cAMP protects endothelial barrier function independent of inhibiting MLC20-dependent tension development

Cortactin is in a complex with VE-cadherin and is required for endothelial adherens junction stability through Rap1/Rac1 activation

Vascular permeability is mediated by Cortactin (Cttn) and regulated by several molecules including cyclic-adenosine-monophosphate, small Rho family GTPases and the actin cytoskeleton. However, it is unclear whether Cttn directly interacts with any of the junctional components or if Cttn intervenes with signaling pathways affecting the intercellular contacts and the cytoskeleton. To address these questions, we employed immortalized microvascular myocardial endothelial cells derived from wild-type and Cttn-knock-out mice. We found that lack of Cttn compromised barrier integrity due to fragmented membrane distribution of different junctional proteins. Moreover, immunoprecipitations revealed that Cttn is within the VE-cadherin-based adherens junction complex. In addition, lack of Cttn slowed-down barrier recovery after Ca2+ repletion. The role of Cttn for cAMP-mediated endothelial barrier regulation was analyzed using Forskolin/Rolipram. In contrast to Cttn-KO, WT cells reacted with increased transendothelial electrical resistance. Absence of Cttn disturbed Rap1 and Rac1 activation in Cttn-depleted cells. Surprisingly, despite the absence of Cttn, direct activation of Rac1/Cdc42/RhoA by CN04 increased barrier resistance and induced well-defined cortical actin and intracellular actin bundles. In summary, our data show that Cttn is required for basal barrier integrity by allowing proper membrane distribution of junctional proteins and for cAMP-mediated activation of the Rap1/Rac1 signaling pathway.

Suppression of CCL2 angiocrine function by adrenomedullin promotes tumor growth

Within the tumor microenvironment, tumor cells and endothelial cells regulate each other. While tumor cells induce angiogenic responses in endothelial cells, endothelial cells release angiocrine factors, which act on tumor cells and other stromal cells. We report that tumor cell-derived adrenomedullin has a pro-angiogenic as well as a direct tumor-promoting effect, and that endothelium-derived CC chemokine ligand 2 (CCL2) suppresses adrenomedullin-induced tumor cell proliferation. Loss of the endothelial adrenomedullin receptor CALCRL or of the G-protein Gs reduced endothelial proliferation. Surprisingly, tumor cell proliferation was also reduced after endothelial deletion of CALCRL or Gs. We identified CCL2 as a critical angiocrine factor whose formation is inhibited by adrenomedullin. Furthermore, CCL2 inhibited adrenomedullin formation in tumor cells through its receptor CCR2. Consistently, loss of endothelial CCL2 or tumor cell CCR2 normalized the reduced tumor growth seen in mice lacking endothelial CALCRL or Gs. Our findings show tumor-promoting roles of adrenomedullin and identify CCL2 as an angiocrine factor controlling adrenomedullin formation by tumor cells.

Passing the Vascular Barrier: Endothelial Signaling Processes Controlling Extravasation

A central function of the vascular endothelium is to serve as a barrier between the blood and the surrounding tissue of the body. At the same time, solutes and cells have to pass the endothelium to leave or to enter the bloodstream to maintain homeostasis. Under pathological conditions, for example, inflammation, permeability for fluid and cells is largely increased in the affected area, thereby facilitating host defense. To appropriately function as a regulated permeability filter, the endothelium uses various mechanisms to allow solutes and cells to pass the endothelial layer. These include transcellular and paracellular pathways of which the latter requires remodeling of intercellular junctions for its regulation. This review provides an overview on endothelial barrier regulation and focuses on the endothelial signaling mechanisms controlling the opening and closing of paracellular pathways for solutes and cells such as leukocytes and metastasizing tumor cells.

Dynamic Regulation of Vascular Permeability by Vascular Endothelial Cadherin-Mediated Endothelial Cell-Cell Junctions

Endothelial cells lining blood vessels regulate vascular barrier function, which controls the passage of plasma proteins and circulating cells across the endothelium. In most normal adult tissues, endothelial cells preserve basal vascular permeability at a low level, while they increase permeability in response to inflammation. Therefore, vascular permeability is tightly controlled by a number of extracellular stimuli and mediators to maintain tissue homeostasis. Accordingly, impaired regulation of endothelial permeability causes various diseases, including chronic inflammation, asthma, edema, sepsis, acute respiratory distress syndrome, anaphylaxis, tumor angiogenesis, and diabetic retinopathy. Vascular endothelial (VE)-cadherin, a member of the classical cadherin superfamily, is a component of cell-to-cell adherens junctions in endothelial cells and plays an important role in regulating vascular permeability. VE-cadherin mediates intercellular adhesion through trans-interactions formed by its extracellular domain, while its cytoplasmic domain is anchored to the actin cytoskeleton via α- and β-catenins, leading to stabilization of VE-cadherin at cell-cell junctions. VE-cadherin-mediated cell adhesions are dynamically, but tightly, controlled by mechanisms that involve protein phosphorylation and reorganization of the actomyosin cytoskeleton. Phosphorylation of VE-cadherin, and its associated-catenins, results in dissociation of the VE-cadherin/catenin complex and internalization of VE-cadherin, leading to increased vascular permeability. Furthermore, reorganization of the actomyosin cytoskeleton by Rap1, a small GTPase that belongs to the Ras subfamily, and Rho family small GTPases, regulates VE-cadherin-mediated cell adhesions to control vascular permeability. In this review, we describe recent progress in understanding the signaling mechanisms that enable dynamic regulation of VE-cadherin adhesions and vascular permeability. In addition, we discuss the possibility of novel therapeutic approaches targeting the signaling pathways controlling VE-cadherin-mediated cell adhesion in diseases associated with vascular hyper-permeability.

Divergent Regulation of Actin Dynamics and Megakaryoblastic Leukemia-1 and -2 (Mkl1/2) by cAMP in Endothelial and Smooth Muscle Cells

Proliferation and migration of vascular smooth muscle cells (VSMCs) or endothelial cell (ECs) promote or inhibit, respectively, restenosis after angioplasty, vein graft intimal thickening and atherogenesis. Here we investigated the effects of cAMP-induced cytoskeletal remodelling on the serum response factor (SRF) co-factors Megakaryoblastic Leukemia-1 and -2 (MKL1 and MKL2) and their role in controlling VSMC and EC proliferation and migration. Elevation of cAMP using forskolin, dibutyryl-cAMP (db-cAMP), BAY60-6583 or Cicaprost induced rapid cytoskeleton remodelling and inhibited proliferation and migration in VSMCs but not EC. Furthermore, elevated cAMP inhibited mitogen-induced nuclear-translocation of MKL1 and MKL2 in VSMCs but not ECs. Forskolin also significantly inhibited serum response factor (SRF)-dependent reporter gene (SRE-LUC) activity and mRNA expression of pro-proliferative and pro-migratory MKL1/2 target genes in VSMCs but not in ECs. In ECs, MKL1 was constitutively nuclear and MKL2 cytoplasmic, irrespective of mitogens or cAMP. Pharmacological or siRNA inhibition of MKL1 significantly inhibited the proliferation and migration of VSMC and EC. Our new data identifies and important contribution of MKL1/2 to explaining the strikingly different response of VSMCs and ECs to cAMP elevation. Elucidation of these pathways promises to identify targets for specific inhibition of VSMC migration and proliferation.

