Introduction to Scavenger Receptors (home next)
The term 'scavenger receptor' was coined to describe the activity of macrophages which mediates the uptake of modified forms of Low Density Lipoprotein (LDL) in cell culture. The resultant lipid laden macrophages closely resemble the macrophage derived foam cells which are a central feature of the pathology of atherosclerosis (1). Other cell types such as endothelial cells and smooth muscle cells have been shown to have different receptors for modified LDLs and the scavenger receptor family has grown to include cell surface receptors which mediate cholesterol transport by 'scavenging' cholesterol from HDL. Scavenger receptors bind a range of polyanionic ligands other than modified LDL and the true physiological ligands for most scavenger receptors remain to be identified.
The molecular characterisation of scavenger receptors began with the cloning of the bovine Macrophage Scavenger Receptor (MSR) in 1990 (2). This class A scavenger receptor was shown to be a trimeric Type II membrane protein with a broad range of polyanionic ligands. The Macrophage Scavenger Receptor gene was shown to produce two distinct forms of SR-A protein through alternative mRNA splicing, the larger type I receptor (SR-AI) differs from the type II (SR-AII) receptor in having a cysteine linked C terminal extension of 110 amino acids. We have described a third naturally occurring splice variant of the human SR-A gene which encodes a third isoform of the SR-A receptor, hSR-AIII. This novel splice variant encodes a protein with an altered C- terminal amino acid sequence which is trapped within the endoplasmic reticulum and can act as a dominant negative mutant of SR-A activity in transfected cells (3). We have recently shown that a similar type III SR-A splice variant lacking exon 10 is found in aortic atherosclerotic plaques of cholesterol fed rabbits (4).
A structurally related molecule expressed by murine macrophages was cloned in 1995 and termed MARCO (MAcrophage Receptor with COllagenous structure)(5). MARCO shows the same overall domain structure as the SR-A type I receptor but differs in having a longer extracellular domain and completely lacking an alpha-helical coiled coil domain. MARCO has been demonstrated to bind bacteria but not acetylated or oxidised LDL (6). Despite binding a similar range of ligands, including modified LDLs, other macrophage scavenger receptors have very different structures to SR-A and MARCO. Macrophages express the class B scavenger receptors CD36 and SR-B1, the class D scavenger recptor CD68 and the class E scavenger receptor LOX-1. The current roster of macrophage scavenger receptors is listed in the table below:
| Scavenger Receptor |
|
Comment |
|
| Human SR-AI/II |
|
Expressed by foam cells in athero lesions, binds acetylated and oxLDL |
|
| Murine SR-AI/II |
|
Expressed by foam cells in athero lesions, binds acetylated and oxLDL |
|
| Human CD36 |
|
Binds oxLDL, CD36 is expressed by foam cells in athero lesions |
|
| Murine CD36 |
|
Binds oxLDL, CD36 knockout mice have less atherosclerosis |
|
| Mouse SR-B1 |
|
Mediates reverse cholesterol transport |
|
| Human CLA-1 |
|
Human homologue of rodent SR-B1 |
|
| Human CD68 |
|
Stains all macrophages in athero lesions, binds oxLDL |
|
| Murine Cd68 (macrosialin) |
|
Stains all macrophages in athero lesions, binds oxLDL |
|
| Human LOX-1 |
|
Binds oxLDL, expressed by endothelial cells and macrophages |
|
| Human SR-PSOX |
|
Recently described scavenger receptor cloned from human macrophages |
|
Anti SR-A Antibodies (top, home)
We had previously developed a monoclonal antibody (2F8) that specifically recognises the murine SR-A receptor and partially inhibits SR-A mediated macrophage adhesion and uptake of acetylated LDL (24). This monoclonal antibody only recognises the murine SR-A receptor and does not cross react with human SR-A. In order to study the in vivo expression profile of SR-A in human tissues we developed polyclonal antisera by immunising SR-A knockout mice with transfected cells expressing high levels of human SR-A (7). These specific anti human SR-A antisera were used to show that macrophages were the predominant cell type expressing SR-A in human aortic atherosclerotic lesions.
The Role of SR-A in Atherogenesis (top, next, home)
In order to study the role of SR-A in macrophage function in vivo, the laboratory of Professor Tat Kodama generated an SR-A knockout mouse (25). Macrophages of SR-A knockout mice show no staining with the monoclonal antibody 2F8 and exhibit 70-80% reduced uptake of acetylated LDL compared with macrophages from wild type animals. Despite this clear difference in uptake by isolated macrophages, plasma clearance rates of injected acetylated LDL were shown to be similar in gene knockout and wild type animals (26).
