Speciation of Human Plasma High-Density Lipoprotein (HDL): HDL Stability and Apolipoprotein A-I Partitioning

Understanding HDL Metabolism and Biology Through In Vivo Tracer Kinetics

HDL (high-density lipoprotein), owing to its high protein content and small size, is the densest circulating lipoprotein. In contrast to lipid-laden VLDL (very-low-density lipoprotein) and LDL (low-density lipoprotein) that promote atherosclerosis, HDL is hypothesized to mitigate atherosclerosis via reverse cholesterol transport, a process that entails the uptake and clearance of excess cholesterol from peripheral tissues. This process is mediated by APOA1 (apolipoprotein A-I), the primary structural protein of HDL, as well as by the activities of additional HDL proteins. Tracer-dependent kinetic studies are an invaluable tool to study HDL-mediated reverse cholesterol transport and overall HDL metabolism in humans when a cardiovascular disease therapy is investigated. Unfortunately, HDL cholesterol-raising therapies have not been successful at reducing cardiovascular events suggesting an incomplete picture of HDL biology. However, as HDL tracer studies have evolved from radioactive isotope- to stable isotope-based strategies that in turn are reliant on mass spectrometry technologies, the complexity of the HDL proteome and its metabolism can be more readily addressed. In this review, we outline the motivations, timelines, advantages, and disadvantages of the various tracer kinetics strategies. We also feature the metabolic properties of select HDL proteins known to regulate reverse cholesterol transport, which in turn underscore that HDL lipoproteins comprise a heterogeneous particle population whose distinct protein constituents and kinetics likely determine its function and potential contribution to cholesterol clearance.

Serum opacity factor rescues fertility among female Scarb1-/- mice by reducing HDL-free cholesterol bioavailability

Human female infertility, 20% of which is idiopathic, is a public health problem for which better diagnostics and therapeutics are needed. A novel cause of infertility emerged from studies of female mice deficient in the HDL receptor gene (Scarb1). These mice are infertile and have high plasma HDL cholesterol (C) concentrations, due to elevated HDL-free cholesterol (FC), which transfers from HDL to all tissues. Previous studies have indicated that oral delivery of probucol, an HDL-lowering drug, to female Scarb1-/- mice reduces plasma HDL-C concentrations and rescues fertility. Additionally, serum opacity factor (SOF), a bacterial virulence factor, disrupts HDL structure, and bolus SOF injection into mice reduces plasma HDL-C concentrations. Here, we discovered that delivering SOF to female Scarb1-/- mice with an adeno-associated virus (AAVSOF) induces constitutive SOF expression, reduces HDL-FC concentrations, and rescues fertility while normalizing ovary morphology. Although AAVSOF did not alter ovary-FC content, the ovary-mol% FC correlated with plasma HDL-mol% FC in a fertility-dependent way. Therefore, reversing the abnormal plasma microenvironment of high plasma HDL-mol% FC in female Scarb1-/- mice rescues fertility. These data provide the rationale to search for similar mechanistic links between HDL-mol% FC and infertility and the rescue of fertility in women by reducing plasma HDL-mol% FC.

Different Pathways of Cellular Cholesterol Efflux

Cholesterol efflux is the first and rate-limiting step of reverse cholesterol transport (RCT) from peripheric cells to the liver. The involvement of high-density lipoprotein (HDL) in RCT determines the atheroprotective properties of HDL. Cholesterol efflux from different membrane pools includes both passive and energy-dependent processes. The first type of route consists of cholesterol desorption from the cell membrane into the unstirred layer adjacent to the cell surface and diffusion in the water phase. Moreover, the selective uptake and facilitated diffusion of cholesterol and cholesteryl ester molecules through the hydrophobic tunnel in the scavenger receptor BI molecule does not require energy consumption. The second type of route includes active cholesterol export by the ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1). Several cholesterol acceptors specifically bind cholesterol and phospholipid molecules, and cholesterol binding to the albumin molecule, which acts as a shuttle, significantly increases cholesterol movement between acceptors and red blood cells, thus functioning as a sink for cholesterol. Cholesterol and phospholipid molecules effluxed from macrophages by ABCA1 are accepted exclusively by the lipid-free apolipoprotein apoA-I, which is the major protein moiety of HDL, whereas those effluxed by ABCG1 are accepted by HDL. ABCA1- and ABCG1-mediated cholesterol transport, together with cholesterol diffusion, largely determine cholesterol turnover at the physiological level of intracellular cholesterol. However, at cholesterol overload, ABCA1-mediated efflux prevails over other routes. The exchange of apoA-I between lipid-free and lipid-associated states and the synergism of nascent and mature HDL contribute to cholesterol efflux efficiency. Moreover, extracellular cholesterol deposits and microvesicles may be involved in RCT.

Denaturation of human plasma high-density lipoproteins by urea studied by apolipoprotein A-I dissociation

We studied the mechanism of HDL denaturation with concomitant apoA-I dissociation with HDL preparations from 48 patients with a wide range of plasma HDL-C and evaluated the contribution of lipid-free apoA-I into cholesterol efflux from macrophage, in particular, mediated by cholesterol transporter ABCA1. We prepared HDL by precipitation of apoB-containing lipoproteins by polyethylene glycol and used the chaotropic agent urea to denature HDL preparations. Apo-I dissociation from urea-treated HDL was assessed by the increase of preβ-band fraction with agarose gel electrophoresis followed by electro transfer and immunodetection and by the increase of ABCA1-mediated efflux of fluorescent analogue BODIPY-Cholesterol from RAW 264.7 macrophages. The HDL denaturation is governed by a single transition to fully dissociated apoA-I and the transition cooperativity decreases with increasing HDL-C. The apoA-I release depends on phospholipid concentration of HDL preparation and HDL compositional and structural heterogeneity and is well described by apolipoprotein partition between aqueous and lipid phases. Dissociated apoA-I determines the increase of ABCA1-mediated efflux of BODIPY-Cholesterol from RAW 264.7 macrophages to patient HDL. The increase in apoA-I dissociation is associated with the increase of ABCA1 gene transcript in peripheral blood mononuclear cells from patients. The low level of plasma HDL particles may be compensated by their increased potency for apoA-I release, thus suggesting apoA-I dissociation as a new HDL functional property.

Effects of Replacing Dietary Monounsaturated Fat With Carbohydrate on HDL (High-Density Lipoprotein) Protein Metabolism and Proteome Composition in Humans

OBJECTIVE

Clinical evidence has linked low HDL (high-density lipoprotein) cholesterol levels with high cardiovascular disease risk; however, its significance as a therapeutic target remains unestablished. We hypothesize that HDLs functional heterogeneity is comprised of metabolically distinct proteins, each on distinct HDL sizes and that are affected by diet. Approach and Results: Twelve participants were placed on 2 healthful diets high in monounsaturated fat or carbohydrate. After 4 weeks on each diet, participants completed a metabolic tracer study. HDL was isolated by Apo (apolipoprotein) A1 immunopurification and separated into 5 sizes. Tracer enrichment and metabolic rates for 8 HDL proteins-ApoA1, ApoA2, ApoC3, ApoE, ApoJ, ApoL1, ApoM, and LCAT (lecithin-cholesterol acyltransferase)-were determined by parallel reaction monitoring and compartmental modeling, respectively. Each protein had a unique, size-specific distribution that was not altered by diet. However, carbohydrate, when replacing fat, increased the fractional catabolic rate of ApoA1 and ApoA2 on alpha3 HDL; ApoE on alpha3 and alpha1 HDL; and ApoM on alpha2 HDL. Additionally, carbohydrate increased the production of ApoC3 on alpha3 HDL and ApoJ and ApoL1 on the largest alpha0 HDL. LCAT was the only protein studied that diet did not affect. Finally, global proteomics showed that diet did not alter the distribution of the HDL proteome across HDL sizes.

