Macrophage scavenger receptor CD36 is the major receptor for LDL modified by monocyte-generated reactive nitrogen species (original) (raw)
The scavenger receptor CD36 recognizes LDL modified by activated human monocytes. [125]LDL incubated in the presence of activated human monocytes in media containing NO2– was readily converted into a form that bound to 293 cells expressing CD36, but not to control vector–transfected 293 cells (Figure 1a, complete system). In contrast, LDL modified by the complete system of monocytes failed to bind to CHO cells expressing murine SR-AI more than their control vector–transfected counterparts (Figure 1b). Surface expression of CD36 and SR-AI on their respective cells was confirmed by FACS analyses (data not shown) and by demonstrating that Cu2+-oxidized [125I]LDL (Cu2+-oxLDL) and acetylated LDL (AcLDL), prototypic ligands for CD36 and SR-AI, respectively, bound to CD36- and SR-AI–transfected cells, but not their control vector–transfected counterparts (Figure 1). Neither native LDL nor LDL modified by activated human monocytes in the absence of NO2– bound significantly to cells transfected with either CD36 or SR-AI (Figure 1). Monocyte-dependent conversion of LDL into a ligand for CD36 required cell activation and was inhibited by either the H2O2 scavenger catalase or by peroxidase inhibitors such as 3-aminotriazole (Figure 1). These results suggest that monocytes employ the MPO-H2O2-NO2– system for conversion of LDL into a ligand for CD36, but not SR-AI.
[![Binding of [125I]LDL by CD36- and SR-AI–transfected cells after modificatio](https://dm5migu4zj3pb.cloudfront.net/manuscripts/8000/8574/small/JCI0008574.f1.gif)
](/articles/view/8574/figure/1)Figure 1
Binding of [125I]LDL by CD36- and SR-AI–transfected cells after modification by activated human monocytes. [125I]LDL was incubated with NO2– and phorbol ester-stimulated human monocytes at 37°C for 8 hours (complete system) as described in Methods. Additions or deletions to the complete system were as indicated. Reactions were stopped by addition of BHT (40 μM) and catalase (300 nM), the cells were pelleted, and then lipoproteins (5 μg/mL) were incubated with either (a) CD36- or vector-transfected 293 cells or (b) murine SR-AI– or vector-transfected CHO cells at 37°C for 5 hours in the appropriate media containing additional catalase (300 nM) and BHT (20 μM). Cellular binding of lipoproteins at 4°C was subsequently determined as described in Methods. The final concentrations of additions to the complete system were catalase (300 nM) and 3-aminotriazole (1 mM). Data represent the mean ± SD of triplicate determinations of a representative experiment performed 3 times. A_P_ < 0.001 for complete system versus complete system–NO2–. Atz, 3-aminotriazole; Mono, human peripheral blood monocytes.
Reactive nitrogen species formed by the MPO-H2O2-NO2– system convert LDL into a ligand for CD36. Because monocyte-dependent conversion of LDL into a ligand for CD36 demonstrated a requirement for NO2– in media (Figure 1), the results suggest that either NO2Cl (60) or the MPO-H2O2-NO2– system (15, 21, 22, 28) was responsible for lipoprotein conversion into a ligand for CD36. They also demonstrate that under the conditions employed, monocyte-generated ONOO– and HOCl are not sufficient to convert the lipoprotein into a stable ligand for either the scavenger receptor CD36 or SR-AI. To more fully explore the mechanism(s) through which monocytes transform LDL into a ligand for CD36, we modified LDL using a model system comprised of purified MPO and an H2O2-generating system. [125I]LDL exposed to MPO isolated from human leukocytes, a H2O2-generating system (GGOx), and NO2– was transformed into a form (NO2-LDL) that readily bound to CD36-transfected 293 cells, but not to their control vector–transfected counterparts (Figure 2a). Conversion of LDL into a ligand for CD36 by isolated MPO had an absolute requirement for NO2– and occurred in both the presence (not shown) and absence of chloride in buffer (Figure 2a). Examination of the NO2– concentration dependence for MPO-dependent conversion of LDL into a ligand for CD36 (in the presence of plasma levels of chloride) demonstrated that levels of NO2– that approximate those commonly observed in normal plasma and inflammatory tissues and fluids (up to 50 μM) converted the lipoprotein into a ligand for the scavenger receptor (Figure 2b and inset). Collectively, these results suggest that under conditions in which MPO-generated oxidants are formed by a physiological flux of H2O2, the reactive nitrogen species formed by the MPO-H2O2-NO2– system are sufficient to convert LDL into a ligand for CD36. Because MPO-generated HOCl can modify lipid and protein components of lipoproteins (26, 61–63), but is not required for MPO-dependent conversion of LDL into a ligand for the scavenger receptor CD36, subsequent studies characterizing the biological consequences of NO2-LDL engagement of CD36 and the structural nature of the ligand(s) responsible for CD36 recognition were performed on LDL modified under chloride-free conditions, unless otherwise indicated.
