Guanine-nucleotide-releasing factor hSos1 binds to Grb2 and links receptor tyrosine kinases to Ras signalling

MAP kinase pathways

MAP kinases help to mediate diverse processes ranging from transcription of protooncogenes to programmed cell death. More than a dozen mammalian MAP kinase family members have been discovered and include, among others, the well studied ERKs and several stress-sensitive enzymes. MAP kinases lie within protein kinase cascades. Each cascade consists of no fewer than three enzymes that are activated in series. Cascades convey information to effectors, coordinates incoming information from other signaling pathways, amplify signals, and allow for a variety of response patterns. Subcellular localization of enzymes in the cascades is an important aspect of their mechanisms of action and contributes to cell-type and ligand-specific responses. Recent findings on these properties of MAP kinase cascades are the major focus of this review.

Signal transduction pathways in normal human monocytes stimulated by cytokines and mediators: comparative study with normal human neutrophils or transformed cells and the putative roles in functionality and cell biology

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin (IL) -3 induced tyrosine phosphorylation of 92-kDa protein in normal human monocytes. We identified this 92-kDa protein as STAT5, but not as STATs1, 3, and 6 nor c-fes and vav protooncogene products, and demonstrated its translocation to the nucleus, enhancement of specific DNA binding capacity, and potentiation of trancriptional activity by GM-CSF. N-formyl-methionyl-leucyl-phenylalanine (FMLP) and phorbol myristate acetate (PMA) induced tyrosine phosphorylation of 42- and 44-kDa proteins, which were identified as extracellular signal-regulated kinase (ERK), in human monocytes. In marked contrast to neutrophils and MO7e cells, GM-CSF did not induce tyrosine phosphorylation and activation of ERK in monocytes. Among upstream signaling molecules of ERK, Shc was constitutively associated with Grb2 and was not tyrosine-phosphorylated by GM-CSF and FMLP, and Sos1 and c-Raf-1 were not phosphorylated by GM-CSF, IL-3, TNF, and FMLP in monocytes, whereas all these signaling molecules were affected and/or utilized by GM-CSF in MO7e cells. In contrast to neutrophils, p38 was constitutively phosphorylated and agonist-dependent phosphorylation and activation was not detected in human monocytes. Superoxide release stimulated by FMLP was inhibited partially by PD98059 or SB203580, a specific inhibitor of ERK or p38 pathway, and was almost completely inhibited by the combination of both inhibitors, whereas PMA-induced superoxide release was resistant to these two inhibitors in monocytes. PD98059 inhibited GM-CSF-dependent proliferation of MO7e cells. Present results indicate trancriptional roles of STAT5 and functional roles of ERK and/or p38 in normal human monocytes stimulated by physiological receptor-mediated agonists GM-CSF and FMLP. Possible roles of ERK in proliferation of transformed cells were also suggested.

The insulin receptor tyrosine kinase domain in a chimaeric epidermal growth factor-insulin receptor generates Ca2+ signals through the PLC-gamma1 pathway

The receptors for insulin (IR) and epidermal growth factor (EGFR) are members of the tyrosine kinase receptor (TKR) family. Despite homology of their cytosolic TK domains, both receptors induce different cellular responses. Tyrosine phosphorylation of insulin receptor substrate (IRS) molecules is a specific IR post-receptor response. The EGFR specifically activates phospholipase C-gamma1 (PLC-gamma1). Recruitment of substrate molecules with Src homology 2 (SH2) domains or phosphotyrosine binding (PTB) domains to phosphotyrosines in the receptor is one of the factors creating substrate specificity. In addition, it has been shown that the TK domains of the IR and EGFR show preferences to phosphorylate distinct peptides in vitro, suggesting additional mechanisms of substrate recognition. We have examined to what extent the substrate preference of the TK domain contributes to the specificity of the receptor in vivo. For this purpose we determined whether the IR TK domain, in situ, is able to tyrosine-phosphorylate substrates normally used by the EGFR. A chimaeric receptor, consisting of an EGFR in which the juxtamembrane and tyrosine kinase domains were exchanged by their IR counterparts, was expressed in CHO-09 cells lacking endogenous EGFR. This receptor was found to activate PLC-gamma1, indicating that the IR TK domain, in situ, is able to tyrosine phosphorylate substrates normally used by the EGFR. These findings suggest that the IR TK domain, in situ, has a low specificity for selection and phosphorylation of non-cognate substrates.

hSiah2 is a new Vav binding protein which inhibits Vav-mediated signaling pathways

The hematopoietic proto-oncogene vav has been characterized as a Rac1-GDP/GTP exchanger protein which regulates cytoskeletal reorganization as well as signaling pathways leading to the activation of stress-activated protein kinases (SAPK/JNKs). Furthermore, vav overexpression enhances basal and T-cell receptor (TCR)-mediated stimulation of the nuclear factor of activated T cells (NFAT). We report here the interaction between Vav and hSiah2, a mammalian homolog of Drosophila Seven in absentia (Sina) that has been implicated in R7 photoreceptor cell formation during Drosophila eye development via the proteasome degradation pathway. Vav and hSiah2 interact in vitro and in vivo and colocalize in the cytoplasm of hematopoietic cells. The Src homology domain of Vav and the C-terminal region of hSiah2 are required for this interaction. We provide evidence for a negative regulation by hSiah2 of Vav-induced basal and TCR-mediated NFAT-dependent transcription. Overexpression of hSiah2 also inhibits the onco-Vav-induced JNK activation. Although the Vav-interacting domain is located in the C-terminal portion of hSiah2, the N-terminal region of hSiah2 is necessary for the inhibitory role that seems to be independent of the proteasome degradation.

The role of G proteins in insulin signalling

Insulin modulates many intracellular processes including cellular metabolism, cell proliferation and cell differentiation. Some of these processes involve significant changes in the traffic of intracellular vesicles or in the structural organization of the cell. These phenomena have been linked to the activity of regulatory GTP-binding proteins. Most, if not all functions, of the insulin receptor are associated with its tyrosine kinase activity. Thus, over the past few years, a significant effort has been dedicated to elucidate the cross-talk between the tyrosine kinase activity of the receptor and the regulation of G protein-mediated pathways. Recent progress indicates that G proteins may mediate the control of several of insulin's intracellular functions. These include the regulation of the MAP kinase pathway, the activation of phospholipase D and the regulation of glucose uptake. This article discusses some recent advances in this area.

