Binding of lecithin:cholesterol acyltransferase to reconstituted high density lipoproteins is affected by their lipid but not apolipoprotein composition

Structural analysis of lecithin:cholesterol acyltransferase bound to high density lipoprotein particles

Lecithin:cholesterol acyltransferase (LCAT) catalyzes a critical step of reverse cholesterol transport by esterifying cholesterol in high density lipoprotein (HDL) particles. LCAT is activated by apolipoprotein A-I (ApoA-I), which forms a double belt around HDL, however the manner in which LCAT engages its lipidic substrates and ApoA-I in HDL is poorly understood. Here, we used negative stain electron microscopy, crosslinking, and hydrogen-deuterium exchange studies to refine the molecular details of the LCAT-HDL complex. Our data are consistent with LCAT preferentially binding to the edge of discoidal HDL near the boundary between helix 5 and 6 of ApoA-I in a manner that creates a path from the lipid bilayer to the active site of LCAT. Our results provide not only an explanation why LCAT activity diminishes as HDL particles mature, but also direct support for the anti-parallel double belt model of HDL, with LCAT binding preferentially to the helix 4/6 region.

A retractable lid in lecithin:cholesterol acyltransferase provides a structural mechanism for activation by apolipoprotein A-I

Lecithin:cholesterol acyltransferase (LCAT) plays a key role in reverse cholesterol transport by transferring an acyl group from phosphatidylcholine to cholesterol, promoting the maturation of high-density lipoproteins (HDL) from discoidal to spherical particles. LCAT is activated through an unknown mechanism by apolipoprotein A-I (apoA-I) and other mimetic peptides that form a belt around HDL. Here, we report the crystal structure of LCAT with an extended lid that blocks access to the active site, consistent with an inactive conformation. Residues Thr-123 and Phe-382 in the catalytic domain form a latch-like interaction with hydrophobic residues in the lid. Because these residues are mutated in genetic disease, lid displacement was hypothesized to be an important feature of apoA-I activation. Functional studies of site-directed mutants revealed that loss of latch interactions or the entire lid enhanced activity against soluble ester substrates, and hydrogen-deuterium exchange (HDX) mass spectrometry revealed that the LCAT lid is extremely dynamic in solution. Upon addition of a covalent inhibitor that mimics one of the reaction intermediates, there is an overall decrease in HDX in the lid and adjacent regions of the protein, consistent with ordering. These data suggest a model wherein the active site of LCAT is shielded from soluble substrates by a dynamic lid until it interacts with HDL to allow transesterification to proceed.

Arginine 123 of apolipoprotein A-I is essential for lecithin:cholesterol acyltransferase activity

ApoA-I activates LCAT that converts lipoprotein cholesterol to cholesteryl ester (CE). Molecular dynamic simulations suggested earlier that helices 5 of two antiparallel apoA-I molecules on discoidal HDL form an amphipathic tunnel for migration of acyl chains and unesterified cholesterol to the active sites of LCAT. Our recent crystal structure of Δ(185-243)apoA-I showed the tunnel formed by helices 5/5, with two positively charged residues arginine 123 positioned at the edge of the hydrophobic tunnel. We hypothesized that these uniquely positioned residues Arg123 are poised for interaction with fatty acids produced by LCAT hydrolysis of the sn-2 chains of phosphatidylcholine, thus positioning the fatty acids for esterification to cholesterol. To test the importance of Arg123 for LCAT phospholipid hydrolysis and CE formation, we generated apoA-I[R123A] and apoA-I[R123E] mutants and made discoidal HDL with the mutants and WT apoA-I. Neither mutation of Arg123 changed the particle composition or size, or the protein conformation or stability. However, both mutations of Arg123 significantly reduced LCAT catalytic efficiency and the apparent V_max for CE formation without affecting LCAT phospholipid hydrolysis. A control mutation, apoA-I[R131A], did not affect LCAT phospholipid hydrolysis or CE formation. These data suggest that Arg123 of apoA-I on discoidal HDL participates in LCAT-mediated cholesterol esterification.

Lipoprotein electrostatic properties regulate hepatic lipase association and activity

The effect of lipoprotein electrostatic properties on the catalytic regulation of hepatic lipase (HL) was investigated. Enrichment of serum or very low density lipoprotein (VLDL) with oleic acid increased lipoprotein negative charge and stimulated lipid hydrolysis by HL. Similarly, enrichment of serum or isolated lipoproteins with the anionic phospholipids phosphatidylinositol (PI), phosphatidic acid, or phosphatidylserine also increased lipoprotein negative charge and stimulated hydrolysis by HL. Anionic lipids had a small effect on phospholipid hydrolysis, but significantly stimulated triacylglyceride (TG) hydrolysis. High density lipoprotein (HDL) charge appears to have a specific effect on lipolysis. Enrichment of HDL with PI significantly stimulated VLDL-TG hydrolysis by HL. To determine whether HDL charge affects the association of HL with HDL and VLDL, HL-lipoprotein interactions were probed immunochemically. Under normal circumstances, HL associates with HDL particles, and only small amounts bind to VLDL. PI enrichment of HDL blocked the binding of HL with HDL. These data indicate that increasing the negative charge of HDL stimulates VLDL-TG hydrolysis by reducing the association of HL with HDL. Therefore, HDL controls the hydrolysis of VLDL by affecting the interlipoprotein association of HL. Lipoprotein electrostatic properties regulate lipase association and are an important regulator of the binding and activity of lipolytic enzymes.

Modulation of lipoprotein metabolism by inhibition of sphingomyelin synthesis in ApoE knockout mice

Plasma sphingomyelin (SM) has been suggested as a risk factor for coronary heart disease independent of cholesterol levels. A decrease of SM in lipoproteins is known to improve the activities of lecithin:cholesterol acyltransferase (LCAT) and lipoprotein lipase (LPL) in vitro. Inhibition of SM biosynthesis may reduce lipoprotein SM content and thus improve cholesterol distribution in lipoproteins by enhancing reverse cholesterol transport and clearance of triglyceride-rich lipoproteins. To examine this hypothesis, ApoE KO mice were fed a western diet and treated for 4 weeks with various concentrations of myriocin, a specific inhibitor of serine palmitoyltransferase. Myriocin treatment lowered plasma cholesterol and TG levels in a dose-dependent manner. In addition, myriocin treatment reduced cholesterol contents in VLDL and LDL and elevated HDL-cholesterol. Observed lipid-lowering effects of myriocin were associated with suppression of HMG CoA reductase and fatty acid synthase via reduced levels of SREBP-1 RNA and protein. Induction of apoAI and lecithin:cholesterol acytransferase (LCAT) in the liver by myriocin was associated with an increased HDL. Lesion area and macrophage area were also diminished in the cuffed femoral artery of ApoE KO mice. In conclusion, inhibition of sphingolipid biosynthesis can be a novel therapeutic target for dyslipidemia and atherosclerosis.

Effect of 4-hydroxynonenal on phosphatidylethanolamine containing condensed monolayer and on its interaction with apolipoprotein A-I

4-Hydroxynonenal (4HNE), generated during polyunsaturated fatty acid oxidation, is present in atherosclerotic lesions. As 4HNE is able to react with phosphatidylethanolamine (PE), we investigated, using AC polarography, whether it may alter the physico-chemical state of a condensed PE-containing phospholipid monolayer and its interaction with apoA-I. The stability of a phospholipid monolayer relative to potential (around the potential of zero charge) is dependent on lipid composition (PE>PC>PE/PC). ApoA-I insertion into PE/PC monolayer is easier than in PC monolayer. Pre-treatment of PE/PC monolayer by 4HNE does not alter monolayer stability, but decreases apo A-I insertion into the monolayer.

