Chylomicron Remnant Particles Research Articles

Deletion of Lymphangiogenic and Angiogenic Growth Factor VEGF-D Leads to Severe Hyperlipidemia and Delayed Clearance of Chylomicron Remnants.

Objective- Dyslipidemia is one of the key factors behind coronary heart disease. Blood and lymphatic vessels play pivotal roles in both lipoprotein metabolism and development of atherosclerotic plaques. Recent studies have linked members of VEGF (vascular endothelial growth factor) family to lipid metabolism, but the function of VEGF-D has remained unexplored. Here, we investigated how the deletion of VEGF-D affects lipid and lipoprotein metabolism in atherogenic LDLR-/- ApoB100/100 mice. Approach and Results- Deletion of VEGF-D (VEGF-D-/-LDLR-/-ApoB100/100) led to markedly elevated plasma cholesterol and triglyceride levels without an increase in atherogenesis. Size distribution and hepatic lipid uptake studies confirmed a delayed clearance of large chylomicron remnant particles that cannot easily penetrate through the vascular endothelium. Mechanistically, the inhibition of VEGF-D signaling significantly decreased the hepatic expression of SDC1 (syndecan 1), which is one of the main receptors for chylomicron remnant uptake when LDLR is absent. Immunohistochemical staining confirmed reduced expression of SDC1 in the sinusoidal surface of hepatocytes in VEGF-D deficient mice. Furthermore, hepatic RNA-sequencing revealed that VEGF-D is also an important regulator of genes related to lipid metabolism and inflammation. The lack of VEGF-D signaling via VEGFR3 (VEGF receptor 3) led to lowered expression of genes regulating triglyceride and cholesterol production, as well as downregulation of peroxisomal β-oxidation pathway. Conclusions- These results demonstrate that VEGF-D, a powerful lymphangiogenic and angiogenic growth factor, is also a major regulator of chylomicron metabolism in mice.

Postprandial Hyperlipidemia and Remnant Lipoproteins.

Fasting hypertriglyceridemia is positively associated with the morbidity of coronary heart disease (CHD), and postprandial (non-fasting) hypertriglyceridemia is also correlated with the risk status for CHD, which is related to the increase in chylomicron (CM) remnant lipoproteins produced from the intestine. CM remnant particles, as well as oxidized low density lipoprotein (LDL) or very low density lipoprotein (VLDL) remnants, are highly atherogenic and act by enhancing systemic inflammation, platelet activation, coagulation, thrombus formation, and macrophage foam cell formation. The cholesterol levels of remnant lipoproteins significantly correlate with small, dense LDL; impaired glucose tolerance (IGT) and CHD prevalence. We have developed an assay of apolipoprotein (apo)B-48 levels to evaluate the accumulation of CM remnants. Fasting apoB-48 levels correlate with the morbidity of postprandial hypertriglyceridemia, obesity, type III hyperlipoproteinemia, the metabolic syndrome, hypothyroidism, chronic kidney disease, and IGT. Fasting apoB-48 levels also correlate with carotid intima-media thickening and CHD prevalence, and a high apoB-48 level is a significant predictor of CHD risk, independent of the fasting TG level. Diet interventions, such as dietary fibers, polyphenols, medium-chain fatty acids, diacylglycerol, and long-chain n-3 polyunsaturated fatty acids (PUFA), ameliorate postprandial hypertriglyceridemia, moreover, drugs for dyslipidemia (n-3 PUFA, statins, fibrates or ezetimibe) and diabetes concerning incretins (dipeptidyl-peptidase IV inhibitor or glucagon like peptide-1 analogue) may improve postprandial hypertriglyceridemia. Since the accumulation of CM remnants correlates to impaired lipid and glucose metabolism and atherosclerotic cardiovascular events, further studies are required to investigate the characteristics, physiological activities, and functions of CM remnants for the development of new interventions to reduce atherogenicity.

