Insulin stimulation of phosphatidylinositol 3-kinase activity maps to insulin receptor regions required for endogenous substrate phosphorylation.

We have studied the phosphatidylinositol 3-kinase (PtdIns 3-kinase) in insulin-stimulated Chinese hamster ovary (CHO) cells expressing normal (CHO/IR) and mutant human insulin receptors. Insulin stimulation of CHO/IR cells results in an increase in PtdIns 3-kinase activity associated with anti-phosphotyrosine (alpha PY) immunoprecipitates, which has been previously shown to correlate with the in vivo production of PtdIns(3,4)P2, and PtdIns(3,4,5)P3 (Ruderman, N., Kapeller, R., White, M.F., and Cantley, L.C. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 1411-1415). Stimulation was maximal within 1 min and showed a dose response identical to that of insulin receptor autophosphorylation. The PtdIns 3-kinase also associated with the insulin receptor in an insulin-stimulated manner, as approximately 50% of the total alpha PY-precipitable activity could be specifically immunoprecipitated with anti-insulin receptor antibody. Mutant insulin receptors displayed variable ability to stimulate the PtdIns 3-kinase, but in all cases the presence of PtdIns 3-kinase in alpha PY immunoprecipitates correlated closely with the tyrosyl phosphorylation of the endogenous substrate pp185. In CHO cells expressing a kinase-deficient mutant (IRA1018), there was no observable insulin stimulation of PtdIns 3-kinase activity in alpha PY immunoprecipitates and no tyrosyl phosphorylation of pp185. Substitution of Tyr1146 in the insulin receptor regulatory region with phenylalanine partially impaired receptor autophosphorylation, pp185 phosphorylation, and insulin-stimulated increases in alpha PY-precipitable PtdIns 3-kinase activity. In contrast, a deletion mutant lacking 12 amino acids from the juxtamembrane region (IR delta 960) displayed normal in vivo autophosphorylation but failed to stimulate the PtdIns 3-kinase or phosphorylate pp185. Finally, a mutant receptor from which the C-terminal 43 amino acids had been deleted (IR delta CT) exhibited normal insulin-stimulated autophosphorylation, pp185 phosphorylation, and stimulation of the PtdIns 3-kinase activity in alpha PY immunoprecipitates. These data suggest that the PtdIns 3-kinase is itself a substrate of the insulin receptor kinase or associates preferentially with a substrate. A comparison of the biological activities of the mutant receptors with their activation of the PtdIns 3-kinase furthermore suggests that the PtdIns 3-kinase may be linked to insulin's ability to regulate DNA synthesis and cell growth.     

Activation of phosphatidylinositol-3 kinase by nerve growth factor involves indirect coupling of the trk proto-oncogene with src homology 2 domains.

Growth factor receptor tyrosine kinases can form stable associations with intracellular proteins that contain src homology (SH) 2 domains, including the p85 regulatory subunit of phosphatidylinositol (PI)-3 kinase. The activation of this enzyme by growth factors is evaluated in PC12 pheochromocytoma cells and NIH 3T3 fibroblasts expressing the pp140c-trk nerve growth factor (NGF) receptor (3T3-c-trk). NGF causes the rapid stimulation of PI-3 kinase activity detected in anti-phosphotyrosine, but not in anti-trk, immunoprecipitates. This effect coincides with the tyrosine phosphorylation of two proteins, with molecular masses of of 100 kd and 110 kd, that coimmunoprecipitate with p85. Similar phosphorylation patterns are induced when an immobilized fusion protein containing the amino-terminal SH2 domain of p85 is used to precipitate tyrosine-phosphorylated proteins. Thus, although NGF produces the rapid activation of PI-3 kinase through a mechanism that involves tyrosine phosphorylation, there is no evidence for tyrosine phosphorylation of p85, or for its ligand-dependent association with the NGF receptor. Perhaps another phosphoprotein may link the NGF receptor to this enzyme.     

Regulation of phosphoinositide kinases in T cells. Evidence that phosphatidylinositol 3-kinase is not a substrate for T cell antigen receptor-regulated tyrosine kinases.

