Lipoprotein-mediated inhibition of endothelial cell production of platelet-derived growth factor-like protein depends on free radical lipid peroxidation

CpG-induced tyrosine phosphorylation occurs via a TLR9-independent mechanism and is required for cytokine secretion

Toll-like receptors (TLRs) recognize molecular patterns preferentially expressed by pathogens. In endosomes, TLR9 is activated by unmethylated bacterial DNA, resulting in proinflammatory cytokine secretion via the adaptor protein MyD88. We demonstrate that CpG oligonucleotides activate a TLR9-independent pathway initiated by two Src family kinases, Hck and Lyn, which trigger a tyrosine phosphorylation–mediated signaling cascade. This cascade induces actin cytoskeleton reorganization, resulting in cell spreading, adhesion, and motility. CpG-induced actin polymerization originates at the plasma membrane, rather than in endosomes. Chloroquine, an inhibitor of CpG-triggered cytokine secretion, blocked TLR9/MyD88-dependent cytokine secretion as expected but failed to inhibit CpG-induced Src family kinase activation and its dependent cellular responses. Knock down of Src family kinase expression or the use of specific kinase inhibitors blocked MyD88-dependent signaling and cytokine secretion, providing evidence that tyrosine phosphorylation is both CpG induced and an upstream requirement for the engagement of TLR9. The Src family pathway intersects the TLR9–MyD88 pathway by promoting the tyrosine phosphorylation of TLR9 and the recruitment of Syk to this receptor.

Oxidized but not acetylated low-density lipoprotein reduces preproinsulin mRNA expression and secretion of insulin from HIT-T15 cells

We examined the effect of oxidized low-density lipoprotein (oxLDL) on the insulin secretion in the culture of HIT-T15 cell line, an islet beta-cell line derived from a hamster pancreatic tumor. In order to check the uptake of modified LDL by HIT-T15 cells, we prepared DiI-labeled native LDL (nLDL), acetylated LDL (AcLDL), and oxLDL. After the addition of each LDL into the cultures of HIT-T15 cells, fluorescence microscopic study was done. It was suggested that AcLDL and oxLDL were taken up by HIT-T15 cells, as well as nLDL. mRNA expression of the LDL receptor, CD36, and SR-B1 was detected in HIT-T15 by RT-PCR. The medium insulin level was measured in the culture of HIT-T15 cells with each LDL. oxLDL significantly reduced the insulin secretion stimulated by various concentrations of glucose, the intracellular content of insulin, and the expression of preproinsulin mRNA compared to the control cultures without LDL addition. In contrast, nLDL and AcLDL had no effect on the insulin secretion, the intracellular insulin level, or the expression of preproinsulin mRNA. MTT assay findings (reflecting cell numbers) were not different between cultures with and without LDLs. These results indicated that oxLDL disturbed the insulin metabolism of HIT-T15 cells.

Effect of alcohol on lipids and lipoproteins in relation to atherosclerosis

Several studies indicate that light-to-moderate alcohol consumption is associated with a low prevalence of coronary heart disease. An increase in high-density lipoprotein (HDL) cholesterol is associated with alcohol intake and appears to account for approximately half of alcohol's cardioprotective effect. In addition to changes in the concentration and composition of lipoproteins, alcohol consumption may alter the activities of plasma proteins and enzymes involved in lipoprotein metabolism: cholesteryl ester transfer protein, phospholipid transfer protein, lecithin:cholesterol acyltransferase, lipoprotein lipase, hepatic lipase, paraoxonase-1 and phospholipases. Alcohol intake also results in modifications of lipoprotein particles: low sialic acid content in apolipoprotein components of lipoprotein particles (e.g., HDL apo E and apo J) and acetaldehyde modification of apolipoproteins. In addition, "abnormal" lipids, phosphatidylethanol, and fatty acid ethyl esters formed in the presence of ethanol are associated with lipoproteins in plasma. The effects of lipoproteins on the vascular wall cells (endothelial cells, smooth muscle cells, and monocyte/macrophages) may be modulated by ethanol and the alterations further enhanced by modified lipids. The present review discusses the effects of alcohol on lipoproteins in cholesterol transport, as well as the novel effects of lipoproteins on vascular wall cells.

The role of chemokines in human cardiovascular pathology: enhanced biological insights

A growing body of experimental evidence supports the pivotal role of chemokines in the pathogenesis of vascular disease. The endothelial expression of monocyte chemoattractant protein-1 (MCP-1) is apparently essential for the earliest cellular responses of atherogenesis. Many atherogenic and anti-atherogenic stimuli can be construed to exert their effects predominantly upon MCP-1 expression within the vascular wall. The atherogenic effects of interleukin-8 (IL-8) seem to be mediated through the down-regulation of the tissue inhibitor of metalloproteinase-1 (TIMP-1). Biological expression of these two important vascular chemokines is further modulated by NF-kappaB. The delineation of these molecular forces that drive atherogenesis increasingly underscores the pivotal role of various chemokines. It is anticipated that more precise delineation of these patterns of gene expression will help to identify molecular targets for the prevention and treatment of atherosclerosis.

Calcium-activated potassium channels mask vascular dysfunction associated with oxidized LDL exposure in rabbit aorta

Endothelium-dependent vasodilation is impaired in atherosclerosis. Oxidized low density lipoprotein (ox-LDL) plays an important role, possibly through alterations in G-protein activation. We examined the effect of acute exposure to ox-LDL on the dilator responses of isolated rabbit aorta segments. We sought also to evaluate the specificity of this dysfunction for dilator stimuli that traditionally operate through a Gi-protein mechanism. Aortic segments were prepared for measurement of isometric tension. After contraction with prostaglandin F2alpha, relaxation to thrombin, adenosine diphosphate (ADP), or the endothelium-independent agonists, sodium nitroprusside (SNP) or papaverine was examined. Maximal relaxation to thrombin was impaired in the presence of ox-LDL (17.7+/-3.7% p<0.05) compared to control (no LDL) (52.6+/-4.0%). Ox-LDL did not affect maximal relaxation to ADP or SNP. However, in the presence of charybdotoxin (CHTX: calcium-activated potassium channel inhibitor) ox-LDL impaired relaxation to ADP (17.4+/-3.2%). CHTX did not affect control (no LDL) responses to ADP (69.6+/-5.0%) or relaxation to thrombin or papaverine. In conclusion, ox-LDL impairs relaxation to thrombin, but in the case of ADP, calcium-activated potassium channels compensate to maintain this relaxation.

