Modification of the 85-kilodalton subunit of phosphatidylinositol-3 kinase in platelet-derived growth factor-stimulated cells

Idiopathic Pulmonary Fibrosis and Lung Cancer: Mechanisms and Molecular Targets

Idiopathic pulmonary fibrosis (IPF) is the most common idiopathic interstitial pulmonary disease with a median survival of 2–4 years after diagnosis. A significant number of IPF patients have risk factors, such as a history of smoking or concomitant emphysema, both of which can predispose the patient to lung cancer (LC) (mostly non-small cell lung cancer (NSCLC)). In fact, IPF itself increases the risk of LC development by 7% to 20%. In this regard, there are multiple common genetic, molecular, and cellular processes that connect lung fibrosis with LC, such as myofibroblast/mesenchymal transition, myofibroblast activation and uncontrolled proliferation, endoplasmic reticulum stress, alterations of growth factors expression, oxidative stress, and large genetic and epigenetic variations that can predispose the patient to develop IPF and LC. The current approved IPF therapies, pirfenidone and nintedanib, are also active in LC. In fact, nintedanib is approved as a second line treatment in NSCLC, and pirfenidone has shown anti-neoplastic effects in preclinical studies. In this review, we focus on the current knowledge on the mechanisms implicated in the development of LC in patients with IPF as well as in current IPF and LC-IPF candidate therapies based on novel molecular advances.

Exploration of a potent PI3 kinase/mTOR inhibitor as a novel anti-fibrotic agent in IPF

Rationale

Idiopathic pulmonary fibrosis (IPF) is the most rapidly progressive and fatal of all fibrotic conditions with no curative therapies. Common pathomechanisms between IPF and cancer are increasingly recognised, including dysfunctional pan-PI3 kinase (PI3K) signalling as a driver of aberrant proliferative responses. GSK2126458 is a novel, potent, PI3K/mammalian target of rapamycin (mTOR) inhibitor which has recently completed phase I trials in the oncology setting. Our aim was to establish a scientific and dosing framework for PI3K inhibition with this agent in IPF at a clinically developable dose.

Methods

We explored evidence for pathway signalling in IPF lung tissue and examined the potency of GSK2126458 in fibroblast functional assays and precision-cut IPF lung tissue. We further explored the potential of IPF patient-derived bronchoalveolar lavage (BAL) cells to serve as pharmacodynamic biosensors to monitor GSK2126458 target engagement within the lung.

Results

We provide evidence for PI3K pathway activation in fibrotic foci, the cardinal lesions in IPF. GSK2126458 inhibited PI3K signalling and functional responses in IPF-derived lung fibroblasts, inhibiting Akt phosphorylation in IPF lung tissue and BAL derived cells with comparable potency. Integration of these data with GSK2126458 pharmacokinetic data from clinical trials in cancer enabled modelling of an optimal dosing regimen for patients with IPF.

Conclusions

Our data define PI3K as a promising therapeutic target in IPF and provide a scientific and dosing framework for progressing GSK2126458 to clinical testing in this disease setting. A proof-of-mechanism trial of this agent is currently underway.

Trial registration number

NCT01725139, pre-clinical.

Platelet-derived growth factor and transforming growth factor beta synergistically potentiate inflammatory mediator synthesis by fibroblast-like synoviocytes

Introduction

The objective of this study was to model the effects of transforming growth factor beta (TGF-β) and platelet-derived growth factor (PDGF), both present in rheumatoid arthritis (RA) synovia, on the behavior of fibroblast-like synoviocytes (FLS) in response to pro-inflammatory cytokine (interleukin (IL)1β, tumor necrosis factor-alpha (TNFα)) challenge.

Methods

Gene and protein expression by fibroblast-like synoviocytes in vitro was studied by quantitative Polymerase Chain Reaction (qPCR), ELISA and multiplex bead cytokine assays. Intracellular signaling pathway activation was determined by Western blot for phospho-kinases and the use of specific inhibitors.

Results

In combination, TGF-β and PDGF (2GF) synergistically augmented TNFα- or IL1β-induced matrix metalloproteinase 3 (MMP3), IL6, IL8, and macrophage inflammatory protein 1 alpha (MIP1α) secretion by FLS. Other FLS-derived mediators remained unaffected. Individually, neither growth factor significantly potentiated TNFα or IL1β-induced MMP3 secretion, and only slightly enhanced IL6. The effect of 2GF on TNFα-induced gene expression was transcriptionally mediated; blocked by imatinib mesylate; and occurred even if 2GF was added as much as four hours prior to TNFα. In addition, a 15-minute pulse of 2GF four hours prior to TNFα stimulation yielded a synergistic response. The extracellular-signal-regulated kinase (ERK) and phosphoinositide 3-kinase (PI3K) signaling pathways were induced for at least four hours by 2GF, as demonstrated by persistently upregulated levels of phospho-Akt and phospho-ERK. However, pharmacologic inhibitor studies demonstrated that the potentiating action of 2GF was dependent on PI3 kinase only, and not on ERK.

Conclusions

The combination of PDGF and TGF-β dramatically potentiates FLS response to cytokines in a receptor-mediated and PI3 kinase-dependent fashion. These data suggest that 2GF contribute to synovitis by directing synovial fibroblasts toward a more aggressive phenotype in response to TNFα. Therefore, inhibition of growth factor signaling may constitute a complementary therapeutic approach to cytokine-targeted treatments for RA.

Computational models of tandem SRC homology 2 domain interactions and application to phosphoinositide 3-kinase

Intracellular signal transduction proteins typically utilize multiple interaction domains for proper targeting, and thus a broad diversity of distinct signaling complexes may be assembled. Considering the coordination of only two such domains, as in tandem Src homology 2 (SH2) domain constructs, gives rise to a kinetic scheme that is not adequately described by simple models used routinely to interpret in vitro binding measurements. To analyze the interactions between tandem SH2 domains and bisphosphorylated peptides, we formulated detailed kinetic models and applied them to the phosphoinositide 3-kinase p85 regulatory subunit/platelet-derived growth factor beta-receptor system. Data for this system from different in vitro assay platforms, including surface plasmon resonance, competition binding, and isothermal titration calorimetry, were reconciled to estimate the magnitude of the cooperativity characterizing the sequential binding of the high and low affinity SH2 domains (C-SH2 and N-SH2, respectively). Compared with values based on an effective volume approximation, the estimated cooperativity is 3 orders of magnitude lower, indicative of significant structural constraints. Homodimerization of full-length p85 was found to be an alternative mechanism for high avidity binding to phosphorylated platelet-derived growth factor receptors, which would render the N-SH2 domain dispensable for receptor binding.

Membrane-binding/modification model of signaling protein activation and analysis of its control by cell morphology

A mechanism for cell shape control of intracellular signal transduction, whereby the average concentration of activated proteins in the cytosol increases as the height of the cell decreases, has been described recently. An important modification of this analysis is offered, recognizing that signaling proteins are not only activated at the plasma membrane but must first form complexes with signaling molecules that reside there, such as receptors and lipids. With these more realistic boundary conditions, it is shown that the region of parameter space where cell shape amplifies the average cytosolic activity is greatly expanded. Moreover, this model allows for amplification of the activated protein bound at the membrane, which is considered more relevant for certain, spatially driven signaling processes in cell migration.

