Role of phosphatidylinositol kinase in PDGF receptor signal transduction

Phosphoinositide 3-kinase: the key switch mechanism in insulin signalling

Insulin plays a key role in regulating a wide range of cellular processes. However, until recently little was known about the signalling pathways that are involved in linking the insulin receptor with downstream responses. It is now apparent that the activation of class 1a phosphoinositide 3-kinase (PI 3-kinase) is necessary and in some cases sufficient to elicit many of insulin's effects on glucose and lipid metabolism. The lipid products of PI 3-kinase act as both membrane anchors and allosteric regulators, serving to localize and activate downstream enzymes and their protein substrates. One of the major ways these lipid products of PI 3-kinase act in insulin signalling is by binding to pleckstrin homology (PH) domains of phosphoinositide-dependent protein kinase (PDK) and protein kinase B (PKB) and in the process regulating the phosphorylation of PKB by PDK. Using mechanisms such as this, PI 3-kinase is able to act as a molecular switch to regulate the activity of serine/threonine-specific kinase cascades important in mediating insulin's effects on endpoint responses.

Full activation of the platelet-derived growth factor beta-receptor kinase involves multiple events

Activation of receptor tyrosine kinases is thought to involve ligand-induced dimerization, which promotes receptor transphosphorylation and thereby increases the receptor's phosphotransferase activity. We used two platelet-derived growth factor beta-receptor (beta-PDGFR) mutants to identify events that are required for full engagement (autophosphorylation and activation of the kinase activity) of the beta-PDGFR kinase. The F79/81 receptor (Tyr to Phe substitution at 579 and 581 in the juxtamembrane domain of the receptor) was capable of only very modest ligand-dependent autophosphorylation and also failed to associate with numerous SH2 domain-containing proteins. Furthermore, stimulation with platelet-derived growth factor (PDGF) did not increase the kinase activity of the F79/81 mutant toward exogenous substrates. However, the F79/81 receptor had basal kinase activity and could be artificially stimulated by incubation with ATP. Because the low kinase activity of the F857 mutant (Tyr to Phe substitution at 857 in the putative activation loop) could not be increased by incubation with ATP, failure to phosphorylate Tyr-857 may be the reason why the F79/81 mutant has low kinase activity. Surprisingly, the F857 mutant underwent efficient PDGF-dependent autophosphorylation. Thus the PDGF-dependent increase in the kinase activity of the receptor is not required for autophosphorylation. We conclude that full activation of the beta-PDGFR kinase requires at least two, apparently distinct events.

Modification of phosphatidylinositol 3-kinase SH2 domain binding properties by Abl- or Lck-mediated tyrosine phosphorylation at Tyr-688

In cells expressing the oncogenic Bcr-Abl tyrosine kinase, the regulatory p85 subunit of phosphatidylinositol 3-kinase is phosphorylated on tyrosine residues. We report that this phosphorylation event is readily catalyzed by the Abl and Lck protein-tyrosine kinases in vitro, by Bcr-Abl or a catalytically activated Lck-Y505F in co-transfected COS cells, and by endogenous kinases in transfected Jurkat T cells upon triggering of their T cell antigen receptor. Using these systems, we have mapped a major phosphorylation site to Tyr-688 in the C-terminal SH2 domain of p85. Tyrosine phosphorylation of p85 in vitro or in vivo was not associated with detectable change in the enzymatic activity of the phosphatidylinositol 3-kinase heterodimer, but correlated with a strong reduction in the binding of some, but not all, phosphoproteins to the SH2 domains of p85. This provides an additional candidate to the list of SH2 domains regulated by tyrosine phosphorylation and may explain why association of phosphatidylinositol 3-kinase with some cellular ligands is transient or of lower stoichiometry than anticipated.

Granulocyte-macrophage colony-stimulating factor and IL-5 activate mitogen-activated protein kinase through Jak2 kinase and phosphatidylinositol 3-kinase in human eosinophils

Mitogen-activated protein (MAP) kinases are activated by the sequential activation of Ras, Raf, and MEK (MAP kinase kinase) and regulate a wide variety of cell functions. To determine the kinase cascade for granulocyte-macrophage colony-stimulating factor (GM-CSF)- and IL-5-induced MAP kinase activation in eosinophils, we studied the effect of inhibitors of Jak2 kinase, tyrosine kinases, phosphatidylinositol 3-kinase, and protein kinase C on GM-CSF- and IL-5-induced MAP kinase activation in human eosinophils. GM-CSF and IL-5 activated 40, 42, and 44 kilodalton MAP kinase isoforms in eosinophils. This was indicated by the electrophoretic mobility shift of the three isoforms of MAP kinase in immunoblotting with anti-MAP kinase antibody and also by in-gel MAP kinase assay. MAP kinase activation was time- and dose-dependent, becoming maximal 3 to 15 minutes after stimulation. A Jak2 kinase inhibitor AG-490, a tyrosine kinase inhibitor genistein, and a phosphatidylinositol 3-kinase inhibitor wortmannin inhibited GM-CSF- and IL-5-induced MAP kinase activation in eosinophils, whereas a protein kinase C inhibitor staurosporine had a weak inhibitory effect. Furthermore, AG-490 and genistein prevented GM-CSF-induced tyrosine phosphorylation of Jak2 kinase in eosinophils. Taken together, these results indicate that GM-CSF and IL-5 activate MAP kinases through the signaling pathway of Jak2 kinase-tyrosine phosphorylated beta chain-phosphatidylinositol 3-kinase-Ras in eosinophils.

Transformation of hematopoietic cells by BCR/ABL requires activation of a PI-3k/Akt-dependent pathway

The BCR/ABL oncogenic tyrosine kinase activates phosphatidylinositol 3-kinase (PI-3k) by a mechanism that requires binding of BCR/ABL to p85, the regulatory subunit of PI-3k, and an intact BCR/ABL SH2 domain. SH2 domain BCR/ABL mutants deficient in PI-3k activation failed to stimulate Akt kinase, a recently identified PI-3k downstream effector with oncogenic potential, but did activate p21 RAS and p70 S6 kinase. The PI-3k/Akt pathway is essential for BCR/ABL leukemogenesis as indicated by experiments demonstrating that wortmannin, a PI-3k specific inhibitor at low concentrations, suppressed BCR/ABL-dependent colony formation of murine marrow cells, and that a kinase-deficient Akt mutant with dominant-negative activity inhibited BCR/ABL-dependent transformation of murine bone marrow cells in vitro and suppressed leukemia development in SCID mice. In complementation assays using mouse marrow progenitor cells, the ability of transformation-defective SH2 domain BCR/ABL mutants to induce growth factor-independent colony formation and leukemia in SCID mice was markedly enhanced by expression of constitutively active Akt. In retrovirally infected mouse marrow cells, the BCR/ABL mutant lacking the SH2 domain was unable to upregulate the expression of c-Myc and Bcl-2; in contrast, expression of a constitutively active Akt mutant induced Bcl-2 and c-Myc expression, and stimulated the transcription activation function of c-Myc. Together, these data demonstrate the requirement for the BCR/ABL SH2 domain in PI-3k activation and document the essential role of the PI-3k/Akt pathway in BCR/ABL leukemogenesis.