Regulation of cell growth by oxidized LDL

The first reports of the influences of oxidized LDL (oxLDL) on cell function pertained to negative effects on cell growth-growth arrest, injury, and toxicity. Since these studies, it has become apparent that sublethal levels of oxLDL cause some, but not all, cells to proliferate. This review highlights the growth-promoting effects of oxLDL rather than its inhibitory or injurious effects. Smooth muscle cells (SMCs) and monocyte-macrophages proliferate after exposure to oxLDL; endothelial cells do not. Scavenger receptors are involved in the proliferative effects on monocyte-macrophages, whereas the effects of oxLDL on SMCs appear to be receptor independent. Lysophosphatidylcholine (lysoPC), and structurally related lipids are among the growth-promoting constituents of oxLDL. OxLDL exerts at least a part of its effects by inducing expression or causing the release of growth factors. OxLDL (or lysoPC) can cause the release of basic fibroblast growth factor (bFGF) from SMCs; oxLDL (or lysoPC) can induce heparin binding EGF-like growth factor (HB-EGF) synthesis and release from macrophages. An imposing array of changes in cytokine and growth factor expression and/or release can be imposed by oxLDL on a wide variety of cell types. These effects and the studies probing the cell signaling events leading to them are described.

The oxidative modification hypothesis of atherogenesis: an overview

The literature relating lipid and lipoprotein oxidation to atherosclerosis has expanded enormously in recent years. Papers on the "oxidative modification hypothesis" of atherogenesis have ranged from the most basic studies of the chemistry and enzymology of LDL oxidation, through studies of the biological effects of oxidized LDL on cultured cells, and on to in vivo studies of the effects of antioxidants on atherosclerosis in animals and humans. The data in support of this theory are mounting but many key questions remain unanswered. For example, while it is generally agreed that LDL undergoes oxidation and that oxidized LDL is present in arterial lesions, it is still not known how and where LDL gets oxidized in vivo nor which of its many biological effects demonstrable in vitro are relevant to atherogenesis in vivo. This brief review is not intended to be comprehensive but rather to offer a perspective and a context for this Forum. We discuss the strengths and weaknesses of each line of evidence, try to identify areas in which further research is needed, assess the relevance of the hypothesis to the human disease, and point to some of the potential targets for therapy.

Gliclazide decreases low-density lipoprotein oxidation and monocyte adhesion to the endothelium

Increasing evidence implicates oxidized low-density lipoprotein (LDL) and advanced glycation end products (AGE) in the atherogenesis associated with diabetes mellitus. In the present study, we examined the in vitro effects of gliclazide on LDL oxidation and monocyte adhesion to endothelial cells induced by oxidized LDL and glycated albumin. To assess the clinical relevance of our in vitro findings, we also measured the effect on monocyte adhesion of gliclazide administration to type 2 diabetic patients. Incubation of human monocytes and endothelial cells with increasing concentrations of gliclazide (0 to 10 microg/mL) and native LDL (100 microg/mL) induced a dose-dependent diminution of cell-mediated LDL oxidation. Pretreatment of endothelial cells with gliclazide (0 to 10 microg/mL) before addition of native LDL (100 microg/mL) or glycated albumin (100 microg/mL) resulted in a dose-dependent diminution of oxidized LDL- and glycated albumin-induced monocyte adhesion to endothelial cells. In type 2 diabetic patients, administration of gliclazide inhibits the increased adhesiveness of monocytes to levels similar to those observed in control subjects. These results indicate that gliclazide is an antioxidant and suggest a beneficial effect of this drug in the prevention of atherosclerosis associated with type 2 diabetes.

Human endothelium as a source of multifunctional cytokines: molecular regulation and possible role in human disease

Endothelial cells, by virtue of their capacity to express adhesion molecules and cytokines, are intricately involved in inflammatory processes. Endothelial cells have been shown to express interleukin-1 (IL-1), IL-5, IL-6, IL-8, IL-11, IL-15, several colony-stimulating factors (CSF), granulocyte-CSF (G-CSF), macrophage CSF (M-CSF) and granulocyte-macrophage CSF (GM-CSF), and the chemokines, monocyte chemotactic protein-1 (MCP-1), RANTES, and growth-related oncogene protein-alpha (GRO-alpha). IL-1 and tumor necrosis factor-alpha (TNF-alpha) produced by infiltrating inflammatory cells can induce endothelial cells to express several of these cytokines as well as adhesion molecules. Induction of these cytokines in endothelial cells has been demonstrated by such diverse processes as hypoxia and bacterial infection. Recent studies have demonstrated that adhesive interactions between endothelial cells and recruited inflammatory cells can also signal the secretion of inflammatory cytokines. This cross-talk between inflammatory cells and the endothelium may be critical to the development of chronic inflammatory states. Endothelial-derived cytokines may be involved in hematopoiesis, cellular chemotaxis and recruitment, bone resorption, coagulation, and the acute-phase protein synthesis. As many of these processes are critical to the maturation of an inflammatory and reparative state, it appears likely that endothelial-derived cytokines play a crucial role in several diseases, including atherosclerosis, graft rejection, asthma, vasculitis, and sepsis. Genetic and pharmacologic manipulation of endothelial-derived cytokines provides an additional approach to the management of chronic inflammatory diseases.

Cholesteryl hydroperoxyoctadecadienoate from oxidized low density lipoprotein inactivates platelet-derived growth factor

Both oxidized low density lipoprotein (ox-LDL) and platelet-derived growth factor (PDGF) have been implicated in the genesis of various inflammatory responses, including atherosclerosis. We demonstrate here a novel interaction between specific oxidized lipids derived from ox-LDL and PDGF. The lipid moieties of ox-LDL caused concentration-dependent inactivation of PDGF as measured by loss of its mitogenic activity and its binding to high affinity receptors. Reverse-phase and normal-phase HPLC were used to purify the inactivating component in the lipid mixture. By fast atom bombardment mass spectrometry and infrared spectroscopy, we identified the inactivating lipids as the 9- and 13-hydroperoxy derivatives of cholesteryl linoleate, cholesteryl hydroperoxyoctadecadienoate. When a series of cholesteryl esters were subjected to oxidizing conditions, only those containing two or more double bonds caused inactivation of PDGF; the extent of inactivation increased with increased levels of oxidation. Exposing PDGF to cumene hydroperoxide, t-butyl hydroperoxide, or hydrogen peroxide did not affect the activity of the mitogen. The oxidized lipid had no effect on the mitogenic activity of epidermal growth factor but did abolish the mitogenic activity of basic fibroblast growth factor and the antiproliferative activity of transforming growth factor beta1. The inactivation of PDGF and other cytokines by lipid hydroperoxides may occur in such processes as vascular disease, inflammation, and wound healing.

