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J Biol Chem, Vol. 274, Issue 33, 23311-23315, August 13, 1999
From the Department of Oncology, Novartis Pharma A.G.,
CH-4002 Basel, Switzerland
Cell motility has been correlated both with
oncogenic invasiveness and metastatic potential. The development of
selective inhibitors of motility has thus great potential importance.
Grb2 is a SH2/SH3 domain-containing adaptor protein that links growth factor receptor tyrosine kinases to the Ras signaling pathway. We have
developed specific small molecule inhibitors of the Grb2 SH2 domain as
potential leads for drug discovery. Synthesis of the inhibitors and
their effects on growth factor-induced growth in cells have been
reported previously. In the current study, we establish that these
inhibitors inhibit hepatocyte growth factor/scatter factor-induced A431
and Madin-Darby canine kidney cell motility and various cell
motility-related events, including epidermal growth factor-induced
ruffling of A431 cells and epidermal growth factor-induced
translocation of the small GTPase Rac in these cells. We demonstrate
for the first time a direct role for Grb2 in cell motility and indicate
a new avenue for cancer therapeutics.
Acquisition of cell motility is a prerequisite to biological
processes taking place in tissue remodeling. It has been described as a
major event in morphogenesis and in pathological situations such as
invasion and metastasis of tumor cells (1, 2). Most cell motility
factors are also growth factors
(GFs),1 a most intriguing
duality of function that implies the possibility of common or related
signaling transduction pathways (3). Hepatocyte growth factor (HGF),
also known as scatter factor (SF), is a pleiotropic GF that, besides
promoting cell survival and proliferation, has the ability to
dissociate epithelial sheets and to stimulate cell motility (3). HGF/SF
predominantly acts as a paracrine factor. Nearly 90% of human
malignant tumors arise from epithelial tissue, and most carcinoma cells
express the c-Met/HGF receptor and are likely to use HGF/SF produced
from stromal tissue (4, 5). On the other hand, GFs such as the
epidermal growth factor (EGF) can enhance motility in an autocrine
manner in several tumor cell lines (6). The EGF receptor (EGFR) has
also been extensively studied. GF activation of EGFR influences a
number of phenotypic properties in malignant cells in vitro,
including mitogenesis and cell motility stimulation (7). Interestingly,
overexpression of EGFR is correlated more strongly with metastasis and
invasion than with tumor size (7).
Growth/motility factors such as EGF and HGF/SF, initiate a response by
binding to receptors and thereby activating a tyrosine kinase domain
located in the cytoplasmic portion of the receptor (8). This event
leads to the autophosphorylation of the receptors on tyrosine residues.
The presence of phosphotyrosine at specific sites on receptors is
crucial for downstream signaling: within a specific sequence context,
phosphotyrosines act as binding sites for many signal-transducing
molecules within the cells (9, 10). The common element that confers the
specific property of phosphotyrosine-containing sequence recognition to
all these target molecules is the Src homology 2 (SH2) domain (11).
Grb2 is an adaptor protein made of one SH2 domain flanked by two SH3
domains. Grb2 interacts via its SH2 domain with activated receptors and
via its SH3 domains to the nucleotide exchange factor Sos, which thus
becomes activated as a positive regulator of Ras (12). Design of
molecules that block the interaction between the
phosphotyrosine-containing activated receptors and the SH2 domain of
Grb2 should interrupt the Ras signaling pathway and may promise
therapeutic leads for diseases like cancer, in which the Ras signaling
pathway plays a major role (13). Grb2 may have other functions as well.
Micro-injection studies with anti-Grb2 antibodies in rat kidney cells
suggest that Grb2 may play a role in signaling from receptor tyrosine
kinases to the small GTPase Rac, a protein involved in motility-related
events (14).