Inhibition of platelet-derived growth factor receptor tyrosine kinase by atrial natriuretic peptide

Atrial natriuretic peptide (ANP) is known to suppress platelet-derived growth factor (PDGF)-stimulated proliferation of rat cultured vascular smooth muscle cells. The present study examined whether ANP inhibits the PDGF receptor (PDGFR) tyrosine kinase activation, an initial event for PDGF cellular signaling. ANP reduced the in vivo tyrosine phosphorylation of PDGFR stimulated by PDGF in a dose-dependent manner. This effect was not due to the reduction in PDGFR protein as detected by immunoblot analysis. 8-Bromo-cyclic GMP, a membrane-permeable 3',5'-cyclic monophosphate (cGMP) derivative, mimicked the action of ANP. HS-142-1, an antagonist for guanylate cyclase A (GC-A) and B, co-incubated with ANP, restored the PDGF-induced PDGFR autophosphorylation. The effect of ANP was also observed in the presence of a protein tyrosine phosphatase inhibitor, sodium orthovanadate. To confirm that ANP exerts its action by inhibiting protein tyrosine kinase (PTK), an in vitro kinase assay was performed. Cyclic GMP inhibited PTK activity of PDGFR partially purified by lectin affinity chromatography. In contrast, PTK activity in immobilized PDGFR immunocomplexes was not inhibited by cGMP. However, exogenous cGMP dependent protein kinase (PKG) reduced the PTK activity in the presence of cGMP. These results demonstrate that ANP suppresses PDGFR PTK through GC-A probably by activating PKG. This may be an important mechanism by which ANP exerts its anti-proliferative action antagonizing PDGF.

Defective signaling through the B cell antigen receptor in Epstein-Barr virus-transformed ataxia-telangiectasia cells

A characteristic series of immunological abnormalities are observed in the human genetic disorder ataxia-telangiectasia (A-T). The recent cloning of a gene mutated in this syndrome provides additional evidence for a defect in intracellular signaling in A-T. We have investigated the possibility that signaling through the B cell antigen receptor is one manifestation of the A-T defect. In response to cross-linking of the B cell receptor, several A-T cell lines were defective in their mitogenic response; in addition Ca2+ mobilization from internal stores was either absent or considerably reduced in these cell lines in response to cross-linking. The defect in signaling was not due to difference in expression of surface immunoglobulin. The defective response in A-T cells was also evident in several arms of the intracellular cascade activated by B cell cross-linking. Tyrosine phosphorylation of phospholipase Cgamma1, a key step in activation of the enzyme, was reduced or negligible in some A-T cell lines. This defect in signaling was also seen at the level of Lyn tyrosine kinase activation and its association with and activation of phosphatidylinositol 3-kinase. Our results provide evidence for a role for the ATM gene product in intracellular signaling which may account at least in part for the abnormalities in B cell function in A-T.

Reovirus infection and tissue injury in the mouse central nervous system are associated with apoptosis

Reovirus serotype 3 strains infect neurons within specific regions of the neonatal mouse brain and produce a lethal meningoencephalitis. Viral replication and pathology colocalize and have a predilection for the cortex, hippocampus, and thalamus. We have shown previously that infection of cultured fibroblasts and epithelial cells with reovirus type 3 Dearing (T3D) and other type 3 reovirus strains results in apoptotic cell death, suggesting that apoptosis is a mechanism of cell death in vivo. We now report that T3D induces apoptosis in infected mouse brain tissue. To determine whether reovirus induces apoptosis in neural tissues, newborn mice were inoculated intracerebrally with T3D, and at various times after inoculation, brain tissue was assayed for viral antigen by immunostaining and apoptosis was identified by DNA oligonucleosomal laddering and in situ terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling. Cells were also stained with cresyl violet to detect morphological changes characteristic of apoptosis, including chromatin condensation and cell shrinkage. DNA laddering was detected in T3D- but not in mock-infected brain tissue. Apoptotic cells were restricted to the same regions of the brain in which infected cells and tissue damage were observed. These findings suggest that virus-induced apoptosis is a mechanism of cell death, tissue injury, and mortality in reovirus-infected mice. The correlation between apoptosis and pathogenesis in vivo identifies apoptosis as a potential target for molecular and pharmacological strategies designed to curtail or prevent diseases resulting from induction of this cell death pathway.

Thrombopoietin Enhances the αIIbβ3-Dependent Adhesion of Megakaryocytic Cells to Fibrinogen or Fibronectin Through PI 3 Kinase

The effect of thrombopoietin (TPO) on the functional activity of surface αIIbβ3 (GPIIbIIIa) was investigated in both primary human megakaryocytic cells, derived from peripheral blood CD34+ cells, and HEL hematopoietic cell line. TPO (100 ng/mL) induced a sixfold to ninefold enhancement of adhesion of both primary megakaryocytic and HEL cells to plates coated with either fibrinogen or fibronectin and a parallel increase of immunoreactivity to the PAC1 monoclonal antibody (MoAb) and fluorescein isothiocyanate-fibrinogen, both of which recognize an activated state of αIIbβ3 . The enhanced adhesion to fibrinogen or fibronectin was mediated by the Arg-Gly-Asp (RGD) recognition sequence of αIIbβ3 , as it was abolished by pretreatment of cells with saturating concentrations of RGDS peptide. A MoAb specific for the αIIb subunit of αIIbβ3 also inhibited cell attachment to fibrinogen or fibronectin, while MoAb to anti-αvβ3 or anti-α5 integrins were completely ineffective, clearly indicating that αIIbβ3 participates in this association. A role for PI 3 kinase (PI 3-K) in the TPO-mediated increase in αIIbβ3 function in megakaryocytic cells was suggested by the ability of the PI 3-K inhibitor wortmannin (100 nmol/L) and antisense oligonucleotides directed against the p85 regulatory subunit of PI 3-K to completely block the TPO-induced increase in αIIbβ3 integrin activity upon TPO stimulation. The modulation of adhesiveness to extracellular matrix proteins containing the RGD motif mediated by TPO likely plays a physiologic role in megakaryocytopoiesis, as pretreatment of CD34+ cells with RGDS or anti-αIIb MoAb significantly reduced the number of megakaryocytic colonies obtained in a fibrinclot semisolid assay.

Selective recognition of phosphatidylinositol 3,4,5-trisphosphate by a synthetic peptide

The present study takes a novel approach to explore the mode of action of phosphoinositide 3-kinase lipid products by identifying a synthetic peptide W-NG(28-43) (WAAKIQASFRGHMARKK) that displays discriminative affinity with phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3). This PtdIns(3,4,5)P3-binding peptide was discovered by a gel filtration-based binding assay and exhibits a high degree of stereochemical selectivity in phosphoinositide recognition. It forms a 1:1 complex with PtdIns(3,4,5)P3 with Kd of 2 microM, but binds phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) and phosphatidylinositol 3,4-bisphosphate (PtdIns(3,4)P2) with substantially lower affinity (5- and 40-fold, respectively) despite the largely shared structural motifs with PtdIns(3,4,5)P3. Other phospholipids examined, including phosphatidylserine, phosphatidylcholine, phosphatidylinositol, and phosphatidylethanolamine, show low or negligible affinity with the peptide. Several lines of evidence indicate that this phosphoinositide-peptide interaction is not due to nonspecific electrostatic interactions or phospholipid aggregation, and requires a cooperative action among the hydrophobic and basic residues to exert the selective recognition. CD data suggest that the peptide acquires an ordered structure upon binding to PtdIns(3,4,5)P3. Further, we demonstrate that PtdIns(3,4,5)P3 enhances the phosphorylation rate of this binding peptide by protein kinase C (PKC)-alpha in a dose-dependent manner. In view of the findings that this stimulatory effect is not noted with other PKC peptide substrates lacking affinity with PtdIns(3,4,5)P3 and that PKC-alpha is not susceptible to PtdIns(3,4,5)P3 activation, the activity enhancement is thought to result from the substrate-concentrating effect of the D-3 phosphoinositide, i.e. the presence of PtdIns(3,4,5)P3 allows the peptide to bind to the same vesicles/micelles to which PKC is bound. Moreover, it is noteworthy that neurogranin, the full-length protein of W-NG(28-43) and a relevant PKC substrate in the forebrain, binds PtdIns(3,4,5)P3 with high affinity. Taken together, it is plausible that, in addition to PKC activation, PtdIns(3,4,5)P3 provides an alternative mechanism to regulate PKC activity in vivo by recruiting and concentrating its target proteins at the interface to facilitate the subsequent PKC phosphorylation.

