The complete sequence and structural analysis of human apolipoprotein B-100: relationship between apoB-100 and apoB-48 forms

Association of mutations in the apolipoprotein B gene with hypercholesterolemia and the risk of ischemic heart disease

BACKGROUND

Familial hypercholesterolemia leads to premature ischemic heart disease and is often caused by mutations in the gene for the low-density lipoprotein receptor. Mutations in the apolipoprotein B gene, which encodes a ligand for this receptor, may also result in this phenotype.

METHODS

We studied the genotypes of 9255 women and men from the general population, 948 patients with ischemic heart disease, and 36 patients with familial hypercholesterolemia, all from Denmark, for three mutations in the apolipoprotein B gene: Arg3500Gln, Arg3531Cys, and Arg3500Trp.

RESULTS

The prevalence of heterozygotes in the general population was 0.08 percent (95 percent confidence interval, 0.03 to 0.16 percent) for both the Arg3500Gln and the Arg3531Cys mutations, and 0.00 percent (95 percent confidence interval, 0.00 to 0.18 percent) for the Arg3500Trp mutation. Among carriers of the Arg3500Gln mutation, cholesterol levels were significantly higher than among noncarriers in the general population - by 100 mg per deciliter (2.6 mmol per liter) among carriers in the general population, 154 mg per deciliter (4.0 mmol per liter) among patients with ischemic heart disease, and 172 mg per deciliter (4.5 mmol per liter) among patients with familial hypercholesterolemia. Heterozygous carriers of the Arg3500Gln mutation were significantly more common among patients with ischemic heart disease (odds ratio, 7.0; 95 percent confidence interval, 2.2 to 22; P=0.003) and patients with familial hypercholesterolemia (odds ratio, 78; 95 percent confidence interval, 16 to 388; P=0.001) than in the general population. Heterzygous carriers of the Arg3531Cys mutation in the general population did not have higher-than-normal plasma cholesterol levels or an increased risk of ischemic heart disease (odds ratio; 1.4; 95 percent confidence interval, 0.2 to 11; P=0.54).

CONCLUSIONS

The Arg3500Gln mutation in the apolipoprotein B gene, which is responsible for familial defective apolipoprotein B-100 and is present in approximately 1 in 1000 persons in Denmark, causes severe hypercholesterolemia and increases the risk of ischemic heart disease.

Role of plasma lipoproteins in modifying the biological activity of hydrophobic drugs

The plasma lipoprotein distribution of potential drug candidates is not commonly studied. For some hydrophobic drug candidates, attainment of similar plasma free drug levels has not been associated with uniform production of pharmacological activity in different animal species. It is well-known that plasma lipoprotein lipid profiles vary considerably between different animal species. In addition, human disease states can significantly influence plasma lipoprotein profiles, resulting in altered therapeutic outcomes. Current research has shown that lipoprotein binding of drug compounds can significantly influence not only the pharmacological and pharmacokinetic properties of the drug, but the relative toxicity as well. Elucidation of drug distribution among plasma lipoproteins is expected to yield valuable insight into factors governing the pharmacological activity and potential toxicity of the drug. This paper will present an historical perspective and summarize the latest research in the area of lipoprotein-drug interactions.

Human mucin genes MUC2, MUC3, MUC4, MUC5AC, MUC5B, and MUC6 express stable and extremely large mRNAs and exhibit a variable length polymorphism. An improved method to analyze large mRNAs

Of the nine mucin genes that have been characterized, only MUC1 and MUC7 have been fully sequenced, and their transcripts can be detected as distinct bands of predicted size by Northern blot analysis. In contrast, the RNA patterns observed for each of the other MUC genes have usually shown a very high degree of polydispersity. This polydispersity has been believed to be one of the typical features of the mucin mRNAs, but until now, its origin has remained unexplained. In the work described in the present paper, we investigated two possible kinds of explanation for this phenomenon: namely that the extensive polydispersity results from a biological mechanism or that it is artifactual in origin. The data obtained, as a result of improving the purification and blotting methods, allowed us to show that in all of the tissues analyzed, each of the genes, MUC2-6, expresses mRNAs that are stable and are of an unusually large size to be found in eukaryotes (14-24 kilobases). Moreover, allelic variations in length of these mucin transcripts were observed. We demonstrate that these variations are directly related to the variable number of tandem repeat polymorphisms seen at the DNA level.