Phosphatidylinositol 3-kinase mediates activation of ATM by high NaCl and by ionizing radiation: Role in osmoprotective transcriptional regulation

High NaCl causes DNA double-strand breaks and activates the transcription factor, TonEBP/OREBP, resulting in increased transcription of several protective genes, including those involved in accumulation of compatible organic osmolytes. Several kinases are known to contribute to signaling activation of TonEBP/OREBP, including ATM, which is a member of the phosphatidylinositol 3-kinase (PI3K)-like kinase family and is activated by DNA double-strand breaks. The purpose of the present studies was to investigate a possible role of PI3K Class IA (PI3K-IA). We found that high NaCl increases PI3K-IA lipid kinase activity. Inhibiting PI3K-IA either by expressing a dominant negative of its regulatory subunit, p85, or by small interfering RNA-mediated knockdown of its catalytic subunit, p110alpha, reduces high NaCl-induced increases in TonEBP/OREBP transcriptional activity and transactivation, but not nuclear translocation of TonEBP/OREBP, or increases in its abundance. Further, suppression of PI3K-IA inhibits the activation of ATM that is caused by either ionizing radiation or high NaCl. High NaCl-induced increase in TonEBP/OREBP activity is reduced equally by inhibition of ATM or PI3K-IA, and the effects are not additive. The conclusions are as follows: (i) PI3K-IA activity is necessary for both high NaCl- and ionizing radiation-induced activation of ATM and (ii) high NaCl activates PI3K-IA, which, in turn, contributes to full activation of TonEBP/OREBP via ATM.

Corneal cell survival in adenovirus type 19 infection requires phosphoinositide 3-kinase/Akt activation

Adenovirus type 19 is a major cause of epidemic keratoconjunctivitis, the only ocular adenoviral infection associated with prolonged corneal inflammation. In this study, we investigated the role of phosphoinositide 3-kinase (PI3K) and Akt and their downstream targets in adenovirus infection, and here we report the novel finding that adenovirus type 19 utilizes the PI3K/Akt pathway to maintain corneal fibroblast viability in acute infection. We demonstrate phosphorylation of GSK-3beta and nuclear translocation of the p65 subunit of NF-kappaB, both downstream targets of the PI3K/Akt pathway, in adenovirus-infected corneal fibroblasts in a PI3K-dependent manner. Inhibition of PI3K had no effect on early viral gene expression, suggesting normal viral internalization, but pretreatment with the PI3K inhibitor LY294002 or overexpression of dominant negative Akt induced early cytopathic effect and caspase-mediated cell death in adenovirus-infected cells. Early cell death could be circumvented despite LY294002 by overexpression of constitutively active Akt. Furthermore, we show an interaction between cSrc and the p85 regulatory subunit of PI3K in infected cells through a phosphorylation-dependent mechanism. The results presented in this paper provide the first direct evidence that PI3K-mediated Akt activation in adenovirus-infected corneal cells may contribute to viral pathogenesis by the prolongation of cell viability.

Light regulation of the insulin receptor in the retina

The peptide hormone insulin binds its cognate cell-surface receptors to activate a coordinated biochemical-signaling network and to induce intracellular events. The retina is an integral part of the central nervous system and is known to contain insulin receptors, although their function is unknown. This article, describes recent studies that link the photobleaching of rhodopsin to tyrosine phosphorylation of the insulin receptor and subsequent activation of phosphoinositide 3- kinase (PI3K). We recently found a light-dependent increase in tyrosine phosphorylation of the insulin receptor-beta-subunit (IR beta) and an increase in PI3K enzyme activity in isolated rod outer segments (ROS) and in anti-phosphotyrosine (PY) and anti-IR beta immunoprecipitates of retinal homogenates. The light effect, which was localized to photoreceptor neurons, is independent of insulin secretion. Our results suggest that light induces tyrosine phosphorylation of IR beta in outer-segment membranes, which leads to the binding of p85 through its N-terminal SH2 domain and the generation of PI-3,4,5-P3. We suggest that the physiological role of this process may be to provide neuroprotection of the retina against light damage by activating proteins that protect against stress-induced apoptosis. The studies linking PI3K activation through tyrosine phosphorylation of IR beta now provide physiological relevance for the presence of these receptors in the retina.

SIP/SHIP inhibits Xenopus oocyte maturation induced by insulin and phosphatidylinositol 3-kinase

SIP (signaling inositol phosphatase) or SHIP (SH2-containing inositol phosphatase) is a recently identified SH2 domain-containing protein which has been implicated as an important signaling molecule. SIP/SHIP becomes tyrosine phosphorylated and binds the phosphotyrosine-binding domain of SHC in response to activation of hematopoietic cells. The signaling pathways and biological responses that may be regulated by SIP have not been demonstrated. SIP is a phosphatidylinositol- and inositol-polyphosphate 5-phosphatase with specificity in vitro for substrates phosphorylated at the 3' position. Phosphatidylinositol 3'-kinase (PI 3-kinase) is an enzyme which is involved in mitogenic signaling and whose phosphorylated lipid products are predicted to be substrates for SIP. We tested the hypothesis that SIP can modulate signaling by PI 3-kinase in vivo by injecting SIP cRNAs into Xenopus oocytes. SIP inhibited germinal vesicle breakdown (GVBD) induced by expression of a constitutively activated form of PI 3-kinase (p110*) and blocked GVBD induced by insulin. SIP had no effect on progesterone-induced GVBD. Catalytically inactive SIP had little effect on insulin- or PI 3-kinase-induced GVBD. Expression of SIP, but not catalytically inactive SIP, also blocked insulin-induced mitogen-activated protein kinase phosphorylation in oocytes. SIP specifically and markedly reduced the level of phosphatidylinositol (3,4,5) triphosphate [PtdIns(3,4,5)P3] generated in oocytes in response to insulin. These results demonstrate that a member of the phosphatidylinositol polyphosphate 5-phosphatase family can inhibit signaling in vivo. Further, our data suggest that the generation of PtdIns(3,4,5)P3 by PI 3-kinase is necessary for insulin-induced GVBD in Xenopus oocytes.

Platelet-derived growth factor-dependent activation of phosphatidylinositol 3-kinase is regulated by receptor binding of SH2-domain-containing proteins which influence Ras activity

Upon binding of platelet-derived growth factor (PDGF), the PDGF beta receptor (PDGFR) undergoes autophosphorylation on distinct tyrosine residues and binds several SH2-domain-containing signal relay enzymes, including phosphatidylinositol 3-kinase (PI3K), phospholipase C gamma (PLC gamma), the GTPase-activating protein of Ras (RasGAP), and the tyrosine phosphatase SHP-2. In this study, we have investigated whether PDGF-dependent PI3K activation is affected by the other proteins that associate with the PDGFR. We constructed and characterized a series of PDGFR mutants which contain binding sites for PI3K as well as one additional protein, either RasGAP, SHP-2, or PLC gamma. While all of the receptors had wild-type levels of PDGF-stimulated tyrosine kinase activity and associated with comparable amounts of PI3K activity, their abilities to trigger accumulation of PI3K products in vivo differed dramatically. The wild-type receptor, as well as receptors that recruited PI3K or PI3K and SHP-2, were all capable of fully activating PI3K. In contrast, receptors that associated with PI3K and RasGAP or PI3K and PLC gamma displayed a greatly reduced ability to stimulate production of PI3K products. When this series of receptors was tested for their ability to activate Ras, we observed a strong positive correlation between Ras activation and PI3K activation. Further investigation of the relationship between Ras and PI3K indicated that Ras was upstream of PI3K. Thus, activation of PI3K requires not only binding of PI3K to the tyrosine-phosphorylated PDGFR but accumulation of GTP-bound Ras as well. Furthermore, PLC gamma and RasGAP negatively modulate PDGF-dependent PI3K activation. Finally, PDGF-stimulated signal relay can be regulated by altering the ratio of SH2-domain-containing enzymes that are recruited to the PDGFR.

