Role of individual amino acids of apolipoprotein A-I in the activation of lecithin:cholesterol acyltransferase and in HDL rearrangements

Deciphering structural aspects of reverse cholesterol transport: mapping the knowns and unknowns

Atherosclerosis is a major contributor to the onset and progression of cardiovascular disease (CVD). Cholesterol-loaded foam cells play a pivotal role in forming atherosclerotic plaques. Induction of cholesterol efflux from these cells may be a promising approach in treating CVD. The reverse cholesterol transport (RCT) pathway delivers cholesteryl ester (CE) packaged in high-density lipoproteins (HDL) from non-hepatic cells to the liver, thereby minimising cholesterol load of peripheral cells. RCT takes place via a well-organised interplay amongst apolipoprotein A1 (ApoA1), lecithin cholesterol acyltransferase (LCAT), ATP binding cassette transporter A1 (ABCA1), scavenger receptor-B1 (SR-B1), and the amount of free cholesterol. Unfortunately, modulation of RCT for treating atherosclerosis has failed in clinical trials owing to our lack of understanding of the relationship between HDL function and RCT. The fate of non-hepatic CEs in HDL is dependent on their access to proteins involved in remodelling and can be regulated at the structural level. An inadequate understanding of this inhibits the design of rational strategies for therapeutic interventions. Herein we extensively review the structure-function relationships that are essential for RCT. We also focus on genetic mutations that disturb the structural stability of proteins involved in RCT, rendering them partially or completely non-functional. Further studies are necessary for understanding the structural aspects of RCT pathway completely, and this review highlights alternative theories and unanswered questions.

A Study on Multiple Facets of Apolipoprotein A1 Milano

For several strategies formulated to prevent atherosclerosis, Apolipoprotein A1 Milano (ApoA1M) remains a prime target. ApoA1M has been reported to have greater efficiency in reducing the incidence of coronary artery diseases. Furthermore, recombinant ApoA1M based mimetic peptide exhibits comparatively greater atheroprotective potential, offers a hope in reducing the burden of atherosclerosis in in vivo model system. The aim of this review is to emphasize on some of the observed ApoA1M structural and functional effects that are clinically and therapeutically meaningful that might converge on the basic role of ApoA1M in reducing the chances of glycation assisted ailments in diabetes. We also hypothesize that the nonenzymatic glycation prone arginine amino acid of ApoA1 gets replaced with cysteine residue and the rate of ApoA1 glycation may decrease due to change substitution of amino acid. Therefore, to circumvent the effect of ApoA1M glycation, the related mechanism should be explored at the cellular and functional levels, especially in respective experimental disease model in vivo.

Human Serum Amyloid a Impaired Structural Stability of High-Density Lipoproteins (HDL) and Apolipoprotein (Apo) A-I and Exacerbated Glycation Susceptibility of ApoA-I and HDL

Human serum amyloid A (SAA) is an exchangeable apolipoprotein (apo) in high-density lipoprotein (HDL) that influences HDL quality and functionality, particularly in the acute phase of inflammation. On the other hand, the structural and functional correlations of HDL containing SAA and apoA-I have not been reported. The current study was designed to compare the change in HDL quality with increasing SAA content in the lipid-free and lipid-bound states in reconstituted HDL (rHDL). The expressed recombinant human SAA1 (13 kDa) was purified to at least 98% and characterized in the lipid-free and lipid-bound states with apoA-I. The dimyristoyl phosphatidylcholine (DMPC) binding ability of apoA-I was impaired severely by the addition of SAA, while SAA alone could not bind with DMPC. The recombinant human SAA1 was incorporated into the rHDL (molar ratio 95:5:1, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC): cholesterol: apoA-I) with various apoA-I:SAA molar ratios from 1:0 to 1:0.5, 1:1 and 1:2. With increasing SAA1 content, the rHDL particle size was reduced from 98 Å to 93 Å, and the α-helicity of apoA-I:SAA was decreased from 73% to 40% for (1:0) and (1:2), respectively. The wavelength maximum fluorescence (WMF) of tryptophan in rHDL was red-shifted from 339 nm to 345 nm for (1:0) and (1:2) of apoA-I:SAA, respectively, indicating that the addition of SAA to rHDL destabilized the secondary structure of apoA-I. Upon denaturation by urea treatment from 0 M to 8 M, SAA showed only a 3 nm red-shift in WMF, while apoA-I showed a 16 nm red-shift in WMF, indicating that SAA is resistant to denaturation and apoA-I had higher conformational flexibility than SAA. The glycation reaction of apoA-I in the presence of fructose was accelerated up to 1.8-fold by adding SAA in a dose-dependent manner than that of apoA-I alone. In conclusion, the incorporation of SAA in rHDL impaired the structural stability of apoA-I and exacerbated glycation of HDL and apoA-I.

Probing the Structure-Function relationship and amyloidogenic propensities in natural variants of apolipoprotein A-I

Background

Apolipoprotein A-I (apoA-I) protects against atherosclerosis and participates in the removal of excess cellular cholesterol from peripheral organs. Several naturally occurring apoA-I mutations are associated with familial systemic amyloidosis, with deposition of amyloid aggregates in peripheral organs, resulting in multiple organ failure. Systematic studies on naturally occurring variants are needed to delineate their roles and involvement in pathogenesis.

Methods

We performed a comparative structure–function analysis of five naturally occurring apoA-I variants and the wild-type protein. Circular dichroism, Fourier-transform infrared spectroscopy, thioflavin T and congo red fluorescence assays, thermal, chemical, and proteolytic stability assays, and 1,2-Dimyristoyl-sn-glycero-3-phosphocholine clearance analyses were used to assess the effects of mutations on the structure, function, stability, aggregation, and proteolytic susceptibility of the proteins to explore the mechanisms underlying amyloidosis and hypercholesterolemia.

