Low-density lipoprotein aggregation is inhibited by apolipoprotein J-derived mimetic peptide D-[113–122]apoJ

Mimetic peptides are promising therapeutic agents for atherosclerosis prevention. A 10-residue class G* peptide from apolipoprotein J (apoJ), namely, D-[113–122]apoJ, possesses anti-inflammatory and anti-atherogenic properties. This prompted us to determine its effect on the aggregation process of low-density lipoprotein (LDL) particles, an early event in the development of atherosclerosis. LDL particles with and without [113–122]apoJ peptide were incubated at 37 °C with sphingomyelinase (SMase) or were left to aggregate spontaneously at room temperature. The aggregation process was analyzed by size-exclusion chromatography (SEC), native gradient gel electrophoresis (GGE), absorbance at 405 nm, dynamic light scattering (DLS), and transmission electronic microscopy (TEM). In addition, circular dichroism was used to determine changes in the secondary structure of apoB, and SDS-PAGE was performed to assess apoB degradation. At an equimolar ratio of [113–122]apoJ peptide to apoB-100, [113–122]apoJ inhibited both SMase-induced or spontaneous LDL aggregation. All methods showed that [113–122]apoJ retarded the progression of SMase-induced LDL aggregation at long incubation times. No effect of [113–122]apoJ on apoB secondary structure was observed. Binding experiments showed that [113–122]apoJ presents low affinity for native LDL but binds readily to LDL during the first stages of aggregation. Laurdan fluorescence experiments showed that mild aggregation of LDL resulted in looser lipid packaging, which was partially prevented by D-[113–122]apoJ. These results demonstrate that [113–122]apoJ peptide prevents SMase-induced LDL aggregation at an equimolar ratio and opens the possibility for the use of this peptide as a therapeutic tool.     

Aggregated Electronegative Low Density Lipoprotein in Human Plasma Shows a High Tendency toward Phospholipolysis and Particle Fusion

Aggregation and fusion of lipoproteins trigger subendothelial retention of cholesterol, promoting atherosclerosis. The tendency of a lipoprotein to form fused particles is considered to be related to its atherogenic potential. We aimed to isolate and characterize aggregated and nonaggregated subfractions of LDL from human plasma, paying special attention to particle fusion mechanisms. Aggregated LDL was almost exclusively found in electronegative LDL (LDL(−)), a minor modified LDL subfraction, but not in native LDL (LDL(+)). The main difference between aggregated (agLDL(−)) and nonaggregated LDL(−) (nagLDL(−)) was a 6-fold increased phospholipase C-like activity in agLDL(−). agLDL(−) promoted the aggregation of LDL(+) and nagLDL(−). Lipoprotein fusion induced by α-chymotrypsin proteolysis was monitored by NMR and visualized by transmission electron microscopy. Particle fusion kinetics was much faster in agLDL(−) than in nagLDL(−) or LDL(+). NMR and chromatographic analysis revealed a rapid and massive phospholipid degradation in agLDL(−) but not in nagLDL(−) or LDL(+). Choline-containing phospholipids were extensively degraded, and ceramide, diacylglycerol, monoacylglycerol, and phosphorylcholine were the main products generated, suggesting the involvement of phospholipase C-like activity. The properties of agLDL(−) suggest that this subfraction plays a major role in atherogenesis by triggering lipoprotein fusion and cholesterol accumulation in the arterial wall.     

GLP-1 Receptor Activation Inhibits VLDL Production and Reverses Hepatic Steatosis by Decreasing Hepatic Lipogenesis in High-Fat-Fed APOE*3-Leiden Mice

Objective

In addition to improve glucose intolerance, recent studies suggest that glucagon-like peptide-1 (GLP-1) receptor agonism also decreases triglyceride (TG) levels. The aim of this study was to evaluate the effect of GLP-1 receptor agonism on very-low-density lipoprotein (VLDL)-TG production and liver TG metabolism.