CONCLUSIONS

This study demonstrates that HDL in humans is composed of a complex system of proteins, each with its own unique size distribution, metabolism, and diet regulation. The carbohydrate-induced hypercatabolic state of HDL proteins may represent mechanisms by which carbohydrate alters the cardioprotective properties of HDL.

Effects of Disease-Causing Mutations on the Conformation of Human Apolipoprotein A-I in Model Lipoproteins

Plasma high-density lipoproteins (HDLs) are protein-lipid nanoparticles that transport lipids and protect against atherosclerosis. Human apolipoprotein A-I (apoA-I) is the principal HDL protein whose mutations can cause either aberrant lipid metabolism or amyloid disease. Hydrogen-deuterium exchange (HDX) mass spectrometry (MS) was used to study the apoA-I conformation in model discoidal lipoproteins similar in size to large plasma HDL. We examined how point mutations associated with hereditary amyloidosis (F71Y and L170P) or atherosclerosis (L159R) influence the local apoA-I conformation in model lipoproteins. Unlike other apoA-I forms, the large particles showed minimal conformational heterogeneity, suggesting a fully extended protein conformation. Mutation-induced structural perturbations in lipid-bound protein were attenuated compared to the free protein and indicated close coupling between the two belt-forming apoA-I molecules. These perturbations propagated to distant lipoprotein sites, either increasing or decreasing their protection. This HDX MS study of large model HDL, compared with previous studies of smaller particles, ascertained that apoA-I's central region helps accommodate the protein conformation to lipoproteins of various sizes. This study also reveals that the effects of mutations on lipoprotein conformational dynamics are much weaker than those in a lipid-free protein. Interestingly, the mutation-induced perturbations propagate to distant sites nearly 10 nm away and alter their protection in ways that cannot be predicted from the lipoprotein structure and stability. We propose that long-range mutational effects are mediated by both protein and lipid and can influence lipoprotein functionality.

High-density lipoprotein function is associated with atherosclerotic burden and cardiovascular outcomes in type 2 diabetes

BACKGROUND AND AIMS

Measures of HDL function are emerging tools for assessing cardiovascular disease (CVD) event risk. HDL-apoA-I exchange (HAE) reflects HDL capacity for reverse cholesterol transport.

METHODS

HAE was measured in 93 participants with type 2 diabetes (T2D) and at least one additional CVD risk factor in the Asker and Bærum Cardiovascular Diabetes study. At baseline and after seven years, the atherosclerotic burden was assessed by invasive coronary angiography. Major CVD events were registered throughout the study.

RESULTS

Linear regression analysis demonstrated a significant inverse association between HAE and atherosclerotic burden. Cox proportional hazard regression analysis showed a significant association between HAE and a composite of major CVD events when controlling for waist-hip ratio, HR = 0.89, 95% CI = 0.80-1.00 and p=0.040.

CONCLUSIONS

Despite the relatively small size of the study population and the limited number of CVD events, these findings suggest that HAE provides valuable information in determining CVD risk.

Rethinking reverse cholesterol transport and dysfunctional high-density lipoproteins

Human plasma high-density lipoprotein cholesterol concentrations are a negative risk factor for atherosclerosis-linked cardiovascular disease. Pharmacological attempts to reduce atherosclerotic cardiovascular disease by increasing plasma high-density lipoprotein cholesterol have been disappointing so that recent research has shifted from HDL quantity to HDL quality, that is, functional vs dysfunctional HDL. HDL has varying degrees of dysfunction reflected in impaired reverse cholesterol transport (RCT). In the context of atheroprotection, RCT occurs by 2 mechanisms: one is the well-known trans-hepatic pathway comprising macrophage free cholesterol (FC) efflux, which produces early forms of FC-rich nascent HDL (nHDL). Lecithin:cholesterol acyltransferase converts HDL-FC to HDL-cholesteryl ester while converting nHDL from a disc to a mature spherical HDL, which transfers its cholesteryl ester to the hepatic HDL receptor, scavenger receptor B1 for uptake, conversion to bile salts, or transfer to the intestine for excretion. Although widely cited, current evidence suggests that this is a minor pathway and that most HDL-FC and nHDL-FC rapidly transfer directly to the liver independent of lecithin:cholesterol acyltransferase activity. A small fraction of plasma HDL-FC enters the trans-intestinal efflux pathway comprising direct FC transfer to the intestine. SR-B1^-/- mice, which have impaired trans-hepatic FC transport, are characterized by high plasma levels of a dysfunctional FC-rich HDL that increases plasma FC bioavailability in a way that produces whole-body hypercholesterolemia and multiple pathologies. The design of future therapeutic strategies to improve RCT will have to be formulated in the context of these dual RCT mechanisms and the role of FC bioavailability.

Effects of aspirin in combination with EPA and DHA on HDL-C cholesterol and ApoA1 exchange in individuals with type 2 diabetes mellitus

BACKGROUND/SYNOPSIS

Low-dose aspirin is an effective drug for the prevention of cardiovascular disease (CVD) events but individuals with diabetes mellitus can be subject to 'aspirin resistance'. Thus, aspirin's effect in these individuals is controversial. Higher blood levels of seafood-derived omega-3 polyunsaturated fatty acids (ω3) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) also have beneficial effects in reducing risk of CVD events but few studies have examined the interaction of plasma EPA and DHA with aspirin ingestion.

OBJECTIVE/PURPOSE

Our study examined the combinatory effects of EPA, DHA, and aspirin ingestion on HDL-cholesterol (HDL-C) and apoA-I exchange (shown to be associated with CVD event risk).

METHODS

30 adults with Type 2 diabetes mellitus ingested aspirin (81mg/day) for 7 consecutive days, EPA+DHA (2.6g/day) for 28 days, then both for 7 days. Plasma was collected at baseline and at 5 subsequent visits including 4h after each aspirin ingestion. Mixed model methods were used to determine HDL-C-concentrations and apoA-I exchange compared to the baseline visit values. LOWESS curves were used for non-linear analyses of outcomes to help discern change patterns, which was followed by piecewise linear functions for formal testing of curvilinear relationships.

RESULTS

Significant changes (p < 0.05) compared to baseline in both HDL-C-concentrations and apoA-I exchange were present at different times. After 7 days of aspirin-only ingestion, apoA-I exchange was significantly modified by increasing levels of DHA concentration, with increased apoA-I exchange observed up until log(DHA) of 4.6 and decreased exchange thereafter (p = 0.03). These LOWESS curve effects were not observed for EPA or HDL-C (p > 0.05). Aspirin's effects on apoA-I exchange were the greatest when EPA or DHA concentrations were moderate compared to high or low. Comparison of EPA, DHA, and EPA+DHA LOWESS curves, demonstrated that the majority of the effect is due to DHA.

CONCLUSION

Our results strongly suggest that plasma concentrations of EPA and DHA influence aspirin effects on lipid mediators of CVD event risk where their concentrations are most beneficial when moderate, not high or low. These effects on HDL-C cholesterol and apoA-I exchange are novel. Personalized dosing of DHA in those who take aspirin may be a beneficial option for patients with type 2 diabetes mellitus.

