Phosphatidylinositol 3-kinase activation is required for insulin stimulation of pp70 S6 kinase, DNA synthesis, and glucose transporter translocation

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

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

Mechanism of action of the immunosuppressant rapamycin

Rapamycin has potent immunosuppressive properties reflecting its ability to disrupt cytokine signaling that promotes lymphocyte growth and differentiation. In IL-2-stimulated T cells, rapamycin impedes progression through the G1/S transition of the proliferation cycle, resulting in a mid-to-late G1 arrest. Two major biochemical alterations underlie this mode of action. The first one affects the phosphorylation/activation of the p70 S6 kinase (p70s6k), an early event of cytokine-induced mitogenic response. By inhibiting this enzyme, whose major substrate is the 40S ribosomal subunit S6 protein, rapamycin reduces the translation of certain mRNA encoding for ribosomal proteins and elongation factors, thereby decreasing protein synthesis. A second, later effect of rapamycin in IL-2-stimulated T cells is an inhibition of the enzymatic activity of the cyclin-dependent kinase cdk2-cyclin E complex, which functions as a crucial regulator of G1/S transition. This inhibition results from a prevention of the decline of the p27 cdk inhibitor, that normally follows IL-2 stimulation. To mediate these biochemical alterations, rapamycin needs to bind to intracellular proteins, termed FKBP, thereby forming a unique effector molecular complex. However, neither(p70s6k) inhibition, nor p27-induced cdk2-cyclin E inhibition are directly caused by the FKBP-rapamycin complex. Instead, this complex physically interacts with a novel protein, designated "mammalian target of rapamycin" (mTOR), which has sequence homology with the catalytic domain of phosphatidylinositol kinases and may therefore be itself a kinase. mTOR may act upstream of (p70s6K) and cdk2-cyclin E in a linear or bifurcated pathway of growth regulation. Molecular dissection of this pathway should further unravel cytokine-mediated signaling processes and help devise new immunosuppressants.

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.

Wortmannin inhibits transcytosis of dimeric IgA by the polymeric immunoglobulin receptor

Phosphatidyl inositol 3-kinase (PI3K) plays an essential role in numerous signaling events, and increasingly has been implicated in regulation of certain membrane traffic events. The polymeric immunoglobulin receptor (pIgR) transcytoses dimeric IgA (dIgA) across epithelial cells and into external secretions, where the dIgA forms the first specific immunological defense against infection. We show here that wortmannin, a highly specific inhibitor of PI3K, inhibits transcytosis of dIgA by the pIgR. Instead, the dIgA is recycled back to the basolateral surface of the epithelial cell. PI3K therefore plays an essential role in regulating the transcytosis of dIgA, a key step in the mucosal immune response.

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits transcytosis in polarized epithelial cells

Wortmannin, an inhibitor of phosphoinositide 3-kinase, inhibits both basolateral to apical and apical to basolateral transcytosis of ricin in Fisher rat thyroid (FRT) cells by 50% at 100 nM in a continuous transcytosis assay. In MDCK cells, a similar effect of wortmannin on basolateral to apical transcytosis of ricin was found, whereas apical to basolateral transcytosis was inhibited to a lesser degree. Transcytosis of dimeric IgA in MDCK cells expressing the polymeric immunoglobulin receptor was also reduced to 50% of controls, suggesting that wortmannin inhibits membrane translocation rather than sorting of specific proteins in the transcytotic pathway. This effect of wortmannin is selective, however, in that endocytosis at the basolateral domain and recycling at both the basolateral and apical membrane domains are unaffected, and apical endocytosis and apical secretion are only moderately reduced. We have shown previously that cAMP stimulates a late stage in basolateral to apical transcytosis in MDCK cells through activation of protein kinase A (Hansen, S. H., and Casanova, J.E. (1994) J. Cell Biol. 126, 677-687). Elevation of cellular cAMP still induced a 100% increase in transcytosis in wortmannin-treated cells, but transcytosis was no longer increased when compared to cells which received no drugs. In contrast, in experiments using a 17 degrees C block to accumulate ricin internalized from the basolateral surface in the apical compartment of MDCK cells, wortmannin had little effect on the stimulation of transcytosis by activators of protein kinase A observed under these conditions. The data thus suggest the existence of a wortmannin-sensitive step in the transcytotic pathway, positioned after endocytosis but prior to translocation into the protein kinase A-sensitive apical compartment, implying a role for phosphoinositide 3-kinase in an intermediate step in transcytosis in polarized epithelial cells.

