A role for macrophage scavenger receptors in atherosclerosis and susceptibility to infection

Macrophage phenotype in mice deficient in both macrophage-colony-stimulating factor (op) and apolipoprotein E

Mice deficient in both macrophage-colony-stimulating factor (M-CSF, op) and apolipoprotein E (apoE) have elevated cholesterol levels but are protected from atherosclerosis. To assess the contribution of macrophage (Mphi) phenotypic heterogeneity and scavenger receptor (SR-A) expression to this seeming paradox, we characterized the Mphi phenotype by immunohistochemistry in these animals. Lesion size was determined in animals fed a chow or Western-type diet, and lipoprotein clearance studies were performed in vivo. Op0/E0 mice have fourfold smaller aortic root lesions than op2/E0 animals despite 2.5-fold higher total plasma cholesterol levels. Mphis in atherosclerotic lesions of op2/E0 mice constitute a predominantly recruited and M-CSF-dependent population. In addition, Mphis in different locations in plaques show phenotypic heterogeneity. SR-A expression in op0/E0 mice is reduced in proportion to the decrease in Mphi numbers, and M-CSF is thus not an essential requirement for SR-A expression in vivo. M-CSF-deficient mice degrade injected AcLDL , showing an adequate level of SR-A activity present in vivo. In contrast, beta-VLDL clearance in op0/E0 mice is decreased, implicating monocytes/Mphis in its catabolism. There is prominent lipid accumulation in op2/E0 Kupffer cells and hepatocytes but not in M-CSF-independent Kupffer Mphis from op0/E0 mice. SR-A, while abundantly expressed on both Kupffer cells and sinusoidal endothelial cells in op2/E0 mice, remains mainly on sinusoidal endothelial cells in op0/E0 mice. This may explain preservation of SR-A activity in these animals. Our findings clearly illustrate the importance of both M-CSF and M-CSF-dependent monocytes/Mphis in maintaining cholesterol homeostasis and in atherogenesis.

Expression of lipoprotein receptors in atherosclerotic lesions

Atherosclerosis is characterized by the presence of lipid-loaded cells which are derived from macrophages and smooth muscle cells. Several lipoprotein receptors may be involved in cellular lipid uptake. These receptors include: scavenger receptor(s); LDL receptor-related protein/alpha2-macroglobulin receptor (LRP); LDL receptor; and VLDL receptor. With the exception of the LDL receptor, all of these receptors are expressed in atherosclerotic lesions. While scavenger receptors are mostly expressed in macrophages, the LRP and VLDL receptor may play an important role in mediating lipid uptake in smooth muscle cells. It is evident that no single receptor pathway is solely responsible for the increased lipid uptake in lesion cells but several redundant mechanisms may contribute to the uptake and degradation of lipoproteins in atherosclerotic lesions.

Expression of LDL receptor, VLDL receptor, LDL receptor-related protein, and scavenger receptor in rabbit atherosclerotic lesions: marked induction of scavenger receptor and VLDL receptor expression during lesion development

BACKGROUND

Atherosclerotic lesions contain foam cells that arise from monocyte-macrophages and smooth muscle cells (SMCs) by excessive uptake of lipoproteins. There are many candidate receptors for the lipid accumulation, such as LDL receptor (LDLR), VLDL receptor (VLDLR), LDL receptor-related protein (LRP), and scavenger receptors (SRs). However, little quantitative information exists on the expression of these receptors in normal and atherosclerotic arteries.

METHODS AND RESULTS

Competitive reverse transcription-polymerase chain reaction and in situ hybridization were used for the studies in New Zealand White (NZW) and Watanabe heritable hyperlipidemic (WHHL) rabbit aortic intima-medias. NZW rabbits were fed a 1% cholesterol diet for 0 (control group), 3, 6, or 14 weeks. LDLR mRNA expression was low in aortic intima-medias of all groups. Of the analyzed receptors, LRP had the highest expression in the control group, and its mRNA was induced threefold in the 14-week group, the aortas of which had extensive lesions. SR expression was low and VLDLR expression moderate in the control group. Both receptors were highly induced during cholesterol feeding (SRs, 3-fold and 270-fold induction; VLDLR, 15-fold and 100-fold induction in the 3-week and 14-week groups, respectively). Comparable results were obtained from WHHL rabbits: high basal LRP mRNA in normal intima-medias; moderate induction of LRP and marked induction of SRs and VLDLR in fatty streaks and fatty plaques. In situ hybridization indicated that LRP and VLDLR were expressed in SMCs and macrophages. VLDLR expression was also observed in endothelial cells. SR expression was detected only in macrophages.

