Insulin receptor substrate-1 mediates phosphatidylinositol 3'-kinase and p70S6k signaling during insulin, insulin-like growth factor-1, and interleukin-4 stimulation

Molecular markers of IGF-I-mediated mitogenesis

The aim of these investigations was to identify a number of molecular markers that correlate to growth stimulation by IGF-I. For this purpose, we have selected four cell lines that respond equally well to growth stimulation by serum, but differ in their proliferative response to IGF-I. Two cell lines (R503 and R600 cells) respond to IGF-I with both DNA synthesis and cell division, a third cell line (R508 cells) can enter S phase after IGF-I, but the cells do not divide, and a fourth one (R12 cells) totally fails to respond to IGF-I with growth. Using these cell lines, all of which had an intact mitogenic response program to serum, we show that: (1) an increase in GTP/GDP ratio is an early event that distinguishes cells capable of entering S phase after IGF-I from cells that do not; (2) all cells that are induced to synthesize DNA by IGF-I have increased phosphorylation of MAP kinases, regardless of their ability to divide; (3) the same cell lines display a similar increase in cyclin A and B expression at early times after stimulation; and (4) cyclin levels and cyclin B-associated cdc2 kinase activity remain elevated at later times only in cells that undergo cell division. These results establish certain parameters of IGF-I-mediated mitogenesis and clearly separate the occurrence of DNA synthesis from cell division in certain situations.

Regulation of p70s6k/p85s6k and its role in the cell cycle

Two to three-fold increases in the rate of protein synthesis are required both to enter the G1 phase of the cell cycle from G0 and to proceed to S phase in response to growth factors and mitogens. This increase is in part regulated via multiple phosphorylation of the 40S ribosomal protein S6 by the mitogen-stimulated p70s6k/p85s6k. At the protein synthesis level this event appears to be involved in specifically increasing the efficiency of translation of a family of essential mRNAs containing a polypyrimidine tract at their 5' transcriptional start site. The activation of p70s6k/p85s6k and maintenance of its activity throughout G1 is controlled via multiple phosphorylation events mediated by a complex signalling network acting on distinct sets of phosphorylation sites.

IGF I induction of p53 requires activation of MAP kinase in cardiac muscle cells

The aim of this study was to investigate whether IGF I induction of p53 expression and p21 promoter require activation of MAP kinase in cardiac muscle cells. Compared to cardiomyocytes transfected with control vector, activation of MAP kinase by IGF I was decreased by approximately 60-70% in the cells transfected with dominant negative MAP kinase Y185. Transfection with Y185 also resulted in decreased induction of p53 mRNA by IGF I (70% reduction). In the cells transfected with a wildtype p21WAF1/CIP1 promoter construct, activation of luciferase reporter gene by IGF I was decreased in the cells co-transfected with Y185. To further confirm these findings, cells were preincubated with PD98059, a specific MAP kinase kinase inhibitor. As expected, PD98059 inhibited induction of p53 mRNA and p21WAF1/CIP1 promoter by IGF I. These data indicate that transcriptional activation of p53 and p21WAF1/CIP1 by IGF I involves MAP kinase pathway in cardiomyocytes, and thus link MAP kinase to negative modulation of the cell cycle in cardiac muscle cells.

Regulation of gene expression, growth, and cell survival by IL-4: contribution of multiple signaling pathways

Interleukin-4 is a cytokine produced by activated T cells, mast cells, and basophils that elicits many important biological responses[1] (see Tab 1). These responses range from the regulation of helper T cell differentiation[2] and the production of IgE[3] to the regulation of the adhesive properties of endothelial cells via VCAM-1[4]. In keeping with these diverse biological effects, high-affinity binding sites for IL-4 (Kd 20 to 300 pM) have been detected on many hematopoietic and non-hematopoietic cell types at levels ranging from 50 to 5000 sites per cell[5]. This review will focus on the discrete signal transduction pathways activated by the IL-4 receptor and the coordination of these individual pathways in the regulation of a final biological outcome.

Insulin receptor substrate 1 antisense expression in an hepatoma cell line reduces cell proliferation and induces overexpression of the Src homology 2 domain and collagen protein (SHC)

In mammalian cells, the insulin receptor substrate 1 protein (IRS-1) is a specific substrate for insulin and IGF-1 receptor tyrosine kinases which is involved in mediating metabolic and mitogenic actions of insulin and IGFs. In order to determine if IRS-1 is also essential in a chicken derived hepatoma cell line (LMH cells), IRS-1 gene has been invalidated in these cells. For this, we subcloned chicken IRS-1 gene in an antisense orientation into a mammalian expression vector driven by the cytomegalovirus early promoter. LMH cells were stably transfected with this construct or with the empty vector carrying only the neomycin resistance gene and selected for cIRS-1 expression. One subclone, C2, showed a complete repression of cIRS-1 expression at both protein and mRNA levels. Proliferation of C2 cells was dramatically reduced (54%) compared with Neo(r) cells. Furthermore this reduction was accompanied by a decrease in insulin-dependent [3H]thymidine incorporation, indicating a reduction in DNA synthesis. Insulin-dependent [U-14C]glucose incorporation into cellular lipids was also significantly reduced in C2 cell line suggesting an alteration in lipogenesis. In wild type LMH cells, SHC which is involved in Ras pathway, also served as a substrate for insulin receptor tyrosine kinase. In C2 cells, SHC expression, its association with the insulin receptor and its tyrosine phosphorylation were largely increased. Two forms of the regulatory subunit of PI 3-kinase were present: p85 and p55 forms. Furthermore, C2 cells displayed increased basal phosphatidylinositol (PI) 3'-kinase activity. This report demonstrates a role for cIRS-1 in the metabolic and mitogenic actions of insulin in LMH cells. However, the overexpression of cIRS-1 antisense did not completely abolish cell proliferation. This may be explained by the exacerbation of an alternative pathway that only partly compensate for the knocking out of cIRS-1 gene: the overexpression of SHC.