Amino acids 149 and 294 of human lecithin:cholesterol acyltransferase affect fatty acyl specificity

We identified two regions of human LCAT (hLCAT) that when mutated separately to the corresponding rat sequence (E149A and Y292H/W294F) and transiently expressed in COS-1 cells increased phospholipase A2 (PLA2) activity by 5.5- and 2.8-fold, respectively, and increased cholesteryl ester (CE) formation by 2.9- and 1.4-fold, respectively, relative to hLCAT using substrate particles containing 1-16:0,2-20:4-sn-glycero-3-phosphocholine (PAPC). In contrast, both activities with 1-16:0,2-18:1-sn-glycero-3-phosphocholine (POPC) substrate were similar among the three LCAT proteins. The triple mutant (E149A/Y292H/W294F) had increased PLA2 activity with PAPC similar to that observed with the E149A mutation alone; however, unlike E149A, the triple mutant demonstrated a 50% decrease in activity with POPC for both PLA2 activity and CE formation, suggesting an interaction between the two regions of LCAT. Additional mutagenesis studies demonstrated that W294F, but not Y292H, increased PLA2 activity by 3-fold with PAPC without affecting activity with POPC. The E149A/W294F double mutation mimicked the LCAT activity phenotype of the triple mutant (more activity with PAPC, less with POPC). In conclusion, separate mutation of two amino acids in hLCAT to the corresponding rat sequence increases activity with PAPC, whereas the combined mutations increase PAPC and decrease POPC activity, suggesting that these amino acids participate in the LCAT PC binding site and affect fatty acyl specificity.

Evidence That Hepatic Lipase and Endothelial Lipase Have Different Substrate Specificities for High-Density Lipoprotein Phospholipids

Hepatic lipase (HL) and endothelial lipase (EL) are both members of the triglyceride lipase gene family. HL hydrolyzes phospholipids and triglycerides in triglyceride-rich lipoproteins and high-density lipoproteins (HDL). EL hydrolyzes HDL phospholipids and has low triglyceride lipase activity. The aim of this study was to determine if HL and EL hydrolyze different HDL phospholipids and whether HDL phospholipid composition regulates the interaction of EL and HL with the particle surface. Spherical, reconstituted HDL (rHDL) containing either 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), 1-palmitoyl-2-linoleoylphosphatidylcholine (PLPC), 1-palmitoyl-2-arachidonylphosphatidylcholine (PAPC), or 1-palmitoyl-2-docosahexanoylphosphatidylcholine (PDPC) as the only phospholipid, apolipoprotein A-I as the only apolipoprotein, and either cholesteryl esters (CE) only or mixtures of CE and triolein (TO) in their core were prepared. The rHDL were similar in size and had comparable core lipid/apoA-I molar ratios. The CE-containing rHDL were used to determine the kinetics of HL- and EL-mediated phospholipid hydrolysis. For HL the V(max) of phospholipid hydrolysis for (POPC)rHDL > (PLPC)rHDL approximately (PDPC)rHDL > (PAPC)rHDL, while the K(m)(app) for (POPC)rHDL > (PDPC)rHDL > (PLPC)rHDL > (PAPC)rHDL. For EL the V(max) for (PDPC)rHDL > (PAPC)rHDL > (PLPC)rHDL approximately (POPC)rHDL, while the K(m)(app) for (PAPC)rHDL approximately (PLPC)rHDL > (POPC)rHDL > (PDPC)rHDL. The kinetics of EL- and HL-mediated TO hydrolysis was determined using rHDL that contained TO in their core. For HL the V(max) of TO hydrolysis for (PLPC)rHDL > (POPC)rHDL > (PAPC)rHDL > (PDPC)rHDL, while the K(m)(app) for (PLPC)rHDL > (POPC)rHDL approximately (PAPC)rHDL > (PDPC)rHDL. For EL the V(max) and K(m)(app) for (PAPC)rHDL > (PDPC)rHDL > (PLPC)rHDL > (POPC)rHDL. These results establish that EL and HL have different substrate specificities for rHDL phospholipids and that their interactions with the rHDL surface are regulated by phospholipids.

Deletion of N-terminal amino acids from human lecithin:cholesterol acyltransferase differentially affects enzyme activity toward alpha- and beta-substrate lipoproteins

Lecithin:cholesterol acyltransferase (LCAT) is the enzyme responsible for generation of the majority of the cholesteryl esters (CE) in human plasma. Although most plasma cholesterol esterification occurs on high-density lipoprotein (HDL), via alpha-LCAT activity, esterification also occurs on low-density lipoprotein (LDL) via the beta-activity of the enzyme. Computer threading techniques have provided a three-dimensional model for use in the structure-function analysis of the core and catalytic site of the LCAT protein, but the model does not extend to the N-terminal region of the enzyme, which may mediate LCAT interaction with lipoprotein substrates. In the present study, we have examined the functional consequences of deletion of the highly conserved hydrophobic N-terminal amino acids (residues 1-5) of human LCAT. Western blot analysis showed that the mutant proteins (Delta 1-Delta 5) were synthesized and secreted from transfected COS-7 cells at levels approximately equivalent to those of wild-type hLCAT. The secreted proteins had apparent molecular weights of 67 kDa, indicating that they were correctly processed and glycosylated during cellular transit. However, deletion of the first residue of the mature LCAT protein (Delta 1 mutant) resulted in a dramatic loss of alpha-LCAT activity (5% of wild type using reconstituted HDL substrate, rHDL), although this mutant retained full beta-LCAT activity (108% of wild-type using human LDL substrate). Removal of residues 1 and 2 (Delta 2 mutant) abolished alpha-LCAT activity and reduced beta-LCAT activity to 12% of wild type. Nevertheless, LCAT Delta 1 and Delta 2 mutants retained their ability to bind to rHDL and LDL lipoprotein substrates. The dramatic loss of enzyme activity suggests that the N-terminal residues of LCAT may be involved in maintaining the conformation of the lid domain and influence activation by the alpha-LCAT cofactor apoA-I (in Delta 1) and/or loss of enzyme activity (in Delta 1-Delta 5). Since the Delta 1 and Delta 2 mutants retain their ability to bind substrate, other factor(s), such as decreased access to the substrate binding pocket, may be responsible for the loss of enzyme activity.

