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J Biol Chem, Vol. 273, Issue 36, 22899-22903, September 4, 1998
5
1 Signaling Pathways*
From the Program in Molecular Medicine and Department of Biochemistry and Molecular Biology, University of Massachusetts Medical Center, Worcester, Massachusetts 01605
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ABSTRACT |
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The ligation and clustering of cell surface

heterodimeric integrins enhances cell adhesion and initiates
signaling pathways that regulate such processes as cell spreading,
migration, differentiation, proliferation and apoptosis. Here we show
that insulin treatment of Chinese hamster ovary cells expressing
insulin receptors (CHO-T) markedly promotes cell adhesion onto a
fibronectin matrix, but not onto bovine serum albumin or poly-lysine.
Incubation of cells with a GRGDSP peptide that specifically binds
integrins (but not the nonspecific GRADSP peptide) abolishes this
insulin effect, as does the potent phosphoinositide 3-kinase (PI
3-kinase) inhibitor wortmannin. Moreover, a specific blocking
monoclonal anti-
5
1 integrin antibody,
PB-1, blocks insulin-stimulated cell adhesion onto fibronectin.
Conversely, activating
5
1 integrins on
CHO-T cells by adherence onto fibronectin markedly potentiates the
action of insulin to enhance insulin receptor and insulin receptor
substrate (IRS)-1 tyrosine phosphorylation. Activation of
5
1 integrin also markedly potentiates the
recruitment of p85-associated PI 3-kinase activity to IRS-1 in response
to submaximal levels of insulin in CHO-T cells. These data indicate
that insulin potently activates integrin
5
1 mediated CHO-T cell adhesion, while
integrin
5
1 signaling in turn enhances
insulin receptor kinase activity and formation of complexes containing
IRS-1 and PI 3-kinase. These findings raise the hypothesis that insulin
receptor and
5
1 integrin signaling act
synergistically to enhance cell adhesion.
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INTRODUCTION |
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The tyrosine kinase activity of the cell surface insulin receptor
is required to mediate its many biological actions. Insulin receptor
activation promotes the rapid autophosphorylation of its
subunit,
as well as tyrosine phosphorylation of proteins involved in insulin
signaling, such as insulin receptor substrate (IRS)1 and Shc proteins
(1-4). Insulin-mediated phosphorylation of these proteins is thought
to provide tyrosine phosphate docking sites for the recruitment of
signaling proteins containing Src homology 2 domains (SH2) (1, 5, 6).
One SH2 domain-containing family of proteins which associates with IRS
proteins in response to insulin are isoforms of the p85 regulatory
subunit of the p110-type PI 3-kinases (1-4, 7). PI 3-kinase activity
in such signaling complexes appears to be required for insulin action
on many cellular processes, including glucose transport (8, 9),
glycogen synthesis (9, 10), stress fiber breakdown (11), and membrane ruffling (12). Thus, inhibition of PI 3-kinase activity by wortmannin or disruption of PI 3-kinase recruitment to IRS proteins by dominant inhibitory constructs of p85 subunits ablate the actions of insulin on
these processes (11-13). Our understanding of the downstream elements
that mediate the action of the 3'-phosphoinositide products of the PI
3-kinases is incomplete, but appear to include protein kinases such as
PDK1 and Akt/protein kinase B (PKB) (14), a family of proteins
containing Sec7 homology domains (15), and the zinc finger containing
protein EEA1 (16).
Insulin action also promotes dephosphorylation of tyrosine phosphates
on such proteins as focal adhesion kinase (FAK) and paxillin (17-19),
thought to be involved in cell regulation by integrins. Integrins are

heterodimeric transmembrane receptors that mediate interactions
between the cell surface and the extracellular matrix and also initiate
signaling events (20-22), including tyrosine phosphorylation of FAK
and paxillin (23-25), cytoskeletal reorganization (20-22, 25),
activation of mitogen-activated protein kinase cascades (26), and
regulation of gene expression (22, 27). These biological actions of
integrins can overlap with those of growth factors, and there is
evidence that signaling pathways initiated by integrins synergize
functionally with those triggered by growth factors (22, 28). Thus,
cell adhesion has been shown to greatly enhance autophosphorylation of
epidermal growth factor and platelet-derived growth factor (PDGF)
receptors (25, 29). This in turn potentiates the action of these growth
factors in activating mitogen-activated protein kinases, PI 3-kinase,
and the downstream protein kinases PDK1 and Akt/PKB (29, 30). The
mitogenic effects of insulin have also been shown to be enhanced by
interactions of extracellular vitronectin with cell surface
v
3
integrins, which associate with insulin receptor and IRS-1 in response
to insulin (31, 32). However, the mechanism of such synergism in the
actions of insulin and integrins is still unclear.