Vascular risk factors, vascular disease, lipids and lipid targets in patients with familial dysbetalipoproteinemia: A European cross-sectional study

BackgroundFamilial dysbetalipoproteinemia (FD), also known as type III hyperlipoproteinemia, is a genetic dyslipidemia characterized by elevated very low density lipoprotein (VLDL) and chylomicron remnant particles that confers increased risk of cardiovascular disease (CVD). The objective of this study was to evaluate the prevalence of vascular risk factors, CVD, lipid values, treatment and lipid targets in patients with FD across Europe. MethodsThis cross-sectional study was performed in 305 patients with FD from seven academic hospitals in four European countries. Information was collected from clinical records. ResultsPatients mean (± standard deviation) age was 60.9 ± 14.4 years, 201 (66%) were male, 69 (23%) had diabetes mellitus (DM) and 87 (29%) had a prior history of CVD. Mean body mass index was 28.5 ± 5.0 kg/m2. Lipid-lowering medication was used by 227 (74%) patients (27% usual dose (theoretical low-density lipoprotein cholesterol (LDL-C) reduction ≤40%) and 46% intensive dose (theoretical LDL-C reduction >40%)). Non high-density lipoprotein cholesterol (non-HDL-C) levels below treatment target (<3.3 mmol/L) were present in 123 (40%) patients and 163 patients (53%) had LDL-C levels below target (<2.5 mmol/L). No significant determinants were found for having non-HDL-C levels below target, while a prior history of CVD (OR 1.90, 95%CI 1.05–3.47) and presence of DM (OR 2.00, 95%CI 1.08–3.70) were associated with having LDL-C levels below treatment target. ConclusionThe majority of FD patients had non-HDL-C levels above the treatment target of 3.3 mmol/L. Intensive dose lipid-lowering medication was used by only half of the patients, leaving them at increased cardiovascular risk.

Triglyceride-rich lipoproteins and coronary artery disease risk: new insights from human genetics.

Despite ample success in reducing coronary artery disease (CAD) risk through reduction of low-density lipoprotein cholesterol (LDL-C), there remains substantial residual risk.1–4 Recent prospective studies have demonstrated that elevated triglycerides (TGs) are independent predictors of CAD risk.5–9 Furthermore, TGs are strongly associated with incident CAD events in patients with low LDL-C levels treated with statin.10 Thus, triglyceride-rich lipoproteins (TRLs) offer a potentially orthogonal risk factor to LDL-C for lowering CAD risk, but only if TRLs are causally associated with atherosclerotic disease.11 Human genetics has the potential to reveal the causal relationships of biomarkers found to be associated with disease outcomes.12–15 For example, genetic variants associated with plasma LDL-C levels are consistently associated with CAD risk in the right direction,15–18 consistent with a causal relationship. Importantly, similar studies have causally implicated the key TG-regulating enzyme lipoprotein lipase (LPL) in CAD risk. A common gain-of-function LPL variant, S447X, confers an antiatherogenic lipid profile characterized by low levels of TGs, and in several studies, it has been associated with lower incidence of vascular disease or myocardial infarction (MI).19–25 Conversely, several loss-of-function (LOF) LPL variants associated with elevated TG levels have been reported to be associated with increased CAD risk.21,26 Furthermore, multiple genome-wide association studies in the last 5 years have identified common noncoding variants at the LPL gene locus associated with both TG and CAD risk in the same direction.27–29 Beyond LPL itself, common variants that influence TG levels are significantly associated with CAD risk even after adjusting for their effects on other lipid traits.30 Do et al30 surveyed 185 single-nucleotide polymorphisms (SNPs) that were genome-wide significantly associated with ≥1 plasma lipid trait and identified a subset of …

Diabetische Dyslipoproteinämie: jenseits von LDL

Diabetes mellitus type 2 is reaching epidemic proportions in western societies. The treatment of diabetic dyslipidemia to prevent cardiovascular disease is of increasing clinical and scientific interest. In the pathogenesis of this disease plasma triglycerides play a central role. Triglyceride rich particles by themselves are not considered atherogenic; however, they are hydrolysed to chylomicron and VLDL remnant particles. Furthermore, mediated by cholesteryl ester transfer protein (CETP), atherogenic small dense LDL particles (sdLDL) emerge, and HDL cholesterol decreases. All these factors yield into a significantly increased atherogenesis and cardiovascular risk. Weight reduction and low fat diets have shown positive effects in general, but a specific therapy to treat diabetic dyslipidemia is still missing. Studies so far have failed to show a reliable benefit for fibrates and for nicotinic acid. Thus, statin therapy to decrease LDL cholesterol to target is the essential treatment for diabetic dyslipidemia to reduce cardiovascular risk. Other lipid lowering drugs can be added optionally.

