cDNA cloning of a novel 85 kd protein that has SH2 domains and regulates binding of PI3-kinase to the PDGF beta-receptor

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.

Controlled overexpression of selected domains of the P85 subunit of phosphatidylinositol 3-kinase reverts v-Ha-Ras transformation

Selected domains of the regulatory p85 subunit of phosphatidylinositol 3-kinase have been expressed under the control of the tetracycline transactivator in NIH 3T3 fibroblasts transformed by the v-Ha-Ras oncogene. The domains expressed were the SH3 domain, the BCR homology domain, the region between the two SH2 domains which contains the p110 binding site (the inter SH2 (IS) domain), and the C-terminal (CT) domain (containing both SH2 domains and the IS domain). The levels of IS or SH3 domain expressed in the presence of tetracycline were sufficient to reverse the transforming effects of v-Ha-Ras, and no further inhibition of proliferation was observed when expression was increased 7-fold by removal of tetracycline. In contrast inhibition of proliferation by the CT domain was observed only when the level of expression was increased 5-fold by removal of tetracycline. Overexpression of the BCR domain of p85 had no effect on v-Ha-Ras transformation. Expression of the IS domain disrupted the interaction of the p85 regulatory subunit with the p110 catalytic subunit. These results indicate that the association of the p85 subunit of PI 3-kinase with the p110 subunit is necessary for v-Ha-Ras-induced transformation in NIH 3T3 cells.

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.

Synergistic activation of PtdIns 3-kinase by tyrosine-phosphorylated peptide and beta gamma-subunits of GTP-binding proteins

Stimulation of differentiated THP-1 cells by insulin led to rapid accumulation of PtdIns(3,4,5)P3, a product of PtdIns 3-kinase. Stimulation of the GTP-binding-protein-linked receptor by N-formylmethionyl-leucyl-phenylalanine (fMLP) also induced the accumulation of PtdIns(3,4,5)P3 in the cells. The effect of insulin was, while that of fMLP was not, accompanied by increased PtdIns 3-kinase activity in the anti-phosphotyrosine immuno-precipitate. The combination of insulin and fMLP induced more PtdIns(3,4,5)P3 production than the sum of the individual effects. The insulin-induced recruitment of PtdIns 3-kinase activity in the anti-phosphotyrosine immunoprecipitate was unaffected by the combined treatment with fMLP. To investigate the mechanism underlying the synergistic accumulation of PtdIns(3,4,5)P3, we separated the cytosolic proteins of THP-1 cells on a Mono Q column. PtdIns 3-kinase activities were eluted in two peaks, and one of the peaks markedly increased on the addition of beta gamma-subunits of GTP-binding proteins (G beta gamma). The other peak was affected only slightly by G beta gamma, but was synergistically increased by G beta gamma and a tyrosine-phosphorylated peptide which was synthesized accordingly to the amino acid sequence of insulin receptor substrate-1. The activity in the latter fraction was completely immunoprecipitated by an antibody against the regulatory subunit of PtdIns 3-kinase (p85). These results suggest that the conventional PtdIns 3-kinase (p85/p110), which has been implicated in insulin-induced cellular events, or a closely related isoenzyme is controlled by a combination of a tyrosine-phosphorylated protein and a GTP-binding protein in intact cells.

Alternative binding of p56lck and phosphatidylinositol 3-kinase in T cells by sulfhydryl oxidation: implication of aberrant signaling due to oxidative stress in T lymphocytes

Recent studies of the physiological effects induced by oxidative stress have revealed that not only does oxidative stress causes random and indiscriminate injury on cells or tissues but it may evoke a cascade of signaling, by which cells may manage themselves to counter the stress. We have previously reported that sulfhydryl oxidation induces tyrosine phosphorylation and activation of a src family protein tyrosine kinase, p56lck, in T lymphocytes (Nakamura et al., 1993, Oncogene 8, 3133-3139). However, the possible difference between receptor-mediated signals and oxidative stress-mediated signals is not clear yet. In this study using cultured peripheral blood T lymphocytes (PBL blasts), we show that upon the sulfhydryl oxidation-induced tyrosine phosphorylation of p56lck, the kinase associates with phosphatidylinositol (PI) 3-kinase p85 subunit via the binding of the C-terminal SH2 domain of p85 to the tyrosine-phosphorylated p56lck. This is in contrast to the association of these two molecules in the case of CD4-p56lck cross-linking or interleukin-2 stimulation, where PI 3-kinase p85 subunit binds to the SH3 or SH3/SH2 domain(s) of p56lck. Thus our results indicate the possibility that T cells may utilize an alternative signaling machinery upon an oxidative stress-induced activation of a src family protein tyrosine kinase, p56lck.

Wortmannin inhibits mitogen-activated protein kinase activation by platelet-activating factor through a mechanism independent of p85/p110-type phosphatidylinositol 3-kinase

We have shown previously that wortmannin partially inhibits mitogen-activated protein kinase (MAPK) activated by platelet-activating factor (PAF) in guinea pig neutrophils (Ferby, M. I., Waga, I., Sakanaka, C., Kume, K., and Shimizu, T. (1994) J. Biol. Chem. 269, 30485-30488). To identify whether p85-dependent phosphatidylinositol 3-kinase is a target molecule of wortmannin in this inhibitory process, we established a murine macrophage cell line (P388D1), inducibly expressing a dominant-negative p85, delta p85. Upon induction of delta p85 by isopropyl-beta-D-thiogalactopyranoside, PAF still induced unaltered activation of MAPK, which was inhibited completely by wortmannin and 1,2-bis-(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester in an additive manner. Thus, PAF activates MAPK in P388D1 cells via two distinct pathways, one calcium-dependent and another calcium-independent, but wortmannin-sensitive. The inhibition of calcium-independent activation of MAPK by wortmannin does not involve p85-dependent phosphatidylinositol 3-kinase.

