Articles |
From the Molecular Disease Branch, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Md.
Correspondence to Daniel Rader, MD, University of Pennsylvania Medical Center, BRB-1 Rm 409, 422 Curie Blvd, Philadelphia, PA 19104-6069.
| Abstract |
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Key Words: apoA-I apoA-II kinetics radiotracer HDLs
| Introduction |
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HDL is heterogeneous in apolipoprotein composition. The major HDL subclasses are those containing only apoA-I (LpA-I) and those containing both apoA-I and apoA-II (LpA-I:A-II).9 Substantial evidence indicates that these subclasses differ in their metabolism and effect on atherosclerosis. LpA-I and LpA-I:A-II have different rates of catabolism in normolipidemic subjects10 and in patients with genetic lecithin:cholesterol acyltransferase defects,11 indicating distinct metabolic pathways for these two HDL subclasses. Cholesteryl ester transfer protein interacts preferentially with LpA-I,12 whereas hepatic lipase interacts preferentially with LpA-I:A-II.13 Some but not all clinical studies support the concept that LpA-I may be a more specific "antiatherogenic" lipoprotein particle than LpA-I:A-II.14 15 16
A number of kinetic studies have been conducted to investigate factors regulating plasma apoA-I and apoA-II steady-state levels. Several studies have concluded that apoA-I levels are regulated primarily by the rate of apoA-I catabolism17 18 19 ; others found that apoA-I production rates also affect apoA-I levels.20 21 In contrast, apoA-II levels appear to be regulated solely by apoA-II production rates.17 18 19 However, the metabolic regulation of LpA-I and LpA-I:A-II levels is not understood. In the present study, we investigated apoA-I and apoA-II kinetics in normolipidemic subjects to determine the major kinetic factors regulating LpA-I and LpA-I:A-II levels.
| Methods |
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Isolation and Iodination of Apolipoproteins
ApoA-I and apoA-II were isolated from normal HDL by
gel-permeation chromatography and ion-exchange
chromatography23 and stored at -20°C. Purified
apolipoproteins were redissolved in a buffer of 6 mol/L guanidine-HCl
and 1 mol/L glycine, pH 8.5, and iodinated with 125I and
131I by a modification of the iodine monochloride
method.10 Iodinated apoA-I and apoA-II were reassociated
with separate aliquots of autologous plasma and extensively dialyzed
against phosphate-buffered saline containing 0.01% EDTA at 4°C
overnight.10 The samples were sterile-filtered through a
0.22-µm Millipore filter and tested for pyrogens and sterility prior
to injection.
Study Protocol
Three days prior to the study, subjects were placed on an
isoweight diet containing 47% carbohydrate, 37% fat, 16% protein,
200 mg cholesterol/1000 kcal, and a polyunsaturated/saturated fat ratio
of 0.3. Meals were given three times per day, and the diet was
continued during the metabolic study. One day prior to the study, the
subjects were started on potassium iodide (900 mg) in divided doses;
this regimen was continued throughout the study period. After a 12-hour
fast, the subjects were injected with 125I-apoA-I (50
µCi) and 131I-apoA-II (25 µCi). All 50 subjects
received the radiolabeled apoA-I, and 35 of the subjects also received
radiolabeled apoA-II. Blood samples were obtained 10 minutes after the
injection and then at 1, 3, 6, 12, 18, 24, and 36 hours; daily through
day 5; and on days 7, 9, 11, and 14. Urine was collected continuously
throughout the study.
Blood samples (20 mL) were drawn into tubes containing EDTA (final concentration, 0.1%). The blood was kept on ice, and the plasma was immediately separated by centrifugation at 2300 rpm for 30 minutes at 4°C. Sodium azide and aprotinin were added to the plasma at final concentrations of 0.05% and 200 KIU/mL, respectively. Radioactivity in plasma and urine was quantified in a Packard Cobra gamma counter (Packard Instrument Co).
Analytical Methods
Plasma decay curves were constructed as the fraction of injected
dose by dividing the plasma radioactivity at each time point by the
radioactivity in the 10-minute plasma sample. Multiexponential
functions were fit to the plasma decay curves using SAAM
31.24 Residence times (RT) were obtained from the areas
under the curves. Production rates (PR) were calculated from the
formula
PR=(Plasma Apolipoprotein Concentration)x (Plasma Volume)/(RT)/(Body Weight)
Plasma volume was determined by the radioactivity in the 10-minute plasma sample.
Plasma total cholesterol and triglyceride levels were determined by automated enzymatic techniques on an Abbott VPSS analyzer (Abbott Labs). HDL-C was measured by dextran sulfate precipitation.25 Plasma apoA-I and apoA-II concentrations were quantified using an immunoturbidometric assay (Boehringer-Mannheim). ApoB concentration was measured by a competitive enzyme-linked immunoassay.26 The plasma LpA-I concentration was measured by the method described by Parra et al27 and expressed as the apoA-I mass (in milligrams) in LpA-I per unit of volume (in deciliters). The apoA-I concentration in LpA-I:A-II was obtained by subtracting the LpA-I value from the total plasma apoA-I concentration. The mean values of the male and female groups were compared by using Student's t test. Regression analysis was performed using the SPSS statistical program.
| Results |
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The kinetic parameters of apoA-I and apoA-II are summarized in Table 2
. The mean residence time of apoA-I was 4.47±0.69 days
and of apoA-II, 5.10±0.62 days. Mean residence times of apoA-I and
apoA-II were not significantly different by gender; however, the women
tended toward longer apoA-I residence times (P=.08).
Production rates of apoA-I and apoA-II were not different between the
two groups.
