Lipoprotein lipase enhances the interaction of low density lipoproteins with artery-derived extracellular matrix proteoglycans

Triglycerides and remnant cholesterol associated with risk of aortic valve stenosis: Mendelian randomization in the Copenhagen General Population Study

Aims

We tested the hypothesis that higher levels of plasma triglycerides and remnant cholesterol are observationally and genetically associated with increased risk of aortic valve stenosis.

Methods and results

We included 108 559 individuals from the Copenhagen General Population Study. Plasma triglycerides, remnant cholesterol (total cholesterol minus low-density lipoprotein and high-density lipoprotein cholesterol), and 16 genetic variants causing such increased or decreased levels were determined. Incident aortic valve stenosis occurred in 1593 individuals. Observationally compared to individuals with triglycerides <1 mmol/L (<89 mg/dL), the multifactorially adjusted hazard ratio for aortic valve stenosis was 1.02 [95% confidence interval (CI) 0.87–1.19] for individuals with triglycerides of 1.0–1.9 mmol/L (89–176 mg/dL), 1.22 (1.02–1.46) for 2.0–2.9 mmol/L (177–265 mg/dL), 1.40 (1.11–1.77) for 3.0–3.9 mmol/L (266–353 mg/dL), 1.29 (0.88–1.90) for 4.0–4.9 mmol/L (354–442 mg/dL), and 1.52 (1.02–2.27) for individuals with triglycerides ≥5 mmol/L (≥443 mg/dL). By age 85, the cumulative incidence of aortic valve stenosis was 5.1% for individuals with plasma triglycerides <2.0 mmol/L (77 mg/dL), 6.5% at 2.0–4.9 mmol/L (177–442 mg/dL), and 8.2% for individuals with plasma triglycerides ≥5.0 mmol/L (443 mg/dL). The corresponding values for remnant cholesterol categories were 4.8% for <0.5 mmol/L (19 mg/dL), 5.6% for 0.5–1.4 mmol/L (19–57 mg/dL), and 7.4% for ≥1.5 mmol/L (58 mg/dL). Genetically, compared to individuals with allele score 13–16, odds ratios for aortic valve stenosis were 1.30 (95% CI 1.20–1.42; Δtriglycerides +12%; Δremnant cholesterol +11%) for allele score 17–18, 1.41 (1.31–1.52; +25%; +22%) for allele score 19–20, and 1.51 (1.22–1.86; +51%; +44%) for individuals with allele score 21–23.

Conclusion

Higher triglycerides and remnant cholesterol were observationally and genetically associated with increased risk of aortic valve stenosis.

So Much Cholesterol: the unrecognized importance of smooth muscle cells in atherosclerotic foam cell formation

PURPOSE OF REVIEW

Smooth muscle cells (SMCs) form the thickened intimal layer in atherosclerosis-prone arteries in early life, and provide the initial site for retention and uptake of atherogenic lipoproteins. Here we review current knowledge regarding the importance of SMCs in the deposition of cholesterol in atherosclerotic plaque.

RECENT FINDINGS

SMCs were found to comprise at least 50% of total foam cells in human coronary artery atherosclerosis, and exhibit a selective loss of expression of the cholesterol efflux promoter ATP-binding cassette transporter A1. Cholesterol loading induced a loss of SMC gene expression and an increase in macrophage and proinflammatory marker expression by cultured mouse and human arterial SMCs, with reversal of these effects upon removal of the excess cholesterol. Mice engineered to track all cells of SMC lineage indicated that, at most, SMCs make up about one-third of total cells in atherosclerotic plaque in these animals.

SUMMARY

SMCs appear to be the origin of the majority of foam cells in human atherosclerotic plaque. Recent studies suggest a renaissance of research on the role of SMCs in atherosclerosis is needed to make the next leap forward in the prevention and treatment of this disease.

Molecular biology of calcific aortic valve disease: towards new pharmacological therapies

Calcific aortic valve disease (CAVD) is a chronic process leading to fibrosis and mineralization of the aortic valve. Investigations in the last several years have emphasized that key underlying molecular processes are involved in the pathogenesis of CAVD. In this regard, the processing of lipids and their retention has been underlined as an important mechanism that triggers inflammation. In turn, inflammation promotes/enhances the mineralization of valve interstitial cells, the main cellular component of the aortic valve. On the other hand, transformation of valve interstitial cells into myofibroblasts and osteoblast-like cells is determined by several signaling pathways having reciprocal cross-talks. In addition, the mineralization of the aortic valve has been shown to rely on ectonucleotidase and purinergic signaling. In this review, the authors have highlighted key molecular underpinnings of CAVD that may have significant relevance for the development of novel pharmaceutical therapies.

Lipoprotein lipase in aortic valve stenosis is associated with lipid retention and remodelling

BACKGROUND

Calcific aortic valve disease (CAVD) is a chronic disorder characterized by a fibrocalcific remodelling. It is suspected that lipid retention within the aortic valve may be one important mechanism participating to aortic valve remodelling. Lipoprotein lipase (LPL) is implicated in lipid metabolism and may play a role in lipid retention within the aortic valve.

