Functional importance of amino-terminal domain of Shc for interaction with insulin and epidermal growth factor receptors in phosphorylation-independent manner

Role of Caveolin-1 in Diabetes and Its Complications

It is estimated that in 2017 there were 451 million people with diabetes worldwide. These figures are expected to increase to 693 million by 2045; thus, innovative preventative programs and treatments are a necessity to fight this escalating pandemic disorder. Caveolin-1 (CAV1), an integral membrane protein, is the principal component of caveolae in membranes and is involved in multiple cellular functions such as endocytosis, cholesterol homeostasis, signal transduction, and mechanoprotection. Previous studies demonstrated that CAV1 is critical for insulin receptor-mediated signaling, insulin secretion, and potentially the development of insulin resistance. Here, we summarize the recent progress on the role of CAV1 in diabetes and diabetic complications.

Novel roles for insulin receptor (IR) in adipocytes and skeletal muscle cells via new and unexpected substrates

The insulin signaling pathway regulates whole-body glucose homeostasis by transducing extracellular signals from the insulin receptor (IR) to downstream intracellular targets, thus coordinating a multitude of biological functions. Dysregulation of IR or its signal transduction is associated with insulin resistance, which may culminate in type 2 diabetes. Following initial stimulation of IR, insulin signaling diverges into different pathways, activating multiple substrates that have roles in various metabolic and cellular processes. The integration of multiple pathways arising from IR activation continues to expand as new IR substrates are identified and characterized. Accordingly, our review will focus on roles for IR substrates as they pertain to three primary areas: metabolism/glucose uptake, mitogenesis/growth, and aging/longevity. While IR functions in a seemingly pleiotropic manner in many cell types, through these three main roles in fat and skeletal muscle cells, IR multi-tasks to regulate whole-body glucose homeostasis to impact healthspan and lifespan.

Systems-level interactions between insulin-EGF networks amplify mitogenic signaling

Crosstalk mechanisms have not been studied as thoroughly as individual signaling pathways. We exploit experimental and computational approaches to reveal how a concordant interplay between the insulin and epidermal growth factor (EGF) signaling networks can potentiate mitogenic signaling. In HEK293 cells, insulin is a poor activator of the Ras/ERK (extracellular signal-regulated kinase) cascade, yet it enhances ERK activation by low EGF doses. We find that major crosstalk mechanisms that amplify ERK signaling are localized upstream of Ras and at the Ras/Raf level. Computational modeling unveils how critical network nodes, the adaptor proteins GAB1 and insulin receptor substrate (IRS), Src kinase, and phosphatase SHP2, convert insulin-induced increase in the phosphatidylinositol-3,4,5-triphosphate (PIP₃) concentration into enhanced Ras/ERK activity. The model predicts and experiments confirm that insulin-induced amplification of mitogenic signaling is abolished by disrupting PIP₃-mediated positive feedback via GAB1 and IRS. We demonstrate that GAB1 behaves as a non-linear amplifier of mitogenic responses and insulin endows EGF signaling with robustness to GAB1 suppression. Our results show the feasibility of using computational models to identify key target combinations and predict complex cellular responses to a mixture of external cues.

Indirect recruitment of the signalling adaptor Shc to the fibroblast growth factor receptor 2 (FGFR2)

The adaptor protein Shc (Src homology and collagen-containing protein) plays an important role in the activation of signalling pathways downstream of RTKs (receptor tyrosine kinases) regulating diverse cellular functions, such as differentiation, adhesion, migration and mitogenesis. Despite being phosphorylated downstream of members of the FGFR (fibroblast growth factor receptor) family, a direct interaction of Shc with this receptor family has not been described to date. Various studies have suggested potential binding sites for the Shc PTB domain (phosphotyrosine-binding domain) and/or the SH2 (Src homology 2) domain on FGFR1, but no interaction of full-length Shc with these sites has been reported in vivo. In the present study, we investigated the importance of the SH2 domain and the PTB domain in recruitment of Shc to FGFR2(IIIc) to characterize the interaction of these two proteins. Confocal microscopy revealed extensive co-localization of Shc with FGFR2. The PTB domain was identified as the critical component of Shc which mediates membrane localization. Results from FLIM (fluorescence lifetime imaging microscopy) revealed that the interaction between Shc and FGFR2 is indirect, suggesting that the adaptor protein forms part of a signalling complex containing the receptor. We identified the non-RTK Src as a protein which potentially mediates the formation of such a ternary complex. Although an interaction between Src and Shc has been described previously, in the present study we implicate the Shc SH2 domain as a novel mediator of this association. The recruitment of Shc to FGFR2 via an indirect mechanism provides new insight into the regulation of protein assembly and activation of various signalling pathways downstream of this RTK.

