Endothelial cell PAF synthesis following thrombin stimulation utilizes Ca(2+)-independent phospholipase A(2)

Cigarette smoking promotes bladder cancer via increased platelet-activating factor

Cigarette smoking is the number one risk factor for bladder cancer development and epidemiological data suggest that nearly half of all bladder cancer patients have a history of smoking. In addition to stimulating the growth of a primary tumor, it has been shown that there is a correlation between smoking and tumor metastasis. Platelet activating factor (PAF) is expressed on the cell surface of the activated endothelium and, through binding with the PAF-receptor (PAF-R), facilitates transendothelial migration of cells in the circulation (McHowat et al. Biochemistry 40:14921-14931; 2001). In this study, we show that the exposure of bladder cancer cells to cigarette smoke extract (CSE) results in increased PAF accumulation and increased expression of the PAF-R. Furthermore, treatment with CSE increases adherence of bladder cancer cells to bladder endothelial cells and could be abrogated by pretreatment with ginkgolide B. Immunohistochemical analysis of tumor biopsy samples from bladder cancer patients who smoked revealed increased PAF and the PAF-R in tumor regions when compared to normal tissue. These data highlight a pathway in bladder cancer that is influenced by CSE which could facilitate primary tumor growth and increase metastatic potential. Targeting of the PAF-PAFR interaction could serve as a beneficial therapeutic target for managing further growth of a developing tumor.

Cigarette smoke-induced urothelial cell damage: potential role of platelet-activating factor

Cigarette smoking is an environmental risk factor associated with a variety of pathologies including cardiovascular disease, inflammation, and cancer development. Interstitial cystitis/bladder pain syndrome (IC/BPS) is a chronic inflammatory bladder disease with multiple etiological contributors and risk factors associated with its development, including cigarette smoking. Previously, we determined that cigarette smoking was associated with bladder wall accumulation of platelet activating factor (PAF), a potent inflammatory mediator that facilitates transendothelial cell migration of inflammatory cells from the circulation. PAF has been shown to reduce expression of tight junctional proteins which could ultimately lead to increased urothelial cell permeability. In this study, we observed that cigarette smoke extract (CSE) treatment of human urothelial cells increases PAF production and PAF receptor expression and reduces wound healing ability. After exposure to cigarette smoke for 6 months, wild-type C57BL/6 mice displayed urothelial thinning and destruction which was not detected in iPLA₂β^-/- (enzyme responsible for PAF production) animals. We also detected increased urinary PAF concentration in IC/BPS patients when compared to controls, with an even greater increase in urinary PAF concentration in smokers with IC/BPS These data indicate that cigarette smoking is associated with urothelial cell damage that may be a result of increased PAF-PAF receptor interaction. Inhibition of iPLA₂β activity or blocking of the PAF-PAF receptor interaction could serve as a potential therapeutic target for managing cigarette smoke-induced bladder damage.

Ischemic Stroke After Wasp Sting

BACKGROUND

Despite the common occurrence of hymenopteran stings worldwide, primary neurologic manifestations including stroke are rare. We report a case of a healthy male who developed a right middle cerebral artery (MCA) territory ischemic stroke after getting stung by a wasp.

CASE REPORT

A 44-year-old man with hypertension presented to the hospital with sudden-onset left hemiparesis, left facial weakness, and dysarthria after being stung by a wasp. Magnetic resonance imaging (MRI) and magnetic resonance angiography (MRA) scans of the brain revealed a right MCA territory infarct and a lack of flow in the distal right internal carotid artery and MCA. He was treated with intravenous tissue plasminogen activator. A computed tomography angiography scan of the brain performed 24 hours later revealed multiple regions of vasoconstriction in the territory of the bilateral MCA. Evaluations for causes of stroke, including echocardiography and telemetry, were not revealing. Immunologic testing showed significantly elevated levels of serum wasp immunoglobulin E. Therapy with aspirin and atorvastatin was started. At discharge, the patient had a mild left facial droop but normal strength in his left arm and leg. WHY SHOULD AN EMERGENCY PHYSICIAN BE AWARE OF THIS?: Emergency physicians encounter large numbers of hymenopteran sting cases each year. These patients typically present with local reactions, such as itching, pain, and erythema. Systemic manifestations, such as anaphylaxis causing severe hypotension and bronchospasm, are less common but deadly. Neurologic complications, such as ischemic stroke, are extremely rare. This manuscript highlights the pathophysiology and management of stroke after a hymenopteran sting. There are no guidelines for the management of stroke after a hymenopteran sting, and therefore we intend to provide some guidance to physicians for treating stroke after a hymenopteran sting.

Roles of specific lipid species in the cell and their molecular mechanism

Thousands of different molecular species of lipids are present within a single cell, being involved in modulating the basic processes of life. The vast number of different lipid species can be organized into a number of different lipid classes, which may be defined as a group of lipids with a common chemical structure, such as the headgroup, apart from the nature of the hydrocarbon chains. Each lipid class has unique biological roles. In some cases, a relatively small change in the headgroup chemical structure can result in a drastic change in function. Such phenomena are well documented, and largely understood in terms of specific interactions with proteins. In contrast, there are observations that the entire structural specificity of a lipid molecule, including the hydrocarbon chains, is required for biological activity through specific interactions with membrane proteins. Understanding of these phenomena represents a fundamental change in our thinking of the functions of lipids in biology. There are an increasing number of diverse examples of roles for specific lipids in cellular processes including: Signal transduction; trafficking; morphological changes; cell division. We are gaining knowledge and understanding of the underlying molecular mechanisms. They are of growing importance in both basic and applied sciences.

