Inflammatory Interaction Between LIGHT and Proteinase-Activated Receptor-2 in Endothelial Cells
Potential Role in Atherogenesis
The interaction between inflammatory cytokines and endothelial cells is a critical step in atherogenesis leading to endothelial dysfunction and inflammation. We have previously reported that the tumor necrosis factor superfamily member LIGHT could be involved in atherogenesis through its ability to promote vascular inflammation. In the present study we identified proteinase-activated receptor (PAR)-2 as an inflammatory mediator that was markedly enhanced by LIGHT in endothelial cells. We also found that LIGHT acted synergistically with PAR-2 activation to promote enhanced release of the proatherogenic chemokines interleukin-8 and monocyte chemoattractant protein-1, underscoring that the interaction between LIGHT and PAR-2 is biologically active, promoting potent inflammatory effects. We showed that the LIGHT-mediated upregulation of PAR-2 in endothelial cells is mediated through the HVEM receptor, involving Jun N-terminal kinase signaling pathways. A LIGHT-mediated upregulation of PAR-2 mRNA levels was also found in human monocytes when these cells were preactivated by tumor necrosis factor α. We have previously demonstrated increased plasma levels of LIGHT in unstable angina patients, and here we show a similar pattern for PAR-2 expression in peripheral blood monocytes. We also found that LIGHT, LIGHT receptors, and PAR-2 showed enhanced expression, and, to some degree, colocalization in endothelial cells and macrophages, in the atherosclerotic plaques of ApoE−/− mice, suggesting that the inflammatory interaction between LIGHT and PAR-2 also may be operating in vivo within an atherosclerotic lesion. Our findings suggest that LIGHT/PAR-2–driven inflammation could be a pathogenic loop in atherogenesis potentially representing a target for therapy in this disorder.
The concept that atherosclerosis is an inflammatory disease is no longer controversial. More recent research has focused on understanding what drives this inflammation and how it is regulated. An increasing number of mediators have been suggested to be involved in this process, but there are still components to be clarified, and the precise mechanisms of action of each of these inflammatory mediators, as well as their interactive role in atherogenesis, have not been elucidated.1,2
LIGHT (lymphotoxin-like, exhibits inducible expression, and competes with herpes simplex virus [HSV] glycoprotein D [gD] for HSV entry mediator [HVEM], a receptor expressed by T lymphocytes; TNFSF14) is a cytokine in the tumor necrosis factor (TNF) superfamily that is involved in innate and adaptive immune responses as well as in regulation of cell survival and proliferation. LIGHT is signaling through 2 distinct members of the TNF receptor superfamily, ie, HVEM and the lymphotoxin β receptor (LTβR), and can also bind to the soluble decoy receptor 3. Studies in animal models indicate that LIGHT may be important for the development of various autoimmune disorders (eg, inflammatory bowel disease and rheumatoid arthritis) through effects on T cells and T-cell homing into inflamed tissues.3 More recently, this cytokine has been suggested to promote atherogenesis at least partly by inducing matrix metalloproteinase activity in macrophages and inflammation in endothelial cells.4,5 Lately, LIGHT was also shown to be involved in regulation of lipid homeostasis.6,7
The interaction between inflammatory cytokines and endothelial cells is a critical step in atherogenesis leading to endothelial cell dysfunction and inflammation within the atherosclerotic lesion, promoting additional recruitment of inflammatory cells in to the vessel wall.8 To further elucidate the potential pathogenic role of LIGHT in atherogenesis, we used high-density oligonucleotide microarrays to identify genes regulated by LIGHT in endothelial cells. These screening experiments identified proteinase-activated receptor (PAR)-2 as a gene that was markedly enhanced by LIGHT. PAR-2, a heptahelical G protein–coupled receptor, has been identified in various cell types including endothelial cells.9 PAR-2 has putative inflammatory roles and was recently proposed to be involved in atherogenesis by contributing to vascular inflammation.10 In the present study, we further examined the possible pathogenic role of the LIGHT-mediated PAR-2 response in endothelial cells by several approaches, including experimental as well as clinical studies.
