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Biochemistry. Author manuscript; available in PMC Jul 6, 2011.
Published in final edited form as:
PMCID: PMC2938187
NIHMSID: NIHMS231251

Endothelial Cell Prostaglandin I2 and Platelet-Activating Factor Production Are Markedly Attenuated in the Calcium-Independent Phospholipase A2β Knockout Mouse

Abstract

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 A2 (iPLA2), and this leads to arachidonic acid release and production of prostaglandin I2 (PGI2) and platelet-activating factor (PAF). We stimulated lung endothelial cells isolated from iPLA2β-knockout (KO) and wild type (WT) mice with thrombin and tryptase to determine the role of iPLA2β in endothelial cell membrane phospholipid hydrolysis. Thrombin or tryptase stimulation of WT lung endothelial cells resulted in increased arachidonic acid release and production of PGI2 and PAF. Arachidonic acid release and PGI2 production by stimulated iPLA2β-KO endothelial cells were significantly reduced compared to WT. Measured PLA2 activity and PGI2 production by iPLA2β-KO cells were suppressed by pretreatment with (R)-bromoenol lactone (R-BEL), which is a selective inhibitor of iPLA2γ. 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 iPLA2β activity. Because inflammatory cell recruitment involves the interaction of endothelial cell PAF with PAF receptors on circulating cells, these data suggest that iPLA2β may be a suitable therapeutic target for the treatment of inflammatory lung diseases.

Airway inflammation is involved in the pathogenesis of several acute and chronic lung diseases that include asthma, chronic obstructive pulmonary disease, acute respiratory distress syndrome, emphysema, cystic fibrosis, pneumonia, and interstitial fibrosis. Exposure to injurious stimuli activates a variety of cells, including eosinophils, macrophages, mast cells, fibroblasts, smooth muscle cells, and endothelial cells, and this results in the release of vasoactive mediators, toxic metabolites, and cytokines that are involved in acute and chronic bronchoconstriction (1, 2). Lung endothelial injury can result in interstitial edema which contributes to increased morbidity and mortality in pulmonary diseases (3). In addition, neutrophil infiltration facilitated by endothelial cell barrier dysfunction contributes to tissue damage in the acute phase of lung injury (46).

Serine proteases such as thrombin and tryptase are released in inflammatory lung diseases. Increased numbers of mast cells are frequently observed in terminal airways, bronchoalveolar lavage fluid, and sputum of asthmatic patients (7). Allergen inhalation activates resident mast cells that release a variety of mediators, including arachidonic acid, PAF,1 histamine, and tryptase (810). Inflammatory plasma exudates contain thrombin, which can cause smooth muscle vasoconstriction and increased pulmonary microvascular endothelial permeability (11). Thrombin and tryptase stimulate endothelial cell protease-activated receptor (PAR)-1 and PAR-2 respectively, which results in inflammatory metabolite production (12). We have previously demonstrated that stimulation of human pulmonary vascular endothelial cells (HMVEC-L) with thrombin and tryptase activates calcium-independent phospholipase A2 (iPLA2), which results in increased arachidonic acid release and production of prostaglandin I2 (PGI2) and platelet-activating factor (PAF) (13). PAF induces bronchoconstriction, bronchial hyperresponsiveness, inflammatory infiltration, mucus hypersecretion, and impaired gas exchange, and this contributes to the pathogenesis of bronchial asthma (14, 15). Additionally, PAF associated with endothelial cells assists in the tethering and transendothelial migration of circulating inflammatory cells, and this results in increased pulmonary microvascular permeability and sequestration of neutrophils, platelets, and fibrin (1618).