Therapeutic benefits of phosphodiesterase 4B inhibition after traumatic brain injury

Traumatic brain injury (TBI) initiates a deleterious inflammatory response that exacerbates pathology and worsens outcome. This inflammatory response is partially mediated by a reduction in cAMP and a concomitant upregulation of cAMP-hydrolyzing phosphodiesterases (PDEs) acutely after TBI. The PDE4B subfamily, specifically PDE4B2, has been found to regulate cAMP in inflammatory cells, such as neutrophils, macrophages and microglia. To determine if PDE4B regulates inflammation and subsequent pathology after TBI, adult male Sprague Dawley rats received sham surgery or moderate parasagittal fluid-percussion brain injury (2 ± 0.2 atm) and were then treated with a PDE4B - selective inhibitor, A33, or vehicle for up to 3 days post-surgery. Treatment with A33 reduced markers of microglial activation and neutrophil infiltration at 3 and 24 hrs after TBI, respectively. A33 treatment also reduced cortical contusion volume at 3 days post-injury. To determine whether this treatment paradigm attenuated TBI-induced behavioral deficits, animals were evaluated over a period of 6 weeks after surgery for forelimb placement asymmetry, contextual fear conditioning, water maze performance and spatial working memory. A33 treatment significantly improved contextual fear conditioning and water maze retention at 24 hrs post-training. However, this treatment did not rescue sensorimotor or working memory deficits. At 2 months after surgery, atrophy and neuronal loss were measured. A33 treatment significantly reduced neuronal loss in the pericontusional cortex and hippocampal CA3 region. This treatment paradigm also reduced cortical, but not hippocampal, atrophy. Overall, these results suggest that acute PDE4B inhibition may be a viable treatment to reduce inflammation, pathology and memory deficits after TBI.

Ascorbic acid prevents VEGF-induced increases in endothelial barrier permeability

Vascular endothelial growth factor (VEGF) increases endothelial barrier permeability, an effect that may contribute to macular edema in diabetic retinopathy. Since vitamin C, or ascorbic acid, can tighten the endothelial permeability barrier, we examined whether it could prevent the increase in permeability due to VEGF in human umbilical vein endothelial cells (HUVECs). As previously observed, VEGF increased HUVEC permeability to radiolabeled inulin within 60 min in a concentration-dependent manner. Loading the cells with increasing concentrations of ascorbate progressively prevented the leakage caused by 100 ng/ml VEGF, with a significant inhibition at 13 µM and complete inhibition at 50 µM. Loading cells with 100 µM ascorbate also decreased the basal generation of reactive oxygen species and prevented the increase caused by both 100 ng/ml VEGF. VEGF treatment decreased intracellular ascorbate by 25%, thus linking ascorbate oxidation to its prevention of VEGF-induced barrier leakage. The latter was blocked by treating the cells with 60 µM L-NAME (but not D-NAME) as well as by 30 µM sepiapterin, a precursor of tetrahydrobiopterin that is required for proper function of endothelial nitric oxide synthase (eNOS). These findings suggest that VEGF-induced barrier leakage uncouples eNOS. Ascorbate inhibition of the VEGF effect could thus be due either to scavenging superoxide or to peroxynitrite generated by the uncoupled eNOS, or more likely to its ability to recycle tetrahydrobiopterin, thus avoiding enzyme uncoupling in the first place. Ascorbate prevention of VEGF-induced increases in endothelial permeability opens the possibility that its repletion could benefit diabetic macular edema.

Reinforcing endothelial junctions prevents microvessel permeability increase and tumor cell adhesion in microvessels in vivo

Tumor cell adhesion to the microvessel wall is a critical step during tumor metastasis. Vascular endothelial growth factor (VEGF), a secretion of tumor cells, can increase microvessel permeability and tumor cell adhesion in the microvessel. To test the hypothesis that inhibiting permeability increase can reduce tumor cell adhesion, we used in vivo fluorescence microscopy to measure both microvessel permeability and adhesion rates of human mammary carcinoma MDA-MB-231 cells in post-capillary venules of rat mesentery under the treatment of VEGF and a cAMP analog, 8-bromo-cAMP, which can decrease microvessel permeability. By immunostaining adherens junction proteins between endothelial cells forming the microvessel wall, we further investigated the structural mechanism by which cAMP abolishes VEGF-induced increase in microvessel permeability and tumor cell adhesion. Our results demonstrate that 1) Pretreatment of microvessels with cAMP can abolish VEGF-enhanced microvessel permeability and tumor cell adhesion; 2) Tumor cells prefer to adhere to the endothelial cell junctions instead of cell bodies; 3) VEGF increases microvessel permeability and tumor cell adhesion by compromising endothelial junctions while cAMP abolishes these effects of VEGF by reinforcing the junctions. These results suggest that strengthening the microvessel wall integrity can be a potential approach to inhibiting hematogenous tumor metastasis.

Ascorbate reverses high glucose- and RAGE-induced leak of the endothelial permeability barrier

High glucose concentrations due to diabetes increase leakage of plasma constituents across the endothelial permeability barrier. We sought to determine whether vitamin C, or ascorbic acid (ascorbate), could reverse such high glucose-induced increases in endothelial barrier permeability. Human umbilical vein endothelial cells and two brain endothelial cell lines cultured at 25 mM glucose showed increases in endothelial barrier permeability to radiolabeled inulin compared to cells cultured at 5mM glucose. Acute loading of the cells for 30-60 min with ascorbate before the permeability assay prevented the high glucose-induced increase in permeability and decreased basal permeability at 5mM glucose. High glucose-induced barrier leakage was mediated largely by activation of the receptor for advanced glycation end products (RAGE), since it was prevented by RAGE blockade and mimicked by RAGE ligands. Intracellular ascorbate completely prevented RAGE ligand-induced increases in barrier permeability. The high glucose-induced increase in endothelial barrier permeability was also acutely decreased by several cell-penetrant antioxidants, suggesting that at least part of the ascorbate effect could be due to its ability to act as an antioxidant.