Identification of cysteine pairs within the amino-terminal 5% of apolipoprotein B essential for hepatic lipoprotein assembly and secretion

There is growing evidence that the amino-terminal globular domain of apolipoprotein B (apoB) is essential for lipoprotein particle formation in the hepatic endoplasmic reticulum. To identify the structural requirements for its function in lipoprotein assembly, cysteine (Cys) pairs required to form the seven disulfide bonds within the amino-terminal 21% of apoB were replaced in groups or individually by serine. Substitution of Cys pairs required for formation of disulfide bonds 1-3 or 4-7 (numbered from amino to carboxyl terminus) completely blocked the secretion of apoB28 in transfected HepG2 cells. To identify the specific disulfide bonds required for secretion, Cys pairs were mutated individually. Substitution of Cys pairs required for disulfide bonds 1, 3, 5, 6, or 7 had little or no impact on apoB28 secretion or buoyant density. In contrast, individual substitution of Cys pair 2 (amino acid residues 51 and 70) or 4 (218 and 234) severely inhibited apoB28 secretion and its capacity to undergo intracellular assembly with lipid. The same assembly and secretion defects were observed when these mutations were expressed as part of apoB50. These studies provide direct evidence that the ability of the internal lipophilic regions of apoB to engage in the recruitment and sequestration of lipid during translation is critically dependent upon a structural configuration contained within or affected by the amino-terminal 5% of the protein.

Endothelial cells synthesize and process apolipoprotein B

We reported previously that a 116-kDa lipoprotein lipase (LPL)-binding protein from endothelial cells has sequence homology to the amino-terminal region of apolipoprotein (apo) B. We now tested whether endothelial cells synthesize apoB mRNA and protein. Primers were designed to the human apoB cDNA sequence and reverse transcription polymerase chain reaction was performed using total RNA isolated from bovine and human endothelial cells. With primers to the 5' region of the apoB mRNA (amino-terminal region of apoB protein) expected size PCR products were generated from both bovine and human endothelial cells as well as from mouse liver RNA, which was used as a control. Primers designed to the 3' region of apoB mRNA generated PCR products from human endothelial cells and HepG2 cells but not from bovine or mouse cells. These data suggest that endothelial cells contain full-length apoB mRNA and that the 5' or the amino-terminal region of apoB is highly conserved from mouse to human. This was confirmed by direct sequencing of the mouse and bovine PCR products. To test whether apoB protein was produced, bovine endothelial cell proteins were metabolically labeled with [35S]methionine/cysteine or [3H]leucine and immunoprecipitated with anti-human apoB antibodies. Using extracts from cells labeled for 1 h, monoclonal antibody 47, directed to the low density lipoprotein receptor binding region of apoB, precipitated a protein of approximate molecular mass 550,000, the size of full-length apoB. Immunoprecipitation of the 550-kDa protein was abolished in the presence of added unlabeled low density lipoprotein. From cells labeled for 16 h, a 116-kDa protein was immunoprecipitated by polyclonal anti-apoB antibodies. This protein was partly released from cells by heparin treatment. Pulse-chase analysis showed that the 116-kDa fragment appeared at the same time as the full-length apoB began disappearing. The immunoprecipitated 116-kDa fragment also bound labeled LPL on ligand blot, further suggesting that it is an amino-terminal fragment of apoB. Incubation of endothelial cells with oleic acid (0.25 and 0.5 mM) did not significantly alter the production of either the full-length apoB or the 116-kDa fragment. These data show that endothelial cells synthesize apoB. The full-length apoB appears to be cleaved to form a 116-kDa fragment that can function as a LPL-binding protein.

Chylomicron assembly and catabolism: role of apolipoproteins and receptors

Chylomicrons are lipoproteins synthesized exclusively by the intestine to transport dietary fat and fat-soluble vitamins. Synthesis of apoB48, a translational product of the apob gene, is required for the assembly of chylomicrons. The apob gene transcription in the intestine results in 14 and 7 kb mRNAs. These mRNAs are post-transcriptionally edited creating a stop codon. The edited mRNAs chylomicrons from the shorter apoB48 peptide remains to be elucidated. In addition, the roles of proteins involved in the assembly pathway, e.g. apobec-1, MTP and apoA-IV, needs to be studied. Cloning of enzymes involved in the intestinal biosynthesis of triglycerides will be crucial to fully appreciate the assembly of chylomicrons. There is a need for cell culture and transgenic animal models that can be used for intestinal lipoprotein assembly. The catabolism of chylomicrons is far more complex and efficient than the catabolism of VLDL. Even though the major steps involved in the catabolism of chylomicrons are now known, the determinants for apolipoprotein exchange, processing of remnants in the space of Disse, as well as the mechanism of uptake of these particles by extra-hepatic tissue needs further exploration.

