Group V secretory phospholipase A2-modified low density lipoprotein promotes foam cell formation by a SR-A- and CD36-independent process that involves cellular proteoglycans

Accumulating evidence indicates that secretory phospholipase A2 (sPLA2) enzymes promote atherogenic processes. We have previously showed the presence of Group V sPLA2 (GV sPLA2) in human and mouse atherosclerotic lesions, its hydrolysis of low density lipoprotein (LDL) particles, and the ability of GV sPLA2-modified LDL (GV-LDL) to induce macrophage foam cell formation in vitro. The goal of this study was to investigate the mechanisms involved in macrophage uptake of GV-LDL. Peritoneal macrophages from C57BL/6 mice (wild type (WT)), C57BL/6 mice deficient in LDL receptor (LDLR-/-), or SR-A and CD36 (DKO) were treated with control LDL, GV-LDL, oxidized LDL (ox-LDL) or LDL aggregated by vortexing (vx-LDL). As expected, ox-LDL induced significantly more cholesterol ester accumulation in WT and LDLR-/- compared with DKO macrophages. In contrast, there was no difference in the accumulation of GV-LDL or vx-LDL in the three cell types. 125I-ox-LDL exhibited high affinity, saturable binding to WT cells that was significantly reduced in DKO cells. Vx-LDL and GV-LDL showed low affinity, non-saturable binding that was similar for both cell types, and significantly higher compared with control LDL. GV-LDL degradation in WT and DKO cells was similar. Analyses by confocal microscopy indicated a distinct intracellular distribution of Alexa-568-labeled GV-LDL and Alexa-488-labeled ox-LDL. Uptake of GV-LDL (but not ox-LDL or vx-LDL) was significantly reduced in cells preincubated with heparin or NaClO3, suggesting a role for proteoglycans in GV-LDL uptake. Our data point to a physiological modification of LDL that has the potential to promote macrophage foam cell formation independent of scavenger receptors.     

Induction of interleukin 1 beta expression from human peripheral blood monocyte-derived macrophages by 9-hydroxyoctadecadienoic acid.

Oxidatively modified low density lipoproteins (LDL) have recently been proposed to play a role in atherogenesis by promoting foam cell formation and endothelial cell toxicity. The purpose of the present study was to determine whether modified LDL could also induce macrophage release of interleukin 1 beta (IL-1 beta), a cytokine which enhances vascular smooth muscle cell proliferation, another feature of the atherosclerotic process. LDL were oxidatively modified by incubation with either Cu2+ (Cu(2+)-LDL) or human peripheral blood monocyte-derived macrophages (M-LDL). Incubation of these modified LDL with macrophages (6 x 10(6) cells/culture) resulted in a dose-dependent induction of IL-1 beta release. At 300 micrograms protein/ml, Cu(2+)-LDL and M-LDL induced 422 and 333 pg of IL-1 beta/culture, respectively. Saponified Cu(2+)-LDL and M-LDL were shown to contain 9- and 13-hydroxyoctadecadienoic acid (HODE), lipid oxidation products of linoleate. When tested for activity in macrophage culture (3 x 10(6) cells/culture), it was found that 9-HODE and 13-HODE (final concentration 33 microM) induced the release of 122 and 43 pg of IL-1 beta/culture, respectively, whereas untreated cells released only 4 pg of IL-1 beta/culture. Incubation of macrophages with cholesteryl-9-HODE also induced IL-1 beta release; however, the degree of induction of IL-1 beta release by 9-HODE or its cholesteryl ester relative to modified LDL suggests that other components in oxidized LDL may also contribute to IL-1 beta induction. 9-HODE was rapidly taken up by macrophages, and the kinetics were similar to IL-1 beta release. A 1.5- to 6-fold increase in the level of IL-1 beta mRNA was detected as little as 3-h post-9-HODE treatment. The induction of IL-1 beta release from human monocyte-derived macrophages by 9-HODE and cholesteryl-9-HODE suggests a role for modified LDL, and its associated linoleate oxidation products, in vascular smooth muscle cell proliferation.     

