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

A tripartite array of transcription factor binding sites mediates cAMP induction of phosphoenolpyruvate carboxykinase gene transcription and its inhibition by insulin

Transcription of the phosphoenolpyruvate carboxykinase (PEPCK) gene is induced upon activation of protein kinase A by cAMP and phosphorylation of Ser-133 in the transcription factor, cAMP-response element binding protein (CREB), and this induction is inhibited by insulin. We show here that insulin does not act by dephosphorylating CREB or by affecting heterologous kinases that phosphorylate Ser-129 or Ser-142 in CREB. In addition, insulin inhibition of minimal PEPCK promoter activity induced by CREB-GAL4 + protein kinase A was equivalent to inhibition of basal transcription, and thus cAMP-independent. On the other hand, nearly complete insulin inhibition is observed with the full PEPCK promoter (-600/+69), indicating that other factors are involved. The additional promoter elements required for induction by protein kinase A lie within -271 nucleotides of the start site and correspond to putative binding sites for activator protein-1 and CAAT/enhancer-binding protein (C/EBP), first identified by Roesler et al. (Roesler, W. J., McFie, P. J., and Puttick, D. M., (1993) J. Biol. Chem. 268, 3791-3796). This tripartite array of binding sites for CREB, C/EBP, and activator protein-1 (AP-1) factors forms a cAMP response unit that, together with the minimal promoter, can mediate both induction by cAMP and inhibition by insulin. Thus, for the PEPCK gene with a single CREB site, the CREB.CBP.RNA polymerase II complex cannot mediate either induction by cAMP or inhibition by insulin.

The phylogeny of the insulin-like growth factors

The insulin-like growth factors are major regulators of growth and development in mammals and their presence in lower vertebrates suggests that they played a similarly fundamental role throughout vertebrate evolution. While originally perceived simply as mediators of growth hormone, on-going research in mammals has revealed several hierarchical layers of complexity in the regulation of ligand bioavailability and signal transduction. Our understanding of the biological role and mechanisms of action of these important growth factors in mammals patently requires further elucidation of the IGF hormone system in the simple model systems that can be found in lower vertebrates and protochordates. This review contrasts our knowledge of the IGF hormone system in mammalian and nonmammalian models through comparison of tissue and developmental distributions and gene structures of IGF system components in different taxa. We also discuss the evolutionary origins of the system components and their possible evolutionary pathways.

Replacement of tyrosine 1251 in the carboxyl terminus of the insulin-like growth factor-I receptor disrupts the actin cytoskeleton and inhibits proliferation and anchorage-independent growth

Insulin-like growth factor (IGF)-I signaling through the IGF-I receptor modulates cellular adhesion and proliferation and the transforming ability of cells overexpressing the IGF-I receptor. Tyrosine phosphorylation of intracellular proteins is essential for this transduction of the IGF-I-induced mitogenic and tumorigenic signals. IGF-I induces specific cytoskeletal structure and the phosphorylation of proteins in the associated focal adhesion complexes. The determination of the exact pathways emanating from the IGF-I receptor that are involved in mediating these signals will contribute greatly to the understanding of IGF-I action. We have previously shown that replacement of tyrosine residues 1250 and 1251 in the carboxyl terminus of the IGF-I receptor abrogates IGF-I-induced cellular proliferation and tumor formation in nude mice. In this study, replacement of either tyrosine 1250 or 1251 similarly reduces the cells ability to grow in an anchorage-independent manner. The actin cytoskeleton and cellular localization of vinculin are disrupted by replacement of tyrosine 1251. Tyrosine residues 1250 and 1251 are not essential for tyrosine phosphorylation of two known substrates; insulin receptor substrate-1 and SHC, nor association of known downstream adaptor proteins to these substrates. In addition, these mutant IGF-I receptors do not affect IGF-I-stimulated p42/p44 mitogen-activated protein kinase activation or phosphatidylinositol (PI) 3'-kinase activity. Thus, it appears that in fibroblasts expressing tyrosine 1250 and 1251 mutant IGF-I receptors, the signal transduction pathways impacting on mitogenesis and tumorigenesis do not occur exclusively through the PI 3'-kinase or mitogen-activated protein kinase pathways.

Evidence that IRS-2 phosphorylation is required for insulin action in hepatocytes

Insulin receptor substrates (IRSs) are tyrosine-phosphorylated following stimulation with insulin, insulin-like growth factors (IGFs), and interleukins. A key question is whether different IRSs play different roles to mediate insulin's metabolic and growth-promoting effects. In a novel system of insulin receptor-deficient hepatocytes, insulin fails to (i) stimulate glucose phosphorylation, (ii) enhance glycogen synthesis, (iii) suppress glucose production, and (iv) promote mitogenesis. However, insulin's ability to induce IRS-1 and gab-1 phosphorylation and binding to phosphatidylinositol (PI) 3-kinase is unaffected, by virtue of the compensatory actions of IGF-1 receptors. In contrast, phosphorylation of IRS-2 and generation of IRS-2/PI 3-kinase complexes are markedly reduced. Thus, absence of insulin receptors selectively reduces IRS-2, but not IRS-1 phosphorylation, and the impairment of IRS-2 activation is associated with lack of insulin effects. To address whether phosphorylation of additional IRSs is also affected, we analyzed phosphotyrosine-containing proteins in PI 3-kinase immunoprecipitates from insulin-treated cells. However, these experiments indicate that IRS-1 and IRS-2 are the main PI 3-kinase-bound proteins in hepatocytes. These data identify IRS-2 as the main effector of both the metabolic and growth-promoting actions of insulin through PI 3-kinase in hepatocytes, and IRS-1 as the main substrate mediating the mitogenic actions of IGF-1 receptors.

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) stimulation of GLUT4 translocation is tyrosine kinase-dependent

Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) treatment of permeabilized adipocytes results in GLUT4 translocation similar to that elicited by insulin treatment. However, although the selective phosphatidylinositol 3-kinase inhibitor, wortmannin, completely prevented insulin-stimulated GLUT4 translocation, it was without effect on GTPgammaS-stimulated GLUT4 translocation. In addition, insulin was an effective stimulant, whereas GTPgammaS was a very weak activator of the downstream Akt serine/threonine kinase. Consistent with an Akt-independent mechanism, guanosine 5'-O-2-(thio)diphosphate inhibited insulin-stimulated GLUT4 translocation without any effect on the Akt kinase. Surprisingly, two functionally distinct tyrosine kinase inhibitors, genistein and herbimycin A, as well as microinjection of a monoclonal phosphotyrosine specific antibody, inhibited both GTPgammaS- and insulin-stimulated GLUT4 translocation. Phosphotyrosine immunoblotting and specific immunoprecipitation demonstrated that GTPgammaS did not elicit tyrosine phosphorylation of insulin receptor or insulin receptor substrate-1. In contrast to insulin, proteins in the 120-130-kDa and 55-75-kDa range were tyrosine-phosphorylated following GTPgammaS stimulation. Several of these proteins were identified and include protein-tyrosine kinase 2 (also known as CAKbeta, RAFTK, and CADTK), pp125 focal adhesion tyrosine kinase, pp130 Crk-associated substrate, paxillin, and Cbl. These data demonstrate that the GTPgammaS-stimulated GLUT4 translocation utilizes a novel tyrosine kinase pathway that is independent of both the phosphatidylinositol 3-kinase and the Akt kinase.

Guanosine triphosphatase-activating protein-associated protein, but not src-associated protein p68 in mitosis, is a part of insulin signaling complexes

The insulin receptor, following insulin stimulation of cells, triggers formation of various signaling complexes. In rat HTC hepatoma cells overexpressing normal human insulin receptors (HTC-IR), p85 regulatory subunit of phosphatidylinositol-3-kinase (PI3K) forms signaling complexes containing the insulin receptor, insulin receptor substrate 1 (IRS-1), guanosine triphosphatase-activating protein (GAP) and 60-70 kDa phosphotyrosine proteins (p60-70). In the present study, we demonstrate that p60-70 interacts directly with the p85 subunit via src homology 2 domain of the latter. Employing antibodies specific to two p85 isoforms, p85alpha and p85beta, we demonstrate that HTC-IR cells express both p85 isoforms, and these isoforms induce the formation of similar signaling complexes in response to insulin. p60-70, present in both alpha-p85alpha and alpha-p85beta immunoprecipitates, is a GAP-associated protein, but is distinct from the p68 src-associated protein in mitosis (Sam68) by several criteria. These data suggest that 1) GAP-associated protein, but not Sam68, is a part of insulin signaling complexes; and 2) p85alpha and p85beta form similar, but distinct, insulin receptor signaling complexes.

Membrane-permeant esters of phosphatidylinositol 3,4,5-trisphosphate

Phosphoinositide 3-OH kinases and their products, D-3 phosphorylated phosphoinositides, are increasingly recognized as crucial elements in many signaling cascades. A reliable means to introduce these lipids into intact cells would be of great value for showing the physiological roles of this pathway and for testing the specificity of pharmacological inhibitors of the kinases. We have stereospecifically synthesized di-C8-PIP3/AM and di-C12-PIP3/AM, the heptakis(acetoxymethyl) esters of dioctanoyl- and dilauroylphosphatidylinositol 3,4,5-trisphosphate, in 14 steps from myo-inositol. The ability of these uncharged lipophilic derivatives to deliver phosphatidylinositol 3,4,5-trisphosphate across cell membranes was demonstrated on 3T3-L1 adipocytes and T84 colon carcinoma monolayers. Insulin stimulation of hexose uptake into adipocytes was inhibited by the kinase inhibitor wortmannin and was largely restored by di-C8-PIP3/AM, which had no effect in the absence of insulin. Thus phosphatidylinositol 3,4,5-trisphosphate or a metabolite was necessary but not sufficient for stimulation of hexose transport. In T84 epithelial monolayers, di-C12-PIP3/AM mimicked epidermal growth factor in inhibiting chloride secretion and potassium efflux, suggesting that phosphatidylinositol 3,4, 5-trisphosphate was sufficient to modulate these fluxes and mediate epidermal growth factor's action.