[![Binding of [125I]LDL by CD36-transfected cells after modification by MPO-ge](https://dm5migu4zj3pb.cloudfront.net/manuscripts/8000/8574/small/JCI0008574.f2.gif)
](/articles/view/8574/figure/2)Figure 2
Binding of [125I]LDL by CD36-transfected cells after modification by MPO-generated nitrating intermediates. (a) [125I]LDL was incubated with isolated human MPO (30 nM), glucose (100 μg/mL), glucose oxidase (20 ng/mL), and NO2– (0.5 mM) in sodium phosphate buffer supplemented with DTPA (100 μM) for 8 hours at 37°C (complete system, NO2-LDL) as described in Methods. Reactions were stopped by addition of BHT (40 μM) and catalase (300 nM), and then lipoproteins (5 μg/mL) were incubated with CD36- or vector-transfected 293 cells at 4°C for 3 hours in the appropriate media containing additional catalase (300 nM) and BHT (20 μM). LDL modified by dialysis against copper for 5 hours (Cu2+ oxLDL, 5 μg/mL) was used as a positive control. Cellular binding of lipoproteins was subsequently determined as described in Methods. A_P_ < 0.001 for comparison versus LDL modified in the presence of MPO- and a H2O2-generating system (MPO/GGOx/LDL). (b and inset) [125I]LDL (0.2 mg/mL) was incubated with isolated human MPO (30 nM), glucose (100 μM), glucose oxidase (20 nM), and the indicated concentrations of NO2– in sodium phosphate buffer supplemented with DTPA (100 μM) and NaCl (100 mM) overnight at 37°C. Cellular binding of lipoproteins (10 μg/mL) was subsequently determined as described in Methods. Data represent the mean ± SD of triplicate determinations of a representative experiment performed 3 times.
Additional evidence for the specificity of CD36-mediated recognition of NO2-LDL was obtained from competition experiments. Excess amounts of unlabeled NO2-LDL effectively competed with [125I]NO2-LDL formed by either activated human monocytes or isolated MPO for binding to CD36-transfected 293 cells (Figure 3). In contrast, a 40-fold molar excess of nonlabeled LDL modified by activated monocytes or MPO in the absence of NO2– (–NO2LDL, Figure 3) failed to block binding of monocyte- and MPO-generated [125I]NO2-LDL to CD36-transfected 293 cells. Similarly, a large molar excess of native LDL did not attenuate NO2-LDL binding to CD36-transfected cells (data not shown). Finally, the anti-CD36 blocking mAb, FA6 (64), significantly inhibited binding of monocyte- and MPO-generated NO2-LDL to CD36-transfected cells, whereas isotype-matched nonimmune antibody had no effect (Figure 3).
Effect of CD36-blocking mAbs on the binding of LDL modified by monocyte- or MPO-generated reactive nitrogen species to CD 36-transfected cells. [125I]LDL was modified by reactive nitrogen species generated by (solid bars) activated human monocytes (as in complete system, Figure 1) or by (open bars) isolated human MPO (as in complete system, Figure 2a). Reactions were stopped by addition of BHT (40 μM) and catalase (300 nM), and then lipoproteins (5 μg/mL) were incubated with CD36-transfected 293 cells for 3 hours at 4°C either in the absence or presence of the indicated competitors. Cellular binding of lipoproteins was subsequently determined as described in Methods. Data represent the mean ± SD of triplicate determinations of a representative experiment performed 3 times. A_P_ < 0.001 for comparison versus control (NA, no addition). Final concentrations of competitors are 200 μg/mL for LDL and 10 μg/mL for immunoglobulins.
