A 13C NMR characterization of lysine residues in apolipoprotein B and their role in binding to the low density lipoprotein receptor

Low-density lipoprotein: a versatile nanoscale platform for targeted delivery

Low-density lipoprotein (LDL) is a small lipoprotein that plays a vital role in controlling lipid metabolism. LDL has a delicate nanostructure with unique physicochemical properties: superior payload capacity, long residence time in circulation, excellent biocompatibility, smaller size, and natural targeting. In recent decades, the superiority and feasibility of LDL particles as targeted delivery carriers have attracted much attention. In this review, we introduce the structure, composition, advantages, defects, and reconstruction of LDL delivery systems, summarize their research status and progress in targeted diagnosis and therapy, and finally look forward to the clinical application of LDL as an effective delivery vehicle.

Elucidating the structural organization of a novel low-density lipoprotein nanoparticle reconstituted with docosahexaenoic acid

Low-density lipoprotein nanoparticles reconstituted with unesterified docosahexaenoic acid (LDL-DHA) is promising nanomedicine with enhanced physicochemical stability and selective anticancer cytotoxic activity. The unique functionality of LDL-DHA ultimately relates to the structure of this nanoparticle. To date, however, little is known about the structural organization of this nanoparticle. In this study chemical, spectroscopic and electron microscopy analyses were undertaken to elucidate the structural and molecular organization of LDL-DHA nanoparticles. Unesterified DHA preferentially incorporates into the outer surface layer of LDL, where in this orientation the anionic carboxyl end of DHA is exposed to the LDL surface and imparts an electronegative charge to the nanoparticles surface. This negative surface charge promotes the monodisperse and homogeneous distribution of LDL-DHA nanoparticles in solution. Further structural analyses with cryo-electron microscopy revealed that the LDL-DHA nanostructure consist of a phospholipid bilayer surrounding an aqueous core, which is distinctly different from the phospholipid monolayer/apolar core organization of plasma LDL. Lastly, apolipoprotein B-100 remains strongly associated with this complex and maintains a discrete size and shape of the LDL-DHA nanoparticles similar to plasma LDL. This preliminary structural assessment of LDL-DHA now affords the opportunity to understand the important structure-function relationships of this novel nanoparticle.

Ligand-coupled lipoprotein for ovarian cancer-specific drug delivery

Lipoproteins are natural nanosized delivery vehicles within the circulatory system of all mammals. Scientists have long been interested in utilizing these endogenous macromolecules to transport exogenous imaging or therapeutic agents to specific cells or tissues in the body. The broad distribution of lipoprotein receptors throughout the body however has limited the utility of this approach for targeted delivery of medicinal agents. In recent years lipoprotein rerouting strategies have been developed wherein lipoproteins can be redirected from their natural lipoprotein receptors to an alternate receptor of choice. In this chapter we describe the basic methods of preparing folic acid-conjugated high-density lipoprotein nanoparticles for targeted delivery of imaging or chemotherapeutic agents to ovarian cancer cells.

Lipoprotein-inspired nanoparticles for cancer theranostics

Over hundreds of millions of years, animals have evolved endogenous lipoprotein nanoparticles for shuttling hydrophobic molecules to different parts of the body. In the last 70 years, scientists have developed an understanding of lipoprotein function, often in relationship to lipid transport and heart disease. Such biocompatible, lipid–protein complexes are also ideal for loading and delivering cancer therapeutic and diagnostic agents, which means that lipoprotein and lipoprotein-inspired nanoparticles also offer opportunities for cancer theranostics. By mimicking the endogenous shape and structure of lipoproteins, the nanocarrier can remain in circulation for an extended period of time, while largely evading the reticuloendothelial cells in the body’s defenses. The small size (less than 30 nm) of the low-density (LDL) and high-density (HDL) classes of lipoproteins allows them to maneuver deeply into tumors. Furthermore, lipoproteins can be targeted to their endogenous receptors, when those are implicated in cancer, or to other cancer receptors.

In this Account, we review the field of lipoprotein-inspired nanoparticles related to the delivery of cancer imaging and therapy agents. LDL has innate cancer targeting potential and has been used to incorporate diverse hydrophobic molecules and deliver them to tumors. Nature’s method of rerouting LDL in atherosclerosis provides a strategy to extend the cancer targeting potential of lipoproteins beyond its narrow purview. Although LDL has shown promise as a drug nanocarrier for cancer imaging and therapy, increasing evidence indicates that HDL, the smallest lipoprotein, may also be of use for drug targeting and uptake into cancer cells. We also discuss how synthetic HDL-like nanoparticles, which do not include human or recombinant proteins, can deliver molecules directly to the cytoplasm of certain cancer cells, effectively bypassing the endosomal compartment. This strategy could allow HDL-like nanoparticles to be used to deliver drugs that have increased activity in the cytoplasm. Lipoprotein nanoparticles have evolved to be ideal delivery vehicles, and because of that specialized function, they have the potential to improve cancer theranostics.

Enhancing the contrast of ApoB to locate the surface components in the 3D density map of human LDL

A 26 Å resolution map of the structure of human low-density lipoprotein (LDL) was obtained from electron cryomicroscopy and single-particle image reconstruction. The structure showed a discoidal-shaped LDL particle with high-density regions mainly distributed at the edge of the particle and low-density regions at the flat surface that covers the core region. To determine the chemical components that correspond to these density regions and to delineate the distribution of protein and phospholipid located at the particle surface at the resolution of the map, we used Mono-Sulfo-NHS-Undecagold labeling to increase preferentially the contrast of the apolipoprotein B component on the LDL particle. In the three-dimensional map from the image reconstruction of the undecagold-labeled LDL particles, the high-density region from the undecagold label was distributed mainly at the edge of the particle, and lower density regions were found at the flat surfaces that cover the neutral lipid core. This suggests that apolipoprotein B mainly encircles LDL at the edge of the particle and the phospholipid monolayers are located at the flat surfaces, which are parallel to the cholesterol ester layers in the core and may interact with the core lipid layers through the acyl chains.

2D-NMR reveals different populations of exposed lysine residues in the apoB-100 protein of electronegative and electropositive fractions of LDL particles

Several potentially atherogenic LDL subfractions present low affinity for the LDL receptor, which result in impaired plasma clearance. Electronegative LDL [LDL(-)] is one of these minor subfractions and the molecular basis for its reduced receptor affinity is not well understood. In the present study, high-resolution 2D-NMR spectroscopy has been employed to characterize the surface-exposed lysine residues of the apolipoprotein (apo)B-100 protein in both LDL(-) and LDL(+) subfractions. LDL(+) showed two populations of lysine residues, similar to those previously described in total LDL. "Normal" Lys have a pk(a) of 10.4 whereas "active" Lys have a pk(a) of 8.8 and have been suggested to be involved in receptor binding. In contrast to LDL(+), the LDL(-) subfraction presented a third type of Lys, named as "intermediate" Lys, with a different microenvironment and higher basicity (pk(a) 10.7). These intermediate Lys cannot be reliably identified by 1D-NMR. Because the abundance of normal Lys is similar in LDL(+) and LDL(-), the intermediate Lys in the apoB-100 molecule of LDL(-) should come from a group of active Lys in LDL(+) particles that have a less basic microenvironment in the LDL(-) particle. These differences between LDL(+) and LDL(-) are indicative of a distinct conformation of apoB-100 that could be related to loss of affinity of LDL(-) for the LDL receptor.