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 receptor substrate 1 binds two novel splice variants of the regulatory subunit of phosphatidylinositol 3-kinase in muscle and brain

We have identified two novel alternatively spliced forms of the p85alpha regulatory subunit of phosphatidylinositol (PI) 3-kinase by expression screening of a human skeletal muscle library with phosphorylated baculovirus- produced human insulin receptor substrate 1. One form is identical to p85alpha throughout the region which encodes both Src homology 2 (SH2) domains and the inter-SH2 domain/p110 binding region but diverges in sequence from p85alpha on the 5' side of nucleotide 953, where the entire break point cluster gene and SH3 regions are replaced by a unique 34-amino-acid N terminus. This form has an estimated molecular mass of approximately 53 kDa and has been termed p85/AS53. The second form is identical to p85 and p85/AS53 except for a 24-nucleotide insert between the SH2 domains that results in a replacement of aspartic acid 605 with nine amino acids, adding two potential serine phosphorylation sites in the vicinity of the known serine autophosphorylation site (Ser-608). Northern (RNA) analyses reveal a wide tissue distribution of p85alpha, whereas p85/AS53 is dominant in skeletal muscle and brain, and the insert isoforms are restricted to cardiac muscle and skeletal muscle. Western blot (immunoblot) analyses using an anti-p85 polyclonal antibody and a specific anti-p85/AS53 antibody confirmed the tissue distribution of p85/AS53 protein and indicate a approximately 7-fold higher expression of p85/AS53 protein than of p85 in skeletal muscle. Both p85 and p85/AS53 bind to p110 in coprecipitation experiments, but p85alpha itself appears to have preferential binding to insulin receptor substrate 1 following insulin stimulation. These data indicate that the gene for the p85alpha regulatory subunit of PI 3-kinase can undergo tissue-specific alternative splicing. Two novel splice variants of the regulatory subunit of PI 3-kinase are present in skeletal muscle, cardiac muscle, and brain; these variants may have important functional differences in activity and may play a role in tissue-specific signals such as insulin-stimulated glucose transport or control of neurotransmitter secretion or action.

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.

Rac GTPase interacts specifically with phosphatidylinositol 3-kinase

The Rac GTP-binding proteins are members of the Rho family and regulate growth factor-stimulated actin assembly in a variety of cells. The formation of phosphorylated inositol lipids has been implicated in control of the processes initiating and regulating such actin polymerization. Associations of Rho family GTP-binding proteins with enzymes involved in lipid metabolism have been described. Here we demonstrate a direct and specific interaction of Rac proteins with phosphatidylinositol (PI) 3-kinase. This interaction is dependent upon Rac being in a GTP-bound state and requires an intact Rac effector domain. In contrast, direct binding of RhoA to PI 3-kinase could not be detected. Rac-GTP also bound to PI 3-kinase in Swiss 3T3 fibroblast and human neutrophil lysates, and increased PI 3-kinase activity became associated with Rac-GTP in platelet-derived growth factor-stimulated cells. Interaction of Rac-GTP with PI 3-kinase in vitro stimulated the activity of the enzyme by 2-9-fold. A specific interaction of active Rac with PI 3-kinase might be important in regulation of the actin cytoskeleton.

Wortmannin inactivates phosphoinositide 3-kinase by covalent modification of Lys-802, a residue involved in the phosphate transfer reaction

Wortmannin at nanomolar concentrations is a potent and specific inhibitor of phosphoinositide (PI) 3-kinase and has been used extensively to demonstrate the role of this enzyme in diverse signal transduction processes. At higher concentrations, wortmannin inhibits the ataxia telangiectasia gene (ATM)-related DNA-dependent protein kinase (DNA-PKcs). We report here the identification of the site of interaction of wortmannin on the catalytic subunit of PI 3-kinase, p110alpha. At physiological pH (6.5 to 8) wortmannin reacted specifically with p110alpha. Phosphatidylinositol-4,5-diphosphate, ATP, and ATP analogs [adenine and 5'-(4-fluorosulfonylbenzoyl)adenine] competed effectively with wortmannin, while substances containing nucleophilic amino acid side chain functions had no effect at the same concentrations. This suggests that the wortmannin target site is localized in proximity to the substrate-binding site and that residues involved in wortmannin binding have an increased nucleophilicity because of their protein environment. Proteolytic fragments of wortmannin-treated, recombinant p110alpha were mapped with anti-wortmannin and anti-p110alpha peptide antibodies, thus limiting the target site within a 10-kDa fragment, colocalizing with the ATP-binding site. Site-directed mutagenesis of all candidate residues within this region showed that only the conservative Lys-802-to-Arg mutation abolished wortmannin binding. Inhibition of PI 3-kinase occurs, therefore, by the formation of an enamine following the attack of Lys-802 on the furan ring (at C-20) of wortmannin. The Lys-802-to-Arg mutant was also unable to bind FSBA and was catalytically inactive in lipid and protein kinase assays, indicating a crucial role for Lys-802 in the phosphotransfer reaction. In contrast, an Arg-916-to-Pro mutation abolished the catalytic activity whereas covalent wortmannin binding remained intact. Our results provide the basis for the design of novel and specific inhibitors of an enzyme family, including PI kinases and ATM-related genes, that play a central role in many physiological processes.

