The GTPase-activating protein of Ras suppresses platelet-derived growth factor beta receptor signaling by silencing phospholipase C-gamma 1

RasGAP Promotes Autophagy and Thereby Suppresses Platelet-Derived Growth Factor Receptor-Mediated Signaling Events, Cellular Responses, and Pathology

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) make profound contributions to both physiology and pathology. While it is widely believed that direct (PDGF-mediated) activation is the primary mode of activating PDGFRs, the discovery that they can also be activated indirectly begs the question of the relevance of the indirect mode of activating PDGFRs. In the context of a blinding eye disease, indirect activation of PDGFRα results in persistent signaling, which suppresses the level of p53 and thereby promotes viability of cells that drive pathogenesis. Under the same conditions, PDGFRβ fails to undergo indirect activation. In this paper, we report that RasGAP (GTPase-activating protein of Ras) prevented indirect activation of PDGFRβ. RasGAP, which associates with PDGFRβ but not PDGFRα, reduced the level of mitochondrion-derived reactive oxygen species, which are required for enduring activation of PDGFRs. Furthermore, preventing PDGFRβ from associating with RasGAP allowed it to signal enduringly and drive pathogenesis of a blinding eye disease. These results indicate a previously unappreciated role of RasGAP in antagonizing indirect activation of PDGFRβ, define the underlying mechanism, and raise the possibility that PDGFRβ-mediated diseases involve indirect activation of PDGFRβ.

Imatinib mesylate for the treatment of pulmonary arterial hypertension

INTRODUCTION

Despite recent advances, pulmonary arterial hypertension (PAH) remains a devastating disease which harbors a poor prognosis. Novel therapeutic approaches directly targeting pulmonary vascular remodeling are warranted.

AREAS COVERED

This review delineates the current limitations in the management of PAH and focuses on a novel, anti-proliferative therapeutic concept. It will help readers understand the mechanisms of receptor tyrosine kinase signaling, with a special focus on platelet-derived growth factor (PDGF) receptors and their role in the pathobiology of PAH. Furthermore, it provides a comprehensive summary regarding the rationale, efficacy and safety of the tyrosine kinase inhibitor imatinib mesylate , which potently inhibits the PDGF receptor, as an additional treatment option in PAH.

EXPERT OPINION

PDGF is a potent mitogen for pulmonary vascular smooth muscle cells and represents an important mediator of pulmonary vascular remodeling. Imatinib mesylate, a compound that inhibits the Bcr-Abl kinase and was developed for the treatment of chronic myeloid leukemia, also targets PDGF receptors. Both experimental and clinical data indicate that it reverses the vascular remodeling process even when it is fully established. Results from Phase II and III clinical trials suggest potent and prolonged efficacy in patients with severe PAH (i.e., pulmonary vascular resistance > 800 dynes*s*cm(-5)). Future studies should evaluate the long-term clinical efficacy and safety of imatinib, including patients with less impaired hemodynamics. Based on the current knowledge, this compound is likely to become an additional treatment option for patients with PAH and has the potential to at least partially correct the pathology of the disease.

Expression of an uncleavable N-terminal RasGAP fragment in insulin-secreting cells increases their resistance toward apoptotic stimuli without affecting their glucose-induced insulin secretion

Apoptosis of pancreatic beta cells is implicated in the onset of type 1 and type 2 diabetes. Consequently, strategies aimed at increasing the resistance of beta cells toward apoptosis could be beneficial in the treatment of diabetes. RasGAP, a regulator of Ras and Rho GTPases, is an atypical caspase substrate, since it inhibits, rather than favors, apoptosis when it is partially cleaved by caspase-3 at position 455. The antiapoptotic signal generated by the partial processing of RasGAP is mediated by the N-terminal fragment (fragment N) in a Ras-phosphatidylinositol 3-kinase-Akt-dependent, but NF-kappaB-independent, manner. Further cleavage of fragment N at position 157 abrogates its antiapoptotic properties. Here we demonstrate that an uncleavable form of fragment N activates Akt, represses NF-kappaB activity, and protects the conditionally immortalized pancreatic insulinoma betaTC-tet cell line against various insults, including exposure to genotoxins, trophic support withdrawal, and incubation with inflammatory cytokines. Fragment N also induced Akt activity and protection against cytokine-induced apoptosis in primary pancreatic islet cells. Fragment N did not alter insulin cell content and insulin secretion in response to glucose. These data indicate that fragment N protects beta cells without affecting their function. The pathways regulated by fragment N are therefore promising targets for antidiabetogenic therapy.

Impaired Akt activity down-modulation, caspase-3 activation, and apoptosis in cells expressing a caspase-resistant mutant of RasGAP at position 157

RasGAP bears two caspase-3 cleavage sites that are used sequentially as caspase activity increases in cells. When caspase-3 is mildly activated, RasGAP is first cleaved at position 455. This leads to the production of an N-terminal fragment, called fragment N, that activates the Ras-PI3K-Akt pathway and that promotes cell survival. At higher caspase activity, RasGAP is further cleaved at position 157 generating two small N-terminal fragments named N1 and N2. We have now determined the contribution of this second cleavage event in the regulation of apoptosis using cells in which the wild-type RasGAP gene has been replaced by a cDNA encoding a RasGAP mutant that cannot be cleaved at position 157. Our results show that cleavage of fragment N at position 157 leads to a marked reduction in Akt activity. This is accompanied by efficient processing of caspase-3 that favors cell death in response to various apoptotic stimuli. In nontumorigenic cells, fragments N1 and N2 do not modulate apoptosis. Therefore, the role of the second caspase-mediated cleavage of RasGAP is to allow the inactivation of the antiapoptotic function of fragment N so that caspases are no longer hampered in their ability to kill cells.

PDGF signaling in cells and mice

Since its discovery over three decades ago, platelet-derived growth factor (PDGF) has been a model system for learning how growth factors regulate biological processes. For the first several decades investigators used cells grown in tissue culture. More recently, PDGF signaling has also been investigated in mice. This review outlines the advances in these two systems, and highlights some of the directions for future investigation.

