Expression of lipoprotein receptors in atherosclerotic lesions

The impact of the PCSK-9/VLDL-Receptor axis on inflammatory cell polarization

BACKGROUND

Studies have shown that lipoproteins, such as LDL and VLDL, as well as its major protein component ApoE2 impact on macrophage polarization important in atherosclerosis. Proprotein convertase subtilisin/kexin 9 (PCSK9) is a key regulator of lipoprotein receptor expression. The present study investigated the effect of the VLDL/VLDL-receptor (VLDL-R) axis on mononuclear cell polarization, as well as the role of PCSK9 and PCSK9 inhibitors (PCSK9i) within this network.

METHODS

Human monocytic THP-1 cells and human monocyte-derived macrophages isolated from peripheral blood mononuclear cells (PBMC) were treated with either LPS/IFN-γ to induce a pro-inflammatory phenotype, or with IL-4/IL-13 to induce an anti-inflammatory phenotype. Cells were then subjected to further treatments by lipoproteins, PCSK9, PCSK9i and lipoprotein receptor blockers.

RESULTS

LPS/IFN-γ treatment promoted a pro-inflammatory state with an increased expression of pro-inflammatory mediators such as TNF-α, CD80 and IL-1β. VLDL co-treatment induced a switch of this pro-inflammatory phenotype to an anti-inflammatory phenotype. In pro-inflammatory cells, VLDL significantly decreased the expression of pro-inflammatory markers e.g., TNF-α, CD80, and IL-1β. These effects were eliminated by PCSK9 and restored by co-incubation with a specific anti-PCSK9 monoclonal antibody (PCSK9i). Migration assays demonstrated that pro-inflammatory cells displayed a significantly higher invasive capacity when compared to untreated cells or anti-inflammatory cells. Moreover, pro-inflammatory cell chemotaxis was significantly decreased by VLDL-mediated acquisition of the anti-inflammatory phenotype. PCSK9 significantly lessened this VLDL-mediated migration inhibition, which was reversed by the PCSK9i.

CONCLUSION

VLDL promotes mononuclear cell differentiation towards an anti-inflammatory phenotype. PCSK9, via its capacity to inhibit VLDL-R expression, reverses the VLDL-mediated anti-inflammatory action, thereby promoting a pro-inflammatory phenotype. Thus, PCSK9 targeting therapies may exert anti-inflammatory properties within the vessel wall.

PCSK9: A Key Target for the Treatment of Cardiovascular Disease (CVD)

Proprotein convertase subtilisin/kexin type 9 (PCSK9), as a vital modulator of low-density lipoprotein cholesterol (LDL-C) , is raised in hepatocytes and released into plasma where it binds to LDL receptors (LDLR), leading to their cleavage. PCSK9 adheres to the epidermal growth factor-like repeat A (EGF-A) domain of the LDLR which is confirmed by crystallography. LDLR expression is adjusted at the transcriptional level through sterol regulatory element binding protein 2 (SREBP-2) and at the post translational stages, specifically through PCSK9, and the inducible degrader of the LDLR PCSK9 inhibition is an appealing new method for reducing the concentration of LDL-C. In this review the role of PCSK9 in lipid homeostasis was elucidated, the effect of PCSK9 on atherosclerosis was highlighted, and contemporary therapeutic techniques that focused on PCSK9 were summarized. Several restoration methods to inhibit PCSK9 have been proposed which concentrate on both extracellular and intracellular PCSK9, and they include blockage of PCSK9 production by using gene silencing agents and blockage of it's binding to LDLR through antibodies and inhibition of PCSK9 autocatalytic processes by tiny molecule inhibitors.

Bioactive Compounds of Cooked Tomato Sauce Modulate Oxidative Stress and Arachidonic Acid Cascade Induced by Oxidized LDL in Macrophage Cultures

Sofrito is a mix of tomato, onion, garlic, and olive oil, which contains phenolic compounds and carotenoids. Consumption of tomato-based sofrito has been related to a lower risk of cardiovascular events, but the mechanisms behind such beneficial effects remain unclear. This study aimed to analyze the effects of representative sofrito compounds such as naringenin, hydroxytyrosol, lycopene, and β-carotene on mechanisms involved in the pathogenesis of atherosclerosis. We demonstrated that both phenolic compounds and both carotenoids studied were able to inhibit low density lipoproteins (LDL) oxidation, as well as oxidative stress and eicosanoid production induced by oxidized LDL (oxLDL) in macrophage cultures. These effects were not the consequences of disturbing oxLDL uptake by macrophages. Finally, we observed an additive effect of these sofrito compounds, as well as the activity of a main naringenin metabolite, naringenin 7-O-β-d-glucuronide on LDL oxidation and oxidative stress.

Local effects of human PCSK9 on the atherosclerotic lesion

Proprotein convertase subtilisin/kexin type 9 (PCSK9) promotes atherosclerosis by increasing low-density lipoprotein (LDL) cholesterol levels through degradation of hepatic LDL receptor (LDLR). Studies have described the systemic effects of PCSK9 on atherosclerosis, but whether PCSK9 has local and direct effects on the plaque is unknown. To study the local effect of human PCSK9 (hPCSK9) on atherosclerotic lesion composition, independently of changes in serum cholesterol levels, we generated chimeric mice expressing hPCSK9 exclusively from macrophages, using marrow from hPCSK9 transgenic (hPCSK9tg) mice transplanted into apoE(-/-) and LDLR(-/-) mice, which were then placed on a high-fat diet (HFD) for 8 weeks. We further characterized the effect of hPCSK9 expression on the inflammatory responses in the spleen and by mouse peritoneal macrophages (MPM) in vitro. We found that MPMs from transgenic mice express both murine (m) Pcsk9 and hPCSK9 and that the latter reduces macrophage LDLR and LRP1 surface levels. We detected hPCSK9 in the serum of mice transplanted with hPCSK9tg marrow, but did not influence lipid levels or atherosclerotic lesion size. However, marrow-derived PCSK9 progressively accumulated in lesions of apoE(-/-) recipient mice, while increasing the infiltration of Ly6C(hi) inflammatory monocytes by 32% compared with controls. Expression of hPCSK9 also increased CD11b- and Ly6C(hi) -positive cell numbers in spleens of apoE(-/-) mice. In vitro, expression of hPCSK9 in LPS-stimulated macrophages increased mRNA levels of the pro-inflammatory markers Tnf and Il1b (40% and 45%, respectively) and suppressed those of the anti-inflammatory markers Il10 and Arg1 (30% and 44%, respectively). All PCSK9 effects were LDLR-dependent, as PCSK9 protein was not detected in lesions of LDLR(-/-) recipient mice and did not affect macrophage or splenocyte inflammation. In conclusion, PCSK9 directly increases atherosclerotic lesion inflammation in an LDLR-dependent but cholesterol-independent mechanism, suggesting that therapeutic PCSK9 inhibition may have vascular benefits secondary to LDL reduction.