Functional interactions of phosphatidylinositol 3-kinase with GTPase-activating protein in 3T3-L1 adipocytes

The role of phosphatidylinositol (PI) 3-kinase in specific aspects of insulin signaling was explored in 3T3-L1 adipocytes. Inhibition of PI 3-kinase activity by LY294002 or wortmannin significantly enhanced basal and insulin-stimulated GTPase-activating protein (GAP) activity in 3T3-L1 adipocytes. Furthermore, removal of the inhibitory influence of PI 3-kinase on GAP resulted in dose-dependent decreases in the ability of insulin to stimulate p21ras. This effect was specific to adipocytes, as inhibition of PI 3-kinase did not influence GAP in either 3T3-L1 fibroblasts, Rat-1 fibroblasts, or CHO cells. Immunodepletion of either of the two subunits of the PI 3-kinase (p85 or p110) yielded similar activation of GAP, suggesting that catalytic activity of p110 plays an important role in controlling GAP activity in 3T3-L1 adipocytes. Inhibition of PI 3-kinase activity in 3T3-L1 adipocytes resulted in abrogation of insulin-stimulated glucose uptake and thymidine incorporation. In contrast, effects of insulin on glycogen synthase and mitogen-activated protein kinase activity were inhibited only at higher concentrations of LY294002. It appears that in adipocytes, P1 3-kinase prevents activation of GAP. Inhibition of PI 3-kinase activity or immunodepletion of either one of its subunits results in activation of GAP and decreases in GTP loading of p21ras.

Crystal structure of the PI 3-kinase p85 amino-terminal SH2 domain and its phosphopeptide complexes

Crystal structures of the amino-terminal SH2 domain of the p85alpha subunit of phosphatidylinositol (PI) 3-kinase, alone and in complex with phosphopeptides bearing pTyr-Met/Val-Xaa-Met motifs, show that phosphopeptides bind in the two-pronged manner seen in high-affinity Lck and Src SH2 complexes, with conserved interactions between the domain and the peptide segment from phosphotyrosine to Met+3. Peptide binding requires the rearrangement of a tyrosyl side chain in the BG loop to create the hydrophobic Met+3 binding pocket. The structures suggest a mechanism for the biological specificity exhibited by PI 3-kinase in its interactions with phosphoprotein partners.

Association of phosphatidylinositol 3-kinase, via the SH2 domains of p85, with focal adhesion kinase in polyoma middle t-transformed fibroblasts

Focal adhesion kinase (FAK), a non-receptor protein tyrosine kinase, becomes activated and phosphorylated on tyrosine in cells transformed with v-src. By cytoimmunofluorescence a sub-fraction of the p85 subunit of phosphoinositide 3-kinase (PI 3-kinase) localized in focal adhesion plaques. We examined the possibility that FAK associates with PI 3-kinase. In fibroblasts transformed with polyoma middle t, PI 3-kinase activity co-immunoprecipitated with pp125FAK using two different antibodies against this protein. PP125FAK from middle t-transformed cells associated with a glutathione-S-transferase fusion protein containing the 85-kDa subunit of phosphatidylinositol 3-kinase. Both of the SH2 domains and the SH3 domain of p85 also formed complexes with pp125FAK in vitro. Phosphopeptides that bind to the SH2 domains completely blocked the binding of full-length p85 to pp125FAK, while a peptide that binds to the SH3 domain was ineffective, indicating that the association between p85 and pp125FAK is mediated by the SH2 domains of p85.

Phosphoinositide 3-kinase gamma and p85/phosphoinositide 3-kinase in platelets. Relative activation by thrombin receptor or beta-phorbol myristate acetate and roles in promoting the ligand-binding function of alphaIIbbeta3 integrin

Platelets exposed to thrombin or thrombin receptor agonist peptide (SFLLRN) activate phospholipase C and protein kinase C (PKC), and accumulate 3-phosphorylated phosphoinositides (3-PPI) as a function of the activation and relocalization of two cytoskeletally-associated phosphoinositide 3-kinases (PI 3-K): p85/PI 3-K and PI 3-Kgamma. We now report that exposure of platelets to PKC-activating beta-phorbol myristate acetate (betaPMA) does not stimulate PI 3-Kgamma, but rather stimulates p85/PI 3-K, which associates with the cytoskeleton. Wortmannin is an inhibitor of both PI 3-Ks, known to act with more potency on p85/PI 3-K. betaPMA-stimulated 3-PPI accumulation is more sensitive to wortmannin (IC50 = 1.3 nM) than is SFLLRN- or thrombin-stimulated 3-PPI accumulation (IC50 = 10 nM). The activity of p85/PI 3-K in immunoprecipitates or in cytoskeletal fractions is inhibited more potently by exposure of platelets to wortmannin than is the activity of PI 3-Kgamma. betaPMA or SFLLRN promotes the conversion of platelet integrin alphaIIb/beta3 into a fibrinogen-binding form required for platelet aggregation. Activation of alphaIIb/beta3 in response to betaPMA or SFLLRN is inhibited by wortmannin with an IC50 of 1 nM in each case. Wortmannin inhibits neither activation of alphaIIb/beta3 by ligand-induced binding site antibody (anti-LIBS6 Fab) nor anti-LIBS6 Fab-induced platelet aggregation in the presence of fibrinogen, indicating that this type of "outside-in" signaling by alphaIIb/beta3 is largely PI 3-K-independent. We conclude that p85/PI 3-K, in preference to PI 3-Kgamma, contributes to activation of alphaIIb/beta3 when the thrombin receptor or PKC is stimulated.

