Atherosclerosis and Lipoproteins |
From the Lipid Metabolism Laboratory (P.C., C.L., E.J.S., J.M.O.), Jean MayerUSDA Human Nutrition Research Center on Aging at Tufts University, Boston, Mass; the Department of Biochemistry (J.D.O.), North Carolina State University, Raleigh; Boston University School of Public Health (L.A.C.) and the Framingham Heart Study (P.W.F.W.), National Heart, Lung, and Blood Institute, Framingham, Mass.
Correspondence to Dr Jose M. Ordovas, Lipid Metabolism Laboratory. Jean MayerUSDA Human Nutrition Research Center on Aging at Tufts University, 711 Washington St, Boston, MA 02111. E-mail ordovas{at}hnrc.tufts.edu
| Abstract |
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Key Words: hepatic lipase HDL gene polymorphisms lipoprotein particle size
| Introduction |
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Hepatic lipase is a lipolytic enzyme that is synthesized in the
hepatocytes, secreted, and bound extracellularly to the
liver.15 Hepatic lipase participates in the
metabolism of IDL and large LDL to form smaller, denser LDL
particles and in the conversion of HDL2 to
HDL3. In addition, hepatic lipase can mediate the
unloading of cholesterol from HDL to the plasma membrane in
the liver.16 It has also been suggested that hepatic
lipase may act as a ligand protein with cell-surface proteoglycans in
the uptake of lipoproteins by cell-surface receptors.15
The lipid profile of individuals with complete hepatic lipase
deficiency is characterized by elevated plasma cholesterol
and triglyceride levels, triglyceride
enrichment of lipoprotein fractions with a density >1.006 g/mL, the
presence of ß-VLDL, and an impaired metabolism of
postprandial triglyceride-rich
lipoproteins.17 18 Four polymorphism in the
5'-flanking region of the hepatic lipase gene (G
A at position -250,
C
T at -514, T
C at -710, and A
G at -763) with respect to the
transcription start site13 were observed to be in
complete linkage disequilibrium13 19 and were found to be
associated with a lowered hepatic lipase activity and higher HDL-C
levels.12 13 20 A number of recent studies have also shown
an association between low hepatic lipase activity and more buoyant,
less atherogenic LDL particles21 22 and suggest that
variants in the hepatic lipase promoter may contribute significantly to
the prevalence of the atherogenic small, dense, LDL particle
phenotype associated with an increased CHD
risk.20 23 24 25
In recent studies, the C-514T polymorphism in the promoter region of the hepatic lipase gene has been shown to be associated with significant variations in hepatic lipase activity, plasma HDL-C levels, and LDL particle size. However, data from the general population with regard to the effect of this common hepatic lipase variant on lipid and lipoprotein levels, as well as the heterogeneity and possible atherogenicity of LDL and HDL lipoprotein particles, are clearly missing. The purpose of the current study, therefore, was to examine the frequency, phenotypic effect on lipoprotein levels and lipoprotein subclass profiles, and the potential modulation of CHD risk in the Framingham Offspring Study (FOS) by the C-514T polymorphism in the hepatic lipase gene.
| Methods |
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Plasma Lipid, Lipoprotein, and Apoprotein Measurements
Twelve-hour fasting venous blood samples were collected in tubes
containing 0.1% EDTA. Plasma was separated from blood cells by
centrifugation and immediately used for the measurement
of lipids. Plasma total cholesterol, HDL-C, and
triglyceride levels were measured as previously
described.29 HDL-C was measured after precipitation of
apoB-containing lipoproteins with
heparin-MnCl2.30 LDL-C
concentrations were estimated with the equation of Friedewald et
al.31 Coefficients of variation for total
cholesterol, HDL-C, and triglyceride
measurements were each <5%. Plasma levels of apoAI and apoB were
measured by noncompetitive ELISA with the use of
affinity-purified polyclonal antibodies.32 33
HDL subclass distributions were determined by proton nuclear magnetic resonance (NMR) spectroscopy as previously described.34 35 Each profile displays the concentrations of 5 HDL subclasses and their weighted-average particle sizes. The 5 HDL lipoprotein subclass categories used were the following: large HDL (8.8 to 13.0 nm), intermediate HDL (7.8 to 8.8 nm), and small HDL (7.3 to 7.7 nm). Levels of HDL subclasses are expressed in units of cholesterol (mg/dL). HDL subclass distributions determined by gradient gel electrophoresis and NMR have also been shown to be closely correlated.34
LDL subclasses were separated by subjecting whole plasma to 2% to 16% gradient gel electrophoresis and were visualized by using Sudan black to stain LDL particles, as previously described.36 37 Each subject was assigned an LDL type, with the largest, LDL1, being found in the density range 1.019 to 1.033 g/mL; LDL2 and LDL3 in the range 1.033 to 1.038 g/mL; LDL4 and LDL5 in the range 1.038 to 1.050 g/mL; and the smallest, LDL6 and LDL7, in the range 1.050 to 1.063 g/mL. Because we found that most of the subjects had 1 major LDL peak and 1 or 2 minor peaks, we estimated the percent relative area of each LDL peak after scanning. To take into consideration the presence of secondary peaks, an LDL score for each subject was calculated as the sum of the relative areas under all LDL peaks present. A smaller LDL particle score corresponds to a larger LDL particle diameter.
