Proteins derived from platelet alpha granules modulate the uptake of oxidized low density lipoprotein by macrophages.

Activated platelets secrete from their alpha granules a protein-like factor which stimulates the uptake of oxidized low-density lipoprotein (Ox-LDL) by macrophages. The aim of the present study was to evaluate the effect of three purified proteins obtained from platelet alpha granules: platelet-derived growth factor (PDGF), platelet factor-4 (PF-4), and beta-thromboglobulin (B-TG), on the uptake of Ox-LDL by macrophages. Cellular degradation of Ox-LDL by the J-774 A.1 macrophage-like cell line, that was preincubated for 18 h at 37 degrees C, with increasing concentrations of partially purified PDGF, (designated PDGF-CMS-III) was increased by up to 36% in comparison to control cells preincubated without PDGF. This effect was due to PDGF-mediated increase in the number of macrophage receptors for Ox-LDL. The enhanced uptake of Ox-LDL by PDGF resulted in an increase in cellular cholesterol content. Preincubation of macrophages with two types of recombinant PDGF dimers (10 ng/ml), revealed that PDGF-BB stimulated Ox-LDL cellular degradation by 64%, whereas PDGF-AB demonstrated only 34% stimulation, in comparison to control cells that were not treated with PDGF. The stimulatory effect of PDGF-CMS-III and PDGF-AB were reduced by 20% and 28%, respectively, when incubated in the presence of H-7, a specific protein kinase C inhibitor. When macrophages were preincubated with B-TG, cellular uptake of Ox-LDL was reduced by up to 30% at 100 ng B-TG/ml. This effect, however, was obtained only when B-TG was present in the incubation medium. Cellular degradation of Ox-LDL was not affected by preincubation of the cells with PF-4. Pretreatment of PCM with anti-PDGF or anti-B-TG antibodies abolished the effects of PCM on Ox-LDL degradation by macrophages. PDGF, thus, may represent the protein-like factor present in PCM which stimulates Ox-LDL degradation by macrophages, whereas B-TG may have a role in the recognition of PCM particles by the macrophage scavenger receptor. Modulation of macrophage cholesterol content by proteins secreted from activated platelets may have an important role in foam cell formation and atherosclerosis.     

Apolipoprotein E4 in macrophages enhances atherogenesis in a low density lipoprotein receptor-dependent manner

Apolipoprotein E (apoE) and the low density lipoprotein receptor (LDLr) are well recognized determinants of atherosclerosis. In addition to hepatocytes, where both are highly expressed and contribute to plasma lipoprotein clearance, they are expressed in vascular cells and macrophages. In this study, we examined the effects of human apoE isoforms and LDLr levels in atherogenic pathways in primary macrophages ex vivo and atherosclerosis development after bone marrow transfer in vivo using mice expressing human apoE isoforms and different levels of LDLr expression. Increases in LDLr expression significantly increased cholesterol delivery into macrophages in culture, and the effects were more prominent with lipoproteins containing apoE4 than those containing apoE3. Conversely, increased LDLr expression reduced cholesterol efflux in macrophages expressing apoE4 but not in macrophages expressing apoE3. Furthermore, apoE3 protected VLDL from oxidation in vitro more than did apoE4. In LDLr-deficient mice expressing the human apoE4 isoform, Apoe4/4 Ldlr-/-, the replacement of bone marrow cells with those expressing LDLr increased atherosclerotic lesions in a dose-dependent manner compared with mice transplanted with cells having no LDLr. In contrast, atherosclerosis in Apoe3/3 Ldlr-/- mice, expressing the human apoE3 isoform, did not differ by the levels of macrophage LDLr expression. Our results demonstrate that apoE4, but not apoE3, in macrophages enhances atherosclerotic plaque development in mice in an LDLr-dependent manner and suggests that this interaction may contribute to the association of apoE4 with an increased cardiovascular risk in humans.     