Increased susceptibility to bladder inflammation in smokers: targeting the PAF-PAF receptor interaction to manage inflammatory cell recruitment

Chronic bladder inflammation can result in a significant reduction in quality of life. Smoking remains a leading preventable risk factor in many diseases. Despite the large amount of evidence supporting the risks of smoking, roughly 45 million people in the United States remain smokers. The impact of cigarette smoking on inflammation is well established, but how smoking promotes bladder inflammation is currently unknown. The aim of this study was to determine if cigarette smoke exposure impacts inflammatory cell adherence to bladder endothelial cells and if targeting the platelet‐activating factor (PAF)–PAF receptor (PAFR) interaction could be beneficial in managing bladder inflammation. In response to cigarette smoke extract (CSE) incubation, bladder endothelial cells from human or mouse displayed increased PAF accumulation, decreased PAF‐AH activity, and increased inflammatory cell adherence. Inhibition of endothelial cell calcium‐independent phospholipase A₂β (iPLA₂β) with (S)‐BEL, to block PAF production, prevented adherence of inflammatory cells. Pretreatment of inflammatory cells with PAFR antagonists, ginkgolide B or WEB2086 significantly reduced the number of adhered cells to bladder endothelium. Wild‐type mice exposed to cigarette smoke displayed increased presence of inflammatory infiltration which was absent in iPLA₂β^−/− mice and those exposed to room air. In conclusion, cigarette smoke exposure increases endothelial cell PAF accumulation and increased inflammatory cell adherence. Inhibition of PAF accumulation or PAFR antagonism markedly attenuated inflammatory cell adherence to bladder endothelial cells. The results detailed in this study highlight to potential therapeutic targets for managing bladder inflammation.

Cigarette smoke induces cell motility via platelet-activating factor accumulation in breast cancer cells: a potential mechanism for metastatic disease

Most cancer deaths are a result of metastasis rather than the primary tumor. Although cigarette smoking has been determined as a risk factor for several cancers, its role in metastasis has not been studied in detail. We propose that cigarette smoking contributes to metastatic disease via inhibition of breast cancer cell platelet-activating factor acetylhydrolase (PAF-AH), resulting in PAF accumulation and a subsequent increase in cell motility. We studied several breast cell lines, including immortalized mammary epithelial cells (MCF-10A), luminal A hormone positive MCF-7, basal-like triple negative MDA-MB-468, and claudin-low triple-negative highly metastatic MDA-MB-231 breast tumor cells. We exposed cells to cigarette smoke extract (CSE) for up to 48 h. CSE inhibited PAF-AH activity, increased PAF accumulation, and increased cell motility in MDA-MB-231 metastatic triple negative breast cancer cells. The calcium-independent phospholipase A₂ (iPLA₂) inhibitor, (S) bromoenol lactone ((S)-BEL) was used to prevent the accumulation of PAF and further prevented the increase in cell motility seen previously when cells were exposed to CSE. Thus, iPLA₂ or PAF may represent a therapeutic target to manage metastatic disease, particularly in triple-negative breast cancer patients who smoke.

Absence of calcium-independent phospholipase A2 β impairs platelet-activating factor production and inflammatory cell recruitment in Trypanosoma cruzi-infected endothelial cells

Both acute and chronic phases of Trypanosoma cruzi (T. cruzi) infection are characterized by tissue inflammation, mainly in the heart. A key step in the inflammatory process is the transmigration of inflammatory cells across the endothelium to underlying infected tissues. We observed increased arachidonic acid release and platelet-activating factor (PAF) production in human coronary artery endothelial cells (HCAEC) at up to 96 h of T. cruzi infection. Arachidonic acid release is mediated by activation of the calcium-independent phospholipase A2 (iPLA2) isoforms iPLA2 β and iPLA2 γ, whereas PAF production was dependent upon iPLA2 β activation alone. Trypanosoma cruzi infection also resulted in increased cell surface expression of adhesion molecules. Increased adherence of inflammatory cells to T. cruzi-infected endothelium was blocked by inhibition of endothelial cell iPLA2 β or by blocking the PAF receptor on inflammatory cells. This suggests that PAF, in combination with adhesion molecules, might contribute to parasite clearing in the heart by recruiting inflammatory cells to the endothelium.

The absence of myocardial calcium-independent phospholipase A2γ results in impaired prostaglandin E2 production and decreased survival in mice with acute Trypanosoma cruzi infection

Cardiomyopathy is a serious complication of Chagas' disease, caused by the protozoan parasite Trypanosoma cruzi. The parasite often infects cardiac myocytes, causing the release of inflammatory mediators, including eicosanoids. A recent study from our laboratory demonstrated that calcium-independent phospholipase A2γ (iPLA2γ) accounts for the majority of PLA2 activity in rabbit ventricular myocytes and is responsible for arachidonic acid (AA) and prostaglandin E2 (PGE2) release. Thus, we hypothesized that cardiac iPLA2γ contributes to eicosanoid production in T. cruzi infection. Inhibition of the isoform iPLA2γ or iPLA2β, with the R or S enantiomer of bromoenol lactone (BEL), respectively, demonstrated that iPLA2γ is the predominant isoform in immortalized mouse cardiac myocytes (HL-1 cells). Stimulation of HL-1 cells with thrombin, a serine protease associated with microthrombus formation in Chagas' disease and a known activator of iPLA2, increased AA and PGE2 release, accompanied by platelet-activating factor (PAF) production. Similarly, T. cruzi infection resulted in increased AA and PGE2 release over time that was inhibited by pretreatment with (R)-BEL. Further, T. cruzi-infected iPLA2γ-knockout (KO) mice had lower survival rates and increased tissue parasitism compared to wild-type (WT) mice, suggesting that iPLA2γ-KO mice were more susceptible to infection than WT mice. A significant increase in iPLA2 activity was observed in WT mice following infection, whereas iPLA2γ-KO mice showed no alteration in cardiac iPLA2 activity and produced less PGE2. In summary, these studies demonstrate that T. cruzi infection activates cardiac myocyte iPLA2γ, resulting in increased AA and PGE2 release, mediators that may be essential for host survival during acute infection. Thus, these studies suggest that iPLA2γ plays a cardioprotective role during the acute stage of Chagas' disease.

