Human plasma lecithin-cholesterol acyltransferase. An elucidation of the catalytic mechanism

Human HDL subclasses modulate energy metabolism in skeletal muscle cells

In addition to its antiatherogenic role, HDL reportedly modulates energy metabolism at the whole-body level. HDL functionality is associated with its structure and composition, and functional activities can differ between HDL subclasses. Therefore, we studied if HDL2 and HDL3, the two major HDL subclasses, are able to modulate energy metabolism of skeletal muscle cells. Differentiated mouse and primary human skeletal muscle myotubes were used to investigate the influences of human HDL2 and HDL3 on glucose and fatty uptake and oxidation. HDL-induced changes in lipid distribution and mRNA expression of genes related to energy substrate metabolism, mitochondrial function, and HDL receptors were studied with human myotubes. Additionally, we examined the effects of apoA-I and discoidal, reconstituted HDL particles on substrate metabolism. In mouse myotubes, HDL subclasses strongly enhanced glycolysis upon high and low glucose concentrations. HDL3 caused a minor increase in ATP-linked respiration upon glucose conditioning but HDL2 improved complex I-mediated mitochondrial respiration upon fatty acid treatment. In human myotubes, glucose metabolism was attenuated but fatty acid uptake and oxidation were markedly increased by both HDL subclasses, which also increased mRNA expression of genes related to fatty acid metabolism and HDL receptors. Finally, both HDL subclasses induced incorporation of oleic acid into different lipid classes. These results, demonstrating that HDL subclasses enhance fatty acid oxidation in human myotubes but improve anaerobic metabolism in mouse myotubes, support the role of HDL as a circulating modulator of energy metabolism. Exact mechanisms and components of HDL causing the change, require further investigation.

Children with familial hypercholesterolemia display changes in LDL and HDL function: A cross-sectional study

BACKGROUND

The functional status of lipoprotein particles contributes to atherogenesis. The tendency of plasma low-density lipoprotein (LDL) particles to aggregate and the ability of igh-density lipoprotein (HDL) particles to induce and mediate reverse cholesterol transport associate with high and low risk for cardiovascular disease in adult patients, respectively. However, it is unknown whether children with familial hypercholesterolemia (FH) display lipoprotein function alterations.

HYPOTHESIS

We hypothesized that FH children had disrupted lipoprotein functions.

METHODS

We analyzed LDL aggregation susceptibility and HDL-apoA-I exchange (HAE), and activity of four proteins that regulate lipoprotein metabolism (cholesteryl ester transfer protein, lecithin-cholesterol acyltransferase, phospholipid transfer protein, and paraoxonase-1) in plasma samples derived from children with FH (n = 47) and from normocholesterolemic children (n = 56). Variation in lipoprotein functions was further explored using an nuclear magnetic resonance-based metabolomics profiling approach.

RESULTS

LDL aggregation was higher, and HAE was lower in FH children than in normocholesterolemic children. LDL aggregation associated positively with LDL cholesterol (LDL-C) and negatively with triglycerides, and HAE/apoA-I associated negatively with LDL-C. Generally, the metabolomic profile for LDL aggregation was opposite of that of HAE/apoA-I.

CONCLUSIONS

FH children displayed increased atherogenicity of LDL and disrupted HDL function. These newly observed functional alterations in LDL and HDL add further understanding of the risk for atherosclerotic cardiovascular disease in FH children.

Phenol-Enriched Virgin Olive Oil Promotes Macrophage-Specific Reverse Cholesterol Transport In Vivo

The intake of olive oil (OO) enriched with phenolic compounds (PCs) promotes ex vivo HDL-mediated macrophage cholesterol efflux in humans. We aimed to determine the effects of PC-enriched virgin OO on reverse cholesterol transport (RevCT) from macrophages to feces in vivo. Female C57BL/6 mice were given intragastric doses of refined OO (ROO) and a functional unrefined virgin OO enriched with its own PC (FVOO) for 14 days. Our experiments included two independent groups of mice that received intragastric doses of the phenolic extract (PE) used to prepare the FVOO and the vehicle solution (saline), as control, for 14 days. FVOO intake led to a significant increase in serum HDL cholesterol and its ability to induce macrophage cholesterol efflux in vitro when compared with ROO group. This was concomitant with the enhanced macrophage-derived [3H]cholesterol transport to feces in vivo. PE intake per se also increased HDL cholesterol levels and significantly promoted in vivo macrophage-to-feces RevCT rate when compared with saline group. PE upregulated the expression of the main macrophage transporter involved in macrophage cholesterol efflux, the ATP binding cassettea1. Our data provide direct evidence of the crucial role of OO PCs in the induction of macrophage-specific RevCT in vivo.

LDL Receptor Regulates the Reverse Transport of Macrophage-Derived Unesterified Cholesterol via Concerted Action of the HDL-LDL Axis: Insight From Mouse Models

RATIONALE

The HDL (high-density lipoprotein)-mediated stimulation of cellular cholesterol efflux initiates macrophage-specific reverse cholesterol transport (m-RCT), which ends in the fecal excretion of macrophage-derived unesterified cholesterol (UC). Early studies established that LDL (low-density lipoprotein) particles could act as efficient intermediate acceptors of cellular-derived UC, thereby preventing the saturation of HDL particles and facilitating their cholesterol efflux capacity. However, the capacity of LDL to act as a plasma cholesterol reservoir and its potential impact in supporting the m-RCT pathway in vivo both remain unknown.

OBJECTIVE

We investigated LDL contributions to the m-RCT pathway in hypercholesterolemic mice.

METHODS AND RESULTS

Macrophage cholesterol efflux induced in vitro by LDL added to the culture media either alone or together with HDL or ex vivo by plasma derived from subjects with familial hypercholesterolemia was assessed. In vivo, m-RCT was evaluated in mouse models of hypercholesterolemia that were naturally deficient in CETP (cholesteryl ester transfer protein) and fed a Western-type diet. LDL induced the efflux of radiolabeled UC from cultured macrophages, and, in the simultaneous presence of HDL, a rapid transfer of the radiolabeled UC from HDL to LDL occurred. However, LDL did not exert a synergistic effect on HDL cholesterol efflux capacity in the familial hypercholesterolemia plasma. The m-RCT rates of the LDLr (LDL receptor)-KO (knockout), LDLr-KO/APOB100, and PCSK9 (proprotein convertase subtilisin/kexin type 9)-overexpressing mice were all significantly reduced relative to the wild-type mice. In contrast, m-RCT remained unchanged in HAPOB100 Tg (human APOB100 transgenic) mice with fully functional LDLr, despite increased levels of plasma APO (apolipoprotein)-B-containing lipoproteins.

CONCLUSIONS

Hepatic LDLr plays a critical role in the flow of macrophage-derived UC to feces, while the plasma increase of APOB-containing lipoproteins is unable to stimulate m-RCT. The results indicate that, besides the major HDL-dependent m-RCT pathway via SR-BI (scavenger receptor class B type 1) to the liver, a CETP-independent m-RCT path exists, in which LDL mediates the transfer of cholesterol from macrophages to feces. Graphical Abstract: A graphical abstract is available for this article.

