Structure of apolipoprotein B-100 of human low density lipoproteins

Quantification of LDL-Cholesterol Corrected for Molar Concentration of Lipoprotein(a)

PURPOSE

Cholesterol in lipoprotein(a) [Lp(a)-C] is commonly estimated as 30% of the measured Lp(a) mass. However, difficulties in the accurate measurement of Lp(a) mass, along with the inaccuracy of the 30% assumption, produce erroneous values when LDL-C is corrected for Lp(a) [LDL-CLp(a)corr]. Our aim was to develop a new formula for LDL-CLp(a)corr to reduce this error.

METHODS

We developed a new formula to calculate Lp(a)-C from the molar measurement of Lp(a), which is Lp(a) nmol/L × 0.077 = Lp(a)-C mg/dL. The calculated Lp(a)-C is subtracted from LDL-C to obtain LDL-CLp(a)corr. The results obtained with our novel formula versus the conventional formula were compared in 440 samples from 239 participants enrolled in the BANTING study.

RESULTS

With the conventional formula, approximately 7% of samples with low LDL-C resulted in negative LDL-CLp(a)corr values. With the new formula, no negative LDL-CLp(a)corr values occurred. Among groups with the highest Lp(a)/apoB ratio (p < 0.001) and smaller apolipoprotein(a) isoform size (p < 0.006), LDL-CLp(a)corr was significantly underestimated by the conventional formula, which may result in the undertreatment of some patients.

CONCLUSION

The new formula provides more reliable estimates of LDL-CLp(a)corr than the conventional formula.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02739984.

Lipoprotein(a) and the Risk for Coronary Heart Disease and Ischemic Stroke Events Among Black and White Adults With Cardiovascular Disease

Background

It is unclear whether lipoprotein(a) is associated with coronary heart disease (CHD) and ischemic stroke events in White and Black adults with atherosclerotic cardiovascular disease (ASCVD).

Methods and Results

We conducted a case‐cohort analysis, including Black and White REGARDS (Reasons for Geographic and Racial Differences in Stroke) study participants ≥45 years of age with prevalent ASCVD (ie, CHD or stroke) at baseline between 2003 and 2007. Baseline lipoprotein(a) molar concentration was measured in participants with ASCVD who experienced a CHD event by December 2017 (n=1166) or an ischemic stroke by September 2019 (n=492) and in a random subcohort of participants with prevalent ASCVD (n=1948). The hazard ratio (HR) for CHD events per 1 SD (1.5 units) higher log‐transformed lipoprotein(a) was 1.26 (95% CI, 1.02–1.56) among Black participants and 1.16 (95% CI, 1.02–1.31) among White participants (P value comparing HRs, 0.485). The HR for CHD events per 1 SD higher log‐lipoprotein(a) within subgroups with hs‐CRP (high‐sensitivity C‐reactive protein) ≥2 and <2 mg/L was 1.31 (95% CI, 0.99–1.73) and 1.23 (95% CI, 0.85–1.80), respectively (P value comparing HRs, 0.836), among Black participants, and 1.07 (95% CI, 0.91–1.27) and 1.36 (95% CI, 1.10–1.70), respectively (P value comparing HRs, 0.088), among White participants. There was no evidence that the association between lipoprotein(a) and CHD events differed by statin use. There was no evidence of an association between lipoprotein(a) and ischemic stroke events among Black or White participants.

Conclusions

Higher lipoprotein(a) levels were associated with an increased risk for CHD events in Black and White adults with ASCVD.

Atherosclerosis severity in patients with familial hypercholesterolemia: The role of T and B lymphocytes

Background and aims

Familial hypercholesterolemia (FH) is characterized by lifelong exposure to high LDL-c concentrations and premature atherosclerotic cardiovascular disease; nevertheless, disease severity can be heterogeneous.We aimed at evaluating if the immune-inflammatory system could modulate atherosclerosis burden in FH.

Methods

From a cohort of subjects with confirmed FH (Dutch Lipid Clinic Network and genotype), 92 patients receiving high-intensity lipid-lowering therapy (statin ± ezetimibe) were included. The extension and severity of coronary atherosclerosis was assessed by standardized reporting systems (CAD-RADS) for coronary computed tomography angiography (CCTA) and coronary artery calcium (CAC) scores. Lipids, apolipoproteins, anti-oxLDL and anti-apolipoprotein B-D peptide (anti-ApoB-D) autoantibodies (IgM and IgG), lymphocytes subtypes, platelet, monocyte and endothelial microparticles (MP), IgM levels (circulating or produced by B1 cells) and cytokines in the supernatant of cultured cells were determined. Multiple linear regression models evaluated associations of these biomarkers with CAC and CAD-RADS scores.

Results

In univariate analysis CAC correlated with age, systolic blood pressure, TCD4+ cells, and titers of IgM anti-ApoB-D. In multiple linear regression [ANOVA F = 2.976; p = 0.024; R² = 0.082), CD4+T lymphocytes (B = 35.289; beta = 0.277; p = 0.010; 95%CI for B 8.727 to 61.851), was independently associated with CAC. CAD-RADS correlated with age, systolic blood pressure, titers of IgM anti-ApoB-D, and endothelial MP in univariate analysis. In multiple linear regression, [ANOVA F = 2.790; p = 0.032; R² = 0.119), only age (B = 0.027; beta = 0.234; p = 0.049; 95% CI for B 0.000 to 0.053) was independent predictor.

Conclusions

In subjects with FH, under high-intensity lipid-lowering therapy, age and CD4+T cells were associated to atherosclerosis burden.

The Targeting of Native Proteins to the Endoplasmic Reticulum-Associated Degradation (ERAD) Pathway: An Expanding Repertoire of Regulated Substrates

All proteins are subject to quality control processes during or soon after their synthesis, and these cellular quality control pathways play critical roles in maintaining homeostasis in the cell and in organism health. Protein quality control is particularly vital for those polypeptides that enter the endoplasmic reticulum (ER). Approximately one-quarter to one-third of all proteins synthesized in eukaryotic cells access the ER because they are destined for transport to the extracellular space, because they represent integral membrane proteins, or because they reside within one of the many compartments of the secretory pathway. However, proteins that mature inefficiently are subject to ER-associated degradation (ERAD), a multi-step pathway involving the chaperone-mediated selection, ubiquitination, and extraction (or “retrotranslocation”) of protein substrates from the ER. Ultimately, these substrates are degraded by the cytosolic proteasome. Interestingly, there is an increasing number of native enzymes and metabolite and solute transporters that are also targeted for ERAD. While some of these proteins may transiently misfold, the ERAD pathway also provides a route to rapidly and quantitatively downregulate the levels and thus the activities of a variety of proteins that mature or reside in the ER.

HIV-Infected Naïve Patients Exhibit Endothelial Dysfunction Concomitant with Decreased Natural Antibodies Against Defined Apolipoprotein B Autoantigens

Fundamento:

Fatores de risco definidos para HIV e tradicionais podem estar associados a um aumento de eventos cardiovasculares. Estudos recentes sugerem que a resposta imune humoral à LDL modificada pode estar associada ao processo de aterosclerose.

Objetivos:

Avaliar a presença de anti-LDL oxidada e de peptídeos derivados da Apolipoproteína B no sangue, bem como sua associação à função endotelial na infecção por HIV.

Métodos:

Este estudo incluiu consecutivamente sujeitos com idade, sexo e dados demográficos correspondentes em dois grupos: (1) indivíduos infectados com HIV e naïve para terapia antiviral e (2) indivíduos não infectados. A aterosclerose subclínica foi avaliada pela espessura íntima-média, utilizando-se a ultrassonografia das artérias carótidas. A função endotelial foi determinada pela dilatação mediada por fluxo (DMF) da artéria braquial por ultrassonografia. Os níveis de autoanticorpos (IgM, IgG) de lipoproteínas de baixa densidade antioxidadas (LDL-ox), fragmentos de peptídeos antiapolipoproteína B (peptídeos ApoB-D e 0033G-Cys), e citocina foram avaliados por meio de ELISA.

