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

Phosphatidylinositol 3-kinase-independent signal transduction pathway for platelet-derived growth factor-induced chemotaxis

Platelet-derived growth factor (PDGF)-BB is a potent chemoattractant for mesenchymal cells. Intracellular signal transduction for PDGF-induced chemotactic response has been reported to be dependent on phosphatidylinositol 3-kinase (PI3K) activation. Here, we report a PI3K-independent pathway operating for PDGF-induced chemotaxis in vascular smooth muscle cells and other cell types. Two different PI3K inhibitors, wortmannin (WT, 1 nM-1 microM) and LY294002 (100 nM-10 microM), did not inhibit PDGF-induced chemotaxis in smooth muscle cells and Swiss 3T3 cells, whereas WT inhibited activity of PI3K that were immunopurified from PDGF-stimulated cells as well as PI3K purified from cells that were stimulated with PDGF in the presence of the same concentrations of WT. Similarly, WT (100 nM) abolished the increase in intracellular phosphatidylinositol 3,4,5-triphosphate after PDGF stimulation. Furthermore, Chinese hamster ovary/Deltap85 cells overexpressing a dominant negative p85 subunit of PI3K showed a chemotactic response comparable to that of parental cells while showing a remarkable decrease in PI3K activity. Rapamycin, a specific inhibitor of pp70 S6 kinase, which is one of the well characterized downstreams of PI3K, did not inhibit PDGF-induced chemotaxis. Both WT and LY294002 inhibited PDGF-induced amino acid uptake and actin-stress fiber reorganization and partly inhibited PDGF-induced glucose incorporation in Swiss 3T3 cells. Our findings indicate that, in vascular smooth muscle cells and other cell types, the signal transduction for PDGF-induced chemotaxis is independent of PI3K activity while the signal transduction for PDGF-induced amino acid uptake, glucose incorporation, and cytoskeletal reorganization is dependent on PI3K.

Growth hormone, interferon-gamma, and leukemia inhibitory factor utilize insulin receptor substrate-2 in intracellular signaling

In this report, we demonstrate that insulin receptor substrate-2 (IRS-2) is tyrosyl-phosphorylated following stimulation of 3T3-F442A fibroblasts with growth hormone (GH), leukemia inhibitory factor and interferon-gamma. In response to GH and leukemia inhibitory factor, IRS-2 is immediately phosphorylated, with maximal phosphorylation detected at 15 min; the signal is substantially diminished by 60 min. In response to interferon-gamma, tyrosine phosphorylation of IRS-2 was prolonged, with substantial signal still detected at 60 min. Characterization of the mechanism of signaling utilized by GH indicated that tyrosine residues in GH receptor are not necessary for tyrosyl phosphorylation of IRS-2; however, the regions of GH receptor necessary for IRS-2 tyrosyl phosphorylation are the same as those required for JAK2 association and tyrosyl phosphorylation. The role of IRS-2 as a signaling molecule for GH is further demonstrated by the finding that GH stimulates association of IRS-2 with the 85-kDa regulatory subunit of phosphatidylinositol 3'-kinase and with the protein-tyrosine phosphatase SHP2. These results are consistent with the possibility that IRS-2 is a downstream signaling partner of multiple members of the cytokine family of receptors that activate JAK kinases.

The inhibitory effect of staurosporine on insulin action is prevented by okadaic acid. Evidence for an important role of serine/threonine phosphorylation in eliciting insulin-like effects

The serine/threonine phosphatase inhibitor, okadaic acid (OA), exerted several insulin-like effects in rat adipose cells and was, in part, synergistic with insulin. OA stimulated glucose transport activity, altered the electrophoretic mobility of IRS-1, increased the phosphorylation of the MAP-kinases ERK 1 and 2 on tyrosine sites, markedly increased MAP kinase activity and also acted synergistically with insulin in activating these enzymes. However, OA did not increase PI 3-kinase activity or the tyrosine phosphorylation of key upstream proteins in insulin's signaling cascade. Staurosporine virtually completely inhibited the insulin-stimulated glucose transport and MAP kinase activation in spite of a maintained high PI 3-kinase activity. In contrast, the effects of OA alone or in the presence of insulin were less, or not at all, affected. These data suggest that OA exerts an insulin-like effect through a serine/threonine-related pathway which is distinct from, but converges with, that of insulin downstream PI 3-kinase and upon which staurosporine exerts an inhibitory effect.

Mutant of insulin receptor substrate-1 incapable of activating phosphatidylinositol 3-kinase did not mediate insulin-stimulated maturation of Xenopus laevis oocytes

Insulin receptor substrate-1 (IRS-1) is rapidly phosphorylated on multiple tyrosine residues in response to insulin and binds several Src homology 2 domain-containing proteins, thereby initiating downstream signaling. To assess the tyrosine phosphorylation sites that mediate relevant downstream signaling and biological effects, we created site-directed mutants of IRS-1 and overexpressed them in the Xenopus laevis oocyte. In oocytes overexpressing IRS-1 or IRS-1-895F (Tyr-895 replaced with phenylalanine), insulin activated phosphatidylinositol (PI) 3-kinase, p70 S6 kinase, and mitogen-activated protein kinase and induced oocyte maturation. In contrast, in oocytes overexpressing IRS-1-4F (Tyr-460, Tyr-608, Tyr-939, and Tyr-987 of IRS-1 replaced with phenylalanine), insulin did not activate PI 3-kinase, p70 S6 kinase, and mitogen-activated protein kinase and failed to induce oocyte maturation. These observations indicate that in X. laevis oocytes overexpressing IRS-1, the association of PI 3-kinase rather than Grb2 (growth factor-bound protein 2) with IRS-1 plays a major role in insulin-induced oocyte maturation. Activation of PI 3-kinase may lie upstream of mitogen-activated protein kinase activation and p70 S6 kinase activation in response to insulin.

Different pathways of postreceptor desensitization following chronic insulin treatment and in cells overexpressing constitutively active insulin receptors

We have reported previously that substitution of the transmembrane domain of the insulin receptor with that of the erbB-2 oncogene (IRerbV-->E) results in constitutive activation of the insulin receptor kinase. Compared to NIH3T3 cells overexpressing wild-type insulin receptors (IRwt), cells overexpressing IRerbV-->E displayed a decrease in IRS-1 protein content by 55%, but basal tyrosine phosphorylation of IRS-1 was increased. This resulted in an increased association of IRS-1 with the p85 subunit of phosphatidylinositol 3-kinase, increased phosphatidylinositol 3-kinase activity in anti-IRS-1 immunoprecipitates, constitutive activation of p70 S6 protein kinase, and an increased association of Grb2 with Shc in the absence of ligand. However, Grb2 association with IRS-1 could not be detected in the basal or insulin-stimulated states, and mitogen-activated protein kinase (MAPK) activity could not be stimulated by insulin, epidermal growth factor, or platelet-derived growth factor. In contrast to IRerbV-->E, the insulin receptor content and its tyrosine phosphorylation were significantly decreased in IRwt cells chronically stimulated (>24 h) with insulin. With decreased IRS-1 content, tyrosine phosphorylation of IRS-1 was decreased by over 75%, leading to decreased IRS-1-associated PI 3-kinase and Grb2. In addition, Grb2 association with Shc and activation of MAPK and the p70 S6 kinase were insensitive to insulin stimulation. By contrast, association of Grb2 with Shc and activation of MAPK, but not the p70 S6 kinase, could be stimulated by epidermal growth factor or platelet-derived growth factor. These data suggest that there are multiple levels of postreceptor desensitization to insulin action. These are used somewhat differently in these two different models, probably due in part to the difference in receptor down-regulation.