DNA Analysis
Genomic DNA was isolated from peripheral blood
leukocytes by standard methods.38 Hepatic lipase
genotyping was performed as described by Guerra et al.13 A
285-bp sequence of the hepatic lipase gene was amplified by polymerase
chain reaction (PCR) in a DNA thermal cycler (PTC-100, M.J. Research,
Inc) by using oligonucleotide primers
5'-TCTAGGATCACCTCTCAATGGGTCA-3' and 5'-GGTGGCTTCCACGTGG-CTGCCTAAG-3'.
DNA templates were denatured at 95°C for 3 minutes, and then each PCR
was subjected to 35 cycles, each consisting of 1 minute of denaturation
at 95°C, 0.5 minute of annealing at 63°C, and 0.5 minute of
extension at 72°C. The PCR products were digested with 10 U of
NlaIII and the fragments separated by electrophoresis on a
1.5% agarose gel. After electrophoresis, the gel was treated with
ethidium bromide for 20 minutes, and DNA fragments were visualized by
UV illumination. The resulting fragments are 215 and 70 bp for the
T allele and 285 bp for the uncut C
allele.
Statistical Analyses
To compare men and women who participated in the study, we used
2 tests for categorical measures and a
2-sample t test for continuous measures. We estimated the
frequency of both the T and apoE alleles with the
chromosome counting method and used a
2 test
to compare these frequencies in men and women. To evaluate the
relationship between the hepatic lipase genotypes
(CC, CT, and TT) and lipid levels, we
used ANCOVA techniques, which accounted for the familial relationships
among the members of the study (mostly siblings and cousins). We used 2
approaches to accomplish these analyses. First, we employed a
repeated-measures approach that assumed an exchangeable correlation
structure among all members of a family by using PROC
MIXED in the SAS package. Because this approach does not
accurately represent the true correlation structure within
these pedigrees, we also employed a measured-genotype
approach39 as implemented in SOLAR, a
variance component analysis computer package for quantitative
traits measured in pedigrees of arbitrary size.40 The
latter approach fully accounts for the different types of relationships
within a pedigree in performing an ANOVA on the defined
genotypes. In these analyses, we used several different
models to adjust for potential confounders. First, we obtained
essentially crude results, which accounted only for the family
structure; second, we adjusted for age, body mass index (BMI), smoking,
alcohol consumption, use of ß-blockers, and menopausal status and
hormonal replacement therapy in women. In our final analysis,
we added apoE genotypes to the model, with
E2/E2 and E2/E3 in 1 group,
E3/E4 and E4/E4 in a second
group, and E3/E3 as the reference group. Subjects
with the apo E2/E4 genotype, of
which there were very few, were excluded.
| Results |
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Association of the C-514T Polymorphism With Variations in
Plasma Levels of Lipids, Lipoproteins, and Apoproteins
Table 2
shows that in men and women,
the 3 genetic groups were equivalent with respect to age and BMI.
However, male and female carriers of a T allele had
higher HDL-C and apoAI concentrations compared with noncarriers. The
higher HDL-C levels associated with the T allele were
due to an increase in the HDL2-C subfraction.