Resultados:

Os resultados deste estudo não mostraram diferenças na aterosclerose subclínica entre os grupos. Entretanto, os sujeitos infectados com HIV apresentaram uma DMF mais baixa, em comparação com os sujeitos não infectados. Portanto, os sujeitos infectados com HIV apresentaram níveis mais altos de citocinas inflamatórias, títulos de IgG anti-LDL-ox, e IgG anti-ApoB-D. Em contraste, títulos de IgM anti-ApoB-D foram mais baixos em indivíduos infectados com HIV e associados a funções endoteliais diminuídas.

Conclusões:

Os resultados deste estudo mostram que a infecção por HIV, em sujeitos naïve, está associada à disfunção endotelial e à diminuição de anticorpos naturais para antígenos Apo-B.

Association of apolipoprotein B XbaI (rs693) polymorphism and gallstone disease risk based on a comprehensive analysis

BACKGROUND

Our aim was to investigate the association between XbaI gene polymorphisms in the apolipoprotein B (APOB) gene and gallstone disease (GD) risk through a comparison of the allele and genotype distribution frequencies at this site using meta-analysis.

METHODS

A literature search was performed using PubMed and Wanfang through Jun 1, 2020. Odds ratios (ORs) and 95 % confidence intervals (CIs) were used to assess the strength of associations.

RESULTS

After a comprehensive search, 14 different articles that met the inclusion criteria were selected, with 1583 cases and 1794 controls. Individuals carrying the A-allele or AA genotype of the rs693 polymorphism were determined to possibly have an increased risk of GD. For example, there was a significant relationship between the rs693 polymorphism and increased GD risk in the whole group (OR: 1.40, 95 % CI: 1.05-1.87 in the allelic contrast model), the Asian population (OR: 1.58, 95 % CI: 1.48-2.84 in the heterozygote model), and the hospital-based source of the control (OR: 1.79, 95 % CI: 1.13-2.84 in the dominant model).

CONCLUSIONS

This study suggests that the APOB rs693 polymorphism is potentially associated with GD susceptibility, which might offer a detection marker for use in future large scale clinic research.

Host Defence Cryptides from Human Apolipoproteins: Applications in Medicinal Chemistry

Several eukaryotic proteins with defined physiological roles may act as precursors of cryptic bioactive peptides released upon protein cleavage by the host and/or bacterial proteases. Based on this, the term "cryptome" has been used to define the unique portion of the proteome encompassing proteins with the ability to generate bioactive peptides (cryptides) and proteins (crypteins) upon proteolytic cleavage. Hence, the cryptome represents a source of peptides with potential pharmacological interest. Among eukaryotic precursor proteins, human apolipoproteins play an important role, since promising bioactive peptides have been identified and characterized from apolipoproteins E, B, and A-I sequences. Human apolipoproteins derived peptides have been shown to exhibit antibacterial, anti-biofilm, antiviral, anti-inflammatory, anti-atherogenic, antioxidant, or anticancer activities in in vitro assays and, in some cases, also in in vivo experiments on animal models. The most interesting Host Defence Peptides (HDPs) identified thus far in human apolipoproteins are described here with a focus on their biological activities applicable to biomedicine. Altogether, reported evidence clearly indicates that cryptic peptides represent promising templates for the generation of new drugs and therapeutics against infectious diseases.

Association of Common Single Nucleotide Polymorphisms of Candidate Genes with Gallstone Disease: A Meta-Analysis

Numerous studies have investigated the relationship between various candidate gene polymorphisms and gallbladder stone disease (GSD) across ethnic populations; however, the results are often inconsistent. This meta-analysis aims to comprehensively evaluate the influence of common ABCG8 T400K, ABCG8 D19H, ABCG8 C54Y, ApoB100 EcoRI, ApoB100 XbaI, ApoE HhaI, CETP TaqI, CYP7A1 Bsa, LRPAP1 I/D and TNF-α A308G polymorphisms on the risk of gallbladder stone disease. 33 Full-text articles with 9250 cases and 12,029 healthy controls (total 21,279 subjects) were analyzed using the RevMan software (V5.1) and the Comprehensive Meta-analysis software (Version 2.0, BIOSTAT, Englewood, NJ) a Random-effects model was applied. Begg's funnel plots, Fail-safe number, Egger's regression intercept and Begg and Mazumdar rank correlation tests were performed for the potential publication bias and sensitivity analysis. The studies were also sub-grouped into European and non-European groups to find out role of ethnicity, if any, on GSD risk. Studies included in quantitative synthesis were ABCG8 T400K rs4148217 (cases/controls, n = 671/1416) (4 studies), ABCG8 D19H rs11887534 (n = 1633/2306) (8 studies), ABCG8 C54Y rs4148211 (n = 445/1194) (3 studies), ApoB100 EcoRI rs1042031 (n = 503/390) (4 studies), ApoB100 XbaI rs693 (n = 1214/1389) (9 studies), ApoE HhaI rs429358 (n = 1335/1482) (12 studies), CETP TaqI rs708272 (n = 1038/1025) (5 studies), CYP7A1 Bsa rs3808607 (n = 565/514) (3 studies), LRPAP1 I/D rs11267919 (n = 849/900) (3 studies), TNF-α A308G rs1800629 (n = 997/1413) (3 studies). The combined results displayed significant association of ABCG8 D19H (GC + CC) [OR with 95%CI = 2.2(1.7-2.8); p < 0.00001], ABCG8 Y54C (GA + GG) [OR with 95%CI = 0.65(0.5-0.9); p = 0.01]. APOB100 EcoRI (GG vs. AA) [OR with 95%CI = 0.51(0.3-0.9); p = 0.05], (GG vs. GA) [OR with 95%CI = 0.6(0.4-0.9); p = 0.04], (GA + AA) [OR with 95%CI = 0.6(0.4-0.9); p = 0.006]. APOB Xba I (X^- vs. X⁺) [OR with 95%CI = 0.53(0.3-0.8); p = 0.006. APOE Hha I (E4/E4 vs. E3/E3) [OR with 95%CI = 3.5(1.1-14.9); p = 0.04] and LRPAP1 I/D (ID + II) [OR with 95%CI = 1.27(1.0-1.6); p = 0.03] with the GSD risk. It was found that ABCG D19H was significantly associated with GSD in both European and Non-European populations. While APOB XbaI and LRPAP1 I/D markers were associated with gallstone disease only in Non- European population. Additionally, APOE HhaI and APOB 100 ECoRI were found to be associated with GSD only in European population. The results of quantitative synthesis suggest that the ABCG8 D19H polymorphism was associated with the increased risk of GSD in both European and Non-European populations, APOE Hha I and LRPAP1 I/D polymorphisms were associated with the increased risk of GSD in European and Non-European population respectively. However, no association was found in ABCG8 T400K, CETP Taq1, CYP7A1 Bsa and TNF-A308G polymorphisms with Gallstone Disease.

Apolipoprotein B100/Low-Density Lipoprotein Regulates Proteolysis and Functions of von Willebrand Factor under Arterial Shear

BACKGROUND

 Proteolytic cleavage of von Willebrand factor (VWF) by a plasma a disintegrin and metalloproteinase with a thrombospondin type 1 motifs, member 13 (ADAMTS13) is regulated by shear stress and binding of coagulation factor VIII, platelets or platelet glycoprotein 1b, and ristocetin to VWF.

OBJECTIVE

 Current study aims to identify novel VWF binding partners that may modulate VWF functions under physiological conditions.

METHODS

 A deoxyribonucleic acid aptamer-based affinity purification of VWF, followed by tandem mass spectrometry, functional, and binding assays was employed.

RESULTS

 Apolipoprotein B100/low-density lipoprotein (apoB100/LDL) was identified as a novel VWF-binding partner. Purified apoB100/LDL was able to accelerate the proteolytic cleavage of VWF by ADAMTS13 under shear in a concentration-dependent manner. This rate-enhancing activity was dramatically reduced when apoB100/LDL was oxidized. More interestingly, the oxidized apoB100/LDL appeared to compete with native apoB100/LDL for its enhancing activity on VWF proteolysis under shear. As a control, a purified apoA1/high-density lipoprotein (apoA1/HDL) or apoB48 exhibited a minimal or no activity enhancing VWF proteolysis by ADAMTS13 under the same conditions. Both VWF and ADAMTS13 were able to bind native or oxidized apoB100/LDL with high affinities. No binding interaction was detected between VWF (or ADAMTS13) and apoA1/HDL (or apoB48). Moreover, apoB100/LDL but not its oxidized products inhibited the adhesion of platelets to ultra large VWF released from endothelial cells under flow. Finally, significantly reduced ratios of high to low molecular weight of VWF multimers with increased levels of plasma VWF antigen were detected in LDLR-/- mice fed with high cholesterol diet.