Partial cleavage of RasGAP by caspases is required for cell survival in mild stress conditions

Tight control of apoptosis is required for proper development and maintenance of homeostasis in multicellular organisms. Cells can protect themselves from potentially lethal stimuli by expressing antiapoptotic factors, such as inhibitors of apoptosis, FLICE (caspase 8)-inhibitory proteins, and members of the Bcl2 family. Here, we describe a mechanism that allows cells to survive once executioner caspases have been activated. This mechanism relies on the partial cleavage of RasGAP by caspase 3 into an amino-terminal fragment called fragment N. Generation of this fragment leads to the activation of the antiapoptotic Akt kinase, preventing further amplification of caspase activity. Partial cleavage of RasGAP is required for cell survival under stress conditions because cells expressing an uncleavable RasGAP mutant cannot activate Akt, cannot prevent amplification of caspase 3 activity, and eventually undergo apoptosis. Executioner caspases therefore control the extent of their own activation by a feedback regulatory mechanism initiated by the partial cleavage of RasGAP that is crucial for cell survival under adverse conditions.

Tyrosine kinase oncogenes in normal hematopoiesis and hematological disease

Tyrosine kinase oncogenes are formed as a result of mutations that induce constitutive kinase activity. Many of these tyrosine kinase oncogenes that are derived from genes, such as c-Abl, c-Fes, Flt3, c-Fms, c-Kit and PDGFRbeta, that are normally involved in the regulation of hematopoiesis or hematopoietic cell function. Despite differences in structure, normal function, and subcellular location, many of the tyrosine kinase oncogenes signal through the same pathways, and typically enhance proliferation and prolong viability. They represent excellent potential drug targets, and it is likely that additional mutations will be identified in other kinases, their immediate downstream targets, or in proteins regulating their function.

Effect of platelet-derived growth factor isoforms in rat metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain or PDGF beta-receptor-deficient mice lack mesangial cells. To explore potential mechanisms for failure of PDGF A-chain to rescue mesangial cell phenotype, we investigated the biological effects and signaling pathways of PDGF AA and PDGF BB in metanephric mesenchymal (MM) cells isolated from rat kidney. PDGF AA caused modest cell migration but had no effect on DNA synthesis, unlike PDGF BB, which potently stimulated migration and DNA synthesis. PDGF AA and PDGF BB significantly increased the activities of phosphatidylinositol 3-kinase (PI 3-K) and mitogen-activated protein kinase (MAPK). PDGF BB was more potent than PDGF AA in activating PI 3-K or MAPK in these cells. Pretreatment of MM cells with the MAPK kinase (MEK) inhibitor PD-098059 abrogated PDGF BB-induced DNA synthesis, whereas the PI 3-K inhibitor wortmannin had a very modest inhibitory effect on DNA synthesis (approximately Delta20%). On the other hand, wortmannin completely blocked PDGF AA- and PDGF BB-induced migration, whereas PD-098059 had a modest inhibitory effect on cell migration. These data demonstrate that activation of MAPK is necessary for the mitogenic effect of PDGF BB, whereas PI 3-K is required for the chemotactic effect of PDGF AA and PDGF BB. Although PDGF AA stimulates PI 3-K and MAPK activity, it is not mitogenic and only modestly chemotactic. Collectively, the data may have implications related to the failure of PDGF AA to rescue mesangial cell phenotype in PDGF B-chain or PDGF-beta-receptor deficiency.

Regulation of Raf by Akt controls growth and differentiation in vascular smooth muscle cells

The stimulation of platelet-derived growth factor (PDGF) receptors shifts vascular smooth muscle (VSM) cells toward a more proliferative phenotype. Thrombin activates the same signaling cascades in VSM cells, namely the Ras/Raf/MEK/ERK and the phosphatidylinositol 3-kinase (PI 3-kinase)/Akt pathways. Nonetheless, thrombin was not mitogenic, but rather increased the expression of the smooth muscle-specific myosin heavy chain (SM-MHC) indicative of an in vitro re-differentiation of VSM cells. A more detailed analysis of the temporal pattern and relative signal intensities revealed marked differences. The strong and biphasic phosphorylation of ERK1/2 in response to thrombin correlated with its ability to increase the activity of the SM-MHC promoter whereas Akt was only partially and transiently phosphorylated. By contrast, PDGF, a potent mitogen in VSM cells, induced a short-lived ERK1/2 phosphorylation but a complete and sustained phosphorylation of Akt. The phosphorylated form of Akt physically interacted with Raf. Moreover, Akt phosphorylated Raf at Ser(259), resulting in a reduced Raf kinase activity and a termination of MEK and ERK1/2 phosphorylation. Disruption of the PI 3-kinase signaling prevented the PDGF-induced Akt and Raf-Ser(259) phosphorylation. Under these conditions, PDGF elicited a more sustained MEK and ERK phosphorylation and increased SM-MHC promoter activity. Consistently, in cells that express dominant negative Akt, PDGF increased SM-MHC promoter activity. Furthermore, expression of constitutively active Akt blocked the thrombin-stimulated SM-MHC promoter activity. Thus, we present evidence that the balance and cross-regulation between the PI 3-kinase/Akt and Ras/Raf/MEK signaling cascades determine the temporal pattern of ERK1/2 phosphorylation and may thereby guide the phenotypic modulation of vascular smooth muscle cells.

Antiapoptotic signaling generated by caspase-induced cleavage of RasGAP

Activation of caspases 3 and 9 is thought to commit a cell irreversibly to apoptosis. There are, however, several documented situations (e.g., during erythroblast differentiation) in which caspases are activated and caspase substrates are cleaved with no associated apoptotic response. Why the cleavage of caspase substrates leads to cell death in certain cases but not in others is unclear. One possibility is that some caspase substrates generate antiapoptotic signals when cleaved. Here we show that RasGAP is one such protein. Caspases cleave RasGAP into a C-terminal fragment (fragment C) and an N-terminal fragment (fragment N). Fragment C expressed alone induces apoptosis, but this effect could be totally blocked by fragment N. Fragment N could also block apoptosis induced by low levels of caspase 9. As caspase activity increases, fragment N is further cleaved into fragments N1 and N2. Apoptosis induced by high levels of caspase 9 or by cisplatin was strongly potentiated by fragment N1 or N2 but not by fragment N. The present study supports a model in which RasGAP functions as a sensor of caspase activity to determine whether or not a cell should survive. When caspases are mildly activated, the partial cleavage of RasGAP protects cells from apoptosis. When caspase activity reaches levels that allow completion of RasGAP cleavage, the resulting RasGAP fragments turn into potent proapoptotic molecules.

