Susan Acton

ACE2, the first known human homologue of angiotensin-converting enzyme (ACE), was identified from 5' sequencing of a human heart failure ventricle cDNA library. ACE2 has an apparent signal peptide, a single metalloprotease active site, and a transmembrane domain. The metalloprotease catalytic domains of ACE2 and ACE are 42% identical, and comparison of the genomic structures indicates that the two genes arose through duplication. In contrast to the more ubiquitous ACE, ACE2 transcripts are found only in heart, kidney, and testis of 23 human tissues examined. Immunohistochemistry shows ACE2 protein predominantly in the endothelium of coronary and intrarenal vessels and in renal tubular epithelium. Active ACE2 enzyme is secreted from transfected cells by cleavage N-terminal to the transmembrane domain. Recombinant ACE2 hydrolyzes the carboxy terminal leucine from angiotensin I to generate angiotensin 1-9, which is converted to smaller angiotensin peptides by ACE in vitro and by cardiomyocytes in culture. ACE2 can also cleave des-Arg bradykinin and neurotensin but not bradykinin or 15 other vasoactive and hormonal peptides tested. ACE2 is not inhibited by lisinopril or captopril. The organ- and cell-specific expression of ACE2 and its unique cleavage of key vasoactive peptides suggest an essential role for ACE2 in the local renin-angiotensin system of the heart and kidney. The full text of this article is available at http://www. circresaha.org.

High density lipoprotein (HDL) and low density lipoprotein (LDL) are cholesterol transport particles whose plasma concentrations are directly (LDL) and inversely (HDL) correlated with risk for atherosclerosis. LDL catabolism involves cellular uptake and degradation of the entire particle by a well-characterized receptor. HDL, in contrast, selectively delivers its cholesterol, but not protein, to cells by unknown receptors. Here it is shown that the class B scavenger receptor SR-BI is an HDL receptor. SR-BI binds HDL with high affinity, is expressed primarily in liver and nonplacental steroidogenic tissues, and mediates selective cholesterol uptake by a mechanism distinct from the classic LDL receptor pathway.

Human angiotensin-converting enzyme-related carboxypeptidase (ACE2) is a zinc metalloprotease whose closest homolog is angiotensin I-converting enzyme. To begin to elucidate the physiological role of ACE2, ACE2 was purified, and its catalytic activity was characterized. ACE2 proteolytic activity has a pH optimum of 6.5 and is enhanced by monovalent anions, which is consistent with the activity of ACE. ACE2 activity is increased approximately 10-fold by Cl(-) and F(-) but is unaffected by Br(-). ACE2 was screened for hydrolytic activity against a panel of 126 biological peptides, using liquid chromatography-mass spectrometry detection. Eleven of the peptides were hydrolyzed by ACE2, and in each case, the proteolytic activity resulted in removal of the C-terminal residue only. ACE2 hydrolyzes three of the peptides with high catalytic efficiency: angiotensin II () (k(cat)/K(m) = 1.9 x 10(6) m(-1) s(-1)), apelin-13 (k(cat)/K(m) = 2.1 x 10(6) m(-1) s(-1)), and dynorphin A 1-13 (k(cat)/K(m) = 3.1 x 10(6) m(-1) s(-1)). The ACE2 catalytic efficiency is 400-fold higher with angiotensin II () as a substrate than with angiotensin I (). ACE2 also efficiently hydrolyzes des-Arg(9)-bradykinin (k(cat)/K(m) = 1.3 x 10(5) m(-1) s(-1)), but it does not hydrolyze bradykinin. An alignment of the ACE2 peptide substrates reveals a consensus sequence of: Pro-X((1-3 residues))-Pro-Hydrophobic, where hydrolysis occurs between proline and the hydrophobic amino acid.