A phosphoinositide kinase that can phosphorylate phosphatidylinositol (PtdIns) is present in 4G10 monoclonal antibody (mAb) phosphotyrosine immunoprecipitates isolated from T cells activated via the T cell antigen receptor (TCR).CD3 complex. This PtdIns kinase is not the PtdIns 3-kinase that associates with activated protein tyrosine kinases in fibroblasts, since Western blotting and immunoprecipitation experiments with antibodies specific for the p85 alpha subunit of the PtdIns 3-kinase indicate that this polypeptide is not immunoprecipitated by the 4G10 mAb from TCR.CD3-activated Jurkat cells. Moreover, immunoprecipitated PtdIns 3-kinase isolated from T cells with p85 antibodies is inhibited when PtdIns is presented in Nonidet P-40, whereas the PtdIns kinase activity present in 4G10 mAb phosphotyrosine immunoprecipitates is enhanced in the presence of Nonidet P-40. In vitro kinase assays of PtdIns 3-kinase immunoprecipitated with p85 antibodies from T cells indicate that it associates with a serine kinase that can phosphorylate a p85 polypeptide. However, no protein tyrosine kinase activity capable of tyrosine phosphorylating p85 in vitro associates with p85 alpha immunoprecipitates in quiescent or TCR.CD3-activated T cells. These data suggest that the TCR.CD3 complex does not regulate PtdIns 3-kinase activity by a mechanism that involves protein tyrosine kinases.     

Stimulation of tyrosine kinase activity in anti-phosphotyrosine immune complexes of Swiss 3T3 cell lysates occurs rapidly after addition of bombesin, vasopressin, and endothelin to intact cells.

Treatment of quiescent Swiss 3T3 cells with the mitogenic peptides bombesin, vasopressin, endothelin/vasoactive intestinal contractor (VIC), and bradykinin strikingly increased the initial rate of tyrosine phosphorylation measured in anti-phosphotyrosine immunoprecipitates of a major band of Mr 115,000 (p115) and two minor components of Mr 90,000 and 75,000. Neuropeptides increased the labeling of p115 within seconds and with great potency; half-maximum concentrations were 0.1, 0.2 and 0.3 nM for bombesin, vasopressin, and VIC, respectively. Immunoblotting and peptide mapping showed that the p115 band phosphorylated in anti-phosphotyrosine immunoprecipitates is identical to a major Mr 115,000 substrate for neuropeptide-stimulated tyrosine phosphorylation in intact Swiss 3T3 cells. Furthermore, bombesin, vasopressin, and VIC markedly increased the rate of phosphorylation of Raytide, a broad specificity tyrosine kinase peptide substrate, by decreasing (8 +/- 1.3-fold) the apparent Km of the kinase for the substrate. Phorbol 12,13-dibutyrate and the Ca2+ ionophore A23187 had a weaker effect on tyrosine protein kinase activity in immune complexes compared with bombesin. Furthermore, down-regulation of protein kinase C blocked the small effect of phorbol esters but did not impair bombesin-stimulated tyrosine kinase activity. These results provide direct evidence for neuropeptide activation of a tyrosine kinase in cell-free preparations and identify a novel event in the action of this class of growth factors in Swiss 3T3 cells.     

Membrane Ig cross-linking regulates phosphatidylinositol 3-kinase in B lymphocytes.

