Lecithin:cholesterol acyltransferase: old friend or foe in atherosclerosis?
2012; Elsevier BV; Volume: 53; Issue: 9 Linguagem: Inglês
10.1194/jlr.r024513
ISSN1539-7262
AutoresSandra Kunnen, Miranda Van Eck,
Tópico(s)Peroxisome Proliferator-Activated Receptors
ResumoLecithin:cholesterol acyltransferase (LCAT) is a key enzyme that catalyzes the esterification of free cholesterol in plasma lipoproteins and plays a critical role in high-density lipoprotein (HDL) metabolism. Deficiency leads to accumulation of nascent preβ-HDL due to impaired maturation of HDL particles, whereas enhanced expression is associated with the formation of large, apoE-rich HDL1 particles. In addition to its function in HDL metabolism, LCAT was believed to be an important driving force behind macrophage reverse cholesterol transport (RCT) and, therefore, has been a subject of great interest in cardiovascular research since its discovery in 1962. Although half a century has passed, the importance of LCAT for atheroprotection is still under intense debate. This review provides a comprehensive overview of the insights that have been gained in the past 50 years on the biochemistry of LCAT, the role of LCAT in lipoprotein metabolism and the pathogenesis of atherosclerosis in animal models, and its impact on cardiovascular disease in humans. Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme that catalyzes the esterification of free cholesterol in plasma lipoproteins and plays a critical role in high-density lipoprotein (HDL) metabolism. Deficiency leads to accumulation of nascent preβ-HDL due to impaired maturation of HDL particles, whereas enhanced expression is associated with the formation of large, apoE-rich HDL1 particles. In addition to its function in HDL metabolism, LCAT was believed to be an important driving force behind macrophage reverse cholesterol transport (RCT) and, therefore, has been a subject of great interest in cardiovascular research since its discovery in 1962. Although half a century has passed, the importance of LCAT for atheroprotection is still under intense debate. This review provides a comprehensive overview of the insights that have been gained in the past 50 years on the biochemistry of LCAT, the role of LCAT in lipoprotein metabolism and the pathogenesis of atherosclerosis in animal models, and its impact on cardiovascular disease in humans. cholesterol ester transfer protein carotid intima-media thickness cardiovascular disease fish-eye disease familial LCAT deficiency lipoprotein X nascent discoidal HDL reverse cholesterol transport scavenger receptor BI In 1935, Sperry was the first to recognize that when human plasma was incubated at 37°C, a marked esterification of free cholesterol occurred (1Sperry W.M. Cholesterol esterase in blood.J. Biol. Chem. 1935; 111: 467-478Abstract Full Text PDF Google Scholar). He attributed this to enzymatic activity, as the effect was abolished when the plasma was heated to 55–60°C. Subsequent work by Glomset led in 1962 to the identification of the lecithin:cholesterol acyltransferase (LCAT) enzyme, which accounts for the synthesis of most of the cholesterol esters in plasma (2Glomset J.A. The mechanism of the plasma cholesterol esterification reaction: plasma fatty acid transferase.Biochim. Biophys. Acta. 1962; 65: 128-135Crossref PubMed Google Scholar). Four years later, Glomset identified LCAT as an important driving force behind the reverse cholesterol transport (RCT) pathway (3Glomset J.A. Janssen E.T. Kennedy R. Dobbins J. Role of plasma lecithin:cholesterol acyltransferase in the metabolism of high density lipoproteins.J. Lipid Res. 1966; 7: 638-648Abstract Full Text PDF PubMed Google Scholar), a process that describes the HDL-mediated removal of excess cholesterol from macrophages in the arterial wall and subsequent delivery to the liver for biliary excretion. Interest in the enzyme increased even further when in 1967 the first family with three sisters with familial LCAT deficiency was described (4Norum K.R. Gjone E. Familial plasma lecithin:cholesterol acyltransferase deficiency. Biochemical study of a new inborn error of metabolism.Scandinavian Journal of Clinical & Laboratory Investigation. 