* Department of Pharmacology, Department of Cell Biology, § Department of Pediatrics,
Department of Anesthesiology,
School of Medicine, and ** Department of Periodontics, School of Dentistry, University of North Carolina, Chapel Hill,
North Carolina 27599
Integrin-mediated cell adhesion causes activation of MAP kinases and increased tyrosine phosphorylation of focal adhesion kinase (FAK). Autophosphorylation of FAK leads to the binding of SH2-domain
proteins including Src-family kinases and the Grb2-Sos complex. Since Grb2-Sos is a key regulator of the Ras
signal transduction pathway, one plausible hypothesis
has been that integrin-mediated tyrosine phosphorylation of FAK leads to activation of the Ras cascade and
ultimately to mitogen activated protein (MAP) kinase activation. Thus, in this scenario FAK would serve as
an upstream regulator of MAP kinase activity. However, in this report we present several lines of evidence
showing that integrin-mediated MAP kinase activity in
fibroblasts is independent of FAK. First, a 1 integrin subunit deletion mutant affecting the putative FAK
binding site supports activation of MAP kinase in adhering fibroblasts but not tyrosine phosphorylation of
FAK. Second, fibroblast adhesion to bacterially expressed fragments of fibronectin demonstrates that robust activation of MAP kinase can precede tyrosine
phosphorylation of FAK. Finally, we have used FRNK,
the noncatalytic COOH-terminal domain of FAK, as a
dominant negative inhibitor of FAK autophosphorylation and of tyrosine phosphorylation of focal contacts.
Using retroviral infection, we demonstrate that levels
of FRNK expression sufficient to completely block
FAK tyrosine phosphorylation were without effect on
integrin-mediated activation of MAP kinase. These results strongly suggest that integrin-mediated activation
of MAP kinase is independent of FAK and indicate the
probable existence of at least two distinct integrin signaling pathways in fibroblasts.
The integrins are a major class of receptors used by
cells to interact with other cells and with the extracellular matrix (Ruoslahti, 1991 Recent evidence has indicated that integrins act not only
as simple mediators of cell adhesion but can also transduce
biochemical signals across the cell membrane (Juliano and
Haskill, 1993 The observation that integrin receptor clustering leads
to autophosphorylation of a tyrosine kinase has suggested
that integrin signaling may resemble the signal transduction pathway initiated by peptide mitogens. In that case,
mitogen binding triggers dimerization and autophosphorylation of a receptor tyrosine kinase, binding of adaptor
proteins such as Grb2 or Shc, recruitment of guanine nucleotide exchange proteins such as Sos, and subsequent
activation of the Ras/Raf/MAP kinase kinase (MEK)/MAP
kinase signal transduction cascade (Egan and Weinberg,
1993 We have now explored possible relationships between
integrin-mediated activation of FAK and of MAP kinase.
Our data indicate that integrin-mediated activation of these
two kinases represents distinct sets of events, and that
MAP kinase activation is not dependent on FAK activation. This conclusion is based on several different types of
observations. First, in studies with cells transfected with
wild-type or mutated integrin Materials
Chicken Preparation of Ligand-coated Dishes or Flasks
Anti-mouse IgG-precoated MicroCellector flasks (Applied Immune Sciences, Inc., Santa Clara, CA) were incubated with the 20 µg/ml anti-
chicken Cell Culture, Cell Adherence, and Preparation of
Cell Lysate
NIH 3T3 cells were maintained in DMEM containing 10% bovine calf
serum, 50 U/ml penicillin, and 50 µg/ml streptomycin. Confluent cells
were serum starved for 4 h before detachment with 0.05% trypsin and
0.33 mM EDTA; trypsin activity was neutralized by soybean trypsin inhibitor (1 mg/ml). Cells were suspended in DMEM with 2% BSA and incubated nonadherently at 37°C for 45 min in a rotator to allow kinases to
become quiescent. Cells were then plated onto dishes coated with fibronectin (Fn), Fn fragments, or antibodies and incubated at 37°C for the indicated times. After incubations, cells were washed twice with cold PBS
and then lysed in a modified RIPA buffer containing 50 mM Tris, pH 7.5, 1% NP-40, 0.1% sodium deoxycholate, 150 mM NaCl, 50 mM NaF, 1 mM
sodium pyrophosphate, 1 mM sodium vanadate, 1 mM nitrophenolphosphate, 5 mM benzamidine, 0.2 µM calyculin A, 2 mM PMSF, and 10 µg/ml aprotinin. Total cell lysates were cleared by centrifugation at 16,000 g for 5 min at 4°C. Protein concentration in the lysates was determined using the
bicinchonic acid assay (Pierce, Rockford, IL).
Plasmid Transfections
Lipofectamine (GIBCO BRL, Gaithersberg, MD) was used for transfection of NIH 3T3 cells according to the manufacturer's instructions. NIH
3T3 cells were transfected with vectors expressing the wild-type chicken
Retroviral Infections
Chicken embryo (CE) cells were prepared as described (Reynolds et al.,
1989 Flow Cytometry
Cells (1 × 106) were detached by trypsin/EDTA and pretreated with normal goat IgG (2 mg/ml) for 15 min on ice. Cells were then incubated with
primary antibodies (2 µg/ml) for 30 min on ice, washed, and treated with
goat anti-mouse or anti-rat IgG coupled to fluorescein (20 µg/ml; Cappel
Laboratories, Malvern, PA) for 30 min on ice. After three washes, cells
were resuspended in PBS and analyzed for fluorescence using a Becton
Dickinson (Bedford, MA) flow cytometer. Background staining was assessed by using an irrelevant monoclonal antibody (anti-macrophage NO
synthetase, clone 6; Transduction Laboratories) as the primary antibody.
