Integrative Physiology |
From the Gaubius Laboratory (J.H.P.L., D.J.M.D., M.R.d.V., H.M.G.P., L.M.H., V.W.M.v.H., P.H.A.Q.) and Leiden University Medical Center (J.H.P.L., M.M.L.D., L.M.H., J.H.v.B.), Leiden, the Netherlands.
Correspondence to Dr P.H.A. Quax, Gaubius Laboratory TNO-PG, PO Box 2215, 2301 CE Leiden, the Netherlands. E-mail pha.quax{at}pg.tno.nl
| Abstract |
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smooth muscle cell actinpositive cells, was
observed after 14 days. A mildly cholesterol-rich diet
(mean plasma-cholesterol level, 10.5 mmol/L) resulted
in a 2.7-fold increase of total intimal area, and a highly
cholesterol-rich diet (mean plasma cholesterol
level 28.6 mmol/L), in a 7.8-fold increase. In the
high-cholesterol group, the intima consisted predominantly
of lipid-loaded foam cells and
smooth muscle cell actinpositive
cells. Foam cell accumulation could be observed by as early as 3 days,
resulting in a near-total occlusion of the lumen after 14 days.
Hypercholesterolemia resulted in a rapid,
cholesterol-dependent induction of foam cellrich intimal
hyperplasia in cuffed femoral arteries of ApoE*3Leiden mice. In
conclusion, the present data show that the combination of a local
(cuff placement) and a systemic (hypercholesterolemic)
risk factor of atherosclerosis results in a rapid
induction (within 14 days) of atherosclerotic-like lesions in
ApoE*3Leiden mice.
Key Words: atherosclerosis foam cell accumulation transgenic mice intimal hyperplasia
| Introduction |
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smooth muscle cell actinpositive cells, as is also reported for
rabbits by several other groups.6 8 12 However, the
species (mice, rats, and rabbits) used in these studies were
predominantly young, healthy, and normocholesterolemic
animals. Mice are one of the most useful experimental animals because of their small size, easy maintenance, short breeding time, and the possibilities for carrying out advanced genetic studies such as transgenesis and gene targeting. Transgenic technologies have provided numerous different murine strains to study hyperlipidemia and atherosclerosis. The ApoE*3Leiden transgenic mice13 develop diet-dependent hyperlipidemia and are highly susceptible to diet-induced atherosclerosis. In these mice, when fed a mildly cholesterol-rich, high-fat diet, early fatty-streak formation (stage I, American Heart Association [AHA] classification,14 ) can be observed in the aortic arch after 3 months of treatment, and late complex atherosclerotic lesions, consisting of plaques with a necrotic core and a fibrous cap (stages IV and V, AHA classification14 ), can be registered after 3 to 6 months of feeding a severe cholesterol-rich diet. This animal model, in which the atherosclerotic lesions have many features in common with human atherosclerotic lesions,15 16 is currently considered as one of the animal models of atherosclerosis closest to that occurring in humans.
In the current study, this animal model of cholesterol-induced atherosclerosis has been used to evaluate the effect of plasma cholesterol on the development of intimal thickening by placing a polyethylene cuff around the femoral artery of ApoE*3Leiden mice.
We found that the combination of systemic and local risk factors resulted in an accelerated formation of atherosclerotic-like lesions in mice. After the placement of a nonconstrictive cuff, a plasma cholesterol leveldependent intimal thickening was induced within 14 days. This intimal thickening started within 3 days after cuff placement with monocyte adhesion and foam cell accumulation and progressed until near-total occlusion of the lumen after 14 days.
| Materials and Methods |
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Diets
During the experimental period, animals were fed a chow diet; a
cholesterol-enriched high-fat diet containing 0.5% cholate
to improve intestinal cholesterol uptake and suppress bile
acid synthesis, both leading to increased plasma
cholesterol levels (igh at and
holateenriched [HFC] diet 0.5%: casein 20%, choline
chloride 1%, methionine 0.2%, cocoa butter 15%, cholate 0.5%,
cholesterol 1%, sucrose 40.5%, cornstarch 10%, corn-oil
1%, cellulose 5.1%, and mineral mixture 5.1%); or a
cholesterol-enriched, high-fat diet containing 0.05%
cholate (HFC 0.05%: casein 20%, choline chloride 1%, methionine
0.2%, cocoa butter 15%, cholate 0.05%, cholesterol 1%,
sucrose 40.5%, cornstarch 10%, corn oil 1%, cellulose 4.7%, and
mineral mixture 5.1%) 4 weeks before operation and continued
after operation.