Experimental Approach

The GLP-1 peptide analogues CNTO3649 and exendin-4 were continuously administered subcutaneously to high fat diet-fed APOE*3-Leiden transgenic mice. After 4 weeks, hepatic VLDL production, lipid content, and expression profiles of selected genes involved in lipid metabolism were determined.

Results

CNTO3649 and exendin-4 reduced fasting plasma glucose (up to −30% and −28% respectively) and insulin (−43% and −65% respectively). In addition, these agents reduced VLDL-TG production (−36% and −54% respectively) and VLDL-apoB production (−36% and −43% respectively), indicating reduced production of VLDL particles rather than reduced lipidation of apoB. Moreover, they markedly decreased hepatic content of TG (−39% and −55% respectively), cholesterol (−30% and −55% respectively), and phospholipids (−23% and −36% respectively), accompanied by down-regulation of expression of genes involved in hepatic lipogenesis (Srebp-1c, Fasn, Dgat1) and apoB synthesis (Apob).

Conclusion

GLP-1 receptor agonism reduces VLDL production and hepatic steatosis in addition to an improvement of glycemic control. These data suggest that GLP-receptor agonists could reduce hepatic steatosis and ameliorate dyslipidemia in patients with type 2 diabetes mellitus.     

Molecular variation at the apolipoprotein B gene locus in relation to lipids and cardiovascular disease: a systematic meta-analysis

Apolipoprotein B (apoB) is the sole protein component of low-density lipoprotein (LDL) and is thought to play an important role in atherogenesis. We performed a meta-analysis of the associations between the three most frequently investigated polymorphisms (XbaI, signal peptide insertion/deletion, EcoRI) in the apolipoprotein B (APOB) gene, lipid parameters, and the risk of ischemic heart disease (IHD). We restricted our analysis to Caucasians. Homozygotes for the XbaI X+ allele had significantly elevated levels of LDL cholesterol (LDL-C) and apoB, but a decreased risk (OR=0.80; 95%CI: 0.66–0.96) of IHD. Homozygosity for the signal peptide deletion allele was associated with similarly increased levels of LDL-C and apoB, and with an increased risk of IHD (OR=1.30; 95%CI: 1.08–1.58). Subjects homozygous for the rare EcoRI allele had significantly decreased levels of total and LDL cholesterol, but unaltered risk of IHD. We conclude that all three polymorphic apoB sites are associated with altered lipid levels, but not necessarily with a consistently altered risk of IHD. These data suggest that the relationship between apoB levels, hypercholesterolemia and IHD risk cannot have a simple molecular basis in the apoB gene.     

A Western-Fed Diet Increases Plasma HDL and LDL-Cholesterol Levels in ApoD–/– Mice

Objective

Plasma apolipoprotein (apo)D, a ubiquitously expressed protein that binds small hydrophobic ligands, is found mainly on HDL particles. According to studies of human genetics and lipid disorders, plasma apoD levels positively correlate with HDL-cholesterol and apoAI levels. Thus, we tested the hypothesis that apoD was a regulator of HDL metabolism.

Methods & Results

We compared the plasma lipid and lipoprotein profiles of wild-type (WT) C57BL/6 mice with apoD−/− mice on a C57BL/6 background after receiving a high fat-high cholesterol diet for 12 weeks. ApoD−/− mice had higher HDL-cholesterol levels (61±13-apoD−/− vs. 52±10-WT-males; 37±11-apoD−/− vs. 22±2 WT-female) than WT mice with sex-specific changes in total plasma levels of cholesterol and other lipids. Compared to WT, the HDL of apoD−/− mice showed an increase in large, lipid-rich HDL particles and according to size various quantities and sizes of LDL particles. Plasma levels of lecithin:cholesterol acyltransferase in the control and apoD−/− mice were not different, however, plasma phospholipid transfer protein activity was modestly elevated (+10%) only in male apoD−/− mice. An in vivo HDL metabolism experiment with isolated Western-fed apoD−/− HDL particles showed that female apoD−/− mice had a 36% decrease in the fractional catabolic rate of HDL cholesteryl ester. Hepatic SR-BI and LDLR protein levels were significantly decreased; accordingly, LDL-cholesterol and apoB levels were increased in female mice.

