Recombinant Lecithin:cholesterol Acyltransferase Research Articles

A-213 Assessing the Function of High-Density Lipoprotein and the Risk of Cardiovascular Disease Using Fully Automated Immunoassay Analyzer, HISCLTM

Abstract Background High-density lipoprotein (HDL) function has earned an attention as a way of assessing the risk of cardiovascular disease. Conventionally, cholesterol efflux capacity (CEC) has been used to measure the function of the HDL, but because of its time-consuming and complex procedure it has been withheld from being widely used in clinical fields. We have previously introduced an immunoassay-based technique that measures cholesterol uptake capacity (CUC) as a novel measure of HDL functionality. There we have demonstrated that CUC is capable of risk stratification in patients with coronary artery disease, in which coronary lesions was inversely associated with CUC (Fujimoto et al., Atherosclerosis 2022; Murakami et al., Scientific Reports 2023). However, since this method uses a specialized washing solution with mild surfactant to prevent delipidation from HDL rather than using the conventional washing solution, it had to be operated as a dedicated device. In other words, CUC could not be measured with automatic analyzer along with the existing immunoassay items. Here we propose an upgraded version that intends for IVD use. Method Automated immunoassay system, HISCL™ was used to measure CUC in serum. This rapid and sensitive immunoassay is based on anti-ApoA1 antibody coated magnetic beads that captures HDL particles from the sample, while HDL simultaneously takes in the PEGylated biotin-labeled cholesterol. After the cholesterol uptake reaction, magnetic beads are washed with the HISCL™ washing solution, a commercially available reagent used in conventional immunoassay. Then, using alkaline phosphatase labeled streptavidin and specific substrate, signals are detected. As for the samples, myeloperoxidase-mediated oxidized HDL, recombinant Lecithin-cholesterol acyltransferase (rLCAT) added HDL, and commercially available serum (as control) were measured using above mentioned method to evaluate functional change in HDL. Results Despite using the conventional washing solution, we were able to maintain specific signals corresponding to the dynamics of cholesterol probe by optimizing the reaction sequence and reducing the number of washing process. In this condition, the assay was linear between 0 and 50 nL of serum (R2 > 0.99), and CUC correlated with CEC (r = 0.897, P < 0.01). Furthermore, statistically significant difference (P < 0.05) in CUC level was seen with HDL in different functional states. In specific, CUC dropped with HDL oxidation while an opposite movement was found in HDL that’s been treated with rLCAT, an enzyme that esterifies cholesterol thereby improves cholesterol uptake of HDL. This result suggests that this assay is capable of accurately depicting the function of HDL. In our previous measuring system, CUC demonstrated its utility in coronary risk stratification. Here, we were able to see a significant correlation (r = 0.973, P < 0.01) between our previous data and our current data with high reproducibility (Coefficient of Variation < 10%). Conclusion Our results suggest that we are inching ever closer to expanding the research of HDL functionality to the clinical settings and diagnosing patients with high risk of cardiovascular disease.

Fully automated immunoassay for cholesterol uptake capacity to assess high-density lipoprotein function and cardiovascular disease risk

High-density lipoprotein (HDL) cholesterol efflux capacity (CEC), which is a conventional metric of HDL function, has been associated with coronary heart disease risk. However, the CEC assay requires cultured cells and takes several days to perform. We previously established a cell-free assay to evaluate cholesterol uptake capacity (CUC) as a novel measure of HDL functionality and demonstrated its utility in coronary risk stratification. To apply this concept clinically, we developed a rapid and sensitive assay system based on a chemiluminescent magnetic particle immunoassay. The system is fully automated, providing high reproducibility. Measurement of CUC in serum is completed within 20 min per sample without HDL isolation, a notably higher throughput than that of the conventional CEC assay. CUC decreased with myeloperoxidase-mediated oxidation of HDL or in the presence of N-ethylmaleimide, an inhibitor of lecithin: cholesterol acyltransferase (LCAT), whereas CUC was enhanced by the addition of recombinant LCAT. Furthermore, CUC correlated with CEC even after being normalized by ApoA1 concentration and was significantly associated with the requirement for revascularization due to the recurrence of coronary lesions. Therefore, our new assay system shows potential for the accurate measurement of CUC in serum and permits assessing cardiovascular health.

LCAT protects against Lipoprotein-X formation in a murine model of drug-induced intrahepatic cholestasis.

