Phalloidin enhances endothelial barrier function and reduces inflammatory permeability in vitro

Histamine activates p38 MAP kinase and alters local lamellipodia dynamics, reducing endothelial barrier integrity and eliciting central movement of actin fibers

The role of the actin cytoskeleton in endothelial barrier function has been debated for nearly four decades. Our previous investigation revealed spontaneous local lamellipodia in confluent endothelial monolayers that appear to increase overlap at intercellular junctions. We tested the hypothesis that the barrier-disrupting agent histamine would reduce local lamellipodia protrusions and investigated the potential involvement of p38 mitogen-activated protein (MAP) kinase activation and actin stress fiber formation. Confluent monolayers of human umbilical vein endothelial cells (HUVEC) expressing green fluorescent protein-actin were studied using time-lapse fluorescence microscopy. The protrusion and withdrawal characteristics of local lamellipodia were assessed before and after addition of histamine. Changes in barrier function were determined using electrical cell-substrate impedance sensing. Histamine initially decreased barrier function, lamellipodia protrusion frequency, and lamellipodia protrusion distance. A longer time for lamellipodia withdrawal and reduced withdrawal distance and velocity accompanied barrier recovery. After barrier recovery, a significant number of cortical fibers migrated centrally, eventually resembling actin stress fibers. The p38 MAP kinase inhibitor SB203580 attenuated the histamine-induced decreases in barrier function and lamellipodia protrusion frequency. SB203580 also inhibited the histamine-induced decreases in withdrawal distance and velocity, and the subsequent actin fiber migration. These data suggest that histamine can reduce local lamellipodia protrusion activity through activation of p38 MAP kinase. The findings also suggest that local lamellipodia have a role in maintaining endothelial barrier integrity. Furthermore, we provide evidence that actin stress fiber formation may be a reaction to, rather than a cause of, reduced endothelial barrier integrity.

Role of [Ca(2+)]i and F-actin on mesothelial barrier function

The mesothelial layer acts as a biological barrier between the organ and the enveloping serous cavity and may have functions of transport, equilibrium maintenance, and protection. However, the role of the mesothelial cells in regulation of pleural permeability remains essentially undefined. The present study was designed to clarify the effects of bradykinin, histamine, and thrombin on permeability in pleural mesothelial cells. Rat pleural mesothelial cells were cultured in vitro, and the permeability of mesothelial monolayers was evaluated by transmesothelial albumin diffusion and electrical resistance measurements. Furthermore, the temporal relationship between changes in the levels of [Ca(2+)]i and the mesothelial permeability was examined. Bradykinin (10 μM), histamine (1 mM), and thrombin (10 U) caused albumin diffusion within 5 min. The electrical resistance of mesothelial monolayer began falling within 5 min of adding each agent. Time and concentration dependency of changes in electrical resistance were almost the same as that in albumin diffusion. Each agent also induced a biphasic elevation of [Ca(2+)]i in pleural mesothelial cells. The concentration-dependency of the [Ca(2+)]i responses were almost similar to that noted for each agent induced albumin diffusion and electrical resistance fall. The increase in permeability occurred with reorganization of F-actin cytoskeleton and increased actin polymerization. These results suggest that the Ca(2+)- dependency of increases induced by these agents in mesothelial permeability have been related to the regulatory role of Ca(2+) in the F-actin cytoskeletal reorganization in pleural mesothelial cells.

Studies on the cell biology of interendothelial cell gaps

Pain, redness, heat, and swelling are hallmarks of inflammation that were recognized as early as the first century AD. Despite these early observations, the mechanisms responsible for swelling, in particular, remained an enigma for nearly two millennia. Only in the past century have scientists and physicians gained an appreciation for the role that vascular endothelium plays in controlling the exudation that is responsible for swelling. One of these mechanisms is the formation of transient gaps between adjacent endothelial cell borders. Inflammatory mediators act on endothelium to reorganize the cytoskeleton, decrease the strength of proteins that connect cells together, and induce transient gaps between endothelial cells. These gaps form a paracellular route responsible for exudation. The discovery that interendothelial cell gaps are causally linked to exudation began in the 1960s and was accompanied by significant controversy. Today, the role of gap formation in tissue edema is accepted by many, and significant scientific effort is dedicated toward developing therapeutic strategies that will prevent or reverse the endothelial cell gaps that are present during the course of inflammatory illness. Given the importance of this field in endothelial cell biology and inflammatory disease, this focused review catalogs key historical advances that contributed to our modern-day understanding of the cell biology of interendothelial gap formation.

Analysis of endothelial barrier function in vitro

Increased microvascular solute permeability underlies many forms of pathophysiological conditions, including inflammation. Endothelial monolayer cultures provide an excellent model system which allows systemic and mechanistic study of endothelial barrier function and paracellular permeability in vitro. The endothelial-specific complexus adherens junction protein VE-cadherin and their intracellular complex form pericellular structures along the cell borders which are critical to regulate endothelial barrier function by controlling pericellular permeability of vasculature. Here, we describe methods for both visualizing and quantifying junctional permeability and barrier changes in endothelial monolayers in vitro.

Nitric oxide/cGMP protects endothelial cells from hypoxia-mediated leakiness

Leakiness of the endothelial bed is attributed to the over-perfusion of the pulmonary bed, which leads to high altitude pulmonary edema (HAPE). Inhalation of nitric oxide has been successfully employed to treat HAPE patients. We hypothesize that nitric oxide intervenes in the permeability of the pulmonary macrovascular endothelial bed to rectify the leaky bed under hypoxia. Our present work explores the underlying mechanism of 'hypoxia-mediated' endothelial malfunction by using human umbilical cord-derived immortalized endothelial cells, ECV-304, and bovine pulmonary artery primary endothelial cells. The leakiness of the endothelial monolayer was increased by two-fold under hypoxia in comparison to cells under normoxia, while optical tweezers-based tethering assays reported a higher membrane tension of endothelial cells under hypoxia. Phalloidin staining demonstrated depolymerization of F-actin stress fibers and highly polarized F-actin patterns in endothelial cells under hypoxia. Nitric oxide, 8-Br-cGMP and sildenafil citrate (phosphodiesterase type 5 inhibitor) led to recovery from hypoxia-induced leakiness of the endothelial monolayers. Results of the present study also suggest that 'hypoxia-induced' cytoskeletal rearrangements and membrane leakiness are associated with the low nitric oxide availability under hypoxia. We conclude that nitric oxide-based recovery of hypoxia-induced leakiness of endothelial cells is a cyclic guanosine monophosphate (cGMP)-dependent phenomenon.

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.