Preparation and characterization of a lovastatin-loaded protein-free nanostructured lipid carrier resembling high-density lipoprotein and evaluation of its targeting to foam cells

This study was designed to investigate whether a non-protein nanostructured lipid carrier (NLC) resembling high-density lipoprotein (HDL) could deliver a hydrophobic anti-atherogenic drug, lovastatin, to foam cells. Lovastatin-loaded NLC (LT-NLC) was prepared by a nanoprecipitation/solvent diffusion method. The LT-NLC-apoprotein (LT-NLC-apo) was prepared by incubating LT-NLC with native HDL. The physicochemical parameters of LT-NLC were characterized in terms of particle size, zeta potential, morphology, entrapment efficiency, and crystallization behavior. Targeting behavior and mechanism were demonstrated by the incubation of LT-NLC-apo with a RAW 264.7 macrophage-derived foam cell model in the presence or absence of very-low-density lipoprotein (VLDL) and lipase. The results showed that LT-NLC was solid spherical or oval in shape with an average diameter of 13.8 ± 2.2 nm, zeta potential of −29.3 ± 0.2 mV and entrapment efficiency of 96.2 ± 1.3%. Phagocytosis studies showed that uptake of LT-NLC-apo by macrophages was significantly lower than LT-NLC (p < 0.01), suggesting that LT-NLC-apo could possibly escape recognition from macrophages in vivo. The uptake was increased twofold when LT-NLC-apo was incubated with transfected foam cells containing VLDL and lipase. These results indicated that non-protein NLC resembling HDL could be a useful tool to deliver lipophilic anti-atherogenic drugs to foam cells, and that uptake could be enhanced by the VLDL receptor pathway.

Protein kinase CK1alphaLS promotes vascular cell proliferation and intimal hyperplasia

Protein kinase CK1alpha regulates several fundamental cellular processes including proliferation and differentiation. Up to four forms of this kinase are expressed in vertebrates resulting from alternative splicing of exons; these exons encode either the L-insert located within the catalytic domain or the S-insert located at the C terminus of the protein. Whereas the L-insert is known to target the kinase to the nucleus, the functional significance of nuclear CK1alphaLS has been unclear. Here we demonstrate that selective L-insert-targeted short hairpin small interfering RNA-mediated knockdown of CK1alphaLS in human vascular endothelial cells and vascular smooth muscle cells impairs proliferation and abolishes hydrogen peroxide-stimulated proliferation of vascular smooth muscle cells, with the cells accumulating in G(0)/G(1). In addition, selective knockdown of CK1alphaLS in cultured human arteries inhibits vascular activation, preventing smooth muscle cell proliferation, intimal hyperplasia, and proteoglycan deposition. Knockdown of CK1alphaLS results in the harmonious down-regulation of its target substrate heterogeneous nuclear ribonucleoprotein C and results in the altered expression or alternative splicing of key genes involved in cellular activation including CXCR4, MMP3, CSF2, and SMURF1. Our results indicate that the nuclear form of CK1alpha in humans, CK1alphaLS, plays a critical role in vascular cell proliferation, cellular activation, and hydrogen peroxide-mediated mitogenic signal transduction.

Controllable inhibition of cellular uptake of oxidized low-density lipoprotein: structure-function relationships for nanoscale amphiphilic polymers

A family of anionic nanoscale polymers based on amphiphilic macromolecules (AMs) was developed for controlled inhibition of highly oxidized low-density lipoprotein (hoxLDL) uptake by inflammatory macrophage cells, a process that triggers the escalation of a chronic arterial disease called atherosclerosis. The basic AM structure is composed of a hydrophobic portion formed from a mucic acid sugar backbone modified at the four hydroxyls with lauroyl groups conjugated to hydrophilic poly(ethylene glycol) (PEG). The AM structure-activity relationships were probed by synthesizing AMs with six key variables: length of the PEG chain, carboxylic acid location, type of anionic charge, number of anionic charges, rotational motion of the anionic group, and PEG architecture. All AM structures were confirmed by nuclear magnetic resonance spectroscopy and their ability to inhibit hoxLDL uptake in THP-1 human macrophage cells was compared in the absence and presence of serum. We report that AMs with one, rotationally restricted carboxylic acid within the hydrophobic portion of the polymer was sufficient to yield the most effective AM for inhibiting hoxLDL internalization by THP-1 human macrophage cells under serum-containing conditions. Further, increasing the number of charges and altering the PEG architecture in an effort to increase serum stabilization did not significantly impair the ability of AMs to inhibit hoxLDL internalization, suggesting that selected modifications to the AMs could potentially promote multifunctional characteristics of these nanoscale macromolecules.

Structure-activity relations of nanolipoblockers with the atherogenic domain of human macrophage scavenger receptor A

Oxidized low density lipoprotein (oxLDL) uptake by macrophages is mediated by scavenger receptors and leads to unregulated cholesterol accumulation. Micellar nanolipoblockers (NLBs) consist of alkyl chains and polyethylene glycol on mucic acid. NLBs functionalized with anionic groups inhibit oxLDL uptake via the scavenger receptor A (SR-A). Molecular modeling and docking approaches were used to understand the structure-activity relationship (SAR) between NLBs and SR-A. Six NLB models were docked to the SR-A homology model to investigate charge placement and clustering. NLB models with the most favorable binding energy were also the most effective oxLDL inhibitors in THP-1 macrophages. Mutant SR-A models were generated by replacing charged residues with alanine. All charged residues in the region were necessary, with Lys60, Lys63, and Lys66 having the greatest effect on binding. We hypothesize that structural studies aided by theoretical modeling and docking can be used to design promising NLB candidates with optimal binding properties.

Effect of resveratrol, tyrosol and beta-sitosterol on oxidised low-density lipoprotein-stimulated oxidative stress, arachidonic acid release and prostaglandin E2 synthesis by RAW 264.7 macrophages

Oxidation of LDL is hypothesised as an early and critical event in atherogenesis. Oxidised LDL (oxLDL) favour the transformation of macrophages into foam cells, an important cell involved in atherosclerosis. Furthermore, oxLDL cause multiple changes in macrophage functions. Thus, oxLDL induces certain genes, suppresses others and alters cell lipid metabolism. Consumption of a Mediterranean diet is associated with a low incidence of atherosclerotic disease, but data about the specific dietary constituents involved and mechanisms conferring cardioprotection are still sparse. The aim of the present study was to determine the effect of representative minor components of wine and olive oil on reactive oxygen species and eicosanoid synthesis induced by oxLDL-stimulated macrophages. We observed that exposure to non-toxic oxLDL concentrations leads to the production of H2O2 by RAW 264.7 macrophages and this effect was reverted by apocynin, a NADPH oxidase inhibitor. Moreover, oxLDL induced arachidonic acid (AA) release, cyclo-oxygenase-2 overexpression and subsequent PGE2 release. We observed that resveratrol and tyrosol revert H2O2 production induced by oxLDL as well as AA release and PGE2 synthesis and that these effects were not as a consequence of these compounds interfering with the oxLDL binding to their receptors. Interestingly, beta-sitosterol presence enhances these polyphenol actions. Thus, we found a synergistic action of polyphenols of olive oil and wine and beta-sitosterol of olive oil led to the modulation of the effects of oxLDL on oxidative stress and PGE2 synthesis.

