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(Arteriosclerosis, Thrombosis, and Vascular Biology. 2002;22:710.)
© 2002 American Heart Association, Inc.

Letters to the Editor

Expression of the Novel Scavenger Receptor SR-PSOX in Cultured Aortic Smooth Muscle Cells and Umbilical Endothelial Cells

Oliver Hofnagel; Birgit Luechtenborg; Gabriele Plenz; Horst Robenek

Institute for Arteriosclerosis Research
University of Muenster
Muenster, Germany

To the Editor:

In the November issue of Arteriosclerosis, Thrombosis, and Vascular Biology, Minami et al¹ demonstrated expression of the novel scavenger receptor for phosphatidylserine and oxidized lipoprotein (SR-PSOX) in lipid-laden macrophages accumulated in the intima of human atherosclerotic lesions. Because SR-PSOX seems to be identical to the membrane-anchored chemokine CXCL16,^2,3 which may play a dual role in inflammation and homeostasis, Minami et al¹ discussed the potential regulation of SR-PSOX by pro-inflammatory cytokines. Although the authors did not detect SR-PSOX in smooth muscle cells (SMCs) and endothelial cells (ECs), they did discuss the possible expression of SR-PSOX in these cell types. Until now, only the expression of the scavenger receptors SR-AI/II,⁴ CD36,⁵ and LOX-1⁶ in SMCs has been described.

In our studies on the formation of SMC-derived foam cells during atherogenesis, we have focused on the expression of scavenger receptors,⁷ including SR-PSOX, in SMCs and ECs. We have also investigated the influence of cytokines on the expression of SR-PSOX in SMCs. Reverse transcriptase–polymerase chain reaction (PCR; primers for human SR-PSOX: 5'-TACACGAGGTTCCAGCTCCT-3' and 5'-GGGGGCTGGT- AGGAAGTAAA-3', porcine SR-PSOX: 5'-TATGTGGAGGCAGCAG- TGAC-3' and 5'-CTGCAGGGTAGATGGCAGAT-3') was performed on total RNA from cultured human and porcine aortic SMCs and human umbilical vein endothelial cells (HUVECs). PCR was performed at 94°C (45 seconds), 58°C (60 seconds), and 72°C (60 seconds) for 20 to 40 cycles in the linear area of amplification. The sequences of SR-PSOX products were confirmed by sequence analysis. ß-Actin served as the internal standard.

Thus, reverse transcriptase–PCR demonstrated the expression of SR-PSOX mRNA in porcine and human aortic SMCs and HUVECs as well as in human monocyte-derived macrophages, which were used as a positive control.⁸ In human SMCs, SR-PSOX was more strongly expressed than LOX-1 and SR-AI/II (29 to 31 versus 38 to 40 cycles). The SR-PSOX–mediated uptake of oxidized LDL in synthetic SMCs is possibly as strong as or even stronger than that mediated by LOX-1 or SR-AI/II.

Because SR-PSOX/CXCL16 possibly plays a role in inflammation^2,3 and because several scavenger receptors are regulated by pro-inflammatory cytokines,⁹ we investigated the influence of tumor necrosis factor- (TNF-), interleukin-1 (IL-1), and interferon- (IFN-) on the expression of SR-PSOX in SMCs. However, in contrast to the scavenger receptor SR-AI/II, which is stimulated by TNF-, IL-1, and IFN-,^10,11 and to LOX-1, which is stimulated by TNF-,¹² SR-PSOX mRNA expression was not influenced by these cytokines (Figure, IFN- not shown). The reported induction of LOX-1 expression by TNF-¹² was confirmed (data not shown) and served as a positive control. SR-PSOX does not share any homology with other scavenger receptors, except a mucin-like domain also found in SR-CI¹³ and CD68/macrosialin,¹⁴ and obviously SR-PSOX is regulated via different mechanism than those described for SR-AI/II and LOX-1.

View larger version (39K): [in this window] [in a new window]   SR-PSOX mRNA expression in porcine aortic SMCs after incubation with IL-1 or TNF-. Cells were incubated with the indicated concentrations of IL-1 or TNF- for 14 hours (A) or with 10 ng/mL IL-1 or TNF- for the indicated time period (B). Top, Result of a representative experiment. Bottom, Summary of data from 3 independently performed experiments (mean±SEM).

In summary, we demonstrate that the scavenger receptor SR-PSOX, which is expressed in human atherosclerotic lesions and may be involved in foam cell formation, is not only expressed in macrophages, but also in cultured SMCs and HUVECs. Moreover, our data indicate that SR-PSOX is governed by pathways other than those reported for SR-AI/II and LOX-1. Further studies will elucidate the functional role of SR-PSOX and its regulation in SMCs and ECs.