ABCA1-Derived Nascent High-Density Lipoprotein-Apolipoprotein AI and Lipids Metabolically Segregate

OBJECTIVE

Reverse cholesterol transport comprises cholesterol efflux from ABCA1-expressing macrophages to apolipoprotein (apo) AI, giving nascent high-density lipoprotein (nHDL), esterification of nHDL-free cholesterol (FC), selective hepatic extraction of HDL lipids, and hepatic conversion of HDL cholesterol to bile salts, which are excreted. We tested this model by identifying the fates of nHDL-[³H]FC, [¹⁴C] phospholipid (PL), and [¹²⁵I]apo AI in serum in vitro and in vivo.

APPROACH AND RESULTS

During in vitro incubation of human serum, nHDL-[³H]FC and [¹⁴C]PL rapidly transfer to HDL and low-density lipoproteins (t_1/2=2-7 minutes), whereas nHDL-[¹²⁵I]apo AI transfers solely to HDL (t_1/2<10 minutes) and to the lipid-free form (t_1/2>480 minutes). After injection into mice, nHDL-[³H]FC and [¹⁴C]PL rapidly transfer to liver (t_1/2=≈2-3 minutes), whereas apo AI clears with t_1/2=≈460 minutes. The plasma nHDL-[³H]FC esterification rate is slow (0.46%/h) compared with hepatic uptake. PL transfer protein enhances nHDL-[¹⁴C]PL but not nHDL-[³H]FC transfer to cultured Huh7 hepatocytes.

CONCLUSIONS

nHDL-FC, PL, and apo AI enter different pathways in vivo. Most nHDL-[³H]FC and [¹⁴C]PL are rapidly extracted by the liver via SR-B1 (scavenger receptor class B member 1) and spontaneous transfer; hepatic PL uptake is promoted by PL transfer protein. nHDL-[¹²⁵I]apo AI transfers to HDL and to the lipid-free form that can be recycled to nHDL formation. Cholesterol esterification by lecithin:cholesterol acyltransferase is a minor process in nHDL metabolism. These findings could guide the design of therapies that better mobilize peripheral tissue-FC to hepatic disposal.

Apolipoprotein A-I exchange is impaired in metabolic syndrome patients asymptomatic for diabetes and cardiovascular disease

OBJECTIVE

We tested the hypothesis that HDL-apolipoprotein A-I exchange (HAE), a measure of high-density lipoprotein (HDL) function and a key step in reverse cholesterol transport (RCT), is impaired in metabolic syndrome (MetSyn) patients who are asymptomatic for diabetes and cardiovascular disease. We also compared HAE with cell-based cholesterol efflux capacity (CEC) to address previous reports that CEC is enhanced in MetSyn populations.

METHODS

HAE and ABCA1-specific CEC were measured as tests of HDL function in 60 MetSyn patients and 14 normolipidemic control subjects. Predictors of HAE and CEC were evaluated with multiple linear regression modeling using clinical markers of MetSyn and CVD risk.

RESULTS

HAE was significantly reduced in MetSyn patients (49.0 ± 10.9% vs. 61.2 ± 6.1%, P < 0.0001), as was ABCA1-specific CEC (10.1 ± 1.6% vs. 12.3 ± 2.0%, P < 0.002). Multiple linear regression analysis identified apoA-I concentration as a significant positive predictor of HAE, and MetSyn patients had significantly lower HAE per mg/dL of apoA-I (P = 0.004). MetSyn status was a negative predictor of CEC, but triglyceride (TG) was a positive predictor of CEC, with MetSyn patients having higher CEC per mg/dL of TG, but lower overall CEC compared to controls.

CONCLUSIONS

MetSyn patients have impaired HAE that contributes to reduced capacity for ABCA1-mediated CEC. MetSyn status is inversely correlated with CEC but positively correlated with TG, which explains the contradictory results from earlier MetSyn studies focused on CEC. HAE and CEC are inhibited in MetSyn patients over a broad range of absolute apoA-I and HDL particle levels, supporting the observation that this patient population bears significant residual cardiovascular disease risk.

Structural Stability of Streptococcal Serum Opacity Factor

Streptococcal serum opacity factor (SOF) is a protein that clouds the plasma of multiple mammalian species by disrupting high density lipoprotein (HDL) structure. Intravenous infusion of low dose SOF (4 µg) into mice reduces their plasma cholesterol concentrations ~ 40% in 3 h. Here we investigated the effects of pH, ionic strength, temperature, and denaturation with guanidinium chloride (GdmCl) on SOF stability and its reaction vs HDL. SOF stability was tested by pre-incubation of SOF at various temperatures, pH's, and GdmCl concentrations and measuring the SOF reaction rate at pH 7.4 and 37 °C. SOF retained activity at temperatures up to 58 °C, at pH 4 to 10, and in 8.5 M GdmCl after being returned to standard buffer conditions. The effects of GdmCl, pH, and ionic strength on the SOF reaction rates were also measured. SOF was inactive at GdmCl ≥ 1 M; SOF was most active at pH 5, near its isoelectric point and at an ionic strength of 3 (in NaCl). These data reveal that SOF is a stable protein and suggest that its activity is determined, in part, by the effects of pH and ionic strength on its overall charge relative to that of its reaction target, HDL.

Scavenger receptor B1 (SR-B1) profoundly excludes high density lipoprotein (HDL) apolipoprotein AII as it nibbles HDL-cholesteryl ester

Reverse cholesterol transport (transfer of macrophage-cholesterol in the subendothelial space of the arterial wall to the liver) is terminated by selective high density lipoprotein (HDL)-cholesteryl ester (CE) uptake, mediated by scavenger receptor class B, type 1 (SR-B1). We tested the validity of two models for this process: "gobbling," i.e. one-step transfer of all HDL-CE to the cell and "nibbling," multiple successive cycles of SR-B1-HDL association during which a few CEs transfer to the cell. Concurrently, we compared cellular uptake of apoAI with that of apoAII, which is more lipophilic than apoAI, using HDL-[³H]CE labeled with [¹²⁵I]apoAI or [¹²⁵I]apoAII. The studies were conducted in CHO-K1 and CHO-ldlA7 cells (LDLR^-/-) with (CHO-SR-B1) and without SR-B1 overexpression and in human Huh7 hepatocytes. Relative to CE, both apoAI and apoAII were excluded from uptake by all cells. However, apoAII was more highly excluded from uptake (2-4×) than apoAI. To distinguish gobbling versus nibbling mechanisms, media from incubations of HDL with CHO-SR-B1 cells were analyzed by non-denaturing PAGE, size-exclusion chromatography, and the distribution of apoAI, apoAII, cholesterol, and phospholipid among HDL species as a function of incubation time. HDL size gradually decreased, i.e. nibbling, with the concurrent release of lipid-free apoAI; apoAII was retained in an HDL remnant. Our data support an SR-B1 nibbling mechanism that is similar to that of streptococcal serum opacity factor, which also selectively removes CE and releases apoAI, leaving an apoAII-rich remnant.

