Purine Nucleotides Induce Regulated Secretion of von Willebrand Factor: Involvement of Cytosolic Ca2+ and Cyclic Adenosine Monophosphate–Dependent Signaling in Endothelial Exocytosis

Venous valve hypoxia as a possible mechanism of deep vein thrombosis: a scoping review

INTRODUCTION

The pathogenesis of deep vein thrombosis (DVT) has been explained by an interplay between a changed blood composition, vein wall alteration, and blood flow abnormalities. A comprehensive investigation of these components of DVT pathogenesis has substantially promoted our understanding of thrombogenesis in the venous system. Meanwhile, the process of DVT initiation remains obscure. This systematic review aims to collect, analyze, and synthesize the published evidence to propose hypoxia as a possible trigger of DVT.

EVIDENCE ACQUISITION

An exhaustive literature search was conducted across multiple electronic databased including PubMed, EMBASE, Scopus, and Web of Science to identify studies pertinent to the research hypothesis. The search was aimed at exploring the connection between hypoxia, reoxygenation, and the initiation of deep vein thrombosis (DVT). The following key words were used: "deep vein thrombosis," "venous thrombosis," "venous thromboembolism," "hypoxia," "reoxygenation," "venous valve," and "venous endothelium." Reviews, case reports, editorials, and letters were excluded.

EVIDENCE SYNTHESIS

Based on the systematic search outcome, 156 original papers relevant to the issue were selected for detailed review. These studies encompassed a range of experimental and observational clinical research, focusing on various aspects of DVT, including the anatomical, physiological, and cellular bases of the disease. A number of studies suggested limitations in the traditional understanding of Virchow's triad as an acceptable explanation for DVT initiation. Emerging evidence points to more complex interactions and additional factors that may be critical in the early stages of thrombogenesis. The role of venous valves has been recognized but remains underappreciated, with several studies indicating that these sites may act as primary loci for thrombus formation. A collection of studies describes the effects of hypoxia on venous endothelial cells at the cellular and molecular levels. Hypoxia influences several pathways that regulate endothelial cell permeability, inflammatory response, and procoagulation activity, underpinning the endothelial dysfunction noted in DVT.

CONCLUSIONS

Hypoxia of the venous valve may serve as an independent hypothesis to outline the DVT triggering process. Further research projects in this field may discover new molecular pathways responsible for the disease and suggest new therapeutic targets.

The Use of Fluorescently Labeled ARC1779 Aptamer for Assessing the Effect of H2O2 on von Willebrand Factor Exocytosis

Here, we propose a new approach for quantitative estimation of von Willebrand factor (vWF) exposed on the surface of endothelial cells (ECs) using the ARC1779 aptamer that interacts with the vWF A1 domain. To visualize complex formation between vWF and the aptamer, the latter was conjugated with the Cy5 fluorescent label. Cultured human umbilical vein endothelial cells (HUVEC) were stained with the ARC1779-Cy5 conjugate and imaged with a fluorescence microscope. The images were analyzed with the CellProfiler software. vWF released from the Weibel-Palade bodies was observed as bright dot-like structures of round and irregular shape, the number of which increased several times after HUVEC exposure to histamine or thrombin. Staining with ARC1779-Cy5 also revealed long filamentous vWF structures on the surface of activated HUVEC. vWF secretion by ECs is activated by the second messengers cAMP and Ca2+. There is evidence that hydrogen peroxide also acts as a second messenger in ECs. In addition, exogenous H2O2 formed in leukocytes can enter ECs. The aim of our study was to determine the effect of H2O2 on the vWF exposure at the surface of HUVEC using the proposed method. It was shown that hydrogen peroxide at concentration 100 µM, which is lower than the cytotoxicity threshold of H2O2 for cultured HUVEC, increased several times the number of dot-like structures and total amount of vWF exposed on plasma membrane of HUVEC, which suggest that H2O2 acts as a mediator that activates exocytosis of Weibel-Palade bodies and vWF secretion in the vascular endothelium during inflammation and upon elevated generation of endogenous reactive oxygen species in ECs.

Purinergic Regulation of Endothelial Barrier Function

Increased vascular permeability is a hallmark of several cardiovascular anomalies, including ischaemia/reperfusion injury and inflammation. During both ischaemia/reperfusion and inflammation, massive amounts of various nucleotides, particularly adenosine 5'-triphosphate (ATP) and adenosine, are released that can induce a plethora of signalling pathways via activation of several purinergic receptors and may affect endothelial barrier properties. The nature of the effects on endothelial barrier function may depend on the prevalence and type of purinergic receptors activated in a particular tissue. In this review, we discuss the influence of the activation of various purinergic receptors and downstream signalling pathways on vascular permeability during pathological conditions.

Tuning the Thromboinflammatory Response to Venous Flow Interruption by the Ectonucleotidase CD39

Objective- Leukocyte flux contributes to thrombus formation in deep veins under pathological conditions, but mechanisms that inhibit venous thrombosis are incompletely understood. Ectonucleotide di(tri)phosphohydrolase 1 ( ENTPD1 or Cd39), an ectoenzyme that catabolizes extracellular adenine nucleotides, is embedded on the surface of endothelial cells and leukocytes. We hypothesized that under venous stasis conditions, CD39 regulates inflammation at the vein:blood interface in a murine model of deep vein thrombosis. Approach and Results- CD39-null mice developed significantly larger venous thrombi under venous stasis, with more leukocyte recruitment compared with wild-type mice. Gene expression profiling of wild-type and Cd39-null mice revealed 76 differentially expressed inflammatory genes that were significantly upregulated in Cd39-deleted mice after venous thrombosis, and validation experiments confirmed high expression of several key inflammatory mediators. P-selectin, known to have proximal involvement in venous inflammatory and thrombotic events, was upregulated in Cd39-null mice. Inferior vena caval ligation resulted in thrombosis and a corresponding increase in both P-selectin and VWF (von Willebrand Factor) levels which were strikingly higher in mice lacking the Cd39 gene. These mice also manifest an increase in circulating platelet-leukocyte heteroaggregates suggesting heterotypic crosstalk between coagulation and inflammatory systems, which is amplified in the absence of CD39. Conclusions- These data suggest that CD39 mitigates the venous thromboinflammatory response to flow interruption.

Rab46 integrates Ca2+ and histamine signaling to regulate selective cargo release from Weibel-Palade bodies

It is unclear how a plethora of stimuli evoke differential cargo secretion from endothelial cells to produce stimulus-appropriate responses. Miteva et al. show that Rab46 integrates histamine signaling and Ca²⁺ signals to regulate selective cargo release from Weibel-Palade bodies.

Endothelial cells selectively release cargo stored in Weibel-Palade bodies (WPBs) to regulate vascular function, but the underlying mechanisms are poorly understood. Here we show that histamine evokes the release of the proinflammatory ligand, P-selectin, while diverting WPBs carrying non-inflammatory cargo away from the plasma membrane to the microtubule organizing center. This differential trafficking is dependent on Rab46 (CRACR2A), a newly identified Ca²⁺-sensing GTPase, which localizes to a subset of P-selectin–negative WPBs. After acute stimulation of the H₁ receptor, GTP-bound Rab46 evokes dynein-dependent retrograde transport of a subset of WPBs along microtubules. Upon continued histamine stimulation, Rab46 senses localized elevations of intracellular calcium and evokes dispersal of microtubule organizing center–clustered WPBs. These data demonstrate for the first time that a Rab GTPase, Rab46, integrates G protein and Ca²⁺ signals to couple on-demand histamine signals to selective WPB trafficking.

