Context-dependent and invariant associations between lipids, lipoproteins, and apolipoproteins and apolipoprotein E genotype

Variation in apolipoprotein (apo)E genotypes predicts variation in plasma cholesterol and apoB; however, the context-dependent associations between high density lipoprotein (HDL) cholesterol, apoA-I, triglycerides, and lipoprotein[a] (Lp[a]) and this polymorphism remain unsettled. We genotyped 5,025 women and 4,035 men sampled to represent a white general population in the age range 20 to 80+ years (mean ages 58 and 57 years for women and men, respectively). The relative frequencies of the varepsilon22, varepsilon32, varepsilon42, varepsilon33, varepsilon43, and varepsilon44 genotypes were 0.005, 0.127, 0.027, 0.564, 0.251, and 0. 027, respectively. Variations in apoE genotype (in the order listed above) predicted stepwise increases in cholesterol and apoB in both genders (all ANOVAs: P < 0.001), and stepwise decreases in HDL cholesterol and apoA-I in women (both ANOVAs: P < 0.001), but not in men. In both genders varepsilon33 individuals had the lowest levels of nonfasting triglycerides, whereas the highest levels were found in individuals with varepsilon22 and varepsilon44 genotypes (both ANOVAs: P < 0.001). Finally, a stepwise increase in Lp[a] was seen in women (ANOVA: P < 0.001), but not in men. In women, the association between variation in nonfasting triglycerides and Lp[a], and variation in apoE genotypes was mainly seen in those with the highest alcohol consumption, similar to the consumption of most men. Variations in apoE genotype predicted 5% and 11% in women, and 2% and 6% in men, of the total variation in plasma cholesterol and apoB, respectively. Variation in levels of plasma lipoproteins is associated with variation in apoE genotypes in the population at large, with the most pronounced association in women, except for nonfasting triglycerides, for which the association is most pronounced in men.Whereas the associations between variation in plasma cholesterol and apoB and the variation in apoE genotypes seem invariant, the associations with variation in plasma HDL cholesterol, apoA-I, nonfasting triglycerides, and Lp[a] seem context dependent.     

Apolipoprotein E and apolipoprotein CI polymorphisms in the Czech population: almost complete linkage disequilibrium of the less frequent alleles of both polymorphisms

Apolipoproteins E and CI are the predominant components of triglyceride-rich lipoproteins. The genes are located in one gene cluster and both are polymorphic. Three allelic (epsilon2, epsilon3 and epsilon4) polymorphisms of the APOE gene influence plasma cholesterol levels. The distribution of these alleles differ between ethnic groups. PCR genotyping was used to determine the APOE and APOCI allele incidence in a representative group of 653 probands (302 men and 351 women) of Czech origin. The observed relative frequencies for the epsilon2, epsilon3 and epsilon4 alleles were 7.1 %, 82.0 % and 10.9 %, respectively, and are similar to other middle European populations. APO epsilon4 carriers have the highest and APO epsilon2 carriers the lowest levels of plasma total cholesterol (p<0.0001) and LDL cholesterol (p<0.0001). The frequency of the insertion (I) allele (HpaI restriction site present) of the APOCI polymorphism was 18.5 %. APOCI I/I homozygotes have the highest level of triglycerides (p<0.003). An almost complete linkage disequilibrium of the insertion allele of APOCI with the APOE alleles epsilon2 and epsilon4 has been detected and suggests that the deletion in the APOCI gene probably follows the deriving of all three APOE alleles on the APO epsilon3 allele background.     

Lateral distribution of phospholipid and cholesterol in apolipoprotein A-I recombinants

