Lipoprotein lipase-enhanced binding of human triglyceride-rich lipoproteins to heparan sulfate: modulation by apolipoprotein E and apolipoprotein C

Effect of Inorganic Dietary Selenium Supplementation on Selenoprotein and Lipid Metabolism Gene Expression Patterns in Liver and Loin Muscle of Growing Lambs

Effect of selenium (Se) supplementation on the selenoprotein and lipid metabolism gene expression patterns in ruminants, especially in lambs is not yet fully understood. The aim of study was to evaluate the effect of Se supplementation on the messenger RNA (mRNA) expression patterns of selected selenoproteins and genes related to lipid metabolism in growing lambs. The experiment was conducted on 48 Polish Merino lambs divided into two groups (n = 24): control (C)-lambs fed with a basal diet (BD) with no Se supplementation, and supplemented (S)-lambs fed with a BD, supplemented with 0.5 mg Se/kg as sodium selenate for 8 weeks. Expression of 12 selenoproteins and six genes related to lipid metabolism was analyzed in the liver and longissimus dorsi (LD) muscle of growing lambs by qPCR. Significant differences were found in the expression of GPX1, GPX2, SEPM, SEPW1, SEP15, SEPGS2, and TXNRD1 in the liver, and GPX1, SEPP1, SEPN1, SEPW1, SEP15, and MSRB1 in the LD muscle between S and C lambs. Se supplementation mainly upregulated SEPW1, SEP15 (P < 0.001; P < 0.01) mRNA expression in the liver, and GPX1, SEPP1, SEPN1, SEPW1 (P < 0.001; P < 0.01) in the muscle of S group. On the other hand, significant decrease in GPX2 (P < 0.01), SEPM (P < 0.001), and SEPHS2 (P < 0.01) mRNA expression levels were observed in the liver of S group of lambs. Se supplementation did not affect PON1, LXRα, and PPARα mRNA expression levels, but a significant increase in mRNA levels of APOE and LPL in the LD muscle (P < 0.05) as well as LPL (P < 0.05) in the liver were noticed in the group of Se supplemented lambs. Our study confirmed that, in lambs, similarly to other species, mRNA expression patterns of several selenoproteins highly depend on dietary Se levels, and their expression is ruled by hierarchical principles and tissue-specific mechanisms. Moreover, the study showed that changes Se intake leads to different levels of genes expression related with lipid metabolism.

Impaired postprandial lipemic response in chronic kidney disease

Dyslipidemia in chronic kidney disease (CKD) is usually characterized by hypertriglyceridemia. Here we studied postprandial lipemia in children and young adults to determine whether an increasing degree of CKD results in a proportional increase in triglyceride and chylomicron concentration. Secondary goals were to determine whether subnephrotic proteinuria, apolipoprotein (apo)C-III and insulin resistance modify the CKD effect. Eighteen fasting participants (mean age of 15 years, mean glomerular filtration rate (GFR) of 50 ml/min/1.73 m(2)) underwent a postprandial challenge with a high fat milkshake. Triglycerides, apoB-48, insulin, and other markers were measured before and 2, 4, 6, and 8 hours afterward. Response was assessed by the incremental area under the curve of triglycerides and of apoB-48. The primary hypothesis was tested by correlation to estimated GFR. Significantly, for every 10 ml/min/1.73 m(2) lower estimated GFR, the incremental area under the curve of triglycerides was 17% greater while that of apoB-48 was 16% greater. Univariate analyses also showed that the incremental area under the curve of triglycerides and apoB-48 were significantly associated with subnephrotic proteinuria, apoC-III, and insulin resistance. In multivariate analysis, CKD and insulin resistance were independently associated with increased area under the curve and were each linked to increased levels of apoC-III. Thus, postprandial triglyceride and chylomicron plasma excursions are increased in direct proportion to the degree of CKD. Independent effects are associated with subclinical insulin resistance and increased apoC-III is linked to both CKD and insulin resistance.