Exocytosis of Weibel-Palade bodies: how to unpack a vascular emergency kit

The blood vessel wall has a number of self-healing properties, enabling it to minimize blood loss and prevent or overcome infections in the event of vascular trauma. Endothelial cells prepackage a cocktail of hemostatic, inflammatory and angiogenic mediators in their unique secretory organelles, the Weibel-Palade bodies (WPBs), which can be immediately released on demand. Secretion of their contents into the vascular lumen through a process called exocytosis enables the endothelium to actively participate in the arrest of bleeding and to slow down and direct leukocytes to areas of inflammation. Owing to their remarkable elongated morphology and their secretory contents, which span the entire size spectrum of small chemokines all the way up to ultralarge von Willebrand factor multimers, WPBs constitute an ideal model system for studying the molecular mechanisms of secretory organelle biogenesis, exocytosis, and content expulsion. Recent studies have now shown that, during exocytosis, WPBs can undergo several distinct modes of fusion, and can utilize fundamentally different mechanisms to expel their contents. In this article, we discuss recent advances in our understanding of the composition of the WPB exocytotic machinery and how, because of its configuration, it is able to support WPB release in its various forms.

Endothelial Cells

Endothelial cells are a constitutive part of the heart and vasculature and form a crucial link between the cardiovascular system and the immune system. Besides their commonly accepted roles in angiogenesis, hemostasis, and the regulation of vascular tone, they are an essential and active component of immune responses. Expression of a range of innate pattern recognition receptors allows them to respond to inflammatory stimulation, and they control immune cell recruitment and extravasation into target tissues throughout the body.In this chapter, I will therefore summarize classical endothelial cell properties and functions and their cross talk with the immune system as well as the operational immunological role of endothelial cells in facilitating immune responses.

The pharmacodynamic impact of apremilast, an oral phosphodiesterase 4 inhibitor, on circulating levels of inflammatory biomarkers in patients with psoriatic arthritis: substudy results from a phase III, randomized, placebo-controlled trial (PALACE 1)

Apremilast, an oral phosphodiesterase 4 inhibitor, demonstrated effectiveness (versus placebo) for treatment of active psoriatic arthritis in the psoriatic arthritis long-term assessment of clinical efficacy (PALACE) phase III clinical trial program. Pharmacodynamic effects of apremilast on plasma biomarkers associated with inflammation were evaluated in a PALACE 1 substudy. Of 504 patients randomized in PALACE 1, 150 (placebo: n = 51; apremilast 20 mg BID: n = 51; apremilast 30 mg BID: n = 48) provided peripheral blood plasma samples for analysis in a multiplexed cytometric bead array assay measuring 47 proteins associated with systemic inflammatory immune responses. Association between biomarker levels and achievement of 20% improvement from baseline in modified American College of Rheumatology (ACR20) response criteria was assessed by logistic regression. At Week 24, IL-8, TNF-α, IL-6, MIP-1β, MCP-1, and ferritin were significantly reduced from baseline with apremilast 20 mg BID or 30 mg BID versus placebo. ACR20 response correlated with change in TNF-α level with both apremilast doses. At Week 40, IL-17, IL-23, IL-6, and ferritin were significantly decreased and IL-10 and IL-1 receptor antagonists significantly increased with apremilast 30 mg BID versus placebo. In patients with active psoriatic arthritis, apremilast reduced circulating levels of Th1 and Th17 proinflammatory mediators and increased anti-inflammatory mediators.

Loss of DGKε induces endothelial cell activation and death independently of complement activation

Atypical hemolytic uremic syndrome (aHUS) is classically described to result from a dysregulation of the complement alternative pathway, leading to glomerular endothelial cell (EC) damage and thrombosis. However, recent findings in families with aHUS of mutations in the DGKE gene, which is not an integral component of the complement cascade, led us to consider other pathophysiologic mechanisms for this disease. Here, we demonstrate that loss of DGKε expression/activity in EC induces an increase in ICAM-1 and tissue factor expression through the upregulation of p38-MAPK-mediated signals, thus highlighting a proinflammatory and prothrombotic phenotype of DGKε-deficient ECs. More interestingly, DGKE silencing also increases EC apoptosis and impairs EC migration and angiogenesis in vitro, suggesting that DGKE loss-of-function mutations impair EC repair and angiogenesis in vivo. Conversely, DGKE knockdown moderately decreases the expression of the complement inhibitory protein MCP on quiescent EC, but does not induce complement deposition on their surface in vitro. Collectively, our data strongly suggest that in DGKE-associated aHUS patients, thrombotic microangiopathy results from impaired EC proliferation and angiogenesis rather than complement-mediated EC lesions. Our study expands the current knowledge of aHUS mechanisms and has implications for the treatment of patients with isolated DGKE mutations.

Essential role of vesicular nucleotide transporter in vesicular storage and release of nucleotides in platelets

Nucleotides are stored in the dense granules of platelets. The release of nucleotides triggers one of the first steps in a series of cascades responsible for blood coagulation. However, the mechanism of how the nucleotides are accumulated in the granules is still far less understood. The transporter protein responsible for storage of nucleotides in the neuroendocrine cells has been identified and characterized. We hypothesized that the vesicular nucleotide transporter (VNUT) is also involved in the vesicular storage of nucleotides in platelets. In this article, we present three lines of evidence that VNUT is responsible for the vesicular storage of nucleotides in platelets and that vesicular ATP transport is crucial for platelet function, detection and characterization of VNUT activity in platelets isolated from healthy humans and MEG‐01 cells, RNA interference experiments on MEG‐01 cells, and studies on nucleotide transport and release with a selective inhibitor.

VNUT is highly expressed and associated with dense granules in platelets. VNUT plays an essential role in vesicular storage of nucleotide in platelets.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Possible race and gender divergence in association of genetic variations with plasma von Willebrand factor: a study of ARIC and 1000 genome cohorts

The synthesis, secretion and clearance of von Willebrand factor (VWF) are regulated by genetic variations in coding and promoter regions of the VWF gene. We have previously identified 19 single nucleotide polymorphisms (SNPs), primarily in introns that are associated with VWF antigen levels in subjects of European descent. In this study, we conducted race by gender analyses to compare the association of VWF SNPs with VWF antigen among 10,434 healthy Americans of European (EA) or African (AA) descent from the Atherosclerosis Risk in Communities (ARIC) study. Among 75 SNPs analyzed, 13 and 10 SNPs were associated with VWF antigen levels in EA male and EA female subjects, respectively. However, only one SNP (RS1063857) was significantly associated with VWF antigen in AA females and none was in AA males. Haplotype analysis of the ARIC samples and studying racial diversities in the VWF gene from the 1000 genomes database suggest a greater degree of variations in the VWF gene in AA subjects as compared to EA subjects. Together, these data suggest potential race and gender divergence in regulating VWF expression by genetic variations.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Exploring the potential of the platelet membrane proteome as a source of peripheral biomarkers for Alzheimer's disease

Introduction

Peripheral biomarkers to diagnose Alzheimer's disease (AD) have not been established. Given parallels between neuron and platelet biology, we hypothesized platelet membrane-associated protein changes may differentiate patients clinically defined with probable AD from noncognitive impaired controls.

Methods

Purified platelets, confirmed by flow cytometry were obtained from individuals before fractionation by ultracentrifugation. Following a comparison of individual membrane fractions by SDS-PAGE for general proteome uniformity, equal protein weight from the membrane fractions for five representative samples from AD and five samples from controls were pooled. AD and control protein pools were further divided into molecular weight regions by one-dimensional SDS-PAGE, prior to digestion in gel. Tryptic peptides were analyzed by reverse-phase liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS). Ionized peptide intensities were averaged for each identified protein in the two pools, thereby measuring relative protein abundance between the two membrane protein pools. Log₂-transformed ratio (AD/control) of protein abundances fit a normal distribution, thereby permitting determination of significantly changed protein abundances in the AD pool.

Results

We report a comparative analysis of the membrane-enriched platelet proteome between patients with mild to moderate AD and cognitively normal, healthy subjects. A total of 144 proteins were determined significantly altered in the platelet membrane proteome from patients with probable AD. In particular, secretory (alpha) granule proteins were dramatically reduced in AD. Of these, we confirmed significant reduction of thrombospondin-1 (THBS1) in the AD platelet membrane proteome by immunoblotting. There was a high protein-protein connectivity of proteins in other pathways implicated by proteomic changes to the proteins that define secretory granules.