HMG-I(Y) phosphorylation status as a nuclear target regulated through insulin receptor substrate-1 and the I4R motif of the interleukin-4 receptor

Interleukin (IL)-4 is a cytokine that regulates both the growth and differentiation of hematopoietic cells. Its ligand binding specificity and important signal transduction mechanisms are conferred by the IL-4 receptor alpha chain (IL-4Ralpha). The I4R is a tyrosine-containing motif within IL-4Ralpha that is critical for proliferative responses to IL-4. Although the I4R also contributes to gene regulation, nuclear targets directly regulated by this motif have not been described. It is shown here that the tyrosine at position 497 in the I4R is critical for regulation of the phosphorylation status of a set of nuclear proteins that includes HMG-I(Y), small non-histone chromosomal proteins involved in the control of gene expression in hematopoietic cell lines. Moreover, IL-4 is unable to induce HMG-I(Y) phosphorylation in insulin receptor substrate-1-deficient cells, and the inhibitor wortmannin completely blocks IL-4 regulation of HMG-I(Y) phosphorylation status but not activation of an IL-4 Stat protein. Taken together, these data indicate that HMG-I(Y) is a nuclear target whose phosphorylation status is regulated through the I4R motif via insulin receptor substrate proteins, independent of activation of the Stat pathway.

Janus kinase-dependent activation of insulin receptor substrate 1 in response to interleukin-4, oncostatin M, and the interferons

In addition to a role in response to insulin and insulin-like growth factors, insulin receptor substrate 1 (IRS-1) is phosphorylated in response to IL-4, the interferons (IFNs) and oncostatin M (OSM). Here mutant cell lines lacking JAK1, JAK2, or Tyk2 were used to determine the role(s) of the Janus kinase (JAK) family of protein-tyrosine kinases in IRS-1 phophorylation. 32D cells, which do not express IRS proteins, were analyzed for any requirement for these proteins in response to the IFNs. For the mutant human fibrosarcoma cell lines, phosphorylation of IRS-1 through the insulin-like growth factor receptor is independent of JAK1, JAK2, or Tyk2. In contrast, phosphorylation of IRS-1 mediated by the Type I IFNs, IL-4, and OSM is JAK-dependent. For the alphabeta-IFNs, activation of IRS-1 is dependent on JAK1 and Tyk2, consistent with the interdependence of these kinases in the IFN-alphabeta response. Neither IRS-1 nor IRS-2 was detectably activated by IFN-gamma. Consistent with this, activation of neither IRS proteins appears to be an absolute requirement for an antiproliferative or an antiviral response to the IFNs. For IL-4 and OSM phosphorylation of IRS-1 in the human fibrosarcoma cells is largely dependent on JAK1 but can also be mediated through Tyk2 or JAK2. Activation of phosphatidylinositol 3'-kinase in response to IL-4 and OSM, at least, was also JAK-dependent. The JAKs are, therefore, required not only for STAT activation but also for the activation, through a variety of different types of cytokine receptor, of an additional signaling pathway(s) through IRS-1 and phosphatidylinositol 3'-kinase.

Insulin and interleukin-4 induce desensitization to the mitogenic effects of insulin-like growth factor-I. Pivotal role for insulin receptor substrate-2

Insulin-induced desensitization to insulin-like growth factor-I (IGF-I) stimulated mitogenesis in bovine fibroblasts involves steps distal to IGF-I binding to its tyrosine kinase receptor. When quiescent cultures of bovine fibroblasts were stimulated with 10 nM IGF-I and total cell lysates immunoblotted with anti-phosphotyrosine antibody, we observed a band at approximately 97 kDa, representing the beta-subunit of the IGF-I receptor, and a predominant tyrosyl-phosphorylated species migrating as a broad band between 170 and 190 kDa. The majority of proteins in this latter band were immunoprecipitated by antibodies against insulin receptor substrate (IRS)-2 and not by antibodies against IRS-1. Pretreatment of bovine fibroblasts with 10 nM insulin for 48 h blocked subsequent IGF-I-stimulated DNA synthesis and the IGF-I-induced increase in tyrosyl-phosphorylated IRS-2. Insulin pretreatment did not alter IRS-1 or IRS-2 expression by these cells, as assessed by metabolic labeling and direct immunoblotting with IRS antibodies. The interleukin-4 (IL-4) cytokine receptor also has IRS-2 as its major substrate for tyrosine phosphorylation. Although 10 nM IL-4 was as effective as 10 nM IGF-I in stimulating IRS-2 phosphorylation, 10 nM IL-4 did not have comparable mitogenic power in these cells. Nonetheless, pretreatment of bovine fibroblasts with IL-4 inhibited IGF-I-stimulated DNA synthesis by 50-60%, concomitant with a decrease in IGF-I-induced IRS-2 phosphorylation. Insulin-induced desensitization could be prevented if a specific inhibitor of phosphatidylinositol 3-kinase (LY294002), but not an inhibitor of mitogen-activated protein kinase (PD98059), was present during the preincubation period. LY294002 also prevented the shift in IRS-2 molecular mass in response to prolonged incubation of cells with insulin. These data indicate that, in a nontransformed cell system, IRS-2 plays a key role in cellular desensitization to IGF-I-stimulated mitogenesis most likely through a feedback mechanism in the phosphatidylinositol 3-kinase pathway. Furthermore, they suggest that signaling through IRS-2 may provide an important point of integration for hormone, growth factor, and cytokine receptor systems that regulate critical cellular growth responses.

Novel pathway of insulin signaling involving Stat1alpha in Hep3B cells

STAT proteins are important transcription factors that regulate cell growth and differentiation. To elucidate the molecular mechanisms of insulin actions, we have studied how insulin activates STAT proteins in Hep3B cells. Insulin rapidly phosphorylated Stat1alpha at tyrosine residues and increased its specific binding activities to a GAS/ISRE consensus oligonucleotide. IL-4 also phosphorylated Stat1alpha and increased DNA binding activities to the same Stat1alpha responsive element. There was no increase in tyrosine phosphorylation of JAK family of kinases following insulin stimulation. In contrast, IL-4 stimulated tyrosine phosphorylation of JAK1, JAK2 and tyk2 in this cell line. These data indicate that insulin receptor signaling can activate the transcriptional regulatory function of STAT protein, and that insulin actions on Stat1alpha are mediated through signaling pathways independent of JAK family of kinases.