Overexpression of a constitutively active form of phosphatidylinositol 3-kinase is sufficient to promote Glut 4 translocation in adipocytes

Insulin stimulates glucose transport in its target cells by recruiting the glucose transporter Glut 4 from an intracellular compartment to the cell surface. Previous studies have indicated that phosphatidylinositol 3-kinase (PI 3-kinase) is a necessary step in this insulin action. We have investigated whether PI 3-kinase activation is sufficient to promote Glut 4 translocation in transiently transfected adipocytes. Rat adipose cells were cotransfected with expression vectors that allowed transient expression of epitope-tagged Glut 4 and a constitutively active form of PI 3-kinase (p110*). The expression of p110* induced the appearance of epitope-tagged Glut 4 at the cell surface at a level similar to that obtained after insulin treatment, whereas a kinase-dead version of p110* had no effect. The p110* effect was observed over a wide range of the transfected cDNA. When subcellular fractionation of adipocytes was performed, p110* was found, similar to the endogenous PI 3-kinase, enriched in the low density microsomal compartment, which also contains the Glut 4 vesicles. This could suggest that a specific localization of PI 3-kinase in this compartment is required for the action on Glut 4. The observations made with PI 3-kinase are in contrast with those seen with the MAP kinase cascade. Indeed, a constitutively active form of MAP kinase kinase had no effect on Glut 4 translocation in basal conditions. At the highest degree of expression, the constitutively active form of MAP kinase kinase slightly inhibited the insulin stimulation of Glut 4 translocation. Taken together, our results indicate that Glut 4 translocation can be efficiently promoted by an active form of PI 3-kinase but not by the activation of the MAP kinase pathway.

The early intracellular signaling pathway for the insulin/insulin-like growth factor receptor family in the mammalian central nervous system

Several studies support the idea that the polypeptides belonging to the family of insulin and insulin-like growth factors (IGFs) play an important role in brain development and continue to be produced in discrete areas of the adult brain. In numerous neuronal populations within the olfactory bulb, the cerebral and cerebellar cortex, the hippocampus, some diencephalic and brainstem nuclei, the spinal cord and the retina, specific insulin and IGF receptors, as well as crucial components of the intracellular receptor signaling pathway have been demonstrated. Thus, mature neurons are endowed with the cellular machinery to respond to insulin and IGF stimulation. Studies in vitro and in vivo, using normal and transgenic animals, have led to the hypothesis that, in the adult brain, IGF-I not only acts as a trophic factor, but also as a neuromodulator of some higher brain functions, such as long-term potentiation and depression. Furthermore, a trophic effect on certain neuronal populations becomes clearly evident in the ischemic brain or neurodegenerative disorders. Thus, the analysis of the early intracellular signaling pathway for the insulin/IGF receptor family in the brain is providing us with new intriguing findings on the way the mammalian brain is sculpted and operates.

Direct inhibition of the signaling functions of the mammalian target of rapamycin by the phosphoinositide 3-kinase inhibitors, wortmannin and LY294002

The immunosuppressant, rapamycin, inhibits cell growth by interfering with the function of a novel kinase, termed mammalian target of rapamycin (mTOR). The putative catalytic domain of mTOR is similar to those of mammalian and yeast phosphatidylinositol (PI) 3-kinases. This study demonstrates that mTOR is a component of a cytokine-triggered protein kinase cascade leading to the phosphorylation of the eukaryotic initiation factor-4E (eIF-4E) binding protein, PHAS-1, in activated T lymphocytes. This event promotes G1 phase progression by stimulating eIF-4E-dependent translation initiation. A mutant YAC-1 T lymphoma cell line, which was selected for resistance to the growth-inhibitory action of rapamycin, was correspondingly resistant to the suppressive effect of this drug on PHAS-1 phosphorylation. In contrast, the PI 3-kinase inhibitor, wortmannin, reduced the phosphorylation of PHAS-1 in both rapamycin-sensitive and -resistant T cells. At similar drug concentrations (0.1-1 microM), wortmannin irreversibly inhibited the serine-specific autokinase activity of mTOR. The autokinase activity of mTOR was also sensitive to the structurally distinct PI 3-kinase inhibitor, LY294002, at concentrations (1-30 microM) nearly identical to those required for inhibition of the lipid kinase activity of the mammalian p85-p110 heterodimer. These studies indicate that the signaling functions of mTOR, and potentially those of other high molecular weight PI 3-kinase homologs, are directly affected by cellular treatment with wortmannin or LY294002.

Requirement of phospholipase C gamma, the tyrosine phosphatase Syp and the adaptor proteins Shc and Nck for PDGF-induced DNA synthesis: evidence for the existence of Ras-dependent and Ras-independent pathways

We have investigated the roles of the phosphotyrosine phosphatase Syp (also called SH-PTP2), phospholipase C (PLC) gamma1, rasGTPase Activating Protein (rasGAP) and the adapter molecules Nck and Shc in the mitogenic response induced by PDGF in fibroblasts. Two separate approaches were used to inhibit the biological activity of these signalling proteins in vivo. Either glutathione S-transferase (GST) fusion proteins containing the SH2 domains of these proteins, or antibodies specific for these polypeptides, were microinjected into cells. GST-SH2 fusion proteins are expected to act as dominant inhibitors by competing for physiological SH2-mediated interactions, while microinjected antibodies can directly block protein functions. Inhibition of PLCgamma, Syp, Shc and Nck signals blocked PDGF-stimulated cells in G1 showing a requirement for these proteins for S-phase entry. Inhibition of rasGAP, in contrast, had no effect on S-phase entry. We next examined which of these signals were required for PDGF-induced cFos expression, a Ras-dependent event important for signalling. By using the same approaches with cells expressing beta-galactosidase under the control of a c-fos promoter, we showed that PLCgamma, Syp and Shc were necessary for ligand-induced cFos expression whereas Nck and phosphatidylinositol 3-kinase alpha were not. From these results we concluded that PDGF generates Ras-dependent and Ras-independent pathways important for DNA synthesis.

The insulin-induced down-regulation of IRS-1 in 3T3-L1 adipocytes is mediated by a calcium-dependent thiol protease

Insulin receptor substrate-1 (IRS-1) is a protein expressed in 3T3-L1 adipocytes that is involved in most, if not all of the biological responses to insulin. Chronic exposure of these cells to insulin down-regulates IRS-1 by stimulating its degradation (Rice, K.M., Turnbow, M.A. and Garner, C.W. (1993) Biochem. Biophys. Res. Commun. 190, 961-967). This insulin-induced down-regulation of IRS-1 was totally abolished by BAPTA-AM (cell-permeable calcium chelator), E-64d (cell-permeable thiol protease inhibitor), Cbz-Leu-Nleu-H and Cbz-Leu-Leu-Tyr-CHN2 (selective cell-permeable calpain inhibitor peptides). Calpastatin (specific calpain inhibitor protein) also inhibited the insulin-induced down-regulation of IRS-1 in transiently permeabilized cells. In addition, 3T3-L1 adipocytes express endogenous calpain which can degrade IRS-1 in cell-free extracts. These results suggest that the insulin-induced down-regulation of IRS-1 in 3T3-L1 adipocytes is mediated by a calcium-dependent thiol protease which is sensitive to inhibition by calpain inhibitors.