Signaling Mechanisms Regulating Endothelial Permeability

The microvascular endothelial cell monolayer localized at the critical interface between the blood and vessel wall has the vital functions of regulating tissue fluid balance and supplying the essential nutrients needed for the survival of the organism. The endothelial cell is an exquisite “sensor” that responds to diverse signals generated in the blood, subendothelium, and interacting cells. The endothelial cell is able to dynamically regulate its paracellular and transcellular pathways for transport of plasma proteins, solutes, and liquid. The semipermeable characteristic of the endothelium (which distinguishes it from the epithelium) is crucial for establishing the transendothelial protein gradient (the colloid osmotic gradient) required for tissue fluid homeostasis. Interendothelial junctions comprise a complex array of proteins in series with the extracellular matrix constituents and serve to limit the transport of albumin and other plasma proteins by the paracellular pathway. This pathway is highly regulated by the activation of specific extrinsic and intrinsic signaling pathways. Recent evidence has also highlighted the importance of the heretofore enigmatic transcellular pathway in mediating albumin transport via transcytosis. Caveolae, the vesicular carriers filled with receptor-bound and unbound free solutes, have been shown to shuttle between the vascular and extravascular spaces depositing their contents outside the cell. This review summarizes and analyzes the recent data from genetic, physiological, cellular, and morphological studies that have addressed the signaling mechanisms involved in the regulation of both the paracellular and transcellular transport pathways.

Histamine reduces gap junctional communication of human tonsil high endothelial cells in culture

The regulation of gap junctional communication by histamine was studied in primary cultures of human tonsil high endothelial cells (HUTECs). We evaluated intercellular communication, levels, state of phosphorylation, and cellular distribution of gap junction protein subunits, mainly connexin (Cx)43. Histamine induced a time-dependent reduction in dye coupling (Lucifer yellow) associated with reduction in connexin43 localized at cell-cell appositions (immunofluorescence), without changes in levels and phosphorylation state of connexin43 (immunoblots). These effects were prevented with chlorpheniramine, an H1 receptor blocker; indomethacin, a cyclooxygenase blocker; or GF109203X, a protein kinase C inhibitor. Treatment with phorbol myristate acetate, a protein kinase C activator, and 4bromo (4Br)-A23187, a calcium ionophore, mimicked the histamine-induced effects on dye coupling. 8Bromo-cAMP doubled the dye coupling extent and prevented the histamine-induced reduction in incidence of dye coupling. After 24-h histamine treatment, known to desensitize H1 receptors, reapplication of histamine increased cell coupling in a way prevented by ranitidine, an H2 receptor blocker. Thus, activation of H1 and H2 receptors, which increase intracellular levels of free Ca2+ and cAMP, respectively, may affect gap junctional communication in opposite ways. Stabilization of actin filaments with phalloidine diminished but did not totally prevent histamine-induced cell shape changes and reduction in dye coupling. Hence, the histamine-induced reduction in gap junctional communication between HUTEC is mediated by cytoskeleton-dependent and -independent mechanisms and might contribute to modulate endothelial function in lymphoid tissue.

Regulation of actin dynamics is critical for endothelial barrier functions

We tested the hypothesis that the equilibrium between F- and G-actin in endothelial cells modulates the integrity of the actin cytoskeleton and is important for the maintenance of endothelial barrier functions in vivo and in vitro. We used the actin-depolymerizing agent cytochalasin D and jasplakinolide, an actin filament (F-actin) stabilizing and promoting substance, to modulate the actin cytoskeleton. Low doses of jasplakinolide (0.1 microM), which we have previously shown to reduce the permeability-increasing effect of cytochalasin D, had no influence on resting permeability of single-perfused mesenteric microvessels in vivo as well as on monolayer integrity. The F-actin content of cultured endothelial cells remained unchanged. In contrast, higher doses (10 microM) of jasplakinolide increased permeability (hydraulic conductivity) to the same extent as cytochalasin D and induced formation of intercellular gaps in cultured myocardial endothelial (MyEnd) cell monolayers. This was accompanied by a 34% increase of F-actin and pronounced disorganization of the actin cytoskeleton in MyEnd cells. Furthermore, we tested whether an increase of cAMP by forskolin and rolipram would prevent the cytochalasin D-induced barrier breakdown. Conditions that increase intracellular cAMP failed to block the cytochalasin D-induced permeability increase in vivo and the reduction of vascular endothelial cadherin-mediated adhesion in vitro. Taken together, these data support the hypothesis that the state of polymerization of the actin cytoskeleton is critical for maintenance of endothelial barrier functions and that both depolymerization by cytochalasin D and hyperpolymerization of actin by jasplakinolide resulted in an increase of microvessel permeability in vivo. However, cAMP, which is known to support endothelial barrier functions, seems to work by mechanisms other than stabilizing F-actin.

Bradykinin-induced proinflammatory signaling mechanisms

Intravital microscopic techniques were used to examine the mechanisms underlying bradykinin-induced leukocyte/endothelial cell adhesive interactions (LECA) and venular protein leakage (VPL) in single postcapillary venules of the rat mesentery. The effects of bradykinin superfusion to increase LECA and VPL were prevented by coincident topical application of either a bradykinin-B(2) receptor antagonist, a cell-permeant superoxide dismutase (SOD) mimetic or antioxidant, or inhibitors of cytochrome P-450 epoxygenase (CYPE) or protein kinase C (PKC) but not by concomitant treatment with either SOD, a mast cell stabilizer, or inhibitors of nitric oxide synthase, cyclooxygenase, xanthine oxidase, NADPH oxidase, or platelet-activating factor. Immunoneutralizing P-selectin or intercellular adhesion molecule-1 (ICAM-1) completely prevented bradykinin-induced leukocyte adhesion and emigration but did not affect VPL. On the other hand, stabilization of F-actin with phalloidin prevented bradykinin-induced leukocyte emigration and VPL but did not alter leukocyte adhesion. These data indicate that bradykinin induces LECA in rat mesenteric venules via a B(2)-receptor-initiated, CYPE-, oxidant- and PKC-mediated, P-selectin- and ICAM-1-dependent mechanism. Bradykinin also produced VPL, an effect that was initiated by stimulation of B(2) receptors and involved CYPE and PKC activation, oxidant generation, and cytoskeletal reorganization but was independent of leukocyte adherence and emigration.

Contribution of F-actin to barrier properties of the blood-joint pathway

OBJECTIVES

Because fibroblast filamentous actin (F-actin) influences cutaneous interstitial matrix swelling pressure (5), we investigated whether F-actin in fibroblast-derived synoviocytes influences the hydraulic permeability of the trans-synovial interstitial pathway. The study also tested whether F-actin in fenestrated synovial endothelium contributes to the blood-joint barrier in vivo.

METHODS

The clearance of Evans blue-albumin (EVA) from plasma into the knee joint cavity was determined in joint infused with F-actin disrupting cytochalasin D (1-200 microM), latrunculin B (100 microM) or vehicle in anesthetized rabbits. The hydraulic permeability of the lining was determined as the slope relating net trans-synovial flow Q(s) to intra-articular pressure P(j). Synovium was examined histologically after i.v. Monastral blue to assess endothelial leakiness.

RESULTS

EVA permeation in vivo was increased up to 25-fold by cytochalasin (p = 0.0002, ANOVA), with an EC(50) of 23 microM (95% confidence limits 13-43 microM). Washout quickly reversed the increase. Latrunculin had a similar effect. F-actin disruption switched Q(s) from drainage (control) to filtration into the cavity at low P(j) in vivo and raised the conductance Q(s)/dP(j) by 2.13 (p = 0.001, ANOVA). Circulatory arrest abolished these effects. Monastral blue revealed numerous endothelial leaks.