We have reported the rational drug design of CGP78850, a potent and
selective inhibitor of Grb2 SH2 domain (15). We have already
demonstrated that CGP78850 inhibits Grb2 SH2 domain binding and Ras
activation in live cells and prevents anchorage independence of growth
(16). CGP85793 is a prodrug derivative of CGP78850 with improved cell
penetration (16).
In this study, using both compounds, we explored the possibility that
Grb2 may also play a role in cell motility. Significantly, we
demonstrate that inhibition of Grb2 SH2 domain prevents HGF/SF-induced A431 and MDCK cell motility and cytoskeletal rearrangements essential for this process. We also establish that EGF can no longer activate Rac
in A431 cells.
Reagents and Antibodies--
EGF (BT-201) was from Biomedical
Technologies Inc., and recombinant human HGF/SF (249-HGF), was from
R & D systems. Mouse monoclonal antibody specific for Grb2 (clone
C1.4) was from Neomarkers. Rabbit polyclonal antibodies specific for
Rac1 (C-14) were from Santa Cruz. Monoclonal anti pan-Ras antibody
(Pan-Ras Ab-3, OP40) was from Oncogene. GST-RBD and purified
recombinant c-Met, as GST-Met fusion protein from baculoviruses, were
kindly provided by J. Downwards and D. Stover, respectively. Drugs were
dissolved in Me2SO as stock solution and diluted 1:1000 in
culture medium right before use. For all experiments, appropriate
controls were run to test the effects of the solvent in which stock
drug solutions were prepared.
Expression and Purification of Proteins--
For the SH2 ELISA,
the Grb2 SH2 domain (residues 55-152) was expressed and purified as
described previously (17). GST-RBD encodes amino acids 1-149 of
c-Raf-1 fused to GST. GST-RBD fusion proteins were expressed in
Escherichia coli BL-21 in pGEX-KG expression vector
(Amersham Pharmacia Biotech) as already described (16).
SH2 ELISA--
Binding of autophosphorylated GST-Met to purified
Grb2 SH2 was quantitated by ELISA. Grb2 SH2 stock solution was diluted
to 2 µg/ml in 50 mM Tris, pH 7.5, and a 100-µl aliquot
was placed in each well of Nunc Maxisorp plates. After 2 h at room
temperature, the plates were washed with Tris-buffered saline, 0.1%
Tween. The wells were then filled with 300 µl of blocking buffer
(blocking reagent for ELISA; Roche Molecular Biochemicals) and
incubated for 30 min at room temperature. The plates were washed with
Tris-buffered saline, 0.1% Tween, wrapped in aluminum foil, and stored
at Assay for Detection of Activated Ras in Cells--
MDCK cells
were serum-starved for 20 h in Dulbecco's modified Eagle's
medium and 0.1% bovine serum albumin. Compounds were added for 2 h at 37 °C followed by treatment with HGF/SF (100 pM)
for 15 min. MDCK cells were then washed with ice-cold
phosphate-buffered saline and lysed in a solution containing 50 mM Tris, pH 7.5, 100 mM NaCl, 1% Triton X-100,
1 mM EDTA, 20 mM NaF, 1 mM
Na3VO4, 1 mM phenylmethylsulfonyl
fluoride, 80 µg ml Cell Motility Assay--
For the stimulatory effect on cell
motility, cells were seeded at a density of 2 × 104
cells/well for MDCK cells and 4 × 104 cells/well for
A431 cells on 6-well plates (Corning). After the cells had become
attached to the plate, 50 pM HGF/SF were added, and the
cultures were maintained for 24 h. The cells were stained with
0.2% crystal violet in 20% ethanol, photographed, and counted using
an Imaging System from Zeiss (KS-400; Jena, Germany). The substances
being tested were added to the cell culture 90 min before stimulation.
All experiments were performed in duplicate and repeated three times.
Migration was quantified by measuring cell density and counting
isolated cells in 20 randomly selected fields. Mean and S.E. were
calculated, and the proportion of isolated cells was determined.
