Lecithin:cholesterol acyltransferase-induced modifications of liver perfusate discoidal high density lipoproteins from African green monkeys

High-density lipoproteins, inflammation and oxidative stress

Plasma levels of HDL (high-density lipoprotein)-cholesterol are strongly and inversely correlated with atherosclerotic cardiovascular disease. Both clinical and epidemiological studies have reported an inverse and independent association between serum HDL-cholesterol levels and CHD (coronary heart disease) risk. The cardioprotective effects of HDLs have been attributed to several mechanisms, including their involvement in the reverse cholesterol transport pathway. HDLs also have antioxidant, anti-inflammatory and antithrombotic properties and promote endothelial repair, all of which are likely to contribute to their ability to prevent CHD. The first part of this review summarizes what is known about the origins and metabolism of HDL. We then focus on the anti-inflammatory and antioxidant properties of HDL and discuss why these characteristics are cardioprotective.

Initial interaction of apoA-I with ABCA1 impacts in vivo metabolic fate of nascent HDL

We investigated the in vivo metabolic fate of pre-beta HDL particles in human apolipoprotein A-I transgenic (hA-I (Tg)) mice. Pre-beta HDL tracers were assembled by incubation of [(125)I]tyramine cellobiose-labeled apolipoprotein A-I (apoA-I) with HEK293 cells expressing ABCA1. Radiolabeled pre-beta HDLs of increasing size (pre-beta1, -2, -3, and -4 HDLs) were isolated by fast-protein liquid chromatography and injected into hA-I (Tg)-recipient mice, after which plasma decay, in vivo remodeling, and tissue uptake were monitored. Pre-beta2, -3, and -4 had similar plasma die-away rates, whereas pre-beta1 HDL was removed 7-fold more rapidly. Radiolabel recovered in liver and kidney 24 h after tracer injection suggested increased (P < 0.001) liver and decreased kidney catabolism as pre-beta HDL size increased. In plasma, pre-beta1 and -2 were rapidly (<5 min) remodeled into larger HDLs, whereas pre-beta3 and -4 were remodeled into smaller HDLs. Pre-beta HDLs were similarly remodeled in vitro with control or LCAT-immunodepleted plasma, but not when incubated with phospholipid transfer protein knockout plasma. Our results suggest that initial interaction of apoA-I with ABCA1 imparts a unique conformation that partially determines the in vivo metabolic fate of apoA-I, resulting in increased liver and decreased kidney catabolism as pre-beta HDL particle size increases.

N-Glycosylation is Required for Secretion-Competent Human Plasma Phospholipid Transfer Protein

Human plasma phospholipid transfer protein (PLTP) contains six potential N-glycosylation sites (Asn-X-Ser). To study the role of these sites on PLTP structure and function, seven variants in which asparagine (N) residues were converted to glycine (G) were prepared by site-directed mutagenesis. These were N(47)G, N(77)G, N(100)G, N(126)G, N(228)G, N(381)G and N(47, 77, 100, 126, 228, 381)G (N(null)G). These variants and wild-type (WT) PLTP were expressed in COS-7 cells. Intracellular and secreted PLTP mass was analyzed by Western blots and quantitative enzyme-linked immunosorbent assay; PLTP activities in cellular lysates and media were based on the transfer of [(3)H]dipalmitoylphosphatidylcholine from phospholipid single bilayer vesicles to HDL. N(null)G was not detected intracellularly. N(381)G was similar to WT PLTP with respect to specific activity and secretion efficiency. The specific activities of N(47)G, N(77)G, N(100)G, N(126)G, N(228)G and N(381)G were similar in cell lysate (range = 67-90% WT) and medium (range = 65-77% WT). Intracellular masses of these PLTP variants were similar to that of WT (Mean = 103% WT); mean secreted mass was 88% WT. These results suggest that secretion-competent PLTP requires glycosylation but that no single glycosylation site is required.