Anti-tumor activity of cytokines against opportunistic vascular tumors in mice

Polyoma middle T (PmT)-transformed endothelial cells may represent a unique murine model for human opportunistic vascular tumors. The present study was designed to evaluate the anti-tumor potential of a panel of 13 cytokines against murine PmT-transformed endothelial cells. Interferon gamma (IFNgamma) and transforming growth factor beta 1 (TGFbeta1) substantially decreased in a dose-dependent manner the proliferation of a panel of 6 PmT-transformed cell lines. IFNalpha and tumor necrosis factor alpha(TNFalpha) had marginal anti-proliferative activity, whereas other molecules (interleukins-1, -2, -4, -6 and -13, IFNbeta, leukemia inhibitory factor, oncostatin M, granulocyte-macrophage colony-stimulating factor) caused no growth inhibition. IFNgamma and TGFbeta1 were therefore selected for further analysis of their mechanism of action and in vivo relevance. IFNgamma and TGFbeta1 reduced the activity of phosphatidylinositol-3-kinase and the production of phosphatidylinositol 3,4-biphosphate, without modifying the tyrosine kinase(s) activity associated with PmT. IFNgamma and TGFbeta1 were also tested for their ability to modify the in vivo growth of the PmT-transformed endothelial cells H5V in syngeneic C57B1/6 mice. Treatment with IFNnu and TGFbeta1 significantly delayed tumor growth and increased survival time. In contrast, treatment with IFNalpha and TNFalpha failed to prolong survival. In nude mice, IFNgamma and TGFbeta1 had a transient effect on tumor growth but no effect on survival, suggesting a contribution of T cells to the in vivo anti-tumor activity of these cytokines.

Janus kinases in cytokine signalling

Hematopoiesis is largely regulated by the binding of cytokines to receptors of the cytokine receptor superfamily. Although lacking catalytic domains, members of the cytokine receptor superfamily mediate ligand dependent activation of tyrosine phosphorylation which is critical for all receptor functions. Recent studies have demonstrated that this is mediated through the association and activation of members of the Janus kinase (Jak) family of protein tyrosine kinases. The activated Jaks phosphorylate the receptors, creating docking sites for SH2 containing signalling proteins which are tyrosine phosphorylated following their association with the receptor complex. Among the substrates of tyrosine phosphorylation are members of the signal transducers and activators of transcription family of proteins (Stats). Various cytokines induce the tyrosine phosphorylation and activation of one or more of the six family members. The pattern of Stat activation provides a level of cytokine individuality that is not observed in the activation of other signalling pathways. Although not required for mitogenic responses, it is speculated that the Stats may mediate many of the cytokine specific functional responses of hematopoietic cells.

A heterotrimeric GTPase-regulated isoform of PI3K and the regulation of its potential effectors

We have purified two forms of phosphoinositide 3-kinase (PI3K) that are activated by heterotrimeric G-protein beta gamma-subunits. These novel isoforms of PI3K are structurally distinct to those forms of PI3K which have already been cloned. They are both heterodimers made up of a p120 and a p101 and a p117 and a p101 protein. The p101 species in both heterodimers are identical and show no substantial homology with any other proteins or DNA sequences. The p117 and p120 are highly related. The p101 and p120 species have been cloned from a pig neutrophil mRNA library. The p120 has similarities with other known PI3K catalytic subunits. They may be responsible for conferring cells with the capacity to produce phosphatidylinositol (3,4,5)-trisphosphate in response to activation of G-protein-coupled receptors. Activation of both the monomeric G-protein rac and PI3K(s) have been implicated in receptor-stimulated membrane-ruffling. We have observed that agonist-stimulated guanine nucleotide exchange on rac can be inhibited by a variety of PI3K inhibitors. This suggests PI3K may lie upstream of rac in receptor-driven pathways regulating cell movement.

SH2 domain protein interaction and possibilities for pharmacological intervention

SH2 domain containing proteins play a key role in the process of intracellular transmission of signalling events initiated at the cell surface. As a pre-requisite in the fulfillment of this function, these proteins bind to a variety of phospho-tyrosine (pY) containing target molecules. Delineation of these binding sites as essentially short linear peptides (both structurally and functionally) has led to the suggestion that the activity of these signalling complexes may be manipulated by the development of relatively simple peptide reagents. This review examines the range of possibilities open on this approach and the extent to which positive results have already been realised.

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.

A family of phosphoinositide 3-kinases in Drosophila identifies a new mediator of signal transduction

BACKGROUND

Mammalian phosphoinositide 3-kinases (PI 3-kinases) are involved in receptor-mediated signal transduction and have been implicated in processes such as transformation and mitogenesis through their role in elevating cellular phosphatidylinositol (3,4,5)-trisphosphate. Additionally, a PI 3-kinase activity which generates phosphatidylinositol 3-phosphate has been shown to be required for protein trafficking in yeast.