Presence of an extended duplication in the putative low-density-lipoprotein receptor-binding domain of apolipoprotein B. Cloning and characterization of the domain in salmon

The sequence of the C-terminal 1058 amino acids of atlantic salmon (Salmo salar) apolipoprotein (apo) B was deduced from the nucleotide sequence of cloned cDNA. In comparison with chicken or mammals apoB-100, salmon apoB is C-terminally truncated and extended gaps are found. The two clusters of positively charged residues, previously identified as part of the putative low-density-lipoprotein (LDL) receptor-binding domain of apoB, are brought into close proximity in salmon apoB. This is achieved by the absence between the two clusters of the proline-rich area with the potential to form an amphipathic beta sheet, present in higher vertebrates. In addition, analysis of apoB amino acid sequences currently available in vertebrates revealed the presence of an extended internal duplication in the putative LDL receptor-binding domain. Thus, the two basic clusters would have been duplicated resulting in the presence, except for salmon apoB, of two homologous sites in the C-terminal part of the molecule. The results described here together with earlier biochemical and genetic evidence support the view that Arg3500, a residue mutated in familial defective apoB-100, could be included in a folded critical region of the putative LDL receptor-binding domain of human apoB-100. This region possibly brings the two sub-domains that arise from the duplication close to each other.

Tandem mass spectrometric characterization of a specific cysteic acid residue in oxidized human apoprotein B-100

The oxidation of low density lipoprotein (LDL) in vivo may result in its unregulated uptake by macrophages, with the consequent accumulation of cholesterol that is characteristic of the development of atherosclerosis. This paper describes initial experiments to elucidate structural changes that occur in an in vitro model of LDL oxidation. LDL was isolated from human blood and oxidized in the presence of copper ion. Lipid was removed and the isolated apoprotein was subjected to tryptic hydrolysis. The hydrolysate was separated by high performance liquid chromatography and individual fractions were screened by amino acid analysis to detect cysteic acid residues. Appropriate fractions were analyzed by fast atom bombardment mass spectrometry and hybrid tandem mass spectrometry. In this manner a tryptic fragment was identified that corresponded to residues 4187-4195 (EELCTMFIR), in which the cysteine and methionine residues were oxidized to cysteic acid and methionine sulfoxide, respectively. Identical analysis of LDL not subjected to in vitro oxidation revealed no evidence for this oxidized peptide. Earlier work established a surface location for this cysteine residue (Cys24) on the LDL particle, which suggested that its modification may significantly affect the properties of LDL, such as the propensity to intermolecular interaction via disulfide bridges. The analytical protocol developed here (involving proteolysis, screening of peptide fragments, and tandem mass spectrometry analysis) constitutes a strategy of general applicability to the characterization of targeted modifications of large proteins via mass spectrometry.

Phenotypic expression and frequency of familial defective apolipoprotein B-100 in Belgian hypercholesterolemics

DNA screening for apolipoprotein (apo) B mutations causing familial defective apolipoprotein B-100 (FDB) was performed in 87 hyperlipidemic Belgian individuals using heteroduplex analysis. Eighteen FDB heterozygotes from 5 unrelated families were identified. Three of the index cases reported an early family history of premature coronary heart disease (CHD). The frequency of the apo B3500 mutation was 8% in Belgians with type IIa hyperlipidemia, indicating that the prevalence of FDB may be as high as 1 in 250 in the general Belgian population. Plasma lipid levels of the patients identified in the present study are similar to those previously reported for FDB heterozygotes. We compared these data with results obtained in a genotype/phenotype correlation study of heterozygous familial hyper-cholesterolemia (FH) in the Afrikaner population of South Africa. Plasma cholesterol levels in FDB heterozygotes were similar to those reported for FH heterozygotes with defective receptors (Asp206-->Glu, approximately 20% normal receptor activity), but significantly lower than in FH heterozygotes with a mutant protein which virtually lacks receptor activity (Val408-->Met, < 2% normal receptor activity). FDB appears to be a significant genetic cause of hypercholesterolemia in Belgium.

