In vivo metabolism of apolipoprotein A-I on high density lipoprotein particles LpA-I and LpA-I,A-II

Comparison of the structure-function properties of wild-type human apoA-V and a C-terminal truncation associated with elevated plasma triglycerides

Background

Plasma triglycerides (TGs) are causally associated with coronary artery disease and acute pancreatitis. Apolipoprotein A-V (apoA-V, gene APOA5) is a liver-secreted protein that is carried on triglyceride-rich lipoproteins and promotes the enzymatic activity of lipoprotein lipase (LPL), thereby reducing TG levels. Little is known about apoA-V structure-function; naturally occurring human APOA5 variants can provide novel insights.

Methods

We used hydrogen-deuterium exchange mass spectrometry to determine the secondary structure of human apoA-V in lipid-free and lipid-associated conditions and identified a C-terminal hydrophobic face. Then, we used genomic data in the Penn Medicine Biobank to identify a rare variant, Q252X, predicted to specifically eliminate this region. We interrogated the function of apoA-V Q252X using recombinant protein in vitro and in vivo in apoa5 knockout mice.

Results

Human apoA-V Q252X carriers exhibited elevated plasma TG levels consistent with loss of function. Apoa5 knockout mice injected with AAV vectors expressing wildtype and variant APOA5-AAV recapitulated this phenotype. Part of the loss of function is due to reduced mRNA expression. Functionally, recombinant apoA-V Q252X was more readily soluble in aqueous solutions and more exchangeable with lipoproteins than WT apoA-V. Despite lacking the C-terminal hydrophobic region (a putative lipid binding domain) this protein also decreased plasma TG in vivo.

Conclusions

Deletion of apoA-V's C-terminus leads to reduced apoA-V bioavailability in vivo and higher TG levels. However, the C-terminus is not required for lipoprotein binding or enhancement of intravascular lipolytic activity. WT apoA-V is highly prone to aggregation, and this property is markedly reduced in recombinant apoA-V lacking the C-terminus.

TRIB1 regulates LDL metabolism through CEBPα-mediated effects on the LDL receptor in hepatocytes

Genetic variants near the TRIB1 gene are highly significantly associated with plasma lipid traits and coronary artery disease. While TRIB1 is likely causal of these associations, the molecular mechanisms are not well understood. Here we sought to investigate how TRIB1 influences low density lipoprotein cholesterol (LDL-C) levels in mice. Hepatocyte-specific deletion of Trib1 (Trib1Δhep) in mice increased plasma cholesterol and apoB and slowed the catabolism of LDL-apoB due to decreased levels of LDL receptor (LDLR) mRNA and protein. Simultaneous deletion of the transcription factor CCAAT/enhancer-binding protein alpha (CEBPα) with TRIB1 eliminated the effects of TRIB1 on hepatic LDLR regulation and LDL catabolism. Using RNA-seq, we found that activating transcription factor 3 (Atf3) was highly upregulated in the livers of Trib1Δhep but not Trib1Δhep CebpaΔhep mice. ATF3 has been shown to directly bind to the CEBPα protein, and to repress the expression of LDLR by binding its promoter. Blunting the increase of ATF3 in Trib1Δhep mice reduced the levels of plasma cholesterol and partially attenuated the effects on LDLR. Based on these data, we conclude that deletion of Trib1 leads to a posttranslational increase in CEBPα, which increases ATF3 levels, thereby contributing to the downregulation of LDLR and increased plasma LDL-C.

Apolipoprotein A-I modulates HDL particle size in the absence of apolipoprotein A-II

Human high-density lipoproteins (HDL) are a complex mixture of structurally-related nanoparticles that perform distinct physiological functions. We previously showed human HDL containing apolipoprotein A-I (APOA1) but not apolipoprotein A-II (APOA2), designated LpA-I, is composed primarily of two discretely sized populations. Here, we isolated these particles directly from human plasma by antibody affinity chromatography, separated them by high-resolution size exclusion chromatography and performed a deep molecular characterization of each species. The large and small LpA-I populations were spherical with mean diameters of 109 Å and 91 Å, respectively. Unexpectedly, isotope dilution MS/MS with [¹⁵N]-APOA1 in concert with quantitation of particle concentration by calibrated ion mobility analysis demonstrated that the large particles contained fewer APOA1 molecules than the small particles; the stoichiometries were 3.0 and 3.7 molecules of APOA1 per particle, respectively. MS/MS experiments showed that the protein cargo of large LpA-I particles was more diverse. Human HDL and isolated particles containing both APOA1 and APOA2 exhibit a much wider range and variation of particle sizes than LpA-I, indicating that APOA2 is likely the major contributor to HDL size heterogeneity. We propose a ratchet model based on the trefoil structure of APOA1 whereby the helical cage maintaining particle structure has two 'settings' - large and small - that accounts for these findings. This understanding of the determinants of HDL particle size and protein cargo distribution serves as a basis for determining the roles of HDL subpopulations in metabolism and disease states.

Distinct Proteomic Signatures in 16 HDL (High-Density Lipoprotein) Subspecies

Objective- HDL (high-density lipoprotein) in plasma is a heterogeneous group of lipoproteins typically containing apo AI as the principal protein. Most HDLs contain additional proteins from a palate of nearly 100 HDL-associated polypeptides. We hypothesized that some of these proteins define distinct and stable apo AI HDL subspecies with unique proteomes that drive function and associations with disease. Approach and Results- We produced 17 plasma pools from 80 normolipidemic human participants (32 men, 48 women; aged 21-66 years). Using immunoaffinity isolation techniques, we isolated apo AI containing species from plasma and then used antibodies to 16 additional HDL protein components to isolate compositional subspecies. We characterized previously described HDL subspecies containing apo AII, apo CIII, and apo E; and 13 novel HDL subspecies defined by presence of apo AIV, apo CI, apo CII, apo J, α-1-antitrypsin, α-2-macroglobulin, plasminogen, fibrinogen, ceruloplasmin, haptoglobin, paraoxonase-1, apo LI, or complement C3. The novel species ranged in abundance from 1% to 18% of total plasma apo AI. Their concentrations were stable over time as demonstrated by intraclass correlations in repeated sampling from the same participants over 3 to 24 months (0.33-0.86; mean 0.62). Some proteomes of the subspecies relative to total HDL were strongly correlated, often among subspecies defined by similar functions: lipid metabolism, hemostasis, antioxidant, or anti-inflammatory. Permutation analysis showed that the proteomes of 12 of the 16 subspecies differed significantly from that of total HDL. Conclusions- Taken together, correlation and permutation analyses support speciation of HDL. Functional studies of these novel subspecies and determination of their relation to diseases may provide new avenues to understand the HDL system of lipoproteins.

A human APOC3 missense variant and monoclonal antibody accelerate apoC-III clearance and lower triglyceride-rich lipoprotein levels

Recent large-scale genetic sequencing efforts have identified rare coding variants in genes in the triglyceride-rich lipoprotein (TRL) clearance pathway that are protective against coronary heart disease (CHD), independently of LDL cholesterol (LDL-C) levels. Insight into the mechanisms of protection of these variants may facilitate the development of new therapies for lowering TRL levels. The gene APOC3 encodes apoC-III, a critical inhibitor of triglyceride (TG) lipolysis and remnant TRL clearance. Here we report a detailed interrogation of the mechanism of TRL lowering by the APOC3 Ala43Thr (A43T) variant, the only missense (rather than protein-truncating) variant in APOC3 reported to be TG lowering and protective against CHD. We found that both human APOC3 A43T heterozygotes and mice expressing human APOC3 A43T display markedly reduced circulating apoC-III levels. In mice, this reduction is due to impaired binding of A43T apoC-III to lipoproteins and accelerated renal catabolism of free apoC-III. Moreover, the reduced content of apoC-III in TRLs resulted in accelerated clearance of circulating TRLs. On the basis of this protective mechanism, we developed a monoclonal antibody targeting lipoprotein-bound human apoC-III that promotes circulating apoC-III clearance in mice expressing human APOC3 and enhances TRL catabolism in vivo. These data reveal the molecular mechanism by which a missense variant in APOC3 causes reduced circulating TG levels and, hence, protects from CHD. This protective mechanism has the potential to be exploited as a new therapeutic approach to reduce apoC-III levels and circulating TRL burden.