CONCLUSIONS

SR and VLDLR mRNAs were highly induced in atherosclerotic lesions. VLDLR and LRP may be involved in the formation of both SMC-and macrophage-derived foam cells, whereas SRs play an important role in lipid uptake in macrophages.

Transgenic mouse models to study the role of APOE in hyperlipidemia and atherosclerosis

Transgenic technologies have provided a series of very useful mouse models to study hyperlipidemia and atherosclerosis. Normally, mice carry cholesterol mainly in the high density lipoprotein (HDL) sized lipoproteins, and have low density lipoprotein (LDL) and very low density lipoprotein (VLDL) cholesterol levels. These low LDL and VLDL levels are due to the very rapid metabolism of remnant clearance in mice, which hamper metabolic studies. In addition, due to the lack of atherogenic lipoproteins, mice will not readily develop atherosclerosis. This situation has changed completely, because to date, most known genes in lipoprotein metabolism have been used in transgenesis to obtain mice in which genes have been silenced or overexpressed. These experiments have yielded many mouse strains with high plasma lipid levels and a greater susceptibility for developing atherosclerosis. One of the most widely used strains are knock-out mice deficient for apoE, which is one of the central players in VLDL metabolism. Subsequently, a wide variety of other transgenic studies involving APOE have been performed elucidating the role of apoE and apoE mutants in lipolysis, remnant clearance, cellular cholesterol efflux and atherogenesis. In addition, the APOE mouse models are excellent tools for the development of gene therapy for hyperlipidemias.

Appearance of cross linked proteins in human atheroma and rat pre-fibrotic liver detected by a new monoclonal antibody

A new monoclonal antibody against malondialdehyde (MDA)-treated low density lipoprotein (LDL) was raised using homogenate of human atheroma as immunogen. This antibody, DLH2, was obtained by selecting the clones which did not react to native LDL but did react to copper-induced oxidized LDL (OxLDL). DLH2 showed a greater reactivity to MDA-LDL than to OxLDL. When LDL was treated with various aldehyde containing reagents, treatment of LDL with glutaraldehyde or MDA greatly increased the reactivity to the antibody, while LDL treated with 2,4-hexadienal or 4-hydroxynonenal was not reactive. Among many proteins tested, high density lipoprotein, bovine serum albumin and hemoglobin showed significant reactivity to DLH2 after they were treated with MDA or glutaraldehyde. When low density and high density lipoproteins treated with MDA were subjected to immunoblot analysis, newly formed products larger than the original apolipoproteins were detected with the antibody, suggesting that this antibody recognizes aggregated proteins with divalent short chain cross linkers. The antigenic materials were shown by immunohistochemical analysis to be present in foamy macrophages in human atheromatous lesions. DLH2 antigen did not colocalize either with apolipoprotein B. Furthermore, we found a massive accumulation of the antigenic material in Kupffer cells in the liver of rats treated with alcohol and carbonyl iron, a model of hepatic fibrosis due to oxidative stress. These results suggest the presence of cross linked proteins in damaged tissues.

