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Thrombosis |
From the Cardiovascular Research Center (D.T.E., Z.X.) and Division of Molecular Medicine and Human Genetics (D.G.), Department of Internal Medicine, and the Howard Hughes Medical Institute (R.J.W., J.T., D.G.), University of Michigan Medical Center, Ann Arbor.
| Abstract |
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Key Words: carotid artery transgenic mice cholesterol plaque rose bengal
| Introduction |
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We have developed a model of atherosclerotic plaque disruption with the use of atherosclerosis-prone mice that leads to occlusive arterial thrombosis. This model was used to analyze the effect of high fat feeding on the time to occlusive thrombosis.
| Methods |
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Induction of Carotid Arterial Thrombosis at Site
of Atherosclerosis
Male apoE-/- mice
(aged 6 and 30 weeks) were anesthetized with 1.5 mg
intraperitoneal sodium pentobarbital (Butler). Mice
were then secured in the supine position and placed under a dissecting
microscope (Nikon SMZ-2T, Mager Scientific, Inc). After a midline
cervical incision, the right common carotid artery was isolated, and a
Doppler flow probe (model 0.5 VB, Transonic Systems) was applied.
The probe was connected to a flowmeter (Transonic model T106) and
interpreted with a computerized data acquisition program (Windaq, DATAQ
Instruments). Rose bengal (Fisher Scientific) was diluted to 10 mg/mL
in PBS and then injected into the tail vein in a volume of 0.12 mL at a
concentration of 50 mg/kg by use of a 27-gauge Precision Glide needle
with a 1-mL latex-free syringe (Becton Dickinson and Co). Just before
injection, a 1.5-mW green light laser (540 nm, Melles Griot) was
applied immediately proximal to the carotid bifurcation from a distance
of 6 cm for 60 minutes or until occlusive thrombosis occurred. Flow in
the vessel was monitored continuously from the onset of
injury.8
Plasma Lipid Assays
Blood from mice fasted overnight was collected by retro-orbital
bleeding into heparin-coated capillary tubes. Plasma was retrieved
after centrifugation for 10 minutes at
5000g. Plasma lipid levels were assayed within 4 hours of
collection by use of a Vitros analyzer (Ortho
Diagnostics, Inc). Samples for total
cholesterol determination were diluted 5-fold, and
triglycerides levels were measured from undiluted
plasma.
Platelet Aggregation
Whole blood was obtained from the inferior vena cava
of mice anesthetized with pentobarbital. Platelet-rich
plasma was obtained as previously described.9 Samples were
simultaneously isolated and analyzed in pairs, 1
from each group. Platelet reactivity was assessed in response to
ADP by use of a Platelet Aggregation Profiler (BIODATA Corp)
according to the manufacturers instructions. Aggregation baselines
(100% and 0%) were set with corresponding platelet-poor and
platelet-rich plasmas.
Histology
To confirm plaque-associated thrombus, carotid
arterial segments subjected to injury were excised and
embedded in paraffin. Sections were then stained with hematoxylin
and eosin.
Statistical Analysis
The significance of differences between groups was determined by
the Student t test. A value of P<0.05 was
considered significant.
| Results |
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Effect of High Fat Feeding on Plasma Lipids and Time to Occlusive
Thrombus Formation
To determine whether the development of carotid artery occlusion
at sites of atherosclerosis was affected by acute
changes in diet, the mice were either maintained on normal chow or fed
high fat chow for 5 days before carotid injury. This dietary
manipulation increased total cholesterol levels from
587±24 mg/dL (mean±SEM, n=5) to 921±77 mg/dL (n=5,
P<0.005), with no significant change in
triglyceride levels (Figure 2A
). Nine
apoE-/- mice that were
maintained on normal chow formed occlusive thrombus at the site of
light application a mean of 44±5 minutes after initiation of injury,
and 6 apoE-/- mice fed
high fat chow developed occlusive thrombosis a mean of 27±3 minutes
after injury (Figure 2B
, P<0.02). All 30-week-old
animals presented grossly evident lesions involving the carotid
bifurcation, which were confirmed by histology and appeared to be of
similar size. Histological analysis immediately
after injury revealed occlusive thrombi that appeared similar between
the groups. To determine whether the effect of high fat feeding on the
time to occlusive thrombosis was dependent on the presence of an
atherosclerotic plaque, 6-week-old
apoE-/- mice that lack
overt atherosclerosis10 were
subjected to the same dietary manipulation and photochemical injury.
