Synthetic substrates of lecithin: cholesterol acyltransferase

Intracellular Cholesterol Trafficking

Cholesterol is an essential lipid species of mammalian cells. Cells acquire it through synthesis in the endoplasmic reticulum (ER) and uptake from lipoprotein particles. Newly synthesized cholesterol is efficiently distributed from the ER to other organelles via lipid-binding/transfer proteins concentrated at membrane contact sites (MCSs) to reach the trans-Golgi network, endosomes, and plasma membrane. Lipoprotein-derived cholesterol is exported from the plasma membrane and endosomal compartments via a combination of vesicle/tubule-mediated membrane transport and transfer through MCSs. In this review, we provide an overview of intracellular cholesterol trafficking pathways, including cholesterol flux from the ER to other membranes, cholesterol uptake from lipoprotein donors and transport from the plasma membrane to the ER, cellular cholesterol efflux to lipoprotein acceptors, as well as lipoprotein cholesterol secretion from enterocytes, hepatocytes, and astrocytes. We also briefly discuss human diseases caused by defects in these processes and therapeutic strategies available in such conditions.

Interaction of lecithin:cholesterol acyltransferase with lipid surfaces and apolipoprotein A-I-derived peptides

LCAT is an enzyme responsible for the formation of cholesteryl esters from unesterified cholesterol (UC) and phospholipid (PL) molecules in HDL particles. However, it is poorly understood how LCAT interacts with lipoproteins and how apoA-I activates it. Here we have studied the interactions between LCAT and lipids through molecular simulations. In addition, we studied the binding of LCAT to apoA-I-derived peptides, and their effect on LCAT lipid association-utilizing experiments. Results show that LCAT anchors itself to lipoprotein surfaces by utilizing nonpolar amino acids located in the membrane-binding domain and the active site tunnel opening. Meanwhile, the membrane-anchoring hydrophobic amino acids attract cholesterol molecules next to them. The results also highlight the role of the lid-loop in the lipid binding and conformation of LCAT with respect to the lipid surface. The apoA-I-derived peptides from the LCAT-activating region bind to LCAT and promote its lipid surface interactions, although some of these peptides do not bind lipids individually. The transfer free-energy of PL from the lipid bilayer into the active site is consistent with the activation energy of LCAT. Furthermore, the entry of UC molecules into the active site becomes highly favorable by the acylation of SER181.

Single step reconstitution of multifunctional high-density lipoprotein-derived nanomaterials using microfluidics

High-density lipoprotein (HDL) is a natural nanoparticle that transports peripheral cholesterol to the liver. Reconstituted high-density lipoprotein (rHDL) exhibits antiatherothrombotic properties and is being considered as a natural treatment for cardiovascular diseases. Furthermore, HDL nanoparticle platforms have been created for targeted delivery of therapeutic and diagnostic agents. The current methods for HDL reconstitution involve lengthy procedures that are challenging to scale up. A central need in the synthesis of rHDL, and multifunctional nanomaterials in general, is to establish large-scale production of reproducible and homogeneous batches in a simple and efficient fashion. Here, we present a large-scale microfluidics-based manufacturing method for single-step synthesis of HDL-mimicking nanomaterials (μHDL). μHDL is shown to have the same properties (e.g., size, morphology, bioactivity) as conventionally reconstituted HDL and native HDL. In addition, we were able to incorporate simvastatin (a hydrophobic drug) into μHDL, as well as gold, iron oxide, quantum dot nanocrystals or fluorophores to enable its detection by computed tomography (CT), magnetic resonance imaging (MRI), or fluorescence microscopy, respectively. Our approach may contribute to effective development and optimization of lipoprotein-based nanomaterials for medical imaging and drug delivery.

A fluorescence method to detect and quantitate sterol esterification by lecithin:cholesterol acyltransferase

We describe a simple but sensitive fluorescence method to accurately detect the esterification activity of lecithin:cholesterol acyltransferase (LCAT). The new assay protocol employs a convenient mix, incubate, and measure scheme. This is possible by using the fluorescent sterol dehydroergosterol (DHE) in place of cholesterol as the LCAT substrate. The assay method is further enhanced by incorporation of an amphiphilic peptide in place of apolipoprotein A-I as the lipid emulsifier and LCAT activator. Specific fluorescence detection of DHE ester synthesis is achieved by employing cholesterol oxidase to selectively render unesterified DHE nonfluorescent. The assay accurately detects LCAT activity in buffer and in plasma that is depleted of apolipoprotein B lipoproteins by selective precipitation. Analysis of LCAT activity in plasmas from control subjects and sickle cell disease (SCD) patients confirms previous reports of reduced LCAT activity in SCD and demonstrates a strong correlation between plasma LCAT activity and LCAT content. The fluorescent assay combines the sensitivity of radiochemical assays with the simplicity of nonradiochemical assays to obtain accurate and robust measurement of LCAT esterification activity.