Role of binding proteins to IRS-1 in insulin signalling

Insulin elicits its divergent metabolic and mitogenic effects by binding to its specific receptor, which belongs to the family of receptor tyrosine kinases. The activated insulin receptor phosphorylates the intracellular substrate IRS-1, which then binds various signalling molecules that contain SRC homology 2 domains, thereby propagating the insulin signal. Among these IRS-1-binding proteins, the Grb2-Sos complex and the protein tyrosine phosphatase SHP-2 transmit mitogenic signals through the activation of Ras, and phosphoinositide 3-kinase is implicated in the major metabolic actions of insulin. Although substantial evidence indicates the importance of IRS-1 in insulin signal transduction, the generation of IRS-1-deficient mice has revealed the existence of redundant signalling pathways.

Impaired activation of phosphoinositide 3-kinase by insulin in fibroblasts from patients with severe insulin resistance and pseudoacromegaly. A disorder characterized by selective postreceptor insulin resistance

Some patients with severe insulin resistance develop pathological tissue growth reminiscent of acromegaly. Previous studies of such patients have suggested the presence of a selective postreceptor defect of insulin signaling, resulting in the impairment of metabolic but preservation of mitogenic signaling. As the activation of phosphoinositide 3-kinase (PI 3-kinase) is considered essential for insulin's metabolic signaling, we have examined insulin-stimulated PI 3-kinase activity in anti-insulin receptor substrate (IRS)-1 immunoprecipitates from cultured dermal fibroblasts obtained from pseudoacromegalic (PA) patients and controls. At a concentration of insulin (1 nM) similar to that seen in vivo in PA patients, the activation of IRS-1-associated PI 3-kinase was reduced markedly in fibroblasts from the PA patients (32+/-7% of the activity of normal controls, P < 0.01). Genetic and biochemical studies indicated that this impairment was not secondary to a defect in the structure, expression, or activation of the insulin receptor, IRS-1, or p85alpha. Insulin stimulation of mitogenesis in PA fibroblasts, as determined by thymidine incorporation, was indistinguishable from controls, as was mitogen-activated protein kinase phosphorylation, confirming the integrity of insulin's mitogenic signaling pathways in this condition. These findings support the existence of an intrinsic defect of postreceptor insulin signaling in the PA subtype of insulin resistance, which involves impairment of the activation of PI 3-kinase. The PA tissue growth seen in such patients is likely to result from severe in vivo hyperinsulinemia activating intact mitogenic signaling pathways emanating from the insulin receptor.

Interaction in vitro of the product of the c-Crk-II proto-oncogene with the insulin-like growth factor I receptor

The Crk proto-oncogene product is an SH2 and SH3 domain-containing adaptor protein. We have previously demonstrated that Crk-II becomes rapidly tyrosine-phosphorylated in response to stimulation with insulin-like growth factor I (IGF-I) and might be involved in the IGF-I receptor signalling pathway. To determine whether this involvement includes the direct interaction of Crk-II with the cytoplasmic region of the receptor, studies were performed in vitro with glutathione S-transferase (GST) fusion proteins containing various domains of Crk-II. The kinase assay in vitro showed that activated IGF-I receptors efficiently phosphorylated the GST-Crk-II fusion protein. This phosphorylation was dependent on the presence of the SH2 domain and Tyr-221 located in the spacer region between the two SH3 domains. Mutation of Tyr-221 not only prevented phosphorylation of GST-Crk in vitro, but also significantly increased the ability of GST-Crk proteins to co-precipitate activated IGF-I receptors from total cell lysates. Additional binding experiments in vitro showed that Crk-II might interact with the phosphorylated IGF-I receptor through its SH2 domain. To elucidate which region of the IGF-I receptor interacts with Crk-II, a peptide association assay was used in vitro. Different domains of the IGF-I receptor were expressed as (His)6-tagged fusion peptides, phosphorylated with activated wheat germ agglutinin-purified IGF-I receptors and tested for association with GST-Crk-II fusion proteins. Using wild-type as well as mutated peptides, we showed that the SH2 domain of Crk-II preferentially binds the peptide encoding the juxtamembrane region of the IGF-I receptor. Phosphorylation of Tyr-950 and Tyr-943 of the receptor is important for this interaction. These findings allow us to propose a model of direct interaction of Crk-II and the IGF-I receptor in vivo. On activation of the IGF-I receptor, Crk-II binds to phosphorylated tyrosine residues, especially in the juxtamembrane region. As a result of this binding, the IGF-I receptor kinase phosphorylates Tyr-221 of Crk-II, resulting in a change in intramolecular folding and binding of the SH2 domain to the phosphorylated Tyr-221, which causes rapid disassociation of the Crk-II-IGF-I receptor complex.

Insulin-like growth factor-I receptor internalization regulates signaling via the Shc/mitogen-activated protein kinase pathway, but not the insulin receptor substrate-1 pathway

Insulin-like growth factor-I (IGF-I) receptors activate divergent signaling pathways by phosphorylating multiple cellular proteins, including insulin receptor substrate-1 (IRS-1) and the Shc proteins. Following hormone binding, IGF-I receptors cluster into clathrin-coated pits and are internalized via an endocytotic mechanism. This study investigates the relationship between IGF-I receptor internalization and signaling via IRS-1 and Shc. A mutation in the C terminus of the IGF-I receptor decreased both the rate of receptor internalization and IGF-I-stimulated Shc phosphorylation by more than 50%, but did not affect IRS-1 phosphorylation. Low temperature (15 degrees C) decreased IGF-I receptor internalization and completely inhibited Shc phosphorylation. Although receptor and IRS-1 phosphorylation were decreased in accordance with delayed binding kinetics at 15 degrees C, the ratio of IRS-1 to receptor phosphorylation was increased more than 2-fold. Dansylcadaverine decreased receptor internalization and Shc phosphorylation, but did not change receptor or IRS-1 phosphorylation. Consistent with these findings, dansylcadaverine inhibited IGF-I-stimulated Shc-Grb2 association, mitogen-activated protein kinase phosphorylation, and p90 ribosomal S6 kinase activation, but did not affect the association of phosphatidylinositide 3-kinase with IRS-1 or activation of p70 S6 kinase. These data support the concept that Shc/mitogen-activated protein kinase pathway activation requires IGF-I receptor internalization, whereas the IRS-1 pathway is activated by both cell surface and endosomal receptors.

Mitogenic signaling of insulin-like growth factor I in MCF-7 human breast cancer cells requires phosphatidylinositol 3-kinase and is independent of mitogen-activated protein kinase

Addition of insulin-like growth factor I (IGF-I) to quiescent breast tumor-derived MCF-7 cells causes stimulation of cyclin D1 synthesis, hyperphosphorylation of the retinoblastoma protein pRb, DNA synthesis, and cell division. All of these effects are independent of the mitogen-activated protein kinase (MAPK) pathway since none of them is blocked by PD098059, the specific inhibitor of the MAPK activating kinase MEK1. This observation is consistent with the finding that the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), a strong inducer of MAPK activity in MCF-7 cells, effectively inhibits proliferation. The anti-proliferative effect of TPA in these cells may be accounted for, at least in part, by the MAPK-dependent stimulation of the synthesis of p21(WAF1/CIP1), an inhibitor of cyclin/cyclin-dependent kinase complexes. In contrast, all of the observed stimulatory effects of IGF-I on cell cycle progression, cyclin D1 synthesis, and pRb hyperphosphorylation were blocked by the specific phosphatidylinositol 3-kinase inhibitor LY294002, suggesting that phosphatidylinositol 3-kinase activity but not MAPK activity is required for transduction of the mitogenic IGF-I signal in MCF-7 cells.

Early signaling events triggered by peroxovanadium [bpV(phen)] are insulin receptor kinase (IRK)-dependent: specificity of inhibition of IRK-associated protein tyrosine phosphatase(s) by bpV(phen)

Peroxovanadiums (pVs) are potent protein tyrosine phosphatase (PTP) inhibitors with insulin-mimetic properties in vivo and in vitro. We have established the existence of an insulin receptor kinase (IRK)-associated PTP whose inhibition by pVs correlates closely with IRK tyrosine phosphorylation, activation, and downstream signaling. pVs have also been shown to activate various tyrosine kinases (TKs) that could participate in activation of the insulin-signaling pathway. In the present study we have sought to determine whether pV-induced IRK tyrosine phosphorylation requires the intrinsic kinase activity of the IRK, and whether IRK activation is necessary to realize the early steps in the insulin-signaling cascade. To address this we evaluated the effect of a pure pV compound, bis peroxovanadium 1,10-phenanthroline [bpV(phen)], in HTC rat hepatoma cells overexpressing normal (HTC-IR) or kinase-deficient (HTC-M1030) mutant IRKs. We showed that at a dose of 0.1 mM, but not 1 mM, bpV(phen) induced IRK-dependent events. Thus, 0.1 mM bpV(phen) increased tyrosine phosphorylation and IRK activity in HTC-IR but not HTC-M1030 cells. Tyrosine phosphorylation of insulin signal-transducing molecules was promoted in HTC-IR but not HTC-M1030 cells by bpV(phen). The association of p185 and p60 with the src homology-2 (SH2) domains of Syp and the p85-regulatory subunit of phosphatidylinositol 3'-kinase was induced by bpV(phen) in HTC-IR, but not in HTC-M1030 cells, as was insulin receptor substrate-1-associated phosphatidylinositol 3'-kinase activity. Thus autophosphorylation and activation of the IRK by bpV(phen) is effected by the IRK itself, and the early events in the insulin- signaling cascade follow from this activation event. This establishes a critical role for PTP(s) in the regulation of IRK activity. bpV(phen) could be distinguished from insulin only in its ability to activate ERK1 in HTC-M1030 cells, thus indicating that this event is IRK independent, consistent with our previous hypothesis that bpV(phen) inhibits a PTP involved in the negative regulation of mitogen-activated protein kinases.

