The composition, structural properties and binding of very-low-density and low-density lipoproteins to the LDL receptor in normo- and hypertriglyceridemia: relation to the apolipoprotein E phenotype

VLDL receptor gene therapy for reducing atherogenic lipoproteins

Over the past 40 years, there has been considerable research into the management and treatment of atherogenic lipid disorders. Although the majority of treatments and management strategies for cardiovascular disease (CVD) center around targeting low-density lipoprotein cholesterol (LDL-C), there is mounting evidence for the residual CVD risk attributed to high triglyceride (TG) and lipoprotein(a) (Lp(a)) levels despite the presence of lowered LDL-C levels. Among the biological mechanisms for clearing TG-rich lipoproteins, the VLDL receptor (VLDLR) plays a key role in the trafficking and metabolism of lipoprotein particles in multiple tissues, but it is not ordinarily expressed in the liver. Since VLDLR is capable of binding and internalizing apoE-containing TG-rich lipoproteins as well as Lp(a), hepatic VLDLR expression has the potential for promoting clearance of these atherogenic particles from the circulation and managing the residual CVD risk not addressed by current lipid lowering therapies. This review provides an overview of VLDLR function and the potential for developing a genetic medicine based on liver-targeted VLDLR gene expression.

Different Pathways of Cellular Cholesterol Efflux

Cholesterol efflux is the first and rate-limiting step of reverse cholesterol transport (RCT) from peripheric cells to the liver. The involvement of high-density lipoprotein (HDL) in RCT determines the atheroprotective properties of HDL. Cholesterol efflux from different membrane pools includes both passive and energy-dependent processes. The first type of route consists of cholesterol desorption from the cell membrane into the unstirred layer adjacent to the cell surface and diffusion in the water phase. Moreover, the selective uptake and facilitated diffusion of cholesterol and cholesteryl ester molecules through the hydrophobic tunnel in the scavenger receptor BI molecule does not require energy consumption. The second type of route includes active cholesterol export by the ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1). Several cholesterol acceptors specifically bind cholesterol and phospholipid molecules, and cholesterol binding to the albumin molecule, which acts as a shuttle, significantly increases cholesterol movement between acceptors and red blood cells, thus functioning as a sink for cholesterol. Cholesterol and phospholipid molecules effluxed from macrophages by ABCA1 are accepted exclusively by the lipid-free apolipoprotein apoA-I, which is the major protein moiety of HDL, whereas those effluxed by ABCG1 are accepted by HDL. ABCA1- and ABCG1-mediated cholesterol transport, together with cholesterol diffusion, largely determine cholesterol turnover at the physiological level of intracellular cholesterol. However, at cholesterol overload, ABCA1-mediated efflux prevails over other routes. The exchange of apoA-I between lipid-free and lipid-associated states and the synergism of nascent and mature HDL contribute to cholesterol efflux efficiency. Moreover, extracellular cholesterol deposits and microvesicles may be involved in RCT.

Interactions of different lipoproteins with supported phospholipid raft membrane (SPRM) patterns to understand similar in-vivo processes

To better understand how lipoproteins interact and enter endothelium and participate in cellular processes, we investigated preferential lipid partitioning of triglyceride rich lipoproteins (TGRL), chylomicrons (CM), low density lipoproteins (LDL), very low density lipoproteins (VLDL) and their lipolysis products using supported phospholipid raft membrane (SPRM) patterns. We prepared SPRM patterns with Texas red labeled phospholipid patterns and Marina blue labeled raft patterns and added Atto-520 labeled lipoproteins (TGRL, CM, VLDL, LDL) and their lipolysis products in separate experiments and characterized these interactions using fluorescence microscopy. We observed that VLDL and LDL preferentially interacted with raft patterns. In contrast the TGRL and lipolysed products of TGRL interacted with both the patterns, slightly elevated preference for raft patterns and CM and its lipolysis products showed greater affinity to phospholipid patterns. The clear preference of VLDL and LDL for raft patterns suggests that these lipoproteins associate with cholesterol and sphingomyelin rich lipid micro-domains during their early interactions with endothelial cells, leading to atherosclerosis.

Description of a large family with autosomal dominant hypercholesterolemia associated with the APOE p.Leu167del mutation

Apolipoprotein (apo) E mutants are associated with type III hyperlipoproteinemia characterized by high cholesterol and triglycerides levels. Autosomal dominant hypercholesterolemia (ADH), due to the mutations in the LDLR, APOB, or PCSK9 genes, is characterized by an isolated elevation of cholesterol due to the high levels of low-density lipoproteins (LDLs). We now report an exceptionally large family including 14 members with ADH. Through genome-wide mapping, analysis of regional/functional candidate genes, and whole exome sequencing, we identified a mutation in the APOE gene, c.500_502delTCC/p.Leu167del, previously reported associated with sea-blue histiocytosis and familial combined hyperlipidemia. We confirmed the involvement of the APOE p.Leu167del in ADH, with (1) a predicted destabilization of an alpha-helix in the binding domain, (2) a decreased apo E level in LDLs, and (3) a decreased catabolism of LDLs. Our results show that mutations in the APOE gene can be associated with bona fide ADH.

