Phorbol esters increase MLC phosphorylation and actin remodeling in bovine lung endothelium without increased contraction

β-Hydroxybutyrate impairs neutrophil migration distance through activation of a protein kinase C and myosin light chain 2 signaling pathway in ketotic cows

Ketosis in dairy cows often occurs in the peripartal period and is accompanied by immune dysfunction. High concentrations of β-hydroxybutyrate (BHB) in peripheral blood during ketosis are closely related to the impairment of polymorphonuclear neutrophil (PMN) chemotaxis and contribute to immune dysfunction. The specific effect of BHB on PMN chemotaxis in dairy cows and the underlying molecular mechanisms are unclear. Here, 30 multiparous cows (within 3 wk postpartum) classified based on serum BHB as control (n = 15, BHB <0.6 mM) or clinically ketotic (n = 15, BHB >3.0 mM) were used. Blood samples were collected before feeding, and the isolated PMN were treated with platelet-activating factor for 0.5 h to activate their migration. Scanning electron microscopy revealed a longer tail in PMN of ketotic cows. In addition, the phosphorylation and transcription levels of myosin light chain 2 (MLC2) increased in PMN of ketotic cows. Polymorphonuclear neutrophils from control dairy cows were incubated with 3.0 mM BHB for different times in vitro, and 6 h was selected as the proper duration of BHB stimulation according to its inhibition effect on PMN migration using an under-agarose PMN chemotaxis model. Similarly, BHB stimulation in vitro resulted in inhibition of migration distance and deviation of migration direction of PMN, as well as a longer tail in morphology in the scanning electron microscope data, suggesting that BHB-induced PMN migration inhibition may be mediated by impairing the trailing edge contraction. To confirm this hypothesis, sotrastaurin (Sotra)-a specific inhibitor of protein kinase C (PKC), which is the core regulator of cell contraction-was used with or without BHB treatment in vitro. Sotra was pretreated 0.5 h before BHB treatment. Accordingly, BHB treatment increased the phosphorylation level of PKC and MLC2, the protein abundance of RhoA and rho-kinase 1 (ROCK1), and the mRNA abundance of PRKCA, MYL2, RHOA, and ROCK1 in PMN. In contrast, these effects of BHB on PMN were dampened by Sotra. As demonstrated by immunofluorescence experiments in vitro, the BHB-induced inhibition of trailing edge contraction of PMN was relieved by Sotra. In addition, Sotra also dampened the effects of BHB on PMN migration in vitro. Furthermore, as verified by in vivo experiments, compared with the control cows, both abundance and activation of PKC signaling were enhanced in PMN of ketotic cows. Overall, the present study revealed that high concentrations of blood BHB impaired PMN migration distance through inhibition of the trailing edge contraction, mediated by enhancing the activation of PKC-MLC2 signaling. These findings help explain the dysfunctional immune state in ketotic cows and provide information on the pathogenesis of infectious diseases secondary to ketosis.

Stiff Substrates Enhance Endothelial Oxidative Stress in Response to Protein Kinase C Activation

Arterial stiffness, which increases with aging and hypertension, is an independent cardiovascular risk factor. While stiffer substrates are known to affect single endothelial cell morphology and migration, the effect of substrate stiffness on endothelial monolayer function is less understood. The objective of this study was to determine if substrate stiffness increased endothelial monolayer reactive oxygen species (ROS) in response to protein kinase C (PKC) activation and if this oxidative stress then impacted adherens junction integrity. Porcine aortic endothelial cells were cultured on varied stiffness polyacrylamide gels and treated with phorbol 12-myristate 13-acetate (PMA), which stimulates PKC and ROS without increasing actinomyosin contractility. PMA-treated endothelial cells on stiffer substrates increased ROS and adherens junction loss without increased contractility. ROS scavengers abrogated PMA effects on cell-cell junctions, with a more profound effect in cells on stiffer substrates. Finally, endothelial cells in aortae from elastin haploinsufficient mice (Eln+/-), which were stiffer than aortae from wild-type mice, showed decreased VE-cadherin colocalization with peripheral actin following PMA treatment. These data suggest that oxidative stress may be enhanced in endothelial cells in stiffer vessels, which could contribute to the association between arterial stiffness and cardiovascular disease.

Angiotensin II, a unique vasoactive agent dissociates myosin light chain phosphorylation from contraction

Angiotensin II (100 nM) induced bi-phasic increases in cytosolic Ca²⁺ level ([Ca²⁺]_i) through the activation of angiotensin II type 1 receptor. Pharmacological examinations using 10 µM verapamil, 30 µM La³⁺, and 1 µM thapsigargin indicated that the first phase of the [Ca²⁺]_i-increase was mediated by Ca²⁺ release from sarcoplasmic reticulum (SR) and Ca²⁺ influx independently of voltage dependent Ca²⁺ channel (VDC). In contrast, the second phase of [Ca²⁺]_i-increase was mediated by Ca²⁺ influx through VDC. Although both [Ca²⁺]_i and myosin light chain (MLC)-phosphorylation at the first phase was apparently exceeded the threshold for contraction as estimated by high K⁺-induced responses, there was no appreciable contraction, indicating the dissociation between MLC phosphorylation and force during this phase. In contrast, the second phase of [Ca²⁺]_i was associated with the increases in both MLC phosphorylation and force. These results suggest that angiotensin II is a unique agonist which dissociates MLC-phosphorylation from muscle force during the Ca²⁺ releases from SR.