Mammaglobin as a potential molecular target for breast cancer drug delivery

BACKGROUND

Mammaglobin (MAM) has been used as a specific molecular marker for breast cancer diagnosis. Recently, several groups of researchers proposed a number of therapeutic strategies targeting this molecule. Some of the strategies are based upon an essential but not demonstrated hypothesis - mammaglobin is associated with the surface of breast cancer cells, which strongly disputes the therapeutic strategies.

RESULTS

We conducted a computer-based predictive analysis and identified a small fragment at the N-end of MAM as a potential transmembrane domain. We provided several evidences to demonstrate the presence of the membrane-associated MAM. We isolated the membrane protein components from known MAM positive breast cancer cells (MDA-MB361 and MDA-MB415). We showed that about 22-64% of MAM proteins, depending upon the types of the cancer cells, directly attached on the membrane of breast cancer cells, by Western blotting assays. To directly visualize the presence of the membrane-bound MAM protein, we incubated the MAM positive cancer cells with FITC labeled anti-MAM antibody, and observed clear fluorescent signals on the surface of the cells. In studying the MAM protein distribution in human breast cancer tissues, we first identified two immunostain patterns that are associated with the membrane-bound MAM: the membrane stain pattern and luminary surface stain pattern. To test whether the membrane-associated MAM can serve as a molecular target for drug delivery, we conjugated anti-MAM antibody to human low-density lipoprotein (LDL) and loaded doxorubicin (Dox) in the core of LDL. Specific binding and cytotoxicity of the MAM targeted and Dox loaded LDL was tested in the MAM positive breast cancer cells in vitro.

CONCLUSION

We first showed that some of MAM protein directly associated with the surface of breast cancer cells. The membrane-associated MAM protein may be utilized as a useful molecular marker for breast cancer targeted drug delivery.

Lipoprotein nanoplatform for targeted delivery of diagnostic and therapeutic agents

Low-density lipoprotein (LDL) provides a highly versatile natural nanoplatform for delivery of optical and MRI contrast agents, photodynamic therapy agents and chemotherapeutic agents to normal and neoplastic cells that over express LDL receptors (LDLR). Extension to other lipoproteins ranging in diameter from approximately 5-10 nm (high density lipoprotein, HDL) to over a micron (chilomicrons) is feasible. Loading of contrast or therapeutic agents has been achieved by covalent attachment to protein side chains, intercalation into the phospholipid monolayer and extraction and reconstitution of the triglyceride/cholesterol ester core. Covalent attachment of folate to the lysine side chain amino groups was used to reroute the LDL from its natural receptor (LDLR) to folate receptors and could be utilized to target other receptors. A semi-synthetic nanoparticle has been constructed by coating magnetite iron oxide nanoparticles (MIONs) with carboxylated cholesterol and overlaying a monolayer ofphospholipid to which Apo A1, Apo E or synthetic amphoteric alpha-helical polypeptides were adsorbed for targeting HDL, LDL or folate receptors, respectively. These particles can be utilized for in situ loading of magnetite into cells for MRI monitored cell tracking or gene therapy.

Low-density lipoprotein has an enormous capacity to bind (E)-4-hydroxynon-2-enal (HNE): detection and characterization of lysyl and histidyl adducts containing multiple molecules of HNE

(E)-4-Hydroxynon-2-enal (HNE), an electrophilic bifunctional cytotoxic lipid peroxidation product, forms covalent adducts with nucleophilic side chains of amino acid residues. HNE-derived adducts have been implicated in many pathophysiological processes including atherosclerosis, diabetes, and Alzheimer’s disease. Tritium- and deuterium-labeled HNE (d₄-HNE) were used orthogonally to study adduction with proteins and individual nucleophilic groups of histidyl, lysyl, and cysteine residues. Using tritium-labeled HNE, we detected the binding of 486 molecules of HNE per low-density lipoprotein (LDL) particle, significantly more than the total number of all reactive nucleophiles in the LDL particle. This suggests the formation of adducts that incorporate multiple molecules of HNE with some nucleophilic amino acid side chains. We also found that the reaction of a 1:1 mixture of d₄-HNE and d₀-HNE with N-acetylhistidine, N-acetyl-Gly-Lys-OMe, or N-acetyl cysteine generates 1:1, 2:1, and 3:1 adducts, which exhibit unique mass spectral signatures that aid in structural characterization. A domino-like reaction of initial 1:1 HNE Michael adducts of histidyl or lysyl nucleophiles with multiple additional HNE molecules forms 2:1 and 3:1 adducts that were structurally characterized by tandem mass spectrometry.

Molecular dynamics simulations of a lipovitellin-derived amphiphilic beta-sheet homologous to apoB-100 beta-sheets at a hydrophobic decane-water interface

Lipovitellin, an egg-yolk lipoprotein, transports lipids in a pocket surrounded by amphiphilic beta-sheets. Its X-ray structure provides possibilities to study interactions between lipophilic beta-sheets and lipids at the atomic level. Here, we studied a 67-residue-long amphiphilic beta-sheet of lipovitellin previously suggested a suitable working model for studies of the lipid-binding behaviour of amphiphilic beta-sheet regions in apolipoprotein B-100 (apoB-100). We performed four molecular dynamics simulations with different starting configurations to define characteristics of the amphiphilic beta-sheet model at a decane-water interface. In each simulation the model beta-sheet bound keenly to the decane layer via its hydrophobic surface. The structural profiles showed unchanged secondary structure of the beta-sheet during the attachment. Also, aromatic side chains, especially tryptophans and tyrosines, mediated the attachment to the hydrophobic layer and influenced the orientation of the decane molecules that are in contact with the beta-sheet. In conclusion, the present simulations reveal high affinity of a lipovitellin-derived amphiphilic beta-sheet to a hydrophobic decane layer. They lay thereby the basis for further studies of the interaction between amphiphilic beta-sheets and lipids in complex molecular systems, like LDL particles, in which the large apoB-100 is the main protein component.

