Clearance Of Remnant Lipoproteins Research Articles

CREBH normalizes dyslipidemia and halts atherosclerosis in diabetes by decreasing circulating remnant lipoproteins.

Loss-of-function mutations in the transcription factor CREB3L3 (CREBH) associate with severe hypertriglyceridemia in humans. CREBH is believed to lower plasma triglycerides by augmenting the activity of lipoprotein lipase (LPL). However, by using a mouse model of type 1 diabetes mellitus (T1DM), we found that greater liver expression of active CREBH normalized both elevated plasma triglycerides and cholesterol. Residual triglyceride-rich lipoprotein (TRL) remnants were enriched in apolipoprotein E (APOE) and impoverished in APOC3, an apolipoprotein composition indicative of increased hepatic clearance. The underlying mechanism was independent of LPL, as CREBH reduced both triglycerides and cholesterol in LPL-deficient mice. Instead, APOE was critical for CREBH's ability to lower circulating remnant lipoproteins because it failed to reduce TRL cholesterol in Apoe-/- mice. Importantly, individuals with CREB3L3 loss-of-function mutations exhibited increased levels of remnant lipoproteins that were deprived of APOE. Recent evidence suggests that impaired clearance of TRL remnants promotes cardiovascular disease in patients with T1DM. Consistently, we found that hepatic expression of CREBH prevented the progression of diabetes-accelerated atherosclerosis. Our results support the proposal that CREBH acts through an APOE-dependent pathway to increase hepatic clearance of remnant lipoproteins. They also implicate elevated levels of remnants in the pathogenesis of atherosclerosis in T1DM.

Recruiting a transcription factor in the liver to prevent atherosclerosis.

Hypertriglyceridemia is associated with obesity, diabetes, and atherosclerosis. While lipoprotein lipase (LPL) hydrolyzes triglyceride (TG) cargo into remnant lipoproteins with atherogenic properties, how remnant lipoprotein clearance relates to atherosclerosis in people with diabetes remains unclear. In this issue of the JCI, Shimizu-Albergine et al. examined the effects of the basic leucine zipper transcription factor CREBH, which induces genes that activate LPL in mouse models of type I diabetes. Overexpression of a CREBH fragment reduced apolipoprotein C3 (APOC3) levels, which reduced plasma TGs. Notably, the TGs were lowered by a mechanism that was independent of LPL, and atherosclerosis was alleviated by enhanced lipoprotein remnant clearance as opposed to increased lipolysis of TG-rich lipoprotein precursors. A proinflammatory mechanism likely underlies the atherogenicity of remnant lipoproteins. These findings suggest that modifying CREBH expression in the liver may ameliorate atherosclerosis and, perhaps, other diabetes complications.

Triglyceride-rich lipoproteins and their remnants: metabolic insights, role in atherosclerotic cardiovascular disease, and emerging therapeutic strategies-a consensus statement from the European Atherosclerosis Society.

Recent advances in human genetics, together with a large body of epidemiologic, preclinical, and clinical trial results, provide strong support for a causal association between triglycerides (TG), TG-rich lipoproteins (TRL), and TRL remnants, and increased risk of myocardial infarction, ischaemic stroke, and aortic valve stenosis. These data also indicate that TRL and their remnants may contribute significantly to residual cardiovascular risk in patients on optimized low-density lipoprotein (LDL)-lowering therapy. This statement critically appraises current understanding of the structure, function, and metabolism of TRL, and their pathophysiological role in atherosclerotic cardiovascular disease (ASCVD). Key points are (i) a working definition of normo- and hypertriglyceridaemic states and their relation to risk of ASCVD, (ii) a conceptual framework for the generation of remnants due to dysregulation of TRL production, lipolysis, and remodelling, as well as clearance of remnant lipoproteins from the circulation, (iii) the pleiotropic proatherogenic actions of TRL and remnants at the arterial wall, (iv) challenges in defining, quantitating, and assessing the atherogenic properties of remnant particles, and (v) exploration of the relative atherogenicity of TRL and remnants compared to LDL. Assessment of these issues provides a foundation for evaluating approaches to effectively reduce levels of TRL and remnants by targeting either production, lipolysis, or hepatic clearance, or a combination of these mechanisms. This consensus statement updates current understanding in an integrated manner, thereby providing a platform for new therapeutic paradigms targeting TRL and their remnants, with the aim of reducing the risk of ASCVD.