Time course for acquisition of scavenger receptor recognition, and loss of LDL receptor recognition, of LDL modified by the MPO/GGOx/NO2– system. We have demonstrated previously that with increasing exposure time of LDL to the MPO-H2O2-NO2– system, the degree of LDL modification increases (15). We therefore examined the time course of LDL conversion into a high-uptake form after exposure to MPO, NO2–, and a low, steady flux (10 μM/h) of H2O2 by assessing the extent of lipoprotein binding to cells expressing CD36, SR-AI, or the LDL receptor. Incubation of LDL with the complete MPO-H2O2-NO2– system resulted in conversion of LDL into a ligand for CD36 after only a brief (1–2 hour) period (Figure 4a). Recognition of the modified lipoprotein by CD36-transfected cells progressively increased with the time of incubation, reaching 80% of maximal binding by 8 hours. No increase in CD36-dependent binding was observed in LDL incubated with MPO and H2O2 in the absence of NO2– for up to 24 hours of modification (Figure 4a). These results suggest that the ligand for CD36 on NO2-LDL is formed rapidly upon exposure of LDL to MPO-generated reactive nitrogen species.
Time dependence of LDL-ligand properties after exposure to the MPO-H2O2-NO2– system. [125I]LDL (0.2 mg/mL) was incubated with isolated human MPO (30 nM), glucose (100 μM), and glucose oxidase (20 ng/mL) in sodium phosphate buffer (50 mM, pH 7.0) supplemented with DTPA (100 μM) for the indicated times in either the presence (+ NO2–, filled symbols) or absence (– NO2–, open symbols) of NO2– as described in Methods. Binding of the modified lipoprotein by (a) CD36-transfected 293 cells; (b) mSRA-I–expressing CHO cells, and (c) human foreskin fibroblasts was then determined as described in Methods. Note that the time course for the acquisition of scavenger receptor CD36 recognition and loss of LDLR recognition are early events during lipoprotein modification, whereas acquisition of mSR-AI recognition requires more prolonged lipoprotein oxidation. Data represent the mean ± SD for triplicate determinations of a representative experiment performed at least 3 times. The REM of lipoprotein preparations exposed to the MPO-H2O2-NO2– system for 0, 1, 2, 4, 8, 16, and 24 hours were 1.0, 1.02, 1.13, 1.26, 1.44, 2.01, and 2.22, respectively. The lysine content (percent of control) of LDL exposed to the MPO-H2O2-NO2– system for 0, 1, 2, 4, 8, 16, and 24 hours was 100, 93.4, 90.3, 86.4, 83.7, 75.9, and 68.8, respectively.
Neutralization of Nε-amino groups of lysine on apo B-100 of LDL is regarded as critical for recognition of modified forms of LDL by SR-A (65). After exposure to the MPO-H2O2-NO2– system, however, the content of free amino groups on apo B-100 in LDL demonstrates only modest decreases with time (∼13.6 % reduction at t = 4 hours; Figure 4 legend). Consistent with these findings, no SR-AI–mediated binding of NO2-LDL was observed for up to 6–8 hours of LDL modification (Figure 4b). Recognition of NO2-LDL by SR-AI rapidly increased after prolonged (> 8 hours) modification by the MPO-H2O2-NO2– system. The increase in NO2-LDL recognition was accompanied by an increase in relative electrophoretic mobility (REM) (> 1.44 beyond 8 hours’ incubation; Figure 4 legend) and a decrease in free Nε-amino groups of lysine on LDL apo B-100 (> 20% reduction beyond 8 hours’ incubation). No increase in SR-AI–dependent binding was observed in LDL incubated for 24 hours with MPO and H2O2 in the absence of NO2– (Figure 4b). The more prolonged exposure required before conversion of LDL into a ligand for SR-AI suggests that this receptor will play a role in NO2-LDL recognition only after more extensive oxidative modification.
Because the LDL receptor (LDLR) is expressed on hMDMs (66), we also assessed the ability of NO2-LDL to be recognized by this receptor. Exposure of LDL to MPO, NO2–, and an H2O2-generating system, resulted in rapid loss of lipoprotein recognition by human foreskin fibroblasts that had been preincubated for 24 hours in media containing lipoprotein-deficient serum (LPDS) to upregulate the LDL receptor (Figure 4c). Thus, the binding site for LDLR on apo B is very sensitive to oxidation by the MPO-H2O2-NO2– system; furthermore, the LDLR does not contribute to the binding and uptake of NO2-LDL in MPMs and hMDMs. Collectively, these results suggest that brief exposure of LDL to the MPO-H2O2-NO2– system of monocytes will likely result in the prolongation of lipoprotein half-life by rapid loss of LDLR recognition.
