doi: 10.1161/CIRCRESAHA.107.101105
© 2007 American Heart Association, Inc.
Editorials |
LOX-1 and Atherosclerosis
Proof of Concept in LOX-1–Knockout Mice
From the Department of Vascular Endothelium and Microcirculation, Medical Faculty Carl Gustav Carus, University of Technology Dresden, Germany.
Correspondence to Henning Morawietz, PhD, Department of Vascular Endothelium and Microcirculation, Medical Faculty Carl Gustav Carus, University of Technology Dresden, Fetscherstr. 74, D-01307 Dresden, Germany. E-mail Henning.Morawietz{at}tu-dresden.de
See related article, pages 1634–1642
Key Words: atherosclerosis • LOX-1 • oxidized low-density lipoprotein • transgenic models
An increased plasma level of low-density lipoprotein (LDL) is a well-known risk factor for endothelial dysfunction and atherosclerosis.1 The proatherosclerotic potential of LDL may even increase after oxidative modification to oxidized LDL (oxLDL),2 whose uptake by macrophage scavenger receptors is thought to be a key process in the formation of foam cells, the hallmark of atherosclerotic lesions. However, there is less clarity on oxLDL uptake regulation in endothelial cells and its role in early stages of atherosclerosis.
A decade ago, Tatsuya Sawamura and colleagues discovered the lectin-like oxLDL receptor LOX-1 in endothelial cells.3 LOX-1 is a 50-kDa type II membrane glycoprotein and contains a short N-terminal cytoplasmic domain, a single transmembrane domain, a NECK domain or stalk, and an oxLDL-binding C-terminal extracellular C-type lectin-like domain. LOX-1 assembles on the cell surface in hexamer form or larger, comprising 3 homodimeric LOX-1 molecules bound to oxLDL.4
Although LOX-1 is expressed at very low levels in healthy endothelium, several lines of evidence support a role of LOX-1 in the pathogenesis of atherosclerosis.5 LOX-1 may be upregulated by its own ligand oxLDL or by proinflammatory cytokines in endothelial cells.6 Because proatherosclerotic angiotensin II7,8 and endothelin-19 vasoconstrictors increase endothelial LOX-1 expression and oxLDL uptake, LOX-1 has been suggested as a link between hypertension and atherogenesis.10 In line with these findings, a high-cholesterol diet induced intimal thickening and a marked increase in LOX-1 expression in the endothelium and neointima of rabbits, which was successfully reduced by an angiotensin II receptor type 1 antagonist.11,12 Further support for a proatherosclerotic role of LOX-1 arises from increased atheroma-like lesion areas in mice overexpressing LOX-1 in an apolipoprotein E–null background as compared with non–transgenic littermates after a high-fat diet.13 Nevertheless, there is still no proof of concept for a proatherosclerotic role of LOX-1 by a transgenic deletion model.
In this issue of Circulation Research, Mehta, Sawamura, and colleagues report on the first LOX-1 knockout mice model showing reduced formation of atherosclerotic lesions in a proatherogenic genetic background under a high-cholesterol diet.14 LOX-1–knockout mice are homozygous-viable, possessing a preserved endothelial function after oxLDL treatment. After crossing LOX-1–knockout mice with LDL receptor (LDLR)–knockout mice and feeding the latter a high-cholesterol diet for 18 weeks, atherosclerotic lesion formation was significantly reduced in the aorta of double knockout mice compared with the LDLR-knockout mice. Furthermore, luminal obstruction and intima thickness were reduced in the double knockout mice. Proatherosclerotic and prooxidative signaling and inflammatory response, measured as vascular NF-
B and CD68 expression and p38 MAPK phosphorylation, was increased in LDLR–knockout mice, but not in double knockout animals. In contrast, antioxidative superoxide dismutase activity and antiinflammatory cytokine IL-10 were expressed at lower levels in LDLR-knockout mice compared with double knockout mice, whereas endothelial nitric oxide synthase expression (eNOS) was also preserved in the double knockout mice. These data suggest a LOX-1–dependent balance of pro- and antiatherosclerotic processes in the vessel wall (Figure), whereby deletion or low expression of LOX-1 is associated with low levels of atherosclerosis, low intimal thickness, NF-B, CD68 expression, and p38 MAPK activation. Whereas endothelial function, eNOS, IL-10, and SOD activity is preserved in double knockout animals, a high-fat diet with high LOX-1 expression or with LOX-1 overexpression is associated with a proatherosclerotic, prooxidative, and proinflammatory gene expression pattern together with accelerated atherosclerosis. Therefore, this study provides the proof of concept of a proatherosclerotic role of LOX-1.
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In this context, it has to be considered that the human situation differs from the murine atherosclerosis model regarding lipoprotein metabolism and atherosclerotic lesion structure, although additional studies will be necessary to provide additional support for these findings. Indeed, genetic LOX-1 deletion led to detectable uptake of oxLDL in the vessel wall and atherosclerotic lesion formation in the current study, but apart from LOX-1, additional endothelial oxLDL receptors such as SR-AI/II, CD36, SR-BI, macrosialin/CD68, and SREC have also been discussed,15 and may be responsible for the oxLDL uptake observed in double knockout animals.