The Importance of Lipoprotein Lipase Regulation in Atherosclerosis.

Lipoprotein lipase (LPL) plays a major role in the lipid homeostasis mainly by mediating the intravascular lipolysis of triglyceride rich lipoproteins. Impaired LPL activity leads to the accumulation of chylomicrons and very low-density lipoproteins (VLDL) in plasma, resulting in hypertriglyceridemia. While low-density lipoprotein cholesterol (LDL-C) is recognized as a primary risk factor for atherosclerosis, hypertriglyceridemia has been shown to be an independent risk factor for cardiovascular disease (CVD) and a residual risk factor in atherosclerosis development. In this review, we focus on the lipolysis machinery and discuss the potential role of triglycerides, remnant particles, and lipolysis mediators in the onset and progression of atherosclerotic cardiovascular disease (ASCVD). This review details a number of important factors involved in the maturation and transportation of LPL to the capillaries, where the triglycerides are hydrolyzed, generating remnant lipoproteins. Moreover, LPL and other factors involved in intravascular lipolysis are also reported to impact the clearance of remnant lipoproteins from plasma and promote lipoprotein retention in capillaries. Apolipoproteins (Apo) and angiopoietin-like proteins (ANGPTLs) play a crucial role in regulating LPL activity and recent insights into LPL regulation may elucidate new pharmacological means to address the challenge of hypertriglyceridemia in atherosclerosis development.

Triglycerides, hypertension, and smoking predict cardiovascular disease in dysbetalipoproteinemia

Dysbetalipoproteinemia (DBL) is an autosomal recessive lipid disorder associated with a reduced clearance of remnant lipoproteins and is associated with an increased cardiovascular disease (CVD) risk. The genetic cause of DBL is apoE2 homozygosity in 90% of cases. However, a second metabolic hit must be present to precipitate the disease. However, no study has investigated the predictors of CVD, peripheral artery disease and coronary artery disease in a large cohort of patients with DBL. The objectives of this study were to describe the clinical characteristics of a DBL cohort and to identify the predictors of CVD, peripheral artery disease, and coronary artery disease in this population. The inclusion criteria included age ≥ 18years, apoE2/E2, triglycerides (TG)>135mg/dL and VLDL-C/plasma TG ratio>0.30. We studied 221 adult DBL patients, of which 51 (23%) had a history of CVD. We identified 3 independent predictors of CVD, namely hypertension (OR 5.68, 95% CI 2.13-15.16, P=.001), pack year of smoking (OR 1.03, 95% CI 1.01-1.05, P=.01) and TG tertile (OR 1.82, 95% CI 1.09-3.05, P=.02). The CVD prevalence was 51% in patients with hypertension and 18% in those without hypertension (P=.00001), and 30% in the highest TG tertile vs 15% in the lowest tertile (P=.04). Similarly, the CVD prevalence was higher in heavy smokers compared with nonsmokers (36% vs 13%, P=.006). Hypertension, smoking, and TG are independently associated with CVD risk in patients with DBL. Aggressive treatment should be initiated in patients with DBL because of the increased risk of CVD.

Total Cholesterol as an Indicator for Severity of Liver Disease

Introduction: Alcohol and hepatitis-C induced cirrhosis continue to be a highly prevalent problem in our patient population. A retrospective study was subsequently conducted to determine the correlation between the severity of liver disease and total cholesterol in patients with alcohol and/or hepatitis-C induced cirrhosis. Methods: In total, 237 patients above the age of 18 with a diagnosis of cirrhosis secondary to hepatitis C and/or alcohol use were included in the study. The included patients were either admitted to the hospital or seen in the clinic between January 2012 and December 2014. Patients actively taking statins, diagnosed with cholestasis or primary biliary cirrhosis were excluded from the study. Results: Linear regression analysis was performed to elucidate the correlation between total serum cholesterol and each of the validated liver disease severity scores (MELD and MELD-Na scores). There was a statistically significant inverse correlation (P < 0.0001) between serum total cholesterol and both these scores (Figures 1 and 2). Notably, this study also demonstrated a direct correlation between serum sodium levels and serum total cholesterol levels (P= 0.0020) (Figure 3).Figure 1Figure 2Figure 3Conclusion: Severe intrinsic hepatic dysfunction leads to decreased total cholesterol levels. The mechanisms of dysfunctional lipid metabolism in cirrhosis is not yet fully understood, however it has been shown that lipoproteins and plasma lipids tend to decrease with hepatocellular injury. It is thought that fundamental metabolic defects are related to the alteration in activity of enzymes involved in lipid metabolism including lecithin-cholesterol acyltransferase (LCAT) and apoprotein activator deficiencies. Impaired hepatic triglyceride lipase (H-TGL) and lipoprotein lipase (LPL) activity; defective remnant lipoprotein clearance by the liver have also been implicated. It is likely that the mechanisms for deranged lipid metabolism in cirrhosis from alcoholic, non-alcoholic and viral causes involve a complex interplay between enzyme dysregulation, disease-specific hepatic dysfunction, and host-specific factors. The direct correlation between serum sodium levels and serum total cholesterol levels alludes to the pathophysiology of advanced intrinsic hepatic dysfunction where there is splanchnic vasodilatation with arterial hypotension, decreased intravascular volume with impairment of renal function and subsequent water and sodium retention leading to dilutional hyponatremia.