Compartment-specific regulation of phosphoinositide 3-kinase by platelet-derived growth factor and insulin in 3T3-L1 adipocytes

To understand how the stimulation of phosphoinositide 3-kinase (PI 3-kinase) by different growth factors can activate different subsets of downstream responses, growth-factor regulation of PI 3-kinase activity at different intracellular locations was investigated in 3T3-L1 adipocytes. Insulin caused a large stimulation of glucose transport and stimulated recruitment of transferrin receptors to the plasma membrane (PM) in these cells, whereas platelet-derived growth factor (PDGF)-bb was virtually without effect on these responses. Subcellular fractionation studies after stimulation with PDGF-bb or insulin revealed a differential effect of these growth factors on subcellular localization of PI 3-kinase activity. PDGF was more effective than insulin in stimulating PI 3-kinase activity and recruiting the p85 alpha PI 3-kinase adaptor subunit in the fraction containing the PM. However, in the microsomal fraction insulin significantly increased PI 3-kinase activity and p85 alpha levels, whereas PDGF was almost without effect. In the microsomal membrane fraction the insulin-stimulated recruitment of p85 alpha closely matched the increase PI 3-kinase activity, indicating that insulin stimulation of PI 3-kinase in this fraction is largely due to recruitment of PI 3-kinase enzyme rather than alterations in specific activity. Insulin-stimulated recruitment of p85 alpha to the microsomal membranes was not inhibited by wortmannin, indicating that PI 3-kinase activity was not required for this process. A further level of compartment-specific regulation of PI 3-kinase in response to PDGF was revealed by the finding that tyrosine phosphorylation of the p85 alpha adaptor was restricted to the PM-containing fraction. Insulin had no effect on p85 tyrosine phosphorylation in either fraction. In summary, these results suggest a basis by which insulin and PDGF could both use PI 3-kinase signalling cascades but achieve different signalling outcomes.

Inhibition of phosphatidylinositol 3-kinase activity by association with 14-3-3 proteins in T cells

Proteins of the 14-3-3 family can associate with, and/or modulate the activity of, several protooncogene and oncogene products and, thus, are implicated in regulation of signaling pathways. We report that 14-3-3 is associated with another important transducing enzyme, phosphatidylinositol 3-kinase (PI3-K). A recombinant 14-3-3 fusion protein bound several tyrosine-phosphorylated proteins from antigen receptor-stimulated T lymphocytes. PI3-K was identified by immunoblotting and enzymatic assays as one of the 14-3-3-binding proteins in resting or activated cells. Moreover, endogenous 14-3-3 and PI3-K were coimmunoprecipitated from intact T cells. Far-Western blots of gel-purified, immunoprecipitated PI3-K with a recombinant 14-3-3 fusion protein revealed direct binding of 14-3-3 to the catalytic subunit (p110) of PI3-K. Finally, anti-phosphotyrosine immunoprecipitates from activated, 14-3-3-overexpressing cells contained lower PI3-K enzymatic activity than similar immunoprecipitates from control cells. These findings suggest that association of 14-3-3 with PI3-K in hematopoietic (and possibly other) cells regulates the enzymatic activity of PI3-K during receptor-initiated signal transduction.

Interaction of p72syk with the gamma and beta subunits of the high-affinity receptor for immunoglobulin E, Fc epsilon RI

Activation of protein tyrosine kinases is one of the initial events following aggregation of the high-affinity receptor for immunoglobulin E (Fc epsilon RI) on RBL-2H3 cells, a model mast cell line. The protein tyrosine kinase p72syk (Syk), which contains two Src homology 2 (SH2) domains, is activated and associates with phosphorylated Fc epsilon RI subunits after receptor aggregation. In this report, we used Syk SH2 domains, expressed in tandem or individually, as fusion proteins to identify Syk-binding proteins in RBL-2H3 lysates. We show that the tandem Syk SH2 domains selectively associate with tyrosine-phosphorylated forms of the gamma and beta subunits of Fc epsilon RI. The isolated carboxy-proximal SH2 domain exhibited a significantly higher affinity for the Fc epsilon RI subunits than did the amino-proximal domain. When in tandem, the Syk SH2 domains showed enhanced binding to phosphorylated gamma and beta subunits. The conserved tyrosine-based activation motifs contained in the cytoplasmic domains of the gamma and beta subunits, characterized by two YXXL/I sequences in tandem, represent potential high-affinity binding sites for the dual SH2 domains of Syk. Peptide competition studies indicated that Syk exhibits a higher affinity for the phosphorylated tyrosine activation motif of the gamma subunit than for that of the beta subunit. In addition, we show that Syk is the major protein in RBL-2H3 cells that is affinity isolated with phosphorylated peptides corresponding to the phosphorylated gamma subunit motif. These data suggest that Syk associates with the gamma subunit of the high-affinity receptor for immunoglobulin E through an interaction between the tandem SH2 domains of SH2 domains of Syk and the phosphorylated tyrosine activation motif of the gamma subunit and that Syk may be the major signaling protein that binds to Fc epsilon RI tyrosine activation motif of the gamma subunit and that Syk may be the major signaling protein that binds to Dc epsilon tyrosine activation motifs in RBL-2H3 cells.

Biological function of PDGF-induced PI-3 kinase activity: its role in alpha PDGF receptor-mediated mitogenic signaling

The tyrosine phosphorylation sites in the human alpha PDGF receptor (alpha PDGFR) required for association with PI-3 kinase have been identified as tyrosines 731 and 742. Mutation of either tyrosine substantially reduced PDGF-induced PI-3 kinase activity but did not impair the receptor-mediated mitogenic response. We sought to determine whether PDGF-induced PI-3 kinase activity could be further ablated so as to exclude a low threshold requirement for PDGFR signal transduction. Thus, we mutated both tyrosine 731 and 742 and expressed the double mutant (Y731F/Y742F) in 32D hematopoietic cells. In such transfectants, PDGF induced no detectable receptor-associated or anti-P-Tyr recoverable PI-3 kinase activity. Under the same conditions, neither mobility shift of raf-1 nor tyrosine phosphorylation of either PLC gamma or MAP kinase was impaired. 32D transfectants expressing the double mutant showed wild-type alpha PDGFR levels of mitogenic and chemotactic responses to PDGF. To examine the effect of the double mutation in cells that normally respond to PDGF, we generated chimeras in which the cytoplasmic domains of wild-type alpha PDGFR, Y731F, and Y731F/Y742F were linked to the extracellular domain of colony-stimulating factor-1 (CSF-1) receptor (fms). After introduction of the chimeric receptors into mouse NIH/3T3 fibroblasts, the ability of CSF-1 to stimulate growth of these transfectants was examined. Our data show that all these chimeric receptors exhibited similar abilities to mediate CSF-1-stimulated cell growth. These findings lead us to conclude that PDGF-induced PI-3 kinase activity is not required for PDGF-stimulated mitogenic pathway in both NIH/3T3 fibroblasts and 32D hematopoietic cells.

The interaction of small domains between the subunits of phosphatidylinositol 3-kinase determines enzyme activity

Previous studies have suggested that the two subunits of phosphatidylinositol (PI) 3-kinase, p85 and p110, function as localizing and catalytic subunits, respectively. Using recombinant p85 and p110 molecules, we have reconstituted the specific interaction between the two subunits of mouse PI 3-kinase in cells and in vitro. We have previously shown that the region between the two Src homology 2 (SH2) domains of p85 is able to form a functional complex with the 110-kDa subunit in vivo. In this report, we identify the corresponding domain in p110 which directs the binding to p85. We demonstrate that the interactive domains in p85 and p110 are less than 103 and 124 amino acids, respectively, in size. We also show that the association of p85 and p110 mediated by these domains is critical for PI 3-kinase activity. Surprisingly, a complex between a 102-amino-acid segment of p85 and the full-length p110 molecule is catalytically active, whereas p110 alone has no activity. In addition to the catalytic domain in the carboxy-terminal region, 123 amino acids at the amino terminus of p110 were required for catalytic activity and were sufficient for the interaction with p85. These results indicate that the 85-kDa subunit, previously thought to have only a linking role in localizing the p110 catalytic subunit, is an important component of the catalytic complex.

