Phosphatidylinositol 3'-kinase is activated by association with IRS-1 during insulin stimulation

Glucose- and insulin-induced phosphorylation of the insulin receptor and its primary substrates IRS-1 and IRS-2 in rat pancreatic islets

The presence of tyrosine-phosphorylated proteins was studied in cultured rat pancreatic islets. Immunoblotting performed with total extracts of islets cultured in the presence of 1.8 or 5.6 mM glucose revealed at least three distinct tyrosine-phosphorylated bands (25 kDa, 95 kDa and 165-185 kDa). After 12 h incubation in medium containing 1.8 mM glucose, a pulse exposition to 11 or 22 mM glucose or to 10(-7) M insulin led to a substantial increase in the phosphorylation of all three bands, with no appearance of novel bands. Immunoprecipitation with specific antibodies demonstrated that the signal detected at 95 kDa corresponds to the beta subunit of the insulin receptor (IR) while the band at 165-185 kDa corresponds to the early substrates of the insulin receptor, IRS-1 and IRS-2. Immunoprecipitation with IRS-1 or IRS-2 antisera detected their association with the lipid metabolizing enzyme phosphatidylinositol 3-kinase (PI 3-kinase). Thus, this is the first demonstration that elements involved in the insulin-signalling pathway of traditional target tissues are also present in pancreatic islets and are potentially involved in auto- and paracrine-signalling in this organ.

Insulin-induced activation of phosphatidylinositol (PI) 3-kinase. Insulin-induced phosphorylation of insulin receptors and insulin receptor substrate-1 displaces phosphorylated platelet-derived growth factor receptors from binding sites on PI 3-kinase

Phosphatidylinositol (PI) 3-kinase is an enzyme that functions in the signaling pathways downstream from multiple cell surface receptors. The p85 regulatory subunit of PI 3-kinase binds to phosphotyrosine residues of various phosphoproteins including the platelet-derived growth factor (PDGF) receptor, the insulin receptor, and insulin receptor substrate-1 (IRS-1). Using NIH-3T3 cells overexpressing the human insulin receptor, we demonstrate that the p85 regulatory subunit of PI 3-kinase binds to phosphorylated PDGF receptor in cells incubated in the absence of insulin. When insulin is added, p85 is released from phosphorylated PDGF receptors and binds to phosphorylated insulin receptors and insulin receptor substrate-1. Moreover, insulin-induced dissociation of PDGF receptors from binding sites on PI 3-kinase requires a functional insulin receptor and is not prevented by vanadate treatment. In contrast, insulin activation does not displace PDGF receptors from binding sites on Ras GTPase-activating protein. This competition for binding to PI 3-kinase provides a mechanism for cross-talk among signaling pathways initiated by distinct peptide hormones and growth factors such as insulin and PDGF.

Substrate specificities of the insulin and insulin-like growth factor 1 receptor tyrosine kinase catalytic domains

To compare the substrate specificities of the insulin and insulin-like growth factor 1 (IGF-1) receptor tyrosine kinases, the catalytic domains of the enzymes have been expressed in Escherichia coli as fusion proteins. The purified proteins have kinase activity, demonstrating that the catalytic domain of IGF-1 receptor, like that of insulin receptor, is active independent of its ligand-binding and transmembrane domains. The specificities of the two enzymes for the divalent cations Mg2+ and Mn2+ are indistinguishable. A series of peptides has been prepared that reproduces the major phosphorylation sites of insulin receptor substrate-1, a common substrate for the two receptor tyrosine kinases in vivo. Insulin and IGF-1 receptors show distinct preferences for these peptides; whereas insulin receptor prefers peptides based on Tyr-987 or Tyr-727 of insulin receptor substrate-1, the IGF-1 receptor preferentially recognizes the Tyr-895 site. The latter site, when phosphorylated, is a binding site for the SH2 domain-containing adapter protein Grb2. The ability of the two receptor tyrosine kinases to be phosphorylated and activated by v-Src has also been examined. The catalytic activity of IGF-1 receptor is stimulated approximately 3.4-fold by treatment with purified v-Src, while insulin receptor shows very little effect of Src phosphorylation under these conditions. This observation is relevant to recent findings of IGF-1 receptor activation in Src-transformed cells, and may represent one method by which Src amplifies its mitogenic signal. Collectively the data suggest that the catalytic domains of the two receptor kinases possess inherently different substrate specificities and signaling potentials.

The role of the tyrosine kinase domain of the insulin-like growth factor-I receptor in intracellular signaling, cellular proliferation, and tumorigenesis

Insulin and insulin-like growth factor (IGF-I) receptors are heterotetrameric proteins consisting of two alpha-and two beta-subunits and members of the transmembrane tyrosine kinase receptors. Specific ligand binding to the receptor triggers a cascade of intracellular events, which begins with autophosphorylation of several tyrosine residues of the beta-subunit of the receptor. The triple cluster in the tyrosine kinase domain of the beta-subunit is the earliest and major autophosphorylation site. Previous studies have shown that substitutions of these three tyrosines by phenylalanines of both insulin and IGF-I receptors practically abolish any activation of cellular signaling pathways. We have studied the effect of double tyrosine mutations on IGF-I induced receptor autophosphorylation, activation of Shc and IRS-1 pathways, and cell proliferation and tumorigenicity. Substitution of tyrosines 1131/1135 blocks any detectable autophosphorylation, whereas substitution of tyrosines 1131/1136 or 1135/1136 only reduces autophosphorylation levels in some clones by approximately 50%. Nevertheless, all the cells expressing IGF-I receptors with double tyrosine substitutions demonstrated markedly reduced signaling through Shc and IRS-1 pathways. In addition, they were unable to respond to IGF-I-stimulated cell growth in culture, and tumor formation in nude mice was abrogated. These data suggest that the presence of tyrosine 1131 or 1135 essential for receptor autophosphorylation, whereas the presence of each of these tyrosines is necessary for a fully functional receptor.

Phosphatidylinositol (3,4,5)P3 interacts with SH2 domains and modulates PI 3-kinase association with tyrosine-phosphorylated proteins

Src homology 2 (SH2) domains on the regulatory subunit of phosphoinositide 3-kinase (PI 3-kinase) mediate its binding to specific tyrosine-phosphorylated proteins in stimulated cells. Using a pharmacological and genetic approach, we show that the amount of PI 3-kinase associated with tyrosine-phosphorylated proteins inversely correlates with the amount of PI 3-kinase lipid products present in the cell. An explanation for this observation is provided by our finding that phosphatidylinositol (3,4,5)trisphosphate (Ptdlns [3,4,5]P3) binds directly and selectively to the SH2 domains of the 85 kDa subunit of PI 3-kinase and thereby blocks binding to tyrosine-phosphorylated proteins. The SH2 domain of pp60C-STC also specifically bound Ptdlns (3,4,5)P3, and the binding was competed by a phosphopeptide specific for the Src SH2 domain. These results indicate that production of Ptdlns (3,4,5)P3 at the membrane disrupts the binding of PI 3-kinase to phosphoproteins. This lipid may also recruit other SH2-containing proteins to the membrane to initiate downstream signaling.

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.