A specific ligand for beta(2)-glycoprotein I mediates autoantibody-dependent uptake of oxidized low density lipoprotein by macrophages

beta(2)-Glycoprotein I (beta(2)-GPI) is a major antigen for antiphospholipid antibodies (Abs) present in patients with the antiphospholipid syndrome (APS). We previously reported that beta(2)-GPI specifically binds to oxidized low density lipoprotein (oxLDL), but not to native low density lipoprotein (LDL). In the present study, a ligand specific for beta(2)-GPI, oxLig-1, was purified from the extracted lipids of oxLDL. The structure of oxLig-1 was shown to be identical to that of synthesized 7-ketocholesteryl-9-carboxynonanoate by mass spectroscopy and nuclear magnetic resonance analyses. Both purified and synthesized oxLig-1 were recognized by beta(2)-GPI and subsequently by anti-beta(2)-GPI auto-Abs, either in enzyme-linked immunosorbent assay (ELISA) or in ligand blot analysis. Binding of liposomes containing oxLig-1 (oxLig-1-liposomes) to mouse macrophages, J774A.1 cells, was relatively low, as compared with that of phosphatidylserine (PS)-liposomes. In contrast, binding of oxLig-1-liposomes was enhanced more than 10-fold in the presence of both beta(2)-GPI and an anti-beta(2)-GPI auto-Ab (WB-CAL-1), derived from (NZW x BXSB) F1 mouse, an animal APS model. Anti-beta(2)-GPI auto-Abs derived from APS patients with episodes of arterial thrombosis were detected in ELISA, using a solid phase oxLig-1 complexed with beta(2)-GPI. We suggest that autoimmune atherogenesis linked to beta(2)-GPI interaction with oxLDL and Abs may be present in APS.     

Reduced uptake of cholesterol esterase-modified low density lipoprotein by macrophages.

Changes in low density lipoprotein (LDL) lipid composition were shown to alter its interaction with the LDL receptor, thus affecting its cellular uptake. Upon incubation of LDL with 5 units/ml cholesterol esterase (CEase) for 1 h at 37 degrees C, there was a 33% reduction in lipoprotein cholesteryl ester content, paralleled by an increment in its unesterified cholesterol. CEase-LDL, in comparison to native LDL, was smaller in size, possessed fewer free lysine amino groups (by 14%), and demonstrated reduced binding to heparin (by 83%) and reduced immunoreactivity against monoclonal antibodies directed toward epitopes along the LDL apoB-100. Incubation of CEase-LDL with the J-774 macrophage-like cell line resulted in about a 30% reduction in lipoprotein binding and degradation in comparison to native LDL, and this was associated with a 20% reduction in macrophage cholesterol mass. Similarly, CEase-LDL degradation by mouse peritoneal macrophages, human monocyte-derived macrophages, and human skin fibroblasts was reduced by 20-44% in comparison to native LDL. CEase-LDL uptake by macrophages was mediated via the LDL receptor and not the scavenger receptor. CEase activity toward LDL was demonstrated in plasma and in cells of the arterial wall such as macrophages and endothelial cells. Thus, CEase modification of LDL may take place in vivo, and this phenomenon may have a role in atherosclerosis.     

Lysosomal accumulation of oxidized phosphatidylcholine-apolipoprotein B complex in macrophages: intracellular fate of oxidized low density lipoprotein