The interaction of small domains between the subunits of phosphatidylinositol 3-kinase determines enzyme activity

Previous studies have suggested that the two subunits of phosphatidylinositol (PI) 3-kinase, p85 and p110, function as localizing and catalytic subunits, respectively. Using recombinant p85 and p110 molecules, we have reconstituted the specific interaction between the two subunits of mouse PI 3-kinase in cells and in vitro. We have previously shown that the region between the two Src homology 2 (SH2) domains of p85 is able to form a functional complex with the 110-kDa subunit in vivo. In this report, we identify the corresponding domain in p110 which directs the binding to p85. We demonstrate that the interactive domains in p85 and p110 are less than 103 and 124 amino acids, respectively, in size. We also show that the association of p85 and p110 mediated by these domains is critical for PI 3-kinase activity. Surprisingly, a complex between a 102-amino-acid segment of p85 and the full-length p110 molecule is catalytically active, whereas p110 alone has no activity. In addition to the catalytic domain in the carboxy-terminal region, 123 amino acids at the amino terminus of p110 were required for catalytic activity and were sufficient for the interaction with p85. These results indicate that the 85-kDa subunit, previously thought to have only a linking role in localizing the p110 catalytic subunit, is an important component of the catalytic complex.

Signal transduction via CD40 involves activation of lyn kinase and phosphatidylinositol-3-kinase, and phosphorylation of phospholipase C gamma 2

CD40 is a 50-kD glycoprotein that plays an important role in B cell survival, memory, and immunoglobulin isotype switch. Engagement of the CD40 antigen by monoclonal antibodies (mAbs) results in increased protein tyrosine kinase (PTK) activity, which plays an important role in mediating the biologic effects of CD40. We demonstrate, using an in situ phosphorylation technique, that CD40 cross-linking by the anti-CD40 mAb 626.1 resulted within 1 min in increased phosphorylation of the src type kinase, lyn, in Daudi B cell lines and remained sustained for up to 20 min. The activity of lyn kinase, as measured by immune complex kinase assay, was also increased after CD40 engagement, with similar kinetics. In contrast, the phosphorylation and activity of fyn, fgr, and lck kinases demonstrated minimal changes following stimulation of Daudi cells with mAb 626.1 over this same time period. CD40 engagement also resulted in phosphorylation of phospholipase C gamma 2 of phosphatidylinositol (PLC gamma 2) and phosphatidylinositol (PI)-3-kinase. Phosphorylation of PI-3-kinase was shown to be associated with an increase in its enzymatic activity. These results suggest that lyn plays an important role in CD40-mediated PTK activation and identify PLC gamma 2 and PI-3-kinase targets for CD40-mediated phosphorylation, suggesting a role for these two enzymes in CD40 signal transduction.

A 31-amino-acid N-terminal extension regulates c-Crk binding to tyrosine-phosphorylated proteins

Overproduction of v-Crk, but not of c-Crk, in chicken embryo fibroblasts results in cell transformation. The transforming activity of v-Crk mutants correlates with their ability to cause increased tyrosine phosphorylation of specific cellular proteins, a property that depends on the binding of v-Crk to phosphotyrosine residues via its SH2 domain. In this study, proteins translated in rabbit reticulocyte lysates were used to analyze interactions between Crk derivatives and tyrosine-phosphorylated proteins, particularly the epidermal growth factor (EGF) receptor. The results demonstrate that the binding affinity of c-Crk is much lower than that of v-Crk, despite the fact that both proteins contain identical SH2 domains. Moreover, a 31-amino-acid N-terminal extension of c-Crk, resulting from upstream translational initiation at a CUG codon, significantly increases the ability of the resulting protein to bind to phosphotyrosine-containing proteins. Of those 31 amino acids, 24 can be found in the 27-amino-acid region between Gag and Crk sequences in v-Crk, and removal of this region results in a protein with lower affinity toward the EGF receptor. In addition, fusion of Gag to the amino terminus of c-Crk yields a protein with a binding activity that is lower than that of v-Crk but significantly higher than that of c-Crk without the fusion. These data suggest that sequences N terminal to the Crk SH2 regulate binding activity to tyrosine-phosphorylated proteins and that the amino acids encoded immediately 5' to the c-Crk initiator AUG specifically increase binding affinity. In contrast, deletion of one or two SH3 domains of c-Crk proteins did not change their affinity for the EGF receptor. These results were confirmed in vivo by using A431-derived cell lines overproducing either the chicken c-Crk protein or c-Crk with the 31-amino-acid N-terminal extension. Furthermore, the in vivo experiments suggest that binding of Crk proteins to the stimulated EGF receptor results in Crk phosphorylation and subsequent loss of binding affinity.

Phosphatidylinositol (PI) 3-kinase and PI 4-kinase binding to the CD4-p56lck complex: the p56lck SH3 domain binds to PI 3-kinase but not PI 4-kinase

CD4 serves as a receptor for major histocompatibility complex class II antigens and as a receptor for the human immunodeficiency virus type 1 (HIV-1) viral coat protein gp120. It is coupled to the protein-tyrosine kinase p56lck, an interaction necessary for an optimal response of certain T cells to antigen. In addition to the protein-tyrosine kinase domain, p56lck possesses Src homology 2 and 3 (SH2 and SH3) domains as well as a unique N-terminal region. The mechanism by which p56lck generates intracellular signals is unclear, although it has the potential to interact with various downstream molecules. One such downstream target is the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase), which has been found to bind to activated pp60src and receptor-tyrosine kinases. In this study, we verified that PI 3-kinase associates with the CD4:p56lck complex as judged by the presence of PI 3-phosphate generated from anti-CD4 immunoprecipitates and detected by high-pressure liquid chromatographic analysis. However, surprisingly, CD4-p56lck was also found to associate with another lipid kinase, phosphatidylinositol 4-kinase (PI 4-kinase). The level of associated PI 4-kinase was generally higher than PI 3-kinase activity. HIV-1 gp120 and antibody-mediated cross-linking induced a 5- to 10-fold increase in the level of CD4-associated PI 4- and PI 3-kinases. The use of glutathione S-transferase fusion proteins carrying Lck-SH2, Lck-SH3, and Lck-SH2/SH3 domains showed PI 3-kinase binding to the SH3 domain of p56lck, an interaction facilitated by the presence of an adjacent SH2 domain. PI 4-kinase bound to neither the SH2 nor the SH3 domain of p56lck. CD4-p56lck contributes PI 3- and PI 4-kinase to the activation process of T cells and may play a role in HIV-1-induced immune defects.

Cloning of a novel, ubiquitously expressed human phosphatidylinositol 3-kinase and identification of its binding site on p85

Phosphatidylinositol 3-kinase (PI 3-kinase) has been implicated as a participant in signaling pathways regulating cell growth by virtue of its activation in response to various mitogenic stimuli. Here we describe the cloning of a novel and ubiquitously expressed human PI 3-kinase. The 4.8-kb cDNA encodes a putative translation product of 1,070 amino acids which is 42% identical to bovine PI 3-kinase and 28% identical to Vps34, a Saccharomyces cerevisiae PI 3-kinase involved in vacuolar protein sorting. Human PI 3-kinase is also similar to Tor2, a yeast protein required for cell cycle progression. Northern (RNA) analysis demonstrated expression of human PI 3-kinase in all tissues and cell lines tested. Protein synthesized from an epitope-tagged cDNA had intrinsic PI 3-kinase activity and associated with the adaptor 85-kDa subunit of PI 3-kinase (p85) in intact cells, as did endogenous human PI 3-kinase. Coprecipitation assays showed that a 187-amino-acid domain between the two src homology 2 domains of p85 mediates interaction with PI 3-kinase in vitro and in intact cells. These results demonstrate the existence of different PI 3-kinase isoforms and define a family of genes encoding distinct PI 3-kinase catalytic subunits that can associate with p85.

TOR1 and TOR2 are structurally and functionally similar but not identical phosphatidylinositol kinase homologues in yeast

The Saccharomyces cerevisiae genes TOR1 and TOR2 were originally identified by mutations that confer resistance to the immunosuppressant rapamycin. TOR2 was previously shown to encode an essential 282-kDa phosphatidylinositol kinase (PI kinase) homologue. The TOR1 gene product is also a large (281 kDa) PI kinase homologue, with 67% identity to TOR2. TOR1 is not essential, but a TOR1 TOR2 double disruption uniquely confers a cell cycle (G1) arrest as does exposure to rapamycin; disruption of TOR2 alone is lethal but does not cause a cell cycle arrest. TOR1-TOR2 and TOR2-TOR1 hybrids indicate that carboxy-terminal domains of TOR1 and TOR2 containing a lipid kinase sequence motif are interchangeable and therefore functionally equivalent; the other portions of TOR1 and TOR2 are not interchangeable. The TOR1-1 and TOR2-1 mutations, which confer rapamycin resistance, alter the same potential protein kinase C site in the respective protein's lipid kinase domain. Thus, TOR1 and TOR2 are likely similar but not identical, rapamycin-sensitive PI kinases possibly regulated by phosphorylation. TOR1 and TOR2 may be components of a novel signal transduction pathway controlling progression through G1.

