Differential effect of ultracentrifugation on apolipoprotein A-I-containing lipoprotein subpopulations

Deepening our understanding of HDL proteome

Introduction: High-density lipoprotein (HDL) particles are heterogeneous and their proteome is complex and distinct from HDL cholesterol. However, it is largely unknown whether HDL proteins are associated with cardiovascular protection. Areas covered: HDL isolation techniques and proteomic analyses are reviewed. A list of HDL proteins reported in 37 different studies was compiled and the effects of different isolation techniques on proteins attributed to HDL are discussed. Mass spectrometric techniques used for HDL analysis and the need for precise and robust methods for quantification of HDL proteins are discussed. Expert opinion: Proteins associated with HDL have the potential to be used as biomarkers and/or help to understand HDL functionality. To achieve this, large cohorts must be studied using precise quantification methods. Key factors in HDL proteome quantification are the isolation methodology and the mass spectrometry technique employed. Isolation methodology affects what proteins are identified in HDL and the specificity of association with HDL particles needs to be addressed. Shotgun proteomics yields imprecise quantification, but the majority of HDL studies relied on this approach. Few recent studies used targeted tandem mass spectrometry to quantify HDL proteins, and it is imperative that future studies focus on the application of these precise techniques.

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.

Characterization of circulating APOL1 protein complexes in African Americans

APOL1 gene renal-risk variants are associated with nephropathy and CVD in African Americans; however, little is known about the circulating APOL1 variant proteins which reportedly bind to HDL. We examined whether APOL1 G1 and G2 renal-risk variant serum concentrations or lipoprotein distributions differed from nonrisk G0 APOL1 in African Americans without nephropathy. Serum APOL1 protein concentrations were similar regardless of APOL1 genotype. In addition, serum APOL1 protein was bound to protein complexes in two nonoverlapping peaks, herein referred to as APOL1 complex A (12.2 nm diameter) and complex B (20.0 nm diameter). Neither of these protein complexes associated with HDL or LDL. Proteomic analysis revealed that complex A was composed of APOA1, haptoglobin-related protein (HPR), and complement C3, whereas complex B contained APOA1, HPR, IgM, and fibronectin. Serum HPR was less abundant on complex B in individuals with G1 and G2 renal-risk variant genotypes, relative to G0 (P = 0.0002-0.037). These circulating complexes may play roles in HDL metabolism and susceptibility to CVD.

Development of a method to measure preβHDL and αHDL apoA-I enrichment for stable isotopic studies of HDL kinetics

Our understanding of HDL metabolism would be enhanced by the measurement of the kinetics of preβHDL, the nascent form of HDL, since elevated levels have been reported in patients with coronary artery disease. Stable isotope methodology is an established technique that has enabled the determination of the kinetics (production and catabolism) of total HDL apoA-I in vivo. The development of separation procedures to obtain a preβHDL fraction, the isotopic enrichment of which could then be measured, would enable further understanding of the pathways in vivo for determining the fate of preβHDL and the formation of αHDL. A method was developed and optimised to separate and measure preβHDL and αHDL apoA-I enrichment. Agarose gel electrophoresis was first used to separate lipoprotein subclasses, and then a 4-10 % discontinuous SDS-PAGE used to isolate apoA-I. Measures of preβHDL enrichment in six healthy subjects were undertaken following an infusion of L-[1-¹³C-leucine]. After isolation of preβ and αHDL, the isotopic enrichment of apoA-I for each fraction was measured by gas chromatography-mass spectrometry. PreβHDL apoA-I enrichment was measured with a CV of 0.51 % and αHDL apoA-I with a CV of 0.34 %. The fractional catabolic rate (FCR) of preβHDL apoA-I was significantly higher than the FCR of αHDL apoA-I (p < 0.005). This methodology can be used to selectively isolate preβ and αHDL apoA-I for the measurement of apoA-I isotopic enrichment for kinetics studies of HDL subclass metabolism in a research setting.

Integrated approach for the comprehensive characterization of lipoproteins from human plasma using FPLC and nano-HPLC-tandem mass spectrometry

The implication of the various lipoprotein classes in the development of atherosclerotic cardiovascular disease has served to focus a great deal of attention on these particles over the past half-century. Using knowledge gained by the sequencing of the human genome, recent research efforts have been directed toward the elucidation of the proteomes of several lipoprotein subclasses. One of the challenges of such proteomic experimentation is the ability to initially isolate plasma lipoproteins subsequent to their analysis by mass spectrometry. Although several methods for the isolation of plasma lipoproteins are available, the most commonly utilized techniques require large sample volumes and may cause destruction and dissociation of lipoprotein particle-associated proteins. Fast protein liquid chromatography (FPLC) is a nondenaturing technique that has been validated for the isolation of plasma lipoproteins from relatively small sample volumes. In this study, we present the use of FPLC in conjunction with nano-HPLC-ESI-tandem mass spectrometry as a new integrated methodology suitable for the proteomic analysis of human lipoprotein fractions. Results from our analysis show that only 200 microl of human plasma suffices for the isolation of whole high density lipoprotein (HDL) and the identification of the majority of all known HDL-associated proteins using mass spectrometry of the resulting fractions.

