Journal of Leukocyte Biology Myeloid cells, immune suppression, tumor immunology
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Originally published online as doi:10.1189/jlb.1004587 on July 8, 2005

Published online before print July 8, 2005
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(Journal of Leukocyte Biology. 2005;78:976-984.)
© 2005 by Society for Leukocyte Biology

Leukotriene B4 mediates p47phox phosphorylation and membrane translocation in polyunsaturated fatty acid-stimulated neutrophils

Carlos H. C. Serezani*,{dagger}, David M. Aronoff*,{ddagger}, Sonia Jancar{dagger} and Marc Peters-Golden*,1

{ddagger} Divisions of Infectious Diseases and
* Pulmonary and Critical Care Medicine, Department of Internal Medicine, Medical School, University of Michigan, Ann Arbor; and
{dagger} Department of Immunology, Institute of Biomedical Sciences IV, University of São Paulo, Brazil

1Correspondence: Pulmonary and Critical Care Medicine, Department of Internal Medicine, University of Michigan Health System, 6301 MSRB III, Box 0642, 1150 W. Medical Center Drive, Ann Arbor, MI 48109-0642. E-mail: petersm{at}umich.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Polyunsaturated fatty acids (PUFAs) and leukotriene B4 (LTB4) are involved in many inflammatory and physiological conditions. The role of arachidonic acid (AA) and linoleic acid (LA) in promoting the assembly of reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase subunits is well known, but the involvement of LTB4 and other 5-lipoxygenase (5-LO) pathway metabolites of AA in hydrogen peroxide (H2O2) production by PUFA-stimulated polymorphonuclear leukocytes (PMNs) has not been investigated. We examined this question by determining H2O2 production as well as phosphorylation and membrane translocation of the p47phox subunit of NADPH oxidase. Elicited peritoneal PMNs from rats and from 5-LO-deficient or wild-type mice were pretreated with or without inhibitors of LT biosynthesis and antagonists of the receptors for LTB4 and cysteinyl LTs for 20 min before stimulation with AA (at 5 and 20 µM) or LA (at 20 µM). PUFAs elicited H2O2 production in a dose-dependent manner, and pharmacologic or genetic inhibition of LT synthesis decreased H2O2 production by ~40% when compared with untreated controls. LTB4 was the moiety responsible for H2O2 production, as revealed by studies using receptor antagonists and its exogenous addition. LTB4 itself also promoted p47phox phosphorylation and translocation. These results identify a heretofore unrecognized role for activation of 5-LO and subsequent production of LTB4 in stimulation of PMN NADPH oxidase activation by PUFAs.

Key Words: PMN • PUFAs • lipid mediators • NADPH oxidase • BLT1


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Polyunsaturated fatty acids (PUFAs) are involved in many physiological and pathological situations, including alteration of plasma lipid levels, cardiovascular function, insulin action, neuronal development, and activation and regulation of the immune system [1 ]. These lipids are classified according to the position of the double bond nearest their methyl ({omega}) end into those of the n-3 ({omega}-3) and those of the n-6 ({omega}-6) series. PUFAs are essential constituents of mammalian diets, required for maintenance of normal cell structure and function. Examples of n-6 PUFAs are linoleic acid (LA; 18:2n-6) and arachidonic acid (AA; 20:4n-6). AA and LA are liberated from membrane phospholipids by the action of phospholipase A2 (PLA2) enzymes. AA serves the well-characterized role of substrate for the generation of bioactive eicosanoids, including prostaglandins and leukotrienes (LTs). Besides its role as a precursor for eicosanoids, endogenous AA may function as an intracellular signaling molecule [2 , 3 ] or as a cell activator when released into the extracellular space.

