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Type 1 Plasminogen Activator Inhibitor Synthesis of Endothelial Cells
Is Downregulated by Smooth Muscle Cells
By Gilnter Christ, Dietmar Seiffert, Peter Hufnagl, Alois Gessl, Johann Wojta, and Bernd R. Binder
Plasminogen activator inhibitor type 1 (PAI-I), the physiologic inhibitor of both tissue-type plasminogen activator
(tPA) and urokinase-type plasminogen activator (uPA), is a
major biosynthetic product of endothelial cells in vitro; endothelial cells in vivo, in contrast, do not appear t o produce
significant amounts of PAL1 as made evident by in situhybridization studies in normal mice. This suggests that
the high rate.of PAL1 synthesis of endothelial cells in vitro
might be a result of the culture conditions. When human
umbilical vein endothelial cells (HUVEC) were grown on
human amniotic membranes, resemblingthe natural growth
support instead of coated plastic, their morphology was
changed from the cobblestone-like appearance on plastic
t o an in vivo like flagstone pattern. However, this morphological change had no significant effect on the synthesis
and secretion of PAI-1. When smooth muscle cell (SMC)
conditioned media (CM) were added to HUVEC cultures,
PAL1 antigen secretion of HUVEC was reduced by 40%t o
60%as measured by enzyme-linked immunosorbent assay
(ELBA). Immunoprecipitation experiments using 36S-methionine metabolically labeled HUVEC and Northern blot
analysis of HUVEC PAL1 mRNA indicate that this reduction
was attributable t o decreased PAL1 synthesis and reduced
steady-state levels of both the 3.2 kb and 2.2 kb form of
PAI-1 mRNA. This effect was dose-dependent and observed under serum-containing as well as serum-free conditions, in the absenceor presence of endothelial cell growth
supplement (ECGS, 0 t o 100 pg/mL) and attributable t o a
nondialyzable factor. Our data suggest that the high level
of PAL1 biosynthesis of endothelial cells in vitro may be
attributable t o the lack of a soluble factor producedby SMC,
which controls and suppresses PAI-1 biosynthesis of endothelial cells in vivo.
0 1993by The American Society of Hematology.
P
thelia1 cells. Therefore, PAI- 1 synthesis of endothelial cells
in vivo seems to be downregulated possibly by factors originating in the natural microenvironment of endothelial cells
within the vessel wall in vivo.
It was the aim of our study to elucidate the influence of
culture conditions replacing the artificial support (ie, gelatinecoated plastic) by a natural support (ie, human amniotic
membrane) and/or cell-cell interactions (ie, interactions between SMC and EC, the two main cell types of the vascular
wall) on PAI- 1 biosynthesis of human endothelial cells.
LASMINOGEN activator inhibitor type 1 (PAI-I), a
member of the serine protease inhibitor (serpin) superfamily, appears to be the primary physiological inhibitor of
tPA and uPA in blood and, therefore, the regulator of plasminogen activation in
The importance of PAL1 in
regulating the balance between plasminogen activation and
inhibition is supported by clinical studies reporting association
of elevated PAI- 1 activity in blood with thrombotic disorders:
coronary heart disease, and myocardial i n f a r c t i ~ nwhereas
,~~~
decreased PAI- 1 activity causes recurrent bleeding problem~.’.~
Beside the liver and platelets, endothelial cells and/
or smooth muscle cells (SMC) are thought to be the main
source of circulating PAI- 1.I,’
Endothelial cells (EC) in culture have been widely used to
investigate the regulation of PAI- 1 biosynthesis in vitro.