Identification of insulin-like growth factor-binding protein-1 as a potential physiological substrate for human stromelysin-3

To elucidate the physiological role of human stromelysin-3 (hST-3) in tumor progression and/or wound healing, insulin-like growth factor-binding protein-1 (IGFBP-1) was analyzed as a potential physiological substrate. hST-3 proteolysis generates two fragments of 16 and 9 kDa that react with IGFBP-1 monoclonal antibody, although they do not bind insulin-like growth factor-I (IGF-I) in ligand blot. N-terminal sequencing shows that hST-3 cleaves IGFBP-1 at the His140-Val141 bond located in the IGFBP-1 midregion. We show that IGFBP-1 inhibits IGF-I-induced survival and proliferation of BAF/3 cells, as well as IGF-I-mediated activation of phosphatidylinositol 3-kinase (PI 3-K). Co-incubation of the IGF-I. IGFBP-1 complex with hST-3 restores IGF-I-induced proliferation and PI 3-K kinase activity in these cells. BAF/3 proliferation is significantly increased with the hST-3-treated IGF-I.IGFBP-1 complex compared with that obtained using IGF-I alone. To produce this enhanced proliferation, IGF-I must bind to IGFBP-1 before hST-3 proteolysis, demonstrated using an IGF-I variant that does not bind IGFBP. IGFBP-1 also inhibits IGF-I-induced proliferation of the MCF-7 breast adenocarcinoma, and this inhibition was not seen in hST-3-transfected MCF-7 cells. Such proteolysis may thus play a role in in vivo tumor progression. These results indicate that hST-3 may regulate IGF-I bioavailability by proteolyzing IGFBP, thus favoring cell survival and proliferation.

Phosphorylation of insulin receptor substrate 1 by glycogen synthase kinase 3 impairs insulin action

The phosphorylation of insulin receptor substrate 1 (IRS-1) on tyrosine residues by the insulin receptor (IR) tyrosine kinase is involved in most of the biological responses of insulin. IRS-1 mediates insulin signaling by recruiting SH2 proteins through its multiple tyrosine phosphorylation sites. The phosphorylation of IRS-1 on serine/threonine residues also occurs in cells; however, the particular protein kinase(s) promoting this type of phosphorylation are unknown. Here we report that glycogen synthase kinase 3 (GSK-3) is capable of phosphorylating IRS-1 and that this modification converts IRS-1 into an inhibitor of IR tyrosine kinase activity in vitro. Expression of wild-type GSK-3 or an "unregulated" mutant of the kinase (S9A) in CHO cells overexpressing IRS-1 and IR, resulted in increased serine phosphorylation levels of IRS-1, suggesting that IRS-1 is a cellular target of GSK-3. Furthermore, insulin-induced tyrosine phosphorylation of IRS-1 and IR was markedly suppressed in cells expressing wild-type or the S9A mutant, indicating that expression of GSK-3 impairs IR tyrosine kinase activity. Taken together, our studies suggest a new role for GSK-3 in attenuating insulin signaling via its phosphorylation of IRS-1 and may provide new insight into mechanisms important in insulin resistance.

Logical analysis of timing-dependent receptor signalling specificity: application to the insulin receptor metabolic and mitogenic signalling pathways

We present a method for logical analysis of signal-transduction networks, focusing on metabolic and mitogenic signalling by the insulin receptor, with specific emphasis on dependence of the signalling properties on the timing of binding events. We discuss a basic model which demonstrates this dependence (hormone binding leads to activation of the receptor which can lead to a commitment to mitogenic signalling), and show how residence time of the hormone on the receptor can determine the specificity of signalling between the alternative metabolic or mitogenic pathways. The method gives conditions for the selection of specific branches in the signalling pathway expressed in terms of inequalities among the characteristic activation or deactivation times of components of that pathway. In this way, the conditions for mitogenic signalling can be given in terms of a required range of values of the hormone residence time on the receptor, which is directly related to the kinetic dissociation rate.

Cross-talk between the platelet-derived growth factor and the insulin signaling pathways in 3T3-L1 adipocytes

Phosphatidylinositol (PI) 3-kinase is activated by various growth factors such as PDGF (platelet-derived growth factor) and insulin. The aim of the present study was to determine whether PDGF could modulate insulin activation of PI 3-kinase in 3T3-L1 adipocytes. When cells were preincubated for 5-15 min with PDGF, PI 3-kinase activity associated to insulin receptor substrate 1 (IRS 1) in response to insulin was decreased, due to reduced association of the PI 3-kinase p85 subunit with IRS 1. In addition, following this PDGF pretreatment, the tyrosine phosphorylation of IRS 1 in response to insulin and its electrophoretic mobility were diminished. The change in the mobility of IRS 1 could be attributed to PDGF-induced serine/threonine phosphorylation of the protein which was partly inhibited by PI 3-kinase inhibitors. By contrast, epidermal growth factor, which does not stimulate PI 3-kinase, had no effect on the association of PI 3-kinase with IRS 1 in response to insulin. This series of results indicates that the PDGF-induced serine/threonine phosphorylation of IRS 1 could be due to activation of PI 3-kinase pathway. Furthermore, this phosphorylation of IRS 1 is associated with a decrease in its tyrosine phosphorylation by insulin and in its association with the p85 subunit of PI 3-kinase. This study suggests that a cross-talk exists between the different pathways stimulated by PDGF and insulin in intact cells.

Insulin-like growth factor-I rapidly activates multiple signal transduction pathways in cultured rat cardiac myocytes

In response to insulin-like growth factor-I (IGF-I), neonatal rat cardiac myocytes exhibit a hypertrophic response. The elucidation of the IGF-I signal transduction system in these cells remains unknown. We show here that cardiac myocytes present a single class of high affinity receptors (12,446 +/- 3,669 binding sites/cell) with a dissociation constant of 0.36 +/- 0.10 nM. Two different beta-subunits of IGF-I receptor were detected, and their autophosphorylation was followed by increases in the phosphotyrosine content of extracellular signal-regulated kinases (ERKs), insulin receptor substrate 1, phospholipase C-gamma1, and phosphatidylinositol 3-kinase. IGF-I transiently activates c-Raf in cultured neonatal cardiac myocytes, whereas A-raf is activated much less than c-Raf. Two peaks of ERK activity (ERK1 and ERK2) were resolved in cardiac myocytes treated with IGF-I by fast protein liquid chromatography, both being stimulated by IGF-I (with EC50 values for the stimulation of ERK1 and ERK2 by IGF-I of 0.10 and 0. 12 nM, respectively). Maximal activation of ERK2 (12-fold) and ERK1 (8.3-fold) activities was attained after a 5-min exposure to IGF-I. Maximal activation of p90 S6 kinase by IGF-I was achieved after 10 min, and then the activity decreased slowly. Interestingly, IGF-I stimulates incorporation of [3H]phenylalanine (1.6-fold) without any effect on [3H]thymidine incorporation. These data suggest that IGF-I activates multiple signal transduction pathways in cardiac myocytes some of which may be relevant to the hypertrophic response of the heart.

Alterations in the insulin signaling pathway induced by immortalization and H-ras transformation of brown adipocytes

In fetal brown adipocyte primary cultures, insulin rapidly (at 5 min) induced tyrosine phosphorylation of the insulin receptor beta-subunit; this effect was maximal at physiological concentrations (1 nM). Insulin also stimulated insulin receptor substrate-1 tyrosine phosphorylation and subsequently activated phosphatidylinositol 3-kinase. Moreover, a 3-fold increase in the Ras.GTP active form and a 6-fold increase in Raf-1 kinase activity were induced after insulin stimulation. An immortalized brown adipocyte cell line (by permanent simian virus 40 large T antigen and pMEXneo cotransfection) showed a reduced maximal responsiveness to insulin in the same range of insulin concentrations studied (1-100 nM). Transformed brown adipocyte cell line (by permanent simian virus 40 large T antigen and pMEXneo H-ras(lys12) cotransfection) developed insulin resistance upstream from Ras, showing an impairment in the insulin receptor autophosphorylation, and in insulin receptor substrate-1 tyrosine phosphorylation and its association with phosphatidylinositol 3-kinase upon treatment with 1 nM insulin, although insulin receptor number and affinity (Kd) remained unaltered. This lack of effect was ameliorated upon treatment with higher insulin concentrations, in a dose-dependent manner. However, downstream from Ras, events such as formation of the Ras.GTP active form, and Raf-1 kinase and 12-O-tetradecanoylphorbol-13-acetate response element-chloramphenicol transferase (transiently transfected) activities were overstimulated, compared with those in primary and immortalized cells, in an insulin-independent manner. Wheat-germ lectin-purified receptors from H-ras(lys12)-transformed brown adipocytes showed a marked phosphorylation in the basal state, which was suppressed by serine-threonine phosphatase pretreatment. Moreover, alkaline phosphatase pretreatment restored the tyrosine kinase activity of the receptor in response to insulin. We conclude that the decreased tyrosine autophosphorylation rate of the insulin receptor from H-ras(lys12)-transformed brown adipocytes is a consequence of its basal serine/threonine phosphorylation, resulting in severe insulin resistance.