Placement of Cuff
After 4 weeks of chow, HFC 0.05%, or HFC 0.5%, mice were
anesthetized with Hypnorm (fentanyl citrate 0.315
mg/mL/fluanisone 10 mg/mL; Bayer, 25 mg/kg IP) and Dormicum (midazolam
5 mg/mL Roche, 25 mg/kg IP). The right femoral artery was
dissected from its surroundings. A nonconstrictive polyethylene cuff
(Portex, 0.40-mm inner diameter, 0.80-mm outer diameter, and 1.5-mm
length) was placed loosely around the right femoral
artery.6
Lipids and Lipoprotein Analysis
Blood samples were taken under general anesthesia
from the tail vein at the time of operation and euthanization. Total
plasma cholesterol (Boehringer Mannheim GmbH, kit
236691) and triglyceride (Sigma Diagnostics,
kit 337-B) concentrations were measured enzymatically using
commercially available kits.
Histological Assessment of Intimal Lesions
At euthanization, mice were anesthetized with
Hypnorm/Dormicum. The thorax was opened, and mild pressure-perfusion
(100 mm Hg) with 3.7% formaldehyde in 0.9% NaCl (wt/vol) for 10
minutes was performed by cardiac puncture. After perfusion, the femoral
artery was harvested, fixed overnight in 3.7% formaldehyde in PBS, and
paraffin-embedded. Serial cross sections (5 µm thick) were used
throughout the entire length of the cuffed femoral artery for
histological analysis. Cryosections were made
of 2 mice in each condition. All samples were routinely stained with
hematoxylin-phloxine-saffron (HPS). Weigerts elastin staining was
used to visualize elastic laminae. Smooth muscle cells were visualized
with
-smooth muscle cell actin staining (Boehringer
Mannheim), and Mac-3 (Accurate Chemical) macrophage staining
was used to detect monocytes/macrophages. Lipid deposition was
visualized with Oil-red-O (Boehringer Mannheim) staining.
Antiplatelet-endothelial cell adhesion molecule-1
(PECAM)-1 antibodies (Sigma) were used as
endothelial cell marker.
Quantification of Intimal Lesions in Sections of Cuffed
Femoral Artery
Ten equally spaced cross sections were used in all mice to
quantify intimal lesions. Using image analysis software
(Leica), total cross-sectional medial area was measured between the
external and internal elastic lamina; total cross-sectional intimal
area was measured between the endothelial cell
monolayer and the internal elastic lamina.
Statistics
All data are presented as mean±SEM. Overall comparisons
between groups were performed with the Kruskal-Wallis test. If a
significant difference was found, groups were compared with their
respective controls using Mann-Whitney rank-sum tests. A value of
P <0.05 was regarded as significant.
| Results |
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Time Course of Development of Intimal Lesions
To study the effect of
hypercholesterolemia on the development of
intimal hyperplasia in time, 5 mice on the chow diet and 5 mice on the
HFC 0.5% diet were killed at respectively 1, 3, 7, 10, and 14 days
after placement of the cuff. In the animals on a chow diet,
smooth
muscle cell actinpositive cells invaded in the intimal region within
7 days after the placement of the cuff. After 14 days, the intimal
lesion area was
2 to 4 cell layers thick. This is in accordance with
the intimal lesions observed in C57BL/6 and SV 129 mice after cuff
placement on a chow diet published by Moroi et al6
and also found by us (data not shown). However, in the mice fed
the HFC 0.5% diet, as early as 3 days after the placement of the cuff,
intimal thickening was observed on the luminal side of the inner
elastic lamina. This intimal thickening progressed in time until
near-total occlusion of the lumen was achieved after 14 days (Figure 2
). In these
hypercholesterolemic animals, monocyte adhesion to the
endothelial layer was observed as early as 1 day after
the placement of the cuff. After 3 days, monocyte adhesion became more
distinct and the first foam cells could be detected. Seven days after
the cuff placement, macrophages were present in the media,
in addition to the intimal lesion (Figure 3A
). Endothelial
cells, monitored by PECAM-1 expression, were observed 3 and 7 days
after the cuff placement (Figure 3B
).