LDL is converted into a ligand for CD36 in the presence of serum by MPO-generated reactive nitrogen species. To assess the potential physiological relevance of LDL conversion into a ligand for CD36 by the MPO-H2O2-NO2– system of monocytes, we compared the influence of serum constituents on lipoprotein modification by isolated MPO versus Cu2+-catalyzed oxidation. Addition of 30% LPDS to LDL in buffer containing plasma levels of chloride, isolated human MPO, a H2O2-generating system and NO2– only partially attenuated conversion of the lipoprotein into a high-uptake form for MPM (Figure 5a) and the scavenger receptor CD36 (Figure 5b). Similar results were observed in reactions where LDL was oxidized by MPO in buffer free of plasma levels of chloride (data not shown), consistent with the critical role of the MPO-H2O2-NO2– system (and not HOCl or NO2Cl) in rendering LDL a ligand for CD36. In a parallel experiment, addition of up to 80% LPDS to reaction mixtures containing LDL, human MPO, and the H2O2-generating system, failed to block NO2–-dependent conversion of the lipoprotein into a high uptake form for MPM (Figure 5c). In stark contrast, addition of only 5% LPDS to LDL dialyzed against Cu2+ totally prevented conversion of the lipoprotein into a form recognized by either MPM or CD36-transfected cells (Figure 5, a and b). These results suggest that the MPO-H2O2-NO2– system is a physiologically plausible mechanism for converting LDL into a ligand for the scavenger receptor CD36.
[![Binding and uptake of [125I]LDL by MPM and CD36-transfected cells after mod](https://dm5migu4zj3pb.cloudfront.net/manuscripts/8000/8574/small/JCI0008574.f5.gif)
](/articles/view/8574/figure/5)Figure 5
Binding and uptake of [125I]LDL by MPM and CD36-transfected cells after modification by MPO-generated nitrating intermediates or by Cu2+ in the presence of plasma proteins. [125I]LDL was modified in the presence of the indicated concentrations of human LPDS by (filled squares) incubation with isolated human MPO, a H2O2-generating system (GGOx), and NO2– in PBS supplemented with DTPA or by (filled circles) dialysis against Cu2+ as described in Methods. Binding of modified lipoproteins by (a) MPM and (b) CD36-transfected 293 cells was then determined as described in Methods. (c) [125I]LDL was modified in the presence of the indicated concentrations of human LPDS by incubation with isolated human MPO and a H2O2-generating system (GGOx) either in the presence (filled bars) or absence (open bars) of NO2– in 50 mM phosphate buffer (pH 7.40) supplemented with 100 mM NaCl and 100 μM DTPA overnight at 37°C, and then the extent of lipoprotein uptake by MPM was determined as described in Methods. Each experimental point represents the mean ± SD for triplicate determinations of a representative experiment performed at least 3 times.
CD36 plays a major role in the recognition of NO2-LDL by MPM. Several scavenger receptors for modified lipoproteins are present on macrophages including SR-AI, CD36, and others (8, 10). The relative contribution of these receptors to macrophage recognition of NO2-LDL depends on both their relative affinities for the modified lipoprotein and on the extent of their surface expression. To define the relative contribution of CD36 to the recognition of NO2-LDL by macrophages, we compared the binding (4°C), uptake (37°C), and degradation (37°C) of the modified lipoprotein using MPM recovered from mice homozygous for a null mutation in CD36 (CD36–/–) and from wild-type mice possessing isogenetic backgrounds (CD36+/+) (42). Binding (Figure 6a) and degradation (Figure 6b) of LDL modified by only a brief exposure to the MPO-H2O2-NO2– system were significantly greater with CD36+/+ MPM versus CD36–/– MPM. Both binding and degradation of the MPO-modified lipoprotein by CD36+/+ MPM continued to increase for up to 24 hours, with kinetics similar to that observed for NO2-LDL binding to CD36-transfected cells (Figures 4a and Figure 6). In contrast, binding, uptake, and degradation by CD36–/– cells was negligible for the first 8 hours of modification by the MPO-H2O2-NO2– system, and only modest increases were noted after LDL modification for longer periods (Figure 6). This likely reflects a nominal contribution of SR-AI to NO2-LDL recognition by both CD36–/– and CD36+/+ MPM after prolonged modification of the lipoprotein. Modification of LDL by MPO and the H2O2-generating system in the absence of NO2– resulted in negligible binding, uptake, and degradation by either CD36+/+ or CD36–/– MPM (data not shown). To rule out the possibility that eliciting MPM with thioglycolate affected the function of the macrophages, separate studies were performed using nonelicited peritoneal macrophages. Resident peritoneal macrophages were obtained from CD36–/– and CD36+/+ mice, and the binding and degradation of LDL modified for 8 hours by the MPO-H2O2-NO2– system were determined. Again, CD36 played a major role in both binding (0.31 ± 0.11 vs. 0.13 ± 0.06 μg/mg cell protein for CD36+/+ vs. CD36–/– MPM, respectively; mean ± SD, n = 6 each group) and degradation (7.67 + 0.20 vs. 3.45 + 0.03 μg/mg cell protein for CD36+/+ and CD36–/– MPM, respectively; mean ± SD, n = 6 each group) of the modified lipoprotein. Collectively, these results indicate that CD36 plays a major role in the recognition of NO2-LDL by resident and elicited MPM.