Inhibition of Grb2 SH2 Domain/c-Met Association by CGP78850 in
Vitro--
Phosphoprotein recognition by SH2 domain-containing
proteins is thought to derive its specificity from phosphorylation
per se and from the surrounding amino acid sequence (9). The
recognition motif for Grb2 is pYXN (17). Proteins that
interact with Grb2 contain a pYXN motif, including EGFR,
SHC, and c-Met (18, 19). CGP78850 was designed by molecular modeling to
contain essential elements of this Grb2 SH2 recognition motif, along
with other features that enhance Grb2 binding affinity (15). As such
CGP78850 has already been shown to be a potent inhibitor of Grb2/EGFR
and GRB2/SHC interactions in in vitro assays, in cell
extracts, and in live cells (16). We measured here the ability of
CGP78850 to also inhibit the interaction between the activated c-Met,
expressed as a GST-Met fusion protein, with an immobilized Grb2 SH2
domain. GST-Met bound with high affinity to purified Grb2 SH2 in a
phosphorylation- and concentration-dependent manner (data
not shown). As control, experiments using an EGFR ELISA assay were run
in parallel. The linear EGFR1068 peptide Ac-EpYINQ-NH2 was
employed as a reference. Table I shows
that CGP78850 is also a potent inhibitor of GST-Met/Grb2 SH2
interaction.
CGP78850 Inhibits Ras Activity in MDCK Cells--
The motogenic
effects induced by HGF/SF in MDCK cells are initiated by its
interaction with its cell surface receptor, the c-Met protein, which
results in activation of Ras (19-21). Molecular analysis of the
signaling pathways involved in the motogenic activity induced by HGF
showed that Ras activation was essential (22) but that binding of Grb2
to the phosphorylated receptor was dispensable (23). Because SHC is
associated with signaling by the Met protein, activation of Ras could
be accomplished by the SHC-Grb2-Sos complex (20). We tested whether
CGP78850, which is capable of inhibiting both Grb2-Met and Grb2-SHC
interactions, could inhibit Ras activation in cultured MDCK cells. We
used an assay that exploits the known specificity of the interaction
between Ras-GTP and the RBD of Raf-1 in order to detect activated Ras
in HGF/SF-stimulated MDCK cells (24). In this experiment, activated Ras
was inhibited by pretreatment of cells with CGP78850 at 100 µM (Fig. 1). The data imply
that Ras activity is dependent on Grb2 in MDCK cells stimulated with
HGF/SF and that the SHC-Grb2-Sos pathway may be the dominant one
coupling activated Met to Ras.
Grb2-SH2 Inhibitors Inhibit HGF/SF-stimulated Colony
Scattering--
We next examined whether inhibition of Grb2 function
would directly inhibit the motility of various species of cells. HGF/SF has a marked stimulatory activity on motility of MDCK cells but not on
their growth (5). In addition to high expression of EGFR, A431 cells
also express the c-Met receptor (5). Assays were set up by adding
varying concentrations of CGP78850 or its prodrug derivative CGP85793
(data not shown) to established islands of MDCK or A431 cells 90 min
before stimulation with HGF/SF, with subsequent examination for
scattering 24 h later. Fig. 2 shows a typical assay. CGP78850 completely prevented scattering of A431 and
MDCK cells at 100 µM. Cells remained in close apposition
with no separation of cells at the edge of the colonies and were not distinguishable from cells in the unstimulated controls. At 10 µM of CGP78850, there was a higher proportion of smaller
islands and free cells (Fig. 2). At 1 µM, CGP78850 seemed
to have no effect on HGF/SF-stimulated cells, and cell sheets were
dispersed into single cells or small clusters of cells as in the
stimulated controls (Fig. 2). MDCK cells showed the most pronounced
change after addition of HGF/SF, and these cells were chosen as model
target cells for quantification of the effect of CGP78850 on cell
motility. Fig. 3 shows the changes in
terms of cell density in MDCK cells. As expected cell density was high
in the unstimulated controls and dropped after addition of HGF/SF. Fig.