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Characterization and Quantification of Intimal Lesions
To characterize the effect of
hypercholesterolemia on intimal hyperplasia, 20
male ApoE*3Leiden mice were randomized in 2 groups. One group (n=10)
was fed a chow diet, and 1 group (n=10) was fed a HFC 0.5% diet, 4
weeks before placement of the cuff to obtain stable plasma
cholesterol levels. Fourteen days after placement of the
cuff, light microscopy of transverse sections through the cuffed
femoral artery revealed that in the mice on a chow diet a thickening of
the intimal region occurred (Figure 4
),
whereas proximal and distal sections of the cuffed femoral artery
possessed normal histology (data not shown). This intimal thickening
was
2 to 4 cell layers thick and consisted predominantly of
smooth muscle cell actinpositive cells and to a lesser extent
monocytes/macrophages. An intact endothelial
layer and inner elastic lamina was observed in the arteries 14 days
after cuff placement. No foam cells were detected in any of the
sections taken from the cuffed arteries of
normocholesterolemic animals at 14 days.
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In the group of animals fed a high-cholesterol,
cholate-containing diet (HFC 0.5%) for 4 weeks, atherosclerotic-like
lesions, predominantly consisting of a massive accumulation of
macrophage foam cells in the intimal region, were observed 14
days after the placement of the cuff. At the luminal side of this foam
cell accumulation, deposition of
smooth muscle cell actinpositive
cells partly on top of the macrophages could be observed as a
sign of the start of lesion cap formation. The deposition of foam cells
and smooth muscle cells occurred underneath an apparently intact
endothelial layer and on an intact inner elastic
lamina. These spindle-like macrophage foam cells were
lipid-filled and primarily present in the intimal region but also
to a lesser extent in the medial region.
The abundant accumulation of macrophage foam cells and smooth muscle cells in the mice fed HFC 0.5% resulted in a profound luminal narrowing compared with mice on a chow diet. A near-total occlusion of the lumen was frequently observed in the mice on HFC 0.5% 14 days after placement of the cuff. However, a total occlusion due to intimal hyperplasia or thrombosis never occurred in any of the animals.
Correlation Between Plasma Cholesterol Levels and
Intimal Lesion Development
To evaluate the correlation between serum cholesterol
levels and the development of atherosclerotic lesions in the cuffed
femoral artery, 3 groups of 9 ApoE*3Leiden mice received chow diet, HFC
0.05% diet, or HFC 0.5% diet 4 weeks before the cuff placement. At
the time of the placement of the cuff, these mice had a mean serum
cholesterol level of 2.3±1.2, 13±5.6, and 32±3.2
mmol/L, respectively. Quantification of total intimal areas of 3 groups
of ApoE*3Leiden mice revealed a significant 2.3-fold increase in mean
total intimal area in mice fed HFC 0.05% diet (P<0.001)
and a 7.8-fold increase in mice fed an HFC 0.5% diet
(P<0.001) compared with mice on a chow diet. Additionally,
significant differences in intima/media ratio and percentage luminal
stenoses were observed between mice on the 3 different diets
(Figure 5
). Correlation between plasma
cholesterol levels and intimal thickening of each mouse is
depicted in Figure 6
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| Discussion |
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smooth muscle actinpositive
cells only. In the lesions of these
normocholesterolemic animals, no foam cell
macrophages were registered. Placing a cuff on ApoE*3Leiden
mice on a high-cholesterol diet, however, resulted in an
extremely fast and profound induction of atherosclerotic-like lesions.