Apolipoprotein E genotype as a most significant predictor of lipid response at lipid-lowering therapy: mechanistic and clinical studies

APOE alleles and apolipoprotein E isoforms control plasma cholesterol level on population level. Among three ɛ2, ɛ3, ɛ4 alleles, ɛ4 allele is associated with the increase in cholesterol level, risk of atherosclerosis and Alzheimer disease, while ɛ2 allele is associated with the decrease in cholesterol level and risk of atherosclerosis. The increase in plasma triglyceride is an independent risk factor of atherosclerosis and triglyceride-high density lipoprotein coupling determines the efficiency of reverse cholesterol transport. The impairment of this coupling specifically at hypertriglyceridemia may be followed by specific lipoprotein markers. The influence of major lipid-lowering drugs on lipoprotein metabolism and association of apoE isoforms with the efficiency of therapy by statins and fibrates are summarized both at isolated and combined increase in plasma triglyceride and cholesterol. APOE polymorphism seems to be a single genetic variant with a confirmed stratification both at candidate gene and at wide genome analyses.

Composition, structure and substrate properties of reconstituted discoidal HDL with apolipoprotein A-I and cholesteryl ester

To investigate the influence of lipid unsaturation and neutral lipid on the maturation of high density lipoproteins, the discoidal complexes of apoA-I, phosphatidylcholine and cholesteryl ester (CE) were prepared. Saturated dipalmitoylphosphatidylcholine (DPPC) and unsaturated palmitoyllinoleoylphosphatidylcholine (PLPC), palmitoyloleoylphosphatidylcholine (POPC), and fluorescent probe cholesteryl 1-pyrenedecanoate (CPD) that forms in a diffusion- and concentration-dependent manner short-lived dimer of unexcited and excited molecules (excimer) were used. The apoA-I/DPPC/CPD complexes were heterogeneous by size, composition and probe location. CPD molecules incorporated more efficiently into larger complexes and accumulated in a central part of the discs. The apoA-I/POPC(PLPC)/CPD were also heterogeneous, however, probe molecules distributed preferentially into smaller complexes and accumulated at disc periphery. The kinetics of CPD transfer by recombinant cholesteryl ester transfer protein (CETP) to human plasma LDL is well described by two-exponential decay, the fast component with a shorter transfer time being more populated in PLPC compared to DPPC complexes. The presence of CE molecules in discoidal HDL results in particle heterogeneity. ApoA-I influences the CETP activity modulating the properties of apolipoprotein-phospholipid interface. This may include CE molecules accumulation in the boundary lipid in unsaturated phosphatidylcholine and cluster formation in the bulk bilayer in saturated phosphatidylcholine.

A 12-Week, Prospective, Open-Label Analysis of the Effect of Rosuvastatin on Triglyceride-Rich Lipoprotein Metabolism in Patients with Primary Dyslipidemia

BACKGROUND

Although the effect of statins on lowering low-density lipoprotein cholesterol (LDL-C) has been extensively studied, their hypotriglyceridemic capacity is not fully understood.

OBJECTIVE

The present study examined clinical and laboratory factors potentially associated with the triglyceride (TG)-lowering effect of rosuvastatin.

METHODS

Eligible patients had primary dyslipidemia and a moderate risk of heart disease. Patients were prescribed rosuvastatin 10 mg/d in an open-label fashion and kept 3-day food diaries. Laboratory measurements, performed at baseline and 12 weeks, included serum lipid parameters (total cholesterol [TC], TGs, LDL-C, high-density lipoprotein cholesterol [HDL-C], and apolipoprotein [apo] levels), non-lipid metabolic variables (including carbohydrate metabolism parameters and renal, liver, and thyroid function tests), and LDL-subfraction profile (by high-resolution 3% polyacrylamide gel electrophoresis). Tolerability was assessed at each visit.

RESULTS

Participants were 75 hyperlipidemic patients (39 men and 36 women; mean age, 51.7 years). At 12 weeks, TC levels were reduced by 35.1% (P < 0.001), TGs by 15.2% (P < 0.001), LDL-C by 48.5% (P < 0.001), apoE by 35.4% (P < 0.001), and apoE by 17.3% (P < 0.001) from baseline, whereas HDL-C and apoA1 levels were not significantly changed. Stepwise linear regression analysis showed that baseline TG levels were most significantly correlated (R(2) = 42.0%; P < 0.001) with the TG-lowering effect of rosuvastatin, followed by the reduction in apoCIII levels (R(2) = 13.6%; P < 0.01). Rosuvastatin use was associated with a reduction in cholesterol mass of both large LDL particles (mean [SD], from 150.5 [36.6] to 90.5 [24.3] mg/dL; P < 0.001) and small, dense LDL (sdLDL) particles (from 11.5 [8.4] to 6.6 [4.5] mg/dL; P < 0.001). Rosuvastatin had no effect on cholesterol distribution of the LDL subfractions (mean [SD], large particles, from 90.8% [7.0%] to 91.8% [5.1%]; sdLDL, from 7.1% [4.7%] to 7.5% [4.8%]) or the mean LDL particle size (from 26.5 [4.2] to 26.6 [4.0] rim). A significant increase in mean LDL particle size after rosuvastatin treatment (mean [SD], from 26.4 [0.4] to 26.9 [0.4] rim; P = 0.02) was observed only in patients with baseline TG levels > or =120 mg/dL. No serious adverse events requiring study treatment discontinuation were reported. One patient who presented with headache and 2 patients who presented with fatigue quickly recovered without discontinuing rosuvastatin treatment. A posttreatment elevation in aminotransferase levels <3-fold the upper limit of normal (ULN) was recorded in 5 (6.7%) patients, and 2 (2.7%) patients experienced elevated creatine kinase concentrations <5-fold ULN.

CONCLUSION

Baseline TG levels were the most important independent variable associated with the TG-lowering effect of rosuvastatin.