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.

Insulin-stimulated GLUT4 translocation is mediated by a divergent intracellular signaling pathway

Insulin stimulates glucose transport largely by mediating translocation of the insulin-sensitive glucose transporter (GLUT4) from an intracellular compartment to the plasma membrane. Using single cell microinjection of 3T3-L1 adipocytes, coupled with immunofluorescence detection of GLUT4 proteins, we have determined that inhibition of endogenous p21ras or injection of oncogenic p21ras has no effect on insulin-stimulated GLUT4 translocation. On the other hand, microinjection of anti-phosphotyrosine antibodies or inhibition of endogenous phosphatidylinositol 3-kinase by microinjection of a GST-p85 SH2 fusion protein markedly inhibits this biologic effect of insulin. These data suggest that the p21ras/mitogen-activated protein kinase pathway is not involved in this metabolic effect of insulin, whereas tyrosine phosphorylation and stimulation of phosphatidylinositol 3-kinase activity are critical components of this signaling pathway.

Hepatocyte growth factor-induced scatter of Madin-Darby canine kidney cells requires phosphatidylinositol 3-kinase

Hepatocyte growth factor/scatter factor (HGF/SF) is a multifunctional cytokine that induces mitogenesis, motility, invasion, and morphogenesis of several epithelial and endothelial cell lines in culture. The receptor for HGF/SF has been identified as the Met tyrosine kinase. To investigate the signaling pathways that are involved in these events, we have generated chimeric receptors containing the extracellular domain of the colony-stimulating factor-1 (CSF-1) receptor fused to the transmembrane and intracellular domains of the Met receptor (MET). Madin-Darby canine kidney (MDCK) epithelial cells expressing the CSF-MET chimera dissociate and scatter in response to CSF-1. However, cells expressing a mutant CSF-MET receptor containing a phenylalanine substitution for tyrosine 1356 were unable to scatter or form branching tubules following stimulation with CSF-1. Tyrosine 1356 is essential for the recruitment of multiple substrates including the p85 subunit of PI3-kinase, phospholipase C gamma, and Grb2. In this study, we have investigated the role of PI3-kinase and a downstream target of PI3-kinase, pp70S6K, in the induction of MDCK cell scatter in response to HGF/SF. Our results demonstrate that following stimulation with HGF/SF, activation of PI3-kinase but not pp70S6K is essential for MDCK cell scatter.

Phosphotyrosine residues in the nerve-growth-factor receptor (Trk-A). Their role in the activation of inositolphospholipid metabolism and protein kinase cascades in phaeochromocytoma (PC12) cells

