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* Lipids and Atherosclerosis Research Unit, Hospital Reina Sofía University, Córdoba, Spain
Jean Mayer-United States Department of Agriculture Human Nutrition Research Center on Aging at Tufts University, Boston, MA 02111-1524
Published, JLR Papers in Press, August 16, 2003. DOI 10.1194/jlr.M300124-JLR200
1 To whom correspondence should be addressed. e-mail: fperezjimenez{at}uco.es
| ABSTRACT |
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In conclusion, the -219G/T polymorphism may influence TRL metabolism during the postprandial period, thus prolonging postprandial lipemia in subjects with the TT genotype.
Supplementary key words postprandial lipemia triacylglycerols retinyl palmitate cholesterol triacylglycerol-rich lipoproteins
| INTRODUCTION |
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apoE is a polymorphic protein with three common isoforms (apoE2, apoE3, and apoE4) that contribute to susceptibility to atherosclerotic cardiovascular disease (3). Several matched case control studies have shown a modest but significant increased prevalence of the apoE4 allele in coronary heart disease (CHD) patients from various populations (4, 5). Nevertheless, even though the apoE4 allele appears to be a significant genetic risk factor for CHD, individuals with the apoE4 allele do not inevitably develop this disorder. This fact suggests that other genetic or environmental risk factors may interact with the apoE gene in determining CHD risk. It was recently proposed that together with the qualitative modification of the apoE structure due to the apoE alleles, quantitative variations of apoE mRNA expression play a major role in determining risk (6).
In accordance with this hypothesis, polymorphisms in the proximal promoter region of the apoE gene have recently been described at positions -491 A/T, -427 T/C, and -219 G/T (7, 8). In particular, there is experimental evidence, in vitro and in vivo, indicating that the apoE gene promoter (-219G/T) polymorphism produces variations in the transcriptional activity of the gene. Specifically, the -219G allele showed a higher transcriptional activity than -219T. This effect was probably due to the differential binding of nuclear proteins, as indicated by electrophoretic mobility shift assays (7). A study of a European population, including control individuals and multi-infarct patients, has shown that the apoE gene promoter (-219G/T) polymorphism is also associated with differential plasma apoE levels (9), illustrating the fact that this polymorphism influences apoE expression in vivo. Furthermore, the -219T allele is associated with an increased risk of myocardial infarction (MI) (9) and premature CHD (10).
As a result of eating fat-rich meals, subjects in most industrialized countries are predominantly in a postprandial state throughout the day. Since 1979, when Zilversmit (11) proposed that TRL played a role in atherosclerosis, many research teams have shown the important role of postprandial lipoprotein particles in the development of CHD (1215). In the postprandial state, the hepatic uptake of TRL, determined by the interaction of the LDL receptor and the low-density lipoprotein receptor-related protein with apoE, is one of the main components of this TRL metabolism.
Thus, a postprandial study design would be the most suitable procedure for investigating the functional impact of variation in the apoE gene promoter on the catabolism of TRL. A strong linkage disequilibrium between the -219T and apoE4 allele has been described, and apoE genotypes have been implicated in a variable postprandial lipid response (16). Consequently, we have studied the effect of the apoE gene promoter (-219G/T) polymorphism in apoE 3/3 subjects, to the exclusion of other apoE isoforms. The aim of this study was to determine whether the apoE gene promoter (-219G/T) polymorphism could modify the postprandial response of TRL in young normolipemic apoE 3/3 males in order to explain the increased risk of MI and premature CHD associated with homozygote carriers of the T allele.
| MATERIALS AND METHODS |
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Lipoprotein separations
Blood was collected in tubes containing EDTA to give a final concentration of 0.1% EDTA. Plasma was separated from red blood cells by centrifugation at 1,500 g for 15 min at 4°C. The chylomicron fraction of TRL (large TRL) was isolated from 4 ml of plasma overlayered with 0.15 mol/l NaCl and 1 mmol/l EDTA (pH 7.4, d < 1.006 kg/l) by a single ultracentrifugal spin (36,200 g, for 30 min at 4°C) in a type 50 rotor (Beckman Instruments, Fullerton, CA). Chylomicrons contained in the top layer were removed by aspiration after cutting the tubes. The infranatant fluid was centrifuged at a density of 1.019 kg/l for 24 h at 183,000 g in the same rotor. The nonchylomicron fraction (also referred to as small TRL) was removed from the top of the tube. All operations were done in subdued light. Large and small TRL fractions were stored at -70°C until assayed for retinyl palmitate (RP).
Lipid analysis
Cholesterol and triacylglycerols (TGs) in plasma and lipoprotein fractions were assayed by enzymatic procedures (19, 20). apoA-I and apoB were determined by turbidimetry (21). The serum apoE concentration was determined using a kit from Wako Chemicals. HDL-cholesterol was measured by analyzing the supernatant fluid obtained after precipitation of a plasma aliquot with dextran sulfate and Mg2+ as described by Warnick, Benderson, and Albers (22). LDL-cholesterol was obtained as the difference between HDL-cholesterol and the cholesterol from the bottom part of the tube after ultracentrifugation (183,000 g for 24 h at 4°C) at a density of 1.019 kg/l.
RP assay
The RP content of large and small TRL fractions was assayed using a method previously described (23). The RP concentration in each sample was expressed as the ratio of the area under the RP peak to the area under the retinyl acetate peak (24).
Determination of apoB-48 and apoB-100
apoB-48 and apoB-100 were determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as described by Karpe and Hamsten (25). Gels were scanned with a videodensitometer scanner (TDI, Madrid, Spain) connected to a personal computer for integration of the signals. Background intensity was calculated after scanning an empty lane. The coefficient of variation for the SDS-PAGE was 7.3% for apoB-48 and 5.1% for apoB-100.
