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* Institute for Biomedical Aging Research, Austrian Academy of Sciences, Innsbruck; and
Institute for General and Experimental Pathology, University of Innsbruck, Medical School, Innsbruck
1Correspondence: Institute for Biomedical Aging Research, Austrian Academy of Sciences, Rennweg 10, A-6020 Innsbruck, Austria. E-mail: Qingbo.Xu{at}oeaw.ac.at
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: MAPKs • smooth muscle cell • propidium iodide • cytokine
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. Vein grafts
become occluded when abnormal cell proliferation in the smooth muscle
layer and extra tissue production in the inner lining of the vessel
thicken the vessel wall in response to the new high-pressure
circulation that the grafted vein must withstand (2)
. Most
studies of the pathogenesis of vein graft arteriosclerosis focused on
smooth muscle cell (SMC) proliferation (3)
. In fact, an
extensive loss of endothelial cells and a significant loss of SMCs were
observed in the intima and media at the early stage of vein grafts, but
little is known about the molecular mechanisms and the role of cell
death or apoptosis in the pathogenesis of vein graft arteriosclerosis
(4)
.
Apoptosis, or programmed cell death, can be induced by cytokines
(5)
, conflicting signals, and DNA damage (6)
,
but it remains to be clarified whether biomechanical stress itself can
cause apoptosis in vascular SMCs. The grafted veins are subjected to
increased biomechanical forces in the form of stretch stress due to
blood pressure and shear stress due to blood flow (7)
.
While shear stress is primarily sensed by endothelial cells, tension
stress exposes all vascular cell types to cyclic mechanical strain due
to the pulsatile nature of blood flow. The sudden elevation in
mechanical forces could be a strong stimulus to the grafted vessel wall
and may result in activation of intracellular signal pathways leading
to gene expression, cell death, or cell proliferation (7)
.
Mitogen-activated protein kinases (MAPKs), a family of serine/threonine
kinases, encompass the extracellular signal-regulated kinases (ERKs),
c-Jun NH2-terminal protein kinases (JNKs) or
stress-activated protein kinases (SAPKs), and p38 MAPKs
(8
9
10
11)
. They are thought to play a critical role in
regulating cellular events required for cell growth, differentiation,
and apoptosis (8
9
10
11)
. MAPKs are strongly activated in the
arterial wall in response to angioplasty (12)
, acute
hypertension (13)
, and low-density lipoproteins
(14)
, which are known risk factors for vascular diseases.
Recently we provided direct evidence that mechanical stress results in
rapid activation of PDGF receptor–MAPK signal pathways, which could be
important for understanding the role of biomechanical stress in the
pathogenesis of vascular diseases (15)
.
We hypothesize that apoptosis in venous bypass grafts may occur as one of the earliest events after grafting, and that induction of cell death may be determined by mechanical stress. The objectives of the present study were to detect apoptosis in venous bypass grafts in vivo, to determine the role of mechanical stress for SMC apoptosis in vitro, and to establish a direct link between biomechanical stress stimulation and apoptosis in SMCs.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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. Briefly, C57BL/6J mice
were used as donors and recipients. A segment (approx. 1 cm) of vena
cava was grafted end-to-end to carotid arteries using a cuff technique.
The vein grafts were harvested at 1 day or at 1, 2, 4, and 8 wk
postoperatively (four to eight randomly chosen mice at each time point)
by cutting the implanted segments from the native vessels at the cuff
end after mice were anesthetized and perfused with 0.9% NaCl solution
via cardiac puncture in the left ventricle and with 4%
phosphate-buffered formaldehyde (pH 7.2) for 3 and 5 min, respectively.
TUNEL assay
Samples were fixed with 4% phosphate-buffered formaldehyde (pH
7.2) at 4°C for 24 h. After fixation, the grafts were cut in
middle of the vein segments, dehydrated in graded ethanol baths,
cleared in xylol, and embedded in paraffin. Routinely, 7 µm-thick
sections were made throughout the dissected fragments. Accumulated
internucleosomal DNA fragments (apoptosis) were detected using an
in situ apoptosis detection kit (Boehringer Mannheim Corp.,
Mannheim, Germany). Sections were dewaxed, rehydrated, fixed with 4%
paraformaldehyde, and permeabilized by 0.1% Triton X-100/0.1% sodium
citrate. Sections were incubated with fluorescein-conjugated dUTP
and the enzyme terminal deoxyribonucleotide transferase, and stained
with anti-fluorescein antibodies linked with alkaline phosphatase.
