The NH2-terminal region of apolipoprotein B is sufficient for lipoprotein association with glycosaminoglycans

In the Beginning, Lipoproteins Cross the Endothelial Barrier

Atherosclerosis begins with the infiltration of cholesterol-containing lipoproteins into the arterial wall. White blood cell (WBC)-associated inflammation follows. Despite decades of research using genetic and pharmacologic methods to alter WBC function, in humans, the most effective method to prevent the initiation and progression of disease remains low-density lipoprotein (LDL) reduction. However, additional approaches to reducing cardiovascular disease would be useful as residual risk of events continues even with currently effective LDL-reducing treatments. Some of this residual risk may be due to vascular toxicity of triglyceride-rich lipoproteins (TRLs). Another option is that LDL transcytosis continues, albeit at reduced rates due to lower circulating levels of this lipoprotein. This review will address these two topics. The evidence that TRLs promote atherosclerosis and the processes that allow LDL and TRLs to be taken up by endothelial cells leading to their accumulation with the subendothelial space.

Negatively-charged Liposome Nanoparticles Can Prevent Dyslipidemia and Atherosclerosis Progression in the Rabbit Model

BACKGROUND AND AIM

Negatively charged nanoliposomes have a strong attraction towards plasma lipoprotein particles and can thereby regulate lipid metabolism. Here, the impact of such nanoliposomes on dyslipidaemia and progression of atherosclerosis was investigated in a rabbit model.

METHODS

Two sets of negatively-charged nanoliposome formulations including [Hydrogenated Soy Phosphatidylcholine (HSPC)/1,2-distearoyl-sn-glycero-3- phosphoglycerol (DSPG)] and [1,2- Dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC)/1,2-Dimyristoyl-sn-glycero-3-phosphorylcholine (DMPG)/Cholesterol] were evaluated. Rabbits fed a high-cholesterol diet were randomly divided into 3 groups (n=5/group) intravenously administrated with HSPC/DSPG formulation (DSPG group; 100 mmol/kg), DMPC/DMPG formulation (DMPG group; 100 mmol/kg), or the normal saline (control group; 0.9% NaCl) over a 4-week period. The atherosclerotic lesions of the aortic arch wall were studied using haematoxylin and eosin staining.

RESULTS

Both DSPG and DMPG nanoliposome formulations showed a nano-sized range in diameter with a negatively-charged surface and a polydispersity index of <0.1. After 4 weeks administration, the nanoliposome formulations decreased triglycerides (-62±3% [DSPG group] and -58±2% [DMPG group]), total cholesterol (-58±9% [DSPG group] and -37±5% [DMPG group]), and lowdensity lipoprotein cholesterol (-64±6% [DSPG group] and -53±10% [DMPG group]) levels, and increased high-density lipoprotein cholesterol (+67±28% [DSPG group] and +35±19% [DMPG group]) levels compared with the controls. The nanoliposomes showed a significant decrease in the severity of atherosclerotic lesions: mean values of the intima to media ratio in DMPG (0.96±0.1 fold) and DSPG (0.54±0.02 fold) groups were found to be significantly lower than that in the control (1.2±0.2 fold) group (p<0.05).

CONCLUSION

Anionic nanoliposomes containing [HSPC/DSPG] and [DMPC/DMPG] correct dyslipidaemia and inhibit the progression of atherosclerosis.

Apolipoprotein B and Cardiovascular Disease: Biomarker and Potential Therapeutic Target

Apolipoprotein (apo) B, the critical structural protein of the atherogenic lipoproteins, has two major isoforms: apoB48 and apoB100. ApoB48 is found in chylomicrons and chylomicron remnants with one apoB48 molecule per chylomicron particle. Similarly, a single apoB100 molecule is contained per particle of very-low-density lipoprotein (VLDL), intermediate density lipoprotein, LDL and lipoprotein(a). This unique one apoB per particle ratio makes plasma apoB concentration a direct measure of the number of circulating atherogenic lipoproteins. ApoB levels indicate the atherogenic particle concentration independent of the particle cholesterol content, which is variable. While LDL, the major cholesterol-carrying serum lipoprotein, is the primary therapeutic target for management and prevention of atherosclerotic cardiovascular disease, there is strong evidence that apoB is a more accurate indicator of cardiovascular risk than either total cholesterol or LDL cholesterol. This review examines multiple aspects of apoB structure and function, with a focus on the controversy over use of apoB as a therapeutic target in clinical practice. Ongoing coronary artery disease residual risk, despite lipid-lowering treatment, has left patients and clinicians with unsatisfactory options for monitoring cardiovascular health. At the present time, the substitution of apoB for LDL-C in cardiovascular disease prevention guidelines has been deemed unjustified, but discussions continue.

2017 George Lyman Duff Memorial Lecture: Fat in the Blood, Fat in the Artery, Fat in the Heart: Triglyceride in Physiology and Disease

Cholesterol is not the only lipid that causes heart disease. Triglyceride supplies the heart and skeletal muscles with highly efficient fuel and allows for the storage of excess calories in adipose tissue. Failure to transport, acquire, and use triglyceride leads to energy deficiency and even death. However, overabundance of triglyceride can damage and impair tissues. Circulating lipoprotein-associated triglycerides are lipolyzed by lipoprotein lipase (LpL) and hepatic triglyceride lipase. We inhibited these enzymes and showed that LpL inhibition reduces high-density lipoprotein cholesterol by >50%, and hepatic triglyceride lipase inhibition shifts low-density lipoprotein to larger, more buoyant particles. Genetic variations that reduce LpL activity correlate with increased cardiovascular risk. In contrast, macrophage LpL deficiency reduces macrophage function and atherosclerosis. Therefore, muscle and macrophage LpL have opposite effects on atherosclerosis. With models of atherosclerosis regression that we used to study diabetes mellitus, we are now examining whether triglyceride-rich lipoproteins or their hydrolysis by LpL affect the biology of established plaques. Following our focus on triglyceride metabolism led us to show that heart-specific LpL hydrolysis of triglyceride allows optimal supply of fatty acids to the heart. In contrast, cardiomyocyte LpL overexpression and excess lipid uptake cause lipotoxic heart failure. We are now studying whether interrupting pathways for lipid uptake might prevent or treat some forms of heart failure.

