Feedback inhibition by RALT controls signal output by the ErbB network

Abstract

The ErbB-2 interacting protein receptor-associated late transducer (RALT) was previously identified as a feedback inhibitor of ErbB-2 mitogenic signals. We now report that RALT binds to ligand-activated epidermal growth factor receptor (EGFR), ErbB-4 and ErbB-2.ErbB-3 dimers. When ectopically expressed in 32D cells reconstituted with the above ErbB receptor tyrosine kinases (RTKs) RALT behaved as a pan-ErbB inhibitor. Importantly, when tested in either cell proliferation assays or biochemical experiments measuring activation of ERK and AKT, RALT affected the signalling activity of distinct ErbB dimers with different relative potencies. RALT deltaEBR, a mutant unable to bind to ErbB RTKs, did not inhibit ErbB-dependent activation of ERK and AKT, consistent with RALT exerting its suppressive activity towards these pathways at a receptor-proximal level. Remarkably, RALT deltaEBR retained the ability to suppress largely the proliferative activity of ErbB-2.ErbB-3 dimers over a wide range of ligand concentrations, indicating that RALT can intercept ErbB-2.ErbB-3 mitogenic signals also at a receptor-distal level. A suppressive function of RALT deltaEBR towards the mitogenic activity of EGFR and ErbB-4 was detected at low levels of receptor occupancy, but was completely overcome by saturating concentrations of ligand. We propose that quantitative and qualitative aspects of RALT signalling concur in defining identity, strength and duration of signals generated by the ErbB network.

Full text

Feedback inhibition by RALT controls signal output by the ErbB network
Sergio Anastasi
1,6
, Loredana Fiorentino
1,6,7
, Monia Fiorini
1
, Rocco Fraioli
1
, Gianluca Sala
1
,
L Castellani
2,3
, Stefano Alema
`
4
, Maurizio Alimandi
5
and Oreste Segatto
n
,1
1
Regina Elena Cancer Institute, Via Delle Messi d’Oro, 156, Rome 00158, Italy;
2
University of Cassino, Cassino, Italy;
3
INFM Sez.
B, University of Rome Tor Vergata, Rome, Italy;
4
Istituto di Biologia Cellulare, CNR Monterotondo 00016, Italy;
5
Department of
Experimental Medicine, University of Rome La Sapienza, Viale Regina Elena 324, Rome 00161, Italy
The ErbB-2 interacting protein receptor-associated late
transducer (RALT) was previously identified as a feed-
back inhibitor of ErbB-2 mitogenic signals. We now
report that RALT binds to ligand-activated epidermal
growth factor receptor (EGFR), ErbB-4 and ErbB-
2.ErbB-3 dimers. When ectopically expressed in 32D
cells reconstituted with the above ErbB receptor tyrosine
kinases (RTKs) RALT behaved as a pan-ErbB inhibitor.
Importantly, when tested in either cell proliferation assays
or biochemical experiments measuring activation of ERK
and AKT, RALT affected the signalling activity of
distinct ErbB dimers with different relative potencies.
RALT DEBR, a mutant unable to bind to ErbB RTKs, did
not inhibit ErbB-dependent activation of ERK and AKT,
consistent with RALT exerting its suppressive activity
towards these pathways at a receptor-proximal level.
Remarkably, RALT DEBR retained the ability to
suppress largely the proliferative activity of ErbB-
2.ErbB-3 dimers over a wide range of ligand concentra-
tions, indicating that RALT can intercept ErbB-2.ErbB-3
mitogenic signals also at a receptor-distal level. A
suppressive function of RALT DEBR towards the
mitogenic activity of EGFR and ErbB-4 was detected at
low levels of receptor occupancy, but was completely
overcome by saturating concentrations of ligand. We
propose that quantitative and qualitative aspects of RALT
signalling concur in defining identity, strength and
duration of signals generated by the ErbB network.
Oncogene (2003) 22, 4221–4234. doi:10.1038/sj.onc.1206516
Keywords: ErbB; feedback inhibition; Mig-6; negative
signaling; RALT
Introduction
The family of ErbB receptor tyrosine kinases (RTKs)
consists of four members: ErbB-1 (also referred to as
epidermal growth factor receptor, EGFR) through
ErbB-4. In the human genome, a dozen 10 genes encode
ErbB ligands possessing distinct receptor specificity.
Regardless of the latter, all ErbB ligands trigger cognate
receptors via a common mechanism entailing the
assembly of homo- and heterodimeric receptor combi-
nations (reviewed in Olayioye et al., 2000; Yarden and
Sliwkowski, 2001). The ensuing combinatorial reper-
toire of ligand-driven receptor dimers provides the
family of ErbB RTKs with a great deal of signalling
versatility and, consequently, the ability to govern a
wide array of cellular programs. This is best exemplified
by genetic studies, which have shown that defined
combinations of ErbB receptors play nonredundant
roles during development and in postnatal life. In
human pathology, this is mirrored by the selective
derangement of EGFR and ErbB-2 activity during the
pathogenesis of a number of tumors (reviewed in
Olayioye et al., 2000; Yarden and Sliwkowski, 2001).
ErbB-1 and ErbB-4 share canonical features of RTKs
and can generate downstream signals upon ligand-
induced homodimerization. In contrast, ErbB-2 and
ErbB-3 are peculiar in that ligand-driven lateral inter-
actions with other ErbB RTKs represent a sine qua non
for initiation of their signalling activity (reviewed in
Olayioye et al., 2000; Yarden and Sliwkowski, 2001).
ErbB-3 is able to bind ligands, such as neuregulins
(NRGs), directly (Carraway III et al., 1994, 1997), but is
devoid of intrinsic catalytic activity (Guy et al., 1994).
Therefore, ErbB-3 acquires signalling capacity only in
the context of heterodimers with catalytically competent
ErbB RTKs (Pinkas-Kramarski et al., 1996). Conver-
sely, ErbB-2 is unable to bind ligands directly, but
behaves as a coreceptor for several ligands when
complexed with other ErbB RTKs (Graus-Porta et al.,
1995, 1997; Riese et al., 1995; Karunagaran et al., 1996;
Riese and Stern, 1998). In fact, ErbB-2 appears to be the
favorite partner of all other ErbB receptors and emerges
as the hierarchically dominant element in the assembly
of ErbB heterodimers (Graus-Porta et al., 1997). Of
particular relevance is the ErbB-2/ErbB-3 combination
that reconstitutes a high-affinity receptor for NRGs
endowed with potent oncogenic activity (Alimandi et al.,
1995; Wallasch et al., 1995).
Individual ErbB receptors may couple selectively to
some signalling pathways: for example, only EGFR is
Received 1 October 2002; revised 19 February 2003; accepted 20
February 2003
*Correspondence: O Segatto; E-mail: segatto@ifo.it
6
These two authors contributed equally to this work
7
Current address: The Burnham Institute, 10901 N Torrey Pines Rd,
La Jolla, CA 92037, USA
Oncogene (2003) 22, 4221–4234
&
2003 Nature Publishing Group
All rights reserved 0950-9232/03 $25.00
www.nature.com/onc

able to activate STAT1 and STAT3 (Olayioye et al.,
1999). Combinatorial dimerization among ErbB RTKs
generates additional signalling potential via integration
of the signalling competence of individual protomers as
well as the generation of novel signalling profiles
(Olayioye et al., 1998). Finally, ErbB RTKs share some
signalling pathways, such as those leading to Ras–ERK
and PI-3K–AKT activation. Importantly, distinct ErbB
dimers may activate these pathways with diverse profiles
of intensity and duration (Olayioye et al., 2000 and
references therein), which may be themselves critical
determinants of signal specificity (Marshall, 1995;
Ghiglione et al., 1999). This raises the question of how
strength and duration of ErbB signals are proofread in
order to (a) allow the correct execution of cellular
programs governed by precise thresholds of receptor
signals and (b) restrain the oncogenic potential of
ErbB activity.
