(Circulation. 2001;103:525.)
© 2001 American Heart Association, Inc.
Clinical Investigation and Reports |
From Cardiovascular Medicine (X.-M.Z., P.L.), Department of Pathology (Y.H., R.N.M.), Brigham and Womens Hospital and Harvard Medical School, Boston, Mass; and Department of Medicine (G.G.M.), Vanderbilt Medical Center, Nashville, Tenn.
Correspondence to Peter Libby, MD, Vascular Medicine and Atherosclerosis Unit, 221 Longwood Ave, LMRC 307, Boston, MA 02115. E-mail plibby{at}rics.bwh.harvard.edu
| Abstract |
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Methods and
ResultsLevels of TSP-1 mRNA in endomyocardial
biopsy samples of human cardiac allografts substantially exceeded those
in normal hearts. The ratio of TSP to GAPDH mRNA determined with
quantitative RT-PCR was 6.54±1.6 in cardiac allografts versus
0.26±0.02 (P=0.001) in normal
hearts. Analysis in sequential biopsies revealed a strong association
between persistent elevation of TSP-1 in allografts and the development
of cardiac allograft vasculopathy (CAV). The CAV score was 2.4±0.8 in
patients with persistent TSP-1 elevation compared with 0.2±0.2 in
patients without elevation
(P=0.001). Immunohistochemistry
demonstrated intense expression of TSP-1 in cardiac allografts,
predominantly in cardiac myocytes and neointimal SMCs. In vitro
experiments demonstrated that T cells expressed TSP-1, acidic
fibroblast growth factor, and vascular endothelial cell growth factor
on allogeneic stimulation. Cytokines known to be elevated in cardiac
allografts (interleukin-1ß, interferon-
, and tumor necrosis
factor-
) induced TSP-1 in SMCs but inhibited TSP-1 in endothelial
cells.
ConclusionsPersistent elevation of TSP-1 in cardiac allografts correlates with the development of CAV. Allogeneic stimulation induces angiogenic growth factors and TSP-1 in T cells. Cytokines differentially regulate TSP-1 expression in SMCs versus endothelial cells. Increased levels of TSP-1 in human cardiac allografts may alter vascular responses to angiogenic growth factors by inhibiting angiogenesis and promoting SMC proliferation characteristic of CAV.
Key Words: glycoproteins growth factors transplantation coronary disease cytokines
| Introduction |
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Thrombospondin-1 (TSP-1) is a 190-kDa multifunctional matrix
glycoprotein9 that modulates
many important vascular cell
functions.10 It is present
in large amounts in platelet
-granules9 and is
expressed by endothelial cells (ECs) and SMCs in a highly regulated
manner.11 Its expression
rapidly and dramatically increases after vascular
injury12 or exposure of the
cells to platelet-derived growth factor (PDGF) or
bFGF.13 14
Atheroma and balloon-injured vessels contain abundant
TSP-1.12 15
Antibodies to TSP-1 reduce neointima formation in balloon-injured rat
carotid arteries, suggesting an important functional
role.16 In regard to
angiogenesis, TSP-1 potently inhibits angiogenesis in vivo and blocks
microvascular EC proliferation by angiogenic
factors.17 TSP-1 also
enhances SMC proliferation and migration in response to FGFs and
PDGF,18 19 growth
factors overexpressed in human cardiac
allografts.3 7
Thus, we tested the hypothesis that the elevated expression of TSP-1 in
human cardiac allografts associates with the development of CAV. We
further tested the hypothesis that allogeneic stimulation and
inflammatory cytokines regulate TSP-1 expression by ECs and
SMCs.
| Methods |
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Annual coronary angiography was used to assess CAV. Coronary angiograms were reviewed and compared with baseline angiograms independently by 2 cardiologists who were unaware of the results of studies on TSP-1. CAV was assessed according to the criteria established by Gao et al,8 including the presence of focal stenoses, distal tapering or pruning, and loss or tertiary vessels, and were assigned a numerical rating for severity as absent (0), mild (1), moderate (2), or severe (3).
Isolation of RNA From Myocardial Biopsy Samples
and RT-PCR
Total RNA was isolated from myocardial biopsies using
RNAzol B (Tel-Test) and was used as template for cDNA synthesis.