The phosphodiesterase inhibitor SQ 20006 selectively blocks mitogen activation of p70S6k and transition to S phase of the cell division cycle without affecting the steady state phosphorylation of eIF-4E

In quiescent cells high levels of protein synthesis are required in order to re-enter the cell cycle upon stimulation. Initiation of polypeptide synthesis is the step most often subject to regulation, controlled in part by phosphorylation of 40 S ribosomal protein S6 and a number of initiation factors. The kinase responsible for S6 phosphorylation is p70S6k. We now show that the p70S6k pathway can be selectively blocked by the aminopurine analogue, SQ 20006. This agent is known to raise cAMP levels, resulting in activation of protein kinase A. We present evidence that the increase in cAMP is not responsible for the inhibitory effect observed. We also show that SQ 20006 can prevent the activation of p70S6k in a rapid and reversible manner. The compound does not exert its inhibitory activity on p70S6k but can inhibit in vitro two protein kinase C isozymes (alpha and gamma). In a B lymphoblastoid cell line, treatment with SQ 20006 results in inhibition of protein synthesis at the initiation stage. In contrast, when tested directly upon the translational machinery in the reticulocyte lysate, inhibition is manifest at both the level of initiation and elongation. The role of protein kinase A in the modulation of p70S6k and the rate of translation is discussed.

The GTP-binding protein Rac does not couple PI 3-kinase to insulin-stimulated glucose transport in adipocytes

BACKGROUND

In insulin-sensitive cells, such as adipocytes and skeletal muscle, the activation of phosphoinositide 3-kinase (PI 3-kinase) is thought to be critical in allowing insulin to stimulate both the uptake of glucose and the translocation of a specialized glucose transporter, GLUT4, to the plasma membrane. However, the downstream mediators that couple PI 3-kinase to GLUT4 translocation are still not known. Recent studies have shown that the GTP-binding protein Rac mediates some of the biological effects of PI 3-kinase, and these findings have led to the suggestion that Rac may be a common mediator for a variety of responses mediated by PI 3-kinase. To determine whether Rac couples PI 3-kinase to glucose uptake in adipocytes, we produced 3T3-L1 cells expressing either a constitutively active Rac1 (V12 Rac1, containing a valine residue at position 12) or a dominant-inhibitory Rac1 (N17 Rac1, containing an asparagine residue at position 17).

RESULTS

The stable expression of both V12 Rac1 and N17 Rac1 led to observable phenotypes in 3T3-L1 cells; expression of V12 Rac1 resulted in constitutive formation of lamellipodia and constitutive activation of the cJun-N-terminal kinase (JNK), whereas expression of N17 Rac1 inhibited the insulin-stimulated formation of lamellipodia. However, neither basal glucose uptake nor insulin-stimulated glucose uptake was affected by the expression of either mutant Rac protein. In addition, expression of V12 Rac1 did not reverse the inhibition of insulin-stimulated glucose uptake caused by the PI 3-kinase inhibitor wortmannin.

CONCLUSIONS

These findings provide direct evidence that PI 3-kinase does not use Rac to couple the insulin receptor to glucose uptake in adipocytes. Furthermore, the finding that Rac does not mediate glucose uptake in response to insulin is consistent with the idea that PI 3-kinase couples to a variety of different effector molecules in cells, and suggests that some of the specificity in the biological responses elicited by PI 3-kinase may be mediated by the activation of different effector molecules.

Protein-tyrosine phosphatase 1D modulates its own state of tyrosine phosphorylation

The insulin receptor-mediated signal transduction pathway involves insulin receptor substrate 1 and a variety of proteins containing Src homology-2 (SH2) domains, such as phosphatidylinositol 3-kinase, Grb2, and protein-tyrosine phosphatase 1D (PTP1D). Upon insulin stimulation of baby hamster kidney cells overexpressing the IR, the catalytically inactive mutant of PTP1D, C463A, becomes tyrosine-phosphorylated and coprecipitates with Grb2. Tyrosine phosphorylation of this mutant is significantly reduced when wild type PTP1D is coexpressed. Substitution of tyrosine residues 546 and 584 with phenylalanine abrogates tyrosine phosphorylation of the catalytically inactive mutant and abolishes its interaction with Grb2.

Insulin regulation of membrane-associated insulin receptor substrate 1

Insulin stimulation of 3T3-L1 adipocytes results in rapid activation of the insulin receptor tyrosine kinase followed by autophosphorylation of the receptor and phosphorylation of insulin receptor substrate 1 (IRS-1), its major substrate. The insulin receptor resides mostly at the cell surface of 3T3-L1 adipocytes under basal conditions, while about two-thirds of IRS-1 fractionates with intracellular membranes and one-third fractionates with cytosol. To test whether insulin receptor internalization is required for optimal tyrosine phosphorylation of IRS-1, 3T3-L1 adipocytes and CHO-T cells were incubated at 4 degrees C which inhibits receptor endocytosis but not its tyrosine kinase activity. Under these conditions, tyrosine phosphorylation of IRS-1 in the low density microsome fraction in response to insulin was as intense as that observed at 37 degrees C, indicating that endocytosis of insulin receptors is not necessary for tyrosine phosphorylation of IRS-1 to occur. Surprisingly, at 37 degrees C, insulin action on 3T3-L1 adipocytes progressively decreased the amount of IRS-1 protein associated with the low density microsome fraction and increased that in the cytosol. This redistribution of IRS-1 from the low density microsome fraction to the cytosol in response to insulin was accompanied by decreased electrophoretic mobility of IRS-1 on SDS-polyacrylamide gel electrophoresis. Incubation of adipocytes at 4 degrees C blocked the appearance of tyrosine-phosphorylated IRS-1 in the cytosol. Taken together, these data indicate that insulin receptors phosphorylate IRS-1 at the cell surface, perhaps in coated pits which are included in the low density microsome fraction. The results also suggest a desensitization mechanism in which the tyrosine-phosphorylated membrane-bound IRS-1, associated with signaling molecules such as phosphatidylinositol 3-kinase, is released into the cytoplasm in concert with its serine/threonine phosphorylation.

Modulation of early steps in insulin action in the liver and muscle of epinephrine treated rats

Epinephrine is known to produce insulin resistance, but the exact molecular mechanism involved is unknown. In the present study we have examined the levels and phosphorylation state of the insulin receptor and of insulin receptor substrate 1 (IRS-1), as well as the association between IRS-1 and phosphatidylinositol 3-kinase (PI 3-kinase) in the liver and muscle of rats treated with epinephrine. The results demonstrate a decrease in insulin-stimulated receptor and IRS-1 phosphorylation levels which was accompanied by a reduction in the association of IRS-1 with PI 3-kinasein vivo in liver and muscle of epinephrine treated rats. These data suggest that molecular post-receptor defects may explain some aspects of the insulin resistance induced by catecholamines.