Secretion of prebeta HDL increases with the suppression of cholesteryl ester transfer protein in Hep G2 cells

Prebeta HDL are small, protein rich lipoproteins that are predominantly composed of apo A-I, without apo A-II. Prebeta HDL are secreted from the liver as nascent HDL and/or are produced in the incubated plasma by cholesteryl ester transfer protein (CETP). However, the role of CETP in the secretion of HDL from the liver has yet to be determined. In the present study, we examined the effect of the suppression of hepatic CETP by antisense oligodeoxynucleotides (ODNs) against CETP targeted to the liver on the secretion of apo A-I using a Hep G2 cell culture. The ODNs against CETP were coupled to asialoglycoprotein (ASOR) carrier molecules, which serve as an important method for the regulation of liver gene expression. Hep G2 cells were cultured in DMEM supplemented with 10 FBS. After 2 days, the medium was changed to DMEM with EGF and the cells were divided into three groups. The control group received saline, while the sense group was mixed with the sense ODNs complex and the antisense group was mixed with the antisense ODNs complex, respectively, for 2 days. Both the hepatic CETP mRNA and the CETP mass in the medium in the antisense group decreased significantly more than in the sense and the control groups (CETP mass: 1.697 + /- 0.410 ng/mg cell protein vs. 2.367 + /- 0.22 and 2.360 + /- 0.139, n = 3 in each determination). In contrast, both the hepatic apo A-I mRNA and the apo A-I mass in the medium in the antisense group were significantly higher than those in the sense and the control groups (apo A-I mass; 1.877 + /- 0.215 micro/mg cell protein vs. 1.213 + /- 0.282 and 1.097 + /- 0.144, n = 3 in each determination). The increase in apo A-I was mainly due to the increase in prebeta apo A-I. These findings may partly explain why HDL and apo A-I increase in patients with CETP deficiency, while also indicating the possibility that the original level of prebeta HDL is sufficient in such patients.

Amino acid residue 149 of lecithin:cholesterol acyltransferase determines phospholipase A2 and transacylase fatty acyl specificity

Human LCAT prefers phosphatidylcholine (PC) with sn-1-palmitoyl-2-oleoyl PC (POPC) as substrate for cholesteryl ester synthesis, whereas rat LCAT (which is 92% similar in amino acid sequence) prefers sn-1-palmitoyl-2-arachidonoyl PC (PAPC). Six recombinant human LCAT cDNA clones were constructed with unique clusters of rat sequence substitutions in the human background spanning the region encoding amino acids 121-296. Media from transfected COS cells expressing each of the constructs were assayed for LCAT cholesterol esterification (CE) or phospholipase A2 (PLA2) activity using substrate particles containing POPC or PAPC. The PAPC/POPC CE activity ratio of the cluster 1 construct (amino acids 149-158) was 1.3, resembling rat LCAT, whereas cluster 2-5 clones produced CE activity ratios <0.3, unchanged from human LCAT. The cluster 6 clone (Y292H/W294F) had an intermediate ratio (0.6). Similar results were observed for LCAT PLA2 activity. In additional studies, position 149 of human LCAT was changed to the rat sequence (hE149A) and compared to a triple mutation containing the remainder of the cluster 1 changes (G151R/E154D/R158Q). CE and PLA2 activity ratio for the hE149A construct was >1.7, similar to rat LCAT, whereas the triple mutation construct retained a ratio similar to human LCAT (<0.6). Thus, a single amino acid substitution (E149A) was sufficient to alter the fatty acyl specificity of human LCAT to that of rat LCAT, with an increase in activity toward PAPC. This is the first example of a point mutation in an enzyme with PLA2 activity that results in an increase in activity toward arachidonic acid.