Familial lecithin:cholesterol acyltransferase (LCAT) deficiency (FLD) is a rare genetic disease characterized by low HDL‐C levels, low plasma cholesterol esterification, and the formation of Lipoprotein‐X (Lp‐X), an abnormal cholesterol‐rich lipoprotein particle. LCAT deficiency causes corneal opacities, normochromic normocytic anemia, and progressive renal disease due to Lp‐X deposition in the glomeruli. Recombinant LCAT is being investigated as a potential therapy for this disorder. Several hepatic disorders, namely primary biliary cirrhosis, primary sclerosing cholangitis, cholestatic liver disease, and chronic alcoholism also develop Lp‐X, which may contribute to the complications of these disorders. We aimed to test the hypothesis that an increase in plasma LCAT could prevent the formation of Lp‐X in other diseases besides FLD. We generated a murine model of intrahepatic cholestasis in LCAT‐deficient (KO), wild type (WT), and LCAT‐transgenic (Tg) mice by gavaging mice with alpha‐naphthylisothiocyanate (ANIT), a drug well known to induce intrahepatic cholestasis. Three days after the treatment, all mice developed hyperbilirubinemia and elevated liver function markers (ALT, AST, Alkaline Phosphatase). The presence of high levels of LCAT in the LCAT‐Tg mice, however, prevented the formation of Lp‐X and other plasma lipid abnormalities in WT and LCAT‐KO mice. In addition, we demonstrated that multiple injections of recombinant human LCAT can prevent significant accumulation of Lp‐X after ANIT treatment in WT mice. In summary, LCAT can protect against the formation of Lp‐X in a murine model of cholestasis and thus recombinant LCAT could be a potential therapy to prevent the formation of Lp‐X in other diseases besides FLD.

Lecithin:Cholesterol Acyltransferase Activation by Sulfhydryl-Reactive Small Molecules: Role of Cysteine-31.

Lecithin:cholesterol acyltransferase (LCAT) catalyzes plasma cholesteryl ester formation and is defective in familial lecithin:cholesterol acyltransferase deficiency (FLD), an autosomal recessive disorder characterized by low high-density lipoprotein, anemia, and renal disease. This study aimed to investigate the mechanism by which compound A [3-(5-(ethylthio)-1,3,4-thiadiazol-2-ylthio)pyrazine-2-carbonitrile], a small heterocyclic amine, activates LCAT. The effect of compound A on LCAT was tested in human plasma and with recombinant LCAT. Mass spectrometry and nuclear magnetic resonance were used to determine compound A adduct formation with LCAT. Molecular modeling was performed to gain insight into the effects of compound A on LCAT structure and activity. Compound A increased LCAT activity in a subset (three of nine) of LCAT mutations to levels comparable to FLD heterozygotes. The site-directed mutation LCAT-Cys31Gly prevented activation by compound A. Substitution of Cys31 with charged residues (Glu, Arg, and Lys) decreased LCAT activity, whereas bulky hydrophobic groups (Trp, Leu, Phe, and Met) increased activity up to 3-fold (P < 0.005). Mass spectrometry of a tryptic digestion of LCAT incubated with compound A revealed a +103.017 m/z adduct on Cys31, consistent with the addition of a single hydrophobic cyanopyrazine ring. Molecular modeling identified potential interactions of compound A near Cys31 and structural changes correlating with enhanced activity. Functional groups important for LCAT activation by compound A were identified by testing compound A derivatives. Finally, sulfhydryl-reactive β-lactams were developed as a new class of LCAT activators. In conclusion, compound A activates LCAT, including some FLD mutations, by forming a hydrophobic adduct with Cys31, thus providing a mechanistic rationale for the design of future LCAT activators.

Crystal Structure of LCAT Bound to a Small Molecule Allosteric Activator Reveals its Active Conformation

There is an inverse relationship between risk for coronary heart disease (CHD) and the level of high density lipoprotein cholesterol (HDL‐C). As CHD is one of the leading causes of death worldwide, there is great interest in mechanisms to raise HDL‐C for treatment of CHD. Lecithin:cholesterol acyltransferase (LCAT) catalyzes the transfer of an acyl chain from phosphatidylcholine to cholesterol to create a cholesteryl ester (CE) in the process of reverse cholesterol transport. The formation of the more hydrophobic CE leads to the maturation of HDL from discoidal to spherical and promotes the clearance of cholesterol from arterial plaques. Mutations in LCAT are known to cause two genetic diseases: fish eye disease (FED) and familial LCAT deficiency (FLD). Individuals with either FED or FLD have low HDL‐C and corneal opacities, while FLD patients ultimately develop renal failure. Recombinant LCAT and LCAT‐activating compounds are being investigated as treatments for both CHD and FLD. However, only recently has any structural information on LCAT been available to guide development of these therapies. Based on recent crystal structures and hydrogen deuterium exchange data, it is now apparent that LCAT has a dynamic lid that is able to protect the LCAT active site from solvent. This lid likely plays a critical role in activation of LCAT by apolipoprotein A‐I, the major protein component of HDLs. We report on a new structure of LCAT bound to an LCAT‐activating compound as well as an acyl intermediate state mimic, isopropyl dodecylfluorophosphonate. The combination of these two ligands promotes a lid‐open conformation. We confirmed that the compound binds to an allosteric site via site directed mutants that both eliminate binding and increase the thermal stability of LCAT. These studies pave the way for the design of rational drugs or improved biotherapeutics that can be used to treat CHD and FLD and augment reverse cholesterol transport. The new structure also allows for a better understanding of the acyl binding site of LCAT and its catalytic mechanism.Support or Funding InformationThis work was supported by US National Institutes of Health (NIH) grants HL122416 and Postdoctoral Fellowship F32HL131288 (K.A.M.).