Moreover, the association of the T allele with higher
HDL2-C levels was stronger in women compared with
men after adjustment for familial relationships, age, BMI, smoking,
alcohol intake, the use of ß-blockers, apoE genotype, and
menopausal status and estrogen therapy in women. In both sexes, there
was no significant difference between the genetic groups in the plasma
levels of total cholesterol, LDL-C, apoB, and
triglyceride. To better understand the
metabolic basis of the association of higher HDL-C levels
with the T allele in men and women, lipoprotein subclass
profiles were measured by automated NMR spectroscopy. As shown in Table 3
, this association was significant in
women only and was primarily due to an increase in the large HDL
subfraction.
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Association of the C-514T Polymorphism With Variations in
Lipoprotein Particle Size
We also investigated the effect of the hepatic lipase variants on
HDL and LDL particle size. Overall, the T allele was
associated with a significant increase in HDL particle diameter in both
men and women (Table 4
). No significant
association was observed between the hepatic lipase polymorphism
and LDL particle size (Table 5
). The
association of the T allele with increased HDL particle
size was greater in women compared with men after adjustment for
familial relationships, age, BMI, smoking, alcohol intake, the use of
ß-blockers, apoE genotype, and menopausal status and estrogen
therapy in women.
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Hepatic Lipase Genotype and Risk of CHD
CHD was present in 159 men (11.5%, mean age 54.1±10.0 years)
and 60 women (4.3%, mean age 53.9±7.6 years). Of the 506 male
carriers, 58 had a history of CHD compared with 101 of 875 noncarriers
(P=0.3440). In women, 19 of the 509 carriers had a history
of CHD compared with 41 of the 886 noncarriers (P=0.7270).
Moreover, no significant difference in the age of onset of CHD between
carriers and noncarriers was observed in both sexes. In men, carriers
of the -514T mutation had a mean age of onset of CHD of 53.8±9.0
years compared with 54.2±10.5 years in noncarriers
(P=0.5468), whereas in women, the mean age of onset of CHD
was 53.7±7.1 and 54.0±7.8 years for carriers and noncarriers,
respectively (P=0.5292). Our analyses showed that no
increased risk for CHD could be attributed to this
polymorphism.
| Discussion |
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36% of North Americans of white
descent may be carriers of this functional polymorphism in the
hepatic lipase gene. The carrier prevalence for the C-514T
polymorphism in the FOS is thus similar to that reported earlier in
white subjects from European countries12 43 44 or the
United States in general.13 19 These results are
consistent with the ethnic background of the FOS, as the vast
majority of participants are descended from Italian, Irish, French, and
British immigrants to North America over the last 350 years. We have also shown that in women, the T variant was associated with higher HDL-C concentrations due to an increase in the large-HDL subfractions. Presumably, these changes in HDL subfraction distribution are a reflection of changes in hepatic lipase activity.12 20 Compared with homozygous CC women, women with the CT genotype at position -514 had 3.1% and 8.5% higher HDL-C and HDL2-C levels, respectively. Furthermore, only plasma HDL2-C concentrations appear to be associated with the hepatic lipase polymorphism, whereas the HDL3-C concentrations are not, suggesting that the HDL particle size distribution rather than total HDL-C may represent a more sensitive marker for testing the effect of the C-514T polymorphism on HDL metabolism. A similar trend was seen in men, although the differences were not statistically significant. The significance of this finding remains to be established, but several lines of evidence have demonstrated that plasma hepatic lipase activity levels and HDL-C concentrations are modulated by a number of exogenous and endogenous factors in addition to the hepatic lipase promoter polymorphisms. These include estrogen levels,45 visceral fat, insulin resistance,46 47 48 and drugs.49 Further studies will be required to assess the sex-dependent relationship between plasma HDL-C and HDL subclass distribution and the hepatic lipase promoter genotype. At least we can speculate that the hepatic lipase genotype may influence the metabolic relationships that determine how HDL particles are metabolized in women. Furthermore, the variance components analysis performed in the FOS participants indicated that the C-514T polymorphism in the hepatic lipase gene accounted for <1% of the variance in plasma concentrations of HDL-C and HDL2-C, indicating that the genetically determined variability in plasma HDL-C and HDL2-C levels is most likely due to allelic variation in a relatively large number of genes, each of which has a small or moderate influence.