The influence of cholesterol on the assembly and structure of model high-density lipoproteins (HDL) has been investigated. Model HDL composed of apolipoprotein A-I (apoA-I) and 1,2-dimyristoylphosphatidylcholine (DMPC) formed spontaneously at the transition temperature (Tc) of the lipid. Those composed of apoA-I and 1-palmitoyl-2-oleoylphosphatidylcholine were formed by a cholate dialysis method. At low cholesterol/phospholipid ratios both lipids and assembly methods yielded a model HDL whose composition was identical with that of the initial mixture; as the cholesterol/phospholipid ratio of the initial mixture was increased, the fraction of cholesterol appearing in the model HDL decreased, and a negative correlation between the cholesterol and protein contents of the model HDL was observed. At high cholesterol/phospholipid ratios the association of apoA-I and phospholipids appeared to be thermodynamically unfavorable. The effects of cholesterol content on the thermal properties of a model HDL composed of DMPC and apoA-I were further investigated by differential scanning calorimetry, fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene, fluorescence energy transfer, and excimer fluorescence of pyrenyl derivatives of phosphatidylcholine (PC) and cholesterol. The addition of cholesterol decreased the transition enthalpy of DMPC, raised the midpoint of the transition, and modulated motional freedom in the phospholipid matrix. The amount of cholesterol required to produce these effects was lower in the model HDL than in multilamellar liposomes. In a model HDL composed of DMPC and apoA-I, the lateral diffusion of a pyrene-labeled cholesterol was dramatically changed at the Tc whereas little change was observed in that of a pyrene-labeled PC.(ABSTRACT TRUNCATED AT 250 WORDS)     

The interplay between size, morphology, stability, and functionality of high-density lipoprotein subclasses

High-density lipoprotein (HDL) mediates reverse cholesterol transport (RCT), wherein excess cholesterol is conveyed from peripheral tissues to the liver and steroidogenic organs. During this process HDL continually transitions between subclass sizes, each with unique biological activities. For instance, RCT is initiated by the interaction of lipid-free/lipid-poor apolipoprotein A-I (apoA-I) with ABCA1, a membrane-associated lipid transporter, to form nascent HDL. Because nearly all circulating apoA-I is lipid-bound, the source of lipid-free/lipid-poor apoA-I is unclear. Lecithin:cholesterol acyltransferase (LCAT) then drives the conversion of nascent HDL to spherical HDL by catalyzing cholesterol esterification, an essential step in RCT. To investigate the relationship between HDL particle size and events critical to RCT such as LCAT activation and lipid-free apoA-I production for ABCA1 interaction, we reconstituted five subclasses of HDL particles (rHDL of 7.8, 8.4, 9.6, 12.2, and 17.0 nm in diameter, respectively) using various molar ratios of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, free cholesterol, and apoA-I. Kinetic analyses of this comprehensive array of rHDL particles suggest that apoA-I stoichiometry in rHDL is a critical factor governing LCAT activation. Electron microscopy revealed specific morphological differences in the HDL subclasses that may affect functionality. Furthermore, stability measurements demonstrated that the previously uncharacterized 8.4 nm rHDL particles rapidly convert to 7.8 nm particles, concomitant with the dissociation of lipid-free/lipid-poor apoA-I. Thus, lipid-free/lipid-poor apoA-I generated by the remodeling of HDL may be an essential intermediate in RCT and HDL's in vivo maturation.     

Exchange of Apolipoprotein A-I between Lipid-associated and Lipid-free States: A POTENTIAL TARGET FOR OXIDATIVE GENERATION OF DYSFUNCTIONAL HIGH DENSITY LIPOPROTEINS*

An important event in cholesterol metabolism is the efflux of cellular cholesterol by apolipoprotein A-I (apoA-I), the major protein of high density lipoproteins (HDL). Lipid-free apoA-I is the preferred substrate for ATP-binding cassette A1, which promotes cholesterol efflux from macrophage foam cells in the arterial wall. However, the vast majority of apoA-I in plasma is associated with HDL, and the mechanisms for the generation of lipid-free apoA-I remain poorly understood. In the current study, we used fluorescently labeled apoA-I that exhibits a distinct fluorescence emission spectrum when in different states of lipid association to establish the kinetics of apoA-I transition between the lipid-associated and lipid-free states. This approach characterized the spontaneous and rapid exchange of apoA-I between the lipid-associated and lipid-free states. In contrast, the kinetics of apoA-I exchange were significantly reduced when apoA-I on HDL was cross-linked with a bi-functional reagent or oxidized by myeloperoxidase. Our observations support the hypothesis that oxidative damage to apoA-I by myeloperoxidase limits the ability of apoA-I to be liberated in a lipid-free form from HDL. This impairment of apoA-I exchange reaction may be a trait of dysfunctional HDL contributing to reduced ATP-binding cassette A1-mediated cholesterol efflux and atherosclerosis.     

