The pathophysiology of excess plasma-free cholesterol

PURPOSE OF REVIEW

Several large studies have shown increased mortality due to all-causes and to atherosclerotic cardiovascular disease. In most clinical settings, plasma HDL-cholesterol is determined as a sum of free cholesterol and cholesteryl ester, two molecules with vastly different metabolic itineraries. We examine the evidence supporting the concept that the pathological effects of elevations of plasma HDL-cholesterol are due to high levels of the free cholesterol component of HDL-C.

RECENT FINDINGS

In a small population of humans, a high plasma HDL-cholesterol is associated with increased mortality. Similar observations in the HDL-receptor deficient mouse (Scarb1 -/- ), a preclinical model of elevated HDL-C, suggests that the pathological component of HDL in these patients is an elevated plasma HDL-FC.

SUMMARY

Collective consideration of the human and mouse data suggests that clinical trials, especially in the setting of high plasma HDL, should measure free cholesterol and cholesteryl esters and not just total cholesterol.

Serum opacity factor normalizes erythrocyte morphology in Scarb1-/- mice in an HDL-free cholesterol-dependent way

Compared with WT mice, HDL receptor-deficient (Scarb1-/-) mice have higher plasma levels of free cholesterol (FC)-rich HDL and exhibit multiple pathologies associated with a high mol% FC in ovaries, platelets, and erythrocytes, which are reversed by lowering HDL. Bacterial serum opacity factor (SOF) catalyzes the opacification of plasma by targeting and quantitatively converting HDL to neo HDL (HDL remnant), a cholesterol ester-rich microemulsion, and lipid-free APOA1. SOF delivery with an adeno-associated virus (AAVSOF) constitutively lowers plasma HDL-FC and reverses female infertility in Scarb1-/- mice in an HDL-dependent way. We tested whether AAVSOF delivery to Scarb1-/- mice will normalize erythrocyte morphology in an HDL-FC-dependent way. We determined erythrocyte morphology and FC content (mol%) in three groups-WT, untreated Scarb1-/- (control), and Scarb1-/- mice receiving AAVSOF-and correlated these with their respective HDL-mol% FC. Plasma-, HDL-, and tissue-lipid compositions were also determined. Plasma- and HDL-mol% FC positively correlated across all groups. Among Scarb1-/- mice, AAVSOF treatment normalized reticulocyte number, erythrocyte morphology, and erythrocyte-mol% FC. Erythrocyte-mol% FC positively correlated with HDL-mol% FC and with both the number of reticulocytes and abnormal erythrocytes. AAVSOF treatment also reduced FC of extravascular tissues to a lesser extent. HDL-FC spontaneously transfers from plasma HDL to cell membranes. AAVSOF treatment lowers erythrocyte-FC and normalizes erythrocyte morphology and lipid composition by reducing HDL-mol% FC.

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.

Abstract 238: Serum Opacity Factor Normalizes Dysfunctional High Density Lipoproteins In Srbi -/- Female Mice And Rescues Fertility

Although plasma HDL-C levels negatively correlate with atherosclerotic cardiovascular disease (ACVD), attempts to reduce ACVD risk by raising plasma HDL have disappointed. Thus, hypotheses about salutary HDL effects have shifted from higher-is-better to function-is-more-important. The SRBI-/- mouse is an extreme model of HDL dsyfunctionality; compared to WT mice, SRB1-/- mice have higher plasma HDL levels and an HDL surface that is free cholesterol (FC)-rich (60 vs. 15 mol%). This would be expected to increase HDL-FC bioavailability to cytotoxic levels. HDL dysfunctionality among SRB1-/- mice is associated with multiple metabolic abnormalities—impaired cell membrane structure and function and atherosusceptibility, despite having high plasma HDL-C levels, and infertility among female SRB1-/- mice. Dietary probucol, an HDL-lowering drug, partially reverses infertility in SR-B1-/- mice. The mechanisms underlying ovaritoxicity is not known and interventions that fully reverse this state has not been identified. We hypothesized that a bacterial protein, serum opacity factor (SOF), which acts on HDL and lowers cholesterol in WT mice by 43% in 3 hours, would normalize HDL functionality and rescue infertility in SRBI-/- female mice. SOF DNA was cloned into a TBG-AAV8 plasmid and virus produced by UPENN Vector Lab. The AAV-SOF was delivered by IP injection leading to the endogenous expression of SOF. Probucol-fed and AAV-GFP mice were used as controls. SOF expression drastically decreased plasma cholesterol and normalized the size of HDL in SRBI-/- females comparable to that of WT mice. Fertility was restored in female mice injected with SOF while GFP controls remained infertile. Compared to Probucol-fed mice, AAV-SOF females had a higher percentage of fertile females and shorter time to first litter drop. We concluded that SOF performed equally or better than probucol in rescuing fertility and normalizing HDL.

Abstract 499: Serum Opacity Factor Therapy Alters Plasma, Erythrocyte And Tissue Cholesterol Content In The Dysfunctional High Density Lipoprotein Scarb1 -/- Mouse Model

Aim: In humans, very high plasma HDL-cholesterol concentrations are associated with increased all cause- and atherosclerotic cardiovascular disease (ASCVD)-mortality. The HDL receptor-deficient mouse (Scarb1 -/- ) is a robust model of this phenotype having high free cholesterol (FC) bioavailability due to too many FC-rich HDL particles. Clinically, plasma LDL and HDL are quantified according to total cholesterol = FC + cholesteryl esters (CE), which likely contribute to ASCVD pathophysiology differently. Despite higher HDL, Scarb1 -/- mice have more ASCVD on a Western diet, and increased mol% FC in ovaries, erythrocytes, heart, lung, female liver and macrophages, tissues that are associated with female infertility, impaired cell maturation, cardiac dysfunction and atherosclerosis. Bacterial serum opacity factor (SOF) reduces plasma cholesterol ~ 40% by diverting HDL-cholesterol to the hepatic LDLR.

Hypothesis: Adeno-associated virus delivery of SOF (AAV SOF ) normalizes plasma and tissue FC accretion and reverses the pathologies associated with Scarb1 -/- mice.

Methods: The lipid compositions of plasma, HDL, erythrocytes, and tissues of Scarb1 -/- mice treated with AAV SOF at 12-13 weeks of age for three weeks were compared with age- and sex-matched wild type (WT) C57BL6 and Scarb1 -/- mice.

Results: As hypothesized, AAV SOF reduced plasma and HDL-FC and CE, as well as mol% FC in Scarb1 -/- mice towards WT levels. Erythrocyte FC levels also fell, but mol% FC remained elevated. Some changes were sex-specific: AAV SOF reduced the elevated FC only in female livers to WT levels. AAV SOF reduced FC and CE in lungs of females to WT levels, but not among males; the mol% FC remained high in both sexes. In steroidogenic tissues, adrenals, ovaries and testis, AAV SOF treatment increased FC. Unexpectedly, in Scarb1 -/- mice, AAV SOF increased mol% FC and FC in heart beyond already elevated levels.

Conclusions: These findings support the hypothesis that plasma and HDL cholesterol levels determine tissue cholesterol levels that drive the pathologies specific to Scarb1 -/- mice. This evolving model of the role of HDL-FC in RCT provides a rationale for human studies to determine the utility of HDL-FC bioavailability as a risk factor for ASCVD and other pathologies.