Immunofluorescence Microscopy
Cells were fixed in 3.7% formaldehyde in Dulbecco's PBS for 10 min,
rinsed in 150 mM NaCl, 0.1% NaN3, 50 mM Tris HCl, pH 7.6 (TBS), and
permeabilized for 5 min in TBS containing 0.5% Triton X-100 before
staining. Coverslips were then incubated for 30 min with ICN's (Irvine,
CA) PY20 anti-phosphotyrosine antibody and anti-FAK polyclonal antiserum 5158 in TBS. The coverslips were rinsed extensively in TBS and
then stained with combined fluorophore-conjugated, affinity-purified
donkey anti-mouse and anti-rabbit IgG antibodies in TBS for 30 min at
37°C. After the antibody incubations, the coverslips were washed in TBS,
rinsed in deionized water, and mounted in gelvatol or mowiol. Coverslips
were viewed on a Zeiss (Thornwood, NY) Axiophot microscope equipped
for epifluorescence. Fluorescence micrographs were taken on T-max 100 film (Kodak Co., Rochester, NY).
Immunoprecipitation, Western Blot, and Immune
Complex Kinase Assays
Cell lysates were first incubated with antibody recognizing FAK, MAP
kinase, or EE-tagged MEK for 2 h at 4°C, followed by the addition of protein G-Sepharose, and then incubated for an additional 2 h at 4°C. For
Western analysis, the precipitates were washed three times with cold RIPA
buffer, and boiled with SDS-PAGE sample buffer to dissociate the proteins. For immune complex kinase assays, the precipitates were washed three
times with cold washing buffer (0.25 M Tris, pH 7.5, 0.1 M NaCl). The immunocomplexes were resuspended in 40 µl of kinase assay buffer containing
10 mM Tris, pH 7.5, 10 mM MgCl2, 1 mM DTT, 10 µM ATP, 5 µCi
[32P]ATP, and 2 µg of kinase-dead MAP kinase (K Total cell lysates or immunoprecipitated proteins were separated by
SDS-PAGE under reducing conditions. The proteins were transferred
eletrophoretically onto polyvinylidene fluoride membranes (Immobilon P;
Millipore Corp.). The membranes were blocked with 1% BSA and 0.1%
Tween 20 in PBS for 1 h and subsequently incubated with primary antibody (1 µg/ml) in PBS containing 1% BSA and 0.1% Tween 20 for 1 h.
The membranes were washed briefly and incubated with goat anti-mouse
or anti-rabbit IgG peroxidase conjugates for 1 h. Immunoreactivity was
detected on Hyperfilm using enhanced chemiluminescence (Amersham
Corp., Arlington Heights, IL). Bands from Western blots were quantitated using a GS-670 model densitometer (BioRad Laboratories, Richmond, CA).
We had previously noticed that the kinetics of MAP kinase activation and of tyrosine phosphorylation of FAK
were different following fibroblast adhesion to fibronectin.
MAP kinase activation seemed to occur somewhat more
rapidly than FAK activation, suggesting that FAK might
not be responsible for MAP kinase activation. However, early results did not permit firm conclusions to be drawn.
Since fibronectin-coated substrata rapidly induced both cell
adhesion and extensive cell spreading, we decided it might
be informative to examine FAK and MAP kinase under
conditions where integrin-mediated adhesive interactions
could be partially uncoupled from cell spreading and cytoskeletal organization.
Cell Adhesion Via a Mutated In one set of studies we used NIH 3T3 cells that had been
transfected with constructs expressing wild-type (WT) or
mutated versions of the chicken
The cells expressing WT
Cell Adhesion to Recombinant Fibronectin
Fragments Strongly Activates MAP Kinase but Only
Weakly Activates FAK
Recombinant fragments of fibronectin and of other ECM
proteins such as tenascin have proven to be useful in understanding the structural and biological aspects of cell
matrix interactions (Aukhil et al., 1993
When cells attaching on fibronectin fragments were analyzed for MAP kinase activity and FAK tyrosine phosphorylation, some very striking results were observed (Fig. 6).
All of the fragments caused a rapid and robust activation
of MAP kinase, measured either by a mobility shift assay
(Fig. 6 A) or by an in vitro kinase assay (Fig. 6 B). We
could not discern any quantitiative differences in MAP kinase activation for cells adhering to intact fibronectin or to
the various fragments. By contrast, although adhesion and spreading on intact fibronectin resulted in a strong increase in tyrosine phosphorylation of FAK, only modest
increases were observed in cells adhering to the
To confirm that the activation of MAP kinase observed
in cells adhering to fibronectin was primarily an integrinmediated event, we compared cells attaching to substrata
coated with intact fibronectin or the It is possible that when cells adhere to the fibronectin
fragments FAK might be specifically phosphorylated at
Y925, the Grb2 binding site (Schlaepfer et al., 1994 Activation of MAP Kinase Can Precede Activation
of FAK
We noticed that prolonged incubation of 3T3 cells on the
fibronectin fragments gradually led to increased cell spreading. We used this to examine the detailed kinetics of MAK
activation and FAK tyrosine phosphorylation in cells as
they spread. As seen in Fig. 7, A and B, the MAP kinase
kinetic profiles were essentially identical in cells attaching
to intact fibronectin or to the
Expression of FRNK, the COOH-terminal
Domain of FAK, by Retroviral Infection Blocks
Adhesion-dependent FAK Tyrosine Phosphorylation
but Not MAP Kinase Activation
To further explore the relationship between FAK and
MAP kinase we made use of constructs that could express
the COOH-terminal domain of FAK, FRNK. The FRNK
region of FAK contains the site responsible for targeting
intact FAK to focal contacts (Hildebrand et al., 1993
Transient Transfection with FRNK Does Not Block
Activation of an Epitope-tagged MEK