Intracellular delivery of glutathione S-transferase-fused proteins into mammalian cells by polyethylenimine-glutathione conjugates

The glutathione S-transferase (GST)-fused protein expression system has been extensively used to generate a large quantity of proteins and has served for functional analysis in vitro. In this study, we developed a novel approach for the efficient intracellular delivery of GST-fused proteins into living cells to expand their usefulness up to in vivo use. Since protein cationization techniques are powerful strategies for efficient intracellular uptake by adsorptive-mediated endocytosis, GST-fused proteins were cationized by forming a complex with a polycationic polyethylenimine (PEI)-glutathione conjugate. On screening of protein transduction, optimized PEI-glutathione conjugate for protein transduction was characterized by a partly oligomerized mixture of PEI with average molecular masses of 600 (PEI600) modified with multiple glutathiones, which could have sufficient avidity for GST. Furthermore, enhanced endosomal escape of transduced GST-fused proteins was observed when they were delivered with a glutathione-conjugated PEI600 derivative possessing a hydroxybutenyl moiety. These results were confirmed by both intracellular confocal imaging of GST-fused green fluorescent protein and activation of an endogenous growth signal transduction pathway by a GST-fused constitutively active mutant of a kinase protein. These PEI-glutathione conjugates seem to be convenient molecular tools for protein transduction of widely used GST-fused proteins.

Beta-eudesmol suppresses tumour growth through inhibition of tumour neovascularisation and tumour cell proliferation

In the present study, we investigated the potential anti-angiogenic mechanism and anti-tumour activity of beta-eudesmol using in vitro and in vivo experimental models. Proliferation of human umbilical vein endothelial cells (HUVEC) stimulated with vascular endothelial growth factor (VEGF, 30 ng/ml) and basic fibroblast growth factor (bFGF, 30 ng/ml) was significantly inhibited by beta-eudesmol (50-100 microM). Beta-eudesmol (100 microM) also blocked the phosphorylation of cAMP response element binding protein (CREB) induced by VEGF (30 ng/ml) in HUVEC. Beta-eudesmol (10-100 microM) inhibited proliferation of HeLa, SGC-7901, and BEL-7402 tumour cells in a time- and dose-dependent manner. Moreover, beta-eudesmol treatment (2.5-5 mg/kg) significantly inhibited growth of H(22) and S(180) mouse tumour in vivo. These results indicated that beta-eudesmol inhibited angiogenesis by suppressing CREB activation in growth factor signalling pathway. This is the first study to demonstrate that beta-eudesmol is an inhibitor of tumour growth.

Antiangiogenic activity of β-eudesmol in vitro and in vivo

Abnormal angiogenesis is implicated in various diseases including cancer and diabetic retinopathy. In this study, we examined the effect of beta-eudesmol, a sesquiterpenoid alcohol isolated from Atractylodes lancea rhizome, on angiogenesis in vitro and in vivo. Proliferation of porcine brain microvascular endothelial cells and human umbilical vein endothelial cells (HUVEC) was inhibited by beta-eudesmol (50-100 microM). It also inhibited the HUVEC migration stimulated by basic fibroblast growth factor (bFGF) and the tube formation by HUVEC in Matrigel. beta-eudesmol (100 microM) blocked the phosphorylation of extracellular signal-regulated kinase (ERK) 1/2 induced by bFGF or vascular endothelial growth factor. Furthermore, beta-eudesmol significantly inhibited angiogenesis in subcutaneously implanted Matrigel plugs in mice and in adjuvant-induced granuloma in mice. These results indicate that beta-eudesmol inhibits angiogenesis, at least in part, through the blockade of the ERK signaling pathway. We considered that beta-eudesmol may aid the development of drugs to treat angiogenic diseases.

IRS-1 and Vascular Complications in Diabetes Mellitus

The expected explosive increase in the number of patients with diabetes mellitus will increase the stress on health care. Treatment is focused on preventing vascular complications associated with the disorder. In order to develop better treatment regimens, the field of research has made a great effort in understanding this disorder. This chapter summarizes the current views on the insulin signaling pathway with emphasis on intracellular signaling events associated with insulin resistance, which lead to the prothrombotic condition in the vasculature of patience with diabetes mellitus.

