Evolution of low density lipoprotein structure probed with monoclonal antibodies

Composition and electrophoretic mobility of plasma lipoproteins of dromedary camels (Camelus dromedarius)

OBJECTIVE

To determine the lipid composition and electrophoretic pattern of plasma lipoproteins in samples obtained from healthy 1-humped camels (Camelus dromedarius).

ANIMALS

34 healthy camels raised under similar farming and dietary conditions.

PROCEDURES

Plasma samples were subjected to density-gradient ultracentrifugation for separation of plasma lipoproteins, including very-low-density lipoproteins (VLDL), low-density lipoproteins (LDL), and high-density lipoproteins (HDL). Purity of the separation was assessed by use of polyacrylamide gel disk electrophoresis. Concentrations of triglycerides, cholesterol, and phospholipids were measured in each lipoprotein fraction, and lipoprotein electrophoretic patterns were determined in plasma samples.

RESULTS

Phospholipid was the major constituent of VLDL (mean +/- SD concentration, 10.62 +/- 1.2 mg/dL), LDL (24.66 +/- 3.12 mg/dL), and HDL (38.08 +/- 0.76 mg/dL). Low-density lipoprotein, VLDL, and HDL were important plasma lipoprotein carriers for cholesterol (67.94 +/- 9.51%), triglyceride (55.83 +/- 7.81%), and phospholipid (51.91 +/- 1.55%), respectively. On the basis of electrophoresis results, relative percentages of alpha- and beta-lipoproteins were 31.72 +/- 4.88% and 68.3 +/- 4.68%, respectively.

CONCLUSIONS AND CLINICAL RELEVANCE

The lipoprotein profile in 1-humped camels differed substantially from that of other ruminants. Results may be useful in the evaluation of metabolic disorders in camels.

Truncated apo B-70.5-containing lipoproteins bind to megalin but not the LDL receptor

Apo B-100 of LDL can bind to both the LDL receptor and megalin, but the molecular interactions of apo B-100 with these 2 receptors are not completely understood. Naturally occurring mutant forms of apo B may be a source of valuable information on these interactions. Apo B-70.5 is uniquely useful because it contains the NH2-terminal portion of apo B-100, that includes only one of the two putative LDL receptor-binding sites (site A). The lipoprotein containing apo B-70. 5 (Lp B-70.5) was purified from apo B-100/apo B-70.5 heterozygotes by sequential ultracentrifugation combined with immunoaffinity chromatography. Cell culture experiments, ligand blot analysis, and in vivo studies all consistently showed that Lp B-70.5 is not recognized by the LDL receptor. The kidney was identified as a major organ in catabolism of Lp B-70.5 in New Zealand white rabbits. Autoradiographic analysis revealed that renal proximal tubular cells selectively removed Lp B-70.5. On ligand blotting of renal cortical membranes, Lp B-70.5 bound only to megalin. The ability of megalin to mediate cellular endocytosis of Lp B-70.5 was confirmed using retinoic acid/dibutyryl cAMP-treated F9 cells. This study suggests that the putative LDL receptor-binding site A on apo B-100 might not by itself be a functional binding domain and that the apo B-binding sites recognized by the LDL receptor and by megalin may be different. Moreover, megalin may play an important role in renal catabolism of apo B truncations, including apo B-70.5.

Lipoprotein profiles and glucose tolerance in lean and obese chimpanzees

We compared serum lipid profiles and glucose tolerance of obese and lean chimpanzees maintained on a 10.9% fat diet. Seven of 14 obese and 6 of 17 lean chimpanzees were hypercholesterolemic (low density lipoprotein cholesterol > 160 mg/dl), three obese and three lean animals had total cholesterol/high density lipoprotein cholesterol ratios of 5.9-10.7, and two obese and one lean chimpanzee had abnormal glucose tolerance. Useful numbers of captive chimpanzees thus exhibit metabolic abnormalities without recourse to high fat diets and could serve as surrogates in studies of human metabolic diseases.

