Anti-apoprotein B monoclonal antibodies detect human low density lipoprotein polymorphism

Cryopreservation and long-term storage of human low density lipoproteins

A simple and effective technique of long-term storage of human low density lipoproteins (LDL) has been developed. The technique involves the addition of 1.4 mol/l (or 10% by volume) dimethyl sulphoxide directly to a solution of the freshly isolated LDL in high salt buffer, and subsequent freezing and storage for up to 2 years at -70 degrees C. We have shown that freshly isolated LDL, "preserved" as described above, are able to keep their native properties for a long period, i.e.: a) electrophoretic behaviour in non-denaturing (or with sodium dodecyl sulphate, 1 milligram) polyacrylamide 2-16% gradient gel electrophoresis; b) immunoreactivity of apolipoprotein B (analyzed by radial immunodiffusion, electroimmunoassay and immunoturbidimetric assay); c) immunogeneity of apolipoprotein B; d) an average size of LDL particles (analyzed by electron microscopy); e) ability to bind with B,E-receptors of human skin fibroblasts. The technique can also be applied to radiolabelled LDL samples. Taking into consideration the labour- and time-consuming procedure of obtaining and characterizing LDL, and the preferred use of single well-characterized LDL preparation, we recommend that the above technique of LDL long-term storage be applied in various clinical and biomedical studies.

Production of baboon (Papio hamadryas) monoclonal antibodies by herpesvirus papio immortalized baboon lymph node cells

Baboon monoclonal antibodies specific for monomorphic determinants of baboon apolipoprotein B were produced from in vivo primed lymph node cells immortalized by herpesvirus papio (HVP). Two cell lines from the immortalized cells were readily stabilized and grew rapidly. Since Epstein-Barr virus (EBV) cannot be used to immortalize baboon or macaque cells, HVP expands the potential sources of monoclonal antibodies to include baboons, and is the immortalizing agent of choice for macaques as well.

Kinetics of a heterogeneous population of particles in low density lipoprotein apolipoprotein B

This paper examines the kinetics of low density lipoprotein (LDL) metabolism following the in vivo injection of native and chemically-modified lipoproteins in an attempt to assess the relative importance of receptor-dependent and receptor-independent catabolic pathways. The shape of the urinary/plasma ratio curve suggested heterogeneity of the LDL-apolipoprotein B pool and excluded homogeneity. This heterogeneity required the building of a more complex model that allowed the simultaneous fitting of plasma and urinary data. This new model permits the precise quantification of both receptor and non-receptor pathways.

Monoclonal antibodies to human A-I apolipoprotein and characterisation of cyanogen bromide fragments of apoA-I

Several monoclonal antibodies to human A-I apolipoprotein were produced after immunising mice with pure delipidated apoA-I. These monoclonal antibodies were characterised for their ability to react with whole lipoproteins, apolipoproteins and fragments of apoA-I generated by cleavage with cyanogen bromide. The data suggest that production of monoclonal antibodies using apoA-I as antigen was influenced by two major epitopes subsequently localised to cyanogen bromide fragments 1 and 3, and have been designated antibodies 1----5 A-IB and 6----10 A-IB, respectively. Cyanogen bromide fragments were first purified to homogeneity before screening by competitive displacement or immunoblotting procedures. Definitive characterisation of one antibody series (1----5 A-IB) depended ultimately on Western blotting following isoelectric focusing of purified apoA-I fragments. This technique identified the epitope for these antibodies to fragment 1, an identification not fully concluded from competitive displacement studies. These studies have also revealed the presence of microheterogeneity in fragment 1 (as well as in fragment 4) of apoA-I, suggesting that structural variations in several regions may account for the polymorphism observed in this apolipoprotein.

