Synthetic High-density Lipoprotein Research Articles

HDL as a drug and nucleic acid delivery vehicle.

This review is intended to evaluate the research findings and potential clinical applications of drug transport systems, developed based on the concepts of the structure/function and physiological role(s) of high density lipoprotein type nanoparticles. These macromolecules provide targeted transport of cholesteryl esters (a highly lipophilic payload) in their natural/physiological environment. The ability to accommodate highly water insoluble constituents in their core regions enables High density lipoproteins (HDL) type nanoparticles to effectively transport hydrophobic drugs subsequent to systemic administration. Even though the application of reconstituted HDL in the treatment of a number of diseases is reviewed, the primary focus is on the application of HDL type drug delivery agents in cancer chemotherapy. The use of both native and synthetic HDL as drug delivery agents is compared to evaluate their respective potentials for commercial and clinical development. The current status and future perspectives for HDL type nanoparticles are discussed, including current obstacles and future applications in therapeutics.

HDL in sepsis - risk factor and therapeutic approach.

High-density lipoprotein (HDL) is a key component of circulating blood and plays essential roles in regulation of vascular endothelial function and immunity. Clinical data demonstrate that HDL levels drop by 40–70% in septic patients, which is associated with a poor prognosis. Experimental studies using Apolipoprotein A-I (ApoAI) null mice showed that HDL deficient mice are susceptible to septic death, and overexpressing ApoAI in mice to increase HDL levels protects against septic death. These clinical and animal studies support our hypothesis that a decrease in HDL level is a risk factor for sepsis, and raising circulating HDL levels may provide an efficient therapy for sepsis. In this review, we discuss the roles of HDL in sepsis and summarize the efforts of using synthetic HDL as a potential therapy for sepsis.

Abstract 3669: Engineering the biointerface of synthetic high density lipoprotein nanoparticles enables efficient nucleic acid loading, delivery, and target gene regulation in cancer cells

Abstract Local treatment of cancer confined to the primary organ can be curative. However, other than a few success stories, locally advanced and metastatic cancer remains incurable. Ideally, new therapies are needed that specifically target cancer cells and derive efficacy by targeting molecular pathways that are unique to distinct tumor profiles and individual patients. Toward this end, we focus on the use of biomimicry to exploit mechanisms by which cancer cells uptake molecular cargo. Naturally, some cancer cells are dependent upon exogenous cholesterol for cellular signaling, growth, and in some cancers, such as prostate, steroidogenesis. However, cholesterol is a highly hydrophobic molecule that requires a delivery vehicle for systemic circulation and cellular delivery. High-density lipoproteins (HDLs) are natural nanostructures that engage cancer cells to deliver exogenous cholesterol. In addition to cholesterol, HDLs bind and deliver nucleic acids, such as microRNAs. Importantly, HDLs are targeted to a surface receptor, scavenger receptor type B-1 (SR-B1), whereupon binding allows for particle uptake and cargo delivery. SR-B1 has been shown to be overexpressed in human cancers, thus making HDLs a highly attractive delivery vehicle for therapeutic nucleic acid cargo. Because of the propensity of HDL to bind, stabilize, and deliver RNA our group has developed ways to synthesize lipoproteins nanoparticles and use them as therapeutic RNA delivery vehicles. Importantly, natural HDLs are highly dynamic species, which frequently undergo exchange of molecular surface components other lipoprotein species, as well as, HDLs. Recent work is focused on chemically tailoring the surface of synthetic HDL nanoparticles (HDL NPs) to understand RNA binding, stabilization, and cellular delivery. In addition, manipulating the surface chemical features of HDL also provides a way to better understand intermolecular exchange with native serum lipoproteins. Data demonstrate that by tailoring the surface chemistry of the HDL NP one can modulate the efficiency by which siRNAs can be incorporated onto the particle surface and delivered to target cells to reduce target gene expression in cancer cells. Further, data show the tendency for nanoparticle lipids to transfer to natural HDLs; however, siRNA formulated on the surface of the HDL NP remains bound, even when exposed to serum for prolonged periods. In short, exploration of HDL NPs for the systemic delivery of siRNAs may be a promising approach for targeted patient-specific cancer therapies. Citation Format: Kaylin M. McMahon, Don Vander Griend, Olga Volpert, C. Shad Thaxton. Engineering the biointerface of synthetic high density lipoprotein nanoparticles enables efficient nucleic acid loading, delivery, and target gene regulation in cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3669. doi:10.1158/1538-7445.AM2015-3669