RESULTS

We have identified a family of three distinct PI 3-kinases in Drosophila, using an approach based on the polymerase chain reaction to amplify a region corresponding to the conserved catalytic domain of PI 3-kinases. One of these family members, PI3K_92D, is closely related to the prototypical PI 3-kinase, p110 alpha; PI3K_59F is homologous to Vps34p, whereas the third, PI3K_68D, is a novel PI 3-kinase which is widely expressed throughout the Drosophila life cycle. The PI3K_68D cDNA encodes a protein of 210 kDa, which lacks sequences implicated in linking p110 PI 3-kinases to p85 adaptor proteins, but contains an amino-terminal proline-rich sequence, which could bind to SH3 domains, and a carboxy-terminal C2 domain. Biochemical analyses demonstrate that PI3K_68D has a novel substrate specificity in vitro, restricted to phosphatidylinositol and phosphatidylinositol 4-phosphate, and is unable to phosphorylate phosphatidylinositol (4,5)-bisphosphate, the implied in vivo substrate for p110.

CONCLUSIONS

A family of PI 3-kinases in Drosophila, including a novel class represented by PI3K_68D, is described. PI3K_68D has the potential to bind to signalling molecules containing SH3 domains, lacks p85-adaptor-binding sequences, has a Ca(2+)-independent phospholipid-binding domain and displays a restricted in vitro substrate specificity, so it could define a novel signal transduction pathway.

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.

Identification of a new adapter protein that may link the common beta subunit of the receptor for granulocyte/macrophage colony-stimulating factor, interleukin (IL)-3, and IL-5 to phosphatidylinositol 3-kinase

Binding of human granulocyte/macrophage colony-stimulating factor (hGM-CSF) to its receptor induces the rapid activation of phosphatidylinositol-3 kinase (PI 3-kinase). As hGM-CSF receptor (hGMR) does not contain a consensus sequence for binding of PI 3-kinase, hGMR must use a distinct mechanism for its association with and activation of PI 3-kinase. Here, we describe the identification of a tyrosine-phosphorylated protein of 76-85 kDa (p80) that associates with the common beta subunit of hGMR and with the SH2 domains of the p85 subunit of PI 3-kinase in hGM-CSF-stimulated cells. Src/Yes and Lyn were tightly associated with the p80.PI 3-kinase complex, suggesting that p80 and other phosphotyrosyl proteins present in the complex were phosphorylated by Src family kinases. Tyrosine phosphorylation of p80 was only detected in hGM-CSF or human interleukin-3-stimulated cells, suggesting that activation of p80 might be specific for signaling via the common beta subunit. We postulate that p80 functions as an adapter protein that may participate in linking the hGM-CSF receptor to the PI 3-kinase signaling pathway.

In vitro association of the phosphatidylinositol 3-kinase regulatory subunit (p85) with the human insulin receptor

The insulin receptor, as a consequence of ligand binding, undergoes autophosphorylation of critical tyrosyl residues within the cytoplasmic portion of its beta-subunit. The 85 kDa regulatory subunit of phosphatidylinositol (PI) 3-kinase (p85), an SH2 domain protein, has been implicated as a regulatory molecule in the insulin signal transduction pathway. For the present study, glutathione S-transferase (GST) fusion proteins of p85 SH2 domains were used to determine if such motifs associate directly with the autophosphorylated human insulin receptor. The p85 N + C (amino plus carboxyl) SH2 domains were demonstrated to associate with the autophosphorylated beta-subunit, while neither the GTPase activator protein (GAP) N SH2 domain nor the phospholipase C-gamma 1 (PLC gamma 1) N + C SH2 domains exhibited measurable affinity for the activated receptor. The p85 N SH2 domain demonstrated weak association with the insulin receptor, while the p85 C SH2 domain alone formed no detectable complexes with the insulin receptor. The association of p85 N + C SH2 domains with the autophosphorylated receptor was competed efficiently by a 15-residue tyrosine-phosphorylated peptide corresponding to the carboxyl-terminal region of the insulin receptor, but not by phosphopeptides of similar length derived from the juxtamembrane or regulatory regions. The insulin receptor C domain phosphopeptide inhibited the p85 N + C SH2 domain-insulin receptor complex with an IC0.5 of 2.3 +/- 0.35 microM, whereas a 10-residue phosphopeptide derived from the insulin receptor substrate 1 (IRS-1) competed with an IC0.5 of 0.54 +/- 0.10 microM. These results demonstrate that, in vitro, there is an association between the p85 regulatory protein and the carboxyl-terminal region of the activated insulin receptor that requires the presence of both the N and C SH2 domains. Furthermore, formation of the p85/insulin receptor complex may lead to signaling pathways independent of IRS-1.

The inability of phosphatidylinositol 3-kinase activation to stimulate GLUT4 translocation indicates additional signaling pathways are required for insulin-stimulated glucose uptake