Common and distinct elements in insulin and PDGF signaling

The receptors for insulin and PDGF are tyrosine kinases that mediate distinct effects in identical cellular backgrounds. Each receptor must therefore engage a unique subset of the available signaling elements--at least partly through the selection of proteins with src-homology 2 domains (SH2 proteins). Autophosphorylation sites in the PDGFr directly bind SH2 proteins, whereas activation of the insulin receptor leads to phosphorylation of IRS-1, which in turn binds SH2 proteins. In HIR 3.5 cells, which contain similar numbers of PDGF and insulin receptors, insulin, but not PDGF, stimulated tyrosyl phosphorylation of IRS-1. Similarly, insulin, but not PDGF, treatment of HIR 3.5 stimulated the association of IRS-1 with PtdIns 3'-kinase, although PDGF stimulated the association of PtdIns 3'-kinase with the tyrosine-phosphorylated PDGFr. Association with IRS-1 activated PtdIns 3'-kinase more effectively than association with the PDGFr. Whereas the PDGFr associated with PtdIns 3'-kinase, ras-GAP, GRB-2, and phospholipase C gamma, only GRB-2 and PtdIns 3'-kinase associated with IRS-1. Moreover, PDGF, but not insulin, caused tyrosine phosphorylation of phospholipase C gamma in HIR 3.5 cells. Thus, the insulin signal differs from that of PDGF by the insertion of a cytosolic, nonreceptor SH2 domain docking protein (IRS-1). Furthermore, IRS-1 binds a different subset of SH2 domain-containing proteins than does the PDGFr and regulates at least one common element (PtdIns 3'-kinase) differently than the PDGFr. These results support the hypothesis that IRS-1 differentiates the signals generated by the insulin receptor and PDGFr tyrosine kinases by binding and regulating a specific subset of SH2 domain-containing signaling molecules.

Insulin stimulates the kinase activity of RAC-PK, a pleckstrin homology domain containing ser/thr kinase

In the present study, insulin is shown to rapidly stimulate by 8- to 12-fold the enzymatic activity of RAC-PK alpha, a pleckstrin homology domain containing ser/thr kinase. In contrast, activation of protein kinase C by phorbol esters had almost no effect on the enzymatic activity of RAC-PK alpha. Insulin activation was accompanied by a shift in molecular weight of the RAC-PK alpha protein, and the activated kinase was deactivated by treatment with a phosphatase, indicating that insulin activated the enzyme by stimulating its phosphorylation. This insulin-induced shift in RAC-PK was also observed in primary rat epididymal adipocytes, as well as in a muscle cell line called C2C12 cells. The insulin-stimulated increase in RAC-PK alpha activity was inhibited by wortmannin (an inhibitor of phosphatidylinositol 3-kinase) in a dose-dependent manner with a half-maximal inhibition of 10 nM, but not by 20 ng/ml of rapamycin. Activation of RAC-PK alpha activity was also observed in a variant RAC lacking the pleckstrin homology domain. These results indicate that RAC-PK alpha activity can be regulated by the insulin receptor. RAC-PK alpha may therefore play a general role in intracellular signaling mediated by receptor tyrosine kinases.

Wortmannin and LY294002 inhibit the insulin-induced down-regulation of IRS-1 in 3T3-L1 adipocytes

The insulin receptor substrate-1 (IRS-1) is expressed in 3T3-L1 adipocytes and is involved in at least some insulin responses, notably mitogenesis. Chronic exposure to insulin down regulates IRS-1 in these cells by stimulating its degradation (Rice, K.M., Turnbow, M.A. and Garner, C.W. (1993) Biochem. Biophys. Res. Commun. 190, 961-967). This insulin response was completely inhibited by wortmannin and LY294002 (2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one), two inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase). Neither wortmannin nor LY294002 had any effect on the calcium-dependent degradation of IRS-1 in vitro nor did they inhibit the phosphorylation of IRS-1 in vitro. In addition, neomycin, a cationic aminoglycoside antibiotic that binds to phosphoinositides, inhibited the insulin-induced down-regulation of IRS-1 in 3T3-L1 adipocytes and, also, the C8-PIP3-stimulated degradation of IRS-1 in vitro. These results suggest that PI 3-kinase and its 3-phosphoinositide products mediate the insulin-induced down-regulation of IRS-1 in 3T3-L1 adipocytes.

Effect of glucagon on insulin receptor substrate-1 (IRS-1) phosphorylation and association with phosphatidylinositol 3-kinase (PI 3-kinase)

In the present study we have examined the levels and phosphorylation state of the insulin receptor and insulin receptor substrate 1 (IRS-1) as well as the association between IRS-1 and phosphatidylinositol 3-kinase (PI 3-kinase) in the liver and muscle of rats treated with glucagon. There was a decrease in the insulin-stimulated receptor and IRS-1 phosphorylation levels which was paralleled by a reduced association between IRS-1 and PI 3-kinase in vivo in the liver and muscle of glucagon-treated rats. These observations suggest that glucagon, probably acting through cAMP, may impair insulin signaling in the three early steps in insulin action after binding.

Localization of the insulin-like growth factor I receptor binding sites for the SH2 domain proteins p85, Syp, and GTPase activating protein

Potential signaling substrates for the insulin-like growth factor I (IGF-I) receptor are SH2 domain proteins including the p85 subunit of phosphatidylinositol 3-kinase, the tyrosine phosphatase Syp, GTPase activating protein (GAP), and phospholipase C-gamma (PLC-gamma). In this study, we demonstrate an association between the IGF-I receptor and p85, Syp, and GAP, but not with PLC-gamma in lysates of cells overexpressing the human IGF-I receptor. We further investigated these interactions using glutathione S-transferase (GST) fusion proteins containing the amino-terminal SH2 domains of p85 or GAP, or both SH2 domains of Syp or PLC-gamma to precipitate the IGF-I receptor from purified receptor preparations and from whole cell lysates. p85-, Syp-, and GAP-GSTs precipitated the IGF-I receptor, whereas the PLC-gamma-GST did not. Using phosphopeptides corresponding to IGF-I receptor phosphorylation sites, we determined that the p85- and Syp-GST association with the IGF-I receptor could be inhibited by a carboxyl-terminal peptide containing pY1316 and that the GAP-GST association could be inhibited by a NPXY domain peptide. The GAP-GST binding site was confirmed by showing that a mutant IGF-I receptor with a deletion of the NPXY domain including tyrosine 950 was poorly precipitated by the GAP-GST. We conclude that p85 and Syp may bind directly to the IGF-I receptor at tyrosine 1316, and that GAP may bind to the IGF-I receptor at and PLC-gamma was not evident. p85, Syp, and GAP are potential modulators of IGF-I receptor signal transduction.

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.

SH2 domain structure and function

An emerging theme in both the biology of signal transduction and the biochemistry of proteins has been the modular function of small protein domains. In some cases these can directly regulate catalytic activity. In others, they serve to interconnect important regulatory proteins. SH2 (src homology 2) domains represent some of the best studied models. Originally identified on the basis of homology in src and fps [1], SH2s are elements that ordinarily respond to tyrosine phosphorylation by binding the phosphorylated sequence. As such, they are key elements in tyrosine kinase regulation of cellular processes. Because SH2 interactions result from phosphorylation, such elements provide a regulatable circuitry along which signals can be transmitted in a timely manner. Because the regulation is based on a common mechanism, signal generators can target several different proteins coordinately. The PDGF receptor (PDGFr), for example, may interact with as many as ten different elements [2,3]. There are a number of excellent reviews on SH2 domains available [4-11]. This discussion will try to show how genetic, biochemical and biophysical results can be integrated in a satisfying way.