Studies on effects of Lp(a) lipoprotein on gene expression in endothelial cells in vitro

The reason(s) for the atherogenic properties of Lp(a) lipoprotein is still unclear, and several mechanisms have been studied. Alterations in gene expression in endothelial cells (ECs) could be important with respect to risk for coronary heart disease (CHD). We have tested the effects of Lp(a) lipoprotein or the apolipoprotein of Lp(a) lipoprotein (apo(a)) on cultured human umbilical vein endothelial cells (HUVECs) with respect to: (1) the level of endothelin-1 (ET-1) mRNA; (2) release of ET-1 into the culture medium; (3) plasminogen activator inhibitor-1 (PAI-1) secretion into the culture medium and; (4) total gene expression in HUVECs, examined by a polymerase chain reaction (PCR)-based technique, differential display-reverse transcription-PCR (DD-RT-PCR). Lp(a) lipoprotein reduced the level of ET-1 mRNA as well as the release of ET-1. The reduction of ET-1 in the medium was even more pronounced when HUVECs were incubated with apo(a), but we found no effect of apo(a) on ET-1 mRNA level. Neither Lp(a) lipoprotein nor apo(a) had a significant influence on PAI-1 secretion. DD-RT-PCR revealed 11 fragments that could represent differences between cells exposed or not exposed to Lp(a) lipoprotein. Following subcloning and sequencing, 18 sequences that differed between exposed and unexposed cultures were obtained. Four of the subcloned fragments have up to now been used as a probe for northern blot analyses, and one fragment was confirmed to be regulated by Lp(a) lipoprotein. In conclusion, Lp(a) lipoprotein is shown to control ET-1 mRNA levels and the function of at least one more gene, the nature of which is unknown.

Gliclazide decreases cell-mediated low-density lipoprotein (LDL) oxidation and reduces monocyte adhesion to endothelial cells induced by oxidatively modified LDL

Low-density lipoprotein (LDL) oxidation has been suggested to play a key role in the pathogenesis of atherosclerosis, a major complication of diabetes mellitus. Gliclazide, a second-generation sulfonylurea, is widely used in the treatment of type II diabetes mellitus. Recently, a free-radical-scavenging activity of gliclazide has been reported. In the present study, we examined the effects of gliclazide on cell-mediated LDL oxidation and monocyte adhesion to endothelial cells induced by oxidatively modified LDL. Incubation of human monocytes and bovine aortic endothelial cells (BAE cells) with increasing concentrations of gliclazide (0 to 10 micrograms/mL) and native LDL (100 micrograms/mL) resulted in a dose-dependent diminution of cell-mediated LDL oxidation as assayed by measurement of thiobarbituric acid (TBA)-reactive substances (TBARS). In addition, exposure of BAE cells to gliclazide (0 to 10 micrograms/mL) and native LDL (100 micrograms/mL) induced a dose-dependent diminution of the oxidized LDL-induced monocyte adhesion to BAE cells as measured by the myeloperoxidase (MPO) assay. The effects of glyburide, another second-generation sulfonylurea, were also tested on cell-mediated oxidation of LDL and LDL-induced monocyte adhesion to the endothelium. No significant effect of this drug was observed on these two processes. These results therefore demonstrate that gliclazide is effective in vitro in reducing both cell-mediated LDL oxidation and monocyte adhesion to the endothelium. These findings suggest a potential beneficial effect of gliclazide in the prevention of atherosclerosis in diabetic patients.

Lipoprotein trafficking in vascular cells. Molecular Trojan horses and cellular saboteurs

During the pathogenesis of atherosclerosis, inflammatory cells such as the monocyte-derived macrophage accumulate in the vessel wall where they release cytokines. Initially, cytokines may assist in CE removal of lipoprotein-derived cholesterol/CE hydrolysis to clear intracellular lipid. When plasma levels of LDL become elevated, the vessel wall becomes lipid-engorged over time because it is unable to traffick the large amounts of endocytosed LDL-CE from the cell. In addition, lipoprotein entrapment by the extracellular matrix can lead to the progressive oxidation of LDL because of the action of lipoxygenases, reactive oxygen species, peroxynitrite, and/or myeloperoxidase. A range of oxidized LDL species is thus generated, ultimately resulting in their delivery to vascular cells through several families of scavenger receptors (Fig 1). These molecular Trojan horses and cellular saboteurs once formed or deposited in the cell can contribute to, and participate in, formation of macrophage- and smooth muscle-derived foam cells. A lipid-enriched fatty streak along the vessel wall can ensue. In addition to foam cell development, products of LDL peroxidation may activate endothelial cells, increase smooth muscle mitogenesis, or induce apoptosis because of the effects of oxysterols and products of lipid peroxidation (Fig 1). Because antioxidant defenses may be limited in the microenvironment of the cell or within LDL, the oxidation process continues to progress. Enzymes associated with HDL such as PAF acetylhydrolase and paraoxonase can participate in the elimination of biologically active lipids, but diminished cellular antioxidant activity coupled with low levels of HDL may allow acceleration of the clinical course of vascular disease. There is still much to be learned about how modified LDL initiate cellular signals that lead to inflammation, mitosis, or cholesterol accumulation. The present challenges include elucidation of the key signaling events that regulate lipoprotein-derived cholesterol trafficking in the vessel wall, which can impact on the pathogenesis of vascular disease.

High affinity saturable uptake of oxidized low density lipoprotein by macrophages from mice lacking the scavenger receptor class A type I/II

Oxidation of low density lipoproteins (LDL) has been implicated as a causal factor in the pathogenesis of atherosclerosis. Oxidized LDL has been found to exhibit numerous potentially atherogenic properties in vitro, including receptor-mediated uptake by macrophages. Oxidized LDL is a ligand for the class A scavenger receptor type I/II (SR-AI/II), but cross-competition studies with cultured macrophages suggested that there is an additional receptor(s) that is specific for oxidized LDL and that does not interact with acetyl LDL or other chemically modified LDL. A number of macrophage membrane proteins, including CD36, FcgammaRII-B2, scavenger receptor BI, and macrosialin/CD68, have been found to bind to oxidized LDL in vitro and have been proposed as candidate oxidized LDL receptors. However, because of overlapping ligand specificity with the SR-AI/II, it has been difficult to evaluate the relative importance of these proteins in the uptake of oxidized LDL by macrophages. In the present report, we have studied the uptake and degradation of oxidized LDL by macrophages from mice in which the SR-AI/II gene had been disrupted. The uptake of acetyl LDL was reduced by more than 80% in macrophages from scavenger receptor knockout mice, confirming that most of the uptake of acetyl LDL by macrophages can be attributed to this receptor. In contrast, the uptake of extensively oxidized LDL was reduced by only 30% and showed high affinity, saturable uptake with apparent Km of about 5 microg/ml, similar to that of the SR-AI/II. This indicates that about 70% of the uptake of oxidized LDL in macrophages is attributable to an alternate oxidized LDL receptor(s). In contrast to findings reported with CD36, mildly oxidized LDL was internalized much more slowly than extensively oxidized LDL. Unlabeled oxidized LDL, polyinosinic acid, phosphatidylserine-rich liposomes, and LDL or bovine albumin modified by fatty acid oxidation products were effective competitors for the uptake of radioiodinated oxidized LDL by macrophages from knockout mice, whereas acetyl LDL and malondialdehyde-modified LDL were relatively poor competitors. This ligand specificity differs from that of CD36-related (class B) scavenger receptors but is similar to the reported specificity of macrosialin/CD68 in ligand blots. However, the rate of uptake of oxidized LDL by knockout macrophages was not increased by phorbol ester or in thioglycollate-elicited macrophages, both of which are expected to increase the amount of macrosialin on the cell surface. In macrophages from SR-AI/II knockout mice, ligand blots of membrane proteins with iodinated, oxidized, or acetylated LDL revealed several bands, with apparent molecular size on SDS-polyacrylamide gel electrophoresis of 60, 94, 124, and 210 kDa, but none of the bands were specific for oxidized LDL. These results provide direct evidence that a receptor other than SR-AI/II is responsible for most of the uptake of oxidized LDL in murine macrophages, but further studies are needed to identify the receptor(s) involved.