Role of Syndecans in Lipid Metabolism and Human Diseases

Syndecans are transmembrane heparan sulfate proteoglycans involved in the regulation of cell growth, differentiation, adhesion, neuronal development, and lipid metabolism. Syndecans are expressed in a tissue-specific manner to facilitate diverse cellular processes. As receptors and co-receptors, syndecans provide promising therapeutic targets that bind to a variety of physiologically important ligands. Negatively charged glycosaminoglycan chains of syndecans, located in the extracellular compartment, are critical for such binding. Functions of syndecans are as diverse as their ligands. For example, hepatic syndecan-1 mediates clearance of triglyceride-rich lipoproteins. Syndecan-2 promotes localization of Alzheimer's amyloid Aβ peptide to the cell surface, which is proposed to contribute to amyloid plaque formation. Syndecan-3 helps co-localize the appetite-regulating melanocortin-4 receptor with its agonist, leading to an increased appetite. Finally, syndecan-4 initiates the capture of modified low-density lipoproteins by macrophages and thereby promotes the atheroma formation. We hypothesize that syndecan modifications such as desulfation of glycosaminoglycan chains may contribute to a wide range of diseases, from atherosclerosis to type 2 diabetes. At the same time, desulfated syndecans may have beneficial effects, as they can inhibit amyloid plaque formation or decrease the appetite. Despite considerable progress in understanding diverse functions of syndecans, the complex physiological roles of this intriguing family of proteoglycans are far from clear. Additional studies of syndecans may potentially help develop novel therapeutic approaches and diagnostic tools to alleviate complex human diseases such as cardiovascular and Alzheimer's diseases.

The hepatic uptake of VLDL in lrp-ldlr-/-vldlr-/- mice is regulated by LPL activity and involves proteoglycans and SR-BI

LPL activity plays an important role in preceding the VLDL remnant clearance via the three major apolipoprotein E (apoE)-recognizing receptors: the LDL receptor (LDLr), LDL receptor-related protein (LRP), and VLDL receptor (VLDLr). The aim of this study was to determine whether LPL activity is also important for VLDL remnant clearance irrespective of these receptors and to determine the mechanisms involved in the hepatic remnant uptake. Administration of an adenovirus expressing LPL (AdLPL) into lrp(-)ldlr(-/-)vldlr(-/-) mice reduced both VLDL-triglyceride (TG) and VLDL-total cholesterol (TC) levels. Conversely, inhibition of LPL by AdAPOC1 increased plasma VLDL-TG and VLDL-TC levels. Metabolic studies with radiolabeled VLDL-like emulsion particles showed that the clearance and hepatic association of their remnants positively correlated with LPL activity. This hepatic association was independent of the bridging function of LPL and HL, since heparin did not reduce the liver association. In vitro studies demonstrated that VLDL-like emulsion particles avidly bound to the cell surface of primary hepatocytes from lrp(-)ldlr(-/-)vldlr(-/-) mice, followed by slow internalization, and involved heparin-releaseable cell surface proteins as well as scavenger receptor class B type I (SR-BI). Collectively, we conclude that hepatic VLDL remnant uptake in the absence of the three classical apoE-recognizing receptors is regulated by LPL activity and involves heparan sulfate proteoglycans and SR-BI.

ApoE2-associated hypertriglyceridemia is ameliorated by increased levels of apoA-V but unaffected by apoC-III deficiency

Apolipoprotein E2 (apoE2)-associated hyperlipidemia is characterized by a disturbed clearance of apoE2-enriched VLDL remnants. Because excess apoE2 inhibits LPL-mediated triglyceride (TG) hydrolysis in vitro, we investigated whether direct or indirect stimulation of LPL activity in vivo reduces the apoE2-associated hypertriglyceridemia. Here, we studied the role of LPL and two potent modifiers, the LPL inhibitor apoC-III and the LPL activator apoA-V, in APOE2-knockin (APOE2) mice. Injection of heparin in APOE2 mice reduced plasma TG by 53% and plasma total cholesterol (TC) by 18%. Adenovirus-mediated overexpression of LPL reduced plasma TG by 85% and TC by 40%. Both experiments indicate that the TG in apoE2-enriched particles is a suitable substrate for LPL. Indirect activation of LPL activity via deletion of Apoc3 in APOE2 mice did not affect plasma TG levels, whereas overexpression of Apoa5 in APOE2 mice did reduce plasma TG by 81% and plasma TC by 41%. In conclusion, the hypertriglyceridemia in APOE2 mice can be ameliorated by the direct activation of LPL activity. Indirect activation of LPL via overexpression of apoA-V does, whereas deletion of apoC-III does not, affect the plasma TGs in APOE2 mice. These data indicate that changes in apoA-V levels have a dominant effect over changes in apoC-III levels in the improvement of APOE2-associated hypertriglyceridemia.