Abstract 230: Lipoprotein X Causes Renal Disease in LCAT Deficiency

Familial lecithin:cholesterol acyltransferase (LCAT) deficiency (FLD) is characterized by low HDL, accumulation of an abnormal cholesterol-rich multilamellar particle called lipoprotein-X (LpX) in plasma, and renal disease. The aim of our study was to determine if LpX is nephrotoxic and to gain insight into the pathogenesis of FLD renal disease. We administered a synthetic LpX, nearly identical to endogenous LpX in its physical, and chemical properties, to wild-type and Lcat -/- mice. Our in vitro and in vivo studies demonstrated an apoA-I and LCAT-dependent pathway for LpX conversion to HDL-like particles, which likely mediates normal plasma clearance of LpX. Plasma clearance of exogenous LpX was markedly delayed in Lcat -/- mice, which have low HDL but only minimal amounts of endogenous LpX and do not spontaneously develop renal disease. Chronically administered exogenous LpX deposited in all renal glomerular cellular and matrical compartments of Lcat -/- mice, and induced proteinuria and nephrotoxic gene changes, as well as all of the hallmarks of FLD renal disease as assessed by histological, TEM, and SEM analyses. Extensive in vivo EM studies revealed LpX uptake by macropinocytosis into mouse glomerular endothelial cells, podocytes, and mesangial cells and delivery to lysosomes, where it was degraded. Endocytosed LpX appeared to be degraded by both human podocyte and mesangial cell lysosomal PLA 2 and induced podocyte secretion of pro-inflammatory IL-6 in vitro and renal Cxl10 expression in Lcat -/- mice. In conclusion, LpX is a nephrotoxic particle that in the absence of LCAT induces all of the histological and functional hallmarks of FLD and hence may serve as a biomarker for monitoring recombinant LCAT therapy. In addition, our studies suggest that LpX-induced loss of endothelial barrier function and release of cytokines by renal glomerular cells likely plays a role in the initiation and progression of FLD nephrosis.

Structural and Biochemical Studies on the Activation Mechanism of Lecithin:Cholesterol Acyltransferase

Coronary heart disease (CHD) is the leading cause of death in the United States and is strongly associated with low levels of high density lipoprotein‐cholesterol (HDL‐C), thus there is significant clinical interest in methods that raise HDL‐C in order to treat and prevent CHD. Lecithin:cholesterol acyltransferase (LCAT) esterifies cholesterol on high density lipoprotein (HDL), which promotes HDL maturation and reverse cholesterol transport, the process by which cholesterol is transported from arterial plaques to the liver. Mutations in LCAT are known to cause two genetic diseases. Fish eye disease (FED) is characterized by partial loss of LCAT activity, whereas familial LCAT deficiency (FLD) is characterized by complete loss of LCAT. Individuals with either FED or FLD have low HDL‐C levels and corneal opacities, while FLD patients ultimately develop renal failure. Consequently, administration of recombinant LCAT and LCAT‐activating compounds are being investigated as treatments to increase HDL in both CHD and FLD, and have already shown promise in animal models. However, improvement of these therapeutics is limited due to a lack of structural information on LCAT and its interactions with HDL. LCAT is 50% identical to lysosomal phospholipase A2 (LPLA2). We previously solved a 1.84 Å structure of LPLA2, as well as a low‐resolution structure of LCAT (8.7 Å), both of which are in an active “lid open” conformation. In addition, structures of LPLA2 bound to inhibitors that mimic substrate binding led to models for how LPLA2 and LCAT bind to phospholipids and subsequently transfer an acyl group to the appropriate acceptor. Here we present a new “closed” crystal structure of LCAT at 3.3 Å wherein a retractable lid appears to block substrate access to the active site. Interestingly, the lid interacts with residues on the catalytic core that are mutated in LCAT genetic disease and are hypothesized to be important for activation by apolipoprotein A‐I (ApoA‐I), the major protein component of HDL particles. We therefore hypothesized that ApoA‐I binding will compete with the lid and thereby stimulate LCAT activity. We have probed the role of the lid by examining disease‐causing and lid mutations for esterase activity on a soluble substrate, binding to synthetic HDLs, and transacylase activity on HDLs. Overall, our results confirm the importance of the retractable lid and its observed intramolecular contacts in proper HDL binding and cholesterol esterification. Overall, these studies are expected to guide the design of more potent small molecule LCAT activators as well as more catalytically efficient recombinant LCAT that could be used for enzyme replacement therapy.Support or Funding InformationThis work was supported by US National Institutes of Health (NIH) grant F037815, an American Heart Association (AHA) Postdoctoral Fellowship 15POST24870001 (K.A.M.), and a University of Michigan Chemical Biology Postdoctoral Fellowship (A.G.).

