Endothelial monolayer integrity. Perturbation of F-actin filaments and the dense peripheral band-vinculin network

The Endothelial Cytoskeleton: Organization in Normal and Regenerating Endothelium*

The components of the endothelial cell cytoskeleton that have been shown to be important in maintaining endothelial structural integrity and in regulating endothelial repair include F-actin microfilament bundles, including stress fibers, and microtubules, and centrosomes. Endothelial cells contain peripheral and central actin microfilaments. The dense peripheral band (DPB) consists of peripheral actin microfilament bundles which are associated with vinculin adhesion plaques and are most prominent in low or no hemodynamic shear stress conditions. The central microfilaments are very prominent in areas of elevated hemodynamic shear stress. There is a redistribution of actin microfilaments characterized by a decrease of peripheral actin and an increase in central microfilaments under a variety of conditions, including exposure to thrombin, phorbol-esters, and hemodynamic shear stress. During reendothelialization, there is a sequential series of cytoskeletal changes. The DPB remains intact during the rapid lamellipodia mediated repair of very small wounds except at the base of the lamellipodia where it is splayed. The DPB is reduced or absent when cell locomotion occurs to repair a wound. In addition, when cell locomotion is required, the centrosome, in the presence of intact microtubules, redistributes to the front of the cell to establish cell polarity and acts as a modulator of the directionality of migration. This occurs prior to the loss of the DPB but does not occur in very small wounds that close without migration. Thus, the cytoskeleton is a dynamic intracellular system which regulates endothelial integrity and repair and is modulated by external stimuli that are present at the vessel wall-blood interface.

Thrombin Induces Inositol Trisphosphate-Mediated Spatially Extensive Responses in Lung Microvessels

Activation of plasma membrane receptors initiates compartmentalized second messenger signaling. Whether this compartmentalization facilitates the preferential intercellular diffusion of specific second messengers is unclear. Toward this, the receptor-mediated agonist, thrombin, was instilled into microvessels in a restricted region of isolated blood-perfused mouse lungs. Subsequently, the thrombin-induced increase in endothelial F-actin was determined using confocal fluorescence microscopy. Increased F-actin was evident in microvessels directly treated with thrombin and in those located in adjoining thrombin-free regions. This increase was abrogated by inhibiting inositol trisphosphate-mediated calcium release with Xestospongin C (XeC). XeC also inhibited the thrombin-induced increase in the amplitude of endothelial cytosolic Ca²⁺ oscillations. Instillation of thrombin and XeC into adjacent restricted regions increased F-actin in microvessels in the thrombin-treated and adjacent regions but not in those in the XeC-treated region. Thus, inositol trisphosphate, and not calcium, diffused interendothelially to the spatially remote thrombin-free microvessels. Thus, activation of plasma membrane receptors increased the ambit of inflammatory responses via a second messenger different from that used by stimuli that induce cell-wide increases in second messengers. Thrombin however failed to induce the spatially extensive response in microvessels of mice lacking endothelial connexin43, suggesting a role for connexin43 gap junctions. Compartmental second messenger signaling and interendothelial communication define the specific second messenger involved in exacerbating proinflammatory responses to receptor-mediated agonists.

Dynamic regulation of vascular myosin light chain (MYL9) with injury and aging

BACKGROUND

Aging-associated changes in the cardiovascular system increase the risk for disease development and lead to profound alterations in vascular reactivity and stiffness. Elucidating the molecular response of arteries to injury and age will help understand the exaggerated remodeling of aging vessels.

METHODOLOGY/PRINCIPAL FINDINGS

We studied the gene expression profile in a model of mechanical vascular injury in the iliac artery of aging (22 months old) and young rats (4 months old). We investigated aging-related variations in gene expression at 30 min, 3 d and 7 d post injury. We found that the Myosin Light Chain gene (MYL9) was the only gene differentially expressed in the aged versus young injured arteries at all time points studied, peaking at day 3 after injury (4.6 fold upregulation (p<0.05) in the smooth muscle cell layers. We confirmed this finding on an aging aortic microarray experiment available through NCBI's GEO database. We found that Myl9 was consistently upregulated with age in healthy rat aortas. To determine the arterial localization of Myl9 with age and injury, we performed immunohistochemistry for Myl9 in rat iliac arteries and found that in healthy and injured (30 days post injury) arteries, Myl9 expression increased with age in the endothelial layers.