It is increasingly appreciated that negative signalling
is instrumental in defining quantitative aspects of ErbB
outputs. Coupling to internalization/degradation path-
ways is thought to set thresholds of activity of ErbB
RTKs at the inception of receptor triggering (reviewed
in Waterman and Yarden, 2001). Much less studied are
the mechanisms that gauge and eventually extinguish
ErbB signals as they accumulate over time because of
sustained receptor activity. Genetic studies in flies
indicate that expression of Drosophila EGFR (DER)
feedback inhibitors such as Kekkon1,Argos and Sprouty
is controlled transcriptionally by DER via the Ras-ERK
pathway (Perrimon and McMahon, 1999). Importantly,
negative feedback regulation of DER signalling is
critical for ensuring that DER activity be confined
within appropriate spatial and temporal boundaries
(Perrimon and McMahon, 1999; Freeman, 2000). Thus,
it appears that the levels of expression of Aos, Spry and
Kek1 are a means for the cell to gauge the accumulation
of receptor signals, while their activity is uniquely suited
to proofread, tune and extinguish DER activity.
Consequently, it has been proposed that negative
feedback loops provide DER signalling with an essential
element of stability, which prevents perturbation of
DER function and allows for accurate reproduction of
signals responsible for complex developmental patterns
(Freeman, 2000).
Receptor-associated late transducer (RALT) has been
recently identified as a feedback inhibitor of mitogenic
signals propagated by ErbB-2 (Fiorentino et al., 2000)
and EGFR (Hackel et al., 2001). RALT expression is
activated by EGF, TGF-aand NRG-1 in fibroblasty
and epithelial cells (Fiorentino et al., 2000), via a
mechanism entailing transcriptional activation of the
Ralt gene by the Ras–Raf–ERK pathway (Fiorini et al.,
2002). Constitutive overexpression of RALT markedly
inhibits the mitogenic and transforming activity of
EGFR (Hackel et al., 2001) and ErbB-2 (Fiorentino
et al., 2000). Conversely, neutralization of endogenous
RALT function by microinjection of anti-RALT anti-
bodies enhances the mitogenic activity of an EGFR/
ErbB-2 chimeric receptor (Fiorini et al., 2002). These
data indicate that physiological levels of RALT are
involved in the suppression of ErbB-2 signals and imply
that RALT function may in fact play a necessary
role in negative regulation of ErbB-2 activity. Owing to
its ability to complex with ErbB-2- and SH3-containing
proteins, RALT has been proposed to function as a
receptor-proximal inhibitor, which links ErbB-2 to
effectors containing SH3 domains (Fiorentino et al.,
2000). However, the significance of the RALT/ErbB-2
physical interaction in the context of RALT-
mediated inhibition of ErbB-2 signalling has not been
formally addressed, nor is it known whether and to what
extent RALT is able to antagonize mitogenic signals
propagated by the entire ErbB signalling network.
The latter is an especially relevant issue, given the
high degree of signal integration among ErbB
RTKs.
Here, we report that RALT functions as a pan-ErbB
inhibitor. Remarkably, the potency of RALT suppres-
sive activity impacts differentially on individual ErbB
receptors. We propose that RALT activity adds a novel
layer of regulation to the specification of signal output
by the ErbB network.
Results
RALT is not a promiscuous RTK inhibitor
Despite its suppressive activity towards mitogenic and
transforming signals triggered by ErbB-2 and EGFR,
ectopically expressed RALT altered neither mitogenic
responses of murine fibroblasts to serum nor cell
transformation induced by v-ras and v-src (Fiorentino
et al., 2000; Hackel et al., 2001). The issue of RALT
specificity was further addressed in the experiments
shown in Figure 1. NIH-EGFR/ErbB-2 cells were
infected with either Pinco or Pinco–RALT retroviral
stocks. RALT expression was induced in Pinco
cells by EGF (Figure 1a) as well as by bFGF and
PDGF (data not shown, see also Fiorini et al., 2002)
with comparable kinetics. Pinco–RALT cells showed
constitutive expression of ectopic RALT at levels 3–4-
fold higher than the endogenous species (Figure 1a).
Whereas ErbB-2-dependent mitogenic signals were
clearly suppressed by RALT overexpression (42–52%
reduction over a 0.3–5 ng ml EGF concentration range),
proliferative responses to PDGF and bFGF were
comparable in Pinco and Pinco–RALT cells over a
wide range of ligand concentration (Figure 1b); also
proliferative responses to PMA were unaffected by
RALT overexpression (data not shown). These results
indicate that RALT is unlikely to behave as a pan-RTK
inhibitor.
RALT binds in vitro to all members of the ErbB family
Based on the above conclusions, we focused our
attention on the function of RALT within the ErbB
signalling network. Thus, we asked whether RALT
could enter in a complex with ErbB RTKs other than
ErbB-2.
Negative signalling of RALT to the ErbB network
S Anastasi el al
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Previous studies have shown that the ErbB-2-binding
activity of RALT is confined to the RALT COOH
terminus (residues 262–459); within this 197 amino-acid-
long stretch, the sequence comprised between positions
282 and 396 (GST-RALT cl.52) was sufficient to bind
ErbB-2 in GST pull down assays and in yeast two-
hybrid experiments (Fiorentino et al., 2000) (see also
Figures 2a and 3). GST-RALT cl.52 was able to
precipitate ErbB-2 and ErbB-3 from lysates of NIH-
E2.E3 transfectants; ErbB-3 could be precipitated only
from lysates of NRG-1-stimulated cells, whereas ErbB-2
bound to GST-RALT cl.52 irrespective of NRG-1
stimulation (Figure 2a). In the absence of ErbB-2
coexpression, ErbB-3 was not precipitated by GST-
RALT cl.52, even under conditions of optimal stimula-
tion with NRG-1. On the contrary, ErbB-2 bound
efficiently to GST-RALT also in the absence of
ectopically expressed ErbB-3. The ligand-independent
binding of ErbB-2 to GST-RALT cl.52 is most likely a
consequence of the constitutive activation of ErbB-2 in
NIH 3T3 transfectants, as reported previously (Lonardo
et al., 1990) and demonstrated herein by the anti-P-Tyr
immunoblot shown in Figure 2a. Binding to GST-
RALT cl.52 was also observed with EGFR (Figure 2b)
and ErbB-4 (Figure 2c) solubilized from NIH-E1 and
NIH-E4 transfectants, respectively. Binding of ErbB-4
to GST–RALT cl.52 was unaffected by EGF stimula-
tion but enhanced by NRG-1 stimulation. Conversely,
EGF but not NRG-1 stimulation enhanced binding of
EGFR to GST–RALT cl.52.