Phospholipase Cε Modulates Rap1 Activity and the Endothelial Barrier

The phosphoinositide-specific phospholipase C, PLCε, is a unique signaling protein with known roles in regulating cardiac myocyte growth, astrocyte inflammatory signaling, and tumor formation. PLCε is also expressed in endothelial cells, however its role in endothelial regulation is not fully established. We show that endothelial cells of multiple origins, including human pulmonary artery (HPAEC), human umbilical vein (HUVEC), and immortalized brain microvascular (hCMEC/D3) endothelial cells, express PLCε. Knockdown of PLCε in arterial endothelial monolayers decreased the effectiveness of the endothelial barrier. Concomitantly, RhoA activity and stress fiber formation were increased. PLCε-deficient arterial endothelial cells also exhibited decreased Rap1-GTP levels, which could be restored by activation of the Rap1 GEF, Epac, to rescue the increase in monolayer leak. Reintroduction of PLCε rescued monolayer leak with both the CDC25 GEF domain and the lipase domain of PLCε required to fully activate Rap1 and to rescue endothelial barrier function. Finally, we demonstrate that the barrier promoting effects PLCε are dependent on Rap1 signaling through the Rap1 effector, KRIT1, which we have previously shown is vital for maintaining endothelial barrier stability. Thus we have described a novel role for PLCε PIP₂ hydrolytic and Rap GEF activities in arterial endothelial cells, where PLCε-dependent activation of Rap1/KRIT1 signaling promotes endothelial barrier stability.

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

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

Regulation of Thrombin-Induced Lung Endothelial Cell Barrier Disruption by Protein Kinase C Delta

Protein Kinase C (PKC) plays a significant role in thrombin-induced loss of endothelial cell (EC) barrier integrity; however, the existence of more than 10 isozymes of PKC and tissue-specific isoform expression has limited our understanding of this important second messenger in vascular homeostasis. In this study, we show that PKCδ isoform promotes thrombin-induced loss of human pulmonary artery EC barrier integrity, findings substantiated by PKCδ inhibitory studies (rottlerin), dominant negative PKCδ construct and PKCδ silencing (siRNA). In addition, we identified PKCδ as a signaling mediator upstream of both thrombin-induced MLC phosphorylation and Rho GTPase activation affecting stress fiber formation, cell contraction and loss of EC barrier integrity. Our inhibitor-based studies indicate that thrombin-induced PKCδ activation exerts a positive feedback on Rho GTPase activation and contributes to Rac1 GTPase inhibition. Moreover, PKD (or PKCμ) and CPI-17, two known PKCδ targets, were found to be activated by PKCδ in EC and served as modulators of cytoskeleton rearrangement. These studies clarify the role of PKCδ in EC cytoskeleton regulation, and highlight PKCδ as a therapeutic target in inflammatory lung disorders, characterized by the loss of barrier integrity, such as acute lung injury and sepsis.

Cytoskeletal mechanisms regulating vascular endothelial barrier function in response to acute lung injury

Endothelial cells (EC) form a semi-permeable barrier between the interior space of blood vessels and the underlying tissues. In acute lung injury (ALI) the EC barrier is weakened leading to increased vascular permeability. It is widely accepted that EC barrier integrity is critically dependent upon intact cytoskeletal structure and cell junctions. Edemagenic agonists, like thrombin or endotoxin lipopolysaccharide (LPS), induced cytoskeletal rearrangement, and EC contractile responses leading to disruption of intercellular contacts and EC permeability increase. The highly clinically-relevant cytoskeletal mechanisms of EC barrier dysfunction are currently under intense investigation and will be described and discussed in the current review.

Control of protein kinase C activity, phorbol ester-induced cytoskeletal remodeling, and cell survival signals by the scaffolding protein SSeCKS/GRAVIN/AKAP12

The product of the SSeCKS/GRAVIN/AKAP12 gene ("SSeCKS") is a major protein kinase (PK) C substrate that exhibits tumor- and metastasis-suppressing activity likely through its ability to scaffold multiple signaling mediators such as PKC, PKA, cyclins, calmodulin, and Src. Although SSeCKS and PKCα bind phosphatidylserine, we demonstrate that phosphatidylserine-independent binding of PKC by SSeCKS is facilitated by two homologous SSeCKS motifs, EG(I/V)(T/S)XWXSFK(K/R)(M/L)VTP(K/R)K(K/R)X(K/R)XXXEXXXE(E/D) (amino acids 592-620 and 741-769). SSeCKS binding to PKCα decreased kinase activity and was dependent on the two PKC-binding motifs. SSeCKS scaffolding of PKC was increased in confluent cell cultures, correlating with significantly increased SSeCKS protein levels and decreased PKCα activity, suggesting a role for SSeCKS in suppressing PKC activation during contact inhibition. SSeCKS-null mouse embryo fibroblasts displayed increased relative basal and phorbol ester (phorbol 12-myristate 13-acetate)-induced PKC activity but were defective in phorbol 12-myristate 13-acetate-induced actin cytoskeletal reorganization and cell shape change; these responses could be rescued by the forced expression of full-length SSeCKS but not by an SSeCKS variant deleted of its PKC-binding domains. Finally, the PKC binding sites in SSeCKS were required to restore cell rounding and/or decreased apoptosis in phorbol ester-treated LNCaP, LNCaP-C4-2, and MAT-LyLu prostate cancer cells. Thus, PKC-mediated remodeling of the actin cytoskeleton is likely regulated by the ability of SSeCKS to control PKC signaling and activity through a direct scaffolding function.