Konkle and Ginsberg’ reported that, under normal culture
conditions requiring the addition of the bovine neuropeptide
endothelial cell growth factor (ECGF) and heparin for cell
growth,1°-12PAL1 synthesis of HUVEC is already downregulated threefold to IO-fold. Nevertheless, PAI- 1 is still a major biosynthetic product of endothelial cell cultures in vitro,
representing up to 12%of total protein secreted.13
Whereas only little is known about mechanisms or factors
that might downregulate the basal rate of PAL1 synthesis in
endothelial cells (cyclic AMP dependent pathwayd4,”;
ECGF), a variety of stimuli are known to further upregulate
the already high PAI-I expression in cultured endothelial
cells. Among those are lipopolysaccharide (LPS), interleukin1 (IL- I), tumor necrosis factor a (TNFa),I6-” thrombin;0*21
and growth factors like transforming growth factor ,f3 (TGFP)
and basic fibroblast growth factor (bFGF).22-25
In view of these findings and the fact that PAI-I is only
present in trace amounts (= 5 ng/mL) in plasma,26-28one
has to question whether endothelial cells in culture do reflect
the normal PAL 1 synthesis pattern of their counterparts in
vivo. These doubts are supported by observations that neither
in freshly isolated umbilical artery endothelial cellsz9nor in
in situ-hybridizationstudies in normal murine t i s s ~ e ~could
~,~’
significant amounts of PAI-I mRNA be detected in endoBlood, Vol81, No 5 (March I), 1993: pp 1277-1283
MATERIALS AND METHODS
Cell culture. HWEC were isolated by mild collagenase treatment
as described.loCells were seeded into Petri dishes (Costar, Cambridge,
MA) coated with gelatine (BioRad, Richmond, CA) and grown to
confluency in Medium 199 (Sigma, St Louis, MO) containing 20%
supplemented calf serum (SCS; HyClone, Logan, UT), 50 IU/mL
penicillin, 50 pg/mL streptomycin, 250 n&mL amphotericin B (all
JRH Biosciences, Lenexa, KS), 50 p&mL endothelial cell growth
supplement (ECGS)(unless otherwise stated)prepared as described3’
and 5 U/mL heparin (Liquemin Roche IV, Hoffmann-LaRoche,
Basel, Switzerland) in a humidified atmosphere of 95% air and 5%
CO2at 37°C. Cells were confirmed to be endothelial cells by their
cobblestone m~rphology,’~
by positive immunofluorescence using
anti-von Willebrand factor antibodies (Cappel, Cochranville, PA),)4
From the Laboratory for Clinical Experimental Physiology, and
the Department of General and Experimental Pathology, University
of Vienna, Vienna, Austria.
Submitted April 2, 1992; accepted October 29, 1992.
Supported by Grant Nos. P8011 and 7617from the Austrian Fund
for the Promotion of Scientific Research.
Address reprint requests to Bernd R . Binder, MD, Laboratory for
Clinical Experimental Physiology, University of Vienna, Schwarzspanierstraj3e 17, A-1090 Vienna, Austria.
The publication costs of this article were defrayed in part by page
charge payment. This article must therefore be hereby marked
“advertisement” in accordance with I8 U.S.C.section I734 solely to
indicate this fact.
0 1993 by The American Society of Hematology.
0006-4971/93/8105-0027$3.00/0
1277
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CHRIST ET AL
1278
and by uptake of acetylated low-density lipoprotein (LDL).35All
HUVEC used in this study were between passage 3 and 4.
SMC were either isolated by collagenase treatment of normal pulmonary artery obtained by surgery, or from human umbilical vein
by using the explant te~hnique.)~
SMC from pulmonary artery were
separated from endothelial cells by fluorescence-activatedcell sorter
(FACS) analysis (FACS 111; Becton Dickinson, Mountain View, CA)
using acetylated LDL as an endothelial cell-specific marker, and
characterized by staining with monoclonal anti-SMC actin antibodies
(Sigma)37and their “hill and valley” growth pattern.38 Such characterized SMC were grown to confluency under the same culture
conditions as HUVEC. SMC from pulmonary artery used in this
study were between passage 3 and 5, SMC from umbilical vein in
passage 1 or 2.
Preparation of amniotic membranes and silver nitrate staining of
HUVEC. Acellular amniotic membranes were prepared as described
previo~sly.’~
Briefly, pieces of human amniotic membrane were fastened to teflon rings and the amnion epithelium was lysed by incubation in 0.25 mol/L NH,OH for 2 hours followed by scraping with
a rubber policeman. Such teflon rings were placed into 6-well culture
plates (Costar), creating an upper compartment (0.5 mL medium)
and a lower compartment (2.0 mL medium) communicating only
through the membrane. HUVEC were placed onto the stromal surface, facing the upper compartment of the amnion. Confluent HUVEC monolayers were stained with silver nitrate as de~cribed.’~
Cultures were flooded sequentially with the following reagents at room
temperature: 5% glucose for 30 seconds, 0.25% AgN03 for 30 seconds,
5% glucose to rinse, 1% NH4Br for 30 seconds, 5% glucose to rinse,
3% CoBr2for 30 seconds, 5% glucose to rinse, 5% formalin to fix for
15 minutes. Cells were counterstained with Wrights stain (Sigma) for
15 minutes.
