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Short Communications |



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From the Departments of Pathology* andBiochemistry and Molecular Biology,¶ University ofOklahoma Health Sciences Center, the Howard Hughes MedicalInstitute
and the CardiovascularBiology Research Program,|| Oklahoma Medical ResearchFoundation, Oklahoma City, Oklahoma; the Department ofPathology,
University of Texas SouthwesternMedical Center, Dallas, Texas; and the United States and CanadianAcademy of Pathology,
Atlanta, Georgia
| Abstract |
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Thrombomodulin (TM) and the endothelial cell protein C receptor (EPCR) are known critically important molecules in control of the protein C anticoagulant pathway. The physiological importance of these endothelial cell receptors is demonstrated by the observation that gene disruption of either results in early embryonic lethality in mice.2,3 Maximal rates of protein C activation require thrombin binding to TM as well as protein C binding to EPCR.4 The formation of the thrombin-TM complex acts as a molecular switch to limit the thrombus-generating function of thrombin and to prevent cellular activation via protease-activated receptors including activation of the endothelium. Activated protein C and TM have been shown to exhibit anti-inflammatory properties.4 Of particular relevance to the situation in the coronary artery, TM overexpression has been shown to reduce thrombus formation, neointima formation, and macrophage and neutrophil infiltration into mechanically dilated regions of rabbit femoral arteries.5 In support of this hypothesis, patients with heterozygous TM deficiencies have been reported to have an increased risk of early myocardial infarction.4
In cell culture, TM and EPCR expression can be down-regulated by
inflammatory cytokines exemplified by tumor necrosis factor
(TNF)-
.6
It is known that plasma TNF-
levels are
elevated in patients with myocardial infarction.7
Based on
these observations, we postulated that TM and EPCR expression might be
reduced in atherosclerotic coronary arteries. To test this hypothesis,
we have analyzed endothelial TM and EPCR expression by
immunohistochemistry in coronary arteries with and without
atherosclerotic lesions.
| Materials and Methods |
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A total of six patients with severe coronary artery disease and
ischemic cardiomyopathy (ICMP) [mean (SD) age, 53 ± 7 years;
four males and two females] who underwent allograft heart
transplantation at the University of Oklahoma Health Sciences Center
between 1993 and 1996 were included in the study. The hearts were
immersion-fixed and both the left and right coronary arteries were
extensively sampled. Three segments each from the left descending
coronary artery and the right coronary artery with either stable or
vulnerable atherosclerotic plaques were chosen for immunohistochemical
studies. Each of these coronary arteries were associated with >50%,
but <90%, luminal narrowing. Age matched autopsy cases
(n = 6) with no or only insignificant coronary
atherosclerosis (no atherosclerotic plaques or occasional plaques with
<25% luminal narrowing present) served as the first group of
controls. From these control cases, segments of coronary arteries
showing no atherosclerotic lesions were chosen for immunohistochemical
evaluation. A second control group included five patients with mild
overall coronary atherosclerosis, ie, with atherosclerotic coronary
lesions causing <50% luminal narrowing. The age and sex distribution,
as well as the cause of the death of the patients in the control groups
is listed in Table 1
. None of the control
cases had any clinical or morphological evidence of coronary
insufficiency and/or thrombosis. The possibility that some of the
underlying conditions leading to death, such as inflammatory disorders
or malignancy did alter the endothelial TM and/or EPCR expression in
the coronary arteries cannot be excluded. However, many of these
disorders are associated with increased cytokine production. Because
EPCR and TM can be down-regulated by cytokines, it is likely that the
conditions leading to death in the controls would decrease the
expression levels for these receptors. All of the control cases were
autopsied within 16 hours of death. Sampling and fixation of the
coronary arteries from the control cases was similar to that outlined
for the hearts of the ICMP cases. From the control cases, segments of
coronary arteries showing no atherosclerotic lesions were chosen
for immunohistochemical evaluation. Only such samples were selected in
which the morphological integrity of the endothelial cells was
confirmed by the hematoxylin and eosin-stained sections.
