(Circulation. 2001;103:3047.)
© 2001 American Heart Association, Inc.
Brief Rapid Communications |
From the Atherosclerosis Research Center, the Division of Cardiology, Cedars-Sinai Medical Center and UCLA School of Medicine, Los Angeles, Calif and Esperion Therapeutics Inc, Ann Arbor, Mich (C.L.B., S.D.).
Correspondence to Prediman K. Shah, MD, Cedars-Sinai Medical Center, Room # 5347, 8700 Beverly Boulevard, Los Angeles, CA 90048. E-mail shahp{at}cshs.org
| Abstract |
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Methods and ResultsHigh cholesterolfed, 26-week-old apoE-deficient mice received a single intravenous injection of saline (n=16), 1080 mg/kg dipalmitoylphosphatidylcholine (DPPC; n=14), or 400 mg/kg of recombinant apoA-Im complexed with DPPC (1:2.7 weight ratio; n=18). Blood was sampled before and 1 and 48 hours after injection, and aortic root plaques were evaluated for lipid content and macrophage content after oil-red O and immunostaining, respectively. One hour after injection, the plasma cholesterol effluxpromoting capacity was nearly 2-fold higher in recombinant apoA-Imtreated mice compared with saline and DPPC-treated mice (P<0.01). Compared with baseline values, serum free cholesterol, an index of tissue cholesterol mobilization, increased 1.6-fold by 1 hour after recombinant apoA-Im injection, and it remained significantly elevated at 48 hours (P<0.01). Mice receiving recombinant apoA-Im had 40% to 50% lower lipid content (P<0.01) and 29% to 36% lower macrophage content (P<0.05) in their plaques compared with the saline- and DPPC-treated mice, respectively.
ConclusionsA single high dose of recombinant apoA-Im rapidly mobilizes tissue cholesterol and reduces plaque lipid and macrophage content in apoE-deficient mice. These findings suggest that this strategy could rapidly change plaque composition toward a more stable phenotype.
Key Words: apolipoproteins cholesterol atherosclerosis
| Introduction |
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| Methods |
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Tissue Preparation and
Histological Analysis
After anesthesia with enflurane, mice
were euthanized, and their hearts and aortas were perfusion-fixed and
harvested as described
previously.3 The lipid and
macrophage contents of aortic root plaques was measured as
described in detail
elsewhere.3
Statistical Analysis
Data are presented as mean±SD. For group
comparisons, ANOVA was followed by Tukeys test, with
P
0.05 considered
significant.
| Results |
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Circulating Cholesterol and
ApoA-Im Levels
Circulating cholesterol levels before
treatment were similar in the 3 groups. One hour after injection, the
total, free, and esterified cholesterol levels increased
significantly in mice receiving recombinant
apoA-Im, and they remained elevated at 48 hours.
There was no significant change in these levels in mice receiving
saline alone. There was a modest increase in total and free
cholesterol levels at 1 hour after administration of DPPC,
with a return to baseline levels by 48 hours
(Table
).
High-performance gel-filtration chromatography
showed that 1 hour after the administration of recombinant
apoA-Im, the increase in free
cholesterol was associated with the HDL fraction, but by 48
hours, it was associated with LDL and VLDL fractions
(Figure
,
A). ApoA-Im levels averaged 477 mg/dL at 1 hour
but dropped to 55 mg/dL at 48 hours.
|
Lipid Content in Aortic Sinus Plaque
The lipid content in the aortic sinus plaque was 40%
to 50% less in mice receiving recombinant
apoA-Im compared with mice receiving saline
alone or DPPC alone (P<0.001;
Table
and
Figure
,
B).
Macrophage Content in Aortic Sinus
Plaque
The macrophage content was 29% to 36% less in
mice receiving recombinant apoA-Im compared with
mice receiving saline (P<0.05;
Table
and
Figure
,
C).
| Discussion |
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Stimulation of Tissue Cholesterol
Mobilization by ApoA-Im
The favorable effects of HDL and apoA-I have been
attributed, in part, to the promotion of reverse
cholesterol
transport.7 One of the first
steps in reverse cholesterol transport is the mobilization
of free cholesterol from tissues, including the
arterial wall, by nascent phospholipid-rich but
cholesterol-poor apoA-Icontaining
lipoproteins.7 According to a
recent model, apoA-I acts as an acceptor and the phospholipid component
of HDL acts as a sink for the mobilized
cholesterol.8
Thus, the complex of recombinant apoA-Im and
DPPC simulates a nascent HDL-like particle with the ability to mobilize
cholesterol from peripheral tissues. A rapid
increase in circulating free cholesterol levels within 1
hour of the administration of recombinant
apoA-Im, which was largely HDL-associated, is
consistent with the stimulation of cholesterol
mobilization from peripheral tissues into the circulating
blood. These observations are further supported by our in vitro
findings showing a significantly higher cholesterol
effluxpromoting capacity in the plasma of mice receiving recombinant
apoA-Im compared with mice receiving saline or
DPPC alone. The association of increased cholesterol levels
with the LDL and VLDL fraction observed at 48 hours after recombinant
apoA-Im injection may be related to the transfer
of mobilized cholesterol to these lipoproteins or enhanced
uptake and secretion of cholesterol from the
liver.
Although we have not analyzed all of the potential tissues from which cholesterol mobilization is stimulated by recombinant apoA-Im, our findings of a significant reduction in lipid content in aortic atheromatous lesions suggests that at least part of the mobilized cholesterol is originating from the vessel wall. Recently, a bolus dose of human HDL was shown to stimulate cholesterol efflux from the heart, skeletal muscle, lung, spleen, and liver but not from the brain in normal mice.9 Accumulation of lipid and its subsequent modification within atherosclerotic plaques is thought to play a major role in the recruitment and retention of inflammatory cells in atherosclerosis, and thus a reduction in the plaque lipid may explain the significant decrease in macrophage immunoreactivity after recombinant apoA-Im administration.10
It is likely that most of the mobilized cholesterol is delivered to the liver because the liver plays an important role in eliminating cholesterol through biliary sterol excretion. Enhanced cholesterol mobilization in normal subjects and increased fecal sterol excretion in hyperlipidemic patients after HDL administration has recently been demonstrated.11 12
In the present study, DPPC alone had a small effect, although phospholipid-liposomes have been shown to regress fatty streaks in rabbits.13 Because enhanced reverse cholesterol transport by phospholipid liposomes is facilitated by HDL, modest effects of DPPC alone may have resulted from the low endogenous HDL levels in apoE-deficient mice in a manner consistent with the proposed model of HDL-mediated cholesterol efflux.8
Potential Limitations
Additional studies are required to determine the
precise sources and the fate of the mobilized cholesterol
and to compare the effects with wild-type apoA-I.
Potential Clinical Implications
Although statin therapy favorably alters plaque
composition, such changes occur after weeks and months of
therapy.14 15 By
rapidly mobilizing vessel wall lipids, recombinant
apoA-Im has the potential to stabilize plaques
in the
short-term.14
| Acknowledgments |
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| Footnotes |
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Received March 16, 2001; revision received May 1, 2001; accepted May 1, 2001.
| References |
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