Phosphatidylinositol 3-kinase links the interleukin-2 receptor to protein kinase B and p70 S6 kinase

Phosphatidylinositol 3-kinase (PI 3-kinase) is activated by the cytokine interleukin-2 (IL-2). We have used a constitutively active PI 3-kinase to identify IL-2-mediated signal transduction pathways directly regulated by PI 3-kinase in lymphoid cells. The serine/threonine protein kinase B (PKB)/Akt can act as a powerful oncogene in T cells, but its positioning in normal T cell responses has not been explored. Herein, we demonstrate that PKB is activated by IL-2 in a PI 3-kinase-dependent fashion. Importantly, PI 3-kinase signals are sufficient for PKB activation in IL-2-dependent T cells, and PKB is a target for PI 3-kinase signals in IL-2 activation pathways. The present study establishes also that PI 3-kinase signals or PKB signals are sufficient for activation of p70 S6 kinase in T cells. PI 3-kinase can contribute to, but is not sufficient for, activation of extracellular signal-regulated kinases (Erks) and Erk effector pathways. Therefore, PI 3-kinase is a selective regulator of serine/threonine kinase signal transduction pathways in T lymphocytes, and this enzyme provides a crucial link between the interleukin-2 receptor, the protooncogene PKB, and p70 S6 kinase.

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.

Insulin-induced egr-1 and c-fos expression in 32D cells requires insulin receptor, Shc, and mitogen-activated protein kinase, but not insulin receptor substrate-1 and phosphatidylinositol 3-kinase activation

Many studies suggest that insulin utilizes multiple signal transduction pathways. Insulin's effects are initiated by insulin binding to the insulin receptor, resulting in tyrosine phosphorylation of insulin receptor and intracellular substrates, such as insulin receptor substrate-1 (IRS-1), IRS-2, or Shc. We recently demonstrated that immediate-early gene egr-1 transcription was fully induced without phosphorylation of IRS-1 in Chinese hamster ovary cells (Harada, S., Smith, R. M., Smith, J. A., Shah, N. , Hu, D.-Q. & Jarett, L. (1995) J. Biol. Chem. 270, 26632-26638). In the present study, we examined the effects of insulin on immediate-early gene egr-1 and c-fos expression in 32D cells overexpressing the insulin receptor (32D/IR), IRS-1 (32D/IRS), or both (32D/IR+IRS) and compared these effects with insulin-induced tyrosine phosphorylation. Insulin (17 nM) increased egr-1 and c-fos expression in 32D/IR and 32D/IR+IRS cells, but not in parental cells or 32D/IRS cells, as determined by Northern blot analysis. Insulin treatment (5 min at 37 degrees C) markedly increased tyrosine phosphorylation of several proteins, including the insulin receptor, IRS-1, and Shc, in 32D/IR+IRS cells as determined by immunoprecipitation and Western blot analysis with anti-phosphotyrosine antibody. In contrast, only two tyrosine-phosphorylated proteins, i.e. insulin receptor and Shc, were detected in 32D/IR cells. These data suggest that insulin receptor and Shc phosphorylation is necessary for insulin-induced egr-1 and c-fos expression, but IRS-1 phosphorylation is not necessary or sufficient for the expression of these genes. Furthermore, the effect of specific inhibitors on insulin-induced egr-1 expression was examined. Wortmannin (25 nM), a phosphatidylinositol 3-kinase inhibitor, had no effect on insulin-induced egr-1 expression. In contrast, PD 98059 (30 microM), a mitogen-activated protein kinase kinase inhibitor, totally blocked egr-1 expression induced by insulin. These data indicate that mitogen-activated protein kinase activation, but not phosphatidylinositol 3-kinase activation, is involved in insulin-induced egr-1 expression. Taken together, insulin receptor tyrosine phosphorylation, Shc tyrosine phosphorylation, and mitogen-activated protein kinase activation appear to be the signal transduction pathway responsible for insulin-induced egr-1 expression in 32D cells. These data demonstrate that insulin has multiple signal transduction pathways that vary from cell to cell.

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.

Growth and survival signals transmitted via two distinct NPXY motifs within leukocyte tyrosine kinase, an insulin receptor-related tyrosine kinase

Leukocyte tyrosine kinase (LTK) is a receptor tyrosine kinase, which belongs to the insulin receptor family and is mainly expressed in pre-B cells and brain. In this study, we show that LTK utilizes insulin receptor substrate-1 (IRS-1) and Shc as major two substrates and possesses two NPXY motifs for them separately, tyrosine 485 of one NPXY motif at the juxtamembrane domain for IRS-1 and tyrosine 862 of another NPXY motif at the carboxyl-terminal domain for Shc. By using Ba/F3 cells expressing epidermal growth factor receptor-LTK chimeric receptors containing a mutation at each NPXY site, we showed that while both NPXY motifs equally contribute to activation of the Ras pathway and generation of mitogenic signals, only tyrosine 485 of LTK transmits cell survival signals. These data suggest that IRS-1 possesses anti-apoptotic function at least in LTK signaling. Moreover, our data indicate that the survival signaling pathway of LTK is distinct from the Ras pathway and the p70(S6) kinase pathway. Our results provide a useful insight in understanding the distinctive roles of Shc and IRS-1 in the signal transduction system of the insulin receptor family, and this anti-apoptotic function of IRS-1 may explain the survival effects of insulin, IGF-1, and interleukin 4.

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.