Lipase C, Hepatic Type -250A/G (rs2070895) Variant Enhances Carotid Atherosclerosis in Normolipidemic and Asymptomatic Individuals from Brazil

The common genetic variant in the promoter region of the hepatic lipase gene [LIPC -250G/A(rs2070895)] has an ambiguous association with cardiovascular disease. In this context, our study was performed to identify the relationships between the rs2070895 with carotid atherosclerosis, plasma lipids, and parameters of reverse cholesterol transport. A total of 285 normolipidemic and asymptomatic participants from an initial sample of 598,288 individuals (inclusion criteria: LDL-C ≤130 mg/dL and triglycerides ≤150 mg/dL; age: 20-75 years, both genders; confirmation of clinical, anthropometric and laboratory data; attended all visits; DNA was achieved to perform genetic analysis) were enrolled and the rs2070895 variant was genotyped by TaqMan® OpenArray® Plataform. Carotid intima-media thickness and the screening of atherosclerotic plaques were determined by B-mode ultrasonography. The rs2070895 genotype frequencies were 0.44, 0.41, and 0.15 (GG, GA, and AA, respectively). Logistic regression analysis showed that the risk of having plaques was increased in participants carrying the AA or AG genotypes (OR = 3.90; 95% CI = 1.54-10.33), despite an increase in high-density lipoprotein cholesterol levels, HDL diameter and apolipoprotein A-I, as compared to the GG genotype. Hepatic lipase and endogenous lecithin cholesterol acyl transferase activities were reduced (38% and 19%, respectively) and lipoprotein lipase was increased by 30% (AA vs GG). Our results provide evidence that the AA or AG genotypes of the rs2070895 were associated with carotid atherosclerosis in apparently healthy participants, probably as a consequence of reduced reverse cholesterol transport and accumulation of HDL subfraction 2 rich in triglycerides and depleted in cholesteryl esters that could become dysfunctional.

Impaired HDL (High-Density Lipoprotein)-Mediated Macrophage Cholesterol Efflux in Patients With Abdominal Aortic Aneurysm-Brief Report

Objective- The ability of HDL (high-density lipoprotein) to promote macrophage cholesterol efflux is considered the main HDL cardioprotective function. Abdominal aortic aneurysm (AAA) is usually characterized by cholesterol accumulation and macrophage infiltration in the aortic wall. Here, we aim to evaluate the composition of circulating HDL particles and their potential for promoting macrophage cholesterol efflux in AAA subjects. Approach and Results- First, we randomly selected AAA and control subjects from Spain. The AAA patients in the Spanish cohort showed lower plasma apoA-I levels concomitantly associated with low levels of plasma HDL cholesterol and the amount of preβ-HDL particles. We determined macrophage cholesterol efflux to apoB-depleted plasma, which contains mature HDL, preβ-HDL particles and HDL regulatory proteins. ApoB-depleted plasma from AAA patients displayed an impaired ability to promote macrophage cholesterol efflux. Next, we replicated the experiments with AAA and control subjects derived from Danish cohort. Danish AAA patients also showed lower apoA-I levels and a defective HDL-mediated macrophage cholesterol efflux. Conclusions- AAA patients show impaired HDL-facilitated cholesterol removal from macrophages, which could be mechanistically linked to AAA.

Substantial fat mass loss reduces low-grade inflammation and induces positive alteration in cardiometabolic factors in normal-weight individuals

The accumulation of fat, especially in visceral sites, is a significant risk factor for several chronic diseases with altered cardiometabolic homeostasis. We studied how intensive long-term weight loss and subsequent weight regain affect physiological changes, by longitudinally interrogating the lipid metabolism and white blood cell transcriptomic markers in healthy, normal-weight individuals. The current study examined 42 healthy, young (age: 27.5 ± 4.0 years), normal-weight (body mass index, BMI: 23.4 ± 1.7 kg/m²) female athletes, of which 25 belong to the weight loss and regain group (diet group), and 17 to the control group. Participants were evaluated, and fasting blood samples were drawn at three time points: at baseline (PRE); at the end of the weight loss period (MID: 21.1 ± 3.1 weeks after PRE); and at the end of the weight regain period (POST: 18.4 ± 2.9 weeks after MID). Following the weight loss period, the diet group experienced a ~73% reduction (~0.69 kg) in visceral fat mass (false discovery rate, FDR < 2.0 × 10^-16), accompanied by anti-atherogenic effects on transcriptomic markers, decreased low-grade inflammation (e.g., as α₁-acid glycoprotein (FDR = 3.08 × 10^-13) and hs-CRP (FDR = 2.44 × 10^-3)), and an increase in functionally important anti-atherogenic high-density lipoprotein -associated metabolites (FDR < 0.05). This occurred even though these values were already at favorable levels in these participants, who follow a fitness-lifestyle compared to age- and BMI-matched females from the general population (n = 58). Following the weight regain period, most of the observed beneficial changes in visceral fat mass, and metabolomic and transcriptomic profiles dissipated. Overall, the beneficial anti-atherogenic effects of weight loss can be observed even in previously healthy, normal-weight individuals.

A thumbwheel mechanism for APOA1 activation of LCAT activity in HDL

APOA1 is the most abundant protein in HDL. It modulates interactions that affect HDL's cardioprotective functions, in part via its activation of the enzyme, LCAT. On nascent discoidal HDL, APOA1 comprises 10 α-helical repeats arranged in an anti-parallel stacked-ring structure that encapsulates a lipid bilayer. Previous chemical cross-linking studies suggested that these APOA1 rings can adopt at least two different orientations, or registries, with respect to each other; however, the functional impact of these structural changes is unknown. Here, we placed cysteine residues at locations predicted to form disulfide bonds in each orientation and then measured APOA1's ability to adopt the two registries during HDL particle formation. We found that most APOA1 oriented with the fifth helix of one molecule across from fifth helix of the other (5/5 helical registry), but a fraction adopted a 5/2 registry. Engineered HDLs that were locked in 5/5 or 5/2 registries by disulfide bonds equally promoted cholesterol efflux from macrophages, indicating functional particles. However, unlike the 5/5 registry or the WT, the 5/2 registry impaired LCAT cholesteryl esterification activity (P < 0.001), despite LCAT binding equally to all particles. Chemical cross-linking studies suggest that full LCAT activity requires a hybrid epitope composed of helices 5-7 on one APOA1 molecule and helices 3-4 on the other. Thus, APOA1 may use a reciprocating thumbwheel-like mechanism to activate HDL-remodeling proteins.

Chymase released from hypoxia-activated cardiac mast cells cleaves human apoA-I at Tyr192 and compromises its cardioprotective activity

ApoA-I, the main structural and functional protein of HDL particles, is cardioprotective, but also highly sensitive to proteolytic cleavage. Here, we investigated the effect of cardiac mast cell activation and ensuing chymase secretion on apoA-I degradation using isolated rat hearts in the Langendorff perfusion system. Cardiac mast cells were activated by injection of compound 48/80 into the coronary circulation or by low-flow myocardial ischemia, after which lipid-free apoA-I was injected and collected in the coronary effluent for cleavage analysis. Mast cell activation by 48/80 resulted in apoA-I cleavage at sites Tyr¹⁹² and Phe²²⁹, but hypoxic activation at Tyr¹⁹² only. In vitro, the proteolytic end-product of apoA-I with either rat or human chymase was the Tyr¹⁹²-truncated fragment. This fragment, when compared with intact apoA-I, showed reduced ability to promote migration of cultured human coronary artery endothelial cells in a wound-healing assay. We propose that C-terminal truncation of apoA-I by chymase released from cardiac mast cells during ischemia impairs the ability of apoA-I to heal damaged endothelium in the ischemic myocardium.