Insulin activates a p21-activated kinase in muscle cells via phosphatidylinositol 3-kinase

Insulin activates rapidly a complex cascade of lipid and protein kinases leading to stimulation of mitogenic and metabolic events. Here we describe a renaturable kinase of 65 kDa (PK65) that becomes rapidly activated by insulin in differentiated L6 muscle cells (myotubes) and can phosphorylate histones immobilized in polyacrylamide gels. Insulin activation of PK65 was abolished by the tyrosine kinase inhibitor erbstatin and by the phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor wortmannin, but was unaffected by inhibitors of protein kinase C or of the activation of p70(S6K). Recently, a number of protein kinases have been described which become activated through interaction with the small GTP-binding proteins Rac and Cdc42 (21-ctivated inases, or PAKs) and lead to activation of the stress-induced mitogen-activated protein kinase (MAPK) p38 MAPK. Two different polyclonal antibodies recognizing the carboxyl-terminal or the Rac-binding domain of a 65-kDa PAK (PAK65) immunoprecipitated the myotube PK65. The insulin-induced activation of PK65 in myotubes was detectable following immunoprecipitation of the kinase. Furthermore, PK65 associated with and became activated by glutathione S-transferase-Cdc42Hs in the presence of GTPgammaS (guanosine 5'-3-O-(thio)triphosphate). In myotubes insulin also induced tyrosine phosphorylation of p38 MAPK. However, this phosphorylation was insensitive to wortmannin, indicating that p38 MAPK is not activated by PK65 in insulin-stimulated cells. The results suggest that insulin activates in muscle cells a renaturable kinase (PK65) closely related to PAK65. Tyrosine kinases and PI 3-kinase act upstream of PK65 in the insulin signaling cascade. Insulin activates p38 MAPK in myotubes, but this occurs by a pathway independent of PI 3-kinase and PK65.

Inhibition of insulin receptor signaling by TNF: potential role in obesity and non-insulin-dependent diabetes mellitus

Adipocytes produce a variety of molecules that are capable of functioning in both a paracrine and autocrine fashion. Tumor necrosis factor (TNF) is one of the proteins produced by adipocytes that has been shown to regulate adipocyte function. Interestingly, adipocyte expression of TNF increases with increasing adipocyte mass and expression of TNF is increased in adipocytes isolated from several genetic models of rodent obesity and from obese humans. This finding has led to the idea that TNF produced by adipocytes functions as a local "adipostat" to limit fat accumulation. Increased production of TNF by adipocytes, however, may contribute to insulin resistance in obesity and in non-insulin-dependent diabetes mellitus (NIDDM). TNF has been shown to inhibit insulin-simulated tyrosine phosphorylation of both the insulin receptor (IR) and insulin receptor substrate (IRS)-1 and to stimulate downregulation of the insulin-sensitive glucose transporter, GLUT4, in adipocytes. These findings raise the possibility that pharmacological inhibition of TNF may provide a novel therapeutic target to treat patients with NIDDM.

Receptor signalling and the regulation of endocytic membrane transport

Vesicle-mediated membrane traffic has long been considered to be a constitutive process that is not burdened by layers of regulation. This contrasts with transmembrane signalling systems at the plasma membrane which relay information (i.e. extracellular stimuli) from the cell surface to the cytoplasm via a myriad of different protein-protein interactions and second messenger cascades. An accumulation of recent evidence, however, now suggests that signal-transduction pathways also play a critical role in the regulation of protein and membrane trafficking. In particular, the analysis of the signalling pathways initiated by receptor tyrosine kinases at the plasma membrane has yielded new insights into the molecular mechanisms of endocytosis. In addition, recent evidence has suggested potential new roles for two previously characterized vesicle coat proteins in a membrane traffic route that is regulated via cell surface receptor signalling.

Understanding and controlling the cell cycle with natural products

Small molecule natural products have aided in the discovery and characterization of many proteins critical to the progression and maintenance of the cell cycle. Identification of the direct target of a natural product gives scientists a tool to control a specific aspect of the cell cycle, thus facilitating the study of the cell-cycle machinery.

Activated phosphatidylinositol 3-kinase is sufficient to mediate actin rearrangement and GLUT4 translocation in 3T3-L1 adipocytes

Insulin stimulation of 3T3-L1 adipocytes causes rapid translocation of actin and the GLUT4 glucose transporter to the plasma membrane. Both processes depend on the activity of phosphatidylinositol 3-kinase. Using single cell microinjection, we have transiently expressed a constitutively activated mutant of phosphatidylinositol 3-kinase, p110*, in 3T3-L1 adipocytes. Fluorescent detection of GLUT4 protein and actin within these cells demonstrates that expression of p110* is sufficient to cause translocation of GLUT4 to the plasma membrane and the formation of actin membrane ruffles. These effects are inhibited by wortmannin in the p110*-expressing cells, indicating that the phosphatidylinositol 3-kinase activity of the protein is required. Overexpression of an identical protein containing a point mutation in the kinase domain, p110*Deltakin, was incapable of mediating either action, confirming that neither the microinjection process nor a nonspecific effect of the protein was responsible for the observed effects. These data suggest that although insulin is capable of inducing numerous signaling pathways, the isolated activation of phosphatidylinositol 3-kinase can initiate the signaling cascade leading to both actin rearrangement and GLUT4 translocation in the absence of insulin stimulation.

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

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

Overexpression of catalytic subunit p110alpha of phosphatidylinositol 3-kinase increases glucose transport activity with translocation of glucose transporters in 3T3-L1 adipocytes

To elucidate the mechanisms of phosphatidylinositol (PI) 3-kinase involvement in insulin-stimulated glucose transport activity, the epitope-tagged p110alpha subunit of PI 3-kinase was overexpressed in 3T3-L1 adipocytes using an adenovirus-mediated gene transduction system. Overexpression of p110alpha was confirmed by immunoblot using anti-tagged epitope antibody. p110alpha overexpression induced a 2.5-fold increase in PI 3-kinase activity associated with its regulatory subunits in the basal state, an increase exceeding that of the maximally insulin-stimulated control cells, while PI 3-kinase activity associated with phosphotyrosyl protein was only modestly elevated. Overexpression of p110alpha induced an approximately 14-fold increase in the basal glucose transport rate, which was also greater than that observed in the stimulated control. No apparent difference was observed in the cellular expression level of either GLUT1 or GLUT4 proteins between control and p110alpha-overexpressing 3T3-L1 adipocytes. Subcellular fractionation revealed translocation of glucose transporters from intracellular to plasma membranes in basal p110alpha-overexpressing cells. The translocation of GLUT4 protein to the plasma membrane was further confirmed using a membrane sheet assay. These findings indicate that an increment in PI 3-kinase activity induced by overexpression of p110alpha of PI 3-kinase stimulates glucose transport activity with translocation of glucose transporters, i.e., mimics the effect of insulin.

Analysis of the signaling pathway involved in the regulation of hexokinase II gene transcription by insulin

The hexokinases, by converting glucose to glucose 6-phosphate, help maintain the glucose concentration gradient that results in the movement of glucose into cells through the facilitative glucose transporters. Hexokinase II (HKII) is the major hexokinase isoform in skeletal muscle, heart, and adipose tissue. Insulin induces HKII gene transcription in L6 myotubes, and this, in turn, increases HKII mRNA and the rates of HKII protein synthesis and glucose phosphorylation in these cells. Inhibitors of distinct insulin signaling pathways were used to dissect the molecular mechanism by which HKII gene expression is induced by insulin in L6 myotubes. Treatment with wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI 3-kinase), or with rapamycin, an inhibitor of the pathway from the insulin receptor to p70/p85 ribosomal S6 protein kinase (p70(s6k)), prevented the induction of HKII mRNA by insulin. In contrast, treatment with PD98059, an inhibitor of mitogen-activated protein kinase activation, had no effect on insulin-induced HKII mRNA. In addition, rapamycin blocked the insulin-induced expression of an HKII promoter-chloramphenicol acetyltransferase fusion gene transiently transfected into L6 myotubes, whereas PD98059 had no such effect. These results suggest that a phosphatidylinositol 3-kinase/p70(s6k)-dependent pathway is required for regulation of HKII gene transcription by insulin and that the Ras-mitogen-activated protein kinase-dependent pathway is probably not involved.