Phagocytosis of cholesteryl ester is amplified in diabetic mouse macrophages and is largely mediated by CD36 and SR-A

Type 2 diabetes (T2D) is associated with accelerated atherosclerosis, which accounts for approximately 75% of all diabetes-related deaths. Here we investigate the link between diabetes and macrophage cholesteryl ester accumulation. When diabetic (db/db) mice are given cholesteryl ester intraperitoneally (IP), peritoneal macrophages (PerMΦs) recovered from these animals showed a 58% increase in intracellular cholesteryl ester accumulation over PerMΦs from heterozygote control (db/+) mice. Notably, PerMΦ fluid-phase endocytosis and large particle phagocytosis was equivalent in db/+and db/db mice. However, IP administration of CD36 and SR-A blocking antibodies led to 37% and 25% reductions in cholesteryl ester accumulation in PerMΦ. Finally, in order to determine if these scavenger receptors (SRs) were part of the mechanism responsible for the increased accumulation of cholesteryl esters observed in the diabetic mouse macrophages, receptor expression was quantified by flow cytometry. Importantly, db/db PerMΦs showed a 43% increase in CD36 expression and an 80% increase in SR-A expression. Taken together, these data indicate that direct cholesteryl ester accumulation in mouse macrophages is mediated by CD36 and SR-A, and the magnitude of accumulation is increased in db/db macrophages due to increased scavenger receptor expression.

Engineered polymeric nanoparticles for receptor-targeted blockage of oxidized low density lipoprotein uptake and atherogenesis in macrophages

Strategies to prevent the uptake of modified low density lipoproteins (LDLs) by immune cells, a major trigger of inflammation and atherogenesis, are challenged by complex interfacial factors governing LDL receptor-mediated uptake. We examine a new approach based on a family of "nanoblockers", which are designed to examine the role of size, charge presentation, and architecture on inhibition of highly oxidized LDL (hoxLDL) uptake in macrophages. The nanoblockers are macromolecules containing mucic acid, lauryl chloride, and poly(ethylene glycol) that self-assemble into 15-20 nm nanoparticles. We report that the micellar configuration of the macromolecules and the combined display of anionic (carboxylate) groups in the hydrophobic region of the nanoblockers caused the most effective inhibition in the uptake of hoxLDL by IC21 macrophages. The nanoblockers primarily targeted SR-A and CD36, the major scavenger receptors and modulated the "atherogenic" phenotype of cells in terms of the degree of cytokine secretion, accumulation of cholesterol, and "foam cell" formation. These studies highlight the promise of synthetically engineered nanoblockers against oxidized LDL uptake.

Chondroitin sulfate-modified LDL induces increased cholesteryl ester synthesis and down-regulation of LDL receptors in smooth muscle cells and macrophages

Hypercholesterolemia induces increased transcytosis and accumulation of plasma lipoproteins in the arterial intima, where they interact with matrix proteins and become modified and reassembled lipoproteins. Chondroitin 6-sulfate-modified LDL (CS-mLDL) induces migration, proliferation, and lipid accumulation in human aortic smooth muscle cells (SMCs). To search for the mechanism(s) responsible for lipid accumulation, cultured SMC and macrophages were exposed to CS-mLDL, minimally modified LDL (mmLDL), and native LDL (as a control). Then the cellular uptake, degradation and expression of the LDL receptor (LDL-R) was determined using radioiodinated ligands, ACAT activity assay, fluorescence microscopy and RT-PCR. The uptake of CS-mLDL was 2-fold higher in SMC and 3-to 4-fold higher in macrophages as compared to LDL and mmLDL; the lysosomal degradation of CS-mLDL was slower in SMCs and considerably diminished in macrophages. Compared with LDL, CS-mLDL induced increased synthesis and accumulation of esterified cholesterol in SMCs (∼2-fold) and macrophages (∼10-fold) within an expanded acidic compartment. CS-mLDL and mmLDL down-regulate the gene expression of the LDL-R in the both cell types. Mechanisms of CS-mLDL-induced lipid accumulation in SMC and macrophages involve increased cellular uptake, and diminished cellular degradation that stimulates cholesterol ester synthesis and accumulation in cytoplasmic inclusions and in the lysosomal compartment in an undegraded form; modified lipoproteins induce down-regulation of LDL-R.

Nanoscale Anionic Macromolecules Can Inhibit Cellular Uptake of Differentially Oxidized LDL

Nanoscale particles could be synthetically designed to potentially intervene in lipoprotein matrix retention and lipoprotein uptake in cells, processes central to atherosclerosis. We recently reported on lipoprotein interactions of nanoscale micelles self-assembled from amphiphilic scorpion-like macromolecules based on a lauryl chloride-mucic acid hydrophobic backbone and poly(ethylene glycol) shell. These micelles can be engineered to present varying levels of anionic chemistry, a key mechanism to induce differential retentivity of low-density lipoproteins (LDL) (Chnari, E.; Lari, H. B.; Tian, L.; Uhrich, K. E.; Moghe, P. V. Biomaterials 2005, 26, 3749). In this study, we examined the cellular interactions and the ability of carboxylate-terminated nanoparticles to modulate cellular uptake of differentially oxidized LDL. The nanoparticles were found to be highly biocompatible with cultured IC21 macrophages at all concentrations examined. When the nanoparticles as well as LDL were incubated with the cells over 24 h, a marked reduction in cellular uptake of LDL was observed in a nanoparticle concentration-dependent manner. Intermediate concentrations of nanoparticles (10(-6) M) elicited the most charge-specific reduction in uptake, as indicated by the difference in uptake due to anionic and uncharged nanoparticles. At these concentrations, anionic nanoparticles reduced LDL uptake for all degrees of oxidation (no oxidation, mild, high) of LDL, albeit with qualitative differences in the effects. The anionic nanoparticles were particularly effective at reducing the very high levels of uptake of the most oxidized level of LDL. Since complexation of LDL with anionic nanoparticles is reduced at higher degrees of LDL oxidation, our results suggest that anionic nanoparticles interfere in highly oxidized (hox) LDL uptake, likely by targeting cellular/receptor uptake mechanism, but control unoxidized LDL uptake by mechanisms related to direct LDL-nanoparticle complexation. Thus, anionically functionalized nanoparticles can modulate the otherwise unregulated internalization of differentially oxidized LDL.

Selective uptake and efflux of cholesteryl linoleate in LDL by macrophages expressing 12/15-lipoxygenase

Oxidation of low density lipoprotein (LDL) is a critical step for atherogenesis, and the role of the 12/15-lipoxygenase (12/15-LOX) as well as LDL receptor-related protein (LRP) expressed in macrophages in this process has been suggested. The oxygenation of cholesteryl linoleate in LDL by mouse macrophage-like J774A.1 cells overexpressing 12/15-LOX was inhibited by an anti-LRP antibody but not by an anti-LDL receptor antibody. When the cells were incubated with LDL double-labeled by [3H]cholesteryl linoleate and [125I]apoB, association with the cells of [3H]cholesteryl linoleate expressed as LDL protein equivalent exceeded that of [125I]apoB, indicating selective uptake of [3H]cholesteryl linoleate from LDL to these cells. An anti-LRP antibody inhibited the selective uptake of [3H]cholesteryl ester by 62% and 81% with the 12/15-LOX-expressing cells and macrophages, respectively. Furthermore, addition of LDL to the culture medium of the [3H]cholesteryl linoleate-labeled 12/15-LOX-expressing cells increased the release of [3H]cholesteryl linoleate to the medium in LDL concentration- and time-dependent manners. The transport of [3H]cholesteryl linoleate from the cells to LDL was also inhibited by an anti-LRP antibody by 75%. These results strongly suggest that LRP contributes to the LDL oxidation by 12/15-LOX in macrophages by selective uptake and efflux of cholesteryl ester in the LDL particle.