Vav family proteins couple to diverse cell surface receptors

Vav proteins are guanine nucleotide exchange factors for Rho family GTPases which activate pathways leading to actin cytoskeletal rearrangements and transcriptional alterations. Vav proteins contain several protein binding domains which can link cell surface receptors to downstream signaling proteins. Vav1 is expressed exclusively in hematopoietic cells and tyrosine phosphorylated in response to activation of multiple cell surface receptors. However, it is not known whether the recently identified isoforms Vav2 and Vav3, which are broadly expressed, can couple with similar classes of receptors, nor is it known whether all Vav isoforms possess identical functional activities. We expressed Vav1, Vav2, and Vav3 at equivalent levels to directly compare the responses of the Vav proteins to receptor activation. Although each Vav isoform was tyrosine phosphorylated upon activation of representative receptor tyrosine kinases, integrin, and lymphocyte antigen receptors, we found unique aspects of Vav protein coupling in each receptor pathway. Each Vav protein coprecipitated with activated epidermal growth factor and platelet-derived growth factor (PDGF) receptors, and multiple phosphorylated tyrosine residues on the PDGF receptor were able to mediate Vav2 tyrosine phosphorylation. Integrin-induced tyrosine phosphorylation of Vav proteins was not detected in nonhematopoietic cells unless the protein tyrosine kinase Syk was also expressed, suggesting that integrin activation of Vav proteins may be restricted to cell types that express particular tyrosine kinases. In addition, we found that Vav1, but not Vav2 or Vav3, can efficiently cooperate with T-cell receptor signaling to enhance NFAT-dependent transcription, while Vav1 and Vav3, but not Vav2, can enhance NFkappaB-dependent transcription. Thus, although each Vav isoform can respond to similar cell surface receptors, there are isoform-specific differences in their activation of downstream signaling pathways.

Platelet-derived growth factor-dependent association of the GTPase-activating protein of Ras and Src

Here we report that the platelet-derived growth factor beta receptor (betaPDGFR) is not the only tyrosine kinase able to associate with the GTPase-activating protein of Ras (RasGAP). The interaction of non-betaPDGFR kinase(s) with RasGAP was dependent on stimulation with platelet-derived growth factor (PDGF) and seemed to require tyrosine phosphorylation of RasGAP. Because the tyrosine phosphorylation site of RasGAP is in a sequence context that is favoured by the Src homology 2 ('SH2') domain of Src family members, we tested the possibility that Src was the kinase that associated with RasGAP. Indeed, Src interacted with phosphorylated RasGAP fusion proteins; immunodepletion of Src markedly decreased the recovery of the RasGAP-associated kinase activity. Thus PDGF-dependent tyrosine phosphorylation of RasGAP results in the formation of a complex between RasGAP and Src. To begin to address the relevance of these observations, we focused on the consequences of the interaction of Src and RasGAP. We found that a receptor mutant that did not activate Src was unable to efficiently mediate the tyrosine phosphorylation of phospholipase Cgamma (PLCgamma). Taken together, these observations support the following hypothesis. When RasGAP is recruited to the betaPDGFR, it is phosphorylated and associates with Src. Once bound to RasGAP, Src is no longer able to promote the phosphorylation of PLCgamma. This hypothesis offers a mechanistic explanation for our previously published findings that the recruitment of RasGAP to the betaPDGFR attenuates the tyrosine phosphorylation of PLCgamma. Finally, these findings suggest a novel way in which RasGAP negatively regulates signal relay by the betaPDGFR.

Platelet-derived growth factor-dependent association of the GTPase-activating protein of Ras and Src

Here we report that the platelet-derived growth factor beta receptor (betaPDGFR) is not the only tyrosine kinase able to associate with the GTPase-activating protein of Ras (RasGAP). The interaction of non-betaPDGFR kinase(s) with RasGAP was dependent on stimulation with platelet-derived growth factor (PDGF) and seemed to require tyrosine phosphorylation of RasGAP. Because the tyrosine phosphorylation site of RasGAP is in a sequence context that is favoured by the Src homology 2 ('SH2') domain of Src family members, we tested the possibility that Src was the kinase that associated with RasGAP. Indeed, Src interacted with phosphorylated RasGAP fusion proteins; immunodepletion of Src markedly decreased the recovery of the RasGAP-associated kinase activity. Thus PDGF-dependent tyrosine phosphorylation of RasGAP results in the formation of a complex between RasGAP and Src. To begin to address the relevance of these observations, we focused on the consequences of the interaction of Src and RasGAP. We found that a receptor mutant that did not activate Src was unable to efficiently mediate the tyrosine phosphorylation of phospholipase Cgamma (PLCgamma). Taken together, these observations support the following hypothesis. When RasGAP is recruited to the betaPDGFR, it is phosphorylated and associates with Src. Once bound to RasGAP, Src is no longer able to promote the phosphorylation of PLCgamma. This hypothesis offers a mechanistic explanation for our previously published findings that the recruitment of RasGAP to the betaPDGFR attenuates the tyrosine phosphorylation of PLCgamma. Finally, these findings suggest a novel way in which RasGAP negatively regulates signal relay by the betaPDGFR.

Integrins enhance platelet-derived growth factor (PDGF)-dependent responses by altering the signal relay enzymes that are recruited to the PDGF beta receptor

Since the extracellular matrix (ECM) can promote platelet-derived growth factor (PDGF)-dependent responses, we hypothesized that the ECM mediates this effect by preventing the PDGF beta receptor (betaPDGFR) from associating with the negative regulator, RasGAP (the GTPase-activating protein of Ras). We found that binding of RasGAP to the wild-type betaPDGFR was decreased; the activation of Ras and Erk was enhanced, and [3H]thymidine uptake was better in cells cultured on fibronectin than in cells cultured on polylysine. To investigate the mechanism by which culturing cells on fibronectin diminished the recruitment of RasGAP to the betaPDGFR, we focused on SHP-2 since it dephosphorylates the betaPDGFR at the phosphotyrosine required for binding of RasGAP. Culturing cells on fibronectin increased the amount of SHP-2 that associated with the betaPDGFR. Furthermore, cells expressing receptor mutants that failed to associate with SHP-2 were insensitive to fibronectin. The ECM enhances PDGF-dependent responses by increasing the association of SHP-2 with the betaPDGFR, which in turn decreases the time that RasGAP interacts with the receptor. Thus, fibronectin changes PDGF-dependent signaling and biological responses by altering the signal relay enzymes that are recruited to the receptor.