Materials and Methods
An expanded Materials and Methods section is available in the online data supplement at http://circres.ahajournals.org.
Isolation of Cells
Peripheral blood mononuclear cells were obtained from heparinized blood by Isopaque-Ficoll (Lymphoprep; Nycomed, Oslo, Norway) gradient centrifugation. Further separation of monocytes and CD3+ T cells by monodisperse immunomagnetic beads (Dynal, Oslo, Norway) was performed as described elsewhere.11,12
Cell Culture Experiments
Human umbilical vein endothelial cells (HUVECs) were obtained from umbilical cord veins by digestion with 0.1% collagenase A (Roche Diagnostics GmbH, Mannheim, Germany)13 and cultured as described previously,5 with or without recombinant human (rh) LIGHT (R&D Systems, Minneapolis, Minn). The human monocytic cell line THP-1 (American Type Culture Collection, Rockville, Md) and freshly isolated human monocytes were cultured for 4 days in the presence of rhTNFα (5 ng/mL, R&D Systems) before further incubation with or without rhLIGHT.
RNA was isolated from HUVECs using RNeasy (Quiagen, Hilden, Germany). Three micrograms of total RNA were used to generate cRNA, subsequently hybridized to Genechip Human Genome U133A Array (Affymetrix, Santa Clara, Calif) according to standard protocols (http://www.affymetrix.com/support/technical/manual/expression_manual.affx).
Real-Time Quantitative PCR
Quantification of mRNA was performed using ABI Prism 7500 Fast Realtime PCR System (Applied Biosystems, Foster City, Calif).14
Western blotting was performed as previously described,15 separating equal amounts of protein from each sample by SDS-PAGE (10%) before transferring it onto polyvinyl difluoride membranes (NEN Life Science, Boston, Mass).
Preparation and Transfection of Small Interfering RNA
Predesigned small interfering (si)RNA against HVEM (siRNA ID no. 13868) and LTβR (siRNA ID no. 143461) were purchased from Ambion (Austin, Tex), and certificated nonsilencing scramble control siRNA (cat No: 1022076) was obtained from Qiagen (Hilden, Germany).
Protein extracts from HUVECs, stimulated with rhLIGHT (100 ng/mL) or vehicle for 15 and 120 minutes, were subjected to phosphoprotein detection array (multiplexable beads: phosphorylated-Akt; p-activating transcription factor-2 [ATF-2]; p-extracellular signal regulated kinase (ERK) [ERK]1/2; p-inhibitory [I]κB-α; p-Jun N-terminal kinase [JNK]; p-p38 mitogen-activated protein kinase [MAPK]; p-signal transducer and activator of transcription [STAT]3; Bio-Rad, Hercules, Calif) using the Bioplex (Bio-Rad) suspension array technology.
Levels of macrophage chemoattractant protein (MCP)-1, interleukin (IL)-6, IL-8, IL-10, TNFα, and LIGHT were measured by EIAs (R&D Systems).
Female ApoE−/− and wild-type C57BL/6 mice were obtained from Taconic Europe A/S (Lille Skensved, Denmark) and fed standard chow and water ad libitum as previously described.16
Acetone-fixed sections of the ascending aorta of the ApoE−/− and wild-type C57BL/6 mice were further fixed in formalin and stained using affinity-purified polyclonal goat antihuman LIGHT, HVEM, LTβR, or PAR-2 IgG (all Santa Cruz Biotechnology), rat antimouse CD68 IgG (macrophage marker; Serotec Ltd, Oxford, UK), or sheep antirat von Willebrand factor (vWF) IgG (endothelial cell marker; Cedarlane, Ontario, Canada).
Patients and Controls
In a separate experiment, we analyzed PAR-2 expression in monocytes and T cells from 14 patients with unstable angina, 14 patients with stable angina, and 10 healthy controls.