Recent experimental evidence has focused attention to the role of two molecules, insulin receptor substrate 1 (IRS-1) and phosphatidylinositol 3-kinase (PI3-kinase), in linking the insulin receptor to glucose uptake; IRS-1 knockout mice are insulin resistant, and pharmacological inhibitors of PI3-kinase block insulin-stimulated glucose uptake. To investigate the role of PI3-kinase and IRS-1 in insulin-stimulated glucose uptake we examined whether stimulation of insulin-sensitive cells with platelet-derived growth factor (PDGF) or with interleukin 4 (IL-4) stimulates glucose uptake; the activated PDGF receptor (PDGFR) directly binds and activates PI3-kinase, whereas the IL-4 receptor (IL-4R) activates PI3-kinase via IRS-1 or the IRS-1-related molecule 4PS. We found that stimulation of 3T3-L1 adipocytes with PDGF resulted in tyrosine phosphorylation of the PDGFR and activation of PI3-kinase in these cells. To examine whether IL-4 stimulates glucose uptake, L6 myoblasts were engineered to overexpress GLUT4 as well as both chains of the IL-4R (L6/IL-4R/GLUT4); when these L6/IL-4R/GLUT4 myoblasts were stimulated with IL-4, IRS-1 became tyrosine phosphorylated and associated with PI3-kinase. Although PDGF and IL-4 can activate PI3-kinase in the respective cell lines, they do not possess insulin's ability to stimulate glucose uptake and GLUT4 translocation to the plasma membrane. These findings indicate that activation of PI3-kinase is not sufficient to stimulate GLUT4 translocation to the plasma membrane. We postulate that activation of a second signaling pathway by insulin, distinct from PI3-kinase, is necessary for the stimulation of glucose uptake in insulin-sensitive cells.

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.

Interaction of p85 subunit of PI 3-kinase with insulin and IGF-1 receptors analysed by using the two-hybrid system

Interaction of the p85 subunit of PI 3-kinase with the insulin receptor (IR) and the IGF-1 receptor (IGF-1R) was investigated using the two-hybrid system by assessing for his3 and lacZ activation in S. cerevisiae. The experiments were performed with the cytoplasmic beta domain (wild type or mutated) of IR and IGF-1R and p85 or its subdomains (N + C-SH2, N-SH2, C-SH2, SH3 + N-SH2). The results of his3 activation indicated that p85, N + C-SH2 and C-SH2 interact with both IR beta and IGF-1R beta, whereas N-SH2 and SH3 + N-SH2 interact only with IR beta. Interaction of p85 and N+C-SH2 with IR beta (delta C-43) or IGF-1R beta(delta C-43) in which the C-terminal 43 amino acids (including the YXXM motif) were deleted, persisted. The internal binding site thus revealed was not altered by further mutating Y960/F for IR or Y950/F for IGF-1R. Activation of lacZ upon interaction of p85 with IR beta(delta C-43) was 4-fold less as compared to IR beta. This activation with p85 and IGF-1R beta was 4-fold less as compared to IR beta and was somewhat increased (2-fold) for IGF-1R beta (delta C-43). Thus, the C-terminal domain in IGF-1R appears to exert a negative control on binding of p85 thereby providing a possible regulatory mechanism for direct activation of the PI 3-kinase pathway.

Wiskott-Aldrich syndrome protein physically associates with Nck through Src homology 3 domains

In the second of a series of experiments designed to identify p47nck-Src homology 3 (SH3)-binding molecules, we report the cloning of SAKAP II (Src A box Nck-associated protein II) from an HL60 cDNA expression library. This molecule has been identified as a cDNA encoding the protein product of WASP, which is mutated in Wiskott-Aldrich syndrome patients. Studies in vivo and in vitro demonstrated a highly specific interaction between the SH3 domains of p47nck and Wiskott-Aldrich syndrome protein. Furthermore, anti-Wiskott-Aldrich syndrome protein antibodies recognized a protein of 66 kDa by Western blot (immunoblot) analysis. In vitro translation studies identified the 66-kDa protein as the protein product of WASP, and subcellular fractionation experiments showed that p66WASP is mainly present in the cytosol fraction, although significant amounts are also present in membrane and nuclear fractions. The main p47nck region implicated in the association with p66WASP was found to be the carboxy-terminal SH3 domain.

An alternative role for the src-homology-domain-containing phosphotyrosine phosphatase (SH-PTP2) in regulating epidermal-growth-factor-dependent cell growth

The association of the src homology 2 (SH2) domain-containing tyrosine phosphatase (SH-PTP2) with the activated epidermal growth factor (EGF) and platelet-derived growth factor receptors, as well as the insulin receptor substrate 1 and growth-factor-receptor-bound protein 2 and its intrinsic tyrosine phosphatase activity suggests an important role for this phosphatase in signal transduction. Previous studies have shown a positive role for SH-PTP2 in growth-factor-mediated cell signaling. We show here that SH-PTP2 can also function to negatively regulate EGF-mediated signal transduction in the human glioma cell line SNB19. We demonstrate this by showing that, in SNB19 cells, which lack the ability to proliferate in response to EGF but retain the ability to bind EGF and also activate the EGF receptor as well as allow for the association of SH-PTP2 with the phosphorylated receptor, stable overexpression of an interfering SH-PTP2 mutant can restore the ability of these cells to proliferate in response to EGF.

Effects of wortmannin on glucose uptake and protein kinase C activity in rat adipocytes

Wortmannin is known to be an inhibitor of myosin light chain kinase and phosphatidylinositol 3-kinase (PI 3-kinase) (J. Biol. Chem. 268, 25846, 1993). We studied the effects of wortmannin on insulin- and 12-O-tetradecanoylphorbol 13-acetate (TPA)-induced glucose uptake, purified PKC activity and in vitro 80 kDa protein phosphorylation to elucidate the relationship between insulin-induced PI 3-kinase and PKC activations. Pretreatment with 10(-12)-10(-6) M wortmannin for 60 min resulted in a dose-responsive reduction of 10 nM insulin-stimulated glucose uptake in rat adipocytes. Pretreatment with 10(-6) M wortmannin resulted in 80% and 20% decreases of glucose uptake stimulated by insulin and TPA, respectively. Partially purified rat brain PKC activity and 80 kDa protein in vitro phosphorylation of rat adipocyte cytosol by addition of Ca2+ and phospholipid were dose-dependently decreased by 10(-8)-10(-6) M wortmannin; 20% decrease of PKC activity and 50% decrease of 80 kDa protein phosphorylation by 10(-6) M wortmannin were observed. These results suggest that wortmannin has a potent inhibitory effect on PI 3-kinase and a weak inhibitory effect on PKC activity, and both effects cause a significant inhibition of insulin-stimulated glucose uptake in rat adipocytes.