Oxidized phosphatidylcholine (OxPC) formed in oxidized low density lipoprotein (OxLDL) is thought to be involved in the development of atherosclerosis. OxPC has been found in foam cells in atherosclerotic lesions and suggested to be the epitope for OxLDL recognition by macrophages. OxPC is present as a complex with apolipoprotein B (apoB) in OxLDL, since some OxPC can bind with proteins. In the current study, the intracellular fate of OxPC-apoB complexes after internalization of OxLDL by macrophages was investigated. Murine macrophage cell line J774.1 was incubated with either OxLDL or acetylated LDL for 24 h, then the cells were further incubated for up to 24 h in new medium without lipoprotein. Modified apoB in the cells was quantitated by sandwich ELISA using monoclonal antibodies against OxPC and apoB. Intracellular OxLDL decreased rapidly for the first 4 h to approx. 20% of that before medium change, with the apparent metabolism of OxPC-apoB complex ceasing. OxPC-apoB complexes that remained in the cells after 24 h chasing increased as the period of OxLDL loading in macrophages prolongs. Acetylated LDL in the cells decreased quickly and disappeared after 4 h of chasing. Subcellular fractionation using sucrose density gradient ultracentrifugation of macrophages, which had already accumulated OxPC-apoB complexes by 24 h of incubation with OxLDL and further 24 h chasing, showed that the complex was co-localized with endosomal and lysosomal markers. Immunohistochemical double staining studies demonstrated that OxPC and apoB co-localize in foam cells in early atherosclerotic lesions obtained from human coronary artery. These results suggest that OxPC-apoB complexes originating from OxLDL accumulate in foam cells in human atherosclerotic lesions as well as in macrophages in vitro.     

Differential uptake of proteoglycan-selected subfractions of low density lipoprotein by human macrophages

Macrophages and arterial chondroitin sulfate proteoglycans (CSPG) are probably associated with extracellular and intracellular lipoprotein deposition during atherogenesis. We found that human arterial CSPG can be used to select subclasses from low density lipoprotein (LDL) with different structural properties and capacities to interact with human monocyte-derived macrophages (HMDM). Four subclasses, LDL(PG)1 to LDL(PG)4, in order of decreasing CSPG-complexing capacity, were prepared and characterized in terms of their ability to interact with HMDM. The LDL subclasses with highest avidity for CSPG, LDL(PG)1 and LDL(PG)2, were bound, internalized, and degraded more efficiently than those of lower avidity for CSPG. From LDL(PG)1 to LDL(PG)4, the gradual decrease in uptake by HMDM and decreasing avidity for CSPG were associated with a gradual decrease in isoelectric point (from 5.93 to 5.68) and an augmented ratio of surface polar lipid to core nonpolar components (from 0.35 to 0.54). Competition experiments indicated that the proteoglycan-selected subfractions shared the binding sites and uptake mechanisms of native LDL. The results suggest the existence of a structurally related gradation in the avidity of LDL subpopulations for cells and matrix components. The presence within LDL subpopulations of a differential capacity to interact with intimal extracellular and cellular elements could be associated with a similar heterogeneity in their atherogenic potential.     

Multiple receptors for modified low density lipoproteins in mouse peritoneal macrophages: different uptake mechanisms for acetylated and oxidized low density lipoproteins

Receptor-mediated incorporations of two modified low density lipoproteins (LDL), acetylated LDL (acetyl-LDL) and oxidized LDL were compared in vitro in mouse peritoneal macrophages by cross-competition experiments. Excess amount of oxidized LDL inhibits the binding of [125I]acetyl-LDL only partially, and excess amount of acetyl-LDL inhibits that of [125I]oxidized LDL also only partially, suggesting that the uptake of the two LDL by macrophages is mediated by partially overlapped yet different mechanisms. Scatchard analysis of [125I]acetyl-LDL binding showed a linear plot and addition of excess amount of oxidized LDL partially displaced the binding sites without changing the affinity, suggesting that there are two classes of receptors with similar affinity; one is specific for acetyl-LDL and the other is common. And the plot of [125I]oxidized LDL binding showed a curvilinear plot and excess amount of acetyl-LDL partially displaced the binding sites of the low affinity, suggesting that there are two classes of binding sites with different affinities and the low affinity one is shared with acetyl-LDL. These results indicate that macrophage receptors for modified LDL consist of at least three receptors, two of which are specific for each LDL and the rest is a common receptor.     