Lipoprotein X Causes Renal Disease in LCAT Deficiency.

Human familial lecithin:cholesterol acyltransferase (LCAT) deficiency (FLD) is characterized by low HDL, accumulation of an abnormal cholesterol-rich multilamellar particle called lipoprotein-X (LpX) in plasma, and renal disease. The aim of our study was to determine if LpX is nephrotoxic and to gain insight into the pathogenesis of FLD renal disease. We administered a synthetic LpX, nearly identical to endogenous LpX in its physical, chemical and biologic characteristics, to wild-type and Lcat-/- mice. Our in vitro and in vivo studies demonstrated an apoA-I and LCAT-dependent pathway for LpX conversion to HDL-like particles, which likely mediates normal plasma clearance of LpX. Plasma clearance of exogenous LpX was markedly delayed in Lcat-/- mice, which have low HDL, but only minimal amounts of endogenous LpX and do not spontaneously develop renal disease. Chronically administered exogenous LpX deposited in all renal glomerular cellular and matrical compartments of Lcat-/- mice, and induced proteinuria and nephrotoxic gene changes, as well as all of the hallmarks of FLD renal disease as assessed by histological, TEM, and SEM analyses. Extensive in vivo EM studies revealed LpX uptake by macropinocytosis into mouse glomerular endothelial cells, podocytes, and mesangial cells and delivery to lysosomes where it was degraded. Endocytosed LpX appeared to be degraded by both human podocyte and mesangial cell lysosomal PLA2 and induced podocyte secretion of pro-inflammatory IL-6 in vitro and renal Cxl10 expression in Lcat-/- mice. In conclusion, LpX is a nephrotoxic particle that in the absence of Lcat induces all of the histological and functional hallmarks of FLD and hence may serve as a biomarker for monitoring recombinant LCAT therapy. In addition, our studies suggest that LpX-induced loss of endothelial barrier function and release of cytokines by renal glomerular cells likely plays a role in the initiation and progression of FLD nephrosis.

Abstract 128: Recombinant Lecithin Cholesterol Acyltransferase Generates Mature High Density Lipoprotein with Increased Capacity to Efflux Cholesterol via ABCG1 in Cynomolgus Monkeys

Introduction: Lecithin cholesterol acyltransferase (LCAT) esterifies free cholesterol to cholesteryl esters on the HDL particle and is considered a key component of reverse cholesterol transport. We have developed a recombinant human LCAT fusion protein with high LCAT activity and characterized the effects of this molecule on lipoprotein metabolism and cholesterol efflux in cynomolgus monkeys. Methods: Cynomolgus monkeys received a single injection of rhLCAT at 0, 0.5, 1, or 5 mpk. Blood samples were drawn for analysis at pre-dose and up to 5 days after administration. HDL-C and apoAI were measured using standard clinical analyzers, apoE was measured by ELISA, and rhLCAT was measured by MSD immunoassay. ApoAI containing HDL subfractions were determined by two-dimensional non-denaturing gradient gel electrophoresis as described in the literature. Cholesterol efflux was measured ex vivo from ABCA1, ABCG1, or SRB1 transfected BHK cells, or J774 mouse macrophages using apoB depleted serum. Results: After administration, serum rhLCAT concentrations increased dose proportionately. HDL-C increased rapidly, beginning within the first 6 hours after dosing, and was dose dependent with maximum increases of 87% observed in the 1 and 5 mpk dose groups. ApoAI concentrations increased by as much as 15% and 21% in the 0.5 and 1 mpk groups, respectively, but decreased in the 5 mpk group by as much as 23%. The ratio of HDL-C to apoAI increased dose dependently indicating the formation of HDL particles containing increasingly more cholesterol. 2D gels confirmed the formation of large, primarily α-1, HDL particles. ApoE on HDL particles increased dose dependently with large increases observed following the 1 mpk (169%) and 5 mpk (910%) doses. Changes in the HDL particle were accompanied by changes in the ability of HDL to efflux cholesterol from cells. ABCG1 mediated efflux increased by as much as 21% and 24% in the 1 and 5 mpk dose groups, respectively. Conversely, efflux via ABCA1, SRB1, and from J774 cells was reduced dose dependently. Conclusions: The administration of rhLCAT in cynos resulted in the formation of large HDL particles that were enriched in cholesterol and ApoE. These larger particles showed increased capacity to efflux cholesterol via the ABCG1 transporter.