METHODS

In 57 patients, CAVD were analysed for the expression of LPL by q-PCR and immunohistochemistry. Expression of oxidized-LDL (ox-LDL) and decorin was also documented. In addition, a complete blood profile, including the size of LDL and high-density lipoprotein (HDL) particles, were performed to find associations between the blood lipid profile and expression of ox-LDL and LPL within CAVD.

RESULTS

Immunohistochemistry studies revealed that LPL was expressed in stenotic aortic valves as a diffuse staining and also in dense cellular areas where macrophages were abundant. Expression of LPL co-localized with decorin and ox-LDL. In turn, valves with higher amount of ox-LDL had elevated number of LPL transcripts. In addition, we documented that the small, dense HDL phenotype was associated with an elevated amount of ox-LDL and LPL transcripts within CAVD. Furthermore, expression of LPL was associated with several indices of fibrocalcific remodelling of the aortic valve.

CONCLUSION

Expression of LPL within CAVD is related to the amount of ox-LDL, which is, in turn, associated with the small, dense HDL phenotype. Lipid retention associated with smaller HDL particles may participate in the expression of LPL, whereby a fibrocalcific remodelling of the aortic valve is promoted.

Lipoprotein lipase is a novel amyloid beta (Abeta)-binding protein that promotes glycosaminoglycan-dependent cellular uptake of Abeta in astrocytes

Lipoprotein lipase (LPL) is a member of a lipase family known to hydrolyze triglyceride molecules in plasma lipoprotein particles. LPL also plays a role in the binding of lipoprotein particles to cell-surface molecules, including sulfated glycosaminoglycans (GAGs). LPL is predominantly expressed in adipose and muscle but is also highly expressed in the brain where its specific roles are unknown. It has been shown that LPL is colocalized with senile plaques in Alzheimer disease (AD) brains, and its mutations are associated with the severity of AD pathophysiological features. In this study, we identified a novel function of LPL; that is, LPL binds to amyloid β protein (Aβ) and promotes cell-surface association and uptake of Aβ in mouse primary astrocytes. The internalized Aβ was degraded within 12 h, mainly in a lysosomal pathway. We also found that sulfated GAGs were involved in the LPL-mediated cellular uptake of Aβ. Apolipoprotein E was dispensable in the LPL-mediated uptake of Aβ. Our findings indicate that LPL is a novel Aβ-binding protein promoting cellular uptake and subsequent degradation of Aβ.

Collagen-bound LDL modifies endothelial cell adhesion to type V collagen: Implications for atherosclerosis

Low density lipoprotein (LDL) is retained in the extracellular matrix of the arterial wall where it is considered to be atherogenic, but little is known about how cell adhesion to the matrix is affected by collagen-bound LDL. We tested the effect of native, oxidized and acetylated LDL reacted with adsorbed monomeric type I, III and V collagen on endothelial cell adhesion to collagen using a colorimetric adhesion assay. We found that none of the LDL species affected adhesion to type I and III collagen, but that collagen-bound native and acetylated LDL enhanced attachment to type V collagen, whereas bound oxidized LDL inhibited adhesion to this collagen. We therefore suggest that oxidized LDL associated with type V collagen in the arterial wall would favor de-endothelialization and contribute to atherogenesis and thrombosis.

Endothelial lipase is synthesized by hepatic and aorta endothelial cells and its expression is altered in apoE-deficient mice

Both LPL and HL are synthesized in parenchymal cells, are secreted, and bind to endothelial cells. To learn where endothelial lipase (EL) is synthesized in adult animals, the localization of EL in mouse and rat liver was studied by immunohistochemical analysis. Furthermore, to test whether EL could play a role in atherogenesis, the expression of EL in the aorta and liver of apolipoprotein E knockout (EKO) mice was determined. EL in both mouse and rat liver was colocalized with vascular endothelial cells but not with hepatocytes. In contrast, HL was present in both hepatocytes and endothelial cells. By in situ hybridization, EL mRNA was present only in endothelial cells in liver sections. EL was also present at low levels in aorta of normal mice. We fed EKO mice and wild-type mice a variety of diets and determined EL expression in liver and aorta. EKO mice showed significant expression of EL in aorta. EL expression was lower in the liver of EKO mice than in normal mice. Cholesterol feeding decreased EL in liver of both types of mice. In the aorta, EL was higher in EKO than in wild-type mice, and cholesterol feeding had no effect. Together, these data suggest that EL may be upregulated at the site of atherosclerotic lesions and thus could supply lipids to the area.

Biosensing of arteriosclerotic nanoplaque formation and interaction with an HMG-CoA reductase inhibitor