Conclusion

In the context of a high fat-high cholesterol diet, apoD deficiency in female mice is associated with increases in both plasma HDL and LDL-cholesterol levels, reflecting changes in expression of SR-BI and LDL receptors, which may impact diet-induced atherosclerosis.     

Protein Hydroperoxides are a Major Product of Low Density Lipoprotein Oxidation During Copper,Peroxyl Radical and Macrophage-mediated Oxidation

Damage to apoB100 on low density lipoprotein (LDL) has usually been described in terms of lipid aldehyde derivatisation or fragmentation. Using a modified FOX assay, protein hydroperoxides were found to form at relatively high concentrations on apoB100 during copper, 2,2′-azobis(amidinopropane) dihydrochloride (AAPH) generated peroxyl radical and cell-mediated LDL oxidation. Protein hydroperoxide formation was tightly coupled to lipid oxidation during both copper and AAPH-mediated oxidation. The protein hydroperoxide formation was inhibited by lipid soluble α-tocopherol and the water soluble antioxidant, 7,8-dihydroneopterin. Kinetic analysis of the inhibition strongly suggests protein hydroperoxides are formed by a lipid-derived radical generated in the lipid phase of the LDL particle during both copper and AAPH mediated oxidation. Macrophage-like THP-1 cells were found to generate significant protein hydroperoxides during cell-mediated LDL oxidation, suggesting protein hydroperoxides may form in vivo within atherosclerotic plaques. In contrast to protein hydroperoxide formation, the oxidation of tyrosine to protein bound 3,4-dihydroxyphenylalanine (PB-DOPA) or dityrosine was found to be a relatively minor reaction. Dityrosine formation was only observed on LDL in the presence of both copper and hydrogen peroxide. The PB-DOPA formation appeared to be independent of lipid peroxidation during copper oxidation but tightly associated during AAPH-mediated LDL oxidation.     

ApoB100-LDL Acts as a Metabolic Signal from Liver to Peripheral Fat Causing Inhibition of Lipolysis in Adipocytes

Background

Free fatty acids released from adipose tissue affect the synthesis of apolipoprotein B-containing lipoproteins and glucose metabolism in the liver. Whether there also exists a reciprocal metabolic arm affecting energy metabolism in white adipose tissue is unknown.

Methods and Findings

We investigated the effects of apoB-containing lipoproteins on catecholamine-induced lipolysis in adipocytes from subcutaneous fat cells of obese but otherwise healthy men, fat pads from mice with plasma lipoproteins containing high or intermediate levels of apoB100 or no apoB100, primary cultured adipocytes, and 3T3-L1 cells. In subcutaneous fat cells, the rate of lipolysis was inversely related to plasma apoB levels. In human primary adipocytes, LDL inhibited lipolysis in a concentration-dependent fashion. In contrast, VLDL had no effect. Lipolysis was increased in fat pads from mice lacking plasma apoB100, reduced in apoB100-only mice, and intermediate in wild-type mice. Mice lacking apoB100 also had higher oxygen consumption and lipid oxidation. In 3T3-L1 cells, apoB100-containing lipoproteins inhibited lipolysis in a dose-dependent fashion, but lipoproteins containing apoB48 had no effect. ApoB100-LDL mediated inhibition of lipolysis was abolished in fat pads of mice deficient in the LDL receptor (Ldlr^−/−Apob ^100/100).

Conclusions

Our results show that the binding of apoB100-LDL to adipocytes via the LDL receptor inhibits intracellular noradrenaline-induced lipolysis in adipocytes. Thus, apoB100-LDL is a novel signaling molecule from the liver to peripheral fat deposits that may be an important link between atherogenic dyslipidemias and facets of the metabolic syndrome.     

