Lack of relationship in humans of the parameters of body cholesterol metabolism with plasma levels of subfractions of HDL or LDL, or with apoE isoform phenotype

The factors involved in regulating parameters of whole body cholesterol metabolism in humans have been explored in a series of investigations. Several physiological variables have been identified (weight, excess weight, plasma cholesterol, and age) that can predict 53-76% of the variation in production rate (PR) and in the sizes of the rapidly exchanging pool of body cholesterol (M1) and of the minimum estimates of the slowly exchanging pool of body cholesterol (M3min) and of total body cholesterol (Mtotmin). Surprisingly, measurements of the plasma levels of HDL cholesterol and of the major HDL apolipoproteins (apoA-I, A-II, and E) did not provide additional information useful in predicting parameters of whole body cholesterol metabolism. A study was therefore conducted to investigate possible relationships of the plasma levels of subfractions of lipoproteins, determined by analytic ultracentrifugation, and of apoprotein E phenotype, with the parameters of whole body cholesterol metabolism. Ultracentrifugal analysis of plasma lipoprotein subfractions was performed at the Donner Laboratory in 49 subjects; all of these subjects were currently undergoing whole body cholesterol turnover studies or had previously had such studies and were in a similar metabolic state as judged by plasma lipid and lipoprotein values. Apoprotein E phenotyping was carried out in 71 subjects. Differences in model parameters were sought among subjects with various apoprotein E phenotypes. Ultracentrifugal LDL subfractions Sof 0-2 (the region of LPa), Sof 0-7 (smaller LDL), Sof 7-12 (larger LDL), Sof 12-20 (IDL), and ultracentrifugal HDL subfractions Fo1.20 0-1.5 (smaller HDL3), Fo1.20 2-9 (larger HDL3 plus HDL2), and Fo1.20 5-9 (larger HDL2 or HDL2b) were examined for correlations with each other and with parameters of whole body cholesterol metabolism.     

The effects of dietary fat and cholesterol on the metabolism of plasma low density lipoprotein apoproteins in squirrel monkeys.

Low density lipoprotein apoproteins from squirrel monkeys (Saimiri sciureus) had characteristic 2-phase die-away curves in plasma. The kinetic constants were similar with three methods of labeling: in vitro with 125I by the iodine monochloride or the Bolton-Hunter methods or in vivo by the injection of [3H]-leucine into a donor animal. Dietary cholesterol and the type of dietary fat influenced the concentration of plasma cholesterol and low density lipoproteins. The fractional turnover of low density lipoprotein apoprotein was greaterin monkeys fed semipurified diets with safflower oil than in those on butter but was not influenced by dietary cholesterol. The total low density lipoprotein apoprotein turnover (the product of fractional turnover and plasma lipoprotein concentration) was highest in monkeys fed butter plus added cholesterol and lowest in those on safflower oil without cholesterol. Dietary safflower oil resulted in a smaller proportion of the total low density lipoprotein pool in the intravascular compartment than did butter, regardless of whether cholesterol was added.     

Alterations of the plasma lipoproteins and apoproteins following cholesterol feeding in the rat.

The feeding of cholesterol to rats resulted in marked alterations in the type and distribution of the plasma lipoproteins and their apoproteins. The hyperlipoproteinemia was characterized by an increase in the d < 1.006 lipoproteins (B-VLDL and VLDL), an increase in the intermediate and low density lipoproteins (LDL), and the appearance of HDL(c). Associated with these lipoproteins was a prominence of the arginine-rich apoprotein. The high density lipoproteins (HDL) were decreased. A two-dimensional immunoelectrophoretic procedure was adapted to quantitate the changes in distribution of the arginine-rich apoprotein in the plasma and various ultracentrifugal fractions obtained from control and cholesterol-fed rats. In rats fed the cholesterol diet, the total plasma arginine-rich apoprotein increased from a control value of approximately 29 mg/dl to 47 mg/dl. The method of ultracentrifugation, however, was found to markedly alter the quantitative results. When the 60 Ti rotor was used at maximum speed to isolate the ultracentrifugal fractions, less than 50% of the total plasma arginine-rich apoprotein was associated with the lipoproteins in the d < 1.006 or the d 1.006-1.02, 1.02-1.063, or 1.063-1.21 ultracentrifugal fractions. By contrast, after limited ultracentrifugation with the 40 rotor, much less arginine-rich apoprotein was lost, with approximately 20% of the arginine-rich apoprotein in control rats and 10% in cholesterol-fed rats found in the d > 1.21 fraction. Significant alterations in the arginine-rich apoprotein quantitation notwithstanding, the observations of increased arginine-rich apoprotein in the B-VLDL, intermediate fraction, and HDL(c) following cholesterol feeding remained valid. However, precise quantitation awaits refinements in lipoprotein isolation techniques.     