Time dependence of LDL recognition by macrophages from wild-type mice and CD36-null mice after exposure to the MPO-H2O2-NO2– system. [125I]LDL was incubated with isolated human MPO, a H2O2-generating system (GGOx), and NO2– in sodium phosphate buffer (50 mM, pH 7.0) supplemented with DTPA (100 μM) for the indicated times as described in Methods. Binding (a) at 4°C and degradation (b) at 37°C of the modified lipoprotein by macrophages from wild-type mice (CD36+/+, filled squares) and CD36-null mice (CD36–/–, filled circles) were then determined as described in Methods. Data represent the mean ± SD for triplicate determinations of a representative experiment performed at least 3 times.
CD36-mediated recognition of NO2-LDL by macrophages induces cholesteryl ester synthesis and foam cell formation. To determine whether CD36-mediated uptake of NO2-LDL might promote cholesterol accumulation in macrophages, we first compared the ability of NO2-LDL to stimulate cholesteryl ester (CE) biosynthesis in CD36+/+ and CD36–/– MPM. NO2-LDL markedly stimulated [14C]oleate incorporation into cellular CE pools in CD36+/+ MPM, whereas in CD36–/– cells, incorporation of [14C]oleate was comparable to that of native LDL (Table 1). In a parallel set of experiments, we examined whether stimulation of CE biosynthesis in macrophages mediated by CD36-dependent recognition of NO2-LDL resulted in the formation of lipid-laden foam cells, an early cellular event in the atherosclerotic process. MPMs from CD36+/+ and CD36–/– mice were incubated for 72 hours with either native LDL (negative control), acetylated LDL (positive control), NO2-LDL, or LDL exposed to MPO and H2O2 in the absence of NO2–, and then examined microscopically after neutral lipid staining with oil red-O. LDL modified by the MPO-H2O2-NO2– system promoted cholesterol deposition and foam cell formation in CD36+/+ MPM (Figure 7, a and c). However, foam cell formation in similarly treated CD36–/– MPM was almost completely inhibited (Figure 7, b and d). As anticipated, LDL modified by MPO and H2O2 in the absence of NO2– showed no ability to stimulate neutral lipid accumulation in MPMs (Table 1) and failed to promote macrophage foam cell formation (data not shown). CD36–/– MPMs exposed to acetylated LDL still formed foam cells (not shown), further demonstrating that NO2-LDL–dependent formation of macrophage foam cells was specifically mediated by the macrophage scavenger receptor CD36.
LDL modified by the MPO-H2O2-NO2– system induces lipid loading of macrophages from wild-type mice but not from CD36-null mice. Thioglycollate-elicited MPM from wild-type mice (a and c) and from CD36-null mice (b and d) were grown for 24 hours in RPMI-1640 containing 10% FBS. Cells were then incubated for 72 hours in the same media containing catalase (100 nM), BHT (20 μM), and vitamin E (20 μM) in the presence of the following lipoprotein preparations (75 μg/mL): LDL incubated with isolated human MPO, a H2O2-generating system (GGOx), and NO2– in sodium phosphate buffer (50 mM, pH 7.0) supplemented with DTPA (100 μM) at 37°C for either 8 hours (a and b) or 24 hours (c and d) as described in Methods. Cells were fixed with 4% formaldehyde and stained with hematoxylin and oil red-O. ×300.
[14C]oleate incorporation into cholesteryl esters of macrophages from wild-type and CD36-knockout mice.