Results

We observed structural changes in the mutants independent of fibril formation, suggesting the influence of the surrounding environment. The mutants were involved in aggregate formation to varying degree; L170P, R173P, and V156E showed an increased propensity to aggregate under different physiological conditions. β sheet formation indicates that L170P and R173P participate in amyloid formation. Compared to WT, V156E and L170P exhibited higher capacity for lipid clearance.

Conclusions

The selected point mutations, including those outside the hot spot regions of apoA-I structure, perturb the physiochemical and conformational behavior of the protein, influencing its function.

General significance

The study provides insights into the structure–function relationships of naturally occurring apoA-I variants outside the hot spot mutation sites.

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Several apoA-I mutants are associated with systemic amyloidosis.

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Structure–function analysis of five apoA-I variants and wild-type protein was done.

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Point mutations alter the physicochemical behavior and conformation of the variants.

A thumbwheel mechanism for APOA1 activation of LCAT activity in HDL

APOA1 is the most abundant protein in HDL. It modulates interactions that affect HDL's cardioprotective functions, in part via its activation of the enzyme, LCAT. On nascent discoidal HDL, APOA1 comprises 10 α-helical repeats arranged in an anti-parallel stacked-ring structure that encapsulates a lipid bilayer. Previous chemical cross-linking studies suggested that these APOA1 rings can adopt at least two different orientations, or registries, with respect to each other; however, the functional impact of these structural changes is unknown. Here, we placed cysteine residues at locations predicted to form disulfide bonds in each orientation and then measured APOA1's ability to adopt the two registries during HDL particle formation. We found that most APOA1 oriented with the fifth helix of one molecule across from fifth helix of the other (5/5 helical registry), but a fraction adopted a 5/2 registry. Engineered HDLs that were locked in 5/5 or 5/2 registries by disulfide bonds equally promoted cholesterol efflux from macrophages, indicating functional particles. However, unlike the 5/5 registry or the WT, the 5/2 registry impaired LCAT cholesteryl esterification activity (P < 0.001), despite LCAT binding equally to all particles. Chemical cross-linking studies suggest that full LCAT activity requires a hybrid epitope composed of helices 5-7 on one APOA1 molecule and helices 3-4 on the other. Thus, APOA1 may use a reciprocating thumbwheel-like mechanism to activate HDL-remodeling proteins.

Interactions of six SNPs in APOA1 gene and types of obesity on low HDL-C disease in Xinjiang pastoral area of China

BACKGROUND

This study aims to investigate association between six single nucleotide polymorphisms(SNPs) in APOA1 gene and types of obesity with the risk of low level HDL-C in the pastoral area of northwest China.

METHODS

A total of 1267 individuals including 424 patients with low HDL-C disease and 843 health subjects were analyzed based on matched for age, sex. SNPShot technique was used to detect the genotypes of rs670, rs5069, rs5072, rs7116797, rs2070665 and rs1799837 in APOA1 gene. The relationship between above six SNPs and types of obesity with low HDL-C disease was analyzed by binary logistic regression.

RESULTS

Carriers with rs670 G allele were more likely to get low HDL-C disease (OR = 1.46, OR95%CI: 1.118-1.915; P = 0.005); The genotypic and allelic frequencies of rs5069, rs5072, rs7116797, rs2070665, rs1799837 revealed no significant differences between cases and controls (P < 0.05); with reference to normal weight, Waist circumference (WC), Waist-to-hip ratio (WHR) individuals, respectively, general obesity measured by BMI had 2.686 times (OR95%CI: 1.695-4.256; P < 0.01), abdominal obesity measured by WC had 1.925 times (OR95%CI: 1.273-2.910; P = 0.002) and abdominal obesity measured by WHR had 1.640 times (OR95%CI: 1.114-2.416; P = 0.012) risk to get low HDL-C disease; APOA1 rs670 interacted with obesity (no matter general obesity or abdominal obesity) on low HDL-C disease.

CONCLUSIONS

APOA1 gene may be associated with low HDL-C disease in the pastoral area of northwest China; obesity was the risk factor for low HDL-C disease; the low HDL-C disease is influenced by APOA1, obesity, and their interactions.

Lipid-free apoA-I structure - Origins of model diversity

Apolipoprotein A-I (apoA-I) is a prominent member of the exchangeable apolipoprotein class of proteins, capable of transitioning between lipid-bound and lipid-free states. It is the primary structural and functional protein of high density lipoprotein (HDL). Lipid-free apoA-I is critical to de novo HDL formation as it is the preferred substrate of the lipid transporter, ATP Binding Cassette Transporter A1 (ABCA1) Remaley et al. (2001) [1]. Lipid-free apoA-I is an important element in reverse cholesterol transport and comprehension of its structure is a core issue in our understanding of cholesterol metabolism. However, lipid-free apoA-I is highly conformationally dynamic making it a challenging subject for structural analysis. Over the past 20years there have been significant advances in overcoming the dynamic nature of lipid-free apoA-I, which have resulted in a multitude of proposed conformational models.

A Systematic Investigation of Structure/Function Requirements for the Apolipoprotein A-I/Lecithin Cholesterol Acyltransferase Interaction Loop of High-density Lipoprotein

The interaction of lecithin-cholesterol acyltransferase (LCAT) with apolipoprotein A-I (apoA-I) plays a critical role in high-density lipoprotein (HDL) maturation. We previously identified a highly solvent-exposed apoA-I loop domain (Leu(159)-Leu(170)) in nascent HDL, the so-called "solar flare" (SF) region, and proposed that it serves as an LCAT docking site (Wu, Z., Wagner, M. A., Zheng, L., Parks, J. S., Shy, J. M., 3rd, Smith, J. D., Gogonea, V., and Hazen, S. L. (2007) Nat. Struct. Mol. Biol. 14, 861-868). The stability and role of the SF domain of apoA-I in supporting HDL binding and activation of LCAT are debated. Here we show by site-directed mutagenesis that multiple residues within the SF region (Pro(165), Tyr(166), Ser(167), and Asp(168)) of apoA-I are critical for both LCAT binding to HDL and LCAT catalytic efficiency. The critical role for possible hydrogen bond interaction at apoA-I Tyr(166) was further supported using reconstituted HDL generated from apoA-I mutants (Tyr(166) → Glu or Asn), which showed preservation in both LCAT binding affinity and catalytic efficiency. Moreover, the in vivo functional significance of NO2-Tyr(166)-apoA-I, a specific post-translational modification on apoA-I that is abundant within human atherosclerotic plaque, was further investigated by using the recombinant protein generated from E. coli containing a mutated orthogonal tRNA synthetase/tRNACUA pair enabling site-specific insertion of the unnatural amino acid into apoA-I. NO2-Tyr(166)-apoA-I, after subcutaneous injection into hLCAT(Tg/Tg), apoA-I(-/-) mice, showed impaired LCAT activation in vivo, with significant reduction in HDL cholesteryl ester formation. The present results thus identify multiple structural features within the solvent-exposed SF region of apoA-I of nascent HDL essential for optimal LCAT binding and catalytic efficiency.