Apolipoprotein E and lipoprotein lipase secreted from human monocyte-derived macrophages modulate very low density lipoprotein uptake

We investigated the roles of lipoprotein lipase and apolipoprotein E (apoE) secreted from human monocyte-derived macrophages in the uptake of very low density lipoproteins (VLDL). ApoCII-deficient VLDL were isolated from a patient with apoCII deficiency. The lipolytic conversion to higher density and the degradation of the apoCII-deficient VLDL by macrophages were very slight, whereas the addition of apoCII enhanced both their conversion and degradation. This suggests that the lipolysis and subsequent conversion of VLDL to lipoproteins of higher density are essential for the VLDL uptake by macrophages. VLDL incubated with macrophages obtained from subjects with E3/3 phenotype (E3/3-macrophages) showed a 17-fold greater affinity in inhibiting the binding of 2 micrograms/ml 125I-low density lipoprotein (LDL) to fibroblasts than native VLDL, whereas the incubation of VLDL with macrophages obtained from a subject with E2/2 phenotype (E2/2-macrophages) did not cause any increase in their affinity. Furthermore, 3 micrograms/ml 125I-VLDL obtained from a subject with E3/3 phenotype were degraded by E3/3-macrophages to a greater extent than by E2/2-macrophages (2-fold), indicating that VLDL uptake is influenced by the phenotype of apoE secreted by macrophages. From these results, we conclude that both lipolysis by lipoprotein lipase and incorporation of apoE secreted from macrophages alter the affinity of VLDL for the LDL receptors on the cells, resulting in facilitation of their receptor-mediated endocytosis.     

Human platelets exclusively bind oxidized low density lipoprotein showing no specificity for acetylated low density lipoprotein.

The widely studied macrophage scavenger receptor system is known to bind both acetylated low density lipoprotein and oxidized low density lipoprotein. Although only the latter ligand has been shown to occur in vivo, acetylated low density lipoprotein is often used to evaluate the contribution of scavenger receptors to different (patho)physiologic processes, assuming that all existing subtypes of scavenger receptors recognise both lipoproteins. In the present work, we identify human platelets as the first natural cell type to bind oxidized low density lipoprotein without showing specificity for acetylated low density lipoprotein. Consequently, platelets possess exclusive receptor(s) for oxidized low density lipoprotein distinct from the 'classical' scavenger receptor AI/AII. From the data presented in this work, we conclude that the class B scavenger receptor CD36 (GPIV) is responsible for this exclusive oxidized low density lipoprotein binding.     

Regulation of the uptake and degradation of beta-very low density lipoprotein in human monocyte macrophages

In normal human monocyte macrophages 125I-labeled beta-migrating very low density lipoproteins (125I-beta-VLDL), isolated from the plasma of cholesterol-fed rabbits, and 125I-human low density lipoprotein (LDL) were degraded at similar rates at protein concentrations up to 50 micrograms/ml. The high affinity degradation of 125I-labeled human LDL saturated at approximately 50 micrograms/ml; however, 125I-labeled rabbit beta-VLDL high affinity degradation saturated at 100-120 micrograms/ml. The activity of the beta-VLDL receptor was 3-fold higher than LDL receptor activity on freshly isolated normal monocyte macrophages, but with time-in-culture both receptor activities decreased and were similar after several days. The degradations of both beta-VLDL and LDL were Ca2+ sensitive, were markedly down regulated by sterols, and were up regulated by preincubation of the cells in a lipoprotein-free medium. The beta-VLDL receptor is genetically distinct from the LDL receptor as indicated by its presence on monocyte macrophages from a familial hypercholesterolemic homozygote. Human thoracic duct lymph chylomicrons as well as lipoproteins of Sf 20-5000 from fat-fed normal subjects inhibited the degradation of 125I-labeled rabbit beta-VLDL as effectively as nonradioactive rabbit beta-VLDL. We conclude: 1) the beta-VLDL receptor is genetically distinct from the LDL receptor, and 2) intestinally derived human lipoproteins are recognized by the beta-VLDL receptor on macrophages.     