Protein hydroperoxides are a major product of low density lipoprotein oxidation during copper, peroxyl radical and macrophage-mediated oxidation

Damage to apoB100 on low density lipoprotein (LDL) has usually been described in terms of lipid aldehyde derivatisation or fragmentation. Using a modified FOX assay, protein hydroperoxides were found to form at relatively high concentrations on apoB100 during copper, 2,2'-azobis(amidinopropane) dihydrochloride (AAPH) generated peroxyl radical and cell-mediated LDL oxidation. Protein hydroperoxide formation was tightly coupled to lipid oxidation during both copper and AAPH-mediated oxidation. The protein hydroperoxide formation was inhibited by lipid soluble alpha-tocopherol and the water soluble antioxidant, 7,8-dihydroneopterin. Kinetic analysis of the inhibition strongly suggests protein hydroperoxides are formed by a lipid-derived radical generated in the lipid phase of the LDL particle during both copper and AAPH mediated oxidation. Macrophage-like THP-1 cells were found to generate significant protein hydroperoxides during cell-mediated LDL oxidation, suggesting protein hydroperoxides may form in vivo within atherosclerotic plaques. In contrast to protein hydroperoxide formation, the oxidation of tyrosine to protein bound 3,4-dihydroxyphenylalanine (PB-DOPA) or dityrosine was found to be a relatively minor reaction. Dityrosine formation was only observed on LDL in the presence of both copper and hydrogen peroxide. The PB-DOPA formation appeared to be independent of lipid peroxidation during copper oxidation but tightly associated during AAPH-mediated LDL oxidation.     

Differences in carbohydrate content of low density lipoproteins associated with low density lipoprotein subclass patterns

The neutral carbohydrate content of both the protein (apoB) and lipid fractions of low density lipoproteins (LDL) from subjects with a predominance of small, dense LDL (subclass pattern B) was found to be lower than in subjects with larger LDL (subclass pattern A): 45 +/- 12 versus 64 +/- 13 mg/g apoLDL, and 58 +/- 8 versus 71 +/- 8 mg/g apoLDL (P less than 0.0005 for both). Sialic acid content of LDL lipids, but not apoB, was also reduced in subclass pattern B. ApoB and glycolipid carbohydrate content of total LDL and LDL density subfractions declined with increasing LDL density and decreasing particle diameter. Moreover, in LDL subfractions from pattern B subjects, carbohydrate content of LDL apoB, but not LDL glycolipid, was significantly lower in comparison with particles of similar size from pattern A subjects. Thus, in LDL subclass pattern B, reductions in LDL carbohydrate content are associated both with reduced concentrations of larger carbohydrate-enriched LDL subclasses, and with reduced glycosylation of apoB in all LDL particles. LDL glycolipids may vary with overall lipid content of LDL particles, but variation in apoB glycosylation may indicate differences in pathways for LDL production, and reduced apoB glycosylation may reflect the altered metabolic state responsible for LDL subclass pattern B.     

Inheritance pattern of familial hypercholesterolemia and markers of cardiovascular risk

Studies in children and adults have resulted in conflicting evidence in the quest for the answer to the hypothesis that offspring from hypercholesterolemic mothers might have an increased cardiovascular risk. Previous studies might have suffered from limitations such as cohort size and clinical sampling bias. We therefore explored this hypothesis in large cohorts of both subjects with familial hypercholesterolemia (FH) and unaffected siblings in a wide age range. In three cohorts (cohort 1: n = 1,988, aged 0–18 years; cohort 2: n = 300, 8–30 years; cohort 3: n = 369, 18–60 years), we measured lipid and lipoproteins as well as carotid intima-media thickness (c-IMT) in offspring from FH mothers versus FH fathers. For LDL cholesterol, triglycerides (TGs), and c-IMT, we performed a pooled analysis. No significant differences could be observed in c-IMT, lipid, or lipoprotein levels from offspring of FH mothers versus FH fathers. Pooled analyses showed no significant differences for either LDL cholesterol [mean difference 0.02 (−0.06,0.11) mmol/l, P = 0.60], TGs [mean difference 0.07 (0.00,0.14) mmol/l, P = 0.08], or c-IMT [mean difference −0.00 (−0.01,0.01) mm, P = 0.86]. Our data do not support the hypothesis that cardiovascular risk markers are different between offspring from FH mothers and FH fathers.     