Phospholipid transfer protein mediated conversion of high density lipoproteins generates pre beta 1-HDL

High density lipoproteins (HDL) subclasses can be differentiated by two-dimensional non-denaturing polyacrylamide gradient gel electrophoresis (2D-PAGGE) and subsequent immunoblotting. The quantitatively minor HDL-subclasses pre beta 1-LpA-I and gamma-LpE are initial acceptors of cell-derived cholesterol into the plasma compartment. In this study we analysed the effect of phospholipid transfer protein (PLTP) on the electrophoretic distribution of HDL-subclasses in plasma as well as the ability of plasma, pre beta 1-LpA-I, and gamma-LpE to take up [3H]cholesterol from labeled fibroblasts. Pre beta 1-LpA-I but not gamma-LpE disappeared during a 16 hours incubation in the absence of PLTP. During a one minute incubation pre beta 1-LpA-I of pre-incubated plasma released 75% less [3H]cholesterol from radiolabeled fibroblasts than pre beta 1-LpA-I of control plasma. Pre-incubation of plasma reduced the uptake of [3H]cholesterol by gamma-LpE by 40%. Totally, the cholesterol efflux capacity of plasma decreased by 10% compared to the original sample. The amount of immunodetectable pre beta 1-LpA-I increased when plasma was incubated in the presence of PLTP while the amount of immunodetectable gamma-LpE did not change. After one minute incubation of PLTP-conditioned plasma with [3H]cholesterol-labeled fibroblasts, the amount of radioactive cholesterol taken up by pre beta 1-LpA-I was twice as high as in control plasma whereas the amount of [3H]cholesterol taken up by gamma-LpE remained unchanged. As a net result, treatment with PLTP increased the cholesterol efflux into total plasma by 40%. Together with results of previous studies our data suggest that the conversion of alpha-LpA-I3 into alpha-LpA-I2 by PLTP generates pre beta 1-LpA-I but not gamma-LpE. PLTP helps to enhance the uptake of cell-derived cholesterol by pre beta 1-LpA-I and, thereby, the cholesterol efflux capacity of normal plasma.

Isolation of nascent high-density lipoprotein from rat liver perfusates by immunoaffinity chromatography: effects of oleic acid infusion

Immunoaffinity chromatography on a column of rabbit IgG anti-rat apolipoprotein (apo) A-I covalently bonded to agarose was used to isolate nascent high-density lipoprotein (nHDL) from recirculated perfusates of rat livers. After passage through the affinity column, the bound material was eluted with sodium thiocyanate and analyzed for apolipoproteins and lipids. The protein content was 52% and the lipid composition was 37% triglyceride, 40% phospholipid, and 23% cholesterol. Apolipoproteins E and A-I each comprised approximately one third of the total, and very little apo B was detectable as judged by SDS-PAGE analysis. The affinity-isolated particles were therefore similar in composition to the major apo A-I:apo E-rich subfraction of nHDL isolated by ultracentrifugation in earlier work. It is concluded that the apo E in this class of nHDL (containing both apo E and apo A-I) is present in the secreted particle and is not a consequence of a loss of apo E from very-low-density lipoprotein (VLDL) during ultracentrifugation. The high triglyceride content in the virtual absence of apo B confirms and extends previous analyses and reinforces the conclusion that nHDL particles are enriched in triglyceride compared to plasma HDL. The inclusion of 4% albumin in the perfusion medium did not significantly change the total triglyceride output of 115 micrograms/g liver/h, but it decreased the triglyceride output isolated by anti-apo A-I affinity chromatography from 3.2 to 0.48 micrograms/g liver/h. The addition of oleic acid complexed to albumin increased the total triglyceride output by 70% and that associated with the immunoaffinity column increased from 0.48 to 2.7 micrograms/g liver/h.(ABSTRACT TRUNCATED AT 250 WORDS)

Physical and chemical characteristics of apolipoprotein A-I-lipid complexes produced by Chinese hamster ovary cells transfected with the human apolipoprotein A-I gene