To extend our observations, the effect of FRNK expression was analyzed in NIH 3T3 cells under transient transfection conditions. Cell populations transfected with FRNK
alone and plated on fibronectin were analyzed by doublelabel epifluorescence microscopy. The FRNK-expressing cells were identified by very bright and prominent focal
adhesion staining using antibody 5158 (Fig. 9 A). Nontransfected cells demonstrated numerous lamellae and
filopodia with focal adhesions that stained brightly for
phosphotyrosine. However, FRNK transfectants exhibited
greatly diminished phosphotyrosine content in focal adhesions as well as an altered morphology. Cells were cotransfected with EE-MEK and increasing amounts of FRNK or
with empty vector. FRNK expression was detected by Western blotting of cell lysates (Fig. 9 B) as a 41-43-kD doublet
in agreement with previously published observations (Richardson and Parsons, 1996
The fact that integrin-mediated cell adhesion is clearly involved in anchorage-dependent control of the cell cycle
(Fang et al., 1996 Another set of events that has attracted considerable attention lately relates to integrin-mediated activation of MAP
kinase and Jun kinase (Chen et al., 1994 The fact that integrin-mediated adhesion (or clustering)
can activate both the FAK tyrosine kinase and MAP kinase, raises the possibility that FAK might be "upstream"
of MAP kinase, and that Grb2/Sos and Ras might be intermediates in the integrin signaling pathway (Schlaepfer et
al., 1994 It is apparent that the control of cell growth involves a
complex interplay between signaling pathways triggered
by mitogens and integrin-mediated events including assembly of focal contacts and the actin cytoskeleton (Plopper et al., 1995 As a further step in that direction, we have carefully examined the relationship between two important integrin-
dependent events, namely MAP kinase activation, which
may play a role in mitogenesis, and FAK tyrosine phosphorylation, which is clearly involved in cytoskeletal organization. Some degree of cytoskeletal assembly is necessary
for both FAK tyrosine phosphorylation and MAP kinase
activation. Thus, disruption of focal structures and actin filaments with cytochalasin D competely prevents integrinmediated activation of both FAK and MAP kinase (Bockholt and Burridge, 1993; Hynes, 1992
). Integrins are comprised of noncovalently linked
and
chains
that can associate in various combinations and thus determine the ligand-binding specificities of the intact heterodimer (Hynes, 1992
; Loftus et al., 1994
). A number of ECM proteins including fibronectin, collagens, laminins, and vitronectin are ligands for various integrins. The binding of
integrins to insoluble arrays of these proteins serves to
anchor cells at specialized sites of cell-matrix adhesion
termed focal contacts (Burridge et al., 1988
). Interactions
between integrins and the extracellular matrix help regulate a variety of fundamental biological processes including cell growth (Clarke et al., 1995
; Meredith et al., 1995
; Fang et al., 1996
; Zhu et al., 1996
), cell death (Frisch and
Francis, 1994
; Ruoslahti and Reed, 1994
; O'Brien et al.,
1996
), differentiation (Yurochko et al., 1992
; Huhtala et al.,
1995
; Riikonen et al., 1995
; Streuli et al., 1995
), cell morphology and motility (Chan et al., 1992
; Bauer et al., 1993
),
and tumor growth and metastasis (Chan et al., 1991
; Agrez
et al., 1994
; Brooks et al., 1994
; Varner et al., 1995
).
; Clark and Brugge, 1995
; Rosales et al., 1995
).
Attachment of integrins to their ligands leads to a profound increase in tyrosine phosphorylation of several cellular proteins, including most prominently a molecule termed focal adhesion kinase, pp125FAK (FAK)1 (Hanks
et al., 1992
; Schaller et al., 1992
). FAK is a cytoplasmic protein tyrosine kinase that is tyrosine phosphorylated
and colocalized at focal adhesion sites upon engagement
and clustering of integrins (Burridge et al., 1992
; Kornberg
et al., 1992
; Romer et al., 1994
). FAK can also be tyrosine
phosphorylated in response to a variety of other stimuli
(Zachary et al., 1993
; Seckl et al., 1995
). The precise mechanism that links integrin clustering to FAK activation remains to be determined; however it is clear that integrinmediated adhesion induces autophosphorylation of FAK
predominantly at Y397 (Schaller et al., 1994
, 1995). In addition to clustered integrins and FAK, focal contacts contain
a number of specialized cytoplasmic proteins, including
talin, vinculin,
-actinin, paxillin, and others, that help to
bridge between integrins and actin filaments of the cytoskeleton (Burridge et al., 1988
; Parsons, 1996
). Focal contacts
are also enriched in a variety of signal transducing molecules including Src-family kinases, guanine nucleotide exchange factors, Ras-family proteins, and mitogen activated
protein (MAP) kinases, as can be observed by immunostaining for these proteins (Miyamoto et al., 1995
; Plopper
et al., 1995
). This suggests that focal contacts, and possibly
other sites of integrin-mediated adhesive interactions, may
be important parts of the cellular signal transduction machinery. Tyrosine phosphorylation during integrin-mediated adhesion seems to be prerequisite for focal adhesion
formation and actin stress fiber assembly (Burridge et al.,
1992
; Romer et al., 1994
). Recent data indicate, however, that the regulation of cytoskeletal organization and focal
adhesion formation is independent of FAK-mediated tyrosine phosphorylation in focal adhesions (Wilson et al.,
1995
; Gilmore and Romer, 1996
). Instead, FAK may be
implicated in adhesion-dependent responses to growth
factors (Gilmore and Romer, 1996
).