A macrophage receptor that recognizes oxidized low density lipoprotein but not acetylated low density lipoprotein

The formation of cholesterol-loaded macrophage foam cells in arterial tissue may occur by the uptake of modified lipoproteins via the scavenger receptor pathway. The macrophage scavenger receptor, also called the acetylated low density lipoprotein (Ac-LDL) receptor, has been reported to recognize Ac-LDL as well as oxidized LDL species such as endothelial cell-modified LDL (EC-LDL). We now report that there is another class of macrophage receptors that recognizes EC-LDL but not Ac-LDL. We performed assays of 0 degrees C binding and 37 degrees C degradation of 125I-Ac-LDL and 125I-EC-LDL by mouse peritoneal macrophages. Competition studies showed that unlabeled Ac-LDL could compete for only 25% of the binding and only 50% of the degradation of 125I-EC-LDL. Unlabeled EC-LDL, however, competed for greater than 90% of 125I-EC-LDL binding and degradation. Unlabeled Ac-LDL was greater than 90% effective against 125I-Ac-LDL; EC-LDL competed for about 80% of 125I-Ac-LDL binding and degradation. Copper-oxidized LDL behaved the same as EC-LDL in all the competition studies. Copper-mediated oxidation of Ac-LDL produced a superior competitor which could now displace 90% of 125I-EC-LDL binding. After 5 h at 37 degrees C in the presence of ligand, macrophages accumulated six times more cell-associated radioactivity from 125I-EC-LDL than from 125I-Ac-LDL, despite approximately equal amounts of degradation to trichloroacetic acid-soluble products, which may imply different intracellular processing of the two lipoproteins. Our results suggest that 1) there is more than one macrophage "scavenger receptor" for modified lipoproteins; and 2) oxidized LDL and Ac-LDL are not identical ligands with respect to macrophage recognition and uptake.     

Regulation of acetylated low density lipoprotein uptake in macrophages by pertussis toxin-sensitive G proteins

Class A scavenger receptors (SR-A) mediate the uptake of modified low density lipoprotein (LDL) by macrophages. Although not typically associated with the activation of intracellular signaling cascades, results with peritoneal macrophages indicate that the SR-A ligand acetylated LDL (AcLDL) promotes activation of cytosolic kinases and phospholipases. These signaling responses were blocked by the treatment of cells with pertussis toxin (PTX) indicating that SR-A activates G(i/o)-linked signaling pathways. The functional significance of SR-A-mediated G(i/o) activation is not clear. In this study, we investigated the potential role of G(i/o) activation in regulating SR-A-mediated lipoprotein uptake. Treatment of mouse peritoneal macrophages with PTX decreased association of fluorescently labeled AcLDL with cells. This inhibition was dependent on the catalytic activity of the toxin confirming that the decrease in AcLDL uptake involved inhibiting G(i/o) activation. In contrast to the inhibitory effect on AcLDL uptake, PTX treatment did not alter beta-VLDL-induced cholesterol esterification or deposition of cholesterol. The ability of polyinosine to completely inhibit AcLDL uptake, and the lack of PTX effect on beta-VLDL uptake, demonstrated that the inhibitory effect is specific for SR-A and not the result of non-specific effects on lipoprotein metabolism. Despite having an effect on an SR-A-mediated lipoprotein uptake, there was no change in the relative abundance of SR-A protein after PTX treatment.These results demonstrate that activation of a PTX-sensitive G protein is involved in a feedback process that positively regulates SR-A function.     

