Original Contributions |
From the Department of Internal Medicine III and Biochemistry, Erasmus University, Rotterdam, The Netherlands (H.J.); the Department of Medicine, Rayne Institute, University College Medical School, London, UK (G.C., P.J.T.); the Department of Biochemistry, National Public Health Institute, Helsinki, Finland (C.E.); Unité INSERM-325, Pasteur Institute, Lille, France (J.D.); and INSERM U258, Hôpital Broussais, Paris, France (V.N.).
Correspondence to Hans Jansen, PhD, Department of Biochemistry, Ee 671, Erasmus University Rotterdam, POB 1738, 3000 DR Rotterdam, Netherlands. E-mail jansen{at}bc1.fgg.eur.nl
| Abstract |
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Key Words: hepatic lipase gene promoter polymorphisms postprandial lipoproteins
| Introduction |
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Altogether, HL is an important enzyme with multiple functions affecting the metabolism, composition, and concentration of several lipoproteins. A deficiency of HL often leads to a mild phenotype with elevation of HDL and/or an increase in LDL concentration and buoyancy.13 19 20 In conjunction with other hyperlipidemia-causing genes, HL deficiency influences plasma lipoprotein levels more strongly, with triglyceride enrichment of lipoprotein fractions with a d>1.006 g/mL, the presence of ß-VLDL, and impaired metabolism of postprandial triglyceride-rich lipoproteins.19 Thus, because of the multiple effects of HL on lipoprotein metabolism, genetic variation in HL expression may influence atherosclerotic risk.
Several variant sites have been identified in the HL gene, but only a few lead to decreased postheparin plasma HL activity.21 22 23 Recently, low HL activity was found to be associated with a common C-to-T base substitution in the HL promoter.24 25 26 27 This transition is at position -480 based on the sequence published by Cai et al28 and at position -514 based on the sequence of Ameis and coworkers.29 The -514 C-to-T polymorphism was recently shown to be in complete linkage disequilibrium with 3 other polymorphisms in the HL promoter (-250 G to A, -710 T to C, and -763 A to G).30 To date no effects of any of these substitutions on HL promoter activity in vitro have been reported. In addition to low HL activity, the T allele was found to be associated with an increased HDL cholesterol content in males in the Netherlands, Finland, and the United States.25 31 30 In a Dutch sample of normolipidemic men with coronary artery disease (CAD), the frequency of the T allele was higher than in nonsymptomatic controls.25 In contrast, in a Finnish study of subjects with a family history of myocardial infarction, the occurrence of the T allele was the same as in healthy subjects.31 We carried out our investigation on subjects taking part in the European Atherosclerosis Research Study (EARS) II, a postprandial trial of young, male, university students from 11 European countries (L. Tiret et al, unpublished observations, 1998) whose fathers had documented CAD (designated as cases) and age-matched controls, thus enabling us to examine populations of different European regions. Moreover, because HL deficiency may result in impaired postprandial lipoprotein metabolism,19 we studied whether the HL polymorphism influenced the lipid response to an oral fat load.
| Methods |
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Postprandial Tests
At the first visit each participant underwent a standard 75-g
oral glucose tolerance test (OGTT) after a 12-hour overnight fast.
Venous blood was withdrawn at 0, 30, 60, 90, and 120 minutes for the
determination of glucose and insulin concentrations.
One week later the oral fat tolerance test was performed. The standard meal had an energy content of 1493 kcal (6186 kJ) consisting of 21.6 g protein, 65.5 g fat (of which 41.64 g was saturated), 56.2 g carbohydrate, and 416.6 mg cholesterol. The drink was made from 1 can of cream (Nestlé double cream), sugar (sucrose), and milk protein (Marvel). Blood samples were withdrawn at 0, 2, 3, 4, and 6 hours for the determination of triglyceride concentration. Levels of particles containing apoC-III and apoB (LpC-III:B) were measured at 0 and 4 hours.