1967; 20: 231-243Crossref Scopus (0) Google Scholar). To date, approximately 60 isolated cases and 70 small families with partial or complete LCAT deficiency have been described with 86 different molecular defects in the LCAT gene (5Calabresi, L., Simonelli, S., Gomaraschi, M., Franceschini, G., . Genetic lecithin:cholesterol acyltransferase deficiency and cardiovascular disease. Atherosclerosis., Epub ahead of print. November 28, 2011; doi:10.1016/j.atherosclerosis.2011.11.034.Google Scholar) (http://www.hgmd.org). In addition, numerous animal models lacking or overexpressing LCAT, including mice (6Vaisman B.L. Klein H.G. Rouis M. Bérard A.M. Kindt M.R. Talley G.D. Meyn S.M. Hoyt Jr., R.F. Marcovina S.M. Albers J.J. et al.Overexpression of human lecithin cholesterol acyltransferase leads to hyperalphalipoproteinemia in transgenic mice.J. Biol. Chem. 1995; 270: 12269-12275Abstract Full Text Full Text PDF PubMed Scopus (105) Google Scholar–10Ng D.S. Francone O.L. Forte T.M. Zhang J. Haghpassand M. Rubin E.M. Disruption of the murine lecithin:cholesterol acyltransferase gene causes impairment of adrenal lipid delivery and up-regulation of scavenger receptor class B type I.J. Biol. Chem. 1997; 272: 15777-15781Abstract Full Text Full Text PDF PubMed Scopus (120) Google Scholar), hamsters (11Zhang A.H. Gao S. Fan J.L. Huang W. Zhao T.Q. Liu G. Increased plasma HDL cholesterol levels and biliary cholesterol excretion in hamster by LCAT overexpression.FEBS Lett. 2004; 570: 25-29Crossref PubMed Scopus (25) Google Scholar), rabbits (12Hoeg J.M. Santamarina-Fojo S. Bérard A.M. Cornhill J.F. Herderick E.E. Feldman S.H. Haudenschild C.C. Vaisman B.L. Hoyt Jr., R.F. Demosky Jr., S.J. et al.Overexpression of lecithin: cholesterol acyltransferase in transgenic rabbits prevents diet-induced atherosclerosis.Proc. Natl. Acad. Sci. USA. 1996; 93: 11448-11453Crossref PubMed Scopus (203) Google Scholar), and monkeys (13Amar M.J. Shamburek R.D. Vaisman B. Knapper C.L. Foger B. Hoyt Jr., R.F. Santamarina-Fojo S. Brewer Jr., H.B. Remaley A.T. Adenoviral expression of human lecithin-cholesterol acyltransferase in nonhuman primates leads to an antiatherogenic lipoprotein phenotype by increasing high-density lipoprotein and lowering low-density lipoprotein.Metabolism. 2009; 58: 568-575Abstract Full Text Full Text PDF PubMed Scopus (29) Google Scholar) have been generated to gain better insight in the complex role of LCAT in modulating lipoprotein metabolism, RCT, and atherosclerosis. The human LCAT gene is localized in the q21–22 region of chromosome 16. It consists of 6 exons separated by 5 introns and encompasses a total of 4.2 kb (14Teisberg P. Gjone E. Olaisen B. Genetics of LCAT (lecithin: cholesterol acyltransferase) deficiency.Ann. Hum. Genet. 