The T111I mutation in the EL gene modulates the impact of dietary fat on the HDL profile in women

The objective of the present study was to examine the impact of the T111I missense mutation in exon 3 of the endothelial lipase (EL) gene on HDL and its potential interaction effect with dietary fat. The study sample included 281 women and 216 men aged between 17 and 76 years from the Québec Family Study. Plasma HDL3-C levels of I111I homozygote women were higher compared with those of women carrying the wild-type allele (P = 0.03). These differences were not attenuated when adjusted for levels of obesity and were not observed among men. Dietary PUFA interacted with the T111I mutation to modulate apolipoprotein A-I (apoA-I) and HDL3-C levels among women. Specifically, a diet rich in PUFA was associated with increased apoA-I levels among women carriers of the I111 allele and with decreased apoA-I among women homozygotes for the wild-type allele (P = 0.002). A similar interaction was observed with plasma HDL3-C levels (P = 0.003). These interactions were not observed among men. In conclusion, the EL T111I mutation appears to have a modest effect on plasma HDL levels. The gene-diet interaction among women, however, suggests that the T111I missense mutation may confer protection against the lowering effect of a high dietary PUFA intake on plasma apoA-I and HDL3-C levels.     

Polymorphisms in the ABCG5 and ABCG8 genes associate with cholesterol absorption and insulin sensitivity

The roles of polymorphisms of the sitosterolemia genes ABCG5 and ABCG8 in the regulation of cholesterol metabolism and insulin sensitivity were studied in mildly hypercholesterolemic noncoronary subjects (n = 263, 144 men and 119 women) divided into tertiles by baseline serum cholestanol-to-cholesterol ratio (< or = 118.3 and > or = 147.7 10(2) x mmol/mol cholesterol), a surrogate marker of cholesterol absorption efficiency. The lowest cholestanol tertile was associated with high body mass index (BMI), plasma glucose, serum insulin and triglycerides, and cholesterol synthesis markers (cholestenol, desmosterol, lathosterol) and low HDL cholesterol and cholesterol absorption markers (campesterol, sitosterol) (P < 0.01 for all). The 19H allele of the ABCG8 gene accumulated in the lowest cholestanol tertile (P < 0.001) and was associated with low total and LDL cholesterol and absorption markers and with high synthesis markers (P < 0.05 for all). The 604E allele of the ABCG5 gene in men was associated with high BMI, plasma insulin, low serum sitosterol, and high serum cholestenol levels (P < 0.05 for all). In a subgroup of 71 men, the 604E allele was associated with insulin resistance measured with the hyperinsulinemic euglycemic clamp. In conclusion, low cholesterol absorption efficiency was associated with characteristics of the metabolic syndrome. Low serum cholesterol and cholesterol absorption were linked to the D19H polymorphism of the ABCG8 gene, and characteristics of the insulin resistance syndrome in men were linked with the Q604E polymorphism of the ABCG5 gene.     

Deletions of helices 2 and 3 of human apoA-I are associated with severe dyslipidemia following adenovirus-mediated gene transfer in apoA-I-deficient mice

The objective of this study was to determine the effect of two amino-terminal apolipoprotein A-I (apoA-I) deletions on high-density lipoprotein (HDL) biosynthesis and lipid homeostasis. Adenovirus-mediated gene transfer showed that the apoA-I[Delta(89-99)] deletion mutant caused hypercholesterolemia, characterized by increased plasma cholesterol and phospholipids, that were distributed in the very low density/intermediate density/low-density lipoprotein (VLDL/IDL/LDL) region, and normal triglycerides. The capacity of the mutant protein to promote ATP-binding cassette transporter A1- (ABCA1-) mediated cholesterol efflux and to activate lecithin:cholesterol acyltranserase (LCAT) was approximately 70-80% of the wild-type (WT) control. The phospholipid transfer protein (PLTP) activity of plasma containing the apoA-I[Delta(89-99)] mutant was decreased to 32% of the WT control. Similar analysis showed that the apoA-I[Delta(62-78)] deletion mutant in apoA-I-deficient mice caused combined hyperlipidemia characterized by increased triglycerides, cholesterol, and phospholipids in the VLDL/IDL region. There was enrichment of the VLDL/IDL with mutant apoA-I that resulted in reduction of in vitro lipolysis. The capacity of this mutant to promote ABCA1-mediated cholesterol efflux was normal, and the capacity to activate LCAT in vitro was reduced by 53%. The WT apoA-I and the apoA-I[Delta(62-78)] mutant formed spherical HDL particles, whereas the apoA-I[Delta(89-99)] mutant formed discoidal HDL particles. We conclude that alterations in apoA-I not only may have adverse effects on HDL biosynthesis but also may promote dyslipidemia due to interference of the apoA-I mutants on the overall cholesterol and triglycerides homeostasis.     

