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Thematic Review Series: Lipid Transfer Proteins Scavenger receptor B type 1: expression, molecular regulation, and cholesterol transport function

  • Wen-Jun Shen
    Affiliations
    Geriatric Research, Education, and Clinical Research Center (GRECC), Veterans Affairs Palo Alto Health Care System, Palo Alto, CA 94304 and Division of Endocrinology, Gerontology, and Metabolism, Stanford University School of Medicine, Stanford, CA 94305
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  • Shailendra Asthana
    Affiliations
    Drug Discovery Research Center (DDRC), Translational Health Science and Technology Institute (THSTI), NCR Biotech Science Cluster, Faridabad 121001, Haryana, India
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  • Fredric B. Kraemer
    Affiliations
    Geriatric Research, Education, and Clinical Research Center (GRECC), Veterans Affairs Palo Alto Health Care System, Palo Alto, CA 94304 and Division of Endocrinology, Gerontology, and Metabolism, Stanford University School of Medicine, Stanford, CA 94305
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  • Author Footnotes
    Salman Azhar
    Correspondence
    To whom correspondence should be addressed.
    Affiliations
    Geriatric Research, Education, and Clinical Research Center (GRECC), Veterans Affairs Palo Alto Health Care System, Palo Alto, CA 94304 and Division of Endocrinology, Gerontology, and Metabolism, Stanford University School of Medicine, Stanford, CA 94305
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  • Author Footnotes
    2 For simplicity, we use the term dimerization here to include the multiple forms of the SR-B1 protein; i.e., dimers and high order oligomers.
Open AccessPublished:May 02, 2018DOI:https://doi.org/10.1194/jlr.R083121
      Cholesterol is required for maintenance of plasma membrane fluidity and integrity and for many cellular functions. Cellular cholesterol can be obtained from lipoproteins in a selective pathway of HDL-cholesteryl ester (CE) uptake without parallel apolipoprotein uptake. Scavenger receptor B type 1 (SR-B1) is a cell surface HDL receptor that mediates HDL-CE uptake. It is most abundantly expressed in liver, where it provides cholesterol for bile acid synthesis, and in steroidogenic tissues, where it delivers cholesterol needed for storage or steroidogenesis in rodents. SR-B1 transcription is regulated by trophic hormones in the adrenal gland, ovary, and testis; in the liver and elsewhere, SR-B1 is subject to posttranscriptional and posttranslational regulation. SR-B1 operates in several metabolic processes and contributes to pathogenesis of atherosclerosis, inflammation, hepatitis C virus infection, and other conditions. Here, we summarize characteristics of the selective uptake pathway and involvement of microvillar channels as facilitators of selective HDL-CE uptake. We also present the potential mechanisms of SR-B1-mediated selective cholesterol transport; the transcriptional, posttranscriptional, and posttranslational regulation of SR-B1; and the impact of gene variants on expression and function of human SR-B1. A better understanding of this unique pathway and SR-B1's role may yield improved therapies for a wide variety of conditions.
      Cholesterol is required for the maintenance of plasma membrane fluidity and integrity, as well as many cellular functions. Tissues such as liver, adrenal gland, and gonads have a special requirement for cholesterol, which is used as a substrate for product biosynthesis (
      • Russell D.W.
      Fifty years of advances in bile acid synthesis and metabolism.
      ,
      • Miller W.L.
      Early steps in steroidogenesis: intracellular cholesterol trafficking.
      ). In many species, this cholesterol is obtained from plasma lipoproteins by a unique pathway in which circulating lipoproteins bind to the surface of the biosynthetic cells and contribute their cholesteryl esters (CEs) to the cells by a process known as the “selective” cholesterol uptake pathway (
      • Glass C.
      • Pittman R.C.
      • Weinstein D.B.
      • Steinberg D.
      Dissociation of tissue uptake of cholesterol ester from that of apoprotein A-I of rat plasma high density lipoprotein: selective delivery of cholesterol ester to liver, adrenal, and gonad.
      ). This pathway differs from the classic endocytic LDL receptor pathway (
      • Brown M.S.
      • Goldstein J.L.
      A receptor-mediated pathway for cholesterol homeostasis.
      ) in that CEs, predominantly contained within HDL, are taken into the cell without the uptake and lysosomal degradation of the HDL particle itself (
      • Glass C.
      • Pittman R.C.
      • Weinstein D.B.
      • Steinberg D.
      Dissociation of tissue uptake of cholesterol ester from that of apoprotein A-I of rat plasma high density lipoprotein: selective delivery of cholesterol ester to liver, adrenal, and gonad.
      ,
      • Quarfordt S.
      • Hanks J.
      • Jones R.S.
      • Shelburne F.
      The uptake of high density lipoprotein cholesteryl ester in the perfused rat liver.
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      • Spicher M.
      • Azhar S.
      Morphological evidence that high density lipoproteins are not internalized by steroid-producing cells during in situ organ perfusion.
      ,
      • Glass C.
      • Pittman R.C.
      • Civen M.
      • Steinberg D.
      Uptake of high-density lipoprotein-associated apoprotein A-I and cholesterol esters by 16 tissues of the rat in vivo and by adrenal cells and hepatocytes in vitro.
      ,
      • Reaven E.
      • Chen Y.D.
      • Spicher M.
      • Hwang S.F.
      • Mondon C.E.
      • Azhar S.
      Uptake of low density lipoproteins by rat tissues. Special emphasis on the luteinized ovary.
      ,
      • Azhar S.
      • Stewart D.
      • Reaven E.
      Utilization of cholesterol-rich lipoproteins by perfused rat adrenals.
      ,
      • Stein Y.
      • Dabach Y.
      • Hollander G.
      • Halperin G.
      • Stein O.
      Metabolism of HDL-cholesteryl ester in the rat, studied with a nonhydrolyzable analog, cholesteryl linoleyl ether.
      ). As described, the selective pathway is a high-capacity physiologically regulated bulk-cholesterol delivery system operating in steroidogenic tissues of a variety of animals to selectively internalize cholesterol to produce steroid hormones (
      • Gwynne J.T.
      • Mahaffee D.D.
      Rat adrenal uptake and metabolism of high density lipoprotein cholesteryl ester.
      ,
      • Azhar S.
      • Nomoto A.
      • Leers-Sucheta S.
      • Reaven E.
      Simultaneous induction of an HDL receptor protein (SR-BI) and the selective uptake of HDL-cholesteryl esters in a physiologically relevant steroidogenic cell model.
      ,
      • Azhar S.
      • Tsai L.
      • Medicherla S.
      • Chandrasekher Y.
      • Giudice L.
      • Reaven E.
      Human granulosa cells use high density lipoprotein cholesterol for steroidogenesis.
      ,
      • Reaven E.
      • Zhan L.
      • Nomoto A.
      • Leers-Sucheta S.
      • Azhar S.
      Expression and microvillar localization of scavenger receptor class B, type I (SR-BI) and selective cholesteryl ester uptake in Leydig cells from rat testis.
      ), and in the liver to mediate the transfer of cholesterol into bile and provide cholesterol for bile acid production (
      • Scobey M.W.
      • Johnson F.L.
      • Rudel L.L.
      Delivery of high-density lipoprotein free and esterified cholesterol to bile by the perfused monkey liver.
      ,
      • Pieters M.N.
      • Schouten D.
      • Bakkeren H.F.
      • Esbach B.
      • Brouwer A.
      • Knook D.L.
      • van Berkel T.J.
      Selective uptake of cholesteryl esters from apolipoprotein-E-free high-density lipoproteins by rat parenchymal cells in vivo is efficiently coupled to bile acid synthesis.
      ,
      • Fluiter K.
      • Vietsch H.
      • Biessen E.A.
      • Kostner G.M.
      • van Berkel T.J.
      • Sattler W.
      Increased selective uptake in vivo and in vitro of oxidized cholesteryl esters from high-density lipoprotein by rat liver parenchymal cells.
      ,
      • Guertin F.
      • Brunet S.
      • Lairon D.
      • Levy E.
      Oxidative tyrosylation of high density lipoprotein impairs biliary sterol secretion in rats.
      ). The selective pathway also operates in isolated hepatocytes (
      • Glass C.
      • Pittman R.C.
      • Civen M.
      • Steinberg D.
      Uptake of high-density lipoprotein-associated apoprotein A-I and cholesterol esters by 16 tissues of the rat in vivo and by adrenal cells and hepatocytes in vitro.
      ,
      • Pittman R.C.
      • Glass C.K.
      • Atkinson D.
      • Small D.M.
      Synthetic high density lipoprotein particles. Application to studies of the apoprotein specificity for selective uptake of cholesterol esters.
      ,
      • Rinninger F.
      • Kaiser T.
      • Windler E.
      • Greten H.
      • Fruchart J.C.
      • Castro G.
      Selective uptake of cholesteryl esters from high-density lipoprotein-derived LpA-I and LpA-I:A-II particles by hepatic cells in culture.
      ), fibroblasts (
      • Pittman R.C.
      • Glass C.K.
      • Atkinson D.
      • Small D.M.
      Synthetic high density lipoprotein particles. Application to studies of the apoprotein specificity for selective uptake of cholesterol esters.
      ,
      • Rinninger F.
      • Kaiser T.
      • Windler E.
      • Greten H.
      • Fruchart J.C.
      • Castro G.
      Selective uptake of cholesteryl esters from high-density lipoprotein-derived LpA-I and LpA-I:A-II particles by hepatic cells in culture.
      ), adipocytes (
      • Despres J.P.
      • Fong B.S.
      • Jimenez J.
      • Julien P.
      • Angel A.
      Selective uptake of HDL cholesterol ester by human fat cells.
      ,
      • Schorsch F.
      • Malle E.
      • Sattler W.
      Selective uptake of high density lipoprotein-associated cholesterylesters by differentiated Ob1771 adipocytes is modulated by endogenous and exogenous lipoprotein lipase.
      ), macrophages (
      • Stein O.
      • Israeli A.
      • Leitersdorf E.
      • Halperin G.
      • Stein Y.
      Preferential uptake of cholesteryl ester-HDL by cultured macrophages.
      ,
      • Rinninger F.
      • Deichen J.T.
      • Jackle S.
      • Windler E.
      • Greten H.
      Selective uptake of high-density lipoprotein-associated cholesteryl esters and high-density lipoprotein particle uptake by human monocyte-macrophages.
      ), and adrenal (
      • Glass C.
      • Pittman R.C.
      • Civen M.
      • Steinberg D.
      Uptake of high-density lipoprotein-associated apoprotein A-I and cholesterol esters by 16 tissues of the rat in vivo and by adrenal cells and hepatocytes in vitro.
      ,
      • Gwynne J.T.
      • Mahaffee D.D.
      Rat adrenal uptake and metabolism of high density lipoprotein cholesteryl ester.
      ,
      • Pittman R.C.
      • Glass C.K.
      • Atkinson D.
      • Small D.M.
      Synthetic high density lipoprotein particles. Application to studies of the apoprotein specificity for selective uptake of cholesterol esters.
      ,
      • Leitersdorf E.
      • Israeli A.
      • Stein O.
      • Eisenberg S.
      • Stein Y.
      The role of apolipoproteins of HDL in the selective uptake of cholesteryl linoleyl ether by cultured rat and bovine adrenal cells.
      ), ovarian (
      • Azhar S.
      • Nomoto A.
      • Leers-Sucheta S.
      • Reaven E.
      Simultaneous induction of an HDL receptor protein (SR-BI) and the selective uptake of HDL-cholesteryl esters in a physiologically relevant steroidogenic cell model.
      ,
      • Azhar S.
      • Tsai L.
      • Medicherla S.
      • Chandrasekher Y.
      • Giudice L.
      • Reaven E.
      Human granulosa cells use high density lipoprotein cholesterol for steroidogenesis.
      ) and testicular Leydig (
      • Reaven E.
      • Zhan L.
      • Nomoto A.
      • Leers-Sucheta S.
      • Azhar S.
      Expression and microvillar localization of scavenger receptor class B, type I (SR-BI) and selective cholesteryl ester uptake in Leydig cells from rat testis.
      ) cells, though its function in fibroblasts is less clear. Scavenger receptor B type 1 (SR-B1) is the physiologically relevant cell surface HDL receptor responsible for selective HDL-CE uptake (
      • Acton S.
      • Rigotti A.
      • Landschulz K.T.
      • Xu S.
      • Hobbs H.H.
      • Krieger M.
      Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.
      ).