Genistein inhibits TNF-α-induced endothelial inflammation through the protein kinase pathway A and improves vascular inflammation in C57BL/6 mice

Genistein, a soy isoflavone, has received wide attention for its potential to improve vascular function, but the mechanism of this effect is unclear. Here, we report that genistein at physiological concentrations (0.1 μM-5 μM) significantly inhibited TNF-α-induced adhesion of monocytes to human umbilical vein endothelial cells, a key event in the pathogenesis of atherosclerosis. Genistein also significantly suppressed TNF-α-induced production of adhesion molecules and chemokines such as sICAM-1, sVCAM-1, sE-Selectin, MCP-1 and IL-8, which play key role in the firm adhesion of monocytes to activated endothelial cells (ECs). Genistein at physiologically relevant concentrations didn't significantly induce antioxidant enzyme activities or scavenge free radicals. Further, blocking the estrogen receptors (ERs) in ECs didn't alter the preventive effect of genistein on endothelial inflammation. However, inhibition of protein kinase A (PKA) significantly attenuated the inhibitory effects of genistein on TNF-α-induced monocyte adhesion to ECs as well as the production of MCP-1 and IL-8. In animal study, dietary genistein significantly suppressed TNF-α-induced increase in circulating chemokines and adhesion molecules in C57BL/6 mice. Genistein treatment also reduced VCAM-1 and monocytes-derived F4/80-positive macrophages in the aorta of TNF-α-treated mice. In conclusion, genistein protects against TNF-α-induced vascular endothelial inflammation both in vitro and in vivo models. This anti-inflammatory effect of genistein is independent of the ER-mediated signaling machinery or antioxidant activity, but mediated via the PKA signaling pathway.

Phytoestrogen genistein protects against endothelial barrier dysfunction in vascular endothelial cells through PKA-mediated suppression of RhoA signaling

The soy-derived phytoestrogen genistein has received attention for its potential to improve vascular function, but its mechanism remains unclear. Here, we report that genistein at physiologically relevant concentrations (0.1-10 μM) significantly inhibited thrombin-induced increase in endothelial monolayer permeability. Genistein also reduced the formation of stress fibers by thrombin and suppressed thrombin-induced phosphorylation of myosin light chain (MLC) on Ser(19)/Thr(18) in endothelial cells (ECs). Genistein had no effect on resting intracellular [Ca(2+)] or thrombin-induced increase in Ca(2+) mobilization. Addition of the inhibitors of endothelial nitric oxide synthase or estrogen receptor did not alter the protective effect of genistein. RhoA is a small GTPase that plays an important role in actin-myosin contraction and endothelial barrier dysfunction. RhoA inhibitor blocked the protective effect of genistein on endothelial permeability and also ablated thrombin-induced MLC-phosphorylation in ECs. Inhibition of PKA significantly attenuated the effect of genistein on thrombin-induced EC permeability, MLC phosphorylation, and RhoA membrane translocation in ECs. Furthermore, thrombin diminished cAMP production in ECs, which were prevented by treatment with genistein. These findings demonstrated that genistein improves thrombin-induced endothelial barrier dysfunction in ECs through PKA-mediated suppression of RhoA signaling.

Regulation of the inflammatory response of vascular endothelial cells by EPAC1

Life-threatening diseases of the cardiovascular system, like atherosclerosis, are exacerbated by unwanted inflammation within the structures of large blood vessels. This inflammation involves increased permeability of the vascular endothelial cells (VECs) that form the lining of blood vessels, leading to exaggerated extravasation of blood components and accumulation of fluid in the extravascular space. This results in tissue dysfunction and increased secretion of chemokines that attract leukocytes and monocytes to the inflamed endothelium. Cyclic AMP is synthesized in VECs in response to endogenous Gs-coupled receptors and is known to limit cytokine action and reduce endothelial hyperpermeability induced by multiple pro-inflammatory stimuli. The mechanisms underlying this anti-inflammatory action of cyclic AMP are now being elucidated and it is becoming clear that the cyclic AMP sensor, exchange protein activated by cyclic AMP (EPAC1), appears to play a key role in suppressing unwanted inflammation. EPAC1 mediates at least three anti-inflammatory pathways in VECs by down-regulating inflammatory signalling through the induction of the suppressors of cytokine signalling 3 (SOCS-3) gene, limiting integrin-dependent vascular permeability and enhancing endothelial barrier function through the stabilization of VE-cadherin junctions. Given that manipulation of cellular cyclic AMP levels currently forms the basis of many effective pharmaceuticals and that EPAC1 is involved in multiple anti-inflammatory protective processes in VECs, does this make EPAC1 an attractive target for the development of activators capable of eliciting a coordinated programme of 'protection' against the development of endothelial dysfunction? Here we discuss whether EPAC1 represents an attractive therapeutic target for limiting endothelial dysfunction associated with cardiovascular diseases like atherosclerosis. LINKED ARTICLES This article is part of a themed section on Novel cAMP Signalling Paradigms. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.166.issue-2.

Bacillus anthracis-derived edema toxin (ET) counter-regulates movement of neutrophils and macromolecules through the endothelial paracellular pathway

BACKGROUND

A common finding amongst patients with inhalational anthrax is a paucity of polymorphonuclear leukocytes (PMNs) in infected tissues in the face of abundant circulating PMNs. A major virulence determinant of anthrax is edema toxin (ET), which is formed by the combination of two proteins produced by the organism, edema factor (EF), which is an adenyl cyclase, and protective antigen (PA). Since cAMP, a product of adenyl cyclase, is known to enhance endothelial barrier integrity, we asked whether ET might decrease extravasation of PMNs into tissues through closure of the paracellular pathway through which PMNs traverse.

RESULTS

Pretreatment of human microvascular endothelial cell(EC)s of the lung (HMVEC-L) with ET decreased interleukin (IL)-8-driven transendothelial migration (TEM) of PMNs with a maximal reduction of nearly 60%. This effect required the presence of both EF and PA. Conversely, ET did not diminish PMN chemotaxis in an EC-free system. Pretreatment of subconfluent HMVEC-Ls decreased transendothelial 14 C-albumin flux by ~ 50% compared to medium controls. Coadministration of ET with either tumor necrosis factor-α or bacterial lipopolysaccharide, each at 100 ng/mL, attenuated the increase of transendothelial 14 C-albumin flux caused by either agent alone. The inhibitory effect of ET on TEM paralleled increases in protein kinase A (PKA) activity, but could not be blocked by inhibition of PKA with either H-89 or KT-5720. Finally, we were unable to replicate the ET effect with either forskolin or 3-isobutyl-1-methylxanthine, two agents known to increase cAMP.

CONCLUSIONS

We conclude that ET decreases IL-8-driven TEM of PMNs across HMVEC-L monolayers independent of cAMP/PKA activity.