Lipid and Lipoprotein Particle Measures
All fasting lipids and apolipoproteins were measured at baseline
of the OGTT, and fasting triglycerides, insulin, and
glucose were remeasured at baseline of the OGTT. LpC-III:B's were
measured by a noncompetitive (sandwich) ELISA as described
previously.32 In brief, polystyrene microtiter plates were
coated with affinity-purified polyclonal antibodies to human apoC-III
(1 mg/mL). Duplicate plasma samples were diluted 1:1000, 1:2000, and
1:5000 with 100 mmol/L PBS containing 1% albumin.
The samples were added to the wells along with the standards and
controls and incubated for 2 hours at 37°C. After incubation, the
plates were washed 4 times with PBS, and a rabbit polyclonal apoB
antibody conjugated to peroxidase was added. The plates were incubated
for 2 hours at 37°C and then washed. Color development was performed
for 30 minutes by addition of a peroxidase substrate
(o-phenylenediamine dichloride, Sigma
Chemical Co). The plates were read at 492 nm on an automated
microplate reader model EL340 (Bio-Tek Instruments). A pool of 400
different plasma samples was used as a secondary standard. This pool
was calibrated using immunopurified LpC-III:B. The apoB concentration
of the particles was determined by nephelometry using the BNA system
and apoB standards (Behringwerke). The values are expressed as the
amount of apoB associated with apoC-III.
DNA Extraction and Genotyping
DNA was extracted by the salting-out method.33 The
DNA was stored diluted in 96-well Beckman plates,34
divided into aliquots into 96-well Omnigene plates, and dried. The
polymerase chain reaction (PCR) mixture consisted of 50 mmol/L
KCl, 10 mmol/L Tris (pH 8.3), 0.1 g/L gelatin (Sigma), 0.2
mmol/L of each dNTP, 1.5 mmol/L MgCl2,
0.02% W-1 (GIBCO), 0.1 U of Taq polymerase (Gibco BRL)/10
mL PCR, and 0.2 nmol/L of each of the PCR primers described by Jansen
et al25 for the C-480T variant.
The PCR products were subjected to restriction enzyme analysis by digestion with 2 U of the restriction endonuclease NlaIII (NEB) per 7 mL of the PCR sample at 37°C for 4 hours and in the buffer recommended by the manufacturer. The samples were then loaded on to an ethidium bromideprestained microplate array diagonal gel electrophoresis gel as described,34 electrophoresed for 1 hour and 20 minutes, and marked for the genotypes.
Statistical Analysis
The data were analyzed using the SAS
statistical software package (SAS Institute Inc). Observed numbers of
each genotype were compared with those expected for
Hardy-Weinberg equilibrium in cases and controls from the 4 regions by
using the
2 test. Allele frequencies were
compared between cases and controls by the Mantel-Haenszel test after
adjustment for region. Statistical significance for this and all other
tests was taken to be P<0.05. Triglycerides and
LpC-III:B were logarithmically transformed before the tests to remove
positive skewness. For postprandial analyses of
triglycerides, 2 parameters were calculated:
the area under the curve (AUC, in mmol-h/L) above the fasting
concentration, calculated by the trapezoidal rule, and the peak
(mmol/L), calculated as the highest value minus the fasting value. The
C-480T genotype was tested after assuming additive effects of
alleles in a codominant model. For lipid analysis, data for
cases and controls were pooled because no significant interaction
between case/control status and the HL -480 variant could be detected.