; Khosravi-Far and Der, 1994
). In a somewhat parallel
set of events, following FAK autophosphorylation, additional tyrosines are phosphorylated through the action of Src-family kinases that bind to FAK at Y397 via their SH2
domains (Schaller et al., 1994
; Calab et al., 1995
). This leads
to the binding of other SH2 domain proteins such as PI-3kinase (Bachelot et al., 1996
) and the Grb2/Sos complex
(Schlaepfer et al., 1994
). The binding of Grb2/Sos at Y925 of
FAK raises the possibility that FAK activation may be
able to trigger the Ras signal transduction cascade
(Schlaepfer et al., 1994
). Further, several groups have shown
that MAP kinases are activated in response to integrinmediated adhesion (Chen et al., 1994
; Schlaepfer et al.,
1994
; Zhu and Assoian, 1995
), and we have recently shown
that MEK and Raf-1 are also activated in this way (Chen
et al., 1996
). Other workers have suggested a role for Ras
in integrin signaling (Kapron-Bras et al., 1993
; Clark and
Hynes, 1996
). All of the data above suggest a link between integrins, FAK, and the consensus Ras/MAP kinase pathway. However, when we directly tested the relationship of
integrin-mediated MAP kinase activation to the Ras pathway, using transfection with dominant negative constructs
for that pathway, we found no evidence for Ras involvement
(Chen et al., 1996
). Thus, we find at least one notable difference in the integrin- and mitogen-mediated pathways leading to MAP kinase activation, with a lack of Ras involvement in the integrin-mediated events.
1 subunits, we found that
adhesion mediated by WT
1 resulted in activation of both MAP kinase and FAK, but adhesion mediated by a
1 mutant resulted in activation of MAP kinase but not
FAK. Second, we have examined cells adhering to recombinant fragments of fibronectin. Some of these fragments
sustained cell adhesion that resulted in robust activation of
MAP kinase but barely perceptible activation of FAK. Third,
under some conditions, the time course of MAP kinase activation and FAK activation are completely distinct. Fourth, we have used constructs that express FAK-related nonkinase (FRNK), the COOH-terminal domain of FAK, as a
dominant negative modulator of FAK function. Transfection with FRNK constructs interfered with integrin-mediated tyrosine phosphorylation of FAK and reduced the
amount of phosphotyrosine in focal contacts but had essentially no effect on the integrin-mediated activation of
MEK and MAP kinase. These findings strongly suggest
that integrin-mediated activation of MEK and MAP kinase are independent of FAK; this idea has a number of
important implications for current models of integrin- mediated signal transduction.
Materials and Methods
1 cDNAs and W1B10 anti-chicken
1 mouse monoclonal were
from A.R. Horwitz (University of Illinois, Urbana, IL). Anti-mouse
5
(MFR5) and
1 (9EG7) rat monoclonals were obtained from Pharmingen (San Diego, CA). Antibodies to FAK (clone 77) and MEK were from Transduction Laboratories (Lexington, KY); antibodies to MAP kinase (Sc-93,
94) were from Santa Cruz Biotechnology (Santa Cruz, CA), while antiphosphotyrosine antibodies were obtained from ICN Biochemicals (PY20,
Costa Mesa, CA) or Upstate Biotechnologies Inc. (4G10, Lake Placid, NY).
Fluorophore conjugated second antibodies were from Chemicon International, Inc. (Temecula, CA). Antibody 5158 was raised (L. Romer) against
a GST fusion protein containing the COOH terminus of FAK.
1 integrin antibody W1B10 or 20 µg/ml anti-mouse CD44 antibody (MCA1014; Serotec Ltd., Washington, DC) in PBS at 4°C overnight.
Tissue culture dishes were incubated with fibronectin or with appropriate
concentrations of fibronectin fragments in PBS at 4°C overnight. The
coated flasks and dishes were blocked with 2% BSA in Dulbecco's minimal essential medium (DMEM) for 1 h at room temperature and washed
twice with PBS before use. The fibronectin fragments used were comprised of
a series of fibronectin type III domains from domain 7 to 12, either including
or excluding the two alternatively spliced domains, EIIIB and EIIIA; these were cloned and expressed using the prokaryotic expression vector pET15b
(Novagen Inc., Madison, WI) as described elsewhere (Aukhil et al., 1993
).
1 integrin subunit or a mutant chicken
1 integrin subunit with a deletion
at residues 759-771 (Reszka et al., 1992
). Cells expressing chicken
1 integrins were selected using magnetic beads (Dynal, Inc., Great Neck, NY)
coated with the anti-chicken
1 integrin antibody W1B10 (Reszka et al.,
1992
). The selected cells were allowed to grow in DMEM with 10% bovine calf serum and 250 µg/ml of G418. Cells were further selected twice
using MicroCELLector flasks (Applied Immune Sciences, Inc.) coated with
W1B10 and then expanded before use. In some cases cells were transfected with constructs expressing an epitope-tagged MEK (EE-MEK;
Chen et al., 1996
) and/or the noncatalytic domain of FAK, FRNK (Schaller et al., 1993
). For the latter, chicken FRNK cDNA was ligated into the
EcoRI site under the control of the cytomegalovirus promoter in the
mammalian expression vector pcDNA3 (Invitrogen Corp., San Diego, CA).
). The FRNK cDNA was inserted into the RCAS-A replication competent avian retroviral vector (Hughes et al., 1987
) and the plasmid transfected into CE cells. 7-10 d later FRNK expression was maximal, and virtually all of the cells were transfected. Cell adhesion experiments were
performed as above.
MAPK; for EE-MEK),
or 10 µg of myelin basic protein (for MAP kinase), and incubated for 30 min at room temperature (Chen et al., 1996
). Reactions were stopped by
adding 40 µl of 2× sample buffer and boiling 3 min. The samples were
subjected to SDS-PAGE, and the gels were dried. The dried gels were exposed to X-ray films, or the 32P-labeled substrate bands were quantitated
using a Storm 840 Phosphorimager with Image-QuaNT software (Molecular Dynamics, Inc., Sunnyvale, CA).