Cross-linking of the B cell AgR results in activation of mature B cells and tolerization of immature B cells. The initial signaling events stimulated by membrane immunoglobulin (mIg) cross-linking are tyrosine phosphorylation of a number of proteins. Among the targets of mIg-induced tyrosine phosphorylation are the tyrosine kinases encoded by the lyn, blk, fyn, and syk genes, the mIg-associated proteins MB-1 and Ig-beta, phospholipase C-gamma 1 and -gamma 2, as well as many unidentified proteins. In this report we show that mIg cross-linking also regulates phosphatidylinositol 3-kinase (PtdIns 3-kinase), an enzyme that phosphorylates inositol phospholipids and plays a key role in mediating the effects of tyrosine kinases on growth control in fibroblasts. Cross-linking mIg on B lymphocytes greatly increased the amount of PtdIns 3-kinase activity which could be immunoprecipitated with anti-phosphotyrosine (anti-tyr(P) antibodies. This response was observed after mIg cross-linking in mIgM- and mIgG-bearing B cell lines and after cross-linking either mIgM or mIgD in murine splenic B cells. Thus, regulation of PtdIns 3-kinase is a common feature of signaling by several different isotypes of mIg. This response was rapid and peaked 2 to 3 min after the addition of anti-Ig antibodies. The anti-Ig-stimulated increase in PtdIns 3-kinase activity associated with anti-Tyr(P) immunoprecipitates could reflect increased tyrosine phosphorylation of PtdIns 3-kinase, increased activity of the enzyme, or both. In favor of the first possibility, the tyrosine kinase inhibitor herbimycin A blocked the increase in ant-Tyr(P)-immunoprecipitated PtdIns 3-kinase activity as well as the anti-Ig-induced tyrosine phosphorylation. Moreover, this response was not secondary to phospholipase C activation but rather seemed to be a direct consequence of mIg-induced tyrosine phosphorylation. Activation of the phosphoinositide pathway by a transfected M1 muscarinic acetylcholine receptor expressed in WEHI-231 B lymphoma cells did not increase the amount of PtdIns 3-kinase activity which could be precipitated with anti-Tyr(P) antibodies. Similarly, inhibition of the phosphoinositide pathway did not abrogate the ability of mIg cross-linking to stimulate this response. Thus, mIg-induced tyrosine phosphorylation regulates PtdIns 3-kinase, an important mediator of growth control in fibroblasts and potentially an important regulatory component in B cells as well.     

Insulin treatment stimulates the tyrosine phosphorylation of the alpha-type 85-kDa subunit of phosphatidylinositol 3-kinase in vivo.

After adding insulin to cells overexpressing the insulin receptor, the activity of phosphatidylinositol (PI) 3-kinase in the anti-phosphotyrosine immunoprecipitates was rapidly and greatly increased. This enzyme may therefore be a substrate for the insulin receptor tyrosine kinase and may be one of the mediators of insulin signal transduction. However, it is unclear whether or not activated tyrosine kinase of the insulin receptor directly phosphorylates PI 3-kinase at tyrosine residue(s) and whether insulin stimulates the specific activity of PI 3-kinase. We reported previously that the 85-kDa subunit of purified PI 3-kinase was phosphorylated at tyrosine residue(s) by the insulin receptor in vitro. To examine the tyrosine phosphorylation of PI 3-kinase and change of its activity by insulin treatment in vivo, we used a specific antibody to the 85-kDa subunit of PI 3-kinase. The activity of PI 3-kinase in immunoprecipitates with the antibody against the p85 subunit of PI 3-kinase was increased about 3-fold by insulin treatment of cells overexpressing insulin receptors. Insulin treatment also stimulated the tyrosine, serine, and threonine phosphorylation of the alpha-type 85-kDa subunit of PI 3-kinase in vivo. Phosphatase treatment of the immunoprecipitates abolished the increase in PI 3-kinase activity. The phosphorylation(s) of the kinase itself, tyrosine phosphorylation(s) of associated protein(s), or the complex formation of the phosphorylated PI 3-kinase with associated proteins may increase the activity of PI 3-kinase.     

Interleukin 2- and polyomavirus middle T antigen-induced modification of phosphatidylinositol 3-kinase activity in activated T lymphocytes.

Stimulation of activated T lymphocytes with interleukin 2 (IL-2) results in rapid increases in intracellular protein tyrosine phosphorylation. Both the identity of the protein tyrosine kinase (PTK) activated by IL-2 receptor ligation and the identities of the critical target proteins for this PTK remain largely undefined. In this article, we demonstrate that stimulation of activated murine or human T cells with IL-2 for 10 to 30 min induces two- to threefold increases in the level of phosphatidylinositol (PtdIns) 3-kinase activity present in antiphosphotyrosine (p-Tyr) antibody immunoprecipitates from these cells. Furthermore, substantial levels of PtdIns 3-kinase activity were coprecipitated from IL-2-deprived T cells by antibodies to the src-related PTK p59fyn. Cellular stimulation with IL-2 induced a two- to threefold increase in the level of p59fyn-associated PtdIns 3-kinase activity. To examine the effect of a constitutive increase in PtdIns 3-kinase activity on the growth factor responsiveness of activated T cells, murine CTLL-2 cells were transfected with a polyomavirus middle T antigen (MTAg) expression vector. Anti-p-Tyr and anti-p59fyn immunoprecipitates from MTAg-transfected CTLL-2 cells contained three- to sixfold higher levels of PtdIns 3-kinase activity than wild-type cells. Immune complex kinase assays revealed that MTAg expression concomitantly induced a constitutive threefold increase in the PTK activity of p59fyn in these cells. However, stable MTAg expression did not abrogate the dependence of CTLL-2 cells on exogenous IL-2 for continued growth and proliferation.     