Interleukin-1beta attenuates beta-very low-density lipoprotein uptake and its receptor expression in vascular smooth muscle cells

The very low-density lipoprotein (VLDL) receptor is a member of the low-density lipoprotein (LDL) receptor gene family with distinct tissue distribution and function. VLDL receptors are also expressed in vascular smooth muscle cells (VSMCs) and have been shown to be upregulated in atherosclerotic lesions. In the present study, we examined the effects of interleukin-1beta (IL-1beta) on the uptake of betaVLDL and its receptor expression in rat VSMCs. IL-1beta downregulated expression of the VLDL receptor in a time and dose-dependent manner as shown by Western blotting, Northern blotting, and reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. Treatment with IL-1beta significantly reduced the uptake of beta-VLDL but not LDL in VSMCs. Use of specific pharmacologic inhibitors indicated that the tyrosine kinase inhibitors, herbimycin A and geldanamycin, completely reversed IL-1beta-induced downregulation of the VLDL receptor expression. Another tyrosine kinase inhibitor, genistein, the protein kinase C inhibitors, GF109203X and H7, the mitogen-activated protein (MAP) kinase inhibitors (MEK inhibitor PD098059 for [MEK] and SB203580 for p38-MAP kinase), and the protein kinase A inhibitor, KT5270 all had no effect on receptor expression. In addition, the c-Src specific inhibitor PP2 or adenoviral-mediated gene transfer of kinase inactive (KI)-c-Src failed to reverse IL-1beta-induced downregulation of VLDL receptor expression. These results indicate that IL-1beta attenuates uptake of VLDL through downregulation of its receptor in VSMCs, and that this downregulation is mediated through a benzoquinone ansamycin-dependent but c-Src-independent pathway.

Essential roles of lipoxygenases in LDL oxidation and development of atherosclerosis

Oxidative modification of low-density lipoprotein (LDL) is one of the critical steps for the development of atherosclerosis. Accumulating studies have indicated that 12/15-lipoxygenase highly expressed in macrophages plays an essential role in the oxidation of circulating LDL. It has been demonstrated that LDL needs to bind the LDL receptor-related protein (LRP), a cell-surface receptor, prior to its oxidation by 12/15-lipoxygenase expressed in macrophages. LRP is suggested to mediate the selective transfer of cholesteryl ester in LDL to the plasma membrane of macrophages without endocytosis and degradation of the LDL particle. At the same time, binding of LDL to LRP translocates the 12/15-lipoxygenase from the cytosol to the plasma membrane. It is also demonstrated that 5-lipoxygenase localized in macrophages generates leukotrienes, which exhibit strong proinflammatory activities in cardiovascular tissues and contribute to lesion development. Therefore, the inhibition of these lipoxygenases may be effective in the prevention and treatment of the inflammatory diseases.

Mécanismes des hyperlipidémies dues aux immunosuppresseurs

This article summarises the mechanisms responsible for the hyperlipidaemia observed after immunosuppressive treatment. Much progress has been achieved in the treatment of organ transplantation over the last 10 years, in particular because of the use of new immunosuppressive drugs with less nephrotoxicity. However, hypercholesterolaemia and hypertriglyceridaemia persist among many patients, who are thus more likely to develop cardiovascular diseases. We first reviewed the effects of immunosuppressive drugs on biliary acid biosynthesis, which is the main pathway of cholesterol degradation. The inhibition of this biosynthesis pathway, and especially of some key cytochrome P450s (CYP) such as CYP27A1, could contribute to the increased cholesterolaemia. Immunosuppressive drugs may also modify the activity of lipoprotein receptors or the expression of different apolipoproteins involved in cholesterol and triglyceride transport by lipoproteins. Finally, the fact that hypertriglyceridaemia is more frequently observed after certain immunosuppressive treatments may be partly caused by changes in the synthesis and elimination of triglycerides involving lipoprotein lipase or some apolipoproteins which serve as its cofactors (apoCII or apoCIII).

Lipoprotein receptors in the vascular wall

PURPOSE OF REVIEW

We will discuss the diverse roles of lipoprotein receptors that contribute to the maintenance and integrity of the vascular wall.

RECENT FINDINGS

Lipoprotein receptors function not only as transporters for cholesterol and other lipids. They also act as sensors and signal transducers through which the endothelium, macrophages and smooth muscle cells communicate with their environment.

SUMMARY

Traditionally, lipoprotein receptors were thought of merely as transporters of cholesterol and triglycerides to specific target cells, either for the purpose of delivery and redistribution of nutrients, or for the destruction or clearance of modified (oxidized) lipids by macrophages. Only recently have we begun to appreciate that the same receptors engage in a much more sophisticated and multi-faceted interaction with their environment. Inasmuch, they not only act as mere transporters, but as surprisingly versatile and adaptive signal transducers and modulators throughout the vessel wall. These recent findings now begin to reshape our thinking of how such structurally different and evolutionarily unrelated lipoprotein receptors orchestrate the response of the vessel wall to mechanical or metabolic damage.

Essential role of the low density lipoprotein receptor-related protein in vascular smooth muscle cell migration

The low density lipoprotein receptor-related protein (LRP) is a multifunctional cell surface receptor highly expressed in human aortic smooth muscle cells. In the present study, we used the short interfering RNA (siRNA) technique to explore the role of LRP in smooth muscle cell migration. We identified an LRP-specific siRNA that selective silences LRP expression in human aortic smooth muscle cells. As a consequence, LRP-mediated ligand degradation was significantly reduced. More important, we found that platelet-derived growth factor-dependent cell migration was inhibited in cells transfected with LRP siRNA. These results demonstrate an important role of LRP in smooth muscle cell migration.

The cytoplasmic domain of the low density lipoprotein (LDL) receptor-related protein, but not that of the LDL receptor, triggers phagocytosis

The macrophage LDL receptor and LDL receptor-related protein (LRP, CD91) mediate the phagocytic-like uptake of atherogenic lipoproteins and apoptotic cells, yet the structural basis of their phagocytic functions is not known. To address this issue, we transfected macrophages with chimeric proteins containing the cytoplasmic tails and transmembrane regions of the LDL receptor or LRP and the ectodomain of CD2, which can bind non-opsonized sheep red blood cells (SRBCs). Macrophages expressing receptors containing the LDL receptor domains were able to bind but not internalize SRBCs. In contrast, macrophages expressing receptors containing the cytoplasmic tail of LRP were able to bind and internalize SRBCs. Chimeras in which the LRP cytoplasmic tail was mutated in two di-leucine motifs and a tyrosine in an NPXYXXL motif were able to endocytose anti-CD2 antibody and bind SRBCs, but SRBC phagocytosis was decreased by 70%. Thus, the phagocytic-like functions of LRP, but not those of the LDL receptor, can be explained by the ability of the LRP cytoplasmic tail to trigger phagocytosis. These findings have important implications for atherogenesis and apoptotic cell clearance and for a fundamental cell biological understanding of how the LDL receptor and LRP function in internalization processes.

Fatty acids modulate the effect of darglitazone on macrophage CD36 expression

BACKGROUND

Scavenger receptor-mediated uptake of cholesterol by macrophages in the arterial wall is believed to be proatherogenic. Thiazolidinediones are peroxisome proliferator-activated receptor gamma (PPARgamma)-agonists, which are used in the treatment of type II diabetes. They reduce atherogenesis in LDL receptor deficient and ApoE knockout mice, but up-regulate CD36, which may contribute to foam cell formation. The dyslipidaemia in type II diabetes is characterized by high levels of nonesterified fatty acids. Therefore we tested the effect of fatty acids and how fatty acids and the thiazolidinedione darglitazone interact in their effect on CD36 expression in human monocytes and macrophages.