Differences between group X and group V secretory phospholipase A(2) in lipolytic modification of lipoproteins

Secretory phospholipases A(2) (sPLA(2)s) are a diverse family of low molecular mass enzymes (13-18 kDa) that hydrolyze the sn-2 fatty acid ester bond of glycerophospholipids to produce free fatty acids and lysophospholipids. We have previously shown that group X sPLA(2) (sPLA(2)-X) had a strong hydrolyzing activity toward phosphatidylcholine in low-density lipoprotein (LDL) linked to the formation of lipid droplets in the cytoplasm of macrophages. Here, we show that group V sPLA(2) (sPLA(2)-V) can also cause the lipolysis of LDL, but its action differs remarkably from that of sPLA(2)-X in several respects. Although sPLA(2)-V released almost the same amount of fatty acids from LDL, it released more linoleic acid and less arachidonic acid than sPLA(2)-X. In addition, the requirement of Ca(2+) for the lipolysis of LDL was about 10-fold higher for sPLA(2)-V than sPLA(2)-X. In fact, the release of fatty acids from human serum was hardly detectable upon incubation with sPLA(2)-V in the presence of sodium citrate, which contrasted with the potent response to sPLA(2)-X. Moreover, sPLA(2)-X, but not sPLA(2)-V, was found to specifically interact with LDL among the serum proteins, as assessed by gel-filtration chromatography as well as sandwich enzyme-immunosorbent assay using anti-sPLA(2)-X and anti-apoB antibodies. Surface plasmon resonance studies have revealed that sPLA2-X can bind to LDL with high-affinity (K(d) = 3.1 nM) in the presence of Ca(2+). Selective interaction of sPLA(2)-X with LDL might be involved in the efficient hydrolysis of cell surface or intracellular phospholipids during foam cell formation.

Immuno-electron cryo-microscopy imaging reveals a looped topology of apoB at the surface of human LDL

A single copy of apoB is the sole protein component of human LDL. ApoB is crucial for LDL particle stabilization and is the ligand for LDL receptor, through which cholesterol is delivered to cells. Dysregulation of the pathways of LDL metabolism is well documented in the pathophysiology of atherosclerosis. However, an understanding of the structure of LDL and apoB underlying these biological processes remains limited. In this study, we derived a 22 Å-resolution three-dimensional (3D) density map of LDL using cryo-electron microscopy and image reconstruction, which showed a backbone of high-density regions that encircle the LDL particle. Additional high-density belts complemented this backbone high density to enclose the edge of the LDL particle. Image reconstructions of monoclonal antibody-labeled LDL located six epitopes in five putative domains of apoB in 3D. Epitopes in the LDL receptor binding domain were located on one side of the LDL particle, and epitopes in the N-terminal and C-terminal domains of apoB were in close proximity at the front side of the particle. Such image information revealed a looped topology of apoB on the LDL surface and demonstrated the active role of apoB in maintaining the shape of the LDL particle.

Immunochemical analysis of the electronegative LDL subfraction shows that abnormal N-terminal apolipoprotein B conformation is involved in increased binding to proteoglycans

Electronegative LDL (LDL(-)) is a minor subfraction of modified LDL present in plasma. Among its atherogenic characteristics, low affinity to the LDL receptor and high binding to arterial proteoglycans (PGs) could be related to abnormalities in the conformation of its main protein, apolipoprotein B-100 (apoB-100). In the current study, we have performed an immunochemical analysis using monoclonal antibody (mAb) probes to analyze the conformation of apoB-100 in LDL(-). The study, performed with 28 anti-apoB-100 mAbs, showed that major differences of apoB-100 immunoreactivity between native LDL and LDL(-) concentrate in both terminal extremes. The mAbs Bsol 10, Bsol 14 (which recognize the amino-terminal region), Bsol 2, and Bsol 7 (carboxyl-terminal region) showed increased immunoreactivity in LDL(-), suggesting that both terminal extremes are more accessible in LDL(-) than in native LDL. The analysis of in vitro-modified LDLs, including LDL lipolyzed with sphingomyelinase (SMase-LDL) or phospholipase A(2) (PLA(2)-LDL) and oxidized LDL (oxLDL), suggested that increased amino-terminal immunoreactivity was related to altered conformation due to aggregation. This was confirmed when the aggregated subfractions of LDL(-) (agLDL(-)) and oxLDL (ag-oxLDL) were isolated and analyzed. Thus, Bsol 10 and Bsol 14 immunoreactivity was high in SMase-LDL, ag-oxLDL, and agLDL(-). The altered amino-terminal apoB-100 conformation was involved in the increased PG binding affinity of agLDL(-) because Bsol 10 and Bsol 14 blocked its high PG-binding. These observations suggest that an abnormal conformation of the amino-terminal region of apoB-100 is responsible for the increased PG binding affinity of agLDL(-).

Hepatic heparan sulfate proteoglycans and endocytic clearance of triglyceride-rich lipoproteins

Hypertriglyceridemia, characterized by the accumulation of triglyceride-rich lipoproteins in the blood, affects 10-20% of the population in western countries and increases the risk of atherosclerosis, coronary artery disease, and pancreatitis. The etiology of hypertriglyceridemia is complex, and much interest exists in identifying and characterizing the biological and environmental factors that affect the synthesis and turnover of plasma triglycerides. Genetic studies in mice have recently identified that heparan sulfate proteoglycans are a class of receptors that mediate the clearance of triglyceride-rich lipoproteins in the liver. Heparan sulfate proteoglycans are expressed by endothelial cells that line the hepatic sinusoids and the underlying hepatocytes, and are present in the perisinusoidal space (space of Disse). This chapter discusses the dependence of lipoprotein binding on heparan sulfate structure and the identification of hepatocyte syndecan-1 as the primary proteoglycan that mediates triglyceride-rich lipoprotein clearance.