Oxidized LDL inhibits vascular endothelial cell morphogenesis in culture

Human umbilical cord vein endothelial cells can be induced to undergo morphogenesis (tube formation) by phorbol ester (TPA) when cultured on or in three-dimensional collagen gels. Induction of morphogenesis by TPA is accompanied by increased activity of the collagenase gene transcription factors, ETS1 and API, and the elaboration of collagenase by the endothelial cells. In the present study, we used endothelial cell elongation as a measure of morphogenesis and showed that oxidized low density lipoprotein (oxLDL) inhibited endothelial cell migration in monolayer cultures and TPA-induced morphogenesis in collagen gels in a dose-dependent manner. Moreover, the inhibition was positively correlated with the extent of LDL oxidation. In contrast, native LDL stimulated cell migration and TPA-induced morphogenesis under the same culture conditions. However, in the absence of TPA, LDL showed no effect on EC morphogenesis. Further studies showed that inhibition of TPA-induced endothelial cell morphogenesis by oxLDL is correlated with suppression of the protein kinase C (PKC) and ETS1/AP1 activities. The results indicated that the inhibition of endothelial cell morphogenesis by oxLDL is probably mediated through inhibition of the TPA-activated PKC pathway and its subsequent suppression of the ETS1/AP1 activity. The results also indicated that EC migration can be mediated through PKC-dependent and independent pathways and only the former pathway can induce EC morphogenesis as well.

Selective resistance of LDL core lipids to iron-mediated oxidation. Implications for the biological properties of iron-oxidized LDL

Although the nature and consequences of oxidative changes in the chemical constituents of low density lipoproteins (LDLs) have been extensively examined, the physical dynamics of LDL oxidation and the influence of physical organization on the biological effects of oxidized LDLs have remained relatively unexplored. To address these issues, in the present studies we monitored surface- and core-specific peroxidative stress relative to temporal changes in conjugated dienes (CDs), particle charge (an index of oxidative protein modification), and LDL-macrophage interactions. Peroxidative stress in LDL surface and core compartments was evaluated with the site-specific, oxidation-labile fluorescent probes parinaric acid (PnA) and PnA cholesteryl ester (PnCE), respectively. When oxidation was initiated by Cu2+, oxidative loss of the core probe (PnCE) closely followed that of the surface probe (PnA), as indicated by the time to 50% probe depletion (t1/2; 15.5 +/- 7.8 and 30.4 +/- 12 minutes for PnA and PnCE, respectively). Both probes were more resistant in LDL exposed to Fe3+ (t1/2, 53.2 +/- 8.1 and 346.7 +/- 155.4 minutes), although core probe resistance was much greater with this oxidant (PnCE t1/2/PnA t1/2 5.8 vs 2.0 for Cu2+). Despite differences in the rate and extent of oxidative changes in Cu(2+)- versus Fe(3+)-exposed LDLs, PnCE loss occurred in close correspondence with CD formation and appeared to precede changes in particle charge under both conditions. Exposure of LDLs to hemin, a lipophilic Fe(3+)-containing porphyrin that becomes incorporated into the LDL particle, resulted in rapid loss of PnCE and simultaneous changes in particle, charge, even at concentrations that yielded increases in CDs and thiobarbituric acid-reactive substances similar to those obtained with free Fe3+. These results suggest that oxidation of the LDL hydrophobic core occurs in conjunction with accelerated formation of CDs and may be essential for LDL protein modification. In accordance with the known effects of oxidative protein modifications on LDL receptor recognition, exposure of LDLs to Cu2+ and hemin but not Fe3+ produced particles that were readily processed by macrophages. Thus, the physical site of oxidative injury appears to be a critical determinant of the chemical and biological properties of LDLs, particularly when oxidized by Fe3+.

Effect of hypercholesterolaemia on platelet growth factors

Evidence from several sources suggests that important interactions occur between platelets and low-density lipoproteins. This study was undertaken to find out if diet-induced hypercholesterolaemia affects the growth factor content in circulating platelets. Minipigs were fed either normal diet supplemented with 2% cholesterol (n = 12) or normal diet alone (n = 12). After 4 months, mean platelet volume was significantly lower (P < 0.05) and monocyte count was significantly higher (P < 0.05) in the cholesterol group. Serum and intraplatelet levels of platelet-derived growth factor (BB homodimer) and transforming growth factor beta 1 were statistically unchanged after diet. Hypercholesterolaemia did not affect the proliferative effect of either serum or platelet lysates on porcine vascular smooth muscle cells and Swiss-3T3 cells in culture. A significant positive correlation between Swiss-3T3 and smooth muscle cell proliferation was present in both groups. These results suggest that the atherosclerosis-promoting effect of hypercholesterolaemia cannot be explained by its direct effect on smooth muscle cell proliferation or by changes in serum or intraplatelet concentrations of growth factors.

Modified lipoproteins, cytokines and macrovascular disease in non-insulin-dependent diabetes mellitus

The processes of glycation and oxidation play a significant role in the acceleration of atherosclerosis in diabetes mellitus. Glycation is thought not only to increase the susceptibility of low-density lipoprotein (LDL) to oxidation but also to enhance the propensity of vessel wall structural proteins to bind extravasated plasma proteins, including LDL, and thus to contribute to a more marked oxidative modification of LDL. Glycated and oxidized lipoproteins induce cholesteryl ester accumulation in human macrophages and may promote platelet and endothelial cell dysfunction. Furthermore, these modified lipoproteins have the ability to trigger an autoimmune response that leads to the formation of autoantibodies and subsequently to the formation of immune complexes containing LDL. Both the modified lipoproteins and the immune complexes formed with autoantibodies reactive with modified lipoproteins may be responsible for several alternative and not mutually exclusive pathways leading to foam cell formation, macrophage activation and endothelial cell damage and may thus be of potential significance in initiating and/or contributing to the acceleration of the development of atherosclerosis. In this review we discuss how modified LDL affects lipoprotein metabolism, how immune complexes containing LDL induce the transformation of macrophages into foam cells and promote macrophage activation leading to the release of cytokines and thus initiating a sequence of events leading to endothelial cell damage and to the recruitment and activation of leucocytes. We also summarize our work showing that macrophage activation by LDL containing immune complexes leads to a paradoxical increase in LDL-receptor expression thus further impairing cholesterol homeostasis and enhancing the development of atheromatous lesions.