Complement-mediated activation of calcium-independent phospholipase A2γ: role of protein kinases and phosphorylation

In experimental membranous nephropathy, complement C5b-9-induces glomerular epithelial cell (GEC) injury and proteinuria. The effects of C5b-9 are mediated via signaling pathways, including calcium-independent phospholipase A(2)γ (iPLA(2)γ), and mitogen-activated protein kinases (MAPKs) such as extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38. The iPLA(2)γ pathway is cytoprotective. This study addresses the mechanisms of iPLA(2)γ activation. iPLA(2)γ activity was monitored by quantifying prostaglandin E(2) (PGE(2)) production. In GECs, iPLA(2)γ localized at the endoplasmic reticulum and mitochondria. Complement-mediated production of PGE(2) was amplified in GECs that overexpress iPLA(2)γ, compared with control cells, and was blocked by the iPLA(2)γ inhibitor bromoenol lactone in both iPLA(2)γ-overexpressing and control GECs. In GECs that overexpress iPLA(2)γ, complement-mediated PGE(2) production was reduced by inhibitors of MAP/ERK kinase 1 (MEK1) and p38 but not JNK. In COS-1 cells that overexpress iPLA(2)γ and cyclooxygenase-1, PGE(2) production was induced by co-expression of constitutively active MEK1 or MAPK-interacting kinase 1 (MNK1) as well as by stimulation with epidermal growth factor (EGF) + ionomycin. Complement- and EGF + ionomycin-stimulated iPLA(2)γ activity was attenuated by the S511A/S515A double mutation. Moreover, complement and EGF + ionomycin enhanced phosphorylation of Ser-511. Thus, complement-mediated activation of iPLA(2)γ is mediated via ERK and p38 pathways, and phosphorylation of Ser-511 and/or Ser-515 plays a key role in the catalytic activity and signaling of iPLA(2)γ. Defining the mechanisms by which complement activates iPLA(2)γ provides opportunities for development of novel therapeutic approaches to GEC injury and proteinuria.

Platelet-activating factor and metastasis: calcium-independent phospholipase A2β deficiency protects against breast cancer metastasis to the lung

We determined the contribution of calcium-independent phospholipase A(2)β (iPLA(2)β) to lung metastasis development following breast cancer injection into wild-type (WT) and iPLA(2)β-knockout (iPLA(2)β-KO) mice. WT and iPLA(2)β-KO mice were injected in the mammary pad with 200,000 E0771 breast cancer cells. There was no difference in primary tumor size between WT and iPLA(2)β-KO mice at 27 days postinjection. However, we observed an 11-fold greater number of breast cancer cells in the lungs of WT mice compared with iPLA(2)β-KO animals (P < 0.05). Isolated WT lung endothelial cells demonstrated a significant increase in platelet-activating factor (PAF) production when stimulated with thrombin [1 IU/ml, 10 min, 4,330 ± 555 vs. 15,227 ± 1,043 disintegrations per minute (dpm), P < 0.01] or TNF-α (10 ng/ml, 2 h, 16,532 ± 538 dpm, P < 0.01). Adherence of E0771 cells to WT endothelial cells was increased by thrombin (4.8 ± 0.3% vs. 70.9 ± 6.3, P < 0.01) or TNF-α (60.5 ± 4.3, P < 0.01). These responses were blocked by pretreatment with the iPLA(2)β-selective inhibitor (S)-bromoenol lactone and absent in lung endothelial cells from iPLA(2)β-KO mice. These data indicate that endothelial cell iPLA(2)β is responsible for PAF production and adherence of E0771 cells and may play a role in cancer cell migration to distal locations.

Lipid oxidation by hypochlorous acid: chlorinated lipids in atherosclerosis and myocardial ischemia

Leukocytes, containing myeloperoxidase (MPO), produce the reactive chlorinating species, HOCl, and they have important roles in the pathophysiology of cardiovascular disease. Leukocyte-derived HOCl can target primary amines, alkenes and vinyl ethers of lipids, resulting in chlorinated products. Plasmalogens are vinyl ether-containing phospholipids that are abundant in tissues of the cardiovascular system. The HOCl oxidation products derived from plasmalogens are α-chlorofatty aldehyde and unsaturated molecular species of lysophosphatidylcholine. α-chlorofatty aldehyde is the precursor of both α-chlorofatty alcohol and α-chlorofatty acid. Both α-chlorofatty aldehyde and α-chlorofatty acid accumulate in activated neutrophils and have disparate chemotactic properties. In addition, α-chlorofatty aldehyde increases in activated monocytes, human atherosclerotic lesions and rat infarcted myocardium. This article addresses the pathways for the synthesis of these lipids and their biological targets.

Endothelial cell prostaglandin I(2) and platelet-activating factor production are markedly attenuated in the calcium-independent phospholipase A(2)beta knockout mouse

Damage and activation of lung endothelium can lead to interstitial edema, infiltration of inflammatory cells into the interstitium and airways, and production of inflammatory metabolites, all of which propagate airway inflammation in a variety of diseases. We have previously determined that stimulation of human microvascular endothelial cells from lung (HMVEC-L) results in activation of a calcium-independent phospholipase A(2) (iPLA(2)), and this leads to arachidonic acid release and production of prostaglandin I(2) (PGI(2)) and platelet-activating factor (PAF). We stimulated lung endothelial cells isolated from iPLA(2)beta-knockout (KO) and wild type (WT) mice with thrombin and tryptase to determine the role of iPLA(2)beta in endothelial cell membrane phospholipid hydrolysis. Thrombin or tryptase stimulation of WT lung endothelial cells resulted in increased arachidonic acid release and production of PGI(2) and PAF. Arachidonic acid release and PGI(2) production by stimulated iPLA(2)beta-KO endothelial cells were significantly reduced compared to WT. Measured PLA(2) activity and PGI(2) production by iPLA(2)beta-KO cells were suppressed by pretreatment with (R)-bromoenol lactone (R-BEL), which is a selective inhibitor of iPLA2gamma. In contrast to the increase in PAF production induced by stimulation of WT endothelial cells, none was observed for KO cells, and this suggests that endothelial PAF production is entirely dependent on iPLA(2)beta activity. Because inflammatory cell recruitment involves the interaction of endothelial cell PAF with PAF receptors on circulating cells, these data suggest that iPLA(2)beta may be a suitable therapeutic target for the treatment of inflammatory lung diseases.