MATERIALS AND METHODS

Flow cytometry and reverse transcription-polymerase chain reaction were used to study CD36 expression. Cellular lipids were analyzed with high performance liquid chromatography.

RESULTS

Darglitazone increased CD36 mRNA and protein expression in human macrophage cells. In the presence of 5% human serum, darglitazone increased the accumulation of triglycerides, but did not affect cholesterol ester levels. In the presence of albumin-bound oleic or linoleic acid, darglitazone did not increase CD36 mRNA, cell-surface CD36 protein or triglyceride content. Fatty acids per se increased CD36 mRNA and protein.

DISCUSSION

The increase in CD36 in macrophages suggests a role for fatty acids in the regulation of foam cell formation. The results also suggest that the potentially proatherogenic CD36 up-regulating effect of thiazolidinediones in macrophages might not be present when the cells have access to physiological levels of albumin-bound fatty acids.

Low density lipoprotein receptor-related protein-mediated membrane translocation of 12/15-lipoxygenase is required for oxidation of low density lipoprotein by macrophages

Oxidation of low density lipoprotein (LDL) is the key step for the development of atherosclerosis. The 12/15-lipoxygenase expressed in macrophages is capable of oxygenating linoleic acid esterified to cholesterol in the LDL particle, and thus this enzyme is presumed to initiate LDL oxidation. We recently reported that LDL receptor-related protein (LRP) was required for the enzyme-mediated LDL oxidation by macrophages and suggested the selective uptake of cholesterol ester from LDL to the plasma membrane (Xu, W., Takahashi, Y., Sakashita, T., Iwasaki, T., Hattori, H., and Yoshimoto. T. (2001) J. Biol. Chem. 276, 36454-36459). To elucidate precise mechanisms of lipoxygenase-mediated LDL oxidation, we investigated the intracellular localization of 12/15-lipoxygenase. The 12/15-lipoxygenase was predominantly detected in cytosol of resting peritoneal macrophages and of macrophage-like J774A.1 cells permanently transfected with the cDNA for the enzyme. When the cells were treated with LDL and subjected to subcellular fractionation, the 12/15-lipoxygenase was detected in the membranes with a concomitant decrease in cytosol as shown by Western blot analysis. The levels of the enzyme associated with the membrane reached maximum in 15 min after LDL addition and then decreased. However, the enzymatic activity of 12/15-lipoxygenase in the membrane fraction was very weak even after LDL treatment. This fact supports the suicide inactivation of the enzyme by the oxygenation of cholesterol ester transferred from the LDL particle to the plasma membrane. Immunohistochemical analysis using an antibody against 12/15-lipoxygenase revealed that the plasma membrane was the major site of the enzyme translocation by the LDL treatment. LDL-dependent 12/15-lipoxygenase translocation was inhibited by a blocking antibody against LRP. Furthermore, an enzyme translocation inhibitor, L655238, inhibited the LDL oxidation caused by the 12/15-lipoxygenase. We propose that cholesterol ester selectively transferred from the LDL particle to the plasma membrane via LRP is oxygenated by 12/15-lipoxygenase translocated to this membrane.

Lipoprotein receptors in the nervous system

The low-density-lipoprotein (LDL) receptor family is an evolutionarily ancient gene family of structurally closely related cell-surface receptors. Members of the family are involved in the cellular uptake of extracellular ligands and regulate diverse biological processes including lipid and vitamin metabolism and cell-surface protease activity. Some members of the family also participate in cellular signaling and regulate the development and functional maintenance of the nervous system. Here we review the roles of this family of multifunctional receptors in the nervous system and focus on recent advances toward the understanding of the mechanisms by which lipoprotein receptors and their ligands transmit and modulate signals in the brain.

The autoantibody expression against different source of oxidized low density lipoprotein in patients with acute myocardial infarction

The aim of this study was to examine the expression of antibodies against two different sources of low density lipoprotein (LDL) that were oxidized by CuSO(4), in patients with early stage of acute myocardial infarction (AMI). When LDL purified from sera with high level of LDL was used as a modified antigen, the results indicated that the titers of antibodies against the oxidized LDL in 30 patients were increased by 135% compared to those in normal subjects; however, the titers of antibody against modified LDL purified from normal-range LDL in the same patients were only slightly increased by 52%. Comparing the levels of autoantibody expressed in the high LDL sera group, high triglyceride sera group, and AMI patients sera group (total of 41; in addition to 30 AMI patients, 11 more sera of AMI patients were collected), the amount of autoantibody against the oxLDL purified from high LDL sera in AMI patients sera group was significantly increased up to 195%. In contrast to AMI patients, the sera titers against the same antigen in two subject groups with either high LDL or high triglyceride are only 50% higher than normal subjects. Moreover, the ratio of thromboxane B(2) over 6-keto-prostaglandin F(1alpha) (6-keto-PG F(1alpha)) in the acute myocardial infarction patients was 1.79, which is much lower than the normal subjects, 4.19. Concluding from the above observations, we suggest that the expression level of anti-oxidized LDL antibody may play a role on the pathogenesis of acute myocardial infarction disease, but is independent with the levels of thromboxane A(2) and prostacyclin in the examined sera.

Variation in the lipoprotein receptor-related protein, alpha2-macroglobulin and lipoprotein receptor-associated protein genes in relation to plasma lipid levels and risk of early myocardial infarction

BACKGROUND

The lipoprotein receptor-related protein (LRP) is an endocytic receptor for several ligands, such as alpha2-macroglobulin (alpha2 M) and apolipoprotein E. LRP is involved in the clearance of lipids from the bloodstream and is expressed in the atherosclerotic plaque. The LRP-associated protein (LRPAP in humans, RAP in mice) acts as a chaperone protein, stabilizing the nascent LRP peptide in the endoplasmic reticulum and Golgi complex. In mice, the amount of LRP activity was modulated by RAP, and RAP-null mice showed higher levels of total cholesterol.

OBJECTIVE

To evaluate the association between DNA polymorphisms at the LRP, LRPAP and alpha2 M genes and early myocardial infarction (MI).

METHODS

We genotyped 210 patients with early MI (<55 years) and 200 healthy control participants for three polymorphisms in the LRP, LRPAP and alpha2 M genes.

RESULTS

No association was found between these polymorphisms and plasma lipid levels in patients and control participants. Only the LRPAP-intron 1 polymorphism (a 21 bp insertion/deletion) was associated with MI (P = 0.0065; odds ratio = 2.18, 95% confidence intervals = 1.22-3.90).

CONCLUSIONS

According to our data, the variation at the LRPAP1 gene could contribute to the risk of developing an early episode of MI.

Oxidized low-density lipoprotein receptor LOX-1 and apoptosis in human atherosclerotic lesions

BACKGROUND

Lectin-like oxidized LDL receptor-1 (LOX-1), a novel receptor for oxidized low-density lipoprotein, mediates oxidized low-density lipoprotein-induced apoptosis of endothelial cells, monocyte adhesion to endothelium, and phagocytosis of aged cells. The present study examined the role of LOX-1 and apoptosis in human atherosclerotic lesions.