PC12 cells, which lack platelet derived-growth-factor (PDGF) receptors, have been stably transfected with a chimaera consisting of the extracellular domain of the beta-PDGF receptor and the intracellular and transmembrane domains of the nerve-growth-factor receptor Trk-A (termed PT-R). Mutation of the Trk-A residue Tyr490 to phenylalanine prevents the association with Shc, while similar mutations at Tyr751 or Tyr785 are reported to prevent interaction of Trk-A with the p85 subunit of inositol phospholipid 3-kinase and phospholipase C-gamma 1, respectively. The strong and sustained activation of p42 and p44 mitogen-activated-protein kinases induced by PDGF-B/B in PC12/PT-R cells was unaffected by mutation of Tyr785 or Tyr751 to phenylalanine, but was smaller and transient after mutation of Tyr490, and almost abolished by the double mutation of Tyr490 and Tyr785. Mutation of Tyr490 reduced by 70% the PDGF-induced increase in inositol phospholipid 3-kinase activity immunoprecipitated from cell extracts with antiphosphotyrosine monoclonal antibodies and greatly suppressed the PDGF-induced increase in the intracellular products of inositol phospholipid 3-kinase, while mutation of Tyr751 or Tyr785 had no effect. Mutation of Tyr785 (but not mutation of Tyr490 or Tyr751) abolished PDGF-stimulated hydrolysis of phosphatidylinositol 4,5-bisphosphate. Mutation of Tyr490, alone or in combination with mutation of Tyr751 and Tyr785, had no effect on the PDGF-induced activation of p70 S6 kinase (p70S6K). However, the activation of p70S6K by PDGF (or nerve growth factor), but not the activation of mitogen-activated-protein kinase, was prevented by two structurally unrelated inhibitors of inositol phospholipid 3-kinase, wortmannin or LY294002. Our results demonstrate the following: (1) the phosphorylation of Tyr490 plays a major role in the activation of inositol phospholipid 3-kinase and formation of 3-phosphorylated inositol lipids and confirm that the phosphorylation of Tyr 785 triggers the activation of phospholipase C-gamma 1 in vivo. (2) Tyr490 phosphorylation (but not inositol phospholipid 3-kinase activation) is also required for strong and sustained activation of mitogen-activated-protein kinase and neuronal differentiation, while the smaller and more transient activation of mitogen-activated-protein kinase, produced by the activation of phospholipase C-gamma 1 is insufficient to trigger the neuronal differentiation of PT-R cells. (3) Inositol phospholipid 3-kinase is required for the activation of p70S6K, but only a small increase in inositol phospholipid 3-kinase activity and the level of 3-phosphorylated inositol lipids is required for maximal p70S6K activation.

Activation of p70 S6 kinase and erk-encoded mitogen-activated protein kinases is resistant to high cyclic nucleotide levels in Swiss 3T3 fibroblasts

Treatment of Swiss mouse 3T3 fibroblasts with certain cyclic nucleotide phosphodiesterase inhibitors (theophylline, SQ 20,006, and MY-5445) prevents the activation of the M(r) 70,000 S6 kinase (p70S6k) induced by a variety of external stimuli. Concentrations giving half-maximal inhibition were 800, 50, and 25 microM, respectively. Western blot analysis and immunocomplex kinase assays showed that these compounds inhibit the phosphorylation and activation of p70S6k without affecting the erk-encoded mitogen-activated protein (MAP) kinases or the rsk-encoded S6 kinase (p90rsk). A distinct collection of cAMP and cGMP agonists and analogues did not suppress p70S6k activation, indicating that 1) high intracellular cyclic nucleotide concentrations do not antagonize the p70S6k pathway and 2) phosphodiesterase inhibitors block p70S6k activation by a mechanism that is independent of cAMP or cGMP production. The effect of theophylline and SQ 20,006, but not MY-5445, on p70S6k signaling may be due in part to the inhibition of a phosphatidylinositol 3-kinase that acts upstream of p70S6k. Finally, in contrast to many other cell types, cAMP and cGMP were also found to have no inhibitory effect on the MAP kinase/p90rsk signaling pathway in Swiss 3T3 fibroblasts.

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.

Effect of the activation of phosphatidylinositol 3-kinase by a thiophosphotyrosine peptide on glucose transport in 3T3-L1 adipocytes