Collagen-homology domain 1 deletion mutant of Shc suppresses transformation mediated by neu through a MAPK-independent pathway

Shc proteins are implicated in coupling receptor tyrosine kinase to the mitogen-activated protein kinase (MAPK) pathway by recruiting Grb2/SOS to the plasma membrane. To better understand the role of Shc in the oncogenesis by point-mutation activated neu (p185*), we transfected a Shc mutant (ShcdeltaCHI), which lacks the Grb2 binding site Y317 by deletion of collagen-homology domain 1, into p185*-transformed NIH3T3 cells. The cellular transformation phenotypes were found to be largely suppressed by expression of ShcdeltaCH1. Although ShcdeltaCH1 still retained another Grb2 binding site (Y239/240), we did not detect its physical association with Grb2. We also found that ShcdeltaCH1 could associate with p185*; however, this association did not interfere with the endogenous Shc-p185* interaction or the Shc-Grb2 interaction. In addition, p185*-mediated MAPK and Elk activation likewise were not inhibited by ShcdeltaCH1 expression. Taken together, these data demonstrate that ShcdeltaCH1 suppresses the transformation induced by activated neu through a MAPK-independent pathway, indicating that Shc may be involved in other signal pathway(s) critical for cellular transformation in addition to the MAPK pathway.

Increased mitogenicity of an alphabeta heterodimeric PDGF receptor complex correlates with lack of RasGAP binding

The different platelet-derived growth factor (PDGF) isoforms cause activation of their alpha and beta protein tyrosine kinase receptors through dimerization. Homodimerization as well as heterodimerization of receptors occur. It has been shown previously that the heterodimeric receptor complex mediates a stronger mitogenic response than either of the homodimeric complexes. In this report, we show that in cells expressing both PDGF alpha- and beta-receptors, stimulation with PDGF-AB, which leads to preferential heterodimer formation, leads to a very low degree of phosphorylation of Tyr771 in the beta-receptor. In contrast, Tyr771 is phosphorylated in a homodimeric complex of beta-receptors. Phosphorylated Tyr771 is a binding site for RasGAP; an analogous site is not present in the alpha-receptor, which lacks the ability to associate with RasGAP. The lowered phosphorylation of Tyr771 in the heterodimeric receptor complex correlates with lowered association with RasGAP, as well as with a more efficient activation of Ras and MAP kinase, which is consistent with the increased mitogenicity elicited by PDGF-AB, compared to PDGF-AA or PDGF-BB.

The p21-Ras signal transduction pathway and growth regulation in human high-grade gliomas

Deregulated p21-Ras function, as a result of mutation, overexpression or growth factor-induced overactivation, contributes to at least 30% of human cancer. This article reviews the potential role of the p21-Ras family of GTPases in the regulation of growth of high-grade gliomas and describes how targeting this oncoprotein clinically may provide a novel strategy to counteract glioma proliferation. The application of strategies directed at selectively opposing the deregulated signal transduction pathway of high-grade gliomas may be of potential therapeutic benefit and may offer a whole new arsenal of antineoplastic agents to be included in the multimodal treatment of these challenging neoplasms.

Cloning and characterization of mPAL, a novel Shc SH2 domain-binding protein expressed in proliferating cells

Shc adaptor proteins play a role in linking activated cell surface receptors to the Ras signaling pathway in response to receptor mediated tyrosine kinase activation. While the function of Shc in the activation of the Ras pathway via binding to Grb2 has been well characterized, it is becoming increasingly apparent that Shc participates in additional signaling pathways through interactions with other cytoplasmic proteins. Using the yeast two-hybrid system, we have identified a unique Shc binding protein designated PAL (Protein expressed in Activated Lymphocytes) with no similarity to other known proteins. mPAL binds specifically to the Shc SH2 domain and unlike previously described Shc SH2 domain-protein interactions, the association of mPAL and Shc is phosphotyrosine-independent. Both mPAL RNA and protein expression are restricted to tissues containing actively dividing cells and proliferating cells in culture. mPAL expression is induced upon growth factor stimulation and is down-regulated upon growth inhibition. This pattern, and timing of mPAL expression and its association with the Shc adaptor molecule suggests a role for this protein in signaling pathways governing cell cycle progression.

Isoform-specific insertion near the Grb2-binding domain modulates the intrinsic guanine nucleotide exchange activity of hSos1

Two human hSos1 isoforms (Isf I and Isf II; Rojas et al., Oncogene 12, 2291-2300, 1996) defined by the presence of a distinct 15 amino acid stretch in one of them, were compared biologically and biochemically using representative NIH3T3 transfectants overexpressing either one. We showed that hSos1-Isf II is significantly more effective than hSos1-Isf I to induce proliferation or malignant transformation of rodent fibroblasts when transfected alone or in conjunction with normal H-Ras (Gly12). The hSos1-Isf II-Ras cotransfectants consistently exhibited higher saturation density, lower cell-doubling times, increased focus-forming activity and higher ability to grow on semisolid medium and at low serum concentration than their hSos1-Isf I-Ras counterparts. Furthermore, the ratio of GTP/GDP bound to cellular p21ras was consistently higher in the hSos1-Isf II-transfected clones, both under basal and stimulated conditions. However, no significant differences were detected in vivo between Isf I- and Isf II-transfected clones regarding the amount, stability and subcellular localization of Sos1-Grb2 complex, or the level of hSos1 phosphorylation upon cellular stimulation. Interestingly, direct Ras guanine nucleotide exchange activity assays in cellular lysates showed that Isf II transfectants consistently exhibited about threefold higher activity than Isf I transfectants under basal, unstimulated conditions. Microinjection into Xenopus oocytes of purified peptides corresponding to the C-terminal region of both isoforms (encompassing the 15 amino acid insertion area and the first Grb2-binding motif) showed that only the Isf II peptide, but not its corresponding Isf I peptide, was able to induce measurable rates of meiotic maturation, and synergyzed with insulin, but not progesterone, in induction of GVBD. Our results suggest that the increased biological potency displayed by hSos1-Isf II is due to higher intrinsic guanine nucleotide exchange activity conferred upon this isoform by the 15 a.a. insertion located in proximity to its Grb2 binding region.

Activation of stress-activated protein kinases correlates with neurite outgrowth induced by protease inhibition in PC12 cells

PC12 cells are well characterized for their ability to differentiate into neuronal-like cells when challenged with nerve growth factor. It has been reported that the calpain and proteasome inhibitor N-acetyl-Leu-Leu-norleucinal (CI) is also able to induce neurite outgrowth in PC12 cells. In this study, we report that the inhibitor of proteasomal chymotrypsin-like activity, carbobenzoxy-Ile-Glu-(O-tert-butyl)-Ala-Leu-aldehyde (PSI), can also induce differentiation of PC12 cells. Induction of neurite outgrowth with PSI, CI, or its close analogue, carbobenzoxy-Leu-Leu-leucinal (MG132), was associated with stress-activated protein kinase (SAPK) activation. Neurite formation induced by protease inhibition was independent of mitogen-activated protein kinase/extracellular signal-regulated kinase, p38/reactivating kinase, or phosphatidylinositol 3-kinase activities. The exact mechanism by which protease inhibition activates SAPKs remains to be elucidated; however, our results suggest that the SAPK signal transduction cascade may be an alternative and/or parallel pathway in the regulation of neuronal differentiation.

