IRS-1 activates phosphatidylinositol 3'-kinase by associating with src homology 2 domains of p85

Role of IRS-1-GRB-2 complexes in insulin signaling

GRB-2 is a small SH2- and SH3 domain-containing adapter protein that associates with the mammalian SOS homolog to regulate p21ras during growth factor signaling. During insulin stimulation, GRB-2 binds to the phosphorylated Y895VNI motif of IRS-1. Substitution of Tyr-895 with phenylalanine (IRS-1F-895) prevented the IRS-1-GRB-2 association in vivo and in vitro. The myeloid progenitor cell line, 32-D, is insensitive to insulin because it contains few insulin receptors and no IRS-1. Coexpression of IRS-1 or IRS-1F-895 with the insulin receptor was required for insulin-stimulated mitogenesis in 32-D cells, while expression of the insulin receptor alone was sufficient to mediate insulin-stimulated tyrosine phosphorylation of Shc and activation of p21ras and mitogen-activated protein (MAP) kinase. The Shc-GRB-2 complex formed during insulin stimulation is a possible mediator of p21ras and MAP kinase activation in IRS-1-deficient 32-D cells. Interestingly, IRS-1, but not IRS-1F-895, enhanced the stimulation of MAP kinase by insulin in 32-D cells expressing insulin receptors. Thus, IRS-1 contributes to the stimulation of MAP kinase by insulin, probably through formation of the IRS-1-GRB-2 complex at Tyr-895. Our results suggest that the Shc-GRB-2 complex and the activation of p21ras-dependent signaling pathways, including MAP kinase, are insufficient for insulin-stimulated mitogenesis and that the essential function(s) of IRS-1 in proliferative signaling is largely unrelated to IRS-1-GRB-2 complex formation.

Role of IRS-1-GRB-2 complexes in insulin signaling

GRB-2 is a small SH2- and SH3 domain-containing adapter protein that associates with the mammalian SOS homolog to regulate p21ras during growth factor signaling. During insulin stimulation, GRB-2 binds to the phosphorylated Y895VNI motif of IRS-1. Substitution of Tyr-895 with phenylalanine (IRS-1F-895) prevented the IRS-1-GRB-2 association in vivo and in vitro. The myeloid progenitor cell line, 32-D, is insensitive to insulin because it contains few insulin receptors and no IRS-1. Coexpression of IRS-1 or IRS-1F-895 with the insulin receptor was required for insulin-stimulated mitogenesis in 32-D cells, while expression of the insulin receptor alone was sufficient to mediate insulin-stimulated tyrosine phosphorylation of Shc and activation of p21ras and mitogen-activated protein (MAP) kinase. The Shc-GRB-2 complex formed during insulin stimulation is a possible mediator of p21ras and MAP kinase activation in IRS-1-deficient 32-D cells. Interestingly, IRS-1, but not IRS-1F-895, enhanced the stimulation of MAP kinase by insulin in 32-D cells expressing insulin receptors. Thus, IRS-1 contributes to the stimulation of MAP kinase by insulin, probably through formation of the IRS-1-GRB-2 complex at Tyr-895. Our results suggest that the Shc-GRB-2 complex and the activation of p21ras-dependent signaling pathways, including MAP kinase, are insufficient for insulin-stimulated mitogenesis and that the essential function(s) of IRS-1 in proliferative signaling is largely unrelated to IRS-1-GRB-2 complex formation.

Tumor necrosis factor alpha inhibits signaling from the insulin receptor

Insulin resistance is a common problem associated with infections and cancer and, most importantly, is the central component of non-insulin-dependent diabetes mellitus. We have recently shown that tumor necrosis factor (TNF) alpha is a key mediator of insulin resistance in animal models of non-insulin-dependent diabetes mellitus. Here, we investigate how TNF-alpha interferes with insulin action. Chronic exposure of adipocytes to low concentrations of TNF-alpha strongly inhibits insulin-stimulated glucose uptake. Concurrently, TNF-alpha treatment causes a moderate decrease in the insulin-stimulated autophosphorylation of the insulin receptor (IR) and a dramatic decrease in the phosphorylation of IR substrate 1, the major substrate of the IR in vivo. The IR isolated from TNF-alpha-treated cells is also defective in the ability to autophosphorylate and phosphorylate IR substrate 1 in vitro. These results show that TNF-alpha directly interferes with the signaling of insulin through its receptor and consequently blocks biological actions of insulin.