Proteoheparan sulphate can be adsorbed to a methylated silica surface in a monomolecular layer via its transmembrane hydrophobic protein core domain. As a result of electrostatic repulsion, its anionic glycosaminoglycan side chains are stretched out into the blood substitute solution, thereby representing one receptor site for specific lipoprotein binding through basic amino acid-rich residues within their apolipoproteins. The binding process was studied by ellipsometric techniques suggesting that high-density lipoprotein (HDL) has a high binding affinity and a protective effect on interfacial heparan sulphate proteoglycan layers with respect to low-density lipoprotein (LDL) and Ca2+ complexation. Low-density lipoprotein was found to deposit strongly at the proteoheparan sulphate-coated surface, particularly in the presence of Ca2+, apparently through complex formation 'proteoglycan-LDL-calcium'. This ternary complex build-up may be interpreted as arteriosclerotic nanoplaque formation on the molecular level responsible for the arteriosclerotic primary lesion. On the other hand, HDL bound to heparan sulphate proteoglycan protected against LDL deposition and completely suppressed calcification of the proteoglycan-lipoprotein complex. In addition, HDL was able to decelerate the ternary complex deposition. Therefore, HDL attached to its proteoglycan receptor sites is thought to raise a multidomain barrier, selection and control motif for transmembrane and paracellular lipoprotein uptake into the arterial wall. Although much remains unclear regarding the mechanism of lipoprotein depositions at proteoglycan-coated surfaces, it seems clear that the use of such systems offers possibilities for investigating lipoprotein deposition at a 'nanoscopic' level under close to physiological conditions. In particular, Ca2+-promoted LDL deposition and the protective effect of HDL even at high Ca2+ and LDL concentrations agree well with previous clinical observations regarding risk and beneficial factors for early stages of atherosclerosis. Considering this, the system was tested on its reliability in a biosensor application in order to unveil possible acute pleiotropic effects of the lipid lowering drug fluvastatin. The very low-density lipoprotein (VLDL)/intermediate-density lipoprotein (IDL)/LDL plasma fraction from a high risk patient with dyslipoproteinaemia and type 2 diabetes mellitus showed beginning arteriosclerotic nanoplaque formation already at a normal blood Ca2+ concentration, with a strong increase at higher Ca2+ concentrations. Fluvastatin, whether applied to the patient (one single 80 mg slow release matrix tablet) or acutely in the experiment (2.2 micromol L-1), markedly slowed down this process of ternary aggregational nanoplaque complexation at all Ca2+ concentrations used. This action resulted without any significant change in lipid concentrations of the patient. Furthermore, after ternary complex build-up, fluvastatin, similar to HDL, was able to reduce nanoplaque adsorption and size. These immediate effects of fluvastatin have to be taken into consideration while interpreting the clinical outcome of long-term studies.

Apolipoprotein E mediates the retention of high-density lipoproteins by mouse carotid arteries and cultured arterial smooth muscle cell extracellular matrices

Lipoprotein retention in the vascular extracellular matrix (ECM) plays a critical role in atherogenesis. Previous studies demonstrated the presence of apo A-I and E in atherosclerotic lesions, suggesting that HDL may be trapped by the artery wall. We sought to determine mechanisms by which HDL could be bound and retained by the arterial wall, and whether apo E was a principal determinant of this binding. We evaluated in situ accumulation of fluorescently labeled DiI-human HDL+/-apo E in perfused carotid arteries from apo E-null mice. Apo E was important in mediating HDL binding to the vascular wall, with a 48+/-16% increase in accumulation of DiI-labeled apo E-containing HDL (HDL3+E) compared with DiI-apo E-free HDL (HDL3-E) (P=0.003). To investigate possible mechanisms responsible for retention, we assessed binding of unlabeled HDL3-E and HDL3+E to ECM generated by cultured arterial smooth muscle cells. Similar to the in situ carotid artery data, HDL3+E bound better to the ECM than did HDL3-E (3-fold lower K(a) and 3.5-fold higher B(max) for HDL3+E versus HDL3-E). These differences were eliminated after either neutralization of arginine residues on apo E or digestion of matrix with chondroitin ABC lyase, suggesting that chondroitin and/or dermatan sulfate proteoglycans were responsible for apo E-mediated increased binding. These findings demonstrate that HDL can bind to both intact murine carotid arteries and smooth muscle cell-derived ECM, and that apo E is a principal determinant in mediating the ability of HDL to be trapped and retained via its interaction with ECM proteoglycans.

Lipoprotein lipase in the arterial wall: linking LDL to the arterial extracellular matrix and much more

For low density lipoprotein (LDL) particles to be atherogenic, increasing evidence indicates that their residence time in the arterial intima must be sufficient to allow their modification into forms capable of triggering extracellular and intracellular lipid accumulation. Recent reports have confirmed the longstanding hypothesis that the major determinant(s) of initial LDL retention in the preatherosclerotic arterial intima is the proteoglycans. However, once the initial atherosclerotic lesions have formed, a shift to retention facilitated by macrophage-derived lipoprotein lipase (LPL) appears, leading to the progression of the lesions. Here, we review recent findings on the mechanisms enabling LPL to promote LDL retention and extracellular lipid accumulation in the arterial intima, and we describe the structures in the extracellular matrix that are held to be important in this process. Finally, the potentially harmful consequences of LDL linking by LPL and of other LPL actions in the arterial intima are briefly reviewed.