Abstract 3671: Loading and molecular labeling of cell-specific exosomes by HDL-like AuNPs

Abstract Exosomes are 30-100 nm diameter lipid vesicles produced by most cells. Exosomes carry diverse cargo, such as RNA and proteins. Cancer cells produce an abundant amount of exosomes, which are released from cells of the primary tumor into the tumor microenvironment and the circulation. Data show that exosomes can promote cancer progression and metastasis by delivering, for instance, pro-metastatic factors to pre-metastatic sites. It is not currently possible to specifically separate tumor-derived exosomes from complex matrices. This ability would provide a better understanding of their contribution to tumorigenesis and cancer progression. Furthermore, the targeting properties of exosomes make them appealing vehicles for encapsulating and delivering therapeutic cargo. However, it is challenging to isolate and then load exosomes with specific drugs. Uniquely, our data reveal that synthetic high-density lipoprotein nanoparticles (HDL NPs) can be tailored to achieve cell-specific exosome labeling and loading of therapeutic cargo. HDL NPs are synthesized using a 5 nm diameter spherical gold nanoparticle. The gold NP serves as a template to control size and shape. Apolipoprotein A-I and phospholipids are conjugated to the gold surface, yielding a synthetic HDL NP similar in size, shape, surface chemistry, and function to some natural HDLs. Our data demonstrate that, upon addition of HDL NPs to cultured CWR 22Rv1 prostate cancer cells, they are packaged into exosomes that are secreted from these cells. Furthermore, HDL NPs made using phospholipids functionalized with either biotin or fluorophore molecules leads to the incorporation of the phospholipids into the exosomes. Thus, the specific cell-derived exosomes are labeled with gold, a molecular fluorophore label, and/or biotin. In each case, the specific exosomes are easy to isolate using methods such as fluorescent isolation or by the robust streptavidin-biotin interaction. Isolated exosomes retain the same size and molecular markers as exosomes from untreated cells. Overall, this unique method may provide control over exosomes for a myriad of diagnostic and therapeutic uses. Citation Format: Nicholas L. Angeloni, Kaylin M. McMahon, C. Shad Thaxton. Loading and molecular labeling of cell-specific exosomes by HDL-like AuNPs. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 3671. doi:10.1158/1538-7445.AM2015-3671

Abstract 335: Atheroma-specific Delivery of Synthetic High-density Lipoprotein Containing Sphingosine-1-phosphate for Modulation of Vascular Inflammation.