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The correlations between plasma apoA-I levels and apoA-I kinetic
parameters are shown in Fig 2
. ApoA-I levels were
strongly correlated with apoA-I residence time (r=.54,
P<.001) and with apoA-I production rate to a relatively
weaker degree (r=.30, P<.05). In contrast,
apoA-II levels were strongly correlated with apoA-II production rate
(r=.82, P<.001) but not with apoA-II residence
time (r=-.14, P=.43; Fig 3
).
LpA-I levels were highly positively correlated with both apoA-I and
apoA-II residence times (r=.59, P<.001 and
r=.47, P<.01, respectively) and negatively
correlated with apoA-II production rate (r=-.41,
P<.05; Fig 4
). However, LpA-I:A-II levels
did not correlate with apoA-II residence time and were only weakly
correlated with apoA-I residence time (Fig 5
). In
contrast, LpA-I:A-II levels were strongly correlated with both apoA-I
(r=.42, P<.01) and apoA-II (r=.49,
P<.01) production rates. Thus, the distribution of apoA-I
in LpA-I was strongly inversely correlated with apoA-II production rate
(r=-.67, P<.001), weakly correlated with apoA-I
production rate and apoA-II residence time, and not correlated with the
apoA-I residence time (Fig 6
).
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Multiple regression analysis was performed using the apoA-I and
apoA-II kinetic parameters as the independent variables (Table 3
). In whole-group analysis, the apoA-II production
rate was found to be the most significant factor correlated with the
percentage of apoA-I in LpA-I (P=.003). Table 4
presents the lipid, apolipoprotein, and kinetic
parameters between two groups based on their apoA-II production rates.
The mean apoA-II production rate of 2.68
mg · kg-1 · d-1 was used for the
grouping. HDL levels did not differ between the two groups. However, in
the group with high apoA-II production rates, LpA-I was significantly
lower (P<.01) and LpA-I:A-II higher (P=.06),
resulting in a decreased fraction of apoA-I in LpA-I.
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| Discussion |
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Consistent with previous reports,17 18 19 we confirmed that the rate of apoA-I catabolism is the major metabolic factor regulating apoA-I levels. However, the apoA-I production rate was also correlated (albeit more weakly) with apoA-I concentration. The most likely explanation for these apparent discrepancies is that this study and that of Gylling et al21 excluded individuals with low HDL levels, whereas the other studies17 18 19 included some individuals with HDL levels in the bottom tenth percentile. Although apoA-I catabolism is clearly the major determinant of apoA-I levels across the entire range of HDL levels, apoA-I production rates also appear to have some effect in determining apoA-I levels when the extreme 10th percentiles are excluded. In contrast, we found that apoA-II levels are entirely determined by the rate of apoA-II production.
Levels of apoA-I in LpA-I were most strongly correlated with the catabolic rate of apoA-I, consistent with a major effect of apoA-I turnover on LpA-I levels. However, we also found that LpA-I levels were inversely correlated with apoA-II production rates. Consistent with this was the observation that the levels of apoA-I in LpA-I:A-II were most strongly correlated with apoA-II production rates. The most straightforward interpretation of these findings is that higher rates of apoA-II production result in a shift of apoA-I from LpA-I to LpA-I:A-II. This would be expected to result in lower levels of LpA-I, higher levels of LpA-I:A-II, and a substantially smaller fraction of apoA-I in LpA-I. In fact, on multivariate analysis, the distribution of apoA-I among the two subclasses was most strongly influenced by the apoA-II production rate. This indicates that the apoA-II production rate plays a key role in the distribution of apoA-I between LpA-I and LpA-I:A-II in subjects with normal HDL-C. The concept that variation in apoA-II production could influence HDL composition is supported by the earlier observation that a common apoA-II polymorphism was associated with differences in HDL composition28 as well as a recent linkage analysis linking the apoA-II locus to differences in HDL composition.29
The mechanism for the effect of apoA-II production rate on the distribution of apoA-I likely involves simple mass action of additional apoA-II associating with a limited amount of apoA-I. However, the location of this association is unknown: it could occur within the cell prior to secretion, within the hepatic sinusoids, or within the plasma compartment following secretion from the liver. We have no data to directly support or refute any of these possibilities. However, apoA-II can be secreted without apoA-I in vivo30 31 and in vitro,32 and we therefore favor the concept that the association of apoA-II with apoA-I occurs after secretion from the hepatocyte. Further investigation will be required to address this interesting question.
If LpA-I is a more specific antiatherosclerotic lipoprotein than LpA-I:A-II, then factors that result in redistribution of plasma apoA-I from LpA-I to LpA-I:A-II would be expected to cause increased susceptibility to atherosclerosis. This concept is consistent with the finding that naturally selected mouse strains with elevated apoA-II production rates develop more atherosclerosis.6 In addition, overexpression of mouse apoA-II resulted in increased atherosclerosis.7 Finally, overexpression of human apoA-II in mice also overexpressing apoA-I resulted in a shift of apoA-I to LpA-I:A-II particles and elimination of the protective effect of the apoA-I.8 In light of these reports, our finding here may be interpreted as an indication that the apoA-II production rate may be one metabolic determinant of susceptibility to atherosclerosis in humans.
In summary, our results establish that in normolipidemic humans, the distribution of apoA-I between LpA-I and LpA-I:A-II is strongly influenced by the production rate of apoA-II. This result, together with the proposition that LpA-I but not LpA-I:A-II is antiatherogenic, indicates that the apoA-II production rate modulates HDL composition and may influence human susceptibility to atherosclerosis.
| Acknowledgments |
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Received August 28, 1994; accepted December 12, 1994.
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