Postprandial triglyceride-rich lipoproteins in insulin resistance and Type 2 diabetes

Recent evidence from prospective epidemiological large-scale studies have providedconvincing proof for the role of nonfasting triglycerides, carried in VLDL and chylomicrons,as an independent risk factor for cardiovascular disease and death. Overproduction oflarge VLDL1 by the liver on one hand, and accumulation of apoB-48-containing triglyceriderichlipoproteins (TRLs) on the other hand are present before overt hyperglycemia,highlighting the role of insulin resistance as an early pathogenic defect. Multiple insulinaction sites demonstrate insulin resistance in the liver, adipose tissue and muscle, as well asin the intestine. Together, these perturbations of insulin action contribute to theoverproduction of atherogenic VLDL and chylomicron remnant particles. Consequently, thevascular wall endothelium is exposed to exaggerated cholesterol influx resulting inendothelial dysfunction, oxidative stress and prothrombotic state during subsequent mealsover the 24-h period. This overview summarizes current understanding of diabeticpostprandial ‘dysmetabolism’ and reviews recent data on TRL secretion and intravascularprocessing and removal accumulated during this decade. Various therapeutic strategiesand future opportunities are also discussed.

Genetic evidence that apolipoprotein E4 is not a relevant susceptibility factor for cholelithiasis in two high-risk populations

Apolipoprotein E (apoE) isoforms are genetic determinants of interindividual variations in lipid metabolism. To assess whether apoE is a genetic risk factor for cholesterol gallstone disease (GD), we analyzed apoE variants in populations from Chile and Germany, two countries with very high prevalence rates of this disease. ApoE genotypes were determined in Chilean gallstone patients (n = 117) and control subjects (n = 122) as well as in German gallstone patients (n = 184) and matched controls (n = 184). In addition, we studied apoE variants in subgroups of Chilean patients with strong differences in their susceptibility to acquire gallstones: 50 elderly subjects without gallstones in spite of well-known risk factors for this disease (gallstone-resistant) and 32 young individuals with gallstones but without risk factors (gallstone-susceptible). Furthermore, correlation analysis of apoE genotypes with cholesterol crystal formation times, biliary cholesterol saturation index (CSI), and gallstone cholesterol contents was performed in 81 cholecystectomized patients. In this study analyzing the largest sample set available, apoE4 genotype was not associated with an increased frequency of GD in either population. Moreover, in the Chilean population after adjusting for risk factors such as gender, age, body mass index, serum lipids, and glucose, the odds ratio for the association of the apoE4 allele and GD was significantly (P < 0.05) <1. Also, genotypes were not correlated with cholesterol crystal formation time, CSI, or gallstone cholesterol content. In contrast to previous smaller studies, apoE polymorphisms were not associated with susceptibility to cholesterol GD in high-risk populations.

Novel Insights Into Lp(a) Physiology and Pathogenicity: More Questions Than Answers?

Lipoprotein(a) (Lp(a)) is of interest to both basic researchers who endeavour to understand the mechanism of action of this unique lipoprotein, as well as to clinicians who are interested in the contribution of Lp(a) to cardiovascular risk profiles. The Lp(a) particle contains a moiety that is indistinguishable from circulating LDL, covalently linked to the unique glycoprotein component apolipoprotein(a) (apo(a)). Since the 1970s, epidemiological data have been accumulated that, on balance, indicate that elevated plasma Lp(a) concentrations are an independent risk factor for vascular diseases. Apo(a) is highly homologous to the fibrinolytic proenzyme plasminogen, containing many tandemly-repeated kringle motifs similar to several of those found in the plasminogen molecule; the size of the kringle domain in apo(a) gives rise to Lp(a) isoform size heterogeneity which is a hallmark of this lipoprotein. The similarity between Lp(a) and plasminogen led to speculation of a bridging role for Lp(a) in atherothrombotic disease based on the duality of the structure of this lipoprotein. In this scenario, LDL would contribute to the proatherosclerotic properties of the particle, while apo(a) would interfere with the normal fibrinolytic functions of plasminogen, thereby inhibiting the breakdown of thrombi formed in the vasculature. Many in vitro and in vivo studies have suggested a prothrombotic role for Lp(a) which is attributable to the apo(a) component of the particle. However, there are a number of unique properties that apo(a) confers to Lp(a) which are independent of its similarity to plasminogen. These include the ability of apo(a)/Lp(a) to affect platelet function, to contribute to endothelial dysfunction, and to inhibit the clearance of chylomicron remnant particles in a transgenic mouse model. Very recent data have revealed a potential role for Lp(a) in the preferential binding of oxidized phospholipid adducts through one of the kringle motifs in apo(a). Many questions remain to be answered regarding the role of Lp(a) in atherothrombotic disease. This article will review the relevant literature concerning the contribution of Lp(a) to risk for both atherosclerotic and purely thrombotic disorders, as well as the proposed mechanisms of Lp(a) pathogenicity related to the structure of this lipoprotein. Emerging areas of interest in the field including the role of apo(a) isoform size as a risk factor for CHD - independent of Lp(a) levels - will also be discussed, as will speculation as to the possible physiological role of Lp(a). Future directions in the field as well as recommendations for the use of Lp(a) in clinical contexts will also be addressed.