CD36 plays a significant role in the recognition of NO2-LDL in hMDMs. To determine the relative contribution of the macrophage scavenger receptor CD36 in the recognition of NO2-LDL by hMDMs, we initially examined NO2-LDL binding and uptake to cells in the presence and absence of the anti-CD36 blocking mAb FA6 (64). Incubation of NO2-LDL with hMDMs in the presence of FA6 resulted in a 41% (± 2%) reduction in binding and 32% (± 3%) reduction in uptake of NO2-LDL, whereas isotype-specific nonimmune antibody had no effect (Figure 8a). Addition of a 40-fold excess of –NO2LDL failed to block NO2-LDL binding or uptake by hMDM (data not shown). To further confirm the biological significance of CD36-mediated uptake of NO2-LDL in hMDMs, we assessed the effect of FA6 on the rate of CE accumulation, as measured by the incorporation of [14C]oleate into the cellular CE pool. Addition of NO2-LDL, but not -NO2LDL, significantly stimulated cholesteryl[14C]oleate synthesis in hMDMs over levels observed with native LDL. Addition of FA6, but not isotype-specific nonimmune control antibody, to media containing NO2-LDL inhibited the increase in [14C]oleate incorporation by 51% (± 3%) (Figure 8b)—similar to the reduction observed in [125I]NO2-LDL binding (Figure 8a). Similar results were observed when analyzing the effect of FA6 on NO2-LDL binding, uptake, degradation, and [14C]oleate incorporation into the CE pool of human monocytic cell lines (THP-1 and U937; data not shown). Taken together, these results demonstrate that the scavenger receptor CD36 is a major pathway for LDL recognition by murine and human macrophages after modification by monocyte- and MPO-generated reactive nitrogen species.
Effect of anti-CD36 blocking mAb on (a) binding and uptake of NO2-LDL and (b) cholesteryl oleate synthesis in hMDM exposed to NO2-LDL. (a) [125I]LDL or (b) unlabeled LDL was incubated with isolated human MPO, a H2O2-generating system (GGOx), and NO2– in sodium phosphate buffer (50 mM, pH 7.0) supplemented with DTPA as described in Methods. Either (a) [125I]NO2-LDL or (b) unlabeled NO2-LDL was then incubated with hMDM alone (a, NO2LDL; and b, no addition), or in the presence of the indicated additions. (a) Binding (filled bars) at 4°C and uptake (open bars) at 37°C of modified lipoproteins were then determined as described in Methods. (b) Lipoproteins were incubated overnight with hMDM in the presence of [14C]oleate and the indicated additions, and cholesteryl [14C]oleate formation was determined as described in Methods. Data represent the mean ± SD of triplicate determinations of a representative experiment performed 3 times. A_P_ < 0.05 for comparison versus NO2-LDL. NA, No addition.
Identification of potential ligands for CD36 on NO2-LDL. In a final series of experiments, we sought to gain insights into the chemical nature of the ligand(s) for CD36 that was formed during even brief exposure of LDL to the MPO-H2O2-NO2– system. Both monocyte- (28) and MPO-generated (15) reactive nitrogen species oxidatively modify multiple protein and lipid components on LDL. We therefore reasoned that the component(s) in NO2-LDL recognized by CD36 could be a modified lipid, a protein oxidation product, or a lipid-protein adduct. To distinguish among these potential ligands, we initially performed competition experiments with [125I]NO2-LDL binding to CD36-transfected 293 cells as the screening assay. As expected, neither native LDL (Figure 9a) nor LDL modified by the MPO-H2O2 system in the absence of NO2– (data not shown) effectively blocked [125I]NO2-LDL binding to CD36-transfected cells. In contrast, a 40-fold molar excess of unlabeled NO2-LDL was an effective competitive ligand, inhibiting binding of [125I]NO2-LDL to the level of binding observed in vector-transfected cells (Figure 9a). Liposomes generated from lipid extracts of NO2-LDL were also potent competitive ligands for [125I]NO2-LDL binding to CD36-transfected 293 cells (Figure 9a), in contrast to liposomes generated from either native LDL (Figure 9a) or LDL modified by the MPO-H2O2 system in the absence of NO2– (data not shown). Thus, at least 1 ligand for CD36 formed in NO2-LDL is an oxidized lipid moiety. The concentration dependence for inhibition of CD36 recognition of [125I]NO2-LDL by intact NO2-LDL and liposomes derived from lipid extracts of NO2-LDL are shown in Figure 9b.