Direct association of p110 beta phosphatidylinositol 3-kinase with p85 is mediated by an N-terminal fragment of p110 beta

Phosphatidylinositol (PI) 3-kinase is a heterodimeric enzyme of 85-kDa (p85) and 110-kDa (p110) subunits implicated in mitogenic signal transduction by virtue of its activation in cells transformed by diverse viral oncoproteins and treated with various growth factors. We have identified a domain in p110 that mediates association with p85 in vitro and in intact cells. A glutathione S-transferase fusion protein containing the N-terminal 171 amino-acids of p110 beta bound to free p85 in cell lysates. This fusion protein also bound directly to p85 immobilized on nitrocellulose filters. An epitope-tagged fragment containing amino acids 31 to 150 of p110 beta associated with p85 upon expression in intact cells. Expression of either an N-terminal fragment of p110 beta or the p85 inter-SH2 domain, which mediates association with p110, reduced the association of endogenous PI 3-kinase activity with the activated platelet-derived growth factor receptor in intact cells. Hence, these defined regions of p85 and p110 mediate the interaction between the two subunits of PI 3-kinase.

Direct association of p110 beta phosphatidylinositol 3-kinase with p85 is mediated by an N-terminal fragment of p110 beta

Phosphatidylinositol (PI) 3-kinase is a heterodimeric enzyme of 85-kDa (p85) and 110-kDa (p110) subunits implicated in mitogenic signal transduction by virtue of its activation in cells transformed by diverse viral oncoproteins and treated with various growth factors. We have identified a domain in p110 that mediates association with p85 in vitro and in intact cells. A glutathione S-transferase fusion protein containing the N-terminal 171 amino-acids of p110 beta bound to free p85 in cell lysates. This fusion protein also bound directly to p85 immobilized on nitrocellulose filters. An epitope-tagged fragment containing amino acids 31 to 150 of p110 beta associated with p85 upon expression in intact cells. Expression of either an N-terminal fragment of p110 beta or the p85 inter-SH2 domain, which mediates association with p110, reduced the association of endogenous PI 3-kinase activity with the activated platelet-derived growth factor receptor in intact cells. Hence, these defined regions of p85 and p110 mediate the interaction between the two subunits of PI 3-kinase.

Autophosphorylation of the focal adhesion kinase, pp125FAK, directs SH2-dependent binding of pp60src

The phosphorylation of protein tyrosine kinases (PTKs) on tyrosine residues is a critical regulatory event that modulates catalytic activity and triggers the physical association of PTKs with Src homology 2 (SH2)-containing proteins. The integrin-linked focal adhesion kinase, pp125FAK, exhibits extracellular matrix-dependent phosphorylation on tyrosine and physically associates with two nonreceptor PTKs, pp60src and pp59fyn, via their SH2 domains. Herein, we identify Tyr-397 as the major site of tyrosine phosphorylation on pp125FAK both in vivo and in vitro. Tyrosine 397 is located at the juncture of the N-terminal and catalytic domains, a novel site for PTK autophosphorylation. Mutation of Tyr-397 to a nonphosphorylatable residue dramatically impairs the phosphorylation of pp125FAK on tyrosine in vivo and in vitro. The mutation of Tyr-397 to Phe also inhibits the formation of stable complexes with pp60src in cells expressing Src and FAK397F, suggesting that autophosphorylation of pp125FAK may regulate the association of pp125FAK with Src family kinases in vivo. The identification of Tyr-397 as a major site for FAK autophosphorylation provides one of the first examples of a cellular protein containing a high-affinity binding site for a Src family kinase SH2 domain. This finding has implications for models describing the mechanisms of action of pp125FAK, the regulation of the Src family of PTKs, and signal transduction through the integrins.