Platelet-derived growth factor stimulates sodium-dependent Pi transport in osteoblastic cells via phospholipase Cgamma and phosphatidylinositol 3' -kinase

Inorganic phosphate (Pi) is a major regulator of cell metabolism. The Pi transport activity in the plasma membrane is a main determinant of the intracellular level of this ion. In bone-forming cells, Pi transport is important for the calcification of the bone matrix. In this study, the effect of platelet-derived growth factor (PDGF) on Pi transport activity and the signaling mechanism involved in this cellular response were analyzed. The results indicate that PDGF is a potent and selective stimulator of sodium-dependent Pi transport in the mouse calvaria-derived MC3T3-E1 osteoblast-like cells. The change in Pi transport induced by PDGF-BB was dependent on translational processes and affected the Vmax of the Pi transport system. These observations suggested that enhanced Pi transport activity in response to PDGF resulted from insertion of newly synthesized Pi transporters in the plasma membrane. The role of activation of mitogen activated protein (MAP) kinase, phospholipase C (PLC)gamma or phosphatidylinositol 3-kinase (PI-3-kinase), in mediating this effect of PDGF, was investigated. A selective inhibitor of the PDGF receptor tyrosine kinase activity (CGP 53716) completely blocked PDGF-induced protein tyrosine phosphorylation of several proteins including the PDGF receptor, PLCgamma, MAP kinase, and association of the p85 subunit of PI-3'-kinase. Associated with this effect, the increase in Pi transport induced by PDGF was completely blunted by 5 microM CGP 53716. Inhibition of MAP kinase activity by cAMP agonists did not influence Pi transport stimulation induced by PDGF. However, inhibitors of protein kinase C completely blocked this response. A selective inhibitor of PI-3-kinase, LY294002, also significantly reduced this effect of PDGF. In summary, these results indicate that PDGF is a potent and selective stimulator of Pi transport in osteoblastic cells. The mechanism responsible for this effect is not mediated by MAP kinase but involves tyrosine phosphorylation-dependent activation of PLCgamma and PI-3-kinase.

Evidence that thrombin-stimulated DNA synthesis in pulmonary arterial fibroblasts involves phosphatidylinositol 3-kinase-dependent p70 ribosomal S6 kinase activation

We have examined the regulation of the 70 kDa ribosomal S6 kinase (p70s6k) by the G-protein-coupled receptor agonist alpha-thrombin and the role of this signalling molecule in the mitogenic effect of thrombin in cultured bovine pulmonary arterial (PA) fibroblasts. Thrombin stimulated p70s6k activity in a time and concentration-dependent manner which was abolished by the macrolide rapamycin. The phosphatidylinositol (PI) 3-kinase inhibitor wortmannin also completely blocked p70s6k activity in response to thrombin but did not affect p70s6k activity evoked by platelet-derived growth factor (PDGF) at a concentration that abrogated PDGF-stimulated PI 3-kinase activity. Activation of p70s6k by thrombin, but not PDGF, was also inhibited (by 48.3 +/- 5.4%) by pre-incubation of cells with pertussis toxin (PTX). Downregulation of protein kinase C (PKC) alpha and epsilon isoforms by pretreatment of fibroblasts for 48 h with phorbol 12-myristate 13-acetate (PMA), markedly attenuated both thrombin and PDGF-stimulated p70s6k activation (by 74.8 +/- 4.4% and 82.3 +/- 7.9% respectively). Thrombin also strongly stimulated (over 100 fold) the incorporation of [3H]thymidine into growth arrested PA fibroblasts which was inhibited by rapamycin (by 33.6 +/- 2.0%). From these results we propose that in PA fibroblasts: 1) thrombin stimulates the activation of p70s6k in a manner consistent with an involvement of a heterotrimeric G protein of the G(i) family, a PI 3-kinase other than the PI 3-kinase involved in signalling by PDGF, and PKC. 2) a p70s6k-dependent pathway plays a role in mitogenic signalling by thrombin.

Analysis of mutant platelet-derived growth factor receptors expressed in PC12 cells identifies signals governing sodium channel induction during neuronal differentiation

The mechanisms governing neuronal differentiation, including the signals underlying the induction of voltage-dependent sodium (Na+) channel expression by neurotrophic factors, which occurs independent of Ras activity, are not well understood. Therefore, Na+ channel induction was analyzed in sublines of PC12 cells stably expressing platelet-derived growth factor (PDGF) beta receptors with mutations that eliminate activation of specific signalling molecules. Mutations eliminating activation of phosphatidylinositol 3-kinase (PI3K), phospholipase C gamma (PLC gamma), the GTPase-activating protein (GAP), and Syp phosphatase failed to diminish the induction of type II Na+ channel alpha-subunit mRNA and functional Na+ channel expression by PDGF, as determined by RNase protection assays and whole-cell patch clamp recording. However, mutation of juxtamembrane tyrosines that bind members of the Src family of kinases upon receptor activation inhibited the induction of functional Na+ channels while leaving the induction of type II alpha-subunit mRNA intact. Mutation of juxtamembrane tyrosines in combination with mutations eliminating activation of PI3K, PLC gamma, GAP, and Syp abolished the induction of type II alpha-subunit mRNA, suggesting that at least partially redundant signaling mechanisms mediate this induction. The differential effects of the receptor mutations on Na+ channel expression did not reflect global changes in receptor signaling capabilities, as in all of the mutant receptors analyzed, the induction of c-fos and transin mRNAs still occurred. The results reveal an important role for the Src family in the induction of Na+ channel expression and highlight the multiplicity and combinatorial nature of the signaling mechanisms governing neuronal differentiation.

Ischemia-induced transplant arteriosclerosis in the rat. Induction of peptide growth factor expression

Peptide growth factors have been reported to contribute to the atherogenic process, and they are known to mediate signals for vascular remodeling. Using syngeneic and allogeneic rat aorta transplant models, we analyzed the impact of cold ischemia time up to 24 hours and reperfusion injury on development of transplant arteriosclerosis during the first 2 months after transplantation. The expression of the transforming growth factor-beta (TGF-beta) family as well as the platelet-derived growth factor (PDGF) and its receptors was studied by use of immunohistochemistry, followed by semiquantitative evaluation and multivariate analysis. In the syngeneically transplanted aortas, the expression of TGF-beta 1, PDGF, and the two PDGF receptors in the neointima increased significantly with the extent of cold ischemia time. Furthermore, there was a significant induction of the latent TGF-beta binding protein in the neointima as well as TGF-beta 2 in the media, both correlating with the observation time after transplantation. In the allogeneic grafts, all examined proteins were already induced strongly 2 weeks after transplantation, even at the shortest ischemic period studied (1 hour). However, no positive correlation between growth factor expression and cold ischemia or observation time could be found. Double immunohistochemistry revealed that macrophages express PDGF and its receptors as well as TGF-beta 1. Smooth muscle cells express both types of PDGF receptors, and a few T cells express TGF-beta 1 as well as PDGF receptors. In summary, TGF-beta and PDGF are induced by allogeneic as well as ischemic stimuli in transplanted aortas, suggesting a role in the pathogenesis of transplant arteriosclerosis and representing a potential target for therapeutic intervention.