The studies were conducted according to the ethical guidelines at our hospital according to the Declaration of Helsinki and were approved by the local ethical committee. All animal experiments were in accordance with national guidelines and approved by the local ethical committee.
Probability values (2-sided) were considered significant at a value of <0.05.
LIGHT Increases PAR-2 Expression in Endothelial Cells
To screen for inflammatory effects of LIGHT in endothelial cells, we used Affymetrix microarrays encoding 22 000 human genes to analyze gene expression profile of HUVECs undergoing 3 hours of rhLIGHT stimulation (100 ng/mL). Data analysis identified PAR-2 as 1 of the genes that were markedly upregulated by LIGHT. This LIGHT-mediated effect on PAR-2 mRNA level was confirmed by real-time RT-PCR that showed a time- and dose-dependent pattern (Figure 1A and 1B). Thus, whereas there was no increase in PAR-2 expression in unstimulated HUVECs, rhLIGHT induced a rapid (maximum effect after 2 hours) and persistent increase in PAR-2 expression showing significantly enhanced mRNA levels of PAR-2 even after 24 hours of stimulation (Figure 1A). The LIGHT-induced expression of PAR-2 was also confirmed at the protein level as assessed by Western blotting, which showed significantly higher protein levels after 20 hours of stimulation (Figure 1C).
In addition to PAR-2, the microarray screening experiment showed that chemokines (ie, MCP-1, growth-related oncogene [GRO]α, and IL-8), the IL-7 receptor, and the TNF superfamily ligand 10 was markedly upregulated (>2-fold increase), and the antithrombotic mediator thrombomodulin was significantly downregulated (50% reduction) in LIGHT-activated HUVECs. However, LIGHT stimulation of HUVECs had no effect on the expression of TNFα, IL-6, or IL-10, as assessed by real-time RT-PCR and enzyme immunoassay, all being important endothelial-related cytokines, suggesting some degree of selectivity.
PAR-2 Signaling Enhances LIGHT-Mediated Induction of Proatherogenic Cytokines
Both IL-8 and MCP-1 are critically involved in atherogenesis and plaque destabilization,17,18 and we have previously shown that LIGHT is a potent inducer of these chemokines in endothelial cells.5 PAR-2 has also been reported to exert inflammatory effects in these cells, including the induction of IL-6 and IL-8.10 To examine the biological relevance of the LIGHT-mediated upregulation of PAR-2, we examined the effect of rhLIGHT (100 ng/mL), the PAR-2–activating peptide SLIGKV (10 μmol/L and 100 μmol/L), or a combination thereof on the release of IL-8 and MCP-1 in HUVECs after 8 and 20 hours of stimulation. As shown in Figure 2A and 2B, although SLIGKV show no or only modest effects on MCP-1 on its own, it markedly enhanced the LIGHT-stimulated release of this chemokine at both time points in a dose-dependent manner (≈2-fold increase when these stimuli were combined, 100 μmol/L SLIGKV). Furthermore, although PAR-2 activation induced a significant release of IL-8 on its own, comparable to the effect of LIGHT, again, the combination of these stimuli dose-dependently enhanced the release of IL-8 at both time points (≈3-fold increase when these stimuli were combined, 100 μmol/L SLIGKV) (Figure 2C and 2D). It is noteworthy that even if the microarray screening experiment identified MCP-1 and IL-8 as 2 of the most prominent LIGHT responses (see above), this LIGHT-mediated response was markedly enhanced when costimulated with the PAR-2 agonist, underscoring the inflammatory potential of the interaction between LIGHT and PAR-2 in endothelial cells.