Scavenger receptors mediate adhesion of activated B lymphocytes

The scavenger receptor (SR-A) is considered to play a role in host defense by scavenging endotoxins, oxidized proteins, and denatured or otherwise modified self components, which are routed toward degradation in macrophages. Recent data suggest that SR-A also functions as an adhesion molecule. Our previous finding of SR-A expression by high endothelial cells of venules and on follicular dendritic cells in peripheral lymph nodes prompted us to investigate whether SR-A can act as an addressin for lymphocytes. We describe here that activated B cells adhere to CHO cells transfected with either the type I or type II isoform of SR-A. In contrast, resting B cells isolated from peripheral blood did not adhere to SR-A transfected CHO cells. Other types of leukocytes did not bind to SR-A. The adhesive properties of B lymphocytes in different stages of activation were further explored using lymphoma cell lines of the B cell lineage. The in vitro EBV-transformed B cell line IARC171 showed enhanced adhesiveness to SR-A, whereas the Burkitt lymphoma cell lines, BL41, Rael, and Bl16 did not. The SR-A-dependent adhesion of B-lymphoblasts occurred both at 37 and 4 degrees C, suggesting that it was not dependent on cell metabolism. The known polyanionic ligands for SR-A, fucoidan, and acetylated low density lipoprotein, which bind to a positively charged portion of the collagen-like domain of SR-A, did not inhibit adhesion. This finding suggests that SR-A mediates adhesion of activated B lymphocytes through a binding site that differs from the one that binds polyanionic ligands. Together, our data suggest that SR-A plays a role in the recruitment of activated B cells into lymph nodes and inflammatory lesions by acting as an adhesion molecule for such cells.

Structure of the human macrophage MARCO receptor and characterization of its bacteria-binding region

The primary structure of human macrophage receptor with collagenous structure (MARCO) was determined from cDNA clones and shown to be highly similar to that of mouse (Elomaa, O., Kangas, M., Sahlberg, C. , Tuukkanen, J., Sormunen, R., Liakka, A., Thesleff, I., Kraal, G., and Tryggvason, K. (1995) Cell 80, 603-609). Features such as potential carbohydrate attachment sites in the extracellular spacer domain III and the interruption of Gly-Xaa-Yaa repeats in the collagenous domain IV were conserved between the two species. However, the human MARCO polypeptide chain lacked the intracellular cysteine present in mouse, as well as two extracellular cysteines that form interchain disulfide bonds in the murine protein. In situ hybridization showed MARCO to be strongly expressed in macrophages of several tissues of human individuals with sepsis. No expression was observed in other cell types. The bacteria-binding region of MARCO was determined in binding studies with full-length and truncated variants of MARCO, and localized to a region proximal to the cysteine-rich part of the COOH-terminal domain V. The intrachain disulfide bond pattern of domain V was established showing that these bonds are between cysteine pairs C1-C5, C2-C6, and C3-C4.

Evidence that the lipid moiety of oxidized low density lipoprotein plays a role in its interaction with macrophage receptors

The binding of oxidatively damaged red blood cells (OxRBCs) to resident mouse peritoneal macrophages correlates with an increase in phosphatidylserine on the external leaflet of the plasma membrane. Liposomes rich in phosphatidylserine can inhibit this binding and also the binding of certain apoptotic cells. We have shown previously that oxidized low density lipoproteins (OxLDL) also can inhibit the binding of OxRBCs to resident mouse peritoneal macrophages. The present studies show that microemulsions prepared from the lipids extracted from OxLDL are very effective in inhibiting the binding of OxRBCs and also, to a lesser extent, of apoptotic thymocytes to macrophages. OxRBC binding was also inhibited by cholesterol phospholipid liposomes containing oxidized 1-stearoyl-2-linoleoyl-phosphatidylcholine. The binding and uptake of 125I-labeled OxLDL were also strongly inhibited by microemulsions of the lipids extracted from OxLDL and by cholesterol phospholipid liposomes containing oxidized 1-palmitoyl-2-arachidonoyl-phosphatidylcholine. Earlier studies have shown that the delipidated protein moiety of OxLDL can competitively inhibit macrophage binding of intact OxLDL, implicating the protein moiety as an effective receptor-binding domain of OxLDL with respect to some macrophage scavenger receptors. The present studies suggest that the lipid moiety of OxLDL may also play a role.