Incorporation of cholesterol into apolipoprotein A-I-dimyristoylphosphatidylcholine recombinants

Apolipoprotein A-I (apoA-I) spontaneously associates with dimyristoylphosphatidylcholine (DMPC) liposomes to form discoidal high-density lipoprotein (HDL) recombinants. The uptake of cholesterol by this model HDL was studied by incubation with Celite-dispersed cholesterol. Separation of the resulting complexes by gradient centrifugation and gel filtration showed a heterogeneous distribution of particle size and composition as a consequence of the disruption and rearrangement of the recombinants. Quantitation of the amount of cholesterol taken up gave values between about 28 and 40 mol% cholesterol for the fractions within the protein peaks; the fractions with the lowest DMPC/apoA-I ratios had the lowest cholesterol contents. In another set of experiments, the association of apoA-I with DMPC-cholesterol liposomes was shown to result in complexes with characteristics similar to those obtained by the cholesterol-uptake experiments. Low concentrations of cholesterol in the liposomes enhanced the rate of lipid-protein association, but larger amounts decreased the yield of complexes by making the process thermodynamically and kinetically unfavorable. The enthalpy of recombinant formation increased with decreasing lipid/protein ratio and increasing cholesterol content, and became endothermic at about 23 mol% cholesterol. The effect of cholesterol on the thermal properties of HDL recombinants suggests that cholesterol is partially excluded from the boundary region adjacent to apoA-I. It is concluded that discoidal HDL recombinants, as a model for 'nascent' HDL, can acquire substantial amounts of cholesterol, which may be of great physiological importance for the reverse cholesterol transport and prevention of atherosclerosis.     

Differential involvement of thrombin receptors in Ca2+ release from two different intracellular stores in human platelets

In the present study we have used adenovirus-mediated gene transfer of apoA-I (apolipoprotein A-I) mutants in apoA-I-/- mice to investigate how structural mutations in apoA-I affect the biogenesis and the plasma levels of HDL (high-density lipoprotein). The natural mutants apoA-I(R151C)Paris, apoA-I(R160L)Oslo and the bioengineered mutant apoA-I(R149A) were secreted efficiently from cells in culture. Their capacity to activate LCAT (lecithin:cholesterol acyltransferase) in vitro was greatly reduced, and their ability to promote ABCA1 (ATP-binding cassette transporter A1)-mediated cholesterol efflux was similar to that of WT (wild-type) apoA-I. Gene transfer of the three mutants in apoA-I-/- mice generated aberrant HDL phenotypes. The total plasma cholesterol of mice expressing the apoA-I(R160L)Oslo, apoA-I(R149A) and apoA-I(R151C)Paris mutants was reduced by 78, 59 and 61% and the apoA-I levels were reduced by 68, 64 and 55% respectively, as compared with mice expressing the WT apoA-I. The CE (cholesteryl ester)/TC (total cholesterol) ratio of HDL was decreased and the apoA-I was distributed in the HDL3 region. apoA-I(R160L)Oslo and apoA-I(R149A) promoted the formation of prebeta1 and alpha4-HDL subpopulations and gave a mixture of discoidal and spherical particles. apoA-I(R151C)Paris generated subpopulations of different sizes that migrate between prebeta and alpha-HDL and formed mostly spherical and a few discoidal particles. Simultaneous treatment of mice with adenovirus expressing any of the three mutants and human LCAT normalized plasma apoA-I, HDL cholesterol levels and the CE/TC ratio. It also led to the formation of spherical HDL particles consisting mostly of alpha-HDL subpopulations of larger size. The correction of the aberrant HDL phenotypes by treatment with LCAT suggests a potential therapeutic intervention for HDL abnormalities that result from specific mutations in apoA-I.     