Insulin causes the activation of phosphatidylinositol 3-kinase (PI 3-kinase) through complexation of tyrosine-phosphorylated YMXM motifs on insulin receptor substrate 1 with the Src homology 2 domains of PI 3-kinase. Previous studies with inhibitors have indicated that activation of PI 3-kinase is necessary for the stimulation of glucose transport in adipocytes. Here, we investigate whether this activation is sufficient for this effect. Short peptides containing two tyrosine-phosphorylated or thiophosphorylated YMXM motifs potently activated PI 3-kinase in the cytosol from 3T3-L1 adipocytes. Introduction of the phosphatase-resistant thiophosphorylated peptide into 3T3-L1 adipocytes through permeabilization with Staphylococcus aureus alpha-toxin stimulated PI 3-kinase as strongly as insulin. However, under the same conditions the peptide increased glucose transport into the permeabilized cells only 20% as well as insulin. Determination of the distribution of the glucose transporter isotype GLUT4 by confocal immunofluorescence showed that GLUT4 translocation to the plasma membrane can account for the effect of the peptide. These results suggest that one or more other insulin-triggered signaling pathways, besides the PI 3-kinase one, participate in the stimulation of glucose transport.

The insulin-dependent biosynthesis of GLUT1 and GLUT3 glucose transporters in L6 muscle cells is mediated by distinct pathways. Roles of p21ras and pp70 S6 kinase

Insulin binding results in rapid phosphorylation of insulin receptor substrate-1 to activate p21ras and mitogen-activated protein kinase. Insulin also activates the ribosomal protein S6 kinase (pp70 S6 kinase) independently of the Ras pathway. Chronic (18 h) treatment of L6 muscle cells with insulin increases glucose transport activity severalfold due to biosynthetic elevation of the GLUT1 and GLUT3 but not the GLUT4 glucose transporters. Here we investigate the roles of p21ras and pp70 S6 kinase in the insulin-mediated increases in GLUT1 and GLUT3 expression. L6 cells were transfected with the dominant negative Ras(S17N) under the control of a dexamethasone-inducible promoter. Induction of Ras(S17N) failed to block the insulin-mediated increase in GLUT1 glucose transporter protein and mRNA; however, it abrogated the insulin-mediated increase in GLUT3 glucose transporter protein and mRNA. Inhibition of pp70 S6 kinase by rapamycin, on the other hand, eliminated the insulin-mediated increase in GLUT1 but had no effect on that of GLUT3 in both parental and Ras(S17N) transfected L6 cells. These results suggest that the biosynthetic regulation of glucose transporters is differentially determined, with pp70 S6 kinase and p21ras playing active roles in the insulin-stimulated increases in GLUT1 and GLUT3, respectively.

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

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

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.

Cyclic AMP-increasing agents interfere with chemoattractant-induced respiratory burst in neutrophils as a result of the inhibition of phosphatidylinositol 3-kinase rather than receptor-operated Ca2+ influx

Superoxide anion and arachidonic acid were produced in guinea pig neutrophils in response to a chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP). Both responses were markedly, but the former response to a phorbol ester was not at all, inhibited when the cellular cAMP level was raised by prostaglandin E1 combined with a cAMP phosphodiesterase inhibitor. Increasing cAMP was also inhibitory to fMLP-induced activation of phosphatidylinositol (PI) 3-kinase and Ca2+ influx without any effect on the cation mobilization from intracellular stores. The fMLP-induced respiratory burst was abolished when PI 3-kinase was inhibited by wortmannin or LY294002, but was not affected when Ca2+ influx was inhibited. On the contrary, fMLP released arachidonic acid from the cells treated with the PI 3-kinase inhibitors as well as from non-treated cells, but it did not so when cellular Ca2+ uptake was prevented. The chemotactic peptide activated PI 3-kinase even in cells in which the receptor-mediated intracellular Ca2+ mobilization and respiratory burst were both abolished by exposure of the cells to a permeable Ca(2+)-chelating agent. Thus, stimulation of fMLP receptors gave rise to dual effects, activation of PI 3-kinase and intracellular Ca2+ mobilization; both effects were necessary for the fMLP-induced respiratory burst. Increasing cellular cAMP inhibited the respiratory burst and arachidonic acid release as a result of the inhibitions of PI 3-kinase and Ca2+ influx, respectively, in fMLP-treated neutrophils.