Different effects of insulin and platelet-derived growth factor on phosphatidylinositol 3-kinase at the subcellular level in 3T3-L1 adipocytes. A possible explanation for their specific effects on glucose transport

Insulin stimulates glucose uptake by induction of the translocation of vesicles that contain the glucose transporter Glut 4 to the plasma membrane. Phosphatidylinositol 3-kinase (PtdIns 3-kinase), which is thought to be involved in intracellular trafficking, could play a critical role in insulin-induced glucose transport. In 3T3-L1 adipocytes, insulin and platelet-derived-growth-factor (PDGF) stimulated glucose uptake by 5.8-fold and 2.4-fold, respectively, but PDGF had no significant effect on Glut 4 translocation. Nevertheless, both hormones activated PtdIns 3-kinase activity in total cell extracts. However, insulin and PDGF had different effects on the stimulation of PtdIns 3-kinase activity in several subcellular fractions, and the movements of insulin-receptor substrate (IRS) 1 and the p85 subunit of PtdIns 3-kinase between subcellular compartments. PDGF stimulated PtdIns 3-kinase activity almost exclusively in the plasma membrane, and induced translocation of the p85 subunit from the cytosol to the plasma membrane, where the PDGF receptor was phosphorylated on tyrosine residues. In contrast, insulin stimulated PtdIns 3-kinase activity in the plasma membrane, in low-density microsomes (LDM) and in cytosol. Furthermore, insulin induced the translocation of p85 from the cytosol to LDM and the translocation of IRS 1 from LDM to the cytosol. These data indicate that insulin and PDGF have different effects on the activation of PtdIns 3-kinase and on the movement of IRS 1 and PtdIns 3-kinase between subcellular compartments. We would like to suggest that a crucial event in the stimulation of glucose uptake by insulin could be that insulin, but not PDGF, induces activation of PtdIns 3-kinase in the cytosol and in LDM, the compartment enriched in Glut-4-containing vesicles.

Compartmentalization of protein traffic in insulin-sensitive cells

Insulin-sensitive cells, adipocytes and myocytes, translocate a number of intracellular proteins to the cell surface in response to insulin. Among these proteins are glucose transporters 1 and 4 (GLUT-1 and GLUT-4, respectively), receptors for insulin-like growth factor II (IGF-II)/mannose 6-phosphate (Man-6-P) and transferrin, the aminopeptidase gp 160, caveolin, and a few others. In the case of insulin-activated glucose transport, this translocation has been proven to be the major, if not the only regulatory mechanism of this process. It seems likely that the cell surface recruitment of the IGF-II/Man-6-P and transferrin receptors also serves the nutritional needs of cells, whereas the physiological role of the aminopeptidase gp160 remains uncertain. Analysis of the compartmentalization and trafficking pathways of translocatable proteins in fat cells identified more than one population of recycling vesicles, although all have identical sedimentation coefficients and buoyant densities in vitro. GLUT-4-containing vesicles include essentially all the intracellular GLUT-4, gp160, and the acutely recycling populations of receptors for IGF-II/Man-6-P and transferrin. Besides these proteins, which can be considered as vesicle "cargo", GLUT-4-containing vesicles have other components, like secretory carrier-associated membrane proteins (SCAMP), Rab(s), and vesicle-associated membrane protein (VAMP)/cellubrevin, which are ubiquitous to secretory vesicles and granules from different tissues. GLUT-1 and caveolin are excluded from GLUT-4-containing vesicles and form different vesicular populations of unknown polypeptide composition. In skeletal muscle, two independent populations of GLUT-4-containing vesicles are found, insulin sensitive and exercise sensitive, which explains the additive effect of insulin and exercise on glucose uptake. Both vesicular populations are similar to each other and to analogous vesicles in fat cells.

Oncogenic Raf-1 activates p70 S6 kinase via a mitogen-activated protein kinase-independent pathway

Cell proliferation requires the co-ordinate triggering of several protein kinases of Ser/Thr specificity such as p70 S6 kinase (S6K), which phosphorylates the ribosomal S6 protein and thus increases translation of mRNAs with polypyrimidine tracts. The multiplicity of signaling pathways leading to p70 S6K activation are not fully elucidated. However, several reports have indicated that the activation of p70 S6K is independent of mitogen-activated protein kinase (MAPK) activation. Interestingly, we and others have shown that constitutive activation of the MAPK pathway promotes cell proliferation, suggesting that this cascade is able to activate p70 S6K, a key step to trigger cell cycle entry. In this report we demonstrate that transfection of constitutively active mitogen-activated protein kinase kinase 1 in CCL 39 cells leads to activation of p70 S6K. Furthermore, we have established a cell line that stably expresses DeltaRaf-1:ER, an estradiol-regulated form of oncogenic Raf-1. The addition of estradiol to these cells was sufficient to elicit rapid activation of mitogen-activated protein kinase kinase 1, MAPK, and p70 S6K. Surprisingly, the activation of p70 S6K is not mediated by MAPK because blocking MAPK activation by expression of the phosphatase MKP-1 did not prevent p70 S6K activation by DeltaRaf-1:ER. In conclusion, we have demonstrated that activation of p70 S6K by DeltaRaf-1:ER is mediated by a new MAPK-independent pathway. This pathway is resistant to low nanomolar concentrations of wortmannin, indicating that it does not involve membrane-bound phosphatidylinositol-trisphosphate kinase activation.

Studies with wortmannin and cytochalasins suggest a pivotal role of phosphatidylinositols in the regulation of tight junction integrity

Wortmannin, a selective inhibitor of phosphatidylinositol 3-kinase (P13K), was found to give a dose and time-dependent, bimodal effect-initial increase, followed by decrease on the tight junction integrity of MDCK-1 monolayers, as assessed by electrical resistance measurement of the epithelia. Moreover, dihydrocytochalasin B inhibited the wortmannin-induced alteration, whereas cytochalasin B had a negligible influence on the wortmannin effect. Wortmannin was also found to cause changes in the cytoskeleton structure. These alterations were also seen when wortmannin was combined with cytochalasin B. However, in accordance with the electrical resistance measurements, dihydrocytochalasin B was able to abolish wortmannin-induced filamentous (F-) actin changes. These findings suggest that the P13K, phosphatidylinositols, and filamentous actin rearrangements, in combination, play an important role in the modulation of the junctional integrity.

Purification and characterization of the phosphatidylinositol-3,4,5-trisphosphate phosphatase in bovine thymus

Using phosphatidylinositol 3,4,5-trisphosphate [PtdIns(3,4,5)P3] prepared from phosphatidylinositol 4,5-bisphosphate and inositolphospholipid 3-kinase, we identified in bovine thymus extracts the enzyme activity which catalyzed dephosphorylation of PtdIns(3,4,5)P3, to produce phosphatidylinositol biphosphate. Since bovine thymus exhibited the highest level of activity among tissues screened, we tried to purify this enzyme PtdINs(3,4,5)P3 phosphatase from bovine thymus. After sequential chromatographies using S-Sepharose, heparin-Sepharose, blue Sepharose, and Toyopearl HW55, the enzyme was purified 1875-fold with a yield of 10%. SDS/PAGE analysis revealed that a 120-kDA protein band copurified with the enzyme activity. The apparent molecular mass of the active protein was 120 kDa on size-exclusion chromatography, suggesting that the 120-kDa band on SDS/PAGE is the PtdIns(3,4,5)P3 phosphatase. Since PtdIns(3,4,5)P3 phosphatase seemed to be the only activity that metabolized PtdIns(3,4,5)P3, and the enzyme did not hydrolyze phosphatidylinositol 4,5-biphosphate, the enzyme may play a critical role in the inositolphospholipid 3-kinase signalling.

