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.

Diperoxovanadate alters endothelial cell focal contacts and barrier function: role of tyrosine phosphorylation.

Diperoxovanadate (DPV), a potent tyrosine kinase activator and protein tyrosine phosphatase inhibitor, was utilized to explore bovine pulmonary artery endothelial cell barrier regulation. DPV produced dose-dependent decreases in transendothelial electrical resistance (TER) and increases in permeability to albumin, which were preceded by brief increases in TER (peak TER effect at 10-15 min). The significant and sustained DPV-mediated TER reductions were primarily the result of decreased intercellular resistance, rather than decreased resistance between the cell and the extracellular matrix, and were reduced by pretreatment with the tyrosine kinase inhibitor genistein but not by inhibition of p42/p44 mitogen-activating protein kinases. Immunofluorescent analysis after DPV challenge revealed dramatic F-actin polymerization and stress-fiber assembly and increased colocalization of tyrosine phosphoproteins with F-actin in a circumferential pattern at the cell periphery, changes that were abolished by genistein. The phosphorylation of focal adhesion and adherens junction proteins on tyrosine residues was confirmed in immunoprecipitates of focal adhesion kinase and cadherin-associated proteins in which dramatic dose-dependent tyrosine phosphorylation was observed after DPV stimulation. We speculate that DPV enhances endothelial cell monolayer integrity via focal adhesion plaque phosphorylation and produces subsequent monolayer destabilization of adherens junctions initiated by adherens junction protein tyrosine phosphorylation catalyzed by p60(src) or Src-related tyrosine kinases.

Regulation of endothelial barrier function by the cAMP-dependent protein kinase.

Elevation of cAMP promotes the endothelial cell (EC) barrier and protects the lung from edema development. Thus, we tested the hypothesis that both increases and decreases in PKA modulate EC function and coordinate distribution of regulatory, adherence, and cytoskeletal proteins. Inhibition of PKA activity by RpcAMPS and activation by cholera toxin was verified by assay of kemptide phosphorylation in digitonin permeabilized EC. Inhibition of PKA by RpcAMPS or overexpression of the endogenous inhibitor, PKI, decreased monolayer electrical impedance and exacerbated the decreases produced by agonists (thrombin and PMA). RpcAMPS directly increased F-actin content and organization into stress fibers, increased co-staining of actin with both phosphatase 2B and myosin light chain kinase (MLCK), caused reorganization of focal adhesions, and decreased catenin at cell borders. These findings are similar to those evoked by thrombin. In contrast, cholera toxin prevented the agonist-induced resistance decrease and protein redistribution. Although PKA activation attenuated thrombin-induced myosin light chain (MLC) phosphorylation, PKA inhibition per se did not cause MLC phosphorylation or affect [Ca2+]i. These studies indicate that a decrease in PKA activity alone can produce disruption of barrier function via mechanisms not involving MLCK and support a central role for cAMP/PKA in regulation of cytoskeletal and adhesive protein function in EC which correlates with altered barrier function.

Role of tyrosine phosphorylation in thrombin-induced endothelial cell contraction and barrier function.

Thrombin-induced endothelial cell (EC) barrier dysfunction is highly dependent upon phosphorylation of serine and threonine residues present on myosin light chains (MLC) catalyzed by a novel EC myosin light chain kinase (MLCK) isoform. In this study, we examined the participation of tyrosine protein phosphorylation in EC contraction, gap formation and barrier dysfunction. We first determined that thrombin significantly increases protein tyrosine kinase activity and protein tyrosine phosphorylation in bovine pulmonary artery EC. Tyrosine kinase inhibitors, genistein and 2,5 DHC, reduced EC tyrosine kinase activities, however, only genistein significantly attenuated thrombin-mediated increases in albumin clearance and reductions in transendothelial electrical resistance. Similarly, genistein but not 2,5 DHC, decreased basal and thrombin-induced Ca2+ increases and MLC phosphorylation in the absence of alterations in Type 1 or 2A serine/threonine phosphatase activities. Immunoprecipitation of the EC MLCK isoform revealed a 214 kD immunoreactive phosphotyrosine protein and genistein pretreatment significantly reduced MLCK activity in MLCK immunoprecipitates. Although thrombin induced the translocation of p60src from the cytosol to the EC cytoskeleton, a detectable increase in the level of MLCK tyrosine phosphorylation was not noted after thrombin challenge. Taken together, our data suggest that genistein-sensitive tyrosine kinase activities are involved in thrombin-mediated EC MLCK activation, MLC phosphorylation, and barrier dysfunction.

Thrombin-mediated focal adhesion plaque reorganization in endothelium: role of protein phosphorylation.