Activation of phosphatidylinositol-3' kinase by Src-family kinase SH3 binding to the p85 subunit

Engagement of antigen receptor complexes induces rapid activation of Src-family kinases and association with phosphatidylinositol-3' kinase (PI-3 kinase). Here it was found that the Src homology 3 (SH3) domain of Lyn and Fyn bound to a proline-rich region (residues 84 to 99) within the 85-kilodalton subunit (p85) of PI-3 kinase. The binding of SH3 to the purified kinase led to a five- to sevenfold increase in the specific activity of PI-3 kinase. Ligand-induced receptor stimulation activated PI-3 kinase, and this activation was blocked by a peptide containing residues 84 to 99 of p85. These data demonstrate a mechanism for PI-3 kinase activation and show that binding of SH3 domains to proline-rich target sequences can regulate enzymatic activity.

Autophosphorylation of the focal adhesion kinase, pp125FAK, directs SH2-dependent binding of pp60src

The phosphorylation of protein tyrosine kinases (PTKs) on tyrosine residues is a critical regulatory event that modulates catalytic activity and triggers the physical association of PTKs with Src homology 2 (SH2)-containing proteins. The integrin-linked focal adhesion kinase, pp125FAK, exhibits extracellular matrix-dependent phosphorylation on tyrosine and physically associates with two nonreceptor PTKs, pp60src and pp59fyn, via their SH2 domains. Herein, we identify Tyr-397 as the major site of tyrosine phosphorylation on pp125FAK both in vivo and in vitro. Tyrosine 397 is located at the juncture of the N-terminal and catalytic domains, a novel site for PTK autophosphorylation. Mutation of Tyr-397 to a nonphosphorylatable residue dramatically impairs the phosphorylation of pp125FAK on tyrosine in vivo and in vitro. The mutation of Tyr-397 to Phe also inhibits the formation of stable complexes with pp60src in cells expressing Src and FAK397F, suggesting that autophosphorylation of pp125FAK may regulate the association of pp125FAK with Src family kinases in vivo. The identification of Tyr-397 as a major site for FAK autophosphorylation provides one of the first examples of a cellular protein containing a high-affinity binding site for a Src family kinase SH2 domain. This finding has implications for models describing the mechanisms of action of pp125FAK, the regulation of the Src family of PTKs, and signal transduction through the integrins.

Activation of PI 3-kinase in 3T3-L1 adipocytes by association with insulin receptor substrate-1

Insulin treatment of adipocytes causes the rapid phosphorylation of the insulin receptor substrate-1 (IRS-1) on tyrosine. The phosphotyrosine [Tyr(P)] form of IRS-1 then complexes with the enzyme phosphatidylinositol (PI) 3-kinase. In this study, we have investigated the effect of this association on PI 3-kinase activity in 3T3-L1 adipocytes. Insulin stimulated cytosolic PI 3-kinase activity about sevenfold. This stimulation was maximal after 1 min of exposure of cells to insulin, persisted for at least 1 h, and occurred over the range of insulin concentrations that saturate its receptor. By means of immunoprecipitation of IRS-1, it was shown that virtually all of the enhanced activity was due to PI 3-kinase complexed with IRS-1. Moreover, the purified Tyr(P) form of IRS-1, either isolated from 3T3-L1 adipocytes or obtained by phosphorylation of the recombinant protein with the insulin receptor, markedly stimulated the activity of purified rat liver PI 3-kinase. These results show that the association of Tyr(P) IRS-1 with PI 3-kinase activates the enzyme and thereby can explain the elevation of PI 3,4-bisphosphate and PI 3,4,5-trisphosphate in vivo observed upon treatment of adipocytes with insulin.