The original SR-A gene knockout was performed in Sv129 mouse derived ES cells. To study the effect of SR-AI and SR-AII gene expression on atherogenesis in apoE-deficient mice the SR-A knockout mice had to be bred onto a C57BL6J background before intercrossing to generate mice deficient in both apoE and SR-A. SR-A/apoE double knockout animals exhibit elevated levels of plasma cholesterol as compared to apoE -/- animals but the average size of atherosclerotic lesions in double knockout animals is 60% smaller than that seen in age matched apoE -/- mice suggesting an important proatherogenic role for SR-A in atherosclerotic lesion development in vivo (25).
Analysis of the effect of SR-A gene deletion in other mouse models of atherosclerosis such as the LDL receptor knockout and the ApoE Leiden transgenic mouse suggested that the effect of SR-A gene deletion may not be as marked in other mouse backgrounds (27, 28). However a recent paper from Kodama and collaborators clearly shows that SR-A is proatherogenic in both LDLR-/- and ApoE-/- mice on a C57 background and that the effects of SR-A deletion on atherogenesis can be seen in irradiated animals reconstituted with haematopoietic stem cells of SR-A-/- mice (29, reviewed in 30)
The Role of SR-A in Innate and Acquired Immunity (top, next, home)
The SR-A receptor has a wide range of polyanionic ligands in addition to modified forms of LDL including the lipopolysaccharide of Gram negative bacteria (31) and components of Gram positive bacterial cell walls (32). In the initial description of the SR-A knockout mice it was reported that SR-A-/- mice were more susceptible to infection with the bacterial pathogen Listeria monocytogenes (25). Work from our laboratory has shown that SR-A knockout mice are more susceptible to endotoxic shock than wild type mice, strongly suggesting a role for SR-A in the clearence of bacterial endotoxin from the circulation (33). We have recently demonstrated that a number of bacterial pathogens can bind to SR-A and that SR-A can act as a phagocytic receptor for their uptake by murine macrophages (34, Leanne Peiser and Siamon Gordon, unpublished data).
In addition to its roles in innate immunity there is some evidence that SR-A may contribute to acquired immune responses. SR-A has been reported to be expressed by some dendritic cells in vivo and a recent study by the group of Goran Hansson showed that SR-A-/- mice did not mount an efficient T cell response to maleyated murine serum albumin, a known SR-A ligand (35). It will be interesting to see if SR-A or other macrophage scavenger receptors are involved in the generation of inappropriate acquired immune responses to self antigens. For a recent review of the role of SR-A in innate and acquired immunity see 36.
Adenoviral Expression of Soluble SR-A (top, home)
We have developed expression vectors that encode a secreted soluble form of the human SR-AI protein. We have used these vectors to purify soluble SR-AI protein and to study the effect of soluble SR-AI on atherogenesis, immunity and macrophage adhesion. In collaboration with Johanna Laukkanene and Seppo Yla-Herttuala of the AI Virtanen Institute in Kuopio, Finland, we developed an adenovirus driving expression of soluble human SR-AI. Using this recombinant adenovirus we were able to block acetylated and oxidised LDL uptake by murine macrophages. Importantly, we were able to block modified LDL uptake by macrophages from SR-A knockout mice demonstrating that the secreted soluble form of human SR-A was able to bind modified LDL in such a way as to prevent its uptake by other scavenger receptors (37). Experiments are in progress to determine if overexpression of soluble forms of SR-A can slow the rate of progression of atherosclerosis in vivo.
Summary
Macrophages express at least six structurally different cell surface receptors for modified forms of LDL.
Among the scavenger receptor family the SR-A receptor has the widest range of identified ligands.
SR-A ligands include bacterial cell surface proteins and SR-A knockout mice have been shown to be more susceptible to endotoxic shock.
SR-A knockout mice are less susceptible to diet induced atherosclerosis suggesting a proatherogenic role for SR-A in early atherosclerotic lesions.
1. Brown MS, Goldstein JL: Lipoprotein metabolism in the macrophage: implications for cholesterol deposition in atherosclerosis. Ann Review Biochem 1983; 52: 223-261. (back)
2. Kodama T, Freeman T, Rohrer L, Zabrecky J, Matsudaira P, Krieger M: Type I macrophage scavenger receptor contains alpha-helical and collagen-like coiled coils. Nature 1990; 343: 531-535. (back)
3. Gough PJ, Greaves DR, Gordon S: A naturally occurring isoform of the human macrophage scavenger receptor (SR-A) gene generated by alternative splicing blocks modified LDL uptake. J Lipid Res 1998; 39: 531-543. (back)
4. Hiltunen
TP, Gough PJ, Greaves DR, Gordon S, Ylä-Herttuala
S. Rabbit atherosclerotic lesions express scavenger receptor AIII
mRNA, a naturally occurring splice variant that encodes a non-functional,
dominant negative form of the macrophage scavenger receptor.