Overexpression of Rad inhibits glucose uptake in cultured muscle and fat cells

Rad is a Ras-like GTPase that was isolated by subtraction cloning of human muscle and shown to have increased expression in some individuals with Type II diabetes. To ascertain the potential role of Rad in insulin-mediated signaling, we have overexpressed Rad in myocyte and adipocyte cell lines. Expression of Rad resulted in a 50-90% reduction in insulin-stimulated 2-deoxyglucose glucose uptake in C2C12 murine myotubes, L6 rat myotubes, and 3T3-L1 adipocytes and a 25% reduction in 3-O-methylglucose uptake in 3T3-L1 adipocytes. This occurred despite unaltered levels of glucose transporter expression, with no detectable change in Glut4 translocation and with no alteration in insulin receptor or substrate phosphorylation or phosphatidylinositol 3-kinase activity. These data indicate that Rad is a negative regulator of glucose uptake and that this effect may be due to a decrease in the intrinsic activity of the transporter molecules, rather than an effect on the translocation of Glut4.

Phosphatidylinositol 3-kinase inhibitors block differentiation of skeletal muscle cells

Skeletal muscle differentiation involves myoblast alignment, elongation, and fusion into multinucleate myotubes, together with the induction of regulatory and structural muscle-specific genes. Here we show that two phosphatidylinositol 3-kinase inhibitors, LY294002 and wortmannin, blocked an essential step in the differentiation of two skeletal muscle cell models. Both inhibitors abolished the capacity of L6E9 myoblasts to form myotubes, without affecting myoblast proliferation, elongation, or alignment. Myogenic events like the induction of myogenin and of glucose carrier GLUT4 were also blocked and myoblasts could not exit the cell cycle, as measured by the lack of mRNA induction of p21 cyclin-dependent kinase inhibitor. Overexpresssion of MyoD in 10T1/2 cells was not sufficient to bypass the myogenic differentiation blockade by LY294002. Upon serum withdrawal, 10T1/2-MyoD cells formed myotubes and showed increased levels of myogenin and p21. In contrast, LY294002-treated cells exhibited none of these myogenic characteristics and maintained high levels of Id, a negative regulator of myogenesis. These data indicate that whereas phosphatidylinositol 3-kinase is not indispensable for cell proliferation or in the initial events of myoblast differentiation, i.e. elongation and alignment, it appears to be essential for terminal differentiation of muscle cells.

Signaling via the insulin-like growth factor-I receptor: does it differ from insulin receptor signaling?

The insulin and insulin-like growth factor (IGF-I) receptors while similar in structure and function serve different physiological functions in vivo. In non-disease states the insulin receptor is primarily involved in metabolic functions whereas the IGF-I receptor mediates growth and differentiation. The separation of these functions is controlled by a number of factors including the tissue distribution of the respective receptors. Modulation of the binding of the ligands insulin or IGF-I and IGF-II to their respective receptors by the local environment of the cell also offers signaling specificity mediated via the receptors. Each ligand bind to its respective receptor with high affinity. This high affinity binding is dictated by the primary sequence of both the ligand and the receptor. Furthermore IGF-binding proteins are specific for IGF-I and IGF-II thereby modulating the binding of the IGFs to the IGF-I receptor. In contrast insulin circulates unbound to any proteins and interacts in the free state with the insulin receptor. It has been postulated that downstream substrates of the activated receptors differ in their specificity for the receptors, thus lending further specificity to the actions mediated by the receptors. While a number of known endogenous substrates such as IRS-1, IRS-2 and She are utilized by both receptors, the structural differences in the beta subunits of the two receptors has lead investigators to suggest that certain substrates may be unique to each receptor. Candidate substrates which show this specificity of action have been and are being described. Full eludication of the specificities of the insulin and IGF-I signaling pathways is of interest of course for a better understanding of intercellular communication. In addition, because the closely related proteins insulin and IGF-I are used clinically, a clear understanding of the pathways activated by these agents is essential if more specific therapeutic modalities are to be developed for use in disease states.

Okadaic acid exerts a full insulin-like effect on glucose transport and glucose transporter 4 translocation in human adipocytes. Evidence for a phosphatidylinositol 3-kinase-independent pathway

The effects of the serine/threonine phosphatase inhibitor, okadaic acid, and insulin on glucose transport activity, glucose transporter 4 translocation to the plasma membrane, and the signaling pathway of insulin were examined in human adipocytes. Okadaic acid consistently produced a greater increase than insulin in the rate of glucose transport, and both agents together had a partial additive effect. Both insulin alone and okadaic acid alone stimulated the translocation of glucose transporter 4 to the plasma membrane. Insulin, but not okadaic acid, stimulated phosphatidylinositol 3-kinase (PI 3-kinase) activity, and wortmannin completely inhibited the effect of insulin on glucose transport. When the cells were incubated with both agents, okadaic acid inhibited insulin-stimulated PI 3-kinase activity but did not block the association of the p85 or p110 subunits of PI 3-kinase with insulin receptor substrate 1. Insulin-stimulated tyrosine phosphorylation of insulin receptor substrate 1 was only slightly reduced (15-30%) by okadaic acid. These data demonstrate that okadaic acid exerts a full insulin-like effect independent of the activation of PI 3-kinase. Thus, PI 3-kinase lipid kinase is not essential for glucose transporter 4 translocation in human adipocytes, and different pathways exist that lead to glucose transporter 4 translocation and increased glucose transport.

Interleukin-10 stimulation of phosphatidylinositol 3-kinase and p70 S6 kinase is required for the proliferative but not the antiinflammatory effects of the cytokine

Interleukin-10 (IL-10) is a powerful suppressor of the proinflammatory monokine production by lipopolysaccharide-stimulated monocytes as well as a T- and B-cell growth cofactor. The signal transduction cascades initiated by IL-10 ligation to its cognate receptor remain to be elucidated. Here, we demonstrate that in both primary monocytes and the D36 cell line, IL-10 rapidly and transiently stimulated phosphatidylinositol 3-kinase activity associated with the p85 subunit of the enzyme. IL-10 also activated p70 S6 kinase in both cell types. The activation of both of these kinases was sensitive to wortmannin, an inhibitor of phosphatidylinositol 3-kinase. The activation of p70 S6 kinase was also inhibited by the immunosuppressive drug rapamycin. Both rapamycin and wortmannin inhibited the IL-10-induced proliferation of D36 cells but in contrast had no effect on the antiinflammatory effects of the cytokine on lipopolysaccharide-stimulated monocytes. Similar results on D36 proliferation and lipopolysaccharide-stimulated monocyte inhibition by IL-10 were obtained with another phosphatidylinositol 3-kinase inhibitor, LY294002. This suggests that the activation of phosphatidylinositol 3-kinase and p70 S6 kinase is involved in the proliferative functions of IL-10 and that other as yet uncharacterized pathways affect the suppressive effects on monocytes, indicating that multiple and distinct signaling pathways mediate the various pleiotropic activities of IL-10. Furthermore, these findings suggest that it may be possible in the future to modulate the antiinflammatory effects of IL-10 for therapeutic benefit without disrupting other functions of the cytokine.