Although endothelial LOX-1 is considered to play a major role in the initial phase of atherosclerosis, LOX-1 expression is not completely restricted to endothelial cells. LOX-1 can also be found on a lower but inducible level in vascular smooth muscle cells and macrophages in vitro.16 Additionally, macrophages and smooth muscle cells in the intima of advanced atherosclerotic plaques are positive for LOX-1 expression.17 LOX-1 has indeed been found to promote vascular smooth muscle cell proliferation and foam cell formation in macrophages at later stages of atherosclerosis,18 bind additional ligands such as aged and apoptotic cells, and support adhesion of platelets, leukocytes, and bacteria.5 These additional roles may promote the infiltration of the vessel wall with inflammatory cells even further, leading to vascular complications at later stages of atherosclerosis.
If increased LOX-1 expression promotes atherosclerosis, what therapeutic strategies exist toward reducing LOX-1 expression and atherosclerosis? Pretreatment of endothelial cells with statins reduces oxLDL-induced LOX-1 expression. Upregulation of LOX-1 by high-cholesterol diet in the neointima of rabbit aortas is decreased by treatment with AT1 receptor blocker losartan.12 Although ß1-receptor blockers have been shown to reduce increased vascular LOX-1 expression and accelerate decreased eNOS expression in Dahl salt-sensitive rats,19 a more recent study using the same model has demonstrated that AT1 receptor blockade and NAD(P)H oxidase inhibitor gp91ds-tat normalizes endothelial dysfunction, reactive oxygen species production, and LOX-1 expression.20
What evidence is there for a role of LOX-1 in human atherosclerosis? Increased expression of LOX-1 has been shown in atherosclerotic plaques from human samples.17 Whereas mainly endothelial cells express LOX-1 in the intimal neovasculature of advanced lesions, macrophages and smooth muscle cells in the intima of advanced atherosclerotic plaques have been found positive for LOX-1 expression. Reports on the impact of available pharmacological therapies on LOX-1 expression in patients are still limited, but one report has demonstrated reduced LOX-1 expression in internal mammary arteries in patients receiving ACE inhibitors.8 In the recent prospective clinical Endothelial Protection, AT1 blockade and Cholesterol-Dependent Oxidative Stress (EPAS) trial, statin and AT1 blocker therapy independently and in combination improved the endothelial expression quotient of anti- and proatherosclerotic genes (including eNOS, NADPH oxidase subunit gp91phox, and LOX-1) and endothelial function in internal mammary arteries of patients with coronary artery disease undergoing elective coronary artery bypass surgery.21 However, LOX-1 expression itself was not significantly regulated by either medication.
In view of these findings, antihypertensive and lipid-lowering drugs currently available may not be sufficient to reduce LOX-1 expression and atherosclerosis in a clinically relevant manner. Even if LOX-1 blocking antibodies have been shown to preserve endothelial function in response to oxLDL in the current study, their application in clinical settings is limited. Because the recently defined basic spine structure of LOX-1–mediating ligand recognition accelerates the development of specific nonpeptide LOX-1 inhibitors,22 inhibition of LOX-1 might be an interesting and novel therapeutic strategy in the treatment of atherosclerosis and its clinical implications.
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Acknowledgments |
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Sources of Funding
H.M. is supported by the German Federal Ministry of Education and Research program, NBL3, of the University of Technology Dresden (Professorship of Vascular Endothelium and Microcirculation), the Dr Robert Pfleger Foundation, and the DFG Research Center for Regenerative Therapies Dresden.
Disclosures
None.
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Footnotes |
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The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.
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References |
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 Top References |
|---|
1. Lusis AJ. Atherosclerosis. Nature. 2000; 407: 233–241.[CrossRef][Medline] [Order article via Infotrieve]
2. Stocker R, Keaney JF Jr. Role of oxidative modifications in atherosclerosis. Physiol Rev. 2004; 84: 1381–1478.
3. 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.[CrossRef][Medline] [Order article via Infotrieve]
4. Xie Q, Matsunaga S, Niimi S, Ogawa S, Tokuyasu K, Sakakibara Y, Machida S. Human lectin-like oxidized low-density lipoprotein receptor-1 functions as a dimer in living cells. DNA Cell Biol. 2004; 23: 111–117.[CrossRef][Medline] [Order article via Infotrieve]
5. Mehta JL, Chen J, Hermonat PL, Romeo F, Novelli G. Lectin-like, oxidized low-density lipoprotein receptor-1 (LOX-1): a critical player in the development of atherosclerosis and related disorders. Cardiovasc Res. 2006; 69: 36–45.
6. Kume N, Murase T, Moriwaki H, Aoyama T, Sawamura T, Masaki T, Kita T. Inducible expression of lectin-like oxidized LDL receptor-1 in vascular endothelial cells. Circ Res. 1998; 83: 322–327.