Figure 1 Biological analysis of RALT signalling in mitogenic
programs activated by PDGF and bFGF. NIH-EGFR/ErbB-2
cells were seeded in 24-well plates and infected with either Pinco or
Pinco–RALT retrovirus stocks. Infection efficiency, as monitored
by FACS assessment of GFP-positive cells was 495%. (a)
Following infection, cells were made quiescent by 24 h serum
deprivation and subsequently challenged with 5 ng/ml EGF for the
indicated time. Cell lysates were subjected to immunoblot analysis
with either the S1 antiserum that recognizes mouse, rat and human
RALT species or MoAb 19C5/4 specific for the ectopically
expressed rat RALT species. (b) Following infection, cells were
switched to either serum-free medium (SFM) or SFM containing
escalating concentrations of EGF (0.3, 1 and 5 ng/ml), bFGF (0.3,
1 and 5 ng/ml) or PDGF (0.2, 2 and 10 ng/ml). Assays were
performed in quadruplicate wells. After 44 h, cultures were labelled
for 4 h with 1 mCi/ml [
3
H]methyl-thymidine and cell proliferation
measured as fold increase of TCA-precipitated radioactivity in
growth factor-treated wells versus control cultures. The s.d. was
o15%. Data are representative of one out of three experiments
that gave similar results. White bars indicate Pinco cells, gray bars
indicate Pinco–RALT cells
Figure 2 In vitro analysis of RALT interaction with ErbB RTKs.
(a) Recombinant GST-RALT cl.52 fusion protein (referred to as
GST–RALT) spanning positions 282–396 of RALT was purified
onto glutathione–sepharose beads and used in pull-down assays
against ErbB receptors solubilized from the indicated NIH 3T3
transfectants. Lysates of parental NIH 3T3 were used as control.
Where indicated, cells were treated with 20 ng/ml recombinant
NRG-1bfor 5 min prior to lysis. Protein complexes were subjected
to immunoblot analysis with the indicated antibodies. The bottom
portion of the gel was stained with Coomassie blue to control for
equal input of GST–RALT. Reference lysates (immunoblotted
with anti-ErbB-3 and anti-P-Tyr antibodies) corresponded to 5%
of input lysate in GST pull-down experiments. Control experiments
(data not shown, see also Figure 3b) indicated that GST alone did
not bind to ErbB RTKs. (band c) NIH-EGFR (b) and NIH-ErbB-
4(c) transfectants were serum starved for 16 h and lysed either
without further treatment or after stimulation for 5 min with 20 ng/
ml of the indicated ligands. Lysates were subjected to pull-down
assays with GST–RALT and immunoblotted with anti-EGFR (b)
and anti-ErbB-4 antibodies (c). Reference lysates corresponded to
5% of input lysate in GST pull-down experiments. Input GST–
RALT was visualized by Coomassie stain
Negative signalling of RALT to the ErbB network
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Identification of the minimal ErbB-2-binding region of
RALT
In order to define the minimal ErbB-2-binding region
(EBR) of RALT, we generated a panel of GST–RALT
fusion proteins (Figure 3a) and assayed their ability to
precipitate ErbB-2 from lysates of NIH-ErbB-2 trans-
fectants. Neither the NH2 (aa. 282–343) nor the COOH
(aa. 335–396) halves of GST–RALT cl.52 were capable
of binding to ErbB-2 (data not shown), whereas
sequences comprised between positions 282 and 312
were dispensable for directing the interaction of GST–
RALT cl.52 with ErbB-2 (Figure 3b). Further deletion
analysis defined the sequence comprised between posi-
tions 323 and 372 of RALT as the minimal EBR
(Figure 3b). We took advantage of a conveniently
located StuI site in the Ralt cDNA sequence to generate
a RALT mutant lacking residues 315–361 (RALT
DEBR, Figure 3a), which could be expressed in various
cell lines as efficiently as full-length RALT (see below).
Analysis of RALT interaction with ErbB receptors in
living cells
We next asked whether RALT interacts with ErbB
RTKs in living cells in an EBR-dependent fashion. To
address these questions, we performed coimmunopreci-
pitation assays using lysates of either 293 or NIH 3T3
cells engineered to express ErbB RTKs along with either
RALT or RALT DEBR (Figures 4 and 5). Neither
RALT nor RALT DEBR had detrimental consequences
on steady-state levels of coexpressed ErbB receptors
(Figures 4 and 5). Importantly, the DEBR mutant was
able to interact with Grb-2 as efficiently as RALT
(Figure 4b). We infer that the DEBR mutation is
unlikely to cause gross conformational alterations of
the RALT protein, since the RALT/Grb-2 interaction
requires that Grb-2 SH3 domains bind to Pro-rich
sequences located in close proximity to the RALT EBR
module (Fiorentino et al., 2000).
RALT, but not RALT DEBR, was found in a
complex with ErbB-2 in 293 cells, as revealed by
probing anti-RALT immunoprecipitates with anti-
ErbB-2 antibodies. This immunoreactivity was contin-
gent on coexpression of RALT and ErbB-2 (Figure 4a).
ErbB-3 could be coimmunoprecipitated with anti-
RALT antibodies only if coexpressed with ErbB-2 and
solubilized from cells stimulated with NRG-1. Coim-
munoprecipitation of RALT and ErbB-3 required
integrity of the EBR (Figure 4a). Thus, it appears that
ErbB-3 coimmunoprecipitates with RALT only when
recruited to ErbB-2.ErbB-3 dimers generated by NRG-1
stimulation. Interestingly, in E2.E3/RALT transfectants
the amount of ErbB-2 immunoreactivity recovered in
anti-RALT immunoprecipitates from NRG-stimulated
cells was consistently lower than that obtained from
unstimulated cells (Figure 4a). We infer that in
unstimulated ErbB-2.ErbB-3 transfectants, RALT is
likely to be complexed to activated ErbB-2 homodimers
generated by ErbB-2 overexpression. NRG-1 stimula-
tion drives redistribution of activated ErbB-2 molecules
into complexes with ErbB-3, thus causing preferential
association of RALT with ErbB-2.ErbB-3 heterodimers
rather than with ErbB-2 homodimers.
EGFR (Figure 5a, c) and ErbB-4 (Figure 5b) were
also able to associate with RALT. These interactions
were largely dependent on ligand stimulation of the
receptors and absolutely contingent on the presence of
the EBR module in the RALT protein. A catalytically
inactive EGFR mutant carrying the K-A mutation at
position 721 was unable to recruit RALT in a physical
complex both in anti-EGFR immunoprecipitations
followed by anti-RALT immunoblot (Figure 5c), as
well as in anti-RALT immunoprecipitations followed by
anti-EGFR immunoblot (data not shown). Moreover,
we did not detect RALT/EGFR K721A complexes even
upon marked receptor overexpression, that is, under
conditions causing constitutive, ligand-indepen-
dent RALT/EGFR association (data not shown). In
Figure 3 Mapping of the minimal EBR in the RALT protein. (a,
top) Schematic outline of domain organization of RALT and
RALT DEBR mutant. NLS indicates a putative nuclear localiza-
tion signal. (Bottom) Schematic representation of RALT fragments
used as GST fusion products in pull-down assays reported in panel
b. The ability of each recombinant polypeptide to interact in vitro
with the ErbB-2 protein is also indicated. Note that RALT
fragments are drawn to a scale larger than the one used to draw
RALT and RALT DEBR in the top portion of the panel. (b) GST
fusion proteins (5 mg for each pull-down experiment) spanning the
indicated portions of the RALT protein were produced in E. coli,
purified onto glutathione–agarose beads and assayed for their
ability to capture ErbB-2 protein solubilized from NIH-ErbB-2
transfectants. Protein complexes were analysed by immunoblot
with anti-ErbB-2 antibodies. GST was used as control; reference
lysate corresponded to 5% of input lysate in pull-down assays
Negative signalling of RALT to the ErbB network
S Anastasi el al
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contrast, the deletion of the EGFR COOH tail, which
contains all the major autophosphorylation sites of the
receptor (DC mutant), did not affect RALT/EGFR
complex formation (Figure 5c).