High Free Cholesterol Bioavailability Drives the Tissue Pathologies in Scarb1-/- Mice

Objective: Overall and atherosclerosis-associated mortality is elevated in humans with very high HDL (high-density lipoprotein) cholesterol concentrations. Mice with a deficiency of the HDL receptor, Scarb1 (scavenger receptor class B type 1), are a robust model of this phenotype and exhibit several additional pathologies. We hypothesized that the previously reported high plasma concentration of free cholesterol (FC)-rich HDL in Scarb1-/- mice produces a state of high HDL-FC bioavailability that increases whole-body FC and dysfunction in multiple tissue sites. Approach and Results: The higher mol% FC in Scarb1-/- versus WT (wild type) HDL (41.1 versus 16.0 mol%) affords greater FC bioavailability for transfer to multiple sites. Plasma clearance of autologous HDL-FC mass was faster in WT versus Scarb1-/- mice. FC influx from Scarb1-/- HDL to LDL (low-density lipoprotein) and J774 macrophages was greater ([almost equal to]4x) than that from WT HDL, whereas FC efflux capacity was similar. The higher mol% FC of ovaries, erythrocytes, heart, and macrophages of Scarb1-/- versus WT mice is associated with previously reported female infertility, impaired cell maturation, cardiac dysfunction, and atherosclerosis. The FC contents of other tissues were similar in the two genotypes, and these tissues were not associated with any overt pathology. In addition to the differences between WT versus Scarb1-/- mice, there were many sex-dependent differences in tissue-lipid composition and plasma FC clearance rates. Conclusions: Higher HDL-FC bioavailability among Scarb1-/- versus WT mice drives increased FC content of multiple cell sites and is a potential biomarker that is mechanistically linked to multiple pathologies.

Abstract MP30: Excess HDL Free Cholesterol Bioavailability Drives Free Cholesterol Accretion Into Macrophages And Erythrocytes In Scarb1 -/- Mice

Aim: In humans, very high plasma HDL-cholesterol concentrations are associated with increased all cause- and atherosclerotic cardiovascular disease (ASCVD)-mortality. The HDL receptor-deficient mouse (Scarb1 -/- ), a robust model of this phenotype, is characterized by high free cholesterol (FC) bioavailability due to too many HDL particles that are FC-rich. Clinically, plasma LDL and HDL are quantified according to total cholesterol content, the sum of FC and esterified cholesterol, which likely contribute to ASCVD pathophysiology differently. A Western diet induces ASCVD in Scarb1 -/- mice, despite an attendant increase in HDL. We tested the hypothesis that high HDL-FC bioavailability contributes to ASCVD in Scarb1 -/- mice by increasing FC flux into macrophage cells, erythrocytes and other major tissues.

Methods: Influx of HDL-FC and efflux of macrophage FC were determined between WT and Scarb1 -/- HDL and J774 macrophage cells. HDL of both genotypes were radiolabelled with [ 3 H]FC, injected into autologous mice, and the rates of plasma clearance and erythrocyte uptake were determined.

Results: The magnitude of FC transfer from Scarb1 -/- HDL to LDL is greater than that from WT HDL; APOB-containing lipoproteins from Scarb1 -/- vs. WT mice are FC-enriched due likely to greater HDL-FC transfer. While macrophage efflux to HDL of Scarb1 -/- vs. WT HDL was not different, FC influx from Scarb1 -/- vs. WT HDL to macrophages was three-fold greater, a net effect that increased the FC burden of macrophages. In vivo studies showed that compared to WT mice, in Scarb1 -/- mice, autologous HDL-FC cleared more slowly and more FC transferred to erythrocytes. We compared the FC, CE, PL, and TG contents of all major tissues and determined that FC accretion by some tissues is higher among Scarb1 -/- vs. WT mice whereas in other tissues FC homeostasis is maintained. Lastly, we determined that the tissue compositions and plasma FC clearance kinetics varied according to sex, particularly among Scarb1 -/- mice.

Conclusions: These findings are relevant to pathologies specific to Scarb1 -/- mice and to the evolving model of the role of HDL-FC in RCT. They provide a rationale for human studies to determine the utility of HDL-FC bioavailability as a risk factor for ASCVD and other pathologies.

Physico-chemical and physiological determinants of lipo-nanoparticle stability

Liposome-based nanoparticles (NPs) comprised mostly of phospholipids (PLs) have been developed to deliver diagnostic and therapeutic agents. Whereas reassembled plasma lipoproteins have been tested as NP carriers of hydrophobic molecules, they are unstable because the components can spontaneously transfer to other PL surfaces-cell membranes and lipoproteins-and can be degraded by plasma lipases. Here we review two strategies for NP stabilization. One is to use PLs that contain long acyl-chains: according to a quantitative thermodynamic model and in vivo tests, increasing the chain length of a PL reduces the spontaneous transfer rate and increases plasma lifetime. A second strategy is to substitute ether for ester bonds which makes the PLs lipase resistant. We conclude with recommendations of simple ex vivo and in vitro tests of NP stability that should be conducted before in vivo tests are begun.

Highly conserved amino acid residues in apolipoprotein A1 discordantly induce high density lipoprotein assembly in vitro and in vivo

OBJECTIVE

Apolipoprotein A1 (APOA1) is essential to reverse cholesterol transport, a physiologically important process that protects against atherosclerotic cardiovascular disease. APOA1 is a 28 kDa protein comprising multiple lipid-binding amphiphatic helices initialized by proline residues, which are conserved across multiple species. We tested the hypothesis that the evolutionarily conserved residues are essential to high density lipoprotein (HDL) function.

APPROACH

We used biophysical and physiological assays of the function of APOA1_P➔A variants, i.e., rHDL formation via dimyristoylphosphatidylcholine (DMPC) microsolubilization, activation of lecithin: cholesterol acyltransferase, cholesterol efflux from human monocyte-derived macrophages (THP-1) to each variant, and comparison of the size and composition of HDL from APOA1^-/- mice receiving adeno-associated virus delivery of each human variant.

RESULTS

Differences in microsolubilization were profound and showed that conserved prolines, especially those in the C-terminus of APOA1, are essential to efficient rHDL formation. In contrast, P➔A substitutions produced small changes (-25 to +25%) in rates of cholesterol efflux and no differences in the rates of LCAT activation. The HDL particles formed following ectopic expression of each variant in APOA1^-/- mice were smaller and more heterogeneous than those from control animals.

CONCLUSION

Studies of DMPC microsolubilization show that proline residues are essential to the optimal interaction of APOA1 with membranes, the initial step in cholesterol efflux and HDL production. In contrast, P➔A substitutions modestly reduce the cholesterol efflux capacity of APOA1, have no effect on LCAT activation, but according to the profound reduction in the size of HDL formed in vivo, P➔A substitutions alter HDL biogenesis, thereby implicating other cellular and in vivo processes as determinants of HDL metabolism and function.

The Alcohol-High-Density Lipoprotein Athero-Protective Axis

Ingestion of alcohol is associated with numerous changes in human energy metabolism, especially that of plasma lipids and lipoproteins. Regular moderate alcohol consumption is associated with reduced atherosclerotic cardiovascular disease (ASCVD), an effect that has been attributed to the concurrent elevations of plasma high-density lipoprotein-cholesterol (HDL-C) concentrations. More recent evidence has accrued against the hypothesis that raising plasma HDL concentrations prevents ASCVD so that other metabolic processes associated with alcohol consumption have been considered. This review explored the roles of other metabolites induced by alcohol consumption-triglyceride-rich lipoproteins, non-esterified free fatty acids, and acetate, the terminal alcohol metabolite in athero-protection: Current evidence suggests that acetate has a key role in athero-protection but additional studies are needed.