Results
1 Subunit Activates
MAP Kinase but Not FAK
1 subunit (Reszka et al.,
1992
). These cells were then allowed to adhere to substrata coated with an antibody directed against the
1 subunit. Adhesion to substrata coated with antibodies to the
abundant nonintegrin surface protein CD44 served as a
negative control, while adhesion to fibronectin was used as
a positive control. One mutated
1 construct (here termed
1
) proved particularly interesting; this construct gives rise to a subunit with a membrane proximal deletion in the
cytoplasmic domain (amino acids 759-771) that overlaps
with the putative binding site for FAK (Reszka et al., 1992
;
Schaller et al., 1995
). Fig. 1 depicts flow cytometry profiles
for expression of endogenous and transfected integrin subunits, and expression of endogenous CD44, in NIH 3T3
cells. Similar levels of chicken
1 integrins were expressed
on the cell surface in both
1WT and
1
transfected cells
(Fig. 1 A). The expression of the exogenous chicken
1 integrin subunit was not as homogeneous as that of the endogenous integrin subunits (Fig. 1 B). CD44, a nonintegrin
cell surface protein (Aruffo et al., 1990
), was abundantly
expressed on the cell surface (Fig. 1 B). As seen in Fig. 2,
cells expressing WT
1 showed substantial spreading on
an anti-
1 antibody-coated surface, while cells expressing
1
remained completely round, although firmly attached.
This result is consistent with previous observations that
deletion of amino acid 759-771 resulted in inhibition of cell
spreading (Reszka et al., 1992
). Cells adhering on fibronectin spread extensively, while cells firmly attached on
anti-CD44 remained round. In these experiments, all of
the cells were adherent to the various substrata and could
resist vigorous washing of the plates, even though differences in appearance were obvious.
Fig. 1.
Expression of endogenous and transfected integrin subunits, and expression of endogenous CD44 in NIH 3T3 cells. NIH
3T3 cells were transfected with constructs expressing chicken
1WT or
1
integrin subunit. Cells expressing chicken
1 integrin subunits were selected and expanded as described in Materials and Methods. (A) Untransfected cells or
1WT or
1
transfected cells were incubated with the anti-chicken
1 integrin
antibody W1B10 or with anti-macrophage nitric oxide synthase
antibody as a control antibody. (B)
1WT transfected cells were
incubated with W1B10, with antibodies recognizing mouse
1 or
5 integrin subunits or CD44, or the irrelevant antibody to macrophage nitric oxide synthase. Antibodies to mouse integrins are
marked with the prefix m. In both A and B, cells were washed,
and incubated with goat anti-mouse IgG or goat anti-rat IgG
coupled to fluorescein. After further washing, cells were resuspended and analyzed using a flow cytometer.
[View Larger Version of this Image (20K GIF file)]
Fig. 2.
Cell morphology: cell adherence on anti-chicken 1 antibody, anti-CD44 antibody, or Fn-coated surfaces.
1WT or
1
transfected cells were plated onto MicroCELLector flasks coated
with anti-chicken
1 integrin antibody or anti-CD44 antibody, or
onto tissue culture dishes coated with Fn (10 µg/ml), in each case
for 30 min at 37°C. Cells were washed with PBS, fixed with 2%
formaldehyde, and photographed at 300× magnification.
[View Larger Version of this Image (119K GIF file)]
1 or
1
were tested for FAK
tyrosine phosphorylation and for MAP kinase activation.
As shown in Fig. 3, A and B, adherence of WT
1 cells or
1
cells to anti-
1-coated surfaces caused similar degrees of activation of MAP kinase (twofold). However,
while adherence of WT cells resulted in a modest but clear
increase in FAK tyrosine phosphorylation, no increase
was detected in the
1
cells. Cell adhesion to an Fn substratum triggered a strong increase in MAP kinase activity
(fourfold) and strong tyrosine phosphorylation of FAK in
both
1WT and
1
transfected cells. This result ruled out
the possibility that the machinery to trigger FAK tyrosine
phosphorylation was defective in
1
transfected cells.
The activation of FAK and MAP kinases is clearly mediated by integrins, since attachment of cells to an antiCD44 antibody-coated surface did not induce FAK or
MAP kinase activities. These results suggested that FAK
and MAP kinase activations might be uncoupled in the
cells expressing
1
. It is interesting to note that, while at
the level of light microscope morphology, the cells attaching on anti-CD44 and the
1
cells attaching on anti-
1
were similarly rounded in appearance, they were clearly
different in terms of their biochemical capabilities.
Fig. 3.
Effect of integrin ligation on FAK tyrosine phosphorylation and MAP kinase activity in 1Wt and
1
transfected
cells.
1WT or
1
transfected cells were detached by trypsin
and incubated in suspension for 45 min at 37°C. Cells were plated
onto MicroCELLector flasks coated with anti-chicken
1 integrin antibody or anti-CD44 antibody, onto tissue culture dishes
coated with Fn (10 µg/ml), or held in suspension (NAD), in each
case for 30 min at 37°C. After incubating, cells were washed and
lysed in modified RIPA buffer. MAP kinase activity was analyzed by a band shift assay (A) and by an immune complex kinase
assay with quantitation using a Phosphorimager (B). FAK was
immunoprecipitated and analyzed for tyrosine phosphorylation
by anti-phosphotyrosine immunoblotting and blotting with anti-
FAK (C).
[View Larger Version of this Image (49K GIF file)]
; Leahy et al., 1996
).
