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Cardiovascular Research Advance Access originally published online on October 13, 2008
Cardiovascular Research 2009 81(1):216-225; doi:10.1093/cvr/cvn277
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Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2008. For permissions please email: journals.permissions@oxfordjournals.org

Leukotriene B4 enhances the activity of nuclear factor-{kappa}B pathway through BLT1 and BLT2 receptors in atherosclerosis

Eva Sánchez-Galán1,2, Almudena Gómez-Hernández1,2, Cristina Vidal1,2, José Luis Martín-Ventura1,2, Luis Miguel Blanco-Colio1,2, Begoña Muñoz-García1, Luis Ortega3, Jesús Egido1,2 and José Tuñón2,4,*

1 Vascular Research Laboratory, Madrid, Spain
2 Autónoma University, Madrid, Spain
3 Department of Pathology, Hospital Clínico, Madrid, Spain
4 Department of Cardiology, Fundación Jiménez Díaz, Avenida Reyes Católicos 2, 28040 Madrid, Spain

* Corresponding author. Tel: +34 915504816; fax: +34 915497033. E-mail address: j.tunon{at}wanadoo.es

Received 11 March 2008; revised 27 September 2008; accepted 6 October 2008

Time for primary review: 27 days


   Abstract
Top
 Abstract
1. Introduction
 2. Methods
 3. Results
 4. Discussion
 Funding
 References
 
Aims: Leukotriene B4 (LTB4) is a powerful chemoattractant and pro-inflammatory mediator in several inflammatory diseases, including atherosclerosis. It acts through its two membrane receptors, BLT1 and BLT2. The aim of this study was to determine the molecular mechanism involved in the proatherogenic effect of LTB4, BLT1 and BLT2 in atherosclerosis. Moreover, we characterized the expression of 5-lipoxygenase (5-LO) pathway and LTB4 receptors in blood and plaques from patients with carotid atherosclerosis.

Methods and results: In cultured monocytic cells, LTB4 induced a rapid phosphorylation of mitogen-activated protein kinases (MAPKs ERK1/2 and JNK1/2) and PI3K/Akt via BLT1 and BLT2 in a pertussis toxin (PTX)-dependent mechanism (assessed via western blotting) and also increased nuclear factor-{kappa}B (NF-{kappa}B) DNA binding activity (assessed via EMSA) in a MAPK- and reactive oxygen species-dependent mechanism. Furthermore, LTB4 elicited interleukin-6, monocyte chemoattractant protein-1 and tumour necrosis factor-{alpha} mRNA overexpression also via BLT1 and BLT2 by a PTX- and NF-kB-dependent mechanism (assessed by real-time PCR), promoting an inflammatory environment. When compared with healthy subjects, patients with carotid atherosclerosis showed a significant increase in the expression of all the components of the 5-LO pathway and BLT1 and BLT2 mRNA (real-time PCR) in peripheral blood mononuclear cells and LTB4 plasma levels (ELISA). In these patients, an overexpression of 5-LO, leukotriene A-4 hydroxylase (LTA4-H) and BLT1 was noted in the inflammatory region of carotid plaques when compared with the fibrous cap (assessed by immunohistochemistry).

Conclusion: The 5-LO pathway is enhanced in patients with carotid atherosclerosis. Furthermore, its product LTB4 phosphorylates MAPKs and stimulates NF-{kappa}B-dependent inflammation via BLT1 and BLT2 receptors in cultured monocytic cells. The blockade of this pathway could be a novel and potential therapeutic target in atherothrombosis.

KEYWORDS Leukotriene B4; BLT1; BLT2; Inflammation; NF-{kappa}B; Atherosclerosis


   1. Introduction
Top
 Abstract
 1. Introduction
2. Methods
 3. Results
 4. Discussion
 Funding
 References
 
Leukotrienes are inflammatory lipid mediators derived from the 5-lipoxygenase (5-LO) cascade of arachidonic acid.1 Leukotriene B4 (LTB4), one of the final products of this pathway, is a potent chemoattractant and proinflammatory mediator in several inflammatory diseases, including atherosclerosis.2 It is secreted by several cells such as monocytes, macrophages, mast cells, or neutrophils. LTB4 is present in human atherosclerosis3 along with the components of the 5-LO pathway, 5-LO, 5-LO activating protein (FLAP) and LTA4 hydrolase (LTA4-H), which are involved in the pathogenesis of this disorder.2,4 LTB4 binds to cell surface G-protein-coupled receptors BLT1 and BLT2, generating a variety of intracellular signals, such us calcium mobilization or adenylate cyclase inhibition. BLT1 and BLT2 are, respectively, the high- and the low-affinity receptors. BLT1 is preferentially expressed in leukocytes and monocytes, while BLT2 is ubiquitously expressed.5,6