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.

Prolonged sleep restriction induces changes in pathways involved in cholesterol metabolism and inflammatory responses

Sleep loss and insufficient sleep are risk factors for cardiometabolic diseases, but data on how insufficient sleep contributes to these diseases are scarce. These questions were addressed using two approaches: an experimental, partial sleep restriction study (14 cases and 7 control subjects) with objective verification of sleep amount, and two independent epidemiological cohorts (altogether 2739 individuals) with questions of sleep insufficiency. In both approaches, blood transcriptome and serum metabolome were analysed. Sleep loss decreased the expression of genes encoding cholesterol transporters and increased expression in pathways involved in inflammatory responses in both paradigms. Metabolomic analyses revealed lower circulating large HDL in the population cohorts among subjects reporting insufficient sleep, while circulating LDL decreased in the experimental sleep restriction study. These findings suggest that prolonged sleep deprivation modifies inflammatory and cholesterol pathways at the level of gene expression and serum lipoproteins, inducing changes toward potentially higher risk for cardiometabolic diseases.

Agonistic Human Antibodies Binding to Lecithin-Cholesterol Acyltransferase Modulate High Density Lipoprotein Metabolism

Drug discovery opportunities where loss-of-function alleles of a target gene link to a disease-relevant phenotype often require an agonism approach to up-regulate or re-establish the activity of the target gene. Antibody therapy is increasingly recognized as a favored drug modality due to multiple desirable pharmacological properties. However, agonistic antibodies that enhance the activities of the target enzymes are rarely developed because the discovery of agonistic antibodies remains elusive. Here we report an innovative scheme of discovery and characterization of human antibodies capable of binding to and agonizing a circulating enzyme lecithin cholesterol acyltransferase (LCAT). Utilizing a modified human LCAT protein with enhanced enzymatic activity as an immunogen, we generated fully human monoclonal antibodies using the XenoMouse^TM platform. One of the resultant agonistic antibodies, 27C3, binds to and substantially enhances the activity of LCAT from humans and cynomolgus macaques. X-ray crystallographic analysis of the 2.45 Å LCAT-27C3 complex shows that 27C3 binding does not induce notable structural changes in LCAT. A single administration of 27C3 to cynomolgus monkeys led to a rapid increase of plasma LCAT enzymatic activity and a 35% increase of the high density lipoprotein cholesterol that was observed up to 32 days after 27C3 administration. Thus, this novel scheme of immunization in conjunction with high throughput screening may represent an effective strategy for discovering agonistic antibodies against other enzyme targets. 27C3 and other agonistic human anti-human LCAT monoclonal antibodies described herein hold potential for therapeutic development for the treatment of dyslipidemia and cardiovascular disease.

The orientation and dynamics of estradiol and estradiol oleate in lipid membranes and HDL disc models

Estradiol (E2) and E2 oleate associate with high-density lipoproteins (HDLs). Their orientation in HDLs is unknown. We studied the orientation of E2 and E2 oleate in membranes and reconstituted HDLs, finding that E2 and E2 oleate are membrane-associated and highly mobile. Our combination of NMR measurements, molecular dynamics simulation, and analytic theory identifies three major conformations where the long axis of E2 assumes a parallel, perpendicular, or antiparallel orientation relative to the membrane's z-direction. The perpendicular orientation is preferred, and furthermore, in this orientation, E2 strongly favors a particular roll angle, facing the membrane with carbons 6, 7, 15, and 16, whereas carbons 1, 2, 11, and 12 point toward the aqueous phase. In contrast, the long axis of E2 oleate is almost exclusively oriented at an angle of ∼60° to the z-direction. In such an orientation, the oleoyl chain is firmly inserted into the membrane. Thus, both E2 and E2 oleate have a preference for interface localization in the membrane. These orientations were also found in HDL discs, suggesting that only lipid-E2 interactions determine the localization of the molecule. The structural mapping of E2 and E2 oleate may provide a design platform for specific E2-HDL-targeted pharmacological therapies.

Distant homology modeling of LCAT and its validation through in silico targeting and in vitro and in vivo assays

LCAT (lecithin:cholesterol acyltransferase) catalyzes the transacylation of a fatty acid of lecithin to cholesterol, generating a cholesteryl ester and lysolecithin. The knowledge of LCAT atomic structure and the identification of the amino acids relevant in controlling its structure and function are expected to be very helpful to understand the enzyme catalytic mechanism, as involved in HDL cholesterol metabolism. However - after an early report in the late ‘90 s - no recent advance has been made about LCAT three-dimensional structure. In this paper, we propose an LCAT atomistic model, built following the most up-to-date molecular modeling approaches, and exploiting newly solved crystallographic structures. LCAT shows the typical folding of the α/β hydrolase superfamily, and its topology is characterized by a combination of α-helices covering a central 7-strand β-sheet. LCAT presents a Ser/Asp/His catalytic triad with a peculiar geometry, which is shared with such other enzyme classes as lipases, proteases and esterases. Our proposed model was validated through different approaches. We evaluated the impact on LCAT structure of some point mutations close to the enzyme active site (Lys218Asn, Thr274Ala, Thr274Ile) and explained, at a molecular level, their phenotypic effects. Furthermore, we devised some LCAT modulators either designed through a de novo strategy or identified through a virtual high-throughput screening pipeline. The tested compounds were proven to be potent inhibitors of the enzyme activity.

Prevention of arterial stiffening by using low-dose atorvastatin in diabetes is associated with decreased malondialdehyde

Introduction

Without affecting the lipid profile, a low-dose treatment with atorvastatin contributes to the reduction of oxidative stress, inflammation, and adverse cardiovascular events in diabetes. In this study, we investigated whether low-dose atorvastatin exerts any beneficial effect on vascular dynamics in streptozotocin (STZ)-induced diabetes in male Wistar rats.

Methods

Diabetes was induced using a single tail-vein injection of STZ at 55 mg kg⁻¹. The diabetic rats were treated daily with atorvastatin (10 mg kg⁻¹ by oral gavage) for 6 weeks. They were also compared with untreated age-matched diabetic controls. Arterial wave reflection was derived using the impulse response function of the filtered aortic input impedance spectra. A thiobarbituric acid reactive substances measurement was used to estimate the malondialdehyde content.

Results

The high plasma level of total cholesterol in the diabetic rats did not change in response to this low-dose treatment with atorvastatin. Atorvastatin resulted in a significant increase of 15.4% in wave transit time and a decrease of 33.5% in wave reflection factor, suggesting that atorvastatin may attenuate the diabetes-induced deterioration in systolic loads imposed on the heart. This was in parallel with its lowering of malondialdehyde content in plasma and aortic walls in diabetes. Atorvastatin therapy also prevented the diabetes-related cardiac hypertrophy, as evidenced by the diminished ratio of left ventricular weight to body weight.