The multisubstrate docking site of the MET receptor is dispensable for MET-mediated RAS signaling and cell scattering

The scatter factor/hepatocyte growth factor regulates scattering and morphogenesis of epithelial cells through activation of the MET tyrosine kinase receptor. In particular, the noncatalytic C-terminal tail of MET contains two autophosphorylation tyrosine residues, which form a multisubstrate-binding site for several cytoplasmic effectors and are thought to be essential for signal transduction. We show here that a MET receptor mutated on the four C-terminal tyrosine residues, Y1311F, Y1347F, Y1354F, and Y1363F, can induce efficiently a transcriptional response and cell scattering, whereas it cannot induce cell morphogenesis. Although the mutated receptor had lost its ability to recruit and/or activate known signaling molecules, such as GRB2, SHC, GAB1, and PI3K, by using a sensitive association-kinase assay we found that the mutated receptor can still associate and phosphorylate a approximately 250-kDa protein. By further examining signal transduction mediated by the mutated MET receptor, we established that it can transmit efficient RAS signaling and that cell scattering by the mutated MET receptor could be inhibited by a pharmacological inhibitor of the MEK-ERK (MAP kinase kinase-extracellular signal-regulated kinase) pathway. We propose that signal transduction by autophosphorylation of the C-terminal tyrosine residues is not the sole mechanism by which the activated MET receptor can transmit RAS signaling and cell scattering.

The gene structure of the human growth factor bound protein GRB2

The growth factor bound protein GRB2, a 25-kDa cytosolic protein, plays a key role in two separate pathways of the insulin signal transduction system leading from the insulin receptor to the Ras proteins and thus affecting mitogenic signaling. GRB2 regulates Ras activation through association with the guanine nucleotide exchange factor Sos. The GRB2/Sos complex can connect with insulin receptor substrate 1 (IRS-1), which is one of the primary targets of the insulin and insulin-like growth factor receptors. In a second pathway, independent of IRS-1, GRB2 links the insulin receptor to Ras signaling through another adapter protein, called Shc. In protooncogenic and other noninsulin signaling systems, GRB2 appears to link receptor tyrosine kinases to Ras in similar pathways as well. This study presents the exon-intron organization of the human GRB2 gene. After primers were placed across the known mRNA sequence, Long PCR products spanning introns and their adjacent splice sites were amplified and adequately sequenced to establish the splice sites and flanking regions. The gene was found to consist of five exons (ranging from 78 to 186 bp) and of four introns (from approximately 1 to approximately 7 kb). Intron primers for the respective exons were generated using the newly found flanking sequences. All exons were successfully amplified and sequenced, and the data obtained from Long PCR sequencing were confirmed.

C-terminal Src kinase associates with ligand-stimulated insulin-like growth factor-I receptor

Increased expression of the insulin-like growth factor-I receptor (IGF-IR) protein-tyrosine kinase occurs in several kinds of cancer and induces neoplastic transformation in fibroblast cell lines. The transformed phenotype can be reversed by interfering with the function of the IGF-IR. The IGF-IR is required for transformation by a number of viral and cellular oncoproteins, including SV40 large T antigen, Ras, Raf, and Src. The IGF-IR is a substrate for Src in vitro and is phosphorylated in v-Src-transformed cells. We observed that the IGF-IR and IR associated with the C-terminal Src kinase (CSK) following ligand stimulation. We found that the SH2 domain of CSK binds to the tyrosine-phosphorylated form of IGF-IR and IR. We determined the tyrosine residues in the IGF-IR and in the IR responsible for this interaction. We also observed that fibroblasts stimulated with IGF-I or insulin showed a rapid and transient decrease in c-Src tyrosine kinase activity. The results suggest that c-Src and CSK are involved in IGF-IR and IR signaling and that the interaction of CSK with the IGF-IR may play a role in the decrease in c-Src activity following IGF-I stimulation.

Growth inhibition of breast cancer cells by Grb2 downregulation is correlated with inactivation of mitogen-activated protein kinase in EGFR, but not in ErbB2, cells

Increased breast cancer growth has been associated with increased expression of epidermal growth factor receptor (EGFR) and ErbB2 receptor tyrosine kinases (RTKs). Upon activation, RTKs may transmit their oncogenic signals by binding to the growth factor receptor bound protein-2 (Grb2), which in turn binds to SOS and activates the Ras/Raf/MEK/mitogen-activated protein (MAP) kinase pathway. Grb2 is important for the transformation of fibroblasts by EGFR and ErbB2; however, whether Grb2 is also important for the proliferation of breast cancer cells expressing these RTKs is unclear. We have used liposomes to deliver nuclease-resistant antisense oligodeoxynucleotides (oligos) specific for the GRB2 mRNA to breast cancer cells. Grb2 protein downregulation could inhibit breast cancer cell growth; the degree of growth inhibition was dependent upon the activation and/or endogenous levels of the RTKs. Grb2 inhibition led to MAP kinase inactivation in EGFR, but not in ErbB2, breast cancer cells, suggesting that different pathways might be used by EGFR and ErbB2 to regulate breast cancer growth.

Activation of Vav and Ras through the nerve growth factor and B cell receptors by different kinases

Engagement of the B-cell antigen receptor (BCR) or the nerve growth factor receptor (NGFR/TrkA) induces activation of multiple tyrosine kinases, resulting in phosphorylation of numerous intracellular substrates. We show that addition of NGF or anti-IgM antibody leads to the early tyrosine phosphorylation of p95(vav), which is expressed exclusively in hematopoietic cells; NGF, similar to crosslinking the BCR, also results in the rapid activation of Ras. The phosphorylation of Vav and activation of Ras triggered by NGF is mediated through Trk tyrosine kinase, whereas signaling through the BCR uses a different tyrosine kinase. We also show that NGF induces tyrosine phosphorylation of Shc and its association with Grb2. Vav and Ras with the adaptor proteins Shc and Grb2 appear to serve as a link between different receptor-mediated signaling pathways and, in human B cells, may play an important regulatory role in neuroimmune interactions.

Multiple binding sites in the growth factor receptor Xmrk mediate binding to p59fyn, GRB2 and Shc

Melanoma formation in Xiphoporus is initiated by overexpression of the EGFR-related receptor tyrosine kinase Xmrk (Xiphoporus melanoma receptor kinase). This receptor is activated in fish melanoma as well as in a melanoma-derived cell line (PSM) resulting in constitutive Xmrk-mediated mitogenic signaling. In order to define the underlying signaling pathway(s), triggered by the activated Xmrk receptor, we attempted to identify its physiological substrates. Examination of the Xmrk carboxyterminus for putative tyrosine autophosphorylation sites revealed the presence of potential binding motifs for GRB2 as well as for Shc. Binding of these adaptor proteins to the Xmrk receptor was detected in vitro and in cells expressing the mrk kinase. The GRB2 and Shc interactions with the receptor could be disrupted individually by phosphotyrosine peptides containing putative Xmrk autophosphorylation sites, indicating direct binding of both proteins. Recruitment of GRB2 by the constitutively activated Xmrk receptor led to strong MAP kinase activation in Xiphoporus melanoma cells. We also identified a high-affinity binding site for src-kinases (pYEDL) in the Xmrk carboxyterminus. Competition experiments with phosphopeptides comprising this site confirmed that it is used for high-affinity binding of Xiphoporus fyn (Xfyn) to Xmrk in melanoma cells. Thus, Xmrk can initiate different signaling pathways by using multiple substrate-binding sites to trigger proliferation of pigment cells.