The Ras activator RasGRP3 mediates diabetes-induced embryonic defects and affects endothelial cell migration

RATIONALE

Fetuses that develop in diabetic mothers have a higher incidence of birth defects that include cardiovascular defects, but the signaling pathways that mediate these developmental effects are poorly understood. It is reasonable to hypothesize that diabetic maternal effects are mediated by 1 or more pathways activated downstream of aberrant glucose metabolism, because poorly controlled maternal glucose levels correlate with the frequency and severity of the defects.

OBJECTIVE

We investigated whether RasGRP3 (Ras guanyl-releasing protein 3), a Ras activator expressed in developing blood vessels, mediates diabetes-induced vascular developmental defects. RasGRP3 is activated by diacylglycerol, and diacylglycerol is overproduced by aberrant glucose metabolism in diabetic individuals. We also investigated the effects of overactivation and loss of function for RasGRP3 in primary endothelial cells and developing vessels.

METHODS AND RESULTS

Analysis of mouse embryos from diabetic mothers showed that diabetes-induced developmental defects were dramatically attenuated in embryos that lacked Rasgrp3 function. Endothelial cells that expressed activated RasGRP3 had elevated Ras-ERK signaling and perturbed migration, whereas endothelial cells that lacked Rasgrp3 function had attenuated Ras-ERK signaling and did not migrate in response to endothelin-1. Developing blood vessels exhibited endothelin-stimulated vessel dysmorphogenesis that required Rasgrp3 function.

CONCLUSIONS

These findings provide the first evidence that RasGRP3 contributes to developmental defects found in embryos that develop in a diabetic environment. The results also elucidate RasGRP3-mediated signaling in endothelial cells and identify endothelin-1 as an upstream input and Ras/MEK/ERK as a downstream effector pathway. RasGRP3 may be a novel therapeutic target for the fetal complications of diabetes.

Thy-1-integrin alphav beta5 interactions inhibit lung fibroblast contraction-induced latent transforming growth factor-beta1 activation and myofibroblast differentiation

Myofibroblasts, key effector cells in tissue fibrosis, are specialized contractile cells. Lung myofibroblast contraction induces integrin alpha(v)beta(5)-dependent latent transforming growth factor (TGF)-beta1 activation suggests that myofibroblast contractility may be a driving force for the persistent myofibroblast differentiation observed in fibrotic lungs. Understanding the mechanisms that regulate fibroblast contraction and mechanotransduction will add new insights into the pathogenesis of lung fibrosis and may lead to new therapeutic approaches for treating fibrotic lung diseases. We and others previously demonstrated that lung fibroblast expression of Thy-1 prevents lung fibrosis. The mechanisms underlying the anti-fibrotic effect of Thy-1 are not well understood. In this study, we showed that Thy-1 interacts with integrin alpha(v)beta(5), both in a cell-free system and on the cell surface of rat lung fibroblasts. Thy-1-integrin alpha(v)beta(5) interactions are RLD-dependent because mutated Thy-1, in which RLD is replaced by RLE, loses the ability to bind the integrin. Furthermore, Thy-1 expression prevents fibroblast contraction-induced, integrin alpha(v)beta(5)-dependent latent TGF-beta1 activation and TGF-beta1-dependent lung myofibroblast differentiation. In contrast, lack of Thy-1 expression or disruption of Thy-1-alpha(v)beta(5) interactions renders lung fibroblasts susceptible to contraction-induced latent TGF-beta1 activation and myofibroblast differentiation. These data suggest that Thy-1-integrin alpha(v)beta(5) interactions inhibit contraction-induced latent TGF-beta1 activation, presumably by blocking the binding of extracellular matrix-bound latent TGF-beta1 with integrin alpha(v)beta(5). Our studies suggest that targeting key mechanotransducers to inhibit mechanotransduction might be an effective approach to inhibit the deleterious effects of myofibroblast contraction on lung fibrogenesis.

ARF6-regulated shedding of tumor cell-derived plasma membrane microvesicles

BACKGROUND

Increased mitogen-activated protein kinase (MAPK) signaling, small GTPase activation, cytoskeletal rearrangements, and the directed targeting of proteases to sites of extracellular matrix degradation all accompany the process of tumor cell invasion. Several studies have implicated the small GTP-binding protein ARF6 in tumor cell invasion, although the molecular basis by which ARF6 facilitates this process is unclear.

