Characterization of nascent high density lipoprotein subfractions from perfusates of rat liver

Lipidated apolipoprotein E4 structure and its receptor binding mechanism determined by a combined cross-linking coupled to mass spectrometry and molecular dynamics approach

Apolipoprotein E (apoE) is a forefront actor in the transport of lipids and the maintenance of cholesterol homeostasis, and is also strongly implicated in Alzheimer’s disease. Upon lipid-binding apoE adopts a conformational state that mediates the receptor-induced internalization of lipoproteins. Due to its inherent structural dynamics and the presence of lipids, the structure of the biologically active apoE remains so far poorly described. To address this issue, we developed an innovative hybrid method combining experimental data with molecular modeling and dynamics to generate comprehensive models of the lipidated apoE4 isoform. Chemical cross-linking combined with mass spectrometry provided distance restraints, characterizing the three-dimensional organization of apoE4 molecules at the surface of lipidic nanoparticles. The ensemble of spatial restraints was then rationalized in an original molecular modeling approach to generate monomeric models of apoE4 that advocated the existence of two alternative conformations. These two models point towards an activation mechanism of apoE4 relying on a regulation of the accessibility of its receptor binding region. Further, molecular dynamics simulations of the dimerized and lipidated apoE4 monomeric conformations revealed an elongation of the apoE N-terminal domain, whereby helix 4 is rearranged, together with Arg172, into a proper orientation essential for lipoprotein receptor association. Overall, our results show how apoE4 adapts its conformation for the recognition of the low density lipoprotein receptor and we propose a novel mechanism of activation for apoE4 that is based on accessibility and remodeling of the receptor binding region.

Among the proteins involved in the transport of lipids and their distribution to the cells, apolipoprotein E (apoE) mediates the internalization of cholesterol rich lipoproteins by acting as a ligand for cell-surface receptors. In the central nervous system, while apoE is the major cholesterol transport protein, a dysfunction of apoE in the transport and metabolism of lipids is associated with Alzheimer’s disease. A molecular understanding of the mechanisms underlying the receptor binding abilities of apoE is crucial to address its biological functions, but is so far hindered by the dynamic and complex nature of these assemblies. We have designed an original hybrid approach combining experimental data and bioinformatics tools to generate high resolution models of lipidated apoE. Based on these models, we can propose how apoE adapts its conformation at the surface of lipid nanoparticles. Further, we propose a novel mechanism of regulation of the activation and receptor recognition of apoE that could prove valuable to interpret its role in Alzheimer and apoE-related cardiovascular diseases.

HDL lipids and insulin resistance

There is renewed interest in high-density lipoproteins (HDLs) due to recent findings linking atherosclerosis to the formation of dysfunctional HDL. This article focuses on the universe of HDL lipids and their potential protective or proinflammatory roles in vascular disease and insulin resistance. HDL carries a wide array of lipids including sterols, triglycerides, fat-soluble vitamins, and a large number of phospholipids, including phosphatidylcholine, sphingomyelin, and ceramide with many biological functions. Ceramide has been implicated in the pathogenesis of insulin resistance and has many proinflammatory properties. In contrast, sphingosine-1-phosphate, which is transported mainly in HDL, has anti-inflammatory properties that may be atheroprotective and may account for some of the beneficial effects of HDL. However, the complexity of the HDL lipidome is only beginning to reveal itself. The emergence of new analytical technologies should rapidly increase our understanding of the function of HDL lipids and their role in disease states.