Finally, sections were developed with Fast Red substrate and
counterstained with hematoxylin. Percentages of positive stained cells
were determined by counting the numbers of labeled and total cells
using a light microscope (Axiophot, Zeiss). Positive cells of whole
intima and media of control veins and 1 wk grafts were counted and two
regions of each section of 4 and 8 wk grafts. Morphological features of
apoptosis were assessed on light and laser microscope in transmission
scan mode (Microradiance, Bio-Rad).
Cell culture
SMCs were isolated by enzymatic digestion of aortas from rat,
mice and humans, as described previously (17
, 18)
, and
cultured in DMEM (PAA Laboratories GmbH, Linz, Austria) supplemented
with 20% fetal calf serum, penicillin (100 U/ml), and streptomycin
(100 µg/ml). Cells were incubated at 37°C in a humidified
atmosphere of 5% CO2 and passaged by treatment
with 0.05% trypsin/0.02% EDTA solution. Experiments were conducted on
SMCs achieving subconfluence at passages 7 to 15.
Cyclic strain stress
SMCs were seeded on silicone elastomer-bottomed culture plates
(Flexcell, McKeesport, Pa.) at 1.5 x 105
cells per well, grown for 48 h in medium with 20% fetal calf
serum, and subjected to cyclic strain stress. The Cyclic Stress Unit, a
modification of the unit initially described by Banes et al.
(19)
, consisted of a computer-controlled vacuum unit and a
base plate to hold the culture plates (FX3000 AFC-CTL, Flexcell).
Vacuum (15 to 20 kPa) was repetitively applied to the
elastomer-bottomed plates via the base plate. Cyclic deformation (60
cycles/min) with elongations ranging from 5 to 30% was applied for 10
min up to 6 h in a humidified incubator with 5%
CO2 at 37°C (15)
.
Annexin V staining
Annexin staining was performed according to the manufacturer’s
instructions (PharMingen, San Diego, Calif.). In brief, adherent SMCs
were trypsinized 16 h after mechanical stress, washed twice with
cold phosphate-buffered saline (PBS) (4°C), resuspended in binding
buffer containing 10 mM HEPES/NaOH (pH 7.4), 140 mM NaCl, 2.5 mM
CaCl2 and incubated with 5 µl FITC-labeled
annexin V and 10 µl propidium iodide (PI, stock solution 50 µg/ml)
for 15 min in the dark at room temperature. SMCs were analyzed by flow
cytometry. After appropriate markings for negative and positive
populations were set, the percentage of annexin
V+/PI- cells was
determined and compared with unstressed controls.
PI staining
SMCs were harvested by trypsinization 24 h after mechanical
stress. Cell pellets were resuspended in 200 µl 0.1% Triton
X-100/0.1% NaN3/H2O and
stained with 50 µg/ml propidium iodide (Sigma, St. Louis, Mo.) at
4°C for 30 min. Cellular PI fluorescence signal was recorded on the
FL2 channel of a FACS scan flow cytometer (Program Cell Quest, Becton
Dickinson, Mountain View, Calif.). The number of apoptotic cells in the
sub-G1 peak was indicated by lower fluorescence intensity and expressed
as percentage of total events (5000 events/measurement).
Stable transfection
MKP-1 cDNA was isolated from a rat lung cDNA library by Liu et
al. (20)
; plasmids expressing dominant negative rac1 were
provided by Dr. G. Baier (Institute for Medical Biology and Human
Genetics, University of Innsbruck, Austria). Rat SMCs were stably
transfected with N17 rac (pEF-rac1 N17) and rMKP-1 (pSG5-rMKP1)
plasmids, respectively, using a SuperFect Kit (Qiagen, Chatsworth,
Calif.). Transfected cells were cultured overnight, divided one to
four, and placed in culture medium supplemented with 150 µg/ml G418
(Sigma) to guarantee selection of cells carrying a neomycin-resistant
plasmid. MKP-1- and rac N17-transfected SMCs were identified by
Northern blotting and/or Western blotting, as described previously
(21)
. Transfected cells were used for experiments on
passages 20 to 30.