Neo High-Density Lipoprotein Produced by the Streptococcal Serum Opacity Factor Activity against Human High-Density Lipoproteins Is Hepatically Removed via Dual Mechanisms

Injection of streptococcal serum opacity factor (SOF) into mice reduces the plasma cholesterol level by ∼40%. In vitro, SOF converts high-density lipoproteins (HDLs) into multiple products, including a small HDL, neo HDL. In vitro, neo HDL accounts for ∼60% of the protein mass of the SOF reaction products; in vivo, the accumulated mass of neo HDL is <1% of that observed in vitro. To identify the underlying cause of this difference, we determined the fate of neo HDL in plasma in vitro and in vivo. Following incubation with HDL, neo HDL-PC rapidly transfers to HDL, giving a small remnant, which fuses with HDL. An increased level of SR-B1 expression in Huh7 hepatoma cells and a reduced level of LDLR expression in CHO cells had little effect on neo HDL-[³H]CE uptake. Thus, the dominant receptors for neo HDL uptake are not LDLR or SR-B1. The in vivo metabolic fates of neo HDL-[³H]CE and HDL-[³H]CE were different. Thirty minutes after the injection of neo HDL-[³H]CE and HDL-[³H]CE into mice, plasma [³H]CE counts were 40 and 53%, respectively, of injected counts, with 10 times more [³H]CE appearing in the livers of neo HDL-[³H]CE-injected than in those of HDL-[³H]CE-injected mice. These data support a model of neo HDL-[³H]CE clearance by two parallel pathways. At early post-neo HDL-[³H]CE injection times, some neo HDL is directly removed by the liver; the remainder transfers its PC to HDL, leaving a remnant that fuses with HDL, which is also hepatically removed more slowly. Given that SR-B1 and SOF both remove CE from HDL, this novel mechanism may also underlie the metabolism of remnants released by hepatocytes following selective SR-B1-mediated uptake of HDL-CE.

Acylation of lysine residues in human plasma high density lipoprotein increases stability and plasma clearance in vivo

Although human plasma high density lipoproteins (HDL) concentrations negatively correlate with atherosclerotic cardiovascular disease, underlying mechanisms are unknown. Thus, there is continued interest in HDL structure and functionality. Numerous plasma factors disrupt HDL structure while inducing the release of lipid free apolipoprotein (apo) AI. Given that HDL is an unstable particle residing in a kinetic trap, we tested whether HDL could be stabilized by acylation with acetyl and hexanoyl anhydrides, giving AcHDL and HexHDL respectively. Lysine analysis with fluorescamine showed that AcHDL and HexHDL respectively contained 11 acetyl and 19 hexanoyl groups. Tests with biological and physicochemical perturbants showed that HexHDL was more stable than HDL to perturbant-induced lipid free apo AI formation. Like the reaction of streptococcal serum opacity factor against HDL, the interaction of HDL with its receptor, scavenger receptor class B member 1 (SR-B1), removes CE from HDL. Thus, we tested and validated the hypothesis that selective uptake of HexHDL-[³H]CE by Chinese Hamster Ovary cells expressing SR-B1 is less than that of HDL-[³H]CE; thus, selective SR-B1 uptake of HDL-CE depends on HDL instability. However, in mice, plasma clearance, hepatic uptake and sterol secretion into bile were faster from HexHDL-[³H]CE than from HDL-[³H]CE. Collectively, our data show that acylation increases HDL stability and that the reaction of plasma factors with HDL and SR-B1-mediated uptake are reduced by increased HDL stability. In vivo data suggest that HexHDL promotes charge-dependent reverse cholesterol transport, by a mechanism that increases hepatic sterol uptake via non SR-B1 receptors, thereby increasing bile acid output.

New Insights into the High-Density Lipoprotein Dilemma

Although high-density lipoprotein-cholesterol (HDL-C) concentration is a negative risk factor for atherosclerotic cardiovascular disease (CVD), efforts to reduce CVD risk by raising HDL-C have not been uniformly successful. Many studies have shown that alcohol consumption, that increases plasma HDL-C concentration, reduces CVD incidence. However, recent genetic studies in large populations have not only removed HDL-C from the causal link between plasma HDL-C concentration and reduced CVD risk, but also suggest that the association is weak. We propose here that the cardioprotective effects of alcohol are mediated by the interaction of its terminal metabolite, acetate, with the adipocyte free fatty acid receptor 2 (FFAR2), which elicits a profound antilipolytic effect that may increase insulin sensitivity without necessarily raising plasma HDL-C concentration.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

HDL-apolipoprotein A-I exchange is independently associated with cholesterol efflux capacity

HDL is the primary mediator of cholesterol mobilization from the periphery to the liver via reverse cholesterol transport (RCT). A critical first step in this process is the uptake of cholesterol from lipid-loaded macrophages by HDL, a function of HDL inversely associated with prevalent and incident cardiovascular disease. We hypothesized that the dynamic ability of HDL to undergo remodeling and exchange of apoA-I is an important and potentially rate-limiting aspect of RCT. In this study, we investigated the relationship between HDL-apoA-I exchange (HAE) and serum HDL cholesterol (HDL-C) efflux capacity. We compared HAE to the total and ABCA1-specific cholesterol efflux capacity of 77 subjects. We found that HAE was highly correlated with both total (r = 0.69, P < 0.0001) and ABCA1-specific (r = 0.47, P < 0.0001) efflux, and this relationship remained significant after adjustment for HDL-C or apoA-I. Multivariate models of sterol efflux capacity indicated that HAE accounted for approximately 25% of the model variance for both total and ABCA1-specific efflux. We conclude that the ability of HDL to exchange apoA-I and remodel, as measured by HAE, is a significant contributor to serum HDL efflux capacity, independent of HDL-C and apoA-I, indicating that HDL dynamics are an important factor in cholesterol efflux capacity and likely RCT.

Apolipoprotein AI deficiency inhibits serum opacity factor activity against plasma high density lipoprotein via a stabilization mechanism

The reaction of Streptococcal serum opacity factor (SOF) against plasma high-density lipoproteins (HDL) produces a large cholesteryl ester-rich microemulsion (CERM), a smaller neo HDL that is apolipoprotein (apo) AI-poor, and lipid-free apo AI. SOF is active versus both human and mouse plasma HDL. In vivo injection of SOF into mice reduces plasma cholesterol ∼40% in 3 h while forming the same products observed in vitro, but at different ratios. Previous studies supported the hypothesis that labile apo AI is required for the SOF reaction vs HDL. Here we further tested that hypothesis by studies of SOF against HDL from apo AI-null mice. When injected into apo AI-null mice, SOF reduced plasma cholesterol ∼35% in 3 h. The reaction of SOF vs apo AI-null HDL in vitro produced a CERM and neo HDL, but no lipid-free apo. Moreover, according to the rate of CERM formation, the extent and rate of the SOF reaction versus apo AI-null mouse HDL were less than that against wild-type (WT) mouse HDL. Chaotropic perturbation studies using guanidine hydrochloride showed that apo AI-null HDL was more stable than WT HDL. Human apo AI added to apo AI-null HDL was quantitatively incorporated, giving reconstituted HDL. Both SOF and guanidine hydrochloride displaced apo AI from the reconstituted HDL. These results support the conclusion that apo AI-null HDL is more stable than WT HDL because it lacks apo AI, a labile protein that is readily displaced by physicochemical and biochemical perturbations. Thus, apo AI-null HDL is less SOF-reactive than WT HDL. The properties of apo AI-null HDL can be partially restored to those of WT HDL by the spontaneous incorporation of human apo AI. It remains to be determined what other HDL functions are affected by apo AI deletion.