Genetic ablation of calcium-independent phospholipase A(2)beta causes hypercontractility and markedly attenuates endothelium-dependent relaxation to acetylcholine

Activation of phospholipases leads to the release of arachidonic acid and lysophospholipids that play prominent roles in regulating vasomotor tone. To identify the role of calcium-independent phospholipase A(2)beta (iPLA(2)beta) in vasomotor function, we measured vascular responses to phenylephrine (PE) and ACh in mesenteric arterioles from wild-type (WT; iPLA(2)beta(+/+)) mice and those lacking the beta-isoform (iPLA(2)beta(-/-)) both ex vivo and in vivo. Vessels isolated from iPLA(2)beta(-/-) mice demonstrated increased constriction to PE, despite lower basal smooth muscle calcium levels, and decreased vasodilation to ACh compared with iPLA(2)beta(+/+) mice. PE constriction resulted in initial intracellular calcium release with subsequent steady-state constriction that depended on extracellular calcium influx. Endothelial denudation had no effect on vessel tone or PE-induced constriction although the dilation to ACh was significantly reduced in iPLA(2)beta(+/+) vessels. In contrast, vessels from iPLA(2)beta(-/-) constricted by 54% after denudation, indicating smooth muscle hypercontractility. In vivo, blood pressure, resting vessel diameter, and constriction of mesenteric vessels to PE were not different in iPLA(2)beta(-/-) vessels compared with WT mouse vessels. However, relaxation after ACh administration in situ was attenuated, indicating an endothelial inability to induce dilation in response to ACh. In cultured endothelial cells, inhibition of iPLA(2)beta with (S)-(E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one (BEL) decreased endothelial nitric oxide synthase phosphorylation and reduced endothelial agonist-induced intracellular calcium release as well as extracellular calcium influx. We conclude that iPLA(2)beta is an important mediator of vascular relaxation and intracellular calcium homeostasis in both smooth muscle and endothelial cells and that ablation of iPLA(2)beta causes agonist-induced smooth muscle hypercontractility and reduced agonist-induced endothelial dilation.

Polymorphonuclear leukocytes isolated from umbilical cord blood as a useful research tool to study adherence to cell monolayers

One of the initial steps in the inflammatory process involves the adherence and transmigration of circulating polymorphonuclear leukocytes (PMN) across the endothelial cell monolayer. One of the main constituents of the neutrophil phagosome that contributes to bacterial killing is myeloperoxidase (MPO) which can be measured spectrophotometrically, using hydrogen peroxide as a substrate, and hence can be used as an index to quantify neutrophil adherence. To evaluate whether PMN isolated from umbilical cord blood could be used for in vitro experiments to monitor neutrophil adherence, we compared the adherence to confluent endothelial and epithelial cell monolayers using PMN isolated from umbilical cord and adult peripheral blood. The extent of PMN adherence was assessed by measuring MPO activity. In initial experiments, we isolated PMN from umbilical cord and adult peripheral blood and measured MPO activity with respect to cell number and assay incubation times. Our data demonstrate that PMN obtained from either source had similar MPO activity and similar adherence to endothelial or epithelial cells. In conclusion, our data suggest that umbilical cord blood is a suitable source of leukocytes to examine PMN adherence in the setting of inflammation in a variety of disease processes.

Prostaglandin production in human coronary artery endothelial cells is modulated differentially by selective phospholipase A(2) inhibitors

Atherosclerotic plaque formation is a dynamic process involving repeated injury and inflammation of the endothelium. We have demonstrated previously that thrombin and tryptase stimulation of human coronary artery endothelial cells (HCAEC) leads to increased phospholipase A(2) (PLA(2)) activity and generation of membrane phospholipid derived inflammatory metabolites, including eicosanoids and platelet activating factor. Thus, our hypothesis is that selective PLA(2) inhibitors have therapeutic potential as anti-inflammatory agents. Stimulation of confluent HCAEC monolayers with thrombin or tryptase resulted in a concentration and time-dependent increase in both prostaglandin E(2) (PGE(2)) and prostacyclin (PGI(2)) production. Pretreatment with PX-18 to inhibit secretory PLA(2) or BEL to inhibit calcium-independent PLA(2) prior to thrombin or tryptase stimulation resulted in a significant inhibition of both PGI(2) and PGE(2) release. However, pretreatment with methyl arachidonyl fluorophosphonate (MAFP), a widely used inhibitor of cytosolic PLA(2) isoforms, resulted in a significant potentiation of both thrombin and tryptase stimulated PGI(2) and PGE(2) release as a consequence of increased free arachidonic acid production. We conclude that the use of selective PLA(2) inhibitors may be of therapeutic benefit in the development and progression of atherosclerosis, however, the development of such an agent requires rigorous screening.

Calcium-independent phospholipase A2-catalyzed plasmalogen hydrolysis in hypoxic human coronary artery endothelial cells

Thrombin stimulation of human coronary artery endothelial cells (HCAEC) results in activation of a membrane-associated, calcium-independent phospholipase A2(iPLA2) that selectively hydrolyzes membrane plasmalogen phospholipids. Rupture of an atherosclerotic plaque and occlusion of the coronary vasculature results in a coronary ischemic event in which HCAEC in the ischemic area would be exposed to dramatic decreases in oxygen tension in addition to thrombin exposure. We exposed HCAEC to hypoxia in the presence or absence of thrombin stimulation and measured iPLA2activation, membrane phospholipid hydrolysis, and the accumulation of biologically active phospholipid metabolites. HCAEC exposed to hypoxia, thrombin stimulation, or a combination of the two conditions demonstrated an increase in iPLA2activity and an increase in arachidonic acid release from plasmenylcholine. Thrombin stimulation of normoxic HCAEC did not result in an accumulation of choline lysophospholipids, but hypoxia alone and in combination with thrombin stimulation led to a significant accumulation of lysoplasmenylcholine (LPlsCho). We propose that the presence of hypoxia inhibits LPlsCho catabolism, at least in part, as a result of the accumulation of long-chain acylcarnitines. The combination of increased production and decreased catabolism of LPlsCho is necessary for its accumulation. Pretreatment with bromoenol lactone to inhibit iPLA2blocked membrane phospholipid hydrolysis and production of membrane phospholipid-derived metabolites. The increase in iPLA2activity and the subsequent accumulation of membrane phospholipid-derived metabolites in HCAEC exposed to hypoxia or thrombin stimulation alone, and particularly in combination, have important implications in inflammation and arrhythmogenesis in atherosclerosis/thrombosis and subsequent myocardial ischemia.