High dose of simvastatin normalizes postprandial remnant-like particle response in patients with heterozygous familial hypercholesterolemia.

Familial hypercholesterolemia (FH) and disturbances in postprandial lipoprotein metabolism are both associated with premature atherosclerosis. The effect of beta-hydroxy-beta-methylglutaryl coenzyme A reductase inhibitors on plasma cholesterol levels in patients with FH is well established; however, it is not known whether postprandial lipoproteins are also influenced. In this case-controlled intervention study, we investigated the effects of high-dose simvastatin on postprandial lipoproteins. We used a new method to analyze remnant lipoproteins based on the immunoseparation principle (remnant-like particle cholesterol [RLP-C] assay) and the well-established measurement of retinyl ester (RE) analysis in plasma and in the Svedberg flotation unit (Sf)<1000 fraction. Seven heterozygous FH patients and 7 control subjects matched for sex, age, body mass index, triglycerides, and apolipoprotein E genotype were enrolled in the study. An oral vitamin A (RE) fat-loading test was performed at baseline in both groups and after 3 months of high-dose simvastatin (80 mg/d) treatment in the FH patients. Before treatment, FH patients had significantly higher fasting and postprandial concentrations of lipoprotein remnants (plasma RLP-C 42+/-19 mg/dL and area under the RLP-C curve 415+/-82 mg. L(-1). h(-1), respectively) than did control subjects (7+/-3 mg/dL and 101+/-35 mg. L( -1). h(-1), respectively; P<0.05), suggesting a delayed clearance of chylomicron remnant particles in the FH patients. Treatment with simvastatin significantly reduced fasting and postprandial remnant lipoprotein cholesterol concentrations (13+/-3 mg/dL and 136+/-53 mg. L(-1). h(-1), respectively; P<0.05 for both). Postprandial RE in the Sf<1000 fraction, not total RE in plasma, was also significantly higher in FH patients than in control subjects (24+/-10 versus 6.3+/-5.9 mg. L( -1). h(-1), P<0.05), but treatment with simvastatin did not result in improvement of the postprandial RE response, either in the Sf<1000 fraction or in plasma. It is concluded that heterozygous FH patients have increased fasting and postprandial remnant lipoprotein concentrations. Treatment with simvastatin significantly reduced the fasting and postprandial RLP-C concentrations but did not result in improved postprandial RE response.

The Interaction between Oxidised Chylomicron Remnants and the Aorta of Rats Fed a Normocholesterolaemic or Hypercholesterolaemic Diet

The effects of oxidised chylomicron remnants on endothelium-dependent relaxation and lipoprotein uptake were studied in both the normocholesterolaemic and hypercholesterolaemic rat aorta in vitro. Incubation of aortic rings taken from normocholesterolaemic animals with oxidised (by treatment with copper sulphate) chylomicron remnant particles resulted in a reduction in both vessel sensitivity and maximum percent relaxation in response to carbachol (CCh) and ATP, without affecting responses to A23187 and S-nitroso-N-acetylpenicillamine (SNAP). Studies comparing control vessels and those taken from fat-fed rats confirmed that hypercholesterolaemia significantly decreased relaxations in response to CCh and potentiated contractions in response to phenylephrine (PE) via a nitric oxide (NO)-dependent mechanism. Perfusion of the aorta of these hypercholesterolaemic rats for 2 h with ¹²⁵I-labelled oxidised chylomicron remnants showed that significantly greater amounts of lipoprotein became associated with the artery wall, as compared to control normocholesterolaemic animals. However, there was no significant difference in the uptake of native chylomicron remnant particles between control and hypercholesterolaemic vessels. Taken together, the findings of this study suggest that incorporation of lipoproteins of dietary origin into the arterial wall may contribute to endothelial dysfunction and that their contribution may be enhanced by hypercholesterolaemia. These data also support the putative involvement of oxidative modification in the atherosclerotic process, although the presence of oxidised chylomicron remnants in vivo and their role in atherogenesis remains to be established.