In previous studies,12 13 no functional mutations were detected in the coding sequence of the hepatic lipase gene that could account for the observed relationship between hepatic lipase activity, plasma HDL-C levels, and genetic polymorphism of the hepatic lipase promoter. Because we did not measure hepatic lipase activity directly in the FOS participants, we can only speculate on the mechanisms underlying the association between the C-514T polymorphism and variations in plasma HDL-C and HDL2-C concentrations. In fact, it is possible that this polymorphism does not directly affect hepatic lipase expression and HDL-C levels but that another linked polymorphism within the hepatic lipase gene affects its expression. Finally, we cannot exclude the possibility that the C-514T polymorphism is in linkage disequilibrium with another unidentified gene responsible for variations in plasma HDL-C concentrations, but so far, no other genes affecting plasma HDL-C levels have been identified in close proximity to the hepatic lipase locus.
Epidemiological studies have suggested that both small, dense LDL23 24 and low HDL-C1 50 are associated with an increased risk of developing CHD. In fact, the change in LDL particle size appears to be mediated by a complex network of genetic, metabolic, and hormonal factors. The factors associated with decreased LDL particle size include male sex; decreased HDL-C and apoAI levels; elevated triglycerides; low-fat, high-carbohydrate diets; and ß-adrenergic blocker use.23 Thus, it has been suggested that LDL particle size distribution is a marker for a series of metabolic alterations that are probably influenced by similar mechanisms.51 In the current study, the increased HDL particle size associated with the T-514 variant was not associated with smaller, denser LDL particles after adjustment for familial relationships and other covariates. This observation is in contrast with previous studies demonstrating a significant relationship between hepatic lipase promoter polymorphism, hepatic lipase activity, plasma HDL2-C levels, and LDL buoyancy.20 22 52 Differences in the phenotypic expression of the hepatic lipase variants could be related to factors such as sample size, admixture or population stratification, differences in environmental factors, and/or differences in genetic background of the populations studied. It should be noted that different dietary habits across populations may influence the genotype-lipid associations. Moreover, the older age of participants in our study may be responsible, at least in part, for these discrepancies, because as previously indicated, the hormonal profile in women might modulate the effect of the hepatic lipase polymorphism on HDL subclass distribution.
Several studies in humans and animals have yielded conflicting results concerning the potential role of hepatic lipase in the pathogenesis of atherosclerosis.53 Some of the previously described hepatic lipasedeficient individuals have suffered from CHD,18 54 55 but the precise contribution of hepatic lipase deficiency to the presence of CHD may be obscured by the presence of other concomitant lipoprotein disorders.53 Hepatic lipase deficiency results in the elevation of plasma HDL2-C levels, a change that is considered antiatherogenic but that is also associated with the impaired clearance of lipoprotein remnants,17 56 decreased production of pre-ß-HDL,57 and decreased delivery of HDL-C to the liver.15 All these changes are potentially atherogenic. From epidemiological studies, it is known that a low plasma HDL-C level is a major risk factor for CHD1 58 and, consistent with its role in the hydrolysis of triglycerides and phospholipids of HDL, hepatic lipase activity is inversely correlated with HDL-C concentrations.59 60 Thus, it has been hypothesized that the lower hepatic lipase activity and higher HDL-C levels present in premenopausal women would be responsible for their lower risk of heart disease.61 62 Likewise, Blades et al63 have demonstrated that an increase in hepatic lipase activity is the major change in lipolytic enzymes observed in hypoalphalipoproteinemic subjects with or without hypertriglyceridemia. In contrast, several studies have provided evidence for the protective role of hepatic lipase in atherosclerosis. It has been shown that hepatic lipase activity is inversely correlated with the degree of calcific atherosclerosis in patients homozygous for familial hypercholesterolemia.64 Similarly, hepatic lipase activity is significantly reduced in normolipidemic men with CHD, a finding also associated with delayed clearance of postprandial lipoproteins.65 The current study clearly shows that the C-514T polymorphism in the hepatic lipase promoter is associated with significant variations in plasma HDL2-C levels in women and HDL particle size in both sexes, but the cumulative effects of these variations on atherosclerotic risk remain uncertain, probably owing to the small numbers of CHD patients in this young cohort. This underscores the need for larger, prospective studies in the assessment of common genetic polymorphism with mild effects on lipoprotein subclass profiles and outcome such CHD. Consequently, the role of hepatic lipase deficiency in the pathogenesis of CHD remains controversial, and more studies at the population level are needed.
| Acknowledgments |
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Received April 26, 1999; accepted October 25, 1999.
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G mutation in intron I of the
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