1975; 38: 327-331Crossref PubMed Google Scholar, 15Jonas A. Lecithin cholesterol acyltransferase.Biochim. Biophys. Acta. 2000; 1529: 245-256Crossref PubMed Scopus (281) Google Scholar). In 1986, it was sequenced and cloned for the first time (16McLean J. Fielding C. Drayna D. Dieplinger H. Baer B. Kohr W. Henzel W. Lawn R. Cloning and expression of human lecithin-cholesterol acyltransferase cDNA.Proc. Natl. Acad. Sci. USA. 1986; 83: 2335-2339Crossref PubMed Google Scholar, 17McLean J. Wion K. Drayna D. Fielding C. Lawn R. Human lecithin-cholesterol acyltransferase gene: complete gene sequence and sites of expression.Nucleic Acids Res. 1986; 14: 9397-9406Crossref PubMed Google Scholar). The gene encodes for a polypeptide chain, consisting of 416 amino acid residues with an apparent molecular mass of approximately 60 kDa. LCAT is a glycoprotein with four N-glycosylation (Asn20, 84, 272, and 384) and two O-glycosylation sites (Thr407 and Ser409) (18Schindler P.A. Settineri C.A. Collet X. Fielding C.J. Burlingame A.L. Site-specific detection and structural characterization of the glycosylation of human plasma proteins lecithin:cholesterol acyltransferase and apolipoprotein D using HPLC/ electrospray mass spectrometry and sequential glycosidase digestion.Protein Sci. 1995; 4: 791-803Crossref PubMed Google Scholar). The carbohydrate content is ∼25% of its total mass, with the majority being N-linked (19Collet X. Fielding C.J. Effects of inhibitors of N-linked oligosaccharide processing on the secretion, stability, and activity of lecithin:cholesterol acyltransferase.Biochemistry. 1991; 30: 3228-3234Crossref PubMed Scopus (0) Google Scholar). Removal of the carbohydrate moieties of isolated human LCAT by neuraminidase is associated with a 60% increase in the enzymatic activity (20Doi Y. Nishida T. Microheterogeneity and physical properties of human lecithin-cholesterol acyltransferase.J. Biol. Chem. 1983; 258: 5840-5846Abstract Full Text PDF PubMed Google Scholar). However, inhibition of glycosylation in Chinese hamster ovary (CHO) cells reduced the enzymatic activity without affecting LCAT protein secretion (19Collet X. Fielding C.J. Effects of inhibitors of N-linked oligosaccharide processing on the secretion, stability, and activity of lecithin:cholesterol acyltransferase.Biochemistry. 1991; 30: 3228-3234Crossref PubMed Scopus (0) Google Scholar). The biological significance of the two O-glycosylation sites is largely unclear. Site-directed mutagenesis studies in transfected COS-6 cells by substitution of Asn with Thr showed that N-linked glycosylation at Asn272 is indispensible for secretion of active LCAT, whereas Asn84 is critical for its full activity but not for intracellular processing (21Qu S.J. Fan H.Z. Blanco-Vaca F. Pownall H.J. Effects of site-directed mutagenesis on the N-glycosylation sites of human lecithin:cholesterol acyltransferase.Biochemistry. 1993; 32: 8732-8736Crossref PubMed Google Scholar). In another study by Karmin and colleagues, the effect of substitution of the Asn residues with Gln was investigated in COS-1 cells (22Karmin O. Hill J.S. Wang X. McLeod R. Pritchard P.H. Lecithin:cholesterol acyltransferase: role of N-linked glycosylation in enzyme function.Biochem. J. 1993; 294: 879-884Crossref PubMed Scopus (0) Google Scholar). These studies showed that glycosylation at all four sites is required to generate the full-size mature LCAT protein, but deletion of only one of the N-linked glycosylation sites does not affect intracellular processing and secretion. The pattern of N-linked glycosylation, however, has profound effects on the catalytic activity of the enzyme. Substitution of Asn84 or Asn272 with Gln led to 82% and 62% decrease in activity, respectively, whereas replacement of Asn384 led to substantially increased activity (21Qu S.J. Fan H.Z. Blanco-Vaca F. Pownall H.J. Effects of site-directed mutagenesis on the N-glycosylation sites of human lecithin:cholesterol acyltransferase.Biochemistry. 