      MOLECULAR AND BIOCHEMICAL CHARACTERISTICS OF SR-B1

      The SR-B1 gene (Scarb1) was almost simultaneously cloned as a cluster determinant 36 (CD36)-related class B scavenger receptor by expression cloning using cDNA libraries from a Chinese hamster ovary (CHO) cell line, Var-261, with a mutated LDL receptor gene (
      • Acton S.L.
      • Scherer P.E.
      • Lodish H.F.
      • Krieger M.
      Expression cloning of SR-BI, a CD36-related class B scavenger receptor.
      ), and as CD36 and lysosomal integral membrane protein (LIMP) II analogous-I (CLA-1) from human erythroleukemia (HEL) cells based on its sequence similarity to CD36 and LIMP-2 (
      • Calvo D.
      • Vega M.A.
      Identification, primary structure, and distribution of CLA-1, a novel member of the CD36/LIMPII gene family.
      ). CLA-1 is a human homolog of SR-B1. Subsequently, the isolation and characterization of SR-B1 from mouse (
      • Acton S.
      • Rigotti A.
      • Landschulz K.T.
      • Xu S.
      • Hobbs H.H.
      • Krieger M.
      Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.
      ), rat (
      • Mizutani T.
      • Sonoda Y.
      • Minegishi T.
      • Wakabayashi K.
      • Miyamoto K.
      Cloning, characterization, and cellular distribution of rat scavenger receptor class B type I (SRBI) in the ovary.
      ), rabbit (
      • Ritsch A.
      • Tancevski I.
      • Schgoer W.
      • Pfeifhofer C.
      • Gander R.
      • Eller P.
      • Foeger B.
      • Stanzl U.
      • Patsch J.R.
      Molecular characterization of rabbit scavenger receptor class B types I and II: portal to central vein gradient of expression in the liver.
      ), pig (
      • Kim J.G.
      • Vallet J.L.
      • Nonneman D.
      • Christenson R.K.
      Molecular cloning and endometrial expression of porcine high density lipoprotein receptor SR-BI during the estrous cycle and early pregnancy.
      ), cow (
      • Rajapaksha W.R.
      • McBride M.
      • Robertson L.
      • O'Shaughnessy P.J.
      Sequence of the bovine HDL-receptor (SR-BI) cDNA and changes in receptor mRNA expression during granulosa cell luteinization in vivo and in vitro.
      ), and Northern tree shrew (
      • Tong Y.
      • Zhu Y.
      • Xia X.
      • Liu Y.
      • Feng Y.
      • Hua X.
      • Chen Z.
      • Ding H.
      • Gao L.
      • Wang Y.
      • et al.
      Tupaia CD81, SR-BI, claudin-1, and occludin support hepatitis C virus infection.
      ) have been described. Further studies by Monty Krieger's group revealed that SR-B1 binds HDL and mediates the selective delivery of HDL-CE (
      • Acton S.
      • Rigotti A.
      • Landschulz K.T.
      • Xu S.
      • Hobbs H.H.
      • Krieger M.
      Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.
      ). This led to identification of SR-B1 as the first bona fide HDL receptor (
      • Acton S.
      • Rigotti A.
      • Landschulz K.T.
      • Xu S.
      • Hobbs H.H.
      • Krieger M.
      Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.
      ).
      SR-B1 is a member of the class B family of scavenger receptors, which is also known as the CD36 superfamily of scavenger receptors, which are plasma membrane receptors that recognize and take up macromolecules that have a negative charge. Although classified as a group, the scavenger receptor family members are structurally heterogeneous; specifically, the scavenger receptor B family members have a very different molecular structure from other scavenger receptors by having two transmembrane domains with both the N and C termini of the proteins residing intracellularly (
      • PrabhuDas M.R.
      • Baldwin C.L.
      • Bollyky P.L.
      • Bowdish D.M.E.
      • Drickamer K.
      • Febbraio M.
      • Herz J.
      • Kobzik L.
      • Krieger M.
      • Loike J.
      • et al.
      A consensus definitive classification of scavenger receptors and their roles in health and disease.
      ). Proteins in this group are cell surface-associated glycoproteins, which include SR-B1 and its splicing variant, SR-B2 (SR-BII), its human homolog, CLA-1/human SR-B1, and spliced variant, CLA-2/human SR-B2, CD36, and LIMP-2 (
      • Calvo D.
      • Dopazo J.
      • Vega M.A.
      The CD36, CLA-1 (CD36L1), and LIMPII (CD36L2) gene family: cellular distribution, chromosomal location, and genetic evolution.
      ,
      • Webb N.R.
      • de Villiers W.J.
      • Connell P.M.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Alternative forms of the scavenger receptor BI (SR-BI).
      ,
      • Krieger M.
      Charting the fate of the “good cholesterol”: identification and characterization of the high-density lipoprotein receptor SR-BI.
      ,
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ,
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ,
      • Silverstein R.L.
      • Febbraio M.
      CD36, a scavenger receptor involved in immunity, metabolism, angiogenesis, and behavior.
      ,
      • Gonzalez A.
      • Valeiras M.
      • Sidransky E.
      • Tayebi N.
      Lysosomal integral membrane protein-2: a new player in lysosome-related pathology.
      ). Full-length SR-B1 encodes a 509 amino acid protein, which, like other family members, contains a short N-terminal cytoplasmic domain of 9 amino acids (N-terminal domain), a transmembrane spanning domain of 22 amino acids, a large extracellular domain of 408 amino acids containing a cysteine-rich region and multiple sites for N-linked glycosylation, a second transmembrane domain of 23 amino acids, and a cytoplasmic C-terminal domain of 44 amino acids (
      • Krieger M.
      Charting the fate of the “good cholesterol”: identification and characterization of the high-density lipoprotein receptor SR-BI.
      ,
      • Williams D.L.
      • Connelly M.A.
      • Temel R.E.
      • Swarnakar S.
      • Phillips M.C.
      • de la Llera-Moya M.
      • Rothblat G.H.
      Scavenger receptor BI and cholesterol trafficking.
      ). SR-B1 is posttranslationally glycosylated, and mutational analysis showed the importance of Asn-108 and Asn-173 for plasma membrane localization of SR-B1 and for the ability to transfer lipid from HDL to cells (
      • Viñals M.
      • Xu S.
      • Vasile E.
      • Krieger M.
      Identification of the N-linked glycosylation sites on the high density lipoprotein (HDL) receptor SR-BI and assessment of their effects on HDL binding and selective lipid uptake.
      ). The mouse SR-B1 gene is located on chromosome 5 with 13 exons and 12 introns. Due to alternative splicing, the second isoform of SR-B1, SRB2 (SR-BII), has only 12 exons and a different C terminal with 41 amino acids (
      • Webb N.R.
      • de Villiers W.J.
      • Connell P.M.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Alternative forms of the scavenger receptor BI (SR-BI).
      ). Using a specific antibody for SR-B2 isoform, SR-B2 protein was detected in mouse liver, testis, and adrenal glands at about 5–12% of immunodetectable SR-B1 (
      • Webb N.R.
      • de Villiers W.J.
      • Connell P.M.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Alternative forms of the scavenger receptor BI (SR-BI).
      ,
      • Webb N.R.
      • Connell P.M.
      • Graf G.A.
      • Smart E.J.
      • de Villiers W.J.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      SR-BII, an isoform of the scavenger receptor BI containing an alternate cytoplasmic tail, mediates lipid transfer between high density lipoprotein and cells.
      ). Recent genomic analysis revealed a third SR-B1 variant with another different alternative splicing that results in 11 exons and 520 amino acids; however, no functional significance of this variant has been reported yet. The mouse SR-B1 gene exon organization and the C termini of the three variants, as well as a cartoon of the structural features of SR-B1, are shown in Fig. 1. The predicted molecular mass of SR-B1 is ∼57 kDa, but, because the protein is heavily glycosylated (
      • Viñals M.
      • Xu S.
      • Vasile E.
      • Krieger M.
      Identification of the N-linked glycosylation sites on the high density lipoprotein (HDL) receptor SR-BI and assessment of their effects on HDL binding and selective lipid uptake.
      ), it runs ∼83–84 kDa on Western blot (
      • Krieger M.
      Charting the fate of the “good cholesterol”: identification and characterization of the high-density lipoprotein receptor SR-BI.
      ,
      • Williams D.L.
      • Connelly M.A.
      • Temel R.E.
      • Swarnakar S.
      • Phillips M.C.
      • de la Llera-Moya M.
      • Rothblat G.H.
      Scavenger receptor BI and cholesterol trafficking.
      ). SR-B1 is also myristoylated and palmitoylated, but fatty acid acylation of the protein does not affect its selective HDL-CE transport function (
      • Babitt J.
      • Trigatti B.
      • Rigotti A.
      • Smart E.J.
      • Anderson R.G.
      • Xu S.
      • Krieger M.
      Murine SR-BI, a high density lipoprotein receptor that mediates selective lipid uptake, is N-glycosylated and fatty acylated and colocalizes with plasma membrane caveolae.
      ).
      Figure thumbnail gr1
      Fig. 1Gene exon organization (A) and C-termini sequences (B) of mouse SR-B1 variants and structural features of SR-B1 (C). A: Exon organization. Mouse SR-B1 variant 1 has 13 exons and 509 amino acids. Variant 2 arises from alternative splicing at the C-terminal end resulting in 12 exons and 506 amino acids. Genomic analysis shows that variant 3 has 11 exons and 520 amino acids. B: C-terminal sequences of mouse SR-B1 variants. C: Structural features of SR-B1. SR-B1 has an N-terminal and a C-terminal intracellular domain and an extended extracellular ectodomain. Several structural features are important for the function of SR-B1. An N-terminal transmembrane glycine motif (G15_G18_G25) was shown to be required for oligomerization and lipid transport. The C terminal of SR-B1 also contains sequences important for oligomerization. Six conserved cysteine residues (C251, C280, C321, C323, C334, and C384) found in the ectodomain of SR-B1 were also demonstrated to be involved in dimer/oligomer formation. Analysis of human SR-B1 revealed 11 putative N-linked glycosylation sites. Mutational analysis showed the importance of Asn-108 and Asn-173 for plasma membrane localization and for the ability to transfer lipid from HDL to cells. Structural analysis of SR-B1 homolog, LIMP-2, showed that there are eight amino acids that work coordinately to form a tunnel cavity that spans the entire length of the ectodomain allowing facilitated lipid transfer. E96, R98, K100, K117, W258, Q260, N383, and E418 are the amino acids that comprise the tunnel cavity to facilitate lipid transfer of SR-B1. The C-terminal intracellular domain contains an interacting domain (VLQEAKL) for binding with PDZ domain-containing proteins, through which the function of SR-B1 is regulated in a tissue-specific manner.