Apolipopotein E knockout (ApoE–/–) and low density lipoproteins receptors knockout (LDLr–/–) mice treated with BLT1 antagonists showed a reduced lipid accumulation, monocyte infiltration, and size of atheroma.7 Moreover, levels of adhesion molecules, such us CD11b, were reduced in mice treated with BLT1 antagonists.7 Macrophage cell lines from BLT1–/– mice expressed BLT2 receptor and exhibited chemotaxis to LTB4.8 In rat basophilic cells, LTB4 promotes conversion of monocytes to foam cells through an enhanced expression of scavenger receptors (CD36) and subsequent fatty acid accumulation.9 From these studies, it was concluded that effects of LTB4 in chemotaxis and in promoting foam cells are mediated by BLT1 and BLT2 receptors. Other studies in smooth muscle cells show that BLT1 expression was blunted by dominant-negative I{kappa}kinase-B, indicating a role for nuclear factor-{kappa}B (NF-{kappa}B). The involvement of BLT1 and BLT2 in atherosclerosis is not clear.

In this study, we have investigated the intracellular mechanisms involved in the atherogenic effects of LTB4 and their possible regulation in a monocytic cell line (U-937). Moreover, we have examined LTB4 plasma levels and the 5-LO pathway in peripheral blood mononuclear cells (PBMC) and plaques from subjects with carotid atherosclerosis to assess the in vivo relevance of our in vitro studies.


   2. Methods
Top
 Abstract
 1. Introduction
 2. Methods
3. Results
 4. Discussion
 Funding
 References
 
2.1 Cell culture
U-937 monocytic cells [ATCC (TIB 202)] were cultured in RPMI supplemented with 10% heat inactived foetal bovine serum, 2 mmol/L glutamine, 100 U/mL penicillin (GIBCO, BRL), at 37°C in 5% CO2. Before the experiments, cells were growth-arrested by incubation in serum-free medium for 48 h and then replaced with fresh serum-free medium with or without (control cells) treatment. For the inhibition experiments, U-75302 and Ly-2552837 (BLT1 and BLT2 selective antagonists, respectively) (Cayman Chemical, Ann Arbor, MI, USA), Pertussin Toxin (PTX, G{alpha}i/0 protein inhibitor) (Sigma, Saint Louis, MO, USA), MG-132, Bay117032, and parthenolide (NF-{kappa}B inhibitors) (Calbiochem, La Jolla, CA, USA), PD98059, a ERK1/2 inhibitor, SB203580, a p-38 MAPK inhibitor, SP-600125, a JNK inhibitor, Wortmannin, a AKT inhibitor, and two different antioxidants, DPI and TIRON (Sigma, Saint Louis, MO, USA) were added to the cultured medium 1 h before treatment with LTB4 (Cayman Chemical, Ann Arbor). All the drugs used were prepared and stored in dimethylsulfoxide (DMSO) at a final concentration of 10–2 or 10–3 mol/L at –20°C. To achieve the concentration used for the different compounds, further serial 10-fold dilutions were done in serum-free RPMI-1640 cell culture medium, which was instantly added to cells yielding a final DMSO concentration <0.1%. At the doses tested, neither vehicle nor the drugs produced significant cell toxicity or apoptotic cell death (analysed by cell morphology and flow cytometry; data not shown).

2.2 Analysis of nuclear factor-{kappa}B DNA binding activity
NF-{kappa}B DNA-binding activity was determined as described.10 Briefly, 5-µg nuclear protein extracts binds to a [{gamma}-32P]- ATP-labelled oligonucleotide containing the NF-{kappa}B sequence (5'-AGTTGAGGGGACTTTCCCAGGC-3') (Promega, Madison, WI, USA). Complexes were analysed by electrophoretic mobility shift assay (EMSA).10 Competition assays were performed by adding 100-fold excess of cold probe before the labelled probe. For supershift, nuclear extracts were incubated with 1.0 µg of p50 and p65 antibodies from Sta. Cruz Biotechnology (Santa Cruz, CA, USA) 1 h before incubation with labelled oligonucleotide.

2.3 Immunofluorescence
NF-{kappa}B localization was performed by indirect immunofluorescence. Cells were fixed in paraformaldehyde 2% (Sigma, Saint Louis, MO, USA), treated with 0.1% Triton X-100, and incubated with anti-p65 antibody at a final concentration of 1:50 (sc-372X, Santa Cruz Biotechnology) followed by FITC-conjugated antibody at a final concentration of 1:200 (Sigma, Saint Louis, MO, USA). The integrity of the nuclei was confirmed with propidium iodide (IP) staining (1 µg/mL) (Sigma, Saint Louis, MO, USA). Controls were stained with the secondary antibody alone (not shown). Coverslips were mounted in FluorSave (Calbiochem, La Jolla, CA, USA) and examined by a laser scanning confocal microscope (Leika).