Structural Insights into High Density Lipoprotein: Old Models and New Facts

The physiological link between circulating high density lipoprotein (HDL) levels and cardiovascular disease is well-documented, albeit its intricacies are not well-understood. An improved appreciation of HDL function and overall role in vascular health and disease requires at its foundation a better understanding of the lipoprotein's molecular structure, its formation, and its process of maturation through interactions with various plasma enzymes and cell receptors that intervene along the pathway of reverse cholesterol transport. This review focuses on summarizing recent developments in the field of lipid free apoA-I and HDL structure, with emphasis on new insights revealed by newly published nascent and spherical HDL models constructed by combining low resolution structures obtained from small angle neutron scattering (SANS) with contrast variation and geometrical constraints derived from hydrogen-deuterium exchange (HDX), crosslinking mass spectrometry, electron microscopy, Förster resonance energy transfer, and electron spin resonance. Recently published low resolution structures of nascent and spherical HDL obtained from SANS with contrast variation and isotopic labeling of apolipoprotein A-I (apoA-I) will be critically reviewed and discussed in terms of how they accommodate existing biophysical structural data from alternative approaches. The new low resolution structures revealed and also provided some answers to long standing questions concerning lipid organization and particle maturation of lipoproteins. The review will discuss the merits of newly proposed SANS based all atom models for nascent and spherical HDL, and compare them with accepted models. Finally, naturally occurring and bioengineered mutations in apoA-I, and their impact on HDL phenotype, are reviewed and discuss together with new therapeutics employed for restoring HDL function.

Dysfunctional High-Density Lipoprotein: An Innovative Target for Proteomics and Lipidomics

High-Density Lipoprotein-Cholesterol (HDL-C) is regarded as an important protective factor against cardiovascular disease, with abundant evidence of an inverse relationship between its serum levels and risk of cardiovascular disease, as well as various antiatherogenic, antioxidant, and anti-inflammatory properties. Nevertheless, observations of hereditary syndromes featuring scant HDL-C concentration in absence of premature atherosclerotic disease suggest HDL-C levels may not be the best predictor of cardiovascular disease. Indeed, the beneficial effects of HDL may not depend solely on their concentration, but also on their quality. Distinct subfractions of this lipoprotein appear to be constituted by specific protein-lipid conglomerates necessary for different physiologic and pathophysiologic functions. However, in a chronic inflammatory microenvironment, diverse components of the HDL proteome and lipid core suffer alterations, which propel a shift towards a dysfunctional state, where HDL-C becomes proatherogenic, prooxidant, and proinflammatory. This heterogeneity highlights the need for further specialized molecular studies in this aspect, in order to achieve a better understanding of this dysfunctional state; with an emphasis on the potential role for proteomics and lipidomics as valuable methods in the search of novel therapeutic approaches for cardiovascular disease.

Neuronal response in Alzheimer's and Parkinson's disease: the effect of toxic proteins on intracellular pathways

Accumulation of protein aggregates is the leading cause of cellular dysfunction in neurodegenerative disorders. Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease, Prion disease and motor disorders such as amyotrophic lateral sclerosis, present with a similar pattern of progressive neuronal death, nervous system deterioration and cognitive impairment. The common characteristic is an unusual misfolding of proteins which is believed to cause protein deposition and trigger degenerative signals in the neurons. A similar clinical presentation seen in many neurodegenerative disorders suggests the possibility of shared neuronal responses in different disorders. Despite the difference in core elements of deposits in each neurodegenerative disorder, the cascade of neuronal reactions such as activation of glycogen synthase kinase-3 beta, mitogen-activated protein kinases, cell cycle re-entry and oxidative stress leading to a progressive neurodegeneration are surprisingly similar. This review focuses on protein toxicity in two neurodegenerative diseases, AD and PD. We reviewed the activated mechanisms of neurotoxicity in response to misfolded beta-amyloid and α-synuclein, two major toxic proteins in AD and PD, leading to neuronal apoptosis. The interaction between the proteins in producing an overlapping pathological pattern will be also discussed.

Genetic polymorphisms in the APOA1 gene and their relationship with serum HDL cholesterol levels

Low high density lipoprotein cholesterol (HDL-C) is a known risk factor of coronary artery disease. Apolipoprotein A1 (APOA1) is the most abundant component of HDL-C. This study aimed at identifying sequence variations (rare and common) in the APOA1 gene and its association with serum HDL-C levels. This study was conducted from April 2012 to February 2013 on 79 Tehranians (participants of Tehran Lipid and Glucose Study) with extremely low HDL-C (within the 5th percentile) and 63 individuals with extremely high HDL-C (within the 95th percentile) levels. After DNA amplification by PCR, DNA sequencing of all three exons and 700 bps of promoter region of the APOA1 gene was performed. Sequence results were analyzed and interpreted using the appropriate software and variants were identified. After sequencing 42 common and rare variants were identified, 11 of which were known variants and the others had been unreported so far. Of the exonic variants, 11 were missense, 6 were synonymous and 1 was nonsense. There was a significant association between serum HDL-C and variant of rs2070665 as well as variants Chr.11:116707788, Chr.11:116708059, Chr.11:116708036, Chr.11:116707729, rs201148448, Chr.11:116707018, Chr.11:116707801, Chr.11:116708530, Chr.11:116708088, rs121912724 and Chr.11:116706966 (p < 0.001). Variants Chr.11:116707018, rs121912724 and 2070665 were independent predictors of the HDL-C level (p < 0.001). SNP Chr.11:116707018 was the strongest predictor of the HDL-C level (OR 7.527, p < 0.001). This study identified 42 variants in APOA1 gene, 31 of which were new variants. Three variants of rs2070665, rs121912724 and Chr.11:116707018 could predict the HDL-C level independently. Variant rs2070665 was protective against low-HDL-C levels while variants rs121912724 and Chr.11:116707018 were risk factors for that in our population.

