From the Department of Biological Sciences and the
Karmanos Cancer Institute, Wayne State University,
Detroit, Michigan 48202 and the Section on Molecular Biology, Joslin
Diabetes Center, Harvard Medical School,
Boston, Massachusetts 02215
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ABSTRACT |
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Insulin receptor (IR) and the related insulin-like growth factor-I (IGF-I) receptor (IGF-IR) mediate a variety of metabolic and mitogenic cellular responses, some of which may involve unidentified receptor targets. A Src homology-2 (SH2) domain-coding region of a mouse protein was cloned based on its interaction with IR. It was designated mSH2-B based on its high similarity to an earlier reported rat sequence SH2-B. A role of mSH2-B in IGF-I and insulin action was suggested by the interaction of the SH2 domain with activated IGF-IR and IR catalytic fragments but not with an inactive IR catalytic fragment in the yeast two-hybrid system in vivo and by the hormone-dependent association of a glutathione S-transferase (GST) SH2 domain fusion protein of mSH2-B with both receptors in cell extracts. A comparison of IGF-IR and IR mutants lacking individual Tyr autophosphorylation sites for association with GST mSH2-B showed that homologous juxtamembrane (IR960/IGF-IR950) and C-terminal (IR1322/IGF-IR1316) receptor motifs were required. Synthetic phosphopeptides representing IR960 and IR1322 competed for GST mSH2-B binding to the receptor, suggesting that both motifs participate in the association with mSH2-B. Antibodies raised against GST mSH2-B identified a cellular protein of 92 kDa that was not found to be phosphorylated on Tyr. It co-immunoprecipitated with IGF-IR or IR, which was strictly dependent on receptor activation. IR and IGF-IR Tyr phosphorylation motifs were not identified in the complete SH2-B primary structure, suggesting that it may participate as an adapter rather than a substrate in the IGF-I and insulin signaling pathways.
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INTRODUCTION |
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Insulin-like growth factor-I (IGF-I)1 has been implicated particularly in mitogenic functions and of insulin more in metabolic cellular functions (1, 2). The insulin receptor (IR) and the closely related IGF-I receptor (IGF-IR) have served as a model system for the elucidation of receptor Tyr kinase-mediated signaling mechanisms (3). The nature for the physiologic differences between the IGF-I and insulin signals is unclear and may involve cellular targets such as Grb10 and pp120 that exhibit a preference for one of the receptors (4, 5). Major cellular targets that are shared by both receptors include the insulin receptor substrates IRS-1, IRS-2, IRS-3, and Shc, most of which also play established roles in other signaling pathways (6-10).
Some of the metabolic responses to insulin must be mediated by
mechanisms that parallel PI 3-kinase, which suggests a role of an
undefined alternative IR signaling pathway (11). A role of IRS-1 and PI
3
-kinase has been proposed in the insulin-stimulated glucose uptake by
GLUT4 glucose transporter translocation (12, 13); however, an IRS-1-
and PI 3
-kinase-independent, unidentified pathway has been described
in the insulin stimulation of glucose uptake in independent studies
(14, 15). Insulin activates glycogen synthase in CHO cells by an
unidentified, Ras- and PI 3
-kinase-independent
(wortmannin-independent) mechanism (16). Despite normal Ras binding to
Raf-1, truncation of the IR C terminus impairs Raf-1, mitogen-activated
protein kinase kinase, and MAPK activities, glucose transport, glycogen
synthesis, PI 3
-kinase, and phosphoprotein phosphatase-1 activities,
whereas mitogenic responses remain largely unimpaired (17, 18).
Overexpression of IRS-1 was shown to restore the mitogenic response of
a defective IR (Y960A, which fails to activate IRS-1) including
activation of PI 3
-kinase but restored only partial glycogen synthesis
and failed to restore MAPK activation (19). IR mutants in the tyrosine kinase region (R1174Q and L1178P) suggest a role of unidentified signaling mediators in the impaired glycogen synthesis, DNA synthesis, and MAPK activation in response to insulin in CHO cells, because IRS-1
appears to be normally activated (20). Mitogen-activated protein kinase
and PI 3
-kinase signaling pathways are not sufficient for
IGF-I-induced mitogenesis and tumorigenesis, suggesting a role of
unidentified signaling mediators in these important IGF-I actions (21).