RESULTS

We show that the ARF6 GTP/GDP cycle regulates the release of protease-loaded plasma membrane-derived microvesicles from tumor cells into the surrounding environment. To enable microvesicle shedding, ARF6-GTP-dependent activation of phospholipase D promotes the recruitment of the extracellular signal-regulated kinase (ERK) to the plasma membrane where, in turn, ERK phosphorylates and activates myosin light-chain kinase (MLCK). MLCK-mediated MLC phosphorylation is required for microvesicle release. Inhibition of ARF6 activation is accompanied by PKC-mediated phosphorylation of MLC, which blocks microvesicle shedding. Protein cargo appears to be selectively sorted into microvesicles, and adhesion to the extracellular matrix (ECM) is facilitated by microvesicle-associated integrin receptors.

CONCLUSIONS

Microvesicle shedding in tumor cells occurs via an actomyosin-based membrane abscission mechanism that is regulated by nucleotide cycling on ARF6. Microvesicle shedding appears to release selected cellular components, particularly those involved in cell adhesion and motility, into the surrounding environment. These findings suggest that ARF6 activation and the proteolytic activities of microvesicles, both of which are thought to correlate directly with tumor progression, could potentially serve as biomarkers for disease.

Protein kinase C activation disrupts epithelial apical junctions via ROCK-II dependent stimulation of actomyosin contractility

BACKGROUND

Disruption of epithelial cell-cell adhesions represents an early and important stage in tumor metastasis. This process can be modeled in vitro by exposing cells to chemical tumor promoters, phorbol esters and octylindolactam-V (OI-V), known to activate protein kinase C (PKC). However, molecular events mediating PKC-dependent disruption of epithelial cell-cell contact remain poorly understood. In the present study we investigate mechanisms by which PKC activation induces disassembly of tight junctions (TJs) and adherens junctions (AJs) in a model pancreatic epithelium.

RESULTS

Exposure of HPAF-II human pancreatic adenocarcinoma cell monolayers to either OI-V or 12-O-tetradecanoylphorbol-13-acetate caused rapid disruption and internalization of AJs and TJs. Activity of classical PKC isoenzymes was responsible for the loss of cell-cell contacts which was accompanied by cell rounding, phosphorylation and relocalization of the F-actin motor nonmuscle myosin (NM) II. The OI-V-induced disruption of AJs and TJs was prevented by either pharmacological inhibition of NM II with blebbistatin or by siRNA-mediated downregulation of NM IIA. Furthermore, AJ/TJ disassembly was attenuated by inhibition of Rho-associated kinase (ROCK) II, but was insensitive to blockage of MLCK, calmodulin, ERK1/2, caspases and RhoA GTPase.

CONCLUSION

Our data suggest that stimulation of PKC disrupts epithelial apical junctions via ROCK-II dependent activation of NM II, which increases contractility of perijunctional actin filaments. This mechanism is likely to be important for cancer cell dissociation and tumor metastasis.

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

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

Reprint of "The role of cytoskeleton in the regulation of vascular endothelial barrier function" [Microvascular Research 76 (2008) 202-207]

The cytoskeleton is vital to the function of virtually all cell types in the organism as it is required for cell division, cell motility, endo- or exocytosis and the maintenance of cell shape. Endothelial cells, lining the inner surface of the blood vessels, exploit cytoskeletal elements to ensure the integrity of cell monolayer in quiescent endothelium, and to enable the disintegration of the formed barrier in response to various agonists. Vascular permeability is defined by the combination of transcellular and paracellular pathways, with the latter being a major contributor to the inflammation-induced barrier dysfunction. This review will analyze the cytoskeletal elements, which reorganization affects endothelial permeability, and emphasize signaling mechanisms with barrier-protective or barrier-disruptive potential.

The actin cytoskeleton in endothelial cell phenotypes

Endothelium forms a semi-permeable barrier that separates blood from the underlying tissue. Barrier function is largely determined by cell-cell and cell-matrix adhesions that define the limits of cell borders. Yet, such cell-cell and cell-matrix tethering is critically reliant upon the nature of adherence within the cell itself. Indeed, the actin cytoskeleton fulfills this essential function, to provide a strong, dynamic intracellular scaffold that organizes integral membrane proteins with the cell's interior, and responds to environmental cues to orchestrate appropriate cell shape. The actin cytoskeleton is comprised of three distinct, but inter-related structures, including actin cross-linking of spectrin within the membrane skeleton, the cortical actin rim, and actomyosin-based stress fibers. This review addresses each of these actin-based structures, and discusses cellular signals that control the disposition of actin in different endothelial cell phenotypes.

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

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

The role of cytoskeleton in the regulation of vascular endothelial barrier function

The cytoskeleton is vital to the function of virtually all cell types in the organism as it is required for cell division, cell motility, endo- or exocytosis and the maintenance of cell shape. Endothelial cells, lining the inner surface of the blood vessels, exploit cytoskeletal elements to ensure the integrity of cell monolayer in quiescent endothelium, and to enable the disintegration of the formed barrier in response to various agonists. Vascular permeability is defined by the combination of transcellular and paracellular pathways, with the latter being a major contributor to the inflammation-induced barrier dysfunction. This review will analyze the cytoskeletal elements, which reorganization affects endothelial permeability, and emphasize signaling mechanisms with barrier-protective or barrier-disruptive potential.