METHODS AND RESULTS

Grafted vein (n = 8), human carotid artery endarterectomy (n = 11), and normal human internal mammary artery (n = 8) specimens were used to study the expression of LOX-1 and apoptosis. LOX-1 expression was determined by reverse transcriptase-polymerase chain reaction, Western analysis, and immunostaining. Presence of apoptosis was determined by fluorescent in situ nick end-labeling staining and by the presence of poly (ADP-ribose) polymerase protein (an apoptotic marker). Expression of LOX-1 was significantly increased in atherosclerotic grafted vein and carotid artery specimens compared with that in normal arteries. LOX-1 was expressed in endothelial cells, macrophages, and smooth muscle cells. LOX-1 was extensively expressed in the new blood vessels in the core of advanced atherosclerotic lesions. Double immunostaining showed LOX-1 expression to be colocalized with apoptotic cells. Fluorescent in situ nick end-labeling staining showed that the apoptotic cells were present mostly in the rupture-prone regions of the atherosclerotic plaque.

CONCLUSION

These observations indicate that LOX-1 is extensively expressed in the proliferated intima of grafted veins and in advanced atherosclerotic carotid arteries. Further, LOX-1 is colocalized with apoptotic cells. These observations may relate to the phenomenon of plaque rupture, and provide targets for developing new therapies.

Inhibitory effects of N-acetylcysteine on scavenger receptor class A expression in human macrophages

OBJECTIVE

The formation of foam cells from monocyte-derived macrophages involves the uptake of modified lipoproteins by scavenger receptors. Antioxidants inhibit lipoprotein oxidation and may also modulate gene expression. We investigated the effect of the antioxidant N-acetylcysteine on the expression of the class A scavenger receptor (SR-A) types I and II in human macrophages.

DESIGN

Monocytes and macrophages from healthy blood donors and plaque-derived macrophages from patients undergoing carotid endarterectomy were used for experiments. SR-A mRNA was analysed with quantitative and semiquantitative reverse transcription-polymerase chain reaction, and ligand binding and uptake were assessed with 125I-labelled acetylated low-density lipoprotein (LDL).

RESULTS

Incubation of monocytes and monocyte-derived macrophages with N-acetylcysteine decreased both SR-A I and II mRNA expression. N-Acetylcysteine also reduced SR-A mRNA in lesion-derived cells. Binding and uptake of 125I-acetylated LDL was decreased after brief incubation with N-acetylcysteine. After longer periods of incubation with N-acetylcysteine we observed an increased degradation of lipoproteins.

CONCLUSIONS

Our results imply that N-acetylcysteine leads to a decrease in SR-A mRNA and initially also to an attenuated uptake of modified lipoproteins. This adds more to the knowledge about the cellular actions of this drug.

VLDL receptor deficiency enhances intimal thickening after vascular injury but does not affect atherosclerotic lesion area

The very low density lipoprotein receptor (VLDLR) has been shown to modulate cell migration and foam cell formation in vitro. This suggests a role for the VLDLR in vascular pathology associated with intimal thickening and atherogenesis. In the present paper both intimal thickening and atherosclerosis were studied using VLDLR knockout and transgenic mouse models. The role of the VLDLR in intimal thickening was established in an in vivo model for vascular injury. A non-restrictive cuff was placed around the femoral artery of VLDLR deficient (VLDLR-/-), heterozygous deficient (VLDLR+/-) and wild type (WT) mice. Intimal thickening was assessed after 3 weeks by determining the intima to media (I/M) volume ratio. Both VLDLR-/- (I/M ratio 42%) and VLDLR+/- (I/M ratio 40%) mice showed a significant increase as compared with WT littermates (I/M ratio 25%). The effect of VLDLR deficiency on atherosclerosis was examined in VLDLR-/- mice on an LDLR deficient (LDLR-/-) background. In addition, we assessed whether increased endothelial VLDLR expression levels affect atherosclerotic lesion formation. Therefore, atherosclerosis was studied in LDLR deficient mice that over express the VLDLR in endothelial cells (PVL, LDLR-/-). Both VLDLR deficiency and endothelial VLDLR over expression did not affect the atherosclerotic lesion size. Interestingly, VLDLR-/-, LDLR-/- mice showed a high incidence of necrosis in both fatty streaks and atherosclerotic plaques as compared with LDLR-/- mice (75 vs. 0% and 76 vs. 45%, respectively). In conclusion, deficiency for the VLDLR profoundly increased intimal thickening after vascular injury.

Oxidative stress increases the expression of the angiotensin-II receptor type 1 in mouse peritoneal macrophages

Angiotensin II (Ang II) has been shown to accelerate atherogenesis, and the cellular Ang II type 1 (AT(1))-receptor mediates most of Ang II-induced pro-atherogenic effects. In this study we have examined the effect of macrophage oxidative stress on cellular AT(1)-receptor expression. Mouse peritoneal macrophages (MPM) from apolipoprotein-E deficient (E(0)) mice at increasing ages (1 6 months) demonstrated an age-dependent increase in cellular lipid-peroxides (PD) content. In parallel, the AT(1)-receptor mRNA and protein levels both increased by up to 3.7-fold and 1.7-fold, respectively, in MPM from 6-month old mice compared with 1-month old mice. Vitamin E supplementation to E(0) mice significantly decreased the MPM PD content and macrophage AT(1)-receptor mRNA expression compared with placebo-treated mice. The role of oxidative stress in the cellular expression of AT(1)-receptors was further demonstrated by manipulation of macrophage glutathione content. Buthionine-sulfoximine, a glutathione synthesis inhibitor, increased MPM PD content and AT(1)-receptor mRNA expression, whereas L-2-oxothiazolidine-4-carboxylic acid, that contributes to glutathione synthesis, reduced macrophage PD and AT(1)-receptor mRNA expression. Incubation of MPM with oxidised low-density lipoproteins (LDL) led to a significant, dose-dependent and time-dependent increase in macrophage AT(1)-receptor mRNA and protein expression, compared with control cells. In contrast, native LDL or acetylated LDL did not significantly affect macrophage AT(1)-receptor mRNA expression. In conclusion, our findings suggest that oxidative stress in macrophages induces AT(1)-receptor expression. This phenomenon can stimulate the interaction of Ang II with macrophages and hence accelerate macrophage foam cell formation and early atherogenesis.

Upregulation of collagens detected by gene array in a model of flow-induced pulmonary vascular remodeling

We recently reported localized increased pulmonary arterial resistance, neointimal lesions, and medial thickening induced by aortopulmonary anastomosis in young pigs. This model was used to investigate changes in expression of genes potentially involved in pulmonary vascular remodeling employing a high throughput Atlas Human Cardiovascular Array carrying approximately 600 cardiovascular-related cDNA sequences. Data were confirmed by Northern analysis, Western blots, and histological examination. With the use of lower stringency conditions for hybridization, 56% of the 588 human genes on the array showed visible signal after autoradiography. Approximately 10% of the genes with visible hybridization were altered by shunt-induced high flow. Extracellular matrix and cell adhesion molecules were the most highly represented group of upregulated genes. To our knowledge, our data are the first to demonstrate flow-induced changes in gene expression using a combination of cross species cDNA arrays, homologous hybridization, immunospecific protein, and histology. Our observations expand the list of genes as putative candidates in pulmonary vascular remodeling and support the utility of cross-species microarray analysis in such applications.