HDL antielastase activity prevents smooth muscle cell anoikis, a potential new antiatherogenic property

Various studies using proteomic approaches have shown that HDL can carry many proteins other than its constitutive apolipoprotein A-I (apoA-I). Using mass spectrometry and Western blotting, we showed the presence of alpha(1)-antitrypsin (AAT) (SERPINA1, serpin peptidase inhibitor, clade A, an elastase inhibitor) in HDL, isolated either by ultracentrifugation or by selected-affinity immunosorption using an anti-apoA-I column. Furthermore, we report that HDL possesses potent antielastase activity. We further showed that only HDL but not LDL is able to bind AAT. HDL-associated AAT was able to inhibit extracellular matrix degradation, cell detachment, and apoptosis induced by elastase in human vascular smooth muscle cells (VSMCs) and in mammary artery cultured ex vivo. Degradation of fibronectin by elastase used as a marker of pericellular proteolysis was prevented by addition of HDL. Elastase present in aortic abdominal aneurysm (AAA) thrombus samples was also able to induce apoptosis of VSMCs in culture. This phenomenon was prevented by addition of HDL but not of LDL. Finally, we report that the proportion of AAT in HDL isolated from patients with an AAA is decreased relative to that from matched control subjects, suggesting a reduced capacity of HDL to inhibit elastase in these patients. In conclusion, our data provide evidence of a new potential antiatherogenic property of HDL attributable to AAT and its antielastase activity.

Shotgun proteomics implicates protease inhibition and complement activation in the antiinflammatory properties of HDL

HDL lowers the risk for atherosclerotic cardiovascular disease by promoting cholesterol efflux from macrophage foam cells. However, other antiatherosclerotic properties of HDL are poorly understood. To test the hypothesis that the lipoprotein carries proteins that might have novel cardioprotective activities, we used shotgun proteomics to investigate the composition of HDL isolated from healthy subjects and subjects with coronary artery disease (CAD). Unexpectedly, our analytical strategy identified multiple complement-regulatory proteins and a diverse array of distinct serpins with serine-type endopeptidase inhibitor activity. Many acute-phase response proteins were also detected, supporting the proposal that HDL is of central importance in inflammation. Mass spectrometry and biochemical analyses demonstrated that HDL3 from subjects with CAD was selectively enriched in apoE, raising the possibility that HDL carries a unique cargo of proteins in humans with clinically significant cardiovascular disease. Collectively, our observations suggest that HDL plays previously unsuspected roles in regulating the complement system and protecting tissue from proteolysis and that the protein cargo of HDL contributes to its antiinflammatory and antiatherogenic properties.

Polyacrylamide gradient gel electrophoresis of lipoprotein subclasses

High-density (HDL), low-density (LDL), and very-low-density (VLDL) lipoproteins are heterogeneous cholesterol-containing particles that differ in their metabolism, environmental interactions, and association with disease. Several protocols use polyacrylamide gradient gel electrophoresis (GGE) to separate these major lipoproteins into known subclasses. This article provides a brief history of the discovery of lipoprotein heterogeneity and an overview of relevant lipoprotein metabolism, highlighting the importance of the subclasses in the context of their metabolic origins, fates, and clinical implications. Various techniques using polyacrylamide GGE to assess HDL and LDL heterogeneity are described, and how the genetic and environmental determinations of HDL and LDL affect lipoprotein size heterogeneity and the implications for cardiovascular disease are outlined.

HDLs in apoA-I transgenic Abca1 knockout mice are remodeled normally in plasma but are hypercatabolized by the kidney

Patients homozygous for Tangier disease have a near absence of plasma HDL as a result of mutations in ABCA1 and hypercatabolize normal HDL particles. To determine the relationship between ABCA1 expression and HDL catabolism, we investigated intravascular remodeling, plasma clearance, and organ-specific uptake of HDL in mice expressing the human apolipoprotein A-I (apoA-I) transgene in the Abca1 knockout background. Small HDL particles (7.5 nm), radiolabeled with (125)I-tyramine cellobiose, were injected into recipient mice to quantify plasma turnover and the organ uptake of tracer. Small HDL tracer was remodeled to 8.2 nm diameter particles within 5 min in human apolipoprotein A-I transgenic (hA-I(Tg)) mice (control) and knockout mice. Decay of tracer from plasma was 1.6-fold more rapid in knockout mice (P < 0.05) and kidney uptake was twice that of controls, with no difference in liver uptake. We also observed 2-fold greater hepatic expression of ABCA1 protein in hA-I(Tg) mice compared with nontransgenic mice, suggesting that overexpression of human apoA-I stabilized hepatic ABCA1 protein in vivo. We conclude that ABCA1 is not required for in vivo remodeling of small HDLs to larger HDL subfractions and that the hypercatabolism of normal HDL particles in knockout mice is attributable to a selective catabolism of HDL apoA-I by the kidney.

Prebeta high density lipoprotein has two metabolic fates in human apolipoprotein A-I transgenic mice

We compared the in vivo metabolism of prebeta HDL particles isolated by anti-human apolipoprotein A-I (apoA-I) immunoaffinity chromatography (LpA-I) in human apoA-I transgenic (hA-I Tg) mice with that of lipid-free apoA-I (LFA-I) and small LpA-I. After injection, prebeta LpA-I were removed from plasma more rapidly than were LFA-I and small LpA-I. Prebeta LpA-I and LFA-I were preferentially degraded by kidney compared with liver; small LpA-I were preferentially degraded by the liver. Five minutes after tracer injection, 99% of LFA-I in plasma was found to be associated with medium-sized (8.6 nm) HDL, whereas only 37% of prebeta tracer remodeled to medium-sized HDL. Injection of prebeta LpA-I doses into C57Bl/6 recipients resulted in a slower plasma decay compared with hA-I Tg recipients and a greater proportion (>60%) of the prebeta radiolabel that was associated with medium-sized HDL. Prebeta LpA-I contained one to four molecules of phosphatidylcholine per molecule of apoA-I, whereas LFA-I contained less than one. We conclude that prebeta LpA-I has two metabolic fates in vivo, rapid removal from plasma and catabolism by kidney or remodeling to medium-sized HDL, which we hypothesize is determined by the amount of lipid associated with the prebeta particle and the particle's ability to bind to medium-sized HDL.

