Integrative Physiology |
From the First Department of Internal Medicine, Niigata University School of Medicine (M.I., M.K., M.M., M.Y., K.F., Y.U., S.H., Y.O., K.K., Y.A.), and Research Laboratory for Molecular Genetics, Niigata University (Y.H., T.H., R.K.), Niigata, Japan.
Correspondence to Masahiro Ito, MD, First Department of Internal Medicine, Niigata University School of Medicine, 1-757 Asahimachi, Niigata 951-8510, Japan.
| Abstract |
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Key Words: coxsackievirus and adenovirus receptors myocarditis cardiomyocytes
| Introduction |
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CAR structurally belongs to the immunoglobulin superfamily, similar to intercellular adhesion molecule-1 (ICAM-1) and neural cell adhesion molecule (N-CAM). Both ICAM-1 and N-CAM have been demonstrated to be related to either the normal development of the heart or the pathogenesis of cardiovascular diseases.5 6 7 The physiological functions of CAR have not yet been elucidated.3 Whether CAR plays a role in left ventricular remodeling after myocardial injury has not yet been determined. In the present study, we demonstrated that CAR was reexpressed in the hearts of adult rats with rat experimental autoimmune myocarditis, which is an animal model of human giant cell myocarditis leading to dilated cardiomyopathy. Furthermore, we examined the expression of CAR in cultured rat cardiomyocytes supplemented with conditioned medium of rat splenocytes activated by concanavalin A (Con A) to elucidate the mechanisms of the induction of CAR, accompanied by inflammation.
| Materials and Methods |
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Sampling
Newborn rats were killed on day 1 after birth, and their hearts
were removed. Myosin-immunized rats were killed on days 9, 12, 15, 18,
21, 24, 27, 30, 33, 36, 39, 42, 49, and 56 after immunization.
Macroscopic findings were scored in 3 grades as previously
described.8 Hearts were then removed and weighed to
calculate the heart weight/body weight ratio. The
ventricular muscle was used for the following
analysis.
Preparation of Antibody
Seventeen amino acid residues (KTQYNQVPSEDFERAPQ) in an
intracellular domain of CAR were selected. Antibodies against CAR were
prepared as previously described.9
Histopathology and Immunohistochemistry
CAR was immunohistochemically detected by use of purified rabbit
anti-murine CAR antibodies. Positive findings for CAR produced a brown
color. The severity of inflammation was graded from 0 (no inflammation)
to 3 (severe inflammation) as previously described.10 The
expression of CAR was graded as follows: -, negative; ±, weakly
positive; +, moderately positive; and ++, strongly positive.
RT-PCR Analysis
Total RNA was extracted from rat hearts, and reverse
transcription (RT)polymerase chain reaction (PCR) was performed with
primers (ATGGATCCTACACCCGAACAGAGGATCG [sense] and
GCGAATTCGCGTCGCCAGACTTGACAT [antisense]). ß-Actin cDNA was
amplified as the control. Thirty-three cycles of amplification
reactions (94°C denaturation, 53°C annealing, and 72°C extension)
were performed.
Preparation of Conditioned Medium of Rat Splenocytes
Activated by Con A
Single-splenocyte suspensions of adult rat11 were
cultured for 2 days in the presence of 10 µg/mL of Con A, and the
supernatant was obtained (Con A medium).
Preparation of Cultured Cardiomyocytes
Cardiomyocytes were prepared from neonatal rat ventricles by the
modified method of Libby.12 The cardiomyocytes
were divided into 4 groups. Group 1 was not cultured. Group 2 was
cultured for 24 hours. Groups 3 and 4 were cultured for 96 hours. Group
4 was cultured in standard medium for an initial 24 hours and then
cultured in Con A medium (10% of culture medium) for 72 hours.
Western Blot Analysis
Total protein was prepared from the left ventricle of 1-day-old
and adult rats and from cultured cardiomyocytes with cell
lysis buffer. Protein (15 µg) from total cell lysates13
was separated by SDS-PAGE and analyzed by
immunoblotting with the polyclonal anti-CAR antibodies.
The bands of CAR were analyzed with NIH Image and expressed in
relation to the control values.
Statistical Analysis
Data are presented as mean±SD. Statistical assessment
of the significance among groups was made by 1-way ANOVA followed by
the Bonferroni-Dunn method. A value of P<0.05 was
considered significant.
An expanded Materials and Methods section is available online at http://www.circresaha.org.
| Results |
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Expression of CAR in Rat Hearts With Myocarditis
Clinical Course of Myosin-Immunized Rats
One of the myosin-immunized rats died on day 18, during the active
phase of myocarditis. This rat showed macroscopic evidence of severe
myocarditis and was thought to have died of heart failure. No control
rats died spontaneously throughout the period of this experiment
(Table
).