Structural stability and functional remodeling of high-density lipoproteins

Lipoproteins are protein-lipid nanoparticles that transport lipids in circulation and are central in atherosclerosis and other disorders of lipid metabolism. Apolipoproteins form flexible structural scaffolds and important functional ligands on the particle surface and direct lipoprotein metabolism. Lipoproteins undergo multiple rounds of metabolic remodeling that is crucial to lipid transport. Important aspects of this remodeling, including apolipoprotein dissociation and particle fusion, are mimicked in thermal or chemical denaturation and are modulated by free energy barriers. Here we review the biophysical studies that revealed the kinetic mechanism of lipoprotein stabilization and unraveled its structural basis. The main focus is on high-density lipoprotein (HDL). An inverse correlation between stability and functions of various HDLs in cholesterol transport suggests the functional role of structural disorder. A mechanism for the conformational adaptation of the major HDL proteins, apoA-I and apoA-II, to the increasing lipid load is proposed. Together, these studies help understand why HDL forms discrete subclasses separated by kinetic barriers, which have distinct composition, conformation and functional properties. Understanding these properties may help improve HDL quality and develop novel therapies for cardiovascular disease.

Kinetic and thermodynamic analyses of spontaneous exchange between high-density lipoprotein-bound and lipid-free apolipoprotein A-I

It is thought that apolipoprotein A-I (apoA-I) spontaneously exchanges between high-density lipoprotein (HDL)-bound and lipid-free states, which is relevant to the occurrence of preβ-HDL particles in plasma. To improve our understanding of the mechanistic basis for this phenomenon, we performed kinetic and thermodynamic analyses for apoA-I exchange between discoidal HDL-bound and lipid-free forms using fluorescence-labeled apoA-I variants. Gel filtration experiments demonstrated that addition of excess lipid-free apoA-I to discoidal HDL particles promotes exchange of apoA-I between HDL-associated and lipid-free pools without alteration of the steady-state HDL particle size. Kinetic analysis of time-dependent changes in NBD fluorescence upon the transition of NBD-labeled apoA-I from HDL-bound to lipid-free state indicates that the exchange kinetics are independent of the collision frequency between HDL-bound and lipid-free apoA-I, in which the lipid binding ability of apoA-I affects the rate of association of lipid-free apoA-I with the HDL particles and not the rate of dissociation of HDL-bound apoA-I. Thus, C-terminal truncations or mutations that reduce the lipid binding affinity of apoA-I strongly impair the transition of lipid-free apoA-I to the HDL-bound state. Thermodynamic analysis of the exchange kinetics demonstrated that the apoA-I exchange process is enthalpically unfavorable but entropically favorable. These results explain the thermodynamic basis of the spontaneous exchange reaction of apoA-I associated with HDL particles. The altered exchangeability of dysfunctional apoA-I would affect HDL particle rearrangement, leading to perturbed HDL metabolism.

HDL-cholesterol in coronary artery disease risk: function or structure?

High-density lipoproteins (HDL) are inversely related with coronary artery disease (CAD) and HDL-cholesterol is the only standardized and reproducible parameter available to estimate plasma concentration of these lipoproteins. However, pharmacological interventions intended to increase HDL-cholesterol have not been consistently associated to an effective CAD risk reduction. Among patients with a myocardial infarction, 43 and 44% of men and women, respectively, had normal plasma levels of HDL-cholesterol, whereas genetic studies have failed to show a causal association between HDL-cholesterol and CAD risk. Instead, HDL functionality seems to be the target to be evaluated, but the existing methods are still poorly reproducible and far to be adapted to the clinical laboratory. HDL subclasses rise as a potential alternative for the evaluation of CAD risk; HDL subclasses are a surrogate of intravascular metabolism of these lipoproteins and probably of their functionality. Low levels of large HDL and increased proportions of small particles are the most remarkable features associated to an increased risk of type 2 diabetes mellitus (T2DM) or CAD. However, inflammation and other environmental factors are related with abnormal HDL structure, and, as a consequence, more prospective studies are needed to better support the clinical usefulness of HDL subclasses. New insights from proteome and lipidome profiles of HDL will provide potential HDL-related biomarkers in the coming years.

HDL-apoA-I exchange: rapid detection and association with atherosclerosis

High density lipoprotein (HDL) cholesterol levels are associated with decreased risk of cardiovascular disease, but not all HDL are functionally equivalent. A primary determinant of HDL functional status is the conformational adaptability of its main protein component, apoA-I, an exchangeable apolipoprotein. Chemical modification of apoA-I, as may occur under conditions of inflammation or diabetes, can severely impair HDL function and is associated with the presence of cardiovascular disease. Chemical modification of apoA-I also impairs its ability to exchange on and off HDL, a critical process in reverse cholesterol transport. In this study, we developed a method using electron paramagnetic resonance spectroscopy (EPR) to quantify HDL-apoA-I exchange. Using this approach, we measured the degree of HDL-apoA-I exchange for HDL isolated from rabbits fed a high fat, high cholesterol diet, as well as human subjects with acute coronary syndrome and metabolic syndrome. We observed that HDL-apoA-I exchange was markedly reduced when atherosclerosis was present, or when the subject carries at least one risk factor of cardiovascular disease. These results show that HDL-apoA-I exchange is a clinically relevant measure of HDL function pertinent to cardiovascular disease.

Impaired lipoprotein processing in HIV patients on antiretroviral therapy: aberrant high-density lipoprotein lipids, stability, and function

OBJECTIVE

HIV patients on antiretroviral therapy (HIV/ART) exhibit a unique atherogenic dyslipidemic profile with hypertriglyceridemia (HTG) and low plasma concentrations of high-density lipoprotein (HDL) cholesterol. In the Heart Positive Study of HIV/ART patients, a hypolipidemic therapy of fenofibrate, niacin, diet, and exercise reduced HTG and plasma non-HDL cholesterol concentrations and raised plasma HDL cholesterol and adiponectin concentrations. We tested the hypothesis that HIV/ART HDL have abnormal structures and properties and are dysfunctional.

APPROACH AND RESULTS

Hypolipidemic therapy reduced the TG contents of low-density lipoprotein and HDL. At baseline, HIV/ART low-density lipoproteins were more triglyceride (TG)-rich and HDL were more TG- and cholesteryl ester-rich than the corresponding lipoproteins from normolipidemic (NL) subjects. Very-low-density lipoproteins, low-density lipoprotein, and HDL were larger than the corresponding lipoproteins from NL subjects; HIV/ART HDL were less stable than NL HDL. HDL-[(3)H]cholesteryl ester uptake by Huh7 hepatocytes was used to assess HDL functionality. HIV/ART plasma were found to contain significantly less competitive inhibition activity for hepatocyte HDL-cholesteryl ester uptake than NL plasma were found to contain (P<0.001).

CONCLUSIONS

Compared with NL subjects, lipoproteins from HIV/ART patients are larger and more neutral lipid-rich, and their HDL are less stable and less receptor-competent. On the basis of this work and previous studies of lipase activity in HIV, we present a model in which plasma lipolytic activities or hepatic cholesteryl ester uptake are impaired in HIV/ART patients. These findings provide a rationale to determine whether the distinctive lipoprotein structure, properties, and function of HIV/ART HDL predict atherosclerosis as assessed by carotid artery intimal medial thickness.