Phosphorylation of tyrosine 503 in the erythropoietin receptor (EpR) is essential for binding the P85 subunit of phosphatidylinositol (PI) 3-kinase and for EpR-associated PI 3-kinase activity

We recently reported that phosphatidylinositol (PI) 3-kinase becomes associated with the activated erythropoietin receptor (EpR), most likely through the Src homology 2 (SH2) domains within the p85 subunit of PI-3 kinase and one or more phosphorylated tyrosines within the EpR. We have now investigated this interaction in more detail and have found, based on both blotting studies with glutathione S-transferase-p85-SH2 fusion proteins and binding of these fusion proteins to SDS-denatured EpRs, that this binding is direct. Moreover, both in vitro competition studies, involving phosphorylated peptides corresponding to the amino acid sequences flanking the eight tyrosines within the intracellular domain of the EpR, and in vivo studies with mutant EpRs bearing tyrosine to phenylalanine substitutions, indicate that phosphorylation of Tyr503 within the EpR is essential for the binding of PI 3-kinase. The presence of PI 3-kinase activity in EpR immunoprecipitates from DA-3 cells infected with wild-type but not Y503F EpRs confirms this finding. Our results demonstrate that the SH2 domains of p85 can bind, in addition to their well established Tyr-Met/Val-X-Met consensus binding sequence, a Tyr-Val-Ala-Cys motif that is present in the EpR. A comparison of erythropoietin-induced tyrosine phosphorylations and proliferation of wild-type and Y503F EpR-infected DA-3 cells revealed no differences. However, the PI-3 kinase inhibitor, wortmannin, markedly inhibited the erythropoietin-induced proliferation of both cell types, suggesting that PI 3-kinase is activated in Y503F EpR expressing cells. This was confirmed by carrying out PI 3-kinase assays with anti-phosphotyrosine immunoprecipitates from erythropoietin-stimulated Y503F EpR-infected DA-3 cells and suggested that PI 3-kinase has a role in regulating erythropoietin-induced proliferation, but at a site distinct from the EpR.

Effected of insulin and insulin-like growth factor-I on glucose transport and its transporters in soleus muscle of lean and obese mice

The mechanisms underlying insulin and insulin-like growth factor-I (IGF-I) action on glucose transport share similar processes leading to Glut 4 translocation after respective receptor activation. Among these steps are phosphorylation of insulin receptor substrate-1 (IRS-1) and activation of phosphatidylinositol-3-kinase (P13-kinase). This enzyme could be involved in stimulated glucose transport in muscle, since its inhibitor, wortmannin, blocks the hormonal effect in muscle. P13-kinase is activated by insulin and IGF-I in a rapid and transient manner in incubated soleus muscles. When P13-kinase activation was studied in muscle of obese insulin-resistant mice, there was a marked alteration in the response to insulin both in vivo and in vitro. P13-kinase activation by IGF-I was also altered in obese mice, although to a lesser degree.

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.

Alterations in insulin signalling pathway induced by prolonged insulin treatment of 3T3-L1 adipocytes

Insulin-induced glucose transport stimulation, which results from the translocation of glucose transporter 4 (GLUT 4)-containing vesicles, is completely blocked after prolonged insulin treatment of 3T3-L1 adipocytes. Since GLUT 4 expression was reduced by only 30%, we looked at the insulin signaling pathway in this insulin-resistant model. Insulin-induced tyrosine phosphorylation of the major insulin receptor substrate IRS 1 was reduced by 50 +/- 7%, while its expression was decreased by 70 +/- 4%. When cells were treated with worthmannin (a PI3-kinase inhibitor) together with insulin, the expression of IRS 1 diminished to a much lower extent. Associated with the decrease in IRS 1 expression and phosphorylation, the activation by insulin of anti-phosphotyrosine immunoprecipitable PI3-kinase activity and of p44mapk activities was altered. However, the expression of these proteins was normal and p44mapk activity remained responsive to the tumour promoter TPA. Those results indicate that prolonged insulin treatment of 3T3-L1 adipocytes induces an insulin-resistant state with a reduced ability of insulin to stimulate the PI3-kinase and the MAP-kinases and a blockade of glucose transporter translocation.