Effect of various competitors on the binding of NO2-LDL to CD36- transfected cells. [125I]LDL was modified by isolated MPO, a H2O2-generating system, and NO2– as described for the complete system in Figure 2a. [125I]-labeled lipoproteins ([125I]NO2-LDL, 5 μg/mL) were then incubated with vector-transfected cells (hatched bar) (a) or CD36-expressing 293 cells (filled bars) (a) and (b), for 3 hours at 4°C either in the absence (NA) or presence of the indicated competitors. Cellular binding of lipoproteins was subsequently determined as described in Methods. Liposomes generated from lipid extracts of native LDL (LDL lipids) and NO2-LDL (NO2-LDL lipids) were prepared as described in Methods. NO2-LDL protein was prepared as described in Methods. BSA was exposed to the same MPO-H2O2-NO2– system as LDL (NO2-BSA) and used as a competitive ligand as well. (a) The final concentrations of competitors used were 200 μg protein/mL for LDL, NO2-LDL, NO2-LDL protein, BSA, and NO2-BSA. The concentration of LDL lipids and NO2-LDL lipids used was equivalent to that present in 200 μg LDL protein/mL. Synthetic tri-peptides were used at 200 μM final concentration. (b) The concentration of intact LDL and LDL lipids used was equivalent to the amount present in the indicated amount of LDL (μg protein/mL). Data represent the mean ± SD of triplicate determinations (a) or means of triplicate determinations (b) of a representative experiment that was performed 3 times. A_P_ < 0.001 for comparison versus control (NA).
To determine whether an oxidation product of a protein or a protein-lipid adduct was also a potential ligand for CD36 on NO2-LDL, we examined the ability of apo B-100 from delipidated NO2-LDL to either bind to CD36-transfected cells (using [125I]apo B-100) or to block [125I]NO2-LDL binding to CD36-transfected cells. This approach has been used successfully when looking for potential ligands on Cu2+-oxLDL for hMDM (57, 59, 67). We were able to resolubilize delipidated apo B-100 from NO2-LDL only after exposure to the MPO-H2O2-NO2– system for approximately 24 hours (i.e., extensive oxidation). Apo B-100 from NO2-LDL failed to block the binding of [125I]NO2-LDL (Figure 9a), and [125I]NO2-apo B-100 recovered from 24-hour modified [125I]NO2-LDL failed to bind to the CD36-expressing cells (data not shown). Because of the potential that detergent in the microemulsions of resolubilized apo B-100 might interfere with the binding assay, we also studied model proteins exposed to the MPO-H2O2-NO2– system and synthetic peptides containing nitrotyrosine as potential competitive ligands. When BSA was modified by MPO-H2O2 in the presence of NO2–, extensive nitration of tyrosine residues was observed, as determined by stable isotope dilution GC/MS analyses (nitrotyrosine/tyrosine = 34 ± 2 mmol/mol)—nearly a 100-fold higher level of tyrosine nitration than that observed with NO2-LDL under similar conditions (15). However, a 500-fold molar excess of “nitrated BSA” had no effect on the binding of NO2-LDL to CD36 (Figure 9a). Similar results were obtained using a vast molar excess of a synthetic tri-peptide consisting of glycine-nitrotyrosine-alanine, a potent ligand for antinitrotyrosine antibodies (68). In separate binding experiments, we found that nitration of several irrelevant proteins ([125I]BSA, [125I]ovalbumin, and [125I]ribonuclease A) by the MPO-H2O2-NO2– system, did not convert them into ligands for either CD36-transfected cells or MPM (data not shown).