Endothelial cell (EC) gap formation and barrier function are subject to dual regulation by (1) axial contractile forces, regulated by myosin light chain kinase activity, and (2) tethering forces, represented by cell-cell and cell-substratum adhesions. We examined whether focal adhesion plaque proteins (vinculin and talin) and focal adhesion kinase, p125FAK (FAK), represent target regulatory sites involved in thrombin-mediated EC barrier dysfunction. Histologically, thrombin produced dramatic rearrangement of EC actin, vinculin, and FAK in parallel with the evolution of gap formation and barrier dysfunction. Vinculin and talin were in vitro substrates for phosphorylation by EC PKC, a key effector enzyme involved in thrombin-induced EC barrier dysfunction. Although vinculin and talin were phosphorylated in situ under basal conditions in 32P-labeled EC, thrombin failed to alter the basal level of phosphorylation of these proteins. Phosphotyrosine immunoblotting showed that neither vinculin nor talin was significantly phosphorylated in situ on tyrosine residues in unstimulated ECs, and this was not further increased after thrombin. In contrast, both thrombin and the thrombin receptor-activating peptide (TRAP) produced an increase in FAK phosphotyrosine levels (corrected for immunoreactive FAK content) present in EC immunoprecipitates. Ionomycin, which produces EC barrier dysfunction in a myosin light chain kinase-independent manner, was used to increase intracellular Ca2+ and evaluate the Ca2+ sensitivity of this observation. In contrast to thrombin, ionomycin effected a dramatic decrease in the phosphotyrosine-to-immunoreactive FAK ratios, suggesting distinct effects of the two agents on FAK phosphorylation and function. These data indicate that modulation of cell tethering via phosphorylation of focal adhesion proteins is complex, agonist-specific, and may be a relevant mechanism of EC barrier dysfunction in permeability models that do not depend on an increase in myosin 20-kD regulatory light chain phosphorylation.

Mechanisms of ionomycin-induced endothelial cell barrier dysfunction.

Myosin light chain (MLC) phosphorylation catalyzed by the Ca(2+)- calmodulin-dependent MLC kinase (MLCK) is critical to thrombin-mediated endothelial cell gap formation and barrier dysfunction. We have tested the hypothesis that the Ca2+ ionophore ionomycin stimulates MLCK-dependent endothelial cell contraction and permeability. Ionomycin significantly increased albumin clearance and decreased electrical resistance across confluent bovine pulmonary microvascular and macrovascular endothelial cell monolayers in a concentration-dependent manner that was temporally similar to that produced by thrombin. In contrast, however, ionomycin produced a significant Ca(2+)-dependent reduction in the levels of phosphorylated MLC with evidence of serine/threonine phosphatase activation. Potential MLCK-independent mechanisms of endothelial cell permeability were examined with little evidence to support a role for stimulated nitric oxide synthase or phospholipase A2 activities. Importantly, ionomycin produced 1) reductions in the activities of the barrier protective adenylate cyclase and the adenosine 3',5'-cyclic monophosphate-dependent protein kinase A, 2) dramatic dose- and time-dependent inhibition of endothelial cell tyrosine kinase activities, and 3) marked decreases in the phosphotyrosine content of the p125 focal adhesion kinase. These data indicate that ionomycin produces endothelial cell barrier dysfunction by mechanisms that are independent of MLCK activation and may involve reductions in endothelial cell tethering forces via inhibition of protein kinase A and tyrosine kinase activities, especially the p125 focal adhesion kinase.

Regulation of thrombin-mediated endothelial cell contraction and permeability.

A variety of physical forces exist in a dynamic equilibrium in the vascular endothelium (EC) monolayer and serve to maintain EC responsiveness while preserving the integrity of the EC monolayer and barrier properties. Thrombin has potent effects on EC permeabilities disrupting the equilibrium between tethering forces (cadherins, focal adhesion plaque) and forces that increase centripetal tension primarily via myosin light chain (MLC) phosphorylation. Like other EC effects, thrombin-induced MLC kinase (MLCK) activation is dependent upon receptor proteolysis, Ca2+ mobilization, and activation of protein kinase C (PKC). While EC gap formation is central to barrier dysfunction and dependent upon activation of MLCK, (which phosphorylates MLC) an obligatory event in smooth muscle cell contraction, little is known regarding the events that reverse inflammatory responses, halt the contractile response, and initiate relaxation. However, as these events likely include MLC dephosphorylation, further examination of the processes that regulate MLC protein phosphatase activity, focal intercellular junctions, and extracellular matrix adhesions is needed. These investigations should yield new information as to how receptor occupancy is transduced into specific cellular responses, such as increased permeability, which promotes pathological vascular processes such as tissue edema formation and organ dysfunction.

Mechanisms of pertussis toxin-induced barrier dysfunction in bovine pulmonary artery endothelial cell monolayers.