The effects of sterol structure upon sterol esterification

Cholesterol is esterified in mammals by two enzymes: LCAT (lecithin cholesterol acyltransferase) in plasma and ACAT(1) and ACAT(2) (acyl-CoA cholesterol acyltransferases) in the tissues. We hypothesized that the sterol structure may have significant effects on the outcome of esterification by these enzymes. To test this hypothesis, we analyzed sterol esters in plasma and tissues in patients having non-cholesterol sterols (sitosterolemia and Smith-Lemli-Opitz syndrome). The esterification of a given sterol was defined as the sterol ester percentage of total sterols. The esterification of cholesterol in plasma by LCAT was 67% and in tissues by ACAT was 64%. Esterification of nine sterols (cholesterol, cholestanol, campesterol, stigmasterol, sitosterol, campestanol, sitostanol, 7-dehydrocholesterol and 8-dehydrocholesterol) was examined. The relative esterification (cholesterol being 1.0) of these sterols by the plasma LCAT was 1.00, 0.95, 0.89, 0.40, 0.85, 0.82 and 0.80, 0.69 and 0.82, respectively. The esterification by the tissue ACAT was 1.00, 1.29, 0.75, 0.49, 0.45, 1.21 and 0.74, respectively. The predominant fatty acid of the sterol esters was linoleic acid for LCAT and oleic acid for ACAT. We compared the esterification of two sterols differing by only one functional group (a chemical group attached to sterol nucleus) and were able to quantify the effects of individual functional groups on sterol esterification. The saturation of the A ring of cholesterol increased ester formation by ACAT by 29% and decreased the esterification by LCAT by 5.9%. Esterification by ACAT and LCAT was reduced, respectively, by 25 and 11% by the presence of an additional methyl group on the side chain of cholesterol at the C-24 position. This data supports our hypothesis that the structure of the sterol substrate has a significant effect on its esterification by ACAT or LCAT.

Saccharomyces cerevisiae phospholipid:diacylglycerol acyl transferase (PDAT) devoid of its membrane anchor region is a soluble and active enzyme retaining its substrate specificities

A N-terminal deleted version of the Saccharomyces cerevisiae phospholipid:diacylglycerol acyltransferase (ScPDAT), lacking the predicted membrane-spanning region, was fused in frame with alpha-factor secretion signal and expressed in Pichia pastoris under the control of the methanol inducible alcohol oxidase promoter. This resulted in a truncated, soluble and highly active PDAT protein secreted into the culture medium of the recombinant cells. The soluble as well as native membrane bound enzymes was shown to be glycosylated and extensive deglycosylation severely lowered the activity. The production of a soluble and extracellular PDAT allowed us to investigate substrate preferences of the enzyme without interference of endogenous lipids and enzymes. Similar to the membrane bound counterpart, the highest activity was achieved with acyl groups at sn-2 position of phosphatidylethanolamine as acyl donor and 1,2-diacylglycerols as acyl acceptor. The soluble enzyme was also able to catalyze, at a low rate, a number of transacylation reactions between various neutral lipids and between polar lipids and neutral lipids others than diacylglycerols, including acylation of long chain alcohols.

Moderate dietary intake of myristic and alpha-linolenic acids increases lecithin-cholesterol acyltransferase activity in humans

Cholesterol removal from tissues into HDL depends on the activity of lecithin-cholesterol acyltransferase (LCAT; E.C. 2.3.1.43) that is associated with lower cardiovascular diseases risk. HDL cholesterol concentration and LCAT activity can be modulated by dietary fatty acids. Original data with substrate models have shown a positive effect of myristic acid (MA) on the esterification rate of cholesterol. The purpose of this study was to examine the effect of moderate intakes of MA associated with recommended intake of alpha-linolenic acid (ALA) on LCAT activity in humans. Two experimental diets were tested for 3 months each. Diet 1-MA 1.2% of total energy (TE) and ALA 0.9% TE, diet 2-MA 1.8% and ALA 0.9% TE; a control diet (MA 1.2% and ALA 0.4% TE) was given 3 months before diet 1 and diet 2. The endogenous activity of LCAT was determined at completion of each diet. Compared with the control diet (13.2 +/- 3.1 micromol CE/(L x h)), LCAT activity increased significantly (P < 0.001) with diet 1 (24.2 +/- 3.6 micromol CE/(L x h)) and diet 2 (33.3 +/- 7.4 micromol CE/(L x h)); the increase observed with diet 2 was significantly (P < 0.001) greater than that due to diet 1. These results suggest that ALA (from rapeseed oil, mainly in sn-2 position) and MA (from dairy fat, mainly in sn-2 position) favor LCAT activity, by respective increases of 83 and 38%. When they are supplied together, a complementary effect was observed (average increase of 152%). Moreover, these observations were associated with a decrease of the ratio of total to HDL-cholesterol. In conclusion, our results suggest that moderate supply of MA (1.8% TE) associated with the recommended intake of ALA (0.9% TE) contributes to improve LCAT activity.

Evidence for altered positional specificity of LCAT in vivo: studies with docosahexaenoic acid feeding in humans

The percentage of saturated cholesteryl esters (CEs) synthesized by human LCAT is several times higher than expected from the sn-2 acyl composition of plasma phosphatidylcholine (PC), whereas the synthesis of 20:4 CE and 22:6 CE is much lower than expected. To explain these discrepancies, we proposed that LCAT transfers some saturated fatty acids from the sn-1 position of PC species that contain 20:4 or 22:6 at sn-2. The present studies provide in vivo evidence for this hypothesis. We determined the composition and synthesis of CE species in plasma of volunteers before and after a 6 week dietary supplementation with docosahexaenoic acid (22:6; DHA). In addition to an increase in the DHA content of all plasma lipids, there was a significant (+12%; P <0.005) increase of 16:0 CE, although there was no increase in 16:0 at sn-2 of PC. The increase of DHA in CE was much lower than its increase at sn-2 of PC. Ex vivo synthesis of CE species in plasma showed a significant (+24%; P <0.005) increase in the synthesis of 16:0 CE after DHA supplementation, which correlated positively with the increase of 22:6, but not of 16:0, at sn-2 of PC. These results show that the positional specificity of human LCAT is altered when the concentration of 16:0-22:6 PC is increased by DHA supplementation.