2.4 Protein studies
To quantify protein levels, western blot analyses were also performed. Protein extracts from cultured cells were obtained by homogenization and centrifugation as previously described.11 Then, samples were separated using a 8–12% SDS–polyacrylamide gel electrophoresis and were transferred to polyvinylidene difluoride membranes (Millipore, Bedford, MA, USA). They were blocked in PBS containing 0.1% Tween-20, 7.5% dry skimmed milk for 1 h at room temperature and incubated in the same buffer with specific antibodies phospho-IKK{alpha}β, IKK{alpha}, IKKβ, phospho-I{kappa}B{alpha}, I{kappa}B{alpha}, phospho-ERK1/2, ERK1/2, phospho-JNK1/2, JNK1/2, phospho-AKT, AKT, phospho-p38, and p38 (Santa Cruz Biotechnology) overnight at 4°C. After washing, detection was made by incubation with peroxidase-conjugated secondary antibody, and developed using an ECL chemiluminiscence kit (Amersham Bioscience, Arlington Heights, IL, USA). All the first antibodies were diluted 1:1000 and secondary antibodies 1:5000. Proteins were quantified in all samples by the BCA method in accordance with the manufacturer's directions (Pierce, Rockford, IL, USA) and a fixed amount of protein (50 µg) was added in each lane to normalize for protein loading. The quality of proteins and efficacy of protein transfer were evaluated by Red Ponceau staining (not shown). To evaluate loading control, total isoform of antibody in all cases was used.

2.5 Patients
Seventeen patients undergoing carotid endarterectomy in our Institution were studied. Patients with malignancies, inflammatory diseases, coagulation disorders, or those needing additional therapy for chronic conditions different than atherosclerosis or its risk factors were excluded. Informed consent was obtained before enrollment. Table 1 shows the baseline characteristics of the patients. Blood samples were collected the day of endarterectomy before surgery, and from 19 healthy volunteers without significant difference in age (55 ± 9 years) and sex (12 men/7 women). During surgery, carotid endarterectomy specimens were collected for the study. The researchers who performed the studies at the laboratory were blind to the origin of the samples. The study was approved by the Ethical Committee of the Fundación Jiménez Díaz in accordance with the principles outlined in the Declaration of Helsinki.


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  		Table 1 Baseline characteristics of patients with atherosclerosis

 
2.6 Isolation of peripheral blood mononuclear cells
Twenty millilitres of blood were obtained from patients and healthy controls. PBMC isolation was performed as described previously.10 Briefly, blood samples were diluted in phosphate-buffered saline 1:1, and the cells were separated in 5 mL Ficoll gradient (lymphocyte isolation solution; Rafer SL, Zaragoza, Spain) by centrifugation at 2000 g for 30 min. PBMCs were collected, washed twice with cold phosphate-buffered saline, and resuspended in appropriate buffer. Approximately 95% of the cells were mononuclear cells (by flow cytometry; data not shown).

2.7 ELISA
Plasma was obtained from blood drawn on EDTA after centrifugation (2500 g for 15 min) and aliquoted to avoid freeze/thaw cycles. LTB4 was measured by ELISA (R&D Systems, Minneapolis, MN, USA), and its concentrations were determined according to the manufacturer's directions. The sensitivity of the LTB4 assay is <20 pg/mL. Intra- and inter-assay coefficients of variation were <12%.

2.8 Gene expression studies
Total RNA from PBMC was isolated with TRIzolTM (Invitrogen, Carlsbad, CA, USA) according to the manufacturer's specifications and quantified by absorbance at 260 nm in duplicate. cDNA was synthesized with a high capacity cDNA Archive Kit (Applied Biosystems, Foster City, CA, USA) using 2 µg of total RNA primed with ramdon hexamer primers, following the manufacturer's instructions. Real time-PCR was performed using a fluorogenic TaqMan MGB probes and primers designed by Assay-on-DemandTM gene expression products (Applied Byosystem, Foster City, CA, USA). Assays on demand used were: human 5-LO (Hs00167536_m1), human FLAP (Hs00233463_m1), human LTA4H (Hs00251637_m1), human BLT1 (Hs00175124_m1), human BLT2 (Hs001885851_s1), human MCP-1 (Hs00234140_m1), human IL-6 (Hs00174131_m1), and human TNF-{alpha} (Hs00174128_m1). Quantitative RT–PCR was performed by 7500 Real Time PCR System, and the relative quantification was carried out with the Prism 7000 System SDS Software (Applied Biosystems).

The mRNA copy numbers were calculated for each sample with the instrument software using Ct value (‘arithmetic fit point analysis for the lightcycler’). Results were expressed in copy numbers, calculated in relation to control cells, after normalization against GAPDH (Hs99999905_m1), and 18S eukaryotic ribosomal (4310893E), as previously described.11 All primers, probes, software, and reagents were obtained from Applied Biosystems. All measurements were performed in duplicate.