Protease activation of calcium-independent phospholipase A2 leads to neutrophil recruitment to coronary artery endothelial cells

INTRODUCTION

Thrombin or tryptase cleavage of protease-activated receptors (PAR) on human coronary artery endothelial cells (HCAEC) results in activation of a membrane-associated, calcium-independent phospholipase A2 (iPLA2) that selectively hydrolyzes plasmalogen phospholipids. Atherosclerotic plaque rupture results in a coronary ischemic event in which HCAEC in the ischemic area would be exposed to increased thrombin concentrations in addition to tryptase released by activated mast cells present in the plaque.

MATERIALS AND METHODS

HCAEC were stimulated with thrombin or tryptase in the absence or presence of bromoenol lactone (BEL), a selective iPLA2 inhibitor, and iPLA2 activation, accumulation of biologically active membrane phospholipid-derived metabolites, upregulation of cell surface P-selectin expression and neutrophil adherence were measured.

RESULTS

HCAEC exposed to thrombin or tryptase stimulation demonstrated an increase in iPLA2 activity and arachidonic acid release. Additionally, stimulated HCAEC demonstrated increased platelet-activating factor (PAF) production and cell surface P-selectin expression, resulting in increased adhesion of neutrophils to HCAEC monolayers. Pretreatment with bromoenol lactone to inhibit iPLA2, blocked membrane phospholipid-derived metabolite production, increased cell surface P-selectin expression and neutrophil adherence.

CONCLUSIONS

The similar biochemical and cellular responses in HCAEC exposed to thrombin or tryptase stimulation suggest that the cleavage of two separate PAR serve to extend the range of proteases to which the cells respond rather than resulting in separate intracellular events. This suggests that in conditions such as thrombosis and atherosclerosis that multiple mechanisms can activate the inflammatory response.

Group VIA Calcium-independent Phospholipase A2 Mediates Endothelial Cell S Phase Progression

Arachidonic acid and its metabolites have been previously implicated in the regulation of endothelial cell proliferation. Arachidonic acid may be liberated from cellular phospholipids by the action of group VIA calcium-independent phospholipase A2 (iPLA2-VIA). Consequently, we tested the hypothesis that iPLA2-VIA activity is linked to the regulation of endothelial cell proliferation. Inhibition of iPLA2 activity by bromoenol lactone (BEL) was sufficient to entirely block endothelial cell growth. BEL dose-dependently inhibited endothelial cell DNA synthesis in a manner that was reversed upon the exogenous addition of arachidonic acid. DNA synthesis was inhibited by the S-isomer and not by the R-isomer of BEL, demonstrating that endothelial cell proliferation is mediated specifically by iPLA2-VIA. iPLA2-VIA activity was critical to the progression of endothelial cells through S phase and is required for the expression of the cyclin A/cdk2 complex. Thus, inhibition of iPLA2-VIA blocks S phase progression and results in exit from the cell cycle. Inhibition of iPLA2-VIA-mediated endothelial cell proliferation is sufficient to block angiogenic tubule formation in co-culture assays. Consequently, iPLA2-VIA is a novel regulator of endothelial cell S phase progression, cell cycle residence, and angiogenesis.

Potential role for mast cell tryptase in recruitment of inflammatory cells to endothelium

Recent research suggests that activation of protease-activated receptors (PARs) on the surface of endothelial and epithelial cells may play a role in general mechanisms of inflammation. We hypothesized that mast cell tryptase activation of endothelial cell PAR-2 is coupled to increased calcium-independent PLA2(iPLA2) activity and increased platelet-activating factor (PAF) production that may play a role in inflammatory cell recruitment at sites of vascular injury. Stimulation of human coronary artery endothelial cells (HCAEC) with 20 ng/ml tryptase increased iPLA2activity, arachidonic acid release, and PAF production. These tryptase-stimulated responses were inhibited by pretreatment with the iPLA2-selective inhibitor bromoenol lactone (BEL; 5 μM, 10 min). Similar patterns of increased iPLA2activity and PAF production were also seen when HCAEC were treated with SLIGKV, which represents the tethered ligand sequence for the human PAR-2 once the receptor is cleaved by tryptase. Tryptase stimulation also increased cell surface expression of P-selectin, decreased electrical resistance, and increased neutrophil adherence to the endothelial cell monolayer. The tryptase-stimulated increases in both cell surface P-selectin expression and neutrophil adhesion were also inhibited with BEL pretreatment. We conclude that tryptase stimulation of HCAEC contributes importantly to early inflammatory events after vascular injury by activation of iPLA2, leading to arachidonic acid release, PAF production, cell surface P-selectin expression, and increased neutrophil adherence.