Chinese hamster ovary cells transfected with the human apolipoprotein A-I gene linked to the human metallothionein gene promoter region secrete large quantities of apolipoprotein A-I (7.1 +/- 0.4% total secreted protein) in the presence of zinc. Approx. 16% of the secreted apolipoprotein A-I is complexed with lipid and can be isolated ultracentrifugally at d less than or equal to 1.21 g/ml. The latter complexes are composed of discs and vesicles as judged by electron microscopy and can be further separated by column chromatography into three fractions: fraction I, mostly vesicles (60-260 nm) and large discs (18-20 nm diameter); fraction II, discs 14.2 +/- 2.6 nm diameter; and fraction III, nonresolvable by electron microscopy. The latter fraction is extremely lipid-poor (94% protein, 6% phospholipid); in contrast, the protein, phospholipid and unesterified cholesterol content for the other fractions are 43, 33 and 24%, respectively, for fraction I and 53, 33 and 14%, respectively, for fraction II. Fraction II particles contain three and four apolipoprotein A-Is per particle as determined by protein crosslinking while large structures in fraction I contain primarily six to seven apolipoprotein A-Is per particle. Following incubation with purified lecithin: cholesterol acyltransferase, discoidal particles were transformed into apparent spherical particles 12.9 +/- 3.4 nm diameter; this transformation coincided with 19-21% conversion of unesterified cholesterol to esterified cholesterol. The apolipoprotein A-I-lipid complexes isolated from Chinese hamster ovary cell media are similar to nascent HDL found in plasma of lecithin:cholesterol acyltransferase-deficient patients and those secreted by the human hepatoma line, Hep G2. The ability of the Chinese hamster ovary cell nascent HDL-like particles to undergo transformation in the presence of purified lecithin:cholesterol acyltransferase indicates that they are functional particles.

Hepatic apo B-100 lipoproteins and plasma LDL heterogeneity in African green monkeys

The contribution of hepatic apolipoprotein (apo) B-100 lipoproteins to plasma low-density lipoprotein (LDL) metabolic heterogeneity was examined in African green monkeys. Hepatic 3H-labeled very low-density lipoproteins (VLDL) (d less than 1.006, where d is density in g/ml) or hepatic 131I-labeled LDL (1.030 less than d less than 1.063) were isolated from perfused livers and injected simultaneously with autologous plasma 125I-LDL into African green monkeys. Serial blood samples were taken, and the distribution of radioactivity among various subfractions of apo B-100 lipoproteins was determined using density-gradient ultracentrifugation. Compartmental models were developed to describe simultaneously the kinetics of hepatic lipoproteins and plasma LDL. In five of seven studies, the metabolic behavior of LDL derived from radiolabeled hepatic lipoprotein precursors differed from the metabolic behavior of radiolabeled autologous plasma LDL. These differences could be described by different models supporting two hypotheses with different physiological interpretations: 1) lipoproteins of donor and recipient animals are kinetically distinct, and/or 2) plasma LDL derived from various potential sources are kinetically distinct. Compartmental modeling was used to test these hypotheses, which were not accessible to testing by conventional experimental methodologies. The kinetic analyses of these studies suggest that plasma LDL may be derived from a variety of precursors, including hepatic VLDL and hepatic LDL, with each source giving rise to metabolically distinct plasma LDL.

Discoidal complexes containing apolipoprotein E and their transformation by lecithin-cholesterol acyltransferase