Binding and uptake of differently oxidized low density lipoprotein in mouse peritoneal macrophages and THP-1 macrophages: involvement of negative charges as well as oxidation-specific epitopes

Oxidatively modified low-density lipoprotein (LDL) has been found in vivo, and oxidized LDL (oxLDL) could bind to scavenger receptors, leading to foam cell formation. Macrophages bear a number of different scavenger receptors for oxLDL, and macrophages of different origins may have a different scavenger receptor repertoire. In addition, LDL oxidized to different degrees may differ in the ability to bind macrophage scavenger receptors. In this study, we characterized the patterns of the binding and uptake of differently oxidized LDL in mouse peritoneal macrophages (MPM) and human THP-1 macrophages, and the influence of negative charge and oxidation-specific epitopes in oxLDL on these processes. Thresholds of increased binding and uptake in MPM were found when LDL was oxidized to the degrees with a relative electrophoretic mobility (REM) of 2.6 (minor threshold) and 3.0 (major threshold), corresponding to 49 and 57%, respectively, of the loss of free amino groups in these oxLDL. There was no threshold for the binding of oxLDL to THP-1 macrophages, while for uptake, a major threshold with REM of 3.0 (57% free amino groups lost) was found. The presence of the F(ab')(2) fragments of the monoclonal antibody OB/04, which was raised against copper-oxidized LDL, led to the reduction of the binding and uptake, respectively, of Eu(3+)-oxLDL (REM:3.6) in MPM by 31 and 29%, and by 19 and 22% in THP-1 macrophages. It is concluded that LDL oxidized to different degrees binds differently to macrophages, and the patterns of binding and uptake are different for MPM and human THP-1 macrophages. Both, the negative charge and the oxidation-specific epitopes of oxLDL are involved in these processes.     

Molecular cloning of a novel scavenger receptor for oxidized low density lipoprotein, SR-PSOX, on macrophages

Receptor-mediated endocytosis of oxidized low density lipoprotein (OxLDL) by macrophages has been implicated in foam cell transformation in the process of atherogenesis. Although several scavenger receptor molecules, including class A scavenger receptors and CD36, have been identified as OxLDL receptors on macrophages, additional molecules on macrophages may also be involved in the recognition of OxLDL. From a cDNA library of phorbol 12-myristate 13-acetate-stimulated THP-1 cells, we isolated a cDNA encoding a novel protein designated SR-PSOX (scavenger receptor that binds phosphatidylserine and oxidized lipoprotein), which acts as a receptor for OxLDL. SR-PSOX was a type I membrane protein consisting of 254 amino acids, expression of which was shown on human and murine macrophages with a molecular mass of 30 kDa. SR-PSOX could specifically bind with high affinity, internalize, and degrade OxLDL. The recognition of OxLDL was blocked by polyinosinic acid and dextran sulfate but not by acetylated low density lipoprotein. Taken together, SR-PSOX is a novel class of molecule belonging to the scavenger receptor family, which may play important roles in pathophysiology including atherogenesis.     

Variations in transmembrane Ca2+ gradient and apoptosis of macrophages induced by oxidized low density lipoprotein

While Ca²⁺ has been proposed to be a messenger in OxLDL-induced cell death, few studies have addressed the possibility that it may influence the occurrence of apoptosis and necrosis of macrophages induced by OxLDL in virtue of change of transmembrane Ca²⁺ gradient including that across plasma membrane and intracellular organelle membranes. In this paper, various lipophilic Ca²⁺ fluorescent indicators and specific organelle markers were used to study the relationship between the changes of the transmembrane Ca²⁺ gradients and the OxLDL induced apoptosis of macrophages. Our results showed that following exposure of low dose OxLDL to macrophages, the transmembrane Ca²⁺ gradient across the plasma membrane, as well as the membrane-proximal Ca²⁺ gradient, the transnuclear, and the transmitochondrial membrane Ca²⁺ gradient were all changed significantly. These data suggested that changes in transmembrane Ca²⁺ gradients might be involved in the apoptosis of macrophages induced by OxLDL.     