Dietary Alcohol and Fat Differentially Affect Plasma Cholesteryl Ester Transfer Activity and Triglycerides in Normo- and Hypertriglyceridemic Subjects

Moderate alcohol consumption is associated with increased plasma high-density lipoprotein (HDL)-cholesterol concentrations and reduced risk for cardiovascular disease. Plasma cholesteryl ester transfer activity (CETA) mediates the exchange of HDL-cholesteryl ester (CE) for the triacylglycerol (TAG) of very-low-density lipoproteins. We compared the effects of oral challenges of Alcohol, saturated fat (SAT), and (Alcohol + SAT) on plasma CETA, cholesterol, nonesterified fatty acids (NEFA), and TAG among normo-triglyceridemic (NTG) and mildly hypertriglyceridemic (HTG) volunteers having a range of plasma TAG concentrations. The major changes were (1) CETA increased more after ingestion of SAT and (Alcohol + SAT) in the HTG group versus the NTG group; (2) after all three challenges, elevation of plasma TAG concentration persisted longer in the HTG versus NTG group. Plasma cholesterol was not affected by the three dietary challenges, while Alcohol increased NEFA more in the HTG group than the NTG group. Plasma TAG best predicted plasma CETA, suggesting that intestinally derived lipoproteins are acceptors of HDL-CE. Unexpectedly, ingestion of (Alcohol + SAT) reduced the strength of the correlation between plasma TAG and CETA, that is the effects of (SAT and Alcohol) on plasma CETA are not synergistic nor additive but rather mutually suppressive. The alcohol-mediated inhibition of CE-transfer to chylomicrons maintains a higher plasma HDL-cholesterol concentration, which is athero-protective, although the suppressive metabolite underlying this correlation could be acetate, the terminal alcohol metabolite, other factors, including CETA inhibitors, are also likely important.

Somatic Editing of Ldlr With Adeno-Associated Viral-CRISPR Is an Efficient Tool for Atherosclerosis Research

Objective- Atherosclerosis studies in Ldlr knockout mice require breeding to homozygosity and congenic status on C57BL6/J background, a process that is both time and resource intensive. We aimed to develop a new method for generating atherosclerosis through somatic deletion of Ldlr in livers of adult mice. Approach and Results- Overexpression of PCSK9 (proprotein convertase subtilisin/kexin type 9) is currently used to study atherosclerosis, which promotes degradation of LDLR (low-density lipoprotein receptor) in the liver. We sought to determine whether CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats-associated 9) could also be used to generate atherosclerosis through genetic disruption of Ldlr in adult mice. We engineered adeno-associated viral (AAV) vectors expressing Staphylococcus aureus Cas9 and a guide RNA targeting the Ldlr gene (AAV-CRISPR). Both male and female mice received either (1) saline, (2) AAV-CRISPR, or (3) AAV-hPCSK9 (human PCSK9)-D374Y. A fourth group of germline Ldlr-KO mice was included for comparison. Mice were placed on a Western diet and followed for 20 weeks to assess plasma lipids, PCSK9 protein levels, atherosclerosis, and editing efficiency. Disruption of Ldlr with AAV-CRISPR was robust, resulting in severe hypercholesterolemia and atherosclerotic lesions in the aorta. AAV-hPCSK9 also produced hypercholesterolemia and atherosclerosis as expected. Notable sexual dimorphism was observed, wherein AAV-CRISPR was superior for Ldlr removal in male mice, while AAV-hPCSK9 was more effective in female mice. Conclusions- This all-in-one AAV-CRISPR vector targeting Ldlr is an effective and versatile tool to model atherosclerosis with a single injection and provides a useful alternative to the use of germline Ldlr-KO mice.

Revisiting Reverse Cholesterol Transport in the Context of High-Density Lipoprotein Free Cholesterol Bioavailability

Dysregulated free cholesterol (FC) metabolism has been implicated in nearly all stages of atherosclerosis, the underlying cause of most cardiovascular disease. According to a widely cited model, the burden of macrophage FC in the arterial wall is relieved by transhepatic reverse cholesterol transport (RCT), which comprises three successive steps: (1) macrophage FC efflux to high-density lipoprotein (HDL) and/or its major protein, apolipoprotein AI; (2) FC esterification by lecithin:cholesterol acyltransferase (LCAT); and (3) HDL-cholesteryl ester (CE) uptake via the hepatic HDL-receptor, scavenger receptor class B type 1 (SR-B1). Recent studies have challenged the validity of this model, most notably the role of LCAT, which appears to be of minor importance. In mice, most macrophage-derived FC is rapidly cleared from plasma (t_1/2 < 5 min) without esterification by hepatic uptake; the remainder is taken up by multiple tissue and cell types, especially erythrocytes. Further, some FC is cleared by the nonhepatic transintestinal pathway. Lastly, FC movement among lipid surfaces is reversible, so that a higher-than-normal level of HDL-FC bioavailability-defined by high plasma HDL levels concurrent with a high mol% HDL-FC-leads to the transfer of excess FC to cells in vivo. SR-B1^-/- mice provide an animal model to study the mechanistic consequences of high HDL-FC bioavailability that provokes atherosclerosis and other metabolic abnormalities. Future efforts should aim to reduce HDL-FC bioavailability, thereby reducing FC accretion by tissues and the attendant atherosclerosis.

Abstract 394: Bacterial Serum Opacity Factor Rescues HDL Functionality in SR-B1 -/- Mice

Although plasma HDL-C levels negatively correlate with atherosclerotic cardiovascular disease (ACVD), attempts to reduce ACVD risk by raising plasma HDL have disappointed. Thus, hypotheses about salutary HDL effects have shifted from higher-is-better to function-is-more-important. The SR-B1 -/- mouse is an extreme model of HDL dsyfunctionality; compared to WT mice, SR-B1 -/- mice have higher plasma HDL levels and an HDL surface that is free cholesterol (FC)-rich (60 vs. 15 mol%). This would be expected to increase HDL-FC bioavailability to cytotoxic levels. HDL dysfunctionality among SR-B1 -/- mice is associated with multiple metabolic abnormalities—impaired cell membrane structure and function and atherosusceptibility, despite having high plasma HDL-C levels; moreover, female SR-B1 -/- mice are infertile. Liver-specific SR-B1 expression in SR-B1 -/- mice normalizes HDL size and FC content. Thus, the SR-B1 -/- mouse phenotype is due to lack of hepatic clearance of lipids from dysfunctional HDL. Serum opacity factor (SOF) is a bacterial protein that catalyzes the quantitative disproportionation of HDL into a cholesteryl ester-rich micro emulsion (CERM), neo HDL, and lipid-free apo AI. The CERM contains apo E and all HDL-CE. Injection of SOF (4 μg) into WT mice lowers plasma cholesterol by diverting the CERM to hepatic LDLR. Thus, we began testing whether adeno-viral delivery of SOF (AAV SOF ) to SR-B1 -/- mice rescues HDL functionality. A plasmid encoding the SOF gene was synthesized; SOF DNA was isolated by restriction enzyme digestion and cloned into a pAAV-TBG-mcs plasmid that was submitted to the PENN Vector Lab for virus production and isolation. AAV GFP plasmid (UPENN) was used as control. Good SOF production and secretion was confirmed by transfection of Huh7 hepatocytes. AAV SOF injection into SR-B1 -/- mice induced constitutive plasma SOF activity and reduced HDL-C levels to nil. Superposition of high plasma HDL levels and a high mol% FC in SR-B1 -/- is expected to increase HDL-FC bioavailablity that contributes to whole-body FC-toxicity and the observed metabolic abnormalities. Future tests will determine whether ablation of dysfunctional HDL in SR-B1 -/- mice rescues their pathological phenotype, especially atherosclerosis.