Lipoprotein metabolism in chronic renal insufficiency

Chronic renal insufficiency (CRI) is associated with a characteristic dyslipidemia. Findings in children with CRI largely parallel those in adults. Moderate hypertriglyceridemia, increased triglyceride-rich lipoproteins (TRL) and reduced high-density lipoproteins (HDL) are the most usual findings, whereas total and low-density lipoprotein cholesterol (LDL-C) remain normal or modestly increased. Qualitative abnormalities in lipoproteins are common, including small dense LDL, oxidized LDL, and cholesterol-enriched TRL. Measures of lipoprotein lipase and hepatic lipase activity are reduced, and concentrations of apolipoprotein C-III are markedly elevated. Still an active area of research, major pathophysiological mechanisms leading to the dyslipidemia of CRI include insulin resistance and nonnephrotic proteinuria. Sources of variability in the severity of this dyslipidemia include the degree of renal impairment and the modality of dialysis. The benefits of maintaining normal body weight and physical activity extend to those with CRI. In addition to multiple hypolipidemic pharmaceuticals, fish oils are also effective as a triglyceride-lowering agent, and the phosphorous binding agent sevelamer also lowers LDL-C. Emerging classes of hypolipidemic agents and drugs affecting sensitivity to insulin may impact future treatment. Unfortunately, cardiovascular benefit has not been convincingly demonstrated by any trial designed to study adults or children with renal disease. Therefore, it is not possible at this time to endorse general recommendations for the use of any agent to treat dyslipidemia in children with chronic kidney disease.

Apolipoprotein A-V Deficiency Results in Marked Hypertriglyceridemia Attributable to Decreased Lipolysis of Triglyceride-Rich Lipoproteins and Removal of Their Remnants

OBJECTIVE

ApoAV, a newly discovered apoprotein, affects plasma triglyceride level. To determine how this occurs, we studied triglyceride-rich lipoprotein (TRL) metabolism in mice deficient in apoAV.

METHODS AND RESULTS

No significant difference in triglyceride production rate was found between apoa5(-/-) mice and controls. The presence or absence of apoAV affected TRL catabolism. After the injection of 14C-palmitate and 3H-cholesterol labeled chylomicrons and (125)I-labeled chylomicron remnants, the disappearance of 14C, 3H, and (125)I was significantly slower in apoa5(-/-) mice relative to controls. This was because of diminished lipolysis of TRL and the reduced rate of uptake of their remnants in apoa5(-/-) mice. Observed elevated cholesterol level was caused by increased high-density lipoprotein (HDL) cholesterol in apoa5(-/-) mice. VLDL from apoa5(-/-) mice were poor substrate for lipoprotein lipase, and did not bind to the low-density lipoprotein (LDL) receptor as well as normal very-low-density lipoprotein (VLDL). LDL receptor levels were slightly elevated in apoa5(-/-) mice consistent with lower remnant uptake rates. These alterations may be the result of the lower apoE-to-apoC ratio found in VLDL isolated from apoa5(-/-) mice.

CONCLUSIONS

These results support the hypothesis that the absence of apoAV slows lipolysis of TRL and the removal of their remnants by regulating their apoproteins content after secretion.

Increased apoC-III production is a characteristic feature of patients with hypertriglyceridemia