Conclusion

These findings indicate that low-dose atorvastatin might protect diabetic vasculature against diabetes-associated deterioration in aorta stiffness and cardiac hypertrophy, possibly through its decrease of lipid oxidation-derived malondialdehyde.

Osbpl8 deficiency in mouse causes an elevation of high-density lipoproteins and gender-specific alterations of lipid metabolism

OSBP-related protein 8 (ORP8) encoded by Osbpl8 is an endoplasmic reticulum sterol sensor implicated in cellular lipid metabolism. We generated an Osbpl8^−/− (KO) C57Bl/6 mouse strain. Wild-type and Osbpl8KO animals at the age of 13-weeks were fed for 5 weeks either chow or high-fat diet, and their plasma lipids/lipoproteins and hepatic lipids were analyzed. The chow-fed Osbpl8KO male mice showed a marked elevation of high-density lipoprotein (HDL) cholesterol (+79%) and phospholipids (+35%), while only minor increase of apolipoprotein A-I (apoA-I) was detected. In chow-fed female KO mice a less prominent increase of HDL cholesterol (+27%) was observed, while on western diet the HDL increment was prominent in both genders. The HDL increase was accompanied by an elevated level of HDL-associated apolipoprotein E in male, but not female KO animals. No differences between genotypes were observed in lecithin:cholesterol acyltransferase (LCAT) or hepatic lipase (HL) activity, or in the fractional catabolic rate of fluorescently labeled mouse HDL injected in chow-diet fed animals. The Osbpl8KO mice of both genders displayed reduced phospholipid transfer protein (PLTP) activity, but only on chow diet. These findings are consistent with a model in which Osbpl8 deficiency results in altered biosynthesis of HDL. Consistent with this hypothesis, ORP8 depleted mouse hepatocytes secreted an increased amount of nascent HDL into the culture medium. In addition to the HDL phenotype, distinct gender-specific alterations in lipid metabolism were detected: Female KO animals on chow diet showed reduced lipoprotein lipase (LPL) activity and increased plasma triglycerides, while the male KO mice displayed elevated plasma cholesterol biosynthetic markers cholestenol, desmosterol, and lathosterol. Moreover, modest gender-specific alterations in the hepatic expression of lipid homeostatic genes were observed. In conclusion, we report the first viable OsbplKO mouse model, demonstrating a HDL elevating effect of Osbpl8 knock-out and additional gender- and/or diet-dependent impacts on lipid metabolism.

Lecithin:cholesterol acyltransferase: old friend or foe in atherosclerosis?

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme that catalyzes the esterification of free cholesterol in plasma lipoproteins and plays a critical role in high-density lipoprotein (HDL) metabolism. Deficiency leads to accumulation of nascent preβ-HDL due to impaired maturation of HDL particles, whereas enhanced expression is associated with the formation of large, apoE-rich HDL(1) particles. In addition to its function in HDL metabolism, LCAT was believed to be an important driving force behind macrophage reverse cholesterol transport (RCT) and, therefore, has been a subject of great interest in cardiovascular research since its discovery in 1962. Although half a century has passed, the importance of LCAT for atheroprotection is still under intense debate. This review provides a comprehensive overview of the insights that have been gained in the past 50 years on the biochemistry of LCAT, the role of LCAT in lipoprotein metabolism and the pathogenesis of atherosclerosis in animal models, and its impact on cardiovascular disease in humans.

Spontaneous remodeling of HDL particles at acidic pH enhances their capacity to induce cholesterol efflux from human macrophage foam cells

HDL particles may enter atherosclerotic lesions having an acidic intimal fluid. Therefore, we investigated whether acidic pH would affect their structural and functional properties. For this purpose, HDL(2) and HDL(3) subfractions were incubated for various periods of time at different pH values ranging from 5.5 to 7.5, after which their protein and lipid compositions, size, structure, and cholesterol efflux capacity were analyzed. Incubation of either subfraction at acidic pH induced unfolding of apolipoproteins, which was followed by release of lipid-poor apoA-I and ensuing fusion of the HDL particles. The acidic pH-modified HDL particles exhibited an enhanced ability to promote cholesterol efflux from cholesterol-laden primary human macrophages. Importantly, treatment of the acidic pH-modified HDL with the mast cell-derived protease chymase completely depleted the newly generated lipid-poor apoA-I, and prevented the acidic pH-dependent increase in cholesterol efflux. The above-found pH-dependent structural and functional changes were stronger in HDL(3) than in HDL(2). Spontaneous acidic pH-induced remodeling of mature spherical HDL particles increases HDL-induced cholesterol efflux from macrophage foam cells, and therefore may have atheroprotective effects.

Conversion of lipid transfer inhibitor protein (apolipoprotein F) to its active form depends on LDL composition

Lipid transfer inhibitor protein (LTIP) exists in both active and inactive forms. Incubation (37°C) of plasma causes LTIP to transfer from a 470 kDa inactive complex to LDL where it is active. Here, we investigate the mechanisms underlying this movement. Inhibiting LCAT or cholesteryl ester transfer protein (CETP) reduced incubation-induced LTIP translocation by 40-50%. Blocking both LCAT and CETP completely prevented LTIP movement. Under appropriate conditions, either factor alone could drive maximum LTIP transfer to LDL. These data suggest that chemical modification of LDL, the 470 kDa complex, or both facilitate LTIP movement. To test this, LDL and the 470 kDa fraction were separately premodified by CETP and/or LCAT activity. Modification of the 470 kDa fraction had no effect on subsequent LTIP movement to native LDL. Premodification of LDL, however, induced spontaneous LTIP movement from the native 470 kDa particle to LDL. This transfer depended on the extent of LDL modification and correlated negatively with changes in the LDL phospholipid + cholesterol-to-cholesteryl ester + triglyceride ratio. We conclude that LTIP translocation is dependent on LDL lipid composition, not on its release from the inactive complex. Compositional changes that reduce the surface-to-core lipid ratio of LDL promote LTIP binding and activation.

Substrate specificity of plasma lecithin: cholesterol acyltransferase in abetalipoproteinemia

The lecithin: cholesterol acyltransferase (LCAT) activity of lipoprotein-depleted plasma from a patient with abetalipoproteinemia has been assayed in a modified Glomset-Wright incubation system with three different normal lipoprotein substrates consisting of an authentic mixture of very low (VLDL), low (LDL) and high (HDL) density lipoproteins for the assay of total LCAT activity, HDL to assay alpha-LCAT activity and combined VLDL and LDL to assay beta-LCAT activity, respectively. Although reduced to about half the normal control values, both alpha- and beta-LCAT activities were present in the patient's plasma. It has been shown earlier that secretion of LCAT is linked to that of VLDL, but since patients with abetalipoproteinemia cannot form either chylomicrons or VLDL, our results suggest that a secretion of these triglyceride-rich lipoproteins do not seem to be a prerequisite for a basal secretion of beta-LCAT.