Counter regulatory effects of PKCbetaII and PKCdelta on coronary endothelial permeability

OBJECTIVE

The aim of this study was to examine the endothelial distribution and activity of selected PKC isoforms in coronary vessels with respect to their functional impact on endothelial permeability under the experimental conditions relevant to diabetes.

METHODS AND RESULTS

En face immunohistochemistry demonstrated a significant increase of PKC(betaII) and decrease of PKCdelta expression in coronary arterial endothelium of Zucker diabetic rats. To test whether changes in PKC expression alter endothelial barrier properties, we measured the transcellular electric resistance in human coronary microvascular endothelial monolayers and found that either PKC(betaII) overexpression or PKCdelta inhibition disrupted the cell-cell adhesive barrier. Three-dimensional fluorescence microscopy revealed that hyperpermeability was caused by altered PKC activity in association with distinct translocation of PKC(betaII) to the cell-cell junction and PKCdelta localization to the cytosol. Further analyses in fractionated endothelial lysates confirmed the differential redistribution of these isozymes. Additionally, FRET analysis of PKC subcellular dynamics demonstrated a high PKC(betaII) activity at the cell surface and junction, whereas PKCdelta activity is concentrated in intracellular membrane organelles.

CONCLUSIONS

Taken together, these data suggest that PKC(betaII) and PKCdelta counter-regulate coronary endothelial barrier properties by targeting distinctive subcellular sites. Imbalanced PKC(betaII)/PKCdelta expression and activity may contribute to endothelial hyperpermeability and coronary dysfunction in diabetes.

Heat shock protein 90 inhibitors protect and restore pulmonary endothelial barrier function

Heat shock protein 90 (hsp90) inhibitors inactivate and/or degrade various client proteins, including many involved in inflammation. Increased vascular permeability is a hallmark of acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). Thus, we tested the hypothesis that hsp90 inhibitors may prevent and/or restore endothelial cell (EC) permeability after injury. Exposure of confluent bovine pulmonary arterial endothelial cell (BPAEC) monolayer to TGF-beta1, thrombin, bacterial lipopolysaccharide (LPS), or vascular endothelial growth factor (VEGF) increased BPAEC permeability, as revealed by decreased transendothelial electrical resistance (TER). Treatment of injured endothelium with hsp90 inhibitors completely restored TER of BPAEC. Similarly, preincubation of BPAEC with hsp90 inhibitors prevented the decline in TER induced by the exposure to thrombin, LPS, VEGF, or TGF-beta1. In addition, hsp90 inhibitors restored the EC barrier function after PMA or nocodazole-induced hyperpermeability. These effects of the hsp90 inhibitors were associated with the restoration of TGF-beta1- or nocodazole-induced decrease in VE-cadherin and beta-catenin expression at EC junctions. The protective effect of hsp90 inhibitors on TGF-beta1-induced hyperpermeability was critically dependent upon preservation of F-actin cytoskeleton and was associated with the inhibition of agonist-induced myosin light chain (MLC) and myosin phosphatase target subunit 1 (MYPT1) phosphorylation, F-actin stress fibers formation, microtubule disassembly, increase in hsp27 phosphorylation, and association of hsp90 with hsp27, but independent of p38MAPK activity. We conclude that hsp90 inhibitors exert barrier protective effects on BPAEC, at least in part, via inhibition of hsp27-mediated, agonist-induced cytoskeletal rearrangement, and therefore may have useful therapeutic value in ALI, ARDS, and other pulmonary inflammatory disease.

Effects of fluid shear stress on adventitial fibroblast migration: implications for flow-mediated mechanisms of arterialization and intimal hyperplasia

The involvement of vascular fibroblasts (FBs) and smooth muscle (SM)-like cells in physiological and pathological processes in large vessels (intimal hyperplasia) and microvessels (capillary arterialization), and the realization that these cells are exposed to interstitial flow shear stress (SS), motivate this study of SS on FB migratory activity. Rat adventitial FBs were grown to either 30-50% confluence (subconfluent FBs; SFBs) or full confluence (confluent FBs; CFBs) in culture. Immunofluorescence and Western blotting assays were conducted to evaluate the expression of two phenotype markers: SM alpha-actin and SM myosin heavy chain (MHC). Both assays indicated a significant increase in SM alpha-actin expression in CFBs compared with SFBs, suggesting a phenotype difference between the two cell populations. SFBs and CFBs both expressed minimal SM MHC. Both cell populations were seeded on Matrigel-coated cell culture inserts and exposed to 4 h of either 1 or 20 dyn/cm(2) SS via a rotating disk apparatus in the presence of the chemoattractant platelet-derived growth factor-BB to quantify the effect of SS on SFB and CFB migration. Four hours of 20 dyn/cm(2) SS significantly enhanced SFB migration while it suppressed CFB migratory activity. Four hours of 1 dyn/cm(2) SS did not significantly alter either SFB or CFB migration levels. Because of the distinct migratory responses of SFBs and CFBs in response to SS, phenotype modulation appears to be one way to regulate their involvement in both physiological and pathological remodeling processes.

