(Circulation. 2000;102:2528.)
© 2000 American Heart Association, Inc.
Basic Science Reports |
From the Second Department of Internal Medicine, Gunma University School of Medicine (Y.H., M.K., T.K., A.H., H.S., K.K.), and the Department of Cardiovascular Medicine, Saiseikai Maebashi Hospital (A.N., S.T.), Gunma; the First Department of Internal Medicine, Shinsyu University School of Medicine, Nagano (N.W.); and the Department of Cardiovascular Medicine, Graduate School of Medicine, University of Tokyo, Tokyo (I.M., T.S., R.N.), Japan; and the Department of Medicine, Emory University, Atlanta, Ga (E.O., J.N.W.)
Correspondence to Ryozo Nagai, MD, The Department of Cardiovascular Medicine, Graduate School of Medicine, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8655, Japan. E-mail nagai-tky{at}umin.ac.jp
| Abstract |
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Methods and ResultsImmunohistochemistry showed developmentally regulated expression of BTEB2 with abundant expression in fetal but not in adult aortic SMCs of humans and rabbits. In balloon-injured aortas, predominant expression of BTEB2 was seen in neointimal SMCs. Atherectomy specimens obtained from primary and restenotic lesions showed predominant expression of BTEB2 to stellate SMCs. The incidence of restenosis in primary lesions was significantly higher in lesions containing BTEB2-positive cells than in lesions without (55.6% versus 25.0%, P=0.01).
ConclusionsThe present study shows that BTEB2 expression is developmentally and pathologically regulated. BTEB2 is preferentially expressed in dedifferentiated or activated SMCs. Examination of human coronary artery specimens suggests that primary lesions containing BTEB2-positive cells are associated with higher risk of restenosis than BTEB2-negative lesions. These results suggest that BTEB2 can serve as a molecular marker for phenotypic modulation of vascular SMCs.
Key Words: angioplasty muscle, smooth restenosis genes
| Introduction |
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In our studies to understand the mechanisms underlying phenotypic modulation of SMCs, we have recently identified the DNA-binding BTEB2 transcription factor to regulate the SMemb/NMHC-B gene, which is associated with the dedifferentiated or activated SMC phenotype.5 In vitro experiments indicate that BTEB2 expression is restricted to proliferating vascular SMCs, and BTEB2 mRNA levels are rapidly increased on mitogenic stimulation.5 These data have led us to propose that BTEB2 represents a marker for activated SMCs in the vascular wall.
The aim of the present study was to determine the expression profile of BTEB2 in developmental and pathological vascular processes and, in addition, to examine the expression of BTEB2 in human atherectomy specimens. Our results show that BTEB2 is expressed in fetal and dedifferentiated SMCs in humans and in rabbits and for expression of BTEB2 to correlate with subsequent development of restenosis after angioplasty.
| Methods |
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Thoracic aortas of normal fetuses and adults were used as controls for normal tissues. Aortas were obtained from three 9- to 10-week-old fetuses at autopsy. Three adult aortas without atherosclerosis were obtained from 30- to 40-year-old patients during a valvular replacement procedure. This research project was approved by the Ethics Committee of the Gunma University School of Medicine.
Collection of Human Coronary Atherectomy Samples
Samples were obtained from patients undergoing directional
coronary atherectomy (DCA) between January 1993 and December
1994 at Saiseikai Maebashi Hospital (80 primary lesions and 136
restenotic lesions). Clinical variables documented include
age, sex, presence of current angina pectoris and risk factors for
coronary artery disease (history of hypertension,
hypercholesterolemia, diabetes mellitus, and
cigarette smoking), and type of angina (stable versus unstable) (Table 1
).
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Atherectomy Procedures and Angiographic Analysis
DCA procedures and criteria for successful DCA were done as
described.6 Follow-up angiography was performed at 4
months after atherectomy. Cineangiograms were evaluated
with digital calipers by 3 cardiologists blinded to the patients
backgrounds. The minimal lumen diameter (MLD) was determined from
magnified images obtained in the single most stenotic view
before and after coronary atherectomy, and the reference
diameter (RD) was defined as the diameter of the proximal or distal
segment without stenosis. A 10F guiding catheter was used as
the reference object. Angiographic parameters were
calculated as follows: % diameter stenosis=MLD/RD; acute
gain=MLD (at DCA)-MLD (immediately after DCA); gain index=acute
gain/RD; late loss=MLD (immediately after DCA)-MLD (follow-up); and
loss index=late loss/acute gain. Restenosis was considered to
be present if the treated vascular segment contained a lesion of
>50% stenosis or if >50% of the acute gain was lost.