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

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

Osmotic loading of neutralizing antibodies demonstrates a role for protein-tyrosine phosphatase 1B in negative regulation of the insulin action pathway

Protein-tyrosine phosphatases (PTPases) have been postulated to balance the steady-state phosphorylation and the activation state of the insulin receptor and its substrate proteins. To explore whether PTP1B, a widely expressed, non-receptor-type PTPase, regulates insulin signaling, we used osmotic shock to load rat KRC-7 hepatoma cells with affinity-purified neutralizing antibodies that immunoprecipitate and inactivate the enzymatic activity of recombinant rat PTP1B in vitro. In cells loaded with PTP1B antibody, insulin-stimulated DNA synthesis and phosphatidylinositol 3'-kinase activity were increased by 42% and 38%, respectively, compared with control cells loaded with preimmune IgG (p < 0.005). In order to characterize the potential site(s) of action of PTP1B in insulin signaling, we also determined that insulin-stimulated receptor autophosphorylation and insulin receptor substrate 1 tyrosine phosphorylation were increased 2.2- and 2.0-fold, respectively, and that insulin-stimulated receptor kinase activity toward an exogenous peptide substrate was increased by 57% in the PTP1B antibody-loaded cells. Osmotic loading did not alter the cellular content of PTP1B protein, suggesting that the antibody acts in the cell by sterically blocking catalytic interactions between PTP1B and its physiological substrates. These studies demonstrate that PTP1B has a role in the negative regulation of insulin signaling and acts, at least in part, directly at the level of the insulin receptor. These results also show that insulin signaling can be enhanced by the inhibition of specific PTPases, a maneuver that has potential clinical relevance in the treatment of insulin resistance and Type II diabetes mellitus.

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.

Insulin-like growth factor I receptor activated by a transmembrane mutation

We constructed mutant receptors by mutating transmembrane Val922 of the human insulin-like growth factor I receptor (IGF-IR). Assays of receptor kinase and autophosphorylation revealed constitutively augmented tyrosine kinase activity of V922E IGF-IR in both transient and stable expression. The constitutively active tyrosine kinase of this mutant was verified by promoted tyrosine phosphorylation of insulin receptor substrate-1 (IRS-1) in the absence of IGF-I. In CHO cells stably increasing V922E IGF-IR, both IRS-1 phosphorylation and the IRS-1 associated phosphoinositide 3-kinase activity were stimulated in the absence of IGF-I to the level attained by 1 nM IGF-I stimulation of wild-type IGF-IR, whereas the Ras-mitogen-activated protein kinase pathway was not activated under the same condition. In these CHO cells, V922E IGF-IR significantly stimulated glucose uptake but did not promote mitogenesis in the absence of IGF-I. We thus conclude that the V922E mutation of IGF-IR switches on the intrinsic tyrosine kinase and differentially activates the downstream pathways. This mutant is extremely useful in clarifying the turning-on mechanism of IGF-IR as well as the differential roles of individual downstream pathways of receptor tyrosine kinases.

Wortmannin as a unique probe for an intracellular signalling protein, phosphoinositide 3-kinase

Wortmannin is a fungal metabolite that so far has been shown to act as a selective inhibitor of phosphoinositide 3-kinase. It can therefore be used to investigate the convergence between two major cellular signalling systems: those involving G-protein-coupled receptors and those involving receptor tyrosine kinases. Importantly, wortmannin can enter intact cells, making whole-cell studies of the above signalling pathways possible.

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.

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.

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.

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.