Involvement of microtubules, p38, and Rho kinases pathway in 2-methoxyestradiol-induced lung vascular barrier dysfunction

2-Methoxyestradiol (2ME), a promising anti-tumor agent, is currently tested in phase I/II clinical trial to assess drug tolerance and clinical effects. 2ME is known to affect microtubule (MT) polymerization rather than act through estrogen receptors. We hypothesized that 2ME, similar to other MT inhibitors, disrupts endothelial barrier properties. We show that 2ME decreases transendothelial electrical resistance and increases FITC-dextran leakage across human pulmonary artery endothelial monolayer, which correlates with 2ME-induced MT depolymerization. Pretreatment of endothelium with MT stabilizer taxol significantly attenuates the decrease in transendothelial resistance. 2ME treatment results in the induction of F-actin stress fibers, accompanied by the increase in myosin light chain (MLC) phosphorylation. The experiments with Rho kinase (ROCK) and MLC kinase inhibitors and ROCK small interfering RNA (siRNA) revealed that increase in MLC phosphorylation is attributed to the ROCK activation rather than MLC kinase activation. 2ME induces significant ERK1/2, p38, and JNK phosphorylation and activation; however, only p38 activation is relevant to the 2ME-induced endothelial hyperpermeability. p38 activation is accompanied by a marked increase in MAPKAP2 and 27-kDa heat shock protein (HSP27) phosphorylation level. Taxol significantly decreases p38 phosphorylation and activation in response to 2ME stimulation. Vice versa, p38 inhibitor SB203580 attenuates MT rearrangement in 2ME-challenged cells. Together, these results indicate that 2ME-induced barrier disruption is governed by MT depolymerization and p38- and ROCK-dependent mechanisms. The fact that certain concentrations of 2ME induce endothelial hyperpermeability suggests that the issue of the maximum-tolerated dose of 2ME for cancer treatment should be addressed with caution.

HIV-1 gp120 compromises blood-brain barrier integrity and enhances monocyte migration across blood-brain barrier: implication for viral neuropathogenesis

Human immunodeficiency virus-1 (HIV-1) encephalitis is characterized by brain infiltration of virus-infected monocytes and macrophages. Cellular products and viral proteins secreted by infected cells likely play an important role in blood-brain barrier (BBB) impairment and the development of HIV-1-associated dementia (HAD). We previously demonstrated that HIV-1 envelope glycoprotein gp120 induces toxicity and alters expression of tight junction proteins in human brain microvascular endothelial cells (HBMECs). Here, we delineate the mechanisms of gp120-induced BBB dysfunction. Human brain microvascular endothelial cells expressed HIV-1 co-receptors (CCR5 and CXCR4). Exposure of HBMECs to gp120 derived from macrophage (CCR5) or lymphocyte (CXCR4)-tropic viruses decreased BBB tightness, increased permeability, and enhanced monocyte migration across in vitro BBB models. Blood-brain barrier integrity was restored after gp120 removal. CCR5 antibodies and inhibitors of myosin light chain kinase or protein kinase C (PKC) blocked gp120-enhanced monocyte migration and permeability of BBB in vitro. Exposure of HBMECs to gp120 induced release of intracellular calcium ([Ca(2+)](i)) that was prevented by CCR5 antibody and partially blocked by CXCR4 antagonist. Human immunodeficiency virus-1 gp120 activated three PKC isoforms in HBMECs [PKC-alpha/betaII, PKC(pan)-betaII and PKC-zeta/lambda]. Furthermore, specific PKC inhibitors (acting at the ATP-binding and calcium release site) blocked gp120-induced PKC activation and prevented increase in BBB permeability, supporting the biologic significance of these results. Thus, gp120 can cause dysfunction of BBB via PKC pathways and receptor mediated [Ca(2+)](i) release leading to cytoskeletal alterations and increased monocyte migration.

Actin-based features negatively regulated by protein kinase C-epsilon

Cells exposed to phorbol 12-myristate 13-acetate (PMA) undergo a choreographed sequence of morphological changes. Some of these, including stimulation of membrane ruffles and the later appearance of stress fibers, rely on remodeling of the actin cytoskeleton. Although this process is poorly understood, it is important, because the same features are affected during oncogenic transformation. PMA also activates protein kinase C (PKC). Enzyme activation is followed by degradation. Either process might affect the remodeling of actin. The present studies determined whether any PKC isozymes were subject to degradation in tracheal epithelial cells by quantifying the amount of each isozyme present after PMA exposure. PKC-epsilon was the only isozyme to show declining content correlated with increased stress fiber accumulation. Stress fibers increased between 5 and 10 h, whereas PKC-epsilon declined to 38% of its starting value (95% confidence interval, 10-68%). The relationship could be fit by the function F(x) = 0.683 x exp[-0.841(x - 0.387)], where F is the frequency of fiber-containing cells and x is PKC-epsilon content. Fiber accumulation was further investigated after knockdown of PKC-epsilon with RNA interference and antisense oligodeoxynucleotide. Knockdown enhanced stress fibers in cells not yet exposed to PMA as well as the final frequency of fiber-containing cells after PMA exposure. With knockdown at both transcriptional and protein levels, approximately 15% of the original content was predicted and achieved, as judged from real-time PCR and PKC-epsilon content measurements. The results suggest that PKC-epsilon negatively regulates stress fibers, either by directly turning over one of their components or by regulating an upstream step affecting fiber organization.