CONCLUSION

 These results indicate that apoB100/LDL may be a novel physiological regulator for ADAMTS13-VWF functions.

The cell biology of the hepatocyte: A membrane trafficking machine

The liver performs numerous vital functions, including the detoxification of blood before access to the brain while simultaneously secreting and internalizing scores of proteins and lipids to maintain appropriate blood chemistry. Furthermore, the liver also synthesizes and secretes bile to enable the digestion of food. These diverse attributes are all performed by hepatocytes, the parenchymal cells of the liver. As predicted, these cells possess a remarkably well-developed and complex membrane trafficking machinery that is dedicated to moving specific cargos to their correct cellular locations. Importantly, while most epithelial cells secrete nascent proteins directionally toward a single lumen, the hepatocyte secretes both proteins and bile concomitantly at its basolateral and apical domains, respectively. In this Beyond the Cell review, we will detail these central features of the hepatocyte and highlight how membrane transport processes play a key role in healthy liver function and how they are affected by disease.

Can modulators of apolipoproteinB biogenesis serve as an alternate target for cholesterol-lowering drugs?

Understanding the molecular defects underlying cardiovascular disease is necessary for the development of therapeutics. The most common method to lower circulating lipids, which reduces the incidence of cardiovascular disease, is statins, but other drugs are now entering the clinic, some of which have been approved. Nevertheless, patients cannot tolerate some of these therapeutics, the drugs are costly, and/or the treatments are approved for only rare forms of disease. Efforts to find alternative treatments have focused on other factors, such as apolipoproteinB (apoB), which transports cholesterol in the blood stream. The levels of apoB are regulated by endoplasmic reticulum (ER) associated degradation as well as by a post ER degradation pathway in model systems, and we suggest that these events provide novel therapeutic targets. We discuss first how cardiovascular disease arises and how cholesterol is regulated, and then summarize the mechanisms of action of existing treatments for cardiovascular disease. We then review the apoB biosynthetic pathway, focusing on steps that might be amenable to therapeutic interventions.

A correlation between secondary structure and rheological properties of low-density lipoproteins at air/water interfaces

The secondary structure of apolipoprotein B-100 is studied within the bulk phase and at the air/water interface. In these "in viro" experiments, infrared reflection absorption spectroscopy (IRRAS) study was performed at the air/water interface while circular dichroism (CD) was conducted in the bulk phase. In the bulk phase, the conformational structure containing a significant amount of β-structure, whereas varying amount of α-helix, unordered structures, and β-sheet were observed at the air/water interface depending on the low-density lipoprotein (LDL) film interfacial pressure. The present IRRAS results demonstrate the importance of interfacial pressure-induced structural conformations on the apoB-100. A correlation between the secondary structure of the apoB-100 protein and the monomolecular film elasticity at the air/water interface was also established. The orientation of apoB-100 with respect to the LDL film-normal was found to depend on the interfacial pressure exhibited by the monomolecular film. These results may shed light on LDL's pivotal role in the progression of atherosclerotic coronary artery disease as demonstrated previously by clinical trials.

Novel human bioactive peptides identified in Apolipoprotein B: Evaluation of their therapeutic potential

Host defence peptides (HDPs) are short, cationic amphipathic peptides that play a key role in the response to infection and inflammation in all complex life forms. It is increasingly emerging that HDPs generally have a modest direct activity against a broad range of microorganisms, and that their anti-infective properties are mainly due to their ability to modulate the immune response. Here, we report the recombinant production and characterization of two novel HDPs identified in human Apolipoprotein B (residues 887-922) by using a bioinformatics method recently developed by our group. We focused our attention on two variants of the identified HDP, here named r(P)ApoB_L and r(P)ApoB_S, 38- and 26-residue long, respectively. Both HDPs were found to be endowed with a broad-spectrum antimicrobial activity while they show neither toxic nor haemolytic effects towards eukaryotic cells. Interestingly, both HDPs were found to display a significant anti-biofilm activity, and to act in synergy with either commonly used antibiotics or EDTA. The latter was selected for its ability to affect bacterial outer membrane permeability, and to sensitize bacteria to several antibiotics. Circular dichroism analyses showed that SDS, TFE, and LPS significantly alter r(P)ApoB_L conformation, whereas slighter or no significant effects were detected in the case of r(P)ApoB_S peptide. Interestingly, both ApoB derived peptides were found to elicit anti-inflammatory effects, being able to mitigate the production of pro-inflammatory interleukin-6 and nitric oxide in LPS induced murine macrophages. It should also be emphasized that r(P)ApoB_L peptide was found to play a role in human keratinocytes wound closure in vitro. Altogether, these findings open interesting perspectives on the therapeutic use of the herein identified HDPs.

Hepatitis C virus G1b infection decreases the number of small low-density lipoprotein particles

AIM: To investigate how hepatitis C virus (HCV) G1b infection influences the particle number of lipoproteins.

METHODS: The numbers of lipoprotein particles in fasting sera from 173 Japanese subjects, 82 with active HCV G1b infection (active HCV group) and 91 with cleared HCV infection (SVR group), were examined. Serum lipoprotein was fractionated by high-performance liquid chromatography into twenty fractions. The cholesterol and triglyceride concentrations in each fraction were measured using LipoSEARCH. The number of lipoprotein particles in each fraction was calculated using a newly developed algorithm, and the relationship between chronic HCV G1b infection and the lipoprotein particle number was determined by multiple linear regression analysis.

RESULTS: The median number of low-density lipoprotein (LDL) particles was significantly lower in the active HCV group [1182 nmol/L, interquartile range (IQR): 444 nmol/L] than in the SVR group (1363 nmol/L, IQR: 472 nmol/L, P < 0.001), as was that of high-density lipoprotein (HDL) particles (14168 nmol/L vs 15054 nmol/L, IQR: 4114 nmol/L vs 3385 nmol/L, P = 0.042). The number of very low-density lipoprotein (VLDL) particles was similar between the two groups. Among the four LDL sub-fractions, the number of large LDL particles was similar between the two groups. However, the numbers of medium (median: 533.0 nmol/L, IQR: 214.7 nmol/L vs median: 633.5 nmol/L, IQR: 229.6 nmol/L, P < 0.001), small (median: 190.9 nmol/L, IQR: 152.4 nmol/L vs median: 263.2 nmol/L, IQR: 159.9 nmol/L; P < 0.001), and very small LDL particles (median: 103.5 nmol/L, IQR: 66.8 nmol/L vs median: 139.3 nmol/L, IQR: 67.3 nmol/L, P < 0.001) were significantly lower in the active HCV group than in the SVR group, respectively. Multiple linear regression analysis indicated an association between HCV G1b infection and the decreased numbers of medium, small, and very small LDL particles. However, active HCV infection did not affect the number of large LDL particles or any sub-fractions of VLDL and HDL particles.

CONCLUSION: HCV G1b infection decreases the numbers of medium, small, and very small LDL particles.

Adsorption kinetics of low-density lipoproteins with Langmuir monolayer

The present work utilizes the Langmuir monolayer technique to detect the adsorption kinetics of native low-density lipoproteins and their oxidized form with the lipid monolayer. We found that low-density lipoproteins and oxidized low-density lipoproteins are able to penetrate the LM up to pressure π = 9.9 and 11.6 mN/m. Also, the adsorption constants of both particles were found to depend strongly on the monolayer initial pressure. It is found that less compressed lipid monolayers could accommodate more native low-density lipoproteins than the oxidized ones due their higher binding affinity toward monolayers. The probable α-helical regions along the apoproteinB-100 secondary structure and average hydrophobicity could explain partially their adsorption kinetics into lipid monolayers. This simplified 'in vitro' study of low-density lipoprotein-monolayer interaction may serve as a step further to understand the mechanism and bioactivity of the atherosclerotic process. Also, it may shed light on the oxidized low-density lipoprotein's role in plaque formation in the innermost arterial wall in blood vessels.