Oncogenes, Protein Tyrosine Kinases, and Signal Transduction

Many oncogenes encode protein tyrosine kinases (PTKs). Oncogenic mutations of these genes invariably result in constitutive activation of these PTKs. Autophosphorylation of the PTKs and tyrosine phosphorylation of their cellular substrates are essential events for transmission of the mitogenic signal into cells. The recent discovery of the characteristic amino acid sequences, of the src homology domains 2 and 3 (SH2 and SH3), and extensive studies on proteins containing the SH2 and SH3 domains have revealed that protein tyrosine-phosphorylation of PTKs provides phosphotyrosine sites for SH2 binding and allows extracellular signals to be relayed into the nucleus through a chain of protein-protein interactions mediated by the SH2 and SH3 domains. Studies on oncogenes, PTKs and SH2/SH3-containing proteins have made a tremendous contribution to our understanding of the mechanisms for the control of cell growth, oncogenesis, and signal transduction. This review is intended to provide an outline of the most recent progress in the study of signal transduction by PTKs. Copyright 1994 S. Karger AG, Basel

The insulin receptor: structure, function, and signaling

The insulin receptor is a member of the ligand-activated receptor and tyrosine kinase family of transmembrane signaling proteins that collectively are fundamentally important regulators of cell differentiation, growth, and metabolism. The insulin receptor has a number of unique physiological and biochemical properties that distinguish it from other members of this large well-studied receptor family. The main physiological role of the insulin receptor appears to be metabolic regulation, whereas all other receptor tyrosine kinases are engaged in regulating cell growth and/or differentiation. Receptor tyrosine kinases are allosterically regulated by their cognate ligands and function as dimers. In all cases but the insulin receptor (and 2 closely related receptors), these dimers are noncovalent, but insulin receptors are covalently maintained as functional dimers by disulfide bonds. The initial response to the ligand is receptor autophosphorylation for all receptor tyrosine kinases. In most cases, this results in receptor association of effector molecules that have unique recognition domains for phosphotyrosine residues and whose binding to these results in a biological response. For the insulin receptor, this does not occur; rather, it phosphorylates a large substrate protein that, in turn, engages effector molecules. Possible reasons for these differences are discussed in this review. The chemistry of insulin is very well characterized because of possible therapeutic interventions in diabetes using insulin derivatives. This has allowed the synthesis of many insulin derivatives, and we review our recent exploitation of one such derivative to understand the biochemistry of the interaction of this ligand with the receptor and to dissect the complicated steps of ligand-induced insulin receptor autophosphorylation. We note possible future directions in the study of the insulin receptor and its intracellular signaling pathway(s).

Phosphatidylinositol-3 kinase activation induced upon Fc gamma RIIIA-ligand interaction

Induced activation of protein tyrosine kinase(s) is a central event in signal transduction mediated via the low affinity receptor for IgG (Fc gamma RIIIA, CD16) in natural killer (NK) cells. Tyrosine phosphorylation may affect the function of several protein directly, or indirectly by inducing their association with other tyrosine phosphorylated proteins. Here, we report that Fc gamma RIII stimulation induces activation of phosphatidylinositol (PI)-3 kinase in NK cells. Phosphotyrosine immunoprecipitates from Fc gamma RIII-stimulated NK cells contain PI-kinase activity and PI-3 kinase can be directly precipitated from them. Conversely, a series of tyrosine-phosphorylated proteins is coprecipitated with PI-3 kinase from the stimulated, but not from control cells. Analogous results obtained using Jurkat T cells expressing transfected Fc gamma RIIIA alpha ligand binding chain in association with gamma 2 or zeta 2 homodimers indicate that both complexes transduce this effect, although the Fc gamma RIIIA-zeta 2 complexes do so with greater efficiency. Accumulation of phosphoinositide D3 phosphorylated products in stimulated cells confirms PI-3 kinase activation, indicating the participation of this enzyme in Fc gamma RIIIA-mediated signal transduction.

Association of the insulin receptor and phosphatidylinositol 3-kinase requires a third component