Polyomavirus large T antigen induces alterations in cytoplasmic signalling pathways involving Shc activation

It has been extensively demonstrated that growth factors play a key role in the regulation of proliferation. Several lines of evidence support the hypothesis that for the induction of cell cycle progression in the absence of exogenous growth factors, oncogenes must either induce autocrine growth factor secretion or, alternatively, activate their receptors or their receptor substrates. Cells expressing polyomavirus large T antigen (PyLT) display reduced growth factor requirements, but the mechanisms underlying this phenomenon have yet to be explored. We conducted tests to see whether the reduction in growth factor requirements induced by PyLT was related to alterations of growth factor-dependent signals. To this end, we analyzed the phosphorylation status of a universal tyrosine kinase substrate, the transforming Shc adapter protein, in fibroblasts expressing the viral oncogene. We report that the level of Shc phosphorylation does not decrease in PyLT-expressing fibroblasts after growth factor withdrawal and that this PyLT-mediated effect does not require interaction with protein encoded by the retinoblastoma susceptibility gene. We also found that the chronic activation of the adapter protein is correlated with the binding of Shc to Grb-2 and with defects in the downregulation of mitogen-activated protein kinases. In fibroblasts expressing the nuclear oncoprotein, we also observed the formation of a PyLT-Shc complex that might be involved in constitutive phosphorylation of the adapter protein. Viewed comprehensively, these results suggest that the cell cycle progression induced by PyLT may depend not only on the direct inactivation of nuclear antioncogene products but also on the indirect induction, through the alteration of cytoplasmic pathways, of growth factor-dependent nuclear signals.

Fyn associates with Cbl and phosphorylates tyrosine 731 in Cbl, a binding site for phosphatidylinositol 3-kinase

We have investigated the interaction between Cbl and the Src-related tyrosine kinase Fyn. Fyn was observed to be constitutively associated with Cbl in lysates of several different cell types including the interleukin-3-dependent murine myeloid cell line 32Dcl3, and the prolactin-dependent rat thymoma cell line Nb2. Binding studies indicated that Cbl could bind to glutathione S-transferase (GST) fusion proteins encoding the unique, Src homology domain 3 (SH3), and SH2 domains of Fyn, Hck, or Lyn. Fusion proteins encoding either the SH3 or SH2 domains of Fyn bound to Cbl as effectively as the fusion protein encoding the unique, SH3, and SH2 domains of Fyn. The Fyn SH2 domain bound to both tyrosine-phosphorylated and nonphosphorylated Cbl, implying that this interaction might be phosphotyrosine-independent. Binding of the Fyn SH2 domain to Cbl was not disrupted by the addition of phosphotyrosine, phosphoserine, or phosphothreonine. A GST fusion protein encoding the proline-rich region of Cbl bound to Fyn present in a total cell lysate. Far Western blot analysis also indicated that the SH3 domain of Fyn bound preferentially to the proline-rich region of Cbl. The addition of [gamma-32P]ATP to either anti-Cbl immunoprecipitates or anti-Fyn immunoprecipitates resulted in the phosphorylation of both Cbl and Fyn as demonstrated by immunoprecipitation of the phosphorylated proteins with specific antisera. Fyn directly phosphorylated a GST fusion protein containing the C-terminal region of Cbl (GST-CBL-LZIP). In contrast, immunoprecipitated JAK2 was not able to phosphorylate this same region of Cbl. The GST-CBL-LZIP fusion protein contains a binding site for the SH2 domain of the p85 subunit of phosphatidylinositol 3-kinase, which mapped to Tyr731, which is present in the sequence YEAM. Mutation of Tyr731 in GST-CBL-LZIP eliminated binding of the p85 subunit of phosphatidylinositol 3-kinase and substantially reduced the phosphorylation of this fusion protein by Fyn, despite the presence of four other tyrosine residues in this fusion protein. These data are consistent with the hypothesis that Cbl represents a substrate for Src-like kinases that are activated in response to the engagement of cell surface receptors, and that Src-like kinases are responsible for the phosphorylation of a tyrosine residue in Cbl that may regulate activation of phosphatidylinositol 3-kinase.

Activation of ERK by Ca2+ store depletion in rat liver epithelial cells

In rat liver epithelial (WB) cells, Ca2+ pool depletion induced by two independent methods resulted in activation of extracellular signal-regulated protein kinase (ERK). In the first method, Ca2+ pool depletion by thapsigargin increased the activity of ERK, even when rise in cytosolic Ca2+ was blocked with the Ca2+ chelator BAPTA-AM. For the second method, addition of extracellular EGTA at a concentration shown to deplete intracellular Ca2+ pools also increased ERK activity. In each instance, ERK activation, as measured by an immunocomplex kinase assay, was greatly reduced by the tyrosine kinase inhibitor genistein, suggesting that Ca2+ store depletion increased ERK activity through a tyrosine kinase pathway. The intracellular Ca2+-releasing agent thapsigargin increased Fyn activity, which was unaffected by BAPTA-AM pretreatment, suggesting that Fyn activity was unaffected by increased cytosolic free Ca2+. Furthermore, depletion of intracellular Ca2+ with EGTA caused inactivation of protein phosphatase 2A and protein tyrosine phosphatases. ANG II-induced activations of Fyn, Raf-1, and ERK were augmented in cells pretreated with BAPTA-AM, but ANG II-induced expression of the dual-specificity phosphatase mitogen-activated protein kinase phosphatase-1 was blocked by BAPTA-AM pretreatment. Together these results indicate that ERK activity is regulated by the balance of phosphorylation vs. dephosphorylation reactions in intact cells and that the amount of Ca2+ stored in intracellular pools plays an important role in this regulation.

A Sos-derived peptidimer blocks the Ras signaling pathway by binding both Grb2 SH3 domains and displays antiproliferative activity

With the aim of interrupting the growth factor-stimulated Ras signaling pathway at the level of the Grb2-Sos interaction, a peptidimer, made of two identical proline-rich sequences from Sos linked by a lysine spacer, was designed using structural data from Grb2 and a proline-rich peptide complexed with its SH3 domains. The peptidimer affinity for Grb2 is 40 nM whereas that of the monomer is 16 microM, supporting the dual recognition of both Grb2 SH3 domains by the dimer. At 50 nM, the peptidimer blocks selectively Grb2-Sos complexation in ER 22 (CCL 39 fibroblasts overexpressing epidermal growth factor receptor) cellular extracts. The peptidimer specifically recognizes Grb2 and does not interact with PI3K or Nck, two SH3 domain-containing adaptors. The peptidimer was modified to enter cells by coupling to a fragment of Antennapedia homeodomain. At 10 microM, the conjugate inhibits the Grb2-Sos interaction (100%) and MAP kinase (ERK1 and ERK2) phosphorylation (60%) without modifying cellular growth of ER 22 cells. At the same concentration, the conjugate also inhibits both MAP kinase activation induced by nerve growth factor or epidermal growth factor in PC12 cells, and differentiation triggered by nerve growth factor. Finally, when tested for its antiproliferative activity, the conjugate was an efficient inhibitor of the colony formation of transformed NIH3T3/HER2 cells grown in soft agar, with an IC50 of around 1 microM. Thus, the designed peptidimers appear to be interesting leads to investigate signaling and intracellular processes and for designing selective inhibitors of tumorigenic Ras-dependent processes.