      SR-B1 LIGANDS

      Scavenger receptors are cell surface membrane proteins that can bind chemically modified lipoproteins, including acetylated LDL (AcLDL), oxidized LDL (OxLDL), and often many other types of ligands (
      • PrabhuDas M.R.
      • Baldwin C.L.
      • Bollyky P.L.
      • Bowdish D.M.E.
      • Drickamer K.
      • Febbraio M.
      • Herz J.
      • Kobzik L.
      • Krieger M.
      • Loike J.
      • et al.
      A consensus definitive classification of scavenger receptors and their roles in health and disease.
      ,
      • Zani I.A.
      • Stephen S.L.
      • Mughal N.A.
      • Russell D.
      • Homer-Vanniasinkam S.
      • Wheatcroft S.B.
      • Ponnambalam S.
      Scavenger receptor structure and function in health and disease.
      ). SR-B1 was first identified in scavenger receptor expression cloning experiments using AcLDL as a ligand. Further studies have demonstrated that SR-B1 or its human homolog, CLA-1, bind an array of ligands (Table 1), including both native (VLDL, LDL, and HDL) (
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ,
      • Calvo D.
      • Gomez-Coronado D.
      • Lasuncion M.A.
      • Vega M.A.
      CLA-1 is an 85-kD plasma membrane glycoprotein that acts as a high-affinity receptor for both native (HDL, LDL, and VLDL) and modified (OxLDL and AcLDL) lipoproteins.
      ) and modified lipoproteins (OxLDL and AcLDL) (
      • Acton S.L.
      • Scherer P.E.
      • Lodish H.F.
      • Krieger M.
      Expression cloning of SR-BI, a CD36-related class B scavenger receptor.
      ,
      • Calvo D.
      • Gomez-Coronado D.
      • Lasuncion M.A.
      • Vega M.A.
      CLA-1 is an 85-kD plasma membrane glycoprotein that acts as a high-affinity receptor for both native (HDL, LDL, and VLDL) and modified (OxLDL and AcLDL) lipoproteins.
      ). SR-B1 can also bind maleylated BSA (
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ); advanced glycation end-product modified proteins, such as advanced glycation end product BSA (
      • Ohgami N.
      • Nagai R.
      • Miyazaki A.
      • Ikemoto M.
      • Arai H.
      • Horiuchi S.
      • Nakayama H.
      Scavenger receptor class B type I-mediated reverse cholesterol transport is inhibited by advanced glycation end products.
      ); hypochlorite-modified LDL (
      • Marsche G.
      • Hammer A.
      • Oskolkova O.
      • Kozarsky K.F.
      • Sattler W.
      • Malle E.
      Hypochlorite-modified high density lipoprotein, a high affinity ligand to scavenger receptor class B, type I, impairs high density lipoprotein-dependent selective lipid uptake and reverse cholesterol transport.
      ,
      • Marsche G.
      • Zimmermann R.
      • Horiuchi S.
      • Tandon N.N.
      • Sattler W.
      • Malle E.
      Class B scavenger receptors CD36 and SR-BI are receptors for hypochlorite-modified low density lipoprotein.
      ); discoidal reconstituted phospholipid/unesterified cholesterol particles containing apoA-I, apoA-II, apoC-III, or apoE (
      • Xu S.
      • Laccotripe M.
      • Huang X.
      • Rigotti A.
      • Zannis V.I.
      • Krieger M.
      Apolipoproteins of HDL can directly mediate binding to the scavenger receptor SR-BI, an HDL receptor that mediates selective lipid uptake.
      ,
      • Li X.
      • Kan H.Y.
      • Lavrentiadou S.
      • Krieger M.
      • Zannis V.
      Reconstituted discoidal ApoE-phospholipid particles are ligands for the scavenger receptor BI. The amino-terminal 1-165 domain of ApoE suffices for receptor binding.
      ); reconstituted HDL particles containing apoA-I or apoA-II (
      • de Beer M.C.
      • Durbin D.M.
      • Cai L.
      • Mirocha N.
      • Jonas A.
      • Webb N.R.
      • de Beer F.C.
      • van Der Westhuyzen D.R.
      Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI.
      ); and lipid-free apoE (
      • de Beer M.C.
      • Durbin D.M.
      • Cai L.
      • Mirocha N.
      • Jonas A.
      • Webb N.R.
      • de Beer F.C.
      • van Der Westhuyzen D.R.
      Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI.
      ). However, SR-B1 exhibits much higher affinity for native HDL (
      • Acton S.
      • Rigotti A.
      • Landschulz K.T.
      • Xu S.
      • Hobbs H.H.
      • Krieger M.
      Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.
      ,
      • Krieger M.
      Charting the fate of the “good cholesterol”: identification and characterization of the high-density lipoprotein receptor SR-BI.
      ). The facts that HDL consists of particles of varying sizes and different lipid and protein compositions and densities (
      • Phillips M.C.
      New insights into the determination of HDL structure by apolipoproteins: Thematic review series: high density lipoprotein structure, function, and metabolism.
      ), and that apoA-I is the principal component of HDL, are in accordance with observations that the physical characteristics of HDL as well as the conformation/organization of apoA-I in HDL particles are critical for optimal binding of HDL to SR-B1 (
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ). Indeed, it has been demonstrated that spherical α-HDL particles, which are larger in size, cholesterol rich, and lower density, bind more strongly to SR-B1 as compared with high-density lipid-poor preβ-HDL (
      • Liadaki K.N.
      • Liu T.
      • Xu S.
      • Ishida B.Y.
      • Duchateaux P.N.
      • Krieger J.P.
      • Kane J.
      • Krieger M.
      • Zannis V.I.
      Binding of high density lipoprotein (HDL) and discoidal reconstituted HDL to the HDL receptor scavenger receptor class B type I. Effect of lipid association and APOA-I mutations on receptor binding.
      ,
      • de Beer M.C.
      • Durbin D.M.
      • Cai L.
      • Jonas A.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Apolipoprotein A-I conformation markedly influences HDL interaction with scavenger receptor BI.
      ). Interestingly, reconstituted discoidal apoA-I complexes bind SR-B1 with even much greater affinity than native α-HDL (
      • Liadaki K.N.
      • Liu T.
      • Xu S.
      • Ishida B.Y.
      • Duchateaux P.N.
      • Krieger J.P.
      • Kane J.
      • Krieger M.
      • Zannis V.I.
      Binding of high density lipoprotein (HDL) and discoidal reconstituted HDL to the HDL receptor scavenger receptor class B type I. Effect of lipid association and APOA-I mutations on receptor binding.
      ). Furthermore, amphipathic helices of apoA-I have been demonstrated to be critical for its binding to SR-B1 (
      • Williams D.L.
      • de La Llera-Moya M.
      • Thuahnai S.T.
      • Lund-Katz S.
      • Connelly M.A.
      • Azhar S.
      • Anantharamaiah G.M.
      • Phillips M.C.
      Binding and cross-linking studies show that scavenger receptor BI interacts with multiple sites in apolipoprotein A-I and identify the class A amphipathic alpha-helix as a recognition motif.
      ). The relative levels of apoA-I and apoA-II in spherical α-HDL particles also dictate the association with SR-B1 (
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ,
      • de Beer M.C.
      • Durbin D.M.
      • Cai L.
      • Mirocha N.
      • Jonas A.
      • Webb N.R.
      • de Beer F.C.
      • van Der Westhuyzen D.R.
      Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI.
      ,
      • Pilon A.
      • Briand O.
      • Lestavel S.
      • Copin C.
      • Majd Z.
      • Fruchart J.C.
      • Castro G.
      • Clavey V.
      Apolipoprotein AII enrichment of HDL enhances their affinity for class B type I scavenger receptor but inhibits specific cholesteryl ester uptake.
      ). SR-B1 can also bind anionic phospholipids (
      • Rigotti A.
      • Acton S.L.
      • Krieger M.
      The class B scavenger receptors SR-BI and CD36 are receptors for anionic phospholipids.
      ,
      • Cai L.
      • de Beer M.C.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Serum amyloid A is a ligand for scavenger receptor class B type I and inhibits high density lipoprotein binding and selective lipid uptake.
      ,
      • Kawasaki Y.
      • Nakagawa A.
      • Nagaosa K.
      • Shiratsuchi A.
      • Nakanishi Y.
      Phosphatidylserine binding of class B scavenger receptor type I, a phagocytosis receptor of testicular Sertoli cells.
      ), serum amyloid A (
      • Cai L.
      • de Beer M.C.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Serum amyloid A is a ligand for scavenger receptor class B type I and inhibits high density lipoprotein binding and selective lipid uptake.
      ), silica (
      • Tsugita M.
      • Morimoto N.
      • Tashiro M.
      • Kinoshita K.
      • Nakayama M.
      SR-B1 is a silica receptor that mediates canonical inflammasome activation.
      ), oxidized phospholipids (
      • Ashraf M.Z.
      • Kar N.S.
      • Chen X.
      • Choi J.
      • Salomon R.G.
      • Febbraio M.
      • Podrez E.A.
      Specific oxidized phospholipids inhibit scavenger receptor bi-mediated selective uptake of cholesteryl esters.
      ,
      • Gao D.
      • Sayre L.M.
      • Podrez E.A.
      Analysis of relationship between oxidized phospholipid structure and interaction with the class B scavenger receptors.
      ), and bacterial cell-wall components, such as endotoxin [lipopolysaccharides (LPSs)] of gram-negative bacteria and lipoteichoic acid of gram-positive bacteria (
      • Vishnyakova T.G.
      • Bocharov A.V.
      • Baranova I.N.
      • Chen Z.
      • Remaley A.T.
      • Csako G.
      • Eggerman T.L.
      • Patterson A.P.
      Binding and internalization of lipopolysaccharide by Cla-1, a human orthologue of rodent scavenger receptor B1.
      ,
      • Bocharov A.V.
      • Baranova I.N.
      • Vishnyakova T.G.
      • Remaley A.T.
      • Csako G.
      • Thomas F.
      • Patterson A.P.
      • Eggerman T.L.
      Targeting of scavenger receptor class B type I by synthetic amphipathic alpha-helical-containing peptides blocks lipopolysaccharide (LPS) uptake and LPS-induced pro-inflammatory cytokine responses in THP-1 monocyte cells.
      ). Additionally, SR-B1/CLA-1 can bind and internalize other bacterial and mammalian proteins containing amphipathic helices (
      • Cai L.
      • de Beer M.C.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Serum amyloid A is a ligand for scavenger receptor class B type I and inhibits high density lipoprotein binding and selective lipid uptake.
      ,
      • Bocharov A.V.
      • Baranova I.N.
      • Vishnyakova T.G.
      • Remaley A.T.
      • Csako G.
      • Thomas F.
      • Patterson A.P.
      • Eggerman T.L.
      Targeting of scavenger receptor class B type I by synthetic amphipathic alpha-helical-containing peptides blocks lipopolysaccharide (LPS) uptake and LPS-induced pro-inflammatory cytokine responses in THP-1 monocyte cells.
      ,
      • Vishnyakova T.G.
      • Kurlander R.
      • Bocharov A.V.
      • Baranova I.N.
      • Chen Z.
      • Abu-Asab M.S.
      • Tsokos M.
      • Malide D.
      • Basso F.
      • Remaley A.
      • et al.
      CLA-1 and its splicing variant CLA-2 mediate bacterial adhesion and cytosolic bacterial invasion in mammalian cells.
      ). Finally, SR-B1 has been shown to bind several types of apoptotic cells (
      • Vishnyakova T.G.
      • Bocharov A.V.
      • Baranova I.N.
      • Chen Z.
      • Remaley A.T.
      • Csako G.
      • Eggerman T.L.
      • Patterson A.P.
      Binding and internalization of lipopolysaccharide by Cla-1, a human orthologue of rodent scavenger receptor B1.
      ,
      • Fukasawa M.
      • Adachi H.
      • Hirota K.
      • Tsujimoto M.
      • Arai H.
      • Inoue K.
      SRB1, a class B scavenger receptor, recognizes both negatively charged liposomes and apoptotic cells.
      ,
      • Murao K.
      • Terpstra V.
      • Green S.R.
      • 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.
      ,
      • Shiratsuchi A.
      • Kawasaki Y.
      • Ikemoto M.
      • Arai H.
      • Nakanishi Y.
      Role of class B scavenger receptor type I in phagocytosis of apoptotic rat spermatogenic cells by Sertoli cells.
      ,
      • Svensson P.A.
      • Johnson M.S.
      • Ling C.
      • Carlsson L.M.
      • Billig H.
      • Carlsson B.
      Scavenger receptor class B type I in the rat ovary: possible role in high density lipoprotein cholesterol uptake and in the recognition of apoptotic granulosa cells.
      ,
      • Imachi H.
      • Murao K.
      • Hiramine C.
      • Sayo Y.
      • Sato M.
      • Hosokawa H.
      • Ishida T.
      • Kodama T.
      • Quehenberger O.
      • Steinberg D.
      • et al.
      Human scavenger receptor B1 is involved in recognition of apoptotic thymocytes by thymic nurse cells.
      ). From the perspective of displaying a broad spectrum of ligand recognition, the most closely related proteins are from the LDL receptor family (
      • Herz J.
      • Strickland D.K.
      LRP: a multifunctional scavenger and signaling receptor.
      ,
      • Neels J.G.
      • van Den Berg B.M.
      • Lookene A.
      • Olivecrona G.
      • Pannekoek H.
      • van Zonneveld A.J.
      The second and fourth cluster of class A cysteine-rich repeats of the low density lipoprotein receptor-related protein share ligand-binding properties.
      ). Interestingly, the structure of the SR-B1 family is distinct from that of the LDL receptor family proteins, again by having two transmembrane domains with both the N and C termini of the protein residing intracellularly, while LDL receptor family members have a single transmembrane domain.
      TABLE 1SR-B1 ligands
      Ligands
      Native VLDL, LDL, and HDL
      OxLDL and AcLDL
      Maleylated BSA
      Liposomes containing anionic phospholipids
      Hypochlorite-modified LDL
      Lipoprotein (a)
      Discoidal POPC-apoE particles
      Discoidal reconstituted phospholipid/unesterified cholesterol particles containing apoA-I, apoA-II, or apoC-III
      Advanced glycation end product BSA
      Lipid free apoE
      Oxidized phosphatidylcholine CD36 and other oxidized phospholipids
      Myeloperoxidase-H2O2-NO2-modified LDL
      Serum amyloid A
      Silica
      Endotoxin (LPSs of gram-negative bacteria and lipoteichoic acid of gram-positive bacteria)
      Amphipathic helices containing bacterial and mammalian proteins
      Apoptotic cells