Adaptor ShcA protein binds tyrosine kinase Tie2 receptor and regulates migration and sprouting but not survival of endothelial cells

Angiopoietin-1 can promote migration, sprouting, and survival of endothelial cells through activation of different signaling pathways triggered by the Tie2 tyrosine kinase receptor. ShcA adapter proteins are targets of activated tyrosine kinases and are implicated in the transmission of activation signals to the Ras/mitogen-activated protein kinase pathway. Here we report the identification of an interaction between the adapter protein ShcA and the cytoplasmic domain of Tie2 through in vitro co-immunoprecipitation analysis. Stimulation of endogenous Tie2 in endothelial cells with its ligand angiopoietin-1 increased its association with ShcA and phosphorylation of the adapter protein. The interaction requires the SH2 domain of ShcA and the tyrosine phosphorylation of Tie2 as shown by pull-down experiments. Furthermore, Tyr-1101 of Tie2 was identified as the primary binding site for the SH2 domain of ShcA. Overexpression of a dominant-negative form of ShcA affects angiopoietin-1-induced chemotaxis and sprouting, although it has no effect on survival of endothelial cells. Furthermore, this mutant partially reduces the tyrosine phosphorylation of the regulatory p85 subunit of phosphatidylinositol 3-kinase. Together, our results identified a novel interaction between Tie2 with the adapter molecule ShcA and suggested that this interaction may play a role in the regulation of migration and three-dimensional organization of endothelial cells induced by angiopoietin-1.

Tyrosine phosphorylation-dependent and -independent role of Shc in the regulation of IGF-1-induced mitogenesis and glycogen synthesis

To examine the functional role of Shc tyrosine phosphorylation in IGF-1 signaling, wild-type (WT)-Shc and Y239,240,317F (3F)-Shc were transiently transfected into L6 myoblasts. IGF-1 signaling was compared among the transfected cells. IGF-1-induced tyrosine phosphorylation of Shc and its subsequent association with Grb2 were increased in WT-Shc cells, whereas they were decreased in 3F-Shc cells compared with those in parental L6 cells. Consistent with their changes, IGF-1-induced MAPK activation and thymidine incorporation were enhanced in WT-Shc cells, whereas they were again decreased in 3F-Shc cells. It is possible that Shc and insulin receptor substrate (IRS)-1 can interact competitively, via their phosphotyrosine binding (PTB) domains, with the activated IGF-1 receptor. In this regard, IGF-1-induced tyrosine phosphorylation of IRS-1 was decreased by overexpressing both WT-Shc and 3F-Shc cells. Consistent with the decrease, IGF-1-induced IRS-1 association with the p85 subunit of PI3K and activation of PI3K and Akt were reduced in both WT-Shc and 3F-Shc cells. As a result, IGF-1-induced glycogen synthesis was also decreased in both cells. Furthermore, expression of Shc PTB domain alone inhibited IGF-1 stimulation of Akt and glycogen synthesis. These results indicate that tyrosine phosphorylation of Shc is important for IGF-1 stimulation of MAPK leading to mitogenesis and that Shc, via its PTB domain, negatively regulates IGF-1-induced glycogen synthesis by competing with IRS-1, which is not relevant to Shc tyrosine phosphorylation.

Stimulation of mitogenic pathways through kinase-impaired mutants of the epidermal growth factor receptor

Two residues have been shown to be critical for the kinase activity of the receptor for epidermal growth factor (EGF): lysine-721, which functions in the binding of ATP by correctly positioning the gamma-phosphate for phosphoryl transfer, and aspartate-813, which functions as the catalytic base of the kinase. Mutation of either of these two residues has been shown to disrupt kinase activity of the receptor. However, studies performed in different laboratories had suggested that while EGF receptors mutated at lysine-721 are unable to stimulate significant increases of [(3)H]thymidine incorporation into DNA in response to EGF treatment, cells expressing EGF receptors mutated at aspartate-813 do stimulate significant incorporation of [(3)H]thymidine into DNA in response to EGF. In the present study, EGF receptors mutated at lysine-721 or aspartate-813 (K721R and D813A, respectively), as well as wild-type EGF receptors, were expressed in the same cellular background, Chinese hamster ovary cells, and side-by-side experiments were performed to investigate possible signaling-related differences. Our results indicate that while there are measurable differences in the abilities of the two mutant receptors to stimulate [(3)H]thymidine incorporation between 20 and 24 h after addition of EGF, these differences cannot be correlated with significant differences in EGF-stimulated tyrosine phosphorylation of mutant EGF receptor and endogenous ErbB2, the extent of receptor internalization, EGF-stimulated ion uptake, stimulation of SHC activity, or receptor association with Grb2. Flow cytometric data suggest that populations of cells expressing either kinase-impaired mutant EGF receptor progress similarly into S phase in response to addition of EGF. These observations suggest that D813A and K721R retain similar ability to stimulate mitogenic signaling events through transactivation of ErbB2 with only subtle temporal differences, and they emphasize the importance of expressing mutant receptors in an identical cellular context to make valid comparisons of functions.