Mechanism of uptake of copper-oxidized low density lipoprotein in macrophages is dependent on its extent of oxidation

Several investigators have reported nonreciprocal cross-competition between unlabeled acetyl low density lipoprotein (LDL) and oxidized LDL for the degradation of the corresponding labeled LDLs. The failure of acetyl LDL to compete fully for the degradation of oxidized LDL has been interpreted as evidence for additional receptor(s) specific for oxidized LDL. In the present study, it is demonstrated that the ability of oxidized LDL to compete for the degradation of acetyl LDL is determined largely by its extent of oxidation. Extensively oxidized LDL competed for 90% of acetyl LDL degradation in murine macrophages, and hence there appears to be no pathway in these cells that is specific for acetyl LDL but not oxidized LDL. The reciprocal situation (competition by acetyl LDL for uptake and degradation of oxidized LDL) proved to be more complicated. Oxidized LDL is known to be susceptible to aggregation, and less than half of the aggregates found in the present experiments were large enough to be removed by filtration or centrifugation at 10,000 x g. When oxidized LDL was prepared under conditions that resulted in minimal aggregation, acetyl LDL competed for greater than 80% of oxidized LDL degradation. With more extensive oxidation and aggregation of LDL, acetyl LDL only competed for about 45% of oxidized LDL degradation, while polyinosinic acid remained an effective competitor. Individual preparations of oxidized LDL that differed in degree of oxidation were separated into aggregated and nonaggregated fractions, and it was shown that both fractions were competed to a similar degree by acetyl LDL in mouse peritoneal macrophages and in Chinese hamster ovary cells transfected with human scavenger receptor type I cDNA. Hence, aggregation by itself did not alter the apparent rate of uptake by the scavenger receptor pathway. These results indicate that the extent of oxidation of LDL affects its mechanism of uptake and that about half of the uptake of very extensively oxidized LDL appears to be via a pathway distinct from the scavenger receptor type I/II. The uptake of very extensively oxidized LDL was not affected by cytochalasin D, an inhibitor of phagocytosis. As well, it was not affected by an antibody to CD36 in human monocyte-derived macrophages or in THP-1 cells, suggesting that this alternate pathway does not involve CD36.

Signals controlling the expression of PDGF

PDGF is an important polypeptide growth factor that plays an essential role during early vertebrate development and is associated with tissue repair and wound healing in the adult vertebrate. Moreover, PDGF is thought to play a role in a variety of pathological phenomena, such as cancer, fibrosis and atherosclerosis. PDGF is expressed as a dimer of A and/or B chains, the precursors of which are encoded by two single copy genes. Although the PDGF genes are expressed coordinately in a number of cell types, they are independently expressed in a majority of cell types. The expression of either PDGF gene can be affected by very diverse extracellular stimuli and the type of response is dependent on the cell type that is exposed to the stimulus. Expression of the PDGF chains can be modulated at every imaginable level: by regulating accessibility of the transcription start site, by varying the transcription initiation rate, by using alternative transcription start sites, by alternative splicing, by using alternative polyadenylation signals, by varying mRNA decay rates, by regulating efficiency of translation, by protein modification, and by regulating secretion. Even upon secretion, the activity of PDGF can be modulated by non-specific or specific PDGF-binding proteins. This review provides an overview of the cell types in which the PDGF genes are expressed, of the factors that are known to affect the expression of PDGF, and of the various levels at which the expression of PDGF genes can be regulated.

Lipoproteins regulate C-type natriuretic peptide secretion from cultured vascular endothelial cells

We have shown that oxidized low-density lipoprotein (Ox-LDL) modulates various endothelial cell (EC) functions. C-type natriuretic peptide (CNP), the third member of the natriuretic peptide family to be discovered, is secreted from peripheral vascular ECs and regulates body fluid homeostasis, vascular tone, and vascular growth. This study was designed to investigate the effects of lipoproteins on CNP secretion from cultured ECs. Treatment of bovine carotid ECs with OX-LDL and its extracted lipids resulted in a concentration-dependent suppression of the spontaneous and transforming growth factor-beta 1-stimulated secretion CNP. Native LDL, its extracted lipids, and acetylated LDL were inactive. OX-LDL depleted of its amphiphilic lipids, which was prepared by incubation with defatted albumin, lost its suppressive effect on CNP secretion. 7-Ketocholesterol, one of the amphiphilic lipids in OX-LDL that is transferable from OX-LDL to defatted albumin, suppressed CNP secretion by ECs, thus mimicking the effect of OX-LDL. Coincubation with high-density lipoprotein (HDL), which alone had no effect on CNP release, significantly prevented OX-LDL-induced inhibition of CNP secretion by ECs. Analysis by thin-layer chromatography demonstrated that oxysterols, including 7-ketocholesterol, in OX-LDL were transferred from OX-LDL to HDL during coincubation of these two lipoproteins. These results indicate that OX-LDL suppresses CNP secretion from ECs by 7-ketocholesterol or other transferable hydrophilic lipids in OX-LDL, and the suppressive effect of OX-LDL is reversed by HDL. Lipoproteins thus may regulate CNP secretion from the endothelium of atherosclerotic arteries.

Oxidized low density lipoprotein stimulates collagen production in cultured arterial smooth muscle cells

We examined the interactive effect of oxidized low density lipoprotein (LDL) and ascorbic acid on collagen production in cultured smooth muscle cells (SMCs). Porcine aortic SMCs were incubated with 50-200 micrograms/ml of human LDL with/without 5 microM Cu2+ for 24 h. Collagen production was assayed by successive salt precipitation at acidic and neutral pH after pepsin digestion of 3H-proline-labeled collagenous protein. Oxidation of LDL was evaluated by electrophoresis and by the level of thiobarbituric acid reactive substances (TBARS). Ascorbic acid reduced the oxidation of LDL + Cu2+ (53% reduction). In the presence of ascorbic acid, no differences were noted in collagen production between LDL and LDL + Cu2+. Without ascorbic acid, collagen production with LDL + Cu2+ was increased dose-dependently up to 6-fold with 150 micrograms/ml LDL, while no such effects were observed at any doses of native LDL. The addition of butylated hydroxytoluene to LDL + Cu2+ strongly suppressed oxidation (88% reduction), and significantly reduced collagen production close to that seen with native LDL. These results indicate that oxidized LDL stimulates collagen production in SMCs, while native LDL does not. Therefore, oxidized LDL may play a direct role in stimulating collagen production in SMCs, which could lead to collagenosis in atherosclerosis.

