Primary structure of apoB-100

ApoB100 and Atherosclerosis: What's New in the 21st Century?

ApoB is the main protein of triglyceride-rich lipoproteins and is further divided into ApoB48 in the intestine and ApoB100 in the liver. Very low-density lipoprotein (VLDL) is produced by the liver, contains ApoB100, and is metabolized into its remnants, intermediate-density lipoprotein (IDL) and low-density lipoprotein (LDL). ApoB100 has been suggested to play a crucial role in the formation of the atherogenic plaque. Apart from being a biomarker of atherosclerosis, ApoB100 seems to be implicated in the inflammatory process of atherosclerosis per se. In this review, we will focus on the structure, the metabolism, and the function of ApoB100, as well as its role as a predictor biomarker of cardiovascular risk. Moreover, we will elaborate upon the molecular mechanisms regarding the pathophysiology of atherosclerosis, and we will discuss the disorders associated with the APOB gene mutations, and the potential role of various drugs as therapeutic targets.

Apolipoprotein B compared with low-density lipoprotein cholesterol in the atherosclerotic cardiovascular diseases risk assessment

The subendothelial retention of apolipoprotein B (apoB)-containing lipoproteins is a critical step in the initiation of pro-atherosclerotic processes. Recent genetic and clinical evidence strongly supports the concept that the lipid content of the particles is secondary to the number of circulating atherogenic particles that are trapped within the arterial lumen. Since each low-density lipoproteins (LDL) particle contains one apoB molecule, as do intermediate density lipoprotein (IDL) and very low-density lipoprotein (VLDL) particles, apoB level represents the total number of atherogenic lipoproteins, which is independent of particle density, and not affected by the heterogeneity of particle cholesterol content (clinically evaluated by LDL-cholesterol level). From this perspective, apoB is proposed as a better proxy to LDL-cholesterol for assessing atherosclerotic cardiovascular disease risk, especially in specific subgroups of patients, including subjects with diabetes mellitus, with multiple cardiometabolic risk factors (obesity, metabolic syndrome, insulin resistance, and hypertension) and with high triglyceride levels and very low LDL-cholesterol levels. Therefore, given the causal role of LDL-cholesterol in atherosclerotic cardiovascular disease (ASCVD) development, routine measurement of both LDL-cholesterol and apoB is of utmost importance to properly estimate global cardiovascular risk and to determine the 'residual' risk of ASCVD in patients receiving therapy, as well as to monitor therapeutic effectiveness.

Sex-Biased Expression of Genes Allocated in the Autosomal Chromosomes: Blood LC-MS/MS Protein Profiling in Healthy Subjects

Background

Sex and gender have a large impact in human health and disease prediction. According to genomic/genetics, men differ from women by a limited number of genes in Y chromosome, while the phenotypes of the 2 sexes differ markedly.

Methods

In this study, serum samples from six healthy Bahraini men and women were analyzed by liquid chromatography-mass spectrometry (LC-MS/MS). Bioinformatics databases and tools were used for protein/peptide (PPs) identification and gene localization. The PPs that differed significantly (p < 0.05, ANOVA) in abundance with a fold change (FC) of ≥1.5 were identified.

Results

Revealed 20 PPs, 11 were upregulated in women with very high FC (up to 8 folds), and 9 were upregulated in men but with much lower FC. The PPs are encoded by genes located in autosomal chromosomes, indicative of sex-biased gene expression. The only PP related to sex, the sex hormone-binding globulin, was upregulated in women. The remaining PPs were involved in immunity, lipid metabolism, gene expression, connective tissue, and others, with some overlap in function.

Conclusions

The upregulated PPs in men or women are mostly reflecting the functon or risk/protection provided by the PPs to the specific sex, e.g., Apo-B100 of LDLC. Finally, the basis of sex-biased gene expression and sex phenotypic differences needs further investigation.

Homocysteine Modification in Protein Structure/Function and Human Disease

Epidemiological studies established that elevated homocysteine, an important intermediate in folate, vitamin B₁₂, and one carbon metabolism, is associated with poor health, including heart and brain diseases. Earlier studies show that patients with severe hyperhomocysteinemia, first identified in the 1960s, exhibit neurological and cardiovascular abnormalities and premature death due to vascular complications. Although homocysteine is considered to be a nonprotein amino acid, studies over the past 2 decades have led to discoveries of protein-related homocysteine metabolism and mechanisms by which homocysteine can become a component of proteins. Homocysteine-containing proteins lose their biological function and acquire cytotoxic, proinflammatory, proatherothrombotic, and proneuropathic properties, which can account for the various disease phenotypes associated with hyperhomocysteinemia. This review describes mechanisms by which hyperhomocysteinemia affects cellular proteostasis, provides a comprehensive account of the biological chemistry of homocysteine-containing proteins, and discusses pathophysiological consequences and clinical implications of their formation.