Three classes of phospholipase A2 coexist in mammalian cells, secretory (sPLA2), cytosolic (cPLA2), and iPLA2 (for review, see refs 1922). The enzymes within each class have been further divided into groups and subgroups based on their amino acid sequences (23). Secretory PLA2 isoforms require the presence of millimolar concentrations of calcium for phospholipid hydrolysis, demonstrate no preference for the sn-2 fatty acid, and are proposed to play a role in inflammatory conditions such as rheumatoid arthritis and ulcerative colitis. Cytosolic PLA2 is expressed constitutively in most cells, demonstrates a preference for arachidonylated phospholipids, and has been demonstrated to play a critical role in agonist-induced eicosanoid production in several cells and tissues. However, in several previous studies, we have demonstrated that the majority of endothelial cell PLA2 activity is iPLA2 and that agonist stimulation results in iPLA2 activation, accelerated membrane phospholipid hydrolysis, and the subsequent production of PGI2 and PAF (13, 2427). Most iPLA2 activity in mammalian cells resides in the Group VIA and VIB enzymes designated iPLA2β and iPLA2γ (2830). Homology between iPLA2β and iPLA2γ includes an ATP binding motif, a consensus serine lipase catalytic center, and a region of nine amino acids of currently unknown functional significance (31). These two enzymes exhibit differential sensitivity to inhibition by enantiomers of the suicide substrate designated bromoenol lactone (BEL). Racemic BEL inhibits iPLA2 activity at concentrations over 1000-fold lower than those required to inhibit cPLA2 and sPLA2 enzymes (32). In addition, (S)-BEL inhibits iPLA2β preferentially over iPLA2γ, and the converse is true for (R)-BEL (33). BEL also inhibits phosphatidate phosphohydrolase which converts phosphatidic acid to diacylglycerol (34), and hydrolysis of BEL by iPLA2 generates a diffusible bromomethyl keto acid product that can alkylate thiol groups and that might inhibit neighboring enzymes such as those with active cysteine residues (35). Such “off target” effects complicate the interpretation of studies in which BEL is used as a pharmacologic inhibitor of iPLA2 and have motivated studies of genetic manipulations of iPLA2 enzymes to elucidate their roles in biological processes (3646).

Mice that do not express iPLA2β have been generated by homologous recombination (36), and these iPLA2β-KO mice have been used to identify roles for iPLA2β in insulin secretion and glucose homeostasis (41, 44), in macrophage functions (37, 39, 40), and in vascular myocyte biology (38, 42, 46). Here, we have used iPLA2β-KO mice to study the role of this enzyme in production of the phospholipid-derived inflammatory mediators arachidonic acid, PGI2, and PAF by isolated pulmonary endothelial cells upon stimulation with thrombin and tryptase.

EXPERIMENTAL PROCEDURES

iPLA2β Knockout Mice

The generation of mice deficient in iPLA2β has been described previously (36). Mice were housed in a pathogen-free facility and studies were conducted under protocols approved by Saint Louis University Animal Care and Use Committee.

Endothelial Cells

Human microvascular endothelial cells-lung (HMVEC-L) were obtained from Lonza (Walkersville, MD). HMVEC-L were grown to confluence in EGM-2MV media (Lonza) and incubated at 37 °C, with an atmosphere of 95% O2, 5% CO2. Cells were passaged using subculture pack (Lonza) in a 1:3 ratio. Cells from passage 3–4 were used for experiments.

Endothelial cells were isolated from mouse lung by collagenase digestion. The diced lung tissue was incubated in 1 mg/mL collagenase for 1 h @ 37 °C and the digested tissue was passed through a cell strainer. A single cell suspension was obtained by incubating in trypsin-EDTA for 10 min. Endothelial cells were isolated by incubating with murine immunoglobulins to block Fc receptors and then incubating with rat antimouse CD31, rat antimouse CD105, and biotinylated isolectin B4. Cells were washed, incubated with rat antimouse Ig, and streptavidin-conjugated microbeads and separated using an AutoMACs cell separator. The eluted cells were washed, resuspended in EGM-2MV cell culture medium (Lonza), and plated in 25 cm2 culture flasks. Nonadherent cells were removed the next day, and cells were grown to confluence and passaged at a 1–3 dilution. Cells from passage 3–4 were used for experiments.

