Roles of calcium, cyclic nucleotides, and protein kinase C in regulation of endothelial permeability

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.

Bixa orellana leaf extract suppresses histamine-induced endothelial hyperpermeability via the PLC-NO-cGMP signaling cascade

BACKGROUND

Histamine is established as a potent inflammatory mediator and it is known to increased endothelial permeability by promoting gap formation between endothelial cells. Previous studies have shown that aqueous extract of Bixa orellana leaves (AEBO) exhibits antihistamine activity in vivo, yet the mechanism of its action on endothelial barrier function remains unclear. Therefore, the current study aimed to determine the protective effect of AEBO against histamine-induced hyperpermeability in vitro.

METHODS

The endothelial protective effect of AEBO was assess using an in vitro vascular permeability assay kit. Human umbilical vein endothelial cells (HUVEC) were used in the current study. HUVEC were pre-treated with AEBO for 12 h before histamine induction. Vascular permeability was evaluated by the amount of FITC-dextran leakage into the lower chamber. In order to elucidate the mechanism of action of AEBO, phospholipase C (PLC) activity, intracellular calcium level, nitric oxide (NO) concentration, cyclic guanosine monophosphate (cGMP) production and protein kinase C (PKC) activity were determined following histamine challenge.

RESULTS

Histamine-induced increased HUVEC permeability was significantly attenuated by pretreatment with AEBO in a time- and concentration-dependent manner. Upregulation of PLC activity caused by histamine in HUVEC was suppressed by pretreatment with AEBO. Pretreatment with AEBO also blocked the production of intracellular calcium induced by histamine in HUVEC. In addition, AEBO suppressed the NO-cGMP signaling cascade when HUVEC were challenged with histamine. Moreover, PKC activity was significantly abolished by pretreatment with AEBO in HUVEC under histamine condition.

CONCLUSION

In conclusion, the present data suggest that AEBO could suppress histamine-induced increased endothelial permeability and the activity may be closely related with the inhibition of the PLC-NO-cGMP signaling pathway and PKC activity.

Intracellular Ascorbate Prevents Endothelial Barrier Permeabilization by Thrombin

Intracellular ascorbate (vitamin C) has previously been shown to tighten the endothelial barrier and maintain barrier integrity during acute inflammation in vitro. However, the downstream effectors of ascorbate in the regulation of endothelial permeability remain unclear. In this study, we evaluated ascorbate as a mediator of thrombin-induced barrier permeabilization in human umbilical vein endothelial cells and their immortalized hybridoma line, EA.hy926. We found that the vitamin fully prevented increased permeability to the polysaccharide inulin by thrombin in a dose-dependent manner, and it took effect both before and after subjection to thrombin. Thrombin exposure consumed intracellular ascorbate but not the endogenous antioxidant GSH. Likewise, the antioxidants dithiothreitol and tempol did not reverse permeabilization. We identified a novel role for ascorbate in preserving cAMP during thrombin stimulation, resulting in two downstream effects. First, ascorbate maintained the cortical actin cytoskeleton in a Rap1- and Rac1-dependent manner, thus preserving stable adherens junctions between adjacent cells. Second, ascorbate prevented actin polymerization and formation of stress fibers by reducing the activation of RhoA and phosphorylation of myosin light chain. Although ascorbate and thrombin both required calcium for their respective effects, ascorbate did not prevent thrombin permeabilization by obstructing calcium influx. However, preservation of cAMP by ascorbate was found to depend on both the production of nitric oxide by endothelial nitric-oxide synthase, which ascorbate is known to activate, and the subsequent generation cGMP by guanylate cyclase. Together, these data implicate ascorbate in the prevention of inflammatory endothelial barrier permeabilization and explain the underlying signaling mechanism.

ADAM10 regulates endothelial permeability and T-Cell transmigration by proteolysis of vascular endothelial cadherin

Vascular endothelial (VE)-cadherin is the major adhesion molecule of endothelial adherens junctions. It plays an essential role in controlling endothelial permeability, vascular integrity, leukocyte transmigration, and angiogenesis. Elevated levels of soluble VE-cadherin are associated with diseases like coronary atherosclerosis. Previous data showed that the extracellular domain of VE-cadherin is released by an unknown metalloprotease activity during apoptosis. In this study, we used gain-of-function analyses, inhibitor studies, and RNA interference experiments to analyze the proteolytic release of VE-cadherin in human umbilical vein endothelial cells (HUVECs). We found that VE-cadherin is specifically cleaved by the disintegrin and metalloprotease ADAM10 in its ectodomain, releasing a soluble fragment and generating a carboxyl-terminal membrane-bound stub, which is a substrate for a subsequent gamma-secretase cleavage. This ADAM10-mediated proteolysis could be induced by Ca(2+) influx and staurosporine treatment, indicating that ADAM10-mediated VE-cadherin cleavage contributes to the dissolution of adherens junctions during endothelial cell activation and apoptosis, respectively. In contrast, protein kinase C activation or inhibition did not modulate VE-cadherin processing. Increased ADAM10 expression was functionally associated with an increase in endothelial permeability. Remarkably, our data indicate that ADAM10 activity also contributes to the thrombin-induced decrease of endothelial cell-cell adhesion. Moreover, knockdown of ADAM10 in HUVECs as well as in T cells by small interfering RNA impaired T-cell transmigration. Taken together, our data identify ADAM10 as a novel regulator of vascular permeability and demonstrate a hitherto unknown function of ADAM10 in the regulation of VE-cadherin-dependent endothelial cell functions and leukocyte transendothelial migration.

Astragaloside IV improved barrier dysfunction induced by acute high glucose in human umbilical vein endothelial cells

The purpose of the present study was to examine the effects of astragaloside IV, a saponin isolated from Astragalus membranaceus (Fisch) Bge, on the impairment of barrier function induced by acute high glucose in cultured human vein endothelial cells. High glucose (27.8 mM) induced a decrease in transendothelial electrical impedance and an increase in cell monolayer permeability in human umbilical vein endothelial cells. Endothelial barrier dysfunction stimulated by high glucose was accompanied by translocation and activation of protein kinase C (PKC), the redistribution of F-actin and formation of intercellular gaps, suggesting that increases in PKC activity and rearrangement of F-actin could be associated with endothelial barrier dysfunction induced by acute high glucose. Application of astragaloside IV inhibited high glucose-induced endothelial barrier dysfunction in a dose-dependent manner, which is compatible with inhibition of PKC translocation and improvement of F-actin rearrangements. Western blot analysis revealed that high glucose-induced PKC alpha and beta2 overexpression in the membrane fraction were significantly reduced by astragaloside IV. These findings indicate that astragaloside IV protected endothelial cells from high glucose-induced barrier impairment by inhibiting PKC activation, as well as improving cytoskeleton remodeling.

Role of protein kinase Cζ in thrombin-induced endothelial permeability changes: inhibition by angiopoietin-1

Endothelial cell leakiness is regulated by mediators such as thrombin, which promotes endothelial permeability, and anti-inflammatory agents, such as angiopoietin-1. Here we define a new pathway involved in thrombin-induced permeability that involves the atypical protein kinase C isoform, PKCζ. Chemical inhibitor studies implicated the involvement of an atypical PKC isoform in thrombin-induced permeability changes in human umbilical vein endothelial cells. Thrombin stimulation resulted in PKCζ, but not the other atypical PKC isoform, PKCλ, translocating to the membrane, an event known to be critical to enzyme activation. The involvement of PKCζ was confirmed by overexpression of constitutively active PKCζ, resulting in enhanced basal permeability. Dominant-negative PKCζ prevented the thrombin-mediated effects on endothelial cell permeability and inhibited thrombin-induced activation of PKCζ. Rho activation does not appear to play a role, either upstream or downstream of PKCζ, as C3 transferase does not block thrombin-induced PKCζ activation and dominant-negative PKCζ does not block thrombin-induced Rho activation. Finally, we show that angiopoietin-1 inhibits thrombin-induced PKCζ activation, Rho activation, and Ca++ flux, thus demonstrating that the powerful antipermeability action of angiopoietin-1 is mediated by its action on a number of signaling pathways induced by thrombin and implicated in permeability changes. (Blood. 2004; 104:1716-1724)

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.