Diverse signaling pathways activated by growth factor receptors induce broadly overlapping, rather than independent, sets of genes

We sought to explore the relationship between receptor tyrosine kinase (RTK) activated signaling pathways and the transcriptional induction of immediate early genes (IEGs). Using global expression monitoring, we identified 66 fibroblast IEGs induced by platelet-derived growth factor beta receptor (PDGFRbeta) signaling. Mutant receptors lacking binding sites for activation of the PLCgamma, PI3K, SHP2, and RasGAP pathways still retain partial ability to induce 64 of these IEGs. Removal of the Grb2-binding site further broadly reduces induction. These results suggest that the diverse pathways exert broadly overlapping effects on IEG induction. Interestingly, a mutant receptor that restores the RasGAP-binding site promotes induction of an independent group of genes, normally induced by interferons. Finally, we compare the PDGFRbeta and fibroblast growth factor receptor 1; each induces essentially identical IEGs in fibroblasts.

Disruption of gap junctional communication by the platelet-derived growth factor is mediated via multiple signaling pathways

The platelet-derived growth factor (PDGF) mediates its cellular functions via activation of its receptor tyrosine kinase followed by the recruitment and activation of several signaling molecules. These signaling molecules then initiate specific signaling cascades, finally resulting in distinct physiological effects. To delineate the PDGF signaling pathway responsible for the disruption of gap junctional communication (GJC), wild-type PDGF receptor beta (PDGFRbeta) and a series of PDGFRbeta mutants were expressed in T51B rat liver epithelial cells. In cells expressing wild-type PDGFRbeta, PDGF induced disruption of GJC and phosphorylation of a gap junctional protein, connexin-43 (Cx43), which required activation of mitogen-activated protein kinase, although involvement of additional factors was also evident. In the F5 mutant lacking binding sites for phosphatidylinositol 3-kinase, GTPase-activating protein, SHP-2, and phospholipase Cgamma1 (PLCgamma1), PDGF induced mitogen-activated protein kinase, but failed to affect GJC or Cx43, indicating involvement of additional signals presumably initiated by one or more of the mutated binding sites. Examination of the single-site mutants revealed that PDGF effects were not mediated via a single signaling component. This was confirmed by the "add-back" mutants, which showed that restoration of either SHP-2 or PLCgamma1 binding was sufficient to propagate the GJC inhibitory actions of PDGF. Further analysis showed that activation of PLCgamma1 is involved in Cx43 phosphorylation, which surprisingly failed to correlate with GJC blockade. The results of our study demonstrate that PDGF-induced disruption of GJC can be mediated by multiple signaling pathways and requires participation of multiple components.

Tyrosine phosphatase SHP-2 dephosphorylates the platelet-derived growth factor receptor but enhances its downstream signalling

SHP-2 is a widely distributed Src homology 2 (SH2) domain-containing tyrosine phosphatase that is recruited to growth factor receptors on stimulation. We have transiently co-expressed several catalytically active and inactive forms of the enzyme with the platelet-derived growth factor (PDGF) receptor in human embryonic kidney 293 cells. The catalytically active forms of SHP-2 decreased the tyrosine phosphorylation of the receptor, whereas the catalytically inactive forms increased the phosphorylation. However, PDGF-induced activation of the mitogen-activated protein (MAP) kinase pathway was enhanced by the active forms of SHP-2 but decreased by the inactive forms. The results suggest that the PDGF receptor is a physiological substrate of SHP-2 and that SHP-2 has a positive role in the PDGF-stimulated activation of MAP kinase. The dissociation of the receptor phosphorylation from the activation of MAP kinase suggests that signalling through growth factor receptors does not depend merely on their tyrosine phosphorylation.

APS, an adaptor protein containing PH and SH2 domains, is associated with the PDGF receptor and c-Cbl and inhibits PDGF-induced mitogenesis

Previously we cloned a novel adaptor protein, APS (adaptor molecules containing PH and SH2 domains) which was tyrosine phosphorylated in response to c-kit or B cell receptor stimulation. Here we report that APS was expressed in some human osteosarcoma cell lines, markedly so in SaOS-2 cells, and was tyrosine-phosphorylated in response to several growth factors, including platelet derived growth factor (PDGF), insulin-like growth factor (IGF), and granulocyte-macrophage colony stimulating factor (GM-CSF). Ectopic expression of the wild type APS, but not C-terminal truncated APS, in NIH3T3 fibroblasts suppressed PDGF-induced MAP kinase (Erk2) activation, c-fos and c-myc induction as well as cell proliferation. In vitro binding experiments suggest that APS bound to the beta type PDGF receptor, mainly via phosphotyrosine 1021 (pY1021). Indeed, tyrosine phosphorylation of PLC-gamma, which has been demonstrated to bind to pY1021, but not that of PI3 kinase and associated proteins, was reduced in APS transformants. PDGF induced phosphorylation of the tyrosine residue of APS close to the C-terminal end. In vitro and in vivo binding experiments indicate that the tyrosine phosphorylated C-terminal region of APS bound to c-Cbl, which has been shown to be a negative regulator of tyrosine kinases. Since coexpression of c-Cbl with wild type APS, but not C-terminal truncated APS, synergistically inhibited PDGF-induced c-fos promoter activation, c-Cbl could be a mechanism of inhibitory action of APS on PDGF receptor signaling.

A single amino acid substitution in a WW-like domain of diverse members of the PDGF receptor subfamily of tyrosine kinases causes constitutive receptor activation

Platelet-derived growth factor beta receptor (PDGFbetaR) is a transmembrane receptor tyrosine kinase involved in a variety of cellular functions. We have generated a constitutively activated murine PDGFbetaR containing a valine to alanine substitution at residue 536, located in the cytoplasmic juxtamembrane domain. When this mutant receptor (PR-V536A) was expressed in Ba/F3 cells, it allowed the cells to survive and proliferate in the absence of IL-3 or PDGF, and tyrosine phosphorylation of PR-V536A was increased markedly compared with that of the wild-type PDGFbetaR in the absence of ligand and similar to that observed in ligand-activated PDGFbetaR. PR-V536A displayed increased tyrosine kinase activity in vitro toward an exogenous substrate, and the tyrosine kinase activity of the receptor was required for the constitutive activation of the mutant. This valine to alanine substitution also activated a PDGFbetaR mutant unable to bind PDGF. Alanine substitutions at positions homologous to V536 of the murine PDGFbetaR also activated other members of the PDGF receptor subfamily. The amino acid sequence of this region revealed a strong similarity to WW domains present in other signal transduction proteins. Furthermore, GST fusion proteins containing the juxtamembrane region of the PDGFR specifically associated with peptides containing the WW domain consensus recognition sequence PPXY. The results suggest that the cytoplasmic juxtamembrane domain plays a role in the regulation of receptor activity and function, perhaps by participating in protein-protein interactions.