Protein extraction
After strain stress, SMCs were washed twice with cold PBS
(4°C, pH 7.4) and harvested on ice in buffer A containing 20 mM HEPES
(pH 7.4), 2 mM EDTA, 50 mM ß-glycerophosphate, 1 mM DTT, 1 mM
Na3VO4, 1% Triton X-100,
10% glycerol, 1 µg/ml leupeptin, 1 µg/ml aprotinin, and 100 µM
PMSF. The suspension was incubated on ice for 20 min with vortexing
every 5 min. Cellular debris was pelleted by centrifugation for 30 min
at 13,000 rpm (Heraeus centrifuge) at 4°C, supernatants were
collected, and protein concentration was measured by the Bio-Rad assay
(Bio-Rad Laboratories, Hercules, Calif.). Membrane protein isolation
for rac expression was performed according to this method with slight
modifications for membrane preparation, as described (21)
.
Briefly, SMCs were washed in cold PBS (4°C), scraped, pelleted, and
resuspended in 500 µl of homogenizing buffer (HB) (25 mM HEPES, 1 mM
EDTA, 1 µg/ml leupeptin, pepstatin A, and 0.1 mM PMSF). Cells were
sonicated for 10 s and centrifuged at 2,000 rpm for 10 min to
remove debris. After centrifugation at 55,000 x g for
1 h at 4°C, the supernatant was removed and the membrane pellet
was resuspended in 50 µl of HB, sonicated for 10 s, and protein
concentration was measured.
Western blot analysis
The procedure used for Western blot analysis was similar to that
described elsewhere (21)
. Briefly, 50–150 µg of
proteins were separated by electrophoresis through a 10–12% sodium
dodecyl sulfate (SDS)-polyacrylamide gel and transferred onto
nitrocellulose membranes. For Western blot of rac, 30–100 µg
membrane protein was used in reduced conditions (2.5% SDS/250 µM DTT
for 5 min at 90°C). Blots were probed with antibodies against
phosphorylated p38 MAPK, pan p38 and MKP-1 (Santa Cruz Biochemicals,
Santa Cruz, Calif.) or myc-tagged rac1 (a gift from Dr. G. Baier,
Innsbruck). Reactions were visualized by ECL Western blot Detection Kit
(Amersham) after incubation with peroxidase conjugates.
Kinase assay
The procedure for measuring kinase activity was also described
previously (22)
. Briefly, p38 proteins were
immunoprecipitated from the protein extracts. The p38 antibody used in
our experiments was not cross-reactive with other MAPKs, as confirmed
by its single band in Western blot analyses. p38 kinase activity was
measured based on phosphorylation of myelin basic protein (MBP; 6 µg,
Upstate Biotech. Inc., Lake placid, N.Y.) and
-P32 ATP (5 µCi) for 20 min, which has
proved to be similar to HSP27 as a substrate (23)
. To stop
the reaction, 15 µl of 4x Laemmli buffer was added and the mixture
was boiled for 5 min. Protein kinase reactions were resolved by
SDS-polyacrylamide gel electrophoresis (15% gel) and subjected to
autoradiography.
Statistical analysis
Statistical analyses were performed on a Macintosh computer with
StatView SE+ Graphics software. Analysis of variance was
performed for multiple comparisons. An unpaired Student’s t
test was used to assess differences between two groups. Results are
given in means ± standard deviations (SD). A
P value of less than 0.05 was considered statistically
significant.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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, the
intima became largely acellular 1 wk after grafting, implicating a
considerable loss of cells in grafted vein segments. Figure 1
shows typical examples of TUNEL+ cells.
Quantitative analysis revealed a significant increase of
TUNEL+ nuclei in venous bypass grafts compared to
normal venous vessels 1 wk after implantation (13% vs. 2%). This
marked increase in TUNEL+ cells peaked 4 wk
(29%) and declined 8 wk (21%) after grafting (Fig. 2A
). When apoptosis was corroborated by morphology such as
concomitant condensation, lobulation, or fragmentation of the labeled
nucleus on light and laser microscope levels, the maximal rate of
apoptosis diminished to 6.4% (Fig. 2B
) but was
significantly higher than in freshly harvested veins (0.5%).
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To clarify the role of mechanical stress on SMC apoptosis, vein
segments were grafted either to jugular veins or carotid arteries.