Protein kinase C delta specifically associates with phosphatidylinositol 3-kinase following cytokine stimulation

Phosphatidylinositol (PI) 3-kinase is activated as a result of cytokine-induced association of the enzyme with specific tyrosine-phosphorylated proteins. PI 3-kinase lipid products, PI 3, 4-P2 and PI 3,4,5-P3, have been shown, in vitro, to directly activate novel and atypical protein kinase C (PKC) isozymes. However, the mechanism by which PI 3-kinase may be involved in regulation of PKC isoforms in vivo is presently unknown. We investigated a possible relationship by looking for associations between these enzymes. We found that in a human erythroleukemia cell line, as well as in rabbit platelets, PI 3-kinase and PKCdelta associate in a specific manner that is modulated by cell activation. Granulocyte-macrophage colony-stimulating factor treatment of cells caused increased association of PKCdelta and PI 3-kinase as did treatment of platelets with platelet-activating factor. Results using two PI 3-kinase inhibitors, wortmannin and LY-294002, showed that the former inhibited this association, while the latter did not, suggesting that PI 3-kinase lipid products may not be a prerequisite for the PI 3-kinase/PKCdelta association. Our results also suggest that tyrosine phosphorylation of PKCdelta is not involved in its association with PI 3-kinase.

The Fyn tyrosine kinase binds Irs-1 and forms a distinct signaling complex during insulin stimulation

Irs-proteins link the receptors for insulin/IGF-1, growth hormones, and several interleukins and interferons to signaling proteins that contain Src homology-2 (SH2). To identify new Irs-1-binding proteins, we screened a mouse embryo expression library with recombinant [32P]Irs-1, which revealed a specific association between p59fyn and Irs-1. The SH2 domain in p59fyn bound to phosphorylated Tyr895 and Tyr1172, which are located in YXX(L/I) motifs. Mutation of p59fyn at the COOH-terminal tyrosine phosphorylation site (Tyr531) enhanced its binding to Irs-1 during insulin stimulation. Binding experiments with various SH2 protein revealed that Grb-2 was largely excluded from Irs-1 complexes containing p59fyn, whereas Grb-2 and p85 occurred in the same Irs-1 complex. By comparison with the insulin receptor, p59fyn kinase phosphorylated a unique cohort of tyrosine residues in Irs-1. These results outline a role for p59fyn or other related Src-kinases during insulin and cytokine signaling.

IL-4 function can be transferred to the IL-2 receptor by tyrosine containing sequences found in the IL-4 receptor alpha chain

IL-4 binds to a cell surface receptor complex that consists of the IL-4 binding protein (IL-4R alpha) and the gamma chain of the IL-2 receptor complex (gamma c). The receptors for IL-4 and IL-2 have several features in common; both use the gamma c as a receptor component, and both activate the Janus kinases JAK-1 and JAK-3. In spite of these similarities, IL-4 evokes specific responses, including the tyrosine phosphorylation of 4PS/IRS-2 and the induction of CD23. To determine whether sequences within the cytoplasmic domain of the IL-4R alpha specify these IL-4-specific responses, we transplanted the insulin IL-4 receptor motif (I4R motif) of the huIL-4R alpha to the cytoplasmic domain of a truncated IL-2R beta. In addition, we transplanted a region that contains peptide sequences shown to block Stat6 binding to DNA. We analyzed the ability of cells expressing these IL-2R-IL-4R chimeric constructs to respond to IL-2. We found that IL-4 function could be transplanted to the IL-2 receptor by these regions and that proliferative and differentiative functions can be induced by different receptor sequences.

Synergistic effects of insulin and phorbol ester on mitogen-activated protein kinase in Rat-1 HIR cells

Regulation of the activity of the extracellular signal regulated kinase (ERK) mitogen-activated protein kinases was examined in Rat-1 HIR, a fibroblast cell line overexpressing the human insulin receptor. Insulin or phorbol ester induced partial activations of ERKs, while a combination of insulin and phorbol ester resulted in a synergistic activation. Preincubation with phorbol ester increased the subsequent response to insulin. Phorbol ester did not enhance tyrosine phosphorylation of the insulin receptor. Insulin did not enhance activation of phospholipase D in response to phorbol ester. Lysophosphatidic acid also acted synergistically with insulin to induce ERK activation. Lysophosphatidic acid alone had little effect on ERK, and did not activate phospholipase D. The combination of phorbol ester and insulin maintained tyrosine phosphorylation of focal adhesion kinase, while insulin alone decreased its tyrosine phosphorylation. Phorbol ester induced phosphorylation of She on serine/threonine, while insulin induced tyrosine phosphorylation of She and She-Grb2 binding. These results suggest that full activation of ERKs in fibroblasts can require the cooperation of at least two signaling pathways, one of which may result from a protein kinase C-dependent phosphorylation of effectors regulating ERK activation. In this manner, phorbol esters may enhance mitogenic signals initiated by growth factor receptors.