Systematic comparison of two novel, thiol-reactive prosthetic groups for 18F labeling of peptides and proteins with the acylation agent succinimidyl-4-[18F]fluorobenzoate ([18F]SFB)

A systematic comparison of 4-[18F]fluorobenzaldehyde-O-(2-{2-[2-(pyrrol-2,5-dione-1-yl)ethoxy]-ethoxy}-ethyl)oxime ([18F]FBOM) and 4-[18F]fluorobenzaldehyde-O-[6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl]oxime ([18F]FBAM) as prosthetic groups for the mild and efficient 18F labeling of cysteine-containing peptides and proteins with the amine-group reactive acylation agent, succinimidyl-4-[18F]fluorobenzoate ([18F]SFB), is described. All three prosthetic groups were prepared in a remotely controlled synthesis module. Synthesis of [18F]FBOM and [18F]FBAM was accomplished via oxime formation through reaction of appropriate aminooxy-functionalized labeling precursors with 4-[18F]fluorobenzaldehyde. The obtained radiochemical yields were 19% ([18F]FBOM) and 29% ([18F]FBAM), respectively. Radiolabeling involving [18F]FBAM and [18F]FBOM was exemplified by the reaction with cysteine-containing tripeptide glutathione (GSH), a cysteine-containing dimeric neurotensin derivative, and human native low-density lipoprotein (nLDL) as model compounds. Radiolabeling with the acylation agent [18F]SFB was carried out using a dimeric neurotensin derivative and nLDL. Both thiol-group reactive prosthetic groups show significantly better labeling efficiencies for the peptides in comparison with the acylation agent [18F]SFB. The obtained results demonstrate that [18F]FBOM is especially suited for the labeling of hydrophilic cysteine-containing peptides, whereas [18F]FBAM shows superior labeling performance for higher molecular weight compounds as exemplified for nLDL apolipoprotein constituents. However, the acylation agent [18F]SFB is the preferred prosthetic group for labeling nLDL under physiological conditions.

Low-density lipoprotein nanoparticles as magnetic resonance imaging contrast agents

Low-density lipoproteins (LDLs) are a naturally occurring endogenous nanoplatform in mammalian systems. These nanoparticles (22 nm) specifically transport cholesterol to cells expressing the LDL receptor (LDLR). Several tumors overexpress LDLRs presumably to provide cholesterol to sustain a high rate of membrane synthesis. Amphiphilic gadolinium (Gd)-diethylenetriaminepentaacetic acid chelates have been incorporated into the LDL to produce a novel LDLR-targeted magnetic resonance imaging (MRI) contrast agent. The number of Gd chelates per LDL particle ranged between 150 and 496 Gd(III). In vitro studies demonstrated that Gd-labeled LDL retained a similar diameter and surface charge as the native LDL particle. In addition, Gd-labeled LDL retained selective cellular binding and uptake through LDLR-mediated endocytosis. Finally, Gd-labeled LDLs exhibited significant contrast enhancement 24 hours after administration in nude mice with human hepatoblastoma G2 xenografts. Thus, Gd-labeled LDL demonstrates potential use as a targeted MRI contrast agent for in vivo tumor detection.

Rerouting lipoprotein nanoparticles to selected alternate receptors for the targeted delivery of cancer diagnostic and therapeutic agents

We report that a lipoprotein-based nanoplatform generated by conjugating tumor-homing molecules to the protein components of naturally occurring lipoproteins reroutes them from their normal lipoprotein receptors to other selected cancer-associated receptors. Multiple copies of these targeting moieties may be attached to the same nanoparticle, or a variety of different targeting moieties can be attached. Such a diverse set of tumor-homing molecules could be used to create a variety of conjugated lipoproteins as multifunctional, biocompatible nanoplatforms with a broad application to both cancer imaging and treatment. The same principle can be applied to imaging and treatment of other diseases and for monitoring specific tissues. To validate this concept, we prepared a low-density lipoprotein (LDL)-based folate receptor (FR)-targeted agent by conjugating folic acid to the Lys residues of the apolipoprotein B (apoB)-100 protein. To demonstrate the ability of the lipoprotein-based nanoplatform to deliver surface-loaded and core-loaded payloads, the particles were labeled either with the optical reporter 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine that was intercalated in the phospholipid monolayer or with the lipophilic photodynamic therapy agent, tetra-t-butyl-silicon phthalocyanine bisoleate, that was reconstituted into the lipid core. Cellular localization of the labeled LDL was monitored by confocal microscopy and flow cytometry in FR-overexpressing KB cells, in FR-nonexpressing CHO and HT-1080 cells, and in LDL receptor-overexpressing HepG2 cells. These studies demonstrate that the folic acid conjugation to the Lys side-chain amino groups blocks binding to the normal LDL receptor and reroutes the resulting conjugate to cancer cells through their FRs.

High payload delivery of optical imaging and photodynamic therapy agents to tumors using phthalocyanine-reconstituted low-density lipoprotein nanoparticles

To improve the labeling efficiency of a low-density lipoprotein (LDL)-based photosensitizer (PS) for achieving high probe to protein payload, a tetra-t-butyl silicon phthalocyanine bearing two oleate moieties at its axial positions, SiPcBOA, is designed and synthesized. Using this novel strategy, SiPcBOA reconstituted LDL (r-SiPcBOA-LDL) with a very high payload (SiPcBOA to LDL molar ratio >3000 to 35001:1) is obtained. Using electron microscopy, we find reconstituted LDL (rLDL) with such a high payload essentially retains the mean particle size of native LDL. Since acetylated LDL binds to scavenger receptors of endothelial and microglial cells instead of LDLR, SiPcBOA reconstituted acetylated LDL (r-SiPcBOA-AcLDL) is also prepared to serve as a negative control to validate the LDL receptor (LDLR) targeting specificity. Confocal microscopy studies demonstrate that the internalization of r-SiPcBOA-LDL by human hepatoblastoma G2 (HepG2) tumor cells is mediated by LDLR pathway. The in vitro photodynamic therapy (PDT) response of HepG2 cells to r-SiPcBOA-LDL is compared to SiPcBOA (free drug control) using a clonogenic assay. The slopes of the linear regression fit to the logarithmic data for these two plots are significantly different from each other (p=0.0007), indicating greatly enhanced efficacy of LDLR-targeted PDT.

MR and fluorescent imaging of low-density lipoprotein receptors

RATIONALE AND OBJECTIVES

Over-expression of low-density lipoprotein receptors (LDLRs) occurs in many types of malignancies and is related to the requirement for lipids for rapid proliferation of the tumors. On the other hand, LDLRs that are unable to bind LDL are found on hepatocytes of patients with familial hypercholesterolemia (FH), a genetic disease that leads to premature atherosclerosis and death. The highly selective binding of LDL to LDLR makes these particles ideal carriers of therapeutic and diagnostic contrast agents into the targeted cells. The objectives of this paper are to examine whether a prototype contrast agent (PTIR267) with dual detection properties is suitable for labeling of LDL particles for in vivo detection of LDLR by magnetic resonance imaging (MRI) and for in vitro monitoring of cellular localization by confocal fluorescence microscopy.

