Topographic regulation of cytoskeletal protein phosphorylation by multimeric complexes in the squid giant fiber system

In mammalian and squid nervous systems, the phosphorylation of neurofilament proteins (NFs) seems to be topographically regulated. Although NFs and relevant kinases are synthesized in cell bodies, phosphorylation of NFs, particularly in the lys‐ser‐pro (KSP) repeats in NF‐M and NF‐H tail domains, seem to be restricted to axons. To explore the factors regulating the cellular compartmentalization of NF phosphorylation, we separated cell bodies (GFL) from axons in the squid stellate ganglion and compared the kinase activity in the respective lysates. Although total kinase activity was similar in each lysate, the profile of endogenous phosphorylated substrates was strikingly different. Neurofilament protein 220 (NF220), high‐molecular‐weight NF protein (HMW), and tubulin were the principal phosphorylated substrates in axoplasm, while tubulin was the principal GFL phosphorylated substrate, in addition to highly phosphorylated low‐molecular‐weight proteins. Western blot analysis showed that whereas both lysates contained similar kinases and cytoskeletal proteins, phosphorylated NF220 and HMW were completely absent from the GFL lysate. These differences were highlighted by P13^suc1 affinity chromatography, which revealed in axoplasm an active multimeric phosphorylation complex(es), enriched in cytoskeletal proteins and kinases; the equivalent P13 GFL complex exhibited six to 20 times less endogenous and exogenous phosphorylation activity, respectively, contained fewer cytoskeletal proteins and kinases, and expressed a qualitatively different cdc2‐like kinase epitope, 34 kDa rather than 49 kDa. Cell bodies and axons share a similar repertoire of molecular consitutents; however, the data suggest that the cytoskeletal/kinase phosphorylation complexes extracted from each cellular compartment by P13 are fundamentally different. © 1999 John Wiley & Sons, Inc. J Neurobiol 40: 89–102, 1999     

Association of the tyrosine phosphatase SHP-2 with transducin-alpha and a 97-kDa tyrosine-phosphorylated protein in photoreceptor rod outer segments

Increasing evidence indicates that tyrosine phosphorylation, controlled by the concerted action of tyrosine kinases and protein tyrosine phosphatases (PTPs), plays important roles in retinal photoreceptor rod outer segments (ROS). We characterized PTP activity in isolated bovine ROS that is significantly inhibited by orthovanadate. Incubating ROS in the presence of exogenous Mg2+, ATP, and orthovanadate dramatically enhanced the tyrosine phosphorylation of several endogenous proteins. SHP-2, a PTP with two SH2 domains, was identified in ROS by immunoblot analysis and was found to associate with ROS membranes. Immunocytochemistry showed localization of SHP-2 in photoreceptor outer segments and possibly in the outer plexiform, inner nuclear, and inner plexiform cell layers of the retina as well. SHP-2 associated with transducin-alpha and a 97-kDa tyrosine-phosphorylated protein in ROS, suggesting the formation of a multimeric signaling complex. Based on its association with transducin-alpha and a 97-kDa protein, SHP-2 may regulate the tyrosine phosphorylation of endogenous proteins, including transducin-alpha, and may play a significant role in a novel signaling pathway in photoreceptors.     

Cell-to-cell transfer of glial proteins to the squid giant axon: The glia- neuron protein transfer hypothesis

The hypothesis that glial cells synthesize proteins which are transferred to adjacent neurons was evaluated in the giant fiber of the squid (Loligo pealei). When giant fibers are separated from their neuron cell bodies and incubated in the presence of radioactive amino acids, labeled proteins appear in the glial cells and axoplasm. Labeled axonal proteins were detected by three methods: extrusion of the axoplasm from the giant fiber, autoradiography, and perfusion of the giant fiber. This protein synthesis is completely inhibited by puromycin but is not affected by chloramphenicol. The following evidence indicates that the labeled axonal proteins are not synthesized within the axon itself. (a) The axon does not contain a significant amount of ribosomes or ribosomal RNA. (b) Isolated axoplasm did not incorporate [(3)H]leucine into proteins. (c) Injection of Rnase into the giant axon did not reduce the appearance of newly synthesized proteins in the axoplasm of the giant fiber. These findings, coupled with other evidence, have led us to conclude that the adaxonal glial cells synthesize a class of proteins which are transferred to the giant axon. Analysis of the kinetics of this phenomenon indicates that some proteins are transferred to the axon within minutes of their synthesis in the glial cells. One or more of the steps in the transfer process appear to involve Ca++, since replacement of extracellular Ca++ by either Mg++ or Co++ significantly reduces the appearance of labeled proteins in the axon. A substantial fraction of newly synthesized glial proteins, possibly as much as 40 percent, are transferred to the giant axon. These proteins are heterogeneous and range in size from 12,000 to greater than 200,000 daltons. Comparisons of the amount of amino acid incorporation in glia cells and neuron cell bodies raise the possibility that the adaxonal glial cells may provide an important source of axonal proteins which is supplemental to that provided by axonal transport from the cell body. These findings are discussed with reference to a possible trophic effect of glia on neurons and metabolic cooperation between adaxonal glia and the axon.     