The role of galectin-3 in endocytosis of advanced glycation end products and modified low density lipoproteins

Galectin-3, a member of beta-galactoside-binding lectin family, is suggested to be an AGE-receptor. To examine this possibility, we prepared CHO cells overexpressing human galectin-3 (galectin-3-CHO cells). Galectin-3-CHO cells showed a specific and saturable binding to (125)I-AGE-BSA with Kd of 3.1 microg/ml. (125)I-AGE-BSA was endocytosed by galectin-3-CHO cells and underwent lysosomal degradation. The endocytosis of (125)I-AGE-BSA was inhibited not only by unlabeled AGE-BSA but also by acetylated LDL and oxidized LDL, ligands for the scavenger receptor family. Furthermore, (125)I-oxidized LDL and (125)I-acetylated LDL were actively endocytosed by galectin-3-CHO cells and the incubation with acetyl-LDL led to intracellular accumulation of cholesteryl esters, indicating the role of galectin-3 in endocytosis of AGE-proteins and modified LDLs. Since galectin-3 was localized and up-regulated in foam cells at human atherosclerotic lesions, the present results suggest that galectin-3 plays an important role in formation of atherosclerotic lesions in vivo, by modulating endocytic uptake of AGE-proteins and modified LDLs.     

Platelet factor 4 enhances the binding of oxidized low-density lipoprotein to vascular wall cells

Accumulation of low-density lipoprotein (LDL)-derived cholesterol by macrophages in vessel walls is a pathogenomic feature of atherosclerotic lesions. Platelets contribute to lipid uptake by macrophages through mechanisms that are only partially understood. We have previously shown that platelet factor 4 (PF4) inhibits the binding and degradation of LDL through its receptor, a process that could promote the formation of oxidized LDL (ox-LDL). We have now characterized the effect of PF4 on the binding of ox-LDL to vascular cells and macrophages and on the accumulation of cholesterol esters. PF4 bound to ox-LDL directly and also increased ox-LDL binding to vascular cells and macrophages. PF4 did not stimulate ox-LDL binding to cells that do not synthesize glycosaminoglycans or after enzymatic cleavage of cell surface heparan and chondroitin sulfates. The effect of PF4 on binding ox-LDL was dependent on specific lysine residues in its C terminus. Addition of PF4 also caused an approximately 10-fold increase in the amount of ox-LDL esterified by macrophages. Furthermore, PF4 and ox-LDL co-localize in atherosclerotic lesion, especially in macrophage-derived foam cells. These observations offer a potential mechanism by which platelet activation at sites of vascular injury may promote the accumulation of deleterious lipoproteins and offer a new focus for pharmacological intervention in the development of atherosclerosis.     

Different Effects of Homocysteine and Oxidized Low Density Lipoprotein on Methylation Status in the Promoter Region of the Estrogen Receptor α Gene

We investigated the effects of homocysteine (Hcy) and oxidized low density lipoprotein (ox-LDL) on DNA methylation in the promoter region of the estrogen receptor α (ERos) gene,and its potentialmechanism in the pathogenesis of atherosclerosis.Cultured smooth muscle cells (SMCs) of humans weretreated by Hcy and ox-LDL with different concentrations for different periods of time.The DNA methylationstatus was assayed by nested methylation-specific polymerase chain reaction,the lipids that accumulated inthe SMCs and foam cell formations were examined with Oil red O staining.The proliferation of SMCs wasassayed by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide method.The results showedthat ox-LDL in moderate concentrations (10-40 mg/L) induced de novo methylation in the promoter regionof the ERα gene of SMCs.However,high concentrations (50 mg/L) of ox-LDL,resulted in demethylation ofERα.The Hcy treatment resulted in de novo methylation in the promoter region of the ERα gene with aconcentration- and treating time-dependent manner,and a dose-dependent promoting effect on SMCproliferation.These data indicated that the two risk factors for atherosclerosis had the function of inducingde novo methylation in the promoter region of the ERα gene of SMCs. However,high concentrations (50rag/L) of ox-LDL induced demethylation,indicating that different risk factors of atherosclerosis with differentpotency might cause different aberrant methylation patterns in the promoter region of the ERα gene.Theatherogenic mechanism of Hcy might involve the hypermethylation of the ERα gene,leading to the proliferationof SMCs in atherosclerotic lesions.     