A moderate reduction in extracellular pH protects macrophages against apoptosis induced by oxidized low density lipoprotein

We investigated the effect of pH on macrophage apoptosis induced by oxidized low density lipoprotein (OxLDL), as human atherosclerotic lesions have regions of low pH. Hydroperoxide-rich and oxysterol-rich LDL caused 38% and 74% apoptosis of J774 macrophages, respectively, at 24 h, as measured by the externalization of phosphatidylserine. Native LDL, however, did not cause apoptosis. Reducing the pH of the culture medium from 7.4 to 7.0 inhibited apoptosis induced by hydroperoxide-rich or oxysterol-rich OxLDL by 61% and 46%, respectively (P < 0.001). These data were confirmed by semiquantitative analysis of cytochrome c release from mitochondria. Decreasing the extracellular pH to 7.0 reduced the uptake of hydroperoxide-rich and oxysterol-rich (125)I-labeled LDL by 82% and 42%, respectively, and reduced cell surface binding of oxysterol-rich LDL by 31%. This may explain the reduced apoptosis. Additionally, low pH did not affect OxLDL-induced apoptosis of human monocytes, which do not possess scavenger receptors for OxLDL, but reduced apoptosis of human monocyte-derived macrophages, which do possess them. Our investigations suggest that the presence of areas of low pH within atherosclerotic lesions may reduce the uptake of OxLDL and reduce macrophage apoptosis, thus affecting lesion progression.     

Macrophage binding and the uptake of oxidized low density lipoprotein are regulated by intracellular protein phosphorylation

The involvement of intracellular protein phosphorylation in macrophages in the binding and uptake of oxidized low density lipoprotein (oxLDL) was investigated. The treatment of fibronectin-unstimulated and stimulated mouse thioglycolate-induced macrophages with inhibitors of myosin light chain kinase, protein kinase C and protein tyrosine kinase resulted in decreased macrophage binding of oxLDL, macrophage foam cell formation, and whole intracellular protein phosphorylation. The treatment of fibronectin-unstimulated and stimulated macrophages with inhibitors of protein serine/threonine and tyrosine phosphatases caused enhanced macrophage binding of oxLDL, macrophage foam cell formation, and whole intracellular protein phosphorylation. Fibronectin, which stimulates macrophage activity, enhanced macrophage intracellular protein phosphorylation. Myosin light chain phosphorylation may be involved in the fibronectin stimulation of macrophages. Treatment of fibronectin-unstimulated and stimulated macrophages with thiophosphate, which forms thiophosphate esters of intracellular proteins that are not so susceptible to protein phosphatases, enhanced macrophage binding of oxLDL. The above results indicate that intracellular protein phosphorylation maintains and enhances macrophage binding and the uptake of oxLDL.     

Decomposition of lipid hydroperoxides enhances the uptake of low density lipoprotein by macrophages.

This study examined the roles of low-density lipoprotein (LDL) lipid oxidation and peroxide breakdown in its conversion to a form rapidly taken up by mouse peritoneal macrophages. Oxidation of the LDL without decomposition of the hydroperoxide groups was performed by exposure to gamma radiation in air-saturated solutions. Virtually complete decomposition of the hydroperoxides was achieved by treatment of the irradiated LDL with Cu2+ under strictly anaerobic conditions. No uncontrolled LDL uptake by macrophages occurred when the lipoprotein contained less than 150 hydroperoxide groups per particle. More extensively oxidized LDL was taken up and degraded by mouse macrophages significantly faster than the native lipoprotein. The uptake was greatly enhanced by treatment of the oxidized LDL with Cu2+. A significant proportion of the LDL containing intact or copper-decomposed LDL hydroperoxide groups accumulated within the macrophages without further degradation. Treatment of the radiation-oxidized LDL with Cu2+ was accompanied by aggregation of the particles. Competition studies showed that the oxidized LDL was taken up by macrophages via both the LDL and the scavenger receptors, whereas the copper-treated lipoprotein entered the cells only by the scavenger pathway. Phagocytosis also played an important role in the metabolism of all forms of the extensively modified LDL. Our results suggest that minimally-oxidized LDL is not recognized by the macrophage scavenger receptors unless the lipid hydroperoxide groups are decomposed to products able to derivatize the apo B protein.