Tyrosine-phosphorylated caveolin is a physiological substrate of the low M(r) protein-tyrosine phosphatase

Low M(r) phosphotyrosine-protein phosphatase is involved in the regulation of several tyrosine kinase growth factor receptors. The best characterized action of this enzyme is on the signaling pathways activated by platelet-derived growth factor, where it plays multiple roles. In this study we identify tyrosine-phosphorylated caveolin as a new potential substrate for low M(r) phosphotyrosine-protein phosphatase. Caveolin is tyrosine-phosphorylated in vivo by Src kinases, recruits into caveolae, and hence regulates the activities of several proteins involved in cellular signaling cascades. Our results demonstrate that caveolin and low M(r) phosphotyrosine-protein phosphatase coimmunoprecipitate from cell lysates, and that a fraction of the enzyme localizes in caveolae. Furthermore, in a cell line sensitive to insulin, the overexpression of the C12S dominant negative mutant of low M(r) phosphotyrosine-protein phosphatase (a form lacking activity but able to bind substrates) causes the enhancement of tyrosine-phosphorylated caveolin. Insulin stimulation of these cells induces a strong increase of caveolin phosphorylation. The localization of low M(r) phosphotyrosine-protein phosphatase in caveolae, the in vivo interaction between this enzyme and caveolin, and the capacity of this enzyme to rapidly dephosphorylate phosphocaveolin, all indicate that tyrosine-phosphorylated caveolin is a relevant substrate for this phosphatase.

Insulin signals to prenyltransferases via the Shc branch of intracellular signaling

We assessed the roles of insulin receptor substrate-1 (IRS-1) and Shc in insulin action on farnesyltransferase (FTase) and geranylgeranyltransferase I (GGTase I) using Chinese hamster ovary (CHO) cells that overexpress wild-type human insulin receptors (CHO-hIR-WT) or mutant insulin receptors lacking the NPEY domain (CHO-DeltaNPEY) or 3T3-L1 fibroblasts transfected with adenoviruses that express the PTB or SAIN domain of IRS-1 and Shc, the pleckstrin homology (PH) domain of IRS-1, or the Src homology 2 (SH2) domain of Shc. Insulin promoted phosphorylation of the alpha-subunit of FTase and GGTase I in CHO-hIR-WT cells, but was without effect in CHO-DeltaNPEY cells. Insulin increased FTase and GGTase I activities and the amounts of prenylated Ras and RhoA proteins in CHO-hIR-WT (but not CHO-DeltaNPEY) cells. Overexpression of the PTB or SAIN domain of IRS-1 (which blocked both IRS-1 and Shc signaling) prevented insulin-stimulated phosphorylation of the FTase and GGTase I alpha-subunit activation of FTase and GGTase I and subsequent increases in prenylated Ras and RhoA proteins. In contrast, overexpression of the IRS-1 PH domain, which impairs IRS-1 (but not Shc) signaling, did not alter insulin action on the prenyltransferases, but completely inhibited the insulin effect on the phosphorylation of IRS-1 and on the activation of phosphatidylinositol 3-kinase and Akt. Finally, overexpression of the Shc SH2 domain completely blocked the insulin effect on FTase and GGTase I activities without interfering with insulin signaling to MAPK. These data suggest that insulin signaling from its receptor to the prenyltransferases FTase and GGTase I is mediated by the Shc pathway, but not the IRS-1/phosphatidylinositol 3-kinase pathway. Shc-mediated insulin signaling to MAPK may be necessary (but not sufficient) for activation of prenyltransferase activity. An additional pathway involving the Shc SH2 domain may be necessary to mediate the insulin effect on FTase and GGTase I.

Epidermal growth factor-stimulated tyrosine phosphorylation of caveolin-1. Enhanced caveolin-1 tyrosine phosphorylation following aberrant epidermal growth factor receptor status

Caveolin-1 is the major coat protein of caveolae and has been reported to interact with various intracellular signaling molecules including the epidermal growth factor (EGF) receptor. To investigate the involvement of caveolin-1 in EGF receptor action, we used mouse B82L fibroblasts transfected with (a) wild type EGF receptor, (b) a C-terminally truncated EGF receptor at residue 1022, (c) a C-terminally truncated EGF receptor at residue 973, or (d) a kinase-inactive EGF receptor (K721M). Following EGF treatment, there was a distinct electrophoretic mobility shift of the caveolin-1 present in cells expressing the truncated forms of the EGF receptor, but this shift was not detectable in cells bearing either normal levels of the wild type EGF receptor or a kinase-inactive receptor. This mobility shift was also not observed following the addition of other cell stimuli, such as platelet-derived growth factor, insulin, basic fibroblast growth factor, or phorbol 12-myristate 13-acetate. Analysis of caveolin-1 immunoprecipitates from EGF-stimulated or nonstimulated cells demonstrated that the EGF-induced mobility shift of caveolin-1 was associated with its tyrosine phosphorylation in cells expressing truncated EGF receptors. Maximal caveolin-1 phosphorylation was achieved within 5 min after exposure to 10 nM EGF and remained elevated for at least 2 h. Additionally, several distinct phosphotyrosine-containing proteins (60, 45, 29, 24, and 20 kDa) were co-immunoprecipitated with caveolin-1 in an EGF-dependent manner. Furthermore, the Src family kinase inhibitor, PP1, does not affect autophosphorylation of the receptor, but it does inhibit the EGF-induced mobility shift and phosphorylation of caveolin-1. Conversely, the MEK inhibitors PD98059 and UO126 could attenuate EGF-induced mitogen-activated protein kinase activation, they do not affect the EGF-induced mobility shift of caveolin-1. Because truncation and overexpression of the EGF receptor have been linked to cell transformation, these results provide the first evidence that the tyrosine phosphorylation of caveolin-1 occurs via an EGF-sensitive signaling pathway that can be potentiated by an aberrant activity or expression of various forms of the EGF receptor.