Introduction: Sphingosine-1-phosphate (S1P) is a potent anti-inflammatory signaling lipid carried in the body by circulating HDL. HDL has been shown to exhibit anti-inflammatory activities through activation of endothelial nitric oxide synthase (eNOS) and subsequent production and release of nitric oxide (NO) by endothelial cells. Objective: The aim of this study is to use synthetic HDL particles to selectively deliver S1P to the site of arterial plaques in order to exert anti-inflammatory activity and modulate the progression of atherosclerosis. Methods/Results: Synthetic HDL (sHDL) particles were prepared using the ApoA1 mimetic peptide 22A (PVLDLFRELLNELLEALKQKLK), dipalmitoylphosphatidylcholine (DPPC) and sphingomyelin. We also prepared sHDL containing either the hydrophobic dye, DiD, or S1P to assess the capability of sHDL to effectively reach atheroma site and induce nitric oxide (NO) release, respectively. The purity of all particles was determined to be > 97% and average particle size was 9.6 ± 0.4 nm for all preparations. To measure sHDL accumulation in the plaque, ApoE -/- mice were intravenously injected with 0.2 mg/kg HDL-DiD. Whole aortas were excised and analysed by IVUS imaging system, revealing significant accumulation of sHDL-DiD in the atherosclerotic lesions. We then tested the ability of sHDL to deliver S1P in vitro and induce NO production by treating human umbilical vein endothelial cells (HUVEC) with 1 mg/mL of 22A-DPPC-sHDL containing 0, 0.05, 0.5, or 5 nmol/mL of S1P using free 22A peptide (1 mg/mL) and saline as controls, and analyzing media by ozone chemiluminescence. Blank sHDL particles increased NO production two-fold over controls (0.27 ± 0.02 μM for 22A-DPPC-sHDLDL, 0.13 ± 0.01 μM PBS and 0.14 ± 0.02 μM for 22A peptide), while HDL-S1P further increased NO release: 0.35 ± 0.03, 0.44 ± 0.01, and 0.59 ± 0.01 μM for HDL with 0.05, 0.5, and 5 nmol/mL S1P, respectively. Conclusions: Our studies show that HDL is capable of delivering hydrophobic cargo to atherosclerotic plaques, making HDL a promising platform to deliver S1P for modulation vascular inflammation and atherosclerosis. In vitro studies have revealed that HDL-S1P is able to increase NO production 2 to 4-fold over saline control setting the basis for future in vivo studies.

High density lipoproteins and ischemia reperfusion injury: the therapeutic potential of HDL to modulate cell survival pathways.

The clinical importance of high density lipoproteins has grown in recent years with demonstrations of their impact on diverse pathological mechanisms implicated not only in vascular disease, but also in other physiological systems. This is related to the multiple functions associated with high-density lipoproteins (HDL), notably their ability to limit oxidant and inflammatory processes, which are common to different disease states. A second feature of particular clinical relevance is the possibility of synthesising a simplified form of HDL that exhibits some of the functions of the mature lipoprotein. The therapeutic potential of synthetic HDL is already under clinical scrutiny. To illustrate these points, the present chapter will discuss the role of HDL in limiting damage to the heart consequent to myocardial ischemia. It will review molecular survival pathways stimulated by HDL to combat oxidative stress and the potential of synthetic HDL to activate such pathways.

Reply to Cormode et al.: High-density lipoprotein mimicking synthetic nanoparticle

We thank Cormode et al. (1) for their interest and comments on our recent article in PNAS (2). We would like to emphasize that throughout our article, we call our nanoparticle as “synthetic high-density lipoprotein (HDL) mimic” not “HDL-like.” The very definition of the term “synthetic mimic” implies that the nanoparticle, itself, imitates the actions of the natural source but is not exactly the same. We would like to stress that the goal of this work was to produce a synthetic HDL-mimicking particle, as evidenced from the title of the article. The rationale for creating a completely synthetic HDL-mimicking nanoparticle was to overcome problems associated with reconstitution of natural HDL, such as possible immune responses, and scale-up challenges.

Biomimetic, synthetic HDL nanostructures for lymphoma

New therapies that challenge existing paradigms are needed for the treatment of cancer. We report a nanoparticle-enabled therapeutic approach to B-cell lymphoma using synthetic high density lipoprotein nanoparticles (HDL-NPs). HDL-NPs are synthesized using a gold nanoparticle template to control conjugate size and ensure a spherical shape. Like natural HDLs, biomimetic HDL-NPs target scavenger receptor type B-1, a high-affinity HDL receptor expressed by lymphoma cells. Functionally, compared with natural HDL, the gold NP template enables differential manipulation of cellular cholesterol flux in lymphoma cells, promoting cellular cholesterol efflux and limiting cholesterol delivery. This combination of scavenger receptor type B-1 binding and relative cholesterol starvation selectively induces apoptosis. HDL-NP treatment of mice bearing B-cell lymphoma xenografts selectively inhibits B-cell lymphoma growth. As such, HDL-NPs are biofunctional therapeutic agents, whose mechanism of action is enabled by the presence of a synthetic nanotemplate. HDL-NPs are active in B-cell lymphomas and potentially, other malignancies or diseases of pathologic cholesterol accumulation.