Comparison of apoA-I helical structure and stability in discoidal and spherical HDL particles by HX and mass spectrometry

Elucidation of apoA-I secondary structure in spherical plasma HDL particles is essential for understanding HDL structure and function at the molecular level. To provide this information, we have applied hydrogen exchange (HX) and mass spectrometry methods to compare apoA-I secondary structure in discoidal (two apoA-I molecules/particle) and spherical (five apoA-I molecules/particle) HDL particles. The HX kinetics indicate that the locations of helical segments within the apoA-I molecules are the same in both discoidal and spherical HDL particles (approximately 10 nm hydrodynamic diameter). Helix stabilities in both types of particles are 3–5 kcal/mol, consistent with the apoA-I molecules being in a highly dynamic state with helical segments unfolding and refolding in seconds. For the spherical HDL, apoA-I fragments corresponding to residues 115–158 exhibit bimodal HX kinetics consistent with this segment adopting an inter-converting (on the timescale of tens of minutes) helix-loop configuration. The segment adopting this configuration in the 10 nm disc is shorter because the surface area available to each apoA-I molecule is apparently larger. Loop formation in the central region of the apoA-I molecule contributes to the ability of the protein to adapt to changes in available space on the HDL particle surface. Overall, apoA-I secondary structure is largely unaffected by a change in HDL particle shape from disc to sphere.

Interactions of apolipoprotein A-I with high-density lipoprotein particles

Although the partitioning of apolipoprotein A-I (apoA-I) molecules in plasma between high-density lipoprotein (HDL)-bound and -unbound states is an integral part of HDL metabolism, the factors that control binding of apoA-I to HDL particles are poorly understood. To address this gap in knowledge, we investigated how the properties of the apoA-I tertiary structure domains and surface characteristics of spherical HDL particles influence apoA-I binding. The abilities of (14)C-labeled human and mouse apoA-I variants to associate with human HDL and lipid emulsion particles were determined using ultracentrifugation to separate free and bound protein. The binding of human apoA-I (243 amino acids) to HDL is largely mediated by its relatively hydrophobic C-terminal domain; the isolated N-terminal helix bundle domain (residues 1-190) binds poorly. Mouse apoA-I, which has a relatively polar C-terminal domain, binds to human HDL to approximately half the level of human apoA-I. The HDL binding abilities of apoA-I variants correlate strongly with their abilities to associate with phospholipid (PL)-stabilized emulsion particles, consistent with apoA-I-PL interactions at the particle surface being important. When equal amounts of HDL2 and HDL3 are present, all of the apoA-I variants partition preferentially to HDL3. Fluorescence polarization measurements using Laurdan-labeled HDL2 and HDL3 indicate that PL molecular packing is looser on the more negatively charged HDL3 particle surface, which promotes apoA-I binding. Overall, it is clear that both apoA-I structural features, especially the hydrophobicity of the C-terminal domain, and HDL surface characteristics such as the availability of free space influence the ability of apoA-I to associate with HDL particles.

New insights into the determination of HDL structure by apolipoproteins: Thematic review series: high density lipoprotein structure, function, and metabolism

Apolipoprotein (apo)A-I is the principal protein component of HDL, and because of its conformational adaptability, it can stabilize all HDL subclasses. The amphipathic α-helix is the structural motif that enables apoA-I to achieve this functionality. In the lipid-free state, the helical segments unfold and refold in seconds and are located in the N-terminal two thirds of the molecule where they are loosely packed as a dynamic, four-helix bundle. The C-terminal third of the protein forms an intrinsically disordered domain that mediates initial binding to phospholipid surfaces, which occurs with coupled α-helix formation. The lipid affinity of apoA-I confers detergent-like properties; it can solubilize vesicular phospholipids to create discoidal HDL particles with diameters of approximately 10 nm. Such particles contain a segment of phospholipid bilayer and are stabilized by two apoA-I molecules that are arranged in an anti-parallel, double-belt conformation around the edge of the disc, shielding the hydrophobic phospholipid acyl chains from exposure to water. The apoA-I molecules are in a highly dynamic state, and they stabilize discoidal particles of different sizes by certain segments forming loops that detach reversibly from the particle surface. The flexible apoA-I molecule adapts to the surface of spherical HDL particles by bending and forming a stabilizing trefoil scaffold structure. The above characteristics of apoA-I enable it to partner with ABCA1 in mediating efflux of cellular phospholipid and cholesterol and formation of a heterogeneous population of nascent HDL particles. Novel insights into the structure-function relationships of apoA-I should help reveal mechanisms by which HDL subclass distribution can be manipulated.

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

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

The structure and function of serum opacity factor: a unique streptococcal virulence determinant that targets high-density lipoproteins

Serum opacity factor (SOF) is a virulence determinant expressed by a variety of streptococcal and staphylococcal species including both human and animal pathogens. SOF derives its name from its ability to opacify serum where it targets and disrupts the structure of high-density lipoproteins resulting in formation of large lipid vesicles that cause the serum to become cloudy. SOF is a multifunctional protein and in addition to its opacification activity, it binds to a number of host proteins that mediate adhesion of streptococci to host cells, and it plays a role in resistance to phagocytosis in human blood. This article will provide an overview of the structure and function of SOF, its role in the pathogenesis of streptococcal infections, its vaccine potential, its prevalence and distribution in bacteria, and the molecular mechanism whereby SOF opacifies serum and how an understanding of this mechanism may lead to therapies for reducing high-cholesterol concentrations in blood, a major risk factor for cardiovascular disease.

Exchange of apolipoprotein A-I between lipid-associated and lipid-free states: a potential target for oxidative generation of dysfunctional high density lipoproteins

An important event in cholesterol metabolism is the efflux of cellular cholesterol by apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins (HDL). Lipid-free apoA-I is the preferred substrate for ATP-binding cassette A1, which promotes cholesterol efflux from macrophage foam cells in the arterial wall. However, the vast majority of apoA-I in plasma is associated with HDL, and the mechanisms for the generation of lipid-free apoA-I remain poorly understood. In the current study, we used fluorescently labeled apoA-I that exhibits a distinct fluorescence emission spectrum when in different states of lipid association to establish the kinetics of apoA-I transition between the lipid-associated and lipid-free states. This approach characterized the spontaneous and rapid exchange of apoA-I between the lipid-associated and lipid-free states. In contrast, the kinetics of apoA-I exchange were significantly reduced when apoA-I on HDL was cross-linked with a bi-functional reagent or oxidized by myeloperoxidase. Our observations support the hypothesis that oxidative damage to apoA-I by myeloperoxidase limits the ability of apoA-I to be liberated in a lipid-free form from HDL. This impairment of apoA-I exchange reaction may be a trait of dysfunctional HDL contributing to reduced ATP-binding cassette A1-mediated cholesterol efflux and atherosclerosis.

HDL superphospholipidation enhances key steps in reverse cholesterol transport

HDL-phospholipids (HDL-PL) play an important role in reverse cholesterol transport (RCT). Phosphatidylcholine (PC) is the most important phospholipid in RCT because it is the essential cholesterol-binding component of lipoproteins and is the acyl donor in the esterification of FC by lecithin:cholesterol acyltransferase (LCAT). FC efflux to sera is a positive anti-atherogenic function of HDL-PL. Although PC has long been recognized as an anti-atherogenic agent, development of new HDL therapies based on PC has been fraught with issues of efficacy, cost, and safety. Moreover, some methods to increase HDL-PC perturb HDL and release lipid-free apolipoproteins (apo) A-I. We developed a new method, HDL SPLn (SPLn) using a modified detergent removal method that obviates these concerns. SPLn can incorporate PC into HDL and increase HDL-PC>10-fold. This is achieved with no loss of apo A-I. According to size exclusion chromatography and native gradient gel electrophoresis, SPLn raises the HDL particle weight in a dose-dependent way, from approximately 120 to approximately 350kDa. Kinetic analysis of FC efflux to the resulting SPLn particles shows that K(m) and V(max) for SPLn HDL are lower and higher respectively than for native HDL. As a consequence, the catalytic efficiency, V(max)/K(m), increases by more than 400%. Clinically, small increases in serum HDL-PL are associated with significant and profound increases in FC efflux to serum. Treatment of relatively small amounts of plasma by SPLn is a potential method of improving at least one step in RCT.