Mona, a novel hematopoietic-specific adaptor interacting with the macrophage colony-stimulating factor receptor, is implicated in monocyte/macrophage development

The production, survival and function of monocytes and macrophages are regulated by the macrophage colony-stimulating factor (M-CSF or CSF-1) through its tyrosine kinase receptor Fms. Binding of M-CSF results in Fms autophosphorylation on specific tyrosines that act as docking sites for intracellular signaling molecules containing SH2 domains. Using a yeast two-hybrid screen, we cloned a novel adaptor protein which we called 'Mona' for monocytic adaptor. Mona contains one SH2 domain and two SH3 domains related to the Grb2 adaptor. Accordingly, Mona interacts with activated Fms on phosphorylated Tyr697, which is also the Grb2-binding site. Furthermore, Mona contains a unique proline-rich region located between the SH2 domain and the C-terminal SH3 domain, and is apparently devoid of any catalytic domain. Mona expression is restricted to two hematopoietic tissues: the spleen and the peripheral blood mononuclear cells, and is induced rapidly during monocytic differentiation of the myeloid NFS-60 cell line in response to M-CSF. Strikingly, overexpression of Mona in bone marrow cells results in strong reduction of M-CSF-dependent macrophage production in vitro. Taken together, our results suggest an important role for Mona in the regulation of monocyte/macrophage development as controlled by M-CSF.

Mammalian Grb2 regulates multiple steps in embryonic development and malignant transformation

Proteins with SH2 and SH3 domains link tyrosine kinases to intracellular pathways. To investigate the biological functions of a mammalian SH2/SH3 adaptor, we have introduced a null mutation into the mouse gene for Grb2. Analysis of mutant embryonic stem cells, embryos, and chimeras reveals that Grb2 is required during embyrogenesis for the differentiation of endodermal cells and formation of the epiblast. Grb2 acts physiologically as an adaptor, since replacing the C terminus of the Ras activator Sos1 with the Grb2 SH2 domain yields a fusion protein that largely rescues the defects caused by the Grb2 mutation. Furthermore, Grb2 is rate limiting for mammary carcinomas induced by polyomavirus middle T antigen. These data provide genetic evidence for a mammalian Grb2-Ras signaling pathway, mediated by SH2/SH3 domain interactions, that has multiple functions in embryogenesis and cancer.

Innovative treatment programs against cancer. I. Ras oncoprotein as a molecular target

Modulation of Ras function may provide a novel means by which cancer cells with oncogenic mutations can be sensitized to chemotherapeutic or radiotherapeutic regimens. Moreover, cancer cells without ras oncogene mutations can also be eliminated by compounds that interfere with the mevalonate pathway, which is more fundamental to mitogenesis because it allows the synthesis of sterol and nonsterol lipids and without which many Ras-related proteins and nuclear lamins would not be prenylated and functional.

Grb2 interaction with MEK-kinase 1 is involved in regulation of Jun-kinase activities in response to epidermal growth factor

Epidermal growth factor (EGF) receptor was shown to be involved in the activation pathway of the stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) cascade not only by EGF, but also by UV radiation or osmotic stress. This paper describes a specific interaction between the COOH-terminal SH3 domain of Grb2 and the NH2-terminal regulatory domain of MEKK1 in ER22 cells overexpressing the EGF receptor. This interaction results in the formation of a constitutive complex between Grb2 and MEKK1 in both proliferating and resting cells. EGF stimulation causes this complex to be rapidly and transiently recruited by Shc proteins. The subsequent release of the Grb2-MEKK1 complex from Shc proteins correlates with JNK activation. Transfection of the NH2-terminal regulatory domain of MEKK1 specifically inhibits EGF-dependent JNK activation indicating that Grb2 is involved in MEKK1 activation. Thus, adaptor proteins have a new role in the regulation of the SAPK/JNK cascade after EGF stimulation.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

Serum and insulin induce a Grb2-dependent shift in agonist affinity of beta-adrenergic receptors

Beta-adrenergic receptors transduce catecholamine binding to activation of adenylylcyclase, a response counter-regulated by insulin. Insulin stimulates tyrosine phosphorylation of Tyr 350/354, which abolishes the catecholamine response. Phosphorylation of Try 350/354 creates a Src homology 2 (SH2) domain on the beta2-adrenergic receptor and the binding of adaptor protein Grb2 to this SH2 domain of the beta-adrenergic receptor takes place in an insulin-dependent manner. In membranes from serum-deprived S49 mouse lymphoma cells, GTPgammaS yields the well-known agonist-specific shift in agonist affinity for beta2-adrenergic receptors. The agonist-specific shift is observed in cell membranes either in the absence or in the presence of exogenously added purified Grb2. In membranes for serum-fed cells, in contrast, the addition of Grb2 induces an agonist-specific shift in receptor affinity that mimics addition of GTPgammaS to the membranes. The ability of the Grb2 to induce an agonist-specific shift in the membranes from serum-fed cells was abolished equally effectively either by competition with phosphopeptide harbouring the (p)YVNV motif or by disruption of the SH2 domain of added Grb2. Challenging Chinese hamster ovary cells with insulin (100 nM) for 30 min enabled Grb2 to induce an agonist-specific shift in agonist affinity for beta2-adrenergic receptors, suggestive of uncoupling of the receptors from G proteins. The insulin-dependent Grb2 effect on receptor-G-protein coupling was sensitive to competition by the pYVNY phosphopeptide and to disruption of the SH2 domain of Grb2. These data provide a biochemical link between the ability of insulin to counter-regulate catecholamine stimulation of cyclic AMP accumulation and the phosphorylation of the beta-adrenergic receptor, consequent biding of the adaptor molecule Grb2 and disruption of receptor-G-protein coupling.