CONCLUSIONS

F-actin is crucial to the barrier function of fenestrated endothelium in situ. No significant effect of synoviocyte F-actin on matrix permeability was detected.

Platelet adherence to isolated rat hepatic sinusoidal endothelial cells after cold preservation

BACKGROUND

Platelet adhesion to hepatic sinusoidal endothelial cells (SEC) is a major mechanism of cold preservation injury. This study was performed to determine whether cold preservation leads to adhesion between SEC and platelets in the absence of other cell types, whether adherent platelets become activated upon adhesion, and whether there is increased expression of the platelet receptor von Willebrand factor (vWF) on cold preserved SEC. Because we previously showed that cold causes actin disassembly and matrix metalloprotease (MMP) secretion by SEC, we also questioned whether these events are related to increased adhesion of platelets to SEC after preservation.

METHODS

Isolated SEC were cold preserved for 8 hr and rewarmed briefly. Biotinylated platelets were added to the plates and adhesion was assessed. Activation of platelets was determined by staining with antibody to P-selectin. Expression of vWF was assessed with a specific antibody.

RESULTS

Cold preservation induced increased adhesion of platelets to SEC in the absence of other cell types. Adherent platelets were activated. Preservation increased the expression of vWF on SEC. Pretreatment with phalloidin or treatment with MMP inhibitors partly prevented platelet adhesion and activation, as well as vWF expression. Treatment of SEC at 37 degrees C with recombinant human MMPs for 24 hr also rendered the SEC more adherent for unactivated platelets.

CONCLUSION

Cold preservation of SEC results in increased expression of the platelet receptor vWF, increased platelet adhesion, and platelet activation. This is at least partly mediated by actin disassembly and MMP secretion.

Extracellular matrix, junctional integrity and matrix metalloproteinase interactions in endothelial permeability regulation

Vascular endothelial permeability is maintained by the regulated apposition of adherens and tight junctional proteins whose organization is controlled by several pharmacological and physiological mediators. Endothelial permeability changes are associated with: (1) the spatial redistribution of surface cadherins and occludin, (2) stabilization of focal adhesive bonds and (3) the progressive activation of matrix metalloproteinases (MMPs). In response to peroxide, histamine and EDTA, endothelial cells sequester VE-cadherin and alter its cytoskeletal binding. Simultaneously, these mediators enhance focal adhesion to the substratum. Oxidants, cytokines and pharmacological mediators also trigger the activation of matrix metalloproteinases (MMPs) in a cytoskeleton and tyrosine phosphorylation dependent manner to degrade occludin, a well-characterized tight junction element. These related in vitro phenomena appear to co-operate during inflammation, to increase endothelial permeability, structurally stabilize cells while also remodelling cell junctions and substratum.

The role of p38 MAP kinase in hydrogen peroxide mediated endothelial solute permeability

OBJECTIVE

The purpose of this study was to determine the contribution of p38 MAP kinase activity during hydrogen peroxide mediated increased endothelial solute permeability. We also sought to identify the role of p38 MAP kinase-mediated changes in endothelial cell architecture due to hydrogen peroxide challenge.

METHODS

Hydrogen peroxide mediated permeability of HUVEC was determined with and without inhibition of p38 MAP kinase by SB202190. Hydrogen peroxide mediated rearrangement of the endothelial actin cytoskeleton and junctional proteins occludin and ZO-1 were observed by immunofluorescence microscopy.

RESULTS

Hydrogen peroxide treatment of endothelial monolayers caused a significant increase in solute permeability over a ninety-minute time period. Oxidant-mediated permeability and phosphorylation of p38 MAP kinase was significantly attenuated by SB 202190. Immunofluorescent staining for the tight junctional proteins occludin and ZO-1 demonstrated that oxidant challenge caused a loss of endothelial tight junction organization. Rhodamine phalloidin staining of the actin cytoskeleton showed that hydrogen peroxide stimulated increased stress fiber formation with concomitant gap formation between adjacent endothelial cells. Inhibition of p38 MAP kinase during oxidant challenge significantly attenuated actin stress fiber formation and prevented gap formation.

CONCLUSIONS

These data demonstrate that p38 MAP kinase activity is involved in hydrogen peroxide mediated permeability, stress fiber formation, and intracellular gap formation.

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.

Influence of actin cytoskeleton on intra-articular and interstitial fluid pressures in synovial joints

Fibroblast microfilamentous actin (F-actin) influences interstitial fluid pressure via linkages to collagen in rat skin (Berg et al., 2001). The present aims were to determine whether the actin cytoskeleton of synovial endothelium, fibroblasts, and synoviocytes influences in vivo (i) fluid exchange between a joint cavity and synovial microcirculation and (ii) extracellular fluid pressures in joints. Rabbit knee joints were treated intra-articularly with the F-actin disrupting drugs cytochalasin D and latrunculin B while joint fluid pressure P(j) was recorded. In joints injected with small volumes of control solution, P(j) fell with time (-0.05 +/- 0.01 cm H2O x min(-1), mean +/- SEM, n = 9, equivalent drainage rate 3.9 microl x min(-1)). Cytochalasin or latrunculin reversed this in approximately 4 min in vivo; P(j) increased with time, e.g., +0.12 +/- 0.04 cm H2O x min(-1) at 200 microM cytochalasin (equivalent filtration rate into joint 6.6-12.5 microl x min(-1), n = 4), with a cytochalasin EC50 of 45 microM. Plasma gamma-globulin clearance into the joint cavity was also increased. Post mortem, cytochalasin did not reverse dP(j)/dt and had no more effect on P(j) than did control solution. Also, when synovial interstitial fluid pressures were measured by servonull micropipette post mortem (control -0.95 +/- 0.37 cmH2O, n = 18) cytochalasin had no significant effect on interstitial pressure over 60 min, even at 1 mM. It was concluded that synovial endothelial F-actin has an important role in the normal synovial microvascular resistance to fluid filtration and plasma gamma-globulin permeation and is thus a potential link between pro-inflammatory mediators and arthritic joint effusions. The results provided no support for the hypothesis that synoviocyte F-actin influences the swelling tendency of synovial matrix and hence extracellular fluid pressures, in contrast to the findings of Berg et al. (2001) in rat dermis.

Regulation of PGE(2) and PGI(2) release from human umbilical vein endothelial cells by actin cytoskeleton

Disruption of microfilaments in human umbilical vein endothelial cells (HUVEC) with cytochalasin D (cytD) or latrunculin A (latA) resulted in a 3.3- to 5.7-fold increase in total synthesis of prostaglandin E(2) (PGE(2)) and a 3.4- to 6.5-fold increase in prostacyclin (PGI(2)) compared with control cells. Disruption of the microtubule network with nocodazole or colchicine increased synthesis of PGE(2) 1.7- to 1.9-fold and PGI(2) 1.9- to 2.0-fold compared with control cells. Interestingly, however, increased release of PGE(2) and PGI(2) from HUVEC into the media occurred only when microfilaments were disrupted. CytD treatment resulted in 6.7-fold more PGE(2) and 3.8-fold more PGI(2) released from HUVEC compared with control cells; latA treatment resulted in 17.7-fold more PGE(2) and 11.2-fold more PGI(2) released compared with control cells. Both increased synthesis and release of prostaglandins in response to all drug treatments were completely inhibited by NS-398, a specific inhibitor of cyclooxygenase-2 (COX-2). Disruption of either microfilaments using cytD or latA or of microtubules using nocodazole or colchicine resulted in a significant increase in COX-2 protein levels, suggesting that the increased synthesis of prostaglandins in response to drug treatments may result from increased activity of COX-2. These results, together with studies demonstrating a vasoprotective role for prostaglandins, suggest that the cytoskeleton plays an important role in maintenance of endothelial barrier function by regulating prostaglandin synthesis and release from HUVEC.