Characterization of a cyclic nucleotide- and calcium-independent neurofilament protein kinase activity in axoplasm from the squid giant axon

The phosphorylation activity associated with a neurofilament-enriched cytoskeletal preparation isolated from the squid giant axon has been studied and compared to the phosphorylation activities in intact squid axoplasm. The high molecular weight (greater than 300 kDa) and 220-kDa neurofilament proteins are the major endogenous substrates for the kinases in the axoplasm and the neurofilament preparation, whereas 95- and less than 60-kDa proteins are the major phosphoproteins in the ganglion cell preparation. The squid axon neurofilament (SANF) protein kinase activity appeared to be both cAMP and Ca2+ independent and could phosphorylate both casein (Km = 40 microM) and histone (Km = 180 microM). The SANF protein kinase could utilize either ATP or GTP in the phosphotransferase reaction, with a Km for ATP of 58 microM and 129.4 microM for GTP when casein was used as the exogenous substrate; and 25 and 98.1 microM for ATP and GTP, respectively, when the endogenous neurofilament proteins were used as substrates. The SANF protein kinase activity was only slightly inhibited by 2,3-diphosphoglycerate and various polyamines at high concentrations and was poorly inhibited by heparin (34% inhibition at 100 micrograms/ml). The failures of heparin to significantly inhibit and the polyamines to stimulate the SANF protein kinase indicate that it is not a casein type II kinase. The relative efficacy of GTP as a phosphate donor indicates that SANF protein kinase differs from known casein type I kinases. Phosphorylated (32P-labeled) neurofilament proteins were only slightly dephosphorylated in the presence of axoplasm or stellate ganglion cell supernatants, and the neurofilament-enriched preparation did not dephosphorylate 32P-labeled neurofilament proteins. The axoplasm and neurofilament preparations had no detectable protein kinase inhibitor activity, but a strong inhibitor activity, which was not dialyzable but was heat inactivatable, was found in ganglion cells. This inhibitor activity may account for the low phosphorylation activity found in the stellate ganglion cells and may indicate inhibitory regulation of SANF protein kinase activity in the ganglion cell bodies.     

Vanadate protects human neuroblastoma SH-SY5Y cells against peroxynitrite-induced cell death

We investigated the effect of vanadate, a tyrosine phosphatase inhibitor, on cell death induced by peroxynitrite in human neuroblastoma SH-SY5Y cells. Vanadate prevented cell death induced by 3-morpholinosydnonimine (SIN-1), a peroxynitrite donor; whereas SIN-1-induced cell death was not prevented by neither okadaic acid, an inhibitor of serine/threonine phosphatases 1 and 2A, nor cyclosporin A, an inhibitor of serine/threonine phosphatase 2B. Vanadate did not prevent cell death induced by N-ethyl-2-(1-ethyl-hydroxy-2-nitrosohydrazino)-ethanamine, a nitric oxide donor. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase (PI3-kinase), did not block the protective effect of vanadate, suggesting that the protective effect of vanadate is independent on PI3-kinase. Vanadate increased tyrosine phosphorylation of several proteins including the focal adhesion protein p130 Crk-associated substrate (p130(cas)). By the treatment with SIN-1, the endogenous association of p130(cas) and Crk was disrupted, and the association was restored by vanadate treatment. These results suggest that disruption of tyrosine phosphorylation signaling may be critical for peroxynitrite-induced cell death, and that vanadate prevents cell death at least in part through the enhancement in tyrosine phosphorylation of the proteins including p130(cas).     