Abstract 551: Is Free Cholesterol Bioavailability a Determinant of Dysfunctional, Atherogenic High Density Lipoproteins? Refining the Model of Reverse Cholesterol Transport

Although cardiovascular disease (CVD) negatively correlates with high-density lipoprotein-cholesterol (HDL-C) levels, to date HDL-raising therapies have not reduced CVD; thus, in the context of reverse cholesterol transport (RCT), HDL quality, i.e., functionality, may be more important to atheroprotection than HDL quantity. The current RCT model comprises free cholesterol (FC) efflux from macrophages to apo AI to give nHDL, LCAT-mediated nHDL-FC esterification forming mature HDL, SR-B1-mediated hepatic uptake of HDL lipids, followed by sterol metabolism and excretion. Our recent studies challenge this model: We observed that nHDL apo AI, FC and phospholipid (PL) metabolically segregate. In mice, plasma nHDL FC and PL are hepatically cleared with t 1/2 ~3 min; nHDL-apo AI is cleared more slowly with t 1/2 = 460 min. FC esterification is 100X slower, and thus a minor RCT step. These results and the observation that nHDL is FC-rich (~64 mol%) led to a revised model of RCT, with a focus on FC bioavailabilty rather than CE uptake. HDL from SR-B1 -/- mice is also FC-rich and associated with atherosclerosis. Moreover, the magnitude of HDL-C content/particle is associated with carotid artery atheroprogression (Qi et al JACC 2015 65:355-363), and HDL from hyperlipidemic HIV patients, which are at increased CVD risk, also have a high mol% FC compared to controls (29 vs 16 mol%, p<0.05). Thus, we hypothesize that high HDL-FC bioavailability, measured as the product of mol% HDL-FC and HDL particle number, is a metric for dysfunctional, atherogenic HDL. We studied SRB1 -/- mice, which are atherosusceptible and a model of dysfunctional HDL. Compared to WT mice, SR-B1 -/- mice have a higher HDL particle number and mol% FC (~58 vs. 15). Compared to WT HDL, SRB1 -/- HDL is more resistant to disruption by GdmHCl and serum opacity factor, indicating a resistance to remodeling. We plan to compare FC bioavailability of WT and SRB1 -/- HDL according to the kinetics of HDL-FC transfer to LDL. Our studies will determine if increased HDL-FC bioavailability in dysfunctional FC-rich HDL supports whole-body hypercholesterolemia that could increase 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.

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.

Native and Reconstituted Plasma Lipoproteins in Nanomedicine: Physicochemical Determinants of Nanoparticle Structure, Stability, and Metabolism

Although many acute and chronic diseases are managed via pharmacological means, challenges remain regarding appropriate drug targeting and maintenance of therapeutic levels within target tissues. Advances in nanotechnology will overcome these challenges through the development of lipidic particles, including liposomes, lipoproteins, and reconstituted high-density lipoproteins (rHDL) that are potential carriers of water-soluble, hydrophobic, and amphiphilic molecules. Herein we summarize the properties of human plasma lipoproteins and rHDL, identify the physicochemical determinants of lipid transfer between phospholipid surfaces, and discuss strategies for increasing the plasma half-life of lipoprotein- and liposome-associated molecules.

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.

High-Density Lipoprotein Processing and Premature Cardiovascular Disease

High plasma concentrations of low-density lipoprotein-cholesterol (LDL-C) are a well-accepted risk factor for cardiovascular disease (CVD), and the statin class of hypolipidemic drugs has emerged as an effective means of lowering LDL-C and reducing CVD risk. In contrast, the role of plasma high-density lipoproteins (HDL) in protection against atherosclerotic vascular disease is the subject of considerable controversy. Although the inverse correlation between plasma HDL-C and CVD is widely acknowledged, reduction of CVD risk by interventions that increase HDL-C have not been uniformly successful. Several studies of large populations have shown that the first step in reverse cholesterol transport (RCT), the transfer of cholesterol from the subendothelial space of the arterial wall via the plasma compartment to the liver for disposal, is impaired in patients with CVD. Here we review HDL function, the mechanisms by which HDL supports RCT, and the role of RCT in preventing CVD.

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.

Abstract 635: Cholesterol Loading Increases the Size and Cholesterol Content of Nascent HDL Formed via the Interaction of Apo AI With Cellular ABCA1

Cardiovascular disease (CVD) is a major cause of mortality and morbidity and development of new therapies that address its underlying causes is an important public health priority. Although CVD negatively correlates with high-density lipoprotein-cholesterol (HDL-C), HDL-raising therapies have not reduced CVD risk, particularly with statin co-therapy. Current data suggests that improving HDL functionality is more important to cardioprotection than increasing plasma HDL-C. Thus, it is important to identify structure-function relationships for various HDL and their precursors, nascent HDL. Nascent HDL is formed via the interaction of apo AI with ATP-binding cassette transporter A1 (ABCA1); rHDL formed via the interaction of apo AI with multilamellar membranes are in vitro models of nascent HDL. Both rHDL and nascent HDL are discoidal and their sizes increase with the cholesterol content of membranes from which they are derived. Given that very large rHDL, ~36 nm are formed at high cholesterol concentrations, ~15 mol%, we tested the hypothesis that similar large nascent HDL form via the interaction of apo AI with cells after hyperloading with cholesterol. To test this hypothesis, we incubated BHK-ABCA1 cells with cholesterol-loaded methyl β-cyclodextrin (CDX) for two hours. The free cholesterol (FC) content of the cells increased with increasing CDX:cholesterol concentration, from 29 ± 2 to 52 ± 6 μg FC/ mg cell protein. Concurrently, the size and cholesterol content of the nascent HDL produced by the interaction of apo AI with the cells also increased, with sizes ranging from ~ 7 to ~20 nm. These data suggest that the same membrane qualities and their modification by cholesterol determine the size of both rHDL and nascent HDL. Future studies will identify their structure in greater detail by a combination of physicochemical methods.

Abstract 429: Structural Stability of Streptococcal Serum Opacity Factor

Despite its putative cardioprotective qualities, raising plasma high density lipoprotein-cholesterol (HDL-C) levels via pharmacologic means has failed to add protection against atherosclerosis, particularly when used as co-therapy with a statin. Two scenarios argue against the raising-plasma-HDL-is-better hypothesis and suggest that enhancing the final RCT step, hepatic cholesterol disposal, is a better cardioprotective strategy. First, probucol prevents CVD events and increases survival in humans and reduces CVD in SR-BI/apo E DKO mice. Despite its anti atherogenic properties, probucol lowers plasma HDL-C levels. Second, SR-BI over expressing vs. WT mice have lower plasma HDL-C but less atherosclerosis whereas the converse is true in SR-BI KO mice, suggesting that increasing HDL-C disposal is a rational cardioprotective strategy. One possible agent for therapeutic development is streptococcal serum opacity factor (SOF), a 100 kDa protein that clouds human serum via a novel HDL-targeting mechanism. SOF diverts HDL-CE to the LDL receptor and to bile acid secretion in vitro and in vivo, increases plasma HDL-C clearance in mice in an apo E-, LDLR-dependent mechanism thereby increasing hepatic CE uptake and reducing plasma cholesterol levels. SOF is active at 10 -14 M. Given its novel mechanism and potent reduction of plasma cholesterol levels in mice, we studied the structure and stability of SOF using its activity as a marker of its integrity versus several physicochemical challenges—extremes of pH, the denaturant, guanidinium chloride, heat, and ionic strength. SOF was highly resistant to all of these challenges. SOF has only one cysteine so it cannot be stabilized by internal disulfide bonds. Thus, SOF is an unusually stable protein that undergoes reversible unfolding-folding when challenged with a variety of physicochemical perturbants. These studies have helped us identify optimal conditions for crystallizing SOF for X-ray structure analysis.