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Formalin-fixed paraffin-embedded sections were immunostained for TM, EPCR, and endothelial cell markers (CD31 and CD34) using the streptavidin-biotin-peroxidase method.8 In brief, 3-µm sections were incubated with 3% hydrogen peroxide to quench endogenous peroxidase activity. The sections were then incubated for 1 hour at room temperature with mouse monoclonal antibodies to detect TM (TM 1009, 7.8 µg/ml), EPCR (1489, 0.2 mg/ml),9 CD31 (DAKO, Carpinteria, CA), and CD34 (DAKO) expression, respectively. Microwave heat-induced antigen retrieval in citrate buffer, pH 6.0, was required for optimal staining with the anti-CD31 and anti-CD34 antibodies.10 Primary antibody incubation was followed sequentially by biotinylated horse anti-mouse antibody (Vector Laboratories, Burlingame, CA) for 20 minutes, then streptavidin-peroxidase complex (DAKO) for 30 minutes. For negative controls, a monoclonal mouse IgG1 (Bethyl Laboratories Inc., Montgomery, TX) was used at equivalent concentration. Diaminobenzidine was used as chromogen and hematoxylin was used for nuclear counterstain. Each section was assessed by severity of atherosclerosis: severe, >75% luminal narrowing because of atherosclerotic plaque; moderate, 50 to 75% luminal narrowing; and mild, <50% luminal narrowing.
| Results |
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In the left descending coronary arteries of the ICMP group, luminal narrowing was 75 to 90% in 5 samples and 50 to 74% in 13 other samples. In the right coronary arteries of the ICMP group, luminal narrowing was 75 to 90% in 6 samples and 50 to 74% in 12 other samples. None of the plaques was either ulcerated or associated with thrombosis. In five cases, the atherosclerotic plaques had large lipid cores and a thin fibrous capsule.11 In the control group with mild coronary atherosclerosis but no clinical or morphological evidence of coronary insufficiency, all of the atherosclerotic plaques were associated with <50% luminal narrowing.
Immunoperoxidase Studies for CD31 and CD34
The endothelial cell staining intensity of both CD31 and CD34
appeared to be lower in the coronary arteries with significant
atherosclerosis of ICMP cases than in either of the controls (Figures 1 and 2)
.
Semiquantitative analysis confirmed this with good concordance between
the two independent reviewers. In vessels where the sclerotic plaques
were not circumferential, the endothelial staining of CD31 and CD34
appeared to be substantially weaker in the atherosclerotic portions of
the vessel wall.
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In normal control coronary arteries, the endothelial staining for
both EPCR and TM was moderate to strong and uniform. Small arterioles
serving as internal controls in the normal myocardium showed moderate
to strong endothelial staining for both EPCR and TM, similar to that
seen in the large coronary arteries. Staining in arteries with severe
atherosclerosis was uniformly less intense (Figures 3, 4, 5, and 6)
than in those vessels with either mild atherosclerosis or without
atherosclerosis. These differences were compared semiquantitatively
(Table 2)
. No significant differences in
staining intensity were observed between segments from the left
descending coronary arteries versus right coronary arteries
in the atherosclerotic vessels of the ICMP cases. Furthermore, no
significant differences were observed in either EPCR or TM expression
in the ICMP cases between arterial segments with more severe
versus less severe atherosclerosis (ie, arterial segments
with 75 to 90% stenosis versus segments with 50 to 75%
stenosis). In vessels where the sclerotic plaques were not
circumferential, the endothelial staining intensity of EPCR and TM was
substantially weaker in the atherosclerotic regions of the vessel wall.