Apolipoprotein (apo) C-III plays an important role in regulating plasma triglyceride (TG) metabolism. In order to further investigate the plasma metabolism of apoC-III in hypertriglyceridemic subjects, we have studied the plasma kinetics of VLDL apoC-III, HDL apoC-III and total plasma apoC-III with a primed constant intravenous infusion of deuterated leucine in a group of male patients with mixed hyperlipidemia (type IIb hyperlipoproteinemia, HLP, n=6) and in a group with type III HLP (n=6). Compared to normolipidemic control subjects (n=5), patients with type IIb and type III HLP had significantly higher levels of plasma TG (0.89 +/- 0.15 mmol/l vs 2.56 +/- 0.40 mmol/l vs 8.76 +/- 1.39 mmol/l, respectively, P <0.01), plasma apoC-III (9.5 +/- 0.8 mg/dl vs 20.8 +/- 2.5 mg/dl vs 41.7 +/- 5.6 mg/dl, P <0.01) and VLDL apoC-III (3.6 +/- 0.8 mg/dl vs 14.6 +/- 2.2 mg/dl vs 35.4 +/- 5.1 mg/dl, P <0.01). VLDL apoC-III production rates were significantly elevated in type IIb and type III patients (1.35 +/- 0.23 mg kg(-1) day(-1) vs 3.53 +/- 0.43 mg kg(-1) day(-1) vs 5.60 +/- 0.78 mg kg(-1) day(-1), P <0.01), as were total plasma apoC-III production rates (1.80 +/- 0.22 mg kg(-1) day(-1) vs 4.16 +/- 0.44 mg kg(-1) day(-1) vs 7.26 +/- 0.74 mg kg(-1) day(-1), P <0.01). VLDL apoC-III but not total plasma apoC-III fractional catabolic rates were reduced in type IIb and type III patients. Together with our previous results showing an increase of apoC-III production in patients with type IV HLP, and in overweight subjects with reduced insulin sensitivity, our data suggest that increased apoC-III production is a characteristic feature of patients with hypertriglyceridemia.

Effects of atorvastatin on fasting plasma and marginated apolipoproteins B48 and B100 in large, triglyceride-rich lipoproteins in familial combined hyperlipidemia

Large triglyceride (TG)-rich lipoproteins (TRLs) circulate in the blood, but they may also be present in a marginated pool, probably attached to the endothelium. It is unknown whether statins can influence this marginated pool in vivo in humans. Intravenous fat tests were performed in familial combined hyperlipidemia (FCHL) subjects before and after atorvastatin treatment and in controls to investigate whether acute increases in apoB in TRL fractions would occur, potentially reflecting the release of this TRL from a marginated pool. After a 12-h fast, a bolus injection of 10% Intralipid was given to 12 FCHL patients before and after 16-wk treatment with atorvastatin. Twelve carefully matched controls were included. For 60 min postinjection, apoB48, apoB100, and lipids were measured in TRLs. Fasting apoB100 in all TRL fractions were 2- to 3-fold higher in untreated FCHL compared with controls. ApoB48 concentrations in chylomicron fractions increased significantly within 10 min in FCHL before and after treatment, but not in controls. ApoB100 increased significantly in the chylomicron fractions in untreated FCHL and in controls, but not in FCHL after treatment. In very low density lipoprotein 1, apoB100 increased only in untreated FCHL. In very low density lipoprotein 2, apoB100 did not change in any group. These data show that increasing the number of circulating TRLs by chylomicron-like particles, results in increased plasma apoB-TRLs, probably by acute release from a marginated pool. This is a physiological process occurring in FCHL and in healthy normolipidemic subjects, but it is more pronounced in the former. Decreased marginated TRL particles in FCHL is a novel antiatherogenic property of atorvastatin.

Arterial permeability and efflux of apolipoprotein B-containing lipoproteins assessed by in situ perfusion and three-dimensional quantitative confocal microscopy

OBJECTIVES

There is accumulating evidence that an increased risk of cardiovascular disease (CVD) is not simply caused by the degree of arterial exposure to plasma lipoproteins but, in addition, is determined by the affinity of the vasculature for different lipoprotein phenotypes. In this study we compare the delivery and efflux of 2 atherogenic lipoproteins to further understand the factors that regulate cholesterol accumulation in early atherogenesis.

METHODS AND RESULTS

Lipoproteins containing apolipoprotein (apo) B100 (a low-density lipoprotein [LDL]) and apoB48 (chylomicron remnants) were isolated and differentially conjugated with fluorophores and simultaneously perfused at equivalent concentrations in situ through rabbit carotid vessels. Perfusion systems were established to quantify and differentiate between lipoprotein arterial delivery and efflux. The total average rate of delivery for LDL particles (23 nm) compared with chylomicron remnants (50 nm) was 4427 particles/min(-1) per microm3 and 452 particles/min(-1) per microm3, respectively. In contrast, the average rate of efflux was 3195 particles/min(-1) per microm3 and 163 particles/min(-1) per microm3 for LDL and chylomicron remnants, respectively.