Vanadate inhibition of protein tyrosine phosphatases in Jurkat cells: modulation by redox state

 Vanadate is a potent reversible inhibitor of protein tyrosine phosphatases (PTP) in vitro. Vanadate has been shown to increase the phosphotyrosine levels in some cell types whereas in others, like the Jurkat T-lymphoma, vanadate has no effect. The reason for the apparent lack of effect of vanadate in Jurkat cells was investigated in this study. Alteration of the redox state of these cells by reducing the glutathione level with 1-chloro-2,4-dinitrobenzene (DnpCl) had no effect on phosphotyrosine levels. However, the cells became sensitive to vanadate, as measured by an increase in phosphotyrosine levels on a wide range of proteins including the MAP kinases. The increase in phosphotyrosine levels most likely results from inhibition of cellular PTP and suggests that protein tyrosine kinases are constitutively active in cells, resulting in a dynamic phosphorylation-dephosphorylation cycle. The mode of inhibition of PTP by vanadate was investigated by measuring the PTP activity of Jurkat membranes isolated after treatment of cells with vanadate and DnpCl. In contrast to the reversible inhibition of PTP in vitro, the effect of vanadate in the presence of DnpCl was irreversible, raising the possibility that it is peroxovanadate formed in situ that is responsible for the inhibition of PTP in intact cells. Received: 4 December 1998 · Accepted: 22 March 1999     

Neural cell adhesion molecules modulate tyrosine phosphorylation of tubulin in nerve growth cone membranes.

Triggering neural cell adhesion molecules of the immunoglobulin superfamily with specific ligands or antibodies inhibited the phosphorylation of tryosyl residues in a subpopulation of alpha- and beta-tubulin associated with membranes from a subcellular fraction of nerve growth cones from fetal rat brain. Preincubation of these membranes with purified extracellular fragments of L1, N-CAM, or myelin-associated glycoprotein, or with antibodies directed against the extracellular domains of L1 or N-CAM, inhibited pp60c-src-dependent phosphorylation of tubulin in an endogenous membrane kinase reaction. Other proteins that affect neurite outgrowth (fibronectin, laminin, antibodies against N-cadherin) had no effect. The results suggest that cell adhesion molecules transduce cell surface events to intracellular signals by modulating the activity of protein tyrosine kinases or phosphatases in axonal membranes to influence cytoskeletal dynamics at the growth cone.     

Calcium-Activated Proteolysis of Neurofilament Proteins in the Squid Giant Neuron

The phosphorylation and proteolysis of squid neurofilament proteins by endogenous kinase and calcium-activated protease activities, respectively, were studied. When axoplasm was incubated in the presence of [gamma-32P]ATP, most of the phosphate was incorporated into two neurofilament proteins: a 220-kilodalton (NF-220) and a high-molecular-weight (HMW) protein. When this phosphorylated axoplasm was subjected to endogenous calcium-activated proteolysis, two significant phosphorylated fragments were generated, i.e., a soluble 110K fragment and a pelletable 100K fragment. Immunochemical and other analyses suggest that the pelletable 100K fragment contains the common helical neurofilament rod region and that the soluble 110K protein is the putative side arm of the NF-220. In contrast, neither the HMW or the NF-220 was detected in the region of the stellate ganglion which contains the cell bodies of the giant axon. However, this region did contain a number of proteins that were sensitive to calcium-activated proteolysis and reacted with a monoclonal intermediate filament antibody. This intermediate filament antibody reacts with most of the axoplasmic proteins that copurify with neurofilaments, i.e., in the order of their intermediate filament antibody staining intensity, a 60K, 65K, 220K, and 74K protein. In the cell body preparation, the intermediate filament antibody labeled, in order of their staining intensity, a 65K, 60K, 74K, and 180K protein. In both the axoplasmic and cell body preparations, endogenous calcium-activated proteolysis generated characteristic fragments that could be labeled with the anti-intermediate filament antibody.     

Protein tyrosine phosphatases in disease processes

Given the importance of tyrosine phosphorylation of proteins in signalling pathways, it is perhaps not surprising that protein tyrosine phosphatases (PTPs) are involved in the pathogenesis of certain human diseases. A PTP produced by the Yersinia bacteria (which can cause bubonic plague, septicemia and enteric diseases) is thought to be used as a ‘weapon’ against host cell functions. In addition, dysfunction of cells' endogenous PTPs may contribute to defective immune function, to cancer and to diabetes.     

