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Circulation Research. 2007;100:946-949
Published online before print March 22, 2007, doi: 10.1161/01.RES.0000264498.60702.0d
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(Circulation Research. 2007;100:946.)
© 2007 American Heart Association, Inc.


Report

5-Lipoxygenase–Activating Protein

A Potential Link Between Innate and Adaptive Immunity in Atherosclerosis and Adipose Tissue Inflammation

Magnus Bäck, Ariane Sultan, Olga Ovchinnikova, Göran K. Hansson

From the Department of Medicine, Karolinska Institutet and Center for Molecular Medicine, Karolinska University Hospital, Stockholm, Sweden.

Correspondence to Magnus Bäck, MD, PhD, Center for Molecular Medicine, Karolinska University Hospital, L8:03, 171 76 Stockholm, Sweden. E-mail Magnus.Back{at}cmm.ki.se


*    Abstract
up arrowTop
*Abstract
down arrowIntroduction
down arrowMaterials and Methods
down arrowResults and Discussion
down arrowReferences
 
Transforming growth factor-ß (TGF-ß) is a major antiinflammatory mediator in atherosclerosis. Transgenic ApoE–/– mice with a dominant-negative TGFß type II receptor (dnTGFßRII) on CD4+ and CD8+ T cells display aggravated atherosclerosis. The aim of the present study was to elucidate the mechanisms involved in this enhanced inflammatory response. Gene array analyses identified the 5-lipoxygenase–activating protein (FLAP) among the most upregulated genes in both the aorta and adipose tissue of dnTGFßRII transgenic ApoE–/– mice compared with their ApoE–/– littermates, a finding that was confirmed by real-time quantitative RT-PCR. Aortas from the former mice in addition produced increased amounts of the lipoxygenase product leukotriene B4 after ex vivo stimulation. FLAP protein expression in both the aorta and adipose tissue was detected in macrophages, but not in T cells. Four weeks of treatment with the FLAP inhibitor MK-886 (10 mg/kg in 1% tylose delivered by osmotic pumps) significantly reduced atherosclerotic lesion size and T-cell content. Finally, FLAP mRNA levels were upregulated approximately 8-fold in adipose tissue derived from obese ob/ob mice. In conclusion, the results of the present study suggest a key role for mediators of the 5-lipoxygenase pathway in inflammatory reactions of atherosclerosis and metabolic disease.


Key Words: atherosclerosis • cytokines • inflammation • leukotriene • lipid metabolites • lipoxygenase • metabolism


*    Introduction
up arrowTop
up arrowAbstract
*Introduction
down arrowMaterials and Methods
down arrowResults and Discussion
down arrowReferences
 
Atherosclerosis is regulated by both pro- and antiinflammatory stimuli.1 Among the latter, transforming growth factor-ß (TGF-ß) may play a major role in atheroprotective signaling. For example, selective expression of a dominant-negative TGFß type II receptor on CD4+ and CD8+ T cells (CD4dnTßRII) leads to aggravated atherosclerosis and more vulnerable lesions in hyperlipidemic mice.2,3 Furthermore, the recent discovery that a CD25-depleting antibody increases lesion size in ApoE–/– but not ApoE–/– xCD4dnTßRII mice has implicated TGF-ß in the atheroprotective effects exerted by CD4+CD25+ regulatory T cells.4 Whereas those studies suggest a key role for T-cell TGF-ß signaling in dampening inflammatory responses in atherosclerosis, the aim of the present study was to establish the proatherogenic mediators involved. In this process, we discovered the 5-lipoxygenase–activating protein (FLAP) as a main upregulated gene in the aorta from the ApoE–/– xCD4dnTßRII mice, compared with their ApoE–/– littermates. Furthermore, we also found the adipose tissue to be a major site of inflammation in these mice, with FLAP also among the most upregulated genes in this tissue. The latter observation is of particular interest because evidence is emerging linking cardiovascular risk factors such as obesity and insulin resistance to infiltration of inflammatory cells in adipose tissue.

FLAP is necessary for the production of leukotrienes through the 5-lipoxygenase (5-LO) pathway of arachidonic acid metabolism. Importantly, the gene encoding FLAP was recently associated with increased risk of myocardial infarction and stroke,5 and mechanistic studies have supported the notion of a major role of leukotriene signaling in atherosclerosis.6 Leukotriene B4 (LTB4), for example, is among the most potent chemotactic mediators formed within the atherosclerotic lesion, inducing migration of a number different cell types, such as monocytes,7 vascular smooth muscle cells,8 and T cells,9 suggesting LTB4 as a potential link between innate and adaptive immunological reactions. However, the latter notion has not previously been assessed in the context of atherosclerosis and adipose tissue inflammation. The beneficial effects observed in ApoE–/–xCD4dnTßRII mice after FLAP inhibition in the present study point to an important role of the FLAP/5-LO/leukotriene pathway in the regulation of T-cell-driven inflammatory responses in atherosclerosis and metabolic disease.