Neutrophil Adherence to Bladder Microvascular Endothelial Cells following Platelet-Activating Factor Acetylhydrolase Inhibition

Interstitial cystitis (IC) is an inflammatory bladder condition of unknown etiology. Tryptase released from elevated numbers of activated mast cells is a proposed mediator of the inflammatory process in IC. We have previously shown that tryptase increases human bladder microvascular endothelial cell (HBMEC) calcium-independent phospholipase A(2) (iPLA(2)) activity, resulting in the production of multiple biologically active phospholipid metabolites, including platelet-activating factor (PAF), that can mediate inflammation. Because the design of selective PLA(2) inhibitors may provide a useful therapeutic strategy to reduce the inflammatory process in IC, we tested several frequently used PLA(2) inhibitors on PAF production in tryptase-stimulated HBMEC. Among the inhibitors tested, methyl arachidonyl fluorophosphonate (MAFP) was found to be a potent inhibitor of PAF-acetylhydrolase activity. Pretreatment of HBMEC with MAFP significantly increased PAF production in both unstimulated and tryptase-stimulated cells. In addition, MAFP pretreatment of tryptase-stimulated HBMEC increased both surface expression of P-selectin and polymorphonuclear leukocyte adherence to the HBMEC monolayer. These effects suggest that MAFP has a proinflammatory effect, irrespective of its ability to inhibit PLA(2).

Arachidonic acid cascade in endothelial pathobiology

Arachidonic acid (AA) and its metabolites (eicosanoids) represent powerful mediators, used by organisms to induce and suppress inflammation as a part of the innate response to disturbances. Several cell types participate in the synthesis and release of AA metabolites, while many cell types represent the targets for eicosanoid action. Endothelial cells (EC), forming a semi-permeable barrier between the interior space of blood vessels and underlying tissues, are of particular importance for the development of inflammation, since endothelium controls such diverse processes as vascular tone, homeostasis, adhesion of platelets and leukocytes to the vascular wall, and permeability of the vascular wall for cells and fluids. Proliferation and migration of endothelial cells contribute significantly to new vessel development (angiogenesis). This review discusses endothelial-specific synthesis and action of arachidonic acid derivatives with a particular focus on the mechanisms of signal transduction and associated intracellular protein targets.

Calcium-independent phospholipase A2 is regulated by a novel protein kinase C in human coronary artery endothelial cells

We demonstrated previously that thrombin stimulation of endothelial cells activates a membrane-associated, Ca(2+)-independent phospholipase A2 (iPLA2) that selectively hydrolyzes arachidonylated plasmalogen phospholipids. We report that incubation of human coronary artery endothelial cells (HCAEC) with phorbol 12-myristate 13-acetate (PMA) to activate protein kinase C (PKC) resulted in hydrolysis of cellular phospholipids similar to that observed with thrombin stimulation (0.05 IU/ml; 10 min). Thrombin stimulation resulted in a decrease in arachidonylated plasmenylcholine (2.7 +/- 0.1 vs. 5.3 +/- 0.4 nmol PO4/mg of protein) and plasmenylethanolamine (7.5 +/- 1.0 vs. 12.0 +/- 0.9 nmol PO4/mg of protein). Incubation with PMA resulted in decreases in arachidonylated plasmenylcholine (3.2 +/- 0.3 nmol PO4/mg of protein) and plasmenylethanolamine (6.0 +/- 1.0 nmol PO4/mg of protein). Incubation of HCAEC with the selective iPLA2 inhibitor bromoenol lactone (5 mM; 10 min) inhibited accelerated plasmalogen phospholipid hydrolysis in response to both PMA and thrombin stimulation. Incubation of HCAEC with PMA (100 nM; 5 min) resulted in increased arachidonic acid release (7.1 +/- 0.3 vs. 1.1 +/- 0.1%) and increased production of lysoplasmenylcholine (1.4 +/- 0.2 vs. 0.6 +/- 0.1 nmol PO4/mg of protein), similar to the responses observed with thrombin stimulation. Downregulation of PKC by prolonged exposure to PMA (100 nM; 24 h) completely inhibited thrombin-stimulated increases in arachidonic acid release (7.1 +/- 0.6 to 0.5 +/- 0.1%) and lysoplasmenylcholine production (2.0 +/- 0.1 to 0.2 +/- 0.1 nmol PO4/mg of protein). These data suggest that PKC activates iPLA2 in HCAEC, leading to accelerated plasmalogen phospholipid hydrolysis and increased phospholipid metabolite production.

Signalling pathways regulating human neutrophil migration induced by secretory phospholipases A2

This study was designed to elucidate the signalling pathways by which secretory phospholipases A2 (sPLA2s) induce in vitro neutrophil migration. The cell migration assays were performed with Naja mocambique venom PLA2 (sPLA2 with high catalytic activity), bothropstoxin-I (sPLA2 devoid of catalytic activity) and platelet-activating factor (PAF), using a 48-well microchemotaxis chamber. Both the non-selective protein kinase inhibitor staurosporine (30-300 nM) and the selective protein kinase C (PKC) inhibitor 1-(5-isoquinolinesulfonyl)-2-methylpyperazine (H7; 50-200 microM) as well as the Gi inactivator pertussis toxin (30-300 nM) caused a concentration-dependent inhibition of the neutrophil migration induced by either N. mocambique venom PLA2 (100 microg/ml) or bothropstoxin-I (100 microg/ml). Pertussis toxin nearly abolished PAF-induced migration, while staurosporine and H7 partly (but significantly) inhibited the chemotactic responses to PAF. The dual inhibitor of cytosolic PLA2 and Ca2+ -independent PLA2 (iPLA2), arachidonil-trifluoromethyl-ketone (ATK; 0.2-20 microM), or the specific iPLA2 inhibitor bromoenol lactone (1-30 microM) caused a concentration-dependent inhibition of the migration induced by either sPLA2s. At the maximal concentration used for each compound, the migration was almost suppressed. In contrast, both of these compounds caused only slight inhibitions of PAF-induced migration. No rise in intracellular Ca2+ was observed in neutrophil-stimulated sPLA2, as determined in cells preloaded with fura 2-AM. In the experimental condition used, pertussis toxin, staurosporine, H7, ATK or bromoenol lactone did not induce cytotoxic effects, according to MTT assay. Our results suggest that activation of an endogenous PLA2 through activation of GTP-binding protein and PKC is the main mechanism by which exogenous sPLA2s cause neutrophil migration.