Polymorphisms of the HDL receptor gene associated with HDL cholesterol levels in diabetic kindred from three populations

OBJECTIVE: We examined polymorphisms in the HDL receptor, SR-BI, for association with plasma HDL cholesterol levels. METHODS: Study subjects, including 847 women and 725 men, were from families originally ascertained for type 2 diabetes from Finland, Sweden and Israel. Four common polymorphisms were examined in linear regression analysis: an exon 1 missense (EX1), exon 8 silent (EX8), intron 5 (IVS5) and intron 10 (IVS10) variants. RESULTS: Genotype combinations for the three polymorphisms in linkage disequilibrium (IVS5, EX8 and IVS10) were found to be associated with HDL-C among women from the Israeli (p = 0.01) and Swedish (p = 0.06) populations. In Finnish women, the association was only apparent after taking into account effect modification by triglyceride levels (p = 0.04). One specific pattern of genotypes, denoted by presence of the IVS5_T and EX8_C alleles, and absence of the IVS10_G allele, was consistently associated with the lowest mean levels of HDL-C in women from all three populations. These same associations were not found in men. CONCLUSIONS: Polymorphic variation of the SR-BI gene may influence HDL-C levels and act in a sex-dependent manner.     

Characterization of baboon plasma high-density lipoproteins and of their major apoproteins.

Baboon high-density lipoproteins (HDL) were isolated by preparative ultracentrifugation between d = 1.063 and 1.215 g/mL. The HDL contains 48.8% protein and a lipid distribution similar to human HDL. The phospholipid distribution shows a low sphingomyelin value (5.9%), and the fatty acid composition of HDL is comparable to the human data except for the 18:1/18:2 ratio as a result of a higher 18:1 content in the CE and a lower 18:2 concentration in the PL. The major HDL apoproteins isolated on diethylaminoethyl-cellulose had a mobility on sodium dodecyl sulfate--polyacrylamide gel electrophoresis and a molecular weight and an amino acid composition similar to human apoA-I. However, the amino acid sequence of the first 30 residues of baboon apoA-I differed from the human apoprotein in residues 15 and 21. Treatment of apoA-I with carboxypeptidase A indicated a carboxyl-terminal sequence of Leu-Ser-Thr-Gln. Baboon apoHDL contained monomeric apoA-II with the mobility of monomeric human apoA-II and a molecular weight of 8500. The amino acid composition differed from the human apoA-II by the presence of arginine and by the absence of half-cystine and isoleucine. The circular dichroic spectra of apoA-I and apoA-II demonstrated a higher helicity compared to the human apoproteins. Recombination studies by microcalorimetry of apoHDL with dimyristoylphosphatidylcholine (DMPC) indicated similarities in the thermodynamic binding properties of the HDL apoproteins from man and baboon. The maximal-binding enthalpies of DMPC to apoHDL, apoA-I, and apoA-II were lower for the baboon than for the human apoprotein.     

Emerging role of HDL in brain cholesterol metabolism and neurodegenerative disorders

High-density lipoproteins (HDL) play a key role in cholesterol homeostasis maintenance in the central nervous system (CNS), by carrying newly synthesized cholesterol from astrocytes to neurons, to support their lipid-related physiological functions. As occurs for plasma HDL, brain lipoproteins are assembled through the activity of membrane cholesterol transporters, undergo remodeling mediated by specific enzymes and transport proteins, and finally deliver cholesterol to neurons by a receptor-mediated internalization process. A growing number of evidences indicates a strong association between alterations of CNS cholesterol homeostasis and neurodegenerative disorders, in particular Alzheimer's disease (AD), and a possible role in this relationship may be played by defects in brain HDL metabolism. In the present review, we summarize and critically examine the current state of knowledge on major modifications of HDL and HDL-mediated brain cholesterol transport in AD, by taking into consideration the individual steps of this process. We also describe potential and encouraging HDL-based therapies that could represent new therapeutic strategies for AD treatment. Finally, we revise the main plasma and brain HDL modifications in other neurodegenerative disorders including Parkinson's disease (PD), Huntington's disease (HD), and frontotemporal dementia (FTD).     