Interaction of lecithin:cholesterol acyltransferase (LCAT).alpha 2-macroglobulin complex with low density lipoprotein receptor-related protein (LRP). Evidence for an alpha 2-macroglobulin/LRP receptor-mediated system participating in LCAT clearance

The reaction of lecithin:cholesterol acyltransferase (LCAT) with high density lipoproteins (HDL) is of critical importance in reverse cholesterol transport, but the structural and functional pathways involved in the regulation of LCAT have not been established. We present evidence for the direct binding of LCAT to alpha(2)-macroglobulin (alpha(2)M) in human plasma to form a complex 18.5 nm in diameter. Forty percent of plasma LCAT-HDL was associated with alpha(2)M; moreover, most of the LCAT in cerebrospinal fluid and in the medium of cultured human hepatoma cell line was associated with alpha(2)M. Purified recombinant human LCAT (rLCAT) labeled with (125)I bound to native and methylamine-activated alpha(2)M (alpha(2)M-MA) in vitro in a time- and concentration-dependent manner, and this binding did not depend on the presence of lipid. rLCAT bound to alpha(2)M-MA with greater affinity than to alpha(2)M. Furthermore, rLCAT did not activate alpha(2)M as phosphatidylcholine-specific phospholipase C does. Reconstituted HDL particles (LpA-I) inhibited the binding of rLCAT to alpha(2)M more efficiently than native HDL(3) did. LCAT associated with alpha(2)M was enzymatically inactive under both endogenous and exogenous assay conditions. Purified rLCAT alone did not bind to low density lipoprotein receptor-related protein (LRP) as lipoprotein lipase (LPL) does; however, when rLCAT was combined with alpha(2)M-MA to form a complex, binding, internalization, and degradation of rLCAT took place in LRP-expressing cells (LRP (+/+)) but not in cells deficient in LRP (LRP (-/-)). It is concluded that the binding of LCAT to alpha(2)M inhibits its enzymatic activity. Furthermore, the finding supports the possibility that the LRP receptor can act in vivo to mediate clearance of the LCAT-alpha(2)M complex and may significantly influence the bioavailability of LCAT.

Apolipoprotein A-I regulates lipid hydrolysis by hepatic lipase

Association of hepatic lipase (HL) with pure heparan sulfate proteoglycans (HSPG) has little effect on hydrolysis of high density lipoprotein (HDL) particles, but significantly inhibits (>80%) the hydrolysis of low (LDL) and very low density lipoproteins (VLDL). Lipolytic inhibition is associated with a differential ability of the lipoproteins to remove HL from the HSPG. LDL and VLDL are unable to displace HL, whereas HDL readily displaces HL from the HSPG. These data show that HSPG-bound HL is inactive. Purified apolipoprotein (apo) A-I is more efficient than HDL at liberating HL from HSPG, and HL displacement is associated with the direct binding of apoA-I to HSPG. However, displacement of HL by apoA-I does not enhance hydrolysis of VLDL particles. This appears due to the direct inhibition of HL by apoA-I. Both apoA-I and HDL are able to inhibit VLDL lipid hydrolysis by up to 60%. Inhibition of VLDL hydrolysis is associated with the binding of apoA-I to the surface of the VLDL particle and a concomitant decreased affinity for HL. These data show that apoA-I can regulate lipid hydrolysis by HL by liberating/activating the enzyme from cell surface proteoglycans and by directly modulating lipoprotein binding and hydrolysis.

High density lipoprotein oxidation: in vitro susceptibility and potential in vivo consequences

Elevated levels of plasma high density lipoprotein (HDL) are strongly predictive of protection against atherosclerotic vascular disease. HDL particles likely have several beneficial actions in vivo, including the initiation of reverse cholesterol transport. The apparent importance of oxidative modification of low density lipoprotein in atherogenesis raises the question of how oxidative modification of HDL might affect its cardioprotective actions. HDL is readily oxidized using numerous models of lipoprotein oxidation. In vitro evidence suggests oxidation might impair some protective actions, but actually enhance other mechanisms induced by HDL that prevent the accumulation of cholesterol in the artery wall. This article reviews the current literature concerning the relative oxidizability of HDL, the structural changes induced in HDL by oxidation in vitro, and the potential consequences of oxidative modification on the protective actions of HDL in vivo.

Surface plasmon resonance biosensor studies of human wild-type and mutant lecithin cholesterol acyltransferase interactions with lipoproteins

Binding of lecithin cholesterol acyltransferase (LCAT) to lipoprotein surfaces is a key step in the reverse cholesterol transport process, as the subsequent cholesterol esterification reaction drives the removal of cholesterol from tissues into plasma. In this study, the surface plasmon resonance method was used to investigate the binding kinetics and affinity of LCAT for lipoproteins. Reconstituted high-density lipoproteins (rHDL) containing apolipoprotein A-I or A-II, (apoA-I or apoA-II), low-density lipoproteins (LDL), and small unilamellar phosphatidylcholine vesicles, with biotin tags, were immobilized on biosensor chips containing streptavidin, and the binding kinetics of pure recombinant LCAT were examined as a function of LCAT concentration. In addition, three mutants of LCAT (T123I, N228K, and (Delta53-71) were examined in their interactions with LDL. For the wild-type LCAT, binding to all lipid surfaces had the same association rate constant, k(a), but different dissociation rate constants, k(d), that depended on the presence of apoA-I (k(d) decreased) and different lipids in LDL. Furthermore, increased ionic strength of the buffer decreased k(a) for the binding of LCAT to apoA-I rHDL. For the LCAT mutants, the Delta53-71 (lid-deletion mutant) exhibited no binding to LDL, while the LCAT-deficiency mutants (T123I and N228K) had nearly normal binding to LDL. In conclusion, the association of LCAT to lipoprotein surfaces is essentially independent of their composition but has a small electrostatic contribution, while dissociation of LCAT from lipoproteins is decreased due to the presence of apoA-I, suggesting protein-protein interactions. Also, the region of LCAT between residues 53 and 71 is essential for interfacial binding.

Binding affinity and reactivity of lecithin cholesterol acyltransferase with native lipoproteins

The first step in the reaction of lecithin cholesterol acyltransferase (LCAT) with lipoproteins is the interfacial binding of the enzyme to the lipid surfaces. In this study the equilibrium dissociation constants (Kds) for the interaction of pure human plasma LCAT with LDL, HDL2, HDL3, and a reconstituted discoidal HDL (rHDL) were determined by the activity-inhibition method. In addition, enzyme kinetics were measured with each of the lipoprotein substrates. Based on phospholipid concentrations, the Kd values (0.9 x 10(-5) to 4.6 x 10(-5) M) increased in the order rHDL = HDL3 </= HDL2 < LDL while the relative reactivities (app Vmax/app Km) with LCAT were 100, 16, 1, 6%, respectively, for the different lipoproteins. These quantitative measures were used to predict the distribution of LCAT in plasma and to explain cholesterol esterification when HDL are absent or ineffective as substrates.

Inhibition of lecithin cholesterol acyltransferase by phosphatidylcholine hydroperoxides

To gain insight into the nature of the lecithin-cholesterol acyltransferase inhibitory factor(s), we separated and collected the oxidation products from oxidized lipoproteins after lipoxygenase treatment. Isolated fractions identified by chemiluminescence, as hydroperoxides of phosphatidylcholine, were found to produce a significant reduction of lecithin-cholesterol acyltransferase activity. The reaction kinetics of lecithin-cholesterol acyltransferase with reconstitued high density lipoproteins were studied in the presence of 0.6 and 1.2 microM hydroperoxides of phosphatidylcholine. No significant changes in the apparent Vmax were observed but a concentration-dependent increase in slope of the reciprocal plots and in the apparent Km values was observed with increasing hydroperoxide concentrations. These results show that the active site of lecithin-cholesterol acyltransferase is not affected by the presence of phosphatidylcholine hydroperoxides. Nevertheless, hydroperoxides of phosphatidylcholine altered the reactivity of lecithin-cholesterol acyltransferase for reconstitued high density lipoproteins suggesting either an alteration of the binding of lecithin-cholesterol acyltransferase to the reconstitued high density lipoproteins or a competitive inhibition mechanism.