Hypoglycemic effect of insulin-like growth factor-1 in mice lacking insulin receptors

We have investigated the metabolic actions of recombinant human IGF-1 in mice genetically deficient of insulin receptors (IR-/-). After intraperitoneal administration, IGF-1 caused a prompt and sustained decrease of plasma glucose levels in IR-/- mice. Plasma free fatty acid concentrations were unaffected. Interestingly, the effects of IGF-1 were identical in normal mice (IR+/+) and in IR-/- mice. Despite decreased glucose levels, IR-/- mice treated with IGF-1 died within 2-3 d of birth, like sham-treated IR-/- controls. In skeletal muscle, IGF-1 treatment caused phosphorylation of IGF-1 receptors and increased the levels of the phosphatidylinositol-3-kinase p85 subunit detected in antiphosphotyrosine immunoprecipitates, consistent with the possibility that IGF-1 stimulates glucose uptake in a phosphatidylinositol-3-kinase-dependent manner. IGF-1 receptor phosphorylation and coimmunoprecipitation of phosphatidylinositol3-kinase by antiphosphotyrosine antibodies was also observed in liver, and was associated with a decrease in mRNA levels of the key gluconeogenetic enzyme phosphoenolpyruvate carboxykinase. Thus, the effect of IGF-1 on plasma glucose levels may be accounted for by increased peripheral glucose use and by inhibition of hepatic gluconeogenesis. These data indicate that IGF-1 can mimic insulin's effects on glucose metabolism by acting through its own receptor. The failure of IGF-1 to rescue the lethal phenotype due to lack of insulin receptors suggests that IGF-1 receptors cannot effectively mediate all the metabolic actions of insulin receptors.

CSF-1 receptor/insulin receptor chimera permits CSF-1-dependent differentiation of 3T3-L1 preadipocytes

A chimeric growth factor receptor (CSF1R/IR) was constructed by splicing cDNA sequences encoding the extracellular ligand binding domain of the human colony stimulating factor-1 (CSF-1) receptor to sequences encoding the transmembrane and cytoplasmic domains of the human insulin receptor. The addition of CSF-1 to cells transfected with the CSF1R/IR chimera cDNA stimulated the tyrosine phosphorylation of a protein that was immunoprecipitated by an antibody directed against the carboxyl terminus of the insulin receptor. Phosphopeptide maps of the 32P-labeled CSF1R/IR protein revealed the same pattern of phosphorylation observed in 32P-labeled insulin receptor beta subunits. CSF-1 stimulated the tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and Shc in cells expressing the CSF1R/IR chimera. Lipid accumulation and the expression of a differentiation-specific marker demonstrated that 3T3-L1 preadipocytes undergo CSF-1-dependent differentiation when transfected with the CSF1R/IR chimera cDNA but not when transfected with the expression vector alone. A 12-amino acid deletion within the juxtamembrane region of the CSF1R/IR (CSF1R/IRDelta960) blocked CSF-1-stimulated phosphorylation of IRS-1 and Shc but did not inhibit CSF-1-mediated differentiation of 3T3-L1 preadipocytes. These observations indicate that adipocyte differentiation can be initiated by intracellular pathways that do not require tyrosine phosphorylation of IRS-1 or Shc.

Phosphatidylinositol 3-kinase is a requirement for insulin-like growth factor I-induced differentiation, but not for mitogenesis, in fetal brown adipocytes

In the present study we have examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the insulin-like growth factor I (IGF-I)-signaling pathways involved in differentiation and in mitogenesis in fetal rat brown adipocytes. Activation of PI 3-kinase in response to IGF-I was markedly inhibited by two PI 3-kinase inhibitors (wortmannin and LY294002) in a dose-dependent manner. IGF-I-stimulated glucose uptake was also inhibited by both compounds. The expression of adipogenic-related genes such as fatty acid synthase, malic enzyme, glycerol 3-phosphate dehydrogenase, and acetylcoenzyme A carboxylase induced by IGF-I was totally prevented in the presence of IGF-I and any of those inhibitors, resulting in a marked decrease of the cytoplasmic lipid content. Moreover, the expression of the thermogenic marker uncoupling protein induced by IGF-I was also down-regulated in the presence of wortmannin/LY294002. IGF-I-induced adipogenic- and thermogenic-related gene expression was only partly inhibited by the p70S6k inhibitor rapamycin. In addition, pretreatment of brown adipocytes with either wortmannin or LY294002, but not with rapamycin, blocked protein kinase C zeta activation by IGF-I. In contrast, IGF-I-induced fetal brown adipocyte proliferation was PI 3-kinase-independent. Our results show for the first time an essential requirement of PI 3-kinase in the IGF-I-signaling pathways leading to fetal brown adipocyte differentiation, but not leading to mitogenesis. In addition, protein kinase C zeta seems to be a signaling molecule also involved in the IGF-I differentiation pathways downstream from PI 3-kinase.

Insulin receptor substrate (IRS) 1 is reduced and IRS-2 is the main docking protein for phosphatidylinositol 3-kinase in adipocytes from subjects with non-insulin-dependent diabetes mellitus

The large docking protein IRS-1 is a major substrate for the insulin receptor and other tyrosine kinases. It plays a key role in eliciting many of insulin's actions, including binding and activation of phosphatidylinositol (PI) 3-kinase and the subsequent increase in glucose transport. Gene disruption of IRS-1 in mice is associated with an impaired insulin-stimulated glucose disposal in vivo and glucose transport in vitro, but the survival of the animals and residual insulin sensitivity is dependent on the presence of the alternative docking protein IRS-2. We examined the expression and function of IRS-1 and IRS-2 in adipocytes from healthy and diabetic individuals. Cells from subjects with non-insulin-dependent diabetes mellitus (NIDDM), but not with insulin-dependent diabetes mellitus, had an impaired insulin effect and a marked reduction (70 +/- 6%) in the expression of IRS-1 protein, whereas IRS-2 was unchanged. In normal cells, IRS-1 was the main docking protein for the binding and activation of insulin-stimulated PI 3-kinase; IRS-2 was also functional but required a higher insulin concentration for a similar binding and activation of PI 3-kinase. In contrast in NIDDM cells with a low IRS-1 content, IRS-2 became the main docking protein. These findings may provide important reasons for the insulin resistance in NIDDM.

Specific increase in p85alpha expression in response to dexamethasone is associated with inhibition of insulin-like growth factor-I stimulated phosphatidylinositol 3-kinase activity in cultured muscle cells

The stimulation of phosphatidylinositol (PI) 3-kinase by insulin-like growth factor I (IGF-I) in L6 cultured skeletal muscle cells is inhibited by the glucocorticoid dexamethasone. The objective of this study was to investigate the mechanism of dexamethasone action by determining its effects on the expression of the p85alpha and p85beta regulatory subunit isoforms of PI 3-kinase, their coupling with the p110 catalytic subunit, and their association with insulin receptor substrate 1 (IRS-1) in response to IGF-I stimulation. Dexamethasone induced a 300% increase in p85alpha protein content in the L6 cultured myoblast cell line, whereas it increased p110 content by only 38% and had no effect on p85beta. The increase in p85alpha protein was associated with a coordinate increase in p85alpha mRNA. Stimulation with IGF-I induced the association of p85alpha and p85beta with IRS-1, and this was accompanied by increased amounts of the p110 catalytic subunit and markedly increased PI 3-kinase activity in IRS-1 immunoprecipitates. In cells treated with dexamethasone, greater amounts of p85alpha and lower amounts of p85beta, respectively, were found in IRS-1 immunoprecipitates, such that the alpha/beta ratio was markedly higher than in control cells. In spite of the increase in both total and IRS-1-associated p85alpha following dexamethasone treatment, IRS-1-associated p110 catalytic subunit and PI 3-kinase activity were decreased by approximately 50%. Thus, dexamethasone induces a specific increase in expression of the p85alpha regulatory subunit that is not associated with a coordinate increase in the p110 catalytic subunit of PI 3-kinase. As a consequence, in dexamethasone-treated cells, p85alpha that is not coupled with p110 competes with both p85alpha.p110 and p85beta.p110 complexes for association with IRS-1, leading to increased p85alpha but decreased p85beta, p110, and PI 3-kinase activity in IRS-1 immunoprecipitates.

The IRS-2 gene on murine chromosome 8 encodes a unique signaling adapter for insulin and cytokine action

Signal transduction by insulin and IGF-1, several interleukins (IL-2, IL-4, IL-9, IL-13), interferons, GH, and other cytokines involves IRS proteins, which link the receptors for these factors to signaling molecules with Src homology-2 domains (SH2-proteins). We recently reported the amino acid sequence of murine IRS-2; in order to examine a potential genetic role for this molecule in disease, we isolated the murine IRS-2 gene and compared the expression pattern of IRS-2 against IRS-1. Like IRS-1, IRS-2 is encoded by a single exon. Whereas IRS-1 is located on murine chromosome 1, IRS-2 is located on murine chromosome 8 near the insulin receptor. IRS-2 is expressed together with IRS-1 in many cells and tissues; however, IRS-2 predominates in murine hematopoietic cells where it may be essential for cytokine signaling; IRS-1 predominates in adipocytes and differentiated 3T3-L1 cells where it contributes to the normal insulin response. In 32D cells, IRS-1 and IRS-2 undergo differential tyrosine phosphorylation during insulin or IL-4 stimulation, as assessed indirectly by interaction with various recombinant SH2 domains. Thus, signaling specificity through the IRS proteins may be accomplished by specific expression patterns and distinct phosphorylation patterns during interaction with various activated receptors.