MATERIALS AND METHODS

PTIR267 is a lipophilic GdDTPA derivative conjugated to a fluorescent dye. The conjugated dye molecule makes the probe sufficiently water soluble to allow labeling of LDL by a brief incubation of LDL with PTIR267 dissolved in PBS at 37 degrees C (mole ratio LDL: PTIR267 = 0.09:1). The molar relaxivity of PTIR267 in saline is 26 mM(-1)s(-1). Specific LDLR-mediated uptake of PTIR267-labeled LDL was demonstrated in vitro by confocal fluorescence imaging of B16 melanoma cells using confocal fluorescence imaging. In vivo uptake of PTIR267-labeled LDL by a subcutaneously implanted B16 melanoma in mice leads to 30% decrease in longitudinal relaxation time (T(1)) in the tumor. In vivo uptake of PTIR267-labeled LDL leads to 70% decrease in T(1) in a normal C57BL/6 mouse liver; however, in the liver of LDL receptor gene knockout (LDLr-/-) mice with C57BL/6 background, only 12% decrease in T(1) is observed.

CONCLUSIONS

The dual fluorescence and MR imaging properties of PTIR267, combined with the ease of LDL labeling, suggest that it will be a useful tool for optimization of LDLR-targeted cancer diagnosis or therapy and for monitoring the efficacy of gene therapy of FH.

Characterization of the heparin binding sites in human apolipoprotein E

Apolipoprotein (apo) E mediates lipoprotein remnant clearance via interaction with cell-surface heparan sulfate proteoglycans. Both the 22-kDa N-terminal domain and 10-kDa C-terminal domain of apoE contain a heparin binding site; the N-terminal site overlaps with the low density lipoprotein receptor binding region and the C-terminal site is undefined. To understand the molecular details of the apoE-heparin interaction, we defined the microenvironments of all 12 lysine residues in intact apoE3 and examined their relative contributions to heparin binding. Nuclear magnetic resonance measurements showed that, in apoE3-dimyristoyl phosphatidylcholine discs, Lys-143 and -146 in the N-terminal domain and Lys-233 in the C-terminal domain have unusually low pK(a) values, indicating high positive electrostatic potential around these residues. Binding experiments using heparin-Sepharose gel demonstrated that the lipid-free 10-kDa fragment interacted strongly with heparin and a point mutation K233Q largely abolished the binding, indicating that Lys-233 is involved in heparin binding and that an unusually basic lysine microenvironment is critical for the interaction with heparin. With lipidated apoE3, it is confirmed that the Lys-233 site is completely masked and the N-terminal site mediates heparin binding. In addition, mutations of the two heparin binding sites in intact apoE3 demonstrated the dominant role of the N-terminal site in the heparin binding of apoE even in the lipid-free state. These results suggest that apoE interacts predominately with cell-surface heparan sulfate proteoglycans through the N-terminal binding site. However, Lys-233 may be involved in the binding of apoE to certain cell-surface sites, such as the protein core of biglycan.

NMR studies of lipoprotein structure

Early NMR structural studies of serum lipoproteins were based on (1)H, (13)C, (31)P, and (2)H studies of lipid components. From the early studies information on composition, lipid chain dynamics and order parameters, and monolayer organization resulted. More recently, selective or complete isotopic labeling techniques, combined with multidimensional NMR spectroscopy, have resulted in structural information of apoprotein fragments. Finally, use of heteronuclear three- and four-dimensional experiments have yielded solution structures and protein-lipid interactions of intact apolipoproteins C-I, C-II, and A-I.

New insights into the heparan sulfate proteoglycan-binding activity of apolipoprotein E

Defective binding of apolipoprotein E (apoE) to heparan sulfate proteoglycans (HSPGs) is associated with increased risk of atherosclerosis due to inefficient clearance of lipoprotein remnants by the liver. The interaction of apoE with HSPGs has also been implicated in the pathogenesis of Alzheimer's disease and may play a role in neuronal repair. To identify which residues in the heparin-binding site of apoE and which structural elements of heparan sulfate interact, we used a variety of approaches, including glycosaminoglycan specificity assays, (13)C nuclear magnetic resonance, and heparin affinity chromatography. The formation of the high affinity complex required Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147 from apoE and N- and 6-O-sulfo groups of the glucosamine units from the heparin fragment. As shown by molecular modeling, using a high affinity binding octasaccharide fragment of heparin, these findings are consistent with a binding mode in which five saccharide residues of fully sulfated heparan sulfate lie in a shallow groove of the alpha-helix that contains the HSPG-binding site (helix 4 of the four-helix bundle of the 22-kDa fragment). This groove is lined with residues Arg-136, Ser-139, His-140, Arg-142, Lys-143, Arg-145, Lys-146, and Arg-147. In the model, all of these residues make direct contact with either the 2-O-sulfo groups of the iduronic acid monosaccharides or the N- and 6-O-sulfo groups of the glucosamine sulfate monosaccharides. This model indicates that apoE has an HSPG-binding site highly complementary to heparan sulfate rich in N- and O-sulfo groups such as that found in the liver and the brain.

Different susceptibility to oxidation of proline and arginine residues of apolipoprotein B-100 among subspecies of low density lipoproteins

gamma-Glutamyl semialdehyde is a primary oxidation product of apolipoprotein (apo) B-100 proline (Pro) and arginine (Arg) side chain residues. By reduction gamma-glutamyl semialdehyde forms 5-hydroxy-2-aminovaleric acid (HAVA). Here we describe the application of sensitive and specific HAVA measurement to characterize the formation of gamma-glutamyl semialdehyde in several domains of apoB-100 in LDL(1) (S(f) 7-12) and LDL(2) (S(f) 0-7) subfractions subjected to oxidative damage in the presence of iron in vitro. Results suggest that susceptibility of apoB-100 Pro and Arg residues toward oxygen radicals drastically changes along the lipoprotein metabolic cascade.

Effects of lipid interaction on the lysine microenvironments in apolipoprotein E

Lysines in apolipoprotein (apo) E are key factors in the binding of apoE to the low density lipoprotein receptor, and high affinity binding requires that apoE be associated with lipid. To gain insight into this effect, we examined the microenvironments of the eight lysines in the 22-kDa fragment of apoE3 (residues 1-191) in the lipid-free and lipid-associated states. As shown by (1)H,(13)C heteronuclear single quantum coherence nuclear magnetic resonance, lysine resonances in the lipid-free fragment were poorly resolved over a wide pH range, whereas in apoE3.dimyristoyl phosphatidylcholine (DMPC) discs, the lysine microenvironments and protein conformation were significantly altered. Sequence-specific assignments of the lysine resonances in the spectrum of the lipidated 22-kDa fragment were made. In the lipid-free protein, six lysines could be resolved, and all had pK(a) values above 10. In apoE3.DMPC complexes, however, all eight lysines were resolved, and the pK(a) values were 9.2-11.1. Lys-143 and Lys-146, both in the receptor binding region in helix 4, had unusually low pK(a) values of 9.5 and 9.2, respectively, likely as a result of local increases in positive electrostatic potential with lipid association. Shift reagent experiments with potassium ferricyanide showed that Lys-143 and Lys-146 were much more accessible to the ferricyanide anion in the apoE3.DMPC complex than in the lipid-free state. The angle of the nonpolar face of helix 4 is smaller than the angles of helices 1, 2, and 3, suggesting that helix 4 cannot penetrate as deeply into the DMPC acyl chains at the edge of the complex and that its polar face protrudes from the edge of the disc. This increased exposure and the greater positive electrostatic potential created by interaction with DMPC may explain why lipid association is required for high affinity binding of apoE to the low density lipoprotein receptor.