Direct Measurement of the Structure of Reconstituted High-Density Lipoproteins by Cryo-EM

Early forms of high-density lipoproteins (HDL), nascent HDL, are formed by the interaction of apolipoprotein AI with macrophage and hepatic ATP-binding cassette transporter member 1. Various plasma activities convert nascent to mature HDL, comprising phosphatidylcholine (PC) and cholesterol, which are selectively removed by hepatic receptors. This process is important in reducing the cholesterol burden of arterial wall macrophages, an important cell type in all stages of atherosclerosis. Interaction of apolipoprotein AI with dimyristoyl (DM)PC forms reconstituted (r)HDL, which is a good model of nascent HDL. rHDL have been used as an antiathersclerosis therapy that enhances reverse cholesterol transport in humans and animal models. Thus, identification of the structure of rHDL would inform about that of nascent HDL and how rHDL improves reverse cholesterol transport in an atheroprotective way. Early studies of rHDL suggested a discoidal structure, which included pairs of antiparallel helices of apolipoprotein AI circumscribing a phospholipid bilayer. Another rHDL model based on small angle neutron scattering supported a double superhelical structure. Herein, we report a cryo-electron microscopy-based model of a large rHDL formed spontaneously from apolipoprotein AI, cholesterol, and excess DMPC and isolated to near homogeneity. After reconstruction we obtained an rHDL structure comprising DMPC, cholesterol, and apolipoprotein AI (423:74:1 mol/mol) forming a discoidal particle 360 Å in diameter and 45 Å thick; these dimensions are consistent with the stoichiometry of the particles. Given that cryo-electron microscopy directly observes projections of individual rHDL particles in different orientations, we can unambiguously state that rHDL particles are protein bounded discoidal bilayers.

Streptococcal serum opacity factor promotes cholesterol ester metabolism and bile acid secretion in vitro and in vivo

Plasma high density lipoprotein-cholesterol (HDL-C) concentrations negatively correlate with atherosclerotic cardiovascular disease. HDL is thought to have several atheroprotective functions, which are likely distinct from the epidemiological inverse relationship between HDL-C levels and risk. Specifically, strategies that reduce HDL-C while promoting reverse cholesterol transport (RCT) may have therapeutic value. The major product of the serum opacity factor (SOF) reaction versus HDL is a cholesteryl ester (CE)-rich microemulsion (CERM), which contains apo E and the CE of ~400,000 HDL particles. Huh7 hepatocytes take up CE faster when delivered as CERM than as HDL, in part via the LDL-receptor (LDLR). Here we compared the final RCT step, hepatic uptake and subsequent intracellular processing to cholesterol and bile salts for radiolabeled HDL-, CERM- and LDL-CE by Huh7 cells and in vivo in C57BL/6J mice. In Huh7 cells, uptake from LDL was greater than from CERM (2-4X) and HDL (5-10X). Halftimes for [(14)C]CE hydrolysis were 3.0±0.2, 4.4±0.6 and 5.4±0.7h respectively for HDL, CERM and LDL-CE. The fraction of sterols secreted as bile acids was ~50% by 8h for all three particles. HDL, CERM and LDL-CE metabolism in mice showed efficient plasma clearance of CERM-CE, liver uptake and metabolism, and secretion as bile acids into the gall bladder. This work supports the therapeutic potential of the SOF reaction, which diverts HDL-CE to the LDLR, thereby increasing hepatic CE uptake, and sterol disposal as bile acids.

Abstract 326: Streptococcal Serum Opacity Factor Product, Cholesteryl Ester Rich Particles, Induce Cholesterol Ester Metabolism and Bile Acid Secretion after Uptake by Human Huh7 Hepatocytes

Plasma concentrations of HDL-cholesterol (C) negatively correlate with atherosclerotic cardiovascular disease. Nevertheless, current evidence suggests that this correlation is not axiomatic and that some forms of HDL are dysfunctional and atherogenic. Interventions that raise HDL-C have not been uniformly successful suggesting that mechanisms by which HDL-C is increased determine its anti atherogenic potency. Additionally, mice overexpressing the HDL receptor, SR-BI, have lower plasma HDL-C levels and less atherosclerosis. Thus, new strategies that reduce HDL-C while promoting reverse cholesterol transport (RCT) may have therapeutic value. Serum opacity factor (SOF) disrupts HDL and forms three products, including a cholesteryl ester-rich microemulsion (CERM) containing apo E and the CE of ~400,000 HDL particles. Hepatic CE uptake in Huh7 cells is faster when delivered by CERM than by HDL, and cleared both in vitro and in vivo, in part, via the LDL-receptor (LDLR). We investigated the therapeutic potential of SOF in promoting RCT by comparing the final RCT steps, hepatic uptake, CE metabolism to cholesterol and bile salts, and secretion, of [ 14 C]-CE-HDL, -CERM and [[Unable to Display Character: &#8211;]]LDL. Cells were pulse-labeled for 2 h with 20-50 ug/mL HDL protein (6-17 ug/mL [ 14 C]-CE), or the CE-equivalent as CERM or LDL, and chased for 0, 2 or 6 h. Cells, pulse media and chase media were analyzed for sterols by beta-counting, TLC and solvent partitioning. [ 14 C]CE uptake from LDL was greater than from CERM (2-4X) and HDL (5-10X). Halftimes for [ 14 C]CE hydrolysis were 3.0 ± 0.2, 4.4 ± 0.6 and 5.4 ± 0.7 h respectively for HDL, CERM and LDL-CE. Bile acids in cells after uptake from HDL or CERM were low, <0.2% of total sterol but higher in cells after uptake from LDL, 4.2 ± 2.2 % of total sterol ( p = 0.03 ) at the 2 h chase time. The fraction of sterols secreted as bile acids was comparable for all three donor particles. After the 2 h chase, ~40% of the secreted sterols were bile acids, and after 6 h, ~ 50% were bile acids. These ratios were the same for cells treated with 14 C-CE-HDL, CERM or LDL. Thus, the rates of hepatic metabolism of CERM-CE to free cholesterol, to bile acids and secretion are intermediate between those for HDL- and LDL-CE, supporting the therapeutic potential of SOF as a promoter of 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.

Alcohol: a nutrient with multiple salutary effects

Numerous studies have shown that cardiovascular disease is lower among alcohol consumers than among nonconsumers. Many of the metabolic effects of alcohol are mediated by its terminal metabolite, acetate, which has reported insulinemic properties. There have been few rational metabolic targets that underly its cardioprotective effects until it was reported that acetate, the terminal product of alcohol metabolism, is the ligand for G-protein coupled receptor 43 (GPCR43), which is highly expressed in adipose tissue. Here, we recast much of some of the major lipid and lipoprotein effects of alcohol in the context of this newly discovered G-protein and develop a mechanistic model connecting the interaction of acetate with adipose tissue-GPCR43 with these effects. According to our model, ingestions of acetate could replace alcohol as a means of improving plasma lipid risk factors, improving glucose disposal, and reducing cardiovascular disease. Future studies should include biochemical, cell, animal, and human tests of acetate on energy metabolism.