Heritable allele-specific differences in amounts of apoB and low-density lipoproteins in plasma

Low-density lipoprotein (LDL) concentrations correlate with risk of coronary heart disease, and genetic variation affecting LDL levels influences atherosclerosis susceptibility. The principal LDL protein is apolipoprotein B (apoB); apoB is not exchangeable between lipoprotein particles and there is only one apoB per LDL particle. Plasma LDL therefore consists of two populations, one containing apoB derived from the maternal and one from the paternal apoB alleles. Products of the apob gene with high or low affinity for the MB-19 monoclonal antibody can be distinguished, and this antibody was used to identify heterozygotes with allele-specific differences in the amount of apoB in their plasma. A family study confirmed that the unequal expression phenotype was inherited in an autosomal dominant manner and was linked to the apob gene locus. Significant apoB genetic variation affecting plasma LDL levels may be more common than previously appreciated.

The apolipoprotein multigene family: structure, expression, evolution, and molecular genetics

The plasma apolipoproteins can be classified into two subgroups: the soluble apolipoproteins including apolipoprotein (apo) A-I, A-II, A-IV, C-I, C-II, C-III, and E, and the apoBs including apoB-100 and apoB-48. The soluble apolipoproteins have very similar genomic structures, each having a total of three introns at the same locations; apoA-IV is an exception in that it has lost its first intron. Using the exon/intron junctions as reference points, we can obtain an alignment of the coding regions of all the soluble apolipoprotein genes. The mature peptide regions of the genes are almost completely made up of tandem repeats of 11 codons. The part of mature peptide region encoded by exon 3 contains a common block of 33 codons, whereas the part encoded by exon 4 contains a much more variable number of internal repeats of 11 codons. On the basis of the degree of homology of the various sequences, and the pattern of the internal repeats in these genes, an evolutionary tree has been proposed for the soluble apolipoprotein genes. ApoB-100 differs considerably from the soluble apolipoproteins. It is the largest apolipoprotein containing 4536 amino acid residues. Two types of internal repeats are identified in apoB-100: amphipathic alpha-helical repeats and proline-containing repeats with high beta-sheet content. The apoB gene contains 29 exons and 28 introns. Its evolutionary relationship to the soluble apolipoprotein genes is unclear. The 3' end of the apoB gene contains a region of variable number of tandem 12-16-base pair repeats.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular modeling of protein-glycosaminoglycan interactions

Forty-nine regions in 21 proteins were identified as potential heparin-binding sites based on the sequence organizations of their basic and nonbasic residues. Twelve known heparin-binding sequences in vitronectin, apolipoproteins E and B-100, and platelet factor 4 were used to formulate two search strings for identifying potential heparin-binding regions in other proteins. Consensus sequences for glycosaminoglycan recognition were determined as [-X-B-B-X-B-X-] and [-X-B-B-B-X-X-B-X-] where B is the probability of a basic residue and X is a hydropathic residue. Predictions were then made as to the heparin-binding domains in endothelial cell growth factor, purpurin, and antithrombin-III. Many of the natural sequences conforming to these consensus motifs show prominent amphipathic periodicities having both alpha-helical and beta-strand conformations as determined by predictive algorithms and circular dichroism studies. The heparin-binding domain of vitronectin was modeled and formed a hydrophilic pocket that wrapped around and folded over a heparin octasaccharide, yielding a complementary structure. We suggest that these consensus sequence elements form potential nucleation sites for the recognition of polyanions in proteins and may provide a useful guide in identifying heparin-binding regions in other proteins. The possible relevance of protein-glycosaminoglycans interactions in atherosclerosis is discussed.

Human apolipoprotein B. Evidence for its immunochemical heterogeneity using monoclonal antibodies and an immunoenzymometric assay

Predefined monoclonal antibodies (Mabs) were used in an immunoenzymometric assay to study the immunochemical heterogeneity of lipoproteins and to search for potential epitopes with pathological importance. By measuring apolipoprotein B (apo B) epitopes in patients with and without angiographically documented coronary artery disease and in patients with type IIa hyperlipoproteinemia, we have found that both types of patients have a significant increase in Apo B-containing particles specifically recognized by one Mab (BL3). We have also observed that the effects of fenofibrate on type IIa patients vary greatly depending on the plasma concentrations of various Apo B-containing lipoproteins. The greatest effects occurred in patients with epitopes recognized by BL3. Lastly, by sequential precipitation of specific epitopes by BL3, we have obtained evidence that the residual epitope(s) may be related to one or more lipoprotein particles.