Tyrosine phosphorylation of insulin receptor substrate-1 in vivo depends upon the presence of its pleckstrin homology region

To characterize the structural basis for the interactions between the insulin receptor (IR) and its major substrate, insulin receptor substrate-1 (IRS-1), a segment of the NH2-terminal region of IRS-1 (Pro5-Pro65) was deleted. This region contains the first four conserved boxes of a pleckstrin homology (PH) domain, located at the NH2-terminal part of IRS-1. COS-7 cells were then cotransfected with the genes coding for IR and a wild-type (WT) or a mutated form of IRS-1. IRS-1 delta PH underwent significantly reduced insulin-dependent tyrosine phosphorylation compared with WT IRS-1. The reduced in vivo tyrosine phosphorylation of IRS-1 delta PH was accompanied by reduced association between IRS-1 delta PH and its downstream effector p85 regulatory subunit of phosphatidylinositol-3 kinase. In contrast, both WT IRS-1 and IRS-1 delta PH underwent comparable insulin-dependent tyrosine phosphorylation in vitro when incubated with partially purified insulin receptor kinase. These findings suggest that the overall structure of IRS-1 is not altered by deletion of its PH domain and that the PH domain is not the main site for protein-protein interactions between the insulin receptor and IRS-1, at least in vitro. In conclusion, the PH region might facilitate in vivo binding of IRS-1 to membrane phospholipids or other cellular constituents in close proximity to the IR, whereas the actual interactions with the IR are presumably mediated through other domains of the IRS-1 molecule. This could account for the fact that partial deletion of the PH domain selectively impairs the in vivo interactions between the insulin receptor and IRS-1, whereas their in vitro interactions remain unaffected.

Insulin and insulin-like growth factor I stimulate expression of the primary response gene cMG1/TIS11b by a wortmannin-sensitive pathway in RIE-1 cells

The addition of insulin or insulin-like growth factor I (IGF-I) to RIE-1 cells increased the expression of the primary response gene cMG1; dose-response analysis suggested that this effect was mediated largely through type 1 IGF receptors. Insulin/IGF-I did not affect the expression of the cMG1-related genes TIS11 and TIS11d, whereas epidermal growth factor, angiotensin II or 12-O-tetradecanoyl phorbol-13-acetate stimulated the expression of all three genes. Incubation with wortmannin (WM) prevented the insulin/IGF-I-induced elevation of cMG1 mRNA, but not that induced by the other mitogens or the stimulation of mitogen-activated protein kinase by insulin. We conclude that WM-sensitive phosphatidylinositol 3-kinase may be involved in the specific stimulation of cMG1 expression by insulin/IGF-I.

Interferon-alpha engages the insulin receptor substrate-1 to associate with the phosphatidylinositol 3'-kinase

Interferon-alpha (IFN alpha) induces rapid tyrosine phosphorylation of the insulin receptor substrate-1 (IRS-1), a docking protein with multiple tyrosine phosphorylation sites that bind to the Src homology 2 (SH2) domains of various signaling proteins. During IFN alpha stimulation, the p85 regulatory subunit of the phosphatidylinositol 3'-kinase binds via its SH2 domains to tyrosine-phosphorylated IRS-1, and phosphatidylinositol 3'-kinase activity is detected in association with IRS-1. Thus, IFN alpha responses occur by activation of the IRS signaling system, which it shares with insulin, insulin-like growth factor-1, and interleukin-4.

Combination of insulinomimetic agents H2O2 and vanadate enhances insulin receptor mediated tyrosine phosphorylation of IRS-1 leading to IRS-1 association with the phosphatidylinositol 3-kinase

To analyze the mechanism of action of the insulinomimetic agents H2O2, vanadate, and pervanadate (H2O2 and vanadate), CHO cells or CHO cells that overexpress wild-type or mutant insulin receptor and/or the insulin receptor substrate (IRS-1) were used. H2O2 or vanadate treatment alone had little or no effect on tyrosine phosphorylation of cellular proteins; however, pervanadate treatment dramatically enhanced tyrosine phosphorylation of a number of proteins including the insulin receptor and IRS-1. However, the insulin receptor and IRS-1 coimmunoprecipitate from insulin-treated but not from pervanadate-treated cells. Pervanadate-induced tyrosine phosphorylation of the insulin receptor led to an increase in insulin receptor tyrosine kinase activity toward IRS-1 in vivo and IRS-1 peptides in vitro equal to that induced by insulin treatment. Pervanadate-enhanced phosphorylation of IRS-1 led to a fifteenfold increase in IRS-1-associated phosphatidylinositol (PtdIns) 3-kinase activity. However, insulin receptor-associated PtdIns 3-kinase activity from pervanadate-treated cells was not detectable, while insulin receptor-associated PtdIns 3-kinase activity from insulin-treated cells was 20% of the IRS-1-associated activity. Thus, pervanadate but not H2O2 or vanadate alone under these conditions mimics many of insulin actions, but pervanadate treatment does not induce insulin receptor/IRS-1 association.

Phosphatidylinositol 3-kinase, but not p70/p85 ribosomal S6 protein kinase, is required for the regulation of phosphoenolpyruvate carboxykinase (PEPCK) gene expression by insulin. Dissociation of signaling pathways for insulin and phorbol ester regulation of PEPCK gene expression

Phosphoenolpyruvate carboxykinase (PEPCK) catalyzes the rate-limiting step in hepatic gluconeogenesis. Glucagon (via the second messenger cAMP) and glucocorticoids stimulate the transcription of the PEPCK gene, whereas insulin and phorbol esters inhibit, in a dominant fashion, these effects. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, prevents the stimulation of glycogen synthesis, glucose transport, mitogen-activated protein kinase, and p70/p85 ribosomal S6 protein kinase by insulin. We now show that wortmannin can also block the inhibition of glucocorticoid- and cAMP-stimulated PEPCK gene expression by insulin. PEPCK-chloramphenicol acetyltransferase fusion gene experiments demonstrate that wortmannin blocks an activity that is required for insulin signaling to elements within the PEPCK promoter. Phorbol esters mimic the action of insulin on the regulation of PEPCK gene expression, but wortmannin does not block the effect of these agents. Thus, phosphatidylinositol 3-kinase is required for the regulation of PEPCK gene expression by insulin, but not by phorbol esters. The immunosuppressant rapamycin, a potent inhibitor of insulin or phorbol ester stimulation of p70/p85 ribosomal S6 protein kinase, has no significant effect on the regulation of PEPCK gene expression by insulin or phorbol esters. Thus, p70/p85 ribosomal S6 protein kinase does not have a role in signaling to the PEPCK promoter by insulin or phorbol esters.