ROCK and cAMP promote lymphatic endothelial cell barrier integrity and modulate histamine and thrombin-induced barrier dysfunction

BACKGROUND

There is recent evidence that inflammatory signals can modulate lymphatic vessel permeability, but current understanding of the mechanisms regulating lymphatic endothelial barrier function is limited. The objectives of this study were to 1) investigate whether inflammatory mediators that increase microvascular permeability also cause barrier dysfunction of lymphatic endothelial cell monolayers, and 2) determine the roles of signaling pathways that affect intercellular junctions and cell contraction in lymphatic endothelial barrier function.

METHODS AND RESULTS

Transendothelial electrical resistance (TER) of confluent adult human microlymphatic endothelial cells of dermal origin (HMLEC-d) served as an indicator of lymphatic endothelial barrier function. Human umbilical vein endothelial cells (HUVEC) were used to model blood-tissue barrier function. The inflammatory mediators histamine and thrombin each caused a decrease in TER of HMLEC-d and HUVEC monolayers, with notable differences between the two cell types. Treatment with 8-Br-cAMP enhanced HMLEC-d barrier function, which limited histamine and thrombin-induced decreases in TER. Blockade of myosin light chain kinase (MLCK) with ML-7 did not affect histamine or thrombin-induced decreases in TER. Treatment with the Rho kinase (ROCK) inhibitor Y-27632 caused a decrease in HMLEC-d barrier function.

CONCLUSIONS

These data show that inflammatory mediators can cause lymphatic endothelial barrier dysfunction, although the responses are not identical to those seen with blood endothelial cells. ROCK and cAMP both promote lymphatic endothelial barrier function, however ROCK appears to also serve as a mediator of histamine and thrombin-induced barrier dysfunction.

Nitric oxide mediates tightening of the endothelial barrier by ascorbic acid

Vitamin C, or ascorbic acid, decreases paracellular endothelial permeability in a process that requires rearrangement of the actin cytoskeleton. To define the proximal mechanism of this effect, we tested whether it might involve enhanced generation and/or sparing of nitric oxide (NO) by the vitamin. EA.hy926 endothelial cells cultured on semi-porous filter supports showed decreased endothelial barrier permeability to radiolabeled inulin in response to exogenous NO provided by the NO donor spermine NONOATE, as well as to activation of the downstream NO pathway by 8-bromo-cyclic GMP, a cell-penetrant cyclic GMP analog. Inhibition of endothelial nitric oxide synthase (eNOS) with N(ω)-nitro-l-arginine methyl ester increased endothelial permeability, indicating a role constitutive NO generation by eNOS in maintaining the permeability barrier. Inhibition of guanylate cyclase by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one also increased endothelial permeability and blocked barrier tightening by spermine NONOATE. Loading cells with what are likely physiologic concentrations of ascorbate decreased endothelial permeability. This effect was blocked by inhibition of either eNOS or guanylate cyclase, suggesting that it involved generation of NO by eNOS and subsequent NO-dependent activation of guanylate cyclase. These results show that endothelial permeability barrier function depends on constitutive generation of NO and that ascorbate-dependent tightening of this barrier involves maintaining NO through the eNOS/guanylate cyclase pathway.

Phosphodiesterase 4 inhibition attenuates atrial natriuretic peptide-induced vascular hyperpermeability and loss of plasma volume

Inhibition of phosphodiesterase 4 (PDE4) to increase endothelial cAMP and stabilize the endothelial barrier attenuates acute inflammatory increases in vascular permeability.We extended this approach to attenuate physiological increases in vascular permeability in response to atrial natriuretic peptide (ANP), which acts with the kidney to regulate plasma volume. We measured blood-to-tissue albumin clearance and changes in plasma volume in isoflurane-anaesthetized mice (C57BL/6J) pre-treated with rolipram (8 mg kg(-1) I.P., 30 min). Rolipram significantly reduced albumin permeability, measured using a dual-label fluorescence method, in skin and skeletal muscle compared with ANP alone (500 ng kg(-1) min(-1)). Skin and muscle tissue accounted for 70% of the reduction in whole body albumin clearance taking into account albumin clearance in gastrointestinal (GI) tissue, heart and kidney. The action of ANP and rolipram to modify albumin clearances in duodenum and jejunum could be accounted for by local increases in vascular perfusion to increase surface area for exchange. ANP increased haematocrit from 40.6% to 46.8%, corresponding to an average loss of 22% plasma fluid volume (227 μl), and this was almost completely reversed with rolipram. Renal water excretion accounted for less than 30% of plasma fluid loss indicating that reduced albumin permeability and reduced filtration into vasodilated GI tissue were the predominant actions of PDE4 inhibition. Similar fluid retention was measured in mice with endothelial-restricted deletion of the guanylyl cyclase-A receptor for ANP. Stabilizing the endothelial barrier to offset ANP-induced increases in vascular permeability may be part of a strategy to maintain plasma volume.

Abscisic acid ameliorates atherosclerosis by suppressing macrophage and CD4+ T cell recruitment into the aortic wall

Abscisic acid (ABA) is a natural phytohormone which improves insulin sensitivity and reduces adipose tissue inflammation when supplemented into diets of obese mice. The objective of this study was to investigate the mechanisms by which ABA prevents or ameliorates atherosclerosis. apolipoprotein E-deficient (ApoE(-/-)) mice were fed high-fat diets with or without ABA for 84 days. Systolic blood pressure was assessed on Days 0, 28, 56 and 72. Gene expression, immune cell infiltration and histological lesions were evaluated in the aortic root wall. Human aortic endothelial cells were used to examine the effect of ABA on 3',5'-cyclic adenosine monophosphate (cAMP) and nitric oxide (NO) production in vitro. We report that ABA-treated mice had significantly improved systolic blood pressure and decreased accumulation of F4/80(+)CD11b(+) macrophages and CD4(+) T cells in aortic root walls. At the molecular level, ABA significantly enhanced aortic endothelial nitric oxide synthase (eNOS) and tended to suppress aortic vascular cell adhesion molecule-1 (VCAM-1) and monocyte chemoattractant protein-1 (MCP-1) expression and plasma MCP-1 concentrations. ABA also caused a dose-dependent increase in intracellular concentrations of cAMP and NO and up-regulated eNOS mRNA expression in human aortic endothelial cells. This is the first report showing that ABA prevents or ameliorates atherosclerosis-induced hypertension, immune cell recruitment into the aortic root wall and up-regulates aortic eNOS expression in ApoE(-/-) mice.