The differential apoA-I enrichment of prebeta1 and alphaHDL is detectable by gel filtration separation

The aim of the study was to assess the isolation of HDL by fast protein liquid chromatography (FPLC) to perform kinetics studies of apolipoprotein (apo)A-I-HDL labelled with a stable isotope. Comparison between FPLC and ultracentrifugation has been made. ApoA-I-HDL kinetics were studied by infusion of [5.5.5-(2)H(3)]leucine for 14 h in five subjects. Using FPLC, prebeta(1) HDL and alphaHDL (HDL(2) and HDL(3)) were separated from 200 microl of plasma samples. Total HDL was isolated by sequential ultracentrifugation (HDL-UC). The tracer-to-tracee ratio was higher in prebeta(1) HDL than in total HDL-UC. The higher leucine enrichment found in total HDL-UC compared to alphaHDL suggested the existence of a mixture of apoA-I-HDL sub-classes. From this difference in enrichments, the turnover rate of total HDL-UC, usually assumed to be alphaHDL, was probably overestimated in previous studies. To our knowledge, this study is the first report which provides a convenient tool to distinguish enrichments of apoA-I in prebeta(1) HDL and alphaHDL from total HDL previously used for kinetic measurements. This original and new method should help to understand the kinetics of HDL in humans and the reverse cholesterol transport dynamics.

Distribution and kinetics of lipoprotein-bound endotoxin

Lipopolysaccharide (LPS), the major glycolipid component of gram-negative bacterial outer membranes, is a potent endotoxin responsible for pathophysiological symptoms characteristic of infection. The observation that the majority of LPS is found in association with plasma lipoproteins has prompted the suggestion that sequestering of LPS by lipid particles may form an integral part of a humoral detoxification mechanism. Previous studies on the biological properties of isolated lipoproteins used differential ultracentrifugation to separate the major subclasses. To preserve the integrity of the lipoproteins, we have analyzed the LPS distribution, specificity, binding capacity, and kinetics of binding to lipoproteins in human whole blood or plasma by using high-performance gel permeation chromatography and fluorescent LPS of three different chemotypes. The average distribution of O111:B4, J5, or Re595 LPS in whole blood from 10 human volunteers was 60% (+/-8%) high-density lipoprotein (HDL), 25% (+/-7%) low-density lipoprotein, and 12% (+/-5%) very low density lipoprotein. The saturation capacity of lipoproteins for all three LPS chemotypes was in excess of 200 microg/ml. Kinetic analysis however, revealed a strict chemotype dependence. The binding of Re595 or J5 LPS was essentially complete within 10 min, and subsequent redistribution among the lipoprotein subclasses occurred to attain similar distributions as O111:B4 LPS at 40 min. We conclude that under simulated physiological conditions, the binding of LPS to lipoproteins is highly specific, HDL has the highest binding capacity for LPS, the saturation capacity of lipoproteins for endotoxin far exceeds the LPS concentrations measured in clinical situations, and the kinetics of LPS association with lipoproteins display chemotype-dependent differences.

Very Small Apolipoprotein A-I-containing Particles from Human Plasma: Isolation and Quantification by High-Performance Size-Exclusion Chromatography

Background: Very small apolipoprotein (apo) A-I-containing lipoprotein (Sm LpA-I) particles with pre-β electrophoretic mobility may play key roles as “nascent” and/or “senescent” HDL; however, methods for their isolation are difficult and often semiquantitative.

Methods: We developed a preparative method for separating Sm LpA-I particles from human plasma by high-performance size-exclusion chromatography (HP-SEC), using two gel permeation columns (Superdex 200 and Superdex 75) in series and measuring apo A-I content in column fractions in 30 subjects with HDL-cholesterol (HDL-C) concentrations of 0.4–3.83 mmol/L.

Results: Three major sizes of apo A-I-containing particles were detected: an ∼15-nm diameter (∼700 kDa) species; a 7.5–12 nm (100–450 kDa) species; and a 5.8–6.3 nm species (40–60 kDa, Sm LpA-I particles), containing 0.2–3%, 80–96%, and 2–15% of plasma total apo A-I, respectively. Two subjects with severe HDL deficiency had increased relative apo A-I content in Sm LpA-I: 25% and 37%, respectively. The percentage of apo A-I in Sm LpA-I correlated positively with fasting plasma triglyceride concentrations (r = 0.581; P &lt;0.0005) and inversely with total apo A-I (r = −0.551; P &lt;0.0013) and HDL-C concentrations (r = −0.532; P &lt;0.0017), although the latter two relationships were largely attributable to extremely hypoalphalipoproteinemic subjects. The percentage of apo A-I in Sm LpA-I correlated with that in pre-β-migrating species by crossed immunoelectrophoresis (r = 0.98; P &lt;0.0001; n = 24) and with that in the d &gt;1.21 kg/L fraction by ultracentrifugation (r = 0.86; P &lt;0.001; n = 20). Sm LpA-I particles, on average, appear to contain two apo A-I and four phospholipid molecules but little or no apo A-II, triglyceride, or cholesterol.

Conclusions: We present a new HP-SEC method for size separation of native HDL particles from plasma, including Sm Lp A-I, which may play important roles in the metabolism of HDL and in its contribution(s) to protection against atherosclerosis. This method provides a basis for further studies of the structure and function of Sm Lp A-I.