Redox regulation of the protein tyrosine phosphatase PTP1B in cancer cells

The oxidation and inactivation of protein tyrosine phosphatases is one mechanism by which reactive oxygen species influence tyrosine phosphorylation-dependent signaling events and exert their biological functions. In the present study, we determined the redox status of endogenous protein tyrosine phosphatases in HepG2 and A431 human cancer cells, in which reactive oxygen species are produced constitutively. We used mass spectrometry to assess the state of oxidation of the catalytic cysteine residue of endogenous PTP1B and show that this residue underwent both reversible and irreversible oxidation to high stoichiometry in response to intrinsic reactive oxygen species production. In addition, our data show that the oxidation of PTP1B is specific to the active site Cys, with the other Cys residues in the protein remaining in a reduced state. Treatment of these cells with diphenyleniodonium, an inhibitor of NADPH oxidases, decreased reactive oxygen species levels. This resulted in inhibition of protein tyrosine phosphatase oxidation, concomitant with decreased tyrosine phosphorylation of cellular proteins and inhibition of anchorage-independent cell growth. Therefore, our data also suggest that the high level of intrinsic reactive oxygen species may contribute to the transformed phenotype of HepG2 and A431 cells via constitutive inactivation of cellular protein tyrosine phosphatases.     

蛋白酪氨酸磷酸酶-PEST在生理调节及相关疾病中的作用

蛋白质分子中酪氨酸残基可逆性的磷酸化是细胞内信号分子传导的基本方式。两类作用相反的酶参与磷酸化的调节:蛋白酪氨酸激酶(protein tyrosinekinase,PTK)和蛋白酪氨酸磷酸酶(protein tyrosine phosphatase,PTP)。含脯氨酸-谷氨酸-丝氨酸-苏氨酸(P-E-S-T)结构域的蛋白酪氨酸磷酸酶(PTP-PEST)属于非受体型酪氨酸磷酸酶类,其本身能与多种蛋白质相互作用,并在细胞迁移、免疫细胞活化和胚胎发育等生理过程中发挥重要作用。本文对PTP-PEST的结构特点、生理功效、介导的信号传导途径和近年来PTP-PEST在疾病中的作用作一综述。     

Stimulation of tyrosine-specific protein phosphorylation and phosphatidylinositol phosphorylation by orthovanadate in rat liver plasma membrane

Orthovanadate stimulated the incorporation of 32P from [gamma-32P]ATP by Triton X-100-solubilized rat liver plasma membrane into endogenous, trichloroacetic acid-precipitable materials as well as added (Glu4:Tyr1) copolymers. Extraction of incubation mixture with chloroform-methanol-HCl revealed that the increase in 32P incorporation by vanadate was predominantly into endogenous phospholipids. [32P]Phosphatidylinositol 4-phosphate (PtdIns-4-P) was identified by thin-layer chromatography as the major phosphorylated product of vanadate stimulation, which also resulted in elevated 32P, predominantly in P-Tyr in endogenous membrane proteins. Vanadate effects on protein tyrosine and phosphatidylinositol phosphorylation were concomitant and exhibited similar sensitivity. These effects of vanadate were enhanced by the presence of either dithiothreitol or NAD(P)H. Phosphatidylinositol phosphorylation could also be stimulated by a substrate of and inhibited by a synthetic inhibitory copolymer of tyrosine kinase. These results suggest that vanadate, an oxygen radical producer, stimulates a tyrosine kinase-PtdIns kinase coupled system much like those described for a number of growth factors and oncogene encoded products.     

Vanadate-treated baby hamster kidney fibroblasts show cytoskeleton and adhesion patterns similar to their Rous sarcoma virus-transformed counterparts