CONCLUSIONS

Results indicate that although LDL particles have a higher rate of delivery, they efflux more readily from arterial tissue compared with the larger chylomicron remnants. Collectively, our findings highlight that lipoproteins permeate through arterial tissue differently and may be dependent on the phenotype and potential interactions with extracellular matrix components.

Mechanism of triglyceride lowering in mice expressing human apolipoprotein A5

Overexpression of human APOA5 in mice results in dramatically decreased plasma triglyceride levels. In this study we explored the mechanism underlying this hypotriglyceridemic effect. Initially we found that triglyceride turnover was faster in hAPOA5 transgenic mice compared to controls, and this strongly correlated with increased LPL activity in postheparin plasma. Furthermore, we show that in vitro recombinant apoAV interacts physically with lipoprotein lipase and significantly increased its activity. We show that both apoB and apoCIII are decreased in hAPOA5 transgenic mice indicating a decrease in VLDL number. To further investigate the mechanism of hAPOA5 in a hyperlipidemic background, we inter-crossed hAPOA5 and hAPOC3 transgenic mice. We found a marked decrease in VLDL triglyceride and cholesterol, as well as apolipoprotein B and CIII levels. These data indicated that apoAV induces a decrease in VLDL size by activating lipolysis and an increase of VLDL clearance. In a postprandial state, the normal triglyceride response found in wild-type mice was significantly reduced in hAPOA5 transgenics. In addition, we demonstrated that in response to this fat load in hAPOA5xhAPOC3 mice, apoAV, but not apoCIII, was redistributed from primarily HDL to VLDL. This shift of apoAV in VLDL appears to limit the increase of triglyceride by activating the lipoprotein lipase.

The lipoprotein lipase S447X polymorphism and plasma lipids

We studied 4,058 subjects from a representative sample of the Singapore population 1) to determine the association between the S447X polymorphism at the LPL locus and serum lipid concentration in Chinese, Malays, and Asian Indians living in Singapore and 2) to explore any interactions with apolipoprotein E (APOE) genotype, exercise, obesity, cigarette smoking, and alcohol intake. Information on obesity, lifestyle factors (including smoking, alcohol consumption, and exercise frequency), glucose tolerance, and fasting lipids was obtained. Male and female carriers of the X447 allele had lower serum triglyceride concentrations and higher HDL cholesterol (HDL-C) concentrations. The association between the X447 allele and serum HDL-C concentration was modulated by APOE genotype in males and cigarette smoking and alcohol intake in females. The effect of the X447 allele was greatest in men who carried the E4 allele and women who smoked or consumed alcohol. The X447 allele at the LPL locus is common and associated with a less atherogenic lipid profile in Asian populations. Interactions with APOE genotype, cigarette smoking, and alcohol intake reinforce the importance of examining genetic associations, such as this one, in the context of the population of interest.

Apolipoprotein C-III, a strong discriminant of coronary risk in men and a determinant of the metabolic syndrome in both genders

AIMS

Apolipoprotein C-III (apoC-III) has been recognized as a useful marker of triglyceride-rich lipoproteins (TRLs) metabolism and proposed as an indicator of prognosis for coronary risk in healthy subjects. We studied cross-sectionally in a population having low cholesterol levels, but a high prevalence of the metabolic syndrome, whether serum levels of total apoC-III or its sub-fractions were independent markers of prevalent coronary heart disease (CHD) or were related to variables reflecting the metabolic syndrome.