A receptor-based biosensor for lipoprotein docking at the endothelial surface and vascular matrix

Proteoheparan sulfate can be adsorbed to a methylated silica surface in a monomolecular layer via its transmembrane hydrophobic protein core domain. Due to electrostatic repulsion, its anionic glycosaminoglycan side chains are stretched out into the blood substitute solution, representing a receptor site for specific lipoprotein binding through basic amino acid-rich residues within their apolipoproteins. The binding process was studied by ellipsometric techniques showing that HDL has a high binding affinity to the receptor and a protective effect on interfacial heparan sulfate proteoglycan layers, with respect to LDL and Ca(2+) complexation. LDL was found to deposit strongly at the proteoheparan sulfate, particularly in the presence of Ca(2+), thus creating the complex formation "proteoglycan-low density lipoprotein-calcium". This ternary complex build-up may be interpreted as arteriosclerotic nanoplaque formation on the molecular level responsible for the arteriosclerotic primary lesion. On the other hand, HDL bound to heparan sulfate proteoglycan protected against LDL docking and completely suppressed calcification of the proteoglycan-lipoprotein complex. In addition, HDL and aqueous garlic extract were able to reduce the ternary complex deposition and to disintegrate HS-PG/LDL/Ca(2+) aggregates. Although much remains unclear regarding the mechanism of lipoprotein depositions at proteoglycan-coated surfaces, it seems clear that the use of such systems offers possibilities for investigating lipoprotein deposition at a "nanoscopic" level under close to physiological conditions. In particular, Ca(2+)-promoted LDL deposition and the protective effect of HDL, even at high Ca(2+) and LDL concentrations, agree well with previous clinical observations regarding risk and beneficial factors for early stages of atherosclerosis. Therefore, we believe that the system can be of some use in investigations, e.g. of the interplay between different lipoproteins in arteriosclerotic plaque formation, as well as in high throughput screening of candidate drugs to atherosclerosis in a biosensor application.

Macrophage-specific expression of human lipoprotein lipase accelerates atherosclerosis in transgenic apolipoprotein e knockout mice but not in C57BL/6 mice

Transgenic mice with macrophage-specific expression of human (hu) lipoprotein lipase (LPL) were generated to determine the contribution of macrophage LPL to atherogenesis. Macrophage specificity was accomplished with the scavenger receptor A promoter. Complete characterization demonstrated that macrophages from these mice expressed huLPL mRNA and secreted enzymatically active huLPL protein. Expression of huLPL was macrophage specific, because total RNA isolated from heart, thymus, lung, liver, muscle, and adipose tissues was devoid of huLPL mRNA. Macrophage-specific expression of huLPL did not exacerbate lesions in aortas of C57BL/6 mice even after 32 weeks on an atherosclerotic diet. However, when expressed in apolipoprotein E knockout background, the extent of occlusion in the aortic sinus region of male huLPL+ mice increased 51% (n=9 to 11, P<0.002) compared with huLPL- mice after they had been fed a Western diet for 8 weeks. The proatherogenic effect of macrophage LPL was confirmed in serial sections of the aorta obtained after mice had been fed a Western diet for 3 weeks. By immunohistochemical analysis, huLPL protein was detected in the lesions of huLPL+ mice but not in huLPL- mice. Our results establish that macrophage LPL accelerates atherosclerosis in male apolipoprotein E knockout mice.

Binding of low density lipoproteins to lipoprotein lipase is dependent on lipids but not on apolipoprotein B

Lipoprotein lipase (LPL) efficiently mediates the binding of lipoprotein particles to lipoprotein receptors and to proteoglycans at cell surfaces and in the extracellular matrix. It has been proposed that LPL increases the retention of atherogenic lipoproteins in the vessel wall and mediates the uptake of lipoproteins in cells, thereby promoting lipid accumulation and plaque formation. We investigated the interaction between LPL and low density lipoproteins (LDLs) with special reference to the protein-protein interaction between LPL and apolipoprotein B (apoB). Chemical modification of lysines and arginines in apoB or mutation of its main proteoglycan binding site did not abolish the interaction of LDL with LPL as shown by surface plasmon resonance (SPR) and by experiments with THP-I macrophages. Recombinant LDL with either apoB100 or apoB48 bound with similar affinity. In contrast, partial delipidation of LDL markedly decreased binding to LPL. In cell culture experiments, phosphatidylcholine-containing liposomes competed efficiently with LDL for binding to LPL. Each LDL particle bound several (up to 15) LPL dimers as determined by SPR and by experiments with THP-I macrophages. A recombinant NH(2)-terminal fragment of apoB (apoB17) bound with low affinity to LPL as shown by SPR, but this interaction was completely abolished by partial delipidation of apoB17. We conclude that the LPL-apoB interaction is not significant in bridging LDL to cell surfaces and matrix components; the main interaction is between LPL and the LDL lipids.

Lipoprotein lipase mediates the uptake of glycated LDL in fibroblasts, endothelial cells, and macrophages