Chylomicron remnants potentiate phenylephrine-induced contractions of rat aorta by an endothelium-dependent mechanism

The effects of chylomicron remnants on endothelium-dependent contraction of rat aorta were studied in vitro. Chylomicron remnant particles were prepared in vivo from male Wistar rats and were incubated with aortic rings for 45 min before concentration contraction response curves were constructed to phenylephrine. Both native and oxidised chylomicron remnants significantly increased vessel sensitivity to this agonist. Oxidised chylomicron remnants also significantly increased the maximum response. This potentiation was abolished by endothelial removal, but was still evident in the presence of N ω-nitro- l-arginine, with or without cyclo ( d-α-aspartyl- l-prolyl- d-valyl- l-leucyl- d-tryptophyl) (BQ-123), indomethacin or superoxide dismutase. The study demonstrates, for the first time, that lipoprotein particles of dietary origin potentiate vascular contractions. This effect is endothelium-dependent, but is not due to inhibition of basal nitric oxide production or to stimulation of endothelin, superoxide or a cyclo-oxygenase-derived product.

Comparison of short- and long-term effects of different dietary fats on the hepatic uptake and metabolism of chylomicron remnants in rats.

The uptake and metabolism of [14C]oleate-labelled chylomicron remnants derived from olive oil, maize oil, palm oil, fish oil or butter fat was investigated using perfused livers from rats fed on the corresponding fat-supplemented diet (providing 40% of the dietary energy) or a low-fat diet for 21 d. The percentage of added [14C]oleate-labelled remnant removed from the perfusate was similar for livers from rats fed on the fat-supplemented diets irrespective of the type of fat fed, whereas livers from rats fed on the low-fat diet removed more labelled fish oil and butter fat remnants than olive, maize or palm oil remnants. Following hepatic uptake in the fat-supplemented groups, the oxidation of [14C]oleate-labelled remnant lipid from maize oil, fish oil, and butter fat remnants was greater than that of the lipids from olive and palm oil remnants, although only the oxidation of lipids from maize and palm oil remnants was increased by prior fat-supplementation of the diet. In addition, the livers from rats fed on the fish-oil-supplemented diet incorporated more [14C]oleate-labelled remnant lipid into phospholipid compared with the livers from rats fed on the other fat-supplemented diets or the low-fat diets. These investigations show that both prior fat feeding and the composition of the fat fed, as well as the fatty acid composition of the chylomicron remnant particles themselves, influence the uptake and metabolism of chylomicron remnants by the liver.

In vivo gene delivery to the liver using reconstituted chylomicron remnants as a novel nonviral vector.

Lipoproteins are emulsion particles that consist of lipids and apolipoproteins. Their natural function is to transport lipids and/or cholesterol to different tissues. We have taken advantage of the hydrophobic interior of these natural emulsions to solubilize DNA. Negatively charged DNA was first complexed with cationic lipids containing a quaternary amine head group. The resulting hydrophobic complex was extracted by chloroform and then incorporated into reconstituted chylomicron remnant particles ( approximately 100 nm in diameter) with an efficiency approximately 65%. When injected into the portal vein of mice, there were approximately 5 ng of a transgene product (luciferase) produced per mg of liver protein per 100 microg injected DNA. This level of transgene expression was approximately 100-fold higher than that of mice injected with naked DNA. However, such a high expression was not found after tail vein injection. Histochemical examination revealed that a large number of parenchymal cells and other types of cells in the liver expressed the transgene. Gene expression in the liver increased with increasing injected dose, and was nearly saturated with 50 microg DNA. At this dose, the expression was kept at high level in the liver for 2 days and then gradually reduced and almost disappeared by 7 days. However, by additional injection at day 7, gene expression in the liver was completely restored. By injection of plasmid DNA encoding human alpha1-antitrypsin, significant concentrations of hAAT were detected in the serum of injected animals. This is the first nonviral vector that resembles a natural lipoprotein carrier.