Shc binding to nerve growth factor receptor is mediated by the phosphotyrosine interaction domain

Shc is an adaptor protein that contains two phosphotyrosine-binding domains, a Src homology 2 (SH2) domain and the newly described phosphotyrosine interaction (PI) domain. Shc interacts with several tyrosine-phosphorylated proteins and is itself tyrosine-phosphorylated in cells stimulated with a variety of growth factors and cytokines. Upon phosphorylation, Shc binds to the Grb2.Sos complex leading to the activation of the Ras signaling pathway. Mutational analysis of the nerve growth factor (NGF) receptor (TrkA) suggested that the binding of Shc to the activated receptor is required for NGF-induced neuronal differentiation of PC12 cells. Here we report that the PI domain of Shc directly binds to tyrosine 490 on the autophosphorylated NGF receptor. The PI domain specifically recognizes an I/LXN-PXpY motif (where p indicates phosphorylation) as determined by phosphopeptide competition assay. In addition, the PI domain is able to efficiently compete for binding of full-length Shc proteins to the NGF receptor. In PC12 cells, the Shc SH2 domain interacts with an unidentified tyrosine-phosphorylated protein of 115 kDa but not with the activated NGF receptor. The ability of Shc to interact with different tyrosine-phosphorylated proteins via its PI and SH2 domains may allow Shc to play a unique role in tyrosine kinase signal transduction pathways.

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.

Selective inhibition of p70 S6 kinase activation by phosphatidylinositol 3-kinase inhibitors

Treatment of fibroblasts with wortmannin or demethoxyviridin, two potent inhibitors of phosphatidylinositol 3-kinase, prevents the activation of ribosomal protein S6 kinase, which is induced by a variety of external stimuli. Concentrations giving 50% inhibition of 45 nM (wortmannin) and 400 nM (demethoxyviridin) were obtained when epidermal growth factor was used as an S6 kinase activator; with platelet-derived growth factor, the concentrations giving 50% inhibition were about three-times higher. Western-blot analysis and immunocomplex kinase assays showed that wortmannin and demethoxyviridin specifically block the phosphorylation and activation of p70 S6 kinase without affecting the M(r) 90,000 ribosomal S6 kinase (p90rsk) or mitogen-activated protein kinases. Consistent with the irreversible nature of the inhibition of phosphatidylinositol 3-kinase by these compounds, treatment of cells with wortmannin, followed by washing out of the inhibitor, still led to inhibition of p70 S6 kinase activation. Several S6 kinase agonists not previously known to activate phosphatidylinositol 3-kinase (A23187, bombesin and phorbol 12-myristate 13-acetate) were found to increase the production of phosphatidylinositol 3,4,5-trisphosphate in a wortmannin-sensitive manner. These results support a model in which phosphatidylinositol 3-kinase acts upstream of p70 S6 kinase in a mitogenic signalling pathway; the existence of a phosphatidylinositol 3-kinase-independent pathway is also evident.

Ras-independent and wortmannin-sensitive activation of glycogen synthase by insulin in Chinese hamster ovary cells

Activation of glycogen synthase is one of the major metabolic events triggered by exposure of cells to insulin. The molecular mechanism by which insulin activates glycogen synthase was investigated. The possible role of Ras and mitogen-activated protein kinase cascade was investigated with a stable cell line, CHO-IR-C/S 46, that overexpresses insulin receptors and a catalytically inactive SH-PTP 2 protein phosphatase and in which insulin does not induce the formation of the Ras-GTP complex or the subsequently activation of the mitogen-activated protein kinase cascade. Insulin activated glycogen synthase in this cell line to a similar extent as in parental CHO-IR cells. The importance of heteromeric phosphoinositide (PI) 3-kinase in insulin activation of glycogen synthase was examined in a stable cell line, CHO-IR/delta p85, that overexpresses insulin receptors and a dominant negative mutant (delta p85) of the 85-kDa subunit of PI 3-kinase that lacks the binding site for the catalytic 110-kDa subunit. Insulin-dependent activation of PI-3 kinase and glucose transport, but not the formation of the Ras-GTP complex, are markedly attenuated in this cell line. In CHO-IR/delta p85 cells, insulin activated glycogen synthase to a similar extent as in parental CHO-IR cells. The failure of overproduction of the mutant (delta p85) protein to inhibit insulin activation of glycogen synthase was also confirmed by transient expression in Rat 1 cells with the use of a recombinant vaccinia virus. However, wortmannin abolished insulin activation of glycogen synthase in all cell lines. These data suggest that existence of a Ras-independent and wortmannin-sensitive pathway for activation of glycogen synthase by insulin.

Syk is activated by phosphotyrosine-containing peptides representing the tyrosine-based activation motifs of the high affinity receptor for IgE

Engagement of the high affinity receptor for immunoglobulin E (Fc epsilon RI) on the surface of mast cells induces tyrosine phosphorylation of numerous cellular proteins. Syk, one of several non-receptor protein tyrosine kinases implicated in Fc epsilon RI signaling, is activated following receptor cross-linking and associates with phosphorylated gamma subunits of Fc epsilon RI. We previously showed that the Src homology 2 (SH2) domains of Syk bind with high affinity to the conserved tyrosine-based activation motif (TAM) of the gamma subunit in vitro. In this report, we show that a tyrosine-phosphorylated gamma TAM peptide induced tyrosine phosphorylation of Syk in RBL-2H3 cell lysates and stimulated Syk kinase activity 10-fold in vitro, with half-maximal activation at 1-2 microM. A similar beta subunit TAM peptide showed much lower stimulation of Syk tyrosine phosphorylation and kinase activity. Phosphopeptide-induced activation was inhibited by an antiserum to the carboxyl-terminal tail of Syk, suggesting that those amino acids are also involved in Syk activation. These results indicate that the catalytic domain of Syk may be regulated by intramolecular interactions with adjacent domains and suggest that Syk binding to phosphorylated gamma subunits following Fc epsilon RI engagement in vivo stimulates Syk kinase activity.

Mechanisms of enhanced transmembrane signaling by an insulin receptor lacking a cytoplasmic beta-subunit domain

We have recently characterized a mutant insulin receptor (HIR delta 978) in which the insulin receptor beta-subunit was truncated at amino acid residue 978. Compared with parental Rat1 cells, the cells expressing the truncated receptor exhibited enhanced sensitivity to insulin's biologic actions. All of these effects are now extended to transcriptional events, since we now show enhanced sensitivity to insulin stimulation of c-fos mRNA expression. These effects were insulin-specific, since insulin-like growth factor-1 stimulation of glucose incorporation into glycogen, alpha-aminoisobutyric acid uptake, and thymidine incorporation into DNA were normal. In addition, the truncated receptor exhibited enhanced sensitivity only in vivo, but not in vitro, since the kinase activity of wheat germ agglutinin-purified receptor preparations was comparable between HIR delta 978 and parental Rat1 insulin receptors. Parental rat endogenous insulin-like growth factor-1 receptors and transfected human insulin receptors form hybrid receptors as well as homologous tetrameric receptors. The normal heterotetrameric receptors possess kinase activity in vivo leading to tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) and its association with the p85 regulatory subunit of phosphatidyl inositol 3-kinase. Interestingly, preincubation with human-specific anti-insulin receptor antibody abolished the increased insulin sensitivity in glucose incorporation into glycogen in HIR delta 978 cells. Furthermore, microinjection of anti-IRS-1 antibody into HIR delta 978 cells inhibited insulin stimulation of DNA synthesis. In summary: 1) truncated receptors on the cell surface confer enhanced insulin sensitivity in vivo; 2) the normal heterotetrameric receptors are functionally active and couple to IRS-1 efficiently; and 3) IRS-1 is an important molecule transmitting insulin's biological signals in HIR delta 978 cells.

