In vivo visualization and attenuation of oxidized lipid accumulation in hypercholesterolemic zebrafish.

Oxidative modification of LDL is an early pathological event in the development of atherosclerosis. Oxidation events such as malondialdehyde (MDA) formation may produce specific, immunogenic epitopes. Indeed, antibodies to MDA-derived epitopes are widely used in atherosclerosis research and have been demonstrated to enable cardiovascular imaging. In this study, we engineered a transgenic zebrafish with temperature-inducible expression of an EGFP-labeled single-chain human monoclonal antibody, IK17, which binds to MDA-LDL, and used optically transparent zebrafish larvae for imaging studies. Feeding a high-cholesterol diet (HCD) supplemented with a red fluorescent lipid marker to the transgenic zebrafish resulted in vascular lipid accumulation, quantified in live animals using confocal microscopy. After heat shock-induced expression of IK17-EGFP, we measured the time course of vascular accumulation of IK17-specific MDA epitopes. Treatment with either an antioxidant or a regression diet resulted in reduced IK17 binding to vascular lesions. Interestingly, homogenates of IK17-EGFP-expressing larvae bound to MDA-LDL and inhibited MDA-LDL binding to macrophages. Moreover, sustained expression of IK17-EGFP effectively prevented HCD-induced lipid accumulation in the vascular wall, suggesting that the antibody itself may have therapeutic effects. Thus, we conclude that HCD-fed zebrafish larvae with conditional expression of EGFP-labeled oxidation-specific antibodies afford an efficient method of testing dietary and/or other therapeutic antioxidant strategies that may ultimately be applied to humans.

The CC chemokine MCP-1 stimulates surface expression of CX3CR1 and enhances the adhesion of monocytes to fractalkine/CX3CL1 via p38 MAPK.

The membrane-anchored form of CX3CL1 has been proposed as a novel adhesion protein for leukocytes. This functional property of CX3CL1 is mediated through CX3CR1, a chemokine receptor expressed predominantly on circulating white blood cells. Thus far, it is still uncertain at what stage of the trafficking process CX3CR1 becomes importantly involved and how the CX3CR1-dependent adhesion of leukocytes is regulated during inflammation. The objective of this study was to examine the functional effects of chemokine stimulation on CX3CR1-mediated adhesion of human monocytes. Consistent with previous reports, our data indicate that the activity of CX3CR1 on resting monocytes is sufficient to mediate cell adhesion to CX3CL1. However, the basal, nonstimulated adhesion activity is low, and we hypothesized that like the integrins, CX3CR1 may require a preceding activation step to trigger firm leukocyte adhesion. Compatible with this hypothesis, stimulation of monocytes with MCP-1 significantly increased their adhesion to immobilized CX3CL1, under both static and physiological flow conditions. The increase of the adhesion activity was mediated through CCR2-dependent signaling and obligatory activation of the p38 MAPK pathway. Stimulation with MCP-1 also induced a rapid increase of CX3CR1 protein on the cell surface. Inhibition of the p38 MAPK pathway prevented this increase of CX3CR1 surface expression and blunted the effect of MCP-1 on cell adhesion, indicating a causal link between receptor surface density and adhesion activity. Together, our data suggest that a chemokine signal is required for firm CX3CR1-dependent adhesion and demonstrate that CCR2 is an important regulator of CX3CL1-dependent leukocyte adhesion.

The amino terminus and the third extracellular loop of CX3CR1 contain determinants critical for distinct receptor functions.

The G protein-coupled receptor CX3CR1 is a specific receptor for the CX3C chemokine fractalkine (CX3CL1 according to the new chemokine nomenclature). The aim of this study was to identify receptor elements that contribute independently to agonist binding and receptor activation. Targeted mutation of selected acidic amino acid residues demonstrated that the binding activity of CX3CR1 was critically dependent on the two negatively charged residues Asp25 and Glu254 located on the N-terminal domain and third extracellular loop, respectively. In addition, mutation of the uncharged polar residue Tyr14 in the amino terminus caused a reduction in the ligand binding affinity. In contrast, the three acidic residues Glu13, Asp16, and Asp266 did not contribute to ligand binding but were crucial for receptor activation. The mutant receptors E13A, D16A, and D266A bound fractalkine with high affinity but were unable to induce signaling events necessary to support chemotaxis. These acidic residues may engage in electrostatic interactions with basic residues on fractalkine that are necessary for receptor function but not for binding. Our data are consistent with a model of chemokine receptor activation consisting of a multi-step mechanism. Step one mediates the high-affinity fractalkine binding involving Tyr14, Asp25, and Glu254. The initial interaction then triggers the engagement of Glu13, Asp16, and Asp266, which are necessary for CX3CR1 activation.