Brk, a breast tumor-derived non-receptor protein-tyrosine kinase, sensitizes mammary epithelial cells to epidermal growth factor

brk (breast tumor kinase) shows homology to the src family of non-receptor protein-tyrosine kinases and is expressed in breast carcinomas. In order to investigate the role of brk in breast tumor development, we have examined the growth and transformation properties of human mammary epithelial cells engineered to overexpress Brk. Interestingly, like c-Src, overexpression of Brk leads to sensitization to EGF, and also results in a partially transformed phenotype. Further investigation of the latter activity was attempted by mutational analysis, targeting key residues known to affect tyrosine kinase activity in Src-like kinases. Mutation of amino acid residue Lys-219 to Met, by analogy to Src, abolished both kinase activity and transformation capacity. Mutation of amino acid residue Tyr-447 to Phe, however, resulted in a decrease in transforming potential without affecting kinase activity. These results suggest that while Src and Brk share some functional properties, they act differently during transformation. These differences are discussed in the context of the mechanisms underlying breast cancer development.

Effect of glucose, insulin, and insulin-like growth factor-I on glucose transport activity in cultured rat vascular smooth muscle cells

Glucose transport activity in cultured rat vascular smooth muscle cells (VSMCs) was examined under various concentrations of D-glucose, insulin, and insulin-like growth factor-I (IGF-I). Confluent cultures of VSMCs were incubated with serum-free medium containing 0-25 mmol/l of D-glucose for 24-49 h. The basal rate of 2-deoxyglucose uptake was reduced in association with increasing concentrations of D-glucose. Uptake of 2-deoxyglucose into the cells was linear between 0 and 15 min of incubation regardless of the glucose concentration. The uptake was inhibited by the addition of 10 mumol/l cytochalasin B or 100 mmol/l D-glucose indicating that the effects were mediated by specific integral glucose carriers. The effect of D-glucose was time-dependent and reversible. Insulin increased the uptake of 2-deoxyglucose in a dose-dependent manner, and its effect was dependent on the preincubation dose of D-glucose. Insulin-stimulated uptake was lower in the cells pre-exposed to 25 mmol/l D-glucose than in the cells pre-exposed to concentrations of D-glucose below 5.5 mmol/l. After a long-term incubation with insulin, the insulin-stimulated glucose transport was inhibited. Recovery of glucose transport activity was assessed by incubating cells with D-glucose for 24-48 h to induce desensitization. After a 24 h glucose conditioning, the uptake of 2-deoxyglucose was lower in the cells preincubated with 25 mmol/l glucose than in the cells preincubated with 5.5 mmol/l glucose. The effect of the glucose conditioning was reversible and dependent on the preincubation dose of D-glucose. IGF-I was a more potent stimulator of glucose transport than insulin. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), inhibited the uptake of glucose stimulated by insulin or IGF-I in a dose-dependent manner. Our results suggest that D-glucose regulates its own uptake independently of insulin and modulates the ability of insulin to induce insulin resistance in the cultured rat VSMCs. Glucose attenuated the effect of insulin, and led to a progressive decrease in the activity of the glucose transport effector system. Activation of wortmannin-sensitive PI3-kinase may be involved in the signaling pathways of the insulin- and IGF-I-stimulated glucose uptake in VSMCs. This mechanism of insulin resistance may be relevant to the formation of cellular defects in the vascular wall in patients with diabetes mellitus.

The E5 gene product of rhesus papillomavirus is an activator of endogenous Ras and phosphatidylinositol-3'-kinase in NIH 3T3 cells

We examined the effect of two rhesus papillomavirus 1 (RhPV) oncogenes on cytokine-induced signal transduction pathways leading to the possible activation of Ras protein (p21ras) and phosphatidylinositol kinase. p21ras in both the activated (GTP-bound) and inactivated (GDP-bound) states were quantitated. NIH 3T3 cell lines expressing the RhPV 1 E5 gene or epidermal growth factor receptor cDNA had about a sixfold higher ratio of p21ras-bound GTP to p21ras-bound GDP as compared with parental NIH 3T3 cells or a cell line expressing the RhPV 1 E7 gene under normal culture conditions, yet expressed similar levels of p21ras. Quiescent cells had dramatically reduced levels of activated p21ras, except those containing RhPV 1 E7. Levels were restored by stimulation with epidermal growth factor or platelet-derived growth factor. Both epidermal growth factor and platelet-derived growth factor receptor of RhPV 1 E5- and E7-containing cells responded to cytokine stimulation. Endogenous phosphatidylinositol-3'-kinase was up-regulated in NIH 3T3 cells transformed with the E5 genes of RhPV 1 and bovine papillomavirus 1. These results suggest that E5 genes of papillomaviruses play a major role in the regulation of transduction pathways.

D-3 phosphoinositide metabolism in cells treated with platelet-derived growth factor

Despite extensive analysis of phosphoinositide 3-hydroxykinases (PI 3-kinases) at the molecular level, comparatively little is known about the mechanisms by which products of these enzymes exert their expected second-messenger functions. This study examines the metabolism of D-3 phosphoinositides in mouse Ph-N2 fibroblasts lacking the platelet-derived growth factor (PDGF) alpha-receptor. Treatment of these cultures with BB PDGF, but not AA PDGF, resulted in transient activation of PI 3-kinase activity measured in vitro. Treatment of myo-[3H]inositol-labelled Ph-N2 cells with BB PDGF resulted in the rapid induction of PtdIns(3,4)P2 and PtdIns(3,4,5)P3 and, to a smaller extent, PtdIns3P. The appearance of PtdIns(3,4,5)P3 preceded that of PtdIns(3,4)P2 and PtdIns3P after the addition of PDGF, suggesting that PtdIns(4,5)P2 is the preferred substrate of the agoniststimulated PI 3-kinase in intact cells. Treatment of both resting and PDGF-stimulated cells with the fungal metabolite wortmannin resulted in pronounced, selective effects on the levels of all D-3 phosphoinositides. Kinetic studies with this PI 3-kinase inhibitor revealed the presence of at least two independent routes for the biosynthesis of D-3 phosphoinositides in PDGF-treated cells.

BCR/ABL regulation of PI-3 kinase activity

The ability of BCR-ABL oncoproteins to induce leukemic transformation of hematopoietic cells depends on their tyrosine kinase activity, which is essential for recruitment and activation of multiple pathways that transduce oncogenic signals. Although it is unknown yet whether activation of PI 3-kinase is required for transformation, the colony-forming ability of Philadelphia cells is dependent on PI 3-kinase activity, as indicated by the results of studies using a number of strategies to interfere with the synthesis and/or the function of the regulatory and catalytic subunits of this kinase. In particular, wortmannin, a specific PI 3-kinase inhibitor, preferentially affected colony formation of Philadelphia cells over that of normal marrow hematopoietic progenitors. The mechanism(s) of such effects are unknown, but PI 3-kinase inhibitors may represent a novel class of therapeutic agents for the ex vivo and/or in vivo treatment of Philadelphia leukemias.

Gastrin stimulates tyrosine phosphorylation of insulin receptor substrate 1 and its association with Grb2 and the phosphatidylinositol 3-kinase

The growth-promoting effects of gastrin on normal and neoplastic gastrointestinal tissues have been shown to be mediated by the gastrin/CCKB receptor, which belongs to the family of G protein-coupled receptors. However, the downstream signaling pathways activated by gastrin are not well characterized. In the present study, we demonstrate that gastrin stimulates tyrosine phosphorylation of insulin receptor substrate 1 (IRS-1), the major cytoplasmic substrate of the insulin receptor. The gastrin-induced phosphorylation of IRS-1 was rapid and transient, occurring within 30 s of treatment and diminishing thereafter. IRS-1 binds several proteins containing Src homology 2 domains through its multiple tyrosine phosphorylation sites. Following gastrin stimulation, we observed a time- and dose-dependent association of IRS-1 with the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI 3-kinase). In addition, activation of PI 3-kinase was detected in anti-IRS-1 immunoprecipitates from gastrin-treated cells, suggesting that tyrosine phosphorylation of IRS-1, which leads to the rapid recruitment of p85, might be one mechanism used by gastrin to activate PI 3-kinase. We have previously reported that tyrosine phosphorylation of Shc and its association with the Grb2-Sos complex may contribute to the activation of the mitogen-activated protein kinase pathway by gastrin. We report here that Grb2 also interacts with tyrosine-phosphorylated IRS-1 in response to gastrin. Taken together, our results suggest that IRS-1 may serve as a converging target in the signaling pathways stimulated by receptors that belong to different families, such as the gastrin/CCKB G protein-coupled receptor and the insulin receptor.