Abstract 241: Anti-Inflammatory Properties of Cynomolgus Monkey and Rabbit High-Density Lipoprotein Following Treatment With Recombinant Lecithin Cholesterol Acyltransferase

Introduction: Lecithin cholesterol acyltransferase (LCAT) modulates HDL formation and increases HDL-C. Anti-inflammatory properties of HDL may be atheroprotective. We evaluated HDL from rabbits and cynomolgus monkeys treated with recombinant LCAT (rLCAT) to determine its anti-inflammatory properties. Methods: HDL from serum was isolated by ultracentrifugation. HUVECs were treated with HDL, stimulated with TNFα, and mRNA expression of VCAM-1, E-Selectin, and ICAM-1 was measured. Results: TNFα induced adhesion molecule expression in HUVECs treated with HDL (0.3 mg/mL) from rLCAT treated rabbits was reduced (E-Selectin) or similar (VCAM-1, ICAM-1) compared to pre-dose HDL. Using HDL (0.3 mg/mL) from rLCAT treated cynomolgus monkeys reduced expression of E-Selectin, VCAM-1, and ICAM-1 compared to control (pre-dose or vehicle) HDL in a rLCAT dose-dependent manner. In HUVECs treated with cynomolgus monkey HDL at concentrations normalized to serum HDL-C, adhesion molecule expression was reduced but showed a different profile. HDL after treatment with 1 or 5 mpk rLCAT reduced expression more than HDL from controls, while HDL after treatment with 20 or 150 mpk rLCAT blocked expression less than controls. Conclusions: HDL from rabbits and cynomolgus monkeys after rLCAT treatment was equally or more anti-inflammatory compared to control HDL when tested at a fixed concentration, a measure of HDL quality. To account for treatment changes to both HDL quality and quantity, HDL was tested relative to serum HDL-C. In this case, HDL from the 1 mpk group proved most efficacious in reducing TNFα stimulated adhesion molecule expression compared to vehicle HDL or HDL from higher doses.

A kindred with fish eye disease, corneal opacities, marked high-density lipoprotein deficiency, and statin therapy

A kindred affected with fish eye disease (FED) from Oklahoma is reported. Two probands with corneal opacification had mean levels of high-density lipoprotein (HDL) cholesterol (C), apolipoprotein (apo) A-I, and apoA-I in very large alpha-1 HDL particles that were 9%, 17%, and 5% of normal, whereas their parents and 1 sibling had values that were 61%, 77%, and 72% of normal. The probands had no detectable lipoprotein-X, and had mean low-density lipoprotein cholesterol (LDL-C) and triglyceride levels that were elevated. Their mean lecithin cholesterol acyltransferase (LCAT) activities, cholesterol esterification rates, and free cholesterol levels were 8%, 42%, and 258% of normal, whereas their parents and 1 sibling had values that were 55%, 49%, and 114% of normal. The defect was due to 1 common variant in the LCAT gene in exon 1: c101t causing a proline34leucine substitution and a novel mutation c1177t causing a threonine37methionine substitution, with the former variant being found in the father and 1 sibling, and the latter mutation being found in the mother, and both mutations being present in the 2 probands. FED is distinguished from familial LCAT deficiency (FLD) by the lack of anemia, splenomegaly, and renal insufficiency as well as normal or increased LDL-C. Both FLD and FED cases have marked HDL deficiency and corneal opacification, and FED cases may have premature coronary heart disease in contrast to FLD cases. Therapy, using presently available agents, in FED should be to optimize LDL-C levels, and 1 proband responded well to statin therapy. The investigational use of human recombinant LCAT as an enzyme source is ongoing.