Clearance of lipoprotein remnant particles in adipose tissue and muscle in humans

A major proportion of triglycerides in plasma triglyceride-rich lipoproteins (TRL) are removed in peripheral tissues by lipoprotein lipase, and hypothetically a minor proportion can also be removed by whole-lipoprotein particle uptake. This second removal pathway has not previously been directly demonstrated in humans. Simultaneous blood samples were drawn from arterialized blood, a vein draining the subcutaneous abdominal adipose tissue, and a deep antecubital vein of the forearm to provide arterio-venous gradients from blood-draining adipose tissue and muscle in seven male subjects. The men were given a fat-rich mixed meal containing vitamin A and the triglyceride and retinyl palmitate (RP) concentrations were quantified in the plasma. Density gradient ultracentrifugation was used to isolate TRL fractions, in which triglycerides, RP, apoB-48, and apoB-100 were quantified. There was clearance of triglycerides in muscle and adipose tissue and, in addition, removal of RP. By analysis of the TRL subfractions, the RP removal was likely to be confined to the largest chylomicron remnant particles. For the Sf > 400 fraction, the area under curve (AUC) relative to arterial for triglycerides were 79% (66-91%) and 81% (72-89%) in adipose tissue and muscle venous outflow, respectively (each P < 0.02 versus arterial). The corresponding values for RP were 87% (73-101%) and 85% (69-100%), respectively, (each P < 0.05 versus arterial). In the Sf 60-400 fraction there was further uptake of triglycerides, but not of RP. We hypothesize that the periphery could be of importance for removal of the largest chylomicron remnants, as their size might partially exclude them penetrating the fenestrated hepatic sinusoidal endothelium to reach the hepatic chylomicron remnant receptors.

Features of cholesterol structure that regulate the clearance of chylomicron-like lipid emulsions.

Cholesterol is an absolute requirement for the clearance from plasma of the remnants of triglyceride-rich lipoproteins. Our laboratory previously established that cholesterol was essential for the hepatic uptake of remnant particles after intravenous injection of chylomicron-like lipid emulsions (1). The aim of the present study was to determine the structural features of the cholesterol molecule that regulate the metabolism of chylomicrons. Chylomicron-like lipid emulsions, which reflect the size and composition and mimic the physiology of lymph chylomicrons, were prepared with tracer amounts of labeled triolein ([14C]TO) and cholesteryl oleate ([3H]CO) to follow the hydrolysis of triglyceride and the uptake of chylomicron remnant particles by the liver. Sterols selected as cholesterol congeners with functional group variations were incorporated into the emulsions in place of cholesterol and injected intravenously in rats. Control emulsions contained either no cholesterol or approximately 1% (by weight) cholesterol. The effects of the different sterol structures on lipolysis and hepatic remnant uptake were compared with controls to determine the significance of various functional groups. Clearance of emulsion CO was impaired when cholesterol was absent or replaced by cholesteryl chloride, cholesteryl formate, or 3-keto-cholesterol. Clearance of emulsions containing epicholesterol, where the OH group at the 3-position is in the alpha configuration, was similar to control emulsions containing cholesterol. Congeners with an additional hydroxyl group, viz. 7 alpha-hydroxycholesterol, 7 beta-hydroxycholesterol, or 25-hydroxycholesterol, reduced CO clearance. Androstenol, which lacks the side chain at the C17-position, also retarded CO clearance from plasma. In contrast, emulsions incorporating congeners with side chain variations such as campesterol, beta-sitosterol, stigmasterol, or saturated congeners of cholesterol such as cholestanol, coprostanol and its epimer, epicoprostanol, all were cleared similarly to emulsions containing cholesterol. In conclusion, for physiological clearance of a chylomicron-like emulsion, the presence of a hydroxyl (-OH) group at the 3-position and an alkyl side chain at the C17-position of cholesterol are essential, while the structure of the side chain and the saturation of the ring structure are not critical. The mechanism of the specificity of sterols on the metabolism of protein-free emulsions is unclear, but does not relate to changes in microfluidity of the surface lipids, nor to the amount or isoform of associated apolipoproteins.