Heparan sulfate 2-O-sulfotransferase is required for triglyceride-rich lipoprotein clearance

Hepatic clearance of triglyceride-rich lipoproteins depends on heparan sulfate and low density lipoprotein receptors expressed on the basal membrane of hepatocytes. Binding and uptake of the lipoproteins by way of heparan sulfate depends on the degree of sulfation of the chains based on accumulation of plasma triglycerides and delayed clearance of triglyceride-rich lipoproteins in mice bearing a hepatocyte-specific alteration of N-acetylglucosamine (GlcNAc) N-deacetylase-N-sulfotransferase 1 (Ndst1) (MacArthur, J. M., Bishop, J. R., Stanford, K. I., Wang, L., Bensadoun, A., Witztum, J. L., and Esko, J. D. (2007) J. Clin. Invest. 117, 153-164). Inactivation of Ndst1 led to decreased overall sulfation of heparan sulfate due to coupling of uronyl 2-O-sulfation and glucosaminyl 6-O-sulfation to initial N-deacetylation and N-sulfation of GlcNAc residues. To determine whether lipoprotein clearance depends on 2-O-and 6-O-sulfation, we evaluated plasma triglyceride levels in mice containing loxP-flanked conditional alleles of uronyl 2-O-sulfotransferase (Hs2st(f/f)) and glucosaminyl 6-O-sulfotransferase-1 (Hs6st1(f/f)) and the bacterial Cre recombinase expressed in hepatocytes from the rat albumin (Alb) promoter. We show that Hs2st(f/f)AlbCre(+) mice accumulated plasma triglycerides and exhibited delayed clearance of intestinally derived chylomicrons and injected human very low density lipoproteins to the same extent as observed in Ndst1(f/f)AlbCre(+) mice. In contrast, Hs6st1(f/f)AlbCre(+) mice did not exhibit any changes in plasma triglycerides. Chemically modified heparins lacking N-sulfate and 2-O-sulfate groups did not block very low density lipoprotein binding and uptake in isolated hepatocytes, whereas heparin lacking 6-O-sulfate groups was as active as unaltered heparin. Our findings show that plasma lipoprotein clearance depends on specific subclasses of sulfate groups and not on overall charge of the chains.

A high mobility group B-1 box A peptide combined with an artery wall binding peptide targets delivery of nucleic acids to smooth muscle cells

The TAT-high mobility group box-1 A box peptide (TAT-HMGB1A) has been reported previously to be able to deliver DNA into cells without cytotoxicity. In this study, an artery wall smooth muscle cell-targeting carrier was developed using TAT-HMGB1A combined with an artery wall binding peptide (ABP). For the production of ABP linked TAT-HMGB1A (TAT-HMGB1A-ABP), pET15b-TAT-HMGB1A-ABP was constructed by inserting the ABP cDNA into pET15b-TAT-HMGB1A. TAT-HMGB1A-ABP was expressed in E. coli and purified by Nickel chelate chromatography. Gel retardation assays showed that TAT-HMGB1A-ABP formed a complex with the plasmid at or above a 5:1 weight ratio (peptide:plasmid). At a 20:1 weight ratio, the zeta-potential was approximately 25 mV and the particle size was approximately 120 nm. TAT-HMGB1A-ABP had the highest transfection efficiency in A7R5 smooth muscle cells at a weight ratio of 20:1. TAT-HMGB1A-ABP exhibited higher transfection efficiency in A7R5 cells than PLL or TAT-HMGB1A, while TAT-HMGB1A-ABP had lower transfection efficiencies in Hep3B hepatoma, 293 kidney, NIH3T3 fibroblast, and Raw264.7 macrophage cells compared with PLL. Together, these results suggest that the ABP moiety of the peptide increased transfection efficiency specifically in smooth muscle cells. In a competition assay, the transfection efficiency of TAT-HMGB1A-ABP in A7R5 cells was reduced by the addition of free ABP. MTT assays showed that TAT-HMGB1A-ABP did not produce any cytotoxicity in A7R5 cells. Therefore, TAT-HMGB1A-ABP may be useful for a targeting gene delivery to smooth muscle cells.

Upregulation of decorin by FXR in vascular smooth muscle cells

Decorin is a member of the family of small leucine-rich proteoglycans that are present in blood vessels and synthesized by vascular smooth muscle cells (VSMCs). Decorin plays complex roles in both normal vascular physiology and the pathogenesis of various types of vascular disorders. However, the mechanisms of regulation of decorin expression in vasculature are not clearly understood. Particularly little information is available about a role of nuclear receptors in the regulation of decorin expression. In the present study, we report that activation of vascular FXR by a specific ligand resulted in upregulation of decorin at the levels of both mRNA and protein. FXR appears to induce decorin expression at a transcriptional level because (1) upregulation of decorin mRNA expression was abolished by the treatment of a transcription inhibitor, actinomycin D; and (2) decorin promoter activity was significantly increased by activation of FXR. Functional analysis of human decorin promoter identified an imperfect inverted repeat DNA motif, IR8 (-2313TGGTCAtagtgtcaTGACCT-2294), as a likely FXR-responsive element that is involved in decorin regulation.

Heparan sulfate proteoglycans and triglyceride-rich lipoprotein metabolism

PURPOSE OF REVIEW

Clearance of triglyceride-rich lipoprotein remnants by the liver is a key step in preventing hypertriglyceridemia, an independent risk factor for cardiovascular disease. We review recent genetic evidence that heparan sulfate proteoglycans work in concert with the LDL receptor in the liver to facilitate binding and clearance of both triglyceride and cholesterol-rich lipoproteins from the circulation.

RECENT FINDINGS

Partial reduction of sulfation of liver heparan sulfate using the Cre-loxP system caused accumulation of hepatic and dietary triglyceride-rich lipoprotein particles due to delayed clearance. Compounding the mutation with LDL receptor deficiency caused enhanced accumulation of both cholesterol and triglyceride-rich particles compared with mice lacking only LDL receptors. These findings provide the first genetic evidence that hepatic heparan sulfate proteoglycans play a central role in the clearance of lipoproteins by the liver and work independently of LDL receptors.

