(Circulation. 2001;103:2514.)
© 2001 American Heart Association, Inc.
Basic Science Reports |
From the Departments of Medicine (R.R.S., A.A., P.J.C.), Physiology (J.H.), Pathology (M.J.S., E.A., J.Z., M.M.), and Surgery (N.M.), Columbia University College of Physicians and Surgeons, New York, NY.
Correspondence to Paul J. Cannon, MD, Department of Medicine, Division of Cardiology, Columbia University, 630 W 168th St, New York, NY 10032. E-mail pjc4{at}columbia.edu
| Abstract |
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Methods and ResultsIn the first experiments, hearts from C3H donor mice were transplanted into NOS-2-/- and NOS-2+/+ C57BL/6J.129J recipients. A second series of experiments included NOS-2-/- donor hearts transplanted into NOS-2-/- recipients and wild-type NOS-2+/+ donor hearts transplanted into wild-type NOS-2+/+ recipients. (All donors were C57BL/6J and recipients were C57BL/6J.129J.) In the first series of experiments, no significant differences were observed in allograft survival, rejection score, total number of apoptotic nuclei (TUNEL), total number of apoptotic cardiomyocytes, or graft NOS-2 mRNA and protein. Positive NOS-2 immunostaining occurred in endothelial cells and cardiomyocytes in the allografts; the inflammatory infiltrate was NOS-2 positive only when recipients were NOS-2+/+. In the second series of experiments, cardiac allograft survival was significantly increased in the NOS-2-/- mice (26±13 versus 17±8 days, P<0.05), along with significant reductions in inflammatory infiltrate, rejection score, and total number of apoptotic nuclei (23.5±9.5 versus 56.4±15.3, P<0.01) and of apoptotic cardiomyocytes (2.9±1.6 versus 6.9±2.7, P<0.05). No NOS-2 or nitrotyrosine, a marker of peroxynitrite exposure, was detected in NOS-2-/- allografts transplanted into NOS-2-/- recipients.
ConclusionsThe data suggest that NO derived from NOS-2 contributes to the inflammatory response and to cardiomyocyte damage and apoptosis during acute cardiac allograft rejection.
Key Words: apoptosis rejection nitric oxide synthase transplantation
| Introduction |
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, interferon-
, and
interleukin-1ß.6 7 8
Increased NOS-2 expression during acute cardiac allograft rejection has
been associated with impaired contractility, with
pathological changes, and with the death of cardiac muscle cells by
necrosis and
apoptosis.1 2 3 4
In the studies by Szabolcs et
al3 in a rat heterotopic
heart transplantation model, there was a parallelism between the time
and extent of NOS-2 expression in the allografts and apoptosis
of cardiac muscle cells. The presence of NOS-2 and nitrotyrosine, a
marker of exposure to peroxynitrite that is formed by interaction of NO
with superoxide, has also been observed in association with
apoptosis of cardiomyocytes in acute experimental
and human heart transplant
rejection.3 5 In
contrast, NOS-2 expression in chronic cardiac allograft rejection has
been associated with an amelioration of the transplant-associated
vasculopathy that occurs in this
situation.9 10 The
present study was designed to investigate the role of NOS-2 in the
pathological changes and cardiomyocyte apoptosis
associated with acute cardiac allograft rejection in strains of mice in
which the NOS-2 gene was functionally deleted
(NOS-2-/-). In the first series of
experiments, NOS-2+/+ hearts that differed
in major histocompatibility antigens from the recipients were
transplanted into recipient animals that were either
NOS-2+/+ or
NOS-2-/-. In the second series of
experiments, cardiac transplantation was performed with
NOS-2-/- donors and recipients and
NOS-2+/+ donors and recipients. The donor
and recipient mice differed in minor histocompatibility antigens. In
both series, cardiac allograft survival, the
histological degree of rejection, the extent of
apoptosis, NOS-2 mRNA, and NOS-2 protein were measured along
with immunostaining for NOS-2 and for nitrotyrosine in
the different cells within the rejecting
allografts. | Methods |
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Mouse Genotyping by Polymerase Chain
Reaction
Female C3H/J(H-2 k) mice 6 to 8 weeks old (obtained
from Jackson Laboratory, Bar Harbor, Me) were used as heart donors.