SLP-76-Cbl-Grb2-Shc Interactions in FcγRI Signaling

SLP-76 and Cbl are complex adapter proteins that have the capacity to bind to smaller adapter proteins, such as Grb2, which subsequently binds the nucleotide exchange protein Sos in the transmission of intracellular signals. SLP-76, Cbl, Shc, and Grb2 have been implicated in immunoreceptor tyrosine-based activation motif (ITAM) signaling, leading to activation of Ras. However, their mechanism of action has not been determined. To date, there have been no reports of SLP-76 involvement in FcγRI-receptor signaling and no data exist for an interaction between Cbl, Shc, and SLP-76 in vivo. We provide evidence that SLP-76, Cbl, and Shc are tyrosine phosphorylated on FcγRI-receptor stimulation and are associated with the adapter protein Grb2 in γ-interferon–differentiated U937 cells (U937IF). The interactions between SLP-76 and Cbl and SLP-76 and Grb2 are present in resting U937IF cells. However, the interaction between SLP-76 and Grb2 becomes augmented twofold on FcγRI-receptor aggregation. Our results provide the first evidence for a phosphorylation-dependent interaction between SLP-76 and Shc, induced at least 10-fold on FcγRI receptor stimulation. Our data indicate that a significant portion of a multimolecular complex containing Cbl, SLP-76, Shc, and Grb2 is distinct from a trimolecular complex containing the Ras guanine nucleotide exchanger Sos, Shc, and Grb2. FcγRI-induced tyrosine phosphorylation of SLP-76, Cbl, Shc, and the highly induced SLP-76-Shc interaction provide the first evidence that SLP-76 and Cbl are involved in FcγRI signaling and suggest a functional significance for these interactions in FcγRI signal relay in the control of Ras in myeloid cells.

© 1998 by The American Society of Hematology.

Grb2 forms an inducible protein complex with CD28 through a Src homology 3 domain-proline interaction

CD28 provides a costimulatory signal that results in optimal activation of T cells. The signal transduction pathways necessary for CD28-mediated costimulation are presently unknown. Engagement of CD28 leads to its tyrosine phosphorylation and subsequent binding to Src homology 2 (SH2)-containing proteins including the p85 subunit of phosphatidylinositol 3'-kinase (PI3K); however, the contribution of PI3K to CD28-dependent costimulation remains controversial. Here we show that CD28 is capable of binding the Src homology 3 (SH3) domains of several proteins, including Grb2. The interaction between Grb2 and CD28 is mediated by the binding of Grb2-SH3 domains to the C-terminal diproline motif present in the cytoplasmic domain of CD28. While the affinity of the C-terminal SH3 domain of Grb2 for CD28 is greater than that of the N-terminal SH3 domain, optimal binding requires both SH3 domains. Ligation of CD28, but not tyrosine-phosphorylation, is required for the SH3-mediated binding of Grb2 to CD28. We propose a model whereby the association of Grb2 with CD28 occurs via an inducible SH3-mediated interaction and leads to the recruitment of tyrosine-phosphorylated proteins such as p52(shc) bound to the SH2 domain of Grb2. The inducible interaction of Grb2 to the C-terminal region of CD28 may form the basis for PI3K-independent signaling through CD28.

Roles of Lck, Syk and ZAP-70 tyrosine kinases in TCR-mediated phosphorylation of the adapter protein Shc

The adapter protein Shc has been implicated in mitogenic signaling via growth factor receptors, antigen receptors and cytokine receptors. Recent studies have suggested that tyrosine phosphorylation of Shc may play a key role in T lymphocyte proliferation via interaction of phosphorylated Shc with downstream molecules involved in activation of Ras and Myc proteins. However, the sites on Shc that are tyrosine phosphorylated in response to TCR engagement and the ability of different T cell tyrosine kinases to phosphorylate Shc have not been defined. In this report, we show that during TCR signaling, the tyrosines Y239, Y240 and Y317 of Shc are the primary sites of tyrosine phosphorylation. Mutation of all three tyrosines completely abolished tyrosine phosphorylation of Shc following TCR stimulation. Our data also suggest that multiple T cell tyrosine kinases contribute to tyrosine phosphorylation on Shc. In T cells, CD4/Lck-dependent tyrosine phosphorylation on Shc was markedly diminished when Y317 was mutated, suggesting a preference of Lck for the Y317 site. The syk-family kinases (Syk and ZAP-70) were able to phosphorylate the Y239 and Y240 sites, and less efficiently the Y317 site. Moreover, co-expression of Syk or ZAP-70 with Lck resulted in enhanced phosphorylation of Shc on all three sites, suggesting a synergy between the syk-family and scr-family kinases. Of the two potential Grb2 binding sites (Y239 and Y317), Y239 appears to play a greater role in recruiting Sos through Grb2. These studies have implications for Ras activation and mitogenic signaling during T cell activation.

Growth Factor Receptor-Bound Protein 2 (Grb2) Association with Hemopoietic Specific Protein 1: Linkage Between Lck and Grb2

To analyze the growth factor receptor-bound protein 2 (Grb2) signaling pathway in lymphoid cells, we used expression cloning to isolate the genes encoding proteins that associate with Grb2. We find that the Src homology 3 domains of Grb2 directly associate, in vitro and in vivo, with murine hemopoietic specific protein 1 (HS1), a protein identical to Lck-binding protein 1. Because HS1 associates with the p56lck and p59lyn tyrosine kinases in vitro and in vivo, and becomes tyrosine phosphorylated upon various receptor stimulations, our present data suggest that HS1 mediates linkage between Lck or Lyn and Grb2 in lymphoid lineage cells.

Biochemical studies of the mechanism of action of the Cdc42-GTPase-activating protein

The small GTP-binding proteins Rac, Rho, and Cdc42 were shown to mediate a variety of signaling pathways including cytoskeletal rearrangements, cell-cycle progression, and transformation. Key to the proper function of these GTP-binding proteins is an efficient shut-off mechanism that ensures the decay of the signal. Regulatory proteins termed GAPs (GTPase-activating proteins) enhance the intrinsic GTP hydrolysis of the GTP-binding proteins, thereby ensuring signal termination. We have used site-specific mutagenesis to elucidate the limit domain for GAP activity in Cdc42-GAP, and show that in addition to the known GAP-homology domain (three conserved boxes), a C-terminal region outside that domain is also essential for GAP activity. In addition, we have replaced the conserved arginine (Arg305), which was suggested by structural studies to be a key catalytic residue, with an alanine and found that the R305A Cdc42-GAP mutant has a greatly diminished catalytic capacity but is still able to bind Cdc42 with high affinity. Thus, a key catalytic role for this residue is confirmed. However, we also present evidence for the involvement of an additional residue(s), since the R305A Cdc42-GAP mutant still exhibits measurable activity. Some of this residual activity might result from a neighboring arginine, since a double mutant R305A/R306A shows a further decrease in catalytic activity.