Conversely, it also has been shown that growth factor receptors, such
as KIT (33) and PDGF (34) receptors, stimulate integrin-mediated cell
adhesion onto fibronectin through a PI 3-kinase-dependent pathway. Thus, in the present studies, we addressed the questions whether insulin may directly activate integrin-mediated cell adhesion and if integrin activation modulates insulin signaling. Here, we show
that insulin markedly promotes CHO-T cell adhesion onto a fibronectin
matrix by a mechanism that is mediated by
5
1 integrin. Activation of this integrin
in turn enhances insulin receptor and IRS-1 tyrosine phosphorylation,
as well as recruitment of PI 3-kinase activity to IRS-1 in response to
insulin. This cross-talk between insulin and integrin receptor pathways
is likely to play an important role in biological processes regulated
by insulin and which depend on integrin engagement.
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EXPERIMENTAL PROCEDURES |
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Materials--
Anti-phosphotyrosine (anti-Tyr(p)) mouse
monoclonal 4G10, anti-p85 polyclonal, and laminin were purchased from
Upstate Biotechnology. Rabbit polyclonal anti-IRS-1 immunoglobulin used
for immunoprecipitation was prepared as described previously (35).
Anti-insulin receptor monoclonal (CT-1) and polyclonal antibodies were
from mouse ascites and from Santa Cruz Biotechnology, respectively.
Plasma human fibronectin was from Boehringer. GRGDSP and GRADSP
peptides for the adhesion competition assay were from Life
Technologies, Inc. Specific anti
5
1
integrin receptor monoclonal antibody, used in CHO-T cell adhesion
blocking assay, was a gift from Dr. Rudolph Juliano.
[
-32P]ATP and 35S-labeled protein labeling
mix were from NEN Life Science Products. Phosphatidylinositol (PI) was
from Avanti Polar Lipids.
Cell Culture-- CHO-T cells were maintained in Ham's F-12 medium, 10% fetal bovine serum, and 50 µg/ml streptomycin/penicillin and grown to confluence before use.
Cell Adhesion Assay--
To assay cell adhesion on different
matrices, cell culture dishes (12-well plate) were coated with
fibronectin (0.5 µg/ml), laminin (10 µg/ml), and poly-lysine (0.5 µg/ml) at 4 °C overnight and blocked with 0.1% bovine serum
albumin (BSA) in phosphate-buffered saline (PBS) for 1 h, prior to
plating the cells. Confluent CHO-T cells were serum starved for 12 h in F-12 serum-free medium containing 0.5% BSA and labeled with
35S-labeled protein labeling mix (1 µCi/150
cm2 plate) at 37 °C and washed twice with PBS. Cells
labeled with 35S were detached by adding EDTA-trypsin (0.05 mM, 0.025%). Cells were held in suspension for 5 min, and
additions were made for the times indicated in the figure legends.
Cells were plated in dishes coated with different substrata, as
indicated. Nonadhered cells were removed, and attached cells were
washed twice with PBS containing 25 mM MgCl2.
Adhered cells were quantified by adding 0.5 ml SDS (0.1%), and samples
were counted by a
-counter.
Cell Lysis, Immunoprecipitation, and Immunodetection-- Cells held in suspension or attached on a substrata were lysed by adding detergent lysis buffer (50 mM HEPES, pH 7.5, 100 mM NaF, 10 mM NaPPi, 2 mM Na3VO4, 1% Nonidet P-40, 1 mM phenylmethylsulfonyl fluoride, 10 µg/ml leupeptin, 10 µg/ml aprotinin) and kept on ice for 20 min. Detergent lysates were precleared by centrifugation at 14,000 × g for 15 min in the cold, and protein concentration was determined by the Bicinchoninic Acid (BCA) protein assay protocol from Pierce. Appropriate antibodies were then added to the cleared cell lysate standardized for total cell protein, and the lysates were incubated overnight at 4 °C by end-over-end mixing. Protein A-Sepharose was added and the samples incubated for 2 h. Immunoprecipitated proteins were then washed four times in PBS with 1% Nonidet P-40, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and transferred to nitrocellulose membranes for immunoblotting with appropriate antibody. Bound primary monoclonal or polyclonal antibodies were visualized by the Renaissance Chemiluminescence Detection System (NEN Life Science Products) with horseradish peroxidase-conjugated detection antibody at 1:15,000 dilution. The tyrosine phosphorylation levels of immunoprecipitated insulin receptor and IRS-1 were quantified, where indicated, by scanning densitometry.