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.

Oxidant stress and endothelial cell dysfunction

Reactive oxygen species (ROS) are generated at sites of inflammation and injury, and at low levels, ROS can function as signaling molecules participating as signaling intermediates in regulation of fundamental cell activities such as cell growth and cell adaptation responses, whereas at higher concentrations, ROS can cause cellular injury and death. The vascular endothelium, which regulates the passage of macromolecules and circulating cells from blood to tissues, is a major target of oxidant stress, playing a critical role in the pathophysiology of several vascular diseases and disorders. Specifically, oxidant stress increases vascular endothelial permeability and promotes leukocyte adhesion, which are coupled with alterations in endothelial signal transduction and redox-regulated transcription factors such as activator protein-1 and nuclear factor-kappaB. This review discusses recent findings on the cellular and molecular mechanisms by which ROS signal events leading to impairment of endothelial barrier function and promotion of leukocyte adhesion. Particular emphasis is placed on the regulation of cell-cell and cell-surface adhesion molecules, the actin cytoskeleton, key protein kinases, and signal transduction events.

Burn-activated neutrophils and tumor necrosis factor-alpha alter endothelial cell actin cytoskeleton and enhance monolayer permeability

BACKGROUND

This study examined the hypothesis that exposure of an endothelial cell (EC) monolayer to tumor necrosis factor-alpha (TNF-alpha) and that burn-activated neutrophils alter EC actin cytoskeleton and enhance the permeability of the monolayer.

METHODS

Neutrophils were harvested from rats that had undergone a 45% surface area burn (BURN-neutrophil) or uninjured control rats. ECs were grown on polyester filters or fibronectin-coated glass slides and exposed for 4 hours to media, TNF-alpha (100 ng/mL), or TNF-alpha plus BURN-neutrophil or uninjured control rats (10(7) cells). Monolayer permeability was assessed by measuring the flux of albumin across the cells. EC surface area and microfilament number and length were determined by the staining of actin microfilaments with rhodamine phalloidin followed by fluorescent microscopy.

RESULTS

The amount of albumin that moved across the monolayer in response to TNF-alpha plus BURN-neutrophil was twice that of media alone (P <.05) or TNF-alpha alone (P <.05). The number and length of actin microfilaments in ECs exposed to TNF-alpha plus BURN-neutrophil were significantly less than that of cells exposed to media alone or TNF-alpha alone.

CONCLUSIONS

These data are consistent with a hypothesis that TNF-alpha plus BURN-neutrophil affect endothelial monolayer permeability by altering EC actin cytoskeletal organization.

Inflammatory mediators induce sequestration of VE-cadherin in cultured human endothelial cells

The mechanisms through which inflammatory mediators modify endothelial junctional structure are not well understood. Endothelial cells exposed to 1 mM H2O2, 0.1 mM histamine or 4 mM EDTA displayed decreased amounts of VE-cadherin on the cell surface in a time-dependent manner. H2O2 and EDTA-treated cells showed a sustained reduction in surface VE-cadherin, but histamine (0.1 mM) decreased cell surface VE-cadherin only at 5 and 15 min, not at 30 and 60 min. Sequestering of VE-cadherin could also be visualized as a decrease in immunofluorescent labeling of endothelial junctions in fixed, non-extracted monolayers. However, junctional staining was observed in these cells after membrane extraction. This decreased surface expression of VE-cadherin was actin-filament, but not PKC/MAP kinase dependent. VE-cadherin binding to the cytoskeleton was decreased by EDTA, but was not diminished by histamine or H2O2. Therefore, by promoting sequestration of junctional cadherins, inflammatory mediators may decrease adhesive bonds between apposed endothelial cells and increase solute permeability.

Cytoskeletal alterations in lipopolysaccharide-induced bovine vascular endothelial cell injury and its prevention by sodium arsenite

Morphological changes, especially cytoskeletal alterations, in lipopolysaccharide (LPS)-induced vascular endothelial cell injury were studied by using LPS-susceptible bovine aortic endothelial cells (BAEC). BAEC in cultures with LPS showed cell rounding, shrinking, and intercellular gap formation. In those cells, LPS caused the disorganization of actin, tubulin, and vimentin. LPS also induced a reduction in the F-actin pool and an elevation in the G-actin pool. Cytoskeletal disorganization affected transendothelial permeability across the endothelial monolayer. Pretreatment of BAEC with sodium arsenite (SA) prevented alterations in LPS-induced BAEC injury. However, posttreatment with SA had no protective effect on them. SA upregulated the expression of heat shock protein in the presence of LPS. The role of SA in prevention of LPS-induced BAEC injury is discussed.

Evidence that actin disassembly is a requirement for matrix metalloproteinase secretion by sinusoidal endothelial cells during cold preservation in the rat

Cold preservation induces the secretion of matrix metalloproteinases (MMPs) by hepatic sinusoidal endothelial cells (SECs). These enzymes are important mediators of cold preservation injury. The purpose of this study was to determine if low temperature caused actin disassembly in SECs and whether actin disassembly was required for secretion of MMPs under these conditions. To establish the basis of interpreting the effect of low temperature, isolated SECs were exposed to cytochalasin B with or without pretreatment with phalloidin. Cytochalasin B produced actin disassembly and resulted in the secretion of MMPs. Both were retarded by phalloidin pretreatment. Low temperature (4 degrees C) also induced actin disassembly and MMP secretion and pretreatment with phalloidin again retarded actin disassembly and MMP secretion. Cycloheximide had no effect on these results. Actin disassembly began with 30 minutes of exposure of isolated SECs to cold and reached a final state at 8 hours, at which time no actin stress fibers were visible, and the normally fusiform SECs were fully rounded. Increased MMP activity in the supernatant was also present at 30 minutes and continued to rise sharply in the first hour; thereafter the rate of rise diminished. The study shows that secretion of MMPs during cold preservation is dependent on the induction of actin disassembly by low temperature. The rapid appearance of increased MMP activity after exposure to cold and the studies using cycloheximide indicate that the MMPs originate from preformed MMPs rather than newly synthesized MMPs.

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.