1993; 32: 8732-8736Crossref PubMed Google Scholar). Furthermore, N-glycosylation is important for determining substrate specificity toward native HDL and LDL (23Karmin O. Hill J.S. Pritchard P.H. Role of N-linked glycosylation of lecithin:cholesterol acyltransferase in lipoprotein substrate specificity.Biochim. Biophys. Acta. 1995; 1254: 193-197Crossref PubMed Scopus (0) Google Scholar). No X-ray structure of the LCAT protein has been published. By use of selective chemical modification and stoichiometric analysis, it was shown that a single serine and a single histidine mediate lecithin cleavage and that the two free cysteines participate as transient fatty acyl acceptors in cholesterol esterification (24Jauhiainen M. Dolphin P.J. Human plasma lecithin-cholesterol acyltransferase. An elucidation of the catalytic mechanism.J. Biol. Chem. 1986; 261: 7032-7043Abstract Full Text PDF PubMed Google Scholar–26Jauhiainen M. Stevenson K.J. Dolphin P.J. Human plasma lecithin-cholesterol acyltransferase. The vicinal nature of cysteine 31 and cysteine 184 in the catalytic site.J. Biol. Chem. 1988; 263: 6525-6533Abstract Full Text PDF PubMed Google Scholar). The first structural model of LCAT was proposed by Yang and colleagues in 1987 based upon the primary structure, chemical modification, homology with other proteins, and enzymatic assays (27Yang C.Y. Manoogian D. Pao Q. Lee F.S. Knapp R.D. Gotto Jr., A.M. Pownall H.J. Lecithin:cholesterol acyltrans-ferase. Functional regions and a structural model of the enzyme.J. Biol. Chem. 1987; 262: 3086-3091Abstract Full Text PDF PubMed Google Scholar). Furthermore, 3D modeling based on its homology with other members of the αβ-hydrolase superfamily has shown that LCAT contains a catalytic triad consisting of three amino acid residues, serine (Ser), aspartic acid (Asp), and histidine (His) at positions 181, 345, and 377 (28Francone O.L. Fielding C.J. Effects of site-directed mutagenesis at residues cysteine-31 and cysteine-184 on lecithin-cholesterol acyltransferase activity.Proc. Natl. Acad. Sci. USA. 1991; 88: 1716-1720Crossref PubMed Scopus (60) Google Scholar–30Peelman F. Vinaimont N. Verhee A. Vanloo B. Verschelde J.L. Labeur C. Seguret-Mace S. Duverger N. Hutchinson G. Vandekerckhove J. et al.A proposed architecture for lecithin cholesterol acyl transferase (LCAT): identification of the catalytic triad and molecular modeling.Protein Sci. 1998; 7: 587-599Crossref PubMed Google Scholar). Recently, using a combination of several new and improved fold-recognition methods, Holleboom and colleagues confirmed this model with, according to the new protein nomenclature, residues Ser205, Asp369, and His401 forming the catalytic triad of LCAT (31Francone O.L. Fielding C.J. Structure-function relationships in human lecithin:cholesterol acyltransferase. Site-directed mutagenesis at serine residues 181 and 216.Biochemistry. 1991; 30: 10074-10077Crossref PubMed Google Scholar). LCAT contains six cysteine residues, of which four are located at the active site of the protein and are used to form two disulfide bridges (Cys50–Cys74 and Cys313–Cys356) (27Yang C.Y. Manoogian D. Pao Q. Lee F.S. Knapp R.D. Gotto Jr., A.M. Pownall H.J. Lecithin:cholesterol acyltrans-ferase. Functional regions and a structural model of the enzyme.J. Biol. Chem. 1987; 262: 3086-3091Abstract Full Text PDF PubMed Google Scholar, 32Holleboom A.G. Kuivenhoven J.A. Peelman F. Schimmel A.W. Peter J. Defesche J.C. Kastelein J.J. Hovingh G.K. Stroes E.S. Motazacker M.M. High prevalence of mutations in LCAT in patients with low HDL cholesterol levels in The Netherlands: identification and characterization of eight novel mutations.Hum. Mutat. 