Surface plasmon resonance analysis of the mechanism of binding of apoA-I to high density lipoprotein particles

The partitioning of apolipoprotein A-I (apoA-I) molecules in plasma between HDL-bound and -unbound states is an integral part of HDL metabolism. We used the surface plasmon resonance (SPR) technique to monitor in real time the reversible binding of apoA-I to HDL. Biotinylated human HDL(2) and HDL(3) were immobilized on a streptavidin-coated SPR sensor chip, and apoA-I solutions at different concentrations were flowed across the surface. The wild-type (WT) human and mouse apoA-I/HDL interaction involves a two-step process; apoA-I initially binds to HDL with fast association and dissociation rates, followed by a step exhibiting slower kinetics. The isolated N-terminal helix bundle domains of human and mouse apoA-I also exhibit a two-step binding process, consistent with the second slower step involving opening of the helix bundle domain. The results of fluorescence experiments with pyrene-labeled apoA-I are consistent with the N-terminal helix bundle domain interacting with proteins resident on the HDL particle surface. Dissociation constants (K(d)) measured for WT human apoA-I interactions with HDL(2) and HDL(3) are about 10 microM, indicating that the binding is low affinity. This K(d) value does not apply to all of the apoA-I molecules on the HDL particle but only to a relatively small, labile pool.

Apolipoproteins A-I, A-II and E are independently distributed among intracellular and newly secreted HDL of human hepatoma cells

Whereas hepatocytes secrete the major human plasma high density lipoproteins (HDL)-protein, apo A-I, as lipid-free and lipidated species, the biogenic itineraries of apo A-II and apo E are unknown. Human plasma and HepG2 cell-derived apo A-II and apo E occur as monomers, homodimers and heterodimers. Dimerization of apo A-II, which is more lipophilic than apo A-I, is catalyzed by lipid surfaces. Thus, we hypothesized that lipidation of intracellular and secreted apo A-II exceeds that of apo A-I, and once lipidated, apo A-II dimerizes. Fractionation of HepG2 cell lysate and media by size exclusion chromatography showed that intracellular apo A-II and apo E are fully lipidated and occur on nascent HDL and VLDL respectively, while only 45% of intracellular apo A-I is lipidated. Secreted apo A-II and apo E occur on small HDL and on LDL and large HDL respectively. HDL particles containing both apo A-II and apo A-I form only after secretion from both HepG2 and Huh7 hepatoma cells. Apo A-II dimerizes intracellularly while intracellular apo E is monomeric but after secretion associates with HDL and subsequently dimerizes. Thus, HDL apolipoproteins A-I, A-II and E have distinct intracellular and post-secretory pathways of hepatic lipidation and dimerization in the process of HDL formation. These early forms of HDL are expected to follow different apolipoprotein-specific pathways through plasma remodeling and reverse cholesterol transport.

Disruption of human plasma high-density lipoproteins by streptococcal serum opacity factor requires labile apolipoprotein A-I

Human plasma high-density lipoproteins (HDL), the primary vehicle for reverse cholesterol transport, are the target of serum opacity factor (SOF), a virulence determinant of Streptococcus pyogenes that turns serum opaque. HDL comprise a core of neutral lipidscholesteryl esters and some triglyceridesurrounded by a surface monolayer of cholesterol, phospholipids, and specialized proteins [apolipoproteins (apos) A-I and A-II]. A HDL is an unstable particle residing in a kinetic trap from which it can escape via chaotropic, detergent, or thermal perturbation. Recombinant (r) SOF catalyzes the transfer of nearly all neutral lipids of approximately 100,000 HDL particles (D approximately 8.5 nm) into a single, large cholesteryl ester-rich microemulsion (CERM; D > 100 nm), leaving a new HDL-like particle [neo HDL (D approximately 5.8 nm)] while releasing lipid-free (LF) apo A-I. CERM formation and apo A-I release have similar kinetics, suggesting parallel or rapid consecutive steps. By using complementary physicochemical methods, we have refined the mechanistic model for HDL opacification. According to size exclusion chromatography, a HDL containing nonlabile apo A-I resists rSOF-mediated opacification. On the basis of kinetic cryo-electron microscopy, rSOF (10 nM) catalyzes the conversion of HDL (4 microM) to neo HDL via a stepwise mechanism in which intermediate-sized particles are seen. Kinetic turbidimetry revealed opacification as a rising exponential reaction with a rate constant k of (4.400 +/- 0.004) x 10(-2) min(-1). Analysis of the kinetic data using transition state theory gave an enthalpy (DeltaH()), entropy (DeltaS(++)), and free energy (DeltaG()) of activation of 73.9 kJ/mol, -66.87 J/K, and 94.6 kJ/mol, respectively. The free energy of activation for opacification is nearly identical to that for the displacement of apo A-I from HDL by guanidine hydrochloride. We conclude that apo A-I lability is required for HDL opacification, LF apo A-I desorption is the rate-limiting step, and nearly all HDL particles contain at least one labile copy of apo A-I.

Differential stability of high-density lipoprotein subclasses: effects of particle size and protein composition

High-density lipoproteins (HDLs) are complexes of proteins (mainly apoA-I and apoA-II) and lipids that remove cholesterol and prevent atherosclerosis. Understanding the distinct properties of the heterogeneous HDL population may aid the development of new diagnostic tools and therapies for atherosclerosis. Mature human HDLs form two major subclasses differing in particle diameter and metabolic properties, HDL(2) (large) and HDL(3) (small). These subclasses are comprised of HDL(A-I) containing only apoA-I, and HDL(A-I/A-II) containing apoA-I and apoA-II. ApoA-I is strongly cardioprotective, but the function of the smaller, more hydrophobic apoA-II is unclear. ApoA-II is thought to counteract the cardioprotective action of apoA-I by stabilizing HDL particles and inhibiting their remodeling. To test this notion, we performed the first kinetic stability study of human HDL subclasses. The results revealed that the stability of plasma spherical HDL decreases with increasing particle diameter; which may facilitate preferential cholesterol ester uptake from large lipid-loaded HDL(2). Surprisingly, size-matched plasma HDL(A-I/A-II) showed comparable or slightly lower stability than HDL(A-I); this is consistent with the destabilization of model discoidal HDL observed upon increasing the A-II to A-I ratio. These results clarify the roles of the particle size and protein composition in HDL remodeling, and help reconcile conflicting reports regarding the role of apoA-II in this remodeling.