The role of adherens junctions and VE-cadherin in the control of vascular permeability

Endothelial cells control the passage of plasma constituents and circulating cells from blood to the underlying tissues. This specialized function is lost or impaired in several pathological conditions - including inflammation, sepsis, ischemia and diabetes - which leads to severe, and sometimes fatal, organ dysfunction. Endothelial permeability is regulated in part by the dynamic opening and closure of cell-cell adherens junctions (AJs). In endothelial cells, AJs are largely composed of vascular endothelial cadherin (VE-cadherin), an endothelium-specific member of the cadherin family of adhesion proteins that binds, via its cytoplasmic domain, to several protein partners, including p120, beta-catenin and plakoglobin. Endogenous pathways that increase vascular permeability affect the function and organization of VE-cadherin and other proteins at AJs in diverse ways. For instance, several factors, including vascular endothelial growth factor (VEGF), induce the tyrosine phosphorylation of VE-cadherin, which accompanies an increase in vascular permeability and leukocyte diapedesis; in addition, the internalization and cleavage of VE-cadherin can cause AJs to be dismantled. From the knowledge of how AJ organization can be modulated, it is possible to formulate several pharmacological strategies to control the barrier function of the endothelium. We discuss the possible use of inhibitors of SRC and other kinases, of agents that increase cAMP levels, and of inhibitors of lytic enzymes as pharmacological tools for decreasing endothelial permeability.

Epac/Rap1 pathway regulates microvascular hyperpermeability induced by PAF in rat mesentery

Experiments in cultured endothelial cell monolayers demonstrate that increased intracellular cAMP strongly inhibits the acute permeability responses by both protein kinase A (PKA)-dependent and -independent pathways. The contribution of the PKA-independent pathways to the anti-inflammatory mechanisms of cAMP in intact mammalian microvessels has not been systematically investigated. We evaluated the role of the cAMP-dependent activation of the exchange protein activated by cAMP (Epac), a guanine nucleotide exchange factor for the small GTPase Rap1, in rat venular microvessels exposed to the platelet-activating factor (PAF). The cAMP analog 8-pCPT-2'-O-methyl-cAMP (O-Me-cAMP), which stimulates the Epac/Rap1 pathway but has no effect on PKA, significantly attenuated the PAF increase in microvessel permeability as measured by hydraulic conductivity (Lp). We also demonstrated that PAF induced a rearrangement of vascular endothelial (VE)-cadherin seen as numerous lateral spikes and frequent short breaks in the otherwise continuous peripheral immunofluorescent label. Pretreatment with O-Me-cAMP completely prevented the PAF-induced rearrangement of VE-cadherin. We conclude that the action of the Epac/Rap1 pathway to stabilize cell-cell adhesion is a significant component of the activity of cAMP to attenuate an acute increase in vascular permeability. Our results indicate that increased permeability in intact microvessels by acute inflammatory agents such as PAF is the result of the decreased effectiveness of the Epac/Rap1 pathway modulation of cell-cell adhesion.

Polar head groups are important for barrier-protective effects of oxidized phospholipids on pulmonary endothelium

We have previously described protective effects of oxidized 1-palmitoyl-2-arachidonoyl- sn-glycero-3-phosphocholine (OxPAPC) on pulmonary endothelial cell (EC) barrier function and demonstrated the critical role of cyclopentenone-containing modifications of arachidonoyl moiety in OxPAPC protective effects. In this study we used oxidized phosphocholine (OxPAPC), phosphoserine (OxPAPS), and glycerophosphate (OxPAPA) to investigate the role of polar head groups in EC barrier-protective responses to oxidized phospholipids (OxPLs). OxPAPC and OxPAPS induced sustained barrier enhancement in pulmonary EC, whereas OxPAPA caused a transient protective response as judged by measurements of transendothelial electrical resistance (TER). Non-OxPLs showed no effects on TER levels. All three OxPLs caused enhancement of peripheral EC actin cytoskeleton. OxPAPC and OxPAPS completely abolished LPS-induced EC hyperpermeability in vitro, whereas OxPAPA showed only a partial protective effect. In vivo, intravenous injection of OxPAPS or OxPAPC (1.5 mg/kg) markedly attenuated increases in the protein content, cell counts, and myeloperoxidase activities detected in bronchoalveolar lavage fluid upon intratracheal LPS instillation in mice, although OxPAPC showed less potency. All three OxPLs partially attenuated EC barrier dysfunction induced by IL-6 and thrombin. Their protective effects against thrombin-induced EC barrier dysfunction were linked to the attenuation of the thrombin-induced Rho pathway of EC hyperpermeability and stimulation of Rac-mediated mechanisms of EC barrier recovery. These results demonstrate for the first time the essential role of polar OxPL groups in blunting the LPS-induced EC dysfunction in vitro and in vivo and suggest the mechanism of agonist-induced hyperpermeability attenuation by OxPLs via reduction of Rho and stimulation of Rac signaling.

Signaling pathways involved in OxPAPC-induced pulmonary endothelial barrier protection

Increased tissue or serum levels of oxidized phospholipids have been detected in a variety of chronic and acute pathological conditions such as hyperlipidemia, atherosclerosis, heart attack, cell apoptosis, acute inflammation and injury. We have recently described signaling cascades activated by oxidized 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (OxPAPC)in the human pulmonary artery endothelial cells (EC) and reported potent barrier-protective effects of OxPAPC, which were mediated by small GTPases Rac and Cdc42. In this study we have further characterized signal transduction pathways involved in the OxPAPC-mediated endothelial barrier protection. Inhibitors of small GTPases, protein kinase A (PKA), protein kinase C (PKC), Src family kinases and general inhibitors of tyrosine kinases attenuated OxPAPC-induced barrier-protective response and EC cytoskeletal remodeling. In contrast, small GTPase Rho, Rho kinase, Erk-1,2 MAP kinase and p38 MAP kinase and PI3-kinase were not involved in the barrier-protective effects of OxPAPC. Inhibitors of PKA, PKC, tyrosine kinases and small GTPase inhibitor toxin B suppressed OxPAPC-induced Rac activation and decreased phosphorylation of focal adhesion kinase (FAK) and paxillin. Barrier-protective effects of OxPAPC were not reproduced by platelet activating factor (PAF), which at high concentrations induced barrier dysfunction, but were partially attenuated by PAF receptor antagonist A85783. These results demonstrate for the first time upstream signaling cascades involved in the OxPAPC-induced Rac activation, cytoskeletal remodeling and barrier regulation and suggest PAF receptor-independent mechanisms of OxPAPC-mediated endothelial barrier protection.

Rap1: a key regulator in cell-cell junction formation

Rap1 is a Ras-like small GTPase that is activated by many extracellular stimuli and strongly implicated in the control of integrin-mediated cell adhesion. Recent evidence indicates that Rap1 also plays a key role in formation of cadherin-based cell-cell junctions. Indeed, inhibition of Rap1 generates immature adherens junctions, whereas activation of Rap1 tightens cell-cell junctions. Interestingly, Rap1 guanine nucleotide exchange factors, such as C3G and PDZ-GEF, are directly linked to E-cadherin or to other junction proteins. Furthermore, several junction proteins, such as afadin/AF6 and proteins controlling the actin cytoskeleton, function as effectors of Rap1. These findings point to a role of Rap1 in spatial and temporal control of cell-cell junction formation.