Immunofluorescence Microscopy for Factor VIII in Mouse Endothelial Cells

To determine purity of mouse endothelial cell isolations, cells were fixed with ice-cold methanol for 15 min, washed, and permeabilized for 2 min with 0.5% Triton X-100 in (in mM) 10 piperazine ethane sulfonic acid, 50 NaCl, 300 sucrose, and 3 MgCl2 (pH 6.8). After incubation in blocking solution (1% albumin and normal goat serum in PBS) with rabbit antifactor VIII antibody (AbCam, Cambridge, MA), cultures were washed and treated with Alexa Fluor 568 goat antirabbit IgG (Molecular Probes, Eugene, OR). ProLong Gold antifade reagent with 4′,6′-diamidino-2-phenylindole (Molecular Probes) was used for mounting. Images were viewed using a LOMO PLC fluorescent microscope with attached Sony 3CCD camera, saved as TIFF files, and processed using Image Pro Plus software (MediaCybernetics, Silver Spring, MD).

Prostaglandin I2 Release

Endothelial cells were grown to confluence in 16 mm tissue culture dishes. Cells were washed twice with Hank’s balanced salt solution (HBSS) containing (in mmol/L) 135 NaCl, 0.8 MgSO4, 10 HEPES (pH=7.6), 1.2 CaCl2, 5.4 KCl, 0.4 KH2PO4, 0.3 Na2HPO4, and 6.6 glucose. After washing, 0.5mL of HBSS with 0.36% bovine serum albumin was added to each culture well. Endothelial cells were stimulated with the appropriate human recombinant skin β-tryptase (Promega, Madison, WI) and thrombin (Sigma Chemical Co., St. Louis, MO) concentrations. The surrounding buffer was removed after selected time intervals and PGI2 release was measured immediately using an immunoassay kit (R&D Systems, Minneapolis, MN).

Arachidonic Acid Release

Endothelial cells were grown to confluence in 35 mm tissue culture dishes. Arachidonic acid release was determined by measuring [3H] arachidonic acid released into the surrounding medium from endothelial cells prelabeled with 1 µCi of [3H] arachidonic acid (specific activity 100 Ci/mmol; Perkin-Elmer Life Sciences, Boston, MA) per culture dish for 18 h. Cells were washed three times with HEPES buffer containing (in mmol/L) 133.5 NaCl, 4.8 KCl, 1.2 CaCl2, 1.2 MgCl2, 1.2 KH2PO4, 10 HEPES (pH 7.4), 10 glucose, and 0.36% bovine serum albumin and incubated at 37 °C for 15 min before experimental conditions. At the end of the stimulation period, the surrounding medium was transferred to a scintillation vial and the remaining cells were lysed in 10% sodium dodecyl sulfate and the lysate was then transferred to a separate vial. Radioactivity in the medium and cells was quantified by liquid scintillation spectrometry. Arachidonic acid mobilized from cellular phospholipids was expressed as the percentage of total incorporated radioactivity.

Phospholipase A2 Activity Measurement

Cells were grown to confluence in 100 mm culture dishes. At the end of each stimulation period, media was removed and immediately replaced with ice cold buffer containing (mmol/L): 250 sucrose, 10 KCl, 10 imidazole, 5 EDTA, 2 dithiothreitol, with 10% glycerol (pH = 7.8). The cells were removed from the tissue culture plate by scraping and the suspension was sonicated on ice for six bursts of 10 s. PLA2 activity in the lysates was assessed by incubating the cellular protein with 100 µM 1-palmitoyl-2-oleoyl plasmenylcholine [oleoyl-9,10-3H] or 1-palmitoyl-2-oleoyl phosphatidylcholine [oleoyl-9,10-3H] substrate (specific activity approximately 150 dpm/pmol) in assay buffer containing 10 mM Tris, 10% glycerol with 4 mM EGTA or 1 mM calcium, pH = 7.0 at 37 °C for 5 min in a total volume of 200 µL. Reactions were initiated by adding the radiolabeled phospholipid substrate as a concentrated stock solution in ethanol. Reactions were terminated by the addition of 100 µL of butanol. The radiolabeled fatty acid released in the above reaction was isolated by application of 25 µL of the butanol phase to channeled Silica Gel G plates and then developed in petroleum ether/diethyl ether/ acetic acid (70/30/1,v/v/v). Results were quantified by liquid scintillation spectrometry and normalized for protein content in each sample.