Insulin regulation of triacylglycerol-rich lipoprotein synthesis and secretion

This review has considered a number of observations obtained from studies of insulin in perfused liver, hepatocytes, transformed liver cells and in vivo and each of the experimental systems offers advantages. The evaluation of insulin effects on component lipid synthesis suggests that overall, lipid synthesis is positively influenced by insulin. Short-term high levels of insulin through stimulation of intracellular degradation of freshly translated apo B and effects on synthesis limit the ability of hepatocytes to form and secrete TRL. The intracellular site of apo B degradation may involve membrane-bound apo B, cytoplasmic apo B and apo B which has entered the ER lumen. How insulin favors intracellular apo B degradation is not known. An area of recent investigation is in insulin-stimulated phosphorylation of intracellular substrates such as IRS-1 which activates insulin specific cellular signaling molecules [245]. Candidate molecules to study insulin action on apo B include IRS-1 and SH2-containing signaling molecules. Insulin dysregulation in carbohydrate metabolism occurs in non-insulin-dependent diabetes mellitus due to an imbalance between insulin sensitivity of tissue and pancreatic insulin secretion (reviewed in Refs. [307,308]). Insulin resistance in the liver results in the inability to suppress hepatic glucose production; in muscle, in impaired glucose uptake and oxidation and in adipose tissue, in the inability to suppress release of free FA. This lack of appropriate sensitivity towards insulin action leads to hyperglycemia which in turn stimulates compensatory insulin secretion by the pancreas leading to hyperinsulinemia. Ultimately, there may be failure of the pancreas to fully compensate, hyperglycemia worsens and diabetes develops. The etiology of insulin resistance is being intensively studied for the primary defect may be over secretion of insulin by the pancreas or tissue insulin resistance and both of these defects may be genetically predetermined. We suggest that, in addition to effects in carbohydrate metabolism, insulin resistance in liver results in the inability of first phase insulin to suppress hepatic TRL production which results in hypertriglyceridemia leading to high levels of plasma FA which accentuate insulin resistance in other target organs. As recently reviewed [17,254] the role of insulin as a stimulator of hepatic lipogenesis and TRL production has been long established. Several lines of evidence support that insulin is stimulatory to the production of hepatic TRL in vivo. First, population based studies support a positive relationship between plasma insulin and total TG and VLDL [253]. Second, there is a strong association between chronic hyperinsulinemia and VLDL overproduction [309].(ABSTRACT TRUNCATED AT 400 WORDS)

apoB-100 has a pentapartite structure composed of three amphipathic alpha-helical domains alternating with two amphipathic beta-strand domains. Detection by the computer program LOCATE

Due to the great length of apolipoprotein (apo) B-100, the localization of lipid-associating domains in this protein has been difficult. To address this question, we developed a computer program called Locate that searches amino acid sequences to identify potential amphipathic alpha-helixes and beta-strands by using sets of rules for helix and strand termination. A series of model chimeric protein test datasets were created by tandem linking of amino acid sequences of multiple proteins containing four different secondary structural motifs: motif A (exchangeable plasma apolipoproteins); motif G (globular alpha-helical proteins); motif C (coiled-coil alpha-helical proteins); and motif B (beta pleated-sheet proteins). These four test datasets, as well as randomly scrambled sequences of each dataset, were analyzed by Locate using increasingly stringent parameters. Using intermediately stringent parameters under which significant numbers of amphipathic helixes were found only in the unscrambled motif A, two dense clusters of putative lipid-associating amphipathic helixes were located precisely in the middle and at the C-terminal end of apoB-100 (a sparse cluster of class G* helixes is located at the N-terminus). The dense clusters are located between residues 2103 through 2560 and 4061 through 4338 and have densities of 2.4 and 2.2 amphipathic helixes per 100 residues, respectively; under these conditions, motif A has a density of 1.4 amphipathic helixes per 100 residues. These two domains correspond closely to the two major apoB-100 lipid-associated domains at residues 2100 through 2700 and 4100 through 4500 using the principle of releasability of tryptic peptides from trypsin-treated intact low-density lipoprotein. The classes of amphipathic helixes identified within these two putative lipid-associating domains are considerably more diverse than those found in the exchangeable plasma apolipoproteins. Interestingly, apoB-48 terminates at the N-terminal edge of the middle cluster. By using a similar strategy for analysis of amphipathic beta-strands, we discovered that the two gap regions between the three amphipathic helix clusters are highly enriched in putative amphipathic beta-strands, while the three amphipathic helical domains are essentially devoid of this putative lipid-associating motif. We propose, therefore, that apoB-100 has a pentapartite structure, NH2-alpha 1-beta 1-alpha 2-beta 2-alpha 3-COOH, with alpha 1 representing a globular domain.