Induction of platelet-derived growth factor beta-receptor in focal ischemia of rat brain

Our previous study on the ischemia-induced expression of platelet-derived growth factor (PDGF)-B chain in the rat brain prompted us to examine expression of PDGF beta-receptor in the ischemic brain. Focal ischemia was induced by permanent tandem occlusion of middle cerebral and common carotid arteries in spontaneously hypertensive rats. Northern analysis revealed that ischemia significantly increased expression of the receptor in the ischemic neocortex at 4 and 7 days (328 +/- 109%; 323 +/- 119%, respectively, over control: n = 4, p < 0.05 versus sham). Neurons in infarct transiently showed increased immunostaining for the receptor at 1 day, whereas neurons in periinfarct area showed sustained and increased immunoreactivity from 1 to 14 days post-ischemia. Reactive glial cells in the external capsule and in molecular layer of the neocortex adjacent to infarct possessed enhanced immunoreactivity from 1 to 21 days. Furthermore, marked immunoreactivity was observed on brain macrophages in infarct and on the abluminal side of capillaries surrounding infarct from 4 to 7 days. These results demonstrated that ischemic insult increases expression of the PDGF beta-receptor at both the mRNA and protein level in the brain, suggesting its important role in cellular cascade of the ischemic brain.

Binding to the platelet-derived growth factor receptor transiently activates the p85alpha-p110alpha phosphoinositide 3-kinase complex in vivo

Ligand stimulation of the platelet-derived growth factor (PDGF) receptor results in its association with phosphoinositide 3-kinase activity and a corresponding synthesis of 3'-phosphorylated lipids. Early studies that examined this interaction in vivo employed anti-phosphotyrosine antiserum or antiserum against the PDGF receptor. The recent identification of multiple isoforms of both the regulatory and the catalytic subunit of the enzyme have led us to utilize antisera against p85alpha and p110alpha to characterize the association of this particular phosphoinositide 3-kinase complex with the PDGF receptor following ligand stimulation of murine fibroblasts. Both the p85alpha and p110alpha subunits rapidly associated with the ligand-activated receptor resulting in a transient, 2-fold increase in the total pool of p110alpha lipid kinase activity. This association was stable for 15 min after initial stimulation. Subsequently, both subunits began to dissociate from the receptor with similar kinetics. By 60 min this process was complete, demonstrating that p85alpha and p110alpha both associate with the receptor and dissociate from the receptor as a dimeric complex. At this time, marked PDGF receptor down-regulation was observed. Immunoprecipitation from metabolically labeled cells revealed that p85alpha is constitutively phosphorylated on serine residues in quiescent cultures. Upon PDGF stimulation, this phosphorylation upon serine residues was maintained in addition to tyrosine phosphorylation of this subunit. No phosphorylation of the p110alpha subunit was detected in either quiescent or PDGF-stimulated cells. Quantitation of Western blot analysis demonstrated that only 5% of the total pool of p85alpha associated with the PDGF receptor upon ligand stimulation. The 2-fold increase in the lipid kinase activity measured in immunoprecipitates using either anti-p85alpha or anti-p110alpha antiserum therefore reflects a far greater increase in the specific activity of the enzyme upon its association with the PDGF receptor.

Association of phosphatidylinositol 3 kinase to protein kinase C zeta during interleukin-2 stimulation

Interleukin-2 induces a serine-phosphorylated phosphatidylinositol 3 kinase activity in the mouse T cell line TS1 alpha beta. Moreover, protein kinase C (PKC) zeta directly or indirectly associates with the phosphatidylinositol 3 kinase and the association appears to be necessary for the serine-phosphorylated phosphatidylinositol 3 kinase activity, since release of zeta PKC by competition of binding with peptides spanning the p110 sequence from amino acids 907 to 925 abolishes the serine-phosphorylated phosphatidylinositol 3 kinase activity. This kinase activity is also blocked when zeta PKC expression is inhibited by antisense oligonucleotide. Inhibition of phosphatidylinositol 3 kinase activity by wortmannin does not abolish zeta PKC association.

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.

Oxidative stress induces tyrosine phosphorylation of PDGF alpha-and beta-receptors and pp60c-src in mesangial cells

Reactive oxygen species are autocrine and paracrine modulators of cell behavior. Hydrogen peroxide, a cellular oxidant, has been shown to stimulate mesangial cell proliferation. In the present study we analyzed the H2O2-induced early signaling events. Immunofluorescence analysis revealed a H2O2 induced dose-dependent increase in tyrosine phosphorylation. Short treatment (2 or 5 min) with 5 mM H2O2 induced a mitogenic response and a significant (P < 0.01) increase in the number of cells compared to non-treated controls. Proteins extracted from H2O2 (0.1 to 10 mM) treated cells were separated on SDS-PAGE and subjected to immunoblot analysis with anti-phosphotyrosine. A dose-dependent induction of tyrosine phosphorylation of 180 kDa, 120 kDa and 60 kDa proteins was observed within 1 to 10 minutes. By sequentially using immunoprecipitation and immunoblotting the 180 kDa tyrosine phosphorylated band was shown to represent both PDGF alpha- and beta-receptors. The tyrosine phosphorylated 60 kDa protein was identified as the cytoplasmic protein tyrosine kinase pp60c-src. The c-src phosphorylation was associated with an inhibition of c-src kinase activity, suggesting phosphorylation of tyrosine 527 in the c-src regulatory domain. Pretreatment with catalase completely abrogated the H2O2-induced PDGF receptor and c-src tyrosine phosphorylation. These data support the notion that the activation of a signaling pathway involving the PDGF receptors and c-src contributes to the mitogenic effects of reactive oxygen species.

In vitro characterization of a novel series of platelet-derived growth factor receptor tyrosine kinase inhibitors

In this report, we describe the discovery and characterization of a novel biarylhydrazone series of platelet-derived growth factor (PDGF) receptor tyrosine kinase inhibitors typified by the prototype WIN 41662 (3-phenyl-N1-[1-(4-pytidyl)pyrimidine]hydrazone). WIN 41662 inhibited PDGF-stimulated autophosphorylation of PDGF receptors from human vascular smooth muscle cells (hVSMC) with an IC50 value of 60 nM. The inhibitor appeared to be competitive with respect to substrate (Mn(2+)-ATP), having a calculated Ki of 15 +/- 5 nM. WIN 41662 was approximately 500-fold more potent in inhibiting the PDGF receptor tyrosine kinase than the p56lck tyrosine kinase. It was inactive against other serine/threonine and tyrosine kinases tested. WIN 41662 produced concentration-dependent inhibition of PDGF-stimulated receptor autophosphorylation in intact hVSMC with an IC50 < 100 nM. Intracellular Ca2+ mobilization and cell proliferation were events that occurred in hVSMC subsequent to PDGF receptor activation. WIN 41662 inhibited PDGF-stimulated Ca2+ mobilization and cell proliferation ([3H]TdR incorporation) with IC50 values of 430 nM and 2.3 microM, respectively. These effects appeared to be specifically related to PDGF receptor tyrosine kinase inhibition since WIN 41662 was not cytotoxic (in vitro) and since WIN 72039, a close structural analog that does not inhibit PDGF receptor tyrosine kinase, also did not inhibit PDGF-stimulated receptor autophosphorylation, Ca2+ mobilization, or hVSMC proliferation. Thus, WIN 41662 is representative of a novel class of selective PDGF receptor tyrosine kinase inhibitors that inhibit PDGF-regulated secondary events in intact cells.