Ox-LDL和天然LDL诱导人动脉平滑肌细胞增殖中蛋白激酶A作用的初步研究

汪浩川等研究表明一定量Ｏｘ－ＬＤＬ能刺激培养人动脉ＳＭＣ细胞的增殖［１］,Ｄｅｊａｇｅｒ等采用交叉抑制实验证明兔ＳＭＣ细胞膜上有能结合Ｏｘ－ＬＤＬ的清道夫受体［２］,因此Ｏｘ－ＬＤＬ诱导培养人ＳＭＣ细胞增殖可能是Ｏｘ－ＬＤＬ作用于ＳＭＣ膜清道夫受体后...     

Modified low density lipoprotein isolated from atherosclerotic lesions does not cause lipid accumulation in aortic smooth muscle cells

Foam cells in atherosclerotic lesions are derived not only from blood monocytes but also from smooth muscle cells (SMC). To better understand the mechanisms by which SMC may become lipid-laden, we have studied the catabolism by cultured rabbit aortic SMC of LDL derived from atherosclerotic lesions (A-LDL) previously shown to be chemically modified. A-LDL was isolated either from homogenates of atherosclerotic plaques in human aortas by affinity chromatography and gel filtration, or from nonhomogenized extracts of plaque minces by ultracentrifugation and gel filtration. Internalization of A-LDL by SMC or fibroblasts appeared to be mediated primarily via the LDL receptor since: 1) either unlabeled LDL or A-LDL could inhibit the degradation of 125I-labeled A-LDL or of 125I-labeled LDL, 2) the uptake of both A-LDL and LDL, as estimated by their abilities to stimulate cholesterol esterification, was reduced in cells in which LDL receptor expression was down-regulated; and 3) the uptake of both [3H]cholesteryl ether-labeled A-LDL and LDL by normal fibroblasts was significant and could be inhibited by excess LDL, but was negligible in receptor-negative fibroblasts. At saturating concentrations of lipoproteins, maximum cholesterol esterification in SMC was greater for LDL than for A-LDL. Over a 48-h incubation, A-LDL, like LDL, was unable to induce cellular cholesteryl ester accumulation. Cross-competition studies suggested that either the affinity of A-LDL for the LDL receptor was less than that of LDL, or that some particles in A-LDL are not internalized by SMC. The latter alternative was supported by the observations that some A-LDL particles had undergone aggregation, especially at high concentrations, and that aggregated forms of A-LDL or plasma LDL failed to be internalized and degraded by SMC. Collectively, these results are consistent with recognition of some of the A-LDL particles by the LDL receptor, but also suggest that, at least under in vitro conditions, A-LDL is unlikely to induce lipid accumulation in SMC resulting in SMC-derived foam cells.     

天然及氧化修饰脂蛋白对人动脉平滑肌细胞原癌基因表达的影响

动脉平滑肌细胞（ＳＭＣ）的增殖在动脉粥样硬化（ＡＳ）的形成过程中极其重要。我们在建立人主动脉ＳＭＣ体外培养方法的基础上,观察了ＬＤＬ,ＶＬＤＬ及ＨＤＬ和相应的氧化修饰型脂蛋白对培养人ＳＭＣｓｉｓ,ｊｕｎ,Ｈ－ｒａｓ原癌基因及Ｒｂ抗癌基因转录表达的影响。结果表明：（１）ＨＤＬ对ＳＭＣｓｉｓ,ｊｕｎ,ｒａｓ基因表达无影响;（２）ＬＤＬ和ＶＬＤＬ有使这些基因表达增加的趋势;（３）ｏｘ－ＬＤＬ,ｏｘ－ＶＬＤＬ和ｏｘ－ＨＤＬ具有使ＳＭＣｓｉｓ,ｊｕｎ,和ｒａｓ基因表达显著增强的作用（Ｐ＜０．０１）,且其作用较相应的天然脂蛋白大（Ｐ＜０．０１）;（４）天然和氧化修饰型脂蛋白对Ｒｂ基因表达均无影响。据上述结果推测：ＬＤＬ,ＶＬＤＬ,ｏｘ－ＬＤＬ,ｏｘ－ＶＬＤＬ和ｏｘ－ＨＤＬ的致ＡＳ作用可能与刺激ＳＭＣｓｉｓ,ｊｕｎ和ｒａｓ原癌基因表达增加有关。     