DNA amplification and genotyping
Genomic DNA extraction and apoE E2/E3/E4 (26) and -219G/T (7, 8) genotypes were determined as previously described. Digested DNA was separated by electrophoresis on an 8% nondenaturing polyacrylamide gel at 150 V for 2 h. Bands were visualized by silver staining. Samples containing the T allele of the -219G/T polymorphism were amplified a second time to verify the genotype.
Statistical analysis
Several variables were calculated to characterize the postprandial responses of the plasma TGs large TRL and small TRL to the test meal. The area under the curve (AUC) was defined as the area between the plasma concentration versus the time curve and a line drawn parallel to the horizontal axis through the 0 h concentration. Because the latter time points for plasma apoE were lower than the 0 h level, the apoE AUC was calculated as the area between the plasma apoE concentration and the horizontal axis. These areas were calculated by a computer program using the trapezoidal rule. Other variables were the normalized peak concentration above baseline and the peak time, which was the average of the time of peak concentration and the time to the second highest concentration. Data were tested for statistical significance between genotypes by ANOVA and the Kruskal-Wallis test, and between genotypes and time by ANOVA for repeated measures. In this analysis, we studied the statistical effects of the genotype alone, independent of time, in the postprandial study, the effect of time alone or change in the variable after ingesting fatty food over the entire lipemic period, and the effect of the interaction of both factors (genotype and time), which is indicative of the magnitude of the postprandial response in each group of subjects. When statistical significance was found, the Tukey posthoc comparison test was used to identify group differences. A probability value of less than 0.05 was considered significant. All data presented in the text and tables are expressed as means ± SD. SPSS 7.5 for WINDOWS (SPSS Inc., Chicago, IL) was used for the statistical comparisons.
| RESULTS |
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The postprandial response of triglycerides, RP, and cholesterol in large TRL is shown in Fig. 1 . The fat-loading test significantly increased triglycerides (P < 0.001), RP (P < 0.009), and cholesterol (P < 0.001) in both large and small TRL, with respect to baseline levels, indicating an increase of these parameters in the different groups of subjects during the postprandial period.
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| DISCUSSION |
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Several studies have demonstrated that the presence of polymorphisms located in the AI-CIII-AIV complex (17, 18) and in other gene loci determines variations in the postprandial response. Moreover, the effects of variations in the apoE gene on the absorption and clearance of dietary fats are well known (16). For example, clearance of postprandial particles is delayed in carriers of the E2 allele compared with carriers of the E4 allele. However, in recent years, it has been suggested that in addition to the qualitative effect induced by the apoE gene polymorphisms, the quantitative changes in gene expression could also be an important factor in the variability of the postprandial state. This hypothesis is backed by the recent discovery of a new polymorphism, -219G/T, located in the apoE gene promoter region that determines levels of gene expression.
The influence of this polymorphism on postprandial lipemic response in healthy subjects remains unknown. In our study, we have observed that subjects homozygous for the T allele have a higher postprandial response, with elevated levels of intestinal TRL, as demonstrated by a higher AUC and higher levels of apoB-48 and RP. Additionally, we have found a lower AUC and lower serum apoE levels in subjects with the TT genotype, with significant differences observed at 4 h, coinciding with the highest peak of postprandial lipemia. These differences are in accordance with the results obtained in other studies that demonstrated that apoE gene expression was modified by the -219G/T polymorphism (7). This can be explained by the fact that the presence of a G instead of a T significantly increases promoter activity by as much as 169%, as compared with the controls. Furthermore, a study with a European population that included controls and multi-infarct patients has shown that subjects homozygous for the T allele had significantly lower baseline plasma levels of apoE than other subjects (9).
The mechanisms via which the -219T allele of the apoE gene enhances atherothrombosis are yet to be elucidated. In the study by Lambert et al. (9), the -219G/T polymorphism did not modify baseline plasma lipid or lipoprotein concentrations, as in our study. The authors speculated that the -219TT genotype could increase the risk of MI at a local level by modifying the macrophage apoE expression, but experimental data to corroborate this hypothesis do not exist. Unlike Lambert et al., we designed a postprandial study to investigate the functional impact of the -219G/T polymorphism because of the important role of apoE in the uptake of TRL that occurs in the postprandial state. Several studies have shown that the apoE in the TRL interacts with different receptors in the liver, inducing the uptake of these particles from the bloodstream and their hepatic metabolism (1, 2). It is probable that lower apoE plasma levels observed in subjects homozygous for the T allele are also associated with lower apoE-TRL levels, thus reducing clearance by hepatic receptors. This phenomenon could explain the higher postprandial response observed in our study, as well as the increased risk of MI and premature CHD (9, 10) associated with homozygous carriers of the T allele, owing to the importance of triglyceride-rich postprandial particles as a risk factor for atherosclerosis (26, 27).
A limitation of our study is that it includes a relatively small number of subjects. However, we wanted to include in our study only young normolipemic apoE 3/3 males in order to avoid the variable postprandial lipid response of the other apoE isoforms or gender. Therefore, only 51 healthy apoE 3/3 male volunteers were selected from over 100 subjects.
In conclusion, our data suggest that the 219G/T polymorphism located in the apoE gene promoter region may influence TRL metabolism during the postprandial period, thus prolonging postprandial lipemia in subjects with the TT genotype.
| ACKNOWLEDGMENTS |
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Manuscript received March 24, 2003 and in revised form July 31, 2003.
| REFERENCES |
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