The nonenzymatic glycation of LDL is a naturally occurring chemical modification of apolipoprotein (apo)-B lysine residues by glucose. Once glycated, LDL is only poorly recognized by lipoprotein receptors including the LDL receptor (LDL-R), the LDL-R-related protein (LRP), and scavenger receptors. Glycated LDL (gLDL) is a preferred target for oxidative modifications. Additionally, its presence initiates different processes that can be considered "proatherogenic." Thus, LDL glycation might contribute to the increased atherosclerotic risk of patients with diabetes and familial hypercholesterolemia. Here we investigate whether lipoprotein lipase (LPL) can mediate the cellular uptake of gLDL. The addition of exogenous LPL to the culture medium of human skin fibroblasts, porcine aortic endothelial cells, and mouse peritoneal macrophages enhanced the binding, uptake, and degradation of gLDL markedly, and the relative effect of LPL on lipoprotein uptake increased with the degree of apoB glycation. The efficient uptake of gLDL by LDL-R-deficient fibroblasts and LRP-deficient Chinese hamster ovary cells in the presence of LPL suggested a mechanism that was independent of the LDL-R and LRP. In macrophages, the uptake of gLDL was also correlated with their ability to produce LPL endogenously. Mouse peritoneal macrophages from genetically modified mice, which lacked LPL, exhibited a 75% reduction of gLDL uptake compared with normal macrophages. The LPL-mediated effect required the association of the enzyme with cell surface glycosaminoglycans but was independent of its enzymatic activity. The uptake of gLDL in different cell types by an LPL-mediated process might have important implications for the cellular response after gLDL exposure as well as the removal of gLDL from the circulation.

Marked neointimal lipoprotein lipase increase in distinct models of proclivity to atherosclerosis: a feature independent of endothelial layer integrity

Lipoprotein lipase (LPL) in the arterial wall has been proposed to enhance the retention of apoB-containing lipoproteins, an early event in atherosclerosis. As the neointima is considered the primary site of lipid accumulation in atherogenesis, the arterial expression and location of LPL was investigated in distinct experimental models of neointimal formation in normolipidemic rabbits and rats. Neointima elicited by balloon aortic denudation or raised beneath an anatomically intact endothelial layer by placing a silastic collar around the common carotid artery, both showed a striking LPL immunostaining that mostly co-localized with neointimal smooth muscle cells. Besides, increased LPL protein and mRNA in deendothelialized aortas was demonstrated by Western and Northern blot analysis, respectively, suggesting an enhanced expression of LPL in injured arteries. It was concluded that LPL is increased in neointima developed in either denuded vessels or arteries with a preserved endothelium, a finding which suggests that LPL abundance may be an attribute of the neointima, whatever the stimulus that promotes its formation. On the basis of former evidence concerning the role of LPL in lipid retention, this study provides a possible explanation for the injury-induced vessel susceptibility to atherosclerosis, and the particular proneness of the neointimal layer to lipid accretion.

High-Energy Diets, Fatty Acids and Endothelial Cell Function: Implications for Atherosclerosis

Diets high in fat and/or calories can lead to hypertriglyceridemia and postprandial lipemia and thus are considered a risk factor for the development of atherosclerosis. Plasma chylomicron levels are elevated in humans after consuming a high-fat meal, and hepatic synthesis of VLDL is increased when caloric intake is in excess of body needs. High lipoprotein lipase activity and subsequent hydrolysis of triglyceride-rich lipoproteins may be an important source of elevated concentrations of fatty acid anions in the proximity to the endothelium and hence a major risk factor for atherosclerosis. We have shown that selected fatty acids, as well as lipoprotein lipase-derived remnants of lipoproteins isolated from hypertriglyceridemic subjects, can activate vascular endothelial cells and disrupt endothelial integrity. Our studies suggest that omega-6 fatty acids, and especially linoleic acid, cause endothelial cell dysfunction most markedly as well as can potentiate TNF-mediated endothelial cell injury. We propose that high-energy diets, and especially diets rich in linoleic acid, are atherogenic by contributing to an imbalance in cellular oxidative stress/antioxidant status of the endothelium, which can lead to activation of oxidative stress-responsive transcription factors, inflammatory cytokine production and the expression of adhesion molecules. Our data also suggest that nutrients, which have antioxidant and/or membrane stabilizing properties, can protect endothelial cells. These findings contribute to the understanding of the interactive role of high fat/calorie diets and subsequent hypertriglyceridemia with inflammatory components and nutrients that exhibit antiatherogenic properties in the development of atherosclerosis. Moreover, results from our research further support the concept that high-fat/calorie diets and associated postprandial hypertriglyceridemia are significant risk factors for atherosclerosis.

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.

Role of macrophage-derived lipoprotein lipase in lipoprotein metabolism and atherosclerosis

Lipoprotein lipase (LPL) synthesis by macrophages is upregulated in early atherogenesis, implicating the possible involvement of LPL in plaque formation. However, it is still unclear whether macrophage-derived LPL displays a proatherosclerotic or an antiatherosclerotic role in atherosclerotic lesion development. In this study, the role of macrophage-derived LPL on lipid metabolism and atherosclerosis was assessed in vivo by transplantation of LPL-deficient (LPL-/-) and wild-type (LPL+/+) bone marrow into C57BL/6 mice. Eight weeks after bone marrow transplantation (BMT), serum cholesterol levels in LPL-/--->C57BL/6 mice were reduced by 8% compared with those in LPL+/+-->C57BL/6 mice (P:<0.05, n=16), whereas triglycerides were increased by 33% (P:<0.05, n=16). Feeding the mice a high-cholesterol diet increased serum cholesterol levels in LPL-/--->C57BL/6 and LPL+/+-->C57BL/6 mice 5-fold and 9-fold, respectively, resulting in a difference of approximately 50% (P:<0. 01) after 3 months on the diet. No effects on triglyceride levels were observed under these conditions. Furthermore, serum apolipoprotein E levels were reduced by 50% in the LPL-/--->C57BL/6 mice compared with controls under both dietary conditions. After 3 months on a high-cholesterol diet, the atherosclerotic lesion area in LPL-/--->C57BL/6 mice was reduced by 52% compared with controls. It can be concluded that macrophage-derived LPL plays a significant role in the regulation of serum cholesterol, apolipoprotein E, and atherogenesis, suggesting that specific blockade of macrophage LPL production may be beneficial for decreasing atherosclerotic lesion development.