Assay of IRS-1-associated PI 3-Kinase
Activity--
IRS-1-associated PI 3-kinase activity assays were
performed in vitro by immunoprecipitation of IRS-1 from
total cell lysates as described above. The IRS-1 immunoprecipitates
were washed three times in PBS, 1% Nonidet P-40, twice in buffer
containing 10 mM Tris-HCl, pH 7.5, 100 mM NaCl,
1 mM EDTA, and once in PI 3-kinase assay buffer (20 mM Tris-HCl, pH 7.5, 100 mM NaCl, 0.1 mM EGTA, 10 mM MgCl2). The IRS-1
immunoprecipitates were resuspended in 0.2 ml of PI 3-kinase assay
buffer containing 50 µg of PI and [
-32P]ATP (100 µCi at final concentration of 0.05 mM), and the reaction was incubated for 20 min at room temperature and quenched with HCl 1 N. Phospholipids were extracted with chloroform:methanol (1:1), washed
twice with methanol:HCl 1 N (1:1), and resolved by thin layer
chromatography (TLC), and the radioactive products were quantified as
described previously (35).
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RESULTS AND DISCUSSION |
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Insulin Stimulates Cell Adhesion Mediated by Integrins-- To determine whether insulin signaling can activate integrin-mediated cell adhesion, CHO-T cells were detached from culture dishes, treated with 100 nM insulin for 10 min at 37 °C, and plated onto dishes coated with bovine serum albumin, poly-lysine, or fibronectin. As shown in Fig. 1A, adhesion of CHO-T cells onto a fibronectin matrix was stimulated 2-3-fold by prior treatment with insulin. A similar result was observed when a laminin matrix was used (Fig. 2, right). Nonspecific adhesion processes were not found to be affected by insulin since insulin treatment did not have any effect on CHO-T cell adhesion to dishes coated with bovine serum albumin or poly-lysine. Thus, insulin can modulate CHO-T cell adhesion, similar to the effects described for other growth factor receptors, such as those that bind PDGF, and KIT (33, 34).
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5
1 integrin receptors (37). We thus
examined the effect of a monoclonal
anti-
5
1 blocking antibody, PB-1 (37-39), on CHO-T cell adhesion to fibronectin in the presence and absence of
insulin. As seen in Fig. 2 (right), addition of PB-1 to
assay media markedly inhibited the adhesion of CHO-T cells to a
fibronectin matrix under both basal and insulin-stimulated conditions.
In contrast, no inhibition of CHO-T cell adhesion onto laminin could be
detected by adding PB-1 to the assay media (Fig. 2, left). These data are consistent with a previous report showing PB-1 inhibition of CHO cell adhesion onto fibronectin, but not onto laminin
(37). Taken together, the data in Figs. 1 and 2 demonstrate a marked
effect of insulin to enhance CHO-T cell adhesion onto fibronectin
through
5
1 integrin.
Insulin-stimulated Cell Adhesion Is Inhibited by Wortmannin-- To determine whether insulin-stimulated and integrin-mediated cell adhesion is dependent on activated PI-3-kinase, CHO-T cells were detached from the culture dishes, treated with or without 50 nM of the PI 3-kinase inhibitor wortmannin for 15 min prior to incubation with or without insulin for 10 min, and then replated onto fibronectin or laminin-coated plates. As depicted in Fig. 3, wortmannin treatment of the cells markedly inhibited insulin-stimulated, fibronectin-dependent CHO-T cell adhesion. Similar results were observed when cells were plated onto laminin-coated dishes (Fig. 3). These results suggest that insulin-stimulated cell adhesion in this system is dependent, at least in part, on PI 3-kinase activity. The mechanism of this insulin action on cell adhesion may thus share common elements with the actions of growth factors such as PDGF on adhesion, which are also blocked by wortmannin (33, 34, 40).
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5
1 Integrin-mediated CHO-T Cell
Adhesion Increases Insulin Receptor and IRS-1 Tyrosine Phosphorylation
in Response to Insulin--
Attachment of cells to fibronectin results
in increased tyrosine phosphorylation of FAK (Mr = 125,000) and paxillin (Mr = 68,000-75,000)
proteins that are associated with focal adhesion complexes (23-25).
Also, it has been shown that integrin-mediated cell anchorage promotes
increases in tyrosine phosphorylation of epidermal growth factor and
PDGF receptors (28, 29). To examine whether the
5
1 integrin engagement modulates insulin receptor and IRS-1 tyrosine phosphorylation in response to insulin, we
performed experiments in which serum-starved CHO-T cells were detached
from the culture dish, held in suspension for 30 min at 37 °C,
treated with or without 100 nM insulin for 10 min, and then
either kept in suspension or plated onto fibronectin-coated plates for
20 min. As shown in Fig. 4A
and B, CHO-T cell attachment to fibronectin markedly
increased tyrosine phosphorylation of the 125-kDa protein that
corresponds to FAK as well as 68-kDa proteins. Surprisingly, cell
attachment to fibronectin also markedly increased insulin receptor and
IRS-1 tyrosine phosphorylation under basal conditions and in response
to insulin by 2-3-fold (Fig. 4, A and B). Fig. 4
also shows that insulin treatment of these cells caused
dephosphorylation of FAK, consistent with previous reports that insulin
induces FAK tyrosine dephosphorylation (17-19).