SUMMARY

A role for hepatocyte heparan sulfate in lipoprotein metabolism has now been genetically established in mice. Given this finding, mild, but clinically relevant, hyperlipidemias in human patients may be a result of alterations in heparan sulfate structure or possible genetic polymorphisms in the relevant biosynthetic genes.

Subendothelial lipoprotein retention as the initiating process in atherosclerosis: update and therapeutic implications

The key initiating process in atherogenesis is the subendothelial retention of apolipoprotein B-containing lipoproteins. Local biological responses to these retained lipoproteins, including a chronic and maladaptive macrophage- and T-cell-dominated inflammatory response, promote subsequent lesion development. The most effective therapy against atherothrombotic cardiovascular disease to date--low density lipoprotein-lowering drugs--is based on the principle that decreasing circulating apolipoprotein B lipoproteins decreases the probability that they will enter and be retained in the subendothelium. Ongoing improvements in this area include more aggressive lowering of low-density lipoprotein and other atherogenic lipoproteins in the plasma and initiation of low-density lipoprotein-lowering therapy at an earlier age in at-risk individuals. Potential future therapeutic approaches include attempts to block the interaction of apolipoprotein B lipoproteins with the specific subendothelial matrix molecules that mediate retention and to interfere with accessory molecules within the arterial wall that promote retention such as lipoprotein lipase, secretory sphingomyelinase, and secretory phospholipase A2. Although not the primary focus of this review, therapeutic strategies that target the proatherogenic responses to retained lipoproteins and that promote the removal of atherogenic components of retained lipoproteins also hold promise. The finding that certain human populations of individuals who maintain lifelong low plasma levels of apolipoprotein B lipoproteins have an approximately 90% decreased risk of coronary artery disease gives hope that our further understanding of the pathogenesis of this leading killer could lead to its eradication.

Decorin overexpression reduces atherosclerosis development in apolipoprotein E-deficient mice

Atherosclerosis results from accumulation of macrophages and extracellular matrix in the arterial wall. Decorin, a small matrix proteoglycan, is able to regulate cell proliferation, migration and growth factors' activity. We investigated the effect of decorin overexpression on atherosclerosis progression in apolipoprotein E-deficient (ApoE(-/-)) mice. Female ApoE(-/-) mice, 10 weeks old (early treatment, n = 20) and 20 weeks old (delayed treatment, n = 20) were administered intravenously with either an adenovirus (2.5 x 10(9) plaque-forming units/mouse) containing human decorin gene (Ad-Dcn) or beta-galactosidase (LacZ), or PBS. Transgenic decorin was mainly expressed in the liver, and was secreted in the plasma up to 4 weeks. Six weeks after treatment, no significant difference in aortic root lesion size was observed between LacZ- and PBS-control groups. In contrast, Ad-Dcn-treated mice showed significantly reduced atherosclerotic lesions as compared to controls in both early and delayed treatment groups (2.9 +/- 1.1% versus 5.5 +/- 0.4%; p = 0.004 and 13.4 +/- 1.3% versus 19.9 +/- 1.41%; p = 0.009, respectively). In parallel, macrophage, gelatinase activity and collagen plaque content were also reduced. Interestingly, plasma triglycerides were reduced and decorin formed complexes with transforming growth factor-beta1 (TGF-beta1) that resulted in reduced circulating free-TGF-beta1. In conclusion, systemic overexpression of decorin reduces inflammation, triglycerides and fibrosis in atherosclerotic plaques of ApoE(-/-) mice resulting in slowing down of disease progression.

Antioxidants inhibit the ability of lysophosphatidylcholine to regulate proteoglycan synthesis

OBJECTIVE

We previously have shown that lysophosphatidylcholine (lysoPC) regulates proteoglycan synthesis by vascular smooth muscle cells (SMCs). Given the accumulating evidence for reactive oxygen species (ROS) as mediators of a variety of effects of lysoPC, the present study evaluates the potential role of ROS as intermediate molecules in the regulation of proteoglycan synthesis by lysoPC.

METHODS AND RESULTS

LysoPC (10 micromol/L) was found to stimulate rapid and sustained generation of ROS by SMC, as indicated using a fluorescent probe for measuring intracellular oxidants and fluorescence-activated cell sorting. This was not associated with cytotoxicity, as evaluated by fluorescence microscopy using MitoTracker Red or propidium iodide, cell number, cell protein, or lactate dehydrogenase release. Pretreatment with catalase or superoxide dismutase, specific scavengers of hydrogen peroxide and superoxide, respectively, blocked the ability of lysoPC to stimulate both accumulation of ROS and proteoglycan synthesis. Most importantly, these enzymatic antioxidants prevented lysoPC from stimulating the synthesis of proteoglycans with enhanced lipoprotein-binding properties, as quantified by a gel shift binding assay.

CONCLUSIONS

These findings strongly suggest that ROS are key mediators in the ability of lysoPC to regulate proteoglycan synthesis and that these effects can be inhibited by antioxidants.

Lipid composition influences the shape of human low density lipoprotein in vitreous ice

Earlier cryo-electron microscopic studies have indicated that the normal low density lipoprotein (N-LDL) has a discoid shape when its core is in the liquid-crystalline state. In the present study, we investigated whether the shape of LDL depends on the physical state and/or the lipid composition of the lipoprotein core. Using a custom-built freezing device, we vitrified NLDL samples from either above or below the phase-transition temperature of the core (42 and 24 degrees C, respectively). Cryo-electron microscopy revealed no differences between these samples and indicated a discoid shape of the N-LDL particle. In contrast, TG-enriched LDL (T-LDL) did not have discoid features and appeared to be quasi-spherical in preparations that were vitrified from either 42 or 24 degrees C. These results suggest that the shape of NLDL is discoid, regardless of the physical state of its core, whereas T-LDL is more spherical. Aspects that may influence the shape of LDL are discussed.