Such mediators may act in parallel to IRS-1 and IRS-2 and may resemble
other adapters known to associate with other receptor tyrosine kinases
(22). In this study we have characterized the association of a newly
identified protein target of 92 kDa, designated mSH2-B, with IGF-IR and
IR that is strictly dependent on ligand stimulation for both receptors
and was found in a yeast two-hybrid screen (23). We find that the SH2
domain of mSH2-B as well as the juxtamembrane regions and the
C-terminal regions of both receptors are required for the association
and that synthetic peptides representing these receptor motifs compete with receptor binding to mSH2-B. These characteristics suggest a role
of mSH2-B as a Pro-rich and SH2 domain-containing mediator in IGF-I and
insulin action.
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EXPERIMENTAL PROCEDURES |
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Antibodies and Phosphopeptides--
Rabbit polyclonal antibodies
directed against the IGF-IR or IR subunit were obtained from
Upstate Biotechnology, Inc., monoclonal anti-phospho Tyr antibody PY20
was from Transduction Laboratories, and horseradish peroxidase-coupled
anti-IgG antibody was from Kirkegaard and Perry Laboratories. mSH2-B
antiserum was produced by HRP Inc. (Denver, PA) in rabbits against a
GST fusion protein containing the SH2 domain of mSH2-B (GST mSH2-B).
Phosphopeptides representing pY953 (LpYASSNPEYLSASDV), pY960
(SSNPEpYLSASD), pY1146 (DIpYETDYYRKG), pY1150 (DIYETDpYYRKG), pY1316
(KRSpYEEHIPY), or pY1322 (HIPpYTHMNGG) were incubated with GST mSH2-B
during the immunoprecipition step as described below (24).
Yeast Two-hybrid Screen and Interaction Specificity--
Yeast
two-hybrid plasmid pGBT9 (25) containing the C terminus of the IR subunit (26) was used as a bait to screen a 9.5-10.5-day post
conception mouse embryo cDNA library in plasmid pVP16 (27). Library
(prey) plasmids were isolated from His prototroph and
-galactosidase-positive yeast colonies (24). Plasmids were introduced into Escherichia coli strain DH5
, sequenced,
and re-introduced into Saccharomyces cerevisiae strain L40.
Mating with strains (AMR70) carrying various test baits followed by the
-galactosidase color assay evaluated the specificity of the
underlying bait-prey interactions (27).
GST Fusion Protein--
The cloned cDNA insert encoding the
SH2 domain of mSH2-B and surrounding coding sequences was released from
the pVP16 library plasmid at its NotI cloning site and
inserted into the NotI cloning site of pGEX-4T-1 (Pharmacia
Biotech Inc.). GST mSH2-B fusion and control GST protein were expressed
in E. coli strain DH5 and purified on a
glutathione-agarose column (Pharmacia), eluted in 10 mM
reduced glutathione in 50 mM Tris-HCl, pH 8.0, and stored after addition of 10 mM dithiothreitol, 1 mM
EDTA as described by the manufacturer.
Cell Culture, Immunoprecipitation, and
Immunoblotting--
Subconfluent wild-type or mutant IR overexpressing
CHO fibroblasts (kindly provided by Drs. Yousuke Ebina, Takashi
Kadowaki, and Morris F. White) or wild-type or mutant IGF-IR
overexpressing R fibroblasts (kindly provided by Dr.
Renato Baserga) were employed in various experiments (28-33). Cells
were incubated in serum-free medium for 16 h and stimulated with
100 ng/ml insulin or 100 ng/ml IGF-I for 15 min. Cells were washed
twice with phosphate-buffered saline and harvested in ice-cold lysis
buffer containing 50 mM HEPES, pH 7.4, 1% Triton X-100,
10% glycerol, 137 mM NaCl, 2 mM EDTA, 10 mM NaF, 100 mM Na3VO4,
10 mM sodium pyrophosphate, 10 µg/ml leupeptin, 10 µg/ml aprotinin, and 1 mM phenylmethylsulfonyl fluoride.