Glycosyltransferases and non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease and its incidence is increasing worldwide. However, the underlying mechanisms leading to the development of NAFLD are still not fully understood. Glycosyltransferases (GTs) are a diverse class of enzymes involved in catalyzing the transfer of one or multiple sugar residues to a wide range of acceptor molecules. GTs mediate a wide range of functions from structure and storage to signaling, and play a key role in many fundamental biological processes. Therefore, it is anticipated that GTs have a role in the pathogenesis of NAFLD. In this article, we present an overview of the basic information on NAFLD, particularly GTs and glycosylation modification of certain molecules and their association with NAFLD pathogenesis. In addition, the effects and mechanisms of some GTs in the development of NAFLD are summarized.

Human plasma protein N-glycosylation

Glycosylation is the most abundant and complex protein modification, and can have a profound structural and functional effect on the conjugate. The oligosaccharide fraction is recognized to be involved in multiple biological processes, and to affect proteins physical properties, and has consequentially been labeled a critical quality attribute of biopharmaceuticals. Additionally, due to recent advances in analytical methods and analysis software, glycosylation is targeted in the search for disease biomarkers for early diagnosis and patient stratification. Biofluids such as saliva, serum or plasma are of great use in this regard, as they are easily accessible and can provide relevant glycosylation information. Thus, as the assessment of protein glycosylation is becoming a major element in clinical and biopharmaceutical research, this review aims to convey the current state of knowledge on the N-glycosylation of the major plasma glycoproteins alpha-1-acid glycoprotein, alpha-1-antitrypsin, alpha-1B-glycoprotein, alpha-2-HS-glycoprotein, alpha-2-macroglobulin, antithrombin-III, apolipoprotein B-100, apolipoprotein D, apolipoprotein F, beta-2-glycoprotein 1, ceruloplasmin, fibrinogen, immunoglobulin (Ig) A, IgG, IgM, haptoglobin, hemopexin, histidine-rich glycoprotein, kininogen-1, serotransferrin, vitronectin, and zinc-alpha-2-glycoprotein. In addition, the less abundant immunoglobulins D and E are included because of their major relevance in immunology and biopharmaceutical research. Where available, the glycosylation is described in a site-specific manner. In the discussion, we put the glycosylation of individual proteins into perspective and speculate how the individual proteins may contribute to a total plasma N-glycosylation profile determined at the released glycan level.

ApoB gene SpIns/Del, XbaI polymorphisms and myocardial infarction: a meta-analysis of 7169 participants

BACKGROUND

Apolipoprotein B (ApoB) gene signal peptide insertion/deletion (SpIns/Del, I/D) and XbaI polymorphisms have been associated with susceptibility to myocardial infarction (MI). However, the results of studies on this association are still controversial.

OBJECTIVE AND METHODS

This study explored reports published from 1986 to 2008 regarding the association of ApoB gene SpIns/Del and XbaI polymorphisms with MI. A meta-analysis including 7169 participants from 19 individual studies was performed. The pooled odds ratios (ORs) and their corresponding 95% confidence intervals (95% CIs) were evaluated by fixed-effect or random-effect models.

RESULTS

A significant relationship between ApoB SpIns/Del gene polymorphism and MI was found under allelic (OR: 1.270, 95% CI: 1.090-1.480, P = 0.002), recessive (OR: 1.360, 95% CI: 1.130-1.630, P = 0.0009), dominant (OR: 1.091, 95% CI: 1.037-1.146, P = 0.001), homozygous (OR: 1.610, 95% CI: 1.330-1.950, P <0.00001) and heterozygous (OR: 1.081, 95% CI: 1.020-1.146, P = 0.009) genetic models. A marginal relationship between ApoB XbaI polymorphism and MI was found under a dominant genetic model (OR: 1.083, 95% CI: 1.004-1.168, P = 0.039). No significant association was detected under other genetic models (P >0.05). However, in the non-European subgroup analysis, increased MI risk emerged under all genetic models (P <0.05).

CONCLUSION

ApoB SpIns/Del gene polymorphism was positively associated with increased MI risk. D allele and DD genotype carriers might be predisposed to MI susceptibility. The ApoB XbaI gene polymorphism locus had a significant positive association with increased MI risk only in the non-European population. T allele and TT genotype carriers might be susceptible to MI in the non-European population. On the contrary, the ApoB gene XbaI restriction fragment length polymorphism was not associated with increased MI risk in the entire population, particularly in the European population.

Modified low density lipoprotein and lipoprotein-containing circulating immune complexes as diagnostic and prognostic biomarkers of atherosclerosis and type 1 diabetes macrovascular disease

In atherosclerosis; blood low-density lipoproteins (LDL) are subjected to multiple enzymatic and non-enzymatic modifications that increase their atherogenicity and induce immunogenicity. Modified LDL are capable of inducing vascular inflammation through activation of innate immunity; thus, contributing to the progression of atherogenesis. The immunogenicity of modified LDL results in induction of self-antibodies specific to a certain type of modified LDL. The antibodies react with modified LDL forming circulating immune complexes. Circulating immune complexes exhibit prominent immunomodulatory properties that influence atherosclerotic inflammation. Compared to freely circulating modified LDL; modified LDL associated with the immune complexes have a more robust atherogenic and proinflammatory potential. Various lipid components of the immune complexes may serve not only as diagnostic but also as essential predictive markers of cardiovascular events in atherosclerosis. Accumulating evidence indicates that LDL-containing immune complexes can also serve as biomarker for macrovascular disease in type 1 diabetes.

Impact of myeloperoxidase-LDL interactions on enzyme activity and subsequent posttranslational oxidative modifications of apoB-100

Oxidation of LDL by the myeloperoxidase (MPO)-H2O2-chloride system is a key event in the development of atherosclerosis. The present study aimed at investigating the interaction of MPO with native and modified LDL and at revealing posttranslational modifications on apoB-100 (the unique apolipoprotein of LDL) in vitro and in vivo. Using amperometry, we demonstrate that MPO activity increases up to 90% when it is adsorbed at the surface of LDL. This phenomenon is apparently reflected by local structural changes in MPO observed by circular dichroism. Using MS, we further analyzed in vitro modifications of apoB-100 by hypochlorous acid (HOCl) generated by the MPO-H2O2-chloride system or added as a reagent. A total of 97 peptides containing modified residues could be identified. Furthermore, differences were observed between LDL oxidized by reagent HOCl or HOCl generated by the MPO-H2O2-chloride system. Finally, LDL was isolated from patients with high cardiovascular risk to confirm that our in vitro findings are also relevant in vivo. We show that several HOCl-mediated modifications of apoB-100 identified in vitro were also present on LDL isolated from patients who have increased levels of plasma MPO and MPO-modified LDL. In conclusion, these data emphasize the specificity of MPO to oxidize LDL.

Low-density lipoprotein modified by myeloperoxidase in inflammatory pathways and clinical studies

Oxidation of low-density lipoprotein (LDL) has a key role in atherogenesis. Among the different models of oxidation that have been studied, the one using myeloperoxidase (MPO) is thought to be more physiopathologically relevant. Apolipoprotein B-100 is the unique protein of LDL and is the major target of MPO. Furthermore, MPO rapidly adsorbs at the surface of LDL, promoting oxidation of amino acid residues and formation of oxidized lipoproteins that are commonly named Mox-LDL. The latter is not recognized by the LDL receptor and is accumulated by macrophages. In the context of atherogenesis, Mox-LDL accumulates in macrophages leading to foam cell formation. Furthermore, Mox-LDL seems to have specific effects and triggers inflammation. Indeed, those oxidized lipoproteins activate endothelial cells and monocytes/macrophages and induce proinflammatory molecules such as TNFα and IL-8. Mox-LDL may also inhibit fibrinolysis mediated via endothelial cells and consecutively increase the risk of thrombus formation. Finally, Mox-LDL has been involved in the physiopathology of several diseases linked to atherosclerosis such as kidney failure and consequent hemodialysis therapy, erectile dysfunction, and sleep restriction. All these issues show that the investigations of MPO-dependent LDL oxidation are of importance to better understand the inflammatory context of atherosclerosis.