A casein kinase I activity is constitutively associated with Nck

Nck is a 47-kDa cytosolic protein devoid of intrinsic catalytic activity and consisting of Src homology 2 and 3 (SH2 and SH3) domains organized as follows: SH3-SH3-SH3-SH2. Nck is believed to act as an adaptor protein mediating signal transduction initiated by receptor tyrosine kinases (RTKs). Through its SH2 domain, Nck recognizes a specific phosphotyrosine residue on RTKs or on protein substrates of RTKs like insulin receptor substrate-1, the major substrate of the insulin receptor, and through its SH3 domains it interacts with poorly characterized effector molecules. To identify novel proteins that might interact with Nck, we have used the amino-terminal segment of Nck encompassing its three SH3 domains in the yeast two-hybrid system. Among the polypeptides that associate with Nck, we have identified the gamma2 isoform of the serine/threonine casein kinase I (CKI-gamma2). In transformed rat hepatocytes overexpressing the insulin receptor (HTC-IR cells), serine/threonine protein kinase activity coimmunoprecipitates with Nck, an interaction mediated mainly by the third SH3 domain of Nck. This kinase activity is not apparently modulated by insulin, nor is it sensitive to staurosporine or heparin, and it does not use GTP as a phosphate donor. However the kinase activity coimmunoprecipitated with Nck is completely abolished by N-(2-aminoethyl)-5-chloroisoquinoline-8-sulfonamide, a specific inhibitor of casein kinase I. In an in vitro renaturation gel kinase assay, a protein kinase of 70-75 kDa was detected associated with the SH3 domains of Nck. Far Western analysis demonstrated that the SH3 domains of Nck bound directly to a cytosolic protein of 70-75 kDa. A rabbit polyclonal antibody raised against the C-terminal region of CKI-gamma2 protein kinase immunoprecipitated a single specific protein of 70-75 kDa from HTC-IR cell lysates and detected CKI-gamma2 among the proteins coimmunoprecipitated with Nck. These results support an in vivo interaction between Nck and CKI-gamma2 and suggest that CKI-gamma2 could be involved in signaling pathways downstream of RTKs.

Mutations of Ros differentially effecting signal transduction pathways leading to cell growth versus transformation

The signaling functions of the oncogenic protein-tyrosine kinase v-Ros were studied by systematically mutating the tyrosine residues in its cytoplasmic domain. The carboxyl mutation of Tyr-564 produces the most pronounced inhibitory effect on v-Ros autophosphorylation and interaction with phospholipase Cgamma. A cluster of 3 tyrosine residues, Tyr-414, Tyr-418, and Tyr-419, within the PTK domain of v-Ros plays an important role in modulating its kinase activity. The mutant F419 and the mutant DI, deleting 6-amino acids near the catalytic loop, retain wild type protein tyrosine kinase and mitogenic activities, but have dramatically reduced oncogenicity. Both mutant proteins are able to phosphorylate or activate components in the Ras/microtubule-associated protein kinase signaling pathway. However, F419 mutant protein is unable to phosphorylate insulin receptor substrate 1 (IRS-1) or promote association of IRS-1 with phosphatidylinositol 3-kinase. This tyrosine residue in the context of the NDYY motif may define a novel recognition site for IRS-1. Both F419 and DI mutants display impaired ability to induce tyrosine phosphorylation of a series of cytoskeletal and cell-cell interacting proteins. Thus the F419 and DI mutations define v-Ros sequences important for cytoskeleton signaling, the impairment of which correlates with the reduced cell transforming ability.

Effect of dimerization on signal transduction and biological function of oncogenic Ros, insulin, and insulin-like growth factor I receptors

The avian sarcoma virus UR2 codes for an oncogenic Gag-Ros fusion protein-tyrosine kinase (PTK). We have previously derived two retroviruses, T6 and NM1, coding for oncogenic Gag-insulin receptor and Gag-insulin-like growth factor I receptor (IGFR) fusion proteins, respectively. The Gag-IGFR fusion protein dimerizes, whereas Gag-Ros does not. To identify sequences affecting dimerization and the effect of dimerization on signaling and biological functions, we generated recombinants exchanging the extracellular and transmembrane sequences among the three fusion receptors. The presence of multiple cysteines in the Gag sequence appears to preclude dimerization, since deletion of the 3' cysteine residue allows for dimerization. Most of the chimeric receptors retain high PTK activity and induce transformation regardless of their configuration on the cell surface. UT, a UR2/T6 chimera, retained mitogenic activity but has a markedly reduced transforming ability, while UN7, a UR2/NM1 recombinant, which also harbors Y950F and F951S mutations in IGFR, exhibits dramatic reductions in both activities. All of the fusion receptors can phosphorylate insulin receptor substrate 1 and activate PI 3-kinase. UT protein induces Shc phosphorylation, whereas UN7 protein does not, but both are unable to activate mitogen-activated protein kinase. Our results show that overexpressed oncogenic Gag-fusion receptors do not require dimerization for their signaling and transforming functions and that the extracellular and transmembrane sequences of a receptor PTK can affect its specific substrate interactions.

A synthetic peptide derived from a COOH-terminal domain of the insulin receptor specifically enhances insulin receptor signaling

The role of the insulin receptor COOH-terminal domain in the regulation of insulin signal transduction was explored with a variety of synthetic peptides. One of the peptides, termed peptide HC, whose structure corresponds to residues 1293-1307 of the insulin proreceptor sequence, enhanced insulin-stimulated autophosphorylation of the insulin receptor in cell-free systems and in semipermeabilized Chinese hamster ovary (CHO) cells that had been transfected with an expression plasmid encoding the human insulin receptor (CHO/HIRc) at concentrations where there was no detectable effect on basal autophosphorylation levels or on receptor dephosphorylation. A lipophilic analogue of peptide HC, stearyl peptide HC, added to intact CHO/HIRc cells enhanced significantly insulin-stimulated insulin receptor autophosphorylation while having no effect on ligand-stimulated receptor phosphorylation in CHO cells overexpressing either the IGF-1 receptor or epidermal growth factor receptor. Addition of stearyl peptide HC to CHO/HIRc cells resulted in a 2.4 +/- 0.3-fold increase in the amount of insulin-stimulated phosphatidylinositol 3-kinase detected in anti-IRS-1 immunoprecipitates and a 2.1 +/- 0.6-fold increase in the levels of tyrosine phosphorylation of mitogen-activated protein kinase in response to insulin. Finally, a derivative of peptide HC coupled to a biotin moiety was prepared and showed to bind with the beta-subunit of the wild-type insulin receptor and a truncated receptor that lacks 43 amino acids from its carboxyl terminus. However, there was little binding, if any, of the peptide with the IGF-1 receptors or the epidermal growth factor receptors. Taken together, our data demonstrate that a pentadecapeptide related to the carboxyl terminus of the insulin receptor binds to the insulin receptor beta-subunit and that this interaction may contribute to the increased receptor's intrinsic activity and signal transduction.

Regulation of phosphatidylinositol 3,4,5-trisphosphate 5'-phosphatase activity by insulin

Polyphosphoinositides are thought to be mediators of cellular signaling pathways as well as regulators of cytoskeletal elements and membrane trafficking events. It has recently been demonstrated that a class of phosphatidylinositol (PI) 3,4,5-P3 5'-phosphatases contains SH2 domains and proline-rich regions, which are present in many signaling proteins. We report here that insulin stimulation of Chinese hamster ovary cells (CHO-T) expressing human insulin receptors causes an 8-10-fold increase in PI 3,4,5-P3 5'-phosphatase activity in anti-phosphotyrosine immunoprecipitates of the cell lysates. This insulin-sensitive polyphosphoinositide 5'-phosphatase did not catalyze dephosphorylation of PI 4,5-P2. No change in 5'-phosphatase activity was detected in insulin receptor or IRS-1 immune complexes in response to insulin. However, insulin treatment of CHO-T cells markedly increased the PI 3,4,5-P3 5'-phosphatase activity associated with Shc and Grb2. The insulin-regulated polyphosphoinositide 5'-phosphatase was not immunoreactive with antibody raised against the recently cloned SHIP 5'-phosphatase reported to associate with Shc and Grb2 in B lymphocytes. These data demonstrate that insulin causes formation of complexes containing a PI 3,4,5-P3 5'-phosphatase, and Shc or Grb2, or both, suggesting an important role of this enzyme in insulin signaling.

Mutant of insulin receptor substrate-1 incapable of activating phosphatidylinositol 3-kinase did not mediate insulin-stimulated maturation of Xenopus laevis oocytes

Insulin receptor substrate-1 (IRS-1) is rapidly phosphorylated on multiple tyrosine residues in response to insulin and binds several Src homology 2 domain-containing proteins, thereby initiating downstream signaling. To assess the tyrosine phosphorylation sites that mediate relevant downstream signaling and biological effects, we created site-directed mutants of IRS-1 and overexpressed them in the Xenopus laevis oocyte. In oocytes overexpressing IRS-1 or IRS-1-895F (Tyr-895 replaced with phenylalanine), insulin activated phosphatidylinositol (PI) 3-kinase, p70 S6 kinase, and mitogen-activated protein kinase and induced oocyte maturation. In contrast, in oocytes overexpressing IRS-1-4F (Tyr-460, Tyr-608, Tyr-939, and Tyr-987 of IRS-1 replaced with phenylalanine), insulin did not activate PI 3-kinase, p70 S6 kinase, and mitogen-activated protein kinase and failed to induce oocyte maturation. These observations indicate that in X. laevis oocytes overexpressing IRS-1, the association of PI 3-kinase rather than Grb2 (growth factor-bound protein 2) with IRS-1 plays a major role in insulin-induced oocyte maturation. Activation of PI 3-kinase may lie upstream of mitogen-activated protein kinase activation and p70 S6 kinase activation in response to insulin.