Reduced expression of the macrophage scavenger receptors in macrophage-like cell mutants resistant to brefeldin A

Brefeldin A (BFA)-resistant mutants, JB15, JB23 and JB33, were isolated from mutagenized murine macrophage-like (J774) cells and their modified low density lipoprotein (LDL) metabolism was studied. When JB23 cells, the most resistant clone, were incubated with acetylated LDL, intracellular accumulation of cholesteryl esters (CE) was reduced by 31% as compared with J774 cells. The cell-association of 125I-acetyl-LDL with, and subsequent endocytic degradation by JB23 cells were reduced by 40-60% compared with J774 cells. Western and Northern blot analyses showed that the protein and mRNA levels of the macrophage scavenger receptors (MSR) were reduced by 68% and 55% respectively in JB23 cells as compared with those in J774 cells. These results indicate that a putative BFA-target molecule(s) might regulate MSR gene expression as well as macrophage-derived foam cell formation.

Liver uptake of phosphodiester oligodeoxynucleotides is mediated by scavenger receptors

The therapeutic activity of antisense oligodeoxynucleotides (ODNs) often is impaired due to premature degradation and poor ability to reach the (intra)cellular target. In this study, we addressed the in vivo fate of ODNs and characterized the major sites responsible for the clearance of intravenously injected phosphodiester ODN. On injection into rats, 32P-ODNs (miscellaneous sequences and GT-containing ODNs with variable G content) are rapidly cleared from the bloodstream (t1/2 = 0.6-0.7 min), with the liver being the main site of elimination. The contribution of the liver to ODN clearance depended on its sequence and varied considerably. Hepatic uptake tended to be lower for G-rich ODNs as a result of increased bone marrow uptake. Within the liver, both Kupffer cells (KC) and endothelial cells (EC) were responsible for 32P-ODN uptake. To elucidate the mechanism of liver uptake, 32P-ODN binding studies using isolated EC and KC were performed. Binding to both cell types seemed to be saturable, of moderate affinity, and mediated by a membrane-bound protein. The inhibition profiles of 32P-ODN binding to EC and KC by various (poly)anions were essentially equal and corresponded closely to those of 125I-acetylated low-density lipoprotein. In summary, the results indicate that scavenger receptors on nonparenchymal liver and bone marrow cells contribute to the elimination of ODNs from the bloodstream. Minor changes in ODN sequence markedly affect receptor recognition, resulting in considerable shifts in the biodistribution of antisense ODNs.

Ligand binding to macrophage scavenger receptor-A induces urokinase-type plasminogen activator expression by a protein kinase-dependent signaling pathway

Macrophage scavenger receptor-type A (MSR-A) has been implicated in the transmission of cell signals and the regulation of diverse cellular functions (Falcone, D. J., and Ferenc, M. J. (1988) J. Cell. Physiol. 135, 387-396; Falcone, D. J., McCaffrey, T. A., and Vergilio, J. A. (1991) J. Biol. Chem. 266, 22726-22732; Palkama, T. (1991) Immunology 74, 432-438; Krieger, M., and Herz, J. (1994) Annu. Rev. Biochem. 63, 601-637); however, the signaling mechanisms are unknown. In studies reported here, we demonstrate that binding of both lipoprotein and non-lipoprotein ligands to MSR-A induced protein tyrosine phosphorylation and increased protein kinase C (PKC) activity leading to up-regulated urokinase-type plasminogen activator (uPA) expression. Specifically, the binding of acetylated low density lipoprotein and fucoidan to MSR-A in human THP-1 macrophages triggered tyrosine phosphorylation of many proteins including phospholipase C-gamma1 and phosphatidylinositol-3-OH kinase. Inhibitors of tyrosine kinase dramatically reduced MSR-induced protein tyrosine phosphorylation and PKC activity. Moreover, inhibitors of tyrosine kinase and PKC reduced uPA activity expressed by THP-1 macrophages exposed to MSR-A ligands. The intracellular signaling response for tyrosine phosphorylation following ligand binding was further demonstrated by using the stable MSR-transfected Bowes cells that express surface MSR-A. These findings establish for the first time a signaling pathway induced by ligand binding to MSR-A and suggest a molecular model for the regulation of macrophage uPA expression by specific ligands of the MSR-A.