Numerous signaling components have been reported to be activated by exogenous AA. These include guanosine 5'-triphosphate-binding proteins [4 ], plasma membrane Ca++ channels [5 ], extracellular signal-regulated kinase 1/2 (ERK 1/2) [6 ], Raf-1/mitogen-activated protein kinase kinase (MEK) [7 ], c-Jun N-terminal kinase [8 ], phosphatidylinositol 3-kinase [9 ], PLC [9 ], nuclear factor-{kappa} B [10 ], and protein kinase C (PKC) [11 ]. In neutrophils [polymorphonuclear leukocytes (PMNs)], it has been suggested that activation of ERK 1/2 and Raf-1/MEK by AA could be mediated by formation of its lipoxygenase (LO) metabolic products [6 , 7 ]. One of the most well-studied actions of AA is the activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase in cell-free systems as well as intact PMNs [4 , 5 , 12 13 14 15 16 17 18 19 ]. The NADPH oxidase complex is formed by the cytosolic proteins p47phox, p40phox, p67phox, and Rac-1 or -2 and the membrane-associated components gp91phox, p22phox, and Rap-1 [20 ]. Upon cell activation, the cytosolic components are phosphorylated by specific serine kinases. This leads to structural modification of the phox proteins, allowing their translocation to the cell membrane [20 ]. Most activators of NADPH oxidase, such as cytokines, lipids, and bacteria, act via receptor-mediated signaling pathways; however, lipids, such as AA, LA, diacylglycerol, or phosphatidic acid, may also interact directly with p47phox or p67phox proteins by unmasking their membrane-binding domains [19 , 21 ] or may interact with other relevant regulatory components [22 23 24 25 26 27 ].

AA is converted to LTs by 5-LO, and the major products formed in PMNs are LTB4 and 5-hydroxy-eicosatetraynoic acid (5-HETE) [28 ]. It is important that LTB4 has been shown to activate NADPH oxidase [29 , 30 ], although the mechanism of this activation remains unknown. However, we recently demonstrated that in rat alveolar macrophages, LTB4 activates NADPH oxidase through phosphorylation and translocation of p47phox to the membrane, a process that was itself dependent on PKC-{delta} activity [31 ]. It is unclear whether the ability of AA to activate NADPH oxidase depends, in part, on its metabolism to LTB4. Conversely, as LTB4 activates cytosolic PLA2 (cPLA2) [32 ], it is possible that NADPH oxidase activation depends on LTB4-stimulated AA. As AA and LTB4 can stimulate Ca++ release [33 , 34 ] as well as PLC and PKC activities [9 , 11 , 30 , 35 ], these actions may affect NADPH oxidase indirectly as well. Thus, the goals of this work were to evaluate the role of 5-LO metabolites in PUFA-induced NADPH oxidase activation in PMNs and to determine the molecular mechanisms involved in these actions. Herein, we report that LTB4 is a significant mediator of the ability of AA to activate NADPH oxidase in PMNs. LTB4 participates in NADPH oxidase activation by modulating p47phox phosphorylation and translocation to the membrane. To our knowledge, this is the first report showing that LTB4 is able to directly affect the assembly of NADPH oxidase components in PMNs.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Reagents
Dulbecco’s modified Eagle’s medium (DMEM) without phenol red was purchased from Gibco-Invitrogen (Carlsbad, CA). Type IV horseradish peroxidase (HRP) was purchased from Sigma Chemical Co. (St. Louis, MO). AA, LA, LTB4, LTC4, LTD4, phorbol myristate acetate (PMA), MK886 [5-LO-activating protein (FLAP) inhibitor], AA-861 (5-LO inhibitor), MK571 [cysteinyl LT receptor 1 (cysLT1) antagonist], and diphenyleneiodonium (DPI; NADPH oxidase-like flavoprotein inhibitor) were purchased from Biomol (Palo Alto, CA). CP105,696 [LTB4 receptor 1 (BLT1) antagonist] was a generous gift of Dr. Henry Showell (Pfizer, Groton, CT). Abbott Laboratories (Abbott Park, IL) provided zileuton. Amplex Red (10-acetyl-3,7-dihydroxyphenoxazine) was purchased from Molecular Probes (Eugene, OR). Compounds requiring reconstitution were dissolved in ethanol or dimethyl sulfoxide. Required dilutions of all compounds were prepared immediately before use, and equivalent quantities of vehicle were added to the appropriate controls.

Animals
5-LO knockout (KO; 129-Alox5tm1Fun) [36 ] and strain-matched wild-type (WT) sv129 mice were bred in the University of Michigan Unit for Laboratory Animal Medicine (Ann Arbor) from breeders obtained from Jackson Laboratories (Bar Harbor, ME), and female Wistar rats were obtained from Charles River Laboratories (Portage, MI). The University Committee on Use and Care of Animals approved animal protocols.

