(Circulation. 2001;103:207.)
© 2001 American Heart Association, Inc.
Clinical Investigation and Reports |
Agonists Inhibit Tissue Factor Expression in Human Monocytes and Macrophages
From the Département dAthérosclérose, U.325 INSERM, Institut Pasteur de Lille, and the Faculté de Pharmacie, Université de Lille II (B.P.N., G.C., J.-C.F., P.D., B.S.), and the Laboratoire d Hématologie, Hôpital Cardiologique (D.C., C.Z., B.J.), Lille, France. The first 2 authors contributed equally to this article.
Correspondence to Brigitte Jude, Laboratoire dHématologie, Hôpital Cardiologique, Boulevard du Professeur J. Leclercq, 59037 Lille CEDEX, France. E-mail b-jude{at}chru-lille.fr
| Abstract |
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B. Both these signaling pathways are modulated by
peroxisome proliferatoractivated receptor-
(PPAR
). Therefore,
we have studied the effects of fibrates and other PPAR
agonists on
the expression of TF.
Methods and ResultsWe
show that PPAR
protein, like primary human monocytes, is also
expressed in the human monocytic THP-1 cell line. Fenofibric acid,
WY14643, and GW2331 inhibited TF mRNA upregulation after stimulation of
THP-1 cells with lipopolysaccharide or interleukin-1ß. In primary
human monocytes and macrophages, the lipopolysaccharide- or
interleukin-1ßmediated induction of TF activity was also inhibited
by fenofibric acid, WY14643, or GW2331.
ConclusionsThese data
indicate that activation of PPAR
results in the downregulation of
the TF gene. Our results suggest a novel role for PPAR
in the
control of atherosclerotic plaque thrombogenicity through its effects
on TF expression in monocytes and
macrophages.
Key Words: tissue factor lipopolysaccharide receptors monocytes fibrates
| Introduction |
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), oxidized LDLs, and
endotoxin.2 Monocytic TF
activity has been implicated in several diseases associated with
inflammation.3 In particular,
TF antigen and mRNA have been localized in atheromatous
plaques.4 5 Both
monocytes and macrophages are involved in the progression of
atherosclerosis and in the pathogenesis of
thrombosis.6 Monocytic TF
expression may contribute to thrombogenicity associated with plaque
rupture.7 Moreover, TF from
monocytes present in peripheral blood may propagate thrombus formation
at the site of a vascular
lesion.8
Peroxisome proliferatoractivated receptor-
(PPAR
) is a ligand-dependent transcription factor that, on
heterodimerization with the retinoid X receptor, binds to specific
peroxisome proliferator response elements (PPREs) in the promoter of
target genes, thus regulating the transcription of these genes.
Transcription of affected genes may also be modulated by PPAR
via
interference with other transcription factor pathways. Activation of
PPAR
negatively interferes with nuclear factor-
B (NF-
B),
signal transducer and activator of transcription (STAT), and activator
protein-1 (AP-1)
pathways.9 10 11
PPAR
, which plays an important role in the metabolism of
fatty acids, lipids, and lipoproteins, has also been implicated in
interference with atherogenic and inflammatory processes.
PPAR
-deficient mice show a prolonged response to inflammatory
stimuli.12 PPAR
has been
shown to inhibit transcription of several inflammatory response genes,
which also occur in atherosclerotic plaques. In human aortic smooth
muscle cells, fibrates inhibit the IL-1ßinduced expression of
cyclooxygenase (COX)-2 and IL-6 by inhibiting the NF-
B and AP-1
signaling
pathway.9 10 In
human vascular endothelial cells, PPAR
inhibits the
thrombin-mediated activation of endothelin-1 via negative interference
with the AP-1 signaling
pathway.13 Moreover, PPAR
activators prevent TNF-
induced VCAM-1 expression in human
saphenous vein endothelial cells, partly via inhibition of the NF-
B
pathway.14 This PPAR
action may lead to a decreased recruitment of monocytes to early
atherosclerotic lesions. In addition, PPAR
is present in primary
human monocytes, and its expression increases on differentiation into
macrophages.15 Furthermore,
PPAR
activators induce apoptosis of TNF-
activated
macrophages,15 most likely
by inhibiting the antiapoptotic NF-
B
pathway.16
Although the TF promoter does not contain a PPRE, it
contains Sp1-, Egr-1, AP-1, and NF-
Bresponsive
elements.17 Induction of
monocytic TF expression by endotoxin is mediated by the interaction of
transcription factors such as AP-1 and NF-
B with its
promoter.17 18 19
Because both these signaling pathways are modulated by PPAR
, we
hypothesized that PPAR
may modulate TF expression in human monocytes
and macrophages. In the present report, the effect of several PPAR
agonists on the endotoxin- and IL-1ßmediated induction of TF
expression was studied. We demonstrate that PPAR
is also expressed
in human monocytic THP-1 cells and that LPS- and IL-1ßinduced TF
expression in THP-1, primary human monocytes, and macrophages is
downregulated by activation of PPAR
.