Systematic comparison of two novel, thiol-reactive prosthetic groups for 18F labeling of peptides and proteins with the acylation agent succinimidyl-4-[18F]fluorobenzoate ([18F]SFB)

A systematic comparison of 4-[18F]fluorobenzaldehyde-O-(2-{2-[2-(pyrrol-2,5-dione-1-yl)ethoxy]-ethoxy}-ethyl)oxime ([18F]FBOM) and 4-[18F]fluorobenzaldehyde-O-[6-(2,5-dioxo-2,5-dihydro-pyrrol-1-yl)-hexyl]oxime ([18F]FBAM) as prosthetic groups for the mild and efficient 18F labeling of cysteine-containing peptides and proteins with the amine-group reactive acylation agent, succinimidyl-4-[18F]fluorobenzoate ([18F]SFB), is described. All three prosthetic groups were prepared in a remotely controlled synthesis module. Synthesis of [18F]FBOM and [18F]FBAM was accomplished via oxime formation through reaction of appropriate aminooxy-functionalized labeling precursors with 4-[18F]fluorobenzaldehyde. The obtained radiochemical yields were 19% ([18F]FBOM) and 29% ([18F]FBAM), respectively. Radiolabeling involving [18F]FBAM and [18F]FBOM was exemplified by the reaction with cysteine-containing tripeptide glutathione (GSH), a cysteine-containing dimeric neurotensin derivative, and human native low-density lipoprotein (nLDL) as model compounds. Radiolabeling with the acylation agent [18F]SFB was carried out using a dimeric neurotensin derivative and nLDL. Both thiol-group reactive prosthetic groups show significantly better labeling efficiencies for the peptides in comparison with the acylation agent [18F]SFB. The obtained results demonstrate that [18F]FBOM is especially suited for the labeling of hydrophilic cysteine-containing peptides, whereas [18F]FBAM shows superior labeling performance for higher molecular weight compounds as exemplified for nLDL apolipoprotein constituents. However, the acylation agent [18F]SFB is the preferred prosthetic group for labeling nLDL under physiological conditions.

Factors Affecting S-Homocysteinylation of LDL Apoprotein B

Background: Hyperhomocysteinemia is an important risk factor for vascular disease and atherosclerosis, but the mechanisms by which homocysteine exerts its deleterious effects are not known. Because oxidation and/or homocysteinylation may increase atherogenicity of LDL, we investigated S-homocysteinylation of LDL as a possible contributor to atherosclerosis pathogenesis.

Methods: We used capillary electrophoresis to measure LDL-bound thiols [homocysteine, cysteine (Cys), cysteinylglycine, glutathione, and glutamylcysteine] in 104 healthy study participants We also assessed total plasma thiol concentrations and lipid profiles.

Results: Our data suggest that apoprotein B (apoB)-cysteinylglycine (CysGly), apoB-Hcy, and apoB-Cys concentrations are markedly higher in men than in women. The percentage of CysGly and glutathione on apoB was higher than that of the same thiols in plasma, whereas the other thiols were markedly less prevalent in lipoprotein than in plasma. Pearson correlation showed that among all thiols, only total plasma Hcy is related to apoB-Hcy concentrations. Multiple correlation analysis confirmed that total Hcy was the most important determinant of apoB-Hcy. Age and LDL cholesterol also showed positive associations, but Cys and, mainly, CysGly were negatively associated with apoB-Hcy concentrations.

Conclusions: apoB-Hcy derivative formation is mainly dependent on total homocysteine concentration. Increased cholesterol concentrations are related to increased apoB-Hcy. CysGly seems to compete with Hcy for binding to LDL apoprotein, suggesting that CysGly may protect against atherosclerosis by decreasing the concentrations of Hcy transferred by LDL from plasma to endothelial and subendothelial spaces.