Monocyte colony-stimulating factor stimulates binding of phosphatidylinositol 3-kinase to Grb2.Sos complexes in human monocytes

Monocyte colony-stimulating factor (M-CSF) is required for the proliferation of mononuclear phagocytes. The activated M-CSF receptor associates with phosphatidylinositol 3-kinase (PI 3-kinase). In the present studies, we demonstrate that M-CSF also induces direct interaction of PI 3-kinase (p85 alpha subunit) with the SH2/SH3 adaptor protein Grb2. Tyrosine-phosphorylated PI 3-kinase interacts with the SH2 domain of Grb2. A pYRNE (pY408) site in PI 3-kinase is potentially involved in this interaction. The results also demonstrate that the PI 3-kinase.Grb2 complex associates with the guanine nucleotide exchange protein Sos. Since Sos binds to the SH3 domains of Grb2 and thereby associates with Ras at the cell membrane, formation of the PI 3-kinase.Grb2.Sos complex provides a potential mechanism for growth factor-induced interactions of PI 3-kinase and Ras.

Effects of growth factors on phosphatidylinositol-3 kinase in astroglial cells

Growth factors differently regulate astroglial cell differentiation and proliferation. In an effort to understand the early intracellular events promoted by growth factors in astroglial cells, we have determined the effects of insulin-like growth factor I (IGF1), insulin, platelet-derived growth factor (PDGF), epidermal growth factor (EGF) and fibroblast growth factors (FGFs) on phosphatidylinositol-3 kinase (PI(3)-kinase). In astroglial cells cultured in serum-free medium, IGF1, PDGF, and EGF, which stimulate cell proliferation, increased PI(3)-kinase activity immunoprecipitated with anti-phosphotyrosine antibodies as shown by thin layer chromatography and high performance liquid chromatography. FGFa and FGFb, which strongly stimulate proliferation, glutamine synthetase, and deiodinase activities and modify cell morphology, have no effect on PI(3)-kinase activity. Addition of 1 nM PDGF, 10 nM IGF1, or 100 nM EGF to the culture medium rapidly stimulated PI(3)-kinase activity which declined slowly after 2 min. The stimulation of PI(3)-kinase increased with growth factor concentration. The maximum increase in PI(3)-kinase activity occurred with 50 nM IGF1, 1 nM PDGF, or 100 nM EGF. Since insulin was active only at high concentration (1 microM), its effect was probably mediated through IGF1 receptors and not through insulin receptors. IGF1 and PDGF, to a lesser degree, also increased the PI(3)-kinase activity associated with pp60c-src protein. Immunoblots performed with an antibody directed against the p85-subunit of the PI(3)-kinase confirmed that IGF1 increased the number of PI(3)-kinase molecules associated with phosphotyrosine-containing proteins or with c-src protein. Each growth factor affects in a different manner the association of PI(3)-kinase with phosphotyrosine-containing proteins and with pp60c-src and thus probably modulates intracellular signals downstream of PI(3)-kinase in astroglial cells.

Effect of phosphotyrosyl-IRS-1 level and insulin receptor tyrosine kinase activity on insulin-stimulated phosphatidylinositol 3, MAP, and S6 kinase activities

The role of tyrosine phosphorylation of the insulin receptor substrate 1 (IRS-1) was studied utilizing parental CHO cells or CHO cells that overexpress IRS-1, the insulin receptor, or both IRS-1 and the insulin receptor. Insulin stimulation of these four cell lines led to progressive levels of IRS-1 tyrosine phosphorylation of one, two, four, and tenfold. Maximal insulin-stimulated IRS-1 associated PtdIns 3'-kinase activit in these cells was 1-, 1.5-, 3-, and 3-fold, while insulin sensitivity, as determined by ED50, was 1-, 2.5-, 10-, and 10-fold. Both sensitivity and maximal response paralleled the increased level of phosphotyrosyl-IRS-1; however, the increased level of phosphotyrosyl-IRS-1 seen in CHO/IR/IRS-1 cells did not further increase these responses. Likewise, maximal insulin-stimulated MAP kinase activity in these cell lines increased in parallel with IRS-1 tyrosine phosphorylation except in the CHO/IR/IRS-1 cell lines with activity levels of one-, five-, nine-, and ninefold. However, insulin sensitivity of the MAP and S6 kinases and maximal insulin-stimulated S6 kinase activity was not changed by a twofold increase in phosphotyrosyl-IRS-1, but an increase was observed with insulin-stimulated receptor autophosphorylation and kinase activity in CHO/IR cells which led to a tenfold increase in insulin receptor autophosphorylation and a fourfold increase in IRS-1 tyrosine phosphorylation. Thus, these three kinase activities may be differentially coupled to the activation of the insulin receptor kinase activity via IRS-1 and other possible cellular substrates.

PDGF stimulates an increase in GTP-Rac via activation of phosphoinositide 3-kinase

BACKGROUND

Phosphoinositide 3-kinases (PI 3-kinases) are thought to play an important role in coordinating the responses elicited by a variety of growth factors, oncogene products and inflammatory stimuli. These responses include activation of membrane ruffling, chemotaxis, glucose transport, superoxide production, neurite outgrowth and pp70 S6 kinase. Some of these responses are also known to be regulated by Rac, a small GTP-binding protein related to Ras. Neither the transducing elements upstream of Rac, nor those downstream of PI 3-kinase, have been defined.

RESULTS

We show here that platelet-derived growth factor (PDGF) can stimulate an increase in the level of GTP-Rac by at least two distinct mechanisms: firstly, by increased guanine nucleotide exchange; and secondly, by inhibition of a Rac GTPase activity. The first of these mechanisms is essential for the activation of Rac, and we show that it is dependent upon PDGR-stimulated synthesis of phosphatidylinositol (3,4,5)-trisphosphate.

CONCLUSIONS

These results suggest that Rac activation lies downstream of PI 3-kinase activation on a PDGF-stimulated signalling pathway. Furthermore, as Rac has been implicated in at least two diverse cellular responses that are also though to require activation of PI 3-kinase--a reorganization of the actin cytoskeleton known as membrane ruffling and the neutrophil oxidative burst--these results suggest that Rac may be a major effector protein for the PI 3-kinase signalling pathway in many cell types.

Insulin-dependent tyrosine phosphorylation of the vav protooncogene product in cells of hematopoietic origin

Insulin activates the ras signaling pathway and promotes hematopoietic cell proliferation. One possible mediator in such signaling is the vav proto-oncogene product (p95vav), which is specifically expressed in cells of hematopoietic origin and contains domains typical of guanine nucleotide exchange factors as well as Src homology 2 and Src homology 3 domains. We studied the tyrosine phosphorylation of p95vav in hematopoietic cells expressing insulin receptors. Immunoblotting experiments with an antiphosphotyrosine monoclonal antibody disclosed that insulin induces rapid and transient tyrosine phosphorylation of p95vav in the human U-266 myeloma cell line. These findings were confirmed by immunoprecipitation experiments performed with 32P-labeled cells and phosphoamino acid analysis of the bands corresponding to p95vav. Similarly, insulin-dependent tyrosine phosphorylation of p95vav was observed in the human IM-9 and mouse J558L hematopoietic cell lines. Furthermore, insulin treatment of cells led to the association of the Src homology 2 domain of p95vav with the activated beta-subunit of the insulin receptor in vitro. Altogether, these data suggest that p95vav is a substrate for the insulin receptor tyrosine kinase and may be involved in an insulin signaling pathway linking receptor-generated signals to Ras or other GTP-binding proteins in cells of hematopoietic origin.