Measurement of PAF Production

Endothelial cells grown in 35-mm culture dishes were washed twice with Hanks’ balanced salt solution containing (in mM) 135 NaCl, 0.8 MgSO4, 10 HEPES (pH 7.4), 1.2 CaCl2, 5.4 KCl, 0.4 KH2PO4, 0.3 Na2HPO4, and 6.6 glucose. Cells were incubated with 10 µCi [3H] acetic acid/well for 20 min. After stimulation with thrombin or tryptase, lipids were extracted from the cells using the method of Bligh and Dyer (47). The chloroform layer was concentrated by evaporation under nitrogen, resuspended in 9:1 CHCl3/ CH3OH, applied to a silica gel 60 TLC plate, and developed in chloroform–methanol–acetic acid–water (50:25:8:4 vol/vol/vol/vol). The region corresponding to [3H]PAF was scraped, and radioactivity was quantified by liquid scintillation spectrometry. Loss of PAF during extraction and chromatography was corrected by adding a known amount of [14C] PAF as an internal standard.

Adherence of RAW 264.7 Cells to Endothelial Cell Monolayers

Murine monocyte/macrophage RAW 264.7 cells were labeled with calcein-AM (4 µg/mL, Alexis Biochemicals, Lausen, Switzerland) for 45 min at 37 °C. After washing three times, 2 × 106 cells were layered onto confluent endothelial cell monolayers. Medium and unbound cells were removed and discarded. Adherent RAW 264.7 and endothelial cells were washed with Dulbecco’s phosphate buffered saline and lysed with 1 mL of 0.2% Triton. Samples were sonicated (550 Sonic Dismembrator, Fisher Scientific, Pittsburgh, PA) for 10 s. The amount of calcein-AM fluorescence was measured using a Synergy 2 microplate reader (Biotek, Winooski, VT) at an excitation wavelength of 485 nm and emission wavelength of 530 nm. The percent of RAW cell adherence was calculated from the amount of calcein-AM fluorescence measured in 2 × 106 cells.

RESULTS

In previous studies, we have demonstrated that stimulation of HMVEC-L with thrombin and tryptase activates iPLA2 resulting in arachidonic acid release and production of PGI2 and PAF. These responses were inhibited by pretreatment with racemic BEL, and we have now examined the effects of BEL enantiomers (Figure 1). Stimulation of HMVEC-L with thrombin or tryptase resulted in a significant increase in PAF production (Figure 1, black bars). Pretreating HMVEC-L with 5 µM (R)-BEL resulted in no significant inhibition of thrombin- or tryptase-stimulated PAF production (Figure 1, open bars). In contrast, pretreatment with 5 µM (S)-BEL resulted in complete inhibition of thrombin- or tryptase-stimulated PAF production (Figure 1, gray bars), suggesting that iPLA2β activity is required for these responses and that stimulation of HMVEC-L with thrombin or tryptase results in activation of iPLA2β.

FIGURE 1
Effect of pretreatment with (R)-bromoenol lactone (5 µM, 10 min, (R)-BEL) or (S)-BEL (5 µM, 10 min) on platelet-activating factor (PAF) production in human pulmonary microvascular endothelial cells stimulated with thrombin (1 IU/mL, 10 ...

Similarly, stimulation of HMVEC-L with thrombin or tryptase resulted in a significant increase in prostaglandin I2 (PGI2) release (Figure 2, black bars). Pretreating HMVEC-L with 5 µM (R)-BEL resulted in no significant inhibition of thrombin- or tryptase-stimulated PGI2 production and release (Figure 2, open bars). In contrast, pretreatment with 5 µM (S)-BEL resulted in complete inhibition of thrombin- or tryptase-stimulated PGI2 production (Figure 2, gray bars), suggesting that iPLA2β activity is required for PGI2 production and release and that stimulation of HMVEC-L with thrombin or tryptase results in activation of iPLA2β.

FIGURE 2
Effect of pretreatment with (R)-bromoenol lactone (5 µM, 10 min, (R)-BEL) or (S)-BEL (5 µM, 10 min) on prostaglandin I2 (PGI2) release from human pulmonary microvascular endothelial cells stimulated with thrombin (1 IU/mL, 10 min) or tryptase ...