CONCLUSIONS/SIGNIFICANCE

The consistent upregulation of the myosin light chain protein (Myl9) with age and injury in arterial tissue draws attention to the increased vascular permeability and to the age-caused predisposition to arterial constriction after balloon angioplasty.

Rho and basic fibroblast growth factor involvement in centrosome redistribution and actin microfilament remodeling during early endothelial wound repair

OBJECTIVE

We have shown that centrosome redistribution to the front of the cell and actin microfilament remodeling occurs during the initiation of early porcine aortic endothelial wound repair even before cell migration. Because Ras homologous protein (Rho) induces actin microfilament polymerization, interacts with microtubules, and is believed to be activated by growth factors, we set forth to study the regulatory roles of basic fibroblast growth factor (bFGF) and Rho signaling on centrosome redistribution and actin microfilament remodeling in endothelial cells at an in vitro wound edge.

STUDY DESIGN

With double immunofluorescent confocal microscopy, we studied the distribution of various cytoskeletal proteins in wounded porcine aortic endothelial cells in response to bFGF and exoenzyme C3 treatments.

RESULTS

We showed that the addition of 10 ng/mL bFGF for 3 hours after wounding resulted in a significant increase (P <.05) in cells at the wound edge with central microfilaments oriented perpendicular to the wound. Rho inhibition with 2 microg/mL C3 resulted in the reduction of phosphotyrosine, paxillin, and central microfilament staining. Centrosome redistribution and endothelial cell elongation also were significantly inhibited (P <.05) with C3, resulting in decreased wound closure. However, inhibition was reduced with coincubation of bFGF with C3, which also returned the rate of endothelial wound closure toward control values. This Rho-independent bFGF-induced centrosome redistribution was associated with the cells showing a significant increase (P <.05) in acetylated microtubules that extended from the centrosome to the posterior cell border.

CONCLUSION

We conclude that Rho regulates centrosome redistribution and central microfilament remodeling during early endothelial wound repair, and bFGF promotes actin remodeling through a downstream Rho-dependent pathway and promotes centrosome redistribution, at least in part, with a Rho-independent pathway.

Update on pulmonary edema: the role and regulation of endothelial barrier function

Discovery of the pathophysiologic mechanisms leading to pulmonary edema and identification of effective strategies for prevention remain significant clinical concerns. Endothelial barrier function is a key component for maintenance of the integrity of the vascular boundary in the lung, particularly since the gas exchange surface area of the alveolar-capillary membrane is large. This review is focused on new insights in the pulmonary endothelial response to injury and recovery, reversible activation by edemagenic agents, and the biochemical/structural basis for regulation of endothelial barrier function. This information is discussed in the context of fundamental concepts of lung fluid balance and pulmonary function.

Cytoskeletal regulation of pulmonary vascular permeability

The endothelial cell (EC) lining of the pulmonary vasculature forms a semipermeable barrier between the blood and the interstitium of the lung. Disruption of this barrier occurs during inflammatory disease states such as acute lung injury and acute respiratory distress syndrome and results in the movement of fluid and macromolecules into the interstitium and pulmonary air spaces. These processes significantly contribute to the high morbidity and mortality of patients afflicted with acute lung injury. The critical importance of pulmonary vascular barrier function is shown by the balance between competing EC contractile forces, which generate centripetal tension, and adhesive cell-cell and cell-matrix tethering forces, which regulate cell shape. Both competing forces in this model are intimately linked through the endothelial cytoskeleton, a complex network of actin microfilaments, microtubules, and intermediate filaments, which combine to regulate shape change and transduce signals within and between EC. A key EC contractile event in several models of agonist-induced barrier dysfunction is the phosphorylation of regulatory myosin light chains catalyzed by Ca(2+)/calmodulin-dependent myosin light chain kinase and/or through the activity of the Rho/Rho kinase pathway. Intercellular contacts along the endothelial monolayer consist primarily of two types of complexes (adherens junctions and tight junctions), which link to the actin cytoskeleton to provide both mechanical stability and transduction of extracellular signals into the cell. Focal adhesions provide additional adhesive forces in barrier regulation by forming a critical bridge for bidirectional signal transduction between the actin cytoskeleton and the cell-matrix interface. Increasingly, the effects of mechanical forces such as shear stress and ventilator-induced stretch on EC barrier function are being recognized. The critical role of the endothelial cytoskeleton in integrating these multiple aspects of pulmonary vascular permeability provides a fertile area for the development of clinically important barrier-modulating therapies.