Regulation of blood-brain barrier permeability

The blood-brain barrier minimizes the entry of molecules into brain tissue. This restriction arises by the presence of tight junctions (zonulae occludens) between adjacent endothelial cells and a relative paucity of pinocytotic vesicles within endothelium of cerebral arterioles, capillaries, and venules. Many types of stimuli can alter the permeability characteristics of the blood-brain barrier. Acute increases in arterial blood pressure beyond the autoregulatory capacity of cerebral blood vessels, application of hyperosmolar solutions, application of various inflammatory mediators known to be elevated during brain injury, and/or activation of blood-borne elements such as leukocytes can produce changes in permeability of the blood-brain barrier. The second messenger systems that account for increases in permeability of the blood-brain barrier during pathophysiologic conditions, however, remain poorly defined. This review will summarize studies that have examined factors that influence disruption of the blood-brain barrier, and will discuss the contribution of various cellular second messenger pathways in disruption of the blood-brain barrier during pathophysiologic conditions.

Regulation of endothelial CD73 by adenosine: paracrine pathway for enhanced endothelial barrier function

During episodes of inflammation, multiple cell types release adenine nucleotides in the form of ATP, ADP, 5'-AMP, and adenosine. In particular, following activation, polymorphonuclear leukocytes release larger quantities of 5'-AMP. Extracellular 5'-AMP is metabolized to adenosine by surface-expressed 5'-ectonucleotidase (CD73). Adenosine liberated by this process activates surface adenosine A(2B) receptors, results in endothelial junctional reorganization, and promotes barrier function. We hypothesized that adenosine signaling to endothelia provides a paracrine loop for regulated expression of CD73 and enhanced endothelial barrier function. Using an in vitro microvascular endothelial model, we investigated the influence of 5'-AMP; adenosine; and adenosine analogues on CD73 transcription, surface expression, and function. Initial experiments revealed that adenosine and adenosine analogues induce CD73 mRNA (RT-PCR), surface expression (immunoprecipitation of surface biotinylated CD73), and function (HPLC analysis of etheno-AMP conversion to ethenoadenosine) in a time- and concentration-dependent fashion. Subsequent studies revealed that similar exposure conditions increase surface protein through transcriptional induction of CD73. Analysis of DNA-binding activity by EMSA identified a functional role for CD73 cAMP response element and, moreover, indicated that multiple cAMP agonists induce transcriptional activation of functional CD73. Induced CD73 functioned to enhance 5'-AMP-mediated promotion of endothelial barrier (measured as a paracellular flux of 70-kDa FITC-labeled tracer). These results provide an example of transcriptional induction of enzyme (CD73) by enzymatic product (adenosine) and define a paracrine pathway for the regulated expression of vascular endothelial CD73 and barrier function.

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

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

Effect of vascular endothelial growth factor on cultured endothelial cell monolayer transport properties

Vascular endothelial growth factor (VEGF) is a potent enhancer of microvascular permeability in vivo. To date, its effects on hydraulic conductivity (L(p)) and diffusive albumin permeability (P(e)) of endothelial monolayers have not been thoroughly assessed in vitro. We hypothesized that VEGF affects endothelial transport properties differently depending on vessel location and endothelial phenotype. Using three well-established endothelial cell culture models-human umbilical vein endothelial cells (HUVECs), bovine aortic endothelial cells (BAECs), and bovine retinal microvascular cells (BRECs)-grown on porous, polycarbonate filters we were able to produce baseline transport properties characteristic of restrictive barriers. Our results show 3.1-fold and 5.7-fold increases in endothelial L(p) for BAEC and BREC monolayers, respectively, at the end of 3 h of VEGF (100 ng/ml) exposure. HUVECs, however, showed no significant alteration in L(p) after 3 h (100 ng/ml) or 24 h (25 ng/ml) of incubation with VEGF even though they were responsive to the inflammatory mediators, thrombin (1 U/ml; 27-fold increase in L(p) in 25 min) and bradykinin (10 microM; 4-fold increase in L(p) in 20 min). Protein kinase C (PKC) and nitric oxide (NO) are downstream effectors of VEGF signaling. BAEC L(p) was responsive to activation of NO (SNAP) and PKC (PMA), whereas these agents had no effect in altering HUVEC L(p). Moreover, BAECs exposed to the PKC inhibitor, staurosporine (50 ng/ml), exhibited significant attenuation of VEGF-induced increase in L(p), but inhibition of nitric oxide synthase (NOS) with L-NMMA (100 microM) had no effect in altering the VEGF-induced increase in L(p). These data provide strong evidence that in BAECs, the VEGF-induced increase in L(p) is mediated by a PKC-dependent mechanism. Regarding diffusive albumin P(e), at the end of 3 h, BAECs and BRECs showed 6.0-fold and 9. 9-fold increases in P(e) in response to VEGF (100 ng/ml), whereas VEGF had no significant effect after 3 h (100 ng/ml) or 24 h (25 ng/ml) in changing HUVEC P(e). In summary, these data indicate that VEGF affects endothelial transport properties differently depending on the vessel type and that differences in cell signaling pathways underlie the differences in VEGF responsiveness.

Stimulated neutrophils evoke signal transduction to increase vascular permeability in rat lungs

The mechanisms by which stimulated neutrophils (PMNs) damage pulmonary vascular endothelium were investigated using twenty-four perfused lung preparations isolated from rats. We tested the ability of unstimulated and mechanically stimulated PMNs to adhere to pulmonary endothelial cells and, thereby, alter pulmonary vascular permeability (measured as the pulmonary filtration coefficient) and hemodynamics. To stimulate PMNs, they were gently agitated in a glass vial for 10 seconds. Perfusing lungs with the stimulated PMNs (stimulated group) elicited a 3-fold increase in the filtration coefficient as compared to lungs perfused with unstimulated cells (unstimulated group). This increase in filtration was completely blocked by preincubation of stimulated PMNs with CD18 monoclonal antibody (MoAb group). This increase in filtration coefficient was also completely blocked by GF109203X, a protein kinase C inhibitor (GF group). Pulmonary vascular resistance increased when the stimulated PMNs were injected to the isolated lungs. Although, preincubation of stimulated PMNs with CD18 MoAb successfully blocked and GF109203X partly blocked this increase in pulmonary vascular resistance. The accumulation of stimulated PMNs within the lungs, as assessed by myeloperoxidase (MPO) levels, was blocked by preincubation of stimulated PMNs with CD18 MoAb. However, GF109203X did not decrease MPO levels. These findings suggest that stimulated PMN-induced increases in pulmonary vascular filtration, resulted from endothelial cell injury caused by adhesion to the endothelial cells, evoke intracellular signaling within the endothelial cells.