Rous sarcoma virus-transformed baby hamster kidney fibroblasts (RSV/B4-BHK) adhere to a fibronectin-coated substratum by means of dot-like adhesion sites called podosomes in view of their shape and function as cellular feet (Tarone et al.: Exp Cell Res 159:141, 1985). Podosomes concentrate tyrosine-phosphorylated proteins, including pp60v-src, and appear in many cells transformed by oncogenes coding for tyrosine kinases. In this paper we used orthovanadate, an inhibitor of phosphotyrosine phosphatases, in order to increase the cellular concentration of phosphotyrosine and to study whether this treatment induced the cytoskeleton remodeling leading to the formation of podosomes. Indeed, orthovanadate (10-100 microM) induced in a time- and dose-dependent manner the redistribution of F-actin and the formation of podosomes in BHK cells. Cytoskeleton remodeling occurred along with a marked increase of tyrosine phosphorylated proteins. The vanadate effect on the cytoskeletal phenotype was enhanced by the simultaneous treatment of cells with a phorbol ester. Under the latter conditions almost all BHK cells showed podosomes. The vanadate effect was reversible insofar as podosomes and tyrosine-phosphorylated proteins disappeared. Then, vanadate treatment of normal cells induced the cascade of events leading to the cytoskeletal changes typical of transformation and suggested that the transformed cytoskeletal phenotype may be primarily induced by the tyrosine phosphorylation of unknown target(s) operated by endogenous kinases.     

A Loss-of-Function Screen for Phosphatases that Regulate Neurite Outgrowth Identifies PTPN12 as a Negative Regulator of TrkB Tyrosine Phosphorylation

Alterations in function of the neurotrophin BDNF are associated with neurodegeneration, cognitive decline, and psychiatric disorders. BDNF promotes axonal outgrowth and branching, regulates dendritic tree morphology and is important for axonal regeneration after injury, responses that largely result from activation of its tyrosine kinase receptor TrkB. Although intracellular neurotrophin (NT) signaling presumably reflects the combined action of kinases and phosphatases, little is known about the contributions of the latter to TrkB regulation. The issue is complicated by the fact that phosphatases belong to multiple independently evolved families, which are rarely studied together. We undertook a loss-of-function RNA-interference-based screen of virtually all known (254) human phosphatases to understand their function in BDNF/TrkB-mediated neurite outgrowth in differentiated SH-SY5Y cells. This approach identified phosphatases from diverse families, which either positively or negatively modulate BDNF-TrkB-mediated neurite outgrowth, and most of which have little or no previously established function related to NT signaling. “Classical” protein tyrosine phosphatases (PTPs) accounted for 13% of the candidate regulatory phosphatases. The top classical PTP identified as a negative regulator of BDNF-TrkB-mediated neurite outgrowth was PTPN12 (also called PTP-PEST). Validation and follow-up studies showed that endogenous PTPN12 antagonizes tyrosine phosphorylation of TrkB itself, and the downstream activation of ERK1/2. We also found PTPN12 to negatively regulate phosphorylation of p130cas and FAK, proteins with previously described functions related to cell motility and growth cone behavior. Our data provide the first comprehensive survey of phosphatase function in NT signaling and neurite outgrowth. They reveal the complexity of phosphatase control, with several evolutionarily unrelated phosphatase families cooperating to affect this biological response, and hence the relevance of considering all phosphatase families when mining for potentially druggable targets.     

Cultured Giant Fiber Lobe of Squid Expresses Three Distinct Potassium Channel Activities in Selective Combinations

Neurons from the giant fiber lobe (GFL) of squid Loligo bleekeri were dissociated and cultured. The ionic currents were recorded using whole-cell patch clamp methods. The sodium current and the noninactivating potassium current like those elicited by the giant axon were among the currents expressed in axonal bulbs and bulblike structures upon dissociation. Meanwhile axonless cell bodies did not elicit such currents. Axonless cell bodies and some bulblike structures elicited two kinds of inactivating potassium currents, the slow- and the fast-inactivating current, which differed in their inactivation kinetics and pharmacology. Within 24 hr of plating, the current composition remained the same. While the noninactivating current was not sensitive to 4-aminopyridine, the two inactivating currents were sensitive, the slow-inactivating current being more sensitive. Selective combinations of the sodium current and the three potassium currents expressed in different structures of the acutely dissociated GFL could have resulted from cellular control of synthesis and transportation of the channel proteins to the somatic and the axonal membrane. The sodium current and the noninactivating potassium current could be recorded from some axonless cell bodies maintained in culture for over three days, indicating that the separation of the giant axon from its somata could result in the transportation of the channels normally expressed on the giant axon membrane to the somatic membrane. Received: 24 October 1995/Revised: 5 March 1996     