Activation of extracellular signal-regulated kinase 2 by a novel Abl-binding protein, ST5

The human ST5 gene encodes three proteins with predicted molecular masses of 126, 82, and 70 kDa. These widely expressed proteins share a C-terminal region that bears significant sequence homology to a group of GDP/GTP exchange proteins for the Rab3 family of small GTP binding proteins. The N-terminal region of the largest ST5 protein, p126, contains two proline-rich sequences, PR1 and PR2, with consensus motifs similar to Src homology 3 (SH3) binding regions and to mitogen-activated protein kinase (MAPK) phosphorylation sites. Based on these properties, we sought to investigate the activity of ST5 proteins in signal transduction pathways. In vitro, p126 displayed preferential binding to c-Abl SH3, as compared with other SH3 domains. This interaction was mediated by the PR2 sequence. In vivo, expression of p126, but not p82 or p70, activated MAPK/ERK2 in response to EGF in COS-7 cells. Expression of c-Abl with p126 greatly enhanced this activity. Deletion of PR1 blocked the ability of p126 to activate ERK2. Deletion of PR2 did not affect the basal activity, but blocked the stimulatory effect of c-Abl. Whereas p82 expression had no effect on ERK2 activation by p126, p70 completely abrogated this activity. These observations suggest that ST5 can function as a signaling protein and can provide a link between c-Abl and ERK2.

Identification and characterization of the major phosphorylation sites of the B-type natriuretic peptide receptor

C-type natriuretic peptide (CNP) is a newly discovered factor that stimulates vasorelaxation and inhibits cell proliferation. Natriuretic peptide receptor-B (NPR-B) is the primary signaling molecule for CNP. Recently, the guanylyl cyclase activity of NPR-B was shown to correlate with its phosphorylation state, and it was suggested that receptor dephosphorylation is a mechanism of desensitization. We now report the identification and characterization of the major NPR-B phosphorylation sites. Mutagenesis and comigration studies using synthetic phosphopeptides were employed to identify five residues (Ser-513, Thr-516, Ser-518, Ser-523, and Ser-526) within the kinase homology domain that are phosphorylated when NPR-B is expressed in human 293 cells. Mutation of any of these residues to alanine reduced the receptor's phosphorylation state and CNP-dependent guanylyl cyclase activity. The reductions were not explained by decreases in receptor protein level as indicated by immunoblot analysis and determinations of cyclase activity in the absence of CNP or in the presence of detergent. Elimination of all of the phosphorylation sites resulted in a completely dephosphorylated receptor whose CNP-dependent cyclase activity was decreased by >90%. However, unlike NPR-A, the dephosphorylated receptor was not completely unresponsive to hormone. Finally, two additional residues (Gly-521 and Ser-522) were identified that when mutated to alanine reduced the overall phosphorylation state and hormone responsiveness of the receptor without abolishing the phosphorylation of a specific site. These data indicate that phosphorylation of the kinase homology domain is a critical event in the regulation of NPR-B.

Cbl: complex formation and functional implications

Chbl, a 120-kDa proto-oncogene product, whose gene was first identified as part of a transforming gene of a murine retrovirus and whose expression is predominant in haematopoietic cells, consists of an amino-terminal transforming region, a zinc Ring finger, multiple proline-rich stretches, and several potential phosphotyrosine-containing motifs. Cbl is rapidly tyrosine-phosphorylated in response to stimulation of a variety of cell-surface receptors and becomes associated with a number of intracellular signalling molecules such as protein tyrosine kinases, phosphatidylinositol 3-kinase, Crk, and 14-3-3 proteins through different protein-interacting modules, leading to the formation of multimolecular signalling complexes. Cbl and its transforming mutants have been shown to display both negative and positive regulatory activities in protein tyrosine kinase- and Ras-mediated signalling pathways. Nevertheless, the exact biological function of this adaptor protein remains largely unknown. The present review summarizes recent progress in our understanding of the structure, regulation and biological function of Chl and defines open questions for future research.

Involvement of growth factor receptor-bound protein-2 in rat hepatocyte growth

Growth factor receptor-bound protein-2 (GRB-2) is a protein linking receptor tyrosine kinase and Sos (Son of Sevenless gene; Ras GDP/GTP exchange protein), leading to activation of the Ras-mitogen-activated protein kinase (MAPK) cascade. So far, it remains unclear how GRB-2 plays a role in signal transduction pathways evoked by hepatotrophic factors. This study was attempted to evaluate the involvement of GRB-2 in signalling in rat hepatocyte growth. Using rat cultured hepatocytes stimulated by hepatotrophic factors and regenerating livers after partial hepatectomy (PH) we examined GRB-2-mediated linkage of hepatotrophic factor receptors to signal transducing molecules such as Sos or dynamin-II by immunoprecipitation and western blot analysis. In primary cultured hepatocytes stimulated with hepatocyte growth factor (HGF) or epidermal growth factor (EGF), GRB-2 linked HGF receptor or EGF receptor, respectively, to Sos which activated the mitogen-activated protein kinase (MAPK) cascade. In contrast, in primary cultured hepatocytes stimulated with insulin, GRB-2 linked insulin receptor substrate-1 (IRS-1) to dynamin-II as well as Sos. In the early phase after PH, GRB-2 activated the Ras-MAPK cascade by linking HGF receptor, IRS-1, or EGF receptor to Sos. In the late phase after PH, a complex of IRS-1-GRB-2 associated with dynamin-II, indicating that GRB-2 may transduce signals from IRS-1 to dynamin-II. We conclude that GRB-2 may play a role in transmitting signals from hepatotrophic factors to not only MAPK but also to other signalling pathways in hepatocyte growth.

Insulin receptor-mediated dissociation of Grb2 from Sos involves phosphorylation of Sos by kinase(s) other than extracellular signal-regulated kinase

The Ras signaling pathway is rapidly activated and then down-regulated following stimulation of multiple cell-surface receptors including the insulin receptor (IR). Much recent attention has focused on elucidating the mechanism of Ras inactivation following IR engagement. Previous data suggest that IR-mediated serine/threonine phosphorylation of the Ras guanine nucleotide exchange factor Sos correlates with its decreased affinity for the adapter protein Grb2. This phosphorylation-induced disassembly of the Grb2.Sos complex is thought to be responsible, at least in part, for diminishing Ras activity in Chinese hamster ovary cells. In this report, we confirm the causal relationship between Sos phosphorylation and Grb2/Sos dissociation. We then examine several putative phosphorylation sites of Sos that could potentially regulate this event. Since a number of reports suggest that extracellular signal-regulated kinase (ERK) phosphorylates Sos, we generated a Sos mutant lacking all seven canonical phosphorylation sites for ERK. This mutant is a poor substrate of activated ERK in vitro and fails to undergo a change in its electrophoretic mobility following IR stimulation. It is, however, phosphorylated after IR stimulation when expressed in Chinese hamster ovary cells. Interestingly, the mutant protein still dissociates from Grb2 following insulin stimulation, suggesting that ERK is not the kinase responsible for regulating the stability of the Grb2.Sos complex.