Use of monoclonal antibodies in genetic research with nonhuman primates

Monoclonal antibodies, because of their specificity and unlimited availability, have become one of the most powerful experimental tools available to the biological sciences. It is possible to make monoclonal antibodies that bind to determinants that are monomorphic in one or more species or to determinants that are polymorphic within a species. Few monoclonal antibodies have been made using immunogens derived from nonhuman primates. However, some monoclonal antibodies that recognize monotypic markers in humans can be used to detect polymorphic markers in nonhuman primates. Thus, the rapid development of monoclonal antibodies specific for human proteins significantly increases the potential number of immunogenetic markers useful for studying phylogenetic relationships and for identifying genetic polymorphisms among nonhuman primates.

Isolation and characterization of four heparin-binding cyanogen bromide peptides of human plasma apolipoprotein B

Apolipoprotein B-100 (apoB-100) is the major protein constituent of human plasma low-density lipoproteins (LDL). On the basis of its amino acid sequence [Chen, S.-H., Yang, C.-Y., Chen, P.-F., Setzer, D., Tanimura, M., Li, W.-H., Gotto, A. M., Jr., & Chan, L. (1986) J. Biol. Chem. 261, 12918-12921], apo B-100 is one of the largest monomeric proteins known with a calculated molecular weight of 512937. Heparin binds to the LDL surface by interacting with positively charged amino acid residues of apoB-100, forming soluble complexes in the absence of divalent metals and insoluble complexes in their presence. The purpose of this study was to isolate and characterize the heparin-binding domain(s) of apoB-100. Human plasma LDL were fragmented with cyanogen bromide (CNBr). After delipidation and reduction-carboxymethylation, the CNBr peptides were fractionated by sequential chromatography on DEAE-Sephacel, Mono S, and high reactive heparin (HRH) AffiGel-10; HRH was purified by chromatography of crude bovine lung heparin on LDL AffiGel-10. Heparin-binding peptides were further purified by reverse-phase high-performance liquid chromatography. Heparin-binding activity was monitored by a dot-blot assay with 125I-HRH. The amino-terminal sequences of four CNBr heparin-binding peptides (CNBr-I-IV) were determined. CNBr-I-IV correspond to residues 2016-2151, 3109-3240, 3308-3394, and 3570-3719, respectively, of the amino acid sequence of apoB-100. Each CNBr peptide contains a domain(s) of basic amino acid residues which we suggest accounts for their heparin-binding activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural studies of apolipoprotein B: physical properties of the protein in guanidine hydrochloride

Apolipoprotein B was isolated from human plasma low-density-lipoprotein without precipitation by diethyl ether/ethanol extraction of the protein in 6 M guanidine hydrochloride. The physical properties of this protein, which contained a residuum of approximately 7% phospholipid, were examined in 6 M guanidine solution under reducing conditions. The circular dichroism spectrum was indistinguishable from that of a random coil protein. Sedimentation equilibrium analyses of apolipoprotein B by the meniscus depletion method of Yphantis (1984, Biochemistry 3, 297-317) were complicated by heterogeneity and nonideality despite the low concentrations employed. 63 analyses of the weight average (Mw) and z average (Mz) molecular weight were made on the apolipoprotein B from 12 subjects. The Mw observed was a function of initial concentration, rotor speed, and a heterogeneity index (Mz/Mw). Multiple linear regression of apolipoprotein B molecular mass against these parameters suggested that an Mw of 540,000 +/- 110,000 would be observed under apparently ideal and homogeneous conditions. The sedimentation coefficient and intrinsic viscosity of the reduced protein at 25 degrees C in 6 M guanidine were 2.13 S and 116 ml/g, respectively; these values predict molecular weights of 640,000 and 250,000, respectively, if apolipoprotein B was fully denatured into a random coil. Lack of agreement between these estimates and with the sedimentation equilibrium analysis can best be explained by compactness of structure and incomplete denaturation to a random coil state. Furthermore, an irreversible temperature dependence of apolipoprotein B reduced viscosity indicated that residual structure remained in solutions of 6 M guanidine hydrochloride/20 mM dithiothreitol. Taken together, the physical data demonstrate that apolipoprotein is a single polypeptide of approximately 540 kDa, whose structure resists denaturation under conditions where most proteins exist as random coils.