Expression and purification of recombinant human apolipoprotein A-II in Pichia pastoris

Apolipoprotein A-II (ApoA-II) is the second most abundant protein constituent of high-density lipoprotein (HDL). The physiologic role of ApoA-II is poorly defined. ApoA-II may inhibit lecithin:cholesterol acyltransferase and cholesteryl-ester-transfer protein activities, but may increase the hepatic lipase activity. ApoA-II may also inhibit the hepatic cholesteryl uptake from HDL probably through the scavenger receptor class B type I depending pathway. Interpretation of data from transgenic and knockout mice of genes involved in lipoprotein metabolism has been often complicated as clinical implications because of species difference. So it is important to obtain human ApoA-II for further studies about its functions. In our studies, Pichia pastoris expression system was first used to express a high-level secreted recombinant human ApoA-II (rhApoA-II). We have cloned the cDNA encoding human ApoA-II and achieved its high-level secreting expression with a yield of 65 mg/L of yeast culture and the purification process was effective and easy to handle. The purified rhApoA-II can be used to further study its biological activities.

Conservation of apolipoprotein A-I's central domain structural elements upon lipid association on different high-density lipoprotein subclasses

The antiatherogenic properties of apolipoprotein A-I (apoA-I) are derived, in part, from lipidation-state-dependent structural elements that manifest at different stages of apoA-I's progression from lipid-free protein to spherical high-density lipoprotein (HDL). Previously, we reported the structure of apoA-I's N-terminus on reconstituted HDLs (rHDLs) of different sizes. We have now investigated at the single-residue level the conformational adaptations of three regions in the central domain of apoA-I (residues 119-124, 139-144, and 164-170) upon apoA-I lipid binding and HDL formation. An important function associated with these residues of apoA-I is the activation of lecithin:cholesterol acyltransferase (LCAT), the enzyme responsible for catalyzing HDL maturation. Structural examination was performed by site-directed tryptophan fluorescence and spin-label electron paramagnetic resonance spectroscopies for both the lipid-free protein and rHDL particles 7.8, 8.4, and 9.6 nm in diameter. The two methods provide complementary information about residue side chain mobility and molecular accessibility, as well as the polarity of the local environment at the targeted positions. The modulation of these biophysical parameters yielded new insight into the importance of structural elements in the central domain of apoA-I. In particular, we determined that the loosely lipid-associated structure of residues 134-145 is conserved in all rHDL particles. Truncation of this region completely abolished LCAT activation but did not significantly affect rHDL size, reaffirming the important role of this structural element in HDL function.

Clinical role of apolipoproteins A and B

The assessment and correction of the traditional parameters of atherogenic dyslipidemia are important, but not exclusive methods in the management of atherosclerosis, including coronary artery atherosclerosis. More accurate diagnostic and therapeutic assessment requires the measurement of apolipoprotein (Apo) A, ApoB, and their ratio.Lower ApoB/ApoAI ratio values denote lower levels of cardiovascular risk.

Analysis of apolipoprotein A-I as a substrate for matrix metalloproteinase-14

Substrates for matrix metalloproteinase (MMP)-14 were previously identified in human plasma using proteomic techniques. One putative MMP-14 substrate was apolipoprotein A-I (apoA-I), a major component of high-density lipoprotein (HDL). In vitro cleavage assays showed that lipid-free apoA-I is a more accessible substrate for MMP-14 compared to lipid-bound apoA-I, and that MMP-14 is more prone to digest apoA-I than MMP-3. The 28-kDa apoA-I was cleaved into smaller fragments of 27, 26, 25, 22, and 14-kDa by MMP-14. ApoA-I sites cleaved by MMP-14 were determined by isotope labeling of C-termini derived from the cleavage and analysis of the labeled peptides by mass spectrometry, along with N-terminal sequencing of the fragments. Cleavage of apoA-I by MMP-14 resulted in a loss of ability to form HDL. Our results suggest that cleavage of lipid-free apoA-I by MMP-14 may contribute to reduced HDL formation, and this may be occurring during the development of various vascular diseases as lipid metabolism is disrupted.

Expression and recovery of biologically active recombinant Apolipoprotein AI(Milano) from transgenic safflower (Carthamus tinctorius) seeds

Apolipoprotein AI Milano (ApoAI(Milano) ) was expressed as a fusion protein in transgenic safflower seeds. High levels of expression corresponding to 7 g of ApoAI(Milano) per kilogram of seed have been identified in a line selected for commercialization. The ApoAI(Milano) fusion protein was extracted from seed using an oilbody-based process and matured in vitro prior to final purification. This yielded a Des-1,2-ApoAI(Milano) product which was confirmed by biochemical characterization including immunoreactivity against ApoAI antibodies, isoelectric point, N-terminal sequencing and electrospray mass spectrometry. Purified Des-1,2-ApoAI(Milano) readily associated with dimyristoylphosphatidylcholine in clearance assays comparable to Human ApoAI. Its biological activity was assessed by cholesterol efflux assays using Des-1,2-ApoAI(Milano) :1-palmitoyl-2-oleoyl phosphatidylcholine complexes in vitro and in vivo. This study has established that high levels of biologically functional ApoAI(Milano) can be produced using a plant-based expression system.