Opposing actions of CSW and RasGAP modulate the strength of Torso RTK signaling in the Drosophila terminal pathway

In Drosophila, specification of embryonic terminal cells is controlled by the Torso receptor tyrosine kinase. Here, we analyze the molecular basis of positive (Y630) and negative (Y918) phosphotyrosine (pY) signaling sites on Torso. We find that the Drosophila homolog of RasGAP associates with pY918 and is a negative effector of Torso signaling. Further, we show that the tyrosine phosphatase Corkscrew (CSW), which associates with pY630, specifically dephosphorylates the negative pY918 Torso signaling site, thus identifying Torso to be a substrate of CSW in the terminal pathway. CSW also serves as an adaptor protein for DRK binding, physically linking Torso to Ras activation. The opposing actions of CSW and RasGAP modulate the strength of the Torso signal, contributing to the establishment of precise boundaries for terminal structure development.

Involvement of phospholipase Cgamma1 in mouse egg activation induced by a truncated form of the C-kit tyrosine kinase present in spermatozoa

Microinjection of a truncated form of the c-kit tyrosine kinase present in mouse spermatozoa (tr-kit) activates mouse eggs parthenogenetically, and tr-kit- induced egg activation is inhibited by preincubation with an inhibitor of phospholipase C (PLC) (Sette, C., A. Bevilacqua, A. Bianchini, F. Mangia, R. Geremia, and P. Rossi. 1997. Development [Camb.]. 124:2267-2274). Co-injection of glutathione-S-transferase (GST) fusion proteins containing the src-homology (SH) domains of the gamma1 isoform of PLC (PLCgamma1) competitively inhibits tr-kit- induced egg activation. A GST fusion protein containing the SH3 domain of PLCgamma1 inhibits egg activation as efficiently as the whole SH region, while a GST fusion protein containing the two SH2 domains is much less effective. A GST fusion protein containing the SH3 domain of the Grb2 adaptor protein does not inhibit tr-kit-induced egg activation, showing that the effect of the SH3 domain of PLCgamma1 is specific. Tr-kit-induced egg activation is also suppressed by co-injection of antibodies raised against the PLCgamma1 SH domains, but not against the PLCgamma1 COOH-terminal region. In transfected COS cells, coexpression of PLCgamma1 and tr-kit increases diacylglycerol and inositol phosphate production, and the phosphotyrosine content of PLCgamma1 with respect to cells expressing PLCgamma1 alone. These data indicate that tr-kit activates PLCgamma1, and that the SH3 domain of PLCgamma1 is essential for tr-kit-induced egg activation.

Signal transduction via platelet-derived growth factor receptors

Platelet-derived growth factor (PDGF) exerts its stimulatory effects on cell growth and motility by binding to two related protein tyrosine kinase receptors. Ligand binding induces receptor dimerization and autophosphorylation, allowing binding and activation of cytoplasmic SH2-domain containing signal transduction molecules. Thereby, a number of different signaling pathways are initiated leading to cell growth, actin reorganization migration and differentiation. Recent observations suggest that extensive cross-talk occurs between different signaling pathways, and that stimulatory signals are modulated by inhibitory signals arising in parallel.

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

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

Activation of Src family members is not required for the platelet-derived growth factor beta receptor to initiate mitogenesis

The basal activity of Src family kinases is readily detectable throughout the cell cycle and increases by two- to fivefold upon acute stimulation of cells with growth factors such as platelet-derived growth factor. Previous reports have demonstrated a requirement for Src activity for the G1/S and G2/M transitions. With a chimeric alpha-beta PDGF receptor (PDGFR) expressed in fibroblasts, we have investigated the importance of the PDGF-mediated increase in Src activity at the G0/G1 transition for subsequent cell cycle events. A mutant PDGFR chimera that was not able to detectably associate with or activate Src was compromised in its ability to mediate tyrosine phosphorylation of receptor-associated signaling molecules and initiated a submaximal activation of Erk. In contrast to these early cell cycle events, later responses such as entry of cells into S phase and cell proliferation proceeded normally when Src activity did not increase following acute stimulation with PDGF. We conclude that the initial burst of Src activity is required for efficient tyrosine phosphorylation of receptor-associated proteins such as PLCgamma, RasGAP, Shc, and SHP-2 and for maximal activation of Erk. Surprisingly, these events are not required for PDGF-dependent cell proliferation. Finally, later cell cycle events do not require that Src be activated at the G0/G1 transition and leave open the possibility that events such as the G1/S transition require the basal Src activity and/or activation of Src at later times in G1.

Caspase-dependent cleavage of signaling proteins during apoptosis. A turn-off mechanism for anti-apoptotic signals

Caspases are activated during apoptosis and cleave specific proteins, resulting in the irreversible commitment to cell death. The signal transduction proteins MEKK1, p21-activated kinase 2, and focal adhesion kinase are caspase substrates that contribute to the cell death response when cleaved. Thirty additional signaling proteins were screened for their ability to be cleaved during apoptosis. Twenty-two of these proteins were not affected in Jurkat cells stimulated to undergo apoptosis by Fas ligation, exposure to ultraviolet-C or incubation with etoposide. Ras GTPase-activating protein was found to be a caspase substrate whose cleavage followed the same time course as that for activation of caspase activity and the cleavage of MEKK1 and focal adhesion kinase. Four additional proteins, Cbl, Cbl-b, Raf-1, and Akt-1, were cleaved later in the apoptotic response. These signaling proteins were similarly cleaved in U937 cells undergoing apoptosis. Cleavage of the proteins was blocked by caspase inhibitors in Jurkat cells or in U937 cells expressing BclxL, demonstrating that the cleavage was dependent on caspase activation. Cleavage of Raf-1 and Akt correlated with the loss of extracellular signal-regulated kinase and Akt activities in apoptotic cells. Neither c-Jun N-terminal kinase nor p38 mitogen-activated protein kinase was cleaved in cells undergoing apoptosis, and the activation of the c-Jun N-terminal kinase and p38 mitogen-activated protein kinase pathways was not compromised in apoptotic cells. These results indicate that caspase-dependent cleavage of specific proteins induces the turn off of survival pathways, such as the extracellular signal-regulated kinase and phosphatidylinositol-3 kinase/Akt pathways, that could otherwise interfere with the apoptotic response.