Endogenously produced glycosaminoglycans affecting the release of lipoprotein lipase from macrophages and the interaction with lipoproteins

Macrophages are intimately involved in the pathogenesis of atherosclerotic diseases. A key feature of this process is their uptake of various lipoproteins and subsequent transformation to foam cells. Since lipoprotein lipase (LPL) is believed to play a role in foam cell formation, we investigated if endogenously produced proteoglycans (PGs) affect the release of this enzyme from macrophages. The human leukaemic cell line THP-1 which differentiates into macrophages by treatment with phorbol ester (phorbol 12-myristate 13-acetate) served as a model. The differentiation of THP-1 macrophages promoted the release of PGs into the cell medium which caused the detachment of LPL activity from the cell surface, and prevented LPL re-uptake and inactivation. These PGs were mainly composed of chondroitin sulfate type and exerted a heparin-like effect on LPL release. LPL is known to increase the cell association of lipoproteins by the well known bridging function. Exogenous bovine LPL at a concentration of 1 microg/ml enhanced low density lipoprotein (LDL)-binding 10-fold. Endogenously produced PGs reduced LPL-mediated binding of LDL. It is proposed that the differentiation-dependent increase in the release of PGs interferes with binding of LPL and reduces lipoprotein-binding to macrophages.

Lipoprotein lipase enhances the binding of native and oxidized low density lipoproteins to versican and biglycan synthesized by cultured arterial smooth muscle cells

Retention of low density lipoproteins (LDL) by vascular proteoglycans and their subsequent oxidation are important in atherogenesis. Lipoprotein lipase (LPL) can bind LDL and proteoglycans, although the effect of different proteoglycans to influence the ability of LPL to act as a bridge in the formation of LDL-proteoglycan complexes is unknown. Using an electrophoretic gel mobility shift assay, [(35)S]SO(4)-labeled versican and biglycan, two extracellular proteoglycans secreted by vascular cells, bound native LDL in a saturable fashion. The addition of bovine milk LPL dose-dependently increased the binding of native LDL to both versican and biglycan, approaching saturation at 30-40 microgram/ml LPL for versican and 20 microgram/ml LPL for biglycan. LDL was oxidized by several methods, including copper, 2, 2-azo-bis(2-amidinopropane)-2HCl and hypochlorite. Extensively copper- and hypochlorite-oxidized LDL bound poorly to versican and biglycan. Proteoglycan binding to LDL was correlated inversely with the extent of LDL; however, the addition of LPL to oxidized LDL together with biglycan or versican allowed the oxidized LDL to bind the proteoglycans in an LPL dose-dependent manner. Addition of LPL had a greater relative effect on the binding of extensively oxidized LDL to proteoglycans compared with native LDL. LPL had a slightly greater effect on increasing the binding of native and oxidized LDL to biglycan than versican. Thus, LPL in the artery wall might increase the atherogenicity of oxidized LDL, since it enables its binding to vascular biglycan and versican.

Sphingomyelinase, an enzyme implicated in atherogenesis, is present in atherosclerotic lesions and binds to specific components of the subendothelial extracellular matrix

Atherosclerotic lesions contain an extracellular sphingomyelinase (SMase) activity that hydrolyzes the sphingomyelin of subendothelial low density lipoprotein (LDL). This SMase activity may promote atherosclerosis by enhancing subendothelial LDL retention and aggregation, foam cell formation, and possibly other atherogenic processes. The results of recent cell-culture studies have led to the hypothesis that a specific molecule called secretory SMase (S-SMase) is responsible for the SMase activity known to be in lesions, although its presence in atheromata had not been examined directly. Herein we provide immunohistochemical and biochemical support for this hypothesis. First, 2 different antibodies against S-SMase detected extracellular immunoreactive protein in the intima of mouse, rabbit, and human atherosclerotic lesions. Much of this material in lesions appeared in association with the subendothelial matrix. Second, binding studies in vitro demonstrated that (125)I-S-SMase adheres to the extracellular matrix of cultured aortic smooth muscle and endothelial cells, specifically to the laminin and collagen components. Third, in its bound state, S-SMase retains substantial enzymatic activity against lipoprotein substrates. Overall, these data support the hypothesis that S-SMase is an extracellular arterial wall SMase that contributes to the hydrolysis of the sphingomyelin of subendothelial LDL. S-SMase may therefore be an important participant in atherogenesis through local enzymatic effects that stimulate subendothelial retention and aggregation of atherogenic lipoproteins.