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.

Transfection efficiency and uptake process of polyplexes in human lung endothelial cells: a comparative study in non-polarized and polarized cells

BACKGROUND

Following systemic administration, polyplexes must cross the endothelium barrier to deliver genes to the target cells underneath. To design an efficient gene delivery system into lung epithelium, we evaluated capture and transfection efficiencies of DNA complexed with either Jet-PEI (PEI-polyplexes) or histidylated polylysine (His-polyplexes) in human lung microvascular endothelial cells (HLMEC) and tracheal epithelial cells.

METHODS

After optimizing growth conditions to obtain a tight HLMEC monolayer, we characterized uptake of polyplexes by flow cytometry and evaluated their transfection efficiency. Polyplexes were formulated as small particles. YOYO-labelled plasmid fluorescence intensity and luciferase activity were used as readouts for uptake and gene expression, respectively.

RESULTS

PEI-polyplexes were more efficiently taken up than His-polyplexes by both non-polarized (2-fold) and polarized HLMEC (10-fold). They were mainly internalized by a clathrin-dependent pathway whatever the cell state. In non-polarized cells, His-polyplexes entered also mainly via a clathrin-dependent pathway but with an involvement of cholesterol. The cell polarization decreased this way and a clathrin-independent pathway became predominant. PEI-polyplexes transfected more efficiently HLMEC than His-polyplexes (10(7) vs. 10(5) relative light units (RLU)/mg of proteins) with a more pronounced difference in polarized cells. In contrast, no negative effect of the cell polarization was observed with tracheal epithelial cells in which both polyplexes had comparable efficiency.

CONCLUSIONS

We show that the efficiency of polyplex uptake by HLMEC and their internalization mechanism are polymer-dependent. By contrast with His-polyplexes, the HLMEC polarization has little influence on the uptake process and on the transfection efficiency of PEI-polyplexes.

Differing roles of protein kinase C-zeta in disruption of tight junction barrier by enteropathogenic and enterohemorrhagic Escherichia coli

BACKGROUND & AIMS

Enteropathogenic Escherichia coli and enterohemorrhagic E. coli harbor highly homologous pathogenicity islands yet show key differences in their mechanisms of action. Both disrupt host intestinal epithelial tight junctions, but the effects of enteropathogenic E. coli are more profound than those of enterohemorrhagic E. coli. The basis for this is not understood. The atypical protein kinase C isoform, protein kinase C-zeta, associates with and regulates the tight junction complex. The aim of this study was to compare the role of protein kinase C-zeta in the disruption of tight junctions after infection with enteropathogenic E. coli and enterohemorrhagic E. coli.

METHODS

Model intestinal epithelial monolayers infected by enteropathogenic E. coli or enterohemorrhagic E. coli were used for these studies.

RESULTS

Neither bisindolylmaleimide nor Gö6976, which block several protein kinase C isoforms but not protein kinase C-zeta, protected against the decrease in transepithelial electrical resistance after enteropathogenic E. coli infection. Rottlerin at concentrations that block novel and atypical isoforms, including protein kinase C-zeta, significantly attenuated the decrease in transepithelial electrical resistance. The specific inhibitory peptide, myristoylated protein kinase C-zeta pseudosubstrate, also significantly decreased the enteropathogenic E. coli -associated decrease in transepithelial electrical resistance and redistribution of tight junction proteins. In contrast to enteropathogenic E. coli, the level of protein kinase C-zeta enzyme activity stimulated by enterohemorrhagic E. coli was transient and minor, and protein kinase C-zeta inhibition had no effect on the decrease in transepithelial electrical resistance or the redistribution of occludin.

CONCLUSIONS

The differential regulation of protein kinase C-zeta by enteropathogenic E. coli and enterohemorrhagic E. coli may in part explain the less profound effect of the latter on the barrier function of tight junctions.

Vascular smooth muscle cell glycocalyx influences shear stress-mediated contractile response

This study addressed the influence of the rate of shear stress application on aortic smooth muscle cell (SMC) contraction and the role of specific glycosaminoglycans in this mechanotransduction. Rat aortic SMCs were exposed to either a step increase in shear stress (0 to 25 dyn/cm(2)) or a ramp increase in shear stress (0 to 25 dyn/cm(2) over 5 min) in a parallel plate flow chamber, and cell contraction was characterized by cell area reduction. SMCs contracted at levels similar to those reported previously and equally in response to both a step and ramp increase in shear stress. When the cells were pretreated with heparinase III or chondroitinase ABC to remove the glycosaminoglycans heparan sulfate and chondroitin sulfate, respectively, from the glycocalyx, the contraction response to increases in shear stress was significantly inhibited. These studies indicate that specific components of the SMC glycocalyx play an important role in the mechanotransduction of shear stress into a contractile response and that the rate of application of shear stress does not affect the SMC contraction.