The role of the mouse macrophage scavenger receptor in myelin phagocytosis

Myelin phagocytosis during Wallerian degeneration and immune-mediated demyelination depends on the action of mononuclear cells of the monocyte/macrophage system. The present study investigated the role of the macrophage scavenger receptor, a trimeric membrane glycoprotein, in myelin uptake by macrophages. Two in vitro models of myelin phagocytosis were used: an organ culture model of mouse peripheral nerves exposed to cocultured macrophages and a quantitative flow cytometric assay. Different concentrations of the monoclonal rat anti-mouse scavenger receptor antibody (2F8) were applied to these systems to selectively block the macrophage scavenger receptor. Concentration-dependent effects on macrophage migration and myelin uptake were seen when the macrophage scavenger receptor was blocked by the antibody 2F8. Low concentrations reduced myelin phagocytosis by the invading macrophages; higher concentrations completely abolished macrophage invasion of the nerves. Using a quantitative flow cytometric assay it was also shown that the 2F8 antibody inhibits phagocytosis of myelin in a dose-dependent manner. These data indicate that the macrophage scavenger receptor is involved in myelin phagocytosis by macrophages.

The macrophage scavenger receptor type A is expressed by activated macrophages and protects the host against lethal endotoxic shock

During gram-negative bacterial infections, lipopolysaccharide (LPS) stimulates primed macrophages (Mphi) to release inflammatory mediators such as tumor necrosis factor (TNF)-alpha, which can cause hypotension, organ failure, and often death. Several different receptors on Mphi have been shown to bind LPS, including the type A scavenger receptor (SR-A). This receptor is able to bind a broad range of polyanionic ligands such as modified lipoproteins and lipoteichoic acid of gram-positive bacteria, which suggests that SR-A plays a role in host defense. In this study, we used mice lacking the SR-A (SRKO) to investigate the role of SR-A in acquired immunity using a viable bacillus Calmette Guérin (BCG) infection model. We show that activated Mphi express SR-A and that this molecule is functional in assays of adhesion and endocytic uptake. After BCG infection, SRKO mice are able to recruit Mphi to sites of granuloma formation where they become activated and restrict BCG replication. However, infected mice lacking the SR-A are more susceptible to endotoxic shock and produce more TNF-alpha and interleukin-6 in response to LPS. In addition, we show that an antibody which blocks TNF-alpha activity reduces LPS-induced mortality in these mice. Thus SR-A, expressed by activated Mphi, plays a protective role in host defense by scavenging LPS as well as by reducing the release by activated Mphi of proinflammatory cytokines. Modulation of SR-A may provide a novel therapeutic approach to control endotoxic shock.

Heritability analysis of lipids and three gene loci in twins link the macrophage scavenger receptor to HDL cholesterol concentrations

We studied 100 healthy monozygotic and 72 dizygotic twin pairs (mean age, 34 +/- 14 years) to test for genetic influences on blood lipids and to examine relevant gene loci. Total cholesterol (TC), LDL cholesterol (LDL-C), HDL cholesterol (HDL-C), and triglyceride (TG) levels were determined after a 12-hour fast. Zygosity was determined with the use of microsatellite markers. Heritability estimates were conducted by using the lisrel 8 program; a sib-pair analysis was conducted by using the sibpal program. Linear regression analyses were carried out between identical-by-descent status and squared within-pair differences of TC, LDL-C, HDL-C, and TG values. Heritability estimates of the lipid serum concentrations ranged from .58 to .66. A significant linkage relationship was found for HDL-C (P = .008) and TGs (P = .05) with D8S261 on chromosome 8p. However, no linkage was found between any of the lipid variables and the lipoprotein lipase gene locus (LPL GZ14/15 and D8S282). Because D8S261 is located approximately halfway between the LPL and macrophage scavenger receptor genes, we examined the nearby markers D8S549 and D8S1731. Linkage was found for HDL-C and D8S549 (P = .001) and for HDL-C and D8S1731 (P = .04). On the other hand, we found no linkage between the LDL receptor gene locus and LDL-C serum concentrations nor between the LPL gene locus and the various other lipid fractions. Our data suggest a significant influence of the macrophage scavenger receptor gene locus on HDL-C and weak influence on TG levels. We suggest that inherited variability in the macrophage scavenger receptor gene has an influence on serum lipid concentrations.