Abstract 259: Generation of Model HDL Complexes to Dissect the Role of Apolipoprotein A-II on Antioxidative and Anti-inflammatory Properties of HDL

Human high density lipoproteins (HDL) are divided into two major subpopulations based on lipoprotein composition. The predominant subpopulation contains both major apolipoproteins apoA-I and apoA-II (termed LpA-I/A-II), while the minor subpopulation contains only apoA-I (termed LpA-I). It is speculated that LpA-I/A-II could demonstrate different anti-oxidative and anti-inflammatory properties compared to LpA-I. The goal of the current work was to dissect out any contributions from apoA-II on such beneficial functions of HDL. We generated a series of reconstituted (r)LpA-I/A-II in which major apolipoprotein ratios mimic native LpA-I/A-II (apoA-I: apoA-II; 2:1, 2:2 and 2:3) but lack the anti-oxidative enzymes in native HDL. First, we generated the precursor rLpA-I by reacting discoidal HDL with recombinant lecithin cholesterol acyl transferase (LCAT) in the presence of low density lipoproteins. The rLpA-I consist of a single population of ∼93 Å particles. Cross-linking followed by MALDI-MS indicated exactly three apoA-I per rLpA-I. Different rLpA-I/A-II complexes were generated by adding the required amount of apoA-II into rLpA-I followed by isolation of rLpA-I:A-II using size exclusion chromatography. All rLpA-I/A-II showed comparable particle diameter to rLpA-I. Electron microscopy clearly captured the spherical nature of these particles and agarose gel electrophoresis demonstrated the presence of a neutral lipid core in rLpA-I and rLpA-I/A-II compared to discoidal HDL. All particles contain comparable amounts of total protein 36-40%, phospholipids 40-45%, cholesteryl esters 17-22%, free cholesterol &lt;1% and triglycerides &lt;1%. In vitro anti-oxidative functional assays utilizing copper-mediated oxidation demonstrated ∼20% better anti-oxidative capacity for rLpA-I/A-II (apoA-I: apoA-II, 2:1) compared to rLpA-I. We are currently in the process of testing anti-oxidative properties of other rLpA-I/A-II complexes. Experiments are currently underway to assess differences of anti-inflammatory properties of these complexes by quantitative measurements of adhesion molecule VCAM, ICAM and E-selection using flow cytometry in cell culture.

Lecithin Cholesterol Acyltransferase: An Anti- or Pro-atherogenic Factor?

Lecithin cholesterol acyl transferase (LCAT) is a plasma enzyme that esterifies cholesterol and raises high-density lipoprotein cholesterol, but its role in atherosclerosis is not clearly established. Studies of various animal models have yielded conflicting results, but studies done in rabbits and non-human primates, which more closely simulate human lipoprotein metabolism, indicate that LCAT is likely atheroprotective. Although suggestive, there are also no biomarker studies that mechanistically link LCAT with cardiovascular disease. Imaging studies of patients with LCAT deficiency have also not yielded a clear answer to the role of LCAT in atherosclerosis. Recombinant LCAT, however, is currently being developed as a therapeutic product for enzyme replacement therapy of patients with genetic disorders of LCAT for the prevention and/or treatment of renal disease, but it may also have value for the treatment of acute coronary syndrome.

Molecular Analysis of a Novel LCAT Mutation (Gly179 → Arg) Found in a Patient with Complete LCAT Deficiency

Lecithin-cholesterol acyltransferase (LCAT) is an important enzyme involved in the esterification of cholesterol. Here, we report a novel point mutation in the LCAT gene of a 63-year-old female with characteristics of classic familial LCAT deficiency. The patient's clinical manifestations included corneal opacity, mild anemia, mild proteinuria and normal renal function. She had no sign of coronary heart disease. Her LCAT activity was extremely low. DNA sequencing revealed a point mutation in exon 5 of the LCAT gene: a G to C substitution converting Gly(179) to an Arg, located in one of the catalytic triads of the enzyme. In vitro expression of recombinant LCAT proteins in HEK293 cells showed that the mutant G179R protein was present in the cell lysate, but not the culture medium. LCAT activity was barely detectable in the cell lysate or medium of the cells expressing the G179R mutant. This novel missense mutation seems to cause a complete loss of catalytic activity of LCAT, which is also defective in secretion.

Effect of Recombinant Human Lecithin Cholesterol Acyltransferase Infusion on Lipoprotein Metabolism in Mice