The type I interferon receptor mediates tyrosine phosphorylation of insulin receptor substrate 2

Binding of interferon alpha (IFN alpha) to its receptor induces activation of the Tyk-2 and Jak-1 tyrosine kinases and tyrosine phosphorylation of multiple downstream signaling elements, including the Stat components of the interferon-stimulated gene factor 3 (ISGF-3). IFN alpha also induces tyrosine phosphorylation of IRS-1, the principle substrate of the insulin receptor. In this study we demonstrate that various Type I IFNs rapidly stimulate tyrosine phosphorylation of IRS-2. This is significant since IRS-2 is the major IRS protein found in hematopoietic cells. The IFN alpha-induced phosphorylated form of IRS-2 associates with the p85 regulatory subunit of the phosphatidylinositol 3'-kinase, suggesting that this kinase participates in an IFN alpha-signaling cascade downstream of IRS-2. We also provide evidence for an interaction of IRS-2 with Tyk-2, suggesting that Tyk-2 is the kinase that phosphorylates this protein during IFN alpha stimulation. A conserved region in the pleckstrin homology domain of IRS-2 may be required for the interaction of IRS-2 with Tyk-2, as shown by the selective binding of glutathione S-transferase (GST) fusion proteins containing the IRS-2-IH1PH or IRS-1-IH1PH domains to Tyk-2 but not other Janus kinases in vitro.

Insulin stimulation of mitogen-activated protein kinase, p90rsk, and p70 S6 kinase in skeletal muscle of normal and insulin-resistant mice. Implications for the regulation of glycogen synthase

Skeletal muscles from mice stimulated with insulin in vivo were used to evaluate relationships between the insulin receptor tyrosine kinase, mitogen-activated protein (MAP) kinase, p90rsk, p70 S6 kinase (p70S6k), and glycogen synthase. Two models of insulin resistance were also evaluated: (a) transgenic mice with a severe insulin receptor defect and (b) gold thioglucose (GTG) mice (obesity with minimal insulin receptor dysfunction). In normal mice, insulin stimulated MAP kinase (6-fold), p90rsk (RSK2, 5-fold), p70S6k (10-fold), and glycogen synthase (30-50% increase in fractional velocity). In transgenic mice, stimulation of MAP kinase and RSK2 were not detectable, whereas activation of p70S6k and glycogen synthase were preserved. In GTG mice, activation of MAP kinase, RSK2, p70S6k, and glycogen synthase were impaired. Since p70S6k and glycogen synthase were correlated, rapamycin was used to block p70S6k, and glycogen synthase activation was unaffected in normal mice; however, it was partially impaired in transgenic mice.

CONCLUSIONS

(a) stimulation of p70S6k and glycogen synthase are selectively preserved in muscles with a severe insulin receptor kinase defect, indicating signal amplification in pathways leading to these effects; (b) MAP kinase-RSK2 and p70S6k activation are impaired in obese mice, suggesting multiple loci for postreceptor insulin resistance; (c) glycogen synthase was dissociated from MAP kinase and RSK2, indicating that they are not required for this effect of insulin; and (d) p70S6k is not essential for glycogen synthase activation, but it may participate in redundant signaling pathways leading to this effect of insulin.

The 70 kDa S6 kinase: regulation of a kinase with multiple roles in mitogenic signalling

The activation of the 70kDa S6 kinase, pp70S6k, is a well documented mitogenic response, yet until recently little was known of how pp70S6k is activated, or of the identities of its crucial targets. The past year has revealed the complexity of pp70S6k regulation, with the overriding theme being that enzymes which have proven or putative roles in phospholipid metabolism mediate its activation. Studies also indicate that pp70S6k may regulate many more pathways than previously recognized.

Differential activation of mitogen-activated protein kinase and S6 kinase signaling pathways by 12-O-tetradecanoylphorbol-13-acetate (TPA) and insulin. Evidence for involvement of a TPA-stimulated protein-tyrosine kinase

AG-18, an inhibitor of protein-tyrosine kinases, was employed to study the role of tyrosine-phosphorylated proteins in insulin- and phorbol ester-induced signaling cascades. When incubated with Chinese hamster ovary cells overexpressing the insulin receptor, AG-18 reversibly inhibited insulin-induced tyrosine phosphorylation of insulin receptor substate-1, with minimal effects either on receptor autophosphorylation or on phosphorylation of Shc64. Under these conditions, AG-18 inhibited insulin-stimulated phosphorylation of the ribosomal protein S6, while no inhibition of insulin-induced activation of mitogen-activated protein kinase (MAPK) kinase or MAPK was detected. In contrast, 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced activation of MAPK kinase and MAPK and phosphorylation of S6 were inhibited by AG-18. This correlated with inhibition of TPA-stimulated tyrosine phosphorylation of several proteins, the most prominent ones being pp114 and pp120. We conclude that Tyr-phosphorylated insulin receptor substrate-1 is the main upstream regulator of insulin-induced S6 phosphorylation by p70s6k, whereas MAPK signaling seems to be activated in these cells primarily through the adaptor molecule Shc. In contrast, TPA-induced S6 phosphorylation is mediated by the MAPK/p90rsk cascade. A key element of this TPA-stimulated signaling pathway is an AG-18-sensitive protein-tyrosine kinase.

4PS/insulin receptor substrate (IRS)-2 is the alternative substrate of the insulin receptor in IRS-1-deficient mice

Insulin receptor substrate-1 (IRS-1) is the major cytoplasmic substrate of the insulin and insulin-like growth factor (IGF)-1 receptors. Transgenic mice lacking IRS-1 are resistant to insulin and IGF-1, but exhibit significant residual insulin action which corresponds to the presence of an alternative high molecular weight substrate in liver and muscle. Recently, Sun et al. (Sun, X.-J., Wang, L.-M., Zhang, Y., Yenush, L. P., Myers, M. G., Jr., Glasheen, E., Lane, W.S., Pierce, J. H., and White, M. F. (1995) Nature 377, 173-177) purified and cloned 4PS, the major substrate of the IL-4 receptor-associated tyrosine kinase in myeloid cells, which has significant structural similarity to IRS-1. To determine if 4PS is the alternative substrate of the insulin receptor in IRS-1-deficient mice, we performed immunoprecipitation, immunoblotting, and phosphatidylinositol (PI) 3-kinase assays using specific antibodies to 4PS. Following insulin stimulation, 4PS is rapidly phosphorylated in liver and muscle, binds to the p85 subunit of PI 3-kinase, and activates the enzyme. Insulin stimulation also results in the association of 4PS with Grb 2 in both liver and muscle. In IRS-1-deficient mice, both the phosphorylation of 4PS and associated PI 3-kinase activity are enhanced, without an increase in protein expression. Immunodepletion of 4PS from liver and muscle homogenates removes most of the phosphotyrosine-associated PI 3-kinase activity in IRS-1-deficient mice. Thus, 4PS is the primary alternative substrate, i.e. IRS-2, which plays a major role in physiologic insulin signal transduction via both PI 3-kinase activation and Grb 2/Sos association. In IRS-1-deficient mice, 4PS/IRS-2 provides signal transduction to these two major pathways of insulin signaling.