We have studied the interactions between the insulin receptor and PtdIns 3-kinase by a reconstitution system in vitro composed of highly purified PtdIns 3-kinase from rat liver and highly purified insulin receptors bound to insulin-agarose or to antibodies against insulin receptors. As a positive control, receptors for platelet-derived growth factor, which bind and phosphorylate PtdIns 3-kinase, were studied in parallel with insulin receptors. Our results indicate that the insulin receptor, regardless of its phosphorylation state, does not directly associate with purified PtdIns 3-kinase, whereas the autophosphorylated receptor does associate with PtdIns 3-kinase present in the crude CHO-cell lysate. Also, we could not detect phosphorylation of PtdIns 3-kinase by the insulin receptor, even through the receptor readily underwent autophosphorylation and phosphorylated an insulin-receptor substrate, poly(Glu-Tyr) (4:1). These findings argue that one or more cytosolic components link the receptor and the enzyme. Insulin-receptor substrate-1 (IRS-1) was evaluated as a potential linking protein. In the absence of ATP, IRS-1 did not facilitate the coupling of the phosphorylated insulin receptor to PtdIns 3-kinase. Thus IRS-1 is unlikely to be the component in crude CHO-cell lysate that couples PtdIns 3-kinase to the phosphorylated insulin receptor. However, the addition of ATP, which allows phosphorylation of IRS-1 by the insulin receptor, also enhances the coupling of PtdIns 3-kinase to the insulin receptor. In support of this idea, immunoprecipitates of IRS-1 from insulin-treated CHO cells were found to contain both the insulin receptor and PtdIns 3-kinase. In conclusion, the insulin receptor does not appear to phosphorylate or bind directly to PtdIns 3-kinase, regardless of the receptor's state of phosphorylation. Association of PtdIns 3-kinase with the insulin receptor is mediated by one or more components, one of which may involve an unidentified factor in cell lysate and another that apparently involves phosphorylated IRS-1.

Pleiotropic insulin signals are engaged by multisite phosphorylation of IRS-1

IRS-1 (insulin receptor substrate 1) is a principal insulin receptor substrate that undergoes tyrosine phosphorylation during insulin stimulation. It contains over 20 potential tyrosine phosphorylation sites, and we suspect that multiple insulin signals are enabled when the activated insulin receptor kinase phosphorylates several of them. Tyrosine-phosphorylated IRS-1 binds specifically to various cellular proteins containing Src homology 2 (SH2) domains (SH2 proteins). We identified some of the tyrosine residues of IRS-1 that undergo insulin-stimulated phosphorylation by the purified insulin receptor and in intact cells during insulin stimulation. Automated sequencing and manual radiosequencing revealed the phosphorylation of tyrosine residues 460, 608, 628, 895, 939, 987, 1172, and 1222; additional sites remain to be identified. Immobilized SH2 domains from the 85-kDa regulatory subunit (p85 alpha) of the phosphatidylinositol 3'-kinase bind preferentially to tryptic phosphopeptides containing Tyr(P)-608 and Tyr(P)-939. By contrast, the SH2 domain in GRB2 and the amino-terminal SH2 domain in SHPTP2 (Syp) specifically bind to Tyr(P)-895 and Tyr(P)-1172, respectively. These results confirm the p85 alpha recognizes YMXM motifs and suggest that GRB2 prefers a phosphorylated YVNI motif, whereas SHPTP2 (Syp) binds to a phosphorylated YIDL motif. These results extend the notion that IRS-1 is a multisite docking protein that engages various downstream regulatory elements during insulin signal transmission.

Cloning of a novel, ubiquitously expressed human phosphatidylinositol 3-kinase and identification of its binding site on p85

Phosphatidylinositol 3-kinase (PI 3-kinase) has been implicated as a participant in signaling pathways regulating cell growth by virtue of its activation in response to various mitogenic stimuli. Here we describe the cloning of a novel and ubiquitously expressed human PI 3-kinase. The 4.8-kb cDNA encodes a putative translation product of 1,070 amino acids which is 42% identical to bovine PI 3-kinase and 28% identical to Vps34, a Saccharomyces cerevisiae PI 3-kinase involved in vacuolar protein sorting. Human PI 3-kinase is also similar to Tor2, a yeast protein required for cell cycle progression. Northern (RNA) analysis demonstrated expression of human PI 3-kinase in all tissues and cell lines tested. Protein synthesized from an epitope-tagged cDNA had intrinsic PI 3-kinase activity and associated with the adaptor 85-kDa subunit of PI 3-kinase (p85) in intact cells, as did endogenous human PI 3-kinase. Coprecipitation assays showed that a 187-amino-acid domain between the two src homology 2 domains of p85 mediates interaction with PI 3-kinase in vitro and in intact cells. These results demonstrate the existence of different PI 3-kinase isoforms and define a family of genes encoding distinct PI 3-kinase catalytic subunits that can associate with p85.