Remnant-like lipoproteins stimulate whole blood platelet aggregation in vitro

We have developed a simple, rapid assay method to measure remnant- like lipoproteins by using an immunoaffinity gel mixture of anti apo B-100 and apoA-1 antobodies to Sepharose 4B. Characterization of the unbound lipoproteins has shown that they represent chylomicron and VLDL remnant particles (RLP). Preincubation of whole blood with RLP resulted in the enhanced activation of aggregation with ADP and collagen. Such enhancement was not observed in the presence of lipoprotein deficient serum or alubumin preparation. The extent of enhancement was 2.78 times by 7.5 μM of ADP and 44 times by 0.5 μg/ml of collagen in the presence of RLP-prepalation 1 (RLP-1), respectively. In the presence of RLP-2, the enhancement was 5.37 times by 7.5 μM of ADP and 102 times by 0.5 μg/ml of collagen, respectively. On the other hand RLP slightly inhibited PRP aggregation by these agonists. Inhibitions were 19% by 7.5 μM of ADP and 18 % by 1.0 μg/of collagen in the presence of RLP-1, respectively. Incubation of whole blood with RLP did not result in the release of factors to stimulate platelets or ADP- or collagen-induced platelet aggregation in vitro. The extents of enhanced aggregation in whole blood or inhibition in PRP were not correlated with RLP-cholesterol nor RLP-protein concentrations of RLP preparations used. These results may indicate that RLP not only interact with platelets but with erythrocytes or leukocytes. Our findings support the hypothesis that the postprandial increase in remnant lipoproteins is an atherosclerotic risk factor and may be a part of the reasons of thrombotic complications by stimulating platelets in patients with remnant hyperlipoproteinemia.

17 beta-Estradiol improves postprandial lipid metabolism in postmenopausal women.

We studied the effect of 2 mg micronized 17 beta-estradiol replacement therapy, administered orally during 6 weeks, on postprandial lipid and retinyl palmitate (RP) metabolism. In the human postprandial state, atherogenic chylomicron remnant particles are produced. RP is incorporated into the core of newly synthesized chylomicrons and can be used as a marker of chylomicrons and chylomicron remnants. Six normolipidemic (plasma cholesterol, 5.63 +/- 0.83 mmol/L; plasma triglycerides, 1.47 +/- 0.69 mmol/L) postmenopausal women (amenorrhea > 1 yr; aged 55.5 +/- 4.0 yr) received an oral fat load (50 g/m2 fat as cream, with 60,000 IU RP/m2) before and after 6 weeks of 17 beta-estradiol treatment. Plasma RP areas under the curve decreased significantly from 27.1 +/- 15.9 to 16.6 +/- 13.2 mg/h.L-1 (P = 0.01). Fasting cholesterol concentrations in intermediate density lipoproteins decreased significantly. Fasting and postprandial plasma triglyceride levels did not change. These findings indicated that 17 beta-estradiol improved the postprandial elimination of potentially atherogenic lipoprotein remnants.

Triglyceride-rich lipoproteins and atherosclerosis, from fast to feast.

There is increasing evidence that fasting hypertriglyceridaemia identifies individuals who are at increased risk for coronary artery disease. It is probable that this reflects the effects of the smaller triglyceride-bearing lipoproteins. It may also reflect increased levels of postprandial lipoproteins. There is considerable theoretical and experimental evidence to support an atherogenic role for the smaller, triglyceride-rich postprandial lipoproteins. Cross-sectional studies in humans also indicate that those with coronary disease have increased levels of postprandial lipids, probably in the remnants of chylomicrons. Recently we have completed a study examining the progression of coronary atherosclerosis angiographically. The study showed that the progression was linearly related to the numbers of chylomicron remnant particles, as reflected by the quantity of apoB48 in Sf20-60 lipoproteins, in the postprandial circulation. This paper reviews these studies, the physiology of postprandial lipoprotein metabolism and the therapeutic implications of this information.

Interrelationship of triglycerides with lipoproteins and high-density lipoproteins