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Macroscopic Findings
The extent of discolored areas of the hearts and the appearance of
pericardial effusion are summarized in the Table
. From the 15th
day after immunization, discolored areas were observed on the cardiac
surface of the myosin-immunized rats. Hearts of the myosin-immunized
rats were markedly enlarged, as shown by heart weight/body weight
ratios >4.0; control rat heart weight/body weight ratios ranged from
2.89 to 3.83, with a mean of 3.16. Pericardial effusion was observed in
myosin-immunized rats on days 18, 21, and 24 (Table
).
Histological Study
Microscopic findings are summarized in the Table
. There
were no microscopic abnormalities in the hearts of either the control
rats or the myosin-immunized rats on day 9. Small lesions of
mononuclear cell infiltration were observed in the hearts of the
myosin-immunized rats on day 12. Thereafter, inflammatory lesions
spread extensively, and interstitial edema became evident.
From day 21, in addition to mononuclear cell infiltration, myocardial
necrosis was observed. Fibrosis became prominent on day 36. Most of the
infiltrated cells disappeared on day 49, and the inflammatory lesions
were replaced by fibrosis.
RT-PCR Analysis
CAR mRNA was identified in the hearts of the myosin-immunized rats
on days 18 to 30 and peaked on days 21 and 24. CAR mRNA was not
detectable after day 33 by RT-PCR (Figure 3
). CAR mRNA was not detected in the
control group throughout the experiment by our RT-PCR (data are not
shown).
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Immunohistochemical Analysis
Immunohistochemical staining for CAR was negative in the hearts of
control rats. Hearts of the rats with experimental autoimmune
myocarditis were stained with anti-CAR antibodies from day 24.
Immunoreactivity of CAR was found in the cardiomyocytes of
rats with experimental autoimmune myocarditis, not only in the
inflammatory lesions but also in the intact area, where infiltration
cells and myocardial degeneration were not found. Scar tissue and the
vascular wall were not stained. The staining was most intense in the
hearts obtained on day 33, after which immunoreaction for CAR gradually
decreased (Figure 4
).
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Induction of CAR in Cultured Cardiomyocytes
Although CAR was expressed abundantly in the hearts of the newborn
rats, only a slight expression of CAR was detected in cultured rat
cardiomyocytes after isolation from the newborn rats by
Western blot analysis (Figure 5A
). The expression of CAR was suppressed
after isolation, but then it gradually increased according to the
creation of clusters and beating of cardiomyocytes.
Furthermore, the induction of CAR was more significantly enhanced in
cultured cardiomyocytes treated with Con A medium than
those treated with culture medium (Figure 5B
).
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| Discussion |
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Rat experimental autoimmune myocarditis is inducible by purified cardiac myosin and is an animal model of human giant cell myocarditis, which leads to dilated cardiomyopathy. 17 T cells play an important role in inducing myocarditis in this model.11 We examined the expression of CAR in experimental autoimmune myocarditis of rats to clarify whether CAR was reenhanced in diseased hearts. In the course of experimental autoimmune myocarditis, the expression of CAR in the hearts was low or undetectable before the onset of the disease, and then its expression became apparent during the active phase of myocarditis. Next, the expression of CAR decreased in the chronic phase. ICAM-1, which also belongs to the immunoglobulin superfamily, was also induced in myocardial cells of mice with acute viral myocarditis.7 Its expression coincided with cell infiltration. ICAM-1 on cardiac myocytes, together with myosin heavy chain antigens, was demonstrated to be associated with cell-mediated myocardial injuries by activated lymphocytes.7 On the other hand, the expression of rat CAR was preceded by several days of massive cell infiltration. Accordingly, the expression of CAR in myocardial cells might not be related to myocardial injuries in acute myocarditis. CAR may be associated with the healing phase of myocarditis or regeneration of damaged myocardium.
The mechanism responsible for the induction of CAR had not been
elucidated. In the present study, CAR was induced when myocytes
were treated with conditioned medium of rat splenocytes
activated by Con A. This result implied that the expression of
CAR was induced by inflammatory mediators. We previously revealed the
time course of the expression of various cytokine mRNA in
experimental autoimmune myocarditis.18 19 mRNA of
proinflammatory cytokines, such as interferon-
, tumor
necrosis factor-
, and interleukin-1ß, and inducible nitric oxide
synthase were expressed in the myocardium of experimental
autoimmune myocarditis from the beginning of the acute inflammatory
phase. CAR mRNA was reexpressed after the expression of these
cytokines. The present study suggests that inflammatory
mediators play a role in the induction of CAR in experimental
autoimmune myocarditis. Further study is necessary to identify the
specific mediators inducing CAR.
In conclusion, the expression of CAR was enhanced in experimental autoimmune myocarditis. Our results suggest that CAR is induced by inflammatory mediators. This molecule may contribute to cell-to-cell contact and to the adhesion of cardiomyocytes and act on myocardial regeneration.
Received August 19, 1999; accepted November 5, 1999.
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