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.

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 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.

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.

High-density lipoproteins, reverse cholesterol transport and atherogenesis.

Plasma HDL-cholesterol concentrations correlate negatively with the risk of atherosclerotic cardiovascular disease (ASCVD). According to a widely cited model, HDL elicits its atheroprotective effect through its role in reverse cholesterol transport, which comprises the efflux of cholesterol from macrophages to early forms of HDL, followed by the conversion of free cholesterol (FCh) contained in HDL into cholesteryl esters, which are hepatically extracted from the plasma by HDL receptors and transferred to the bile for intestinal excretion. Given that increasing plasma HDL-cholesterol levels by genetic approaches does not reduce the risk of ASCVD, the focus of research has shifted to HDL function, especially in the context of macrophage cholesterol efflux. In support of the reverse cholesterol transport model, several large studies have revealed an inverse correlation between macrophage cholesterol efflux to plasma HDL and ASCVD. However, other studies have cast doubt on the underlying reverse cholesterol transport mechanism: in mice and humans, the FCh contained in HDL is rapidly cleared from the plasma (within minutes), independently of esterification and HDL holoparticle uptake by the liver. Moreover, the reversibility of FCh transfer between macrophages and HDL has implicated the reverse process - that is, the transfer of FCh from HDL to macrophages - in the aetiology of increased ASCVD under conditions of very high plasma HDL-FCh concentrations.

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 APOA1P➔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.

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 (t1/2 < 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.

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.

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 (t1/2=2-7 minutes), whereas nHDL-[125I]apo AI transfers solely to HDL (t1/2<10 minutes) and to the lipid-free form (t1/2>480 minutes). After injection into mice, nHDL-[3H]FC and [14C]PL rapidly transfer to liver (t1/2=≈2-3 minutes), whereas apo AI clears with t1/2=≈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.

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.

Somatic genome editing with CRISPR/Cas9 generates and corrects a metabolic disease.

Germline manipulation using CRISPR/Cas9 genome editing has dramatically accelerated the generation of new mouse models. Nonetheless, many metabolic disease models still depend upon laborious germline targeting, and are further complicated by the need to avoid developmental phenotypes. We sought to address these experimental limitations by generating somatic mutations in the adult liver using CRISPR/Cas9, as a new strategy to model metabolic disorders. As proof-of-principle, we targeted the low-density lipoprotein receptor (Ldlr), which when deleted, leads to severe hypercholesterolemia and atherosclerosis. Here we show that hepatic disruption of Ldlr with AAV-CRISPR results in severe hypercholesterolemia and atherosclerosis. We further demonstrate that co-disruption of Apob, whose germline loss is embryonically lethal, completely prevented disease through compensatory inhibition of hepatic LDL production. This new concept of metabolic disease modeling by somatic genome editing could be applied to many other systemic as well as liver-restricted disorders which are difficult to study by germline manipulation.

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.

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.

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.

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.