Collectively, the experiments described in Figures 3–5
indicate that RALT interacts with different ErbB RTKs
via the EBR module. Importantly, when comparing the
amount of ErbB receptors precipitated by either GST–
RALT or anti-RALT antibodies to the amount of ErbB
receptors present in 5% of input lysate, we consistently
observed that RALT interacts with EGFR, ErbB-4 and
ErbB-2.ErbB-3 dimers with comparable efficiencies.
RALT is spatially controlled by its interaction with
ligand-activated ErbB RTKs
EGF-dependent activation of an EGFR/ErbB-2 chi-
meric receptor leads to the redistribution of RALT from
a cytosolic location to intracellular membranes (Fior-
entino et al., 2000). The inability of the DEBR mutant to
associate with ErbB RTKs allowed us to test whether
physical interaction with ErbB receptors is implicated in
spatial control of the RALT protein. MoAb 19C5/4
allows immunohistochemical detection of RALT and
RALT DEBR with similar efficiency. Since Ab 19C5/4
does not crossreact with murine RALT (Fiorentino
et al., 2000, see also Figure 1a), we could selectively
image ectopic RALT proteins in NIH-EGFR and NIH-
EGFR/ErbB-2 cells. Of note, the degree of RALT
overexpression in these cells was modest (3–4-fold
higher than that of the endogenous species, see
Figure 1a for reference). Immunofluorescence imaging
of RALT in quiescent cells indicated that both RALT
(Figure 6e, m) and RALT DEBR (Figure 6g, o) were
distributed in the cytosol. RALT relocated to a vesicular
compartment following activation of EGFR for 15 min
(Figure 6n) and EGFR/ErbB-2 for 15 min (not shown)
or 60 min (Figure 6f), showing extensive colocalization
with ErbB receptors (Figure 6j). Strikingly, ligand
activation of neither EGFR nor EGFR/ErbB-2 caused
relocation of RALT DEBR (Figure 6h, p). We conclude
that RALT relocation induced by activation of ErbB
RTKs requires that RALT forms a physical complex
with ligand-bound ErbB RTKs. We also infer that the
DEBR mutation does not abolish coimmunoprecipita-
tion between RALT and ErbB receptors by simply
causing a reduction of the affinity of RALT DEBR for
ErbB RTKs.
RALT inhibits ErbB signals leading to activation of ERKs
and AKT
ErbB family members share the ability to trigger the
Ras-ERK and the PI-3-AKT pathways (Yarden and
Sliwkowski, 2001). Thus, we surmised that evaluation of
ERK and AKT activities triggered by engagement of
ErbB RTKs could provide a convenient biochemical
readout for comparative analysis of RALT signalling to
distinct ErbB dimers.
For these studies we used 32D cells reconstituted with
defined ErbB RTKs. 32D.ErbB transfectants are re-
lieved from IL-3 dependency if cultured in media
containing ErbB ligands, thus providing a sensitive
assay of the signalling activity of defined ErbB dimers
(Alimandi et al., 1997). 32D.ErbB transfectants were
further engineered to express either RALT or RALT
DEBR. Of note, 32D cells do not express endogenous
RALT protein, as indicated by immunoblot analysis of
lysates prepared from 32D and 32D.E1 cells triggered
with either IL-3 (growth medium) or EGF (Figure 7c).
Figure 4 In vivo analysis of physical interaction between RALT
and the ErbB-2.ErbB-3 NRG-1 receptor. (a) ErbB-2 and ErbB-3
were transiently expressed in 293 cells either alone or in
combination. Where indicated, either RALT or RALT DEBR
were coexpressed with ErbB RTKs. After 2 h serum starvation,
cells were stimulated at 371C with either carrier solution or 20 ng/
ml NRG-1bbefore lysis. Equal amounts of each lysate were
analysed by immunoblot with anti-ErbB-2 and anti-ErbB-3
antibodies to control for receptor expression. Lysates were
subjected to immunoprecipitation with anti-RALT 19C5/4 mono-
clonal antibody followed by blot analysis with anti-RALT and
anti-ErbB-3 antibodies. Filters were subsequently reprobed with
anti-ErbB-2 antibodies. (b) The DEBR mutation does not affect the
ability of RALT to interact with Grb-2. HA-tagged GRB-2 was
expressed in 293 cells either alone or in combination with RALT or
RALT DEBR. Equal amounts of each lysate and anti-RALT
immunoprecipitates were immunoblotted with anti-HA tag 12CA/5
and anti-RALT 19C5/4 monoclonal antibodies
Negative signalling of RALT to the ErbB network
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This was confirmed by Northern blot analysis (data not
shown), in agreement with the absence of Mig-6 mRNA
expression in cells of the hemopoietic lineage reported in
the Genecard database (http://bioinfo.weizmann.ac.il/
cards/). Thus, by expressing RALT proteins in
32D.ErbB transfectants we could (a) assess whether
reconstitution of RALT expression is per se capable of
restoring a pathway of negative signalling to ErbB
RTKs and (b) study the activity of the DEBR mutant
without the possible interference of endogenous RALT
protein.
Stable RALT and RALT DEBR derivatives of
32D.E1, 32D.E2.E3 and 32D.E4 cells were generated
by transfection with Pinco–RALT and Pinco–RALT
DEBR vectors, respectively. Control cells were drug-
selected after transfection with the backbone Pinco
Figure 5 In vivo analysis of RALT interaction with EGFR and ErbB-4. (a) NIH 3T3 and NIH-EGFR cells were infected with
recombinant retrovirus directing the expression of either RALT or RALT DEBR. Control cells were infected with empty virus. After
2 h serum starvation, cells were stimulated for 5 min with either carrier solution or EGF (10 ng/ml) prior to lysis. Equal amounts of
each lysate were analysed by immunoblot with anti-receptor and anti-P-Tyr antibodies. Lysates were subjected to immunoprecipitation
with anti-RALT 19C5/4 monoclonal antibody followed by blot analysis with anti-RALT and anti-receptor antibodies. (b) ErbB-4 was
transiently expressed in 293 cells along with either RALT or RALT DEBR. At 48 h after transfection, cells were lysed after stimulation
for 5 min with either carrier solution or 20ng/ml NRG-1. Equal amounts of each lysate were analysed by immunoblot with anti-ErbB-4
and anti-P-Tyr antibodies. Anti-RALT 19C5/4 immunoprecipitates were resolved on SDS–PAGE and immunoblotted with anti-
RALT and anti-ErbB-4 antibodies. (c) RALT was coexpressed in 293 cells with either wt EGFR or EGFR mutants depicted in the left
portion of the panel, namely EGFR K721A (KD, i.e. kinase-defective) and EGFR DC, that lacks the COOH-terminal region. At 48 h
after transfection, cells were stimulated for 5 min with either carrier solution or 10 ng/ml EGF. Equal amounts of each lysate were
analysed by immunoblot with anti-RALT and anti-P-Tyr antibodies. Anti-EGFR immunoprecipitates were resolved on SDS–PAGE
and immunoblotted with anti-RALT and anti-EGFR antibodies
Negative signalling of RALT to the ErbB network
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vector. Neither RALT nor RALT DEBR affected the
steady-state levels of ErbB RTKs in 32D transfectants
(Figure 7a). For each of the 32D.ErbB transfectants
mentioned above, we selected cell populations expres-
sing comparable amounts of RALT and RALT DEBR.
Levels of RALT proteins were similar in 32D.E2.E3 and
32D.E4 cells. When compared to 32D.E4 and
32D.E2.E3 cells, 32D.E1 transfectants contained lower
levels of ectopic RALT and RALT DEBR (Figure 7b).