Functional variants in the lipoprotein lipase gene and risk cardiovascular disease

The current report is a quantitative review of the relationship between lipoprotein lipase gene variants and cardiovascular disease based on published population-based studies. Sixteen studies, representing 17,630 individuals, report allelic distribution for lipoprotein lipase gene variants among patients and control individuals. Patient outcomes included clinical cardiovascular disease events, documented coronary disease based on angiography, or intimal media thickening by B-mode ultrasonography. Mantel-Haenszel stratified analysis was used to compute a summary odds ratio and 95% confidence intervals for the association between rare allele in the lipoprotein lipase gene and disease status. Because of potential differing effects associated with different lipoprotein lipase variants, each lipoprotein lipase mutant allele was considered separately. The lipoprotein lipase D9N/-93G to T allele has a summary odds ratio of 2.03 (95% confidence interval 1.30-3.18), indicating a twofold increase in risk of coronary disease for carriers with this allelic variant. The summary odds ratio for the relationship of the rare lipoprotein lipase G188E variant with cardiovascular disease is 5.25 (95% confidence interval 1.54-24.29). The lipoprotein lipase N291S allele is associated with a marginal increase in cardiovascular disease (summary odds ratio 1.25, 95% confidence interval 0.99-1.60, P = 0.07). However, there is stronger evidence for a positive association in certain populations. The summary odds ratio for lipoprotein lipase S447X allele is 0.81 (95% confidence interval 0.65-1.0), which indicates a cardioprotective effect of this lipoprotein lipase gene variant. Thus, lipoprotein lipase gene variants are associated with differential susceptibility to cardiovascular disease.

Mild oxidation of lipoproteins increases their affinity for surfaces covered by heparan sulfate and lipoprotein lipase

Lipoprotein lipase (LPL) is present in cells involved in development of atherosclerosis (endothelial cells, smooth muscle cells, and macrophages). A direct involvement of LPL in atherogenesis has been suggested. Previously we used the surface plasmon resonance technique to study the interaction of lipoproteins with surfaces covered by heparan sulfate proteoglycans (HSPG) and LPL [A. Lookene et al. (1997) Biochemistry 36, 5267-5275]. The binding was much increased by the presence of LPL. Here we demonstrate that mild oxidation of low-density-lipoprotein (LDL) and very-low-density lipoprotein (VLDL) in vitro increases their binding to surfaces covered by HSPG and LPL, while extensive oxidation decreases it. Similar results were obtained with a lipid emulsion (Intralipid), indicating that oxidation-induced changes of the lipid part could explain the effects. LPL increased binding and uptake of the mildly oxidized (compared to nonoxidized) LDL by THP-I monocyte-derived macrophages. Our studies indicate that LPL has the highest affinity for mildly oxidized LDL and support its involvement in development of atherosclerosis.

Human monocyte-derived macrophages secrete two forms of proteoglycan-macrophage colony-stimulating factor that differ in their ability to bind low density lipoproteins

This study evaluated whether human monocyte-derived macrophages synthesize specific types of proteoglycans with lipoprotein-binding capability that could contribute to lipid retention in the arterial wall. After labeling with either [35S]SO4 or [35S]methionine, macrophages secreted a high molecular mass proteoglycan, with glycosaminoglycan chains of approximately 18 kDa and core protein bands of approximately 100 and 55 kDa. Both core protein bands were recognized by an antibody to PG-100, an antibody that recognizes the proteoglycan form of macrophage colony-stimulating factor (PG-100/PG-MCSF). The interaction between PG-100/PG-MCSF and low density lipoproteins (LDL) was examined by gel mobility shift. In this system, PG-100/PG-MCSF was resolved further into two forms. The two forms had the same core proteins but differed in their overall size and glycosaminoglycan content. The larger form contained glycosaminoglycan chains that were entirely chondroitin ABC lyase-sensitive, whereas the smaller form contained chains that were sensitive to both chondroitin ABC lyase and heparinase. Both forms bound native LDL with high affinity, but the larger form bound LDL with higher affinity than the smaller form. The glycosaminoglycan chains of PG-100/PG-MCSF, but not the core proteins, were responsible for binding to native LDL. Mildly oxidized LDL and methyl-LDL, which have an electrophoretic charge similar to that of native LDL, also bound PG-100/PG-MCSF. In contrast, extensively oxidized LDL and acetyl-LDL, which are more electronegative than native LDL, did not bind to either form of PG-100/PG-MCSF. The demonstration of two forms of human monocyte-derived macrophage PG-100/PG-MCSF which bind LDL may represent an additional role for macrophages in the extracellular trapping of lipoproteins in atherosclerosis.