Mutations at residues Tyr771 and Tyr783 of phospholipase C-gamma1 have different effects on cell actin-cytoskeleton organization and cell proliferation in CCL-39 cells

Currently a central question that remains unresolved is the potential importance of phospholipase C (PLC)-gamma1 in cell mitogenesis. In this study, we introduced wild-type PLC-gamma1 plasmid and mutants Y771F and Y783F into CCL-39 fibroblasts and investigated their effect on host cell functions. To our surprise, Y771F and Y783F plasmids appeared to have opposite effects on CCL-39 cell actin-cytoskeleton organization and cell proliferation. Y771F transfectants increased cell proliferation by two-fold. Y783F transfectants showed much thicker actin filaments and decreased cell growth rate by 50%. These results suggest that PLC-gamma1 mutations have an essential impact on cell mitogenesis.

A negative regulatory function for the protein tyrosine phosphatase PTP2C revealed by reconstruction of platelet-derived growth factor receptor signalling in Schizosaccharomyces pombe

We have exploited reconstitution in the fission yeast Schizosaccharomyces pombe to investigate how activation of phospholipase Cgamma (PLCgamma) by the platelet-derived growth factor-beta receptor (PDGFbetaR) is regulated by the SH2 domain-containing protein tyrosine phosphatase PTP2C (also known as SHP-2). When co-expressed in S. pombe, PTP2C abolished PDGFbetaR autophosphorylation as well as its ability to phosphorylate and activate PLCgamma. Inhibition of PDGFbetaR signalling by PTP2C appears specific insofar that PTPIC, a close homologue of PTP2C, does not suppress activation of either PDGFbetaR or PLCgamma. Surprisingly, an inactive PTP2C mutant (C459S), which dephosphorylates neither PDGFbetaR nor PLCgamma, remains fully effective as an inhibitor of [3H]inositol phosphate generation indicating that negative regulation is at least in part independent of catalytic activity. This contrasts with PLCgamma activation by c-Src which, although blocked by active PTP2C, is not inhibited by the mutant PTP2C C459S. These observations indicate that in addition to a reported positive role relaying trophic signals, PTP2C can also exert a negative effect on the PDGFbetaR and its signalling to PLCgamma.

SHP-1 associates with both platelet-derived growth factor receptor and the p85 subunit of phosphatidylinositol 3-kinase

The Src homology 2 (SH2)-containing protein tyrosine phosphatase 1, SHP-1, is highly expressed in all hematopoietic cells as well as in many non-hematopoietic cells, particularly in some malignant epithelial cell lines. In hematopoietic cells, SHP-1 negatively regulates multiple cytokine receptor pathways. The precise function and the targets of SHP-1 in non-hematopoietic cells, however, are largely unknown. Here we demonstrate that SHP-1 associates with both the tyrosine-phosphorylated platelet-derived growth factor (PDGF) receptor and the p85 subunit of phosphatidylinositol 3-kinase in MCF-7 and TRMP cells. Through the use of mutant PDGF receptors and performing peptide competition for immunoprecipitation, it was determined that SHP-1 independently associates with the PDGF receptor and p85 and that its N-terminal SH2 domain is directly responsible for the interactions. Overexpression of SHP-1 in TRMP cells transfected with the PDGF receptor markedly inhibited PDGF-induced c-fos promoter activation, whereas the expression of three catalytically inactive SHP-1 mutants increased the c-fos promoter activation in response to PDGF stimulation. These results indicate that SHP-1 might negatively regulate PDGF receptor-mediated signaling in these cells. Identification of the association of SHP-1 with the PDGF receptor and p85 in MCF-7 and TRMP cells furthers our understanding of the function of SHP-1 in non-hematopoietic cells.

Wortmannin-sensitive phosphorylation, translocation, and activation of PLCgamma1, but not PLCgamma2, in antigen-stimulated RBL-2H3 mast cells

In RBL-2H3 tumor mast cells, cross-linking the high affinity IgE receptor (FcepsilonRI) with antigen activates cytosolic tyrosine kinases and stimulates Ins(1,4,5)P3 production. Using immune complex phospholipase assays, we show that FcepsilonRI cross-linking activates both PLCgamma1 and PLCgamma2. Activation is accompanied by the increased phosphorylation of both PLCgamma isoforms on serine and tyrosine in antigen-treated cells. We also show that the two PLCgamma isoforms have distinct subcellular localizations. PLCgamma1 is primarily cytosolic in resting RBL-2H3 cells, with low levels of plasma membrane association. After antigen stimulation, PLCgamma1 translocates to the plasma membrane where it associates preferentially with membrane ruffles. In contrast, PLCgamma2 is concentrated in a perinuclear region near the Golgi and adjacent to the plasma membrane in resting cells and does not redistribute appreciably after FcepsilonRI cross-linking. The activation of PLCgamma1, but not of PLCgamma2, is blocked by wortmannin, a PI 3-kinase inhibitor previously shown to block antigen-stimulated ruffling and to inhibit Ins(1,4,5)P3 synthesis. In addition, wortmannin strongly inhibits the antigen-stimulated phosphorylation of both serine and tyrosine residues on PLCgamma1 with little inhibition of PLCgamma2 phosphorylation. Wortmannin also blocks the antigen-stimulated translocation of PLCgamma1 to the plasma membrane. Our results implicate PI 3-kinase in the phosphorylation, translocation, and activation of PLCgamma1. Although less abundant than PLCgamma2, activated PLCgamma1 may be responsible for the bulk of antigen-stimulated Ins(1,4,5)P3 production in RBL-2H3 cells.