METHODS AND RESULTS

In 857 unselected participants of the representative population sample of the Turkish Adult Risk Factor Survey in 2001, apoC-III as well as other risk variables were evaluated, and CHD was diagnosed based on clinical findings and Minnesota coding of resting electrocardiograms. The sample consisted of men and women, aged 33-82 years, having a mean waist circumference of 89.4 and 92.9 cm, respectively, 42% of whom had the metabolic syndrome identified by criteria of the ATP III. ApoC-III values were measured by turbidimetric immunoassay. Mean concentrations for non-high-density lipoprotein (nonHDL) apoC-III in men and women were 6.4 and 6.2 mg/dl, respectively, and for apoC-III in HDL were 6.2 and 6.3 mg/dl, respectively. NonHDL apoC-III was similar to, and apoC-III in HDL was higher than that in Western populations. Both fractions of apoC-III were significantly correlated with lipids, lipoproteins, apoB, anthropometric measures, and blood pressures in both genders. Correlations for both were high with serum triglycerides (r(s)=around 0.70) and apoB (r(p)=around 0.37). Total apoC-III as well as both fractions were significantly correlated in women also with levels of inflammatory risk markers: strongly (r=0.40-0.45, P<0.001) with complement C3, and weakly (r(s)=around 0.20, P<0.001) with C-reactive protein. A cutoff of >7.0 mg/dl as opposed to lower levels of nonHDL apoC-III indicated the presence of hypertriglyceridemic hyperapo B with an age-adjusted odds ratio (OR) of 13.8; it indicated the presence of metabolic syndrome with 4.66-fold likelihood. Total apoC-III and nonHDL apoC-III proved to be significantly (P-trend <0.05 and 0.002) and strongly associated with prevalent CHD in men even after adjustment for age, low-density lipoprotein-cholesterol (LDL-C) and high-density lipoprotein-cholesterol (HDL-C): OR gradients across upper and lower quartiles were 3.88-fold (CI: 1.3; 11.4) and 8.8-fold (CI: 2.6; 29.8), respectively.

CONCLUSIONS

In a population among whom the metabolic syndrome prevails, total- and nonHDL apoC-III are each a determinant in both genders of the metabolic syndrome and of hypertriglyceridemic hyperapo B. Each is a powerful significant marker of prevalent CHD in men independent of LDL- and HDL-C levels. In women, despite being correlated with inflammatory risk markers, the significant association of elevated levels of apoC-III with CHD did not prove to be independent of age.

Lipoprotein lipase affects the survival and differentiation of neural cells exposed to very low density lipoprotein

Lipoprotein lipase (LPL) is a key enzyme involved in the metabolism of lipoproteins, providing tissues like adipose tissue or skeletal muscle with fatty acids. LPL is also expressed in the brain, fulfilling yet unknown functions. Using a neuroblastoma cell line transfected with a NEO- or a LPL-expression vector, we have developed a model to study the function of LPL in neurons exposed to native or copper-oxidized lipoproteins. The addition to the culture media of VLDL with 10 microm copper sulfate led to a significant reduction in the viability of NEO transfectants whereas LPL-transfectants were protected from this injury. In the presence of VLDL and CuSO(4), LPL transfectants were even able to display significant neurite extension. This neuritogenic effect was also observed in LPL transfectants exposed to native lipoproteins. However, addition of VLDL particles oxidized with CuSO(4) prior to their addition to the culture media resulted in neurotoxic effects on LPL transfectants. These findings suggest that the presence of LPL in cultured neuronal cells modulates the physiological response of neurons following exposure to native or oxidized lipoproteins. LPL could thus play a key role in the differentiation of Neuro-2A cells and in the pathophysiological effects of oxidative stress in several neurodegenerative disorders.

Apolipoprotein E and lipoprotein lipase increase triglyceride-rich particle binding but decrease particle penetration in arterial wall

OBJECTIVE

Liver-derived apolipoprotein E (apoE) decreases atherosclerosis without altering the circulating concentrations of plasma lipoproteins. We evaluated the effects of apoE and lipoprotein lipase (LpL) on the interactions of triglyceride-rich particles (TGRPs) in the arterial wall.

METHODS AND RESULTS

Quantitative fluorescence microscopy was used to study the interactions of TGRPs (25- to 35-nm diameter) in the arterial wall. Carotid arteries were harvested from rats, placed in a perfusion chamber, and perfused with fluorescently labeled TGRPs. In the absence of apoE or LpL, 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine-TGRP (100 microg neutral lipid/mL) was poorly retained in the arterial wall. The addition of either apoE (10 microg/mL) or LpL (10 microg/mL) increased TGRP accumulation 220% and 100%, respectively. This effect was attenuated by heparin (10.0 IU/mL). Histological analyses of cross sections from these vessels demonstrate that in the absence of apoE or LpL, there is deep penetration of lipid into the arterial wall. With the addition of either apoE or LpL, arterial wall penetration of TGRP is blocked.

CONCLUSIONS

These results demonstrate that although apoE and LpL increase arterial wall accumulation of TGRPs, these proteins also reduce the penetration of TGRPs into the arterial wall. We postulate that this may represent a novel antiatherogenic property of apoE and LpL.