Apolipoprotein A5: Extracellular and Intracellular Roles in Triglyceride Metabolism.

This review addresses two major functions of apolipoprotein (apo) A5 including (1) its role in maintaining normal plasma levels of circulating triglyceride (TG) and (2) its role as a component of hepatic lipid droplets. ApoA5 is synthesized solely in the liver and circulating concentrations are extremely low. In the plasma, ApoA5 associates with TG-rich lipoproteins and enhances TG hydrolysis and remnant lipoprotein clearance. ApoA5 loss-of-function single nucleotide polymorphisms are associated with reduced lipolysis, poor remnant clearance and concomitantly, hypertriglyceridemia. Although there have been substantial breakthroughs in understanding pathophysiology associated with secreted ApoA5, there is a paucity of knowledge on the functionality of intracellular ApoA5. However, recent studies indicate that overexpression of intracellular ApoA5 is positively associated with accumulation of TG-rich lipid droplets in hepatocytes. It is thought that ApoA5 may have a causal role in non-alcoholic fatty liver disease (NAFLD) and thus, may serve as a target for developing therapeutics for NAFLD.

Decreased Expression of Hepatic Low-Density Lipoprotein Receptor–Related Protein 1 in Hypothyroidism: A Novel Mechanism of Atherogenic Dyslipidemia in Hypothyroidism

The atherogenic effects of hypothyroidism on lipid metabolism could result, in part, from the reduced clearance of remnant lipoproteins. In this study, we investigated the expression of hepatic low-density lipoprotein receptor-related protein 1 (LRP1), a receptor for remnant lipoproteins, in hypothyroidism and the effect of 3,3',5-triiodo-L-thyronine (T3) treatment on hepatic LRP1 expression. C57BL/6 mice were fed a normal diet (control group) or a low-iodine diet supplemented with 0.15% propylthiouracil (PTU/LI group) for 4 weeks. Mice in the PTU/LI group were injected intraperitoneally with T3 (0, 30, and 150 μg/kg of body weight) for 7 days. HepG2 cells were incubated in fetal bovine serum or charcoal-stripped fetal bovine serum with various concentrations of T3. The expression and function of LRP1 in liver samples and cells were analyzed. Hypothyroidism was successfully induced in PTU/LI mice. Hepatic LRP1 protein expression was lower in the PTU/LI group than in the control group. T3 treatment upregulated hepatic LRP1 protein expression in PTU/LI mice. LRP1 expression in HepG2 cells was reduced after incubation in the medium containing charcoal-stripped fetal bovine serum, which mimics hypothyroidism in vitro, and was recovered by T3 treatment. The protein expression of LRP1 in HepG2 cells was increased by T3 treatment in a dose-dependent manner up to 2.0 nM T3. However, LRP1 mRNA transcription was not affected by hypothyroidism conditions or T3 treatment, either in liver samples or in HepG2 cells. T3 treatment on HepG2 cells increased cellular uptake of lipid-conjugated apolipoprotein E through LRP1. Our data demonstrate that hepatic LRP1 expression and function decrease in hypothyroidism and are regulated by the thyroid hormone. These results suggest that in hypothyroidism, decreased expression of hepatic LRP1 may be associated with reduced clearance of circulating remnant lipoproteins.