Phosphatidylinositol 3-kinase is required for the regulation of hepatitis B surface antigen production and mitogen-activated protein kinase activation by insulin but not by TPA

Insulin suppresses hepatitis B surface antigen (HBsAg) gene expression and stimulates cell proliferation in human hepatoma Hep3B cells. 12-O-tetradecanoyl phorbol-13-acetate, TPA, has been demonstrated to mimic insulin actions in these cells. We examined the role of phosphatidylinositol 3-kinase (PI 3-kinase) in the signaling pathways of insulin and TPA towards these two biological phenomena in Hep3B cells. The pre-treatment of 5 microM of wortmannin diminished insulin suppressed HBsAg production and completely abolished insulin stimulated cell proliferation. However, wortmannin had no effect on TPA actions in both HBsAg suppression and cell growth stimulation. We further investigated the effect of wortmannin in mitogen-activated protein kinases (MAPKs) activation induced by insulin or TPA. After the pretreatment of wortmannin, insulin activated MAPKs was completely blocked, but TPA was still capable to activate MAPKs. These results suggest that PI 3-kinase is involved in insulin actions but not in TPA effects, and allow us to dissociate the signaling pathways of insulin and TPA in human hepatoma Hep3B cells.

What does insulin do to Ras?

The Ras pathway lies in the center of signalling cascades of numerous growth-promoting factors. The Ras pathway appears to connect signalling events that begin at the plasma membrane with nuclear events. Insulin is one of the major stimulants of the Ras signalling pathway. The influence of insulin on this pathway consists of five important events: (1) p21Ras activation is promoted by insulin stimulation of the guanine nucleotide exchange factor, Sos, resulting in increased GTP-loading of p21Ras; (2) p21Ras deactivation involves the hyperphosphorylation of Sos; (3) insulin increases farnesyltransferase (FTase) activity that farnesylates p21Ras; (4) increased amounts of farnesylated p21Ras translocate to the plasma membrane where they can be activated by other growth-promoting agents; and (5) cellular responses to other growth factors are potentiated by insulin-stimulated pre-loading of the plasma membrane with farnesylated p21Ras.

PDGF-induced glycosaminoglycan synthesis is mediated via phosphatidylinositol 3-kinase

Platelet-derived growth factor (PDGF)-BB has been shown previously to increase glycosaminoglycan (GAG) synthesis but not DNA synthesis in freshly isolated fetal lung fibroblasts. In the present study, we found that PDGF-BB also enhanced 35SO4 incorporation into the small, soluble proteoglycan biglycan without affecting biglycan's core protein mRNA expression, suggesting that PDGF-BB mainly affects GAG chain elongation and/or sulfation. PDGF-BB-stimulated GAG synthesis was abrogated by tyrphostin 9, a PDGF receptor-associated tyrosine kinase inhibitor, implying that the stimulatory effect is mediated via the PDGF beta-receptor (PDGFR). The intracellular signal transduction pathways that mediate PDGF-BB-stimulated GAG synthesis in fetal lung fibroblasts were investigated. On ligand-induced tyrosine phosphorylation, PDGFR associated with phospholipase C (PLC)-gamma 1, Ras GTPase activating protein (RasGAP), and phosphatidylinositol 3-kinase (PI3K) but not with the Syp-growth factor receptor-bound protein 2-Son of Sevenless complex. Association of PDGFR with PLC-gamma 1 and RasGAP followed by their tyrosine phosphorylation failed, however, to activate PLC-gamma 1, protein kinase C (PKC), and Ras. Neither a PLC-gamma inhibitor, U-73122; a PKC inhibitor, calphostin C; nor a mitogen-activated protein kinase kinase inhibitor, PD-98059, inhibited PDGF-BB-induced GAG synthesis. In contrast, PDGF-BB stimulation triggered PDGFR-associated PI3K activity. Both PDGF-BB-induced PI3K activation and GAG synthesis were abolished by the PI3K inhibitors wortmannin and LY-294002. The results suggest that PI3K is a downstream mediator of PDGF-BB-stimulated GAG synthesis in fetal rat lung fibroblasts.

Genetic engineering of proteins with cell membrane permeability

The discovery of methods for generating proteins with inherent cell membrane-translocating activity will expand our ability to study and manipulate various intracellular processes in living systems. We report a method to engineer proteins with cell-membrane permeability. After a 12-amino acid residue membrane-translocating sequence (MTS) was fused to the C-terminus of glutathione S-transferase (GST), the resultant GST-MTS fusion proteins were efficiently imported into NIH 3T3 fibroblasts and other cells. To explore the applicability of this nondestructive import method to the study of intracellular processes, a 41-kDa GST-Grb2SH2-MTS fusion protein containing the Grb2 SH2 domain was tested for its effect on the epidermal growth factor (EGF)-stimulated signaling pathway. This fusion protein entered cells, formed a complex with phosphorylated EGF receptor (EGFR), and inhibited EGF-induced EGFR-Grb2 association and mitogen-activated protein kinase activation.

Interaction of phospholipase C gamma 1 via its COOH-terminal SRC homology 2 domain with synaptojanin

The role of the phospholipase C gamma 1 (PLC gamma 1) in signal transduction was investigated by characterizing its interactions with proteins that may represent components of a novel signalling pathway. A 145-kDa protein that binds SH2 domain of PLC gamma 1 was purified from rat brain. The sequence of peptide derived from the purified binding protein now identify it as synaptojanin, a nerve terminal protein that has been implicated in the endocytosis of fused synaptic vesicles and shown to be a member of the inositol polyphosphate 5-phosphatase family. We demonstrate here stable association of PLC gamma 1 with synaptojanin, a protein that not only binds carboxyl terminal SH2 domain of PLC gamma 1, but also inhibits PLC gamma 1 activity.

Epidermal growth factor receptor and the adaptor protein p52Shc are specific substrates of T-cell protein tyrosine phosphatase

T-cell protein tyrosine phosphatase (TCPTP) exists as two forms generated by alternative splicing: a 48-kDa endoplasmic reticulum (ER)-associated form (TC48) and a 45-kDa nuclear form (TC45). To identify TCPTP substrates, we have generated substrate-trapping mutants, in which the invariant catalytic acid of TCPTP (D182) is mutated to alanine. The TCPTP D182A substrate-trapping mutants were transiently overexpressed in COS cells, and their ability to form complexes with tyrosine-phosphorylated (pTyr) proteins was assessed. No pTyr proteins formed complexes with wild-type TCPTP. In contrast, TC48-D182A formed a complex in the ER with pTyr epidermal growth factor receptor (EGFR). In response to EGF, TC45-D182A exited the nucleus and accumulated in the cytoplasm, where it bound pTyr proteins of approximately 50, 57, 64, and 180 kDa. Complex formation was disrupted by vanadate, highlighting the importance of the PTP active site in the interaction and supporting the characterization of these proteins as substrates. Of these TC45 substrates, the approximately 57- and 180-kDa proteins were identified as p52Shc and EGFR, respectively. We examined the effects of TC45 on EGFR signaling and observed that it did not modulate EGF-induced activation of p42Erk2. However, TC45 inhibited the EGF-induced association of p52Shc with Grb2, which was attributed to the ability of the PTP to recognize specifically p52Shc phosphorylated on Y239. These results indicate that TC45 recognizes not only selected substrates in a cellular context but also specific sites within substrates and thus may regulate discrete signaling events.