      CELLULAR AND TISSUE DISTRIBUTION OF SR-B1

      SR-B1 is expressed in a wide variety of tissues and cell types, including adipocytes, macrophages, endothelial cells, intestine, keratinocytes, epithelial cells, smooth muscle cells, monocytes, placenta, gallbladder, and ocular tissues (
      • Krieger M.
      Charting the fate of the “good cholesterol”: identification and characterization of the high-density lipoprotein receptor SR-BI.
      ,
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ,
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ,
      • Azhar S.
      • Reaven E.
      Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis.
      ,
      • Azhar S.
      • Leers-Sucheta S.
      • Reaven E.
      Cholesterol uptake in adrenal and gonadal tissues: the SR-BI and ‘selective’ pathway connection.
      ,
      • Provost A.C.
      • Pequignot M.O.
      • Sainton K.M.
      • Gadin S.
      • Salle S.
      • Marchant D.
      • Hales D.B.
      • Abitbol M.
      Expression of SR-BI receptor and StAR protein in rat ocular tissues.
      ). However, in rodents, SR-B1 is expressed most abundantly in the liver and steroidogenic cells of the adrenal gland, testis, and ovary (
      • Acton S.
      • Rigotti A.
      • Landschulz K.T.
      • Xu S.
      • Hobbs H.H.
      • Krieger M.
      Identification of scavenger receptor SR-BI as a high density lipoprotein receptor.
      ), tissues that efficiently utilize the selective pathway in deriving CEs for use in bile acid synthesis and biliary cholesterol secretion (
      • Ji Y.
      • Wang N.
      • Ramakrishnan R.
      • Sehayek E.
      • Huszar D.
      • Breslow J.L.
      • Tall A.R.
      Hepatic scavenger receptor BI promotes rapid clearance of high density lipoprotein free cholesterol and its transport into bile.
      ,
      • Mardones P.
      • Quinones V.
      • Amigo L.
      • Moreno M.
      • Miquel J.F.
      • Schwarz M.
      • Miettinen H.E.
      • Trigatti B.
      • Krieger M.
      • VanPatten S.
      • et al.
      Hepatic cholesterol and bile acid metabolism and intestinal cholesterol absorption in scavenger receptor class B type I-deficient mice.
      ,
      • Yuan Q.
      • Bie J.
      • Wang J.
      • Ghosh S.S.
      • Ghosh S.
      Cooperation between hepatic cholesteryl ester hydrolase and scavenger receptor BI for hydrolysis of HDL-CE.
      ,
      • Wiersma H.
      • Gatti A.
      • Nijstad N.
      • Oude Elferink R.P.
      • Kuipers F.
      • Tietge U.J.
      Scavenger receptor class B type I mediates biliary cholesterol secretion independent of ATP-binding cassette transporter g5/g8 in mice.
      ) or steroid hormone production (
      • Azhar S.
      • Nomoto A.
      • Leers-Sucheta S.
      • Reaven E.
      Simultaneous induction of an HDL receptor protein (SR-BI) and the selective uptake of HDL-cholesteryl esters in a physiologically relevant steroidogenic cell model.
      ,
      • Reaven E.
      • Zhan L.
      • Nomoto A.
      • Leers-Sucheta S.
      • Azhar S.
      Expression and microvillar localization of scavenger receptor class B, type I (SR-BI) and selective cholesteryl ester uptake in Leydig cells from rat testis.
      ,
      • Temel R.E.
      • Trigatti B.
      • DeMattos R.B.
      • Azhar S.
      • Krieger M.
      • Williams D.L.
      Scavenger receptor class B, type I (SR-BI) is the major route for the delivery of high density lipoprotein cholesterol to the steroidogenic pathway in cultured mouse adrenocortical cells.
      ,
      • Reaven E.
      • Nomoto A.
      • Leers-Sucheta S.
      • Temel R.
      • Williams D.L.
      • Azhar S.
      Expression and microvillar localization of scavenger receptor, class B, type I (a high density lipoprotein receptor) in luteinized and hormone-desensitized rat ovarian models.
      ,
      • Azhar S.
      • Nomoto A.
      • Reaven E.
      Hormonal regulation of adrenal microvillar channel formation.
      ,
      • Connelly M.A.
      • Williams D.L.
      SR-BI and cholesterol uptake into steroidogenic cells.
      ,
      • Rao R.M.
      • Jo Y.
      • Leers-Sucheta S.
      • Bose H.S.
      • Miller W.L.
      • Azhar S.
      • Stocco D.M.
      Differential regulation of steroid hormone biosynthesis in R2C and MA-10 Leydig tumor cells: role of SR-B1-mediated selective cholesteryl ester transport.
      ). In humans, CLA-1/human SR-B1 shows a similar tissue distribution (
      • Calvo D.
      • Gomez-Coronado D.
      • Lasuncion M.A.
      • Vega M.A.
      CLA-1 is an 85-kD plasma membrane glycoprotein that acts as a high-affinity receptor for both native (HDL, LDL, and VLDL) and modified (OxLDL and AcLDL) lipoproteins.
      ,
      • Cao G.
      • Garcia C.K.
      • Wyne K.L.
      • Schultz R.A.
      • Parker K.L.
      • Hobbs H.H.
      Structure and localization of the human gene encoding SR-BI/CLA-1. Evidence for transcriptional control by steroidogenic factor 1.
      ). Interestingly, variable expression levels of SR-B2 mRNA are also detected in mouse liver, adrenal gland, and testis, with SR-B2 transcripts ranging from 13% to 40% of total SR-B1 in mouse liver (
      • Webb N.R.
      • de Villiers W.J.
      • Connell P.M.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Alternative forms of the scavenger receptor BI (SR-BI).
      ,
      • Pal R.
      • Ke Q.
      • Pihan G.A.
      • Yesilaltay A.
      • Penman M.L.
      • Wang L.
      • Chitraju C.
      • Kang P.M.
      • Krieger M.
      • Kocher O.
      Carboxy-terminal deletion of the HDL receptor reduces receptor levels in liver and steroidogenic tissues, induces hypercholesterolemia, and causes fatal heart disease.
      ), from 6% to 30% in adrenal (
      • Webb N.R.
      • de Villiers W.J.
      • Connell P.M.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Alternative forms of the scavenger receptor BI (SR-BI).
      ,
      • Pal R.
      • Ke Q.
      • Pihan G.A.
      • Yesilaltay A.
      • Penman M.L.
      • Wang L.
      • Chitraju C.
      • Kang P.M.
      • Krieger M.
      • Kocher O.
      Carboxy-terminal deletion of the HDL receptor reduces receptor levels in liver and steroidogenic tissues, induces hypercholesterolemia, and causes fatal heart disease.
      ), and 80% in whole testis (
      • Webb N.R.
      • de Villiers W.J.
      • Connell P.M.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Alternative forms of the scavenger receptor BI (SR-BI).
      ). Moreover, of the total hepatic SR-B1/SR-B2 mRNA, only 12% of SR-B1/SR-B2 protein is detected in mouse liver by Western blotting, possibly because of less efficient translation of the SR-B2 protein (
      • Webb N.R.
      • de Villiers W.J.
      • Connell P.M.
      • de Beer F.C.
      • van der Westhuyzen D.R.
      Alternative forms of the scavenger receptor BI (SR-BI).
      ).
      It is of interest that steroidogenic tissues, which express high levels of SR-B1 in vivo, are equipped with an intricate microvillar system for the trapping of lipoproteins (
      • Reaven E.
      • Chen Y.D.
      • Spicher M.
      • Azhar S.
      Morphological evidence that high density lipoproteins are not internalized by steroid-producing cells during in situ organ perfusion.
      ,
      • Reaven E.
      • Chen Y.D.
      • Spicher M.
      • Hwang S.F.
      • Mondon C.E.
      • Azhar S.
      Uptake of low density lipoproteins by rat tissues. Special emphasis on the luteinized ovary.
      ,
      • Reaven E.
      • Zhan L.
      • Nomoto A.
      • Leers-Sucheta S.
      • Azhar S.
      Expression and microvillar localization of scavenger receptor class B, type I (SR-BI) and selective cholesteryl ester uptake in Leydig cells from rat testis.
      ,
      • Reaven E.
      • Nomoto A.
      • Leers-Sucheta S.
      • Temel R.
      • Williams D.L.
      • Azhar S.
      Expression and microvillar localization of scavenger receptor, class B, type I (a high density lipoprotein receptor) in luteinized and hormone-desensitized rat ovarian models.
      ,
      • Azhar S.
      • Nomoto A.
      • Reaven E.
      Hormonal regulation of adrenal microvillar channel formation.
      ,
      • Reaven E.
      • Boyles J.
      • Spicher M.
      • Azhar S.
      Evidence for surface entrapment of cholesterol-rich lipoproteins in luteinized ovary.
      ). This general region of steroidogenic cells is referred to as the microvillar compartment, and the specialized spaces created between adjacent microvilli are called microvillar channels. It is the microvillar channels where the various lipoproteins are trapped prior to the selective uptake of lipoprotein-CEs into cells. Use of electron microscopic immunocytochemistry techniques revealed heavy labeling of SR-B1 specifically in these regions (corresponding to microvilli and microvillar channels), and current evidence suggests that these microvillar compartments express high levels of SR-B1 to facilitate the selective uptake of HDL-CE in steroidogenic tissues. Figure 2 displays photomicrographs demonstrating the microvillar system. In this context, we previously demonstrated that expression of SR-B1 in heterologous (Sf9 insect cells) (
      • Reaven E.
      • Leers-Sucheta S.
      • Nomoto A.
      • Azhar S.
      Expression of scavenger receptor class B type 1 (SR-BI) promotes microvillar channel formation and selective cholesteryl ester transport in a heterologous reconstituted system.
      ) or homologous (Y1-BS1 mouse adrenocortical cells) (
      • Reaven E.
      • Nomoto A.
      • Cortez Y.
      • Azhar S.
      Consequences of over-expression of rat Scavenger Receptor, SR-BI, in an adrenal cell model.
      ) cell systems promotes microvillar channel formation and selective HDL-CE uptake. Furthermore, examination of SR-B1 knockout mice provided further evidence that SR-B1 is required for formation of microvilli and microvillar channels. Unlike their wild-type counterparts, the adrenal zona fasciculata cells from SR-B1-null mice exhibit disorganized microvilli, devoid of microvillar channels with an absence of HDL particles (
      • Williams D.L.
      • Wong J.S.
      • Hamilton R.L.
      SR-BI is required for microvillar channel formation and the localization of HDL particles to the surface of adrenocortical cells in vivo.
      ).
      Figure thumbnail gr2
      Fig. 2Microvillar channels in murine adrenocortical cells. A: Ultrastructure appearance of microvilli (denoted by arrows). B: High magnification of microvillar channels containing HDL (denoted by arrowheads). C: Localization of immunogold-labeled HDL in microvillar channels. This research was originally published in the Journal of Lipid Research (
      • Azhar S.
      • Nomoto A.
      • Reaven E.
      Hormonal regulation of adrenal microvillar channel formation.
      ). © The American Society for Biochemistry and Molecular Biology.
      SR-B1 expression has also been reported in several nonmammalian vertebrates, as well as in several insect species. High expression of SR-B1 is reported in the liver, ovary, heart, and blood vessels in the turtle and in the liver of chickens, frogs, goldfish, sharks, and skates (
      • Duggan A.E.
      • Marie Jr., R.S.
      • Callard I.P.
      Expression of SR-BI (scavenger receptor class B type I) in turtle (Chrysemys picta) tissues and other nonmammalian vertebrates.
      ). SR-B1 expression is reported in the liver of Atlantic salmon (Salmo salar L.) (
      • Kleveland E.J.
      • Syvertsen B.L.
      • Ruyter B.
      • Vegusdal A.
      • Jorgensen S.M.
      • Gjoen T.
      Characterization of scavenger receptor class B, type I in Atlantic salmon (Salmo salar L.).
      ) and in several organs of prawns (Macrobrachium nipponense) (
      • Ding Z.
      • Luo N.
      • Kong Y.
      • Li J.
      • Zhang Y.
      • Cao F.
      • Ye J.
      Scavenger receptor class B, type I, a CD36 related protein in Macrobrachium nipponense: characterization, RNA interference, and expression analysis with different dietary lipid sources.
      ). Twelve to fourteen candidate SNMP/CD36 homologs from each of the genomes of Drosophila melanogaster, Drosophila pseudoobscura, Anopheles gambiae, and Aedes aegypti (Diptera), eight candidate homologs from Apis mellifera (Hymenoptera), and 15 from Tribolium castaneum (Coleoptera) have been identified (
      • Nichols Z.
      • Vogt R.G.
      The SNMP/CD36 gene family in Diptera, Hymenoptera and Coleoptera: Drosophila melanogaster, D. pseudoobscura, Anopheles gambiae, Aedes aegypti, Apis mellifera, and Tribolium castaneum.
      ,
      • Vogt R.G.
      • Miller N.E.
      • Litvack R.
      • Fandino R.A.
      • Sparks J.
      • Staples J.
      • Friedman R.
      • Dickens J.C.
      The insect SNMP gene family.
      ). The 14 D. melanogaster SNMP/CD36 homologs include CG10345, CG1887, CG2736, CG31741, CG3829, CG40006, CG7227, CG7422, crq, emp, ninaD, pes, santa-maria, and Snmp (
      • Herboso L.
      • Talamillo A.
      • Perez C.
      • Barrio R.
      Expression of the scavenger receptor class B type I (SR-BI) family in Drosophila melanogaster.
      ). In the silkworm, two genes, Cameo1 and Cameo2, encode proteins homologous to SR-B1 (
      • Vogt R.G.
      • Miller N.E.
      • Litvack R.
      • Fandino R.A.
      • Sparks J.
      • Staples J.
      • Friedman R.
      • Dickens J.C.
      The insect SNMP gene family.
      ,
      • Sakudoh T.
      • Iizuka T.
      • Narukawa J.
      • Sezutsu H.
      • Kobayashi I.
      • Kuwazaki S.
      • Banno Y.
      • Kitamura A.
      • Sugiyama H.
      • Takada N.
      • et al.
      A CD36-related transmembrane protein is coordinated with an intracellular lipid-binding protein in selective carotenoid transport for cocoon coloration.
      ). There are 13 other genes homologous to Cameo1 and Cameo2. These genes are SCRB5, SCRB6, SCRB7, SCRB8, SCRB9, SCRB10, SCRB11, SCRB12, SCRB13, SCRB14, SCRB15, SNMP1, and SNMP2 (
      • Duggan A.E.
      • Marie Jr., R.S.
      • Callard I.P.
      Expression of SR-BI (scavenger receptor class B type I) in turtle (Chrysemys picta) tissues and other nonmammalian vertebrates.
      ).

