Articles |
Presented in part at the joint meeting of the 16th Congress of the European Society of Cardiology and the 12th World Congress of Cardiology, Berlin, Germany, September 10-14, 1994.
From the Center for Internal Medicine, Department of Rehabilitation, Prevention and Sports Medicine, Freiburg University Hospital, Germany.
Correspondence to Dr Martin Halle, Medizinische Klinik, Abt. Rehabilitative und Präventive Sportmedizin, Hugstetter Strasse 55, D-79106 Freiburg, Germany. E-mail mh@msm1.ukl.uni-freiburg.de.
| Abstract |
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Key Words: LDL subfractions fibrinogen coronary risk factors
| Introduction |
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To investigate whether FIB levels are associated with HDL and LDL subfraction phenotypes independent of signs of inflammation and smoking habits, we examined young, healthy nonsmoking men without evidence of acute infection or CAD and assessed their serum FIB concentrations and lipoprotein profiles. By measuring the CHOL and APO concentrations of VLDL, IDL, and subfractions of LDL and HDL and the concentrations of Lp(a), a detailed evaluation of the possible link between FIB and lipoproteins was possible.
| Methods |
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The study was approved by an institutional review committee, and all subjects gave informed consent for participation in the study.
Obesity Index and Ergometry
BMI (weight in kilograms divided
by the square of the height in
meters) was calculated for all participants in the study. A
symptom-limited bicycle exercise stress test with a stepwise
increment in workload of 50 W every 3 minutes was performed, with
continuous ECG recording.
Density Gradient
Ultracentrifugation
EDTA plasma was obtained after an overnight fast.
VLDL
(d<1.006 g/mL), IDL (d=1.006 to 1.019 g/mL), LDL
(d=1.019 to 1.063 g/mL), and HDL (d=1.063 to
1.210 g/mL) were prepared by sequential
flotation.4 18
Total LDL was separated into six and HDL into two density classes by
equilibrium density-gradient
centrifugation.4 The density ranges of LDL
subfractions, as determined by precision refractometry18
of blank gradients, were LDL-1: 1.019 to 1.031 g/mL; LDL-2: 1.031 to
1.034 g/mL; LDL-3: 1.034 to 1.037 g/mL; LDL-4: 1.037 to 1.040 g/mL;
LDL-5: 1.040 to 1.044 g/mL; LDL-6: 1.044 to 1.063 g/mL;
HDL2: 1.063 to 1.125 g/mL; and HDL3:
1.125 to 1.210 g/mL. All centrifugation steps were
performed at a temperature of 18°C using partially filled 6-mL
polycarbonate bottles in a 50 Ti rotor (Beckman). The within-assay
coefficient of variation for the determination of LDL subfraction
concentrations was between 2.2% and 4.5% for CHOL and between 3.0%
and 5.8% for APOB, depending on the subfraction.
Chemical Analysis
FIB was determined by end-point
nephelometry to detect immune
complexes of FIB and specific antibodies (Behring).
In all HDL and LDL subfractions, total CHOL was measured by automated (EPOS, Eppendorf) enzymatic methods (Boehringer Mannheim; bioMérieux). The APOA-I, APOB, and APOA-II were measured by end-point nephelometry (Behring).
Lp(a) was determined by a commercial ELISA with polyclonal anti-APO(a) antibodies (Immunozym Lp(a); Immuno GmbH).
Fasting INS concentrations were determined by an ELISA (Boehringer Mannheim), and FFA by an enzymatic colorimetric method (Wako Chemicals).
IR was calculated from fasting blood GLUC in mmol/L and serum INS concentrations by using a computer-solved homeostasis model assessment method19 20 21 : IR=fasting insulinxfasting glucose/22.5.
Statistical Analysis
An ANOVA was used to test the hypothesis
that lipoprotein values
were equal in all FIB quartiles. A conservative multiple comparison
test (Scheffé's test) was chosen for pairwise comparisons of
means between the FIB quartiles (Tables 1 through
3). All values for each
FIB quartile are expressed as mean±SD if not otherwise indicated.
A univariate correlation analysis (Spearman's correlation) was performed between age, BMI, FIB, parameters of IR (GLUC, INS, and FFA), uric acid, BP, and lipoprotein subfractions (small, dense LDL [LDL-6] APOB and CHOL; HDL2 CHOL and APOA-I). This analysis was followed by a stepwise multiple regression analysis, in which terms that were significantly associated with lipoprotein subfractions in the univariate correlation analysis were entered according to their level of significance, the APOs of LDL and HDL subfractions (LDL-6 APOB; HDL2 APOA-I) being the dependent variable. For this procedure, all variables were logarithmically transformed to reduce the skew of the distribution.
Data were analyzed using the Statistical Package for the Social Sciences (SPSS/PC+, SPSS Inc). All values of P<.05 were considered to indicate statistical significance.
| Results |
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Concentrations of CHOL and APOs of total LDL and total HDL did not vary
between FIB quartiles (Tables 1
and 2
). However,
when dividing LDL and
HDL into subfractions, it became evident that the FIB groups differed
significantly in their CHOL and APO subfraction profiles. In
particular, men with FIB >2.90 g/L had twofold higher concentrations
of small, dense LDL particles (LDL-6 APOB) than men with FIB levels
<2.55 g/L. They also had the lowest concentrations of
HDL2 CHOL and HDL2 APOA-I (Tables 1
and
2
).
