Accumulation of Apolipoprotein C-I–Rich and Cholesterol-Rich VLDL Remnants During Exaggerated Postprandial Triglyceridemia in Normolipidemic Patients With Coronary Artery Disease
Background—Exaggerated postprandial triglyceridemia is common in normolipidemic patients with coronary artery disease (CAD). Alterations in the composition of triglyceride-rich lipoproteins (TRLs) are likely to underlie this metabolic disturbance. However, the composition of very-low-density lipoproteins (VLDLs), which are the most abundant postprandial TRLs, has never been defined in CAD patients.
Methods and Results—We examined postprandial changes in the number and composition of VLDLs in middle-aged, normolipidemic CAD patients and control subjects. TRLs from 14 patients and 14 control subjects aged 45 to 55 years were subfractionated by density gradient ultracentrifugation into Svedberg flotation rate (Sf) fractions >400, 60 to 400, and 20 to 60. The VLDLs were separated from chylomicron remnants by immunoaffinity chromatography. In CAD patients, the postprandial concentrations of triglycerides and large (Sf 60 to 400) VLDL particles were elevated. In addition, their postprandial large VLDLs were enriched in apolipoprotein (apo) C-I and their postprandial small (Sf 20 to 60) VLDL remnants were enriched with apo C-I and cholesterol.
Conclusions—Perturbed handling of postprandial triglycerides in normolipidemic CAD patients involves the accumulation of apo C-I–rich large VLDL particles and the generation of small, apo C-I– and cholesterol-rich VLDL remnants.
Exaggerated postprandial triglyceridemia has been reported to be common in humans with coronary artery disease (CAD).1 A significant fraction of the postprandial triglycerides are carried on intestinally derived chylomicrons. However, most of the triglyceride-rich lipoproteins (TRLs) that accumulate in the plasma after a meal are actually VLDLs.2 The composition of postprandial VLDL is likely to be important, both for understanding the mechanisms of the accumulation of TRLs and for understanding how these lipoproteins might contribute to the pathogenesis of atherosclerosis. An accumulation of VLDL after a meal might be quite relevant to the pathogenesis of atherosclerotic disease, because several clinical studies have suggested a link between the fasting levels of VLDLs and coronary atherosclerosis.3 4 In addition, the VLDLs that accumulate postprandially have an altered apolipoprotein (apo) and lipid composition.5 In recent studies,6 specific families of TRLs, as defined by their apolipoprotein content, appeared to be better predictors of CAD progression than the total plasma concentrations of apolipoproteins or lipids.
Perturbations in the numbers and composition of postprandial VLDL subfractions in CAD patients have never been carefully defined. In the present study, we sought to test the hypothesis that exaggerated postprandial triglyceridemia in normolipidemic CAD patients might be due to an elevated number of postprandial VLDL particles from the liver. We also sought to define whether there were abnormalities in the composition of postprandial VLDLs from CAD patients. Our studies revealed abnormalities in the numbers and composition of postprandial VLDLs in CAD patients. These abnormalities are likely to be relevant to the pathogenesis of postprandial hyperlipidemia and to the development of atherosclerotic disease.
The subjects were 45- to 55-year-old male myocardial infarction survivors (n=14) without congestive heart failure who had normal fasting lipid and lipoprotein levels (LDL cholesterol <4.5 mmol/L and plasma triglycerides <2.0 mmol/L; ie, <90th percentile). The control subjects were 14 healthy normolipidemic men matched for age, smoking habits, alcohol consumption, waist-hip circumference ratio, and blood pressure (Table 1⇓). All subjects gave informed consent to the study, which was approved by the ethics committee of the Karolinska Hospital.
TRL Separation and Lipid and Apolipoprotein Determination
Participants underwent a mixed meal-type oral fat-tolerance test.5 TRLs were subfractionated by cumulative flotation in a density gradient into Svedberg flotation rate fractions (Sf) >400, 60 to 400, and 20 to 60. Within these fractions, VLDLs and VLDL remnants were purified from chylomicron remnants by immunoaffinity chromatography with apo B-100–specific monoclonal antibodies 4G3 and 5E11.5 The recovery of TRLs from the immunoaffinity chromatography was 90±5.3% (mean±SD, n=8). The lipids were determined enzymatically.5 Apo B and apo E were quantified by SDS-PAGE, and apo Cs were determined by urea gel electrophoresis.5
Differences between CAD patients and the control group were assessed by Mann-Whitney tests. Within-group comparisons of measurements from the fasting state to various time points during the fat-tolerance test were assessed by Wilcoxon signed rank test. Associations between lipoprotein parameters were assessed by calculation of Spearman correlation coefficients.
Plasma Triglycerides and Apo B-48 and B-100 Concentrations in TRL Fractions
Although postprandial plasma triglyceride levels were elevated in the CAD patients (Figure 1⇓), they returned toward fasting levels at 6 hours. The large (Sf 60 to 400) VLDL and chylomicron remnants remained elevated in the patients 6 hours after the meal compared with fasting levels (Table 2⇓; P<0.05 for apo B-48, P<0.005 for apo B-100). The CAD patients had significantly more large VLDL particles in their plasma 6 hours after the test meal than the control subjects (P<0.05).
Composition of Fasting and Postprandial VLDL
The most striking aspect of VLDL composition was the postprandial increase in apo C-I in VLDLs from CAD patients (Figure 2⇓). At 6 hours, large and small (Sf 20 to 60) VLDLs from CAD patients had 50% to 100% more apo C-I than VLDLs from controls (P<0.005, P<0.05).