Purification and characterization of the lipid A 1-phosphatase LpxE of Rhizobium leguminosarum

LpxE, a membrane-bound phosphatase found in Rhizobium leguminosarum and some other Gram-negative bacteria, selectively dephosphorylates the 1-position of lipid A on the outer surface of the inner membrane. LpxE belongs to the family of lipid phosphate phosphatases that contain a tripartite active site motif and six predicted transmembrane helices. Here we report the purification and characterization of R. leguminosarum LpxE. A modified lpxE gene, encoding a protein with an N-terminal His6 tag, was expressed in Escherichia coli. The protein was solubilized with Triton X-100 and purified to near-homogeneity. Gel electrophoresis reveals a molecular weight consistent with the predicted 31 kDa. LpxE activity is dependent upon Triton X-100, optimal near pH 6.5, and Mg2+-independent. The H197A and R133A substitutions inactivate LpxE, as does treatment with diethyl pyrocarbonate. In a mixed micelle assay system, the apparent Km for the precursor lipid IV(A) is 11 microm. Substrates containing the 3-deoxy-d-manno-oct-2-ulosonic acid disaccharide are dephosphorylated at similar rates to lipid IV(A), whereas glycerophospholipids like phosphatidic acid or phosphatidylglycerol phosphate are very poor substrates. However, an LpxE homologue present in Agrobacterium tumefaciens is selective for phosphatidylglycerol phosphate, demonstrating the importance of determining substrate specificity before assigning the functions of LpxE-related proteins. The availability of purified LpxE will facilitate the preparation of novel 1-dephosphorylated lipid A molecules that are not readily accessible by chemical methods.

A dual role for lecithin:cholesterol acyltransferase (EC 2.3.1.43) in lipoprotein oxidation

Lecithin:cholesterol acyltransferase (LCAT) is a key enzyme involved in lipoprotein metabolism. It mediates the transesterification of free cholesterol to cholesteryl ester in an apoprotein A-I-dependent process. We have isolated purified LCAT from human plasma using anion-exchange chromatography and characterized the extracted LCAT in terms of its molecular weight, molar absorption coefficient, and enzymatic activity. The participation of LCAT in the oxidation of very low density lipoproteins (VLDL) and low-density lipoproteins (LDL) was examined by supplementing lipoproteins with exogenous LCAT over a range of protein concentrations. LCAT-depleted lipoproteins were also prepared and their oxidation kinetics examined. Our results provide evidence for a dual role for LCAT in lipoprotein oxidation, whereby it acts in a dose-responsive manner as a potent pro-oxidant during VLDL oxidation, but as an antioxidant during LDL oxidation. We believe this novel pro-oxidant effect may be attributable to the LCAT-mediated formation of oxidized cholesteryl ester in VLDL, whereas the antioxidant effect is similar to that of chain-breaking antioxidants. Thus, we have demonstrated that the high-density lipoprotein-associated enzyme LCAT may have a significant role to play in lipoprotein modification and hence atherogenesis.

Effect of apolipoprotein M on high density lipoprotein metabolism and atherosclerosis in low density lipoprotein receptor knock-out mice

To investigate the role of apoM in high density lipoprotein (HDL) metabolism and atherogenesis, we generated human apoM transgenic (apoM-Tg) and apoM-deficient (apoM(-/-)) mice. Plasma apoM was predominantly associated with 10-12-nm alpha-migrating HDL particles. Human apoM overexpression (11-fold) increased plasma cholesterol concentration by 13-22%, whereas apoM deficiency decreased it by 17-21%. The size and charge of apoA-I-containing HDL in plasma were not changed in apoM-Tg or apoM(-/-) mice. However, in plasma incubated at 37 degrees C, lecithin:cholesterol acyltransferase-dependent conversion of alpha- to pre-alpha-migrating HDL was delayed in apoM-Tg mice. Moreover, lecithin: cholesterol acyltransferase-independent generation of pre-beta-migrating apoA-I-containing particles in plasma was increased in apoM-Tg mice (4.2 +/- 1.1%, p = 0.06) and decreased in apoM(-/-) mice (0.5 +/- 0.3%, p = 0.03) versus controls (1.8 +/- 0.05%). In the setting of low density lipoprotein receptor deficiency, apoM-Tg mice with approximately 2-fold increased plasma apoM concentrations developed smaller atherosclerotic lesions than controls. The effect of apoM on atherosclerosis may be facilitated by enzymatic modulation of plasma HDL particles, increased cholesterol efflux from foam cells, and an antioxidative effect of apoM-containing HDL.

Long-term effects of fenofibrate on VLDL and HDL subspecies in participants with type 2 diabetes mellitus

AIMS/HYPOTHESIS

Low HDL-cholesterol (HDL-C) is frequently accompanied by high triacylglycerol levels in diabetic dyslipidaemia, increasing the risk of CHD. In the Fenofibrate Intervention and Event Lowering in Diabetes (FIELD) study, fenofibrate treatment lowered triacylglycerol levels, but the initial 5% increase in HDL-C attenuated over 5 years. We explored the changes in VLDL and HDL subspecies during fenofibrate treatment in a statin-free FIELD cohort.

METHODS

We randomised 171 participants with type 2 diabetes mellitus, who had been recruited to the FIELD study in Helsinki, to micronised fenofibrate (200 mg/day) or placebo in double-blind study design. VLDL and HDL subspecies were separated by ultracentrifugation at baseline and at the second and fifth year. Apolipoprotein (apo)A-I and apoA-II were measured by immunoturbidometric methods and lipoprotein (Lp)A-I and LpAI-AII particles by differential immunoassay.

RESULTS

Fenofibrate reduced plasma triacylglycerol levels by 26%, resulting from a marked reduction in VLDL1 triacylglycerol (0.62 vs 0.29 mmol/l, p < 0.001). Fenofibrate caused an increase in LDL size (Delta 0.80 nm, p < 0.001). HDL-C was similar between the groups. HDL2-C was decreased by fenofibrate (-27.5% at 5th year, p < 0.001) and HDL3-C increased (13.0% at 5th year, p < 0.001). Fenofibrate had no effect on apoA-I, whereas apoA-II increased. Thus, LpA-I decreased while LpAI-AII increased. Activities of cholesteryl ester transfer protein, phospholipids transfer protein and lecithin:cholesterylacyl transferase were unchanged by fenofibrate. High homocysteine levels were associated with a slight decrease in HDL-C and apoA-I.

CONCLUSIONS/INTERPRETATION

Fenofibrate markedly reduced large VLDL particles and produced a clear shift in HDL subspecies towards smaller particles. The HDL3-C increase in conjunction with unchanged apoA-I [corrected] levels is a dilemma with regard to cardiovascular disease.