      REGULATION OF SR-B1 EXPRESSION

      Promoter analysis

      In humans, the SCARB1 gene is located on chromosome 12, and analysis of its 5′-flanking promoter region shows that it is very guanine-cytosine rich with consensus binding sites for many nuclear transcription factors, such as SREBP-1, which regulates SCARB1 expression in response to altered intracellular sterol levels (
      • Cao G.
      • Garcia C.K.
      • Wyne K.L.
      • Schultz R.A.
      • Parker K.L.
      • Hobbs H.H.
      Structure and localization of the human gene encoding SR-BI/CLA-1. Evidence for transcriptional control by steroidogenic factor 1.
      ,
      • Lopez D.
      • McLean M.P.
      Sterol regulatory element-binding protein-1a binds to cis elements in the promoter of the rat high density lipoprotein receptor SR-BI gene.
      ). In steroidogenic cells, trophic hormones stimulate SCARB1 expression through changes of cellular cAMP levels (
      • Lopez D.
      • Sandhoff T.W.
      • McLean M.P.
      Steroidogenic factor-1 mediates cyclic 3′,5′-adenosine monophosphate regulation of the high density lipoprotein receptor.
      ), and steroidogenic factor-1 (SF-1) is one of the major transcription factors involved in this pathway through binding to its consensus sequence in the SCARB1 promoter (
      • Lopez D.
      • Sandhoff T.W.
      • McLean M.P.
      Steroidogenic factor-1 mediates cyclic 3′,5′-adenosine monophosphate regulation of the high density lipoprotein receptor.
      ). Analysis of both the human and rat promoter 5′-flanking regions revealed the presence of many consensus sequences for binding transcription factors responsible for positive and negative regulation of the SCARB1 promoter, although the exact location of the regulatory elements varies in the human and rat promoters. Figure 3 illustrates the presence of the binding sites for positive and negative regulatory factors in the 5′ proximal 2.2 kb region of the rat promoter. There are two binding sites for SREBP-1 that mediate positive regulation of Scarb1 expression in steroidogenic tissue and parenchymal liver cells when cellular cholesterol is depleted (
      • Lopez D.
      • McLean M.P.
      Sterol regulatory element-binding protein-1a binds to cis elements in the promoter of the rat high density lipoprotein receptor SR-BI gene.
      ). There is one SF-1 binding site for regulation mediated through alterations in cellular cAMP levels (
      • Lopez D.
      • Sandhoff T.W.
      • McLean M.P.
      Steroidogenic factor-1 mediates cyclic 3′,5′-adenosine monophosphate regulation of the high density lipoprotein receptor.
      ). There are three estrogen response element binding sites (ERE half-sites) through which estrogen receptor regulates Scarb1 expression in an E2-dependent manner in coordination with mixed-lineage leukemia histone methylases (
      • Ansari K.I.
      • Kasiri S.
      • Hussain I.
      • Bobzean S.A.
      • Perrotti L.I.
      • Mandal S.S.
      MLL histone methylases regulate expression of HDLR-SR-B1 in presence of estrogen and control plasma cholesterol in vivo.
      ) in human liver and placenta cells. In the adrenal, prolactin regulatory element-binding protein (PREB) can bind to the PREB-binding core element at −321 of the human Scarb1 promoter and can induce SR-B1 protein expression under conditions that increase cellular cAMP levels (
      • Murao K.
      • Imachi H.
      • Yu X.
      • Cao W.M.
      • Muraoka T.
      • Dobashi H.
      • Hosomi N.
      • Haba R.
      • Iwama H.
      • Ishida T.
      The transcriptional factor prolactin regulatory element-binding protein mediates the gene transcription of adrenal scavenger receptor class B type I via 3′,5′-cyclic adenosine 5′-monophosphate.
      ). In both human and rodent preadipocytes and liver cells, LXRα/RXR and LXRβ/RXR were shown to bind the Scarb1 promoter and induce Scarb1 transcription (
      • Malerød L.
      • Juvet L.K.
      • Hanssen-Bauer A.
      • Eskild W.
      • Berg T.
      Oxysterol-activated LXRalpha/RXR induces hSR-BI-promoter activity in hepatoma cells and preadipocytes.
      ). In the rat Scarb1 promoter, there is a PPRE motif located at −1622, and PPARα and RXRα were shown to bind to the SR-B1 PPRE motif and result in increased expression of Scarb1 in hepatic and macrophage cell lines (
      • Lopez D.
      • McLean M.P.
      Activation of the rat scavenger receptor class B type I gene by PPARalpha.
      ). Estrogen receptors α and β bind to the three estrogen-responsive elements in the rat SR-B1 promoter and liver receptor homolog 1 binds to an LRH binding site in the promoter, both upregulating Scarb1 (
      • Lopez D.
      • Sanchez M.D.
      • Shea-Eaton W.
      • McLean M.P.
      Estrogen activates the high-density lipoprotein receptor gene via binding to estrogen response elements and interaction with sterol regulatory element binding protein-1A.
      ). There are two binding sites for Yin Yang 1 transcription factor in the rat Scarb1 promoter at −1329 and −1211, through which YY1 binds and represses Scarb1 promoter activity under basal conditions (
      • Shea-Eaton W.
      • Lopez D.
      • McLean M.P.
      Yin yang 1 protein negatively regulates high-density lipoprotein receptor gene transcription by disrupting binding of sterol regulatory element binding protein to the sterol regulatory element.
      ). Using two-hybrid studies, Shea-Eaton and colleagues also confirmed that YY1 can bind to SREBP-1a, therefore inhibiting the interaction of SREBP-1a binding to the SRE and negatively regulating the expression of Scarb1 (
      • Shea-Eaton W.
      • Lopez D.
      • McLean M.P.
      Yin yang 1 protein negatively regulates high-density lipoprotein receptor gene transcription by disrupting binding of sterol regulatory element binding protein to the sterol regulatory element.
      ). Other negative regulators for the Scarb1 promoter include the nuclear receptor, NR0B1 (DAX-1), a protein that plays an important role in adrenal development. DAX-1 was shown to repress the expression of the Scarb1 promoter through interaction with SREBP-1a and SF-1 (
      • Lopez D.
      • Shea-Eaton W.
      • Sanchez M.D.
      • McLean M.P.
      DAX-1 represses the high-density lipoprotein receptor through interaction with positive regulators sterol regulatory element-binding protein-1a and steroidogenic factor-1.
      ). In addition, pregnane X receptor, which is a regulator of detoxification processes, was shown to repress the expression of human SCARB1 in response to the pregnane X receptor agonists, rifampicin and lithocholic acid (
      • Sporstøl M.
      • Tapia G.
      • Malerød L.
      • Mousavi S.A.
      • Berg T.
      Pregnane X receptor-agonists down-regulate hepatic ATP-binding cassette transporter A1 and scavenger receptor class B type I.
      ).
      Figure thumbnail gr3
      Fig. 3Regulation of SR-B1 expression. The 5′ proximal 2.2 kb region of the rat Scarb1 promoter contains binding sites for various regulatory factors for positive and negative regulation of Scarb1 expression. The 3′ UTR of the rat Scarb1 gene contains binding sites for miR-185, miR-96, and miR-223 for negative regulation of SR-B1 expression in liver and macrophages, as well as binding sites for miR-125a and miR-455 for negative regulation of SR-B1expression in steroidogenic tissues.

      The 3′ untranslated region

      Recently, small noncoding RNAs, including microRNAs, were demonstrated to regulate expression of many genes. These microRNAs are about 22 nucleotides and can bind to specific motifs, mostly at the 3′ untranslated region (3′ UTR) of a gene, silencing gene expression via mRNA degradation or preventing mRNA from being translated. Analysis of the sequence of the SR-B1 3′ UTR revealed potential binding sites for several miRNAs. Further analysis demonstrated negative regulation of SR-B1 in liver and macrophages by miR-185, miR-96, and miR-223 (
      • Wang L.
      • Jia X.J.
      • Jiang H.J.
      • Du Y.
      • Yang F.
      • Si S.Y.
      • Hong B.
      MicroRNAs 185, 96, and 223 repress selective high-density lipoprotein cholesterol uptake through posttranscriptional inhibition.
      ). The 3′ UTR of SR-B1 also possesses binding sites for miR-125a and miR-455. Studies show that both miR-125a and miR-455 are present in steroidogenic cells and their expression was suppressed by ACTH and cAMP treatment. Mutational analysis of their binding sites in the 3′ UTR of SR-B1 revealed that both miRNA-125a and miRNA-455 bind to those specific sites in the 3′ UTR of SR-B1 mRNA and negatively regulate SR-B1 expression and inhibit steroidogenesis (
      • Hu Z.
      • Shen W.J.
      • Kraemer F.B.
      • Azhar S.
      MicroRNAs 125a and 455 repress lipoprotein-supported steroidogenesis by targeting scavenger receptor class B type I in steroidogenic cells.
      ).