Scavenger receptors are integral membrane proteins that mediate the endocytosis of modified lipoproteins. The first of these to be purified and cloned were the type I and II macrophage scavenger receptors (SR-AI and SR-AII; class A scavenger receptors). Subsequently, the cell surface protein CD36 was shown to bind oxidized low density lipoprotein (oxidized LDL). From a Chinese hamster ovary (CHO) cell variant we have cloned by expression the cDNA for a new member of the CD36 family of membrane proteins, SR-BI, whose predicted protein sequence of 509 amino acids is approximately 30% identical to those of the four previously identified family members. Both SR-BI and CD36 displayed high affinity binding for acetylated LDL with an apparent dissociation constant on the order of approximately 5 micrograms of protein/ml. The ligand binding specificities of CD36 and SR-BI, determined by direct binding or competition assays, were similar, but not identical; both bind modified proteins (acetylated LDL, oxidized LDL, maleylated bovine serum albumin), but not the broad array of other polyanions (e.g. fucoidin, polyguanosinic acid, carrageenan) which are ligands of the class A receptors. Thus, SR-BI and CD36 define a second class of scavenger receptors, designated class B. Native LDL, which does not bind to either class A receptors or CD36, unexpectedly bound with high affinity to SR-BI. Northern blot analysis of murine tissues showed that SR-BI was most abundantly expressed in fat and was present at moderate levels in lung and liver. Furthermore, SR-BI mRNA expression was induced upon differentiation of 3T3-L1 cells into adipocytes. Thus, the tissue distribution of expression and ligand binding properties of SR-BI raise the possibility that this cell surface receptor may play an important role in lipid metabolism.

The specific recognition of anionic phospholipids in the outer leaflets of cell membranes and lipoproteins by cell surface receptors may play an important role in a variety of physiologic and pathophysiologic processes (e.g. recognition of damaged or senescent cells by the reticuloendothelial system or lipoprotein homeostasis). Several investigators have described anionic phospholipid binding to cells, and phosphatidylserine (PS) binding to a partially purified ~95-kDa membrane protein has recently been reported (Sambrano, G. R., and Steinberg, D.(1995) Proc. Natl. Acad. Sci. U. S. A. 92, 1396-1400). Using both direct binding and ligand competition assays in transfected cells, we have found that two class B scavenger receptors, SR-BI and CD36, can tightly bind PS and phosphatidylinositol (PI)-containing liposomes (Kd for PS liposome binding to SR-BI is ~15 μg phospholipid/ml or 0.18 nM (mol PS liposomes/l)), but not phosphatidylcholine, phosphatidylethanolamine, or sphingomyelin liposomes. PS and PI liposomes, but not the others, could effectively compete with PS liposomes and modified or native lipoproteins for binding to these receptors. Phosphatidic acid, another anionic phospholipid, could also compete, but was not as effective as PS or PI. Class B scavenger receptors are the first molecularly well-defined, specific cell surface receptors for anionic phospholipids to be described. The specific recognition of anionic phospholipids in the outer leaflets of cell membranes and lipoproteins by cell surface receptors may play an important role in a variety of physiologic and pathophysiologic processes (e.g. recognition of damaged or senescent cells by the reticuloendothelial system or lipoprotein homeostasis). Several investigators have described anionic phospholipid binding to cells, and phosphatidylserine (PS) binding to a partially purified ~95-kDa membrane protein has recently been reported (Sambrano, G. R., and Steinberg, D.