Coating of open tubular capillaries with discoidal and spherical high-density lipoprotein particles in electrochromatography

A new method was developed for the coating of fused-silica capillaries with human high-density lipoproteins (HDLs) for use in electrochromatography. The HDL particles used for the coating differed in particle shape and composition. Both discoidal and spherical particles formed a monolayer on the inner silica wall as confirmed by atomic force microscopy. The effect of coating conditions, such as HDL concentration and coating time, was investigated with spherical HDL particles. Examination of the influence of pH on the coating stability also allowed the determination of pI values for the HDL particles attached to the capillary wall. The pI values for spherical and discoidal HDL particles were close to 5.0. The repeatabilities of the EOF mobility and the retention factors of the uncharged steroid hormones used as model compounds were exploited in the evaluation of the coating stability. The optimal coating was achieved with 0.1 mg/mL HDL protein and 50 min flushing with coating solution followed by 15 min standing time. Electrochromatography with HDL-coated open tubular capillaries offers a new tool for the study of HDL particle structure and transformations.

Macrophage Phospholipid Transfer Protein Contributes Significantly to Total Plasma Phospholipid Transfer Activity and Its Deficiency Leads to Diminished Atherosclerotic Lesion Development

Objective— Systemic phospholipid transfer protein (PLTP) deficiency in mice is associated with a decreased susceptibility to atherosclerosis, whereas overexpression of human PLTP in mice increases atherosclerotic lesion development. PLTP is also expressed by macrophage-derived foam cells in human atherosclerotic lesions, but the exact role of macrophage PLTP in atherosclerosis is unknown.

Methods and Results— To clarify the role of macrophage PLTP in atherogenesis, PLTP was selectively disrupted in hematopoietic cells, including macrophages, by transplantation of bone marrow from PLTP knockout (PLTP −/− ) mice into irradiated low-density lipoprotein receptor knockout mice. Selective deficiency of macrophage PLTP (PLTP −M/−M ) resulted in a 29% ( P <0.01 for difference in lesion area) reduction in aortic root lesion area as compared with mice possessing functional macrophage PLTP (384±36*10 3 μm 2 in the PLTP −M/−M group (n=10), as compared with 539±35*10 3 μm 2 in the PLTP +M/+M group (n=14)) after 9 weeks of Western-type diet feeding. The decreased lesion size in the PLTP −M/−M group coincided with significantly lower serum total cholesterol, free cholesterol, and triglyceride levels in these mice. Furthermore, plasma PLTP activity in the PLTP −M/−M group was 2-fold ( P <0.001) lower than that in the PLTP +M/+M group.

Conclusion— Macrophage PLTP is a significant contributor to plasma PLTP activity and deficiency of PLTP in macrophages leads to lowered atherosclerotic lesion development in low-density lipoprotein receptor knockout mice on Western-type diet.

Estradiol fatty acid esterification is increased in high density lipoprotein subclass 3 isolated from hypertriglyceridemic subjects

Estrogen fatty acid esters are potent lipophilic estrogens transported exclusively in lipoproteins. They are formed in HDL in a reaction catalyzed by LCAT which is considered a prerequisite for their antioxidative action. Our previous studies in normotriglyceridemic (NTG) individuals demonstrated that estradiol (E2) esterification occurred mainly in HDL3 causing accumulation of esterified, but not of unesterified E2 in the lipoprotein particles. Using HDL obtained from hypertriglyceridemic (HTG) patients, we now investigated the effect of altered HDL composition on E2 esterification. Ultracentrifugally isolated HDL2 and HDL3 from NTG- and HTG-males were incubated in an in vitro model system with radioactive and with supraphysiological concentrations of non-radioactive E2 with and without exogenous LCAT. After purification, copper-induced oxidation of HDL was measured by monitoring conjugated diene formation. The results demonstrated that (i) E2 esterification occurring mainly in HDL3 was significantly more efficient in HTG-HDL3 compared to NTG-HDL3, (ii) triglyceride content in HDL3 correlated positively with E2 esterification rate, and (iii) addition of both exogenous LCAT and E2 into the incubation prolonged lag time of HDL3 oxidation. Thus, HDL composition regulates LCAT-facilitated E2 esterification but the in vivo role of this finding can be verified only in experiments using physiological hormone concentrations.

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.

Periodontitis decreases the antiatherogenic potency of high density lipoprotein

Periodontitis, a consequence of persistent bacterial infection and chronic inflammation, has been suggested to predict coronary heart disease (CHD). The aim of this study was to investigate the impact of periodontitis on HDL structure and antiatherogenic function in cholesterol efflux in vitro. HDL was isolated from 30 patients (age 43.6 +/- 6.1 years, mean +/- SD) with periodontitis before and after (3.2 +/- 1.4 months) periodontal treatment. The capacity of HDL for cholesterol efflux from macrophages (RAW 264.7), HDL composition, and key proteins of HDL metabolism were determined. After periodontal treatment, phospholipid transfer protein (PLTP) activity was 6.2% (P<0.05) lower, and serum HDL cholesterol concentration, PLTP mass, and cholesteryl ester transfer protein activity were 10.7% (P<0.001), 7.1% (P=0.078), and 19.4% (P<0.001) higher, respectively. The mean HDL2/HDL3 ratio increased from 2.16 +/- 0.87 to 3.56 +/- 0.48 (P<0.05). HDL total phospholipid mass and sphingomyelin-phosphatidylcholine ratio were 7.4% (P<0.05) and 36.8% (P<0.001) higher, respectively. The HDL-mediated cholesterol efflux tended to be higher after periodontal treatment; interestingly, this increase was significant (P<0.05) among patients whose C-reactive protein decreased (53.7% reduction, P=0.015) and who were positive by PCR for Actinobacillus actinomycetemcomitans. These results suggest that periodontitis causes similar, but milder, changes in HDL metabolism than those that occur during the acute-phase response and that periodontitis may diminish the antiatherogenic potency of HDL, thus increasing the risk for CHD.

Enhancement of scavenger receptor class B type I-mediated selective cholesteryl ester uptake from apoA-I(-/-) high density lipoprotein (HDL) by apolipoprotein A-I requires HDL reorganization by lecithin cholesterol acyltransferase

The severe depletion of cholesteryl ester (CE) in adrenocortical cells of apoA-I(-/-) mice suggests that apolipoprotein (apo) A-I plays an important role in the high density lipoprotein (HDL) CE selective uptake process mediated by scavenger receptor BI (SR-BI) in vivo. A recent study showed that apoA-I(-/-) HDL binds to SR-BI with the same affinity as apoA-I(+/+) HDL, but apoA-I(-/-) HDL has a decreased V(max) for CE transfer from the HDL particle to adrenal cells. The present study was designed to determine the basis for the reduced selective uptake of CE from apoA-I(-/-) HDL. Variations in apoA-I(-/-) HDL particle diameter, free cholesterol or phospholipid content, or the apoE or apoA-II content of apoA-I(-/-) HDL had little effect on HDL CE selective uptake into Y1-BS1 adrenal cells. Lecithin cholesterol acyltransferase treatment alone or addition of apoA-I to apoA-I(-/-) HDL alone also had little effect. However, addition of apoA-I to apoA-I(-/-) HDL in the presence of lecithin cholesterol acyltransferase reorganized the large heterogeneous apoA-I(-/-) HDL to a more discrete particle with enhanced CE selective uptake activity. These results show a unique role for apoA-I in HDL CE selective uptake that is distinct from its role as a ligand for HDL binding to SR-BI. These data suggest that the conformation of apoA-I at the HDL surface is important for the efficient transfer of CE to the cell.