Urethral Dysfunction in Female Mice with Estrogen Receptor β Deficiency

Estrogen has various regulatory functions in the growth, development, and differentiation of the female urogenital system. This study investigated the roles of ERβ in stress urinary incontinence (SUI). Wild-type (ERβ^+/+) and knockout (ERβ^−/−) female mice were generated (aged 6–8 weeks, n = 6) and urethral function and protein expression were measured. Leak point pressures (LPP) and maximum urethral closure pressure (MUCP) were assessed in mice under urethane anesthesia. After the measurements, the urethras were removed for proteomic analysis using label-free quantitative proteomics by nano-liquid chromatography–mass spectrometry (LC-MS/MS) analysis. The interaction between these proteins was further analysed using MetaCore. Lastly, Western blot was used to confirm the candidate proteins. Compared with the ERβ^+/+ group, the LPP and MUCP values of the ERβ^−/− group were significantly decreased. Additionally, we identified 85 differentially expressed proteins in the urethra of ERβ^−/− female mice; 57 proteins were up-regulated and 28 were down-regulated. The majority of the ERβ knockout-modified proteins were involved in cell-matrix adhesion, metabolism, immune response, signal transduction, nuclear receptor translational regelation, and muscle contraction and development. Western blot confirmed the up-regulation of myosin and collagen in urethra. By contrast, elastin was down-regulated in the ERβ^−/− mice. This study is the first study to estimate protein expression changes in urethras from ERβ^−/− female mice. These changes could be related to the molecular mechanism of ERβ in SUI.     

HDL and electronegative LDL exchange anti- and pro-inflammatory properties

Electronegative LDL [LDL(–)] is a minor modified LDL subfraction present in blood with inflammatory effects. One of the antiatherogenic properties of HDL is the inhibition of the deleterious effects of in vitro modified LDL. However, the effect of HDL on the inflammatory activity of LDL(–) isolated from plasma is unknown. We aimed to assess the putative protective role of HDL against the cytokine released induced in monocytes by LDL(–). Our results showed that LDL(–) cytokine release was inhibited when LDL(–) was coincubated with HDL and human monocytes and also when LDL(–) was preincubated with HDL and reisolated prior to cell incubation. The addition of apoliprotein (apo)AI instead of HDL reproduced the protective behavior of HDL. HDL preincubated with LDL(–) promoted greater cytokine release than native HDL. Incubation of LDL(–) with HDL decreased the electronegative charge, phospholipase C-like activity, susceptibility to aggregation and nonesterified fatty acid (NEFA) content of LDL(–), whereas these properties increased in HDL. NEFA content in LDL appeared to be related to cytokine production because NEFA-enriched LDL induced cytokine release. HDL, at least in part through apoAI, inhibits phospholipase-C activity and cytokine release in monocytes, thereby counteracting the inflammatory effect of LDL(–). In turn, HDL acquires these properties and becomes inflammatory.     