Conclusions

Depletion of secretory granule proteins is consistent with a preponderance of post-activated platelets in circulation in AD. Significantly changed pathways implicate additional AD-related defects in platelet glycoprotein synthesis, lipid homeostasis, amyloidogenic proteins, and regulators of protease activity, many of which may be useful plasma membrane-expressed markers for AD. This study highlights the utility of LC-MS/MS to quantify human platelet membrane proteins and suggests that platelets may serve as a source of blood-based biomarkers in neurodegenerative disease.

cAMP-induced secretion of endothelial von Willebrand factor is regulated by a phosphorylation/dephosphorylation switch in annexin A2

The large multimeric glyocoprotein von Willebrand factor (VWF) is a crucial component of both primary and secondary hemostasis. It is stored in secretory granules of vascular endothelial cells, the Weibel-Palade bodies (WPBs), and is released following stimulation by agonists that raise intracellular Ca(2+) or cyclic adenosine monophosphate (cAMP) levels. cAMP-induced exocytosis of WPBs requires protein kinase A activity, but downstream factors that are regulated by phosphorylation/dephosphorylation are not known. Here we identify the complex consisting of the lipid-binding protein annexin A2 (AnxA2) and S100A10 as such a factor. Knockdown and specific rescue approaches reveal that a functional AnxA2-S100A10 complex is required for the forskolin-induced, cAMP-dependent release of VWF. Forskolin triggers dephosphorylation of AnxA2 that is mediated by a calcineurin-like phosphatase and stabilizes the AnxA2-S100A10 complex, thereby promoting VWF release. Serine 11 of AnxA2 was identified as the target residue of this phosphorylation switch because a phosphomimicking mutation at this site prevents complex formation with S100A10 and, in contrast to wild-type or S11A-AnxA2, is unable to restore cAMP-dependent VWF secretion in AnxA2-depleted cells. Thus, complex formation of AnxA2 with S100A10 is a central regulatory mechanism in the acute release of VWF in response to cAMP-elevating agonists.

Enzymes that hydrolyze adenine nucleotides in platelets and polymorphisms in the alpha2 gene of integrin alpha2beta1 in patients with von Willebrand disease

Von Willebrand disease (VWD) is one of the most common inherited bleeding diseases caused by a qualitative or quantitative deficiency of the von Willebrand factor (FvW). FvW is a multimeric glycoprotein synthesized by megakaryocytes and endothelial cells and it is present in the subendothelial matrix, blood plasma, platelets, and endothelium. This glycoprotein plays an important role in thrombus formation by initiating platelet adhesion to sites of injury as well as platelet aggregation. The aim of this study was to evaluate the activities of enzymes that hydrolyze adenine nucleotides in platelets, ristocetin-induced platelet aggregation (RIPA), and polymorphisms of the alpha2 gene of alpha2beta1 integrin from VWD patients. Platelet nucleoside triphosphate diphosphohydrolase (NTPDase), 5'-nucleotidase, and ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) activities were verified in 14 VWD patients. For RIPA determination, a final concentration of 1.25 mg/ml of ristocetin was used. Polymorphisms of the alpha2 gene were analyzed through PCR. Platelet NTPDase and E-NPP were decreased in VWD patients. 5'-Nucleotidase activity was not statistically significant between controls and VWD patients. RIPA was significantly reduced, with an allelic frequency of 78.57% for 807C in VWD patients. Our results indicated reduced platelet NTPDase and E-NPP activities which might be related to the low platelet adhesiveness. The prevalence of the 807C allele might account for the variability in bleeding in VWD.

Glucose-induced endothelial heparanase secretion requires cortical and stress actin reorganization

AIMS

Heparanase, which specifically cleaves carbohydrate chains of heparan sulfate, has been implicated in the pathology of diabetes-associated complications. Using high glucose (HG) to replicate hyperglycaemia observed following diabetes, the present study was designed to determine the mechanism by which HG initiates endothelial heparanase secretion.

METHOD AND RESULTS

To examine the effect of HG on endothelial heparanase, bovine coronary artery endothelial cells were incubated with 25 mM glucose. Strategies using different agonists and antagonists were used to determine the mechanism behind HG-induced heparanase secretion. In endothelial cells, heparanase colocalized with lysosomes predominately around the nucleus, and HG caused its dispersion towards the plasma membrane for subsequent secretion. ATP release, purinergic receptor activation, cortical actin disassembly, and stress actin formation were essential for this HG-induced heparanase secretion. With HG, phosphorylation of filamin likely contributed to the cortical actin disassembly, whereas Ca(2+)/calmodulin-dependent protein kinase II and p38 mitogen-activated protein kinase /heat shock protein 25 phosphorylation mediated stress actin formation. The endothelial secreted heparanase in response to HG demonstrated endoglucuronidase activity, cleaved heparan sulfate, and released attached proteins like lipoprotein lipase and basic fibroblast growth factor.

CONCLUSION

Our results suggest that HG is a potent stimulator of endothelial heparanase secretion. These data may assist in devising new therapeutic strategies to prevent or delay the cardiovascular complications associated with diabetes.

Endothelial cell confluence regulates Weibel-Palade body formation

Secretory granules called Weibel-Palade bodies (WPBs) containing Von Willebrand factor (VWF) are characteristic of the mammalian endothelium. We hypothesized that vascular-specific antigens such as VWF are linked to endothelial identity and proliferation in vitro. To test this idea, the cellular accumulation of VWF in WPBs was monitored as a function of cell proliferation, confluence and passage number in human umbilical vein endothelial cells (HUVECs). We found that as passage number increased the percentage of cells containing VWF in WPBs was reduced significantly, whilst the protein was still detected within the secretory pathway at all times. However, the endothelial-specific marker protein, PECAM-1, is present on all cells even when WPBs are absent, indicating partial maintenance of endothelial identity. Biochemical studies show that a significant pool of immature pro-VWF can be detected in sub-confluent HUVECs; however, a larger pool of mature, processed VWF is detected in confluent cells. Newly synthesized VWF must thus be differentially sorted and packaged along the secretory pathway in semi-confluent versus confluent endothelial cells. Our studies thus show that WPB formation is linked to the formation of a confluent endothelial monolayer.

Hemostatic implications of endothelial cell apoptosis in obstructive sleep apnea

Patients with obstructive sleep apnea (OSA) are at increased risk of atherothrombosis independent of the Framingham risk factors. Studies on hemostasis factors in OSA are scarce and inconsistent. We sought to understand the variation in atherothrombotic propensity as a function of apoptotic circulating endothelial cells (CECs) in OSA by investigating the relationship between CEC apoptosis and plasma levels of hemostatic factors tissue factor (TF) and von Willebrand Factor (vWF) in apneic subjects. Apoptotic CECs were detected by flow cytometry in 35 male subjects free of cardiovascular diseases (AHI range 8-43) and 12 healthy male controls (AHI range 2-5) before and after 8 weeks of nasal continuous positive airway pressure (nCPAP). Quantitative determination of TF and vWF was performed using an enzyme-linked immunosorbent assay (ELISA) kit. The mean levels of TF (66.78 +/- 41.59 pg/ml) and vWF (189.70 +/- 69.24 IU/dl) were significantly higher in OSA patients compared with those in healthy subjects (42.83 +/- 14.18 pg/ml; and 124.48 +/- 31.43 IU/dl). Apoptotic CECs were elevated in patients with OSA and correlated strongly with TF and vWF levels (p = 0.02 and p < 0.001; respectively). There were no correlations between TF, vWF and apnea hypopnea index, or arousal index. Only the percentage of time spent <90% oxygen saturation was inversely associated with TF (r = 0.38; p = 0.02). Following nCPAP therapy, there was significant decrease in TF levels that correlated with decrease in apoptotic CECs. In patients with OSA, increased prothrombotic factors are strongly determined by apoptotic CECs. Treatment with nCPAP may alleviate the coagulation propensity.

Short-term or long-term treatments with a phosphodiesterase-4 (PDE4) inhibitor result in opposing agonist-induced Ca(2+) responses in endothelial cells

BACKGROUND AND PURPOSE

We previously reported that agonist-induced rises in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) in human umbilical vein endothelial cells (HUVEC) were inhibited after a short-term (2 min) pre-treatment with cAMP-elevating agents. The aim of this work was to study the effects of longer term (8 h) pre-treatment with dibutyryl-cAMP (db-cAMP) or rolipram, a specific inhibitor of phosphodiesterase-4 (PDE4), on [Ca(2+)](i), cAMP levels and PDE activity and expression in HUVEC.