Biomimetic Synthetic High Density Lipoprotein Nanostructures Target the SR-B1 Receptor and Differentially Manipulate Cellular Cholesterol Flux in Lymphoma Cells: A Novel Treatment Paradigm

Abstract 157We report a nanoparticle-enabled therapeutic approach to B cell lymphoma using synthetic, high-density lipoprotein nanoparticles (HDL-NP). Like natural HDLs, biomimetic HDL-NPs target scavenger receptor type B-1 (SR-B1), a high-affinity HDL receptor expressed by lymphoma cells. Functionally, and unlike natural HDL, a gold nanoparticle template used to control HDL-NP synthesis enables differential manipulation of cellular cholesterol flux through SR-B1. Recent evidence in lymphoblasts and myeloblasts from patients with acute lymphocytic leukemia (ALL) and acute myeloid leukemia (AML) demonstrates enhanced uptake of cholesterol through high-density lipoprotein (HDL) carriers, which may result in increased cell proliferation. We therefore hypothesized that by targeting SR-B1, we could manipulate cholesterol flux in lymphoma cells thereby targeting cellular signaling pathways that would lead to cell death and offer an innovative approach to the treatment of lymphoma and other cancers. Methods:To accomplish this, we developed a biomimetic spherical nanoparticle (HDL-NP) with surface chemical properties similar to natural HDL, including the ability to sequester cholesterol. Biomimetic HDL-NPs are synthesized using a 5 nm diameter gold (Au) nanoparticle (NP) as a size- and shape-restrictive template on which to assemble the surface chemical components of natural HDLs, including phospholipids and the HDL-defining apolipoprotein A1 (Apo A1). Importantly, the core AuNP template occupies the real estate in natural cholesterol-rich HDLs reserved for esterified cholesterol, which inherently limits the ability of HDL-NPs to deliver cholesterol. We incubated the HDL-NPs with various lymphoma cell lines, and similarly tested the HDL-NPs in a xenograft model. Results:We first examined gene expression profiles of diffuse large B-cell lymphoma (DLBCL), Burkitt Lymphoma (BL) and normal B cells from patient samples in a database generated using Affymetrix U133plus 2.0 arrays in order to establish the prevalence of SR-B1 expression. We compared the expression of SR-B1 in BL cases (n=20), and DLBCL cases (n=40) that were further subdivided as activated B-cell (ABC)-like DLBCL (n=20), and germinal center (GC)-like DLBCL (n=20) to normal naive (n=3) and memory (n=3) B cells obtained from healthy donors. We found that SR-B1 was expressed at two to four-fold higher levels in the lymphomas (ABC and GC) compared with normal B cells. Next, we determined the expression of SR-B1 in lymphoma cell lines and normal peripheral lymphocytes by immunoblotting, and we found that SR-B1 is expressed in multiple B cell lymphoma cell lines, but not in Jurkat, a T-cell line, and is not expressed by normal human lymphocytes. Incubation of HDL-NP with Ramos, LY-3 and SUDHL-4 resulted in a dose-dependent decrease in cell viability and apoptosis (Figure 1) of the Ramos and SUDHL-4 cells, less so in LY-3 cells, and not in the Jurkat line. This required the nanoparticle construct and could not be duplicated by individual components of that construct, and was reversible with addition of acetylated low-density lipoprotein, indicating that the SR-B1 receptor was targeted. Xenograft experiments with SCID beige mice (C.B-Igh-1b/GbmsTac-Prkdcscid-Lystbg N7) bearing Ramos and Jurkat flank tumor xenografts confirmed the activity of the HDL-NP (Figure 2). Conclusion:We report a template-directed and bio-functional therapeutic nanostructure that could shift the paradigm for treating lymphoma and other cancers. A combination of SR-B1 binding and manipulation of cholesterol flux is responsible for selective induction of apoptosis in B cell lymphoma. [Display omitted] [Display omitted] Disclosures:Thaxton:Aurasense: Employment, Equity Ownership.