The 70 kDa S6 kinase complexes with and is activated by the Rho family G proteins Cdc42 and Rac1

The 70 kDa ribosomol S6 kinase (pp70S6k) plays an important role in the progression of cells through G1 phase of the cell cycle. However, little is known of the signaling molecules that mediate its activation. We demonstrate that Rho family G proteins regulate pp70S6k activity in vivo. Activated alleles of Cdc42 and Rac1, but not RhoA, stimulate pp70S6k activity in multiple cell types. Activation requires an intact effector domain and isoprenylation of Cdc42 and Rac1. Coexpression of Dbl, an exchange factor for Cdc42, also activates pp70S6k. Growth factor-induced activation of pp70S6k is abrogated by dominant negative alleles of Cdc42 and Rac1. In addition, Cdc42 and Rac1 form GTP-dependent complex with the catalytically inactive form of pp70S6k in vitro and in vivo, suggesting a mechanism by which these G proteins activate pp70S6k.

Cytokines and insulin induce cationic amino acid transporter (CAT) expression in cardiac myocytes. Regulation of L-arginine transport and no production by CAT-1, CAT-2A, and CAT-2B

Cytokine-dependent production of nitric oxide (NO) by rat cardiac myocytes is a consequence of increased expression of the inducible isoform of nitric oxide synthase (iNOS or NOS2) and, in the presence of insulin, depresses the contractile function of these cells in vivo and in vitro. Experiments reported here show that L-lysine, a competitive antagonist of L-arginine uptake, suppressed NO production (detected as nitrite accumulation) by interleukin (IL)-1beta and interferon (IFN) gamma-pretreated cardiac myocytes by 70%, demonstrating that NO production is dependent on L-arginine uptake. Cardiac myocytes constitutively exhibit a high-affinity L-arginine transport system (Km = 125 microM; Vmax = 44 pmol/2 X 10(5) cells/min). Following a 24-h exposure to IL-1beta and IFNgamma, arginine uptake increases Vmax = 167 pmol/2 X 10(5) cells/min) and a second low-affinity L-arginine transporter activity appears (Km = 1.2 mM). To examine the molecular basis for these cytokine-induced changes in arginine transport, we examined expression of three related arginine transporters previously identified in other cell types. mRNA for the high-affinity cationic amino acid transporter-1 (CAT-1) is expressed in resting myocytes and steady-state levels increase by 10-fold following exposure to IL-1beta and IFNgamma. Only cytokine-pretreated myocytes expressed a second high-affinity L-arginine transporter, CAT-2B, as well as a low-affinity L-arginine transporter, CAT-2A. In addition, insulin, which potentiated cytokine-dependent NO production independent of any change in NOS activity, increased myocyte L-arginine uptake by 2-fold and steady-state levels of CAT-1, but not CAT-2A or CAT-2B mRNA. Thus, NO production by cardiac myocytes exposed to IL-1beta plus IFNgamma appears to be dependent on the coinduction of CAT-1, CAT-2A, and CAT-2B, while insulin independently augments L-arginine transport through CAT- 1.

Overexpression of Ha-ras selectively in adipose tissue of transgenic mice. Evidence for enhanced sensitivity to insulin

To determine the role of Ras-dependent signaling pathways in adipocyte function, we created transgenic mice that overexpress Ha-ras in adipocytes using the aP2 fatty acid-binding protein promoter/enhancer ligated to the human genomic ras sequence. ras mRNA was increased 8-17-fold and Ras protein 4-5-fold in white and brown fat, with no overexpression in other tissues. The subcellular distribution of overexpressed Ras paralleled that of endogenous Ras. [U-14C]Glucose uptake into isolated adipocytes was increased approximately 2-fold in the absence of insulin, and the ED50 for insulin was reduced 70%, with minimal effect on maximally stimulated glucose transport. Expression of Glut4 protein was unaltered in transgenic adipocytes, but photoaffinity labeling of transporters in intact cells with [3H]2-N-[4-(1-azi-Z,Z,Z-trifluoroethyl)benzoyl]-1,3-bis-(D-mann os-4- yloxy)-2-propylamine revealed 1.7-2.6-fold more cell-surface Glut 4 in the absence of insulin and at half-maximal insulin concentration (0.3 nM) compared with nontransgenic adipocytes. With maximal insulin concentration (80 nM), cell-surface Glut4 in nontransgenic and transgenic adipocytes was similar. Glut1 expression and basal cell-surface Glut1 were increased 2-2.9-fold in adipocytes of transgenic mice. However, Glut1 was much less abundant than Glut4, making its contribution to transport negligible. These in vitro changes were accompanied by in vivo alterations including increased glucose tolerance, decreased plasma insulin levels, and decreased adipose mass. We conclude that ras overexpression in adipocytes leads to a partial translocation of Glut4 in the absence of insulin and enhanced Glut4 translocation at physiological insulin concentration, but no effect with maximally stimulating insulin concentrations.

Wortmannin-sensitive trafficking pathways in Chinese hamster ovary cells. Differential effects on endocytosis and lysosomal sorting

Phosphatidylinositol (PI) 3'-kinases are a family of lipid kinases implicated in the regulation of cell growth by oncogene products and tyrosine kinase growth factor receptors. The catalytic subunit of the p85/p110 PI 3'-kinase is homologous to VPS-34, a phosphatidylinositol-specific lipid kinase involved in the sorting of newly synthesized hydrolases to the yeast vacuole. This suggests that PI 3'-kinases may play analogous roles in mammalian cells. We have measured a number of secretory and endocytic trafficking events in Chinese hamster ovary cells in the presence of wortmannin, a potent inhibitor of PI 3'-kinase. Wortmannin caused a 40-50% down-regulation of surface transferrin receptors, with a dose dependence identical to that required for maximal inhibition of the p85/p110 PI 3'-kinase in intact cells. The redistribution of transferrin receptors reflected a 60% increase in the internalization rate and a 35% decrease in the recycling rate. Experiments with fluorescent transferrin showed that entry of transferrin receptors into the recycling compartment and efflux of receptors out of the compartment were slowed by wortmannin. Wortmannin altered the morphology of the recycling compartment, which was more vesiculated than in untreated cells. Using Semliki Forest virus as a probe, we also found that delivery of the endocytosed virus to its lysosomal site of degradation was slowed by wortmannin, whereas endosomal acidification was unaffected. In contrast to these effects on endocytosis and recycling, wortmannin did not affect intracellular processing of newly synthesized viral spike proteins. Wortmannin did induce missorting of the lysosomal enzyme cathepsin D to the secretory pathway, but only at a dose 20-fold greater than that required to inhibit p85/p110 PI 3'-kinase activity or to redistribute transferrin receptors. Our data demonstrate the presence of wortmannin-sensitive enzymes at three distinct steps of the endocytic cycle in Chinese hamster ovary cells: internalization, transit from early endosomes to the recycling and degradative compartments, and transit from the recycling compartment back to the cell surface. The wortmannin-sensitive enzymes critical for endocytosis and recycling are distinct from those involved in sorting newly synthesized lysosomal enzymes.