Detection of human apolipoprotein B polymorphic species with one monoclonal antibody (BIP 45) against low density lipoprotein. Influence of this polymorphism on lipid levels and coronary artery stenosis

The immunoreactivity of apolipoprotein B (apo B) in plasma samples obtained from a variety of subjects was analysed by non-competitive ELISA with a polyclonal and a monoclonal (BIP 45) anti-LDL antibody. Three populations were tested: the first, comprising 244 healthy male volunteers, provided reference values; the second consisted of a population undergoing coronary angiography (n = 88) and was divided into a subgroup with (n = 64) and without (n = 24) coronary artery disease (CAD); the third was made up of 56 patients with heterozygous familial hypercholesterolemia. Total apo B (measured with the polyclonal antibody) was increased in the populations with CAD and in the heterozygous familial hypercholesterolemic subjects compared to the reference population. When monoclonal antibody BIP 45 was used in the non-competitive ELISA, three different patterns emerged in each population, corresponding to weak, intermediate and strong binding of the particles containing apo B to the monoclonal antibody. This may result from genetic polymorphism of apo B, and in the reference population the data fit a model consisting of two co-dominant apo B alleles (BIP(-) and BIP(+]; the 3 subpopulations then correspond to the 2 homozygotes and the heterozygote. The number of patients whose particles bound weakly to monoclonal BIP 45 antibody was low in the CAD population, while intermediate binding was increased in this group. Nevertheless, when the analysis of variance of allele BIP(-) was studied no significant difference between groups was established. This finding indicates that the genetic difference in apo B detected by BIP 45 may not be significant in the development of CAD. Furthermore, the apo B genetic polymorphism detected by BIP 45 is not associated with a particular lipoprotein level in the reference population.

Detection of two apolipoprotein B species (apoBc and apoBg) by a monoclonal antibody

A monoclonal antibody (MB-19) was used to investigate the polymorphism of apolipoprotein B in a large East Finnish family and in unrelated subjects. Apolipoprotein B was shown to exhibit high, intermediate or low affinity binding to this antibody. Thus, MB-19 bound strongly to the Ag(c) epitope, an Ag antigenic domain previously characterized by human antisera, while it bound only weakly to the allelic epitope Ag(g). It proved useful for the detection of the two corresponding allelic apoB species designated apoBc (= high affinity binding) and apoBg (= low affinity binding), and for confirming their co-dominant transmission. Intermediate binding resulted from the presence of a mixture of both apoB populations in heterozygous subjects.

The production and use of monoclonal antibodies to measure apolipoprotein B by a competitive enzyme-linked immunoassay

The production of monoclonal antibodies to human lipoproteins is described. One of these antibodies, which was shown to be specific to apolipoprotein B, was used to develop a competitive enzyme-linked immunoassay for apolipoprotein B in serum samples. The antibody selected recognises apolipoprotein B in both low density and very low density lipoprotein particles, but there is no cross-reactivity with high density lipoprotein. There is no requirement for labelling of antigen or antibody used in the assay, and results obtained correlate well (r = 0.88) with measurements on serum samples using a radial immunodiffusion assay for apolipoprotein B.