The role of cadherin endocytosis in endothelial barrier regulation: involvement of protein kinase C and actin-cadherin interactions

We have previously reported that exposure of endothelial monolayers to low (0.12 mM) extracellular calcium significantly decreased the endothelial solute barrier, and that this effect was reversed by restoring 'normal' (1.2 mM) calcium (1). This effect was shown to be dependent on cadherins, however the molecular mechanisms through which barrier was altered by low calcium were not characterized. Here we investigated the mechanism of increased endothelial permeability produced by low calcium exposure. Endothelial permeability was significantly increased by exposure to low (0.12 mM) calcium; this effect was attenuated by pre-treatment with the protein kinase C (PKC) inhibitor, staurosporine (2 x 10(-7) M) for 30 min. Cell border retraction and gap formation produced by low calcium was also prevented by staurosporine. Treatment of monolayers with 0.12 mM calcium also stimulated the endocytosis of endothelial cadherins. This low calcium mediated cadherin endocytosis was also prevented by pretreatment with staurosporine. Low calcium mediated endocytosis was also prevented by the actin filament toxin, cytochalasin D (1 ug/ml, 30 min). We conclude that the mechanism of low calcium mediated loss of endothelial barrier function is mediated in part by a PKC dependent endocytosis of endothelial cadherins, which may involve interactions with the actin cytoskeleton. Physiological regulation of the in vivo endothelial barrier may also involve PKC dependent-actin mediated endocytosis of cadherin junctional elements.

Neutrophil elastase promotes lung microvascular injury and proteolysis of endothelial cadherins

Intestinal ischemia-reperfusion (I-R) is associated with lung injury and the acute respiratory distress syndrome. The hypothesis of this study was that intestinal I-R activates circulating neutrophils to promote elastase-mediated lung injury. Isolated rat lungs were perfused with blood or plasma obtained after intestinal I-R, and lung neutrophil retention and injury and bronchoalveolar lavage (BAL) elastase were measured. Perfusion with I-R blood caused lung neutrophil accumulation and injury and increased BAL elastase. These effects were attenuated by the elastase inhibitor L-658758. Interference with neutrophil adherence before gut reperfusion blocked BAL elastase accumulation. The role of endothelial junction proteins (cadherins) in I-R-elicited lung damage was also evaluated. Activated human neutrophils proteolyzed cadherins in human umbilical vein endothelial cells. Furthermore, plasma of patients with acute respiratory distress syndrome contained soluble cadherin fragments. The results of this study suggest that the elastase released by systemically activated neutrophils contributes to lung neutrophil accumulation and pulmonary microvascular injury. Elastase-mediated proteolysis of endothelial cell cadherins may represent the mechanism through which lung microvascular integrity is disrupted after intestinal I-R.

Inflammatory responses to ischemia and reperfusion in skeletal muscle

Skeletal muscle ischemia and reperfusion is now recognized as one form of acute inflammation in which activated leukocytes play a key role. Although restoration of flow is essential in alleviating ischemic injury, reperfusion initiates a complex series of reactions which lead to neutrophil accumulation, microvascular barrier disruption, and edema formation. A large body of evidence exists which suggests that leukocyte adhesion to and emigration across postcapillary venules plays a crucial role in the genesis of reperfusion injury in skeletal muscle. Reactive oxygen species generated by xanthine oxidase and other enzymes promote the formation of proinflammatory stimuli, modify the expression of adhesion molecules on the surface of leukocytes and endothelial cells, and reduce the bioavailability of the potent antiadhesive agent nitric oxide. As a consequence of these events, leukocytes begin to form loose adhesive interactions with postcapillary venular endothelium (leukocyte rolling). If the proinflammatory stimulus is sufficient, leukocytes may become firmly adherent (stationary adhesion) to the venular endothelium. Those leukocytes which become firmly adherent may then diapedese into the perivascular space. The emigrated leukocytes induce parenchymal cell injury via a directed release of oxidants and hydrolytic enzymes. In addition, the emigrating leukocytes also exacerbate ischemic injury by disrupting the microvascular barrier during their egress across the vasculature. As a consequence of this increase in microvascular permeability, transcapillary fluid filtration is enhanced and edema results. The resultant increase in interstitial tissue pressure physically compresses the capillaries, thereby preventing microvascular perfusion and thus promoting the development of the no-reflow phenomenon. The purpose of this review is to summarize the available information regarding these mechanisms of skeletal muscle ischemia/reperfusion injury.

Platelet-derived lysophosphatidic acid decreases endothelial permeability in vitro

We previously reported that platelets release a soluble factor that decreases the solute permeability of cultured bovine aortic endothelial monolayers. This factor was characterized as heat stable, tryspsin sensitive, and not serotonin, adenosine, ADP, or ATP [F. R. Haselton and J. S. Alexander. Am. J. Physiol. 263 (Lung Cell Mol. Physiol. 7): L670-L678, 1992]. We now report its identity as lysophosphatidic acid (LPA). Endothelial permeability decreases rapidly, reversibly, and repeatedly when exposed to platelet supernatants. Continuous exposure produces a sustained decrease in permeability. Methanol extracts of platelet supernatants also decrease endothelial permeability. Treatment of methanol extracts of platelet supernatants with phospholipase B or alkaline phosphatase, which modify the structure of LPA, abolishes the permeability-decreasing activity. However, activity is unaffected by treatment with phospholipase A2. This pattern of enzyme inactivation is consistent with the structure of LPA. Furthermore, synthetic 1-oleoyl-LPA rapidly and significantly decreases endothelial permeability in a concentration-dependent manner. Platelet activation does not appear to be required to produce activity in supernatants from platelet isolations, since P-selectin expression is not increased and thromboxane B2 is < 14 pg/6,000 platelets. Our data show that platelets release a methanol-extractable compound with an enzyme degradation profile consistent with LPA, which decreases the permeability of endothelial monolayers in vitro. In vivo, LPA derived from platelets may be an important mediator of the transport barrier formed by the vascular endothelium.

Activation of endothelial cell kinin receptors leads to intracellular calcium increases and filamin translocation: regulation by protein kinase C

Membrane-associated cytoskeletal proteins provide support for endothelial cell (EC) junctional cell adhesion molecules. Nonmuscle filamin is a dimeric actin cross-linking protein that interacts with F-actin and membrane glycoproteins. Both bradykinin and des-Arg9-bradykinin cause filamin redistribution from the plasma membrane to the cytosol of confluent EC. Kinin-induced filamin translocation parallels the dynamics of intracellular Ca2+ increases. Pretreatment with kinin receptor antagonists blocks the Ca2+ response as well as filamin translocation induced by kinins. Protein kinase C activation prior to kinin stimulation attenuates intracellular Ca2+ increases and filamin translocation. BAPTA, a cell-permeable Ca2+ chelator, attenuates bradykinin-induced intracellular Ca2+ increases and filamin translocation. This study demonstrates that bovine pulmonary artery ECs express both kinin B1 and B2 receptors, and that activation of either receptor leads to intracellular Ca2+ increases. This Ca2+ signalling, which is downregulated by protein kinase C activation, is essential for kinin-induced filamin translocation.