      STRUCTURAL FEATURES OF SR-B1

      Various approaches, including site-directed mutagenesis, “domain swap,” and chemical cross-linking, have been utilized and have identified several structural features to be important for the function of SR-B1 (Fig. 1C). The C-terminal intracellular domain contains an interacting domain (VLQEAKL) for binding with PDZ domain-containing protein, through which the function of SR-B1 is regulated in a tissue-specific manner (for detail see below). SR-B1 was shown to form oligomers, which display improved function (
      • Azhar S.
      • Nomoto A.
      • Reaven E.
      Hormonal regulation of adrenal microvillar channel formation.
      ,
      • Sahoo D.
      • Darlington Y.F.
      • Pop D.
      • Williams D.L.
      • Connelly M.A.
      Scavenger receptor class B Type I (SR-BI) assembles into detergent-sensitive dimers and tetramers.
      ,
      • Sahoo D.
      • Peng Y.
      • Smith J.R.
      • Darlington Y.F.
      • Connelly M.A.
      Scavenger receptor class B, type I (SR-BI) homo-dimerizes via its C-terminal region: fluorescence resonance energy transfer analysis.
      ,
      • Gaidukov L.
      • Nager A.R.
      • Xu S.
      • Penman M.
      • Krieger M.
      Glycine dimerization motif in the N-terminal transmembrane domain of the high density lipoprotein receptor SR-BI required for normal receptor oligomerization and lipid transport.
      ,
      • Landschulz K.T.
      • Pathak R.K.
      • Rigotti A.
      • Krieger M.
      • Hobbs H.H.
      Regulation of scavenger receptor, class B, type I, a high density lipoprotein receptor, in liver and steroidogenic tissues of the rat.
      ,
      • Reaven E.
      • Cortez Y.
      • Leers-Sucheta S.
      • Nomoto A.
      • Azhar S.
      Dimerization of the scavenger receptor class B type I: formation, function, and localization in diverse cells and tissues.
      ), with several portions involved in oligomerization. An N-terminal transmembrane glycine motif (G15_G18_G25) was shown to be required for oligomerization and lipid transport (
      • Gaidukov L.
      • Nager A.R.
      • Xu S.
      • Penman M.
      • Krieger M.
      Glycine dimerization motif in the N-terminal transmembrane domain of the high density lipoprotein receptor SR-BI required for normal receptor oligomerization and lipid transport.
      ), whereas studies using fluorescence resonance energy transfer techniques to visualize SR-B1 homo-oligomerization revealed that the C termini of SR-B1 appeared to be involved in its homo-dimerization (
      • Sahoo D.
      • Peng Y.
      • Smith J.R.
      • Darlington Y.F.
      • Connelly M.A.
      Scavenger receptor class B, type I (SR-BI) homo-dimerizes via its C-terminal region: fluorescence resonance energy transfer analysis.
      ). Meanwhile, using a sulfhydryl-reactive reagent, the six conserved cysteine residues (C251, C280, C321, C323, C334, and C384) found in the ectodomain of SR-B1 were also demonstrated to be involved in dimer/oligomer formation (
      • Hu J.
      • Zhang Z.
      • Shen W-J.
      • Nomoto A.
      • Azhar S.
      Differential roles of cysteine residues in the cellular trafficking, dimerization, and function of the high-density lipoprotein receptor, SR-BI.
      ). Experiments using chimeric SR-B1 and CD36 revealed that the N terminal of the extracellular domain is important for unesterified cholesterol efflux to HDL (
      • Kartz G.A.
      • Holme R.L.
      • Nicholson K.
      • Sahoo D.
      SR-BI/CD36 chimeric receptors define extracellular subdomains of SR-BI critical for cholesterol transport.
      ,
      • Papale G.A.
      • Nicholson K.
      • Hanson P.J.
      • Pavlovic M.
      • Drover V.A.
      • Sahoo D.
      Extracellular hydrophobic regions in scavenger receptor BI play a key role in mediating HDL-cholesterol transport.
      ).
      Several recent studies have also examined the impact of cysteine residues on cellular trafficking and selective HDL-CE transport function (
      • Hu J.
      • Zhang Z.
      • Shen W-J.
      • Nomoto A.
      • Azhar S.
      Differential roles of cysteine residues in the cellular trafficking, dimerization, and function of the high-density lipoprotein receptor, SR-BI.
      ,
      • Yu M.
      • Romer K.A.
      • Nieland T.J.
      • Xu S.
      • Saenz-Vash V.
      • Penman M.
      • Yesilaltay A.
      • Carr S.A.
      • Krieger M.
      Exoplasmic cysteine Cys384 of the HDL receptor SR-BI is critical for its sensitivity to a small-molecule inhibitor and normal lipid transport activity.
      ,
      • Guo L.
      • Chen M.
      • Song Z.
      • Daugherty A.
      • Li X.A.
      C323 of SR-BI is required for SR-BI-mediated HDL binding and cholesteryl ester uptake.
      ,
      • Papale G.A.
      • Hanson P.J.
      • Sahoo D.
      Extracellular disulfide bonds support scavenger receptor class B type I-mediated cholesterol transport.
      ,
      • Yu M.
      • Lau T.Y.
      • Carr S.A.
      • Krieger M.
      Contributions of a disulfide bond and a reduced cysteine side chain to the intrinsic activity of the high-density lipoprotein receptor SR-BI.
      ). These studies were achieved primarily by mutating cysteine residues in mouse, rat, or human SR-B1 individually or in certain combinations to either Ser or Gly, and subsequent expression of individual constructs in cultured cells followed by determination of receptor activity, subcellular distribution of receptor protein, or selective lipid transport function. Two reports demonstrated that four cysteine residues, C280, C321, C323, and C334, may be involved in disulfide bond formation and critically involved in the expression and function of SR-B1 (
      • Hu J.
      • Zhang Z.
      • Shen W-J.
      • Nomoto A.
      • Azhar S.
      Differential roles of cysteine residues in the cellular trafficking, dimerization, and function of the high-density lipoprotein receptor, SR-BI.
      ,
      • Papale G.A.
      • Hanson P.J.
      • Sahoo D.
      Extracellular disulfide bonds support scavenger receptor class B type I-mediated cholesterol transport.
      ). In addition, Yu et al. (
      • Yu M.
      • Lau T.Y.
      • Carr S.A.
      • Krieger M.
      Contributions of a disulfide bond and a reduced cysteine side chain to the intrinsic activity of the high-density lipoprotein receptor SR-BI.
      ), using a mass spectrometry technique, have reported the identification of two disulfide bonds in SR-B1 that connect cysteine residues within the conserved C321-P322-C323 (CPC) motif and connect C280 to C334 and a reduced cysteine side chain that contribute to the functional expression of SR-B1. Using a C323G mutant, a blocking antibody against C323 region, and a C323G mutant transgenic mouse model, Guo et al. (
      • Guo L.
      • Chen M.
      • Song Z.
      • Daugherty A.
      • Li X.A.
      C323 of SR-BI is required for SR-BI-mediated HDL binding and cholesteryl ester uptake.
      ) demonstrated that C323 plays a critical role in HDL binding to cell surface SR-B1 and SR-B1-mediated selective HDL-CE uptake. An exoplasmic cysteine, C384, of SR-B1 has also been implicated in SR-B1-mediated selective HDL-CE transport activity (
      • Yu M.
      • Romer K.A.
      • Nieland T.J.
      • Xu S.
      • Saenz-Vash V.
      • Penman M.
      • Yesilaltay A.
      • Carr S.A.
      • Krieger M.
      Exoplasmic cysteine Cys384 of the HDL receptor SR-BI is critical for its sensitivity to a small-molecule inhibitor and normal lipid transport activity.
      ).
      The crystal structure of LIMP-2 was obtained recently and showed that the main ectodomain of the protein contains an antiparallel β-barrel core with many short α-helical segments (
      • Neculai D.
      • Schwake M.
      • Ravichandran M.
      • Zunke F.
      • Collins R.F.
      • Peters J.
      • Neculai M.
      • Plumb J.
      • Loppnau P.
      • Pizarro J.C.
      • et al.
      Structure of LIMP-2 provides functional insights with implications for SR-BI and CD36.
      ). Two disulphide bridges stabilize the fold. The disulphide bridge pattern for LIMP-2 (C274-C329 and C312-C318) is similar to that predicted for SR-B1 (C321-C323, C274-C329) and for CD36 (C313-C322, C272-C333), and is consistent with experimental data (
      • Yu M.
      • Lau T.Y.
      • Carr S.A.
      • Krieger M.
      Contributions of a disulfide bond and a reduced cysteine side chain to the intrinsic activity of the high-density lipoprotein receptor SR-BI.
      ,
      • Rasmussen J.T.
      • Berglund L.
      • Rasmussen M.S.
      • Petersen T.E.
      Assignment of disulfide bridges in bovine CD36.
      ,
      • Conrad K.S.
      • Cheng T.W.
      • Ysselstein D.
      • Heybrock S.
      • Hoth L.R.
      • Chrunyk B.A.
      • Am Ende C.W.
      • Krainc D.
      • Schwake M.
      • Saftig P.
      • et al.
      Lysosomal integral membrane protein-2 as a phospholipid receptor revealed by biophysical and cellular studies.
      ). Most of the proteins in this family have lipid transport activity, and the crystal structure of LIMP-2 showed that eight amino acids work coordinately to form a tunnel cavity that spans the entire length of the ectodomain, allowing facilitated lipid transfer. These eight acidic and basic amino acids form a network of hydrogen and ionic bonds and contribute to the lining of the cavity, which is predominately hydrophobic to accommodate lipid moieties. Figure 4 displays the structure of SR-B1, both as a monomer and as a dimer, modeled on the crystal structure of LIMP-2, with the eight amino acids comprising the tunnel cavity highlighted. As illustrated, there are conformational changes in the tunnel cavity that are predicted to occur during the transition between monomer and dimer. Further experimental studies will be needed to explore the functional significance of the structural transition between monomer to dimer.
      Figure thumbnail gr4
      Fig. 4Structure of human SR-B1, both as a monomer and as a dimer. The monomer and dimer forms of human SR-B1 were built separately through homology modeling using MODELLER (
      • Webb B.
      • Sali A.
      Protein structure modeling with MODELLER.
      ). From 200 generated models, the top ranked models were chosen for energy minimization through Amber molecular dynamics simulation (
      • Case D.A.
      • Cheatham 3rd, T.E.
      • Darden T.
      • Gohlke H.
      • Luo R.
      • Merz Jr., K.M.
      • Onufriev A.
      • Simmerling C.
      • Wang B.
      • Woods R.J.
      The Amber biomolecular simulation programs.
      ). The energy-minimized structures were used for the structural analysis. The residues comprising the tunnel cavity are labeled, and the regions where conformational differences are observed between monomers and dimers are highlighted in yellow squares.
      Recently, a fragment of SR-B1 (residues 405-475), which includes the C-terminal transmembrane domain, was purified and reconstituted in detergent micelles (
      • Yu M.
      • Lau T.Y.
      • Carr S.A.
      • Krieger M.
      Contributions of a disulfide bond and a reduced cysteine side chain to the intrinsic activity of the high-density lipoprotein receptor SR-BI.
      ). Analysis of this fragment using NMR has generated high-resolution structural information of the C-terminal transmembrane domain of SR-B1 (
      • Chadwick A.C.
      • Jensen D.R.
      • Peterson F.C.
      • Volkman B.F.
      • Sahoo D.
      Expression, purification and reconstitution of the C-terminal transmembrane domain of scavenger receptor BI into detergent micelles for NMR analysis.
      ), which revealed that there are two short α-helices (residues 409-419 and 427-436) and a long α-helix spanning the entire transmembrane domain (residues 438-469). There is a potential GXXXG glycine dimerization motif starting at G420 together with a putative leucine zipper motif spanning from amino acid 413 to amino acid 455. Mutational analysis showed that mutants L413A and L448A resulted in decreased receptor efflux of unesterified cholesterol to HDL. G420 was previously shown to be important for SR-B1 to function for CE delivery (
      • Parathath S.
      • Sahoo D.
      • Darlington Y.F.
      • Peng Y.
      • Collins H.L.
      • Rothblat G.H.
      • Williams D.L.
      • Connelly M.A.
      Glycine 420 near the C-terminal transmembrane domain of SR-BI is critical for proper delivery and metabolism of high density lipoprotein cholesteryl ester.
      ).When a smaller SR-B1 fragment containing residues 405-445, which lacks the hydrophobic transmembrane domain, was purified and examined by NMR, this fragment was shown to require a hydrophobic environment to fold properly, suggesting the existence of a potential membrane-interacting juxtamembrane domain.
      As mentioned above, oligomerization of SR-B1 improves its function. Recently a crystal structure of LIMP-2 dimer bound with cholesterol and phosphatidylcholine was obtained. Further analysis revealed that LIMP-2 switches from a monomeric form to a dimeric form when binding to different ligands. When a monomer, it binds glucocerebrosidase; whereas as a dimer, it preferentially binds anionic phosphatidylserine (
      • Conrad K.S.
      • Cheng T.W.
      • Ysselstein D.
      • Heybrock S.
      • Hoth L.R.
      • Chrunyk B.A.
      • Am Ende C.W.
      • Krainc D.
      • Schwake M.
      • Saftig P.
      • et al.
      Lysosomal integral membrane protein-2 as a phospholipid receptor revealed by biophysical and cellular studies.
      ). This shifting of ligand specificity through oligomerization could be a mechanism of regulation shared with other proteins in the family, such as SR-B1 and CD36, but awaits experimental evidence.