Activation of mitogen activated protein (MAP) kinases by vanadate is independent of insulin receptor autophosphorylation

Treatment of Chinese hamster ovary (CHO) cells over-expressing the human insulin receptor (CHO-HIRc) with the insulin mimetic agent, vanadate, resulted in a dose- and time-dependent tyrosine phosphorylation of two proteins with apparent molecular sizes of 42 kDa (p42) and 44 kDa (p44). However, vanadate was unable to stimulate the tyrosyi phosphorylation of theβ-subunit of the insulin receptor. By using myelin basic protein (MBP) as the substrate to measure mitogen-activated protein (MAP) kinase activity in whole cell lysates, vanadate-stimulated tyrosyl phosphorylation of p42 and p44 was associated with a dose- and time-dependent activation of MAP kinase activity. Furthermore, affinity purification of cell lysates on anti-phosphotyrosine agarose column followed by immunoblotting with a specific antibody to MAP kinases demonstrated that vanadate treatment increased the tyrosyl phosphorylation of both p44^mapk and p42^mapk by several folds, as compared to controls, in concert with MAP kinase activation. In addition, retardation in gel mobility further confirmed that vanadate treatment increased the phosphorylation of p44^mapk and p42^mapk in CHO-HIRc. A similar effect of vanadate on MAP kinase tyrosyl phosphorylation and activation was also observed in CHO cells over-expressing a protein tyrosine kinase-deficient insulin receptor (CHO-1018). These results demonstrate that the protein tyrosine kinase activity of the insulin receptor may not be required in the signaling pathways leading to the vanadate-mediated tyrosyl phosphorylation and activation of MAP kinases.     

Nerve growth factor promotes the activation of phosphatidylinositol 3-kinase and its association with the trk tyrosine kinase.

We investigated the involvement of phosphatidylinositol 3-kinase (PtdIns 3-kinase) in the initiation of signal transduction by nerve growth factor (NGF) in the rat pheochromocytoma PC12 cell line. PtdIns 3-kinase catalyzes the formation of phosphoinositides with phosphate in the D-3 position of the inositol ring and previously has been found to associate with other activated protein tyrosine kinases, including growth factor receptor tyrosine kinases. Anti-phosphotyrosine immunoprecipitates had PtdIns 3-kinase activity that reached a maximum (9 times the basal activity) after a 5-min exposure of PC12 cells to NGF (100 ng/ml). Since NGF activates the tyrosine kinase activity of gp140trk, the protein product of the trk proto-oncogene, we also examined the association of PtdIns 3-kinase with gp140trk. Anti-gp140trk immunoprecipitates from NGF-stimulated PC12 cells had increased PtdIns 3-kinase activity compared to that of unstimulated cells, and larger increases were detected in cells overexpressing gp140trk, indicating that PtdIns 3-kinase associates with gp140trk. NGF produced large increases in [32P]phosphatidylinositol 3,4-bisphosphate and [32P]phosphatidylinositol 3,4,5-trisphosphate in PC12 cells labeled with [32P]orthophosphate, indicating an increase in PtdIns 3-kinase activity in intact cells. Using an anti-85-kDa PtdIns 3-kinase subunit antibody, we found that NGF promoted the tyrosine phosphorylation of an 85-kDa protein and two proteins close to 110 kDa. These studies demonstrate that NGF activates PtdIns 3-kinase and promotes its association with gp140trk and also show that NGF promotes the tyrosine phosphorylation of the 85-kDa subunit of PtdIns 3-kinase. Thus, PtdIns 3-kinase activation appears to be involved in differentiation as well as mitogenic responses.     