3 also gives the percentage of isolated cells separated from the
colonies. This was uniformly low throughout in the unstimulated
controls, but the numbers of isolated cells rose sharply after addition
of HGF/SF with the sudden disintegration of the cell sheets. However,
in the presence of CGP78850, there was no significant difference in the
number of individual cells or in cell density between unstimulated
control cells and sequential treatment with 100 µM
CGP78850 followed by HGF/SF. At 10 µM of compound, cell
density was the same as in the unstimulated control cells, but the
number of isolated cells rose. At 1 µM of CGP78850, there
was no significant difference in the number of individual cells or in
cell density compared with the stimulated control cells (Fig. 3).
Requirement of Grb2 for EGF-induced Membrane Ruffling and Rac
Activation--
EGF and HGF/SF have been shown to cause the formation
of membrane ruffles (21, 25). Membrane ruffling formation is due to
extensive actin polymerization and is one of the cytoskeletal changes
associated with cell motility. To see whether inhibition of Grb2 SH2
domain was accompanied by changes in membrane ruffling formation, the
subcellular localization of Grb2 and actin was determined in A431 cells
by double immunofluorescence using Grb2 antibody and phalloidin,
respectively (Fig. 4). In serum-deprived A431 cells, Grb2 labeling was found mostly associated with the plasma
membrane in areas of cell-cell contacts, and F-actin was localized
around the cell periphery. Upon treatment with EGF for 2 min,
spectacular bursts of ruffling activity occurred as seen by actin
staining, and Grb2 was confined to the membrane ruffles. In cells
pretreated with CGP85793, addition of EGF did not lead to membrane
ruffle formation, and Grb2 localization was the same as in
serum-deprived cells (Fig. 4). The inability of EGF to induce membrane
ruffles in A431 cells treated with CGP85793 suggests that EGFR
signaling to the actin cytoskeleton is compromised in these cells.
As previously demonstrated, the reorganization of actin fiber assembly
is under the control of the Rac and Rho proteins, the former being
indispensable for the formation of ruffles and the latter for the
formation of stress fibers (26). In mouse dermal fibroblasts Rac has
been shown to be recruited from the plasma membrane upon GF stimulation
concomitant with its activation (27). In serum-deprived A431 cells Rac
as detected by immunofluorescence was found only in the cytosol (Fig.
5). Upon EGF stimulation, Rac colocalized
with Grb2 in the membrane ruffles but not in cells treated with
CGP85793 where Rac remained cytosolic (Fig. 5), indicating that EGF
stimulation does not activate Rac in cells treated with Grb2 SH2
inhibitors. The results of the present experiments show that Grb2
functions in Rac-related pathways downstream from EGFR and suggest that
Grb2 controls EGF-induced membrane ruffle formation through Rac.
A parameter often associated with epithelial invasiveness is
motility. Cell motility requires several distinct steps that must occur
in a coordinated fashion for cellular translocation to occur. Following
the establishment of adhesion to the underlying substratum, the cell
must be able to form protrusions, establishing new adhesions, and be
able to break older adhesions for translocation to occur (3). The fact
that so many GFs can, at least in some cells, stimulate movement as
well as growth suggests that there may be some link in the early stage
of signal transduction. Grb2 is a SH2/SH3 domain-containing adaptor
protein that links receptor tyrosine kinases to the Ras signaling
pathway and mitogenesis (18). Earlier studies have shown that the
injection of an antibody against Grb2 into normal rat kidney-derived
fibroblasts had an effect on the cytoskeletal rearrangements induced by
EGF and platelet-derived growth factor (14), suggesting the possibility
that Grb2 functions in pathways involving cell motility.
