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J. Biol. Chem., Vol. 278, Issue 29, 26323-26326, July 18, 2003
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ACCELERATED PUBLICATIONS




¶
From the
Cancer Research UK Institute for Cancer
Studies and
School of Biosciences, The
University of Birmingham, Edgbaston, Birmingham B15 2TA, United Kingdom
Received for publication, May 16, 2003 , and in revised form, May 29, 2003.
| ABSTRACT |
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| INTRODUCTION |
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A number of integrin-dependent signaling pathways have been linked to cell motility. These include: association with, and activation of, receptor tyrosine kinases (9); activation of Src-family non-receptor tyrosine kinases (10); and activation of focal adhesion kinase (FAK)1 (11). All of these signaling events, in turn, trigger a network of parallel and intersecting downstream signaling reactions (12, 13). Proteins of the Ras family of small GTPases take one of the central positions in this signaling map, by regulating the activities of Erk1/2 and Rac1 (13). It has been demonstrated that several pathways could be responsible for the adhesion-dependent activation of Ras proteins. Two of them (i.e. the assembly of the phospho-FAK-(Tyr925)-Grb2-Sos and phospho-Shc-Grb2-Sos complexes) require the upstream activation of Src kinases (14, 15). In addition, it has been reported that cell-ECM adhesion triggers the assembly of the p120RasGAP-p62Dok complex, an event that suppresses the activity of RasGAP, which, consequently, leads to the increase in active, GTP-bound, Ras in cells (16).
Recently, it has been shown that the tetraspanin CD151 is involved in regulation of migration of neutrophils (17), endothelial cells (18), and various tumor cell lines (19, 20). Furthermore, it appeared that CD151 could manifest its activity only on the FAK-positive cellular background, suggesting a role for FAK in CD151-dependent migration (19). However, no biochemical signaling events have been ascribed to the activity of CD151.
In this report we investigated in detail how expression of CD151 affects integrin-dependent signaling in non-transformed fibroblasts. We show that CD151 specifically attenuates adhesion-dependent activation of Ras and the corresponding downstream activation of ERK1/2 and PKB/c-Akt. Furthermore, we establish that this attenuation arises from the marked effect of CD151 on the inactivation of Ras proteins upon cell detachment. Finally, detailed analysis of upstream signaling events indicates that CD151 affects the adhesion-dependent Ras activation via a novel mechanism.
| MATERIALS AND METHODS |
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C and
Rat-1/CD151
N cell lines were generated by transfecting
pZeoSV/CD151
C and pZeoSV/CD151
N plasmids, respectively, into
Rat-1 cells. Zeocin-resistant colonies in each transfection experiment were
pooled together
(2030
individual colonies) and further selected in two cycles of cell sorting using
a mixture of anti-CD151 mAbs, 5C11
(22) and 11G1B4 (kindly
provided by Dr. L. Ashman). Mouse mAbs against FAK, Ras, Rac1, and p120RasGAP
were purchased from BD Biosciences. Mouse mAb to RhoA and rabbit polyclonal Ab
to Cdc42 were from Santa Cruz Biotechnology. Rabbit anti-phospho specific Abs
to FAK and rabbit anti-Src polyclonal serum were from BioSource International.
Rabbit anti-FAK Ab was purchased from Autogen Bioclear. All antibodies to PKB,
anti-pTyr mAb, and anti-pSrc416 were purchased from Cell Signaling
Technology. Mouse anti-Src mAb, GD11, was purchased from Upstate
Biotechnology. All other reagents were purchased from Sigma.
Construction of CD151 MutantsThe CD151
N and
CD151
C mutants were engineered by a standard PCR protocol on the
pZeoCD151 template. The N-terminal Gly2Leu13 and
the C-terminal Ser248Tyr253 regions of human
CD151 were substituted for the hemagglutinin tag sequence (YPYDVPDYA) to
generate CD151
N and CD151
C fragments, respectively. The PCR
fragments were subcloned (HindIII-EcoRI) into the pZeoSV
plasmid.
Small G-protein Pull-down AssaysCells (24 x 106) were washed twice with PBS and then scraped into RIPA buffer (50 mM Tris, pH 7.2, 1% Triton X-100, 0.5% sodium deoxycholate, 0.1% SDS, 500 mM NaCl, 10 mM MgCl2, 10 µg/ml leupeptin, 10 µg/ml aprotinin, 1 mM phenylmethylsulfonyl fluoride). Lysates were subsequently incubated with the appropriate GST-RBD beads for 45min at 4 °C. After the washes with 50 mM Tris, pH 7.2, 1% Triton X-100, 150 mM NaCl 10 mM MgCl2, 10 µg/ml leupeptin, 10 µg/ml aprotinin, 0.1 mM phenylmethylsulfonyl fluoride, the captured material was eluted into Laemmli buffer and resolved in 11% SDS-PAGE. The proteins were transferred to the nitrocellulose membrane and probed with the appropriate Ab. The pGEX-2TK-Rhotekin(RBD), pGEX-2TK-PAK(crib), and pGEX-2TK-Raf1(RBD) plasmids were kindly provided by Dr. M. Schwartz, Dr. J. Collard and Dr. R. Marais, respectively.