PAR2 activation interrupts E-cadherin adhesion and compromises the airway epithelial barrier: protective effect of beta-agonists

The airway epithelium is an important barrier between the environment and subepithelial tissues. The epithelium is also divided into functionally restricted apical and basolateral domains, and this restriction is dependent on the elements of the barrier. The protease-activated receptor-2 (PAR2) receptor is expressed in airway epithelium, and its activation initiates multiple effects including enhanced airway inflammation and reactivity. We hypothesized that activation of PAR2 would interrupt E-cadherin adhesion and compromise the airway epithelial barrier. The PAR2-activating peptide (PAR2-AP, SLIGRL) caused an immediate approximately 50% decrease in the transepithelial resistance of primary human airway epithelium that persisted for 6-10 min. The decrease in resistance was accompanied by an increase in mannitol flux across the epithelium and occurred in cystic fibrosis transmembrane conductance receptor (CFTR) epithelium pretreated with amiloride to block Na and Cl conductances, confirming that the decrease in resistance represented an increase in paracellular conductance. In parallel experiments, activation of PAR2 interrupted the adhesion of E-cadherin-expressing L cells and of primary airway epithelial cells to an immobilized E-cadherin extracellular domain, confirming the hypothesis that activation of PAR2 interrupts E-cadherin adhesion. Selective interruption of E-cadherin adhesion with antibody to E-cadherin decreased the transepithelial resistance of primary airway epithelium by >80%. Pretreatment of airway epithelium or the E-cadherin-expressing L cells with the long-acting beta-agonist salmeterol prevented PAR2 activation from interrupting E-cadherin adhesion and compromising the airway epithelial barrier. Activation of PAR2 interrupts E-cadherin adhesion and compromises the airway epithelial barrier.

Growth factor- and heparin-dependent regulation of constitutive and agonist-mediated human endothelial barrier function

We report functional differences in constitutive and agonist-mediated endothelial barrier function between cultured primary and Clonetics human umbilical vein endothelial cells (pHUVEC and cHUVEC) grown in soluble growth factors and heparin. Basal transendothelial resistance (TER) was much lower in pHUVEC than in cHUVEC grown in medium supplemented with growth factors, such as basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), and human epithelial growth factor (EGF), and heparin. On the basis of a numerical model of TER, the increased basal TER in cHUVEC was due to effects on cell-matrix adhesion and membrane capacitance. Heparin and bFGF increased constitutive TER in cultured pHUVEC, and heparin mediated additional increases in constitutive TER in pHUVEC supplemented with bFGF. EGF attenuated bFGF-mediated increases in TER. On the basis of the numerical model, in contrast to cHUVEC, heparin and bFGF augmented TER through effects on cell-cell adhesion and membrane capacitance in pHUVEC. Thrombin mediated quantitatively greater amplitude and a more sustained decline in TER in cultured cHUVEC than pHUVEC. Thrombin-mediated barrier dysfunction was attenuated in pHUVEC conditioned in EGF in the presence or absence of heparin. Thrombin-mediated barrier dysfunction was also attenuated when monolayers were exposed to low concentrations of heparin and further attenuated in the presence of bFGF. cAMP stimulation mediated differential attenuation of thrombin-mediated barrier dysfunction between pHUVEC and cHUVEC. VEGF displayed differential effects in TER in serum-free medium. Taken together, these data demonstrate marked differential regulation of constitutive and agonist-mediated endothelial barrier function in response to mitogens and heparin stimulation.

Involvement of ROCK-mediated endothelial tension development in neutrophil-stimulated microvascular leakage

Neutrophil-induced coronary microvascular barrier dysfunction is an important pathophysiological event in heart disease. Currently, the precise cellular and molecular mechanisms of neutrophil-induced microvascular leakage are not clear. The aim of this study was to test the hypothesis that rho kinase (ROCK) increases coronary venular permeability in association with elevated endothelial tension. We assessed permeability to albumin (P(a)) in isolated porcine coronary venules and in coronary venular endothelial cell (CVEC) monolayers. Endothelial barrier function was also evaluated by measuring transendothelial electrical resistance (TER) of CVEC monolayers. In parallel, we measured isometric tension of CVECs grown on collagen gels. Transference of constitutively active (ca)-ROCK protein into isolated coronary venules or CVEC monolayers caused a significant increase in P(a) and decreased TER in CVECs. The ROCK inhibitor Y-27632 blocked the ca-ROCK-induced changes. C5a-activated neutrophils (10(6)/ml) also significantly elevated venular P(a), which was dose-dependently inhibited by Y-27632 and a structurally distinct ROCK inhibitor, H-1152. In CVEC monolayers, activated neutrophils increased permeability with a concomitant elevation in isometric tension, both of which were inhibited by Y-27632 or H-1152. Treatment with ca-ROCK also significantly increased CVEC monolayer permeability and isometric tension, coupled with actin polymerization and elevated phosphorylation of myosin regulatory light chain on Thr18/Ser19. The data suggest that during neutrophil activation, ROCK promotes microvascular leakage in association with actin-myosin-mediated tension development in endothelial cells.

Modeling error and stability of endothelial cytoskeletal membrane parameters based on modeling transendothelial impedance as resistor and capacitor in series

Transendothelial impedance across an endothelial monolayer grown on a microelectrode has previously been modeled as a repeating pattern of disks in which the electrical circuit consists of a resistor and capacitor in series. Although this numerical model breaks down barrier function into measurements of cell-cell adhesion, cell-matrix adhesion, and membrane capacitance, such solution parameters can be inaccurate without understanding model stability and error. In this study, we have evaluated modeling stability and error by using a χ2evaluation and Levenberg-Marquardt nonlinear least-squares (LM-NLS) method of the real and/or imaginary data in which the experimental measurement is compared with the calculated measurement derived by the model. Modeling stability and error were dependent on current frequency and the type of experimental data modeled. Solution parameters of cell-matrix adhesion were most susceptible to modeling instability. Furthermore, the LM-NLS method displayed frequency-dependent instability of the solution parameters, regardless of whether the real or imaginary data were analyzed. However, the LM-NLS method identified stable and reproducible solution parameters between all types of experimental data when a defined frequency spectrum of the entire data set was selected on the basis of a criterion of minimizing error. The frequency bandwidth that produced stable solution parameters varied greatly among different data types. Thus a numerical model based on characterizing transendothelial impedance as a resistor and capacitor in series and as a repeating pattern of disks is not sufficient to characterize the entire frequency spectrum of experimental transendothelial impedance.