Cell harvest
Glycogen-elicited rat or murine PMNs were obtained following injection of 20 mL 4% glycogen (Sigma Chemical Co.) into the peritoneal cavity. After 5–6 h, the peritoneal exudates were harvested by lavage with phosphate-buffered saline (PBS). Contaminating red blood cells were lysed with H2O, and the cells were washed 2x with PBS. The percentage of PMNs was determined microscopically using a modified Wright-Giemsa stain, and a typical experiment yielded ~90% PMNs.

Cell viability
No experimental compounds or vehicle showed any adverse effects on PMN viability as determined by a cell-based 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay (data not shown).

Hydrogen peroxide (H2O2) detection
Rat PMNs (5x105/well) were plated in 96-well dishes as described above. H2O2 production was assessed in a HRP-coupled reaction using Amplex Red as a probe, according to the manufacturer (Molecular Probes). To assess the role of 5-LO metabolites in H2O2 production stimulated by PUFAs, the cells were first pretreated with zileuton, AA-861, MK886, MK571, or CP105,696 for 20 min followed by the addition of the Amplex Red solution containing AA or LA. The solution containing 50 µM Amplex Red reagents and 10 U/mL HRP was prepared in PBS, 0.1 mL was added to PMNs at 37°C for 30 min, and the H2O2 levels were determined colorimetrically (absorbance at 560 nm).

LTB4 measurement
Rat PMNs (5x105 cells/well) were cultured in 96-well plates in DMEM. Cultures were then incubated for 5, 15, and 30 min with 20 µM AA to stimulate LT production. Culture supernatants were collected, and LTB4 levels were quantified by enzyme immunoassay, according to the manufacturer (Assay Designs, Ann Arbor, MI).

Fractionation, immunoprecipitation, and Western blotting
PMNs (1x107) were plated in six-well tissue-culture dishes and pretreated with AA-861 or CP105,696 for 20 min, followed by stimulation for 5 min with 20 µM AA or vehicle control. After this, PMNs were lysed by sonication in ice-cold lysis buffer containing 150 mM Tris-HCl (pH 8.0), 100 mM NaCl, 1 mM sodium orthovanodate, 1 mM phenylmethylsulfonyl fluoride, 50 mM NaF, and 1 µg/mL leupeptin, followed by ultracentrifugation at 100,000 g for 20 min at 4°C. The cytosolic (soluble) fraction was harvested, and the membrane (insoluble) fraction was washed and subjected to another ultracentrifugation step as described above. The resulting pellet was resuspended in lysis buffer and sonicated. Protein concentrations were determined by a modified Coomassie dye-binding assay (Pierce Chemical Co., Rockford, IL). The cytosolic fraction was used for immunoprecipitation as described previously [37 ] with some modifications. The fraction was incubated overnight at 4°C with anti-p47phox antibody (1:80; Upstate Biotechnology, Lake Placid, NY). Protein A-Sepharose was added to each sample and incubated for 3 h with rotation at 4°C. The beads were washed briefly 3x with lysis buffer without Triton X-100, and samples containing 20 µg protein were separated on 8% sodium dodecyl sulfate-polyacrylamide gel electrophoresis gels and then transferred to nitrocellulose membranes. Following blocking with 5% nonfat milk, membranes were probed with antibodies directed against p47phox (1:500 dilution; Upstate Biotechnology) or antiphosphoserine (1:900; KI-191, clone 1C8; Biomol) for 90 min, followed by peroxidase-conjugated goat anti-rabbit (Amersham, Piscataway, NJ) or anti-mouse secondary (1:5000; Zymed, South San Francisco, CA) and developed using enhanced chemiluminescence detection (Amersham).