Potential role of MCP-1 in endothelial cell tight junction 'opening': signaling via Rho and Rho kinase

The expression of the monocyte chemoattractant protein-1 (MCP-1) receptor CCR2 by brain endothelial cells suggests that MCP-1 may have other functions than purely driving leukocyte migration into brain parenchyma during inflammation. This study examines one of these potential novel roles of MCP-1 regulation of endothelial permeability using primary cultures of mouse brain endothelial cells. MCP-1 induces reorganization of actin cytoskeleton (stress fiber formation) and redistribution of tight junction proteins, ZO-1, ZO-2 occludin and claudin-5, from the Triton X-100-soluble to the Triton X-100-insoluble fractions. These morphological changes are associated with a decrease in transendothelial electrical membrane resistance and an increase in [14C]inulin permeability. MCP-1 did not induce these events in brain endothelial cells prepared from mice genotype CCR2-/-. The Rho kinase inhibitor Y27632 and inhibition of Rho (C3 exoenzyme, and dominant negative mutant of Rho, RhoT19N) prevented MCP-1-induced stress fiber assembly, reorganization of tight junction proteins and alterations in endothelial permeability. In all, this suggests that a small GTPase Rho and Rho kinase have a pivotal role in MCP-1-induced junction disarrangement. These data are the first to strongly suggest that MCP-1, via CCR2 present on brain endothelial cells, contributes to increased brain endothelial permeability.

Phorbol ester-mediated pulmonary artery endothelial barrier dysfunction through regulation of actin cytoskeletal mechanics

The mechanisms of phorbol ester- and thrombin-mediated pulmonary artery endothelial barrier dysfunction were compared. Phorbol ester dibutyrate (PDBU) mediated slow force velocity and less force than thrombin. Taxol did not attenuate PDBU-mediated tension, while it reversed nocodazole-mediated tension. PDBU-mediated tension was not affected by acrylamide; PDBU increased cell stiffness and produced greater declines in transendothelial resistance (TER) than acrylamide. Thus PDBU caused a net increase in tension and did not unload microtubule or intermediate filaments. Microfilament remodeling, determined on the basis of immunocytochemistry and actin solubility, lacked the sensitivity and specificity to predict actin-dependent mechanical properties. Thrombin increased myosin light chain (MLC) kinase site-specific MLC phosphorylation, according to peptide map analysis, whereas PDBU did not increase PKC-specific MLC phosphorylation. The initial PDBU-mediated tension development temporally correlated with PDBU-mediated decline in TER and increased low-molecular-weight caldesmon (l-CaD) phosphorylation. PDBU-mediated tension development and decreases in TER were associated with a temporal loss of endothelial cell-matrix adhesion, based on a numerical model of TER. Although, on the basis of immunocytochemistry, thrombin-mediated tension was associated with actin insolubility, actin reorganization, and gap formation, these changes did not predict thrombin-mediated gap formation, based on TER and time-lapse differential interference contrast microscopy. These data suggest that PDBU may disrupt endothelial barrier function through loss of cell-matrix adhesion through l-CaD-dependent actin contraction.

Signal transduction pathways activated in human pulmonary endothelial cells by OxPAPC, a bioactive component of oxidized lipoproteins

The bioactive component of mildly oxidized low-density lipoproteins, oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine (OxPAPC), activates tissue factor expression and monocyte adhesion to endothelial cells (EC) from systemic circulation, but blocks expression of inflammatory adhesion molecules (VCAM, E-selectin) and neutrophil adhesion associated with EC acute inflammatory response to bacterial lypopolysacharide (LPS). Due to constant exposure to oxygen free radicals, lipids in the injured lung are especially prone to oxidative modification and increased OxPAPC generation. In this study, we focused on OxPAPC-mediated intracellular signaling mechanisms that lead to physiological responses in pulmonary endothelial cells. Our results demonstrate that OxPAPC treatment activated in a time-dependent fashion protein kinase C (PKC), protein kinase A (PKA), Raf/MEK1,2/Erk-1,2 MAP kinase cascade, JNK MAP kinase and transient protein tyrosine phosphorylation in human pulmonary artery endothelial cells (HPAEC), whereas nonoxidized PAPC was without effect. Pharmacological inhibition of PKC and tyrosine kinases blocked activation of Erk-1,2 kinase cascade upstream of Raf. OxPAPC did not affect myosin light chain (MLC) phosphorylation, but increased phosphorylation of cofillin, a molecular regulator of actin polymerization. Finally, OxPAPC induced p60Src-dependent tyrosine phosphorylation of focal adhesion proteins paxillin and FAK. Our results suggest a critical involvement of PKC and tyrosine phosphorylation in OxPAPC-induced activation of Erk-1,2 MAP kinase cascade associated with regulation of specific gene expression, and demonstrate rapid phosphorylation of cytoskeletal proteins, which indicates OxPAPC-induced EC remodeling.