The primary objectives of this study were to determine whether analogs to native discoidal apolipoprotein (apo)E-containing high-density lipoproteins (HDL) could be prepared in vitro, and if so, whether their conversion by lecithin-cholesterol acyltransferase (LCAT; EC 2.3.1.43) produced particles with properties comparable to those of core-containing, spherical, apoE-containing HDL in human plasma. Complexes composed of apoE and POPC, without and with incorporated unesterified cholesterol, were prepared by the cholate-dialysis technique. Gradient gel electrophoresis showed that these preparations contain discrete species both within (14-40 nm) and outside (10.8-14 nm) the size range of discoidal apoE-containing HDL reported in LCAT deficiency. The isolated complexes were discoidal particles whose size directly correlated with their POPC:apoE molar ratio: increasing this ratio resulted in an increase in larger complexes and a reduction in smaller ones. At all POPC:apoE molar ratios, size profiles included a major peak corresponding to a discoidal complex 14.4 nm long. Preparations with POPC:apoE molar ratios greater than 150:1 contained two distinct groups of complexes, also in the size range of discoidal apoE-containing HDL from patients with LCAT deficiency. Incorporation of unesterified cholesterol into preparations (molar ratio of 0.5:1, unesterified cholesterol:POPC) resulted in component profiles exhibiting a major peak corresponding to a discoidal complex 10.9 nm long. An increase of unesterified cholesterol and POPC (at the 0.5:1 molar ratio) in the initial mixture, increased the proportion of larger complexes in the profile. Incubation of isolated POPC-apoE discoidal complexes (mean sizes, 14.4 and 23.9 nm) with purified LCAT and a source of unesterified cholesterol converted the complexes to spherical, cholesteryl ester-containing products with mean diameters of 11.1 nm and 14.0 nm, corresponding to apoE-containing HDL found in normal plasma. Conversion of smaller cholesterol-containing discoidal complexes (mean size, 10.9 nm) under identical conditions resulted in spherical products 11.3, 13.3, and 14.7 nm across. The mean sizes of these conversion products compared favorably with those (mean diameter, 12.3 nm) of apoE-containing HDL of human plasma. This conversion of cholesterol-containing complexes is accompanied by a shift of some apoE to the LDL particle size interval. Our study indicates that apoE-containing complexes formed by the cholate-dialysis method include species similar to discoidal apoE-containing HDL and that incubation with LCAT converts most of them to spherical core-containing particles in the size range of plasma apoE-containing HDL. Plasma HDL particles containing apoE may arise in part from direct conversion of discoidal apoE-containing HDL by LCAT.

Discharge of newly-synthesized dolichol and ubiquinone with lipoproteins to rat liver perfusate and to the bile

An effective system for perfusing rat liver using complete tissue culture medium and washed calf erythrocytes as oxygen carriers was devised. Infusion of taurocholate and glucose proved necessary to maintain stable metabolic activity and bile secretion during a 6-hr period. Perfusate pO2, pCO2 and pH values were monitored continuously and found to be stable. Electron microscopic examination revealed the maintenance of normal hepatic structure, even after 6 hr. Normal rates of protein and urea synthesis, no leakage of cytoplasmic enzymes, and continuous bile acid production demonstrated the functional integrity of this system. Using [3H]mevalonic acid as precursor, dolichol, dolichyl phosphate, ubiquinone and cholesterol were found to be continuously synthesized in this perfused liver system. All these lipids appeared in the perfusate, indicating discharge through the ER-Golgi system. The lipoproteins of the perfusate were isolated by ultracentrifugation and characterized with respect to size distribution and lipid composition. Dolichol was found in VLDL, LDL and HDL fractions, with the highest concentration present in the latter. In rat and human blood plasma this lipid was mainly associated with HDL. The ubiquinone in the perfusate was primarily associated with the VLDL fraction, while in rat plasma it was found more evenly distributed among all the three lipoprotein fractions studied. Dolichol, ubiquinone and cholesterol were also discharged to the bile, whereas dolichyl phosphate was not. Thus, newly-synthesized dolichol and ubiquinone are transported out of the hepatocyte to the blood and to the bile.

Conversion of apolipoprotein-specific high-density lipoprotein populations during incubation of human plasma