Immunological heterogeneity of rat apolipoprotein B epitope expression. A study using monoclonal antibodies to rat apolipoprotein B

Epitope expression of rat apolipoprotein B on lipoproteins was investigated with the help of six monoclonal antibodies produced from mice. Through a variety of techniques, which include cotitrations, ELISAs and quantitative immunoadsorption precipitation, we concluded that the six monoclonal antibodies recognize five different epitopes. LRB 110 and LRB 260 recognize epitopes that may be overlapping. LRB 240 and LRB 250 recognize epitopes that are preferentially expressed in triacylglycerol-rich particles. LRB 220 recognizes an epitope that is expressed by all apolipoprotein-B-containing lipoproteins. We have also determined that apolipoprotein B epitope expression in rat lipoproteins is very similar to its human counterpart. Both rat and human apolipoprotein B epitope expression on lipoproteins showed heterogeneities even in homologous lipoprotein preparations. We concluded that a variety of techniques are necessary to fully characterize monoclonal antibodies to apolipoproteins. The possible implications of epitope expression in pathophysiology are also discussed.

Immunogenetic polymorphism of apolipoprotein B in humans: studies with a monoclonal anti-Ag(c) antibody

In studies that use a monoclonal antibody (MB-19), apolipoprotein B exhibited one of three immunophenotypes: high, intermediate, or low affinity binding to this antibody. The distribution of these immunophenotypes (allotypes) in families was compatible with a codominant transmission of two alleles, one coding for the high and the other for the low affinity binding allotype. The high affinity binding allotype coincided with antigen Ag(c) and the low affinity binding allotype with Ag(g), two allelic antigenic determinants previously defined by human antisera. Preliminary studies did not reveal differences in plasma lipid levels in association with apolipoprotein B allotypes. Young Finnish men with low affinity binding apolipoprotein B had slightly lower plasma apolipoprotein B levels than those with the intermediate affinity binding phenotype.

Lipoprotein mutations in pigs are associated with elevated plasma cholesterol and atherosclerosis

A strain of pigs bearing three immunogenetically defined lipoprotein-associated markers (allotypes), designated Lpb5, Lpr1, and Lpu1, has marked hypercholesterolemia on a low fat, cholesterol-free diet. Unlike individuals with familial hypercholesterolemia or WHHL rabbits, the affected pigs have normal low density lipoprotein receptor activity. The animals, by 7 months of age, have extensive atherosclerotic lesions in all three coronary arteries. This strain of pig represents an animal model for atherosclerosis and hypercholesterolemia associated with mutations affecting the structures of plasma lipoproteins. One of the variant apolipoproteins, Lpb5, is apolipoprotein-B. A second variant apolipoprotein (Lpr1), termed apo-R, is a 23-kilodalton protein present in both the very low density (d less than 1.006 g/ml) and the very high density (d greater than 1.21 g/ml) fractions of pig plasma. Isoforms of this protein correlate with two Lpr alleles, Lpr1 and Lpr2. The Lpr genes segregate independently of the Lpb5 and Lpu1 alleles. The Lpu1 allotype is a component of low density lipoprotein and is genetically linked to Lpb5.

Kinetic heterogeneity of low density lipoproteins in primary hypertriglyceridemia

The kinetics of two subfractions of low density lipoproteins (LDL) were examined in nine patients with primary hypertriglyceridemia. LDL was subjected to equilibrium ultracentrifugation, and three patterns of LDL subfractions were noted. The LDL of five patients with moderate hypertriglyceridemia (plasma triglycerides (TG) ranging from 333 to 580 mg/dl) appeared to contain two distinct subfractions. One was less dense and had a high TG content; the other was more dense and had a reduced content of all lipids, particularly cholesterol. Each subfraction was labeled separately and was reinjected into the patient. Of the two subfractions, the more dense LDL usually had a higher fractional catabolic rate (FCR), although the turnover rates of both subfractions for these hypertriglyceridemic patients were higher than normal. Two other patients with mild hypertriglyceridemia had only a single LDL after gradient equilibrium ultracentrifugation. This fraction was divided into less dense and more dense subfractions, and their FCR was determined. In both patients, turnover rates of the two subfractions were similar and both were in the normal range. Finally, two more patients with mildly elevated TG had a very dense LDL, besides having a single, less dense band. For both patients, the FCR for the less dense and very dense subfractions were similar, although the denser LDL had a greater fraction in the extravascular compartment. Thus, patients with primary moderate hypertriglyceridemia often have distinct subfractions that have different turnover rates. For patients with mild hypertriglyceridemia, the LDL is more homogeneous, and its subfractions are kinetically similar.