Intensive lifestyle modification reduces Lp-PLA2 in dyslipidemic HIV/HAART patients

PURPOSE

Patients with dyslipidemia associated with HIV-1 infection and highly active antiretroviral therapy (HAART) have elevated levels of Lp-PLA2 and CCL5/regulated on activation, normal T-cell expressed and secreted (RANTES), which may increase the risk of cardiovascular disease.

PURPOSE

This study aimed to determine whether an intensive diet and exercise (D/E) program, independently or combined with fenofibrate or niacin, could reduce Lp-PLA2 or RANTES.

METHODS

Patients with hypertriglyceridemic HIV on stable HAART (n = 107) were randomized to one of five interventions: 1) usual care, 2) D/E with placebos, 3) D/E with fenofibrate and placebo, 4) D/E with niacin and placebo, or 5) D/E with fenofibrate and niacin for 24 wk. Lp-PLA2 and RANTES concentrations were measured in fasting plasma samples at baseline and postintervention. General linear models were used to compare Lp-PLA2 and RANTES levels between the five groups postintervention, controlling for baseline levels, age, body mass index, CD4 T-cell count, viral load, duration of infection, and HAART.

RESULTS

At baseline, fasting plasma Lp-PLA2 (388.5 ± 127.5 ng·mL) and RANTES (43.8 ± 25.5 ng·mL) levels were elevated when compared with healthy controls. Posttreatment Lp-PLA2 mass was lower in patients who received D/E only (323.0 ± 27.2 ng·mL), D/E plus fenofibrate (327.2 ± 25.9 ng·mL), and D/E plus niacin (311.1 ± 27.8 ng·mL) when compared with patients receiving usual care (402.2 ± 25.3 ng·mL). RANTES concentrations were not significantly affected by any intervention.

CONCLUSIONS

Elevated plasma Lp-PLA2 mass can be reduced by an intensive D/E program in patients with HIV/HAART-associated dyslipidemia. RANTES is elevated but is not reduced by lifestyle modification, fenofibrate, or niacin.

Setting the course for apoAII: a port in sight?

ApoAII, the second most abundant protein of the human plasma HDLs, was discovered nearly 50 years ago. Over the subsequent years, nearly 2000 studies - epidemiological, cell-based, biochemical, mouse and human - have attempted to unravel its role in human lipid metabolism. On the basis of these studies, apoAII has been described as an activator and inhibitor of various plasma activities, and as both pro- and anti-atherogenic. Here, we summarize the studies of apoAII, use the preponderance of evidence to propose that the apoAII compass can be reset towards an antiatherogenic course, and suggest ways to stay the course.

Free cholesterol determines reassembled high-density lipoprotein phospholipid phase structure and stability

Reassembled high-density lipoproteins (rHDL) of various sizes and compositions containing apo A-I or apo A-II as their sole protein, dimyristoylphosphatidylcholine (DMPC), and various amounts of free cholesterol (FC) have been isolated and analyzed by differential scanning calorimetry (DSC) and by circular dichroism to determine their stability and the temperature dependence of their helical content. Our data show that the multiple rHDL species obtained at each FC mole percent usually do not have the same FC mole percent as the starting mixture and that the size of the multiple species increases in a quantized way with their respective FC mole percent. DSC studies reveal multiple phases or domains that can be classified as virtual DMPC, which contains a small amount of DMPC that slightly reduces the melting temperature (Tm), a boundary phase that is adjacent to the apo A-I or apo A-II that circumscribes the discoidal rHDL, and a mixed FC/DMPC phase that has a Tm that increases with FC mole percent. Only the large rHDL contain virtual DMPC, whereas all contain boundary phase and various amounts of the mixed FC/DMPC phase according to increasing size and FC mole percent. As reported by others, FC stabilizes the rHDL. For rHDL (apo A-II) compared to rHDL (apo A-I), this occurs in spite of the reduced number of helical regions that mediate binding to the DMPC surface. This effect is attributed to the very high lipophilicity of apo A-II and the reduction in the polarity of the interface between DMPC and the aqueous phase with an increasing FC mole percent, an effect that is expected to increase the strength of the hydrophobic associations with the nonpolar face of the amphipathic helices of apo A-II. These data are relevant to the differential effects of FC and apolipoprotein species on intracellular and plasma membrane nascent HDL assembly and subsequent remodeling by plasma proteins.

Modest diet-induced weight loss reduces macrophage cholesterol efflux to plasma of patients with metabolic syndrome

BACKGROUND

Obesity-linked metabolic syndrome (MetS) is associated with a dyslipidemic profile that includes hypertriglyceridemia and low plasma high-density lipoprotein (HDL) cholesterol. HDL initiates reverse cholesterol transport via macrophage cholesterol efflux (MCE). Some hypothesize that dyslipidemic patients have impaired reverse cholesterol transport. MCE to patient plasma, a metric of HDL function, inversely correlates with atherosclerotic burden. Paradoxically, MCE to plasma of hypertriglyceridemic subjects is higher than that to normolipidemic (NL) plasma.

OBJECTIVE

Although weight loss reduces dyslipidemia, its effect on MCE to the plasma of obese patients with MetS is unknown. Thus, we tested the hypothesis that reducing dyslipidemia with weight loss reduces the MCE capacity of MetS plasma to that of NL plasma.

METHODS

Cholesterol efflux (MCE) from THP-1 macrophages to plasma from NL controls and to obese patients with MetS before and after weight loss was measured.

RESULTS

MCE to plasma of obese patients with MetS was higher than that of control plasma (P = .006). Weight loss in patients with MetS (mean, -9.77 kg) reduced dyslipidemia, insulin resistance, and systolic blood pressure. HDL cholesterol was unchanged, and apolipoprotein A-I decreased with weight loss. Weight loss in patients with MetS normalized MCE (P < .001) to that of NL subjects. MCE correlated with apolipoprotein B levels (r² = 0.13-0.38). Chromatography showed that macrophage cholesterol initially associates with HDL but accumulates in apolipoprotein B-containing lipoproteins at later times.

CONCLUSIONS

Although the initial acceptor of MCE is HDL, the elevated apolipoprotein B lipoproteins are a cholesterol sink that increases MCE in patients with MetS. Weight loss results in decreased apolipoprotein B lipoproteins and decreased MCE to plasma of patients with MetS.

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.

