(Circulation. 1999;100:5-8.)
© 1999 American Heart Association, Inc.
Brief Rapid Communication |
From Cardiovascular Research (Z.Y., N.K., T.F.L.), Institute of Physiology, University Zürich-Irchel and Cardiology (Z.Y., T.F.L.), University Hospital Zürich, Switzerland.
Correspondence to Thomas F. Lüscher, MD, FRCP, FACC, Professor and Head of Cardiology, University Hospital, CH-8091 Zürich, Switzerland. E-mail cardiotfl{at}compuserve.com
| Abstract |
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Methods and ResultsHuman aortic SMCs were cultured and cell proliferation was assayed by [3H]thymidine incorporation. PDGF receptor expression, activation of mitogen-activated protein kinase (MAPK), cell cycle regulators such as cyclin-dependent kinase 2 (Cdk2), Cdk inhibitor (p27Kip1), and retinoblastoma protein (pRb) were analyzed by immunoblotting. ET-1 on its own was unable to stimulate [3H]thymidine incorporation but dramatically potentiated the effect of PDGF-BB up to 6-fold (P<0.001). Most of the potentiating effects (88%) were blocked by the ETA receptor antagonist LU135252 and slightly further blocked by the ETA/B receptor antagonist bosentan (P<0.05). ET-1 stimulated MAPK, but it neither potentiated PDGF-induced MAPK activation nor overexpressed PDGF receptors. In contrast to PDGF-BB, ET-1 had no regulatory effects on Cdk2, p27Kip1, and pRb.
ConclusionsIn human SMCs, ET-1 activates MAPK but has no mitogenic effects on its own. However, ET-1 markedly potentiates proliferation to PDGF, mainly via ETA receptors. This may represent an important function of ET-1 for vascular structural changes in patients and provide new therapeutic opportunities for ET-1 receptor antagonists.
Key Words: cells growth substances kinases platelet-derived factors
| Introduction |
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In contrast to ET-1, platelet-derived growth factor (PDGF) is a well-established mitogen for SMCs and contributes to the pathogenesis of proliferative vascular disease.7 Although the mechanisms of cell growth by PDGF are not yet completely understood, much of the evidence indicates that the c-Raf/MEK/mitogen-activated protein kinase (MAPK) cascade activated by PDGF tyrosine kinase receptors is important for transmitting extracellular growth signals into the cell nucleus, thereby regulating downstream gene expression and cell cycle progression.8
Cell cycle progression is positively regulated by the orderly activation of cyclin-dependent kinases (Cdks) and negatively regulated by several cell-cycle inhibitors known as cyclin-dependent kinase inhibitors.9 Our previous study showed that Cdk inhibitor (p27Kip1) seems most likely involved in cell cycle control in human SMCs.10 Cyclin DCdk4/Cdk6 regulates G1 progression, and cyclin ECdk2 is essential for G1/S transition by phosphorylating and inactivating the tumor suppressor gene retinoblastoma protein (pRB),9 which causes release and activation of E2F transcription factor, and in turn regulating several proteins required for cell proliferation.11
In this study, we clarified the proliferative role of ET-1 in human SMCs and demonstrated that a marked growth-potentiating effect of the peptide to PDGF represents an important mechanism of ET-1induced human SMC proliferation.
| Methods |
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-smooth muscle actin and all
chemicals for immunoblotting were purchased from Sigma
Chemical Co; all tissue culture materials were from Gibco;
[3H]methylthymidine was from Amersham;
recombinant human PDGF-BB was from R&D Systems GmbH; rabbit polyclonal
anti-human p42mapk (C14),
p27Kip1 (C19), Cdk2 (M2), and PDGF
- (C20) and
ß- (958) receptors were from Santa Cruz Biotechnology Inc; and the
mouse monoclonal anti-human pRB (G3-245) was from PharMingen.
Antiphospho-p44/42mapk was from New England
BioLabs Inc; LU135252 was supplied by Knoll AG; and bosentan was
supplied by Hoffman La Roche.
Cell Culture
Human aortic SMCs were purchased from Clonetics Corp. The cells
were cultured in DMEM containing 10% FCS supplemented with 20
mmol/L L-glutamine and HEPES buffer solution, 100 U/mL
penicillin, and 100 µg/mL streptomycin in a humidified atmosphere
(37°C; 95% air/5% CO2). Culture medium was
replaced every 3 days. Cells were passaged by 0.01%
EDTA-trypsin.10
[3H]Thymidine Incorporation and Cell
Division
SMCs in DMEM containing 10% FCS were seeded on 12-well plates
(density, 2x104/mL) overnight to allow
attachment. Culture media were then replaced with serum-free DMEM
containing all ingredients as described above and 0.2% BSA instead of
FCS without any possible growth factors, such as insulin. Cells in
serum-free DMEM were incubated for 48 hours to achieve a quiescent
stage. Quiescent cells were stimulated with PDGF-BB or ET-1 or both
together at various concentrations in the presence or absence of the
ET-1 receptor antagonists LU135252 or bosentan
10-6 to 10-5 mol/L.
[3H]thymidine incorporation was then assayed as
described.10 In another set of experiments, quiescent SMCs
(104/well) in 12-well plates were stimulated by
PDGF-BB 1 ng/mL or ET-1 10-7 mol/L or both
together over 8 days. Cell number was counted every 2 days (Coulter
Counter).