Expression of lipoprotein receptors in the aortic walls of diabetic APA hamsters

Syrian hamsters of the APA strain (APA hamsters) have recently been demonstrated to develop atheromatous lesions in the aortic arches under the diabetic condition induced by a single injection of streptozotocin (SZ). Various lipoprotein receptors are reported to play important roles in atherogenesis mainly in vitro, while there are few reports on the relative expressions of these receptors in vivo. In this study, we therefore examined messenger RNA (mRNA) expressions of several lipoprotein receptors on the aortic arches of diabetic APA hamsters at 6, 14 and 26 weeks after the injection (WAI) of SZ. In semi-quantitative RT-PCR, scavenger receptor (SR)-AI, macrosialin (MS)/CD68, and receptor for advanced glycation end-products (RAGE) mRNAs showed significant increases at 6 WAI of SZ, and SR-AI and CD36 mRNA obviously increased until 26 WAI, as compared with the control. Low-density lipoprotein receptor mRNA showed a significant decrease at 14 and 26 WAI, and SR-BI mRNA significantly decreased at 6 and 14 WAI, as compared with the control. Very low-density lipoprotein receptor mRNA was at the same level as the control. By means of in situ hybridization, SR-AI, MS/CD68 and RAGE mRNA were detected in the foam cells of the fatty streaks at 6 WAI, which suggested that SR-AI, MS/CD68 and RAGE play crucial roles in the formation of the fatty streaks, the initial lesions of atherogenesis in diabetic APA hamsters. SR-AI and CD36 were also believed to be related to the progression of atherogenesis in this model.

Caveolin-1 negatively regulates SR-BI mediated selective uptake of high-density lipoprotein-derived cholesteryl ester

The class B, type I scavenger receptor (SR-BI) mediates the selective uptake of high density lipoprotein (HDL) cholesteryl esters and the efflux of free cholesterol. SR-BI is predominantly associated with caveolae in Chinese hamster ovary cells. The caveola protein, caveolin-1, binds to cholesterol and is involved in intracellular cholesterol trafficking. We previously demonstrated a correlative increase in caveolin-1 expression and the selective uptake of HDL cholesteryl esters in phorbol ester-induced differentiated THP-1 cells. The goal of the present study was to determine if the expression of caveolin-1 is the causative factor in increasing selective cholesteryl ester uptake in macrophages. To test this, we established RAW and J-774 cell lines that stably expressed caveolin-1. Transfection with caveolin-1 cDNA did not alter the amount of 125I-labeled HDL that associated with the cells, although selective uptake of HDL [3H]cholesteryl ether was decreased by approximately 50%. The amount of [3H]cholesterol effluxed to HDL was not affected by caveolin-1. To directly address whether caveolin-1 inhibits SR-BI-dependent selective cholesteryl ester uptake, we overexpressed caveolin-1 by adenoviral vector gene transfer in Chinese hamster ovary cells stably transfected with SR-BI. Caveolin-1 inhibited the selective uptake of HDL [3H]cholesteryl ether by 50-60% of control values without altering the extent of cell associated HDL. We next used blocking antibodies to CD36 and SR-BI to demonstrate that the increase in selective [3H]cholesteryl ether uptake previously seen in differentiated THP-1 cells was independent of SR-BI. Finally, we used beta-cyclodextrin and caveolin overexpression to demonstrate that caveolae depleted of cholesterol facilitate SR-BI-dependent selective cholesteryl ester uptake and caveolae containing excess cholesterol inhibit uptake. We conclude that caveolin-1 is a novel negative regulator of SR-BI-dependent selective cholesteryl ester uptake.

Low density lipoprotein receptor-related protein is required for macrophage-mediated oxidation of low density lipoprotein by 12/15-lipoxygenase

The oxidative modification of low density lipoprotein (LDL) has been implicated in the early stage of atherosclerosis through multiple potential pathways, and 12/15-lipoxygenase is suggested to be involved in this oxidation process. We demonstrated previously that the 12/15-lipoxygenase overexpressed in mouse macrophage-like J774A.1 cells was required for the cell-mediated LDL oxidation. However, the mechanism of the oxidation of extracellular LDL by the intracellular 12/15-lipoxygenase has not yet been elucidated. In the present study, we found that not only the LDL receptor but also LDL receptor-related protein (LRP), both of which are cell surface native LDL-binding receptors, were down-regulated by the preincubation of the cells with cholesterol or LDL and up-regulated by lipoprotein-deficient serum. Moreover, 12/15-lipoxygenase-expressing cell-mediated LDL oxidation was decreased by the preincubation of the cells with LDL or cholesterol and increased by the preincubation with lipoprotein-deficient serum. Heparin-binding protein 44, an antagonist of the LDL receptor family, also suppressed the cell-mediated LDL oxidation in a dose-dependent manner. The cell-mediated LDL oxidation was dose-dependently blocked by an anti-LRP antibody but not by an anti-LDL receptor antibody. Furthermore, antisense oligodeoxyribonucleotides against LRP reduced the cell-mediated LDL oxidation under the conditions in which the expression of LRP was decreased. The results taken together indicate that LRP was involved essentially for the cell-mediated LDL oxidation by 12/15-lipoxygenase expressed in J774A.1 cells, suggesting an important pathophysiological role of this receptor-enzyme system as the initial trigger of the progression of atherosclerosis.

Expression of very low density lipoprotein receptor mRNA in circulating human monocytes: its up-regulation by hypoxia

Although very low density lipoprotein (VLDL) receptor expression by macrophages has been shown in the vascular wall, it is not clear whether or not circulating monocytes express the VLDL receptor. We investigated the expression of VLDL receptor mRNA in human peripheral blood monocytes and monocyte-derived macrophages by reverse transcriptase polymerase chain reaction (RT-PCR) and nucleotide sequencing after subcloning of PCR product. VLDL receptor mRNA was detected both in peripheral blood monocytes and monocyte-derived macrophages. Expression of VLDL receptor mRNA was upregulated by hypoxia in monocytes, whereas treatment with oxidized LDL, interleukin-1beta or monocyte chemoattractant protein-1 did not affect the levels of VLDL receptor mRNA in monocytes and macrophages. The present study shows a novel response of VLDL receptor mRNA to hypoxia, suggesting a role for VLDL receptor in the metabolism of lipoproteins in the vascular wall and the development of atherosclerosis.

An olive oil-rich diet reduces scavenger receptor mRNA in murine macrophages

During atherogenesis, a pathological accumulation of lipids occurs within aortic intimal macrophages through uptake of oxidised LDL via scavenger receptors. Here we investigated whether some of the anti-atherosclerotic effects ascribed to an olive oil rich-diet are mediated through effects on macrophage scavenger receptors (MSR). Male C57 Bl6 mice aged 6 weeks were fed for 12 weeks on a low-fat diet (containing 25 g corn oil/kg) or on high-fat diets containing 200 g coconut oil, olive oil or safflower oil/kg. Thioglycollate-elicited peritoneal macrophages were analysed for fatty acid composition by GC and the levels of mRNA coding for three MSR (MSRA type I, MSRA type II and CD36) were measured by reverse-transcription polymerase chain reaction. Feeding mice diets enriched with different fats resulted in significant differences in the fatty acid profile of macrophages, which reflected the fatty acid compositions of the diets. These differences were accompanied by a lower level of mRNA for MSRA type I, MSRA type II and CD36 in macrophages from mice fed an olive-oil-enriched diet compared with the mice fed on the low-fat diet. These data suggest that part of the protective effect of olive oil against atherosclerosis might be via reducing macrophage uptake of oxidised LDL. Whether this effect is due to the downregulation of gene transcription directly by unsaturated fatty acids or is the result of the effect of monounsaturated fatty acids or other components of olive oil on LDL composition and oxidation remains to be ascertained.