Perlecan heparan sulfate proteoglycan: a novel receptor that mediates a distinct pathway for ligand catabolism

Cell surface heparan sulfate proteoglycans (HSPGs) participate in the catabolism of many physiologically important ligands. We previously reported that syndecan HSPGs directly mediate endocytosis, independent of coated pits. We now studied perlecan, a major cell surface HSPG genetically distinct from syndecans. Cells expressing perlecan but no other proteoglycans bound, internalized, and degraded atherogenic lipoproteins enriched in lipoprotein lipase. Binding was blocked by heparitinase, and degradation by chloroquine. Antibodies against beta(1) integrins reduced initial ligand binding, consistent with their roles as cell surface attachment sites for perlecan. By several criteria, catabolism via perlecan was distinct from either coated pits or the syndecan pathway. The kinetics of internalization (t(12) = 6 h) and degradation (t(12) approximately 18 h) were remarkably slow, unlike the other pathways. Blockade of the low density lipoprotein receptor-related protein did not slow perlecan-dependent internalization. Internalization via perlecan was inhibited by genistein but unaffected by cytochalasin D, a pattern distinct from coated pits or syndecan-mediated endocytosis. Finally, we examined cooperation between perlecan and low density lipoprotein receptors and found limited synergy. Our results demonstrate that perlecan mediates internalization and lysosomal delivery that is kinetically and biochemically distinct from other known uptake pathways and is consistent with a very slow component of HSPG-dependent ligand processing found in vitro and in vivo.

Native and oxidized low density lipoprotein induction of tissue factor gene expression in smooth muscle cells is mediated by both Egr-1 and Sp1

Tissue factor, in association with factor VIIa, initiates the coagulation cascade. We studied the influences of two pathophysiological stimuli, native (unmodified) and oxidized low density lipoprotein, on tissue factor gene expression in a cell important in vascular remodeling and vascular diseases, the smooth muscle cell. Our results demonstrated that both lipoproteins significantly induced tissue factor gene expression in rat aortic smooth muscle cells; oxidized low density lipoprotein was slightly more potent. Both lipoproteins increased tissue factor mRNA in a concentration- and time-dependent manner. Results from nuclear run-on assays and mRNA stability experiments indicated that increased tissue factor mRNA accumulation in response to the lipoproteins was principally controlled at the transcriptional level. By using lipid extracts of low density lipoprotein or methylation of the intact lipoprotein to block receptor recognition, we showed that this lipoprotein induced tissue factor mRNA via both receptor-independent and receptor-augmented pathways. Transfection studies using a series of deleted tissue factor promoters revealed that a -143- to +106-base pair region of the rat tissue factor promoter contained regulatory elements required for lipoprotein-mediated induction. Electrophoretic mobility shift assays showed that the binding activities of the transcription factor Egr-1, but not Sp1, were markedly elevated in response to these lipoproteins. Transfection of site-directed mutants of the tissue factor (TF) promoter demonstrated that not only Egr-1 but also Sp1 cis-acting elements in the TF (-143) promoter construct were necessary for optimal TF gene induction. Our data show for the first time that both low density lipoprotein and oxidized low density lipoprotein induce tissue factor gene expression in smooth muscle cells and that this tissue factor gene expression is mediated by both Egr-1 and Sp1 transcription factors.

Sphingomyelinase induces aggregation and fusion, but phospholipase A2 only aggregation, of low density lipoprotein (LDL) particles. Two distinct mechanisms leading to increased binding strength of LDL to human aortic proteoglycans

During atherogenesis, low density lipoprotein (LDL) particles bind to extracellular matrix proteoglycans in the arterial wall, become modified, and appear as aggregated and fused particles. Sphingomyelinase (SMase) and phospholipase A2 (PLA2) have been found in the arterial wall, and, moreover, lesional LDL shows signs of hydrolysis of both sphingomyelin and phosphatidylcholine. We have now studied the effects of these two lipolytic modifications on the aggregation and fusion of LDL particles by hydrolyzing the particles with Bacillus cereus SMase or bee venom PLA2. In addition, the binding strengths of the modified LDL to human aortic proteoglycans (PG) were analyzed on an affinity column. We found that SMase induced aggregation and fusion of LDL, but PLA2 induced only aggregation of the particles. In addition, the SMase-induced aggregation and fusion of LDL was promoted by pretreatment of LDL with PLA2. Determination of the binding strengths of the hydrolyzed LDL revealed that mere lipolysis of LDL without aggregation or fusion, either by SMase or PLA2, did not affect the binding of the particles to PG. Aggregation and fusion of lipolyzed LDL particles, however, increased their strength of binding to PG. Active lysine residues in apolipoprotein B-100 (apoB-100) appear to be involved in the binding of LDL to PG, and, in fact, quantitative 13C NMR analysis revealed that, in the fused LDL particles, the number of active lysine residues per apoB-100 moiety was increased. Moreover, aggregation and fusion of LDL increased the number of apoB-100 copies and, consequently, the number of active lysine residues per aggregate or fused particle. Our present findings therefore (i) show that treatment of LDL with SMase and PLA2 generates modified LDL particles, which then bind to human aortic PG with increased strength, and (ii) suggest that SMase- and PLA2-induced aggregation and fusion of LDL are potential mechanisms leading to focal retention of extracellular lipid in the arterial wall.