EXPERIMENTAL APPROACH

[Ca(2+)](i) changes were measured in isolated HUVEC by Fura-2 imaging. Intracellular cAMP levels and PDE4 activity were assessed by enzyme-immunoassay and radio-enzymatic assay, respectively. PDE expression was measured by northern and western blot analysis.

KEY RESULTS

Long-term pre-treatment of HUVEC with rolipram or db-cAMP significantly increased ATP-, histamine- and thrombin-induced [Ca(2+)](i) rises. Short-term pre-treatment with rolipram was associated with an increase in cAMP, whereas long-term pre-treatment was associated with a decrease in cAMP. Long-term pre-treatment with rolipram or db-cAMP induced a significant increase in PDE4 activity and the expression of 74 kDa-PDE4A and 73 kDa-PDE4B was specifically enhanced. All these effects were suppressed by cycloheximide.

CONCLUSIONS AND IMPLICATIONS

Our data suggest that sustained inhibition of PDE4 by rolipram induced an increase in PDE4 activity, possibly as a compensatory mechanism to accelerate cAMP degradation and that PDE4A and PDE4B were implicated in the regulation of [Ca(2+)](i). Thus, isozyme-specific PDE4 inhibitors might be useful as therapeutic agents in diseases where [Ca(2+)](i) handling is altered, such as atherosclerosis, hypertension and tolerance to beta-adrenoceptor agonists.

Adenosine receptor-mediated adhesion of endothelial progenitors to cardiac microvascular endothelial cells

Intracoronary delivery of endothelial progenitor cells (EPCs) is an emerging concept for the treatment of cardiovascular disease. Enhancement of EPC adhesion to vascular endothelium could improve cell retention within targeted organs. Because extracellular adenosine is elevated at sites of ischemia and stimulates neovascularization, we examined the potential role of adenosine in augmenting EPC retention to cardiac microvascular endothelium. Stimulation of adenosine receptors in murine embryonic EPCs (eEPCs) and cardiac endothelial cells (cECs) rapidly, within minutes, increased eEPC adhesion to cECs under static and flow conditions. Similarly, adhesion of human adult culture-expanded EPCs to human cECs was increased by stimulation of adenosine receptors. Furthermore, adenosine increased eEPC retention in isolated mouse hearts perfused with eEPCs. We determined that eEPCs and cECs preferentially express functional A1 and A2B adenosine receptor subtypes, respectively, and that both subtypes are involved in the regulation of eEPC adhesion to cECs. We documented that the interaction between P-selectin and its ligand (P-selectin glycoprotein ligand-1) plays a role in adenosine-dependent eEPC adhesion to cECs and that stimulation of adenosine receptors in cECs induces rapid cell surface expression of P-selectin. Our results suggest a role for adenosine in vasculogenesis and its potential use to stimulate engraftment in cell-based therapies.

Rate, extent and concentration dependence of histamine-evoked Weibel-Palade body exocytosis determined from individual fusion events in human endothelial cells

The rate, concentration dependence and extent of histamine-evoked Weibel-Palade body (WPB) exocytosis were investigated with time-resolved fluorescence microscopy in cultured human umbilical vein endothelial cells expressing WPB-targeted chimeras of enhanced green fluorescent protein (EGFP). Exocytosis of single WPBs was characterized by an increase in EGFP fluorescence, morphological changes and release of WPB contents. The fluorescence increase was due to a rise of intra-WPB pH from resting levels, estimated as pH 5.45+/-0.26 (s.d., n=144), to pH 7.40. It coincided with uptake of extracellular Alexa-647, indicating the formation of a fusion pore, prior to loss of fluorescent contents. Delays between the increase in intracellular free calcium ion concentration evoked by histamine and the first fusion event were 10.0+/-4.42 s (n=9 cells) at 0.3 microM histamine and 1.57+/-0.21 s (n=15 cells) at 100 microM histamine, indicating the existence of a slow process or processes in histamine-evoked WPB exocytosis. The maximum rates of exocytosis were 1.20+/-0.16 WPB s(-1) (n=9) at 0.3 microM and 3.66+/-0.45 WPB s(-1) at 100 microM histamine (n=15). These occurred 2-5 s after histamine addition and declined to lower rates with continued stimulation. The initial delays and maximal rate of exocytosis were unaffected by removal of external Ca2+ indicating that the initial burst of secretion is driven by Ca2+ release from internal stores, but sustained exocytosis required external Ca2+. Data were compared to exocytosis evoked by a maximal concentration of the strong secretagogue ionomycin (1 microM), for which there was a delay between calcium elevation and secretion of 1.67+/-0.24 s (n=6), and a peak fusion rate of approximately 10 WPB s(-1).

Delay of acute intracellular pH recovery after acidosis decreases endothelial cell activation

Reperfusion after ischemic conditions induces massive endothelial cell (EC) activation, an initial step of reperfusion injury. Reperfusion is characterized by reoxygenation, realkalinization and a localized increase of inflammatory stimuli. In this study, we focused on the influence of extracellular realkalinization on human umbilical vein endothelial cell (HUVEC) activation. We examined intracellular pH (pH(in)) and intracellular free calcium concentration ([Ca(2+)](in)), a second messenger known to mediate von Willebrand factor (VWF) exocytosis in endothelium, upon realkalinization. Furthermore, we measured the agonist-stimulated exocytosis of VWF, Interleukin-8 and soluble P-selectin (sP-Selectin) as markers of EC activation. To verify a morphological correlate of EC activation, we finally observed platelet-endothelial adherence during realkalinization using shear flow. Realkalinization of HUVEC was simulated by switching from bicarbonate buffered Ringer solution of an acidotic pH(ex) of 6.4 to a physiologic pH(ex) of 7.4. Extracellular realkalinization was accompanied by pH(in) recovery from 6.5 to 7.2 within 10 min. Application of cariporide, an inhibitor of the Na(+)/H(+) exchanger subtype 1 (NHE), during extracellular realkalinization significantly delayed the early kinetics of intracellular realkalinization. Histamine stimulated [Ca(2+)](in) was significantly increased upon realkalinization compared to control cells. Also agonist-stimulated release of VWF, Interleukin-8 and sP-Selectin was massively enhanced during pH(in) recovery in comparison to control. Furthermore, we observed an increased platelet binding to endothelium. Interestingly, each of these realkalinization-induced effects were significantly reduced by early application of cariporide. Therefore, delay of acute NHE-dependent pH(in) recovery may represent a promising mechanism for inhibition of EC activation upon reperfusion.

Dopamine modulates von Willebrand factor secretion in endothelial cells via D2-D4 receptors

OBJECTIVE

von Willebrand factor (VWF) is acutely released from endothelial cells in response to numerous calcium-raising agents (e.g. thrombin, histamine) and cAMP-raising agents (e.g. epinephrine, adenosine, vasopressin). In contrast, very few inhibitors of endothelial VWF secretion have been described. The neurotransmitter dopamine is a modulator of exocytosis in several endocrine cells, and is possibly involved in the regulation of several endothelial cell functions. We therefore investigated the effect of dopamine on endothelial VWF secretion.

RESULTS

Dopamine, D2/D3- and D4-specific agonists inhibited histamine- but not thrombin-induced VWF secretion. Expression of dopamine D2, D3 and D4 receptors was demonstrated by reverse transcription polymerase chain reaction (RT-PCR) in both human aortic (HAEC) and umbilical vein (HUVEC) endothelial cells. D2-D4 agonists did not inhibit histamine-induced rise in [Ca(2+)](i): they inhibited histamine-induced secretion even in the absence of extracellular calcium. Thus, the dopamine effects are not mediated by [Ca(2+)](i)-dependent signalling. D2/D3- and D4-specific agonists inhibited neither the rise in cAMP nor VWF secretion in response to epinephrine and adenosine, arguing against an effect on cAMP-mediated signalling. D1 and D5 receptors were not detected in HAEC or HUVEC by RT-PCR, and the D1/D5-specific agonist SKF 38 393 failed to modulate VWF secretion, arguing against a role for these receptors in endothelial exocytosis.