The time to occlusion was 78±8 minutes in mice fed normal chow (n=6)
and 63±4 minutes in mice fed high fat chow (n=6) for 5 days before
injury (P=0.13).
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Effect of High Fat Feeding on Platelet Aggregation
To determine whether the shortened time to occlusion after high
fat feeding was associated with enhanced platelet reactivity,
platelet aggregation studies were performed on 30-week-old
apoE-/- mice with and
without the high fat dietary manipulation. Platelet counts were
similar between the 2 groups, with a mean count of
1.4x109 platelets per milliliter in the high
fat chow group and 1.7x109 platelets per
milliliter in the normal chow group. Platelet counts were adjusted
to 2.5x108 per milliliter by the addition of
citrated platelet-poor plasma. Mean maximal platelet
aggregation in response to 10 µmol/L ADP was increased in mice
after the brief period of high fat feeding (44±11%, n=5) compared
with mice on normal chow (24±10%, n=5; P<0.003).
| Discussion |
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Atherosclerosis-prone mice are a useful model for
assessing the role of various determinants of thrombosis at the site of
an atherosclerotic lesion.
ApoE-/- mice develop
hyperlipidemia and extensive
atherosclerosis, both of which are enhanced by high fat
feeding. Previous studies have demonstrated that high fat chow produces
a marked elevation in plasma cholesterol levels in
apoE-/- mice, from a
baseline of
600 mg/dL for mice on regular chow to a peak of
2700
mg/dL, which plateaus at
2 weeks after the initiation of high fat
feeding.18 Therefore, by dietary manipulation, the effect
of high fat chow can be studied. Photochemical injury is a relatively
noninvasive method of inducing endothelial injury and
thrombosis that is particularly useful for small animals such as
mice.8 19 20 In addition, the endothelial
injury elicited by rose bengal is mediated by a superoxide
anion,21 a type of injury that may occur
endogenously in the progression of
atherosclerosis22 and contribute to plaque
disruption.
Our data indicate that hyperlipidemia induced by high
fat feeding in apoE-/-
mice promotes occlusive thrombus formation after vascular injury. The
brief period of high fat feeding in these experiments produced a
relatively modest elevation of cholesterol, approximately
one third the level attained with more prolonged feeding. This milder
elevation may be more relevant to humans with
hyperlipidemia. These studies indicate that acute
changes in cholesterol are especially relevant in the
setting of an atherosclerotic plaque. Although a trend toward shorter
occlusion times with high fat feeding was noted in young
apoE-/- mice before the
development of overt atherosclerosis, this did not
achieve statistical significance. The occlusion times of young
apoE-/- mice were
actually slightly longer than occlusion times previously reported for
wild-type C57BL/6J mice,8 even though the mean
cholesterol level is
94 mg/dL in wild-type C57BL/6J mice
compared with 587 mg/dL in
apoE-/- mice maintained
on regular chow.18 Although we cannot exclude an
additional antiplatelet effect of apoE deficiency, this supports
the hypothesis that thrombi forming at the site of a preexisting
atherosclerotic plaque are more susceptible to the effects of acute
lipid alterations than thrombi forming in normal arteries. However, it
is also possible that a more prolonged period of high fat feeding would
produce a greater effect on nonatherosclerotic vessels.
The mechanism of these acute changes in plasma cholesterol on the development of thrombosis is unclear. It may be that the enhanced platelet aggregation observed in these studies after high fat feeding is especially relevant in the thrombotic milieu of an atherosclerotic plaque. The enhanced thrombogenicity of the plaque is apparent by the shortened occlusion times in 30-week-old versus 6-week-old mice, regardless of diet. Additional mechanisms may also be playing important roles. For example, monocyte-derived macrophages, which accumulate in atherosclerotic plaques, express tissue factor that is positively regulated by oxidized LDL and cholesterol.23 In addition, LDL, which increases in apoE-/- mice after high fat feeding,18 has been shown to bind and inhibit the anticoagulant function of an important regulator of tissue factor, tissue factor pathway inhibitor.24 Thus, acute changes in LDL could lead to enhanced tissue factor expression, which would be particularly relevant at the site of an atherosclerotic plaque.
Although there are important differences in the hyperlipidemia observed in this mouse model compared with common human hyperlipidemia, this model may be useful in further elucidating the factors involved in atherothrombosis.
| Acknowledgments |
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| Footnotes |
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Received February 28, 2000; accepted April 5, 2000.
| References |
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