Triglycerides are transported by the largest and most lipid-rich of the lipoprotein particles, namely, chylomicrons and very low density lipoproteins (VLDL). These particles are buoyant because of the high triglyceride content, which makes up approximately 90% by weight of the chylomicron and 70% by weight of the VLDL. The chylomicron transports exogenous or dietary fat and cholesterol, whereas VLDL transports endogenous triglyceride and cholesterol in lipoproteins synthesized and secreted by the liver. Both chylomicrons and VLDL are hydrolyzed at the capillary surface by the enzyme lipoprotein lipase. Lipoprotein lipase catalyzes the hydrolysis of triglyceride in the lipid core of these particles, producing smaller particles known as remnants. We currently believe the remnants are atherogenic and that this is one reason why hypertriglyceridemia may predispose to coronary artery disease. Chylomicron remnants are recognized and removed by hepatic receptors that contain apolipoprotein (apo) E. The rate of clearance of remnant particles depends on which subfraction of apo E is present. Particles containing apo EII are removed more slowly than those with apo EIII and EIV. The dietary cholesterol from the chylomicron remnant particles is thought to down-regulate the hepatic low-density lipoprotein (LDL) receptors. VLDL remnants, also called intermediate-density lipoprotein (IDL), contain apo E and may be removed by the liver through the LDL or B/E receptor. The decrease in activity of these receptors results in apparent oversynthesis of LDL, the end-product of VLDL and IDL metabolism. LDL is the major cholesterol carrier, followed by high-density lipoprotein (HDL). LDL carries approximately up to three-fourths of the cholesterol in the Mood, and about two-thirds of LDL is removed by specific LDL receptors as it circulates. The other portion is removed by low-affinity pathways. HDL contains approximately 50% cholesterol and phospholipid. HDL is synthesized in the liver, the intestine and also originates from the surface of chylomicrons and VLDL during lipolysis. HDL acquires cholesterol from peripheral tissues and is esterified by the lecithin cholesterol acyltransferase or LCAT to cholesteryl ester. The cholesteryl ester may then be transferred from HDL to LDL, IDL or VLDL by the cholesteryl ester transfer protein. Thus, cholesterol can be transported to the liver indirectly by HDL, and HDL fractions containing apo E may directly transport HDL to the liver. HDL exists in a nascent form before being converted into spherical particles. There is some evidence that apo AI may play a particularly important role in this reverse cholesterol transport process. HDL may be subdivided in several ways: one based on its apoprotein content and one based on its density (e.g., HDL 2 and HDL 3). In hypertriglyceridemia, the formation of HDL from the hydrolysis of triglyceride-rich particles may be diminished. In addition, when postprandial lipemia occurs, or when there is a prolonged elevation of triglyceride-rich lipoproteins, an increased transfer of triglyceride from triglyceride-rich lipoprotein particles into LDL and HDL may occur in exchange for cholesteryl ester. The metabolic changes associated with hypertriglyceridemia may decrease the concentration of HDL and result in small, dense particles of LDL and HDL.

Mechanism of the hypertriglyceridemia induced by tumor necrosis factor administration to rats

4 h after intravenous injection of recombinant HuTNF-α to fed rats, an increase in heart, diaphragm, and plasma lipoprotein lipase activity was observed. At the same time, a 40–60% decrease in enzymic activity in epididymal fat pad and kidney and 40% decrease in hepatic lipase activity in liver had occurred. Similar results were obtained 20 h after injection of recombinant HuTNF-α into fasted rats. Pretreatment with Indomethacin did not affect the changes in tissue lipoprotein lipase activity observed following recombinant HuTNF-α administration. Serum triacylglycerol concentration increased by 2- and 6-fold; 4 and 20 h after recombinant HuTNF-α administration. Disappearance of 14C-labeled triacylglycerol from the circulation after injection of small chylomicrons, biosynthetically labeled in their triacylglycerol and cholesterol moieties, was lower in TNF-treated than in control rats. However, the clearance rate of triacylglycerol was the same or even higher in recombinant HuTNF-α treated rats (assuming that 14C-labeled chylomicron triacylglycerol represents the serum triacylglycerol pool). The livers of recombinant HuTNF-α-treated rats and controls contained similar amounts of 14C-labeled lipids, but less [ 3H]cholesterol, suggesting that in recombinant HuTNF-α-treated rats, the liver took up chylomicron remnant particles enriched with triacylglycerol. Separation of the d < 1.04 g/ml fraction of serum obtained from control and recombinant HuTNF-α treated rats by zonal ultracentrifugation revealed that in recombinant HuTNF-α-treated rats the lipoprotein particles were less lipolyzed than in controls. The secretion rate of [ 3H]triacylglycerol into the serum was determined 90 min after injection of [ 3H]palmitate albumin complex and Triton WR 1339. In recombinant HuTNF-α-treated rats, the secretion of [ 3H]triacylglycerol into plasma was 48% higher than in controls. It is suggested that the increase in lipoprotein lipase activity of heart and diaphragm resulted from an indirect effect of TNF. It is concluded that the increase in serum triacylglycerol in the recombinant HuTNF-α-treated rats is due mainly to an increased secretion of triacylglycerol by the liver. Impaired lipolysis, probably due to a fall in hepatic lipase could also contribute to the rise in plasma triacylglycerol.