Ability of the LDL receptor from several animal species to recognize the human apo B binding domain: studies with LDL from familial defective apo B-100

To verify whether the LDL receptors from different animal species recognize the binding domain of human apo B-100 we studied the interaction of LDL from control and familial binding defective apo B-100 (FDB) with cultured cells. Human, monkey, bovine, guinea pig and rabbit LDL receptors distinguish between normal and binding defective LDL with a displacement ratio (defective/normal) of 3.3, 2.6, 3.4, 3.1 and 2.0, respectively. Guinea pig and rabbit receptors, however, showed affinities 2-3-fold lower than the human receptor. Hamster, rat and mouse cells failed to differentiate between normal and FDB LDL with a ratio of 1.2, 0.8, and 1.4; the apparent affinities were 4-8 times lower than that of the human receptor. The data from the latter species suggest that the LDL receptor recognizes an area of human apo B different from the human receptor binding domain. The ability of antibody Mb47 to inhibit the binding of human LDL to human, rabbit and guinea pig but not to mouse cells further stresses this concept. Moreover, in 17 alpha-ethinyl estradiol-treated rats the rate of disappearance from plasma of FDB and control 125I-labelled LDL was identical, thus confirming the in vitro observations. These data suggest that the binding domain of the LDL receptor is functionally conserved in man, monkey, cow, rabbit and guinea pig, but is quite distinct in rat, mouse and hamster.

Monoclonal antibody studies of the antigenic determinants of human plasma retinol-binding protein

A battery of monoclonal antibodies (MoAbs) against human retinol-binding protein (RBP) was produced to obtain useful probes for the study of the antigenic determinants of RBP. The 12 antibodies all reacted with human RBP by immunoblotting. Based on antibody cross-competition radioimmunoassays, four distinct and different groups of antibodies were identified: group I, 1A4 and 2F4; group II, 1G10, 5C5, 6F4, and 7G3; group III, 5H6, 6C7, 10G5, and 14E3; and group IV, 5H9 and 13A1. Information about the epitopes of RBP recognized by these MoAbs was obtained by testing the reactivity of each antibody with human, rabbit, and rat RBPs by immunoblotting. Group I and group IV antibodies reacted to a similar extent with human, rabbit, and rat RBPs. Group II antibodies reacted strongly with human and rabbit RBPs, but reacted very weakly with rat RBP. Group III antibodies reacted strongly with human RBP, but did not react with rabbit or rat RBP. Thus, the epitopes for group I and group IV antibodies appear to be regions of the RBP molecule that are conserved across the three species, whereas group III antibodies recognized only human RBP. In a preliminary study, the reactivity of each antibody with purified cyanogen bromide fragments of RBP was tested by slot immunoblotting. None of the MoAbs reacted with any of the cyanogen bromide fragments. This study shows that MoAbs specific for at least four different regions of the RBP molecule can be produced; hence, RBP contains at least four major antigenic domains.

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.

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.

Conservation of the low density lipoprotein receptor-binding domain of apoprotein B. Demonstration by a new monoclonal antibody, MB47

The fact that low density lipoprotein (LDL) from multiple animal species binds to the human LDL receptor suggested that the LDL-receptor binding domain of apoprotein (apo)B must be evolutionarily conserved. To determine if a common receptor domain epitope existed on apo B, we generated a monoclonal antibody that was specific for the LDL-receptor domain of apo B. This was accomplished by using a screening procedure that selected for a hybridoma supernatant that could block specific cellular uptake and degradation of LDL. Western blots showed that this antibody, termed MB47, was specific for apo B-100. Fluid phase assays indicated a high binding affinity (Ka = 4 X 10(9) M-1) and demonstrated that all human LDL particles expressed the MB47 epitope. Scatchard analysis indicated that a maximum of one MB47 molecule bound to each LDL particle. In solid phase assays, antibody MB47 bound to plasma or LDL of multiple mammalian species, including guinea pig, rabbit, pig, dog, cat, seal, whale, bear, and lion, but it did not bind to mouse or rat LDL. In contrast, a rabbit antiserum to LDL and two other anti-apo B monoclonal antibodies, MB3 and MB19, which do not bind to the receptor domain, were specific only for human LDL. LDL from multiple species, including mouse LDL, competed effectively with 125I-human LDL for binding to human fibroblasts. MB47 effectively inhibited uptake and degradation of labeled human, guinea pig, and rabbit LDL by both human and guinea pig fibroblasts. We conclude that antibody MB47 binds to a single receptor domain on LDL and identifies a vital region conserved through mammalian evolution.