2011; 32: 1290-1298Crossref PubMed Scopus (29) Google Scholar). The disulfide-linked Cys50–Cys74 residues span the lid region of LCAT that covers the catalytic site of LCAT and opens upon binding to lipoprotein surfaces (33Jonas A. Regulation of lecithin cholesterol acyltransferase activity.Prog. Lipid Res. 1998; 37: 209-234Crossref PubMed Scopus (94) Google Scholar). The human LCAT protein is synthesized primarily by the liver, although it is also expressed in small amounts in the testes and in astrocytes in the brain, where it is involved in the esterification of cholesterol in glia-derived apoE-containing lipoproteins (15Jonas A. Lecithin cholesterol acyltransferase.Biochim. Biophys. Acta. 2000; 1529: 245-256Crossref PubMed Scopus (281) Google Scholar, 27Yang C.Y. Manoogian D. Pao Q. Lee F.S. Knapp R.D. Gotto Jr., A.M. Pownall H.J. Lecithin:cholesterol acyltrans-ferase. Functional regions and a structural model of the enzyme.J. Biol. Chem. 1987; 262: 3086-3091Abstract Full Text PDF PubMed Google Scholar, 34Warden C.H. Langner C.A. Gordon J.I. Taylor B.A. McLean J.W. Lusis A.J. Tissue-specific expression, developmental regulation, and chromosomal mapping of the lecithin: cholesterol acyltransferase gene. Evidence for expression in brain and testes as well as liver.J. Biol. Chem. 1989; 264: 21573-21581Abstract Full Text PDF PubMed Google Scholar, 35Hirsch-Reinshagen V. Donkin J. Stukas S. Chan J. Wilkinson A. Fan J. Parks J.S. Kuivenhoven J.A. Lütjohann D. Pritchard H. et al.LCAT synthesized by primary astrocytes esterifies cholesterol on glia-derived lipoproteins.J. Lipid Res. 2009; 50: 885-893Abstract Full Text Full Text PDF PubMed Scopus (31) Google Scholar). The plasma concentration of LCAT is about 6 μg/ml and varies little in adult humans with age, gender, and smoking (36Albers J.J. Bergelin R.O. Adolphson J.L. Wahl P.W. Population-based reference values for lecithin-cholesterol acyltransferase (LCAT).Atherosclerosis. 1982; 43: 369-379Abstract Full Text PDF PubMed Scopus (0) Google Scholar). The half-life of human LCAT in plasma has been estimated to be 4–5 days (37Stokke K.T. Bjerve K.S. Blomhoff J.P. Oystese B. Flatmark A. Norum K.R. Gjone E. Familial lecithin:cholesterol acyltransferase deficiency. Studies on lipid composition and morphology of tissues.Scand. J. Clin. Lab. Invest. Suppl. 1974; 137: 93-100Crossref PubMed Scopus (42) Google Scholar) LCAT reversibly binds to lipoproteins and is primarily found on HDL, which likely prevents its rapid clearance from the circulation (38Rousset X. Vaisman B. Auerbach B. Krause B.R. Homan R. Stonik J. Csako G. Shamburek R. Remaley A.T. Effect of recombinant human lecithin cholesterol acyltransferase infusion on lipoprotein metabolism in mice.J. Pharmacol. Exp. Ther. 2010; 335: 140-148Crossref PubMed Scopus (60) Google Scholar). ApoAI is the most potent activator of LCAT, which enables it to convert free cholesterol into cholesteryl esters on HDL by a transesterification reaction involving the transfer of a fatty acid at the sn-2 position of phosphatidylcholine (or lecithin) to the free hydroxyl group of cholesterol (15Jonas A. Lecithin cholesterol acyltransferase.Biochim. Biophys. Acta. 2000; 1529: 245-256Crossref PubMed Scopus (281) Google Scholar, 39Fielding C.J. Fielding P.E. Molecular physiolology of reverse cholesterol transport.J. Lipid Res. 1995; 36: 211-228Abstract Full Text PDF PubMed Google Scholar). During this reaction, lecithins are converted into lysophosphatidylcholines. The transfer process occurs in multiple steps. First, apoAI "activates" the phospholipid-cholesterol bilayer by concentrating the lipid substrates near LCAT and presenting it in an optimal conformation to LCAT. The conformation of these apoAI complexes is affected by the fluidity of the lipid bilayer (40Laccotripe M. Makrides S.C. Jonas A. Zannis V.I. The carboxyl-terminal hydrophobic residues of apolipoprotein A-I affect its rate of phospholipid binding and its association with high density lipoprotein.J. Biol. Chem. 1997; 272: 17511-17522Abstract Full Text Full Text PDF PubMed Scopus (94) Google Scholar, 41Sparks D.L. Frank P.G. Nevill T.A. Effect of the surface lipid composition of reconstituted