Phosphatidylinositol-3-kinase is a non-tyrosine phosphorylated member of the insulin receptor signalling complex.

In rat HTC cells expressing a large number of human insulin receptors, insulin stimulated phosphatidylinositol-3-kinase (PI-3-kinase) activity. This activity was more effectively immunoprecipitated with anti-phosphotyrosine antibody (alpha-PY) than with anti-insulin receptor antibody (alpha-IR), suggesting that PI-3-kinase was not directly associated with the insulin receptor. alpha-PY immunoprecipitable PI-3 kinase activity, which was regulated by insulin, corresponded to a small pool of the total cellular PI-3-kinase activity. PI-3-kinase was not directly tyrosine phosphorylated by insulin treatment. A comparison of both catalytic activity and content of PI-3-kinase in alpha-PY immunoprecipitates indicated that after insulin treatment PI-3-kinase activity was enhanced by its association with tyrosine phosphorylated proteins. These studies suggest therefore that PI-3-kinase is a non-tyrosine phosphorylated member of the insulin receptor signalling complex.     

Insulin stimulates phosphatidylinositol-3-kinase activity in rat adipocytes.

Phosphatidylinositol (PtdIns) 3-kinase is thought to participate in the signal transduction pathways initiated by the activation of receptor tyrosine kinases including the insulin receptor. To approach the physiological relevance of this enzyme in insulin signaling, we studied the activation of PtdIns-3-kinase in adipocytes, a major insulin target tissue for glucose transport and utilisation. To analyze possible interactions of the enzyme with cellular proteins, immunoprecipitations with the following antibodies were performed: (a) anti-phosphotyrosine antibodies, (b) two antibodies to the 85-kDa subunit of PtdIns-3-kinase (p85) and (c) an antibody to the 185-kDa major insulin receptor substrate (p185). We show that in cell extracts from adipocytes exposed to insulin, and after immunoprecipitation with an anti-phosphotyrosine antibody and an antibody to p85, we are able to detect a PtdIns-3-kinase activity stimulated by the hormone. Similarly, after immunoprecipitation with an antibody to p185, an increase in the PtdIns-3-kinase activity could be demonstrated. Taken together these results suggest that, upon insulin stimulation of fat cells, PtdIns-3-kinase itself is tyrosine phosphorylated and/or associated with an insulin receptor substrate, such as p185, which could function as a link between the insulin receptor and PtdIns-3-kinase. The PtdIns-3-kinase was activated within 1 min of exposure to insulin, and the half-maximal effect was reached at the same concentration, i.e. 3 nM, as for stimulation of the insulin receptor kinase. Subcellular fractionation showed that PtdIns-3-kinase activity was found both in the membranes and in the cytosol. Further, immunoprecipitation with an antibody to p85, which possesses the capacity to activate PtdIns-3-kinase, suggests that the presence of the enzyme in the membrane may be due to an insulin-induced recruitment of the PtdIns-3-kinase from the cytosol to the membrane. Finally, we used isoproterenol, which exerts antagonistic effects on insulin action. This drug was found to inhibit both the PtdIns-3-kinase and the insulin receptor activation by insulin, suggesting that the activation of the PtdIns-3-kinase was closely regulated by the insulin receptor tyrosine kinase. The occurrence of an insulin-stimulated PtdIns-3-kinase in adipocytes leads us to propose that this enzyme might be implicated in the generation of metabolic responses induced by insulin.     

Evidence for two distinct phosphatidylinositol kinases in fibroblasts. Implications for cellular regulation.