Cocaine-induced increase in the permeability function of human vascular endothelial cell monolayers

The effects of cocaine on endothelial cell macromolecular transport, electrical resistance, and morphology were assessed. In confluent endothelial monolayers grown on microporus filters, cocaine (0.01 to 1 mmol/L) induced a rapid concentration-dependent increase in permeability to peroxidase and low density lipoprotein. Along with increased transport, the cocaine effect was paralleled by a decrease in transendothelial electrical resistance. Alterations in membrane resistance were fully reversible following washout of the drug, providing evidence that cocaine does not cause permanent injury to the integrity of the monolayer. Cocaines major metabolites, benzoylecgonine and ecgonine methyl ester, had minimal effect on electrical resistance properties, whereas monolayer impedance was markedly depressed by the novel cocaine/alcohol metabolite, cocaine ethyl ester (cocaethylene). Morphologic studies of cocaine-treated endothelial cells revealed a marked disruption of F-actin and the formation of intercellular gaps; no evidence of cell lysis and/or detachment was noted. Forskolin, a potent activator of adenylate cyclase known to promote the endothelial cell barrier function, impaired cocaine-induced changes in electrical resistance and morphology. Cocaine, however, had no effect on resting levels of intracellular adenosine 3',5'-cyclic monophosphate (cAMP) in confluent endothelial monolayers. In summary, the results indicate that cocaine directly induces structural defects in the endothelial cell barrier which enhance the transport of macromolecular tracers, the mechanism does not appear to involve intracellular cAMP.

Transient and prolonged increase in endothelial permeability induced by histamine and thrombin: role of protein kinases, calcium, and RhoA

In the present study, we differentiated between short- and long-term effects of vasoactive compounds on human endothelial permeability in an in vitro model. Histamine induced a rapid and transient (<3 minutes) decrease in barrier function, as evidenced by a decreased transendothelial electrical resistance and an increased passage of 22Na ions. This increase in permeability was inhibited completely by chelation of intracellular calcium ions by BAPTA-AM and inhibition of calmodulin activity and myosin light chain (MLC) phosphorylation. The presence of serum factors prolonged the barrier dysfunction induced by histamine. Thrombin by itself induced a prolonged barrier dysfunction (>30 minutes) as evidenced by an increased passage of peroxidase and 40 kDa dextran. It was dependent only partially on calcium ions and calmodulin. The protein tyrosine kinase inhibitors genistein and herbimycin A, but not the inactive analogue daidzein, inhibited to a large extent the increase in permeability induced by thrombin. Genistein and BAPTA-AM inhibited the thrombin-induced permeability in an additive way, causing together an almost complete prevention of the thrombin-induced increase in permeability. Inhibition of protein tyrosine kinase was accompanied by a decrease in MLC phosphorylation and a reduction in the extent of F-actin fiber and focal attachment formation. Inhibition of RhoA by C3 transferase toxin reduced both the thrombin-induced barrier dysfunction and MLC phosphorylation. Genistein and C3 transferase toxin did not elevate the cellular cAMP levels. No evidence was found for a significant role of protein kinase C in the thrombin-induced increase in permeability or in the accompanying MLC phosphorylation. These data indicate that in endothelial cell monolayers that respond to histamine in a physiological way, thrombin induces a prolonged increase in permeability by "calcium sensitization," which involves protein tyrosine phosphorylation and RhoA activation.

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

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

Regulation of cerebral microvascular permeability by histamine in the anaesthetized rat

1. The permeability response of slightly leaky pial venular capillaries to histamine was investigated using the single microvessel occlusion technique. 2. Histamine dose-response curves showed that concentrations between 5 nm and 5 microM increased permeability, while concentrations from 50 microM to 5 mM reduced it. 3. The H2 receptor antagonist cimetidine (2 microM) blocked the effects of lower concentrations of histamine, while the H1 receptor antagonist mepyramine (3 nM) blocked those of higher concentrations of histamine. 4. The effects of lower doses of histamine were mimicked by the H2 receptor agonist dimaprit, and the effects of higher doses of histamine were mimicked by the H1 receptor agonist alpha-2-(2-aminoethyl)pyridine (AEP). 5. Low concentrations of histamine, which normally increase the permeability of Lucifer Yellow (PLY), reduced it when co-applied with the phosphodiesterase 4 (PDE4) inhibitor rolipram. Rolipram also potentiated the response to AEP, but had no effect on that to dimaprit. 6. The effects of dimaprit were blocked by reducing extracellular Ca2+ from 2.5 mM to nominally Ca2+ free, or by applying the calcium entry blocker SKF 96365.

Platelet-derived lysophosphatidic acid decreases endothelial permeability in vitro

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

Cyclic GMP accumulation in pulmonary microvascular endothelial cells measured by intact cell prelabeling

Cyclic GMP accumulation in cultured rat pulmonary microvascular endothelial cells (RPMVEC) was studied with a new prelabeling method developed using intact platelets and smooth muscle cells (1). [3H]-hypoxanthine was used to radiolabel the cellular guanine nucleotide pool. Neutral alumina and Dowex-50 double column chromatography was used to purify and quantitate the levels of [3H]-cyclic GMP. Changes in cyclic GMP metabolism in short and long term RPMVEC cultures were studied using rat atrial naturetic factor 8-33 (ANF) and sodium nitroprusside (SNP) in the presence and absence of cyclic nucleotide (CN) phosphodiesterase (PDE) inhibitors. In RPMVEC exogenous hypoxanthine was incorporated into both low (65% uptake) and high (34% uptake) passage cells in a time-dependent manner reaching maximum incorporation near 8 hours. Basal cyclic GMP values in both groups were 0.003% of the total cellular tritium (9 x 10(6) and 4 x 10(6) cpm/10(6) cells, respectively). ANF treatment of prelabeled RPMVEC resulted in a 10- to 12-fold increase in [3H]-cyclic GMP in the absence of CN PDE inhibitors (EC50 = 5.4 nM). However, incubation with SNP showed no changes in cellular cyclic GMP accumulation. Several relatively selective CN PDE inhibitors had no effect on ANF or SNP induced cyclic GMP accumulation in RPMVEC. The ANF induced cGMP accumulation was verified by radioimmunoassay. These studies confirm the utility of the hypoxanthine prelabeling technique to monitor intact microvascular EC cyclic GMP accumulation. Cultured RPMVEC show little or no functional soluble guanylate cyclase or cyclic GMP PDE activity.

Direct interactions of plasminogen activators with human aortic and pulmonary artery endothelial cells in vitro: implications for thrombolytic therapy

Direct interactions of plasminogen activators with arterial endothelial cells are important in the pathogenesis of vascular complications associated with thrombolytic therapy. We investigated the direct effects of various plasminogen activators on human aortic and pulmonary artery endothelial cell functions in vitro. The effects of plasminogen activators on endothelial cells were not caused by generation of plasmin, as shown by the absence of plasminogen and alpha(2)-plasmin inhibitor-plasmin complex both before and after addition of plasminogen activators to endothelial cells. High concentrations of plasminogen activators increased the permeability of aortic endothelial cells to albumin. Alteplase (50 x 10(3) IU/ml), a recombinant tissue-type plasminogen activator (t-PA), increased prostaglandin I(2) (PGI(2)) production by aortic endothelial cells from 175.5 +/- 13.8 to 870.8 +/- 131.0 pg/mg cellular protein during a 2-h incubation; other plasminogen activators increased PGI(2) production to a lesser extent. Alteplase (100 x 10(3) IU/ml) also increased PGI(2) production from 152.0 +/- 16.2 to 1,080 +/- 95.1 pg/mg cellular protein in human pulmonary artery endothelial cells. High concentrations of urokinases decreased the amount of endothelin-1 in the medium of aortic or pulmonary artery endothelial cells by as much as 93%; part of this decrease was attributable to degradation of endothelin-l by urokinases. Other plasminogen activators either had no effect on or slightly increased the production of endothelin-1. These changes in the function of human arterial endothelial cells induced by plasminogen activators may affect regional vascular tone, endothelial permeability, and platelet aggregability, all of which are important in the efficacy of thrombolysis and in the pathogenesis of such vascular complications as rethrombosis and hemorrhage.