(1995) Proc. Natl. Acad. Sci. U. S. A. 92, 1396-1400). Using both direct binding and ligand competition assays in transfected cells, we have found that two class B scavenger receptors, SR-BI and CD36, can tightly bind PS and phosphatidylinositol (PI)-containing liposomes (Kd for PS liposome binding to SR-BI is ~15 μg phospholipid/ml or 0.18 nM (mol PS liposomes/l)), but not phosphatidylcholine, phosphatidylethanolamine, or sphingomyelin liposomes. PS and PI liposomes, but not the others, could effectively compete with PS liposomes and modified or native lipoproteins for binding to these receptors. Phosphatidic acid, another anionic phospholipid, could also compete, but was not as effective as PS or PI. Class B scavenger receptors are the first molecularly well-defined, specific cell surface receptors for anionic phospholipids to be described. Phospholipids are key structural components of cell membranes and lipoproteins. Based on studies of the red blood cell's and other membranes (reviewed in 39Schroit A.J. Zwaal R.F.A. Biochim. Biophys. Acta. 1991; 1071: 313-329Crossref PubMed Scopus (288) Google Scholar), it is generally assumed that there is an asymmetric distribution of phospholipids in the plasma membranes of eukaryotic cells ( 35Roelofsen B. Op den Kamp J.A.F. Hoekstra D. Current Topics in Membranes. Academic Press, San Diego1994: 7-46Google Scholar). The outer leaflet appears to be composed predominantly of neutral zwitterionic phospholipids, e.g. phosphatidylcholine (PC)1( 1The abbreviations used are: PCphosphatidylcholinePAphosphatidic acidPEphosphatidylethanolaminePIphosphatidylinositolPSphosphatidylserineSMsphingomyelinSR-Aclass A scavenger receptorsSR-Bclass B scavenger receptorsSR-Cclass C scavenger receptorshaSR-BIhamster scavenger receptor class B type ILDLlow density lipoproteinAcLDLacetylated LDLOxLDLoxidized LDLM-BSAmaleylated bovine serum albuminFAF-BSAfatty acid free bovine serum albuminDPPCdipalmitoyl phosphatidylcholineCHO cellsChinese hamster ovary cellscpmcounts/min. )and sphingomyelin (SM), while the inner leaflet is greatly enriched in negatively charged phospholipids, such as phosphatidylserine (PS) and phosphatidylinositol (PI). Breakdown of this asymmetry, especially the exposure of increased levels of PS on the outer cell surface, occurs in a variety of physiologic and pathologic states. These include platelet activation (8Bevers E.M. Comfurius P. Zwaal R.F.A. Biochim. Biophys. Acta. 1983; 736: 57-66Crossref PubMed Scopus (501) Google Scholar), cell aging (41Shukla S.D. Hanahan D.J. Arch. Biochem. Biophys. 1982; 214: 335-341Crossref PubMed Scopus (38) Google Scholar), programmed cell death (apoptosis) (12Fadok V.A. Voelker D.R. Campbell P.A. Cohen J.J. Bratton D.L. Henson P.M. J. Immunol. 1992; 148: 2207-2216Crossref PubMed Google Scholar, 13Fadok V.A. Savill J.S. Haslett C. Bratton D.L. Doherty D.E. Campbell P.A. Henson P.M. J. Immunol. 1992; 149: 4029-4035Crossref PubMed Google Scholar), sickle cell anemia (23Kuypers F.A. van den Berg J.J.M. Lubin B.H. Embury S.H. Hebbel R.P. Mohandas N. Steinberg M.H. Sickle Cell Disease: Basic Principles and Clinical Practice. Raven Press, New York1994: 139-152Google Scholar), and Plasmodium falciparum infection of erythrocytes (18Joshi P. Gupta C.M. Br. J. Hematol. 1988; 68: 255-259Crossref PubMed Scopus (32) Google Scholar). Exposure of anionic phospholipids at the external surface of cells can stimulate blood coagulation (Schroit and Zwaal, 1991) and has been proposed to play a critical role in the recognition of damaged or senescent cells by the reticuloendothelial system (37Savill J. Fadok V. Henson P. Haslett C. Immunol. Today. 1993; 14: 131-136Abstract Full Text PDF PubMed Scopus (989) Google Scholar). phosphatidylcholine phosphatidic acid phosphatidylethanolamine phosphatidylinositol phosphatidylserine sphingomyelin class A scavenger receptors class B scavenger receptors class C scavenger receptors hamster scavenger receptor class B type I low density lipoprotein acetylated LDL oxidized LDL maleylated bovine serum albumin fatty acid free bovine serum albumin dipalmitoyl phosphatidylcholine Chinese hamster ovary cells counts/min. Several investigators have described specific anionic phospholipid binding to cells, especially to macrophages, using either direct binding or indirect ligand-competition assays (38Schroit A.J. Fidler I.J. Cancer Res. 1982; 42: 161-167PubMed Google Scholar; 34Ratner S. Schroit A.J. Vinson S.B. Fidler I.J. Proc. Soc. Exp. Biol. Med. 1986; 182: 272-276Crossref PubMed Scopus (22) Google Scholar; 4Allen T.M. Williamson P. Schlegel R.A. Proc. Natl. Acad. Sci. U. S. A. 1988; 85: 8067-8071Crossref PubMed Scopus (139) Google Scholar; 29Nishikawa K. Arai H. Inoue K. J. Biol. Chem. 1990; 265: 5226-5231Abstract Full Text PDF PubMed Google Scholar; 24Lee K.