2.9 Histological analysis
2.9.1 Tissue sampling
Specimens were stored in paraformaldehyde for 18 h and later in ethanol until being paraffin-embedded. The region of atheroma covering the bifurcation of the common carotid artery was chosen. We studied the atherosclerotic plaques in two different areas: the shoulder and the fibrous cap.10

2.9.2 Immunohistochemistry
Paraffin-embedded atherosclerotic plaques were dewaxed and rehydrated. The following polyclonal antibodies were used at a final concentration of 1:100 in PBS: FLAP (sc-28815) (Santa Cruz, CA, USA), 5-LO (160402), LTA4-H (160250), BLT1 (120114), and BLT2 (120124) (Cayman Chemical, Ann Arbor, MI, USA). A biotin-labelled secondary antibody was used at a final concentration of 1:200 in PBS (Amersham Bioscience, Arlington Heights). Then, ABComplex/HRP (DAKO) was added and sections were stained with 3,3'-diaminobenzidine (DAKO) and counterstained with haematoxylin. Negative controls using the corresponding IgG were included to check for non-specific staining (not shown). Results are expressed as a percentage of positive staining area in the shoulder and cap regions and quantified as described previously.10

2.9.3 Southwestern histochemistry
The distribution and DNA-binding activity of NF-{kappa}B in situ was detected using a digoxigenin-labelled double-stranded DNA probe (Roche Molecular Biochemicals, Indianapolis, IN, USA) with a specific NF-{kappa}B consensus sequence.10 Competition assays with 100-fold excess of unlabelled probe were used as negative controls.

2.10 Statistical analysis
Statistical analysis was performed with SPSS 11.0 (SPSS Inc., Chicago, IL, USA). Data were expressed as mean ± SEM and analysed by the Mann–Whitney U-test for in vivo studies and the t-Student (Bonferroni) for in vitro experiments. The Kolmogorov–Smirnov test was used to establish the parametric or non-parametric distribution of LTB4 plasma levels. To meet the distributional assumptions, LTB4 plasma levels were log-transformed and expressed as median (interquartile range). Statistical significance was defined as P < 0.05.


   3. Results
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
4. Discussion
 Funding
 References
 
3.1 5-LO pathway in the blood of patients with carotid atherosclerosis
First, we assessed the expression of 5-LO, FLAP, LTA4H, BLT1, and BLT2 proteins in PBMC isolated from the blood of atherosclerotic patients. The results showed a significant increase of 5-LO, FLAP, LTA4-H, BLT1, and BLT2 mRNA expression in PBMC of atherosclerotic patients in relation to healthy controls (Figure 1A). We also observed an increase of LTB4 levels in the plasma from patients compared to the control group (290 ± 48 vs. 131 ± 16 pg/mL; P < 0.005) (Figure 1B).


Figure 1
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  		Figure 1 The 5-LO pathway in the blood of atherosclerotic patients. (A) 5-LO/LTB4 pathway gene expression in peripheral blood mononuclear cells. 5-LO, LTA4-H, FLAP, BLT1, and BLT2 gene expression was greater in patients with carotid atherosclerosis (n = 17) than in healthy subjects (n = 19) (real time-PCR). Results are expressed as mean ± SEM. *P < 0.05 vs. healthy. (B) LTB4 plasma levels (ELISA) were also higher in patients than in healthy controls. Black and white bars represent patients and healthy subjects, respectively. Results are expressed as median (interquartile range).

 
3.2 Intracellular signalling induced by LTB4 through BLT1 and BLT2 in human monocytic cells
To investigate the intracellular signalling pathways that involve LTB4 activation, we analysed the MAPK and PI3K pathways as well as G-proteins. LTB4 induced phosphorylation of ERK1/2, JNK1/2, AKT, and p38 in a time-dependent mechanism (data not shown), peaking at around 15 min for all kinases in monocytes (western blot). Pre-treatment with U-75302 and Ly-255283 (BLT1 and BLT2 antagonists, respectively) diminished phosphorylation of ERK1/2, JNK1/2 (Figure 2A), and AKT (Figure 2B), but did not modify the p38 phosphorylation levels (Figure 2A). Moreover, we also examined by western blot the effect of PTX, a specific inhibitor of Gi/0-type G-proteins on LTB4-induced MAPK and AKT pathway activation. Pre-treatment with PTX prior to LTB4-stimulation, inhibited ERK1/2, JNK1/2 and AKT activation (Figure 2C). These results indicate that LTB4 induces ERK1/2, JNK1/2, and AKT phosphorylation via BLT1 and BLT2 in a PTX-sensitive mechanism.