Lecithin cholesterol acyl transferase (LCAT) deficiency is associated with low high-density lipoprotein (HDL) and the presence of an abnormal lipoprotein called lipoprotein X (Lp-X) that contributes to end-stage renal disease. We examined the possibility of using LCAT an as enzyme replacement therapy agent by testing the infusion of human recombinant (r)LCAT into several mouse models of LCAT deficiency. Infusion of plasma from human LCAT transgenic mice into LCAT-knockout (KO) mice rapidly increased HDL-cholesterol (C) and lowered cholesterol in fractions containing very-low-density lipoprotein (VLDL) and Lp-X. rLCAT was produced in a stably transfected human embryonic kidney 293f cell line and purified to homogeneity, with a specific activity of 1850 nmol/mg/h. Infusion of rLCAT intravenously, subcutaneously, or intramuscularly into human apoA-I transgenic mice showed a nearly identical effect in increasing HDL-C approximately 2-fold. When rLCAT was intravenously injected into LCAT-KO mice, it showed a similar effect as plasma from human LCAT transgenic mice in correcting the abnormal lipoprotein profile, but it had a considerably shorter half-life of approximately 1.23 ± 0.63 versus 8.29 ± 1.82 h for the plasma infusion. rLCAT intravenously injected in LCAT-KO mice crossed with human apolipoprotein (apo)A-I transgenic mice had a half-life of 7.39 ± 2.1 h and increased HDL-C more than 8-fold. rLCAT treatment of LCAT-KO mice was found to increase cholesterol efflux to HDL isolated from mice when added to cells transfected with either ATP-binding cassette (ABC) transporter A1 or ABCG1. In summary, rLCAT treatment rapidly restored the normal lipoprotein phenotype in LCAT-KO mice and increased cholesterol efflux, suggesting the possibility of using rLCAT as an enzyme replacement therapy agent for LCAT deficiency.

Compound heterozygosity (G71R/R140H) in the lecithin:cholesterol acyltransferase (LCAT) gene results in an intermediate phenotype between LCAT-deficiency and fish-eye disease

The esterification of free cholesterol (FC) in plasma, catalyzed by the enzyme lecithin:cholesterol acyltransferase (LCAT; EC 2.3.1.43), is a key process in lipoprotein metabolism. The resulting cholesteryl esters (CE) represent the main core lipids of low (LDL) and high density lipoproteins (HDL). Primary (familial) LCAT-deficiency (FLD) is a rare autosomal recessive genetic disease caused by the complete or near absence of LCAT activity. In fish-eye disease (FED), residual LCAT activity is still detectable. Here, we describe a 32-year-old patient with corneal opacity, very low LCAT activity, reduced amounts of CE (low HDL-cholesterol level), and elevated triglyceride (TG) values. The lipoprotein pattern was abnormal with regard to lipoprotein composition and concentration, but distinct lipoprotein classes were still present. Despite of typical features of glomerular proteinuria, creatinine clearance was normal. DNA sequencing and restiction fragment analyses revealed two separate mutations in the patient's LCAT gene: a previously described G to A transition in exon 4 converting Arg 140 to His, inherited from his mother, and a novel G to C transversion in exon 2 converting Gly 71 to Arg, inherited from his father, indicating that M.P. was a compound heterozygote. Determination of enzyme activities of recombinant LCAT proteins obtained upon transfection of COS-7 cells with plasmids containing G71R-LCAT or wild-type LCAT cDNA revealed very low α- and absence of β-LCAT activity for the G71R mutant. The identification of the novel G71R LCAT mutation supports the proposed molecular model for the enzyme implying that the “lid” domain at residues 50–74 is involved in enzyme:substrate interaction. Our data are in line with the hypothesis that a key event in the etiology of FLD is the loss of distinct lipoprotein fractions.

Apolipoprotein E Is the Major Physiological Activator of Lecithin−Cholesterol Acyltransferase (LCAT) on Apolipoprotein B Lipoproteins

Our previous studies have indicated that lecithin-cholesterol acyltransferase (LCAT) contributes significantly to the apoB lipoprotein cholesteryl ester (CE) pool. Cholesterol esterification rate (CER) in apoA-I(-)(/)(-) apoE(-)(/)(-) mouse plasma was <7% that of C57Bl/6 (B6) mouse plasma, even though apoA-I(-)(/)(-) apoE(-)(/)(-) plasma retained (1)/(3) the amount of B6 LCAT activity. This suggested that lack of LCAT enzyme did not explain the low CER in apoA-I(-)(/)(-) apoE(-)(/)(-) mice and indicated that apoE and apoA-I are the only major activators of LCAT in mouse plasma. Deleting apoE on low-density lipoprotein (LDL) reduced CER (1% free cholesterol (FC) esterified/h) compared to B6 (6% FC esterified/h) and apoA-I(-)(/)(-) (11% FC esterified/h) LDL. Similar sized LDL particles from all four genotypes were isolated by fast protein liquid chromatography (FPLC) after radiolabeling with [(3)H]-free cholesterol (FC). LDLs (1 microg FC) from each genotype were incubated with purified recombinant mouse LCAT; LDL particles from B6 and apoA-I(-)(/)(-) plasma were much better substrates for CE formation (5.7% and 6.3% CE formed/30 min, respectively) than those from apoE(-)(/)(-) and apoE(-)(/)(-) apoA-I(-)(/)(-) plasma (1.2% and 1.1% CE formed/30 min). Western blot analysis showed that the amount of apoA-I on apoE(-)(/)(-) LDLs was higher compared to B6 LDL. Adding apoE to incubations of apoA-I(-)(/)(-) apoE(-)(/)(-) very low density lipoprotein (VLDL) resulted in a 3-fold increase in LCAT CER, whereas addition of apoA-I resulted in a more modest 80% increase. We conclude that apoE is a more significant activator of LCAT than apoA-I on mouse apoB lipoproteins.