Hugh sinclair lecture: the regulation and remodelling of HDL by plasma factors

High density lipoproteins (HDLs) originate as lipid-free or lipid-poor apolipoproteins that acquire most of their lipid in the extracellular space. They accept phospholipids from cells in a process promoted by the ATP binding cassette A1 transporter to form prebeta-migrating discoidal HDL that are efficient acceptors of cholesterol released from cell membranes. The cholesterol in discoidal HDL is esterified by lecithin:cholesterol acyltransferase (LCAT) in a process that converts the prebeta-migrating disc into an alpha-migrating, spherical HDL. Spherical HDL are further remodelled by cholesteryl ester transfer protein (CETP) that transfers cholesteryl esters from HDL to other lipoproteins and by hepatic lipase that hydrolyses HDL triglyceride in processes that reduce HDL size and lead to the dissociation of prebeta-migrating, lipid-poor apolipoprotein (apo)A-I from the particle. Prebeta-migrating, lipid-poor apoA-I is also generated as a product of the remodelling of HDL by phospholipid transfer protein. Thus, apoA-I cycles between lipid-poor and lipid associated forms as part of a highly dynamic metabolism of HDL. The other main HDL apolipoprotein, apoA-II is incorporated into apoA-I-containing particles in a process of particle fusion mediated by LCAT. Extracellular assembly and remodelling of HDL not only plays a major role in HDL regulation but also provides potential targets for therapeutic intervention. One example of this is the development of inhibitors of CETP.

Transgenic overexpression of human lecithin: cholesterol acyltransferase (LCAT) in mice does not increase aortic cholesterol deposition

Results from several atherosclerosis studies using morphometric procedures have proven controversial with regard to whether over-expression of human LCAT in transgenic (Tg) mice is atherogenic. The purpose of the present study was to determine the effect of 10-fold over-expression of human LCAT on aortic free and esterified cholesterol (EC) deposition as well as plasma lipoprotein cholesteryl ester (CE) fatty acid composition in mice fed an atherogenic diet containing cholic acid. C57Bl/6 (control) and human LCAT-Tg mice were fed chow or an atherogenic diet (15% of calories from palm oil, 1.0% cholesterol and 0.5% cholic acid) for 24 weeks before measurement of aortic cholesterol content. Compared with the chow diet, control and LCAT-Tg mice fed the atherogenic diet had a 2-fold increase in plasma total, free and EC, a 7-fold increase in plasma apoB lipoprotein cholesterol, and a 40-50-fold increase in hepatic cholesterol content. The aortic EC content was increased in control (0.7 vs. 1.2 mg/g protein) and LCAT-Tg (0.3 vs. 1.5 mg/g protein) mice fed the atherogenic diet compared with those consuming the chow diet; however, there was no difference in aortic free (14.4+/-6.8 vs. 18.5+/-7.7 mg/g protein) or esterified (1.2+/-1.0 vs. 1.5+/-1.2 mg/g protein) cholesterol content between atherogenic diet-fed control and LCAT-Tg mice, respectively. LCAT-Tg mice fed the atherogenic diet had a 2-fold increase in the ratio of saturated+monounsaturated to polyunsaturated CE species in plasma apoB lipoproteins compared with control mice (9.4+/-2.4 vs. 4.9+/-0.7). We conclude that over-expression of human LCAT in Tg mice fed an atherogenic diet containing cholic acid does not result in increased aortic cholesterol deposition compared with control mice, even though the CE fatty acid saturation index of plasma apoB lipoproteins was doubled.

Down-regulation of hepatic lecithin:cholesterol acyltransferase gene expression in chronic renal failure

BACKGROUND

Chronic renal failure (CRF) is associated with premature arteriosclerosis, impaired high-density lipoprotein (HDL) maturation, increased pre-beta HDL (a lipid-poor HDL species), reduced HDL/total cholesterol ratio, hypertriglyceridemia, and depressed lipolytic activity. The latter has been, in part, attributed to elevated pre-beta HDL, which is a potent inhibitor of lipoprotein lipase (LPL). Accumulation of cholesterol in the arterial wall is a critical step in atherogenesis, and HDL-mediated cholesterol removal from peripheral tissues mitigates atherosclerosis. Lecithin:cholesterol acyltransferase (LCAT) is essential for maturation of HDL and cholesterol removal by HDL from peripheral tissues. Earlier studies have revealed depressed plasma LCAT enzymatic activity in patients with CRF. This study was conducted to determine whether impaired LCAT activity can be confirmed in CRF animals and if so whether it is due to down-regulation of hepatic LCAT expression.

METHODS

Hepatic tissue LCAT mRNA and plasma LCAT enzymatic activity were measured in male Sprague-Dawley rats six weeks after excisional 5/6 nephrectomy or sham operation.

RESULTS

Compared with the controls, the CRF group exhibited a significant reduction of hepatic tissue LCAT mRNA abundance. The reduction in hepatic LCAT mRNA was accompanied by a marked reduction of plasma LCAT activity and elevation of serum-free cholesterol in the CRF animals. LCAT activity correlated positively with the HDL/total cholesterol ratio and inversely with free cholesterol and triglyceride concentrations.