We next sought to characterize the nature of lipid oxidation products that potentially contribute to CD36 recognition using a model vesicle system. Small unilamellar vesicles comprised of synthetic homogeneous phosphatidylcholine molecular species were exposed to the MPO-H2O2 system (± NO2–), and then tested for their ability to compete for [125I]NO2-LDL binding to CD36-transfected 293 cells. PAPC vesicles that were modified by the complete MPO-H2O2-NO2– system (NO2-PAPC) completely inhibited binding of [125I]NO2-LDL (Figure 10a). Significant inhibition was observed at concentrations of less than 2 μg lipid/mL, with half-maximal inhibition at less than 5 μg/mL (Figure 10b). Similar results (inhibition of [125I]NO2-LDL binding) were observed in control experiments where CD36-transfected cells were first preincubated with NO2-PAPC for 2 hours at 4°C, washed, and then [125I] NO2-LDL binding was determined (data not shown). Thus, NO2-PAPC likely inhibits NO2-LDL recognition by CD36-transfected cells by direct binding to CD36 and not through interaction with [125I]NO2-LDL. Moreover, the inhibitory effect of NO2-PAPC was specific for CD36 because NO2-PAPC failed to inhibit the binding of either [125I]AcLDL or LDL modified by the MPO-H2O2-NO2– system for 24 hours to mSR-AI–transfected CHO cells, as well as the binding of LDL to LDLR on human foreskin fibroblasts (data not shown). The possibility that trace levels of preexisting hydroperoxides are required for conversion of PAPC into a ligand for CD36 was excluded by demonstrating that identical results were observed with hydroperoxide-free preparations of parent PAPC vesicles (see Methods) oxidized by MPO-generated reactive nitrogen species (Figure 10b). PLPC vesicles oxidized in the presence of NO2– demonstrated an intermediate inhibitory activity (Figure 10a), and modified POPC vesicles failed to compete for [125I]NO2-LDL recognition by CD36-positive cells (Figure 10). None of the vesicles (including PAPC) modified in the absence of NO2– were able to inhibit [125I]NO2-LDL binding to CD36-transfected cells (Figure 10). The ability of vesicles to block binding of [125I]NO2-LDL paralleled their extent of lipid peroxidation, as assessed by LC/MS/MS quantification of the content of esterified 9-H(P)ETE or 9-H(P)ODE formed (data not shown). Finally, oxidation of vesicles in the presence of BSA (to assess the possible participation of putative lipid-protein adducts in the recognition by CD36) had no effect on the observed results (Figure 10a). Collectively, these results support a role for oxidized lipids as the major moiety in NO2-LDL that confers CD36-dependent receptor recognition.
Effect of lipid competitors on the binding of NO2-LDL to CD36-transfected cells. [125I]LDL was modified as described for the complete system in Figure 2a. [125I]-NO2LDL (5 μg/mL) was then incubated with CD36-expressing 293 cells for 3 hours at 4°C in the presence of (a) 20 μg lipid/mL or (b) the indicated concentrations (μg lipid/mL) of competitors. PAPC, PAPC(SnCl2), PLPC, and POPC unilamellar vesicles were oxidized for 8 hours at 37°C as described for the complete system in Figure 2 in the presence (+NO2–, filled symbols) or absence (–NO2–, open symbols) of NO2–. Where indicated, BSA (0.2 mg protein/mL final concentration) was also included during liposome preparation as described in Methods (hatched bars). PAPC (SnCl2), hydroperoxide-free PAPC generated by reduction of PAPC with SnCl2, and then reisolation of PAPC under argon atmosphere before use were used as described in Methods. Data represent the mean ± SD of triplicate determinations (a) or means of triplicate determinations (b) of a representative experiment performed 3 times. A_P_ < 0.001 for comparison versus control (no competitor).
Finally, we evaluated the ability of NO2-PAPC vesicles to block NO2-LDL binding to MPM and hMDM because it appears to be a specific and potent ligand for CD36-dependent recognition (Figure 11). We found that NO2-PAPC inhibited the binding of [125I]NO2-LDL to MPM by at least 80% and to hMDM by 50%. In contrast, large molar excess of –NO2 PAPC failed to inhibit MPM and hMDM recognition of [125I]NO2-LDL. Finally, addition of FA6 to NO2-PAPC failed to further suppress [125I]NO2-LDL binding to hMDM (Figure 11b). These results are in agreement with our previous data with the CD36-specific blocking antibody FA6 (Figures 3 and 8) and CD36–/– MPM (Figures 5 and 6). These findings confirm that CD36 is a major macrophage scavenger receptor responsible for the recognition of NO2-LDL in murine and human macrophages.
Effect of oxidized PAPC vesicles on the binding of NO2-LDL to hMDM and MPM. [125I]LDL was modified as described for the complete system in Figure 2. [125I]NO2-LDL (5 μg/mL) was then incubated with (a) MPM or with (b) hMDM for 3 hours at 4°C either in the absence (NA) or presence of the indicated additions. Cellular binding of lipoproteins was subsequently determined as described in Methods. The concentrations of competitors used were 200 μg protein/mL for lipoproteins, 20 μg lipid/mL for vesicles, and 20 μg/mL for antibody. NO2-LDL, LDL modified by the MPO-H2O2-NO2– system as in the complete system of Figure 2a; –NO2LDL, LDL modified by the MPO-H2O2 system but in the absence of NO2–; NO2-PAPC, PAPC vesicles modified by the complete MPO-H2O2-NO2– system; –NO2 PAPC, PAPC modified by the MPO-H2O2 system but in the absence of NO2–. Data represent the mean ± SD of triplicate determinations of a representative experiment performed at least 3 times. A_P_ < 0.001 for comparison versus control (NA).