Conformational flexibility of the N-terminal domain of apolipoprotein a-I bound to spherical lipid particles

Lipid binding of human apolipoprotein A-I (apoA-I) occurs initially through the C-terminal alpha-helices followed by conformational reorganization of the N-terminal helix bundle. This led us to hypothesize that apoA-I has multiple lipid-bound conformations, in which the N-terminal helix bundle adopts either open or closed conformations anchored by the C-terminal domain. To investigate such possible conformations of apoA-I at the surface of a spherical lipid particle, site-specific labeling of the N- and C-terminal helices in apoA-I by N-(1-pyrene)maleimide was employed after substitution of a Cys residue for Val-53 or Phe-229. Neither mutagenesis nor the pyrene labeling caused discernible changes in the lipid-free structure and lipid interaction of apoA-I. Taking advantage of a significant increase in fluorescence when a pyrene-labeled helix is in contact with the lipid surface, we monitored the behaviors of the N- and C-terminal helices upon binding of apoA-I to egg PC small unilamellar vesicles. Comparison of the binding isotherms for pyrene-labeled apoA-I as well as a C-terminal helical peptide suggests that an increase in surface concentration of apoA-I causes dissociation of the N-terminal helix from the surface leaving the C-terminal helix attached. Consistent with this, isothermal titration calorimetry measurements showed that the enthalpy of apoA-I binding to the lipid surface under near saturated conditions is much less exothermic than that for binding at a low surface concentration, indicating the N-terminal helix bundle is out of contact with lipid at high apoA-I surface concentrations. Interestingly, the presence of cholesterol significantly induces the open conformation of the helix bundle. These results provide insight into the multiple lipid-bound conformations that the N-terminal helix bundle of apoA-I can adopt on a lipid or lipoprotein particle, depending upon the availability of space on the surface and the surface composition.

Properties of the products formed by the activity of serum opacity factor against human plasma high-density lipoproteins

Serum opacity factor from Streptococcus pyogenes transfers the cholesteryl esters (CE) of approximately 100,000 plasma high-density lipoprotein particles (HDL) to a CE-rich microemulsion (CERM) while forming neo HDL, a cholesterol-poor HDL-like particle. HDL, neo HDL, and CERM are distinct. Neo HDL is lower in free cholesterol and has lower surface and total microviscosities than HDL; the surface polarity of neo HDL and HDL are similar. CERM is much larger than HDL and richer in cholesterol and CE. Although the surface microviscosity of HDL is higher than that of CERM, they have similar total microviscosities because cholesterol partitions into the neutral lipid core. Because of its unique surface properties apo E preferentially associates with the CERM. In contrast, the composition and properties of neo HDL make it a potential acceptor of cellular cholesterol and its esterification. Thus, neo HDL and CERM are possible vehicles for improving cholesterol transport to the liver.

Apolipoprotein A-I mimetic peptide helix number and helix linker influence potentially anti-atherogenic properties

We hypothesize that apolipoprotein A-I (apoA-I) mimetic peptides better mimicking the punctuated alpha-helical repeats of full-length apoA-I are more anti-inflammatory and anti-atherogenic. This study compares a monomeric apoA-I mimetic helix to three different tandem helix peptides in vitro: 4F (18 mer), 4F-proline-4F (37 mer, Pro), 4F-alanine-4F (37 mer, Ala), and 4F-KVEPLRA-4F [the human apoA-I 4/5 interhelical sequence (IHS), 43 mer]. All peptides cleared turbid lipid suspensions, with 4F being most effective. In contrast to lipid clearance, tandem peptides were more effective at remodeling mouse HDL. All four peptides displaced apoA-I and apoE from the HDL, leaving a larger particle containing apoA-II and peptide. Peptide-remodeled HDL particles show no deficit in ABCG1 cholesterol efflux despite the loss of the majority of apoA-I. Tandem peptides show greater ability to efflux cholesterol from lipid-loaded murine macrophages, compared with 4F. Although 4F inhibited oxidation of purified mouse LDL, the Ala tandem peptide increased oxidation. We compared several tandem 4F-based peptides with monomeric 4F in assays that correlated with suggested anti-inflammatory/anti-atherogenic pathways. Tandem 4F-based peptides, which better mimic full-length apoA-I, exceed monomeric 4F in HDL remodeling and cholesterol efflux but not LDL oxidation protection. In addition, apoA-I mimetic peptides may increase reverse cholesterol transport through both ABCA1 as well as ABCG1 pathways.

Serum opacity factor unmasks human plasma high-density lipoprotein instability via selective delipidation and apolipoprotein A-I desorption

Human plasma high-density lipoproteins (HDL) are important vehicles in reverse cholesterol transport, the cardioprotective mechanism by which peripheral tissue-cholesterol is transported to the liver for disposal. HDL is the target of serum opacity factor (SOF), a substance produced by Streptococcus pyogenes that turns mammalian serum cloudy. Using a recombinant (r) SOF, we studied opacification and its mechanism. rSOF catalyzes the partial disproportionation of HDL into a cholesteryl ester-rich microemulsion (CERM) and a new HDL-like particle, neo HDL, with the concomitant release of lipid-free (LF)-apo A-I. Opacification is unique; rSOF transfers apo E and nearly all neutral lipids of approximately 100,000 HDL particles into a single large CERM whose size increases with HDL-CE content (r approximately 100-250 nm) leaving a neo HDL that is enriched in PL (41%) and protein (48%), especially apo A-II. rSOF is potent; within 30 min at 37 degrees C, 10 nM rSOF opacifies 4 microM HDL. At respective low and high physiological HDL concentrations, LF-apo A-I is monomeric and tetrameric. CERM formation and apo A-I release have similar kinetics suggesting parallel or rapid sequential steps. According to the reaction products and kinetics, rSOF is a heterodivalent fusogenic protein that uses a docking site to displace apo A-I and bind to exposed CE surfaces on HDL; the resulting rSOF-HDL complex recruits additional HDL with its binding-delipidation site and through multiple fusion steps forms a CERM. rSOF may be a clinically useful and novel modality for improving reverse cholesterol transport. With apo E and a high CE content, CERM could transfer large amounts of cholesterol to the liver for disposal via the LDL receptor; neo HDL is likely a better acceptor of cellular cholesterol than HDL; LF-apo A-I could enhance efflux via the ATP-binding casette transporter ABCA1.

Structural requirements for antioxidative and anti-inflammatory properties of apolipoprotein A-I mimetic peptides

Recently, attention has been focused on pharmacological treatments that increase HDL cholesterol to prevent coronary artery disease. Despite three decades of extensive research of human apolipoprotein A-I (apoA-I), the major protein component of HDL, the molecular basis for its antiatherogenic and anti-inflammatory functions remain elusive. Another protein component of HDL, apoA-II, has structural features similar to those of apoA-I but does not possess atheroprotective properties. To understand the molecular basis for the effectiveness of apoA-I, we used model synthetic peptides. We designed analogs of the class A amphipathic helical motif in apoA-I that is responsible for solubilizing phospholipids. None of these analogs has sequence homology to apoA-I, but all are similar in their lipid-associating structural motifs. Although all of these peptide analogs interact with phospholipids to form peptide:lipid complexes, the biological properties of these analogs are different. Physical-chemical and NMR studies of these peptides have enabled the delineation of structural requirements for atheroprotective and anti-inflammatory properties in these peptides. It has been shown that peptides that interact strongly with lipid acyl chains do not have antiatherogenic and anti-inflammatory properties. In contrast, peptides that associate close to the lipid head group (and hence do not interact strongly with the lipid acyl chain) are antiatherogenic and anti-inflammatory. Understanding the structure and function of apoA-I and HDL through studies of the amphipathic helix motif may lead to peptide-based therapies for inhibiting atherosclerosis and other related inflammatory lipid disorders.