We next isolated endothelial cells from the lungs of WT and iPLA2β-KO mice by selecting cells that expressed CD31 and CD105, and the isolated cells were grown to confluence. Confluent monolayers were stained for factor VIII and found to consist of > 80% endothelial cells (Figure 3). Phospholipase A2 activity in human and mouse lung endothelial cells was determined with radiolabeled phospholipid substrate [100 µM 1-palmitoyl-2-oleoyl plasmenylcholine [oleoyl-9,10-3H] (PlsCho) or phosphatidylcholine (PtdCho)] under Ca2+-replete (1 mM Ca2+) or Ca2+-chelated (4 mM EGTA) conditions by measuring the release of [3H] oleate (Figure 4). Phospholipase A2 activity in mouse endothelial cells was found to be significantly lower than that in HMVEC-L under all conditions studied (Figure 4). In human and mouse endothelial cells, PLA2 activity was maximal when Ca2+ was chelated (4 mM EGTA) with both PtdCho and PlsCho substrates (Figure 4). In iPLA2β-KO lung endothelial cells, PLA2 activity was significantly lower than that in WT cells under all conditions (Figure 4). No PLA2 activity was detectable in iPLA2β-KO cells under Ca2+-replete conditions with either PtdCho or PlsCho substrate (Figure 4). iPLA2 activity from iPLA2β-KO lung endothelial cells measured under Ca2+-chelated conditions was about 60% of that from WT cells (Figure 4). This residual iPLA2 activity from iPLA2β-KO cells appears to be attributable to iPLA2γ because it is inhibited by pretreatment with (R)-BEL at a concentration of 0.5 µM (Figure 5). More than 10-fold higher BEL concentrations were required to inhibit iPLA2 activity from WT or human endothelial cells. Thus, PLA2 activity in WT mouse lung endothelial cells is comparable to that in HMVEC-L cells with respect to substrate selectivity, Ca2+-dependence, and sensitivity to inhibition by BEL. In contrast, residual iPLA2 activity in iPLA2β-KO mouse lung endothelial cells is attributable to iPLA2γ.

FIGURE 3
Lung endothelial cell cultures isolated from wild type (left panel) and knockout (right panel) mice. Cultures were stained with rabbit antifactor VIII antibody followed by goat antirabbit Alexa Fluor 568 (red) and with DAPI (blue) to localize cell nuclei. ...
FIGURE 4
Phospholipase A2 (PLA2) activity in human pulmonary vascular endothelial cells (HMVEC-L) and endothelial cells isolated from the lungs of wild type (WT) and iPLA2β knockout (KO) mice. Activity was measured using 100 µM 1-palmitoyl-2-oleoyl ...
FIGURE 5
Calcium-independent phospholipase A2 (iPLA2) activity in human pulmonary vascular endothelial cells (HMVEC-L) and endothelial cells isolated from the lungs of wild type (WT) and iPLA2β knockout (KO) mice. Protein was incubated with indicated concentrations ...

Lung endothelial cells isolated from WT or iPLA2β-KO mice were prelabeled with [3H] arachidonic acid, washed to remove unincorporated radiolabel, and then stimulated with thrombin (0.1 IU/mL) or tryptase (20 ng/mL) for up to 30 min. Release of [3H] arachidonate into the incubation medium was measured after various time intervals for 30 min, and both agents were found to induce arachidonic acid release from WT cells after 2 min that continued up to 10 min and then achieved a stable plateau (Figure 6). Under these conditions, iPLA2β-KO cells stimulated with thrombin or tryptase released amounts of [3H] arachidonate that were significantly smaller than those from WT cells at all time points between 5 and 30 min (Figure 6).

FIGURE 6
Arachidonic acid release from wild type (WT, open symbols) and iPLA2β knockout (KO, filled symbols) mouse lung endothelial cells stimulated with thrombin (open and filled squares, 0.1 IU/mL) or tryptase (open and filled circles, 20 ng/mL). Arachidonic ...