Statistical analysis
Data are represented as mean ± SE and were analyzed with the Prism 3.0 statistical program (GraphPad Software, San Diego, CA). Comparisons between two experimental groups were performed with Student’s t-test. Comparisons among more than or equal to three experimental groups were performed with ANOVA followed by the Bonferroni analysis. Differences were considered significant if P ≤ 0.05. All experiments were performed on more than or equal to three separate occasions unless otherwise specified.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
PUFAs induce H2O2 release in glycogen-elicited PMNs via NADPH oxidase
To determine if PUFAs activate NADPH oxidase (as determined by H2O2 production), we performed dose-response experiments using AA or LA. As shown in Figure 1A and 1B , AA and LA induced H2O2 generation in a dose-dependent manner. At the plateau dose (20 µM) of either PUFA, we observed the production of approximately five times more H2O2 than by untreated cells. As concentrations of AA above 20 µM can induce cell necrosis [38 ], we used 20 µM of both PUFAs for subsequent experiments.



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  		Figure 1. PUFAs stimulate PMN H2O2 release in a dose-dependent manner. H2O2 from glycogen-elicited rat PMNs was measured 30 min after stimulation with AA (A) or LA (B). H2O2 concentrations were determined using Amplex Red as a probe as described in Materials and Methods. Data represent mean ± SE from quadruplicate values from one experiment representative of a total of four. *, P< 0.05,versus control by ANOVA.

 
Reactive oxygen intermediates (ROI) may be generated by a variety of sources, including enzymes other than NADPH oxidase [39 , 40 ]. To determine if ROI produced during PUFA activation is from NADPH oxidase activation, we pretreated PMNs with DPI, a flavoprotein inhibitor that has relative specificity for NADPH oxidase at a concentration of 10 µM, followed by addition of 20 µM AA or LA. Our data show that the major source of ROI during AA or LA stimulation is through activation of NADPH oxidase, as H2O2 production was abolished completely in DPI-treated as compared with untreated cells (Fig. 2 ).



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  		Figure 2. PMN H2O2 production in response to PUFAs is dependent on NADPH oxidase activity. PMNs were treated with the NADPH oxidase inhibitor DPI (10 µM) for 20 min and then stimulated for 30 min with 20 µM AA or 20 µM LA. H2O2 production was determined using Amplex Red as a probe, as described in Materials and Methods. Data represent mean ± SE from quadruplicate values from one experiment representative of a total of four. *, P < 0.05, versus control and #, P< 0.05 versus PUFA alone by ANOVA.

 
NADPH oxidase activation by PUFAs is partially dependent on 5-LO activity
The role of cPLA2 activation and AA release in NADPH oxidase activation is well-established [18 , 41 ]. As AA is the substrate for 5-LO, and 5-LO metabolites are themselves able to induce NADPH oxidase activation [29 , 30 , 42 ], we sought to determine if such products are important in mediating the effects of PMN stimulation by PUFAs.

Our first approach was to determine whether endogenous 5-LO metabolites play a role in PUFA-stimulated H2O2 production. Rat PMNs were pretreated with either of two inhibitors of LT synthesis, namely, the 5-LO inhibitor zileuton (10 µM) or the FLAP inhibitor MK886 (1 µM), for 20 min before activation with AA or LA (20 µM). As observed in Figure 3A and 3C , zileuton and MK886 treatment decreased by ~40% the AA- or LA-induced H2O2 production as compared with untreated cells. Similar experiments using cells from 5-LO-deficient mice confirmed these results (Fig. 3B and 3D) . It is interesting that when the PMNs were stimulated with a lower dose of AA (5 µM; a concentration of AA that is perhaps more relevant to its possible function as an endogenous second messenger), the role of endogenous 5-LO metabolites in inducing H2O2 generation was even more pronounced. For example, stimulation of H2O2 generation with 20 µM AA was inhibited by ~40% by the 5-LO inhibitor AA-861, but with 5 µM AA, the inhibition was ~75% when compared with untreated control (control, 2.93±0.129; AA, 11.23±0.134; AA+AA-861, 5.080±0.240 nM H2O2/2x105 PMN/30 min; P<0.05 for AA+AA-861 vs. AA alone). Taken together, these data indicate that H2O2 production stimulated by PUFAs in glycogen-elicited PMNs is dependent, in part, on endogenous 5-LO products.