Bradykinin- and thrombin-induced increases in endothelial permeability occur independently of phospholipase C but require protein kinase C activation

We determined whether activation of phosphatidylinositol-specific phospholipase C (PI-PLC) and a subsequent increase in cytosolic calcium concentration ([Ca2+]i) was an obligatory signaling event mediating the increase in transendothelial permeability induced by bradykinin (BK) and alpha-thrombin (alpha-T). Both BK and alpha-T (each at a concentration range of 0.01-1 microM) caused dose-dependent increases in transendothelial 125I-albumin permeability in cultured bovine pulmonary artery endothelial cell monolayers. Both agonists also produced a rise in inositol (1,4,5)-trisphosphate [Ins(1,4,5)P3] by 10 sec that was followed by a prolonged increase in [Ca2+]i. Pretreatment of endothelial cells with the PLC inhibitor, 1-(6-((17 beta-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1 H-pyrrole-2,5-dion [(U73122) at 10 microM for 15 min], prevented the increases in Ins(1,4,5)P3 and [Ca2+]i induced by both BK and alpha-T. However, inhibition of PLC with U73122 or another PLC inhibitor, neomycin, did not prevent the increase in endothelial permeability induced by either agonist. In contrast, depletion of cellular protein kinase C (PKC) with phorbol-12-myristate 13-acetate (0.01 microM for 20 hr) increased both BK- and alpha-T-induced phosphoinositide turnover but inhibited the agonist-induced increase in permeability. A PKC inhibitor, staurosporine (5 microM) likewise inhibited the BK-induced increase in endothelial cell permeability to albumin. We conclude that increases in endothelial permeability induced by the inflammatory mediators, BK and thrombin, can occur independently of PLC activation and increased [Ca2+]i but that a PKC-dependent pathway is required for the permeability response.

Nitric oxide enhances hydrogen peroxide-mediated endothelial permeability in vitro

The objective of this study was to evaluate the effects of nitric oxide (NO) on H2O2-mediated endothelial permeability. H2O2 (0.1 mM) increased permeability at 90 min to 298% of baseline. Spermine NONOate (SNO), an NO donor, at 0.1 or 1 mM did not alter permeability. However, 0.1 mM H2O2 + 1 mM SNO increased permeability to 764%, twice that of 0.1 mM H2O2 alone. These treatments were not directly toxic to endothelial cells. This NO effect was concentration dependent, inasmuch as 0.1 mM SNO did not significantly change H2O2-mediated permeability. The NO-enhanced, H2O2-dependent permeability required the simultaneous presence of NO and H2O2, inasmuch as preincubation with SNO for 30 min followed by 0.1 mM H2O2 did not alter permeability. Staining of endothelial junctions showed widening of the intercellular space only in junctions of cells exposed to H2O2 (0.1 mM) + SNO (1 mM). Furthermore, NO did not affect H2O2 metabolism by endothelial cells but significantly depleted intracellular glutathione. This reduction of cell glutathione produced by NO exposure recovered 15-30 min after removal of the NO donor. NO-enhanced permeability was completely blocked by methionine (1 mM), a scavenger of reactive oxygen species, and by the iron chelator desferrioxamine (0.1 mM). These results suggest that NO may exacerbate the effects of H2O2-dependent increase in endothelial monolayer permeability via the iron-catalyzed formation of reactive oxygen metabolites.

Histamine reduces ZO-1 tight-junction protein expression in cultured retinal microvascular endothelial cells

We examined ZO-1 protein content in cultured retinal vascular endothelial cells to test the hypothesis that histamine alters tight-junction-protein expression. Histamine (10(-9) -10(-4) M) causes a reversible concentration-dependent reduction of ZO-1 protein content, mediated by both H1 and H2 receptors. Histamine reduces ZO-1 expression within the time associated with increased paracellular permeability. Tight-junction-protein alterations may be a novel explanation for the mechanism by which vasoactive agents increase microvascular permeability.

Filamin translocation is an early endothelial cell inflammatory response to bradykinin: regulation by calcium, protein kinases, and protein phosphatases

Endothelial cell (EC) cytoskeletal proteins are one of the earliest primary targets of second messenger cascades generated in response to inflammatory agonists. Actin binding proteins, by modulating actin gelation-solation state and membrane-cytoskeleton interactions, in part regulate cell motility and cell-cell apposition. This in turn can also modulate interendothelial junctional diameter and permeability. Nonmuscle filamin (ABP-280), a dimeric actin-crosslinking protein, promotes orthogonal branching of F-actin and links microfilaments to membrane glycoproteins. In the present study, immunoblot analysis demonstrates that filamin protein levels are low in sparse EC cultures, increase once cell-cell contact is initiated and then decrease slightly at post-confluency. Both bradykinin and ionomycin cause filamin redistribution from the peripheral cell border to the cytosol of confluent EC. Forskolin, an activator of adenylate cyclase, blocks filamin translocation. Bradykinin activation of EC is not accompanied by significant proteolytic cleavage of filamin. Instead, intact filamin is recycled back to the membrane within 5-10 min of bradykinin stimulation. Inhibitors of calcium/calmodulin dependent protein kinase (KT-5926 and KN-62) attenuate bradykinin-induced filamin translocation. H-89, an inhibitor of cAMP-dependent protein kinase, causes translocation of filamin in unstimulated cells. Calyculin A, an inhibitor of protein phosphatases, also causes translocation of filamin in the absence of an inflammatory agent. ML-7, an inhibitor of myosin light chain kinase and phorbol myristate acetate, an activator of protein kinase C, do not cause filamin movement into the cytosol, indicating that these pathways do not modulate the translocation. Pharmacological data suggest that filamin translocation is initiated by the calcium/calmodulin-dependent protein kinase whereas the cAMP-dependent protein kinase pathway prevents translocation. Inflammatory agents therefore may increase vascular junctional permeability by increasing cytoplasmic calcium, which disassembles the microfilament dense peripheral band by releasing filamin from F-actin.

Early albumin leakage in pulmonary endothelial monolayers exposed to varying levels of hyperoxia

We assessed the effect of varying levels of hyperoxia on 14C-albumin flux across bovine pulmonary artery endothelial cell (BPAEC) monolayers. Endothelialized nitrocellulose filters were mounted in Ussing-type chambers which were filled with cell culture medium (M 199). Equimolar amounts of 14C-labeled and unlabeled albumin were added to the "hot" and "cold" chambers, respectively, and the monolayers were exposed to 3 hours of varying levels of oxygen (16%, 30%, 40%, 60%, and 95%). When compared to 16% O2, exposure to hyperoxic gas mixtures of 40% or greater progressively increased albumin permeability across endothelial monolayers within 3 hours to a value 2.5 times higher at 95% O2 compared to 16% O2 (p < 0.001). Hyperoxia-induced permeability increases were prevented by catalase, superoxide dismutase, desferrioxamine, and allopurinol. Our data indicate that hyperoxia induces endothelial permeability changes more rapidly than previously reported even at O2 concentrations as low as 40%.