| Methods |
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THP-1 Culture
Suspensions of human monocytic leukemia THP-1
cells were maintained in RPMI 1640 medium containing 25 mmol/L HEPES
buffer and 10% FCS. The cell suspensions were grown at 37°C in a
humidified 5% CO2 atmosphere. Suspensions were
diluted 1:1 when they reached a concentration of
1.5x106 cells/mL. Cells were incubated 1
hour before LPS stimulation with either 100 µmol/L fenofibric acid,
10 µmol/L WY14643, 0.5 µmol/L GW2331, or 0.5 µmol/L ATRA. After
addition of 10 µg/mL LPS, cells were further incubated for 2 hours.
Then 5 mL of cell suspension was centrifuged (15 seconds,
10 000g), washed with PBS, and
used for further RNA extraction.
Isolation and Culture of Human
Monocytes
Venous blood obtained from healthy donors was
anticoagulated with EDTA, and mononuclear cells were isolated by
gradient centrifugation (separation medium MSL,
d=1.077±0.001, Eurobio),
washed 2 times with PBS, and resuspended in RPMI 1640
(1x106 cells/mL). Monocytes were isolated
from lymphocytes by adherence (1 hour at 37°C in a humidified 5%
CO2 atmosphere) to 96-well microplates. In
separate experiments, monocytes were differentiated by culturing for 12
days in the presence of 10% human serum at 37°C in a humidified 5%
CO2 atmosphere. Cells were either preincubated
with 100 µmol/L fenofibric acid, 10 or 100 µmol/L WY14643, 1
µmol/L GW2331, or 1 µmol/L ATRA for 4 hours before a 16-hour
stimulation with LPS (0.4 ng/mL) or IL-1ß (10 ng/mL) at 37°C in a
humidified 5% CO2 atmosphere. At the end of the
incubation period, the medium was removed, and plates were washed with
cold PBS and assayed for TF activity. All reagents and culture supplies
used were free of endotoxin (chromogenic limulus amoebocyte lysate
assay; sensitivity, 0.025 endotoxin units/mL).
TF mRNA Analysis
Total RNA was prepared from THP-1 cells by acid
guanidinium thiocyanatephenol-chloroform
extraction.20 Fifteen
micrograms of RNA was separated by electrophoresis and transferred to
nylon membranes. Northern blots were hybridized at 68°C with
radiolabeled TF or
36B415 cDNA probes in
ExpressHyb according to the manufacturers instructions (Clontech
Laboratories). For the TF probe, a 641-bp cDNA product identical to the
probe reported in the
literature21 was isolated
after reverse transcriptionpolymerase chain reaction amplification of
RNA from human monocytes (primers,
5'-CTAGAATTCTACAAATACTGTGGCAGCATA-3'and
5'-ACGGAATTCCCCTTTCTCCTGGCCC-3'). The fragment was cloned into a pBSKS
vector and its identity verified by sequence
analysis.
TF Activity Assay
TF activity was determined by a modified amidolytic
assay.22 Briefly, cells were
mixed with 0.25 mol/L CaCl2 (50 µL) and
prothrombin concentrate complex (Laboratoire de Fractionnement et des
Biotechnologies) as a source of factor VII (50 µL, 3 IU/mL). After
addition of 50 µL of the chromogenic substrate S2765 (Biogenic), the
change in optical density at 405 nm was quantified with a microplate
reader and converted to units of TF activity by being plotted log to
log with readings from standard dilutions of tissue thromboplastin.
Arbitrarily, 1 mL of thromboplastin was assigned a value of 1000 U/mL
of TF activity.