CONCLUSIONS

Dopamine inhibits histamine-induced endothelial exocytosis by activating D2-D4 receptor, via a mechanism distinct from [Ca(2+)](i)-or cAMP-mediated signaling. In contrast, D1 and D5 receptors are not functionally expressed in cultured endothelial cells. Dopamine agonists may be useful as inhibitors of endothelial activation in inflammation and cardiovascular disease.

The C242T CYBA polymorphism of NADPH oxidase is associated with essential hypertension

OBJECTIVE

Oxidative stress is implicated in hypertension. The reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidases are the main source of superoxide in phagocytic and vascular cells. The C242T polymorphism of CYBA, the human gene that encodes p22phox, has been found to be functionally associated with vascular NADPH oxidase activity in atherosclerotic patients. We investigated the association of the C242T polymorphism with hypertension and its potential impact on NADPH oxidase activity. We also analysed the interaction of C242T polymorphism with the -930A/G CYBA variant.

DESIGN

Case-control study in a random sample of 623 subjects (326 hypertensive patients and 297 normotensive controls) from the general population.

METHODS

CYBA polymorphisms were determined by restriction fragment length polymorphism (RFLP) or allelic discrimination. NADPH oxidase activity and p22phox expression were quantified in phagocytic cells by chemiluminescence and by northern and western blots, respectively.

RESULTS

The prevalence of the CC genotype and the C allele frequency were significantly higher (P < 0.05) in hypertensives than in normotensives. CC genotype remained associated with hypertension after adjusting for potential confounders in a logistic regression analysis. Increased phagocytic NADPH oxidase activity was observed in CC hypertensives compared with CT and TT hypertensives (P < 0.05). Enhanced plasma levels of von Willebrand factor were found in CC hypertensives compared with TT hypertensives (P < 0.05). The C242T polymorphism was not in linkage disequilibrium with the -930A/G CYBA promoter variation, which also associates with hypertension.

CONCLUSION

The C242T CYBA polymorphism is associated with essential hypertension. Furthermore, hypertensives carrying the CC genotype of this polymorphism exhibit features of NADPH oxidase-mediated oxidative stress and endothelial damage.

Reconstitution of CD39 in liposomes amplifies nucleoside triphosphate diphosphohydrolase activity and restores thromboregulatory properties

BACKGROUND

CD39 (nucleoside triphosphate diphosphohydrolase [NTPDase-1]) expressed on the luminal surface of endothelial cells rapidly metabolizes extracellular adenosine triphosphate (ATP) and adenosine diphosphate (ADP) to adenosine monophosphate (AMP), and abrogates platelet reactivity. Optimization of CD39 enzymatic activity appears dependent upon the expression of both transmembrane domains within plasma membranes. Thus, motivation exists to examine therapeutic antiplatelet formulations that consist of liposomal CD39.

METHODS

Full-length human CD39 was produced by using a yeast expression system, purified, and reconstituted within lipid vesicles. The catalytic efficiency (kcat/Km) of CD39-mediated phosphohydrolysis of ADP and ATP was determined both for detergent-solubilized and protein-reconstituted CD39 within lipid membranes. The capacity of CD39-containing lipid vesicles to inhibit platelet activation induced by ADP, collagen, or thrombin was determined in vitro by platelet aggregometry. A murine model of thromboplastin-induced thromboembolism was used to determine the effectiveness of intravenous liposomal CD39 in limiting platelet consumption and mortality.

RESULTS

Reconstitution of human CD39 in lipid vesicles was associated with a decrease in Km of nearly an order of magnitude over the detergent-solubilized form. There was a concomitant increase in both ADPase and ATPase catalytic efficiencies (kcat/Km ADPase: sol CD39: 2.7 x 10(6) vs liposomal CD39: 1.4 x 10(7) min/ M; kcat/Km ATPase: sol CD39: 7.2 x 10(6) vs liposomal CD39: 2.0 x10(7) min/M). Furthermore, CD39 lipid vesicles effectively inhibited platelet aggregation when activated by ADP, collagen, or thrombin, and also promoted platelet disaggregation (60.4% +/- 6.1%). Treatment with CD39 lipid vesicles preserved platelet counts after thromboplastin injection (pretreatment, 906.8 +/- 42.9 platelets/microm3; empty vesicles, 278.6 +/- 34.8 platelets/microm3; CD39 vesicles, 563.6 +/- 42.2 platelets/microm3; n = 10 mice/test group; P < .0001). In parallel survival studies, liposomal CD39 reduced mortality from 73% to 33% (P < or = .05; n = 12 mice/experimental test group, n = 15 mice/control test group).

CONCLUSIONS

Incorporation of solubilized CD39 into a lipid bilayer restores enzyme activity and optimizes thromboregulatory potential. Treatment with CD39 in liposomal formulations decreased mortality in a murine model of thromboplastin-induced thromboembolism by limiting intravascular platelet aggregation and thrombosis.

Dynamics and plasticity of Weibel-Palade bodies in endothelial cells

Agonist-induced release of endothelial cell specific storage granules, designated Weibel-Palade bodies (WPBs), provides the endothelium with the ability to rapidly respond to changes in its micro-environment. Originally being defined as an intracellular storage pool for von Willebrand factor (VWF), it has recently been shown that an increasing number of other components, including P-selectin, interleukin (IL)-8, eotaxin-3, endothelin-1, and angiopoietin-2, is present within this subcellular organelle, implicating a role for WPB exocytosis in inflammation, hemostasis, regulation of vascular tone and angiogenesis. Recent studies emphasize that WPBs provide a dynamic storage compartment whose contents can be regulated depending on the presence of inflammatory mediators in the vascular micro-environment. Additionally, release of WPBs is tightly regulated and feedback mechanisms have been identified that prevent excessive release of bioactive components from this subcellular organelle. The ability to regulate both contents and exocytosis of WPBs endows these endothelial cell specific organelles with a remarkable plasticity. This is most likely needed to allow for controlled delivery of bioactive components into the circulation on vascular perturbation.

Differential regulation of endothelial exocytosis of P-selectin and von Willebrand factor by protease-activated receptors and cAMP

Thrombin-mediated endothelial-cell release of von Willebrand factor (VWF) and P-selectin functionally links protease-activated receptors (PARs) to thrombosis and inflammation. VWF release can be stimulated by both Ca2+ and cAMP, and, although both VWF and P-selectin are found in Weibel-Palade bodies (WPBs), we found that their release could be differentially regulated. In these studies, human umbilical vein endothelial cells stimulated with cAMP or PAR2-AP led to a delayed release of VWF and significantly less P-selectin release compared with histamine, thrombin, or PAR1-AP. Dose-response studies revealed that PAR2-AP was significantly less efficacious in promoting the release of P-selectin compared with VWF. PAR2-AP-induced robust stimulation of intracellular Ca2+ coupled with a significantly greater inhibitory effect of calcium chelation on release of VWF compared with cell-surface expression of P-selectin, suggests an additional Ca2+-independent pathway involved in release of P-selectin. PAR2-AP failed to increase global cAMP levels; however, inhibition of protein kinase A led to a significant attenuation of PAR2-AP-mediated release of VWF. Confocal microscopy studies revealed that PAR2 and forskolin caused preferential release of a population of Weibel-Palade bodies (WPBs) consisting of only VWF. Thus, WPBs are pharmacologically and morphologically heterogeneous, and distinct granule populations are susceptible to differential regulation.

Regulation of Weibel–Palade Body Exocytosis

Weibel-Palade bodies (WPBs) are endothelial granules that store von Willebrand factor (VWF), P-selectin, and other vascular modulators. Endothelial cells secrete WPBs in response to vascular injury, releasing VWF, which triggers platelet rolling, and externalizing P-selectin, which activates leukocyte trafficking. Endothelial exocytosis is one of the earliest responses to vascular damage and plays a pivotal role in thrombosis and inflammation. This review examines the regulation of WPB exocytosis-the exocytic machinery, activators, and inhibitors of exocytosis-and speculates about the development of novel anti-exocytic drugs.