Atherosclerosis 2001, in press. (back)
5. Elomaa O, Kangas M, Sahlberg C, Tuukkanen J, Sormunen R, Liakka A, Thesleff I, Kraal G, Tryggvason K: Cloning of a novel bacteria-binding receptor structurally related to scavenger receptors and expressed in a subset of macrophages. Cell 1995 80: 603-609. (back)
6. Elshourbagy NA, Li X, Terrett J, Vanhorn S, Gross MS, Adamou JE, Anderson KM, Webb CL, Lysko PG. Molecular characterization of a human scavenger receptor, human MARCO. Eur J Biochem 2000; 267: 919-926.
7.
Gough PJ, Greaves DR, Suzuki H, Hakkinen
T, Hiltunen MO, Turunen M, Ylä Herttuala S, Kodama T and
Gordon,S.
Analysis of human Macrophage Scavenger Receptor (SR-A) expression
in human atherosclerotic lesions. Arterioscler Thromb Vasc
Biol 1999; 19: 461-471. (back)
8. de Villiers WJ, Smith JD, Miyata M, Dansky HM, Darley E, Gordon S. Macrophage phenotype in mice deficient in both macrophage-colony-stimulating factor (op) and apolipoprotein E. Arterioscler Thromb Vasc Biol 1998; 18: 631-640. (back)
9. Ramprasad MP, Fischer W, Witztum JL, Sambrano GR, Quehenberger O, Steinberg D: The 94- to 97-kDa mouse macrophage membrane protein that recognizes oxidized low density lipoprotein and phosphatidylserine-rich liposomes is identical to macrosialin, the mouse homologue of human CD68. Proc Natl Acad Sci USA 1995; 92: 9580 (back)
10. Pearce SF, Roy P, Nicholson AC, Hajjar DP, Febbraio M, Silverstein RL. Recombinant glutathione S-transferase/CD36 fusion proteins define an oxidized low density lipoprotein-binding domain. J Biol Chem 1998; 273: 34875-34881. (back)
11. Nakata A, Nakagawa Y, Nishida M, Nozaki S, Miyagawa J, Nakagawa T, Tamura R, Matsumoto K, Kameda-Takemura K, Yamashita S, Matsuzawa Y. CD36, a novel receptor for oxidized low-density lipoproteins, is highly expressed on lipid-laden macrophages in human atherosclerotic aorta. Arterioscler Thromb Vasc Biol 1999;19:1333-1339. (back)
12. Boullier A, Gillotte KL, Horkko S, Green SR, Friedman P, Dennis EA, Witztum JL, Steinberg D, Quehenberger O. The binding of oxidized low density lipoprotein to mouse CD36 is mediated in part by oxidized phospholipids that are associated with both the lipid and protein moieties of the lipoprotein. J Biol Chem 2000; 275: 9163-9169. (back)
13. Febbraio M, Podrez EA, Smith JD, Hajjar DP, Hazen SL, Hoff HF, Sharma K, Silverstein RL. Targeted disruption of the class B scavenger receptor CD36 protects against atherosclerotic lesion development in mice. J Clin Invest 2000;105:1049-1056. (back)
14. Huszar D, Varban ML, Rinninger F, Feeley R, Arai T, Fairchild-Huntress V, Donovan MJ, Tall AR. Increased LDL cholesterol and atherosclerosis in LDL receptor-deficient mice with attenuated expression of scavenger receptor B1. Arterioscler Thromb Vasc Biol 2000; 20:1068-1073. (back)
15. Acton S, Rigotti A, Landschulz KT, Xu S, Hobbs HH, Krieger M. Identification of scavenger receptor SR-BI as a high density lipoprotein receptor. Science 1996; 271: 518-520. (back)
16. Krieger M. Charting the fate of the "good cholesterol": identification and characterization of the high-density lipoprotein receptor SR-BI. Annu Rev Biochem 1999; 68: 523-558. (back)
17. Murao K, Terpstra V, Green SR, Kondratenko N, Steinberg D, Quehenberger O. Characterization of CLA-1, a human homologue of rodent scavenger receptor BI, as a receptor for high density lipoprotein and apoptotic thymocytes. J Biol Chem 1997; 272:17551-17557. (back)
18. Ramprasad MP, Terpstra V, Kondratenko N, Quehenberger O, Steinberg D. Cell surface expression of mouse macrosialin and human CD68 and their role as macrophage receptors for oxidized low density lipoprotein. Proc Natl Acad Sci USA 1995; 93: 14833- (back)
19. Ramprasad MP, Fischer W, Witztum JL, Sambrano GR, Quehenberger O, Steinberg D: The 94- to 97-kDa mouse macrophage membrane protein that recognizes oxidized low density lipoprotein and phosphatidylserine-rich liposomes is identical to macrosialin, the mouse homologue of human CD68. Proc Natl Acad Sci (USA) 1995; 92: 9580- (back)