Identification of a 190-kDa protein as a novel substrate for the insulin receptor kinase functionally similar to insulin receptor substrate-1

Recently, we generated mice with a targeted disruption of the insulin receptor substrate-1 (IRS-1) gene and demonstrated that they exhibited growth retardation and mild insulin resistance, suggesting the presence of IRS-1-independent pathway that partially substitutes for IRS-1 in IRS-1-deficient mice (Tamemoto, H., Kadowaki, T., Tobe, K., Yagi, T., Sakura, H., Hayakawa, T., Terauchi, Y., Ueki, K., Kaburagi, Y., Satoh, S., Sekihara, H., Yoshioka, S., Horikoshi, H., Furuta, Y., Ikawa, Y., Kasuga, M., Yazaki, Y., and Aizawa, S. (1994) Nature 372, 182-186). We have examined the insulin-stimulated tyrosine-phosphorylated proteins in livers of wild type and IRS-1-deficient mice. Tyrosine phosphorylation of an 190-kDa protein (pp190) by insulin was significantly stimulated in livers of IRS-1-deficient mice, which was weakly observed in wild type mice in addition to IRS-1. We also demonstrated that pp190 was immunologically distinct from IRS-1 and was associated with both the 85-kDa subunit of phosphatidylinositol 3-kinase and the Grb2/Ash molecule as IRS-1. We identified pp190 as a novel substrate for insulin receptor kinase (IRS-2), which can bind both PI3-kinase and Ash/Grb2, and whose tyrosine phosphorylation is specifically induced in IRS-1-deficient mice. These data suggested that pp190 may play some physiological roles in insulin's signal transduction; furthermore, induction of tyrosine phosphorylation of pp190 may be one of the compensatory mechanisms that substitute for IRS-1 in IRS-1-deficient mice.

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.

Insulin-like growth factor-I stimulates tyrosine phosphorylation of endogenous c-Crk

Crk, a cellular homolog of v-crk, is an SH2 and SH3 domain-containing adaptor protein related to Grb2 and Nck, two proteins which have been shown to be involved in growth factor signal transduction. Crk proteins have recently been found to associate with two guanine nucleotide releasing proteins, mSos and C3G, and thus appear to lie on the Ras pathway. We investigated whether Crk is a target for the insulin-like growth factor I (IGF-I) receptor tyrosine kinase. We show that IGF-I stimulates tyrosine phosphorylation of Crk II via stimulation of endogenous IGF-I receptors in both 293 cells and NIH-3T3 cells. IGF-I stimulated tyrosine phosphorylation of Crk II in a dose- and time-dependent manner. In 293 cells, which express both IGF-I and insulin receptors, insulin also induced a dose-dependent tyrosine phosphorylation of Crk II, but with somewhat reduced sensitivity, compared to IGF-I. In NIH 3T3 cells, IGF-I also stimulated tyrosine phosphorylation of a 45- kDa protein which co-immunoprecipitated with Crk II. These findings indicate that Crk II is an endogenous substrate of the IGF-I receptor tyrosine kinase and provide the first demonstration that a mitogenic growth factor induces tyrosine phosphorylation of endogenous c-Crk.

The Drosophila insulin receptor contains a novel carboxyl-terminal extension likely to play an important role in signal transduction

The nucleic acid and deduced amino acid sequence of the Drosophila insulin receptor homologue (dir) has been determined. The coding sequence of dir is contained within 10 exons spanning less than 8 kilobase pairs of genomic DNA. The deduced amino acid sequence of the dir encodes a protein of 2148 amino acids, larger than the human insulin receptor due to amino- and carboxyl-terminal extensions. The overall level of amino acid identity between the DIR and human insulin and insulin-like growth factor-I receptors is 32.5 and 33.3%, respectively. Higher levels of identity are found in exon 2 (45 and 43%, respectively) and in the beta subunit (50 and 48%, respectively), and the positions of most cysteine residues in the alpha subunit cysteine-rich domain are conserved. A novel, 400-amino acid, carboxyl-terminal extension contains 9 tyrosine residues, four of which are present in YXXM or YXXL motifs, suggesting that they function as binding sites for SH2 domain-containing signaling proteins. The presence of multiple putative SH2 domain binding sites in the DIR represents a significant difference from its mammalian homologues and suggests that, unlike the human insulin and insulin-like growth factor-I receptors, the DIR forms stable complexes with signaling molecules as part of its signal transduction mechanism.

Growth hormone stimulates the tyrosine phosphorylation of the insulin receptor substrate-1 and its association with phosphatidylinositol 3-kinase in primary adipocytes

Insulin receptor substrate-1 (IRS-1) is tyrosine-phosphorylated in response to insulin resulting in association with and activation of phosphatidylinositol 3-kinase (PI 3-kinase), thereby initiating some of the effects of insulin. We have recently shown that the insulin-like effects of growth hormone (GH) in adipocytes can be inhibited by the selective PI 3-kinase inhibitor wortmannin (Ridderstråle, M., and Tornqvist, H. (1994) Biochem. Biophys. Res. Commun. 203, 306-310), suggesting a similar role for PI 3-kinase in GH action. Here we show that IRS-1 is tyrosine-phosphorylated in a time- and dose-dependent manner in response to GH in primary rat adipocytes. This phosphorylation coincided with the extent of interaction between IRS-1 and the 85-kDa subunit of PI 3-kinase as evidenced by coimmunoprecipitation. Stimulation with 23 nM GH increased the PI 3-kinase activity associated with IRS1 4-fold. Our data suggest that GH-induced tyrosine phosphorylation of IRS-1 and the subsequent docking of PI 3-kinase are important postreceptor events in GH action. The mechanism for the phosphorylation of IRS-1 induced by GH is unknown, but involvement of JAK2, the only known GH receptor-associated tyrosine kinase, seems possible.