Roles of ApoB-100 gene polymorphisms and the risks of gallstones and gallbladder cancer: a meta-analysis

Background

Gallstones (GS) is the major manifestation of gallbladder disease, and is the most common risk factor for gallbladder cancer (GBC). Previous studies investigating the association between ApoB-100 gene polymorphisms and the risks of GS and GBC have yielded conflicting results. Therefore, we performed a meta-analysis to clarify the effects of ApoB-100 gene polymorphisms on the risks of GS and GBC.

Methods

A computerized literature search was conducted to identify the relevant studies from PubMed and Embase. Fixed or random effects model was selected based on heterogeneity test. Publication bias was estimated using Begg’s funnel plots and Egger’s regression test.

Results

A total of 10, 3, and 3 studies were included in the analyses of the association between ApoB-100 XbaI, EcoRI, or insertion/deletion (ID) polymorphisms and the GS risks, respectively, while 3 studies were included in the analysis for the association between XbaI polymorphism and GBC risk. The combined results showed a significant association in Chinese (X+ vs. X−, OR = 2.37, 95%CI 1.52–3.70; X+X+/X+X- vs. X+X+, OR = 2.47, 95%CI 1.55–3.92), but not in Indians or Caucasians. Null association was observed between EcoRI or ID polymorphisms and GS risks. With regard to the association between XbaI polymorphism and GBC risk, a significant association was detected when GBC patients were compared with healthy persons and when GBC patients were compared with GS patients. A significant association was still detected when GBC patients (with GS) were compared with the GS patients (X+X+ vs. X-X−, OR = 0.33, 95%CI 0.12–0.90).

Conclusion

The results of this meta-analysis suggest that the ApoB-100 X+ allele might be associated with increased risk of GS in Chinese but not in other populations, while the ApoB-100 X+X+ genotype might be associated with reduced risk of GBC. Further studies with larger sample sizes are needed to confirm these results.

Conformation and stability properties of B17: I. Analytical investigations using circular dichroism

Structural characterization of B17, the 17% N-terminal domain of apo B, was carried out using circular dichroic (CD) spectroscopy, where secondary and tertiary structures were studied as a function of temperature and pH. Mild acidic conditions that correlate with histidine protonation invoked a change in the α-helix and random coil contents of the protein, with no apparent change in the β-sheet structural content. Specific changes in the structure of the protein that occur in response to temperature were also investigated to understand the stability and conformational changes of B17. Far- and near-UV CDs were used to probe the thermal changes in the protein. The protonation of some histidine residues was attributed to underlie the increase in the helical content of the protein.

Protein disulfide isomerases contribute differentially to the endoplasmic reticulum-associated degradation of apolipoprotein B and other substrates

ER-associated degradation (ERAD) rids the early secretory pathway of misfolded or misprocessed proteins. Some members of the protein disulfide isomerase (PDI) family appear to facilitate ERAD substrate selection and retrotranslocation, but a thorough characterization of PDIs during the degradation of diverse substrates has not been undertaken, in part because there are 20 PDI family members in mammals. PDIs can also exhibit disulfide redox, isomerization, and/or chaperone activity, but which of these activities is required for the ERAD of different substrate classes is unknown. We therefore examined the fates of unique substrates in yeast, which expresses five PDIs. Through the use of a yeast expression system for apolipoprotein B (ApoB), which is disulfide rich, we discovered that Pdi1 interacts with ApoB and facilitates degradation through its chaperone activity. In contrast, Pdi1's redox activity was required for the ERAD of CPY* (a misfolded version of carboxypeptidase Y that has five disulfide bonds). The ERAD of another substrate, the alpha subunit of the epithelial sodium channel, was Pdi1 independent. Distinct effects of mammalian PDI homologues on ApoB degradation were then observed in hepatic cells. These data indicate that PDIs contribute to the ERAD of proteins through different mechanisms and that PDI diversity is critical to recognize the spectrum of potential ERAD substrates.

Iron-catalyzed oxidation of Trp residues in low-density lipoprotein

The mechanisms of oxidation of low-density lipoproteins (LDLs) are not well defined, but epidemiological and experimental studies suggest that iron-catalyzed processes may contribute to atherogenesis. The aim of this study was to test the hypothesis that iron-catalyzed oxidations of LDLs in vitro produce diagnostic biomarkers of oxidation of the apolipoprotein that could be applied to studies in vivo. LDLs were oxidized in the presence of Fe2+, EDTA, and ascorbic acid for up to 40 h. Following delipidation and trypsin digestion, the peptides were separated by HPLC, with four peaks detected at 365 nm, whereas none were observed in peptides from unoxidized LDLs. The peptides were identified by MALDI-QTOF mass spectrometry as IVQILP(W+4) EQNEQVK, IYSL(W+4)EHSTK, FEGLQE(W+4)EGK, and YH(W+4)EHTGLTLR, with (W+4) rather than the W residues of the unoxidized protein. The mass gains (+4 increase in m/z in tryptophan, W) and absorbance at 365 nm indicate kynurenines, which were trypsin-releasable peptides that are on the surface of LDL particles. All four peptides thus characterized share the sequence of WE. The preferential oxidation of W residues in WE sequences suggest contributions from the C-proximate glutamate residues in chelation of the iron species, thereby influencing site selectivities of oxidation. These kynurenine-containing peptides might serve as biomarkers of iron-mediated oxidations in vivo.

Immuno-electron cryo-microscopy imaging reveals a looped topology of apoB at the surface of human LDL

A single copy of apoB is the sole protein component of human LDL. ApoB is crucial for LDL particle stabilization and is the ligand for LDL receptor, through which cholesterol is delivered to cells. Dysregulation of the pathways of LDL metabolism is well documented in the pathophysiology of atherosclerosis. However, an understanding of the structure of LDL and apoB underlying these biological processes remains limited. In this study, we derived a 22 Å-resolution three-dimensional (3D) density map of LDL using cryo-electron microscopy and image reconstruction, which showed a backbone of high-density regions that encircle the LDL particle. Additional high-density belts complemented this backbone high density to enclose the edge of the LDL particle. Image reconstructions of monoclonal antibody-labeled LDL located six epitopes in five putative domains of apoB in 3D. Epitopes in the LDL receptor binding domain were located on one side of the LDL particle, and epitopes in the N-terminal and C-terminal domains of apoB were in close proximity at the front side of the particle. Such image information revealed a looped topology of apoB on the LDL surface and demonstrated the active role of apoB in maintaining the shape of the LDL particle.

Core fucosylation is required for midline patterning during zebrafish development

Complex carbohydrates represent one of the most polymorphic classes of macromolecules, but their functions during embryonic development remain poorly defined. Herein, we show that knockdown of FucT8, the fucosyltransferase responsible for adding an α1,6 fucosyl residue to the core region of N-linked oligosaccharides, results in defective midline patterning during zebrafish development. Reduced FucT8 expression leads to mild cyclopia, small forebrains, U-shaped somites, among other midline patterning defects. One of the principal FucT8 substrates was identified as Apolipoprotein B (ApoB), the major scaffold protein that is responsible for assembly and secretion of lipoprotein particles in vertebrates. In Drosophila, lipoprotein particles are thought to facilitate cell signaling by serving as a transport vehicle for lipid-modified cell signaling proteins, such as hedgehog. In this regard, knockdown of ApoB expression in zebrafish embryos leads to similar midline patterning defects as those seen in FucT8 morphant embryos. Furthermore, preliminary studies suggest that ApoB facilitates Sonic hedgehog signaling during zebrafish development, analogous to the function of lipoprotein particles during hedgehog signaling in Drosophila.