Increased frequency of apolipoprotein B signal peptide sp24/24 in patients with coronary artery disease. General allele survey in the population of Taiwan and comparison with Caucasians

Apolipoprotein B (apoB) signal peptide (sp) polymorphism was characterized by polymerase chain reaction in blood samples of 58 coronary artery disease (CAD) patients and 319 control individuals of Chinese Han ethnic origin in Taiwan. In the CAD group, 77% of the observed alleles were sp27 (sp with 27 amino acids), and the remaining 23% sp24 (sp with 24 amino acids). The frequency distributions of the apoB sp allele in the control group were 0.81 for sp27 and 0.19 for sp24. The genotype distributions were 0.64 sp27/27, 0.26 sp27/24 and 0.10 sp24/24 in the CAD group; 0.64 sp27/27, 0.33 sp27/24 and 0.03 sp24/24 in the control group. The frequency of sp24/24 was significantly higher (p = 0.012) in the CAD group than in the control group. Several studies have shown that the frequency of sp24/24 is higher in hyperlipidemic than in normolipidemic groups. This marker is probably in linkage disequilibrium with some other atherogenic genes. Our study shows that the differences in both apoB signal peptide alleles and sp27/27 and sp27/24 genotype distributions are statistically significant between the Taiwanese and Caucasians.

The effect of variation in the apolipoprotein B gene on plasmid lipid and apolipoprotein B levels. I. A likelihood-based approach to cladistic analysis

A new method is described for employing family data to test for significant haplotype effects on continuously distributed variables, using likelihood-ratio tests of linear models in which haplotype effects are parameterized and familial correlations taken into account. The method is applied to the apolipoprotein B (Apo B) gene, using 5 polymorphisms (Insertion/deletion, Bsp1286I, XbaI, MspI, EcoRI) to define haplotypes in 121 French nuclear families. Eleven haplotypes were found, five of which, combined, account for over 95% of the sample. A haplotype phylogeny is proposed, and is used to define a nested set of models for testing the effects of Apo B variation on total-, low-density-lipoprotein (LDL)-, and high-density-lipoprotein (HDL)-cholesterol, triglyceride, and Apo B levels. Apo B haplotype effects account for about 10% of the genetic variance and 5% of the total variance in HDL-cholesterol and triglyceride levels. Clusters of evolutionarily-related haplotypes with similar phenotypic effects are identified for HDL-cholesterol and triglycerides. Single haplotypes with statistically significant effects are identified for cholesterol, LDL-cholesterol, and Apo B levels.

Apolipoprotein B and low-density lipoprotein structure: implications for biosynthesis of triglyceride-rich lipoproteins

ApoB100 is a very large glycoprotein essential for triglyceride transport in vertebrates. It plays functional roles in lipoprotein biosynthesis in liver and intestine, and is the ligand recognized by the LDL receptor during receptor-mediated endocytosis. ApoB100 is encoded by a single gene on chromosome 2, and the message undergoes a unique processing event to form apoB48 message in the human intestine, and, in some species, in liver as well. The primary sequence is relatively unique and appears unrelated to the sequences of other serum apolipoproteins, except for some possible homology with the receptor recognition sequence of apolipoprotein E. From its sequence, structure prediction shows the presence of both sheet and helix scattered along its length, but no transmembrane domains apart from the signal sequence. The multiple carbohydrate attachment sites have been identified, as well as the locations of most of its disulfides. ApoB is the single protein found on LDL. These lipoproteins are emulsion particles, containing a core of nonpolar cholesteryl ester and triglyceride oil, surrounded by an emulsifying agent, a monolayer of phospholipid, cholesterol, and a single molecule of apoB100. An emulsion particle model is developed to predict accurately the physical and compositional properties of an LDL of any given size. A variety of techniques have been employed to map apoB100 on the surface of the LDL, and all yield a model in which apoB surrounds the LDL like a belt. Moreover, it is concluded that apoB100 folds into a long, flexible structure with a cross-section of about 20 x 54 A2 and a length of about 585 A. This structure is embedded in the surface coat of the LDL and makes contact with the core. During lipoprotein biosynthesis in tissue culture, truncated fragments of apoB100 are secreted on lipoproteins. Here, it was found that the lipoprotein core circumference was directly proportional to the apoB fragment size. A cotranslational model has been porposed for the lipoprotein assembly, which includes these structural features, and it is concluded that in permanent hepatocyte cell lines, apoB size determines lipoprotein core circumference.