Apolipoprotein A-II alters the proteome of human lipoproteins and enhances cholesterol efflux from ABCA1

HDLs are a family of heterogeneous particles that vary in size, composition, and function. The structure of most HDLs is maintained by two scaffold proteins, apoA-I and apoA-II, but up to 95 other "accessory" proteins have been found associated with the particles. Recent evidence suggests that these accessory proteins are distributed across various subspecies and drive specific biological functions. Unfortunately, our understanding of the molecular composition of such subspecies is limited. To begin to address this issue, we separated human plasma and HDL isolated by ultracentrifugation (UC-HDL) into particles with apoA-I and no apoA-II (LpA-I) and those with both apoA-I and apoA-II (LpA-I/A-II). MS studies revealed distinct differences between the subfractions. LpA-I exhibited significantly more protein diversity than LpA-I/A-II when isolated directly from plasma. However, this difference was lost in UC-HDL. Most LpA-I/A-II accessory proteins were associated with lipid transport pathways, whereas those in LpA-I were associated with inflammatory response, hemostasis, immune response, metal ion binding, and protease inhibition. We found that the presence of apoA-II enhanced ABCA1-mediated efflux compared with LpA-I particles. This effect was independent of the accessory protein signature suggesting that apoA-II induces a structural change in apoA-I in HDLs.

Predictive value of different HDL particles for the protection against or risk of coronary heart disease

The inverse relationship between plasma HDL levels and the risk of developing coronary heart disease is well established. The underlying mechanisms of this relationship are poorly understood, largely because HDL consist of several functionally distinct subpopulations of particles that are continuously being interconverted from one to another. This review commences with an outline of what is known about the origins of individual HDL subpopulations, how their distribution is regulated, and describes strategies that are currently available for isolating them. We then summarise what is known about the functionality of specific HDL subpopulations, and how these findings might impact on cardiovascular risk. The final section highlights major gaps in existing knowledge of HDL functionality, and suggests how these deficiencies might be addressed. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945-2010).

HDL Measures, Particle Heterogeneity, Proposed Nomenclature, and Relation to Atherosclerotic Cardiovascular Events

BACKGROUND

A growing body of evidence from epidemiological data, animal studies, and clinical trials supports HDL as the next target to reduce residual cardiovascular risk in statin-treated, high-risk patients. For more than 3 decades, HDL cholesterol has been employed as the principal clinical measure of HDL and cardiovascular risk associated with low HDL-cholesterol concentrations. The physicochemical and functional heterogeneity of HDL present important challenges to investigators in the cardiovascular field who are seeking to identify more effective laboratory and clinical methods to develop a measurement method to quantify HDL that has predictive value in assessing cardiovascular risk.

CONTENT

In this report, we critically evaluate the diverse physical and chemical methods that have been employed to characterize plasma HDL. To facilitate future characterization of HDL subfractions, we propose the development of a new nomenclature based on physical properties for the subfractions of HDL that includes very large HDL particles (VL-HDL), large HDL particles (L-HDL), medium HDL particles (M-HDL), small HDL particles (S-HDL), and very-small HDL particles (VS-HDL). This nomenclature also includes an entry for the pre-β-1 HDL subclass that participates in macrophage cholesterol efflux.

SUMMARY

We anticipate that adoption of a uniform nomenclature system for HDL subfractions that integrates terminology from several methods will enhance our ability not only to compare findings with different approaches for HDL fractionation, but also to assess the clinical effects of different agents that modulate HDL particle structure, metabolism, and function, and in turn, cardiovascular risk prediction within these HDL subfractions.

Apolipoprotein modulation of streptococcal serum opacity factor activity against human plasma high-density lipoproteins

Human plasma HDL are the target of streptococcal serum opacity factor (SOF), a virulence factor that clouds human plasma. Recombinant (r) SOF transfers cholesteryl esters (CE) from approximately 400,000 HDL particles to a CE-rich microemulsion (CERM), forms a cholesterol-poor HDL-like particle (neo HDL), and releases lipid-free (LF) apo A-I. Whereas the rSOF reaction requires labile apo A-I, the modulation effects of other apos are not known. We compared the products and rates of the rSOF reaction against human HDL and HDL from mice overexpressing apos A-I and A-II. Kinetic studies showed that the reactivity of various HDL species is apo-specific. LpA-I reacts faster than LpA-I/A-II. Adding apos A-I and A-II inhibited the SOF reaction, an effect that was more profound for apo A-II. The rate of SOF-mediated CERM formation was slower against HDL from mice expressing human apos A-I and A-II than against WT mice HDL and slowest against HDL from apo A-II overexpressing mice. The lower reactivity of SOF against HDL containing human apos is due to the higher hydropathy of human apo A-I, particularly its C-terminus relative to mouse apo A-I, and the higher lipophilicity of human apo A-II. The SOF-catalyzed reaction is the first to target HDL rather than its transporters and receptors in a way that enhances reverse cholesterol transport (RCT). Thus, effects of apos on the SOF reaction are highly relevant. Our studies show that the "humanized" apo A-I-expressing mouse is a good animal model for studies of rSOF effects on RCT in vivo.

Computational lipidology: predicting lipoprotein density profiles in human blood plasma

Monitoring cholesterol levels is strongly recommended to identify patients at risk for myocardial infarction. However, clinical markers beyond "bad" and "good" cholesterol are needed to precisely predict individual lipid disorders. Our work contributes to this aim by bringing together experiment and theory. We developed a novel computer-based model of the human plasma lipoprotein metabolism in order to simulate the blood lipid levels in high resolution. Instead of focusing on a few conventionally used predefined lipoprotein density classes (LDL, HDL), we consider the entire protein and lipid composition spectrum of individual lipoprotein complexes. Subsequently, their distribution over density (which equals the lipoprotein profile) is calculated. As our main results, we (i) successfully reproduced clinically measured lipoprotein profiles of healthy subjects; (ii) assigned lipoproteins to narrow density classes, named high-resolution density sub-fractions (hrDS), revealing heterogeneous lipoprotein distributions within the major lipoprotein classes; and (iii) present model-based predictions of changes in the lipoprotein distribution elicited by disorders in underlying molecular processes. In its present state, the model offers a platform for many future applications aimed at understanding the reasons for inter-individual variability, identifying new sub-fractions of potential clinical relevance and a patient-oriented diagnosis of the potential molecular causes for individual dyslipidemia.

Dose-dependent regulation of high-density lipoprotein metabolism with rosuvastatin in the metabolic syndrome

BACKGROUND

Low plasma concentration of high-density lipoprotein (HDL) cholesterol is a risk factor for cardiovascular disease and a feature of the metabolic syndrome. Rosuvastatin has been shown to increase HDL cholesterol concentration, but the mechanisms remain unclear.

METHODS AND RESULTS

Twelve men with the metabolic syndrome were studied in a randomized, double-blind, crossover trial of 5-wk therapeutic periods with placebo, 10 mg/d rosuvastatin, or 40 mg/d rosuvastatin, with 2-wk placebo washout between each period. Compared with placebo, there was a significant dose-dependent increase in HDL cholesterol, HDL particle size, and concentration of HDL particles that contain apolipoprotein A-I (LpA-I). The increase in LpA-I concentration was associated with significant dose-dependent reductions in triglyceride concentration and LpA-I fractional catabolic rate, with no changes in LpA-I production rate. There was a significant dose-dependent reduction in the fractional catabolic rate of HDL particles containing both apolipoprotein A-I and A-II (LpA-I:A-II), with concomitant reduction in LpA-I:A-II production rate, and hence no change in LpA-I:A-II concentration.

CONCLUSIONS

Rosuvastatin dose-dependently increased plasma HDL cholesterol and LpA-I concentrations in the metabolic syndrome. This could relate to reduction in plasma triglycerides with remodeling of HDL particles and reduction in LpA-I fractional catabolism. The findings contribute to understanding mechanisms for the HDL-raising effect of rosuvastatin in the metabolic syndrome with implications for reduction in cardiovascular disease.

Plasma phospholipid transfer protein activity, a determinant of HDL kinetics in vivo

OBJECTIVE

Phospholipid transfer protein (PLTP) is an important regulator in the transport of surface components of triglyceride-rich lipoprotein (TRL) to high density lipoprotein (HDL) during lipolysis and may therefore play an important role in regulating HDL transport. In this study we investigated the relationship of plasma PLTP activity with HDL metabolism in men.

DESIGN AND METHODS

The kinetics of HDL LpA-I and LpA-I:A-II were measured using intravenous administration of [D3]-leucine, gas chromatography-mass spectrometry (GCMS) and a new multicompartmental model for HDL subpopulation kinetics (SAAM II) in 31 men with wide-ranging body mass index (BMI 18-46 kg/m2). Plasma PLTP activity was determined as the transfer of radiolabelled phosphatidylcholine from small unilamellar phosphatidylcholine vesicles to ultracentrifugally isolated HDL.

RESULTS

PLTP activity was inversely associated with LpA-I concentration and production rate (PR) after adjusting for insulin resistance (P < 0.05). No significant associations were observed between plasma PLTP activity and LpA-I fractional catabolic rate (FCR). In multivariate analysis, including homeostasis model assessment score (HOMA), triglyceride, cholesteryl ester transfer protein (CETP) activity and PLTP activity, PLTP activity was the only significant determinant of LpA-I concentration and PR (P = 0.020 and P = 0.016, respectively).