Assessment of TUNEL+ nuclei on day 1 after the
surgical procedure revealed significant differences (Fig. 3
). Levels of apoptosis, as indicated by TUNEL staining, increased to
20% in vein to artery grafts, whereas vein-to-vein grafts manifested
preserved cell viability (3.6%) in their normal low pressure
environment compared to normal venous vessels (2%). By quantitating
apoptotic cells with additional morphological criteria, the difference
in the level of apoptosis remained significant (9% vs. 0.8% or
0.5%). Less TUNEL-positive staining in vein-to-vein grafts underscores
the possible role of mechanical stress in vascular cell apoptosis and
suggests that the surgical procedure per se has little
effect on overestimation of apoptosis.
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Cyclic strain stress-induced SMC apoptosis
To explore the possibility that mechanical stress directly
stimulates SMC apoptosis, rat aortic SMCs were cultivated on a flexible
membrane and subjected to cyclic strain stress. Apoptotic cells were
identified by double labeling with annexin V and PI. PI labels all dead
cells, including necrosis and final stages of apoptosis, whereas cells
entering apoptosis are only stained by annexin V. Data shown in
Fig. 4
indicate a time- and intensity-dependent induction of cell death in
response to mechanical stress. Time course analysis (60 cycles/min,
15% elongation) revealed a two- to threefold increase in annexin
V-FITC+/PI- cells 16 h after mechanical stress (Fig. 4A
), reflecting an early
stage of apoptosis. Furthermore, apoptosis increased in SMCs subjected
to cyclic strain stress (60 cycles/min, 6 h) in a
strength-dependent manner, reaching significant differences from
unstressed controls above 5% elongation (Fig. 4B
). Changes
in the rate of apoptosis were comparable in vascular SMCs of mouse,
rat, and human (Fig. 5
). After 24 h, apoptosis almost doubled in strain-stressed cells
(60 cycles/min, 6 h, 10% elongation) compared to unstressed
controls, although significant differences in spontaneous
apoptosis/necrosis were observed among untreated SMCs of different
species cultivated on elastomer bottomed plates.
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In addition, mechanical stress-induced apoptosis was demonstrated
by various techniques. PI-stained nuclei were analyzed by flow
cytometry for DNA content. Apoptotic cells appeared in the
sub-G1 region compared to viable cells due to DNA
fragmentation (Fig. 6A
). Consistent with these results, TUNEL positivity was
observed only in stressed SMCs, but not in unstressed controls after
6 h of 15% stretch stress (Fig. 6B
). One of the
earliest features of cells undergoing apoptosis are changes in plasma
membrane asymmetry, such as phosphatidylserine exposure. Therefore,
annexin V staining can identify apoptosis at an earlier stage than
assays based on nuclear changes, such as TUNEL and PI staining.
Representative dot plots of annexin V staining are shown in Fig. 6C
. Untreated cells were primarily annexin
V-FITC-/PI- (lower left
quadrant), indicating viable cells. After mechanical stress, a
significant number of cells became annexin
V-FITC+/PI- (lower right
quadrant), reflecting early stages of apoptosis. A population of cells
progressing to late stages of apoptosis became double positive (upper
right quadrant).
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Involvement of p38 MAPK in mechanical stress-induced apoptosis
To elucidate the potential regulatory mechanisms that may
lead to acute cell death after mechanical stress, levels of p38 MAPKs
were analyzed in stretch-stressed SMCs by Western blot analyses using
anti-phosphorylated-p38 MAPK antibodies. As shown in Fig. 7A
, tensile stimuli resulted in increased p38 phosphorylation
in SMCs stressed with elongation ranging from 5% to 30% of original
size. Antibodies against pan-p38 indicated that similar amounts of
proteins were loaded (bottom panel). Angiotensin II, a potent activator
of all MAPKs in SMCs (24)
, served as positive control. A
similar p38 activation was measured in kinase assays on phosphorylation
of MBP as substrate. This p38 activity was markedly inhibited by 1 µM
of the specific inhibitor SB 202190 (Fig. 7B
). Pretreatment
with SB 202190 prevented mechanical stress-induced apoptosis, as
identified by annexin V staining (Fig. 7C
). No elevation of
apoptosis was observed in unstressed cells in the presence of 1 µM
SB202190 for 24 h. Thus, p38 MAPKs are required for the
transduction of apoptotic signals in response to mechanical stress.
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To provide further evidence for the importance of p38 MAPK
pathways, we investigated the effects of upstream and downstream signal
transducers in MAPK signaling and apoptosis: Rac, a member of the ras
superfamily of small GTP binding proteins (25)
, is
considered to be a key transducer in various MAPK signaling events.