Protein tyrosine phosphatases--endogenous markers of oxidative stress

The reversible phosphorylation of structural and regulatory proteins in eucaryotic cells is one of the most important regulatory mechanisms. Protein tyrosine phosphatases (PTP) regulate a wide range of signal transduction pathways that control many cellular processes such as cell proliferation, differentiation and growth. Disorder in PTP gene expression is implicated in the development of cancer, autoimmune and neurodegenerative diseases. The active sites of these enzymes are characterized by the consensus sequence containing cysteine which is essential for enzyme activity and highly susceptible to oxidation. Reversible oxidation of the catalytic cysteine is becoming recognized as a general mechanism for regulation of PTP enzymatic activity. These findings suggest that protein tyrosine phosphatases may be considered as very sensitive markers of oxidative stress. Many studies have demonstrated that the production of reactive oxygen species during oxidative stress can inactivate protein tyrosine phosphatases.     

Subcellular Localization of Functionally Differentiated Microtubules in Squid Neurons: Regional Distribution of Microtubule-Associated Proteins and β-Tubulin Isotypes

The subcellular localization of microtubule proteins in the neurons of squid (Doryteuthis bleekeri) was immunologically studied using monoclonal antibodies against the microtubule proteins. We found that (1) the squid neurons contained three kinds of high-molecular-weight microtubule-associated proteins [MAP A of approximately 300 kilodaltons (kD), MAP B of 260 kD, and axolinin of 260 kD] and two kinds of beta-tubulin isotypes (beta 1 and beta 2); (2) the cell body of the squid giant neuron contained MAP A, MAP B, and the two beta-tubulin isotypes (beta 1 and beta 2); (3) axolinin and the beta 1 isotype were present exclusively in the peripheral axoplasm of the giant axon; and (4) a small amount of axolinin, MAP A, and the beta 1 isotype was found in the insoluble aspect of the central axoplasm, whereas the soluble aspect of the central axoplasm contained an abundant amount of MAP A along with the modified form of the beta 1 isotype. The regional difference of the distribution of the microtubule protein components may explain the differences in stability among axonal microtubules. Microtubules in the soluble aspect of the central axoplasm are sensitive to any treatment with colchicine, cold temperature, and high ionic strength but those both in the insoluble aspect of the central axoplasm and in the peripheral axoplasm are highly insensitive to the treatment.     

The control of phosphatidylinositol 3,4-bisphosphate concentrations by activation of the Src homology 2 domain containing inositol polyphosphate 5-phosphatase 2, SHIP2

Activation of class Ia PI3K (phosphoinositide 3-kinase) produces PtdInsP3, a vital intracellular mediator whose degradation generates additional lipid signals. In the present study vanadate analogues that inhibit PTPs (protein tyrosine phosphatases) were used to probe the mechanisms which regulate the concentrations of these molecules allowing their independent or integrated function. In 1321N1 cells, which lack PtdInsP3 3-phosphatase activity, sodium vanadate or a cell permeable derivative, bpV(phen) [potassium bisperoxo(1,10-phenanthroline)oxovanadate (V)], increased the recruitment into anti-phosphotyrosine immunoprecipitates of PI3K activity and of the p85 and p110a subunits of class Ia PI3K and enhanced the recruitment of PI3K activity stimulated by PDGF (platelet-derived growth factor). However, neither inhibitor much increased cellular PtdInsP3 concentrations, but both diminished dramatically the accumulation of PtdInsP3 stimulated by PDGF or insulin and markedly increased the control and stimulated concentrations of PtdIns(3,4)P2. These actions were accounted for by the ability of PTP inhibitors to stimulate the activity of endogenous PtdInsP3 5-phosphatase(s), particularly SHIP2 (Src homology 2 domain containing inositol polyphosphate 5-phosphatase 2) and to inhibit types I and II PtdIns(3,4)P2 4-phosphatases. Thus bpV(phen) promoted the translocation of SHIP2 from the cytosol to a Triton X-100-insoluble fraction and induced a marked (5-10-fold) increase in SHIP2 specific activity mediated by enhanced tyrosine phosphorylation. The net effect of these inhibitors was, therefore, to switch the signal output of class I PI3K from PtdInsP3 to PtdIns(3,4)P2. A key component controlling this shift in the balance of lipid signals is the activation of SHIP2 by increased tyrosine phosphorylation, an effect observed in HeLa cells in response to both PTP inhibitors and epidermal growth factor.