Figure 2
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  		Figure 2 LTB4 activates ERK1/2, JNK1/2, and PI3K/AKT via BLT1 and BLT2 in a PTX-dependent mechanism in U-937 cells. Cells were serum deprived for 24 h, pre-treated with the BLT1 antagonist U-75302 (U; 10–7 mol/L), the BLT2 antagonist Ly-255283 (Ly; 10–7 mol/L) and Pertussis Toxin (PTX; 100nmol/L, a Gi/0 type G-proteins inhibitor) for 1 h. Later, they were stimulated with LTB4 10–7 mol/L for 15 min. The presence of increased phosphorylated proteins is considered as MAPK pathway activation. Total isoform of antibody in all cases was used to evaluate loading control. Control cells refer to unstimulated cells, without treatment. (A) shows data, expressed as mean ± SEM of five experiments (left panel), and a representative western blot (right panel). (B) shows data as mean ± SEM of five experiments (top panel) and a representative western blot (bottom). (C) shows mean ± SEM of five western blot experiments. *P < 0.001 vs. control; #P < 0.005 vs. LTB4.

 
3.3 LTB4 increases NF-{kappa}B activity through BLT1 and BLT2 in human monocytic cells
To further investigate the mechanism involved in the inflammatory effects of LTB4, we studied NF-{kappa}B activation, which is highly involved in the regulation of several pro-inflammatory genes involved in atherosclerosis.

LTB4 increased NF-{kappa}B DNA-binding activity in a dose- and time-dependent mechanism with maximal effect observed at 10–7 mol/L, peaking at 90 min (data not shown). Pre-treatment with U-75302 and Ly-255283 reduced LTB4-induced NF-{kappa}B activation (Figure 3A). No additive effect was observed when both antagonists were combined in cell culture medium. Supershift assays confirmed that LTB4 activated NF-{kappa}B complex as a p50/p65 heterodimer (Figure 3B).


Figure 3
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  		Figure 3 LTB4 increases NF-{kappa}B activity via BLT1 and BLT2 receptors in monocytic cells. (A) displays mean ± SEM values from four different experiments and a representative EMSA. Cells were pre-treated for 1 h with U-75302 (BLT1 antagonist), Ly-255283 (BLT2 antagonist), or both at doses of 10–7 mol/L in all cases and later stimulated with LTB4 at 10–7 mol/L for 90 min. Control refers to cells without treatment. *P < 0.001 vs. control; #P< 0.005 vs. LTB4. (B) shows supershift analysis of LTB4-induced p65/p50 NF-{kappa}B subunits. (C) shows a representative confocal picture of NF-{kappa}B p65 nuclear translocation induced by LTB4 (10–7 mol/L; 90 min) and the effect of U-75302 (BLT1 antagonist) and Ly-255283 (BLT2 antagonist) (10–7 mol/L for both). Indirect immunofluorescence was performed using p65 antibody and FITC-labelled secondary antibody (green staining). Nuclei were stained with propidium iodure (IP, in red). Merge refers to an overlapping of the same images to stain for p65 antibody and for propidium iodure. Protein levels of p-IKK{alpha}β, IKK{alpha}, IKKβ (D) and p-I{kappa}B{alpha} and I{kappa}B{alpha} (E) were determined by western blot. The upper panel shows data (mean ± SEM) of five experiments. The low panel displays a representative western blot. *P < 0.001 vs. control; #P < 0.005 vs. LTB4.

 
Additional assays with immunofluorescence techniques showed that, in control cells, a diffuse cytoplasmic immunofluorescence was seen with p65 antibody (Figure 3C). When cells were treated with LTB4 (10–7 mol/L) for 90 min, an intense nuclear fluorescence was observed, showing nuclear translocation of NF-{kappa}B. Pre-treatment with U-75302 and Ly-255283 diminished the nuclear translocation of the p-65 subunit of NF-{kappa}B.

NF-{kappa}B activation involves phosphorylation of I{kappa}B by the I{kappa}B kinase (IKK) complex, which results in I{kappa}B degradation. In control cells, I{kappa}B{alpha} was found in the cytosolic fraction. After 90 min of LTB4 stimulation, an increase in IKK{alpha}β and IkB{alpha} phosphorylation was observed (Figure 3D and E, respectively). BLT1- and BLT2-specific antagonists reduced IKK{alpha}β and I{kappa}B{alpha} phosphorylation in monocytes (Figure 3D and E, respectively). Moreover, the specific NF-{kappa}B inhibitors MG-132, Bay-117032, and parthenolide reduced I{kappa}B{alpha} phosphorylation (data not shown).

Furthermore, we observed crosstalk between ERK and JNK pathways in this process (Figure 4A). ERK activation was affected by inhibition of JNK with SP600125. The same happened with JNK, which was inhibited by the ERK1/2 inhibitor PD98059. Moreover, MAPK and NF-{kappa}B pathway are regulated by redox mechanisms.1,12 In this sense, we pre-treated the monocytes with two different antioxidants, DPI (an inhibitor of flavoprotein-containing enzymes such as NADPH/NADPH oxidase) and Tiron (an O2– scavenger) to determine the role of reactive oxygen species (ROS). As shown in Figure 4A and B, DPI and Tiron abolished LTB4-induced MAPK pathways activation and NF-{kappa}B activation, respectively. These results show that LTB4 regulates MAPK and NF-{kappa}B pathways in a ROS-dependent mechanism.