In vivo hepatic lipid quantification using MRS at 7 Tesla in a mouse model of glycogen storage disease type 1a

The assessment of liver lipid content and composition is needed in preclinical research to investigate steatosis and steatosis-related disorders. The purpose of this study was to quantify in vivo hepatic fatty acid content and composition using a method based on short echo time proton magnetic resonance spectroscopy (MRS) at 7 Tesla. A mouse model of glycogen storage disease type 1a with inducible liver-specific deletion of the glucose-6-phosphatase gene (L-G6pc^−/−) mice and control mice were fed a standard diet or a high-fat/high-sucrose (HF/HS) diet for 9 months. In control mice, hepatic lipid content was found significantly higher with the HF/HS diet than with the standard diet. As expected, hepatic lipid content was already elevated in L-G6pc^−/− mice fed a standard diet compared with control mice. L-G6pc^−/− mice rapidly developed steatosis which was not modified by the HF/HS diet. On the standard diet, estimated amplitudes from olefinic protons were found significantly higher in L-G6pc^−/− mice compared with that in control mice. L-G6pc^−/− mice showed no noticeable polyunsaturation from diallylic protons. Total unsaturated fatty acid indexes measured by gas chromatography were in agreement with MRS measurements. These results showed the great potential of high magnetic field MRS to follow the diet impact and lipid alterations in mouse liver.     

Isolation of pure LpB from human serum

Low density lipoproteins (LDL), even after isolation from a narrow density cut and after several washes by preparative ultracentrifugation, are contaminated by 3-5% non-apoB proteins. Incubation of these LDL with artificial triglyceride-rich lipid emulsions (TGRP) removed all contaminating apoC and also, under certain conditions, apoA proteins. TGRP treatment did not, however, change the lipid composition and the flotation behavior of LDL. Residual apoE and albumin, amounting up to 0.5% of the apoB mass, were resistant to removal by TGRP treatment as well as by heparin-Sepharose column chromatography. ApoE and albumin could only be removed by immunoabsorption.     

Isolation, characterization, and uptake in human fibroblasts of an apo(a)-free lipoprotein obtained on reduction of lipoprotein(a)

Treatment of native human Lp(a) under nondenaturing conditions with dithiothreitol yielded both a lipoprotein particle and a lipid-free protein component that could be separated by either ultracentrifugation at d 1.063 g/ml or heparin-Sepharose chromatography. The protein component only showed antigenicity against anti-Lp(a) but not against anti-B. It was heterogeneous according to SDS polyacrylamide gel electrophoresis (PAGE) consisting of two bands, a major band with molecular weight similar to apoB and a minor band with slightly lower molecular weight. The lipoprotein particle was similar to LDL with regard to its electrophoretic mobility, lipid-protein composition, its apparent molecular weight according to gel-exclusion chromatography, and its apoprotein content; only apoB was found to be present by SDS-PAGE and immunochemical analysis. This lipoprotein also proved to be identical to LDL in its uptake by the receptor-mediated LDL-pathway in cultured human fibroblasts as shown by the similarity of the concentration-dependent binding, internalization, and degradation curves at 37 degrees C of the 125I-labeled lipoproteins. Normal Lp(a) was not taken up as readily as either its reduced lipoprotein component or LDL in the various steps of the receptor-mediated pathway. The maximal capacity for Lp(a) in the degradation assay was only 25% of that of LDL and it had a fourfold higher Km. It is therefore probable that the LDL-receptor-mediated pathway is not a major route for the clearance of Lp(a) in vivo. These studies suggest that Lp(a) is, in essence, an LDL-particle to which the protein (a) is attached through disulfide bonds to apoB.     

Mice with Chimeric Livers Are an Improved Model for Human Lipoprotein Metabolism

Objective

Rodents are poor model for human hyperlipidemias because total cholesterol and low density lipoprotein levels are very low on a normal diet. Lipoprotein metabolism is primarily regulated by hepatocytes and we therefore assessed whether chimeric mice extensively repopulated with human cells can model human lipid and bile acid metabolism.

Design

FRG [ F ah(−/−) R ag2(−/−)Il2r g (−/−)]) mice were repopulated with primary human hepatocytes. Serum lipoprotein lipid composition and distribution (VLDL, LDL, and HDL) was analyzed by size exclusion chromatography. Bile was analyzed by LC-MS or by GC-MS. RNA expression levels were measured by quantitative RT-PCR.