Statistics
Statistically significant differences between groups
were reported when P
0.05
through an ANOVA test followed by a Bonferroni
correction.
| Results |
|---|
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|
|---|
activators, the LPS-mediated induction of TF expression was
monitored in monocytic THP-1 cells. As in other
studies,19 21 23
LPS increased TF mRNA expression in these cells several-fold. After 2
hours of incubation with LPS, both the 2.2- and 3.4-kb mRNA species of
TF were observed
(Figure 1A
|
To examine whether specific effects of PPAR
activators
could be expected in THP-1 cells, we analyzed the expression of PPAR
in these cells. Both PPAR
mRNA (not shown) and protein
(Figure 1B
) were detected in control and LPS-stimulated THP-1
cells. LPS had no effect on the level of PPAR
protein
expression.
Because PPAR
is expressed in THP-1 cells, we further
examined the effects of several PPAR
activators on TF mRNA
expression. Incubation of THP-1 cells with PPAR
activators 1 hour
before LPS stimulation for 2 hours resulted in a decreased level of TF
mRNA compared with cells incubated with only LPS
(Figure 2A
). The PPAR
agonists fenofibric acid and WY14643
inhibited LPS-mediated induction of TF mRNA to 61% and 46%,
respectively, of the mRNA levels of cells incubated with LPS alone
(Figure 2A
). Moreover, coincubation of THP-1 cells with LPS
and the potent PPAR
agonist GW2331 (EC50, 50
nmol/L24 ) decreased TF mRNA
expression to 39% of the level of LPS-stimulated cells.
|
To verify the extent of the TF mRNA inhibition, we
compared the effects of the PPAR
activators with those of ATRA, a
previously identified negative regulator of TF
expression.25 ATRA inhibited
the LPS-mediated induction of TF expression in THP-1 cells to 21% of
the control level.
To investigate whether PPAR
activators could also inhibit
TF mRNA induction by other inflammatory stimuli, we studied their
effects on THP-1 cells stimulated with IL-1ß. Like LPS-induced TF
mRNA expression, PPAR
agonists and ATRA inhibited IL-1ßinduced
TF expression
(Figure 2B
). Both WY14643 and GW2331 decreased TF mRNA levels
to
63% and 48%, respectively, of levels in cells incubated with
IL-1ß alone. ATRA diminished the mRNA level to 19% of the
IL-1ßstimulated control.
Next, we studied the influence of PPAR
agonists on TF
activity in primary human monocytes. Incubation of unstimulated
monocytes with fenofibric acid, WY14643, GW2331, or ATRA did not
influence basal TF activity
(Figure 3A
). Incubation of monocytes with LPS or IL-1ß
resulted in a 10- and 5-fold increase of TF activity, respectively.
Preincubation with different PPAR
agonists significantly inhibited
both the LPS- and IL-1ßinduced TF activity
(Figure 3B
and 3C
). The TF activity in LPS-stimulated
monocytes decreased to 67%, 57%, and 56% on preincubation with
fenofibric acid, WY14643, or GW2331, respectively. In
IL-1ßstimulated monocytes, the TF expression after fenofibric acid,
WY14643, or GW2331 preincubation was decreased to 72%, 60%, and 45%,
respectively, compared with IL-1ß only. The
inhibitory effects of PPAR
agonists on TF activity
levels at the doses used correlated well with their relative potency
for PPAR
activation (GW2331>WY14643>fenofibric acid). ATRA
inhibited IL-1ßinduced TF activity to
33% of the activity in
stimulated monocytes. ATRA had no detectable effect on LPS-induced TF
activity when preincubated, however, analogous to the PPAR
agonists,
for 4 hours
(Figure 3C
). Preincubation with ATRA for 30 minutes followed
by an incubation for 16 hours with LPS, however, resulted in a
pronounced downregulation (to 11%) of TF activity compared with LPS
incubation alone.
|
Finally, we studied the effects of PPAR
agonists on
TF activity in human monocytederived macrophages
(Figure 4
). Preincubation of differentiated macrophages with
the PPAR
agonists fenofibric acid, WY14643, and GW2331 resulted in
an inhibition of LPS-stimulated TF activity to 78%, 35%, and 13%,
respectively, of the activity in cells incubated with LPS alone
(Figure 4
). Like the effects in LPS-stimulated monocytes,
only a 30-minute and not a 4-hour preincubation of the LPS-stimulated
macrophages with ATRA decreased the TF activity level (12% of
control).