Influence of insulin sensitivity and the TaqIB cholesteryl ester transfer protein gene polymorphism on plasma lecithin:cholesterol acyltransferase and lipid transfer protein activities and their response to hyperinsulinemia in non-diabetic men.

Lecithin:cholesteryl acyl transferase (LCAT), cholesteryl ester transfer protein (CETP), phospholipid transfer protein (PLTP), and lipoprotein lipases are involved in high density lipoprotein (HDL) metabolism. We evaluated the influence of insulin sensitivity and of the TaqIB CETP gene polymorphism (B1B2) on plasma LCAT, CETP, and PLTP activities (measured with exogenous substrates) and their responses to hyperinsulinemia. Thirty-two non-diabetic men without hyperlipidemia were divided in quartiles of high (Q(1)) to low (Q(4)) insulin sensitivity. Plasma total cholesterol, very low + low density lipoprotein cholesterol, triglycerides, and apolipoprotein (apo) B were higher in Q(4) compared to Q(1) (P < 0.05 for all), whereas HDL cholesterol and apoA-I were lowest in Q(4) (P < 0.05 for both). Plasma LCAT activity was higher in Q(4) than in Q(1) (P < 0. 05) and PLTP activity was higher in Q(4) than in Q(2) (P < 0.05). Insulin sensitivity did not influence plasma CETP activity. Postheparin plasma lipoprotein lipase activity was highest and hepatic lipase activity was lowest in Q(1). Insulin infusion decreased PLTP activity (P < 0.05), irrespective of the degree of insulin sensitivity. The CETP genotype exerted no consistent effects on baseline plasma lipoproteins and LCAT, CETP, and PLTP activities. The decrease in plasma PLTP activity after insulin was larger in B1B1 than in B2B2 homozygotes (P < 0.05). These data suggest that insulin sensitivity influences plasma LCAT, PLTP, lipoprotein lipase, and hepatic lipase activities in men. As PLTP, LCAT, and hepatic lipase may enhance reverse cholesterol transport, it is tempting to speculate that high levels of these factors in association with insulin resistance could be involved in an antiatherogenic mechanism. A possible relationship between the CETP genotype and PLTP lowering by insulin warrants further study.     

The role of the ABCA1 transporter and cholesterol efflux in familial hypoalphalipoproteinemia

Defects in the gene encoding for the ATP binding cassette (ABC) transporter A1 (ABCA1) were shown to be one of the genetic causes for familial hypoalphalipoproteinemia (FHA). We investigated the role of ABCA1-mediated cholesterol efflux in Dutch subjects suffering from FHA. Eighty-eight subjects (mean HDL cholesterol levels 0.63 +/- 0.21 mmol/l) were enrolled. Fibroblasts were cultured and loaded with [3H]cholesterol. ABCA1 and non-ABCA1-mediated efflux was studied by using apolipoprotein A-I (apoA-I), HDL, and methyl-beta-cyclodextrin as acceptors. Efflux to apoA-I was decreased in four patients (4/88, 4.5%), and in all cases, a mutation in the ABCA1 gene was found. In the remaining 84 subjects, no correlation between efflux and apoA-I or HDL cholesterol was found. Efflux to both HDL and cyclodextrin, in contrast, did correlate with HDL cholesterol plasma levels (r = 0.34, P = 0.01; and r = 0.27, P = 0.008, respectively). The prevalence of defects in ABCA1-dependent cholesterol efflux in Dutch FHA patients is low. The significant correlation between plasma HDL cholesterol levels and methyl-beta-cyclodextrin-mediated efflux in the FHA patients with normal ABCA1 function suggests that non-ABCA1-mediated efflux might also be important for plasma HDL cholesterol levels in these individuals.     