The means were adjusted for status, age, and center.
| Results |
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Relation to Plasma Lipids, Lipoproteins, and
Apolipoproteins
The association of the rare -480T allele with
plasma lipid and lipoprotein levels in cases and controls is shown in
Table 3
. There was no significant interaction
between case/control status and genotype, so cases and controls
were pooled to acquire sufficient numbers for T/T
carriers. In the pooled data, the -480T allele was
associated with higher fasting plasma total cholesterol,
triglyceride, and HDL cholesterol. These
results conform to a codominant model, because the homozygous
T/T carriers had the highest values followed by
the C/T carriers, and the lowest values were
found in the C/C homozygotes. LDL
cholesterol did not differ among the different
genotypes. Although in carriers of the T allele
both plasma triglyceride and HDL were increased, the
inverse correlation between fasting triglyceride level and
HDL cholesterol concentration was significantly stronger in
the -480T homozygotes than in heterozygotes or
-480C homozygotes (correlation coefficients of -0.65,
-0.26, and -0.32, respectively; P<0.05). The strongest
association of the C-480T polymorphism was with apoA-I. In
T/T carriers the apoA-I concentration was 10%
higher than in -480C/C homozygotes
(P<0.001). ApoB was slightly higher in T
allele carriers than in C/C homozygotes, but
there was no difference between the C/T and
T/T genotypes. There was no
genotype effect on body mass index, waist-hip ratio, or plasma
glucose or insulin concentrations.
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Postprandial Triglyceride and LpC-III:B
Response
Because HL is involved in postprandial lipid clearance, we studied
the effects of the C-480T polymorphism on postprandial
triglyceride concentrations after a fat tolerance test
(Figure 1
). There were no significant differences
in the mean triglyceride concentrations between the
different genotypes when tested either directly or after
adjustment for fasting triglyceride. Differences in the
AUC, peak height minus fasting concentration, or the time at which the
triglyceride value reached its peak did not reached
statistical significance among genotypes (Table 4
). The association of the polymorphism with
the concentration of LpC-III:B was also studied before and after the
fat load. Before the fat load, the concentration of LpC-III:B was
dose-dependently higher in the -480T allele carriers
than in noncarriers. Homozygous T allele carriers had a
23% higher value than did individuals homozygous for the
-480C allele (P<0.05; Figure 2
). After the fat load this difference in
LpC-III:B concentration remained, with -480T homozygotes
having the highest levels of LpC-III:B (P<0.05).
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| Discussion |
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0.2) as those previously
reported in the Netherlands, Finland, and the United
States.25 30 31 The study population consisted of
university students recruited in northern (the Baltic), central, and
southern Europe and the United Kingdom. When all 4 regions were pooled,
no significant difference in the frequency of the T
allele between cases and controls was found (P=0.08).
The T allele was associated with an increased
concentration in plasma triglyceride, total
cholesterol, HDL cholesterol, apoA-I, and apoB.
Whether or to what extend the changes in plasma lipids are related to
atherosclerosis is not clear. On the one hand, HDL
cholesterol and apoA-I were increased in T
allele carriers, suggesting that the T allele may be
associated with a reduced atherogenic risk. On the other hand, the
T allele was also associated with higher plasma
triglyceride and cholesterol. Moreover, in
carriers of the T allele, the concentration of LpC-III:B
was significantly increased both preprandially and postprandially.
ApoC-III is an important determinant of triglyceride-rich
lipoprotein clearance. In vitro studies have shown that apoC-III
inhibits lipoprotein lipase35 and HL
activities36 and displaces apoE from the surface of
triglyceride-rich lipoproteins. Therefore, in
metabolic conditions in which lipolytic activity is
decreased, lipoprotein particles that are enriched in apoC-III tend to
accumulate. LpC-III:B is associated with CAD.37 38 39 40 Thus
overall, the T allele is associated with a number of
potentially antiatherogenic and proatherogenic lipoprotein changes in
our population. In a population of normolipidemic men with
CAD25 and in white Americans,30 the
T allele was also associated with enhanced HDL, but not
with total cholesterol and triglyceride. In our
population, why effects of the T allele on total
cholesterol and triglyceride were also found is
not clear. Possibly, differences in selection criteria (age,
normolipidemia) between the different studies may play a role. Although
we did not determine HL activity in our population, the changes in
plasma lipids are presumably related to a lower HL activity in the
T allele carriers, as has been shown to exist in a
number of other populations.25 26 27 In which way the
C-480T substitution or other polymorphisms lead to lowered HL
expression is not known. To date, no effects of any of these
substitutions on HL promoter activity in vitro have been reported. It
is interesting to note that the C-to-T substitution at -480 disrupts a
potential upstream stimulatory factor binding site. Upstream
stimulatory factor is an insulin-inducible transcription factor
involved in the regulation of hepatic lipid and carbohydrate
metabolism.41 42 43 However, whether this is
related to or causes the lowered HL activity in the T
allele carriers is not clear. Studies on this subject are in
progress.