Regulation of phosphatidylinositol 3'-kinase by tyrosyl phosphoproteins. Full activation requires occupancy of both SH2 domains in the 85-kDa regulatory subunit

Phosphatidylinositol 3'-kinase (PI 3'-kinase) is activated in insulin-stimulated cells by the binding of the SH2 domains in its 85-kDa regulatory subunit to insulin receptor substrate-1 (IRS-1). We have previously shown that both tyrosyl-phosphorylated IRS-1 and mono-phosphopeptides containing a single YXXM motif activate PI 3'-kinase in vitro. However, activation by the monophosphopeptides was significantly less potent than activation by the multiply phosphorylated IRS-1. We now show that the increased potency of PI 3'-kinase activation by IRS-1 relative to phosphopeptide is not due to tertiary structural features IRS-1, as PI 3'-kinase is activated normally by denatured, reduced, and carboxymethylated IRS-1. Furthermore, activation of PI 3'-kinase by bis-phosphorylated peptides containing two YXXM motifs is 100-fold more potent than the corresponding mono-phosphopeptides and similar to activation by IRS-1. These data suggest that tyrosyl-phosphorylated IRS-1 or bis-phosphorylated peptides bind simultaneously to both SH2 domains of p85. However, these data cannot differentiate between an activation mechanism that requires two-site occupancy for maximal activity as opposed to one in which bivalent binding enhances the occupancy of a single activating site. To distinguish between these possibilities, we produced recombinant PI 3'-kinase containing either wild-type p85 or p85 mutated in its N-terminal, C-terminal, or both SH2 domains. We find that mutation of either SH2 domains significantly reduced phosphopeptide binding and decreased PI 3'-kinase activation by 50%, whereas mutation of both SH2 domains completely blocked binding and activation. These data provide the first direct evidence that full activation of PI 3'-kinase by tyrosylphosphorylated proteins requires occupancy of both SH2 domains in p85.

Activation of mitogen-activated protein kinase and phosphatidylinositol 3'-kinase is not sufficient for the hormonal stimulation of glucose uptake, lipogenesis, or glycogen synthesis in 3T3-L1 adipocytes

The precise mechanism by which insulin regulates glucose metabolism is not fully understood. However, it is known that insulin activates two enzymes, phosphatidylinositol 3'-kinase (PI 3'-K) and mitogen-activated protein kinase (MAPK), which may be involved in stimulating the metabolic effects of insulin. The role of these enzymes in glucose metabolism was examined by comparing the effects of insulin, platelet-derived growth factor (PDGF) and epidermal growth factor (EGF) in 3T3-L1 adipocytes. Treatment of the cells with PDGF or EGF for 5 min increased the MAPK activity 3-5-fold, while insulin treatment produced a 2.5-fold increase. The MAPK activity remained elevated for 1 h after either PDGF or insulin treatment. PDGF and insulin, but not EGF, caused a transient increase in the amount PI 3'-K activity coprecipitated with tyrosine phosphorylated proteins. Although PDGF and insulin caused a similar increase in the activities of these two enzymes, only insulin caused substantial increases in glucose utilization. Insulin increased the transport of glucose and the synthesis of lipid 4- and 17-fold, respectively, while PDGF did not affect these processes significantly. Glycogen synthesis was increased 15-fold in response to insulin and only 3-fold in response to PDGF. Thus, the activation of MAPK and PI 3'-K are not sufficient for the complete stimulation of glucose transport, lipid synthesis, or glycogen synthesis by hormones in 3T3-L1 adipocytes, suggesting a requirement for other signaling mechanisms that may be uniquely responsive to insulin.

Upstream mechanisms of glycogen synthase activation by insulin and insulin-like growth factor-I. Glycogen synthase activation is antagonized by wortmannin or LY294002 but not by rapamycin or by inhibiting p21ras

This study was undertaken to define intracellular signaling pathways upstream to glycogen synthase activation. First, we examined the role of the two pathways of insulin signaling, Ras-dependent and wortmannin/LY294002-sensitive, in glycogen synthase activation. Although negative dominant Ras (Ras17N) induction in PC12 cells markedly decreased activities of mitogen-activated protein kinase (MAP) and pp90 S6 kinase in response to insulin or insulin-like growth factor I (IGF-I), activation of glycogen synthase by these agents was unaffected by negative dominant Ras induction. In contrast, wortmannin and 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002), inhibitors of phosphatidylinositol 3-kinase, antagonized glycogen synthase activation in response to insulin or IGF-I. Next, we examined the contribution of pp70 S6 kinase, one of the wortmannin/LY294002-sensitive signaling molecules on glycogen synthase activation. Immunosuppressant rapamycin completely blocked activation of pp70 S6 kinase by insulin or IGF-I, but rapamycin alone or in combination with induction of negative dominant Ras failed to antagonize glycogen synthase activation by these hormones. These data suggest that 1) activation of Ras-MAP kinase is not necessary for stimulation of glycogen synthase and 2) activation of wortmannin/LY294002-sensitive pathway, independent of pp70 S6 kinase, plays a key role in glycogen synthase regulation in PC12 cells.

Insulin receptor signaling is augmented by antisense inhibition of the protein tyrosine phosphatase LAR

Considerable evidence has shown that most physiologic responses to insulin require activation of the intrinsic tyrosine kinase of the insulin receptor. Biochemical studies have also supported the hypothesis that receptor kinase activity can be modulated by cellular protein tyrosine phosphatases (PTPases), which have not yet been identified. To test the hypothesis that the transmembrane PTPase LAR can modulate insulin receptor signaling in vivo, antisense RNA expression was used to specifically suppress LAR protein levels by 63% in the rat hepatoma cell line, McA-RH7777. Hormone-dependent autophosphorylation of the insulin receptor was increased by approximately 150% in the antisense-expressing cells at all insulin concentrations tested. This increase in autophosphorylation was paralleled by a 35% increase in insulin receptor tyrosine kinase activity. Reduced LAR levels did not alter non-hormone-dependent tyrosine phosphorylation nor basal insulin receptor tyrosine phosphorylation and kinase activity. Most significantly, reduced LAR levels resulted in a 350% increase in insulin-dependent phosphatidylinositol 3-kinase activity. These studies provide unique in vivo evidence that LAR is involved in the modulation of insulin receptor signaling in intact cells.

Specificity of receptor tyrosine kinase signaling: transient versus sustained extracellular signal-regulated kinase activation

A number of different intracellular signaling pathways have been shown to be activated by receptor tyrosine kinases. These activation events include the phosphoinositide 3-kinase, 70 kDa S6 kinase, mitogen-activated protein kinase (MAPK), phospholipase C-gamma, and the Jak/STAT pathways. The precise role of each of these pathways in cell signaling remains to be resolved, but studies on the differentiation of mammalian PC12 cells in tissue culture and the genetics of cell fate determination in Drosophila and Caenorhabditis suggest that the extracellular signal-regulated kinase (ERK-regulated) MAPK pathway may be sufficient for these cellular responses. Experiments with PC12 cells also suggest that the duration of ERK activation is critical for cell signaling decisions.

Signal transduction by the antigen receptors of B and T lymphocytes

B and T lymphocytes of the immune system recognize and destroy invading microorganisms but are tolerant to the cells and tissues of one's own body. The basis for this self/non-self-discrimination is the clonal nature of the B and T cell antigen receptors. Each lymphocyte has antigen receptors with a single unique antigen specificity. Multiple mechanisms ensure that self-reactive lymphocytes are eliminated or silenced whereas lymphocytes directed against foreign antigens are activated only when the appropriate antigen is present. The key element in these processes is the ability of the antigen receptors to transmit signals to the interior of the lymphocyte when they bind the antigen for which they are specific. Whether these signals lead to activation, tolerance, or cell death is dependent on the maturation state of the lymphocytes as well as on signals from other receptors. We review the role of antigen receptor signaling in the development and activation of B and T lymphocytes and also describe the biochemical signaling mechanisms employed by these receptors. In addition, we discuss how signal transduction pathways activated by the antigen receptors may alter gene expression, regulate the cell cycle, and induce or prevent programmed cell death.