Ag(c): recognition by a monoclonal antibody

A monoclonal antibody directed against human apolipoprotein B, which was previously shown in family studies to detect allelic variations (J Biol Chem 1984; 259:6423-6430), has now been identified with the Ag(c) factor. This identification allows the location of the Ag system on the structural gene for apolipoprotein B and on the short arm of human chromosome 2. The epitope corresponding to Ag(c) is located within the amino acid sequence common to apolipoproteins B-100 and B-48. Since a single molecule of apolipoprotein B-100 is present on human LDL, individual LDL possesses either the epitope corresponding to Ag(c) or that corresponding to Ag(g). These studies on allelic variation among human apolipoprotein B species parallel similar studies in animals in which a relationship to atherosclerosis was found.

Human apolipoprotein B: chromosomal mapping and DNA polymorphisms of hepatic and intestinal species

Apolipoprotein B (apoB) is the major protein component of low-density and very-low-density lipoproteins. We have recently isolated nonoverlapping cDNA clones for apoB and confirmed their identity by sequence comparisons. We now report the mapping of the human apoB gene (APOB) to the p23-p24 region of chromosome 2 by examination of human-mouse somatic cell hybrids and by in situ hybridization to human chromosomes. Thus, APOB is unlinked to members of the dispersed gene family encoding other apolipoprotein species or to the gene encoding the low-density lipoprotein receptor. Hybridization analysis with genomic DNA and liver and intestinal mRNA suggests that APOB encodes both the high-molecular-weight form of apoB (apoB100) incorporated into very-low-density lipoproteins in liver and the lower-molecular-weight form (apoB48) incorporated into chylomicrons in intestine. Restriction fragment length polymorphisms of APOB have been identified and should prove useful in examining the possibility that genetic variations of APOB are involved in dyslipoproteinemias and atherosclerosis.

Characterization of a monoclonal antibody that binds equally to all apolipoprotein and lipoprotein forms of human plasma apolipoprotein B. I. Specificity and binding studies

A stable mouse hybridoma cell line has been developed that produces monoclonal antibody to human plasma apolipoprotein B. This antibody was proven to be specific for apolipoprotein B immunoblotting and an enzyme immunoassay using apolipoprotein B and other apolipoproteins. The antibody bound with comparable affinities to soluble apolipoprotein B, chylomicrons, very-low-density (VLDL) and low-density lipoproteins (LDL). Coupled to agarose, this antibody allowed complete removal of apolipoprotein B-containing lipoproteins from normolipidemic, hypertriglyceridemic and hypercholesterolemic plasma. Desialyzation and deglycosylation had no effect on its binding to LDL. The described antibody had no effect on the receptor-mediated binding of radiolabeled LDL to the human hepatoma cells (HepG2) in culture. Analysis of 25 different samples of human plasma indicated identical expression of the corresponding epitope in these individuals. The described monoclonal antibody, most likely, binds to a rather stable domain of apolipoprotein B that is not altered by the interaction with lipids or polymorphism of the apolipoprotein B. We propose that this antibody be called 'Pan B' antibody.

The human apolipoprotein B-100 gene: a highly polymorphic gene that maps to the short arm of chromosome 2

Using a cloned cDNA of apolipoprotein B-100 as hydridization probe, we have found high frequence polymorphisms in the apoB-100 gene involving sites for the restriction enzymes EcoRI, BamHI, and HindIII. The major EcoRI polymorphisms involved a 17 kb vs 15 kb variant. The incidence of the various phenotypes was estimated. In addition, other complex polymorphisms involving MspI and TaqI sites were also noted. [32P]-labeled apoB-100 cDNA was used as a probe in chromosome mapping studies to detect the human apoB-100 structural gene sequence in human-Chinese hamster and human-mouse cell hybrids. Southern blot analysis of 14 hybrids localized the gene to the short arm of human chromosome 2.