Histological Definitions
Atherectomized tissues were histologically
classified into 3 types: (1) diffuse intimal thickening (DIT), (2)
atherosclerotic tissue, and (3) lesion containing stellate cells.
Lesions classified as DIT were defined as having proliferation of the
intima without atherosclerotic change; lesions classified as
atherosclerotic tissue included samples consisting of fibrous
connective tissue, with or without associated foam cells,
cholesterol clefts, lipid cores of pale granular material,
or lymphocytes; and lesions containing stellate cells were
characterized by the proliferation of stellate-tospindle-shaped cells
with loose to mildly fibrotic connective tissues.
Immunohistochemistry
All samples were immediately fixed in 10% buffered formalin and
embedded in paraffin. Immunoenzymatic staining was carried out as
previously described.7 All tissues were stained with
antibodies against SM1 smooth muscle myosin heavy chain isoform
(hereafter referred to as SM1) (dilution 1:1000 in
PBS),7 8 9 10 von Willebrand factor (vWF) (1:100
dilution, Atlantic Antibodies),11 12 or BTEB2 (1:1000
dilution).5 Negative control sections were stained with
normal mouse IgG (dilution 1:100 in PBS, Santa Cruz Biotechnology) in
lieu of BTEB2 antisera.
Double-Labeling Immunohistochemistry
Atherectomized specimens were stained by anti-BTEB2 antibody and
monoclonal antibody against human proliferating cell nuclear antigen
(PCNA) (1:50 dilution in PBS, Santa Cruz Biotechnology). Pretreatment
of atherectomized tissues by microwave irradiation (400 W, 5
minutesx2) was performed after deparaffinization. After incubation
with anti-PCNA antibody, specimens were covered with
peroxidase-conjugated anti-mouse immunoglobulin (1:100 dilution in PBS,
Santa Cruz Biotechnology). After being stained with DAB, sections were
treated with 0.1 mol/L glycinehydrochloric acid buffer (pH 2.2) for
120 minutes with 4 changes. Next, samples were incubated for 60 minutes
with monoclonal antibody against BTEB2. After being washed with
Tris-buffered saline (pH 7.6), sections were treated by the alkaline
phosphatase/antialkaline phosphatase (APAAP) method (APAAP kit,
DAKO). Sections were visualized by addition of a solution of 0.2
mmol/L naphthol AS-MX phosphate, 1 mmol/L Fast Red TR, and 1
mmol/L levamisole in 0.1 mol/L Tris-HCl buffer (pH 8.2) for 20 minutes.
After being counterstained with methyl green, samples were mounted in
glycerol-gelatin medium.
In Situ Hybridization
In situ hybridization using rabbit-specific
35S-labeled riboprobes was performed as
previously described.13 14 A 660-bp fragment (full length)
from the rabbit BTEB2 cDNA was subcloned into the pSP72 vectors
(Promega). Sense and antisense riboprobes were prepared by
linearization of the constructs with the respective restriction
endonucleases EcoRI and XhoI and then use of
either SP6 or T7 RNA polymerase. In situ results were evaluated by
polarized light epiluminescence microscopy (Leitz).
Statistical Analysis
Statistical analysis of frequency was performed with the
2 test or Fishers exact test for small
samples, and means were compared by unpaired 2-tailed t
test. The odds ratio was used as a measure of risk. Modeling of
dichotomous variables was done by logistic-regression modeling.
Values are reported as mean±SD. A value of P<0.05 was
considered statistically significant.
| Results |
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BTEB2 Expression Is Pathologically Regulated With Induction in
Neointima
We next investigated whether BTEB2 is expressed in
neointimal cells containing dedifferentiated SMCs in
balloon-injured rabbit aortas. Neointimal cells which
developed after denudation were positive for BTEB2 (Figure 2A
). SMCs in neointima as
well as endothelial cells were BTEB2-positive as
assessed by SM1 (Figure 2A
) and vWF expression (data not shown).