Abstract 102: Profound Exclusion of Apolipoprotein A-II from Selective Hepatic Cholesteryl Ester Uptake

Reverse cholesterol transport (RCT), the transfer of cholesterol from peripheral tissues, including the subendothelial space of the arterial wall, to the liver for disposal, is a current model of HDL atheroprotection. The final RCT step, selective hepatic HDL-cholesteryl ester (CE) uptake, is mediated by scavenger receptor class B type I (SR-BI). The net receptor reaction of SR-BI vs. HDL is distinct from that of LDL vs. the LDL receptor. LDL holo particle uptake is succeeded by steps that breakdown apo B-100 and hydrolyze and recycle the CE. In contrast, HDL-CE uptake is selective, occurring without a concomitant net uptake of the major HDL protein, apo A-I and even though apo E and apo A-I bind equally well to SR-BI, apoA-I-containing particles mediate 2-fold more selective CE uptake. The reaction of HDL with SR-BI is similar to the activity of a streptococcal serum opacity factor (SOF) against HDL_both reactions selectively remove CE from HDL leaving remnants. In addition, SOF catalyzes the displacement of apo A-I leaving an apo A-II-rich neo HDL, an effect that was assigned to the greater lipophilicity of apo A-II vs. apo A-I. Thus, we tested the hypothesis that the same occurs during the interaction of HDL with SR-BI, i.e., that apo A-II vs. apo A-I is selectively excluded from cellular uptake via SR-BI. Herein, we compare the selective uptake of HDL-CE vs. HDL-apo A-I and apo A-II. Cellular uptake of HDL-[ 3 H]CE labeled with [ 125 I]apo A-I or [ 125 I]apo A-II was compared in CHO-K1 and CHO-ldlA7 cells (LDL-R -/- ) with and without over expression of mouse SR-BI, and Huh7 human hepatocytes. Cell-associated 125 I and 3 H were determined by γ- and β-counting respectively. Uptake of CE, apo A-I, and apo A-II SR-BI-over expressing CHO cells was 32,800 ± 4800, 9.3 ± 2.7, and 2.5 ± 0.2 nmol/mg cell protein. The corresponding values for Huh7 cells were 9,700 ± 1,800, 15 ± 2.4, and 7.6 ± 0.9 nmol/mg cell protein. Relative to CE, both apo A-I and apo A-II were excluded from uptake by all cells. However, relative to the apo A-I and apo A-II contents of HDL, uptake of apo A-I was twice that of apo A-II, thus supporting the hypothesis that the more lipophilic apo A-II is selectively excluded from cellular uptake via SR-BI and retained in the neo HDL remnant.

Abstract 113: Streptococcal Serum Opacity Factor Activity Diverts Human High Density Lipoprotein (HDL)-Cholesteryl Esters (CE) to the LDL-Receptor Pathway in Human Huh7 Hepatocytes

Plasma concentrations of HDL-cholesterol (C) are negatively correlated with atherosclerotic cardiovascular disease. Nevertheless, current evidence suggests that this correlation is not axiomatic and that some forms of HDL are dysfunctional and atherogenic. Moreover, clinical trials of interventions that raise HDL-C have not been uniformly successful suggesting that mechanisms by which HDL-C is increased determine its anti atherogenic potency. Additionally, mice overexpressing the HDL receptor, SR-BI, have lower plasma HDL-C concentrations and less atherosclerosis. Thus, new strategies that reduce HDL-C while promoting reverse cholesterol transport (RCT) may have therapeutic value. Serum opacity factor (SOF) disrupts HDL and forms three products_lipid-free apo A-I, a CE-depleted remnant neo HDL, and a cholesteryl ester-rich microemulsion (CERM) containing apo E and the CE of up to ~400,000 HDL particles. Hepatic CE uptake in Huh7 and HepG2 cells is faster when delivered by CERM than from the parent HDL, and cleared in vitro , in part, via the LDL-receptor (LDLR). We investigated the therapeutic potential of SOF in promoting RCT by comparing the uptake of HDL-, LDL- and CERM-CE in the final RCT steps_hepatocyte uptake, CE metabolism to cholesterol and bile salts, and their secretion and transfer to bile. HDL and LDL with [ 14 C]CE were labeled with [ 14 C]CE by incubation with lipoprotein deficient serum and the lipoproteins recovered by ultracentrifugation. CERM-labeled with [ 14 C]CE was obtained from SOF activity vs. HDL labeled with [ 14 C]CE. Incorporation efficiencies for HDL and LDL were 67% and 52% while [ 14 C]CE specific activities were 3.93 x 10 -4 and 3.04 x 10 -4 mCi/mg CE, respectively. [ 14 C]Metabolites from the hepatic uptake of HDL, LDL, and CERM were quantified by a two-solvent TLC system that resolves CE, C, and the major bile salts. The [ 14 C]CE of all three particles was taken up by cells and converted to free cholesterol with half-times of 2.7, 3.8, and 3.7 h respectively from HDL, CERM and LDL. Rates of bile salt synthesis and secretion are currently being investigated. These data show that SOF diverts potentially toxic HDL-CE to the hepatic LDLR pathway where it is hydrolyzed.

Abstract 394: Metabolic Insights into Age and Gender: A New Algorithm for Regional Body Mass Distribution

Metabolic syndrome (MetS), a disorder of energy metabolism associated with increased risk of cardiovascular disease (CVD), comprises a cluster of risk factors—low HDL-C, hypertension, hypertriglyceridemia, hyperglycemia and central (abdominal) obesity. Although other investigators noted that non central fat loss, especially femoral gluteal depots, are associated with CVD, the association was not as strong as that with central obesity. We hypothesized that age- and gender-dependent changes in body fat distribution would uncover an underlying cause of MetS. However, it is impossible to compare fat distribution based on anatomical circumferences (C) and skin folds (SF) because of the confounding effects of body mass index (BMI), which rises in the early decades and declines thereafter. We developed an algorithm that adjusts skin folds and circumference data for BMI to obtain a BMI-corrected adiposity index (BAI = C/BMI) for each depot. For comparison over time, we normalized the data set to the value in the third decade of life (BAI 3 = 100) and calculated a normalized BAI. as BAI N = [BAI D /BAI 3 ] X 100. Application of this algorithm to the NHANES I data set of C and SF values, reveals BMI-independent changes in body fat distribution as a function of gender and age, with the most profound changes between the second and eighth decades of life occurring in waist C, which increases with time and buttock C, which decreases over the same time interval. In men the changes begin in the third decade of life. In women, the change does not occur until the beginning of the fifth decade of life, which is approximately the perimenopausal period. We propose a mechanistic model in which the underlying defect in MetS, is impaired fat storage in non-central depots, which results in the transfer of non-esterified fatty acids to skeletal muscle thereby impairing glucose disposal, to liver where it drives VLDL-TG synthesis and secretion, and to central fat depots giving the MetS waistline. The defect is not known but could be tissue-specific apoptosis or insulin resistance. If valid, this model would suggest that therapies that enhanced adipogenesis and/or insulin sensitivity in non-central depots could prevent or delay the onset of MetS.

Cholesterol determines and limits rHDL formation from human plasma apolipoprotein A-II and phospholipid membranes

Apolipoprotein (apo) A-II, the second most abundant protein after apo A-I of human plasma high-density lipoproteins (HDL), is the most lipophilic of the exchangeable apolipoproteins. The rate of microsolubilization of dimyristoylphosphatidylcholine (DMPC) membranes by apo A-I to give rHDL increases as the level of membrane free cholesterol (FC) increases up to 20 mol % when the level of reaction decreases to nil. Given its greater lipophilicity, we tested the hypothesis that apo A-II and its reduced and carboxymethylated monomer (rcm apo A-II) would form rHDL at a membrane FC content of >20 mol %. According to turbidimetric titrations, the DMPC/apo A-II stoichiometry is 65/1 (moles to moles). At this stoichiometry, apo A-II forms rHDL from DMPC and FC. Contrary to our hypothesis, apo A-II, like apo A-I, reacts poorly with DMPC containing ≥20 mol % FC. The rate of formation of rHDL from rcm apo A-II and DMPC at all FC mole percentages is faster than that of apo A-II but nil at 20 mol % FC. In parallel reactions, monomeric and dimeric apo A-II form large FC-rich rHDL coexisting with smaller FC-poor rHDL; increasing the FC mole percentage increases the number and size of FC-rich rHDL. On the basis of the compositions of coexisting large and small rHDL, the free energy of transfer of FC from the smallest to the largest particle is approximately −1.2 kJ. On the basis of our data, we propose a model in which apo A-I and apo A-II bind to DMPC via surface defects that disappear at 20 mol % FC. These data suggest apo A-II-containing HDL formed intrahepatically are likely cholesterol-rich compared to the smaller intracellular lipid-poor apo A-I HDL.