Effect of LIGHT/PAR-2 Costimulation on Pro-/Antithrombotic Mediators and Nitric Oxide Synthase
We next examined the ability of LIGHT/PAR-2 activation to modulate other endothelial related mediators with relevance to atherosclerosis. We have previously shown that LIGHT enhances the expression of tissue factor and plasminogen activator inhibitor (PAI)-1 and downregulates the expression of thrombomodulin in endothelial cells.19 Whereas costimulation with PAR-2 agonist (100 μmol/L) had no effect on the LIGHT-mediated (100 ng/mL) expression of tissue factor and thrombomodulin (data not shown), it enhanced the LIGHT-mediated upregulation of PAI-1 mRNA levels in HUVECs (supplemental Figure I, A), suggesting that costimulation with LIGHT/PAR-2 agonist may contribute to a prothrombotic phenotype in endothelial cells. Moreover, whereas either LIGHT or PAR-2 activation modulated the expression of inducible nitric oxide synthase (NOS), LIGHT significantly downregulated the mRNA levels of endothelial NOS, but in contrast to the effect of PAI-1, there was no additional effect of costimulation with SLIGKV (supplemental Figure I, B).
LIGHT Mediates PAR-2 Expression Through HVEM
LIGHT is signaling through 2 distinct members of the TNF receptor superfamily: HVEM and LTβR. Both of these receptors, but not the LIGHT-binding decoy receptor 3, have been reported previously to be expressed in HUVECs,20 and we confirmed strong mRNA transcript of HVEM and LTβR in these cells (data not shown). To examine which receptor that mediates the LIGHT-induced expression of PAR-2, HUVECs were transfected with siRNA probes silencing each of these receptors. We found successful silencing of these genes, as assessed by Western blotting (≈90%; Figure 3A) and real-time RT-PCR (≈85%; data not shown), 8 and 5 hours posttransfection, respectively. As shown in Figure 3B, silencing HVEM totally abolished the LIGHT-mediated PAR-2 upregulation after 5 hours, whereas LTβR silencing had no effect compared to scramble control.
LIGHT-Mediated PAR-2 Expression in HUVECs Involves JNK/AP1 and NF-κB Activation
LIGHT signaling has been shown to involve activation of the NF-κB and JNK/AP-1 pathways.3,21 Such an intracellular pattern was also seen in endothelial cells when analyzing the effect of LIGHT on HUVECs by multiplex suspension array technology with enhanced phosphorylation of IκBα, resulting in dissociation of NF-κB from its inhibitor, and of JNK (Figure 4A and 4B). Blocking JNK with 100 μmol/L SP600125 and blocking NF-κB with 20 μmol/L NF-κB activation inhibitor II (JSH-23) completely abolished LIGHT-mediated PAR-2 expression (Figure 4C and 4D).
LIGHT-Mediated JNK Activation Is Mediated Through HVEM
Our findings suggest that HVEM as well as JNK and NF-κB activation are involved in the LIGHT-mediated induction of PAR-2 in HUVECs, and we next examined the relation between HVEM and the activation of JNK and NF-κB signaling pathways. By using siRNA of HVEM and LTβR, we show that silencing of HVEM, but not of LTβR, markedly downregulated the LIGHT-mediated activation of JNK (supplemental Figure II). This finding suggests a direct link between HVEM, JNK activation, and the LIGHT-induced PAR-2 expression. In contrast, either silencing of HVEM or LTβR significantly downregulated NF-κB activation in LIGHT-activated HUVECs (data not shown), suggesting that blocking of both receptors is necessary for totally abolishing the LIGHT-mediated NF-κB activation in these cells.