Hepatic Remnant Lipoprotein Clearance by Heparan Sulfate Proteoglycans and Low-Density Lipoprotein Receptors Depend on Dietary Conditions in Mice

Chylomicron and very low-density lipoprotein remnants are cleared from the circulation in the liver by heparan sulfate proteoglycan (HSPG) receptors (syndecan-1), the low-density lipoprotein receptor (LDLR), and LDLR-related protein-1 (LRP1), but the relative contribution of each class of receptors under different dietary conditions remains unclear. Triglyceride-rich lipoprotein clearance was measured in AlbCre(+)Ndst1(f/f), Ldlr(-/-), and AlbCre(+)Lrp1(f/f) mice and mice containing combinations of these mutations. Triglyceride measurements in single and double mutant mice showed that HSPGs and LDLR dominate clearance under fasting conditions and postprandial conditions, but LRP1 contributes significantly when LDLR is absent. Mice lacking hepatic expression of all 3 receptors (AlbCre(+)Ndst1(f/f) Lrp1(f/f) Ldlr(-/-)) displayed dramatic hyperlipidemia (870 ± 270 mg triglyceride/dL; 1300 ± 350 mg of total cholesterol/dL) and exhibited persistent elevated postprandial triglyceride levels because of reduced hepatic clearance. Analysis of the particles accumulating in mutants showed that HSPGs preferentially clear a subset of small triglyceride-rich lipoproteins (≈ 20-40 nm diameter), whereas LDLR and LRP1 clear larger particles (≈ 40-60 nm diameter). Finally, we show that HSPGs play a major role in clearance of triglyceride-rich lipoproteins in mice fed normal chow or under postprandial conditions but seem to play a less significant role on a high-fat diet. These data show that HSPGs, LDLR, and LRP1 clear distinct subsets of particles, that HSPGs work independently of LDLR and LRP1, and that HSPGs, LDLR, and LRP1 are the 3 major hepatic triglyceride-rich lipoprotein clearance receptors in mice.

Hepatic lipase activity is decreased in Japanese patients with type III hyperlipoproteinemia

BackgroundType III hyperlipoproteinemia (HLP), a disorder associated with a high incidence of premature cardiovascular diseases, is characterized by the accumulation of remnant lipoproteins in the plasma. The primary genetic defect in patients with type III HLP is the presence of apolipoprotein E2 (apoE2), an isoform of apoE, and accumulation of remnant lipoproteins in the plasma has been thought to be attributable to the presence of apoE2, which bind poorly to low density lipoprotein receptors, resulting in defective remnant lipoprotein clearance. On the other hand, the activity of hepatic lipase (HL), the enzyme that plays a pivotal role in the removal of remnant lipoproteins, in type III HLP has not been investigated. MethodsWe examined post-heparin plasma lipoprotein lipase (LPL) and HL activities in 7 patients with type III HLP. The activities of HL and LPL in post-heparin plasma were measured separately by an immunochemical method using antiserum specifically directed against HL. ResultsThe post-heparin plasma HL activity was significantly reduced, while the LPL activity was normal. ConclusionsReduced HL activity may account, at least in part, for the accumulation of remnant lipoproteins in the plasma, a characteristic feature of type III HLP.

Presence of Apolipoprotein C‐III Attenuates Apolipoprotein E‐Mediated Cellular Uptake of Cholesterol‐Containing Lipid Particles by HepG2 Cells

Apolipoprotein C-III (apoC-III) decreases the apolipoprotein E (apoE)-mediated uptake of lipoprotein remnants by the liver, and a high plasma concentration of apoC-III in VLDL is associated with hypertriglyceridemia and the risk of coronary heart disease. In this study, we prepared lipid emulsions containing triolein, phosphatidylcholine and cholesterol as model particles of lipoproteins, and examined the roles of apoC-III in apoE-mediated uptake of emulsions by HepG2 cells. Cholesterol in emulsion particles enhanced the apoE-mediated uptake via heparan sulfate proteoglycan and LDL receptor-related protein pathways. The amount of apoE bound to emulsion particles was increased by the presence of cholesterol at the particle surface, whereas cholesterol had no effect on the binding amount of apoC-III. Surface cholesterol alleviated the inhibitory effect of apoC-III on apoE incorporation into the emulsion surface. However, ApoC-III almost completely inhibited the apoE-mediated uptake of cholesterol-containing emulsions despite sufficient binding of apoE to emulsions. These findings suggest that apoC-III attenuates the binding of apoE to the lipoprotein surface and apoE-mediated cellular uptake of lipoprotein remnants. Furthermore, cholesterol may affect these functions of apoC-III and apoE involved in the clearance of lipoprotein remnants.