Insulin-induced activation of phosphoinositide 3-kinase in Fao cells

Phosphoinositide 3-kinase (PI3-kinase) plays a crucial role in insulin signal transduction. We studied the molecular mechanism of the insulin-induced activation of PI3-kinase in rat hepatoma Fao cells using an antibody against the 110-kDa catalytic subunit (p110) and two against the 85-kDa regulatory subunit (p85 alpha). PI3-kinase activity increased 1.6-fold in anti-p85 immunoprecipitates after insulin stimulation, whereas it did not increase when cell lysates were first immunoprecipitated with anti-phosphotyrosine or anti-insulin receptor substrate-1 (IRS-1), then with anti-p85, suggesting that the PI3-kinase which associates with tyrosyl phosphoproteins including IRS-1 is responsible for the increase in kinase activity. The activated PI3-kinase molecules constituted 4-6% of the total PI3-kinase, and their specific activity was 11-14 times higher than that of the basal state. Anti-p110 recognized the catalytically active form of p110, and immunoprecipitated p110 only after exposure to insulin. Hence, the epitope of anti-p110, P200-C215, seems to be included in the portion of p110, the conformation of which is changed by insulin stimulation. We conclude that, in response to insulin stimulation, only a small fraction of p85 in the PI3-kinase pool associates with tyrosyl phosphoproteins including IRS-1, and that the specific activity of p110 is increased presumably through a conformational change including the P200-C215 region.

Insulin-mediated targeting of phosphatidylinositol 3-kinase to GLUT4-containing vesicles

Phosphatidylinositol (PI) 3-kinase is hypothesized to be a signaling element in the acute redistribution of intracellular GLUT4 glucose transporters to the plasma membrane in response to insulin. However, some receptors activate PI 3-kinase without causing GLUT4 translocation, suggesting specific cellular localization may be critical to this PI 3-kinase function. Consistent with this idea, complexes containing PI 3-kinase bound to insulin receptor substrate 1 (IRS-1) in 3T3-L1 adipocytes are associated with intracellular membranes (Heller-Harrison, R., Morin, M. and Czech, M. (1995) J. Biol. Chem. 270, 24442-24450). We report here that in response to insulin, activated complexes of IRS-1.PI 3-kinase can be immunoprecipitated with anti-IRS-1 antibody from detergent extracts of immunoadsorbed GLUT4-containing vesicles prepared from 3T3-L1 adipocytes. The targeting of PI 3-kinase to rat adipocyte GLUT4-containing vesicles using vesicles prepared by sucrose velocity gradient ultracentrifugation was also demonstrated. Insulin treatment caused a 2.3-fold increase in immunoreactive p85 protein in these GLUT4-containing vesicles while anti-p85 immunoprecipitates of PI 3-kinase activity in GLUT4-containing vesicle extracts increased to a similar extent. HPLC analysis of the GLUT4 vesicle-associated PI 3-kinase activity showed insulin-mediated increases in PI 3-P, PI 3,4-P2, and PI 3,4,5-P3 when PI, PI 4-P, and PI 4,5-P2 were used as substrates. Our data demonstrate that insulin directs the association of PI 3-kinase with GLUT4-containing vesicles in 3T3-L1 and rat adipocytes, consistent with the hypothesis that PI 3-kinase is involved in the insulin-regulated movement of GLUT4 to the plasma membrane.

Wortmannin-sensitive activation of p70s6k by endogenous and heterologously expressed Gi-coupled receptors

In order to study the regulation of the ribosomal protein S6 kinase, p70s6k, by G protein-coupled receptors, Rat-1 fibroblasts were stably transfected with two versions of the alpha2 adrenergic receptor. Stimulation of clone 1C cells, which express 3.5 pmol/mg of protein of the human alpha2C10 receptor, with the alpha2 agonist UK 14304 led to a transient increase in p70s6k activity. UK 14304 also activated p70s6k in a clone expressing the porcine alpha2A receptor (400 fmol/mg of protein). Lysophosphatidic acid (LPA), acting through endogenous G protein-coupled receptors, also activated p70s6k in alpha2 receptor-transfected and in nontransfected cells. Activation of p70s6k by both UK 14304 and LPA was accompanied by increased phosphorylation of the protein. Rapamycin completely blocked the activation of p70s6k by both agents. Activation of p70s6k by UK 14304 and by LPA, but not by platelet-derived growth factor (PDGF), was blocked by preincubation of cells with pertussis toxin. Wortmannin, a selective inhibitor of phosphoinositide (PI) 3-OH kinase, prevented activation of p70s6k by UK 14304, LPA, and PDGF. These data indicate that p70s6k is regulatable by Gi-coupled receptor agonists in a pertussis toxin-sensitive fashion in Rat-1 fibroblasts and that activation of p70s6k by such agents appears to involve an isoform of PI 3-kinase.

Phosphoinositide kinases

Recently, a number of cDNA clones with homology to the catalytic subunit of phosphoinositide 3-kinase have been identified, and the sequence of the first cDNA clone encoding a phosphatidylinositol 4-phosphate 5-kinase has been published. Use of both dominant-negative mutants of phosphoinositide 3-kinase and the inhibitors wortmannin and LY294002 has identified a number of processes in which phosphoinositide 3-kinase participates, including cell motility, the Ras pathway, vesicle trafficking and secretion, and apoptosis. Several possible biochemical targets of phosphoinositides have been found.

A novel 55-kDa regulatory subunit for phosphatidylinositol 3-kinase structurally similar to p55PIK Is generated by alternative splicing of the p85alpha gene

Phosphatidylinositol 3-kinase, which is composed of a 110-kDa catalytic subunit and a regulatory subunit, plays important roles in various cellular signaling mechanisms. We screened a rat brain cDNA expression library with 32P-labeled human IRS-1 protein and cloned cDNAs that were very likely to be generated by alternative splicing of p85alpha gene products. These cDNAs were demonstrated to encode a 55-kDa protein (p55alpha) containing two SH2 domains and an inter-SH2 domain of p85alpha but neither a bcr domain nor a SH3 homology domain. Interestingly, p55 alpha contains a unique 34-amino acid sequence at its NH2 terminus, which is not included in the p85alpha amino acid sequence. This 34-amino acid portion was revealed to be comparable with p55PIK (p55gamma) in length, with a high homology between the two, suggesting that these NH2-terminal domains of p55alpha and p5 gamma may have a specific role that p85 does not. The expression of p55alpha mRNA is most abundant in the brain, but expression is ubiquitous in most rat tissues. Furthermore, it should be noted that the expression of p85alpha mRNA in muscle is almost undetectably low by Northern blotting with a cDNA probe coding for the p85alpha SH3 domain, while the expression of p55alpha can be readily detected. These results suggest that p55 alpha may play an unique regulatory role for phosphatidylinositol 3-kinase in brain and muscle.