7. Li DY, Zhang YC, Philips MI, Sawamura T, Mehta JL. Upregulation of endothelial receptor for oxidized low-density lipoprotein (LOX-1) in cultured human coronary artery endothelial cells by angiotensin II type 1 receptor activation. Circ Res. 1999; 84: 1043–1049.
8. Morawietz H, Rueckschloss U, Niemann B, Duerrschmidt N, Galle J, Hakim K, Zerkowski HR, Sawamura T, Holtz J. Angiotensin II induces LOX-1, the human endothelial receptor for oxidized low-density lipoprotein. Circulation. 1999; 100: 899–902.
9. Morawietz H, Duerrschmidt N, Niemann B, Galle J, Sawamura T, Holtz J. Induction of the oxLDL receptor LOX-1 by endothelin-1 in human endothelial cells. Biochem Biophys Res Commun. 2001; 284: 961–965.[CrossRef][Medline] [Order article via Infotrieve]
10. Kita T. LOX-1, a possible clue to the missing link between hypertension and atherogenesis. Circ Res. 1999; 84: 1113–1115.
11. Chen M, Kakutani M, Minami M, Kataoka H, Kume N, Narumiya S, Kita T, Masaki T, Sawamura T. Increased expression of lectin-like oxidized low density lipoprotein receptor-1 in initial atherosclerotic lesions of Watanabe heritable hyperlipidemic rabbits. Arterioscler Thromb Vasc Biol. 2000; 20: 1107–1115.
12. Chen H, Li D, Sawamura T, Inoue K, Mehta JL. Upregulation of LOX-1 expression in aorta of hypercholesterolemic rabbits: modulation by losartan. Biochem Biophys Res Commun. 2000; 276: 1100–1104.[CrossRef][Medline] [Order article via Infotrieve]
13. Inoue K, Arai Y, Kurihara H, Kita T, Sawamura T. Overexpression of lectin-like oxidized low-density lipoprotein receptor-1 induces intramyocardial vasculopathy in apolipoprotein E-null mice. Circ Res. 2005; 97: 176–184.
14. Mehta JL, Sanada N, Chen J, Hu C, Dandapat A, Sugawara F, Inoue K, Takeya M, Kawase Y, Jishage K, Suzuki H, Satoh H, Schnackenberg L, Beger R, Hermonat PL, Thomas M, Sawamura T. Deletion of LOX-1 reduces atherogenesis in LDLR knockout mice fed high cholesterol diet. Circ Res. 2007; 100: 1634–1642.
15. Chen M, Masaki T, Sawamura T. LOX-1, the receptor for oxidized low-density lipoprotein identified from endothelial cells: implications in endothelial dysfunction and atherosclerosis. Pharmacol Ther. 2002; 95: 89–100.[CrossRef][Medline] [Order article via Infotrieve]
16. Aoyama T, Chen M, Fujiwara H, Masaki T, Sawamura T. LOX-1 mediates lysophosphatidylcholine-induced oxidized LDL uptake in smooth muscle cells. FEBS Lett. 2000; 467: 217–220.[CrossRef][Medline] [Order article via Infotrieve]
17. Kataoka H, Kume N, Miyamoto S, Minami M, Moriwaki H, Murase T, Sawamura T, Masaki T, Hashimoto N, Kita T. Expression of lectinlike oxidized low-density lipoprotein receptor-1 in human atherosclerotic lesions. Circulation. 1999; 99: 3110–3117.
18. Tanigawa H, Miura S, Matsuo Y, Fujino M, Sawamura T, Saku K. Dominant-negative lox-1 blocks homodimerization of wild-type Lox-1–induced cell proliferation through extracellular signal regulated kinase 1/2 activation. Hypertension. 2006; 48: 294–300.
19. Kobayashi N, Yoshida K, Mita S, Honda T, Hara K, Nakano S, Tsubokou Y, Matsuoka H. Betaxolol stimulates eNOS production associated with LOX-1 and VEGF in Dahl salt-sensitive rats. J Hypertens. 2004; 22: 1397–1402.[CrossRef][Medline] [Order article via Infotrieve]
20. Zhou MS, Hernandez Schulman I, Pagano PJ, Jaimes EA, Raij L. Reduced NAD(P)H oxidase in low renin hypertension: link among angiotensin II, atherogenesis, and blood pressure. Hypertension. 2006; 47: 81–86.
21. Morawietz H, Erbs S, Holtz J, Schubert A, Krekler M, Goettsch W, Kuss O, Adams V, Lenk K, Mohr FW, Schuler G, Hambrecht R. Endothelial Protection, AT1 blockade and Cholesterol-Dependent Oxidative Stress: the EPAS trial. Circulation. 2006; 114: I296–301.[Medline] [Order article via Infotrieve]
22. Ohki I, Ishigaki T, Oyama T, Matsunaga S, Xie Q, Ohnishi-Kameyama M, Murata T, Tsuchiya D, Machida S, Morikawa K, Tate S. Crystal structure of human lectin-like, oxidized low-density lipoprotein receptor 1 ligand binding domain and its ligand recognition mode to OxLDL. Structure. 2005; 13: 905–917.[Medline] [Order article via Infotrieve]
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