Characterization of a Novel Chemokine-Containing Storage Granule in Endothelial Cells: Evidence for Preferential Exocytosis Mediated by Protein Kinase A and Diacylglycerol

We have recently shown that several proinflammatory chemokines can be stored in secretory granules of endothelial cells (ECs). Subsequent regulated exocytosis of such chemokines may then enable rapid recruitment of leukocytes to inflammatory sites. Although IL-8/CXCL8 and eotaxin-3/CCL26 are sorted to the rod-shaped Weibel-Palade body (WPB), we found that GROalpha/CXCL1 and MCP-1/CCL2 reside in small granules that, similarly to the WPB, respond to secretagogue stimuli. In the present study, we report that GROalpha and MCP-1 colocalized in 50- to 100-nm granules, which occur throughout the cytoplasm and at the cell cortex. Immunofluorescence confocal microscopy revealed no colocalization with multimerin or tissue plasminogen activator, i.e., proteins that are released from small granules of ECs by regulated exocytosis. Moreover, the GROalpha/MCP-1-containing granules were Rab27-negative, contrasting the Rab27-positive, WPB. The secretagogues PMA, histamine, and forskolin triggered distinct dose and time-dependent responses of GROalpha release. Furthermore, GROalpha release was more sensitive than IL-8 release to inhibitors and activators of PKA and PKC but not to an activator of Epac, a cAMP-regulated GTPase exchange factor, indicating that GROalpha release is regulated by molecular adaptors different from those regulating exocytosis of the WPB. On the basis of these findings, we designated the GROalpha/MCP-1-containing compartment the type 2 granule of regulated secretion in ECs, considering the WPB the type 1 compartment. In conclusion, we propose that the GROalpha/MCP-1-containing type 2 granule shows preferential responsiveness to important mediators of EC activation, pointing to the existence of selective agonists that would allow differential release of selected chemokines.

Rab3D and annexin A2 play a role in regulated secretion of vWF, but not tPA, from endothelial cells

von-Willebrand factor (vWF) and tissue-type plasminogen activator (tPA) are products of endothelial cells acutely released into the vasculature following cell activation. Both factors are secreted after intraendothelial Ca2+ mobilization, but exhibit opposing physiological effects with vWF inducing coagulation and tPA triggering fibrinolysis. To identify components that could regulate differentially the release of pro- and antithrombogenic factors, we analyzed the contribution of Rab3D and the annexin A2/S100A10 complex, proteins implicated in exocytotic events in other systems. We show that mutant Rab3D proteins interfere with the formation of bona fide Weibel-Palade bodies (WPbs), the principal storage granules of multimeric vWF, and consequently the acute, histamine-induced release of vWF. In contrast, neither appearance nor exocytosis of tPA storage granules is affected. siRNA-mediated downregulation of annexin A2/S100A10 and disruption of the complex by microinjection of peptide competitors result in a marked reduction in vWF but not tPA secretion, without affecting the appearance of WPbs. This indicates that distinct mechanisms underlie the acute secretion of vWF and tPA, enabling endothelial cells to fine-regulate the release of thrombogenic and fibrinolytic factors.

Small GTP-binding protein Ral is involved in cAMP-mediated release of von Willebrand factor from endothelial cells

OBJECTIVE

von Willebrand factor (vWF) is synthesized by endothelial cells and stored in specialized vesicles called Weibel-Palade bodies (WPBs). Recently, we have shown that the small GTP-binding protein Ral is involved in thrombin-induced exocytosis of WPBs. In addition to Ca2+-elevating secretagogues such as histamine and thrombin, release of WPB is also observed after administration of cAMP-raising substances such as epinephrine and vasopressin. In the present study, we investigated whether Ral is also involved in cAMP-mediated vWF release.

METHODS AND RESULTS

Activation of Ral was observed 15 to 20 minutes after stimulation of endothelial cells with epinephrine, forskolin, or dibutyryl-cAMP. A cell-permeable peptide comprising the carboxy-terminal part of the Ral protein reduced both thrombin-induced and epinephrine-induced vWF secretion supporting a crucial role for Ral in this process. Furthermore, inhibition of protein kinase A by H-89 resulted in a marked reduction of vWF release and greatly diminished levels of GTP-Ral on stimulation with epinephrine. Activation of Ral was independent of the activation of Epac, a cAMP-regulated exchange factor for the small GTPases Rap1 and Rap2.

CONCLUSIONS

These results suggest that protein kinase A-dependent activation of Ral regulates cAMP-mediated exocytosis of WPB in endothelial cells.

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Desmopressin (DDAVP) induces NO production in human endothelial cells via V2 receptor- and cAMP-mediated signaling

The hemostatic agent desmopressin (DDAVP) also has strong vasodilatory effects. DDAVP is a selective agonist for the vasopressin V2 receptor (V2R), which is coupled to cAMP-dependent signaling. DDAVP-induced vasodilation may be due to endothelial NO synthase (eNOS) activation. This hypothesis implies cAMP-mediated eNOS activation. It also implies wide extrarenal, endothelial V2R expression. We show that in human umbilical vein endothelial cells (HUVECs) the cAMP-raising agents forskolin and epinephrine increase NO production, as measured by a l-NMMA-inhibitable rise in cellular cGMP content. They also increase eNOS enzymatic activity, in a partly calcium-independent manner. cAMP-mediated eNOS activation is associated with phosphorylation of residue Ser1177, in a phosphatidyl inositol 3-kinase (PI3K)-independent manner. HUVECs do not express V2R. However, after heterologous V2R expression, DDAVP induces cAMP-dependent eNOS activation via Ser1177 phosphorylation. We have previously found V2R expression in cultured lung endothelial cells. By real time quantitative RT-PCR, we now find a wide V2R distribution notably in heart, lung and skeletal muscle. These results indicate that DDAVP and other cAMP-raising agents can activate eNOS via PI3K-independent Ser1177 phosphorylation in human endothelial cells. This mechanism most likely accounts for DDAVP-induced vasodilation.

Cellular mechanisms of the hemostatic effects of desmopressin (DDAVP)

The synthetic analog of vasopressin desmopressin (DDAVP) is widely used for the treatment of patients with von Willebrand disease (VWD), hemophilia A, several platelet disorders, and uremic bleeding. DDAVP induces an increase in plasma levels of von Willebrand factor (VWF), coagulation factor VIII (FVIII), and tissue plasminogen activator (t-PA). It also has a vasodilatory action. In spite of its extensive clinical use, its cellular mechanism of action remains incompletely understood. Its effect on VWF and t-PA as well as its vasodilatory effect are likely explained by a direct action on the endothelium, via activation of endothelial vasopressin V2R receptor and cAMP-mediated signaling. This leads to exocytosis from Weibel Palade bodies where both VWF and t-PA are stored, as well as to nitric oxide (NO) production via activation of endothelial NO synthase. The mechanism of action of DDAVP on FVIII plasma levels remains to be elucidated. The hemostatic effect of DDAVP likely involves additional cellular effects that remain to be discovered.

Ca2+ -regulated secretion of tissue-type plasminogen activator and von Willebrand factor in human endothelial cells

von Willebrand factor (vWF) and tissue-type plasminogen activator (tPA) are products of endothelial cells which are secreted into the bloodstream upon a stimulus-induced rise in intracellular Ca(2+). Although the release of both factors appears to be regulated similarly, they exhibit opposing physiological effects in the vasculature with vWF inducing coagulation and platelet aggregation and tPA triggering fibrinolysis and thrombolysis. To analyze possible differences in the regulated secretion of vWF and tPA in more detail, we recorded the Ca(2+)-triggered exocytosis of both factors in cultured human endothelial cells. We demonstrate that vWF and tPA which are stored in different granules within endothelial cells are released with different kinetics following endothelial stimulation with histamine or the Ca(2+) ionophore A23187. While the stimulus-induced release of vWF increases with time over a course of 30 min, maximal acute secretion of tPA is observed 5 min following stimulation and subsequently drops to background levels. In the case of vWF, secretion can also be monitored indirectly through an antibody-reinternalization assay which indicates an incomplete release of vWF during single exocytotic fusion events. Our data thus point to differences in the Ca(2+)-triggered secretion of vWF and tPA which could allow a fine-tuning of their release thereby ensuring a balanced physiological action.