Male mice deficient in inducible NOS
(NOS-2-/-) in a C57BL/6J.129J(H-2b)
background were used as organ recipients. Results of the C3H/J heart
transplantations into the NOS-2-/-
recipients were compared with the results of C3H/J heart
transplantations into NOS-2+/+ wild-type
C57BL/6J.129J(H-2b) mice. The NOS-2deficient and the wild-type mice
were obtained at the F3 generation from Dr V.
Laubach.12 Both the
NOS-2-/- and wild-type
NOS-2+/+ C57BL/6J.129J(H-2b) mice were
back-crossed for >10 generations in our laboratory for the second set
of experiments. For these experiments, additional
NOS-2-/- mice in a C57BL/6J background
(ie, back-crossed for 10 generations) and
NOS-2+/+ wild-type C57BL/6J mice were also
obtained from Jackson Laboratory. The genotypes of the
NOS-2-/- and wild-type mice were
confirmed by polymerase chain reaction (PCR) analysis of mouse
tail DNA and by measurement of
NO3- synthesis by
peritoneal macrophages from the mice incubated with
lipopolysaccharide and
interferon-
.12
PCR for NOS-2
NOS-2 mRNA in myocardial tissue or cells was measured
with a semiquantitative reverse transcription (RT)PCR system using a
recombinant mRNA (rcRNA) internal standard that contained target mRNA
primer sequences that were 60 bp shorter than the target gene (NOS-2)
sequences.3 Semiquantitative
competitive RT-PCR was carried out with the rcRNA as a competitive
template. For each sample, a dilution series of the rcRNA internal
standard was spiked into each aliquot of sample. RT-PCR products
generated from the internal standard were distinguished from the
products generated from the target gene mRNA by their size
difference and by use of different probes in the
fluorescein-antifluoresceinbased
enzyme-linked oligonucleotide sorbent assay. An RNA
sample that was not subjected to RT was also run as a control for
contamination by genomic DNA.
Western Analysis for NOS-2
Myocardial homogenates were lysed in a
buffer containing 150 mmol/L NaCl, 1.0% SDS, 1 mmol/L EDTA,
and 50 mmol/L Tris (pH 7.7), supplemented with protease
inhibitors (10 µg/mL of antipain and leupeptin, and
1 mmol/L PMSF), and centrifuged at 14 000 rpm for 20
minutes at 4°C. The cytosolic proteins (60 µg/lane) were
electrophoresed on a 75% SDS-polyacrylamide gel, transferred
to a nitrocellulose filter, and immunoblotted with rabbit
polyclonal antiserum raised against the amino acid 117 to 128 sequence
of the (mouse) macrophage-inducible NOS at a 1:2000 dilution.
Anti-rabbit horseradish peroxidaseconjugated Ab was used as a
secondary Ab. Blots were detected with the enhanced chemiluminescence
method. The lysates of cytokine-treated macrophages of
NOS-2+/+ and
NOS-2-/- mice were used as
controls.
Immunohistochemistry and Apoptosis
Labeling: Antibodies
Two antibodies (Abs) were used to detect NOS-2. The
rabbit polyclonal Ab raised against the amino acid sequence 117 to 128
of the mouse macrophage-inducible NO synthase was used for
Western blot (dilution 1:2000) and immunohistochemistry (1:500). A
second polyclonal antiNOS-2 Ab was obtained from Transduction
Laboratories and was used for immunohistochemistry (1:50). A polyclonal
anti-nitrotyrosine Ab (1:100; Upstate Biotechnology) was used to
determine whether cells had been exposed to peroxynitrite. Cardiac
myocytes were highlighted with polyclonal anti-desmin Abs, 1 produced
in rabbit (1:200; Sigma Chemical Co) for immunoperoxidase staining and
1 produced in goat (1:20, Santa Cruz Biotechnology) for
immunofluorescence labeling with FITC.