Apolipoprotein B-100 conformation and particle surface charge in human LDL subspecies: implication for LDL receptor interaction

The plasma low-density lipoprotein (LDL) profile in coronary artery disease patients is characterized by a predominance of small, dense LDL. Small, dense LDL exhibit both high susceptibility to oxidation and low binding affinity for the LDL receptor, suggesting that these particles may be of elevated atherogenic potential. Here we examine whether the variation in biological function is due to differences in apo B-100 conformation that alter the interaction with the cellular LDL receptor. The microenvironments (pKa) of Lys residues in apo B-100 in small, dense, intermediate, and light human LDL subspecies have been compared by 13C NMR, and the net surface charge of these particles has been characterized. Relative to the total LDL fraction, small, dense, and light LDL subspecies have a decreased number of pKa 8.9 Lys, while intermediate density LDL has a consistently higher number of pKa 8.9 Lys. It follows that differences in protein conformation, as reflected in the Lys microenvironments, exist in the different LDL subspecies. Electrophoretic mobility measurements revealed that the light LDL subfractions exhibit a surface charge at pH 8.6 that is from -26 to -34e more negative than the intermediate density LDL subfraction. For the small, dense LDL particles the increments in negative charge range from -7 to -17e relative to the intermediate density LDL subfraction. These results suggest that differences in the conformation of apo B-100 and surface charge between LDL subspecies are major determinants of their catabolic fate. The lower number of pKa 8.9 Lys leads to a reduction in binding of small, dense, and light LDL to the cellular LDL receptor and prolongs their plasma residence time, thereby elevating the atherogenicity of these particles. These data support the proposal that the intermediate LDL subspecies constitute the optimal ligand for the LDL receptor among human LDL particle subpopulations.

Lysine and arginine residues in the N-terminal 18% of apolipoprotein B are critical for its binding to microsomal triglyceride transfer protein

Apolipoprotein B (apoB) and microsomal triglyceride transfer protein (MTP) are essential for the efficient assembly and secretion of triglyceride-rich lipoproteins. We have presented evidence for a high-affinity interaction between these proteins [Hussain, M. M., et al. (1997) Biochemistry 36, 13060-13067]. In this study, we used chemically modified low-density lipoproteins (LDL) and recombinant human apoB18 to identify amino acid residues in apoB that are critical for its interactions with MTP. Acetoacetylation of 74% of lysine residues and cyclohexanedione modification of 54% of arginine residues completely abolished the interactions between LDL and MTP. Regeneration of lysine and arginine residues by hydroxylamine treatment completely restored the binding of modified LDL to MTP. Carboxyethylation of all the histidine residues decreased, but did not abolish, apoB-MTP interactions. In contrast, glycine methyl ester modifications of aspartic and glutamic acid residues, up to 38-44%, had no effect on LDL-MTP interactions. Furthermore, modification of lysine and arginine, but not the aspartic and glutamic acid, residues in apoB18 also completely abolished its interactions with MTP. These studies indicated that lysine and arginine, but not aspartic and glutamic acid, residues are critical for apoB-MTP interactions, whereas histidine residues are not as critical. Since lysine and arginine residues in apoB are known to interact with the LDL receptors and heparin, we studied the effect of different glycosaminoglycans on apoB-MTP interactions. Glycosaminoglycans had no significant inhibitory effect on apoB-MTP interactions, suggesting that the lysine and arginine residues crucial for apoB-MTP interactions are different from those that interact with the LDL receptor and heparin. The lysine and arginine residues in apoB18 may directly interact with negatively charged residues in the MTP molecule, or they may function to maintain the conformation of the recognition site.

Thermal stability of apolipoprotein B100 in low-density lipoprotein is disrupted at early stages of oxidation while neutral lipid core organization is conserved

The time course of the unfolding characteristics of the protein moiety and of the thermotropic behavior of the core-located apolar lipids of highly homogeneous low-density lipoprotein (LDL) subspecies (d 1.030-1.040 g/mL) have been evaluated during transition metal- and azo radical-induced oxidation using differential scanning calorimetry. Apolipoprotein B100 (apo-B100) structure was highly sensitive to oxidative modification; indeed, a significant loss of thermal stability was observed at initial stages irrespective of whether oxidation was mediated by site-specific binding of copper ions or by free radicals generated during decomposition of azo compounds. Subsequently, thermal protein integrity was destroyed, as a result of potentially irreversible protein unfolding, cross-linking reactions, and aggregation. Our results suggest that even minimal oxidative modification of apo-B100 has a major impact on the stability of this large monomeric protein. By contrast, the core lipids, which consist primarily of cholesteryl esters and triglycerides and play a determinant role in the thermal transition occurring near physiological temperature, preserved features of an ordered arrangement even during propagation of lipid peroxidation.

Oxidation of low density lipoprotein particles decreases their ability to bind to human aortic proteoglycans. Dependence on oxidative modification of the lysine residues

Oxidation of low density lipoprotein (LDL) leads to its rapid uptake by macrophages in vitro, but no detailed studies have addressed the effect of oxidation on the binding of LDL to proteoglycans. We therefore treated LDL with various substances: copper sulfate, 2,2'-azobis(2-amidinopropane)hydrochloride (AAPH), soybean lipoxygenase, and mouse peritoneal macrophages, and determined the extent to which the oxidatively modified LDL bound to human aortic proteoglycans in an affinity column. Oxidation of LDL with copper, AAPH, or macrophages, all of which increased its electrophoretic mobility, was associated with reduced binding to proteoglycans, until strongly oxidized LDL was totally unable to bind to them. After treatment of LDL with soybean lipoxygenase, the change in electrophoretic mobility was small, and the amount of binding to proteoglycans was only slightly decreased. The increased electrophoretic mobility of oxidized LDL reflects modification of the lysine residues of apolipoprotein B-100 (apoB-100). To mimic the oxidative modification of lysines, we treated LDL with malondialdehyde. This treatment also totally prevented the binding of LDL to proteoglycans. In contrast, if the lysine residues of apoB-100 were methylated to shield them against oxidative modification, subsequent treatment of LDL with copper sulfate failed to reduce the degree of LDL binding to proteoglycans. Finally, the active lysine residues in the oxidized LDL particles, which are thought to be involved in this binding, were quantified with NMR spectroscopy. In oxidized LDL, the number of these residues was found to be decreased. The present results show that, after modification of the lysine residues of apoB-100 during oxidation, the binding of LDL to proteoglycans is decreased, and suggest that oxidation of LDL tends to lead to intracellular rather than extracellular accumulation of LDL during atherogenesis.

The genetic determinants of plasma cholesterol and response to diet

In general, risk factors for multifactorial disorders such as atherosclerosis and hyperlipidaemia show a continuous distribution in the population, and this is the result of both interaction between genetic variation at genetic loci, and genetic and environmental interaction. Therefore, the investigation of the genetics of intermediate phenotypes such as levels of plasma lipid traits is likely to be particularly informative. Once the genes involved in determining the levels of these phenotypes have been identified, it should be possible to use the information to obtain a better understanding of the way these genetic variations determine the clinical end points. In the population it will be possible to identify a number of polygenes that are having a small effect on determining the trait, but for a particular individual, or the relatives of that individual, only a subset of all these polygenes will determine the level of the trait and therefore the risk of developing the disorder. In general, mutations with a large effect on the trait are rare in the population, By contrast, polymorphisms with a small effect on the trait may be common, such as is found with the effect of the apoE alleles and variation at the apoB gene locus on lipid levels. In the field of hyperlipidaemia and atherosclerosis research, molecular techniques have already given a great deal of information on how specific sequence variations in some of the candidate genes are involved in determining levels of plasma apoproteins, lipoproteins and lipids. As more mutations and sequence variations are identified, this will not only aid our understanding of the underlying pathology, but should be useful for identifying individuals who are at risk of developing atherosclerosis because of their particular genotype or combination of genotypes.