Lipid packing determines protein-membrane interactions: challenges for apolipoprotein A-I and high density lipoproteins

Protein and protein-lipid interactions, with and within specific areas in the cell membrane, are critical in order to modulate the cell signaling events required to maintain cell functions and viability. Biological bilayers are complex, dynamic platforms, and thus in vivo observations usually need to be preceded by studies on model systems that simplify and discriminate the different factors involved in lipid-protein interactions. Fluorescence microscopy studies using giant unilamellar vesicles (GUVs) as membrane model systems provide a unique methodology to quantify protein binding, interaction, and lipid solubilization in artificial bilayers. The large size of lipid domains obtainable on GUVs, together with fluorescence microscopy techniques, provides the possibility to localize and quantify molecular interactions. Fluorescence Correlation Spectroscopy (FCS) can be performed using the GUV model to extract information on mobility and concentration. Two-photon Laurdan Generalized Polarization (GP) reports on local changes in membrane water content (related to membrane fluidity) due to protein binding or lipid removal from a given lipid domain. In this review, we summarize the experimental microscopy methods used to study the interaction of human apolipoprotein A-I (apoA-I) in lipid-free and lipid-bound conformations with bilayers and natural membranes. Results described here help us to understand cholesterol homeostasis and offer a methodological design suited to different biological systems.

Composition and lipid spatial distribution of HDL particles in subjects with low and high HDL-cholesterol

A low level of high density lipoprotein cholesterol (HDL-C) is a powerful risk factor for cardiovascular disease. However, despite the reported key role of apolipo-proteins, specifically, apoA-I, in HDL metabolism, lipid molecular composition of HDL particles in subjects with high and low HDL-C levels is currently unknown. Here lipidomics was used to study HDL derived from well-characterized high and low HDL-C subjects. Low HDL-C subjects had elevated triacylglycerols and diminished lysophosphatidylcholines and sphingomyelins. Using information about the lipid composition of HDL particles in these two groups, we reconstituted HDL particles in silico by performing large-scale molecular dynamics simulations. In addition to confirming the measured change in particle size, we found that the changes in lipid composition also induced specific spatial distributions of lipids within the HDL particles, including a higher amount of triacylglycerols at the surface of HDL particles in low HDL-C subjects. Our findings have important implications for understanding HDL metabolism and function. For the first time we demonstrate the power of combining molecular profiling of lipoproteins with dynamic modeling of lipoprotein structure.

Dynamics of activation of lecithin:cholesterol acyltransferase by apolipoprotein A-I

The product of transesterification of phospholipid acyl chains and unesterified cholesterol (UC) by the enzyme lecithin:cholesterol acyltransferase (LCAT) is cholesteryl ester (CE). Activation of LCAT by apolipoprotein (apo) A-I on nascent (discoidal) high-density lipoproteins (HDL) is essential for formation of mature (spheroidal) HDL during the antiatherogenic process of reverse cholesterol transport. Here we report all-atom and coarse-grained (CG) molecular dynamics (MD) simulations of HDL particles that have major implications for mechanisms of LCAT activation. Both the all-atom and CG simulations provide support for a model in which the helix 5/5 domains of apoA-I create an amphipathic "presentation tunnel" that exposes methyl ends of acyl chains at the bilayer center to solvent. Further, CG simulations show that UC also becomes inserted with high efficiency into the amphipathic presentation tunnel with its hydroxyl moiety (UC-OH) exposed to solvent; these results are consistent with trajectory analyses of the all-atom simulations showing that UC is being concentrated in the vicinity of the presentation tunnel. Finally, consistent with known product inhibition of CE-rich HDL by CE, CG simulations of CE-rich spheroidal HDL indicate partial blockage of the amphipathic presentation tunnel by CE. These results lead us to propose the following working hypothesis. After attachment of LCAT to discoidal HDL, the helix 5/5 domains in apoA-I form amphipathic presentation tunnels for migration of hydrophobic acyl chains and amphipathic UC from the bilayer to the phospholipase A2-like and esterification active sites of LCAT, respectively. This hypothesis is currently being tested by site-directed mutagenesis.

Cleavage and functional loss of human apolipoprotein E by digestion of matrix metalloproteinase-14

By means of a degradomic approach applying proteomic techniques, we previously suggested that apolipoprotein E (apoE) is a substrate of matrix metalloproteinase-14 (MMP-14). Here we confirm that apoE is, in fact, a substrate of MMP-14 and also of MMP-7 and MMP-2 to a lesser extent. The 34 kDa apoE protein was initially processed by MMP-14 into fragments with molecular masses of 28, 23, 21, and 11 kDa. MMP-14 cleavage sites within the apoE protein were determined by C-terminal labeling of MMP-14-digested apoE fragments with isotope ((18)O/(16)O = 1:1) and identification of the doublet fragments or peptides showing 2 Da difference by MS, along with N-terminal sequencing of the fragments. It was determined that the primary MMP-14 cleavage sites were A(176)-I(177), P(183)-L(184), P(202)-L(203), and Q(249)-I(250). The MMP-14-mediated cleavage of apoE was consistent regardless of whether apoE existed in its lipid-bound or lipid-free form. Upon digestion with MMP-14, apoE loses its ability to suppress the platelet-derived growth factor-induced migration of rat vascular smooth muscle cells. Considering the important role of apoE for lipid metabolism and atherosclerosis protection, our findings suggest that MMP-14 plays an essential role for the development of hyperlipidemia and atherosclerosis as a result of degradation of apoE.