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5
1 integrin engagement
potentiates signaling by insulin at submaximal doses, the effect of
different concentrations of insulin on insulin receptor and IRS-1
tyrosine phosphorylation was assessed in cells held in suspension or
allowed to attach onto fibronectin (Fig.
5). CHO-T cell attachment onto a
fibronectin matrix markedly potentiated the ability of insulin to
enhance tyrosine phosphorylation of the insulin receptor at all
concentrations employed. Similar to this effect observed on insulin
receptor phosphorylation, IRS-1 tyrosine phosphorylation in response to
all doses of insulin tested was enhanced by CHO-T cell attachment onto
fibronectin (Fig. 6A).
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5
1 Integrin Engagement Potentiates
Insulin Stimulation of IRS-1-associated PI-3-Kinase Activity--
The
increase in insulin-stimulated tyrosine phosphorylation of IRS-1 by
CHO-T cell adhesion onto fibronectin suggests increased binding of
signaling proteins such as PI 3-kinase to IRS-1 may be elicited (1-4,
7). Thus, we tested the effect of
5
1
integrin engagement on the association of the p85 subunit of PI
3-kinase with IRS-1. As seen in Fig. 6A, CHO-T cell adhesion
onto fibronectin promotes the increased association of p85 with IRS-1
at both submaximal and maximal concentrations of insulin.
5
1 integrin receptor was associated with a significant, severalfold increase in basal CHO-T cell
PI 3-kinase activity detected in IRS-1 immunoprecipitates (Fig. 6,
B and C). Elevated PI 3-kinase activity
associated with IRS-1 was also observed when submaximal or maximal
doses of insulin were incubated with adherent cells compared with
nonadherent cells. These data are consistent with the hypothesis that
activation of
5
1 integrin through cell
adhesion onto fibronectin causes enhanced recruitment of p85/p110-type
PI 3-kinases to IRS-1, resulting in enhanced catalytic activity of
these enzymes.
The findings presented here demonstrating the potentiation of insulin
receptor modulation of IRS-1 by
5
1
integrin engagement suggests the concommittant enhancement of signaling
events downstream of IRS-1. One function proposed to be regulated by
IRS-1/PI 3-kinase signaling complexes is cell proliferation (41), and
recent evidence indeed indicates that
V
3 integrin-mediated cell
adhesion enhances this insulin action (31). As PI 3-kinase activation
appears to also be required for the metabolic actions of insulin
(8-10, 13), it will be important in future studies to determine
whether integrin ligation may influence such insulin effects. The
present work shows for the first time that insulin stimulates cell
adhesion, and this effect is dependent upon PI 3-kinase activity (Fig.
3). Cell adhesion is also enhanced upon ligation and clustering of integrins on the cell surface. Thus, our findings are consistent with
the hypothesis that potentiation of the insulin signaling pathway
through IRS-1/PI 3-kinase by integrins reflects an indirect mechanism
to further enhance adhesion over that caused directly by the integrins.
Testing this hypothesis will require determining whether IRS-1/PI
3-kinase complexes formed in response to integrin activation actually
mediate increased cell adhesion. Such studies will also be important in
ultimately probing the full physiological implications of insulin
regulation of cell adhesion.
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ACKNOWLEDGEMENTS |
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We thank Dr. Rudolph Juliano for a gift of
anti-
5
1 integrin receptor
monoclonal blocking antibody and Jane Erickson for expert
assistance in the preparation of this manuscript.
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FOOTNOTES |
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* This work was supported by Grant DK 30648 (to M. P. C.) from the National Institutes of Health.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.
To whom correspondence should be addressed. Tel.: 508-856-2254;
Fax: 508-856-1617; E-mail: Michael.Czech{at}ummed.edu.
The abbreviations used are: IRS, insulin receptor substrate; PI 3-kinase, phosphoinositide 3-kinase; FAK, focal adhesion kinase, PI, phosphatidylinositol; CHO, Chinese hamster ovary; BSA, bovine serum albumin; PBS, phosphate-buffered saline; PAGE, polyacrylamide gel electrophoresis; TLC, thin layer chromatography; PDGF, platelet-derived growth factor; PKB, protein kinase B.
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