CONCLUSIONS

CRF leads to a marked down-regulation of hepatic LCAT mRNA expression and plasma LCAT activity. This abnormality can impair HDL-mediated cholesterol uptake from the vascular tissue and contribute to cardiovascular disease. In addition, LCAT deficiency can, in part, account for elevated serum-free cholesterol, reduced HDL/total cholesterol, and elevated pre-beta HDL in CRF. The latter can, in turn, depress lipolytic activity and hinder triglyceride-rich lipoprotein clearance in CRF.

Surface properties of native human plasma lipoproteins and lipoprotein models

Plasma lipoprotein surface properties are important but poorly understood determinants of lipoprotein catabolism. To elucidate the relation between surface properties and surface reactivity, the physical properties of surface monolayers of native lipoproteins and lipoprotein models were investigated by fluorescent probes of surface lipid fluidity, surface lateral diffusion, and interfacial polarity, and by their reactivity to Naja melanoleuca phospholipase A2 (PLA2). Native lipoproteins were human very low, low-, and subclass 3 high-density lipoproteins (VLDL, LDL, and HDL3); models were 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) or its ether analog in single-bilayer vesicles, large and small microemulsions of POPC and triolein, and reassembled HDL (apolipoprotein A-I plus phospholipid). Among lipoproteins, surface lipid fluidity increased in the order HDL3 < LDL < VLDL, varying inversely with their (protein + cholesterol)/phospholipid ratios. Models resembled VLDL in fluidity. Both lateral mobility in the surface monolayer and polarity of the interfacial region were lower in native lipoproteins than in models. Among native lipoproteins and models, increased fluidity in the surface monolayer was associated with increased reactivity to PLA2. Addition of cholesterol (up to 20 mol%) to models had little effect on PLA2 activity, whereas the addition of apolipoprotein C-III stimulated it. Single-bilayer vesicles, phospholipid-triolein microemulsions, and VLDL have surface monolayers that are quantitatively similar, and distinct from those of LDL and HDL3. Surface property and enzymatic reactivity differences between lipoproteins and models were associated with differences in surface monolayer protein and cholesterol contents. Thus differences in the surface properties that regulate lipolytic reactivity are a predictable function of surface composition.

Conformational studies of the N-terminal lipid-associating domain of human apolipoprotein C-I by CD and 1H NMR spectroscopy

A peptide comprising the N-terminal 38 residues of human apolipoprotein C-I (apoC-I(1-38)) was synthesized using solid-phase methods and its solution conformation studied by CD and 1H NMR spectroscopy. The CD data indicate that apoC-I(1-38) has a similar helical content (55%) in the presence of saturating amounts of SDS or egg yolk lysophosphatidylcholine. A structural ensemble of SDS-bound apoC-I(1-38) was calculated from 464 NOE-based distance restraints using distance geometry methods. ApoC-I(1-38) adopts a helical structure between residues V4 and K30 and an extended C-terminus from Q31 when associated with SDS. The region K12-G15 undergoes slow conformational exchange as indicated by above-average amide resonance linewidths, large temperature coefficients, and fast exchange (< 2 h) of backbone amide protons with deuterium. The mobility of K12-G15 is reflected in the poorly defined dihedral angles of K12 and E13 in the calculated ensemble of structures. The average structure of apoC-I(1-38) is curved toward its hydrophobic face with bends of 125 degrees, centered at K12/E13, and 150 degrees, centered at K21. This curvature appears to be driven by the interaction of two hydrophobic clusters, one formed by residues L8, L11, F14, and L18, and the other by L25, I26, and I29, with the amphiphile SDS. Based on our present structural definition of apoC-I(1-38) and the previously obtained structure of the fragment apoC-I(35-53), we propose the secondary structure of intact apolipoprotein C-I.

Disparate effects of oxidation on plasma acyltransferase activities: inhibition of cholesterol esterification but stimulation of transesterification of oxidized phospholipids

Oxidation of lipoproteins results in the formation of several polar phospholipids with pro-inflammatory and pro-atherogenic properties. To examine the possible role of lecithin/cholesterol acyltransferase (LCAT) in the metabolism of these oxidized phospholipids, we oxidized whole plasma with either Cu(2+) or a free-radical generator, and determined the various activities of LCAT. Oxidation caused a reduction in plasma phosphatidylcholine (PC), an increase in a short-chain polar PC (SCP-PC), and an inhibition of the transfer of long-chain acyl groups to cholesterol (LCAT activity) or to lyso PC (lysolecithin acyltransferase (LAT) I activity). However, the transfer of short-chain acyl groups from SCP-PC to lyso PCLAT II activity) was stimulated several fold, in direct correlation with the degree of oxidation. LAT II activity was not stimulated by oxidation in LCAT-deficient plasma, showing that it is carried out by LCAT. Oxidized normal plasma exhibited low LCAT activity even in the presence of exogenous proteoliposome substrate, indicating that the depletion of substrate PC was not responsible for the loss of activity. Oxidation of isolated LDL or HDL abolished their ability to support LCAT and LAT I activities of exogenous enzyme, but promoted the LAT II activity. Purified LCAT lost its LCAT and LAT I functions, but not its LAT II function, when oxidized in vitro. These results show that while oxidation of plasma causes a loss of LCAT's ability to transfer long-chain acyl groups, its ability to transfer short-chain acyl groups, from SCP-PC is retained, and even stimulated, suggesting that LCAT may have a physiological role in the metabolism of oxidized PC in plasma.