Effect of apolipoprotein A-I lipidation on the formation and function of pre-beta and alpha-migrating LpA-I particles

A unique class of lipid-poor high-density lipoprotein, pre-beta1 HDL, has been identified and shown to have distinct functional characteristics associated with intravascular cholesterol transport. In this study we have characterized the structure/function properties of poorly lipidated HDL particles and the factors that mediate their conversion into multimolecular lipoprotein particles. Studies were undertaken with homogeneous recombinant HDL particles (LpA-I) containing apolipoprotein (apo) A-I and various amounts of palmitoyloleoylphosphatidylcholine (PC) and cholesterol. Complexation of apoA-I with small amounts of PC and cholesterol results in the formation of discrete lipoprotein structures that have a hydrated diameter of about 6 nm but contain only one molecule of apoA-I (Lp1A-I). While the molecular charge and alpha-helix content of apoA-I are unaffected by lipidation, the thermodynamic stability of the protein is reduced significantly (from 2.4 to 0.9 kcal/mol of apoA-I). Evaluation of apoA-I conformation by competitive radioimmunoassay with monoclonal antibodies shows that addition of small amounts of PC and cholesterol to apoA-I significantly increases the immunoreactivity of a number of domains over the entire molecule. Increasing the ratio of PC:apoA-I to 10:1 in the Lp1A-I complex is associated with increases in the alpha-helix content and stability of apoA-I. However, incorporation of 10-15 mol of PC destabilizes the Lp1A-I complex and promotes the formation of more thermodynamically stable (1.8 kcal/mol of apoA-I) bimolecular structures (Lp2A-I) that are approximately 8 nm in diameter. The formation of an Lp2A-I particle is associated with an increased immunoreactivity of most of the epitopes studied, with the exception of one central domain (residues 98-121), which becomes significantly less exposed. This structural change parallels a significant increase in the net negative charge on the complex. Characterization of the ability of these lipoproteins to act as substrates for lecithin:cholesterol acyltransferase (LCAT) shows that unstable Lp1A-I complexes stimulate a higher rate of cholesterol esterification by LCAT than the small but more stable Lp2A-I particles (Vmax values are 5.8 and 0.3 nmol of free cholesterol esterified/h, respectively). The ability of LCAT to interact with lipid-poor apoA-I suggests that LCAT does not need to bind to the lipid interface on an HDL particle but that LCAT may directly interact with apoA-I. The data suggests that lipid-poor HDL particles may be metabolically reactive particles because they are thermodynamically unstable.

Structural and functional properties of two mutants of lecithin-cholesterol acyltransferase (T123I and N228K)

Two naturally occurring mutants of human lecithin-cholesterol acyltransferase (LCAT), T123I and N228K, were expressed in COS-1 and Chinese hamster ovary cells, overproduced, and purified to homogeneity in order to study the structural and functional defects that lead to the LCAT deficiency phenotypes of these mutations. The mutants were expressed and secreted by transfected cells normally and had molecular weights and levels of glycosylation similar to wild type LCAT. The purified proteins (>98% purity) had almost indistinguishable structures and stabilities as determined by CD and fluorescence spectroscopy. Enzymatic activities and kinetic analysis of the pure enzyme forms showed that wild type LCAT and both mutants were reactive with the water-soluble substrate, p-nitrophenyl butyrate, indicating the presence of an intact core active site and catalytic triad. Both the T123I and N228K mutants had markedly depressed reactivity with reconstituted HDL (rHDL), but T123I retained activity with low density lipoprotein. To determine whether defective binding to rHDL was responsible for the low activity of both mutants with rHDL, the equilibrium binding constants were measured directly with isothermal titration calorimetry and surface plasmon resonance (SPR) methods. The results indicated that the affinities of the mutants for rHDL were only about 2-fold lower than the affinity of wild type LCAT (Kd = 2.3 x 10(-7) M). Together, the activity and equilibrium binding results suggest that the T123I mutant is defective in activation by apolipoprotein A-I, and the N228K mutant has impaired binding of lipid substrate to the active site. In addition, the kinetic binding rate constants determined by the SPR method indicate that normal LCAT dissociates from rHDL, on average, after one catalytic cycle.

Importance of central alpha-helices of human apolipoprotein A-I in the maturation of high-density lipoproteins

We have studied the role of amphipathic alpha-helices in the ability of apoA-I to promote cholesterol efflux from human skin fibroblasts and activate lecithin:cholesterol acyltransferase (LCAT). Three apoA-I mutants were designed, each by deletion of a pair of predicted adjacent central alpha-helices [Delta(100-143), Delta(122-165), Delta(144-186)], and expressed in Escherichia coli. This strategy was used to minimize disruption of the predicted secondary structure of the resulting protein. These three central deletion mutants have been previously shown to be expressed as stable folded proteins but to exhibit altered phospholipid-binding properties. When recombined with phospholipids to form homogeneous LpA-I containing equivalent amounts of POPC and tested for their ability to promote diffusional cholesterol efflux from normal [3H]cholesterol-labeled fibroblasts, each mutant and the wild-type recombinant protein (Rec.-apoA-I) promoted cholesterol efflux with very similar rates at all the concentrations tested. These experiments showed that all LpA-I could acquire cellular cholesterol with similar affinity and binding capacity. However, when the cell-incubated LpA-I were incubated with purified LCAT, two mutants, Delta(122-165) and Delta(144-186), appeared incapable of activating the enzyme. To directly determine their ability to activate LCAT, each mutant and the control were recombined with equivalent amounts of cholesterol and phospholipid and incubated with the purified enzyme. The results show that whereas deletion of residues 100-143 has little effect on LCAT activation, deletion of residues 122-165 or 144-186 results in an inability of the mutants to promote cholesterol esterification. In conclusion, our results show that no specific sequence in the central domain of apoA-I is required for efficient diffusional cholesterol efflux from normal fibroblasts; however, residues 144-186 appear critical for optimum LCAT activation and cholesteryl ester accumulation. Since deletion of residues 144-186 also perturbs phospholipid association and prevents the formation of large LpA-I particles [Frank, P. G., Bergeron, J., Emmanuel, F., Lavigne, J. P., Sparks, D. L., Denèfle, P., Rassart, E., and Marcel, Y. L. (1997) Biochemistry 36, 1798-1806], the data show that this pair of alpha-helices plays an important role in the maturation of HDL. Sequence analysis of these apoA-I helices further identifies specific residues that appear essential to this activity.

Evidence against alterations in Lecithin:cholesterol acyltransferase (LCAT) activity in familial combined hyperlipidemia

Elevated concentrations of plasma cholesterol and triglycerides are characteristic of familial combined hyperlipidemia (FCHL) which may also present with reduced high density lipoprotein (HDL) cholesterol concentrations. Lecithin:cholesterol acyltransferase (LCAT) plays a key role in reverse cholesterol transport by converting unesterified cholesterol to cholesterol ester in the process of maturation of HDL in the presence of its activator, apolipoprotein (apo) A-I. We hypothesised that alterations in LCAT activity or plasma concentrations or gene sequence of apo A-I could influence HDL metabolism in these patients. We studied cholesterol concentrations of high density lipoprotein subfractions and LCAT activity in 25 FCHL subjects and 48 controls. Total HDL (p=0.018) and HDL2 (p=0.008) were significantly decreased in the FCHL group compared with controls. After analyses with adjusted data only HDL2 remained significantly decreased in the FCHL group (p=0.050). The LDLc/HDLc and A-I/HDLc ratios were significantly elevated in the FCHL group (p <0.0001), the latter suggesting the existence of compositional differences in the HDL particles of the FCHL individuals. LCAT activity assessed in the FCHL (19.94+/-3.95 nmol/ml per h) and control (20.13+/-6.86 nmol/ml per h) groups showed no statistically significant differences. A significant positive correlation of LCAT activity with total HDL (r=0.42), HDL3 cholesterol (r=0.46) and apolipoprotein A-I (r=0.47) was observed in affected subjects but not in controls. An association between a Ga(-75)-A variation in the promoter region of the apo A-I gene and elevated concentrations of apo A-I (p=0.009) and apo C-III (p=0.041) was observed. This association was strongly influenced by the status of the subject providing further evidence for a regulatory role of this genetic region in the expression of FCHL. Our data suggests that LCAT activity is normal in FCHL and, therefore, does not account for the abnormalities observed in these patients essentially with regard to the HDL2 subfraction.