Proteins were directly subjected to SDS-PAGE or were first mixed with
GST mSH2-B fusion protein or antibodies directed against mSH2-B, IR, or
IGF-IR and co-precipitated with glutathione-agarose or protein
A-Sepharose beads, respectively. Precipitates were washed with lysis
buffer, separated by SDS-PAGE, and analyzed by immunoblotting with
specific antibodies using the ECL (Amersham Corp.) detection system
(24).
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RESULTS AND DISCUSSION |
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Two-hybrid Analysis of IGF-IR and IR Association with the SH2
Domain of mSH2-B in Vivo--
We employed the cytoplasmic domains of
the IR and IGF-IR as baits in a yeast two-hybrid screen to identify
unknown signaling mediators in both receptor pathways from mouse
libraries (24, 4). Most identified sequences represented SH2 domains of
known IR and IGF-IR targets (Fig. 1)
including the C-terminal SH2 domain of the regulatory subunit p85 of PI
3-kinase, the SH2 domain of the signaling mediator Grb10/IR, and the
SH2 domain of the transforming protein Vav (4, 34-39). In addition, we
identified an SH2 domain-encoding sequence with the two-hybrid system
(23) that was scanned against the sequence data base. It was designated mSH2-B because one sequence with high similarity was identified that
had earlier been reported in rat as SH2-B (40). SH2-B (GenBankTM data
base accession number U57391) had been found based on its association
with the Fc epsilon RI receptor in a modified yeast two-hybrid screen,
and its complete protein-coding region was isolated and shown to
contain several Pro-rich sequences (40). The C-terminal fragment
encoding the SH2 domain and adjacent sequences of mSH2-B that was
identified in our two-hybrid screen displayed only few nucleotide
replacements C1900T, C1918T, G1945A, C1981T, A1982G, A2023G, T2065C,
T2077C, T2098A, T2257C, and A2261G, which resulted in a single aa
change S547G when compared with the rat SH2-B sequence, which is
represented by the first letter and its position number (40). These
changes are likely explained by species-specific differences. A second
related human sequence (GSDB, DDBJ, EMBL, and NCBI data base accession
number AB000520) was identified that encodes a putative adapter
designated APS that appears to represent a related but distinct gene
based on a considerable number of aa replacements when compared with
mSH2-B and rat SH2-B (41).
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Biochemical Analysis of IGF-IR and IR Association with the SH2 Domain of mSH2-B in Vitro-- We investigated this association biochemically by testing a GST mSH2-B fusion protein for its association with IGF-IR and IR in cell extracts. A GST mSH2-B fusion construct was prepared, and the protein was expressed in and partially purified from E. coli and mixed with detergent extracts of IGF-IR or IR overexpressing cells. IGF-IR and IR co-precipitated with glutathione-Sepharose beads directed against GST as detected in immunoblots with receptor-specific antibodies (Fig. 2). The association was strictly dependent on receptor activation by the respective ligand. Similarly, the association of the SH2 domain of mSH2-B in the two-hybrid screen had been observed with an active but not with an inactivated IR cytoplasmic domain under in vivo conditions in the yeast nucleus. If the observed associations were indirect the same adaptor protein would need to be present and active in the yeast nucleus and in mammalian detergent cell extracts under very different experimental conditions. Because this is unlikely our data suggest that mSH2-B binds directly to the receptors without the involvement of an intermediate protein.