Scattering of MDCK and A431 cells is indeed prevented by CGP78850 or
its prodrug derivative CGP85793. Because regulated changes in actin
microfilament organization underlie cell motility, we have also
monitored early changes in the cytoskeleton of A431 cells. Membrane
ruffles and lamellipodia are found at the leading edges of motile cells
and are believed to play a fundamental role in migration (28, 29).
Membrane ruffling formation is due to extensive actin polymerization.
In this respect it is of interest to mention that EGF has been
demonstrated as causing actin polymerization (30). The strong
colocalization of Grb2 to membrane ruffles and the observation that
Grb2 SH2 inhibitors prevented membrane ruffle formation support a role
for Grb2 in cell motility and provide evidence that Grb2 is essential
for tyrosine kinase signaling to the cytoskeleton.
The GTPase that appears most likely to control cell surface protrusive
activity associated with translational locomotion is Rac (21).
Overexpression of constitutively active Rac promotes the formation of
ruffling lamellae in fibroblasts (26). In quiescent A431 cells, we
observed Rac in the cytosol, correlating with the inactive state and
its translocation to the cell periphery to regions of ruffling activity
upon stimulation. CGP85793 completely blocked this translocation,
indicating that Rac translocation is dependent on Grb2 binding to
phosphoproteins. However, the different localization of Grb2 (mostly
membranous) and Rac (cytosolic) before stimulation suggests that Grb2
effect on Rac translocation is indirect and may occur through Ras.
Moreover, our demonstration that Rac and Grb2 are recruited and
colocalize upon stimulation to the sites of ruffling activity suggests
that Grb2 may be involved in the two aspects of GF regulation of Rac
activity, i.e. not only in the recruitment of Rac from the
cytosol to the membrane but also, as for Ras, in its guanine nucleotide
exchange activity, possibly, as recently proposed, directly through Sos
Dbl homology domain (31).
We establish for the first time that Grb2 is involved directly in the
regulation of cell motility. The present study also demonstrates that,
at least in A431 cells, Grb2 plays a role in cell motility-related
phenomena, including Rac regulation and cytoskeletal rearrangements
downstream of receptor tyrosine kinases. Our Grb2 SH2 inhibitors are
now prototypes for a new class of therapeutic agents that may control
the ability of tumor cells to move and traverse interstitial barriers.
*
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement" in
accordance with 18 U.S.C. Section
1734 solely to indicate this fact.
The abbreviations used are:
GF, growth factor;
EGF, epidermal growth factor;
HGF/SF, hepatocyte growth factor/scatter
factor;
GST, glutathione S-transferase;
SH2, Src homology 2;
EGFR, EGF receptor;
RBD, Ras-binding domain;
MDCK, Madin-Darby canine
kidney;
ELISA, enzyme-linked immunosorbent assay.
Effect of Potent and Selective Inhibitors of the Grb2 SH2
Domain on Cell Motility*
,
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ABSTRACT
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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EXPERIMENTAL PROCEDURES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
20 °C for up to 3 weeks. GST-Met was diluted to 25 ng/ml in
dilution buffer (50 mM Tris, pH 7.5, 0.1% bovine serum
albumin, 0.1% Tween) and 50 µl were added with control buffer or the
substance to be tested (50 µl) in triplicate to wells of GRB2 SH2
coated plates. The plates were incubated for 2 h at room
temperature with gentle agitation, then rinsed with Tris-buffered
saline, 0.1% Tween. The presence of bound GST-Met was detected with
rabbit polyclonal anti-GST antibody diluted 1:30 000 in 50 mM Tris pH 7.5, 3% bovine serum albumin, 0.1% Tween.
After 1 h of incubation at room temperature, bound anti-GST
antibody was detected with horseradish peroxidase goat anti-rabbit IgG
diluted 1:5000 in dilution buffer, and the plates were washed as
described above. Peroxidase activity was monitored at 655 nm on a plate
reader. The EGFR ELISA assay has already been described (28). The
results are expressed as the concentration at which half-maximal
competition was observed (IC50, µM). The
errors quoted correspond to the standard error in the fits of the data.
