doi: 10.1161/01.RES.0000215626.34470.e6
© 2006 American Heart Association, Inc.
Editorials |
A Rancid Culprit in Vascular Inflammation Acts on the Prostaglandin Receptor EP2
From the Robert M. Berne Cardiovascular Research Center and Department of Pharmacology, University of Virginia, Charlottesville.
Correspondence to Norbert Leitinger, PhD, Robert M. Berne Cardiovascular Research Center, University of Virginia, P.O. Box 801394, Charlottesville, VA 22908. E-mail nl2q{at}virginia.edu
See related article, pages 642–650
Key Words: oxidized phospholipids • prostaglandin receptors • atherosclerosis • endothelial cells • monocytes • inflammation
Although our knowledge about the mechanisms underlying atherosclerosis and its complications has dramatically increased, questions about the initiating factors of atherogenesis remain. Accumulating evidence suggests retention of low-density lipoprotein (LDL) particles in the subendothelial space with subsequent oxidative modification as key steps in atherogenesis. Oxidative modification initially gives rise to minimally oxidized LDL (MM-LDL), which was shown by Judy Berliner in 1990 to activate endothelial cells to specifically bind monocytes but not neutrophils.1 It was subsequently shown by the same group that the biological activity of MM-LDL primarily results from oxidation of phospholipids such as 1-palmitoyl-2-arachidonoyl-sn-3-glycero-phosphorylcholine (PAPC), yielding a series of structurally defined oxidation products (OxPAPC). The advances that have been made in dissecting the molecular components of MM-LDL responsible for its proatherogenic effect now allow for the experimental use of defined compounds rather than complex lipoproteins. One such biologically active oxidized phospholipid was structurally identified by Watson et al as 1-palmitoyl-2-epoxyisoprostane-sn-glycero-3-phosphorylcholine (PEIPC; Figure 1).2,3 Oxidized ("rancid") phospholipids were shown to accumulate in atherosclerotic lesions,4 and thus could be regarded as "culprits" in chronic inflammation. Although intracellular signaling pathways induced by various oxidized phospholipids had been studied, target receptors that are activated by these lipids remained unknown. Indications that oxidized phospholipids may act by binding to a G protein–coupled receptor (GPCR) came from studies by Parhami et al, who demonstrated that MM-LDL stimulates a putative G
s-coupled receptor, increasing cyclic AMP (cAMP) levels in endothelial cells.5,6
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In this issue of Circulation Research, Li et al7 demonstrate that the oxidized phospholipid PEIPC induces monocyte adhesion to endothelial cells by activating the prostaglandin E2 (PGE2) receptor EP2. Activation of EP2 by PEIPC increased cAMP levels, thus mimicking effects of an EP2-specific PGE2 analogue, butaprost. Furthermore, PEIPC was also recognized by the PGD2 receptor DP.
There are 4 subtypes of the PGE2 receptor, termed EP1 to EP4. EP2 is a Gs-linked GPCR whose activation results in an increase of cAMP levels and activation of protein kinase A. Activation of this pathway in endothelial cells would, as previously shown, result in activation of R-Ras and ß1-integrins, leading to surface expression of CS-1 fibronectin, which then could readily interact with VLA-4 on monocytes (Figure 2). In monocytes, upregulation of cAMP levels via activation of EP2 by PEIPC resulted in downregulation of TNF and concomitant upregulation of IL-10 expression, characteristics of alternatively activated macrophages. Induction of this antiinflammatory phenotype would also prevent apoptosis, which is necessary for the monocyte/macrophage to become a foam cell (Figure 2). These findings imply that initiation of monocytic vascular inflammation can occur via activation of EP2 even in the absence of cyclooxygenase (COX)-derived PGE2, through formation of the oxidized phospholipid PEIPC. In a more advanced stage of atherogenesis, where monocytes/macrophages accumulate in the subendothelial space, activation of the EP2 receptor may occur by PEIPC, but also by COX-derived PGE2.
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There are a number of intriguing questions that remain to be addressed regarding EP receptor activation by PEIPC versus PGE2. The authors show that PEIPC competes with PGE2 for binding to EP2, and while PEIPC seems to be specific for EP2, PGE2 would also bind to EP1, 3, and 4. Activation of EP4 in macrophages stimulates antiinflammatory pathways via increasing cAMP levels,8 whereas in T-cells, for instance, EP4 can also stimulate proinflammatory cAMP-independent pathways.9 Consequently, the presence of both PGE2 and PEIPC would result in dual activation of EP2 and EP4 receptors in macrophages, likely potentiating antiinflammatory effects. However, in other cell types the relative abundance of the two EP ligands may determine stimulation of individual receptor subtypes and thus the outcome of cell activation. Moreover, it is conceivable that specific activation of EP2 by PEIPC would buffer proinflammatory effects elicited by EP1 and EP3 receptor activation by PGE2.