Differential metabolism of human VLDL according to content of ApoE and ApoC-III

We studied the metabolism of very low density lipoprotein (VLDL) and intermediate density lipoprotein (IDL) particles that did or did not have apolipoprotein E (apoE) in 12 normolipidemic women by endogenously labeling plasma apolipoprotein B. The plasma was separated into bound (E+) and unbound (E-) fractions by use of a monoclonal antibody (1D7), and the fractions were ultracentrifuged to yield E+ and E- subfractions of light and dense VLDL and IDL. VLDL E+ and IDL E+ were produced mainly by the liver. VLDL E+ and IDL E+ had lower fractional catabolic rates and much higher apolipoprotein C-III (apoC-III) content than did the corresponding E- particles. Most light VLDL apoE+ underwent lipolysis to dense VLDL E+ with reduced apoC-III content, which was removed from the circulation without conversion to IDL. In contrast, most light VLDL apoE-, poor in apoC-III, was removed from the circulation, and a smaller proportion underwent lipolysis to dense VLDL E-. Most dense VLDL E- underwent lipolysis to IDL E-. The rate constant for lipolysis of dense VLDL to IDL was greater for E- than for E+, and the rate constant for clearance from plasma was greater for dense VLDL E+ than for E-. In conclusion, metabolism of human VLDL particles is influenced by their content of apoE, further modulated by the coexistence of apoC-III.

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.

Lipoprotein lipase compensates for the defective function of apo E variants in vitro by interacting with proteoglycans and lipoprotein receptors

Lipoprotein lipase (LPL) and apolipoprotein E (apo E) independently enhance binding and uptake of lipoproteins to cells. A coordinate effect of LPL and apo E has been previously described in human hepatozytes where simultaneous addition of both proteins resulted in an additive increase of chylomicron binding and uptake. The role of lipoprotein receptors and proteoglycans in this coordinate effect was now analysed using various cell types and heparinase treatment. To investigate a pathophysiological relevance, the effect of LPL and normal apo E-3 was compared to LPL and four apo E variants, associated with type III hyperlipoproteinemia (HLP). Apo E-3 and LPL increased the binding and uptake of chylomicrons and beta-very low density lipoproteins (VLDL) in an additive way in all cell types analysed, except proteoglycan deficient Chinese hamster ovary (CHO)-cells. Heparinase treatment almost completely abolished the effect of apo E and LPL. Addition of LPL to the apo E variants resulted in significant compensation of their defective function in mediating beta-VLDL binding to low density lipoprotein (LDL)-receptor defective fibroblasts. These findings indicate that the coordinate effect of apo E and LPL is mediated by proteoglycans and lipoprotein receptors, independent of the LDL receptor. LPL may compensate for the defective function of apo E variants by enhancing lipoprotein binding to these receptors. Defects in this mechanism may explain how mutations in the LPL molecule contribute to the manifestation of type III HLP in addition to the presence of a defective apo E.

Lipoprotein lipase-enhanced binding of lipoprotein(a) [Lp(a)] to heparan sulfate is improved by apolipoprotein E (apoE) saturation: secretion-capture process of apoE is a possible route for the catabolism of Lp(a)

Recently, it has been recognized that cell-bound heparan sulfate (HS) proteoglycans (HSPG) are able to bind and subsequently initiate degradation of lipoproteins. Two mediators of lipoprotein catabolism, both with HS binding capacity, lipoprotein lipase (LPL) and apolipoprotein E (apoE), are involved in this process. This mechanism is known as the secretion-capture process of apoE. Lipoprotein(a) [Lp(a)] was shown to have a strong binding capacity to cell-associated HSPG. This binding capacity was increased by LPL addition. We investigated the effects of recombinant apoE (r-apoE) enrichment of Lp(a) on the binding to HS. Lp(a), isolated by ultracentrifugation and gel filtration, was incubated with r-apoE and reisolated by ultracentrifugation, resulting in r-apoE-enriched Lp(a). ApoE-enriched Lp(a) and control Lp(a) were coated to microtiter plates. The capacity to bind biotin-conjugated HS (b-HS) in the presence or absence of inactivated bovine LPL was studied. R-apoE-enriched Lp(a) showed increased b-HS binding capacity versus control Lp(a). Addition of LPL resulted in an increased b-HS binding capacity of both control and r-apoE-enriched Lp(a). To investigate whether binding of Lp(a) to endothelial cell HSPG occurred in vivo, 39 volunteers were injected with heparin (50 U/kg) and plasma lipid and Lp(a) levels were determined before and 20 minutes after heparin injection. No significant increase in plasma Lp(a) concentrations was found. The results showed that Lp(a) can be enriched with apoE and that this resulted in increased LPL-enhanced binding to HSPG. From the in vitro studies, it can be concluded that the secretion-capture process of apoE is a possible catabolic route for Lp(a). However, whether this also occurs in vivo remains to be confirmed.