1 aprotinin, and 50 µg
ml
1 leupeptin, 12 mM MgCl2. For
affinity precipitation lysates were incubated with GST-RBD prebound to
glutathione-Sepharose for 30 min at 4 °C with rocking. Bound
proteins were eluted with SDS-polyacrylamide gel electrophoresis sample
buffer, resolved on 12.5% acrylamide gels, and subjected to Western
blotting. Blots were probed with anti pan-Ras antibodies. Proteins were
detected using peroxidase-conjugated anti-mouse antibodies and
visualized by ECL.
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RESULTS
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ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
Relative affinities of compound/phosphopeptide for Grb2 SH2 domain

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Fig. 1.
Serum-starved MDCK cells were treated without
(lane 2) or with 100 pM HGF/SF
(lanes 3-6) for 10 min before cell lysis.
CGP78850 was added 2 h before stimulation (lanes 4-6).
Lysates were subjected to affinity precipitation with GST-RBD. Ras
proteins were detected by immunoblotting with monoclonal anti-Ras
antibodies. Lane 1 contains whole cell lysate
(WCL).

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Fig. 2.
Regulation of HGF/SF-induced scattering by
CGP78850. A-F, A431 cells were plated at a density of
4 × 104 cells/well on 6-well plates and cultured for
24 h in the absence (A and B; B,
Me2SO control) or presence (C-F) of 50 pM HGF/SF. CGP78850 was added at different concentrations
2 h before stimulation (D-F; D, 100 µM; E, 10 µM; F, 1 µM). G-J, regulation of HGF/SF induced
scattering by CGP78850. MDCK cells were plated at a density of 2 × 104 cells/well on 6-well plates and cultured for 24 h in the absence (G) or presence (H-J) of 50 pM HGF/SF. CGP78850 was added at different concentrations
2 h before stimulation (I and J;
I, 10 µM; J, 1 µM).

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Fig. 3.
Effect of CGP78850 on cell density and
proportion of cells separated from neighbors. A-F,
MDCK cells were cultured for 24 h in the absence (A and
B; B, Me2SO control) or presence
(C-F) of 50 pM HGF/SF. CGP78850 was added at
different concentrations 2 h before stimulation (D-F;
D, 100 µM; E, 10 µM;
F, 1 µM).

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Fig. 4.
Inhibition of Grb2 SH2 domain prevents
EGF-induced membrane ruffling in A431 cells. Fluorescence
micrographs of serum-starved A431 cells (A and B)
and serum-starved cells stimulated with 50 ng/ml of EGF for 2 min
(C-F) are shown. CGP85793 was added at 100 µM
2 h before stimulation (E-F). Actin filaments were
localized by staining with fluorescently-tagged phalloidin
(A, C, and E). Grb2 (B,
D, and F) was visualized by indirect
immunofluorescence.

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Fig. 5.
Distribution of Grb2 and Rac in
EGF-stimulated A431 cells treated with CGP85793. Fluorescence
micrographs of serum-starved A431 cells (A-C) and
serum-starved cells stimulated with 50 ng/ml EGF for 2 min
(D-I) are shown. CGP85793 was added at 100 µM
2 h before stimulation (G-I). Grb2 (A,
D, and G) and Rac (B, E,
and H) were visualized by indirect immunofluorescence.
Nuclei are also shown (C, F, and
I).
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DISCUSSION
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
REFERENCES
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FOOTNOTES
To whom correspondence should be addressed: Dept. of Oncology,
K-136-4-26, Novartis Pharma AG, CH-4002 Basel, Switzerland. E-mail:
brigitgay@aol.com.
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ABBREVIATIONS
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REFERENCES
TOP
ABSTRACT
INTRODUCTION
EXPERIMENTAL PROCEDURES
RESULTS
DISCUSSION
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