Glycosylation and Sialylation of Macrophage-derived Human Apolipoprotein E Analyzed by SDS-PAGE and Mass Spectrometry: EVIDENCE FOR A NOVEL SITE OF GLYCOSYLATION ON SER290

Apolipoprotein E (apoE) is a 34-kDa glycoprotein secreted from various cells including hepatocytes and macrophages and plays an important role in remnant lipoprotein clearance, immune responses, Alzheimer disease, and atherosclerosis. Cellular apoE and plasma apoE exist as multiple glycosylated and sialylated glycoforms with plasma apoE being less glycosylated/sialylated than cell-derived apoE. Some of the glycan structures on plasma apoE are characterized; however, the more complicated structures on plasma and cellular/secreted apoE remain unidentified. We investigated glycosylation and sialylation of cellular and secreted apoE from primary human macrophages by one- and two-dimensional gel electrophoresis and mass spectrometry. Our results identify eight different glycoforms with (HexNAc)(2)-Hex(2)-(NeuAc)(2) being the most complex glycan detected on Thr(194) in both cellular and secreted apoE. Four additional glycans were identified on apoE(283-299), and using beta-elimination/alkylation by methylamine in vitro, we identified Ser(290) as a novel site of glycan attachment. Comparison of plasma and cellular/secreted apoE from the same donor confirmed that cell-derived apoE is more extensively sialylated than plasma apoE. Given the importance of the C terminus of apoE in regulating apoE solubility, stability, and lipid binding, these results may have important implications for our understanding of apoE biochemistry.

MS407 REDUCED MYOCARDIAL GLUCOSE UTILIZATION AFTER FASTING AND HEPARIN INJECTION IN RATS

Type III hyperlipoproteinemia (HLP), a disorder associated with a high incidence of premature cardiovascular diseases, is characterized by the accumulation of remnant lipoproteins in the plasma. The primary genetic defect in patients with type III HLP is the presence of apolipoprotein E2 (apoE2), an isoform of apoE, and accumulation of remnant lipoproteins in the plasma has been thought to be attributable to the presence of apoE2, which bind poorly to low density lipoprotein receptors, resulting in defective remnant lipoprotein clearance. On the other hand, the activity of hepatic lipase (HL), the enzyme that plays a pivotal role in the removal of remnant lipoproteins, in type III HLP has not been investigated.We examined post-heparin plasma lipoprotein lipase (LPL) and HL activities in 7 patients with type III HLP. The activities of HL and LPL in post-heparin plasma were measured separately by an immunochemical method using antiserum specifically directed against HL.The post-heparin plasma HL activity was significantly reduced, while the LPL activity was normal.Reduced HL activity may account, at least in part, for the accumulation of remnant lipoproteins in the plasma, a characteristic feature of type III HLP.

Apolipoprotein E: structure determines function, from atherosclerosis to Alzheimer's disease to AIDS

Apolipoprotein (apo) E has roles beyond lipoprotein metabolism. The detrimental effects of apoE4 in cardiovascular, neurological, and infectious diseases correlate with its structural features (e.g., domain interaction) that distinguish it from apoE3 and apoE2. Structure/function studies revealed that apoE2 is severely defective in LDL receptor binding because of a structural difference that alters the receptor binding region and helped unravel the mechanism of type III hyperlipoproteinemia. ApoE4 is the major genetic risk factor for Alzheimer's disease and sets the stage for neuropathological disorders precipitated by genetic, metabolic, and environmental stressors. ApoE also influences susceptibility to parasitic, bacterial, and viral infections. In HIV-positive patients, apoE4 homozygosity hastens progression to AIDS and death and increases susceptibility to opportunistic infections. The next phase in our understanding of apoE will be characterized by clinical intervention to prevent or reverse the detrimental effects of apoE4 by modulating its structure or blocking the pathological processes it mediates.