Results

Chimeric mice displayed increased LDL and VLDL fractions and a lower HDL fraction compared to wild type, thus significantly shifting the ratio of LDL/HDL towards a human profile. Bile acid analysis revealed a human-like pattern with high amounts of cholic acid and deoxycholic acid (DCA). Control mice had only taurine-conjugated bile acids as expcted, but highly repopulated mice had glycine-conjugated cholic acid as found in human bile. RNA levels of human genes involved in bile acid synthesis including CYP7A1, and CYP27A1 were significantly upregulated as compared to human control liver. However, administration of recombinant hFGF19 restored human CYP7A1 levels to normal.

Conclusion

Humanized-liver mice showed a typical human lipoprotein profile with LDL as the predominant lipoprotein fraction even on a normal diet. The bile acid profile confirmed presence of an intact enterohepatic circulation. Although bile acid synthesis was deregulated in this model, this could be fully normalized by FGF19 administration. Taken together these data indicate that chimeric FRG-mice are a useful new model for human lipoprotein and bile-acid metabolism.     

Analysis of lipid transfer activity between model nascent HDL particles and plasma lipoproteins: implications for current concepts of nascent HDL maturation and genesis

The specifics of nascent HDL remodeling within the plasma compartment remain poorly understood. We developed an in vitro assay to monitor the lipid transfer between model nascent HDL (LpA-I) and plasma lipoproteins. Incubation of α-¹²⁵I-LpA-I with plasma resulted in association of LpA-I with existing plasma HDL, whereas incubation with TD plasma or LDL resulted in conversion of α-¹²⁵I-LpA-I to preβ-HDL. To further investigate the dynamics of lipid transfer, nascent LpA-I were labeled with cell-derived [³H]cholesterol (UC) or [³H]phosphatidylcholine (PC) and incubated with plasma at 37°C. The majority of UC and PC were rapidly transferred to apolipoprotein B (apoB). Subsequently, UC was redistributed to HDL for esterification before being returned to apoB. The presence of a phospholipid transfer protein (PLTP) stimulator or purified PLTP promoted PC transfer to apoB. Conversely, PC transfer was abolished in plasma from PLTP^−/− mice. Injection of ¹²⁵I-LpA-I into rabbits resulted in a rapid size redistribution of ¹²⁵I-LpA-I. The majority of [³H]UC from labeled r(HDL) was esterified in vivo within HDL, whereas a minority was found in LDL. These data suggest that apoB plays a major role in nascent HDL remodeling by accepting their lipids and donating UC to the LCAT reaction. The finding that nascent particles were depleted of their lipids and remodeled in the presence of plasma lipoproteins raises questions about their stability and subsequent interaction with LCAT.     

The elevation of apoB in hypercholesterolemic patients is primarily attributed to the relative increase of apoB/Lp-PLA2

Lipoprotein-associated phospholipase A₂ (Lp-PLA₂) is a risk factor of cardiovascular disease. Plasma Lp-PLA₂ is mainly associated with apolipoprotein (apo)B-containing lipoproteins, primarily with low density lipoproteins (LDLs). Importantly, only a proportion of circulating lipoproteins contain Lp-PLA₂. We determined the plasma levels of Lp-PLA₂-bound apoB (apoB/Lp-PLA₂) in patients with primary hypercholesterolemia. The effect of simvastatin therapy was also addressed. The plasma apoB/Lp-PLA₂ concentration in 50 normolipidemic controls and 53 patients with primary hypercholesterolemia at baseline and at 3 months posttreatment with simvastatin (40 mg/day) was determined by an enzyme-linked immunosorbent assay. The concentration of the apoB-containing lipoproteins that do not bind Lp-PLA₂ [apoB/Lp-PLA₂(−)] was calculated by subtracting the apoB/Lp-PLA₂ from total apoB. The apoB/Lp-PLA₂ levels were 3.6-fold higher, while apoB/Lp-PLA₂(−) were 1.3-fold higher in patients compared with controls. After 3 months of simvastatin treatment apoB/Lp-PLA₂ and apoB/Lp-PLA₂(−) levels were reduced by 52% and 33%, respectively. The elevation in apoB-containing lipoproteins in hypercholesterolemic patients is mainly attributed to the relative increase in the proatherogenic apoB/Lp-PLA₂, while simvastatin reduces these particles to a higher extent compared with apoB/Lp-PLA₂(−). Considering that Lp-PLA₂ is proatherogenic, the predominance of apoB/Lp-PLA₂ particles in hypercholesterolemic patients may contribute to their higher atherogenicity and incidence of cardiovascular disease.     