MAPK Activation
p42/44mapk activation or
phosphorylation was analyzed by
immunoblotting as described.10
Cell Cycle Regulatory Proteins
In another series of experiments, the cells were stimulated
either with ET-1 10-7 to
10-6 mol/L or PDGF-BB 10 ng/mL for 24 hours.
p27Kip1, Cdk2, and pRb were then analyzed
by immunoblotting on 12% or 8% SDS-PAGE.
PDGF Receptors
To study the effects of ET-1 on PDGF receptor proteins, the
cells were stimulated with ET-1 10-7 to
10-6 mol/L for 24 hours and then harvested for
PDGF
- and ß-receptor protein analysis by
immunoblotting on 8% SDS-PAGE.
Statistics
Data are mean±SEM. [3H]thymidine
incorporation was expressed as percent increase of control. n equals
the number of different independent experiments. Student's
t test for paired observations and ANOVA followed by
Scheffé's test with repeated measurements were used. A 2-tailed
P value <0.05 was considered significant.
| Results |
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Potentiation of Cell Growth by ET-1 Mainly Through
ETA Receptor
A low concentration of PDGF-BB (1 ng/mL) stimulated
[3H]thymidine incorporation (1103±4% increase
of control, n=3), which was dramatically potentiated by ET-1
10-7 mol/L (Figure 1C
; 6868±358%
increase above control, corresponding to 6.2-fold;
P<0.001). The potentiating effects of ET-1 were
concentration-dependently prevented by the selective
ETA-receptor antagonist LU135252
10-6 to 10-5 mol/L
(1820±24%, n=3; P<0.001 versus PDGF-BB plus ET-1) and
slightly but significantly further prevented by the
ETA/B-receptor antagonist bosentan
10-6 to 10-5 mol/L
(Figure 1C
, 1595±12% increase, n=3, P<0.05 versus
LU135252).
ET-1 Had No Effects on Cell Cycle Regulators
Although both ET-1 10-7 mol/L and PDGF-BB
10 ng/mL time-dependently stimulated p42mapk, as
demonstrated by a slower mobility on immunoblotting
(data not shown), only PDGF-BB 10 ng/mL was capable of activating Cdk2,
accompanied by an increase in its electrophoretic mobility on SDS-PAGE
(Figure 2A
) due to Cdk2
phosphorylation on Thr160,9 downregulating
p27Kip1 and hyperphosphorylating pRb (Figure 2B
).
|
ET-1 Had No Effects on PDGF Receptors and PDGF-Stimulated
MAPK
Treatment of the cells with ET-1 10-7 to
10-6 mol/L for 24 hours influenced neither PDGF
- nor ß-receptor protein level (Figure 2C
). Furthermore, no
potentiation effects of PDGF-BBinduced
p42/44mapk activation by ET-1
10-7 mol/L were observed at 2 minutes (Figure 2B
).
| Discussion |
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Recent studies using animal models or human tissue indicate that ET-1 contributes to vascular structural changes in proliferative cardiovascular disease.12 13 However, in vitro studies with cultured SMCs did not always find such mitogenic effects of ET-1.5 6 It has been postulated that the putative proliferative effects of ET-1 may depend on the species differences or different culture conditions in particular ingredients of the culture medium such as serum.5 6 To clarify the issue, we therefore used human aortic SMCs in medium devoid of any growth factors. Under these conditions, we clearly demonstrated that ET-1 on its own was unable to stimulate DNA synthesis even in the µmol/L range, although the peptide activated MAPK. The dissociation of MAPK activation from cell growth is in line with our previous study, which suggested that MAPK activation alone is necessary but not sufficient for cell proliferation.10 The ineffectiveness of ET-1 to stimulate SMC growth was further confirmed by the fact that in contrast to PDGF, ET-1 was unable to activate Cdk2, downregulate p27Kip1, or hyperphosphorylate pRb, which is crucial for cell cycle progression.
The most important and intriguing finding of the present study is that ET-1 dramatically potentiated [3H]thymidine incorporation in human SMCs in response to PDGF-BB, with a potency reaching almost 6-fold. The potentiating effects of ET-1 were inhibited mainly by the selective ETA-receptor antagonist LU135252 and slightly but significantly further inhibited by the ETA/B-receptor antagonist bosentan.1 This indicates that activation of ETA receptors is primarily responsible for the potentiating effects of ET-1. Furthermore, our results demonstrated that the potentiating effect of cell growth by ET-1 is due to a mechanism other than overexpression of PDGF receptors or potentiation of PDGF-induced MAPK activation by the peptide. The crosstalk of G-proteincoupled receptors, such as ET-1 receptors, with tyrosine kinase pathways has been documented.14 However, which tyrosine kinasemediated pathways are influenced by PDGF deserves further investigation.
The data presented in this study may explain the contradictory reports on the mitogenic effects of ET-1 in several in vitro studies. The dramatic potentiation of proliferative responses to other growth factors by ET-1 may be crucial in the development of structural vascular changes in patients with atherosclerosis, restenosis, and venous bypass graft disease. The present study also provides strong support for the clinical use of ET-1 antagonists in those clinical situations.
| Acknowledgments |
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Received November 20, 1998; revision received May 12, 1999; accepted May 12, 1999.
| References |
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