-D. Hong K. Papahadjopoulus D. Biochim. Biophys. Acta. 1992; 1103: 185-197Crossref PubMed Scopus (204) Google Scholar; 14Fukasawa M. Hirota K. Adachi H. Mimura K. Murakami-Murofushi K. Tsujimoto M. Arai H. Inoue K. J. Biol. Chem. 1995; 270: 1921-1927Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar; 36Sambrano G.R. Steinberg D. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1396-1400Crossref PubMed Scopus (278) Google Scholar). Several studies have suggested that scavenger receptors which bind modified lipoproteins, such as oxidized low density lipoprotein (OxLDL) or acetylated LDL (AcLDL), may in some cases function as anionic phospholipid receptors (29Nishikawa K. Arai H. Inoue K. J. Biol. Chem. 1990; 265: 5226-5231Abstract Full Text PDF PubMed Google Scholar; 14Fukasawa M. Hirota K. Adachi H. Mimura K. Murakami-Murofushi K. Tsujimoto M. Arai H. Inoue K. J. Biol. Chem. 1995; 270: 1921-1927Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar; for a review of scavenger receptors see 22Krieger M. Herz J. Annu. Rev. Biochem. 1994; 63: 601-637Crossref PubMed Scopus (1071) Google Scholar). The only receptor for anionic phospholipids to be identified to date is a partially purified ~95-kDa membrane protein from macrophages that was shown to directly bind PS and OxLDL using a ligand blotting assay (9de Rijke Y.B. van Berkel T.J.C. J. Biol. Chem. 1994; 269: 824-827Abstract Full Text PDF PubMed Google Scholar; 32Ottnad E. Parthasarathy S. Sambrano S.R. Ramprasad M.P. Quehenberger O. Kondratenko N. Green S. Steinberg D. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1391-1395Crossref PubMed Scopus (140) Google Scholar; 36Sambrano G.R. Steinberg D. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 1396-1400Crossref PubMed Scopus (278) Google Scholar). The primary sequence of this protein has not been reported; thus, it is not known if it is a member of one of the three classes of scavenger receptors, A, B, and C, which have been previously defined based on their distinctive binding activities and primary sequences (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar; 33Pearson A. Lux A. Krieger M. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 4056-4060Crossref PubMed Scopus (204) Google Scholar). Two of these, class A and class C scavenger receptors (SR-A, SR-C), are expressed almost exclusively on mammalian and embryonic Drosophila melanogaster macrophages, respectively (27Naito M. Kodama T. Matsumoto A. Doi T. Takahashi K. Am. J. Pathol. 1991; 139: 1411-1423PubMed Google Scholar; 10Elomaa O. Kangas M. Sahlberg C. Tuukkanen J. Sormunen R. Liakka A. Thesleff I. Kraal G. Tryggvason K. Cell. 1995; 80: 603-609Abstract Full Text PDF PubMed Scopus (413) Google Scholar; 7Bell M.D. Lopez-Gonzalez R. Lawson L. Hughes D. Fraser I. Gordon S. Perry V.H. J. Neurocytol. 1994; 23: 605-613Crossref PubMed Scopus (99) Google Scholar; 33Pearson A. Lux A. Krieger M. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 4056-4060Crossref PubMed Scopus (204) Google Scholar). 25Lee K.-D. Pitas R.E. Papahadjopoulus D. Biochim. Biophys. Acta. 1992; 1111: 1-6Crossref PubMed Scopus (42) Google Scholar and 14Fukasawa M. Hirota K. Adachi H. Mimura K. Murakami-Murofushi K. Tsujimoto M. Arai H. Inoue K. J. Biol. Chem. 1995; 270: 1921-1927Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar have shown that at least two types of class A receptors are unlikely to be involved in the cellular uptake of anionic phospholipid liposomes, and we have recently obtained data suggesting that the Drosophila SR-C (33Pearson A. Lux A. Krieger M. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 4056-4060Crossref PubMed Scopus (204) Google Scholar) may not be able to recognize anionic phospholipids.2( 2A. Pearson and A. Rigotti, unpublished data. ) In the current study, we have examined the binding of phospholipids to class B scavenger receptors (SR-B). SR-Bs are members of the CD36 superfamily of proteins, including CD36 itself and SR-BI (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). SR-Bs are expressed in a variety of cells and tissues, including macrophages and endothelial cells (1Abumrad N.A. El-Maghrabi M.R. Amri E.-Z. Lopez E. Grimaldi P.A. J. Biol. Chem. 1993; 268: 17665-17668Abstract Full Text PDF PubMed Google Scholar; and see 16Greenwalt D.I. Lipsky R.H. Ockenhouse C.F. Ikeda H. Tandon N.N. Jamieson G.A. Blood. 1992; 80: 1105-1115Crossref PubMed Google Scholar for review). The highest levels of expression have been reported to be in adipose tissue (1Abumrad N.A. El-Maghrabi M.R. Amri E.-Z. Lopez E. Grimaldi P.A. J. Biol. Chem. 1993; 268: 17665-17668Abstract Full Text PDF PubMed Google Scholar; 3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). In addition to binding modified LDL (11Endemann G. Stanton L.W. Madden K.S. Bryant C.M. White R.T. Protter A.A. J. Biol. Chem. 1993; 268: 11811-11816Abstract Full Text PDF PubMed Google Scholar; 3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar; 28Nicholson A.C. Frieda S. Pearce A. Silverstein R.L. Arterioscl. Thromb. Vasc. Biol. 1995; 15: 269-275Crossref PubMed Scopus (226) Google Scholar), CD36 binds M-BSA (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar), thrombospondin (5Asch A.S. Barnwell J. Silverstein R.L. Nachman R.L. J. Clin. Invest. 1987; 79: 1054-1061Crossref PubMed Scopus (398) Google Scholar), collagen (43Tandon N.N. Kralisz U. Jamieson G.A. J. Biol. Chem. 1989; 264: 7576-7583Abstract Full Text PDF PubMed Google Scholar), long chain fatty acids (1Abumrad N.A. El-Maghrabi M.R. Amri E.-Z. Lopez E. Grimaldi P.A. J. Biol. Chem. 1993; 268: 17665-17668Abstract Full Text PDF PubMed Google Scholar; 25Lee K.-D. Pitas R.E. Papahadjopoulus D. Biochim. Biophys. Acta. 1992; 1111: 1-6Crossref PubMed Scopus (42) Google Scholar), and P. falciparum-infected erythrocytes (31Oquendo P. Hundt E. Lawler J. Seed B. Cell. 1989; 58: 95-101Abstract Full Text PDF PubMed Scopus (412) Google Scholar). Although its physiologic functions are unknown, CD36 may serve as an adhesion molecule, a component in fatty acid transport (1Abumrad N.A. El-Maghrabi M.R. Amri E.-Z. Lopez E. Grimaldi P.A. J. Biol. Chem. 1993; 268: 17665-17668Abstract Full Text PDF PubMed Google Scholar), a signal transduction molecule (30Ockenhouse C.F. Magowan C. Chulay J.D. J. Clin. Invest. 1989; 84: 468-475Crossref PubMed Scopus (109) Google Scholar; 17Huang M.-M. Bolen J.B. Barnwell J.W. Shattil S.J. Brugge J.S. Proc. Natl. Acad. Sci. U. S. A. 1991; 88: 7844-7848Crossref PubMed Scopus (368) Google Scholar), and a receptor for senescent neutrophils (37Savill J. Fadok V. Henson P. Haslett C. Immunol. Today. 1993; 14: 131-136Abstract Full Text PDF PubMed Scopus (989) Google Scholar). A striking feature of SR-BI, not shared by CD36 or other well-defined scavenger receptors, is its ability to bind native LDL with high affinity (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). Thus, SR-BI may play an important role in lipid metabolism. Here, we show that SR-BI and CD-36 specifically bind anionic PS and PI liposomes with high affinity, and are, therefore, the first molecularly well-defined cell surface receptors for anionic phospholipids to be described. Reagents (and sources) were: acetic anhydride (Mallinckrodt, Inc., Paris, KY); egg phosphatidylcholine, egg phosphatidic acid, liver phosphatidylinositol, brain phosphatidylserine, egg phosphatidylethanolamine, and brain sphingomyelin (Avanti Polar Lipids, Inc., Alabaster, AL); polycarbonate membrane filters (Poretics Corp., Livermore, CA); sodium [125I]iodide and 1,2-dipalmitoyl-L-3-phosphatidyl[N-methyl-3H]choline ([3H]DPPC) (Amersham Corp.); DEAE-dextran (Pharmacia Biotech Inc.); Ham's F-12 medium, Dulbecco's modified Eagle's medium, fetal bovine serum, and trypsin/EDTA (JRH Biosciences, Lenexa, KS); and penicillin/streptomycin, glutamine, and G418 sulfate (Life Technologies, Inc.). All other reagents and supplies were purchased from Sigma or were obtained as described previously (Krieger, 1983). Human LDL, AcLDL, 125I-labeled LDL, and 125I-labeled AcLDL (90-300 cpm/ng protein) were prepared essentially as described previously (15Goldstein J.L. Basu S.K. Brown M.S. Methods Enzymol. 