In 32D.E1 Pinco cells EGF stimulation (3 ng/ml) led
to sustained activation of ERKs throughout the 6 h time
interval of the experiment. AKT was also activated
robustly in the initial 60 min of EGF stimulation, after
which time anti-phospho-AKT reactivity declined
(Figure 8a). In EGF-treated 32D.E1 RALT DEBR cells
strength and duration of ERK and AKT activation were
quite similar to those observed in Pinco controls. In
contrast, expression of RALT in 32D.E1 cells led to
premature termination of ERK and AKT activation,
with no reactivity to anti-P-ERK and anti-P-AKT being
observed past the 1 h and 15 min time point, respectively
(Figure 8a). These differences were not accounted for by
changes in either EGFR expression or EGFR phos-
phorylation on tyrosine residues (Figure 8a).
Stimulation of Pinco and RALT DEBR derivatives of
32D.E2.E3 cells with 10 ng/ml NRG-1 generated rather
similar kinetics of activation of AKT and ERKs
(Figure 8b). While the profile of ERK activity in
32D.E1 Pinco and 32D.E2.E3 Pinco transfectants were
comparable, activated ErbB-2.ErbB-3 dimers generated
a more sustained AKT signal (compare Figure 8a, b).
This observation is consistent with ErbB-3 providing
powerful activation of PI-3K once transphosphorylated
by catalytically competent ErbB RTKs. Expression of
ectopic RALT in 32D.E2.E3 cells caused premature
extinction of both ERK and AKT activity (Figure 8b).
ERK activity in E2.E3 RALT cells returned to back-
ground levels past the 3 h time point, that is, later than
what we observed in E1 RALT transfectants (Figure 8a).
AKT activity was not detectable past the 3 h time point
Figure 6 Distinct spatial distribution of RALT and RALT DEBR in response to activation of ErbB receptors. Subconfluent cultures
of serum-deprived NIH-EGFR/ErbB-2 (a–l) and NIH-EGFR (m–p) cells ectopically expressing either RALT or RALT DEBR were
processed for immunocytochemistry before (a,e,i,c,g,k,m,o) or after stimulation with EGF (10 ng/ml) for 60 (b,f,j,d,h,l) or 15 min
(n,p). NIH-EGFR/ErbB-2 cells (a–l) were double stained for ErbB-2 (a–d, green) and either RALT (e,f, red) or RALT DEBR (g,h,
red). Upon EGF stimulation, RALT (f) relocates to vesicular perinuclear structures where it colocalizes with the EGFR/ErbB-2
chimera (b, merge in j), while RALT DEBR (h) does not show significant relocation and does not colocalizes with EGFR/ErbB-2 (d,
merge in l). Also in EGF-treated NIH-EGFR cells (m–p), RALT (m,n) relocates from the cytosol to a vesicular compartment, whereas
spatial distribution RALT DEBR (o,p) is not detectably altered by EGFR activation. Bars: 10 mm.
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in RALT cells, whereas it was still high after 9 h of
NRG-1 stimulation in both E2.E3 Pinco and E2.E3
RALT DEBR cells. Neither RALT nor RALT DEBR
altered significantly the profile of expression and
activation of ErbB-2 and ErbB-3 (Figure 8b).
At variance with what observed in E1 and E2.E3
transfectants, expression of RALT in 32D.E4 cells led to
a severe inhibition of NRG-1-dependent ERK activity
since the inception of receptor triggering and through-
out the 6 h time frame of the experiment (Figure 8c).
Pronounced attenuation of anti-P-AKT reactivity in
32D.E4 RALT cell lysates was also observed at the
earliest time point of NRG-1 stimulation, with P-AKT
levels being reduced by 80% past the 60 min time point
(Figure 8c). These drastic effects of RALT activity on
ErbB-4 signalling could not be accounted for by obvious
alterations in levels of ErbB-4 expression and tyrosine
phosphorylation (Figure 8c). The DEBR mutation
caused RALT to lose its ability to suppress ErbB-4-
dependent activation of ERKs and AKT (Figure 8c). In
fact, the activity of ERKs and AKT was reproducibly
enhanced by the expression of RALT DEBR, raising the
possibility that the DEBR mutant exerts a dominant-
negative function (see the Discussion section).
Collectively, the data presented in Figure 8 indicate
that genetic reconstitution of RALT expression in
32D.E1, 32D.E4 and 32D.E2.E3 cells is sufficient to
suppress ErbB-dependent activation of ERKs and AKT.
RALT suppressed AKT and ERK activities with kinetics
that were significantly different from one another in
distinct 32D.ErbB transfectants. However, such inhibi-
tory function of RALT was in all cases strictly dependent
on the integrity of the EBR module and, by inference,
required physical interaction and spatial segregation of
RALT with activated ErbB receptors.
Impact of RALT on mitogenic signals generated by ErbB
RTKs
We next studied the impact of RALT and RALT DEBR
signalling on mitogenic programs governed by ErbB
RTKs. Ectopically expressed RALT did not influence
Figure 7 Analysis of expression of RALT, RALT DEBR and ErbB RTKs in 32D transfectants. (a) 32D transfectants expressing
EGFR, ErbB-4 and the ErbB-2.ErbB-3 combination were supertransfected with Pinco, Pinco–RALT and Pinco–RALT DEBR
expression vectors. Stable Pinco, RALT and RALT DEBR derivatives were established by puromycin selection. Lysates prepared from
each of the transfectants were immunoblotted with antibodies specific for each of the ErbB family members. Filters were also probed
with anti-SHC antibodies to control for equal loading of lysates. (b) lysates from 32D transfectants described in (a) were analysed
simultaneously for RALT expression by immunoblot in order to compare relative levels of RALT expression. Filters were stripped and
reprobed with anti-SHC antibodies to control for sample loading. (c) 32D and 32D.E1 cells were deprived of IL-3 for 2 h and then
stimulated for the indicated time with either IL-3 (growth medium) or EGF (10 ng/ml). Equal amounts of lysates were immunoblotted
with the indicated antibodies. A lysate from 32D.E1 RALT cells was used as positive control for RALT immunodetection
Negative signalling of RALT to the ErbB network
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the ability of 32D derivatives to proliferate in response
to IL-3 (data not shown). In contrast, EGF-dependent
proliferation of 32D.E1 RALT cells was severely
reduced in comparison to their Pinco counterpart
(Figure 9a). The suppressive effect of RALT on EGFR
mitogenic signals was almost complete at suboptimal
concentrations of EGF; saturating doses of EGF
alleviated RALT-mediated suppression, rescuing mito-
genic activity to 25–30% of that of Pinco cells
(Figure 9a). Similar results were obtained with TGF-a
(data not shown). Annexin V staining indicated that
EGF stimulation rescued 32D.E1 Pinco cells from
apoptotic cell death induced by IL-3 deprivation. In
contrast, EGF-treated 32D.E1 RALT cells underwent
apoptosis at rates similar to those of 32D.E1 Pinco and
32D.E1 RALT cells cultured in medium devoid of both
EGF and IL-3 (data not shown). Thus, in 32D.E1 cells
RALT inhibits EGFR signals necessary for both cell
survival and cell proliferation, but does not exert a
direct proapoptotic activity. The RALT DEBR mutant
showed a bimodal behavior. At concentrations of EGF
equal to or lower than 3 ng/ml, expression of RALT
DEBR partially suppressed the mitogenic responses of
32D.E1 cells (Figure 9a). On the other hand, at EGF
concentrations higher than 3 ng/ml, which in this assay
corresponded approximately to the ED
50
of EGF, the
DEBR mutant enhanced EGF-dependent proliferation
of 32D.E1 cells.