Endothelial cell expression of vasoconstrictors and growth factors is regulated by smooth muscle cell-derived carbon monoxide

CO is produced in vascular smooth muscle cells (VSMC) by heme oxygenase-1 (HO-1). CO increases cGMP levels in VSMC; however, its possible additional roles in the vasculature have not been examined. We report that a product of HO, released from VSMC and inhibited by hemoglobin, has paracrine effects on endothelial cells: it increases endothelial cGMP content and decreases the expression of the mitogens, endothelin-1 (ET-1) and platelet-derived growth factor-B (PDGF-B). This product has the characteristics of CO, and its production is increased sevenfold under hypoxia. The VSMC-derived CO caused a fourfold rise in endothelial cell cGMP. In addition, it inhibited the hypoxia-induced increases in mRNA levels of the ET-1 and PDGF-B genes. Inhibitors of HO, and hemoglobin, a scavenger of CO, prevented the rise in cGMP and also restored the hypoxic response of these genes. The inhibition of ET-1 and PDGF-B mRNA by CO resulted in decreased production of these endothelial-derived mitogens, and in turn, inhibition of VSMC proliferation. These findings suggest an important physiologic role for VSMC-derived CO in modulating cell-cell interaction and cell proliferation in the vessel wall during hypoxia.

An in vitro model for endothelial permeability: assessment of monolayer integrity

An essential component of any in vitro model for endothelial permeability is a confluent cell monolayer. The model reported here utilizes primary human umbilical vein endothelial cells (HUVEC) cultured on recently developed polyethylene terephthalate micropore membranes. Using a modification of the Wright-Giemsa stain, confluent HUVEC monolayers grown on micropore membranes were routinely assessed using light microscopy. Determination of confluence using this method was confirmed by scanning electron microscopy. Transendothelial electrical resistance of HUVEC monolayers averaged 27.9 +/- 11.4 omega.cm2, 10 to 21% higher than literature values. Studies characterizing the permeability of the endothelial cell monolayer to 3H-inulin demonstrated a linear relationship between the luminal concentration of 3H-inulin and its flux across HUVEC monolayers. The slope of the flux versus concentration plot, which represents endothelial clearance of 3H-inulin, was 2.01 +/- 0.076 x 10(-4) ml/min (r2 = .9957). The permeability coefficient for the HUVEC monolayer-micropore membrane barrier was 3.17 +/- 0.427 x 10(-6) cm/s with a calculated permeability coefficient of the HUVEC monolayer alone of 4.07 +/- 0.617 x 10(-6) cm/s. The HUVEC monolayer reduced the permeability of the micropore membrane alone to 3H-inulin (1.43 +/- 0.445 x 10(-5) cm/s) by 78%. Evans blue dye-labeled bovine serum albumin could not be detected on the abluminal side without disruption of the HUVEC monolayer. These results demonstrate a model for endothelial permeability that can be extensively assessed for monolayer integrity by direct visualization, transendothelial electrical resistance, and the permeability of indicator macromolecules.

Expression of cGMP-dependent protein kinase I and phosphorylation of its substrate, vasodilator-stimulated phosphoprotein, in human endothelial cells of different origin

Previous studies demonstrated that the thrombin-induced permeability of endothelial cell monolayers is reduced by the elevation of cGMP. In the present study, the presence of cGMP-dependent protein kinase (cGMP-PK) immunoreactivity and activity in various types of human endothelial cells (ECs) and the role of cGMP-PK in the reduction of thrombin-induced endothelial permeability was investigated. cGMP-PK type I was demonstrated in freshly isolated ECs from human aorta and iliac artery as well as in cultured ECs from human aorta, iliac vein, and foreskin microvessels. Addition of the selective cGMP-PK activator 8-(4-chlorophenylthio)-cGMP (8-pCPT-cGMP) to these ECs caused phosphorylation of the vasodilator-stimulated phosphoprotein (VASP), an established cGMP-PK substrate, which is localized at cell-cell contact sites of confluent ECs. cGMP-PK type I expression decreased during serial passage of ECs, which correlated with a diminished ability of 8-pCPT-cGMP to induce VASP phosphorylation. Preincubation of aorta and microvascular EC monolayers with 8-pCPT-cGMP caused a 50% reduction of the thrombin-stimulated permeability, as determined by measuring the peroxidase passage through EC monolayers on porous filters. Furthermore, the thrombin-induced rise in cytoplasmic [Ca2+]i was strongly attenuated by the cGMP-PK activator in fura 2-loaded aorta ECs. In contrast, cGMP-PK could not be demonstrated in freshly isolated and cultured human umbilical vein ECs. Incubation of umbilical vein ECs with 8-pCPT-cGMP did not cause VASP phosphorylation and had no effect on the thrombin-induced increases in cytoplasmic Ca2+ and endothelial permeability.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of protein kinase C does not participate in disruption of the blood-brain barrier to albumin during acute hypertension

The blood-brain barrier minimizes the entry of macromolecules into brain tissue. During acute increases in arterial blood pressure, disruption of the blood-brain barrier occurs primarily in cerebral venules and veins. Mechanisms by which increases in cerebral venous pressure produce disruption of the blood-brain barrier during acute hypertension are not clear. The goal of this study was to determine the role of activation of protein kinase C in disruption of the blood-brain barrier during acute hypertension. We examined the microcirculation of the cerebrum in vivo. Permeability of the blood-brain barrier was quantitated by the formation of venular leaky sites and clearance of fluorescent-labeled albumin (FITC-albumin) before and during phenylephrine-induced acute hypertension. In addition, we examined changes in pial arteriolar and pial venular pressure before and during phenylephrine-induced acute hypertension. We compared responses of the blood-brain barrier to acute hypertension in control (untreated) rats and in rats treated with inhibitors of protein kinase C; calphostin C (0.1 microM) or sphingosine (1.0 microM). Under control conditions, no venular leaky sites were visible and clearance of FITC-albumin was minimal in all groups. Phenylephrine infusion increased systemic arterial, pial arteriolar and pial venular pressures, and increased the formation of venular leaky sites and clearance of FITC-albumin by a similar magnitude in all groups. The findings of the present study suggest that inhibition of protein kinase C does not significantly alter the formation of venular leaky sites and/or clearance of FITC-albumin during acute hypertension. Thus, disruption of the blood-brain barrier during acute hypertension does not appear to be influenced by activation of protein kinase C.