*    Materials and Methods
up arrowTop
up arrowAbstract
up arrowIntroduction
*Materials and Methods
down arrowResults and Discussion
down arrowReferences
 
An expanded Materials and Methods section is available in the online data supplement at http://circres.ahajournals.org. In brief, female ApoE–/– and male ApoE–/–xCD4dnTßRII mice were bred to obtain littermates that were either ApoE–/–xCD4dnTßRII or ApoE–/– mice lacking the dominant-negative transgene. For treatment protocols, either MK-886 or vehicle (1% tylose) was administered for 4 weeks by Alzet osmotic minipumps implanted into the intrascapular subcutaneous space in 8-week-old ApoE–/– xCD4dnTßRII mice.


*    Results and Discussion
up arrowTop
up arrowAbstract
up arrowIntroduction
up arrowMaterials and Methods
*Results and Discussion
down arrowReferences
 
The expression of a dominant-negative TGF-ß type II receptor under the control of the CD4 promoter in ApoE–/– mice has provided a model of atherosclerosis with a predominantly T-cell-driven inflammation.2,4 In the present study, microarray analysis revealed candidate genes for inflammatory mechanisms involved in the aggravated atherosclerosis of ApoE–/–xCD4dnTßRII mice (Table I in the online data supplement). In addition to genes reflecting increased T-cell infiltration (T-cell–specific GTPase, CD52, and T-cell receptor {alpha} chain) and interferon {gamma} (IFN-{gamma}) signaling (IFN-{gamma} inducible protein, ubiquitin D, and Stat1), FLAP, being among the 20 most upregulated, should be considered a proatherogenic candidate gene. The 2- to 3-fold increase of FLAP mRNA levels in ApoE–/–xdnTGFßRII mice compared with their ApoE–/– littermates lacking the dominant-negative transgene confirmed by RT-PCR and immunohistochemistry was in contrast to the lack of aortic FLAP upregulation in ApoE–/– compared with wild-type mice (Figure 1A, 1B, and supplemental Figure I). Furthermore, ApoE–/–xdnTGFßRII mice also expressed increased FLAP mRNA levels when compared with 18-week-old ApoE–/– mice (supplemental I and Figure 1A), which have approximately the same lesion size as 12-week-old ApoE–/–xCD4dnTßRII, supporting a selective upregulation of FLAP transcription rather than a consequence of the increased atherosclerosis burden.


Figure 1
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Figure 1. Upregulation of FLAP expression and LTB4 production in aortas from ApoE–/–xCD4dnTßRII mice. A, Real-time quantitative RT-PCR analysis of FLAP mRNA levels (normalized to hypoxanthine guanine phosphoribosyl transferase mRNA) in aorta from ApoE–/–xCD4dnTßRII compared with either age- or lesion size–matched ApoE–/– mice (n=6 to 8). B, Immunohistochemical quantification of FLAP expression (n=5). D, Fluorescent labeling of FLAP in aorta from ApoE–/–xCD4dnTßRII mice (12 weeks) revealed colocalization of FLAP with the monocyte/macrophage marker CD68 but not with the T-lymphocyte marker CD3. C, The LTB4 concentrations in supernatants of calcium ionophore–stimulated aortas from ApoE–/–xCD4dnTßRII were significantly increased compared with lesion-matched ApoE–/– mice (n=5). *P<0.05. w indicates weeks.

Importantly, FLAP expression in ApoE–/–xCD4dnTßRII mice colocalized with the monocyte/macrophage marker CD68, but not with the T-cell marker CD3 (Figure 1D). These results indicate that abrogating the dampening effects of TGF-ß selectively on T-cells induced paracrine effects on macrophages. The increased FLAP expression led to an increased LTB4 formation, measured after ex vivo stimulation of the aorta (Figure 1C). Because the mRNA levels of other enzymes involved in LTB4 biosynthesis, ie, 5-LO and LTA4 hydrolase, were not significantly altered in these mice (supplemental Figure I), the present study suggests that FLAP may be rate limiting in leukotriene biosynthesis during atherosclerosis.

Because the expression pattern suggested FLAP as a potential proatherogenic mediator, we subsequently evaluated the effect of the FLAP inhibitor MK-886 in ApoE–/– xCD4dnTßRII and found significantly reduced aortic root atherosclerotic lesions (Figure 2A and 2B), without altered lipid levels (supplemental Table II). These findings provide the first reported successful treatment strategy capable of reducing atherosclerosis in this particular model. The latter observation also extends the somewhat conflicting results from previous studies of either genetic or pharmacological targeting of leukotriene synthesis in different strains of hyperlipidemic mice having reported both beneficial10,11 and neutral effects12 on atherosclerosis burden. The present study suggests that the FLAP/leukotriene signaling may have its predominant effects in T-cell-driven inflammatory responses. This notion is further extended by the observation that although the macrophage density in the lesions was not significantly changed (supplemental Table III), MK886 decreased aortic CD3+ T cells and reduced levels of mRNA encoding the T-cell chemokine IFN-{gamma} (Figure 2C and 2D)