A 31-amino-acid N-terminal extension regulates c-Crk binding to tyrosine-phosphorylated proteins

Overproduction of v-Crk, but not of c-Crk, in chicken embryo fibroblasts results in cell transformation. The transforming activity of v-Crk mutants correlates with their ability to cause increased tyrosine phosphorylation of specific cellular proteins, a property that depends on the binding of v-Crk to phosphotyrosine residues via its SH2 domain. In this study, proteins translated in rabbit reticulocyte lysates were used to analyze interactions between Crk derivatives and tyrosine-phosphorylated proteins, particularly the epidermal growth factor (EGF) receptor. The results demonstrate that the binding affinity of c-Crk is much lower than that of v-Crk, despite the fact that both proteins contain identical SH2 domains. Moreover, a 31-amino-acid N-terminal extension of c-Crk, resulting from upstream translational initiation at a CUG codon, significantly increases the ability of the resulting protein to bind to phosphotyrosine-containing proteins. Of those 31 amino acids, 24 can be found in the 27-amino-acid region between Gag and Crk sequences in v-Crk, and removal of this region results in a protein with lower affinity toward the EGF receptor. In addition, fusion of Gag to the amino terminus of c-Crk yields a protein with a binding activity that is lower than that of v-Crk but significantly higher than that of c-Crk without the fusion. These data suggest that sequences N terminal to the Crk SH2 regulate binding activity to tyrosine-phosphorylated proteins and that the amino acids encoded immediately 5' to the c-Crk initiator AUG specifically increase binding affinity. In contrast, deletion of one or two SH3 domains of c-Crk proteins did not change their affinity for the EGF receptor. These results were confirmed in vivo by using A431-derived cell lines overproducing either the chicken c-Crk protein or c-Crk with the 31-amino-acid N-terminal extension. Furthermore, the in vivo experiments suggest that binding of Crk proteins to the stimulated EGF receptor results in Crk phosphorylation and subsequent loss of binding affinity.

Phosphatidylinositol (PI) 3-kinase and PI 4-kinase binding to the CD4-p56lck complex: the p56lck SH3 domain binds to PI 3-kinase but not PI 4-kinase

CD4 serves as a receptor for major histocompatibility complex class II antigens and as a receptor for the human immunodeficiency virus type 1 (HIV-1) viral coat protein gp120. It is coupled to the protein-tyrosine kinase p56lck, an interaction necessary for an optimal response of certain T cells to antigen. In addition to the protein-tyrosine kinase domain, p56lck possesses Src homology 2 and 3 (SH2 and SH3) domains as well as a unique N-terminal region. The mechanism by which p56lck generates intracellular signals is unclear, although it has the potential to interact with various downstream molecules. One such downstream target is the lipid kinase phosphatidylinositol 3-kinase (PI 3-kinase), which has been found to bind to activated pp60src and receptor-tyrosine kinases. In this study, we verified that PI 3-kinase associates with the CD4:p56lck complex as judged by the presence of PI 3-phosphate generated from anti-CD4 immunoprecipitates and detected by high-pressure liquid chromatographic analysis. However, surprisingly, CD4-p56lck was also found to associate with another lipid kinase, phosphatidylinositol 4-kinase (PI 4-kinase). The level of associated PI 4-kinase was generally higher than PI 3-kinase activity. HIV-1 gp120 and antibody-mediated cross-linking induced a 5- to 10-fold increase in the level of CD4-associated PI 4- and PI 3-kinases. The use of glutathione S-transferase fusion proteins carrying Lck-SH2, Lck-SH3, and Lck-SH2/SH3 domains showed PI 3-kinase binding to the SH3 domain of p56lck, an interaction facilitated by the presence of an adjacent SH2 domain. PI 4-kinase bound to neither the SH2 nor the SH3 domain of p56lck. CD4-p56lck contributes PI 3- and PI 4-kinase to the activation process of T cells and may play a role in HIV-1-induced immune defects.

Cloning of a novel, ubiquitously expressed human phosphatidylinositol 3-kinase and identification of its binding site on p85

Phosphatidylinositol 3-kinase (PI 3-kinase) has been implicated as a participant in signaling pathways regulating cell growth by virtue of its activation in response to various mitogenic stimuli. Here we describe the cloning of a novel and ubiquitously expressed human PI 3-kinase. The 4.8-kb cDNA encodes a putative translation product of 1,070 amino acids which is 42% identical to bovine PI 3-kinase and 28% identical to Vps34, a Saccharomyces cerevisiae PI 3-kinase involved in vacuolar protein sorting. Human PI 3-kinase is also similar to Tor2, a yeast protein required for cell cycle progression. Northern (RNA) analysis demonstrated expression of human PI 3-kinase in all tissues and cell lines tested. Protein synthesized from an epitope-tagged cDNA had intrinsic PI 3-kinase activity and associated with the adaptor 85-kDa subunit of PI 3-kinase (p85) in intact cells, as did endogenous human PI 3-kinase. Coprecipitation assays showed that a 187-amino-acid domain between the two src homology 2 domains of p85 mediates interaction with PI 3-kinase in vitro and in intact cells. These results demonstrate the existence of different PI 3-kinase isoforms and define a family of genes encoding distinct PI 3-kinase catalytic subunits that can associate with p85.

Internalization of activated platelet-derived growth factor receptor-phosphatidylinositol-3' kinase complexes: potential interactions with the microtubule cytoskeleton

Phosphatidylinositol (PI)-3' kinase catalyzes the formation of PI 3,4-diphosphate and PI 3,4,5-triphosphate in response to stimulation of cells by platelet-derived growth factor (PDGF). Here we report that tyrosine-phosphorylated PDGF receptors, the p85 subunit of PI-3' kinase (p85), and activated PI-3' kinase are found in isolated clathrin-coated vesicles within 2 min of exposure of cells to PDGF, indicating that both receptor and activated PI-3' kinase enter the endocytic pathway. Immunofluorescence analysis of p85 in serum-starved cells revealed a punctate/reticular staining pattern, concentrated in the perinuclear region and displaying high focal concentration at the centrosome. In addition, partial coalignment of p85 with microtubules was observed after optical sectioning microscopy and image reconstruction. The association of p85 with the microtubule network was further evidenced by the microtubule-depolymerizing drug nocodazole, which caused a redistribution of p85 from the perinuclear region to the cell periphery. Interestingly, the most significant effect of PDGF on the distribution of p85 was an increase in the staining intensity of this protein in the perinuclear region, and this effect was eliminated by prior treatment of cells with nocodazole. These results suggest that PDGF receptor-p85 complexes internalize and transit in association with the microtubule cytoskeleton. In addition, the high concentration of p85 in intracellular structures in the absence of PDGF stimulation suggests additional roles for this protein independent of its association with receptor tyrosine kinases.

Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Flk2/Flt3 is a recently identified receptor tyrosine kinase expressed in brain, placenta, testis, and primitive hematopoietic cells. The mitogenic signalling potential and biochemical properties of Flk2/Flt3 have been analyzed by using a chimeric receptor composed of the extracellular domain of the human colony-stimulating factor 1 receptor and the transmembrane and cytoplasmic domains of murine Flk2/Flt3. We demonstrate that colony-stimulating factor 1 stimulation of the Flk2/Flt3 kinase in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a full proliferative response in the absence of other growth factors. In transfected interleukin 3 (IL-3)-dependent Ba/F3 lymphoid cells, activation of the chimeric receptor can abrogate IL-3 requirement and sustain long-term proliferation. We show that phospholipase C-gamma 1, Ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, Vav, Fyn, and Src are components of the Flk2/Flt3 signal transduction pathway. In addition, we demonstrate that phospholipase C-gamma 1, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, and Src family tyrosine kinases, but not Ras GTPase-activating protein, Vav, or Nck, physically associate with the Flk2/Flt3 cytoplasmic domain. Cell-type-specific differences in tyrosine phosphorylation of p85 and Shc are observed. A comparative analysis of the Flk2/Flt3 signal cascade with those of the endogenous platelet-derived growth factor and IL-3 receptors indicates that Flk2/Flt3 displays specific substrate preferences. Furthermore, tyrosine phosphorylation of p85 and Shc is similarly affected by totally different growth factors in the same cellular background.

Mitogenic signalling and substrate specificity of the Flk2/Flt3 receptor tyrosine kinase in fibroblasts and interleukin 3-dependent hematopoietic cells

Flk2/Flt3 is a recently identified receptor tyrosine kinase expressed in brain, placenta, testis, and primitive hematopoietic cells. The mitogenic signalling potential and biochemical properties of Flk2/Flt3 have been analyzed by using a chimeric receptor composed of the extracellular domain of the human colony-stimulating factor 1 receptor and the transmembrane and cytoplasmic domains of murine Flk2/Flt3. We demonstrate that colony-stimulating factor 1 stimulation of the Flk2/Flt3 kinase in transfected NIH 3T3 fibroblasts leads to a transformed phenotype and generates a full proliferative response in the absence of other growth factors. In transfected interleukin 3 (IL-3)-dependent Ba/F3 lymphoid cells, activation of the chimeric receptor can abrogate IL-3 requirement and sustain long-term proliferation. We show that phospholipase C-gamma 1, Ras GTPase-activating protein, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, Vav, Fyn, and Src are components of the Flk2/Flt3 signal transduction pathway. In addition, we demonstrate that phospholipase C-gamma 1, the p85 subunit of phosphatidylinositol 3'-kinase, Shc, Grb2, and Src family tyrosine kinases, but not Ras GTPase-activating protein, Vav, or Nck, physically associate with the Flk2/Flt3 cytoplasmic domain. Cell-type-specific differences in tyrosine phosphorylation of p85 and Shc are observed. A comparative analysis of the Flk2/Flt3 signal cascade with those of the endogenous platelet-derived growth factor and IL-3 receptors indicates that Flk2/Flt3 displays specific substrate preferences. Furthermore, tyrosine phosphorylation of p85 and Shc is similarly affected by totally different growth factors in the same cellular background.

Internalization of activated platelet-derived growth factor receptor-phosphatidylinositol-3' kinase complexes: potential interactions with the microtubule cytoskeleton

Phosphatidylinositol (PI)-3' kinase catalyzes the formation of PI 3,4-diphosphate and PI 3,4,5-triphosphate in response to stimulation of cells by platelet-derived growth factor (PDGF). Here we report that tyrosine-phosphorylated PDGF receptors, the p85 subunit of PI-3' kinase (p85), and activated PI-3' kinase are found in isolated clathrin-coated vesicles within 2 min of exposure of cells to PDGF, indicating that both receptor and activated PI-3' kinase enter the endocytic pathway. Immunofluorescence analysis of p85 in serum-starved cells revealed a punctate/reticular staining pattern, concentrated in the perinuclear region and displaying high focal concentration at the centrosome. In addition, partial coalignment of p85 with microtubules was observed after optical sectioning microscopy and image reconstruction. The association of p85 with the microtubule network was further evidenced by the microtubule-depolymerizing drug nocodazole, which caused a redistribution of p85 from the perinuclear region to the cell periphery. Interestingly, the most significant effect of PDGF on the distribution of p85 was an increase in the staining intensity of this protein in the perinuclear region, and this effect was eliminated by prior treatment of cells with nocodazole. These results suggest that PDGF receptor-p85 complexes internalize and transit in association with the microtubule cytoskeleton. In addition, the high concentration of p85 in intracellular structures in the absence of PDGF stimulation suggests additional roles for this protein independent of its association with receptor tyrosine kinases.

A region of the 85-kilodalton (kDa) subunit of phosphatidylinositol 3-kinase binds the 110-kDa catalytic subunit in vivo

Phosphatidylinositol (PI) 3-kinase is a heterodimer consisting of an 85-kDa subunit (p85) and 110-kDa subunit (p110). The 85-kDa noncatalytic subunit, which contains two Src homology 2 (SH2) domains, one SH3 domain, and a domain homologous to the carboxy terminus of the breakpoint cluster region gene product, is known to mediate the association of the PI 3-kinase complex with activated growth factor receptors. We previously demonstrated that the C-terminal SH2 domain of p85 is responsible for the interaction of PI 3-kinase with phosphorylated platelet-derived growth factor receptor. To define the region in p85 that directs the complex formation with the PI 3-kinase catalytic subunit, a series of truncated p85 mutants was analyzed for association with p110 in vivo. We found that a fragment of p85 containing the region between the two SH2 domains was sufficient to promote the interaction with p110 in vivo. The complex between the fragment of p85 and p110 had PI 3-kinase activity that was comparable in magnitude to the activity of p110 associated with full-length p85. The binding with p110 was abolished when this domain in p85 was disrupted. These results identify a novel structural and functional element that is responsible for localizing the catalytic subunit of PI 3-kinase.

A region of the 85-kilodalton (kDa) subunit of phosphatidylinositol 3-kinase binds the 110-kDa catalytic subunit in vivo

Phosphatidylinositol (PI) 3-kinase is a heterodimer consisting of an 85-kDa subunit (p85) and 110-kDa subunit (p110). The 85-kDa noncatalytic subunit, which contains two Src homology 2 (SH2) domains, one SH3 domain, and a domain homologous to the carboxy terminus of the breakpoint cluster region gene product, is known to mediate the association of the PI 3-kinase complex with activated growth factor receptors. We previously demonstrated that the C-terminal SH2 domain of p85 is responsible for the interaction of PI 3-kinase with phosphorylated platelet-derived growth factor receptor. To define the region in p85 that directs the complex formation with the PI 3-kinase catalytic subunit, a series of truncated p85 mutants was analyzed for association with p110 in vivo. We found that a fragment of p85 containing the region between the two SH2 domains was sufficient to promote the interaction with p110 in vivo. The complex between the fragment of p85 and p110 had PI 3-kinase activity that was comparable in magnitude to the activity of p110 associated with full-length p85. The binding with p110 was abolished when this domain in p85 was disrupted. These results identify a novel structural and functional element that is responsible for localizing the catalytic subunit of PI 3-kinase.

A novel recognition motif for phosphatidylinositol 3-kinase binding mediates its association with the hepatocyte growth factor/scatter factor receptor

The pleiotropic effects (mitogenesis, motogenesis, and morphogenesis) elicited by hepatocyte growth factor/scatter factor (HGF/SF) are mediated by the activation of the tyrosine kinase receptor encoded by the MET proto-oncogene. Following autophosphorylation, the receptor associates with the p85/110 phosphatidylinositol (PI) 3-kinase complex in vivo and in vitro. By a combination of two complementary approaches, competition with synthetic phosphopeptides and association with Tyr-Phe receptor mutants, we have identified Y-1349 and Y-1356 in the HGF/SF receptor as the binding sites for PI 3-kinase. Y-1349VHV and Y-1356VNV do not conform to the canonical consensus sequence YXXM for PI 3-kinase binding and thus define YVXV as a novel recognition motif. Y-1349 and Y-1356 are located within the C-terminal portion of the HGF/SF receptor and are phosphorylation sites. The affinity of the N- and C-terminal src homology region 2 (SH2) domains of p85 for the phosphopeptides including Y-1349 and Y-1356 is 2 orders of magnitude lower than that measured for Y-751 in the platelet-derived growth factor receptor binding site. However, the closely spaced duplication of the novel recognition motif in the native HGF/SF receptor may allow binding with both SH2 domains of p85, thus generating an efficient docking site for PI 3-kinase. In agreement with this model, we have observed that a phosphopeptide including both Y-1349 and Y-1356 activates PI 3-kinase in vitro.