Incubation of WT lung endothelial cells with thrombin or tryptase also stimulated PGI2 production and release into the medium that was detectable after 2 min and continued for up to 30 min, and these responses were significantly smaller for iPLA2β-KO cells at each tested time point (Figure 7). Pretreatment of WT endothelial cells with (S)-BEL significantly inhibited thrombin- and tryptase-induced PGI2 production, but pretreatment with (R)-BEL had no significant effect on these responses (Figure 8). In view of the fact that (S)-BEL preferentially inhibits iPLA2β and that (R)-BEL preferentially inhibits iPLA2γ, the data in Figure 8 suggest that iPLA2β activity is required for thrombin- or tryptase-stimulated PGI2 production by WT cells but that iPLA2γ activity is not. In contrast, the modest PGI2 production by thrombin- or tryptase-stimulated iPLA2β-KO endothelial cells was prevented by pretreatment with (R)-BEL but was not significantly affected by pretreatment with (S)-BEL (Figure 8). This indicates that iPLA2β is not involved in stimulated PGI2 production in KO cells, as expected, but that the modest increases in PGI2 production induced by stimulating KO cells with thrombin or tryptase involves the action of iPLA2γ.

FIGURE 7
Prostaglandin I2 (PGI2) release from wild type (WT, open symbols) and iPLA2β knockout (KO, filled symbols) mouse lung endothelial cells stimulated with thrombin (open and filled squares, 0.1 IU/mL) or tryptase (open and filled circles, 20 ng/mL). ...
FIGURE 8
Prostaglandin I2 release from wild type and iPLA2β knockout mouse lung endothelial cells stimulated with thrombin (0.1 IU/mL, 15 min) or tryptase (2 ng/mL, 15 min). Cells were pretreated with the iPLA2 inhibitors (R)-bromoenolactone ((R)-BEL, ...

Incubation of lung endothelial cells isolated from WT mice with thrombin or tryptase induced about a 5-fold rise in PAF production, and these responses were completely prevented by pretreating the cells with racemic BEL (Figure 9), which is consistent with the involvement of an iPLA2 in the responses. In contrast, stimulation of iPLA2β-KO endothelial cells with neither thrombin nor tryptase induced a significant increase in PAF production (Figure 9), which is consistent with a requirement for iPLA2β in thrombin- and tryptase-stimulated PAF production by pulmonary endothelial cells. PAF expressed by endothelial cells binds to its cognate receptors on circulating inflammatory cells, leading to cell adherence to an activated endothelial cell monolayer. In this study, we used the murine monocyte/macrophage cell line RAW 264.7 as a cell model to study endothelial cell adherence. As shown in Figure 10, thrombin or tryptase stimulation of lung endothelial cells isolated from WT mice resulted in a 4-fold increase in RAW cell adherence. Pretreatment with BEL inhibited RAW cell adherence after either tryptase or thrombin stimulation. In contrast, stimulation of lung endothelial cells from iPLA2β-KOmice with thrombin or tryptase failed to increase RAW cell adherence to the endothelial cell monolayer (Figure 10). These results are consistent with a requirement for iPLA2β in thrombin and tryptase-stimulated endothelial cell PAF production and inflammatory cell adherence.

FIGURE 9
Platelet-activating factor (PAF) production in wild type (WT) and iPLA2β knockout (KO) mouse lung endothelial cells stimulated with thrombin (thr, 1.0 IU/mL) or tryptase (try, 20 ng/mL). Cells were pretreated with BEL (filled bars, 5 µM, ...
FIGURE 10
Adherence of calcein-labeled RAW 264.7 cells to wild type (WT) and iPLA2β knockout (KO) mouse lung endothelial cell monolayers stimulated with thrombin (thr, 1.0 IU/mL) or tryptase (try, 20 ng/mL). Cells were pretreated with BEL (filled bars, ...

DISCUSSION

We have previously reported that stimulation of HMVEC-L with thrombin or tryptase results in release of arachidonic acid and production of PGI2 and PAF, and we suggested that iPLA2 activation was required to initiate these responses because they were blocked by the iPLA2 inhibitor BEL when administered as a racemic mixture (13). In our previous studies, we used racemic BEL pretreatment to demonstrate the activation of iPLA2 (13). Here, we have examined which iPLA2 family members might participate in these responses by examining the effects of BEL enantiomers that discriminate between iPLA2β and iPLA2γ and by determining the responses of pulmonary endothelial cells isolated from iPLA2β-KO mice. In the case of HMVEC-L cells, arachidonic acid release and production of PGI2 and PAF induced by stimulation with thrombin or tryptase are all blocked by pretreatment with (S)-BEL but not with (R)-BEL, which is consistent with a requirement for iPLA2β activity but not for iPLA2γ activity in these responses.