Modification of low density lipoproteins by secretory granules of rat serosal mast cells

When low density lipoprotein (LDL) is incubated with granules isolated from rat serosal mast cells, a fraction of LDL is bound to the granule heparin proteoglycan. If incubation is continued at 37 degrees C, the bound LDL, but not the unbound LDL, is degraded by granule neutral proteases. In the early stage of incubation, all the granule-bound LDL can be released by 0.3 M NaCl (the "salt-sensitive" fraction of LDL). With time, an increasing proportion of the granule-bound LDL requires 0.5 M NaCl for release (the "salt-resistant" fraction of LDL). Chemical analysis showed that, on average, 20% of the apolipoprotein B LDL was lost from the salt-sensitive fraction and 60% from the salt-resistant fraction, without any change in the composition of the lipid portion. Electron microscopic analysis disclosed large fused particles of LDL (diameters up to 100 nm) in the highly proteolyzed salt-resistant fraction, but no fused particles could be found in the less proteolyzed salt-sensitive fraction. We conclude that both binding and extensive degradation of LDL by mast cell granules is required for fusion of LDL particles on the granule surface. As compared with native LDL, the mast cell granule-modified LDL particles exhibit (i) increased particle size, (ii) selective loss of protein (apoB), (iii) a decrease in hydrated density, and (iv) stronger ionic interaction between apoB and heparin proteoglycan. The particles resemble the extracellular lipid droplets found in atherosclerotic lesions of both man and animals. Modification of LDL by mast cells may therefore provide a model of how these lipid structures are formed.     

MicroRNA-590 Inhibits Lipoprotein Lipase Expression and Prevents Atherosclerosis in apoE Knockout Mice

Recent studies have suggested that miR-590 may play critical roles in cardiovascular disease. This study was designed to determine the effects of miR-590 on lipoprotein lipase (LPL) expression and development of atherosclerosis in apolipoprotein E knockout (apoE^−/−) mice and explore the potential mechanisms. En face analysis of the whole aorta revealed that miR-590 significantly decreased aortic atherosclerotic plaque size and lipid content in apoE^−/− mice. Double immunofluorescence staining in cross-sections of the proximal aorta showed that miR-590 agomir reduced CD68 and LPL expression in macrophages in atherosclerotic lesions. MiR-590 agomir down-regulated LPL mRNA and protein expression as analyzed by RT-qPCR and western blotting analyses, respectively. Consistently, miR-590 decreased the expression of CD36 and scavenger receptor A1 (SRA1) mRNA and protein. High-performance liquid chromatography (HPLC)analysis confirmed that treatment with miR-590 agomir reduced lipid levels either in plasma orinabdominal cavity macrophages of apoE^−/− mice. ELISA analysis showed that miR-590 agomir decreased plasma levels of pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α), monocyte chemotactic protein-1 (MCP-1), interleukin-1β (IL-1β)and interleukin-6 (IL-6). In contrast, treatment with miR-590 antagomir prevented or reversed these effects. Taken together, these results reveal a novel mechanism of miR-590 effects, and may provide new insights into the development of strategies for attenuating lipid accumulation and pro-inflammatory cytokine secretion.