Protein tyrosine phosphorylation induced by epidermal growth factor and insulin-like growth factor-I in a rat clonal dental pulp-cell line

Both epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I) produce a dose-dependent stimulation in the rate of cell division in a rat clonal dental pulp-cell line (RDP 4-1). To elucidate the initial mitogen-induced cellular events that may mediate mitogenic action, the effects of EGF and IGF-I on cellular protein tyrosine phosphorylation were examined. In a dose-dependent manner, EGF (1-100 ng/ml) transiently stimulated tyrosine phosphorylation in four major proteins with apparent molecular weights of 220, 180, 140 and 120 kDa, and in five other more minor proteins (90, 80, 65, 55 and 44 kDa). IGF-I (1-100 ng/ml) dose-dependently stimulated the tyrosine phosphorylation of 160- and 140-kDa proteins, and had a smaller effect on the 80-, 65- and 44 kDa proteins. In contrast to the action of EGF, IGF-I-induced tyrosine phosphorylation was sustained for more than 60 min, particularly that of the 160-kDa phosphoprotein. From the results of specific immunoprecipitation/Western-blot analyses, the 180-kDa EGF-sensitive protein could be identified as the EGF receptor (EGF-R). Among the IGF-I-sensitive pulp cell proteins, the 160-kDa protein was identified as insulin-receptor substrate-1. Both mitogenic treatments stimulated the tyrosine phosphorylation of a weak, 44-kDa protein, which we have identified as the extracellular signal-regulated kinase-1. Despite the presence of phosphoproteins of the correct size, neither the IGF-I receptor (IGF-I-R) nor the phospholipase C gamma-isoform could be identified as tyrosine kinase substrates in either treatment. Pretreatment with the tyrosine kinase inhibitor genistein (20 micrograms/ml) significantly inhibited EGF- and IGF-I-induced tyrosine phosphorylation in permeabilized RDP 4-1 cells, and the tyrosine phosphatase inhibitor orthovanadate (1 mM) significantly prolonged the duration of the mitogen-stimulated tyrosine phosphorylation in both intact or permeabilized cells. Phorbol 12-myristate 13-acetate (100 nM), which activates protein kinase C (PKC), inhibited the tyrosine phosphorylation induced by either growth factor. This action was blocked by pretreatment with staurosporine (200 nM, 15 min), a selective PKC inhibitor. However, neither removing external Ca2+ with EGTA (1 mM) nor inducing Ca2+ influx with A23187 ionophore (2 microM) significantly altered EGF- or IGF-I-induced phosphorylation. These findings strongly suggest that authentic EGF-R and IGF-I-R on RDP 4-1 cells are coupled to complex, tyrosine kinase-mediated, intracellular signalling systems that are sensitive to a PKC-dependent mechanism. EGF- and IGF-I-induced tyrosine phosphorylation cascades may have important roles in vivo in the regulation of dental pulp-cell proliferation and ultimately may affect dentine formation.

Growth-dependent translation of IGF-II mRNA by a rapamycin-sensitive pathway

Insulin-like growth factor (IGF)-II is important for fetal growth and development. The human IGF-II gene generates multiple mature transcripts with different 5' untranslated regions (5'UTRs) but identical coding regions and 3'UTRs. We have previously shown that a minor 4.8-kilobase messenger RNA was engaged in the synthesis of preproIGF-II, and a major 6.0-kb mRNA was untranslated and stored in a 100S ribonucleoprotein particle. Here we demonstrate that the 6.0-kb mRNA is selectively mobilized and translated in dispersed exponentially growing cells. Translational activation is prevented by rapamycin and mimicked by anisomycin, which suggests that translation of the 6.0-kb mRNA is regulated by the p70S6k/85S6k kinase signalling pathway. Therefore, the minor 4.8-kb mRNA generates a constitutive production of prepro-IGF-II, whereas the major 6.0-kb mRNA provides a post-transcriptionally regulated species.

Role of IRS-2 in insulin and cytokine signalling

The protein IRS-1 acts as an interface between signalling proteins with Src-homology-2 domains (SH2 proteins) and the receptors for insulin, IGF-1, growth hormone, several interleukins (IL-4, IL-9, IL-13) and other cytokines. It regulates gene expression and stimulates mitogenesis, and appears to mediate insulin/IGF-1-stimulated glucose transport. Thus, survival of the IRS-1-/- mouse with only mild resistance to insulin was surprising. This dilemma is provisionally resolved with our discovery of a second IRS-signalling protein. We purified and cloned a likely candidate called 4PS from myeloid progenitor cells and, because of its resemblance to IRS-1, we designate it IRS-2. Alignment of the sequences of IRS-2 and IRS-1 revealed a highly conserved amino terminus containing a pleckstrin-homology domain and a phosphotyrosine-binding domain, and a poorly conserved carboxy terminus containing several tyrosine phosphorylation motifs. IRS-2 is expressed in many cells, including tissues from IRS-1-/- mice, and may be essential for signalling by several receptor systems.