The lysine cluster in the collagen-like domain of the scavenger receptor provides for its ligand binding and ligand specificity

Scavenger receptors bind modified low-density lipoproteins (LDL) on a collagen-like domain which possesses a lysine cluster at the carboxy end. We previously constructed a receptor model peptide containing the lysine cluster. In the present study, we evaluated the ligand specificity of the receptor model peptide. It selectively bound modified-LDLs, and not LDL. The binding of acetylated-LDL (Ac-LDL) was inhibited by dextran sulfate, fucoidan, and sulfatides in a manner similar to that of the natural receptor. Both polyguanylic and polyinosinic acids inhibit the Ac-LDL binding whereas polycytidylic acid did not. These results indicate that the lysine cluster in the collagen-like domain has important roles in both ligand binding and ligand specificity.

Listeria monocytogenes infection of P388D1 macrophages results in a biphasic NF-kappaB (RelA/p50) activation induced by lipoteichoic acid and bacterial phospholipases and mediated by IkappaBalpha and IkappaBbeta degradation

As previously reported, Listeria monocytogenes infection of P388D1 macrophages results in a rapid induction of NF-kappaB DNA-binding activity. Here we show that this induction of NF-kappaB activity occurs in a biphasic mode: first, a transient, IkappaBalpha degradation-dependent phase of activity, also induced by the nonvirulent species Listeria innocua, which is mediated by binding of the bacteria to the macrophage, or by adding Listeria-derived lipoteichoic acid to the macrophage; the second persistent phase of activation is only markedly induced when the bacteria enter the cytoplasm of the host cell and express the virulence genes plcA and plcB, encoding two phospholipases. We suggest that products of the enzymatic activity of phospholipases directly interfere with host cell signal transduction pathways, thus leading to persistent NF-kappaB activation via persistent IkappaBbeta degradation.

Listeria monocytogenes in laboratory mice: a model of short-term infectious and pathogenic processes controllable by regulated protective immune responses

The mammalian immune system is an integrated network of tissue, leukocytes and effector and regulatory molecules. All these components operate i) to maintain the proper structure of and processes expressed by each tissue. and ii) to protect the hosts from those microorganisms that generally invade them as part of their life cycle. Among the invading microorganisms. Listeria monocytogenes (L. monocytogenes) can persist as a live organism independent of the host, and is, thus, able to drive short-term infectious and pathogenic processes that are controlled by integrated innate and adaptive protective immune responses driven by CD8 and CD4 type 1 T lymphocytes acting on non-T non-B leukocytes. Although the effector functions and the fine specificity of T lymphocytes have been more and more characterized, an understanding of the precise regulation of both leukocyte traffic and T-lymphocyte migration depends on knowledge of the early tissue distribution of L. monocytogenes, points that are addressed in this review.

Macrophages lacking scavenger receptor A show a decrease in binding and uptake of acetylated low-density lipoprotein and of apoptotic thymocytes, but not of oxidatively damaged red blood cells

Macrophage binding of oxidatively damaged red blood cells (OxRBC) and apoptotic thymocytes correlates in many instances with a loss of phospholipid bilayer asymmetry, i.e., with an increase in expression of phosphatidylserine on the outer leaflet of the plasma membrane. Oxidatively modified LDL (OxLDL) can compete for the binding of these ligands to macrophages. However, the receptor(s) responsible remains to be identified. The present studies show that mouse peritoneal macrophages totally lacking scavenger receptor A (SRA) bound OxRBC just as effectively as wild-type macrophages, whereas their binding and uptake of acetyl LDL was reduced by more than 80%. Binding of apoptotic thymocytes and binding of OxLDL were also reduced, but only by 20-30%. We conclude that SRA is not involved in the recognition of phosphatidylserine-rich membranes but contributes to the binding of OxLDL and apoptotic thymocytes. The binding of OxRBC was almost totally calcium-dependent, whereas the binding of apoptotic thymocytes was not, suggesting that the mechanisms involved in their uptake by macrophages under these conditions were different.