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.

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.

The effect of argatroban on injured endothelial cells by thrombin

When endothelial cells are exposed to thrombin, they become perturbed and acquire thrombogenic properties. Argatroban is an arginine derivative, synthetic small molecule that binds to the active site of thrombin and inhibits its catalytic activity. Therefore, the effects of argatroban on endothelial cells, which had been injured by thrombin, were investigated. The established endothelial cell line, TKM-33, which had been cloned from human umbilical vein endothelial cells, was used. Endothelial cells produce plasminogen activator (PA) to prevent thrombosis and maintain the blood flow. When the endothelial cells were injured by thrombin, secretion of plasminogen activator inhibitor-1 (PAI-1) increased and then the PA activity proportionally decreased. The treatment of endothelial cells with argatroban after thrombin injury did not restore their reduced PA activity. However, the treatment of endothelial cells with argatroban prior to thrombin injury resulted in inhibiting the induction of PAI-1 secretion. Thus, pretreatment of endothelial cells with argatroban suppresses the inhibition of their PA activity by thrombin. Since the effect of thrombolytic agent may be modified by the fibrinolytic factors produced by the endothelial cells, the activity of staphylokinase (SAK) was measured in the presence of endothelial cells that had been injured by thrombin. SAK is a newly developed thrombolytic agent. SAK activity in the presence of injured endothelial cells by thrombin was lower than that in the presence of endothelial cells without thrombin injury. However, treatment of endothelial cells with argatroban prior to thrombin injury revealed higher SAK activity than that after thrombin injury. These findings indicate that argatroban pretreatment prevents thrombin injury of endothelial cells, which may then maintain their physiological function.

Heat shock protein 27 kDa expression and phosphorylation regulates endothelial cell migration

The effects of enhanced HSP27 expression or expression of a nonphosphorylatable form of HSP27 on the migration of bovine arterial endothelial cells was assessed. Expression of the wild-type protein enhanced migration by twofold compared to control transfectants, whereas expression of the mutant protein retarded migration by 40%. Since homologs of the small heat shock protein inhibit F-actin polymerization in vitro and may alter basolateral F-actin content in vivo, it was postulated that the 27 kDa heat shock protein affects microfilament extension essential for cell motility. Expression of the wild-type protein promoted the generation of long cellular extensions, whereas expression of the dominant negative mutant protein resulted in a marked reduction of lamellipodia and generated aberrant microfilament morphology at the wound edge. Immunofluorescence combined with phalloidin staining demonstrated the colocalization of the HSP27 gene products with lamellipodial microfilament structures. These data suggest that the 27 kDa heat shock protein regulates migration by affecting the generation lamellipodia microfilaments.

2-Phenyl-4-quinolone prevents serotonin-induced increases in endothelial permeability to albumin

To investigate the role of 2-phenyl-4-quinolone in enhancing endothelial monolayer paracellular barrier function and preventing the disturbance of paracellular barrier function by vasoactive agents, the study examined the effect of 2-phenyl-4-quinolone on serotonin-mediated macromolecule transfer and microfilament changes in cultured rat heart endothelial cells. Serotonin-treated endothelial cells induced concentration-dependent increases in the passage of Evans blue dye-bound bovine serum albumin. Incubation of the endothelial monolayers with 2-phenyl-4-quinolone antagonized serotonin- and cytochalasin B-induced macromolecular permeability. 2-Phenyl-4-quinolone also opposed the effect of serotonin or cytochalasin B on the distribution and quantity of actin filaments in the endothelial cytoskeleton. Furthermore, 2-phenyl-4-quinolone alone led to an apparent quantitative increase in F actin fluorescence in endothelial cells. The addition of 10(-7) M 2-phenyl-4-quinolone had an effect on serotonin-induced changes in the myosin and distribution of myosin were comparable to that on serotonin monolayers. In conclusion, 2-phenyl-4-quinolone attenuated the serotonin-induced permeability of rat heart endothelial cells and this was associated with stabilization of F actin microfilaments and changes in the myosin organization. This result suggests that influences on cytoskeletal assembly may be involved in this process.