Human leukemia inhibitory factor upregulates LDL receptors on liver cells and decreases serum cholesterol in the cholesterol-fed rabbit

In a previous study, we found that the cytokine (human) leukemia inhibitory factor (hLIF) significantly reduced plasma cholesterol levels and the accumulation of lipid in aortic tissues of cholesterol-fed rabbits after 4 weeks of treatment. The mechanisms by which this occurs were investigated in the present study. This involved examining the effect of hLIF on (1) the level of plasma cholesterol at different times throughout the 4-week treatment and diet period; (2) smooth muscle cell (SMC) and macrophage-derived foam cell formation in vitro; and (3) LDL receptor expression and uptake in the human hepatoma cell line HepG2. At time zero, an osmotic minipump (2-mL capacity; infusion rate, 2.5 microL/h; 28 days) containing either hLIF (30 micrograms kg-1.d-1) or saline was inserted into the peritoneal cavity of New Zealand White rabbits (N = 24). Rabbits were divided into four groups of six animals each. Group 1 received a normal diet/saline; group 2, a normal diet/hLIF; group 3, a 1% cholesterol diet/saline; and group 4, a 1% cholesterol diet/hLIF. hLIF had no effect on the plasma lipids or artery wall of group 2 rabbits (normal diet). However, in group 4 rabbits, plasma cholesterol levels and the percent surface area of thoracic aorta covered by fatty streaks was decreased by approximately 30% and 80%, respectively, throughout all stages of the 4-week treatment period. In vitro, hLIF failed to prevent lipoprotein uptake by either SMCs or macrophages (foam cell formation) when the cells were exposed to beta-VLDL for 24 hours. In contrast, hLIF (100 ng/mL) added to cultured human hepatoma HepG2 cells induced a twofold or threefold increase in intracellular lipid accumulation in the medium containing 10% lipoprotein-deficient serum or 10% fetal calf serum, respectively. This was accompanied by a significant non-dose-dependent increase in LDL receptor expression in hLIF-treated HepG2 cells incubated with LDL (20 micrograms/mL) when compared with controls (P < .05) incubated in control medium alone (P < .05). We suggest that the hLIF-induced lowering of plasma cholesterol and tissue cholesterol levels (inhibition of fatty streak formation) in the hyperlipidemic rabbit is due in part to upregulation of hepatic LDL receptors, with resultant increased clearance of lipoprotein-associated cholesterol from the circulation. There is an additional and as-yet-unknown mechanism acting at the level of the vessel wall that appears to be affecting the process of arterial cholesterol accumulation.

Possible functional interactions of apolipoprotein B-100 segments that associate with cell proteoglycans and the ApoB/E receptor

The interaction of apoE lipoproteins with cells appears to be mediated by an association with basic sequences of proteoglycans and the apoB/E receptor. ApoB-100 has basic sequences, homologous with those of apoE, that form part of the apoB/E receptor-binding domain. These sequences of apoB-100 also interact with proteoglycans. We investigated whether such segments, in analogy with apoE, could act cooperatively on LDL interactions with proteoglycans and the receptor. As a model we used the two most basic regions of apoB-100, 3147 through 3157 and 3359 through 3367, connected by three glycines (3145-3157-GGG-3359-3367). Such segments may be proximal in LDL by the presence of a disulfide bridge between Cys(3167) and Cys(3297). The apoB heterodimer but not the separated monomers inhibited 125I-LDL degradation in fibroblasts and THP-1 cells by 50% at approximately 11 mumol/L. The heterodimer affinity with arterial proteoglycans was closer to that of LDL and higher than that of the individual peptides. The heterodimer appears to bind specifically to THP-1 cells, with a Kd of 6.2 x 10(-8) mol/L and a Bmax of 1.3 x 10(6) molecules/cell. Monoclonal antibody C-7, which recognizes the apoB receptor, inhibited the binding to cells. Treatment of fibroblasts with chondroitinase ABC or chlorate decreased 125I-LDL degradation markedly. Hydrolysis of pericellular proteoglycans of fibroblasts by chondroitinases reduced mostly the low-affinity, high-capacity component of LDL binding. This compartment appears to hold 70% of the cell-associated LDL when internalization is inhibited at 4 degrees C. Therefore, cell-surface chondroitin sulfate/dermatan sulfate proteoglycans appear to modulate binding and receptor-mediated internalization of LDL. This may be caused, at least in part, by the association of proteoglycans with the apoB-100 segments 3145 through 3157 and 3359 through 3367.

Differentiated macrophages synthesize a heparan sulfate proteoglycan and an oversulfated chondroitin sulfate proteoglycan that bind lipoprotein lipase

Lipoprotein lipase (LpL), which facilitates lipoprotein uptake by macrophages, associates with the cell surface by binding to proteoglycans (PGs). Studies were designed to identify and characterize specific PGs that serve as receptors for LpL and to examine effects of cell differentiation on LpL binding. PG synthesis was examined by radiolabeling THP-1 monocytes and macrophages (a cell line originally derived from a patient with acute monocytic leukemia) with [35S]sodium sulfate and [3H]serine or [3H]glucosamine. Radiolabeled PGs isolated from the cell surface were purified by chromatography and identified as chondroitin-4-sulfate (CS) PG and heparan sulfate (HS) PG. A sixfold increase in CSPG and an 11-fold increase in HSPG accompanied cell differentiation. Whereas HS glycosaminoglycan chains from both monocytes and macrophages were 7.5 kD in size, CS chains increased in size from 17 kD to 36 kD with cell differentiation, and contained hexuronyl N-acetylgalactosamine-4,6-di-O sulfate disaccharides. LpL binding was sevenfold higher to differentiated cells, and affinity chromatography demonstrated that two cell surface PGs bound to LpL: HSPG and the oversulfated CSPG produced only by differentiated cells. We conclude that differentiation-associated changes in cell surface PG of human macrophages have functional consequences that could increase the atherogenic potential of the cells.