Therefore, SMCs were stably transfected with expression vectors
encoding a myc-tagged form of a dominant negative rac 1 (rac1 N17). The
expression vector also contained the selectable marker gene, neo, to
select multiple G418-resistant colonies. Transfected clones stably
expressing rac-1 N17 were identified by Western blotting using
anti-myc-tag antibodies (Fig. 8A
). As shown in Fig. 8B
, increased phosphorylation
of p38 was observed in vector controls, whereas phosphorylated p38
levels were almost undetectable in rac1 N17-transfected cell lines,
indicating significant blocking of p38 activation induced by strain
stress. Concomitantly, the production of a large quantity of a potent
dominant negative mutated form of rac1 markedly reduced mechanical
stress-induced apoptosis compared to vector-transfected SMCs (Fig. 8C, D
). Since MAPK phosphatase-1 (MKP-1) specifically
inactivates all three MAPKs, including p38, we assessed the effects of
MKP-1 overexpression on SMC apoptosis. Transfected clones stably
expressing MKP-1 were identified by Western blotting and showed high
MKP-1 expression (Fig. 9A
). In comparison to cells transfected with vector, p38
phosphorylation was significantly reduced in MKP-1-transfected SMCs
(Fig. 9B
). Quantification of optical densities from
autoradiograms of three experiments revealed a 13-fold increase of
phosphorylated p38 in vector cells after cyclic strain stress compared
to unstressed controls, but only a minor change in stressed
MKP-1-transfected cells. Inhibition of p38 activation was again
associated with a marked decrease in apoptosis, as evidenced by annexin
V/PI staining, suggesting that MAPK activation is a critical
determinant for SMC apoptosis stimulated by biomechanical stress (Fig. 9C, D
).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Investigations on the pathogenesis of vein graft disease have been
conducted primarily on cell proliferation (3)
, and little
is known about the role of cell death in disease development. Our
findings provide solid evidence that one of the earliest cellular
events in neointima formation in vein grafts is cell apoptosis, and our
in vivo and in vitro experiments demonstrate for
the first time that mechanical stress is a critical initiator of SMC
apoptosis. Based on our observations in the murine vein graft model, a
large portion of cells within grafted vessels become apoptotic in the
first week. Concomitantly, a marked loss of SMCs has been observed in
early lesions of human vein grafts (4
, 16
, 26)
. Necrosis
cannot be excluded in this process, particularly in vivo,
because cell death by nonapoptotic mechanisms is also TUNEL positive
when extensive DNA breaks have occurred (27)
. Thus,
elevated biomechanical force is primarily responsible for cell death,
an early event in the pathogenesis of vein graft disease.
Massive mononuclear cell infiltration into the vessel wall follows cell
death between 1 and 4 wk after engraftment. Recently, we found that
intimal lesions in vein isografts performed in ICAM-1 -/- mice were
reduced up to 50% compared to wild-type controls, implicating the
importance of mononuclear cell recruitment for the development of
neointimal hyperplasia (Y. Zou et al., unpublished observations). The
mechanism by which monocytes/macrophages are continuously recruited to
the neointima of vein bypass grafts is not fully understood (28
, 29)
. Although apoptosis is believed to rarely evoke inflammation
responses in tissues (30
, 31)
, apoptotic cells release the
S19 ribosomal protein dimer (32)
, a chemokine for
macrophages/monocytes, by molecular mimicry to complement C5a
(33)
. The infiltrating cells might remove apoptotic bodies
(34
, 35)
, and paracrine stimuli such as cytokine secretion
during phagocytosis could enhance infiltration and initiate an
overwhelming repair mechanism. If cell death in vein grafts is
partially induced by necrosis, cell death may be a primary force for
the induction of inflammatory responses in grafted veins.
Abundant evidence indicates that SMC accumulation and matrix protein
deposition are major components of neointimal lesions in late-stage
vein grafts (36)
. The molecular mechanisms of SMC
proliferation in vivo remain to be elucidated. Given the
fact that SMC accumulation in the intima occurs 4 wk postoperatively,
we postulate three possibilities. 1) Phenotypic selection:
the phenotype of neointimal SMCs differs from medial SMCs, increased
resistance against apoptosis being one of their striking features
(37)
. Elevated biomechanical forces result in apoptosis of
a large portion of SMCs. The small remaining population of SMCs can
sustain the arterial blood pressure and may rapidly expand to form
neointima. 2) Inflammatory response: monocytes/macrophages
in vein grafts can release a panel of cytokines and growth factors,
such as transforming growth factor-ß and fibroblast growth factor,
which lead to SMC migration and proliferation (38)
.