CONCLUSIONS

Plasma PLTP activity may be a significant, independent determinant of LpA-I kinetics in men, and may contribute to the maintenance of the plasma concentration of these lipoprotein particles in setting of hypercatabolism of HDL.

High-density lipoprotein apolipoprotein A-I kinetics: comparison of radioactive and stable isotope studies

To compare the kinetic determinants of high-density lipoprotein (HDL) apolipoprotein A-I (apoA-I) concentration in lean normolipidaemic subjects using radioisotope and stable isotope studies. We pooled data from 16 radioisotope and 13 stable isotope studies to investigate the kinetics of apoA-I in lean normolipidemic individuals. We also examined the associations of HDL kinetic parameters with age, sex, body mass index (BMI) and concentrations of apoA-I, triglycerides, HDL cholesterol and low-density lipoprotein (LDL) cholesterol. Lean subjects from radioisotope and stable isotope studies were matched for age, gender, BMI and lipid profile. The apoA-I concentration was significantly lower in the radioisotope group than the stable isotope group (P = 0.031). There was no significant difference in HDL apoA-I fractional catabolic rate (FCR) and production rate (PR) between the groups. In the radioisotope group, HDL apoA-I FCR was significantly associated with apoA-I and HDL cholesterol concentrations (r = -0.681, P < 0.001 and r = -0.542, P < 0.001, respectively), whereas in the stable isotope group, only HDL apoA-I PR was significantly associated with apoA-I concentration (r = 0.455, P = 0.004). Our findings suggest that HDL apoA-I FCR is the primary determinant of apoA-I concentrations in lean subjects in studies using radiotracer techniques. By contrast, HDL apoA-I PR is the primary determinant of apoA-I concentration in lean subject in studies employing stable isotope methods. These discrepancies may be reconciled by differences in methodologies and/or study population characteristics.

LpA-I, LpA-I:A-II HDL and CHD-risk: The Framingham Offspring Study and the Veterans Affairs HDL Intervention Trial

OBJECTIVE

We tested the hypothesis that concentrations of LpA-I and/or LpA-I:A-II HDL subclasses are significantly associated with CHD prevalence and recurrent cardiovascular events.

METHODS

LpA-I levels were determined by differential electroimmunoassay in male participants with (n = 169) and without CHD (n = 850) from the Framingham Offspring Study (FOS) and in male participants with CHD from the placebo arm of the Veterans Affairs HDL Intervention Trial (VA-HIT) (n = 741). Data were analyzed cross-sectionally (FOS) and prospectively (VA-HIT) and were adjusted for established lipid and non-lipid CHD risk factors.

RESULTS

We observed slightly but significantly higher LpA-I levels in CHD cases compared to all or to HDL-C-matched controls and slightly but significantly higher LpA-I:A-II levels in CHD cases compared to HDL-C-matched controls it the FOS. Neither LpA-I nor LpA-I:A-II levels were significantly different between groups with and without recurrent cardiovascular events in the VA-HIT. No significant differences were observed in LpA-I and LpA-I:A-II levels in low HDL-C (< or = 40 mg/dl) subjects with CHD (VA-HIT, n = 711) and without CHD (FOS, n = 373). Plasma LpA-I concentration had a positive correlation with the large LpA-I HDL particle (alpha-1) but no correlation with the small LpA-I HDL particle (prebeta-1). LpA-I:A-II concentration had a positive correlation with the large (alpha-2) and an inverse correlation with the small (alpha-3) LpA-I:A-II HDL particles.

CONCLUSION

Our data do not support the hypothesis that CHD prevalence (FOS) or recurrence of cardiovascular events (VA-HIT) are associated with significant reductions in the concentrations of LpA-I and/or LpA-I:A-II HDL subclasses.

Thematic review series: Patient-Oriented Research. Design and analysis of lipoprotein tracer kinetics studies in humans

Lipoprotein tracer kinetics studies have for many years provided new and important knowledge of the metabolism of lipoproteins. Our understanding of kinetics defects in lipoprotein metabolism has resulted from the use of tracer kinetics studies and mathematical modeling. This review discusses all aspects of the performance of kinetics studies, including the development of hypotheses, experimental design, statistical considerations, tracer administration and sampling schedule, and the development of compartmental models for the interpretation of tracer data. In addition to providing insight into new metabolic pathways, such models provide quantitative information on the effect of interventions on lipoprotein metabolism. Compartment models are useful tools to describe experimental data but can also be used to aid in experimental design and hypothesis generation. The SAAM II program provides an easy-to-use interface with which to develop and test compartmental models against experimental models. The development of a model requires that certain checks be performed to ensure that the model describes the experimental data and that the model parameters can be estimated with precision. In addition to methodologic aspects, several compartment models of apoprotein and lipid metabolism are reviewed.

Human Lecithin:Cholesterol Acyltransferase Deficiency

Objectives— Lecithin:cholesterol acyltransferase deficiency (LCAT-def) is characterized by low levels of high-density lipoprotein (HDL) and low-density lipoprotein (LDL) and the accumulation of lipoprotein-X (LpX). Despite the low HDL, atherosclerosis is uncommon in LCAT-def. The decreased LDL would be a possible explanation but the underlying mechanism is not clear. In addition, the mechanism(s) for LpX accumulation is not known. The aim of the present study is to elucidate the mechanism(s) responsible for the low LDL and determine the plasma kinetics of LpX in LCAT-def.

Methods and Results— We conducted a radiotracer study in LCAT-def (n=2) and normal controls (n=10) and a stable isotope study in one patient and other controls (n=7). LCAT-def LDL was catabolized faster than control LDL in the control subjects as well as in LCAT-def patients. Control LDL was catabolized faster in LCAT-def patients than the controls. The production rate of LDL apolipoprotein B-100 was normal in LCAT-def. The increased LDL apoB-100 catabolism was confirmed by a stable isotope study. LpX was catabolized more slowly in LCAT-def.

Conclusions— The decreased LDL in LCAT-def is attributable to an increased catabolism caused by a rapid catabolism of abnormal LDL and an upregulation of LDL receptor pathway. The decreased catabolism of LpX contributes to its accumulation in LCAT-def.

Thematic review series: patient-oriented research. What have we learned about HDL metabolism from kinetics studies in humans?

Plasma measurements of lipids, lipoproteins, and apolipoproteins provide information on the static levels of these fractions without providing key information on the dynamic fluxes of lipoproteins in the circulation. Kinetics studies, in contrast, provide additional information on the production and clearance rates of lipoproteins and the flow of lipids and apolipoproteins through lipoprotein fractions. This information is crucial in accurately delineating the metabolism of HDL in plasma, because plasma concentrations of HDL are the net result of the de novo production and catabolism of HDL as well as the recycling of HDL particles and the contribution to HDL from components of other lipoproteins. Studies aimed at measuring the metabolism of HDL particles have shown that HDL metabolism in vivo is complex and consists of multiple components. Kinetics studies provide a window into the metabolism of HDL, allowing us to better understand the mechanisms of HDL decrease in human conditions and the functionality of HDL particles. Here, we review the progress in our understanding of HDL metabolism derived from in vivo kinetics studies, focusing primarily on studies in humans but also reviewing key studies in animal models.

High-density lipoprotein apolipoprotein A-I kinetics in obesity

OBJECTIVE

Low plasma concentrations of high-density lipoprotein (HDL)-cholesterol and apolipoprotein A-I (apoA-I) are independent predictors of coronary artery disease and are often associated with obesity and the metabolic syndrome. However, the underlying kinetic determinants of HDL metabolism are not well understood.

RESEARCH METHODS AND PROCEDURES

We pooled data from 13 stable isotope studies to investigate the kinetic determinants of apoA-I concentrations in lean and overweight-obese individuals. We also examined the associations of HDL kinetics with age, sex, BMI, fasting plasma glucose, fasting insulin, Homeostasis Model Assessment score, and concentrations of apoA-I, triglycerides, HDL-cholesterol and low-density lipoprotein-cholesterol.

RESULTS

Compared with lean individuals, overweight-obese individuals had significantly higher HDL apoA-I fractional catabolic rate (0.21+/-0.01 vs. 0.33+/-0.01 pools/d; p<0.001) and production rate (PR; 11.3+/-4.4 vs. 15.8+/-2.77 mg/kg per day; p=0.001). In the lean group, HDL apoA-I PR was significantly associated with apoA-I concentration (r=0.455, p=0.004), whereas in the overweight-obese group, both HDL apoA-I fractional catabolic rate (r=-0.396, p=0.050) and HDL apoA-I PR (r=0.399, p=0.048) were significantly associated with apoA-I concentration. After adjustment for fasting insulin or Homeostasis Model Assessment score, HDL apoA-I PR was an independent predictor of apoA-I concentration.