Platelet adhesion to aortic endothelial cells in vitro after thrombin treatment: observation with video-enhanced contrast microscopy

Secondary thrombus formation following arterial occlusion is suggested to play an important role in the exacerbation of ischemic organ damage. We investigated the effect of thrombin on endothelial cells from the aspect of morphological changes and induction of platelet adhesion to the endothelial cells. Using a video-enhanced contrast microscopy, we observed human aortic endothelial cells (HAEC) following perfusion of human alpha-thrombin of 1.0 U/ml (n = 7) or vehicle (n = 7) for 30 minutes. The endothelial cells began to shrink 15 minutes after thrombin administration. Gaps between the cells were formed. The cells became rearranged orderly in the same direction 30 minutes later. In another study, following pretreatment with human alpha-thrombin 1.0 U/ml (n = 10) or vehicle (n = 7) for 20 minutes and washout, platelets were perfused over HAEC for 30 minutes. Platelets adhered directly to thrombin-treated endothelial cells and became flat on the endothelial cells. Then other platelets were observed to approach to the flattened platelets and aggregated onto it. After washout of floating platelets, adhesion of platelets was further confirmed. These results suggest that thrombin may be involved in the endothelial damage and formation of platelet thrombi on the endothelial cells after blood flow disturbance.

Hydrogen peroxide-induced cytoskeletal rearrangement in cultured pulmonary endothelial cells

Although the signaling pathways leading to hydrogen peroxide (H2O2)-induced endothelial monolayer permeability remain ambiguous, cytoskeletal proteins are known to be essential for maintaining endothelial integrity and regulating solute flux through the monolayer. We have recently demonstrated that thrombin-induced actin reorganization in bovine pulmonary artery endothelial cells (BPAEC) requires activation of both myosin light chain kinase (MLCK) and protein kinase C (PKC). Therefore, the present study was designed to investigate the effects of H2O2 on actin reorganization in BPAEC. H2O2 initiated sustained recruitment of actin to the cytoskeleton and transient myosin recruitment in a time- and concentration-dependent manner. The H2O2-induced actin recruitment was significantly inhibited by the calmodulin antagonists, W7 and TFP, but not by the MLCK inhibitor, KT5926, nor the PKC inhibitors, H7 and calphostin C. H2O2 also caused actin filament rearrangement in BPAEC with disruption of the dense peripheral bands and formation of stress fibers. These alterations occurred prior to actin translocation to the cytoskeleton and are prevented by inhibition of either MLCK or PKC. High concentrations of H2O2 transiently attenuated PKC activity but slightly increased the phosphorylation of the prominent PKC substrate and actin-binding protein, myristoylated alanine-rich C kinase substrate (MARCKS), by 5 min. However, MARCKS phosphorylation was reduced to below basal levels by 30 min. On the other hand, H2O2 induced a time- and dose-dependent phosphorylation of myosin light chains which was eliminated by both MLCK and PKC inhibitors. These data suggest that MLCK contributes to H2O2-induced myosin light chain phosphorylation and actin rearrangement and that PKC may play a permissive role. Neither of these enzymes appears to be involved in the H2O2-induced recruitment of actin to the cytoskeleton.

Alterations in endothelial F-actin microfilaments in rabbit aorta in hypercholesterolemia

The current study tests whether hypercholesterolemia influences the distribution of endothelial cell microfilaments during the initiation and growth of fatty streak-type lesions. We classified the lesions occurring over a 20-week period into four types based on the location and extent of macrophage infiltration observed microscopically. The earliest lesion was characterized by leukocytes adherent to the endothelial surface. Minimal lesions were characterized by a few cells in the subendothelium. Intermediate lesions consisted of numerous subendothelial leukocytes in a minimally raised lesion. Advanced fatty streak lesions were elevated, with several layers of leukocytes. The organization of peripheral junctional actin (the dense peripheral band) and of central endothelial cell actin microfilament bundles was studied in each of these lesions by using fluorescent microscopy. We found that in the aorta away from branch sites and in areas away from lesions, the central microfilament distribution was unaffected by hypercholesterolemia. The macrophages entered the wall without any identifiable reorganization in the microfilaments. During the accumulation of subendothelial macrophages in minimal and intermediate lesions, stress fibers were initially increased in comparison to lesion-free areas. In raised advanced lesions, the central microfilaments became thinner and disappeared. However, at flow dividers, where central stress fibers are normally prominent, endothelial cells on the surface of intermediate lesions showed a reduction in central fibers, and peripheral bands became prominent. This finding was associated with changes in cell shape from elongated to cobblestone type. Thus, actin microfilament bundles in endothelial cells underwent substantial changes in distribution during the accumulation of subendothelial macrophages, forming hypercholesterolemia-induced fatty streak-type lesions. These changes may influence endothelial substrate adhesion, permeability, or repair after injury.