Nck associates with the SH2 domain-docking protein IRS-1 in insulin-stimulated cells

Nck, an oncogenic protein composed of one SH2 and three SH3 domains, is a common target for various cell surface receptors. Nck is thought to function as an adaptor protein to couple cell surface receptors to downstream effector molecules that regulate cellular responses induced by receptor activation. In this report, we show that Nck forms a stable complex in vivo with IRS-1 in insulin-stimulated cells. The interaction between IRS-1 and Nck is mediated by the binding of the SH2 domain of Nck to tyrosine-phosphorylated IRS-1. Although Nck associates with IRS-1, Nck phosphorylation is not affected by insulin stimulation. Furthermore, in vitro and in vivo studies show that the SH2 domains of Nck, GRB2, and p85 bind distinct phosphotyrosine residues in IRS-1. After insulin stimulation all three signaling molecules can be found complexed to a single IRS-1 molecule. These findings provide further evidence that, in response to insulin stimulation, IRS-1 acts as an SH2 docking protein that coordinates the regulation of various different signaling pathways activated by the insulin receptor.

Pleiotropic insulin signals are engaged by multisite phosphorylation of IRS-1

IRS-1 (insulin receptor substrate 1) is a principal insulin receptor substrate that undergoes tyrosine phosphorylation during insulin stimulation. It contains over 20 potential tyrosine phosphorylation sites, and we suspect that multiple insulin signals are enabled when the activated insulin receptor kinase phosphorylates several of them. Tyrosine-phosphorylated IRS-1 binds specifically to various cellular proteins containing Src homology 2 (SH2) domains (SH2 proteins). We identified some of the tyrosine residues of IRS-1 that undergo insulin-stimulated phosphorylation by the purified insulin receptor and in intact cells during insulin stimulation. Automated sequencing and manual radiosequencing revealed the phosphorylation of tyrosine residues 460, 608, 628, 895, 939, 987, 1172, and 1222; additional sites remain to be identified. Immobilized SH2 domains from the 85-kDa regulatory subunit (p85 alpha) of the phosphatidylinositol 3'-kinase bind preferentially to tryptic phosphopeptides containing Tyr(P)-608 and Tyr(P)-939. By contrast, the SH2 domain in GRB2 and the amino-terminal SH2 domain in SHPTP2 (Syp) specifically bind to Tyr(P)-895 and Tyr(P)-1172, respectively. These results confirm the p85 alpha recognizes YMXM motifs and suggest that GRB2 prefers a phosphorylated YVNI motif, whereas SHPTP2 (Syp) binds to a phosphorylated YIDL motif. These results extend the notion that IRS-1 is a multisite docking protein that engages various downstream regulatory elements during insulin signal transmission.

Cloning of a novel, ubiquitously expressed human phosphatidylinositol 3-kinase and identification of its binding site on p85

Phosphatidylinositol 3-kinase (PI 3-kinase) has been implicated as a participant in signaling pathways regulating cell growth by virtue of its activation in response to various mitogenic stimuli. Here we describe the cloning of a novel and ubiquitously expressed human PI 3-kinase. The 4.8-kb cDNA encodes a putative translation product of 1,070 amino acids which is 42% identical to bovine PI 3-kinase and 28% identical to Vps34, a Saccharomyces cerevisiae PI 3-kinase involved in vacuolar protein sorting. Human PI 3-kinase is also similar to Tor2, a yeast protein required for cell cycle progression. Northern (RNA) analysis demonstrated expression of human PI 3-kinase in all tissues and cell lines tested. Protein synthesized from an epitope-tagged cDNA had intrinsic PI 3-kinase activity and associated with the adaptor 85-kDa subunit of PI 3-kinase (p85) in intact cells, as did endogenous human PI 3-kinase. Coprecipitation assays showed that a 187-amino-acid domain between the two src homology 2 domains of p85 mediates interaction with PI 3-kinase in vitro and in intact cells. These results demonstrate the existence of different PI 3-kinase isoforms and define a family of genes encoding distinct PI 3-kinase catalytic subunits that can associate with p85.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