Effect of the surface lipid composition of reconstituted LPA-I on apolipoprotein A-I structure and lecithin: cholesterol acyltransferase activity

Characterization of the factors that regulate plasma cholesterol esterification shows that the increased activity of lecithin:cholesterol acyltransferase (LCAT) in the plasma of hyperlipidemic subjects is due to enhanced interactions with a preferred substrate. The details of how the physical properties of high density lipoproteins (HDL) may affect their ability to stimulate cholesterol esterification by LCAT have been investigated in homogeneous reconstituted HDL particles containing two molecules of apolipoprotein (apo) A-I (Lp2A-I) and palmitoyl-oleoyl phosphatidylcholine (POPC). Increasing the POPC or sphingomyelin (SPH) content in an Lp2A-I complex increases particle size and stability but decreases the negative surface charge of apoA-I. Increasing Lp2A-I POPC or SPH content also significantly inhibits cholesterol esterification by LCAT. Increase in the maximum rate of CE production (Vmax) by LCAT is directly related to an increased negative charge on the different Lp2A-I particles and to a reduced amount and stability of amphipathic alpha-helices in apoA-I. In contrast, increasing the Lp2A-I complex negative charge directly by addition of a charged lipid, phosphatidylinositol (PI), has minimal effect on apoA-I conformation and LCAT activation. While variations in Lp2A-I PI content have little effect on the interfacial binding of LCAT, increasing POPC content appears to directly increase the binding affinity of LCAT for the different Lp2A-I particles. These results show that LCAT is stimulated by an apoA-I conformation-dependent increase in negative charge but is less sensitive to electrostatic changes in the lipid interface of discoidal Lp2A-I. The activation of LCAT appears to be dependent on the exposure of both central (residues 98-132) and N-terminal (residues 2-8) domains in apoA-I. A strong relationship between the immunoreactivity of two specific mAbs, 4H1 and A11, and LCAT reactivity suggests that the N-terminus of apoA-I may interact with a central domain in a manner that may regulate the accessibility of LCAT to the edge of the disc. This indicates that the conformation and charge of apoA-I are sensitive to the surface-lipid composition of HDL particles and play a central role in regulating LCAT activation. Since alterations in the surface lipid composition of HDL particles from hyperlipidemic subjects also modify the charge and structure of these particles, this may stimulate the rates of cholesterol esterification by making these lipoproteins preferred LCAT substrates.

The effect of apolipoprotein A-II on the structure and function of apolipoprotein A-I in a homogeneous reconstituted high density lipoprotein particle

In this study we examined the effects of apoA-II on the structure and function of apoA-I in homogeneous reconstituted HDL (rHDL). First, we measured the binding of apoA-II to apoA-I-rHDL, containing dipalmitoylphosphatidylcholine or palmitoyloleoylphosphatidylcholine, and the degree of apoA-I displacement at various ratios of apolipoproteins. Using fluorescence methods, we determined that apoA-II binding is rapid, irreversible, and associated with apoA-I displacement only when the molar ratio of apoA-II/apoA-I is greater than 1:2. Next, we used the stable apoA-II/apoA-I-rHDL complex at the apoA-II/apoA-I ratio of 1:2 to examine its physical properties, apoA-I structure, and reactivity with lecithin:cholesterol acyltransferase (LCAT). Using chemical cross-linking in conjunction with fluorescence and electrophoretic methods, we demonstrated that the conformation of apoA-I must be flexible to allow apoA-II binding to the apoA-I-rHDL particles and showed that the hybrid particles have an unchanged Stokes diameter. Fluorescence and circular dichroism measurements revealed little or no change in the secondary structure or in the N-terminal domain of apoA-I, but showed a marked destabilization of apoA-I to denaturation by guanidine hydrochloride. Limited tryptic digestion indicated that the central region of apoA-I becomes accessible to proteolysis in the hybrid particles. Together, these results suggest that amphipathic alpha-helices of apoA-II replace four central helices of one apoA-I molecule (residues approximately 99-187) in the complex and in the process destabilize apoA-I. Thus, apoA-II binding at physiologic ratios may not completely displace apoA-I from HDL, but may provide a reservoir of easily exchangeable apoA-I. Finally, we showed that the reaction of the hybrid HDL with LCAT was inhibited 2-5-fold, relative to apoA-I-rHDL, due to a corresponding increase in the apparent Km value. This suggests that LCAT binding to the hybrid particles is sterically hindered by the excess protein (portions of apoA-I and apoA-II not bound to lipid). Therefore, apoA-II can modulate the reaction of HDL with LCAT by decreasing LCAT binding to hybrid particles and making the enzyme available for reaction with other substrates.

The carboxyl-terminal hydrophobic residues of apolipoprotein A-I affect its rate of phospholipid binding and its association with high density lipoprotein

We performed a series of mutations in the human apolipoprotein A-I (apoA-I) gene designed to alter specific amino acid residues and domains implicated in lecithin:cholesterol acyltransferase (LCAT) activation or lipid binding. We used the mutant apoA-I forms to establish nine stable cell lines, and developed strategies for the large scale production and purification of the mutated apoA-I proteins from conditioned media. HDL and dimyristoyl phosphatidylcholine binding assays using the variant apoA-I forms have shown that replacement of specific carboxyl-terminal hydrophobic residues Leu222, Phe225, and Phe229 with lysines, as well as replacement of Leu211, Leu214, Leu218, and Leu219 with valines, diminished the ability of apoA-I to bind to HDL and to lyse dimyristoyl phosphatidylcholine liposomes. The findings indicate that Leu222, and Phe225, Phe229 located in the putative random coil region, and Leu211, Leu214, Leu218, and Leu219 located in the putative helix 8, are important for lipid binding. In contrast, substitutions of alanines for specific charged residues in putative helices 7, 8, or 9 as well as various point mutations in other regions of apoA-I, did not affect the ability of the variant apoA-I forms to bind to HDL or to lyse dimyristoyl phosphatidylcholine liposomes. Cross-linking experiments confirmed that the carboxyl-terminal domain of apoA-I participates in the self-association of the protein, as demonstrated by the inability of the carboxyl-terminal deletion mutants delta185-243 and delta209-243 to form higher order aggregates in solution. Lecithin:cholesterol acyltransferase analysis, using reconstituted HDL particles prepared by the sodium cholate dialysis method, has shown that mutants (Pro165-->Ala,Gln172-->Glu) (Leu211-->Val,Leu214-->Val, Leu218-->Val,Leu219-->Val), Leu222-->Lys,Phe225-->Lys, Phe229-->Lys) and delta209-243 reduced LCAT activation (38-68%). Mutant (Glu191-->Ala,His193-->Ala,Lys195-->Ala) enhanced LCAT activation (131%), and mutant (Ala152-->Leu, Leu159-->Trp) exhibited normal LCAT activation as compared with the wild type proapoA-I and plasma apoA-I forms [corrected]. The apparent catalytic efficiency (Vmax(app)/Km(app)) of the apoA-I mutants ranged from 17.8 to 107.2% of the control and was the result of variations in both the Km and the Vmax in the different mutants. These findings indicate that putative helices 6 and 7, and the carboxyl-terminal helices 8 and 9 contribute to the optimum activation of lecithin:cholesterol acyltransferase. In addition to their use in the present study, the variant apoA-I forms generated will serve as valuable reagents for the identification of the domains and residues of apoA-I involved in binding the scavenger receptor BI, and facilitating cholesterol efflux from cells as well as aid in the structural analysis of apoA-I.