Primers used in PCR are listed in the
Table
.
Quantitative RT-PCR was performed with
32P-labeled dCTP to generate radioactively
labeled PCR products.4 PCR
products were run on 2% agarose gel, dried, and exposed to a
PhosphorImager (Molecular Dynamics) for quantification. Standard curves
within the exponential range of amplification for each gene were
generated with known amounts of cDNA template. The concentration of
cDNA in each sample was calculated from the standards run at the same
time.
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RNA from myocardial biopsy samples could not be quantified because of the small size of the samples. Therefore, the amount of cDNA for each gene was normalized to the amount of cDNA for GAPDH, a constitutively expressed gene, in each sample. The ratio between each gene of interest and GAPDH is used for comparison.
Immunohistochemistry
Twenty specimens were obtained from 3 explanted
cardiac allografts during autopsy. Normal hearts were obtained from
patients who died of noncardiac diseases. Specimens were fixed in 10%
formalin and embedded in paraffin for processing. After
deparaffinization, slides were incubated sequentially in proteinase
K/PBS (5 mg/mL; Boehringer Mannheim) for 20 minutes, 0.3%
H2O2/PBS for 20 minutes,
5% horse serum/PBS for 1 hour, and first antibodies in 5% horse
serum/PBS for 2 hours. The first antibodies used were goat anti-human
TSP-1 (Neo Markers), goat anti-human VEGF (Santa Cruz), and mouse
anti-human CD31 (DAKO). Goat or mouse IgG (Santa Cruz) was used as
first antibody in negative controls. After incubation and washing in
PBS, slides were incubated with biotinylated second antibodies (horse
anti-goat or mouse; Vector) and developed with use of a Vectastain ABC
kit (Vector) and a DAB substrate kit (Vector).
Cell Cultures
Human SMCs were isolated from saphenous veins and
grown in Medium 199 (BioWhittaker) supplemented with 10% FCS. These
experiments used 80% to 90% confluent cells from passages 2 to 3. To
induce quiescent SMCs, cells were cultured in IT
medium20 for 24 hours. To
study the regulation of TSP-1 expression in SMCs, cells were treated
with recombinant human interleukin (IL)-1ß (10 ng/mL; R&D Systems),
interferon (IFN)-
(10 ng/mL; R&D Systems), or tumor necrosis factor
(TNF)-
(5 ng/mL; R&D Systems), for 4, 8, 12, or 24
hours.
Human ECs were isolated from human saphenous veins and grown on gelatin-coated plates in Medium 199 (BioWhittaker), supplemented with 10% FCS. To induce quiescence in ECs, cells were cultured in Medium 199 supplemented with 0.5% BSA for 24 hours. Quiescent cells were treated with cytokines as described earlier.
Mixed Lymphocyte Cultures
Peripheral blood mononuclear cells were isolated from
normal healthy donors with lymphocyte separation medium (ICN).
Stimulator cells were prepared with irradiation (45 Gy). Responder
cells (5x106/well) were cultured with
irradiated stimulator cells (5x106/well) in
6-well plates at 37°C in a 5% CO2 humidified
incubator. After 5 days of coincubation, cells were collected and T
cells were isolated with a T-cell enrichment column (R&D Systems).
Purified T cells typically had 85% to 90% CD3-positive cells on flow
cytometry (data not shown). Those cells were used for RNA extraction
(RT-PCR analysis) or cytospinning
(immunohistochemistry).
Cytospin Preparation
T cells before and after allostimulation were
isolated with T-cell enrichment column (R&D Systems) and
cytocentrifuged onto slides. The slides were fixed in acetone for 5
minutes, dried, and stained with goat anti-aFGF (Santa Cruz), goat
anti-bFGF (Santa Cruz), goat anti-VEGF (Santa Cruz), or goat anti-TSP-1
(Neo Marker). After being washed in PBS, slides were incubated with
biotinylated horse anti-goat antibody and developed with a Vectastain
ABC kit (Vector) and a DAB substrate kit
(Vector).