In contrast to the apo C-I content, the apo C-II and C-III contents of VLDL were unaffected by the oral fat load. The apo E content of large VLDLs increased after the fatty meal in both groups (Figure 2⇑) but remained elevated only in the CAD group at 6 hours (P<0.005 versus fasting values).
The cholesterol content of small VLDLs in CAD patients had increased significantly at the end of the postprandial period and was then significantly higher in patients than in controls (P<0.05). Interestingly, the cholesterol content correlated positively to the apo C-I content of small postprandial VLDL particles in the CAD patients (r=0.73, n=14, P<0.005 at 6 hours).
Twenty years ago, Zilversmit7 suggested that postprandial lipoproteins might be particularly atherogenic. Since then, much of the research in this area has focused on intestinally derived chylomicrons and chylomicron remnants. However, most of the TRLs that accumulate in postprandial plasma are not chylomicrons but VLDLs, which originate in the liver. In the present study, we sought to define compositional differences in the postprandial VLDL subfractions in normolipidemic CAD patients and controls. Using a combination of ultracentrifugation techniques and immunoaffinity chromatography, we were able to purify and analyze VLDL subfractions after an oral fat-tolerance test in both CAD patients and matched controls. In our study, the CAD patients had an expected exaggerated postprandial triglyceridemia compared with the controls. In addition, the CAD patients manifested a postprandial accumulation of large, apo C-I–rich VLDLs, and their small VLDL remnants were enriched with apo C-I and cholesterol.
Interestingly, we found no significant differences in the apo C-II and apo C-III concentrations in the VLDLs of CAD patients and controls, even though both of those apolipoproteins are known to influence the metabolism of TRLs. The fact that apo C-I was uniquely elevated in the postprandial large VLDL of CAD patients suggests that it could be pivotal in causing the postprandial accumulation of those lipoproteins. We suspect that the enrichment in apo C-I causes a delay in VLDL clearance, even though these lipoproteins contained large amounts of apo E, the key ligand that mediates the uptake of hepatic lipoproteins. This idea is supported by in vitro studies, which have shown that apo C-I enrichment of TRLs inhibits apo E-mediated uptake by both the LDL receptor8 and the LDL receptor–related protein pathways.9 Interestingly, in vivo studies have suggested that apo C-I enrichment of VLDLs has no effect on the binding capacity of these particles to proteoglycans in the arterial wall and does not interfere with TRL hydrolysis by lipoprotein lipase.10 In agreement with those studies, it is likely that the small postprandial VLDLs rich in apo C-I were generated in part from lipoprotein lipase–mediated hydrolysis of apo C-I–rich large VLDL particles.
The small postprandial VLDLs from CAD patients were, in addition to apo C-I, enriched in cholesterol. On the basis of the strong positive correlation between the number of apo C-I and cholesterol molecules on these particles, we strongly suspect that an apo C-I–mediated delay in VLDL clearance contributes to the cholesterol enrichment of smaller VLDL particles, simply because it prolongs the VLDL circulation time and extends the time during which cholesterol transfer from HDL could occur. The cholesterol enrichment of small VLDLs could be very relevant to the development of premature coronary atherosclerosis in CAD patients, because small VLDL remnants, like LDLs, are thought to be very susceptible to retention within the arterial intima.11
This study was supported by the Swedish Medical Research Council (8691), the Swedish Heart-Lung Foundation, and the Marianne and Marcus Wallenberg Foundation. We thank Dr Stephen G. Young for editorial assistance and Dr Peter Bacchetti for statistical advice.
- Received July 16, 1999.
- Revision received November 9, 1999.
- Accepted November 18, 1999.
- Copyright © 2000 by American Heart Association
Björkegren J, Packard CJ, Hamsten A, Bedford D, Caslake M, Shepherd J, Foster L, Stewart P, Karpe F. Accumulation of large very low density lipoprotein in plasma during intravenous infusion of a chylomicron-like triglyceride emulsion reflects competition for a common lipolytic pathway. J Lipid Res. 1996;37:76–86.
Tornvall P, Båvenholm P, Landou C, deFaire U, Hamsten A. Relation of plasma levels and composition of apolipoprotein B-containing lipoproteins to angiographically defined coronary artery disease in young patients with myocardial infarction. Circulation. 1993;88:2180–2189.
Phillips NR, Waters D, Havel RJ. Plasma lipoproteins and progression of coronary artery disease evaluated by angiography and clinical events. Circulation. 1993;88:2762–2770.
Björkegren J, Hamsten A, Milne RW, Karpe F. Alterations of VLDL composition during alimentary lipemia. J Lipid Res. 1997;38:301–314.
Hodis HN, Mack WJ. Triglyceride-rich lipoproteins and progression of atherosclerosis. Eur. Heart J. 1998;19(suppl A):A40–A44.
Zilversmit DB. Atherogenesis: a postprandial phenomenon. Circulation. 1979;60:473–485.
Sehayek E, Eisenberg S. Mechanisms of inhibition by apolipoprotein C of apolipoprotein E-dependent cellular metabolism of human triglyceride-rich lipoproteins through the low density lipoprotein receptor pathway. J Biol Chem. 1991;266:18259–18267.
Kowal RC, Herz J, Weisgraber KH, Mahley RW, Brown MS, Goldstein JL. Opposing effects of apolipoproteins E and C on lipoprotein binding to low density lipoprotein receptor-related protein. J Biol Chem. 1990;265:10771–10779.
Shaikh M, Wootton R, Nordestgaard BG, Baskerville P, Lumely JS, Ville AEL, Quiney J, Lewis B. Quantitative studies of transfer in vivo of low density, Sf 12–60, and Sf 60–400 lipoproteins between plasma and arterial intima in humans. Arterioscler Thromb. 1991;11:569–577.