Alphavbeta3 integrin associates with activated insulin and PDGFbeta receptors and potentiates the biological activity of PDGF

Integrin-mediated cell attachment modulates growth responses and growth factors regulate cell attachment. Moreover, both cell attachment to extracellular matrix and mitogenic signaling by growth factors are necessary for the proliferation of most types of normal cells, suggesting that integrin and growth factor receptor signaling pathways meet at some downstream point. We report here that a small, highly tyrosine-phosphorylated fraction of PDGFbeta and insulin receptors co-immunoprecipitates with the alphavbeta3 integrin from cells. The integrin association requires growth factor stimulation of the receptors. Several signaling molecules that are known to be associated with activated growth factor receptors were present in the alphavbeta3 integrin complexes. Mitogenicity and chemotaxis induced by PDGF-BB were enhanced in cells plated on the alphavbeta3 ligand vitronectin compared with cells plated on the beta1 integrin ligand collagen. Thus, the engagement of the alphavbeta3 integrin in cell-matrix interactions appears to coordinate an intense response to growth factors, helping to explain the importance of this integrin for tissue regeneration, angiogenesis and tumor metastasis.

A new function for phospholipase C-gamma1: coupling to the adaptor protein GRB2

Epidermal growth factor (EGF)-induced autophosphorylation of the EGF receptor results in high-affinity binding of the adaptor protein GRB2, which serves as a convergence point for multiple signaling pathways. Present studies demonstrate that EGF induces the co-immunoprecipitation of phospholipase C (PLC)-gamma1 with the adaptor protein GRB2 and the guanine nucleotide exchange factor Sos, but not with the adaptor protein SHC, in WB cells. Inhibition of PLC-gamma1 tyrosine phosphorylation by phenylarsine oxide reduces the co-immunoprecipitation of PLC-gamma1 with GRB2. Furthermore, angiotensin II, a G protein-coupled receptor agonist, also induces the tyrosine phosphorylation of PLC-gamma1 and its co-immunoprecipitation with GRB2 in WB cells. Interestingly, angiotensin II stimulation also causes tyrosine phosphorylation of the EGF receptor, suggesting that angiotensin II-induced PLC-gamma1 tyrosine phosphorylation in WB cells may be via EGF receptor tyrosine kinase activation. In addition, there is some level of association between PLC-gamma1 and GRB2 that is independent of the tyrosine phosphorylation of PLC-gamma1 in both in vivo and in vitro studies. In vitro studies further demonstrate that the Tyr771 and Tyr783 region of PLC-gamma1 and the SH2 domain of GRB2 are potentially involved in the tyrosine phosphorylation-dependent association between PLC-gamma1 and GRB2. The association of PLC-gamma1 with GRB2 and Sos suggests that PLC-gamma1 may be directly involved in the Ras signaling pathway and that GRB2 may be involved in the translocation of PLC-gamma1 from cytosol to the plasma membrane as a necessary step for its effect on inositol lipid hydrolysis.

Differential effects of B cell receptor and B cell receptor-FcgammaRIIB1 engagement on docking of Csk to GTPase-activating protein (GAP)-associated p62

The stimulatory and inhibitory pathways initiated by engagement of stimulatory receptors such as the B cell receptor for antigen (BCR) and inhibitory receptors such as Fcgamma receptors of the IIB1 type (FcgammaRIIB1) intersect in ways that are poorly understood at the molecular level. Because the tyrosine kinase Csk is a potential negative regulator of lymphocyte activation, we examined the effects of BCR and FcgammaRIIB1 engagement on the binding of Csk to phosphotyrosine-containing proteins. Stimulation of a B lymphoma cell line, A20, with intact anti-IgG antibody induced a direct, SH2-mediated association between Csk and a 62-kD phosphotyrosine-containing protein that was identified as RasGTPase-activating protein-associated p62 (GAP-A.p62). In contrast, stimulation of A20 cells with anti-IgG F(ab')2 resulted in little increase in the association of Csk with GAP-A.p62. The effect of FcgammaRIIB1 engagement on this association was abolished by blockade of FcgammaRIIB1 with the monoclonal antibody 2.4G2. Furthermore, the increased association between Csk and GAP-A.p62 seen upon stimulation with intact anti-Ig was abrogated in the FcgammaRIIB1-deficient cell line IIA1.6 and recovered when FcgammaRIIB1 expression was restored by transfection. The differential effects of BCR and BCR-FcgammaRIIB1-mediated signaling on the phosphorylation of GAP-A.p62 and its association with Csk suggest that docking of Csk to GAP-A.p62 may function in the negative regulation of antigen receptor-mediated signals in B cells.

Positive and negative tissue-specific signaling by a nematode epidermal growth factor receptor

The major determinants of receptor tissue tyrosine kinase (RTK) signaling specificity have been proposed to be Src homology 2 (SH2) binding sites, phosphotyrosine-containing oligopeptides in the cytoplasmic domain of the receptor. The Caenorhabditis elegans epidermal growth factor receptor homologue LET-23 has multiple functions during development and has eight potential SH2-binding sites in a region carboxyl terminal to its kinase domain. By analyzing transgenic nematodes for three distinct LET-23 functions, we show that six of eight potential sites function in vivo and that they are required for most, but not all, of LET-23 activity. A single site is necessary and sufficient to promote wild-type fertility. Three other sites activate the RAS pathway and are involved only in viability and vulval differentiation. A fifth site is promiscuous and can mediate all three LET-23 functions. An additional site mediates tissue-specific negative regulation. Putative SH2 binding sites are thus key effectors of both cell-specific and negative regulation in an intact organism. We suggest two distinct mechanisms for tissue-specific RTK-mediated signaling. A positive mechanism would promote RTK function through effectors present only in certain cell types. A negative mechanism would inhibit RTK function through tissue-specific negative regulators.

Platelet-derived growth factor-dependent cellular transformation requires either phospholipase Cgamma or phosphatidylinositol 3 kinase

Although it has been well established that constitutive activation of receptor tyrosine kinases leads to cellular transformation, the signal relay pathways involved have not been systematically investigated. In this study we used a panel of platelet-derived growth factor (PDGF) beta receptor mutants (beta-PDGFR), which selectively activate various signal relay enzymes to define which signaling pathways are required for PDGF-dependent growth of cells in soft agar. The host cell line for these studies was Ph cells, a 3T3-like cell that expresses normal levels of the beta-PDGFR but no PDGF-alpha receptor (alpha-PDGFR). Hence, this cell system can be used to study signaling of mutant alphaPDGFRs or alpha/beta chimeras. We constructed chimeric receptors containing the alphaPDGFR extracellular domain and the betaPDGFR cytoplasmic domain harboring various phosphorylation site mutations. The mutants were expressed in Ph cells, and their ability to drive PDGF-dependent cellular transformation (growth in soft agar) was assayed. Cells infected with an empty expression vector failed to grow in soft agar, whereas introduction of the chimera with a wild-type beta-PDGFR cytoplasmic domain gave rise to a large number of colonies. In contrast, the N2F5 chimera, in which the binding sites for phospholipase Cgamma (PLC-gamma), RasGTPase-activating protein, phosphatidylinositol 3 kinase (PI3K), and SHP-2 were eliminated, failed to trigger proliferation. Restoring the binding sites for RasGTPase-activating protein or SHP-2 did not rescue the PDGF-dependent response. In contrast, receptors capable of associating with either PLC-gamma or PI3K relayed a growth signal that was comparable to wild-type receptors in the soft agar growth assay. These findings indicate that the PDGF receptor activates multiple signaling pathways that lead to cellular transformation, and that either PI3K or PLC-gamma are key initiators of such signal relay cascades.