Minimally oxidized LDL inhibits macrophage selective cholesteryl ester uptake and native LDL-induced foam cell formation

Scavenger receptor-mediated uptake of oxidized LDL (oxLDL) is thought to be the major mechanism of foam cell generation in atherosclerotic lesions. Recent data has indicated that native LDL is also capable of contributing to foam cell formation via low-affinity receptor-independent LDL particle pinocytosis and selective cholesteryl ester (CE) uptake. In the current investigation, Cu²⁺-induced LDL oxidation was found to inhibit macrophage selective CE uptake. Impairment of selective CE uptake was significant with LDL oxidized for as little as 30 min and correlated with oxidative fragmentation of apoB. In contrast, LDL aggregation, LDL CE oxidation, and the enhancement of scavenger receptor-mediated LDL particle uptake required at least 3 h of oxidation. Selective CE uptake did not require expression of the LDL receptor (LDL-R) and was inhibited similarly by LDL oxidation in LDL-R^−/− versus WT macrophages. Inhibition of selective uptake was also observed when cells were pretreated or cotreated with minimally oxidized LDL, indicating a direct inhibitory effect of this oxLDL on macrophages. Consistent with the effect on LDL CE uptake, minimal LDL oxidation almost completely prevented LDL-induced foam cell formation. These data demonstrate a novel inhibitory effect of mildly oxidized LDL that may reduce foam cell formation in atherosclerosis.     

Sphingomyelinase enhances low density lipoprotein uptake and ability to induce cholesteryl ester accumulation in macrophages.

Cholesteryl ester-loaded macrophages, or foam cells, are a prominent feature of atherosclerotic lesions. Low density lipoprotein (LDL) receptor-mediated endocytosis of native LDL is a relatively poor inducer of macrophage cholesteryl ester accumulation. However, the data herein show that in the presence of a very small amount of sphingomyelinase, LDL receptor-mediated endocytosis of 125I-LDL was enhanced and led to a 2-6-fold increase in 125I-LDL degradation and up to a 10-fold increase in cholesteryl ester accumulation in macrophages. The enhanced lipoprotein uptake and cholesterol esterification was seen after only approximately 12% hydrolysis of LDL phospholipids, was specific for sphingomyelin hydrolysis, and appeared to be related to the formation of fused or aggregated spherical particles up to 100 nm in diameter. Sphingomyelinase-treated LDL was bound by the macrophage LDL receptor. However, when unlabeled acetyl-LDL, a scavenger receptor ligand, was present during or after sphingomyelinase treatment of 125I-LDL, 125I-LDL binding and degradation were enhanced further through the formation of LDL-acetyl-LDL mixed aggregates. Experiments with cytochalasin D suggested that endocytosis, not phagocytosis, was involved in internalization of sphingomyelinase-treated LDL. Nonetheless, the sphingomyelinase effect on LDL uptake was macrophage-specific. These data illustrate that LDL receptor-mediated endocytosis of fused LDL particles can lead to foam cell formation in cultured macrophages. Furthermore, since both LDL and sphingomyelinase are present in atherosclerotic lesions and since some lesion LDL probably is fused or aggregated, there is a possibility that sphingomyelinase-treated LDL is a physiologically important atherogenic lipoprotein.     