20. Sawamura T, Kume N, Aoyama T, Moriwaki H, Hoshikawa H, Aiba Y, Tanaka T, Miwa S, Katsura Y, Kita T, Masaki T. An endothelial receptor for oxidized low-density lipoprotein. Nature 1997; 386: 73-77. (back)
21. Yoshida H, Kondratenko N, Green S, Steinberg D, Quehenberger O. Identification of the lectin-like receptor for oxidized low-density lipoprotein in human macrophages and its potential role as a scavenger receptor. Biochem J 1998; 334( Pt 1): 9-13. (back)
22. Moriwaki H, Kume N, Kataoka H, Murase T, Nishi E, Sawamura T, Masaki T, Kita T. Expression of lectin-like oxidized low density lipoprotein receptor-1 in human and murine macrophages: upregulated expression by TNF-alpha. FEBS Lett. 1998; 440: 29-32. (back)
23. Shimaoka T, Kume N, Minami M, Hayashida K, Kataoka H, Kita T, Yonehara S. Molecular cloning of a novel receptor for oxidized low density lipoprotein, SR-PSOX, on macrophages. J Biol Chem 2001; in press. (back)
24. Fraser I , Hughes D, Gordon S.Divalent cation-independent macrophage adhesion inhibited by monoclonal antibody to murine scavenger receptor. Nature 1993; 364: 343-346. (back)
25. Suzuki H et al. A role for macrophage scavenger receptors in atherosclerosis and susceptibility to infection. Nature 1997; 386: 292-296. (back)
26. Van Berkel TJ, Van Velzen A, Kruijt JK, Suzuki H, Kodama T. Uptake and catabolism of modified LDL in scavenger-receptor class A type I/II knock-out mice. Biochem J 1998; 331: 29-35. (back)
27. Sakaguchi H, Takeya M, Suzuki H, Hakamata H, Kodama T, Horiuchi S, Gordon S, van der Laan LJ, Kraal G, Ishibashi S, Kitamura N, Takahashi K. Role of macrophage scavenger receptors in diet-induced atherosclerosis in mice. Lab Invest 1998; 78: 423-434. (back)
28. de Winther MP, Gijbels MJ, van Dijk KW, van Gorp PJ, suzuki H, Kodama T, Frants RR, Havekes LM, Hofker MH. Scavenger receptor deficiency leads to more complex atherosclerotic lesions in APOE3Leiden transgenic mice. Atherosclerosis 1999; 144: 315-321. (back)
29. Babaev VR, Gleaves LA, Carter KJ, Suzuki H, Kodama T, Fazio S, Linton MF. Reduced atherosclerotic lesions in mice deficient for total or macrophage-specific expression of scavenger receptor-A. Arterioscler Thromb Vasc Biol 2000; 20: 2593-2599. (back)
30. Mazzone T. Scavenger receptors in atherosclerosis, new answers, new questions. Arterioscler Thromb Vasc Biol 2000; 20: 2506-2508. (back)
31. Hampton RY, Golenbock DT, Penman M, Krieger M, Raetz CR. Recognition and plasma clearance of endotoxin by scavenger receptors. Nature 1991; 352: 342-344. (back)
32. Dunne DW, Resnick D, Greenberg J, Krieger M, Joiner KA. The type I macrophage scavenger receptor binds to gram-positive bacteria and recognizes lipoteichoic acid. Proc Natl Acad Sci U S A 1994; 91:1863-1867. (back)
33. Haworth R, Platt N, Keshav S, Hughes D, Darley E, Suzuki H, Kurihara Y, Kodama T, Gordon S. The macrophage scavenger receptor type A is expressed by activated macrophages and protects the host against lethal endotoxic shock. J Exp Med 1997; 186: 1431-1439. (back)
34. Peiser L, Gough PJ, Kodama T, Gordon S. Macrophage class A scavenger receptor-mediated phagocytosis of Escherichia coli: role of cell heterogeneity, microbial strain, and culture conditions in vitro. Infect Immun 2000; 68:1953-1963. (back)
35. Nicoletti A, Caligiuri G, Tornberg, I, Kodama T, Stemme S, Hansson GK. The macrophage scavenger receptor type A directs modified proteins to antigen presentation. Eur J Immunol 1999; 29: 512-521. (back)
36. Gough PJ, Gordon S. The role of scavenger receptors in the innate immune system. Microbes Infect 2000; 2: 305-311. (back)
37. Laukkanen J, Lehtolainen P, Gough PJ, Greaves DR, Gordon S, Yla-Herttuala S. Adenovirus-mediated gene transfer of a secreted form of human macrophage scavenger receptor inhibits modified low-density lipoprotein degradation and foam-cell formation in macrophages. Circulation 2000; 101:1091-1096. (back)
| http://users.path.ox.ac.uk/~greaves/ | Last updated 3 January 2001 | Copyright David R. Greaves 2001 |