Role of the Arg123-Tyr166 paired helix of apolipoprotein A-I in lecithin:cholesterol acyltransferase activation

The Arg123-Tyr166 central and Ala190-Gln243 carboxyl-terminal pairs of helices of apoA-I were substituted with the pair of helices of apoA-II, resulting in the apoA-I(Delta(Arg123-Tyr166), nablaA-II(Ser12-Ala75)) and apoA-I(Delta(Ala190-Gln243), nablaA-II(Ser12-Gln77)) chimeras, respectively. The structures of these chimeras in aqueous solution and in reconstituted high density lipoproteins (rHDL) and the lecithin:cholesterol acyltransferase (LCAT) activation properties of the rHDL were studied. Recombinant human apoA-I and the chimeras were expressed in Escherichia coli and purified from the periplasmic space. Binding of the apolipoproteins with palmitoyloleoylphosphatidylcholine was associated with a similar shift of Trp fluorescence maxima from 337 to 332 nm, from 339 to 334 nm, and from 337 to 333 nm, respectively. All rHDL had a Stokes radius of 4.8 nm and contained 2 apolipoprotein molecules/particle. Circular dichroism measurements revealed eight alpha-helices per apoA-I and per chimera molecule. The catalytic efficiencies of LCAT activation were 1.5 +/- 0.33 (mean +/- S.D.; n = 3), 0.054 +/- 0.009 (p < 0.001 versus apoA-I), and 1.3 +/- 0.32 (p = not significant versus apoA-I) nmol of cholesteryl ester/h/microM, respectively. The lower LCAT activity of the central domain chimera was due to a 27-fold reduced Vmax with unaltered Km. Binding of radiolabeled LCAT to rHDL of apoA-I and apoA-I(Delta(Arg123-Tyr166), nablaA-II(Ser12-Ala75)) was very similar. In conclusion, although substitution of the Arg123-Tyr166 central or Ala190-Gln243 carboxyl-terminal pair of helices of apoA-I with the pair of helices of apoA-II yields chimeras with structure similar to that of native apoA-I, exchange of the central domain (but not the carboxyl-terminal domain) of apoA-I reduces the rate of LCAT activity that is independent of binding to rHDL.

Structural organization of lipid phase and protein-lipid interface in apolipoprotein-phospholipid recombinants: influence of cholesterol

The complexes of individual human plasma apolipoproteins (apo) A-I, E and A-II with dipalmitoylphosphatidylcholine (DPPC) in the absence or in the presence of cholesterol (Chol) were prepared with initial DPPC/Chol/protein weight ratio as 3:0.15:1. ApoA-I/DPPC/Chol complexes with different protein content (initial DPPC/apoA-I weight ratios were changed from 10.5:1 to 2.6:1) but with a fixed initial DPPC/Chol weight ratio of 20:1 were also prepared. The complexes were isolated by gel-filtration and characterized by size and composition. ApoA-I- and apoA-II-complexes had the same size (80-84 A) and the complexes became more heterogeneous upon Chol inclusion; apoE-complexes were larger (97-100 A) and more homogeneous and Chol addition had no effect on their hydrodynamic properties. Chol seems to be excluded partially in the following manner for isolated complexes with different apo's: A-II > E > A-I. The possible existence of two lipid regions in the complexes differing in lipid dynamics - the lipid shell in the vicinity of apolipoprotein (boundary lipid) opposite to the remaining part of the lipid bilayer - has been studied by absorbance and fluorescence spectroscopy with cis-parinaric acid (cis-PA) and trans-parinaric acid (trans-PA) embedded into the complexes. Their application is based on a strong preference of trans-PA for solid lipid while cis-PA distributes more equally between co-existing fluid and solid lipid regions (Sklar et al. (1979) Biochemistry 18, 1707-1716). (1) For apoA-I-complexes, the partition of cis-PA between water and lipid phase at temperatures below and above the transition temperature of DPPC (T(t)) was insensitive to Chol and temperature, while partition of trans-PA into the lipid phase of Chol-containing complex was increased at high temperature and decreased at low temperature. These results seem to be related to trans-PA redistribution between Chol-rich and protein-rich lipid domains, the latter being more disordered at T < T(t) and more immobilized at T > T(t) compared to the bulk bilayer; cis-PA localizes preferentially in boundary lipid. This hypothesis was directly confirmed by measurements of energy transfer between apoA-I tryptophanyls and probe molecules. (2) The relative response of trans-PA fluorescence intensity to temperature-induced phase transition of DPPC in apoA-I/DPPC/Chol complexes was decreased as a function of apolipoprotein content in a non-monotonic fashion with a transition midpoint at a mol ratio DPPC/A-I of 250:1, probably indicating two different modes of apolipoprotein/DPPC interaction in different sized complexes. (3) The comparative study of lipid dynamics in apoA-I-, apoE- and apoA-II-containing complexes with temperature response to phospholipid phase transition with fluorescence parameters such as intensity and anisotropy of cis-PA and trans-PA revealed the presence of boundary lipid in all three complexes without Chol. In contrast to apoA-I-containing complexes, in apoA-II/DPPC/Chol complexes, trans-PA seems to move preferentially into boundary lipid and cis-PA to distribute between two different regions probably as a result of more ordering action induced by apoA-II compared to apoA-I on the nearest phospholipid molecules in Chol-containing complexes; the apoE action on trans-PA and cis-PA distribution could be intermediate. Based on these results, the degree of Chol exclusion from the boundary lipid region for complexes with different apo's increasing in the order A-II > E > A-I can be suggested. Different Chol distributions between two lipid regions in the complexes seems not to be a function of complex size, but rather is an inherent property of the particular apolipoprotein molecule.

Activation of lecithin cholesterol acyltransferase by a disulfide-linked apolipoprotein A-I dimer

Apolipoprotein A-IMilano is a molecular variant of apoA-I, containing the Arg173-->Cys substitution that forms a disulfide linked homodimer (A-IM/A-IM). To assess the effect of this structural modification on a major function of the apolipoprotein, its activation of lecithin cholesterol acyltransferase (LCAT), we prepared well-defined complexes of A-IM/A-IM and apoA-I with phospholipids and cholesterol and compared their reactivities with LCAT. Particles with A-IM/A-IM had very similar diameters to apoA-I particles (7.8 and 12.5 nm) but had distinct apolipoprotein and phospholipid contents and protein secondary structures; they bound LCAT with comparable affinities, but were less efficient substrates for the enzyme (40 to 70% less reactive). We conclude that the local structural constraints in A-IM/A-IM do not prevent the formation of well-defined complexes with phospholipids and do not influence the binding of the enzyme to the particles, but have an inhibitory effect on LCAT activation.