Neuregulin-increased expression of acetylcholine receptor epsilon-subunit gene requires ErbB interaction with Shc

Selective transcription of acetylcholine receptor (AChR) subunit genes by neuregulin is one of the mechanisms involved in the synaptic localization of AChRs to the neuromuscular junction. Neuregulin stimulates ErbB receptor tyrosine kinases and subsequently activates the Ras/ERK pathway, which is required for neuregulin-mediated induction of AChR subunit genes in muscle cells and synapse-specific expression in vivo. Here we investigated the neuregulin transduction mechanism that leads to ERK activation after ErbB receptor tyrosine phosphorylation. Neuregulin increases the association of the adaptor proteins Grb2 and Shc with both ErbB2 and ErbB3 in C2C12 muscle cells. Dephosphorylation of the tyrosine-phosphorylated ErbB proteins abolished their association with both Grb2 and Shc, suggesting a tyrosine phosphorylation-dependent interaction. The interaction of Shc with the ErbB receptors is mediated by Shc's phosphotyrosine-binding domain. In addition, neuregulin increased tyrosine phosphorylation of Shc. Mutagenesis approaches demonstrated that tyrosine phosphorylation of Shc is required for neuregulin induction of AChR subunit gene expression. Taken together, these data indicate that the interaction of ErbB receptors with Grb2 alone is insufficient for neuregulin-activated transcription, but that ErbB receptor signaling via Shc is necessary and important.

Role of the Src homology 2 (SH2) domain and C-terminus tyrosine phosphorylation sites of SH2-containing inositol phosphatase (SHIP) in the regulation of insulin-induced mitogenesis

To examine the role of SHIP in insulin-induced mitogenic signaling, we used a truncated SHIP lacking the SH2 domain (deltaSH2-SHIP) and a Y917/1020F-SHIP (2F-SHIP) in which two tyrosines contributing to Shc binding were mutated to phenylalanine. Wild-type (WT)-, deltaSH2-, and 2F-SHIP were transiently transfected into Rat1 fibroblasts overexpressing insulin receptors (HIRc). Insulin-stimulated tyrosine phosphorylation of WT-SHIP and deltaSH2-SHIP, whereas tyrosine phosphorylation of 2F-SHIP was not detectable, indicating that 917/1020-Tyr are key phosphorylation sites on SHIP. Although SHIP can bind via its 917/1020-Tyr residues and SH2 domain to Shc PTB domain and 317-Tyr residue, respectively, insulin-induced SHIP association with Shc was more greatly decreased in 2F-SHIP cells than that in deltaSH2-SHIP cells. Insulin stimulation of Shc association with Grb2, which is important for p21ras-MAP kinase activation, was decreased by overexpression of WT- and 2F-SHIP. Importantly, insulin-induced Shc x Grb2 association was not detectably reduced in deltaSH2-SHIP cells. In accordance with the extent of Shc association with Grb2, insulin-induced MAP kinase activation was relatively decreased in both WT-SHIP and 2F-SHIP cells, but not in deltaSH2-SHIP cells. To examine the functional role of SHIP in insulin's biological action, insulin-induced mitogenesis was compared among these transfected cells. Insulin stimulation of thymidine incorporation and bromodeoxyuridine incorporation was decreased in WT-SHIP cells compared with that of control HIRc cells. Expression of 2F-SHIP also significantly reduced insulin-induced mitogenesis, whereas it was only slightly affected by overexpression of deltaSH2-SHIP. Furthermore, the reduction of insulin-induced mitogenesis in WT-SHIP cells was partly compensated by coexpression of Shc. These results indicate that SHIP plays a negative regulatory role in insulin-induced mitogenesis and that the SH2 domain of SHIP is important for its negative regulatory function.