Structural studies of a peptide activator of human lecithin-cholesterol acyltransferase

The synthetic lipid-associating peptide, LAP-20 (VSSLLSSLKEYWSSLKESFS), activates lecithin-cholesterol acyltransferase (LCAT) despite its lack of sequence homology to apolipoprotein A-I, the primary in vivo activator of LCAT. Using SDS and dodecylphosphocholine (DPC) to model the lipoprotein environment, the structural features responsible for LAP-20's ability to activate LCAT were studied by optical and two-dimensional 1H NMR spectroscopy. A large blue shift in the intrinsic fluorescence of LAP-20 with the addition of detergent suggested that the peptide formed a complex with the micelles. Analysis of the CD data shows that LAP-20 lacks well defined structure in aqueous solution but adopts helical, ordered conformations upon the addition of SDS or DPC. The helical nature of the peptides in the presence of both lipids was confirmed by upfield H alpha NMR secondary shifts relative to random coil values. Average structures for both peptides in aqueous solutions containing SDS and DPC were generated using distance geometry methods from 329 (SDS) and 309 (DPC) nuclear Overhauser effect-based distance restraints. The backbone (N, Calpha, C=O) RMSD from the average structure of an ensemble of 17 out of 20 calculated structures was 0.41 +/- 0.15 Angstrom for LAP-20 in SDS and 0.41 +/- 0.12 A for an ensemble of 20 out of 20 calculated structures for LAP-20 in DPC. In the presence of SDS, the distance geometry and simulated annealing calculations show that LAP-20 adopts a well defined class A amphipathic helix with distinct hydrophobic and hydrophilic faces. A similar structure was obtained for LAP-20 in the presence of DPC, suggesting that both detergents may be used interchangeably to model the lipoprotein environment. Conformational features of the calculated structures for LAP-20 are discussed relative to models for apolipoprotein A-I activation of LCAT.

Role of sn-2 acyl group of phosphatidylcholine in determining the positional specificity of lecithin-cholesterol acyltransferase

Although human plasma lecithin-cholesterol acyltransferase (LCAT) is believed to be specific for the sn-2 position of phosphatidylcholine (PC), our recent studies showed that it derives a significant percent of acyl groups from the sn-1 position of certain PC species. To understand the physicochemical basis for this altered positional specificity, we determined the effect of sn-2 acyl group of PC on the enzyme activity and utilization of 16:0 from the sn-1 position by purified human and rat LCATs. Positional isomers of PC containing 16:0 at sn-2 were better substrates for human LCAT than the corresponding sn-1-16:0 isomers, whereas the reverse was true for rat LCAT. The positional specificity of human LCAT varied greatly depending on the nature of the acyl group at sn-2. The sn-1 contribution from various sn-1-16:0-2-acyl PCs for cholesteryl ester (CE) synthesis was 1.0% from 16:0-16:0, 1.4% from 16:0-20:5, 7.3% from 16:0-18:1, 47.0% from 16:0-20:3, 49.9% from 16:0-20:4, 54.9% from 16:0-22:6, and 72.3% from 16:0-18:0. There was a linear relationship between the percentage of 16:0 CE formed (from sn-1 position) and the acyl chain length at sn-2 position (r = 0.94). Rat LCAT also transferred some 16:0 from sn-1 position of 16:0-22:6, 16:0-20:3, and 16:0-18:0 PCs, but not from the other natural PCs tested. The phospholipase A activity of both LCATs in the presence of 16:0-20:4 PC showed the same positional specificity as CE synthesis, indicating that the specificity is determined at the formation of acyl-enzyme intermediate. These results show that the positional specificity of LCAT is influenced by the structure of PC, especially the chain length of the sn-2 acyl group.

Analysis of the selective uptake of the cholesteryl ester of human intermediate density lipoproteins by HepG2 cells

We have recently shown by the analysis of the association to HepG2 cells of human intermediate density lipoproteins (IDL) either labeled in cholesteryl ester (CE) with [3H]cholesteryl ethers (CEt) or in proteins by iodine-125 that the CE of IDL are selectively taken up by these cells. Our results also revealed that the addition of a sufficient quantity of HDL3 abolishes the binding of IDL to the 'Lipoprotein binding site' (LBS) and also the CE-selective uptake. This suggested that the LBS mediates this uptake (Brissette and Falstrault (1992) Biochim. Biophys. Acta. 1165, 84-92). This study was undertaken to analyze further the mechanism of the selective uptake of the CE of IDL by HepG2 cells to determine if the LBS is directly or indirectly involved. We show that the different labeling had no effect on the binding affinity of IDL to HepG2 cells. To verify that the apolipoprotein moiety of HDL3 was responsible for the abolishment of the CE selective uptake, we have studied the effect of free apoA-I and apoA-II on the association of IDL. Our results demonstrate that apoA-I and apoA-II are approximately 10-times better than HDL3 or apoA-I liposomes in abolishing the selective uptake of the CE from IDL. We also show that this correlates with a more efficient reduction of the binding of 125I-IDL to HepG2 cells by free apoA-I compared to apoA-I associated with lipids. Thus free apoA-I interfere with the binding of IDL to the LBS and free apolipoproteins have a better capacity to saturate the LBS than lipoproteins. Also, we found no evidence for the transfer of CE from the labeled IDL to HDL3 or to apolipoproteins used to abolish the interaction of IDL to the LBS. Thus our results indicate that the LBS is directly responsible for the selective uptake of CE.