Proteolysis and fusion of low density lipoprotein particles strengthen their binding to human aortic proteoglycans

Lipid droplets resembling those seen in the extracellular space of the arterial intima were generated in vitro when granule proteases of rat serosal mast cells degraded the apolipoprotein B-100 (apoB-100) component of granule-bound low density lipoprotein (LDL), and the particles fused on the granule surface (Paanenen, K., and Kovanen, P. T. (1994) J. Biol. Chem. 269, 2023-2031). Moreover, the binding of the fused particles to the heparin proteoglycan component of the granules was found to be strengthened. We have now treated LDL particles with alpha-chymotrypsin and examined the strength with which the proteolytically modified LDL binds to human aortic proteoglycans on an affinity column. We found that chymotryptic degradation of the LDL particles triggered particle fusion. The higher the degree of proteolytic degradation, the higher were the degree of fusion and the strength of binding to the aortic proteoglycans. Separation of the proteolyzed particles by size exclusion chromatography into two fractions, unfused and fused particles, and analysis of their binding strengths revealed that not only the fused but also the unfused proteolyzed particles bound more tightly to the proteoglycans than did the native LDL particles. To investigate the mechanism underlying this increase in binding strength, we attached [13C]dimethyl groups to the lysines and used NMR spectroscopy to quantify the active lysine residues of apoB-100, which are thought to be located in basic areas of apoB-100 and involved in binding of LDL to proteoglycans. Analysis of the 13C-labeled particles showed that, despite loss of apoB-100 fragments from the particles, the number of active lysine residues in the unfused proteolyzed particles had not decreased. In the fused proteolyzed particles, the number of active lysine residues was markedly increased. Thus, proteolytic fusion appears to increase the number of basic domains of apoB-100, which would explain the observed increase in the strength of binding of the modified LDL particles to arterial proteoglycans. Since the fused particles resemble the small lipid droplets found in the atherosclerotic arterial intima, this LDL modification offers a plausible mechanism for the focal accumulation of lipid droplets in the extracellular proteoglycan matrix during atherogenesis.

Identification of the major site of apolipoprotein B modification by advanced glycosylation end products blocking uptake by the low density lipoprotein receptor

Advanced glycosylation end products (AGEs) arise from glucose-derived Amadori products and have been implicated in the pathogenesis of diabetic vascular disease. We recently reported the presence of an AGE-modified form of low density lipoprotein (LDL) that circulates in high amounts in patients with diabetes or renal insufficiency and that exhibits impaired plasma clearance kinetics. We utilized AGE-specific antibodies to identify the major sites of AGE modification within protease-digested preparations of apolipoprotein B that impair the binding of the AGE-modified form of LDL by human fibroblast LDL receptors. The predominant site of AGE immunoreactivity was found to lie within a single, 67-amino acid region located 1791 residues NH2-terminal of the putative LDL receptor binding domain. These data point to the high reactivity and specificity of this site for AGE formation and provide further evidence for important structural interactions between the LDL receptor binding domain and remote regions of the apolipoprotein B polypeptide.

Characterization of native and drug-loaded human low density lipoproteins

Low-density lipoproteins (LDLs), the physiological vehicles for lipids, are potentially useful drug delivery devices for (hydrophobic) drugs. The physicochemical characteristics of LDL loaded with the adriamycin derivative AD 32 or the N-mustard derivative WB 4291 were compared to that of native and reconstituted LDL at different temperatures. X-ray solution scattering indicates that loading with AD 32 has no detectable effect on the particle structure at room temperature, in contrast to WB 4291. According to 19F NMR data, AD 32 molecules are located in two distinct chemical environments with restricted motional freedom of the CF3 groups in samples stored as lyophilisates. 1H NMR signals from AD 32 were not observed, while those from WB 4291 could be distinguished from those of LDL constituents. WB 4291 molecules are in an environment with a higher motional freedom than AD 32 molecules. 1H NMR data suggest a higher fluidity of the core components for the WB-loaded LDLs compared to the other LDL preparations. While the motional freedom of the phospholipid head groups seems to be temperature independent, there is an increase in the mobility of the lipid components in the core region of the LDL particles with temperature.

Apolipoprotein B and low-density lipoprotein structure: implications for biosynthesis of triglyceride-rich lipoproteins

ApoB100 is a very large glycoprotein essential for triglyceride transport in vertebrates. It plays functional roles in lipoprotein biosynthesis in liver and intestine, and is the ligand recognized by the LDL receptor during receptor-mediated endocytosis. ApoB100 is encoded by a single gene on chromosome 2, and the message undergoes a unique processing event to form apoB48 message in the human intestine, and, in some species, in liver as well. The primary sequence is relatively unique and appears unrelated to the sequences of other serum apolipoproteins, except for some possible homology with the receptor recognition sequence of apolipoprotein E. From its sequence, structure prediction shows the presence of both sheet and helix scattered along its length, but no transmembrane domains apart from the signal sequence. The multiple carbohydrate attachment sites have been identified, as well as the locations of most of its disulfides. ApoB is the single protein found on LDL. These lipoproteins are emulsion particles, containing a core of nonpolar cholesteryl ester and triglyceride oil, surrounded by an emulsifying agent, a monolayer of phospholipid, cholesterol, and a single molecule of apoB100. An emulsion particle model is developed to predict accurately the physical and compositional properties of an LDL of any given size. A variety of techniques have been employed to map apoB100 on the surface of the LDL, and all yield a model in which apoB surrounds the LDL like a belt. Moreover, it is concluded that apoB100 folds into a long, flexible structure with a cross-section of about 20 x 54 A2 and a length of about 585 A. This structure is embedded in the surface coat of the LDL and makes contact with the core. During lipoprotein biosynthesis in tissue culture, truncated fragments of apoB100 are secreted on lipoproteins. Here, it was found that the lipoprotein core circumference was directly proportional to the apoB fragment size. A cotranslational model has been porposed for the lipoprotein assembly, which includes these structural features, and it is concluded that in permanent hepatocyte cell lines, apoB size determines lipoprotein core circumference.