A novel recognition motif for phosphatidylinositol 3-kinase binding mediates its association with the hepatocyte growth factor/scatter factor receptor

The pleiotropic effects (mitogenesis, motogenesis, and morphogenesis) elicited by hepatocyte growth factor/scatter factor (HGF/SF) are mediated by the activation of the tyrosine kinase receptor encoded by the MET proto-oncogene. Following autophosphorylation, the receptor associates with the p85/110 phosphatidylinositol (PI) 3-kinase complex in vivo and in vitro. By a combination of two complementary approaches, competition with synthetic phosphopeptides and association with Tyr-Phe receptor mutants, we have identified Y-1349 and Y-1356 in the HGF/SF receptor as the binding sites for PI 3-kinase. Y-1349VHV and Y-1356VNV do not conform to the canonical consensus sequence YXXM for PI 3-kinase binding and thus define YVXV as a novel recognition motif. Y-1349 and Y-1356 are located within the C-terminal portion of the HGF/SF receptor and are phosphorylation sites. The affinity of the N- and C-terminal src homology region 2 (SH2) domains of p85 for the phosphopeptides including Y-1349 and Y-1356 is 2 orders of magnitude lower than that measured for Y-751 in the platelet-derived growth factor receptor binding site. However, the closely spaced duplication of the novel recognition motif in the native HGF/SF receptor may allow binding with both SH2 domains of p85, thus generating an efficient docking site for PI 3-kinase. In agreement with this model, we have observed that a phosphopeptide including both Y-1349 and Y-1356 activates PI 3-kinase in vitro.

Interactions between SH2 domains and tyrosine-phosphorylated platelet-derived growth factor beta-receptor sequences: analysis of kinetic parameters by a novel biosensor-based approach

The interaction between SH2 domains and phosphotyrosine-containing sequences was examined by real-time measurements of kinetic parameters. The SH2 domains of the p85 subunit of the phosphatidylinositol 3-kinase as well as of other signaling molecules were expressed in bacteria as glutathione S-transferase fusion proteins. Phosphotyrosine-containing peptides, corresponding to two autophosphorylation sites on the human platelet-derived growth factor beta-receptor that are responsible for phosphatidylinositol 3-kinase binding, were synthesized and used as capturing molecules, immobilized on a biosensor surface. The association and dissociation rate constants for binding to both sites were determined for intact p85 and the recombinant SH2 domains. High association rates were found to be coupled to very fast dissociation rates for all interactions studied. A binding specificity was observed for the two SH2 domains of p85, with the N-terminal SH2 binding with high affinity to the Tyr-751 site but not to the Tyr-740 site, and the C-terminal SH2 interacting strongly with both sites. This approach should be generally applicable to the study of the specificity inherent in the assembly of signaling complexes by activated protein-tyrosine kinase receptors.

Platelet-derived growth factor receptor mediates activation of ras through different signaling pathways in different cell types

A series of pieces of evidence have shown that Ras protein acts as a transducer of the platelet-derived growth factor (PDGF) receptor-mediated signaling pathway: (i) formation of Ras.GTP is detected immediately on PDGF stimulation, and (ii) a dominant inhibitory mutant Ras, as well as a neutralizing anti-Ras antibody, can interfere with PDGF-induced responses. On the other hand, several signal transducing molecules including phosphatidylinositol 3-kinase (PI3-K), GTPase-activating protein (GAP), and phospholipase C gamma (PLC gamma) bind directly to the PDGF receptor and become tyrosine phosphorylated. Recently, it was shown that specific phosphorylated tyrosines of the PDGF receptor are responsible for interaction between the receptor and each signaling molecule. However, the roles of these signaling molecules have not been elucidated, and it remains unclear which molecules are implicated in the Ras pathway. In this study, we measured Ras activation in cell lines expressing mutant PDGF receptors that are deficient in coupling with specific molecules. In fibroblast CHO cells, a mutant receptor (Y708F/Y719F [PI3-K-binding sites]) was unable to stimulate Ras, whereas another mutant (Y739F [the GAP-binding site]) could do so, suggesting an indispensable role of PI3-K or a protein that binds to the same sites as PI3-K for PDGF-stimulated Ras activation. By contrast, both of the above mutants were capable of stimulating Ras protein in a pro-B-cell line, BaF3. Furthermore, a mutant receptor (Y977F/Y989F [PLC gamma-binding sites]) could fully activate Ras, and the direct activation of protein kinase C and calcium mobilization had almost no effect on the GDP/GTP state of Ras in this cell line. These results suggest that, in the pro-B-cell transfectants, each of the above pathways (PI3-K, GAP, and PLC gamma) can be eliminated without a loss of Ras activation. It remains unclear whether another unknown essential pathway which regulates Ras protein exists within BaF3 cells. Therefore, it is likely that several different PDGF receptor-mediated signaling pathways function upstream of Ras, and the extent of the contribution of each pathway for the regulation of Ras may differ among different cell types.

Tyrosine kinases and phosphoinositide metabolism in thrombin-stimulated human platelets

In this study we have examined the implication of tyrosine kinase activities in aggregation, 5-hydroxytryptamine secretion and mainly phosphoinositide metabolism in response to human platelet stimulation by thrombin. Using the potent tyrosine kinase inhibitor tyrphostin AG-213, we have observed a significant inhibition of aggregation and 5-hydroxytryptamine release; however, this percentage inhibition was lower at high thrombin concentrations. On the other hand, tyrphostin treatment of metabolically 32P-labelled platelets significantly inhibited the thrombin-dependent accumulation of PtdIns(3,4)P2, which involves at least a PtdIns 3-kinase and/or a PtdIns3P 4-kinase, whereas the synthesis of phosphatidic acid (PtdOH), a good reflection of the phospholipase C (PLC) activation in platelets, was partially blocked. Inositol phosphate production was also inhibited by about 40% when tyrphostin-treated platelets were stimulated with thrombin. In addition, we show by Western-blot analysis that PLC gamma 1, as well as the regulatory subunit (p85) of the PtdIns 3-kinase, were present in the anti-phosphotyrosine immunoprecipitate isolated from thrombin-stimulated platelets. Furthermore, tyrphostin treatment clearly decreased the PLC gamma 1 and p85 contents in such an anti-phosphotyrosine immunoprecipitate. Our results provide the first evidence for a direct or indirect regulation of PtdIns(3,4)P2 accumulation and PLC gamma 1 activity by tyrosine phosphorylation during thrombin stimulation of human platelets.

Platelet-derived growth factor receptor mediates activation of ras through different signaling pathways in different cell types

A series of pieces of evidence have shown that Ras protein acts as a transducer of the platelet-derived growth factor (PDGF) receptor-mediated signaling pathway: (i) formation of Ras.GTP is detected immediately on PDGF stimulation, and (ii) a dominant inhibitory mutant Ras, as well as a neutralizing anti-Ras antibody, can interfere with PDGF-induced responses. On the other hand, several signal transducing molecules including phosphatidylinositol 3-kinase (PI3-K), GTPase-activating protein (GAP), and phospholipase C gamma (PLC gamma) bind directly to the PDGF receptor and become tyrosine phosphorylated. Recently, it was shown that specific phosphorylated tyrosines of the PDGF receptor are responsible for interaction between the receptor and each signaling molecule. However, the roles of these signaling molecules have not been elucidated, and it remains unclear which molecules are implicated in the Ras pathway. In this study, we measured Ras activation in cell lines expressing mutant PDGF receptors that are deficient in coupling with specific molecules. In fibroblast CHO cells, a mutant receptor (Y708F/Y719F [PI3-K-binding sites]) was unable to stimulate Ras, whereas another mutant (Y739F [the GAP-binding site]) could do so, suggesting an indispensable role of PI3-K or a protein that binds to the same sites as PI3-K for PDGF-stimulated Ras activation. By contrast, both of the above mutants were capable of stimulating Ras protein in a pro-B-cell line, BaF3. Furthermore, a mutant receptor (Y977F/Y989F [PLC gamma-binding sites]) could fully activate Ras, and the direct activation of protein kinase C and calcium mobilization had almost no effect on the GDP/GTP state of Ras in this cell line. These results suggest that, in the pro-B-cell transfectants, each of the above pathways (PI3-K, GAP, and PLC gamma) can be eliminated without a loss of Ras activation. It remains unclear whether another unknown essential pathway which regulates Ras protein exists within BaF3 cells. Therefore, it is likely that several different PDGF receptor-mediated signaling pathways function upstream of Ras, and the extent of the contribution of each pathway for the regulation of Ras may differ among different cell types.