Interestingly, a large number of BTEB2-positive cells were found in the
adventitia after balloon injury, although these cells stained only
weakly for BTEB2 in normal rabbit aorta (data not shown). In situ
hybridization experiments using the rabbit balloon-injured aorta at 2
weeks after injury showed that the silver grain dots on the
neointima yielded by antisense BTEB2 probe seemed to be
stronger than those formed by sense BTEB2 probe (Figure 2B
).
Although some grains were present in the media, they seemed to be
nonspecific because many of them were not localized to the cytoplasm
where mRNAs should be present but rather were distributed over
nuclei.
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Human Coronary Lesions Express BTEB2
We further investigated BTEB2 expression in coronary
lesions by use of atherectomized tissues. All atherectomy specimens,
including restenotic lesions and primary lesions, contained
atherosclerotic tissue and/or DIT lesions. As shown in Figure 3A
and 3B
, both restenotic and
primary lesions contained stellate cells, which we refer to as stellate
SMCs (St-SMCs) on the basis of their being SM1-positive (Figure 3
). One hundred twenty of 136 restenotic lesions (88%)
and 32 of 80 primary lesions (42%) were positive for these cells. In
addition, in both restenotic and primary lesions, BTEB2 was
exclusively positive in St-SMCs; few if any cells in DIT lesions and
atherosclerotic tissues were positively stained with BTEB2 antibody
(Figure 3C
). Furthermore, double-labeling immunohistochemistry
showed St-SMCs to be positive for both PCNA and BTEB2, in contrast to
DIT and atherosclerotic lesions, in which either protein was barely
detectable (Figure 3C
). These findings show that BTEB2 is
preferentially expressed in PCNA-positive St-SMCs. In contrast, neither
PCNA nor BTEB2 was detectable in DIT lesions (Figure 3C
).
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High Incidence of Restenosis in Patients With
BTEB2-Positive Lesion
To assess the clinical relevance of BTEB2 expression in human
coronary disease, we further correlated the restenosis
rate after DCA with the immunoreactivity of BTEB2 staining. We
classified patients into two groups depending on the basis of the
presence of BTEB2-positive lesions. Of 80 primary lesions, 32 showed
presence of St-SMC lesions with positive BTEB2 staining, in contrast to
48 patients for those without (the lesions have absolutely no St-SMCs).
The restenosis rate was significantly higher at 55.6% for the
BTEB2-positive group compared with 25.0% for the BTEB2-negative group
(P=0.01) (Table 2
and Figure 4A
). Interestingly, among the 25 patients
with restenosis, 7 patients, all of whom had BTEB2-positive
lesions, repeatedly developed restenosis and needed
coronary intervention more than three times. The incidence of
recurrent restenosis was remarkably higher in the
BTEB2-positive than in the BTEB2-negative group (46.7% versus 0%,
P=0.02) (Table 2
and Figure 4A
).
Univariate analysis showed the presence of
BTEB2-positive cells to be a significant predictor of
restenosis. Other than the presence of BTEB2-positive cells,
the presence of a left anterior descending coronary artery
lesion was also a significant predictor of restenosis. Odds
ratios for restenosis and recurrent restenosis
associated with the presence of a BTEB2-positive lesion were 3.75
(P=0.01) and infinity (P=0.02), respectively
(Table 3
). The relation between the
presence of BTEB2-positive lesions and the risk of restenosis
did not change in the multivariate logistic regression
model (odds ratio 4.79, 95% CI 1.73 to 13.24, P=0.003).
MLD, percent diameter stenosis, late loss, and loss index also
differed significantly between the BTEB2-positive and BTEB2-negative
groups (Table 2
and Figure 4B
). Baseline patient profiles
and angiographic results before and immediately after atherectomy did
not differ between the two groups (Tables 1
and 2
). In
contrast, there was no correlation between restenotic lesions
containing BTEB2-positive cells and recurrent restenosis
(data not shown).
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| Discussion |
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Developmental Change in BTEB2 Expression
BTEB2 was expressed in fetal aortic SMCs but not in adult rabbits
or humans. Expression of BTEB2 was regulated in a developmental manner.
Although BTEB2 expression in the present study was examined in two
distinct states (fetal and adult stages), prominent expression in the
fetal stages suggests that BTEB2 may play a role in aortic development.