Figure 4
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  		Figure 4 LTB4 induced ERK, JNK, and NF-kB activation in a MAPK- and ROS-dependent mechanism. Quiescent U-937 cells were pre-incubated for 1 h with PD98059 (ERK1/2 inhibitor) at 10–6mol/L, SB203580 (p-38 MAPK inhibitor) at 10–6 mol/L, SP-600125 (JNK inhibitor) at 10–6 mol/L, Wortmannin (AKT inhibitor) at 10–6 mol/L, and two different antioxidants, DPI and TIRON at 10–5 mol/L for both. Later, monocytes were stimulated with LTB4 10–7 mol/L for 15 min (A) or 90 min (B). (A) Evaluation of interrelation among ERK, JNK, and ROS activation caused by LTB4. The figure shows data (mean ± SEM) from four western blot experiments. (B) LTB4 induced NF-kB activation in a MAPK- and ROS-dependent mechanism. The figure shows data as mean ± SEM of four experiments and a representative EMSA. The specificity of the reaction was established using competition assays with excess of unlabelled NF-{kappa}B oligonucleotide (Cold NF-kB; C). *P < 0.001 vs. control and #P < 0.005 vs. LTB4.

 
3.4 LTB4 induces proinflammatory factors through BLT1 and BLT2 by a PTX- and NF-{kappa}B-dependent mechanism in monocytic cells
In cultured monocytes, LTB4 upregulated IL-6, MCP-1, and TNF-{alpha} expression at 3 h (Real Time-PCR). U-75302, Ly-2552837, and PTX significantly diminished LTB4-mediated gene overexpression (Figure 5A). No additive effect was observed when both antagonists were combined. Pre-treatment with NF-{kappa}B inhibitors MG-132, Bay-117032, and parthenolide significantly decreased IL-6, MCP-1, and TNF-{alpha} mRNA induction caused by LTB4 in monocytes (Figure 5B). These results suggest that LTB4 induced the expression of pro-inflammatory cytokines IL-6, MCP-1, and TNF-{alpha}, via BLT1 and BLT2 in a PTX- and NF-{kappa}B-dependent mechanism.


Figure 5
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  		Figure 5 LTB4 induces pro-inflammatory cytokines through BLT1 and BLT2 in a NF-{kappa}B-dependent mechanism in U-937. (A) Treatment with U-75302 (U, 10–7 mol/L), Ly-255283 (Ly, 10–7 mol/L), and PTX (100 nmol/L) reduces the expression of IL-6, MCP-1, and TNF-{alpha}. (B) NF-{kappa}B inhibition with MG-132 (MG), Bay117032 (Bay), and parthenolide (10–7 mol/L, 1 h for all) significantly reduces LTB4-induced expression of IL-6, MCP-1, and TNF-{alpha}. Data are mean ± SEM of four real-time PCR experiments.*P < 0.001 vs. control; #P < 0.05 vs. LTB4.

 
3.5 Expression of 5-LO/LTB4 pathway are augmented in the shoulder region of carotid atherosclerotic plaques
To assess the in vivo relevance of our studies, we analysed 5-LO/LTB4 pathway in human atherosclerotic plaques. It has been shown previously that the shoulder region of human atherosclerotic plaques is characterized by an increase in macrophage infiltration, COX-2 expression, and NF-{kappa}B activation.10,13 A significant increase of 5-LO, LTA4-H, and BLT1 expression in the shoulder when compared with the cap of the lesions (Figure 6A) was observed. In contrast, FLAP and BLT2 expression was similar in both areas. Proteins of the 5-LO pathway and BLT1 and BLT2 receptors expression colocalized with active NF-{kappa}B (Figure 6B), suggesting that they could all modulate the transcriptional activity of NF-{kappa}B in human atheroma.


Figure 6
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  		Figure 6 Expression of 5-LO pathway in human carotid atheroma. (A) 5-LO, LTA4-H, and BLT1 expression are augmented in the shoulder of atherosclerotic plaques (n = 17). Representative examples of immunohistochemistry (Magnification x200). Results are expressed as mean ± SEM. Black and white bars represent shoulder and cap, respectively, *P < 0.005 and {dagger}P < 0.05 vs. shoulder. (B) Active NF-{kappa}B colocalizes with LTA4-H and BLT2. Blue colour shows active nuclear NF-{kappa}B. Brown colour displays LTA4-H and BLT2 expression (Magnification x400).