Effect of end group blockage on the properties of a class A amphipathic helical peptide

In a recent classification of biologically active amphipathic alpha-helixes, the lipid-associating domains in exchangeable plasma apolipoproteins have been classified as class A amphipathic helixes (Segrest, J.P., De Loof, H., Dohlman, J.G., Brouillette, C.G., Anantharamaiah, G.M. Proteins 8:103-117, 1990). A model peptide analog with the sequence, Asp Trp Leu Lys Ala Phe Tyr Asp Lys Val Ala Glu Lys Leu Lys Glu Ala Phe (18A), possesses the characteristics of a class A amphipathic helix. The addition of an acetyl group at the alpha-amino terminus and an amide at the alpha-carboxyl terminus, to obtain Ac-18A-NH2, produces large increases in helicity for the peptide both in solution and when associated with lipid (for 18A vs Ac-18A-NH2, from 6 to 38% helix in buffer and from 49 to 92% helix when bound to dimyristoyl phosphatidylcholine in discoidal complexes). Blocking of the end-groups of 18A stabilizes the alpha-helix in the presence of lipid by approximately 1.3 kcal/mol. There is also an increase in the self-association of the blocked peptide in aqueous solution. The free energy of binding to the PC-water interface is increased only by about 3% (from -8.0 kcal/mol for 18A to -8.3 kcal/mol for Ac-18A-NH2). The Ac-18A-NH2 has a much greater potency in raising the bilayer to hexagonal phase transition temperature of dipalmitoleoyl phosphatidylethanolamine than does 18A. In this regard Ac-18A-NH2 more closely resembles the behavior of the apolipoprotein A-I, which is the major protein component of high-density lipoprotein and a potent inhibitor of lipid hexagonal phase formation. The activation of the plasma enzyme lecithin: cholesterol acyltransferase by the Ac-18A-NH2 peptide is greater than the 18A analog and comparable to that observed with the apo A-I. In the case of Ac-18A-NH2, the higher activating potency may be due, at least in part, to the ability of the peptide to micellize egg PC vesicles.

LCAT activation properties of apo A-I CNBr fragments and conversion of discoidal complexes into spherical particles

We studied the substrate properties of the phospholipid-cholesterol-apolipoprotein complexes generated with apo A-I, apo A-I-CNBr fragments, apo A-II and apo A-IV for cholesterol esterification by the enzyme lecithin-cholesterol acyltransferase (LCAT). The kinetic parameters determined with the different complexes as substrates, showed that the complexes containing apo A-I and apo A-IV were about 40-times more efficient than those generated with the apo A-I fragments. In this system, the substrates containing apo A-II had the lowest efficiency. In spite of the differences in the kinetic parameters observed with the various apolipoprotein-lipid complexes, the cholesterol inserted in the complexes was esterified for more than 90% after 24 h in all systems studied. Based upon the results of the kinetic experiments, we followed the transformation of the discoidal complexes into spherical particles, due to the formation of a cholesteryl esters core, in the presence of low-density lipoproteins as an external source of cholesterol. We observed the formation of spherical particles by electron microscopy, after incubation of the discoidal complexes with LCAT for 24 h. The average percentage of cholesteryl esters in the converted particles was around 60% of the total cholesterol, varying between 40% for the apo A-I-CNBr-1-DPPC-cholesterol complex and up to 86% for the apo A-I-DPPC-cholesterol complex. The secondary structure of protein in the complexes was not significantly modified. However, the phospholipid phase transition disappeared, together with the parallel orientation of the phospholipid acyl chains with the helical segments of the apolipoproteins, as the phospholipids are organized in a monolayer at the surface of the spheres.

Altered positional specificity of human plasma lecithin-cholesterol acyltransferase in the presence of sn-2 arachidonoyl phosphatidyl cholines. Mechanism of formation of saturated cholesteryl esters

The positional specificity of purified human lecithin-cholesterol acyltransferase (LCAT) was studied by analyzing the labeled cholesteryl ester (CE) species formed in the presence of proteoliposome substrates containing mixed chain phosphatidylcholine (PC) species, labeled cholesterol and apoprotein A-I. Whereas over 90% of the acyl groups used for CE synthesis were derived from the sn-2 position of most of the naturally occurring PC substrates, about 75% of the CE species formed in the presence of sn-1-myristoyl 2-arachidonoyl PC, sn-1-palmitoyl-2-arachidonoyl (PAPC) and sn-1-palmitoyl 2-docosahexaenoyl PC were derived from the sn-1-position. On the other hand, rat LCAT utilized mostly sn-2-acyl group from either PAPC or from sn-1-palmitoyl 2-linoleoyl PC. The positional specificity of the human enzyme was not affected by the alteration in the matrix fluidity, type of the apoprotein activator used, or by the free cholesterol/PC ratio in the substrate. These results show that the positional specificity of human plasma LCAT is altered in the presence of sn-2-arachidonoyl PC, or sn-2-docosahexaenoyl PC, probably due to steric restrictions at the active site, and this may account for the formation of disproportionately high concentrations of saturated CE, and low concentrations of long-chain polyunsaturated CE in human plasma, relative to the composition of sn-2-acyl groups in plasma PC.