Apolipoprotein AV: gene expression,physiological role in lipid metabolism and clinical relevance

The apolipoprotein APOA5 gene, a member of the gene cluster on chromosome 11q23 that includes APOA1, APOC3 and APOA4, has gained considerable interest as it encodes ApoAV, a key determinant of circulating levels of potentially atherogenic triglyceride-rich lipoproteins (TRL). Indeed, strong associations between genetic variants of the APOA5 gene sequence and elevated triglyceride (TG) levels have been established. This apolipoprotein may potentiate lipolysis of TRL through facilitation of lipoprotein interaction with lipoprotein lipase. In addition, ApoAV may enhance clearance of remnant lipoproteins by mediating their interaction with the LDL receptor-related protein (LRP)1. The implication of ApoAV in intravascular TRL metabolism is further supported by studies that have demonstrated upregulation of APOA5 gene expression by nuclear receptors (PPARα, FXR and HNF4α) and hormones (thyroxine) involved in hypotriglyceridemic pathways. APOA4 expression may equally be modulated by nutritional status and, more ...

Atherogenic remnant lipoproteins: role for proteoglycans in trapping, transferring, and internalizing

Unraveling the mechanisms controlling remnant lipoprotein clearance is important, as these lipoproteins are highly atherogenic. The most critical molecule in this process is apoE, which mediates high-affinity binding of remnant lipoproteins to members of the LDL receptor (LDLR) family and cell-surface heparan sulfate proteoglycans (HSPGs), which have been shown to play major independent as well as cooperative roles in remnant lipoprotein clearance. While all the players may have been identified, our understanding of how they interact and function together continues to evolve. In this issue of the JCI, MacArthur et al. (see the related article beginning on page 153) demonstrated that HSPGs under normal physiological conditions are critically important in the clearance of remnant lipoproteins, independent of LDLR family members. The complexity of VLDL and chylomicron remnant clearance was exemplified by the studies of Jones et al., also in this issue (see the related article beginning on page 165). Despite defective clearance of LDL in mice with a deficiency in the adaptor protein controlling internalization of the LDLR, called autosomal recessive hypercholesterolemia (ARH), remnant lipoprotein clearance was not grossly abnormal. A likely explanation is that the abnormal LDLRs bind the remnants and then transfer them to another acceptor for internalization. While the studies clearly demonstrate that the LDLR-related protein 1 is not involved and suggest a role for an additional unidentified receptor, it remains a possibility that HSPGs are responsible for remnant uptake by hepatocytes in the presence of defective LDLR internalization.

Influence of Aging and Menopause on Postprandial Lipoprotein Responses in Healthy Adult Women

To analyze the influence of menopause and age on postprandial lipoprotein responses in healthy adult women. Twenty-seven healthy young and middle-aged pre- and postmenopausal female volunteers aged 21-53 y were enrolled. They ingested OFTT cream(Jomo, Takasaki, Japan). Fasting and postprandial blood samples were obtained for up to 6 h, and serum concentrations of lipoproteins were analyzed. In the postprandial phase, serum triglycerides(TG), remnant-like particle(RLP)-TG(RLP-TG), RLP-cholesterol(RLP-C), and TG-rich lipoprotein-TG(TRL-TG)concentrations in all groups peaked after 2 h. After 4 h, the TG, RLP-C, RLP-TG and TRL-TG concentrations in the young women returned to the fasting concentrations. However, at 6 h, these parameters in the pre- and postmenopausal women had barely returned to the fasting concentrations. The present results suggest that:(1)the magnitude of postprandial TG concentrations is dependent on age, but not on menopause;(2)clearance of remnant lipoproteins is delayed with age in pre- and postmenopausal women compared to young women, and(3)menopause is associated with an increase of RLP-C, but may not influence LDL particle size.

Remnant Lipoprotein Density Profiling by CsBiEDTA Density Gradient Ultracentrifugation

Remnant lipoproteins (RLPs) are now considered a strong marker of the triglyceride-rich lipoprotein (TRL) class for cardiovascular heart disease. The purpose of this research is to demonstrate the efficacy of a novel method that combines an established immunoseparation assay used to measure the RLP class in human serum with ultracentrifugal density gradient separation. These two methods are combined to obtain an RLP density profile. The immunoseparation effectively removes the non-RLP lipoproteins from serum. The RLPs obtained from the immunoseparation are separated into two density-distinct fractions by ultracentrifugal density gradient separation in CsBiEDTA. It is now clear that IDL is distinct in density and immunoreactivity from the two RLP classes isolated by the immunoseparation and ultracentrifugation. This methodology defines the RLP by density and measures their relative prevalence in the TRL class. When applied to clinical samples, variations in the RLP subclasses in different patients are examined. The differences in the RLP density profile are also examined in fasting and postprandial samples. The RLP density profile significantly increases in the postprandial state versus the fasting state. However, the overall quantity of TRL does not appreciably increase in the postprandial state. This work demonstrates the feasibility of measuring the postprandial state in clinical samples to provide insight into the clearance of RLP by the liver as well as the general atherogenicity of these particles. The major outcome of this research is a novel analytical method that couples immunoseparation and density gradient ultracentrifugation to separate and differentially profile the RLP subclass against its nascent counterparts in the TRL class.