To further pursue the relationships between cell spreading, FAK tyrosine phosphorylation, and MAP kinase activation, we made use of several recombinant fragments of
fibronectin that had been expressed in prokaryotic systems (Fig. 4). A series of fibronectin type III domains from domain 7 to 12, either including or excluding the two alternatively spliced domains EIIIB and EIIIA were cloned and
expressed as described in Materials and Methods. As shown
in Fig. 4, each of the recombinant fibronectin fragments
contained the cell-adhesive RGD site in type III domain
10, as well as the recently identified PHSRN sequence in
the "synergy" domain 9 (Aota et al., 1994
; Bodwitch et al., 1994
). As seen in Fig. 5, NIH 3T3 cells rapidly adhered
and spread on fibronectin-coated substrata. The cells also
adhered rapidly but spread less well on substrata coated
with equimolar amounts of the recombinant fibronectin
fragments. In particular, cells adhering to the
B
A fragment displayed very little spreading during the first 30 min; cells on the
B+A or +B
A fragments also showed
modest spreading, while the +B+A fragment supported
the greatest degree of spreading, although still far less than
for intact fibronectin. Adhesion of cells to the fibronectin
fragments was almost completely blocked by 1 mM RGD
peptide but not by RGE peptide (data not shown). Thus,
adhesion to the recombinant fibronectin fragments seems
to be primarily an integrin-mediated process.
Fig. 4.
Linear structure of
Fn and recombinant Fn fragments. Fn consists of three
types of repeating motifs:
types I, II, and III. Two alternatively spliced variants with
additional type III repeats, IIIA and IIIB, are indicated
at the top. The underlines
represent two putative cell
binding sites. The central cell
binding site contains the
RGD sequence in the 10th
type III repeat. The recombinant Fn fragments used in
these studies contain the 7th
to 12th type III repeats, with
one or two or none of the alternative splicing motifs.
[View Larger Version of this Image (19K GIF file)]
Fig. 5.
Cell morphology:
cell adherence on tissue culture dishes coated with Fn or
recombinant Fn fragments.
Tissue culture dishes were
coated with 5 nM of intact Fn
or recombinant Fn fragments overnight at 4°C. NIH 3T3
cells were plated onto these
intact Fn or Fn fragment substrata for 30 min at 37°C.
Cells were washed with PBS,
fixed with 2% formaldehyde,
and photographed at 300×.
[View Larger Version of this Image (83K GIF file)]
B+A,
+B
A, and +B+A fragments, while there was no detectable increase in cells adhering to the
B
A fragment
(Fig. 6 C). This result suggests that it is possible to obtain
quite strong activation of MAP kinase in the absence of
significant tyrosine phosphorylation of FAK.
Fig. 6.
Differential activation of FAK and MAP kinase upon
cell adhesion to intact Fn or recombinant Fn fragment-coated
dishes. NIH 3T3 cells were detached and incubated nonadherently for 45 min at 37°C (NAD). Cells were then allowed to adhere to intact Fn or Fn fragment-coated dishes for 30 min at
37°C. MAP kinase activity and FAK tyrosine phosphorylation
were determined as described in Materials and Methods. (A)
MAP kinase band shift assay. (B) MAP kinase immune complex
kinase assay. (C) FAK tyrosine phosphorylation and anti-FAK
Western blotting.
[View Larger Version of this Image (40K GIF file)]
B
A fragment to
cells attaching to substrata coated with the nonspecific adhesive substance poly-L-lysine. Preliminary observations
showed a weak MAP kinase activation in 3T3 cells adhering to poly-L-lysine and a more robust activation in cells adhering to
B
A or to intact fibronectin (data not
shown). This suggests that integrins play an important role
in the MAP kinase activation events studied here, consistent with our previous observations (Chen et al., 1994
).
), thus
leading to activation of the MAP kinase pathway even
though overall tyrosine phosphorylation of FAK is low.
However, when we used a Grb2-GST fusion protein to
bind tyrosine phosphorylated FAK, we found that the reduced FAK tyrosine phosphorylation observed in cells adhering to the fibronectin fragments was accompanied by
reduced binding of FAK to Grb 2 (data not shown). Thus,
it seems likely that the tyrosine phosphorylation status of
FAK is not related to adhesion-mediated MAP kinase activation. It is also striking that robust MAP kinase activation occurs in adhering cells that are completely round by
light microscopic criteria (cells on
B
A), suggesting that extensive actin stress fiber organization is not required for integrin-mediated MAP kinase activation.
B
A fragment. There was
a rapid activation to ~4× basal levels that reached a maximum between 10 and 20 min and then a gradual decline between 30 and 90 min. In contrast, the kinetic profiles of
FAK activation were very different on the two substrata.
In cells attaching to fibronectin, a distinct increase in FAK
tyrosine phosphorylation was observed by 10 min; this
continued to increase up to 45 min and then remained constant. In cells attaching on
B
A, tyrosine phosphorylation of FAK was barely above background until 30 min; it
then progressively increased during the remainder of the
experiment, thus paralleling spreading behavior (not shown). These results indicate that MAP kinase activation can kinetically precede FAK activation, and that MAP kinase activation can decline even as FAK activation is increasing.
Fig. 7.
Time course of MAP kinase activation and FAK tyrosine phosphorylation upon cell adhesion to intact Fn or B
A
Fn fragment-coated dishes. NIH 3T3 cells were trypsinized and
kept in suspension for 45 min at 37°C. Cells were then plated onto
intact Fn or the
B
A Fn fragment-coated dishes at 37°C for
the time indicated. After incubation, cell lysates were analyzed
for MAP kinase activity and FAK tyrosine phosphorylation as described in Materials and Methods. (A) MAP kinase band shift assay. (B) MAP kinase immune complex kinase assay. (C) FAK tyrosine phosphorylation and anti-FAK Western blotting.