Cyclic AMP potentiates vascular endothelial cadherin-mediated cell-cell contact to enhance endothelial barrier function through an Epac-Rap1 signaling pathway

Cyclic AMP (cAMP) is a well-known intracellular signaling molecule improving barrier function in vascular endothelial cells. Here, we delineate a novel cAMP-triggered signal that regulates the barrier function. We found that cAMP-elevating reagents, prostacyclin and forskolin, decreased cell permeability and enhanced vascular endothelial (VE) cadherin-dependent cell adhesion. Although the decreased permeability and the increased VE-cadherin-mediated adhesion by prostacyclin and forskolin were insensitive to a specific inhibitor for cAMP-dependent protein kinase, these effects were mimicked by 8-(4-chlorophenylthio)-2'-O-methyladenosine-3', 5'-cyclic monophosphate, a specific activator for Epac, which is a novel cAMP-dependent guanine nucleotide exchange factor for Rap1. Thus, we investigated the effect of Rap1 on permeability and the VE-cadherin-mediated cell adhesion by expressing either constitutive active Rap1 or Rap1GAPII. Activation of Rap1 resulted in a decrease in permeability and enhancement of VE-cadherin-dependent cell adhesion, whereas inactivation of Rap1 had the counter effect. Furthermore, prostacyclin and forskolin induced cortical actin rearrangement in a Rap1-dependent manner. In conclusion, cAMP-Epac-Rap1 signaling promotes decreased cell permeability by enhancing VE-cadherin-mediated adhesion lined by the rearranged cortical actin.

Gene delivery of l-caldesmon protects cytoskeletal cell membrane integrity against adenovirus infection independently of myosin ATPase and actin assembly

The cytoskeleton is critical to the viral life cycle. Agents like cytochalasin inhibit viral infections but cannot be used for antiviral therapy because of their toxicity. We report the efficacy, safety, and mechanisms by which gene delivery of human wild-type low-molecular-weight caldesmon (l-CaD) protects cell membrane integrity from adenovirus infection in a DF-1 cell line, an immortalized avian fibroblast that is null for l-CaD. Transfection with an adenovirus (Ad)-controlled construct mediated a dose-dependent decline in transcellular resistance. In accordance with a computational model of cytoskeletal membrane properties, Ad disturbed cell-cell and cell-matrix adhesion and membrane capacitance. Transfection with the Ad-l-CaD construct attenuated adenovirus-mediated loss in transcellular resistance. Quantitation of vinculin-stained plaques revealed an increase in total focal contact mass in monolayers transfected with the Ad-l-CaD construct. Expression of l-CaD protected transcellular resistance through primary effects on membrane capacitance and independently of actin solubility and effects on prestress, as measured by the decline in isometric tension in response to cytochalasin D. Expression of l-CaD exhibited less Trypan blue cell toxicity than cytochalasin, and, unlike cytochalasin, it did not interfere with wound closure or adversely effect transcellular resistance. These findings demonstrate the gene delivery of wild-type human l-CaD as a potentially efficacious and safe agent that inhibits some of the cytopathic effects of adenovirus.

Histamine selectively interrupts VE-cadherin adhesion independently of capacitive calcium entry

Histamine is an important agent of innate immunity, transiently increasing the flux of immune-competent molecules from the vascular space to the tissues and then allowing rapid restoration of the integrity of the endothelial barrier. In previous work we found that histamine alters the endothelial barrier by disrupting cell-cell adhesion and identified VE-cadherin as an essential participant in this process. The previous work did not determine whether histamine directly interrupted VE-cadherin adhesion, whether the effects of histamine were selective for cadherin adhesion, or whether capacitive calcium flux across the cell membrane was necessary for the effects of histamine on cell-cell adhesion. In the current work we found that histamine directly interrupts adhesion of L cells expressing the type 1 histamine (H1) receptor and VE-cadherin to a VE-cadherin-Fc fusion protein. In contrast, integrin-mediated adhesion to fibronectin of the same L cells expressing the H1 receptor was not affected by histamine, demonstrating that the effects of histamine are selective for cadherin adhesion. Some of the effects of many edemagenic agonists on endothelium are dependent on the capacitive flux of calcium across the endothelial cell membrane. Blocking capacitive calcium flux with LaCl3 did not prevent histamine from interrupting VE-cadherin adhesion of transfected L cells, nor did it prevent histamine from interrupting cell-cell adhesion of human umbilical vein endothelial cells. These data support the contentions that histamine directly and selectively interrupts cadherin adhesion and this effect on cadherin adhesion is independent of capacitive calcium flux.

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.

PKA inhibits RhoA activation: a protection mechanism against endothelial barrier dysfunction

Much evidence indicates that cAMP-dependent protein kinase (PKA) prevents increased endothelial permeability induced by inflammatory mediators. We investigated the hypothesis that PKA inhibits Rho GTPases, which are regulator proteins believed to mediate endothelial barrier dysfunction. Stimulation of human microvascular endothelial cells (HMEC) with thrombin (10 nM) increased activated RhoA (RhoA-GTP) within 1 min, which remained elevated approximately fourfold over control for 15 min. The activation was accompanied by RhoA translocation to the cell membrane. However, thrombin did not activate Cdc42 or Rac1 within similar time points, indicating selectivity of activation responses by Rho GTPases. Pretreatment of HMEC with 10 micro M forskolin plus 1 micro M IBMX (FI) to elevate intracellular cAMP levels inhibited both thrombin-induced RhoA activation and translocation responses. FI additionally inhibited thrombin-mediated dissociation of RhoA from guanine nucleotide dissociation inhibitor (GDI) and enhanced in vivo incorporation of (32)P by GDI. HMEC pretreated in parallel with FI showed >50% reduction in time for the thrombin-mediated resistance drop to return to near baseline and inhibition of approximately 23% of the extent of resistance drop. Infection of HMEC with replication-deficient adenovirus containing the protein kinase A inhibitor gene (PKA inhibitor) blocked both the FI-mediated protective effects on RhoA activation and resistance changes. In conclusion, the results provide evidence that PKA inhibited RhoA activation in endothelial cells, supporting a signaling mechanism of protection against vascular endothelial barrier dysfunction.

Tensegrity II. How structural networks influence cellular information processing networks

The major challenge in biology today is biocomplexity: the need to explain how cell and tissue behaviors emerge from collective interactions within complex molecular networks. Part I of this two-part article, described a mechanical model of cell structure based on tensegrity architecture that explains how the mechanical behavior of the cell emerges from physical interactions among the different molecular filament systems that form the cytoskeleton. Recent work shows that the cytoskeleton also orients much of the cell's metabolic and signal transduction machinery and that mechanical distortion of cells and the cytoskeleton through cell surface integrin receptors can profoundly affect cell behavior. In particular, gradual variations in this single physical control parameter (cell shape distortion) can switch cells between distinct gene programs (e.g. growth, differentiation and apoptosis), and this process can be viewed as a biological phase transition. Part II of this article covers how combined use of tensegrity and solid-state mechanochemistry by cells may mediate mechanotransduction and facilitate integration of chemical and physical signals that are responsible for control of cell behavior. In addition, it examines how cell structural networks affect gene and protein signaling networks to produce characteristic phenotypes and cell fate transitions during tissue development.