Mechanisms of increased endothelial permeability

The increase in endothelial permeability in response to inflammatory mediators such as thrombin and histamine is accompanied by reversible cell rounding and interendothelial gap formation, suggesting that the predominant transport pathway is a diffusive one (i.e., via cellular junctions (paracellular transport)). However, vesicle-mediated transport (i.e., via albumin-binding protein gp60) may also contribute significantly to the overall increase in permeability. Regulation of paracellular transport in endothelial cells is associated with modulation of actin-based systems, which anchor the cell to its neighbor or extracellular matrix, thus maintaining endothelial integrity. At the cell-cell junctions, actin is linked indirectly to the plasma membrane by linking proteins (e.g., vinculin, catenins, alpha-actinin) to cadherins, which function in homophilic intercellular adhesion. At endothelial focal contacts, the transmembrane receptors (integrins) for matrix proteins are linked to actin via linking proteins (i.e., vinculin, talin, alpha-actinin). In response to inflammatory mediators, second messengers signal two regulatory pathways, which modulate the actin-based systems, and can thus lead to impairment of the endothelial barrier integrity. One critical signal may be based on protein kinase C isoenzyme specific phosphorylation of linking proteins at the cell-cell and cell-matrix junctions. The increased phosphorylation is associated with actin reorganization, cell rounding, and increased paracellular transport. Another important event is the activation of myosin light chain kinase (MLCK), which causes an actin-myosin-based contraction that may lead to centripetal retraction of endothelial cells. Current research is being conducted at identification of protein substrates of protein kinase C isoenzymes, the specific role of their phosphorylation in barrier function, and determination of the precise role of MLCK in modulation of endothelial barrier function. Since mechanisms by which the increased permeability is returned to normal may be regulated at multiple levels (e.g., receptor desensitization, protein kinase C mediated negative feedback pathways, activation of protein phosphatases), it is also important to determine these cellular "off-switch" mechanisms.

Actin cytoskeletal isoforms in human endothelial cells in vitro: alteration with cell passage

The microfilamentous actin component of the cytoskeleton is crucial to endothelial angiogenesis and vascular permeability. Differences in actin cytoskeletal profiles in cultured human endothelial cells were explored: when first isolated, both primary human umbilical vein endothelial cells (HUVEC) and primary human placental microvascular endothelial cells (HPMEC) expressed F-actin, but not beta-actin or alpha-smooth muscle actin. A similar endothelial actin profile was observed in cryo-sections of freshly delivered term umbilical cord and placenta. In subsequent cell culture, although the actin cytoskeleton of HUVEC remained unchanged, the actin profiles of HPMEC altered after the second passage with the induction of alpha-smooth muscle actin expression, which was intercellularly heterogeneous and increased to 20% at P4. This behaviour occurred in HPMEC monolayers cultured on a variety of extracellular matrices. Comparisons with a spontaneously immortalized human microvascular cell-line, HGTEN 21, revealed that in prolonged passage, both alpha-smooth muscle actin and beta-actin were expressed, whereas HPMEC at P4 showed a lower level of beta-actin expression. Therefore, in comparison with large vessels, microvascular cells are more likely to dedifferentiate. This may reflect the ability of microvascular cells to remodel according to changing requirements for new vessel formation. In conclusion, passage of human microvascular endothelial cells, but not of larger vessel endothelial cells, alters the expression of actin isoforms. This may be important in relation to comparisons of in vitro and in vivo vascular permeability; higher passage microvascular endothelial cells should thus be used with caution in such studies.

Transcellular gaps in microvascular walls of frog and rat when permeability is increased by perfusion with the ionophore A23187

1. The experiments described in this paper aimed to determine whether the gaps which develop in microvascular endothelium in association with increases in permeability are located in the intercellular clefts or are openings passing through the endothelial cells. 2. Hydraulic permeability (Lp) was estimated in frog mesenteric capillaries and single rat venules using a microperfusion-micro-occlusion technique before and during perfusion with solutions containing the ionophore A23187 at a concentration of 10 microM. When Lp was seen to have increased, the tissues were fixed in situ with 2.5% glutaraldehyde. 3. The increases in Lp varied considerably from vessel to vessel. In six frog vessels Lp increased from 2.6 +/- 0.9 x 10(-7) to 266 +/- 159 x 10(-7) cm s-1 cmH2O-1 and in three rat venules Lp rose from 0.94 +/- 0.09 x 10(-7) to 16.4 +/- 4.9 x 10(-7) cm s-1 cmH2O-1 (means +/- S.E.M.). 4. Forty openings or gaps were completely reconstructed from electron micrographs of serial ultrathin sections of the six frog vessels. Thirty-nine of these gaps passed through the endothelial cells and did not communicate with the intercellular clefts; one was intercellular. Similarly, fifteen out of sixteen gaps reconstructed from electron micrographs of the rat venules were transcellular and clearly separated from the intercellular clefts. 5. The increased Lp and associated ultrastructural changes induced by A23187 were reversed by perfusion with ionophore-free solutions.

Regulation of endothelial cell gap formation and barrier dysfunction: role of myosin light chain phosphorylation

Endothelial cell (EC) contraction results in intercellular gap formation and loss of the selective vascular barrier to circulating macromolecules. We tested the hypothesis that phosphorylation of regulatory myosin light chains (MLC) by Ca2+/calmodulin-dependent myosin light chain kinase (MLCK) is critical to EC barrier dysfunction elicited by thrombin. Thrombin stimulated a rapid (< 15 sec) increase in [Ca2+]i which preceded maximal MLC phosphorylation (60 sec) with a 6 to 8-fold increase above constitutive levels of phosphorylated MLC. Dramatic cellular shape changes indicative of contraction and gap formation were observed at 5 min with maximal increases in albumin permeability occurring by 10 min. Neither the Ca2+ ionophore, A23187, nor phorbol myristate acetate (PMA), a direct activator of protein kinase C (PKC), alone or in combination, produced MLC phosphorylation. The combination was synergistic, however, in stimulating EC contraction/gap formation and barrier dysfunction (3 to 4-fold increase). Down-regulation or inhibition of PKC activity attenuated thrombin-induced MLC phosphorylation (approximately 40% inhibition) and both thrombin- and PMA-induced albumin clearance (approximately 50% inhibition). Agents which augmented [cAMP]i partially blocked thrombin-induced MLC phosphorylation (approximately 50%) and completely inhibited both thrombin- and PMA-induced EC permeability (100% inhibition). Furthermore, cAMP produced significant reduction in the basal levels of constitutive MLC phosphorylation. Finally, MLCK inhibition (with either ML-7 or KT 5926) or Ca2+/calmodulin antagonism (with either trifluoperazine or W-7) attenuated thrombin-induced MLC phosphorylation and barrier dysfunction. These results suggest a model wherein EC contractile events, gap formation and barrier dysfunction occur via MLCK-dependent and independent mechanisms and are significantly modulated by both PKC and cAMP-dependent protein kinase A activities.

Membrane model of endothelial cells and leukocytes. A proposal for the origin of a cortical stress

Previous models of the erythrocyte membrane have been based on the assumption that the resting curvature of the membrane is either flat or has a small curvature relative to the overall cell dimension. In contrast, several recent experimental observations, both in leukocytes and in endothelial cells, suggest that local regions of the membrane may have high membrane curvature in the resting state. The resting curvature may be of the order of plasmalemmal vesicles in endothelial cells or surface membrane folds on leukocytes. A tension is required to unfold the membrane with strain energy which depends largely on mean curvature. It is proposed that the tendency of endothelial or leukocyte membranes to wrinkle in the unstressed state may provide a restoring force, i.e. a cortical tension.