Absorption and lipoprotein transport of sphingomyelin

Dietary sphingomyelin (SM) is hydrolyzed by intestinal alkaline sphingomyelinase and neutral ceramidase to sphingosine, which is absorbed and converted to palmitic acid and acylated into chylomicron triglycerides (TGs). SM digestion is slow and is affected by luminal factors such as bile salt, cholesterol, and other lipids. In the gut, SM and its metabolites may influence TG hydrolysis, cholesterol absorption, lipoprotein formation, and mucosal growth. SM accounts for approximately 20% of the phospholipids in human plasma lipoproteins, of which two-thirds are in LDL and VLDL. It is secreted in chylomicrons and VLDL and transferred into HDL via the ABCA1 transporter. Plasma SM increases after periods of large lipid loads, during suckling, and in type II hypercholesterolemia, cholesterol-fed animals, and apolipoprotein E-deficient mice. SM is thus an important amphiphilic component when plasma lipoprotein pools expand in response to large lipid loads or metabolic abnormalities. It inhibits lipoprotein lipase and LCAT as well as the interaction of lipoproteins with receptors and counteracts LDL oxidation. The turnover of plasma SM is greater than can be accounted for by the turnover of LDL and HDL particles. Some SM must be degraded via receptor-mediated catabolism of chylomicron and VLDL remnants and by scavenger receptor class B type I receptor-mediated transfer into cells.

High density lipoproteins (HDLs) and atherosclerosis; the unanswered questions

The concentration of high density lipoprotein-cholesterol (HDL-C) has been found consistently to be a powerful negative predictor of premature coronary heart disease (CHD) in human prospective population studies. There is also circumstantial evidence from human intervention studies and direct evidence from animal intervention studies that HDLs protect against the development of atherosclerosis. HDLs have several documented functions, although the precise mechanism by which they prevent atherosclerosis remains uncertain. Nor is it known whether the cardioprotective properties of HDL are specific to one or more of the many HDL subpopulations that comprise the HDL fraction in human plasma. Several lifestyle and pharmacological interventions have the capacity to raise the level of HDL-C, although it is not known whether all are equally protective. Indeed, despite the large body of information identifying HDLs as potential therapeutic targets for the prevention of atherosclerosis, there remain many unanswered questions that must be addressed as a matter of urgency before embarking wholesale on HDL-C-raising therapies as strategies to prevent CHD. This review summarises what is known and highlights what we still need to know.

ATP-binding cassette transporter A1 (ABCA1) functions as a cholesterol efflux regulatory protein

ABCA1, an ATP-binding cassette transporter mutated in Tangier disease, promotes cellular phospholipid and cholesterol efflux by loading free apoA-I with these lipids. This process involves binding of apoA-I to the cell surface and phospholipid translocation by ABCA1. The goals of this study were to examine the relationship between ABCA1-mediated lipid efflux and apolipoprotein binding and to determine whether phospholipid and cholesterol efflux are coupled. Inhibition of lipid efflux by glybenclamide treatment or by mutation of the ATP-binding cassette of ABCA1 showed a close correlation between lipid efflux, the binding of apoA-I to cells, and cross-linking of apoA-I to ABCA1. The data suggest that a functionally important apoA-I binding site exists on ABCA1 and that the binding site could also involve lipids. After using cyclodextrin preincubation to deplete cellular cholesterol, ABCA1-mediated cholesterol efflux was abolished but phospholipid efflux and the binding of apoA-I were unaffected. The conditioned media from cyclodextrin-pretreated, ABCA1-expressing cells readily promoted cholesterol efflux when added to fresh cells not expressing ABCA1, indicating that cholesterol efflux can be dissociated from phospholipid efflux. Further, using a photoactivatable cholesterol analog, we showed that ABCA1 did not bind cholesterol directly, even though several other cholesterol-binding proteins specifically bound the cholesterol analog. The data suggest that the binding of apoA-I to ABCA1 leads to the formation of phospholipid-apoA-I complexes, which subsequently promote cholesterol efflux in an autocrine or paracrine fashion.