Phosphatidylinositol (PtdIns) kinase activities from non-transformed and polyoma-middle-T-transformed murine fibroblasts were examined. Both normal and transformed 3T3 fibroblasts have two PtdIns kinases, which can be separated by anion-exchange chromatography. One of these activities (Type I) has a Km for ATP of 10 microM, is resistant to inhibition by adenosine, AMP or ADP, and is inhibited by non-ionic detergents. The other activity (Type II) has a somewhat higher Km for ATP (35 microM) and is inhibited competitively by ADP, AMP and adenosine at concentrations suggesting regulation of this activity by the energy charge of the cell. The Type II PtdIns kinase is activated by non-ionic detergents. We have previously reported the specific association of a PtdIns kinase activity with polyoma-middle-T immunoprecipitates [Whitman, Kaplan, Schaffhausen, Cantley & Roberts (1985) Nature (London) 315, 239-242; Kaplan, Whitman, Schaffhausen, Raptis, Garcea, Pallas, Roberts & Cantley (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 3624-3628]. Comparison of the immunoprecipitated PtdIns kinase with the activities identified by ion-exchange chromatography indicates that it is the Type I enzyme which specifically associates with the middle-T/pp60c-src complex. This PtdIns kinase activity is separable from both middle T and pp60c-src. Type I PtdIns kinase also associates with pp60v-src immunoprecipitates from Rous-sarcoma-virus-transformed cells. Furthermore, this PtdIns kinase appears to co-precipitate with partially purified platelet derived growth factor (PDGF) receptor. The amount of this activity found in anti-phosphotyrosine immunoprecipitates or in wheat-germ-lectin-agarose precipitates is increased 50-fold by stimulation of quiescent Balb/C 3T3 fibroblasts with PDGF. These results suggest that the Type I PtdIns kinase is regulated by agents which affect cell growth and transformation, whereas the Type II PtdIns kinase may be regulated by the local [ATP]/[ADP] ratio.     

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 Ptdlns 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. © 1995 Wiley-Liss, Inc.     

Mutations in the juxtamembrane region of the insulin receptor impair activation of phosphatidylinositol 3-kinase by insulin.

CHO/IRF960/T962 cells express a mutant human insulin receptor in which Tyr960 and Ser962 in the juxtamembrane region of the receptor's beta-subunit are replaced by Phe and Thr, respectively. The mutant insulin receptor undergoes autophosphorylation normally in response to insulin; however, insulin fails to stimulate thymidine incorporation into DNA, glycogen synthesis, and tyrosyl phosphorylation of an endogenous substrate pp185 in these cells. Another putative substrate of the insulin receptor tyrosine kinase is phosphatidylinositol 3-kinase (Ptdlns 3-kinase). We have previously shown that Ptdlns 3-kinase activity in Chinese hamster ovary cells expressing the wild-type human insulin receptor (CHO/IR) increases in both antiphosphotyrosine [anti-Tyr(P)] immunoprecipitates and intact cells in response to insulin. In the present study a new technique (detection of the 85-kDa subunit of Ptdlns 3-kinase using [32P]phosphorylated polyoma virus middle T-antigen as probe) is used to monitor the Ptdlns 3-kinase protein. The 85-kDa subunit of Ptdlns 3-kinase is precipitated by anti-Tyr(P) antibodies from insulin-stimulated CHO/IR cells, but markedly less protein is precipitated from CHO/IRF960/T962 cells. The amount of Ptdlns 3-kinase activity in the immunoprecipitates was also reduced in the CHO/IRF960/T962 cells compared to CHO/IR cells. In intact CHO/IRF960/T962 cells, insulin failed to stimulate phosphate incorporation into one of the products of activated Ptdlns 3-kinase, phosphatidylinositol-3,4-bisphosphate [Ptdlns(3,4)P2], whereas it caused a 12-fold increase in CHO/IR cells. In contrast, phosphate incorporation into another product, phosphatidylinositol trisphosphate [PtdlnsP3], was only partially depressed in the CHO/IRF960/T962 cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Common elements in growth factor stimulation and oncogenic transformation: 85 kd phosphoprotein and phosphatidylinositol kinase activity

The phosphorylation of proteins on tyrosine in vivo and in vitro was examined in 3T3 cells stimulated by platelet-derived growth factor (PDGF) and transformed by polyoma middle T antigen (MTAg) by using an antibody directed against phosphotyrosine (P-tyr). Two common events were observed upon PDGF stimulation or MTAg transformation of cells: the appearance in the immunoprecipitates of an 85 kd phosphoprotein, and increased phosphatidylinositol (PI) kinase activity. In PDGF-stimulated cells, the 85 kd phosphoprotein and PI kinase activity appeared rapidly, within 1 min of growth factor addition. The PI kinase activity and 85 kd phosphorylation were also increased in anti-P-tyr immunoprecipitates from cells transformed by v-fms and v-sis, but not by SV40 T antigen. The presence of the tyrosine-phosphorylated 85 kd protein correlated with PI kinase activity during several purification steps. These results suggest that the 85 kd phosphoprotein, a putative PI kinase, is a substrate for both the PDGF receptor and MTAg/pp60c-src tyrosine kinase activities.     