Mitogenic inhibition by phorbol esters is associated with decreased phosphatidylinositol-3 kinase activation

In contrast to their role as potent tumor promoters, phorbol esters can cause inhibition of cell growth. Because the effect of phorbol esters occurs through activation of protein kinase C (PKC) and because activated PKC is translocated to the membrane placing it in a position to act on the intracellular portion of the growth factor receptor, we asked whether this inhibitory effect is mediated through the action of phorbol 12-myristate 13-acetate (PMA) on receptor association with the signal transfer proteins. When added to rat vascular smooth muscle (VSM) cells concurrently with basic fibroblast growth factor (bFGF), PMA at 100 ng/ml completely inhibits bFGF-stimulated DNA synthesis. Under the same growth-inhibitory conditions of PMA addition, aggregation of phosphatidylinositol 3-kinase (PI3K) to the fibroblast growth factor receptor and tyrosine phosphorylation of the 85-kDa regulatory component of the signal transfer protein PI3K are reduced by 94 and 79%, respectively. PI3K catalytic activity, as measured by conversion of phosphatidylinositol to phosphatidylinositol 3-phosphate, is decreased 88% by PMA addition. This effect is not specific to PI3K, since aggregation of phospholipase C-gamma 1 to the activated bFGF receptor is also decreased by PMA treatment. In addition, the PI3K inhibitor wortmannin markedly attenuates bFGF-stimulated VSM cell growth in a dose-dependent manner. These data suggest that the site of growth inhibition by PMA in VSM cells lies upstream of signal transfer particle aggregation and that such growth arrest may be mediated through inhibition of activation of PI3K.

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.

Platelet-derived growth factor activates protein kinase C epsilon through redundant and independent signaling pathways involving phospholipase C gamma or phosphatidylinositol 3-kinase

Protein kinase C (PKC), a major cellular receptor for tumor-promoting phorbol esters and diacylglycerols (DGs), appears to be involved in a variety of cellular functions, although its activation mechanism in vivo is not yet fully understood. To evaluate the signaling pathways involved in the activation of PKC epsilon upon stimulation by platelet-derived growth factor (PDGF) receptor (PDGFR), we used a series of PDGFR "add-back" mutants. Activation of a PDGFR mutant (Y40/51) that binds and activates phosphatidylinositol 3-kinase (PI 3-kinase) caused translocation of PKC epsilon from the cytosol to the membrane in response to PDGF. A PDGFR mutant (Y1021) that binds and activates phospholipase C gamma (PLC gamma), but not PI 3-kinase, also caused the PDGF-dependent translocation of PKC epsilon. The translocation of PKC epsilon upon stimulation of PDGFR (Y40/51) was inhibited by wortmannin, an inhibitor of PI 3-kinase. Activation of PKC epsilon was further confirmed in terms of PKC epsilon-dependent expression of a phorbol 12-tetradecanoate 13-acetate response element (TRE)-luciferase reporter. Further, purified PKC epsilon was activated in vitro by either DG or synthetic phosphatidylinositol 3,4,5-trisphosphate. These results clearly demonstrate that PKC epsilon is activated through redundant and independent signaling pathways which most likely involve PLC gamma or PI 3-kinase in vivo and that PKC epsilon is one of the downstream mediators of PI 3-kinase whose downstream targets remain to be identified.

Mitogenic signals and transforming potential of Nyk, a newly identified neural cell adhesion molecule-related receptor tyrosine kinase

Nyk/Mer is a recently identified receptor tyrosine kinase with neural cell adhesion molecule-like structure (two immunoglobulin G-like domains and two fibronectin III-like domains) in its extracellular region and belongs to the Ufo/Axl family of receptors. The ligand for Nyk/Mer is presently unknown, as are the signal transduction pathways mediated by this receptor. We constructed and expressed a chimeric receptor (Fms-Nyk) composed of the extracellular domain of the human colony-stimulating factor 1 receptor (Fms) and the transmembrane and cytoplasmic domains of human Nyk/Mer in NIH 3T3 fibroblasts in order to investigate the mitogenic signaling and biochemical properties of Nyk/Mer. Colony-stimulating factor 1 stimulation of the Fms-Nyk chimeric receptor in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a proliferative response in the absence of other growth factors. We show that phospholipase C gamma, phosphatidylinositol 3-kinase/p70 S6 kinase, Shc, Grb2, Raf-1, and mitogen-activated protein kinase are downstream components of the Nyk/Mer signal transduction pathways. In addition, Nyk/Mer weakly activates p90rsk, while stress-activated protein kinase, Ras GTPase-activating protein (GAP), and GAP-associated p62 and p190 proteins are not activated or tyrosine phosphorylated by Nyk/Mer. An analysis comparing the Nyk/Mer signal cascade with that of the epidermal growth factor receptor indicates substrate preferences by these two receptors. Our results provide a detailed description of the Nyk/Mer signaling pathways. Given the structural similarity between the Ufo/Axl family receptors, some of the information may also be applied to other members of this receptor tyrosine kinase family.

Effect of lovastatin and fluoromevalonate on phosphatidylinositol 3-kinase activity stimulated with PDGF

1. Phosphatidylinositol 3-kinase (PI3K) appears to have a crucial role in cellular proliferation induced by platelet-derived growth factor (PDGF). However, the mode of activation of the enzyme has been unclear so far. In the present study, we investigated the effects of a cholesterol lowering drug on [3H]-thymidine ([3H]-TdR) incorporation and PI3K activity in cultured vascular smooth muscle cells (VSMC) stimulated with PDGF. 2. PDGF stimulated both [3H]-TdR incorporation and PI3K activity immunoprecipitated with antiphosphotyrosine antibody in a dose dependent manner (ED50 was 4 ng/mL for [3H]-TdR uptake and 3 ng/mL for PI3K activity). Lovastatin inhibited serum-stimulated [3H]-TdR incorporation dose dependently. PI3K activity induced by PDGF was also inhibited in a dose dependent manner; however, its activity was 61% at 10(-6) mol/L, 72% at 10(-5) mol/L and 8% at 10(-4) mol/L of the control value. 3. These inhibitory effects of lovastatin were completely abolished by adding 1 mmol/L mevalonic acid (MVA), suggesting that MVA metabolites had some important role on the PI3K activation and cellular proliferation. 4. Fluoromevalonate (Fmev), a competitive inhibitor of mevalonate diphosphate (MVA-PP) decarboxylase, inhibited [3H]-TdR incorporation at concentrations more than 10(-6) mol/L. Moreover, marked inhibitory effect was observed at concentrations of 10(-7) and 10(-8) mol/L (76% of control). PI3K activity was also reduced by 10(-3) mol/L Fmev (0.2% of control). However, in contrast to [3H]-TdR uptake, there was no inhibitory effect detected at concentrations up to 10(-4) mol/L. 5. These results suggest that PDGF-stimulated PI3K activity as well as cellular proliferation was modified by protein isoprenylation.

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.