Distribution of low-density lipoprotein-bound low-molecular-weight thiols: A new analytical approach

We have recently demonstrated that low-density lipoprotein (LDL) apoprotein is able to bind the most concentrated plasma thiols such as cysteine, cysteinylglycine, and homocysteine by disulfide linkage. However, the LIF CE assay employed to measure linked thiols was not sensitive enough to verify whether low concentrated plasma thiols as glutathione and glutamylcysteine are also linked to apoprotein. By modifying sample treatment and electrophoretic parameters we set up a new method with an LOQ of about 1.5 nmol/L, by which we demonstrate that LDL apoprotein binds all physiological plasma thiols. The increased sensitivity was obtained by drying released apoB thiols after reduction treatment, dissolving them directly in a low volume of derivatization buffer and decreasing the dilution factor of derivatized sample before CE injection. Moreover, by increasing the concentration of the electrolyte buffer, we improved the selectivity of peaks, in particular between glutathione (GSH) and the impurity peak derived from unreacted 5-iodoacetamidofluorescein, which in the previous electrophoretic conditions were overlapped. The method optimization, reached by searching the best combination between sample matrix and CE run buffer, is fully described. Given the potential pathologic significance of protein thiolation, the proposed method may be useful to understand the mechanisms and the balances that regulate the interaction between thiols and -SH free groups of proteins.

Degradation of apolipoprotein B-100 by lysosomal cysteine cathepsins

Although the degradation of cellular or endocytosed proteins comprises the normal function of lysosomal proteinases, these enzymes were also detected extracellularly during diseases such as atherosclerosis. Since lysosomal cysteine cathepsins were demonstrated to transform native LDL particles into a proatherogenic type, the following study was undertaken to characterize the modification of LDL particles and the degradation of apolipoprotein B-100 in more detail. LDL was incubated with cathepsins B, F, K, L, S, and V at pH 5.5 and under physiological conditions (pH 7.4) for 2 h to mimic conditions of limited proteolysis. Gel electrophoretic analysis of the degradation products revealed that cathepsin-mediated proteolysis of apolipoprotein B-100 is a fast process carried out by all enzymes at pH 5.5, and by cathepsin S also at pH 7.4. Electron microscopic analysis showed that cathepsin-mediated degradation of apolipoprotein B-100 rendered LDL particles fusion-competent compared to controls. N-Terminal sequencing of cathepsin cleavage fragments from apolipoprotein B-100 revealed an abundance of enzyme-specific cleavage sites located in almost all structurally and functionally essential regions. Since the cleavage sites superimpose well with results from substrate specificity studies, they might be useful for the development of cathepsin-specific inhibitors and substrates.

Identification and analysis of products formed from phospholipids in the free radical oxidation of human low density lipoproteins

Phospholipids reside in the surface layer of LDLs and constitute approximately 20-25% of the particle by weight. We report a study of the primary products generated from the most abundant molecular species of phosphatidylcholines present in LDL during in vitro free radical oxidations. The 13-hydroperoxides of 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLPC) and 1-stearoyl-2-linoleoyl-sn-glycero-phosphocholine (SLPC) and the 15-hydroperoxides of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (PAPC) and 1-stearoyl-2-arachidonoyl-sn-glycero-phosphocholine (SAPC) were found to increase in a time-dependent manner and in significant amounts even in the presence of alpha-tocopherol. Phospholipid alcohols also formed during the course of the oxidations. Early in the LDL oxidations, while alpha-tocopherol was still present, the thermodynamically favored trans,trans products of PLPC and SLPC were found to form in significantly larger quantities than those formed from cholesteryl linoleate. Additionally, quantities of PAPC 11-hydroperoxide (11-OOH) decreased over time relative to PAPC 15-OOH, even while alpha-tocopherol was still present in the oxidation, presumably as a result of further oxidation of PAPC 11-OOH to form cyclic peroxide oxidation products. These results suggest that alpha-tocopherol is more closely associated with the inner cholesteryl ester-rich hydrophobic core of an LDL particle and is not as effective as an antioxidant in the outer phospholipid layer as it is in the lipid core.

Structural aspects of thiol-specific spin labeling of human plasma low density lipoprotein

A novel thiol-specific spin labeling procedure for the protein component (apoprotein B, apoB) of low density lipoproteins (LDLs) is presented. A methanethiosulfonate spin label was used to probe the free cysteine residues of apoB with electron paramagnetic resonance (EPR) spectroscopy. The results indicated that the spin labeled sites are predominantly buried in the LDL particle in two distinct environments that differ in their mobility restrictions. The suitability of thiol-specific labeling for the study of the stability and conformation of apoB was demonstrated in experiments with denaturing agents. The results presented in this work offer a new approach for the matching of EPR data with the primary structure of apoB.