Expression of RALT in 32D.E2.E3 cells led to the
suppression of mitogenic responses elicited by NRG-1
(Figure 9b). The suppressive activity of RALT was
strongest at concentrations of NRG-1 equal to or lower
Figure 8 Interaction of RALT with ErbB receptors affects strength and duration of ERK and AKT activation. Pinco, RALT and
RALT DEBR derivatives of 32D.E1 (a), 32D.E2.E3 (b) and 32D.E4 (c) transfectants described in Figure 7 were deprived of IL-3 for
2 h. Following stimulation with 3 ng/ml EGF (a) or 10 ng/ml NRG-1b(b,c) for the indicated time, cells were harvested and lysed in
Laemmli buffer. Lysates were analysed by immunoblot with the indicated antibodies. (a–c) Show a representative experiment of at
least three independent assays for E1, E2.E3 and E4 transfectants
Negative signalling of RALT to the ErbB network
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than its ED
50
, with higher NRG-1 doses somehow
overcoming the inhibitory activity of RALT. RALT
activity prevented the NRG-1-dependent rescue of
32D.E2.E3 cells from the apoptotic effect of IL-3
deprivation, but was not proapoptotic per se (data not
shown). Strikingly, RALT DEBR was as efficient as
RALT in suppressing NRG-1-driven proliferation of
32D.E2.E3 cells at all of the NRG-1 concentrations
tested, including doses in excess of those yielding
optimal proliferation of 32D.E2.E3 Pinco cells
(Figure 9b). Expression of RALT in 32D.E4 cells led
to complete suppression of NRG-1-dependent cell
proliferation (Figure 9c), even at high ligand concentra-
tions. Similar results were obtained with b-cellulin (data
not shown). As observed with E1 and E2.E3 transfec-
tants, RALT did not exert a direct proapoptotic activity
(data not shown). The DEBR mutation deprived RALT
of the ability to suppress proliferation of 32D.E4 cells
over a wide range of NRG-1 concentrations (Figure 9c).
In fact, at optimal ligand concentrations, RALT DEBR
significantly enhanced NRG-1-dependent proliferation
of 32D.E4 cells.
Discussion
Quantitative aspects of RTK outputs are important in
establishing the identity of receptor signals and restrain-
ing their oncogenic potential. They are defined by the
balanced interplay between mechanisms of signal gen-
eration and signal extinction. Experimental manipula-
tions leading to either ablation (Fiorini et al., 2002) or
enhancement of RALT activity (Fiorentino et al., 2000)
in murine fibroblasts are consistent with RALT being
physiologically involved in negative signalling to the
ErbB-2 receptor. Strikingly, RALT appears to be a
rather selective inhibitor, as its overexpression is unable
to suppress cell proliferation driven by PDGF, bFGF
(Figure 1) and serum (Fiorentino et al., 2000), or cell
transformation induced by v-src and v-ras (Fiorentino
et al., 2000). This contrasts with the relative promiscuity
of other receptor-proximal feedback inhibitors, such as
mSprouty and SOCS proteins (reviewed in Fiorini et al.,
2001). The aim of this study was to analyse how RALT
function impacts on the activity of the ErbB network.
Physical interaction of RALT with ErbB receptors
RALT is able to form a physical complex in living cells
with ligand-activated ErbB RTKs, including EGFR,
ErbB-4 and the high-affinity NRG-1 receptor generated
by ErbB-2.ErbB-3 dimerization. The ErbB-binding
activity of RALT can be reconstituted in vitro using a
minimal 47 amino-acid region that binds distinct ErbB
family members with comparable affinities.
Binding of RALT to the EGFR in intact cells requires
that the receptor be catalytically active, as demonstrated
by the failure of kinase-inactive EGFR to form a
complex with RALT. Furthermore, the most distal 214
residues of the EGFR are dispensable for the RALT/
EGFR interaction. Hence, autophosphorylation is not
required for EGFR to recruit RALT and RALT must
bind to the EGFR upstream of position 972.
The requirements of ErbB-1 for RALT binding
recapitulate those described for ErbB-2/RALT complex
formation (Fiorentino et al., 2000) and may in fact
Figure 9 Impact of RALT signalling on mitogenic activity of
defined ErbB dimers. Pinco, RALT and RALT DEBR derivatives
of 32D.E1 (a), 32D.E2.E3 (b) and 32D.E4 (c) transfectants
described in Figure 6 were seeded at 1 10
4
cells/well in IL-3-free
medium and either kept in IL-3-free medium or supplemented with
either EGF (a) or NRG-1b(b,c) to the indicated final
concentrations; control wells were added IL-3. After 44 h, cells
were pulse-labelled for 4 h with 1 mCi/ml [
3
H]methyl-thymidine.
EGF- and NRG-1b-dependent cell proliferation was calculated as
percentage over IL-3-dependent proliferation. A representative
experiment of four independent assays for E1, E2.E3 and E4
transfectants is shown. Each experimental point was determined in
quadruplicate wells, with standard deviation not exceeding 15%
Negative signalling of RALT to the ErbB network
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underlie the entire spectrum of ErbB/RALT interac-
tions. Thus, the interaction of RALT with ErbB-4 is also
dependent on ligand activation of the receptor, while
ErbB-1 and ErbB-2 may bind RALT in a ligand-
independent fashion when constitutively activated.
Conversely, catalytically impaired ErbB-3 cannot bind
RALT directly and is found in a complex with RALT
only if recruited into a heterodimer with ErbB-2.
Recently, in structure–function studies based on the
yeast two-hybrid assay Hackel et al. (2001) found that
RALT is unable to bind to kinase-inactive EGFR.
These authors also suggested that RALT binds to the
sequence comprised between positions 985–995 of
activated EGFR. This conclusion was based on the
observation that EGFR baits spanning positions 646–
995 were able to trans-activate reporter genes in a
RALT-dependent fashion, an activity that was lost in
the 646–985 and 640–940 EGFR baits (Hackel et al.,
2001). In contrast, our coimmunoprecipitation experi-
ments show that EGFR truncation at position 972 still
allows efficient recruitment of RALT (Figure 5c). The
latter result is compatible with yeast two-hybrid experi-
ments that provisionally delimited the RALT-binding
surface of ErbB-2 to the NH2 half of its kinase domain,
that is, well apart from the ErbB-2 COOH tail
(Fiorentino et al., 2000). Clearly, only the precise
mapping of the RALT-binding surface in different ErbB
RTKs will allow to solve these discrepancies. Based on
our data, we propose that RALT binds to the catalytic
domain of kinase-active ErbB RTKs following a ligand-
induced conformational change of the receptor. This
model is consistent with the inability of RALT to be
recruited to catalytically impaired ErbB-3 in the absence
of ErbB-2. Upon triggering of ErbB RTKs, RALT
molecules undergo redistribution from a cytosolic
location to a vesicular compartment, which likely
corresponds to endocytic vesicles. This process shows
an absolute requirement for the integrity of the EBR
module of RALT. Spatial segregation with ligand-
engaged ErbB receptors appears to be directly linked
to some aspects of RALT signalling to the ErbB
network, that is, suppression of activation of ERKs
and AKT (see below for further discussion).