Specific association of the beta isoform of the p85 subunit of phosphatidylinositol-3 kinase with the proto-oncogene c-cbl

Phosphatidylinositol-3 kinase (PI-3 kinase) has been implicated in cellular events such as mitogenic signaling, actin organization, and receptor sorting. The p85 subunit of PI-3 kinase contains multiple domains capable of protein-protein interactions that may contribute to mediate the multiple physiological functions of this enzyme. Here, we demonstrate that antibodies raised against the p85 subunit of PI-3 kinase immunoprecipitate a single tyrosine-phosphorylated protein of 120 kDa (pp120) from lysates of activated Jurkat T cells and A20 B cells. This protein is the only significant phosphotyrosine-containing protein in p85 immunoprecipitates from these cells, and it cannot be detected in immunoprecipitates of other signaling proteins such as PLC gamma. Furthermore, antibodies specific for the beta isoform of p85 but not antibodies specific for the alpha isoform immunoprecipitate this tyrosine-phosphorylated protein. pp120 completely comigrates with the proto-oncogene c-cbl, which is a 120 kDa protein product abundant in lymphoid cells. Furthermore, immunoblots of p85 immunoprecipitates using antibodies raised against c-cbl detect a band at exactly the position of pp120. In addition, p85 can be detected in immunoblots of c-cbl immunoprecipitates. Thus, pp120 appears to correspond to c-cbl. A direct association between c-cbl and p85 can be observed in vitro using a fusion protein comprising the Src homology 2 (SH2) domains of p85, and this binding is abolished by phenyl phosphate, suggesting that the interaction is mediated through phosphotyrosine-SH2 domain interactions. Thus, these results show important functional differences between the alpha and beta isoforms of p85 in vivo and point to c-cbl as a potentially important mediator of some of the functions of PI-3 kinase in intact cells.

Role for phosphatidylinositol 3-kinase in the sorting and transport of newly synthesized lysosomal enzymes in mammalian cells

Previous work with the yeast Saccharomyces cerevisiae has demonstrated a role for a phosphatidylinositol-specific PI 3-kinase, the product of the VPS34 gene, in the targeting of newly synthesized proteins to the vacuole, an organelle functionally equivalent to mammalian lysosomes (Schu, P. V., K. Takegawa, M. J. Fry, J. H. Stack, M. D. Waterfield, and S. D. Emr. 1993. Science [Wash. DC]. 260:88-91). The activity of Vps34p kinase is significantly reduced by the PI 3-kinase inhibitors wortmannin, a fungal metabolite, and LY294002, a quercetin analog (Stack, J. H., and S. D. Emr. 1994. J. Biol. Chem. 269:31552-31562). We show here that at concentrations which inhibit VPS34-encoded PI 3-kinase activity, wortmannin also inhibits the processing and delivery of newly synthesized cathepsin D to lysosomes in mammalian cells with half-maximal inhibition of delivery occurring at 100 nM wortmannin. As a result of wortmannin action, newly synthesized, unprocessed cathepsin D is secreted into the media. Moreover, after accumulation in the trans-Golgi network (TGN) at 20 degrees C, cathepsin D was rapidly missorted to the secretory pathway after addition of wortmannin and shifting to 37 degrees C. At concentrations that inhibited lysosomal enzyme delivery, both wortmannin and LY294002 caused a highly specific dilation of mannose 6-phosphate receptor (M6PR)-enriched vesicles of the prelysosome compartment (PLC), which swelled to approximately 1 micron within 15 min after treatment. With increasing time, the inhibitors caused a significant yet reversible change in M6PR distribution. By 3 h of treatment, the swollen PLC vacuoles were essentially depleted of receptors and, in addition, there was a fourfold loss of receptors from the cell surface. However, M6PRs were still abundant in the TGN. These results are most consistent with the interpretation that PI 3-kinase regulates the trafficking of lysosomal enzymes by interfering with a M6PR-dependent sorting event in the TGN. Moreover, they provide evidence that trafficking of soluble hydrolases to mammalian lysosomes and yeast vacuoles rely on similar regulatory mechanisms.

Novel functions of phosphatidylinositol 3-kinase in terminally differentiated cells

Importance of phosphatidylinositol 3-kinase (PI 3-kinase) in signalling pathways leading to growth stimulation has already been reviewed in this journal and others. Evidence has now been accumulating that PI 3-kinase is involved in transmission of activation signals in terminally differentiated cells, especially signals starting from receptors which have no intrinsic tyrosine kinase domain. The pioneer works showed the presence of PI 3-kinase activity and the accumulation of the reaction products of PI 3-kinase correlated with the cell responses. However, these studies were done in only limited cell responses such as respiratory burst in neutrophils and degranulation in platelets. Recent finding of a potent and selective inhibitor of PI 3-kinase, wortmannin, reported from three independent groups including us, gave a new and powerful tool not only to confirm the suggested functions but also to reveal new functions of PI 3-kinase such as histamine release from antigen-stimulated mast cells/basophils and glucose uptake in insulin-stimulated adipocytes. Nearly one hundred papers which describe the action of wortmannin on various cells have been reported during one year after the publication of the discovery of wortmannin as PI 3-kinase inhibitor, suggesting possible involvement of the enzyme in the diverse cell responses besides cell proliferation.