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Analysis of Receptor Mutant and Phosphopeptide Association with the SH2 Domain of mSH2-B-- Various IR and IGF-IR mutants in the juxtamembrane or C-terminal receptor regions were employed to test their impact on the association with the SH2 domain of mSH2-B. Cell lines overexpressing individual mutants were ligand-stimulated, and detergent cell extracts were mixed with GST mSH2-B. Precipitation with glutathione-Sepharose failed to precipitate juxtamembrane mutants IGF-IR Y950F and IR Y960F as well as C-terminal mutants IGF-IR Y1316F, IR Y1322C, IR Y1316F/Y1322F (YF2), or dCT lacking 43 aa of the IR C terminus (Fig. 3, A and B). To test whether these motifs are in fact able to bind to mSH2-B, synthetic phosphopeptides representing these regions were tested for competition with normal IR binding to GST mSH2-B. In accordance with the receptor mutant analysis, peptides representing Tyr960 and Tyr1322 abolished IR binding in contrast to peptides representing other receptor regions (Fig. 3C). These data suggest that homologous sites of both receptors at IGF-IR Tyr950/IR Tyr960 in the juxtamembrane region and at IGF-IR Tyr1316/IR Tyr1322 at the C terminus are involved in the interaction with the SH2 domain of mSH2-B.
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Ligand-dependent Association of Cellular SH2-B with IGF-IR and IR-- To learn more about the cellular SH2-B protein, a rabbit antiserum was raised against GST mSH2-B that identified a 92-kDa protein in immunoblots of CHO and mouse fibroblast cell extracts (Fig. 4, A and B). To evaluate the association of this protein with the IR and IGF-IR, fibroblasts overexpressing either receptor were ligand stimulated, and detergent cell extracts were immunoprecipitated with receptor-specific antibodies followed by SDS-PAGE and immunoblotting with mSH2-B antiserum. A 92-kDa protein of identical migration was specifically co-precipitated with either receptor, strictly dependent on ligand activation (Fig. 4, A and B). When cell extracts were precipitated with mSH2-B antiserum in complementary experiments, IR (Fig. 4A) and IGF-IR (Fig. 4B) were co-precipitated from the respective cell lysates in a ligand-dependent fashion. Our data indicate that mSH2-B associates with IR and IGF-IR upon receptor activation in three independent approaches based on 1) two-hybrid constructs, 2) GST mSH2-B fusion protein, and 3) authentic cellular SH2-B. These findings strongly suggest that mSH2-B plays an as yet unidentified role in insulin and IGF-I action as a direct receptor target.
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ACKNOWLEDGEMENTS |
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We are grateful to Dr. Heping Dai for expert help with the preparation of mSH2-B antiserum, Dr. Youhou Kang for expert technical support in sequencing and SH2 domain association experiments, Dr. Nasim Yousaf for critical support in the characterization of the mSH2-B sequence and of cellular SH2-B, and Dr. Yuyuan Zhao for contributing to the sequence analysis. We thank Drs. Hans Hansen, Jianwei Zhu, and Gert Wolf for reagents, advice, and continued support, Drs. Renato Baserga, Yosuke Ebina, Takashi Kadowaki, and Masato Kasuga for cell lines, and Dr. Nora Riedel for critical discussion of the data.
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FOOTNOTES |
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* This work was supported in part by National Science Foundation Grants MCB-9316997 and MBC-9696090, by a Virtual Discovery Grant of the Karmanos Cancer Institute, and by Juvenile Diabetes Foundation International Research Grant Award 197048 (to H. R.).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 nucleotide sequence(s) reported in this paper has been submitted to the GenBankTM/EMBL Data Bank with accession number(s) AF036355.
§ To whom correspondence should be addressed: Dept. of Biological Sciences, 2171 BSB, Wayne State University, 5047 Gullen Mall, Detroit, MI 48202-3917. Tel.: 313-577-7870; Fax: 313-577-6891; E-mail: hriedel{at}sun.science.wayne.edu.
1 The abbreviations used are: IGF-I, insulin-like growth factor-I; GST, glutathione S-transferase; IGF-IR, IGF-I receptor; IR, insulin receptor; IRS, insulin receptor substrate; PAGE, polyacrylamide gel electrophoresis; PI, phosphatidylinositol; SH2, Src homology-2; CHO, Chinese hamster ovary; aa, amino acid; MAPK, mitogen-activated protein kinase.
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REFERENCES |
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