Localization of mRNA for phosphatidylinositol 3-kinase in brain of developing and mature rats

In the adult brain, the gene expression for phosphatidylinositol 3-kinase was weak in almost all neurons with relatively high levels in the olfactory bulb, hippocampus, and cerebellum. On the embryonic days high levels of the gene expression were found throughout the entire neuraxis, then decreased gradually to adult levels during postnatal development. The present finding suggests that phosphatidylinositol 3-kinase may contribute not only to neurogenesis but also to some important physiological roles in differentiating and mature neurons.

Sodium butyrate inhibits platelet-derived growth factor-induced proliferation of vascular smooth muscle cells

Sodium butyrate (SB), a naturally occurring short-chain fatty acid, was investigated for its therapeutic value as an antiproliferative agent for vascular smooth muscle cells (SMCs). At 5-mmol/L concentration, SB had no significant effect on rat SMC proliferation. However, at the same concentration, SB inhibited platelet-derived growth factor (PDGF)-AA-, -AB-, and -BB-induced proliferation of SMCs. Exposure of SMCs to PDGF-BB resulted in activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of beta-PDGF-receptor (beta-PDGFR). The activated beta-PDGFR physically associated and phosphorylated signaling molecules such as ras-GTPase activating protein (GAP) and phospholipase C gamma (PLC gamma). SB, in the absence of PDGF-BB, caused neither beta-PDGFR tyrosine phosphorylation nor phosphorylation and association of GAP and PLC gamma with beta-PDGFR. PDGF-BB-enhanced activation of receptor intrinsic tyrosine kinase activity and autophosphorylation of tyrosine residues of beta-PDGFR were unaffected by SB irrespective of whether SMCs were preincubated with SB before exposure to PDGF-BB plus SB or incubated concomitantly with PDGF-BB plus SB. Likewise, phosphorylation and association of GAP and PLC gamma with PDGF-BB-activated beta-PDGFR were unaffected. In addition, SB did not block PDGF-BB-stimulated, PLC gamma-mediated production of inositol triphosphate. Similarly, PDGF-BB-induced beta-PDGFR degradation was unaffected when SMCs were exposed to PDGF-BB plus SB, and SB by itself had no influence on beta-PDGFR degradation. Unlike beta-PDGFR kinase activity, mitogen-activated protein kinase (MAP-kinase) activity was stimulated by SB by about 2.7-fold. Exposure of SMCs to PDGF-BB caused an approximately 11.4-fold increase in MAP-kinase activity and this increase in activity was not significantly affected when cells were coincubated with PDGF-BB and SB (10.3-fold). However, pretreatment of SMCs with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB abolished most of the PDGF-BB-induced MAP-kinase activity (4.6-fold). Transcription of growth response genes such as c-fos, c-jun, and c-myc were induced by PDGF-BB, and their induction was suppressed, particularly c-myc, by incubating SMCs with PDGF-BB plus SB. Similarly, preincubation of cells with SB for 30 minutes and subsequent incubation in PDGF-BB plus SB diminished PDGF-BB-induced transcription of c-fos, c-jun, and c-myc. However, SB by itself had no significant effect on c-fos, c-jun, and c-myc transcription.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth factor-induced tyrosine phosphorylation of Hrs, a novel 115-kilodalton protein with a structurally conserved putative zinc finger domain

The activation of growth factor receptor tyrosine kinases leads to tyrosine phosphorylation of many intracellular proteins which are thought to play crucial roles in growth factor signaling pathways. We previously showed that tyrosine phosphorylation of a 115-kDa protein is rapidly induced in cells treated with hepatocyte growth factor. To clarify the structure and possible function of the 115-kDa protein (designated Hrs for hepatocyte growth factor-regulated tyrosine kinase substrate), we purified this protein from B16-F1 mouse melanoma cells by anti-phosphotyrosine immunoaffinity chromatography and determined its partial amino acid sequences. On the basis of the amino acid sequences, we molecularly cloned the cDNA for mouse Hrs. The nucleotide sequence of the cDNA revealed that Hrs is a novel 775-amino-acid protein with a putative zinc finger domain that is structurally conserved in several other proteins. This protein also contained a proline-rich region and a proline- and glutamine-rich region. The expression of Hrs mRNA was detected in all adult mouse tissues tested and also in embryos. To analyze the Hrs cDNA product, we prepared a polyclonal antibody against bacterially expressed Hrs. Using this antibody, we showed by subcellular fractionation that Hrs is localized to the cytoplasm; we also showed that that tyrosine phosphorylation of Hrs is induced in cells treated with epidermal growth factor or platelet-derived growth factor. These results suggest that Hrs plays a unique and important role in the signaling pathway of growth factors.

A phosphatidylinositol (PI) kinase gene family in Dictyostelium discoideum: biological roles of putative mammalian p110 and yeast Vps34p PI 3-kinase homologs during growth and development

Three groups of phosphatidylinositol (PI) kinases convert PI into PI(3)phosphate, PI(4)phosphate, PI(4,5) bisphosphate, and PI(3,4,5)trisphosphate. These phosphoinositides have been shown to function in vesicle-mediated protein sorting, and they serve as second-messenger signaling molecules for regulating cell growth. To further elucidate the mechanism of regulation and function of phosphoinositides, we cloned genes encoding five putative PI kinases from Dictyostelium discoideum. Database analysis indicates that D. discoideum PIK1 (DdPIK1), -2, and -3 are most closely related to the mammalian p110 PI 3-kinase, DdPIK5 is closest to the yeast Vps34p PI 3-kinase, and DdPIK4 is most homologous to PI 4-kinases. Together with other known PI kinases, a superfamily of PI kinase genes has been defined, with all of the encoded proteins sharing a common highly conserved catalytic core domain. DdPIK1, -2, and -3 may have redundant functions because disruption of any single gene had no effect on D. discoideum growth or development. However, strains in which both of the two most highly related genes, DdPIK1 and DdPIK2, were disrupted showed both growth and developmental defects, while double knockouts of DdPIK1 and DdPIK3 and DdPIK2 and DdPIK3 appear to be lethal. The delta Ddpik1 delta Ddpik2 null cells were smaller than wild-type cells and grew slowly both in association with bacteria and in axenic medium when attached to petri plates but were unable to grow in suspension in axenic medium. When delta Ddpik1 delta Ddpik2 null cells were plated for multicellular development, they formed aggregates having multiple tips and produced abnormal fruiting bodies. Antisense expression of DdPIK5 (a putative homolog of the Saccharomyces cerevisiae VPS34) led to a defect in the growth of D. discoideum cells on bacterial lawns and abnormal development. DdPIK5 complemented the temperature-sensitive growth defect of a Schizosaccharomyces pombe delta Svps34 mutant strain, suggesting DdPIK5 encodes a functional homolog of yeast Vps34p. These observations indicate that in D. discoideum, different PI kinases regulate distinct cellular processes, including cell growth, development, and protein trafficking.

Tyrosine 807 of the v-Fms oncogene product controls cell morphology and association with p120RasGAP

Expression of the v-fms oncogene of feline sarcoma virus in fibroblasts causes surface exposure of an activated receptor tyrosine kinase, v-Fms, that is autophosphorylated at multiple sites within its cytoplasmic domain. Cellular proteins interacting with this part of v-Fms modulate the mitogenic activity and morphology of the cells. We show here that the tyrosine residue in position 807 (Y-807) of the v-Fms molecule constitutes a major autophosphorylation site. The replacement of this residue by phenylalanine (Y807F mutation) allowed us to functionally dissect v-Fms-specific mitogenic and morphogenic cascades. Cells expressing the mutant v-Fms molecule resembled wild-type (wt) v-Fms-transformed (wt-v-Fms) cells in terms of [3H]thymidine uptake rates and activation of the Ras/Raf-1 mitogenic cascade. Such cells showed, however, a flat morphology and contained intact actin cables and fibronectin network. Our studies indicate that the v-Fms molecule controls cell morphology by a cascade that involves a direct interaction with p120RasGAP and p190RhoGAP: (i) in contrast to wt v-Fms molecules, the Y807F v-Fms protein failed to associate with and phosphorylate p120RasGAP; (ii) tight complexes between p120RasGAP and p190RhoGAP as well as detectable RhoGAP activity were present exclusively in wt-v-Fms cells; and (iii) p190RhoGAP was dispersed throughout the cytoplasm of wt-v-Fms cells, whereas its distribution was restricted to perinuclear regions of cells expressing the mutant v-Fms gene.