Plasma phospholipid transfer protein-mediated reactions are impaired by hypochlorite-modification of high density lipoprotein

The two main functions of phospholipid transfer protein (PLTP) are the transfer of phospholipids between plasma lipoproteins and the conversion of high density lipoprotein (HDL), where prebeta-HDL particles are generated. HDL is considered an anti-atherogenic lipoprotein due to its function in the reverse cholesterol transport, where prebeta-HDL accepts cellular membrane cholesterol from peripheral tissues. However, the anti-atherogenic properties of native HDL may be abolished by oxidation/modification. Hypochlorous acid/hypochlorite (HOCl/OCl-)-a potent oxidant generated in vivo only by the myeloperoxidase-H2O2-chloride system of activated phagocytes-alters the physiological properties of HDL by generating a pro-atherogenic lipoprotein particle. Therefore, we have studied the effect of HOCl on the function of HDL subclass 3 (HDL3) and triglyceride-enriched HDL3 (TG-HDL3) in PLTP-mediated processes in vitro. Modification of HDL3 and TG-HDL3 with increasing HOCl concentrations (oxidant:lipoprotein molar ratio between 25:1 and 200:1) decreased the capacity of the corresponding lipoprotein particles to accept phospholipids. Although binding of PLTP to unmodified and HOCl-modified lipoprotein particles was similar, the degree of PLTP-mediated HDL conversion was decreased upon HOCl oxidation. PLTP released apolipoprotein A-I (apoA-I) from HOCl-modified HDL3, but the particles formed displayed no prebeta-mobility. Based on these findings, we conclude that the substrate properties of HOCl-modified HDL3 and TG-HDL3 in PLTP-mediated processes are impaired, which indicates that the anti-atherogenic properties of HDL are impaired.

High intakes of vegetables, berries, and apples combined with a high intake of linoleic or oleic acid only slightly affect markers of lipid peroxidation and lipoprotein metabolism in healthy subjects

BACKGROUND

A high consumption of vegetables and fruit is associated with decreased risk of ischemic heart disease and several cancers. The pathophysiology of these diseases involves free radical mechanisms. Diet may either enhance or diminish oxidative stress in the body.

OBJECTIVE

We studied the effects of high and low intakes of vegetables, berries, and apples on markers of lipid peroxidation and lipoprotein metabolism in subjects consuming diets high in linoleic or oleic acid.

DESIGN

For 6 wk, healthy men and women (n = 77; aged 19-52 y) consumed 1 of 4 controlled isoenergetic diets rich in either linoleic acid (11% of energy) or oleic acid (12% of energy) and containing either 815 or 170 g vegetables, berries, and apples/10 MJ. Nineteen healthy volunteers served as control subjects. Several markers of dietary compliance (plasma fatty acids, vitamin C, carotenoids, and quercetin), lipid peroxidation [ex vivo LDL oxidation, plasma and LDL thiobarbituric acid-reactive substances, paraoxonase (EC 3.1.8.1), and urinary 8-iso-prostaglandin F(2)(alpha)], and lipoprotein metabolism (plasma lipids, apolipoproteins, and lipid transfer protein activities) were measured from samples collected before and at the end of the experimental period.

RESULTS

Plasma fatty acid composition and antioxidant concentrations showed that compliance with the diets was good. However, there were no significant differences between the diets in the markers of lipid peroxidation and lipoprotein metabolism.

CONCLUSIONS

In healthy volunteers with adequate vitamin intakes, 6-wk diets differing markedly in the amounts of linoleic and oleic acid and vegetables, berries, and apples did not differ in their effects on lipid peroxidation or lipoprotein metabolism.

Studies on the plasma lipid profiles, and LCAT and CETP activities according to hyperlipoproteinemia phenotypes (HLP)

Hyperlipoproteinemia phenotypes (HLP), one of genetic disorders with an estimated prevalence of 0.5-2% in the general population, is responsible for 10% of premature CHD. After first screening with the high cholesterol (>6.47 mM/l) and triglyceride (TG) (>2.6 mM/l) levels without medication, subjects were typed for HLP classification. Differential metabolic effects of HLP types on plasma lipid profiles and the reverse cholesterol transport system (RCT) were studied in 196 HLP types (91.2%) and 19 non-HLP (8.8%). A total of 45% of subjects had primary HLP and the others had NIDDM (10.7%), hypertension (9.3%) and other chronic diseases. Type IV HLP (58.6%) was most predominant and Types IIa, IIb, III and V comprised 16.7, 12.1, 2.3 and 1.4% of the HLP. Type I was not found. Plasma lipids excluding apo A-I and Lp(a) were significantly different among HLP compared to non-HLP (P<0.001). Since Type V and III impact the clearance of TG-riched lipoproteins, TG and VLDL-C levels were higher in V and III. TG and LDL-C were higher in Type II than those in the others because of defect of LDL receptors. LCAT activity, lower in Type III and Type IV and highest in Type V, was highly associated with plasma free cholesterol levels and the ratio of apoB/apoA and LDL/HDL. CETP activity was highest in Type V due to high VLDL-C and TG and low HDL-C. The ratio of LCAT/CETP was not different among HLP types but was significantly lower in HLP than in non-HLP. CETP increased 2-3 times as well as LCAT decreased among HLP patients compared to non-HLP. We conclude non-HLP subjects with high cholesterol and TG levels do not always mean high risk of CHD and the intervention effects of HLP types would lead to impose the risk of CHD by the impact of RCT.

The role of plasma phospholipid transfer protein (PLTP) in HDL remodeling in acute-phase patients

During reverse cholesterol transport plasma phospholipid transfer protein (PLTP) converts high density lipoprotein(3) (HDL(3)) into two new subpopulations, HDL(2)-like particles and pre-beta-HDL. The acute-phase response is accompanied with dramatic changes in lipid metabolism including alterations in HDL concentration, composition, and thereby its function as a substrate for HDL remodeling proteins in circulation. To evaluate how acute-phase HDL (AP-HDL) functions in PLTP-mediated HDL conversion, we collected plasma samples from patients with severe acute-phase response (n=17), and from healthy controls (n=30). Subsequently, total HDL (1.063<d<1.21 g/ml) was isolated from patients and controls and incubated in the absence and presence of purified PLTP. The results show that HDL isolated from the acute-phase patients is converted by PLTP in vitro in a corresponding manner as normal HDL. In the combined population, C-reactive protein correlated significantly with lecithin-cholesterol acyltransferase (LCAT) activity (r=-0.53), cholesterol ester transfer protein activity (r=-0.80), PLTP activity (r=0.44), and PLTP mass (r=-0.66). When compared to the controls, the patients had 31% higher PLTP activity, but 52% lower PLTP mass leading to a 165% higher PLTP specific activity in the patients. The present data indicate that during the acute-phase response, plasma PLTP activity and mass are strongly affected by the lipoprotein distribution as well as lipid composition. We suggest that the decrease of HDL during the acute phase is caused by reduced LCAT and increased PLTP activities both increasing the plasma levels of lipid-poor apoA-I particles.