Assembly of lipoprotein particles containing apolipoprotein-B: structural model for the nascent lipoprotein particle

Apolipoprotein B (apoB) is the major protein component of large lipoprotein particles that transport lipids and cholesterol. We have developed a detailed model of the first 1000 residues of apoB using standard sequence alignment programs (ClustalW and MACAW) and the MODELLER6 package for three-dimensional homology modeling. The validity of the apoB model was supported by conservation of disulfide bonds, location of all proline residues in turns and loops, and conservation of the hydrophobic faces of the two C-terminal amphipathic beta-sheets, betaA (residues 600-763) and betaB (residues 780-1000). This model suggests a lipid-pocket mechanism for initiation of lipoprotein particle assembly. In a previous model we suggested that microsomal triglyceride transfer protein might play a structural role in completion of the lipid pocket. We no longer think this likely, but instead propose a hairpin-bridge mechanism for lipid pocket completion. Salt-bridges between four tandem charged residues (717-720) in the turn of the hairpin-bridge and four tandem complementary residues (997-1000) at the C-terminus of the model lock the bridge in the closed position, enabling the deposition of an asymmetric bilayer within the lipid pocket.

Molecular interactions leading to lipoprotein retention and the initiation of atherosclerosis

Atherosclerosis is distinguished by the accumulation of lipoprotein lipid within the arterial wall. An ionic interaction of positively charged regions of apolipoprotein (apo) B with matrix proteins, including proteoglycans, collagen, and fibronectin, is thought to initiate this process. Proteoglycans are complex glycoproteins containing highly negatively charged carbohydrate chains. These proteins are abundant in atherosclerosis lesions, and they associate with apoB-containing lipoproteins. Several specific regions of apoB may mediate this process. Other lipoprotein-associated proteins, including apoE and lipases, might also participate in this process. In addition, retention may occur via lipoprotein association with other matrix molecules or as a consequence of intra-arterial lipoprotein aggregation.

Atherosclerosis in perlecan heterozygous mice

The hypothesis that lipoprotein association with perlecan is atherogenic was tested by studying atherosclerosis in mice that had a heterozygous deletion of perlecan, the primary extracellular heparan sulfate proteoglycan in arteries. We first studied the expression of perlecan in mouse lesions and noted that this proteoglycan in aorta was found in the subendothelial matrix. Perlecan was also a major component of the lesional extracellular matrix. Mice with a heterozygous deletion had a reduction in arterial wall perlecan expression. Atherosclerosis in these mice was studied after crossing the defect into the apolipoprotein E (apoE) and LDL receptor knockout backgrounds. At 12 weeks, chow-fed apoE null mice with a heterozygous deletion had less atherosclerosis. However, at 24 weeks and in the LDL receptor heterozygous background, the presence of a perlecan knockout allele did not significantly alter lesion size. Thus, it appears that loss of perlecan leads to less atherosclerosis in early lesions. Although this might be attributable to a decrease in lipoprotein retention, it should be noted that perlecan might mediate multiple other processes that could, in sum, accelerate atherosclerosis.

Mechanism of lipoprotein retention by the extracellular matrix

PURPOSE OF REVIEW

Considerable evidence suggests that the subendothelial retention of atherogenic lipoproteins is a key early step in atherogenesis. In humans and experimental animals, elevated levels of plasma lipoproteins are associated with increased atherosclerosis, and lipoproteins with higher affinity for arterial proteoglycans are more atherogenic. Here we discuss the molecular mechanisms underlying lipoprotein retention in the arterial wall and how this interaction can be modulated.

RECENT FINDINGS

Functional proteoglycan binding sites in lipoproteins containing apolipoprotein B have been identified and shown to have atherogenic potential in vivo. In addition to apolipoprotein B, novel bridging molecules, those that can interact with both proteoglycans and lipoproteins, have been identified that mediate the retention of atherogenic particles in the vessel wall. The interaction between lipoproteins and proteoglycans can be enhanced by the modification of lipoproteins in the circulation and in the arterial wall, by alterations in the subendothelium, and by changes in proteoglycan synthesis that result in a more atherogenic profile. The retention of atherogenic lipoproteins is a potential target for therapies to reverse atherosclerosis, and in-vitro studies have identified compounds that decrease the affinity of proteoglycans for lipoproteins.

SUMMARY

Considerable progress has been made in understanding the association between lipoproteins and cardiovascular disease. This review highlights the importance of the interaction between lipoproteins and the arterial matrix.

Association of hepatic lipase with proteoglycans stimulates the production of proteoglycans in vivo and in vitro

HL is synthesized in hepatocytes and functions while bound to heparan sulfate proteoglycans (HSPGs) in sinusoidal endothelial cells. The HL-mediated uptake of lipoprotein requires cell-surface HSPG. The present study tested whether HL plays a role in the production of HSPG. The production of HSPG in Chinese hamster ovary (CHO) cells was determined by measuring the incorporation of (35)SO(4) into PGs. HL-producing HL-CHO cells showed approximately 30% more cellular PG than did wild-type (WT) cells. In contrast, PG production in cells producing a membrane-anchored HL-glycophosphatidylinositol (GPI) that was not bound to HSPG was virtually identical to that in WT cells. When purified HL was added to the WT- or HL-GPI cells, PG production increased significantly to a level similar to that of the HL-secreting cells, suggesting that the binding of HL to HSPG triggered the increased HSPG production. Heparin reduced PG production in HL-producing cells, confirming that PG production is stimulated only when HL is present as a ligand for HSPG. Real-time PCR and Northern blots demonstrated that PG production was significantly reduced in animals lacking HL. Together, these data suggest that the binding of HL to PG on the cell surface exerts a positive feedback on cellular PG production.

Amino terminal 38.9% of apolipoprotein B-100 is sufficient to support cholesterol-rich lipoprotein production and atherosclerosis

OBJECTIVE

Carboxyl terminal truncation of apolipoprotein (apo)B-100 and apoB-48 impairs their capacity for triglyceride transport, but the ability of the resultant truncated apoB to transport cholesterol and to support atherosclerosis has not been adequately studied. The atherogenicity of apoB-38.9 was determined in this study by using our apoB-38.9-only (Apob38.9/38.9) mice.