A modification of apolipoprotein B accounts for most of the induction of macrophage growth by oxidized low density lipoprotein

It has recently been shown that macrophage proliferation occurs during the progression of atherosclerotic lesions and that oxidized low density lipoprotein (LDL) stimulates macrophage growth. Possible mechanisms for this include the interaction of oxidized LDL with integral plasma membrane proteins coupled to signaling pathways, the release of growth factors and autocrine activation of growth factor receptors, or the potentiation of mitogenic signal transduction by a component of oxidized LDL after internalization. The present study was undertaken to further elucidate the mechanisms involved in the growth-stimulating effect of oxidized LDL in macrophages. Only extensively oxidized LDL caused significant growth stimulation, whereas mildly oxidized LDL, native LDL, and acetyl LDL were ineffective. LDL that had been methylated before oxidation (to block lysine derivatization by oxidation products and thereby prevent the formation of a scavenger receptor ligand) did not promote growth, even though extensive lipid peroxidation had occurred. The growth stimulation could not be attributed to lysophosphatidylcholine (lyso-PC) because incubation of oxidized LDL with fatty acid-free bovine serum albumin resulted in a 97% decrease in lyso-PC content but only a 20% decrease in mitogenic activity. Similarly, treatment of acetyl LDL with phospholipase A2 converted more than 90% of the initial content of phosphatidylcholine (PC) to lyso-PC, but the phospholipase A2-treated acetyl LDL was nearly 10-fold less potent than oxidized LDL at stimulating growth. Platelet-activating factor receptor antagonists partly inhibited growth stimulation by oxidized LDL, but platelet-activating factor itself did not induce growth. Digestion of oxidized LDL with phospholipase A2 resulted in the hydrolysis of PC and oxidized PC but did not attenuate growth induction. Native LDL, treated with autoxidized arachidonic acid under conditions that caused extensive modification of lysine residues by lipid peroxidation products but did not result in oxidation of LDL lipids, was equal to oxidized LDL in potency at stimulating macrophage growth. Albumin modified by arachidonic acid peroxidation products also stimulated growth, demonstrating that LDL lipids are not essential for this effect. These findings suggest that oxidatively modified apolipoprotein B is the main growth-stimulating component of oxidized LDL, but that oxidized phospholipids may play a secondary role.     

Oxysterol efflux from macrophage foam cells: the essential role of acceptor phospholipid.

Oxidized forms of cholesterol (oxysterols) are present in atherosclerotic lesions and may play an active role in lesion development. For example, 7-ketocholesterol (7KC) inhibits cholesterol efflux from macrophage foam cells induced by apolipoprotein A-I (apoA-I). Such oxysterols may promote foam cell formation in atherosclerotic lesions by preventing effective clearance of excess cholesterol. ApoA-I also induces phospholipid (PL) export from foam cells and it has been suggested that cholesterol efflux is dependent upon PL association with the apolipoprotein. In the current study, the effect of oxysterol enrichment of foam cells on phospholipid efflux was measured. Export of cellular PL to apoA-I from 7KC-enriched foam cells was inhibited to the same extent as cholesterol, indicating that the reduced cholesterol export may be a consequence of a decline in the capacity of the foam cells to generate PL/apoA-I particles capable of accepting cellular cholesterol. Incubation of foam cells with pre-formed PL/apoA-I discs increased cholesterol export from 7KC-enriched cells to levels seen in 7KC-free cells. Foam cells produced by uptake of oxidized LDL, which contain similar amounts of 7KC plus other oxidation products, expressed a more profound inhibition of PL export to apoA-I. Cholesterol efflux from these cells improved only partially by provision of PL-containing acceptors. Efflux of 7KC from both foam cell types occurred to PL/apoA-I discs but was only minimal to lipid-free apoA-I, indicating that export of this oxysterol is more dependent than cholesterol upon the presence of extracellular phospholipid.     