Insulin-stimulated oocyte maturation requires insulin receptor substrate 1 and interaction with the SH2 domains of phosphatidylinositol 3-kinase

Xenopus oocytes from unprimed frogs possess insulin-like growth factor I (IGF-I) receptors but lack insulin and IGF-I receptor substrate 1 (IRS-1), the endogenous substrate of this kinase, and fail to show downstream responses to hormonal stimulation. Microinjection of recombinant IRS-1 protein enhances insulin-stimulated phosphatidylinositol (PtdIns) 3-kinase activity and restores the germinal vesicle breakdown response. Activation of PtdIns 3-kinase results from formation of a complex between phosphorylated IRS-1 and the p85 subunit of PtdIns 3-kinase. Microinjection of a phosphonopeptide containing a pYMXM motif with high affinity for the src homology 2 (SH2) domain of PtdIns 3-kinase p85 inhibits IRS-1 association with and activation of the PtdIns 3-kinase. Formation of the IRS-1-PtdIns 3-kinase complex and insulin-stimulated PtdIns 3-kinase activation are also inhibited by microinjection of a glutathione S-transferase fusion protein containing the SH2 domain of p85. This effect occurs in a concentration-dependent fashion and results in a parallel loss of hormone-stimulated oocyte maturation. These inhibitory effects are specific and are not mimicked by glutathione S-transferase fusion proteins expressing the SH2 domains of ras-GAP or phospholipase C gamma. Moreover, injection of the SH2 domains of p85, ras-GAP, and phospholipase C gamma do not interfere with progesterone-induced oocyte maturation. These data demonstrate that phosphorylation of IRS-1 plays an essential role in IGF-I and insulin signaling in oocyte maturation and that this effect occurs through interactions of the phosphorylated YMXM/YXXM motifs of IRS-1 with SH2 domains of PtdIns 3-kinase or some related molecules.

IRS-1: essential for insulin- and IL-4-stimulated mitogenesis in hematopoietic cells

Although several interleukin-3 (IL-3)-dependent cell lines proliferate in response to IL-4 or insulin, the 32D line does not. Insulin and IL-4 sensitivity was restored to 32D cells by expression of IRS-1, the principal substrate of the insulin receptor. Although 32D cells possessed receptors for both factors, they lacked the IRS-1--related protein, 4PS, which becomes phosphorylated by tyrosine in insulin- or IL-4--responsive lines after stimulation. These results indicate that factors that bind unrelated receptors can use similar mitogenic signaling pathways in hematopoietic cells and that 4PS and IRS-1 are functionally similar proteins that are essential for insulin- and IL-4--induced proliferation.

Robert Feulgen Prize Lecture 1993. The journey of the insulin receptor into the cell: from cellular biology to pathophysiology

The data that we have reviewed indicate that insulin binds to a specific cell-surface receptor. The complex then becomes involved in a series of steps which lead the insulin-receptor complex to be internalized and rapidly delivered to endosomes. From this sorting station, the hormone is targeted to lysosomes to be degraded while the receptor is recycled back to the cell surface. This sequence of events presents two degrees of ligand specificity: (a) The first step is ligand-dependent and requires insulin-induced receptor phosphorylation of specific tyrosine residues. It consists in the surface redistribution of the receptor from microvilli where it preferentially localizes in its unoccupied form. (b) The second step is more general and consists in the association with clathrin-coated pits which represents the internalization gate common to many receptors. This sequence of events participates in the regulation of the biological action of the hormone and can thus be implicated in the pathophysiology of diabetes mellitus and various extreme insulin resistance syndromes, including type A extreme insulin resistance, leprechaunism, and Rabson-Mendehall syndrome. Alterations of the internalization process can result either from intrinsic abnormalities of the receptor or from more general alteration of the plasma membrane or of the cell metabolism. Type I diabetes is an example of the latter possibility, since general impairment of endocytosis could contribute to extracellular matrix accumulation and to an increase in blood cholesterol. Thus, better characterization of the molecular and cellular biology of the insulin receptor and of its journey inside the cell definitely leads to better understanding of disease states, including diabetes.