Penetration and damage of endothelial cells by Candida albicans

The mechanisms of phagocytosis of Candida albicans by human vascular endothelial cells and subsequent endothelial cell injury were examined in vitro. Both live and killed C. albicans cells were phagocytized by endothelial cells. This organism specifically induced endothelial cell phagocytosis because neither Candida tropicalis nor Torulopsis glabrata was ingested. Endothelial cell microfilaments polymerized around C. albicans as the organisms were phagocytized. Cytochalasin D inhibited this polymerization of microfilaments around C. albicans and blocked phagocytosis. The blocking of actin depolymerization with phalloidin had no effect on microfilament condensation around the organism, indicating that the microfilaments surrounding C. albicans are formed from a pool of G-actin. Intact microtubules were also necessary for the phagocytosis of C. albicans, since the depolymerizing of endothelial cell microtubules with nocodazole prevented the condensation of actin filaments around the organisms and inhibited phagocytosis. In contrast, microtubule depolymerization was not required for microfilament function because the blocking of microtubule depolymerization with taxol had no effect on microfilament condensation around C. albicans. The phagocytosis of C. albicans was pivotal in the induction of endothelial cell damage, since the blocking of candidal internalization significantly reduced endothelial cell injury. Endothelial cells were not damaged by phagocytosis of dead organisms, indicating that injury was caused by a factor associated with viable organisms. Therefore, C. albicans is uniquely able to induce endothelial cell phagocytosis by comparison with non-albicans species of Candida. Furthermore, at least two components of the endothelial cytoskeleton, microfilaments and microtubules, are necessary for the phagocytosis of C. albicans.

Endocardial endothelium in the rat: junctional organization and permeability

Selective permeability of endocardial endothelium has been suggested as a mechanism underlying the modulation of the performance of subjacent myocardium. In this study, we characterized the organization and permeability of junctional complexes in ventricular endocardial endothelium in rat heart. The length of intercellular clefts viewed en face per unit endothelial cell surface area was lower, and intercellular clefts were deeper in endocardial endothelium than in myocardial vascular endothelium, whereas tight junctions had a similar structure in both endothelia. On this basis, endocardial endothelium might be less permeable than capillary endothelium. However, confocal scanning laser microscopy showed that intravenously injected dextran 10,000 coupled to Lucifer Yellow penetrated first the endocardial endothelium and later the myocardial capillary endothelium. Penetration of dextran 10,000 in myocardium occurred earlier through subepicardial capillary endothelium than through subendocardial capillary endothelium. Penetration of tracer might thus be influenced by hydrostatic pressure. Dextran of MW 40,000 did not diffuse through either endocardial endothelium or capillary endothelium. The ultrastructure of endocardial endothelium may constitute an adaptation to limit diffusion driven by high hydrostatic pressure in the heart. Differences in paracellular diffusion of dextran 10,000, between endocardial endothelium and myocardial vessels, may result from differing permeability properties of the endocardium and underlying myocardium.

Modulation of endothelial cell permeability by lung carcinoma cells: a potential mechanism of malignant pleural effusion formation

This study examined the hypothesis that tumor cells metastatic to the pleura secrete a soluble factor(s) that directly increases endothelial cell permeability. Nitrocellulose filters were endothelialized with bovine pulmonary artery endothelial cells and exposed to conditioned media from either human lung adenocarcinoma (Calu-3), human lung squamous cell carcinoma (SK-MES-1), or control media for 16 h. The diffusional permeability (Pd x 10(-5) cm/sec) to [14C]albumin was then determined for each monolayer with Ussing-type chambers. Both adenocarcinoma conditioned media (ACCM) and squamous cell carcinoma conditioned media (SCCM) caused a two- to threefold increase in endothelial monolayer permeability. The addition of indomethacin (10 micrograms/ml) blocked the observed permeability increase in ACCM but not in SCCM, suggesting that the increase in permeability by ACCM was secondary to the production of prostaglandins. To confirm this, a variety of prostanoids previously shown to be produced by the Calu-3 cell line were added directly to the endothelial monolayer. Prostaglandin F2 alpha (PGF2 alpha) in both low (10 ng/ml) and high (100 ng/ml) concentrations for 16 h resulted in a three- to fourfold increase in permeability. Prostaglandin E2 (PGE2) resulted in a small increase in [14C]albumin permeability but only at high concentrations (100 ng/ml). PGF2 alpha production by the two tumor cell lines was measured using radioimmunoassay. Baseline adenocarcinoma production of PGF2 alpha was 117.5 pmol/10(6) cells and fell to 24.2 pmol/10(6) cells hours following incubation with indomethacin. The decrease in PGF2 alpha occurred in parallel with the changes in permeability. Concomitant, reversible changes in cell shape and F-actin distribution were detected in endothelial cells exposed to ACCM. No significant production of PGF2 alpha by the squamous cell carcinoma cell line was detected. These results suggest that both adenocarcinoma and squamous cell carcinoma secrete a soluble factor(s) that directly increases endothelial cell permeability to albumin and that in the case of adenocarcinoma this soluble factor may be a prostanoid such as PGF2 alpha.

SPARC (secreted protein acidic and rich in cysteine) regulates endothelial cell shape and barrier function

SPARC (secreted protein acidic and rich in cysteine) can be selectively expressed by the endothelium in response to certain types of injury and induces rounding in adherent endothelial cells in vitro. To determine whether SPARC might influence endothelial permeability, we studied the effect of exogenous SPARC on the movement of 14C-labeled bovine serum albumin across postconfluent bovine pulmonary artery endothelial cells. SPARC increased (P < 0.02) transendothelial albumin flux in a dose-dependent manner at concentrations > or = 0.5 microgram/ml. At a fixed dose (15 micrograms/ml), exposure times > or = 1 h augmented (P < 0.005) albumin flux by 1.3- to 3.6-fold; this increase was blocked by anti-SPARC antibodies but not by inhibition of protein synthesis. Barrier dysfunction was not associated with loss of cell viability. Monolayers exposed to SPARC exhibited a rounded morphology and intercellular gaps. Prior stabilization of F-actin with phallicidin protected against the changes in barrier function (P = 0.0001) that were otherwise induced by SPARC. Bovine aortic and retinal microvascular endothelia also responded to SPARC. We propose that SPARC regulates endothelial barrier function through F-actin-dependent changes in cell shape, coincident with the appearance of intercellular gaps, that provide a paracellular pathway for extravasation of macromolecules.

Possible involvement of microtubules in platelet-activating factor-induced increases in microvascular permeability in vitro

The blood vessels of the rat small intestine were perfused in vitro with a gelatin-containing physiological salt solution (GPSS). The addition of platelet-activating factor (PAF, 5 microM), podophyllotoxin (50 microM), colcemid (50 microM), or nocodazole (50 microM) to the GPSS for 5 min caused an increase in vascular permeability. This was manifested as an increased trapping of circulating colloidal carbon (CC) within the walls and was assessed using semiautomated image analysis. Pretreatment for 10 min with taxol (5 microM) in the perfusate significantly reduced the permeability-enhancing effects of all four agonists. Since podophyllotoxin, colcemid, and nocodazole are all microtubule-disrupting agents, and since taxol is a microtubule-stabilizing agent, these results suggest that microtubules are involved in the response of the microvessels to PAF. An explanation based on "tensegrity" or "force-counterbalance" is put forward to account for these findings.