Overexpression of membrane glycoprotein PC-1 can influence insulin action at a post-receptor site

An elevated content of membrane glycoprotein PC-1 has been observed in cells and tissues of insulin resistant patients. In addition, in vitro overexpression of PC-1 in cultured cells induces insulin resistance associated with diminished insulin receptor tyrosine kinase activity. We now find that PC-1 overexpression also influences insulin receptor signaling at a step downstream of insulin receptor tyrosine kinase, independent of insulin receptor tyrosine kinase. In the present studies, we employed Chinese hamster ovary cells that overexpress the human insulin receptor (CHO IR cells; ∼10⁶ receptors per cell), and transfected them with human PC-1 c-DNA (CHO IR PC-1). In CHO IR PC-1 cells, insulin receptor tyrosine kinase activity was unchanged, following insulin treatment of cells. However, several biological effects of insulin, including glucose and amino acid uptake, were decreased. In CHO IR PC-1 cells, insulin stimulation of mitogen-activated protein (MAP) kinase activity was normal, suggesting that PC-1 overexpression did not affect insulin receptor activation of Ras, which is upstream of MAP kinase. Also, insulin-stimulated phosphatidylinositol (PI)-3-kinase activity was normal, suggesting that PC-1 overexpression did not interfere with the activation of this enzyme by insulin receptor substrate-1. In these cells, however, insulin stimulation of p70 ribosomal S6 kinase activity was diminished. These studies suggest, therefore, that, in addition to blocking insulin receptor tyrosine kinase activation, PC-1 can also block insulin receptor signaling at a post-receptor site. J. Cell. Biochem. 68:366–377, 1998. © 1998 Wiley-Liss, Inc.     

Regulation of the insulin receptor kinase by hyperinsulinism

A murine fibroblast cell line transfected with human insulin receptor cDNA, NIH 3T3 HIR3.5, was observed to display insulin-induced down-regulation of insulin-binding activity in a time- and concentration-dependent manner. Maximal inhibition of insulin-binding activity (54%) occurred within 16 h of exposure to 100 nM insulin in vivo, where in vivo refers to intact cells in tissue culture. The decrease in cellular insulin-binding activity was the consequence of a decrease in the number of cell-associated insulin receptors as determined by Scatchard analysis of insulin binding, 125I-insulin affinity cross-linking, and Western blotting of the insulin receptor beta subunit. Acute insulin treatment in vivo (1-60 min) resulted in the activation of the insulin receptor protein tyrosine kinase as determined by in vitro phosphorylation of glutamic acid:tyrosine (4:1), where in vitro refers to broken cell preparations. This acute in vivo insulin activation of the insulin receptor tyrosine kinase resulted in a greater stimulation (1.4-1.9-fold) of tyrosine kinase activity in the glutamic acid:tyrosine (4:1) assay than the maximal stimulation produced by insulin treatment in vitro. In contrast, long term (24 h) insulin treatment in vivo resulted in a 50-70% decrease in intrinsic protein tyrosine kinase activity of the insulin receptors compared with that of acutely activated (1 min) insulin receptors. Under these conditions, the insulin receptor protein kinase activity remained insulin independent in the in vitro substrate kinase assay. Surprisingly, the insulin-independent activated (1 min in vivo insulin-treated) and uncoupled (24 h in vivo insulin-treated) insulin receptors displayed similar stoichiometries of 32P incorporation into the beta subunit by in vitro autophosphorylation when compared with the control insulin receptors, ranging from 1.5 to 1.8 mol of phosphate incorporated/mol of insulin receptor. Phosphoamino acid analysis demonstrated that the phosphoserine/phosphothreonine content of in vivo 32P-labeled insulin receptors increased markedly within a 1-h exposure to insulin in vivo, whereas insulin-induced receptor desensitization was not apparent until 10-24 h after exposure to insulin. These data suggest that insulin treatment in vivo results initially in the activation of the insulin receptor kinase followed by a subsequent uncoupling of protein kinase activity. This insulin-induced desensitization of the insulin receptor kinase does not correlate with the extent of beta subunit serine/threonine phosphorylation.     