p62(dok): a constitutively tyrosine-phosphorylated, GAP-associated protein in chronic myelogenous leukemia progenitor cells

Characteristic of chronic myelogenous leukemia (CML) is the presence of the chimeric p210(bcr-abl) protein possessing elevated protein tyrosine kinase activity relative to normal c-abl tyrosine kinase. Hematopoietic progenitors isolated from CML patients in the chronic phase contain a constitutively tyrosine-phosphorylated protein that migrates at 62 kDa by SDS-PAGE and associates with the p120 ras GTPase-activating protein (GAP). We have purified p62(dok) from a hematopoietic cell line expressing p210(bcr-abl). p62(dok) is a novel protein with features of a signaling molecule. Association of p62(dok) with GAP correlates with its tyrosine phosphorylation. p62(dok) is rapidly tyrosine-phosphorylated upon activation of the c-Kit receptor, implicating it as a component of a signal transduction pathway downstream of receptor tyrosine kinases.

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

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

Differential Ca2+ signaling induced by activation of the epidermal growth factor and nerve growth factor receptors

Stimulation by epidermal growth factor (EGF) of NIH3T3 cells overexpressing the EGF receptor (EGFR) results in a release of Ca2+ from internal stores. Ca2+ release is followed by an influx of extracellular calcium which can be recorded by the influx of the calcium surrogate Mn2+. Both Ca2+ release and Mn2+/Ca2+ influx are inhibited by expression of the dominant negative Asn17-Ras mutant and abrogated by microinjected neutralizing anti-Ras antibody Y13-259, whereas microinjection of the anti-Ras antibody Y13-238 which does not interact with the effector binding domain of Ras is without any effect on the EGF-induced Ca2+ transient. Neither Asn17-Ha-Ras nor the Y13-259 antibody interferes with the thapsigargin-induced Mn2+/Ca2+ influx. The nerve growth factor receptor (Trk)-mediated Ca2+ transient was found to be unaffected by the dominant negative Ras mutant or microinjected neutralizing anti-Ras antibodies. Substitution of the phospholipase Cgamma1 (PLCgamma1) binding site of the EGFR by the PLCgamma binding domain of Trk renders the EGFR-induced Ca2+ influx insensitive to the expression of Asn17-Ha-Ras, whereas the Ca2+ signal induced by Trk carrying the PLC binding site of EGFR is Ras-dependent and abrogated by the dominant negative Ras mutant. It is concluded that the Ca2+ transient induced by the activated EGFR, not, however, the Ca2+ transient elicited by the activated NGFR/Trk, is a Ras-mediated phenomenon and that the role of Ras in regulating EGFR-induced Ca2+ influx depends on the structure of the PLCgamma binding domain.

Grb7 is a downstream signaling component of platelet-derived growth factor alpha- and beta-receptors

Ligand stimulation of the platelet-derived growth factor (PDGF) alpha- or beta-receptors leads to activation of their intrinsic tyrosine kinases and autophosphorylation of tyrosine residues. Grb7 is an SH2 and PH domain-containing molecule that is known to be overexpressed in some breast cancer tissues and cell lines. Here we show that the SH2 domain of Grb7 can directly bind to the autophosphorylated PDGF beta-receptor in vitro. Grb7 association to the PDGF beta-receptor was dramatically reduced by replacement of tyrosine residues 716 or 775 with phenylalanine residues. Synthetic phosphorylated peptides containing Tyr-716 or Tyr-775 inhibited binding of the Grb7 SH2 domain to the autophosphorylated PDGF beta-receptor in a manner similar to but distinct from the binding of the Grb2 SH2 domain. Grb7 associated with activated PDGF beta-receptors in vivo, and the association was dramatically reduced by substitution of Tyr-716 or Tyr-775 with a phenylalanine residue. Furthermore, complex formation between Shc and Grb7 was observed after ligand stimulation of PDGF alpha- or beta-receptors in cells transfected with Grb7 cDNA or in the breast cancer cell line BT-474. Thus, Grb7 is implicated in PDGF signaling pathways in certain cell types by binding to the receptor directly or indirectly via Shc.

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.

Drosophila terminal structure development is regulated by the compensatory activities of positive and negative phosphotyrosine signaling sites on the Torso RTK

Specification of cell fates in the nonsegmented terminal regions of developing Drosophila embryos is under the control of a signal transduction pathway mediated by the receptor tyrosine kinase Torso (Tor). Here, we identify tyrosines (Y) 630 and 918 as the major sites of Tor autophosphorylation. We demonstrate that mutation of Y630, a site required for association with and tyrosine phosphorylation of the tyrosine phosphatase Corkscrew, decreases the efficiency of Tor signaling. In contrast, mutation of Y918, a site capable of binding mammalian rasGAP and PLC-gammal, increases Tor signaling. Interestingly, when receptors contain mutations in both the Y630 and Y918 sites, Tor signaling is restored to wild-type levels. These results identify a novel mechanism whereby Tor function is regulated using compensatory signals generated from distinct autophosphorylation sites and reveal an underlying signaling pathway for terminal development.

Neuregulins and their receptors

The recent identification of an activator for the ErbB2/Neu receptor has uncovered a new family of polypeptide growth factors that undoubtedly play a major role in the regulation of neuronal growth and differentiation. These factors, called the neuregulins, are expressed in neural and mesenchymal tissues, and activate members of the epidermal growth factor family of receptor tyrosine kinases. The identification and characterization of the neuregulins and their receptors will facilitate the dissection of the biochemical pathways regulating nervous system development.

Identification of a putative Syp substrate, the PDGF beta receptor

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