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Molecular and Cellular Biology, April 2001, p. 2659-2670, Vol. 21, No. 8
Institut National de la Santé et de la
Recherche Médicale U363, Institut Cochin de Génétique
Moléculaire,1 and Institut
National de la Santé et de la Recherche Médicale U248,
Institut Curie,2 Paris, and Unité
Mixte de Recherche 146 du Centre National de la Recherche
Scientifique, Institut Curie, Orsay,3 France
Received 2 November 2000/Returned for modification 30 November
2000/Accepted 24 January 2001
Thrombopoietin (TPO) regulates growth and differentiation of
megakaryocytes. We previously showed that extracellular
signal-regulated kinases (ERKs) are required for TPO-mediated full
megakaryocytic maturation in both normal progenitors and a
megakaryoblastic cell line (UT7) expressing the TPO receptor (Mpl). In
these cells, intensity and duration of TPO-induced ERK signal are
controlled by several regions of the cytoplasmic domain of Mpl. In this
study, we explored the signaling pathways involved in this control. We show that the small GTPases Ras and Rap1 contribute together to TPO-induced ERK activation in UT7-Mpl cells and that they do so by
activating different Raf kinases as downstream effectors: a Ras-Raf-1
pathway is required to initiate ERK activation while Rap1 sustains this
signal through B-Raf. Indeed, (i) in cells expressing wild-type or
mutant Mpl, TPO-induced Ras and Rap1 activation correlates with early
and sustained phases of ERK signal, respectively; (ii) interfering
mutants of Ras and Rap1 both inhibit ERK kinase activity and
ERK-dependent Elk1 transcriptional activation in response to TPO; (iii)
the kinetics of activation of Raf-1 and B-Raf by TPO follow those of
Ras and Rap1, respectively; (iv) RasV12-mediated Elk1 activation was
modulated by the wild type or interfering mutants of Raf-1 but not
those of B-Raf; (v) Elk1 activation mediated by a constitutively active
mutant of Rap1 (Rap1V12) is potentiated by B-Raf and inhibited by an
interfering mutant of this kinase. UT7-Mpl cells represent the second
cellular model in which Ras and Rap1 act in concert to modulate the
duration of ERK signal in response to a growth factor and thereby the
differentiation program. This is also, to our knowledge, the
first evidence suggesting that Rap1 may play an active role in
megakaryocytic maturation.
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.8.2659-2670.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Thrombopoietin-Mediated Sustained Activation of
Extracellular Signal-Regulated Kinase in UT7-Mpl Cells Requires
Both Ras-Raf-1- and Rap1-B-Raf-Dependent
Pathways
*
Corresponding author. Mailing address: INSERM U363,
ICGM, Hôpital Cochin, 27 rue du Faubourg Saint Jacques, 75014 Paris, France. Phone: (33) 1 40 51 65 15. Fax: (33) 1 40 51 65 10. E-mail: porteu{at}cochin.inserm.fr.
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