Interactions between SH2 domains and tyrosine-phosphorylated platelet-derived growth factor beta-receptor sequences: analysis of kinetic parameters by a novel biosensor-based approach

The interaction between SH2 domains and phosphotyrosine-containing sequences was examined by real-time measurements of kinetic parameters. The SH2 domains of the p85 subunit of the phosphatidylinositol 3-kinase as well as of other signaling molecules were expressed in bacteria as glutathione S-transferase fusion proteins. Phosphotyrosine-containing peptides, corresponding to two autophosphorylation sites on the human platelet-derived growth factor beta-receptor that are responsible for phosphatidylinositol 3-kinase binding, were synthesized and used as capturing molecules, immobilized on a biosensor surface. The association and dissociation rate constants for binding to both sites were determined for intact p85 and the recombinant SH2 domains. High association rates were found to be coupled to very fast dissociation rates for all interactions studied. A binding specificity was observed for the two SH2 domains of p85, with the N-terminal SH2 binding with high affinity to the Tyr-751 site but not to the Tyr-740 site, and the C-terminal SH2 interacting strongly with both sites. This approach should be generally applicable to the study of the specificity inherent in the assembly of signaling complexes by activated protein-tyrosine kinase receptors.

In vivo binding properties of SH2 domains from GTPase-activating protein and phosphatidylinositol 3-kinase

We have used a transient expression system and mutant platelet-derived growth factor (PDGF) receptors to study the binding specificities of the Src homology 2 (SH2) regions of the Ras GTPase-activator protein (GAP) and the p85 alpha subunit of phosphatidylinositol 3-kinase (PI3 kinase). A number of fusion proteins, each tagged with an epitope allowing recognition by a monoclonal antibody, were expressed at levels comparable to those of endogenous GAP. Fusion proteins containing the central SH2-SH3-SH2 region of GAP or the C-terminal region of p85 alpha, which includes two SH2 domains, bound to PDGF receptors in response to PDGF stimulation. Both fusion proteins showed the same requirements for tyrosine phosphorylation sites in the PDGF receptor as the full-length proteins from which they were derived, i.e., binding of the GAP fusion protein was reduced by mutation of Tyr-771, and binding of the p85 fusion protein was reduced by mutation of Tyr-740, Tyr-751, or both residues. Fusion proteins containing single SH2 domains from either GAP or p85 alpha did not bind detectably to PDGF receptors in this system, suggesting that two SH2 domains in a single polypeptide cooperate to raise the affinity of binding. The sequence specificities of individual SH2 domains were deduced from the binding properties of fusion proteins containing one SH2 domain from GAP and another from p85. The results suggest that the C-terminal GAP SH2 domain specifies binding to Tyr-771, the C-terminal p85 alpha SH2 domain binds to either Tyr-740 or Tyr-751, and each protein's N-terminal SH2 domain binds to unidentified phosphorylation sites.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo binding properties of SH2 domains from GTPase-activating protein and phosphatidylinositol 3-kinase

We have used a transient expression system and mutant platelet-derived growth factor (PDGF) receptors to study the binding specificities of the Src homology 2 (SH2) regions of the Ras GTPase-activator protein (GAP) and the p85 alpha subunit of phosphatidylinositol 3-kinase (PI3 kinase). A number of fusion proteins, each tagged with an epitope allowing recognition by a monoclonal antibody, were expressed at levels comparable to those of endogenous GAP. Fusion proteins containing the central SH2-SH3-SH2 region of GAP or the C-terminal region of p85 alpha, which includes two SH2 domains, bound to PDGF receptors in response to PDGF stimulation. Both fusion proteins showed the same requirements for tyrosine phosphorylation sites in the PDGF receptor as the full-length proteins from which they were derived, i.e., binding of the GAP fusion protein was reduced by mutation of Tyr-771, and binding of the p85 fusion protein was reduced by mutation of Tyr-740, Tyr-751, or both residues. Fusion proteins containing single SH2 domains from either GAP or p85 alpha did not bind detectably to PDGF receptors in this system, suggesting that two SH2 domains in a single polypeptide cooperate to raise the affinity of binding. The sequence specificities of individual SH2 domains were deduced from the binding properties of fusion proteins containing one SH2 domain from GAP and another from p85. The results suggest that the C-terminal GAP SH2 domain specifies binding to Tyr-771, the C-terminal p85 alpha SH2 domain binds to either Tyr-740 or Tyr-751, and each protein's N-terminal SH2 domain binds to unidentified phosphorylation sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Specific changes of Ras GTPase-activating protein (GAP) and a GAP-associated p62 protein during calcium-induced keratinocyte differentiation

Induction of tyrosine phosphorylation occurs as an early and specific event in keratinocyte differentiation. A set of tyrosine-phosphorylated substrates which transduce mitogenic signals by tyrosine kinases has previously been identified. We show here that of these substrates, the Ras GTPase-activating protein, GAP, is specifically affected during calcium-induced keratinocyte differentiation. As early as 10 min after calcium addition to cultured primary mouse keratinocytes, GAP associates with tyrosine-phosphorylated proteins and translocates to the membrane. In addition, a GAP-associated protein of approximately 62 kDa (p62) becomes rapidly and heavily tyrosine phosphorylated in both membrane and cytosolic fractions. This protein corresponds to the major tyrosine-phosphorylated protein that is induced in differentiating keratinocytes as early as 5 min after calcium addition. p62 phosphorylation was not observed after exposure of these cells to epidermal growth factor, phorbol ester, or transforming growth factor beta. In contrast, PLC gamma and P13K were tyrosine phosphorylated after epidermal growth factor, but not calcium, stimulation. Thus, changes of Ras GAP and an associated p62 protein occur as early and specific events in keratinocyte differentiation and appear to involve a calcium-induced tyrosine kinase.

Specific changes of Ras GTPase-activating protein (GAP) and a GAP-associated p62 protein during calcium-induced keratinocyte differentiation

Induction of tyrosine phosphorylation occurs as an early and specific event in keratinocyte differentiation. A set of tyrosine-phosphorylated substrates which transduce mitogenic signals by tyrosine kinases has previously been identified. We show here that of these substrates, the Ras GTPase-activating protein, GAP, is specifically affected during calcium-induced keratinocyte differentiation. As early as 10 min after calcium addition to cultured primary mouse keratinocytes, GAP associates with tyrosine-phosphorylated proteins and translocates to the membrane. In addition, a GAP-associated protein of approximately 62 kDa (p62) becomes rapidly and heavily tyrosine phosphorylated in both membrane and cytosolic fractions. This protein corresponds to the major tyrosine-phosphorylated protein that is induced in differentiating keratinocytes as early as 5 min after calcium addition. p62 phosphorylation was not observed after exposure of these cells to epidermal growth factor, phorbol ester, or transforming growth factor beta. In contrast, PLC gamma and P13K were tyrosine phosphorylated after epidermal growth factor, but not calcium, stimulation. Thus, changes of Ras GAP and an associated p62 protein occur as early and specific events in keratinocyte differentiation and appear to involve a calcium-induced tyrosine kinase.