METHODS AND RESULTS

ApoB-38.9-lipoproteins (Lp-B38.9) circulate at very low levels in Apob38.9/38.9 mice as small LDLs or HDLs. Disruption of apoE gene in these mice caused accumulation of large amounts of betaVLDL-like LpB-38.9 in plasma. These betaVLDL particles were more enriched with cholesteryl esters but poor in triglycerides compared with the apoB-48-betaVLDL of the apoB-wild-type/apoE-null (Apob+/+/Apoe-/-) mice. Likewise, apoB-38.9-VLDL secreted by cultured Apob38.9/38.9 mouse hepatocytes also had higher ratios of total cholesterol to triglycerides than apoB-48-VLDL secreted by the apoB-48-only hepatocytes. Thus, despite its impaired triglyceride-transporting capacity, apoB-38.9 has a relatively intact capacity for cholesterol transport. Spontaneous aortic atherosclerotic lesions were examined in apoB-38.9-only/apoE-null (Apob38.9/38.9/Apoe-/-) mice at ages 9 and 13 months. Extensive lesions were found in the Apob38.9/38.9/Apoe-/- mice as well as in their Apob+/38.9/Apoe-/- and Apob+/+/Apoe-/- littermates.

CONCLUSIONS

Deleting the C-terminal 20% from apoB-48 does not impair its ability to transport cholesterol and to support atherosclerosis, thus narrowing the "atherogenic region" of apoB.

Low density lipoproteins interact with acidic fibroblast growth factor and modify its function

OBJECTIVE

Oxidized LDL (oxLDL) was shown to trigger the release of acidic fibroblast growth factor (FGF-1). Because these components are likely to be present simultaneously in the atherosclerotic milieu, we investigated whether oxLDL interacts with FGF-1 and whether this interaction affects FGF-1 functioning.

METHODS AND RESULTS

Using molecular sieve and electrophoretic mobility shift assays, we found that FGF-1 forms a complex with oxLDL in vitro, in contrast to its low affinity for nonatherogenic, native LDL. The FGF-1/oxLDL complex had a dramatically decreased ability to bind heparin and was nonmitogenic on cultured smooth muscle cells. In human atherosclerotic lesions, the highest FGF-1 immunoreactivity was found in macrophages. With respect to oxLDL accumulation, 2 patterns were distinguished: (1) moderate, intracellular in matrix-rich regions containing viable cells and (2) massive, both cell-associated and extracellular oxLDL deposits in foam cell-rich regions with necrotic areas. The proliferating cell nuclear antigen readings for proliferating cells reflected that the mitogenic activity of FGF-1 was confined to the regions where oxLDL was strictly intracellular and was inhibited in the regions with extracellular oxLDL deposition.

CONCLUSIONS

oxLDL, besides being a bulky component of the atherosclerotic lesion, possibly manifests its pathogenicity by complexing FGF-1 and inhibiting its growth-promoting function during atherogenesis.

Identification of the proteoglycan binding site in apolipoprotein B48

An initial event in atherosclerosis is the retention of lipoproteins within the intima of the vessel wall. Previously we identified Site B (residues 3359-3369) in apolipoprotein (apo) B100 as the proteoglycan binding sequence in low density lipoproteins (LDLs) and showed that the atherogenicity of apoB-containing lipoproteins is linked to their affinity for artery wall proteoglycans. However, both apoB100- and apoB48-containing lipoproteins are equally atherogenic even though Site B lies in the carboxyl-terminal half of apoB100 and is absent in apoB48. If binding to proteoglycans is a key step in atherogenesis, apoB48-containing lipoproteins must bind to proteoglycans via other proteoglycan binding sites in the amino-terminal 48% of apoB. In vitro studies have identified five clusters of basic amino acids in delipidated apoB48 that bind negatively charged glycosaminoglycans. To determine which of these sites is functional on LDL particles, we analyzed the proteoglycan binding activity of recombinant human LDLs from transgenic mice or rat hepatoma cells. Substitution of neutral amino acids for the basic amino acids in Site B-Ib (residues 84-94) abolished the proteoglycan binding activity of recombinant apoB53. Carboxyl-truncated apoB80 bound biglycan with higher affinity than apoB100 and apoB48. ApoB80 in which Site B was mutated had the same affinity for proteoglycans as apoB48. These data support the hypothesis that the carboxyl terminus of apoB100 "masks" Site B-Ib, the amino-terminal proteoglycan binding site, and that this site is exposed in carboxyl-truncated forms of apoB. The presence of a proteoglycan binding site in the amino-terminal region of apoB may explain why apoB48- and apoB100-containing lipoproteins are equally atherogenic.

Subendothelial retention of atherogenic lipoproteins in early atherosclerosis

Complications of atherosclerosis are the most common cause of death in Western societies. Among the many risk factors identified by epidemiological studies, only elevated levels of lipoproteins containing apolipoprotein (apo) B can drive the development of atherosclerosis in humans and experimental animals even in the absence of other risk factors. However, the mechanisms that lead to atherosclerosis are still poorly understood. We tested the hypothesis that the subendothelial retention of atherogenic apoB-containing lipoproteins is the initiating event in atherogenesis. The extracellular matrix of the subendothelium, particularly proteoglycans, is thought to play a major role in the retention of atherogenic lipoproteins. The interaction between atherogenic lipoproteins and proteoglycans involves an ionic interaction between basic amino acids in apoB100 and negatively charged sulphate groups on the proteoglycans. Here we present direct experimental evidence that the atherogenicity of apoB-containing low-density lipoproteins (LDL) is linked to their affinity for artery wall proteoglycans. Mice expressing proteoglycan-binding-defective LDL developed significantly less atherosclerosis than mice expressing wild-type control LDL. We conclude that subendothelial retention of apoB100-containing lipoprotein is an early step in atherogenesis.