Requirement for protein kinase C theta for cell cycle progression and formation of actin stress fibers and filopodia in vascular endothelial cells

Activation of the protein kinase C (PKC) family with phorbol esters induces endothelial proliferation and angiogenesis, but which of the events that constitute angiogenesis are affected by individual members of the PKC family is unknown. In rat capillary endothelial (RCE) cells, serum stimulation increased expression of a single PKC isoenzyme, PKCtheta, and its translocation to the periphery. Conditional overexpression of a dominant-negative mutant of PKCtheta markedly inhibited RCE proliferation, as well as closure of a "wound" by RCE migration and formation of capillary rings and tubules in vitro. PKCtheta inhibition delayed the endothelial cell cycle at the G2/M phase and prevented formation of actin stress fibers and filopodia but not lamellipodia. The defect in cell morphology and wound closure in PKCtheta-kn cells was reversed by overexpressing kinase-active PKCtheta, indicating that these RCE functions depend upon PKCtheta substrates. Thus, PKCtheta is required for multiple processes essential for angiogenesis and wound repair, including endothelial mitosis, maintenance of a normal actin cytoskeleton, and formation of an enclosed tube.

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.

Effects of shear stress on protein kinase C distribution in endothelial cells

We studied the effects of shear stress on protein kinase C (PKC) in cultured human umbilical vein endothelial cells by use of a flow channel and a monoclonal antibody (MAb 1.3) that recognizes the PKC beta-isozyme. The fluorescence intensity (FI) of the secondary antibody, crystalline tetramethylrhodamine isothiocyanate, was determined by image analysis. The results on each of five shearing experiments were normalized by using the paired stationary control. After 30-min shearing at 2 N/m2, FI per cell increased to 1.6 times that of control, as did the mean FI per unit cell area. The FI per unit stained area and the stained area/cell area ratio were also increased significantly by shearing. The distribution of immunostaining in each cell was determined for its cortical, cytoplasmic, perinuclear, and nuclear regions. The normalized FI per unit area in all four regions and the stained area/cell area ratio in cortical and cytoplasmic regions were significantly higher in the sheared cells than in control; the increases were greatest in the cortical area. Double staining with rhodamine-phalloidin and MAb 1.3 showed the association of actin with the PKC isozyme in both stationary and sheared cells.

Shear stress modulates endothelial cell morphology and F-actin organization through the regulation of focal adhesion-associated proteins

Flow-related shear stress has been shown to modulate endothelial cell structure and function including F-actin microfilament organization. Focal adhesion-associated proteins such as vinculin, talin, and specific integrins may play a role in the modulation of these cytoskeletal and morphological changes. Double-label immunofluorescence studies indicated that, in static culture, alpha 5 beta 1 fibronectin receptors (alpha 5 beta 1 FNRs) and alpha v beta 3 vitronectin receptors (alpha v beta 3 VNRs) were found predominantly in the peripheral regions of bovine aortic endothelial cells (BAECs) corresponding to the localization of vinculin, talin, and actin microfilament terminations. In response to shear stress, concomitant with cell elongation and the appearance of stress fibers aligned with the direction of flow, there was a prominent localization of vinculin and alpha v beta 3 VNRs as the "upstream" end of the cells. Stress fiber terminations were clearly evident at these concentrations of focal adhesion-associated proteins. These data suggest that the upstream concentration of these proteins may direct shear stress-induced stress fiber formation and may function in the alignment of the fibers in the direction of flow. Levels of surface alpha v beta 3 VNRs were found to decrease in response to flow, possibly reflecting the decrease in numbers of "downstream" receptors. Unlike the arrangement of vinculin and alpha v beta 3 VNRs observed following exposure to flow, talin and alpha 5 beta 1 FNRs, in addition to being localized at the upstream end of the cell, were also evenly distributed throughout the rest of the cell. Surface levels of alpha 5 beta 1 FNRs increased in response to shear stress, perhaps providing an increased adherence of BAECs to the extracellular matrix through these receptors. These data suggest that focal adhesion-associated proteins play specific roles in the response of BAECs to shear stress.

