearrangement. Our results suggest that PKC-dependent phosphorylation of caldesmon is involved in PMA-mediated complex cytoskeletal changes leading to the EC barrier compromise.