Preparation of conditioned media (CM). SMC were seeded into
24-well culture plates (Costar) and reached confluency within 3 to 5
days at a density of about 1.2 X lo5 cells per well. Confluent cultures
were washed twice with Hanks’ balanced salt solution (HBSS; Sigma)
containing 10 mmol/L HEPES and given either 1 mL Medium 199
(plus 5 U/mL heparin and 50 pg/mL ECGS, unless otherwise stated)
containing 20% SCS or 1% bovine serum albumin (BSA; Behring,
Marburg, Germany), or 1 mL RPMI 1640 (Sigma) (plus 5 U/mL
heparin without ECGS and serum).
After 24 hours the SMC-conditioned Medium 199 was collected,
centrifuged (l,OOOg, 5 minutes) to remove cell debris, and immediately transferred to HUVEC, grown to confluency in 24-well plates
and washed twice with HBSS before experiment. For each experiment,
separate 24 hour SMC CM was collected from cultures (between
passages 3 and 5 ) , either cultured in 24-well plates (n = 16) or in 100
mm Petri dishes (n = 3). SMC CM was transferred to corresponding
HUVEC cultures also grown either in 24-well plates (n = 16) or 100
mm Petri dishes (n = 3). Cultures in 24-well plates were used for
antigen determination (n = 13) as well as immunoprecipitation analysis (n = 3). Cultures in 100 mm Petri dishes were used for RNA
extraction (n = 3). For control experiments, CM from confluent
HUVEC cultures was processed in the same way.
For immunoprecipitation analysis, SMC CM (RPMI 1640) was
processed as above and sequentially dialyzed against phosphate-buffered saline (PBS), pH 7.4 (4 hours, 4°C) and methionine deficient
RPMI 1640 (GIBCO/BRL, Grand Island, NY) plus 5 U/mL heparin
(16 hours, 4 T ) , using a dialyzing membrane with a cutoff of 1,000
daltons (Spectra/Por; Spectrum, Los Angeles, CA). For control experiments fresh RPMI I640 was processed in the same way. Viability
of cells was determined by standard Trypan Blue exclusion.
Metabolic labeling of cells and immunoprecipitation analysis.
This method was performed as described previously.4o Briefly,
HUVEC monolayers were washed twice with HBSS and labeled with
3SS-methionine( 100 pCi/mL) (ICN Radiochemicals, Irvine, CA) in
methionine-free RPMI 1640 prepared as described above. After 6
hours, the CM was collected, cell extracts (CE) were prepared by lysis
with 0.5%Triton X-100 (Serva, Heidelberg, Germany) in PBS, and,
after washing with distilled water, the extracellular matrix (ECM)
was extracted in 0.1% sodium dodecyl sulfate (SDS) (BioRad). Metabolically labeled CM was mixed with an equal volume of immunoprecipitation buffer (PBS, pH 7.4, containing 1%nonfat dry milk,
10 U/L aprotinin [Trasylol, Bayer, Germany], 10 mmol/L benzamidinehydrochloride [Sigma], and 0.1% Triton X-100) and incubated
with an excess (20 pg/mL) of polyclonal rabbit anti PAI-I antibodies
(Technoclone, Vienna, Austria) for 2 hours at 37°C followed by
overnight incubation at 4°C with protein A Sepharose (Beckman,
Fullerton, CA). Unbound proteins were removed from the Sepharose
beads by washing twice with 2 mol/L NaCl containing 0.1%Triton
X-100 and once with distilled water. Bound proteins were released
from the beads by boiling in Laemmli sample buffer for 5 minutes
and fractionated by SDS-polyacrylamide gel electrophoresis (SDSPAGE)‘“; subsequently the gels were fixed (30% methanol, 10%acetic
acid), soaked in Enlightning (NEN, Boston, MA), dried, and exposed
to Kodak XAR x-ray films (Eastman Kodak, Rochester, NY) with
intensifying screens. Additionally, the region containing PAI- 1 was
excised, hydrolyzed in Ready Protein scintillation fluid (Beckman),
and subjected to liquid scintillation counting (Beckman LS 7500).