LPA-I on apolipoprotein A-I and lecithin:cholesterol acyltransferase activity.Biochim. Biophys. Acta. 1998; 1390: 160-172Crossref PubMed Scopus (0) Google Scholar). Furthermore, the binding of LCAT to the apoAI bilayer is influenced by the size and charge of the HDL particles (42Dobiásová M. Frohlich J.J. Advances in understanding of the role of lecithin cholesterol acyltransferase (LCAT) in cholesterol transport.Clin. Chim. Acta. 1999; 286: 257-271Crossref PubMed Scopus (0) Google Scholar). The second step involves the cleavage of the sn-2 ester bond of lecithin, leading to the release of a fatty acyl (24Jauhiainen M. Dolphin P.J. Human plasma lecithin-cholesterol acyltransferase. An elucidation of the catalytic mechanism.J. Biol. Chem. 1986; 261: 7032-7043Abstract Full Text PDF PubMed Google Scholar). This step is mediated by the phospholipase activity of LCAT and depends on the lecithin composition (42Dobiásová M. Frohlich J.J. Advances in understanding of the role of lecithin cholesterol acyltransferase (LCAT) in cholesterol transport.Clin. Chim. Acta. 1999; 286: 257-271Crossref PubMed Scopus (0) Google Scholar). The last step includes the transacylation of the fatty acyl moiety to the sulfur atom of a cystein residue forming a thioester, which subsequently donates its fatty acyl to the 3β-hydroxy group of the cholesterol molecule, thereby forming cholesteryl ester (24Jauhiainen M. Dolphin P.J. Human plasma lecithin-cholesterol acyltransferase. An elucidation of the catalytic mechanism.J. Biol. Chem. 1986; 261: 7032-7043Abstract Full Text PDF PubMed Google Scholar). In addition to apoAI, other apolipoproteins, such as apoAII, apoAIV, apoCI–III, and apoE, can activate LCAT, although less efficiently (43Jonas A. Lecithin-cholesterol acyltransferase in the metabolism of high-density lipoproteins.Biochim. Biophys. Acta. 1991; 1084: 205-220Crossref PubMed Scopus (191) Google Scholar). Two distinct types of LCAT activity can be distinguished: α and β. α-Activity describes the enzymatic activity of LCAT toward cholesterol bound to apoAI-containing lipoproteins (e.g., HDL particles). β-Activity constitutes the enzymatic activity of LCAT toward cholesterol bound to apoB-containing lipoproteins (e.g., VLDL and LDL particles) (44Kuivenhoven J.A. Pritchard H. Hill J. Frohlich J. Assmann G. Kastelein J. The molecular pathology of lecithin:cholesterol acyltransferase (LCAT) deficiency syndromes.J. Lipid Res. 1997; 38: 191-205Abstract Full Text PDF PubMed Google Scholar). The equilibrium dissociation constants (Kd) for the interaction of pure human LCAT with LDL, HDL2, HDL3, and reconstituted discoidal HDL (rHDL) are as follows: rHDL = HDL3≤ HDL2 < LDL with relative reactivities (app. Vmax/app. Km) of 100, 16, 1, 6%, respectively (45Kosek A.B. Durbin D. Jonas A. Binding affinity and reactivity of lecithin cholesterol acyltransferase with native lipoproteins.Biochem. Biophys. Res. Commun. 1999; 258: 548-551Crossref PubMed Scopus (34) Google Scholar). Hence, only a minority of LCAT in the circulation is bound to apoB-containing lipoproteins. Already in 1966, Glomset identified LCAT as an important driving force behind the RCT pathway (3Glomset J.A. Janssen E.T. Kennedy R. Dobbins J. Role of plasma lecithin:cholesterol acyltransferase in the metabolism of high density lipoproteins.J. Lipid Res. 1966; 7: 638-648Abstract Full Text PDF PubMed Google Scholar), a process that describes the HDL-mediated removal of excess cholesterol from peripheral tissues, including macrophages from the arterial wall, and subsequent delivery to the liver for biliary excretion (Fig. 1). The first step of the RCT pathway involves production of apoAI in the liver or intestine that is then released into the plasma (46Ohashi R. Mu H. Wang X. Yao Q. Chen