ImmunoprecipitationProteins were solubilized into 1% Triton X-100/PBS supplemented with 2 mM phenylmethylsulfonyl fluoride, 10 µg/ml aprotinin, 10 µg/ml leupeptin for 216 h at 4 °C. The insoluble material was pelleted at 12,000 rpm for 10 min, and protein lysates were precleared by incubation for 4 h at 4 °C with agarose-conjugated goat anti-mouse antibodies. Immune complexes were collected on the agarose beads prebound with mAbs, followed by four successive washes with the immunoprecipitation buffer. Immune complexes were eluted from the beads with Laemmli sample buffer, and proteins were resolved in 810% SDS-PAGE.
Analysis of Activation of FAK, ERK1/2, and PKB/c-AktAdhesion-dependent activation of FAK, ERK1/2, and PKB/c-Akt was analyzed as described previously (21).
Analysis of Activation of c-SrcSerum-starved cells plated on the laminin-5 matrix were scraped into Triton X-100-based lysis buffer, and the c-Src complexes were immunoprecipitated using GD11 mAb. Equal aliquots of the immunoprecipitated proteins were resolved in 8% SDS-PAGE, transferred to the nitrocellulose membrane, and probed either with the anti-Tyr416-Src Ab or with the polyclonal Ab recognizing total proteins.
| RESULTS AND DISCUSSION |
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3
1, which is a principal integrin partner
for CD151 in Rat-1/CD151 cells. The ectopic expression of CD151 in Rat-1 cells
did not affect adhesion to, and spreading on, LN5M
(Fig. 1, upper
panels). Furthermore, the number and distribution of vinculin-containing
adhesion complexes were similar in Rat-1 and Rat-1/CD151 cells
(Fig. 1, lower
panels). As illustrated in Fig.
2, the expression of CD151 had a negative effect on the kinetics
and amplitude of adhesion-dependent activation of ERK1/2 and PKB/c-Akt
(Fig. 2, A and
B, lower panels, compare lanes
24 with 68). Specifically, we found that, in
control Rat-1 cells, the level of ERK phosphorylation reached the plateau at
10 min (the earliest time point analyzed). By contrast it required 40 min for
the pERK1/2 to reach their maximum in Rat-1/CD151 cells
(Fig. 2A, lower
panel, compare lanes 25 with 710). On the
other hand, adhesion of Rat-1 and Rat-1/CD151 cells to LN5M induced comparable
increases in tyrosine phosphorylation of Tyr397-FAK and
Tyr416-c-Src (Fig. 2, A
and B, top panels). The differences in the
adhesion-dependent activation of ERK1/2 were observed when the cells were
plated on other ECM ligands (e.g. fibronectin, laminin-10/11)
(Fig. 2C, compare
lanes 25 with 710). In contrast, the kinetics
and degree of ERK1/2 activation were comparable when serum-starved Rat-1 and
Rat-1/CD151 cells were stimulated with EGF
(Fig. 2D). These
results indicated that CD151 specifically attenuated adhesion-dependent
activation of ERK1/2 and PKB/c-Akt but had no effect on the EGF-induced
signaling.
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Small GTP-binding proteins of the Ras family are the best characterized
activators of ERK1/2 and PI3K in integrin-dependent signaling
(24,
25). Hence, we analyzed the
role of CD151 in adhesion-dependent activation of Ras. As illustrated in
Fig. 3A cell
attachment increases the levels of GTP-loaded Ras in both Rat-1 and
Rat-1/CD151 cells. However, the level of GTP-Ras in Rat-1 cells was higher
than that in Rat-1/CD151 cells (up to
1.8-fold at 20 min after plating to
LN5M). On the other hand, LN5M-dependent modulation of Rho-proteins was
similar for Rat-1 and Rat-1/CD151 cells (results are not shown). It has been
previously reported that integrin-dependent activation of Ras may involve two
signaling pathways: one requires Fyn-dependent phosphorylation of Shc and
subsequent recruitment of the Grb2-Sos1 complex
(15), and another relies on
Src-dependent phosphorylation of FAK on tyrosine 925 followed by assembly of
the FAK-Grb2-Sos1 signaling complex
(14). Although phosphorylation
of Tyr925-FAK was induced upon cell adhesion, no apparent
differences were observed between Rat-1 and Rat-1/CD151 cells
(Fig. 3B, lower
panel). Furthermore, we were unable to detect adhesion-induced
phosphorylation of Shc above the existing baseline in both cell lines
(Fig. 3B, upper
panel). Therefore, it is highly unlikely that CD151-induced differences
in the adhesion-dependent activation of Ras linked to signaling through Fak or
Shc. Rather, we propose that CD151 affects Ras activation through a novel
pathway.