Metabolism of high density lipoprotein subfractions

Over the past few years, new experimental approaches have reinforced the awareness among investigators that the heterogeneity of HDL particles indicates significant differences in production and catabolism of HDL particles. Recent kinetic studies have suggested that small HDL, containing two apolipoprotein A-I molecules per particle, are converted in a unidirectional manner to medium HDL or large HDL, containing three or four apolipoprotein A-I molecules per particle, respectively. Conversion appears to occur in close physical proximity with cells and not while HDL particles circulate in plasma. The medium and large HDL are terminal particles in HDL metabolism with large HDL, and perhaps medium HDL, being catabolized primarily by the liver. These novel kinetic studies of HDL subfraction metabolism are compelling in-vivo data that are consistent with the proposed role of HDL in reverse cholesterol transport.

Apolipoprotein L, a new human high density lipoprotein apolipoprotein expressed by the pancreas. Identification, cloning, characterization, and plasma distribution of apolipoprotein L

In this study, we have identified and characterized a new protein present in human high density lipoprotein that we have designated apolipoprotein L. Using a combination of liquid-phase isoelectrophoresis and high resolution two-dimensional gel electrophoresis, apolipoprotein L was identified and partially sequenced from immunoisolated high density lipoprotein (Lp(A-I)). Expression was only detected in the pancreas. The cDNA sequence encoding the full-length protein was cloned using reverse transcription-polymerase chain reaction. The deduced amino acid sequence contains 383 residues, including a typical signal peptide of 12 amino acids. No significant homology was found with known sequences. The plasma protein is a single chain polypeptide with an apparent molecular mass of 42 kDa. Antibodies raised against this protein detected a truncated form with a molecular mass of 39 kDa. Both forms were predominantly associated with immunoaffinity-isolated apoA-I-containing lipoproteins and detected mainly in the density range 1.123 < d < 1.21 g/ml. Free apoL was not detected in plasma. Anti-apoL immunoaffinity chromatography was used to purify apoL-containing lipoproteins (Lp(L)) directly from plasma. Nondenaturing gel electrophoresis of Lp(L) showed two major molecular species with apparent diameters of 12.2-17 and 10.4-12.2 nm. Moreover, Lp(L) exhibited both pre-beta and alpha electromobility. Apolipoproteins A-I, A-II, A-IV, and C-III were also detected in the apoL-containing lipoprotein particles.

Abnormal reverse cholesterol transport in controlled type II diabetic patients. Studies on fasting and postprandial LpA-I particles

The high incidence and prevalence of coronary heart disease in diabetes mellitus is clearly established. The usual lipid pattern found in type II diabetic patients is a moderate increase in fasting triglyceride levels associated with low HDL cholesterol levels. These abnormalities are further amplified in the postprandial state. To study the effect of these alterations on reverse cholesterol transport, we isolated lipoprotein containing apoA-I but not apoA-II (LpA-I) particles by immunoaffinity chromatography from the plasma of well-controlled type II diabetic patients and nondiabetic matched control subjects. Different parameters involved in this antiatherogenic pathway were measured in both fasting and postprandial states. Diabetic patients had reduced levels of LpA-I particles that were protein enriched and phospholipid depleted. Gradient gel electrophoresis showed that control LpA-I particles had five distinct populations, whereas diabetic particles lacked the largest one. LpA-I isolated from diabetic plasma exhibited a decreased capacity to induce cholesterol efflux from Ob 1771 adipose cells both in fasting (15.1 +/- 10.0% versus 7.5 +/- 2.7%, P < .05) and postprandial (17.7 +/- 11.2% versus 7.7 +/- 3.9%, P < .05) states, whereas only control particles showed significantly higher ability to promote cholesterol efflux after the test meal (P = .02). Lecithin:cholesterol acyltransferase activity measured with an exogenous substrate showed a 54% increase and an 18% decrease postprandially for control subjects and patients, respectively. Thus, the different abnormalities found in the fasting state were further amplified in the postprandial situation. This resulted in LpA-I particles with aberrant size and composition and decreased ability to accomplish their antiatherogenic role in type II diabetic patients.

Chromatographic techniques for the isolation and purification of lipoproteins

Various modes of chromatography are available for lipoprotein separation. Gel permeation and affinity chromatography are used for preparative purposes and to separate lipoproteins according to size and apolipoprotein content, respectively. Development of rigid supports for gel permeation has led to large improvements in speed and resolution. Reversed-phase high-performance liquid chromatography (HPLC) of apolipoproteins offers the best performance in terms of speed and resolution of structural variants. Due to its high speed and superior resolving power, the recently developed technique of capillary electrophoresis should emerge as an important method for lipoprotein analysis.

Functional expression of human and mouse plasma phospholipid transfer protein: effect of recombinant and plasma PLTP on HDL subspecies

The molecular cloning of mouse plasma phospholipid transfer protein (PLTP) and the eukaryotic cell expression of complementary DNA for mouse and human PLTP are described. Mouse PLTP was found to share 83% amino acid sequence identity with human PLTP. PLTP was produced in baby hamster kidney cells. Conditioned medium from BHK cells expressing PLTP possessed both phospholipid transfer activity and high density lipoprotein (HDL) conversion activity. PLTP mRNA was detected in all 16 human tissues examined by Northern blot analysis with ovary, thymus, and placenta having the highest levels. PLTP mRNA was also examined in eight mouse tissues with the highest PLTP mRNA levels found in the lung, brain, and heart. The effect of purified human plasma-derived PLTP and human recombinant PLTP (rPLTP) on the two human plasma HDL subspecies Lp(A-I) and Lp(A-I/A-II) was evaluated. Plasma PLTP or rPLTP converted the two distinct size subspecies of Lp(A-I) into a larger species, an intermediate species, and a smaller species. Lp(A-I/A-II) particles containing multiple size subspecies were significantly altered by incubation with either plasma or rPLTP with the largest but less prominent subspecies becoming the predominant one, and the smallest subspecies increasing in concentration. Thus, PLTP promoted the conversion of both Lp(A-I) and Lp(A-I/A-II) to populations of larger and smaller particles. Also, both human PLTP and mouse rPLTP were able to convert human or mouse HDL into larger and smaller particles. These observations suggest that PLTP may play a key role in extracellular phospholipid transport and modulation of HDL particles.