Differential modulation of the tyrosine phosphorylation state of the insulin receptor by IRS (insulin receptor subunit) proteins

In response to insulin, tyrosine kinase activity of the insulin receptor is stimulated, leading to autophosphorylation and tyrosine phosphorylation of proteins including insulin receptor subunit (IRS)-1, IRS-2, and Shc. Phosphorylation of these proteins leads to activation of downstream events that mediate insulin action. Insulin receptor kinase activity is requisite for the biological effects of insulin, and understanding regulation of insulin receptor phosphorylation and kinase activity is essential to understanding insulin action. Receptor tyrosine kinase activity may be altered by direct changes in tyrosine kinase activity, itself, or by dephosphorylation of the insulin receptor by protein-tyrosine phosphatases. After 1 min of insulin stimulation, the insulin receptor was tyrosine phosphorylated 8-fold more and Shc was phosphorylated 50% less in 32D cells containing both IRS-1 and insulin receptors (32D/IR+IRS-1) than in 32D cells containing only insulin receptors (32D/IR), insulin receptors and IRS-2 (32D/IR+IRS-2), or insulin receptors and a form of IRS-1 that cannot be phosphorylated on tyrosine residues (32D/IR+IRS-1F18). Therefore, IRS-1 and IRS-2 appeared to have different effects on insulin receptor phosphorylation and downstream signaling. Preincubation of cells with pervanadate greatly decreased protein-tyrosine phosphatase activity in all four cell lines. After pervanadate treatment, tyrosine phosphorylation of insulin receptors in insulin-treated 32D/IR, 32D/ IR+IRS-2, and 32D/IR+IRS-1F18 cells was markedly increased, but pervanadate had no effect on insulin receptor phosphorylation in 32D/IR+IRS-1 cells. The presence of tyrosine-phosphorylated IRS-1 appears to increase insulin receptor tyrosine phosphorylation and potentially tyrosine kinase activity via inhibition of protein-tyrosine phosphatase(s). This effect of IRS-1 on insulin receptor phosphorylation is unique to IRS-1, as IRS-2 had no effect on insulin receptor tyrosine phosphorylation. Therefore, IRS-1 and IRS-2 appear to function differently in their effects on signaling downstream of the insulin receptor. IRS-1 may play a major role in regulating insulin receptor phosphorylation and enhancing downstream signaling after insulin stimulation.

Molecular cloning of rat SH2-containing inositol phosphatase 2 (SHIP2) and its role in the regulation of insulin signaling

SH2-containing inositol 5'-phosphatase (SHIP) plays a negative regulatory role in hematopoietic cells. We have now cloned the rat SHIP isozyme (SHIP2) cDNA from skeletal muscle, which is one of the most important target tissue of insulin action. Rat SHIP2 cDNA encodes a 1183-amino-acid protein that is 45% identical with rat SHIP. Rat SHIP2 contains an amino-terminal SH2 domain, a central 5'-phosphoinositol phosphatase activity domain, and a phosphotyrosine binding (PTB) consensus sequence and a proline-rich region at the carboxyl tail. Specific antibodies to SHIP2 were raised and the function of SHIP2 was studied by stably overexpressing rat SHIP2 in Rat1 fibroblasts expressing human insulin receptors (HIRc). Endogenous SHIP2 underwent insulin-mediated tyrosine phosphorylation and phosphorylation was markedly increased when SHIP2 was overexpressed. Although overexpression of SHIP2 did not affect insulin-induced tyrosine phosphorylation of the insulin receptor beta-subunit and Shc, subsequent association of Shc with Grb2 was inhibited, possibly by competition between the SH2 domains of SHIP2 and Grb2 for the Shc phosphotyrosine. As a result, insulin-stimulated MAP kinase activation was reduced in SHIP2-overexpressing cells. Insulin-induced tyrosine phosphorylation of IRS-1, IRS-1 association with the p85 subunit of PI3-kinase, and PI3-kinase activation were not affected by overexpression of SHIP2. Interestingly, although both PtdIns-(3,4,5)P3 and PtdIns(3,4)P2 have been implicated in the regulation of Akt activity in vitro, overexpression of SHIP2 inhibited insulin-induced Akt activation, presumably by its 5'-inositol phosphatase activity. Furthermore, insulin-induced thymidine incorporation was decreased by overexpression of SHIP2. These results indicate that SHIP2 plays a negative regulatory role in insulin-induced mitogenesis, and regulation of the Shc. Grb2 complex and of the downstream products of PI3-kinase provides possible mechanisms of SHIP2 action in insulin signaling.