| Acknowledgments |
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| Appendix 1 |
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D. St. J. O'Reilly, UK
EARS II Project Management Group
F. Cambien, France; G. De Backer, Belgium; D. St. J. O'Reilly, UK; M. Rosseneu, Belgium; J. Shepherd, UK; L. Tiret, France.
The EARS II Group Collaborating Centers and Their Associated Investigators
Austria: H.J. Menzel, Institute for Medical Biology and Genetics, University of Innsbruck, laboratory.
Belgium: G. De Backer, S. De Henauw, Department of Public Health, University of Ghent, recruitment center; M. Rosseneu, Laboratorium voor Lipoproteïne Chemie/Vakgroep Biochemie, University of Ghent, laboratory.
Denmark: O. Faegerman, C. Gerdes, Medical Department I, Aarhus Amtssygehus, Aarhus, recruitment center.
Estonia: M. Saava, Department of Nutrition and Metabolism, Institute of Cardiology, Tallinn, recruitment center.
Finland: C. Ehnholm, National Public Health Institute, and R. Elovainio, J. Peräsalo, Finnish Student Health Service, Helsinki, recruitment center; Y.A. Kesäniemi, M.J. Savolainen, Department of Internal Medicine and Biocenter Oulu, and P. Palomaa, Finnish Student Health Service, University of Oulu, Oulu, recruitment center and laboratory.
France: L. Tiret, V. Nicaud, J. Boer, R. Rakotovao, INSERM U258, Hôpital Broussais, Paris, EARS data center; S. Visvikis, Center de Médecine Préventive, Nancy, laboratory; J.C. Fruchart, J. Dallongeville, Service de Recherche sur les Lipoprotéines et l'Athérosclérose (SERLIA), INSERM U325, Institut Pasteur, Lille, laboratory.
Germany: U. Beisiegel, C. Dingler, Medizinische Klinik Universitäts-Krankenhaus Eppendorf, Hamburg, recruitment center and laboratory.
Greece: G. Tsitouris, N. Papageorgakis, Department of Medicine and Cardiology, Evangelismos Hospital, Athens, recruitment center.
Italy: E. Farinaro, Institute of Internal Medicine and Metabolic Disease, University of Naples, Naples, recruitment center.
The Netherlands: L.M. Havekes, IVVO-TNO Health Research, Gaubius Institute, Leiden, laboratory.
Portugal: M.J. Halpern, J. Canena, Instituto Superior de Ciencas da Saude, Lisbon, recruitment center.
Spain: L. Masana, J. Ribalta, Unitat Recerca Lipids, University Rovira i Virgili, Reus, recruitment center and laboratory.
Switzerland: F. Gutzwiller, B. Martin, Institute of Social and Preventive Medicine, University of Zurich, Zurich, recruitment center and laboratory.
United Kingdom: D. St. J. O'Reilly, M. Murphy, Institute of Biochemistry, Royal Infirmary, Glasgow, recruitment center and laboratory; S. Humphries, P. Talmud, V. Gudnason, R. Fisher, University College London School of Medicine, London, laboratory; D. Stansbie, A.P. Day, M. Edgar, Department of Chemical Pathology, Royal Infirmary, Bristol, recruitment center and laboratory; F. Kee, Northern Health and Social Services Board, and A. Evans, Department of Epidemiology and Public Health, Queen's University of Belfast, Belfast, recruitment center.
Received January 28, 1998; accepted June 29, 1998.
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