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  		Figure 3. H2O2 production in response to PUFAs in LT-deficient PMNs. Glycogen-elicited rat PMNs were pretreated with the 5-LO inhibitor zileuton (10 µM) or the FLAP inhibitor MK886 (1 µM) for 20 min and then stimulated for 30 min with 20 µM AA (A) or 20 µM LA (C). Glycogen-elicited PMNs from WT or 5-LO KO mice were stimulated with AA (B) or LA (D) for 30 min. H2O2 concentrations were determined using Amplex Red as a probe as described in Materials and Methods. Data represent the mean ± SE of quadruplicate values from one experiment representative of a total of four. *, P< 0.05 versus control and #, P< 0.05 versus PUFA alone by ANOVA.

 
NADPH oxidase activation by endogenous 5-LO products involves the BLT1 receptor
The major products of the 5-LO pathway include LTB4 as well as the cysteinyl LTs C4, D4, and E4. Both act in leukocytes primarily through BLT1 or cysLT1 receptors. To evaluate which of these LTs are involved in the activation of NADPH oxidase by PUFAs, we pretreated cells with the BLT1 antagonist CP105,696 (10 µM), or the cysLT1 antagonist MK571 (10 µM) for 20 min before PUFA stimulation. Antagonism of BLT1 abolished H2O2 secretion in PMNs stimulated with AA or LA. In contrast, PMNs pretreated with the cysLT1 antagonist MK571 did not demonstrate altered H2O2 production when compared with the untreated group (Fig. 4A and 4B ). To exclude the possibility that CP105,696 was merely acting as a H2O2 scavenger in this circumstance, we assayed a range of concentrations of added reagent H2O2 in the presence or absence of 10 µM CP105,696 and detected no influence of the antagonist (data not shown). Furthermore, we examined if the BLT1 antagonist influenced NADPH oxidase activation in response to the strong stimulus PMA, and we observed no effect of the antagonist (data not shown). To investigate the role of BLT1 in PMNs stimulated with 5 µM AA, we pretreated PMNs with the BLT1 antagonists CP105,696 (10 µM) or U75302 (1 µM) followed by AA stimulation. As observed above, the effects of both antagonists were again more evident at this lower AA concentration (control, 2.93±0.129; AA, 11.23±0.134; AA+CP105,696, 2.700±0.385; AA+U75302, 6.725±0.125 nM H2O2/2x105 PMN/30 min. P<0.05 for AA+CP105,696 or AA+U75302 vs. AA alone).



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  		Figure 4. Role of LT receptors in PMN H2O2 production by PUFAs. Glycogen-elicited rat PMNs were treated with the BLT1 antagonist CP105,696 (10 µM) or the cysLT1 antagonist MK571 (10 µM) and then stimulated for 30 min with 20 µM AA (A) or 20 µM LA (B). H2O2 production was determined using Amplex Red as a probe as described in Materials and Methods. Data represent mean ± SE of quadruplicate values from one experiment representative of a total of four. *, P< 0.05 versus control and # P< 0.05, versus PUFA alone by ANOVA.

 
To confirm that LTB4 itself can increase NADPH oxidase activity in rat PMNs, we incubated cells with increasing concentrations of LTB4 or cysteinyl LTs (LTC4 and LTD4). The stimulation of PMNs with 0.01–10 nM LTB4 induced H2O2 secretion in a dose-dependent manner (Fig. 5A ). However, we observed no H2O2 production in PMNs stimulated by cysLTs at any concentration tested (10–1000 nM LTC4 or LTD4; data not shown). Thus, our results show that only exogenous LTB4 can induce NADPH oxidase activation in PMNs.



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  		Figure 5. AA stimulates LTB4 production followed by H2O2 secretion in PMN from rats. (A) H2O2 levels were measured as described in Materials and Methods after stimulation with different LTB4 concentrations. (B) LTB4 and H2O2 levels were measured as described in Materials and Methods in PMN supernatants at different time-points following addition of 20 µM AA. Data represent mean ± SE of quadruplicate values from one experiment representative of a total of four. *, P< 0.05 versus control by ANOVA.