Inhibition by beraprost sodium of thrombin-induced increase in endothelial macromolecular permeability

Effect of beraprost sodium (BPS), a long-acting and orally active stable analogue of PGI2, on the macromolecular permeability of cultured vascular endothelial cells (HUVEC) was detected by the transport of FITC-albumin. Thrombin treatment resulted in induction of FITC-albumin transport across the endothelial cell monolayer. The albumin transport induced by thrombin was not accompanied by any damage to the cells. BPS had no effect on the permeability of resting endothelial monolayers, while BPS inhibited the thrombin-induced increase in the albumin permeability in a dose-dependent manner (30-1000 nM). Treatment of the cells with PGI2 or dibutyryl cAMP caused a significant inhibition of the thrombin-induced increase in the albumin permeability. These results strongly suggested that BPS suppressed the thrombin-induced macromolecular permeability in HUVEC through the elevation of its intracellular cAMP, and that BPS was a suppressor against inflammatory vascular changes such as exudation.

cGMP and nitric oxide modulate thrombin-induced endothelial permeability. Regulation via different pathways in human aortic and umbilical vein endothelial cells

Previous studies have demonstrated that cGMP and cAMP reduce the endothelial permeability for fluids and macromolecules when the endothelial permeability is increased by thrombin. In this study, we have investigated the mechanism by which cGMP improves the endothelial barrier function and examined whether nitric oxide (NO) can serve as an endogenous modulator of endothelial barrier function. Thrombin increased the passage of macromolecules through human umbilical vein and human aortic endothelial cell monolayers and concomitantly increased [Ca]2+ in vitro. Inhibition of these increases by the intracellular Ca2+ chelator BAPTA indicated that cytoplasmic Ca2+ elevation contributes to the thrombin-induced increase in endothelial permeability. The cGMP-dependent protein kinase activators 8-bromo-cGMP (8-Br-cGMP) and 8-(4-chlorophenylthio)cGMP (8-PCPT-cGMP) decreased the thrombin-induced passage of macromolecules. Two pathways accounted for this observation. Activation of cGMP-dependent protein kinase by 8-PCPT-cGMP decreased the accumulation of cytoplasmic Ca2+ in aortic endothelial cells and hence reduced the thrombin-induced increase in permeability. On the other hand, in umbilical vein endothelial cells, cGMP-inhibited phosphodiesterase (PDE III) activity was mainly responsible for the cGMP-dependent reduction of endothelial permeability. The PDE III inhibitors Indolidan (LY195115) and SKF94120 decreased the thrombin-induced increase in permeability by 50% in these cells. Thrombin treatment increased cGMP formation in the majority of, but not all, cell cultures. Inhibition of NO production by NG-nitro-L-arginine methyl ester (L-NAME) enhanced the thrombin-induced increase in permeability, which was restricted to those cell cultures that displayed an increased cGMP formation after addition of thrombin. Simultaneous elevation of the endothelial cGMP concentration by atrial natriuretic factor, sodium nitroprusside, or 8-Br-cGMP prevented the additional increase in permeability induced by L-NAME. These data indicate that cGMP reduces thrombin-induced endothelial permeability by inhibition of the thrombin-induced Ca2+ accumulation and/or by inhibition of cAMP degradation by PDE III. The relative contribution of these mechanisms differs in aortic and umbilical vein endothelial cells. NO can act in vitro as an endogenous permeability-counteracting agent by raising cGMP in endothelial cells of large vessels.

S-nitroso-N-acetylpenicillamine reduces leukocyte adhesion to type I collagen

The initial step in the migration of neutrophils to the extravascular space is adhesion to the endothelium. We examined the effect of nitric oxide on this process by treating human neutrophils with S-nitroso-N-acetylpenicillamine (SNAP), a NO-producing compound. Since NO has been shown to increase the level of cGMP in other cell types, we used 8-Br-cGMP in order to mimic the effects of NO. Indeed, both these treatments resulted in a reduced adhesion of neutrophils to type I collagen coated surfaces. After a prolonged incubation with SNAP, the adhesion was the same as for untreated cells. SNAP incubation reduced the F-actin content in the cells whereas 8-Br-cGMP increased it, demonstrating different mechanisms of action on F-actin. These data suggest that endothelium-derived nitric oxide is an important endogenous modulator of neutrophil adhesion, but the effect is not mediated by a cGMP-dependent regulation of F-actin levels.

Heparin induces endothelial extracellular matrix alterations and barrier dysfunction

We investigated the influence of heparin on the composition of the subendothelial matrix and on endothelial permeability to elucidate the structure-function relationship of matrix composition and permeability. Albumin flux across the confluent endothelial monolayers was used to assess the macromolecular permeability. Lowest values were obtained when 100% serum was used as medium for permeability studies. The endothelial matrix components, fibronectin and basement membrane-associated heparan sulfate proteoglycan (HSPG), were measured by enzyme immunoassay. Treatment of proliferating endothelial cells with heparin (0-900 micrograms/ml) induced a dose-dependent decrease in endothelial HSPG content, whereas the fibronectin content was unaltered. This structural change was accompanied by an increase in albumin permeability. Both heparin effects exhibited similar dose-response curves with half-maximal effects at approximately 5 micrograms/ml heparin. Acute addition of 300 micrograms/ml heparin had no effect on permeability or HSPG content. When endothelial cells were preincubated with an HSPG antiserum, the endothelial permeability increased nearly threefold. Our results indicate that heparin-induced loss of HSPG may cause the increase in endothelial permeability. The data underline the importance of HSPG for the integrity of the endothelial barrier.

Effects of glyceryl trinitrate, nitroprusside and nitric oxide on arterial, venous and capillary functions in cat skeletal muscle in vivo

The aim of the present study was to analyse quantitatively, on a cat gastrocnemius preparation in vivo, the effects of i.a. or i.v. administered glyceryl trinitrate (GTN), sodium nitroprusside (SNP) or nitric oxide (NO dissolved in saline) on vascular resistance (tone) in the following consecutive vascular sections: Large-bore arterial resistance vessels (> 25 microns), small arterioles (< 25 microns), and the veins. Effects on hydrostatic capillary pressure (Pc,v) and transcapillary fluid exchange were simultaneously recorded. Close-arterially infused GTN (1-4096 micrograms kg tissue-1 min-1), SNP (0.5-32 micrograms kg tissue-1 min-1) and NO (0.14-0.82 mg kg tissue-1 min-1) elicited a generalized dose-dependent dilator response in all three sections, though with a preferential action on the arterial side. Further, these agents caused an increase in Pc,v and transcapillary fluid filtration. The sites of action along the vascular bed of these exogenous vasodilators differed from that previously established for endogenous EDNO. Infusion of GTN, SNP and NO during EDNO blockade (L-NAME) could, therefore, not restore the vascular resistance distribution to that prevailing in the initial control state. Myogenic vascular reactivity to standardized transmural pressure stimuli was clearly depressed by GTN and SNP. Intravenously infused GTN (4-512 micrograms kg body wt-1 min-1) and SNP (4-64 micrograms kg body wt-1 min-1) decreased arterial pressure and elicited, via reflex sympathetic activation, a dose-dependent vasoconstriction in skeletal muscle, a decrease in Pc,v, and net transcapillary fluid absorption. The constrictor response thus overruled the direct dilator effect of the drugs. The plasma volume expansion known to result from long-term systematic administration of nitrovasodilators seems in part to be caused by transcapillary fluid absorption in skeletal muscle.

Nutrition, endothelial cell metabolism, and atherosclerosis

The vascular endothelium that forms an interface between the blood and the surrounding tissues is continuously exposed to both physiologic and pathophysiologic stimuli. These stimuli are often mediated by nutrients that can contribute to the overall function of the endothelial cell in the regulation of vascular tone, coagulation and fibrinolysis, cellular growth and differentiation, and immune and inflammatory responses. Therefore, nutrient-mediated functional changes of the endothelium and the underlying tissues may be significantly involved in the atherosclerotic disease process. There is evidence that individual nutrients or nutrient derivatives may either provoke or prevent metabolic and physiologic perturbations of the vascular endothelium. Preservation of nutrients that exhibit antiatherogenic properties may, therefore, be a critical issue in the preparation and processing of foods. This review focuses on selected nutrients as they affect endothelial cell metabolism and their possible implications in atherosclerosis.