[View Larger Version of this Image (50K GIF file)]
). Thus,
it seemed likely that overexpressed FRNK might interfere
with FAK function in a dominant inhibitory manner (Richardson and Parsons, 1996
). As described in Materials and Methods, chicken fibroblasts were infected with a replication competent retrovirus that expresses FRNK from the
LTR promoter. These cells, as well as control transfectants, were removed from the substratum and either maintained in suspension or replated on fibronectin. The cells
were then lysed and tested for adhesion-induced MAP kinase activity and for tyrosine phosphorylation of endogenous FAK. As seen in Fig. 8, the cells transfected with the
FRNK construct expressed substantially higher levels of
FRNK than control transfectants. FRNK-expressing cells displayed dramatically reduced levels of tyrosine phosphorylation of FAK. By contrast, FRNK expression had little effect on the activation of MAP kinase by cell adhesion to
fibronectin. Thus, levels of FRNK expression sufficient to almost completely inhibit FAK tyrosine phosphorylation
were not able to significantly inhibit integrin-mediated
MAP kinase activation.
Fig. 8.
Effect of overexpression of FRNK on MAP
kinase activity and FAK tyrosine phosphorylation in
chicken fibroblasts. Chicken
embryonic fibroblasts were
infected with a retroviral expression vector for FRNK as
described in Materials and
Methods. Expression of FRNK in untransfected cells
(CE) or in the FRNK transfected cells (FRNK) was
examined by anti-FRNK
immunoblotting (A). Untransfected or FRNK-transfected cells were trypsinized
and incubated nonadherently for 45 min at 37°C (NAD).
Cells were then plated onto
Fn-coated dishes for 30 min
at 37°C. After incubating,
cells were washed and lysed
in modified RIPA buffer.
MAP kinase activity was analyzed by a band shift assay
and by a immune complex kinase assay (B); it should be noted that chicken fibroblasts contain only one form of MAP kinase
(MAPK2). FAK was immunoprecipitated and analyzed for tyrosine phosphorylation by anti-phosphotyrosine immunoblotting and by
anti-FAK Western blot (C).
[View Larger Version of this Image (35K GIF file)]
). EE-MEK immunoprecipitated
from transfected cell populations replated on fibronectin
showed an increase in kinase activity compared to EEMEK isolated from suspension cells, as measured by the
ability to phosphorylate K
MAPK. Even at the higher levels of FRNK expression, EE-MEK activity was robustly
activated in response to adhesion to fibronectin (Fig. 9 B).
Quantitation of EE-MEK activity normalized to the protein level of EE-MEK showed no significant decrease of
the integrin-induced activation of EE-MEK due to FRNK
cotransfection (Fig. 9 C). These data are consistent with
the previous experiments in showing that integrin-induced
activation of MAP kinase cascade activity and increased
tyrosine phosphorylation of FAK are independent.
Fig. 9.
Effect of overexpression of FRNK on MAP kinase activity and tyrosine phosphorylation in 3T3 fibroblasts. NIH 3T3
cells were transiently transfected either with different amounts of
pcDNA3-FRNK or empty vector and EE-MEK. Cells were serum starved and replated on Fn or kept in suspension (NAD) as
described in Materials and Methods. Transfection DNA levels were
made up to 2 µg in each condition with empty vector (pCMV5 or
pcDNA3). (A) After adherence to Fn-coated coverslips, transfected cell populations were fixed and coimmunostained with an
anti-FAK/FRNK polyclonal antibody and for phosphotyrosine.
The large arrow indicates a cell transfected with FRNK; small arrows indicate presumed focal contact sites in nontransfected cells
(B) Detergent soluble lysates were analyzed for exogenous FRNK
expression by Western blotting and for EE-MEK activity by immunopreciptation and in vitro kinase assay. Incorporation of 32P
into KMAPK was detected by autoradiography. The level of expressed EE-MEK was determined by Western blotting of EEimmunoprecipitates with anti-MEK antibody. (C) Quantitation
of EE-MEK activity from kinase assays relative to EE-MEK protein levels. The average and standard deviation from the mean is
represented for three separate transfection experiments. Bar,
60 µm.
[View Larger Versions of these Images (53 + 54K GIF file)]
Discussion
; Zhu et al., 1996
), control of apoptosis
(Ruoslahti and Reed, 1994
; O'Brien et al., 1996
), and the
regulation of cell differentiation (Juliano and Haskill,
1993
) has led to substantial interest in detailed mechanisms of integrin-mediated signal transduction. Initial observations on integrin signaling emphasized the importance of tyrosine phosphorylation (Guan et al., 1991
; Kornberg
et al., 1991
) and particularly the autophosphorylation of
FAK, a seemingly unique integrin-regulated cytoplasmic tyrosine kinase (Hanks et al., 1992
; Schaller et al., 1992
). Although FAK homologues have now been identified (Avraham et al., 1995
; Lev et al., 1995
; Sasaki et al., 1995
) and
other tyrosine kinases such as Syk have been shown to be
integrin-responsive (Clark et al., 1994
; Lin et al., 1995
),
FAK is still considered to be a key element in integrin signaling pathway(s) in many cell types. Clearly FAK is implicated in pathways that regulate cell motility and cytoskeletal organization and may possibly be involved in cell
growth control as well (Romer et al., 1994
; Illc et al., 1995
;
Frisch et al., 1996
; Gilmore and Romer, 1996
; Richardson
and Parson, 1996). Tyrosine phosphorylated FAK can bind
to a number of interesting signaling molecules via their SH2
domains, including Src family kinases, PI-3-K, and Grb2/ Sos (Chen and Guan, 1994
; Schlaepfer et al., 1994
; Bachelot et al., 1996
). In addition, FAK can bind other structural
and signal transduction proteins through SH3 domain interactions or other interactions; this includes talin, paxillin,
p130Cas, and a GAP for Rho and CDC42 (Polte and Hanks,
1995
; Vuori and Ruoslahti, 1995
; Chen et al., 1995
; Turner
and Miller, 1994
; Hildebrand et al., 1995
, 1996). Finally,
FAK can help to organize complex networks of structural
and signal transduction proteins at focal contacts, since
many of the binding partners of FAK bind in turn to other proteins (Birge et al., 1993
; Parsons, 1996
). Despite all of
these interesting possibilities, the precise role of FAK in
integrin signaling remains rather poorly defined.
