Oxidized low-density lipoprotein-induced foam cell formation is mediated by formyl peptide receptor 2

Highlights

• oxLDL induces foam cell formation via formyl peptide receptor 2 (FPR2).

• oxLDL stimulates calcium increase via FPR2.

• FPR2 mediates oxLDL-induced macrophage migration and TNF-α production.

• FPR2 is involved in the oxLDL-induced pathogenesis of atherosclerosis.

Abstract

The increased level of LDL and its modification into oxLDL has been regarded as an important risk factor for the development of cardiovascular diseases such as atherosclerosis. Although some scavenger receptors including CD36 and RAGE have been considered as target receptors for oxLDL, involvement of other receptors should be investigated for oxLDL-induced pathological responses. In this study, we found that oxLDL-induced foam cell formation was inhibited by formyl peptide receptor 2 (FPR2) antagonist WRW⁴. oxLDL also stimulated calcium signaling and chemotactic migration in FPR2-expressing RBL-2H3 cells but not in vector-expressing RBL-2H3 cells. Moreover, oxLDL stimulated TNF-α production, which was also almost completely inhibited by FPR2 antagonist. Our findings therefore suggest that oxLDL stimulates macrophages, resulting in chemotactic migration, TNF-α production, and foam cell formation via FPR2 signaling, and thus likely contributes to atherogenesis.

Introduction

Atherosclerosis is a serious chronic inflammatory disorder which is associated with upregulation of inflammatory chemokines, such as CCL2 [1]. Produced CCL2 recruits monocytes from blood vessels into the intima region, where macrophages engulf modified low-density lipoprotein (LDL) including oxidized LDL (oxLDL) [1]. During the process of uptake of oxLDL, macrophages use several different cell surface receptors, including some scavenger receptors such as lectin-like oxLDL receptor 1 (LOX1), CD36, and SR-A [2], [3], [4]. After uptaking oxLDL, macrophages can be differentiated into foam cells, which produce diverse growth factors and proinflammatory cytokines such as tumor necrosis factor (TNF)-α [1]. When upregulated, these factors stimulate proliferation of vascular smooth muscle cells, leading to plaque formation [1]. Since oxLDL is a crucial modified LDL, which induces foam cell formation, mediating the pathological process in atherosclerosis [5], [6], the identification and characterization of target receptor(s) for oxLDL has been an important issue.

Although several cell surface receptors have been reported to act on the scavenger receptor, the possible involvement of different types of receptors has also been suggested [7], [8], [9]. Previously, it was demonstrated that oxLDL stimulates the activation of intracellular signaling molecules, which are inhibited by a pertussis toxin (PTX), such as p38 MAPK and ERK in smooth muscle cells [7]. Since PTX specifically blocks G_i-protein-mediated signaling, it has been suggested that oxLDL may stimulate a G_i-protein-induced signaling cascade. In a previous report we demonstrated that the stimulation of Raw264.7 cells with oxLDL induced foam cell formation, which was markedly inhibited by PTX, suggesting a putative role of PTX-sensitive G-protein or PTX-sensitive G-protein coupled receptor(s) [9]. Considering our previous reports and those by others, additional receptors should be considered as putative receptors for oxLDL, which may be associated with PTX-sensitive G-protein.

Formyl peptide receptor 2 (FPR2) is a classical chemoattractant receptor, which is mainly expressed on leukocytic cells including neutrophils, monocytes, macrophages, natural killer cells, and dendritic cells [10]. Unlike other chemoattractant receptors, FPR2 can recognize such diverse extracellular ligands as formyl peptides derived from Gram negative bacteria and host-derived agonists (serum amyloid A, lipoxin A4, annexin-1, LL-37, and humanin) [10]. The activation of FPR2 by its specific agonists induces a complex signaling cascade including intracellular calcium increase, mitogen-activated protein kinases, phospholipase A₂, C, D, and phosphoinositide 3-kinase and Akt activation [10], [11], [12]. In terms of functional activity, FPR2 mediates innate immunity against invading pathogens and polymicrobial sepsis by stimulating the production of reactive oxygen species and downregulating proinflammatory cytokines [13]. Recently we demonstrated that serum amyloid A, an acute reactant protein which acts on FPR2, stimulates macrophage foam cell formation [14]. We also showed that FPR2 is involved in the serum amyloid A-stimulated upregulation of scavenger receptor, LOX-1, resulting in foam cell formation [9]. However the functional role of FPR2 on the oxLDL-induced pathological process of atherosclerosis has not been investigated. In this study, we demonstrate that FPR2 is involved in the oxLDL-stimulated macrophage foam cell formation. We also showed that oxLDL-stimulated calcium increase, macrophage migration, and inflammatory cytokine production were mediated by FPR2. Collectively we suggest that FPR2 may be crucial for the oxLDL-induced pathological process of atherosclerosis.