DISCUSSION

In overweight-obese subjects, hypercatabolism of apoA-I is paralleled by an increased production of apoA-I, with HDL apoA-I PR being the stronger determinant of apoA-I concentration. This could have therapeutic implications for the management of dyslipidemia in individuals with low plasma HDL-cholesterol.

Formation of high density lipoproteins containing both apolipoprotein A-I and A-II in the rabbit

Human plasma HDLs are classified on the basis of apolipoprotein composition into those that contain apolipoprotein A-I (apoA-I) without apoA-II [(A-I)HDL] and those containing apoA-I and apoA-II [(A-I/A-II)HDL]. ApoA-I enters the plasma as a component of discoidal particles, which are remodeled into spherical (A-I)HDL by LCAT. ApoA-II is secreted into the plasma either in the lipid-free form or as a component of discoidal high density lipoproteins containing apoA-II without apoA-I [(A-II)HDL]. As discoidal (A-II)HDL are poor substrates for LCAT, they are not converted into spherical (A-II)HDL. This study investigates the fate of apoA-II when it enters the plasma. Lipid-free apoA-II and apoA-II-containing discoidal reconstituted HDL [(A-II)rHDL] were injected intravenously into New Zealand White rabbits, a species that is deficient in apoA-II. In both cases, the apoA-II was rapidly and quantitatively incorporated into spherical (A-I)HDL to form spherical (A-I/A-II)HDL. These particles were comparable in size and composition to the (A-I/A-II)HDL in human plasma. Injection of lipid-free apoA-II and discoidal (A-II)rHDL was also accompanied by triglyceride enrichment of the endogenous (A-I)HDL and VLDL as well as the newly formed (A-I/A-II)HDL. We conclude that, irrespective of the form in which apoA-II enters the plasma, it is rapidly incorporated into spherical HDLs that also contain apoA-I to form (A-I/A-II)HDL.

SR-BI-mediated selective lipid uptake segregates apoA-I and apoA-II catabolism

The HDL receptor scavenger receptor class B type I (SR-BI) binds HDL and mediates the selective uptake of cholesteryl ester. We previously showed that remnants, produced when human HDL(2) is catabolized in mice overexpressing SR-BI, become incrementally smaller, ultimately consisting of small alpha-migrating particles, distinct from pre-beta HDL. When mixed with mouse plasma, some remnant particles rapidly increase in size by associating with HDL without the mediation of cholesteryl ester transfer protein, LCAT, or phospholipid transfer protein. Here, we show that processing of HDL(2) by SR-BI-overexpressing mice resulted in the preferential loss of apolipoprotein A-II (apoA-II). Short-term processing generated two distinct, small alpha-migrating particles. One particle (8.0 nm diameter) contained apoA-I and apoA-II; the other particle (7.7 nm diameter) contained only apoA-I. With extensive SR-BI processing, only the 7.7 nm particle remained. Only the 8.0 nm remnants were able to associate with HDL. Compared with HDL(2), this remnant was more readily taken up by the liver than by the kidney. We conclude that SR-BI-generated HDL remnants consist of particles with or without apoA-II and that only those containing apoA-II associate with HDL in an enzyme-independent manner. Extensive SR-BI processing generates small apoA-II-depleted particles unable to reassociate with HDL and readily taken up by the liver. This represents a pathway by which apoA-I and apoA-II catabolism are segregated.

Correlation of high density lipoprotein (HDL)-associated sphingosine 1-phosphate with serum levels of HDL-cholesterol and apolipoproteins

BACKGROUND

Extracellular sphingosine 1-phosphate (S1P) has been shown to contribute to the action of high density lipoprotein (HDL) on endothelial and smooth muscle cells. We examined the relationship of lipoprotein-associated S1P concentrations with cholesterol (C) and apolipoprotein (apo) contents of lipoprotein and lipoprotein subfractions characterized by capillary isotachophoresis (cITP).

METHODS

Blood samples were drawn from 16 volunteers. S1P concentrations were quantified by bioassay based on the ability of S1P to stimulate its receptor. cITP was performed using plasma that had been prestained with NBD-ceramide.

RESULTS

In plasma, S1P was concentrated in HDL and associated with LDL at a much lower concentration. HDL-S1P was the major determinant of the plasma S1P concentration. HDL-S1P was strongly and positively (p<0.001) correlated with serum levels of HDL-C (r=0.82), apo A-I (r=0.91) and apo A-II (r=0.92). HDL-S1P was strongly and positively (p<0.01) correlated with the apo A-I- and apo A-I/apo A-II-containing cITP HDL subfractions [fast HDL-C (r=0.66) and intermediate HDL-C (r=0.80)], but was not significantly correlated with apo E-containing slow HDL, suggesting that S1P is associated with both apo A-I HDL and apo A-I/A-II HDL. LDL-S1P was positively correlated (p<0.01) with levels of LDL-C (r=0.65) and apo B (r=0.85).

CONCLUSION

Lipoprotein-associated S1P was related to the lipoprotein composition of cholesterol and apolipoproteins, suggesting that extracellular S1P may play different roles depending on the particles with which it is associated.

Lipoprotein metabolism in subjects with hepatic lipase deficiency

A heritable deficiency of hepatic lipase (HL) provides insights into the physiologic function of HL in vivo. The metabolism of apolipoprotein B (apoB)-100 in very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) and of apoA-I and apoA-II in high-density lipoprotein (HDL) particles lipoprotein (Lp)(AI) and Lp(AI:AII) was assessed in 2 heterozygous males for compound mutations L334F/T383M or L334F/R186H, with 18% and 22% of HL activity, respectively, compared with 6 control males. Subjects were provided with a standard Western diet for a minimum of 3 weeks. At the end of the diet period, apo kinetics was assessed using a primed-constant infusion of [5,5,5-(2)H(3)] leucine. Mean plasma triglyceride (TG) and HDL cholesterol levels were 55% and 12% higher and LDL cholesterol levels 19% lower in the HL patients than control subjects. A higher proportion of apoB-100 was in the VLDL than IDL and LDL fractions of HL patients than control subjects due to a lower VLDL apoB-100 fractional catabolic rate (FCR) (4.63 v 9.38 pools/d, respectively) and higher hepatic production rate (PR) (33.24 v 10.87 mg/kg/d). Delayed FCR of IDL (2.78 and 6.31 pools/d) and LDL (0.128 and 0.205 pools/d) and lower PR of IDL (3.67 and 6.68 mg/kd/d) and LDL 4.57 and 13.07 mg/kg/d) was observed in HL patients relative to control subjects, respectively. ApoA-I FCR (0.09 and 0.13 pools/d) and PR (4.01 and 6.50 mg/kg/d) were slower in Lp(AI:AII) particles of HL patients relative to control subjects, respectively, accounting for the somewhat higher HDL cholesterol levels. HL deficiency may result in a lipoprotein pattern associated with low heart disease risk.

[Paraoxonase, something more than an enzyme?]

Coronary heart disease is one of the major causes of death in developed countries. The hypothesis that peroxidation of low density lipoproteins (LDL) may be the initial step of the atherosclerotic process has promoted numerous studies aimed at investigating the mechanisms by which the body protects itself from such oxidative phenomena. Among these mechanisms we find the paraoxanase (PON) enzyme, which is quite thriving the last decades. This enzyme is principally associated with high density lipoproteins (HDL) but it also seems to help LDL to recover their antioxidant status. This paper reviews different aspects concerning the mechanisms implicated in the induction and activity of this enzyme, as well as its production, attachment to HDL, and modifications of its activity due to external factors. The use of genetic techniques, the study of the polimorphisms of the PON enzyme and the possibility of increasing paraoxonase activity by means of pharmacotherapy and/or dietary therapy open new perspectives with regard to coronary heart disease treatment and prevention.

ApoA-II modulates the association of HDL with class B scavenger receptors SR-BI and CD36

The class B scavenger receptors SR-BI and CD36 exhibit a broad ligand binding specificity. SR-BI is well characterized as a HDL receptor that mediates selective cholesteryl ester uptake from HDL. CD36, a receptor for oxidized LDL, also binds HDL and mediates selective cholesteryl ester uptake, although much less efficiently than SR-BI. Apolipoprotein A-II (apoA-II), the second most abundant HDL protein, is considered to be proatherogenic, but the underlying mechanisms are unclear. We previously showed that apoA-II modulates SR-BI-dependent binding and selective uptake of cholesteryl ester from reconstituted HDL. To investigate the effect of apoA-II in naturally occurring HDL on these processes, we compared HDL without apoA-II (from apoA-II null mice) with HDLs containing differing amounts of apoA-II (from C57BL/6 mice and transgenic mice expressing a mouse apoA-II transgene). The level of apoA-II in HDL was inversely correlated with HDL binding and selective cholesteryl ester uptake by both scavenger receptors, particularly CD36. Interestingly, for HDL lacking apoA-II, the efficiency with which CD36 mediated selective uptake reached a level similar to that of SR-BI. These results demonstrate that apoA-II exerts a marked effect on HDL binding and selective lipid uptake by the class B scavenger receptors and establishes a potentially important relationship between apoA-II and CD36.