 

   4. Discussion
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
Funding
 References
 
Over the previous years, the leukotriene research field in atherosclerosis has experienced a great advance. Genetic studies have indicated a link between variants of FLAP, LTA4-H, and 5-LO genes and the development of atherosclerosis and myocardial infarction.14,15 Moreover, in patients with a specific risk to variants of FLAP and LTA4H, DG-031, therapy with a FLAP inhibitor, leads to dose-dependent suppression of biomarkers associated to an increased risk of myocardial infarction.16 DG031 is now in a phase III clinical trial for the prevention of myocardial infarction (http://www.decode.com/).

Further in vivo studies performed on mice have shown several connections among 5-LO, LTB4 receptors, hyperlipidaemia, and inflammatory chemokine production.7,17,18 The plaque-forming cells, including macrophages and smooth muscle cells, express the entire cascade of leukotriene biosynthetic proteins and receptors. In the present study, we have identified the first evidence, to our knowledge, that the proteins of the 5-LO pathway and BLT1 and BLT2 receptors are overexpressed in PBMCs of patients with atherosclerosis. We have demonstrated previously that NF-{kappa}B activity, COX-2, and mPGES-1 are enhanced in PBMCs of patients with carotid atherosclerosis when compared with healthy controls.10,11,13,19 In our study, we have also noted that LTB4 plasma levels are increased in these patients. This could suggest the presence of an inflammatory state in the circulating cells of these patients before they migrate into the arterial wall. As a limitation, we must acknowledge that plasma and PBMC data from patients with atherosclerosis could have been influenced by the stress of imminent surgery, a condition that was not present in healthy controls. Nevertheless, to our knowledge, there are no data in the literature supporting that stress by itself may induce significant changes in the studied parameters. On the other hand, we must note that in our study, the LTB4 concentrations measured by ELISA are 1–2 order of magnitude higher compared with those measured by gas chromatography-negative ion chemical ionization-mass spectrometry (GC-MS)12 or immuno high-performance liquid chromatography.20 We believe that this disparity in LTB4 concentration may be due to the different specificity of the different techniques by LTB4. Furthermore, we performed the experimental protocol in accordance with specifications provided by the manufacturers, and the LTB4 concentrations present in our study are in accordance with the results obtained by the manufacturers in their tests.

LTB4 is a potent pro-inflammatory mediator that activates multiple leukocyte subsets leading to cell recruitment, production of ROS, and induction of gene expression.1,5,6,21 It binds to specific heptahelical receptors of the rhodopsin class, BLT1 and BLT2.5,6,22 These receptors interact with G proteins in the cytoplasm, and induce different activities, ranging from motility to transcriptional activation. BLT1 is the high-affinity receptor that mediates most, if not all, of its chemoattractant and proinflammatory actions. BLT2 is the lower-affinity receptor that also binds other lipoxygenase products, but little is known about its functions. In this paper, we investigated LTB4 signalling via BLT1 and BLT2 and their contribution to inflammation in atherosclerosis. The NF-{kappa}B has a key role in inflammation and innate immunity by regulating the expression of many proinflammatory and prothrombotic molecules.23,24 We have observed that LTB4 significantly enhanced NF-{kappa}B DNA-binding activity. We have demonstrated also that LTB4 induced nuclear translocation of p65 subunits. The blockade of BLT1 and BLT2 receptors with specific antagonists inhibited NF-{kappa}B DNA binding activity in monocytes, suggesting that these receptors participate in NF-{kappa}B activation. Moreover, studies with confocal microscopy showed that BLT1 and BLT2 inhibition diminishes nuclear translocation of the p65 subunit. Furthermore, there was not an additive effect of selective BLT1 and BLT2 inhibitors on NF-{kappa}B activation.

The activation of NF-{kappa}B involves phosphorylation of I{kappa}B by the IKK complex, which results in I{kappa}B degradation. Our data showed that the BLT1 and BLT2 antagonists reduced IKK{alpha}β and I{kappa}B{alpha} phosphorylation in monocytes. In addition, NF-{kappa}B inhibitors MG-132, Bay-117032, and parthenolide also reduced I{kappa}B{alpha} phosphorylation in monocytes, suggesting a NF-{kappa}B-dependent mechanism of LTB4 activation in monocytes. Moreover, to study the mechanism by which NF-{kappa}B is activated by LTB4, we investigated whether LTB4 activates MAPK pathways in monocytes cells. Some studies show that LTB4 activates these pathways in several cell types transducing different LTB4 responses such as proliferation and chemotaxis.25–28 In this work, we have found that LTB4 induces a rapid activation of ERK1/2, JNK1/2, p38, and PI3K/AKT, showing that these signalling pathways participate in the modulation of LTB4 actions. The inhibition of BLT1 and BLT2 diminishes the activation of these pathways, implicating both receptors in the modulation of LTB4 responses. Moreover, pre-treatment with PTX, also inhibited ERK1/2, JNK1/2, and AKT activation, suggesting that LTB4 induces ERK1/2, JNK1/2, and AKT phosphorylation via BLT1 and BLT2, in a PTX-sensitive mechanism. In this sense, we also have observed that LTB4 induced NF-{kappa}B activation in a MAPK- and ROS-dependent mechanism. Therefore, we must note that the inhibitory effect of antioxidants (DPI and Tiron) gives indirect evidence on the role of ROS in the effects evoked by LTB4; perhaps the measurement of ROS-production is a more precise indicator of the ROS involvement in NF-{kappa}B activation induced by LTB4.