Glycated LDL concentrations in non-diabetic and diabetic subjects measured with monoclonal antibodies reactive with glycated apolipoprotein B epitopes

The potential importance of the non-enzymatic glycation of low density lipoproteins (LDL) in atherogenesis and in the accelerated atherosclerosis associated with diabetes is well recognized. However, it has been difficult to evaluate LDL glycation in the clinical setting because of the lack of suitable methods. To approach this problem, we produced monoclonal antibodies, designated ES12, that recognize glycated apolipoprotein B epitopes in the LDL complex in human plasma. Here we report the use of these antibodies in a competitive enzyme-linked immunosorbent assay (ELISA) to measure glycated LDL concentrations in plasma from non-diabetic and diabetic subjects. In this assay, glycated LDL in the soluble phase inhibits binding of the ES12 antibody to glycated LDL immobilized to microtitre wells, whereas other glycated proteins and non-glycated LDL do not compete. A linear dose-response relationship for 10-125 ng glycated LDL per well allows the construction of standard curves, from which the concentration of glycated LDL in human plasma can be determined. The mean concentration of glycated LDL in samples from non-diabetic subjects was 21.8 +/- 0.9 mg/l, increasing to 40.8 +/- 2.6 mg/l in samples from patients with type II diabetes, comprising 1.9-4.8% and 3.2-14.8%, respectively, of total apolipoprotein B. Glycated LDL concentrations in samples from diabetic patients correlated positively and significantly with other indices of glycaemic status. The results indicate that circulating glycated LDL, which may have diagnostic and pathophysiologic importance, is increased in diabetes with attendant hyperglycaemia. The results further indicate that the described monoclonal antibody-based competitive ELISA affords a simple and reproducible method for quantitative measurement of glycated LDL.

Immunologic detection and measurement of glycated apolipoprotein B with site specific monoclonal antibodies

Nonenzymatic glycation of apolipoprotein B (apo B) is a post-secretory modification of low density lipoprotein (LDL) that affects its atherogenic potential and is implicated in the accelerated atherosclerosis associated with diabetes. To facilitate assessment of apo B glycation, we produced hybridomas secreting monoclonal antibodies specific for glycated apo B. SP 2/0 myeloma cells were fused with spleen cells from BALB/c mice immunized with purified apo B glycated non-reductively in vitro. Specificity of monoclonal antibodies secreted by the cloned cell line designated ES12 was demonstrated by immunoblotting and by direct ELISA, wherein the antibodies reacted with glycated epitopes residing in LDL but not in other plasma proteins, and did not react with nonglycated apo B or nonglycated LDL. Immunoblotting of human plasma with ES12 monoclonal antibody yielded an approx. 180,000 molecular weight component showing co-identity with apo B, indicating site specificity for glycated epitopes residing in apo B of the LDL complex and absence of reactivity with other nonenzymatically glycated plasma proteins. This reactivity of ES12 with the physiologic form of glycated apo B that occurs in vivo differs from properties of other antibodies raised against glycated lipoproteins, which recognized glycated residues only after reductive conversion to glucitol-lysine and which do not discriminate between different glycated proteins. In a competitive ELISA, mean concentration of glycated LDL, measured as apo B equivalents, in eight separate plasma samples was 19.7 +/- 1.9 micrograms/ml, representing 3.5 +/- 0.3% of total apo B. The ES12 monoclonal antibody allows specific determination of plasma glycated LDL concentrations, which may have diagnostic and pathogenetic importance.

Surface-induced conformational switching in amphiphilic peptide segments of apolipoproteins B and E and model peptides

The conformational and surface-binding properties of a synthetic peptide corresponding to Tyr-apolipoprotein B-100(1000-1016) amide, SP-4, which was previously shown to mimic the focal accumulation pattern of LDL on the healing de-endothelialized rabbit aorta [Shih et al. (1990) Proc. Natl. Acad. Sci. USA 87, 1436-1440], have been investigated. SP-4 behaves as an amphiphilic alpha-helical peptide at the air-water interface and bound to siliconized quartz slides. However, its N alpha-acetylated analogue formed beta-sheet structures at the air-water interface. Nonhomologous peptide models of SP-4 also exhibited mixed alpha-helical and beta-sheet surface-binding behavior. Peptides corresponding to the cationic apolipoprotein (apo) B/E receptor binding regions of apoE (SP-2) and apoB (SP-11) were also studied. SP-2 behaved as an amphiphilic alpha helix, but, surprisingly, SP-11 formed surface-induced beta-sheets. These results demonstrate that all of the peptides studied have surface-binding properties, and suggest further that either alpha-helical or beta-sheet peptide structures may determine the binding of LDL to the arterial wall or the apoB/E receptor.