Oxidized or acetylated low density lipoproteins are rapidly cleared by the liver in mice with disruption of the scavenger receptor class A type I/II gene

Oxidized low density lipoprotein (LDL) and acetyl LDL are recognized by the scavenger receptor class A type I/II (SR-AI/II) on macrophages and liver endothelial cells. Several investigators have suggested that there are additional receptors specific for oxidized LDL, but characterization of these alternate receptors for oxidized LDL and evaluation of their quantitative importance in uptake of oxidized LDL has been difficult because of overlapping ligand specificity with SR-AI/II. The purpose of this study was to determine the importance of SR-AI/II in the removal of modified LDL from the bloodstream in vivo. The clearance rate of oxidized LDL from plasma in normal mice was very rapid, and > 90% of injected dose was removed from the blood within 5 min. Clearance rates of oxidized LDL were equally high in SR-AI/II knockout mice, indicating that this receptor is not required for removal of oxidized LDL from plasma. Surprisingly, there was no difference in the clearance rate of acetyl LDL in wild-type and SR-AI/II knockout animals. The plasma clearance of radioiodinated acetyl LDL was almost fully blocked by a 50-fold excess of unlabeled acetyl LDL, but the latter only inhibited oxidized LDL clearance by approximately 5%. Both modified LDLs were cleared mostly by the liver, and there was no difference in the tissue distribution of modified LDL in control and knockout mice. Studies in isolated nonparenchymal liver cells showed that Kupffer cells accounted for most of the uptake of oxidized LDL. Extensively oxidized LDL and LDL modified by exposure to fatty acid peroxidation products were efficient competitors for the uptake of labeled oxidized LDL by SR-AI/II-deficient Kupffer cells, while acetyl LDL and malondialdehyde-modified LDL were relatively poor competitors.

Regulation of scavenger receptor expression in smooth muscle cells by protein kinase C: a role for oxidative stress

Phorbol esters increase scavenger-receptor mRNA expression and receptor activity in smooth muscle cells (SMCs). Our present results demonstrate that activation of protein kinase C (PKC) mediates this increase in receptor expression. This conclusion is based on the findings that (1) phorbol esters induced translocation of PKC-alpha from the cytosol to the membrane fraction; (2) PKC inhibitors blocked the effect of phorbol esters on receptor expression; (3) diacylglycerol, a physiological PKC agonist, enhanced scavenger-receptor activity; and (4) in cotransfected human SMCs, constitutively active PKC-alpha stimulated the expression of a reporter gene under control of the scavenger-receptor promoter. Phorbol ester treatment of SMCs increased intracellular reactive oxygen, and the increase in receptor activity was reduced 30% by the antioxidant N-acetyl cysteine (NAC), suggesting a role for reactive oxygen in phorbol ester-mediated receptor regulation. Furthermore, direct treatment of SMCs with reactive oxygen species increased scavenger-receptor activity. In rabbit SMCs, 100 micromol/L H2O2 alone slightly increased scavenger-receptor mRNA and protein expression. In combination, 100 micromol/L H2O2 and 10 micromol/L vanadate, which promotes formation of OH and enhances the inhibition of protein tyrosine phosphatase by H2O2, increased scavenger-receptor mRNA expression 25-fold in rabbit SMCs and 8-fold in human SMCs. NAC reduced the effect of H2O2 and vanadate by 93%. The increase in SMC scavenger-receptor expression occurs at the level of gene transcription. Receptor mRNA half-life was unchanged after treatment with either phorbol esters or reactive oxygen (approximately 14.5 hours), and induction by phorbol esters increased SMC scavenger-receptor mRNA transcription, as determined by nuclear run-on assay. Multiple cytokines and growth factors that contribute to the generation of reactive oxygen species are present in atherosclerotic lesions. These factors may all contribute to the upregulation of SMC scavenger-receptor activity and therefore to the formation of smooth muscle foam cells.