P2 receptors: new potential players in atherosclerosis

Atherosclerosis is a focal inflammatory disease of the arterial wall. It starts with the formation of fatty streaks on the arterial wall that evolve to form a raised plaque made of smooth muscle cells (SMCs), and infiltrating leukocytes surrounding a necrotic core. The pathogenesis of the atherosclerotic lesion is incompletely understood, but it is clear that a dysfunction of the endothelium, recruitment and activation of inflammatory cells and SMC proliferation have a pivotal role. Over recent years receptors for extracellular nucleotides, the P2 receptors, have been recognized as fundamental modulators of leukocytes, platelets, SMCs and endothelial cells. P2 receptors mediate chemotaxis, cytokine secretion, NO generation, platelet aggregation and cell proliferation in response to accumulation of nucleotides into the extracellular milieu. Clinical trials have shown the benefit of antagonists of the ADP platelet receptor(s) in the prevention of vascular accidents in patients with atherosclerosis. Therefore, we anticipate that a deeper understanding of the involvement of P2 receptors in atheroma formation will open new avenues for drug design and therapeutic intervention.

ATP induces intracellular calcium increases and actin cytoskeleton disaggregation via P2x receptors

The consequences of purinoceptor activation on calcium signalling, inositol phosphate metabolism, protein secretion and the actin cytoskeleton were demonstrated in the WRK-1 cell line. Extracellular ATP was used as a secretagogue to induce a rise in intracellular Ca(2+) concentration ([Ca(2+)](i)), acting via P2x purinergic receptors, which causes actin skeleton disaggregation and protein secretion. ATP bound specifically to purinergic receptors, with Ki of 0.8 microM. The magnitude order for binding of different nucleotides was alpha beta-Met-ATP >or= dATPalphaS > ATP >or= ADP > UTP > AMP > suramin. No increase in inositol phosphates (IPs) was observed after ATP application suggesting that the purinergic sites in WRK-1 cells are not of a P2y type. ATP (1-100 microM) caused a concentration-dependent increase in [Ca(2+)](i)(EC(50)= 30 microM). The responses were reproducible without any desensitization over several applications. The response to ATP was abolished when extracellular calcium ([Ca(2+)](e)) was reduced to 100 nM. A non-specific purinergic antagonist, suramin, reversibly inhibited the ATP-response suggesting that ATP is able to bind to P2x purinergic sites to trigger Ca(2+) entry and increase of [Ca(2+)](i). ATP induced a concentration-dependent disaggregation of actin and exocytotic release of proteins both, which were dependent upon [Ca(2+)](e). Similarly, alpha,beta-Met-ATP, a potent P2x agonist also stimulated Ca(2+) mobilization, actin network destructuration, and protein release. In the isolated rat neurohypophysial nerve terminals, ATP was shown to act as a physiological stimulus for vasopressin release via Ca(2+) entry through a P2x receptor [6]. Here, we show that in these nerve terminals, ATP is also able to induce actin disaggregation by a Ca(2+) dependent mechanism. Thus, actin cytoskeleton alterations induced by ATP through activation of P2x receptors could be a prelude to exocytosis.

Nucleotide receptors: an emerging family of regulatory molecules in blood cells

Nucleotides are emerging as an ubiquitous family of extracellular signaling molecules. It has been known for many years that adenosine diphosphate is a potent platelet aggregating factor, but it is now clear that virtually every circulating cell is responsive to nucleotides. Effects as different as proliferation or differentiation, chemotaxis, release of cytokines or lysosomal constituents, and generation of reactive oxygen or nitrogen species are elicited upon stimulation of blood cells with extracellular adenosine triphosphate (ATP). These effects are mediated through a specific class of plasma membrane receptors called purinergic P2 receptors that, according to the molecular structure, are further subdivided into 2 subfamilies: P2Y and P2X. ATP and possibly other nucleotides are released from damaged cells or secreted via nonlytic mechanisms. Thus, during inflammation or vascular damage, nucleotides may provide an important mechanism involved in the activation of leukocytes and platelets. However, the cell physiology of these receptors is still at its dawn, and the precise function of the multiple P2X and P2Y receptor subtypes remains to be understood.

Activation of purinergic receptors by ATP inhibits secretion in bovine adrenal chromaffin cells

Autoinhibition is a common mechanism observed in neurons to regulate neurotransmission. Released neurotransmitter interacts with presynaptic autoreceptors to inhibit subsequent release. The requisite elements for autoinhibition are present in chromaffin cells: secretory granules contain millimolar levels of ATP which is coreleased with catecholamines upon stimulation and the cells express purinergic receptors. We were interested to determine whether autoinhibition produced by ATP binding to purinergic receptors plays an important role in catecholamine release from chromaffin cells. In these studies, short depolarizations were used to elicit transmitter release measured by membrane capacitance. We find that stimulation of chromaffin cells results in the release of endogenous ATP which may suppress Ca(2+) channel currents and secretion. In the presence of a maximal concentration of ATP, both the amount of secretion and the maximal rate of release are about half that observed in the absence of ATP. ATP-mediated inhibition of secretion was blocked by Reactive Blue-2 suggesting the involvement of P(2Y) purinergic receptors. Prepulses to positive potentials that relieve the Ca(2+) channel block largely relieve the inhibition of secretion. Furthermore, when secretion is plotted as a function of Ca(2+) influx there is no apparent change in the relationship between control cells and those stimulated in the presence of ATP and prepulses. These results suggest that ATP diminishes secretion by inhibiting Ca(2+) influx into the cells. Our results indicate that feedback inhibition by ATP, mediated primarily by Ca(2+) channels, may be an important regulator of catecholamine release in chromaffin cells.

Genetic induction of a releasable pool of factor VIII in human endothelial cells

Although it is known that factor VIII (FVIII) plasma levels increase rapidly in response to a number of stimuli, the biological stimuli behind this release is less clear. Previously, we showed that FVIII can traffic together with von Willebrand factor (vWF) into storage granules in a pituitary tumor cell line, AtT-20; however, AtT-20 cells could not be used to address the release or functional activity of released FVIII. To investigate the regulated secretion of stored FVIII, endothelial cells with intact agonist-stimulated release pathways were used. Human umbilical vein endothelial cells (HUVECs) were transduced with retroviral FVIII construct [hFVIII(V)] to create a FVIII/vWF storage pool. Immunofluorescent staining of transduced cells demonstrated FVIII in Weibel-Palade bodies. In contrast, the transduction of hFVIII(V) into HT-1080 and HepG2 cells displayed FVIII only in the cytoplasm. We studied the regulated release of both FVIII and vWF from endothelial cells after agonist-induced stimulation and demonstrated a parallel release of FVIII and vWF proteins. This released FVIII was functionally active. Hence, endothelial cells transduced with hFVIII(V) store FVIII together with vWF in Weibel-Palade bodies, creating a releasable storage pool of both proteins. Because FVIII secretion can be physiologically regulated by agonists in culture, this may explain the pharmacological agonist-induced release of FVIII by drugs such as desmopressin in vivo and suggests vascular endothelium as a reasonable target of gene therapy of hemophilia A.