Postnatal reorganization of actin filaments and differentiation of intercellular boundaries in the rat aortic endothelial cells

Postnatal change in the distribution of actin filaments in endothelial cells was studied in the rat aorta by use of rhodamine-phalloidin staining and confocal laser scanning microscopy. Endothelial cells of the rat aorta possessed two populations of actin filament bundles, namely, peripheral bands at the cell border and stress fibers running longitudinally in the cytoplasm. Aortic endothelial cells of the neonatal rat contained only stress fibers, whereas those of the 10-day-old rat developed both peripheral bands and stress fibers. After 20 days of age, aortic endothelial cells had predominantly peripheral bands with occasional stress fibers around the branch orifices. During postnatal development the length density of stress fibers in aortic endothelial cells decreased, whereas individual stress fibers in endothelial cells were shortened. Electron-microscopic observation revealed that the high intercellular boundaries of aortic endothelial cells at birth decreased in height and developed cytoplasmic interdigitations after 20 days of age. The occurrence of peripheral bands at the cell border is thought to be closely related to formation of cytoplasmic interdigitation which strengthens the mechanical connection between endothelial cells against increasing transmural pressure. Expression of stress fibers in aortic endothelial cells of the neonatal rat is supposed to be affected by longitudinal elongation of the developing aorta, whereas their postnatal decrease is thought to be correlated with the change of fluid shear stress loaded on the aortic endothelium.

Three patterns of distribution characterize the organization of endothelial microfilaments at aortic flow dividers

Since actin microfilaments are essential in the maintenance of endothelial integrity and in the repair of injured endothelium, we have carried out a detailed study of the distribution of microfilaments in the immediate vicinity of aortic branches. Branches are of major interest because there is a predilection for atherosclerotic lesions near branch ostia. We made an extensive, systematic examination of branches of the aorta and iliac arteries using in situ staining of perfusion-fixed arteries. Microfilaments were localized using rhodamine phalloidin. Three patterns of staining were observed. Some endothelial cells showed prominent central stress fibers. Others had few central stress fibers but prominent peripheral fibers. Still others showed an intermediate pattern with some central and some peripheral fibers present. At small branch sites, the lip of the divider was more blunt, and there were more cells with peripheral actin. At large branches, cells with peripheral actin were confined mainly to the lip, while there were many more cells with prominent central fibers. We also found that major differences can occur over very small distances, so adjacent cells may have strikingly different patterns of microfilament distribution. These patterns appear to reflect the geometry of the flow divider and local variations in hemodynamic shear stress. The differences in microfilament distribution may reflect differences in endothelial functions which are essential in maintaining endothelial integrity.

Disruption of cytoskeletal structures mediates shear stress-induced endothelin-1 gene expression in cultured porcine aortic endothelial cells

Hemodynamic shear stress alters the architecture and functions of vascular endothelial cells. We have previously shown that the synthesis of endothelin-1 (ET-1) in endothelial cells is increased by exposure to shear stress. Here we examined whether shear stress-induced alterations in cytoskeletal structures are responsible for increases in ET-1 synthesis in cultured porcine aortic endothelial cells. Exposure of endothelial cells to 5 dyn/cm2 of low shear stress rapidly increased monomeric G-actin contents within 5 min without changing total actin contents. The ratio of G- to total actin, 54 +/- 0.8% in quiescent endothelial cells, increased to 87 +/- 4.2% at 6 h and then decreased. Following the disruption of filamentous (F)-actin into G-actin, ET-1 mRNA levels in endothelial cells also increased within 30 min and reached a peak at 6 h. The F-actin stabilizer, phalloidin, abolished shear stress-induced increases in ET-1 mRNA; however, it failed to inhibit increases in ET-1 mRNA secondary to other stimulants. This indicates that shear stress-induced increases in ET-1 mRNA levels may be mediated by the disruption of actin fibers. Furthermore, increases in ET-1 gene expression can be induced by actin-disrupting agents, cytochalasin B and D. Another cytoskeleton-disrupting agent, colchicine, which inhibits dimerization of tubulin, did not affect the basal level of ET-1 mRNA. However, colchicine completely inhibited shear stress- and cytochalasin B-induced increases in ET-1 mRNA levels. These results suggest that shear stress-induced ET-1 gene expression in endothelial cells is mediated by the disruption of actin cytoskeleton and this induction is dependent on the integrity of microtubules.