These lesions predominantly consisted of lipid-loaded foam cells and to
a lesser extent of
smooth muscle cell actinpositive cells. The
accumulation of spindle-like lipid-filled
macrophage-derived foam cells on the luminal side of the
inner elastic lamina is a typical feature of early human,
postintervention atherosclerosis.14 Two weeks after the placement of the cuff, the atherosclerotic-like lesions that developed in mice on a high-cholesterol diet were abundant and resulted in a near-total occlusion of the lumen. The total area of these lesions was significantly larger (7.8-fold) than lesions developed in mice on a chow diet. Quantification of the total intimal area of mice on different diets (chow and mildly and highly cholesterol rich) revealed that there is a correlation between plasma cholesterol and total intimal growth. This correlation has been described earlier in transgenic mice developing atherosclerosis spontaneously on a high-cholesterol diet after several months.15
The development of the atherosclerotic-like lesions started directly after the placement of the cuff as demonstrated by monocyte adhesion, which was already present after 1 day. The first deposition of lipid-filled foam cells on the luminal side of the inner elastic lamina was observed as early as 3 days after the placement of the cuff.
Both phenomena illustrate the extremely fast initiation of the atherosclerotic process. Also, the total area of these lesions after 14 days is far more extensive than lesions developed spontaneously in transgenic mice after a prolonged period on a high-cholesterol HFC 0.5% diet without the placement of a cuff.16
The exact mechanism by which collar-induced intimal thickening is triggered is still unclear. The disturbance of the laminar blood flow, vascular damage, production of cytokines by the granulation tissue, and elimination of transmural flow by the cuff may account for the intimal thickening.19 In the cuffed artery, shear and wall stress is altered, resulting in endothelial damage and activation.7 This leads to intimal hyperplasia predominantly consisting of smooth muscle cells in normocholesterolemic animals. In hypercholesterolemic animals, however, endothelial activation is accompanied by the accumulation of lipids in the infiltrating monocytes, resulting in excessive foam cell formation. Consistent with this observation is the fact that the predilection sites of spontaneous atherosclerosis in transgenic mice (aortic valve, aortic branch, and bifurcation sites) are sites with disturbance in laminar flow and subsequent damage to the endothelial layer.
Human-like atherosclerotic lesions develop spontaneously in
ApoE*3Leiden mice when fed an atherogenic diet for several months.
However, this process of atherosclerotic lesion development is rather
slow, and it takes
3 to 6 months on a severe cholesterol
diet before type IV and type V lesions (nomenclature of AHA
classification used by Stary et al14 ) can be
observed.15 16 Furthermore, the exact lesion location and
onset of atherosclerosis in time is unknown. These
aspects imply that the analysis of the mechanisms of
atherosclerosis and the evaluation of
antiatherosclerotic pharmacological and nonpharmacological strategies
in this mouse model is a time-consuming process. In addition, because
atherosclerotic lesions, which develop spontaneously in transgenic
mice, are frequently located in the central part of the
arterial tree, local antiatherosclerotic strategies are
difficult to apply. Therefore, the induction of atherosclerotic-like
lesions within several weeks, at a predefined, easily accessible site
in the arterial tree, with a known onset in time, would be
preferable for studying factors causing the early onset of
atherosclerotic-like lesions and for assessing the effect of systemic
and local antiatherosclerotic therapies. As shown, the cuff model for
accelerated atherosclerotic-like lesions is suitable for studying the
early steps in atherosclerotic plaque formation. Moreover, the
accelerated atherosclerosis observed in human vein
grafts, with less organized structures lacking typical features such as
the fibrous cap and necrotic core, shows high morphological resemblance
to the atherosclerotic lesions observed in this
model.20
The approach of induction of intimal thickening by placement of a
nonocclusive perivascular cuff in mice, as described above, has several
characteristics in common with collar-induced intimal thickening in
rabbits. As in rabbits, in mice, intima formation occurs without
endothelial cell denudation. However, the intimal
lesions in ApoE*3Leiden mice on a high-cholesterol diet
predominantly consist of lipid-loaded foam cells, whereas
collar-induced lesions in Watanabe rabbits mainly consist of
smooth
muscle cell actinpositive cells.8
In conclusion, in the present study we demonstrated that, by the placement of a nonconstrictive polyethylene cuff around a femoral artery of ApoE*3Leiden mice, highly reproducible, accelerated, atherosclerotic-like lesions can be induced. These lesions display human characteristics, especially in the early phase, and are dependent on plasma cholesterol level. This accelerated, diet-dependent induction of atherosclerotic-like lesions is of value in studying the mechanisms of accelerated atherosclerosis.
| Acknowledgments |
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Received May 15, 2000; accepted June 9, 2000.
| References |
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