 
To determine the kinetic relationship between LTB4 production and H2O2 generation in response to exogenous AA, we performed a time-course experiment in PMNs stimulated with 20 µM AA. LTB4 production was detected within 5 min of AA addition, appeared to plateau at 15 min, and increased once again by 30 min. By contrast, H2O2 demonstrated an initial lag period, until an increase could be observed between 5 and 15 min, which increased further by 30 min (Fig. 5B) . Thus, our results indicate that LTB4 production precedes NADPH oxidase activation in AA-stimulated PMNs, a temporal relationship consistent with the possibility that the former response contributes to the latter.

Next, we wished to investigate whether it was possible to restore H2O2 secretion with exogenous LTB4 in PMNs treated with inhibitors of endogenous LT synthesis and if the LTB4 effect was a result of interaction with the BLT1 receptor. We therefore pretreated PMNs for 20 min with the 5-LO inhibitor AA-861 (10 µM; Fig. 6A ) or the BLT1 antagonist CP105,696 (10 µM; Fig. 6B ), followed by stimulation with AA and/or LTB4. Our results show that exogenous LTB4 restored the inhibition of H2O2 generation in PMNs treated with the 5-LO inhibitor. In contrast, the addition of LTB4 to AA-treated PMNs incubated with the BLT1 antagonist did not overcome this inhibition. These results indicate that endogenous LTB4 is important in NADPH oxidase activation by AA, and this effect is mediated by interaction with the BLT1 receptor.



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  		Figure 6. Role of BLT1 receptor in mediating H2O2 stimulation. Glycogen-elicited rat PMNs were treated with the 5-LO inhibitor AA-861 (10 µM; A) or the BLT1 antagonist CP105,696 (10 µM; B) and then stimulated for 30 min with 20 µM AA and/or 10 nM LTB4. H2O2 production was determined using Amplex Red as a probe as described in Materials and Methods. Data represent mean ± SE of quadruplicate values from one experiment representative of a total of four. *, P< 0.05 versus control; #, P< 0.05 versus AA; and &, P< 0.05 versus LTB4 by ANOVA.

 
LTB4 induces p47phox phosphorylation and translocation in AA-stimulated PMNs
As our data identified a role for LTB4 in AA activation of NADPH oxidase, we sought to determine the relevant molecular mechanism. We first evaluated if LTB4 contributed to p47phox phosphorylation in PMNs stimulated by AA. As can be observed in Figure 7A , AA elicited phosphorylation of p47phox, and pretreatment of PMNs with a 5-LO inhibitor as well as a BLT1 antagonist decreased AA-induced p47phox phosphorylation. In addition, LTB4 itself induced p47phox phosphorylation. As p47phox translocation is dependent on phosphorylation in some cell types [43 ], we evaluated the effects of a 5-LO inhibitor and a BLT1 antagonist on this phenomenon in AA-stimulated PMNs. Our results show that 5-LO inhibition as well as BLT1 antagonism inhibited p47phox translocation to the membrane fraction, indicating that endogenous LTB4 contributes to p47phox translocation in PMNs stimulated by AA (Fig. 7C) . In addition, we observed the same profile in LA-stimulated cells (data not shown). To directly test the capacity of LTB4 to promote NADPH oxidase activation, we stimulated PMNs with 10 nM LTB4 and observed phosphorylation and membrane translocation of p47phox. This finding therefore extends to PMNs what we have recently demonstrated with LTB4 in alveolar macrophages [31 ]. Together, these results suggest that LTB4 is partially responsible for the NADPH oxidase activation induced by AA, through effects on phosphorylation as well as membrane translocation of p47phox.



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  		Figure 7. AA induces phosphorylation and translocation of p47phox in glycogen-elicited rat PMNs in a 5-LO- and BLT1-dependent manner. PMNs (4x106/well) were pretreated or not with CP105,696 or AA-861, stimulated by 20 µM AA or 10 nM LTB4 for 5 min, and then harvested and fractionated as described in Materials and Methods. The cytosolic fraction was immunoprecipited, and immunoprecipited protein was immunoblotted using antiphosphoserine (1:900), and phosphorylated protein is designated p-p47phox (A). The total cytosolic fraction (B) and membrane fraction (C) were probed for total (T) p47phox (1:500). Experiment is representative of a total of two.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
This study establishes a role for LTB4 in NADPH oxidase activation by exogenous PUFAs in elicited rodent PMNs. Specifically, our results show that AA and LA are able to induce secretion of H2O2 in elicited PMNs; NADPH oxidase is the major source of H2O2 in PUFA-stimulated PMNs; PUFA-induced H2O2 generation is dependent on 5-LO activity and LTB4 biosynthesis, and effects of LTB4 are mediated by the BLT1 receptor; and LTB4 contributes to p47phox phosphorylation and membrane translocation in AA-stimulated PMNs.