1983; 98: 241-260Crossref PubMed Scopus (1335) Google Scholar; 21Krieger M. Cell. 1983; 33: 413-422Abstract Full Text PDF PubMed Scopus (52) Google Scholar; 3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). Unilamellar liposomes were made by extrusion through polycarbonate membranes (42Szoka F. Olson F. Heath T. Vail W. Mayhew E. Papahadjopoulos D. Biochim. Biophys. Acta. 1980; 601: 559-571Crossref PubMed Scopus (453) Google Scholar). Phospholipid liposomes were prepared containing the indicated phospholipid, phosphatidylcholine, and free cholesterol in a molar ratio of 1:1:1. The lipids were mixed in chloroform and dried by rotary evaporation for 30 min. For preparation of radiolabeled liposomes, 50-75 μCi of [3H]DPPC (62 Ci/mmol) were added to the lipid mixtures before drying. The dried lipids were resuspended in 150 mM NaCl, 0.1 mM EDTA, 10 mM HEPES, pH 7.5 (Buffer A). Once the samples were fully hydrated, they were extruded through 0.1-μm pore size polycarbonate membranes using a mini-extruder device (Avanti Polar Lipids, Inc., Alabaster, AL). After extrusion, liposomes were dialyzed against Buffer A and then stored under nitrogen at 4°C were used of The phospholipid was by the of G.R. J. Biol. Chem. Full Text PDF PubMed Google The of liposomes, which were from either two or three using with a Inc., were: and The of phospholipid liposome was as for cholesterol and phospholipid in are assumed to be and respectively ( Scopus Google an distribution of the components the liposomes, of the surface was phospholipid, or phospholipid Based on an phospholipid of a liposome of 10 μg phospholipid/ml to nM in liposome and cells were in as described previously M. Cell. 1983; 33: 413-422Abstract Full Text PDF PubMed Scopus (52) Google Scholar; S.L. D. M. L. J. Krieger M. J. Biol. Chem. 1993; 268: Full Text PDF PubMed Google Scholar, 3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). is a cell LDL receptor in an essentially LDL ( Krieger M. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar; Krieger M. Cell. Biol. 1986; PubMed Scopus Google Scholar; K. Krieger M. Cell. Biol. 1986; PubMed Scopus Google Scholar). and cells were in with Ham's F-12 containing fetal bovine serum, and mM A). cells are cells which hamster SR-BI (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar) and were in A with G418 cells were in Dulbecco's modified Eagle's with fetal bovine serum, and mM All with cells were at in a cells were transfected with either 3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar) or P. Hundt E. Lawler J. Seed B. Cell. 1989; 58: 95-101Abstract Full Text PDF PubMed Scopus (412) Google Scholar) as described previously (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). were on transfected on and on in in C with mM sodium binding assays were on and cells were in in A or B, and the assay was on cells were prepared as described assays were as described previously (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar), with the were on for 30 with the indicated radiolabeled or in F-12 containing fatty acid free bovine serum albumin and 10 mM HEPES, pH with or for at 4°C with were then with mM NaCl, pH containing by one with The cells were then with 0.1 and and protein were made as described previously (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). The high affinity ligand binding activities shown the obtained in the and of an of the indicated binding of liposomes