Scavenger receptor-mediated uptake and metabolism of lipid vesicles containing acidic phospholipids by mouse peritoneal macrophages

We studied the mechanism of uptake and metabolism of exogenous phospholipids in mouse peritoneal macrophages using vesicles composed of various phospholipids and cholesterol. Macrophages in culture were found to actively incorporate and metabolize phosphatidylcholine/cholesterol vesicles containing small amounts of acidic phospholipids such as phosphatidylserine, phosphatidylinositol, or phosphatidic acid and to store the fatty acyl chains and cholesterol in triacylglycerol and cholesteryl ester form in their cytosol. These cells exhibited massive amounts of oil red O-positive lipid droplets, a typical feature of foam cells. The metabolism of exogenous phospholipid vesicles was completely inhibited by chloroquine and cytochalasin B, suggesting that vesicle uptake occurs by endocytosis. A similar type of metabolism was observed in guinea pig peritoneal macrophages, macrophage cell line J774.1, but not in Swiss 3T3 fibroblasts. Competition studies using various ligands for the scavenger receptor showed that acetylated low density lipoprotein (acetyl-LDL), dextran sulfate, or fucoidan was able to compete for up to 60% of the binding of phosphatidylserine-containing vesicles, and that copper-oxidized LDL (oxidized LDL) competed for more than 90% of the vesicle binding. On the other hand, phosphatidylserine-containing vesicles was able to compete for more than 90% of the binding of acetyl-LDL. These results indicate that acidic phospholipids are recognized by the scavenger receptors on the surface of macrophages and that more than one scavenger receptor exists on mouse peritoneal macrophages, i.e. one capable of recognizing acetyl-LDL, oxidized LDL, and an array of acidic phospholipids on membranes, and the other recognizing both acidic phospholipids and oxidized LDL but not acetyl-LDL.     

Proliferation and wound healing of vascular cells trigger the generation of extracellular reactive oxygen species and LDL oxidation

Cell proliferation of vascular cells is a key feature in vascular biology, wound healing, and pathophysiological processes such as atherosclerosis and restenosis. In atherosclerotic intima, cell proliferation colocalizes with oxidized LDL that indicate a local oxidative stress. This study aims to investigate whether cell proliferation is causally related with extracellular ROS generation and subsequent LDL oxidation. Sparse proliferating endothelial and smooth muscle cells generate higher levels of extracellular ROS (O₂^•− and H₂O₂) and LDL oxidation than confluent contact-inhibited cells. During wound healing of confluent cell layer, cell proliferation associated with healing also induced enhanced extracellular ROS generation and LDL oxidation. Proliferation-associated extracellular ROS generation is mediated through mitogenic signaling pathways, involving ERK1/2 and PKC, but is independent of de novo DNA synthesis, gene expression and protein synthesis. Data obtained with inhibitors of oxidases suggest that proliferation-associated extracellular ROS are not generated by a single ROS-generating system and are not essential for cell proliferation. In conclusion, our data show that proliferating vascular cells (in sparse culture or during wound healing) generate high levels of extracellular ROS and LDL oxidation through regulation of ROS-generating systems by mitogenic signaling. This constitutes a link between proliferative events and oxidative stress/LDL oxidation in atherosclerotic lesions and restenosis.     

Transforming growth factor-beta 1 inhibits scavenger receptor activity in THP-1 human macrophages.

The macrophage scavenger receptor, a 220-kDa trimeric membrane glycoprotein, mediates the internalization of modified forms of low density lipoprotein (LDL) such as acetyl-LDL and oxidized-LDL and thus is likely to play a key role in atheroma macrophage foam cell formation. In addition, recent evidence suggests that the scavenger receptor may be an important macrophage binding site for lipopolysaccharide involved in lipopolysaccharide scavenging by macrophages. However, little is known about the regulation of this important receptor. We now report that the induction of scavenger receptor activity (as measured by acetyl-LDL stimulation of intracellular cholesterol esterification) seen in phorbol ester-differentiated THP-1 human macrophages was completely suppressed to the level seen in undifferentiated THP-1 monocytes by picomolar concentrations of transforming growth factor-beta 1 (TGF-beta 1). 125I-Acetyl-LDL degradation was inhibited in a dose-dependent manner by TGF-beta 1, with maximal inhibition (approximately 70%) occurring at 24 pM TGF-beta 1. Scatchard analysis revealed that TGF-beta 1 treatment resulted in a approximately 2-fold decrease in receptor number, and Northern blot analysis of RNA isolated from differentiated THP-1 macrophages demonstrated approximately 2-fold less scavenger receptor mRNA in TGF-beta 1-treated cells compared with that in macrophages not treated with TGF-beta 1. Since TGF-beta 1 is thought to be present in both atherosclerotic and inflammatory lesions, the above findings may have physiological relevance regarding the regulation of atheroma foam cell formation and/or the regulation of lipopolysaccharide clearance by macrophages.     