The interplay between size, morphology, stability, and functionality of high-density lipoprotein subclasses

High-density lipoprotein (HDL) mediates reverse cholesterol transport (RCT), wherein excess cholesterol is conveyed from peripheral tissues to the liver and steroidogenic organs. During this process HDL continually transitions between subclass sizes, each with unique biological activities. For instance, RCT is initiated by the interaction of lipid-free/lipid-poor apolipoprotein A-I (apoA-I) with ABCA1, a membrane-associated lipid transporter, to form nascent HDL. Because nearly all circulating apoA-I is lipid-bound, the source of lipid-free/lipid-poor apoA-I is unclear. Lecithin:cholesterol acyltransferase (LCAT) then drives the conversion of nascent HDL to spherical HDL by catalyzing cholesterol esterification, an essential step in RCT. To investigate the relationship between HDL particle size and events critical to RCT such as LCAT activation and lipid-free apoA-I production for ABCA1 interaction, we reconstituted five subclasses of HDL particles (rHDL of 7.8, 8.4, 9.6, 12.2, and 17.0 nm in diameter, respectively) using various molar ratios of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, free cholesterol, and apoA-I. Kinetic analyses of this comprehensive array of rHDL particles suggest that apoA-I stoichiometry in rHDL is a critical factor governing LCAT activation. Electron microscopy revealed specific morphological differences in the HDL subclasses that may affect functionality. Furthermore, stability measurements demonstrated that the previously uncharacterized 8.4 nm rHDL particles rapidly convert to 7.8 nm particles, concomitant with the dissociation of lipid-free/lipid-poor apoA-I. Thus, lipid-free/lipid-poor apoA-I generated by the remodeling of HDL may be an essential intermediate in RCT and HDL's in vivo maturation.

Naturally occurring and bioengineered apoA-I mutations that inhibit the conversion of discoidal to spherical HDL: the abnormal HDL phenotypes can be corrected by treatment with LCAT

In the present study we have used adenovirus-mediated gene transfer of apoA-I (apolipoprotein A-I) mutants in apoA-I-/- mice to investigate how structural mutations in apoA-I affect the biogenesis and the plasma levels of HDL (high-density lipoprotein). The natural mutants apoA-I(R151C)Paris, apoA-I(R160L)Oslo and the bioengineered mutant apoA-I(R149A) were secreted efficiently from cells in culture. Their capacity to activate LCAT (lecithin:cholesterol acyltransferase) in vitro was greatly reduced, and their ability to promote ABCA1 (ATP-binding cassette transporter A1)-mediated cholesterol efflux was similar to that of WT (wild-type) apoA-I. Gene transfer of the three mutants in apoA-I-/- mice generated aberrant HDL phenotypes. The total plasma cholesterol of mice expressing the apoA-I(R160L)Oslo, apoA-I(R149A) and apoA-I(R151C)Paris mutants was reduced by 78, 59 and 61% and the apoA-I levels were reduced by 68, 64 and 55% respectively, as compared with mice expressing the WT apoA-I. The CE (cholesteryl ester)/TC (total cholesterol) ratio of HDL was decreased and the apoA-I was distributed in the HDL3 region. apoA-I(R160L)Oslo and apoA-I(R149A) promoted the formation of prebeta1 and alpha4-HDL subpopulations and gave a mixture of discoidal and spherical particles. apoA-I(R151C)Paris generated subpopulations of different sizes that migrate between prebeta and alpha-HDL and formed mostly spherical and a few discoidal particles. Simultaneous treatment of mice with adenovirus expressing any of the three mutants and human LCAT normalized plasma apoA-I, HDL cholesterol levels and the CE/TC ratio. It also led to the formation of spherical HDL particles consisting mostly of alpha-HDL subpopulations of larger size. The correction of the aberrant HDL phenotypes by treatment with LCAT suggests a potential therapeutic intervention for HDL abnormalities that result from specific mutations in apoA-I.

A Caucasian male with very low blood cholesterol and low apoA-II without evidence of atherosclerosis

BACKGROUND

It is well known that a high level of apolipoprotein (apo) A-II can be associated with familial combined hyperlipidaemia, and that high apolipoprotein profiles can contribute to the development of atherosclerosis. The serum lipoprotein/apolipoprotein profile of a Caucasian patient who had unusually low serum total cholesterol (83 mg dL(-1)) and triglyceride (28 mg dL(-1)) levels despite a high body mass index (33.5 kg m(-2)), is the subject of this report.

MATERIALS AND METHODS

Each lipoprotein was isolated from serum by sequential ultracentrifugation, and serum and lipoprotein lipids and proteins were determined. The cholesteryl ester (CE) conversion ability of lecithin:cholesterol acyltransferase and CE transfer activity of CE transfer protein were assayed, and the composition of apolipoprotein and lipoprotein(-1) was analyzed by electrophoresis and Western blot analysis.

RESULTS

Electrophoresis and immunodetection analyses revealed a 60% decrease in the apoA-II band intensity compared to normal reference serum. The decreased apoA-II was associated with reduced very low density lipoprotein-cholesterol and protein content, as well as a greater high-density lipoprotein (HDL)(2) size with high cholesterol content. The CE conversion activity and CE transfer activity of HDL(3) were almost totally lacking in the hypolipidaemic serum, although the expression level of lecithin:cholesterol acyltransferase was normal. Electron microscopy revealed that the obese patient had larger HDL(2) and HDL(3) particle sizes than those of reference serum.

CONCLUSION

These results suggest that a decreased apoA-II protein in serum and increased HDL-cholesterol and particle size might protect against hyperlipidaemia and the atherosclerotic process, even in a patient with severe obesity.

Anti-Atherosclerotic Effect of a New Synthetic Functional Oil Containing Mono- and Diacylglycerol from Corn Oil

Synthetic oil containing diacylglycerol and monoacylglycerol, called 'functional oil' (FO), was newly produced and evaluated for its putative anti-atherosclerotic potential by in vitro assays and in vivo test using hypercholesterolemic mice (C57BL/6). The FO revealed good inhibitory activities against both liver acyl-CoA:cholesterol acyltransferase and serum lipoprotein-associated phospholipase A2. The FO showed enhanced activities on lipoprotein interaction such as HDL particle rearrangement to produce different sizes of HDL species. In control mice, hypercholesterolemia was induced by consumption of high-cholesterol, high-fat (HCHF) diet that contained 1.25% cholesterol/15% fat/0.5% Na-cholate with or without 5% of corn oil. In experimental mice, 5% of the FO + HCHF diet was fed during the same period. After the 4-week administration of the diet, serum total cholesterol concentration of the FO-fed group decreased by 38 or 20% when compared to the HCHF diet control group or corn oil (99.9% of triacylglycerol) diet group, respectively. The percentage of HDL cholesterol to total cholesterol was 36% of HDL cholesterol in the FO-fed group, while the HCHF control group and corn oil-fed group showed 21 and 25%, respectively. These results indicate that the FO possesses a blood cholesterol-lowering effect in mouse model and inhibition effects against the atherogenic enzymes.