Characterization of lecithin:cholesterol acyltransferase expressed in a human lung cell line

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme for the transfer of mammalian cholesterol from peripheral tissues to the liver. In patients deficient in LCAT, serum cholesterol levels rise and can lead to corneal opacity, proteinuria, anemia, and kidney failure. As early as 1968, relatively low volume transfusion of normal plasma was shown to temporarily correct the abnormal lipoprotein profiles in LCAT-deficient patients. However, despite the cloning, study, and extensive expression of LCAT in mammalian cell lines, there is still no viable, clinical therapy for LCAT deficiency. The current study was initiated to provide a source of recombinant human LCAT for enzyme replacement therapy. Accordingly, human LCAT has been cloned and expressed for the first time in a human cell line. The recombinant LCAT secreted by these cells was purified by phenyl-Sepharose chromatography, analyzed to determine the nature of its glycosylation, and tested for its enzymatic properties. The activity and basic kinetic parameters for the enzyme were determined using both a fluorescent water-soluble substrate and a macromolecular (proteoliposome) substrate. The enzymatic properties and the carbohydrate components of the recombinant LCAT were all sufficiently similar to those of the circulating human plasma enzyme, suggesting that this source of LCAT may be appropriate for use in some form of enzyme replacement therapy.

Characterization of lecithin:cholesterol acyltransferase expressed in a human lung cell line

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme for the transfer of mammalian cholesterol from peripheral tissues to the liver. In patients deficient in LCAT, serum cholesterol levels rise and can lead to corneal opacity, proteinuria, anemia, and kidney failure. As early as 1968, relatively low volume transfusion of normal plasma was shown to temporarily correct the abnormal lipoprotein profiles in LCAT-deficient patients. However, despite the cloning, study, and extensive expression of LCAT in mammalian cell lines, there is still no viable, clinical therapy for LCAT deficiency. The current study was initiated to provide a source of recombinant human LCAT for enzyme replacement therapy. Accordingly, human LCAT has been cloned and expressed for the first time in a human cell line. The recombinant LCAT secreted by these cells was purified by phenyl-Sepharose chromatography, analyzed to determine the nature of its glycosylation, and tested for its enzymatic properties. The activity and basic kinetic parameters for the enzyme were determined using both a fluorescent water-soluble substrate and a macromolecular (proteoliposome) substrate. The enzymatic properties and the carbohydrate components of the recombinant LCAT were all sufficiently similar to those of the circulating human plasma enzyme, suggesting that this source of LCAT may be appropriate for use in some form of enzyme replacement therapy.

Probing the 121–136 Domain of Lecithin:Cholesterol Acyltransferase Using Antibodies

Lecithin:cholesterol acyltransferase (LCAT) catalyzes the esterification of plasma lipoprotein cholesterol in mammals as part of the reverse cholesterol transport pathway. Studies of the natural mutations of LCAT revealed a region that is highly sensitive to mutations (residues 121–136) and it is highly conserved in six animal species. The purpose of these studies was to investigate the reactivity of wild type and several mutated forms of LCAT, with a series polyclonal antibodies to further characterize this specific domain (residues 121–136). Two polyclonal antibodies directed against the whole enzyme, one against human plasma LCAT and the other against purified recombinant LCAT, and one site specific polyclonal antibody, directed against the 121–136 region of LCAT, were employed. All three antibodies reacted with a recombinant form of purified LCAT; however, only the polyclonal antibodies directed against the whole enzyme were able to recognize the LCAT when it was adsorbed to a hydrophobic surface in a solid phase immunoassay, or when bound to HDL in a sink immunoassay. These findings indicate that the epitope(s) of the 121–136 region are not accessible to antibodies under these conditions. Three mutant forms of LCAT, representing alterations in the 121–136 region, were also examined for their immunoreactivity with the same panel of antibodies and compared to the wild-type enzyme. These studies demonstrate that in its native configuration the 121–136 region of LCAT is likely to reside on a surface of LCAT. Furthermore, mutations within this region appear to markedly impact the exposure of epitopes at additional sites. These findings suggest that the 121–136 region could play an important role in enzyme interaction with its hydrophobic lipoprotein substrates as mutations within this region appear to alter enzyme conformation, catalytic activity, and the specificity of LCAT.