; Miyamoto et al.,
1995
). While these events are clearly integrin-mediated
(Chen et al., 1996
), they differ strikingly from FAK responses in that they are quite transient, with the peak of
activation coming immediately after cell attachment and
then rapidly returning to basal levels. The kinetics of the
MAP kinase response to integrin-mediated adhesion is
somewhat similar to MAP kinase activation in response to
mitogen treatment. In that case, the attenuation of MAP
kinase activity is due, in part, to the induction of specific
phosphatases that dephosphorylate critical resides on
MAP kinase (Sun et al., 1994
; Ward et al., 1994
); it is unclear whether this also occurs in connection with integrin
signaling to MAP kinase.
). However, previous data from our laboratory indicate that Ras is not essential for integrin-mediated MAP
kinase activation in 3T3 cells (Chen et al., 1996
). Further, our current data strongly support the concept that FAK is
not involved in integrin-mediated activation of MAP kinase. Several independent lines of investigation including
the use of mutant
1 subunits, the use of recombinant fibronectin fragments, and the use of the FRNK domain of
FAK as a dominant negative mutant, all indicate that FAK
activation can be quantitatively and kinetically uncoupled
from MAP kinase activation. This set of observations is
difficult to reconcile with the simple and appealing model that integrin-mediated FAK activation leads to recruitment
of Grb2/Sos and subsequent activation of the Ras/Raf/
MEK/MAP kinase pathway. Rather, current work is consistent with the possibility that FAK activation and MAP
kinase activation represent distinct integrin-mediated signaling pathways. Another possibility is that MAP kinase activation is upstream of FAK activation; this interesting alternative is currently being investigated. However, even if this
were so, it seems unlikely that the only role for integrinmediated MAP kinase activation would be to regulate FAK,
since during cell adhesion MAP kinase migrates to the nucleus where it potentially may act on key transcription factors (Chen et al., 1994
). The finding that integrin-mediated
MAP kinase activation is largely independent of both Ras
(Chen et al., 1996
) and FAK suggests the existence of a
novel pathway for MAP kinase stimulation, but the components of this pathway remain to be defined.
; Fang et al., 1996
; Zhu et al., 1996
). It seems
likely that small GTPases may be critically important in this
interplay. The function of the Rho family members CDC42,
Rac, and RhoA can be regulated by a variety of mechanisms, including activation of the Ras proto-oncogene. Thus,
the Ras signaling pathway bifurcates to act both on activation of Rho family members and upon the ERK subfamily
of MAP kinases (Khosravi-Far et al., 1995
; Qiu et al., 1995
).
CDC42, Rac, and Rho have been implicated in actin filament formation in filopodia, ruffled membranes, and stress
fibers, respectively (Nobes and Hall, 1995
). Rac and Rho
also cooperate with integrins in the assembly of focal contacts, and both elements are required for the process
(Hotchin and Hall, 1995
). In addition, CDC42 and Rac can
regulate the signaling pathway leading to activation of the JUN kinase subfamily of MAP kinases (Vojtek and Cooper, 1995
), while all three Rho-family proteins regulate
transcriptional events associated with cell cycle traverse
(Hill et al., 1995
; Olson et al., 1995
). Although the relationships between the cytoskeletal and signaling activities
of the Rho proteins are not fully understood, they may
well be essential for growth control. One possible intersection concerns the generation of inositol lipids. A Rhodependent PI-5-kinase may control the availability of PIP2
in cells (Chong et al., 1994
). Levels of PIP2 play a key role
both in mitogenic signal transduction (Liscovitch and
Cantley, 1995
) and in assembly of cytoskeletal structures
(Gilmore and Burridge, 1996
), thus potentially linking these two important processes. Further, recent findings
have demonstrated a role for Rho in integrin-mediated activation of MAP kinase (Renshaw et al., 1996
), possibly by
promoting the assembly of cytoskeletal complexes. Despite these important advances, a detailed mechanistic delineation of connections between adhesion-induced cytoskeletal events and signaling events involved in mitogenesis
remains unknown.
; Chen et al., 1994
). However,
current findings suggest that MAP kinase and FAK may
respond to different levels or degrees of cytoskeletal assembly. Integrin-mediated MAP kinase activation seems
to be an early, transient event that occurs before cell spreading, while FAK activation occurs concurrently with
spreading and is persistent. Our observations on FAK and
MAP kinase are consistent with the concept that integrinmediated cell adhesion creates a hierarchy of structural
and signaling events (Miyamoto et al., 1995
) that can be
kinetically and topologically resolved. However, the precise biological and biochemical roles of the individual components of the integrin-regulated structural/signaling
hierarchy remain to be determined.
Received for publication 23 August 1996 and in revised form 20 November 1996.
The EEEEYMPME epitope-tagged rat MEK-1 construct (EE-MEK) was kindly provided by Dennis J. Templeton (Case Western Reserve University, Cleveland, OH). A.R. Horwitz (University of Illinois, Urbana, IL) generously provided the chickenThis work was supported by National Institutes of Health grants GM26065 and HL45100 to R.L. Juliano and HL03299 to L. Romer.
FAK, pp125 focal adhesion kinase; Fn, fibronectin; FRNK, FAK-related nonkinase; MAP, mitogen activated protein; MEK, MAP kinase kinase; WT, wild type.