Proliferative effect of lipoprotein lipase on human vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) proliferation is a key event in the development and progression of atherosclerotic lesions. Accumulating evidence suggests that lipoprotein lipase (LPL) produced in the vascular wall may exert proatherogenic effects. The aim of the present study was to examine the effect of LPL on VSMC proliferation. Incubation of growth-arrested human VSMCs with purified endotoxin-free bovine LPL for 48 and 72 hours, in the absence of any added exogenous lipoproteins, resulted in a dose-dependent increase in VSMC growth. Addition of VLDLs to the culture media did not further enhance the LPL effect. Treatment of growth-arrested VSMCs with purified human or murine LPL (1 microg/mL) led to a similar increase in cell proliferation. Neutralization of bovine LPL by the monoclonal 5D2 antibody, irreversible inhibition, or heat inactivation of the lipase suppressed the LPL stimulatory effect on VSMC growth. Moreover, preincubation of VSMCs with the specific protein kinase C inhibitors calphostin C and chelerythrine totally abolished LPL-induced VSMC proliferation. In LPL-treated VSMCs, a significant increase in protein kinase C activity was observed. Treatment of VSMCs with heparinase III (1 U/mL) totally inhibited LPL-induced human VSMC proliferation. Taken together, these data indicate that LPL stimulates VSMC proliferation. LPL enzymatic activity, protein kinase C activation, and LPL binding to heparan sulfate proteoglycans expressed on VSMC surfaces are required for this effect. The stimulatory effect of LPL on VSMC proliferation may represent an additional mechanism through which the enzyme contributes to the progression of atherosclerosis.

Dietary fish oil reduces intercellular adhesion molecule 1 and scavenger receptor expression on murine macrophages

During atherogenesis, a pathological accumulation of lipids occurs within aortic intimal macrophages through uptake of oxidised low-density lipoprotein (LDL) via scavenger receptors. Here we investigate whether some of the anti-atherosclerotic effects ascribed to dietary fish oil are mediated through effects on macrophage intercellular adhesion molecule 1 (ICAM-1) and scavenger receptor expression. Mice were fed on a low fat diet (containing 25 g/kg corn oil) or on high fat diets containing 200 g/kg coconut oil, safflower oil or fish oil. Thioglycollate-elicited peritoneal macrophages were analysed for fatty acid composition by gas chromatography. Macrophage scavenger receptor A (MSR-A) type I+type II and ICAM-1 expression were measured by flow cytometry and the levels of mRNA coding for MSR-A type I, MSR-A type II and ICAM-1 were measured by reverse-transcription polymerase chain reaction. Feeding mice diets enriched with different fats resulted in significant changes in the fatty acid profile of macrophages, which reflected the fatty acid compositions of the diets. Macrophages from the fish oil fed mice had the lowest expression of ICAM-1 and MSR-A at the level of both mRNA and cell surface expression. The reduced expression of ICAM-1 and MSR-A on macrophages from mice fed on a fish oil-rich diet supports our hypothesis that part of the protective effect of fish oil against atherosclerosis might be due to an altered macrophage phenotype and function ameliorating macrophage-induced plaque formation.

LDL receptor-related protein mediates uptake of aggregated LDL in human vascular smooth muscle cells

Foam cell formation is a key event in the onset and progression of atherosclerotic lesions. We have previously reported that internalization of aggregated low density lipoproteins (agLDLs) by vascular smooth muscle cells (VSMCs) produces cholesteryl ester (CE) accumulation in these cells. The aim of this study was to analyze whether the low density lipoprotein receptor-related protein (LRP) mediates the uptake of agLDL by VSMCs. First, immunocytochemistry and fluorescence microscopic analysis with the use of anti-LRP antibodies indicated that there was a high expression of LRP in VSMCs. Confocal microscopic analysis with the use of agLDLs labeled with fluorochrome 1,1'-dioctadecyl-3,3,3', 3'-tetramethylindocarbocyanine and anti-LRP antibodies showed the colocalization of agLDL and LRP. The second approach was to analyze the effect of LRP ligands on agLDL internalization; lactoferrin strongly inhibited CE accumulation from agLDLs (85.0+/-5.7% at 25 microg/mL) by impairing agLDL binding. Coincubation of agLDL with anti-LRP antibodies decreased in a dose-dependent manner agLDL-derived CE accumulation (from 20% at 12.5 microg/mL to 80% at 50 microg/mL). The third approach was to evaluate whether antisense LRP oligodeoxynucleotides were able to block agLDL internalization. Treatment of VSMCs with 5 micromol/L antisense LRP oligodeoxynucleotides reduced agLDL-derived CE accumulation by 84+/-2%. In conclusion, these results from immunologic, biochemical, and molecular interventions demonstrate that LRP mediates the binding and internalization of agLDL in human VSMCs. Because LRP is highly expressed in VSMCs and the uptake of 1 LDL aggregate amounts to the deposition of several hundreds of LDL particles, the uptake of agLDL through LRP could have a crucial role for lipid deposition in VSMCs.

Lipoprotein receptors: new roles for ancient proteins

Lipoprotein receptors used to be viewed simply as the means by which cells were supplied with lipids for energy production and membrane synthesis. This perception has now changed dramatically. Megalin, a member of the low density lipoprotein receptor gene family, turns out to mediate the endocytic uptake of retinoids and steroids, thus helping to regulate their biological function. Other members of this receptor family interact with cytosolic signalling proteins, giving this evolutionarily ancient family of receptors an entirely unexpected new role as transducers of extracellular signals.

Metabolism of modified LDL and foam cell formation in murine macrophage-like RAW 264 cells

The uptake of modified low density lipoprotein (LDL) by arterial macrophages is a key event in the atherogenesis. We studied 1) the uptake and degradation of modified LDL, 2) LDL recognition by specific receptors, and 3) the foam cell formation with murine macrophage-like RAW 264 cells in vitro. The cells took up and degraded effectively 125I-labeled acetylated LDL (Ac-LDL) and aggregated LDL (Aggr-LDL). Also oxidized LDL (Ox-LDL) was taken up but it was degraded poorly. The degradation of 125I-Ac-LDL was efficiently competed by both unlabeled Ac-LDL and Ox-LDL, whereas the degradation of 125I-Ox-LDL was partially competed by unlabeled Ox-LDL and Aggr-LDL but not at all by unlabeled Ac-LDL. The incubation with increasing concentrations of Ac-LDL, Aggr-LDL or Ox-LDL resulted in marked foam cell formation in the RAW 264 cells. Ox-LDL was cytotoxic at 500 to 1000 microg/ml concentrations. The results show that RAW 264 cells have at least two classes of receptors for modified lipoproteins: one that recognizes both Ox-LDL and Ac-LDL, and is similar to the scavenger receptors, and another that recognizes Ox-LDL but not Ac-LDL. RAW 264 cells are a convenient model cell line for examining the metabolism of modified lipoproteins, not only that of Ac-LDL but also that of Ox-LDL and Aggr-LDL, and cellular accumulation of lipids derived from modified LDL.

Is oxidized low-density lipoprotein present in vivo?

Basic research has provided strong evidence that oxidation of LDL plays an important role in the pathogenesis of atherosclerosis. Several mechanisms have been identified which can lead to LDL oxidation in vivo. Clinical and epidemiological studies have provided circumstantial evidence that oxidized LDL, as measured by serum autoantibody levels, may be associated with the progression of atherosclerotic vascular disease. This review discusses recent findings regarding the presence of oxidized LDL (ox-LDL) in vivo and the significance of ox-LDL autoantibody measurements as a tool to predict cardiovascular diseases in various patient populations.