Arterial wall chondroitin sulfate proteoglycans: diverse molecules with distinct roles in lipoprotein retention and atherogenesis

Chondroitin sulfate proteoglycans (CSPGs) of the arterial wall are generally considered to be atherogenic because of their ability to trap cholesterol-rich lipoproteins in vitro. Nevertheless, CSPGs are a diverse group of molecules with a long evolutionary history and distinct biologic functions. The three principal CSPGs in the arterial wall are versican, which is part of the hyalectan gene family; and decorin and biglycan, which are members of a separate gene family, the small leucine-rich proteoglycans. Importantly, there is now substantial evidence that the different molecular species of CSPGs participate unequally in lipoprotein retention, and that they exert unequal regulatory effects that are related to atherogenesis. Recently available murine models with genetic manipulations that affect CSPGs now allow causal studies of the roles of these molecules to be conducted in vivo, with occasionally surprising results. Moreover, tools are being developed to examine human genetic variations that are relevant to CSPGs, which may provide additional important insights into the human disease. The era in which proteoglycans are regarded as a nondescript backdrop, playing purely nonspecific structural roles, is over. Studies in manipulated animals and in human populations will continue to reveal precise, dynamic roles for these fascinating and ancient molecules.

Identification of a critical lysine residue in apolipoprotein B-100 that mediates noncovalent interaction with apolipoprotein(a)

We have previously shown that lipoprotein(a) (Lp(a)) assembly involves an initial noncovalent interaction between sequences within apolipoprotein(a) (apo(a)) kringle IV types 5-8 and the amino terminus of apolipoprotein B-100 (sequences between amino acids 680 and 781 in apoB-100), followed by formation of a disulfide bond. In the present study, citraconylation of lysine residues in apoB-100 abolished the ability of the modified low density lipoprotein to associate with apo(a), thereby demonstrating a direct role for lysine residues in apoB in the first step of Lp(a) assembly. To identify specific lysine residues in the amino terminus of apoB that are required for the noncovalent interaction, we initially used an affinity chromatography method in which recombinant forms of apo(a) (r-apo(a)) were immobilized on Sepharose beads. Assessment of the ability of carboxyl-terminal truncations of apoB-18 to bind to r-apo(a)-Sepharose revealed that a 25-amino acid sequence in apoB (amino acids 680-704) bound specifically to apo(a) in a lysine-dependent manner; citraconylation of the lysine residues in the apoB derivative encoding this sequence abolished the binding interaction. Using fluorescence spectrometry, we found that a synthetic peptide corresponding to this sequence bound directly to apo(a); the peptide also reduced covalent Lp(a) formation. Lysine residues present in this sequence (Lys(680) and Lys(690)) were mutated to alanine in the context of apoB-18. We found that the apoB-18 species containing the Lys(680) mutation was incapable of binding to r-apo(a)-Sepharose columns, whereas the apoB-18 species containing the Lys(690) mutation exhibited slightly reduced binding to these columns. Taken together, our data indicate that Lys(680) is critical for the noncovalent interaction of apo(a) and apoB-100 that precedes covalent Lp(a) formation.

LDL induced association of anionic liposomes with cells and delivery of contents as shown by the increase in potency of liposome dependent drugs

PURPOSE

To establish whether anionic liposomes interact with the low-density lipoprotein (LDL) receptor, to determine the role of lipoproteins in this interaction, and whether the association causes functional delivery of encapsulated drugs.

METHODS

The cell lines used were CV1-P and CHO wild type, both of which express the LDL receptor, and CHOldlA7, which lacks the LDL receptor. Cellular association of encapsulated methotrexate and fluorescein, labeled phosphatidylethanolamine in the lipid bilayer, was measured. Potency of three liposome dependent drugs (N-phosphonacetyl-L-aspartic acid, fluoroorotic acid, and methotrexate-gamma-aspartate) was also measured by growth inhibition.

RESULTS

Association of liposomes containing at least 75 mol egg phosphatidylglycerol (ePG)/100 mol phospholipid with cells grown in defined medium supplemented with 1.0 mg/ml LDL was up to 30-fold higher with CV1-P or CHO wild type cells than with CHOldlA7, and 5-fold higher than association in defined medium lacking LDL. The addition of LDL did not yield any elevation of cellular association of distearoylphosphatidylglycerol liposomes. Increased association was paralleled by a corresponding increase in potency of all three liposome dependent drugs tested.

CONCLUSIONS

ePG liposomes interact with the LDL receptor in an LDL-dependent fashion, and the interaction results in the delivery of contents to cells.

TerplexDNA gene carrier system targeting artery wall cells

The development of non-viral gene carrier systems becomes more urgent and important due to the major biosafety considerations involved with application of viral vector systems for clinical gene therapy. We recently developed a novel non-viral gene carrier system, termed TerplexDNA, which showed high gene transfer efficiency when compared to the lipofectamine gene delivery system both in HepG2 and A7R5 cell lines in vitro. In present studies, we demonstrated that the TerplexDNA gene carrier system specifically delivered the reporter genes (LacZ and Luciferase) and therapeutic gene (hrVEGF(165) cDNA) into bovine aortic artery wall cells (endothelial cells and smooth muscle cells) by receptor mediated endocytosis. We found that the transfection efficiency to these primary artery wall cells, when mediated by the TerplexDNA system, was dose-dependent, saturable and was significantly inhibited by excess free LDL. The transfection efficiency of the TerplexDNA gene carrier system was approximately 60-fold higher than that of the lipofectamine gene carrier system. The TerplexDNA gene carrier system is a useful and promising tool for artery wall gene transfer.

Role of extracellular retention of low density lipoproteins in atherosclerosis

The pathogenesis for atherosclerosis is still unclear, and several hypotheses have been articulated to explain the initiating events in atherogenesis. Although these hypotheses are by no means mutually exclusive, there is a growing body of recent evidence that has led to the concept that subendothelial retention of apolipoprotein B100-containing lipoproteins is the initiating event in atherogenesis. Subsequently, a series of biological responses to this retained material leads to specific molecular and cellular processes that promote lesion formation. The present review assesses some of the studies that support this concept.