Computer prediction of antigenic and topogenic domains in HSV-1 and HSV-2 glycoprotein B (gB)

The envelope glycoprotein B (gB) coded for by the herpes simplex virus type 1 (HSV-1) UL27 gene is similar to the amino acid (aa) sequence of the gB coded by a homologous gene in HSV-2 DNA. The putative antigenic domains in HSV-1 and HSV-2 gB glycoproteins were analyzed on a comparative basis by suitable computer programs, which allowed the prediction of putative antigenic and topogenic domains. The computer-derived domains were compared to experimentally reported antigenic domains in HSV-1 gB glycoprotein. The computer-predicted antigenic domains in the HSV-1 gB glycoprotein matched well with the reported experimentally derived antigenic domains. The aa sequence of antigenic domain 1 was noted to resemble the amino acid sequence in ApoE that is involved in the attachment of this protein to LDL receptors. The clusters of hydrophobic aa domains are conserved in the two viral glycoproteins and are signals for transfer of the viral proteins through the cellular membrane.

Low density lipoprotein ligand-receptor interactions in normal healthy individuals characterized by their XbaI apolipoprotein B DNA polymorphism

A DNA restriction fragment length polymorphism (RFLP), observed with the XbaI restriction enzyme digestion of peripheral lymphocyte genomic DNA and a 3.5 kb probe 3' end of the apolipoprotein B gene, was investigated in 228 normal healthy males. Lipoprotein measurements were conducted on fasting plasma and related to the genotype; the X2X2 homozygotes (the X2 allele contains the enzyme cutting site) had significantly higher plasma cholesterol, low density (LDL) cholesterol and LDL apolipoprotein B. Thirty subjects (10 from each of the X1X1, X1X2 and X2X2 groups) were recalled and the LDL receptor activity measurements, conducted on peripheral venous blood lymphocytes, indicated no significant differences between the genotypes. However, when LDLs isolated from these individuals were assayed for ligand-receptor interaction with a human embryonic lung fibroblast cell line, significantly different maximum binding (Bmax) values in the X2 allele-bearing individuals were observed. This paradoxically elevated in vitro binding and degradation of LDL from X2X2 subjects suggests that the elevated concentrations of LDL cholesterol observed with this genotype in vivo does not result from a defective ligand-receptor interaction directly related to this polymorphism.

Lack of association between the apolipoprotein B gene 3' hypervariable region alleles and coronary artery disease in Finnish patients with angiographically documented coronary artery disease

Previous studies have suggested that some apolipoprotein B (apoB) 3' variable number of tandem repeats (3'VNTR) locus alleles are associated with coronary artery disease (CAD). We examined the possible association between the apoB 3'VNTR alleles and CAD in 387 Finnish subjects. Using the polymerase chain reaction and polyacrylamide gel electrophoresis, the 3'VNTR genotype was determined in 187 individuals with severe CAD confirmed by coronary angiography (patients), in 121 individuals with normal coronary angiograms (controls), and in 79 apparently healthy subjects (normals). In contrast to previous reports from other populations, the larger apoB 3'VNTR alleles were not significantly more frequent among CAD patients than among controls or normals. In addition, there was no significant association between the 3'VNTR alleles and serum lipid levels in this Finnish population.

A mechanistic study of the interaction of UVB radiations with human serum lipoproteins

The tryptophan residues of HDL are important chromophores of the interstitial fluid feeding the epidermal cells. The UVB light readily photoionizes tryptophan residues as demonstrated by the increase in their photolysis yield in presence of N2O. Saturation of the HDL solution with a N2O/O2 mixture (80%:20%, v/v) decreases the peroxidation of HDL lipids, thereby implying that lipid peroxidation is at least partly induced by tryptophan photolysis. Addition of EDTA and desferrioxamine to HDL solutions loaded with either Cu2+ or Fe2+ ions suggests that Fe2+ could be the 'contaminating' trace metal ions that are required to explain the occurrence of lipid photoperoxidation induced by photons absorbed by tryptophan residues.