Apolipoprotein A-I structural organization in high-density lipoproteins isolated from human plasma

High density lipoproteins (HDL) mediate cholesterol transport and protection from cardiovascular disease. Although synthetic HDLs have been studied for 30 years, the structure of human plasma-derived HDL, and its major protein apolipoprotein (apo)A-I, is unknown. We separated normal human HDL into 5 density subfractions and then further isolated those containing predominantly apoA-I (LpA-I). Using cross-linking chemistry and mass spectrometry, we found that apoA-I adopts a structural framework in these particles that closely mirrors that in synthetic HDL. We adapted established structural models for synthetic HDL to generate the first detailed models of authentic human plasma HDL in which apoA-I adopts a symmetrical cage-like structure. The models suggest that HDL particle size is modulated via a twisting motion of the resident apoA-I molecules. This understanding offers insights into how apoA-I structure modulates HDL function and its interactions with other apolipoproteins.

Reconstituted Lipoprotein: A Versatile Class of Biologically-Inspired Nanostructures

One of biology's most pervasive nanostructures, the phospholipid membrane, represents an ideal scaffold for a host of nanotechnology applications. Whether engineering biomimetic technologies or designing therapies to interface with the cell, this adaptable membrane can provide the necessary molecular-level control of membrane-anchored proteins, glycopeptides, and glycolipids. If appropriately prepared, these components can replicate in vitro or influence in vivo essential living processes such as signal transduction, mass transport, and chemical or energy conversion. To satisfy these requirements, a lipid-based, synthetic nanoscale architecture with molecular-level tunability is needed. In this regard, discrete lipid particles, including reconstituted high density lipoprotein (HDL), have emerged as a versatile and elegant solution. Structurally diverse, native biological HDLs exist as discoidal lipid bilayers of 5-8 nm diameter and lipid monolayer-coated spheres 10-15 nm in diameter, all belted by a robust scaffolding protein. These supramolecular assemblies can be reconstituted using simple self-assembly methods to incorporate a broad range of amphipathic molecular constituents, natural or artificial, and provide a generic platform for stabilization and transport of amphipathic and hydrophobic elements capable of docking with targets at biological or inorganic surfaces. In conjunction with top-down or bottom-up engineering approaches, synthetic HDL can be designed, arrayed, and manipulated for a host of applications including biochemical analyses and fundamental studies of molecular structure. Also highly biocompatible, these assemblies are suitable for medical diagnostics and therapeutics. The collection of efforts reviewed here focuses on laboratory methods by which synthetic HDLs are produced, the advantages conferred by their nanoscopic dimension, and current and emerging applications.

Nanotechnology for synthetic high-density lipoproteins.

Atherosclerosis is the disease mechanism responsible for coronary heart disease (CHD), the leading cause of death worldwide. One strategy to combat atherosclerosis is to increase the amount of circulating high-density lipoproteins (HDL), which transport cholesterol from peripheral tissues to the liver for excretion. The process, known as reverse cholesterol transport, is thought to be one of the main reasons for the significant inverse correlation observed between HDL blood levels and the development of CHD. This article highlights the most common strategies for treating atherosclerosis using HDL. We further detail potential treatment opportunities that utilize nanotechnology to increase the amount of HDL in circulation. The synthesis of biomimetic HDL nanostructures that replicate the chemical and physical properties of natural HDL provides novel materials for investigating the structure-function relationships of HDL and for potential new therapeutics to combat CHD.