Insulin receptor substrate 2 and Shc play different roles in insulin-like growth factor I signaling

The major substrates for the type I insulin-like growth factor (IGF-I) receptor are Shc and insulin receptor substrate (IRS) proteins. In the current study, we report that IGF-I induces a sustained tyrosine phosphorylation of Shc and its association with Grb2 in SH-SY5Y human neuroblastoma cells. The time course of Shc tyrosine phosphorylation parallels the time course of IGF-I-stimulated activation of extracellular signal-regulated kinase (ERK). Transfection of SH-SY5Y cells with a p52 Shc mutant decreases Shc tyrosine phosphorylation and Shc-Grb2 association. This results in the inhibition of IGF-I-mediated ERK tyrosine phosphorylation and neurite outgrowth. In contrast, IGF-I induces a transient tyrosine phosphorylation of IRS-2 and an association of IRS-2 with Grb2. The time course of IRS-2 tyrosine phosphorylation and IRS-2-Grb2 and IRS-2-p85 association closely resembles the time course of IGF-I-mediated membrane ruffling. Treating cells with the phosphatidylinositol 3'-kinase inhibitors wortmannin and LY294002 blocks IGF-I-induced membrane ruffling. The ERK kinase inhibitor PD98059, as well as transfection with the p52 Shc mutant, has no effect on IGF-I-mediated membrane ruffling. Immunolocalization studies show IRS-2 and Grb2, but not Shc, concentrated at the tip of the extending growth cone where membrane ruffling is most active. Collectively, these results suggest that the association of Shc with Grb2 is essential for IGF-I-mediated neurite outgrowth, whereas the IRS-2-Grb2-phosphatidylinositol 3'-kinase complex may regulate growth cone extension and membrane ruffling.

Activation of the Ras/mitogen-activated protein kinase pathway by kinase-defective epidermal growth factor receptors results in cell survival but not proliferation

Signalling by the epidermal growth factor (EGF) receptor (EGFR) has been studied intensively, but for most cell types the analysis is complicated by the fact that EGFR not only homodimerizes but can also form heterodimers with other EGFR family members. Heterodimerization is a particular problem in the study of EGFR mutants, where the true phenotype of the mutants is confounded by the contribution of the heterodimer partner to signal transduction. We have made use of the murine hemopoietic cell line BaF/3, which does not express EGFR family members, to express wild-type (WT) EGFR, three kinase-defective EGFR mutants (V741G, Y740F, and K721R), or a C-terminally truncated EGFR (CT957) and have measured their responses to EGF. We found that under the appropriate conditions EGF can stimulate cell proliferation of BaF/3 cells expressing WT or CT957 EGFRs but not that of cells expressing the kinase-defective mutants. However, EGF promotes the survival of BaF/3 cells expressing either of the kinase-defective receptors (V741G and Y740F), indicating that these receptors can still transmit a survival signal. Analysis of the early signalling events by the WT, V741G, and Y740F mutant EGF receptors indicated that EGF stimulates comparable levels of Shc phosphorylation, Shc-GRB-2 association, and activation of Ras, B-Raf, and Erk-1. Blocking the mitogen-activated protein kinase (MAPK) signalling pathway with the specific inhibitor PD98059 abrogates completely the EGF-dependent survival of cells expressing the kinase-defective EGFR mutants but has no effect on the EGF-dependent proliferation mediated by WT and CT957 EGFRs. Similarly, the Src family kinase inhibitor PP1 abrogates EGF-dependent survival without affecting proliferation. However blocking phosphatidylinositol-3-kinase or JAK-2 kinase with specific inhibitors does arrest growth factor-dependent cell proliferation. Thus, EGFR-mediated mitogenic signalling in BaF/3 cells requires an intact EGFR tyrosine kinase activity and appears to depend on the activation of both the JAK-2 and PI-3 kinase pathways. Activation of the Src family of kinases or of the Ras/MAPK pathway can, however, be initiated by a kinase-impaired EGFR and is linked to survival.

Relative involvement of Shc tyrosine 239/240 and tyrosine 317 on insulin induced mitogenic signaling in rat1 fibroblasts expressing insulin receptors

Shc is phosphorylated on Tyr-239/240 and/or Tyr-317, which serves as a docking site for Grb2. To clarify the relative involvement of Shc Tyr-239/240 and Tyr-317 in insulin-induced mitogenesis, we generated expression vectors for Y317F (1F)-Shc, Y239/240F (2F)-Shc, and Y239/240/317F (3F)-Shc, and stably transfected them into Rat1 fibroblasts expressing insulin receptors (HIRc). Insulin-induced Shc phosphorylation and subsequent association with Grb2 was enhanced in wild-type (WT)-Shc cell. In contrast, insulin-stimulated Shc phosphorylation and Shc.Grb2 association were significantly decreased in 1F-Shc and 3F-Shc cells, while these were only slightly affected and almost comparable in 2F cells compared with those in parental HIRc cells. The kinetics of MAP kinase activation closely paralleled the kinetics of Shc phosphorylation and Shc.Grb2 association. Thus, insulin stimulation of MAP kinase activation occurred more rapidly in WT-Shc cells, and the activation was delayed in 1F-Shc and 3F-Shc cells, while it was comparable in 2F-Shc cells compared with that in HIRc cells. Furthermore, WT-Shc cells displayed enhanced sensitivity to insulin stimulation of thymidine incorporation. Importantly, the sensitivity was significantly decreased in 1F-Shc and 3F-Shc cells, while it was almost comparable in 2F-Shc cells compared with that in HIRc cells. These results indicate that Shc Tyr-317 is more predominant insulin-induced phosphorylation site than Tyr-239/240 for coupling with Grb2 leading to MAP kinase activation and mitogenesis in Rat1 fibroblasts.