The use of genetically modified mice circumvents the potential nonspecificity of pharmacologic agents such as BEL by selectively eliminating the target gene product, and we have therefore used pulmonary endothelial cells isolated from iPLA2β-KO mice and their WT littermates to characterize further the potential involvement of iPLA2β in lung endothelial cell responses to thrombin and tryptase. The iPLA2β-KO mouse was described originally in 2004 (36) and has been observed to exhibit a number of phenotypic abnormalities at the whole animal and cellular level that impaired male reproductive ability (36), impaired insulin secretory responses (41, 44), and acceleration of age-related loss in bone mass and strength (45). Vascular smooth muscle cells isolated from iPLA2β-KO mice exhibit impaired release of arachidonic acid and production of PGI2 and reduced proliferative and migratory responses (46). We compared iPLA2 activity in HMVEC-L to the activity in WT and iPLA2β-KO mouse lung endothelial cells using plasmenylcholine and phosphatidylcholine in the presence and absence of Ca2+. Maximal activity in HMVEC-L and WT mouse lung endothelial cells was observed in Ca2+-chelated conditions. These data agree with our previous studies demonstrating that the majority of PLA2 activity in endothelial cells is iPLA2 (13, 2427). In a previous study, we measured PLA2 activity using several published assay methods and demonstrated that higher activity measurements were made using shorter times of incubation than are more commonly used (24). Interestingly, we demonstrated increased PLA2 activity in response to thrombin stimulation when using plasmenylcholine as substrate, but not when using phosphatidylcholine substrate (24). We were not able to detect any PLA2 activity in iPLA2β-KO endothelial cells under Ca2+-replete conditions, suggesting that iPLA2γ is not active in the presence of 1 mM Ca2+. These data also suggest that cPLA2 activity is minimal in lung endothelial cells.

In the present study, we have demonstrated that lung endothelial cells isolated from iPLA2β-KO mice and incubated with thrombin or tryptase exhibit reduced arachidonic acid release and production of PGI2 and PAF compared to cells isolated from WT littermates. Pretreatment of WT endothelial cells with (S)-BEL abolished PGI2 production in response to thrombin or tryptase but pretreatment with (R)-BEL did not affect this response significantly. This is consistent with a requirement for iPLA2β activity but not for iPLA2γ activity in endothelial cell PGI2 production in response to thrombin and tryptase, and this is supported by the greatly reduced PGI2 production observed with iPLA2β-KO endothelial cells stimulated with thrombin or tryptase. Interestingly, the small amount of PGI2 produced by the KO cells in response to thrombin or tryptase was eliminated when the iPLA2β-KO cells were pretreated with the iPLA2γ inhibitor (R)-BEL, which could reflect compensatory upregulation of iPLA2γ in iPLA2β-KO cells, as has been observed in vascular smooth muscle cells isolated from iPLA2β-KO mice (46).

In that regard, our observations on the differential sensitivity to BEL enantiomers of Ca2+-independent PLA2 (iPLA2) activities in HMVEC-L cells and WT and KO mouse lung endothelial cells are of interest. The much greater sensitivity of HMVEC-L iPLA2 activity to inhibition by (S)-BEL compared to (R)-BEL is consistent with iPLA2β being the predominant contributor to total iPLA2 activity in those cells, and this is also true for WT mouse lung endothelial cells. In contrast, iPLA2β-KO endothelial cell iPLA2 activity is much more susceptible to inhibition by (R)-BEL, suggesting that iPLA2γ is responsible for the residual iPLA2 activity in iPLA2β-KO cells and may be upregulated in those cells. Although iPLA2 activity in iPLA2β-KO is approximately 60% of that in WT endothelial cells, PAF production is inhibited completely, and PGI2 production is inhibited by at least 70%. Since our data suggest that the residual activity in iPLA2β-KO endothelial cells is due to iPLA2γ, we propose that endothelial cell iPLA2β is primarily responsible for membrane phospholipid hydrolysis in response to stimulation, whereas iPLA2γ may play an alternative role. In previous studies, iPLA2γ has been demonstrated to preferentially hydrolyze the sn-1 fatty acid when the sn-2 fatty acid of phospholipids is polyunsaturated, resulting in the production of polyunsaturated lysophospholipids (48). The generation of an iPLA2γ KO mouse resulted in an animal with growth retardation, cold intolerance, and reduced exercise endurance, suggesting that this isoform maintains efficient bioenergetic mitochondrial function (49). Taken together, these data suggest distinct and separate roles for iPLA2 isoforms.