to cells was generally For for the shown in binding from μg to μg of the For liposomes, binding from μg to μg of the The binding are expressed as of 125I-labeled protein or of phospholipids from of cell if phospholipids could bind to we prepared PS liposomes ratio radiolabeled with of phosphatidylcholine (62 Ci/mmol) and examined their binding at 4°C to cells and transfected cells which that there was high affinity μg and binding to the transfected cells but binding to the cells that the phospholipid and cholesterol were in liposomes containing of we the (mol of PS to be PS binding was of it was not by data not on the phospholipid of the liposomes. In to that of liposomes, the binding of radiolabeled liposomes was low and to binding to These indicated that phospholipids can bind to and that this binding on the of the phospholipid The of the binding was by the competition for binding by liposomes of ratio I that the anionic phospholipids PS and PI were effective while the zwitterionic and as as were another anionic phospholipid, was able to compete, but not as effectively as PS and of and binding to expressed in transfected cells in a previously that can bind both native LDL and AcLDL with high affinity (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). that PS and PI liposomes of the binding of to in transfected cells at μg while at as high as μg also that but not liposomes of the binding of native to in the of liposome of binding was almost to that for the of binding The of PS of binding on the PS of the liposomes. that by μg phospholipid/ml increased as the of PS in mixed liposomes increased from to of phospholipid, with the PS was These competition that anionic phospholipids to at a to or with the of native and modified LDL binding and that of the liposomes with may binding anionic phospholipid of the molar ratio on liposome of binding to at of cell the binding of μg cpm/ng protein) to cells was as described under in the of liposomes μg ratio of with the indicated of phosphatidylserine as of The binding the of The was cell is a member of the CD36 superfamily of (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). and CD36 acid sequence to are class B scavenger receptors which can bind a variety of modified maleylated they bind to of the other which are of the class A and C scavenger receptors (3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar). as was the for the binding of to expressed in cells was by PI and but not by PI was a Thus, CD36, as as can serve as a receptor for anionic for the in the ability to recognize the receptor reported by 14Fukasawa M. Hirota K. Adachi H. Mimura K. Murakami-Murofushi K. Tsujimoto M. Arai H. Inoue K. J. Biol. Chem. 1995; 270: 1921-1927Abstract Full Text Full Text PDF PubMed Scopus (19) Google Scholar is to that of The phospholipid and modified lipoprotein binding of the class B scavenger receptors (11Endemann G. Stanton L.W. Madden K.S. Bryant C.M. White R.T. Protter A.A. J. Biol. Chem. 1993; 268: 11811-11816Abstract Full Text PDF PubMed Google Scholar; 3Acton S.L. Scherer P.E. Lodish H.F. Krieger M. J. Biol. Chem. 1994; 269: 21003-21009Abstract Full Text PDF PubMed Google Scholar; the OxLDL receptor and Steinberg, 1995; Rijke and van and the AcLDL receptor reported by 29Nishikawa K. Arai H. Inoue K. J. Biol. Chem. 1990; 265: 5226-5231Abstract Full Text PDF PubMed Google Scholar are not in addition to SR-BI and CD36 there may be other receptors for the structural and of class B receptors to previously described anionic phospholipid receptor activities and the physiologic of anionic phospholipid binding by class B scavenger receptors, with to the recognition of lipoproteins and damaged or senescent cells, cells were on the binding of liposomes μg cpm/ng or μg cpm/ng protein) at 4°C was as described under in the or of the indicated phospholipid liposomes μg The the of two The of for the binding of liposomes and were and cell for D. and C. for and liposome preparation the of this B. Seed for of the CD36, and A. Pearson and D. for