Figure 2
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Figure 2. Inhibition of atherosclerosis and reduced T-cell inflammation by the FLAP inhibitor MK-886. A, Representative micrographs of hematoxylin/oil red O–stained aortic root lesions of 12-week-old ApoE–/–xCD4dnTßRII mice after 4 weeks of treatment with either vehicle (1% tylose) or the FLAP inhibitor MK-886 (10 mg/kg). B, The mean lesion size at the different levels studied (100 to 800 µm from the aortic cusps) was significantly reduced, measured as either the cross-sectional area covered by lesions or the lesion proportion of total aortic cross-section (n=4 to 8). *P<0.05 vs vehicle (2-way ANOVA). C and D, Immunohistochemical staining of the T-cell marker CD3 (C) showed decreased T-cell count, and RT-PCR revealed lower IFN-{gamma} mRNA levels (D) in the treatment group. *P<0.05 vs vehicle. HPRT indicates hypoxanthine guanine phosphoribosyl transferase.

In addition to atherosclerosis, several findings in the present study also indicate a major role of FLAP/leukotriene signaling in T-cell-driven adipose tissue inflammation. For example, microarray analysis of adipose tissue again revealed FLAP as among the most upregulated genes in ApoE–/– xCD4dnTßRII compared with both 12- and 18-week-old ApoE–/– mice (supplemental Table IV), which was confirmed by RT-PCR (Figure 3A). In comparison with age-matched C57BL/6 mice, FLAP mRNA levels were significantly increased in adipose tissue from ApoE–/–xCD4dnTßRII mice, and even higher levels (almost 8-fold increase) were detected in adipose tissue from age-matched ob/ob mice (Figure 3A). Whereas the adipose tissue levels of LTA4 hydrolase mRNA were not significantly different between the different groups of mice examined, ob/ob mice also expressed 2-fold higher levels of 5-LO compared with C57BL/6 mice (Figure 3A), hence supporting a potential link between leukotrienes and obesity.


Figure 3
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Figure 3. A, Adipose tissue expression of the 5-LO pathway. Real-time quantitative RT-PCR analysis of mRNA levels (normalized to hypoxanthine guanine phosphoribosyl transferase mRNA) of FLAP, 5-LO, and LTA4 hydrolase in gonadal adipose tissue from 12-week-old B57BL/6, ApoE–/–, ApoE–/–xCD4dnTßRII, or ob/ob mice (n=6 to 7). B, Immunohistochemical fluorescent labeling of FLAP protein in adipose tissue from ApoE–/–xCD4dnTßRII mice showed colocalization with the monocyte/macrophage marker CD68. C, Local leukotriene formation was detected in supernatants of calcium ionophore–stimulated adipose tissue, with significantly lower levels from tissues derived from mice treated with MK-886 (n=6 to 7). D, Inflammatory mRNA levels in adipose tissue of 12-week-old ApoE–/–xCD4dnTßRII mice were reduced after 4 weeks of treatment with either vehicle (1% tylose) or the FLAP inhibitor MK-886 (10 mg/kg). *P<0.05.

Immunohistochemical staining revealed adipose tissue FLAP protein localization to macrophages infiltrating adipose tissue of ApoE–/–xCD4dnTßRII mice (Figure 3B). Furthermore, a local LTB4 formation was detected after ex vivo stimulation of adipose tissue from these mice (Figure 3C). This local LTB4 formation hence suggests that increased FLAP expression may be a mediator of proinflammatory circuits in adipose tissue, which is a potential cardiovascular risk factor. Furthermore, the notion of leukotriene-driven adaptive immunological circuits could be extended also to adipose tissue inflammation. For example, microarray analysis revealed increased expression of not only macrophage-associated genes in ApoE–/–xCD4dnTßRII mice but also of T-cell markers, such as CD52 and CD8. Interestingly, FLAP inhibitor treatment significantly reduced IFN-{gamma} expression in adipose tissue (Figure 3D). Taken together, these observations suggest that T cells may be involved in the inflammatory alterations of adipose tissue and that targeting the FLAP/leukotriene pathway may inhibit this response.

In summary, the selective upregulation of FLAP in the hyperinflamed ApoE–/–xCD4dnTßRII mice points to an important role of the FLAP/leukotriene pathway in adaptive immunological circuits in vascular and adipose tissue inflammation. In the absence of the dampening effects of TGF-ß on T cells, FLAP expression in macrophages represented one of the most upregulated genes with as a result, increased production of the T-cell chemoattractant LTB4. Pharmacological targeting of FLAP reduced atherosclerosis burden and vascular as well as adipose tissue T-cell inflammation. In conclusion, the results of the present study suggest that FLAP may be a key regulator of T-cell-driven inflammatory responses and could represent a possible target in the treatment of atherosclerosis and metabolic disease.


*    Acknowledgments
 
Sources of Funding

This study was supported by the Torsten and Ragnar Söderberg Foundation, Swedish Heart and Lung Foundation, European Union FP6, and Karolinska Institutet.

Disclosures

None.


*    Footnotes
 
Original received January 26, 2007; revision received February 22, 2007; accepted March 9, 2007.


*    References
up arrowTop
up arrowAbstract
up arrowIntroduction
up arrowMaterials and Methods
up arrowResults and Discussion
*References
 

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