3) Mechanical stress as mitogen: mechanical forces stimulate
the expression and release of platelet-derived growth factors (PDGFs)
(39
, 40)
and induce PDGF receptor phosphorylation or
activation in SMCs (15
, 41)
. Therefore, PDGF production
and their receptor activation play a pivotal role in initiating SMC
migration and proliferation.
MAPKs have been implicated in both regulation of cell proliferation and
cell death (8
9
10
11)
, which underscores their possible
importance in vein graft disease. We documented previously that PDGF
receptor 
is phosphorylated and activated in response to mechanical
stimuli whereby mechanical forces initiate signaling pathways normally
used by growth factors such as ras-raf-MEK-ERK activation, followed by
enhanced DNA binding activity of transcription factor AP-1
(15)
. In the present study, we describe another mechanical
stress-induced signal pathway involving rac and p38 MAPKs. Activation
of p38 MAPKs occurs in a variety of extracellular stressors including
osmotic shock, UV irradiation, exposure to cytokines, and endotoxin
(7
, 9)
. A novel finding in the present study is that
p38-mediated signaling plays a crucial role in apoptosis after cyclic
strain stress. Mechanical stress-induced apoptosis declined by
pretreatment with p38 MAPK inhibitor or in transfected cell lines that
inactivate or inhibit p38 phosphorylation. It should be noted that the
transfectants failed to result in complete inhibition of apoptosis
induced by mechanical stress. Therefore, other signal transduction
pathways, such as JNK/SAPK or cross talk between ERK and p38 signaling,
may contribute to direct modulation of signal transductions leading to
apoptosis during mechanical stress. Taken together, our findings
suggest that p38 is a crucial signal transducer for SMC apoptosis in
response to mechanical stress and that activation of p38 MAPKs in
grafted vessels might play a significant role in mediating cell
apoptosis.
What initiates p38 activation after mechanical stress? PDGF
receptors cannot be regarded as candidates because the growth factor
receptor antagonist suramin does not block p38 activation, but rather
enhances its activation (41a)
. Integrins that bind to
collagen or flexible membrane might be activated in response to
mechanical stress (42)
, a notion supported by the
observation that integrins are involved in JNK/SAPK activation in
endothelial cells stimulated by shear stress (43
, 44)
.
Further studies to identify other mechanosensors will significantly
enhance our understanding of biomechanical stress-induced vascular
remodeling. In addition, the downstream mediators of
p38-dependent apoptosis are poorly characterized, but there is evidence
that p38 MAPK contributes to phosphorylation of p53 (45)
.
p53 activation has been implicated in cell cycle control, DNA repair,
and apoptosis (46
, 47)
by up-regulating Fas/Apo1
(48)
and Bax expression (49)
.
In summary, our results suggest that biomechanical stress rapidly
induces SMC apoptosis in vivo and in vitro, in
which p38 MAPKs play a pivotal role in signal transduction leading to
apoptosis in response to mechanical stimuli. We hypothesize that cell
death is one of the earliest cellular events that evokes an
inflammatory response, followed by SMC proliferation. Vascular cell
apoptosis and proliferation coincide in the development of
arteriosclerotic lesions. The balance between these two processes,
i.e., proliferation/apoptosis, is a determinant during vessel
remodeling. If the balance can be modified arbitrarily, e.g., by
inhibiting (50)
or enhancing apoptosis (51)
at different stages of vessel grafts, new strategies for prevention and
treatment of vein graft disease may be achieved.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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U. Mayr, Y. Zou, Z. Zhang, H. Dietrich, Y. Hu, and Q. Xu Accelerated Arteriosclerosis of Vein Grafts in Inducible NO Synthase-/- Mice Is Related to Decreased Endothelial Progenitor Cell Repair Circ. Res., February 17, 2006; 98(3): 412 - 420. [Abstract] [Full Text] [PDF]
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 Z. A. Ali, C. A. Bursill, Y. Hu, R. P. Choudhury, Q. Xu, D. R. Greaves, and K. M. Channon Gene Transfer of a Broad Spectrum CC-Chemokine Inhibitor Reduces Vein Graft Atherosclerosis in Apolipoprotein E-Knockout Mice Circulation, August 30, 2005; 112(9_suppl): I-235 - I-241. |