Insulin-like growth factor and epidermal growth factor independence in human mammary carcinoma cells with c-erbB-2 gene amplification and progressively elevated levels of tyrosine-phosphorylated p185erbB-2

Growth factor-independent proliferation is an essential aspect of the transformation process. To study the influence of c-erbB-2 overexpression on the autonomous growth of human mammary cancer cells, we used a series of non-neoplastic and neoplastic human mammary epithelial cell lines isolated from a patient with intraductal and invasive ductal carcinoma of the breast. The non-neoplastic cell line, H16N-2, which expresses a normal level (single gene copy) of c-erbB-2, was used for comparison with the neoplastic cell lines. Both the metastatic tumor cell lines, 21MT-1 and 21 MT-2, showed equivalent amplification of the c-erbB-2 gene; however, 21MT-1 cells showed a higher level of c-erbB-2 overexpression. Therefore, the H16N-2, 21MT-2, and 21MT-1 cell series forms a distinct gradient of progressively increasing c-erbB-2 gene expression. Furthermore, the overexpression of c-erbB-2 in the 21MT cell lines was concordant with increases in the constitutive tyrosine kinase activity of p185erb-2 measured in the absence of exogenous growth factors in culture. Normal mammary epithelial cells require both insulin-like growth factor (IGF)-l (or supraphysiological concentrations of insulin) and epidermal growth factor (EGF) to proliferate under serum-free conditions in culture. By contrast, 21MT-2 cells showed a reduced requirement for IGF but still required EGF to proliferate. 21MT-1 cells did not require either insulin or EGF to proliferate. Therefore, the progressive increases in constitutive p185erbB-2, tyrosine kinase activity in the 21MT-2 and 21MT-1 cell lines was directly correlated with IGF independence and combined IGF and EGF independence under defined conditions in culture. Experiments using conditioned media and anti-IGF-1 receptor and anti-EGF receptor neutralizing antibodies showed that the growth-factor independence of the tumor cells did not involve detectable IGF- or EGF-like autocrine activity expressed by the 21MT cells. Furthermore, neu differentiation factor/heregulin, a ligand that indirectly activates p185erbB-2 by direct binding to erbB-3 receptors, potently stimulated the proliferation of the growth factor-dependent H16N-2 cells (which expressed c-erbB-2 and c-erbB-3 but not c-erbB-4) in the absence of both IGF and EGF. Thus, HRG-induced mitogenesis mimicked the autonomous growth seen in the 21MT cells that have the highest level of constitutive p185erbB-2 activation. These data support the hypothesis that the constitutive activation of p185erbB-2 in human mammary carcinoma cells causes growth-factor independence by directly activating multiple signal-transduction pathways that substitute for both IGF and EGF during proliferation.

Phosphoinositide 3-kinases and membrane traffic

Phosphoinositide 3-kinases (PI 3-kinases) and their 3-phosphoinositide products were identified initially as components of intracellular signalling pathways emanating from cell surface receptors. A new role for 3-phosphoinositides in the constitutive movement o f proteins from one intracellular compartment to another was proposed with the discovery of homology between the product of a yeast gene important for vacuolar sorting, Vps34p, and a mammalian PI 3-kinase. Recent studies have implicated PI 3-kinase as an essential component in membrane traffic at specific steps o f the trans-Golgi-network-endosomal pre-lysosomal system. Evidence largely emerging from the insulin-stimulated glucose transport system suggests that PI 3-kinase may also mediate the effects o f growth factors on membrane traffic events. These studies suggest a possible link between growth-factor-stimulated and constitutive membrane traffic in the endosomal system.

New connections in the regulation of PEPCK gene expression by insulin

Phosphoenolpyruvate carboxykinase (PEPCK) catalyses the rate-limiting step in hepatic gluconeogenesis. Glucagon (via the second messenger cAMP) and glucocorticoids stimulate transcription of the PEPCK gene whereas insulin and phorbol esters have a dominant inhibitory effect. Wortmannin, an inhibitor of 1-phosphatidylinositol 3-kinase (PI 3-kinase), blocks the inhibition of glucocorticoid- and cAMP-stimulated PEPCK gene transcription by insulin. By contrast, although phorbol esters mimic the action of insulin on the regulation of PEPCK gene transcription, wortmannin does not block the effect of these agents. Thus PI 3-kinase is required for the regulation of PEPCK gene expression by insulin but not by phorbol esters. In liver cells, insulin administration stimulates the activity of multiple protein kinases, including the p42/p44 Mitogen Activated Protein (MAP) kinase and the p70/p85 ribosomal protein S6 kinase. Selective inhibition of the activation of either kinase, utilizing the compounds PD98059 and rapamycin respectively, does not affect insulin regulation of PEPCK gene transcription. Thus regulation of PEPCK gene transcription requires PI 3-kinase but does not require the activation of either p42/p44 MAP kinase or p70/p85 ribosomal protein S6 kinase.

EGF or PDGF receptors activate atypical PKClambda through phosphatidylinositol 3-kinase

Overexpression of a TPA-insensitive PKC member, an atypical protein kinase C (aPKClambda), results in an enhancement of the transcriptional activation of TPA response element (TRE) in cells stimulated with epidermal growth factor (EGF) or platelet-derived growth factor (PDGF). EGF or PDGF also caused a transient increase in the in vivo phosphorylation level and a change in the intracellular localization of aPKClambda from the nucleus to the cytosol, indicating the activation of aPKClambda in response to this growth factor stimulation. These immediate signal-dependent changes in aKPClambda were observed for a PDGF receptor add-back mutant (Y40/51) that possesses only two of the five major autophosphorylation sites and binds PI3-kinase, and were inhibited by wortmannin, an inhibitor of PI3-kinase. Furthermore, an N-terminal fragment of the catalytic subunit of PI3-kinase, p110alpha, inhibited aPKClambda-dependent activation of TRE in Y40/51 cells stimulated with PDGF. Overexpression of p110alpha resulted in an enhancement of TRE expression in response to PDGF and the regulatory domain of aPKClambda inhibited this TRE activation in Y40/51 cells. These results provide the first in vivo evidence supporting the presence of a novel signalling pathway from receptor tyrosine kinases to aPKClambda through PI3-kinase.

Inhibition of phosphatidylinositol 3-kinase blocks T cell antigen receptor/CD3-induced activation of the mitogen-activated kinase Erk2

The production of 3-phosphorylated inositol phospholipids is implicated in regulation of cell growth and transformation. To explore the role of these lipids in T cell antigen receptor (TCR)/CD3-induced signaling, we have examined the effects of a specific phosphatidylinositol 3-kinase (PtdIns3K) inhibitor, wortmannin, and overexpression of two PtdIns3K constructs on the activation of down-stream effectors in anti-CD3 treated T cells. We report that treatment of cells with wortmannin blocked anti-CD3-induced activation of the mitogen-activation kinase Erk2 while not affecting phorbol-ester-induced Erk2 activation. An inactive analog of wortmannin, WM12, did not affect TCR/CD3-induced Erk2 activation, and wortmannin had no effect on the activity of Erk2 when added directly to the in vitro assays. Expression of a disruptive PtdIns3K construct also reduced Erk2 activation, while a construct that stimulates PtdIns3K enhanced the activation of Erk2. Receptor-induced activation of other Ser/Thr kinases, such as c-Raf, B-Raf, Mek1, Mek2, Mekk, was not affected by wortmannin. Our results suggest that the production of 3-phosphorylated inositol phospholipids is involved in the activation of Erk2, but does not regulate the enzymes that are thought to be upstream of Erk2.

Initiation and maintenance of NGF-stimulated neurite outgrowth requires activation of a phosphoinositide 3-kinase

Application of nerve growth factor (NGF) to PC12 cells stimulates a programme of physiological changes leading to the development of a sympathetic neuron like phenotype, one aspect of which is the development of a neuronal morphology characterised by the outgrowth of neuritic processes. We have investigated the role of phosphoinositide 3-kinase in NGF-stimulated morphological differentiation through two approaches: firstly, preincubation with wortmannin, a reputedly specific inhibitor of phosphoinositide kinases, completely inhibited initial morphological responses to NGF, the formation of actin filament rich microspikes and subsequent neurite outgrowth. This correlated with wortmannin inhibition of NGF-stimulated phosphatidylinositol(3,4,5)trisphosphate (PtdInsP3) and phosphatidylinositol(3,4)bisphosphate (PtdIns(3,4)P2) production and with inhibition of NGF-stimulated phosphoinositide 3-kinase activity in anti-phosphotyrosine immunoprecipitates. Secondly, the overexpression of a mutant p85 regulatory subunit of the phosphoinositide 3-kinase, which cannot interact with the catalytic p110 subunit, also substantially inhibited the initiation of NGF-stimulated neurite outgrowth. In addition, we found that wortmannin caused a rapid collapse of more mature neurites formed following several days exposure of PC12 cells to NGF. These results indicate that NGF-stimulated neurite outgrowth requires the activity of a tyrosine kinase regulated PI3-kinase and suggest that the primary product of this enzyme, PtdInsP3, is a necessary second messenger for the cytoskeletal and membrane reorganization events which occur during neuronal differentiation.