The HepG2 extracellular matrix contains separate heparinase- and lipid-releasable pools of ApoE. Implications for hepatic lipoprotein metabolism

We have examined the association of apoE with the extracellular matrix (ECM) of HepG2 cells. Comparison of ECM prepared by previously published methods demonstrated that cytochalasin B-prepared material yielded the highest endogenous apoE, representing 23.6% of that in cell monolayers. ECM prepared with EDTA or Triton X-100 exhibited decreased levels of apoE, 3 and 6%, respectively. ECM bound very low density lipoprotein poorly (5-6% of the monolayer capacity); however, these incubations dramatically increased the apoE content of the ECM. Heparinase or suramin decreased apoE of the ECM by 19.6 and 37.3%, respectively, suggesting association with heparin sulfate proteoglycans. EDTA or EGTA also displaced 35% of the apoE, suggesting a Ca2+-dependent association. Incubation with phosphatidylcholine vesicles (PCV) displaced 30% of the apoE, suggesting that lipid content affects association of apoE with the ECM. Data derived from sequential incubations with combinations of suramin, EGTA, and PCV were consistent with the presence of two distinct pools of apoE on the HepG2 ECM, one releasable with suramin and EGTA and the other releasable with lipids. Exogenously applied lipid-free apoE readily bound to the ECM; however, increasing the lipid content decreased its association. Lipid-free apoE could be equally displaced from the ECM with PCV or suramin. When lipid-free apoE adsorbed to microtiter wells was incubated with a triglyceride emulsion or palmitoyloleyl phosphatidylcholine micelles, the immunoreactivity of 3H1 (but not other antibodies), a monoclonal antibody against an epitope in the C-terminal domain of apoE, increased about 4-fold. In a similar manner, incubation of ECM with lipid dramatically increased the immunoreactivity of 3H1, indicating that apoE of the ECM exists in a lipid-poor form. Scatchard analysis demonstrated that the increased immunoreactivity was due to an increase in the number of antibody binding sites. In conclusion, the ECM contains two pools of lipid-poor apoE. One pool associates with the ECM through heparin sulfate proteoglycans- and Ca2+-dependent interactions. A second pool of apoE dissociates from the ECM upon lipidation. The lipid-sensitive pool of apoE may participate in secretion or efflux of lipids or in the capture of lipoproteins by providing the apoE needed for receptor-mediated uptake.

The influence of sphingomyelin on the structure and function of reconstituted high density lipoproteins

The effect of sphingomyelin (SPM) on the structure and function of discoidal and spherical reconstituted high density lipoproteins (rHDL) has been studied. Three preparations of discoidal rHDL with 1-palmitoyl-2-oleoyl phosphatidylcholine (POPC)/SPM/unesterified cholesterol (UC)/apolipoprotein (apo)A-I molar ratios of 99.6/0. 0/10.2/1.0, 86.0/13.6/10.8/1.0, and 72.5/26.3/11.4/1.0 were prepared by cholate dialysis. SPM did not affect discoidal rHDL size or surface charge. Esterification of cholesterol by lecithin:cholesterol acyltransferase (LCAT) was inhibited in the SPM-containing discoidal rHDL. When the discoidal rHDL of POPC/SPM/UC/apoA-I molar ratio 99.6/0.0/10.2/1.0 were incubated with low density lipoproteins (LDL) and LCAT, SPM transferred spontaneously from the LDL to the rHDL (t1/2 = 0.8 h) and spherical particles with a POPC/SPM/UC/CE/apoA-I molar ratio of 24.6/4.9/3. 6/24.9/1.0 were formed. Depleting the spherical rHDL of SPM head groups by incubation with sphingomyelinase increased the negative charge on the surface, but did not change their size. Cholesteryl ester transfer protein (CETP)-mediated transfers of cholesteryl esters and triglyceride between spherical rHDL and Intralipid were not affected by SPM head group depletion. The effect of SPM on rHDL structure was assessed spectroscopically. SPM increased POPC acyl chain and head group packing in the discoidal rHDL. When the spherical rHDL were depleted of SPM head groups, POPC acyl chain packing order decreased, but head group packing order was not affected. SPM inhibited the lipid-water interfacial hydration of discoidal rHDL. This parameter was not affected when the spherical rHDL were depleted of SPM head groups. The SPM molecule and the SPM head group, respectively, inhibited the unfolding of apoA-I in discoidal and spherical rHDL. It is concluded that (i) SPM influences the structure of discoidal and spherical rHDL, (ii) SPM inhibits the LCAT reaction in discoidal rHDL, and (iii) the SPM head group does not affect CETP-mediated lipid transfers into or out of spherical rHDL.