The concept of apolipoprotein-defined lipoprotein families and its clinical significance

Classification of plasma lipoproteins on the basis of apolipoprotein (apo) composition recognizes two lipoprotein (Lp) classes, one of which is characterized by apoA-I and the other by apoB as major protein constituents. The former lipoprotein class consists of three major subclasses referred to (according to their apolipoprotein constituents) as Lp-A-I, Lp-A-I:A-II, and Lp-A-II, and the latter one of five subclasses called Lp-B, Lp-B:E, Lp-B:C, Lp-B:C:E, and Lp-A-II:B:C:D:E. As polydisperse systems of particles, the apoA-I-containing lipoproteins overlap in high-density segments and apoB- containing lipoproteins in low-density segments of the density gradient. Each subclass is characterized by a specific chemical composition and metabolic property. Normolipidemia and dyslipoproteinemias are characterized by quantitative rather than qualitative differences in the levels of apoA- and apoB-containing subclasses. Furthermore, apoA-containing subclasses seem to differ with respect to their relative antiatherogenic capacities, and apoB-containing subclasses regarding their relative atherogenic potentials. Whereas Lp-A-I may have a greater antiatherogenic capacity than other apoA-containing subclasses, the cholesterol-enriched Lp-B:C appears to be the most atherogenic subclass among apoB-containing lipoprotein families. The use of pharmacologic and/or dietary interventions to treat dyslipoproteinemias has already shown that these therapeutic modalities may affect selectively individual apolipoprotein-defined lipoproteins, and thus allow the selection of individualized treatments targeted at decreasing harmful and/or increasing beneficial lipoprotein subclasses.

Lipoprotein lipase and hepatic lipase: their relationship with HDL subspecies Lp(A-I) and Lp(A-I,A-II)

HDL subspecies Lp(A-I) and Lp(A-I,A-II) have different anti-atherogenic potentials. To determine the role of lipoprotein lipase (LPL) and hepatic lipase (HL) in regulating these particles, we measured these enzyme activities in 28 healthy subjects with well-controlled Type 1 diabetes, and studied their relationship with Lp(A-I) and Lp(A-I,A-II). LPL was positively correlated with the apolipoprotein A-I (apoA-I), cholesterol, and phospholipid mass in total Lp(A-I), and with the apoA-I in large Lp(A-I) (r >or= 0.58, P >or= 0.001). HL was negatively correlated with all the above Lp(A-I) parameters plus Lp(A-I) triglyceride (r >or= -0.53, P <or= 0.003). No correlation was detected between LPL and Lp(A-I,A-II). However, HL was inversely correlated with total Lp(A-I,A-II) phospholipid, and with large Lp(A-I,A-II) (r >or= 0.50, P <or= 0.006). Similar studies were performed with phospholipid transfer protein (PLTP). Only total Lp(A-I) triglyceride in women (not men) (r = 0.71, P = 0.009) was significantly correlated with PLTP activity. These observations indicate that LPL and HL play major roles in determining the level and composition of plasma Lp(A-I), particularly large Lp(A-I), but not with Lp(A-I,A-II) level. Furthermore, select correlations of LPL and/or HL with the apoA-I, cholesterol, and triglyceride of Lp(A-I) but not Lp(A-I,A-II) imply that the apoA-I and lipid of Lp(A-I) and Lp(A-I,A-II) are not fully equilibrated.

High density lipoproteins (HDLs) and atherosclerosis; the unanswered questions

The concentration of high density lipoprotein-cholesterol (HDL-C) has been found consistently to be a powerful negative predictor of premature coronary heart disease (CHD) in human prospective population studies. There is also circumstantial evidence from human intervention studies and direct evidence from animal intervention studies that HDLs protect against the development of atherosclerosis. HDLs have several documented functions, although the precise mechanism by which they prevent atherosclerosis remains uncertain. Nor is it known whether the cardioprotective properties of HDL are specific to one or more of the many HDL subpopulations that comprise the HDL fraction in human plasma. Several lifestyle and pharmacological interventions have the capacity to raise the level of HDL-C, although it is not known whether all are equally protective. Indeed, despite the large body of information identifying HDLs as potential therapeutic targets for the prevention of atherosclerosis, there remain many unanswered questions that must be addressed as a matter of urgency before embarking wholesale on HDL-C-raising therapies as strategies to prevent CHD. This review summarises what is known and highlights what we still need to know.

Characterization of lipoproteins from the turtle, Trachemys scripta elegans, in fasted and fed states

The lipid and apolipoprotein composition of VLDL, IDL, LDL, HDL(2) and HDL(3) were examined in the turtle, Trachemys scripta elegans, in fasted and fed states. The lipid composition of turtle lipoproteins was very similar to their human counterparts. The major apolipoprotein found in LDL, IDL and VLDL, which has a molecular weight of approximately 550 kD, is a homologue of apoB100. The major apolipoprotein found in both HDL(2) and HDL(3), has a molecular weight of 28-kD and is homologous to human apoA-I. HDL(3) also contains a 6.5 kD protein that is homologous to apoA-II, while HDL(2) has two low molecular weight proteins of 6 kD and 7 kD which are also found on the triglyceride rich lipoproteins (TRL). The 7 kD protein is homologous to apoC-III, while the 6 kD protein has a similar size and distribution as apoC-II or apoC-I. In addition, HDL(2) also possesses a protein of 15.8 kD that has no obvious mammalian homologue. In both size and apolipoprotein composition, turtle HDL(2) resembles human HDL(2b) while turtle HDL(3) resembles human HDL(3). In the fasted state, turtles contained very little TRL. When fed a high fat diet, the amount of IDL and LDL sized particles increased significantly.

Evaluation of apolipoprotein A-I kinetics in rabbits in vivo using in situ and exogenous radioiodination methods

The kinetics of in vivo clearance of apolipoprotein (apo) A-I radioiodinated by the iodine monochloride (ICI) method of McFarlane [McFarlane, A.S. (1958) Efficient Trace-Labelling of Proteins with Iodine, Nature 182, 53] as modified by Bilheimer and co-workers [Bilheimer, D.W., Eisenberg, S., and Levy, R.I. (1972) The Metabolism of Very Low Density Lipoprotein Proteins. I. Preliminary in vitro and in vivo Observations, Biochim. Biophys. Acta 260, 212-221] and by using the IODO Beads Iodination Reagent were evaluated in rabbits. Both human apoA-I and rabbit HDL radioiodinated by the IODO Beads Iodination Reagent were cleared faster from plasma of rabbits than those radiolabeled by the ICI method. However, the different radiolabeling procedures in the ICI method, i.e., apoA-I radiolabeled either exogenously or in situ as a part of intact HDL, were not associated with a significant difference in the in vivo kinetics of apoA-I in rabbits if apoA-I was prepared by the guanidine HCI method and used fresh. 125I-ApoA-I subjected to delipidation and lyophilization was cleared only slightly faster from the plasma of rabbits than fresh 125I-apoA-I. We also found that apoA-I separated by the guanidine HCI method and used fresh was cleared faster from the plasma of rabbits when it was injected as free apoA-I without adding serum albumin or after in vitro incubation with rabbit HDL than when injected after reassociation with rabbit plasma. We conclude that the ICI method is a more appropriate radioiodination method for studying the in vivo kinetics of HDL than the IODO Beads Iodination Reagent and that the in vitro incubation conditions before injection are important factors that affect the in vivo kinetics of apo A-I.