References

1. Minami M, Kume N, Shimaoka T, Kataoka H, Hayashida K, Akiyama Y, Nagata I, Ando K, Nobuyoshi M, Hanyuu M, Komeda M, Yonehara S, Kita T. Expression of SR-PSOX, a novel cell-surface scavenger receptor for phosphatidylserine and oxidized LDL in human atherosclerotic lesions. Arterioscler Thromb Vasc Biol. 2001; 21: 1796–1800.[Abstract/Free Full Text]
2. Matloubian M, David A, Engel S, Ryan JE, Cyster JG. A transmembrane CXC chemokine is a ligand for HIV-coreceptor Bonzo. Nat Immunol. 2000; 1: 298–304.[CrossRef][Medline] [Order article via Infotrieve]
3. Wilbanks A, Zondlo SC, Murphy K, Mak S, Soler D, Langdon P, Andrew DP, Wu L, Briskin M. Expression cloning of the STRL33/BONZO/TYMSTR ligand reveals elements of CC, CXC, and CX3C chemokines. J Immunol. 2001; 166: 5145–5154.[Abstract/Free Full Text]
4. Pitas RE. Expression of the acetyl low density lipoprotein receptor by rabbit fibroblasts and smooth muscle cells: up-regulation by phorbol esters. J Biol Chem. 1990; 265: 12722–12727.[Abstract/Free Full Text]
5. Matsumoto K, Hirano K, Nozaki S, Takamoto A, Nishida M, Nakagawa-Toyama Y, Janabi MY, Ohya T, Yamashita S, Matsuzawa Y. Expression of macrophage (Mphi) scavenger receptor, CD36, in cultured human aortic smooth muscle cells in association with expression of peroxisome proliferator activated receptor-gamma, which regulates gain of Mphi-like phenotype in vitro, and its implication in atherogenesis. Arterioscler Thromb Vasc Biol. 2000; 20: 1027–1032.[Abstract/Free Full Text]
6. Draude G, Hrboticky N, Lorenz RL. The expression of the lectin-like oxidized low-density lipoprotein receptor (LOX-1) on human vascular smooth muscle cells and monocytes and its down-regulation by lovastatin. Biochem Pharmacol. 1999; 57: 383–386.[CrossRef][Medline] [Order article via Infotrieve]
7. Rommeswinkel M, Severs NJ, Koster M, Robenek H. Repression of the macrophage scavenger receptor in macrophage-smooth muscle cell heterokaryons. Arterioscler Thromb Vasc Biol. 1995; 15: 601–611.[Abstract/Free Full Text]
8. Shimaoka T, Kume N, Minami M, Hayashida K, Kataoka H, Kita T, Yonehara S. Molecular cloning of a novel scavenger receptor for oxidized low density lipoprotein, SR-PSOX, on macrophages. J Biol Chem. 2000; 275: 40663–40666.[Abstract/Free Full Text]
9. Steinbrecher UP. Receptors for oxidized low density lipoprotein. Biochim Biophys Acta. 1999; 1436: 279–298.[Medline] [Order article via Infotrieve]
10. Li H, Freeman MW, Libby P. Regulation of smooth muscle cell scavenger receptor expression in vivo by atherogenic diets and in vitro by cytokines. J Clin Invest. 1995; 95: 122–133.[Medline] [Order article via Infotrieve]
11. Inaba T, Yamada N, Gotoda T, Shimano H, Shimada M, Momomura K, Kadowaki T, Motoyoshi K, Tsukada T, Morisaki N, Saito Y, Yoshida S, Takaku F, Yazaki Y. Expression of M-CSF receptor encoded by c-fms on smooth muscle cells derived from arteriosclerotic lesion. J Biol Chem. 1992; 267: 5693–5699.[Abstract/Free Full Text]
12. Kume N, Moriwaki H, Kataoka H, Minami M, Murase T, Sawamura T, Masaki T, Kita T. Inducible expression of LOX-1, a novel receptor for oxidized LDL, in macrophages and vascular smooth muscle cells. Ann N Y Acad Sci. 2000; 902: 323–327.[Abstract/Free Full Text]
13. Pearson A, Lux A, Krieger M. Expression cloning of dSR-CI, a class C macrophage-specific scavenger receptor from Drosophila melanogaster. Proc Natl Acad Sci U S A. 1995; 92: 4056–4060.[Abstract/Free Full Text]
14. 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 U S A. 1996; 93: 14833–14838.[Abstract/Free Full Text]

Noriaki Kume

Department of Geriatric Medicine
Graduate School of Medicine
Kyoto University
Kyoto, Japan

In Response:

SR-PSOX/CXCL16 is expressed by macrophages, dendritic cells, and CD19⁺ B lymphocytes.^1–3 Its expression can be upregulated, to some extent, by pro-inflammatory stimuli, such as bacterial endotoxin, tumor necrosis factor-, and ligation to CD40.^2,3 Currently, we are trying to elucidate whether other cell types, including vascular endothelial and smooth muscle cells, express SR-PSOX. Our preliminary data, so far, have shown that SR-PSOX mRNA expression is undetectable by northern blot analysis in cultured human vascular smooth muscle or endothelial cells, suggesting that levels of SR-PSOX expression in these cell types seem to be much lower than those in macrophages. In addition, our immunohistochemical studies have demonstrated that expression of SR-PSOX in endothelial cells or smooth muscle cells was undetectable, although SR-PSOX expression in macrophages⁴ and LOX-1 expression in intimal smooth muscle cells⁵ were prominent in adjacent sections of human carotid and coronary atherosclerotic lesions. LOX-1 expression also was observed in endothelial cells covering early atherosclerotic lesions.⁵ Therefore, we understand that endothelial or smooth muscle expression of SR-PSOX, if any, may be much less prominent than that of LOX-1 in human atherogenesis. Our recent studies have suggested a role of LOX-1 in oxidized LDL–induced apoptosis of vascular smooth muscle cells.^6,7 LOX-1 seems to be one of oxidized LDL receptors abundantly expressed by activated smooth muscle cells.^6,7 SR-PSOX expression, in contrast, may be more confined to macrophages in atherogenesis. The reverse transcriptase–polymerase chain reaction data shown in the letter by Hofnagel et al do not include appropriate negative controls, such as omission of reverse transcription. Therefore, contamination of genomic DNA in their RNA preparations might possibly affect their results. We do not agree with the conclusion by Hofnagel et al that SR-PSOX is more strongly expressed than LOX-1 in human smooth muscle cells, which is based on reverse transcriptase–polymerase chain reaction analysis alone. Further studies by use of neutralizing monoclonal antibodies and gene knockout mice would tell us more concerning the roles of SR-PSOX in macrophages and other cell types in a variety of pathophysiological settings including atherogenesis.

References

1. Shimaoka T, Kume N, Minami M, Hayashida K, Kataoka H, Kita T, Yonehara S. Molecular cloning of a novel scavenger receptor for phosphatidylserine and oxidized low density lipoprotein, SR-PSOX, on macrophages. J Biol Chem. 2000; 275: 40663–40666.[Abstract/Free Full Text]
2. Matloubian M, David A, Engel S, Ryan JE, Cyster JG. A transmembrane CXC chemokine is a ligand for HIV-coreceptor Bonzo. Nat Immunol. 2000; 1: 298–304.[CrossRef][Medline] [Order article via Infotrieve]
3. Wilbanks A, Zondlo SC, Murphy K, Mak S, Soler D, Langdon P, Andrew DP, Wu L, Briskin M. Expression cloning of the STRL33/BONZO/TYMSTR ligand reveals elements of CC, CXC, and CX3C chemokines. J Immunol. 2001; 166: 5145–5154.[Abstract/Free Full Text]
4. Minami M, Kume N, Shimaoka T, Kataoka H, Hayashida K, Akiyama Y, Nagata I, Ando K, Nobuyoshi M, Hanyuu M, Komeda M, Yonehara S, Kita T. Expression of SR-PSOX, a novel cell-surface receptor for oxidized low density lipoprotein, in human atherosclerotic lesions. Arterioscler Thromb Vasc Biol. 2001; 21: 1796–1800.[Abstract/Free Full Text]
5. Kataoka H, Kume N, Miyamoto S, Minami M, Moriwaki H, Murase T, Sawamura T, Masaki T, Kita T. Expression of lectin-like oxidized low density lipoprotein receptor-1 in human atherosclerotic lesions. Circulation. 1999; 99: 3110–3117.[Abstract/Free Full Text]
6. Kataoka H, Kume N, Miyamoto S, Minami M, Morimoto M, Hayashida K, Hashimoto N, Kita T. Oxidized low density lipoprotein (Ox-LDL) modulates Bax/Bcl-2 through lectin-like Ox-LDL receptor-1 in vascular smooth muscle cells. Arterioscler Thromb Vasc Biol. 2001; 21: 955–960.[Abstract/Free Full Text]
7. Kume N, Kita T. Roles of lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) and its soluble forms in atherogenesis. Curr Opin Lipidol. 2001; 12: 419–423.[CrossRef][Medline] [Order article via Infotrieve]

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This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Request Permissions Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Hofnagel, O. Articles by Kume, N. Search for Related Content PubMed PubMed Citation Articles by Hofnagel, O. Articles by Kume, N. Related Collections Lipid and lipoprotein metabolism Mechanism of atherosclerosis/growth factors Pathophysiology Smooth muscle proliferation and differentiation