In addition to the lipoprotein subfraction analysis, we
determined other coronary risk factors such as age, BMI, GLUC,
INS, FFA, IR, uric acid, Lp(a), and BP. Other than significant
differences in age and body weight, no significant differences between
FIB quartiles were found for all other parameters in this
healthy population of nonsmoking men (Table 3
).
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To investigate whether other risk factors were also predictors for a
more atherogenic lipoprotein subfraction profile, namely an increased
number of small, dense LDL particles (LDL-6 APOB) and reduced
concentrations of HDL2 CHOL, we performed a
univariate correlation analysis between
coronary risk factors and small, dense LDL APOB and
HDL2 CHOL particles. Serum triglycerides
were the strongest predictor of small, dense LDL particles followed by
CHOL, BMI, FIB, and age (Table 4
).
HDL2 CHOL was also primarily determined by serum
triglycerides (r=-.41;
P<.001), BMI (r=-.34; P<.001),
and FIB (r=-.12; P<.05), whereas age and
CHOL had no effect. In this sample of healthy young nonsmoking men, the
factors relating to IR (GLUC, INS, and FFA), as well as BP, were not
associated with HDL and LDL subfraction distribution. In a
multivariate stepwise regression analysis, only
serum triglycerides on the first step
(r=-.41, R2=.17,
P<.001) and BMI (r=-.44,
R2=.20, P<.05) on the second step
independently influenced HDL2 CHOL. FIB was not
included in the multivariate regression equation. For
the concentration of APOB in small, dense LDL, which corresponds with
the number of circulating small, dense LDL particles, FIB was a
significant and independent predictor (Table 4
).
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| Discussion |
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The reason for the independent association between FIB and concentrations of small, dense LDL particles is unclear. However, evidence for a direct metabolic link between serum lipids and FIB concentrations was reported almost 30 years ago. Animal and in vitro studies by Pilgeram and Pickart35 demonstrated that plasma FFA exert significant control over the biosynthesis of FIB. They showed that the rate of synthesis of FIB by the liver is augmented by GLUC and FFA, particularly by palmitate. Thereby, they provided initial data connecting the metabolism of fatty acids, lipoproteins, and FIB. In our study population, FFA and GLUC levels were always within normal limits and did not differ significantly between FIB quartiles. Serum triglycerides and concentrations of triglyceride-rich lipoprotein particles (VLDL) were, however, higher in men with FIB >2.90 g/L than in men with FIB <2.55 g/L. Since elevated small, dense LDL particles are primarily found in hypertriglyceridemia,36 and hypertriglyceridemia is in turn associated with hyperfibrinogenemia, it may be postulated that similar mechanisms lead to an elevated hepatic synthesis or secretion of VLDL and FIB. In the circulation, VLDL is then converted to small, dense LDL particles, thus explaining our observation that small, dense LDL particles are increased in hyperfibrinogenemia.
A few investigators have also hypothesized that LDL particles are synthesized and secreted directly by the liver.37 38 In hyperfibrinogenemia, metabolic states associated with elevated concentrations of FFA and triglycerides may stimulate the synthesis of both FIB and APOs simultaneously. Further research is necessary to resolve this matter.
Besides its effect on the progression of arteriosclerosis and acute cardiovascular events, FIB is also an acute-phase protein that is found in elevated concentrations in patients with inflammatory disease. Arteriosclerosis is itself an inflammatory process, and parameters of inflammation, such as a raised white blood cell count and ferritin concentrations, have also been observed to be elevated in patients with coronary artery disease and acute myocardial infarction.12 39 Other inflammatory mediators, such as interleukins, have been shown to influence the hepatic metabolism of lipoproteins and FIB in vitro.40 41 Via this mechanism, acute or chronic inflammation could influence FIB and serum lipoprotein concentrations. To exclude the effect of an acute or chronic infection or inflammation on the concentrations of FIB and lipoproteins in our study, we included only men without history of acute infection or chronic inflammatory disease or signs of acute infection (normal white blood cell count). Because smoking has also been shown to increase FIB levels,8 13 we also excluded smokers from the study. Therefore, the distinct association between concentrations of FIB and lipoprotein subfractions found in our study is not owing to underlying inflammatory processes or smoking.
Our study has shown that nonsmoking, clinically healthy men with serum FIB concentrations >2.90 g/L have a significantly unfavorable LDL subfraction profile that is independent of other coronary risk factors, such as BMI, age, IR, total CHOL, serum triglycerides, uric acid, and BP. This is in accordance with epidemiological studies showing that men with FIB levels >3.11 g/L have a significantly higher risk for CAD than subjects with lower levels.13 It may be proposed that in addition to its effect on coagulation, FIB influences atherogenesis by worsening the LDL subfraction profile. Because FIB is independently associated with the expression of a more atherogenic lipoprotein subfraction profile, it should be included in the assessment of coronary risk factors, particularly in patients with dyslipoproteinemia.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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Received August 11, 1995; accepted October 16, 1995.
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