Total protein synthesis was quantified by trichloroaceticacid (TCA)
precipitation of CM, CE, and ECM (equal amounts of sample, 0.25
mol/L methionine, 1%BSA, 100%TCA were mixed, diluted IO-fold
in PBS, and incubated for I hour at 4°C). The precipitate wascollected
on Whatman GF/A filters (Whatman, Maidstone, England) under
light suction and washed twice with ice-cold 10% TCA, once with
70% ethanol, and once with distilled water. The filters were air dried
and subjected to scintillation counting in Beckman Ready Protein
scintillation fluid.
Assayfor PAI-1antigen. PAL 1 antigen in the CM was determined
by specific ELISA using monoclonal antibodies recognizing active,
latent, and complexed PAL 1 (Technoclone).
Preparation of RNA and Northern blots. Total RNA was prepared
from cells cultured in 100 mm Petri dishes (Costar) by acid guanidinium thiocyanate-phenol-chloroform extraction?2 RNA was electrophoretically separated on I .2% agarose gels containing 6% formaldehyde, transferred to nitrocellulose (BioRad) and hybridized ( IO6
cpm [Cerenkov]/mL) for 16 hours at 42°C with random-primed a[32P]dCTP-labeledprobes (Boehringer Mannheim) for either human
PAI-I (a 1.4 kb EcoR1-Bgl I1 fragment of polymerase. chain reaction
[PCRI-amplified cDNA) or rat glyceraldehyde-3-phosphate dehydrogenase (GAPDH, a 1.3 kb PstI cDNA fragment, kindly provided
by Dr Busslinger, Vienna). 50 pL hybridization solution, containing
50% vol/vol formamide, 5.0 X SSC, 5.0 X Denhardt’s solution, 0.01
mol/L sodium phosphate, pH 7.0, 1 mmol/L EDTA, 0.1% SDS, and
50 pg/mL sonicated and heat-denatured salmon sperm DNA (Boehringer Mannheim), was used per cm2 nitrocellulose filter. Blots were
washed twice with 1.0 X SSC, 0.1%SDS, 0.1 mol/L EDTA and 0.01
mol/L sodium phosphate, pH 7.0, and once with 0.1 X SSC, 0.1%
SDS, 0.1 mmol/L EDTA, and 0.01 mol/L sodium phosphate, pH
7.0, at room temperature, air-dried, and exposed to Kodak XAR xray films. Quantitative analysis of films was performed by densitometry (Hirschmann elscript 400, Germany).
Statistical analysis. The results are reported as means f standard
deviation. Student’s unpaired t-test was used for determination of
significance levels.
RESULTS
Effect of growth substratum on PAI-I secretion of HUVEC. Endothelial cells grown on amniotic membranes are
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1279
PAI-1 DOWNREGULATION
C
Fig 1. (A) Silver nitrate staining of confluent
monolayers of HUVEC grown on amniotic membranes, showing the typical in vivo-like "flagstone
pattem," compared with (B) HUVEC grown on gelatine coated plastic lacking this staining pattern.
(C) PAI-1 antigen levels in the 24-hour CM of HUVEC monolayers grown either on gelatine coated
24-well plastic dishes (0).
or on human amniotic
membranes p).Experiments were performed in
complete culture media. PAL1 antigen was determined by specific ELSA (Technoclone), P = NS.
lO0Ot
7
r
1
500
0
plastic
amnion
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1280
CHRIST ET AL
-c
d
cu
1
m
l-4
1000-
al
I
c
\
7
Y
0
0
Ln
0
T
500-
H
a
a.
0,
C
0-
0
5
50
100
ECGS pg/ml
known to display an in vivo-like morphology under silver
nitrate staining.3y the so-called “flagstone pattern” (Fig 1 A).
in contrast to the “cobblestone pattern” of endothelial cells
grown on coated plastic (Fig I B). However, this morphologic
change, resembling the endothelium in vivo. was not accompanied by a significant change of PAI-I secretion ( P = NS)
into the CM (Fig IC). HUVEC cultures grown on amnion
secreted 809 f 124 ng PAI-1/105 cells into the supernatant
within 24 hours, compared with 883 f 101 ng PAI-l/lOs
cells/24 hours detected in the CM of HUVEC grown on gelatine-coated plastic.
F&t
of SA4C CM on PAL1 .svnthe.si.s and secretion of
HUIKC. Incubation of confluent monolayers of HUVEC
Fig 2. PAI-1 antigen levels in the 24-hour C M
of HUVEC incubated with SMC C M (e)
or normal
culture media (0)containing different concentrations of ECGS under serum-containing conditions.