|
| Discussion |
|---|
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|---|
activation. In
the present study, we determined whether incubation with PPAR
agonists modulates TF expression in the human monocytic THP-1 cell line
and in human monocytes and macrophages. It was previously demonstrated
that PPAR
is present in primary human
monocytes.15 Here, we extend
these observations by showing that PPAR
protein is also expressed in
THP-1 cells. Furthermore, our study demonstrates that PPAR
activation can inhibit TF upregulation in THP-1 cells and in primary
human monocytes and macrophages. Thus, these results extend previous
observations identifying a role for PPAR
in human monocytes and,
moreover, for the first time point out a possible role for PPAR
in
the control of atherosclerotic plaque thrombogenicity.
The activity of PPAR
agonists on the downregulation of TF
expression was compared with that of ATRA. Retinoic acid has been shown
to inhibit the LPS-induced TF expression in human THP-1 cells,
monocytes, and
macrophages.25 26 27
We confirmed these effects for LPS- and IL-1ßinduced TF mRNA
expression in THP-1 cells. As previously demonstrated for
macrophages,26 the
inhibitory effect of ATRA on LPS-stimulated TF activity in monocytes
appeared to depend on the time of preincubation with retinoids. Whereas
preincubation of monocytes with ATRA for 4 hours inhibited
IL-1ßinduced TF activity, no inhibition of LPS-induced TF activity
was observed. However, a limited preincubation of 30 minutes with ATRA
completely prevented the LPS induction of TF activity in monocytes and
macrophages. The bases for these apparent kinetic differences, as well
as the mechanism by which ATRA inhibits the induction of TF mRNA
expression, are poorly known.
PPAR
may interfere with proatherogenic processes at
different levels. First, PPAR
exerts beneficial effects on
atherosclerosis by changing plasma lipid and lipoprotein profiles
toward less atherogenic levels. Second, PPAR
interferes with the
development of atherosclerosis by inhibiting inflammatory responses at
the level of the vascular
wall.9 10 12
PPAR
may interfere with the early stages of atherosclerotic lesion
development by affecting monocyte recruitment by inhibiting
TNF-
induced VCAM-1 expression in endothelial
cells.14 Furthermore,
PPAR
may also influence later stages of atherosclerosis by inducing
apoptosis of activated human
macrophages.15 Our results
demonstrate that PPAR
activation also inhibits TF expression, which
is a major initiator of thrombosis. In addition, TF may mediate
adhesion and migration of
monocytes.28 Thus,
inhibition of monocytic TF expression is another way by which PPAR
may modulate atherogenic processes.
We have shown that PPAR
activators inhibit the
expression of TF after induction by the inflammatory stimuli LPS and
IL-1ß. Induction of TF mRNA by LPS has been shown to occur via Jun
phosphorylation and NF-
B translocation in a rapid but transient
way.19 PPAR
inhibits the
proinflammatory AP-1 and NF-
B signaling pathways by repression of
both c-Jun and p65 transcription
activity.9 10 13
Because these factors also control TF promoter transcription, it is
likely that PPAR
modulates TF expression also by interfering
negatively with the AP-1 and/or NF-
B activation pathway. Further
molecular studies are necessary to determine whether the repression of
TF gene expression by PPAR
agonists indeed occurs via cross talk of
PPAR
with other transcription factors, such as Jun-Fos and
NF-
B.
In conclusion, the PPAR
agonists fenofibric acid,
WY14643, and GW2331 all inhibit the upregulation of TF expression,
which occurs after stimulation of THP-1 cells or human monocytes with
LPS or IL-1ß. The effect of PPAR
stimulation on monocyte and
macrophage TF expression suggests a novel role for PPAR
in
atherosclerosis by influencing atherosclerotic plaque thrombogenicity.
In vivo studies using atherosclerotic animal models may elucidate
whether PPAR
is able to reduce the thrombogenicity of
atherosclerotic plaques by lowering TF
expression.
| Acknowledgments |
|---|
Received August 14, 2000; revision received September 26, 2000; accepted September 26, 2000.
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