Structure, oxidant activity, and cardiovascular mechanisms of human ceruloplasmin

Ceruloplasmin is the principal carrier of copper in human plasma. It is an abundant protein that participates in the acute phase reaction to stress, but its physiological function(s) is unknown. An antioxidant activity of ceruloplasmin has been described, but recent evidence suggests that the protein may also exhibit potent pro-oxidant activity and cause oxidative modification of low density lipoprotein (LDL). The pro-oxidant activity is highly dependent on the structure of the protein; removal of a single one of the seven integral copper atoms, or a specific proteolytic cleavage event, completely suppresses LDL oxidation. This newly described pro-oxidant activity may help to explain epidemiological studies indicating that ceruloplasmin is an independent risk factor for cardiovascular disease.

Activation of cytosolic phospholipase A2 by basic fibroblast growth factor via a p42 mitogen-activated protein kinase-dependent phosphorylation pathway in endothelial cells

Basic fibroblast growth factor (FGF) stimulates the proliferation, differentiation, and motility of multiple cell types. Signal transduction by FGF is mediated by high affinity FGF receptors that have autophosphorylating tyrosine kinase activity and also elicit the release of low molecular weight signaling molecules, including inositol 1,4,5-trisphosphate, diacylglycerol, and arachidonate. We have shown previously that basic FGF-stimulated, phospholipase A2 (PLA2)-mediated arachidonate release regulates endothelial cell (EC) motility (Sa, G., and Fox, P.L. (1994) J. Biol. Chem. 269, 3219-3225). Here we identify the phospholipase responsible for basic FGF-mediated arachidonate release as cytosolic PLA2 (cPLA2) by demonstrating in EC lysates a requirement for micromolar Ca2+, dithiothreitol insensitivity, and inactivation by anti-cPLA2 antiserum. The role of cPLA2 is also indicated by the observed mechanisms of activation which show a requirement for p42 mitogen-activated protein kinase activity, cPLA2 phosphorylation, and cPLA2 translocation from cytosol to membranes. Phosphorylation of cPLA2, arachidonate release from prelabeled intact cells, and cell motility all have similar concentration dependencies on basic FGF. Since arachidonate release is required for basic FGF-stimulated motility of EC, our results show that p42 mitogen-activated protein kinase activation of cPLA2 may be a regulatory event in stimulation of cellular release of this important eicosanoid precursor during cellular responses to basic FGF.

Oxidized lipids in the diet are a source of oxidized lipid in chylomicrons of human serum

We examined whether oxidized lipids in the diet determine the levels of oxidized lipid in human postprandial serum chylomicrons. After we fed subjects control corn oil containing low quantities of oxidized lipid, the levels of conjugated dienes in the chylomicron fraction were low (9.67 +/- 0.92 nmol/mumol triglyceride), and no thiobarbituric acid-reactive substances (TBARS) could be detected. However, when subjects were fed a highly oxidized oil, the conjugated diene content in chylomicrons was increased 4.7-fold to 46 +/- 5.63 nmol/mumol triglyceride, with 0.140 +/- 0.03 nmol TBARS/mumol triglyceride. When subjects were fed medium-oxidized oil, the degree of oxidation of the chylomicron lipids was moderately increased (21.86 +/- 2.03 nmol conjugated dienes/mumol triglyceride). Additionally, we found that chylomicrons isolated after ingestion of oxidized oil were more susceptible to CuSO4 oxidation than chylomicrons isolated after ingestion of the control oil. The lag time for oxidation decreased from 4.30 +/- 0.40 to 3.24 +/- 0.51 hours (P < .05). These data demonstrate that in humans dietary oxidized lipids are absorbed by the small intestine, incorporated into chylomicrons, and appear in the bloodstream, where they contribute to the total body pool of oxidized lipid.

Oxidized lipoproteins, altered cell function and atherosclerosis

A correlation between atherogenesis and lipoprotein oxidation was first suggested by experiments showing increased uptake by macrophages of oxidized LDL and oxidized LDL injury to cultured cells. Recent data which demonstrate the existence of oxidized lipoproteins in vivo, combined with studies showing a 'protective' effect of antioxidants against atherosclerosis progression, have greatly increased the interest in theories posing that lipoprotein oxidation is causally related to arterial disease. The fact that dozens of new compounds are produced upon the oxidation of low density lipoprotein has led, perhaps not surprisingly, to numerous discoveries in vitro of altered cell function induced by exposure of cells to oxidized LDL that are distinct from those resulting from exposure to native LDL. This brief overview will describe selected altered cell functions of oxidized lipoproteins and how they may impact on atherosclerosis.

Oxidized LDL enhances lipopolysaccharide-induced tissue factor expression in human adherent monocytes

Oxidized low-density lipoprotein (oxLDL) has been characterized as an atherogenic molecule responsible for the induction of a variety of gene products. One such gene, tissue factor (TF), the cellular initiator of the coagulation cascade, is not expressed in normal vascular tissue but is expressed by monocytes and foam cells in atherosclerotic lesions. Therefore, we examined the effect of oxLDL on TF expression in cultured human adherent monocytes. Endotoxin-free oxLDL alone did not induce TF expression in adherent monocytes. However, oxLDL significantly enhanced TF expression induced by the inflammatory mediator, bacterial lipopolysaccharide (LPS), in a time- and dose-dependent manner. In contrast, oxLDL did not alter LPS-mediated production of interleukin-8 and actually inhibited LPS-induced secretion of tumor necrosis factor-alpha, suggesting that some aspects of the signaling pathways for TF induction differ from those of other LPS-responsive monocyte/macrophage gene products. Thus, this study documents specific modulation of the expression of LPS-inducible genes in monocytic cells by oxLDL. Factors that enhance TF expression in monocyte/macrophage cells present in atheroma may contribute to the severity of thrombotic episodes and complications observed in atherosclerosis.

Oxidation, lipoxygenase, and atherogenesis

Mounting evidence suggests that oxidative processes contribute to the pathogenesis of atherosclerosis and that antioxidants may represent a strategy to complement the lowering of lipids in the therapy of this disease. Although multiple molecular events have been identified in vitro and although it is tempting to ascribe multiple atherogenic properties to oxidized LDL, our understanding of this process remains incomplete. Further research is warranted in several areas. First, it will be important to selectively inhibit different aspects of the process to determine the relative contribution of various biological targets. In this regard pharmacological inhibition of 15-lipoxygenase in vivo in relevant animal models is required to address the question of the contribution of this enzyme to significant oxidative events. The lack of specific inhibitors has made this task more difficult. It will also be important to define the biologically active moiety of oxidized LDL to begin to determine the mechanisms through which it exerts its atherogenic effects. It is likely that alternate protein targets can be identified both downstream and upstream of the oxidative process. Research is only now beginning to elucidate the inflammatory mechanisms that account for the cellular response. Further research into adhesion events, cytokine profiles, and downstream effector molecules of the oxidative process are likely to identify alternate targets for therapeutic intervention.