Stimulation of C2C12 myoblast growth by basic fibroblast growth factor and insulin-like growth factor 1 can occur via mitogen-activated protein kinase-dependent and -independent pathways

It is now well-recognized that the mitogen-activated protein (MAP) kinase cascade facilitates signaling from an activated tyrosine kinase receptor to the nucleus. In fact, an increasing number of extracellular effectors have been reported to activate the MAP kinase cascade, with a significant number of cellular responses attributed to this activation. We set out to explore how two extracellular effectors, basic fibroblast growth factor (bFGF) and insulin-like growth factor 1 (IGF-1), which have both been reported to activate MAP kinase, generate quite distinct cellular responses in C2C12 myoblasts. We demonstrate here that bFGF, which is both a potent mitogen and inhibitor of myogenic differentiation, is a strong MAP kinase agonist. By contrast, IGF-1, which is equally mitogenic for C2C12 cells but ultimately enhances the differentiated phenotype, is a weak activator of the MAP kinase cascade. We further demonstrate that IGF-1 is a potent activator of both insulin receptor substrate IRS-1 tyrosyl phosphorylation and association of IRS-1 with activated phosphatidylinositol 3-kinase (PI 3-kinase). Finally, use of the specific MAP kinase kinase inhibitor, PD098059, and wortmannin, a PI 3-kinase inhibitor, suggests the existence of an IGF-1-induced, MAP kinase-independent signaling event which contributes to the mitogenic response of this factor, whereas bFGF-induced mitogenesis appears to strongly correlate with activation of the MAP kinase cascade.

A carboxy-terminal truncated insulin receptor substrate-1 dominant negative protein reverses the human hepatocellular carcinoma malignant phenotype

Insulin receptor substrate-1 (IRS-1), a substrate of various receptor tyrosine kinases transmits mitogenic signals initiated by extracellular ligands. This protein is involved in normal hepatocyte growth and has been found to be overexpressed in human hepatocellular carcinoma. Expression of a carboxy-terminal truncated IRS-1 molecule containing the pleckstrin homology and phosphotyrosine-binding domains associates with the insulin receptor and prevents tyrosyl phosphorylation of endogenous IRS-1 and Shc proteins. Thus, subsequent activation of downstream signaling molecules induced by insulin and IGF-1 such as phosphatidylinositol-3 kinase and mitogen activated protein kinase is inhibited. The morphologic features of transformed human hepatocellular carcinoma cells change to a differentiated hepatocyte appearance and characteristics of the malignant phenotype as manifested by anchorage independent cell growth and tumor formation in nude mice are lost. These studies demonstrate that signal transduction pathways mediated through or by IRS-1 are important in hepatocyte and human hepatocellular carcinoma cell growth.

The early intracellular signaling pathway for the insulin/insulin-like growth factor receptor family in the mammalian central nervous system

Several studies support the idea that the polypeptides belonging to the family of insulin and insulin-like growth factors (IGFs) play an important role in brain development and continue to be produced in discrete areas of the adult brain. In numerous neuronal populations within the olfactory bulb, the cerebral and cerebellar cortex, the hippocampus, some diencephalic and brainstem nuclei, the spinal cord and the retina, specific insulin and IGF receptors, as well as crucial components of the intracellular receptor signaling pathway have been demonstrated. Thus, mature neurons are endowed with the cellular machinery to respond to insulin and IGF stimulation. Studies in vitro and in vivo, using normal and transgenic animals, have led to the hypothesis that, in the adult brain, IGF-I not only acts as a trophic factor, but also as a neuromodulator of some higher brain functions, such as long-term potentiation and depression. Furthermore, a trophic effect on certain neuronal populations becomes clearly evident in the ischemic brain or neurodegenerative disorders. Thus, the analysis of the early intracellular signaling pathway for the insulin/IGF receptor family in the brain is providing us with new intriguing findings on the way the mammalian brain is sculpted and operates.

Biochemical isolation of a membrane microdomain from resting platelets highly enriched in the plasma membrane glycoprotein CD36

Here we describe the isolation and characterization of a Triton X-100-insoluble fraction isolated from lysates of platelets by flotation in sucrose gradients. Transmission electron microscopy of the insoluble material revealed a heterogeneous population of vesicles ranging in size from 20 to 1000 nm, and Western blot analyses of platelet lysates for the caveolae structural coat protein, caveolin/VIP21, were negative. Biochemical characterization of the Triton X-100-insoluble fraction showed it to be cholesterol-rich, greatly and specifically enriched in the plasma membrane glycoprotein CD36, and also to contain Src and the Src-related kinase, Lyn. CD36 within this fraction is shown to be palmitoylated, but the fraction itself is not generally enriched in palmitoylated platelet proteins. These results suggest that this fraction represents caveolin-negative, CD36-rich microdomains in the resting platelet membrane. CD36 can form associations with certain Src-related kinases and can signal to activate platelets. These results suggest the possibility that such microdomains are implicated in platelet activation.

Glucose-induced tyrosine phosphorylation of p125 in beta cells and pancreatic islets. A novel proximal signal in insulin secretion

In this study, we demonstrate that stimulation of beta cells with carbachol and glucose causes increased tyrosine phosphorylation of a 125-kDa protein concurrently with increased insulin secretion. The effect was observed in two different insulin-secreting cell lines and in rat pancreatic islets. Tyrosine phosphorylation was largely calcium independent and occurred within 2 min after stimulation of beta cells with glucose and the muscarinic agonist carbachol. In islets, the effect of glucose was greatly diminished by the addition of mannoheptulose, a seven-carbon sugar that inhibits glucokinase, suggesting that glucose metabolism is required for tyrosine phosphorylation of the protein to occur. Neither insulin nor insulin-like growth factor I significantly increased tyrosine phosphorylation of the 125-kDa protein, suggesting that it was not an autocrine effect. Depolarization of beta cells with glyburide or 50 m potassium dramatically increased insulin secretion but had no significant effect on tyrosine phosphorylation. Addition of phorbol ester caused a less than 2-fold increase in tyrosine phosphorylation, whereas the calcium ionophore A23187 had no effect. Among the various fuel secretagogues tested, only -glucose stimulated tyrosine phosphorylation, both alone and in combination with carbachol. Finally, the tyrosine kinase inhibitor AG879 inhibited both tyrosine phosphorylation and insulin secretion in a dose-dependent manner. Taken together, these data demonstrate the presence of a novel signaling pathway in glucose-induced insulin secretion: tyrosine phosphorylation of beta cell p125, which is a proximal step in insulin secretion. Our current working hypothesis is that glucose stimulation of beta cell p125 tyrosine phosphorylation is an essential step for insulin secretion.

Insulin-induced tyrosine dephosphorylation of paxillin and focal adhesion kinase requires active phosphotyrosine phosphatase 1D

Insulin stimulation of fibroblasts rapidly induces the tyrosine dephosphorylation of proteins of 68 kDa and 125 kDa, in addition to the tyrosine phosphorylation of the insulin receptor beta-chain, insulin receptor substrates 1 and 2, and Shc. Using specific antibodies, the 68 kDa and 125 kDa proteins were identified as paxillin and focal adhesion kinase (pp125FAK) respectively. We have examined whether dephosphorylation of paxillin and pp125FAK requires interaction of the cells with the extracellular matrix. For this, cells were grown on poly(L-lysine) plates, and the tyrosine phosphorylation of pp125FAK and paxillin was increased by addition of lysophosphatidic acid. Under these conditions, insulin still induced the complete dephosphorylation of pp125FAK and paxillin, indicating that this process can occur independently of the interaction of integrins with extracellular matrix proteins. We also studied whether dephosphorylation of pp125FAK and paxillin results from the action of a phosphotyrosine phosphatase. It was found that phenylarsine oxide, a phosphotyrosine phosphatase inhibitor, prevented the insulin-induced dephosphorylation of pp125FAK and paxillin. Furthermore, this insulin-induced dephosphorylation was also impaired in cells expressing a dominant-negative mutant of phosphotyrosine phosphatase 1D (PTP 1D). Thus we have identified paxillin as a target for dephosphorylation by insulin. In addition, we have obtained evidence that the insulin-mediated dephosphorylation of paxillin and pp125FAK requires active PTP 1D.

Insulin and angiotensin II are additive in stimulating TGF-beta 1 and matrix mRNAs in mesangial cells

Angiotensin II (Ang II) and insulin are implicated in the mesangial cell hypertrophy and excessive accumulation of mesangial matrix seen in glomerulosclerosis. Therefore, the effects of Ang II with and without insulin on mRNA levels of several important extracellular matrix genes and transforming growth factor beta-1 (TGF-beta 1) were examined. Ang II alone (1 microM) added to quiescent, murine mesangial cells in serum-free, insulin-free media slightly but not significantly increased TGF-beta 1, fibronectin, collagen I, collagen IV and laminin message levels. The slight elevations in message expression were reversed by losartan, suggesting that these modest effects are mediated by the AT-1 receptor. Ang II alone also had no significant effects on TGF-beta 1 and extracellular matrix message levels in quiescent rat mesangial cells. In contrast, significant increases in mRNA for collagen 1 (6-fold), collagen IV (4-fold), fibronectin 1 (4-fold) and TGF-beta 1 (2-fold) were seen with insulin alone (10(-6)M) in rat mesangial cells, and a dose-response effect could be demonstrated for insulin (10(-9) to 10(-6)M). Ang II plus insulin further significantly increased collagen I (9-fold), collagen IV (9-fold), fibronectin 1 (5-fold) and TGF-beta 1 (3-fold) message expression. These effects were partially reversed in the presence of losartan. The Northern analyses were supported by measurements of active and total TGF-beta 1 activity (pg/ml/ 5 x 10(6) cells): 1145 +/- 76 and 1960 +/- 199, serum free control; 1121 +/- 92 and 1932 +/- 214, Ang II (10(-6)M); 4589 +/- 103 (P < 0.001 vs. control) and 11071 +/- 1952 (P < 0.01 vs. control), insulin (10(-6)M); and 6881 +/- 183 (P < 0.001 vs. control) and 16626 +/- 1435 (P < 0.01 vs. control), insulin plus Ang II. These results suggest that insulin, itself, significantly increases TGF-beta 1 and extracellular matrix gene expression in rat mesangial cells. Ang II alone has modest effects, while Ang II and insulin have additive effects. To explain the mechanism of these additive effects, we investigated the action of Ang II on insulin signaling and the effect of insulin on Ang II AT1 receptor mRNA expression. Ang II did not enhance insulin-induced insulin receptor substrate-1 (IRS-1) phosporylation or phosphatidylinositol3 (PI-3) kinase activity, but did enhance insulin-induced mitogen activated protein (MAP) kinase activity. Insulin increased message levels of AT1 receptor by twofold. These results suggest that enhancement of MAP kinase activity and AT1 receptor regulation by insulin may contribute to the additive effects of insulin and Ang II in mesangial cells.