Northern Blot Analysis
The regulation of TSP-1 mRNA expression in SMCs was
studied with Northern blot analysis. mRNA was extracted from SMCs with
RNAzol B. Total RNA (20 µg) from each sample was denatured and run on
formaldehyde agarose gels. The RNA was transferred to nylon membrane
and UV cross-linked. The probe used in Northern blot analysis was the
antisense oligonucleotide for TSP-1. A probe for GAPDH was used as
positive control to ensure equal loading of total RNA in each sample.
The probes were labeled with
-32P-dCTP
with terminal deoxynucleotidyl transferase (GIBCO
BRL).
Western Blot Analysis
Cells were washed with cold PBS once, scraped from
the plastic plates, and resuspended in 1 mL of cold PBS. After
centrifugation at 9.5g for 1
minute, cells were solubilized in 100 µL of lysis buffer (1% Triton
X-100, 50 mmol/L HEPES, 10 mmol/L EDTA, 10 mmol/L sodium pyrophosphate,
100 mmol/L sodium fluoride, 1 mmol/L sodium orthovanadate, 1 µg/mL
aprotinin, 1 µg/mL leupeptin, and 2 mmol/L PMSF). Supernatants were
collected after centrifugation at
12.5g for 15 minutes at 4°C.
Protein concentration was determined with BCA protein assay reagent
(Bio-Rad). For Western blot analysis, 50 µg of proteins was separated
by 7.5% SDS-PAGE and transferred to a nylon membrane. After blocking
with 5% serum/PBS, the membrane was subsequently incubated with
monoclonal anti-human TSP-1 antibody (1:1000 dilution; Neo Marker) and
peroxidase-conjugated goat anti-mouse antibody (1:1000; Vector). The
signals were detected by ECL Western blot analysis system (GIBCO
BRL).
Data Analysis
Differences in levels of TSP-1, CD36, and CD47 mRNA
in transplanted hearts with and without severe CAV as well as in normal
hearts were analyzed by unpaired Students
t test. A value of
P<0.05 was considered
statistically significant.
| Results |
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Human Cardiac Allografts Express TSP-1, TSP-1
Receptors, and VEGF
The lack of parenchymal neovascularization in cardiac
allografts in the presence of high levels of angiogenic growth factors
prompted the hypothesis that cardiac allografts expressed inhibitors of
angiogenesis. We therefore investigated the expression of TSP-1, an
angiogenesis inhibitor, in 133 human cardiac allograft biopsy samples.
Levels of TSP-1 mRNA in cardiac allografts substantially exceeded those
in normal hearts. The mean TSP-1to-GAPDH ratio was 6.54±1.61 in
allografts (133 biopsy samples from 11 allografts) versus 0.26±0.21 in
normal hearts (15 endomyocardial specimens from 15 normal hearts,
P=0.001).
Figure 2
shows a representative RT-PCR gel for 5 allograft
and 4 normal hearts samples.
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To test whether elevated TSP-1 mRNA correlates with the development of CAV, we analyzed TSP-1 mRNA levels in sequential biopsy samples from 11 heart transplant recipients who had 2-year posttransplantation coronary angiography. Biopsy samples were obtained from each patient at routine follow-up biopsy or when clinically indicated. On average, 10 to 17 biopsy samples were obtained from each patient during the 2-year period.
TSP-1 mRNA expression generally followed 2 distinct
patterns. Five patients demonstrated persistent elevation of TSP-1 mRNA
in sequential biopsies, after a rise in TSP-1 mRNA levels immediately
after transplantation
(Figure 3A
). Six other patients demonstrated an initial
elevation of TSP-1 mRNA after transplantation, followed by return to
normal levels
(Figure 3A
). Patients with persistent elevation of TSP-1
developed severe CAV as determined with coronary angiography. Their
mean CAV score was 2.4±0.8 compared with 0.2±0.2 for patients with
only transient TSP-1 elevation
(P=0.001)
(Figure 3A
). The mean TSP-1to-GAPDH ratio (over 2 years)
was 14-fold higher in patients with CAV than in those without CAV
(12.43±6.09 versus 0.87±0.25,
P=0.0003)
(Figure 3B
).