The function of GRB2 in linking the insulin receptor to Ras signaling pathways

Insulin-induced activation of extracellular signal-regulated kinases [ERKs, also known as mitogen-activated protein (MAP) kinases] is mediated by Ras. Insulin activates Ras primarily by increasing the rate of guanine nucleotide-releasing activity. Here, we show that insulin-induced activation of ERKs was enhanced by stable overexpression of growth factor receptor-bound protein 2 (GRB2) but not by overexpression of GRB2 proteins with point mutations in the Src homology 2 and 3 domains. Moreover, a dominant negative form of Ras (with Ser17 substituted with Asn) blocked insulin-induced activation of ERKs in cells that overexpressed GRB2. GRB2 overexpression led to increased formation of a complex between the guanine nucleotide-releasing factor Sos (the product of the mammalian homolog of son of sevenless gene) and GRB2. In response to insulin stimulation, this complex bound to tyrosine-phosphorylated IRS-1 (insulin receptor substrate-1) and Shc. In contrast to the activated epidermal growth factor receptor that binds the GRB2-Sos complex directly, activation of the insulin receptor results in the interaction of GRB2-Sos with IRS-1 and Shc, thus linking the insulin receptor to Ras signaling pathways.

Kinetics of p56lck and p60src Src homology 2 domain binding to tyrosine-phosphorylated peptides determined by a competition assay or surface plasmon resonance

Src homology 2 (SH2) domains are phosphotyrosine-binding modules found within various signal-transducing proteins. We have determined by 125I competition assay and surface plasmon resonance that the SH2 domains of Src and Lck bind to a variety of phosphopeptides with similar affinity and specificity. Both bound with highest affinity [Kd(app) approximately 3.7 nM; ka = 2.4 x 10(5) M-1 x s-1; kd = 1.2 x 10(-3) s-1] a phosphopeptide having a Tyr(P)-Glu-Glu-Ile motif found in the hamster polyomavirus middle-sized tumor antigen. Intermediate affinity (5- to 40-fold lower) was observed with phosphopeptides corresponding to the regulatory domains of Src and Lck, containing Tyr527 and Tyr505, respectively. Lowest affinity (80- to 300-fold lower) was observed with phosphopeptides corresponding to phosphorylated tyrosines of GTPase-activating protein, insulin receptor substrate 1, and SH2 domain-containing protein-tyrosine-phosphatase 1.

Phosphatidylinositol 3-kinase p85 SH2 domain specificity defined by direct phosphopeptide/SH2 domain binding

We have developed a competition binding assay to quantify relative affinities of isolated Src-homology 2 (SH2) domains for phosphopeptide sequences. Eleven synthetic 11-12-amino acid phosphopeptides containing YMXM or YVXM recognition motifs bound to a PI 3-kinase p85 SH2 domain with highest affinities, including sequences surrounding phosphorylated tyrosines of the PDGF, CSF-1/c-Fms, and kit-encoded receptors, IRS-1, and polyoma middle T antigens; matched, unphosphorylated sequences did not bind. A scrambled YMXM phosphopeptide or sequences corresponding to the GAP or PLC-gamma SH2 domain binding motifs of the PDGF, FGF, and EGF receptors bound to the p85 SH2 domain with 30-100-fold reduced affinity, indicating that this affinity range confers specificity. Binding specificity was appropriately reversed with an SH2 domain from PLC-gamma: a phosphopeptide corresponding to the site surrounding PDGF receptor Tyr1021 binds with approximately 40-fold higher affinity than a YMXM-phosphopeptide. We conclude that essential features of specific phosphoprotein/SH2 domain interactions can be reconstituted using truncated versions of both the phosphoprotein (a phosphopeptide) and cognate SH2 domain-containing protein (the SH2 domain). SH2 domain binding specificity results from differences in affinity conferred by the linear sequence surrounding phosphotyrosine.