Endothelial cells were labeled with the lectin
fluorescein Ricinus
communis agglutinin I (1:1000, Vector
Laboratories).
Immunohistochemistry
NOS-2 immunolabeling using either polyclonal
antiNOS-2 Abs, immunolabeling for peroxynitrite, or double labeling
for apoptotic nuclei (terminal
deoxynucleotidyl transferasemediated dUTP nick
end-labeling, TUNEL) and desmin (which highlights cardiac myocytes) was
performed as previously
described.5
Immunofluorescence staining for labeling of NOS-2 was performed with Cy3-conjugated donkey anti-rabbit IgG (1:2000, Jackson Immuno Research Laboratories, Inc) yielding a red fluorescence signal. Thereafter, sections were incubated with goat anti-mouse desmin (1:20, Santa Cruz Biotechnology, Inc) followed by fluorescein (FITC)-conjugated rabbit anti-goat IgG (1:100) or by fluorescein-conjugated R communis agglutinin I (1:1000). Sections were mounted in Vectashield mounting medium with DAPI. As negative controls, primary antibodies were replaced with nonspecific immunoglobulins from the respective animals in equal concentrations.
Selected frozen sections fixed in paraformaldehyde were dually labeled with rabbit antiNOS-2 (Transduction Laboratories, 1:50 for 90 minutes) and TUNEL to correlate the distribution of apoptotic nuclei with NOS-2. All reagents for immunofluorescence labeling were obtained from Vector and from Boehringer-Mannheim Co for TUNEL labeling.
Quantitative Evaluation
The total number of apoptotic nuclei and the
number of apoptotic cardiac myocytes were enumerated for an
entire coronal section dually labeled for apoptosis (TUNEL) and
desmin and are expressed as the number of apoptotic cells per
square millimeter. The degree of rejection was evaluated on
hematoxylin-eosinstained sections and quantified by use of the
International Society of Heart and Lung Transplantation (ISHLT) grading
system for cardiac allograft
rejection,13 with ISHLT
grades 0, 1A, 1B, 2, 3A, 3B, and 4 rejection converted to a linear
scale from 0 to 6.
Statistical Analysis
Graft survival times for the different treatment
groups were analyzed by the nonparametric
Kruskal-Wallis test. Analysis of apoptosis and
pathology scores used a multifactorial extension of the Kruskal-Wallis
test to account for the effect of treatment group and time after
transplantation.
| Results |
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Additional C3H cardiac allografts were harvested on
days 1, 3, and 5 after transplantation. The C3H allografts in
NOS-2-/- and in
NOS-2+/+ recipients manifested pathological
changes and ISHLT rejection scores that were not significantly
different at each time point
(Table 1
). The total numbers of apoptotic
nuclei (TUNEL) and of apoptotic cardiac myocytes were also not
significantly different
(Table 1
).