C. Reverse cholesterol transport and cholesterol efflux in atherosclerosis.QJM. 2005; 98: 845-856Crossref PubMed Scopus (205) Google Scholar). Interaction with ATP-binding cassette transporter (ABC)A1 on primarily the liver and intestine induces the formation of nascent discoidal HDL (ndHDL) particles that can stimulate cholesterol efflux from macrophages in the arterial wall (47Van Eck M. Pennings M. Hoekstra M. Out R. Van Berkel T.J.C. Scavenger receptor BI and ATP-binding cassette transporter A1 in reverse cholesterol transport and atherosclerosis.Curr. Opin. Lipidol. 2005; 16: 307-315Crossref PubMed Google Scholar, 48Jessup W. Gelissen I.C. Gaus K. Kritharides L. Roles of ATP binding cassette transporters A1 and G1, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages.Curr. Opin. Lipidol. 2006; 17: 247-257Crossref PubMed Scopus (211) Google Scholar). Upon association of cholesterol with the ndHDL particle, it is esterified by LCAT, leading to partitioning of the cholesterol esters into the core of the particle and conversion of the ndHDL into a more mature HDL3 particle. This particle subsequently is able to induce efflux of cellular cholesterol via ABCG1 and SR-BI (47Van Eck M. Pennings M. Hoekstra M. Out R. Van Berkel T.J.C. Scavenger receptor BI and ATP-binding cassette transporter A1 in reverse cholesterol transport and atherosclerosis.Curr. Opin. Lipidol. 2005; 16: 307-315Crossref PubMed Google Scholar, 48Jessup W. Gelissen I.C. Gaus K. Kritharides L. Roles of ATP binding cassette transporters A1 and G1, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages.Curr. Opin. Lipidol. 2006; 17: 247-257Crossref PubMed Scopus (211) Google Scholar). Upon further enrichment of the HDL particles with cholesteryl ester, they are transformed into larger HDL2 particles (46Ohashi R. Mu H. Wang X. Yao Q. Chen C. Reverse cholesterol transport and cholesterol efflux in atherosclerosis.QJM. 2005; 98: 845-856Crossref PubMed Scopus (205) Google Scholar). Several studies indicate that LCAT activity decreases upon enlargement of the HDL particle, particularly on large apoE-rich HDL1 particles (49Lee J.Y. Badeau R.M. Mulya A. Boudyguina E. Gebre A.K. Smith T.L. Parks J.S. Functional LCAT deficiency in human apolipoprotein A-I transgenic, SR-BI knockout mice.J. Lipid Res. 2007; 48: 1052-1061Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar–52Oliveira H.C. Ma L. Milne R. Marcovina S.M. Inazu A. Mabuchi H. Tall A.R. Cholesteryl ester transfer protein activity enhances plasma cholesteryl ester formation. Studies in CETP transgenic mice and human genetic CETP deficiency.Arterioscler. Thromb. Vasc. Biol. 1997; 17: 1045-1052Crossref PubMed Google Scholar). This might be a direct effect of the fact that LCAT is subject to product inhibition (45Kosek A.B. Durbin D. Jonas A. Binding affinity and reactivity of lecithin cholesterol acyltransferase with native lipoproteins.Biochem. Biophys. Res. Commun. 1999; 258: 548-551Crossref PubMed Scopus (34) Google Scholar), but it has also been suggested that sphingomyelin enrichment of HDL prevents binding of LCAT to the lipoprotein (49Lee J.Y. Badeau R.M. Mulya A. Boudyguina E. Gebre A.K. Smith T.L. Parks J.S. Functional LCAT deficiency in human apolipoprotein A-I transgenic, SR-BI knockout mice.J. Lipid Res. 2007; 48: 1052-1061Abstract Full Text Full Text PDF PubMed Scopus (14) Google Scholar, 53Bolin D.J. Jonas A. Sphingomyelin inhibits the lecithin-cholesterol acyltransferase reaction with reconstituted high density lipoproteins by decreasing enzyme binding.J. Biol. Chem. 1996; 271: 19152-19158Abstract Full Text Full Text PDF PubMed Scopus (103) Google Scholar). Importantly, upon esterification of cholesterol in HDL, LCAT maintains the gradient of free cholesterol between the