The role of cholesterol accumulation in prosthetic vascular graft anastomotic intimal hyperplasia

PURPOSE

To demonstrate that modulation of plasma cholesterol concentrations affects prosthetic vascular graft anastomotic intimal hyperplasia (AIH), aortic grafts were examined histologically and biochemically in 41 rabbits.

METHODS

Twenty-seven rabbits were fed standard rabbit diet, whereas 14 were fed cholesterol-supplemented diet to induce hypercholesterolemia.

RESULTS

A smooth muscle cell proliferative response, similar to AIH in humans, was seen equally at the proximal and distal anastomoses. However, surface area and thickness of AIH were significantly greater in rabbits with hypercholesterolemia. Anastomotic tissue cholesterol concentrations were fifteenfold higher in rabbits with hypercholesterolemia than in rabbits with normal cholesterol concentrations and anastomotic cholesterol concentrations were fivefold higher than in the aorta away from the graft in rabbits with hypercholesterolemia. Preferential deposition of radioiodinated dilactitol tyramine coupled to low-density lipoproteins, but not albumin, was demonstrated in anastomotic areas and grafts of rabbits with normal cholesterol concentrations as well. Surface area and thickness of AIH correlated closely with plasma and tissue cholesterol concentrations.

CONCLUSIONS

Oxidized products of lipoproteins have been shown to stimulate production of growth factors that cause smooth muscle cell proliferation, migration, and synthetic function. It is likely they play an important part in prosthetic vascular graft AIH, similar to their role in atherogenesis.

Transferable lipids in oxidized low-density lipoprotein stimulate plasminogen activator inhibitor-1 and inhibit tissue-type plasminogen activator release from endothelial cells

Decreased fibrinolytic activity has been reported in atherosclerotic cardiovascular diseases. To determine whether oxidized low-density lipoprotein (Ox-LDL), which accumulates in atherosclerotic arteries, modulates the endothelial fibrinolytic system, cultures of human umbilical vein endothelial cells were incubated with low-density lipoproteins or lipids, and levels of plasminogen activator inhibitor-1 (PAI-1) and tissue-type plasminogen activator (t-PA) antigens in the conditioned medium were measured by enzyme-linked immunosorbent assay. Ox-LDL (30 micrograms protein/mL) and its extracted lipid (50 micrograms cholesterol/mL) stimulated PAI-1 release by 42 +/- 3% and 29 +/- 3% of control cultures, respectively, whereas Ox-LDL and its lipid inhibited t-PA release by 42 +/- 4% and 53 +/- 3% of control cultures, respectively. Native LDL and its lipid were inactive on their release. Ox-LDL depleted of hydrophilic lipids, which was prepared by the incubation with defatted albumin (an acceptor for hydrophilic lipids), lost both the stimulatory action on PAI-1 and the inhibitory action on t-PA. The extracted lipid from the incubated albumin, which has been found to accept the hydrophilic lipids from Ox-LDL, gained the stimulatory action on PAI-1 and the inhibitory action on t-PA. Ox-LDL depleted of lysophosphatidylcholine (LPC), which was prepared by the incubation with phospholipase B, lost the stimulatory effect on PAI-1, whereas the inhibitory effect on t-PA remained present in the Ox-LDL depleted of LPC. The incubation with synthetic palmitoyl LPC (10 microM) stimulated PAI-1 release by 85 +/- 7% of control.(ABSTRACT TRUNCATED AT 250 WORDS)

Suppression of endothelin-1 secretion by lysophosphatidylcholine in oxidized low density lipoprotein in cultured vascular endothelial cells

Oxidatively modified low density lipoprotein (oxidized LDL), an atherogenic lipoprotein that exists in the atherosclerotic arteries, has been shown to alter endothelial cell functions. In the present study, we examined the effects of oxidized LDL on the production of endothelin-1-like immunoreactivity (ET-1-LI) by the cultured vascular endothelial cells from porcine aorta and human umbilical vein. Incubation with oxidized LDL resulted in a dose-dependent suppression of ET-1-LI release by both endothelial cells. Oxidized LDL also inhibited thrombin-mediated stimulation of ET-1-LI secretion. However, native LDL had no effects on ET-1-LI secretion. A lipid extract from oxidized LDL, but not from native LDL, inhibited ET-1-LI secretion, indicating that the lipid component of oxidized LDL was required for the inhibition of ET-1-LI secretion. Oxidative modification of LDL was associated with degradation of a substantial amount of phosphatidylcholine to lysophosphatidylcholine (LPC). Pretreatment with defatted albumin, which is an acceptor for hydrophilic lipids including LPC, reduced LPC concentration in oxidized LDL to that in native LDL and simultaneously prevented the inhibitory effects of oxidized LDL on ET-1-LI secretion. Incubation with synthetic LPC (palmitoyl), but not with synthetic phosphatidylcholine (dipalmitoyl), suppressed ET-1-LI secretion by the endothelial cells. No cell death was observed during the incubations as judged by the trypan blue exclusion test, and protein synthesis of the endothelial cells was not affected by lipids or lipoproteins at a concentration at which suppression of ET-1-LI was observed. We concluded that LPC in oxidized LDL causes suppression of ET-1-LI release, which may counteract the vasoconstrictive properties of atherosclerotic arteries.

Vitamin E supplementation, plasma lipids and incidence of restenosis after percutaneous transluminal coronary angioplasty (PTCA)

To test whether alpha-tocopherol prevents restenosis following percutaneous transluminal coronary angioplasty (PTCA), we enrolled patients in a double-blind, placebo-controlled trial. Patients were randomized after successful PTCA to receive vitamin E in the form of dl-alpha-tocopherol, 1200 IU/day, orally vs an inactive placebo for 4 months. Patients' blood was analyzed at baseline and at 4 months post-PTCA for differences in plasma lipids, lipoproteins, apolipoproteins, alpha-tocopherol, retinol, beta-carotene and lipoperoxide concentrations. One hundred patients completed the protocol. No significant difference was found in any parameter except alpha-tocopherol level between the vitamin E group and the placebo group, verifying compliance. Follow-up cardiac catheterization was obtained in 83% of the patients receiving placebo and in 86% of the patients receiving dl-alpha-tocopherol. Including thallium and exercise stress testing, objective information was obtained for practically all the patients receiving dl-alpha-tocopherol or placebo. Restenosis was defined as the presence of a lesion with greater than or equal to 50% stenosis in a previously dilated artery segment and results were analyzed with respect to pre- and post-PTCA artery diameter, vessel diameter at follow-up, and restenosis rate. Patients receiving dl-alpha-tocopherol had a 35.5% restenosis angiographically documented vs 47.5% restenosis in patients receiving the placebo. The overall incidence of restenosis defined by an abnormal angiogram or thallium test or exercise stress test was 34.6% in patients receiving dl-alpha-tocopherol and 50% in patients receiving the placebo. This difference (p = 0.06) did not reach significance because of an inadequate sample size.