To further investigate the proinflammatory effect of LTB4 in monocytic cells, we noted that NF-{kappa}B dependent genes, such as IL-6, TNF-{alpha}, and MCP-1, are upregulated by LTB4, and pre-treatment with NF-{kappa}B inhibitors diminished this overexpression, suggesting a NF-{kappa}B-mediated transcriptional mechanism. Moreover, the blockade of BLT1, BLT2, and G{alpha}i/0 protein, also inhibited IL-6, TNF-{alpha}, and MCP-1 mRNA expression induced by LTB4 in monocytes and no additive effect was observed when combining both antagonists. This suggests that NF-{kappa}B activation and upregulation of NF-{kappa}B-related genes by LTB4 is mediated to some extent via BLT1 and BLT2 receptors in a PTX-sensitive mechanism. Several studies have shown that BLT1 and BLT2 expression depends on the cell type. Back et al.29 demonstrated that BLT1 colocalizes with smooth muscle cells, endothelial cells, and macrophages in human atheroma, whereas BLT2 protein was found only in macrophage areas, suggesting that it may be involved in the vascular damage induced by LTB4. Moreover, it has been reported that macrophages from BLT1–/– mice show LTB4-induced chemotaxis through the activation of a BLT2 receptor.8

It is interesting to note that Lundeen et al.25 and Kitaura et al.30 reported that the addition of both combined antagonists to the culture medium had no additive effect on chemotaxis in mast cells and bone marrow-derived mast cells (BMMC). The concentrations of U-75302 and Ly-255283 used in these works and in our study were selective for BLT1 and BLT2 inhibition, respectively. In this sense, Lundeen et al.25 proposed that BLT1 and BLT2 could function together, as a heterodimer, completely inhibiting chemotactic responses. Our data suggest that both receptors may work together also in the stimulation of cytokine expression, as their combined blockade does not cause an additive effect.

Finally, to assess the in vivo relevance of our findings, we analysed carotid human atheroma, and we observed a significant increase of 5-LO, LTA4-H, and BLT1 expression in the shoulder when compared with the cap of the plaques. In regard to this, we and others have shown previously that the shoulder of human atheroma displays an increase in macrophage infiltration, COX-2 expression, and NF-{kappa}B activation.10,13,31 Additionally, on the one hand, symptomatic atherosclerotic plaques express elevated levels of 5-LO and LTB4, supporting the idea that LTB4 may be a mediator of 5-LO-dependent plaque instability.32,33 On the other hand, atherosclerotic lesions display more NF-{kappa}B activity in coronary plaques responsible for an acute coronary syndrome that in stables ones.34 Here, we have demonstrated that active NF-{kappa}B colocalizes with LTA4-H and with BLT2 in carotid atherosclerotic plaques, suggesting that the 5-LO pathway favours NF-{kappa}B activation in situ, thereby increasing their inflammatory state.

In summary, our results demonstrate that patients with carotid atherosclerosis display an enhancement of the leukotriene cascade in blood and in the vulnerable region of the plaques that could contribute to increase the inflammatory burden in this disease. In cultured monocytes, LTB4 activate ERK1/2, JNK1/2, AKT, and NF-{kappa}B pathway via BLT1 and BLT2 in a PTX-dependent mechanism, upregulating the expression of pro-inflammatory cytokines involved in atherosclerosis. Our data suggest that both receptors play a role in the pro-inflammatory environment in atherosclerosis. Then, this pathway could be a novel therapeutic target in atherosclerosis. Further studies are needed to confirm if its inhibition is useful in the treatment of this disorder.


   Funding
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 Funding
References
 
This work was supported by grants from SAF2004/06109, SAF2007/63648, CAM (S2006/GEN-0247), Ministerio de Sanidad y Consumo, Instituto de Salud Carlos III, Red RECAVA (RD06/0014/0035), Fondo de Investigaciones Sanitarias (PI050451), Mutua Madrileña, Fundación Ramón Areces, Sociedad Española de Arteriosclerosis y Fundación Española del Corazón.


   Acknowledgements
 
E.S.-G. is a fellow of Fundación Conchita Rábago. The authors would like to thank Mar Gonzalez Garcia-Parreño for technical help with the confocal microscopy.

Conflict of interest: none declared.


   References
Top
 Abstract
 1. Introduction
 2. Methods
 3. Results
 4. Discussion
 Funding
 References
 

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