SMC-derived PAI-1, showing also an ECGS concentration dependency (0 pg/mL 173 2 45; 5 pg/
m L 61 1 f 77; 50 pg/mL 369 2 17: 100 pg/mL
564 2 1go), was determined simultaneously and
subtracted from each measured value. Under
serum-free culture conditions (INSET) similar results were obtained. SMC-derived PAL1 (0pg/mL
ECGS: 70 2 5; 50 pg/mL ECGS: 129 2 4) was
subtracted from each measured value, P < .0009.
with the 24 hour SMC CM for 24 hours resulted in a mean
decrease of PAI-I antigen secretion into the supernatant of
56.98 f 9. I % (P < .0009) relative to that of control cells,
under serum-containing conditions (20% SCS, 5 U/mL h e p
ann) in the absence as well as in the presence of up to 100
pg/mL ECGS (Fig 2). Under serum-free conditions (1% BSA,
5 U/mL heparin, 0 to 50 pg/mL ECGS) a mean decrease of
35.7 f 8.990(P = .0009) of PAI-I antigen secretion compared
with control cells can be observed (Fig 2, inset). SMCderived
PAI-I was determined simultaneously (legend to Fig 2) and
subtracted from each measured value. When CM of SMC
derived from human umbilical vein were used, a decrease in
HUVEC PAI-I antigen secretion of 12.8 ? 0.3%(P= .09)
T
1 ooc
75c
c)
0
tc
X
E
Q
0
Fig 3. Dose-dependent downregulation of PAI1 synthesis of HUVEC by 24-hour SMC CM. Immunoprecipitation analysis of the C M of ”S-methionine metabolically labeled HUVEC )-(
shows a 51.9% decrease (P = .0005) of secreted
PAL1 by incubation with 100%(ie, undiluted) SMC
CM, a 30.9%decrease (P = .03)by incubationwith
50% SMC CM, and no effect by incubation with
10%SMC C M compared with controls (0%).Total
protein synthesis remains unaffected as seen by
TCA precipitation of CM, CE, and ECM (-----).
5
I
0 10
.
.
.
.
,
50
100
SMC CM concentration ( X )
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1281
PAI-1 DOWNREGULATION
l
5
t
O
2aplll
E
100-
thbu.s
0
L
0
24
hours
hours
Fig 4. Time-dependent downregulationof PAL1 mRNA levels
in HUVEC by 24-hour SMC CM.
After 4 hours of incubation, a
74.4% decreaseof PAL1 mRNA
is observed under serum-containing conditions (o), a 76.5%
decrease under serum-free conditions (0).Intensity of PAL1
mRNA bands is expressed in arbitrary units relative to the intensity of the GAPDH band, used
as an internal marker.
50:\
0.
0
4
I
4
24
hours
can be observed. PAI-I synthesis levels of such SMC were
higher (1,745 f 63 ng/mL) compared with those observed
in pulmonary artery SMC (564 zk 190 ng/mL).
Incubation of HUVEC with SMC CM had neither visible
influence on the endothelial morphology, as seen under phasecontrast microscopy. nor did any change in viability (9 I %
2.6%) occur as determined by Trypan-Blue exclusion. Freezing (-7O"C, 60 minutes) and thawing of SMC CM did not
abolish the PAI-I downregulatory effect on HUVEC.
In control experiments, in which HUVEC monolayers were
incubated with 24 hour HUVEC CM, no reduction of PAI1 antigen secretion was observed. (1,3 16 f 240 ng/mL PAII versus 1,380 f 28 ng/mL; P = NS).
To assure that the influence of SMC CM on synthesis and
secretion of PAI-I is attributable to effects on de novo-synthesized PAL I , HUVEC were metabolically labeled with "Smethionine. Incubation of HUVEC monolayers with 24 hour
SMC CM (processed as described in Materials and Methods)
for 6 hours resulted in a dose-dependent decrease of PAI-I
antigen secreted into the supernatant with a maximal inhibition of 5 1.9%(P = .0005) by undiluted SMC CM, a 30.9%
reduction (P = .03) by 1:2 diluted SMC CM, and no effect
by 1:lO dilution of SMC CM compared with control cells
(Fig 3). Total protein synthesis remained unaffected as seen
by TCA precipitation of CM, CE. and ECM (Fig 3).