Influence of mitomycin C on endothelial monolayer regeneration in vitro

This study examines the effect of Mitomycin C, a fungal toxin which inhibits DNA synthesis, on the regeneration of partially denuded large vessel endothelium in vitro. Monolayers of bovine pulmonary artery endothelial cells were treated with Mitomycin C prior to or immediately following partial denudation and were incubated in the continuing presence of Mitomycin C; the effects of this treatment on monolayer repair, cell proliferation, and other aspects of endothelial phenotype were monitored. Cell proliferation, DNA, RNA, and protein synthesis were all reduced in a dose dependent manner in treated cultures. Incubation with Mitomycin C for 48 h or longer resulted in reduced cell spreading, and rounding up and loss of cells from both intact and partially denuded cultures. Effects were less severe with lower doses and shorter incubation times. However, significant reductions in monolayer regeneration occurred within 8 h of incubation, sufficiently early to suggest that Mitomycin C may affect aspects of the regeneration process independent of cell proliferation. Polarization/spreading of cells at the denudation edge was monitored by fluorescence staining for golgi with C5-DMB-ceramide, and for centrioles with antibodies to tubulin. Centrioles and golgi rapidly reoriented to a location at the putative leading edge of control cultures. Mitomycin C treatment had no effect on centriole reorientation, but caused a significant delay in golgi localization. These results suggest that Mitomycin C inhibits endothelial monolayer regeneration by mechanisms independent of cell proliferation and DNA synthesis, perhaps by interfering with cell spreading or translocation at the wound edge.

Disruption of microtubules inhibits the stimulation of tissue plasminogen activator expression and promotes plasminogen activator inhibitor type 1 expression in human endothelial cells

The expression of certain proteolytic enzymes involved in cell migration (collagenase, urokinase) can be enhanced by the disruption of cellular cytoskeletal organization, suggesting an association between cell shape and gene expression. We have examined the effect of cytoskeleton-disrupting agents on the production and secretion of another proteolytic enzyme, tissue plasminogen activator (tPA), and its inhibitor, plasminogen activator inhibitor-1 (PAI-1), in human endothelial cells. Addition of 1 x 10(-6) M colchicine, 5 x 10(-6) M cytochalasin B, 10(-6) M nocodazole, or 10(-6) M tubulazole had no effect on the constitutive rate of release of tPA. However, the three microtubule-disrupting agents--colchicine, nocodazole, and tubulazole--depressed the stimulation of tPA secretion by phorbol myristate acetate (PMA) by 50- to 65%. Disruption of microfilament structure by cytochalasin B had no effect. In contrast, microtubule disruption in the absence or presence of PMA stimulated PAI-1 secretion by 2.5 and 2 times, respectively. The depression of tPA secretion was not due to inhibition of the secretory function since tPA did not accumulate intracellularly during colchicine treatment. Nor did colchicine affect the PMA activation of protein kinase C-alpha, upon which stimulation of tPA is dependent; neither translocation of the kinase nor phosphorylation of the protein kinase C substrate protein, P80, was inhibited. Measurement of tPA mRNA levels demonstrated that the increase which precedes PMA-enhanced tPA secretion was also inhibited by colchicine by 50%. However, tPA gene transcriptional activity was only reduced 13%, suggesting that a post-transcriptional event was affected by microtubule disruption. PAI-1 mRNA levels and transcription rates were elevated 3.5 times. This study suggests that the changes that occur in endothelial cells during PMA-induced signal transmission leading to enhanced tPA mRNA levels and tPA antigen production can be partly blocked by agents that disrupt microtubule organization.