Prostaglandin I2 is generated by the pulmonary endothelium and macrophages and binds to the IP receptor, which is coupled to the Gs subunit of a G-protein. In a murine model of pulmonary inflammation, the PGI2–IP complex has been reported to suppress Th2-mediated allergic inflammatory reactions (50). PGI2 also has antithrombotic effects in vivo, and bronchodilator effects on human airways in vitro, although it is less potent than PGE2 in the latter regard (51). In the lungs of IP-deficient mice, increased inflammatory leukocyte infiltrations, and Th2 cytokine levels, have been observed in response to prolonged allergen exposure that are accompanied by goblet cell hyperplasia and subepithelial cells fibrosis, and these responses are greatly augmented compared to WT mice (52). These observations suggest that PGI2 plays regulatory roles in allergen-induced airway inflammation and remodeling and point to the potential utility of pharmacologic PGI2 agonists in the therapy of allergic asthma.

We have demonstrated here that pulmonary endothelial cells isolated from WT mice greatly increase PAF production when stimulated with thrombin or tryptase and that these responses are blocked by pretreatment with BEL. These observations are similar to our previously reported findings with HMVEC-L cells (13) and suggest that an iPLA2 might be involved in these responses. In contrast, we observe no increase in PAF production by iPLA2β-KO endothelial cells stimulated with thrombin or tryptase, and together these observations strongly suggest that pulmonary endothelial cell PAF production in response to thrombin or tryptase is dependent on iPLA2β activity. We have demonstrated previously that HMVEC-L PAF production is associated with increased neutrophil adherence to endothelial cells and that such adherence is prevented by pretreating the endothelial cells with BEL or by blocking the neutrophil PAF receptor with the compound CV3988 (13). Neutrophils are the most abundant cell type in the airways of normal and asthmatic subjects (53), and increased numbers of neutrophils are associated with more severe airway obstruction (4). Neutrophils are also prominent during acute asthma exacerbations (54) and may regulate both initiation and resolution of attacks (53). Platelet-activating factor can play a central role in the propagation of chronic inflammatory conditions by increasing systemic, pulmonary, and microvascular permeability and disrupting vascular integrity (55). Additionally, PAF stimulates migration of eosinophils into the airways (56) and induces airway smooth muscle contraction and hyperreactivity in otherwise healthy subjects (14, 15). Increased numbers of eosinophils in airway secretions are a characteristic feature of asthma and are associated with disease severity (57, 58). Taken together, the data presented here and previously suggest that a selective and systemically bioavailable iPLA2β inhibitor could represent a potentially useful therapeutic tool in inflammatory airway disease. Such an agent might reduce inflammatory cell recruitment to the airways while sparing a component of pulmonary endothelial cell PGI2 production.

ACKNOWLEDGMENT

The authors thank Alice Rickard for endothelial cell staining.

Footnotes

This work was supported by United States Public Health Service Grants R37-DK34388, P41-RR00954, P60-DK20579, and P30-DK56341

1Abbreviations: BEL, bromoenol lactone; EDTA, ethylenediaminete-traacetic acid; EGTA, ethylene glycol tetraacetic acid; HMVEC-L, human microvascular endothelial cells-lung; iPLA2, calcium-independent phospholipase A2; KO, knockout; PAF, platelet-activating factor; PAR-1, protease-activated receptor-1; PAR-2, protease-activated receptor-2; PLA2, phospholipase A2; PGI2, prostaglandin I2; WT, wild type.

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