Alteration in apolipoprotein A-I 22-mer repeat order results in a decrease in lecithin:cholesterol acyltransferase reactivity

Apolipoprotein A-I contains eight 22-amino acid and two 11-amino acid tandem repeats that comprise 80% of the mature protein. These repeating units are believed to be the basic motif responsible for lipid binding and lecithin:cholesterol acyltransferase (LCAT) activation. Computer analysis indicates that despite a fairly high degree of compositional similarity among the tandem repeats, significant differences in hydrophobic and amphipathic character exist. Our previous studies demonstrated that deletion of repeat 6 (143-164) or repeat 7 (165-186) resulted in a 98-99% reduction of LCAT activation as compared with wild-type apoA-I. To determine the effects of substituting one of these repeats with a more hydrophobic repeat we constructed a mutant apoA-I protein in which residues 143-164 (repeat 6) were replaced with repeat 10 (residues 220-241). The cloned mutant protein, 10F6 apoA-I, was expressed and purified from an Sf-9 cell baculoviral system and then analyzed using a number of biophysical and biochemical techniques. Recombinant complexes prepared at a 100:5:1 molar ratio of L-alpha-dimyristoylphosphatidylcholine:cholesterol:wild-type or 10F6 apoA-I showed a doublet corresponding to Stokes diameters of 114 and 108 A on nondenaturing 4-30% polyacrylamide gel electrophoresis. L-alpha-Dimyristoylphosphatidylcholine 10F6 apoA-I complexes had a 5-6-fold lower apparent Vmax/apparent Km as compared with wild-type apoA-I containing particles. As expected, monoclonal antibody epitope mapping of the lipid-free and lipid-bound 10F6 apoA-I confirmed that a domain expressed between residues 143 and 165 normally found in wild-type apoA-I was absent. The region between residues 119 and 144 in 10F6 apoA-I showed a marked reduction in monoclonal antibody binding capacity. Therefore, we speculate that the 5-6-fold lower LCAT reactivity in 10F6 compared with wild-type apoA-I recombinant particles results from increased stabilization within the 121-165 amino acid domain due to more stable apoprotein helix phospholipid interactions as well as from conformational alterations among adjacent amphipathic helix repeats.

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.

Sphingomyelin inhibits the lecithin-cholesterol acyltransferase reaction with reconstituted high density lipoproteins by decreasing enzyme binding

Lecithin-cholesterol acyltransferase (LCAT) catalyzes the formation of cholesterol esters on high density lipoproteins (HDL) and plays a critical role in reverse cholesterol transport. Sphingomyelin, an important constituent of HDL, may regulate the activity of LCAT at any of the key steps of the enzymatic reaction: binding of LCAT to the interface, activation by apo A-I, or inhibition at the catalytic site. In order to clarify the role of sphingomyelin in the regulation of the LCAT reaction and its effects on the structure of apolipoprotein A-I, we prepared reconstituted HDL (rHDL) containing egg phosphatidylcholine, cholesterol, apolipoprotein A-I, and up to 22 mol % sphingomyelin. Because the interfacial properties of substrate particles can dramatically affect LCAT binding and kinetics, we also prepared and analyzed proteoliposome substrates having the same components as the rHDL, except for a 4-fold higher ratio of phospholipid to apolipoprotein A-I. The reaction kinetics of LCAT with the rHDL particles revealed no significant change in the apparent Vmax but showed a concentration-dependent increase in slope of the reciprocal plots and in the apparent Km values with sphingomyelin content. The dissociation constant (Kd) for LCAT with these particles increased linearly with sphingomyelin content up to 22 mol %, changing in parallel with the apparent Km values. No structural changes of apolipoprotein A-I were detected in the particles with increasing content of sphingomyelin, but fluorescence results with lipophilic probes revealed that significant changes in the acyl chain, backbone, and head group regions of the lipid bilayer of the particles are introduced by the addition of sphingomyelin. On the other hand, the proteoliposome substrates also had increased Kdvalues for LCAT at high sphingomyelin contents but compared with the rHDL particles had a 6-10-fold lower affinity for LCAT binding and exhibited kinetics consistent with competitive inhibition by sphingomyelin at the active site. These results show conclusively that the dominant mechanism for the inhibition of LCAT activity with rHDL particles by sphingomyelin is the impaired binding of the enzyme to the interface. The results also underscore the significant differences in the enzyme reaction kinetics with different substrate particles.

Functional lecithin:cholesterol acyltransferase deficiency and high density lipoprotein deficiency in transgenic mice overexpressing human apolipoprotein A-II

The concentration of high density lipoproteins (HDL) is inversely related to the risk of atherosclerosis. The two major protein components of HDL are apolipoprotein (apo) A-I and apoA-II. To study the role of apoA-II in lipoprotein metabolism and atherosclerosis, we have developed three lines of C57BL/6 transgenic mice expressing human apoA-II (lines 25.3, 21.5, and 11.1). Northern blot experiments showed that human apoA-II mRNA was present only in the liver of transgenic mice. SDS-polyacrylamide gel electrophoresis and Western blot analysis demonstrated a 17.4-kDa human apoA-II in the HDL fraction of the plasma of transgenic mice. After 3 months on a regular chow, the plasma concentrations of human apoA-II were 21 +/- 4 mg/dl in the 25.3 line, 51 +/- 6 mg/dl in the 21.5 line, and 74 +/- 4 mg/dl in the 11.1 line. The concentration of cholesterol in plasma was significantly lower in transgenic mice than in control mice because of a decrease in HDL cholesterol that was greatest in the line that expressed the most apoA-II (23 mg/dl in the 11.1 line versus 63 mg/dl in control mice). There was also a reduction in the plasma concentration of mouse apoA-I (32 +/- 2, 56 +/- 9, 91 +/- 7, and 111 +/- 2 mg/dl for lines 11.1, 21.5, 25.3, and control mice, respectively) that was inversely correlated with the amount of human apoA-II expressed. Additional changes in plasma lipid/lipoprotein profile noted in line 11.1 that expressed the highest level of human apoA-II include elevated triglyceride, increased proportion of total plasma, and HDL free cholesterol and a marked (>10-fold) reduction in mouse apoA-II. Total endogenous plasma lecithin:cholesterol acyltransferase (LCAT) activity was reduced to a level directly correlated with the degree of increased plasma human apoA-II in the transgenic lines. LCAT activity toward exogenous substrate was, however, only slightly decreased. The biochemical changes in the 11.1 line, which is markedly deficient in plasma apoA-I, an activator for LCAT, are reminiscent of those in patients with partial LCAT deficiency. Feeding the transgenic mice a high fat, high cholesterol diet maintained the mouse apoA-I concentration at a normal level (69 +/- 14 mg/dl in line 11.1 compared with 71 +/- 6 mg/dl in nontransgenic controls) and prevented the appearance of HDL deficiency. All this happened in the presence of a persistently high plasma human apoA-II (96 +/- 14 mg/dl). Paradoxical HDL elevation by high fat diets has been observed in humans and is reproduced in human apoA-II overexpressing transgenic mice but not in control mice. Finally, HDL size and morphology varied substantially in the three transgenic lines, indicating the importance of apoA-II concentration in the modulation of HDL formation. The LCAT and HDL deficiencies observed in this study indicate that apoA-II plays a dynamic role in the regulation of plasma HDL metabolism.