Binding to the platelet-derived growth factor receptor transiently activates the p85alpha-p110alpha phosphoinositide 3-kinase complex in vivo

Ligand stimulation of the platelet-derived growth factor (PDGF) receptor results in its association with phosphoinositide 3-kinase activity and a corresponding synthesis of 3'-phosphorylated lipids. Early studies that examined this interaction in vivo employed anti-phosphotyrosine antiserum or antiserum against the PDGF receptor. The recent identification of multiple isoforms of both the regulatory and the catalytic subunit of the enzyme have led us to utilize antisera against p85alpha and p110alpha to characterize the association of this particular phosphoinositide 3-kinase complex with the PDGF receptor following ligand stimulation of murine fibroblasts. Both the p85alpha and p110alpha subunits rapidly associated with the ligand-activated receptor resulting in a transient, 2-fold increase in the total pool of p110alpha lipid kinase activity. This association was stable for 15 min after initial stimulation. Subsequently, both subunits began to dissociate from the receptor with similar kinetics. By 60 min this process was complete, demonstrating that p85alpha and p110alpha both associate with the receptor and dissociate from the receptor as a dimeric complex. At this time, marked PDGF receptor down-regulation was observed. Immunoprecipitation from metabolically labeled cells revealed that p85alpha is constitutively phosphorylated on serine residues in quiescent cultures. Upon PDGF stimulation, this phosphorylation upon serine residues was maintained in addition to tyrosine phosphorylation of this subunit. No phosphorylation of the p110alpha subunit was detected in either quiescent or PDGF-stimulated cells. Quantitation of Western blot analysis demonstrated that only 5% of the total pool of p85alpha associated with the PDGF receptor upon ligand stimulation. The 2-fold increase in the lipid kinase activity measured in immunoprecipitates using either anti-p85alpha or anti-p110alpha antiserum therefore reflects a far greater increase in the specific activity of the enzyme upon its association with the PDGF receptor.

YMXM motifs and signaling by an insulin receptor substrate 1 molecule without tyrosine phosphorylation sites

Tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1) by the activated receptors for insulin, IGF-1, and various cytokines creates binding sites for signaling proteins with Src homology 2 domains (SH2 proteins). Determining the role of specific SH2 proteins during insulin signaling has been difficult because IRS-1 possesses as many as 18 potential tyrosine phosphorylation sites, several of which contain redundant motifs. Using 32D cells, which contain no endogenous IRS proteins, we compared the signaling ability of an IRS-1 molecule in which 18 potential tyrosine phosphorylation sites were replaced by phenylalanine (IRS-1(F18)) with two derivative molecules which retained three YMXM motifs (IRS-1(3YMXM)) or the two COOH-terminal SHP2-Fyn binding sites (IRS-1(YCT)). During insulin stimulation, IRS-1(F18) failed to undergo tyrosine phosphorylation or mediate activation of the phosphotidylinositol (PI) 3'-kinase or p70(s6k); IRS-1(YCT) was tyrosine phosphorylated but also failed to mediate these signaling events. Neither IRS-1(3YMXM) nor IRS-1(YCT) mediated activation of mitogen-activated protein kinases. IRS-1(F18) and IRS-1(YCT) partially mediated similar levels of insulin-stimulated mitogenesis at high insulin concentrations, however, suggesting that IRS-1 contains phosphotyrosine-independent elements which effect mitogenic signals, and that the sites in IRS-l(YCT) do not augment this signal. IRS-1(3YMXM) mediated the maximal mitogenic response to insulin, although the response to insulin was more sensitive with wild-type IRS-1. By contrast, the association of IRS-1(3YMXM) with PI 3'-kinase was more sensitive to insulin than the association with IRS-1. Thus, the binding of SH2 proteins (such as PI 3'-kinase) by YMXM motifs in IRS-1 is an important element in the mitogenic response, but other elements are essential for full mitogenic sensitivity.

Neoplastic transformation induced by insulin receptor substrate-1 overexpression requires an interaction with both Grb2 and Syp signaling molecules

The insulin receptor substrate-1 (IRS-1) is the major intracellular substrate of insulin and insulin-like growth factor-I (IGF-I) receptor tyrosine kinase activity, and this protein has been found to be overexpressed in human hepatocellular carcinomas. IRS-1 contains several src homology 2 (SH2) binding motifs that interact following tyrosyl phosphorylation with SH2-containing proteins, and this interaction may be essential for transmitting the growth signal from the cell surface to the nucleus. We have previously reported that overexpression of IRS-1 may induce neoplastic transformation of NIH 3T3 cells. This study examines the role of two SH2-containing molecules, namely the Grb2 adapter and Syp tyrosine phosphatase proteins as important components of the cellular transforming activity of IRS-1. Mutations of tyrosine 897 in the YVNI motif (Y897F) and of tyrosine 1180 in the YIDL motif (Y1180F) reduced the intracellular interaction of IRS-1 with Grb2 and Syp proteins, respectively. Furthermore, a single mutation at either Phe-897 or Phe-1180 substantially but not completely reduced IGF-I-dependent transforming activity of IRS-1, whereas creation of a double mutation of both tyrosine residues (Y897F/Y1180F) strikingly attenuated the transforming activity of IRS-1. Stable expression of the IRS-1 mutant constructs in NIH 3T3 cells was associated with a lower level of activation of the mitogen-activated protein kinase kinase (MAPKK)/MAPK cascade following IGF-I stimulation compared with cells stably transfected with the "wild-type" IRS-1 gene. These results suggest that IRS-1-induced cellular transformation requires an interaction with both Grb2 and Syp signal transduction molecules since neither interaction alone appears to be required, and this event subsequently leads to activation of the MAPKK/MAPK cascade.

Long-term treatment of Swiss 3T3 fibroblasts with dexamethasone attenuates MAP kinase activation induced by insulin-like growth factor-I (IGF-I)

Bone formation is reduced in hyperglucocorticoid states, e.g. Cushing's syndrome or long-term treatment with synthetic glucocorticoids during rheumatic diseases. possibly related to decreased sensitivity of the target to insulin-like growth factor-I (IGF-I). In this study, we have sought to identify postreceptor-mechanisms for glucocorticoid-induced resistance to insulin-like peptides in a model system. Treatment of Swiss 3T3 fibroblasts with 100 nM dexamethasone for 48h reduced IGF-I-induced activation of mitogen-activated protein kinase (MAP kinase). The level of insulin receptor substrate-1 (IRS-1) was reduced in dexamethasone-treated cells, as measured by Western blot; however, the pattern of tyrosine-phosphorylated protein subsequent to stimulation with IGF-I (1 min) was not altered. No inhibitory effect of dexamethasone was observed on the level of phosphotyrosine in IRS-1 in extracts from IGF-I-treated cells. The amount of IGF-I-induced association of insulin receptor substrate-1 and phosphatidylinositol 3-kinase was increased in steroid treated cells. Addition of IGF-I increased the synthesis of lipid, glycogen and protein, and the reduction of a tetrazolium dye, MTS, in untreated cells. The response to IGF-I in terms of glycogen synthesis was blunted, whereas the effect of IGF-I was unaffected for the other three parameters in cells pretreated with dexamethasone. These findings indicate that the activation of MAP kinase may be dissociated from IGF-I-induced anabolic pathways and tyrosine phosphorylation of IRS-1. The results agree with the previously proposed role for the activation of MAP kinase in the regulation of glycogen synthesis. Furthermore, they suggest that dexamethasone-induced reduction of IRS-1 expression may be important for the impaired activation of MAP kinase by insulin-like peptides in steroid-treated cells.

Phosphoinositide 3-kinase and p72syk noncovalently associate with the low affinity Fc gamma receptor on human platelets through an immunoreceptor tyrosine-based activation motif. Reconstitution with synthetic phosphopeptides

Previously, we have demonstrated that the cytoplasmic tyrosine kinase p72syk is coupled to the platelet Fc receptor for IgG (Fc gamma RIIA) (Chacko, G. W., Duchemin, A. M., Coggeshall, K. M., Osborne, J. M., Brandt, J. T., and Anderson, C. L. (1994) J. Biol. Chem. 269, 32435-32440). Further analysis of the platelet activation by Fc gamma RIIA demonstrated that Fc gamma RIIA is also inducibly coupled to the serine/threonine and lipid kinase, phosphoinositide 3-kinase (PI 3-K). activation of platelets with anti-Fc gamma RIIA antibodies resulted in the noncovalent association of PI 3-K with Fc gamma RIIA as well as an increase in Fc gamma RIIA-associated PI 3-K activity. Binding of both p72syk and PI 3-K to Fc gamma RIIA was reconstituted with synthetic phosphopeptides corresponding to the sequence of the atypical immunoreceptor tyrosine-based activation motif (ITAM) in the cytoplasmic domain of Fc gamma RIIA. Our findings demonstrate that coupling of both p72syk and PI. 3-K activities to Fc gamma RIIA is regulated by tyrosine phosphorylation of the ITAM, and we speculate that p72syk might act as an adapter to recruit PI 3-K to activated Fc gamma RIIA.