Apolipoprotein synthesis in normal and abetalipoproteinemic intestinal mucosa

The genetic disease abetalipoproteinemia is characterized by a total absence of apolipoprotein B-containing lipoproteins from plasma. A presumed synthetic defect in apolipoprotein B synthesis was thought to be responsible for this disorder. The present study quantitates apoprotein B synthesis and apolipoprotein B messenger RNA levels in duodenal mucosa from normal patients and four patients with abetalipoproteinemia. After in vitro [3H]leucine incorporation, small intestinal biopsy specimens from three of four patients with abetalipoproteinemia synthesized immunoprecipitable apolipoprotein B of identical mobility (on sodium dodecyl sulfate gel electrophoresis) to normal apolipoprotein B. In abetalipoproteinemia, the apolipoprotein B content of intestinal mucosa by radioimmunoassay was 15% of normal mucosal values, whereas apolipoprotein B messenger RNA quantitation showed 3-20-fold increased levels compared with normal mucosa. In one patient, smaller-molecular-weight fragments of apolipoprotein B were immunoprecipitated from duodenal biopsy specimens. The synthesis rates and messenger RNA levels of two other chylomicron apoproteins (apolipoprotein A-I and apolipoprotein A-IV) were found to be reduced by 50%. These results show the synthesis of immunologically recognizable apolipoprotein B48 in abetalipoproteinemia. The significance of mucosal apolipoprotein B content in abetalipoproteinemia is discussed in terms of factors controlling apolipoprotein B synthesis in normal mucosa and in abetalipoproteinemia.

Variation of apolipoprotein B gene is associated with myocardial infarction and lipoprotein levels in Danes

Three DNA polymorphisms (XbaI, EcoRI, MspI) in the 3'-end of the apolipoprotein B gene were studied in relation to atherosclerosis, lipoprotein levels and age in three groups of atherosclerotic individuals and in nonatherosclerotic controls. The atherosclerotic groups comprised a postmyocardial infarction group with a mean age of 48 years, a group of individuals operated on for carotid stenosis with a mean age of 62 years, and a group of 85-year-olds with clinical coronary artery disease, peripheral arterial disease, or both. All 311 individuals were unrelated Caucasians of Danish ancestry. For the XbaI polymorphism, the X- allele was an independent predictor for myocardial infarction on multivariate analysis, but did not distinguish between patients and controls on univariate analysis. Additionally, this polymorphism was associated with variation in lipoprotein levels, but there was no clear evidence of a gene dosage effect. For the EcoRI polymorphism, the E- allele was associated with elevated levels of VLDL cholesterol, plasma triglycerides and VLDL triglycerides. Similar, but weaker associations were found for the MspI polymorphism. There were no significant differences in allele frequencies as a function of age for any of the DNA polymorphisms. In conclusion, while variation associated with the EcoRI polymorphism appears to be involved in the regulation of VLDL metabolism, variation associated with the XbaI polymorphism may determine susceptibility to coronary artery disease independent of other conventional risk factors, but it also appears to affect variation in lipoprotein levels.

Comparative analysis of sequences at the 5' end of the human and mouse apolipoprotein B genes

A comparison was made between the DNA sequences in two regions of the mouse and the human apolipoprotein B genes: the 5'-flanking sequence and the region between the first exon and the second intron. Considerable homology was observed, particularly in the immediate 5' region and in the second intron. Because promoter and enhancer elements have been previously localized to these regions in the human apolipoprotein B gene, it is proposed that regions of conserved base sequence delineate binding regions for regulatory proteins. In some cases, contiguous regions of homology are longer than expected for regions designed as recognition sites for individual nuclear proteins, and may define regions recognizable by a cluster of interacting proteins. Both the human and mouse genes contain repetitive elements and a hypervariable dinucleotide repeat.

Increased degradation of low density lipoproteins by mononuclear leukocytes associated with coronary artery disease

Low density lipoprotein (LDL) and peripheral blood mononuclear leukocytes (MNL) were isolated from patients with (n = 11) and without (n = 11) angiographically documented coronary artery disease (CAD). LDL degradation rates in MNL were determined in vitro using both autologous and homologous LDL. The mean rate of LDL degradation was 1.7-fold higher in CAD-MNL than in control-MNL (P less than 0.05), independent of the LDL source. The increased LDL degradation rate in CAD-MNL appeared to be due to an increased receptor-mediated LDL degradation rate in CAD-MNL and not to an increased CAD-LDL interaction with the receptor since LDL isolated from patients with and without CAD had similar in vitro degradation rates in HL-60 cells and 1.25-dihydroxyvitamin D3-induced HL-60 macrophages. An increased ratio of apo B to cholesterol, specifically apo B to cholesteryl ester, was observed in LDL isolated from patients with CAD. LDL particles isolated from CAD patients contained 14.8% less cholesteryl ester than LDL from control subjects (P less than 0.01). The data suggest that CAD patients have an increased plasma LDL particle number even though they have similar plasma LDL-cholesterol levels as compared to control subjects. These data indicate that CAD patients with normal plasma LDL cholesterol levels have two metabolic abnormalities: an altered LDL composition resulting in particles with reduced cholesteryl ester content and an increased LDL catabolism resulting in an increased influx of LDL cholesterol into MNL; both of which may play a role in the development of coronary heart disease.