Regulation of hepatic and non-hepatic apolipoprotein A-I and E gene expression during liver regeneration

In this study, we have determined what tissues other than liver express apolipoprotein (apo) A-I and apo E genes during liver regeneration at the level of the specific mRNAs, and have compared these findings with the serum values of high-density lipoprotein (HDL), low-density lipoprotein (LDL) and very-low-density lipoprotein (VLDL). Our results show that liver and intestine express most of the apo A-I mRNA during liver regeneration. Although apo E mRNA is expressed principally by the liver, its expression is reduced in liver during regeneration but is increased markedly in non-hepatic tissues, such as in intestine, kidney, lung and brain. These results suggest that humoral or circulating factors released during liver regeneration influence apolipoprotein E gene expression, not only in hepatic but also in non-hepatic tissue.

Isolation of nascent high-density lipoprotein from rat liver perfusates by immunoaffinity chromatography: effects of oleic acid infusion

Immunoaffinity chromatography on a column of rabbit IgG anti-rat apolipoprotein (apo) A-I covalently bonded to agarose was used to isolate nascent high-density lipoprotein (nHDL) from recirculated perfusates of rat livers. After passage through the affinity column, the bound material was eluted with sodium thiocyanate and analyzed for apolipoproteins and lipids. The protein content was 52% and the lipid composition was 37% triglyceride, 40% phospholipid, and 23% cholesterol. Apolipoproteins E and A-I each comprised approximately one third of the total, and very little apo B was detectable as judged by SDS-PAGE analysis. The affinity-isolated particles were therefore similar in composition to the major apo A-I:apo E-rich subfraction of nHDL isolated by ultracentrifugation in earlier work. It is concluded that the apo E in this class of nHDL (containing both apo E and apo A-I) is present in the secreted particle and is not a consequence of a loss of apo E from very-low-density lipoprotein (VLDL) during ultracentrifugation. The high triglyceride content in the virtual absence of apo B confirms and extends previous analyses and reinforces the conclusion that nHDL particles are enriched in triglyceride compared to plasma HDL. The inclusion of 4% albumin in the perfusion medium did not significantly change the total triglyceride output of 115 micrograms/g liver/h, but it decreased the triglyceride output isolated by anti-apo A-I affinity chromatography from 3.2 to 0.48 micrograms/g liver/h. The addition of oleic acid complexed to albumin increased the total triglyceride output by 70% and that associated with the immunoaffinity column increased from 0.48 to 2.7 micrograms/g liver/h.(ABSTRACT TRUNCATED AT 250 WORDS)

The assembly of lipids into lipoproteins during secretion

The process of assembly and secretion of lipoproteins is discussed with particular reference to the role of lipids. The majority of circulating lipoproteins is produced by the liver (80%) with the remainder being supplied by the intestine. The liver secretes both very low density lipoproteins and high density lipoproteins, but the assembly and secretion of these two types of particles may follow different routes. The major lipid components of lipoproteins are triacylglycerols, cholesterol, cholesterol esters and phospholipids. The biosynthesis of these lipids occurs on membranes of the endoplasmic reticulum, with many of the enzymes also being present in the Golgi; the roles of these two subcellular organelles in the assembly of lipoproteins are discussed. There appears to be a compartmentalization of lipids in cells, such that defined pools, often those newly-synthesized, are preferred, or even required, for lipoprotein assembly. The process of hepatic very low density lipoprotein secretion appears to be regulated by the supply of lipids. Indeed, the synthesis of new lipid may be a major driving force in lipoprotein assembly and secretion.