Role of lipoprotein peroxidation in the pathogenesis of atherosclerosis

Oxidative modification of low-density lipoprotein (LDL) has been shown to alter its properties in a way that tends to increase its atherogenicity. Cultured arterial endothelial cells and smooth muscle cells can promote LDL oxidation in vitro. Several recent studies have provided evidence for the presence of oxidized LDL in atherosclerotic lesions. Furthermore, treatment of atherosclerosis-prone rabbits with an antioxidant was found to slow the progression of aortic lesions. Additional experimental work is required to determine if LDL oxidation is indeed an important causal step in atherogenesis in humans, but the preliminary data are encouraging and offer the potential for a new approach to the prevention and therapy of atherosclerosis.

Arterial wall oxygenation, oxyradicals, and atherosclerosis

The oxygen supply of inner media and thickened intima of atherosclerosis prone arteries depends largely on diffusion from the endothelium. Conditions which increase wall thickness and oxygen diffusion or reduce oxygen transmissibility produce hypoxia and steep PO2 gradients within the wall. Cerebral injury and myocardial reperfusion studies indicate that intermittent hypoxia and steep PO2 gradients lead to oxyradical formation and tissue damage. Products of lipid and sterol peroxidation are found in atherosclerotic plaques and can be generated by arterial wall cells in culture. It is likely that peroxidation occurs directly within the arterial wall. Sufficient oxyradical generation occurs during normal oxygen metabolism that local scavenger mechanisms are required to avoid tissue damage. Experimental hypertension, hyperlipemia and balloon injury produce medial hypoxia with steep PO2 gradients and redistribution of the pattern of arterial wall antioxidant enzymes. This suggests that minor deviations from normal arterial wall anatomy and function can lead to oxyradicals which can be directly injurious and can amplify the atherogenic potential of lipoprotein infiltration.

Antioxidants and atherosclerosis: a current assessment

Numerous recently published studies demonstrate that, once altered by free radical oxidation, plasma lipoproteins undergo dramatic change, both in the manner in which they can interact with cells and in the ways in which they influence cell function. For example, with increasing degrees of oxidation, low-density lipoprotein (LDL) will cease to be recognizable by the LDL receptor and ultimately can become a ligand for "scavenger" receptors on macrophages. Gene expression and production of certain cytokines and growth factors can be modified through the interaction of oxidized LDL with the cell sources of these potent cell regulators. These discoveries have stimulated the formulation of hypotheses of roles played in vivo by oxidized lipoproteins or their various oxidized lipid moieties in cellular regulation and in various disease processes. Among the more detailed of these hypotheses is a putative sequence in which LDL becomes oxidized and subsequently participates in the various facets of atherosclerotic lesion development, including monocyte recruitment, foam cell formation, vascular cell injury, and cellular proliferation. The evidence supporting this scenario makes a compelling story, one that is fed by reports that certain antioxidants favorably alter the course of vascular lesion development. However, other studies suggest that antioxidants do not inhibit lesion progression or that any alleviation is secondary to lipid lowering. This brief accounting examines some of the more recent studies dealing specifically with the effects of antioxidants on atherosclerosis.

Localization of PDGF-B protein in macrophages in all phases of atherogenesis

Lesions of atherosclerosis occur in the innermost layer of the artery wall and consist primarily of proliferated smooth muscle cells surrounded by large amounts of connective tissue, numerous lipid-laden macrophages, and varying numbers of lymphocytes. Growth-regulatory molecules may be involved in intimal accumulation and proliferation of smooth muscle cells responsible for the occlusive lesions of atherosclerosis. Platelet-derived growth factor (PDGF) B-chain protein was found within macrophages in all stages of lesion development in both human and nonhuman primate atherosclerosis. Thus macrophages may play a critical role in the disease by providing PDGF, a potent chemotactic and growth-stimulatory molecule, to the intimal smooth muscle cells.

Platelet-derived growth factor and its role in health and disease

Platelet-derived growth factor (PDGF) was first discovered in platelets because they are the principal source of mitogenic activity in whole blood serum for mesenchymal cells in culture. PDGF is ubiquitous in that it can be formed by a large number of normal cells as well as many varieties of transformed cells. However, its expression and biological activity appear to be controlled at a number of different levels. The molecule consists of two peptide chains (termed 'A' and 'B') and is found as one of at least three possible isoforms, (AB, AA or BB). Each of these isoforms binds to a high-affinity cell-surface receptor that is composed of two different subunits, each of which has specificity for one or the other of the peptide chains of PDGF. The two receptor subunits are present in differing amounts on different cell types, and therefore the capacity of the different isoforms of PDGF to induce mitogenesis depends on the specific PDGF isoform and the relative numbers of receptor subunits present on the responding cell. In addition to inducing cell replication, PDGF elicits a number of intracellular signals related to mitogenesis, is chemotactic, is a vasoconstrictor, activates leukocytes, and modulates extracellular matrix turnover. This growth factor is probably involved in a number of biologically important events including wound repair, embryogenesis and development, and inflammation, leading to fibrosis, atherosclerosis and neoplasia.

Role of oxidatively modified LDL in atherosclerosis

Oxidative modification of LDL is accompanied by a number of compositional and structural changes, including increased electrophoretic mobility, increased density, fragmentation of apolipoprotein B, hydrolysis of phosphatidylcholine, derivatization of lysine amino groups, and generation of fluorescent adducts due to covalent binding of lipid oxidation products to apo B. In addition, oxidation of LDL has been shown to result in numerous changes in its biologic properties that could have pathogenetic importance, including accelerated uptake in macrophages, cytotoxicity, and chemotactic activity for monocytes. The present article summarizes very recent developments related to the mechanism of oxidation of LDL by cells, receptor-mediated uptake of oxidized LDL in macrophages, the mechanism of phosphatidylcholine hydrolysis during LDL oxidation, and other biologic actions of oxidized LDL including cytotoxicity, altered eicosanoid metabolism, and effects on the secretion of growth factors and chemotactic factors. In addition, this review will examine the evidence for the presence of oxidized LDL in vivo and the evidence that oxidized LDL plays a pathogenetic role in atherosclerosis.