Mechanisms of endothelin-induced macromolecular leakage in microvascular beds of rat mesentery

Microvascular responses to endothelin-3 were investigated in the rat mesentery under fluorescence microscopy. Endothelin-3 in a range of 0.1-100 pM induced arteriolar constriction in a dose-dependent manner, and stimulated Ca2+ mobilization, demonstrated by fura-2-associated fluorography, in both arterioles and venules. Cyclo(-D-Trp-D-Asp-Pro-D-Val-Leu-) (BQ123), and endothelin ETA receptor antagonist, at a concentration of 10 microM inhibited the endothelin-3 (100 pM)-induced arteriolar constriction and Ca2+ mobilization in arterioles but not in venules. In venules, an early onset leakage of FITC (fluorescein isothiocyanate)-labeled albumin and subsequent reduction of red blood cell velocity without arteriolar constriction were observed after the superfusion of endothelin-3 with BQ123, suggesting that a non-endothelin ETA receptor mediates macromolecular leakage followed by a decrease in blood flow. Endothelin-3 with BQ123 neither stimulated leukocyte adhesion nor activated luminol-dependent chemiluminescence in venules, showing that endothelin-3-increased permeability may be induced by leukocyte-independent and oxyradical-independent mechanisms. These microvascular alterations of permeability and red blood cell velocity were significantly attenuated by the addition of phalloidin, an F-actin stabilizer, suggesting the involvement of endothelial cell contraction. Nicardipine (1,4-dihydro-2,6-dimethyl-4-[3-nitrophenyl]methyl-2- [methyl(phenylmethyl)amino]-3,5-pyridinedicarboxylic acid ethyl ester), a dihydropyridine-type Ca2+ channel antagonist, eliminated endothelin-3-induced arteriolar constriction; however, it did not affect albumin leakage promoted by endothelin-3 with BQ123, suggesting that a non-voltage-dependent Ca2+ channel(s) is involved in non-endothelin ETA receptor-mediated Ca2+ mobilization and contraction of venular endothelial cells. Overall, it is conceivable that endothelin ETA receptor and voltage-dependent Ca2+ channel are involved in endothelin-3-induced arteriolar constriction. In addition, the present results suggest that Ca2+ mobilization in venular endothelium, which is mediated by a non-endothelin ETA receptor, possibly endothelin ETB receptor and regulated by non-voltage-dependent Ca2+ channel(s), may cause endothelial cell contraction and subsequently increase macromolecular permeability in microvascular beds treated with endothelin-3.

Endothelial cell-to-cell junctions. Structural characteristics and functional role in the regulation of vascular permeability and leukocyte extravasation

Endothelial monolayer forms the main barrier to the passage of macromolecules and circulating cells from blood to tissues. This property is regulated by intercellular junctions. These are complex structures formed by transmembrane adhesive molecules linked to a network of cytoplasmic cytoskeletal proteins. Endothelial junctions vary in number and organization along the vascular tree. Some transmembrane components of endothelial junctions have recently been identified. One is specifically expressed by endothelial cells (cadherin-5) while others (such as PECAM-1 and integrins) are also present in other cell types. The mechanisms that regulate the opening and the closure of endothelial junctions are still obscure. It is possible that inflammatory agents increase permeability by binding to specific receptors on the endothelial membrane. This would lead to the generation of intracellular signals causing cytoskeletal reorganization and opening of interendothelial gaps. This general sequence of events, however, seems to follow specific routes for different stimuli. In fact, permeability-increasing agents differ in the type of intracellular second messenger they trigger, for the time course of their effect, and for their specificity for the endothelium of different vascular districts. Endothelial junctions also regulate leukocyte extravasation. Endothelial cells actively contribute to this process by expressing adhesive molecules on their surface and by releasing chemotactic cytokines. Once leukocytes have adhered to the endothelium, a coordinated opening of interendothelial junctions occurs. The mechanism by which this takes place is unknown but it might resemble that triggered by soluble inflammatory mediators.

Increased prevalence of aortic fatty streaks in cholesterol-fed rabbits administered intravenous cocaine: the role of vascular endothelium

Several recent postmortem studies suggest an increased prevalence of atherosclerosis in young habitual cocaine abusers. However, little is known about the effects of cocaine abuse on the vascular endothelium and its relationship to atherosclerosis. Therefore, the consequence of chronic administration of intravenous cocaine on the induction of aortic sudanophilia was examined. Male New Zealand White rabbits were fed a 0.5% cholesterol diet for 10 wk. During this period, animals were randomized to receive either cocaine-hydrochloride (0.25 mg/kg) intravenously (n = 17) twice daily; or an equivalent volume of 0.9% physiologic saline, control group (n = 16). Mean values for total circulating leukocytes and platelets and total plasma cholesterol and triglycerides were similar in both groups throughout the protocol. At the completion of the study, aortic sudanophilia was measured and expressed as a percentage of regional involvement (R1 = proximal 4 cm, R2 = middle 6 cm, and R3 = distal 10 cm). Statistical significance among groups was achieved in the proximal thoracic aorta (p = 0.057). No significant differences in sudanophilia were noted in the middle and distal segments. When animals were placed in subgroups according to percent total plaque involvement, there was a significant increased distribution of rabbits with a greater extent of sudanophilia in the cocaine-treated group as compared with control (p = 0.01, chi-square analysis). Immunocytochemical studies using the macrophage-specific and muscle actin-specific monoclonal antibodies demonstrated that sudanophilic areas in both groups were predominantly composed of macrophage-derived foam cells. Evaluation of plaque morphology showed an increase in intimal plaque thickness and in the number of macrophages and smooth muscle cells in cocaine-treated animals; however, group differences were not statistically significant. Because no significant differences were found in the cellular composition of atherosclerotic plaques between groups, further studies were performed to assess the effects of cocaine on the permeability function of cultured endothelial cell monolayers as a possible mechanism of increased sudanophilia. Cocaine (100 microM)-treated endothelial cell monolayers demonstrated an increased permeability to horseradish peroxidase during all time intervals studied (0-6 hr). Permeability differences were statistically significant at 30 min and 1 hr (p = 0.003 and 0.02, respectively). Collectively, these observations suggest that administration of cocaine to cholesterol-fed rabbits increases the prevalence of aortic sudanophilia via at least one possible mechanism involving enhanced vascular permeability.

Role of cyclic AMP in promoting the thromboresistance of human endothelial cells by enhancing thrombomodulin and decreasing tissue factor activities

1. The effects of forskolin, prostaglandin E1 (PGE1), dibutyryl cyclic AMP (db cyclic AMP), dibutyryl cyclic GMP (db cyclic GMP) and 3-isobutyl-l-methyl-xanthine (IBMX) were investigated on the expression of tissue factor and thrombomodulin activities on the surface of human saphenous vein endothelial cells (HSVEC) in culture. 2. Forskolin (10(-6) to 10(-4) M), PGE1 (10(-7) to 10(-5) M) and db cyclic AMP (10(-4) to 10(-3) M) caused a concentration-dependent decrease of cytokine-induced tissue factor activity. 3. Similar concentrations of forskolin, PGE1 and db cyclic AMP enhanced significantly constitutive thrombomodulin activity and reversed the decrease of this activity caused by interleukin-1 (IL-1). 4. IBMX (10(-4) M) decreased tissue factor activity and enhanced the effect of forskolin on tissue factor and thrombomodulin activities. 5. Forskolin (10(-4) M) decreased the IL-1-induced tissue factor mRNA and increased the thrombomodulin mRNA level. IL-1 did not change the thrombomodulin mRNA level after 2 h of incubation with HSVEC in culture. 6. Dibutyryl cyclic GMP (10(-4) M to 10(-3) M) did not influence tissue factor or thrombomodulin activity. 7. Our data suggest that elevation of intracellular cyclic AMP levels may participate in the regulation of tissue factor and thrombomodulin expression, thus contributing to promote or restore antithrombotic properties of the endothelium.