Biochemical characterization of the three major subclasses of lipoprotein A-I preparatively isolated from human plasma

Apolipoprotein (apo) A-I is the major protein constituent of plasma high-density lipoproteins (HDL). HDL consist of two major classes of apoA-I-containing lipoproteins: LpA-I and LpA-I:A-II. LpA-I includes heterogeneous lipoprotein particles that differ in size and hydrated density. LpA-I was isolated by immunoaffinity chromatography from the fasting plasma of 24 normal human subjects and separated by gel filtration chromatography. Three major subclasses of LpA-I were eluted: large (Lg-LpA-I), medium (Md-LpA-I), and small LpA-I (Sm-LpA-I). By nondenaturing gradient PAGE, Lg-LpA-I, Md-LpA-I, and Sm-LpA-I had mean Strokes diameters of 10.8 +/- 0.5, 8.9 +/- 0.5, and 7.5 +/- 0.3 nm, respectively. The lipid/protein ratios were 1.25 +/- 0.12 for Lg-LpA-I, 0.75 +/- 0.10 for Md-LpA-I, and 0.38 +/- 0.08 for Sm-LpA-I. Lg-LpA-I was relatively lipid and cholesteryl ester rich compared with Md-LpA-I and Sm-LpA-I. Sm-LpA-I contained phospholipids as the major lipid component. ApoA-I was the major apolipoprotein in all LpA-I subfractions, whereas apoE was present only in Lg-LpA-I and apoA-IV was associated with both Md-LpA-I and Sm-LpA-I. All three LpA-I subclasses exhibited predominantly alpha mobility on agarose electrophoresis. Lg-LpA-I migrated as a diffuse band in the fast alpha position, whereas Md-LpA-I and Sm-LpA-I migrated to the slow alpha position.(ABSTRACT TRUNCATED AT 250 WORDS)

Preparation and characterization of spheroidal, reconstituted high-density lipoproteins with apolipoprotein A-I only or with apolipoprotein A-I and A-II

This study describes the preparation of spheroidal reconstituted HDL which contain apolipoprotein (apo) A-I only, (A-I w/o A-II) r-HDL, or apo A-I and apo A-II, (A-I w A-II) r-HDL. Spheroidal (A-I w/o A-II) r-HDL with diameters of 8.0, 9.2 and 11.2 nm were prepared by incubating discoidal (A-I w/o A-II) r-HDL with lecithin-cholesterol acyltransferase and low-density lipoproteins. Spheroidal (A-I w A-II) r-HDL were prepared by displacing apo A-I from spheroidal (A-I w/o A-II) r-HDL with apo A-II. Modification with apo A-II did not significantly affect the diameters of the 8.0 and 9.2 nm (A-I w/o A-II) r-HDL. When, however, apo A-II was added to the (A-I w/o A-II) r-HDL of diameter 11.2 nm, the size of the particles decreased to 9.4 nm. To determine whether modification of (A-I w/o A-II) r-HDL with apo A-II altered the structure of the r-HDL, the packing of phospholipids in the modified and unmodified particles was compared by steady state fluorescence polarization and the environments of the apo A-I tryptophan residues in (A-I w/o A-II) and (A-I w A-II) r-HDL were compared by fluorescence spectroscopy. The results of these studies suggested that modification of spheroidal (A-I w/o A-II) r-HDL with apo A-II alters the environment of apo A-I tryptophan residues in small, but not large, r-HDL and does not affect the packing of phospholipids.

Effects of high-density lipoprotein particles containing apo A-I, with or without apo A-II, on intracellular cholesterol efflux

Previous reports have shown a differential effect of high-density lipoprotein (HDL) particles which contain apolipoprotein (apo) A-I without apo A-II (Lp A-I) and particles containing both apo A-I and apo A-II (Lp A-I/A-II) on cholesterol efflux from the mouse adipocyte cell line Ob1771, with Lp A-I and Lp A-I/A-II being active and inactive cholesterol efflux promotors, respectively. The present study was conducted to examine the roles of these two populations of apo-specific HDL particles on reverse cholesterol transport from cholesterol-loaded human skin fibroblasts and bovine aortic endothelial cells. The ability of HDL particles to remove intracellular cholesterol was tested by measuring depletion of the substrate pool for acylCoA:cholesterol acyltransferase (ACAT) and efflux of newly synthesized cholesterol, while removal of plasma membrane cholesterol was assessed by measuring efflux of [3H]cholesterol from prelabeled cells. Lp A-I and Lp A-I/A-II isolated from HDL2, HDL3 or plasma by immunoaffinity techniques each decreased esterification of cholesterol by both fibroblasts and endothelial cells. A mixture of Lp A-I and Lp A-I/A-II isolated from HDL3 decreased cholesterol esterification by fibroblasts in an additive manner, thus demonstrating that Lp A-I/A-II did not inhibit Lp A-I-mediated cholesterol efflux. Both Lp A-I and Lp A-I/A-II promoted efflux of sterol newly synthesized by fibroblasts, and no significant differences were observed between the apo-specific particles. Apo-specific particles were also similarly effective at preventing the accumulation of LDL-derived cholesterol in cholesterol-depleted fibroblasts. Efflux of [3H]cholesterol from plasma membranes was stimulated to similar extents by Lp A-I and Lp A-I/A-II isolated from either HDL2, HDL3 or plasma. Thus, the apo-specific HDL particles Lp A-I and Lp A-I/A-II are both effective promoters of cholesterol efflux from fibroblasts and aortic endothelial cells.