Insulin regulation of phosphoenolpyruvate carboxykinase gene expression does not require activation of the Ras/mitogen-activated protein kinase signaling pathway

Expression of phosphoenolpyruvate carboxykinase (PEPCK), the rate-limiting step in hepatic gluconeogenesis, is primarily regulated at the level of gene transcription. Insulin and phorbol esters inhibit basal PEPCK transcription and antagonize the induction of PEPCK gene expression by glucocorticoids and glucagon (or its second messenger cAMP). Insulin activates a signaling cascade involving Ras --> Raf --> p42/p44 mitogen-activated protein (MAP) kinase kinase (MEK) --> p42/p44 MAP kinase (ERK 1 and 2). Recent reports suggest that activation of this Ras/MAP kinase pathway is critical for the effects of insulin on mitogenesis and c-fos transcription but is not required for insulin action on metabolic processes such as glycogen synthesis, lipogenesis, and Glut-4-mediated glucose transport. We have used three distinct approaches to examine the role of the Ras/MAP kinase pathway in the regulation of PEPCK transcription by insulin in H4IIE-derived liver cells: (i) chemical inhibition of Ras farnesylation, (ii) infection of cells with an adenovirus vector encoding a dominant-negative mutant of Ras, and (iii) use of a chemical inhibitor of MEK. Although each of these methods blocks insulin activation of MAP kinase, none alters insulin antagonism of cAMP- and glucocorticoid-stimulated PEPCK transcription. Although phorbol esters activate MAP kinase and mimic the effects of insulin on PEPCK gene transcription, inhibition of MEK has no effect on phorbol ester inhibition of PEPCK gene transcription. Using the structurally and mechanistically distinct phosphatidylinositol 3-kinase (PI 3-kinase) inhibitors, wortmannin and LY 294002, we provide further evidence supporting a role for PI 3-kinase activation in the regulation of PEPCK gene transcription by insulin. We conclude that neither insulin nor phorbol ester regulation of PEPCK gene transcription requires activation of the Ras/MAP kinase pathway and that insulin signaling to the PEPCK promoter is dependent on PI 3-kinase activation.

Cell cycle regulation of p70 S6 kinase and p42/p44 mitogen-activated protein kinases in Swiss mouse 3T3 fibroblasts

We show here using synchronized Swiss mouse 3T3 fibroblasts that p70 S6 kinase (p70S6k) and mitogen-activated protein kinases (p42mapk/p44mapk) are not only activated at the G0/G1 boundary, but also in cells progressing from M into G1. p70S6k activity increases 20-fold in G1 cells released from G0. Throughout G1, S, and G2 it decreases constantly, so that during M phase low kinase activity is measured. The kinase is reactivated 10-fold when cells released from a nocodazole-induced metaphase block enter G1 of the next cell cycle. p42mapk/p44mapk in G0 cells are activated transiently early in G1 and are reactivated late in mitosis after nocodazole release. p70S6k activity is dependent on permanent signaling from growth factors at all stages of the cell cycle. Immunofluorescence studies showed that p70S6k and its isoform p85S6k become concentrated in localized spots in the nucleus at certain stages in the cell cycle. Cell cycle-dependent changes in p70S6k activity are associated with alterations in the phosphorylation state of the protein. However, examination of the regulation of a p70S6k mutant in which the four carboxyl-terminal phosphorylation sites are changed to acidic amino acids suggests that a mechanism independent of these phosphorylation sites controls the activity of the enzyme during the cell cycle.

The 3-phosphorylated phosphoinositide response of 3T3-L1 preadipose cells exposed to insulin, insulin-like growth factor-1, or platelet-derived growth factor

We compared the pattern of 3-phosphorylated phosphoinositides produced by confluent 3T3-L1 preadipose cells upon exposure to growth factors that either induce differentiation (insulin, insulin-like growth factor-1) or do not (platelet-derived growth factor). Following addition of insulin or insulin-like growth factor-1, PI(3,4,5)P3 rapidly rose, on average, to levels tenfold over basal. PI(3,4)P2 either did not change (after insulin) or slightly increased (1.5 fold). Time course studies with insulin demonstrated that the rise in PI(3,4,5)P3 peaked by 1 minute, and levels then remained steady over 30 minutes. Dose-response experiments showed that insulin at a concentration of 1 nM was sufficient for the PI(3,4,5)P3 response. Insulin failed to increase PI(3,4)P2 at any of the time points or at any of the doses used. In contrast, after addition of platelet-derived growth factor, both PI(3,4)P2 and PI(3,4,5)P3 rose concurrently and to comparable extents. These data suggest that one possible mechanism contributing to insulin/insulin-like growth factor-1-induced 3T3-L1 preadipose cell differentiation is a distinct pattern of 3-phosphorylated phosphoinositide accumulation, defined by a prominent increase in PI(3,4,5)P3 with no (in the case of insulin), or a minimal (in the case of IGF-1), rise in PI(3,4)P2.

Purification and biochemical characterization of a mammalian phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase

Characterization of the enzymes involved in metabolism of 3-phosphorylated inositol lipids and their subcellular localization revealed that in vitro a 5-phosphatase activity was responsible for the degradation of phosphatidylinositol 3,4,5-trisphosphate, whereas a 3-phosphatase activity hydrolyzed phosphatidylinositol 3-phosphate and/or phosphatidylinositol 3,4-bisphosphate. All these activities were localized in the cytosol. No phospholipase activities were detected. The cytosolic phosphatidylinositol 3,4,5-trisphosphate 5-phosphatase activity was purified to near homogeneity using ion exchange, affinity, and size exclusion chromatography. Characterization of the purified phosphatase revealed that it is a magnesium-dependent 5-phosphatase that is able to hydrolyze phosphatidylinositol 4,5-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. The enzyme is only partially inhibited by neomycin and vanadate but is strongly inhibited by phosphatidylinositol 4,5-bisphosphate and to a slightly lesser extent by phosphatidylinositol 4-phosphate.

Vanadium salts stimulate mitogen-activated protein (MAP) kinases and ribosomal S6 kinases

Effect of several vanadium salts, sodium orthovanadate, vanadyl sulfate and sodium metavanadate on protein tyrosine phosphorylation and serine/threonine kinases in chinese hamster ovary (CHO) cells overexpressing a normal human insulin receptor was examined. All the compounds stimulated protein tyrosine phosphorylation of two major proteins with molecular masses of 42 kDa (p42) and 44 kDa (p44). The phosphorylation of p42 and p44 was associated with an activation of mitogen activated protein (MAP) kinase as well as increased protein tyrosine phosphorylation of p42mapk and p44mapk. Vanadium salts also activated the 90 kDa ribosomal s6 kinase (p90rsk) and 70 kDa ribosomal s6 kinase (p70s6k). Among the three vanadium salts tested, vanadyl sulfate appeared to be slightly more potent than others in stimulating MAP kinases and p70s6k activity. It is suggested that vanadium-induced activation of MAP kinases and ribosomal s6 kinases may be one of the mechanisms by which insulin like effects of this trace element are mediated.