SH2 domain structure and function

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

The regulation of tyrosine kinase signalling pathways by growth factor and G-protein-coupled receptors

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A neuroprotective compound, aurin tricarboxylic acid, stimulates the tyrosine phosphorylation cascade in PC12 cells

Aurin tricarboxylic acid (ATA), a general nuclease inhibitor, was reported to prevent PC12 cells from cell death caused by serum starvation (1). In our study, ATA also protected PC12 cells, but not NIH3T3 cells, from serum-starved cell death. When we investigated the mechanism of action of ATA on these cells, ATA was found to increase tyrosine phosphorylation in PC12 cells, but not in NIH3T3 cells. Further investigation on tyrosine-phosphorylated proteins revealed that ATA, similar to nerve growth factor and epidermal growth factor, induced tyrosine phosphorylation of mitogen-activated protein kinases. Since the tyrosine phosphorylation of mitogen-activated protein kinases is thought to play an important role inn growth factor-dependent signal pathways, this finding suggests that the action of ATA on PC12 cells is mediated by tyrosine phosphorylation cascade, similar to growth factor signaling. In addition, we found that Shc proteins, phosphatidylinositol 3-kinase, and phospholipase C-gamma were also phosphorylated in ATA-treated PC12 cells. These key proteins in signal transduction pathways are known to associate with ligand-activated growth factor receptors and are phosphorylated on tyrosine. Thus, the phosphorylation of these three proteins by ATA stimulation supports the speculation that ATA activates a certain receptor tyrosine kinase.

Tyrosine kinases in megakaryocytopoiesis

Protein-tyrosine kinases (PTKs) are of vital importance in a variety of cell functions. Recent studies have provided considerable insight into the binding of growth factors to tyrosine kinase receptors and the consequent induction of signal pathways that lead to a biologic response. Future studies will further delineate the signals that result in a proliferative response and those that induce a differentiation response. Current studies, reviewed here, indicate an important biologic role for PTKs in the regulation of megakaryocyte development and maturation. Whether PTKs function in megakaryocytes in signaling pathways that are similar to pathways in other cells will need to be examined in future studies.

Phosphatidylinositol 3-kinase signals activation of p70 S6 kinase in situ through site-specific p70 phosphorylation

The p70 S6 kinase is activated by insulin and mitogens through multisite phosphorylation of the enzyme. One set of activating phosphorylations occurs in a putative autoinhibitory domain in the noncatalytic carboxyl-terminal tail. Deletion of this tail yields a variant (p70 delta CT104) that nevertheless continues to be mitogen regulated. Coexpression with a recombinant constitutively active phosphatidylinositol (PI) 3-kinase (EC 2.7.1.137) gives substantial activation of both full-length p70 and p70 delta CT104 but not Rsk. Activation of p70 delta CT104 by PI 3-kinase and inhibition by wortmannin are each accompanied by parallel and selective changes in the phosphorylation of p70 Thr-252. A Thr or Ser at this site, in subdomain VIII of the catalytic domain just amino-terminal to the APE motif, is necessary for p70 40S kinase activity. The inactive ATP-binding site mutant K123M p70 delta CT104 undergoes phosphorylation of Thr-252 in situ but does not undergo direct phosphorylation by the active PI 3-kinase in vitro. PI 3-kinase provides a signal necessary for the mitogen activation of the p70 S6 kinase, which directs the site-specific phosphorylation of Thr-252 in the p70 catalytic domain, through a distinctive signal transduction pathway.

Phosphatidylinositol 3-kinase activity is required at a postendocytic step in platelet-derived growth factor receptor trafficking

We have previously reported that platelet-derived growth factor (PDGF) receptor mutants that lack high affinity binding sites for phosphatidylinositol 3-kinase (PI 3-kinase) fail to concentrate in juxtanuclear vesicular structures after activation with PDGF. We have now identified the point in the endocytic pathway at which PI 3-kinase binding sites are required. Receptor internalization from the plasma membrane, measured as the acquisition of acid resistance of prebound 125I-PDGF, was only slightly decreased in cells expressing a PDGF receptor mutant (F5) lacking PI 3-kinase, GTPase-activating protein (GAP), phospholipase C gamma, and Syp binding sites but not expressing mutants where any of these individual sites were restored nor expressing a mutant lacking exclusively PI 3-kinase binding sites. In contrast, the extent of down-regulation of PDGF binding sites from the cell surface after prolonged incubation with PDGF as well as the degradation of [35S]methionine-labeled receptor were markedly reduced in cells expressing the F5 mutant, mutants restored in GAP, phospholipase C gamma, or Syp binding sites or expressing the mutant exclusively lacking PI 3-kinase binding sites but not in cells expressing the mutant where PI 3-kinase binding sites were restored. Inhibition of PI 3-kinase activity with wortmannin caused a dramatic decrease in the rates of down-regulation and degradation of wild-type receptors. These results suggest that PI 3-kinase binding sites are not required for internalization of PDGF receptor but are required to divert the PDGF receptor to a degradative pathway. Furthermore, the requirement for PI 3-kinase binding sites on the receptor appears to be due to a requirement for PI 3-kinase catalytic activity.

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

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

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.