Acute endothelial cell contraction in vitro: a comparison with vascular smooth muscle cells and fibroblasts

The contractile responses of cultured rat and calf endothelial cells (EC), vascular smooth muscle cells (VSMC), and fibroblasts (FB) to vasoactive mediators (thrombin, serotonin, bradykinin, and histamine), forskolin, and cytochalasin B were compared. Cells were grown on a pliable silicone membrane, and contraction was assessed, using time-lapse video microscopy, by recording changes in the wrinkling of the silicone as the cells exerted tension on the surface. We found that all cells contracted in the presence of serum or thrombin and that VSMC and FB also contracted with serotonin stimulation. Bradykinin and histamine were not contractants in this system. Discrepancies between these results and reports of changes in permeability of endothelial layers in vitro and in vivo may be due to (1) the vascular segment from which EC were studied or (2) the possibility that certain mediators may provoke a noncontractile response that results in gap formation. Thus changes in vascular permeability, which occur during inflammation, may have both contractile and noncontractile components. Forskolin, known to indirectly inhibit myosin light-chain kinase activity, and cytochalasin B were potent relaxants, suggesting a similar smooth muscle-like contractile mechanism for all three cell types.

Disruption of endothelial actin microfilaments by protein kinase C inhibitors

In this study, we report that the isoquinolinesulfonamide inhibitors of protein kinase C (PKC), H-7 [1-(5-isoquinolinesulfonyl)-2-methylpiperazine] and its related derivatives H-8 and HA-1004, in addition to staurosporine cause depletion and reorganization of microfilament bundles of porcine aortic endothelial cells in both low-density and confluent monolayer cultures. Concomitantly, significant loss of cell adhesion was noted following treatment with H-7. The effects of these compounds were found to be reversible upon wash-out, with restoration of the microfilament network. In addition, longer term incubation with phorbol myristate acetate (PMA) carried out to deplete PKC results in depletion of microfilaments as well. After 24 hr of PMA incubation, however, addition of H-7 or staurosporine is associated with further loss of the remaining microfilaments, suggesting that these agents act, at least in part, through a PKC-independent mechanism.

Morphological response of human endothelial cells subjected to cyclic strain in vitro

Endothelial cells (EC) are subjected to hemodynamic forces in vivo. However, most in vitro studies of EC biology have been performed utilizing stationary culture conditions. To study the morphology and cytoskeletal features of EC under dynamic culture conditions, we utilize a system capable of exerting repetitive strain on cells in culture. Human saphenous vein EC were plated to a subconfluent density in 25-mm wells with a thin flexible bottom and a rat collagen, Type I surface. A -20 kPascals vacuum applied to the bottoms led to a maximum deformation of 24%. EC were exposed to 0.5 sec deformation alternating with 0.5 sec relaxation (60 cycles/min) for 24 hr. EC were fixed with formalin at different time intervals and stained with crystal violet. Actin filaments were stained with rhodamine phalloidin, an F-actin marker, while beta-tubulin and vimentin were visualized by immunofluorescent antibody techniques. Within 15 min after initiation of cyclic strain, actin stress fibers were aligned perpendicular to the force vector. By 12 hr of cyclic strain EC were elongated and oriented in the same direction as the actin filaments. EC elongation and alignment were inhibited by cytochalasin B. Even up to 24 hr of cyclic strain, beta-tubulin and vimentin distributions were unaltered. We propose that cyclic strain of EC in vitro influences cell alignment and elongation by a mechanism dependent on the actin filament system.

Norepinephrine and iloprost improve barrier function of human endothelial cell monolayers: role of cAMP

The barrier function of human artery endothelial cells was improved by addition of agents that increase the cellular adenosine 3',5'-cyclic monophosphate (cAMP) concentration. Together with a decrease in the passage rate of peroxidase, an increase in the transendothelial electrical resistance was observed. A direct correlation was found between the relative increases in cellular cAMP concentration and the relative decrease in peroxidase passage after incubation of the cells with forskolin (0.25 and 2.5 microM), the beta-adrenergic agonist isoproterenol (10 microM), and the stable prostacyclin analogue iloprost (10 microM). Norepinephrine (10 microM) reduced the peroxidase passage to a much larger extent (40% reduction) than might be expected on the basis of a small increase of cAMP concentration. This small increase in cAMP (44%) was the result of interactions of norepinephrine with beta-adrenergic receptors, which increase cAMP, and alpha-adrenergic receptors, which decrease cAMP. The relatively strong reduction in permeability (also found in the presence of the alpha-adrenergic antagonist phentolamine) suggests that an additional cAMP-independent mechanism underlaid the barrier-improving effect of norepinephrine. A marked elevation of cAMP by forskolin was accompanied by a disappearance of F-actin and myosin from stress fibers. They were found diffusely spread over the cell, and F-actin in the cell periphery became prominently visible.