Lipoprotein lipase increases lipoprotein binding to the artery wall and increases endothelial layer permeability by formation of lipolysis products

Mechanisms responsible for the accumulation of low-density lipoprotein (LDL) were investigated in a new model, the perfused hamster aorta. To do this, we developed a method to study LDL flux in real time in individually perfused arteries; each artery served as its own control. Using quantitative fluorescence microscopy, the rates of LDL accumulation and efflux were separately determined. Perfusion of arteries with buffer plus lipoprotein lipase (LpL) increased LDL accumulation 5-fold (0.1 +/- 0.03 mV/min [control] versus 0.5 +/- 0.05 mV/min [LpL]) by increasing LDL retention in the artery wall. This effect was blocked by heparin and monoclonal antibodies directed against the amino-terminal region of apolipoprotein B (apo B). This suggests that specific regions of apo B are involved in LDL accumulation within arteries. Also, the effect of hydrolysis of triglyceride-rich lipoproteins on endothelial barrier function was studied. We compared endothelial layer permeability using a water-soluble reference molecule, fluorescently labeled dextran. When LpL was added to hypertriglyceridemic plasma, dextran accumulation within the artery wall increased > 4-fold (0.024 +/- 0.01 mV/min [control] versus 0.098 +/- 0.05 mV/min [LpL]). Under the same conditions, LpL increased LDL accumulation approximately 3-fold (0.016 +/- 0.003 mV/min [control] versus 0.047 +/- 0.013 mV/min [LpL]). Rapid efflux of LDL from the artery wall indicated that increased endothelial layer permeability was the primary mechanism during periods of increased lipolysis. Our data demonstrate two LpL-mediated effects that may increase the amount of LDL in the artery wall. These findings may pertain to the observed relationship between increased postprandial lipemia and atherosclerosis.

Lipoprotein lipase reduces secretion of apolipoprotein E from macrophages

Macrophages are a significant source of lipoprotein lipase (LPL) and apolipoprotein E (apo E) in the developing arterial wall lesion, and each of these proteins can importantly modulate lipid and lipoprotein metabolism by arterial wall cells. LPL and apo E share a number of cell surface binding sites, including proteoglycans, and we have previously shown that proteoglycans are important for modulating net secretion of apoprotein E from macrophages. We therefore evaluated a potential role for LPL in modulating net secretion of macrophage-derived apo E. In pulse-chase experiments, addition of LPL during the chase period produced a decrease in secretion of apoprotein E from human monocyte-derived macrophages, from the human monocytic THP1 cell line, and from J774 cells transfected to constitutively express a human apo E cDNA. LPL similarly reduced apo E secretion when it was prebound to the macrophage cell surface at 4 degrees C. A native LPL particle was required to modulate apo E secretion; addition of monomers and aggregates did not produce the same effect. Depletion of cell surface proteoglycans by a 72-h incubation in 4-methylumbelliferyl-beta-D-xyloside did not attenuate the ability of LPL to reduce apo E secretion. However, addition of receptor-associated protein attenuated the effect of LPL on apo E secretion. Although LPL could mediate removal of exogenously added apo E from the culture medium, detailed pulse-chase analysis suggested that it primarily prevented release of newly synthesized apo E from the cell layer. Cholesterol loading of cells or antibodies to the low density lipoprotein receptor attenuated LPL effects on apo E secretion. We postulate that LPL sequesters endogenously synthesized apo E at the cell surface by a low density lipoprotein receptor-dependent mechanism. Such post-translational regulation of macrophage apo E secretion by LPL could significantly influence apo E accumulation in arterial vessel wall lesions.