Apolipoprotein A-I assumes a "looped belt" conformation on reconstituted high density lipoprotein

Apolipoprotein A-I (apoA-I) plays a central role in the reverse cholesterol transport pathway; however, the structural basis for its antiatherogenic effects remains poorly understood. Here we employ EPR spectroscopy and fluorescence resonance energy transfer to elucidate the conformation and relative alignment of apoA-I monomers on discoidal (9.4 nm) reconstituted high density lipoprotein (rHDL). EPR spectroscopy provided evidence for an extended helical secondary structure. Position 139 since it was the only residue examined to display a dynamic motional character consistent with a flexible loop structure. The EPR spectra of nitroxide probes at positions 133 and 146 exhibit spin coupling, indicating that these positions are proximal to an apoA-I paired counterpart on the perimeter of rHDL. fluorescence resonance energy transfer studies employing engineered apoA-I variants possessing a single tryptophan (energy donor) and/or a single cysteine (whose thiol moiety was covalently labeled with an extrinsic energy acceptor) provided evidence that paired apoA-I molecules around the perimeter of rHDL align in an extended antiparallel conformation. Taken together with the observation that the EPR spectra of nitroxide probes positioned at intervening sequence positions (134-145) do not exhibit spin coupling, this has led us to propose a "looped belt" model, wherein residues 133-146 comprise a flexible loop segment that confers to apoA-I an intrinsic ability to adapt its structure to accommodate changing particle lipid content. Specifically, in the looped belt model, with the exception of amino acids 134-145, apoA-I aligns with its counterpart in a helix 5-helix 5 registry, centered at position 139.

Cysteine mutants of human apolipoprotein A-I: a study of secondary structural and functional properties

Apolipoprotein A-I(Milano) (A-I(M)) (R173C), a natural mutant of human apolipoprotein A-I (apoA-I), and five other cysteine variants of apoA-I at residues 52 (S52C), 74 (N74C), 107 (K107C), 129 (G129C), and 195 (K195C) were generated. Cysteine residues were incorporated in each of the various helices at the same helical wheel position as for the substitution in A-I(M). The secondary structural properties of the monomeric mutants, their abilities to bind lipid and to promote cholesterol efflux from THP-1 macrophages, and the possibility of antiperoxidation were investigated. Results showed that the alpha helical contents of all of the cysteine mutants were similar to that of wild-type apoA-I (wtapoA-I). The cysteine variant of A-I(M) at residue 173 [A-I(M)(R173C)] exhibited weakened structural stability, whereas A-I(G129C) a more stable structure than wtapoA-I. A-I(G129C) and A-I(K195C) exhibited significantly impaired capabilities to bind lipid compared with wtapoA-I. A-I(K107C) possessed a higher capacity to promote cholesterol efflux from macrophages than wtapoA-I, and A-I(M)(R173C) and A-I(K195C) exhibited an impaired efflux capability. Neither A-I(M)(R173C) nor any other cysteine mutant could resist oxidation against lipoxygenase. In summary, in spite of the similar mutant position on the helix, these variants exhibited different structural features or biological activities, suggesting the potential influence of the local environment of mutations on the whole polypeptide chain.

Structural and functional properties of V156K and A158E mutants of apolipoprotein A-I in the lipid-free and lipid-bound states

Val156 of apolipoprotein A-I (apoA-I) was found to be a key amino acid in the structure and function of high density lipoprotein (HDL) (J. Biol. Chem., 275: 26821-26827, 2000). To determine more precisely the functions of the individual amino acids proximal to Val156, serial point mutants of proapoA-I, including V156K, D157K, and A158E, were overexpressed and purified to at least 95% purity. In the lipid-free state, A158E exhibited the most profound self-associative patterns and the least pronounced dimyristoyl phosphatidylcholine (DMPC) clearance activities. In the lipid-bound state, A158E formed a larger reconstituted HDL (rHDL) with palmitoyloleoyl phosphatidylcholine (POPC), approximately 120 A, whereas other mutants and the wild type (WT) formed 97 A of POPC-rHDL. Cross-linking analysis revealed that A158E-rHDL harbored at least four protein molecules in the particle, while other rHDL conformations contained only two protein molecules. All of the POPC-rHDL produced smaller HDL, around 78 A, after 24 h of incubation in the presence of low density lipoprotein at 37 degrees C. V156K and A158E exhibited decreased lecithin:cholesterol acyltransferase activation activity in the POPC-rHDL state, showing <2% of WT reactivity (apparent Vmax/Km). A158E also displayed markedly different properties in secondary structure, and its accessibility to proteolytic enzymes is different. These results suggest that the two amino acids in helix 6, Val156 and Ala158, are critical to both the structure and function of rHDL.

