Correlation of an immunogenetically defined lipoprotein type with aortic intimal lipidosis in swine

Regulation of plasma lipoprotein levels by dietary triglycerides enriched with different fatty acids

Saturated vegetable oils (coconut, palm, and palm kernel oil) containing predominantly saturated fatty acids, lauric (12:0) or myristic (14:0 and palmitic (16:0), raise plasma total cholesterol (TC) and low density lipoprotein cholesterol (LDL-C) levels in animals and humans, presumably by decreasing LDL receptor activity and/or increasing LDL-C production rate. Although stearic acid (18:0) is chemically a saturated fatty acid, both human and animal studies suggest it is biologically neutral (neither raising nor lowering) blood cholesterol levels. Although earlier studies indicated that medium chain fatty acids (8:0-10:0) were also thought to be neutral, more recent studies in animals and humans suggest otherwise. Unsaturated vegetable oils such as corn, soybean, olive, and canola oil, by virtue of their predominant levels of either linoleic acid (18:2) or oleic acid (18:1), are hypocholesterolemic, probably as a result of their ability to upregulate LDL receptor activity and/or decrease LDL-C production rate. Whether trans fatty acids such as trans oleate (t18:1), in hydrogenated products such as margarine, are hypercholesterolemic remains controversial. Studies in humans suggest that their cholesterol-raising potential falls between the native nonhydrogenated vegetable oil and the more saturated dairy products such as butter. Assessment of the magnitude of the cholesterolemic response of trans 18:1 is difficult because in most diet studies its addition is often at the expense of cholesterol-lowering unsaturated fatty acids, making an independent evaluation almost impossible.

Low-density lipoprotein cholesterol responsiveness to diet in normolipidemic subjects

Both apolipoprotein E genotype (apo E) and diet predict very-low-density (VLDL-C) and low-density lipoprotein cholesterol (LDL-C) levels. In a retrospective pooled analysis of six studies, we sought to identify the predictors of VLDL-C and LDL-C change, or "responsiveness," to a diet crossover. "Response" to diet was studied in 67 normolipidemic subjects of common apo E genotype. Subjects were fed two contrasting, metabolically controlled diets: one had a low polyunsaturated to saturated fatty acid ratio (P:S), and the other had a high P:S ratio. Multiple blood samples were analyzed for VLDL-C and LDL-C levels at the end of each metabolic diet period, and values were averaged and differences were calculated. Despite adjustment for significant predictors across the component studies, a wide range of LDL-C responsiveness was found, with an average decrease of 28 mg/dL. Multivariate regression analysis was used to identify the most significant predictors of LDL-C response to the diet crossover. All dietary and clinical variables were entered by stepwise regression for potential inclusion in a "best-fit" model. The degree of change in saturated fat content and age were the most significant predictors of LDL-C responsiveness. Neither dietary cholesterol nor apo E phenotype were significant predictors of responsiveness. The most LDL-C-responsive subjects were older and required smaller reductions in dietary saturated fat levels than did less-responsive subjects to achieve a comparable reduction in LDL-C levels. Multiple regression analysis suggested a precursor-product relationship between VLDL-C and LDL-C responsiveness.

A monoclonal antibody (BIP 45) detects Ag(c,g) polymorphism of human apolipoprotein B

A monoclonal antibody (BIP 45) against human apolipoprotein B (apo B) was used to study the polymorphism of apo B in families and in unrelated subjects. BIP 45 bound to apo B-containing lipoprotein particles in one of three distinct patterns of immunoreactivity (strong, weak and intermediate). Family studies showed that these binding patterns result from co-dominant transmission of apo B allelic pairs which are temporarily designated allele BIP- and allele BIP+; allele BIP+ would code for the apo B BIP 45 epitope. Analysis of plasma samples from 244 unrelated men randomly chosen from the North French population indicated that 46.7% of them bound BIP 45 with low affinity (weak reactors), 44.7% with intermediate affinity (intermediate reactors) and 8.6% with high affinity (strong reactors). According to the Hardy-Weinberg equilibrium, this corresponds to gene frequencies of 0.690/0.310 for the type BIP-/BIP+ alleles. This corresponds to the gene frequencies of 0.695/0.305 at the Ag(g)/Ag(c) locus previously found in a Caucasian population. Furthermore, the investigation of Ag(c,g) and of monoclonal BIP 45 antibody immunoaffinity for 30 individual plasma samples showed that BIP 45 bound strongly to Ag(c) factor, whereas it bound weakly to the allelic Ag(g) factor. This monoclonal antibody will be useful for the detection of the two corresponding apo B species designated apo B (Ag(c) factor, BIP+) and apo B (Ag(g) factor, BIP-).

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.

Genetic linkage between the antigenic group (Ag) variation and the apolipoprotein B gene: assignment of the Ag locus

The antigenic group (Ag) system of homospecific human serum antigens of low density lipoprotein is detected by antiserum from multiply transfused patients. A complex series of common Ag alleles has been described, but the biochemical nature of this polymorphism is uncertain. Here we report that DNA polymorphisms at the human apolipoprotein B (apoB) locus are very closely linked to alleles of the Ag system. We also show a strong association between Ag(x) and a polymorphism detected with the restriction endonuclease Xba I. We conclude that the immunologically determined Ag system represents protein polymorphism of apoB rather than primary genetic differences in posttranslational processing or lipid binding. These studies therefore demonstrate that the Ag locus is located on the short arm of human chromosome 2 in the region p23-p24 to which the apoB gene has been assigned. Since the Ag(x) antigen is associated with altered plasma lipid levels, this determinant may indicate a functionally important domain of apoB.