Apoprotein A-1 is a cofactor independent substrate of protein kinase C

Apoprotein A-1 (apo A-1), the predominant protein constituent of high density lipoproteins (HDL), was phosphorylated by protein kinase C (PKC). Optimal phosphorylation of lipid-free apo A-1 occurs in the absence of calcium, phosphatidyl serine (PS), and diolein (DO). However, HDL-bound apo A-1 was not phosphorylated by PKC. Furthermore, addition of either native or reconstituted HDL particles to lipid-free apo A-1 resulted in a concentration-dependent inhibition of phosphorylation. It appears that the phosphorylatable sites on apo A-1 are involved in hydrophobic interaction with the lipids of HDL. Apo A-1 is a novel substrate of PKC because it does not require calcium and lipid cofactors for optimal phosphorylation.

Reverse cholesterol transport: physiology and pharmacology

Reverse cholesterol transport identifies a series of metabolic events resulting in the transport of cholesterol from peripheral tissues to the liver and plays a major role in maintaining cholesterol homeostasis in the body. High density lipoproteins (HDL) are the vehicle of cholesterol in this reverse transport, a function believed to explain the inverse correlation between plasma HDL levels and atherosclerosis. An attempt to stimulate, by the use of drugs, this transport process seems to be of great promise in the prevention and treatment of arterial disease. Only few drugs are now known that can modify the activity of the various factors involved in the process. Clofibrate reduces cholesterol esterification, but the newer fibric acids are generally ineffective as anion-exchange resins. Probucol directly increases the activity and mass of cholesteryl ester transfer protein, thus possibly improving the physiological process of cholesterol removal from tissues. The few available data on the effects of drugs on reverse cholesterol transport should stimulate the search for new agents specifically stimulating this antiatherogenic process.

Apolipoprotein AIMilano. Partial lecithin:cholesterol acyltransferase deficiency due to low levels of a functional enzyme

The cholesterol esterification process was analyzed in 19 carriers of the apolipoprotein AIMilano (AIM) variant and in 19 age-sex matched controls by measuring lecithin:cholesterol acyltransferase (LCAT) mass, activity (i.e., cholesterol esterification with a standard proteoliposome substrate) and cholesterol esterification rate (i.e., cholesterol esterification in the presence of the endogenous substrate). The AIM subjects had lower LCAT mass (3.30 +/- 0.85 micrograms/ml), activity (71.1 +/- 36.4 nmol/ml per h) and cholesterol esterification rate (23.6 +/- 12.5 nmol/ml per h) compared to controls (5.22 +/- 0.74 micrograms/ml, 121.6 +/- 54.6 nmol/ml per h and 53.6 +/- 29.9 nmol/ml per h, respectively). The specific LCAT activity, i.e., LCAT activity per microgram of LCAT, was similar in the two groups, indicating that the LCAT protein in the AIM carriers is structurally and functionally normal. However, the specific cholesterol esterification rate was 23% lower in the AIM subjects (8.03 +/- 6.01 nmol/h per microgram) compared to controls (10.49 +/- 5.86 nmol/h per microgram; P less than 0.05). The capacity of HDL3, purified from both AIM and control plasma, to act as substrates for cholesterol esterification was similar, thus suggesting that other mechanism(s) may be in play. Carriers with a relative abundance of abnormal, small HDL3b particles had the most altered cholesterol esterification pattern. Upon evaluating all AIM subjects, a complex relationship between HDL structure, plasma lipid-lipoprotein levels and cholesterol esterification emerged, making the AIMilano condition a unique model for the study of the mechanisms regulating the cholesterol esterification-transfer process in man.

Phosphatidylcholine metabolism after transfer from lipid emulsions injected intravenously in rats. Implications for high-density lipoprotein metabolism

When injected intravenously in rats, emulsion models of triacylglycerol-rich lipoproteins were metabolized like natural lipoproteins and during the hydrolysis of emulsion triacylglycerols, a large fraction of the emulsion phosphatidylcholine was transferred to the plasma high-density lipoproteins. The removal from plasma of emulsion phosphatidylcholine was followed for 2 h in unanaesthetized rats. The half-lives for removal of phospholipid after injection of emulsions stabilized with dioleoylphosphatidylcholine or 1-palmitoyl-2-oleolyphosphatidylcholine were 58-63 min when traced with isologous label. In comparison, the published half-lives of HDL mixed phospholipids in rats are approx. 40 min, indicating that much of the clearance of the emulsion phospholipid could be accounted for by HDL catabolism. Measured LCAT activity was sufficient to account for not more than 2% of the catabolism of the HDL phospholipids labelled by this physiological procedure. Removal from plasma of label was more rapid when the same emulsions were labelled with tracer amounts of the heterologous dipalmitoylphosphatidylcholine, showing that individual phosphatidylcholine species were handled distinctly even when present only in tracer amounts in a bulk of another phosphatidylcholine differing in acyl chains.