|
Densitometric measurements in day 5 C3H cardiac allografts
showed only insignificant reductions of NOS-2 mRNA and protein in the
allografts of NOS-2-/- recipients
compared with those in NOS-2+/+ recipients
(Figure 1A
and 1B
). Immunohistochemistry, however, revealed
differences in the cellular distribution of NOS-2 immunoreactivity. In
C3H allografts in NOS-2+/+ recipients,
positive NOS-2 immunostaining was apparent by dual
immunofluorescence labeling in cardiac myocytes
(Figure 2A
) and endothelial cells
(Figure 2B
); there was also strong NOS-2 labeling of
infiltrating inflammatory cells
(Figure 2A
and 2C
). In C3H allografts in
NOS-2-/- recipients, positive NOS-2
immunostaining was observed in cardiac myocytes and
endothelial cells, but NOS-2 staining was not observed
in the inflammatory infiltrate
(Figure 2D
). In additional control experiments, hearts from
NOS-2-/- and from
NOS-2+/+ C57BL/6J.129 mice were transplanted
into C3H recipients. In these experiments, there were no significant
differences in survival, ISHLT rejection score, or apoptosis
between the NOS-2-/- and
NOS-2+/+ allografts
(Table 2
). There was no immunoreactivity for NOS-2 in the
endothelial cells or cardiomyocytes in the
NOS-2-/- allografts, whereas strong
NOS-2 immunoreactivity was noted in the myocardial inflammatory
infiltrate
(Figure 2E
). Overall, NOS-2 activity was observed in all
allografts of this first series of experiments. In the group in which
the donor was NOS-2+/+ and the recipient was
NOS-2-/-, the donor myocytes and
endothelial cells expressed NOS-2 and the inflammatory
infiltrate lacked NOS-2 staining. In the groups with
NOS-2-/- donors and
NOS-2+/+ recipients, the allografts showed
strong NOS-2 reactivity in the inflammatory infiltrate derived from the
NOS-2+/+ recipient.
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Accordingly, a second series of cardiac transplantations was performed to test the hypothesis that the elimination of NOS-2 in both the donor and recipient would result in less severe rejection than that observed when NOS-2 was expressed in both donor and recipient animals. In these experiments, the differences between the 2 groups of transplantations were restricted to the NOS-2 gene and to differences in minor histocompatibility antigens among mice of the inbred C57BL/6J and C57BL/6J.129J strains. Hearts from NOS-2-/- C57BL/6J donors were transplanted into NOS-2-/- C57BL/6J.129J recipients, and hearts from NOS-2+/+ C57BL/6J donors were transplanted into NOS-2+/+ C57BL/6J.129J recipients. The mean survival of the NOS-2-/- allografts that had been transplanted into NOS-2-/- recipients (26±13 days, n=11) was significantly longer (P<0.05) than the mean survival of the NOS-2+/+ cardiac allografts that had been transplanted into the NOS-2+/+ recipients (17±8 days, n=20).
Additional cardiac transplants were harvested on day
12 after transplantation for biochemical and
histological measurements. At day 12, NOS-2 mRNA and
protein were lower in the NOS-2-/-
cardiac allografts than in the NOS-2+/+
allografts and were comparable to values found in native hearts
(Figure 3A
and 3B
). The magnitudes of the inflammatory
infiltrate, tissue edema, and damage to cardiac muscle cells were also
lower in the NOS-2-/- cardiac allografts
than in the wild-type NOS-2+/+ allografts
(Figure 4A
and 4B
). The ISHLT rejection score and the total
number of apoptotic cells and of apoptotic cardiac
myocytes were significantly lower
(P<0.05) in the
NOS-2-/- cardiac allografts
(Table 3
) than in the wild-type
NOS-2+/+ allografts
(Figure 4C
and 4D
,
Table 3
).
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Positive immunostaining for NOS-2 protein
was observed in endothelial cells, macrophages,
and cardiac myocytes in the NOS-2+/+
allografts. Many apoptotic cells, including cardiac myocytes,
were strongly NOS-2 positive
(Figure 4E
). Allografts in which both the donor and recipient
were NOS-2-/- showed no NOS-2 reactivity
at all
(Figure 4F
). Similarly, immunostaining for
nitrotyrosine, which reflects exposure to peroxynitrite, was strong in
the inflammatory infiltrate and adjacent cardiac myocytes in the
NOS-2+/+ allografts
(Figure 4G
). No labeling for nitrotyrosine was identified in
NOS-2-/- allografts transplanted into
NOS-2-/- recipients
(Figure 4H
).