RALT is a negative regulator of mitogenic signals
generated by the ErbB signalling network
In murine fibroblasts, RALT suppresses mitogenic
signals generated by a recombinant EGFR.ErbB-2
chimeric receptor (Fiorentino et al., 2000). Under
physiological conditions, the signalling activity of
ErbB-2 is dictated by its lateral interactions with other
ErbB family members. We report here that RALT
suppresses the mitogenic activity of NRG-1-activated
ErbB-2.ErbB-3 dimers as well as that of EGFR and
ErbB-4 homodimers. RALT suppresses mitogenic sig-
nalling by ErbB-2.ErbB-4 dimers activated by either
EGF or NRG-1 (our unpublished observation) as well
as the transforming activity of overexpressed EGFR in
Rat1 fibroblasts (Hackel et al., 2001). Thus, RALT
affects signalling by all ErbB RTK combinations tested
so far and it does so in several cellular model systems
and different biological assays. These findings, along
with the demonstration that RALT expression is
induced by growth factors capable of activating
different ErbB RTKs (Fiorentino et al., 2000) are
consistent with RALT being a feedback inhibitor of
the entire ErbB signalling network. Comparable levels
of RALT suppressed the mitogenic activity of ErbB-4
much more efficiently than that of E2.E3 dimers. This
was mirrored by remarkable differences in the ability of
RALT to interfere with E4 and E2.E3 signals leading to
ERK and AKT activation (see below for further
discussion). The potency of RALT signalling to EGFR
could not be directly compared to RALT activity
towards E4 and E2.E3, given the lower level of RALT
expression in 32D.E1 cells. We notice, however, that
NRG-driven proliferation of 32D.E4 cells was ablated
by RALT activity even when cells were cultured with
high concentrations of NRG-1. In contrast, saturating
doses of ligand partially alleviated RALT suppression of
E1 and E2.E3 mitogenic signals. Thus, the sensitivity
of EGFR to RALT suppressive activity resembled that
of E2.E3 dimers rather than that of E4.
The differential sensitivity of ErbB RTKs to RALT
signalling may impact on signal specification by the
ErbB network whenever a promiscuous ligand activates
more than one type of ErbB dimers in the same target
cell. For instance, NRG-1-driven RALT expression may
be used to tune signals generated by the ErbB-2.ErbB-3
combination, while aborting the function of concomi-
tantly activated ErbB-4 homodimers.
Role of RALT in determining strength and duration of
AKT and ERK activity
During development, distinct thresholds of RTK-
induced ERK activity are instrumental in translating
morphogenetic gradients into correspondingly graded
transcriptional responses (reviewed in Hazzalin and
Mahadevan, 2002). In proliferating cells, growth factor
stimulation and ensuing RTK activity are required
throughout the G1 phase of the mitotic cell cycle
(reviewed in Jones and Kazlauskas, 2001). Besides
regulating transcriptional responses (Cook et al., 1999;
Kops et al., 2002; Murphy et al., 2002) the Ras-ERK
and PI-3K-AKT pathways couple RTK activity to the
regulation of the cell cycle machinery (reviewed in
Lawlor and Alessi, 2001; Pruitt and Der, 2001). Thus,
intensity and duration of Ras-ERK and PI-3K-AKT
signals concur in defining the biological outcome of
ErbB activity in normal and transformed cells (Zhou
et al., 2001a, b; Neve et al., 2002; Shin et al., 2002;
Viglietto et al., 2002).
Reconstitution of RALT expression in 32D cells leads
to suppression of EGFR, ErbB-4 and ErbB-2.ErbB-3
signals culminating in activation of ERKs and AKT.
However, the RALT-enforced suppression of ERK and
AKT activity displays temporal profiles that are
different in distinct 32D.ErbB transfectants. RALT
expression in 32D.E4 cells led to an abrupt and robust
inhibition of ERK and AKT activity since the inception
Negative signalling of RALT to the ErbB network
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of receptor triggering. In contrast, suppression of ERK
and AKT activation by RALT in E1 and E2.E3 cells
was clearly observed only at later stages of receptor
stimulation. Additionally, the timing of suppression of
ERKs and AKT was further delayed in E2.E3 cells as
compared to EGFR transfectants. Delayed suppression
of ErbB-dependent signals by RALT is not confined to
the 32D cell background: RALT overexpression caused
delayed suppression of ERK activity triggered by an
EGFR/ErbB-2 chimeric receptor expressed in murine
fibroblasts, while leaving serum- and PMA-driven
responses unaffected (Fiorentino et al., 2000).
Our results indicate that RALT activity impacts in a
selective fashion on strength and duration of ERK and
AKT signals generated by E1, E4 and E2.E3 dimers.
While it is not yet clear how RALT discerns among
different ErbB RTKs, these findings suggest that RALT
may protect cells against perturbations induced by
additive or superadditive activation of these pathways.
As an example, the buffering function of RALT is likely
to prevent further boosting of ERK and AKT activity
generated by E2.E3 dimers, should ErbB-4 be concomi-
tantly activated.
Mechanisms of suppression of ErbB RTKs signalling by
RALT
Initial studies modelled RALT as an adaptor that links
cytosolic effectors to activated ErbB receptors, thereby
suppressing receptor-proximal events (Fiorentino et al.,
2000; Hackel et al., 2001). In order to validate this
model, we have compared the biochemical and biologi-
cal activities of RALT to those of RALT DEBR, a
mutant which retains the property to couple to SH3-
containing effectors but is unable to complex with and
be spatially regulated by ErbB RTKs. Our data indicate
that the DEBR mutation deprives RALT of its ability to
suppress ErbB-dependent activation of ERKs and
AKT. We conclude that RALT must be recruited to
ErbB RTKs in order to interfere with these signalling
pathways.
Since RALT inhibits multiple signalling pathways once
relocated to ErbB RTKs, we infer that it acts as a general
suppressor of receptor-proximal events. Indeed, Hackel
et al. (2001) have reported that overexpression of RALT
enhances ligand-dependent downregulation of the EGFR.
Several SH3-containing proteins play essential roles in
receptor endocytosis, including Grb-2, intersectin, synda-
pin and endophilin (Simpson et al., 1999). Thus, RALT
may suppress ErbB RTKs by coupling them to elements
of the endocytic machinery such as Grb-2 (Fiorentino
et al., 2000) and intersectin (our unpublished data). This
model would provide a rationale for the seemingly
paradoxical enhancement of EGFR and ErbB-4 mito-
genic activity observed upon expression of RALT DEBR
in 32D.E1 and 32D.E4 cells (Figure 9). Such a dominant-
negative effect may be explained by RALT DEBR acting
as a cytosolic trap for spatially regulated components of
the endocytic machinery.
RALT DEBR is able to suppress the mitogenic
activity of ErbB-2.ErbB-3 dimers over a wide range of
ligand concentrations and under conditions that gen-
erate normal profiles of ERK and AKT signals. We
propose that the DEBR mutant intercepts, at a receptor-
distal level, signals yet unidentified but necessary to the
mitogenic activity of E2.E3 dimers. It is likely that these
signals are generated in parallel to the ERK and AKT
pathways and eventually converge with the latter to fully
implement the E2.E3 mitogenic program. Intriguingly,
the EBR-independent component of RALT signalling
impacts on the mitogenic activity of E1 and E4 only at
low levels of receptor occupancy. We hypothesize that
high levels of EGFR and ErbB-4 activity relieve the
dependency for signals intercepted by RALT DEBR,
thus allowing the above-described dominant-negative
activity of RALT DEBR to be unmasked.