Cholesterol and oxysterol metabolism and subcellular distribution in macrophage foam cells. Accumulation of oxidized esters in lysosomes

Cholesterol- and cholesteryl ester-rich macrophage foam cells, characteristic of atherosclerotic lesions, are often generated in vitro using oxidized low density lipoprotein (OxLDL). However, relatively little is known of the nature and extent of sterol deposition in these cells or of its relationship to the foam cells formed in atherosclerotic lesions. The purpose of this study was to examine the content and cellular processing of sterols in OxLDL-loaded macrophages, and to compare this with macrophages loaded with acetylated LDL (AcLDL; cholesteryl ester-loaded cells containing no oxidized lipids) or 7-ketocholesterol-enriched acetylated LDL (7KCAcLDL; cholesteryl ester-loaded cells selectively supplemented with 7-ketocholesterol (7KC), the major oxysterol present in OxLDL). Both cholesterol and 7KC and their esters were measured in macrophages after uptake of these modified lipoproteins. Oxysterols comprised up to 50% of total sterol content of OxLDL-loaded cells. Unesterified 7KC and cholesterol partitioned into cell membranes, with no evidence of retention of either free sterol within lysosomes. The cells also contained cytosolic, ACAT-derived, cholesteryl and 7-ketocholesteryl esters. The proportion of free cholesterol and 7KC esterified by ACAT was 10-fold less in OxLDL-loaded cells than in AcLDL or 7KCAcLDL-loaded cells. This poor esterification rate in OxLDL-loaded cells was partly caused by fatty acid limitation. OxLDL-loaded macrophages also contained large (approximately 40-50% total cell sterol content) pools of oxidized esters, containing cholesterol or 7KC esterified to oxidized fatty acids. These were insensitive to ACAT inhibition, very stable and located in lysosomes, indicating resistance to lysosomal esterases. Macrophages loaded with OxLDL do not accumulate free sterols in their lysosomal compartment, but do accumulate lysosomal deposits of OxLDL-derived cholesterol and 7-ketocholesterol esterified to oxidized fatty acids. The presence of similar deposits in lesion foam cells would represent a pool of sterols that is particularly resistant to removal.     

Low density lipoproteins transactivate EGF receptor: role in mesangial cell proliferation

Hyperlipidemia and the glomerular accumulation of atherogenic lipoproteins (low density lipoprotein, LDL; and its oxidatively-modified variants, ox-LDL) are commonly associated with the development of glomerular mesangial proliferative diseases. However, cellular signaling mechanisms by which atherogenic lipoproteins stimulate mesangial cell proliferation are poorly defined. In this study, we examined the effect of atherogenic lipoproteins on the activation of mesangial cell epidermal growth factor (EGF) receptor, mitogen activated protein kinase (MAP kinase), Ras, and mesangial cell proliferation. Stimulation of mesangial cells with LDL, and with greater activity, ox-LDL, markedly induced the transactivation of EGF receptor within 5 min of stimulation; the effect persisted up to at least 60 min LDL, and with a greater degree, ox-LDL, increased the activation of Ras, MAP kinase, and mesangial cell proliferation. Inhibition of EGF receptor kinase activity and/or MAP kinase activation blocked both LDL- and ox-LDL-induced mesangial cell proliferation. We suggest that the accumulation of LDL and more potently its oxidized forms within the glomerulus, through the transactivation of EGF receptor, stimulate down-stream Ras-MAP kinase signaling cascade leading to mesangial cell proliferation. Regulation of glomerular accumulation of atherogenic lipoproteins and/or EGF receptor signaling may provide protective environment against mesangial hypercellularity seen in glomerular diseases.