Optimization of apolipoprotein-B-100 sequence coverage by liquid chromatography-tandem mass spectrometry for the future study of its posttranslational modifications

Proteomic applications have been increasingly used to study posttranslational modifications of proteins (PTMs). For the purpose of identifying and localizing specific but unknown PTMs on huge proteins, improving their sequence coverage is fundamental. Using liquid chromatography coupled to mass spectrometry (LC-MS/MS), peptide mapping of the native apolipoprotein-B-100 was performed to further document the effects of oxidation. Apolipoprotein-B-100 is the main protein of low-density lipoprotein particles and its oxidation could play a role in atherogenesis. Because it is one of the largest human proteins, the sequence recovery rate of apolipoprotein-B-100 only reached 1% when conventional analysis parameters were used. The different steps of the peptide mapping process-from protein treatment to data analysis-were therefore reappraised and optimized. These optimizations allowed a protein sequence recovery rate of 79%, a rate which has never been achieved previously for such a large human protein. The key points for improving peptide mapping were optimization of the data analysis software; peptide separation by LC; sample preparation; and MS acquisition. The new protocol has allowed us to increase by a factor of 4 the detection of modified peptides in apolipoprotein-B-100. This approach could easily be transferred to any study of PTMs using LC-MS/MS.

Apo B100 similarities to viral proteins suggest basis for LDL-DNA binding and transfection capacity

LDL mediates transfection with plasmid DNA in a variety of cell types in vitro and in several tissues in vivo in the rat. The transfection capacity of LDL is based on apo B100, as arginine/lysine clusters, suggestive of nucleic acid-binding domains and nuclear localization signal sequences, are present throughout the molecule. Apo E may also contribute to this capacity because of its similarity to the Dengue virus capsid proteins and its ability to bind DNA. Synthetic peptides representing two apo B100 regions with prominent Arg/Lys clusters were shown to bind DNA. Region 1 (0014Lys-Ser0160) shares sequence motifs present in DNA binding domains of Interferon Regulatory Factors and Flaviviridae capsid/core proteins. It also contains a close analog of the B/E receptor ligand of apo E. Region 1 peptides, B1-1 (0014Lys-Glu0054) and B1-2 (0055Leu-Ala0096), mediate transfection of HeLa cells but are cytotoxic. Region 2 (3313Asp-Thr3431), containing the known B/E receptor ligand, shares analog motifs with the human herpesvirus 5 immediate-early transcriptional regulator (UL122) and Flaviviridae NS3 helicases. Region 2 peptides, B2-1 (3313Asp-Glu3355), and B2-2 (3356Gly-Thr3431) are ineffective in cell transfection and are noncytotoxic. These results confirm the role of LDL as a natural transfection vector in vivo, a capacity imparted by the apo B100, and suggest a basis for Flaviviridae cell entry.

Surface study of apoB1694-1880, a sequence that can anchor apoB to lipoproteins and make it nonexchangeable

Apolipoprotein B (apoB) is a nonexchangeable apolipoprotein. During lipoprotein assembly, it recruits phospholipids and triacylglycerols (TAG) into TAG-rich lipoprotein particles. It remains bound to secreted lipoproteins during lipid metabolism in plasma. The beta1 region (residues 827-1880) of apoB has a high amphipathic beta strand (AbetaS) content and is proposed to be one region anchoring apoB to lipoproteins. The AbetaS-rich region between apoB37 and apoB41 (residues 1694-1880) was cloned, expressed, and purified. The interfacial properties were studied at the triolein/water (TO/W) and air/water (A/W) interfaces. ApoB[37-41] is surface-active and adsorbs to the TO/W interface. After adsorption the unbound apoB[37-41] was removed from the aqueous phase. Adsorbed apoB[37-41] did not desorb and could not be forced off by increasing the surface pressure up to 23 mN/m. ApoB[37-41] adsorbed on the TO/W interface was completely elastic when compressed and expanded by +/-13% of its area. On an A/W interface, the apoB[37-41] monolayer became solid when compressed to 4 mN/m pressure indicating extended beta-sheet formation. It could be reversibly compressed and expanded between low pressure and its collapse pressure (35 mN/m). Our studies confirm that the AbetaS structure of apoB[37-41] is a lipid-binding motif that can irreversibly anchor apoB to lipoproteins.

Interfacial properties of a complex multi-domain 490 amino acid peptide derived from apolipoprotein B (residues 292-782)

ApolipoproteinB (ApoB) is a lipid binding protein that is a nonexchangeable component of chylomicrons, VLDL, and LDL. In the liver and intestinal cells ApoB recruits lipid to form nascent triacylglycerol rich particles cotranslationally in the endoplasmic reticulum membrane which are then processed and secreted to form plasma lipoproteins. The N-terminal domain, which comprises the first 22% of apoB, recruits lipid in a controlled manner. The first 6% (residues 1-291) of the N-terminus does not bind lipid. The first lipid binding domain, including residues 292-782 (B6-17), forms a lipid binding pocket which is predicted to consist of 17 alpha-helices and 6 beta-strands. A structural model based on the X-ray structure of the homologues protein lipovitellin suggests that the N-terminal 6-8 helices and the beta-sheet interact with lipid while the C-terminal helices form a structural unit stabilizing the beta-sheet. Using isothermal drop tensiometry we showed that ApoB6.4-17 is surface active and binds to a triolein/water interface and exerts 16-19 mN/m of pressure (Pi) on that surface. The protein initially adsorbs slowly from aqueous solution to the surface but following compression and re-expansion it reaches equilibrium much faster. When Pi exceeds 16.9 mN/m part of the protein is ejected from the surface, but when compressed to high Pi the protein is never completely ejected indicating that part of the peptide is irreversibly anchored to the interface. The surface dilation modulus (epsilon) varies between 25-38 mN/m, and is predominantly elastic with a small viscous component. When compressed at an air/water interface ApoB6.4-17 has a limiting area of approximately 11 A2 per amino acid at lift off and only approximately 7 A2 per amino acid at the collapse Pi (28 mN/m). These values are about half the anticipated values if all the residues are at the surface. This suggests that ApoB6.4-17 retains some globular structure at an interface and does not completely denature at the surface, as many other globular proteins do. We suggest that while bound to the surface ApoB6.4-17 exhibits properties of both alpha and beta structure giving it unique and versatile characteristics at a hydrophobic interface.

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

Charged amino acid residues 997-1000 of human apolipoprotein B100 are critical for the initiation of lipoprotein assembly and the formation of a stable lipidated primordial particle in McA-RH7777 cells

We previously demonstrated that a portion, or perhaps all, of the residues between 931 and 1000 of apolipoprotein (apo) B100 are required for the initiation of apoB-containing particle assembly. Based on our structural model of the first 1000 residues of apoB (designated as apoB:1000), we hypothesized that this domain folds into a three-sided lipovitellin-like "lipid pocket" via a hairpin-bridge mechanism. We proposed that salt bridges are formed between four tandem charged residues 717-720 in the turn of the hairpin bridge and four tandem complementary residues 997-1000 located at the C-terminal end of the model. To identify the specific motif within residues 931 and 1000 that is critical for apoB particle assembly, apoB:956 and apoB:986 were produced. To test the hairpin-bridge hypothesis, the following mutations were made: 1) residues 997-1000 deletion (apoB:996), 2) residues 717-720 deletion (apoB:1000Delta717-720), and 3) substitution of charged residues 997-1000 with alanines (apoB:996 + 4Ala). Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. 1) ApoB:956 did not form stable particles and was secreted as large lipid-rich aggregates. 2) ApoB:986 formed both a lipidated particle that was denser than HDL(3) and large lipid-rich aggregates. 3) Compared with wild-type apoB:1000, apoB:1000Delta717-720 displayed the following: (i) significantly diminished capacity to form intact lipidated particles and (ii) increased propensity to form large lipid-rich aggregates. 4) In striking contrast to wild-type apoB:1000, (i) apoB:996 and apoB:996 + 4Ala were highly susceptible to intracellular degradation, (ii) only a small proportion of the secreted proteins formed stable HDL(3)-like lipoproteins, and (iii) a majority of the secreted proteins formed large lipid-rich aggregates. We conclude that the first 1000 amino acid residues of human apoB100 are required for the initiation of nascent apoB-containing lipoprotein assembly, and residues 717-720 and 997-1000 play key roles in this process, perhaps via a hairpin-bridge mechanism.