Protein transfer between A-I-containing lipoprotein subpopulations: evidence of non-transferable A-I in particles with A-II

Transfer of apolipoproteins (apo) between the two subpopulations of apo A-I-containing lipoproteins in human plasma: those with A-II [Lp(AI w AII)] and those without [Lp(AI w/o AII)], were studied by observing the transfer of 125I-apo from a radiolabeled subpopulation to an unlabeled subpopulation in vitro. When Lp(AI w AII) was directly radioiodinated, 50.3 +/- 7.4 and 19.5 +/- 7.7% (n = 6) of the total radioactivity was associated with A-I and A-II, respectively. In radioiodinated Lp(AI w/o AII), 71.5 +/- 6.8% (n = 6) of the total radioactivity was A-I-associated. Time-course studies showed that, while some radiolabeled proteins transferred from one population of HDL particles to another within minutes, at least several hours were necessary for transfer to approach equilibrium. Incubation of the subpopulations at equal A-I mass resulted in the transfer of 51.8 +/- 5.0% (n = 4) of total radioactivity from [125I]Lp(AI w/o AII) to Lp(AI w AII) at 37 degrees C in 24 h. The specific activity (S.A.) of A-I in the two subpopulations after incubation was nearly identical. Under similar incubation conditions, only 13.4 +/- 4.6% (n = 4) of total radioactivity was transferred from [125I]Lp(AI w AII) to Lp(AI w/o AII). The S.A. of A-I after incubation was 2-fold higher in particles with A-II than in particles without A-II. These phenomena were also observed with iodinated high-density lipoproteins (HDL) isolated by ultracentrifugation and subsequently subfractionated by immunoaffinity chromatography. However, when Lp(AI w AII) radiolabeled by in vitro exchange with free [125I]A-I was incubated with unlabeled Lp(AI w/o AII), the S.A. of A-I in particles with and without A-II differed by only 18% after incubation. These data are consistent with the following: (1) in both populations of HDL particles, some radiolabeled proteins transferred rapidly (minutes or less), while others transferred slowly (hours); (2) when Lp(AI w AII) and Lp(AI w/o AII) were directly iodinated, all labeled A-I in particles without A-II were transferable, but some labeled AI in particles with A-II were not; (3) when Lp(AI w AII) were labeled by in vitro exchange with [125I]A-I, considerably more labeled A-I were transferable. These observations suggest the presence of non-transferable A-I in Lp(AI w AII).

Effects of intralipid-induced hypertriglyceridemia on plasma high-density lipoprotein metabolism in the cynomolgus monkey

Low plasma levels of high-density lipoprotein (HDL) and apolipoprotein (apo) A-I often accompany human hypertriglyceridemia. In an animal model of hypertriglyceridemia, the lipoprotein lipase (LPL)-inhibited cynomolgus monkey, we reported that plasma levels of apo A-I were decreased and the fractional catabolic rate (FCR) of HDL apo was increased. To explore whether hypertriglyceridemia alone would alter plasma apo A-I levels and catabolism, hypertriglyceridemia was produced by intravenous (IV) infusion of 20% Intralipid into female cynomolgus monkeys. Baseline plasma triglyceride (TG) levels averaged 106 mg/dL. With infusion of 200 mg/kg/h Intralipid TG, plasma TG levels peaked at 967 mg/dL (range, 413 to 1,069; n = 6). More prolonged or more severe hypertriglyceridemia caused serious complications in several monkeys. Despite the severe hypertriglyceridemia, HDL TG content, HDL apoproteins, and plasma apo A-I levels did not markedly change, suggesting that very little HDL remodeling had occurred. Kinetic studies of HDL protein and apo A-I were performed in four pairs of monkeys. The two tracers were removed from the plasma at identical rates. In five pairs of animals, apo A-I turnover during control and Intralipid-induced hypertriglyceridemia was not significantly different. We hypothesize that apo A-I FCR is a function of HDL composition. Because Intralipid infusion did not alter HDL composition to the same degree as did LPL inhibition, its effects on HDL apo catabolism were not apparent.

Interaction between high-density lipoprotein subpopulations in apo B-free and abetalipoproteinemic plasma

Two populations of high-density lipoprotein (HDL) particles exist in human plasma. Both contain apolipoprotein (apo) A-I, but only one contains apo A-II: Lp(AI w AII) and Lp(AI w/o AII). To study the extent of interaction between these particles, apo B-free plasma prepared by the selective removal of apo B-containing lipoproteins (LpB) from the plasma of three normolipidemic (NL) subjects and whole plasma from two patients with abetalipoproteinemia (ABL) were incubated at 37 degrees C for 24 h. Apo B-free plasma samples were used to avoid lipid-exchange between HDL and LpB. Lp(AI w AII) and Lp(AI w/o AII) were isolated from each apo B-free plasma sample before and after incubation and their protein and lipid contents quantified. Before incubation, ABL plasma had reduced levels of Lp(AI w AII) and Lp(AI w/o AII), (40% and 70% of normals, respectively). Compared to the HDL of apo B-free NL plasma, ABL HDL had higher relative contents of free cholesterol, phospholipid and total lipid, and contained more particles with apparent hydrated Stokes diameter in the 9.2-17.0 nm region. These differences were particularly pronounced in particles without apo A-II. Despite their differences, the total cholesterol contents of Lp(AI w AII) increased, while that of Lp(AI w/o AII) decreased in all five plasma samples and the amount of apo A-I in Lp(AI w AII) increased by 6-8 mg/dl in four during the incubation. These compositional changes were accompanied by a relative reduction of particles in the 7.0-8.2 nm Stokes diameter size region and an increase of particles in the 9.2-11.2 nm region. These data are consistent with intravascular modulation between HDL particles with and without apo A-II. The observed increase in apo A-II-associated cholesterol and apo A-I, could involve either the transfer of cholesterol and apo A-I from particles without apo A-II to those with A-II, or the transfer of apo A-II from Lp(AI w AII) to Lp(AI w/o AII). The exact mechanism and direction of the transfer remain to be determined.