The Drosophila insulin receptor activates multiple signaling pathways but requires insulin receptor substrate proteins for DNA synthesis

The Drosophila insulin receptor (DIR) contains a 368-amino-acid COOH-terminal extension that contains several tyrosine phosphorylation sites in YXXM motifs. This extension is absent from the human insulin receptor but resembles a region in insulin receptor substrate (IRS) proteins which binds to the phosphatidylinositol (PI) 3-kinase and mediates mitogenesis. The function of a chimeric DIR containing the human insulin receptor binding domain (hDIR) was investigated in 32D cells, which contain few insulin receptors and no IRS proteins. Insulin stimulated tyrosine autophosphorylation of the human insulin receptor and hDIR, and both receptors mediated tyrosine phosphorylation of Shc and activated mitogen-activated protein kinase. IRS-1 was required by the human insulin receptor to activate PI 3-kinase and p70s6k, whereas hDIR associated with PI 3-kinase and activated p70s6k without IRS-1. However, both receptors required IRS-1 to mediate insulin-stimulated mitogenesis. These data demonstrate that the DIR possesses additional signaling capabilities compared with its mammalian counterpart but still requires IRS-1 for the complete insulin response in mammalian cells.

Roles of insulin, guanosine 5'-[gamma-thio]triphosphate and phorbol 12-myristate 13-acetate in signalling pathways of GLUT4 translocation

Insulin, guanosine 5'-[gamma-thio]triphosphate (GTP[S] and phorbol 12-myristate 13-acetate (PMA) trigger the translocation of Gl UT4 (type 4 glucose transporter; insulin-sensitive glucose transporter) from an intracellular pool to the cell surface. We have developed a highly sensitive and quantitative method to detect GLUT4 immunologically on the surface of intact 3T3-L1 adipocytes and Chinese hamster ovary (CHO) cells, using c-myc epitope-tagged GLUT4 (GLUT4myc). We examined the roles of insulin, GTP[S] and PMA in the signalling pathways of GLUT4 translocation in the CHO cell system. Among small molecular GTP-binding proteins, ras, rab3D, rad and rho seem to be candidates as signal transmitters of insulin-stimulated GLUT4 translocation. Overexpression of wild-type H-ras and the dominant negative mutant H-rass17N in our cell system respectively enhanced and blocked insulin-stimulated activation of mitogen-activated protein kinase, but did not affect insulin-stimulated GLUT4 translocation. Overexpression of rab3D or rad in the cells did not affect GLUT4 translocation triggered by insulin, GTP[S] or PMA. Treatment with Botulinum C3 exoenzyme, a specific inhibitor of rho, had no effect on GLUT4 translocation induced by insulin, GTP[S] or PMA. Therefore these small molecular GTP-binding proteins are not likely to be involved in GLUT4 translocation. In addition, insulin, GTP[S] and PMA apparently stimulate GLUT4 translocation through independent pathways.

The proto-oncogene product c-Crk associates with insulin receptor substrate-1 and 4PS. Modulation by insulin growth factor-I (IGF) and enhanced IGF-I signaling

The Crk proto-oncogene product is an SH2 and SH3 domain-containing adaptor protein which we have previously shown to become rapidly tyrosine phosphorylated in response to stimulation with insulin-like growth factor I (IGF-I) in NIH-3T3 cells. In order to further characterize the role of Crk in the IGF-I signaling pathway, NIH-3T3 and 293 cells were stably transfected with an expression vector containing the Crk cDNA. The various resultant 3T3-Crk clones expressed Crk at approximately 2-15-fold higher levels than parental 3T3 cells. In 3T3-Crk cells, Crk immunoreactivity was detected in insulin receptor substrate-1 (IRS-1) immunoprecipitates. Stimulation with IGF-I resulted in a dissociation of Crk protein from IRS-1. In contrast, the association of the related adaptor protein Grb2 with IRS-1 was enhanced by IGF-I stimulation. Similar results were obtained in stably transfected 293-Crk cells, which express both IRS-1 and the IRS-1-related signaling protein 4PS. In these cells, IRS-1 and 4PS both associated with Crk, and this association was also decreased by IGF-I treatment, whereas the association of Grb2 with IRS-1 and 4PS was enhanced by IGF-I. Overexpression of Crk also enhanced IGF-I-induced mitogenesis of NIH-3T3 cells, as measured by [3H]thymidine incorporation. The levels of IGF-I-induced mitogenesis were proportional to the level of Crk expression. These results suggest that Crk is a positive effector of IGF-I signaling, and may mediate its effects via interaction with IRS-1 and/or 4PS.

Functional interactions of phosphatidylinositol 3-kinase with GTPase-activating protein in 3T3-L1 adipocytes

The role of phosphatidylinositol (PI) 3-kinase in specific aspects of insulin signaling was explored in 3T3-L1 adipocytes. Inhibition of PI 3-kinase activity by LY294002 or wortmannin significantly enhanced basal and insulin-stimulated GTPase-activating protein (GAP) activity in 3T3-L1 adipocytes. Furthermore, removal of the inhibitory influence of PI 3-kinase on GAP resulted in dose-dependent decreases in the ability of insulin to stimulate p21ras. This effect was specific to adipocytes, as inhibition of PI 3-kinase did not influence GAP in either 3T3-L1 fibroblasts, Rat-1 fibroblasts, or CHO cells. Immunodepletion of either of the two subunits of the PI 3-kinase (p85 or p110) yielded similar activation of GAP, suggesting that catalytic activity of p110 plays an important role in controlling GAP activity in 3T3-L1 adipocytes. Inhibition of PI 3-kinase activity in 3T3-L1 adipocytes resulted in abrogation of insulin-stimulated glucose uptake and thymidine incorporation. In contrast, effects of insulin on glycogen synthase and mitogen-activated protein kinase activity were inhibited only at higher concentrations of LY294002. It appears that in adipocytes, P1 3-kinase prevents activation of GAP. Inhibition of PI 3-kinase activity or immunodepletion of either one of its subunits results in activation of GAP and decreases in GTP loading of p21ras.

Overexpression of human insulin receptor substrate 1 induces cellular transformation with activation of mitogen-activated protein kinases

The receptor insulin substrate 1 protein (IRS-1) is a specific substrate for insulin receptor tyrosine kinase. Expression and tyrosyl phosphorylation of IRS-1 play an important role during normal hepatocyte growth, and the gene is overexpressed in hepatocellular carcinoma tissue. We determined if IRS-1 overexpression directly contributes to cellular transformation. The human IRS-1 gene was subcloned into a mammalian expression vector driven by the cytomegalovirus early promoter. NIH 3T3 cells transiently transfected with this vector subsequently developed transformed foci. Several stably transfected cell lines were established, and they grew efficiently under low-serum conditions and formed colonies when plated in soft agar. Cell lines overexpressing IRS-1 displayed increased tyrosyl phosphorylation of IRS-1 and association with Grb2 but not with the p85 subunit of phosphatidylinositol 3' kinase. Since Grb2 is a component of the son-of-sevenless-Ras pathway and upstream in the mitogen-activated protein kinase (MAPK) cascade, enzymatic activities of the major components of this cascade, such as MAPK kinase and MAPK were evaluated and found to be substantially increased in three independent cell lines with IRS-1 protein overexpression. Such cells, when injected into nude mice, were highly tumorigenic, and there may be a correlation between the degree of MAPK activation and tumor growth rate. This report describes the generation of a transformed phenotype by overexpression of a molecule without a catalytic domain far upstream in the signal transduction cascade and suggests that prolonged activation of MAPKs by this mechanism may be one of the molecular events related to hepatocellular transformation.

Diverse signaling pathways in the cellular actions of insulin

Insulin is one of the most important regulators of glucose and lipid homeostasis. Many of its cellular actions are mediated by changes in protein phosphorylation. The consequences of these phosphorylation events extend from a series of different short-term metabolic actions to longer-term effects of the hormone on cellular growth and differentiation. Although the insulin receptor itself is a tyrosine kinase that is activated upon hormone binding, the ensuing changes in phosphorylation occur predominantly on serine and threonine residues. Moreover, insulin can simultaneously stimulate the phosphorylation of some proteins and the dephosphorylation of others. These paradoxical effects of insulin suggest that separate signal transduction pathways may emanate from the receptor itself to produce the pleiotropic actions of the hormone.

The IRS-signalling system in insulin and cytokine action

IRS-signalling proteins are engaged and phosphorylated on tyrosine residues by the receptors for insulin and IGF-1, and various classes of cytokine receptors, including IL-4, IL-9, and IL-13; IFN alpha/beta and IFN gamma; and growth hormone and LIF. IRS-proteins provide an interface between these receptors and signalling proteins which contain Src homology-2 domains (SH2-proteins). The recent identification of IRS-2 provides new insight into the modular structure and function of the IRS-proteins. The IRS-proteins provide a means for signal amplification by eliminating the stoichiometric constraints encountered by most receptors which directly recruit SH2-proteins to their autophosphorylation sites. Moreover, IRS-proteins dissociate the intracellular signalling complex from the endocytic pathways of the activated receptor. The shared use of IRS-proteins by multiple receptors is likely to reveal important connections between various hormones and cytokines that were previously unrecognized,or observed but unexplained. The existence of additional signalling molecules based on the IRS-paradigm is likely.