Modulation of human endothelial cell permeability by combinations of the cytokines interleukin-1 alpha/beta, tumor necrosis factor-alpha and interferon-gamma

The permeability of human umbilical vein endothelial cell (HUVEC) monolayers to [125I]-labelled bovine serum albumin (BSA) was examined following pretreatment of the cells with various cytokines. The electrical resistance measured across untreated, confluent, intact HUVEC monolayers was 18.2 +/- 3.8 omega.cm2 (mean +/- S.D. of 4 observations). Human recombinant (hr) interleukin-1 alpha/beta (IL-1 alpha/beta), hr tumor necrosis factor-alpha (TNF-alpha), and hr interferon-gamma (IFN-gamma) each increased HUVEC monolayer permeability in a time- and dose-dependent manner. These effects were inhibitable by neutralizing antibodies (nAb) to the corresponding cytokines, and were not due to contamination by endotoxin (abolition of cytokine effect by heat treatment, and no effect on cytokine action of the endotoxin inhibitor polymyxin B). The effects of these cytokines were not due to endothelial cell (EC) interleukin-6 (IL-6) induction (IL-6 shown not to increase permeability) and the effect of hrTNF-alpha could not be accounted for by induction of IL-1 (effect not inhibited by hrIL-1 alpha/beta nAb). The effects of three different combinations of the cytokines (each combination at two different concentrations) on HUVEC monolayer permeability were also examined. hrIFN-gamma with hrTNF-alpha or hrIL-1 alpha/beta gave an increase in permeability (at both concentration combinations) greater than that seen with either cytokine alone. hrTNF-alpha and hrIL-1 alpha/beta in combination however produced an enhanced effect only at low concentrations, high concentrations in combination producing an effect no greater than either agent alone. These results highlight the importance of investigating actions of cytokine combinations on in vitro models of endothelial cell activation.

Modulation of barrier function of bovine aortic and pulmonary artery endothelial cells: dissociation from cytosolic calcium content

1. Barrier function and cytosolic free calcium content [Ca2+]i was measured in monolayers of bovine pulmonary artery endothelial cells (BPAEC) and bovine aortic endothelial cells (BAEC). 2. Thrombin (1 u ml-1) increased albumin transfer across monolayers of BPAEC but not BAEC, yet induced biphasic increases in [Ca2+]i in both endothelial cell types, consisting of a rapid, initial phasic component which decayed to a lower, more sustained plateau phase. 3. 4 beta-Phorbol 12-myristate 13-acetate (PMA; 0.3-3000 nM) increased albumin transfer across monolayers of BPAEC and BAEC, but had no effect on basal levels of [Ca2+]i in either endothelial cell type. 4. Treatment of BPAEC and BAEC with forskolin (30 microM), an activator of adenylate cyclase, had no effect on resting transfer of albumin, but inhibited that stimulated by PMA (600 nM). It also inhibited the thrombin (1 u ml-1)-induced increase in albumin transfer across monolayers of BPAEC, but enhanced the plateau phase of the associated increase in [Ca2+]i. 5. Treatment of BPAEC and BAEC with either atriopeptin II (100 nM), an activator of particulate guanylate cyclase, or 8 bromo cyclic GMP (30 microM) had no effect on resting or PMA (600 nM)-stimulated transfer of albumin. Both agents did, however, inhibit the thrombin (1 u ml-1)-induced increase in albumin transfer across monolayers of BPAEC, but had no effect on the associated increase in [Ca2+]i. 6. These data suggest a dissociation between the ability of agents that increase or decrease albumin transfer and their effects on [Ca2+]i. Consequently, activation of protein kinase C may be the major stimulus for trans-endothelial transfer of macromolecular solutes. Endothelial barrier function is enhanced by elevation of either cyclic AMP or cyclic GMP content. Cyclic AMP appears to act by inhibiting the actions of protein kinase C, while cyclic GMP may act to inhibit a key step proximal to activation of this enzyme.

Inhibition of astroglia-induced endothelial differentiation by inorganic lead: a role for protein kinase C

Microvascular endothelial function in developing brain is particularly sensitive to lead toxicity, and it has been hypothesized that this results from the modulation of protein kinase C (PKC) by lead. We examined the effects of inorganic lead on an in vitro model of central nervous system endothelial differentiation in which astroglial cells induce central nervous system endothelial cells to form capillary-like structures. Capillary-like structure formation within C6 astroglial-endothelial cocultures was inhibited by lead acetate with 50% maximal inhibition at 0.5 microM total lead. Inhibition was independent of effects on cell viability or growth. Under conditions that inhibited capillary-like structure formation, we found that lead increased membrane-associated PKC in both C6 astroglial and endothelial cells. Prolonged exposure of C6 cells to 5 microM lead for up to 16 h resulted in a time-dependent increase in membranous PKC as determined by immunoblot analysis. Membranous PKC increased after 5-h exposures to as little as 50 nM lead and was maximal at approximately 1 microM. Phorbol esters were used to determine whether PKC modulation was causally related to the inhibition of endothelial differentiation by lead. Phorbol 12-myristate 13-acetate (10 nM) inhibited capillary-like structure formation by 65 +/- 5%, whereas 4 alpha-phorbol 12,13-didecanoate was without effect. These findings suggest that inorganic lead induces cerebral microvessel dysfunction by interfering with PKC modulation in microvascular endothelial or perivascular astroglial cells.

Endothelium-derived relaxing factor contributes to the regulation of endothelial permeability

To determine whether endothelium-derived relaxing factor (EDRF) contributes to the regulation of endothelial permeability, the transendothelial flux of 14C-sucrose, a marker for the paracellular pathway across endothelial monolayers (Oliver, J. Cell. Physiol. 145:536-548, 1990), was examined in monolayers of bovine aortic endothelial cells grown on collagen-coated filters. The permeability coefficient of 14C-sucrose was significantly decreased by 10(-3) M 8-Bromoguanosine 3',5'-cyclic monophosphate or by 5 x 10(-6) M glyceryl trinitrate, an activator of soluble guanylate cyclase. Depletion of L-arginine from endothelial monolayers increased 14C-sucrose permeability from 3.21 +/- 0.59 to 3.88 +/- 0.50 x 10(-5) cm.sec-1 (mean +/- SEM; n = 6; P < 0.05). The acute administration of 5 x 10(-4) M L-arginine to monolayers depleted of this amino acid decreased 14C-sucrose permeability from 2.91 +/- 0.27 to 2.52 +/- 0.26 x 10(-5) cm.sec-1 (n = 11; P < 0.05). 14C-sucrose permeability was increased by 10(-7) M bradykinin and this effect was enhanced by the presence of each one of the following compounds: 10(-5) M methylene blue, 4 x 10(-6) M oxyhemoglobin, 5 x 10(-4) M NG-methyl-L-arginine or 5 x 10(-4) M N omega-nitro-L-arginine. These results suggest that EDRF contributes to the sealing of the endothelial monolayer and that EDRF released by bradykinin acts as a feedback inhibitor attenuating the increase in endothelial permeability induced by this peptide. Because endothelial cells have the ability to contract and relax and possess guanylate cyclase responsive to nitric oxide, our results suggest that EDRF decreases 14C-sucrose permeability by relaxing endothelial cells, thereby narrowing the width of endothelial junctions.