Metabolism of HDL apolipoprotein A-I and A-II in type 1 (insulin-dependent) diabetes mellitus

Concentrations of HDL cholesterol and apolipoprotein A-I are commonly increased in Type 1 (insulin-dependent) diabetes mellitus but the mechanisms whereby diabetes influences HDL metabolism have not been studied. We investigated the metabolism of HDL apoproteins A-I and II in normolipidaemic Type 1 diabetic men (n = 17, HbA1 6.4-11.9%) without microalbuminuria but with a wide range of HDL cholesterol (0.85-2.10 mmol/l) and in nondiabetic men (n = 18) matched for body mass index and the range of HDL cholesterol. Input rates and fractional catabolic rates for apolipoproteins A-I and II were determined following injection of 125I-apolipoprotein A-I and 131I-apolipoprotein A-II tracers. Additional multicompartmental analysis was performed using a model to describe the kinetics of HDL particles containing only apolipoprotein A-I (Lp A-I) and apolipoprotein A-I and apolipoprotein A-II (Lp A-I/A-II). No gross differences from normal subjects were observed in the mean levels of lipids, lipoproteins, apoproteins and the lipolytic enzymes in the diabetic men as a result of the selection process. Furthermore, the relationship between apolipoprotein A kinetics and plasma HDL cholesterol levels appeared to be preserved in the diabetic group. However, some normal interrelationships were disrupted in the diabetic men. Firstly, the rate of apolipoprotein A-II synthesis was 22% lower than in control subjects (p less than 0.05). Modelling indicated that this was due to decreased input of Lp A-I/A-II particles whereas the input of Lp A-I particles was similar in the two groups. Secondly, there was no correlation between VLDL triglyceride and HDL cholesterol or VLDL triglyceride and the fractional catabolic rate of apolipoproteins A-I and A-II in diabetic men in contrast to that seen in control subjects. We conclude that there is a disruption in the normal association between VLDL and HDL metabolism in Type 1 diabetic men and postulate that the observed differences may be due to the therapeutic use of exogenous insulin.

The very-high-density lipoprotein fraction of rabbit plasma is rich in tissue-derived cholesterol

When plasma from rabbits, which several weeks earlier had been infused with [3H]cholesterol, was subjected to equilibrium density gradient ultracentrifugation, the specific radioactivity of cholesterol in the very-high-density lipoprotein (VHDL) fraction (d 1.22-1.32 g/ml) was three to 8-fold greater (mean, 5.5-fold; P less than 0.001) than that in high-density lipoproteins (HDL; d 1.06-1.21 g/ml). On size exclusion chromatography of plasma, no increase in specific radioactivity was seen in particles smaller than HDL. These findings suggest that those apolipoprotein-lipid complexes that dissociate from HDL during ultracentrifugation to form the VHDL fraction contain proportionately more tissue-derived cholesterol than do those that are more tightly bound to HDL.

Differential role of apolipoprotein AI-containing particles in cholesterol efflux from adipose cells

Cholesterol efflux was studied in cultured Ob1771 adipose cells after preloading with LDL cholesterol. Exposure to particles containing apo AII (LpAI) and particles containing apo AI and apo AII (LpAI:AII) isolated from native human plasma, and from HDL2 or HDL3, showed that only LpAI were able to promote cholesterol efflux, despite the fact that both kinds of particles were able to bind to receptor sites within the same range of concentrations (apparent Kd values between 10 and 25 micrograms/ml). During this long-term exposure, LpAI:AII demonstrated a concentration-dependent inhibition (10-60 micrograms/ml) of LpAI-mediated cholesterol efflux from adipose cells under conditions where LpAI:AII did not deliver cholesterol to the cells and where no net change in the distribution of apo AI between LpAI and LpAI:AII was observed. The antagonizing and modulating role of LpAI:AII in preventing cholesterol efflux mediated by LpAI appears not to be related to the lipid composition and cholesterol content of the particles but, rather, appears dependent upon the presence of apo AI in LpAI particles and apo AII in LpAI:AII particles. The actual concentrations of these particles in the interstitial fluid bathing peripheral cells might be critical for the in vivo occurrence of cholesterol efflux.