Rapamycin, wortmannin, and the methylxanthine SQ20006 inactivate p70s6k by inducing dephosphorylation of the same subset of sites

Activation of p70s6k in cells stimulated with serum correlates with the phosphorylation of seven sites. Pretreatment of Swiss 3T3 cells with the immunosuppressant rapamycin blocks phosphorylation of four of these sites (Thr229, Thr389, Ser404, and Ser411), whereas phosphorylation proceeds in the remaining three sites (Ser418, Thr421, and Ser424). If rapamycin is added postserum stimulation, the pattern of phosphorylation is qualitatively similar except that Ser411 is still highly phosphorylated. The inhibitory effect of rapamycin on serum-induced p70s6k activation and the phosphorylation of Thr229, Thr389, Ser404, and Ser411 is rescued by FK506, providing further evidence that the inhibitory effect is exerted through a complex of rapamycin-FKBP12. Wortmannin treatment pre- or post-serum stimulation inhibits phosphorylation of the same set of sites as rapamycin, supporting the argument that both agents act on the same pathway. Likewise, methylxanthine phosphodiesterase inhibitors block p70s6k activation and phosphorylation of the same set of sites as wortmannin and rapamycin. However, other agents that raise intracellular cAMP levels have no inhibitory effect, leading to the hypothesis that the inhibitory actions of methylxanthines on p70s6k activity are not through activating protein kinase A but through inhibition of an upstream kinase. Together the results indicate that there are two kinase signaling pathways that must converge to activate p70s6k and that only one of these pathways is sensitive to rapamycin, wortmannin, and methylxanthine inhibition.

New frontiers in insulin receptor substrate signaling

Although most tyrosine kinase growth factor receptors directly bind Src homology 2 (SH2) proteins, the insulin receptor, and a select group of other hormone receptors-including an emerging group of cytokine receptors-phosphorylate intracellular insulin receptor substrate (IRS) proteins, which subsequently bind SH2 proteins. There are currently two members of the IRS family (IRS-1 and IRS-2); these IRS proteins contain elements of substantial similarity, but may also play divergent roles in mammalian physiology. The engagement of IRS proteins by other receptors suggests that IRS proteins mediate diverse biological signals.

Growth hormone, interferon-gamma, and leukemia inhibitory factor promoted tyrosyl phosphorylation of insulin receptor substrate-1

The identification of JAK2 as a growth hormone (GH) receptor-associated, GH-activated tyrosine kinase has established tyrosyl phosphorylation as a signaling mechanism for GH. In the present study, GH is shown to stimulate tyrosyl phosphorylation of insulin receptor substrate 1 (IRS-1), the principle substrate of the insulin receptor. Tyrosyl phosphorylation of IRS-1 is a critical step in insulin signaling and provides binding sites for proteins with the appropriate Src homology 2 domains, including the 85-kDa regulatory subunit of phosphatidylinositol (PI) 3'-kinase. In 3T3-F442A fibroblasts, GH-dependent tyrosyl phosphorylation of IRS-1 was detected by 1 min and at GH concentrations as low as 5 ng/ml (0.23 nM). Tyrosyl phosphorylation of IRS-1 was transient, with maximal stimulation detected at 30 min and diminished signal detected at 60 min. The ability of GH receptor (GHR) to transduce the signal for IRS-1 tyrosyl phosphorylation is mediated by the intracellular region of GHR between amino acids 295 and 380 by a mechanism not involving the two tyrosines in this region. This region of GHR is required for GH-dependent JAK2 association and activation (VanderKuur, J. A., Wang, X., Zhang, L., Campbell, G. S., Allevato, G., Billestrup, N., Norstedt, G., and Carter-Su, C. (1994) J. Biol. Chem. 269, 21709-21717). When other cytokines that activate JAK2 were tested for the ability to stimulate the tyrosyl phosphorylation of IRS-1, stimulation was detected with interferon-gamma and leukemia inhibitory factor. The correlation between JAK2 tyrosyl phosphorylation and IRS-1 tyrosyl phosphorylation in response to GH, interferon-gamma, and leukemia inhibitory factor and in cells expressing different GHR mutants, provides evidence that IRS-1 may interact with JAK2 or an auxiliary molecule that binds to JAK2. GH is also shown to stimulate binding of IRS-1 to the 85-kDa regulatory subunit of PI 3'-kinase. The ability of GH to stimulate tyrosyl phosphorylation of IRS-1 and its association with PI 3'-kinase provides a biochemical basis for responses shared by insulin and GH including the well characterized insulin-like metabolic effects of GH observed in a variety of cell types.

The pleckstrin homology domain in insulin receptor substrate-1 sensitizes insulin signaling

The NH2 terminus of insulin receptor substrate-1 (IRS-1) contains a pleckstrin homology (PH) domain. We deleted the PH domain in IRS-1 (IRS-1 delta PH) and expressed the mutant in Chinese hamster ovary and 32D cells. During insulin stimulation, IRS-1 delta PH is poorly tyrosine-phosphorylated in CHO cells, but undergoes serine/threonine phosphorylation. Similarly, IRS-1 delta PH fails to undergo insulin-stimulated tyrosine phosphorylation in 32D cells, which uncouples the activation of phosphatidylinositol 3'-kinase and p70s6k from the endogenous insulin receptors. Overexpression of the insulin receptor in 32DIR cells, however, restores tyrosine phosphorylation of IRS-1 delta PH and rescues insulin responses including mitogenesis. Thus, while the PH domain is not required for the engagement of downstream signals, it is one of the elements in the NH2 terminus of IRS-1 that is needed for a sensitive coupling to insulin receptors, especially at ordinary receptor levels found in most cells and tissues.

Wortmannin inhibits insulin-stimulated but not contraction-stimulated glucose transport activity in skeletal muscle

In skeletal muscle, glucose transport is stimulated by insulin, contractions and hypoxia. In this study, we used the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin to examine whether (i) PI 3-kinase activity is necessary for stimulation of glucose transport by insulin in muscle, and (ii) PI 3-kinase mediates a step in the pathway by which contractions/hypoxia stimulate glucose transport. Wortmannin completely blocked insulin- and insulin-like growth factor-1-stimulated glucose transport in muscle. In contrast, wortmannin had no effect on the stimulation of glucose transport by contractions or hypoxia, providing evidence that PI 3-kinase activity is not involved in the activation of glucose transport by these stimuli.