Are there structural determinants of RALT that could
account for EBR-independent signals? RALT has been
reported to bind to 14-3-3 proteins (Makkinje et al.,
2000), which are implicated in nuclear–cytoplasmic
traffic. We have found that both RALT and RALT
DEBR accumulate in the nuclei of cells treated with
leptomycin B (unpublished data). Intriguingly, a RALT
fragment spanning positions 1–150 behaves as a potent
transcriptional activator when fused to the DNA-
binding domain of GAL-4 (unpublished data). Hence,
dynamic regulation of its nuclear localization may
confer to RALT the ability to regulate gene activity
during G1 progression in an EBR-independent fashion.
Conclusions
RALT appears to be a versatile and selective feedback
inhibitor of signals propagated by ErbB RTKs. Its
expression is controlled at the transcriptional and post-
translational level: such an integrated control provides
for timely adjustments of RALT levels and, by
inference, of its signalling activity (Fiorini et al., 2002).
The RALT feedback loop is therefore suited to proof-
read and extinguish signals generated by ErbB RTKs.
The ability of RALT to integrate negative signalling to
combinatorial ErbB dimers and to affect in a differential
manner strength and duration of signals generated by
distinct ErbB RTKs is likely to be a mechanism through
which the identity of ErbB signals is preserved and
global output from the ErbB network is tightly
controlled.
Materials and methods
Materials
EGF was from Upstate Biotechnology, NRG-1b, bFGF and
PDGF were from R&D, glutathione–agarose, IPTG, puromy-
cin and transferrin were from Sigma. Tissue culture media and
sera were from Life Technologies.
Recombinant DNA methodologies
pcDNA3 vectors for wt EGFR, EGFR KD and EGFR DC
have been described (Levkowitz et al., 1998) and were obtained
Negative signalling of RALT to the ErbB network
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from Y. Yarden. LTR-2-based expression vectors for ErbB-1–
ErbB-4 have been previously described (Di Fiore et al., 1990;
Alimandi et al., 1995, 1997). cDNA fragments encoding
discrete portions of RALT EBR were generated by polymerase
chain reaction (PCR) amplification using synthetic oligonu-
cleotide primers of 22 bases. Amplification products contained
50BamHI and 30EcoRI tags to allow cloning into the pGEX
4 T vector (Pharmacia) and expression in Escherichia coli. The
DEBR mutant was generated by fusing an EcoRI–StuI fragment
of RALT cDNA encompassing aa. 1–313 to a PCR-generated
StuI–XbaI fragment spanning positions 362–459 of RALT.
Sequence analysis confirmed the fidelity of PCR amplification
and that the fusion maintained the correct ORF skipping aa.
315–361. For expression in mammalian cells, RALT DEBR was
cloned in Pinco vector (Fiorentino et al., 2000).
Cell lines, cell culture conditions and gene transfer procedures
NIH-3T3 cells and their derivatives were grown in Dulbecco’s
minimal essential medium (D-MEM) containing 10% (vol/vol)
newborn calf serum (NCS). 293 human embryonal kidney cells
were grown in D-MEM containing 10% fetal calf serum
(FCS). Transfections of 293 cells were performed as described
(Fiorentino et al., 2000). Cells were processed for immuno-
precipitation experiments 36–48 h post-transfection. NIH 3T3,
NIH-EGFR and NIH-EGFR/ErbB-2 cells were infected with
Pinco, Pinco-RALT and Pinco–RALT DEBR retrovirus
stocks to obtain Pinco, RALT and RALT DEBR derivatives.
Flow-cytometry analysis of GFP-positive cells indicated that
infection efficiencies were routinely 495%. 32D cells and their
derivatives were cultured in RPMI medium supplemented with
10% FCS and 5% (vol/vol) conditioned medium from WEHI
cells as source of IL-3. Expression of ectopic RALT and
RALT DEBR in 32D.E1 (Di Fiore et al., 1990), 32D.E4 (Tang
et al., 1998) and 32D.E2.E3 cells (Alimandi et al., 1997) was
achieved by electroporation of Pinco–RALT and Pinco–
RALT DEBR vectors as described (Alimandi et al., 1997).
Transfected cells were selected in 1 mg/ml puromycin. Prolif-
eration assays with 32D cells were performed as described
(Alimandi et al., 1997). Briefly, cells were seeded at
110
4
cells/well in 96 flat bottom well plates in IL-3-free
medium containing either ErbB ligands or 5% WEHI medium.
Control cells were cultivated in IL-3-free medium. After 44 h,
cells were pulsed for 4 h with 1 mCi/ml [
3
H]methyl-thymidine
and TCA-precipitated radioactivity measured in a scintillation
counter. Assays were performed in quadruplicate wells. ErbB-
driven cell proliferation was calculated as percentage over IL-
3-driven proliferation. Mitogenic assays of NIH-EGFR/ErbB-
2 cells were performed as described (Fiorentino et al., 2000).
Briefly, 5 10
3
cells/well were seeded in 24-well plates and
subjected to three cycles of infection with either Pinco or
PincoRALT retrovirus stocks over a 24 h period. Cells were
then switched to serum-free medium containing defined
growth factors. Control wells were added to serum-free
medium only. Cell proliferation was assessed after 48 h by
pulsing cells for 4 h with 1 mCi/ml [
3
H]methyl-thymidine and
counting TCA-precipitated radioactivity in a scintillation
counter.
Immunochemical procedures
For immunoblot analysis, cells were lysed as described
(Fiorentino et al., 2000). The S1 antiserum and the 19C5/4
monoclonal antibody were used at 2 mg/ml. Anti-ERK, anti-p-
ERK, anti-AKT and anti-p-AKT antibodies (New England
Biolabs), anti-P-Tyr MoAb 4G10 (UBI), anti-SHC antiserum
(Transduction Laboratories) and affinity-purified anti-ErbB
receptors antibodies (Santa Cruz Biotechnology) were used at
1mg/ml. GST pull-down assays were performed as described
(Fiorentino et al., 2000). For coimmunoprecipitation assays,
lysates were incubated for 2 h at 41C with MoAb 19C5/4
bound to ImmunoPure resin (Pierce).
Immunofluorescence and confocal microscopy analysis
Cultures of NIH-EGFR and NIH-EGFR/ErbB-2 cells in-
fected with Pinco–RALT and Pinco–RALT DEBR recombi-
nant retroviruses were deprived of serum for 24 h prior to
stimulation with EGF (10 ng/ml) for different lengths of time.
Cells were fixed with 4% (vol/vol) paraformaldehyde in PBS
for 10 min at 201C and permeabilized with 0.25% (vol/vol)
Triton X-100 in PBS for 10 min. Comparable results were
obtained when cells were fixed and permeabilized with a 1 : 1
methanol/acetone solution for 10 min at 201C. Immunostain-
ing was performed with MoAb 19C5/4 (for RALT proteins),
MoAb 108 (for EGFR) and polyclonal antibody M6 (for the
EGFR/ErbB-2 chimera) as described (Fiorentino et al., 2000).
Secondary antibodies (FITC- and TRITC-conjugated donkey
anti-rabbit, anti-goat and anti-mouse antibodies) were from
Jackson Immunoresearch. Samples were routinely examined
with a Zeiss microscope equipped with 40 and 50 water-
immersion objectives. Confocal analysis was carried out with a
Leica NBT system, equipped with 40 1.00–0.5 and 100 1.3–
0.6 oil-immersion lenses.
Acknowledgements
S Anastasi dedicates this work to the loving memory of his
mother. We thank Y Yarden and J Sap for reagents and C Full
for tissue culture work. MF is the recipient of an AIRC
fellowship. This work was supported by grants from: EC (5th
program) and AIRC to OS, AIRC and CNR PF- Biotecno-
logie to SA, AIRC to MA.
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