Nanotechnology in molecular medicine

Nanotechnology in molecular medicine

The promise of apolipoprotein A-I mimetics

Synthetic high-density lipoprotein (HDL) and apolipoprotein (apo) A-I mimetic peptides emulate many of the atheroprotective biological functions attributed to HDL and can modify atherosclerotic disease processes. Administration of these agents as HDL replacement or modifying therapy has tremendous potential of providing new treatments for cardiovascular disease. Progress in the understanding of these agents is discussed in this review. Prospective, observational, and interventional studies have convincingly demonstrated that elevated serum levels of high-density lipoprotein-cholesterol (HDL-C) are associated with reduced risk for coronary heart disease (CHD). Although traditional pharmacological agents have shown modest utility in raising HDL levels and reducing CHD risk, use of HDL and apo A-I mimetics provides novel therapies to not only increase HDL levels, but to also influence HDL functionality. Evidence developed over the last several years has identified a number of pathways affected by synthetic HDL and apoA-I mimetic peptides, including enhancing reverse cholesterol transport and reducing oxidation and inflammation that directly influence the progression and regression of atherosclerotic disease. Clinical trials of relatively short-term synthetic HDL infusion into patients with CHD demonstrate beneficial effects. Use of apo A-I mimetic peptides could potentially overcome some of the limitations associated with use of the intact apo. Studies to establish the most efficacious peptides, optimal dosing regimens, and routes of administration are needed. Use of apo A-I mimetic peptides shows great promise as a therapeutic modality for HDL replacement and enhancing HDL function in treatment of patients with CHD.

Receptor mediated uptake of paclitaxel from a synthetic high density lipoprotein nanocarrier

The purpose of these studies was to determine the mechanism(s) whereby paclitaxel (PTX), is taken up by cancer cells, once encapsulated into synthetic/reconstituted high density lipoprotein (rHDL). The uptake of PTX was found to be facilitated by the scavenger receptor type B-1 (SR-B1) when drug-loaded rHDL particles were incubated with cells that express the SRB1 receptor. Studies with double-labeled, PTX containing rHDL nanoparticles showed that prostate cancer (PC-3) cells incorporated PTX primarily via a selective (SR-B1 type) uptake mechanism. In the presence of a 10-fold excess of plasma HDL, PTX uptake decreased to 30% of the control. These findings suggest that the incorporation of lipophilic drugs by cancer cells from rHDL nanoparticles is facilitated by a receptor mediated (SR-B1) mechanism.

HDL Therapy for the Treatment of Cardiovascular Diseases

High-density lipoprotein (HDL) therapy is an emerging area of therapeutic development in the cardiovascular field, aimed at supplementing and improving the vascular benefit exerted by current treatments. Several studies have clearly established that HDL-cholesterol (HDL-C) levels are a potent and independent epidemiologic risk factor for cardiovascular diseases; moreover, studies in animal models have established that HDL-C raising interventions, such as over-expression of apolipoprotein A-I (apoA-I), the major HDL protein component, reduce the extent of atherosclerosis. In vitro and in vivo experiments have provided mechanistic explanations for the atheroprotective effects of HDL. HDL not only mediates reverse cholesterol transport, but also exerts antioxidant, anti-inflammatory, antithrombotic and vasodilatory effects. These multiple antiatherosclerotic properties provide an excellent rationale for designing therapeutic interventions targeted at enhancing HDL/apoA-I levels, but also for considering a direct administration of HDL-apoA-I in a variety of cardiovascular diseases. We provide an overview and an update of all therapeutic applications of synthetic HDL tested in animal models or in clinical trials. HDL therapy has proven to be effective in promoting atherosclerosis regression not only in experimental models, but also in humans, whereas applications to other areas of cardiovascular disease have only, up to now, been tested in animal models.