Antagonists of phosphatidylinositol 3-kinase block activation of several novel protein kinases in neutrophils

Several novel protein kinases are known to be rapidly activated in neutrophils stimulated with the chemoattractant fMet-Leu-Phe (fMLP). These kinases include a histone H4 protein kinase and several renaturable kinases with molecular masses of about 69, 63, 49, and 40 kDa. The renaturable kinases can catalyze the phosphorylation of a peptide that corresponds to residues 297-331 of the 47-kDa subunit of the NADPH-oxidase system (p47-phox). Previous studies have indicated that the activation of all of these protein kinases involves an uncharacterized stimulatory pathway and/or novel second messenger. The studies reported herein were undertaken to determine if phosphatidylinositol 3-kinase (PI3-K) is a component of this pathway. We report that certain chromosome derivatives (e.g. 2-(4-morpholinyl)-8-phenylchromone (LY294002)) and wortmannin, which inhibit PI3-K by distinct mechanisms, blocked activation of all of these novel kinases. These antagonists also inhibited the phosphorylation of p47-phox (about 50%) and O2.- release (about 80%) in cells stimulated with fMLP, but not with 4 beta-phorbol 12-myristate 13-acetate. A strong correlation exists between the amounts of these antagonists required to produce 50% inhibition of PI3-K in vitro and O2.- release in vivo. In contrast, a single atom substitution of LY294002 produced a compound (LY303511) that did not inhibit PI3-K. Compound LY303511 did not appreciably inhibit the activation of the novel protein kinases or O2.- generation. These data strongly suggest that PI3-K is involved in the activation of several novel protein kinases in neutrophils, one or more of which may be involved in O2.- release.

The effects of wortmannin, a potent inhibitor of phosphatidylinositol 3-kinase, on insulin-stimulated glucose transport, GLUT4 translocation, antilipolysis, and DNA synthesis

PI 3-kinase, an enzyme that selectively phosphorylates the 3-position of the inositol ring, is acutely activated by insulin and other growth factors. The physiological significance of PI 3-kinase activation and, more specifically, its role in insulin action is an area under intense investigation. In this study, we have examined the role of PI 3-kinase activation in mediating selected metabolic and mitogenic effects of insulin employing the fungal metabolite wortmannin, a potent inhibitor of PI 3-kinase activity. In isolated rat and cultured 3T3-L1 adipocytes, wortmannin inhibited insulin-stimulated glucose transport (IC50 = 9 nM) without a significant effect on basal transport. Insulin-stimulated translocation of GLUT4 in isolated rat adipocytes was markedly inhibited by wortmannin. Wortmannin had no effect on either basal or insulin-stimulated glucose utilization in L6 myocytes, a skeletal muscle cell line in which GLUT1 is the predominant transporter isoform. Wortmannin also partially antagonized the antilipolytic effect of insulin on adenosine deaminase-stimulated lipolysis in isolated rat adipocytes. Furthermore, wortmannin caused a significant reduction in insulin-stimulated DNA synthesis in Fao rat hepatoma cells. We conclude that PI 3-kinase activation is necessary for maximum insulin-stimulated glucose transport, translocation of GLUT4, antilipolysis and DNA synthesis.

Effects of contractile activity on tyrosine phosphoproteins and PI 3-kinase activity in rat skeletal muscle

Insulin stimulates signaling reactions that include insulin receptor autophosphorylation and tyrosine kinase activation, insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation, and phosphatidylinositol 3-kinase (PI 3-kinase) activation. Muscle contraction has metabolic effects similar to insulin, and contraction can increase insulin sensitivity, but little is known about the molecular signals that mediate the effects of contraction. To investigate the effects of muscle contraction on insulin signaling, rats were studied after contraction of hindlimb muscles by electrical stimulation, maximal insulin injection in the absence of contraction, or contraction followed by insulin injection. Insulin increased tyrosine phosphorylation of the insulin receptor and IRS-1, whereas contraction alone had no effect. Contraction before insulin injection decreased the insulin effect on receptor and IRS-1 phosphorylation by 20-25%. Increased tyrosine phosphorylation of other proteins by insulin and/or contraction was not observed. Contraction alone had little effect on PI 3-kinase activity, but contraction markedly blunted the insulin-stimulated activation of IRS-1 and insulin receptor-immunoprecipitable PI 3-kinase. In conclusion, skeletal muscle contractile activity does not result in tyrosine phosphorylation of molecules involved in the initial steps of insulin signaling. Although contractile activity increases insulin sensitivity and responsiveness in skeletal muscle, contraction causes a paradoxical decrease in insulin-stimulated tyrosine phosphorylation and PI 3-kinase activity.