PUFAs are involved in inflammatory conditions, such as modulation of vascular contraction, chemotaxis, cell adhesion and diapedesis, cell activation, and cell death [44 ]. AA is a pleiotropic lipid involved in inflammatory and physiological situations. A role for AA in inflammation has been suggested by studies showing that AA increases chemotaxis, granule release, and effector functions such as phagocytosis, microbicidal activity, and release of ROI in PMNs [45 ]. The role of AA as a second messenger [2 ] and its ability to activate NADPH oxidase are well-known [4 , 5 , 12 13 14 15 16 17 18 19 ]. In addition, AA is the precursor of bioactive lipids including prostanoids and LTs, which are themselves able to induce cell activation. Upon activation, the major AA metabolites produced by PMNs are LTB4 and 5-hydroxy-5-HETE [32 ], but the respective roles of AA and its metabolic products in cell signaling are incompletely understood. Thus, we sought to evaluate the role of these 5-LO products in AA and LA stimulation of NADPH oxidase activation.

We first asked if PUFAs activate NADPH oxidase by themselves or via formation of metabolic products. As expected [18 , 19 ], both PUFAs were able to induce H2O2 release dose-dependently to a similar extent at concentrations similar to those observed previously. To address the role of 5-LO metabolites in AA- or LA-stimulated PMNs, we inhibited 5-LO or FLAP (both essential for LT synthesis [46 ]), and we observed an inhibition in H2O2 production of ~40% when compared with the untreated control. This effect was confirmed using cells obtained from genetically 5-LO-deficient mice. Although ROI can be generated by a variety of enzymatic sources [47 ], the ability of the NADPH oxidase-like flavoprotein inhibitor DPI to completely attenuate ROI production in AA- or LA-stimulated PMNs suggests that PUFA effects occurred exclusively through activation of NADPH oxidase.

Our results strongly suggest that AA and LA act to increase 5-LO activity, which results in NADPH oxidase activation. These results are consistent with studies in AA-stimulated guinea pig eosinophils pretreated with the AA congener eicosatetraynoic acid (ETYA), which prevents the further metabolism of AA [48 ]. By contrast, Pompeia et al. [38 ] demonstrated that O2– production by HL-60 cells stimulated with AA was independent of 5-LO metabolites. Regarding the dependence on AA metabolites observed with exogenous LA stimulation, Alzoghaibi et al. [49 ] showed that LA-induced interleukin-8 production in human intestinal smooth muscle cells was dependent on 5-LO and cyclooxygenase metabolites. To our knowledge, however, this is the first report showing that LA induction of NADPH oxidase activity depends on 5-LO metabolites. That endogenous 5-LO activity is important for activating NADPH oxidase has been demonstrated in PMNs stimulated by myriad agonists. Maridonneau-Parini et al. [50 ] showed that opsonized zymosan, but not PMA, induced O2– generation in a manner dependent on 5-LO and cyclooxygenase products in human PMNs. Furthermore, it was observed that 5-LO metabolites are necessary for NADPH oxidase activation in PMNs stimulated by platelet-activating factor, formyl-Met-Leu-Phe, PMA, and A23187 [51 ]. Although the role of 5-LO metabolites in NADPH oxidase activation is recognized, the importance of individual LTs in PUFA-induced NADPH oxidase activation is not. When we pretreated PMNs with BLT1 or cysLT1 receptor antagonists followed by stimulation with AA or LA, we observed that only the BLT1 antagonist abolished the H2O2 secretion in PUFA-stimulated PMNs. Likewise, only LTB4, but not cysLTs, was capable of stimulating secretion of H2O2 in PMNs. To confirm that the effects of PUFAs were dependent on LTB4 interaction with BLT1, we performed "add back" experiments. The first approach was to pretreat cells with AA-861 and then add AA with or without LTB4. Our experiments clearly show that in 5-LO-inhibited cells, the addition of LTB4 restored the PMN ability to secrete H2O2. We confirmed that the LTB4 effects are mediated by interaction with BLT1, as LTB4 addition was ineffective in BLT1 antagonist-treated cells. At lower (i.e., 5 µM) AA concentrations more relevant to those produced endogenously, which might function as second messengers, we observed an even more prominent dependence on 5-LO products and BLT1 in AA-stimulated PMNs. This could reflect the ability of lower doses of AA to activate classical PKC [11 ] or Ca++ release [9 ], both of which are known to be important in the activation of 5-LO [46 ].