Genistein differentially inhibits postreceptor effects of insulin in rat adipocytes without inhibiting the insulin receptor kinase.

Genistein, an isoflavone putative tyrosine kinase inhibitor, was used to investigate the coupling of insulin receptor tyrosine kinase activation to four metabolic effects of insulin in the isolated rat adipocyte. Genistein inhibited insulin-stimulated glucose oxidation in a concentration-dependent manner with an ID50 of 25 micrograms/ml and complete inhibition at 100 micrograms/ml. Genistein also prevented insulin's (10(-9) M) inhibition of isoproterenol-stimulated lipolysis with an ID50 of 15 micrograms/ml and a complete effect at 50 micrograms/ml. The effect of genistein (25 micrograms/ml) was not reversed by supraphysiological (10(-7) M) insulin levels. In contrast, genistein up to 100 micrograms/ml had no effect on insulin's (10(-9) M) stimulation of either pyruvate dehydrogenase or glycogen synthase activity. We determined whether genistein influenced insulin receptor beta-subunit autophosphorylation or tyrosine kinase substrate phosphorylation either in vivo or in vitro by anti-phosphotyrosine immunoblotting. Genistein at 100 micrograms/ml did not inhibit insulin's (10(-7) M) stimulation of insulin receptor tyrosine autophosphorylation or tyrosine phosphorylation of the cellular substrates pp185 and pp60. Also, genistein did not prevent insulin-stimulated autophosphorylation of partially purified human insulin receptors from NIH 3T3/HIR 3.5 cells or the phosphorylation of histones by the activated receptor tyrosine kinase. In control experiments using either NIH 3T3 fibroblasts or partially purified membranes from these cells, genistein did inhibit platelet-derived growth factor's stimulation of its receptor autophosphorylation. These findings indicate the following: (a) Genistein can inhibit certain responses to insulin without blocking insulin's stimulation of its receptor tyrosine autophosphorylation or of the receptor kinase substrate tyrosine phosphorylation. (b) In adipocytes genistein must block the stimulation of glucose oxidation and the antilipolytic effects of insulin at site(s) downstream from the insulin receptor tyrosine kinase. (c) The inhibitory effects of genistein on hormonal signal transduction cannot necessarily be attributed to inhibition of tyrosine kinase activity, unless specifically demonstrated.     

High specific induction of spermidine/spermine N1-acetyltransferase in a human large cell lung carcinoma.

The effects of insulin and anti-(insulin receptor) monoclonal antibodies on tyrosine phosphorylation were investigated in fibroblasts transfected with human insulin receptor cDNA (NIH 3T3HIR3.5 cells) using anti-phosphotyrosine immunoblotting. Insulin increased levels of tyrosine phosphorylation in two major proteins of molecular mass 97 kDa (pp97, assumed to be the insulin receptor beta-subunit) and 185 kDa (pp185). Insulin-mimetic anti-receptor antibodies also stimulated tyrosine phosphorylation of both pp97 and pp185. The observation of antibody-stimulated pp97 phosphorylation, as detected by immunoblotting, is in contrast with previous data which failed to show receptor autophosphorylation in NIH 3T3HIR3.5 cells labelled with [32P]P1. The effect of insulin on pp97 was maximal within 1 min, but the response to antibody was apparent only after a lag of 1-2 min and rose steadily over 20 min. The absolute level of antibody-stimulated phosphorylation of both pp97 and pp185 after 20 min was only about 20% of the maximum level induced by equivalent concentrations of insulin, even at concentrations of antibody sufficient for full occupancy of receptors. Another insulin-mimetic agent, wheat-germ agglutinin, stimulated receptor autophosphorylation with kinetics similar to those produced by the antibody. It is suggested that the relatively slow responses to both agents may be a function of the dependence on receptor cross-linking. These data are consistent with a role for the insulin receptor tyrosine kinase activity in the mechanism of action of insulin-mimetic anti-receptor antibodies.