Histamine-modulated transdifferentiation of dermal microvascular endothelial cells

Homeostatic and inflammatory functions of skin microvessels are tightly regulated by vasoactive amines. Following stimulation with histamine, dermal microvascular endothelial cells (MEC) undergo a rapid change in phenotype (transdifferentiation) and subsequently exhibit an enhanced rate of growth. To elucidate mechanisms regulating MEC transdifferentiation, this study investigated the functional relationships among vimentin, Ca2+, and protein kinase C (PKC) in histamine-modulated dermal MEC in vitro. Distribution of vimentin and PKC in foreskin-derived MEC cultivated in a modified Iscove's medium was assessed with immunocytochemistry. Calcium ion kinetics in histamine-treated MEC were analyzed using the Ca2+ probe Fluo-3 in conjunction with interactive laser cytometry. Histamine, acting through H-1 receptors, produces a rapid (less than 100 ms) and differential elevation of free calcium in each of three cytological compartments defined by the vimentin cytoskeleton in epithelial MEC. A distinctive compartmentalized and nonuniform distribution of PKC precisely coincides with that observed for free-Ca2+ released in response to histamine. The studies reveal that histamine modulation of the MEC phenotype is associated with a rapid patterned reorganization of the vimentin skeleton. It is hypothesized that histamine induces vimentin post-translational modifications by activating a spatially localized interaction among cytoplasmic free Ca2+, PKC, and the vimentin matrix. The results further suggest that vimentin, in addition to its structural role, may participate in signal transduction and gene regulation processes in effecting MEC transdifferentiation.

A delayed and sustained rise of cytosolic calcium is elicited by oxidized LDL in cultured bovine aortic endothelial cells

Bovine aortic endothelial cells (BAEC) pulsed for 5 h with mildly oxidized low density lipoproteins (LDL), exhibited a broad, sustained and high peak of [Ca2+]i occurring several hours after the end of the pulse and reaching very high [Ca2+]i values (around 2500-3000 nmol/l) and a concomitant drop of cytosolic pH (around 0.2-0.3 pH units) without any loss of cell viability. When BAEC were continuously pulsed with oxidized LDL, the peak of [Ca2+]i was more sustained than in short pulse experiments and was associated with irreversible morphological changes usually associated with cytotoxic events (blebbing) and with a marked loss of viability. The potential involvement of these biochemical and morphological changes in atherogenesis are discussed.

Effects of selected vasoactive substances on adenylate cyclase activity in brain, isolated brain microvessels, and primary cultures of brain microvessel endothelial cells

The specific activity of adenylate cyclase was assayed in homogenates of gray matter, freshly isolated and primary cultured microvessel endothelial cells from bovine cerebral cortex. Specific activities for the tissues were 14.6 +/- 2.1, 15.6 +/- 2.7, and 8.4 +/- 1.5 pmol cAMP/mg protein/min +/- SD for gray matter, cultured microvessels, and freshly isolated microvessels, respectively. Adenylate cyclase associated with gray matter and cultured microvessels was sensitive to histamine and selected catecholamines. Perhaps due to metabolic deficiencies, adenylate cyclase of freshly isolated microvessels exhibited little or no response to either the catecholamines or histamine. Angiotensin II stimulated adenylate cyclase of both freshly isolated and cultured microvessels but had no effect on gray matter. Bradykinin did not stimulate cAMP generation in any of the tissues. Overall results support the role of cAMP in regulating brain microvessel functions and suggest that primary cultures of brain microvessels may be useful in examining cAMP-mediated biochemical pathways at the blood-brain barrier.

A cell culture model of the blood-brain barrier

Endothelial cells that make up brain capillaries and constitute the blood-brain barrier become different from peripheral endothelial cells in response to inductive factors found in the nervous system. We have established a cell culture model of the blood-brain barrier by treating brain endothelial cells with a combination of astrocyte-conditioned medium and agents that elevate intracellular cAMP. These cells form high resistance tight junctions and exhibit low rates of paracellular leakage and fluid-phase endocytosis. They also undergo a dramatic structural reorganization as they form tight junctions. Results from these studies suggest modes of manipulating the permeability of the blood-brain barrier, potentially providing the basis for increasing the penetration of drugs into the central nervous system.

Permeability of human venous endothelial cell monolayers perfused in microcarrier cultures: effects of flow rate, thrombin, and cytochalasin D

We have applied a multiple isotope dilution technique to examine junctional permeability of human umbilical vein endothelial cells (HUVEC) in vitro. Primary cultures were grown to confluence on porous Cytodex-3 microcarrier beads, packed into 0.3 ml columns (3 x 10(6) cells) and perfused at varying flow rates (0.3-1.2 ml/min) with HEPES-buffered Tyrodes solution containing unlabeled cyanocobalamin, insulin, and albumin. Columns were challenged periodically with mixtures of radioactive tracers of different molecular size. Permeability to 22Na+, [57Co]cyanocobalamin (1.3 kD), [125I]insulin (6 kD) or [125I]albumin (66 kD) was assessed relative to [131I]IgG (160 kD, impermeant reference tracer) by comparing column elution profiles. Although the single passage extraction of [125I]albumin by beads alone approximated 40%, the presence of confluent HUVEC rendered these beads effectively impermeable to albumin. High junctional extractions were measured for cyanocobalamin (0.79 +/- 0.02, n = 28) and insulin (0.51 +/- 0.05, n = 14) in cultures perfused at 0.3-0.4 ml/min, and tracer extraction decreased as perfusion rates increased. Permeability coefficients for cyanocobalamin (9.66 x 10(-5) cm/s) and insulin (4.18 x 10(-5) cm/s) increased significantly during perfusion with thrombin (10 U/ml) or cytochalasin D (1 microgram/ml), whereas permeability to albumin (0.39 x 10(-5) cm/s) remained unchanged. Morphological studies, using the glycocalyx stain ruthenium red, revealed that thrombin or cytochalasin D increased the penetration of the stain into junctions between endothelial cells.

Adenylate cyclase and protein kinase C mediate opposite actions on endothelial junctions

To determine whether the endothelial paracellular pathway is regulated, the effect of intracellular messengers on the transendothelial flux of inert radiolabeled molecules of diverse molecular size was examined in bovine aortic endothelial cells grown on collagen-coated filters. The endothelial monolayers showed a modest electrical resistance (21 +/- 10 delta.cm2; m +/- SD) and restricted the passage to 14C-sucrose, 3H-inulin, 14C-dextran (70 kDa), and 125I-polyvinyl pyrrolidone (125I-PVP, 360 kDa) according to their molecular mass. 8-Bromoadenosine 3'-5' cyclic monophosphate (8-Br-cAMP) reduced by more than 30% the permeability coefficients of 14C-sucrose and 3H-inulin but had no effect on the permeability of 125I-PVP. The permeabilities of 14C-sucrose and of 14C-inulin were strikingly increased by activating protein kinase C (PKC) by phorbol 12-myristate-13-acetate or sn-1,2-dioctanoly-glycerol whereas the latter compound had no effect on the permeability of 125I-PVP. In addition, the permeability of 14C-sucrose was unchanged by a phorbol ester that does not activate PKC. Increasing intracellular calcium with ionomycin had no effect on the permeability of 14C-sucrose. None of these maneuvers significantly affected the protein content of the endothelial monolayers. The results indicate that 8-Br-cAMP and PKC activators modulate a pathway across the endothelial monolayer that excludes 125I-PVP (360 kDa) but readily accepts 14C-sucrose and 3H-inulin, suggesting that this pathway is the paracellular pathway. Hence, low molecular weight molecules such as sucrose and inulin can be used to probe the behavior of the paracellular pathway of endothelial monolayers grown in vitro. The results also indicate that the paracellular pathway in endothelium is regulated and suggest that endothelial junctions can be closed by simulating adenylate cyclase and opened by stimulating protein kinase C.