High density lipoprotein subfractions and the risk of coronary heart disease: 9-years follow-up in the Caerphilly Study

Whether the protective effect of high density lipoprotein (HDL) on incident coronary heart disease (CHD) can be attributed to one or both HDL subfractions remains controversial. The associations of HDL(2) and HDL(3) cholesterol with the incidence of CHD in the 9-year follow-up of the Caerphilly study are described. A total of 2398 middle-aged British men were recruited from the general population between 1984 and 1988 and were followed, on average, for 9 years. Total and HDL(3) cholesterol were measured by a two-step precipitation technique on fresh, fasting samples from 2225 men. HDL(2) cholesterol was calculated by subtracting HDL(3) from total HDL cholesterol. Relative odds and 95% confidence intervals (CI) for incident CHD were obtained by use of a logistic regression model. During follow-up, 282 (12%) men developed a major new CHD event. Total HDL and HDL(3) cholesterol were significantly and inversely associated with the risk of incident CHD. When divided into fifths of the distributions of total HDL and HDL(3) cholesterol, multivariate-adjusted relative odds were 1.00, 0.95, 0.72, 0.85, 0.38 and 1.00, 1.05, 0.92, 0.67, 0.39, respectively graded from the least to the most quintile, with the lowest quintile group as referent. Tests for trend were significant (P for trend 0.003 and 0.001, respectively). In a multivariate model, the contribution of HDL(3) was significant (standardized relative odds, 0.76; 95% CI, 0.64-0.91), whereas HDL(2) was not significant. No linear combination of the two subfractions was a better predictor of CHD than total HDL cholesterol alone. HDL(3) cholesterol was an independent predictor of incident CHD and may be more closely related to the development of CHD than HDL(2) cholesterol. The prediction of the risk of CHD from total HDL cholesterol alone could not be improved upon by measurement of the two HDL subfractions. In our view, the only way to improve our understanding of this situation is to measure both subfractions independently of each other and not to calculate one by subtraction.

Tibolone lowers high density lipoprotein cholesterol by increasing hepatic lipase activity but does not impair cholesterol efflux

OBJECTIVE

Androgens and other drugs that reduce plasma concentrations of high density lipoprotein (HDL) cholesterol are often considered to be pro-atherogenic. Tibolone lowers HDL-cholesterol by 20% but the clinical significance of this effect is unknown.

METHODS

In a randomized, double-blind study, 34 women received 2.5 mg tibolone daily and 34 women received placebo. Serum concentrations of lipids, lipoprotein subclasses and apolipoproteins, together with plasma activities of lipid transfer proteins and lipolytic enzymes and the capacity of plasma to induce cholesterol efflux from cultured cells, were measured.

RESULTS

Compared to placebo, tibolone reduced serum concentrations of HDL-cholesterol (-14%), HDL phosphatidylcholine (-14%), apolipoprotein (apo)A-I (-12%), HDL subclasses lipoprotein (Lp)A-I (-20%), HDL-apoE (-16%), pre beta-LpA-I (-10%) and alpha-LpA-I (-12%) and increased hepatic lipase activity (+25%) and HDL sphingomyelin : phosphatidylcholine ratio (10.5%), but did not alter serum concentrations of HDL sphingomyelin, apoA-IV and LpA-I/A-II, lipoprotein lipase, the plasma activities of lecithin : cholesterol acyl transferase, cholesteryl ester transfer protein, phospholipid transfer protein or the plasma capacity to release cholesterol from cultured fibroblasts or Fu5AH hepatocytes.

CONCLUSIONS

Tibolone lowers HDL-cholesterol in part by increasing hepatic lipase activity. Conservation of sphingomyelin and apoA-II in HDL, as well as cholesteryl ester transfer protein activity, preserves the capacity of plasma to release cholesterol, despite the lower concentrations of HDL-cholesterol. This may have important implications for the use of steroid effects on HDL concentrations as surrogates for atherosclerosis.

In vivo metabolism of apolipoprotein E within the HDL subpopulations LpE, LpE:A-I, LpE:A-II and LpE:A-I:A-II

High-density lipoproteins can be separated into distinct particles based on their apolipoprotein content. In the present study, the in vivo metabolism of apoE within the apoE-containing HDL particles LpE, LpE:A-I, LpE:A-II and LpE:A-I:A-II was assessed in control subjects and in patients with abetalipoproteinemia (ABL), in whom HDL are the sole plasma lipoproteins. The metabolism of apoE within these HDL subspecies was investigated in three separate studies which differed by donor or recipient status: (1) particles purified from normolipidemic plasma and reassociated with 125I or 131I-labeled apoE injected into normolipidemic subjects (study 1); (2) particles purified from ABL plasma injected into normolipidemic subjects (study 2); and (3) particles purified from ABL plasma injected into ABL subjects (study 3). The plasma residence times (RT, hours) in study 1 were 14.3+/-2.9, 11.3+/-3.4, and 9.1+/-1.2 for apoE within LpE:A-I:A-II, LpE:A-II and LpE:A-I, respectively, while those in study 2 were 10.1+/-2.2, 9.7+/-2.4, 7.9+/-1.0 and 7.3+/-0.8 for apoE within LpE:A-I:A-II, LpE:A-II, LpE:A-I and LpE, respectively. In study 3, RTs for apoE within LpE:A-I:A-II and LpE were 8.7+/-0.9 and 6.8+/-0.9, respectively. In comparison, RT for apoA-I on LpA-I:A-II has been reported to be 124.1+/-5.5 h and that for apoA-I on LpA-I 105.8+/-6.2 h. Thus, apoE within the different apoE-containing HDL particles was metabolized rapidly and at a similar rate in control and ABL subjects. The plasma RT of apoE was longest when injected on LpE:A-I:A-II particles and shortest when injected on LpE. In summary, our data show that: (1) the plasma RT of apoE within HDL is approximately ten times shorter than that of apoA-I within HDL, and (2) apoE within HDL is metabolized at a slower rate when apoproteins A-I and A-II are present (LpE:A-I:A-II RT>LpE:A-II>LpE:A-I>LpE). These differences were related to the lipid and apolipoprotein composition of the HDL subspecies, and, in control subjects, to the transfer of apoE from HDL subspecies to apoB-containing lipoproteins as well.

High-density lipoproteins and atherosclerosis

High-density lipoproteins (HDLs) are strongly related to risk of atherosclerotic cardiovascular disease. Low levels of HDL cholesterol are a major cardiovascular risk factor, and overexpression of the major HDL protein, apolipoprotein (apo) A-I, markedly inhibits progression and even induces regression of atherosclerosis in animal models. Clinical data regarding the effect of increasing HDL cholesterol on vascular events are limited. HDL remains an important potential target for therapeutic intervention. A variety of gene products are involved in the regulation of HDL metabolism. Yet, the mechanisms by which HDL inhibits atherosclerosis are not yet fully understood. There remains much to be learned about HDL metabolism and its relation to atherosclerosis and other cardiovascular risk factors.

Apolipoprotein A-II, HDL metabolism and atherosclerosis

Apolipoprotein (Apo) A-I and apo A-II are the major apolipoproteins of HDL. It is clearly demonstrated that there are inverse relationships between HDL-cholesterol and apo A-I plasma levels and the risk of coronary heart disease (CHD) in the general population. On the other hand, it is still not clearly demonstrated whether apo A-II plasma levels are associated with CHD risk. A recent prospective epidemiological (PRIME) study suggests that Lp A-I (HDL containing apo A-I but not apo A-II) and Lp A-I:A-II (HDL containing apo A-I and apo A-II) were both reduced in survivors of myocardial infarction, suggesting that both particles are risk markers of CHD. Apo A-II and Lp A-I:A-II plasma levels should be rather related to apo A-II production rate than to apo A-II catabolism. Mice transgenic for both human apo A-I and apo A-II are less protected against atherosclerosis development than mice transgenic for human apo A-I only, but the results of the effects of trangenesis of human apo A-II (in the absence of a co-transgenesis of human apo A-I) are controversial. It is highly suggested that HDL reduce CHD risk by promoting the transfer of peripherical free cholesterol to the liver through the so-called 'reverse cholesterol transfer'. Apo A-II modulates different steps of HDL metabolism and therefore probably alters reverse cholesterol transport. Nevertheless, some effects of apo A-II on intermediate HDL metabolism might improve reverse cholesterol transport and might reduce atherosclerosis development while some other effects might be deleterious. In different in vitro models of cell cultures, Lp A-I:A-II induce either a lower or a similar cellular cholesterol efflux (the first step of reverse cholesterol transport) than Lp A-I. Results depend on numerous factors such as cultured cell types and experimental conditions. Furthermore, the effects of apo A-II on HDL metabolism, beyond cellular cholesterol efflux, are also complex and controversial: apo A-II may inhibit lecithin-cholesterol acyltransferase (LCAT) (potential deleterious effect) and cholesteryl-ester-transfer protein (CETP) (potential beneficial effect) activities, but may increase the hepatic lipase (HL) activity (potential beneficial effect). Apo A-II may also inhibit the hepatic cholesteryl uptake from HDL (potential deleterious effect) probably through the SR-BI depending pathway. Therefore, in terms of atherogenesis, apo A-II alters the intermediate HDL metabolism in opposing ways by increasing (LCAT, SR-BI) or decreasing (HL, CETP) the atherogenicity of lipid metabolism. Effects of apo A-II on atherogenesis are controversial in humans and in transgenic animals and probably depend on the complex effects of apo A-II on these different intermediate metabolic steps which are in weak equilibrium with each other and which can be modified by both endogenous and environmental factors. It can be suggested that apo A-II is not a strong determinant of lipid metabolism, but is rather a modulator of reverse cholesterol transport.