Inhibition of type 4 phosphodiesterase by rolipram and Ginkgo biloba extract (EGb 761) decreases agonist-induced rises in internal calcium in human endothelial cells

The effects of Gingko biloba extract EGb 761 on 5 isolated, vascular, cyclic nucleotide phosphodiesterase (PDE) isoforms were evaluated. EGb 761 preferentially inhibited PDE4 (IC(50)=25.1 mg/L), the isoform that is mainly present in endothelial cells, in a competitive manner (K:(i)=12.5 mg/L). Because changes in cyclic nucleotide levels may affect intracellular calcium ([Ca(2+)](i)) levels in endothelial cells, we examined the effects of EGb 761 on both resting [Ca(2+)](i) levels and agonist-induced rises in [Ca(2+)](i) in single human umbilical vein endothelial cells (HUVECs) in culture. The effects of EGb 761 were compared with those of rolipram, a selective PDE4 inhibitor that increases cellular cAMP levels, and the cAMP analogue dibutyryl cAMP (db-cAMP). EGb 761 (20 and 100 mg/L), rolipram (50 micromol/L), and db-cAMP (100 micromol/L) significantly inhibited histamine-, ATP-, and thrombin-induced [Ca(2+)](i) increases in HUVECs without modifying resting [Ca(2+)](i) levels. Similar results were obtained by using a Ca(2+)-free bath solution. EGb 761 (100 mg/L), but not rolipram (50 micromol/L) or db-cAMP (100 micromol/L), also inhibited Ca(2+) influx into cells having thapsigargin-depleted internal Ca(2+) stores and bathed in a Ca(2+)-free external solution. Our results are consistent with an inhibition of PDE activity that causes a reduction of agonist-induced increases in [Ca(2+)](i) in HUVECs, mainly by inhibition of Ca(2+) mobilization from internal stores. It thus may be that the cardiovascular effects of EGb 761 involve inhibition of PDE4 activity and subsequent modification of Ca(2+) signaling in endothelial cells.

Evaluation of plasma von Willebrand factor as a biomarker for acute arterial damage in rats

Plasma von Willebrand factor (vWF) was evaluated as a potential biomarker of acute arterial damage in rats after a vasotoxic dose of the dopaminergic vasodilator, fenoldopam (FP). Male Sprague-Dawley rats were given FP or isotonic saline by subcutaneous injection, and plasma vWF was measured at 2, 6, and 24 hours after challenge. Mean plasma vWF values increased in FP-treated rats compared to controls at 2 hours (167 vs 122%; p < 0.05) and 6 hours postdose (172 vs 130%; p < 0.01) but were comparable to control values after 24 hours. Mesenteric arterial lesions were observed microscopically in all FP-treated rats 24 hours postdose but were not present in rats at 1, 2, 4, 6, or 8 hours after FP challenge. Further, plasma vWF concentrations increased in saline-treated rats after only the minimal perturbation of repeated venipuncture. These results indicate an early, minimal, and transient release of vWF that precedes the onset of morphologically evident vascular damage. The minimal increases in plasma vWF concentrations were of limited predictive value, may be more reflective of an acute-phase reactant response, and were not considered a reliable biomarker of acute FP-induced arterial damage in the rat.

The mechanisms of aquaporin control in the renal collecting duct

The antidiuretic hormone arginine-vasopressin (AVP) regulates water reabsorption in renal collecting duct principal cells. Central to its antidiuretic action in mammals is the exocytotic insertion of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the apical membrane of principal cells, an event initiated by an increase in cAMP and activation of protein kinase A. Water is then reabsorbed from the hypotonic urine of the collecting duct. The water channels aquaporin-3 (AQP3) and aquaporin-4 (AQP4), which are constitutively present in the basolateral membrane, allow the exit of water from the cell into the hypertonic interstitium. Withdrawal of the hormone leads to endocytotic retrieval of AQP2 from the cell membrane. The hormone-induced rapid redistribution between the interior of the cell and the cell membrane establishes the basis for the short term regulation of water permeability. In addition water channels (AQP2 and 3) of principal cells are regulated at the level of expression (long term regulation). This review summarizes the current knowledge on the molecular mechanisms underlying the short and long term regulation of water channels in principal cells. In the first part special emphasis is placed on the proteins involved in short term regulation of AQP2 (SNARE proteins, Rab proteins, cytoskeletal proteins, G proteins, protein kinase A anchoring proteins and endocytotic proteins). In the second part, physiological and pathophysiological stimuli determining the long term regulation are discussed.

Cyclic nucleotide analogs as biochemical tools and prospective drugs

Cyclic AMP (cAMP) and cyclic GMP (cGMP) are key second messengers involved in a multitude of cellular events. From the wealth of synthetic analogs of cAMP and cGMP, only a few have been explored with regard to their therapeutic potential. Some of the first-generation cyclic nucleotide analogs were promising enough to be tested as drugs, for instance N(6),O(2)'-dibutyryl-cAMP and 8-chloro-cAMP (currently in clinical Phase II trials as an anticancer agent). Moreover, 8-bromo and dibutyryl analogs of cAMP and cGMP have become standard tools for investigations of biochemical and physiological signal transduction pathways. The discovery of the Rp-diastereomers of adenosine 3',5'-cyclic monophosphorothioate and guanosine 3',5'-cyclic monophosphorothioate as competitive inhibitors of cAMP- and cGMP-dependent protein kinases, as well as subsequent development of related analogs, has proven very useful for studying the molecular basis of signal transduction. These analogs exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity. Furthermore, better understanding of signaling pathways and ligand/protein interactions has led to new therapeutic strategies. For instance, Rp-8-bromo-adenosine 3',5'-cyclic monophosphorothioate is employed against diseases of the immune system. This review will focus mainly on recent developments in cyclic nucleotide-related biochemical and pharmacological research, but also highlights some historical findings in the field.

Vasopressin-induced von Willebrand factor secretion from endothelial cells involves V2 receptors and cAMP

Vasopressin and its analogue 1-deamino-8-D-arginine vasopressin (DDAVP) are known to raise plasma von Willebrand factor (vWF) levels. DDAVP is used as a hemostatic agent for the treatment of von Willebrand's disease. However, its cellular mechanisms of action have not been elucidated. DDAVP, a specific agonist for the vasopressin V2 receptor (V2R), exerts its antidiuretic effect via a rise in cAMP in kidney collecting ducts. We tested the hypothesis that DDAVP induces vWF secretion by binding to V2R and activating cAMP-mediated signaling in endothelial cells. vWF secretion from human umbilical vein endothelial cells (HUVECs) can be mediated by cAMP, but DDAVP is ineffective, presumably due to the absence of V2R. We report that DDAVP stimulates vWF secretion in a cAMP-dependent manner in HUVECs after transfection of the V2R. In addition, vasopressin and DDAVP induce vWF secretion in human lung microvascular endothelial cells (HMVEC-L). These cells (but not HUVECs) express endogenous V2R, as shown by RT-PCR. Vasopressin-induced vWF secretion is mimicked by DDAVP and inhibited by the selective V2R antagonist SR121463B. It is mediated by cAMP, since it is inhibited by the protein kinase A inhibitor Rp-8CPT-cAMPS. These results indicate that vasopressin induces cAMP-mediated vWF secretion by a direct effect on endothelial cells. They also demonstrate functional expression of V2R in endothelial cells, and provide a cellular mechanism for the hemostatic effects of DDAVP.

The tetraspanin CD63/lamp3 cycles between endocytic and secretory compartments in human endothelial cells

In the present study, we show that in human endothelial cells the tetraspanin CD63/lamp3 distributes predominantly to the internal membranes of multivesicular-multilamellar late endosomes, which contain the unique lipid lysobisphosphatidic acid. Some CD63/lamp3 is also present in Weibel-Palade bodies, the characteristic secretory organelle of these cells. We find that CD63/lamp3 molecules can be transported from late endosomes to Weibel-Palade bodies and thus that CD63/lamp3 cycles between endocytic and biosynthetic compartments; however, movement of CD63/lamp3 is much slower than that of P-selectin, which is known to cycle between plasma membrane and Weibel-Palade bodies. When cells are treated with U18666A, a drug that mimics the Niemann-Pick type C syndrome, both proteins accumulate in late endosomes and fail to reach Weibel-Palade bodies efficiently, suggesting that P-selectin, like CD63/lamp3, cycles via late endosomes. Our data suggest that CD63/lamp3 partitions preferentially within late endosome internal membranes, thus causing its accumulation, and that this mechanism contributes to CD63/lamp3 retention in late endosomes; however, our data also indicate that the protein can eventually escape from these internal membranes and recycle toward Weibel-Palade bodies to be reused. Our observations thus uncover the existence of a selective trafficking route from late endosomes to Weibel-Palade bodies.