| Discussion |
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In our second series of heart transplantations, the degree of antigenic disparity between donor and recipient mice was less than in the first series. NOS-2-/- C57BL/6J donors were transplanted into NOS-2-/- C57BL/6J.129J recipients, and NOS-2+/+ C57BL/6J donors were transplanted into NOS-2+/+ C57BL/6J.129J recipients. We observed in the NOS-2-/- cardiac allografts placed in NOS-2-/- recipients that (1) allograft survival was significantly prolonged, (2) the intensity of rejection (ISHLT score) was less, and (3) the mean total number of apoptotic nuclei and the mean number of apoptotic cardiac myocytes were significantly less than in allografts in which both the donor and recipient mice were NOS-2+/+. These data are consistent with the findings of Koglin et al during acute rejection.10 14 Our finding that apoptosis of cardiac myocytes was significantly reduced in the total absence of NOS-2 is new and is consistent with the hypothesis that NO-mediated apoptosis of cardiac muscle cells occurs during acute cardiac allograft rejection.3 5 14 15
NO exerts a bifunctional effect on apoptosis depending on the cell type and features of the microenvironment in which it is synthesized.16 NO released from NO-donor drugs or synthesized by activated macrophages has been demonstrated to trigger apoptosis of macrophages, lymphocytes, mast cells, chondrocytes, vascular smooth muscle cells, pancreatic islet cells, and both neonatal and adult cardiac myocytes.15 Pinsky et al,8 Ing et al,17 and Arstall et al18 found that cytokine induction of NOS-2 in adult rat or neonatal cardiac myocytes was followed by an increase in NO-mediated apoptosis of the cardiomyocytes. These results are consistent with a report by Kawaguchi et al,19 who used a viral vector to transfect NOS-3 into the myocardium of rats and found that the increased expression of NOS triggered apoptosis-like cell death of the transfected cardiac muscle cells.
In contrast, NO has been shown to inhibit apoptosis of hepatocytes, of cardiac myocytes subjected to stretch, and of vascular endothelial cells.16 The disparate biological effects of NO are determined by various factors. These include (1) the amount of NO synthesized (influenced by the number and types of cells producing NO and the NOS isoforms present in the tissue), (2) diffusion of NO (influenced by distance and the presence or absence of carrier molecules), (3) competing target molecules within the diffusion range and their affinities for NO, and (4) the redox state of the microenvironment. (In the presence of superoxide, NO combines with it to form peroxynitrite, which is a strong oxidant that can cause lipid peroxidation, nitration of cell proteins, and DNA strand breaks that have the potential to initiate apoptosis.)20
The prolonged survival, reduced ISHLT rejection score, and significantly reduced total and cardiomyocyte apoptosis in the NOS-2deficient cardiac allografts found in our second set of experiments confirm and extend previous work that suggests that the NO produced by NOS-2 contributes to heart muscle damage and impaired contractility during cardiac allograft rejection in humans and rats.1 2 3 4 5 9 10 14 The results are consistent with previous reports that cardiac allograft survival was prolonged, contractility was improved, and the intensity of histological changes during rejection was reduced in cardiac allografts treated with the semiselective NOS-2 inhibitor aminoguanidine. They are also consistent with the modest increase in cardiac allograft survival observed in rats treated with a nonselective inhibitor of NOSs.21 The gradual loss of cardiomyocytes over time by NO-triggered apoptosis may contribute to the decline in ventricular performance and development of heart failure that is observed in some cardiac allografts.4 In addition, the reduced inflammatory response in the rejecting NOS-2-/- cardiac allografts suggests that large amounts of NO, particularly in conjunction with peroxynitrite-induced cell damage, may amplify the inflammatory response in allograft rejection. Such interactions between inflammatory mediators and apoptosis may provide new targets for pharmacological and genetic strategies to ameliorate cardiac allograft rejection.
| Acknowledgments |
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Received September 6, 2000; revision received December 31, 2000; accepted January 16, 2001.
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