Monoclonal antibody MB19 detects genetic polymorphism in human apolipoprotein B

Using a specific monoclonal antibody (MB19) against human apolipoprotein B (apo B), we have detected a genetic polymorphism in human low density lipoprotein (LDL). MB19 bound to LDL from different individuals in one of three distinct patterns of immunoreactivity: strong, weak, and intermediate. Scatchard analysis revealed that LDLs with strong and with weak binding patterns differed 10-fold in their affinity for MB19, but both bound the same total amount of antibody (about one mole of MB19 per mole of apo B). LDL showing the intermediate binding pattern yielded a curvilinear Scatchard plot that could be resolved into two distinct components with affinities similar to those of LDLs exhibiting only the high- or only the low-affinity binding of MB19. LDL chemical composition was similar for all three MB19 binding patterns, and the polymorphism remained after removal of LDL lipid or carbohydrate. Analysis of plasmas from 77 unrelated individuals indicated that 40% of them bound MB19 with low affinity, 23% with high affinity, and 36% with intermediate or "hybrid" affinity. Family studies showed that the three MB19 binding patterns result from codominant transmission of two common apo B alleles, each coding for an allotype with different affinity for MB19, conditionally designated here MB19(1) (high affinity) and MB19(2) (low affinity).

The influence of dietary unsaturated cis and trans and saturated fatty acids on tissue lipids of swine

A feeding trial was conducted to evaluate the effects of dietary trans unsaturated fatty acids (trans fat) and of the interplay of dietary saturated fatty acids (saturated fat), cis unsaturated fatty acids, (cis fat) and trans fat on tissue lipids, particularly those effects suggestive of angiotoxicity. Swine were fed for 10 months a diet containing 17% added fat. Seven blends of varying proportions of the 3 fat components provided sufficient sample points to permit an examination of the interplay. Parameters under study included weight gain, serum cholesterol and triglyceride concentrations, lipoprotein lipid profile, total lipid and cholesterol concentrations of liver, heart and aorta, fatty acid composition of liver and aorta lipids and hepatic fatty acid synthesis and cholesterol synthesis and oxidation. Fat blends containing disproportionately high levels of saturated or cis fat generally elicited responses consistent with results reported by others. The notable exception was the serum cholesterol concentration. Throughout the study, the swine were hypercholesterolemic. Swine fed the high saturated fat blend had serum cholesterol levels equal to those swine fed the high cis fat blend. Serum cholesterol levels in the swine fed the other fat blends were more elevated. Another apparent anomaly was the lower concentration of lipid in the aortas of swine fed the high-saturated fat diet. The impact of the trans fat was modulated by the relative proportions of saturated and cis fat in the diet. The impact of trans fat was of greater magnitude for most parameters when the fat blend was low in saturated fat. The sole parameter suggestive of trans fat-mediated angiotoxicity was the distribution of lipids in lipoprotein fractions. Swine fed diets containing trans fat had lower relative proportions of the alpha-lipoprotein lipids. Although hypercholesterolemic, the high fat diets were not overtly angiotoxic except when fed to swine that carried a specific immunogenetically-defined low density lipoprotein.

Characterization of low density lipoprotein-like particle in the human aorta from grossly normal and atherosclerotic regions

Physical and chemical criteria of lipoproteins containing apolipoprotein B, extracted from human aortic intima, were compared with those of plasma low density lipoproteins (LDL). Homogenates of grossly normal intima and advanced atherosclerotic lesions were subjected to differential ultracentrifugation to isolate a d = 1.006--1.063 g/ml density fraction which was extensively characterized. By electroimmunoassay, over 90% of the recovered apolipoprotein B immunological reactivity was found in isolates from both plaques and normal intima. In isolates of plaque and normal intima, particles of the same size as LDL were found, although a small population of very large structures was also present in plaque fractions. Apolipoprotein composition was similar to that of plasma LDL except for the presence of human serum albumin in aortic isolates. Fractions from aorta demonstrated greater electrophoretic mobilities than LDL. The lipid composition of isolates from normal intima was similar to that of LDL. The lipid composition of plaque fractions showed a significant decrease in the cholesteryl ester to free cholesterol ratio and in the triglyceride content in comparison to LDL and to fractions from normal intima. The fatty acid pattern of the cholesteryl ester fraction from isolates of both normal and plaque aortic homogenates demonstrated a significant decrease in the linoleate to oleate ratio as compared to LDL. Our initial studies suggest that althought aortic fractions are similar to LDL by certain criteria, some differences observed are more pronounced in fractions from lesions than from normal intima.

Lipoprotein immunogenetics and atherosclerosis

The discovery of the first human lipoprotein polymorphism by Allison and Blumberg [Lancet i:634-637, 1961] and the availability of alloimmune sera stimulated us to begin immunogenetic studies on swine in search of lipoprotein diversity and its relationship to biological functions. We found considerable lipoprotein polymorphism, complexity, and heterogeneity in this species. These results and the correlation between immunogenetically defined lipoprotein type and arterial lipidosis in swine, fed a high fat diet, are discussed. Immunogenetic studies of lipoproteins, initiated more recently in rhesus monkeys, will be reviewed also. Preliminary data show similarities between these two species with regard to polymorphism, complexity, phenotypic expression of lipoprotein genes and, most importantly, their serological relationship to human lipoproteins. We also note immunogenetic studies on lipoproteins done by other investigators, or in other species. Brief remarks on implications of the lipoproteins in atherosclerosis, their general classification, immunological properties, and immunological methods used in their study precede the immunogenetic presentation.