Higher alleles of apolipoprotein B gene 3' VNTR: Risk for gallstone disease

BACKGROUND

Imbalance in cholesterol homeostasis may lead to gallstone disease. Apolipoprotein B is sole component of low-density lipoprotein and plays an important role in cholesterol metabolism. The present study was carried out to explore the association of APOB 3' VNTR, exon 26 XbaI and signal peptide insertion/ deletion polymorphisms with gallstone disease. 214 ultrasonographically proven gallstone patients and 322 healthy, age and sex matched controls were taken for the study. Genotyping was done using PCR followed by polyacrylamide gel electrophoresis for VNTR and insertion/ deletion analysis. For APOB XbaI polymorphism PCR product was digested with XbaI restriction enzyme, followed by agarose gel electrophoresis. All statistical analyses were done using SPSS v11.5. Higher repeat alleles of APOB 3' VNTR polymorphism were more frequent in gallstone patients than in controls. Alleles with more than 57 repeats were present only in patient group. Long (L) alleles with repeat higher than 49, were significantly higher (P=0.000; OR=3.705, 95% CI 2.577-5.326) and medium (M) alleles were lower (P=0.000; OR=0.406, 95% CI 0.304-0.542) in patients than in controls. To nullify the effect of gender, data was further stratified into male and female population. APOB 3' VNTR, L alleles were imposing risk and M alleles were protective in both male and female population. APOBXbaI and insertion/deletion polymorphisms were not found to be associated with the gallstone disease. Longer alleles of APOB 3' VNTR occur more frequently in gallstone patients, and may be an important risk factor for the development of gallstone disease. APOB XbaI and signal peptide insertion/deletion polymorphisms may not be contributing to the risk for gallstone disease.

Genetic variation and atherosclerosis

A family history of atherosclerosis is independently associated with an increased incidence of cardiovascular events. The genetic factors underlying the importance of inheritance in atherosclerosis are starting to be understood. Genetic variation, such as mutations or common polymorphisms has been shown to be involved in modulation of a range of risk factors, such as plasma lipoprotein levels, inflammation and vascular calcification. This review presents examples of present studies of the role of genetic polymorphism in atherosclerosis.

Haplotype analysis of signal peptide (insertion/deletion) and XbaI polymorphisms of the APOB gene in gallbladder cancer

PURPOSE

The incidence of gallbladder cancer (GBC) is usually paralleled by the prevalence of gallstone disease, and genes of cholesterol metabolism have been implicated in gallstone disease. The XbaI and insertion/deletion (ins/del) polymorphism of Apolipoprotein B (APOB) appears to influence cholesterol homoeostasis and possibly risk for gallstone disease. We examined the effect of these polymorphisms individually as well as their haplotypes on GBC and gallstone patients in North Indian population.

METHODS

The study comprises 123 consecutive cases of proven GBC, 172 cases of gallstone and 232 healthy subjects of similar age and sex. The genomic DNA was extracted from peripheral blood leucocytes and genotyping was performed using polymerase chain reaction (PCR) and PCR-restriction fragment length polymorphism.

RESULTS

In a case-control study, APOB XbaI and ins/del polymorphisms were not significantly associated with risk of GBC. Using the expectation maximization algorithm, four haplotypes were obtained, and haplotype X(+),D was found to be significantly higher in GBC patients without stone in comparison with healthy subjects [odds ratio (OR) 2.9, 95% confidence interval 1.2-6.6 P=0.012].

CONCLUSIONS

The X(+),D haplotype of APOB is associated with increased risk for development of GBC and the risk is not modified in the presence of gallstones.

Short communication: Identification of two polymorphisms in the goat lipoprotein lipase gene and their association with milk production traits

Lipoprotein lipase (LPL) is a glycoprotein that plays a central role in plasma triglyceride metabolism by hydrolyzing triglyceride-rich chylomicrons and very low density lipoproteins. The activity of milk LPL has been shown to differ among several goat breeds, suggesting the existence of a genetic polymorphism influencing the functional properties of this enzyme. We have characterized the complete coding sequence of the goat LPL gene in 18 individuals belonging to 3 breeds. The coding region of the goat LPL cDNA was 1,437 bp long and encoded a protein of 478 amino acids. Moreover, we have identified 2 single nucleotide polymorphisms (SNP) including a G50C missense mutation, which involved a Ser-->Thr amino acid replacement at position 17 of the signal peptide, and a C2094T substitution in the 3' untranslated region. A univariate mixed model was used to evaluate the association between LPL genotypes and milk production and composition in 130 Murciano-Granadina goats. The G50C SNP was suggestively associated with milk fat content and tended to affect the milk dry weight basis. The C2094T SNP was not associated with any of the measured traits.

The N-terminal domain of apolipoprotein B-100: structural characterization by homology modeling

Background

Apolipoprotein B-100 (apo B-100) stands as one of the largest proteins in humans. Its large size of 4536 amino acids hampers the production of X-ray diffraction quality crystals and hinders in-solution NMR analysis, and thus necessitates a domain-based approach for the structural characterization of the multi-domain full-length apo B.

Results

The structure of apo B-17 (the N-terminal 17% of apolipoprotein B-100) was predicted by homology modeling based on the structure of the N-terminal domain of lipovitellin (LV), a protein that shares not only sequence similarity with B17, but also a functional aspect of lipid binding and transport. The model structure was first induced to accommodate the six disulfide bonds found in that region, and then optimized using simulated annealing.

Conclusion

The content of secondary structural elements in this model structure correlates well with the reported data from other biophysical probes. The overall topology of the model conforms with the structural outline corresponding to the apo B-17 domain as seen in the EM representation of the complete LDL structure.

Lipid–protein interactions in human plasma LDL evidenced by magnetic resonance

Low density lipoprotein (LDL) particles exhibit extremely complex three-dimensional structural organization which is still not understood at the molecular level. The aim of this study was to provide the experimental evidence of a direct non-covalent interaction of the protein part with the lipid matrix. The approach was based on the combination of (1)H NMR (600 MHz) spectroscopy with thiol-specific spin labeling of the protein (apoB). It is shown that the spectral peaks assigned to the methyl head groups of phosphatidylcholine and sphingomyelin in the (1)H spectra of LDL exhibit line broadening when otherwise free thiol groups of apoB are covalently modified by methanethiosulfonate spin label. The effect is similar in the presence of water soluble paramagnetic compound. These results indicate that fragments of apoB, which are part of the receptor binding region, are directly in contact with the solvated phospholipid head groups of the lipid matrix.

Apolipoprotein B is conformationally flexible but anchored at a triolein/water interface: a possible model for lipoprotein surfaces

Apolipoprotein B (apoB) is one of a unique group of proteins that form and bind to fat droplets, stabilize the emulsified fat, and direct their metabolism. ApoB, secreted on lipoproteins (emulsions), remains bound during lipid metabolism yet exhibits conformational flexibility. It has amphipathic beta-strand (AbetaS)-rich domains and amphipathic alpha-helix (AalphaH)-rich domains. We showed that two consensus AbetaS peptides of apoB bound strongly to hydrophobic interfaces [triolein/water (TO/W) and dodecane/water], were elastic, and were not pushed off the interface when the surface was compressed. In contrast, an AalphaH peptide modeling helical parts of apoB was forced off the TO/W interface by compression and readsorbed when the interface was expanded. In this report, the surface behavior of apoB-100 was studied at the TO/W interface. Solubilized apoB lowered the interfacial tension of TO/W in a concentration-dependent fashion. At equilibrium tension, if the surface was compressed, part of apoB was pushed off but quickly readsorbed when the surface was expanded. Even when the surface area was compressed by approximately 55%, part of the apoB molecule remained bound. The maximum surface pressure that apoB could withstand without being partially ejected was 13 mN/m. ApoB showed high elasticity at the TO/W interface. Based on studies of the consensus AbetaS and AalphaH peptides, we suggest that AbetaSs anchor apoB and are its nonexchangeable motif, whereas its conformational flexibility arises from both the elastic nature of the AbetaS and the ability of AalphaH domains of the molecule to desorb and readsorb rapidly in response to surface pressure changes.