The mouse CCR2 gene is regulated by two promoters that are responsive to plasma cholesterol and peroxisome proliferator-activated receptor gamma ligands.

We have previously shown that the expression of monocyte CCR2, the receptor for monocyte chemoattractant protein-1, is induced by plasma cholesterol. The present study examines the mechanisms that regulate monocyte CCR2 expression in hypercholesterolemia using a mouse model. Our data demonstrate that in the mouse, CCR2 expression in circulating monocytes is controlled by two promoters P1 and P2. The two distinct transcripts, which encode the same protein, are produced by alternative splicing in the 5'-untranslated region. Both promoters are constitutively active, but only P2 is stimulated by cholesterol. However, both promoters are repressed by peroxisome proliferator-activated receptor gamma.

Phosphocholine as a pattern recognition ligand for CD36.

We have previously shown that CD36 recognizes oxidation products of phospholipids on oxidized LDL (OxLDL) such as 1-palmitoyl-2-(5'-oxovaleroyl)-sn-glycero-3-phosphocholine (POVPC). The current study was designed to examine whether the phosphocholine (PC) headgroup in POVPC constitutes an obligatory binding target for CD36. To examine the contribution of PC in the binding of POVPC to CD36, we used well-defined synthetic oxidized phospholipids (OxPLs) cross-linked to BSA or to a hexapeptide. The OxPL adducts were then tested for their ability to bind to CD36-transfected cells and for their ability to inhibit OxLDL binding to CD36. Both POVPC-BSA and POVPC-peptide adducts were high-affinity ligands for CD36 and potent inhibitors of OxLDL binding. Enzymatic removal of the entire PC moiety of the POVPC-peptide, or of the choline headgroup alone, as well as substitution of the choline headgroup by ethanolamine abrogated the inhibitory activity of POVPC. Interestingly, PC by itself or cross-linked to BSA did not show any intrinsic competition activity. In conclusion, our data demonstrate that the PC headgroup of OxPL alone is sufficient for binding to CD36, but only if presented in the correct conformation as in OxPL of OxLDL or as in POVPC-peptide adducts.

LDL activates signaling pathways leading to an increase in cytosolic free calcium and stimulation of CD11b expression in monocytes.

In the present study, we investigated the mechanisms by which plasma lipoproteins modulate the integrin-dependent adhesion properties of monocytes. LDL induced the expression of the monocyte CD11b in vitro as well as in vivo via intracellular signaling mechanisms involving calcium transients. The effect on CD11b transcription was specific for native LDL and was blocked by a neutralizing anti-LDL receptor antibody. Neither oxidized LDL nor HDL had any effect on CD11b expression. Although LDL stimulated CD11b surface expression, the integrins were not activated. To initiate the CD11b-specific adhesion to the endothelium, the engagement of chemokine receptor CCR2 and intact chemokine-to-integrin signaling was necessary. However, the activation of CCR2 with monocyte chemoattractant protein-1 not only stimulated the integrins preexisting on the cell surface, but also increased the number of CD11b molecules on the cell surface. This was particularly pronounced in THP-1 cells after treatment with LDL. In a previous study, we showed that LDL induces the expression of CCR2 in monocytes. We conclude that this may be the underlying cause of the enhanced chemokine effect on CD11b expression and activation observed with these cells.

Differential expression of the isoforms for the monocyte chemoattractant protein-1 receptor, CCR2, in monocytes.

Two isoforms of human CCR2, the receptor for monocyte chemoattractant protein-1 (MCP-1), have been identified but their relative expression in monocytes and contribution to inflammatory responses mediated by MCP-1 remain uncertain. All available information on CCR2 expression is based on mRNA data because isoform-specific antibodies were not available until now. To analyze the relative expression of each isoform, we made two antibodies that specifically recognized CCR2A and CCR2B. Examination of receptor protein with these isoform-specific antibodies showed that the total expression of CCR2B in monocytes was about 10-fold higher than that of CCR2A with an equal distribution between the cell surface and intracellular pools. A detailed analysis using purified plasma membranes demonstrated that about 90% of all CCR2 on the cell surface were composed of CCR2B. The relatively abundant expression of CCR2B on the cell surface suggests a principal role of this isoform as a mediator of monocyte responses to MCP-1 in inflammation.