An apolipoprotein A-I mimetic dose-dependently increases the formation of prebeta1 HDL in human plasma

Prebeta1 HDL is the initial plasma acceptor of cell-derived cholesterol in reverse cholesterol transport. Recently, small amphipathic peptides composed of D-amino acids have been shown to mimic apolipoprotein A-I (apoA-I) as a precursor for HDL formation. ApoA-I mimetic peptides have been proposed to stimulate the formation of prebeta1 HDL and increase reverse cholesterol transport in apoE-null mice. The existence of a monoclonal antibody (MAb 55201) and a corresponding ELISA method that is selective for the detection of the prebeta(1) subclass of HDL provides a means of establishing a correlation between apoA-I mimetic dose and prebeta1 HDL formation in human plasma. Using this prebeta1 HDL ELISA, we demonstrate marked apoA-I mimetic dose-dependent prebeta1 HDL formation in human plasma. These results correlated with increases in band density of the plasma prebeta1 HDL, when observed by Western blotting, as a function of increased apoA-I mimetic concentration. Increased prebeta1 HDL formation was observed after as little as 1 min and was maximal within 1 h. Together, these data suggest that a high-throughput prebeta1 HDL ELISA provides a way to quantitatively measure a key component of the reverse cholesterol transport pathway in human plasma, thus providing a possible method for the identification of apoA-I mimetic molecules.     

Postheparin plasma lipoprotein and hepatic lipase are determinants of hypo- and hyperalphalipoproteinemia

To study the role of the two postheparin plasma lipolytic enzymes, lipoprotein lipase (LPL) and hepatic lipase (HL) in high density lipoprotein (HDL) metabolism at a population level, we determined serum lipoproteins, apoproteins A-I, A-II, B, and E, and postheparin plasma LPL and HL activities in 65 subjects with a mean HDL-cholesterol of 34 mg/dl and in 62 subjects with a mean HDL-cholesterol of 87 mg/dl. These two groups represented the highest and lowest 1.4 percentile of a random sample consisting 4,970 subjects. The variation in HDL level was due to a 4.1-fold difference in the HDL2 cholesterol (P less than 0.001) whereas the HDL3 cholesterol level was increased only by 32% (P less than 0.001) in the group with high HDL-cholesterol. Serum apoA-levels were 128 +/- 2.2 mg/dl and 210 +/- 2.8 mg/dl (mean +/- SEM) in hypo- and hyper-HDL cholesterolemia, respectively. Serum apoA-II concentration was elevated by 28% (P less than 0.001) in hyperalphalipoproteinemia. The apoA-I/A-II ratio was elevated only in women with high HDL-cholesterol but not in men, suggesting that elevation of apoA-I is involved in hyperalphalipoproteinemia in females, whereas both apoA proteins are elevated in men with high HDL cholesterol. Serum concentration of apoE and its phenotype distribution were similar in the two groups. The HL activity was reduced in the high HDL-cholesterol group (21.2 +/- 1.5 vs. 38.5 +/- 1.8 mumol/h/ml, P less than 0.001), whereas the LPL activity was elevated in the group with high HDL-cholesterol compared to subjects with low HDL-cholesterol (27.8 +/- 1.3 vs. 19.9 +/- 0.8 mumol/h/ml, P less than 0.001). The HL and LPL activities correlated in opposing ways with the HDL2 cholesterol (r = 0.57, P less than 0.001 and r = 0.51, P less than 0.001, respectively), and this appeared to be independent of the relative ponderosity by multiple correlation analysis. The results demonstrate major influence of both HL and LPL on serum HDL cholesterol concentration at a population level.     

Paraoxonase-1 and serum concentrations of HDL-cholesterol and apoA-I

Paraoxonase-1 (PON1) and HDL are tightly associated in plasma, and this is generally assumed to reflect the need for the enzyme to associate with a hydrophobic complex. The association has been examined in coronary cases and age-matched controls. Highly significant (P < 0.0001), positive associations were observed between PON1 activities and concentrations and HDL-cholesterol and apolipoprotein A-I (apoA-I) concentrations in cases and controls. Corrected slopes were significantly different in cases (cases vs. controls: arylesterase, r = 0.19 vs. 0.38, P < 0.02 for apoA-I and r = 0.15 vs. 0.34, P < 0.02 for HDL-cholesterol) such that if PON1 should influence serum HDL, it would be less effective in coronary cases. When examined as a function of the PON1 gene promoter polymorphism C-107 T, highly significant differences (P < 0.001) in HDL-cholesterol and apoA-I were observed between genotypes for controls, with high expresser alleles having the highest HDL concentrations. This relationship was lost in cases with coronary disease. The coding region polymorphisms Q192R and L55M of the PON1 gene showed no association with HDL. The promoter polymorphism was an independent determinant of HDL concentrations in multivariate analyses. These data are consistent with an impact of PON1 on plasma concentrations of HDL, with detrimental modifications to the relationship in coronary cases.     