Substrate specificity of human plasma lecithin-cholesterol acyltransferase towards molecular species of phosphatidylcholine in native plasma

The specificity of human plasma lecithin-cholesterol acyltransferase for molecular species of phosphatidylcholine (PC) was studied by determining the molecular species composition of whole plasma before and after incubation at 37 degrees C. Since the disappearance of PC under the conditions employed is entirely due to the activity of lecithin-cholesterol acyltransferase, its specificity can be determined from the decrease in the concentration of each species after the reaction. The selectivity factor for each species was calculated by dividing its observed contribution by its concentration at zero time. The major species contributing to cholesterol esterification in whole plasma were 16:0-18:2 (46%), 18:0-18:2 (16%), 16:0-18:1 (15%), 16:0-20:4 (10%), 18:0-20:4 (5%) and 18:1-18:2 (5%). The specificity, as determined from the selectivity factors for whole plasma, was in the order: 16:0-18:2 greater than 18:1-18:2 greater than 16:0-18:1 greater than 18:0-18:2 greater than 16:0-22:6 greater than 18:0-20:4 greater than 16:0-20:4. The high-density lipoproteins (HDL) contained a significantly higher percentage of 16:0-20:4 and 18:0-20:4 and a lower percentage of 16:0-18:1 and 18:0-18:1 compared to the very-low and low-density lipoproteins. These differences disappeared after incubation of the plasma for 24 h. Using selectivity factors for HDL PCs only, the specificity of the enzyme was found to be in the order: 16:0-18:2 greater than 18:1-18:2 greater than 18:1-18:1 greater than 16:0-22:6 greater than 18:0-18:2 greater than 16:0-18:1 greater than 16:0-20:4. These results indicate that in native plasma, lecithin-cholesterol acyltransferase prefers 16:0 greater than 18:1 greater than 18:0 at the 1-position and 18:2 greater than 18:1 greater than 22:6 greater than 20:4 at the 2-position of PC.

Alterations in the HDL system after rapid plasma cholesterol reduction by LDL-apheresis

Changes in high density lipoprotein (HDL) subfraction structure and composition were analyzed during and after extracorporeal removal of apo B containing lipoproteins in seven familial hypercholesterolemic (FH) patients. After the apheretic procedure, carried out with dextran-sulfate-cellulose columns, the plasma levels of very low density lipoproteins (VLDL), low density lipoproteins (LDL), and HDL decreased by 72%, 50%, and 19%, respectively. The free cholesterol to esterified cholesterol ratio in plasma increased, with a 26% drop in the lecithin:cholesterol acyl transferase (LCAT) activity. In the ensuing 24 hours, VLDL, HDL, and LCAT activity approached the pretreatment levels. During this phase, possibly as a consequence of increased cholesterol esterification and exchange of cholesteryl esters for triglycerides between HDL and VLDL, HDL2a particles were detected in plasma. However, these metabolic changes did not result in clearcut modifications in the HDL2-HDL3 subfraction distribution. These findings clearly demonstrate that rapid changes in the plasma VLDL-LDL levels affect several processes involved in the HDL metabolism, but confirm that the HDL system, in spite of a considerable plasticity, displays a marked stability of the HDL2-HDL3 subfraction distribution.

Activation of lecithin-cholesterol acyltransferase by apolipoprotein D: comparison of proteoliposomes containing apolipoprotein D, A-I or C-I

To study the activation of lecithin-cholesterol acyl transferase (LCAT) (phosphatidylcholine:sterol O-acyltransferase, EC 2.3.1.43) by apolipoprotein D in comparison to apolipoproteins A-I and C-I, proteoliposomes with a phosphatidylcholine/free cholesterol molar ratio of 24:1, containing 10-300 micrograms/ml of apolipoproteins were used. The proteoliposomes were prepared by the cholate dialysis technique. In all proteoliposome preparations we found rouleaux structures and stacked discs. The particles formed with apolipoprotein A-I were the most homogeneous, followed by apolipoprotein D- and apolipoprotein C-I-containing particles. Apolipoprotein A-I was the most potent LCAT activator in our system followed by apolipoproteins C-I and D. The fractional esterification rate observed with apolipoprotein D-containing substrates amounted to 15-48% that of apolipoprotein A-I-containing ones. Neither apolipoprotein A-I- nor C-I-containing proteoliposomes gave linear reaction kinetics with LCAT. Even during the first 15-30 min of incubation, the kinetics deviated strikingly from linearity at all apolipoprotein concentrations. In contrast, proteoliposomes containing apolipoprotein D exhibited linear reaction kinetics up to 60-90 min. At low apolipoprotein A-I concentrations (5 micrograms/ml), the addition of apolipoprotein D to the incubates resulted in significantly higher esterification rates as compared to substrates containing apolipoprotein A-I only. This was not the case using substrates with high apolipoprotein A-I concentrations (50 micrograms/ml). From our results we speculate that apolipoprotein D may have some stabilizing effect on the enzyme LCAT.

A continuous fluorescence assay for lecithin cholesterol acyltransferase

A continuous fluorescence assay was adapted to the measurement of the phospholipase reaction of lecithin cholesterol acyltransferase (LCAT). The fluorescent phospholipid 1-acyl-2-(N-4-nitrobenzo-2-oxa-1,3-diazole)aminocaproyl phosphatidylcholine (C6-NBD-PC) in micelle form reacted with LCAT to yield NBD-caproic acid, resulting in up to 5-fold increases in fluorescence in 30 min. The reaction rates were optimal in mixtures containing 0.1 M NaCl and 4 mM beta-mercaptoethanol at 37 degrees C. Apolipoprotein A-I did not activate the enzyme and bovine serum albumin bound monomeric substrate and interfered with the fluorescence assay. Under similar reaction conditions, bee venom phospholipase A2 was almost 100-fold more reactive than LCAT.