Lecithin:cholesterol acyl transferase G30S: association with atherosclerosis, hypoalphalipoproteinemia and reduced in vivo enzyme activity

OBJECTIVES

A 69 yr old male was referred for assessment of a very low plasma HDL cholesterol and apolipoprotein AI concentration. At age 65, he had undergone triple vessel coronary bypass graft surgery. He had a strong family history of early coronary heart disease. We analyzed the molecular basis of his clinical and biochemical abnormalities.

DESIGN AND METHODS

We used DNA sequencing to determine whether mutations in LCAT were present. We also evaluated plasma biochemistry and LCAT activity.

RESULTS

DNA sequencing revealed that the patient was a heterozygote for the G30S mutation in the gene encoding lecithin:cholesteol acyl transferase (LCAT). His plasma was found to have half-normal LCAT activity.

CONCLUSIONS

The findings in this patient suggest that rare dysfunctional mutations in candidate genes, such as LCAT, can contribute to the spectrum of patients ascertained because of low HDL cholesterol.

Dynamic changes in mouse lipoproteins induced by transiently expressed human phospholipid transfer protein (PLTP): importance of PLTP in prebeta-HDL generation

The plasma phospholipid transfer protein (PLTP) plays an important role in the regulation of plasma high density lipoprotein (HDL) levels and governs the distribution of HDL sub-populations. In the present study, adenovirus mediated overexpression of human PLTP in mice was employed to investigate the distribution of PLTP in serum and its effect on plasma lipoproteins. Gel filtration experiments showed that the distributions of PLTP activity and mass in serum are different, suggesting that human PLTP circulated in mouse plasma as two distinct forms, one with high and the other with low specific activity. Our study further demonstrates that overexpression of PLTP leads to depletion of HDL and that, as PLTP activity declines, replenishment of the HDL fraction occurs. During this process, the lipoprotein profile displays transient particle populations, including apoA-IV and apoE-rich particles in the LDL size range and small particles containing apoA-II only. The possible role of these particles in HDL reassembly is discussed. The increased PLTP activity enhanced the ability of mouse sera to produce pre(beta)-HDL. The present results provide novel evidence that PLTP is an important regulator of HDL metabolism and plays a central role in the reverse cholesterol transport (RCT) process.

Acute-phase HDL in phospholipid transfer protein (PLTP)-mediated HDL conversion

In reverse cholesterol transport, plasma phospholipid transfer protein (PLTP) converts high density lipoprotein(3) (HDL(3)) into two new subpopulations, HDL(2)-like particles and prebeta-HDL. During the acute-phase reaction (APR), serum amyloid A (SAA) becomes the predominant apolipoprotein on HDL. Displacement of apo A-I by SAA and subsequent remodeling of HDL during the APR impairs cholesterol efflux from peripheral tissues, and might thereby change substrate properties of HDL for lipid transfer proteins. Therefore, the aim of this work was to study the properties of SAA-containing HDL in PLTP-mediated conversion. Enrichment of HDL by SAA was performed in vitro and in vivo and the SAA content in HDL varied between 32 and 58 mass%. These HDLs were incubated with PLTP, and the conversion products were analyzed for their size, composition, mobility in agarose gels, and apo A-I degradation. Despite decreased apo A-I concentrations, PLTP facilitated the conversion of acute-phase HDL (AP-HDL) more effectively than the conversion of native HDL(3), and large fusion particles with diameters of 10.5, 12.0, and 13.8 nm were generated. The ability of PLTP to release prebeta from AP-HDL was more profound than from native HDL(3). Prebeta-HDL formed contained fragmented apo A-I with a molecular mass of about 23 kDa. The present findings suggest that PLTP-mediated conversion of AP-HDL is not impaired, indicating that the production of prebeta-HDL is functional during the ARP. However, PLTP-mediated in vitro degradation of apo A-I in AP-HDL was more effective than that of native HDL, which may be associated with a faster catabolism of inflammatory HDL.

Differential Regulation of Three Catalytic Activities of Platelet-Activating Factor (PAF)-Dependent Transacetylase

We have previously established that PAF-dependent transacetylase (TA) purified to apparent homogeneity from rat kidney membranes and cytosol contains three separate catalytic activities, namely PAF lysophospholipid transacetylase (TAL), PAF sphingosine transacetylase (TAS), and PAF acetylhydrolase (AH). In the present investigation, we studied the biochemical factors and mechanism(s) that differentially regulate these three TA activities of the purified enzymes. We found that only the TAS activity of the TA purified from the membranes was stimulated by phosphatidyl-serine (PS) with optimal concentration of activation occurring at 25 microM. Other acidic phospholipids, such as phosphatidylinositol (PI) and phosphatidylinositol 4-phosphate (PIP), are partially effective, while diacylglycerol and free fatty acids had no effect on the TAS activity. PS exerted its effect on the TAS activity through the increases of both Km and Vmax. In addition, N-ethylmalimide (NEM) and dithiobis-(2-nitro-5-thiobenzoic acid) (DTNB) strongly inhibited the TAS activity and partially decreased the TAL and AH activities of the purified membrane enzyme in a dose-dependent manner. The addition of PS, but not by its substrate, sphingosine, could prevented the inhibition by NEM on the basal level of TAS. On the other hand, the inhibition of TAL by NEM and DTNB were partially protected by the substrate, lysoplasmalogens. Furthermore, PAF fully protects the inhibition of AH, partially protects the inhibition of TAL, and does not protect the inhibition of TAS by NEM. These results suggested that the three individual catalytic activities of TA have different dependencies on the thiol-containing residue(s) of the enzyme, i.e., cysteine. Furthermore, the nonresponsiveness of the purified cytosolic TAS to PS activation is consistent with our previous notions that membrane and cytosolic TA are posttranslationally distinct.

Phospholipid transfer is a prerequisite for PLTP-mediated HDL conversion

Phospholipid transfer protein (PLTP) is an important regulator of high-density lipoprotein (HDL) metabolism. The two main functions of PLTP are transfer of phospholipids between lipoprotein particles and modulation of HDL size and composition in a process called HDL conversion. These PLTP-mediated processes are physiologically important in the transfer of surface remnants from lipolyzed triglyceride-rich lipoproteins to nascent HDL particles and in the generation of prebeta-HDL, the initial acceptor of excess peripheral cell cholesterol. The aim of the study presented here was to investigate the interrelationship between the two functions of PLTP. Plasma PLTP was chemically modified using diethylpyrocarbonate or ethylmercurithiosalicylate. The modified proteins displayed a dose-dependent decrease in phospholipid transfer activity and a parallel decrease in the ability to cause HDL conversion. Two recombinant PLTP mutant proteins, defective in phospholipid transfer activity due to a mutation in the N-terminal lipid-binding pocket, were produced, isolated, and incubated together with radioactively labeled HDL(3). HDL conversion was analyzed using three methods: native gradient gel electrophoresis, ultracentrifugation, and crossed immunoelectrophoresis. The results demonstrate that the mutant proteins (i) are able to induce only a modest increase in HDL particle size compared to the wild-type protein, (ii) are unable to release apoA-I from HDL(3), and (iii) do not generate prebeta-mobile particles following incubation with HDL(3). These data suggest that phospholipid transfer is a prerequisite for HDL conversion and demonstrate the close interrelationship between the two main activities of PLTP.