Ethnic differences in hepatic lipase and HDL in Japanese, black, and white Americans: role of central obesity and LIPC polymorphisms

Hepatic lipase activity (HLA) is a determinant of HDL levels, and a polymorphism in the hepatic lipase gene (LIPC) promoter (C-514T) has been hypothesized to account for higher HDL in blacks and Japanese compared with whites. To determine whether the polymorphism contributes to ethnic differences in HDL, we compared LIPC allele frequencies and HLA in Japanese American (JA; n = 84), black American (BA; n = 94), and white American (WA; n = 110) men and women. The LIPC polymorphism was associated with HLA in all cohorts (BA, P = 0.012; JA, P = 0.008; WA, P = 0.009). WA men had 49% and 58% higher HLA than BA and JA men, respectively (both P < 0.05), yet no differences in HLA were found between the women. The higher HLA in the WA men remained after adjustment for the LIPC polymorphism's effect on HLA (P = 0.037) but was erased after adjustment for waist-to-hip-ratio (P = 0.46). Although the WA men had lower HDL and HDL(3) than the JA and BA men (all P < 0.05), there were no differences in HDL(2), implying that variance in HLA may not underlie the ethnic differences in HDL levels. These results suggest that 1) the LIPC promoter polymorphism contributes to variation in HLA and HDL(2) in the three ethnic groups; 2) WA men had higher HLA than BA and JA men, related to ethnic differences in central adiposity but not LIPC allele frequency; and 3) the higher HLA in WA men did not contribute to the ethnic differences in HDL, as the differences in HDL were made up entirely of differences in HDL(3) and not HDL(2).     

A polymorphism affecting apolipoprotein A-II translational efficiency determines high density lipoprotein size and composition

High density lipoproteins (HDL) are heterogeneous particles consisting of about equal amounts of lipid and protein that are thought to mediate the transport of cholesterol from peripheral tissues to liver. We show that a previously identified polymorphism affecting HDL electrophoretic mobility in mice is due to a monogenic variation controlling HDL size and apolipoprotein composition. Thus, the HDL particles of various inbred strains of mice exhibit a striking difference in the ratio fo the two major apolipoproteins of HDL, apoA-I and apoA-II. HDL particles in all strains examined contain an average of about five apoA-I molecules; however, whereas the strains with small HDL contain two to three apoA-II molecules per particle, the strains with large HDL contain about five apoA-II molecules per particle. This increase in the protein content of the large HDL is also accompanied by increased lipid content. The HDL size polymorphism and apoA-II levels cosegregate with the apoA-II structural gene on mouse chromosome 1, indicating that a mutation of the apoA-II gene locus is responsible. The rates of synthesis of apoA-II are increased in the strains with large HDL and high apoA-II levels as compared to the strains with small HDL and low apoA-II levels. On the other hand, the fractional catabolic rates of both apoA-I and apoA-II among the strains are very similar, confirming that apoA-II concentrations are controlled at the level of synthesis. Despite the difference in rates of apoA-II synthesis between strains, the apoA-II mRNA levels in the strains are not discernibly different, suggesting that a mutation of the apoA-II structural gene controls apoA-II translational efficiency. This was confirmed by translating apoA-II mRNA in vitro using a rabbit reticulocyte lysate system. Sequencing of apoA-II cDNA from the strains revealed a number of nucleotide substitutions, which may affect translational efficiency. We conclude that the assembly of apoA-II into HDL does not have a set stoichiometry but, rather, is controlled by the production of apoA-II. As apoA-II levels increase, the HDL particles become larger and acquire more lipid, but apoA-I content per particle remains unchanged. These studies with mice provide a model for the metabolic relationships between apoA-I, apoA-II, and HDL lipid in humans.