Characterization of human high-density lipoprotein subclasses LP A-I and LP A-I/A-II and binding to HepG2 cells

Plasma HDL can be classified according to their apolipoprotein content into at least two types of lipoprotein particles: lipoproteins containing both apo A-I and apo A-II (LP A-I/A-II) and lipoproteins with apo A-I but without apo A-II (LP A-I). LP A-I and LP A-I/A-II were isolated by immuno-affinity chromatography. LP A-I has a higher cholesterol content and less protein compared to LP A-I/A-II. The average particle mass of LP A-I is higher (379 kDa) than the average particle weight of LP A-I/A-II (269 kDa). The binding of 125I-LP A-I to HepG2 cells at 4 degrees C, as well as the uptake of [3H]cholesteryl ether-labelled LP A-I by HepG2 cells at 37 degrees C, was significantly higher than the binding and uptake of LP A-I/A-II. It is likely that both binding and uptake are mediated by apo A-I. Our results do not provide evidence in favor of a specific role for apo A-II in the binding and uptake of HDL by HepG2 cells.

Immunofractionation of high density lipoprotein subclasses 2 and 3. Similarities and differences of fractions isolated from male and female populations

High density lipoprotein (HDL) subclasses 2 and 3 isolated from male and female populations were further subfractionated by immunoaffinity techniques. Each subclass gave rise to 2 fractions: one contained apolipoprotein (apo) A-I but no apo A-II (LpAI); the other contained apo A-I and apo A-II (LpAI,AII). The bulk fraction (HDL-3(LpAI,AII)) comprised over 70% of total HDL and was present in similar concentrations in both populations. There were, however, significant male-female differences in plasma levels of the minor HDL-3 fraction i.e. HDL-3(LpAI). Females had significantly higher plasma concentrations of both fractions within HDL-2. These fractions also exhibited strong, positive correlations with total HDL cholesterol concentrations, both in males as well as females. It suggests that metabolic activities giving rise to both HDL-2(LpAI) and HDL-2(LpAI,AII) determine plasma HDL cholesterol concentrations. Several similarities were noted between the male and female populations. Triglyceridaemia was negatively correlated with HDL-2 derived fractions and positively correlated with the bulk fraction HDL-3(LpAI,AII). Compositional data showed that the fraction (LpAI) had a lower esterified cholesterol to total cholesterol ratio than the fraction (LpAI,AII), the differences being more apparent at the HDL-3 level. Additionally, analysis of the surface components of HDL-3 fractions suggested that (LpAI,AII) had a greater potential than (LpAI) for absorbing lipoprotein surface material. Finally, the relative concentrations of the individual components of fractions within the same population and defined by the same apolipoprotein criterion showed highly significantly, positive correlations. Such correlations were not apparent for apolipoprotein dissimilar fractions. These observations could reflect a metabolic link between apolipoprotein similar fractions.

Effects of age and physical performance capacity on distribution and composition of high-density lipoprotein subfractions in men

The influences of age and maximal aerobic capacity (VO2max) on serum lipoproteins with special regard to the concentration, composition and distribution of high density lipoprotein (HDL) subfractions were investigated in 51 healthy males of different characteristics: younger than 35 years, untrained (n = 14, mean age 28.2 years, SD 6.0; VO2max, 47.9 ml.kg-1.min-1, SD 5.8) and trained (n = 11, mean age 27.9 years, SD 4.3; VO2max, 61.1 ml.kg-1.min-1, SD 5.1), older than 50 years untrained (n = 14, mean age 58.9 years, SD 5.9, VO2max, 29.3 ml.kg-1.min-1, SD 5.3) and trained (n = 12, mean age 59.3 years, SD 7.2, VO2max, 45.7 ml.kg-1.min-1, SD 7.7). The fasting-state serum concentrations of total cholesterol, tri-acylglycerol and lipoprotein-cholesterol were measured. The HDL-subfractions were separated by density (rho) gradient ultracentrifugation. Concentrations of cholesterol, cholesterylester, tri-acylglycerol, phospholipids, apolipoprotein (apo) A-I and A-II were measured in the subfractions HDL2b: rho = 1.063-1.100 g.ml-1; HDL2al: rho = 1.00-1.110 g.ml-1; HDL2a2: rho = 1.110-1.150 g.ml-1; HDL3: rho = 1.150-1.210 g.ml-1. Elderly untrained subjects showed increased serum concentrations of total-, very low- and low density lipoprotein-cholesterol and elevated tri-acylglycerol levels. The HDL-cholesterol concentration was decreased, due to reduced concentrations of HDL2-subfractions. Significant changes in the composition of HDL2-subfractions were found in elderly untrained subjects. The HDL2-subfractions had more protein, a decreased apoA-I:A-II ratio and less phospholipids in comparison to HDL2-subfractions from younger untrained and trained, and elderly trained subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Immunoaffinity fractionation of high-density lipoprotein subclasses 2 and 3 using anti-apolipoprotein A-I and A-II immunosorbent gels

High-density lipoprotein (HDL) subclasses 2 and 3 prepared by density gradient ultracentrifugation have been further fractionated by immunoaffinity chromatography using antibody affinity gels targetting the major HDL apolipoproteins, A-I and A-II. Fractions containing A-I without A-II (AI w/o AII) and A-I with A-II (AI w AII) were isolated from both density ranges. Whereas there were similar concentrations of the major subfraction (HDL3(AI w AII] in both males and females, the remaining subfractions were present in higher concentrations in females as compared to males, in the order HDL3 (AI w/o AII) less than HDL2(AI w AII) less than HDL2(AI w/o AII). The difference was most marked for HDL2 (AI w/o AII), where plasma concentrations in females were almost 3-fold greater than in males. Compositional analyses indicated that the plasma concentrations of the fractions, rather than their compositions, were the major determinants of male-female differences in HDL levels. In contrast, fractions defined by similar apolipoprotein criteria and isolated from different density subclasses (i.e., HDL2(AI w/o AII) vs. HDL3(AI w/o AII) and HDL2(AI w AII) vs. HDL3(AI w AII] showed major compositional differences. This is suggestive of distinct lipoprotein particles.