Fatty acid saturation of the diet and plasma lipid concentrations, lipoprotein particle concentrations, and cholesterol efflux capacity

BACKGROUND

The fatty acid content and saturation degree of the diet can modulate HDL composition and cholesterol efflux.

OBJECTIVE

We studied the modifications in plasma lipoprotein particles and serum capacity to stimulate cholesterol efflux induced by different fatty acids.

DESIGN

Seventeen women and 24 men followed in the same sequence 4 diets containing 35% of total energy as fat. The saturated fat diet contained 17% palm oil; the monounsaturated fat diet, 20.9% olive oil; the n-6 polyunsaturated fat diet, 12.5% sunflower oil; and the n-3 polyunsaturated fat diet, sunflower oil supplemented with 4-4.5 g fish oil/d. Each phase lasted 4-5 wk.

RESULTS

In both sexes, apolipoprotein (apo) A-I concentrations were significantly lower with unsaturated fat diets than with the saturated fat diet, but concentrations of lipoproteins containing only apo A-I (Lp A-I) were lower only in the men. Concentrations of lipoproteins containing both apo A-I and apo A-II (Lp A-I:A-II) were lower with both polyunsaturated fat diets in the women but significantly higher in the men. Lp E concentrations were significantly higher with the 2 polyunsaturated fat diets. Lp E non-B particle concentrations were not modified in the men but were significantly higher in the women in both polyunsaturated fat phases. Lp C-III concentrations were higher with the saturated fat diet only in the men. The serum samples taken after the n-3 polyunsaturated fat phase were the most efficient for extracting cellular cholesterol in both sexes.

CONCLUSIONS

The monounsaturated and polyunsaturated fat diets were healthier, producing a better lipid profile. The n-3 polyunsaturated fat diet increased the capacity of serum to promote the efflux of cholesterol from cells in culture.

Lowering of HDL cholesterol in post-menopausal women by tibolone is not associated with changes in cholesterol efflux capacity or paraoxonase activity

Low HDL cholesterol increases the risk of coronary heart disease. Treatment of postmenopausal women with tibolone lowers HDL cholesterol. We elucidated the consequences of this unwanted side effect in a randomized, double-blind study, where 12 women received 2.5 mg tibolone per day and 6 women, placebo. Blood samples were collected on days -1 (i.e. baseline), 28, 56, and 84 for the analysis of various parameters of lipid metabolism and HDL function. Compared to placebo, treatment with tibolone led to statistically significant decreases of HDL cholesterol (-22% to -32%), apoA-I (-14% to -22%), and HDL subclass LpA-I (-30% to -40%) but to no significant changes in apoA-II and HDL subclass LpA-I,A-II. These changes were not associated with statistically significant changes in the activity of plasma to release 3H-cholesterol from radiolabeled fibroblasts or in the serum activity of the anti-oxidative enzyme paraoxonase/arylesterase. There were no significant changes in either serum levels of triglycerides, LDL cholesterol, apoB, and leptin, or in LDL size. We conclude that changes in insulin do not contribute to the lowering of HDL cholesterol by tibolone. Despite decreased HDL cholesterol, putatively anti-atherogenic activities of HDL remained unchanged.

Association of sex, adiposity, and diet with HDL subclasses in middle-aged Chinese

BACKGROUND

There is limited information regarding the associations of lifestyle factors and sex with HDL subclasses containing apolipoprotein (apo) A-I (Lp A-I) and both apo A-I and apo A-II (Lp A-I:A-II).

OBJECTIVE

We sought to examine the relations between 2 major HDL subclasses and sex, menopausal status, nutrient intakes, and adiposity.

DESIGN

We conducted interviews and measured blood variables in 409 government employees aged 40-59 y in Taiwan.

RESULTS

Women (n = 203) had significantly higher concentrations of HDL cholesterol, Lp A-I, and Lp A-I:A-II than did men (n = 206). Postmenopausal women (n = 72) had higher concentrations of HDL cholesterol, Lp A-I, and Lp A-I:A-II than did premenopausal women (n = 131). Body mass index and waist-to-hip ratio were strong predictors of and exerted an independent additive effect on Lp A-I concentrations in both men and women. However, body adiposity was associated with Lp A-I:A-II concentrations only in men. Waist-to-hip ratio was an independent determinant of Lp A-I but not of Lp A-I:A-II in men and postmenopausal women after adjustment for age, body mass index, smoking, and diet. Although there were relatively weak associations between dietary factors and both HDL subclasses (r = 0.01-0.26) in men and women according to bivariate analyses, multiple regression models showed that total fat, saturated fat, and cholesterol intakes were significantly correlated with HDL cholesterol and both Lp A-I and Lp A-I:A-II in men, but not in women.

CONCLUSION

Our data suggest that body adiposity and dietary fat consumption affect 2 major HDL subclasses differently depending on subject sex and menopausal status.

Apolipoprotein A-II modulates the binding and selective lipid uptake of reconstituted high density lipoprotein by scavenger receptor BI

High density lipoprotein (HDL) represents a mixture of particles containing either apoA-I and apoA-II (LpA-I/A-II) or apoA-I without apoA-II (LpA-I). Differences in the function and metabolism of LpA-I and LpA-I/A-II have been reported, and studies in transgenic mice have suggested that apoA-II is pro-atherogenic in contrast to anti-atherogenic apoA-I. The molecular basis for these observations is unclear. The scavenger receptor BI (SR-BI) is an HDL receptor that plays a key role in HDL metabolism. In this study we investigated the abilities of apoA-I and apoA-II to mediate SR-BI-specific binding and selective uptake of cholesterol ester using reconstituted HDLs (rHDLs) that were homogeneous in size and apolipoprotein content. Particles were labeled in the protein (with (125)I) and in the lipid (with [(3)H]cholesterol ether) components and SR-BI-specific events were analyzed in SR-BI-transfected Chinese hamster ovary cells. At 1 microg/ml apolipoprotein, SR-BI-mediated cell association of palmitoyloleoylphosphatidylcholine-containing AI-rHDL was significantly greater (3-fold) than that of AI/AII-rHDL, with a lower K(d) and a higher B(max) for AI-rHDL as compared with AI/AII-rHDL. Unexpectedly, selective cholesterol ester uptake from AI/AII-rHDL was not compromised compared with AI-rHDL, despite decreased binding. The efficiency of selective cholesterol ester uptake in terms of SR-BI-associated rHDL was 4-5-fold greater for AI/AII-rHDL than AI-rHDL. These results are consistent with a two-step mechanism in which SR-BI binds ligand and then mediates selective cholesterol ester uptake with an efficiency dependent on the composition of the ligand. ApoA-II decreases binding but increases selective uptake. These findings show that apoA-II can exert a significant influence on selective cholesterol ester uptake by SR-BI and may consequently influence the metabolism and function of HDL, as well as the pathway of reverse cholesterol transport.

Probucol promotes reverse cholesterol transport in heterozygous familial hypercholesterolemia. Effects on apolipoprotein AI-containing lipoprotein particles

In order to investigate the effect of Probucol therapy on reverse cholesterol transport, apo AI-containing lipoprotein particles were isolated and characterized, and their cholesterol effluxing capacity and LCAT activity were assayed in four familial hypercholesterolemia patients before and after 12 weeks of Probucol therapy. Four major subpopulations of apo A-containing lipoprotein particles are separated before and after drug treatment; LpAI, LpAI:AII, LpAIV, LpAI:AIV:AII. Probucol reduces both total plasma and LDL-cholesterol (-17 and -14%, respectively). Apo B decreases slightly (-7.6%). Plasma HDL-cholesterol and apo AI decrease by 36.6 and 34.7%. LpA-I showed a marked decrease (-46%). Moreover, plasma LCAT and CETP activities were markedly increased under Probucol treatment. Analysis of lipoprotein particles showed that Probucol induces a decrease of protein content and an increase of cholesterol and triglycerides contents. Interestingly, Probucol induces an enhancement of LCAT activity in LpAI (4.5-fold). This drug induces a trend toward greater cholesterol efflux from cholesterol-preloaded adipose cells promoted by Lp AI and Lp AIV but not by Lp AI:AII. This study confirms the hypothesis, in addition to the lowering LDL-cholesterol levels and antioxidant effects of Probucol, that HDL reduction was not an atherogenic change in HDL system but may cause an antiatherogenic action by accelerating cholesterol transport through HDL system, promoting reverse cholesterol transport from peripheral tissues.