Reactions of *NO, *NO2 and peroxynitrite in membranes: physiological implications

Nitric oxide (*NO) and nitrogen dioxide (*NO2) are hydrophobic gases. Therefore, lipid membranes and hydrophobic regions of proteins are potential sinks for these species. In these hydrophobic environments, reactive nitrogen species will exhibit different chemistry than in aqueous environments due to higher local concentrations and the lack of hydrolysis reactions. The peroxynitrite anion (ONOO-) and peroxynitrous acid (ONOOH) can freely pass through lipid membranes, making peroxynitrite-mediated reactions in a hydrophobic environment also of extreme relevance. The reactions observed by these reactive nitrogen species in a hydrophobic milieu include oxidation, nitration and even potent chain-breaking antioxidant reactions. The physiological and toxicological relevance of these reactions is discussed.

Nitric oxide and low-density lipoprotein oxidation

Nitric oxide can have both pro-oxidant and antioxidant effects on low-density lipoprotein. Nitric oxide does not appear to react directly with components of LDL. However, in the presence of oxygen (through NO2 and N2O3 formation) or superoxide (through peroxynitrite formation) nitric oxide may cause oxidation of the lipid, protein and antioxidant components of LDL. Conversely, nitric oxide is a potent inhibitor of LDL oxidation when initiated by copper ions or by azo-initiators. The possible implications of these observations to vascular pathology are discussed.

Site-specific mutagenesis demonstrates that cysteine 4326 of apolipoprotein B is required for covalent linkage with apolipoprotein (a) in vivo

In the formation of the lipoprotein(a) (Lp(a)) particle, apolipoprotein(a) (apo(a)) and apolipoprotein B (apoB) are covalently linked via a disulfide bond in both humans and human-apo(a)/apoB transgenic mice. Studies based upon fluorescent labeling of free cysteine residues have suggested that cysteine 3734 of the 4 carboxyl-terminal cysteines of apoB (Cys-3734, Cys-3890, Cys-4190, and Cys-4326) is the most likely candidate to form a disulfide bond with apo(a). However, other recent studies using truncated apoB molecules suggest that Cys-4326, the terminal cysteine of apoB, may be implicated in the binding to apo(a). In order to definitively show which of apoB's carboxyl-terminal cysteines is essential in interacting with apo(a) we have used RecA-assisted restriction enzyme digestion coupled with site-specific mutagenesis to convert Cys-3734 and Cys-4326 to serine within separate 90-kilobase pair apoB P1 phagemid clones. Transgenic mice containing the normal or mutated apoB transgenes were created, and the covalent association of mutated apoB with apo(a) was assessed in mice transgenic for both apoB and apo(a). Analysis by ultracentrifugation and immunoblotting revealed that Cys-4326, but not Cys-3734, was essential in the formation of the covalent bond between apo(a) and apoB in vivo.

Lipoprotein(a): levels in a Swedish population in relation to other lipid parameters and in comparison with a male Sri Lankan population

OBJECTIVE

To evaluate differences in Lipoprotein (a) [Lp(a)] concentrations between a Swedish and Sri Lankan population.

METHODS

The distribution of Lp(a) and its relation to other lipid parameters, measured with an automated turbidimetric method, in 4646 Swedes (1944 females and 2702 males) undergoing health screening and 757 randomly selected Sri Lankan males (667 non-CHD and 80 CHD subjects) was evaluated.

RESULTS

The distribution was highly skewed towards low values in both the Swedish population and the Sri Lankan male population. The Swedish population had a median of 0.16 g/L (reported as total mass) whereas the Sri Lankan population median of 0.06 g/L was much lower. For the Swedes, there was a small significant difference of 0.03 g/L between the sexes (F < M; p < 0.001) and Lp(a) was significantly higher in subjects > 50 years of age in both sexes (p < 0.002(F); p < 0.02(M)). 29% had Lp(a) values > 0.30 g/L. In the Sri Lankan males population Lp(a) was also significantly higher in subjects > 50 years of age (p < 0.009) but only 7% had an Lp(a) concentration of > 0.30 g/L. In the CHD subgroup, though not significant, subjects > 50 years of age had a lower Lp(a) concentration, indicating that Lp(a) may be a more significant risk factor in younger subjects. Both the Swedish female and male hypercholesterolemic subgroups had significantly higher Lp(a) concentrations than normolipemic subgroups and the male hypertriglyceridemic subgroups significantly lower Lp(a) concentrations than normolipemic. Great differences in Lp(a) levels are thus found between the two populations. The differences are similar in normolipemic subjects and probably they reflect mainly genetic differences. Lipid/lipoprotein concentrations were also found to differ. It is being investigated if this reflects differences in CHD prevalence.

CONCLUSION

Our data support the importance of including Lp(a) measurements when assessing the risk profile for premature development of CHD in the individual patient.