Lecithin:cholesterol acyltransferase overexpression generates hyperalpha-lipoproteinemia and a nonatherogenic lipoprotein pattern in transgenic rabbits

Cholesterol esterification within plasma lipoprotein particles is catalyzed by lecithin:cholesterol acyltransferase (LCAT). The impact of the overexpression of this enzyme on plasma concentrations of the different plasma lipoproteins in an animal model expressing cholesteryl ester transfer protein was evaluated by generating rabbits expressing human LCAT. A 6.2-kilobase human genomic DNA construct was injected into the pronuclei of rabbit embryos. Of the 1002 embryos that were injected, 3 founder rabbits were characterized that expressed the human LCAT gene. As in mice and humans, the principal sites of mRNA expression in these rabbits is in the liver and brain, indicating that the regulatory elements required for tissue-specific expression among these species are similar. The alpha-LCAT activity correlated with the number of copies of LCAT that integrated into the rabbit DNA. Compared with controls, the high expressor LCAT-transgenic rabbits total and high density lipoprotein (HDL) cholesterol concentrations were increased 1.5-2.5-fold with a 3.1-fold increase in the plasma cholesterol esterification rate. Analysis of the plasma lipoproteins by fast protein liquid chromatography indicates that these changes reflected an increased concentration of apolipoprotein E-enriched, HDL1-sized particles, whereas atherogenic apolipoprotein B particles disappeared from the plasma. The concentrations of plasma HDL cholesterol were highly correlated with both human LCAT mass (r = 0.93; p = 0.001) and the log LCAT activity (r = 0.94; p < 0.001) in the transgenic rabbits. These results indicate that overexpression of LCAT in the presence of cholesteryl ester transfer protein leads to both hyperalpha-lipoproteinemia and reduced concentrations of atherogenic lipoproteins.

Effects of the neutral lipid content of high density lipoprotein on apolipoprotein A-I structure and particle stability

Alterations in high density lipoprotein (HDL) composition that occur in dyslipidemic states may modulate a number of events involved in cholesterol homeostasis. To elucidate the details of how HDL-core composition can affect the molecular structure of different kinds of HDL particles, the conformation and stability of apoA-I have been investigated in homogeneous recombinant HDL particles (LpA-I) containing palmitoyloleoyl phosphatidylcholine (POPC), triolein (TG), and/or cholesteryl linoleate (CE). In a discoidal particle containing two molecules of apoA-I and 85 molecules of POPC, apoA-I exhibits an alpha-helix content of 70% and a free energy of stability of its alpha-helical segments (delta G0D) of 2.2 kcal/mol. Inclusion of eight molecules of TG into the complex significantly reduces the alpha-helix content and stability of apoA-I, whereas inclusion of four molecules of CE into the complex has an opposite effect in that the alpha-helix content is significantly reduced and the stability of the remaining alpha-helical structure of apoA-I is increased. Neutral lipids have a different effect on apoA-I conformation in spherical LpA-I particles. In a sonicated-spherical LpA-I particle containing two molecules of apoA-I and 70 molecules of POPC, apoA-I exhibits an alpha-helix content of about 60% and a delta G0D of 1.2 kcal/mol apoA-I. Inclusion of either 10 molecules of TG or six molecules of CE into such a particle increases both the alpha-helix content and stability of apoA-I. Increasing the CE/TG ratio in LpA-I particles that contain both neutral lipids enhances the stability of the alpha-helical segments. ApoA-I molecules tend to dissociate and cause particle instability when delta G0D for the lipid-bound alpha-helices is less than that for helices in the lipid-free state. The stabilities of both discoidal and spherical LpA-I particles are relatively low when the only neutral lipid present is TG but the particle stability is enhanced by the presence of CE molecules. Such dissociation of apoA-I molecules from LpA-I particles that have a low CE/TG ratio would be promoted in the hypertriglyceridemic state in vivo.

Properties of an N-terminal proteolytic fragment of apolipoprotein AI in solution and in reconstituted high density lipoproteins

Limited proteolysis was used to study the domain structure and to produce a large N-terminal fragment of human apolipoprotein AI (apoAI). Digestion of reconstituted high density lipoprotein (rHDL) prepared with apoAI and dipalmitoyl phosphatidylcholine or palmitoyloleoyl phosphatidylcholine by chymotrypsin, trypsin, elastase, and subtilisin generated a major fragment of 22 kDa. Under milder conditions proteolysis of lipid-free apoAI produced a fragment of similar size. The fragments shared the same N terminus as intact apoAI, and the chymotryptic fragment had a molecular weight of 22,384 as determined by electrospray ionization mass spectrometry. Thus the fragment consists of the N-terminal 192 amino acid residues of apoAI, and the region around Tyr192 seems to be especially accessible to proteases. In aqueous solution the fragment, apoAI-(1-192), had an alpha-helix content similar to that of apoAI (approximately 52%) but existed only as monomers and dimers. ApoAI-(1-192) lysed dimyristoyl phosphatidylcholine liposomes slowly compared with apoAI but did form rHDL complexes with palmitoyloleoyl phosphatidylcholine or dipalmitoyl phosphatidylcholine when prepared by the sodium cholate dialysis method. ApoAI-(1-192) rHDL exhibited sizes and size distributions distinct from apoAI rHDL but displayed similar stability against denaturation. The isolated apoAI-(1-192) rHDLs retained a high ability to activate lecithin-cholesterol acyltransferase, comparable with the most effective apoAI rHDL. The results suggest that the C-terminal domain of apoAI is crucial for self-association and initial lipid binding but is not involved in specific lecithin-cholesterol acyltransferase activation.

Effect of the cholesterol content of reconstituted LpA-I on lecithin:cholesterol acyltransferase activity

The production of cholesteryl ester (CE) by lecithin: cholesterol acyl transferase (LCAT) is elevated significantly in hyperlipidemic subjects at high risk for coronary artery disease. To elucidate the molecular events involved, the relationship between LCAT activation and apolipoprotein (apo) A-I charge and structure in high density lipoproteins (HDL) has been studied in both native HDL and homogeneous recombinant HDL (Lp2A-I) particles containing apoA-I, palmitoyloleoyl phosphatidylcholine and cholesterol. Increasing the cholesterol content of discoidal Lp2A-I from 4 to 26 molecules/particle raises the maximum rate of cholesterol esterification by LCAT (Vmax) from 3.1 to 9.2 nmol CE/h/unit of LCAT and increases the apparent Km from 0.5 to 3.5 microM cholesterol. Similarly, increasing the cholesterol content in triolein core-containing Lp2A-I (4-18 molecules/particle) and in native HDL3 (12-21 molecules/particle) also significantly increases the Vmax for LCAT (2.8-7.7 and 0.5-3.6 nmol CE/h, respectively) and raises the Km values (7.6-36.9 and 7.3-8.5 microM cholesterol, respectively). In contrast, changes in the cholesterol content of native and recombinant HDL have no significant effect on the apparent Km values when expressed in terms of the concentration of either apoA-I or palmitoyloleoyl phosphatidylcholine. This appears to indicate that interfacial cholesterol content has no effect on the binding affinity of LCAT to different LpA-I particles but directly affects catalysis by modulating the interaction of cholesterol molecules with the active site of LCAT. Increasing the cholesterol content of the different HDL particles progressively increases the particle net negative charge, and these changes in apoA-I charge are strongly correlated with both the Vmax and apparent Km values for LCAT. This suggests that the conformation and charge of apoA-I play a central role in LCAT activation and that these parameters are influenced by the amount of cholesterol in the surface of HDL particles.