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We propose that the attenuation of the adhesion-dependent activation of Ras is reflective of the lower level of the GTP-Ras complexes in the detached Rat-1/CD151 cells (Fig. 3A, lanes 1 and 4). Indeed, in separate experiments we found that detachment of serum-starved Rat-1/CD151 cells induced a greater decrease in the amount of the precipitated GTP-Ras complexes than detachment of Rat-1 cells (Fig. 4A, lanes 2 and 4). The dissociation of the p120RasGAP/p62-Dok complex is thought to be a critical event in the detachment-induced inactivation of Ras proteins (16). However, while the p120RasGAP/p62-Dok complex dissociated upon detachment of both Rat-1 and Rat-1/CD151 cells, no quantitative differences were observed between the cell lines (Fig. 4B, lanes 2 and 4). These results suggest that the effect of CD151 on inactivation of Ras involves an alternative mechanism(s).
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To determine which region(s) of CD151 is responsible for the adhesion-dependent regulation of Ras activity, we carried out cell detachment experiments using the Rat-1 cells expressing various CD151 mutants at the levels similar to that of the wild-type protein. As illustrated in Fig. 5A the deletion of the N-terminal cytoplasmic portion of CD151 did not affect significantly the "Ras-inactivating function" of the tetraspanin. On the other hand, deletion of the C-terminal cytoplasmic portion completely abolished the detachment-dependent activity of CD151 toward Ras (Fig. 5B, lanes 2, 4, and 6). These data indicate that the C-terminal end is essential for the Ras-inactivating function of CD151.
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Activation of Ras proteins is one of the central events in the
attachment-dependent signaling mediated by integrins
(12,
26). Here we demonstrate that
the tetraspanin CD151 plays an important role in this process. Specifically,
we identified CD151 as a new negative regulator of Ras proteins, which
potentiates the decrease in the amount of the GTP-Ras complex in cells upon
detachment. Consequently, adhesion-dependent accumulation of GTP-Ras and
subsequent activation of ERK1/2 and PKB is attenuated in cells expressing
CD151. Importantly, the effect of CD151 on the adhesion-dependent signaling is
specific. Indeed, activation of FAK and c-Src and modulation of the activity
of Rho GTPases were comparable in the Rat-1 and Rat-1/CD151 cells.
Importantly, CD151 exhibits its activity toward Ras only when integrins are
disengaged from their ligands. This function of CD151 may be particularly
relevant for cell migration. In migrating cells the dynamic
adhesion-deadhesion cycles are taking place at the front edge of extending
lamellae. Thus, it is conceivable that CD151 facilitates the GTP·Ras
GDP·Ras cycle and specifically affects this step of the
migratory process.
The molecular mechanisms that link integrin disengagement with the
"Ras-inactivating" function of CD151 remain to be established.
CD151 is directly associated with
3
1 and
6
1 integrins through its large
extracellular loop (27,
28). Furthermore, the
conformation of LECL CD151 can be influenced by these interactions
(29). In addition, CD151 is
indirectly associated with other integrins within the tetraspanin-enriched
microdomains (30). Thus, it is
possible that the initial events in signal transduction involve lateral
transmission of conformational changes from the disengaged integrins to CD151.
Subsequently, the signal may be transmitted allosterically to the cell
interior and further involve the C-terminal cytoplasmic part of CD151.
Our results indicate that detachment-induced disassembly of the RasGAP-Dok complex, the only known pathway that regulates decrease in the amount of the GTP-Ras complex in suspended cells, is not affected by CD151. This suggests that the tetraspanin operates through a novel mechanism. This may involve CD151-dependent membrane recruitment and/or compartmentalization of direct regulators of Ras proteins (e.g. RasGAP proteins of the GAP1 family, Ras guanine nucleotide exchange factors (31, 32)). In this regard, palmitoylation of CD151 and its association with phosphatidylinositol 4-kinase may play an important role in this process.
Future study should establish whether function of other "pro-" or "anti-migratory" tetraspanins (e.g. CD82 and CO-029) also linked to the Ras pathway. CD151 is associated with various other tetraspanins, each of which may recruit a unique subset of regulatory proteins into integrin complexes (2, 5). Hence, by changing a tetraspanin balance in the integrin/CD151 microdomains it may be possible to amplify, negate, or even reverse the effect of CD151 in the adhesion-dependent activation of Ras.
| FOOTNOTES |
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¶ To whom correspondence should be addressed: Cancer Research UK Inst. for Cancer Studies, The University of Birmingham, Egdbaston, Birmingham B15 2TA, UK. Tel.: 44-121-414-2801; Fax: 44-121-414-4486; E-mail: f.berditchevski{at}bham.ac.uk.
1 The abbreviations used are: FAK, focal adhesion kinase; ECM, extracellular
matrix; Ab, antibody; mAb, monoclonal antibody. ![]()
2 S. Sawada, M. Yoshimoto, E. Odintsova, N. A. Hotchin, and F. Berditchevski,
unpublished results. ![]()
| ACKNOWLEDGMENTS |
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| REFERENCES |
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