Carbon-13 NMR studies of the lysine side chains of calmodulin and its proteolytic fragments

The pH-titration and dynamic behaviour of the seven lysine side chains in bovine calmodulin were studied by carbon-13 NMR. The amino groups of the calcium saturated protein and its proteolytic fragments TR1C (1-75) and TR2C (78-148) were dimethylated with carbon-13 labeled formaldehyde; this modification did not alter the protein's structure or its ability to activate the enzyme cyclic nucleotide phosphodiesterase. Tentative assignments for 5 out of the 7 dimethyl lysine resonances could be obtained by comparing spectra of the fully and partially modified protein, with those of the proteolytic fragments. The pKa values measured for calcium saturated calmodulin ranged between 9.5 (Lys 75) and 10.2 (Lys 13); two residues (Lys 94 and Lys 13) showed a biphasic titration curve suggesting their possible involvement in ion-pairs. The dynamic behavior of the lysine side chains was deduced from spin lattice relaxation measurements. All side chains were flexible and this was not influenced by the removal of calcium, or the addition of the calmodulin antagonist trifluoperazine. The latter data suggest that the lysine side chains are not directly involved in calmodulin's target binding sites.

Chemical modification of low-density lipoprotein enhances the number of binding sites for divalent cations

The EPR technique with paramagnetic Mn(II) ions has been used to probe the negatively charged sites on the surface of modified low-density lipoprotein (LDL). LDL modified in five different ways exhibited increased binding capacity for divalent cations. Enhanced binding is caused by the increase in the number of 'strong' binding sites. The 'strong' sites have been identified to be the aspartic acid and/or glutamic acid carboxyl residues and the 'weak' sites are zwitter-ionic phospholipids. In native LDL the negative groups make 'bonds' with the positive lysyl residues, thus stabilizing the structure. Any deprotonation or modification of the lysine amino groups makes the LDL structure more loose and the amino acid carboxyl groups accessible to divalent cations.

Effect of dietary supplementation with n-3 polyunsaturated fatty acids on physical properties and metabolism of low density lipoprotein in humans

The effects of marine n-3 polyunsaturated fatty acids were investigated in relation to the chemical and physical properties of low density lipoprotein (LDL) and how these changes affected LDL metabolism in humans. The subjects received supplements of six capsules daily, each capsule containing 1 g of either highly concentrated ethyl esters of n-3 fatty acids (85% eicosapentaenoic acid and docosahexaenoic acid) (n = 12) or corn oil (56% linoleic and 26% oleic acid) (n = 11). After 4 months of oil supplementation, the following changes were observed in the lipid moiety of the n-3-enriched LDL particles compared with LDL from the corn oil group: LDL cholesteryl ester, as well as the amount of total lipids of LDL, was significantly lower (0.97 +/- 0.12 versus 1.19 +/- 0.23 mg/mg protein and 1.88 +/- 0.40 versus 2.45 +/- 0.31 mg/mg, respectively; mean +/- SD, n = 6, p less than 0.05); the amount of eicosapentaenoic and docosahexaenoic acids and the unsaturation index increased (104.0 versus 29.4 micrograms/mg protein and 6.64 versus 5.49, respectively); and differential scanning calorimetry showed that LDL cholesteryl ester melting temperature was lowered by 2 degrees C (27.6 +/- 0.8 degrees versus 29.5 +/- 0.2 degrees C). The only effect observed on the protein moiety was an increase in the ratio of apolipoprotein (apo) B to cholesterol (0.66 +/- 0.17 versus 0.82 +/- 0.14 mg/mg cholesterol; p less than 0.05). Circular dichroism spectra of LDL indicated an alpha-helix content of 46 +/- 5% in apo B from both groups. No difference was observed by 13C nuclear magnetic resonance spectroscopy in the ratio of "active" to "normal" lysine residues of apo B. No detectable differences in the size of n-3 fatty acid-enriched LDL particles versus control LDL could be measured by either electron microscopy of negatively stained LDL (24.5 +/- 2.0 versus 25.0 +/- 1.5 nm) or dynamic light scattering (24.9 +/- 0.9 versus 24.9 +/- 0.4 nm). LDL from the fish oil and corn oil groups showed similar susceptibility to Cu(2+)-catalyzed lipid peroxidation, as indicated by the amount of lipid peroxides formed during the oxidation time, and degradation of oxidatively modified LDL in J774 macrophages as a function of Cu2+ oxidation time. No effect of n-3 fatty acids was observed on LDL metabolism. Specific uptake and degradation of n-3 fatty acid-enriched LDL were similar to those for control LDL in HepG2 cells as well as in human skin fibroblasts, and they showed the same ability to stimulate cholesteryl ester synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Familial defective apolipoprotein B-100: a common cause of primary hypercholesterolemia

Familial defective apolipoprotein B-100 (FDB) is a recently identified dominantly inherited genetic disorder characterized by a decreased binding of low density lipoprotein (LDL) to the LDL receptor due to defective apo B-100. FDB is caused by a G to A mutation at nucleotide 10,708 in exon 26 of the apo B gene creating a substitution of glutamine for arginine in the codon for amino acid 3500. The arginine (3500)----glutamine mutation has been observed in several populations in North America and Europe with a similar frequency of approximately 1/500 to 1/700. Haplotype analysis has demonstrated that the arginine(3500)----glutamine mutation occurs on the same chromosomal background. The fact that all individuals with FDB are of Caucasian extraction implies that the mutation has its origin in this population. The arginine(3500)----glutamine mutation has a profound impact of varying strength on the plasma LDL cholesterol level, leading to heterogeneous clinical expression comparable to "classic" familial hypercholesterolemia (FH) caused by a defective LDL receptor: tendon xanthoma, premature atherosclerosis and arcus lipoides. The present data suggest that the combination of these clinical features is no longer appropriate for the diagnosis of LDL-receptor-defective FH, but may be a common feature of a defective LDL receptor pathway originating either from defective LDL receptors or from malfunctioning ligand apo B-100.

Modifications of low-density lipoprotein induced by arterial proteoglycans and chondroitin-6-sulfate

Association of low-density lipoproteins (LDL) with arterial chondroitin sulfate proteoglycans (CSPG) appears to contribute to their deposition in the extracellular intimal compartment and to its internalization by macrophages. CSPG and LDL interact by ionic bridges with formation of soluble and insoluble complexes. We studied the alterations on LDL structure induced by its association with arterial CSPG and other glycosaminoglycans (GAG). In soluble complexes, at low and at physiological ionic strength, arterial CSPG and sulfated GAG modify the kinetics of apoB-100 proteolysis by trypsin. However, less marked alterations in the peptide patterns were observed with proteinase V8 and almost none with thermolysin. This is indirect evidence that the presence of CSPG and GAG modified the exposure of polar regions of apoB-100 in LDL. Competitive binding experiments with agarose-bound heparin and soluble GAG also suggest that after formation of insoluble complexes with arterial CSPG and resolubilization the exposure of Lys, Arg-rich segments of apoB-100 is increased. Results from differential scanning calorimetry and differential thermal spectrophotometry showed that the CSPG and GAG-induced modifications reduced the thermal stability of the surface and core in LDL. If present in vivo, the structural alterations of polar segments of the LDL protein moiety may influence the outcome of its interaction with the arterial mesenchyma.