Interplay Between Apolipoprotein E and Scavenger Receptor Class B Type I Controls Coronary Atherosclerosis and Lifespan in the Mouse

The mouse has become a standard model for the study of atherosclerosis induced by extreme dyslipidemia in genetically modified strains such as the apolipoprotein E (apoE)–deficient or the LDL-receptor–deficient mouse. Although the process of plaque formation appears to repeat the fundamental steps of the human disease, it has been notoriously difficult to reproduce in the mouse the clinical consequences of atherosclerosis, such as myocardial infarction and stroke, that so commonly in patients signal plaque rupture or total lumen occlusion.1 The apoE-deficient mouse is a particularly interesting model because of its susceptibility to atherosclerotic lesions spanning from the aortic sinus to the abdominal aorta and involving the brachiocephalic and carotid arteries as well.2,3 It has recently been reported that the brachiocephalic plaque in the apoE-deficient mouse is prone to rupture on feeding a high-fat diet for periods as short as 8 weeks,4 but even in this setting of exaggerated hypercholesterolemia, the aortic sinus and coronary tree do not appear to be susceptible to plaque rupture. Scavenger receptor class B type I (SR-BI) is a scavenger-type receptor expressed by many tissues and accepted by consensus to represent the docking receptor for HDL in the liver. SR-BI-deficient mice have raised HDL levels and increased atherosclerosis susceptibility on fat feeding.5 ApoE and SR-BI are both expressed, among other cells, by the hepatocyte and the macrophage. ApoE is an efficient ligand for receptor-mediated lipoprotein removal by the liver, but it is also a strong acceptor of cellular cholesterol. As such, it may control the rate of foam cell formation in the vessel wall. Similarly, SR-BI channels cholesterol in and out of the cell, and its abundant expression by the macrophage may regulate cholesterol homeostasis in arterial foam cells. Although apoE and SR-BI play crucial roles in lipoprotein metabolism and have been …

Apolipoprotein E-induced cell signaling in the vessel wall.

Apolipoprotein E (apoE) is a 34-kDa glycoprotein that circulates in plasma in association with various lipoproteins, including chylomicrons, chylomicron remnants, very low density lipoproteins, and a cholesterol-rich subclass of high density lipoproteins. In contrast to most other apolipoproteins, apoE is synthesized in many tissues, including the liver, brain, kidney, spleen, adrenal glands, and in macrophages and vascular smooth muscle cells. Normal apoE functions are necessary to protect against vascular occlusive disease. Its most well-known function is in lipid redistribution between different cells, particularly between cells in hepatic and extrahepatic tissues. Hepatic synthesis of apoE promotes VLDL secretion [1], thereby promoting lipid distribution to peripheral cells. The apoE synthesized in the liver is also important in mediating hepatic clearance of cholesterol-rich lipoproteins and preventing their accumulation in plasma circulation. The process of apoE mediated lipoprotein clearance by the liver involves the participation of apoE associated with circulating lipoproteins as well as apoE secreted by the cells and retained on the cell surface through its binding to cell surface receptors and matrix proteins. Lipoproteins containing apoE can be taken up by the liver through its direct interaction with either the LDL receptor or the LDL receptor related protein-1 (LRP-1). ApoE-mediated hepatic lipoprotein clearance can also occur through a twostep process involving locally-derived apoE, LRP-1, and heparan sulfate proteoglycans (HSPG). The first step of this latter process is the secretion of cell-derived apoE and its retention on the cell surface through interaction with HSPG. The cell surface apoE is responsible for capturing circulating remnant lipoproteins and promoting their internalization through enhanced interaction with LRP1 and HSPG (Please see recent reviews in refs. [2,3]). Recent studies revealed that hepatic-derived apoE is more efficient than non-hepatic-derived apoE in mediating remnant lipoprotein clearance [4], suggesting the importance of locally-derived apoE in this process.