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| Discussion |
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The reduction in endothelial cell TM and EPCR expression demonstrated here provides an example of a shift favoring thrombosis. Although the present study cannot directly demonstrate that reduction in TM and EPCR expression contributes causally to coronary thrombosis, this possibility is supported by a growing number of genetic and animal studies. Monkeys fed an atherogenic diet generate lower levels of activated protein C than control monkeys when both groups are infused with low levels of thrombin.12 Patients with heterozygous TM deficiency have been reported to have an increased risk of myocardial infarction13 and specific polymorphisms in the TM gene have been associated with an increased risk of myocardial infarction.14 Chimeric mice have been produced in which specific vascular regions were made TM-deficient. In these mice, fibrin deposition was observed over the deficient regions.15 In preliminary clinical studies of EPCR deficiency, patients with EPCR deficiency seemed to have an increased incidence of myocardial infarction.16 Assuming that the same percent reduction of TM and EPCR levels would occur in the heterozygous patients as occurred in the patients studied here, the heterozygous patients would be nearly devoid of TM or EPCR at the atherosclerotic sites in the vasculature. This situation would approach that seen in the chimeric mice where the TM-deficient endothelium was overlaid with fibrin. The reduction seen in EPCR levels may be particularly favorable to fibrin formation because the rate of protein C activation increases with increasing EPCR concentration even when EPCR is in excess over TM.17
From the rabbit experiments in which TM was overexpressed in injured common femoral arteries,5 it can be inferred that TM plays an important role in preventing leukocyte migration into the vessel wall. Therefore, down-regulation of TM and possibly EPCR would be likely to facilitate the leukocyte influx into the plaques. Activated inflammatory cells have been shown to increase the decay of the plaque cap leading to plaque rupture.18
Because of the retrospective nature of the study, we could not address
the question of the mechanism of endothelial down-regulation of TM and
EPCR. The observation that other endothelial markers, such as CD31 and
CD34, also show decreased expression over atherosclerotic plaques
suggests that EPCR and TM may be nonselectively down-regulated in these
regions. Active processes may contribute to TM and EPCR down-regulation
also. Inflammatory mediators generated locally, such as TNF-
, could
down-regulate both TM and EPCR by blocking gene
transcription.6,19
Alternatively, adherence and
degranulation of neutrophils can lead to proteolytic release of TM from
the endothelium. Endothelial cell stimulation by either thrombin or
interleukin-1ß (IL-1ß) can activate an endothelial cell
metalloproteinase that results in EPCR shedding.20
Because
TNF-
can induce IL-1 secretion from endothelium21
and
this in turn can induce EPCR shedding, the elevated TNF-
levels seen
in myocardial infarction patients could reduce EPCR expression locally
by increasing IL-1. Finally, hypoxia that could result from
cardiomyopathy has been shown to decrease TM expression in cell
culture.22
Regardless of the mechanisms involved, the
present study indicates that EPCR and TM densities are severely
decreased on endothelium overlying atherosclerotic plaques.
Like TM and EPCR, endothelial cell nitric oxide synthase is down-regulated in endothelium overlying the atherosclerotic plaque.23 Not all endothelial cell proteins, however, are down-regulated at sites of atherosclerosis. Other studies have demonstrated previously that adhesion molecules exemplified by ICAM-124 and P selectin25 are expressed at higher levels on the endothelium overlying atherosclerotic plaque. These changes are likely to work in concert to favor thrombosis and the adhesion of leukocytes and platelets.
Preliminary results from our laboratory indicate that EPCR may have an immunoregulatory function.26 Soluble EPCR has been shown to interact with leukocytes27 and blockage of EPCR in a baboon model of sepsis resulted in increased leukocyte infiltration into the tissues.28 One can hypothesize that decreased endothelial expression of EPCR may contribute to the inflammatory cell influx of the atherosclerotic plaques. By reducing the anticoagulant and anti-inflammatory activity of the endothelium overlying the atherosclerotic plaque, the down-regulation of EPCR and TM could facilitate both plaque rupture and increase the size of the resultant thrombus.
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| Footnotes |
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Accepted for publication May 4, 2001.
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