The control of cholesterol metabolism and plasma lipid levels in infant rats.

Infant rats received an i.p. injection of insulin, anti-insulin serum, streptozotocin, antiglucagon serum or dexamethasone. All substances except the antiinsulin serum, raised the plasma triglyceride level. Both antisera decreased plasma cholesterol levels, while streptozotocin, insulin and dexamethasone caused an increase. The activity of 3-hydroxy-3-glutaryl CoA reductase in liver and brown adipose tissue changed inversely to the cholesterol level. However, small intestinal enzyme activity was increased by insulin administration inspite of the rise in plasma cholesterol.     

Role of the hepatic ABCA1 transporter in modulating intrahepatic cholesterol and plasma HDL cholesterol concentrations

The current model for reverse cholesterol transport proposes that HDL transports excess cholesterol derived primarily from peripheral cells to the liver for removal. However, recent studies in ABCA1 transgenic mice suggest that the liver itself may be a major source of HDL cholesterol (HDL-C). To directly investigate the hepatic contribution to plasma HDL-C levels, we generated an adenovirus (rABCA1-GFP-AdV) that targets expression of mouse ABCA1-GFP in vivo to the liver. Compared with mice injected with control AdV, infusion of rABCA1-GFP-AdV into C57Bl/6 mice resulted in increased expression of mouse ABCA1 mRNA and protein in the liver. ApoA-I-dependent cholesterol efflux was increased 2.6-fold in primary hepatocytes isolated 1 day after rABCA1-GFP-AdV infusion. Hepatic ABCA1 expression in C57Bl/6 mice (n = 15) raised baseline levels of TC, PL, FC, HDL-C, apoE, and apoA-I by 150-300% (P < 0.05 all). ABCA1 expression led to significant compensatory changes in expression of genes that increase hepatic cholesterol, including HMG-CoA reductase (3.5-fold), LDLr (2.1-fold), and LRP (5-fold) in the liver. These combined results demonstrate that ABCA1 plays a key role in hepatic cholesterol efflux, inducing pathways that modulate cholesterol homeostasis in the liver, and establish the liver as a major source of plasma HDL-C.     

Apolipoprotein J polymorphisms and serum HDL cholesterol levels in African blacks

Apolipoprotein J (apoJ, protein; APOJ, gene) is found in serum associated with high-density lipoprotein (HDL) subfractions, which also contain apolipoprotein A-I (apoA1) and cholesteryl ester transfer protein. ApoJ has been shown to be involved in a variety of physiological functions, including lipid transport. In earlier studies we reported the existence of a common genetic polymorphism (APOJ*1 and APOJ*2 alleles) using isoelectric focusing (IEF) and immunoblotting. In this study we determined the molecular basis of this polymorphism and together with another polymorphism at codon 328 (G-->A) evaluated its relationship with serum HDL cholesterol and apoA1 levels in 767 African blacks stratified by staff level: junior (less affluent, n = 450) and senior (more affluent, n = 317). The molecular analysis of the cathodally shifted APOJ*2 allele on IEF gels revealed an amino acid substitution of asparagine by histidine resulting from a missense mutation (A-->C) at codon 317 in exon 7. The frequency of the APOJ*2 (C) allele of codon 317 in the total sample was 0.267, whereas that of the less common allele A of codon 328 was 0.04. Despite their close proximity, no linkage disequilibrium was observed between the 2 polymorphisms. The impact of the codon 317 polymorphic variation was significant on serum HDL cholesterol (p = 0.003) and HDL3 cholesterol (p = 0.001) in junior staff. The adjusted mean values of these traits were higher in the codon 317 APOJ*2/*2 genotype than in the *1/*1 and *1/*2 genotypes. Overall, the APOJ codon 317 polymorphism explained 10.2% and 8.3% of the phenotypic variation in HDL cholesterol and HDL3 cholesterol, respectively, in junior staff. The codon 328 polymorphism showed a significant effect on HDL2 cholesterol (p = 0.039) and apoA1 (p = 0.007) only in junior women and accounted for 2.5% and 4.2% of the phenotypic variation in HDL2 cholesterol and apoA1, respectively. We also analyzed the combined effects of these genotypes at the 2 polymorphic sites. Significant effects on HDL cholesterol (p = 0.004) and HDL3 cholesterol (p = 0.008) in junior men and on HDL2 cholesterol (p = 0.003) in junior women were observed in the combined genotype data. The 2-locus genotypes explained 6.0% and 5.3% of the residual phenotypic variation of HDL cholesterol and HDL3 cholesterol in junior men and 10.4% of HDL2 cholesterol in junior women. These data indicate that the effect of the APOJ polymorphism on HDL cholesterol levels is modulated by socioeconomic status, as measured by staff level. Given the association of HDL and its subfractions with cardiovascular disease, these polymorphisms may lead to a better understanding of interracial differences in the risk of cardiovascular disease.     