cellular membrane and the surface of the HDL particle, which is thought to generate a continuous flow of cholesterol from the cell to lipoproteins and prevent the transfer of cholesterol back to the cell (54Glomset J.A. The plasma lecithins:cholesterol acyltransferase reaction.J. Lipid Res. 1968; 9: 155-167Abstract Full Text PDF PubMed Google Scholar–56Czarnecka H. Yokoyama S. Regulation of cellular cholesterol efflux by lecithin:cholesterol acyltransferase reaction through nonspecific lipid exchange.J. Biol. Chem. 1996; 271: 2023-2028Abstract Full Text Full Text PDF PubMed Scopus (98) Google Scholar). The latter ( i.e., prevention of reuptake of cholesterol by the cell) is nowadays considered the most important pathway via which LCAT stimulates HDL-mediated efflux. Importantly, it is also postulated that the effect of LCAT on the flux of cholesterol may depend both on the type and metabolic status of the cells, and on the environment of HDL in the extracellular medium (57Fournier N. Atger V. Paul J.P. de la Llera Moya M. Rothblat G. Moatti N. Fractional efflux and net change in cellular cholesterol content mediated by sera from mice expressing both human apolipoprotein AI and human lecithin:cholesterol acyltransferase genes.Atherosclerosis. 1999; 147: 227-235Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar). Therefore, in addition to its essential role in the first step of the RCT pathway, LCAT is suggested to enhance the delivery of cholesterol to the liver (57Fournier N. Atger V. Paul J.P. de la Llera Moya M. Rothblat G. Moatti N. Fractional efflux and net change in cellular cholesterol content mediated by sera from mice expressing both human apolipoprotein AI and human lecithin:cholesterol acyltransferase genes.Atherosclerosis. 1999; 147: 227-235Abstract Full Text Full Text PDF PubMed Scopus (11) Google Scholar). However, there are several reasons to question whether LCAT is the main driving force for the RCT pathway. First, LCAT-deficient patients only show accumulation of cholesterol in specific cells/organs, including erythrocytes, kidney, and cornea (58Glomset, J. A., Assmann, G., Gjone, E., Norum, K. R., . 1995. Lecithin: cholesterol acyltransferase deficiency and fish-eye disease. In The Metabolic and Molecular Bases of Inherited Disease. 7th edition. C. R. Scriver, A. L Beaudet, W. S. Sly, and D. Valle, editors. McGraw-Hill, New York, NY. 1933–1952.Google Scholar). Second, passive diffusion was previously considered the predominant mechanism of cellular cholesterol efflux, but now facilitated transport via ABCA1, ABCG1, and SR-BI is recognized to be essential (47Van Eck M. Pennings M. Hoekstra M. Out R. Van Berkel T.J.C. Scavenger receptor BI and ATP-binding cassette transporter A1 in reverse cholesterol transport and atherosclerosis.Curr. Opin. Lipidol. 2005; 16: 307-315Crossref PubMed Google Scholar, 48Jessup W. Gelissen I.C. Gaus K. Kritharides L. Roles of ATP binding cassette transporters A1 and G1, scavenger receptor BI and membrane lipid domains in cholesterol export from macrophages.Curr. Opin. Lipidol. 2006; 17: 247-257Crossref PubMed Scopus (211) Google Scholar). Although cellular cholesterol efflux via SR-BI is dependent on the cellular free cholesterol gradient (59Yancey P.G. de la Llera-Moya M. Swarnakar S. Monzo P. Klein S.M. Connelly M.A. Johnson W.J. Williams D.L. Rothblat G.H. High density lipoprotein phospholipid composition is a major determinant of the bi-directional flux and net movement of cellular free cholesterol mediated by scavenger receptor BI.J. Biol. Chem. 2000; 275: 36596-36604Abstract Full Text Full Text PDF PubMed Scopus (236) Google Scholar), active ATP-powered transport via ABCA1 and ABCG1 is not and, henc
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