Results with 5-LO inhibitors and 5-LO KO cells as well as a BLT1 antagonist implicate 5-LO metabolism/BLT1 signaling in NADPH oxidase activation by AA and LA. However, we could only measure immunoreactive LTB4 production in PMNs stimulated with AA and not with LA. These results are consistent with an alternative LT derivative of LA other than LTB4 interacting with BLT1 to affect NADPH oxidase, but we are aware of no precedent regarding such a possibility. It will be of interest to clarify this in future studies.

That PMNs respond predominantly to LTB4, but not cysLTs, is in agreement with previous findings from our laboratory and others. Mancuso et al. [52 ] showed that LTB4, but not cysLTs, increased phagocytosis of opsonized Klebsiella pneumoniae in human PMNs. In addition, Palmblad et al. [53 ] showed that only LTB4 induced NADPH oxidase activation in eosinophils and PMNs. However, Larfars et al. [42 ] demonstrated that LTB4 and cysLTs were able to induce nitric oxide production in human PMNs. In our experimental system, a role for 5-LO products in PUFA stimulation of NADPH oxidase was confined to LTB4 and its interaction with BLT1.

Although AA promotes p47phox translocation in human monocytes, its ability to enhance phosphorylation of this molecule has been the subject of conflicting reports [19 ]. In eosinophils, LTB4 stimulated H2O2 production independent of AA release [29 ], and this effect was dependent on PKC, PLC, and Ca++ [30 ]. The molecular mechanism by which LTB4 activates NADPH oxidase activation in PMNs is unclear. Thus, we sought to determine the effects of LTB4 on p47phox phosphorylation and translocation in AA-stimulated PMNs. Our results show that inhibition of 5-LO metabolism or antagonism of BLT1 decreased phosphorylation and translocation of p47phox in AA-stimulated cells and that exogenous LTB4 was able to increase its phosphorylation and translocation. We have observed similar effects of LA. Together, our results suggest that LTB4 contributes to activation of the p47phox cytosolic component of NADPH oxidase in PUFA-stimulated PMNs.

Activation of p47phox is regulated by the action of a variety of kinases, including PKC [43 ]. PUFAs act as second messengers, in part, by stimulating these kinases [2 ]. Our present data are the first to implicate 5-LO products in PUFA-induced p47phox activation. The plausibility of such a role for LTB4 is emphasized by our recent demonstration in macrophages that LTB4 elicits activation of p47phox via effects on the upstream kinase PKC-{delta} [31 ].

In conclusion, our experiments show that n-6 PUFAs activate NADPH oxidase in glycogen-elicited PMNs, and this activation is partially dependent on 5-LO activity, LTB4 synthesis, and BLT1 signaling. Indeed, we demonstrated that LTB4 is required for two essential aspects of NADPH oxidase activation, namely, phosphorylation and translocation of p47phox, in PMNs stimulated by AA. This study presents a new model of NADPH oxidase activation by PUFAs, and the identification of a role for LTB4 suggests possible therapeutic interventions targeting this lipid mediator for inflammatory conditions in which ROI production is responsible for tissue injury.


   ACKNOWLEDGEMENTS
 
This work was supported by FAPESP and CAPES (Brazil), NIH HL-058897, and the Parker B. Francis Foundation. The authors acknowledge Teresa Marshall for technical contributions and Michael Coffey, Thomas Brock, and Claudio Canetti for helpful discussions.

Received October 14, 2004; revised May 26, 2005; accepted June 6, 2005.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
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