      LIPID UPTAKE FUNCTION OF SR-B1

      A major function of SR-B1 is to facilitate the transport of CEs from HDL or other lipoproteins to cells by a nonendocytic mechanism referred to as selective uptake (
      • Krieger M.
      Charting the fate of the “good cholesterol”: identification and characterization of the high-density lipoprotein receptor SR-BI.
      ,
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ,
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ,
      • Azhar S.
      • Reaven E.
      Scavenger receptor class BI and selective cholesteryl ester uptake: partners in the regulation of steroidogenesis.
      ,
      • Azhar S.
      • Leers-Sucheta S.
      • Reaven E.
      Cholesterol uptake in adrenal and gonadal tissues: the SR-BI and ‘selective’ pathway connection.
      ,
      • Shen W-J.
      • Hu J.
      • Hu Z.
      • Kraemer F.B.
      • Azhar S.
      Scavenger receptor class B type I (SR-BI): a versatile receptor with multiple functions and actions.
      ,
      • Shen W.J.
      • Azhar S.
      • Kraemer F.B.
      SR-B1: a unique multifunctional receptor for cholesterol influx and efflux.
      ,
      • Shen W.J.
      • Azhar S.
      • Kraemer F.B.
      ACTH regulation of adrenal SR-B1.
      ). Numerous studies have demonstrated that SR-B1-mediated selective uptake of CE does not show a specificity toward a single class of lipoproteins. It can utilize a number of lipoproteins, including human and rat HDL, reconstituted synthetic HDL particles containing apoA-I, apoC, or native or modified apoE, IDL, and LDL as donors of CE for SR-B1-mediated selective CE uptake in vitro and/or in vivo (
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ,
      • Azhar S.
      • Leers-Sucheta S.
      • Reaven E.
      Cholesterol uptake in adrenal and gonadal tissues: the SR-BI and ‘selective’ pathway connection.
      ). However, because these donor lipoproteins differ in characteristics, such as apolipoprotein conformation and apolipoprotein and lipid composition, as well as the fact that some of these lipoproteins may serve as ligands for other important pathways (e.g., human LDL and apoE-rich rodent HDL are potent ligands for the LDL receptor endocytic pathway), these lipoproteins do not deliver CEs to cells with similar efficacies. Based on the evidence obtained using CD36/SR-B1 chimeras (
      • Gu X.
      • Trigatti B.
      • Xu S.
      • Acton S.
      • Babitt J.
      • Krieger M.
      The efficient cellular uptake of high density lipoprotein lipids via scavenger receptor class B type I requires not only receptor-mediated surface binding but also receptor-specific lipid transfer mediated by its extracellular domain.
      ,
      • Connelly M.A.
      • Klein S.M.
      • Azhar S.
      • Abumrad N.A.
      • Williams D.L.
      Comparison of class B scavenger receptors, CD36 and scavenger receptor BI (SR-BI), shows that both receptors mediate high density lipoprotein-cholesteryl ester selective uptake but SR-BI exhibits a unique enhancement of cholesteryl ester uptake.
      ), SR-B1 mutant (N173Q) (
      • Viñals M.
      • Xu S.
      • Vasile E.
      • Krieger M.
      Identification of the N-linked glycosylation sites on the high density lipoprotein (HDL) receptor SR-BI and assessment of their effects on HDL binding and selective lipid uptake.
      ), apoA-I mutants (
      • Liu T.
      • Krieger M.
      • Kan H.Y.
      • Zannis V.I.
      The effects of mutations in helices 4 and 6 of ApoA-I on scavenger receptor class B type I (SR-BI)-mediated cholesterol efflux suggest that formation of a productive complex between reconstituted high density lipoprotein and SR-BI is required for efficient lipid transport.
      ), apoA-I−/− HDL particles (
      • Temel R.E.
      • Walzem R.L.
      • Banka C.L.
      • Williams D.L.
      Apolipoprotein A-I is necessary for the in vivo formation of high density lipoprotein competent for scavenger receptor BI-mediated cholesteryl ester-selective uptake.
      ,
      • Temel R.E.
      • Parks J.S.
      • Williams D.L.
      Enhancement of scavenger receptor class B type I-mediated selective cholesteryl ester uptake from apoA-I(−/−) high density lipoprotein (HDL) by apolipoprotein A-I requires HDL reorganization by lecithin cholesterol acyltransferase.
      ), and a chemical inhibitor of selective CE transport (
      • Nieland T.J.
      • Penman M.
      • Dori L.
      • Krieger M.
      • Kirchhausen T.
      Discovery of chemical inhibitors of the selective transfer of lipids mediated by the HDL receptor SR-BI.
      ), it appears that initial binding of HDL to cell surface-associated SR-B1 and subsequent selective CE transport to cells are independent processes (
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ). The selective uptake process itself can be broadly divided into three steps: 1) binding of cholesterol-rich donor lipoprotein particles to the loop domain of SR-B1; 2) SR-B1-mediated transfer of CEs from the lipoprotein particles to the plasma membrane; and 3) the release of the cholesterol-poor lipoprotein particles back into the circulation (
      • Krieger M.
      Charting the fate of the “good cholesterol”: identification and characterization of the high-density lipoprotein receptor SR-BI.
      ,
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ,
      • Hoekstra M.
      SR-BI as target in atherosclerosis and cardiovascular disease - a comprehensive appraisal of the cellular functions of SR-BI in physiology and disease.
      ). Among these steps, the mechanism by which SR-B1 facilitates the transfer of CEs to the plasma membrane is not fully understood. Rodrigueza et al. (
      • Rodrigueza W.V.
      • Thuahnai S.T.
      • Temel R.E.
      • Lund-Katz S.
      • Phillips M.C.
      • Williams D.L.
      Mechanism of scavenger receptor class B type I-mediated selective uptake of cholesteryl esters from high density lipoprotein to adrenal cells.
      ) provided evidence in favor of a model in which SR-B1 forms a nonaqueous channel between lipoprotein particles and the cell membrane through which CEs move down in a concentration gradient manner. The validity of this model is supported by the recent report of the high-resolution crystal structure of the extracellular domain of the lysosomal LIMP-2, and by modeling the structure of SR-B1 and its other family member, CD36 (
      • Neculai D.
      • Schwake M.
      • Ravichandran M.
      • Zunke F.
      • Collins R.F.
      • Peters J.
      • Neculai M.
      • Plumb J.
      • Loppnau P.
      • Pizarro J.C.
      • et al.
      Structure of LIMP-2 provides functional insights with implications for SR-BI and CD36.
      ,
      • Gomez-Diaz C.
      • Bargeton B.
      • Abuin L.
      • Bukar N.
      • Reina J.H.
      • Bartoi T.
      • Graf M.
      • Ong H.
      • Ulbrich M.H.
      • Masson J.F.
      • et al.
      A CD36 ectodomain mediates insect pheromone detection via a putative tunnelling mechanism.
      ). Importantly, the crystal structure shows the existence of a large cavity or tunnel that traverses the entire length of the molecule. The dimensions (with 5 × 5 Å opening and 22 × 11 × 8 Å cavity) of this hydrophobic tunnel are sufficient to accommodate CE and free cholesterol (FC) molecules, and SR-B1 mutagenesis studies provided direct support for the involvement of this hydrophobic tunnel as a facilitator of cholesterol ester transfer from bound donor HDL particles to the cell surface plasma membrane (
      • Neculai D.
      • Schwake M.
      • Ravichandran M.
      • Zunke F.
      • Collins R.F.
      • Peters J.
      • Neculai M.
      • Plumb J.
      • Loppnau P.
      • Pizarro J.C.
      • et al.
      Structure of LIMP-2 provides functional insights with implications for SR-BI and CD36.
      ,
      • Gomez-Diaz C.
      • Bargeton B.
      • Abuin L.
      • Bukar N.
      • Reina J.H.
      • Bartoi T.
      • Graf M.
      • Ong H.
      • Ulbrich M.H.
      • Masson J.F.
      • et al.
      A CD36 ectodomain mediates insect pheromone detection via a putative tunnelling mechanism.
      ,
      • Phillips M.C.
      Molecular mechanisms of cellular cholesterol efflux.
      ). This is further supported by the observation that an inhibitor of selective HDL-CE uptake binds covalently to Cys384, which is located in the lumen of the tunnel. It is predicted that binding of the inhibitor to this site would block cholesterol (ester) transport (
      • Yu M.
      • Romer K.A.
      • Nieland T.J.
      • Xu S.
      • Saenz-Vash V.
      • Penman M.
      • Yesilaltay A.
      • Carr S.A.
      • Krieger M.
      Exoplasmic cysteine Cys384 of the HDL receptor SR-BI is critical for its sensitivity to a small-molecule inhibitor and normal lipid transport activity.
      ,
      • Phillips M.C.
      Molecular mechanisms of cellular cholesterol efflux.
      ). Once selectively delivered to the cell, the intact CEs are either transported to lipid droplets for storage (
      • Azhar S.
      • Nomoto A.
      • Leers-Sucheta S.
      • Reaven E.
      Simultaneous induction of an HDL receptor protein (SR-BI) and the selective uptake of HDL-cholesteryl esters in a physiologically relevant steroidogenic cell model.
      ) or hydrolyzed extra-lysosomally by neutral cholesteryl esterase (
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ,
      • Yuan Q.
      • Bie J.
      • Wang J.
      • Ghosh S.S.
      • Ghosh S.
      Cooperation between hepatic cholesteryl ester hydrolase and scavenger receptor BI for hydrolysis of HDL-CE.
      ) (
      • Sparrow C.P.
      • Pittman R.C.
      Cholesterol esters selectively taken up from high-density lipoproteins are hydrolyzed extralysosomally.
      ,
      • DeLamatre J.G.
      • Carter R.M.
      • Hornick C.A.
      Evidence for extralysosomal hydrolysis of high-density lipoprotein cholesteryl esters in rat hepatoma cells (Fu5AH): a model for delivery of high-density lipoprotein cholesterol.
      ,
      • Shimada A.
      • Tamai T.
      • Oida K.
      • Takahashi S.
      • Suzuki J.
      • Nakai T.
      • Miyabo S.
      Increase in neutral cholesteryl ester hydrolase activity produced by extralysosomal hydrolysis of high-density lipoprotein cholesteryl esters in rat hepatoma cells (H-35).
      ,
      • Connelly M.A.
      • Kellner-Weibel G.
      • Rothblat G.H.
      • Williams D.L.
      SR-BI-directed HDL-cholesteryl ester hydrolysis.
      ) to provide cholesterol substrate for product formation or reesterification. Previously, we provided evidence that hormone-sensitive lipase serves as the neutral cholesterol esterase in adrenal cells and participates in CE turnover and steroidogenesis (
      • Shen W.J.
      • Azhar S.
      • Kraemer F.B.
      ACTH regulation of adrenal SR-B1.
      ,
      • Kraemer F.B.
      • Shen W.J.
      • Harada K.
      • Patel S.
      • Osuga J.
      • Ishibashi S.
      • Azhar S.
      Hormone-sensitive lipase is required for high-density lipoprotein cholesteryl ester-supported adrenal steroidogenesis.
      ).
      Besides CEs, SR-B1 also mediates the selective uptake of other lipid components of receptor bound HDL particles, including FC, triglycerides (TGs), phospholipids, and α-tocopherol (
      • Rigotti A.
      • Miettinen H.E.
      • Krieger M.
      The role of the high-density lipoprotein receptor SR-BI in the lipid metabolism of endocrine and other tissues.
      ,
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ,
      • Thuahnai S.T.
      • Lund-Katz S.
      • Williams D.L.
      • Phillips M.C.
      Scavenger receptor class B, type I-mediated uptake of various lipids into cells. Influence of the nature of the donor particle interaction with the receptor.
      ). Nonpolar FC, CE, and TG are transported five to ten times more efficiently than more polar phospholipid molecules. The relative selective uptake rate constants for CE, FC, TG, and phospholipid (phosphatidylcholine) have been calculated to be 1.0, 1.6, 0.7, and 0.2, respectively (
      • Thuahnai S.T.
      • Lund-Katz S.
      • Williams D.L.
      • Phillips M.C.
      Scavenger receptor class B, type I-mediated uptake of various lipids into cells. Influence of the nature of the donor particle interaction with the receptor.
      ). SR-B1 also promotes the bidirectional flux of FC between cells and lipoproteins (
      • Connelly M.A.
      • Williams D.L.
      Scavenger receptor BI: a scavenger receptor with a mission to transport high density lipoprotein lipids.
      ,
      • Phillips M.C.
      Molecular mechanisms of cellular cholesterol efflux.
      ,
      • Thuahnai S.T.
      • Lund-Katz S.
      • Williams D.L.
      • Phillips M.C.
      Scavenger receptor class B, type I-mediated uptake of various lipids into cells. Influence of the nature of the donor particle interaction with the receptor.
      ,
      • Yancey P.G.
      • Bortnick A.E.
      • Kellner-Weibel G.
      • de la Llera-Moya M.
      • Phillips M.C.
      • Rothblat G.H.
      Importance of different pathways of cellular cholesterol efflux.
      ,
      • Ji Y.
      • Jian B.
      • Wang N.
      • Sun Y.
      • Moya M.L.
      • Phillips M.C.
      • Rothblat G.H.
      • Swaney J.B.
      • Tall A.R.
      Scavenger receptor BI promotes high density lipoprotein-mediated cellular cholesterol efflux.
      ). In addition, in the liver, SR-B1 functions in reverse cholesterol transport (
      • Shen W.J.
      • Azhar S.
      • Kraemer F.B.
      SR-B1: a unique multifunctional receptor for cholesterol influx and efflux.
      ,
      • Phillips M.C.
      Molecular mechanisms of cellular cholesterol efflux.
      ,
      • Kent A.P.
      • Stylianou I.M.
      Scavenger receptor class B member 1 protein: hepatic regulation and its effects on lipids, reverse cholesterol transport, and atherosclerosis.
      ), including selective uptake of CEs from HDL into the liver (one of the later steps in reverse cholesterol transport) and biliary cholesterol secretion (the last step in reverse cholesterol transport), thus promoting conversion of hepatic HDL cholesterol (HDL-C) to bile acids (
      • Pieters M.N.
      • Schouten D.
      • Bakkeren H.F.
      • Esbach B.
      • Brouwer A.
      • Knook D.L.
      • van Berkel T.J.
      Selective uptake of cholesteryl esters from apolipoprotein-E-free high-density lipoproteins by rat parenchymal cells in vivo is efficiently coupled to bile acid synthesis.
      ,
      • Mardones P.
      • Quinones V.
      • Amigo L.
      • Moreno M.
      • Miquel J.F.
      • Schwarz M.
      • Miettinen H.E.
      • Trigatti B.
      • Krieger M.
      • VanPatten S.
      • et al.
      Hepatic cholesterol and bile acid metabolism and intestinal cholesterol absorption in scavenger receptor class B type I-deficient mice.
      ,
      • Wiersma H.
      • Gatti A.
      • Nijstad N.
      • Oude Elferink R.P.
      • Kuipers F.
      • Tietge U.J.
      Scavenger receptor class B type I mediates biliary cholesterol secretion independent of ATP-binding cassette transporter g5/g8 in mice.
      ). SR-B1 also alters the composition of lipid domains of plasma membranes, which then leads to changes in FC flux, changes in membrane cholesterol content, or altered physical/chemical properties of membranes (
      • Kellner-Weibel G.
      • de La Llera-Moya M.
      • Connelly M.A.
      • Stoudt G.
      • Christian A.E.
      • Haynes M.P.
      • Williams D.L.
      • Rothblat G.H.
      Expression of scavenger receptor BI in COS-7 cells alters cholesterol content and distribution.
      ,
      • Silver D.L.
      • Tall A.R.
      The cellular biology of scavenger receptor class B type I.
      ).