Incubation studies were performed on plasma obtained from subjects selected for relatively low levels of high-density lipoprotein cholesterol (HDL-C) (no greater than 30 mg/dl) and particle size distributions enriched in the HDL3 subclass. Incubation (12 h, 37 degrees C) of plasma in the presence or absence of lecithin: cholesterol acyltransferase activity produces marked alteration in size profiles of both major apolipoprotein-specific HDL3 populations (HDL3(AI w AII), HDL3 species containing both apolipoprotein A-I and apolipoprotein A-II, and HDL3(AI w/o AII), HDL3 species containing apolipoprotein A-I) as isolated by immunoaffinity chromatography. In the presence or absence of lecithin: cholesterol acyltransferase activity, plasma incubation results in a shift of HDL3(AI w AII) species (initial mean sizes of major components, approx. 8.8 and 8.0 nm) predominantly to larger particles (mean size, 9.8 nm). A less prominent shift to smaller particles (mean size, 7.8 nm) accompanies the conversion to larger particles only when the enzyme is active. Combined shifts to larger (mean size, 9.8 nm) and smaller (mean size, 7.4 nm) particles are observed for HDL3(AI w/o AII) particles (mean size, 8.3 nm) also only in the presence of enzyme activity. However, in the absence of enzyme activity, HDL3(AI w/o AII) species, unlike the HDL3(AI w AII) species, are converted to smaller (mean size 7.4 nm) rather than to larger particles. Like native HDL2b(AI w/o AII) particles, the larger HDL3(AI w/o AII) conversion products exhibit a protein moiety with molecular weight equivalent to four apolipoprotein A-I molecules per particle; small HDL3(AI w/o AII) products are comprised predominantly of particles with two apolipoprotein A-I per particle. Incubation-induced conversion of HDL3 particles in the presence of lecithin: cholesterol acyltransferase activity is associated with increased binding of both apolipoprotein-specific HDL populations to low-density lipoproteins (LDL). The present studies indicate that, in the absence of lecithin: cholesterol acyltransferase activity, the two HDL3 populations follow different conversion pathways, possibly due to apolipoprotein-specific activities of lipid transfer protein or conversion protein in plasma. Our studies also suggest that lecithin: cholesterol acyltransferase activity may play a role in the origins of large HDL2b(AI w/o AII) species in human plasma by participating in the conversion of HDL3(AI w/o AII) particles, initially with three apolipoprotein A-I, to larger particles with four apolipoprotein A-I per particle.

Lipoproteins and atherosclerosis

The plasma lipoproteins are the primary means of transport of cholesterol among tissues. In particular, the apo B-containing lipoproteins (VLDL, IDL and LDL) are important for the delivery of cholesterol from the liver to peripheral tissues, while HDL appear to mediate the reverse process of movement of cholesterol from tissues back to the liver. Both of these transport processes are necessary for efficient whole body cholesterol homeostasis, because the liver is the major site of both the production and excretion of cholesterol. However, deviations from a proper balance of transport of cholesterol, either increases in LDL levels or decreases in HDL cholesterol flux, may result in accumulation of cholesterol in extrahepatic tissues. Increased risk of atherosclerosis and CHD may be associated with elevation in the number of LDL particles, increase or decrease in LDL particle size, or changes in the composition of plasma LDL. These modifications of plasma LDL may be brought about following perturbation of one of several aspects of LDL metabolism. These include decreased LDL receptor activity, increased VLDL production and cholesterol enrichment of the liver-derived VLDL. The events in the arterial wall that make some LDL particles apparently atherogenic are not well understood. In the case of nonhuman primates, large-size LDL are associated with an increased risk of CHD. One characteristic of these LDL is that their core lipids are rich in saturated cholesteryl esters and their transition temperatures are frequently above body temperature. The liquid crystalline cholesteryl ester cores of such LDL may modulate the conformation of apo B on the surface and thereby affect the interaction of these LDL with cellular receptors or connective tissue matrix proteoglycans. It is likely, though, that changes in LDL particle number, LDL particle size and LDL particle composition may each contribute to progression of atherosclerosis. The presumed metabolic events that make HDL protective against atherosclerosis have been termed reverse cholesterol transport, and suggest that small HDL that are deficient in free cholesterol acquire this lipid from cell membranes. The HDL cholesterol is esterified by LCAT in the circulation, forming large HDL that can then deliver the cholesteryl ester to the liver by both direct and indirect means. In most circumstances, it is assumed that an increase in plasma HDL cholesterol concentration reflects an increase in the rate at which HDL is removing cholesterol from tissues and, consequently, a decrease in atherosclerosis.(ABSTRACT TRUNCATED AT 400 WORDS)