We have previously characterized several G proteins in endothelial cells (EC) as substrates for the ADP-ribosyltransferase activity of both pertussis (PT) and cholera toxin and described the modulation of key EC physiological responses, including gap formation and barrier function, by these toxins. In this study, we investigated the mechanisms involved in PT-mediated regulation of bovine pulmonary artery endothelial cells barrier function. PT caused a dose-dependent increase in albumin transfer, dependent upon action of the holotoxin, since neither the heat-inactivated PT, the isolated oligomer, nor the protomer induced EC permeability. PT-induced gap formation and barrier dysfunction were additive to either thrombin- or thrombin receptor-activating peptide-induced permeability, suggesting that thrombin and PT utilize distinct mechanisms. PT did not result in Ca2+ mobilization or alter either basal or thrombin-induced myosin light chain phosphorylation. However, PT stimulated protein kinase C (PKC) activation, and both PKC downregulation and PKC inhibition attenuated PT-induced permeability, indicating that PKC activity is involved in PT-induced barrier dysfunction. Like thrombin-induced permeability, the PT effect was blocked by prior increases in adenosine 3',5'-cyclic monophosphate. Thus PT-catalyzed ADP-ribosylation of a G protein (possibly other than Gi) may regulate cytoskeletal protein interactions, leading to EC barrier dysfunction.

Regulation of endothelial cell gap formation and paracellular permeability.

Investigation of the regulation of permeability properties of the endothelium has yielded evidence to support the concept of a dual regulation of EC gap formation and barrier function. In this model, the primary determinants of EC permeability are tethering/adhesive properties (Figure 1) and tensile centripetal force generation (Figure 2). The importance of actin-myosin interactions and active cellular contraction and force generation has been reviewed. In the model of thrombin-induced EC barrier dysfunction, there is a strong shift in the MLC species from the unphosphorylated to the diphosphorylated form, indicating activation of MLCK, a key enzyme whose importance in EC contraction has been well established. Although important differences between EC and SMC exist, endothelial cell gap formation involves actomyosin-dependent contractile mechanisms similar to SMC, a cellular system in which MLC phosphorylation correlates with the initial rate of tension development. The increase in MLC phosphorylation and isometric tension is consistent with the hypothesis that activation of signal transduction mediates an increase in isometric tension to a new level of "latch state" through the cytoskeleton. Thus, the available evidence implicates a strong role for cellular force generation and contraction in the evolution of thrombin-induced barrier dysfunction. Accumulating evidence also indicates that modulation of tethering properties, primarily those involving cell-matrix and cell-cell adhesion, is also a key determinant of basal EC barrier properties as well as agonist-mediated barrier dysfunction. Because each of these focal adhesion constituents may be involved in establishing tethering properties in endothelium, they each may be involved in determining barrier permeability and may be involved in the evolution of agonist-mediated barrier dysfunction. Therefore, in addition to MLCK-dependent active tensile force generation, agonist-induced barrier dysfunction may occur via MLCK-independent pathways that rely on basal levels of MLC phosphorylation or by affecting proteins involved in tethering properties of endothelium that contribute to barrier function. Further examination of tethering force properties, combined with elucidation of EC relaxation via MLC dephosphorylation may yield clues as to how this important vascular barrier is maintained and restored after vascular insult.

Mechanisms of cholera toxin prevention of thrombin- and PMA-induced endothelial cell barrier dysfunction.

Thrombin-induced endothelial cell (EC) activation leads to compromise of monolayer barrier function due to cellular retraction/contraction and intercellular gap formation. Cyclic AMP induces relaxation in other contractile cells and promotes barrier function in EC. To investigate mechanisms involved in cAMP protection in thrombin-induced permeability, we pretreated bovine pulmonary arterial EC monolayers with 1 microgram/ml cholera holotoxin which catalyzed ADP ribosylation of Gs and increased synthesis of cAMP. The holotoxin, but not the binding subunit, reduced basal permeability and prevented gap formation and permeability following challenge with 1 microM thrombin, 100 microM thrombin receptor-activating peptide, or 1 microM phorbol myristate acetate (PMA). Furthermore, thrombin-induced gap formation and permeability were reversed by cholera toxin post-treatment. Pretreatment with 5 microM forskolin or 1 mM dibutyryl cAMP, with or without 1 mM isobutyl methylxanthine, but not cGMP analogs, protected against thrombin-induced EC permeability, mimicking the cholera toxin effect. Although downregulation of protein kinase C attenuated both thrombin- and PMA-induced permeability, cholera toxin did not alter either PMA-induced protein kinase C activation or thrombin-induced Ca2+ mobilization. In contrast, cholera toxin attenuated thrombin-induced myosin light chain phosphorylation and largely prevented actin redistribution. These studies suggest that cholera toxin: (1) protects endothelial barrier function and reverses established dysfunction via increased cAMP (2) does not alter thrombin receptor interaction or early signal events such as Ca2+ mobilization and PKC activation, (3) attenuates myosin light chain kinase activation and actomyosin contractile interaction subsequent to thrombin activation, and (4) abrogates contractile processes subsequent to PKC activation, which is also an important mechanism in thrombin-induced permeability but is independent of myosin light chain kinase activation.