Protease-activated receptor stimulation activates a Ca2+-independent phospholipase A2 in bladder microvascular endothelial cells

Increased mast cell numbers and mast cell activation represent one of the prevalent etiologic theories for interstitial cystitis, an inflammatory condition in the bladder. This study was designed primarily to determine whether increased mast cell tryptase in the bladder wall may play a role in activating bladder endothelial cell phospholipase A(2) (PLA(2)), leading to increased inflammatory phospholipid metabolite accumulation, which may propagate the inflammatory process. We stimulated human bladder microvascular endothelial cells with thrombin or tryptase and measured the activation of PLA(2) and the production of multiple membrane phospholipid-derived inflammatory mediators. Thrombin and tryptase stimulation resulted in activation of a Ca(2+)-independent PLA(2), leading to increased release of arachidonic acid and prostacyclin and increased production of platelet-activating factor. These responses were blocked completely by pretreatment of human bladder microvascular endothelial cells with the Ca(2+)-independent PLA(2)-selective inhibitor bromoenol lactone. The combination of increased prostacyclin and platelet-activating factor in the bladder circulation may result in vasodilation and increased polymorphonuclear leukocyte adherence to the endothelium and may facilitate recruitment of polymorphonuclear leukocytes to the bladder wall of patients with interstitial cystitis.

Changes in phospholipid content and myocardial calcium-independent phospholipase A2 activity during chronic anthracycline administration

Despite numerous investigations, the causes underlying anthracycline cardiomyopathy are yet to be established. We have recently reported that acute treatment with anthracyclines inhibits membrane-associated calcium-independent phospholipase A(2) (iPLA(2)) activity both in vitro and in vivo. This study presents data that iPLA(2) activity is also suppressed during chronic drug administration. Adult Sprague-Dawley rats were given weekly 1 mg/kg i.v. injections of doxorubicin for a total of 8 weeks. One week after the last injection, the animals were sacrificed, and heart tissue was assessed for phospholipid content and iPLA(2) activity. Membrane-associated iPLA(2) activity in the myocardium of doxorubicin-treated animals was 40% lower than that in control hearts. In addition, doxorubicin treatment resulted in significant alterations in the distribution of fatty acyl moieties esterified to the sn-2 position of choline glycerophospholipids. The ethanolamine species remained unaffected. Elevation in the amount of arachidonate and linoleate esterified to the sn-2 position of choline plasmalogens was consistent with the hypothesis that iPLA(2) displays selectivity for plasmalogen phospholipids; therefore, enzyme inhibition may affect hydrolysis of these phospholipid subclasses. Notably, the changes in phospholipid content occurred at a low cumulative dose of 8 mg/kg at which appearance of structural lesions was minimal. Therefore, these alterations seem to be both specific and early signs of cardiomyocyte pathology. The results support our hypothesis that myocardial iPLA(2) inhibition may be one of the steps that leads to the functional and structural changes associated with chronic anthracycline treatment.

Cellular Function of Calcium-Independent Phospholipase A2

The catalytic activity of calcium-independent phospholipase A2 (iPLA2), which is classified as a group VI PLA2, is regulated by protein kinase C, calmodulin, and others such as reactive oxygen species. Numerous findings have shown that iPLA2 is involved in stimulus-induced arachidonic acid release and lysophospholipid generation, although the participation is dependent upon the cell type and stimulus. The catalytic action of iPLA2 is known to be responsible for phospholipid remodeling as a housekeeping function. However, it has been widely accepted that arachidonic acid and lysophospholipid generated by iPLA2 act as a signaling molecule in cellular functions. Those include eicosanoid production, glucose-induced insulin secretion, Fas-induced apoptosis, cellular proliferation, membrane traffic in fusion, contribution to myocardial ischemia, and others. In this review, the functional role of iPLA2 in cellular responses upon stimulation is the focus.

Inhibition of Platelet-Activating Factor (PAF) Acetylhydrolase by Methyl Arachidonyl Fluorophosphonate Potentiates PAF Synthesis in Thrombin-Stimulated Human Coronary Artery Endothelial Cells

We have previously demonstrated that thrombin stimulation of endothelial cells results in increased membrane-associated, Ca(2+)-independent phospholipase A2 (iPLA2) activity, accelerated hydrolysis of membrane plasmalogen phospholipids, and production of several biologically active phospholipid metabolites, including prostacyclin and platelet-activating factor (PAF) that is abolished by pretreatment with the iPLA2-selective inhibitor bromoenol lactone. This study was designed to further investigate the role of alternative PLA2 inhibitors, including methyl arachidonyl fluorophosphonate (MAFP, an inhibitor of cytosolic PLA2 isoforms), on phospholipid turnover and PAF production from thrombin-stimulated human coronary artery endothelial cells (HCAECs). Paradoxically, pretreatment of HCAEC with MAFP (5-25 microM) resulted in a significant increase in PAF production in both unstimulated and thrombin-stimulated cells that was found to be a direct result of inhibition of PAF acetylhydrolase (PAF-AH) activity. Pretreatment with MAFP did not significantly inhibit HCAEC PLA2 activity, possibly due to the localization of PLA2 activity in the membrane fraction rather than the cytosol. Bromoenol lactone did not inhibit PAF-AH activity, even at concentrations as high as 20 microM. We conclude that MAFP augments thrombin-stimulated PAF production by inhibition of PAF catabolism without affecting membrane-associated iPLA2 activity.

Calcium-independent phospholipase A2 mediates CREB phosphorylation in double-stranded RNA-stimulated endothelial cells

One of the products of a calcium-independent phospholipase A2 (iPLA2) attack of plasmenylcholine, lysoplasmenylcholine, has previously been shown to activate cAMP-dependent protein kinase (PKA). Because endothelial cells respond to some agonists in part by the activation of iPLA2, the present study was designed to determine whether double-stranded RNA (dsRNA), the primary activator of the antiviral response in endothelial cells, elicits cAMP response element binding protein (CREB) phosphorylation through a mechanism mediated by iPLA2. dsRNA stimulated CREB phosphorylation in bovine pulmonary artery endothelial cells that was inhibited by the iPLA2 inhibitor, bromoenol lactone, and the PKA inhibitor, H-89. Additionally, the product of iPLA2 hydrolysis of plasmenylcholine and lysoplasmenylcholine elicited CREB phosphorylation in bovine pulmonary endothelial cells. Taken together, the present studies suggest that dsRNA as well as other agonists of endothelial cells elicit signaling mechanisms that include in part CREB phosphorylation mediated by iPLA2.