Variants of the microsomal triglyceride transfer protein gene are associated with plasma cholesterol levels and body mass index.

The microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apolipoprotein B (apoB)-containing lipoproteins from liver and intestine. We set out to study the phenotypic modulation of all common genetic variants in the MTP gene. In addition, we aimed at characterizing the association between the various polymorphisms. A total of 564 healthy men were genotyped for the MTP -493 G/T, -400 A/T, and -164 T/C promoter polymorphisms, as well as the Q/H 95, I/T 128, Q/E 244, and H/Q 297 missense polymorphisms. The -493 G/T, -164 T/C, and I/T 128 polymorphisms showed to be in almost complete linkage disequilibrium. Subjects homozygous for the less common -493 T, -164 C, and T 128 alleles showed significantly lower plasma total and LDL cholesterol levels and plasma LDL apoB levels, and also significantly higher body mass index (BMI) and plasma insulin levels compared with carriers of the common alleles. The associations between plasma total cholesterol and MTP -493 genotype was verified in a cohort consisting of 1,117 disease-free control subjects of the West of Scotland Coronary Prevention Study (WOSCOPS). None of the other polymorphisms showed any significant change in either lipid and lipoprotein levels or anthropometric variables.In summary, two promoter polymorphisms and one missense polymorphism in the MTP gene alter plasma total and LDL cholesterol levels, plasma LDL apoB levels, BMI, and insulin levels. This may, in turn, have implications for genetic regulation of cardiovascular risk factors.     

Swine lipoproteins and atherosclerosis. Changes in the plasma lipoproteins and apoproteins induced by cholesterol feeding.

Cholesterol feeding in miniature swine resulted in a hypercholesterolemia with a distinctive hyperlipoproteinemia and the subsequent development of atherosclerosis. Alterations in the type and distribution of plasma lipoproteins induced by cholesterol feeding were as follows: (a) the occurrence of beta-migrating lipoproteins (B-VLDL) as well as very low density lipoproteins in the d less than 1.006 ultracentrifugal fraction; (b) an increased prominence of the intermediate lipoproteins (d = 1.006-1.02); (c) an increased prominence of low density lipoproteins; and (d) the occurrence of a distinctive lipoprotein with alpha mobility which was referred to as HDLc (cholesterol induced). Characterization of the various plasma lipoproteins included chemical composition, size by electron microscopy, and apoprotein content. The B-VLDL resembled the beta-migrating lipoproteins of human Type III hyperlipoproteinemia and contained a prominent protein equivalent to the arginine-rich apoprotein in addition to the B apoprotein, apo-A-I, and the fast-migrating apoproteins (apo-C). The HDLc were rich in cholesterol, ranged in size from 100 to 240 A in diameter, and contained the arginine-rich apoprotein and apo-A0I but lacked the B apoprotein. The arginine-rich apoproteins isolated from B-VLDL and HDLc by gel chromatography were similar in amino acid analyses, with glutamic acid as their amino-terminal residue. The occurrence of a spectrum of cholesterol-rich lipoproteins which contained the arginine-rich apoprotein with the occurrence of accelerated atherosclerosis suggested an interesting, although speculative, association.