LIGHT Induces PAR-2 Expression in Monocytes
In addition to endothelial cells, monocytes/macrophages play an important role in atherogenesis by promoting lipid accumulation and inflammation. To assess whether LIGHT exerts similar effects on PAR-2 expression in these cells, THP-1 monocytes and freshly isolated monocytes from 6 healthy individuals (all with CRP levels <1.5 mg/L) were cultured for 4 days before stimulation with rhLIGHT (100 ng/mL) for additional 5 hours. Whereas rhLIGHT had only modest and nonsignificant effects on PAR-2 expression when these cells were cultured in medium alone (data not shown), LIGHT induced a significant increase in PAR-2 mRNA levels in cells that had been preincubated with rhTNFα (5 ng/mL) for 4 days before LIGHT activation, with the same pattern in THP-1 cells and primary monocytes (Figure 5A and 5B). The functional consequences of the LIGHT-mediated upregulation of PAR-2 seem to differ between monocytes and HUVECs. Hence, whereas PAR-2 activation markedly enhanced the LIGHT-mediated release of MCP-1 and IL-8 in HUVECs (Figure 3), no such effects were seen in THP-1 monocytes (data not shown). However, when examining the effects of LIGHT/PAR-2 activation on other chemokines with relevance to atherogenesis by multiplex suspension array technology, we found that SLIGKV (100 μmol/L) significantly enhanced the LIGHT-mediated (100 ng/mL) release of RANTES in THP-1 monocytes (Figure 5C), whereas no costimulatory effects was seen in HUVECs (data not shown). Either LIGHT, SLIGKV, or a combination thereof had any effects on IP-10 and MIG in either THP-1 monocytes or HUVECs (data not shown).
Costimulation With LIGHT and PAR-2 Agonist Enhances PAR-2 Expression in HUVECs and Monocytes
Although some differences, our findings suggest enhanced inflammatory responses in both monocytes and HUVECs when these cells are coactivated with LIGHT and PAR-2 agonist. To examine whether this enhancing effect of combined activation could involve upregulation of PAR-2 itself, we examined the costimulatory effect of LIGHT/SLIGKV on PAR-2 expression. Although SLIGKV (100 μmol/L) had no effect on PAR-2 when given alone, it markedly increased the LIGHT-mediated (100 ng/mL) upregulation of PAR-2 mRNA levels, with the same pattern in THP-1 monocytes and HUVECs (supplemental Figure III), potentially contributing to the inflammatory response in these cells when coactivated by LIGHT and SLIGKV.
Expression of LIGHT and PAR-2 in Experimental Atherosclerosis
Our data so far demonstrate a LIGHT-mediated upregulation of PAR-2 in endothelial cells and monocytes. To examine the in vivo relevance of these findings to atherosclerotic disease, we first examined the protein expression of LIGHT and its receptors, as well as PAR-2 by immunohistochemistry within the atherosclerotic lesions of aorta in a murine model of atherosclerosis, the ApoE−/− mouse (n=4), and in wild-type control mice (n=4) (Figure 6). In general, ApoE−/− mice showed increased immunostaining of LIGHT, HVEM, and LTβR as compared with the aorta of wild-type mice, at least partly reflecting the lack of macrophages within the vessel wall in control mice. However, significant immunostaining of these mediators was also seen within endothelial cells in ApoE−/− mice, in particular for LTβR. In ApoE−/− mice, PAR-2 immunostaining showed a similar distribution and intensity, colocalized to LIGHT and its receptors in macrophages and in particular in endothelial cells. In contrast to macrophages and endothelial cells, immunostaining of these mediators in vascular smooth muscle cells displayed the same (LIGHT and HVEM) or stronger (LTβR and PAR-2) intensity in control mice as compared with ApoE−/− mice.
To obtain more quantitative data, we also analyzed the mRNA levels of these mediators in aortas from control (n=8) and ApoE−/− (n=8) mice by real-time quantitative RT-PCR, showing a marked and significant upregulation of PAR-2, LIGHT, HVEM, and LTβR in ApoE−/− mice (>2-fold increase for all parameters) (Figure 7).
Expression of PAR-2 in CAD Patients
We have previously reported increased plasma levels of LIGHT in angina patients with the highest levels in those with unstable disease.6 Here, we examined PAR-2 mRNA levels in monocytes and T cells, and observed enhanced expression in monocytes from those with unstable angina, but not in T cells (data not shown), when examining cells from 14 patients with stable angina, 14 patients with unstable angina, and 10 healthy controls (Figure 8), with no correlation to plasma levels of LIGHT (P=0.18).