Kfict (!fSMC'c'M on PA!- I niRNA lcvels in HUVEC. To
determine whether this decrease of de novo synthesis and
secretion of PAL1 protein is also reflected on the steady-state
level of PAI-I mRNA, total RNA of HUVEC under serumcontaining (20%SCS. 5 U/mL heparin) and serum-free (1%
BSA, 5 U/mL heparin) experimental conditions were prepared. The results show that already after 4 hours, under
serum-containing conditions, a 74.4%downregulation and,
under serum-free conditions, a 76.5% downregulation (Fig
*
4) of the steady-state levels of both the 3.2 kb and 2.2 kb
form of PAI-I mRNA occurs.
DISCUSSION
We could show that the in vivo-like morphology of endothelial cells grown on amniotic membranes, serving as a
natural growth support, does not significantly influence the
high PAL I secretion into the conditioned media observed in
endothelial cell cultures in vitro. However we cannot exclude
that possible alterations of PAI-I biosynthesis, although not
affecting PAI-I secretion into the media, might be concealed
by changes of PAI- I deposition into the matrix. The amount
of PAI-I deposited into a natural growth support like the
amniotic membrane, however, is likely to be much higher
than that deposited into an artificial growth support like plastiC.43.44 Therefore, a possible change in PAL1 synthesis of
HUVEC grown on amniotic membranes should rather be an
increase than a downregulation.
Because there is growing evidence for complex regulatory
cell-cell interactions between the two main cell types of the
vascular wall, EC and SMC, concerning cell replication.
growth factor activation, and pr0liferation,4~"* we were interested in possible effects of SMC CM on PAI-I synthesis
and secretion of HUVEC. We found that PAI-I antigen secretion into the CM by HUVEC decreases when the regular
HUVEC media is replaced by media conditioned for 24 hours
by SMC. This effect is seen regardless ofwhether SMC media
is conditioned with or without serum and with or without
ECGS. Immunoprecipitation analysis of the conditioned
media of "S-methionine-labeled HUVEC, incubated with
various dilutions of 24 hours SMC CM. shows a dose-dependent decrease of PAI-I synthesis and secretion. This
downregulation of the synthesized and secreted protein is
preceded by an -70% decrease of the steady-state level of
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1282
CHRIST ET AL
PAI- 1 m R N A after 4 hours of incubation. These data clearly
indicate that media conditioned by SMC contain factor(s)
capable of downregulating PAI- 1 biosynthesis by E C i n vitro.
This effect seems to be SMC-specific, because it was seen
with C M from pulmonary-artery S M C and umbilical-vein
SMC but not with CM from endothelial cells or tumor cells.49
Furthermore, it does not seem to have an unspecific downregulatory effect, because total protein synthesis as well as
G A P D H expression remains unaffected.
PAI- 1 concentrations found in cell culture systems are frequently considerably higher than in p l a ~ m a . ~ . Although
~~.~'
the relatively short half-life of PAL1 in blood ( 5IO minutes)52
suggests a high biosynthetic rate, the high PAI-I production
of EC in culture might not reflect the behavior of their normal
counterparts in vivo. T h e observation of van den Berg et al,29
that freshly isolated endothelial cells from umbilical arteries
do not contain detectable amounts of PAI-I mRNA, supports
the hypothesis that the high PAI-1 synthesis by E C reflects
changes of the biosynthetic properties of EC as they have
adapted t o the culture conditions. Furthermore, in situ hybridization studies in mice3'x3' suggest that the primary site
of PAI-I synthesis in the vessel wall, under normal conditions,
is the SMC layer. I n contrast, n o significant amounts of PAI1 m R N A were detected i n EC. However, under pathologic
conditions (endotoxin treatment), PAI-1 production is
switched from SMC to EC.333'
T h e data presented in our study suggest that the high PAI1 levels observed in endothelial cell cultures in vitro are, at
least partially, attributable to the lack of inhibitory influences
of vascular SMC. Therefore, S M C not only participate in
PAI-I upregulation i n a coculture system via activation of
latent TGF-p,48 but also produce soluble factors capable of
downregulating PAI- 1 biosynthesis in endothelial cells. These
data also indicate that the regulation of PAI- 1 synthesis and
its tissue distribution in vivo may be part of a complex regulatory system involving several paracrine control mechanisms.
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1993 81: 1277-1283
Type 1 plasminogen activator inhibitor synthesis of endothelial cells
is downregulated by smooth muscle cells
G Christ, D Seiffert, P Hufnagl, A Gessl, J Wojta and BR Binder
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