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The expression of 2 known TSP-1 receptors also increased but to a lesser extent. The CD36-to-GAPDH ratio was 0.78±0.08 in cardiac allografts versus 0.53±0.04 in normal hearts (P=0.01). The CD47-to-GAPDH ratio was 0.93±0.06 versus 0.77±0.10 (P=0.05), respectively. The CD36 and CD47 levels did not correlate with the severity of CAV.
These results at the mRNA level supported the hypothesis
that interaction between TSP-1 and angiogenic growth factors plays an
important role in inhibition of angiogenesis and facilitation of SMC
proliferation. The cellular distribution of TSP-1 and angiogenic growth
factor proteins in cardiac allografts may provide clues of their roles
in CAV. Therefore, we studied the expression of TSP-1 and VEGF in human
cardiac allografts with immunohistochemical studies of tissue sections
from explanted cardiac allografts and normal hearts. In accordance with
the mRNA results, cardiac allografts demonstrated markedly increased
TSP-1 proteins in comparison with normal hearts. Cardiac myocytes and
intimal SMCs were the predominant sites of TSP-1 expression in cardiac
allografts
(Figures 4A
and 4C
). SMCs in the luminal (inner) layer of
neointima displayed strong staining for TSP-1, whereas infiltrating
cells did not express high levels of TSP-1 in cardiac allografts
(Figure 4B
). Consistent with RT-PCR data, the expression of
TSP-1 increased early after transplantation. For example, in the heart
of a patient who died of a noncardiac cause 3 weeks after
transplantation, the coronary arterial media already showed intense
TSP-1 expression, predominantly in SMCs
(Figure 4D
). In contrast, normal hearts showed only scattered
TSP-1 expression
(Figure 4E
).
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The cellular distribution of VEGF in cardiac allografts
differs from TSP-1, with cardiac myocytes and infiltrating mononuclear
inflammatory cells containing the most VEGF in cardiac allografts
(Figures 4G
and 4H
). Intimal SMCs did not express high levels
of VEGF
(Figure 4F
). Normal hearts showed only scattered VEGF in ECs
(Figure 4I
).
Allogeneic Response Induces TSP-1, aFGF, and
VEGF Expression in Lymphocytes
We have proposed that allogenic stimulation of
recipient T cells by donor cells contributes to the pathogenesis of
CAV.1 We previously
demonstrated expression of aFGF by T-cell
clones.21 Others have
reported the expression of VEGF and bFGF in T
cells.22 23 Here,
we investigated the hypothesis that allostimulation modulates the
expression of growth factors and TSP-1 in lymphocytes during mixed
lymphocyte reaction (MLR).
Naive T cells contained low or undetectable levels of VEGF,
aFGF, or TSP-1 transcripts
(Figure 5A
) but did express bFGF mRNA (data not shown). After
coculture with stimulator cells for 5 days, T cells accumulated
substantial VEGF, aFGF, and TSP-1 mRNA. The levels of bFGF mRNA in
activated T cells resembled those seen in naive T cells. In accordance
with mRNA results, allostimulated T cells, but not naive T cells,
contained TSP-1, aFGF, and VEGF proteins
(Figure 5B
).
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Cytokines Differentially Regulate TSP-1
Expression in SMCs and ECs
The expression of proinflammatory cytokines increases
in human and murine cardiac
allografts.2 We hypothesized
that those cytokines modulate TSP-1 expression in SMCs and ECs.
Quiescent SMCs expressed TSP-1 mRNA at low levels. The addition of
IL-1ß greatly increased TSP-1 mRNA expression, being apparent at 4
hours and maximal at 12 hours. TSP-1 mRNA levels remained elevated
after 24 hours of IL-1ß stimulation (data not shown). IFN-
stimulation moderately and transiently elevated TSP-1 mRNA, achieving
maximal levels at 4 hours. TNF-
produced a delayed and modest
increase in TSP-1 expression in SMCs
(Figure 6A
).
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Exposure to IL-1ß markedly elevated TSP-1 protein
expression
(Figure 6B
). The time course of TSP-1 protein expression
paralleled mRNA expression. The induction depended on the
concentrations of IL-1ß. Elevation of TSP-1 protein was evident at
0.2 ng/mL IL-1ß and maximal at 5 ng/mL. IFN-
or TNF-
only
moderately induced TSP-1 in SMCs.
