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Molecular and Cellular Biology, April 2000, p. 2783-2793, Vol. 20, No. 8
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Multiple Ras Downstream Pathways Mediate Functional
Repression of the Homeobox Gene Product TTF-1
Caterina
Missero,
Maria
Teresa
Pirro, and
Roberto
Di
Lauro*
Stazione Zoologica "A. Dohrn" Villa
Comunale, 80121 Naples, Italy
Received 15 June 1999/Returned for modification 28 July
1999/Accepted 12 January 2000
Expression of oncogenic Ras in thyroid cells results in loss of
expression of several thyroid-specific genes and inactivation of TTF-1,
a homeodomain-containing transcription factor required for normal
development of the thyroid gland. In an effort to understand how signal
transduction pathways downstream of Ras may be involved in suppression
of the differentiated phenotype, we have tested mutants of the Ras
effector region for their ability to affect TTF-1 transcriptional
activity in a transient-transfection assay. We find that V12S35 Ras, a
mutant known to interact specifically with Raf but not with RalGDS or
phosphatidylinositol 3-kinase (PI3 kinase) inhibits TTF-1 activity.
Expression of an activated form of Raf (Raf-BXB) also inhibits TTF-1
function to a similar extent, while the MEK inhibitors U0126 and
PD98059 partially relieve Ras-mediated inactivation of TTF-1,
suggesting that the extracellular signal-regulated kinase (ERK) pathway
is involved in this process. Indeed, ERK directly phosphorylates TTF-1
at three serine residues, and concomitant mutation of these serines to
alanines completely abolishes ERK-mediated phosphorylation both in
vitro and in vivo. Since activation of the Raf/MEK/ERK pathway accounts
for only part of the activity elicited by oncogenic Ras on TTF-1, other downstream pathways are likely to be involved in this process. We find
that activation of PI3 kinase, Rho, Rac, and RalGDS has no effect on
TTF-1 transcriptional activity. However, a poorly characterized Ras
mutant, V12N38 Ras, can partially repress TTF-1 transcriptional
activity through an ERK-independent pathway. Importantly, concomitant
expression of constitutive activated Raf and V12N38 Ras results in
almost complete loss of TTF-1 activity. Our data indicate that the
Raf/MEK/ERK cascade may act in concert with an as-yet-uncharacterized
signaling pathway activated by V12N38 Ras to repress TTF-1 function and
ultimately to inhibit thyroid cell differentiation.
*
Corresponding author. Mailing address: Stazione
Zoologica "A. Dohrn" Villa Comunale, 80121 Naples, Italy. Phone:
39-081-5833278. Fax: 39-081-5833285. E-mail:
rdilauro{at}unina.it.
Molecular and Cellular Biology, April 2000, p. 2783-2793, Vol. 20, No. 8
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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