The src homology 2 and phosphotyrosine binding domains of the ShcC adaptor protein function as inhibitors of mitogenic signaling by the epidermal growth factor receptor

Upon ligand activation, the epidermal growth factor receptor (EGFR) becomes tyrosine-phosphorylated, thereby recruiting intracellular signaling proteins such as Shc. EGFR binding of Shc proteins results in their tyrosine phosphorylation and subsequent activation of the Ras and Erk pathways. Shc interaction with activated receptor tyrosine kinases is mediated by two distinct phosphotyrosine interaction domains, an NH2-terminal phosphotyrosine binding (PTB) domain and a COOH-terminal Src homology 2 (SH2) domain. The relative importance of these two domains for EGFR binding was examined by determining if expression of the isolated SH2 or PTB domain of ShcC would inhibit EGFR signaling. The SH2 domain potently inhibited numerous aspects of EGFR signaling including activation of Erk2 and the Elk-1 transcription factor as well as EGFR-dependent transformation. Furthermore, the SH2 domain inhibited focus formation by the Neu oncoprotein, another EGFR family member. Surprisingly, inhibition of the EGFR by the SH2 domain did not involve stable association with the receptor. In contrast, the PTB domain associated quite well with the receptor yet had little effect on EGFR signaling. Although the EGFR cytoplasmic tail contains consensus binding sites for the PTB and SH2 domains of ShcC, and both domains of ShcC interact with the receptor in vitro, the SH2 domain is more potent for inhibiting receptor function in vivo. However, inhibition is not due to stable association with the receptor, suggesting that the SH2 domain is binding to a heretofore unknown protein(s) necessary for proper EGFR function.

Differential utilization of ShcA tyrosine residues and functional domains in the transduction of epidermal growth factor-induced mitogen-activated protein kinase activation in 293T cells and nerve growth factor-induced neurite outgrowth in PC12 cells. Identification of a new Grb2.Sos1 binding site

By transient expression of both truncated forms of p52(SHCA) and those with point mutations in 293T cells, it has been shown that, in addition to Tyr-317, Tyr-239/240 is a major site of phosphorylation that serves as a docking site for Grb2.Sos1 complexes. In addition, analysis of epidermal growth factor (EGF)-induced activation of mitogen-activated protein kinase in 293T cells showed that the overexpression Shc SH2 or phosphotyrosine binding (PTB) domains of ShcA alone has a more potent negative effect than the overexpression of the forms of ShcA lacking Tyr-317 or Tyr 239/240 or both. In transiently transfected PC12 cells, the ShcA PTB domain and tyrosine phosphorylation in the CH1 domain, especially on Tyr-239/240, are crucial for mediating nerve growth factor (NGF)-induced neurite outgrowth. These findings suggest that the EGF and NGF (TrkA) receptor can utilize Shc in different ways to promote their activity. For EGF-induced mitogen-activated protein kinase activation in 293T cells, both Shc PTB and SH2 domains are essential for optimal activation, indicating that a mechanism independent of Grb2 engagement with Shc may exist. For NGF-induced neurite outgrowth in PC12 cells, Shc PTB plays an essential role, and phosphorylation on Tyr-239/240, but not on Tyr-317, is required.

Functional importance of heat shock protein 90 associated with insulin receptor on insulin-stimulated mitogenesis

The role of stress proteins on the function of insulin receptor is not well understood. In the rat-1 fibroblasts overexpressing human insulin receptors, heat shock protein (Hsp) 90 was co-immunoprecipitated with insulin receptors and the association was not affected by insulin stimulation. A GST-fusion protein containing the intracellular insulin receptor beta subunit was associated with Hsp 90 in vitro, suggesting the direct interaction of this protein with insulin receptor beta-subunit. Furthermore, microinjection of anti-Hsp 90 antibody into these cells completely inhibited insulin-stimulated mitogenesis. However, neither epidermal growth factor-stimulated nor serum-stimulated mitogenic signal in the cells was affected by the antibody microinjection. These results suggest that Hsp 90 constitutively binds to insulin receptor beta-subunit, which may be necessary for insulin signaling in mitogenesis.