Lipoprotein lipase-mediated interactions of small proteoglycans and low-density lipoproteins

According to numerous studies low-density lipoproteins (LDL) are supposed to interact with the glycosaminoglycan chain(s) of proteoglycans, e.g. with decorin and biglycan, which themselves are subject to receptor-mediated endocytosis. We tested, therefore, whether complexes of LDL and small proteoglycans can be endocytosed by either the LDL- or the small proteoglycan uptake mechanism. However, neither was the endocytosis of LDL significantly influenced by proteoglycans nor that of proteoglycans by LDL. This negative result could be explained by the observation that in vitro complex formation takes place only in buffers of low ionic strength. Under physiological conditions additional molecules may be necessary for complex stabilization. Lipoprotein lipase (LpL) which binds LDL was also able to interact with high affinity with decorin and its glycosaminoglycan-free core protein, both interactions being heparin-sensitive. Regardless of the presence or absence of LDL, LpL stimulated the endocytosis of decorin 1.5-fold, whereas LpL mediated a 4-fold stimulation of LDL uptake in the absence of decorin. No significant additional effect was seen in the presence of small concentrations of proteoglycans whereas in the presence of 1 microM decorin the endocytosis of [125I]LDL was reduced in normal as well as in LDL receptor-deficient fibroblasts. These observations could best be explained by assuming that LpL/LDL complexes are internalized upon binding to membrane-associated heparan sulphate and that small proteoglycans interfere with this process. It could not be ruled out, however, that a small proportion of the complexes is also taken up by the small proteoglycan receptor.

Interaction of native and modified low-density lipoproteins with extracellular matrix

Lipoprotein-matrix interactions play an important role in arterial disease. Extracellular matrix proteoglycans bind and retain specific positively charged domains on apolipoproteins B- and E-containing lipoproteins during atherogenesis. Retained lipoproteins can undergo several modifications, which may alter their interaction with extracellular matrix molecules. Growth factors, cytokines and oxidized low density lipoproteins influence proteoglycan structure, rendering them more likely to bind and retain lipoproteins during atherogenesis. Lipoproteins, native and modified, also can modulate the expression of several of the matrix degrading enzymes present in vascular tissue, thereby influencing plaque stability. Thus, the interaction of atherogenic lipoproteins with arterial wall matrix molecules can influence the genesis and progression of atherosclerosis and its complications.

High affinity binding between lipoprotein lipase and lipoproteins involves multiple ionic and hydrophobic interactions, does not require enzyme activity, and is modulated by glycosaminoglycans

Lipoprotein lipase (LPL) physically associates with lipoproteins and hydrolyzes triglycerides. To characterize the binding of LPL to lipoproteins, we studied the binding of low density lipoproteins (LDL), apolipoprotein (apo) B17, and various apoB-FLAG (DYKDDDDK octapeptide) chimeras to purified LPL. LDL bound to LPL with high affinity (K(d) values of 10(-12) m) similar to that observed for the binding of LDL to its receptors and 1D1, a monoclonal antibody to LDL, and was greater than its affinity for microsomal triglyceride transfer protein. LDL-LPL binding was sensitive to both salt and detergents, indicating the involvement of both hydrophobic and hydrophilic interactions. In contrast, the N-terminal 17% of apoB interacted with LPL mainly via ionic interactions. Binding of various apoB fusion peptides suggested that LPL bound to apoB at multiple sites within apoB17. Tetrahydrolipstatin, a potent enzyme activity inhibitor, had no effect on apoB-LPL binding, indicating that the enzyme activity was not required for apoB binding. LDL-LPL binding was inhibited by monoclonal antibodies that recognize amino acids 380-410 in the C-terminal region of LPL, a region also shown to interact with heparin and LDL receptor-related protein. The LDL-LPL binding was also inhibited by glycosaminoglycans (GAGs); heparin inhibited the interactions by approximately 50% and removal of trace amounts of heparin from LPL preparations increased LDL binding. Thus, we conclude that the high affinity binding between LPL and lipoproteins involves multiple ionic and hydrophobic interactions, does not require enzyme activity and is modulated by GAGs. It is proposed that LPL contains a surface exposed positively charged amino acid cluster that may be important for various physiological interactions of LPL with different biologically important molecules. Moreover, we postulate that by binding to this cluster, GAGs modulate the association between LDL and LPL and the in vivo metabolism of LPL.

Responses to eating: lipoproteins, lipolytic products and atherosclerosis

Several lines of clinical and experimental data suggest that postprandial lipemia is an independent risk factor for atherosclerosis. There are a number of reasons why processes that occur in the period immediately after eating could be deleterious to arteries. By understanding the links between postprandial lipemia and the accumulation of lipid within vessels, a more global understanding of how lipoproteins cause disease may be forthcoming. In this article recent information on the control of postprandial lipemia and the biological effects of chylomicron remnants and lipolysis products will be reviewed. Because this topic is broad, we will focus on the roles played by lipoprotein lipase and proteoglycans in this process.

The role of the LDL receptor in apolipoprotein B secretion

Familial hypercholesterolemia is caused by mutations in the LDL receptor gene (Ldlr). Elevated plasma LDL levels result from slower LDL catabolism and a paradoxical lipoprotein overproduction. We explored the relationship between the presence of the LDL receptor and lipoprotein secretion in hepatocytes from both wild-type and LDL receptor-deficient mice. Ldlr(-/-) hepatocytes secreted apoB100 at a 3.5-fold higher rate than did wild-type hepatocytes. ApoB mRNA abundance, initial apoB synthetic rate, and abundance of the microsomal triglyceride transfer protein 97-kDa subunit did not differ between wild-type and Ldlr(-/-) cells. Pulse-chase analysis and multicompartmental modeling revealed that in wild-type hepatocytes, approximately 55% of newly synthesized apoB100 was degraded. However, in Ldlr(-/-) cells, less than 20% of apoB was degraded. In wild-type hepatocytes, approximately equal amounts of LDL receptor-dependent apoB100 degradation occured via reuptake and presecretory mechanisms. Adenovirus-mediated overexpression of the LDL receptor in Ldlr(-/-) cells resulted in degradation of approximately 90% of newly synthesized apoB100. These studies show that the LDL receptor alters the proportion of apoB that escapes co- or post-translational presecretory degradation and mediates the reuptake of newly secreted apoB-containing lipoprotein particles.