Lipid transfer proteins (LTP) and atherosclerosis

This review deals with four lipid transfer proteins (LTP): three are involved in cholesteryl ester (CE) synthesis or transport, the fourth deals with plasma phospholipid (PL) transfer. Experimental models of atherosclerosis, clinical and epidemiological studies provided information as to the relationship of these LTP(s) to atherosclerosis, which is the main focus of this review. Thus, inhibition of acyl-CoA:cholesterol acyltransferase (ACAT) 1 and 2 decreases cholesterol absorption, plasma cholesterol and aortic cholesterol esterification in the aorta. The discovery that tamoxifen is a potent ACAT inhibitor explained the plasma cholesterol lowering of the drug. The use of ACAT inhibition in humans is under current investigation. As low cholesteryl ester transfer protein (CETP) activity is connected with high HDL-C, several CETP inhibitors were tried in rabbits, with variable results. A new CETP inhibitor, Torcetrapib, was tested in humans and there was a 50-100% increase in HDL-C. Lecithin cholesterol acyl-transferase (LCAT) influences oxidative stress, which can be lowered by transient LCAT gene transfer in LCAT-/- mice. Phospholipid transfer protein (PLTP) deficiency reduced apo B production in apo E-/- mice, as well as oxidative stress in four models of mouse atherosclerosis. In conclusion, the ability to increase HDL-C so markedly by inhibitors of CETP introduces us into a new era in prevention and treatment of coronary heart disease (CHD).

Synthesis of cinnamic acid derivatives and their inhibitory effects on LDL-oxidation, acyl-CoA:cholesterol acyltransferase-1 and -2 activity, and decrease of HDL-particle size

A series of cinnamic acid derivatives were synthesized and their biological abilities on lipoprotein metabolism were examined. Among the tested compounds, 4-hydroxycinnamic acid (l-phenylalanine methyl ester) amide (1) and 3,4-dihydroxyhydrocinammic acid (l-aspartic acid dibenzyl ester) amide (2) inhibited human acyl-CoA:cholesterol acyltransferase-1 and -2 activities with apparent IC(50) around 60 and 95 microM, respectively. Compounds 1 and 2 also served as an antioxidant against copper mediated low-density lipoproteins (LDL) oxidation with apparent IC(50)=52 and 3 microM, compound 1 and 2, respectively. Additionally, decrease of HDL-particle size under presence of LDL was inhibited by the 1 at 307 microM of final concentration. Treatment of the 1 or 2 did not influence normal growth of RAW264.7 without detectable cytotoxic activity from a cell viability test. These results suggest that the new cinnamic acid derivatives possess useful biological activity as an anti-atherosclerotic agent with inhibition of cellular cholesterol storage and transport by the both ACAT, inhibition of LDL-oxidation, HDL particle size rearrangement.

Human plasma high-density lipoproteins are stabilized by kinetic factors

High-density lipoproteins (HDL) are heterogeneous complexes of proteins and lipids that mediate cholesterol removal from the body. Our thermal and chemical denaturation studies of mature spherical HDL isolated from human plasma show that, contrary to the widely held assumption, the particle stability has a kinetic rather than thermodynamic origin. Guanidinum hydrochloride (GdmHCl) concentration jumps at 25 degrees C monitored by circular dichroism (CD) at 222 nm reveal two dominant irreversible kinetic phases in HDL denaturation. The slower phase (relaxation time tau(1) approximately 2 x 10(4) seconds) is observed in 1-6 M GdmHCl, and the faster phase (tau(2) approximately 2 x 10(3) seconds) is detected in 3-6 M GdmHCl. Comparison of the free energy barriers associated with these phases, deltaG* = 16-17 kcal mol(-1), with the near-zero apparent thermodynamic stability inferred from the spectroscopic measurements after prolonged incubation in 0-6 M GdmHCl at 22 degrees C indicates the kinetic origin for HDL stabilization. Electron microscopic analysis of HDL incubated in 0-6 M GdmHCl suggests that the slower kinetic phase involves HDL fusion, while the faster phase involves particle rupture and release of the apolar lipid core. Thermal denaturation experiments indicate high enthalpic barriers for the particle rupture that may arise from the transient disruption of lipid and/or protein packing interactions. These results corroborate our earlier analysis of model discoidal HDL and indicate that a kinetic mechanism provides a universal natural strategy for lipoprotein stabilization. Such a mechanism may facilitate structural integrity of the heterogeneous lipoprotein particles, slow their spontaneous interconversions, and thereby modulate lipoprotein lifetime and functions.

Structure-function relationships of apolipoprotein A-I: a flexible protein with dynamic lipid associations

PURPOSE OF REVIEW

Apolipoprotein A-I is the major structural protein of HDL. Its physicochemical properties maintain a delicate balance between maintenance of stable lipoproteins and the ability to associate with and dissociate from the lipid transported. Here we review the progress made in the last 2-3 years on the structure-function relationships of apolipoprotein A-I, including elements related to the ATP binding cassette transporter A1.

RECENT FINDINGS

Current evidence now supports the so-called 'belt' or 'hairpin' models for apolipoprotein A-I conformation when bound to discoidal lipoproteins. In-vivo expression of apolipoprotein A-I mutant proteins has shown that both the N- and C-terminal domains are important for lipid association as well as for the esterification reaction, particularly binding of cholesteryl esters and formation of mature alpha-migrating lipoproteins. This property is apparently quite distinct from the activation of the enzyme lecithin cholesterol acyl transferase, which requires interaction with the central helix 6. The interaction of apolipoprotein A-I with the ATP binding cassette transporter A1 has been shown to require the C-terminal domain, which is proposed to mediate the opening of the helix bundle formed by lipid-free or lipid-poor apolipoprotein A-I and allow its association with hydrophobic binding sites.

SUMMARY

Significant progress has been made in the understanding of the molecular mechanisms controlling the folding of apolipoprotein A-I and its interaction with lipids and various other protein factors involved in HDL metabolism.

Natural mutations of apolipoprotein A-I impairing activation of lecithin:cholesterol acyltransferase

Five natural mutations of apolipoprotein A-I (apoA-I), apoA-I(A95D), apoA-I(Y100H), apoA-I(E110K), apoA-I(V156E) and apoA-I(H162Q), were studied for their ability to activate lecithin:cholesterol acyltransferase (LCAT). Mutants apoA-I(E110K), apoA-I(V156E) and apoA-I(H162Q) had an impaired ability to activate LCAT. Combined with data on other apoA-I mutants this finding is consistent with the idea that the central region between amino acids 110 and 160 is likely to be the "active site" of apoA-I involved in the interaction with LCAT and that a specific sequence of apoA-I is required for activation of the enzyme.