We next investigated the regulation of TSP-1 expression in
ECs. In contrast to SMCs, IL-1ß inhibited TSP-1 expression in ECs,
with effects apparent at 4 hours and maximal at 12 hours
(Figure 6C
). IFN-
or TNF-
did not affect TSP-1
expression in ECs.
| Discussion |
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Thus, rather than promoting potentially beneficial parenchymal angiogenesis in cardiac allografts, elevated angiogenic growth factors may contribute to the development of CAV. A recent study demonstrates that the persistent elevation of aFGF in cardiac allografts correlates with the development of CAV.4 Nabel et al25 showed that introduction of aFGF into vessel walls substantially augmented intimal neocapillary formation and intimal hyperplasia. VEGF accentuates neointimal thickening at the site of arterial injury but not in normal vessels.26 The biological effects of these growth factors (angiogenesis versus vascular intimal hyperplasia) resulted from a complex balance among growth factors, their inhibitors, extracellular matrix, and other factors present in situ. Here, we provide evidence to support the hypothesis that TSP-1 expressed in transplanted hearts alters vascular responses to angiogenic growth factors by inhibiting angiogenesis and accelerating SMC proliferation.
TSP-1 is a multifunctional protein that inhibits EC proliferation, migration, and angiogenesis17 but stimulates SMC proliferation and migration.18 It also potentiates SMC proliferation in response to other growth factors.19 The current study demonstrates significant elevation of TSP-1 in cardiac allografts, with the strongest expression in neointimal SMCs and cardiac myocytes. Although the SMCs from the luminal (inner) layer of the neointima express the highest levels of TSP-1, the greatest VEGF expression occurs in infiltrating inflammatory cells in the abluminal (outer) layers of the intima. Neovascularization colocalizes with VEGF-producing cells and is sparse in areas with intense TSP-1 expression. These data suggest that TSP-1 inhibits angiogenesis in cardiac allografts.
Intense expression of TSP-1 in neointima indicates a role in SMC proliferation. The association between TSP-1 levels and the development of CAV supports this hypothesis. Persistent elevation of TSP-1 in cardiac allografts correlates with the development of CAV, whereas transient early elevation of TSP-1 after transplantation does not seem to influence CAV development.
Our in vitro studies examined the mechanisms that underlie
the regulation of TSP-1 and angiogenic growth factors in cardiac
allografts. We found that proinflammatory cytokines differentially
regulate TSP-1 expression in SMCs and ECs. Cardiac allografts contain
abundant IL-1ß, TNF-
, and IFN-
, which may induce TSP-1
production by SMCs in cardiac allografts. We and others found that
IL-1ß inhibits TSP-1 expression in
ECs.27 The in vitro data
agree with the observation that SMCs, but not ECs, produce TSP-1 in
cardiac allografts.
The current study also demonstrates that allogeneic stimulation induces aFGF, VEGF, and TSP-1 expression in T cells. Cardiac allografts contain abundant T cells even in the absence of histologically detectable rejection. Thus, growth factors secreted by T cells may contribute to the persistent elevation of endogenous growth factors present in cardiac allografts. Colocalization of neovessels and VEGF-producing infiltrating cells in the neointima suggests an important role for these cells in the development of CAV. The finding that T-cell activation promotes the production of angiogenic growth factors and inhibitors has implication for many immunity-mediated diseases.
In conclusion, high levels of TSP-1 expressed in cardiac allografts may play a critical role in altering vascular responses to endogenous angiogenic growth factors by inhibiting angiogenesis and promoting neointimal SMC proliferation. Indeed, the deliverance of exogenous angiogenic growth factors to cardiac allografts may accelerate vascular intimal hyperplasia. New approaches to inhibition of TSP-1 expression or block of the biological effects of TSP-1 may offer novel ways to increase angiogenesis and reduce neointimal SMC proliferation in cardiac allografts.
| Acknowledgments |
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Received May 23, 2000; revision received September 20, 2000; accepted September 20, 2000.
| References |
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