Increased LIGHT expression has previously been reported in atherosclerotic disorders potentially promoting matrix degradation, lipid accumulation, thrombus formation, and inflammation.4–6 In the present study, we extend these finding by showing that LIGHT is a potent inducer of PAR-2 in endothelial cells, acting synergistically with PAR-2 activation to promote enhanced release of the proatherogenic chemokines IL-8 and MCP-1. A LIGHT-mediated upregulates PAR-2 was also seen in monocytes that had been preactivated by TNFα, and in both HUVECs and monocytes, coactivation with LIGHT and PAR-2 agonist promoted increased expression of PAR-2 in itself, potentially contributing to the inflammatory interaction between these 2 mediators. Finally, whereas previous studies have shown increased LIGHT expression in peripheral blood in unstable angina patients, as well as within atherosclerotic plaques,4,6 our findings in the present study show a similar pattern for PAR-2, underscoring the in vivo relevance of our in vitro findings, suggesting a role for LIGHT/PAR-2 interaction in atherogenesis.
Atherosclerosis depends critically on altered behavior of the intrinsic cells of the arterial wall including endothelial cells, and atherogenesis is characterized by development of an inflammatory phenotype in these cells. A key event in this process is the localized recruitment of various leukocyte subsets into an inflamed endothelium.8 Although most previous studies have focused on the effect of LIGHT on T-cell activation and lymphoid tissue, there are also some reports of LIGHT-mediated effects on endothelial cells promoting increased expression of chemokines (eg, IL-8 and MCP-1) and adhesion molecules (eg, E-selectin and vascular cell adhesion molecule 1).20 Our findings in the present study suggest that enhanced PAR-2 expression should be added to the list of inflammatory responses in LIGHT-activated endothelial cells. More importantly, although LIGHT has been found to be a potent inducer of IL-8 and MCP-1 in endothelial cells,5 costimulation with the PAR-2–activating peptide SLIGKV strongly boosted the LIGHT-mediated chemokine response in these cells. Based on the important role of IL-8 and MCP-1 in atherosclerosis, including their ability to promote leukocyte recruitment into the atherosclerotic lesion,18 this LIGHT/PAR-2 interaction in endothelial cells could contribute to crucial steps in atherogenesis. In fact, we found that LIGHT, LIGHT receptors (ie, HVEM and LTβR), and PAR-2 all showed enhanced expression, and, to some degree, colocalization in endothelial cells and macrophages, in the atherosclerotic plaques of ApoE−/− mice, suggesting that the inflammatory interaction between LIGHT and PAR-2 also may be operating in vivo within an atherosclerotic lesion.
LIGHT interaction with LTβR triggers the production of inflammatory mediators and upregulates adhesion molecule expression in T cells and macrophages, and has also been shown to be crucial for the inflammatory effects of LIGHT in a mouse model of inflammatory bowel disease.22,23 On the other hand, by signaling through HVEM, LIGHT costimulates CD28-independent T-cell activation, preferentially inducing inflammatory T-helper cell type 1 responses.21,24 By silencing the LIGHT receptors, we show that the LIGHT-mediated upregulation of PAR-2 in HUVECs is dependent on signaling through HVEM but not through LTβR. These findings may suggest a role for HVEM not only in LIGHT-driven T cell inflammation, but also in the LIGHT-mediated inflammation in endothelial cells, as also recently suggested by others.20 Furthermore, our blocking experiments suggest that the LIGHT-mediated upregulation of PAR-2 involves activation of NF-κB and JNK signaling pathways. Additionally, silencing of HVEM, but not of LTβR, markedly downregulated the LIGHT-mediated activation of JNK, suggesting that HVEM/JNK activation is crucial for the LIGHT-mediated effect on PAR-2. In contrast, either silencing of HVEM or LTβR significantly downregulated NF-κB activation in LIGHT-activated HUVECs, indicating that blocking of both receptors is necessary for totally abolishing the LIGHT-mediated NF-κB activation in these cells. Although LIGHT-LTβR signaling recently has been shown to involve JNK activation in fibroblasts,25 overexpression of HVEM seems to be of particular importance for the LIGHT-mediated activation of JNK.26 Based on our findings in the present study, it is tempting to hypothesize that the LIGHT/HVEM/JNK interaction could represent an important cellular pathway in the promotion of endothelial cell–related inflammation.