Structural features and dynamics properties of human apolipoprotein A-I in a model of synthetic HDL

High-density lipoproteins (HDL) play a major role in the reverse transport of cholesterol and have antiatherogenic activities. Their major protein component is apolipoprotein (apo) A-I. While apoA-I amphipathic α-helix based secondary structure has been extensively investigated, for its lipid-bound tertiary structure only theoretical models have been proposed. In the past years, experimental approaches aimed at a direct visualization of HDL structure have been exploited, but data obtained through different microscopy techniques are conflicting and do not settle the issue. Here we present a 50 ns molecular dynamics simulation of a synthetic HDL containing two molecules of apoA-I and 101 of l-α-palmitoyl-oleoyl-phosphatidylcholine. Essential dynamics and structural property investigations suggest that the stabilization of the system is obtained through specific motions, whose driving forces are protein–phospholipid interactions. The most important are: the relative sliding of the two apoA-I molecules along their major axes, the relative rotation of the protein chains, and the out-of-plane deformation around proline hinges. The sliding and the out-of-plane deformation allow apoA-I to optimize its interactions with phospholipids, while the rotation is useful to maximize protein–protein salt bridges. The correspondence between computed parameters and their experimental counterparts contributes to validate our model and its dynamic behaviors. Our findings help in defining a molecular model for apoA-I contained in HDL and suggest a possible mechanism through which apoA-I can vary its diameter and accommodate different numbers of phospholipids during the metabolism of HDL.

Receptor mediated uptake of paclitaxel from a synthetic high density lipoprotein nanocarrier

Receptor mediated uptake of paclitaxel from a synthetic high density lipoprotein nanocarrier

HDL-ApoE Content Regulates the Displacement of Hepatic Lipase from Cell Surface Proteoglycans

Human hepatic lipase (HL) is an interfacial enzyme that must be liberated from cell surface proteoglycans to hydrolyze lipoprotein triglyceride. Both high-density lipoprotein (HDL) and apolipoprotein (apo)A-I can displace HL from cell surface proteoglycans, much like heparin. HL displacement is inhibited by HDL-apoE content. Postprandial HDL is approximately twofold better at displacing HL than is fasting HDL, but only has approximately one-half the apoE content. Enriching native HDL with triglyceride decreases HDL-apoE content and increases HL displacement. Incubation of HDL with the anti-apoE antibody, 6C5, also increases HL displacement. In contrast, enrichment of synthetic HDL with apoE significantly inhibits HL displacement. HDL from fasted female normolipidemic subjects displaces HL approximately twofold better than HDL from male subjects. HDL from female subjects also has significantly less apoE than HDL from males. Normolipidemic females have increased circulating HDL-bound HL. Hyperlipidemia has little effect on the HL displacement ability of HDL from men, whereas HDL from hypercholesterolemic females exhibits impaired HL displacement. HL displacement from liver heparan sulfate proteoglycans therefore appears to be linked to interlipoprotein apoE exchange. Decreased HL displacement is associated with higher HDL-apoE levels and may therefore affect vascular triglyceride hydrolysis.

Comparison of Synthetic High Density Lipoprotein (HDL) Contrast Agents for MR Imaging of Atherosclerosis

Determining arterial macrophage expression is an important goal in the molecular imaging of atherosclerosis. Here, we compare the efficacy of two synthetic, high density lipoprotein (HDL) based contrast agents for magnetic resonance imaging (MRI) of macrophage burden. Each form of HDL was labeled with gadolinium and rhodamine to allow MRI and fluorescence microscopy. Either the 37 or 18 amino acid peptide replaced the apolipoprotein A-I in these agents, which were termed 37pA-Gd or 18A-Gd. The diameters of 37pA-Gd and 18A-Gd are 7.6 and 8.0 nm, respectively, while the longitudinal relaxivities are 9.8 and 10.0 (mM s)(-1). 37pA has better lipid binding properties. In vitro tests with J774A.1 macrophages proved the particles possessed the functionality of HDL by eliciting cholesterol efflux and were taken up in a receptor-like fashion by the cells. Both agents produced enhancements in atherosclerotic plaques of apolipoprotein E knockout mice of approximately 90% (n = 7 per agent) and are macrophage specific as evidenced by confocal microscopy on aortic sections. The half-lives of 37pA-Gd and 18A-Gd are 2.6 and 2.1 h, respectively. Despite the more favorable lipid interactions of 37pA, both agents gave similar, excellent contrast for the detection of atherosclerotic macrophages using MRI.