The findings of the present study suggest that PEIPC–EP2 interactions may be important in vascular inflammation, and its pathophysiological relevance could be tested directly in vivo, for example in models of atherosclerosis. Future studies using mice deficient in EP2 and selective inhibitors should demonstrate whether PEIPC uses the EP2 receptor to induce vascular inflammation, and thus will affect the progression of atherosclerotic lesion formation. Moreover, because EP2 plays important roles not only in inflammation but also in blood pressure homeostasis and reproduction, as illustrated by genetic deletion of this receptor in mice,10,11 a role for PEIPC in these settings seems possible and requires further investigation.
Another important aspect of the present study is the discovery of EP2 as a drugable target holding the potential to modulate inflammatory actions induced by oxidized phospholipids. Because both the prevention of their local formation and the selective destruction of biologically active oxidized phospholipids using antioxidants or enzymes, respectively, seem to be difficult undertakings, the discovery of a receptor that mediates the activity of one of these compounds opens new avenues for pharmacological intervention. Moreover, the activation of prostaglandin receptors by molecules that are free radical–derived makes this effect independent of COX. Therefore, the EP2 receptor may turn out to be an attractive pharmacological target in atherosclerosis and other inflammatory diseases.
The authors focused in their study on the EP2 receptor because it is expressed in endothelial cells and monocytes, important players in atherogenesis. However, the finding that PEIPC also activates the PGD2 receptor DP may have important implications. In chronically inflamed tissue, where oxidized phospholipids accumulate, activation of cells bearing the DP receptor may occur by PEIPC, even in the absence of COX-derived prostaglandin production, and result in modulation of the inflammatory response. For instance, cell types that are activated via the DP receptor include bronchial epithelial cells, implying a role for PEIPC-DP interactions in settings of lung inflammation and asthma.
What are the structural implications for oxidized phospholipid (PEIPC)-receptor (EP2) interactions? E-ring isoprostanes were shown to be recognized by the thromboxane receptor (TP), but also EP receptors; however, structural requirements for individual receptor subtype recognition have not been elucidated yet. The epoxyisoprostane in PEIPC contains an E-prostane ring; however, the structural motif conferring specificity for the EP2 receptor is not known. Whether the epoxide in PEIPC plays a role in specific receptor recognition remains to be shown. Another structural question to be answered is whether PEIPC is recognized by EP2 as an intact phospholipid, or whether the epoxyisoprostane moiety needs to be hydrolyzed by a phospholipase A2 (PLA2). The latter would imply a role for secretory PLA2s, which recently had been attributed important roles in atherogenesis.12,13 Even if the intact epoxyisoprostane phospholipid is recognized by EP2, hydrolysis by PLA2 might increase binding affinity. On the other hand, it was demonstrated previously that PLA2 treatment of PEIPC resulted in loss of biological activity.4 In any case, it remains to be shown for which sPLA2 subtype PEIPC would be a good substrate. Whatever the structural requirements for receptor recognition are, oxidation products of phosphatidylserine, phosphatidylcholine, and phosphatidylethanolamine contain homologous functional groups that result in similar biological activities.4 Thus, epoxyisoprostanes that are recognized by EP2 are potentially formed from all kinds of arachidonate-containing phospholipids, indicating accumulation of quite significant amounts of these biologically active compounds in inflamed tissue.
By screening a substantial number of candidate GPCRs, the authors excluded various other known phospholipid as well as orphan receptors as mediators for oxidized phospholipid-induced cAMP-dependent monocyte adhesion. The finding that other biologically active oxidized phospholipids that are present in OxPAPC act neither on EP nor on the other investigated candidate receptors, including the PAF receptor and lysophospholipid receptors, implies the presence of as of yet unidentified receptors. Together, the identification of the prostaglandin E2 receptor EP2 as a receptor for the oxidized phospholipid PEIPC provides new insights into the mechanisms by which monocytes are selectively recruited to chronically inflamed tissue and should ultimately lead to the development of novel therapeutic approaches against chronic inflammatory diseases such as atherosclerosis.
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Acknowledgments |
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This work was supported by a Partner’s Fund Award of the Robert M. Berne Cardiovascular Research Center and a Research and Development Grant from the University of Virginia. The author thanks Dr Alexandra Kadl for artwork and valuable discussions.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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[Abstract/Free Full Text]
Related Article:
- Identification of Prostaglandin E2 Receptor Subtype 2 As a Receptor Activated by OxPAPC
- Rongsong Li, Kevin P. Mouillesseaux, Dennis Montoya, Daniel Cruz, Navid Gharavi, Martin Dun, Lukasz Koroniak, and Judith A. Berliner
Circ. Res. 2006 98: 642-650.[Abstract] [Full Text] [PDF]
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