Identification of a putative Syp substrate, the PDGF beta receptor

Because the protein-tyrosine phosphatase (PTP) Syp associates with the tyrosine-phosphorylated platelet-derived growth factor beta receptor (beta PDGFR), the beta PDGFR is a likely Syp substrate. We tested this hypothesis by determining whether recombinant Syp (rSyp) and a control PTP, recombinant PTP1B (rPTP1B), were able to dephosphorylate the beta PDGFR. The beta PDGFR was phosphorylated at multiple tyrosine residues in an in vitro kinase assay and then incubated with increasing concentrations of rSyp or rPTP1B. While the receptor was nearly completely dephosphorylated by high concentrations of rPTP1B, receptor dephosphorylation by rSyp plateaued at approximately 50%. Two-dimensional phosphopeptide maps of the beta PDGFR demonstrated that rSyp displayed a clear preference for certain receptor phosphorylation sites; the most efficiently dephosphorylated sites were phosphotyrosines (Tyr(P)-771 and -751, followed by Tyr(P)740, while Tyr(P)-1021 and Tyr(P)-1009 were very poor substrates. In contrast, rPTP1B displayed no selectivity for the various rPTP1B displayed no selectivity for the various beta PDGFR tyrosine phosphorylation sites and dephosphorylated all of them with comparable efficiency. A Syp construct that lacked the SH2 domains was still able to discriminate between the various receptor phosphorylation sites, although less effectively than full-length Syp. These in vitro studies predicted that Syp can dephosphorylate the receptor in vivo. Indeed, we found that a beta PDGFR mutant (F1009) that associates poorly with Syp, had a much slower in vivo rate of receptor dephosphorylation than the wild type receptor. In addition, the GTPase-activating protein of Ras (GAP) and phosphatidylinositol 3-kinase were less stably associated with the wild type beta PDGFR than with the F1009 receptor. These findings are consistent with the in vitro experiments showign that Syp prefers to dephosphorylate sites on the beta PDGFR, that are important for binding phosphatidylinositol 3-kinase (Tyr(P)-740 and Tyr(P)-751) and GAP (Tyr(P)-771). These studies reveal that Syp is a substrate-selective PTP and that both the catalytic domain and the SH2 domains contribute to Syp's ability to choose substrates. Furthermore, it appears that Syp plays a role in PDGF-dependent intracellular signal relay by selectively dephosphorylating the beta PDGFR and thereby regulating the binding of a distinct group of receptor-associated signal relay enzymes.

Structure-activity studies of phosphorylated peptide inhibitors of the association of phosphatidylinositol 3-kinase with PDGF-beta receptor

Phosphorylated pentapeptides derived from Tyr751 of the PDGF-beta receptor (pTyr751-Val-Pro-Met-Leu, pTyr = phosphotyrosine) were prepared to examine their ability to inhibit the association of the C-terminal SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) with the PDGF-beta receptor. Peptidic analogs were prepared to examine the importance of the amine and carboxy terminus and specific amino acids via alanine/D-amino acid scans and site specific modifications. Several of these peptides had submicromolar activity. In particular, it was shown that neutralization of the amine and carboxy terminus led to analogs with enhanced activity. In addition, it was determined that only minimal modifications were allowed for pTyr and Met, while the other positions were quite tolerant of modification.

The chemotactic response to PDGF-BB: evidence of a role for Ras

The PDGF receptor-beta mediates both mitogenic and chemotactic responses to PDGF-BB. Although the role of Ras in tyrosine kinase-mediated mitogenesis has been characterized extensively, its role in PDGF-stimulated chemotaxis has not been defined. Using cells expressing a dominant-negative ras, we find that Ras inhibition suppresses migration toward PDGF-BB. Overexpression of either Ras-GTPase activating protein (Ras-GAP) or a Ras guanine releasing factor (GRF) also inhibited PDGF-stimulated chemotaxis. In addition, cells producing excess constitutively active Ras failed to migrate toward PDGF-BB, consistent with the observation that either excess ligand or excess signaling intermediate can suppress the chemotactic response. These results suggest that Ras can function in normal cells to support chemotaxis toward PDGF-BB and that either too little or too much Ras activity can abrogate the chemotactic response. In contrast to Ras overexpression, cells producing excess constitutively active Raf, a downstream effector of Ras, did migrate toward PDGF-BB. Cells expressing dominant-negative Ras were able to migrate toward soluble fibronectin demonstrating that these cells retained the ability to migrate. These results suggest that Ras is an intermediate in PDGF-stimulated chemotaxis but may not be required for fibronectin-stimulated cell motility.

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.

The isolation and characterization of cDNA encoding human and rat brain inositol polyphosphate 4-phosphatase

Inositol polyphosphate 4-phosphatase, an enzyme of the inositol phosphate signaling pathway, catalyzes the hydrolysis of the 4-position phosphate of inositol 3,4-bisphosphate, inositol 1,3,4-trisphosphate, and phosphatidylinositol 3,4-bisphosphate. The amino acid sequences of tryptic and CNBr peptides of the enzyme isolated from rat brain were determined. Degenerate oligonucleotide primers based on this sequence were used to amplify a 74-base pair polymerase chain reaction product. This product was used to isolate a 5607-base pair composite cDNA, which had an open reading frame encoding a protein with 939 amino acids with a predicted molecular mass of 105,588 Da. The rat brain polymerase chain reaction product was used as a probe to isolate a human brain cDNA that predicts a protein with 938 amino acids and a molecular mass of 105,710 Da. Remarkably, the human and rat proteins were 97% identical. Recombinant rat protein expressed in Escherichia coli catalyzed the hydrolysis of all three substrates of the 4-phosphatase. Northern blot hybridization indicates that the 4-phosphatase is widely expressed in rat tissues with the highest levels of expression occurring in brain, heart, and skeletal muscle. Polyclonal antiserum directed against the carboxyl terminus of the 4-phosphatase immunoprecipitated > 95% of the 4-phosphatase activity in crude homogenates of rat brain, heart, skeletal muscle, and spleen, suggesting that this enzyme accounts for the 4-phosphate activity present in rat tissues. This antiserum also immunoprecipitated the 4-phosphatase from human platelet sonicates.

Mitogen-activated protein kinase activation is insufficient for growth factor receptor-mediated PC12 cell differentiation

When expressed in PC12 cells, the platelet-derived growth factor beta receptor (beta PDGF-R) mediates cell differentiation. Mutational analysis of the beta PDGF-R indicated that persistent receptor stimulation of the Ras/Raf/mitogen-activated protein (MAP) kinase pathway alone was insufficient to sustain PC12 cell differentiation. PDGF receptor activation of signal pathways involving p60c-src or the persistent regulation of phospholipase C gamma was required for PC12 cell differentiation. beta PDGF-R regulation of phosphatidylinositol 3-kinase, the GTPase-activating protein of Ras, and the tyrosine phosphatase, Syp, was not required for PC12 cell differentiation. In contrast to overexpression of oncoproteins involved in regulating the MAP kinase pathway, growth factor receptor-mediated differentiation of PC12 cells requires the integration of other signals with the Ras/Raf/MAP kinase pathway.