Mesenteric microvascular inflammatory responses to systemic hypoxia are mediated by PAF and LTB4

Systemic hypoxia produces a rapid microvascular inflammatory response characterized by increased reactive oxygen species (ROS) levels, leukocyte-endothelial adherence and emigration, and increased vascular permeability. The lipid inflammatory mediator leukotriene B(4) (LTB(4)) is involved in the early hypoxia-induced responses (ROS generation and leukocyte adherence). Whether other lipid inflammatory mediators participate in this phenomenon is not known. The objective of these experiments was to study the role of platelet-activating factor (PAF) in the microvascular inflammatory response to hypoxia and its potential interactions with LTB(4) in this response. Intravital microscopy was used to examine mesenteric venules of anesthetized rats. We found that WEB-2086, a PAF receptor antagonist, completely prevented the increase in ROS levels and leukocyte adherence during a brief reduction in inspired Po(2) to anesthetized rats; administration of either WEB-2086 or the LTB(4) antagonist LTB(4)-DMA attenuated leukocyte emigration and the increase in vascular permeability to the same extent during prolonged systemic hypoxia in conscious rats. Furthermore, no additive effect was observed in either response when both antagonists were administered simultaneously. This study demonstrates a role for PAF in the rapid microvascular inflammatory response to hypoxia, as well as contributions of PAF and LTB(4) to the slowly developing responses observed during sustained hypoxia. The incomplete blockade of the hypoxia-induced increases in vascular permeability and leukocyte emigration by combined administration of both antagonists indicates that factors in addition to LTB(4) and PAF participate in these phenomena.

Identification of calcium-independent phospholipase A2 (iPLA2) beta, and not iPLA2gamma, as the mediator of arginine vasopressin-induced arachidonic acid release in A-10 smooth muscle cells. Enantioselective mechanism-based discrimination of mammalian iPLA2s

The agonist-stimulated release of arachidonic acid (AA) from cellular phospholipids in many cell types (e.g. myocytes, beta-cells, and neurons) has been demonstrated to be primarily mediated by calcium-independent phospholipases A(2) (iPLA(2)s) that are inhibited by the mechanism-based inhibitor (E)-6-(bromomethylene)-3-(1-naphthalenyl)-2H-tetrahydropyran-2-one (BEL). Recently, the family of mammalian iPLA(2)s has been extended to include iPLA(2)gamma, which previously could not be pharmacologically distinguished from iPLA(2)beta. To determine whether iPLA(2)beta or iPLA(2)gamma (or both) were the enzymes responsible for arginine vasopressin (AVP)-induced AA release from A-10 cells, it became necessary to inhibit selectively iPLA(2)beta and iPLA(2)gamma in intact cells. We hypothesized that the R- and S-enantiomers of BEL would possess different inhibitory potencies for iPLA(2)beta and iPLA(2)gamma. Accordingly, racemic BEL was separated into its enantiomeric constituents by chiral high pressure liquid chromatography. Remarkably, (S)-BEL was approximately an order of magnitude more selective for iPLA(2)beta in comparison to iPLA(2)gamma. Conversely, (R)-BEL was approximately an order of magnitude more selective for iPLA(2)gamma than iPLA(2)beta. The AVP-induced liberation of AA from A-10 cells was selectively inhibited by (S)-BEL (IC(50) approximately 2 microm) but not (R)-BEL, demonstrating that the overwhelming majority of AA release is because of iPLA(2)beta and not iPLA(2)gamma activity. Furthermore, pretreatment of A-10 cells with (S)-BEL did not prevent AVP-induced MAPK phosphorylation or protein kinase C translocation. Finally, two different cell-permeable protein kinase C activators (phorbol-12-myristate-13-acetate and 1,2-dioctanoyl-sn-glycerol) could not restore the ability of A-10 cells to release AA after exposure to (S)-BEL, thus supporting the downstream role of iPLA(2)beta in AVP-induced AA release.

Calcium-independent phospholipase A(2)-derived arachidonic acid is essential for endothelium-dependent relaxation by acetylcholine

The role of calcium-independent phospholipase A(2) (iPLA(2))-produced arachidonic acid (AA) in acetylcholine (ACh)-mediated, endothelium-dependent vascular relaxation was investigated. ACh-induced relaxation of phenylephrine-constricted isolated rat mesenteric resistance arteries was attenuated following pretreatment with (E)-6-(bromomethylene)tetrahydro-3-(1-naphthalenyl)-2H-pyran-2-one (BEL; 1 microM; p < 0.01), a highly selective suicide substrate inhibitor of iPLA(2). Following BEL, the ACh relaxation could be completely restored following pretreatment with picomolar quantities of the cell-permeant methyl ester analog of AA (arachidonic acid methyl ester, AA-Me). Higher amounts of AA-Me (1 microM) had a direct endothelium-dependent relaxing action, which was inhibited by the nitric-oxide synthase inhibitor (N(omega)-nitro-L-arginine; 100 microM), independent of ACh, and unaffected by BEL. Neither the ACh relaxation restoring action nor the direct relaxing action of AA-Me was affected by preincubation with inhibitors of the lipoxygenase (esculetin, 10 microM) or cytochrome P450 monooxygenase (17-octadecynoic acid; 10 microM) pathways; and both actions of AA-Me were enhanced following preincubation with the cyclooxygenase inhibitor indomethacin (10 microM; p < 0.05). The results of the present study indicate that iPLA(2)-produced AA plays an essential role in ACh-mediated endothelium-dependent relaxation in rat mesenteric resistance arteries.