Abstract 105: Free Cholesterol Determines the Phospholipid Domain Size and Stability of rHDL

Background: Microsolubilization of dimyristoyl phosphatidylcholine (DMPC) by human apolipoprotein A-I is a frequently used model system for the identification of determinants of macrophage cholesterol efflux, the first step in reverse cholesterol transport (RCT) to hepatic disposal. Although many studies have focused on microsolubilization of DMPC, most have been conducted under stoichiometric conditions resulting in ∼9 nm rHDL, and few have included free cholesterol (FC), a key lipid in RCT.

Methods: Various rHDL species (excess DMPC + FC), were prepared and analyzed by size exclusion chromatography (SEC), differential scanning calorimetry (DSC) and circular dichroism (CD) spectroscopy as a function of temperature.

Results: At each mol% of FC/DMPC, multiple rHDL species differing in size from ∼10 to ∼25 nm were formed. In SEC, rHDL size increased with the mol% FC in the microsolubilization reaction and in a given reaction mixture the large rHDL were always more FC-rich than the small rHDL. In DSC, the DMPC transition range of rHDL was broader than that of pure DMPC and although the transition temperature moderately increases with increasing mol% FC, the apparent enthalpy of the transition is diminished. The thermal dependence of the CD spectra showed that increasing mol% FC and rHDL size moderately increases the stability of apo A-I.

Conclusion: Although large FC-rich and small FC-poor domains coexist on DMPC surfaces; the size of these domains increases with mol% FC. This effect of FC on rHDL size decreases the overall cooperativity of the DMPC phase transition and reduces the number of acyl chain interactions. Physiologically, these data suggest that increased FC-loading of macrophages should increase the size of nascent HDL produced by the interaction of apo A-I with cellular ABCA1. The lower stability of FC-poor HDL would likely make it release more lipid-free apo A-I in response to important plasma proteins including LCAT, PLTP, and CETP and in response to serum opacity factor.

Apolipoprotein E mediates enhanced plasma high-density lipoprotein cholesterol clearance by low-dose streptococcal serum opacity factor via hepatic low-density lipoprotein receptors in vivo

OBJECTIVE

Recombinant streptococcal serum opacity factor (rSOF) mediates the in vitro disassembly of human plasma high-density lipoprotein (HDL) into lipid-free apolipoprotein (apo) A-I, a neo-HDL that is cholesterol poor, and a cholesteryl ester-rich microemulsion (CERM) containing apoE. Given the occurrence of apoE on the CERM, we tested the hypothesis that rSOF injection into mice would reduce total plasma cholesterol clearance via apoE-dependent hepatic low-density lipoprotein receptors (LDLR).

METHODS AND RESULTS

rSOF (4 μg) injection into wild-type C57BL/6J mice formed neo-HDL, CERM, and lipid-free apoA-I, as observed in vitro, and reduced plasma total cholesterol (-43%, t(1/2)=44±18 minutes) whereas control saline injections had a negligible effect. Similar experiments with apoE(-/-) and LDLR(-/-) mice reduced plasma total cholesterol ≈0% and 20%, respectively. rSOF was potent; injection of 0.18 μg of rSOF produced 50% of maximum reduction of plasma cholesterol 3 hours postinjection, corresponding to a ≈0.5-mg human dose. Most cholesterol was cleared hepatically (>99%), with rSOF treatment increasing clearance by 65%.

CONCLUSIONS

rSOF injection into mice formed a CERM that was cleared via hepatic LDLR that recognize apoE. This reaction could provide an alternative mechanism for reverse cholesterol transport.

Serum opacity factor enhances HDL-mediated cholesterol efflux, esterification and anti inflammatory effects

Serum opacity factor (SOF) is a streptococcal protein that disrupts the structure of human high density lipoproteins (HDL) releasing lipid-free apo A-I while forming a large cholesteryl ester-rich particle and a small neo HDL. Given its low cholesterol and high phospholipid contents, we tested the hypotheses that neo HDL is a better substrate for cholesterol esterification via lecithin:cholesterol acyltransferase (LCAT), better than HDL as an acceptor of THP-1 macrophage cholesterol efflux, and improves reduction of oxidized LDL-induced production of inflammatory markers. We observed that both cholesterol efflux and esterification were improved by recombinant (r)SOF treatment of whole plasma and that the underlying cause of the improved cholesterol esterification in plasma and macrophage cholesterol efflux to rSOF-treated plasma was due to the rSOF-mediated conversion of HDL to neo HDL. Moreover, the reduction of secretion of TNF-α and IL-6 by THP-1 cells by neo HDL was twice that of HDL. Studies in BHK cells overexpressing cholesterol transporters showed that efflux to neo HDL occurred primarily via ABCA1 not ABCG1. Thus, rSOF improves two steps in reverse cholesterol transport with a concomitant reduction in the release of macrophage markers of inflammation. We conclude that rSOF catalyzes a novel reaction that might be developed as a new therapy that prevents or reverses atherosclerosis via improved reverse cholesterol transport.

Streptococcal serum opacity factor increases the rate of hepatocyte uptake of human plasma high-density lipoprotein cholesterol

Serum opacity factor (SOF), a virulence determinant of Streptococcus pyogenes, converts plasma high-density lipoproteins (HDL) to three distinct species: lipid-free apolipoprotein (apo) A-I, neo HDL, a small discoidal HDL-like particle, and a large cholesteryl ester-rich microemulsion (CERM) that contains the cholesterol esters (CE) of up to ∼400000 HDL particles and apo E as its major protein. Similar SOF reaction products are obtained with HDL, total plasma lipoproteins, and whole plasma. We hypothesized that hepatic uptake of CERM-CE via multiple apo E-dependent receptors would be faster than that of HDL-CE. We tested our hypothesis using human hepatoma cells and lipoprotein receptor-specific Chinese hamster ovary (CHO) cells. The uptake of [(3)H]CE by HepG2 and Huh7 cells from HDL after SOF treatment, which transfers >90% of HDL-CE to CERM, was 2.4 and 4.5 times faster, respectively, than from control HDL. CERM-[(3)H]CE uptake was inhibited by LDL and HDL, suggestive of uptake by both the LDL receptor (LDL-R) and scavenger receptor class B type I (SR-BI). Studies in CHO cells specifically expressing LDL-R and SR-BI confirmed CERM-[(3)H]CE uptake by both receptors. RAP and heparin inhibit CERM-[(3)H]CE but not HDL-[(3)H]CE uptake, thereby implicating LRP-1 and cell surface proteoglycans in this process. These data demonstrate that SOF treatment of HDL increases the rate of CE uptake via multiple hepatic apo E receptors. In so doing, SOF might increase the level of hepatic disposal of plasma cholesterol in a way that is therapeutically useful.

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.

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

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

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.

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.

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.