Because BTEB2 was isolated as a DNA-binding factor that regulates the
SMemb/NMHC-B gene and shows a temporal profile of in vivo expression
levels being most prominent in the fetal stages,5 it is
tempting to envision that developmentally coordinated expression of
BTEB2 regulates the temporal expression of genes associated with the
activated SMC phenotype including the SMemb/NMHC-B
gene, although BTEB2 may not be the sole regulator of the SMemb/NMHC-B
gene. Because transcription is a result of the combinatorial effect of
numerous factors and regulators, it is likely that it is a candidate
for being a dominant regulator of expression of this gene. In fact, the
expression of BTEB2 was not limited to smooth muscle but was also found
in endothelial cells. Conversely, BTEB2 was not
expressed in the medial layer of normal human aorta or coronary
arteries in contrast to SMemb/NMHC-B. These results suggest that BTEB2
expression does not necessarily parallel the expression of SMemb/NMHC-B
and that BTEB2 alone is not responsible for the regulation of the
SMemb/NMHC-B gene. The role of BTEB2 in SMemb/NMHC-B regulation in vivo
deserves further investigation.
Pathological Regulation of BTEB2 Expression
Consistent with the developmentally regulated expression
profile of BTEB2, we further showed that BTEB2 is highly expressed in
dedifferentiated neointimal cells in a rabbit model of
balloon injury. Given that the neointima is the site at
which numerous injury-induced genes are expressed and proliferation of
SMCs occurs during the first 4 weeks after injury,9 18
these findings suggest that BTEB2 expression is strongly correlated
with the activated vascular SMC phenotype.
Studies using human samples also showed expression of BTEB2 in coronary lesions. Interestingly, BTEB2 expression in both restenotic and primary lesions was restricted to St-SMCs. On the basis that these St-SMCs were also positive for PCNA, we consider that these cells represent the proliferative or activated SMCs. In contrast, SMCs in DIT or media were negative for both BTEB2 and PCNA. Preferential expression of BTEB2 in stellate cells may be associated with phenotypic modulation of vascular SMCs which suggests that BTEB2 can serve as a molecular marker for St-SMCs.
St-SMCs are found in a large number of restenotic lesions but are also present in primary lesions.6 19 20 21 In a previous analysis of atherectomy specimens, lesions containing St-SMCs have not been distinguished from other types of atherectomized tissues, and St-SMCs do not appear to be actively proliferative.22 In contrast, we demonstrate that St-SMCs express PCNA as well as BTEB2. However, it should be noted that the frequency of PCNA-positive St-SMCs in this study is much higher than that reported in the previous study.22 One of the major reasons for this discrepancy would be that we used microwave irradiation to enhance the antigenicity of the sample. This technique may cause overestimation of the number of PCNA-positive cells. Thus, the question regarding PCNA expression and restenosis cannot be addressed without further investigation.
Correlation Between BTEB2 Expression and Restenosis
An important clinical implication of our studies is the
correlation between positive BTEB2 staining and an increased risk of
restenosis in primary lesions. The fact that recurrent
restenosis is strongly correlated with BTEB2 expression is
further suggestive that BTEB2 expression may reflect a proliferative
SMC state. Although further studies are needed to understand the
molecular mechanisms underlying the association between
restenosis and BTEB2 expression, it is tempting to envision
that BTEB2 is at least one of the important transcription factors
involved in regulating the specific subset of genes that are
activated in restenosis. Persistent activation of these
genes by continuously elevated levels of BTEB2 may lead to the
pathogenic state associated with recurrent restenosis. This is
reminiscent of cancerous cells in which gene regulation, including
regulation at the transcription level, is not properly coordinated,
leading to an autostimulatory and proliferative state.
In summary, our studies have shown that BTEB2 expression is developmentally and pathologically regulated. BTEB2 appears to be preferentially expressed in activated vascular SMCs in balloon-injured rabbit aortas and in human coronary arteries. BTEB2 expression in primary lesions also correlated with development of restenosis after atherectomy. Further investigation of BTEB2 as a predictive factor for coronary restenosis may provide important insight into targeted therapeutic interventions for coronary restenosis.
| Acknowledgments |
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Received February 17, 2000; revision received June 2, 2000; accepted June 16, 2000.
| References |
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