Tensegrity I. Cell structure and hierarchical systems biology

In 1993, a Commentary in this journal described how a simple mechanical model of cell structure based on tensegrity architecture can help to explain how cell shape, movement and cytoskeletal mechanics are controlled, as well as how cells sense and respond to mechanical forces (J. Cell Sci. 104, 613-627). The cellular tensegrity model can now be revisited and placed in context of new advances in our understanding of cell structure, biological networks and mechanoregulation that have been made over the past decade. Recent work provides strong evidence to support the use of tensegrity by cells, and mathematical formulations of the model predict many aspects of cell behavior. In addition, development of the tensegrity theory and its translation into mathematical terms are beginning to allow us to define the relationship between mechanics and biochemistry at the molecular level and to attack the larger problem of biological complexity. Part I of this two-part article covers the evidence for cellular tensegrity at the molecular level and describes how this building system may provide a structural basis for the hierarchical organization of living systems--from molecule to organism. Part II, which focuses on how these structural networks influence information processing networks, appears in the next issue.

Intracellular signalling involved in modulating human endothelial barrier function

The endothelium dynamically regulates the extravasation of hormones, macromolecules and other solutes. In pathological conditions, endothelial hyperpermeability can be induced by vasoactive agents, which induce tiny leakage sites between the cells, and by cytokines, in particular vascular endothelial growth factor, which increase the exchange of plasma proteins by vesicles and intracellular pores. It is generally believed that the interaction of actin and non-muscle myosin in the periphery of the endothelial cell, and the destabilization of endothelial junctions, are required for endothelial hyperpermeability induced by vasoactive agents. Transient short-term hyperpermeability induced by histamine involves Ca2+/calmodulin-dependent activation of the myosin light chain (MLC) kinase. Prolonged elevated permeability induced by thrombin in addition involves activation of the small GTPase RhoA and Rho kinase, which inhibits dephosphorylation of MLC. It also involves the action of other protein kinases. Several mechanisms can increase endothelial barrier function, depending on the tissue affected and the cause of hyperpermeability. They include blockage of specific receptors, and elevation of cyclic AMP by agents such as beta2-adrenergic agents. Depending on the vascular bed, nitric oxide and cyclic GMP can counteract or aggravate endothelial hyperpermeability. Finally, inhibitors of RhoA activation and Rho kinase represent a potentially valuable group of agents with endothelial hyperpermeability-reducing properties.

Differential effects of histamine and thrombin on endothelial barrier function through actin-myosin tension

We compared temporal changes in isometric tension in cultured human umbilical vein endothelial cells inoculated on a polymerized collagen membrane with changes in cell-cell and cell-matrix adhesion derived by a mathematical model of transendothelial cell resistance. Thrombin and histamine disrupt barrier function by targeting a greater loss in cell-cell adhesion, which preceded losses in overall transendothelial resistance. There were minor losses in cell-matrix adhesion, which was temporally slower than the decline in the overall transendothelial resistance. In contrast, thrombin and histamine restored barrier function by initiating a restoration of cell-matrix adhesion, which occurred before an increase in overall transendothelial resistance. Thrombin mediated a second and slower decline in cell-cell adhesion, which was not observed in histamine-treated cells. This decline in cell-cell adhesion temporally correlated with expressed maximal levels of tension development, suggesting that actin-myosin contraction directly strains cell-cell adhesion sites. Pretreatment of cells with ML-7 mediated more rapid recovery of cell-cell adhesion and had no effect on cell-matrix adhesion. Taken together, expression of actin-myosin contraction affects the restoration of barrier function by straining cell-cell adhesion sites.

Platelet-conditioned medium increases endothelial electrical resistance independently of cAMP/PKA and cGMP/PKG

Platelets release a soluble factor into blood and conditioned medium (PCM) that decreases vascular endothelial permeability. The objective of this study was to determine the signal-transduction pathway that elicits this decrease in permeability. Permeability-decreasing activity of PCM was assessed by the real-time measurement of electrical resistance across cell monolayers derived from bovine pulmonary arteries and microvessels. Using a desensitization protocol with cAMP/protein kinase A (PKA)-enhancing agents and pharmacological inhibitors, we determined that the activity of PCM is independent of PKA and PKG. Genistein, an inhibitor of tyrosine kinases, prevented the increase in endothelial electrical resistance. Because lysophosphatidic acid (LPA) has been proposed to be responsible for this activity of PCM and is known to activate the G(i) protein, inhibitors of the G protein pertussis toxin and of the associated phosphatidylinositol 3-kinase (PI3K) wortmannin were used. Pertussis toxin and wortmannin caused a 10- to 15-min delay in the characteristic rise in electrical resistance induced by PCM. Inhibition of phosphorylation of extracellular signal-regulated kinase with the mitogen-activated kinase kinase inhibitors PD-98059 and U-0126 did not prevent the activity of PCM. Similar findings with regard to the cAMP protocols and inhibition of G(i) and PI3K were obtained for 1-oleoyl-LPA. These results demonstrate that PCM increases endothelial electrical resistance in vitro via a novel, signal transduction pathway independent of cAMP/PKA and cGMP/PKG. Furthermore, PCM rapidly activates a signaling pathway involving tyrosine phosphorylation, the G(i) protein, and PI3K.

Inhibitors of myosin light chain kinase and phosphodiesterase reduce ventilator-induced lung injury

Alveolar overdistension due to high peak inflation pressures (PIP) is associated with an increased capillary filtration coefficient (K(fc)). To determine which signal pathways contribute to this injury, we perfused isolated rat lungs with 5% bovine albumin in Krebs solution and measured K(fc) after successive 30-min periods of ventilation with peak inflation pressures (PIP) of 7, 20, 30, and 35 cmH(2)O. In a high-PIP control group, K(fc) increased significantly after ventilation with 30 and 35 cmH(2)O PIP, but significant increases were prevented by treatment with 100 microM trifluoperazine, an inhibitor of Ca(2+)/calmodulin, 500 nM ML-7, an inhibitor of myosin light chain kinase (MLCK), a combination of isoproterenol (20 microM) and rolipram (10 microM) to enhance intracellular cAMP levels, and a dose of KT-5720 (2 microM), which inhibits MLCK and protein kinase C. These studies suggest that the Ca(2+)/calmodulin-MLCK pathway augments capillary fluid leak after a modest high-PIP injury and that this is attenuated by kinase inhibition and increased intracellular cAMP.