Previous in vivo studies in PAR-2–deficient mice highlight a role of PAR-2 in progression of skin and joint inflammation, as well as sepsis,27,28 and very recently, Tennant et al reported reduced cellular adhesion to injured vessels with a consequent reduction in neointima formation in mice lacking PAR-2.29 Moreover, Seitz et al have recently shown that PAR-2 activation increases IL-6 and IL-8 expression in endothelial cells.10 However, although we found that SLIGKV had a slight effect on IL-8 and MCP-1 release in HUVECs, confirming inflammatory effects of PAR-2 agonists in these cells, our major finding was that PAR-2 activation strongly and dose-dependently boosted the LIGHT-mediated release of these proatherogenic chemokines in endothelial cells. An inflammatory effect of PAR-2 activation was also seen in LIGHT-stimulated monocytes, but the inflammatory interaction between LIGHT and PAR-2 seems to be somewhat less prominent in these cells, depending on preactivation with TNFα. Additionally, the inflammatory profile in LIGHT/PAR-2–activated monocytes was different from that in HUVECs, with enhanced release of RANTES as the significant finding in the former cells. Increased PAR-2 expression has previously been reported in human coronary atherosclerotic lesions,30 and in the present study, we show enhanced expression of PAR-2 in monocytes from angina patients, with particularly high levels in unstable disease. We also found increased mRNA levels of PAR-2 in atherosclerotic lesions of ApoE−/− mice, with strong immunostaining in macrophages and endothelial cells, at least partly colocalized to LIGHT and its receptors. Our findings suggest that the LIGHT/PAR-2 interaction in endothelial cells and, to some degree, also in monocytes may represent a pathogenic loop that could be operating within an atherosclerotic lesion, contributing to local and systemic inflammation, which in turn could induce further inflammatory responses in other endothelial interacting cells such as various leukocyte subpopulations.
It may be argued that the LIGHT concentrations used in the in vitro experiments in the present study were too high. However, it is not inconceivable that within an inflamed atherosclerotic plaque, consisting of activated platelets, macrophages, and endothelium (all important cellular sources of LIGHT), LIGHT levels could be comparable to those used in the in vitro experiments in the present study. Also, we have previously shown that oxidized LDL is a potent inducer of LIGHT in macrophages,6 further underscoring the relevance of LIGHT-mediated activation of endothelial cells in atherogenesis. Moreover, the ability of other inflammatory mediators, operating within an atherosclerotic lesion, to enhance LIGHT-mediated effects was further supported by our data showing that preactivation of monocytes with TNFα augments the LIGHT-mediated PAR-2 response in these cells.
In the present study, we show a potent inflammatory interaction between LIGHT and PAR-2 in endothelial cells and, to some degree, also in monocytes. The demonstration of enhanced expression of PAR-2, LIGHT, and its receptors within atherosclerotic lesions underscore the potential in vivo relevance of these finding in relation to atherosclerosis. Our findings suggest that LIGHT/PAR-2–driven inflammation, which engages HVEM/JNK signaling, could represent a pathogenic loop in atherogenesis and plaque progression, potentially representing a target for therapy in this disorder.
We thank Ellen Lund Sagen and Morten Mattingsdal (Joint Centre for Bioinformatics, Oslo) for excellent technical assistance.
Sources of Funding
This work was supported by grants from the Norwegian Council of Cardiovascular Research, Research Council of Norway, University of Oslo, Medinnova Foundation, Helse Sør, and Rikshospitalet University Hospital.
Received September 24, 2008; revision accepted November 6, 2008.
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