      OTHER MAJOR FUNCTIONS OF SR-B1

      In addition to mediating the selective transport of HDL-CEs and other lipids, SR-B1 also performs many other essential functions. Besides facilitating selective cholesterol influx into cells, SR-B1 enhances the efflux of cellular cholesterol to HDL (
      • Ji Y.
      • Jian B.
      • Wang N.
      • Sun Y.
      • Moya M.L.
      • Phillips M.C.
      • Rothblat G.H.
      • Swaney J.B.
      • Tall A.R.
      Scavenger receptor BI promotes high density lipoprotein-mediated cellular cholesterol efflux.
      ,
      • Jian B.
      • de la Llera-Moya M.
      • Ji Y.
      • Wang N.
      • Phillips M.C.
      • Swaney J.B.
      • Tall A.R.
      • Rothblat G.H.
      Scavenger receptor class B type I as a mediator of cellular cholesterol efflux to lipoproteins and phospholipid acceptors.
      ,
      • de la Llera-Moya M.
      • Rothblat G.H.
      • Connelly M.A.
      • Kellner-Weibel G.
      • Sakr S.W.
      • Phillips M.C.
      • Williams D.L.
      Scavenger receptor BI (SR-BI) mediates free cholesterol flux independently of HDL tethering to the cell surface.
      ,
      • Gu X.
      • Kozarsky K.
      • Krieger M.
      Scavenger receptor class B, type I-mediated [3H]cholesterol efflux to high and low density lipoproteins is dependent on lipoprotein binding to the receptor.
      ). This property of SR-B1 in mediating the bidirectional flux of FC between cells and HDL is different from two other proteins involved in cellular cholesterol efflux, ABCA1 and ABCG1; ABCA1 mediates unidirectional efflux of cholesterol and phospholipids to apolipoprotein acceptors, such apoA-I and apoE (
      • Oram J.F.
      • Lawn R.M.
      • Garvin M.R.
      • Wade D.P.
      ABCA1 is the cAMP-inducible apolipoprotein receptor that mediates cholesterol secretion from macrophages.
      ), whereas ABCG1 facilitates unidirectional efflux of cholesterol to nascent HDL particles (
      • Sankaranarayanan S.
      • Oram J.F.
      • Asztalos B.F.
      • Vaughan A.M.
      • Lund-Katz S.
      • Adorni M.P.
      • Phillips M.C.
      • Rothblat G.H.
      Effects of acceptor composition and mechanism of ABCG1-mediated cellular free cholesterol efflux.
      ). Both gain-of-function and loss-of-function strategies in mice have established that SR-B1 determines the level of plasma-HDL-C by promoting selective transport of HDL-CE to the liver for transfer to bile and that SR-B1 is a key component of reverse cholesterol transport (
      • Rigotti A.
      • Trigatti B.L.
      • Penman M.
      • Rayburn H.
      • Herz J.
      • Krieger M.
      A targeted mutation in the murine gene encoding the high density lipoprotein (HDL) receptor scavenger receptor class B type I reveals its key role in HDL metabolism.
      ,
      • Varban M.L.
      • Rinninger F.
      • Wang N.
      • Fairchild-Huntress V.
      • Dunmore J.H.
      • Fang Q.
      • Gosselin M.L.
      • Dixon K.L.
      • Deeds J.D.
      • Acton S.L.
      • et al.
      Targeted mutation reveals a central role for SR-BI in hepatic selective uptake of high density lipoprotein cholesterol.
      ,
      • Ueda Y.
      • Royer L.
      • Gong E.
      • Zhang J.
      • Cooper P.N.
      • Francone O.
      • Rubin E.M.
      Lower plasma levels and accelerated clearance of high density lipoprotein (HDL) and non-HDL cholesterol in scavenger receptor class B type I transgenic mice.
      ,
      • Yesilaltay A.
      • Morales M.G.
      • Amigo L.
      • Zanlungo S.
      • Rigotti A.
      • Karackattu S.L.
      • Donahee M.H.
      • Kozarsky K.F.
      • Krieger M.
      Effects of hepatic expression of the high-density lipoprotein receptor SR-BI on lipoprotein metabolism and female fertility.
      ). Consistent with its importance in reverse cholesterol transport, several mouse models have shown that SR-B1 overexpression in the liver decreases atherosclerosis (
      • Arai T.
      • Wang N.
      • Bezouevski M.
      • Welch C.
      • Tall A.R.
      Decreased atherosclerosis in heterozygous low density lipoprotein receptor-deficient mice expressing the scavenger receptor BI transgene.
      ,
      • Kozarsky K.F.
      • Donahee M.H.
      • Glick J.M.
      • Krieger M.
      • Rader D.J.
      Gene transfer and hepatic overexpression of the HDL receptor SR-BI reduces atherosclerosis in the cholesterol-fed LDL receptor-deficient mouse.
      ,
      • Ueda Y.
      • Gong E.
      • Royer L.
      • Cooper P.N.
      • Francone O.L.
      • Rubin E.M.
      Relationship between expression levels and atherogenesis in scavenger receptor class B, type I transgenics.
      ,
      • Zhang Y.
      • Da Silva J.R.
      • Reilly M.
      • Billheimer J.T.
      • Rothblat G.H.
      • Rader D.J.
      Hepatic expression of scavenger receptor class B type I (SR-BI) is a positive regulator of macrophage reverse cholesterol transport in vivo.
      ), whereas partial or total loss of SR-B1 increases atherosclerosis (
      • Trigatti B.
      • Rayburn H.
      • Vinals M.
      • Braun A.
      • Miettinen H.
      • Penman M.
      • Hertz M.
      • Schrenzel M.
      • Amigo L.
      • Rigotti A.
      • et al.
      Influence of the high density lipoprotein receptor SR-BI on reproductive and cardiovascular pathophysiology.
      ,
      • Huszar D.
      • Varban M.L.
      • Rinninger F.
      • Feeley R.
      • Arai T.
      • Fairchild-Huntress V.
      • Donovan M.J.
      • Tall A.R.
      Increased LDL cholesterol and atherosclerosis in LDL receptor-deficient mice with attenuated expression of scavenger receptor B1.
      ,
      • Covey S.D.
      • Krieger M.
      • Wang W.
      • Penman M.
      • Trigatti B.L.
      Scavenger receptor class B type I-mediated protection against atherosclerosis in LDL receptor-negative mice involves its expression in bone marrow-derived cells.
      ,
      • Van Eck M.
      • Twisk J.
      • Hoekstra M.
      • Van Rij B.T.
      • Van der Lans C.A.
      • Bos I.S.
      • Kruijt J.K.
      • Kuipers F.
      • Van Berkel T.J.
      Differential effects of scavenger receptor BI deficiency on lipid metabolism in cells of the arterial wall and in the liver.
      ,
      • Huby T.
      • Doucet C.
      • Dachet C.
      • Ouzilleau B.
      • Ueda Y.
      • Afzal V.
      • Rubin E.
      • Chapman M.J.
      • Lesnik P.
      Knockdown expression and hepatic deficiency reveal an atheroprotective role for SR-BI in liver and peripheral tissues.
      ). Thus, SR-B1 can exert atheroprotective actions by impacting HDL metabolism and reverse cholesterol transport, mediating removal of cholesterol from macrophages via cholesterol efflux, and minimizing inflammation and oxidation (
      • Hoekstra M.
      SR-BI as target in atherosclerosis and cardiovascular disease - a comprehensive appraisal of the cellular functions of SR-BI in physiology and disease.
      ,
      • Kent A.P.
      • Stylianou I.M.
      Scavenger receptor class B member 1 protein: hepatic regulation and its effects on lipids, reverse cholesterol transport, and atherosclerosis.
      ,
      • Linton M.F.
      • Tao H.
      • Linton E.F.
      • Yancey P.G.
      SR-BI: a multifunctional receptor in cholesterol homeostasis and atherosclerosis.
      ). In addition, macrophage and endothelial cell SR-B1 could interfere with the development of atherosclerosis by altering cholesterol trafficking to suppress atherosclerotic lesion formation (
      • Linton M.F.
      • Tao H.
      • Linton E.F.
      • Yancey P.G.
      SR-BI: a multifunctional receptor in cholesterol homeostasis and atherosclerosis.
      ). Likewise, platelet SR-B1 has been implicated as a negative regulator in the development of thrombosis (
      • Hoekstra M.
      SR-BI as target in atherosclerosis and cardiovascular disease - a comprehensive appraisal of the cellular functions of SR-BI in physiology and disease.
      ). SR-B1 signaling also helps to minimize inflammation and apoptosis, and promotes efferocytosis of apoptotic cells in atherosclerotic lesions, thus minimizing vulnerable plaque formation (
      • Linton M.F.
      • Tao H.
      • Linton E.F.
      • Yancey P.G.
      SR-BI: a multifunctional receptor in cholesterol homeostasis and atherosclerosis.
      ). In contrast, endothelial cell SR-B1 plays a more complex role, perhaps through involvement in LDL transcytosis, and may contribute to the development of early atherosclerotic lesions (
      • Armstrong S.M.
      • Sugiyama M.G.
      • Fung K.Y.
      • Gao Y.
      • Wang C.
      • Levy A.S.
      • Azizi P.
      • Roufaiel M.
      • Zhu S.N.
      • Neculai D.
      • et al.
      A novel assay uncovers an unexpected role for SR-BI in LDL transcytosis.
      ). SR-B1 has also been shown to regulate fertility (
      • Yesilaltay A.
      • Morales M.G.
      • Amigo L.
      • Zanlungo S.
      • Rigotti A.
      • Karackattu S.L.
      • Donahee M.H.
      • Kozarsky K.F.
      • Krieger M.
      Effects of hepatic expression of the high-density lipoprotein receptor SR-BI on lipoprotein metabolism and female fertility.
      ,
      • Trigatti B.
      • Rayburn H.
      • Vinals M.
      • Braun A.
      • Miettinen H.
      • Penman M.
      • Hertz M.
      • Schrenzel M.
      • Amigo L.
      • Rigotti A.
      • et al.
      Influence of the high density lipoprotein receptor SR-BI on reproductive and cardiovascular pathophysiology.
      ,
      • Yates M.
      • Kolmakova A.
      • Zhao Y.
      • Rodriguez A.
      Clinical impact of scavenger receptor class B type I gene polymorphisms on human female fertility.
      ), modulate erythrocyte development (
      • Holm T.M.
      • Braun A.
      • Trigatti B.L.
      • Brugnara C.
      • Sakamoto M.
      • Krieger M.
      • Andrews N.C.
      Failure of red blood cell maturation in mice with defects in the high-density lipoprotein receptor SR-BI.
      ) and platelet function (
      • Hoekstra M.
      SR-BI as target in atherosclerosis and cardiovascular disease - a comprehensive appraisal of the cellular functions of SR-BI in physiology and disease.
      ,
      • Zheng Z.
      • Ai J.
      • Li X.A.
      Scavenger receptor class B type I and immune dysfunctions.
      ), contribute to the pathogenesis of inflammation (
      • Zheng Z.
      • Ai J.
      • Li X.A.
      Scavenger receptor class B type I and immune dysfunctions.
      ), regulate immunity (
      • Zheng Z.
      • Ai J.
      • Li X.A.
      Scavenger receptor class B type I and immune dysfunctions.
      ), protect against nitric-oxide-induced cytotoxicity, mediate inducible glucocorticoid production and LPS clearance, protect against sepsis, and play a role in lymphocyte homeostasis and autoimmunity (
      • Zheng Z.
      • Ai J.
      • Li X.A.
      Scavenger receptor class B type I and immune dysfunctions.
      ). SR-B1 has been implicated in some pathological processes as well. For instance, SR-B1, in concert with several viral and cellular factors, mediates hepatitis C virus entry into hepatocytes (
      • Tong Y.
      • Zhu Y.
      • Xia X.
      • Liu Y.
      • Feng Y.
      • Hua X.
      • Chen Z.
      • Ding H.
      • Gao L.
      • Wang Y.
      • et al.
      Tupaia CD81, SR-BI, claudin-1, and occludin support hepatitis C virus infection.
      ,
      • Burlone M.E.
      • Budkowska A.
      Hepatitis C virus cell entry: role of lipoproteins and cellular receptors.
      ), enhances dengue virus infection (
      • Li Y.
      • Kakinami C.
      • Li Q.
      • Yang B.
      • Li H.
      Human apolipoprotein A-I is associated with dengue virus and enhances virus infection through SR-BI.
      ), mediates bacterial adhesion and bacterial invasion in cells (
      • Vishnyakova T.G.
      • Bocharov A.V.
      • Baranova I.N.
      • Chen Z.
      • Remaley A.T.
      • Csako G.
      • Eggerman T.L.
      • Patterson A.P.
      Binding and internalization of lipopolysaccharide by Cla-1, a human orthologue of rodent scavenger receptor B1.
      ), potentiates LPS-induced inflammation and acute liver and kidney injury in mice (
      • Baranova I.N.
      • Souza A.C.
      • Bocharov A.V.
      • Vishnyakova T.G.
      • Hu X.
      • Vaisman B.L.
      • Amar M.J.
      • Chen Z.
      • Kost Y.
      • Remaley A.T.
      • et al.
      Human SR-BI and SR-BII potentiate lipopolysaccharide-induced inflammation and acute liver and kidney injury in mice.
      ), and contributes to the pathogenesis of cancer and tumor progression.

      SR-B1 SIGNALING

      In view of the large variety of ligands reported to bind SR-B1 and the large number of functions attributed to SR-B1 in addition to lipid uptake and efflux, the observation that SR-B1 mediated the ability of HDL to stimulate endothelial nitric oxide synthase (eNOS) in cultured endothelial cells suggested that SR-B1 might possess signaling properties (
      • Yuhanna I.S.
      • Zhu Y.
      • Cox B.E.
      • Hahner L.D.
      • Osborne-Lawrence S.
      • Lu P.
      • Marcel Y.L.
      • Anderson R.G.
      • Mendelsohn M.E.
      • Hobbs H.H.
      • et al.
      High-density lipoprotein binding to scavenger receptor-BI activates endothelial nitric oxide synthase.
      ). Although neither cytoplasmic tail of SR-B1 possesses intrinsic signaling properties, experiments identified the interaction of c-Src with SR-B1 following HDL binding (
      • Zhu W.
      • Saddar S.
      • Seetharam D.
      • Chambliss K.L.
      • Longoria C.
      • Silver D.L.
      • Yuhanna I.S.
      • Shaul P.W.
      • Mineo C.
      The scavenger receptor class B type I adaptor protein PDZK1 maintains endothelial monolayer integrity.
      ) with downstream activation of PI3 kinase, Akt, and MAPK, resulting in phosphorylation and activation of eNOS (
      • Mineo C.
      • Yuhanna I.S.
      • Quon M.J.
      • Shaul P.W.
      High density lipoprotein-induced endothelial nitric-oxide synthase activation is mediated by Akt and MAP kinases.
      ). The adaptor protein, PDZK1, which interacts with SR-B1 (see below), was reported to be required for SR-B1 signaling (
      • Zhu W.
      • Saddar S.
      • Seetharam D.
      • Chambliss K.L.
      • Longoria C.
      • Silver D.L.
      • Yuhanna I.S.
      • Shaul P.W.
      • Mineo C.
      The scavenger receptor class B type I adaptor protein PDZK1 maintains endothelial monolayer integrity.
      ); however, a mutation in the C-terminal transmembrane domain that disrupts the ability of SR-B1 to bind plasma membrane cholesterol eliminated SR-B1 signaling to eNOS without altering HDL binding, cholesterol uptake or efflux, or interaction with PDZK1 (
      • Saddar S.
      • Carriere V.
      • Lee W.R.
      • Tanigaki K.
      • Yuhanna I.S.
      • Parathath S.
      • Morel E.
      • Warrier M.
      • Sawyer J.K.
      • Gerard R.D.
      • et al.
      Scavenger receptor class B type I is a plasma membrane cholesterol sensor.
      ), suggesting that the processes are separate. Recent work suggests that the localization of SR-B1 in lipid rafts and its ability to sense plasma membrane cholesterol link it to signal transduction (
      • Morel E.
      • Ghezzal S.
      • Lucchi G.
      • Truntzer C.
      • Pais de Barros J.P.
      • Simon-Plas F.
      • Demignot S.
      • Mineo C.
      • Shaul P.W.
      • Leturque A.
      • et al.
      Cholesterol trafficking and raft-like membrane domain composition mediate scavenger receptor class B type 1-dependent lipid sensing in intestinal epithelial cells.
      ). Whether all signaling via SR-B1 is due to its function as a lipid sensor is currently unclear. In favor of functioning as a lipid sensor, cell signaling that is induced by cellular cholesterol depletion with methyl-β-cyclodextrin is dependent on SR-B1 and, specifically, its ability to bind plasma membrane cholesterol (
      • Saddar S.
      • Carriere V.
      • Lee W.R.
      • Tanigaki K.
      • Yuhanna I.S.
      • Parathath S.
      • Morel E.
      • Warrier M.
      • Sawyer J.K.
      • Gerard R.D.
      • et al.
      Scavenger receptor class B type I is a plasma membrane cholesterol sensor.
      ). Moreover, serum amyloid A binding to SR-B1 induces activation of ERK1/2 and p38 MAPKs leading to increased cytokine production (
      • Baranova I.N.
      • Vishnyakova T.G.
      • Bocharov A.V.
      • Kurlander R.
      • Chen Z.
      • Kimelman M.L.
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      Serum amyloid A binding to CLA-1 (CD36 and LIMPII analogous-1) mediates serum amyloid A protein-induced activation of ERK1/2 and p38 mitogen-activated protein kinases.
      ), while serum amyloid A also promotes SR-B1-mediated cholesterol efflux (
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      Serum amyloid A promotes cholesterol efflux mediated by scavenger receptor B-I.
      ). In addition, SR-B1 can mediate the uptake or efflux of sphingosine-1-phosphate, along with interaction of sphingosine-1-phosphate receptors with SR-B1, leading to activation of signaling via sphingosine-1-phosphate receptors (
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      S1P in HDL promotes interaction between SR-BI and S1PR1 and activates S1PR1-mediated biological functions: calcium flux and S1PR1 internalization.