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.

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.

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.

Differential requirement for p21ras activation in the metabolic signaling by insulin

To evaluate the role of the "Ras pathway" in mediating metabolic signaling by insulin, we employed lovastatin to exhibit isoprenilation of Ras proteins in Rat-1 fibroblasts transfected with human insulin receptors (HIRc cells) and in differentiated 3T3-L1 adipocytes. Lovastatin blocked an ability of insulin to activate p21ras and mitogen-activated protein kinase. Lovastatin also significantly (p < 0.01) reduced insulin effects on thymidine incorporation and glucose incorporation into glycogen. Nevertheless, an effect of insulin on glucose uptake remained unaffected. It appears that in contrast to its mitogenic action and to its effect on glycogenesis, an effect of insulin on glucose uptake does not require p21ras activation.

The effects of wortmannin on rat skeletal muscle. Dissociation of signaling pathways for insulin- and contraction-activated hexose transport

Both the anabolic hormone insulin and contractile activity stimulate the uptake of glucose into mammalian skeletal muscle. In this study, we examined the role of phosphatidylinositol 3-kinase (PI 3-kinase), a putative mediator of insulin actions, in the stimulation of hexose uptake in response to hormone and contraction. Phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-triphosphate accumulate in skeletal muscle exposed to insulin but not hypoxia, which mimics stimulation of the contractile-dependent pathway of hexose transport activation. The fungal metabolite wortmannin, an inhibitor of PI 3-kinase, completely blocks the appearance of 3'-phospholipids in response to insulin. Moreover, wortmannin entirely prevented the increase in hexose uptake in muscle exposed to insulin but was without effect on muscle stimulated by repetitive contraction or hypoxia. These results support the view that PI 3-kinase is involved in the signaling pathways mediating insulin-responsive glucose transport in skeletal muscle but is not required for stimulation by hypoxia or contraction. Furthermore, these data indicate that there exist at least two signaling pathways leading to activation of glucose transport in skeletal muscle with differential sensitivities to wortmannin.

Does mitogen-activated-protein kinase have a role in insulin action? The cases for and against

The discovery of the mitogen-activated protein (MAP) kinase family of protein kinases has sparked off an intensive effort to elucidate their role in the regulation of many cellular processes. These protein kinases were originally identified based on their rapid activation by insulin. In this review we concentrate on examining the evidence for and against a role for the MAP kinases Erk-1 and Erk-2 in mediating the effects of insulin. While there is good evidence in favour of a direct role for MAP kinase in the growth-promoting effects of insulin and the regulation of Glut-1 and c-fos expression, and AP-1 transcriptional complex activity, this is by no means conclusive. MAP kinase may also play a role in the control of mRNA translation by insulin. On the other hand, the evidence suggests that MAP kinase is not sufficient for the acute regulation of glucose transport (Glut-4 translocation), glycogen synthesis, acetyl-CoA carboxylase or pyruvate dehydrogenase activity. The findings suggest that insulin may utilise at least three distinct signalling pathways which do not involve MAP kinase.

Glycogen synthase kinase 3 regulates cell fate in Dictyostelium

Extracellular cyclic AMP (cAMP) induces the formation of prespore cells in Dictyostelium but inhibits stalk cell formation. We have cloned gskA, which encodes the Dictyostelium homolog of glycogen synthase kinase 3 (GSK-3), and discovered that it is required for both cAMP effects. Disruption of gskA creates a mutant that aggregates but forms few spores and an abnormally high number of stalk cells. These stalk cells probably arise from an expanded prestalk B (pstB) cell population, which normally produces the basal disc of the fruiting body. In cultured mutant cells, cAMP neither inhibits pstB cell differentiation nor induces efficient prespore cell differentiation. We propose that cAMP acts through a common pathway that requires GSK-3 and determines the proportion of prespore and pstB cells.