Monoclonal antibodies to human apolipoproteins: application to the study of high density lipoprotein subpopulations

We produced, selected and cloned hybridomas that secrete monoclonal antibodies against human apolipoprotein (apo) A-I. All of the antibodies corresponded to the IgG(1) subclass and were named 1C11, 2B4, 2C10, 7C5, 8A4 and 8A5. The antibodies were characterized by their reactivity with whole lipoproteins, apolipoproteins, synthetic peptides and fragments generated by cleavage of the apo A-I. Three of the monoclonal antibodies studied (2B4, 2C10 and 7C5) were similarly inhibited by an amino-terminal peptide (amino acid sequence 1-20) of apo A-I, whereas antibodies 1C11, 8A4 and 8A5 had no reaction. Other results show that monoclonal antibody 1C11 recognizes an epitope located between amino acids 135-148. We evaluated the monoclonal antibody 8A4 against different HDL subpopulations by competitive displacement analysis and it showed a similar reactivity with the HDL particles: LpA-I and LpA-I:A-II. This antibody was used to standardize a sandwich ELISA to quantitate LpA-I in plasma. We conclude that these monoclonal antibodies are relevant for the study of apo A-I epitope expression and for quantitating apo A-I containing lipoparticles.

Stable isotope turnover of apolipoproteins of high-density lipoproteins in humans

Amino acid precursors labelled with stable isotopes have been successfully used to explore the metabolism of the apolipoproteins of HDL. Some methodological and mathematical modelling problems remain, mainly related to amino acid recycling in a plasma protein such as apolipoprotein A-I with a long residence time (the reciprocal of the fractional catabolic rate) of 4-5 days. Apolipoprotein A-I, apolipoprotein E, and apolipoprotein A-IV in triglyceride-rich lipoproteins (containing chylomicrons, VLDL, and remnants) exhibit more complex kinetics. The small amounts of apolipoprotein A-I and of apolipoprotein A-IV in the triglyceride-rich lipoproteins have a residence time similar to that of the apolipoprotein A-I of HDL. In contrast, the apolipoprotein E in triglyceride-rich lipoproteins has been found to have an average residence time of 0.11 days. Diets low in saturated fat and cholesterol, which lower HDL levels, do so by decreasing the secretion of apolipoprotein A-I, with apolipoprotein A-II kinetics unaffected. Individuals with impaired glucose tolerance have a decreased residence time of apolipoprotein A-I but no change in secretion rate or in apolipoprotein A-II kinetics. This suggests a link between insulin resistance and the risk of atherosclerosis. In heterozygous familial hypercholesterolemia, both the fractional catabolic rate and the secretion rate of apolipoprotein A-I are increased, resulting in no change in the plasma level. Stable isotope studies have strengthened the evidence that triglyceride enrichment of HDL increases its catabolism Laboratory.

Role of the kidney in regulating the metabolism of HDL in rabbits: evidence that iodination alters the catabolism of apolipoprotein A-I by the kidney

To evaluate the factors that regulate HDL catabolism in vivo, we have measured the clearance of human apoA-I from rabbit plasma by following the isotopic decay of (125)I-apoA-I and the clearance of unlabeled apoA-I using a radioimmunometric assay (RIA). We show that the clearance of unlabeled apoA-I is 3-fold slower than that of (125)I-apoA-I. The mass clearance of iodinated apoA-I, as determined by RIA, is superimposable with the isotopic clearance of (125)I-apoA-I. The data demonstrate that iodination of tyrosine residues alters the apoA-I molecule in a manner that promotes an accelerated catabolism. The clearance from rabbit plasma of unmodified apoA-I on HDL(3) and a reconstituted HDL particle (LpA-I) were very similar and about 3-4-fold slower than that for (125)I-apoA-I on the lipoproteins. Therefore, HDL turnover in the rabbit is much slower than that estimated from tracer kinetic studies. To determine the role of the kidney in HDL metabolism, the kinetics of unmodified apoA-I and LpA-I were reevaluated in animals after a unilateral nephrectomy. Removal of one kidney was associated with a 40-50% reduction in creatinine clearance rates and a 34% decrease in the clearance rate of unlabeled apoA-I and LpA-I particles. In contrast, the clearance of (125)I-labeled molecules was much less affected by the removal of a kidney; FCR for (125)I-LpA-I was reduced by <10%. The data show that the kidneys are responsible for most (70%) of the catabolism of apoA-I and HDL in vivo, while (125)I-labeled apoA-I and HDL are rapidly catabolized by different tissues. Thus, the kidney is the major site for HDL catabolism in vivo. Modification of tyrosine residues on apoA-I may increase its plasma clearance rate by enhancing extra-renal degradation pathways.

Overexpression of secretory phospholipase A(2) causes rapid catabolism and altered tissue uptake of high density lipoprotein cholesteryl ester and apolipoprotein A-I

Plasma levels of high density lipoprotein (HDL) cholesterol and its major protein component apolipoprotein (apo) A-I are significantly reduced in both acute and chronic inflammatory conditions, but the basis for this phenomenon is not well understood. We hypothesized that secretory phospholipase A(2) (sPLA(2)), an acute phase protein that has been found in association with HDL, promotes HDL catabolism. A series of HDL metabolic studies were performed in transgenic mice that specifically overexpress human sPLA(2) but have no evidence of local or systemic inflammation. We found that HDL isolated from these mice have a significantly lower phospholipid and cholesteryl ester and significantly greater triglyceride content. The fractional catabolic rate (FCR) of (125)I-HDL was significantly faster in sPLA(2) transgenic mice (4.08 +/- 0.01 pools/day) compared with control wild-type littermates (2.16 +/- 0.48 pools/day). (125)I-HDL isolated from sPLA(2) transgenic mice was catabolized significantly faster than (131)I-HDL isolated from wild-type mice after injection in wild-type mice (p < 0.001). Injection of (125)I-tyramine-cellobiose-HDL demonstrated significantly greater degradation of HDL apolipoproteins in the kidneys of sPLA(2) transgenic mice compared with control mice (p < 0.05). The fractional catabolic rate of [(3)H]cholesteryl ether HDL was significantly faster in sPLA(2)-overexpressing mice (6.48 +/- 0.24 pools/day) compared with controls (4.80 +/- 0.72 pools/day). Uptake of [(3)H] cholesteryl ether into the livers and adrenals of sPLA(2) transgenic mice was significantly enhanced compared with control mice. In summary, these data demonstrate that overexpression of sPLA(2) alone in the absence of inflammation causes profound alterations of HDL metabolism in vivo and are consistent with the hypothesis that sPLA(2) may promote HDL catabolism in acute and chronic inflammatory conditions.

Apolipoprotein A-I and A-II kinetic parameters as assessed by endogenous labeling with [(2)H(3)]leucine in middle-aged and elderly men and women

The purpose of our study was to investigate high density lipoprotein (HDL) apolipoprotein (apo) A-I and apoA-II kinetics in a state of constant feeding after a primed-constant infusion of [5,5, 5-(2)H(3)]L-leucine in 32 normolipidemic older men and postmenopausal women (aged 41 to 79 years). ApoA-I and apoA-II were isolated from plasma HDL, and enrichment was determined by gas chromatography/mass spectrometry. The fractional secretion rate was obtained by using a monoexponential equation calculated with the SAAM II program (Department of Bioengineering, University of Washington, Seattle). Mean HDL cholesterol (HDL-C) and total triglyceride levels were 23% higher and 27% lower, respectively, in women than in men. Mean plasma apoA-I levels were 10% greater in women than in men, whereas mean apoA-II levels were similar. HDL size, estimated by gradient-sizing gels and by the HDL-C/apoA-I+apoA-II ratio, was significantly higher in women than in men. Mean apoA-I secretion rates (SRs) were similar in men and women (12.28+/-3.64 versus 11.96+/-2.92 mg/kg per day), whereas there was a trend toward a lower (-13%) apoA-I fractional catabolic rate (FCR) in women compared with men (0.199+/-0.037 versus 0. 225+/-0.062 pools per day, P=0.11). Mean apoA-II SRs (2.21+/-0.57 versus 2.27+/-0.91 mg/kg per day) and FCRs (0.179+/-0.034 versus 0. 181+/-0.068 pools per day) were similar in men and women. For the group as a whole, there was an inverse association between the HDL-C/apoA-I+apoA-II ratio and apoA-I FCR and between the ratio and triglyceride levels. Plasma levels of apoA-I and apoA-II were correlated with their respective SRs but not FCRs. These data suggest a major role for apoA-I and apoA-II SRs in regulating the plasma levels of these proteins, whereas apoA-I FCR might be an important factor contributing to the differences in apoA-I levels between men and postmenopausal women. Moreover, plasma triglyceride levels are important determinants of HDL size and apoA-I catabolism.