Self-association and lipid binding properties of the lipoprotein initiating domain of apolipoprotein B

The amino-terminal 20.1% of apolipoprotein B (apoB20.1; residues 1-912) is sufficient to initiate and direct the formation of nascent apoB-containing lipoprotein particles. To investigate the mechanism of initial lipid acquisition by apoB, we examined the lipid binding and interfacial properties of a carboxyl-terminal His6-tagged form of apoB20.1 (apoB20.1H). ApoB20.1H was expressed in Sf9 cells and purified by nickel affinity chromatography. ApoB20.1H was produced in a folded state as characterized by formation of intramolecular disulfide bonds and resistance to chemical reduction. Dynamic light scattering in physiological buffer indicated that purified apoB20.1H formed multimers, which were readily dissociable upon the addition of nonionic detergent (0.1% Triton X-100). ApoB20.1H was incapable of binding dimyristoylphosphatidylcholine multilamellar vesicles, unless its multimeric structure was first disrupted by guanidine hydrochloride. However, apoB20.1H multimers spontaneously dissociated and bound to the interface of naked and phospholipid-coated triolein droplets. These data reveal that the initiating domain of apoB contains solvent-accessible hydrophobic sequences, which, in the absence of a hydrophobic lipid interface or detergent, engage in self-association. The high affinity of apoB20.1H for neutral lipid is consistent with the membrane binding and desorption model of apoB-containing lipoprotein assembly.

Site-specific glycosylation analysis of human apolipoprotein B100 using LC/ESI MS/MS

Human apolipoprotein B100 (apoB100) has 19 potential N-glycosylation sites, and 16 asparagine residues were reported to be occupied by high-mannose type, hybrid type, and monoantennary and biantennary complex type oligosaccharides. In the present study, a site-specific glycosylation analysis of apoB100 was carried out using reversed-phase high-performance liquid chromatography coupled with electrospray ionization tandem mass spectrometry (LC/ESI MS/MS). ApoB100 was reduced, carboxymethylated, and then digested by trypsin or chymotrypsin. The complex mixture of peptides and glycopeptides was subjected to LC/ESI MS/MS, where product ion spectra of the molecular ions were acquired data-dependently. The glycopeptide ions were extracted and confirmed by the presence of carbohydrate-specific fragment ions, such as m/z 204 (HexNAc) and 366 (HexHexNAc), in the product ion spectra. The peptide moiety of glycopeptide was determined by the presence of the b- and y-series ions derived from its amino acid sequence in the product ion spectrum, and the oligosaccharide moiety was deduced from the calculated molecular mass of the oligosaccharide. The heterogeneity of carbohydrate structures at 17 glycosylation sites was determined using this methodology. Our data showed that Asn2212, not previously identified as a site of glycosylation, could be glycosylated. It was also revealed that Asn158, 1341, 1350, 3309, and 3331 were occupied by high-mannose type oligosaccharides, and Asn 956, 1496, 2212, 2752, 2955, 3074, 3197, 3438, 3868, 4210, and 4404 were predominantly occupied by mono- or disialylated oligosaccharides. Asn3384, the nearest N-glycosylation site to the LDL-receptor binding site (amino acids 3359-3369), was occupied by a variety of oligosaccharides, including high-mannose, hybrid, and complex types. These results are useful for understanding the structure of LDL particles and oligosaccharide function in LDL-receptor ligand binding.

Lipid Disorders and Mutations in the APOB Gene

Background: Plasma lipoproteins are important determinants of atherosclerosis. Apolipoprotein (apo) B is a large, amphipathic glycoprotein that plays a central role in human lipoprotein metabolism. Two forms of apoB are produced from the APOB gene by a unique posttranscriptional editing process: apoB-48, which is required for chylomicron production in the small intestine, and apoB-100, required for VLDL production in the liver. In addition to being the essential structural component of VLDL, apoB-100 is the ligand for LDL-receptor-mediated endocytosis of LDL particles.

Content: The study of monogenic dyslipidemias has revealed important aspects of metabolic pathways. In this review, we discuss the regulation of apoB metabolism and examine how APOB gene defects can lead to both hypo- and hypercholesterolemia. The key clinical, metabolic, and genetic features of familial hypobetalipoproteinemia and familial ligand-defective apoB-100 are described.

Summary: Missense mutations in the LDL-receptor-binding domain of apoB cause familial ligand-defective apoB-100, characterized by hypercholesterolemia and premature coronary artery disease. Other mutations in APOB can cause familial hypobetalipoproteinemia, characterized by hypocholesterolemia and resistance to atherosclerosis. These naturally occurring mutations reveal key domains in apoB and demonstrate how monogenic dyslipidemias can provide insight into biologically important mechanisms.

Relevance of different apolipoprotein content in binding of homocysteine to plasma lipoproteins

BACKGROUND AND AIMS

In plasma the atherogenic thiol homocysteine (Hcy) circulates either free or bound to proteins (Pb-Hcy). The present study sets out to evaluate the lipoprotein-Hcy (LP-Hcy) binding in vivo and the possible influence of different apolipoprotein content in this binding, being lipoprotein oxidation a possible mechanism of Hcy-induced damage.

METHODS AND RESULTS

In 34 healthy subjects we assayed fasting plasma lipoprotein and correspondent apolipoprotein (apo A-I, apo A-II, apo C-II, apo C-III, apo B, apo(a) and apo E content, and Hcy bound to different plasma protein fractions; moreover ten subjects underwent an oral methionine load in order to evaluate possible "dynamic" modifications of Pb-Hcy and LP-Hcy after induction of hyperhomocysteinemia. Pb-Hcy (mean values 9.22 +/- 1.7 mumol/L) represented about 78% of total plasma Hcy (mean values 11.8 +/- 1.8 mumol/L). Pb-Hcy distribution between the different fractions was as follows (mumol/L): VLDL = 0.25 +/- 0.08 (2.7%); LDL = 0.88 +/- 0.22 (9.5%); HDL = 1.40 +/- 0.36 (15.2%); fractions with density greater than 1.21 g/mL (Lipoprotein-Free Protein Fraction, LPDS) = 6.7 +/- 1.2 (72.6%). Hcy/protein ratios (nmol/mg of protein) in each protein fraction were: VLDL = 0.32 +/- 0.19, LDL = 0.43 +/- 0.37, HDL = 0.26 +/- 0.18, LPDS < 0.1, thus suggesting a higher binding capacity for Hcy by VLDL and LDL. These data were confirmed by the higher increase in Hcy content in LDL and VLDL (76 and 90%, respectively vs 36% and 3.1% for HDL and LPDS fractions) after hyperhomocysteinemia. Lp-Hcy binding significantly correlated with the apo B content of VLDL and LDL and Apo A-I content of HDL.

CONCLUSIONS

An important fraction of plasma Hcy circulates bound to LP (about 27% of Pb-Hcy); VLDL and LDL show the highest binding capacity for Hcy, probably due to their content in Apo B, a possible high capacity binding site for Hcy.

Identification of immune responses against aldehyde-modified peptide sequences in apoB associated with cardiovascular disease

OBJECTIVE

Atherosclerosis is associated with an immune response against oxidized LDL, which modulates the progression of the disease process.

METHODS AND RESULTS

Using a library of polypeptides covering the complete sequence of apoB-100, the only major protein of LDL, we have identified over 100 different human antibodies reacting against aldehyde-modified apoB-100 sequences. IgM antibody titer levels decreased with age and were associated with the intima-media thickness of the carotid artery in subjects younger than 60 years. There were also inverse associations between IgM levels and oxidized LDL in plasma. In prospective clinical studies, antibody levels against several aldehyde-modified apoB-100 sites were associated with cardiovascular disease in this age group. Whether this immune response is adaptive (protective) or maladaptive (causal) in atherosclerosis requires further investigation.

CONCLUSIONS

We have characterized a large number of epitopes within the apoB-100 component of oxidized LDL that provoke an immune response in humans. These findings will make it possible to study the role of immune responses against specific sites in oxidized LDL and to determine whether these immune responses influence the risk for future cardiac events.